JPH0716605A - Manufacture of cold-rolled stainless steel strip - Google Patents

Abstract

PURPOSE:To manufacture a cold-rolled steel strip having excellent surface gloss by manufacturing a cold-rolled base stock under specified conditions and cold-rolling the base stock by specifying the diameter of work roll of the final stand. CONSTITUTION:The strip of austenitic stainless steel which is represented by SUS-304 is annealed after completing hot rolling. Successively, the steel strip is rapidly cooled at the cooling rate of >=25 deg.C/sec just after annealing or from the range of 850-400 deg.C of the temp. of the steel strip after annealing. A cold- rolled base stock is manufactured by pickling after mechanically grinding the surface of the steel strip by 2mum with a grinder or the like. That base stock is cold-rolled by taking the diameter of work roll of at least the final stand of a cold tandem mill as <=phi400mm. In this way, intercrystalline cracking of the stainless steel strip is remarkably reduced.

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 an excellent surface gloss.

[0002]

2. Description of the Related Art Conventionally, cold-rolled stainless steel strips are produced by annealing hot-rolled steel strips and pickling them, and then cold rolling them in a Sendzimir mill with a work roll diameter of φ150 mm or less to finish annealing pickling or bright finish. It was annealed and then finish temper-rolled. In the case of an austenitic stainless steel strip represented by SUS304, the cold-rolled stainless steel strip manufactured through these steps 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, in order to significantly shorten the rolling time and efficiently produce a cold rolled stainless steel strip, a method of cold rolling with a work roll having a large diameter of φ 150 mm or more is adopted.
When cold-rolled by this method, the surface gloss of the product was remarkably reduced as compared with the case where a small-diameter work roll was used, and it could not be applied to any application requiring gloss.

[0004]

SUMMARY OF THE INVENTION According to the present invention, when cold rolling with a large diameter work roll such as a cold tandem mill, or cold rolling with a large diameter work roll followed by cold rolling with a small diameter work roll. In this case, the object is to solve the problem of lowering the surface gloss that has been present in the past.

[0005]

The present invention solves the above-mentioned problems and provides a manufacturing method for manufacturing a stainless cold-rolled steel strip having excellent surface gloss. That is, the present invention anneals the stainless steel strip after completion of hot rolling, and the steel strip temperature immediately after annealing or after annealing is continuous to this.
The steel strip is rapidly cooled from the range of 850 to 400 ° C at a cooling rate of 25 ° C / sec or more, and the steel strip surface is mechanically ground to 2 μm or more and then pickled to produce a cold-rolled material. The work roll diameter of at least the final stand of the tandem mill is φ
It is a method for producing a stainless cold-rolled steel strip characterized by cold rolling to 400 mm or less, and can be advantageously used for an austenitic stainless steel strip.

[0006]

The present invention will be described in detail below. SUS
In an austenitic stainless steel sheet typified by 304, when cold rolling with a large diameter work roll such as a cold tandem mill, or when cold rolling with a large diameter work roll and then cold rolling with a small diameter work roll ,
Conventionally, there has always been a problem that surface gloss decreases.

[0007] Therefore, as a result of diligent investigation of the cause of gloss inhibition, the present inventors have found that network-like microscopic defects remain on the surface of the product steel strip, which causes the surface gloss to decrease. . Moreover, it was found that these mesh defects remained on the surface of the steel strip after cold rolling, which affects the surface texture of the product, and further, the mesh residuals on the surface of the steel strip after cold rolling. It was found that some of the defects formed on the surface of the steel strip before cold rolling, that is, the steel strip annealed and pickled after hot rolling remain until after 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 crystal grain boundaries on the surface of the steel strip, and formed along the crystal 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 cracking is apt to occur (hereinafter, this cracking is referred to as grain boundary cracking). Moreover, the annealed steel strip removes the oxide scale on the surface of the steel strip by pickling, but the intergranular cracks are eroded by the acid during this pickling and further expand and 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.
Intergranular cracks remained after cold rolling and also remained on the surface of the product steel strip, thus reducing the surface gloss. Therefore, in order to fundamentally eliminate the generation of this grain boundary crack, the present inventors have studied 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 is the most hardened and can be carried out at a low cost.

It is known that in order to reduce the chromium deficient layer, its generation should be suppressed, and it is known that cooling the steel sheet heated at high temperature is effective. However, 850-1150
For steel strips annealed at a high temperature of about 1200 ° C after hot rolling at a temperature of ℃, it is completely unknown whether the chromium-deficient layer is formed in the hot rolling stage or in the subsequent annealing stage. The temperature range and the cooling rate for continuously cooling the steel strip were not known at all.

[0012] Therefore, the inventors of the present invention have made diligent studies on these, and found that a deep and large amount of chromium-deficient layer is formed 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. Also, the temperature at which quenching starts
That is, when the cooling start position was examined, for each of the various austenitic stainless steel sheets,
It was found that quenching immediately after annealing is preferable in order to significantly reduce the chromium deficient layer and eliminate grain boundary cracking. But,
In order to quench immediately after annealing, it is easy for water to enter the furnace and it is necessary to take measures to prevent it. Therefore, as a result of examining 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 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 obtained by cold rolling the steel strip was significantly improved. Further, in order to further reduce the grain boundary cracks and improve the surface gloss, a method of mechanically grinding the grain boundary cracks was examined. That is, as a result of collecting the steel strip after the quenching described above and measuring the depth of the intergranular cracks remaining on the steel strip surface,
It was found that most of the intergranular cracks had a depth of 2 μm or less. Therefore, it has been understood that mechanically grinding the surface of the steel strip by 2 μm or more improves the surface gloss of the product steel strip finished by cold rolling the steel strip thereafter. Further, when the position where the mechanical grinding is carried out is examined, when it is carried out immediately after the annealing, the temperature of the steel strip is high and the life of the tool used for grinding is remarkably shortened, which is a problem. If mechanical grinding is performed after rapid cooling after pickling and pickling, the streak pattern when the steel strip surface is ground remains and the pattern does not disappear even when cold-rolled, resulting in uneven gloss. It's a problem. Therefore, by performing mechanical grinding after pickling after annealing and quenching, the steel strip is at a low temperature and the tool life can be kept long, and pickling after grinding reduces streaks. It was understood that there was no problem of uneven luster and that it was good.

As the method of mechanically grinding the intergranular cracks of the present invention, there are grinding with a grinder, polishing with a brush, grinding with a grindstone in which polishing powder is fixed to resin or cloth, and the like. By adopting the method that can be continuously performed in the above, the steel strip can be efficiently manufactured. Here, the present inventors examined a method for further improving the surface gloss. That is, there may be some intergranular cracks remaining on the surface of the cold rolled material produced by the above method, and it can be expected that the surface gloss can be further improved by reducing the intergranular cracks that slightly remain. That is why.

Therefore, the present inventors have paid attention to the work roll diameter of each stand of the cold tandem mill and have made an examination. Generally, it was known that reducing the work roll diameter 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 desired Sendzimir mill, It was not clear at all what work roll diameter should be used, which stand the diameter should be applied to, and so on.

The present inventors cold-rolled by combining various work roll diameters, followed by finish annealing pickling or finish bright annealing and finish temper rolling. The steel strip that had been subjected to finish annealing pickling was subjected to puff polishing 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, work rolls having a diameter of 500 mm or more are used for rolling, but when a work roll having a smaller diameter of 400 mm or less is used, grain boundary cracking can be significantly reduced. Also, φ
The stand that most requires a work roll of 400 mm or less is the final stand, and if a work roll with this diameter is used for other stands, grain boundary cracks can be further reduced, and as a result, surface gloss is improved. is there.

The above-mentioned method of applying these work roll diameters to a predetermined stand by setting the diameter of these work rolls to a predetermined value or less is
According to the present invention, by using a cold-rolled material which has been subjected to annealing pickling after hot rolling and which has significantly reduced intergranular cracking, it shows a remarkably good effect for the first time, and only the work roll diameter of the cold tandem mill is shown. The effect is small if you only change it.

[0018]

EXAMPLES Hereinafter, SUS as an example of austenite type
304 steel strip is annealed and rapidly cooled after hot rolling according to the method of the present invention, mechanically polished and pickled to produce a pretreated steel strip, and then, according to the method of the present invention, while supplying rolling oil. It was rolled in a cold tandem mill using large diameter rolls. Further, a part thereof was cold-rolled with a Sendzimir mill having a work roll with a 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, and then a rolling oil is prepared according to a conventional method. 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 with a 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 according to JIS Z8741 Glossiness Measurement Method 5 (GS20 °), and glossiness of 950 or more is specified as A, 800-
950 was A, 600 to 800 was B, 400 to 600 was C, and 400 or less was D. For 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 further cold rolling with a Sendzimir mill.

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

[0022]

[Table 1]

[0023]

[Table 2]

[0024]

INDUSTRIAL APPLICABILITY As described above, the cold rolled stainless steel strip obtained by cold rolling the material pretreated by the method of the present invention has a remarkably excellent surface as compared with the cold rolled steel strip manufactured 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 obtained only by using a small-diameter roll such as a Sendzimir mill, which has hitherto been unattainable.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Masahiro Yoshioka, 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel Co., Ltd. Chiba Works (72) Inventor Makoto Suzuki, 1 Kawasaki-cho, Chuo-ku, Chiba-shi Kawasaki Steel Works Co., Ltd. Chiba Works (72) Inventor Katsuhiro Kobori 1 Kawasaki-cho, Chuo-ku, Chiba, Chiba Kawasaki Works Chiba Works

Claims (2)

[Claims] 1. A stainless steel strip after hot rolling is annealed, and the temperature of the steel strip immediately after or after annealing is continuously increased.
The steel strip is rapidly cooled from the range of 850 to 400 ° C at a cooling rate of 25 ° C / sec or more, and the steel strip surface is mechanically ground to 2 μm or more and then pickled to produce a cold-rolled material. The work roll diameter of at least the final stand of the tandem mill is φ
A method for manufacturing a stainless cold-rolled steel strip, which comprises cold rolling to 400 mm or less. 2. A method for producing a cold rolled stainless steel strip, wherein the stainless steel strip is an austenitic stainless steel strip.