JPS61276933A - Manufacture of cold rolled steel sheet for deep drawing having uniform quality in lateral direction - Google Patents

Abstract

PURPOSE:To manufacture a cold rolled steel sheet for deep drawing having uniform quality in the lateral direction by carrying out reduction in width in the austenite range where the formation, coagulation and coarsening of a precipitate are easily caused and by utilizing heat generated by the reduction in width. CONSTITUTION:A steel slab having a composition contg., by weight, <0.005% C, <1% Si, <1% Mn, <0.15% P, <0.015% S, 0.01-0.1% Al, <0.005% N and 0.01-0.05% Ti and/or 0.004-0.03% Nb is hot rolled, cold rolled and continuously annealed to manufacture a cold rolled steel sheet for deep drawing. At this time, reduction in width at 5-40% rate is carried out at 900-1,300 deg.C in the hot rolling stage.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to the production of cold-rolled steel sheets for deep drawing suitable for applications requiring good press formability, particularly deep drawability, such as outer panels for automobiles. Regarding the method, it has achieved uniformity of the material, especially in the width direction of the plate.

(Prior Art) With the development of large press equipment, cold rolled steel sheets used for automobile outer panels and the like are increasingly being used in larger sizes than before.

However, there is a natural limit to the width of the strip that can be rolled during hot and cold rolling, and among conventional steel plates, especially cold-rolled steel sheets manufactured by continuous annealing, the width of the coil in the width direction from the edge to the center is limited. Since the mechanical properties are poor over approximately 100 fins, this part often has to be cut away from the product, and a system that can meet the above-mentioned demands for larger steel plates has not yet been established.

The reason why the material quality in the area near the edges in the width direction of the coil is inferior is that during hot rolling, the cooling rate in this area is faster than in the center, so the size of precipitates formed during rolling is fine. As a result, this shadow stage remains even after continuous annealing.
This is thought to be because the formation of a texture that is favorable for the deep drawability of the steel sheet is suppressed, and this tendency was more pronounced in steels to which Ti and Nh were added.

(Problem to be Solved by the Invention) The reason why the material quality in the area near the acid edge in the coil width direction is inferior is that -L As mentioned above, the cooling rate in this area is faster than in the center, and as a result, the material is formed. This is because carbonitrides such as Nb and Ti, as well as precipitates such as MnS and Al, become finer in size and, in extreme cases, exist in a solid solution state. This is further facilitated by the fast cooling rate near the edges during cooling after winding up as a hot-rolled coil, which deteriorates the material quality after continuous annealing, resulting in non-uniformity of the material in the width direction. He was inviting her.

The present invention advantageously solves the above-mentioned problems, and proposes an advantageous method for manufacturing a cold-rolled steel sheet that is uniform over the entire width of the sheet by effectively avoiding material deterioration in the vicinity of the edge portion of the coil width. The purpose is to

(Means for Solving the Problem) As a result of extensive research to solve the problem described in item 1, the inventors have found that the austenite HW region, which is prone to forming precipitates and coarsening of agglomerates, In a temperature range of 900 to 1300°C, the plate width is reduced, that is, the width is killed, and the processing strain introduced by such width killing causes the precipitates to aggregate and coarsen at the edge portions, and due to this width killing, It has been found that the intended purpose can be advantageously achieved by controlling the cooling rate on the hot run table and the cooling rate after winding the hot-rolled coil using processing heat generation.

This invention is based on the above knowledge.

That is, the present invention provides C: 0.0050 wt% or less (hereinafter simply expressed as %), Si: 1.0% or less, Mn
: 1.0% or less, P: 0.15% or less, S: 0.01
Less than 5%, 87! :o, o1o~0.100% and,
Contains N: 0.0050% or less, and Ti:
0.010-0.050% and Nb: 0.00
In manufacturing a cold rolled steel plate for deep drawing by hot rolling a steel slab having a composition containing at least one of the following: 4 to 0.030%, the above-mentioned In the hot rolling process, 900 to 1300
This is a method for manufacturing a cold-rolled steel sheet for deep drawing with uniform material quality in the sheet width direction, characterized by performing width reduction of 5 to 40% in a temperature range of .degree.

This invention will be specifically explained below.

First, the experimental results that formed the basis of this invention will be explained.

C: 0.0021%, Si: 0.02%, Mn
:0.11%.

P: 0.013%, S: 0.009%, ^4
: 0.033%.

N: 0.0028% and Nb: 0.012%
A 1600mm wide steel slab with a composition containing
After heating to 1180°C, in the rough rolling stage of hot rolling, when the temperature at the center in the width direction is 1100°C, 2
After applying 0% width reduction, finishing rolling was completed at 905℃, then coiling at 620℃, and after pickling, 75%
Cold rolling was performed at a reduction ratio of
Cooled to room temperature at a cooling rate of °C/S.

Discs of 35 mmφ are punched out from the edge of the thus obtained cold-rolled sheet, 50+m from the edge, loo++■, and several points near the center (however, the center of the disk at the edge is 18 mm from the edge). position) and a conical cup test (CCV).

Here, CCv is a test for examining drawability, and the lower the value, the better. Note that deep drawability can be evaluated using the T value, but since a test piece with a large size is required to measure the T value, it is not suitable for measuring drawability near the outermost edge.

In FIG. 1, the CCV test results are shown with a mark. In addition, for comparison, the same figure also shows test results for annealed plates obtained by the conventional method in which width reduction is not performed in the hot rolling process.

As is clear from the figure, in the case of the conventional method, CCV = 2 from the highest edge to 100mm.
The drawability was poor at 6.7 to 27.2 (equivalent to 1.3 to 1.5 in 〒), but the CCV was poor across the entire width direction in the case where the width was reduced in the hot rolling process. '=26
．． Excellent drawability of 0 (corresponding to 1.9 below) was obtained.

The reason why the component composition of the material in this invention is limited to the range described in item 1 is as follows.

C: 0.0050% or less In order to improve the 〒 value and elongation, which are the most important values for steel sheets for processing, the smaller the C content, the better. Therefore, the C content is more preferably 0.0035% or less than 0.0050%. limited to the range of

Si: 1.0% or less, Mn: 1.0% or less Si
Both Mn and Mn effectively contribute to improving the strength of the steel plate, but excessive addition causes deterioration of weldability, so it was decided to add each in a range of 1.0% or less.

P: 0.15% or less P, like Si and Mn, effectively contributes to increasing the strength of steel sheets, but if the content exceeds 0.15%, weldability deteriorates, so 0.15% Limited to the following range.

S: 0.015% or less If a large amount of S is added, it will not be fixed by Mn or Ti and will cause cracks during hot rolling.
The content was limited to 0.010% or less.

8#: 0.010 to 0.10% AA is O in steel
By preferentially fixing and bonding with O, Ti and N
It is a useful element that avoids a decrease in the effective amount of b.
If it is less than 0.010%, the effect of the addition is poor;
Even if it is added in excess of 0%, the effect reaches saturation.
．． 01. It was decided to add it in the range of 0 to 0.10%.

N: 0.0050% or less Like C, N causes deterioration of T value and elongation when contained in a large amount, so it was limited to a range of 0.0050% or less.

Ti: 0.010 to 0.050%, Nb:
0.004 - 0.030% Both Ti and Nb fix C and N, which are dissolved in steel and have a negative effect on deep drawability and elongation, improving formability and aging resistance. It is a useful element that However, Ti and Nb are each 0.010%.

If it is less than 0.004%, it is difficult to say that the fixation of C and N is sufficient and satisfactory workability cannot be obtained; on the other hand, if it is 0.050%
If it exceeds 0.030%, carbonitrides of Ti and Nb will increase, resulting in deterioration of workability, and dense oxides will be formed on the surface, making the surface inert and deteriorating chemical conversion treatment properties. Therefore, it was decided to add Ti and Nb within the above ranges.

Note that Ti has superior ductility compared to Nb, while Nb
Since Ti has the advantage of being superior to Ti in improving anisotropy, it is desirable to add both in combination to obtain the best material quality. However, even if Ti and Nb are added individually, a material with much better quality can be obtained compared to low carbon An-killed steel manufactured by conventional box annealing.

Next, the manufacturing method according to the present invention will be specifically explained step by step.

The steel slab adjusted to the preferred composition described above is reheated to 1000 to 1300 ° C in a heating furnace based on a conventional method, or immediately subjected to hot rolling without reheating. In this invention, in the hot rolling step,
The main feature is that the width is reduced by 5 to 40% in the temperature range of 900 to 1300°C.

Such width reduction is performed before the start of finishing hot rolling, that is, during rough rolling before forming a sheet bar with a thickness of about 20 to 50 mm, or at a stage before rough rolling is applied to the slab extracted from the heating furnace. This can be done in a vertical mill.

The reason why the temperature at which the width cutting should be performed is limited to the range of 900 to 1300'C is because below 900C, processing occurs in the ferrite region and the agglomeration effect of precipitates is small.
When the temperature exceeds 00°C, most of the carbonitrides of Ti and Nb are in a solid solution state, so even if the width is reduced, it is possible to improve the material quality at the edge, but rather the material quality at the center will deteriorate. It is a thick shell that may cause damage.

In addition, as is clear from the examples described later, the effect of width killing is achieved when the width killing rate is 5% or more, so a width killing of at least 5% is required. In view of the uniformity of the crown (thickness distribution) and shape, 40% was set as -1- eye.

A steel plate that has been hot rolled with width reduction as described above is wound into a hot rolled coil in accordance with a conventional method. Here, the hot rolling finishing temperature is generally Ar3 or higher (870 to 900°C),
There is almost no deterioration of the material even if the temperature is lowered to about 700°C, which is the ferrite range. Further, the winding temperature is not particularly limited, and winding at a low temperature of 600° C. or lower is also possible.

After that, it is descaled, then cold rolled and then continuously annealed. Although the rolling reduction ratio in cold rolling is not particularly limited, it is preferably 60% or more in order to improve drawing properties. Continuous annealing may be carried out at a temperature considered to be the recrystallization temperature, and there are no particular limitations on the number of heating and cooling steps.

(Function) The reason why the material quality in the region closest to the edge of the steel plate is improved by performing width reduction in the hot rolling process according to the present invention (J. The width reduction causes the agglomeration of precipitates in the region near the edge. *■ This is because, at the same time, the temperature drop in the area near the edge portion can be advantageously prevented from decreasing due to heat generation during processing.

(Example) Molten steels A to C having the compositions shown in Table 1 obtained by performing R1+ degassing treatment after melting in a converter were each made into slabs by continuous casting.

Then, after hot rolling under the conditions shown in Table 2, descaling was performed, and then cold rolling was performed at a rolling reduction of near 8% to obtain a cold rolled sheet with a thickness of Q, 7 mm.

After that, continuous annealing was performed at 800°C for 40s, and then
．． A product sheet was obtained by subjecting it to 5% temper rolling.

501 from the most edge part in the width direction of each product thus obtained.
A No. 11S5 test piece was cut out in the rolling direction from the position (2) and the center, and a tensile test was conducted. In addition, a CCV test was conducted by punching out a disk of 3611 Ilφ from the same position.

The results of each test are summarized in Table 3.

As is clear from the results shown in Table 3, when width killing was not performed in the hot rolling process (floor 3), when there were few width killing stars (ltlo, 4), and when the width killing temperature exceeded the appropriate range. In both cases (tooth 2), the drawability in the region near the edge is poor, and E7! The value was also poor.

On the other hand, the materials obtained according to the present invention (floors 1 and 5 to 8) were uniform in material quality in the width direction, and were excellent in not only drawability but also mechanical properties.

(Effects of the Invention) Thus, according to the present invention, it is possible to effectively prevent the deterioration of the drawability of the material in the region near the edge portion of the steel plate, and to obtain a cold-rolled steel plate that is homogeneous over the entire width direction.

Therefore, it goes without saying that the yield of steel sheets can be improved, and by applying width reduction in the hot rolling process, restrictions on the casting size during continuous rolling are removed, which also contributes to improving productivity.

[Brief explanation of the drawing]

FIG. 1 is a graph showing a comparison of CCv test results when width reduction was performed and when width reduction was not performed in the hot rolling process.

Claims (1)

[Claims] 1. C: 0.0050 wt% or less Si: 1.0 wt% or less Mn: 1.0 wt% or less P: 0.15 wt% or less S: 0.015 wt% or less Al: 0.010 to 0 .100wt% or less and N:0
．． 0.050 wt% or less, and Ti: 0.010 to 0.050 wt% or less and Nb:
In producing a cold rolled steel plate for deep drawing by hot rolling a steel slab having a composition containing at least one of 0.004 to 0.030 wt% or less, followed by cold rolling and continuous annealing. , in the above hot rolling process, in the temperature range of 900 to 1300°C.
A method for producing a cold-rolled steel sheet for deep drawing, the material quality of which is uniform in the sheet width direction, characterized by performing width reduction by 40%.