METHOD FOR PRELIMINARY TREATMENT OF STAINLESS STEEL FOR COLD ROLLING

Abstract: A process of producing cold rolled stainless steel strip, according to the invention, includes preliminary treatment for hot rolled stainless steel strip, in which preliminary treatment, hot rolled stainless steel strip is, at first, treated through annealing and picking process and is subsequently subject cold rolling without introducing rolling mill lubricant at a reduction of more than 5%. The process of preliminary treatment for preparation of cold rolling is useful for producing high surface brightness cold rolled stainless steel strip, which can achieve both of satisfactory quality and high efficiency.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates generally to a preliminary treatment for a stainless steel which is to be processed by a cold rolling mill for producing cold rolled stainless steel with high surface brightness or gloss. More specifically, the invention relates to a method for performing a preliminary treatment for hot rolled stainless steel strip for obtaining high quality cold rolled stainless steel strip with high surface gloss by a cold rolling process. Description of the Background Art It is known, when producing cold rolled stainless steel strip, to perform a preliminary treatment by the process of annealing after hot rolling and pickling. The preliminarily treated stainless steel strip is subject to cold rolling by means of Sendzimir mill having a work roll of 100 mm⊘ or less, and so forth. Rolling mill lubricant is supplied to the mill during cold rolling process. After cold rolling, the stainless steel strip is treated by finishing annealing and pickling. Thereafter, temper rolling is performed with a draft of 0.5% to 1.2%. The cold rolled stainless steel can be generally classified as ferritic stainless steel, such as SUS430 and austenitic stainless steel, such as SUS304. As is well known, ferritic stainless steel strip is generally used as lining material for constructions, surface material for domestic articles, and for various decorative facilities. For these purposes, ferritic stainless steel strip must have satisfactorily high surface gloss or brightness after the temper rolling process. On the other hand, the austenitic stainless steel strip is used for kitchen appliances because of its higher corrosion resistance than ferritic stainless steel strip. Such austenitic stainless steel strip is usually subject to buffing after temper rolling. After the buffing process, the austenitic stainless steel strip must have satisfactorily high surface gloss or brightness. In order to obtain satisfactorily high surface gloss or brightness, various cold rolling processes for ferritic and austenitic stainless steel strip have been proposed. For example, Japanese Patent First (unexamined) Publication (Tokkai) Showa 60-227904 and Japanese Patent First Publication (Tokkai) Showa 61-49705 propose improvements of the cold rolling process by seeking the optimum work roll diameter and surface roughness of the work roll to be utilized in the cold rolling mill. On the other hand, Japanese Patent First Publication (Tokkai) Showa 58-56013 proposes improvement to the intermediate pickling process. Furthermore, Japanese Patent First Publication (Tokkai) Showa 59-107027 proposes the additional step of grinding after the post-hot rolling annealing and pickling process. Such improvements are generally successful in improving the quality of the stainless steel strip and for providing higher surface gloss or brightness. However, such prior proposals are not satisfactory in providing satisfactorily high surface gloss or brightness for products, such as the bright annealed product, which requires very high surface gloss or brightness. In the meanwhile, in recent years, there has been a tendency to seek higher production efficiency in producing high surface brightness stainless steel strip. For this requirement, a tandem cold rolling mill having a greater work roll diameter than that of the Sendzimir mill, tends to be used in place of the Sendzimir mill. Such a tandem mill with increased diameter work roll achieves higher efficiency. That is, a significant reduction of the process time can be achieved by allowing one way rolling utilizing the tandem mill, instead of the bi-directional rolling required by the Sendzimir mill. Furthermore, because a greater amount of rolling mill lubricant can be introduced between the roll surface and the stainless steel strip surface to avoid direct contact of the roll surface onto the strip surface, this successfully prevents the strip from forming heat-streaks. On the other hand, the greater amount of rolling mill lubricant introduced between the mating surfaces of the roll and strip tends to cause difficulty in controlling the surface roughness of the strip. As a result, the surface roughness of the stainless steel strip after the rolling process becomes substantially great. This clearly degrades the surface brightness or gloss of the stainless steel strip. In order to improve this defect when a tandem cold rolling mill is used in the production of high surface brightness stainless steel strip, there have been proposed various improvements. For example, Japanese Patent First Publication (Tokkai) Showa 61-23720 proposes a process to perform annealing after hot rolling and pickling after cold rolling and subsequently to perform cold rolling once again. On the other hand, Japanese Patent First Publication (Tokkai) Showa 61-49701 proposes a cold rolling process in which cold rolling utilizing a small diameter work roll is performed subsequently to cold rolling using a large diameter work roll. Such improvements are successful in improving the surface quality of the stainless steel strip produced by the cold rolling process. However, providing additional steps, i.e. annealing after hot rolling and pickling after cold rolling and additional cold rolling by small diameter work roll, requires additional process time and lowers efficiency. Degradation of the production efficiency is significant because annealing after hot rolling and pickling or cold rolling utilizing a small diameter work roll is performed at substantially reduced line speed in comparison with that when cold rolling utilizing a large diameter work roll. The manufacture of a cold rolled plate of austentite stainless steel possessing the characteristic features of high corrosion resistance and fine surface lustre is described in Japanese patent JP-A-56 158 819. This describes heat treating a hot rolled sheet, mechanical descaling and pickling the sheet and cold rolling the sheet by conventional methods. Roberts in "Cold Rolling of Steel" (vol. 2, 1978, p 249-251, M Dekker Inc, New York, US) introduces the concept of rolling without lubrication in the tempering step. However it is noted that dry rolling may be disadvantageous owing to the accumulation of abraded particulate matter on the mill housings thereby reducing the quality of the surface finish. SUMMARY OF THE INVENTION It is an object of the present invention to provide a method for producing high surface brightness cold rolled stainless steel strip, which can achieve both satisfactory quality and high efficiency. Another object of the invention is to provide a method for performing a preliminary treatment for hot rolled stainless steel strip to be processed by a cold rolling process, which allows production of satisfactory quality and high efficiency. In order to accomplish the aforementioned and other objects, the process of producing cold rolled stainless steel strip, according to the invention, includes a preliminary treatment for hot rolled stainless steel strip, in which preliminary treatment, hot rolled stainless steel strip is, at first annealed and pickled and is subsequently subjected to cold rolling without introducing rolling mill lubricant at a reduction of more than 5%. According to the present invention, there is provided a method of treating a hot rolled stainless steel strip which comprises: (i) subjecting the hot rolled strip to a preliminary treatment comprising: (a) annealing the hot rolled strip; and (b) pickling the annealed strip; and (ii) subjecting the preliminary treated strip to cold rolling, the preliminary treatment including the step of rolling the pickled strip at a reduction of greater than 5% and in the absence of lubricant prior to effecting any cold rolling step. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be understood more fully from the detailed description given herebelow and from the accompanying drawings of the preferred embodiment of the invention, which, however, should not be taken to limit the invention to the specific embodiment but are for explanation and understanding only. In the drawings: Fig. 1 is a fragmentary and diagrammatic illustration of the preferred embodiment of an apparatus for the preliminary treatment of a stainless steel strip for cold rolling in the production of high surface brightness stainless steel strip; and Fig. 2 is a graph showing the variation of surface brightness in relation to reduction in non-lubricating cold rolling. DETAILED DESCRIPTION OF THE INVENTION Referring now to the drawings, particularly to Fig. 1, the preferred preliminary treatment apparatus for performing preliminary treatment on a hot rolled strip for cold rolling in the production of a cold rolled stainless steel strip with high surface brightness or gloss, generally comprises an annealing after hot rolling and pickling stage 1 and a non-lubricating rolling stage. The annealing after hot rolling and pickling stage 1 includes a continuous heating furnace 8 and a pickling bath 11. The non-lubricating rolling mill 2 constituting the non-lubricating rolling stage is provided downstream of the pickling bath 11. The rolling may be a multi-stand tandem mill, such as 2 High, 4 High, 6 High mill, cluster type mill or so forth. Though the shown embodiment employs a single mill in the non-lubricating rolling stage, it is possible to employ more one mill if necessary. The mill to be employed in the non-lubricating rolling stage is required to have the ability of shape control so as to control the flatness of the strip to be rolled. For example, as shown in Fig. 1, a hot rolled stainless steel strip coil 3 wound on a pay off reel 4 is rolled out. The leading edge of the hot rolled stainless steel strip is cut by a shear device 5. Then the leading edge is connected to the trailing edge of a previous strip by welding in a welding device 6 to form a continuous strip. The continuous strip is introduced into the continuous heating furnace 8 which serves as an annealing furnace, via an inlet side looper 7. In the heating furnace 8, the hot rolled stainless steel strip is subjected to an annealing process. The annealed strip is fed from the heating furnace 8 to the pickling bath 11 via a cooling zone 9 provided immediately downstream of the heating furnace and a mechanical descaling equipment 10 for removing oxidation scale on the strip surface. In the pickling bath 11, oxidation scale remaining on the surface of the stainless steel strip is completely removed. Then, the stainless steel strip is fed through a washing and drying device 12. The stainless steel strip leaving the washing and drying device 12 is fed to the non-lubricating rolling stage via an outlet side looper 13. In the non-lubricating rolling stage, tension bridle rolls 14 are provided at orientations upstream and downstream of the non-lubricating rolling mill 2. The non-lubricating rolling mill 2 is designed to perform rolling for the stainless steel strip at a reduction greater than 5%. The preliminary treated stainless steel strip 16 thus treated is wound on a tension reel 15. In general, in order to obtain high surface brightness on the cold rolled stainless steel strip product, it is essential to reduce surface roughness. The surface roughness of the product strip is generally determined by the surface roughness of the material strip produced by the cold rolling process. Namely, the surface roughness in the material strip produced by the cold rolling process determines the quality of the cold rolled stainless steel strip product. In observation, it is found that the surface roughness of the stainless steel strip after the cold rolling process, arises from uneveness originally existing on the stainless steel strip before the cold rolling and maintained even after the cold rolling process. In order to provide satisfactorily high surface brightness on the surface of the stainless steel strip, an average surface roughness Ra, as measured by difference of surface level at projecting peak and bottom of impression, has to be 0.1 µm or less. In observation, it is also found that the average surface roughness Ra of the stainless steel strip after the pickling process is 2 to 4 µm, because uneveness is created by the mechanical descaling process, for example, shot blasting and by corrosion by the presence of acid in the pickling bath 11, such as nitric acid. When such stainless steel strip having substantially great surface roughness, i.e. average roughness Ra of 2 to 4 µm, is directly fed to a cold rolling mill for cold rolling in the presence of rolling mill lubricant, the lubricant tends to be accumulated within the depressions on the surface of the stainless steel strip. The lubricant accumulated within the depressions is carried with the stainless steel strip to the portion where the surface of the strip contacts with the peripheral surface of the work roll. This lubricant held within the depressions of the stainless steel strip impairs the reduction. Therefore, even after cold rolling, uneveness originally present on the surface of the stainless steel strip leaving the pickling process, is substantially unchanged though a little reduction is achieved. The resultant substantial uneveness on the surface degrades the surface brightness of the cold rolled stainless strip product and thus degrades the quality of the product. From the observation set forth above, it has been found to be essential to reduce the surface roughness before feeding the strip to the cold rolling process. As will be appreciated, reduction of the surface roughness can be achieved by performing a smoothing process by way of rolling or by performing a grinding process. In order to obtain sufficient reduction of the surface roughness by the grinding process, a substantially long process time, e.g. about 4 to 5 hours, is required. Therefore, reduction of surface roughness by grinding process is not practically acceptable. Therefore, it is necessary to use the rolling process for reducing the surface roughness of the stainless steel strip before feeding the strip to the cold rolling mill. In the surface roughness reducing rolling process rolling mill lubricant if present, will interfere with the reduction of roughness as discussed with respect to cold rolling. Therefore, it is better if rolling mill lubricant is not supplied to the rolling mill to be used for the reduction of the surface roughness of the stainless steel strip. In addition, reduction in the roughness reducing rolling process is selected to be greater than 5% so as to obtain satisfactory reduction of the surface roughness. The optimum reduction of roughness reducing rolling is sought through experiments by performing rolling at various reductions. The experimentarily rolled strips obtained by the non-lubricating rolling mill are processed by a cold rolling process using rolling mill lubricant, finishing annealing and pickling and a temper rolling process. The surface brightness of the strips obtained by the aforementioned process are checked. Measurement of the surface roughness of respective experimentarily obtained strips is performed by a method according to Japanese Industrial Standard (JIS) Z 8741 : "Method of measurement of brightness (GS20°)". The result of measurement is shown in the graph of Fig. 2. As will be seen from Fig. 2, both in the case of rolling utilizing a tandem mill with a large diameter work roll and in the case of rolling utilizing a Sendzimir mill with a small diameter work roll, surface brightness is substantially unchanged when the reduction is less than or equal to 5% but is significantly enhanced when the reduction exceeds 5%. As will be naturally understood, non-lubricating rolling allows direct contact between the surface of the stainless steel strip and the peripheral surface of the work roll and thus tends to cause burning on the strip surface. In order to avoid the possibility of the formation of burning on the surface of the stainless steel strip, it is necessary to control the line speed to be sufficiently low. However, since the non-lubricating rolling is performed at the outlet side of the annealing after hot rolling and pickling stage or at the inlet side of the cold tandem mill, low line speed at the non-lubricating rolling mill will not significantly affect the line speed of the cold rolling mill and thus the efficiency of the rolling process. In the shown embodiment, the non-lubricating rolling mill is arranged at the outlet side of the annealing after hot rolling and pickling stage 1. EXAMPLES In order to demonstrate the advantages provided by the present invention, experiments have been performed. In the experiments, stainless steel strips having widths and thickness shown in the appended table 1, were processed by non-lubricating rolling under the rolling condition shown in the appended tables 2(A) and 2(B) and table 3(A) and 3(B). After non-lubricating rolling, the strips were subjected to cold rolling by means of a tandem mill and Sendzimir mill. The cold rolled strips were processed by finishing annealing and pickling and subsequently by temper rolling. For the austenitic stainless steel strip in the table 3(A) and 3(B), buffing was performed after the temper rolling process. Visual brightness of ferritic stainless steel strips resulting from the experiments are shown in table 2(A) and 2(B) and visual brightness of austenitic stainless steel strips resulting from the experiments are shown in table 3(A) and 3(B). The visual brightness level indicated in the tables 2(A) and 2(B) and table 3(A) and 3(B) is classified into five levels viz. special A, A, B, C and D in order of high level to low level. Namely, the strip having the highest surface brightness level is classified at special A level and the strip having the lowest surface brightness level is classified at D level. In order to compare the surface brightness with that of strips produced by the process according to the invention, comparative experiments were also performed. In the comparative experiments, stainless steel strips were produced by the conventional process which does not have the non-lubricating rolling step. In addition, other comparative experiments were performed by reducing the draft during non-lubricating rolling to be less than 5%. The results of these comparative experiments are also shown in tables 2(A). 2(B), 3(A) and 3(B). As will be clear from the results of experiments shown in the tables 2(A), 2(B), 3(A) and 3(B), the surface brightness of the stainless steel strips produced by the process according to the present invention, is much higher than that of the comparative examples. The surface brightness level obtained by the process according to the invention exhibits equivalent quality to that produced by cold rolling utilizing the Sendzimir mill which is significantly less efficient than that of the process of the invention. While the present invention has been disclosed in terms of the preferred embodiment in order to facilitate better understanding of the invention, it should be appreciated that the invention can be embodied in various ways without departing from the principle of the invention. Therefore, the invention should be understood to include all possible embodiments and modifications to the shown embodiments which can be embodied without departing from the principle of the invention set out in the appended claims.

Claims: A method of treating a hot rolled stainless steel strip which comprises: (i) subjecting the hot rolled strip to a preliminary treatment comprising: (a) annealing the hot rolled strip; and (b) pickling the annealed strip; and (ii) subjecting the preliminary treated strip to cold rolling, the preliminary treatment including the step of rolling the pickled strip at a reduction of greater than 5% and in the absence of lubricant prior to effecting any cold rolling step. A method according to claim 1 comprising the additional steps after the cold rolling, of finish annealing, pickling and temper rolling. A method as claimed in claims 1 or 2 wherein the non-lubricating rolling is effected at a speed of 60 m/min or less.

Inventors: ABE, HIDEO, FUKUHARA, AKIHIKO, KENMOCHI, KAZUHITO, KISHIDA, AKIRA, KITAHAMA, MASANORI, KOBAYASHI, MAKOTO, KOMATU, TOMIO, YANAGISHIMA, FUMIYA
Applicants: KAWASAKI STEEL CORPORATION
Application year: 1988
Main classification: C21D8
IPC codes: B21B3, B21B45, B21B1, C21D8