Process for production of austenitic stainless steel thin cast strip and strip obtained thereby

Abstract: Disclosed is a process for the production of an austenitic stainless steel thin cast strip having an excellent surface quality, which comprises casting an austenitic stainless steel, in which δ-Fe cal. (%), defined by 3(Cr + 1.5Si + Mo) - 2.8(Ni + 0.5Mn + 0.5Cu) - 84(C + N) - 19.8, is controlled to 5 to 9%, by a continuous casting machine, in which a thin cast strip is moved synchronously with the wall surface of a cooling drum, on which many dimples having a diameter of 0.1 to 1.2 mm and a depth of 50 to 100 µm and having a circular or ellipsoidal opening are distributed so that the distance between every two adjacent dimple edges is smaller than 0.35 mm, into a thin cast strip having a thickness smaller than 10 mm.

Description: The present invention relates to a thin cast strip having a thickness close to the thickness of a product by a synchronous continuous casting process wherein there is no difference in the relative speed of the thin cast strip and the inner wall surface of a casting mold, a process for the production of this thin cast strip, and a process for the production of an austenitic stainless steel strip and sheet by cold rolling this thin cast strip. According to the conventional process for preparing a stainless steel strip and sheet by the continuous casting process, a cast slab having a thickness larger than 100 mm is formed by casting while shaking a casting mold in the casting direction, the surface of the formed cast slab is finished, the cast slab is heated at a temperature higher than 1000°C in a heating furnace, and hot-rolled by a hot strip mill comprising rows of rough mills and finish mills to form a hot strip having a thickness of several millimeters, and according to need, the thin cast strip is then annealed, descaled, cold-rolled and subjected to a final annealing. In this conventional process, since a cast slab having a thickness larger than 100 mm is hot-rolled, a long hot strip mill is necessary, and the process is disadvantageous in that a large quantity of energy is required for heating and rolling the cast slab. As the means for coping with this disadvantage, a process for preparing a thin cast strip having a thickness equal or substantially equal to the thickness of a hot strip, by the continuous casting process, has recently been investigated. For example, there are known a twin-roll method, a twin-belt method and other synchronous continuous casting processes wherein there is no difference in the relative speed of a thin cast strip and the inner wall surface of a casting mold, as introduced in lectures given in the Special Edition, Iron and Steel, ′85-A197-′85-A-256. In the production of a stainless steel strip and sheet product through the synchronous continuous casting process, since the value of the thin cast strip thickness/strip and sheet product thickness ratio is small, to obtain a strip and sheet having a high-quality surface, it is important to stably maintain the surface state of the thin cast strip at a high level. This process for the continuous production of thin cast strip is different from the conventional process for producing slabs by continuous casting equipment in that a formation of a thin cast strip having a high-grade surface is intended even if the reduction of cold rolling at subsequent steps is lowered. Accordingly, if surface cracking appears in a thin cast strip, surface defects appear in the product, resulting in drastic reduction of the commercial value thereof, and thus the intended object cannot be attained. As the means for eliminating the formation of defects such as surface cracks, there has been proposed a process in which a plurality of dimples having a predetermined size and depth are formed on the circumferential face of a cooling drum (Japanese Patent Application No. 62-240479, Japanese Patent Application No.62-240481, and Japanese Patent Application No. 63-202962). According to this process, the formation of surface defects such as surface cracks can be prevented if casting is carried out in the presence of dimples formed on the circumferential face of the cooling drum, as a rapidly cooled area and a slowly cooled area are formed on the surface of the thin cast strip by an air gap generated by the dimples, with the result that, the residual amount of δ-ferrite in these areas is made different and a solidification structure unevenness occurs on the surface of the thin cast strip. This solidification structure unevenness becomes obvious as a gloss unevenness on the surface of cold rolled strip and sheet product. When casting an austenitic stainless steel thin cast strip by a continuous casting machine in which a thin cast strip is synchronously moved with a casting mold surface having randomly arranged dimples differing in size and shape, and this thin cast strip is cold-rolled to form a strip and sheet product, it is an object of the present invention to prevent the occurrence of gloss unevenness generated by a solidification structure unevenness, by controlling in the thin cast strip the solidification structure unevenness caused by a difference of the residual quantity of δ-ferrite due to dimples formed on the wall surface of the casting mold. The present invention specifies chemical compositions of stainless steel and conditions of dimples arranged on the surface of the cooling drum for attaining the above-mentioned object. More specifically, the present invention is characterized in that δ - Fe cal. (%) defined by 3(Cr + 1.5Si + Mo) - 2.8(Ni + 0.5Mn + 0.5Cu) - 84(C + N) - 19.8 is controlled to 5 to 9%, and a melted steel having a high δ-ferrite steel composition readily giving a primary δ-ferrite solidification structure is continuously cast by a cooling drum, many dimples having diameter of 0.1 to 1.2 mm and a depth of 50 to 100 µm and having a circular or ellipsoidal opening being distributed on the cooling drum so that the distance between every two adjacent dimple edges is smaller than 0.35 mm , while controlling the initial solidification cooling rate to a uniformly slow speed. By dint of this characteristic feature, a δ-ferrite solidification structure having no solidification structure unevenness can be imparted to the surface portion of the thin cast strip, and even if a strip and sheet product is prepared by cold-rolling the obtained thin cast strip, no gloss unevenness occurs on the surface of the product, and thus the quality of the product can be improved. The invention will be described in detail in connection with the drawings in which: Fig. 1 is an optical microstructure of the surface of a thin cast strip, corresponding to rapidly cooled and slowly cooled areas formed when casting is carried out while forming dimples on the circumferential face of a cooling drum; Fig. 2 is a sectional phase diagram of the portion corresponding to Cr eq. + Ni eq. ≒ 30% in the Fe-Cr-Ni ternary system equilibrium phase diagram; Figs. 3(a), 3(b), 3(c) and 3(d) are photos showing sections of optical microstructure of thin cast strip obtained by casting melted steels differing in δ-Fe cal. (%) by using twin drums; Fig. 4 is a diagram illustrating the relation between the dimple edge distance on a cooling drum and the solidification structure; Figs. 5(a), 5(b), 5(c) and 5(d) are diagrams illustrating the mechanism of forming a solidification structure having a reduced δ-ferrite content; Figs. 6(a), and 6(b) are optical microstructure showing solidification structure unevenness in thin cast strip and cold rolled product sheet of sample 3 of the present invention and comparative sample 9. The solidification structure unevenness of the surface of an austenitic stainless steel thin cast strip will now be described. This solidification structure unevenness is brought about because the δ-ferrite content is high in the slowly cooled area and is low in the rapidly cooled area, as shown in Fig. 1. If the thin cast strip having this solidification structure unevenness is cold-rolled and subjected to final annealing, the growth of recrystallization grains is inhibited in the area having a high δ-ferrite content to form a fine grain structure while recrystallization grains grow in the area having a low δ-ferrite content to form a relatively coarse structure, whereby the grain size becomes uneven and this results in the appearance of a gloss unevenness on the surface of a product. The steel chemical compositions will now be described. Fig. 2 is a sectional diagram of the portion corresponding to Cr eq. +Ni eq. ≒ 30% in the Fe-Cr- Ni ternary equilibrium phase diagram, which is quoted from Transaction of JWRI, vol. 14, No. 1, 1985, page 125. C eq. and Ni eq. are calculated from the chemical composition according to the following equations: Cr eq. = Cr(%) + 1.5Si(%) + Mo(%) + Nb(%) Ni eq. = Ni(%) + 0.5Mn(%) + 0.5Cu(%) + 30 {C (%) + N (%)} The region of small Cr eq. is a region of complete γ solidification structure (zone I), but it is expected that as the Cr eq. increases, the solidification state will be changed to a primary γ → δ + γ solidification structure (zone I) or primary δ → δ + γ solidification structure (zone III), or to a complete δ solidification structure (zone IV). As the result of experiments made on various steel chemical compositions, it was found that if δ - Fe cal. (%) defined by 3(Cr + 1.5Si + Mo) -2.8 (Ni + 0.5 Mn + 0.5Cu) -84(C + N) -19,8 is controlled below -2%, from -2 to 5%, and above 5%, a complete γ solidification structure (zone I), primary δ → δ + γ solidification structure (zone III), and complete δ solidification structure (zone IV) are obtained. Figures 3(a), 3(b), 3(c) and 3(d) are photos showing sectional microstructure of thin cast strips obtained by changing δ - Fe cal. (%) in the steel chemical composition. As apparent from the drawings, the area (zone I) where [δ - Fe cal. (%)] is - 2.3% is the area of complete γ solidification structure, and unevenness of the solidification structure due to the δ-ferrite content is not observed. The area (zone II) where [δ → Fe cal. (%)] is 2.3% is the area of primary γ → δ + γ solidification structure, and remaining δ-ferrite amount state is apparently different at the slowly cooled portion and at the rapidly cooled portion due to the presence of dimples, and the solidification structure unevenness is conspicuous. The area (zone III) where [δ - Fe cal. (%)] is 6.3% is the area of the primary δ → δ + γ solidification structure, and the portion of a depth of about 150 µm from the surface layer has a δ-solidification structure and the portion of a greater depth has a δ + γ solidification structure. The δ solidification structure of the surface layer is uniform regardless of the slowly cooled portion or the rapidly cooled portion, and no solidification structure unevenness is observed. In the area (zone IV) where [δ - Fe cal. (%)] is 8.8%, a complete δ solidification structure is obtained. As apparent from the foregoing description, a selection of the chemical composition in the Fe-Cr-Ni system has a serious influence on the solidification structure of the thin cast strip, and it has been found that if [δ - Fe cal. (%)] is controlled below - 2% (complete γ solidification structure) or above 5% (primary δ → δ + γ solidification structure and complete δ solidification structure), a cast strip having a relatively small solidification unevenness can be obtained. Nevertheless, in order to control [δ - Fe cal. (%)] below - 2%, it is necessary to increase the contents of Ni eq. elements such as C, N and Ni, and C and N are a risk of a lowering of the corrosion resistance. Moreover, the addition of Ni is restricted for economical reasons. Therefore, this steel chemical composition is not preferable for a practical steel of the 18-8 system. Moreover, in the case of a high δ-ferrite composition, it is feared that delayed cracking will occur at the press forming, and it is judged that the upper limit of [δ-Fe cal. (%)] is 9% and that this value is preferably controlled within a range of from 5 to 9%. Many circular or ellipsoidal dimples are formed on the surface of the cooling drum used in the present invention. When a primary solidification shell is formed on the surface of the cooling drum, the dimples form independent air gaps not interconnected to one another. In the solidification shell, a slowly cooled portion is formed by these air gaps and a portion contiguous to the surface of the cooling drum is rapidly cooled, the δ-ferrite content is reduced in the rapidly cooled portion below the δ-ferrite content in the slowly cooled portion, with the result that a solidification structure unevenness occurs due to the difference of the δ-ferrite content of these portions. To moderate the solidification structure unevenness caused by a non-uniform cooling of the primarily solidification shell, it is necessary to appropriately control the size and depth of dimples formed on the cooling drum while taking the relationship to cracking into consideration, and to narrow the distance between every two adjacent dimples. Figure 4 illustrates the relationship between the distance between the edges of every two adjacent dimples on the surface of the cooling drum and the solidification structure. From Fig. 4 it is understood that, if the distance between the edges is smaller than 0.35 mm, the solidification structure observed in the shell solidificated contiguously to the face between the edges is an ordinary (δ + γ) solidification structure where the δ-ferrite content is uniform, but if the distance between the dimple edges is larger than 0.35 mm, not only an ordinary (δ + γ) solidification structure but also a solidification structure where the δ-ferrite content is low ("γ"-solidification structure) is observed. The reason for this is considered to be as follows. As shown in Fig. 5, at the primary stage of solidification a melted steel is cooled in the state contiguous to edges of dimples [Fig. 5(a)], and therefore, if the distance between adjacent dimples is narrow, the thickness of the solidification shells between adjacent dimples increases, but in the region where the dimple distance is wide, partial delay of the solidification occurs in this region [Fig. 5(b)]. In this partially solidificated portion, the shell is thin and has a low strength, and therefore, the shell is pressed closely to the surface of the cooling drum by the static pressure of the melted steel [Fig. 5(c)]. This portion is rapidly cooled to inhibit the growth of δ-ferrite, and the δ-ferrite is quickly caused to disappear by the subsequent heat diffusion. As a result, the δ-ferrite content becomes lower than in the other portion and a solidification structure unevenness occurs on the surface of the thin cast strip [Fig. 5(d)]. If the diameter of the opening of the dimple is smaller than 0.1 mm, the gradual cooling effect by the air gap is small, and the dimple-forming operation and brush cleaning operation of removing dust or the like are difficult. If the diameter of the opening of the dimple exceeds 1.2 mm, a fine crack often grows from the dimple. If the depth of the dimple is smaller than 50 µm, the gradual cooling effect by the cooling drum as a whole is insufficient, and if the dimple depth exceeds 100 µm, the convex dimple transferred onto the thin cast strip has a large height, and therefore, grinding by a coil grinder or other treatment becomes necessary before the cold rolling, and thus the productivity is reduced. The ratio of the projection height of the convex dimple transferred onto the thin cast strip to the depth of the dimple (dimple filling ratio) is 80 to 100%. Accordingly, the surface projection of the thin cast strip of the present invention has a circular or ellipsoidal shape, a diameter of 1.0 to 1.2 mm, and a height of 40 to 100 µm. A great number of such projections are distributed while the minimum distance is maintained smaller than 0.35 mm between every two adjacent projection edges. The surface of the obtained cast strip is subjected to ordinary descaling, and the cast strip is cold-rolled at rolling reduction of 50 to 85% and annealed at 1050 to 1200°C for 0.5 to 2 minutes and is then cooled and subjected to pickling. As apparent from the foregoing description, according to the present invention, by controlling the chemical composition of a melted steel to be cast to a high δ-ferrite content side and forming dimples on the surface of a cooling drum so that the distance between every two adjacent dimple edges is smaller than 0.35 mm, a thin cast strip in which an occurrence of solidification structure unevenness, as shown in Fig. 1, is controlled, is prepared, whereby a gloss unevenness can be prevented in a strip and sheet product after cold rolling. Examples Thin cast strips were formed by casting various melted stainless steels differing in δ-Fe cal. (%) by a twin-drum continuous casting machine comprising a cooling drum having dimples arranged uniformly or randomly on the surface thereof. The surface of each thin cast strip was polished to a thickness of about 100 µm, and the solidification structure was manifested by electrolytic etching with nitric acid to examine the solidification structure unevenness. The thin cast strip was descaled, cold-rolled at a thickness reduction ratio of 50 to 90% and annealed at 1050 to 1200°C. Then the cold-rolled sheet was subjected to a salt treatment and pickled with a nitric fluoric mixed acid, and the occurrence of gloss unevenness on the surface of the finished sheet was observed. Then the obtained sheet was formed into a cylinder having a diameter of 32 mm from a disc having a diameter of 80 mm (the draw ratio was 2.5), and after 48 hours, the occurrence of delayed cracking was checked. The results are shown in Table 2. As seen from Table 2, the solidification structure of the thin cast strip prepared according to the process of the present invention (runs 1 through 6) had no unevenness, and the cold rolled sheet prepared form this thin cast strip had no gloss unevenness and had a good quality. In comparative runs 7 and 8, however, although the δ-ferrite content was appropriate, because the dimple distance was too wide, the solidification structure unevenness and the gloss unevenness of cold rolled sheet occurred. In comparative runs 9 and 10, since the δ-ferrite content was not correct, the solidification structure unevenness and gloss unevenness occurred. The product obtained in comparative run 11 had a controlled solidification structure unevenness and gloss unevenness of cold rolled sheet, but since the δ-Fe cal. content was higher than 9%, delayed cracking occurred. With respect to each of run 3 of the present invention and comparative run 9, the solidification structure unevenness of the thin cast strip and the gloss unevenness of the cold rolled sheet are illustrated in Figure 6. Figure 6(a) shows an appearance and microstructure of the thin cast strip of run 4 of the present invention and the cold rolled sheet thereof. It is seen that no unevenness occurred. Figure 6(b) shows the solidification structure unevenness of the thin cast strip obtained in comparative run 9, and it is seen that the unevenness of the thin cast strip depended on the difference of content of δ-ferrite, and that gloss unevenness of cold rolled sheet occurred due to the difference of the size of recrystallization grains. Namely, in the comparative run, the growth of recrystallization grains in the cold rolled sheet is controlled by the difference of δ-ferrite content after the casting, and therefore, the difference in the grain size resulting in an occurrence of gloss unevenness. In contrast, in the present invention, since there is no difference in the δ-ferrite content after the casting, no gloss unevenness occurred in the cold rolled sheet.

Claims: A process for the production of an austenitic stainless steel thin cast strip and cold rolled strip and sheet having an excellent surface quality, which comprises casting an austenitic stainless steel, in which δ-Fe cal. (%), defined by 3(Cr + 1.5Si + Mo) - 2.8(Ni + 0.5Mn + 0.5Cu) - 84(C + N) - 19.8, is controlled to 5 to 9%, by a continuous casting machine, in which a thin cast strip is moved synchronously with the wall surface of a cooling drum, on which many dimples having a diameter of 0.1 to 1.2 mm and a depth of 50 to 100 µm and having a circular or ellipsoidal opening are distributed so that the distance between every two adjacent dimple edges is smaller than 0.35 mm, into a thin cast strip having a thickness smaller than 10 mm. The process according to claim 1, comprising the additional steps of descaling the surface of the thin cast strip, cold-rolling the thin cast strip, and annealing the cold-rolled product to form a thin sheet product. An austenitic stainless steel thin cast strip characterized in that the chemical composition of the cast strip is such that δ-Fe cal. (%), defined by 3(Cr + 1.5Si + Mo) - 2.8(Ni + 0.5Mn + 0.5Cu) - 84(C + N) - 19.8, is 5 to 9%, and many projections having a diameter of 0.1 to 1.2 mm and a height of 40 to 100 µm and having a circular or ellipsoidal shape are distributed on the surface of the cast strip so that the distance between every two adjacent projections is smaller than 0.35 mm.

Inventors: SUEHIRO TOSHIYUKI C O NIPPON S, SUICHI ISAO C O NIPPON STEEL C, UEDA MASANORI C O NIPPON STEEL, SUEHIRO, TOSHIYUKI, C/O NIPPON STEEL CORPORATION, SUICHI, ISAO, C/O NIPPON STEEL CORPORATION, UEDA, MASANORI, C/O NIPPON STEEL CORPORATION
Applicants: NIPPON STEEL CORP, NIPPON STEEL CORPORATION
Application year: 1991
Main classification: B22D11
IPC codes: B22D11, C21D8