JPS63100134A - Manufacture of cold rolled steel sheet for extra deep drawing of thick product - Google Patents

Abstract

PURPOSE:To obtain the titled steel sheet having superior deep drawability by cold rolling at a low draft without putting excessive load on a cold rolling mill by restricting the compsn. of a dead soft steel contg. Ti and Nb and by specifying conditions during the hot rolling of the steel. CONSTITUTION:A steel slab having a compsn. contg., by weight, <=0.003% C, <=0.03% Si, <=0.20% Mn, <=0.015% P, <=0.020% S, 0.005-0.1% Al, <=0.0025% N, 0.001-0.015% Nb and [(48/12).%C+(43/14).%N+(43/32).%S]-0.1% Ti is subjected to hot finish rolling at <=1,100 deg.C starting temp., 870-910 deg.C finishing temp., >=85% total draft and 10-50% draft at the final pass. After the end of the hot rolling, the hot rolled steel sheet is coiled at <=600 deg.C, cold rolled to a prescribed thickness of >=1mm and annealed to manufacture the titled steel sheet.

Description

Detailed Description of the Invention (Industrial Field of Application) Regarding the manufacturing method of cold-rolled steel sheets for thick ultra-deep drawing, especially for sheets with a thickness of about 1 mm or more where it is difficult to sufficiently increase the rolling reduction in the cold rolling process. The results of research related to measures that can advantageously improve the workability of thick cold-rolled steel sheets, that is, thick materials, are described below.

Generally, in cold-rolled steel sheets for ultra-deep drawing, in order to obtain grain sizes and crystal orientations that are advantageous for deep drawability, the cold rolling process involves four strains that can produce sufficient recrystallized texture during annealing. It is necessary to give the steel plate.

In particular, in recent years, the deep drawability required for deep drawing steel sheets has become stricter, and as a result, softer steels with extremely low carbon content have been used in place of conventional low carbon steels. However, as a result, the reduction ratio of cold rolling, which is advantageous in terms of material quality, has come to require further investigation.

(Prior art) Aiming to improve material quality by limiting process conditions such as molten steel components, heating or heat retention up to cold rolling, and hot rolling and cooling to ranges that are advantageous in the cold rolling-annealing process. However, there have been many reports including JP-A No. 59-74233, but these are basically based on the optimum rolling reduction rate of 8.
At present, we are struggling with cold rolling under high pressure of 0% or more.

In addition, it has been reported in Japanese Patent Application Laid-Open No. 119621/1983 that the cold rolling reduction can be kept low by introducing warm rolling into part or all of the hot rolling process. Warm rolling is not yet at a technical stage where it can be introduced into the process, and even if it were successfully introduced after investing a considerable amount of technology development costs, the equipment, maintenance, and operating costs would be lower than the conventional hot rolling process. It is expected that it will become more expensive due to increased load and shorter roll life. Therefore, the introduction of warm rolling cannot be a solution to the problem of increased burden on rolling equipment.

(Problems to be Solved by the Invention) Now, in deep drawing applications where the amount of deformation is large, such as oil pans in automobiles, and press forming from crosspiece plates, extremely excellent deep drawing properties are required. Needless to say, the plate thickness is 1 mm or more, preferably 1 mm.
．． Having a thickness of 2 mm or more is an extremely important condition in order to withstand a large amount of processing.

However, if we rely on the above-mentioned conventional technology and apply cold rolling under high pressure, thin products (0.8a++n to 0.1m+m)
The amount of work is several to ten times larger than that at the same rolling reduction rate, which puts a heavy burden on the cold rolling equipment and also increases operating energy costs. Furthermore, due to equipment restrictions, the thickness of the plate at the rolling entry side cannot be increased unnecessarily, so it is difficult to reduce the thickness of the plate to a high degree in the case of thick products.

Therefore, in the production of thick cold-rolled steel sheets for ultra-deep drawing, there is a strong need for the development of a new production technology that does not require high pressure in the cold region.

In manufacturing thick steel sheets for ultra-deep drawing, we have developed a manufacturing process that does not require cold rolling under high reduction conditions, which is a burden on equipment and production energy. It is an object of this invention to obtain a rolled steel plate.

(Means for Solving the Problems) This invention has the following properties: C: 0.003 wt% or less, Si: 0.03 wt% or less, Mn: 0.20 wt% or less, P: 0.015 wtX or less, S : 0.020 wt% or less, Al: 0.005 to 0.1 wtχ, N: 0
．． Using a Ti-Nb composite added low carbon tone slab as a starting material, it has a composition containing 0.025 wt% or less, Nb: 0.001 to 0.015 wtχ, and undergoes hot rolling, cold rolling, and annealing processes to form an ultra-deep In obtaining a cold rolled steel plate for drawing, the hot finish rolling start temperature is 1100°C or less, and the hot finish rolling end temperature is 870 to 9.
The total rolling reduction rate in this hot finish rolling is set at 10°C and is 85°C.
% or more, the reduction rate in the final pass of hot finish rolling is 10%
For ultra-deep drawing of thick materials, the sheet is rolled up at 600°C or less after hot rolling, then cold rolled to a predetermined thickness of 1 m or more, and then annealed. A method for producing a cold rolled steel sheet, and the following: C: 0.003 wt% or less, Si: 0.03 at% or less, Mn: 0.20 wt% or less, P: 0.015 wt% or less, S: 0.020 wt% or less, AN: 0.005-0.1 wtX, N: 0.
0025 wt% or less, Nb: 0.001 to 0.015 wtX plus S
b70.001~0.02wt% and B: 0.00
Starting material is a Ti-Nb composite additive ultra-low carbon steel slab containing at least one of the following: 0.01 to 0.0010%. When obtaining a cold rolled steel plate for deep drawing, the hot finish rolling start temperature is 1100°C or less, and the hot finish rolling end temperature is 870°C.
~910°C, the total rolling reduction in this hot finish rolling is 85% or more, and the rolling reduction in the final pass of hot finishing rolling is 1.
0% or more and 50% or less, and after hot rolling, it is rolled up at 600°C or less, then cold rolled to a predetermined thickness of 1 m or more, and then annealed. This is a method for manufacturing cold-rolled steel sheets.

Here, it is particularly preferable that the annealing is performed at a heating rate of 10° C./s or less.

First, we will describe the experiment that directly led to this invention.

C: 0.0020 wtX, Si: 0.01
wtχ+Mn: 0.10 wtX.

P: 0.011 wtX, S: 0.004
wtX, Al: 0.036 wtLN: 0
．． 0022 wtX, Ti: 0.068 wtX
, Nb: 0.003wtχ.

Sb: 0.009 wt The rolling reduction in the hot finish rolling was set to 88%, and the rolling reduction in the final pass of this hot finishing rolling was set to 5 to 5.
It was varied between 50%.

These hot-rolled sheets were wound up at 550°C, pickled, cold-rolled by 70%, and continuously annealed at 850°C for 1 minute.

The Lankford value (lower value) of the obtained thick cold-rolled steel plate with a thickness of 1.6s+s is shown in FIG.

(222) A copper plate with developed aggregate IJ1m and excellent deep drawability is in the hot finish rolling end temperature and hot finish rolling final pass reduction ratio region of the present invention, that is, the hot finish rolling end temperature 8
70-910°C and hot finish rolling final pass reduction rate O~
It can be seen that at 50%, it is obtained.

It should be noted that if the final pass reduction ratio of hot finish rolling exceeds 50%, a defective shape will occur in the hot rolled sheet during the hot finish rolling process, and this is excluded from the scope of the present invention.

Furthermore, the inventors conducted the following experiment in order to understand the dependence of the rolling reduction rate in cold rolling regarding the effects of the present invention.

C: 0.0020 wtX, Si: 0.01
wtX, Mn: 0.11 wtX.

P: 0.010 wtX, S: 0.004
wtX, Aj! :0.042wtX.

N: 0.0018 wtL Ti: 0.067
wtL Nb: 0.004 wtX.

Sb: 0.008 wtX, a continuous cast steel strip was hot finish rolled at a start temperature of 1050°C, a hot finish rolling end temperature of 880°C, and a rolling reduction rate of 87% in hot finish rolling.
Hot rolling was performed at a final pass reduction ratio of 1 to 50%.

These hot rolled sheets were wound up at 550°C, pickled, cold rolled at a cold rolling reduction of 45 to 96%, and continuously annealed at 850°C for 1 minute.

The lower values of the obtained steel plates with thicknesses of 1 and 6 mm are shown in FIG. According to the method of this invention, even with a cold rolling reduction of 80% or less, it is possible not only to achieve good deep drawability equivalent to that which conventionally required a high reduction of around 90%, but also to achieve deeper drawability than in the high reduction region. It has become clear that there are areas where drawing performance can be improved.

(Function) The mechanism by which the hot finish rolling conditions of this invention bring about such good material quality is that by taking a large amount of reduction in the temperature range just above the Ar3 transformation point, strain is sufficiently accumulated in one grain, resulting in a fine and homogeneous material. This is thought to be due to the generation of α grains with a large grain size. For this reason, by cold rolling at a lower pressure than conventional methods, it is possible to satisfy the amount of strain required to generate a sufficient recrystallized texture, and also to obtain ideally uniform strain in the cold-rolled sheet. be. Furthermore, a fine α-grain structure with many grain boundaries is advantageous for the (222) orientation, which is said to develop from sites located at grain boundaries.

Note that as the cold rolling reduction increases, the deep drawability improves and then begins to deteriorate; however, according to Figure 2, this turning point occurs as the final pass reduction in hot finish rolling increases. Move to the rolling reduction side. It is inferred that this is because the amount of strain accumulated in a hot-rolled sheet made of fine and homogeneous α grains is greater than that of coarse or heterogeneous grains at the same rolling reduction rate. In the present invention, the effect of improving material quality is particularly observed in the region where the cold rolling reduction is 80% or less.

For the reasons mentioned above, the final pass reduction rate in hot finish rolling needs to be 10 to 50%, and the finishing temperature of hot finish rolling is in the temperature range just above the Ar3 transformation point (870°C to 910°C).
). In addition, in order to give a sufficient amount of strain to each grain before transformation, the total hot finishing rolling reduction must be 85%.
The above is necessary.

If the final pass reduction in hot finish rolling is less than 10%, or the end temperature of hot finishing exceeds 910°C, or the total reduction in hot finishing hot rolling is less than 85%, γ grains If a sufficient amount of strain is not applied, and if the finish hot rolling temperature falls below the Ar3 transformation point, extremely inhomogeneous strain will occur within the α grains, resulting in deterioration in deep drawability.

Moreover, if the final pass reduction ratio of hot finishing rolling exceeds 50%, a shape defect will occur in the hot rolled sheet in the finishing hot rolling process, so 10 to 50% is optimal.

The reason why the starting temperature of hot finish rolling is specified to be 1100°C or less is because if some precipitates such as TiS are not present at the start of hot finish rolling, one grain will become coarse and cause coarse α grains. It is.

Also, considering the amount of temperature drop of the steel plate in the current standard hot finish rolling process, the hot finish rolling end temperature should be 870℃.
In order to achieve ~910°C, the hot finish rolling start temperature is 11
The optimum temperature is below 00°C.

Setting the coil winding temperature after hot rolling to 600°C or less has the advantage of shortening the time required for transferring from the hot rolling process to the cold rolling process, as well as reducing α grains due to grain growth that tends to occur in ultra-low carbon steel. It also has the effect of preventing coarsening.

Although the effects of the present invention do not depend on the annealing method, it is desirable to raise the temperature at a rate of 10° C. or higher in order to develop grains with preferred orientations during heating.

The process conditions described above are indispensable for expanding the cold rolling reduction region that provides good deep drawability to the lower reduction side, but it is necessary to fully bring out the effects and to achieve a high level of deep drawability. In order to achieve this, range limits are also required for the alloying components.

The reason for limiting the content range of each compositional component in the present invention will be described below.

Since both C and N exhibit significant solid solution hardening and have a high aging effect, they are extremely disadvantageous elements for steel sheets for ultra-deep drawing. C is 0
．． 003 wt% or less, N is even more severe 0.0025
It is necessary to suppress it to below wt%.

Although Si is a substitutional solid solution element, it also has an adverse effect on workability, so its content is limited to 0.03 wt% or less.

P is an element that causes solid solution hardening and embrittlement, and is 0.01
Must be kept below 5 wt%.

Since S tends to form inclusions and also causes embrittlement, it is limited to 0.020 wt% or less.

Ti is a key element for fixing C, N, and S, and it must contain at least enough matrix to fix the entire amount of C, N, and S. Specifically, the above is necessary. The effective amount in terms of solid solution effect and cost is:
0.1 wt! It is as follows.

Nb has the effect of improving deep drawability, and Nb also has the effect of improving deep drawability.
It also noticeably improves the value. In particular, the improvement effect of Nb is remarkable in deep drawability in the low rolling reduction region of cold rolling.

The effective amount of Nb is 0.001 wt% or more 0.015 wt
% or less. Even if it is added in an amount exceeding this range, there will be little further effect, the cost will increase, and the elongation value will deteriorate due to the formation of fine NbC.

A1 is an element useful for deoxidation and needs to be present in an amount of 0.005 wt% or more, but since addition of a large amount impairs surface properties, the upper limit is set to 0.1 wt% or less.

Mn is an element useful for fixing S, but in Ti-added steel, its role is only an auxiliary one, so there is no need to try to increase the amount. If it is 0.20 wt2 or less, solid solution hardening is slight and there is no problem.

Note that sb may be added for the purpose of further improving moldability. sb is also recognized to have the effect of suppressing rough skin. Both effects are Sb: 0.001 to 0.02 wtχ
It works effectively in the following areas.

Further, for the purpose of softening the cold-rolled steel sheet and preventing secondary processing embrittlement, 0.0OO1 to 0.0O10wtX of B may be added. The lower limit of the amount added is determined by the presence or absence of the effect of addition, and the upper limit of the amount added is determined by saturation of the effect of addition, prevention of solid solution hardening, and deterioration of elongation value.

(Effects of the Invention) According to the present invention, a thick ultra-deep drawing steel plate with excellent deep drawability can be obtained by rolling at a low reduction rate without imposing an excessive burden on cold rolling equipment.

(Example) Examples of the present invention will be described.

Continuously cast steel strips having the compositions listed in Table 1 were subjected to hot rolling, cold rolling, and annealing under the conditions shown in Table 2.

In Tables 1 and 2, boxed items indicate that they are outside the scope of this invention. In addition, the steel type ■ in Table 1 is 4
Finish hot rolling was performed directly from a 5 mm thick thin cast strip without lowering the temperature to room temperature.

Table 3 shows the material properties (YS, Fi, 'F value, ■, secondary work brittleness) obtained by combining each steel type and processing conditions.
shows. The secondary processing brittleness test is CCV (Conic
a 1Cup Value) Using a sample that was subjected to conical cup drawing processing (blank diameter 50111, punch diameter 20mm, die diameter 24.44mm) using a testing machine, 5kg-1m
A lightning test was conducted, and the total length of cracks that occurred in the sample was 1
The temperature exceeding 0 m was defined as the embrittlement temperature.

In treatments 1 to 0, the temperature or rolling reduction is outside the scope of the present invention, and the composition of steel types DH is also outside the scope of the present invention.

Table 3 In all cases, it can be seen that the present invention exhibits extremely good deep drawability compared to the comparative materials. In addition, in the combination of steel type A and treatment 11, the temperature increase rate during continuous annealing is as high as 20° C./s, so the deep drawability is slightly inferior among the materials of the present invention.

sb-added steel (A, I) brings about further improvement in workability, and B-added steel 8 (J, K), although slightly inferior in workability,
Shows remarkable secondary work brittleness. sb.

B composite additive steel (B) is an excellent material with well-balanced workability and resistance to secondary work brittleness.

[Brief explanation of the drawing]

Figure 1 is a graph showing the influence of the Lankford value of 7, the Hiiragi bath reduction rate, and the end temperature of hot finish rolling for cold rolled annealed steel sheets. This figure shows the influence of the final bath reduction ratio in hot finish rolling and the reduction ratio in cold rolling. Figure 1: Mokukaisha upper pressure, lower view, end Ivs pressure lower prison (%) Figure 2: O in F I Wataru

Claims (1)

[Claims] 1. C: 0.003wt% or less, Si: 0.03wt% or less, Mn: 0.20wt% or less, P: 0.015wt% or less, S: 0.020wt% or less, Al: 0.005 to 0.1 wt%, N: 0.0025 wt% or less, Ti: [(48/12・%C) + (48/14・%N)
+(48/32・%S)] ~0.1 wt%, Nb: 0.
Using a Ti-Nb composite additive ultra-low carbon steel slab having a composition containing 0.001 to 0.015 wt% as a starting material, a cold-rolled steel plate for ultra-deep drawing is obtained through the steps of hot rolling, cold rolling, and annealing. In this case, the hot finish rolling start temperature is 1100°C or less, the hot finish rolling end temperature is 870 to 910°C, the total rolling reduction in this hot finishing rolling is 85% or more, and the rolling reduction in the final hot finishing pass is 10. % or more and 50% or less, after hot rolling, it is rolled up at 600°C or less, then cold rolled to a predetermined thickness of 1 mm or more, and then annealed. Method of manufacturing rolled steel plate. 2. As starting materials, C: 0.003 wt% or less, Si: 0.03 wt% or less, Mn: 0.20 wt% or less, P: 0.015 wt% or less, S: 0.020 wt% or less, Al: 0. 005 to 0.1wt%, N: 0.0025wt% or less, Ti: [(48/12・%C) + (48/14・%N)
+(48/32・%S)] ~0.1 wt%, Nb: 0.
001~0.015wt% plus Sb: 0.001~
The method according to claim 1, which uses a Ti-Nb composite-added ultra-low carbon steel slab having a composition containing at least one of 0.02 wt% and B: 0.0001 to 0.0010 wt%. . 3. The method according to claim 1 or 2, wherein the annealing is performed at a heating rate of 10° C./s or less.