JPH0759727B2 - Method for producing austenitic stainless steel sheet - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention has no relative speed difference between a slab and a mold inner wall surface.
The present invention relates to a method for producing an austenitic stainless steel sheet by cold rolling a slab having a thickness close to the product thickness cast by a so-called synchronous continuous casting process.

[Conventional technology]

A conventional method for producing a stainless steel thin plate using a continuous casting method is to cast a cast piece having a thickness of 100 mm or more while vibrating the mold in the casting direction, and perform surface maintenance of the obtained cast piece,
After heating to 1000 ° C. or higher in a heating furnace, hot rolling is performed by a hot strip mill consisting of a rough rolling mill and a finishing rolling mill row to form hot strips having a thickness of several mm, and after annealing if necessary, The final annealing was performed by scaling and cold rolling.

In such a conventional process, in order to hot-roll a slab with a thickness of 100 mm or more, a long hot strip mill is required, and a problem of using a large amount of energy for heating and rolling the slab was there.

For this problem, research on a process for producing a slab having a thickness equal to or close to that of a hot strip by continuous casting is under way. For example, "Iron and Steel"'8
It is a synchronous continuous casting process with no relative speed difference between the slab and the inner wall of the mold, such as the twin roll method and the twin belt method, as introduced in the paper featured in 5-A197 to '85 -A256. .

However, there have been unsolved problems in producing stainless steel sheet products through these synchronous continuous casting processes.

[Problems to be Solved by the Invention]

When a stainless steel sheet is manufactured by adopting the process of continuously casting a slab (thin strip slab) with a thickness equal to or close to that of hot strip, which is being developed as a new process, Since the process up to the product is shortened, there is a problem in the surface properties of the product.

The present inventors cast molten steel of SUS304, which is a typical austenitic stainless steel, into a strip-shaped slab with a plate thickness of 1 to 5 mm by casting with a twin roll continuous casting machine of an internal water cooling system, and cold rolling this. After that, a part was annealed and pickled to give a 2B product, and a part was bright annealed to give a BA product. Also, thickness 10
A continuously cast slab of 0 mm or more was hot-rolled and then cold-rolled to produce a 2B product and a BA product. As a result of a detailed comparison of the surface properties of these products, the products when using the twin roll continuous casting machine showed the characteristic fine crepe-like undulation (unevenness) and uneven gloss. It was In addition, when the product was processed, fine skin roughness called orange peel occurred.

The present invention, by cold rolling a strip-shaped slab having a thickness close to the product thickness cast by a synchronous continuous casting process with no relative velocity difference between the slab and the mold inner wall surface, an austenitic stainless steel thin plate is obtained. During manufacturing, it is possible to obtain a product that is free from defects such as roping and uneven gloss mentioned above that are characteristic of this process, has no problem in surface quality, and does not cause orange peel after processing. It is intended to provide a simple manufacturing method.

[Means and actions for solving the problems]

 The gist of the present invention is as follows.

(1) When casting austenitic stainless steel into a strip-shaped slab by a continuous casting machine in which the mold wall surface moves in synchronism with the slab, the thickness of the slab is set to 10 mm or less and 1200 ° C from the start of solidification. Austenitic stainless steel characterized by cold rolling at a cooling rate of 50 ° C / sec or more and the obtained strip-shaped slab is cold-rolled using a roll having a surface hardness of Vickers hardness of 600 or more. Method for manufacturing thin plate.

(2) When casting austenitic stainless steel into a strip-shaped slab by a continuous casting machine in which the mold wall surface moves in synchronization with the slab, the thickness of the slab is set to 10 mm or less, and 1200 ° C from the start of solidification. At a cooling rate of 50 ° C / sec or more,
Cool the temperature range of 900 ~ 550 ℃ at a cooling rate of 10 ℃ / sec or more, descale the obtained thin strip slab by pickling, cool it using a roll with a Vickers hardness of 600 or more. A method for producing an austenitic stainless steel thin plate, which comprises hot rolling.

(3) When casting austenitic stainless steel into a strip-shaped slab by a continuous casting machine in which the wall surface of the mold moves in synchronization with the slab, the thickness of the slab is set to 10 mm or less, and 1200 ° C from the start of solidification. Up to 50 ° C / sec or more, the obtained strip-shaped slab is annealed in the temperature range of 1050 ° C or more, and the temperature range of 900 to 550 ° C is cooled in the cooling rate of 10 ° C / sec or more. A method for producing an austenitic stainless steel thin plate, which comprises descaling by pickling and cold rolling using a roll having a Vickers hardness of 600 or more.

In the present invention, the continuous casting machine in which the mold wall surface moves in synchronization with the slab is a single roll method, a twin roll method, an internal method as described in "Iron and Steel"'85 -A200 to '85 -A203. This is a continuous casting machine that performs the ring method, roll belt method, twin belt method, moving mold continuous casting method, spray roll method, and the like.

The present inventors have found that the products produced by cold-rolling thin strip-shaped strips of austenitic stainless steel cast by a twin roll continuous casting machine have the unique roping, orange peel and uneven luster as described above. The elucidation of the cause of the occurrence and the solution was clarified.

That is, roping and orange peel are elucidated to occur during cold rolling because the γ grains of the material before cold rolling are large, and the γ grains of thin strip slabs are refined by appropriate casting conditions and cooling conditions. , Solved by using hard rolls in cold rolling. Also,
Gloss unevenness is caused by intergranular corrosion due to precipitation of Cr carbides on the surface of the material pickled before cold rolling, and the size of crystal grains on the material surface before cold rolling is unevenly distributed. It is solved by pickling what is cooled in the Cr carbide precipitation temperature range under proper conditions during cooling of the slab during casting or cooling after annealing the slab during casting. It was Then, these causes and solving means are not limited to the case of using the twin roll continuous casting machine, but are also applied to the case of undergoing various continuous casting processes in which the mold wall surface moves in synchronization with the slab.

The constituent features of the present invention will be described below together with the operation.

First, in order to obtain a thin strip slab having fine γ grains by the continuous casting machine, the thickness of the slab is set to 10 mm or less, and cooling from the start of solidification to 1200 ° C at a cooling rate of 50 ° C / sec or more. It was decided to. If this cooling rate is less than 50 ° C / sec,
The γ grains of the obtained slab become coarse, and it becomes difficult to suppress the occurrence of roping even when cold rolling using a hard roll as described below. After the temperature of the slab becomes lower than 1200 ° C, slow cooling of less than 50 ° C / sec may be performed. Further, if the thickness of the slab exceeds 10 mm, it becomes industrially difficult to set the cooling rate to 50 ° C./sec or more. The casting conditions in the method of the present invention can be industrially made by appropriately selecting the cooling means for the mold and the slab, the cooling medium, and the like. The strip-shaped slab cast under these conditions has a γ grain size of 100 μm in average diameter.
Hereinafter, the fine particles have a particle size number of 4 or more.

Next, when cold rolling the strip-shaped slab, in order to suppress the occurrence of roping, it was decided to use a hard roll having a surface hardness of 600 or more Vickers hardness. Vickers hardness
If a soft roll of less than 600 is used, it will be difficult to suppress the occurrence of roping even with a thin strip slab cast under the above conditions. When the cold rolling is performed twice or more with the intermediate annealing interposed, a hard roll having a Vickers hardness of 600 or more may be used in the first cold rolling. This is because in the second and subsequent cold rolling, recrystallization is refined by intermediate annealing. Incidentally, the cold rolling in the present invention,
What is necessary is just to carry out in the temperature range of the range which does not produce coloring by oxidation, and what is called temperature rolling may be sufficient. After cold rolling to the product thickness, it is made into products such as 2B products and BA products by known means.

According to the method of claim (1), a strip-shaped slab having a thickness of 10 mm or less, which is solidified and cooled under the above conditions, is surface-conditioned as necessary and cold-rolled. The surface is adjusted by grinding, polishing, shot blasting, particle spraying with high-pressure water, brushing, light reduction rolling, or pickling using an acid solution with a small difference in dissolution rate due to the Cr content in the material. , Defects, irregularities, scales, etc. on the surface of the slab should be such that cold rolling is not hindered. Further, the slab may be annealed.

According to the method of claim (1), since the γ grains of the strip-shaped cast piece are fine, the wrinkle-like waviness of the material surface which tends to occur during cold rolling is small, and the occurrence of waviness is suppressed by the hard roll. Therefore, roping does not occur. Further, since the γ grain before cold rolling is fine, the product after rolling also becomes fine grain, and orange peel does not occur when the product is processed. Furthermore, since pickling that causes intergranular corrosion is not performed, uneven gloss does not occur even if the cooling conditions after casting, or when annealing the cast slab, are not limited to the cooling conditions during annealing.

According to the method of claim (2), when cooling the cast strip-shaped slab, the temperature range of 900 to 550 ° C. is cooled at a cooling rate of 10 ° C./sec or more, descaling by pickling and cold rolling. To do.
As a pretreatment for pickling, the above-mentioned surface adjusting means such as shot blasting and particle spraying with high-pressure water may be performed.
The cooling conditions for the thin strip slab were limited to prevent the precipitation of Cr carbide at the grain boundaries.

According to the method of claim (2), roping does not occur due to the same operation as in the method of claim (1), and uneven gloss does not occur due to the following operation. Descaling of austenitic stainless steel before cold rolling is generally performed by pickling with nitric hydrofluoric acid.Since nitric hydrofluoric acid has a large difference in dissolution rate depending on the Cr content in the material,
If Cr carbide is precipitated, intergranular corrosion is likely to occur. When the thin strip-shaped slab targeted by the present invention is cold-rolled, intergranular corrosion sensitively affects uneven glossiness, but since there is no precipitation of Cr carbide under the above cooling conditions after casting, this is not a concern.

According to the method of claim (3), the cast thin strip slab is 1050
Annealed in the temperature range of ℃ or more, 900 ~ 550 ℃ temperature range 10 ℃ / s
Cool at a cooling rate of ec or higher, descale by pickling and cold-roll. The pretreatment of pickling may be performed as in the method of claim (2). The annealing temperature is 1050 ℃ to reduce the δ ferrite remaining in the strip cast as much as possible.
That's it. The δ ferrite phase is reduced by annealing after cold rolling, but since it adversely affects the workability and corrosion resistance of the product, it is reduced in advance at the stage of casting. The cooling conditions were limited for the same reason as in the method of claim (2).

According to the method of claim (3), roping does not occur due to the same operation as in the method of claim (1), and uneven gloss does not occur due to the same operation as in the method of claim (2). Moreover, since the cast strip-shaped slab is annealed, the amount of δ-ferrite remaining in the product is significantly reduced, and the workability and corrosion resistance are improved.

〔Example〕

Four types of austenitic stainless steels having the components shown in Table 1 were cast into strip-shaped slabs by an internal water-cooled vertical twin roll continuous casting machine and cooled and rolled to produce thin plate products. Table 2 shows the surface characteristics of thin plate products manufactured by variously changing the thickness of the slab, the casting conditions, the cooling conditions, the roll surface hardness in cold rolling, and the like.

In Table 2, the cooling rate from the start of solidification to 1200 ° C is changed by roll cooling or water spray cooling of the slab that has left the twin rolls, and the cooling rate between 900 and 550 ° C is changed by water spray cooling. It was The surface hardness of the cold-rolled roll is changed depending on the material of the roll, etc., Hv1200 or higher is tungsten carbide, Hv1000 is SKD hard Cr plated, Hv920 to 650 is SKH, and Hv550 or lower is SKD. The roping of the product was determined by the height of the waviness measured by a roughness meter, and the gloss was determined by visual observation.

In Nos. 1 to 6 of the present invention examples, the average γ grain size before cold rolling was 100 μm or less, and cold rolling was performed using a hard roll having a Vickers hardness of 600 or more, so that roping occurred. There wasn't. It should be noted that those having a waviness height of 0.2 μm or less shown in the roping column of Table 2 are not judged to be roping and there is no problem as a product. Further, in the examples of the present invention, the surface gloss was good and uneven gloss was not recognized.

On the other hand, in Comparative Examples Nos. 7 and 8, although the slab cooling conditions were appropriate and the γ grain size before cold rolling was 100 μm or less,
Since the cold-rolled roll was soft, roping occurred. In Nos. 9 and 10, the rolls of the hot rolling were hard, but roping occurred because the cooling rate of the slab was slow and the γ grains were large. In Nos. 9 to 12, the cooling rate between 900 and 550 ° C was slow, and therefore intergranular corrosion occurred and uneven gloss occurred due to the nitric hydrofluoric acid pickling.

FIG. 1 shows the surface roughness profile of a typical product with or without roping in the direction perpendicular to the rolling direction. Figure 1 (a) is an example of roping, and the swell height is 0.5.
μm (No. 9 in Table 2), FIG. 1 (b) is an example without roping, and the swell height is 0.15 μm (No. 4 in Table 2).

[Effects of the Invention] According to the method of the present invention, when producing an austenitic stainless steel thin plate by cold rolling a thin strip-shaped slab having a thickness close to that of a continuously cast product, a total amount from the slab to the product is obtained. Since the problem of surface quality caused by the small amount of reduction is solved, a hot strip mill becomes unnecessary, and a great effect can be obtained in shortening the process and saving energy. Further, since the total amount of reduction is small, the development of the texture by rolling is suppressed, and there is also an effect that the earrings that occur when the product is drawn are improved.

[Brief description of drawings]

 FIG. 1 is a roping surface roughness profile.

Claims (3)

[Claims] 1. When casting an austenitic stainless steel into a strip-shaped cast piece by a continuous casting machine in which the mold wall surface moves in synchronization with the cast piece, the cast piece has a thickness of 10 mm or less,
In addition, it is characterized by cooling from the start of solidification to 1200 ° C at a cooling rate of 50 ° C / sec or more, and cold-rolling the obtained strip-shaped slab using a roll having a surface hardness of 600 or more Vickers hardness. And a method for producing an austenitic stainless steel sheet. 2. When casting austenitic stainless steel into a strip-shaped cast piece by a continuous casting machine in which the mold wall surface moves in synchronization with the cast piece, the cast piece has a thickness of 10 mm or less,
In addition, from the start of solidification to 1200 ° C at a cooling rate of 50 ° C / sec or more, and in the temperature range of 900 to 550 ° C at a cooling rate of 10 ° C / sec or more, the thin strip-shaped slab obtained is subjected to descaling. A method for producing an austenitic stainless steel thin plate, which comprises scaling and cold rolling using a roll having a Vickers hardness of 600 or more. 3. When casting austenitic stainless steel into a strip-shaped cast piece by a continuous casting machine in which the mold wall surface moves in synchronization with the cast piece, the cast piece has a thickness of 10 mm or less,
Moreover, from the start of solidification to 1200 ° C, it is cooled at a cooling rate of 50 ° C / sec or more, the obtained thin strip-shaped slab is annealed in the temperature range of 1050 ° C or more, and the temperature range of 900 to 550 ° C is 10 ° C / sec. A method for producing an austenitic stainless steel thin plate, which comprises cooling at the above cooling rate, descaling by pickling, and cold rolling using a roll having a surface hardness of 600 or more Vickers hardness.