Title of Invention	"A METHOD OF PRODUCTION OF HIGH STRENGTH HOT DIP GALVANNEALED STEEL SHEET"
Abstract	The present invention provides a method of production of hot dip galvannealed steel sheet excellent in high strength/high ductility and the alloying degree, characterized in that a pickled hot rolled steel sheet or annealed and pickled cold rolled steel sheet containing C: 0.02 to 0.2% and Mn: 0.15 to 2.5% as main ingredients rinsing it, then, Ni preplating it, rapidly heating it in a nonoxidizing atmosphere to a sheet temperature of 430 to 500°C, then hot dip plating it in a galvanizing bath containing Al: 0.05 to 0.2%, then immediately rapidly heating it for alloying treatment for obtaining improved alloying speed and improved plating appearance and plating adhesion.

Title of Invention

"A METHOD OF PRODUCTION OF HIGH STRENGTH HOT DIP GALVANNEALED STEEL SHEET"

Abstract

The present invention provides a method of production of hot dip galvannealed steel sheet excellent in high strength/high ductility and the alloying degree, characterized in that a pickled hot rolled steel sheet or annealed and pickled cold rolled steel sheet containing C: 0.02 to 0.2% and Mn: 0.15 to 2.5% as main ingredients rinsing it, then, Ni preplating it, rapidly heating it in a nonoxidizing atmosphere to a sheet temperature of 430 to 500°C, then hot dip plating it in a galvanizing bath containing Al: 0.05 to 0.2%, then immediately rapidly heating it for alloying treatment for obtaining improved alloying speed and improved plating appearance and plating adhesion.

Full Text	DESCRIPTION A METHOD OF PRODUCTION OF HIGH STRENGTH HOT DIP GALVANNEALED STEEL SHEET TECHNICAL FIELD The present invention relates to a method of production of high strength hot dip galvannealed steel sheet, more particularly relates to a method of production of high strength hot dip galvannealed steel sheet utilizing Ni preplating to keep the deterioration in quality due to the heat treatment in the hot dip galvanization and alloying treatment extremely low and obtain a good plating performance. BACKGROUND ART As part of the measures for reducing the weight of automobiles, high strength, high ductility steel sheets are being used for internal and exterior body panels, chassis parts, etc. For these steel sheets, from the viewpoint of the corrosion resistance, hot dip galvannealed steel sheet is preferably being used, but the C and Mn added to steel as means for increasing strength are known to be alloying retarding elements in galvanization. Obtaining both strength and alloying degree is not easy. In particular, in steel sheet containing Si in an amount of 0.2% or more, with the conventional Senzimir type hot dip galvanizing method, there were the problems that the wettability of the plating was insufficient and the alloying also proceeded extremely difficultly. To deal with this problem, Japanese Patent No. 2526320 discloses a method for producing hot dip galvannealed steel sheet utilizing Ni preplating and using steel sheet containing Si in an amount of 0.2 to 0.5% as a base sheet. Further, the P in steel is known to inhibit and delay the alloying reaction of zinc. An alloying time longer than ordinary steel sheet is required which becomes a factor obstructing productivity. Further, when using the same line to produce both steel sheet with a fast alloying speed (for example, ultralow carbon steel sheet to which Ti or Nb is added) and steel sheet to which P is added, it is necessary to optimally manage the Al concentration in the hot dip galvanizing bath, the alloying treatment conditions, etc. and therefore the operation becomes complicated. Due to the above background, improvement of the alloying speed of P-containing steel sheet is being strongly sought. Various attempts have been made to tackle this. For example, Japanese Patent No. 2526320 proposes to preplate high tension steel sheet containing P with Ni, heat it under predetermined conditions, hot dip galvanize it, then heat alloy it under predetermined conditions. Further, Japanese Patent Publication (B2) No. 7-9055 proposes the method of annealing P-containing steel sheet, pickling it, cleaning the surface, then galvanizing it, plating it, and heat alloying it. DISCLOSURE OF THE INVENTION One of the problems in Japanese Patent No. 2526320 has been the difficulty of producing high strength, high ductility hot dip galvannealed steel sheet of over the 590 MPa class. Further, another problem in this art has been that a long soaking time was required for securing the alloying degree. As a result, both the strength and ductility dropped quite a bit, so there were limits to application to complicatedly shaped automobile internal and exterior body panels, chassis parts, etc. Further, when using P-containing steel sheet as a base sheet, with the method of Japanese Patent No. 2526320, while a certain effect could be expected, the plating appearance easily became uneven and therefore application to automobiles, in particular exterior panels, was difficult. Even with the method of Japanese Patent Publication (B2) No. 7-9055, while a certain effect could be expected, the heat alloying treatment time was still long and, further, obtaining a good enough appearance enabling application to automobiles, in particular external panels, was difficult. In view of the above, the present invention has as its object the provision of a method of production of hot dip galvannealed steel sheet able to achieve both high strength/high ductility and the alloying degree. Further, the present invention has as its object the provision of a method of hot dip galvannealing P-containing steel sheet enabling the alloying speed to be improved and, at the same time, the performance such as the good plating appearance and plating adhesion to be improved. The inventors engaged in repeated studies to solve the above problems and as a result learned that if, as the heat treatment conditions of the alloying in hot dip galvannealing production, rapidly heating at 470 to 550°C by a rate of temperature rise of 30°C/sec or more, holding for soaking for less than 10 seconds-, then cooling, it is possible to prevent a drop in the strength and ductility or keep the drop to a minimum. However, they simultaneously learned that under such alloying conditions, the required alloying degree could not be obtained. In particular, with steel sheet containing Si, the alloying proceeded extremely poorly. The inventors engaged in further repeated studies to achieve both these goals and as a result discovered that the state of the base sheet used and the conditions of the pretreatment of the Ni preplating had serious effects on them and that by optimizing these conditions, high strength/high ductility and alloying degree could both be achieved, and thereby reached the present invention. That is, the present invention has as its gist a method characterized by pickling pickled hot rolled steel sheet containing C: 0.02 to 0.2% and Mn: 0.15 to 2.5% as main ingredients or annealed and pickled cold rolled steel sheet, rinsing it, then, without drying, preplating it with Ni to 0.2 to 2.0 g/m2, rapidly heating it in a nonoxidizing or reducing atmosphere to a sheet temperature of 430 to 500°C by a rate of temperature rise of 30°C/sec or more, then hot dip plating it in a galvanizing bath containing Al: 0.05 to 0.2%, wiping it, then immediately rapidly heating it to 470 to 550°C at a rate of temperature rise of 30°C/sec or more, and cooling it without taking any soaking time or holding it for soaking for less than 10 seconds, then cooling it. The rinsing water after the pickling treatment preferably has a pH of less than 6. Further, in the present invention, after the pickling treatment, it is also possible to preplate Ni without rinsing or drying. Further, the steel sheet of the present invention may also further contain Si in an amount of 0.2 to 3%. Further, for the case where the high steel sheet contains P in an amount of 0.02% or more, the inventors referred to the art disclosed in Japanese Patent Publication (B2) No. 7-9055 and studied various conditions whereby the alloying speed can be improved and a good plating appearance can be obtained even when the Al concentration in the hot dip galvanizing bath is high. As a result, they discovered that pickling P-containing steel sheet after annealing two times is effective. That is, the present invention provides a method of production of high strength hot dip galvannealed steel sheet characterized by pickling annealed high strength steel sheet containing P in an amount of 0.02% or more, drying it, then further pickling it, then preplating it with Ni, heating it in a nonoxidizing atmosphere to 430 to 500°C, plating it in a hot dip galvanizing bath containing Al in an amount of 0.05 to 0.2%, then heat alloying it. Due to the present invention, it is possible to provide a method of production of hot dip galvannealed steel sheet able to achieve both high strength/high ductility and the alloying degree. Further, due to the present invention, P-containing steel sheet can be hot dip galvannealed with a high productivity and a good plating appearance and plating adhesion can also be obtained. BEST MODE FOR WORKING THE INVENTION First, details of the method of production of high strength, high ductility hot dip galvannealed steel sheet will be explained. The present invention covers steel sheet containing C in an amount of 0.02 to 0.2% and Mn in 0.15 to 2.5% as main ingredients. In addition, Si may also be contained in 0.2 to 3%. One of the main points in the present invention is the state of the base sheet used. A pickled hot rolled steel sheet or annealed and pickled cold rolled steel sheet must be used. The pickling of the hot rolled steel sheet is not particularly limited - it is sufficient that a known general method can be used to remove the surface scale. Regarding the pickling of cold rolled steel sheet, sheet passed through a cooling step using water such as vaporization cooling is formed with scale on the surface, so pickling at the back surface in the annealing line is known. Such a sheet can be used as is as the base sheet of the present invention. Sheet passing through gas cooling etc. in the cooling step is usually never pickled at the back surface in the annealing line. Such steel sheet has to be pickled in the present invention. When preplating by Ni the above pickled hot rolled steel sheet or annealed, pickled cold rolled steel sheet, pickling is required as pretreatment. That is, pickling twice when combined with the pickling of the base sheet is one of the main points of the present invention. Due to this, the alloying degree can be secured under conditions not causing deterioration of the strength or ductility. Regarding the concept of the number of times of pickling in the present invention, for example, when assuming passage through a plurality of pickling tanks, if the steel sheet is not dried between one tank and another tank, even if there are a plurality of tanks, the pickling is considered to be a single treatment. This is because by drying (usually rinsing, then drying) after the crystal grains are corroded due to the pickling, the oxygen in the atmosphere causes the surface to be thinly oxidized, repeated pickling of the surface of this steel sheet in the oxidized state enables the C and Mn to be removed uniformly and effectively, and, as a result, a high alloying speed and uniform plating appearance can be obtained. That is, there is meaning in drying of the steel sheet between one pickling and another pickling. As the pickling conditions, treatment by a sulfuric acid or hydrochloric acid aqueous solution is desirable. Other acids would inhibit the alloying, so are not preferred. Note that before the main pickling treatment, if necessary, the sheet may be degreased to remove any dirt. Further, mechanical grinding by brushes etc. may also be combined. The conditions of the rinsing usually performed after the pickling treatment are also important. It is necessary to avoid rinsing then drying before the Ni preplating. Further, the pH of the rinsing water is preferably made less than 6. Further, it is also possible to preplate Ni as is after pickling without rinsing or drying. If the above conditions are not met, the alloying is inhibited. In the present invention, the amount of Ni preplating has to be 0.2 to 2 g/m2. If less than the lower limit, the wettability of the plating becomes insufficient or the alloying degree cannot be obtained. Even if over the upper limit, the effect becomes saturated and thus this is uneconomical. The Ni preplating is not particularly limited in conditions. A sulfuric acid bath, chlorination bath, watt bath, sulfamic acid bath, or other known bath may be used. After the Ni preplating, the sheet is rapidly heated in a nonoxidizing or reducing atmosphere to a sheet temperature of 430 to 500°C by a rate of temperature rise of 30°C/sec or more. This treatment is required for securing the wettability of the hot dip plating and the plating adhesion. After this heating, the sheet is hot dip galvanized and wiped to adjust the basis weight. The concentration of Al in the hot dip galvanizing bath is made 0.05% to 0.2%. If less than 0.05%, the plating adhesion easily deteriorates, while if over 0.2%, achievement of both alloying and quality becomes difficult. The sheet is wiped, then rapidly heated to 470 to 550°C by a temperature rise of 30°C/sec or more, then cooled without taking any soaking time or held for soaking for less than 10 seconds, then cooled so as to alloy it. This provision is important in terms of preventing the deterioration of the strength and ductility and securing the required alloying degree. Next, the details of the hot dip galvannealing method of the P-containing steel sheet will be explained. The P-containing steel sheet of the present invention used may be any of hot rolled, cold rolled, or low carbon steel sheet, ultralow carbon steel sheet, etc. Further, steel sheet containing so-called "trump elements" such as Cr, Cu, Ni, and Sn may also be used. The present invention has as its object obtaining both a high alloying speed and a good plating appearance, so is particularly effective for cold rolled ultralow carbon steel sheet from which a good plating appearance is required. Further, as the amount of addition of P, it is known that when 0.02% or more, the alloying is remarkably retarded and there is a remarkable drop in the productivity, so the invention is particularly effective for steel sheet to which 0.02% or more of P is added. The present invention is characterized by pickling the P-containing steel sheet several times after annealing. Here, the actions and effects of the first pickling treatment after annealing are as described in Japanese Patent Publication (B2) No. 7-9055. Annealing to form crystal grains, then reducing the P present in particularly large amounts at the crystal grains by removal by pickling contributes to improvement of the alloying speed. However, according to the studies of the inventors, when using this step to remove P, in particular only the crystal grains are deeply corroded resulting in a rough surface, so the subsequent plating appearance easily becomes irregular. Further, the effect of removal of P present at the surfaces inside the crystal grains is not sufficient, so the effect of improvement of the alloying speed is small. Therefore, in the present invention, after the above pickling, pickling is further performed. Here, regarding the concept of the number of times of pickling in the present invention, for example, when assuming passage through a plurality of pickling tanks, if the steel sheet is not dried between one tank and another tank, even if there are a plurality of tanks, the pickling is considered to be a single treatment. This is because by drying (usually rinsing, then drying) after the crystal grains are corroded due to the pickling, the oxygen in the atmosphere causes the surface to be thinly oxidized, repeated pickling of the surface of this steel sheet in the oxidized state enables the C and Mn to be removed uniformly and effectively, and, as a result, a high alloying speed and uniform plating appearance can be obtained. That is, there is meaning in drying of the steel sheet between one pickling and another pickling. The method of the pickling is not particularly limited, but a method of treatment under the conditions such as shown in Japanese Patent Publication (B2) No. 7-9055, that is, treatment by a 1 to 5% hydrochloric acid aqueous solution at a temperature of 60 to 90°C for 1 to 10 seconds, is preferably used. Note that the second pickling (when pickling more than two times, the final pickling), is also significant in smoothening the rough surface conditions formed by the first pickling (when pickling more than two times, the immediately previous pickling), so rather than hydrochloric acid treatment, sulfuric acid treatment is more preferable. In this case, the method of treatment in a 5 to 15% sulfuric acid aqueous solution at ordinary temperature to a temperature of 70°C for 1 to 10 seconds is preferably used. After the above pickling and before the hot dip galvanizing, the sheet is preplated with Ni and heated to 430 to 500°C. After the above surface activation, the sheet is plated in a hot dip galvanizing bath containing Al in an amount of 0.05 to 0.2%. The amount of Al was made 0.05 to 0.2% because if less than 0.05%, an extremely large alloying speed can be obtained, but the plating adhesion deteriorates, while .if over 0.2%, even the method of the present invention cannot give a sufficient alloying speed. As a preferable mode of the alloying conditions after plating, the sheet may be rapidly heated to 470 to 600°C by a rate of temperature rise of 20°C/sec or more, then cooled without taking any soaking time or held for soaking for less than 15 seconds, then cooled. According to this treatment, the plating appearance and plating adhesion are good and the productivity is not obstructed. Example 1 First, examples relating to the method of production of high strength, high ductility hot dip galvannealed steel sheet will be explained. Table 1 shows the base sheets used fo-r the tests. The base sheet 1 and base sheet 2 are cold rolled, annealed, pickled steel sheets. The base sheet 3 is a pickled hot rolled steel sheet. Note that Table 3 also shows values of qualities of the materials measured after temper rolling the base sheets. The base sheets were degreasing under the conditions of Table 2, then those that were to be pickled were pickled under the conditions of Table 3. The Ni preplating was performed by electroplating under the conditions of Table 4. After Ni preplating, the sheets were heated in a 3%H2+N2 atmosphere at a rate of temperature rise of •30°C/sec up to 450°C, then were immediately dipped in a hot dip galvanizing bath (containing Al in an amount of 0.15%) held at 450°C, held for 3 seconds, wiped to adjust the basis weight to 50 g/m2, and alloyed right above the wiping by predetermined rates of temperature rise, temperatures, and soaking times. The sheets were cooled by gradual cooling of 2°C/sec for 8 seconds, then rapid cooling by 20°C/sec. After this, the sheets were temper rolled at reduction rates of 0.5%. Table 5 shows the sample production conditions and results of evaluation. Here, for the alloying degree, the plating layer of the sample was dissolved in hydrochloric acid, chemical analysis was used to find the ingredients, and the Fe% in the plating layer was calculated. Samples with an Fe% of 9% or more were deemed "Good", while those with ones of less than 9% were deemed "Poor". Further, for the material quality, each sample was measured to calculate the value of TS x El (Mpa-%). Samples with a drop from the original TS x El of the base sheet shown in Table 1 of less than 10% were evaluated as "Good" and of over 10% as "Poor". Table 1. Test Base Sheet (Table 1 Removed)T able 2. Alkali Degreasing Conditions (Table 2 Removed) Table 3. Pickling Conditions (Table 3 Removed) Table 4. Ni Preplating Conditions (Table 4 Removed) Table 5. Sample Production Conditions and Evaluation Results (Table 5 Removed) In this way, according to the present invention, an excellent alloying degree and material quality were obtained. Example 2 Next, examples relating to the hot dip galvannealing method of a P-containing steel sheet will be explained. In the following examples, cold rolled, annealed steel sheets of the ingredients shown in Table 6 were used. Table 6. Ingredients of Test Base Sheets (Table 6 Removed) (Examples 9 to 11 and Comparative Example 4) Table 7 shows the combinations of the base sheets and treatment conditions used. After the first pickling, the sheets were rinsed and dried. Except for Comparative Example 4, the sheets were pickled the second time, rinsed, then preplated by Ni to a deposition amount of 0.3 g/m2 under the conditions shown in Table 4 by electroplating. The conditions of the pickling are shown in Table 8. After this, the sheets were heated in a 3% hydrogen + 95% nitrogen atmosphere by 40°C/sec to 460°C, then immediately dipped' in a hot dip galvanizing bath held at 455°C and containing Al, and wiped to adjust the plating to a basis weight of 60. The concentrations of Al in the hot dip galvanizing baths are as shown in Table 7. Right after wiping, the sheets were heated by a rate of temperature rise of 50°C/sec to the predetermined temperatures shown in Table 7, soaked for predetermined times, gradually cooled by 10°C/sec for 3 seconds, then cooled by 20°C/sec to ordinary temperature. The evaluation was conducted as follows: Plating appearance: Sheets visually found to have no unevenness in appearance and to be uniform in appearance were evaluated as "Good", while those with unevenness or atterns in appearance and not able to be used (in particular for automobile external panel applications) were evaluated as "Poor". Alloying degree: The plating layer of a sample was dissolved in hydrochloric acid and chemical analysis was used to find its ingredients and thereby calculate the Fe% in the plating layer. Cases of Fe% of 9% or more were deemed "Good" and less than 9% as "Poor". Plating adhesion: The sheet was bent 60° to a V shape, then the plating peeling at the bent part was evaluated by the tape peeling method. A peeling distance of less than 2 mm was evaluated as "Good" and one over that as "Poor". Table 7. Sample Preparation Conditions and Results of Evaluation (Table 7 Removed) Table 8. Pickling Treatment Conditions (Table 8 Removed) Pickling b comprised two seconds each of treatment in two pickling tanks. No rinsing or drying was performed between the pickling tanks. In the above way, according to the present invention, an excellent alloying degree, plating .appearance, and plating adhesion are obtained by a short alloying treatment. INDUSTRIAL APPLICABILITY According to the present invention, hot dip galvannealed steel sheet excellent in quality and alloying degree is obtained, so the value in utilization in industry is tremendous. Further, according to the present invention, it is possible to hot dip galvanneal P-containing steel sheet with a high productivity and further possible to obtain a good plating appearance and plating adhesion, so the value in utilization in industry is tremendous. CLAIMS 1. A method of production of high strength hot dip galvannealed steel sheet characterized by pickling high strength steel sheet, drying it, then further pickling it, then preplating it with Ni to 0.2 to 2.0 g/m2, heating it in a nonoxidizing atmosphere to 430 to 500°C, then plating it in a hot dip galvanizing bath containing Al: 0.05 to 0.2%, then heat alloying it. 2. A method of production of high strength hot dip galvannealed steel sheet characterized by pickling pickled hot rolled steel sheet containing C: 0.02 to 0.2%, Mn: 0.15 to 2.5%, and a balance of Fe and unavoidable impurities or annealed and pickled cold rolled steel sheet, rinsing it, then, without drying, preplating it with Ni to 0.2 to 2.0 g/m2, rapidly heating it in a nonoxidizing or reducing atmosphere to a sheet temperature of 430 to 500°C by a rate of temperature rise of 30°C/sec or more, then hot dip plating it in a galvanizing bath containing Al: 0.05 to 0.2%, wiping it, then immediately rapidly heating it to 470 to 550°C at a .rate of temperature rise of 30°C/sec or more, and cooling it without taking any soaking time or holding it for soaking for less than 10 seconds, then cooling it. 3. A method of production of high strength hot dip galvannealed steel sheet characterized by pickling pickled hot rolled steel sheet containing C: 0.02 to 0.2%, Mn: 0.15 to 2.5%, and a balance of Fe and unavoidable impurities or annealed and pickled cold rolled steel sheet, rinsing it by rinsing water of less than pH 6, then, without drying, preplating it with Ni to 0.2 to 2.0 g/m2, rapidly heating it in a nonoxidizing or reducing atmosphere to a sheet temperature of 430 to 500°C by a rate of temperature rise of 30°C/sec or more, then hot dip plating it in a galvanizing bath containing Al: 0.05 to 0.2%, wiping it, then immediately rapidly heating it to 470 to 550°C at a rate of temperature rise of 30°C/sec or more, and cooling it without taking any soaking time or holding it for soaking for less than 10 seconds, then cooling it. 4. A method of production of high strength hot dip galvannealed steel sheet characterized by pickling pickled hot rolled steel sheet containing C: 0.02 to 0.2%, Mn: 0.15 to 2.5%, and a balance of Fe and unavoidable impurities or annealed and pickled cold rolled steel sheet, then, without rinsing or drying it, preplating it with Ni to 0.2 to 2.0 g/m2, rapidly heating it in a nonoxidizing or reducing atmosphere to a sheet temperature of 430 to 500°C by a rate of temperature rise of 30°C/sec or more, then hot dip plating it in a 'galvanizing bath containing Al: 0.05 to 0.2%, wiping it, then immediately rapidly heating it to 470 to 550°C at a rate of temperature rise of 30°C/sec or more, and cooling it without taking any soaking time or holding it for soaking for less than 10 seconds, then cooling it. 5. A method of production of high strength hot dip galvannealed steel sheet as set forth in any one of claims 1 to 4, characterized in that the pickled hot rolled steel sheet or the annealed and pickled cold rolled steel sheet further contains Si in an amount of 0,2 to 3%. 6. A method of production of high strength hot dip galvannealed steel sheet as set forth in claim 1, characterized in that the high strength steel sheet is annealed steel sheet containing P in an amount of 0.02% or more. 7. A method of production of high strength hot dip galvannealed steel sheet as set forth in claim 6, characterized in that the two pickling methods comprise first pickling by a hydrochloric acid aqueous solution and second pickling by a sulfuric acid aqueous solution. 8. A method of production of high strength hot dip galvannealed steel sheet as set forth in claim 6 or 7, characterized in that the heat alloying method comprises rapidly heating to 470 to 600°C by a rate of temperature rise of 20°C/sec or more and cooling without taking any soaking time or after soaking for less than 15 seconds. 9. A method of production of high strength hot dip galvannealed steel sheet, substantially as herein described in the foregoing disclosure.

Full Text

DESCRIPTION
A METHOD OF PRODUCTION OF HIGH STRENGTH HOT DIP GALVANNEALED STEEL SHEET
TECHNICAL FIELD
The present invention relates to a method of production of high strength hot dip galvannealed steel sheet, more particularly relates to a method of production of high strength hot dip galvannealed steel sheet utilizing Ni preplating to keep the deterioration in quality due to the heat treatment in the hot dip galvanization and alloying treatment extremely low and obtain a good plating performance.
BACKGROUND ART
As part of the measures for reducing the weight of automobiles, high strength, high ductility steel sheets are being used for internal and exterior body panels, chassis parts, etc. For these steel sheets, from the viewpoint of the corrosion resistance, hot dip galvannealed steel sheet is preferably being used, but the C and Mn added to steel as means for increasing strength are known to be alloying retarding elements in galvanization. Obtaining both strength and alloying degree is not easy. In particular, in steel sheet containing Si in an amount of 0.2% or more, with the conventional Senzimir type hot dip galvanizing method, there were the problems that the wettability of the plating was insufficient and the alloying also proceeded extremely difficultly.
To deal with this problem, Japanese Patent No. 2526320 discloses a method for producing hot dip galvannealed steel sheet utilizing Ni preplating and using steel sheet containing Si in an amount of 0.2 to 0.5% as a base sheet.
Further, the P in steel is known to inhibit and delay the alloying reaction of zinc. An alloying time

longer than ordinary steel sheet is required which becomes a factor obstructing productivity. Further, when using the same line to produce both steel sheet with a fast alloying speed (for example, ultralow carbon steel sheet to which Ti or Nb is added) and steel sheet to which P is added, it is necessary to optimally manage the Al concentration in the hot dip galvanizing bath, the alloying treatment conditions, etc. and therefore the operation becomes complicated.
Due to the above background, improvement of the alloying speed of P-containing steel sheet is being strongly sought. Various attempts have been made to tackle this. For example, Japanese Patent No. 2526320 proposes to preplate high tension steel sheet containing P with Ni, heat it under predetermined conditions, hot dip galvanize it, then heat alloy it under predetermined conditions. Further, Japanese Patent Publication (B2) No. 7-9055 proposes the method of annealing P-containing steel sheet, pickling it, cleaning the surface, then galvanizing it, plating it, and heat alloying it.
DISCLOSURE OF THE INVENTION
One of the problems in Japanese Patent No. 2526320 has been the difficulty of producing high strength, high ductility hot dip galvannealed steel sheet of over the 590 MPa class. Further, another problem in this art has been that a long soaking time was required for securing the alloying degree. As a result, both the strength and ductility dropped quite a bit, so there were limits to application to complicatedly shaped automobile internal and exterior body panels, chassis parts, etc.
Further, when using P-containing steel sheet as a base sheet, with the method of Japanese Patent No. 2526320, while a certain effect could be expected, the plating appearance easily became uneven and therefore application to automobiles, in particular exterior panels, was difficult. Even with the method of Japanese Patent Publication (B2) No. 7-9055, while a certain

effect could be expected, the heat alloying treatment time was still long and, further, obtaining a good enough appearance enabling application to automobiles, in particular external panels, was difficult.
In view of the above, the present invention has as its object the provision of a method of production of hot dip galvannealed steel sheet able to achieve both high strength/high ductility and the alloying degree. Further, the present invention has as its object the provision of a method of hot dip galvannealing P-containing steel sheet enabling the alloying speed to be improved and, at the same time, the performance such as the good plating appearance and plating adhesion to be improved.
The inventors engaged in repeated studies to solve the above problems and as a result learned that if, as the heat treatment conditions of the alloying in hot dip galvannealing production, rapidly heating at 470 to 550°C by a rate of temperature rise of 30°C/sec or more, holding for soaking for less than 10 seconds-, then cooling, it is possible to prevent a drop in the strength and ductility or keep the drop to a minimum. However, they simultaneously learned that under such alloying conditions, the required alloying degree could not be obtained. In particular, with steel sheet containing Si, the alloying proceeded extremely poorly. The inventors engaged in further repeated studies to achieve both these goals and as a result discovered that the state of the base sheet used and the conditions of the pretreatment of the Ni preplating had serious effects on them and that by optimizing these conditions, high strength/high ductility and alloying degree could both be achieved, and thereby reached the present invention.
That is, the present invention has as its gist a method characterized by pickling pickled hot rolled steel sheet containing C: 0.02 to 0.2% and Mn: 0.15 to 2.5% as main ingredients or annealed and pickled cold rolled steel sheet, rinsing it, then, without drying, preplating
it with Ni to 0.2 to 2.0 g/m2, rapidly heating it in a nonoxidizing or reducing atmosphere to a sheet temperature of 430 to 500°C by a rate of temperature rise of 30°C/sec or more, then hot dip plating it in a galvanizing bath containing Al: 0.05 to 0.2%, wiping it, then immediately rapidly heating it to 470 to 550°C at a rate of temperature rise of 30°C/sec or more, and cooling it without taking any soaking time or holding it for soaking for less than 10 seconds, then cooling it. The rinsing water after the pickling treatment preferably has a pH of less than 6. Further, in the present invention, after the pickling treatment, it is also possible to preplate Ni without rinsing or drying. Further, the steel sheet of the present invention may also further contain Si in an amount of 0.2 to 3%.
Further, for the case where the high steel sheet contains P in an amount of 0.02% or more, the inventors referred to the art disclosed in Japanese Patent Publication (B2) No. 7-9055 and studied various conditions whereby the alloying speed can be improved and a good plating appearance can be obtained even when the Al concentration in the hot dip galvanizing bath is high. As a result, they discovered that pickling P-containing steel sheet after annealing two times is effective. That is, the present invention provides a method of production of high strength hot dip galvannealed steel sheet characterized by pickling annealed high strength steel sheet containing P in an amount of 0.02% or more, drying it, then further pickling it, then preplating it with Ni, heating it in a nonoxidizing atmosphere to 430 to 500°C, plating it in a hot dip galvanizing bath containing Al in an amount of 0.05 to 0.2%, then heat alloying it.
Due to the present invention, it is possible to provide a method of production of hot dip galvannealed steel sheet able to achieve both high strength/high ductility and the alloying degree. Further, due to the
present invention, P-containing steel sheet can be hot dip galvannealed with a high productivity and a good plating appearance and plating adhesion can also be obtained.
BEST MODE FOR WORKING THE INVENTION First, details of the method of production of high strength, high ductility hot dip galvannealed steel sheet will be explained.
The present invention covers steel sheet containing C in an amount of 0.02 to 0.2% and Mn in 0.15 to 2.5% as main ingredients. In addition, Si may also be contained in 0.2 to 3%.
One of the main points in the present invention is the state of the base sheet used. A pickled hot rolled steel sheet or annealed and pickled cold rolled steel sheet must be used. The pickling of the hot rolled steel sheet is not particularly limited - it is sufficient that a known general method can be used to remove the surface scale. Regarding the pickling of cold rolled steel sheet, sheet passed through a cooling step using water such as vaporization cooling is formed with scale on the surface, so pickling at the back surface in the annealing line is known. Such a sheet can be used as is as the base sheet of the present invention. Sheet passing through gas cooling etc. in the cooling step is usually never pickled at the back surface in the annealing line. Such steel sheet has to be pickled in the present invention.
When preplating by Ni the above pickled hot rolled steel sheet or annealed, pickled cold rolled steel sheet, pickling is required as pretreatment. That is, pickling twice when combined with the pickling of the base sheet is one of the main points of the present invention. Due to this, the alloying degree can be secured under conditions not causing deterioration of the strength or ductility.
Regarding the concept of the number of times of pickling in the present invention, for example, when
assuming passage through a plurality of pickling tanks, if the steel sheet is not dried between one tank and another tank, even if there are a plurality of tanks, the pickling is considered to be a single treatment. This is because by drying (usually rinsing, then drying) after the crystal grains are corroded due to the pickling, the oxygen in the atmosphere causes the surface to be thinly oxidized, repeated pickling of the surface of this steel sheet in the oxidized state enables the C and Mn to be removed uniformly and effectively, and, as a result, a high alloying speed and uniform plating appearance can be obtained. That is, there is meaning in drying of the steel sheet between one pickling and another pickling.
As the pickling conditions, treatment by a sulfuric acid or hydrochloric acid aqueous solution is desirable. Other acids would inhibit the alloying, so are not preferred. Note that before the main pickling treatment, if necessary, the sheet may be degreased to remove any dirt. Further, mechanical grinding by brushes etc. may also be combined.
The conditions of the rinsing usually performed after the pickling treatment are also important. It is necessary to avoid rinsing then drying before the Ni preplating. Further, the pH of the rinsing water is preferably made less than 6. Further, it is also possible to preplate Ni as is after pickling without rinsing or drying. If the above conditions are not met, the alloying is inhibited.
In the present invention, the amount of Ni preplating has to be 0.2 to 2 g/m2. If less than the lower limit, the wettability of the plating becomes insufficient or the alloying degree cannot be obtained. Even if over the upper limit, the effect becomes saturated and thus this is uneconomical. The Ni preplating is not particularly limited in conditions. A sulfuric acid bath, chlorination bath, watt bath, sulfamic acid bath, or other known bath may be used.
After the Ni preplating, the sheet is rapidly heated in a nonoxidizing or reducing atmosphere to a sheet temperature of 430 to 500°C by a rate of temperature rise of 30°C/sec or more. This treatment is required for securing the wettability of the hot dip plating and the plating adhesion. After this heating, the sheet is hot dip galvanized and wiped to adjust the basis weight. The concentration of Al in the hot dip galvanizing bath is made 0.05% to 0.2%. If less than 0.05%, the plating adhesion easily deteriorates, while if over 0.2%, achievement of both alloying and quality becomes difficult.
The sheet is wiped, then rapidly heated to 470 to 550°C by a temperature rise of 30°C/sec or more, then cooled without taking any soaking time or held for soaking for less than 10 seconds, then cooled so as to alloy it. This provision is important in terms of preventing the deterioration of the strength and ductility and securing the required alloying degree.
Next, the details of the hot dip galvannealing method of the P-containing steel sheet will be explained.
The P-containing steel sheet of the present invention used may be any of hot rolled, cold rolled, or low carbon steel sheet, ultralow carbon steel sheet, etc. Further, steel sheet containing so-called "trump elements" such as Cr, Cu, Ni, and Sn may also be used. The present invention has as its object obtaining both a high alloying speed and a good plating appearance, so is particularly effective for cold rolled ultralow carbon steel sheet from which a good plating appearance is required. Further, as the amount of addition of P, it is known that when 0.02% or more, the alloying is remarkably retarded and there is a remarkable drop in the productivity, so the invention is particularly effective for steel sheet to which 0.02% or more of P is added.
The present invention is characterized by pickling
the P-containing steel sheet several times after annealing. Here, the actions and effects of the first pickling treatment after annealing are as described in Japanese Patent Publication (B2) No. 7-9055. Annealing to form crystal grains, then reducing the P present in particularly large amounts at the crystal grains by removal by pickling contributes to improvement of the alloying speed. However, according to the studies of the inventors, when using this step to remove P, in particular only the crystal grains are deeply corroded resulting in a rough surface, so the subsequent plating appearance easily becomes irregular. Further, the effect of removal of P present at the surfaces inside the crystal grains is not sufficient, so the effect of improvement of the alloying speed is small.
Therefore, in the present invention, after the above pickling, pickling is further performed. Here, regarding the concept of the number of times of pickling in the present invention, for example, when assuming passage through a plurality of pickling tanks, if the steel sheet is not dried between one tank and another tank, even if there are a plurality of tanks, the pickling is considered to be a single treatment. This is because by drying (usually rinsing, then drying) after the crystal grains are corroded due to the pickling, the oxygen in the atmosphere causes the surface to be thinly oxidized, repeated pickling of the surface of this steel sheet in the oxidized state enables the C and Mn to be removed uniformly and effectively, and, as a result, a high alloying speed and uniform plating appearance can be obtained. That is, there is meaning in drying of the steel sheet between one pickling and another pickling.
The method of the pickling is not particularly limited, but a method of treatment under the conditions such as shown in Japanese Patent Publication (B2) No. 7-9055, that is, treatment by a 1 to 5% hydrochloric acid aqueous solution at a temperature of 60 to 90°C for 1 to
10 seconds, is preferably used. Note that the second pickling (when pickling more than two times, the final pickling), is also significant in smoothening the rough surface conditions formed by the first pickling (when pickling more than two times, the immediately previous pickling), so rather than hydrochloric acid treatment, sulfuric acid treatment is more preferable. In this case, the method of treatment in a 5 to 15% sulfuric acid aqueous solution at ordinary temperature to a temperature of 70°C for 1 to 10 seconds is preferably used.
After the above pickling and before the hot dip galvanizing, the sheet is preplated with Ni and heated to 430 to 500°C. After the above surface activation, the sheet is plated in a hot dip galvanizing bath containing Al in an amount of 0.05 to 0.2%. The amount of Al was made 0.05 to 0.2% because if less than 0.05%, an extremely large alloying speed can be obtained, but the plating adhesion deteriorates, while .if over 0.2%, even the method of the present invention cannot give a sufficient alloying speed.
As a preferable mode of the alloying conditions after plating, the sheet may be rapidly heated to 470 to 600°C by a rate of temperature rise of 20°C/sec or more, then cooled without taking any soaking time or held for soaking for less than 15 seconds, then cooled. According to this treatment, the plating appearance and plating adhesion are good and the productivity is not obstructed.
Example 1
First, examples relating to the method of production of high strength, high ductility hot dip galvannealed steel sheet will be explained.
Table 1 shows the base sheets used fo-r the tests. The base sheet 1 and base sheet 2 are cold rolled, annealed, pickled steel sheets. The base sheet 3 is a pickled hot rolled steel sheet. Note that Table 3 also shows values of qualities of the materials measured after
temper rolling the base sheets.
The base sheets were degreasing under the conditions of Table 2, then those that were to be pickled were pickled under the conditions of Table 3. The Ni preplating was performed by electroplating under the conditions of Table 4.
After Ni preplating, the sheets were heated in a 3%H2+N2 atmosphere at a rate of temperature rise of •30°C/sec up to 450°C, then were immediately dipped in a hot dip galvanizing bath (containing Al in an amount of 0.15%) held at 450°C, held for 3 seconds, wiped to adjust the basis weight to 50 g/m2, and alloyed right above the wiping by predetermined rates of temperature rise, temperatures, and soaking times. The sheets were cooled by gradual cooling of 2°C/sec for 8 seconds, then rapid cooling by 20°C/sec. After this, the sheets were temper rolled at reduction rates of 0.5%.
Table 5 shows the sample production conditions and results of evaluation. Here, for the alloying degree, the plating layer of the sample was dissolved in hydrochloric acid, chemical analysis was used to find the ingredients, and the Fe% in the plating layer was calculated. Samples with an Fe% of 9% or more were deemed "Good", while those with ones of less than 9% were deemed "Poor". Further, for the material quality, each sample was measured to calculate the value of TS x El (Mpa-%). Samples with a drop from the original TS x El of the base sheet shown in Table 1 of less than 10% were evaluated as "Good" and of over 10% as "Poor".
Table 1. Test Base Sheet

(Table 1 Removed)T
able 2. Alkali Degreasing Conditions

(Table 2 Removed)
Table 3. Pickling Conditions

(Table 3 Removed)
Table 4. Ni Preplating Conditions

(Table 4 Removed)
Table 5. Sample Production Conditions and Evaluation Results

(Table 5 Removed)
In this way, according to the present invention, an excellent alloying degree and material quality were obtained.
Example 2
Next, examples relating to the hot dip galvannealing method of a P-containing steel sheet will be explained.
In the following examples, cold rolled, annealed steel sheets of the ingredients shown in Table 6 were used. Table 6. Ingredients of Test Base Sheets

(Table 6 Removed)
(Examples 9 to 11 and Comparative Example 4) Table 7 shows the combinations of the base sheets and treatment conditions used. After the first pickling, the sheets were rinsed and dried. Except for Comparative Example 4, the sheets were pickled the second time, rinsed, then preplated by Ni to a deposition amount of 0.3 g/m2 under the conditions shown in Table 4 by electroplating. The conditions of the pickling are shown in Table 8. After this, the sheets were heated in a 3% hydrogen + 95% nitrogen atmosphere by 40°C/sec to 460°C, then immediately dipped' in a hot dip galvanizing bath held at 455°C and containing Al, and wiped to adjust the plating to a basis weight of 60. The concentrations of Al in the hot dip galvanizing baths are as shown in Table 7. Right after wiping, the sheets were heated by a rate of temperature rise of 50°C/sec to the predetermined temperatures shown in Table 7, soaked for predetermined times, gradually cooled by 10°C/sec for 3 seconds, then cooled by 20°C/sec to ordinary temperature.
The evaluation was conducted as follows: Plating appearance: Sheets visually found to have no unevenness in appearance and to be uniform in appearance were evaluated as "Good", while those with unevenness or
atterns in appearance and not able to be used (in particular for automobile external panel applications) were evaluated as "Poor".
Alloying degree: The plating layer of a sample was dissolved in hydrochloric acid and chemical analysis was used to find its ingredients and thereby calculate the Fe% in the plating layer. Cases of Fe% of 9% or more were deemed "Good" and less than 9% as "Poor".
Plating adhesion: The sheet was bent 60° to a V shape, then the plating peeling at the bent part was evaluated by the tape peeling method. A peeling distance of less than 2 mm was evaluated as "Good" and one over that as "Poor".

Table 7. Sample Preparation Conditions and Results of Evaluation

(Table 7 Removed)
Table 8. Pickling Treatment Conditions

(Table 8 Removed)
Pickling b comprised two seconds each of treatment in two pickling tanks.
No rinsing or drying was performed between the pickling tanks.
In the above way, according to the present invention, an excellent alloying degree, plating .appearance, and plating adhesion are obtained by a short alloying treatment.
INDUSTRIAL APPLICABILITY
According to the present invention, hot dip galvannealed steel sheet excellent in quality and alloying degree is obtained, so the value in utilization in industry is tremendous. Further, according to the present invention, it is possible to hot dip galvanneal P-containing steel sheet with a high productivity and further possible to obtain a good plating appearance and plating adhesion, so the value in utilization in industry is tremendous.

CLAIMS
1. A method of production of high strength hot dip
galvannealed steel sheet characterized by pickling high
strength steel sheet, drying it, then further pickling
it, then preplating it with Ni to 0.2 to 2.0 g/m2, heating
it in a nonoxidizing atmosphere to 430 to 500°C, then
plating it in a hot dip galvanizing bath containing Al:
0.05 to 0.2%, then heat alloying it.
2. A method of production of high strength hot dip
galvannealed steel sheet characterized by pickling
pickled hot rolled steel sheet containing C: 0.02 to
0.2%, Mn: 0.15 to 2.5%, and a balance of Fe and
unavoidable impurities or annealed and pickled cold
rolled steel sheet, rinsing it, then, without drying,
preplating it with Ni to 0.2 to 2.0 g/m2, rapidly heating
it in a nonoxidizing or reducing atmosphere to a sheet
temperature of 430 to 500°C by a rate of temperature rise
of 30°C/sec or more, then hot dip plating it in a
galvanizing bath containing Al: 0.05 to 0.2%, wiping it,
then immediately rapidly heating it to 470 to 550°C at a
.rate of temperature rise of 30°C/sec or more, and cooling it without taking any soaking time or holding it for soaking for less than 10 seconds, then cooling it.
3. A method of production of high strength hot dip
galvannealed steel sheet characterized by pickling
pickled hot rolled steel sheet containing C: 0.02 to
0.2%, Mn: 0.15 to 2.5%, and a balance of Fe and
unavoidable impurities or annealed and pickled cold
rolled steel sheet, rinsing it by rinsing water of less
than pH 6, then, without drying, preplating it with Ni to
0.2 to 2.0 g/m2, rapidly heating it in a nonoxidizing or
reducing atmosphere to a sheet temperature of 430 to 500°C
by a rate of temperature rise of 30°C/sec or more, then
hot dip plating it in a galvanizing bath containing Al:
0.05 to 0.2%, wiping it, then immediately rapidly heating
it to 470 to 550°C at a rate of temperature rise of
30°C/sec or more, and cooling it without taking any soaking time or holding it for soaking for less than 10 seconds, then cooling it.
4. A method of production of high strength hot dip galvannealed steel sheet characterized by pickling pickled hot rolled steel sheet containing C: 0.02 to 0.2%, Mn: 0.15 to 2.5%, and a balance of Fe and unavoidable impurities or annealed and pickled cold rolled steel sheet, then, without rinsing or drying it, preplating it with Ni to 0.2 to 2.0 g/m2, rapidly heating it in a nonoxidizing or reducing atmosphere to a sheet temperature of 430 to 500°C by a rate of temperature rise of 30°C/sec or more, then hot dip plating it in a 'galvanizing bath containing Al: 0.05 to 0.2%, wiping it, then immediately rapidly heating it to 470 to 550°C at a rate of temperature rise of 30°C/sec or more, and cooling it without taking any soaking time or holding it for soaking for less than 10 seconds, then cooling it.
5. A method of production of high strength hot dip
galvannealed steel sheet as set forth in any one of
claims 1 to 4, characterized in that the pickled hot
rolled steel sheet or the annealed and pickled cold
rolled steel sheet further contains Si in an amount of
0,2 to 3%.
6. A method of production of high strength hot dip
galvannealed steel sheet as set forth in claim 1,
characterized in that the high strength steel sheet is
annealed steel sheet containing P in an amount of 0.02%
or more.
7. A method of production of high strength hot dip
galvannealed steel sheet as set forth in claim 6,
characterized in that the two pickling methods comprise
first pickling by a hydrochloric acid aqueous solution
and second pickling by a sulfuric acid aqueous solution.
8. A method of production of high strength hot dip
galvannealed steel sheet as set forth in claim 6 or 7,

characterized in that the heat alloying method comprises rapidly heating to 470 to 600°C by a rate of temperature rise of 20°C/sec or more and cooling without taking any soaking time or after soaking for less than 15 seconds.
9. A method of production of high strength hot dip galvannealed steel sheet, substantially as herein described in the foregoing disclosure.

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Patent Number

271363

Indian Patent Application Number

8003/DELNP/2007

PG Journal Number

08/2016

Publication Date

19-Feb-2016

Grant Date

17-Feb-2016

Date of Filing

17-Oct-2007

Name of Patentee

NIPPON STEEL & SUMITOMO METAL CORPORATION

Applicant Address

6-1, MARUNOUCHI 2-CHOME, CHIYODA-KU, TOKYO 100-8071, JAPAN

Inventors:

#	Inventor's Name	Inventor's Address
1	ISHIZUKA, KIYOKAZU	C/O NIPPON STEEL CORPORATION HIROHATA WORKS, 1-FUJICHO, HIROHATA-KU, HIMEJI-SHI, HYOGO 671-1188 JAPAN.
2	NISHIMURA,KAZUMI	C/O NIPPON STEEL CORPORATION HIROHATA WORKS, 1-FUJICHO, HIROHATA-KU, HIMEJI-SHI, HYOGO 671-1188 JAPAN
3	KIKUCHI, IKUO	C/O NIPPON STEEL CORPORATION HIROHATA WORKS, 1-FUJICHO, HIROHATA-KU, HIMEJI-SHI, HYOGO 671-1188 JAPAN
4	KAWASAKI, KAORU	C/O NIPPON STEEL CORPORATION HIROHATA WORKS, 1-FUJICHO, HIROHATA-KU, HIMEJI-SHI, HYOGO 671-1188 JAPAN
5	HAJI,JUNJI	C/O NIPPON STEEL CORPORATION HIROHATA WORKS, 1-FUJICHO, HIROHATA-KU, HIMEJI-SHI, HYOGO 671-1188 JAPAN

PCT International Classification Number

C22C

PCT International Application Number

PCT/JP2006/308376

PCT International Filing date

2006-04-14

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	2005-121830	2005-04-20	Japan
2	2005-145023	2005-05-18	Japan