JPH1192829A - Method for manufacturing soft cold-rolled steel sheet excellent in shape in coil longitudinal direction - Google Patents

Abstract

(57) [Problem] To provide a method for stably producing a soft cold-rolled steel sheet having an excellent shape in a coil longitudinal direction by stabilizing a softening effect by adding B. SOLUTION: In terms of% by weight, C ≦ 0.05% and Mn ≦ 0.
5%, Si ≦ 0.1%, P ≦ 0.025%, S ≦
0.03% and sol. Al ≦ 0.1%, O ≦ 0.0
Amount of alumina-based oxide containing 0.05%, N ≦ 0.004%, and B ≦ 0.005%, and satisfying an atomic ratio of B / N = 0.5 to 2, and 0.1 μm or less. Is 2% by weight
0 ppm or less, and the balance is characterized in that when hot-rolling steel consisting of Fe and unavoidable impurities, the steel is wound at 650 ° C. or less, followed by pickling, cold rolling and continuous annealing.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a soft cold-rolled steel sheet having low coil end properties, excellent pickling properties and excellent shape in the longitudinal direction of a coil, which is particularly suitable for processing such as press forming used for automobiles and home electric appliances. A stable production method of

[0002]

2. Description of the Related Art Thin steel sheets used in automobiles and home electric appliances are required to have high formability, and softness and high r-value are being vigorously promoted. When producing such a highly formable thin steel sheet by continuous annealing using a low carbon aluminum killed steel, it is necessary to fix C and N as coarse precipitates by hot rolling of hot rolling. However, even when high temperature winding is performed, the cooling end is in direct contact with air at the longitudinal end of the coil (T: leading end of the coil, B: tail of the coil) and the end in the width direction. Does not precipitate. This undeposited Al
Since N is finely precipitated during continuous annealing, the ends in the longitudinal and width directions are hardened as compared with the central part of the coil, and so-called coil end properties are generated. Furthermore, there is also a problem that the high-temperature winding increases the thickness of the scale and reduces the pickling property. As a method for solving such problems of the coil end property and the pickling property, Japanese Patent Application Laid-Open No.
A method is disclosed in which the precipitation of fine AlN is suppressed by adding N to form coarse BN to increase the grain growth and reduce the winding temperature.

[0003]

However, as described in the above-mentioned Japanese Patent Application Laid-Open No. 48-100314, the effect of improving the coil end property can be obtained uniformly by adding B, but the variation in the material is reduced. There is a problem of increasing.

[0004] In the above-mentioned prior art, the steel becomes harder as the O content in the steel increases, and the material may fluctuate even with the same O content. FIG. 1 shows an example. C = about 0.02%, Si = about 0.01%, Mn
= About 0.2%, P = about 0.0015%, S = about 0.00
8%, Al = about 0.025%, N = about 0.003%, B
= About 0.0018%, only oxygen amount is 0.001
%, With components different from 0.006%, 0.01%
The two steels were heated at 1220 ° C., then hot-rolled, and wound at 620 ° C. After cooling the coil, pickling, cold rolling, and continuous annealing are performed to form a cold-rolled steel sheet, and then the coil T (head) 20 m
Part, M (center) part, and B (tail) 20 m, and a tensile test was performed. As shown in FIG. 1, the TS (tensile strength) level greatly changes in three steels having only different oxygen contents.

In view of the above situation, an object of the present invention is to stabilize the softening effect by adding B,
An object of the present invention is to provide a method for stably producing a soft cold-rolled steel sheet having an excellent shape in a coil longitudinal direction.

[0006]

In order to solve the above problems and achieve the object, the present invention uses the following means. (1) In the production method of the present invention, C ≦ 0.05% by weight
, Mn ≦ 0.5%, Si ≦ 0.1%, P ≦ 0.0
25%, S ≦ 0.03%, sol. Al ≦ 0.1%
, O ≦ 0.005%, N ≦ 0.004%, B ≦
0.005%, and B / N = 0.5 in atomic ratio.
To 2, wherein the amount of the alumina-based oxide of 0.1 μm or less is 20 ppm or less by weight, and the balance is Fe and unavoidable impurities.
This is a method for producing a soft cold-rolled steel sheet having an excellent shape in the longitudinal direction of a coil, characterized in that the steel sheet is rolled at 650 ° C. or lower when hot-rolled, and subsequently pickled, cold-rolled and continuously annealed.

(2) In the production method of the present invention, C
≦ 0.05%, Mn ≦ 0.5%, Si ≦ 0.1%
, P ≦ 0.025%, S ≦ 0.03%, sol.
Al ≦ 0.1%, O ≦ 0.003%, N ≦ 0.00
4% and B ≦ 0.005%, and the atomic ratio of B
/N=0.5 to 2, and the amount of alumina-based oxide of not more than 0.1 μm is not more than 10 ppm by weight, and the balance is Fe and unavoidable impurities. In the direct-rolling after rolling, 1220 ° C
This is a method for producing a soft cold-rolled steel sheet having an excellent shape in the longitudinal direction of a coil, characterized by starting rolling at the following temperature, winding at 650 ° C. or lower, and subsequently performing pickling, cold rolling, and continuous annealing. .

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS As described above, in the prior art, B
The effect of the addition is to replace fine AlN with coarse BN. However, the present inventors have found that BN precipitates fine MnS on the nucleus and becomes a coarse composite precipitate, thereby suppressing the grain growth of fine MnS. Was found to be highly effective in reducing
Further, although it has been reported that fine MnS can be a nucleus for precipitation of BN, it has been found that in addition to fine MnS, a fine alumina-based oxide having a diameter of 0.1 μm or less can be a precipitation nucleus of BN. Steel having a small softening effect due to the addition of B contains a large amount of this alumina-based oxide, and BN contains fine MnS.
It was found that alumina-based oxides were preferentially deposited on nuclei rather than nuclei, and that many fine MnS remained without being BN precipitate nuclei and hindered grain growth. Based on this finding,
The present inventors have conducted intensive research, and by reducing the amount of alumina-based oxides of 0.1 μm or less, the precipitation of BN with fine MnS as the precipitation nucleus is promoted to make MnS a coarse composite precipitate. It has been found that the effect of adding B is stable.
Further, in direct-feed rolling in which hot rolling is performed as it is after continuous casting, all MnS is rolled in a solid solution state, so that the amount of fine MnS increases. It has also been found that in order to reduce the fine MnS content, it is effective to prevent the strain-induced precipitation at a high temperature where the solid solution MnS content is large. Based on the above findings, the present inventors have limited the oxygen content of the B-added low carbon steel to a certain value or less, reduced fine alumina-based oxides, stabilized the softening effect of the B addition, and furthermore The upper limit of the winding temperature during hot rolling is defined to suppress the precipitation of AlN and enhance the grain growth, thereby maximizing the effect of the addition of B to realize low-temperature winding and pickling. Also improve,
The present inventors have found a method for stably producing a soft cold-rolled steel sheet having an excellent shape in the coil longitudinal direction, and have completed the present invention. That is, the present invention can provide a method for stably producing a soft cold-rolled steel sheet having an excellent shape in the coil longitudinal direction by limiting the steel composition and the production conditions to the following ranges.

The reasons for adding the components, the reasons for limiting the components, and the reasons for limiting the production conditions of the present invention are described below. (1) Component composition range C ≦ 0.05% When C exceeds 0.05%, a large amount of carbide precipitates, lowers the r value and El (elongation), and impairs the formability. % Or less.

Mn ≦ 0.5% Mn fixes S in the form of MnS and has a function of improving hot ductility. Therefore, it is desirable to add 0.05% or more of Mn. The upper limit is 0.5% because it causes hardening and deteriorates the formability.

Si ≦ 0.1% Since excessive addition of Si increases the strength and deteriorates the formability, the content of Si is 0.1% or less.

P ≦ 0.025% P is a solid solution strengthening element, and an excessive addition causes hardening of steel, so the upper limit is 0.025%.

S ≦ 0.03% Since S is an element that inhibits hot ductility and formability, M
Fixed as nS. Therefore, lower is desirable.
Further, if the amount of MnS is too large, the EL is lowered, so the upper limit is 0.03%.

Sol. Al ≦ 0.1% Since Al is used as a deoxidizing agent, Al is included to some extent. In the present invention, since a considerable amount of N is fixed as BN by adding B, the precipitation amount of AlN is so small as not to cause a problem. However, if Al is added excessively, BN will be generated during annealing after cold rolling. Changes to AlN and is hardened by the generation of excess B, so the upper limit is 0.1%.

N ≦ 0.004% In the present invention, N is fixed as BN. However, if the amount of BN is large, the workability is reduced.
%.

B ≦ 0.005%, B / N (atomic ratio) =
0.5-2B is an element which plays an important role in the present invention. B
In the case of the present invention in which the amount of alumina-based oxide is limited, fine MnS precipitates as BN in the nucleus, converts MnS into coarse composite precipitates, and fixes N to suppress precipitation of fine AlN. As a result, in the present invention, stable grain growth that has not been achieved in the past can be realized even at low temperature winding. However, if B is present in excess, solid solution B is generated and hardened, so the upper limit of the amount added is 0.005%. However, even if a large amount is added to N, the amount of solid solution B increases and the steel becomes hard, so that the ratio of atomic% of B to N is 0.5 to 2. In order to obtain a particularly stable material, 0.8 to 1.5 is preferable.

O: 0.005% or less, or 0.003%
% Or less (at the time of direct rolling) O in the steel is fixed as Al ₂ O ₃ by Al,
If it exceeds 0.005%, the amount of alumina-based oxide increases, and coarse Al ₂ O ₃ is generated, which causes deterioration in surface properties and material quality. Therefore, the upper limit is 0.005%. Since the amount of fine MnS increases during direct rolling, the amount of alumina-based oxide needs to be further reduced. Therefore, the upper limit of O during direct rolling is 0.003%.

Amount of alumina-based oxide of 0.1 μm or less: 2
0 ppm or less or 10 ppm or less (at the time of direct rolling) The amount of alumina-based oxide is important in the present invention. 0.1 μm
When a large amount of the following alumina-based oxides is generated, BN precipitates with the alumina-based oxide having a diameter of 0.1 μm or less as a nucleus, so that the formation of coarse composite precipitates of fine MnS is not achieved. From this, the upper limit of the amount of oxides of 0.1 μm or less is 20 ppm or less. Further, in direct rolling, MnS hardly becomes coarse and the amount of fine MnS increases. Therefore, in the case of direct rolling, the upper limit is 10 ppm. Experimental results showing this fact are shown below. C = about 0.02%, Si = 0.
01%, P = about 0.015%, S = about 0.01%, Al
= About 0.02%, N = about 0.002%, B = about 0.00
1% of steel containing 15% and varying the amount of alumina-based oxide
It was heated to 250 ° C and rolling started at 1200 ° C. After rolling, hot rolling was performed at 600 ° C. The obtained hot rolled sheet was pickled, cold rolled, and annealed at 750 ° C. The annealed plate was adjusted at a pressure adjustment rate of 1.0%, and a JIS No. 5 test piece was cut out and subjected to a tensile test. Fig. 2 shows the obtained TS (tensile strength).
Shown in The amount of alumina-based oxide of 0.1 μm or less is 20p
It can be seen that the softening effect due to the addition of B significantly appears below pm.

The effect of the present invention is not lost even if Cu, Ni, Cr, Sn, Mo, Pb or the like is added to the steel to be used in the present invention for various purposes. Where Ti,
When elements that form fine nitrides, such as V, Nb, and Zr, are added, these fine precipitates hinder grain growth and further generate solid solution B to cause deterioration of the material. Alternatively, the content is desirably 0.01% or less.

By adjusting to the above component range, a soft cold-rolled steel sheet having an excellent shape in the longitudinal direction of the coil can be stably obtained. A steel sheet having such characteristics can be manufactured by the following manufacturing method.

(2) Steel plate manufacturing process (2-1) Manufacturing conditions of mode 1 (Manufacturing method) After the steel adjusted to the above-mentioned component range is melted in a converter, it is continuously cast into a steel slab. When the hot rolled steel is hot rolled, it is wound at 650 ° C. or lower, and then pickled, cold rolled and continuously annealed.

A. Winding temperature: 650 ° C. or less The winding temperature is important in the present invention. If the winding temperature is high, AlN precipitates together with BN, so that B remains in excess as solid solution B, causing hardening of the steel and deteriorating the pickling properties. Therefore, the upper limit of the winding temperature is 650 ° C.
However, if the temperature is lower than 300 ° C., the grains will not grow sufficiently after winding, and will be finely precipitated in the grains and harden.
It is desirably at least 00 ° C.

In the production conditions of the first embodiment, the rolling start temperature is not particularly limited, but the rolling start temperature is set to 1300 ° C. in order to suppress fine precipitation of MnS due to solid solution again.
The following is preferred.

(2-2) Manufacturing Conditions of Aspect 2 (Manufacturing Method) In the direct-feed rolling in which the steel adjusted to the above-mentioned component range is melted in a converter, then continuously cast and then hot-rolled without cooling. Rolling is started at a temperature of 1220 ° C. or less, wound at a temperature of 650 ° C. or less, and subsequently pickled, cold-rolled,
Continuous annealing.

A. Rolling start temperature: 1220 ° C. or less When performing direct-feed rolling, the rolling start temperature plays an important role. In the present invention, fine MnS is wrapped with BN and coarsened to increase grain growth, thereby realizing low-temperature winding.

Therefore, by limiting the rolling start temperature, the amount of fine MnS induced by strain is limited. If the rolling start temperature exceeds 1220 ° C., the precipitation of fine MnS due to strain induction is remarkable, and the effect of adding B cannot be obtained, so the rolling start temperature is 1220 ° C. or lower.

B. Winding temperature: 650 ° C. or lower Same as the manufacturing conditions of Embodiment 1. As described above, in the present invention, the temperature of each step has an important meaning, and if any one of these temperatures is applied, the effect of the present invention cannot be obtained.

There are no particular restrictions on the heating conditions.
There is no problem even at 20 ° C. or less. Before rolling, heating for the purpose of equalizing the temperature of the slab surface and the inside thereof may be performed. Further, in order to remove skid marks and secure the finishing temperature, after rough rolling, the rough bar may be heated or wound around a coil box. Further, if the conditions of the present invention are satisfied, the effect of the present invention is not changed at all even if thin slab casting is used.

In the cold rolling after pickling, the rolling ratio is preferably 30 to 90% from the viewpoint of workability, particularly deep drawing. Annealing is also performed at 600 ° C. or more for softening and 900 ° C. or less for suppressing coarse grains. The annealing method is continuous annealing. Further, the surface treatment such as hot-dip plating, electroplating, chemical conversion treatment, and organic coating does not affect the effect of the present invention at all. There are no restrictions on the conditions of the temper rolling, but if it is too high, the El (elongation) decreases sharply. Further, both the converter and the electric furnace can be used for adjusting the composition of the steel of the present invention.

It should be noted that the steel sheet according to the present invention is not only stable in the longitudinal direction, but also stable in the width direction. Further, according to the present invention, shape defects due to variations in the material in the width direction, for example, middle elongation due to hardening of both edges in the width direction are also reduced. Hereinafter, examples of the present invention will be described to demonstrate the effects of the present invention.

[0031]

【Example】

(Example 1) Steel having the components shown in Table 1 (steel of the present invention: No. 1)
~ 11, comparative steel: No. 12 to 16) were continuously cast, once cooled to room temperature, inserted into a heating furnace, and hot-rolled at a rolling start temperature and a winding temperature shown in Table 1. Subsequently, the hot rolled sheet is pickled, cold rolled, and continuously annealed at 700 ° C.
A cold-rolled steel sheet was manufactured by adjusting the pressure at a pressure adjustment rate of 1%. The obtained cold-rolled steel sheet was processed into a JIS No. 5 tensile test piece, a tensile test was performed, and TS (tensile strength) at the center and the end in the longitudinal direction of the coil was measured. The steel No. of the present invention. Nos. 1 to 11 are soft, and the coil end property is such that the difference in TS between the center and the end is 30N.
/ Mm ² or less.

On the other hand, the comparative steel No. No. 12 has a low B / N ratio and a large coil end property. Comparative steel No. No. 13 has a high rolling start temperature and the softening effect by the addition of B is not sufficient. Comparative steel No. Comparative Steel No. 15 has a high oxygen content.
Sample No. 16 is hard because the amount of alumina-based oxide of 0.1 μm or less is high. In addition, the comparative steel No. No. 14 has a high winding temperature, is hardened, and has a large coil end property.

[0033]

[Table 1]

(Example 2) Steels having the components shown in Table 2 (inventive steels: Nos. 1 to 11, comparative steels: Nos. 12 to 15) were continuously cast and rolled without cooling as shown in Table 2 Direct feed hot rolling was performed at the starting temperature and the winding temperature. Subsequently, the hot rolled sheet is pickled, cold rolled, and continuously annealed at 750 ° C.
A cold-rolled steel sheet was manufactured by adjusting the pressure at a pressure adjustment rate of 0.8%. The obtained cold-rolled steel sheet was processed into a JIS No. 5 tensile test piece to perform a tensile test, and the TS at the center in the width direction at the center in the longitudinal direction of the coil and the position (edge) at the edge of 25 mm (end) were measured. The steel No. of the present invention. Samples Nos. 1 to 11 were soft, and the coil end properties were good, with a difference in TS between the center part and the end part being 30 N / mm ² or less.

On the other hand, the comparative steel No. 12 has high B and is hardened. Comparative steel No. No. 13 has a high rolling start temperature,
The softening effect by adding B is not sufficient. Comparative steel No.
15 is hard because of high oxygen. In addition, comparative steel N
o. No. 14 has a high winding temperature, is hardened, and has a large coil end property.

[0036]

[Table 2]

[0037]

According to the present invention, by specifying the steel composition and the manufacturing conditions, it is possible to stabilize the effect of adding B and reduce the winding temperature while maintaining the material. A soft cold-rolled steel sheet having good coil end properties and excellent shape in the coil longitudinal direction can be manufactured.

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship between oxygen content in steel and tensile strength (TS) depending on a coil position according to an embodiment of the present invention.

FIG. 2 is a view showing the relationship between the amount of alumina-based oxide in steel and tensile strength (TS) according to the embodiment of the present invention.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Hiroshi Sawada 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (72) Inventor Jun Taniai 1-2-1, Marunouchi, Chiyoda-ku, Tokyo, Japan Honko Co., Ltd.

Claims (2)

[Claims] 1. The method according to claim 1, wherein C ≦ 0.05% and Mn ≦
0.5%, Si ≦ 0.1%, P ≦ 0.025%,
S ≦ 0.03%, sol. Al ≦ 0.1%, O ≦
0.005%, N ≦ 0.004%, and B ≦ 0.005
% And B / N = 0.5 to 2 in atomic ratio, the amount of alumina-based oxide of 0.1 μm or less is 20 ppm or less in weight%, and the balance is Fe and unavoidable impurities. A method for producing a steel sheet comprising: a soft cold roll having excellent shape in the longitudinal direction of a coil, characterized in that when hot rolling the steel, the steel is wound at 650 ° C. or lower, and then pickled, cold rolled, and continuously annealed. Manufacturing method of rolled steel sheet. 2. In% by weight, C ≦ 0.05% and Mn ≦
0.5%, Si ≦ 0.1%, P ≦ 0.025%,
S ≦ 0.03%, sol. Al ≦ 0.1%, O ≦
0.003%, N ≦ 0.004%, B ≦ 0.005
%, And the atomic ratio of B / N = 0.5 to 2 is satisfied, the amount of alumina-based oxide of 0.1 μm or less is 10 ppm or less by weight, and the balance is Fe and unavoidable impurities. In a method for producing a steel sheet, in a direct-feed rolling after casting of steel, rolling is started at a temperature of 1220 ° C. or less, wound at 650 ° C. or less, and subsequently pickled
A method for producing a soft cold-rolled steel sheet having an excellent shape in the longitudinal direction of a coil, characterized by cold rolling and continuous annealing.