JP2532314B2 - Method for producing Cr-Ni type stainless steel thin plate excellent in surface quality and workability - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention uses a so-called synchronous continuous casting process in which there is no relative velocity difference between a slab and an inner wall surface of a mold, and the slab thickness is set close to the product thickness by using a Cr-Ni type stainless steel. The present invention relates to a method for producing a thin plate, in which the structure is refined from the cast piece stage and MnS is precipitated to produce a Cr-Ni-based stainless steel thin plate having excellent surface quality and workability.

[0002]

2. Description of the Related Art Conventionally, in order to produce a stainless steel thin plate using a continuous casting method, a casting having a thickness of 100 mm or more is cast while vibrating the mold in the casting direction, and the surface of the obtained casting is cleaned. After being heated to 1000 ° C. or higher in a heating furnace, hot rolling was performed by a hot strip mill including a row of rough rolling mills and finishing rolling mills to obtain hot strips having a thickness of several mm.

When cold rolling the hot strip thus obtained, the hot strip subjected to strong hot working is softened in order to secure the shape (flatness), material and surface quality required for the final product. The hot-rolled sheet was annealed for this purpose, and the scale on the surface was removed by grinding after the pickling step. This conventional process requires a large amount of energy for heating and processing the material in a long hot rolling facility, and it is difficult to say that the manufacturing process is excellent in terms of productivity. Also, the final product is
Since a slab with a thickness of 100 mm or more is manufactured by many processes, the texture develops, and it is necessary for the user to consider the anisotropy when pressing a product. There were many restrictions on usage.

By the way, recently, in order to solve the problem that a long hot rolling facility, a large amount of energy, and a rolling power are required to roll a slab having a thickness of 100 mm or more into a hot strip, the continuous casting has recently been performed. In the process of (1), research on a process for obtaining a slab (thin band) having a thickness equal to or close to that of hot strip is underway. For example, the paper featured in "Iron and Steel"'85,A197-'85, A256 discloses a process for directly obtaining hot strip by continuous casting. In such a continuous casting process, when the gauge of the cast piece (strip) to be obtained is in the level of 1 to 10 mm, the twin drum system is used, and the gauge of the cast piece is in the level of 20 to 50 mm. Sometimes the twin belt method is being considered.

However, it is said that there is still a problem in the casting stage in these continuous casting processes, and the problem regarding the material and surface quality of the product has not been solved yet. "CAMP ISIJ" Vol. 1, 1
988, P1670-1705, the twin drum casting, S cast in a single cold rolling process.
Since the US304 thin plate product has a fine grain structure compared to the conventional process and the elongation is low, it is stated that as a countermeasure against this, the slab is annealed at high temperature for a long time to eliminate the δ ferrite remaining in the slab. Has been.

In addition, "CAMP ISIJ" Vol. 3,
In 1990 and P770, it was reported that orange peel-like surface defects were generated, and it was reported that the γ grain size before cold rolling was reduced.

[0007]

In a process which is being developed as a new process and is premised on obtaining a slab (thin strip) having a thickness equal to or close to that of a hot strip by continuous casting. Since the steps from casting to products are simplified, the surface characteristics and workability of stainless steel sheet products are sensitively affected by the slab properties. That is, in order to obtain a product having excellent surface quality and workability, it is necessary to obtain an excellent slab.

The inventors of the present invention have already clarified a method for refining the γ grains of a slab and applied for a patent regarding a method for preventing the occurrence of surface defects called the roping phenomenon or the above-mentioned orange peel phenomenon in a new process. Has come. Japanese Patent Application No. 63-221471 Method by slab cooling and hot rolling Japanese Patent Application No. 63-169095 slab cooling and 2CR method Japanese Patent Application No. 63-221472 slab cooling and delta ferrite control Japanese Patent Application No. 63-286680 Slab cooling and δ-ferrite control Japanese Patent Application No. 1-1586 Slab cooling and addition of fine-grained elements However, the elongation and workability of thin plate products, which are the characteristics of Cr-Ni stainless steel, are insufficient only by these methods. It became clear.

The present invention has been made for the purpose of providing a simple manufacturing process capable of obtaining an austenitic stainless steel sheet having no surface defects such as roping and orange peel phenomenon and having good elongation and workability. .

[0010]

The gist of the present invention is as follows. (1) Cr-Ni represented by 18% Cr-8% Ni steel
Using a continuous casting machine in which the wall surface of the mold-based stainless steel moves in synchronization with the slab, the cooling rate during solidification is 100 ° C / s.
ec or more is continuously cast into a strip-shaped slab having a thickness of 6 mm or less, and after completion of solidification of the obtained slab, cooling is started from a high temperature to suppress reheat of the slab to 1250 ° C., Next, the product is held for 5 seconds or more and 2 minutes or less in a temperature range of 1250 to 900 ° C., then cooled and wound, annealed, pickled, and cold rolled into a product, which is a surface quality and processing. A method for producing a Cr-Ni-based stainless steel sheet having excellent properties.

(2) Cooling below 900 ° C. results in 900-6
Cooling the temperature range between 00 ° C at an average cooling rate of 10 ° C / sec or more, winding at 600 ° C or less, omitting annealing,
A method for producing a Cr-Ni-based stainless steel thin plate having excellent surface quality and workability according to the above 1, which is obtained by pickling and cold rolling to obtain a product. (3) The method for producing a Cr-Ni-based stainless steel thin plate having excellent surface quality and workability according to the above 1, wherein the solidification is cooled up to 1250 ° C at a cooling rate of 100 ° C / sec or more.

[0012]

The present inventors have found that the strip continuous casting method is used for C
As a result of examining measures for improving elongation and workability in the r-Ni stainless steel thin plate manufacturing process, in the case of a thin cast piece, Mn
S does not precipitate during cooling but finely precipitates in the final annealing after cold rolling, which strongly suppresses grain growth, forms a fine grain structure, reduces elongation, and deteriorates workability. It was confirmed that it is necessary to deposit MnS in a sufficiently large amount in the cast piece stage to render it harmless.

The present inventors have found out an efficient method of MnS precipitation as follows. That is, the slab was given the thermal history as shown in FIG. (A) is a thermal history of a slab that is air-cooled after being cast by a twin roll for experiment, (b) is a thermal history of a slab that has been strongly cooled by a water-cooling roll from immediately below the casting, (c-1)
Is the thermal history of the slab that was roll-cooled immediately below casting and retained at 1050 ° C. for 10 seconds and then cooled, (c-2) is the thermal history of the slab that was similarly retained for 30 seconds and then cooled, and (c-3) is the same. The heat history of the slab cooled after being held for 60 seconds is shown.

After descaling, these cast pieces were cold-rolled, annealed, pickled and temper-rolled to obtain 0.6 mmt products. The product characteristic results are shown in Table 1. 10 at 1050 ° C
By holding for 60 seconds, the elongation was greatly improved. This retention effect appeared in the temperature range of 1250 to 900 ° C., and the time was observed for 5 seconds or more and 2 minutes or less. This condition corresponds to the condition that MnS starts coarse precipitation during cooling of the slab.
If it exceeds 1250 ° C, MnS is hard to precipitate. Again 90
Even if the temperature is lower than 0 ° C, MnS precipitation is delayed. Even if it exceeds 2 minutes, the effect of improving elongation is saturated.

Therefore, by combining these requirements with the roping measures which have already been clarified, it is possible to provide a method for producing a Cr-Ni type stainless steel thin plate having excellent surface quality and workability. In the present invention, the cooling rate during solidification
The reason for setting 100 ° C / sec or more is that the cooling rate during solidification
This greatly affects the surface quality of the product. Cooling speed
If the degree is less than 100 ° C / sec, the crystal grains of the solidified structure
The diameter becomes coarse and the surface quality of the product deteriorates. Therefore,
Cooling rate during solidification is 100 ° C / sec or more
It is not necessary to set an upper limit on the cooling rate. Also,
Cooling rate of 100 ℃ / sec or more up to 1250 ℃ after solidification
Similarly, the reason why it was cooled down to 1250 ° C after solidification
Because the cooling rate of the product greatly affects the surface quality of the product.
It That is, grain growth is fast in the temperature range of 1250 ° C or higher.
Therefore, if it is not rapidly cooled during this period, the crystal grains will become coarse and the product
Deteriorate surface quality. Therefore, up to 1250 ° C after solidification
It is desirable that the cooling rate of
In addition, there is no need to set an upper limit on the cooling rate. Also, winding
The reason for setting the temperature below 600 ℃ is to improve the surface gloss of the product.
Because of. When wound at a temperature higher than 600 ° C,
Chromium carbide precipitates at grain boundaries during the cooling process of the foil. That
Therefore, a chromium deficient layer is formed around the chromium carbide.
If a thin slab is pickled before cold rolling, the chromium deficient layer will melt.
It becomes a corrosion groove. Cold rolling a material with such corrosion grooves
Then, corrosion grooves will remain on the product, reducing the gloss of the product.
Will be. Therefore, the winding temperature depends on the precipitation of chromium carbide.
In order to avoid this, it is necessary to keep the temperature below 600 ° C.
However, if the winding temperature is too low, the deformation resistance of the material will increase.
The coiling temperature will increase and the coiler will be heavily loaded.
It is desirable that the temperature be 300 ° C. or higher. Also, 900-6
Average cooling rate of 10 ℃ / sec or more in the temperature range between 00 ℃
The reason why we decided to cool with
The reason is the same. Cooling rate is less than 10 ° C / sec
And surface deterioration of product due to chromium carbide precipitation
It It is not necessary to set an upper limit on the cooling rate.

[0016]

【table 1】

[0017]

[Examples] Austenitic stainless steels of various components based on 18Cr-8% Ni steel shown in Table 2 were melted and cast into a slab having a thickness of 3 mm by an internal water-cooled twin-drum casting machine, Immediately after casting, the slab was cooled to 1250 ° C. by a pressing roll method, a method of bringing the slab into contact with the main drum, a roll cooling method, and a N ₂ gas blowing cooling method. Then 900
The retention was carried out within a period of 2 minutes at a temperature of ˜1250 ° C.

The slab is water-cooled from less than 900 ° C. to 600
After winding at below ℃, pickling, cold rolling, bright annealing,
The tempered rolling was performed to obtain a thin plate product, and the surface quality and material quality were evaluated. As a comparative example, cooling conditions immediately after casting, δ-F
Similarly, a thin plate product was manufactured from a slab cast under conditions in which e cal and slab retention conditions were outside the scope of the present invention, and surface quality and material evaluation were performed.

The results are shown in Table 3. 90 pieces of slab
It can be seen that the workability is significantly improved by holding the material at 0 to 1250 ° C. for 2 minutes or less, and the occurrence of roping can be suppressed by cooling from just below the solidification to 1250 ° C. Thus, the thin plates manufactured by the method of the present invention (No. 1 to 9) were excellent in workability and had good surface quality, but the thin plates manufactured by the comparative method (No. 10 to 12) were made of material (elongation) and surface. The quality (roping) was inferior.

[0020]

[Table 2]

[0021]

[Table 3]

[0022]

According to the present invention, an austenitic stainless steel thin plate excellent in surface quality and workability can be obtained by a simple process of continuously casting a thin strip-shaped slab having a thickness close to the product thickness and directly commercializing it by cold rolling. Obtainable. Therefore,
The technical effects of the present invention are extremely large in terms of economy and production purpose.

[Brief description of drawings]

FIG. 1 is a diagram showing a temperature history of a thin cast piece obtained by a continuous casting method.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hideki Oka 3434 Shimada, Hitsu-shi, Yamaguchi Pref., Nippon Steel Co., Ltd., Hikari Steel Works (72) Yuji Yoshimura 3434 Shimada, Hikari-shi, Yamaguchi Pref. Inside the Kogaku Steel Works

Claims (3)

(57) [Claims] 1. C represented by 18% Cr-8% Ni steel
The cooling rate during solidification of r-Ni stainless steel was set to 10 by a continuous casting machine in which the mold wall surface moved in synchronization with the slab.
Continuously cast into a strip-shaped slab having a thickness of 0 mm / sec or more and a thickness of 6 mm or less, and after completion of solidification of the obtained slab, cooling is started from a high temperature to suppress recuperation of the slab to 1250 ° C. Cool, then in the temperature range between 1250 and 900 ° C for 5 seconds or more,
Hold for 2 minutes or less, then cool and wind, anneal,
A method for producing a Cr-Ni-based stainless steel thin plate having excellent surface quality and workability, which comprises pickling and cold rolling to obtain a product. 2. Cooling below 900 ° C. is 900-600
The product is obtained by cooling the temperature range between 0 ° C at an average cooling rate of 10 ° C / sec or more, winding at 600 ° C or less, omitting annealing, pickling and cold rolling. 1. A method for producing a Cr-Ni-based stainless steel sheet having excellent surface quality and workability as described in 1. 3. Up to 1250 ° C. after solidification is 100 ° C./se
The cooling is performed at a cooling rate of c or more.
A method for producing a Cr-Ni-based stainless steel sheet having excellent surface quality and workability as described.