METHOD OF MANUFACTURING Cr-Ni STAINLESS STEEL SHEET EXCELLENT IN SURFACE QUALITY AND MATERIAL THEREOF

Abstract: A method of manufacturing a Cr-Ni stainless steel sheet excellent in surface quality and material thereof, in which a blank in thickness of 6 mm or under is cast of Cr-Ni stainless steel typified by 18% Cr - 8% Ni steel dependently on the process of continuous casting in which the mold is moved synchronously with the cast blank, and is subjected to cold rolling to be made up into a sheet, characterized in that a blank is wound up at a temperature within a range from 800 to 1200°C immediately after casting, and subjected to cold rolling and final annealing so as to be formed into a thin sheet.

Description: TECHNICAL FIELD The present invention relates to a process for manufacturing a thin strip or sheet of a Cr-Ni-base stainless steel, which comprises casting a cast strip having a thickness close to that of a product, by the synchronous continuous casting process wherein a casting mold is moved synchronously with a cast strip, and cold-rolling the strip. BACKGROUND ART A thin sheet of a stainless steel is currently manufactured by a continuous casting process, which comprises casting a cast slab having a thickness-up to about 100 mm while oscillating a casting mold in the direction of casting, surface-treating the slab, heating the treated slab to a temperature of 1000 ° C or above in a heating furnace, hot-rolling the heated slab by using a hot strip mill comprising rough-rolling stands and finish-rolling stands to form a hot strip having a thickness of several millimeters, cold-rolling the hot-strip, and subjecting the cold-rolled strip to necessary treatments, such as annealing, pickling and skin-pass rolling, to form a cold-rolled product. Before the cold-rolling, the hot strip obtained by the hot rolling is annealed to soften the hot strip, which is in a work-hardened state due to heavy hot working, thereby ensuring the shape (flatness), material quality (grain size and mechanical properties), and surface quality (prevention of roping) required of a final product, and further, is pickled and ground to remove oxide scale present on the surface thereof. The above-described conventional process requires lengthy facilities for hot rolling and a vast amount of energy is consumed for heating and working the material, and thus the conventional process is not considered an optimum manufacturing process, from the viewpoint of productivity. Further, since a texture developed during the hot working firmly remains in the final sheet product, the press working of the product sheet in the user is subjected to many limitations, such as the need to take into consideration the anisotropy attributable to the texture. Accordingly, a process wherein the continuous casting step is directly connected to the cold rolling step without the hot rolling step is now under development, to thereby avoid the need to provide lengthy facilities and use a vast amount of energy for manufacturing a hot strip through the hot rolling of a cast strip having a thickness of 100 mm or more, and at the same time, eliminate the limitations on the use of the product derived from the hot worked texture. Specifically, in this process, a cast strip (a thin strip) having a thickness equivalent or close to that of the hot strip obtained by the conventional hot rolling is continuously cast, and the thin cast strip is cold-rolled. Such a process is described in, for example, special reports in "Tetsu-to-Hagane", vol. 85, 1985, pages A197 to A256. The thin sheet product manufactured by the above-described continuous casting/cold rolling process (hereinafter referred to as "strip continuous casting"), however, has a finer grain structure than that of the thin sheet product manufactured by the conventional continuous casting/hot rolling/cold rolling process (hereinafter referred to as "conventional process"), which causes the elongation to be lowered, whereby the workability during a press working or the like by the user is unfavorably lowered. This phenomenon is reported in, for example, "CAMP ISIJ, vol. 1, 1988, 1670 - 1705. In this report, the annealing of the cast strip to cause δ-ferrite remaining in the cast strip to disappear is described as a countermeasure. Detailed studies conducted by the present inventors on the Cr-Ni-base stainless steel manufacturing process by strip continuous casting have revealed that the presence of δ-ferrite and fine MnS remaining in the cast strip inhibits the growth of recrystallized grains during the cold rolling and annealing and is a cause of the formation of the fine structure and the lowering in the elongation of the final product. Therefore, to eliminate the lowering in the elongation of the product manufactured by the strip continuous casting, it is necessary to cause the δ-ferrite to disappear, and at the same time, to conduct a heat treatment for a sufficient coarsening of the MnS. The δ-ferrite can be made to disappear through the annealing of the cast strip. In the annealing for a short period of time conducted for the conventional hot rolled steel strip of an austenitic stainless steel, however, a sufficient transformation into a γ phase cannot be attained, and thus. it becomes necessary to conduct annealing at a high temperature for a long period of time, which renders this method very disadvantageous from the viewpoints of productivity and production costs. Accordingly, the development of a more efficient method of heat treating the strip, and a method of enhancing the rate at which the δ-ferrite is made to disappear during the heat treatment, is desired in the art. The MnS finely precipitated in the cast strip exhibits a stronger inhibiting of the grain growth of the cold-rolled annealing sheet than the δ-ferrite, and thus it is necessary to precipitate MnS in a sufficiently coarse form in the stage of the cast strip, to render the MnS harmless. In the method wherein the cast strip is reheated and annealed, it is necessary to conduct a heat treatment at a high temperature for a long period of time, and accordingly, a method which enables the heat treatment at a high temperature for a long period of time to be efficiently conducted, and facilitates the grain growth, is desired in the art. The SUS304 thin sheet product manufactured by the strip continuous casting has another problem; specifically, the problem resides in the occurrence of fine uneven portion (roping) on the surface of the cold rolled sheet. The roping is a phenomenon attributable to the large γ grain diameter, and accordingly, it was necessary to inhibit the occurrence of roping by refining the γ grain of the cast strip. DISCLOSURE OF THE INVENTION An object of the present invention is to provide a process for manufacturing a thin strip or sheet of a Cr-Ni-base stainless steel having an excellent work-ability and surface quality, comprising casting a cast strip having a size close to the thickness of a final product by a synchronous continuous casting process in which no difference exists in the relative speed of the cast strip and the mold wall, and subjecting the cast strip to cold rolling, wherein the growth of recrystallized grain is promoted during the cold rolling annealing while inhibiting the occurrence of roping through a control of the casting atmosphere, components, and the temperature of the cast strip during the period between completion of the casting and during the coiling. According to the invention, the above-described object can be attained by a process for manufacturing a thin strip or sheet of a Cr-Ni-base stainless steel having an excellent surface quality and material quality, which comprises casting a cast strip having a thickness of 6 mm or less from a Cr-Ni-base stainless steel represented by 18%Cr-8%Ni steel by a continuous casting wherein a casting mold is moved synchronously with the cast strip, and subjecting the cast strip to cold rolling to form a thin sheet product, characterized in that the cast strip immediately after the casting is coiled at a temperature of 800 to 1200 ° C and subjected to cold rolling and final annealing to form a thin sheet product. The present inventors found that the δ-ferrite is caused to disappear more rapidly by conducting the casting in a state such that the percentage solid phase of the cast sheet at the time of release from the casting mold is high. Preferred embodiments are disclosed in the dependent claims 2 and 3. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a graph showing the relationship between the coiling temperature of a cast strip and the roping height and elongation of a thin sheet product; Fig. 2 is a graph showing a representative example of the relationship between the percentage columnar crystal of a cast strip and the elongation of a thin sheet product; BEST MODE OF CARRYING OUT THE INVENTION First the principle of the invention will be described, with reference to the accompanying Figs. 1 and 2. Figure 1 shows a representative example of the relationship between the coiling temperature immediately after the casting and the roping height and elongation of a final cold-rolled product with respect to a JIS SUS304 stainless steel cast strip (thickness: 2 mm) cast according to continuous casting in a twin drum system. As apparent from Fig. 1, to cause the δ ferrite to sufficiently disappear and attain a satisfactory elongation (48% or more) from the practical point of view, it is necessary to coil the cast strip at a temperature of 800 ° C or above. To maintain the roping height at a value that does not cause a problem in practical use (not higher than 0.2 µm) through the inhibition of the growth of γ grain during the coiling of the cast strip, the coiling temperature must be 1200 ° C or below. It has been confirmed that the thin sheet product manufactured from the cast strip coiled in this temperature region has satisfactory material quality (elongation) and surface quality (prevention of roping) from the practical viewpoint. Further, the δ ferrite remaining in the cast strip must have a composition such that it is so unstable as to be easily transformed into a γ phase in this coiling temperature region. Accordingly, it is necessary to conduct a rapid cooling solidification which can prevent ferrite stabilizing elements (Cr, Si, Mo, Ti, etc.) from concentrating in the δ ferrite, and it is important that the percentage solid phase at the time of a release of the cast strip from the casting mold wall is 65% or more, and that the proportion of an equiaxed crystal portion (a portion wherein the solidification rate is, slower than the columnar crystal portion and the stabilization of the δ ferrite is advanced) is reduced after the release of the cast strip from the casting mold. In view of the above, the present inventors cast thin cast strips under various casting conditions and studied various factors associated with the speed at which the δ ferrite disappears, and as a result, found that the speed of the disappearance of the δ ferrite is greatly influenced by the solidified structure of the cast strip. Namely, it was apparent that, under the same cast strip annealing conditions, the columnar crystal portion, which is a rapid cooling-solidified structure, in the cast strip exhibits an considerably higher δ ferrite disappearance speed than that of the equiaxed crystal portion. Figure 2 shows a representative example of the results of a measurement of the elongation of thin sheet products manufactured from strips prepared by casting a JIS SUS304 stainless steel under various casting conditions, to form cast strips having different proportions of columnar crystal (percentage columnar crystal) in the solidified structure of the cast strip, and annealing the resultant cast strips under the same condition (800 ° C x 60 min). As apparent from this drawing, the elongation of the thin sheet products increases with an increase in the percentage columnar crystal of the cast strip, and optimum elongation value can be obtained particularly when the percentage columnar crystal is 65% or more. Specifically, it is preferable to eliminate the δ ferrite by coiling the cast strip at a high temperature, and at the same time, to make the percentage solid phase 65% or more, to thereby increase the disappearing speed of the δ ferrite, and thus cause the δ ferrite to disappear in a short time even when annealing at a lower temperature. The present invention will now be described in more detail by way of the following Examples. [Example 1] Thin sheets of Cr-Ni-base stainless steels were manufactured according to the first invention of the present application. Various austenitic stainless steels comprising 18%Cr-8%Ni stainless steel as a basic composition given in Table 1 were melted and cast to form a cast strip having a thickness of 2 mm, by a continuous casting machine having an internal water-cooling twin drum system. The percentage solid (percentage columnar crystal) at the time of a release of the cast strip from the drum was controlled to 100 to 60%, through a regulation of the drum gap. The cast strips were subjected to annealing, pickling, 50% cold rolling, annealing, and then skin-pass rolling with a 1% elongation, to obtain thin sheet products. [Comparative Example 1] For a comparison with Example 1, thin sheet products were manufactured from a cast strip in the same sequence, except that the percentage solid phase was 60% and the cast strip was coiled at 800 ° C or 400 ° C. With respect to Example 1 (samples A, B, and C) and Comparative Example 1 (samples D and E), the grain size (G.S.N.), elongation, and surface quality of the thin sheet products were evaluated, and the results are given in Table 2. The thin sheet products manufactured according to the present invention had a product grain size (G.S.N.) of 8.0 or less and an elongation of 50% or more, i.e., sufficiently satisfied the elongation requirement (48% or more), and a satisfactory surface quality from the practical viewpoint, i.e., a roping height of 0.2 µ m or less. By contrast, in Comparative Example 1, which does not meet the coiling temperature requirement of the present invention, although the thin sheets had a product grain size (G.S.N.) of 10.5 (D) and 9.6 (E), i.e., a fine grain structure, and a good surface quality due to this small grain size, the elongation was 43% (D) and 45% (E), i.e., unsatisfactory from the practical viewpoint. INDUSTRIAL APPLICABILITY As described above, according to the present invention, in the process for manufacturing a thin strip or sheet through cold-rolling of a cast strip, the control of the cast strip temperature enables a thin sheet of a Cr-Ni-base stainless steel to be manufactured while ensuring a satisfactory surface quality from the practical viewpoint. This contributes to a realization of a process for manufacturing a thin strip or sheet of a Cr-Ni-base stainless steel which has a much lower production cost and a much higher productivity than the conventional process, wherein a thick cast slab up to about 100 mm is hot-rolled.

Claims: A process for manufacturing a thin strip or sheet of a Cr-Ni-base stainless steel having an excellent surface quality and material quality, which comprises casting a cast strip having a thickness of 6 mm or less from a Cr-Ni-base stainless steel represented by 18%Cr-8%Ni steel by a continuous casting wherein a casting mold is moved synchronously with the cast strip, and subjecting the cast strip to cold rolling to form a thin sheet product, characterized in that the cast strip immediately after the casting is coiled at a temperature of 800 to 1200°C and subjected to cold rolling and final annealing to form a thin sheet product. A process according to claim 1, characterized in that said coiled cast strip is held at a temperature in the range of 800 ° C to 1250 ° C for 80 min or less and then subjected to annealing, cold-rolling and final annealing. A process according to claim 1, characterized in that the casting is conducted in a state such that the percentage solid phase of the cast strip at the time of release from the mold wall is 65% or more.

Inventors: SUEHIRO TOSHIYUKI, TERAOKA SHINICHI, UEDA MASANORI, SUEHIRO, TOSHIYUKI, TERAOKA, SHINICHI, UEDA, MASANORI, SUEHIRO, TOSHIYUKI NIPPON STEEL CORPORATION, TERAOKA, Shinichi Nippon Steel Corporation R&amp;D, UEDA, Masanori Nippon Steel Corporation R&amp;D
Applicants: NIPPON STEEL CORP, NIPPON STEEL CORPORATION
Application year: 1991
Main classification: B22D11
IPC codes: C21D8, B22D11, B21B1, B21B3