JPS62287018A - Production of high-strength cold rolled steel sheet having excellent deep drawability - Google Patents

Abstract

PURPOSE:To produce a high-strength cold rolled steel sheet having the small anisotropy of an r value and excellent deep drawability by subjecting a steel contg. specific ratios of C, N, Nb, Ti, Mn, and P successively to hot rolling, cooling, coiling and cold rolling under specific conditions. CONSTITUTION:The steel which contains, by weight %, <=0.05% C, <=0.015 N, >=0.015% Nb, C/12+N/14<1.2 (Ti/48+Nb/93), 0.5-3% Mn, and <=0.2% P is rolled in at least at >=60% in an Ar3 and Ar3+150 deg.C temp. range. The recrystallization of the austenite between passes is thereby substantially prevented and the strong rolling aggregate texture is provided to the steel. The steel is then cooled at 10-50 deg.C/sec average cooling rate from the final rolling down to the coiling and is coiled at 550-750 deg.C. The coiled sheet is subjected to 40-70% cold rolling and recrystallization treatment. The high- strength cold rolled steel sheet having excellent deep drawability is thus produced in good yield.

Description

Detailed Description of the Invention 3. Detailed Description of the Invention [Field of Industrial Application] The present invention relates to a method for manufacturing a high-strength cold-rolled steel sheet with excellent formability.

[Prior Art] A conventional method for manufacturing a deep-drawn high-strength cold-rolled steel sheet involves cold rolling and annealing a hot-rolled sheet with a relatively random orientation. Therefore, in the hot-rolled sheet state, the in-plane anisotropy of the r value is small. However, as the cold rolling rate increases, the in-plane anisotropy of the r value increases, and when a steel sheet with a high r value is produced, Ar tends to increase as well.

Further, in order to obtain a high r value, it is generally necessary to perform cold rolling of 70% or more. This is shown, for example, in Japanese Patent Publication No. 58-57490.

[Problems to be Solved by the Invention] One problem to be solved by the present invention is that it is difficult to produce a steel plate with a high r value by cold rolling at a low reduction rate with the conventional technology. A large-scale cold rolling facility that can achieve this is required, which requires a large capital investment. Furthermore, steel sheets exhibiting a high r value tend to have a large in-plane anisotropy of the r value, and this has the disadvantage that significant selvage occurs during deep drawing, reducing the yield of the product. This phenomenon is particularly frequently observed in materials that have been cold-rolled under high pressure.

[Means for Solving the Problems] In order to solve the problems of the prior art, the present invention
: 0.05% by weight or less, N: 0.01% by weight or less, Nb: 0.015% by weight or more. %) and C/12+N/14<1.2(Ti/4
8+Nb/93), and the Mn content is 0.5% by weight or more and 3% by weight or less, and the P content is 0.2% by weight or less. ,+150℃
After rolling at least 60% in a temperature range of
To produce a cold-rolled steel sheet with excellent deep drawability, which is characterized in that it is rolled at 550°C or more and 750°C or less at 0°C/s or less, and then cold-rolled at 40% or more and JZ95% or less and subjected to recrystallization treatment. The present invention provides a method.

Hereinafter, the manufacturing method of the present invention will be explained in detail. In addition, % in the following description is weight %.

The key point of the present invention is that the r-value is high and the strong inverted V-shaped r
This is a technology that reduces the anisotropy of the r value without lowering the lower value by manufacturing a steel plate exhibiting chain member orientation, cold rolling it, and annealing it.

The present inventors have developed materials containing Nb of 0.015% by weight or more and Mn of 0.5% by weight or more.
R3+ Rolled by 60% or more in a temperature range of 150℃.

It was confirmed that by cooling at an average cooling rate of 10° C./s or more from finishing to rolling, it is possible to produce a hot-rolled sheet exhibiting strong inverted V-shaped r-chain member orientation. Here, it is thought that Nb suppresses austenite recrystallization, and the addition of Mn indirectly suppresses austenite recrystallization mainly by lowering the transformation point. If the rolling temperature is too high, it is difficult to generate a strong rolling texture without recrystallizing austenite as much as possible. In the present invention, the reason why hot rolling is limited to hot rolling at a reduction rate of 60% or more in a temperature range of Ar3 or more and Ar3 + 150°C is that austenite is formed between passes by hot rolling at Ar, +150°C or less by 60% or more. This is because it has a strong rolled texture without recrystallizing. The reason for limiting the rolling to Ara or higher is that, as is well known, ferrite is generated at temperatures below Ara transformation point, and the hot-rolled steel sheet obtained by rolling this ferrite has a high r
This is because it does not indicate a value.

The Ar3 transformation point used in the description of the present invention is Ar5('C)=
910-507C(%)127Sj-(%)-64Mn
Defined in (%). In addition, the reason why the average freezing after rolling until rolling is limited to 10°C or higher is because if the cooling is slower than this, most of the austenite will recrystallize before transformation, and the texture will become random during transformation. Inverted V
This is because a hot rolled sheet having anisotropy of the r value of the die cannot be obtained. Furthermore, the reason why the maximum freezing was set at 50° C./8 is because the r value of the final product decreases as the percentage of transformed structures other than ferrite increases. Further, the lower limit of the winding temperature is set to 550° C. because the ratio of transformed structures other than ferrite increases when the winding temperature is lower than this. The reason why the upper limit temperature is set to 750° C. is to prevent a decrease in yield due to deterioration of pickling properties and non-uniformity of quality.

-Force component conditions: C/12+N/14<1.2(Ti/4
8+Nb/93) is set because a high r value can be obtained under this condition. The reason is that most of the C2N
It is thought that this is because Ti and Nb precipitate and C and N in solid solution are small and do not suppress the formation of texture with a high r value. Also, C and N stars are 0°05% and 0.05%, respectively.
The reason why C9Ni is limited to 01% or less is that if more C9Ni is added, the amount of Nb that satisfies the above conditions will increase, which will not only be economically disadvantageous but also cause a large amount of precipitates to form in the steel. This is because the presence of M n M makes it difficult to form a texture that exhibits a high r value. This is because not only does it become difficult to obtain a hot-rolled steel sheet exhibiting an inverted V-shaped r-chain orientation, but also the addition of alloying elements increases costs. In addition, the content of I) added to improve strength was kept at 0.2% or less since then.
This is because the risk of secondary processing embrittlement increases with the addition of .

In addition, other components of the steel produced by the method of the present invention include components included in high-strength steel, namely 5j (1.5%, S<0.
02%, AQ<0°1%, Cu<1.0%, Cr<
i, 0%; , Nj<1-, 0%, B<0.
01% etc. are added.

Finally, the reason why the cold rolling rate was divided into two terms will be explained. In cold rolling with a rolling reduction of 40% to 70% as set forth in claim 1, the r values in the L and C directions of the annealed plate are in the direction of -1, and as the lower value increases, Δr decreases, and the difference occurs. A steel plate with small orientation can be obtained. The hot rolled steel sheet of the present invention is below 1-01"-
Since the r value in the 45° direction is very high, a high r value can be obtained even in cold rolling under such low pressure. on the other hand,
Reduction rate of 70% or more and 95% as indicated in Clause 2 of the patent claims
In the following cold rolling, very high r values are obtained after annealing.

At this time, Δr is relatively small, and the occurrence of ears after deep drawing is suppressed. If the rolling rate is 95% or more, the formation of ears becomes noticeable, so the limit of the cold rolling rate was set at 95%.

[Effects of the Invention] According to the method of the present invention, not only can a steel plate with a high r value be produced even in cold rolling with a low reduction rate using a small-scale cold rolling facility with a low capital investment cost, but also the yield can be improved because the Δr is small. It also leads to the future and has a large economic effect. In addition, by adjusting the components, especially the content of Mn and P, the tensile strength can be increased to 30ON.
This invention has high industrial utility value as it can produce a steel plate of 45ON/[lll112 from 1/mm'' and a high-strength cold-rolled steel plate with a small Δr and excellent deep drawability with a high drawability.

Claims (2)

[Claims] (1) C: 0.05% by weight or less, N: 0.01% by weight or less, Nb: 0.015% by weight or more, and the content (weight%) of C and N is one or both of Ti and Nb. Addition amount (wt%) and C/12+N/14<1.2(Ti/48+Nb/93
), and the Mn content is 0.5% by weight or more and 3% by weight or less, and the P content is 0.2% by weight or less, at least 60% or more in the temperature range of Ar_3 and Ar_3 + 150°C. After rolling, the average cooling rate from final rolling to winding is 10°C/s or more and 50°C/s or less to 550°C.
A method for producing a high-strength cold-rolled steel sheet with low r-value anisotropy and excellent deep drawability, which is rolled at a temperature of 750°C or less, then cold-rolled by 40% or more and 70% or less, and subjected to recrystallization treatment. (2) C: 0.05% by weight or less, N: 0.01% by weight or less, Nb: 0.015% by weight or more, and the content (weight%) of C and N is one or both of Ti and Nb. Addition amount (wt%) and C/12+N/14<1.2(Ti/48+Nb/93
), and the Mn content is 0.5% by weight or more and 3% by weight or less, and the P content is 0.2% by weight or less, at least 60% or more in the temperature range of Ar_3 and Ar_3 + 150°C. After rolling, the average cooling rate from the final rolling to winding is 10°C/s or more and 50°C/s or less, and the hot rolled sheet is rolled at 550°C or more and 750°C or less, and then this hot rolled sheet is Method for producing high-strength cold-rolled steel sheet with excellent deep drawability, which is cold-rolled at a reduced reduction rate and subjected to recrystallization treatment