JP3121959B2 - 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 and pickled from a hot rolled steel strip, and then subjected to cold rolling in a Sendzimir mill or the like having a work roll diameter of 150 mm or less to finish-finish pickling or finish bright annealing. And further temper rolling. In the case of austenitic stainless steel represented by SUS304, the cold-rolled stainless steel strip produced by this conventional technique is often subjected to buff polishing after finish temper rolling, and it is important to exhibit excellent gloss after this buff polishing. is there.

As another conventional technique, a method of cold rolling with a large-diameter work roll having a diameter of 150 mm or more has been adopted in order to greatly reduce the rolling time and efficiently produce a cold-rolled stainless steel strip. .

[0004]

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

[0005] In particular, in the method of performing cold rolling using a large-diameter work roll having a diameter of 150 mm or more, the surface gloss of a product is significantly reduced as compared with the case of using a small-diameter work roll. Not applicable at all.

[0006] The present invention has been conventionally used for 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. The purpose of the present invention is to solve the problem of reduced surface gloss.

[0007]

SUMMARY OF THE INVENTION According to the present invention, a stainless steel strip after hot rolling is annealed, and the steel strip immediately after or after annealing has a steel strip temperature of 850 ° C. to 400 ° C. Quenched at a cooling rate of 25 ° C./sec or more in the range of 2 to produce a cold rolled material, and cold rolled this material by setting the work roll diameter of at least the final stand of a cold tandem mill to φ400 mm or less. Things.

[0008]

[Action] When austenitic stainless steel sheet represented by SUS304 is cold-rolled with a large-diameter work roll such as a cold tandem mill, or cold-rolled with a small-diameter work roll after being cold-rolled with a large-diameter work roll In this case, there has always been a problem that the surface gloss is reduced.

The inventors of the present invention have conducted intensive investigations on the cause of the gloss inhibition, and as a result, have found that a net-like micro defect remains on the surface of the product steel strip, which causes the surface gloss to be reduced. . Of course, it has been found that this network defect also remains on the surface of the steel strip after cold rolling, which affects the surface properties of the product, and furthermore, the network-like defect remaining on the surface of the steel strip after cold rolling. It has been found that some of the defects formed on the surface of the steel strip before cold rolling, that is, the steel strip that has been hot-rolled and then annealed and pickled, remain after cold rolling.

Further, as a result of a detailed investigation of the surface of the steel strip annealed and pickled after hot rolling, it was found that this network defect coincided with the crystal grain boundaries on the steel strip surface, and the defects were formed along the crystal grain boundaries. 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. This 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). Of course, the annealed steel strip removes oxide scale on the steel strip surface by pickling, but grain boundary cracks are further eroded by the acid during pickling and further enlarged and increased.

When this steel strip is cold-rolled by supplying a large amount of rolling oil, the rolling oil is sealed inside 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. Cracks remained even after cold rolling, and also remained on the surface of the product steel strip, reducing the surface gloss.

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-deficient layer because it is the most effective and can be implemented at low cost.

It is known that the formation of the chromium-deficient layer can be reduced by suppressing the formation of the chromium-deficient layer, and by cooling a high-temperature heated steel sheet.

However, for a stainless steel strip which has been hot-rolled at a temperature of 850 to 1150 ° C. and then annealed at a high temperature of about 1200 ° C., whether a chromium-depleted layer is formed in a hot-rolling stage or in a subsequent annealing stage. And the range of the temperature for continuously cooling the steel strip and the cooling rate in order to effectively suppress the formation thereof have not been clarified at all.

The inventors of the present invention have made 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.

The temperature at which quenching was started, that is, the cooling start timing, was examined. In all of the various austenitic stainless steel sheets, quenching was performed immediately after annealing in order to significantly reduce the chromium-depleted layer and eliminate grain boundary cracking. Then I found a good thing. However, 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 rapid cooling in a steel strip temperature range of 850 ° C to 400 ° C was good.

Further, the observation of grain boundary cracking of the steel strip obtained by variously changing the cooling rate of the steel strip revealed 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 strip was remarkably improved.

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-mentioned method may have a small amount of grain boundary cracks, and it is expected that the surface gloss can be further improved by reducing the slightly remaining grain boundary cracks. You can do it.

Therefore, the present inventors have paid attention to the work roll diameter of each stand of the cold tandem mill and made further studies. In general, it has been known that reducing the work roll diameter improves the surface gloss of the steel strip.However, in order to obtain a gloss equal to or higher than the product finished by rolling with a small-diameter work roll such as an intended Sendzimir mill, It was not clear at all how much the work roll diameter should be, and which stand should apply the diameter.

The present inventors cold rolled various work roll diameters in combination, and then performed finish annealing pickling or finish bright annealing, followed by finish temper rolling. The steel strip subjected to finish annealing pickling was subjected to buffing after temper rolling. Investigation of the surface gloss of these steel strips revealed the following. That is, in the conventional cold tandem mill, rolling is performed using a work roll having a diameter of 500 mm or more, but when a work roll having a diameter of 400 mm or less is used, grain boundaries can be significantly reduced. In addition, the stand that requires the work roll of φ400 mm or less is the final stand, and if a work roll having this diameter is used for other stands, grain boundaries can be further reduced, and as a result, the surface gloss improves. That is.

The above method of reducing the work roll diameter to a predetermined value or less and applying the work roll to a predetermined stand uses the method of the present invention in which a cold-rolled material in which the grain boundaries are significantly reduced by annealing and pickling after hot rolling according to the present invention. By using it, a remarkably good effect is shown for the first time, and the effect is small by simply changing only the work roll diameter of the cold tandem mill.

[0023]

EXAMPLE Using a SUS304 steel strip as an example of an austenitic system, according to the method of the present invention, after hot rolling, annealing and quenching,
The pretreated steel strip was manufactured by pickling, and then rolled by a cold tandem mill using a large-diameter roll while supplying rolling oil according to the method of the present invention. Further, a part thereof was cold-rolled by a Sendzimir mill having a small-diameter work roll.

Further, 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.

Further, these steel strips were subjected to finish annealing pickling or bright annealing under the same conditions, and were subjected to finish temper rolling. still,
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, 600-800 was B, 400-600 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 followed by cold rolling in a Sendzimir mill.

[0028]

[Table 1]

[0029]

[Table 2]

From these tables, it was confirmed that the cold rolled stainless steel strip produced according to the present invention had significantly better gloss than the steel strip produced by the conventional method.

[0031]

As described above, the stainless steel cold-rolled steel strip obtained by cold rolling the material pretreated according to the method of the present invention has a significantly superior surface compared to the cold-rolled 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 gloss equal to or higher than the case of only rolling using a small-diameter roll such as a Sendzimir mill, which has heretofore been impossible at all.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Katsuhiro Kobori, Inventor 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel Corporation Chiba Works (72) Inventor Eisuke Kawasumi 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel (56) References JP-A-5-23704 (JP, A) JP-A-4-52006 (JP, A) JP-A-4-52008 (JP, A) JP-A-3-264102 ( JP, A) JP-A-5-2701 (JP, A) JP-A-5-50104 (JP, A) JP-A-3-42151 (JP, A) JP-A-6-7804 (JP, A) JP JP-A-5-271781 (JP, A) JP-A-5-311243 (JP, A) JP-A-6-15308 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) B21B 1 / 28,3 / 02 B21B 27 / 02,45 / 02 C21D 8/02

Claims (1)

(57) [Claims] 1. A stainless steel strip after the completion of hot rolling is annealed, and the steel strip is continuously and immediately after annealing at a steel strip temperature of 850 ° C. to 400 ° C. at a rate of 25 ° C./sec. The cold rolled material is manufactured by rapidly cooling at the above cooling rate, and the work roll diameter of at least the final stand of the cold tandem mill is set to φ
A method for producing a cold rolled stainless steel strip, comprising cold rolling to 400 mm or less.