JP3121956B2 - 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 by hot-rolled steel strip and pickled, then cold-rolled by a Sendzimir mill or the like having a work roll diameter of 150 mm or less, and subjected to finish annealing pickling or finish bright annealing. And further temper rolling. Stainless steel cold-rolled steel strips manufactured by this conventional technique are 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.

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]

According to the present invention, the stainless steel strip after the completion of hot rolling is annealed, and the steel strip is continuously heated immediately after the annealing 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. Cold rolling is performed while supplying.

According to the present invention, the stainless steel strip after the completion of hot rolling is annealed, and the steel strip immediately after or after the 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 to produce a cold-rolled material.
Cold rolling is performed while supplying a rolling oil emulsion of m or less.

According to a third aspect of the present invention, the 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 to produce a cold-rolled material, and supply this material by a cold tandem mill to supply a rolling oil emulsion at a supply rate of 10 m ³ / min. It is so designed that it is cold rolled.

[0010]

[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 causes of gloss inhibition, and as a result, have found that network-like micro defects remain on the surface of the product steel strip, thereby reducing the surface gloss. . 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 examination 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 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. 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 in the inside of the grain boundary cracks, so that it is hard to be crushed at the contact portion between 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.

[0014] In order to fundamentally eliminate the generation of grain boundary cracks, the present inventors have 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 most effective and can be implemented at low cost.

In order to reduce the chromium deficiency layer, it is better to suppress the formation thereof, and it is known that cooling a high-temperature heated steel sheet is effective.

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 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 conducted intensive studies and have found that a large amount of a chromium-deficient layer is formed deeply on the surface of a steel strip annealed after the hot-rolling step, 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. As for any of various austenitic stainless steel sheets, rapid cooling 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 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 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.

Hereinafter, three methods A, B, and C for further improving the surface gloss will be described. (Method A) First, the present inventors focused on a rolling oil emulsion supplied to a cold tandem mill, and as a result of study, found that grain boundary cracking was reduced by the following action.
That is, an oil film is formed hydrodynamically at the entrance of the roll bite by the speed of the roll and the steel strip. If the oil film is thin, the roll and the steel strip are likely to come into contact with each other, and the grain boundary cracks are easily crushed. At this time, when using a small-diameter work roll such as a Sendzimir mill, the effect of the rolling oil viscosity hardly appears because the oil film is extremely thin, but when using a large-diameter work roll such as a cold tandem mill, the viscosity of the rolling oil is reduced. If it is made smaller, the oil film becomes thinner and grain boundary cracking is reduced. Therefore, since it was not entirely known how much the viscosity of the rolling oil should be reduced, cold rolling was performed while supplying various types of rolling oils having different viscosities as emulsions, and the gloss of the surface of the steel strip was investigated. These rolled steel strips were subjected to finish annealing pickling or finish bright annealing under the same conditions, and subjected to finish temper rolling. The steel strip which had been 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 viscosity at 50 ° C supplied by a conventional cold tandem mill was reduced to 30 ° C.
When rolling oil with cSt or higher is used, grain boundary cracks remain after rolling and the surface gloss decreases, but it is understood that grain boundary cracking is significantly reduced when using a rolling oil with a viscosity at 50 ° C of 15 cSt or less. It is.

The above method of supplying the rolling oil with a viscosity of a predetermined value or less is achieved by using a cold-rolled material which has been subjected to annealing and pickling after hot rolling according to the present invention to significantly reduce grain boundary cracking. For the first time, a remarkably good effect is exhibited, and the effect is small by merely changing the viscosity of the rolling oil to a predetermined value or less.

(Method B) Next, the present inventors paid attention to the particle size of the rolling oil emulsion supplied to the cold tandem mill, and made further studies. That is, if the particle size of the rolling oil emulsion is reduced, the amount of rolling oil that is caught between the roll and the steel strip decreases, so that the roll and the steel strip are more likely to come into contact with each other and the grain boundary cracking is reduced. It is. However, it was not at all clear how much the emulsion particle size should be reduced in order to obtain gloss equal to or higher than that of a product finished by rolling with a small diameter work roll such as a Sendzimir mill.

The present inventors cold rolled the emulsion while supplying it while changing the average particle size of the emulsion, and then subjected to finish annealing pickling or finish bright annealing, followed by finish temper rolling. The steel strip which had been 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, the average particle size of the rolling oil emulsion supplied to the conventional cold tandem mill is 8
Although the grain boundary cracking could not be sufficiently reduced because the particle size was as large as μm or more, if the average particle size was reduced to 7 μm or less, the grain boundary cracking was sufficiently reduced.

The above-mentioned method for reducing the average particle size of these rolling oil emulsions to a predetermined value or less is achieved by using a cold-rolled material which has been subjected to annealing and pickling after hot rolling according to the present invention to significantly reduce grain boundary cracking. For the first time, a remarkably good effect is exhibited, and the effect is small by simply changing only the average particle size of the rolling oil emulsion.

(Method C) Next, the present inventors paid attention to the supply amount of the rolling oil emulsion to be supplied to the cold tandem mill, and made further studies. That is, if the supply amount of the rolling oil emulsion is reduced, the amount of the rolling oil bitten between the roll and the steel strip is reduced, and the roll and the steel strip are easily contacted,
Grain boundary cracking is reduced. However, it was not clear at all what amount of emulsion should be supplied in order to obtain gloss equal to or higher than that of a product finished by rolling with a small-diameter work roll such as a Sendzimir mill.

The present inventors carried out cold rolling while changing the supply amount of the rolling oil emulsion in various manners, and then subjected to finish annealing pickling or finish bright annealing to 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, the supply amount of the rolling oil emulsion to the conventional cold tandem mill is usually as large as 20 m ³ / min.
Although grain boundary cracking could not be sufficiently reduced, grain boundary cracking can be sufficiently reduced if it is reduced to 10 m ³ / min or less.

The above-mentioned method for reducing the supply amount of these rolling oil emulsions to a predetermined value or less is achieved by using a cold-rolled material which has been subjected to annealing and pickling after hot rolling according to the present invention to significantly reduce grain boundary cracking. For the first time, a remarkably good effect is exhibited, and the effect is small by merely changing the supply amount of the rolling oil emulsion.

[0030]

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.

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 finish bright annealing under the same conditions, followed by finish temper rolling.
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.

Tables 1 and 2 show the results of Method A described above, Tables 3 and 4 show the results of Method B, and Tables 5 and 6 show the results of Method C. Tables 1, 3, and 5
Are rolled by a cold tandem mill.
Shows a case where the sample is rolled by a tandem mill and further cold-rolled by a Sendzimir mill.

[0035]

[Table 1]

[0036]

[Table 2]

[0037]

[Table 3]

[0038]

[Table 4]

[0039]

[Table 5]

[0040]

[Table 6]

[0041]

As described above, the cold rolled stainless steel strip pretreated by the method of the present invention and then cold rolled and finished has a significantly superior surface gloss as compared with the steel strip manufactured by the conventional method. Having. In particular, when a large-diameter roll such as a cold tandem mill is used, it has excellent gloss equal to or higher than that of a roll using a small-diameter roll such as a Sendzimir mill, which has not been attained at all.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Eisuke Kawasumi 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Inside the Chiba Works (72) Inventor Katsuhiro Kobori 1-Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture (56) References JP-A-5-23704 (JP, A) JP-A-4-52006 (JP, A) JP-A-4-52008 (JP, A) JP-A-4-55500 ( JP, A) JP-A-5-23701 (JP, A) JP-A-5-50104 (JP, A) JP-A-5-329511 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , (DB name) B21B 1 / 22,3 / 02 B21B 27 / 02,45 / 02 C21D 8/02

Claims (3)

(57) [Claims] The steel strip after the completion of hot rolling is annealed, and the steel strip is successively annealed immediately after or after annealing at a steel strip temperature in the range of 850 ° C to 400 ° C at 25 ° C / sec. A stainless steel characterized by rapidly cooling at the above cooling rate to produce a cold rolled material, and cold rolling this material by a cold tandem mill while supplying a rolling oil emulsion having a viscosity of 50 cSt or less at 50 ° C. Manufacturing method of cold rolled steel strip. 2. The stainless steel strip after the completion of hot rolling is annealed, and subsequently, the steel strip is continuously cooled at a temperature of 850 ° C. to 400 ° C. in a temperature range of 850 ° C. to 400 ° C. at a rate of 25 ° C./sec. A stainless steel characterized by being rapidly cooled at the above cooling rate to produce a cold-rolled material, and cold-rolling the material by a cold tandem mill while supplying a rolling oil emulsion having an average particle size of 7 μm or less. Manufacturing method of cold rolled steel strip. 3. The stainless steel strip after the completion of the hot rolling is annealed, and subsequently, the steel strip is continuously cooled at a rate of 25 ° C./sec immediately after the annealing or at a steel strip temperature in the range of 850 ° C. to 400 ° C. A cold rolled material is manufactured by quenching at the above cooling rate, and the material is cold rolled while supplying a rolling oil emulsion at a supply rate of 10 m ³ / min or less by a cold tandem mill. Of manufacturing cold rolled stainless steel strip.