JP4186665B2 - Method for producing high-strength hot-dip galvanized steel sheet - Google Patents

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]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a hot dip galvanized steel sheet, and more particularly to a method for producing a high strength hot dip galvanized steel sheet having an optimum strength of 780 MPa or more for structural parts such as pillars and roof rails and excellent in product shape after press forming.
[0002]
[Prior art]
For the purpose of protecting the global environment, various industries, including automobile and chemical manufacturers, are making efforts to reduce CO2 gas. In order to reduce CO2 gas emitted from automobiles, automobile companies reduce the fuel consumption of gasoline cars. Conventionally, it has been studied to improve the vehicle body by reducing the weight of the vehicle body.
[0003]
In reducing the weight of the car body, it is effective to reduce the number of parts and reduce the thickness of the steel sheet. However, in the latter case, there is a concern about deterioration of the car body rigidity, 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 traveling, the energy absorption capacity of the member against impact is increased, the impact load on the occupant is reduced, and the safety of the occupant's life is increased) However, the application of high-strength steel sheets to pillars, roof rails, etc. is sometimes required, and the development of high-strength automotive steel sheets is urgently needed.
[0004]
Properties required for high-strength automotive steel sheets include improved press formability such as stretchability and stretch flangeability, as well as shape defects such as springback and wall warpage in press forming operations (press molds and press molded products) It is also important to reduce the deviation of the shape of the steel sheet, and steel plates and the like having a low yield ratio have been developed along with attempts to design a mold that allows for the amount of spring back.
[0005]
Japanese Patent Application Laid-Open Nos. 55-100958 and 55-100535 relate to a technique for producing a high-tensile galvanized steel sheet having excellent shape, and the former has a tensile strength of 56.0 to 62.8 kgf / The manufacturing technology for steel with a mm ² (548.8 to 615.4 MPa) and a yield ratio of 0.47 to 0.58, the latter with a tensile strength of 56.3 to 70.5 kgf / mm ² (551.7). ˜690.9 MPa), and steel manufacturing techniques with a yield ratio of 0.51 to 0.56 are disclosed.
[0006]
[Problems to be solved by the invention]
However, in order to reduce the springback, when the yield strength is reduced and the elastic deformation at the time of press forming is reduced, the tensile strength of the steel plate is also reduced, and the increase in strength of the steel plate is restricted, and in the above prior art Also, the steel sheet strength obtained is less than 780 MPa.
[0007]
Therefore, in the case of high-strength steel sheets, it is important to improve press formability from the viewpoint of press forming technology. As it rises, not only mechanical uniformity but also excellent shape with no warpage is required in the raw steel plate itself.
[0008]
Therefore, an object of the present invention is to provide a technique for producing a high-strength hot-dip galvanized steel sheet that has both high strength of a tensile strength of 780 MPa or more and excellent shapeability and is excellent in press formability.
[0009]
[Means for Solving the Problems]
The inventors of the present invention have made extensive studies on a method for producing a steel sheet that has excellent shape after continuous hot-dip galvanizing treatment for steel having a component composition that provides a high strength of 780 MPa or higher. The steel plate shape is affected by the amount of C, Cr, V, Mn, which affects martensite and bainite formation, and the effect of temper rolling after continuous hot dip galvanizing is large. It was found that the shape of the steel sheet was improved by the appropriate tension during temper rolling. The present invention has been made on the basis of such findings, and the present invention has been made.
1. In mass (%), C: 0.03 to 0.15%, Si ≦ 0.7%, Mn: 2.0 to 3.0%, P ≦ 0.05%, S ≦ 0.01%, sol . Al: 0.01~0.1%, N ≦ 0.005 %, Cr: 0.01~0.5%, V: 0.004~0.5%, the steel consisting of the remaining portion Fe and unavoidable impurities After the continuous hot dip galvanizing treatment, a tensile strength T (kgf / mm ² ) at the time of temper rolling is applied so as to satisfy the following formula: manufacture of high strength hot dip galvanized steel sheet having a tensile strength of 780 MPa or more Is the method.
[0010]
2 ≦ T ≦ 80 (C + (Cr + V) / 10 + Mn / 20) ² +10
However, C, Cr, V, and Mn are set to mass%.
[0011]
2. As the steel composition, Mo: 0.05-0.3%, Nb: 0.005-0.1%, Ti: 0.005-0.1%, B: 0.0002-0.002% It is a manufacturing method of the high intensity | strength hot-dip galvanized steel plate of 1 characterized by including 1 type, or 2 or more types.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
The component composition and production conditions of steel in the present invention will be described in detail below.
[0013]
1. Ingredient composition C
C is effective for strengthening the quenching of steel and is added by 0.03% or more in order to obtain a tensile strength of 780 MPa or more. On the other hand, if it exceeds 0.15%, a hard structure such as martensite and bainite is formed non-uniformly 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 occur. Since it becomes remarkable, it is made 0.03 to 0.15%.
[0014]
Si
Si may be added within a range of 0.7% or less in order to strengthen the steel. If it exceeds 0.7%, the adhesiveness of hot dip galvanizing is lowered. Therefore, when Si is added, the content is made 0.7% or less.
[0015]
Mn
Mn is effective for strengthening the quenching of steel and is added in an amount of 2.0% or more in order to obtain a tensile strength of 780 MPa or more. On the other hand, if it exceeds 3.0%, when casting molten steel into a slab, Mn segregates in the slab, and hard phases such as martensite and bainite, together with the ferrite phase, in the plate thickness direction and plate width direction during annealing after rolling. Therefore, the steel plate shape is remarkably deteriorated, so the content is made 2.0 to 3.0%.
[0016]
P
Since P is effective for strengthening steel, 0.05% or less can be contained in the present invention. If it exceeds 0.05%, peeling of the surface oxide layer (scale) formed during hot rolling becomes excessive, 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 plated layer after hot dip galvanizing, the plated layer is difficult to be alloyed uniformly, and the adhesion of the plating is deteriorated. Therefore, P is set to 0.05% or less.
[0018]
S
S is an impurity and, if present excessively in the steel, segregates at the grain boundaries of austenite during slab heating, and red hot brittleness is likely to occur from the surface portion of the steel sheet during hot rolling. And
[0019]
sol. Al
Al is added in an amount of 0.01% or more for deoxidation of steel. On the other hand, if it exceeds 0.1%, the surface appearance after hot dip galvanization is remarkably deteriorated, so 0.01 to 0.1%.
[0020]
N
If N is excessively present in the steel, the ductility of the steel sheet after hot dip galvanization deteriorates, so 0.005% or less.
[0021]
Cr
Cr is added in an amount of 0.01% or more in order to strengthen the steel by hardening. On the other hand, if it exceeds 0.5%, this effect is saturated, and Cr-based oxide is formed on the surface of the steel sheet during annealing to deteriorate the adhesion of hot dip galvanizing. To do.
[0022]
V
V is added by 0.004% or more in order to strengthen the steel by hardening. On the other hand, if it exceeds 0.5%, V-based nitride is excessively formed, and the grain size of austenite becomes non-uniform during annealing, so the martensite becomes non-uniform in the plate width and rolling direction, and the steel plate shape Is significantly deteriorated, so 0.004 to 0.5% is set.
[0023]
The above is the basic component composition of the present invention, and one or more of Mo, Nb, Ti, and B can be added depending on the desired properties.
[0024]
Mo
Mo is effective for strengthening steel, and is added at 0.05% or more. However, if it exceeds 0.3%, the effect is saturated, so 0.05 to 0.3%.
[0025]
Nb
Nb forms fine carbides with C in steel, dissolves, and refines austenite in the annealing stage to refine the hard structure of martensite and bainite and has 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 at the time of annealing is suppressed, the work structure tends to remain, and the ductility is remarkably deteriorated. Therefore, the content is made 0.005 to 0.1%.
[0027]
Ti
Ti forms fine carbides, makes the low temperature transformation generation phase uniform and fine by improving the fineness of the austenite grains during annealing, and improves the steel plate shape.
[0028]
In order to obtain such an effect, 0.005% or more is added, but if it exceeds 0.1%, the surface appearance after hot dip galvanization is significantly deteriorated, so 0.005 to 0.1% is made.
[0029]
B
B improves the hardenability of steel, segregates at the grain boundaries of austenite during annealing, suppresses grain growth, makes fine martensite and bainite, and improves the steel sheet shape.
[0030]
In order to obtain such an effect, 0.0002% or more is added, but if it exceeds 0.002%, the effect is saturated, so 0.0002 to 0.002%.
[0031]
In addition, the chemical components other than the steel components are not particularly impaired unless they are added excessively. In the present invention, the remainder substantially iron means that other alloy elements or unavoidable impurities may be contained as long as they do not adversely affect the intended characteristics of the present invention.
[0032]
2. Manufacturing conditions Tension T (kgf / mm ² ) during temper rolling after continuous galvanizing treatment
In the present invention, the hot-rolled steel sheet having the above-described component composition is pickled as it is or after pickling, and is further subjected to cold rolling, and after continuous galvanizing treatment, the tension T (kgf / mm ² ) during temper rolling is set. It grants so that the following formula may be satisfy | filled.
[0033]
2 ≦ T ≦ 80 (C + (Cr + V) / 10 + Mn / 20) ² +10
However, C, Cr, V, and Mn are set to mass%.
[0034]
When the tension T is inappropriate, a wavy unevenness (ear wave) is formed in the vicinity of the edge of the sheet width along the rolling direction, and shape defects such as sheet warpage are generated in the sheet width direction.
[0035]
When the tension T is less than 2 kgf / mm ² , the plate warp in the plate 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 large as 9 to 13 mm near the edge in the plate width direction, and the steel plate shape deteriorates. ≦ T ≦ 80 (C + (Cr + V) / 10 + Mn / 20) ² +10.
[0037]
FIG. 1 shows the results of investigating the effects of the tension T on the steel plate shape and the amounts of C, Cr, V, and Mn in the base material components. Cold-rolled steel sheets having a composition within the scope of the present invention (thickness of 1.4 mm, width of 1000 mm) are annealed in a continuous hot dip galvanizing line (held at 820 ° C. for 180 seconds), galvanized, cooled to room temperature, and then into steel sheets The tension T to be applied is changed to 1 to 30 kgf / mm ² , temper rolling is performed at an elongation rate of 1.0%, and the warp amount (mm) of the obtained galvanized steel sheet in the plate width direction and the ear wave in the rolling direction. (Mm) was measured.
[0038]
The warpage in the plate width direction was determined by measuring the amount of warpage of the plate in the plate width direction at five measurement points randomly selected within a range of 2000 mm in the rolling direction, and obtaining an average value.
[0039]
The ear wave was defined as the maximum unevenness at the plate width edge portion within the 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]
A steel plate shape having a tension T of 2 ≦ T ≦ 80 (C + (Cr + V) / 10 + Mn / 20) ² +10 and a warp amount in the sheet width direction of 0 to 12 mm and an ear wave in the rolling direction of 0 to 8 mm is obtained. ing.
[0042]
The reason why the ear wave in the rolling direction occurs is that the sheet width edge part is easier to cool than the sheet width center part, so that ferrite is easily generated during annealing, plastic deformation due to tension, and the elongation of the edge part is increased. Conceivable. On the other hand, warpage in the plate width direction is considered to be due to shrinkage stress in the plate width direction accompanying the formation of hard phases such as martensite and bainite.
[0043]
In addition, in order to obtain a favorable plate shape, the elongation rate in temper rolling is desirably 0.1 to 1.0%.
[0044]
In the practice of the present invention, the steel melting method and the ingot forming method are not particularly specified, the melting method may be either a converter method or an electric furnace method, and the ingot method may be either a continuous casting method or a block method. .
[0045]
Hot rolling may be performed according to a conventional method. Rough rolling, finish rolling, and winding are performed immediately after casting the slab or after reheating. It is desirable that the finishing temperature is Ar3 or higher and the winding temperature is 700 ° C. or lower so as not to form a non-uniform structure.
[0046]
The hot-rolled steel sheet is pickled, cold-rolled, and subjected to continuous hot-dip galvanizing treatment. The heating temperature in the continuous hot dip galvanizing process is 750 ° C. or higher in order to recrystallize ferrite and generate martensite and bainite, and on the other hand, to prevent a coarse structure and a non-uniform structure in the thickness direction. 950 ° C. or lower is desirable.
[0047]
In the present invention, electroplating can be performed after the hot dip galvanizing treatment.
[0048]
【Example】
(Example 1)
The effects of the present invention were confirmed using steels having various component compositions. Table 1 shows the chemical composition of the test steel. Steels 1 to 6 are invention steels, and steels 7 to 12 are comparative steels. Each steel was melted in a laboratory and then cast to produce a slab having a thickness of 60 mm. However, the steel 6 was melted in an electric furnace. The slab was batch-rolled to a plate thickness of 30 mm, subjected to heat treatment 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 equivalent to winding at 600 ° C. × 1 hr was performed to obtain a hot rolled sheet having a thickness of 3.5 mm. After pickling, it is 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 460 ° C. hot dip galvanizing bath, and then zinc plated at 550 ° C. The plating layer was alloyed. Thereafter, temper rolling was performed at an elongation rate 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, the tensile strength (TS) was 780 MPa or more with good properties (indicated by a circle in the table) and the strength was insufficient (indicated by x in the table) by a method based on JISZ2241.
[0050]
The surface shape of the steel sheet was randomly selected in the range of 2000 mm in length while measuring the maximum unevenness (maximum value of the ear wave shape) at the edge of the sheet width in the range of 2000 mm in the rolling direction. At five points in the longitudinal direction, the amount of warpage in the width direction of the plate was measured and evaluated by an average value. When the ear wave was 9 to 13 mm or the plate warpage amount was 13 to 20 mm, the characteristic was deteriorated (indicated by x in the table), the ear wave was 5 to 8 mm, and the tolerance was 0 to 4 mm. The sheet warpage amount was 6 to 12 mm, and the tolerance was 0 to 5 mm. As for the plating surface appearance, a range of 2000 mm in length was visually inspected, and when a defect such as non-plating was observed, it was determined as surface deterioration (x).
[0051]
Table 2 shows the evaluation results. Invention Example No. 1 to 6 (steel 1 to 6) are all within the range of the composition of the present invention, TS is 795 to 1100 MPa, warpage in the plate width direction is 1 to 3 mm, and the acoustic wave in the rolling direction is 2 to 4 mm. It was.
[0052]
Also, the plating surface appearance is good.
[0053]
On the other hand, in Comparative Examples 7 to 12, the chemical components are outside the scope of the present invention, and the strength, surface shape, and plating surface appearance are inferior.
[0054]
Comparative Example 7 has a low strength of TS of 720 MPa, and Comparative Examples 8 and 10 have good strength and steel plate shape, but non-plating is observed on the plated surface and the surface appearance is poor.
[0055]
In Comparative Examples 9, 11, and 12, TS is as high as 1010 MPa or more, but both warpage in the sheet width direction and ear waves in the rolling direction are both large and take a steel plate shape.
[0056]
(Example 2)
The effect of tension during temper rolling on the steel sheet shape was investigated using steel 6. As described above, the steel 6 was melted by an electric furnace. A slab having a plate thickness of 220 mm was manufactured, heated at 1260 ° C. for 1 hr, and then hot-rolled at a finish rolling temperature of 870 ° C. and a winding temperature of 550 ° C. to manufacture a hot rolled material having a plate thickness of 3.0 mm. After pickling, the steel sheet was cold rolled to a thickness of 1.4 mm, heated at 840 ° C., cooled at an average rate 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, temper-rolled at various elongations of 1 to ²⁰ kgf / mm ² at an elongation of 1.0%, and wound around a coil.
[0057]
At the time of temper rolling, the surface shape and the 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 steel plate shape were in accordance with Example 1.
[0058]
Table 3 shows the evaluation results. All the steel plates 22 to 25 had a high tension within the range of the present invention, TS was as high as 810 to 830 MPa, the warpage amount in the plate width direction, the ear waves in the plate longitudinal direction were small, and the plated surface appearance was good.
[0059]
On the other hand, the steel plates 21, 26 and 27 are inferior in the steel plate shape because the steel plate tension is outside the range of the present invention, and either the warp amount in the plate width direction or the ear wave in the plate longitudinal direction is large.
[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 an excellent steel plate shape with a small warp and an ear wave that gives an excellent product shape after press forming and a tensile strength of 780 MPa or more can be adjusted in the component composition of steel and the continuous hot-dip galvanizing process. Since it is obtained by adjusting the tension of quality rolling, it is not necessary to use a stretcher leveler or the like for correcting the shape of the steel sheet, which is extremely useful industrially.
[Brief description of the drawings]
FIG. 1 is a graph showing the influence of tension T and component composition (C, Cr, V, Mn) on steel plate shape (warp in the plate width direction, ear wave in the rolling direction).

Claims (2)

In mass (%), C: 0.03 to 0.15%, Si ≦ 0.7%, Mn: 2.0 to 3.0%, P ≦ 0.05%, S ≦ 0.01%, sol . Al: 0.01~0.1%, N ≦ 0.005 %, Cr: 0.01~0.5%, V: 0.004~0.5%, the steel consisting of the remaining portion Fe and unavoidable impurities After the continuous hot dip galvanizing treatment, a tensile strength T (kgf / mm ² ) at the time of temper rolling is applied so as to satisfy the following formula: manufacture of high strength hot dip galvanized steel sheet having a tensile strength of 780 MPa or more Method.
2 ≦ T ≦ 80 (C + (Cr + V) / 10 + Mn / 20) ² +10
However, C, Cr, V, and Mn are set to mass%.   As the steel composition, Mo: 0.05-0.3%, Nb: 0.005-0.1%, Ti: 0.005-0.1%, B: 0.0002-0.002% The manufacturing method of the high intensity | strength hot-dip galvanized steel plate of Claim 1 characterized by including 1 type, or 2 or more types.

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