JPH05112831A - Manufacture of cold rolled steel sheet for deep drawing excellent in workability - Google Patents

Abstract

PURPOSE:To make the structure of a steel sheet fine in grains ant to improve its drawability by controlling hot rolling and cooling therefor to specified ranges in dead soft steel in which the content of N and S is regulated and mixed with specified amounts of Ti and Nb. CONSTITUTION:The compsn. of steel consists, by weight, of 0.001 to 0.005% C, <=0.005% N, <=1.0% Si, 0.1 to 1.20% Mn, <=0.1% P, <=0.01% S, <=0.1% Al and either or both of Ti and Nb so as to satisfy the condition of 0.4<(C/12+N /14+S/32)/(Ti/48+Nb/93)<1.2 as well as >=0.02% Ti+Nb and the balance Fe with inevitable impurities. This steel is subjected to hot rolling at a finishing temp. of the Ar3 point to the Ar3 +50 deg.C at >=30% final draft. Immediately after the rolling, cooling is started. The cooling is executed in such a manner that the average cooling rate for 3sec from the start is regulated to >=60 deg.C/sec, particularly, the average cooling rate for 1sec from the start is regulated to >=80 deg.C/sec. Next, pickling, cold rolling and annealing are executed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a cold rolled steel sheet having excellent deep drawability.

[0002]

2. Description of the Prior Art Ti and Nb are added to an ultra-low carbon steel, C and N in the steel are fixed in the form of precipitates, and IF steel (Interstitial atom free steel) free of solid solution interstitial elements is used. Many methods have already been disclosed for producing a cold-rolled steel sheet for deep drawing using the method (for example, JP-A-58-107414,
JP-B-44-18066). Further, as a method for improving the deep drawability of the product sheet after cold rolling and annealing, it is known that it is effective to miniaturize the hot rolled sheet, and in order to achieve the miniaturization, after hot rolling, Techniques for cooling as quickly as possible have been disclosed (for example, Japanese Patent Laid-Open No. 58-48).
635, JP-A-61-276930).

On the other hand, it has been confirmed in the composition system of high-strength steel that it is effective to increase the hot rolling reduction in the method of refining the hot rolled sheet (JP-A-59-107023).
(See JP-A-58-212258). By applying this refinement method to ultra-low carbon steel, it is possible to achieve some refinement of the structure, but under the conventional cooling conditions, a significant grain refinement effect cannot be obtained even with large reduction rolling. The current situation.

[0004]

DISCLOSURE OF THE INVENTION The present invention optimizes the hot-rolling cooling conditions for ultra-low carbon steel and refines the microstructure of the hot-rolled steel sheet to produce a cold-rolled steel sheet having excellent deep drawability. It is intended to provide a method.

[0005]

Means for Solving the Problems The inventors of the present invention have conducted research on refining high-strength hot-rolled steel sheets for many years, and an increase in workability, an increase in cooling rate, and a reduction in cooling start time are effective in reducing grain size. I found that it was the target. Based on this finding, we tried to refine the ultra-low carbon steel, and as the composition was refined, the cooling rate increased and the cooling start time was shortened. It was found that coarse particles were formed.

Further, ultra low carbon steel is subjected to large reduction rolling,
Even if it was cooled in the usual pattern, the grain refinement could hardly be achieved. The cause of this was examined in detail in an experiment using a laboratory tester that can accurately control the working rate, cooling rate, and cooling start time.Austenite rapidly recrystallized immediately after large reduction rolling and was introduced by processing. It was found that the ferrite grains generated after transformation cannot be sufficiently miniaturized by causing the disappearance of dislocations and reducing the frequency of nucleation during transformation. Originally based on the increase in processing degree,
Since it was expected that the number of dislocations introduced would increase the refinement, the increase in the degree of processing would also accompany the temperature rise due to the heat generated during processing. It is thought that this was not achieved. The reason why high-strength steel can be fine-grained by large reduction rolling, and ultra-low carbon steel is not able to achieve remarkable microstructuring is
It is considered that the high-purity composition of the ultra-low carbon steel facilitated the elimination of dislocations.

The inventors of the present invention have studied the influences of the components, the hot rolling conditions, the cold speed after hot rolling, and the cooling start time on the grain refinement and deep drawability of ultra-low carbon steel, and as a result, the conditions were limited. It was found that the cold rolling steel sheet having excellent deep drawability can be obtained while achieving remarkable grain refinement of the ultra low carbon steel hot rolling sheet.

The present invention is based on such knowledge, and the gist thereof is as follows: C: 0.001% or more, 0.005% or less, N: 0.005% or less, Si:
1.0% or less, Mn: 0.1% or more, 1.20% or less,
P: 0.1% or less, S: 0.01% or less, Al: 0.1
% Or less of either or both of Ti and Nb 0.4 <(C / 12 + N / 14 + S / 32) / (Ti
It contained so as to satisfy the /48+Nb/93)<1.2 following condition, and steel the total amount of Ti + Nb is the balance being Fe and unavoidable impurities at 0.02% or more A _r3 transformation point or higher, A _r3 transformation point Hot rolling is performed at a finishing temperature of + 50 ° C. or less and a final reduction rate of 30% or more, cooling is started immediately after hot rolling, and an average cooling rate for 3 seconds from the start is 60 ° C./s or more, particularly from the start. A method for producing a cold rolled steel sheet with excellent workability, which comprises performing ordinary pickling, cold rolling and annealing after cooling at an average cold speed of 1 second for 80 ° C./s or more.

The present invention will be described in detail below. The components of the present invention are limited from the viewpoints of both fineness of structure and deep drawability. The upper limits of the amount of C and the amount of N are set to 0.005% because the addition in excess of these deteriorates the deep drawability. The lower limit of the amount of C is set to 0.001% because if it is added below this amount, the fine graining of the hot rolled sheet does not occur sufficiently and the deep drawability of the final product deteriorates. The upper limit of the amount of Si is set to 1.0% because the addition of the amount of Si exceeding this causes deterioration of deep drawability. The upper limit of the amount of Mn is set to 1.2% in order to prevent the deterioration of deep drawability as well. The lower limit of the amount of Mn is set to 0.1% because the addition of less than this amount does not sufficiently reduce the grain size of the hot rolled sheet and deteriorates the deep drawability of the final product. The upper limit of the amount of P, S, Al added is limited by moldability. If P, Al is added in excess of 0.1% and S is added in excess of 0.01%, hot rolling or product This is because there is a high possibility that defects will occur when the plate is pressed.

Either or both of Ti and Nb should be 0.4 <(C / 12 + N / 14 + S / 32) / (Ti /
48 + Nb / 93) <1.2 is added so as to satisfy the relationship that the C and N in the steel are mostly fixed in the form of precipitates, and the cost increases.
This is to minimize the addition of b. Fixing C and N in steel is an advantageous direction for achieving good deep drawability of the product by controlling the texture in hot rolling (1
This is because it is possible to obtain a steel sheet having a highly integrated texture such as 11) <112> and (554) <225>. Further, the lower limit of the total sum of Ti + Nb is set to 0.02% because if the amount added is less than this, grain refinement of the hot rolled sheet does not occur sufficiently and the deep drawability of the final product deteriorates.

In the present invention, as other components, Cr: 1.0% or less, Cu: 1.5% or less, Ni: 1.0% or less, Mo: 0.5% or less for improving strength. You may make it contain. In addition, 0.005 is used to prevent secondary processing cracks.
Addition of 0% or less of B does not impair the gist of the present invention.

Next, the reasons for limiting the process conditions will be described. After rolling at a finishing temperature of A _r3 transformation point or more and A _r3 transformation point + 50 ° C. or less and a final rolling reduction of 30% or more, cooling is started immediately after rolling, and an average cooling speed of 3 seconds from the start is 6
Above 0 ° C / s, especially the average cooling speed of 1 second from the start is 80
The limitation of the process condition of cooling at ℃ / s or more is for refining the structure of the hot rolled sheet.

If the finishing temperature of hot rolling is not higher than the _{Ar 3} transformation point, processed grains or recrystallized grains of ferrite will be formed and sufficient grain refinement cannot be achieved. On the other hand, when the finishing temperature is above the _{Ar 3} transformation point + 50 ° C., the dislocation density in austenite is low and the ferrite structure after transformation does not become fine.

The lower limit of the final rolling reduction is set to 30% because the ferrite structure does not remarkably become fine with a rolling reduction below this range. However, in order to achieve remarkable miniaturization, combination with the cooling conditions described below is essential. That is, by starting cooling immediately after rolling and limiting the cooling rate, the steel of the present invention can be significantly refined. The average cold speed for 1 second from the start of cooling is limited to 80 ° C / s or more because dislocations in austenite are significantly reduced by the transformation at a cooling speed lower than this and the ferrite structure after transformation does not become fine. Is. Further, if the cooling rate is low, the transformation temperature becomes high, and the refinement due to transformation cannot be sufficiently achieved. On the other hand, the reason why the average cooling rate for 3 seconds from the start of cooling is limited to 60 ° C./s or more is to suppress grain growth after transformation and secure a fine ferrite structure.

Use of the steel of the present invention as a surface-treated steel sheet after the cold rolling process does not impair the purpose of the present invention.

[0016]

EXAMPLES Examples of the present invention will be described together with comparative examples.
Steels having the chemical compositions shown in Table 1 were manufactured under various conditions. Here, the transformation point is a value obtained by using Formaster to determine the transformation start temperature when cooled at 1 ° C./s. Table 2 shows the production conditions, particle size, and average r value of each experiment. The particle size number is ASTM-No. Is. Slab heating temperature is 120
At 0 ° C, the coiling temperature of the hot-rolled sheet was 700 ° C or lower. The finished plate thickness is 4 mm. The cold rolling rate was 80%, and annealing was performed in a continuous annealing furnace at 820 ° C. for 100 seconds. However, in Experiment 16, alloy plating was performed in a continuous hot dip galvanizing line at 780 ° C. In Experiment 17, box annealing was performed at 720 ° C.

[0017]

[Table 1]

[0018]

[Table 2]

Experiment numbers 1, 2, 8, which are within the scope of the present invention,
In Nos. 14, 15, 16, and 17, the particle diameter of the hot-rolled sheet is small, and the average r value of the product sheet is also high. In Experiment No. 3 in which the final reduction ratio is outside the range of the present invention, the grain size of the hot-rolled sheet is larger and the average r value is lower than in Experiment Nos. In Experiment No. 4 having a high finishing temperature, the grain size of the hot-rolled sheet was large and the average r value was low accordingly. The same result was obtained in Experiment No. 5 in which the finishing temperature was below the transformation point.

Experiment No. 6 in which the average cold speed for 1 second from the start of cooling is outside the scope of the present invention and Experiment No. 7 in which the average cold speed for 3 seconds after the start of cooling is outside the scope of the present invention are both grain sizes of the hot-rolled sheet. Is not sufficiently fine, and the average r value is lower than that of the steel of the present invention.

In Experiment No. 9 having a high C content, the grain size of the hot-rolled sheet becomes finer, but the average r value is lower than that of the steel of the present invention. In Experiment No. 10 in which a large amount of solute C remained, the deterioration of the average r value was remarkable. Experiment numbers 11 and 1 with low Mn content or C content
In No. 3, the grain size of the hot rolled sheet is large and the average r value of the product sheet is low. In Experiment No. 12 having a high Mn content, the grain size of the hot-rolled sheet becomes finer, but the average r value is lower than that of the steel of the present invention.

Experiment No. 16 within the scope of the present invention through a continuous hot dip galvanizing line and Experiment No. 1 with box annealing.
Even in No. 7, a high average r value was obtained, and in the present invention other than continuous annealing, excellent characteristics can be obtained even in the annealing process.

Although not shown in the table, the steel of the present invention has an average r
The anisotropy of the value also becomes low, and in the steel of the present invention in the table, Δr is generally
The absolute value of was 0.3 or less.

[0024]

According to the present invention, by controlling the conditions of hot rolling and cooling, it is possible to manufacture a super workable steel sheet having an average r value higher than that of the conventional cold rolled steel sheet. This is an industrially valuable invention because it enables the processing of press materials that were thought to be impossible to form.

Claims (1)

[Claims] 1. By weight%, C: 0.001% or more and 0.005% or less, N: 0.005% or less, Si: 1.0% or less, Mn: 0.1% or more, 1.20. % Or less, P: 0.1% or less, S: 0.01% or less, Al: including 0.1% or less, and either or both of Ti and Nb 0.4 <
(C / 12 + N / 14 + S / 32) / (Ti / 48 + N
b / 93) <1.2 and Ti
+ Nb in a total amount of 0.02% or more, and the balance Fe and unavoidable impurities in steel at a finishing temperature of A _r3 transformation point or more and A _r3 transformation point + 50 ° C. or less, and a final reduction rate of 30%.
Hot rolling is performed as described above, and cooling is started immediately after hot rolling. The average cooling rate for 3 seconds from the start is 60 ° C / s or more, and particularly the average cooling rate for 1 second from the start is 80 ° C / s or more. After being cooled as described above, a method for producing a cold rolled steel sheet having excellent workability, which comprises performing ordinary pickling, cold rolling and annealing.