JPS6376849A - Cold rolled steel sheet for extra deep drawing and its manufacture - Google Patents

Abstract

PURPOSE:To obtain the titled cold rolled steel sheet having a high gamma-value, high elongation, low yield strength and superior press workability by providing a compsn. contg. prescribed amounts of Si, Mn, P and Al besides C, S, N and Ti having specified relation among their contents and a very small amount of Sb. CONSTITUTION:This cold rolled steel sheet for extra deep drawing contains, by weight, <=0.0030% C, <=0.05% Si, <=0.5% Mn, <=0.02% P, <=0.02% S, <=0.1% Al, <=0.003% N, <=0.1% Ti and 0.002-0.02% Sb and has relation represented by the formula among the contents of C, S, N and Ti. The yield strength of the steel sheet is <=10kgf/mm<2>. When recrystallization annealing is carried out during the manufacture of the cold rolled steel sheet, the surface roughness of a steel sheet is regulated preferably at the final stand of a cold rolling mill and the steel sheet is box annealed at such a high temp. as 700-850 deg.C. Thus, the desired cold rolled steel sheet for extra deep drawing is obtd.

Description

[Detailed Description of the Invention] (Industrial Application Field) Used for press processing of particularly difficult-to-form parts among automobile parts such as automobile oil pans and various covers for home appliances. The following describes the results of research and development on cold-rolled steel sheets for ultra-deep drawing and their manufacturing methods.

The materials used for press working as described above have excellent deep drawability, that is, low yield strength as a material property, and have r-value,
A high El<elongation) is required.

(Conventional technology) Excellent r value and E by adding Ti to ultra-low carbon steel
It has been disclosed in, for example, Japanese Patent Publication No. 18066/1983 that 1 can be obtained, and it has come to be used in large quantities for press forming of parts that are actually quite difficult to press.
As parts, which were conventionally two parts joined by welding or the like, are increasingly integrated into one piece for the purpose of simplifying the press working process, the demand for materials with even better workability is increasing.

(Problems to be Solved by the Invention) In view of the above-mentioned circumstances, the present invention aims to further improve press workability by providing high R value, high E! ! Increase the yield strength according to
The purpose of the present invention is to propose a cold-rolled steel sheet for ultra-deep drawing that has a low S) and has excellent actual press workability, and a method for producing the same.

Here, YS is the stress at which plastic deformation of the steel plate begins, and the lower the stress, the easier the press working and the easier the inflow of material during deformation, which has an effect equal to or greater than that of a high r value.

Therefore, a steel sheet that has a high r value, a high E1, and a low YS provides better press formability than before.

(Means for solving problems) The above objectives are advantageously achieved by the following.

C: 0.0030-1% or less, Si: 0.05w
Less than t%? In: 0.5 wt% or less, P
: 0.02wt% or less, S: 0.02wt% or less, Al: 0.1wt% or less, N: 0.0
03 wt% or less, Ti: 0.1 wt% or less and Sb: 0.002 to 0.02 wt%, C, S,
For ultra-deep drawing, the content of N and Ti is contained under the relationship of the following formula, the balance is iron and unavoidable impurities, and the yield strength is 10 kgf/mm2 or less Cold rolled steel plate.

(Above, first invention) C: 0.0030 匈L% or less, Si: 0.05w
t% or less, Mn: 0.5 wt% or less, P:
0.02wt% or less, S: 0.02wt% or less,
I: 0.1wt% or less, N: O, QO3-1
% or less, furthermore, 0.1iyt% or less and Sb: 0.002 to 0.02wt%. During recrystallization annealing of a cold rolled steel sheet, the roughness of the steel sheet surface is adjusted in advance, A method for producing a cold-rolled steel sheet for ultra-deep drawing (second invention), which is then subjected to high-temperature box annealing at a temperature in the range of 700° C. or higher and 850° C. or lower.

The inventors conducted a detailed study on Ti-added ultra-low carbon steel in the laboratory and found that by using ultra-low carbon steel with C50,003% and providing ideal annealing conditions, YS≦10 kgf/mm.
We obtained the knowledge that 2 can be obtained.

However, in the case of actual factory production, a YS of only about 14 kgf/mm'' can be achieved at most, and we have further investigated the various factors in the actual manufacturing process that cause the YS value to increase.

According to the results, first, it was found that in continuous annealing, YS increases because the repeated bending and unbending by rolls during sheet passing causes plastic deformation to the steel sheet. It has become clear that the lower the YS of the material, the greater the amount of plastic deformation due to roll bending, and therefore it is essentially impossible to prevent the upper limit of YS in a continuous annealing furnace.

On the other hand, in box annealing, it was found that nitrification occurred in the surface layer of the steel sheet during annealing, hardening the surface layer, and increasing YS. usually,
In order to obtain a high r value and high El, it is necessary to add a larger amount of Ti than is currently available in order to determine the amount of precipitated impurities such as ClN and S. Therefore, excessive solid solution Ti particularly promotes the negative effects of nitriding. This is thought to be for the purpose of

The inventors investigated various ways to effectively prevent nitriding and suppress the YS upper limit even in the presence of such excessive Ti. As a result, the mt! addition is effective, r(direct, E
It has been discovered that a steel plate with ys≦10 kgf/mm" can be obtained by an actual factory manufacturing process without using 1.

Note that the addition of sb to Ti-added steel has already been disclosed in JP-A-59-13654, but the addition of sb or Bi here is intended to improve secondary processing brittleness. Therefore, YS and El, which are essential for the ultra-deep drawability desired in this invention, are insufficient, and it is also preferable to add a larger amount of sb, but this causes linear surface defects in the steel sheet after annealing. It has also been found that there are disadvantages associated with this.

(Function) The reason for limiting the components in the ultra-deep drawing cold rolled steel sheet of the present invention will be explained.

C, N: Since both C and N impair anti-aging properties, they must be fixed as precipitates. Since the C and N content is low, the material becomes soft, so in a steel plate with a significantly low YS as in this invention,
Both need to be 0.003 wt% or less.

Si: Since Si generates scale that is difficult to remove by pickling during hot rolling, it is necessary to reduce it as much as possible, so the upper limit is set to 0.05 wt%.

Mn: Mn can secure the necessary strength without impairing deep drawability, but if it exceeds 0.5 wt%, [E/
The content is set to 0.5 wt% or less in order to reduce the

P, S: P and S remain in steel as inevitable impurities. If P and S are too large, El will be significantly impaired, so the upper limit for both is set at 0.02 wt%.

Al: Al is added to remove oxygen from the steel, but addition of a large amount not only increases cost but also causes deterioration of surface quality, so the content is limited to O.ht% or less.

Ti: Ti fixes impurities such as C, N, and S as precipitates, and is an essential component for improving the r value and El. Therefore, it is necessary to at least exceed the equivalent amount of C, N, and S. In particular, since C, N, and S are reduced in this invention, if they are not added in excess of the equivalent amount, they will become fine precipitates. In order to damage El, the lower limit of Ti ivt% is set. On the other hand, since Ti not only promotes nitriding but also significantly deteriorates the surface properties, the upper limit is set to 0.1 wt%.

sb: sb effectively prevents N attack even when added in a very small amount of 0.002 wt%, and is a feature of this invention that YS≦
Enables 10kgf/mm2. This is slab heating,
It is thought that this is because sb concentrates in the surface layer of the steel sheet during the hot rolling process, but on the other hand, if it exceeds 0.02 wt%, fine intergranular cracks occur during hot rolling, resulting in linear surface defects.

In the ultra-low carbon steel containing the limited amount of Tt in this invention, s
By adding a small amount of b, the increase in YS can be effectively suppressed, and its effective range is 0.002-t%≦sb≦0.
02 匈t%'.

In addition, in order to improve the anisotropy of the material, in this invention, Nb,
It is also possible to add B, Zr, V, Be, etc., but all of them cause an increase in the recrystallization temperature, so
The amount added is at most 0.01 wt%.

Next, regarding the manufacturing process, in order to achieve a low YS, it is necessary to cause the crystal grains of the steel sheet to grow sufficiently, so the annealing temperature during box annealing is set to 700°C or more and 850°C or less.

By the way, conventionally, after annealing, it has been common to perform temper rolling of 0.5 to 1.5% in order to eliminate the yield elongation of the steel sheet. This is to adjust the surface roughness of the steel plate and to prevent the occurrence of strain during processing due to residual solid solution elements.

However, in the steel used in this invention, all solid solution elements (C, N) are fixed as precipitates due to appropriate addition of Ti, so stretcher strain, that is, yield elongation, occurs even without skin pass rolling after annealing. can't happen. Since temper rolling is also essentially plastic working and involves an increase in YS and a decrease in El, the roughness adjustment of the steel sheet, which is omitted in this invention, is performed before annealing.

Roughness adjustment can be performed on a separate line before recrystallization annealing, but from the standpoint of production efficiency, it is preferable to adjust the surface roughness of the steel plate at the final stand of the cold rolling mill.

The surface roughness at this time is set to be 0.5 μm or more in terms of Ra, that is, the center average roughness. It should be noted that if Ra is less than 0.5 μm, sufficient lubrication effect cannot be obtained during pressing and mold galling occurs, which is not preferable.

(Example) Steel having the composition shown in Table 1 was melted in a converter, then continuously cast, hot rolled at a finishing temperature of 890°C, and then cold rolled into a cold rolled plate with a thickness of 1.2 ms+. And so. At this time, a dull roll was used in the final stand of the cold rolling mill, and the surface roughness of the steel plate was adjusted to an Ra value of 1.5 μm.

Cold-rolled coils are heated to 690°C or 740°C in a nitrogen gas atmosphere.
After box annealing at ℃, temper rolling was omitted and the mechanical properties were investigated. Here, the test pieces were JIS No. S test pieces, and samples were taken from the O'', 45'', and 90'' directions in the rolling direction, respectively.

The test values shown below were averaged. (X (average value) = (Xo +Xqo +2X4s)/4)
From the results in Table 2, in all of the examples of the present invention, steel plates for ultra-deep drawing were obtained which had excellent r value and El, and had YS≦lOkgf/mm”.

Table 2 Mechanical Properties and Annealing Temperature (Effects of the Invention) The cold-rolled steel sheet for ultra-deep drawing of the first invention was difficult to press and required some parts to be combined by welding or the like after individual press working. The present invention is suitable for press forming as an integral part of difficult-to-process parts, and the second invention allows the above-mentioned cold-rolled steel sheet to be manufactured into a cylindrical shape.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the influence of the amount of sb added on YS.

Claims (1)

[Claims] 1. C: 0.0030wt% or less, Si: 0.05wt
% or less, Mn: 0.5wt% or less, P: 0.02wt%
Hereinafter, S: 0.02 wt% or less, Al: 0.1 wt% or less, N: 0.003 wt% or less, Ti: 0.1 wt% or less, and Sb: 0.002 to 0.02 wt%, C, S, Ultra-deep steel is characterized in that the N and Ti contents are contained under the relationship of the following formula, the balance is iron and unavoidable impurities, and the yield strength is 10 kgf/mm^2 or less. Cold rolled steel plate for drawing. Note 48/12・%C+48/32・%S+48/14・%
N+0.01≦Ti (wt%)2, C: 0.0030w
t% or less, Si: 0.05wt% or less, Mn: 0.5w
t% or less, P: 0.02wt% or less, S: 0.02wt
% or less, Al: 0.1wt% or less, N: 0.003wt
% or less, and further Ti: (48/12・%C+48/32・%S+48/
14・%N+0.01) During recrystallization annealing of a cold rolled steel sheet having a composition containing 0.1 wt% or more and Sb: 0.002 to 0.02 wt%, the roughness of the steel sheet surface is adjusted in advance. 700℃ or higher8
A method for producing a cold-rolled steel sheet for ultra-deep drawing, characterized by performing high-temperature box annealing at a temperature in the range of 50°C or less.