JPH05230539A - Production of high tensile strength cold rolled steel sheet excellent in deep drawability - Google Patents

Abstract

PURPOSE:To produce a high tensile strength cold rolled steel sheet having deep drawability equal to that of a steel prepared by a conventional process at a low energy cost with superior productivity. CONSTITUTION:A steel having a composition which consists of, by weight, <=0.01% C, <=1.5% Si, 0.1-3.0% Mn, 0.01-0.2% Ti, 0.001-0.2% Nb, 0.0001-0.0030% B, 0.01-0.20% Al, 0.05-0.20% P, <=0.05% S, <=0.01% N, and the balance Fe with inevitable impurities and in which [C], [Ti], [S], and [N] satisfy inequality [Ti]>=48([C]/12+[N]/14+[S]/32)(wt.%), where [C], [Ti], [S], and [N] represent respective contents of the above-mentioned C, Ti, S, and N, is used. This steel is subjected to hot roughing at high temp. in the course between continuous casting and temp. fall down to a temp. not higher than the Ar3 transformation point and this hot roughing is finished at 900-1100 deg.C. After successive hot finish rolling, the resulting steel plate is pickled and cold-rolled at 60-95% draft, followed by recrystallization annealing at 800-980 deg.C.

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 excellent in deep drawability, which is useful for the purpose of using steel sheets for automobiles, etc., and particularly proposes a method which can be produced at a low energy cost. Is what you are trying to do.

[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.

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 automobile bodies.
There has been a rapid increase in the motivation to use higher tensile strength steel sheets. Even with such a high-tensile steel plate,
Needless to say, it is required to exhibit excellent deep drawability during press forming. Therefore, a steel sheet having higher tensile strength and a high r value equal to or higher than that of conventional steel and excellent ductility. Research and development is underway.

By the way, in recent years, continuous casting of slabs with Ar _{3 is} carried out for the purpose of cost reduction by energy saving and process saving.
A direct rolling method, in which hot rolling is performed without lowering the temperature below the transformation point, has been tried.For example, in JP-A-52-105220, low-carbon Al-killed steel is directly rolled at a temperature of 900 ° C. or higher. By starting rough rolling and completing hot rolling within 180 minutes after the center temperature of the continuous cast slab has dropped to 1100 ° C, the material is superior to the Al-killed steel sheet manufactured by the conventional process. It is disclosed that the steel sheet can be obtained economically.

An example in which this direct rolling method is applied to the production of a high-strength cold-rolled steel sheet having excellent deep drawability is disclosed in JP-A-62-287.
Japanese Patent No. 017 discloses a method of directly controlling the rolling time from the cutter position of a continuous casting machine to the start of rolling to a predetermined time or more depending on the steel composition when directly hot rolling a steel in which Ti or the like is added to an ultra-low carbon steel. ing. However, in this method, since the time from the cutter position of the continuous casting machine to the start of rolling must be limited depending on the steel composition, the waiting time from casting to the start of rolling inevitably becomes long and the productivity is remarkably increased. There was a problem that it decreased and was not preferable in operation.

[0006]

SUMMARY OF THE INVENTION The present invention advantageously solves the above problems, and by controlling the steel composition and manufacturing conditions, it is cheaper in energy cost and equivalent to steel obtained in the conventional process. It is an object of the present invention to propose a method capable of producing a high-strength cold-rolled steel sheet having deep drawability with good productivity.

[0007]

Means for Solving the Problems As a result of intensive studies, the inventors have produced a high-strength cold-rolled steel sheet having excellent deep drawability by limiting the steel composition and production conditions as follows. I found it possible.

The gist is as follows. (1) C: 0.01 wt% or less, Si: 1.5 wt% or less, Mn: 0.1-
3.0 wt%, Ti: 0.01 to 0.2 wt%, Nb: 0.001 to 0.2 wt%
%, B: 0.0001 to 0.0030 wt%, Al: 0.01 to 0.20 wt%,
P: 0.05 to 0.20 wt%, S: 0.05 wt% or less and N: 0.01 wt%
% Or less, and the respective contents of C, Ti, S and N [C], [Ti], [S] and [N] are expressed by the following formula:

[Equation 2] [Ti] ≧ 48 ([C] / 12 + [N] / 14 + [S] / 32) (wt%) The basic composition is satisfied, and the balance is steel consisting of Fe and inevitable impurities. After continuous casting, before lowering the temperature to below the Ar ₃ transformation point, hot rough rolling at a high temperature is performed to finish in a temperature range of 900 to 1100 ° C., and then hot finish rolling is performed.
After pickling and then cold rolling with a reduction of 60 to 95%,
A method for producing a high-strength cold-rolled steel sheet excellent in deep drawability, which comprises performing recrystallization annealing at 0 to 980 ° C (first invention).

(2) A method for producing a high-strength cold-rolled steel sheet excellent in deep drawability, which contains Mo: 0.01 to 1.0 wt% in addition to the steel components of the first invention (second invention).

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

The results of the research on which the present invention is based will be described below. Si: 0.5 wt%, Mn: 0.5 wt%, P: 0.07 wt%, S: 0.01
wt%, Al: 0.05 wt%, N: 0.002 wt%, Nb: 0.007 wt%
And B: 0.0008 wt%, C and Ti, C: 0.001 ~
0.01 wt%, Ti: 0 to 0.2 wt% with various changes in the content, the balance is substantially Fe composition After continuous casting, hot casting without lowering the temperature below the Ar ₃ transformation point Rough rolling and hot finish rolling were performed. At this time, the finish temperature of hot rough rolling and the finish temperature of hot finish rolling are 1000 ° C and 89 ° C, respectively.
It is a constant value of 0 ° C. Then, it was pickled and cold-rolled at a reduction rate of 78% to a plate thickness of 0.7 mm, and then recrystallization annealing was carried out by continuous annealing at 850 ° C. for 20 seconds to obtain a sample steel. FIG. 1 shows the result of examining the effect of the steel components on the r value of the test steel thus obtained in relation to Ti-48 (C / 12 + N / 14 + S / 32). For comparison, a steel manufactured under the same conditions as above except that Nb was not included was also examined. As is clear from the figure, the r value of the direct hot rolled material is
It has a strong effect on the steel composition and Ti ≧ 48 (C / 12 + N / 14 + S / 3
The Ti-Nb compound added steel satisfying 2) shows an excellent r value.

Next, C: 0.002 wt%, Si: 0.5 wt%, Mn:
0.5 wt%, S: 0.01 wt%, Al: 0.05 wt%, N: 0.002 wt%
%, Ti: 0.038 wt%, Nb: 0.007 wt% and B: 0.0009 wt%
%, P is variously contained in the range of 0.01 to 0.15 wt%, and the balance is substantially Fe composition. After continuous casting, molten steel is hot-cast without lowering the temperature below the Ar ₃ transformation point. Rough rolling and hot finish rolling were performed. At this time, the finish temperature of hot rough rolling and the finish temperature of hot finish rolling are 1000 ° C and
It is constant at 890 ℃. Then, it was pickled and cold-rolled at a reduction rate of 78% to a plate thickness of 0.7 mm, and then recrystallization annealing was carried out by continuous annealing at 850 ° C. for 20 seconds to obtain a sample steel. FIG. 2 shows the result of examining the effect of the P amount on the r value of the test steel thus obtained. As is clear from the figure, the r value of the directly hot-rolled material strongly depends on the P amount, and P ≧ 0.05.
The Ti-Nb composite added steel satisfying wt% shows an excellent r value.

Further, in order to investigate the effect of the hot rough rolling temperature on the r value, C: 0.002 wt%, Si: 0.5 wt%, Mn:
0.5 wt%, P 0.07 wt%, S: 0.01 wt%, Al: 0.05 wt%
%, N: 0.002 wt%, Ti: 0.036 wt%, Nb: 0.006 wt%
And B: 0.0009 wt% and the balance being substantially Fe composition, after continuous casting, hot rough rolling and hot finish rolling were performed without lowering the temperature below the Ar ₃ transformation point. At this time, the finish temperature of the hot rough rolling was variously changed in the range of 950 to 1200, and the finish temperature of the hot finish rolling was kept constant at 890 ° C.
After that, it is pickled and cold-rolled at a reduction of 78% to obtain a plate thickness of 0.7.
After being made into mm, recrystallization annealing was performed by continuous annealing at 850 ° C. for 20 seconds to obtain a test steel. FIG. 3 shows the influence of the finish temperature of hot rough rolling on the r value of the test steel thus obtained. As is clear from the figure, the r value of the direct hot rolled material is
It strongly depends on the end temperature of hot rough rolling, and this end temperature is set to 11
The r value was improved by manufacturing at a temperature of 00 ° C. or lower.

[0014]

The reason why the r value is improved by making P of the steel component to be 0.05% or more and performing the hot rough rolling at a low temperature according to the present invention is considered as follows. That is, P is effective for refining the cast structure, and the addition of 0.05% or more of P makes the cast structure and the hot-rolled sheet structure fine, and as a result, the r value after cold rolling-recrystallization annealing is improved. .. Further, the hot rough rolling in the low temperature region makes the cast structure fine and promotes precipitation of TiC sufficiently, so that the r value after cold rolling-recrystallization annealing is improved. Also, after continuous casting,
Since hot rolling can be started immediately, there is no problem that productivity is reduced.

As described above, in the present invention, the steel composition is important, and if the composition range of the composition is not satisfied, excellent deep drawability cannot be secured in the direct hot rolling process. Hereinafter, the reason for limiting the range of each component will be described. C: 0.01 wt% or less C is preferable because the smaller the content, the better the deep drawability. However, when the content is 0.01 wt% or less, it does not exert a bad influence so much, so C is limited to 0.01 wt% or less. More preferably, it is 0.008 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 effect, it is preferable to contain about 0.1 wt% or more. Mn: 0.1-3.0 wt% Mn has the effect of strengthening steel and contains the required amount according to the desired strength, but the content is 0.1 wt%.
In the following, the strength becomes insufficient, while if it exceeds 3.0 wt%, the deep drawability is adversely affected, so it was limited to 0.1 to 3.0 wt%.

Ti: 0.01 to 0.2 wt% Ti is a carbonitride forming component and is a solid solution (C, N) in steel.
And has the effect of preferentially forming crystal grains in the {111} orientation, which is advantageous for deep drawability. Its content is
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 the ductility deteriorates, so the content was limited to 0.01 to 0.2 wt%.

Nb: 0.001 to 0.2 wt% Nb is a carbide forming component, which has the effect of reducing the solid solution C in the steel and is also effective for refining the structure before hot finish rolling. 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 0.0030 wt% B is contained in order 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 0.003 wt%, the deep drawability deteriorates, so it was limited to 0.0001 to 0.003 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: 0.05 to 0.20 wt% P is an important component in the present invention, has the effect of strengthening the steel, and greatly affects the material properties after cold rolling-recrystallization annealing. That is, P is effective for refining the cast structure, and the P content of 0.05 wt% or more makes the cast structure and the hot rolled sheet structure fine, and as a result, the r value after cold rolling-recrystallization annealing is improved. To do. On the other hand, if the P content is less than 0.05 wt%, it is not effective for refining the above-mentioned cast structure, and if the P content exceeds 0.20 wt%, the deep drawability is adversely affected. Limited to.
More preferably, it is 0.05 to 0.15 wt%.

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 of C, Ti, S and N [C], [Ti], [S] and [N] are expressed by the following equations.

## EQU3 ## [Ti] ≧ 48 ([C] / 12 + [N] / 14 + [S] / 32) (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 4] [Ti] ≧ 48 ([C] / 12 + [N] / 14 + [S] / 32) (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. Steel after continuous casting is supplied to a rolling facility immediately to start hot rolling, or by starting hot rolling after heat retention treatment, hot rolling without lowering the temperature below the Ar ₃ transformation point. Roll it. The heat insulation conditions of the continuous casting slab may be arbitrary as long as the steel composition and the hot rolling conditions satisfy the present invention.

Hot Rolling Process The hot rolling process is the most important in the present invention. After continuous casting, high temperature hot rough rolling is performed before the temperature is lowered to the Ar ₃ transformation point, and the hot rolling process is finished at 900 to 1100 ° C. It is necessary to continue hot finish rolling. As described above, the low-temperature hot rough rolling is necessary to make the cast structure fine and the precipitation of TiC sufficiently.
If the temperature is higher than 1100 ° C, the refinement of the cast structure by rough rolling is not sufficiently performed, and the precipitation of TiC does not proceed sufficiently during rough rolling. Therefore, excellent deep drawability is obtained after cold rolling-recrystallization annealing is completed. I can't. On the other hand, even if the hot rough rolling temperature is lower than 900 ° C, no further effect can be obtained, and conversely, the hot finishing temperature is much lower than the Ar ₃ transformation point, so that the deep drawability deteriorates. Therefore, the end temperature of hot rough rolling is set to 900 ~ 11
It was limited to the range of 00 ℃. For the above hot rough rolling, the rolling start temperature is preferably 1150 ° C or lower, and the rolling reduction is 80%.
It is preferably about a degree or more.

Next, the hot finish rolling is not particularly limited, but the finish rolling finish temperature is preferably at least the Ar ₃ transformation point. However, there is no problem even if it is slightly below the Ar ₃ transformation point. The coiling temperature after hot finish rolling is not particularly limited and is preferably in the range of 500 to 750 ° C.

Cold Rolling Process The cold rolling process is essential for obtaining a high r value, and it is essential that the cold rolling reduction ratio is 60 to 95%. If the cold rolling reduction is less than 60% 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 by a continuous annealing method. The annealing temperature shall be in the range of 800-980 ° C. In the low temperature range where the annealing temperature is less than 800 ° C, excellent deep drawability cannot be obtained. On the other hand, the annealing temperature is 980 ℃
If it exceeds, the crystal orientation becomes random due to α-γ transformation and the deep drawability deteriorates, so the temperature was limited to 800 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.

[0032]

EXAMPLES Steels having various compositional compositions shown in Table 1 were melted.

[0033]

[Table 1]

After continuous casting of these molten steels, the temperature is maintained without lowering the temperature to the Ar ₃ transformation point and hot rough rolling is performed under the conditions of the rough rolling finish temperature (RDT) and finish rolling finish temperature (FDT) shown in Table 2. , Hot finish rolling was performed.

[0035]

[Table 2]

The obtained hot-rolled sheet 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,
Recrystallization annealing was performed under the conditions shown in Table 2 in a continuous annealing facility. In Tables 1 and 2, examples in which the numerical values are out of the range of the present invention are underlined. The results of examining the material properties of the cold-rolled steel sheet thus obtained are also shown in Table 2. The tensile properties were measured using JIS No. 5 tensile test pieces. In addition, r value is 15% after tensile prestrain,
Measured by the 3-point method, the average value of L direction (rolling direction), D direction (45 ° direction from rolling direction) and C direction (90 ° direction from rolling direction)

## EQU5 ## It was obtained from the equation of 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 apparent from Table 2, all the conforming examples according to the present invention have excellent deep drawability-high tensile strength level as compared with the comparative examples.

[0038]

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

[Brief description of drawings]

FIG. 1 is a graph showing the effect of steel composition on r-value on r-value in relation to Ti-48 (C / 12 + N / 14 + S / 32).

FIG. 2 is a graph showing the effect of P amount on the r value.

FIG. 3 is a graph showing the influence of the end temperature of hot rough rolling on the r value.

Claims (3)

[Claims] 1. C: 0.01 wt% or less, Si: 1.5 wt% or less, Mn: 0.1 to 3.0 wt%, Ti: 0.01 to 0.2 wt%, Nb: 0.001 to 0.2 wt%, B: 0.0001 to 0.0030 wt% , Al: 0.01 to 0.20 wt%, P: 0.05 to 0.20 wt%, S: 0.05 wt% or less and N: 0.01 wt% or less, and each content of the above C, Ti, S and N [C],
[Ti], [S] and [N] satisfy the following formula [Ti] ≧ 48 ([C] / 12 + [N] / 14 + [S] / 32) (wt%) After the continuous casting, steel with Fe and unavoidable impurities as the balance is subjected to high temperature hot rough rolling before the temperature is lowered to below the Ar ₃ transformation point, and finishes in the temperature range of 900 to 1100 ° C. , After performing hot finish rolling,
After pickling and then cold rolling with a reduction of 60 to 95%,
A method for producing a high-strength cold-rolled steel sheet excellent in deep drawability, which comprises performing recrystallization annealing at 0 to 980 ° C. 2. The method for producing a high-strength cold-rolled steel sheet excellent in 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 or 2, which contains Cu: 0.1 to 1.5 wt% and Ni: 0.1 to 1.5 wt% in addition to the basic composition. ..