JP3049104B2 - Manufacturing method of high tensile cold rolled steel sheet for deep drawing - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-tensile cold-rolled steel sheet suitable for use as a steel sheet for automobiles and the like.
It is intended to effectively improve the corrosion resistance.

[0002]

2. Description of the Related Art Cold rolled steel sheets used for panels of automobiles and the like are required to have excellent deep drawability as a characteristic. In order to improve the deep drawability, the mechanical properties of the steel sheet
A high Rankford value (hereinafter simply referred to as r value) and high ductility (El) are required. Conventionally, in the assembly of automobile bodies, a large number of pressed parts were joined by spot welding, but recently, some of these parts have been enlarged and integrated to reduce the number of parts and the number of welds The demand has been growing. For example, an oil pan of an automobile is indispensable to be welded due to its complicated shape, but automobile manufacturers strongly demand for integral molding. On the other hand, in order to meet diversifying needs, the design of automobiles has become more complicated, and as a result, the number of parts that are difficult to form with conventional steel sheets has increased. Therefore, in order to meet these demands, a cold-rolled steel sheet having much better deep drawability than before has been required.

In recent years, for the purpose of reducing the weight of a vehicle body and improving safety, a tensile strength (TS) of 35 to 50 kgf / mm has been used.
Cold-rolled steel sheet having a ² a and excellent deep drawability has come to be required. In the press forming of these high-tensile cold-rolled steel sheets, El is an important factor together with the r value.
An excellent strength-ductility balance of S × El ≧ 1700 is required. On the other hand, the adoption of these high-tensile cold-rolled steel sheets makes it possible to reduce the gauge of automotive steel sheets, but the corrosion resistance associated with the reduction in sheet thickness, especially the problem of pitting corrosion, is important. It needed to be excellent.

Various methods have been proposed to improve the deep drawability of the cold rolled steel sheet as described above. For example, JP-B-44-17268, JP-B-44-17269 or JP-B-44-17270 discloses that the r-value is increased to 2.18 by performing cold rolling and annealing twice on a low carbon rimmed steel. A method for producing a cold-rolled steel sheet is disclosed. However, at such an r value, it can no longer be said that it has sufficient deep drawability.

[0005] In "Iron and steel, (1971), S280", C: 0.008 wt%, Mn: 0.31 wt%, P: 0.012 wt%
%, S: 0.015 wt%, N: 0.0057 wt%, Al: 0.036 wt%
%, Ti: 0.20 wt% steel, primary cold rolling reduction: 50
%, Intermediate annealing: 800 ° C x 10 hours, Secondary cold rolling reduction: 80%, Final annealing Annealing 800 ° C x 10 hours to produce deep drawing steel sheet with r value of about 3.1 Is shown to be possible. However, the above-mentioned method has a disadvantage that the total cold-rolling reduction rate is as high as 90%, so that it is not possible to secure the thickness (0.6 mm or more) of the cold-rolled steel sheet that is normally used. No mention is made of the strength-ductility balance or corrosion resistance.

[0006]

SUMMARY OF THE INVENTION By devising the composition of the steel and the production conditions, it is possible to produce a high-tensile cold-rolled steel sheet having much better deep drawability and corrosion resistance than when the conventional method is applied. It is an object of the present invention to propose a new method.

[0007]

As a result of the inventors' earnest research, a cold-rolled steel sheet having extremely excellent deep drawability and corrosion resistance can be obtained by regulating the steel composition and production conditions as follows. I got the knowledge of that. That is, the present invention provides any one of the following compositions a) to c): a) C: more than 0.005 to 0.03 wt%, Si: 1.5 wt% or less,
n: 2.0 wt% or less, P: 0.03-0.15 wt%, S: 0.05 wt%
Hereinafter, Al: 0.01 to 0.15 wt%, N: 0.01 wt% or less,
And a composition comprising one or more selected from Ti: 0.01 to 0.2 wt%, Nb: 0.001 to 0.2 wt%, and B: 0.0001 to 0.003 wt%, the balance being Fe and unavoidable impurities; b) C: 0.005 wt% or less, Si: more than 0.1 to 0.5 wt%, M
n: 2.0 wt% or less, P: 0.03-0.15 wt%, S: 0.05 wt%
Hereinafter, Al: 0.01 to 0.15 wt%, N: 0.01 wt% or less,
And one or two selected from Ti: 0.01 to 0.2 wt%, Nb: 0.001 to 0.2 wt%, and B: 0.0001 to 0.003 wt%.
A composition containing at least one species, the balance being Fe and unavoidable impurities, c) C: 0.03 wt% or less, Si: 1.5 wt% or less, Mn: 2.
0 wt% or less, P: 0.03 to 0.15 wt%, S: 0.05 wt% or less,
Al: 0.01 to 0.15 wt%, Cu: 0.10 to 1.5 wt%, N: 0.01 wt%
%, And Ti: 0.01-0.2 wt% and Nb: 0.
A steel containing one or more selected from 001 to 0.2 wt% and B: 0.0001 to 0.003 wt% and having a balance of Fe and unavoidable impurities and having a balance of 30% after hot rolling. 700 ° C after first cold rolling at the above reduction
To Ac ₃ subjected to intermediate annealing at a temperature range of the transformation point, and continues to more than 30% of rolling reduction total reduction ratio is more than 78% 2
This is a method for producing a high-tensile cold-rolled steel sheet for deep drawing, which comprises performing a next cold rolling and a final annealing in a temperature range from 700 ° C. to the Ac ₃ transformation point. In the present invention, Ni: 0.01 to 1.5 wt%, Cr: 0.05
~ 1.5 wt%, Mo: 0.001 ~ 0.5 wt%.

Hereinafter, the results of the research on which the present invention is based will be described. C: 0.002 to 0.012 wt% (hereinafter simply referred to as%), Si: 0.1 to 0.5%, Mn: 0.1 to 0.8%, P: 0.0
3 to 0.09%, S: 0.010%, Al: 0.05%, N: 0.002
%, Ti: 0.03 to 0.80%, Nb: 0.003 to 0.08% The sheet bar is heated to 1250 ° C, soaked, then hot-rolled at a finishing temperature of 880 ° C, and once cold-rolled and annealed. For the material, cold rolling was performed at a reduction rate of 75% and then recrystallization annealing was performed at 850 ° C for 20 seconds. On the other hand, for the twice cold-rolled and annealed material, the first reduction was performed at a reduction rate of 67%. After cold rolling 780 ℃, 20
Second annealing was performed, followed by secondary cold rolling at a rolling reduction of 65%, and final annealing at 850 ° C. for 20 seconds. The strength-ductility balance of each of the obtained steel sheets was compared and investigated. The result is shown in FIG. As is clear from the figure, the twice cold-rolled and annealed material was compared with the once cold-rolled and annealed material by this experiment.
TS × El ≧ 1700, indicating an excellent strength-ductility balance.

Further, C: 0.002%, Si: 0.02%, Mn: 0.
1%, P: 0.01 to 0.07%, S: 0.010%, Al: 0.05%,
A sheet bar having a composition of N: 0.002% and Cu: 0.01 to 0.3% was heated and soaked at 1250 ° C, and hot-rolled at a finishing temperature of 880 ° C. And 75% for cold rolled and annealed material
Cold rolling at a reduction ratio of 850 ° C., recrystallization annealing at 850 ° C. for 20 seconds, and twice cold-rolled and annealed material was subjected to primary cold rolling at a rolling reduction of 67% to 780 ° C. The second annealing was followed by a second cold rolling at a rolling reduction of 65%, a final annealing at 850 ° C. for 20 seconds, and the corrosion resistance of each steel sheet obtained was investigated. As a method of evaluating corrosion resistance in this experiment, 0.5%
As a corrosion cycle in which NaCl, 0.5% CaCl ₂ , and 0.125% Na ₂ S ₂ O ₅ were immersed in a corrosive solution for 8 hours and then dried for 16 hours, the maximum pit depth after the corrosion test was measured. FIG. 2 shows the results. In this experiment, the corrosion resistance of the steel strongly depended on the composition of the steel, and was significantly improved by the addition of the P-Cu composite. In particular, the twice-rolled-annealed material showed a greater degree of improvement than the normal once-rolled-annealed material.

The present invention is based on the above-mentioned experiments, and variously changes the composition of the steel, the rolling reduction, etc.
Based on the results of repeated studies, the scope of the present invention was limited as follows. The composition of the steel is important in the present invention. C: more than 0.005 to 0.03%, Si: 1.5% or less, Mn: 2.0% or less, P: 0.03 to 0.15%, S: 0.05% or less, Al: 0.01 to 0.15%, N: 0.01% or less, and
Ti: 0.01 to 0.2%, Nb: 0.001 to 0.2%, and B: 0.00
One or two or more selected from 01 to 0.2%, or C: 0.005% or less, Si: more than 0.1
1.5%, Mn: 2.0% or less, P: 0.03 to 0.15%, S: 0.05
%, Al: 0.01 to 0.15%, N: 0.01% or less, and Ti: 0.01 to 0.2%, Nb: 0.001 to 0.2%, and B:
One or more selected from 0.0001 to 0.003%, or C: 0.03% or less, Si: 1.
5% or less, Mn: 2.0% or less, P: 0.03 to 0.15%, S: 0.
05% or less, Al: 0.01 to 0.15%, Cu: 0.10 to 1.5%, N:
Including 0.01% or less, Ti: 0.01-0.2%, Nb: 0.00
It is necessary to contain one or more selected from 1 to 0.2% and B: 0.0001 to 0.003%. In the present invention, Ni: 0.01 to 1.
At least one selected from 5%, Cr: 0.05 to 1.5%, and Mo: 0.001 to 0.5% can be contained.

The reasons for limiting each component will be described below. C: 0.03% or less C is preferred because the smaller the amount of C, the better the deep drawability is. However, in a), the content of more than 0.005% to 0.03% is intended to increase the strength by precipitation strengthening of carbide. is there. The reason why the content is set to 0.005% or less in b) is to improve the deep drawability. In c), 0.03
When the content is 0.03% or less, there is no significant adverse effect, so the content is limited to 0.03% or less.

Si: 1.5% or less Si has the effect of strengthening steel and is added in a necessary amount depending on the desired strength. However, when the added amount exceeds 1.5% in cases a) and c), Since the drawability and corrosion resistance deteriorate, the content is set to 1.5%. The reason for b) being more than 0.1 to 1.5% is that there is no effect if the solid solution strengthening of Si is less than 0.1%.

Mn: 2.0% or less Mn has the effect of strengthening steel and is added in a required amount according to the desired strength. However, if the added amount exceeds 2.0%, deep drawability deteriorates. Therefore, Mn was limited to 2.0% or less.

P: 0.03 to 0.15% P is an important element in the present invention, and has an effect of strengthening steel and improving corrosion resistance. If the amount is less than 0.03%, there is no effect on the improvement of corrosion resistance, while if it exceeds 0.15%, deep drawability is adversely affected. Therefore, P is limited to the range of 0.03 to 0.15%.

S: 0.05% or less S is preferably as small as possible because the deep drawability is improved. However, if the content of S is 0.05% or less, there is no significant adverse effect, so the content is limited to 0.05% or less.

Al: 0.01 to 0.15% Al is deoxidized and added as needed to improve the yield of carbonitride-forming elements. However, if the content is less than 0.01, there is no effect. On the other hand, even if added in excess of 0.15%, there is no further deoxidizing effect. Therefore, Al was limited to the range of 0.01 to 0.15%.

N: 0.01% or less N is preferably as small as possible because the deep drawability is improved. However, if the content of N is 0.01% or less, there is no adverse effect, so the content of N is limited to 0.01% or less.

Ti: 0.01-0.2% Ti is a carbonitriding element and forms a solid solution (C, N) in steel.
(TiC, TiN) is added to precipitate and fix as (TiC, TiN) and form a {111} orientation which is advantageous for improving the deep drawability.
If the amount of addition is less than 0.01%, there is no effect of addition, while
Even if it is added in excess of 2%, no further effect can be obtained, and on the contrary, the surface properties are deteriorated. Therefore, Ti is limited to the range of 0.01 to 0.2%.

Nb: 0.001 to 0.2% Nb is a carbide forming element and is added to precipitate and fix solid solution C in steel as NbC and preferentially form a {111} orientation which is advantageous for deep drawability. You. 0.001
If it is less than 0.2%, the effect is not obtained, while if it exceeds 0.2%, the ductility deteriorates. Therefore, Nb is limited to the range of 0.001 to 0.2%.

B: 0.0001 to 0.0030% B is a nitride-forming element, which precipitates and fixes solute N in steel as BN and preferentially forms a {111} orientation which is advantageous for improvement of deep drawability. Is added to Further, B is added for improving the resistance to secondary working brittleness of steel. However, if the addition amount is less than 0.0001%, there is no addition effect,
On the other hand, if it is added in excess of 0.0030%, deep drawability is adversely affected. Therefore, B is limited to the range of 0.0001 to 0.0030%.

Cu: 0.10-1.5% Cu is important in the present invention, and is added to improve corrosion resistance. If the amount is less than 0.10%, there is no effect on the improvement of the corrosion resistance, while if it exceeds 1.5%, the deep drawability is adversely affected. Therefore Cu is 0.10-1.5%
Limited to the range.

Ni: 0.01-1.5% Ni is added to improve the surface properties of the steel when Cu is added and to improve the corrosion resistance. However, if the addition amount is less than 0.01%, there is no addition effect. , On the other hand, 1.5%
If added in excess of, the drawability will be adversely affected. Therefore
When Ni is added, the content is limited to the range of 0.01 to 1.5%.

Cr: 0.05-1.5% Cr is added for the purpose of improving the corrosion resistance. However, if the added amount is less than 0.05%, there is no effect of addition, while if added over 1.5%, the deep drawability is adversely affected. give. Therefore, Cr is limited to the range of 0.05 to 1.5%.

Mo: 0.001 to 0.5% Mo is added in order to improve the corrosion resistance. However, if the amount of Mo is less than 0.001, there is no effect of addition, and if it exceeds 1.5%, the deep drawability is adversely affected. give. Therefore, Mo was limited to the range of 0.001 to 0.5%.

In the present invention, first, hot rolling conditions are important as conditions for rolling steel having the above composition. In the hot rolling, from the viewpoint of energy saving, rough rolling can be performed immediately without reheating the continuous casting slab or lowering the temperature below the Ar ₃ transformation point after continuous casting, or after performing a heat retaining treatment. The finishing temperature in the hot rolling is preferably higher than the Ar ₃ transformation point to improve the deep drawability, but low-temperature hot rolling at a temperature lower than the Ar ₃ transformation point is also possible. In the present invention, from the viewpoint of energy saving, the winding temperature is 300 to
A winding temperature of 500 ° C. or higher is suitable for accelerating precipitation and improving deep drawing by coarsening crystal grains.

The cold rolling-annealing step subsequent to the above hot rolling is also important. In the present invention, after the primary cold rolling is performed at a rolling reduction of 30% or more, the temperature between 700 ° C. and the Ac ₃ transformation point is obtained. Intermediate annealing is performed in the range. Subsequently, it is necessary to perform secondary cold rolling at a reduction ratio of 30% or more and a total reduction ratio of 78% or more, and then to perform final annealing in a temperature range from 700 ° C. to the Ac ₃ transformation point.

The reason is that if the rolling reduction in the primary and secondary cold rolling is less than 30%, an appropriate rolling texture cannot be formed at the time of cold rolling, so that deep drawing is performed after intermediate annealing and final annealing. This is because it becomes difficult to form the {111} orientation that is advantageous for the drawability, and as a result, the deep drawability deteriorates. If the total draft in the second cold rolling is not 78% or more, a strong {111} orientation is not formed after the final annealing, so that sufficient deep drawability cannot be secured.

The processing temperature in the intermediate annealing and the final annealing is as follows:
If performed in a temperature range lower than 700 ° C., it is difficult to form a {111} orientation favorable for deep drawability, so that deep drawability deteriorates. On the other hand, if annealing is performed in a temperature range higher than the Ac ₃ transformation point, α → γ Since the texture is randomized by the transformation, the deep drawability deteriorates. Therefore, in the present invention, the intermediate annealing and the final annealing are limited to a temperature range from 700 ° C. to the Ac ₃ transformation point.

It is considered that the improvement of the strength-ductility balance by twice cold rolling and annealing is due to the growth of crystal grains during intermediate annealing and final annealing. Concerning the improvement of the corrosion resistance by the twice cold rolling and annealing, it is considered that the corrosion resistance was improved due to the change in the concentration state of the steel sheet surface during the second annealing, but the details are not clear.

In the present invention, after the final annealing, 5
% Or less can be applied. As the annealing step, a continuous galvanizing line in addition to a continuous annealing line can be applied. As the continuous hot-dip galvanizing method, a non-alloyed hot-dip galvanizing treatment and an alloyed hot-dip galvanizing treatment are advantageously suited. After annealing or galvanizing, a special treatment such as Ni plating may be applied to improve the chemical conversion property, weldability, press formability, corrosion resistance, and the like.

[0031]

EXAMPLE A steel slab having the composition shown in Table 1 was heated to 1250 ° C.
After heating and soaking in, or after continuous casting, without reheating, rough rolling and finish rolling are performed under the conditions of FDT: 880 ° C, CT: 550 ° C, and then, after pickling, as shown in Table 2. Primary cold rolling-intermediate annealing-secondary cold rolling-final annealing were performed under various conditions, and the material properties of the obtained product sheet were investigated. The results are shown in Table 2. The tensile properties are
Measured using a JIS No. 5 tensile test specimen, the r value was measured by a three-point method after giving a 15% tensile prestrain, and the L direction (rolling direction), D (45 degree direction in rolling direction) and The average value in the C direction (90-degree direction in the rolling direction) is calculated by the following equation

(Equation 1) Asked. The corrosion resistance was evaluated by the same method as the corrosion resistance test method in the above-described experiment.

[0032]

[Table 1]

[0033]

[Table 2]

As is clear from Table 2, it was confirmed that the steel sheet obtained according to the present invention was excellent in both deep drawability and corrosion resistance.

[0035]

As described above, according to the present invention, it is possible to manufacture a cold-rolled steel sheet having much better deep drawability and corrosion resistance than before, and it is possible to increase the size of the component and to integrally form it. .

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between tensile strength and elongation.

FIG. 2 is a graph showing the relationship between the number of cycles and the maximum pit depth.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-50-15724 (JP, A) JP-A-2-190443 (JP, A) JP-A-2-197528 (JP, A) JP-A-2-197 294429 (JP, A) JP-A-3-97813 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C21D 9/48, 8/04 C22C 38/00-38/58

Claims (4)

(57) [Claims] 1. C: more than 0.005 to 0.03 wt%, Si: 1.5 wt%
%, Mn: 2.0 wt% or less, P: 0.03 to 0.15 wt%, S:
0.05 wt% or less, Al: 0.01 to 0.15 wt%, N: 0.01 wt% or less, and Ti: 0.01 to 0.2 wt%, Nb: 0.001 to 0.2
wt% and B: A steel containing one or more selected from 0.0001 to 0.003 wt%, the balance consisting of Fe and unavoidable impurities is hot rolled, and after hot rolling at a rolling reduction of 30% or more, After cold rolling, intermediate annealing is performed in a temperature range from 700 ° C. to the Ac ₃ transformation point, and then secondary cold rolling is performed with a reduction of 30% or more and a total reduction of 78% or more, After a while
A method for producing a high-strength cold-rolled steel sheet for deep drawing, wherein a final annealing is performed in a temperature range from 700 ° C. to an Ac ₃ transformation point. 2. C: 0.005 wt% or less, Si: more than 0.1 to 1.5
wt%, Mn: 2.0 wt% or less, P: 0.03-0.15 wt%, S: 0.
05 wt% or less, Al: 0.01 to 0.15 wt%, N: 0.01 wt% or less, and Ti: 0.01 to 0.2 wt%, Nb: 0.001 to 0.2 wt
% And B: 1 selected from 0.0001 to 0.003 wt%
The steel containing one or more kinds and the balance consisting of Fe and unavoidable impurities is subjected to primary cold rolling at a rolling reduction of 30% or more after hot rolling, and then a temperature range of 700 ° C. to Ac ₃ transformation point. , Followed by secondary cold rolling with a reduction of 30% or more and a total reduction of 78% or more, and then 700
A method for producing a high-strength cold-rolled steel sheet for deep drawing, wherein the final annealing is performed in a temperature range of from ℃ to the Ac ₃ transformation point. 3. C: 0.03 wt% or less, Si: 1.5 wt% or less, Mn: 2.0 wt% or less, P: 0.03-0.15 wt%, S: 0.05
wt% or less, Al: 0.01 to 0.15 wt%, Cu: 0.10 to 1.5 wt%,
N: 0.01 wt% or less, and Ti: 0.01 to 0.2 wt%,
Nb: 0.001 to 0.2 wt% and B: 0.0001 to 0.003 wt%
One or two or more selected from
And the steel consisting of unavoidable impurities is subjected to primary cold rolling at a rolling reduction of 30% or more after hot rolling, and then to intermediate annealing at a temperature in the range of 700 ° C. to the Ac ₃ transformation point, followed by 30% or more. Cold rolling at a rolling reduction of 78% or more and a final annealing in the temperature range from 700 ° C. to the Ac ₃ transformation point. Manufacturing method of tension cold-rolled steel sheet. 4. The steel composition according to claim 1, 2 or 3,
Ni: 0.01 to 1.5 wt%, Cr: 0.05 to 1.5 wt% and Mo: 0.
A method for producing a high-tensile cold-rolled steel sheet for ultra-deep drawing, comprising one or more selected from 001 to 0.5 wt%.