JPH0617140A - Production of cold rolled steel sheet for deep drawing - Google Patents

Abstract

PURPOSE:To obtain a cold rolled steel sheet excellent in deep drawability and shape by specifying hot rolling and cooling conditions for an ultra-low carbon steel with specific composition and refining the structure of a hot rolled plate. CONSTITUTION:A steel consisting of, by weight, 0.0005-0.005% C, <=0.005% N, <=0.1% P, <=0.02% S, <=0.1% Al, Ti and/or Nb by the amounts satisfying the conditions in 0.2<(C/12+N/14+S/32)/(Ti/48+Nb/93)<1.4, 0.1-1.5% of one or more elements among Mn, Cr, Cu, Ni, and Mo, and the balance Fe is used. This steel is rolled at a temp. between the Ar3 transformation point and (Ar3 transformation point +100 deg.C) at >=70% total draft and finish annealing is completed at a temp. not lower than the Ar3 transformation point or rolling is done at a finishing temp. between the Ar3 transformation point and (Ar3 transformation point +50 deg.C) at >=30% final draft. This steel is cooled from the point of time directly after rolling down to (Ar3 transformation point S-50 deg.C) at <=50 deg.C/sec average cooling rate, rolled at 0.5-10% draft, coiled at <=750 deg.C, and successively subjected to ordinary pickling, cold rolling, and annealing.

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 cold rolled steel sheet having excellent deep drawability and shape.

[0002]

2. Description of the Related Art IF steel (Interstitial atom) in which Ti and Nb are added to ultra-low carbon steel, C and N in the steel are fixed in the form of precipitates, and solid solution interstitial elements do not exist
Many methods for producing a cold-rolled steel sheet for deep drawing using a free steel have already been disclosed (for example, JP-A-58-107414 and JP-B-44-1806).
No. 6). Further, as a method of 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. Techniques for cooling as quickly as possible have been disclosed (for example, JP-A-58-48635 and JP-A-SHO-6).
No. 1-276930). On the other hand, it has been confirmed in the composition system of high-strength steel that increasing the hot rolling reduction is effective for the method of refining the hot rolled sheet (JP-A-59-107).
023, JP-A-58-212258). By applying this refinement method to ultra-low carbon steel, it is possible to achieve some refinement of the microstructure, but under the conventional cooling conditions, it is impossible to obtain a remarkable grain refining effect by large reduction rolling. Is.

Further, when large reduction rolling is carried out at the final stage, the shape of the hot rolled sheet deteriorates, which may hinder the operation during cold rolling, and often leads to deterioration of the quality of the final rolled sheet. . Therefore, from the viewpoint of the shape, the final stage rolling reduction is generally suppressed to 30% or less. Besides,
In the current continuous hot rolling equipment, since there are sheet shape measuring instruments such as sheet thickness gauges, sheet width gauges and crown measuring devices that are shape control sensors immediately after the finishing rolling mill, start cooling immediately after finishing rolling. I can't. On the other hand, if the cooling device is installed in the vicinity of the finishing rolling mill and the plate-shaped measuring device is installed in the rear, there is a problem in that the responsiveness of shape control deteriorates and the controllability deteriorates.

[0004]

DISCLOSURE OF THE INVENTION The present invention provides a cold-rolled steel sheet excellent in deep drawability and shape by optimizing the hot-rolling cooling conditions of ultra-low carbon steel and refining the structure of the hot-rolled steel sheet. A method of manufacturing is provided.

[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, even if the ultra-low carbon steel was subjected to large reduction rolling and then cooled in an ordinary pattern, grain refinement could hardly be achieved. The cause of this was investigated in detail in an experiment using a laboratory tester capable of accurately controlling the degree of processing, the cooling rate, and the cooling start time.Austenite rapidly recrystallized immediately after large reduction rolling and was introduced by processing. It was found that the dislocation disappeared and the ferrite grain size could not be sufficiently reduced. 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 be accompanied by the temperature rise due to the heat generated by the processing at the same time. 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 present inventors have examined the effects of components, hot rolling conditions, cold speed after hot rolling, and cooling start time on grain refinement and deep drawability of ultra-low carbon steel, and under limited conditions. It was found that the cold rolled steel sheet having excellent deep drawability can be obtained while achieving remarkable grain refinement of the hot rolled sheet of ultra low carbon steel. The gist of the present invention is as follows: (1) C by weight%: 0.0005% or more, 0.005%
Hereinafter, N: 0.005% or less, P: 0.1% or less, S:
0.02% or less, Al: 0.1% or less, and either or both of Ti and Nb 0.2 <(C / 12 +
N / 14 + S / 32) / (Ti / 48 + Nb / 93) <
It is contained so as to satisfy the condition of 1.4, Mn, Si,
Steel containing Cr and Cu, Ni, and Mo in the amount of 0.1% or more and 1.5% or less and the balance Fe and unavoidable impurities is Ar ₃ transformation point or more, Ar ₃ transformation point or more. At least 100% reduction in the temperature range below + 100 ℃
The above rolling is performed, the finish rolling is completed at the Ar ₃ transformation point or higher, the average cooling rate from immediately after rolling to the Ar ₃ transformation point −50 ° C. is 50 ° C./sec or more, and then 0.5% or more,
A method for producing a cold-rolled steel sheet for deep drawing, which comprises rolling at 10% or less, winding at 750 ° C or less, and then performing ordinary pickling, cold rolling, and annealing.

(2) C by weight%: 0.0005% or more,
0.005% or less, N: 0.005% or less, P: 0.1
% Or less, S: 0.02% or less, Al: 0.1% or less, and either or both of Ti and Nb 0.2 <
(C / 12 + N / 14 + S / 32) / (Ti / 48 + N
b / 93) <1.4 to satisfy the condition
If the content of one or more of Mn, Si, Cr, Cu, Ni and Mo is 0.1% or more and 1.5% or less, the balance Fe
And steel consisting of inevitable impurities above the Ar ₃ transformation point,
After rolling at a finishing temperature of Ar ₃ transformation point + 50 ° C. or less and at a final rolling reduction of 30% or more, cooling is started immediately after rolling, and an average cooling rate of 50 ° C. immediately after rolling to Ar ₃ transformation point −50 ° C. / Cool for more than sec, then 0.5% or more, 1
Roll at 0% or less, wind at 750 ° C or less, and then
It is a method for manufacturing a cold-rolled steel sheet for deep drawing, which is characterized by performing ordinary pickling, cold rolling, and annealing.

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 amounts of C and N are set to 0.005%, because the addition of more than this deteriorates the deep drawability. The lower limit of the amount of C is set to 0.0005% 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. Mn, Si, Cr, C
The lower limit of the content of one or more of u, Ni, and Mo is set to 0.1%, because the addition of less than this does not cause sufficient grain refinement of the hot-rolled sheet and the deep drawability of the final product. Is deteriorated. Further, the upper limit is set to 1.5% because the addition of more than this causes the deterioration of the deep drawability. The upper limit of the amount of P, S, Al added is limited by the formability.
If 0.1% or more and S is 0.02% or more, defects are likely to occur at the time of hot rolling or pressing of the product sheet.

Either or both of Ti and Nb should be 0.2 <(C / 12 + N / 14 + S / 32) / (Ti /
48 + Nb / 93) <1.4 is added so as to limit the reason that C and N in the steel can be 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 achieved by controlling the texture in rolling.
This is because it is possible to obtain a steel sheet having a texture with a high degree of integration such as (111) <112> (554) <225>, which is an orientation advantageous for achieving good deep drawability of the product. In the present invention, addition of 0.0050% or less of B to prevent secondary work cracking does not impair the gist of the present invention.

Next, the reasons for limiting the process conditions will be described. First, regarding the conditions for the first invention, Ar
The lower limit of the total rolling reduction in the temperature range of ₃ transformation points or more and Ar ₃ transformation point + 100 ° C. or less is set to 70%. The total rolling reduction below this is that the hot-rolled sheet can satisfy the following cooling conditions. This is because the miniaturization cannot be achieved sufficiently and the deep drawability of the final product deteriorates. Further, the finishing temperature of hot rolling is limited to the Ar ₃ transformation point or higher, because when finishing rolling is performed at a temperature lower than that, worked grains or recrystallized grains of ferrite are generated and sufficient grain refinement can be achieved. This is because the deep drawability of the final product deteriorates.

The average cold speed from immediately after rolling to the Ar ₃ transformation point of −50 ° C. is limited to 50 ° C./sec or more, because if the cooling speed is lower than this, miniaturization of the hot-rolled sheet cannot be sufficiently achieved. This is because the deep drawability of the final product deteriorates. Also,
Subsequent rolling is to correct the shape,
The minimum reduction required for that is 0.5%. On the other hand, the upper limit of the rolling reduction is set to 10%, because if the rolling reduction is more than that, there is a high possibility that the structure becomes coarser in order to alleviate the strain during cooling and in the winding step. And
As a result, the deep drawability of the final product deteriorates. The upper limit of the coiling temperature is set to 750 ° C. because the coiling of the above structure is likely to occur at a coiling temperature higher than that.

Next, regarding the condition for the second invention, A
After rolling at a finishing temperature of r ₃ transformation point or more and Ar ₃ transformation point + 50 ° C. or less and a final rolling reduction of 30% or more, cooling is started immediately after rolling, and Ar ₃ transformation point −50 ° C. immediately after rolling.
The limitation of the process condition of cooling at an average cooling rate of 50 ° C./sec or more is for refining the structure of the hot rolled sheet. If the finishing temperature for hot rolling is not higher than the Ar ₃ transformation point, processed grains or recrystallized grains of ferrite will be generated, 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 become significantly finer 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. Immediately after rolling, the average cooling rate from the Ar ₃ transformation point to −50 ° C. is 50 ° C. /
The reason for limiting to s or more is that the ferrite structure after transformation becomes remarkably fine by cooling at a cooling rate higher than this.

Further, the subsequent rolling is carried out to correct the shape, and the minimum reduction ratio required for the rolling is 0.5%.
Is. On the other hand, the upper limit of the rolling reduction is set to 10%, because if the rolling reduction is more than that, there is a high possibility that the structure becomes coarser in order to alleviate the strain during cooling and in the winding step. As a result, the deep drawability of the final product deteriorates. The upper limit of the coiling temperature is set to 750 ° C. because the coiling of the above structure is likely to occur at a coiling temperature higher than that.

The above shape-correcting rolling is carried out at an Ar ₃ transformation point of −50.
The temperature may be lower than or equal to ℃, at any point until winding, but by using the rolling mill at the final stage of the finishing rolling mill, such as a thickness gauge, a width gauge, and a crown measuring device that are shape control sensors. There is an advantage that a plate-shaped measuring instrument can be used at the current installation position. In this case, a cooling device is installed between the passes before the final stage, and the Ar ₃ transformation point of -50 ° C is reached until the final stage is reached.
Need to be cooled down. Use of the steel of the present invention as a surface-treated steel sheet through a plating process after cold rolling does not impair the gist of the present invention.

[0016]

【Example】

Example 1 An example of the present invention will be described together with a comparative example. 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. Manufacturing conditions for each experiment,
Table 2 shows the grain size of the hot-rolled sheet, the steepness of the hot-rolled sheet, and the r value of the product sheet. The particle size number is ASTM-No. Is. The steepness represents the wave shape in the plate width direction with the denominator as the amplitude and the numerator as the wave height. The slab heating temperature is 1200 ° C. and 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 20, alloy plating was performed in a continuous hot dip galvanizing line at 780 ° C.

Experiment Nos. 1, 7, 9, which are within the scope of the present invention
In Nos. 10, 15, 16, 18, 19, and 20, the grain size of the hot rolled sheet is small, and the r value of the product sheet is high. Also, the steepness of the hot rolled sheet is small. The material of Experiment No. 2 in which the rolling reduction of the shape-controlled rolling was outside the range of the present invention had a large steepness of the hot-rolled sheet, the workability of the cold-rolling was poor, and the defective shape was partially present in the shape of the product sheet.
On the other hand, in the material of Experiment No. 5 in which the rolling reduction of the shape-controlled rolling was larger than the range of the present invention, coarse grains were partially generated in the hot-rolled sheet, and the r value of the product sheet did not become high. Similar coarse grains in the hot-rolled sheet were also found in the material of Experiment No. 4 in which the winding temperature was above the range of the present invention. In the material of Experiment No. 3 in which the average cold speed from the finish rolling to the Ar ₃ transformation point of −50 ° C. is within the range of the present invention, the structure of the hot-rolled sheet does not become sufficiently fine and the r value of the product sheet is high. There wasn't. In the material of Experiment No. 6 in which the finishing temperature was below the transformation point, the hot rolled structure exhibited a partially worked structure, and the r value of the product sheet did not become high.

The material of Experiment No. 8 in which the total rolling reduction in the temperature range of Ar ₃ transformation point + 100 ° C. to Ar ₃ transformation point is within the range of the present invention does not have a sufficiently fine hot-rolled sheet structure, and the product sheet R
The value did not rise. Regarding the material of Experiment No. 11 in which the amount of C exceeded the range of the present invention, the structure of the hot rolled sheet was fine, but the r value of the product sheet did not become high. On the contrary, in the material of Experiment No. 17 in which the amount of C is less than the range of the present invention, the hot-rolled sheet has coarse particles, and the r value of the product sheet is relatively low. (C / 12 + N / 14
For the material of Experiment No. 12, which did not satisfy the relationship of + S / 32) / (Ti / 48 + Nb / 93) <1.4, the r value of the product plate did not become high.

1 of Mn, Si, Cr, Cu, Ni, Mo
In the material of Experiment No. 13 in which the content of one kind or two or more kinds was less than the range of the present invention, the hot-rolled sheet had coarse particles, and the r value of the product sheet did not become high. On the contrary, the material of Experiment No. 14 added in the range of the present invention or more has a fine hot rolled sheet structure, but the r value of the product sheet is low. A high r-value was obtained even for the material of Experiment No. 20 within the scope of the present invention that passed through the continuous hot dip coating line, and the steel of the present invention exhibits excellent properties even in annealing processes other than continuous annealing. Although not shown in the table, the steels of the present invention have a low r-value anisotropy.
The absolute value of r was 0.3 or less.

[0020]

[Table 1]

[0021]

[Table 2]

Example 2 An example of the present invention will be described together with a comparative example. 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. Manufacturing conditions for each experiment,
Table 3 shows the crystal grain size of the hot-rolled sheet, the steepness of the hot-rolled sheet, and the r value of the product sheet. The particle size number is ASTM-No. Is. The steepness represents the wave shape in the plate width direction with the denominator as the amplitude and the numerator as the wave height. The slab heating temperature is 1200 ° C. and 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 21, alloy plating was performed in a continuous hot dip galvanizing line at 780 ° C.

Experiment Nos. 21 and 22, which are within the scope of the present invention,
In Nos. 29, 31, 35, 36, 38, 39, 41, the grain diameter of the hot rolled sheet is small and the r value of the product sheet is high. Also, the steepness of the hot rolled sheet is small. In the material of Experiment No. 23 in which the rolling reduction in the final stage was within the range of the present invention, the structure of the hot-rolled sheet did not become sufficiently fine, and the r value of the product sheet did not become high. The material of Experiment No. 24, in which the rolling reduction of the shape-controlled rolling was outside the range of the present invention, had a large steepness of the hot-rolled sheet, the workability of the cold-rolling was poor, and the defective shape was partially present in the shape of the product sheet. On the other hand, in the material of Experiment No. 27, in which the rolling reduction of the shape-controlled rolling was larger than the range of the present invention, coarse grains were partially generated in the hot-rolled sheet, and r
The value did not rise. Similar coarse grains in the hot-rolled sheet were also found in the material of Experiment No. 26 in which the winding temperature was above the range of the present invention.

In the material of Experiment No. 25, in which the average cold speed from the completion of finish rolling to the Ar ₃ transformation point of -50 ° C. is within the range of the present invention, the structure of the hot-rolled sheet does not become sufficiently fine and the r value of the product sheet Did not get higher. In the material of Experiment No. 28 in which the finishing temperature was below the transformation point, the hot rolled structure partially exhibited a worked structure, and the r value of the product sheet did not increase. Further, in the material of Experiment No. 40 whose finishing temperature exceeded the range of the present invention, the structure of the hot-rolled sheet did not become sufficiently fine, and the r value of the product sheet did not become high. The material of Experiment No. 30 in which the amount of C exceeded the range of the present invention was
The structure of the hot-rolled sheet was fine, but the r-value of the product sheet did not become high. On the contrary, in the material of Experiment No. 37 in which the amount of C is less than the range of the present invention, the hot-rolled sheet has coarse particles, and the r value of the product sheet is relatively low. (C / 12 + N / 14 + S / 32) / (T
For the material of Experiment No. 32, which did not satisfy the relationship of i / 48 + Nb / 93) <1.4, the r value of the product plate did not become high.

1 of Mn, Si, Cr, Cu, Ni, Mo
In the material of Experiment No. 33 in which the content of one kind or two or more kinds was less than the range of the present invention, the hot-rolled sheet had coarse particles, and the r value of the product sheet did not become high. On the contrary, in the material of Experiment No. 34 added in the range of the present invention or more, the structure of the hot rolled sheet becomes fine, but the r value of the product sheet is low. A high r value was obtained even with the material of Experiment No. 41 within the scope of the present invention that passed through the continuous hot dip coating line, and the present invention steel exhibits excellent properties even in annealing processes other than continuous annealing. Although not shown in the table, the steels of the present invention have a low r-value anisotropy.
The absolute value of r was 0.3 or less.

[0026]

[Table 3]

[0027]

EFFECTS OF THE INVENTION 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 a good shape and a r-value higher than that of the conventional cold-rolled steel sheet. It is an industrially valuable invention because it enables the processing of press materials, which was thought to be impossible to mold once, until now.

Claims (2)

[Claims] 1. C: 0.0005% or more by weight%, 0.
005% or less, N: 0.005% or less, P: 0.1% or less, S: 0.02% or less, Al: 0.1% or less including T
One or both of i and Nb is 0.2 <(C
/ 12 + N / 14 + S / 32) / (Ti / 48 + Nb /
93) <1.4 to satisfy the condition, M
Steel containing one or more of n, Si, Cr, Cu, Ni and Mo in a content of 0.1% or more and 1.5% or less and the balance Fe and unavoidable impurities is used as Ar ₃ transformation point or more, A
Rolling with a total reduction of 70% or more is performed in a temperature range of r ₃ transformation point + 100 ° C. or less, finish rolling is completed at an Ar ₃ transformation point or more, and an average cooling from immediately after rolling to Ar ₃ transformation point −50 ° C. It is cooled at a speed of 50 ° C./sec or more and then 0.
A method for producing a cold-rolled steel sheet for deep drawing, which comprises rolling at 5% or more and 10% or less, winding at 750 ° C. or less, and then performing ordinary pickling, cold rolling, and annealing. 2. C: 0.0005% or more by weight%, 0.
005% or less, N: 0.005% or less, P: 0.1% or less, S: 0.02% or less, Al: 0.1% or less including T
One or both of i and Nb is 0.2 <(C
/ 12 + N / 14 + S / 32) / (Ti / 48 + Nb /
93) <1.4 to satisfy the condition, M
Steel containing one or more of n, Si, Cr, Cu, Ni and Mo in a content of 0.1% or more and 1.5% or less and the balance Fe and unavoidable impurities is used as Ar ₃ transformation point or more, A
After rolling at a finishing temperature of r ₃ transformation point + 50 ° C. or lower and a final rolling reduction of 30% or higher, cooling is started immediately after rolling,
Average cold speed 50 immediately after rolling up to Ar ₃ transformation point -50 ° C
Cooling at ℃ / sec or more, then 0.5% or more, 10%
A method for producing a cold-rolled steel sheet for deep drawing, comprising the following rolling, winding at 750 ° C. or lower, and subsequent ordinary pickling, cold rolling, and annealing.