Title of Invention	RESIN-COATED GALVANIZED STEEL SHEET
Abstract	Disclosed is a resin-coated galvanized steel sheet adopted as a member locally in contact with a heat source. This includes a base steel sheet, galvanized coatings each in a mass of 70 g/m2 or more on both sides of the base steel sheet, and a resin film on at least one of the galvanized coatings. The base steel sheet has a C content of 0. 0010% to 0. 04% (by mass) , a Si content of 0.2% or less, a Mn content of 0.1% to 0.80%, and an Al content of 0.01% to 0.1%, with the remainder including iron and inevitable impurities. The resin film has an integrated emissivity of infrared radiation at wavelengths from 4.5 to 15.4 µm of 0. 60 or more when heated at 100°C. This excels in thermal conductivity and heat dissipation ability and is useful as a material for electronic component locally in contact with heat source.

Title of Invention

RESIN-COATED GALVANIZED STEEL SHEET

Abstract

Disclosed is a resin-coated galvanized steel sheet adopted as a member locally in contact with a heat source. This includes a base steel sheet, galvanized coatings each in a mass of 70 g/m2 or more on both sides of the base steel sheet, and a resin film on at least one of the galvanized coatings. The base steel sheet has a C content of 0. 0010% to 0. 04% (by mass) , a Si content of 0.2% or less, a Mn content of 0.1% to 0.80%, and an Al content of 0.01% to 0.1%, with the remainder including iron and inevitable impurities. The resin film has an integrated emissivity of infrared radiation at wavelengths from 4.5 to 15.4 µm of 0. 60 or more when heated at 100°C. This excels in thermal conductivity and heat dissipation ability and is useful as a material for electronic component locally in contact with heat source.

Full Text	RESIN-COATED GALVANIZED STEEL SHEET FIELD OF THE INVENTION The present invention relates to resin-coated galvanized steel sheets that are highly thermally conductive and diffuse heat effectively. Specifically, it relates to a resin-coated galvanized steel sheet advantageously useful as a material for an electronic component that is locally in contact with a heat source and thereby requires high thermal conductivity and satisfactory heat radiation ability. Examples of such ; electronic components (including electrical components and components! of optical devices) include heat sinks, back chassis of plasma display: television sets, and metallic cabinets (casings) housing electronic components containing heat sources. BACKGROUND OF THE INVENTION With the slimming down of liquid crystal display television sets and plasma display panel television sets, thermal problems become more and more serious. Under these circumstances, manufacturers of such electrical apparatuses put much effort into lowering the temperatures: of the product apparatuses during operation typically by 1°C and use! expensive components taking measures against heat problems. Such heati problems are especially serious in components locally in contact with! heat sources, such as back chassis of plasma display panel television; sets (PDP-TV sets) that are locally in contact with a heat source plasma! device with the interposition of a glass panel. Of heat dissipation! properties, heat conduction plays a most important role in such components which are in contact with heat sources and should take measures against heat problems. Aluminum-based components are currently widely used as components taking measures against heat problems, because aluminum conducts heat satisfactorily. On the other hand, cost competition in the market of thin-screen television sets such as PDP-TV sets is becoming fierce, and the replacement of expensive aluminum components by inexpensive steel components is expected to achieve considerable cost reduction of products. However, not every steel sheet will do, because aluminum-based components have higher heat conductivities than those of steel ; components. Specifically, the replacement of aluminum-based components by steel components needs steel sheets that can lower the temperature of heat source more than known steel sheets even if only slightly. Various proposals have been made on steel components to be adopted! in such electronic components. Typically, Japanese Unexamined Patent; Application Publication (JP-A) No. 2006-307260 proposes a technique relating to an inexpensive plasma display panel fixing plate that hasi excellent corrosion resistance or excellent heat dissipation ability. According to this technique, a slab of continuous cast steel is hot-rolled at a high rolling reduction, quenched to form a hot-rolled steel sheet having a structure in which martensite is dispersed in i ferrite, the hot-rolled steel sheet is subjected sequentially to primary cold rolling, annealing, and secondary cold rolling, and the resulting steel sheet is galvanized to form a galvanized coating thereon, and a! layer of chemical conversion coating is formed on the galvanized coating so as to improve the corrosion resistance or heat dissipation ability. This technique, however, does not consider the improvement of heat conductivity of base steel sheet and thereby does not give sufficient advantageous effects for improving the heat dissipation ability, Japanese Unexamined Patent Application Publication (JP-A) No. 2005-222042 proposes a technique relating to a chassis assembly for plasma display device, which assembly includes a chassis having a heat conductivity of from 10 W/mK to 100 W/mK. The document mentions that one having a higher heat conductivity is more advantageous in heat dissipation ability. This technique, however, does not always exhibit sufficient advantageous effects on the improvement of heat dissipation ability, because the technique has been made in order to reduce discharge delay phenomenon caused by temperature drop. SUMMARY OF THE INVENTION The present invention has been made under these circumstances, and an object of the present invention is to provide a resin-coated galvanized steel sheet that exhibits high thermal conductivity and superior heat dissipation ability and is useful as a material for an electronic component which will be locally in contact with a heat source. Specifically, according to an embodiment of the present invention,: there is provided a resin-coated galvanized steel sheet which includes a base steel sheet; a first galvanized coating present on one side of the base steel sheet; a second galvanized coating present on the other side of the base steel sheet; and a resin film present on at least ond of the first and second galvanized coatings, in which the base steel sheet has a carbon (C) content of from 0.0010 to 0.040 percent by mass (hereinafter the contents will be simply expressed in "%"), a silicon (Si) content of 0,2?> or less, a manganese (Mn) content of from 0.1% to 0.80ft, and an aluminum (Al) content of from 0.01% to 0,1%, with the remainder including iron and inevitable impurities, each of the first and second galvanized coatings is present in a mass of coating of 70 g/m~ or more, and the resin film has an integrated emissivity of infrared radiation at wavelengths ranging from 4.5 to 15.4 jim of 0.60 or more when the resin film is heated at 100°C. The base steel sheet in the resin-coated galvanized steel sheet may further contain titanium (Ti) in a content of from 0.001% to 0.20%, and this further improves characteristic properties of the resin-coated galvanized steel sheet. The resin film in the resin-coated galvanized steel sheet preferably has a thickness of 50 \xm or less to further increase the heat dissipation ability. The resin-coated galvanized steel sheet is advantageous when adopted to electronic components. The resin-coated galvanized steel sheet is especially advantageously adopted to back chassis of PDP-TV sets because such back chassis have wide areas to be used. According to the present invention, there is provided a resin-coatedi galvanized steel sheet that exhibits high thermal conductivity and superior heat dissipation ability and is useful as a material for an; electronic component which is to be locally in contact with a heat source.; This is achieved by adequately specifying the chemical composition ofj the base steel sheet, adequately controlling the mass of each of the: first and second galvanized coatings present on surfaces (both surfaces) '> of the base steel sheet, and adequately controlling the emissivity of the resin film covering at least one of the first and second galvanized coatings. BRIEF DESCRIPTION OF THE DRAWINGS FIGS. lA, IB, and IC are a plan view, a front view, and a cross-sectional view along the lines A-A in FIG. IB, respectively, of a test system for the testing of heat dissipation ability; and FIG, 2 is a schematic diagram of the structure of the test system for the testing of heat dissipation ability. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present inventors made intensive investigations from various angles to provide a resin-coated galvanized steel sheet (hereinafter also referred to as a "surface-treated steel sheet") that exhibits high thermal conductivity and superior heat dissipation ability. Initially, they made investigations about the relationship between the types of compositions and the heat conductivity of a base steel sheet and have found that the compositions such as carbon (C), manganese (Mn), and aluminum (Al) affect the heat conductivity of the steel sheet. They; have also found that the radiation performance can be further improved by forming galvanized coatings in larger masses on surfaces (both surfaces) of the base steel sheet; that the radiation performance can be furthermore improved by covering at least one side (the side opposite to a heat source) of the thickly galvanized steel sheet with a resin film having a high emissivity;" and that the resulting surface-treated steel sheet can achieve the above object. The present invention has been made based on these findings. Conditions specified in the present invention will be described below. Chemical Composition of Base Steel Sheet A base steel sheet for use in the present invention should have: adequately controlled chemical compositions. The compositions are specified for the reasons below. Carbon (C) content: 0.0010% to 0.040% Carbon (C) element mostly affects the heat conductivity of the base steel sheet. The heat conductivity increases with a decreasing carbon content, and the carbon content should therefore be 0.040% or less. The carbon content is preferably 0.03% or less, and more preferably 0.02% or less. Carbon element, however, is useful for ensuring satisfactory: strength when the steel is formed into a thin steel sheet. When a steel sheet having an insufficient strength is used as a large-size electronic component such as back chassis, it may be difficult to support the structure or to maintain flatness of the steel sheet. In this case, the strength necessary as back chassis should be ensured by using other elements in combination. To ensure certain strength usable typically! as back chassis, the carbon content should be 0.0010% or more, and iS: preferably 0.0015% or more, and more preferably 0.0020% or more. Silicon (Si) content: 0.2% or less Silicon (Si) element affects the wettability with galvanized i coatings. The Si content should be 0.2% or less to maintain satisfactory} wettability with galvanized coatings. The Si content is preferably I 0.18% or less, and more preferably 0.16% or less. The Si content herein! is preferably minimized to the extent possible. Manganese (Mn) content: 0.10% to 0.80% i Manganese (Mn) element affects the heat conductivity of the base steel sheet. The heat conductivity increases with a decreasing Mn content, and the Mn content should therefore be 0. 80?, or less . Mn element, however, also improves the hardenability, and the Mn content should be 0.10% or more to ensure satisfactory strength of the steel sheet. The: Mn content is preferably 0.12% or more, and more preferably 0.14% or more. Aluminum (Al) content: 0.01% to 0.1% Aluminum (Al) element affects the heat conductivity of the base steel sheet. The Al content should be 0.1% or less to maintain satisfactory heat conductivity. The Al content is preferably 0.08% or less, and more preferably 0.05% or less. However, Al element also acts: as a deoxidizer, and its content should be 0.01% or more to exhibit the effect effectively. The Al content is preferably 0.02% or more, and more preferably 0.03% or more. The preferred fundamental compositions of the base steel sheet are as above, and the remainder includes iron and inevitable impurities. Representative examples of the inevitable impurities include phosphorus (P), sulfur (S), and nitrogen (N) . These inevitable impurities are preferably controlled as follows. Sulfur (S) content: 0.030% or less Sulfur (S) element belongs to inevitable impurities, combines with; manganese (Mn) to adversely affect the ductility of the steel sheet,' and is preferably minimized. From these viewpoints, the sulfur content! is preferably 0.030% or less, more preferably 0.025% or less, and furthermore preferably 0.020% or less. Sulfur, when contained within; this range, may not affect the heat conductivity of the base steel sheet.: Phosphorus (P) content: 0.20% or less Phosphorus (P) element belongs to inevitable impurities, promotes the grain boundary fracture due to grain boundary segregation, and its content is desirably minimized. From this viewpoint, the phosphorus content is preferably 0.20?, or less, more preferably 0.15% or less, and furthermore preferably 0.10% or less. Nitrogen (N) content: 0.020% or less Nitrogen (N) element belongs to inevitable impurities, forms coarse inclusions, such as TiN, to impair the toughness of the base steel sheet, and is preferably minimized. From this viewpoint, the nitrogen content is preferably 0.020% or less, more preferably 0.015% or less, and furthermore preferably 0.010% or less. Nitrogen, when contained within this range, may not affect the heat conductivity of the base steel sheet. Exemplary inevitable impurities other than above include Cr, Ni, Mo, and Cu. These elements may be contained as impurities in the base steel sheet each in a content of 0.1% or less during regular production processes, and within this range, do not affect the heat conductivity. These elements, however, improve the hardenability and can be positively added to improve characteristic properties within such a range not adversely affecting the heat conductivity. For example, these elements may be added each in a content of about 1,0% or less. Where necessary, the base steel sheet for use herein may further contain titanium (Ti) in a content of from 0.001% to 0.20%, in addition to the above-mentioned basic elements, so as to further improve characteristic properties of the base steel sheet. A preferred content of titanium, if contained, and reasons for specifying the preferred ; content are mentioned below. Titanium (Ti) content: 0.001% to 0.20% Titanium (Ti) element forms a carbide with carbon to decrease dissolved carbon (carbon as solid-solution) to thereby improve the ductility of the base steel sheet. To exhibit these advantageous effects, the Ti content is preferably 0.001% or more, more preferably 0.002% or more, and furthermore preferably 0.003% or more. However, titanium, if contained in excess, may impair the strength of the base steel sheet, and the Ti content is preferably 0.20% or less, more preferably 0.15% or less, and furthermore preferably 0.10% or less. Titanium, when contained within this range, may not adversely affect the heat conductivity of the base steel sheet. Galvanized Coatings In the surface-treated steel sheet, a galvanized coating is applied to both sides of the base steel sheet. The mass of galvanized coating should be increased as much as possible so as to increase the heat conductivity to thereby improve the heat dissipation ability. From this viewpoint, the mass of galvanized coating per one surface (one side) should be 70 q/m' or more, is preferably 80 g/m" or more, and more preferably 90 g/m~ or more. However, the galvanized coating, if applied in an excessively large mass, may adversely affect the surface appearance of the galvanized sheet, and the mass of each galvanized coating is preferably 150 g/m2 or less, more preferably 140 g/m' or less, and furthermore preferably 130 g/m' or less. The galvanized coating preferably contains, as its composition, pure zinc (Zn) or zinc with; a trace amount (from about 0.08% to about 0.30%) of aluminum. The resulting article shows higher heat conductivity with decreasing amounts of impurities in the layer of galvanized coating. The galvanized coating is therefore preferably minimized in impurities to improve the radiation performance. The galvanized coating may contain, as its composition. auxiliary ingredients such as Si, Pb, Fe, Ti, Cr, Ni, and rare-earth elements. Resin Film A resin film is applied to cover at least one side (namely a "side opposite to the heat source") of the galvanized steel sheet. The resin film should be one having an integrated eraissivity of surface infrared radiation at wavelengths ranging from 4.5 to 15.4 urn (hereinafter also referred to as "infrared integrated emissivity") of 0.60 or more when the resin film is heated at 100°C. The radiation performance of the surface-treated steel sheet increases with an increasing infrared integrated emissivity, and the infrared integrated emissivity should thereby be 0 . 60 or more and is preferably 0 . 65 or more, and more preferably 0. 70 or more. Covering at least one side of the galvanized steel sheet with the resin film is enough to exhibit functions as the surface-treated steel sheet according to the present invention. The resin film, however, may cover the galvanized coatings on both sides of the steel sheet, so as to provide satisfactory corrosion resistance. The thickness of the resin film may be such a thickness as to satisfy the condition in infrared integrated emissivity. However, a resin film having an excessively ■ large thickness may adversely affect the heat conductivity, because a regular resin film has a heat conductivity lower than that of the base! steel sheet. The thickness of the resin film is therefore preferably 50 m or less, more preferably 45 m or less, and furthermore preferably 40 m or less. As used herein the term "infrared integrated emissivity" means, in other words, tendency of radiating (tendency of absorbing) infrared radiation (heat energy). Accordingly, one having a higher infrared radiation emissivity radiates (absorbs) a larger amount of heat energy J For example, when an article (the resin film herein) radiates 100?, of the heat energy which has been applied thereto, the article has an infrared integrated emissivity of 1. The infrared integrated emissivity specified herein is determined when the resin film is heated at 100°C. The heating temperature is set at 100°C. This is because the surface-treated steel sheet according to the present invention is adopted typically to electronic components, and such electronic components are generally used at an atmospheric temperature of from about 50°C to 70°C and at highest about 100°C, and the heating temperature is set so as to agree with such practical atmospheric temperatures. The infrared integrated emissivity is measured according to the following technique. Apparatus: "Model JIR-55D0 Fourier Transform Infrared Spectrophotometer" and radiation measuring unit "IRR-200" both supplied by JEOT, Ltd. Measurement Wavelength Range: 4.5 to 15.4 fun Measurement Temperature: The heating temperature of sample being set; at 100°C Cumulated Number: 200 Resolution: 16 cm"' The sample's spectral radiant intensity (actually measured value) ; of infrared radiation at wavelengths ranging from 4.5 to 15.4 im is i measured using the apparatus. The sample's actually measured value is measured as a value added with background radiant intensity and instrument function. To correct these, the integrated emissivity is determined by using an emissivity measurement program "Emissivity Measurement Program" supplied by JEOL Ltd. The determination (calculation) will be described in detail below: e(X) represents the spectral emissivity (%) of the sample at wavelength X; E(T) represents the integrated emissivity (%) of the sample at temperature T (°C) ; M(X.,T) represents the spectral radiant intensity (actually measured value) of the sample at wavelength K and temperature T (°C) ; A{X) represents the instrument function at wavelength X,; Kr:'B(X) represents the spectral radiant intensity of stationary background (background that does not vary from a sample to another) at wavelength X; KTB(X, TTB) represents the spectral radiant intensity of trap blackbody at wavelength X and temperature TTB (°C) ; KB(X, T) represents the spectral radiant intensity (value calculated according to the Planck' s formula of radiation) at wavelength X and temperature T (°C) / and Xi, and X2 each represent the range of wavelengths to be integrated. In the above expressions, the parameter A (X: the instrument function at wavelength X) and the parameter KFB (X: the spectral radiant intensity of stationary background at wavelength X) are calculated according to the following expressions based on the actually measured values of spectral radiant intensity of two blackbody furnaces (at 80°C and at 160°C) and the calculated values (calculated according to the Planck's formula of radiation) of spectral radiant intensity of the blackbodies at the temperature ranges: wherein Mieo-cO, 160°C) represents the spectral radiant intensity (actually measured value) of the blackbody furnace at 160°C at wavelength X; M.r--(X, 8n°c) represents the spectral radiant intensity (actually measured value) of the blackbody furnace at 80°C at wavelength X; Ki6o°c(X, 160°C) represents the spectral radiant intensity (value calculated according to the Planck's formula of radiation) of the blackbody furnace at 160°C at wavelength X; and : K8o'c(X, 80°C) represents the spectral radiant intensity (value calculated according to the Planck's formula of radiation) of the blackbody furnace at 80°C at wavelength X. The infrared integrated emissivity E (T = 100°C) is calculated in consideration of the parameter KTB(A, TTB) • This is because the trap blackbodies cooled with water are arranged around the sample during measurement. The arrangement of the trap blackbodies helps a varyinc background radiation to have a controlled low spectral radiant intensity As used herein the term "varying background radiation" refers to a background radiation that varies from a sample to another. The actually measured value of spectral radiant intensity of the sample is a value added with the background radiation, because radiation around the sample is reflected at the sample surface. The trap blackbodies used hereir are pseudo blackbodies (artificial blackbodies) having an emissivit} of 0.96, and the parameter KTB [ ( TTB) : the spectral radiant intensit] of the trap blackbodies at wavelength X and temperature TTB (°C)] is calculated according to the following expression: KTB(?/ TTB) = 0.96 x KB(X, TTB) wherein KB(X, TTB) represents the spectral radiant intensity of the blackbody at wavelength X and temperature TTB (°C) . The type of resin in the resin film covering the surface of the galvanized coatings of the surface-treated steel sheet is not limited from the viewpoint of heat radiation properties. Exemplary resins usable herein include acrylic resins, urethane resins, polyolef in resins polyester resins, fluorocarbon resins, and silicone resins, as well as mixtures of them, and modified resins derived from them. The resin film may further contain one or more crosslinking agents. Exemplary crosslinking agents include melamine compounds and isocyanate compounds Each of different crosslinking agents can be used alone or in combination in a total amount of from 0.5 to 20 percent by weight. The resin film can have an infrared integrated emissivity of 0.60 or more by suitably choosing the type of its material. Where necessary, the resin film preferably further contains one or more additional components to further improve the integrated emissivity of the resin, film surface. Examples of the additional components herein include powdery carbonaceous materials having a high emissivity, such as carbon black and acetylene black; and powdery ceramics having a high emissivity, such as cordierite, spodumene, silicon nitride, silicon carbide, aluminum oxide, and silicon oxide. The surface-treated steel sheet has the above configuration, thereby exhibits high thermal conductivity and superior heat dissipation ability, and can sufficiently exhibit radiation performance when adopted to back chassis of PDP-TV sets and other electronic components, because the electronic components have large areas of radiation surfaces. Examples The present invention will be illustrated in further detail with reference to several working examples below. It should be noted, however, that these examples are never intended to limit the scope of the present invention; various alternations and modifications may be made without departing from the scope and spirit of the present invention and are all included within the technical scope of the present invention. Experimental Example 1 Slabs having chemical compositions given in Table 1 below (Steels; A to I) were hot-rolled at 1200°C, finish-rolled at 900°C, and coiled! at 500°C to 700°C. The resulting hot-rolled steel sheets were acid-pickled, cold-rolled to a rolling reduction of from 30% to 60%, and thereby yielded thin steel sheets of 0.8 mm gage. Elementary analyses were performed such that carbon was analyzed according to combustion-infrared absorption method; nitrogen was analyzed by measurement of heat conductivity through inert gas fusion; and other compositions were analyzed through emission spectroscopy. The heat conductivities of the above-prepared steel sheets were measured according to the laser flash technique. This technique is summarized as follows. The measured heat conductivities are also shown in Table 1. Laser Flash Technique Measuring Apparatus: Laser Flash Thermal Constants Analyzer "TC-7000" supplied by ULVAC-RIKO, Inc. Initially, thermal diffusibility of the respective steel sheets were measured according to the following technique. Measurement of Thermal Diffusibility (1) A tested steel sheet was cut to a diameter of 10 mm to give a sample, and one side (front side) of the sample is colored black with carbon spray. (2) A laser beam was flashed to the blackened side (front side) of the sample, and the change in temperature of the opposite side (back side) was measured with a thermocouple or infrared detector. (3) A time-temperature rise curve was plotted, from which a thermal diffusibility was determined. (4) Specifically, the thermal diffusibility a (m~/sec) is determined according to the half-time method and expressed by the following expression: Thermal Dif fusibility a (m"/sec) = 1. 37 (L/ji) ""l/ti/a wherein L represents the distance (mm) between the laser beam applied point and the temperature detected point (i.e., the distance corresponding to the thickness of the sample steel sheet); and ti/2 represents the time (second) for the measured temperature to reach one half of the maximum temperature at the back side of the sample. Next, the specific heats of the respective steel sheets were measured according to the following technique. Measurement of Specific Heat (1) The specific heat Cp (J/(g'K) ) of a sample is expressed by the following expression: Specific Heat Cp (J/(g'K)) = Q/(M'AT) wherein Q represents the quantity (J/cm") of heat absorbed by the sample upon flash of the laser beam; M represents the mass (gram) of the sample; and AT represents the temperature rise (K), The heat conductivities of the respective steel sheets were determined in the following manner based on the above-determined thermal diffusibility a (m"/sec) and specific heat Cp (J/(g'K) ) . Determination of Heat Conductivity (1) The heat conductivity T (W/(m'K) ) of a sample is expressed by the following expression: Heat Conductivity r\ [W/ (mK) ] = Cp'a'p wherein a represents the thermal diffusibility (m2/sec) of the sample; Cp represents the specific heat (J/ (g'K) ) of the sample; and p represents the density (g/cm2) of the sample, where the density p herein was measured according to the Archimedes' principle. Based on the data in Table 1, the present inventors found that the compositions such as C, Mn, and Al affect the heat conductivity of the base steel sheet. They made further investigations on respective elements and have succeeded to specify the above-mentioned adequate [ ranges of compositional contents. Experimental Example 2 Galvanized steel sheets were prepared by using Steels C, D, and G in Table 1 in base steel sheets and applying galvanization (electrogalvanization or hot-dip galvanization) to both surfaces of the base steel sheets according to the techniques below. The prepared galvanized steel sheets were cut with a shear cutter into steel sheets each 150 mm wide, 250 mm long, and 0.8 mm thick. Preparation of Electrogalvanized Steel Sheets (1) Degreasing through immersion in alkaline aqueous solution: immersion in 3 percent by weight aqueous sodium hydroxide solution at 60°C for 2 seconds; (2) Degreasing through electrolysis in alkaline aqueous solution: electrolysis in 3 percent by weight aqueous sodium hydroxide solution at 60°C for 2 seconds, at 10 to 30 A/dm"; (3) Rinsing with water; (4) Acid pickling: with 3 to 7 percent by weight aqueous sulfuric acid solution at 40°C for 2 seconds; (5) Rinsing with water; (6) Electrogalvanization (under the following conditions); (7) Rinsing with water; and (8) Drying Conditions for Electrogalvanization Plating cell: Horizontal plating cell Plating bath composition: 300 to 400 g/L of ZnS04-7H20 50 to 100 g/L of Na2S04 25 to 35 g/L of H2SO4 Current density: 50 to 200 A/dm" Plating bath temperature: 60°C Plating bath flow rate; 1 to 2 meters per second Electrode (anode) : IrO:- alloy electrode Mass of coating: 20 g/m" (per one surface) Preparation of Hot-dip Galvanized Steel Sheets The cold-rolled steel sheets were directly subjected to hot-dip galvanization without passing through acid pickling process. The hot-dip galvanization was performed using an apparatus in which reduction by heating in a reducing gas atmosphere, immersion in a plating bath, and gas wiping were sequentially conduced. The plating bath composition was Zn-0.2% Al. The reduction was performed at temperatures of from 560°C to 900°C (preferably from 650°C to 800°C) . The heating and reduction of oxided scale layer of hot-rolled steel sheets can be performed in a continuous hot-dip galvanization line by allowing base steel sheets to pass continuously through a reducing gas atmosphere without passing through acid pickling process. The reduction time is not specified herein, but it is generally from about 10 to 80 seconds to be performed in a regular continuous hot-dip galvanization line. After the reduction, the steel sheets were cooled to temperatures-around the plating bath temperature and then subjected to hot-dip galvanization. Conditions for Hot-dip Galvanization Reduction temperature: 780°C to 860°C Plating composition: Zn-0.2% Al Plating bath temperature: 455°C to 465°C Mass of galvanized coating: 80 to 133 q/ut Next, a resin film was applied to the above-prepared galvanized steel sheets according to the following technique. The resin film was; applied to a side (hereinafter referred to as "back side") opposite to the heat source, Coating of Resin Film Surface Treatment Initially, a surface treatment was applied to the respective galvanized steel sheets by coating them with a non-chromate coating ("CTE-203" supplied by Nihon Parkerizing Co., Ltd.) to a mass of coating of 100 mg/m". This coating was very thin, and effects thereof on the heat dissipation ability of the resin-coated steel sheets were trivial. Resin Film An organic-solvent-soluble polyester resin ("VYLON (registered trademark) 650" supplied by Toyobo Co., Ltd.) was used as the material resin. This product has a glass transition temperature (Tg) of 10°C and a number-average molecular weight of 23x 10' as given in the catalogue. Crosslinking Agent A melamine resin ("SUMIMAL (registered trademark) M-40ST": supplied by Sumitomo Chemical Co., Ltd., having a solids content of 80%) was used as a crosslinking agent. Carbon Black The product "Mitsubishi Carbon Black (average particle diameter:I 25 nm) " supplied by Mitsubishi Chemical Corporation was used as a carbon i black. The polyester resin and the crosslinking agent (solid contents of 80%) were mixed in a dry mass ratio of the former to the latter of 100:20 to give a matrix resin, and the matrix resin was combined with the carbon black to a content of 10%. This was diluted with a 1:1 solvent mixture of xylene and cyclohexane, and stirred with a hand homogenizer at a number of revolutions of 10000 rpm for 10 minutes to give a material composition (resin film material composition) having a viscosity of from about 30 to 100 seconds as determined with a Ford viscosity cup No. 4. The resin film material composition was applied to the back side of the respective galvanized steel sheets using a bar coater, the applied composition was baked in a hot air oven at a final steel sheet temperature of 230°C for about 60 seconds to give resin-coated galvanized steel sheets each having a resin film 10 \m thick. The thickness of the resin film was determined by measuring the mass of the film and converting the mass into the thickness based on the specific gravity. Samples were prepared through baking and drying in the above manner. The respective samples were subjected to measurement of emissivity of the resin film surface and subjected to testing for the heat dissipation ability of the surface-treated steel sheet according to the following technique. Testing for Heat Dissipation Ability of Steel Sheet The sample sheets (surface-treated steel sheets) used herein had a size of 150 mm wide and 250 mm long, and a heater used herein had a size of 56 mm wide and 96 mm long. FIGS. lA, IB, and IC are a plan view, a front view, and a cross-sectional view along the lines A-A in FIG. IB, respectively, of a test system for the testing of heat dissipation ability. With reference to FIGS. lA, IB, and IC, a sample sheet 1 was fixed in a perpendicular direction to the ground; a heater 2 was fixed at the center * of front side of the sample sheet 1; and the sample sheet 1 was surrounded by a heat insulator 3. A thermocouple 4 was arranged on the back side of the sample sheet 1 (the right side of FIG. IB) . A maximum temperature was determined with the thermocouple arranged at the center of the sample sheet. The test system illustrated in FIG. 2 was configured by stacking two or more pieces of the member 5 (constituent) shown in FIGS. lA, IB, and IC. The heat dissipation abilities of the respective samples were tested using the test system. In FIG. 2, eight pieces of the member 5 were stacked. An electric heater was used as the heat source, and the thermal output of the heater was adjusted by controlling the voltage applied from a power source (indicated as "100 V Power Source" in FIG. 2) by a power source stabilizer and an output controller. Testing was performed using the test system. Specifically, after waiting for about 5 hours from the beginning of heating till the measured data of the thermocouple became stable, the measured temperature data were incorporated into a measured temperature data logger and indicated by a display device 6. The testing was performed at room temperature and at a heater output of 20 W. Table 2 below shows the temperatures (hereinafter also referred to as "maximum temperatures") determined through the testing, and the specifications of the samples (galvanizationprocess, mass of galvanized coating on the front side and back side, heat conductivity of the base steel sheet, and emissivity of the back side). Test Samples Nos. 1 to 3 are samples having no resin film. These results demonstrate as follows. Test Sample No, 1 has a chemical composition of the base steel sheet out of the range specified in the present invention, has galvanized coatings in excessively small masses, and thereby fails to exhibit high conductivity of the base steel sheet. Additionally, this sample shows a low emissivity of the back side not coated with a resin film and thereby shows a high maximum temperature, indicating that the steel sheet has insufficient heat dissipation ability. Test Samples Nos. 2 and 3 use base steel sheets having chemical compositions within ranges specified in the present invention so as to have higher heat conductivities, and also have galvanized coatings in increased masses as compared to that of Test Sample No. 1. In these samples, however, the maximum temperatures are still high, because the back side is not coated with a resin film and thereby has a low emissivity, although the maximum temperatures are somewhat lowered due to the higher heat conductivities of the base steel sheets. Test Sample No. 4 is one corresponding to Test Sample No. 1, except for further having a resin film having an emissivity of 0.81 on the back side of the base steel sheet. Test Sample No. 4 shows a lower maximum temperature than that of Test Sample No. 1, due to the coverage of the resin film having a high emissivity. However, this steel sheet still shows an insufficient heat conductivity. Test Samples Nos. 5 and 6 are steel sheets corresponding to Test Samples Nos. 2 and 3, expect for further having resin films having emissivities of 0.81 and 0.82, respectively, coated thereon. These test samples exhibit more satisfactory heat dissipation ability than that of Test Sample No. 4, because these test samples use base steel sheets having higher heat conductivities and thereby show further lowered maximum temperatures than that of Test Sample No. 4. The comparison between samples Nos. 7 to 11 and samples Nos. 12 to 16 shows the effect of the emissivity of back side on the maximum temperature, under various conditions. WHAT IS CLAIMED IS: 1. A resin-coated galvanized steel sheet comprising: a base steel sheet; a first galvanized coating present on one side of the base steel sheet; a second galvanized coating present on the other side of the base steel sheet; and a resin film present on at least one of the first and second galvanized coatings, wherein the base steel sheet has a carbon (C) content of from 0. 0010 to 0.040 percent by mass (hereinafter the contents will be simply expressed in "%"), a silicon (Si) content of 0.2% or less, a manganese (Mn) content of from 0.1% to 0.80%, and an aluminum (Al) content of from 0.01% to 0.1%, with the remainder including iron and inevitable impurities, wherein each of the first and second galvanized coatings is present in a mass of coating of 70 g/m" or more, and wherein the resin film has an integrated emissivity of infrared radiation at wavelengths ranging from 4.5 to 15.4 jim of 0.60 or more when the resin film is heated at 100°C. 2. The resin-coated galvanized steel sheet according to claim 1, wherein the base steel sheet further contains titanium (Ti) in a content of from 0.001% to 0.20%. ' 3. The resin-coated galvanized steel sheet according to claim 1, wherein the resin film has a thickness of 50 \m or less.

Full Text

RESIN-COATED GALVANIZED STEEL SHEET
FIELD OF THE INVENTION The present invention relates to resin-coated galvanized steel sheets that are highly thermally conductive and diffuse heat effectively. Specifically, it relates to a resin-coated galvanized steel sheet advantageously useful as a material for an electronic component that is locally in contact with a heat source and thereby requires high thermal conductivity and satisfactory heat radiation ability. Examples of such ; electronic components (including electrical components and components! of optical devices) include heat sinks, back chassis of plasma display: television sets, and metallic cabinets (casings) housing electronic components containing heat sources.
BACKGROUND OF THE INVENTION With the slimming down of liquid crystal display television sets and plasma display panel television sets, thermal problems become more and more serious. Under these circumstances, manufacturers of such electrical apparatuses put much effort into lowering the temperatures: of the product apparatuses during operation typically by 1°C and use! expensive components taking measures against heat problems. Such heati problems are especially serious in components locally in contact with! heat sources, such as back chassis of plasma display panel television; sets (PDP-TV sets) that are locally in contact with a heat source plasma! device with the interposition of a glass panel. Of heat dissipation! properties, heat conduction plays a most important role in such components which are in contact with heat sources and should take

measures against heat problems. Aluminum-based components are currently widely used as components taking measures against heat problems, because aluminum conducts heat satisfactorily.
On the other hand, cost competition in the market of thin-screen television sets such as PDP-TV sets is becoming fierce, and the replacement of expensive aluminum components by inexpensive steel components is expected to achieve considerable cost reduction of products.
However, not every steel sheet will do, because aluminum-based components have higher heat conductivities than those of steel ; components. Specifically, the replacement of aluminum-based components by steel components needs steel sheets that can lower the temperature of heat source more than known steel sheets even if only slightly.
Various proposals have been made on steel components to be adopted! in such electronic components. Typically, Japanese Unexamined Patent; Application Publication (JP-A) No. 2006-307260 proposes a technique relating to an inexpensive plasma display panel fixing plate that hasi excellent corrosion resistance or excellent heat dissipation ability. According to this technique, a slab of continuous cast steel is hot-rolled at a high rolling reduction, quenched to form a hot-rolled steel sheet having a structure in which martensite is dispersed in i ferrite, the hot-rolled steel sheet is subjected sequentially to primary cold rolling, annealing, and secondary cold rolling, and the resulting steel sheet is galvanized to form a galvanized coating thereon, and a! layer of chemical conversion coating is formed on the galvanized coating so as to improve the corrosion resistance or heat dissipation ability.

This technique, however, does not consider the improvement of heat conductivity of base steel sheet and thereby does not give sufficient advantageous effects for improving the heat dissipation ability,
Japanese Unexamined Patent Application Publication (JP-A) No. 2005-222042 proposes a technique relating to a chassis assembly for plasma display device, which assembly includes a chassis having a heat conductivity of from 10 W/mK to 100 W/mK. The document mentions that one having a higher heat conductivity is more advantageous in heat dissipation ability. This technique, however, does not always exhibit sufficient advantageous effects on the improvement of heat dissipation ability, because the technique has been made in order to reduce discharge delay phenomenon caused by temperature drop.
SUMMARY OF THE INVENTION
The present invention has been made under these circumstances, and an object of the present invention is to provide a resin-coated galvanized steel sheet that exhibits high thermal conductivity and superior heat dissipation ability and is useful as a material for an electronic component which will be locally in contact with a heat source.
Specifically, according to an embodiment of the present invention,: there is provided a resin-coated galvanized steel sheet which includes a base steel sheet; a first galvanized coating present on one side of the base steel sheet; a second galvanized coating present on the other side of the base steel sheet; and a resin film present on at least ond of the first and second galvanized coatings, in which the base steel sheet has a carbon (C) content of from 0.0010 to 0.040 percent by mass (hereinafter the contents will be simply expressed in "%"), a silicon

(Si) content of 0,2?> or less, a manganese (Mn) content of from 0.1% to 0.80ft, and an aluminum (Al) content of from 0.01% to 0,1%, with the remainder including iron and inevitable impurities, each of the first and second galvanized coatings is present in a mass of coating of 70 g/m~ or more, and the resin film has an integrated emissivity of infrared radiation at wavelengths ranging from 4.5 to 15.4 jim of 0.60 or more when the resin film is heated at 100°C.
The base steel sheet in the resin-coated galvanized steel sheet may further contain titanium (Ti) in a content of from 0.001% to 0.20%, and this further improves characteristic properties of the resin-coated galvanized steel sheet.
The resin film in the resin-coated galvanized steel sheet preferably has a thickness of 50 \xm or less to further increase the heat dissipation ability.
The resin-coated galvanized steel sheet is advantageous when adopted to electronic components. The resin-coated galvanized steel sheet is especially advantageously adopted to back chassis of PDP-TV sets because such back chassis have wide areas to be used.
According to the present invention, there is provided a resin-coatedi galvanized steel sheet that exhibits high thermal conductivity and superior heat dissipation ability and is useful as a material for an; electronic component which is to be locally in contact with a heat source.; This is achieved by adequately specifying the chemical composition ofj the base steel sheet, adequately controlling the mass of each of the: first and second galvanized coatings present on surfaces (both surfaces) '> of the base steel sheet, and adequately controlling the emissivity of the resin film covering at least one of the first and second galvanized

coatings.
BRIEF DESCRIPTION OF THE DRAWINGS FIGS. lA, IB, and IC are a plan view, a front view, and a cross-sectional view along the lines A-A in FIG. IB, respectively, of a test system for the testing of heat dissipation ability; and
FIG, 2 is a schematic diagram of the structure of the test system for the testing of heat dissipation ability.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present inventors made intensive investigations from various angles to provide a resin-coated galvanized steel sheet (hereinafter also referred to as a "surface-treated steel sheet") that exhibits high thermal conductivity and superior heat dissipation ability. Initially, they made investigations about the relationship between the types of compositions and the heat conductivity of a base steel sheet and have found that the compositions such as carbon (C), manganese (Mn), and aluminum (Al) affect the heat conductivity of the steel sheet. They; have also found that the radiation performance can be further improved by forming galvanized coatings in larger masses on surfaces (both surfaces) of the base steel sheet; that the radiation performance can be furthermore improved by covering at least one side (the side opposite to a heat source) of the thickly galvanized steel sheet with a resin film having a high emissivity;" and that the resulting surface-treated steel sheet can achieve the above object. The present invention has been made based on these findings. Conditions specified in the present invention will be described below.

Chemical Composition of Base Steel Sheet
A base steel sheet for use in the present invention should have: adequately controlled chemical compositions. The compositions are specified for the reasons below.
Carbon (C) content: 0.0010% to 0.040%
Carbon (C) element mostly affects the heat conductivity of the base steel sheet. The heat conductivity increases with a decreasing carbon content, and the carbon content should therefore be 0.040% or less. The carbon content is preferably 0.03% or less, and more preferably 0.02% or less. Carbon element, however, is useful for ensuring satisfactory: strength when the steel is formed into a thin steel sheet. When a steel sheet having an insufficient strength is used as a large-size electronic component such as back chassis, it may be difficult to support the structure or to maintain flatness of the steel sheet. In this case, the strength necessary as back chassis should be ensured by using other elements in combination. To ensure certain strength usable typically! as back chassis, the carbon content should be 0.0010% or more, and iS: preferably 0.0015% or more, and more preferably 0.0020% or more.
Silicon (Si) content: 0.2% or less
Silicon (Si) element affects the wettability with galvanized i coatings. The Si content should be 0.2% or less to maintain satisfactory} wettability with galvanized coatings. The Si content is preferably I 0.18% or less, and more preferably 0.16% or less. The Si content herein! is preferably minimized to the extent possible.
Manganese (Mn) content: 0.10% to 0.80% i
Manganese (Mn) element affects the heat conductivity of the base steel sheet. The heat conductivity increases with a decreasing Mn

content, and the Mn content should therefore be 0. 80?, or less . Mn element, however, also improves the hardenability, and the Mn content should be 0.10% or more to ensure satisfactory strength of the steel sheet. The: Mn content is preferably 0.12% or more, and more preferably 0.14% or more.
Aluminum (Al) content: 0.01% to 0.1%
Aluminum (Al) element affects the heat conductivity of the base steel sheet. The Al content should be 0.1% or less to maintain satisfactory heat conductivity. The Al content is preferably 0.08% or less, and more preferably 0.05% or less. However, Al element also acts: as a deoxidizer, and its content should be 0.01% or more to exhibit the effect effectively. The Al content is preferably 0.02% or more, and more preferably 0.03% or more.
The preferred fundamental compositions of the base steel sheet are as above, and the remainder includes iron and inevitable impurities. Representative examples of the inevitable impurities include phosphorus (P), sulfur (S), and nitrogen (N) . These inevitable impurities are preferably controlled as follows.
Sulfur (S) content: 0.030% or less
Sulfur (S) element belongs to inevitable impurities, combines with; manganese (Mn) to adversely affect the ductility of the steel sheet,' and is preferably minimized. From these viewpoints, the sulfur content! is preferably 0.030% or less, more preferably 0.025% or less, and furthermore preferably 0.020% or less. Sulfur, when contained within; this range, may not affect the heat conductivity of the base steel sheet.:
Phosphorus (P) content: 0.20% or less
Phosphorus (P) element belongs to inevitable impurities, promotes

the grain boundary fracture due to grain boundary segregation, and its content is desirably minimized. From this viewpoint, the phosphorus content is preferably 0.20?, or less, more preferably 0.15% or less, and furthermore preferably 0.10% or less.
Nitrogen (N) content: 0.020% or less
Nitrogen (N) element belongs to inevitable impurities, forms coarse inclusions, such as TiN, to impair the toughness of the base steel sheet, and is preferably minimized. From this viewpoint, the nitrogen content is preferably 0.020% or less, more preferably 0.015% or less, and furthermore preferably 0.010% or less. Nitrogen, when contained within this range, may not affect the heat conductivity of the base steel sheet.
Exemplary inevitable impurities other than above include Cr, Ni, Mo, and Cu. These elements may be contained as impurities in the base steel sheet each in a content of 0.1% or less during regular production processes, and within this range, do not affect the heat conductivity. These elements, however, improve the hardenability and can be positively added to improve characteristic properties within such a range not adversely affecting the heat conductivity. For example, these elements may be added each in a content of about 1,0% or less.
Where necessary, the base steel sheet for use herein may further contain titanium (Ti) in a content of from 0.001% to 0.20%, in addition to the above-mentioned basic elements, so as to further improve characteristic properties of the base steel sheet. A preferred content of titanium, if contained, and reasons for specifying the preferred ; content are mentioned below.
Titanium (Ti) content: 0.001% to 0.20%
Titanium (Ti) element forms a carbide with carbon to decrease

dissolved carbon (carbon as solid-solution) to thereby improve the ductility of the base steel sheet. To exhibit these advantageous effects, the Ti content is preferably 0.001% or more, more preferably 0.002% or more, and furthermore preferably 0.003% or more. However, titanium, if contained in excess, may impair the strength of the base steel sheet, and the Ti content is preferably 0.20% or less, more preferably 0.15% or less, and furthermore preferably 0.10% or less. Titanium, when contained within this range, may not adversely affect the heat conductivity of the base steel sheet.
Galvanized Coatings
In the surface-treated steel sheet, a galvanized coating is applied to both sides of the base steel sheet. The mass of galvanized coating should be increased as much as possible so as to increase the heat conductivity to thereby improve the heat dissipation ability. From this viewpoint, the mass of galvanized coating per one surface (one side) should be 70 q/m' or more, is preferably 80 g/m" or more, and more preferably 90 g/m~ or more. However, the galvanized coating, if applied in an excessively large mass, may adversely affect the surface appearance of the galvanized sheet, and the mass of each galvanized coating is preferably 150 g/m2 or less, more preferably 140 g/m' or less, and furthermore preferably 130 g/m' or less. The galvanized coating preferably contains, as its composition, pure zinc (Zn) or zinc with; a trace amount (from about 0.08% to about 0.30%) of aluminum. The resulting article shows higher heat conductivity with decreasing amounts of impurities in the layer of galvanized coating. The galvanized coating is therefore preferably minimized in impurities to improve the radiation performance. The galvanized coating may contain, as its composition.

auxiliary ingredients such as Si, Pb, Fe, Ti, Cr, Ni, and rare-earth elements.
Resin Film
A resin film is applied to cover at least one side (namely a "side opposite to the heat source") of the galvanized steel sheet. The resin film should be one having an integrated eraissivity of surface infrared radiation at wavelengths ranging from 4.5 to 15.4 urn (hereinafter also referred to as "infrared integrated emissivity") of 0.60 or more when the resin film is heated at 100°C. The radiation performance of the surface-treated steel sheet increases with an increasing infrared integrated emissivity, and the infrared integrated emissivity should thereby be 0 . 60 or more and is preferably 0 . 65 or more, and more preferably 0. 70 or more. Covering at least one side of the galvanized steel sheet with the resin film is enough to exhibit functions as the surface-treated steel sheet according to the present invention. The resin film, however, may cover the galvanized coatings on both sides of the steel sheet, so as to provide satisfactory corrosion resistance. The thickness of the resin film may be such a thickness as to satisfy the condition in infrared integrated emissivity. However, a resin film having an excessively ■ large thickness may adversely affect the heat conductivity, because a regular resin film has a heat conductivity lower than that of the base! steel sheet. The thickness of the resin film is therefore preferably 50 m or less, more preferably 45 m or less, and furthermore preferably 40 m or less.
As used herein the term "infrared integrated emissivity" means, in other words, tendency of radiating (tendency of absorbing) infrared radiation (heat energy). Accordingly, one having a higher infrared

radiation emissivity radiates (absorbs) a larger amount of heat energy J For example, when an article (the resin film herein) radiates 100?, of the heat energy which has been applied thereto, the article has an infrared integrated emissivity of 1.
The infrared integrated emissivity specified herein is determined when the resin film is heated at 100°C. The heating temperature is set at 100°C. This is because the surface-treated steel sheet according to the present invention is adopted typically to electronic components, and such electronic components are generally used at an atmospheric temperature of from about 50°C to 70°C and at highest about 100°C, and the heating temperature is set so as to agree with such practical atmospheric temperatures.
The infrared integrated emissivity is measured according to the following technique.
Apparatus: "Model JIR-55D0 Fourier Transform Infrared Spectrophotometer" and radiation measuring unit "IRR-200" both supplied by JEOT, Ltd.
Measurement Wavelength Range: 4.5 to 15.4 fun
Measurement Temperature: The heating temperature of sample being set; at 100°C
Cumulated Number: 200
Resolution: 16 cm"'
The sample's spectral radiant intensity (actually measured value) ; of infrared radiation at wavelengths ranging from 4.5 to 15.4 im is i measured using the apparatus. The sample's actually measured value is measured as a value added with background radiant intensity and instrument function. To correct these, the integrated emissivity is

determined by using an emissivity measurement program "Emissivity Measurement Program" supplied by JEOL Ltd. The determination (calculation) will be described in detail below:

e(X) represents the spectral emissivity (%) of the sample at wavelength X;
E(T) represents the integrated emissivity (%) of the sample at temperature T (°C) ;
M(X.,T) represents the spectral radiant intensity (actually measured value) of the sample at wavelength K and temperature T (°C) ;
A{X) represents the instrument function at wavelength X,;
Kr:'B(X) represents the spectral radiant intensity of stationary background (background that does not vary from a sample to another) at wavelength X;
KTB(X, TTB) represents the spectral radiant intensity of trap blackbody at wavelength X and temperature TTB (°C) ;
KB(X, T) represents the spectral radiant intensity (value calculated according to the Planck' s formula of radiation) at wavelength X and temperature T (°C) / and
Xi, and X2 each represent the range of wavelengths to be integrated.

In the above expressions, the parameter A (X: the instrument function at wavelength X) and the parameter KFB (X: the spectral radiant intensity of stationary background at wavelength X) are calculated according to the following expressions based on the actually measured values of spectral radiant intensity of two blackbody furnaces (at 80°C and at 160°C) and the calculated values (calculated according to the Planck's formula of radiation) of spectral radiant intensity of the blackbodies at the temperature ranges:

wherein
Mieo-cO, 160°C) represents the spectral radiant intensity (actually measured value) of the blackbody furnace at 160°C at wavelength X;
M.r--(X, 8n°c) represents the spectral radiant intensity (actually measured value) of the blackbody furnace at 80°C at wavelength X;
Ki6o°c(X, 160°C) represents the spectral radiant intensity (value
calculated according to the Planck's formula of radiation) of the
blackbody furnace at 160°C at wavelength X; and :
K8o'c(X, 80°C) represents the spectral radiant intensity (value calculated according to the Planck's formula of radiation) of the blackbody furnace at 80°C at wavelength X.
The infrared integrated emissivity E (T = 100°C) is calculated in consideration of the parameter KTB(A, TTB) • This is because the trap blackbodies cooled with water are arranged around the sample during

measurement. The arrangement of the trap blackbodies helps a varyinc background radiation to have a controlled low spectral radiant intensity As used herein the term "varying background radiation" refers to a background radiation that varies from a sample to another. The actually measured value of spectral radiant intensity of the sample is a value added with the background radiation, because radiation around the sample is reflected at the sample surface. The trap blackbodies used hereir are pseudo blackbodies (artificial blackbodies) having an emissivit} of 0.96, and the parameter KTB [ ( TTB) : the spectral radiant intensit] of the trap blackbodies at wavelength X and temperature TTB (°C)] is calculated according to the following expression:
KTB(?/ TTB) = 0.96 x KB(X, TTB) wherein KB(X, TTB) represents the spectral radiant intensity of the blackbody at wavelength X and temperature TTB (°C) .
The type of resin in the resin film covering the surface of the galvanized coatings of the surface-treated steel sheet is not limited from the viewpoint of heat radiation properties. Exemplary resins usable herein include acrylic resins, urethane resins, polyolef in resins polyester resins, fluorocarbon resins, and silicone resins, as well as mixtures of them, and modified resins derived from them. The resin film may further contain one or more crosslinking agents. Exemplary crosslinking agents include melamine compounds and isocyanate compounds Each of different crosslinking agents can be used alone or in combination in a total amount of from 0.5 to 20 percent by weight.
The resin film can have an infrared integrated emissivity of 0.60 or more by suitably choosing the type of its material. Where necessary, the resin film preferably further contains one or more additional

components to further improve the integrated emissivity of the resin, film surface. Examples of the additional components herein include powdery carbonaceous materials having a high emissivity, such as carbon black and acetylene black; and powdery ceramics having a high emissivity, such as cordierite, spodumene, silicon nitride, silicon carbide, aluminum oxide, and silicon oxide.
The surface-treated steel sheet has the above configuration, thereby exhibits high thermal conductivity and superior heat dissipation ability, and can sufficiently exhibit radiation performance when adopted to back chassis of PDP-TV sets and other electronic components, because the electronic components have large areas of radiation surfaces. Examples
The present invention will be illustrated in further detail with reference to several working examples below. It should be noted, however, that these examples are never intended to limit the scope of the present invention; various alternations and modifications may be made without departing from the scope and spirit of the present invention and are all included within the technical scope of the present invention.
Experimental Example 1
Slabs having chemical compositions given in Table 1 below (Steels; A to I) were hot-rolled at 1200°C, finish-rolled at 900°C, and coiled! at 500°C to 700°C. The resulting hot-rolled steel sheets were acid-pickled, cold-rolled to a rolling reduction of from 30% to 60%, and thereby yielded thin steel sheets of 0.8 mm gage. Elementary analyses were performed such that carbon was analyzed according to combustion-infrared absorption method; nitrogen was analyzed by measurement of heat conductivity through inert gas fusion; and other

compositions were analyzed through emission spectroscopy.

The heat conductivities of the above-prepared steel sheets were measured according to the laser flash technique. This technique is summarized as follows. The measured heat conductivities are also shown in Table 1.
Laser Flash Technique Measuring Apparatus: Laser Flash Thermal Constants Analyzer "TC-7000" supplied by ULVAC-RIKO, Inc.
Initially, thermal diffusibility of the respective steel sheets were measured according to the following technique.
Measurement of Thermal Diffusibility
(1) A tested steel sheet was cut to a diameter of 10 mm to give a sample, and one side (front side) of the sample is colored black with carbon spray.
(2) A laser beam was flashed to the blackened side (front side) of the sample, and the change in temperature of the opposite side (back side) was measured with a thermocouple or infrared detector.
(3) A time-temperature rise curve was plotted, from which a thermal diffusibility was determined.
(4) Specifically, the thermal diffusibility a (m~/sec) is
determined according to the half-time method and expressed by the
following expression:
Thermal Dif fusibility a (m"/sec) = 1. 37 (L/ji) ""l/ti/a wherein L represents the distance (mm) between the laser beam applied point and the temperature detected point (i.e., the distance corresponding to the thickness of the sample steel sheet); and ti/2 represents the time (second) for the measured temperature to reach one half of the maximum temperature at the back side of the sample.

Next, the specific heats of the respective steel sheets were measured according to the following technique.
Measurement of Specific Heat
(1) The specific heat Cp (J/(g'K) ) of a sample is expressed by the following expression:
Specific Heat Cp (J/(g'K)) = Q/(M'AT) wherein Q represents the quantity (J/cm") of heat absorbed by the sample upon flash of the laser beam; M represents the mass (gram) of the sample; and AT represents the temperature rise (K),
The heat conductivities of the respective steel sheets were determined in the following manner based on the above-determined thermal diffusibility a (m"/sec) and specific heat Cp (J/(g'K) ) .
Determination of Heat Conductivity
(1) The heat conductivity T (W/(m'K) ) of a sample is expressed by the following expression:
Heat Conductivity r\ [W/ (mK) ] = Cp'a'p wherein a represents the thermal diffusibility (m2/sec) of the sample; Cp represents the specific heat (J/ (g'K) ) of the sample; and p represents the density (g/cm2) of the sample, where the density p herein was measured according to the Archimedes' principle.
Based on the data in Table 1, the present inventors found that the compositions such as C, Mn, and Al affect the heat conductivity of the base steel sheet. They made further investigations on respective elements and have succeeded to specify the above-mentioned adequate [ ranges of compositional contents.
Experimental Example 2
Galvanized steel sheets were prepared by using Steels C, D, and

G in Table 1 in base steel sheets and applying galvanization (electrogalvanization or hot-dip galvanization) to both surfaces of the base steel sheets according to the techniques below. The prepared galvanized steel sheets were cut with a shear cutter into steel sheets each 150 mm wide, 250 mm long, and 0.8 mm thick.
Preparation of Electrogalvanized Steel Sheets
(1) Degreasing through immersion in alkaline aqueous solution: immersion in 3 percent by weight aqueous sodium hydroxide solution at 60°C for 2 seconds;
(2) Degreasing through electrolysis in alkaline aqueous solution: electrolysis in 3 percent by weight aqueous sodium hydroxide solution at 60°C for 2 seconds, at 10 to 30 A/dm";
(3) Rinsing with water;
(4) Acid pickling: with 3 to 7 percent by weight aqueous sulfuric acid
solution at 40°C for 2 seconds;
(5) Rinsing with water;
(6) Electrogalvanization (under the following conditions);
(7) Rinsing with water; and
(8) Drying
Conditions for Electrogalvanization
Plating cell: Horizontal plating cell
Plating bath composition:
300 to 400 g/L of ZnS04-7H20
50 to 100 g/L of Na2S04
25 to 35 g/L of H2SO4
Current density: 50 to 200 A/dm"

Plating bath temperature: 60°C
Plating bath flow rate; 1 to 2 meters per second
Electrode (anode) : IrO:- alloy electrode
Mass of coating: 20 g/m" (per one surface)
Preparation of Hot-dip Galvanized Steel Sheets
The cold-rolled steel sheets were directly subjected to hot-dip galvanization without passing through acid pickling process. The hot-dip galvanization was performed using an apparatus in which reduction by heating in a reducing gas atmosphere, immersion in a plating bath, and gas wiping were sequentially conduced. The plating bath composition was Zn-0.2% Al.
The reduction was performed at temperatures of from 560°C to 900°C (preferably from 650°C to 800°C) . The heating and reduction of oxided scale layer of hot-rolled steel sheets can be performed in a continuous hot-dip galvanization line by allowing base steel sheets to pass continuously through a reducing gas atmosphere without passing through acid pickling process. The reduction time is not specified herein, but it is generally from about 10 to 80 seconds to be performed in a regular continuous hot-dip galvanization line. After the reduction, the steel sheets were cooled to temperatures-around the plating bath temperature and then subjected to hot-dip galvanization. Conditions for Hot-dip Galvanization
Reduction temperature: 780°C to 860°C
Plating composition: Zn-0.2% Al
Plating bath temperature: 455°C to 465°C
Mass of galvanized coating: 80 to 133 q/ut
Next, a resin film was applied to the above-prepared galvanized

steel sheets according to the following technique. The resin film was; applied to a side (hereinafter referred to as "back side") opposite to the heat source,
Coating of Resin Film Surface Treatment
Initially, a surface treatment was applied to the respective galvanized steel sheets by coating them with a non-chromate coating ("CTE-203" supplied by Nihon Parkerizing Co., Ltd.) to a mass of coating of 100 mg/m". This coating was very thin, and effects thereof on the heat dissipation ability of the resin-coated steel sheets were trivial. Resin Film
An organic-solvent-soluble polyester resin ("VYLON (registered trademark) 650" supplied by Toyobo Co., Ltd.) was used as the material resin. This product has a glass transition temperature (Tg) of 10°C and a number-average molecular weight of 23x 10' as given in the catalogue. Crosslinking Agent
A melamine resin ("SUMIMAL (registered trademark) M-40ST": supplied by Sumitomo Chemical Co., Ltd., having a solids content of 80%) was used as a crosslinking agent. Carbon Black
The product "Mitsubishi Carbon Black (average particle diameter:I 25 nm) " supplied by Mitsubishi Chemical Corporation was used as a carbon i black.
The polyester resin and the crosslinking agent (solid contents of 80%) were mixed in a dry mass ratio of the former to the latter of 100:20 to give a matrix resin, and the matrix resin was combined with the carbon black to a content of 10%. This was diluted with a 1:1 solvent mixture

of xylene and cyclohexane, and stirred with a hand homogenizer at a number of revolutions of 10000 rpm for 10 minutes to give a material composition (resin film material composition) having a viscosity of from about 30 to 100 seconds as determined with a Ford viscosity cup No. 4.
The resin film material composition was applied to the back side of the respective galvanized steel sheets using a bar coater, the applied composition was baked in a hot air oven at a final steel sheet temperature of 230°C for about 60 seconds to give resin-coated galvanized steel sheets each having a resin film 10 \m thick. The thickness of the resin film was determined by measuring the mass of the film and converting the mass into the thickness based on the specific gravity. Samples were prepared through baking and drying in the above manner. The respective samples were subjected to measurement of emissivity of the resin film surface and subjected to testing for the heat dissipation ability of the surface-treated steel sheet according to the following technique.
Testing for Heat Dissipation Ability of Steel Sheet
The sample sheets (surface-treated steel sheets) used herein had a size of 150 mm wide and 250 mm long, and a heater used herein had a size of 56 mm wide and 96 mm long.
FIGS. lA, IB, and IC are a plan view, a front view, and a cross-sectional view along the lines A-A in FIG. IB, respectively, of a test system for the testing of heat dissipation ability. With reference to FIGS. lA, IB, and IC, a sample sheet 1 was fixed in a perpendicular direction to the ground; a heater 2 was fixed at the center * of front side of the sample sheet 1; and the sample sheet 1 was surrounded by a heat insulator 3. A thermocouple 4 was arranged on the back side of the sample sheet 1 (the right side of FIG. IB) . A maximum temperature

was determined with the thermocouple arranged at the center of the sample sheet.
The test system illustrated in FIG. 2 was configured by stacking two or more pieces of the member 5 (constituent) shown in FIGS. lA, IB, and IC. The heat dissipation abilities of the respective samples were tested using the test system. In FIG. 2, eight pieces of the member 5 were stacked. An electric heater was used as the heat source, and the thermal output of the heater was adjusted by controlling the voltage applied from a power source (indicated as "100 V Power Source" in FIG. 2) by a power source stabilizer and an output controller.
Testing was performed using the test system. Specifically, after waiting for about 5 hours from the beginning of heating till the measured data of the thermocouple became stable, the measured temperature data were incorporated into a measured temperature data logger and indicated by a display device 6. The testing was performed at room temperature and at a heater output of 20 W.
Table 2 below shows the temperatures (hereinafter also referred to as "maximum temperatures") determined through the testing, and the specifications of the samples (galvanizationprocess, mass of galvanized coating on the front side and back side, heat conductivity of the base steel sheet, and emissivity of the back side). Test Samples Nos. 1 to 3 are samples having no resin film.

These results demonstrate as follows. Test Sample No, 1 has a chemical composition of the base steel sheet out of the range specified in the present invention, has galvanized coatings in excessively small masses, and thereby fails to exhibit high conductivity of the base steel sheet. Additionally, this sample shows a low emissivity of the back side not coated with a resin film and thereby shows a high maximum temperature, indicating that the steel sheet has insufficient heat dissipation ability.
Test Samples Nos. 2 and 3 use base steel sheets having chemical compositions within ranges specified in the present invention so as to have higher heat conductivities, and also have galvanized coatings in increased masses as compared to that of Test Sample No. 1. In these samples, however, the maximum temperatures are still high, because the back side is not coated with a resin film and thereby has a low emissivity, although the maximum temperatures are somewhat lowered due to the higher heat conductivities of the base steel sheets.
Test Sample No. 4 is one corresponding to Test Sample No. 1, except for further having a resin film having an emissivity of 0.81 on the back side of the base steel sheet. Test Sample No. 4 shows a lower maximum temperature than that of Test Sample No. 1, due to the coverage of the resin film having a high emissivity. However, this steel sheet still shows an insufficient heat conductivity.
Test Samples Nos. 5 and 6 are steel sheets corresponding to Test Samples Nos. 2 and 3, expect for further having resin films having emissivities of 0.81 and 0.82, respectively, coated thereon. These test samples exhibit more satisfactory heat dissipation ability than that of Test Sample No. 4, because these test samples use base steel

sheets having higher heat conductivities and thereby show further lowered maximum temperatures than that of Test Sample No. 4.
The comparison between samples Nos. 7 to 11 and samples Nos. 12 to 16 shows the effect of the emissivity of back side on the maximum temperature, under various conditions.

WHAT IS CLAIMED IS:
1. A resin-coated galvanized steel sheet comprising:
a base steel sheet;
a first galvanized coating present on one side of the base steel sheet;
a second galvanized coating present on the other side of the base steel sheet; and
a resin film present on at least one of the first and second galvanized coatings,
wherein the base steel sheet has a carbon (C) content of from 0. 0010 to 0.040 percent by mass (hereinafter the contents will be simply expressed in "%"), a silicon (Si) content of 0.2% or less, a manganese (Mn) content of from 0.1% to 0.80%, and an aluminum (Al) content of from 0.01% to 0.1%, with the remainder including iron and inevitable impurities,
wherein each of the first and second galvanized coatings is present in a mass of coating of 70 g/m" or more, and
wherein the resin film has an integrated emissivity of infrared radiation at wavelengths ranging from 4.5 to 15.4 jim of 0.60 or more when the resin film is heated at 100°C.
2. The resin-coated galvanized steel sheet according to claim 1, wherein the base steel sheet further contains titanium (Ti) in a content of from 0.001% to 0.20%. '
3. The resin-coated galvanized steel sheet according to claim 1,

wherein the resin film has a thickness of 50 \m or less.

Documents:

http://ipindiaonline.gov.in/patentsearch/GrantedSearch/viewdoc.aspx?id=8hjZXCmUJEylixqEl3kvMQ==&loc=egcICQiyoj82NGgGrC5ChA==

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Patent Number

269787

Indian Patent Application Number

789/CHE/2010

PG Journal Number

46/2015

Publication Date

13-Nov-2015

Grant Date

06-Nov-2015

Date of Filing

24-Mar-2010

Name of Patentee

KABUSHIKI KAISHA KOBE SEIKO SHO (KOBE STEEL, LTD.)

Applicant Address

10-26, WAKINOHAMA-CHO 2-CHOME, CHUO-KU, KOBE-SHI, HYOGO 651-8585

Inventors:

#	Inventor's Name	Inventor's Address
1	KOJIMA, TAKESHI	C/O KAKAGAWA WORKS IN KOBE STEEL, LTD., KANAZAWA-CHO 1, KAKOGAWA-SHI, HYOGO-675-0137
2	HIRANO, YASUO	C/O KAKOGAWA WORKS IN KOBE STEEL, LTD., KANAZAWA-CHO 1, KAKOGAWA-SHI, HYOGO 675-0137
3	MATSUDA, HARUYUKI	C/O KOBE CORPORATE RESEARCH LABORATORIES IN KOBE STEEL, LTD., 5-5, TAKATSUKADAI 1-CHOME, NISHI-KU, KOBE-SHI, HYOGO-651-2271
4	TAKAHASHI, KAZUO	C/O KOBE CORPORATE RESEARCH LABORATORIES IN KOBE STEEL, LTD., 5-5, TAKATSUKADAI 1-CHOME, NISHI-KU, KOBE-SHI, HYOGO-651-2271

PCT International Classification Number

C22C 38/00

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	2009-080476	2009-03-27	Japan