JPH05230541A - Method for producing high-strength cold-rolled steel sheet and galvanized steel sheet having excellent deep drawability - Google Patents

Abstract

(57) [Summary] [Purpose] To manufacture a high-strength cold-rolled steel sheet having deep drawability that is far superior to conventional ones, and a galvanized steel sheet using this cold-rolled steel sheet. [Structure] C: 0.01 wt% or less, Si: 1.5 wt% or less, Mn:
3.0 wt% or less, Ti: 0.01 to 0.2 wt%, Nb: 0.001 to 0.2
wt%, B: 0.0001 to 0.0020 wt%, Al: 0.01 to 0.20 wt%,
P: 0.03 to 0.20 wt%, S: 0.05 wt% or less and N: 0.01 wt%
% Or less, and the respective contents of C, Ti and N [C], [Ti] and [N] are represented by the following formula: [Ti] ≧ 48 ([C] / 12 + [N] / 14 ) (now basic component composition satisfying the wt%), the steel material balance consisting of Fe and unavoidable impurities, while subjected to lubrication at Ar ₃ transformation point 500 ° C. or higher temperature range, the total rolling reduction is 50 % Or more 95%
After the rolling process described below is performed and then the recrystallization process is performed by the winding or annealing process, cold rolling with a reduction rate of 50 to 95% is performed, and then recrystallization annealing is performed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high-strength cold-rolled steel sheet and a galvanized steel sheet having excellent deep drawability, which are useful for use in steel sheets for automobiles.

[0002]

BACKGROUND OF THE INVENTION Cold-rolled steel sheets used for automobile panels and the like are required to have excellent deep drawability. In order for the steel sheet to exhibit excellent deep drawability, the mechanical properties of the steel sheet are high r value (Rankford value) and good ductility (El.
) Is required.

Various methods have been proposed to improve the deep drawability. For example, JP-B-44-17268, JP-B-44
-17269 and JP-B-44-17270 disclose that the r value is 2.18 by subjecting a low carbon rimmed steel to cold rolling-annealing twice.
A method for manufacturing a cold-rolled steel sheet which has been improved to the above is disclosed. However, these methods involve cold rolling and recrystallization annealing.
It has to be performed one by one, and the energy and cost required therefor are enormous. .

On the other hand, in recent years, the tensile strength is 35 to 60 kgf / mm ² for the purpose of weight reduction and safety improvement of automobiles.
As described above, the momentum to use higher strength steel sheets has been rapidly increasing. Even with such a high strength steel plate,
Needless to say, it is required to exhibit excellent deep drawability at the time of press forming. Therefore, a steel sheet having higher strength and a high r value equal to or higher than that of conventional steel and excellent ductility. Research and development is underway.

For the production of such a high strength cold rolled steel sheet for deep drawing, low carbon containing Si, Mn, P, etc. as a strengthening component is used.
In general, Al-killed steel was generally hot-rolled, then cold-rolled, and then recrystallized and annealed. However, in order to obtain high strength, a large amount of the above-mentioned strengthening components must be contained, so that a texture unfavorable for deep drawability is formed, and only a steel sheet having a low r value has been obtained.

[0006]

SUMMARY OF THE INVENTION The present invention advantageously solves the above-mentioned problems, and by controlling the steel composition and manufacturing conditions, it is possible to obtain a high-strength cold drawn steel having a deep drawability that is far superior to conventional ones. It is an object of the present invention to propose a method for producing a rolled steel sheet and a galvanized steel sheet using this cold rolled steel sheet.

[0007]

As a result of intensive studies to improve the deep drawability, the inventors of the present invention have excellent deep drawability by limiting the steel composition and manufacturing conditions as follows. It has been found that a high strength cold rolled steel sheet can be obtained.

The gist of the present invention is as follows. (1) C: 0.01 wt% or less, Si: 1.5 wt% or less, Mn: 3.0 wt%
%, Ti: 0.01 to 0.2 wt%, Nb: 0.001 to 0.2 wt%,
B: 0.0001 to 0.0020 wt%, Al: 0.01 to 0.20 wt%, P: 0.
03 to 0.20 wt%, S: 0.05 wt% or less and N: 0.01 wt% or less, and the respective contents of C, Ti and N [C], [T
i] and [N] are represented by the following formula [Ti] ≧ 48 ([C] / 12 + [N]
/ 14) (wt%) is satisfied, and the balance is
A steel material consisting of Fe and unavoidable impurities is lubricated in a temperature range not lower than the Ar ₃ transformation point and not lower than 500 ° C, and is rolled so that the total reduction ratio is not lower than 50% and not higher than 95%. After the recrystallization treatment by the annealing process, the rolling reduction is 50 ~
A method for producing a high-strength cold-rolled steel sheet excellent in deep drawability, which comprises performing 95% cold rolling and subsequently performing recrystallization annealing (first invention).

(2) In addition to the steel composition of the first invention, Mo: 0.01-
A method for producing a high-strength cold-rolled steel sheet containing 1.0 wt% and having excellent deep drawability (second invention).

(3) Production of a high-strength cold-rolled steel sheet containing Cu: 0.1 to 1.5 wt% and Ni: 0.1 to 1.5 wt% in addition to the steel components of the first or second invention and having excellent deep drawability Method (3rd invention).

(4) Production of high-strength galvanized steel sheet excellent in deep drawability, in which recrystallization annealing after cold rolling in the first invention, second invention or third invention is performed in a continuous hot-dip galvanizing line. Method (4th invention).

The following describes the results of the research on which the present invention was developed. Si: 0.5 wt%, Mn: 0.5 wt%, P: 0.06 wt%, S: 0.01
wt%, Al: 0.05 wt%, N: 0.002 wt%, Nb: 0.001 wt%
And B: 0.001 wt%, C and Ti, C: 0.001 ~
0.01 wt%, Ti: variously contained in the range of 0.01 to 0.2 wt%, with the balance being a hot-rolled steel strip with a composition of substantially 700
After heating and soaking at ℃, 60% lubrication rolling was performed in one pass. After that, it is pickled and cold-rolled at a reduction rate of 75% to obtain a plate thickness of 0.7 mm.
After that, recrystallization annealing was performed at 850 ° C. for 20 s to obtain a test steel.

The effect of the steel components on the r value of the test steel thus obtained was investigated in relation to Ti-48 (C / 12 + N / 14), and the results are shown in FIG. For comparison, a steel manufactured under the same conditions as above except that Nb was not included was also examined. From the figure, the r-value after cold rolling-annealing strongly influences the steel composition, and Ti-Nb composite added steel satisfying Ti ≧ 48 (C / 12 + N / 14) shows excellent r-value. Recognize.

[0014]

As described above, in the present invention, the steel composition is important, and if the above composition range is not satisfied, excellent deep drawability cannot be secured. Hereinafter, the reason for limiting the range of each component will be described.

C: 0.01 wt% or less C is preferably as small as possible because the smaller the content is, the more the deep drawability is improved. However, when the content is 0.01 wt% or less, it does not exert a bad influence. So
It was limited to 0.01 wt% or less.

Si: 1.5 wt% or less Si has a function of strengthening steel, and a necessary amount is contained according to desired strength, but the content is 1.5 wt%.
If it exceeds the range, the deep drawability and surface properties will be adversely affected.
Limited to 1.5 wt% or less. In order to exert the above-mentioned effects, it is preferable to contain about 0.1 wt% or more. Mn: 3.0 wt% or less, Mn has the effect of strengthening steel, and contains the necessary amount according to the desired strength, but its content is 3.0 wt%
If it exceeds 1.0%, the deep drawability is adversely affected, so it was limited to 3.0 wt% or less. In order to exert the above-mentioned action,
It is preferable to contain about 0.5 wt% or more.

Ti: 0.01 to 0.2 wt% Ti is an important component in the present invention, and solid solution (C, N) in steel is precipitated and fixed as carbonitride to reduce it, which is advantageous for deep drawability. This has the effect of preferentially forming crystal grains in the {111} orientation. If the content is less than 0.01 wt%, it has no effect. On the other hand, if the content exceeds 0.2 wt%, not only the effect is not improved, but also ductility deteriorates, so it is limited to 0.01-0.2 wt%. did.

Nb: 0.001 to 0.2 wt% Nb is an important component in the present invention. It has the effect of reducing the solid solution C in the steel and, by containing a Ti-Nb composite, the microstructure before finish rolling. Due to the miniaturization of
It is effective in improving the value. If the Nb content is less than 0.001 wt%, the effect is not exhibited, while if it exceeds 0.2 wt%, the effect is not improved, so the content was limited to 0.001 to 0.2 wt%.

B: 0.0001 to 0.0020 wt% B is contained to improve the secondary processing brittleness resistance. If the content is less than 0.0001 wt%, there is no effect,
On the other hand, if the content exceeds 0.002 wt%, the deep drawability deteriorates, so the content was limited to 0.0001 to 0.002 wt%. Al: 0.01 to 0.20 wt% Al is contained in a necessary amount for deoxidizing and improving the yield of carbonitride forming components. If the content is less than 0.01 wt%, the effect is None, meanwhile 0.20wt
%, The further deoxidizing effect cannot be obtained, so the content was limited to 0.01 to 0.20 wt%.

P: 0.03 to 0.20 wt% P has a function of strengthening steel and contains a necessary amount according to the desired strength, but the content is 0.03 wt%.
If it does not satisfy the above condition, there is no effect of increasing the strength, while if it exceeds 0.20 wt%, the deep drawability is adversely affected.

S: 0.05 wt% or less Since the smaller the content of S is, the deep drawability is improved,
It is preferable to suppress the content as much as possible, but if the content is 0.05 wt% or less, it does not have a bad effect so much.
Limited to 0.05 wt% or less.

N: 0.01 wt% or less N is the less, the deeper drawability is improved.
It is preferable to suppress the content as much as possible, but if the content is 0.01 wt% or less, it does not have a bad effect.
It was limited to 0.01 wt% or less.

In the present invention, the respective contents [C], [Ti] and [N] of C, Ti and N are expressed by the following formulas.

[Equation 2] [Ti] ≧ 48 ([C] / 12 + [N] / 14) (wt%) (1) must be satisfied. Ti is a carbonitride forming component as described above, and has an action of reducing solid solution (C, N) in steel and preferentially forming crystal grains of {111} orientation, which is advantageous for deep drawability. .. However, when the above formula (1) is not satisfied, the effect is not seen as is clear in FIG. 1, and excellent deep drawability cannot be obtained.

[Formula 3] [Ti] ≧ 48 ([C] / 12 + [N] / 14) (wt%)

Mo: 0.01 to 1.0 wt% Mo has the effect of strengthening steel, and is contained according to the desired strength in the second invention, but its content is 0.01.
If it is less than wt%, it has no effect, while if it exceeds 1.0 wt%, it adversely affects the deep drawability, so it was limited to 0.01-1.0 wt%.

Cu: 0.1-1.5 wt% Cu has the effect of strengthening steel, and is contained according to the desired strength in the third invention, but its content is 0.1.
If it is less than wt%, it has no effect. On the other hand, if it exceeds 1.5 wt%, the deep drawability is adversely affected.

Ni: 0.1-1.5 wt% In the third invention, Ni is contained. Ni has the effect of strengthening the steel and also has the effect of improving the surface properties of the steel sheet when it contains Cu. If the content is less than 0.1 wt%, there is no effect,
On the other hand, if it exceeds 1.5 wt%, the deep drawability will be adversely affected.
Limited to 0.1-1.5 wt%.

Next, the reason why the manufacturing process is limited in the present invention will be described. Rolling process The rolling process is the most important in the present invention. In the temperature range below Ar ₃ transformation point and above 500 ℃, the rolling process is carried out to reduce the total rolling reduction to 50% to 95%. It is necessary to perform recrystallization treatment by a winding or annealing process. Here, in the temperature range higher than the Ar ₃ transformation point, the texture is randomized by the γ-α transformation no matter how much rolling is performed, so the {111} texture is not formed in the hot-rolled sheet, and therefore cold rolling-annealing is performed. Later only low r-values are obtained. On the other hand, even if the rolling temperature is lowered to less than 500 ° C., the r value cannot be further improved and the rolling load only increases, so the rolling temperature was limited to 500 ° C. or higher below the Ar ₃ transformation point.

If the rolling reduction of this rolling is less than 50%, the {111} texture is not formed in the hot rolled sheet, while if it exceeds 95%, an unfavorable texture is formed in the r value in the hot rolled sheet. Therefore, it is limited to 50% or more and 95% or less.

Further, if the rolling below the Ar ₃ transformation point is a non-lubricating rolling, due to the shear deformation due to the frictional force between the roll and the steel sheet, the crystal grains in the {110} orientation, which are unfavorable to the deep drawability, are formed in the steel sheet surface layer. Since it is preferentially formed in the part and the improvement of the r value cannot be expected, it is necessary to perform the lubrication rolling in order to secure the deep drawability.

Here, the roll diameter, the roll structure, the type of lubricant and the type of rolling mill in the above rolling may be arbitrary. The above pre-rolling step is not particularly limited, for example, for a rolling material, after reheating or continuous casting the continuous casting slab, immediately without lowering the temperature below the Ar ₃ transformation point, or a heat retention treatment. It is preferable to use a sheet bar obtained by rough rolling the obtained sheet. Further, the above-mentioned rolling may be carried out subsequently to the hot rolling in which the finishing temperature is Ar ₃ or higher. As a rough rolling condition for such hot rolling, it is preferable to set the rough rolling finish temperature to (Ar ₃ transformation point to Ar ₃ transformation point + 100 ° C.) for the purpose of refining the structure before finish rolling.

Next, since the hot rolling temperature of the steel of the present invention is not higher than the Ar ₃ transformation point, the rolled plate has a worked structure. Therefore, the rolled plate was then subjected to recrystallization treatment, {11
It is necessary to form crystal grains of 1} orientation. If the recrystallization treatment is not performed, the crystal grains in the {111} orientation are not formed on the rolled plate, so that the r value cannot be improved even by the subsequent cold rolling-annealing. The recrystallization treatment can be performed by a winding or annealing process. For the recrystallization treatment by the winding process, a winding temperature of 600 ° C or higher is preferable, and at a winding temperature lower than that, recrystallization is not completed, and no improvement in r value can be expected by subsequent cold rolling-annealing. .. When recrystallization is performed by annealing after hot rolling, the coiling temperature during hot rolling may be arbitrary, and recrystallization annealing at 600 ° C or higher is preferable.

Cold Rolling Step This step is essential for obtaining a high r value, and it is essential that the cold rolling reduction is 50 to 95%. If the cold rolling reduction is less than 50% or more than 95%, excellent deep drawability cannot be obtained.

Annealing Step The cold rolled steel strip that has undergone cold rolling needs to be subjected to recrystallization annealing. This recrystallization annealing may be either a box-type annealing method or a continuous-type annealing method. The annealing temperature is preferably in the range of 650 to 980 ° C. Needless to say, the annealed steel strip may be subjected to temper rolling of 10% or less, which is usually performed, in order to adjust the surface roughness and the like. Further, the cold-rolled steel sheet obtained by the present invention can be applied to the original plate of the surface-treated steel sheet for processing. The surface treatment includes zinc plating (including alloy system), tin plating, enamel and the like.

[0034]

EXAMPLES Steel slabs having various compositional compositions shown in Table 1 were prepared.

[0035]

[Table 1]

These slabs were subjected to rough rolling, finish rolling and recrystallization under the hot rolling conditions shown in Table 2. In Tables 1 and 2, those whose numerical values are out of the range of the present invention are underlined.

[0037]

[Table 2]

The hot-rolled sheet thus obtained was pickled, cold-rolled under the conditions shown in Table 2 to obtain a cold-rolled steel sheet having a thickness of 0.7 mm, and
Recrystallization annealing was performed under the conditions shown in Table 2 in a continuous annealing facility. In Table 2, No. 12, the final annealing was performed by recrystallization annealing and plating treatment in a continuous hot dip galvanizing facility. The results of examination of the material properties of the cold rolled steel sheet thus obtained are shown in Table 2.
Also described in. The tensile properties were measured using JIS No. 5 tensile test pieces. The r-value was measured by the 3-point method after applying 15% tensile prestrain, and was measured in the L direction (rolling direction), D direction (45 ° direction from rolling direction) and C direction (90 ° from rolling direction).
Average value

## EQU4 ## It was obtained from the equation r = (r _L + 2r _D + r _C ) / 4. Further, as to the evaluation of the secondary processing brittleness resistance, the sample processed with the limiting drawing ratio of 2.8 was cooled to -50 ° C, and then the crushing test was conducted to evaluate the presence or absence of brittle cracking.

As is clear from Table 2, all the conforming examples according to the present invention have a better deep drawability than the comparative examples.

[0040]

According to the present invention, it is possible to manufacture a high-strength cold-rolled steel sheet excellent in deep drawability by limiting the steel composition and manufacturing conditions.

[Brief description of drawings]

FIG. 1 is a graph showing the effect of steel components on r-value on r-value in relation to Ti-48 (C / 12 + N / 14).

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Internal reference number FI Technical indication C22C 38/00 T 7217-4K 38/14 38/16

Claims (4)

[Claims] 1. C: 0.01 wt% or less, Si: 1.5 wt% or less, Mn: 3.0 wt% or less, Ti: 0.01 to 0.2 wt%, Nb: 0.001 to 0.2 wt%, B: 0.0001 to 0.0020 wt%, Al: 0.01 to 0.20 wt%, P: 0.03 to 0.20 wt%, S: 0.05 wt% or less and N: 0.01 wt% or less, and each content of C, Ti and N [C], [T
i] and [N] are basic component compositions satisfying the following formula [Ti] ≧ 48 ([C] / 12 + [N] / 14) (wt%), and the balance is Fe and unavoidable impurities. Steel material consisting of 50% or more and 95% or less of total rolling reduction while being lubricated in the temperature range of 500 ° C or more below the Ar ₃ transformation point.
The following rolling process is performed, then, after performing recrystallization treatment by a winding or annealing step, cold rolling with a reduction rate of 50 to 95% is performed, and subsequently, recrystallization annealing is performed.
A method for producing a high-strength cold-rolled steel sheet having excellent deep drawability. 2. The method for producing a high-strength cold-rolled steel sheet having excellent deep drawability according to claim 1, which contains Mo: 0.01 to 1.0 wt% in addition to the basic component composition. 3. The method for producing a high-strength cold-rolled steel sheet excellent in deep drawability according to claim 1, which contains Cu: 0.1 to 1.5 wt% and Ni: 0.1 to 1.5 wt% in addition to the basic composition. .. 4. The method for producing a high-strength galvanized steel sheet excellent in deep drawability according to claim 1, 2 or 3, wherein recrystallization annealing after cold rolling is performed in a continuous hot-dip galvanizing line.