JPH06285509A - Manufacture of cold-rolled stainless steel strip - Google Patents

Abstract

PURPOSE:To obtain an excellent surface luster when a stainles steel strip is cold-rolled by large diameter work rolls of a cold tandem mill etc., or when it is cold-rolled by large diameter work rolls, then, by small diameter rolls. CONSTITUTION:A stainless steel strip after completion of hot rolling is annealed, successively, just after the steel strip is annealed or after it is quenched at a range 850-400 deg.C and at a cooling speed >=25 deg.C/sec after annealing to manufacture a cold rolled blank and this blank is cold-rolled by work rolls having an average roughness Ra >=0.5mum of at least the 1st stand of a cold tandem mill.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a production method capable of advantageously producing a cold rolled stainless steel strip having an excellent surface gloss.

[0002]

2. Description of the Related Art Conventionally, cold-rolled stainless steel strip is annealed from hot-rolled steel strip and pickled, and then cold-rolled in a Sendzimir mill with a work roll diameter of φ150 mm or less to finish-anneal pickling or finish bright annealing. It is manufactured by further finishing temper rolling. The stainless cold-rolled steel strip produced by this conventional technique is often austenitic typified by SUS304 and is often subjected to buff polishing after finish temper rolling, and it is important to exhibit excellent gloss after this buff polishing. is there.

Further, as another conventional technique, a method of cold rolling with a work roll having a large diameter of φ150 mm or more is adopted in order to significantly reduce the rolling time and efficiently produce a cold rolled stainless steel strip. .

[0004]

However, in the prior art, in the austenitic stainless steel sheet represented by SUS304, when cold rolling with a large diameter work roll such as a cold tandem mill, or with a small diameter work roll. Regardless of whether it is rolled or a combination thereof, there is always a problem that the surface gloss of the product is lowered.

Particularly, in the method of cold rolling using a work roll having a large diameter of φ150 mm or more, the surface gloss of the product is remarkably reduced as compared with the case where a work roll having a small diameter is used. Not applicable at all.

The present invention has been conventionally used for cold rolling with a large diameter work roll such as a cold tandem mill, or for cold rolling with a large diameter work roll and then with a small diameter work roll. The purpose is to solve the problem of surface gloss reduction.

[0007]

The present invention according to claim 1 anneals a stainless steel strip after the hot rolling is finished, and continuously produces a temperature of the strip immediately after or after the annealing. Is 8
A cold-rolled material is manufactured by quenching at a cooling rate of 25 ° C / sec or more in the range of 50 ° C to 400 ° C, and this material is used to measure the work roll roughness of at least the first stand of a cold tandem mill to obtain an average roughness R.
a 0.5 μm or more and cold rolled.

According to a second aspect of the present invention, in the first aspect, at least the work roll roughness of the final stand is set to an average roughness Ra of 0.15 μm or less and cold rolled.

[0009]

[Operation] When austenitic stainless steel sheet typified by SUS304 is cold-rolled by a large-diameter work roll such as a cold tandem mill, or cold-rolled by a large-diameter work roll and then cold-rolled by a small-diameter work roll. In that case, there has always been a problem that the surface gloss is lowered.

Therefore, as a result of diligent investigations on the cause of gloss inhibition, the present inventors have found that mesh-like microscopic defects remain on the surface of the product steel strip, which causes the surface gloss to decrease. . However, it was found that this mesh defect remains on the surface of the steel strip after cold rolling, which affects the surface texture of the product. It has been found that some of the defects generated on the surface of the steel strip before cold rolling, that is, the steel strip annealed and pickled after hot rolling remain after the cold rolling.

Further, as a result of a detailed examination of the surface of the steel strip which has been annealed and pickled after hot rolling, it was found that the mesh defects coincided with the grain boundaries of the surface of the steel strip, and formed along the grain boundaries. It has been understood that the cause is a chromium deficient layer. That is, when a stainless steel strip is hot rolled and annealed, a chromium deficient layer is formed along the grain boundaries. This chromium-deficient layer becomes martensite and has hot brittleness, so that cracks are apt to occur remarkably (hereinafter, this crack is referred to as intergranular crack). Of course, the annealed steel strip removes the oxide scale on the surface of the steel strip by pickling, but the intergranular cracks are further corroded and eroded by the acid during the pickling, and further increase.

When this steel strip is cold-rolled by supplying a large amount of rolling oil, the rolling oil is enclosed inside the intergranular cracks and is less likely to be crushed at the contact portion with the roll and the steel strip. The cracks remained after cold rolling, and also remained on the surface of the product steel strip, thus lowering the surface gloss.

Therefore, in order to fundamentally eliminate the formation of the grain boundary cracks, the present inventors examined the following method. (1) The chromium-deficient layer, which is the cause of grain boundary cracking, is significantly reduced. (2) Mechanically remove grain boundary cracks. (3) Crush the intergranular cracks.

Among these, the present inventors have adopted the method of reducing the chromium deficient layer because it has the greatest effect and can be implemented at low cost.

It is known that in order to reduce the chromium deficient layer, its production should be suppressed, and that the steel sheet heated at high temperature should be cooled.

However, for the stainless steel strip annealed at a high temperature of about 1200 ° C. after hot rolling at a temperature of 850 to 1150 ° C., is a chromium deficient layer formed in the hot rolling step or in the subsequent annealing step? Is completely unknown, and in order to effectively suppress the formation, the temperature range and the cooling rate at which the steel strip is continuously cooled are not known at all.

Therefore, the inventors of the present invention have diligently studied these facts, and found that a large amount of chromium-deficient layer is formed deeply on the surface of the steel strip annealed after the hot rolling step, resulting in intergranular cracking. I found it. That is, the steel strip after annealing may be rapidly cooled.

Further, when the temperature at which quenching is started, that is, the cooling start timing, was examined, it was found that quenching was performed immediately after annealing in any of various austenitic stainless steel sheets in order to significantly reduce the chromium-deficient layer and eliminate grain boundary cracking. Then I found something good. However, in order to quench immediately after annealing, it is easy for water to enter the inside of the furnace and it is necessary to take preventive measures against it. Therefore, as a result of investigating a method of obtaining a relatively good effect without taking measures against water ingress, it was found that it is preferable to rapidly cool the steel strip in the temperature range of 850 ° C to 400 ° C.

Further, observation of grain boundary cracks of the steel strips obtained by variously changing the cooling rate of the steel strips revealed that the grain boundary cracks were remarkably reduced at a cooling rate of 25 ° C./sec or more. The surface gloss of the product finished by cold rolling the strip was significantly improved.

Here, the present inventors have examined a method for further improving the surface gloss. That is, there may be slight intergranular cracks remaining on the surface of the cold-rolled material produced by the above method, and it is expected that the surface gloss will be further improved by reducing these slightly left intergranular cracks. It can be done.

Therefore, the present inventors paid attention to the work roll roughness of each stand of the cold tandem mill, and made an examination. Generally, it was known that reducing the work roll roughness improves the surface gloss of the steel strip, but in order to obtain the same or higher gloss as the product finished by rolling with a small diameter work roll such as the Sendzimir mill. However, it was not clear at all about which stand and how much roll roughness should be applied.

The present inventors applied various work roll roughnesses to each stand and cold-rolled, followed by finish annealing pickling or finish bright annealing, and finish temper rolling.
The steel strip subjected to finish annealing and pickling was subjected to buffing after temper rolling. As a result of investigating the surface gloss of these steel strips, the following facts were revealed. That is, in the conventional cold tandem mill, each stand was rolled using a work roll having an average roughness Ra of 0.3 μm, but the first stand had a roughness average of Ra of 0.5 μm or more. Rolling with a work roll having a thickness significantly reduces grain boundary,
When the work roll having the main roughness is applied to the second stand and thereafter, the grain boundary is further reduced.

However, if the main roughness is applied to the rear stand, a new problem arises that the surface gloss of the finished product is lowered.

Therefore, the present inventors have conducted studies to solve this problem. That is, if the large roughness of the roll surface used to reduce grain boundary in the former stand is transferred to the steel plate surface and the large roughness of the steel plate surface remains after rolling, the surface gloss is reduced. Therefore,
As a result of various studies on roll roughness used in the latter stand of the cold tandem mill, it was found that the work roll roughness of the final stand should be 0.15 μm or less in terms of average roughness Ra. Also, in addition to the final stand,
It is even better to apply a roughness of 0.15 μm or less.

The above-mentioned method in which the roughness of these work rolls is set to a predetermined value or less and the work roll is applied to a predetermined stand is a cold-rolled material which is subjected to annealing pickling after hot rolling to remarkably reduce grain boundary. However, the effect is small when only the work roll roughness of the cold tandem mill is changed.

[0026]

EXAMPLES Using SUS304 steel strip as an example of austenite, according to the method of the present invention, annealing and quenching after hot rolling,
A pretreated steel strip was produced by pickling and then rolled in a cold tandem mill using a large diameter roll while supplying a rolling oil according to the method of the present invention. Further, a part thereof was cold-rolled with a Sendzimir mill having a work roll of smaller diameter.

Further, as a conventional example, a SUS304 steel strip is annealed after hot rolling, immediately after annealing, gradually cooled, then water cooled, and pickled to produce a pretreated steel strip. Was supplied in a cold tandem mill using a large-diameter roll. Further, a part thereof was cold-rolled with a Sendzimir mill having a work roll of smaller diameter.

Further, these steel strips were subjected to finish annealing pickling or finish bright annealing under the same conditions, and finish temper rolling.
The finish annealed steel strip was buffed 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 °). Glossiness of 950 or higher is special A, 800-9
The evaluation was made in five stages, with 50 as A, 600 to 800 as B, 400 to 600 as C, and 400 or less as D.

Regarding these results, Table 1 shows the case of rolling with a cold tandem mill, and Table 2 shows the case of rolling with a tandem mill and then cold rolling with a Sendzimir mill.

From these tables, it was confirmed that the stainless cold-rolled steel strip produced according to the present invention has a remarkably good gloss as compared with the steel strip produced by the conventional method.

[0032]

[Table 1]

[0033]

[Table 2]

[0034]

As described above, the stainless cold-rolled steel strip pretreated by the method of the present invention and then cold-rolled and finished has a remarkably excellent surface as compared with the steel strip produced by the conventional method. It has a luster. In particular, when a large-diameter roll such as a cold tandem mill is used, it has a surface gloss equal to or better than that of a rolling using a small-diameter roll such as a Sendzimir mill, which has hitherto been unattainable.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Masao Akita, 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Iron & Steel Co., Ltd. Technical Research Division (72) Masahiro Yoshioka 1, Kawasaki-cho, Chuo-ku, Chiba Kawasaki Steel Co., Ltd. Chiba Steel Works (72) Inventor Katsuhiro Kobori 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Kawasaki Steel Co., Ltd. Chiba Works

Claims (2)

[Claims] 1. A stainless steel strip after hot rolling is annealed, and the steel strip is continuously annealed immediately thereafter or at a steel strip temperature of 850 ° C. to 400 ° C. in the range of 25 ° C./sec. It is characterized in that it is rapidly cooled at the above cooling rate to produce a cold-rolled material, and that this material is cold-rolled so that the work roll roughness of at least the first stand of a cold tandem mill is made to have an average roughness Ra of 0.5 μm or more. Method for producing a cold rolled stainless steel strip. 2. The method for producing a stainless cold-rolled steel strip according to claim 1, wherein at least the work roll roughness of the final stand is set to an average roughness Ra of 0.15 μm or less and cold rolling is performed.