JPH0827523A - Production of cold-rolled steel sheet for deep drawing, free from coarse grain and reduced in oxide film - Google Patents

Abstract

PURPOSE:To produce a cold-rolled steel sheet for deep drawing, free from coarse grains and reduced in oxide film, by subjecting a steel having a specific composition to hot rolling and to cold rolling and then applying box annealing to the resultant sheet while specifying annealing conditions. CONSTITUTION:A steel, which has a composition consisting of, by weight, <=0.0050% C, <=0.3% Mn, <=0.02% P, <=0.02% S, <=0.004% N, 0.01-0.08% T, and the balance essentially Fe and satisfying Ti>=4C+3.43N+1.5S, is hot-rolled and cold-rolled by the conventional method. Subsequently, at the time of box- annealing the resulting cold-rolled steel sheet, box annealing is done while specifying soaking temp., temp.-rise time from the temp. at insertion into the furnace up to 300 deg.C in the course of temp. rise up to the soaking temp., and the dew point of an atmospheric gas to 670-780 deg.C, >=8hr, and -40 to 0 deg.C, respectively. By this method, the cold-rolled steel sheet, excellent in mechanical properties and surface condition, can be obtained.

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 for deep drawing, which does not have coarse grains and has a small oxide film, which requires a particularly beautiful surface in a steel sheet used for automobiles and home appliances. It is a thing.

[0002]

2. Description of the Related Art In recent years, the fields of application of cold rolled steel sheets have diversified,
Also, the required characteristics are becoming more severe. Instead of conventional aluminum-killed steel, the amount of C in steel is now tens of pp
The so-called IF steel, which has been reduced to m and added with carbonitride forming elements Ti and Nb, has been produced and used in large quantities in combination with its excellent elongation and deep drawability. In order to improve the mechanical properties of IF steel, not only the elements C contained in the steel but also N, S, P, etc. are reduced,
It is a highly refined steel. Therefore, during annealing after cold rolling, recrystallized grain growth is good, a texture is developed, and mechanical properties are improved.

[0003]

However, the following problems occur when annealing such a steel sheet. That is, since IF steel has a higher recrystallization temperature than conventional aluminum-killed steel, it needs to be annealed at a high temperature. As the degree of purification is further improved and the recrystallized grain growth property is improved, a part of the crystal grains may be rapidly grown to generate abnormally coarse grains. In particular, when a steel sheet is annealed by the box annealing method, a temperature difference is likely to occur at each part of the coil and the heating rate also differs depending on the part, as compared to the continuous annealing method that can uniformly heat the width and the entire length of the coil. Uniform heat treatment is difficult. Abnormally coarse particles having a huge particle size of 3 to 10 times the average particle size are generated from the part locally heated to a high temperature and the part heated to a high temperature for a long time. The presence of such crystal grains causes a failure such as necking because the steel sheet cannot be uniformly deformed when it is deformed by pressing or the like.In addition, the locally heated portion is oxidized on the surface. A film is formed and the surface becomes defective. The cause is considered as follows. That is, after inserting the coil for box annealing into the furnace, purging is performed using an inert gas to make the atmosphere non-oxidizing. Then, while raising the temperature, a predetermined atmosphere gas is fed to perform annealing. When the atmosphere in the furnace is exhausted by using an inert gas, most of the air is released to the outside of the furnace, but the atmosphere stays in a place where the gas flow is bad and slightly remains.
Due to the presence of the gas having the components in the atmosphere, an oxide film is formed on the surface at the portion locally heated to a high temperature during annealing. When such an oxide film is formed on the surface of the steel sheet, the aesthetic appearance of the product is impaired, so that even if the mechanical properties are excellent, its value as a product is almost lost.

When the ultra low carbon IF steel is manufactured by the box annealing method as described above, there is a problem that defective press molding occurs because coarse grains are generated and an oxide film is generated.

The present invention has been made in order to solve such conventional problems, and when an ultra-low carbon IF steel is manufactured by a box annealing method, good characteristics can be obtained by defining the annealing conditions. It is an object of the present invention to provide a method for producing a cold-rolled steel sheet for deep drawing, which has no coarse grains and has a small oxide film.

[0006]

Means for Solving the Problems The gist of the present invention is that, when an ultra-low carbon IF steel is manufactured by a box annealing method, the mechanical properties and surface condition of the steel can be improved by defining the steel composition and annealing conditions. A method for manufacturing a cold rolled steel sheet is provided. That is, C ≦ 0.0050% by weight, Mn
≦ 0.3% by weight, P ≦ 0.02% by weight, S ≦ 0.02% by weight, N ≦ 0.004% by weight, Ti: 0.01 to 0.0
Steel containing 8% by weight of Ti ≧ 4C + 3.43N + 1.5S, and the balance being substantially Fe, hot-rolled and cold-rolled by a conventional method, and then box-annealed. In obtaining a rolled steel sheet, (1) the soaking temperature of box annealing is set to 670 to 780 ° C.

(2) In the temperature rising process to the soaking temperature, the temperature rising time from the furnace insertion temperature to 300 ° C. is set to 8 hours or more.

(3) The dew point of the atmospheric gas is -40 to 0 ° C.

The condition (1) is satisfied.

[0010]

The reasons for limiting the components in this invention are as follows.

C: C is 0.005% by weight or less. If it is added in excess of this range, the elongation and the deep drawability deteriorate. Further, the amount of Ti added to precipitate and fix as carbides also increases, which causes an increase in cost. Mn: Mn is 0.3% by weight or less. Since Mn is a solid solution strengthening element, if it is added in a larger amount, ductility decreases.

P: P is 0.02% by weight or less. Since P is also a solid solution strengthening element, if it is added in a larger amount, ductility decreases.

S: S is 0.02% by weight or less. S is an impurity, and S is precipitated and fixed as TiS in the Ti-added IF steel. Therefore, if a large amount is present in the steel, the amount of Ti required for precipitation and fixation increases, and if the amount of precipitates also increases, the ductility deteriorates.

N: N is 0.004% by weight or less. N
Since TiN precipitates and fixes as TiN, a large amount of TiN increases the amount of Ti necessary for precipitation fixation. Further, if the amount of precipitates increases, the ductility deteriorates.

Ti: 0.01 to 0.08% by weight of Ti
Then, the range is such that Ti ≧ 4C + 3.43N + 1.5S is satisfied.

Ti is C, N and S in the steel, respectively.
It is an element necessary for fixing as a precipitate of C, TiN, TiS. Therefore, the atomic equivalent ratio of C, N, S in the steel is added in excess. Further, the effect is exhibited at an addition amount of 0.01% by weight or more. The upper limit is 0.08% by weight.
If you add more than this, the effect will be saturated,
This leads to unnecessary cost increase.

Steels having the above chemical composition are usually used in converters,
It is melted in an electric furnace. Such steel is made into a slab by continuous casting or slabbing, which is hot-rolled by a usual method, cold-rolled, and then box-annealed.

The box annealing conditions must be specified as follows.

(1) The soaking temperature of box annealing is set to 670 to 780.
℃.

(2) In the temperature rising process to the soaking temperature, the temperature rising time from the furnace insertion temperature to 300 ° C. is set to 8 hours or more.

(3) The dew point of the atmospheric gas is -40 to 0 ° C.

Here, the reason for defining the annealing conditions will be described.

FIG. 1 shows the relationship between the soaking temperature, the mechanical characteristics, and the presence or absence of the occurrence of abnormally coarse particles. C: 0.0
039 wt%, Mn: 0.15 wt%, P: 0.012
%, Ti: 0.051% by weight, S: 0.012% by weight, N: 0.0031% by weight. In the figure, curves 1, 2 and 3 show the characteristic line of elongation with respect to the soaking temperature, the characteristic line of r-value and the characteristic line of grain size, respectively, and by setting the soaking temperature to 670 to 780 ° C, It can be seen that it is possible to manufacture a steel sheet that has excellent physical properties and that does not generate abnormally coarse grains.

Next, in order to clarify the cause of the generation of the oxide film, the coil soaking temperature, the generation state of the oxide film and the composition of the oxide film were investigated. As a result, the following was found. It is generated in the place where the coil edge is locally heated.

The composition is an oxide of Mn and Si,
It is thickened on the surface.

It is considered that the cause of the formation of the oxide film is that the atmospheric gas in the heating furnace contains the gas of the atmospheric component. When an oxide film is generated by the components in the air (water vapor, oxygen), it is considered that the oxide film is generated in the low temperature part in the early stage of heating. Therefore, in order to investigate the process of oxide film formation in detail, we investigated the occurrence state by changing the temperature rise pattern in the low temperature part at the beginning of heating.

Table 1 shows the results of an investigation on the influence of the temperature rising time in the low temperature part in the initial stage of heating.

[0028]

[Table 1]

In Table 1, to determine the degree of formation of an oxide film, after the annealing was completed, samples were taken from the coil for the entire length in the width direction and 5001 in the length direction. It was measured and evaluated. The ∘ mark indicates that the area where the oxide film is generated is 0 to 1.5%, the Δ mark indicates that the area where the oxide film is generated is 1.5 to 10%, and the X mark indicates that the area where the oxide film is generated is 10% or more. From this result, it can be seen that the longer the heating time is, the less the oxide film is generated especially in the heating time of 8 hours or more in the low temperature part at the initial stage of heating up to 300 ° C.

Next, Table 2 shows the results of an investigation on the influence of the dew point of the atmospheric gas in the heating furnace.

[0031]

[Table 2]

Table 2 shows the results of investigating the effects of changing the temperature rise time in the low temperature part and the dew point of the atmospheric gas. The meanings of x, Δ, and ◯ in Table 2 are the same as in Table 1. From this result, it can be seen that when the temperature rising time in the low temperature part is long and the dew point of the atmospheric gas is 0 ° C. or less, the oxide film is less generated.

Although the reason for this is not clear, the formation of an oxide film is caused by moisture and oxygen in the atmosphere remaining in the furnace, and is formed in the portion that is locally heated in the initial stage of heating. it is conceivable that. Therefore, it is considered that the generation of the oxide film was suppressed by lengthening the temperature rising time in the low temperature part in the initial stage of heating to make the heating of the steel sheet uniform and lower the dew point of the atmosphere gas in the furnace.

[0034]

EXAMPLE Steels having the compositions shown in Table 3 below were melted in a converter and subjected to ordinary hot rolling and cold rolling to obtain a plate thickness of 0.7 m.
m cold rolled sheet, annealing was performed by changing the annealing conditions.
The mechanical properties, the grain size and the oxide film on the surface were investigated. The results are shown in Table 4.

[0035]

[Table 3]

[0036]

[Table 4]

From the results shown in Table 4, in each of the examples of the present invention, r
A steel sheet for deep drawing having excellent value and elongation, no coarse grains generated, and a small oxide film was obtained. In addition, × in Table 4
The meanings of the mark, Δ mark, and ◯ mark are the same as in Table 1.

[0038]

As described above, the present invention regulates the annealing conditions when the ultra-low carbon IF steel is manufactured by the box annealing method. As a result, the oxide film free from coarse particles is formed. It is possible to produce a cold-rolled steel sheet for deep drawing having good properties with less heat.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between soaking temperature, mechanical properties, and grain size.

[Explanation of symbols]

 1 Characteristic curve of elongation to soaking temperature 2 Characteristic curve of r value to soaking temperature 3 Characteristic curve of grain size to soaking temperature

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Mitsuru Kitamura No. 1 Kanazawa-machi, Kakogawa-shi, Hyogo Prefecture Kadogawa Works Kakogawa Works

Claims (1)

[Claims] 1. C ≦ 0.0050% by weight, Mn ≦ 0.3
% By weight, P ≦ 0.02% by weight, S ≦ 0.02% by weight, N
≦ 0.004% by weight, Ti: 0.01 to 0.08% by weight
Then, using steel having a composition of Ti ≧ 4C + 3.43N + 1.5S and the balance being substantially Fe, hot rolling and cold rolling are performed by a conventional method, and then box annealing is performed to obtain a cold rolled steel sheet. In obtaining, (1) the soaking temperature of box annealing is 670-780 degreeC. (2) In the process of raising the temperature to the soaking temperature, 30
The temperature rise time to reach 0 ° C. is set to 8 hours or longer. (3) The dew point of the atmospheric gas is -40 to 0 ° C. The method for producing a cold-rolled steel sheet for deep drawing, which does not have coarse grains and has a small oxide film, characterized by satisfying the above condition.