Title of Invention

VERY-HIGH-STRENGTH AND LOW-DENSITY, HOT-ROLLED STEEL SHEET AND MANUFACTURING PROCESS

Abstract

A very-high-strength and low-density, hot-rolled steel sheet, its composition in % by weight comprises: 0.04% <carbon<0.5% 0.05% < manganese < 3% and optionally the following hardening elements: 0.01 % < niobium < 0.1 % 0.01% < titamum < 0.2% 0.01% < vanadium < 0.2%, taken individually or in combination, and optionally elements that act on the transformation temperatures: 0.0005% < boron 0.005% 0.05% < nickel < 2% 0.05% < chromium < 2% 0.05% < molybdenum < 2%, taken individually or in combination, the balance being iron and elements inherent in smelting, characterized in that it comprises silicon between 2 to 3% and aluminum contain between 7 to 10% and provided that silicon and aluminum contains are such that % Si + A1 > 9: 2% < silicon < 10%, 1% < aluminum < 10%, and in that said sheet further has a microstructure consisting of a primary ferrite phase and of a secondary ferrite phase, the mean grain size of said primary ferrite being greater than the mean grain size of said secondary ferrite, said microstructure also containing carbide phases.

Full Text

FORM 2 THE PATENTS ACT, 1970 (39 of 1970) COMPLETE SPECIFICATION (See Section 10, rule 13) VERY-HIGH-STRENGTH AND LOW- DENSITY, HOT-ROLLED STEEL SHEET AND MANUFACTURING PROCESS USINOR of IMMEUBLE "LA PACIFIC" LA DEFENSE 7 -11/13 COURE VALMY 92800 - PUTEAUX, FRANCE, FRENCH Company The following specification particularly describes the nature of the invention and the manner in which it is to be performed : - ORIGIN 461/MUMNP/2004 20/8/2004 GRANTED 28-10-2005 The invention relatess to a very-high-strength and low 5 density/ hot-rolled steel sheet, obtained from a strip rolling mill, and to its manufacturing process. It is becoming a necessity to lighten mocor vehicles because of the requriments to reduce CO2 emissions to 10 140 g/Km by 200B, This lightening may be achieved only by increasing the level of mechanical strength cf steels in order to compensate for the reduction in sheet thickness. It is therefore necessary to increase the mechanical properties while reducing the 15 thickness of the sheat with which the parts used are produced. This approach reaches its lix5its with the reduction in rigidity of the parts and the appearance of noise and. vibrations unacceptable in the intended applications in the automobile field, where noise is a 20 discomfort. In the field of hot-rolled flat steel products, the mechanical properties of which are obtained by controXled rolling on a wide-strip mill, the highest 25 strength level is obtained with very high strength steels of bainitic structure, which allow a mechanical strength level or between 800 MPa and 1000 MPa to be achieved, but their density is that of a standard steel; that is to say a density of 7.8 g/cm3. 30 It is also possible to obtain a steel of lower density using an addition element such as aluminum, in which steel an adaition of 6.5% aluminum allows the density to be lowered to 7 g/cm3. This solution does not allow 35 mechanical strength levels of greater than 480 MPa to be achieved. Adding other addition elements, such as chromium vanadium and niobium, with contents ranging up to 1%, 0.1% and 0.1% respectively, does not allow a mechanical strength level of 580 MPa to be exceeded. In this approach, the reduction in density is cancelled out by the poor mechanical strengt properties obtained. 5 The object of the invention is to offer users of hot-rolled steel sheets a low-density sheet having strength levels comparable to the high-strength steel sheet currently used or even of a higher level, and to 10 do so in order to combine the two advantages of low density and high mechanical strength. The first subject of the invention is a very-high-strength and low-density, hot-rolled steel sheet, 15 characterized in that its composition in % by weight comprises; 0,04% 0.01% 0.0005% 0.05% the balance being iron and elements inherent in smelting, characterized in that it includes: 2% 35 In a preferred embodiment of the invention, the steel includes in its composition, in % by weight: 0.04% 0.05% In another preferred ambodiment, the sheet according to 10 the invention is such that the silicon content ia betwean 2 and 3% and that the aluminum content is between 7 and 10%. In another preferred embodiment, the silicon and 15 aluminum contents of the sheet according to the invention are such that: %Si + %A1 > 9 The sheet according to the invention may also have the 20 'following features, individually or in combination; the sheet has a microstructure consisting of a primary ferrite phase and of a secondary ferrite phase, the mean grain size of said primary ferrite being 25 greater than the mean grain size of said secondary ferrite, said microstructure also containing carbide phases, , the sheet has a primary ferrite phase obtained during the reheating of the steel carried out prior to 30 the hot rolling, and a secondary ferrite phase attained after the hot rolling, and also carbide phases; the sheet comprises a primary ferrite phase, the mean grain size o1' which is greater than 5 um, and a secondary ferrite phase whose mean grain size is of 35 lass than 2 um. The second subject of the invention is a process for nanufacturing a hot-rolled sheet, which comprises the steps consisting in: reheating a slab, the composition of which is in accorcdanca with the invention, thus forming a slab whose microstructure comprises a primary ferrite phase and an austenite phase; and then 5 - hot-rolling said slab, the temperature at the end of hot rolling being above the AR3 temperature of the austenitic phase formed during the reheat, so as to carry out rolling under austenitic condltiona, thus transforming the austenitic phase into a secondary 10 ferrite phase and carbide phases. The invention will be clearly understood from the description that follows, given with reference to the appended figures which represent: 15 - in figure 1, a curve showing the variation in the density of a sceel as a function of the silicon content,the aluminum content and/or the plus gilocon plus aluminum content; - in figure 2, the microstructure of a steel 20 according to the invention containing 0.04% carbon (heat I); in figure 3, the microstructure of a steel according to the invention containing 0,160% carbon (heat J); 25 - in figure 4, the microstructure of a steel according to the invention containing 0.2 68% carbon (heat K); and ' in figures 5, the microstructure of a steel containing 0.505% carbon (heat L), the steel being 30 shown for comparison. The steel according to the invention, hot-rolled on a strip rolling mill, has a high mechanical strength and a low densitv. 35 The steal has the following general composition by 0.04% possibly containing the hardening elements: 5 in combination ' ' and/or the elements that act on the transformation temperatures: 0.0005% 0.05% The carbon content of the sheet according to the invention is batween 0.04 and 0.5% by weight, preferably between 0.04 and 0.3% by weight. The change 20 structure of the steal as a function of the carbon content is shown in figures 2 to 5 and indicates that the structure of the steel according to the invention (figures 2 to 4) consists of coarse-grained primary ferrite and of a mixture of carbide phases and of fine 25 secondary ferrite with smaller grains. If the carbon content falls below 0.04%, the microstructure contains no carbide phases and loses mechanical properties. In contrast, if the carbon content exceeds 0.5% by weight, the structure becomes very brittle and it is observed 30 that the microstructure no longer contains primary ferrite (cf. figure 5). Without wishing to be bound by any theory, it is belived that the formation of this novel 35 microstructure is due to the combination of carbon, silicon and aluminum contents, It makes it possible to achieve excellent mechanical properties. Specifically, the steel according to the invention may reach mechanical strength levels ranging from 620 MPa to more -» ,♦> 5 The machanical properties may be enhanced by addition of a microalloying element, such miobium, titanium or vanadium, the last two being less dense than iron. 10 The sheet according to the invention can be manufactured by any suitable prcess However, At is preferred to use the process according to the invention. This process firstly comprises. 15 reheating the slab to a high temperature (preferably above 900°C), prior to the hot tolling. The present inventors have discovered that, during this reheat step, the slab has a microstructure composed of what is called a primery ferritic phase, which forms at high 20 temperature and coexists with an austenltic phase. By hot tolling in such a way that the end-of-rolling temperature remains above the AR3 value calculated for the austenitic phase alone, rolling takes place under 25 austenitic conditions. It is observed that the austenitic phase then completely transforms into a carbide phase/secondary ferrite mixture, the mean grain size of which is less 30 than that of the primary ferrite phase, which remains. Advanageously, a carbon-manganese pair will be chosen so as to have an AR3 transformation temperature such that rolling under austenitic conditions can be 35 guaranteed. Table . 1 below, giving the various compositions according to the invention shows the influence of the various elements on the properties of the steels. The heats A, C, F,H and L are given for comparison, whereas the heats B, D', E, G, I, J and K are according to the invention. The data presented in table 1 show that aluminum by 10 itself does not make it poissible to obtain both a low density of the steel and a high strength level of said steel. 15 20 25 In Che example of the steel referenced E, the rolling temperature was 8950C and the coiling temperature was 6OO0C, with a cooling rate of 490C/s, giving the steel a mechanical strength of 760 MPa. By lowering the coiling temperature/ it is possible to Increase the mechanical strength level. This is the case of the example of the steel referenced D, the cooling temperature of which was 200C with a cooling rate of 50C/s thereby making it possible to achieve a mechanical strength level of 902 MPa. By increasing the cooling rate in the case of a steel xererenced C, produced by rolling at a temperature of 8700C, coiling at a temparatura of 1200C and with a cooling rate of 1300C/s,steel with a mechanical 5 strength of 1296 MPa is obtained. The mechanical strength level may also be adjusted by carbon and manganese consents and/or by the contents of other additional elements as given above. Certain 10 operations, such as for example re-rolling or a heat treatment such aa an annealing operation, may be used to modify or adjust the level of the mechanical properties. 15 According to the invention, the proposed steel meets two contredictory requirement in the hot-rolled steel field of, on the one hand, high mechanioal properties and, on the other hand, low density. The existing solutions for producing steels of very high mechanical 20 strength levels are based on the use of addition elements that do not allow a substantial change in density, and the existing solutions for producing low-density steels are based on the use of addition elements that do not allow a high mechanical strength 25 level to be achieved. The steel of the invention combines these two properties, namely a high mechanical strength level and a very low density, in order to lighten a part that can 30 be used in automobiles. 461/MUMNP/2004 10973 We Claim: 1. A very-high-strength and low-density, hot-rolled steel sheet, its composition in % by weight comprises: 0.04% 0.05% and optionally the following hardening elements: 0.01 % 0.01% 0.01% elements that act on the transformation temperatures: 0.0005% 0.05% 0.05% 0.05% the balance being iron and elements inherent in smelting, characterized in that it comprises silicon between 2 to 3% and aluminum contain between 7 to 10% and provided that silicon and aluminum contains are such that % Si + A1 > 9: 2% 1% and in that said sheet further has a microstructure consisting of a primary ferrite phase and of a secondary ferrite phase, the mean grain size of said primary ferrite being greater than the mean grain size of said secondary ferrite, said microstructure also containing carbide phases. 2. The sheet as claimed in any one of claim 1, further characterized in that said primary ferrite phase is obtained during the reheating of the steel carried out prior to the hot rolling, and said secondary ferrite phase is obtained after the hot rolling. 3. The sheet as claimed in any one of claim 1, characterized in that said primary ferrite phase has a mean grain size of greater than 5 u.m and in that said secondary ferrite phase has a mean grain size of less than 2 u.m. 4. A process for manufacturing a hot-rolled sheet as claimed in any one of claim 1 to 4, characterized in that it comprises the steps consisting in: reheating a slab, the composition of which is in accordance with those of any one of claims 1 to 4, thus forming a slab whose microstructure comprises a primary ferrite phase and an austenite phase; and then - hot-rolling said slab, the temperature at the end of hot rolling being above the AR3 temperature of the austenitic phase formed during the reheat, so as to carry out rolling under austenitic conditions, thus transforming the austenitic phase into a secondary ferrite phase and carbide phases. Dated this 20th day of August, 2004 HIRAL CHANDRAKANT JOSHI AGENT FOR USINOR

Documents:

461-mumnp-2004-cancelled page(28-10-2005).pdf

461-mumnp-2004-claim(granted)-(28-10-2005).doc

461-mumnp-2004-claim(granted)-(28-10-2005).pdf

461-mumnp-2004-correspondence(28-10-2005).pdf

461-mumnp-2004-correspondence(ipo)-(08-06-2006).pdf

461-mumnp-2004-drawing(24-02-2005).pdf

461-mumnp-2004-form 19(20-08-2004).pdf

461-mumnp-2004-form 1a(20-04-2004).pdf

461-mumnp-2004-form 2(granted)-(28-10-2005).doc

461-mumnp-2004-form 2(granted)-(28-10-2005).pdf

461-mumnp-2004-form 3(20-08-2004).pdf

461-mumnp-2004-form 5(20-08-2004).pdf

461-mumnp-2004-power of attorney(20-08-2004).pdf

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