JPH06299239A - Manufacture of austenitic stainless steel sheet having excellent polishability - Google Patents

Abstract

PURPOSE:To provide a method for manufacturing a sheet product having excellent buff-polishability and mirror-polishability, in the method for manufacturing an austenitic stainless steel strip product by a synchronous type continuous casting process having no relative velocity difference between a cast slab and the inner wall surface of a mold. CONSTITUTION:The molten steel adjusted to the composition having >=6% the value of delta-Fecal defined with 3(Cr+1.5Si+Mo+Nb)-2.8(Ni+0.5Cu+0.5Mn+30 C+30N)-19.8, is cast into the cast strip by the continuous casting synchronously shifting the mold wall surface and the cast slab. Successively, by using rolls having surface roughness of 0.2mum the max. roughness (Rt), the finish cold-rolling having >=5% rolling ratio per one pass is executed at >=2 passes having >=15% accumulated rolling ratio. Further, annealing, pickling or bright annealing treatment are applied and successively, skin pass rolling having 1-2.5% is executed. By this method, the steel strip having excellent surface quality after polishing is obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention cools a stainless steel thin strip slab having a thickness close to the product thickness produced by a synchronous continuous casting process in which there is no relative speed difference between the slab and the inner wall surface of the mold. The present invention relates to a method for producing a thin plate product excellent in buffing property and mirror surface property by hot rolling.

[0002]

2. Description of the Related Art Synchronous continuous casting processes include twin roll method, twin belt method, as described in, for example, a paper featured in "Iron and Steel"'85 -A197 to A256.
It is a synchronous continuous casting process such as a single roll method in which there is no relative speed difference between the slab and the inner wall surface of the mold. The twin roll type continuous casting method, which is one of these synchronous type continuous casting processes, is a continuous casting mold composed of a pair of parallel or inclined cooling rolls of the same diameter or different diameters and side dams that seal both end surfaces thereof. Molten steel is injected into the inside of the chill rolls to generate solidified shells on the circumferential surfaces of both chill rolls, and the solidified shells are united near the closest position (king point) of the rotating chill rolls to form an integrated ribbon. It is a continuous casting method in which it is sent as a slab.

For example, a thin strip slab produced by the twin roll type continuous casting method has a thickness of several mm (usually about 1 to 10 mm).
Therefore, it is possible to manufacture a thin plate product by performing cold rolling without performing hot rolling. Therefore, compared with a manufacturing method (slab slab-hot rolling process) in which a hot-rolling slab having a thickness of 100 mm or more is cast by continuous casting using a vibration mold or the like, and this is hot-rolled and then cold-rolled. Therefore, the production efficiency and the cost are significantly improved.

As the finish of the austenitic stainless steel thin plate by cold rolling, annealing / pickling finish (2B product) or bright annealing finish (BA product) is widely used. Buffing or mirror polishing of these is widely used. Materials with excellent properties are required for building materials and kitchen applications. It is expected that the twin roll type continuous casting method will be applied to the production thereof to bring about higher economic effect.

However, when the present inventor applies a twin roll type continuous casting process to the production of an austenitic stainless steel thin plate, surface irregularities (hereinafter referred to as roping) occur in the cold rolled product along the rolling direction, and polishing occurs. It was found that the image clarity afterward was lowered. As a conventional roping prevention method, there are JP-A-2-133522 and JP-A-2-13.
Japanese Patent No. 3529 discloses a method of reducing the average γ particle diameter of a cast piece. Also, Japanese Patent Laid-Open No. 12402401/1990
In Japanese Patent Laid-Open No. 8 and Japanese Patent Laid-Open No. 3-193246,
A method of increasing the work roll (hardness, Young's modulus) during cold rolling is disclosed. These methods are elemental technologies necessary for improving roping, but they cannot be said to be necessary and sufficient countermeasure technologies for achieving a surface quality that satisfies the required polishability.

[0006]

SUMMARY OF THE INVENTION The present invention is a method for producing an austenitic stainless steel sheet having improved polishability by a so-called synchronous continuous casting process in which there is no relative speed difference between the slab and the inner wall surface of the mold. The purpose is to provide.

[0007]

The present invention provides 3 (Cr +
1.5Si + Mo + Nb) -2.8 (Ni + 0.5Cu)
+ 0.5Mn + 30C + 30N) -19.8 defined as δ-Fe _cal. (%) Value of 6% or more of molten steel, the wall surface of the mold moves in synchronism with the slab to form a thin strip. A slab is cast, and then a roll having a surface roughness of maximum roughness (Rt) specified by JIS B601 of 0.2 μm or less is used, and the rolling rate per pass is 5% or more and the cumulative rolling rate is 5% or more. It is achieved by performing two or more passes of finish cold rolling for 15% or more, followed by annealing / pickling or bright annealing treatment, and then performing temper rolling of 1 to 2.5%.

[0008]

The present inventors have found that when coarse columnar γ grains are formed in the cast slab, roping occurs in the cold rolled product and the image clarity after buffing or mirror polishing is significantly impaired. . As an example, FIG. 1 shows the effect of the roping height on the image clarity after buffing of a 2B product (JIS H 8686, conforming to the image definition measuring method C ₂ , skewer width 1 mm, measured in the direction perpendicular to the rolling direction). Show. As is clear from the figure, it can be said that the lower the roping height, the higher the image definition and the better the image clarity. This tendency also applies to the mirror surface polishing material. The occurrence of roping is caused by the difference in the amount of deformation between crystal grains during cold rolling, and is considered to appear as a variation in the amount of deformation in the plate thickness direction. Therefore, in order to suppress roping, it is desirable that each crystal grain has a random orientation and is made a fine grain.

The present inventors have studied in detail the relationship between the solidification structure and the components in the non-equilibrium state in the case of rapid solidification by the twin roll continuous casting method, etc.
By setting the value of e _{ca l.} (%) to 6% or more, an equiaxed fine solidification structure called "F mode" can be obtained, but if the molten steel composition is out of the specified range, "FA mode" It was found that a coarse solidification structure of columnar crystals, which is referred to as, is formed. Fig. 2 shows 1/4 of "F mode" and "FA mode" solidification
The texture and the γ grain trace diagram shown in the (200) pole figure of the t layer are shown. The "F-mode" solidification structure is a more random orientation and a fine γ-grain structure, which is clearly a more desirable solidification structure than the "FA-mode" solidification structure. However,
Even though the "F mode" solidification structure is a coarse structure compared with the structure of the conventional process material annealed after hot rolling, it must be supplemented by a new rolling method.

It can be understood that the roping phenomenon during rolling occurs because the deformation resistance of the material during cold rolling overcomes the deformation resistance of the rolling rolls unevenly. We have
As a factor causing this non-uniform deformation, it was considered that the oil film drawn into the rolling roll was uniform, that is, the roll roughness was important. Generally, from the viewpoint of productivity, it is desirable that the frequency of replacement of rolling rolls is minimized, and as the rolling rolls, rolls with a rough surface roughness are used from the initial stage to before finishing rolling. Rolls are used. For the purpose of improving the roping observed in the rapidly solidified material, there is no prior art that examined these rolling conditions. As a result of studying the influence of roll roughness and cumulative rolling ratio in finish rolling on roping, the present inventors have determined the roll roughness of finish rolling to be the maximum roughness (R
t) 0.2 μm or less, rolling rate per pass is 5%
As described above, it has been clarified that performing finish rolling of two or more passes with a cumulative rolling ratio of 15% or more is effective for improving roping.

The purpose of performing finish rolling of two or more passes is 1
Adjusting the roughness up to the previous stage in the pass, uniformly pulling the oil film in the second and subsequent passes, suppressing roping, and reducing "scratch flaws" caused by remaining rough rolls in the previous stage. is there. If the rolling ratio in the first pass is less than 5%, the roughness of the rolls up to the preceding stage remains partially and "scratch flaws" are likely to occur, which is particularly problematic as the surface quality of the bright annealed finish. This "scratch flaw" occurs linearly and particularly affects the difference in image definition between the rolling direction after polishing and the direction perpendicular to the rolling direction. Also, the cumulative rolling rate is 15%
If it is less than 1, the improvement of roping is not sufficient and the image clarity after polishing is poor. When the roll roughness to be used is the same, the roll and the material have the same roughness in two or more passes, and therefore the effect of improving roping and "scratch flaws" is saturated when the number of passes is further increased. However, if finishing rolls are made into multiple passes by using finer rolls, it is effective in improving roping and "scratch flaws", but frequent replacement of rolls poses a problem from the viewpoint of productivity, and it is necessary to finish in two passes. Is desirable.

In addition to the γ grain refining technique by controlling the solidification mode during the production of the slab and the roping improvement technique by reducing the roll roughness in cold rolling, the present inventors further As a result of a detailed study on the improvement of the surface quality, it was found that the improvement of roping was remarkable by increasing the temper rolling rate in the refining process. That is, if the temper rolling rate is 1% or more in terms of elongation, the roping level is further improved. But,
If it exceeds 2.5%, the mechanical properties (proof strength, elongation, etc.) are greatly deteriorated, so that the applicable applications are limited.

In the present invention, it is more effective to combine these improving techniques in order to secure stable and excellent surface quality including the variation of the plate thickness in the casting direction and the plate width direction. As a result, it is possible to stably obtain a cold rolled product having excellent abrasiveness.

[0014]

[Example] By the components shown in Table 1, δ-Fe _cal.
Austenitic stainless steel varied in the range of 10% was cast into a strip-shaped slab having a thickness of 3 mm by a vertical twin roll continuous casting machine with an internal water cooling system. The solidified structure of the obtained slab was observed, and after the diffusive heat treatment of δ ferrite at 1200 ° C. for 1 min, the γ grain size was converted into a sphere-equivalent diameter for measurement.

[0015]

[Table 1]

Then, descaling is performed and cold rolling is performed.
The plate thickness was 1 mm. For finish rolling, a roll having a maximum roughness (Rt) of 2 to 4 μm is used from the initial pass to the final two passes, and the maximum roughness (Rt) of 0.15 to 0.
Finishing was performed in 2 passes with a roll of 4 μm, and the cumulative rolling rate in the final 2 passes was changed to 10 to 30%. After rolling, after air annealing / pickling or bright annealing, the temper rolling ratio was changed in the range of 0.3 to 2.5% to obtain 2B and BA products.

2B material (atmosphere annealing / pickling) was # 400 buff polished, and BA material (bright annealing) was mirror polished, and the quality of the surface quality after polishing was evaluated by image sharpness according to JIS H8686. did. The results are shown in Tables 2 and 3.

[0018]

[Table 2]

[0019]

[Table 3]

Δ-Fe _cal. Value, finish rolling roll roughness
Sample No. 1 having both the cumulative rolling ratio and the temper rolling ratio within the range of the present invention. 1 to 8 had good image clarity after polishing. On the other hand, the sample No. in which at least one of the value of δ-Fe _cal. , Finish rolling roll roughness / cumulative rolling ratio, and temper rolling ratio is outside the range of the present invention. Nos. 9 to 17 had poor image definition after polishing.

[0021]

As described above, according to the present invention,
An austenitic stainless steel thin plate having excellent surface quality after polishing can be stably manufactured at low cost.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between roping and image definition after buffing.

FIG. 2 is a diagram tracing the texture and γ grains of the (200) pole figure in the ¼t layer of the cast slab.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Koichiro Ishimaru 3434 Shimada, Hikari City, Yamaguchi Prefecture Shin Nippon Steel Co., Ltd.

Claims (1)

[Claims] 1. 3 (Cr + 1.5Si + Mo + Nb)-
2.8 (Ni + 0.5Cu + 0.5Mn + 30C + 30
N) -19.8 defined δ-Fe _cal. (%) Value adjusted to a composition of 6% or more, by continuous casting in which the wall surface of the mold moves in synchronization with the cast slab And then using a roll having a surface roughness with a maximum roughness (Rt) of 0.2 μm or less, two passes in which the rolling rate per pass is 5% or more and the cumulative rolling rate is 15% or more. Austenitic stainless steel with excellent polishability, characterized by performing the above-mentioned finish cold rolling, followed by annealing / pickling or bright annealing treatment, and subsequently performing 1 to 2.5% temper rolling. Method for manufacturing thin plate.