JP2003290881A - Cr-Ni BASE STAINLESS STEEL THIN SHEET AND ITS PRODUCING METHOD - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a Cr-Ni base stainless steel thin sheet and its producing method with which the unevenness of luster caused by Ni segregation in the center part of the sheet thickness is improved, in the case of casting the thin thickness with a continuous caster where a mold wall synchronously moves with a cast strip. <P>SOLUTION: In the Cr-Ni base stainless steel thin sheet which is cast thin with the continuous caster where the mold wall synchronously moves with the cast strip, this Cr-Ni base stainless steel thin sheet has the peculiarity in which the total oxygen concentration in the steel is made to ≤35 ppm or ≥80 ppm and further, S concentration in the steel is made to ≤20 ppm. The method for producing the Cr-Ni base stainless steel thin sheet has the peculiarity in which the molten steel for continuous casting is produced with an AOD furnace and slag basicity in the furnace after reducing Cr is made to ≥1.8 or ≤1.2. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Cr-Ni type stainless steel thin plate thinly cast by a continuous casting machine in which a mold wall typified by a twin-drum type moves in synchronism with a slab and a manufacturing method thereof. Is.

[0002]

2. Description of the Related Art Synchronous continuous casting processes include twin drum method (also referred to as twin roll method) and twin roll method, as described in, for example, a paper specializing in "Iron and Steel"'85 -A197 to A256. It is a synchronous continuous casting process such as a belt method or 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-drum type continuous casting method, which is one of these synchronous type continuous casting processes, is a continuous casting method that is composed of a pair of parallel or inclined cooling drums of the same or different diameter and side dams that seal both end surfaces of the cooling drums. Molten steel is poured into the mold, solidified shells are generated on the circumferential surfaces of both cooling drums, and the solidified shells are united near the closest positions (so-called "kissing points") of both rotating cooling drums, This is a continuous casting method in which a thin strip-shaped slab is formed.

For example, a thin strip slab cast by the twin drum type continuous casting method has a thickness of several mm (usually about 2 to 5 mm) and is cold-rolled without the conventional hot rolling. Thin sheet products can be manufactured. For this reason, a conventional manufacturing method (slab casting-hot rolling process) in which a slab for hot rolling having a thickness of more than 100 mm is cast by continuous casting using a vibration mold or the like, and this is hot rolled and then cold rolled Compared with, the production efficiency and the cost are significantly advantageous.

A product obtained by cold rolling a strip-shaped slab cast by a twin-drum type continuous casting method or the like without hot rolling is subjected to cold forming (particularly drawing or bulging). It is known that uneven gloss (rough skin) occurs along the rolling direction. Of this uneven gloss, the length is about several hundred mm,
For large flow pattern gloss unevenness with a width of about 3 mm,
Since the Ni segregation (positive segregation or negative segregation) remaining in the final solidified portion of the slab, that is, the center of the thickness of the product plate is unevenly distributed,
This is caused by locally different deformation resistance.

In Japanese Patent Laid-Open No. 7-268556, casting is carried out at a superheat degree ΔT of molten steel of 50 ° C. or less at the time of continuous casting, thereby making it difficult for molten steel to flow in the final solidification portion, and to obtain strong Ni segregation. An invention that alleviates the above is disclosed. Further, in Japanese Patent No. 2851252, it is clarified that the degree of Ni segregation at the center part of the plate thickness depends not only on ΔT but also on the pressing force to the mold wall surface, and the pressing force is set to a value smaller than a predetermined value. Thus, an invention is disclosed in which Ni segregation at the center of the plate thickness is reduced and uneven gloss (rough skin) is improved.

[0006]

The quality requirements for the surface properties of steel sheets are becoming more and more stringent, and good surface properties beyond the level that can be achieved by the invention disclosed heretofore are required. With respect to the unevenness of gloss due to Ni segregation at the center of the plate thickness, improvement over the level conventionally achieved has been required.

According to the present invention, when thin-wall casting is performed by a continuous casting machine in which the mold wall moves in synchronization with the slab, the unevenness of gloss due to Ni segregation at the center of the plate thickness is improved.
An object of the present invention is to provide a stainless steel thin plate and a method for manufacturing the same.

[0008]

That is, the gist of the present invention is as follows. (1) A Cr-Ni-based stainless steel thin plate formed by thin casting by a continuous casting machine in which a mold wall moves in synchronization with a slab, characterized in that the total oxygen concentration in the steel is 35 ppm or less. Cr-Ni type stainless steel thin plate. (2) A Cr-Ni-based stainless steel thin plate obtained by thin-wall casting by a continuous casting machine in which the mold wall moves in synchronization with the slab, characterized in that the total oxygen concentration in the steel is 70 ppm or more. Cr-Ni type stainless steel thin plate. (3) The Cr-Ni-based stainless steel thin plate according to claim 1 or 2, wherein the S concentration in the steel is 20 ppm or less. (4) Molten steel for continuous casting is melted in an AOD furnace, and the slag basicity in the furnace after Cr reduction is set to 1.8 or more, Cr-Ni type stainless steel according to claim 1. Method for manufacturing thin plate. (5) The Cr-Ni-based stainless steel according to claim 2, wherein the molten steel for continuous casting is melted in an AOD furnace, and the slag basicity in the furnace after Cr reduction is 1.2 or less. Method for manufacturing thin plate. (6) A Cr-Ni-based stainless steel thin plate formed by thin casting by a continuous casting machine in which the mold wall moves in synchronization with the slab, and the average crystal grain size in the steel plate after cold rolling and annealing is 14 μm.
A Cr-Ni-based stainless steel thin plate having a thickness of m or more.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION In a cold-rolled steel sheet, annealing after cold rolling causes recrystallization and grain growth centered on the strain introduced during cold rolling, thereby determining the crystal grain size after annealing. . When an ordinary continuous cast material having a slab thickness of 100 mm or more was used and subjected to hot rolling-cold rolling-annealing to obtain a cold rolled steel sheet, the crystal grain size of the steel sheet was about 35 to 50 µm.

In thin-wall continuous casting represented by twin-drum type continuous casting, the solidification rate is extremely high in all parts from the surface of the slab to the center of the plate thickness.
A very fine secondary deoxidation product crystallizes in the slab, and there is a precipitate with the crystallization as a nucleus. When cold rolling and annealing are performed using a slab containing a large number of such fine crystallized substances, during recrystallization during annealing, the fine crystallized substances function as pinning particles of the γ phase, so that the crystal Grain growth is delayed. Therefore, the cold-rolled steel sheet using the thin-walled continuous cast slab has a characteristic that the crystal grain diameter of the steel sheet is about 5 to 10 μm, that is, extremely fine crystal grains.

When cold forming is performed on a cold rolled steel sheet,
Non-directional roughening (orange peel) occurs on the surface of the steel sheet after processing. The degree of rough skin is affected by the crystal grain size of the cold-rolled steel sheet, and the larger the crystal grain size, the greater the degree of rough skin. Since the cold rolled steel sheet using a normal thick continuous casting slab has a large crystal grain size, the rough surface after processing is also to some extent, and as a result, even if there is Ni segregation inside the steel sheet, uneven gloss It is difficult to recognize. On the other hand, in the thin continuously cast steel sheet, since the crystal grain size is fine as described above, there is little non-directional roughening that appears on the surface of the steel sheet after processing, and slight streaky roughening due to Ni segregation ( The situation was such that uneven glossiness) was easily recognized.

In the present invention, in the thin continuously cast steel sheet, the GSN in the annealed steel sheet is 9 or less,
Alternatively, it is characterized in that the unevenness of gloss is improved by setting the average crystal grain size to 14 μm or more. To that end, among the many crystallized substances that were present in the thin continuously cast steel sheet, by reducing the crystallized substances that function as pinning particles of the γ phase, the crystal grain size of the thin continuously cast cold rolled steel sheet Increase the diameter. As a result, the degree of roughening of the surface of the steel sheet after processing is made to be similar to that of a normally cast steel sheet, and thereby the streaky roughness due to Ni segregation is divided. Further, as a result of increasing the crystal grain size of the steel sheet, it promotes the work-induced transformation during cold rolling, promotes the randomization of the orientation to reveal orange peel, and prevents the uneven glossiness due to Ni segregation from being recognized. . As a result, even if the Ni segregation (mainly negative segregation) at the center of the plate thickness is as conventional as possible, it is possible to greatly improve the degree of gloss unevenness that appears on the surface of the steel sheet after processing.

The GSN means a particle size number, which is a number obtained by measuring the particle size by a prescribed method and dividing
The measuring method is specified in JIS G 0551. Further, the measuring method of the average crystal grain size is as follows:
4 parts were photographed at a magnification of 200 using a microscope, and the particle diameter was a value equivalent to a circle equivalent diameter in image processing.

1 to 3 show the results of deep drawing of various cold-rolled steel sheets and measurement of the roughness of the bottom of the processed steel sheets with a three-dimensional roughness meter. (A) of FIGS. 1 to 3 expresses the roughness of the surface of the steel sheet as a difference in lightness, and the bright portion protrudes from the center roughness. Further, (b) of FIGS. 1 to 3 illustrate the unevenness of the steel sheet surface in the direction perpendicular to the rolling direction of the steel sheet surface. FIG. 1 is a conventional thin-wall continuous cast steel sheet having a crystal grain size of about 8 μm, and FIG. 2 is a thin-wall continuous cast steel sheet of the present invention having a crystal grain size of about 20 μm. A thick continuous cast steel sheet is used as a comparative example, and the crystal grain size is about 30 μm. In the case of using the conventional thin continuously cast steel sheet shown in FIG. 1, since the crystal grain size is fine, as shown in FIG. 1 (b), there are very few fine wavelength irregularities.
Large unevenness of wavelength is generated. According to FIG. 1A, the unevenness of large wavelength is continuous in the rolling direction. Next, in the case of using the thin continuously cast steel sheet of the present invention shown in FIG. 2, although the amplitude of the fine wavelength irregularities is rather increased as can be seen from FIG. 2B, the large wavelength irregularities are uneven. It has disappeared. As a result, as is clear from FIG. 2A, the unevenness extending in the rolling direction has disappeared, and as a result, the large uneven luster in the flow pattern has disappeared. It can be seen that the uneven condition of the product of the present invention shown in FIG. 2 is similar to that of the thick continuously cast steel plate shown in FIG.

In the present invention, the first means for reducing the crystallized substances functioning as pinning particles is to set the total oxygen concentration in the steel to 35 ppm or less. Many of the crystallized substances that function as pinning particles are very fine.
Since it is a secondary deoxidation product or a precipitate having it as a nucleus, the number of pinning particles can be reduced by reducing the total oxygen concentration. The relationship between the total oxygen concentration in the low oxygen region and the grain size of the cold rolled annealed steel is as shown in FIG. It can be seen that in the region where the total oxygen concentration is 35 ppm or less, the crystal grain size increases as the total oxygen concentration decreases.

In the present invention, the second means for reducing the crystallized substances functioning as pinning particles is to set the total oxygen concentration in the steel to 70 ppm or more. In the high oxygen region where the total oxygen concentration is 70 ppm or more, the secondary deoxidation product crystallizes during casting, but the crystallized product is coarse and a large amount exists, and the function as pinning particles deteriorates. Or, even if the function is exhibited, the grain size of the steel sheet after annealing becomes coarse because it is coarse. The relationship between the total oxygen concentration in the high oxygen region and the grain size of the cold rolled annealed steel is as shown in FIG. It can be seen that in the region where the total oxygen concentration is 70 ppm or more, the crystal grain size increases as the total oxygen concentration increases.

The total oxygen concentration of the steel corresponds to the oxygen in the oxide taken in the cast.

Annealing Conditions When annealed at 1100 ° C. for 20 seconds, a normal thick continuous cast steel sheet has a GSN of 7.8 (average grain size is 21 μm) and a conventional thin continuous cast steel sheet has a GSN of 9. 6 (average particle size is 12 μm).
On the other hand, the GSN of the thin continuously cast steel sheet of the present invention in which the total oxygen concentration was 15 ppm was 8.7 (the average particle diameter was 15 μm), and the effect of increasing the diameter could be exhibited. Furthermore, the GSN of the thin continuously cast steel sheet of the present invention in which the total oxygen concentration is 88 ppm has reached 7.6 (average particle size is 24 μm),
It was possible to obtain coarser particles than those of ordinary thick continuously cast steel sheets (Fig. 5).

Further, in the case of annealing at 1150 ° C. for 20 seconds, the G of a normal thick continuously cast steel sheet
The SN was 6.5 (the average particle size was 48 μm), and the GSN of the conventional thin continuously cast steel sheet was 9.1 (the average particle size was 14 μm). On the other hand, the GSN of the thin-wall continuous cast steel sheet of the present invention having a total oxygen concentration of 15 ppm is 7.4 (the average particle size is 24 μm).
m), and further, the GSN of the thin continuously cast steel sheet of the present invention in which the total oxygen concentration is 88 ppm is 6.6 (the average grain size is 4
6 μm), and in each case, a good crystal grain size with a GSN of 8.5 or less could be realized (FIG. 5).

At the same time as reducing the total oxygen concentration in the steel,
Further improvement can be achieved by reducing the S concentration in the steel. MnS present in steel is a harmful precipitate that inhibits crystal growth during annealing of the steel sheet. By reducing the S concentration to 20 ppm or less and suppressing the precipitation of MnS, the grain size of the steel sheet after annealing can be increased. In a low-oxygen thin continuously cast steel sheet with an oxygen concentration of 30 ppm, the crystal grain size after annealing is 18 μm when the S concentration is 30 ppm in the past, but when the S concentration is reduced to 18 ppm, the crystal grain size after annealing increases to 22 μm. be able to. Further, in the high oxygen thin-wall continuous cast steel sheet with an oxygen concentration of 85 ppm, the crystal grain size after annealing was 20 μm when the S concentration was 35 ppm in the conventional case, but when the S concentration was reduced to 14 ppm, the crystal grain size after annealing was 40 μm. Can be large. More preferably, the S concentration in the steel is 15 ppm or less.

In the thin continuously cast cold rolled steel sheet of the present invention,
An additional effect of increasing the crystal grain size in the annealed steel sheet is the improvement of ductility. In the conventional thin-wall continuous casting cold-rolled steel sheet, the steel sheet is hard because the crystal grain size is fine, and as a result, the ductility is low. Fine grains have many grain boundaries, which hinder the movement of dislocations, and thus the hardness becomes hard. On the other hand, in the thin-wall continuously cast cold-rolled steel sheet of the present invention, since the crystal grain size is coarse, the steel sheet becomes soft, and as a result, the ductility is improved. The ductility was 48% in the conventional thin continuous cast steel sheet having a crystal grain size of 10 μm, whereas the ductility was 52% in the thin continuous cast steel sheet of the present invention having a crystal grain size of 25 μm.

The Cr-Ni type stainless steel thin plate of the present invention is a thin cast piece obtained by thin wall casting by a continuous casting machine in which the mold wall moves in synchronization with the cast piece, and an in-line hot rolling machine in the continuous casting machine. It is a concept that includes any of the steel sheet rolled in (1) and the cold rolled steel sheet obtained by cold rolling and further annealing the slab or the steel sheet.

Next, a method of manufacturing the Cr-Ni type stainless steel thin plate of the present invention will be described. In the case of producing molten steel for continuous casting in an AOD furnace, Cr is burned and exists as an oxide in the slag during decarburization refining using oxygen gas and Ar gas. After decarburizing and refining, Cr is reduced by mainly introducing Fe-Si. In-furnace slag basicity after reduction of Cr (CaO / S
The total oxygen concentration in the molten steel can be adjusted by adjusting iO ₂ ). As shown in FIG. 6, when the amount of CaO in the slag after Cr reduction in the AOD furnace is large, strong deoxidation occurs, the basicity is high and the molten steel oxygen concentration is low. On the contrary, when the amount of CaO in the slag is small, weak deoxidation occurs and the molten steel oxygen concentration increases. If the total oxygen concentration in the molten steel is low, the total oxygen concentration of the continuously cast steel will be low as a result. Conversely, if the total oxygen concentration in the molten steel is high, the total oxygen concentration of the continuously cast steel will be high. Specifically, by setting the slag basicity in the furnace after Cr reduction to 1.8 or more, the total oxygen concentration in steel can be set to 35 ppm or less. Further, by setting the furnace slag basicity after Cr reduction to 1.2 or less, the total oxygen concentration in the steel can be set to 80 ppm or more.
The basicity is adjusted by adjusting the amount of CaO source (quick lime, CaO—CaF ₂ powder, etc.) to be charged into the AOD furnace. Incidentally, the relationship between the basicity in slag and the grain size of steel sheet after annealing is shown in FIG.

A twin-drum type continuous casting machine can be used as the continuous casting machine of the present invention in which the mold wall moves in synchronization with the slab for performing thin casting. The twin-drum continuous casting method is one in which molten steel is injected into a continuous casting mold composed of a pair of parallel or inclined cooling drums of the same diameter or different diameters and side dams that seal both end faces of the cooling drums. Solidified shells are generated on the respective circumferential surfaces of and the solidified shells are united in the vicinity of the closest points (kissing points) of both rotating cooling drums to form an integral strip-shaped slab.

A thin cast piece produced by the method of the present invention is used as a raw material, cold rolled to a plate thickness of about 0.8 mm, then annealed at, for example, 1100 ° C. for about 20 seconds, further pickled and tempered. A product can be obtained by rolling.

[0026]

INDUSTRIAL APPLICABILITY The present invention reduces or increases the total oxygen concentration of steel in a Cr-Ni type stainless steel thin plate which is thinly cast by a continuous casting machine in which the mold wall moves in synchronization with the slab. It is possible to increase the crystal grain size after annealing and improve the gloss unevenness due to Ni segregation at the center of the plate thickness.

[Brief description of drawings]

FIG. 1 is a diagram showing a rough surface condition of a conventional thin continuously cast steel sheet after processing, wherein (a) is three-dimensional roughness data and (b) is two-dimensional roughness data in a direction perpendicular to rolling.

FIG. 2 is a diagram showing a rough surface condition of a thin continuously cast steel sheet of the present invention after processing, wherein (a) is three-dimensional roughness data and (b) is
Is two-dimensional roughness data in the direction perpendicular to the rolling direction.

FIG. 3 is a diagram showing a rough surface condition after processing of a thick continuously cast steel sheet as a comparative example, in which (a) is three-dimensional roughness data,
(B) is two-dimensional roughness data in the direction perpendicular to rolling.

FIG. 4 is a diagram showing the relationship between the total oxygen concentration in steel and the crystal grain size of a steel sheet.

FIG. 5 is a diagram showing a relationship between a casting method, a total oxygen concentration, an annealing temperature and a crystal grain size.

FIG. 6 is a diagram showing the relationship between slag basicity and total oxygen concentration.

FIG. 7 is a diagram showing a relationship between slag basicity and crystal grain size.

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Claims (6)

[Claims] 1. A Cr-Ni-based stainless steel thin plate obtained by thin-wall casting by a continuous casting machine in which a mold wall moves in synchronization with a slab, wherein the total oxygen concentration in the steel is 35 ppm or less. Cr-Ni system stainless steel thin plate to be. 2. A Cr-Ni type stainless steel thin plate obtained by thin-wall casting by a continuous casting machine in which a mold wall moves in synchronization with a slab, wherein the total oxygen concentration in the steel is 70 ppm or more. Cr-Ni system stainless steel thin plate to be. 3. The Cr-N according to claim 1 or 2, wherein the S concentration in the steel is 20 ppm or less.
i type stainless steel thin plate. 4. The Cr-Ni system according to claim 1, wherein molten steel for continuous casting is melted in an AOD furnace and the slag basicity in the furnace after Cr reduction is set to 1.8 or more. Manufacturing method of stainless steel sheet. 5. The Cr-Ni system according to claim 2, wherein molten steel for continuous casting is melted in an AOD furnace and the slag basicity in the furnace after Cr reduction is 1.2 or less. Manufacturing method of stainless steel sheet. 6. A Cr-Ni type stainless steel thin plate obtained by thin-wall casting by a continuous casting machine in which a mold wall moves in synchronism with a slab, the average grain size of the steel plate after cold rolling and annealing is 14 μm. The above is a Cr-Ni system stainless steel thin plate characterized by the above.