JPH02205657A - Manufacture of cold rolled steel sheet for ultradeep drawing - Google Patents

Abstract

PURPOSE:To obtain the cold rolled steel sheet for ultradeep drawing having excellent surface properties and extremely excellent deep drawability by subjecting a steel having limited compsn. added with Nb to hot and cold rolling under prescribed process conditions. CONSTITUTION:A steel slab constituted of, by weight, <=0.01% C, <=0.1% Si, <=0.3% Mn, <=0.02% P, <=0.015% S and <=0.01% N and added with <=0.2% Nb whose amt. further satisfies (C/12)<(0.6XNb/93) and the balance iron with inevitable impurities is manufactured. The steel slab is subjected to rough rolling at 900 to 1200 deg.C and the carbide of Nb is precipitated to regulate the total amounts of solid soln. C in the steel to <=20ppm, is subjected to hot finish rolling at >=650 deg.C by a hot rolling roll having D1>100(t)1 roll diameter D1 to finish sheet thickness (t)1 at the draft R1 in T( deg.C)=<=880-5500X(C%), is thereafter coiled at <=600 deg.C and is pickled. The steel slab is furthermore subjected to cold rolling by a roll having D2>100(t)2 roll diameter D2 to cold finish sheet thickness (t)2 at >50% draft (where the total draft R of R1 and R2=95>R>75%) and is thereafter annealed to manufacture the above steel sheet.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a method for manufacturing a cold rolled steel sheet having extremely excellent deep drawability.

[Conventional technology]

Conventionally, in the manufacturing process of cold-rolled steel sheets, A. A material that has been hot-rolled at a finishing temperature above the transformation point is pickled, cold-rolled at a cold rolling rate of about 75%, and then sintered. Common.

In such a conventional method, in order to improve the Lankford (!! (hereinafter referred to as r value)) of the cold-rolled steel sheet, it is sufficient to increase the cold rolling rate from about 75% to about 90%. However, in this case, it was difficult to adopt such a high cold rolling rate due to the capacity of the cold rolling mill.Also, if a reverse rolling mill etc. were adopted, the above-mentioned high rolling rate could be achieved. Even if rolling at a cold rolling rate becomes possible, in reality, the finished hot-rolled plate thickness is limited to the maximum thickness that can pass through the pickling line, so from now on, for example,
When producing Elmt cold-rolled steel sheets, only a cold-rolling rate of 87% can be obtained, and the above-mentioned high cold-rolling rate is difficult to achieve, and sufficient cold-rolling texture cannot be expected to develop.As a result, recrystallization and sintering After purification, no effective texture for deep drawability was developed, and the r value was 2.
It was around 0.

[Problem that the invention seeks to solve]

As mentioned above, in the conventional manufacturing method of cold-rolled steel sheets, the ideal cold-rolling ratio cannot be obtained and a sufficient cold-rolled texture cannot be obtained, so there is a limit to the deep drawability that can be obtained, that is, the r value. There was a problem.

The object of the present invention was made in view of the above-mentioned conventional situation, and it is an object of the present invention to provide a method for manufacturing a cold-rolled steel sheet having extremely good deep drawability.

In order to solve the above-mentioned conventional problems, the inventor of the present invention conducted intensive research, created part of the texture of cold rolling at the hot rolling stage,
We perfected it through subsequent cold rolling, and also discovered that the shape, precision, and surface quality required for cold-rolled steel sheets could be achieved during the cold-rolling stage. The present invention was developed based on the idea that the conditions for forming a rolling texture in the hot rolling stage are that unrecrystallized grains are obtained in the ferrite region.

[Means for solving problems]

Therefore, the invention of item 1 of the present application provides C: 0.01% by weight or less, Si: 0.1% by weight or less, Mn: 0.3% by weight or less, P: 0.02% by weight or less, S: 0.015. % by weight or less, Nb: 0.01% by weight or less, Nb:
0.2 layers! 94 or less, and (C/12) < (0,
6×Nb/93) is subjected to rough rolling at 900°C to 1200°C, and the Nb
carbide is precipitated to reduce the solid solute C to 20 ppm or less, and 6
Finished plate thickness at 50℃ or higher 1. The roll diameter D1 is Dt
T ('C
) -880-5500X (C%) or less rolling reduction R1
After finish rolling, winding at 600°C or less, and pickling, the roll diameter D8 is Dt > 100 t with respect to the finished plate thickness t8.
, using a rolling roll with a rolling reduction ratio Rz>50% (however, R.

This is a manufacturing method in which cold rolling is performed such that the total reduction ratio R of R2 and R2 is 95%>R>75%) and then sintered.

Further, the invention of item 2 of the present application is such that the steel having the chemical composition in the invention of item 1 above contains B: 0.0005 to 0.005% by weight, and this is applicable to the above manufacturing method. In addition, the total rolling reduction ratio R is calculated|required here by the following formula.

R-1-(1-R+)x (1-Rg) Furthermore, the rough rolling may be conventional reheat rolling or direct rolling in which rolling is performed immediately after casting.

Here, the reason for limiting the components and manufacturing conditions of the present invention will be explained.

First, the reason for limiting the components in the specific invention will be explained.

When C and N are added in excess of 0.01% each, N
The processability of the product is poor due to the increase in precipitates such as bC, TiN, and AjN.
Since the amount of j increases and becomes expensive, -C1,01
%, N≦0.01%.

Si, Mn, and P each act as solid solution strengthening elements and reduce the r value and elongation, making it impossible to achieve the ultra-deep drawability that is the objective of the present invention, so they must be regulated below their respective upper limit values. , S cause elongated inclusions and reduce local ductility, so they must be kept at 0°015% or less.

By adding Nb, it is possible to form carbon and nitrides to reduce solid solution C in steel, and also to significantly increase the ferrite recrystallization temperature, which also delays recovery during processing. become. As a result, from a metallurgical point of view, it is possible to extend the state similar to cold rolling to a high temperature range, and it is possible to create a part of the cold texture during hot rolling. This effect cannot be achieved with the same amount of Ti or ■, and is a phenomenon unique to Nb. The amount of Nb added is the amount necessary to form carbon and nitrides and fix the solid solution C in the steel, that is, the amount that satisfies (C/12) < (0,6 x Nb /93). , and considering economic efficiency, the Nb content was set to 1.2% by weight or less.

By adding B, the warp cracking resistance can be improved. That is, by adding Nb, solid solution C can be reduced, but this reduction in solid solution C weakens the bonding force of grain boundaries and deteriorates the warp cracking resistance during secondary processing. Therefore, by adding B, it is possible to segregate this B at the grain boundaries and strengthen the bonding force at the grain boundaries. Here, regarding the amount of B added, if it is less than o or oos%, the above effect cannot be obtained, and if it is added in excess of 0.005%, it is economically disadvantageous, and if it is added in excess, it will have a negative effect on the deep drawability of the product. Therefore, it was set as o,oos% to o,oos%.

Next, manufacturing conditions will be explained.

The point of performing Nb carbide precipitation treatment: Even if rolling is performed in the non-recrystallized ferrite region in a state where solid solution C is included at least 20 ppm, the (111) texture parallel to the sheet surface will develop during recrystallization annealing. (200) texture develops, which adversely affects deep drawability. In order to obtain high deep drawability through cold rolling, it is necessary to coil at a high temperature after hot rolling to precipitate carbon and nitrides. Nb before unrecrystallized rolling at a temperature range of
It is necessary to precipitate carbides to reduce solid solution C in the steel.

Here, the following three methods can be considered for sufficiently precipitating NbC before hot rolling finishing.

■ For reheat rolling, increase the slab heating temperature to 900℃~1
A method that uses a low temperature of 100°C to prevent precipitates from forming a solid solution during the slab heating stage.

, ■ The rough rolling temperature is lowered to 900 to 1000°C, and T
I.

A method for rolling-induced precipitation of Nb carbide.

(2) A method in which a period of time is allowed between the end of rough rolling and the finishing of hot rolling, and carbide is precipitated during this period (in this case, the waiting time is preferably 4 minutes or more in the precipitate formation temperature range).

The hot rolling temperature is 650°C or higher, and T(t)
-880-5500x (C%) The total reduction ratio of hot rolling and cold rolling is 75% or more and 95% or less:
When rolling at temperatures exceeding T (℃) -880-5500x (C%), the result is ferrite + austenite or an austenite region, and no rolling texture remains.
T (℃) -880-5500x (C%) and 65
Hot rolling at 0°C is rolling in the unrecrystallized ferrite region, and the rolling reduction rate is the rolling set & [
It is important in forming the l weave, so a total of 75
The amount of pressure reduction in this temperature range is controlled to 95%.

On the other hand, rolling at a temperature below 650° C. is not practical because the deformation resistance of the material is too high.

In addition, in order to obtain a texture that is effective for deep drawability in recrystallization annealing, the cold rolling texture must be sufficiently developed. The sum of the rolling rate and the cold rolling rate is required to be 75 to 95%. That is, the rolling reduction ratio R1 in hot rolling is the total rolling reduction ratio R(=1 (l
R+)X(1-R2)] is 75% or more and 95% or less.

Further, the winding temperature must be kept under conditions that do not cause recrystallization during the winding stage, that is, 600° C. or lower. This is because if it exceeds the expected cumulative effect of cold rolling, it will no longer be possible.

In this case, the cold rolling requires a reduction ratio of 50% or more in order to perfect the texture created by the hot rolling and to adjust the surface shape.

Lubrication during hot rolling: Regarding lubrication during hot rolling, the better the lubrication, the better the properties equivalent to those of cold rolling, but even if the lubrication conditions are poor, as long as the total reduction with cold rolling is within the range of the present invention. Since it is possible to obtain a higher r value than a cold-rolled steel sheet with a rolling reduction of less than , there is no need to specify it in particular.

The point of specifying the roll diameter: If the roll diameter is too small compared to the finished sheet thickness, even if the cold rolling rate is increased, strain in the sheet pressure direction will not reach the center of the sheet, and a sufficient cold rolling texture will not be obtained. I can't do it,
Therefore, the roll diameter should be D>100 for the finished plate thickness t.
't.

Note that this also applies to cold rolling rolls.

[Effect]

In the method for manufacturing a cold rolled steel sheet according to the present invention, solid solution C in the steel is reduced by precipitating Nb carbides, and hot rolling and cold rolling are performed at a predetermined temperature range and reduction rate using rolling rolls having a predetermined roll diameter. By performing inter-rolling, it is possible to extend the state that is almost the same as that obtained by cold rolling from a metallurgical point of view to a high temperature range, and the cold-rolling texture, which affects deep drawability, can be improved by both cold-rolling textures. It can be developed by rolling, resulting in high deep drawability.

Therefore, if hot rolling is performed that satisfies the above conditions, the temperature at the entry side of finishing hot rolling is T (t) -880-5500
There is no problem even if it exceeds X (C%).

Furthermore, in the related invention of the present application, since B is added, in addition to the above-mentioned effects, it is possible to prevent deterioration of warp cracking resistance due to a reduction in solid solution C in the steel.

〔Example〕

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

First, an embodiment of the invention of item 1 of the present application will be described.

Table 1 shows the chemical composition of the steel of the present invention (C, D in the table) and the comparative steel (A, B in the table).

In this example, the four steel types shown in Table 1 were melted in a converter, a slab was manufactured using the normal process, and then hot rolled under the conditions shown in Table 2, wound into a coil, and After washing, this was also cold rolled under the conditions shown in Table 2 at 850°C x 1.
Continuous sintering (CAL) for 5 minutes or batch sintering (BOX) at 750°C for 3 hours was performed to measure the r value of the material.

The results are shown in Table 2.

As is clear from Table 2, the steel of the present invention (Section 3.4)
．． 6, 7.10), the r value is 2.
It can be seen that good deep drawability is obtained, which is as high as 0 or more.

The inventor also conducted an experiment regarding the relationship between rolling reduction and deep drawability (r value).

The figure is a characteristic diagram showing the above experimental results. This experiment
0.0025%C-0,02%Si-0,15%Mn
-0,001%S-0,025%Nb-0,003%
A hot-rolled plate of N was cold-rolled by varying the rolling reduction ratio, and
After recrystallization and annealing for 2 minutes at °C, the r value was measured (in the figure,
・Indicated by mark), and half of the total rolling reduction of the above steel is 860
℃ or less, the remaining half was cold rolled, and after recrystallization and annealing at 850° C. for 2 minutes, the r value was measured (indicated by a circle in the figure).

As is clear from the figure, in the range of the total rolling reduction of 75% to 95%, r values of 2.0 or more are obtained in all cases (marked with ◯ and *).

Next, an embodiment of the invention of Section 2 of the present application will be described.

Table 3 shows the chemical composition of the steel of the present invention (E, F in the table). Reheated and roughly rolled to 30fi thickness in T area, 920℃ after 2 minutes.
Finish rolling was carried out at a temperature of 60% below 855°C to a thickness of 2.9 fl, and the finishing temperature was approximately 770°C for both image scales (E and C). After being wound up at 500°C, it was pickled and cold rolled. The finished plate thickness of this cold rolling is 0.8 fi, so 85
The rolling reduction ratio in a temperature range of 5° C. or less is 89%. Further, the roll diameter for hot rolling is φ800, and the roll diameter for cold rolling is φ580.

Then, the steel plate obtained by the above manufacturing method was heated to 850°C
After continuous sintering for 1.5 minutes, the r value was measured and the warp cracking resistance test during secondary processing was conducted. This vertical cracking resistance test adopted a cup vertical cracking test, in which a steel plate was blanked with a diameter of 145 mm, a test cup was prepared with a drawing ratio of α: 2.0, and the cup was heated at temperatures ranging from liquid nitrogen temperature to room temperature. Place it over a conical punch and apply a load from the bottom of the cup to destroy it.
The transition temperature was measured from the brittle fracture rate at that time.

The experimental results are shown in Table 4.

According to the same table, C remains in 111E, so!
[It can be seen that in F, the brittle transition temperature is lowered by adding B, and both exhibit good warp cracking resistance.

〔Effect of the invention〕

As mentioned above, this application No. 1. According to the method for producing a cold-rolled steel sheet for ultra-deep drawing according to the invention of item 2, Nb carbide is precipitated to reduce solid solution C in the steel, and the cold-rolling texture is improved during the hot-rolling stage. By completing the aggregated weave and adjusting the surface texture during the cold rolling stage, a cold-rolled steel sheet with excellent surface texture and extremely excellent deep drawability can be obtained. effective.

Furthermore, according to the invention in item 2 of the present application, since B is further added, in addition to the above-mentioned effects, it is possible to ensure warp cracking resistance in steel in which Nb is added in an amount greater than the atomic equivalent of C. .

[Brief explanation of the drawing]

The drawing is a characteristic diagram showing the relationship between rolling reduction and r value for explaining the present invention in detail.

Claims (2)

[Claims] (1) C: 0.01% by weight or less, Si: 0.1% by weight or less, Mn: 0.3% by weight or less, P: 0.02% by weight or less, S: 0.015% by weight or less, N: Contains 0.01% by weight or less, Nb: 0.2% by weight or less, and (C/12
) (0.6×Nb/93) for steel with the balance Fe and unavoidable impurities,
Rough rolling is performed at a temperature range of 900°C to 1200°C, and Nb carbide is precipitated to reduce the total amount of solute C in the steel to 2.
0 ppm or less, and in a temperature range of 650°C or higher, the roll diameter D_1 is D_1>100t_ with respect to the finished plate thickness t_1.
T (°C) = 880-550 using a rolling roll of 1
After finish rolling with a rolling reduction R_1 at 0x(C%) or less, it is rolled up at 600°C or less, pickled, and further the roll diameter D_2 is D_2>100t_2 with respect to the finished plate thickness t_2.
Using a rolling roll, the rolling reduction ratio R_2>50% (however, the total rolling reduction ratio R of R_1 and R_2 is 95%>R>75%
) A method for producing a cold-rolled steel sheet for ultra-deep drawing, characterized in that after cold rolling, sintering is performed. (2) C: 0.01% by weight or less, Si: 0.1% by weight or less, Mn: 0.3% by weight or less, P: 0.02% by weight or less, S: 0.015% by weight or less, N: Contains 0.01% by weight or less, B: 0.0005 to 0.005% by weight, Nb
: 0.2% by weight or less, and (C/12)<(0.6×
Nb/93)
and unavoidable impurities, 900℃~12
Rough rolling is performed in a temperature range of 00°C, Nb carbide is precipitated, the total amount of solid solute c in the steel is 20 ppm or less, and roll diameter D_1 is applied to the finished plate thickness t_1 in a temperature range of 650°C or higher. After performing finish rolling at a reduction rate R_1 at T (°C) -880-5500 x (C%) or less using a rolling roll where D_1>100t_1, 600°C
Below, winding and pickling are performed, and further, rolling reduction ratio R_2>50% (however, R_1
and R_2 (total reduction ratio R of 95%>R>75%), and then sintering is performed.