JP3212188B2 - Method of manufacturing cold rolled stainless steel strip - Google Patents

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 stainless steel strip having excellent surface gloss.

[0002]

2. Description of the Related Art Conventionally, a cold rolled stainless steel strip is annealed, pickled, and then cold rolled in a Sendzimir mill or the like having a work roll diameter of 150 mm or less, and then subjected to finish annealing pickling or finish brightening. It was manufactured by annealing and then finish pass rolling. Stainless steel cold-rolled steel strip manufactured through these steps is often subjected to buff polishing after finish temper rolling in the case of austenitic steel represented by SUS304,
It is important to exhibit excellent gloss after this buffing.

[0003] In order to greatly reduce the rolling time and efficiently produce a cold rolled stainless steel strip, a method of cold rolling with a large diameter work roll having a diameter of 150 mm or more has been adopted.
When cold rolling is performed by this method, the surface gloss of the product is significantly reduced as compared with the case where a small-diameter work roll is used, and the product cannot be used at all for applications requiring gloss.

[0004]

SUMMARY OF THE INVENTION The present invention relates to cold rolling with a large diameter work roll such as a cold tandem mill or cold rolling with a small diameter work roll after cold rolling with a large diameter work roll. In this case, an object of the present invention is to solve the conventional problem of surface gloss reduction.

[0005]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned problems and provides a method for producing a cold rolled stainless steel strip having excellent surface gloss. That is, in the present invention, the stainless steel strip after the end of hot rolling is annealed, and the steel strip temperature is continuously cooled to 25 ° C / sec or more from the temperature range of 850 to 400 ° C immediately after or after annealing. After quenching at a high speed and further mechanically grinding the steel strip surface by 2 μm or more, pickling is performed to produce a cold-rolled material, and this material is averaged at least on the work roll surface roughness of at least the first stand of a cold tandem mill. Roughness Ra
A method for producing a cold-rolled stainless steel strip, comprising: performing cold rolling to at least 0.5 μm and at least the work roll surface roughness of the final stand to an average roughness Ra of 0.15 μm or less. It can be used advantageously for steel strip.

[0006]

Hereinafter, the present invention will be described in detail. SUS
When austenitic stainless steel sheet represented by 304 is cold-rolled with a large-diameter work roll such as a cold tandem mill, or when cold-rolled with a small-diameter work roll after being cold-rolled with a large-diameter work roll. ,
Conventionally, there has always been a problem that the surface gloss is reduced.

The inventors of the present invention have conducted intensive studies on the causes of gloss inhibition, and as a result, have found that network-like micro defects remain on the surface of the product steel strip, which causes the surface gloss to be reduced. . In addition, it has been found that this mesh-like defect also remains on the surface of the steel strip after cold rolling, which affects the surface properties of the product. State defects, the steel strip before cold rolling, that is, a part of the defects generated on the surface of the steel strip annealed and pickled after hot rolling was found to remain until after cold rolling. .

Further, as a result of a detailed examination of the surface of the steel strip annealed and pickled after hot rolling, it was found that this mesh defect coincides with the crystal grain boundary on the steel strip surface, and it was formed along the crystal grain boundary. Chromium deficient layer was found to be the cause. That is, when the stainless steel strip is hot-rolled and annealed, a chromium-deficient layer is formed along the crystal grain boundaries. The chromium-depleted layer becomes martensite and has hot brittleness, so that cracks are liable to occur remarkably (hereinafter, this crack is referred to as grain boundary crack). Moreover, the annealed steel strip removes oxide scale on the surface of the steel strip by pickling, but the grain boundary cracks are further eroded by the acid at the time of pickling, and are further enlarged and increased.

When the steel strip is cold-rolled by supplying a large amount of rolling oil, the rolling oil is sealed in the inside of the grain boundary cracks, so that it is difficult for the steel strip to be crushed at the contact portion with the roll and the steel strip.
Grain boundary cracks remained even after cold rolling, and also remained on the surface of the product steel strip, reducing the surface gloss. Then, in order to fundamentally eliminate the generation of the grain boundary cracks, the present inventors studied the following method.

(1) The chromium-deficient layer, which causes grain boundary cracking, is significantly reduced. (2) Grain boundary cracks are mechanically removed. (3) Crush grain boundary cracks. Of these, the present inventors have adopted the method of reducing the chromium deficiency layer because it is the most effective and can be implemented at low cost.

In order to reduce the chromium deficiency layer, it is preferable to suppress the formation thereof, and it is known that cooling a high-temperature heated steel sheet is effective. But 850-1150
It is completely unclear whether the chromium-depleted layer is formed in the hot-rolling stage or in the subsequent annealing stage of the steel strip annealed at a high temperature of about 1200 ° C. after hot rolling at a temperature of ℃ C. Neither was the temperature range for continuously cooling the steel strip and the cooling rate known at all.

The inventors of the present invention have conducted intensive studies on these problems and found that a large amount of a chromium-deficient layer is formed deeply on the surface of the steel strip annealed after the hot-rolling stage, resulting in grain boundary cracking. I found it. That is, the steel strip after annealing may be rapidly cooled. Also, the temperature at which quenching starts,
That is, after examining the cooling start position, for any of various austenitic stainless steel sheets,
It has been found that quenching can be performed immediately after annealing to significantly reduce the chromium deficiency layer and eliminate grain boundary cracking. But,
In order to rapidly cool immediately after annealing, it is necessary to take measures to prevent water from easily entering the furnace. Therefore, as a result of examining a method of obtaining a relatively good effect without taking measures against water intrusion, it was found that it is better to rapidly cool the steel strip temperature in the range of 850 to 400 ° C.

[0013] Further, the observation of grain boundary cracking of the steel strip obtained by changing the cooling rate of the steel strip variously showed that if the cooling rate was 25 ° C / sec or more, the grain boundary cracking was significantly reduced. The surface gloss of the finished product obtained by cold rolling the steel strip was remarkably improved. Further, in order to further improve the surface gloss by further reducing the grain boundary cracks, a method of mechanically grinding the grain boundary cracks was studied. That is, the steel strip after the above-mentioned quenching was collected, and as a result of measuring the depth of grain boundary cracks remaining on the steel strip surface,
It was found that the depth of most grain boundary cracks was 2 μm or less. Therefore, it has been found that by mechanically grinding the steel strip surface by 2 μm or more, the surface gloss of the product steel strip finished by cold rolling this steel strip is improved.

Further, when the position where mechanical grinding is performed is examined, if it is performed immediately after annealing, the temperature of the steel strip is high and the life of the tool used for grinding is significantly shortened. Also, when mechanical grinding is performed after rapid cooling and pickling after annealing, the streak pattern when the steel strip surface is ground remains, and the pattern does not disappear even if cold rolling and becomes gloss unevenness. It is a problem. Therefore, by performing mechanical grinding before pickling after annealing and quenching, the life of the tool can be kept long because the steel strip is low temperature, and the streak pattern is reduced by pickling after grinding. It was found that there was no problem of uneven gloss and that it was good.

The method of mechanically grinding grain boundary cracks of the present invention includes grinding with a grinder, polishing with a brush, and grinding with a grindstone having abrasive powder fixed to a resin or cloth. By adopting a method that can be performed continuously in a steel strip, a steel strip can be manufactured efficiently. Here, the present inventors have studied a method for further improving the surface gloss. That is, the surface of the cold-rolled material produced by the above-described method may have a small amount of grain boundary cracks remaining, and it is expected that the surface gloss can be further improved by reducing the slightly remaining grain boundary cracks. .

Therefore, the present inventors have paid attention to the surface roughness of the work roll of each stand of the cold tandem mill and have made further studies. In general, it has been known that reducing the work roll roughness improves the surface gloss of the steel strip.However, in order to obtain a gloss equal to or higher than that of a product rolled and finished with a small-diameter work roll such as an intended Sendzimir mill. It was completely unclear what roll roughness should be applied at which stand.

The present inventors applied various work roll roughnesses to each stand and cold rolled them, followed by finish annealing pickling or finish bright annealing, and then subjected to finish temper rolling. The steel strip subjected to finish annealing and pickling was subjected to buff polishing after temper rolling. As a result of investigating the surface gloss of these steel strips,
The following became clear. That is, in the conventional cold tandem mill, each stand is rolled using a work roll having a surface roughness of 0.3 μm in average roughness Ra, but the first stand is provided with a surface having a surface roughness of 0.5 μm or more in average roughness Ra. When rolling is performed using a work roll having roughness, grain boundary cracking is significantly reduced, and when rolling is performed using a work roll having this roughness also in the second and subsequent stands, grain boundary cracking is further reduced.

However, if this roughness is applied to the subsequent stand, the surface gloss of the finished product is reduced. Therefore, the present inventors have studied to solve this problem.
That is, the large roughness of the roll surface used for reducing grain boundary cracking in the former stand is transferred to the steel sheet surface, and if the large roughness of the steel sheet surface remains after rolling, the surface gloss is reduced. Therefore, various studies were made on the roll roughness used for the post-stage stand of the post-stage stand of the cold tandem mill. As a result, it was found that the work roll surface roughness of the final stage should be set to an average roughness Ra of 0.15 μm or less. In addition to the final stand, 0.15
It is even better to apply a roughness of less than μm.

The above method of reducing the work roll roughness to a predetermined value or less and applying it to a predetermined stand is as follows.
By using a cold-rolled material that has been significantly reduced in intergranular cracks by annealing and pickling after hot-rolling according to the present invention, it is possible to obtain a remarkably good effect for the first time, and only the work roll roughness of a cold tandem mill is simply obtained. The effect is small by simply changing the method.

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, SUS as an example of an austenitic system will be described.
Using the 304 steel strip, according to the method of the present invention, annealed after quenching after hot rolling, quenching, mechanically polishing and pickling to produce a pre-treated steel strip, and then supplying rolling oil according to the method of the present invention. It rolled by the cold tandem mill using a large diameter roll. Further, a part thereof was cold-rolled by a Sendzimir mill having a small-diameter work roll.

As a conventional example, a SUS304 steel strip is annealed after hot rolling, gradually cooled immediately after annealing, then water-cooled, and pickled to produce a pre-treated steel strip. While rolling in a cold tandem mill using a large-diameter roll. Further, a part thereof was cold-rolled by a Sendzimir mill having a small-diameter work roll. Furthermore, these steel strips were subjected to finish annealing pickling or finish bright annealing under the same conditions, and finish temper rolled. The finish-annealed steel strip was subjected to buff polishing under the same conditions after temper rolling.

Regarding the surface gloss of these finished steel strips,
Measured in accordance with JIS Z8741 Glossiness Measurement Method 5 (GS20 °).
50 was A, 600 to 800 was B, 400 to 600 was C, and 400 or less was D, and evaluated on a 5-point scale. Table 1 shows the results of the rolling in a cold tandem mill and Table 2 shows the results of the rolling in a tandem mill and then cold rolling in a Sendzimir mill.

From these tables, it can be seen that the cold rolled stainless steel strip produced according to the present invention has significantly better gloss than the steel strip produced by the conventional method.

[0024]

[Table 1]

[0025]

[Table 2]

[0026]

As described above, the stainless steel cold-rolled steel strip pretreated by the method of the present invention and then cold-rolled and finished has a significantly superior surface finish as compared with the steel strip manufactured by the conventional method. Has luster. In particular, when a large-diameter roll such as a cold tandem mill is used, it has excellent surface gloss equal to or higher than that of rolling using a small-diameter roll such as a Sendzimir mill, which has heretofore been impossible.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Masahiro Yoshioka 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Inside the Chiba Works, Chiba (72) Inventor Makoto Suzuki 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture (56) References JP-A-5-320771 (JP, A) JP-A-5-50107 (JP, A) JP-A-5-23704 (JP, A) JP-A-4-52008 ( JP, A) JP-A-6-262206 (JP, A) JP-A-6-285509 (JP, A) JP-A-6-15308 (JP, A) JP-A-5-23701 (JP, A) JP Hei 4-17908 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) B21B 1/22 B21B 3/02 B21B 27/02 B21B 45/00 C21D 6/00

Claims (2)

(57) [Claims] 1. The stainless steel strip after the completion of hot rolling is annealed, and the steel strip temperature immediately after the annealing or after the annealing is continuously reduced.
The steel strip is quenched at a cooling rate of 25 ° C / sec or more from 850 to 400 ° C, and the surface of the steel strip is mechanically ground to 2 µm or more, and then pickled to produce a cold rolled material. The work roll surface roughness of at least the first stand of the cold tandem mill is set to an average roughness Ra of 0.5 μm or more, and the work roll surface roughness of the final stand is set to at least the average roughness Ra0.
A method for producing a cold-rolled stainless steel strip, comprising cold rolling to 15 μm or less. 2. The method according to claim 1, wherein the stainless steel strip is an austenitic stainless steel strip.