JP3138487B2 - Manufacturing method of austenitic stainless steel sheet - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to an austenitic stainless steel sheet having excellent surface properties by subjecting a ribbon cast by a twin-roll type thin plate direct casting process to a pretreatment by a combination of light reduction and diffusion heat treatment. The present invention relates to a method for manufacturing a steel sheet.

[0002]

2. Description of the Related Art Conventionally, a thin stainless steel sheet is manufactured by subjecting a slab having a thickness of 100 mm or more obtained by continuous casting to a hot strip having a thickness of several mm by first hot rolling and then cold rolling. I was On the other hand, recently, such a stainless steel sheet has been cast directly from molten steel into a strip having a thickness equal to or close to the above-mentioned hot strip, and the cast strip is cold-rolled to form a thin steel sheet. Manufacturing techniques have been developed and have been widely adopted because of the advantages of simplification of the process and associated cost reduction.

However, in the case of this new development method,
Compared to the known methods up to that time, since the hot rolling step is omitted from the process from casting to the product,
Since the internal quality and the surface quality of the product are not sufficiently controlled and commercialized, it has been difficult to obtain a thin steel sheet having excellent quality. Nevertheless, in the past, it has been recognized that the internal quality of a thin steel sheet is considerably improved, and that the quality of the steel sheet is equal to or higher than that of a product that has been subjected to a conventional ordinary continuous casting-hot rolling process. However, regarding the surface properties of the thin steel sheet, since the cast surface is transferred as it is, there are still many problems, and improvement thereof has been desired.

That is, undesired surface defects observed on the surface of a thin steel sheet include gloss unevenness and fine waviness (roping). These surface defects are phenomena peculiar to only the so-called direct casting process, which are not seen at all in thin steel sheets manufactured by conventional methods, and must be eliminated for commercialization.

[0005] In view of such a problem, Japanese Unexamined Patent Publication No.
-240218 and JP-A-2-240219, a method for reducing surface defects, that is, the above-mentioned gloss unevenness and roping, for example, solution treatment of a ribbon cast to 10 mm or less in a temperature range of 1050 ° C. or more. Temperature range from 900 to 550 ° C to 10 ° C
/ Sec or more, then descaling, cold rolling, and final annealing. Then, a method of further performing temper rolling at an elongation of 0.5 to 2.5% is proposed.

[0006]

However, in these prior arts, since the rolling reduction in the initial cold rolling exceeds 50%, colonies caused by the non-uniformity of the solidification structure of the ribbon are elongated, On the contrary, there is a problem that gloss unevenness becomes clear. Further, although temper rolling is performed in the final step, problems such as a slight loss of elongation and formability have arisen, and it is necessary to solve them.

SUMMARY OF THE INVENTION An object of the present invention is to provide a thin steel sheet by cold rolling a thin strip cast by a twin-roll type thin sheet direct casting process, without the need for temper rolling, and to reduce gloss unevenness. An object of the present invention is to establish a thin steel sheet manufacturing technology capable of completely eliminating surface defects.

[0008]

As a result of intensive studies to realize the above-mentioned object, the present inventors have newly found that the generation of the surface defects is caused by a non-uniformly solidified structure of the cast ribbon. The present inventor has studied various methods of treating a cast ribbon effective in eliminating this non-uniform solidification structure, and has arrived at a manufacturing method as described below.

That is, the present invention firstly casts an austenitic stainless steel ribbon having a thickness of 10 mm or less by a twin-roll type thin plate direct casting machine, descales the ribbon, and then reduces the rolling reduction by 5 to 20%. %, And then maintained at an α-γ transition temperature of 1200 ° C. or more for 3 minutes to 30 minutes, and then processed and heat-treated according to a conventional method to obtain a product. An austenitic stainless steel ribbon with a thickness of 10 mm or less is cast by a twin-roll type thin plate direct casting machine, and this ribbon is heated to an α-γ transition temperature of 1200 ° C or more for 30 minutes to
This is a method for producing an austenitic stainless steel sheet, wherein the product is subjected to processing and heat treatment according to a conventional method to be kept for 2 hours and then processed into a product.

[0010]

The present inventors have investigated the causes of surface defects, such as gloss unevenness, which are observed in thin steel sheets produced by cold rolling austenitic stainless steel strips cast by a twin-roll type thin sheet direct casting machine. The phenomena have been almost clarified and the solution has been clarified.

[0011] Among the surface defects, it has been found that gloss unevenness is particularly caused by non-uniformity of the solidified structure of the ribbon at the time of casting.
That is, when the cast ribbon obtained by the direct casting method is closely observed, the solidified structure on the surface thereof is divided into two types: a δ-ferrite-rich structure in which dendrites predominate and an austenite-rich structure in which dendrites are unclear. It was found that there were two coagulated tissues. The δ-ferrite rich structure in which the dendrite is dominant, even after annealing after cold rolling, δ-ferrite remains and has a fine ditch-like structure on the pickled surface and exhibits a dull luster, The austenite-rich structure in which dendrites appeared indistinctly did not show any remaining δ-ferrite, had a rough ditch-like structure, and exhibited strong luster.

Therefore, in the present invention, such a non-uniformly solidified structure is subjected to a pretreatment by a combination of cold rolling under light pressure and annealing (diffusion heat treatment) for the purpose of diffusion on the cast ribbon. Thus, the generation of surface defects such as gloss unevenness is prevented.

That is, according to the present invention, the cast ribbon is immediately subjected to cold rolling and / or diffusion heat treatment under light pressure, whereby the dendrite structure disappears and the residual δ-ferrite structure becomes an austenitic single phase structure. This eliminated the cause of uneven gloss.

According to such a method, since the rolling reduction at the time of the initial cold rolling is as small as 5 to 20%, it is possible to overcome the drawbacks in the prior art that the elongation and the formability are slightly impaired. . Hereinafter, the production method of the present invention will be described in more detail.

In the present invention, first, a ribbon is cast.
This ribbon must be less than 10 mm thick.
When a ribbon with a thickness exceeding 10 mm is cast by a twin-roll type thin plate direct casting machine, the coarsening of austenite grains (γ grains) after solidification becomes remarkable, and the product after cold rolling is called orange peel. This is because fine skin roughness occurs, which is not preferable. Incidentally, the diameter of the γ-particles in the ribbon is desirably 50 μm or less in average particle diameter.

Next, according to the present invention, cold rolling is performed on such a ribbon as a first method. This cold rolling is intended to be light reduction and must have a reduction of 5 to 20%. When performed at a large rolling reduction, colonies (dendritic-dominant δ-ferrite-rich structure) caused by unevenness of the solidified structure described above are elongated and stretched,
This is because it is difficult to eliminate the sapphire even in the subsequent steps.

In this regard, stretching of the colony can be suppressed by restricting the initial processing applied to the ribbon to a reduction of 5 to 20%.

[0018] In this method, α
-Diffusion heat treatment is performed for 3 to 30 minutes at the γ transition temperature.
By this treatment, the γ-transformation of δ-ferrite is promoted, and colonies caused by the unevenness of the solidified structure observed at the beginning can be completely eliminated. In this case,
Due to the reduction, the strain introduced near the surface becomes δ−
It is considered to be the driving force for the γ transformation of ferrite.

Here, the reason why the rolling reduction at the time of the first cold rolling added to the ribbon is in the range of 5 to 20% is that when the rolling reduction is less than 5%, less distortion is introduced near the surface. , Δ-
This is because γ transformation of ferrite is not performed sufficiently,
%, The effect of colony elongation due to the unevenness of the coagulated structure is caused, and it is not effective in improving the surface gloss unevenness.

The reason why the temperature of the diffusion heat treatment is set to 1200 ° C. or more is to complete the diffusion treatment of component segregation. The reason why the retention time of the diffusion heat treatment is limited to 3 to 30 minutes is that if less than 3 minutes, γ-transformation of δ-ferrite is not completed, and if it exceeds 30 minutes, orange particles are formed due to coarsening of γ grains. This is because fine skin roughness called peel occurs.

Next, the second method of the present invention is to immediately raise the cast ribbon to an α-γ transition temperature of 1200 ° C. or more for 30 minutes or less.
Diffusion heat treatment for 2 hours is performed. By this treatment, colonies caused by unevenness of the coagulated structure can be eliminated, and uneven gloss can be improved.

Here, the retention time of the diffusion heat treatment is set to 30 minutes to 2 minutes.
The reason for the time is that if the time is less than 30 minutes, the strain introduced into the vicinity of the surface by the processing in the first method is not introduced by the present method, so that the γ transformation of δ-ferrite is insufficient, and If it exceeds, the gamma grains are coarsened and fine skin roughness called orange peel occurs at the time of commercialization.

In the case of this method, since the cast ribbon is immediately placed in a heat-retaining furnace, the loss of energy and cost is small, and the solution of Cr carbide can be promoted.

[0024]

EXAMPLES Austenitic stainless steels having the three component compositions shown in Table 1 were cast by a twin-roll type thin plate direct casting machine to obtain a 2 mm-thick ribbon.

This continuous cast strip is subjected to initial cold rolling and diffusion heat treatment satisfying the conditions defined in the present invention (1, 2).
2) A method of immediately performing a diffusion heat treatment (3, 4) and a comparative method of performing an initial cold rolling or a diffusion heat treatment outside the conditions defined in the present invention. To produce a thin steel sheet product.

Table 2 shows the results on the presence or absence of unevenness in the gloss of pickled skin and the occurrence of orange peel of thin steel sheet products manufactured by variously changing the initial cold rolling conditions and heat treatment conditions under each of the above methods. Show. In Table 2, the occurrence of gloss unevenness and orange peel was determined by visual observation.

[0027]

[Table 1] C to N: wt% O: ppm

[0028]

[Table 2]

As is evident from the results shown in Table 2, none of the methods 1 to 4 of the present invention showed uneven surface gloss and no orange peel, and showed good surface properties. Moreover, in the method of the present invention, elongation and formability were not impaired because the pretreatment was performed so that the structure was uniform even when temper rolling and main processing were performed.

On the other hand, in Comparative Methods 5 to 7, gloss unevenness was strongly generated, and in addition, orange peel was partially roughened. That is, in the case of the comparison methods 5 and 6,
Since the initial cold rolling reduction was as high as 50%, the structure caused by uneven gloss was elongated, and did not disappear even after the subsequent heat treatment, and uneven gloss occurred. In the case of Comparative Method 6, since the initial cold rolling reduction was as high as 50% and the heat treatment time exceeded 30 minutes, the gamma grains became coarse and the orange peel became rough. In the case of the comparative method 7, the heat treatment temperature is 12
Since the temperature was lower than 00 ° C., the diffusion treatment of the component segregation became insufficient, and the structure caused by the uneven gloss did not disappear, resulting in uneven gloss.

[0031]

As described above, according to the present invention,
When the strip obtained by twin-roll type thin plate direct casting is cold rolled to produce a product, by performing light reduction rolling and diffusion heat treatment in advance, the uniformity of the strip casting structure is achieved.
An austenitic stainless steel sheet having good surface properties without uneven gloss can be manufactured without deteriorating internal quality.

Further, according to the present invention, there is an effect of improving elongation and formability by suppressing the initial cold rolling reduction rate and omitting the temper rolling in the final step.

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Tatsuya Ito 4-2 Kojimacho, Kawasaki-ku, Kawasaki-shi, Kanagawa Prefecture Nippon Yakin Kogyo Co., Ltd. Research and Development Division Technology Research Laboratory (56) References JP-A-3-204146 ( JP, A) JP-A-2-240219 (JP, A) JP-A-5-70834 (JP, A) JP-A-3-285023 (JP, A) JP-A-2-240218 (JP, A) (58) ) Surveyed field (Int.Cl. ⁷ , DB name) C21D 9 / 46,8 / 02 B21B 1 / 46,3 / 02 B22D 11/06

Claims (2)

(57) [Claims] 1. Thickness of 10 by twin roll type thin plate direct casting machine
austenitic stainless steel ribbon
After descaling this ribbon, cold rolling is performed under a light pressure with a reduction ratio of 5 to 20%, and then maintained at an α-γ transition temperature of 1200 ° C or higher for 3 to 30 minutes, and then processed according to a conventional method. And producing a product by heat treatment. 2. Thickness of 10 by a twin roll type thin plate direct casting machine.
austenitic stainless steel ribbon
A method for producing an austenitic stainless steel sheet, comprising maintaining the ribbon at an α-γ transition temperature of 1200 ° C. or higher for 30 minutes to 2 hours, and then subjecting the ribbon to processing and heat treatment according to a conventional method to obtain a product.