JP2004285436A - Method for producing high strength hot dip galvanized steel sheet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a high strength steel sheet, which is reduced in warpage in the sheet width direction and earing waves in the rolling direction and has an excellent steel sheet shape. <P>SOLUTION: In the method for producing a high strength hot dip galvanized steel sheet, a steel having a composition containing, by mass, 0.03 to 0.15% C, ≤0.7% Si, 2.0 to 3.0% Mn, ≤0.05% P, ≤0.01% S, 0.01 to 0.1% sol.Al, ≤0.005% N, 0.01 to 0.5% Cr and 0.004 to 0.5% V, and further comprising one or more kinds of metals selected from 0.05 to 0.3% Mo, 0.005 to 0.1% Nb, 0.005 to 0.1% Ti and 0.0002 to 0.002% B, and the balance substantially Fe is subjected to hot dip galvanizing treatment, and thereafter, tension T (kgf/mm<SP>2</SP>) on skinpass rolling is imparted thereto so as to satisfy the relation of 2≤T≤80(C+(Cr+V)/10+Mn/20)<SP>2</SP>+10; wherein, C, Cr, V and Mn are mass%. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【００６１】
【表２】

【００６２】
【表３】

【００６３】
【発明の効果】
本発明によれば、プレス成形後に優れた製品形状が得られる反りや耳波の小さい優れた鋼板形状で引張強度７８０ＭＰａ以上の高強度鋼板が、鋼の成分組成と連続溶融亜鉛めっき工程での調質圧延の張力の調整により得られるため、鋼板の形状矯正のため、ストレッチャレベラなど用いる必要がなく、産業上極めて有用である。
【図面の簡単な説明】
【図１】鋼板形状（板幅方向の反り、圧延方向の耳波）に及ぼす張力Ｔと成分組成（Ｃ，Ｃｒ，Ｖ，Ｍｎ）の影響を示す図。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for manufacturing a hot-dip galvanized steel sheet, and more particularly to a method for manufacturing a high-strength hot-dip galvanized steel sheet having an optimum strength of 780 MPa or more for structural components such as pillars and roof rails and having an excellent product shape after press forming.
[0002]
[Prior art]
In order to protect the global environment, various industries, including automobile and chemical manufacturers, are making efforts to reduce CO2 gas. Automobile companies are working to reduce the CO2 gas emitted from automobiles by reducing the fuel consumption of gasoline vehicles. It has been conventionally studied to improve the vehicle weight by reducing the body weight.
[0003]
To reduce the weight of the body, it is effective to reduce the number of parts and to reduce the thickness of the steel sheet.However, in the latter case, there is a concern that the rigidity of the body will deteriorate, so a higher strength steel sheet must be used. On the other hand, from the viewpoint of collision safety performance (when a car collides with an object while driving, it increases the energy absorption capacity of members against impact, reduces the impact load on occupants, and enhances the safety of occupants' lives) In some cases, it is necessary to apply high-strength steel sheets to pillars, roof rails, and the like, and there is an urgent need to develop high-strength steel sheets for automobiles.
[0004]
Properties required for high-strength automotive steel sheets include not only improved press formability such as stretchability and stretch flangeability, but also poor shape such as springback and wall warpage during press forming work (press dies and press-formed products). It is also important to reduce the shape deviation), and a steel plate or the like having a low yield ratio has been developed along with an attempt to design a mold in consideration of the amount of springback.
[0005]
JP-A-55-100958 and JP-A-55-100935 relate to a technique for producing a high-strength galvanized steel sheet having excellent shape properties. The former has a tensile strength of 56.0 to 62.8 kgf /. mm ² (548.8~615.4MPa), steel manufacturing techniques yield ratio from 0.47 to 0.58 is, the latter, tensile strength 56.3~70.5kgf ^/ mm 2 (551.7 ６９690.9 MPa) and a yield ratio of 0.51 to 0.56, respectively.
[0006]
[Problems to be solved by the invention]
However, in order to reduce springback, if the yield strength is reduced and the elastic deformation during press forming is reduced, the tensile strength of the steel sheet will also decrease, and the high strength of the steel sheet will be restricted, and in the above prior art, Also, the obtained steel sheet strength is less than 780 MPa.
[0007]
Therefore, in the case of high-strength steel sheets, it is important to improve the press formability from the viewpoint of press forming technology. As the temperature rises, not only the mechanical uniformity but also the excellent shape of the material steel plate itself without warpage is required.
[0008]
Therefore, an object of the present invention is to provide a technique for producing a high-strength hot-dip galvanized steel sheet having both high strength of 780 MPa or more in tensile strength and excellent shape properties, and excellent press formability.
[0009]
[Means for Solving the Problems]
The present inventors have conducted intensive studies on a method of manufacturing a steel sheet having excellent shape properties after continuous hot-dip galvanizing, with respect to steel having a component composition capable of obtaining high strength of 780 MPa or more in tensile strength. The shape of the steel sheet is affected by the amounts of C, Cr, V, and Mn that affect the formation of martensite and bainite, and the effect of temper rolling after continuous hot-dip galvanizing is greatly affected. It has been found that the shape of the steel sheet is improved by the appropriate tension during the temper rolling. The present invention has been made by further study based on such findings, that is, the present invention,
1. mass (%), C: 0.03 to 0.15%, Si ≦ 0.7%, Mn: 2.0 to 3.0%, P ≦ 0.05%, S ≦ 0.01%, sol . Al: 0.01% to 0.1%, N ≦ 0.005%, Cr: 0.01% to 0.5%, V: 0.004% to 0.5%, the balance being steel consisting essentially of Fe A method for producing a high-strength hot-dip galvanized steel sheet having a tensile strength of 780 MPa or more, characterized by applying a tension T (kgf / mm ² ) during temper rolling after the hot-dip galvanizing treatment so as to satisfy the following formula. is there.
[0010]
2 ≦ T ≦ 80 (C + (Cr + V) / 10 + Mn / 20) ² +10
However, C, Cr, V, and Mn are mass%.
[0011]
2. As the steel composition, Mo: 0.05 to 0.3%, Nb: 0.005 to 0.1%, Ti: 0.005 to 0.1%, B: 0.0002 to 0.002% 2. The method for producing a high-strength hot-dip galvanized steel sheet according to 1, wherein the method includes one or more kinds.
[0012]
BEST MODE FOR CARRYING OUT THE INVENTION
The composition of the steel and the production conditions in the present invention will be described in detail below.
[0013]
1. Component composition C
C is effective for quenching strengthening of steel, and is added in an amount of 0.03% or more to obtain a tensile strength of 780 MPa or more. On the other hand, if it exceeds 0.15%, hard structures such as martensite and bainite are formed unevenly in the rolling direction and the sheet width direction, and shape defects such as ear waves in the rolling direction and warpage in the sheet width direction are caused. Therefore, the content is set to 0.03 to 0.15%.
[0014]
Si
Si may be added in a range of 0.7% or less to strengthen the steel. If it exceeds 0.7%, the adhesiveness of the hot-dip galvanized coating is reduced. Therefore, when adding Si, the content is made 0.7% or less.
[0015]
Mn
Mn is effective for quenching strengthening of steel, and is added in an amount of 2.0% or more to obtain a tensile strength of 780 MPa or more. On the other hand, if the content exceeds 3.0%, when casting molten steel into a slab, Mn segregates in the slab, and during the annealing after rolling, hard phases such as martensite and bainite, together with the ferrite phase, in the sheet thickness direction and the sheet width direction. To 2.0% to 3.0%, since it is formed unevenly and significantly deteriorates the shape of the steel sheet.
[0016]
P
Since P is effective for strengthening steel, it can be contained at 0.05% or less in the present invention. If it exceeds 0.05%, the surface oxide layer (scale) formed at the time of hot rolling is excessively peeled off, and the surface properties after hot rolling and the surface properties after hot-dip galvanizing also deteriorate.
[0017]
Further, in the alloying treatment of the plating layer after the hot-dip galvanizing, it is difficult to uniformly alloy the plating layer, and the adhesion of the plating deteriorates. Therefore, P is set to 0.05% or less.
[0018]
S
S is an impurity, and if present in steel excessively, segregates at austenite grain boundaries during slab heating, and red hot embrittlement easily occurs from the surface layer of the steel sheet during hot rolling. And
[0019]
sol. Al
Al is added in an amount of 0.01% or more to deoxidize steel. On the other hand, if it exceeds 0.1%, the surface appearance after hot-dip galvanizing is significantly deteriorated.
[0020]
N
If N is excessively present in the steel, the ductility of the steel sheet after hot-dip galvanizing deteriorates, so N is set to 0.005% or less.
[0021]
Cr
Cr is added in an amount of 0.01% or more to harden and strengthen the steel. On the other hand, if it exceeds 0.5%, this effect is saturated, and a Cr-based oxide is formed on the steel sheet surface during annealing to deteriorate the adhesion of hot-dip galvanizing. I do.
[0022]
V
V is added in an amount of 0.004% or more to harden and strengthen the steel. On the other hand, if it exceeds 0.5%, V-based nitrides are excessively generated, and the grain size of austenite becomes non-uniform during annealing. Is significantly deteriorated, so that the content is set to 0.004 to 0.5%.
[0023]
The above is the basic component composition of the present invention, but one or more of Mo, Nb, Ti, and B can be added according to desired characteristics.
[0024]
Mo
Mo is effective for strengthening the steel and is added in an amount of 0.05% or more. However, if the content exceeds 0.3%, the effect is saturated.
[0025]
Nb
Nb forms fine carbides with C in steel or forms a solid solution and refines austenite in the annealing step, thereby reducing the hard structure of martensite and bainite to have a uniform structure and excellent shape. Steel plate.
[0026]
In order to obtain such an effect, 0.005% or more is added. However, if it exceeds 0.1%, recrystallization of ferrite and austenite during annealing is suppressed, a processed structure is likely to remain, and ductility is significantly deteriorated. Therefore, the content is set to 0.005 to 0.1%.
[0027]
Ti
Ti forms fine carbides, refines austenite grains during annealing, makes the low-temperature transformation generation phase uniform and fine, and improves the shape of the steel sheet.
[0028]
In order to obtain such an effect, 0.005% or more is added. However, if it exceeds 0.1%, the surface appearance after hot-dip galvanizing is significantly deteriorated.
[0029]
B
B improves the hardenability of steel, segregates at austenite crystal grain boundaries during annealing, suppresses grain growth, turns into fine martensite and bainite, and improves the shape of the steel sheet.
[0030]
In order to obtain such an effect, 0.0002% or more is added. However, if the content exceeds 0.002%, the effect is saturated, so the content is set to 0.0002 to 0.002%.
[0031]
In addition, the effects of the present invention are not impaired unless chemical components other than the above steel components are added in excess. In the present invention, “substantially iron” means that other alloy elements or unavoidable impurities may be contained as long as they do not adversely affect the properties aimed at by the present invention.
[0032]
2. Manufacturing conditions Tension T (kgf / mm ² ) during temper rolling after continuous hot-dip galvanizing
In the present invention, the hot-rolled steel sheet having the above-described composition is subjected to pickling as it is, or cold-rolled after pickling, and after continuous galvanizing treatment, the tension T (kgf / mm ² ) during temper rolling is determined. It is provided so as to satisfy the following equation.
[0033]
2 ≦ T ≦ 80 (C + (Cr + V) / 10 + Mn / 20) ² +10
However, C, Cr, V, and Mn are mass%.
[0034]
When the tension T is inappropriate, shape defects such as wavy irregularities (ear waves) along the rolling direction near the edge of the sheet width and sheet warpage in the sheet width direction occur.
[0035]
When the tension T is less than 2 kgf / mm ² , the sheet warpage in the sheet width direction is not sufficiently corrected, and is as large as 13 to 20 mm.
[0036]
On the other hand, when the tension T exceeds the value obtained from 80 (C + (Cr + V) / 10 + Mn / 20) ² +10, the ear wave is as large as 9 to 13 mm near the edge in the width direction of the plate, and the shape of the steel plate is deteriorated. ≦ T ≦ 80 (C + (Cr + V) / 10 + Mn / 20) ² +10.
[0037]
FIG. 1 shows the results of investigating the effects of the tension T and the amounts of C, Cr, V, and Mn in the base metal components on the shape of the steel sheet. A cold-rolled steel sheet (sheet thickness: 1.4 mm, sheet width: 1000 mm) having a composition within the range of the present invention is annealed (maintained at 820 ° C. for 180 seconds) in a continuous hot-dip galvanizing line, galvanized, and cooled to room temperature. The tension T to be applied was changed to 1 to 30 kgf / mm ² , temper rolling was performed at an elongation of 1.0%, and the obtained galvanized steel sheet was warped in the width direction (mm) and rolled in the rolling direction. (Mm) was measured.
[0038]
The warpage in the sheet width direction was determined by measuring the amount of warpage of the sheet in the sheet width direction at five measurement points randomly selected within a range of 2000 mm in the rolling direction, and calculating an average value.
[0039]
The ear wave was defined as the maximum value of the unevenness at the edge of the sheet width within a range of 2000 mm in the rolling direction.
[0040]
In addition, the tensile strength of the obtained steel plate was 810-1120 MPa. The tension T at the time of temper rolling may be a tension set at the start of temper rolling, and it is not necessary to control the value during rolling.
[0041]
When the tension T is in the range of 2 ≦ T ≦ 80 (C + (Cr + V) / 10 + Mn / 20) ² +10, a steel sheet shape having a warpage amount of 0 to 12 mm in the sheet width direction and an ear wave of 0 to 8 mm in the rolling direction can be obtained. ing.
[0042]
The cause of the generation of ear waves in the rolling direction is that the edge of the sheet width is easier to cool than the center part of the sheet width, so that ferrite is easily generated during annealing, plastically deformed by tension, and the elongation of the edge part increased. Conceivable. On the other hand, it is considered that the warpage in the sheet width direction is due to the shrinkage stress in the sheet width direction due to the formation of a hard phase such as martensite and bainite.
[0043]
The elongation in the temper rolling is desirably 0.1 to 1.0% in order to obtain a good plate shape.
[0044]
In the practice of the present invention, the method of ingot melting and ingot making method of steel are not particularly specified, and the ingot making method may be any of the converter method and the electric furnace method, and the ingot making method may be any of the continuous casting method and the ingot method. .
[0045]
Hot rolling may be performed according to a conventional method. After casting the slab, immediately or after reheating, rough rolling, finish rolling, and winding are performed. The finishing temperature is desirably Ar3 or more and the winding temperature is desirably 700 ° C or less so as not to form an uneven structure.
[0046]
After pickling, the hot-rolled steel sheet is cold-rolled and subjected to continuous hot-dip galvanizing. The heating temperature in the continuous hot-dip galvanizing treatment is set to 750 ° C. or higher in order to recrystallize ferrite and generate martensite and bainite. On the other hand, in order to prevent a coarse structure and a non-uniform structure in the sheet thickness direction. , 950 ° C. or lower.
[0047]
In the present invention, electroplating can be performed after hot-dip galvanizing.
[0048]
【Example】
(Example 1)
The effects of the present invention were confirmed using steels having various component compositions. Table 1 shows the chemical components of the test steel. Steels 1 to 6 are steels of the present invention, and steels 7 to 12 are comparative steels. Each steel was melted and cast in a laboratory to produce a slab having a thickness of 60 mm. However, steel 6 was melted in an electric furnace. The slab was subjected to block rolling to a plate thickness of 30 mm, heat-treated at 1270 ° C. × 1.5 hr in an atmospheric furnace, and subjected to hot rolling. Finish rolling was completed at 860 ° C., and then heat treatment corresponding to winding at 600 ° C. × 1 hr was performed to obtain a hot-rolled sheet having a thickness of 3.5 mm. After pickling, the sheet was cold-rolled to a thickness of 1.4 mm, soaked at 820 ° C for 180 seconds, cooled at an average cooling rate of 8 ° C / sec, immersed in a hot-dip galvanizing bath at 460 ° C, and then zinced at 550 ° C. The plating layer was alloyed. Then, temper rolling was performed at an elongation of 1.0% while applying a tension of 5 kgf / mm ² . The obtained galvanized steel sheet was subjected to a tensile test, surface properties and plating surface appearance evaluation.
[0049]
In the tensile test, a tensile strength (TS) of 780 MPa or more was determined to have good characteristics (indicated by a circle in the table), and a tensile strength (TS) of less than 780 MPa was determined to be insufficient (indicated by a cross in the table) according to JISZ2241.
[0050]
The surface shape of the steel sheet was measured randomly in the rolling direction in the range of 2000 mm in length and the maximum value of the unevenness at the edge of the sheet width (maximum value of the ear wave shape), and was randomly selected in the range of 2000 mm in length. At five points in the longitudinal direction, the amount of sheet warpage in the sheet width direction was measured and evaluated by an average value. The case where the ear wave was 9 to 13 mm or the amount of warpage of the plate was 13 to 20 mm was regarded as characteristic deterioration (indicated by x in the table), the case where the ear wave was 5 to 8 mm was acceptable, and the case where 0 to 4 mm was good. The board warpage amount was set to an allowable range of 6 to 12 mm, and 0 to 5 mm was set to good characteristics. The plating surface appearance was visually inspected in a range of 2000 mm in length, and a case where defects such as non-plating were observed was judged as surface deterioration (x).
[0051]
Table 2 shows the evaluation results. Invention Example No. All of the steels 1 to 6 (steel 1 to 6) were within the range of the present invention, and had a TS of 79 to 1100 MPa, a warp in the width direction of 1 to 3 mm, and an ear wave in the rolling direction of 2 to 4 mm. Was.
[0052]
In addition, the plating surface appearance is all good.
[0053]
On the other hand, Comparative Examples 7 to 12 were inferior in strength, surface shape, and plating surface appearance when the chemical components were outside the range of the present invention.
[0054]
Comparative Example 7 had a low TS of 720 MPa, and Comparative Examples 8 and 10 had good strength and a good steel plate shape. However, non-plating was observed on the plated surface and the surface appearance was poor.
[0055]
In Comparative Examples 9, 11, and 12, the TS is as high as 1010 MPa or more, but both the warp in the sheet width direction and the ear wave in the rolling direction are large, and the sheet takes the shape of a steel sheet.
[0056]
(Example 2)
The effect of the tension during temper rolling on the shape of the steel sheet was investigated using Steel 6. As described above, steel 6 was melted by an electric furnace. A slab having a sheet thickness of 220 mm was produced, heated at 1260 ° C. for 1 hour, and then hot-rolled at a finish rolling temperature of 870 ° C. and a winding temperature of 550 ° C. to produce a hot-rolled material having a sheet thickness of 3.0 mm. After pickling, the sheet was cold-rolled to a thickness of 1.4 mm, heated at 840 ° C, cooled at an average speed of 5 to 10 ° C / s, and immersed in a hot dip galvanizing bath at 460 ° C. The plating layer was alloyed at 550 ° C., cooled, and then temper-rolled at an elongation of 1.0% with various tensions of 1 to ²⁰ kgf / mm ² and wound around a coil.
[0057]
At the time of temper rolling, the surface shape and plating surface appearance at positions corresponding to the positions where the tension was changed were investigated, and tensile test pieces were collected from these positions. The tensile test and the evaluation of the shape of the steel sheet were in accordance with Example 1.
[0058]
Table 3 shows the evaluation results. Each of the steel plates 22 to 25 had a tensile force within the range of the present invention, a high TS of 810 to 830 MPa, a small amount of warpage in the plate width direction and a small ear wave in the plate longitudinal direction, and a good plating surface appearance.
[0059]
On the other hand, the steel plates 21, 26, and 27 have a steel plate tension outside the range of the present invention and have a large amount of warpage in the plate width direction or ear waves in the plate longitudinal direction, and are inferior to the steel plate shape.
[0060]
[Table 1]

[0061]
[Table 2]

[0062]
[Table 3]

[0063]
【The invention's effect】
According to the present invention, a high-strength steel sheet having a tensile strength of 780 MPa or more in an excellent steel sheet shape having a small warpage or an ear wave, which can obtain an excellent product shape after press forming, is obtained by controlling the steel composition and the continuous hot-dip galvanizing process. Since it is obtained by adjusting the tension of temper rolling, there is no need to use a stretcher leveler or the like to correct the shape of the steel sheet, and this is extremely useful in industry.
[Brief description of the drawings]
FIG. 1 is a diagram showing the influence of a tension T and a component composition (C, Cr, V, Mn) on a steel sheet shape (warp in a sheet width direction, ear waves in a rolling direction).

Claims (2)

mass (%), C: 0.03 to 0.15%, Si ≦ 0.7%, Mn: 2.0 to 3.0%, P ≦ 0.05%, S ≦ 0.01%, sol . Al: 0.01% to 0.1%, N ≦ 0.005%, Cr: 0.01% to 0.5%, V: 0.004% to 0.5%, the balance being steel consisting essentially of Fe A method for producing a high-strength hot-dip galvanized steel sheet having a tensile strength of 780 MPa or more, wherein a tension T (kgf / mm ² ) during temper rolling after the hot-dip galvanizing treatment is applied so as to satisfy the following expression.
2 ≦ T ≦ 80 (C + (Cr + V) / 10 + Mn / 20) ² +10
However, C, Cr, V, and Mn are mass%. As the steel composition, Mo: 0.05 to 0.3%, Nb: 0.005 to 0.1%, Ti: 0.005 to 0.1%, B: 0.0002 to 0.002% The method for producing a high-strength hot-dip galvanized steel sheet according to claim 1, wherein one or more kinds are contained.

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