METHOD OF MANUFACTURING STAINLESS STEEL SHEET OF HIGH CORROSION RESISTANCE

Abstract: A method of manufacturing stainless steel sheet having a higher corrosion resistance than convential, by preventing roughness of the surface of steel sheet during the production of stainless steel sheet, especially, stainless steel sheet in which the contents of C, S and O are reduced to extremely low levels, without treating the surface of steel plate after annealing and pickling. A stainless steel material containing not more than 0.01 wt.% of C, not more than 0.005 wt.% of S and not more than 0.005 % of 0 is hot-rolled at a draft of over 30 % at a temperature of not higher than 830°C, and cooled at a cooling speed of not lower than 25°C/sec. The steel sheet is taken up in a roll at not higher than 650°C, and then annealed and pickled.

Description: TECHNICAL FIELD This invention relates to a process for the production of stainless steel, and more particularly to a process for the production of stainless steel sheets having excellent corrosion resistance. BACKGROUND ART Stainless steel sheets are excellent in corrosion resistance under various corrosive environments and are widely used as building materials, materials for automobiles, materials for chemical plants and so on. Recently, the service environments have become more severe and thus stainless steel sheets are required to have even better corrosion resistance. On the other hand, stainless steels which involve much labor in production (even though their corrosion resistance is excellent) are unfavorable from the viewpoint of stainless steel manufacturers. Thus it is desired to obtain stainless steel that has excellent productivity, particularly hot workability. It has recently been found to be possible to reduce impurities in steel by advanced steel-making techniques, so that attempts have been made to improve corrosion resistance and hot workability by decreasing C, S and O in the stainless steel. For example, JP-B-60-57501 discloses a method of improving anti-corrosion in sea water and hot workability by decreasing C, S and O, and JP-B-2-46642 and JP-B-2-14419 disclose methods of mainly improving the hot workability similar to the above method. However, using the above conventional improving techniques may result in marked chapping in the surface of the stainless steel sheet after hot rolling-annealing - pickling. Such chapping falls down during cold rolling and causes a scab-like defect after the cold rolling. This undesirably deteriorates the corrosion resistance of the hot rolled steel sheet and the cold rolled steel sheet. Of course, attempts have been made to trim the chapped surface of the steel sheet by means of a grinder or the like. However this brings about a decrease in productivity and an increase in cost and is thus not an advantageous countermeasure. It is therefore strongly desired to provide a technique which does not result in the above chapping on the surface of the stainless steel sheet after annealing - pickling. DISCLOSURE OF INVENTION It is, therefore, desirable to solve the aforementioned problems in the production of the present stainless steel sheets, particularly stainless steel sheets having extremely low amounts of C, S and O, and to provide a process for the production of stainless steel sheets having improved corrosion resistance as compared with the conventional ones without having to trim the surface of the steel sheet after annealing - pickling. In order to achieve this, various studies have been made with respect to the cause of the chapping on the surface of conventional stainless steel sheet after annealing - pickling and also means for the prevention thereof has been examined. As a result, the following facts have been confirmed. That is, 1) The chapping of the steel sheet surface is caused due to the fact that the Cr-removed layer formed during the annealing is eroded with an acid which forms unevenness on the surface of the steel sheet. 2) The Cr-removed layer grows as the amount of scale (Fe 3 O 4 ) in the hot rolled sheet becomes large. 3) The Cr-removed layer grows as the adhesion of the scale (Fe 3 O 4 ) in the hot rolled sheet to the iron matrix becomes strong. 4) The scale (Fe 3 O 4 ) in the hot rolled sheet is formed at a relatively low temperature below 830°C. From the above facts, the inventors have noticed the following: 5) In order to prevent chapping of the steel sheet surface, it is effective to decrease the amount of scale (Fe 3 O 4 ) and to lower its adhesion to the iron matrix. 6) In order to decrease the amount of scale (Fe 3 O 4 ) and lower the adhesion to the iron matrix, it is effective to control the finish temperature of the hot rolling, and the subsequent cooling rate and coiling temperature. Although the mechanism of forming the Cr-removed layer through the aforementioned scale (Fe 3 O 4 ) is not certain, the following has been noted. In general, the annealing of cold rolled stainless steel sheet is carried out in a relatively high temperature and low oxygen atmosphere. If the stainless steel is annealed in such an atmosphere, it is oxidised to form Cr 2 O 3 , but since this Cr 2 O 3 provides protection against subsequent oxidation, the oxidation rate gradually lowers and finally the Cr-removed layer hardly forms on the surface of the steel sheet. In the hot rolling of the stainless steel (hereinafter abbreviated as hot rolling in some cases), the atmosphere is different from that in the above annealing, so that scale composed mainly of Fe 3 O 4 is formed. When this Fe 3 O 4 scale has strong adhesion to the iron matrix, the scale absorbs Cr from the iron matrix in the annealing according to the following reaction: (3/2) O 2 + Fe 3 O 4 + 2Cr → Fe 2 O 3 + FeCr 2 O 4 or 4O 2 + Fe 3 O 4 + 6Cr → 3FeCr 2 O 4 Thus, when Fe 3 O 4 exists on the surface, Cr is consumed without the formation of Cr 2 O 3 , with its protection against oxidation, and hence it is believed to considerably promote the growth of the Cr-removed layer. Further, the reason why the Fe 3 O 4 scale in the hot rolled sheet grows at a relatively low temperature below 830°C is considered to be due to the fact that when the steel sheet is cooled in air after the hot rolling, Fe is sufficiently rapidly oxidised, while Cr in the steel is slow to diffuse and cannot diffuse up to the surface. Hence the main component of the scale is Fe. Also, the reason why the degree of surface chapping after pickling stainless steel containing extreme-low levels of C, S and O is larger than that of stainless steel containing approximately the usual level of C, S and O, is believed to be due to the fact that the adhesion of the scale to the iron matrix is high in stainless steel containing extreme-low levels of C, S and O. According to one aspect of the present invention there is provided a process for the production of a stainless steel sheet having excellent corrosion resistance, which process comprises providing a ferritic stainless steel consisting of C in an amount of not more than 0.01 wt%, S in an amount of not more than 0.005 wt%, O in an amount of not more than 0.005 wt%, Si in an amount of not more than 3 wt%, Mn in an amount of not more than 5 wt%, Cr in an amount of from 9 to 50 wt%, less than 5 wt% of Ni, and optionally one or more elements selected from the group consisting of Ti: 0.01-1.0 wt%, Nb: 0.01-1.0 wt%, V: 0.01-1.0 wt%, Zr: 0.01-1.0 wt%, Ta: 0.01-1.0 wt%, Co: 0.1-5 wt%, Cu: 0.1-5 wt%, Mo: 0.1-5 wt%, W: 0.1-5 wt%, Al: 0.005-5.0 wt%, Ca: 0.0003-0.01 wt% and B: 0.0003-not more than 0.01 wt%, with the remainder being Fe and inevitable impurities; subjecting the steel to hot rolling at a draft below 830°C of not less than 30%; cooling the resultant hot rolled sheet at a cooling rate of not less than 25°C/sec; coiling the sheet at a temperature of not higher than 650°C; and thereafter subjecting the sheet to annealing and pickling. According to another aspect of the present invention there is provided a process for the production of a stainless steel sheet having excellent corrosion resistance, which process comprises providing an austenitic stainless steel or dual-phase stainless sheet consisting of C in an amount of not more than 0.01 wt%, S in an amount of not more than 0.005 wt%, O in an amount of not more than 0.005 wt%, Si in an amount of not more than 3 wt%, Mn in an amount of not more than 20 wt%, Cr in an amount of 9-50 wt%, Ni in an amount of 5-20 wt%, N in an amount of not more than 0.2 wt%, and optionally one or more elements selected from the group consisting of Ti: 0.01-1.0 wt%, Nb: 0.01-1.0 wt%, V: 0.01-1.0 wt%, Zr: 0.01-1.0 wt%, Ta: 0.01-1.0 wt%, Co: 0.1-5 wt%, Cu: 0.1-5 wt%, Mo: 0.1-5 wt%, W: 0.1-5 wt%, Al: 0.005-5.0 wt%, Ca: 0.0003-0.01 wt% and B: 0.0003-not more than 0.01 wt%, with the remainder being Fe and inevitable impurities; subjecting the steel to hot rolling at a draft below 830°C of not less than 30%; cooling the resultant hot rolled sheet at a cooling rate of not less than 25°C/sec; coiling the sheet at a temperature of not higher than 650°C; and thereafter subjecting the sheet to annealing and pickling. With regard to the optional element which may be present, it is effective to use elements in each group of 1 ○ Ti, Nb, V, Zr, Ta, 2 ○ Co, Cu, 3 ○ Mo, W, 4 ○ A1, 5 ○ Ca and 6 ○ B alone or a combination of two or more elements selected from each group of 1 ○ - 6 ○. The various features of the invention will be described below. Draft below 830°C of not less than 30%; In the extreme-low C, S, O stainless steel, working in the above range acts to lower the adhesion between the scale and the iron matrix by generating cracks in the Fe 3 O 4 scale produced in the hot rolling, whereby the growth of the Cr-removed layer can be controlled during the annealing to enhance the corrosion resistance.Thus, the draft below 830°C particularly promoting the growth of the Fe 3 O 4 scale is important. When the value of the draft is less than 30%, a sufficient amount of strain is not given and hence sufficient cracks for the improvement of the corrosion resistance cannot be introduced. Therefore, the draft below 830°C is to be not less than 30%.Moreover, the term "draft" used herein is the ratio of sheet thickness after hot rolling to thickness of the steel sheet at 830°C and may be attained by multiple rolling or single rolling. Also, it is desirable that the rolling temperature is low. However when the rolling temperature is too low, surface defects in the hot rolling increase and hence the unevenness after the pickling is increased by factors other than the Cr-removed layer produced through oxidation during the annealing. Therefore, it is desirable that the rolling is carried out at a temperature of not lower than 700°C.The influence of the draft below 830°C upon corrosion resistance of each of hot rolled sheets and cold rolled sheets is shown in Fig. 1 using extreme-low C, extreme-low S, extreme-low O steel (hereinafter referred to as extreme-low CSO steel simply, C: 0.0050 wt%, S: 0.0040 wt%, O: 0.0040 wt%) and commercially available steel (C: 0.0500 wt%, S: 0.0082 wt%, O: 0.0068 wt%) as two kinds of SUS 304, and in Fig. 2 using extreme-low CSO steel (C: 0.0020 wt%, S: 0.0038 wt%, O: 0.0030 wt%) and commercially available steel (C: 0.0520 wt%, S: 0.0068 wt%, O.: 0.0065 wt%) as two kinds of SUS 430, respectively. Moreover, the hot rolled sheet is obtained by subjecting to hot rolling (cooling rate: 40°C/sec, coiling temperature: 600°C)-annealing - pickling, and the cold rolled sheet is obtained by subjecting to hot rolling (cooling rate: 45°C/sec, coiling temperature: 600°C) - annealing-pickling - cold rolling (draft at roll diameter of 250 mm: 50%) - annealing - pickling. The corrosion resistance is evaluated by rust generating area ratio after 2 days of CCT test.In these figures, symbol ▪ is a hot rolled sheet of the extreme-low CSO steel, symbol □ is a cold rolled sheet of the extreme-low CSO steel, symbol  is a hot rolled sheet of the commercially available steel, and symbol O is a cold rolled sheet of the commercially available steel. From these figures, it can be seen that when the draft below 830°C is not less than 30%, there is particularly the effect of considerably improved corrosion resistance for the extreme-low CSO steel. Cooling rate of not less than 25°C/sec; When the cooling rate is increased after the completion of the hot rolling, not only is the amount of scale produced after the hot rolling decreased, but also the adhesion between the scale and the iron matrix is decreased because of the difference in thermal expansion of the iron matrix, so that the increase of the cooling rate is effective for peeling of the scale. Thus, the growth of the Cr-removed layer can be controlled in the subsequent annealing to enhance the corrosion resistance.Since such an effect is not obtained at a cooling rate of less than 25°C/sec, the cooling rate is limited to not less than 25°C/sec. Moreover, the preferable cooling rate is not less than 40°C/sec.The influence of the cooling rate after the completion of the hot rolling upon corrosion resistance of each of hot rolled sheets and cold rolled sheets is shown in Fig. 3 using extreme-low CSO steel (C: 0.0050 wt%, S: 0.0040 wt%, O: 0.0040 wt%) and commercially available steel (C: 0.0500 wt%, S: 0.0082 wt%, O: 0.0068 wt%) as two kinds of SUS 304, and in Fig. 4 using extreme-low CSO steel (C: 0.0020 wt%, S: 0.0038 wt%, O: 0.0030 wt%) and commercially available steel (C: 0.0520 wt%, S: 0.0068 wt%, O.: 0.0065 wt%) as two kinds of SUS 430, respectively. Moreover, the hot rolled sheet is obtained by subjecting to hot rolling (draft below 830°C: 30%, coiling temperature: 550°C) annealing - pickling, and the cold rolled sheet is obtained by subjecting it to hot rolling (draft below 830-°C: 35%, coiling temperature: 550°C) - annealing pickling - cold rolling (draft at roll diameter of 300 mm: 50%) - annealing - pickling. The corrosion resistance was evaluated by the rust generating area ratio after 2 days of CCT test.In these figures, symbol ▪ is a hot rolled sheet of the extreme-low CSO steel, symbol □ is a cold rolled sheet of the extreme-low CSO steel, symbol  is a hot rolled sheet of the commercially available steel, and symbol O is a cold rolled sheet of the commercially available steel. From these figures, it can be seen that when the cooling rate after the hot rolling is not less than 25°C/sec, there is particularly a considerably improved corrosion resistance for the extreme-low CSO steel. Coiling temperature of not higher than 650°C; The coiling temperature affects the adhesion between the scale and the iron matrix and the amount of scale produced after the coiling. When the coiling temperature exceeds 650°C, it is insufficient to weaken the adhesion between the scale and the iron matrix and also the amount of scale produced after the coiling is increased. As a consequence, the growth of the Cr-removed layer is promoted at the subsequent annealing to degrade the corrosion resistance. Therefore, in order to control the Cr-removed layer to improve the corrosion resistance, it is necessary to restrict the coiling temperature to not higher than 650°C. Although it is desired for the coiling temperature to be low, if it is too low, the surface defects in the coiling are increased to increase the unevenness after the pickling based on factors other than the Cr-removed layer. Thus it is desirable to carry out coiling at a temperature of not lower than 200°C.The influence of the coiling temperature after the hot rolling upon corrosion resistance of each of hot rolled sheets and cold rolled sheets is shown in Fig. 5 using extreme-low CSO steel (C: 0.0050 wt%, S: 0.0040 wt%, O: 0.0040 wt%) and commercially available steel (C: 0.0500 wt%, S: 0.0082 wt%, O: 0.0068 wt%) as two kinds of SUS 304, and in Fig. 6 using extreme-low CSO steel (C: 0.0020 wt%, S: 0.0038 wt%, O: 0.0030 wt%) and commercially available steel (C: 0.0520 wt%, S: 0.0068 wt%, O: 0.0065 wt%) as two kinds of SUS 430, respectively. Moreover, the hot rolled sheet is obtained by subjecting to hot rolling (draft below 830°C: 40%, cooling rate: 40°C/sec) - annealing-pickling, and the cold rolled sheet is obtained by subjecting to hot rolling (draft below 830°C: 40%, cooling rate: 45°C/sec) - annealing - pickling - cold rolling (draft at roll diameter of 250 mm: 45%)-annealing - pickling. The corrosion resistance was evaluated by the rust generating area ratio after 2 days of CCT test.In these figures, symbol ▪ is a hot rolled sheet of the extreme-low CSO steel, symbol □ is a cold rolled sheet of the extreme-low CSO steel, symbol  is a hot rolled sheet of the commercially available steel, and symbol O is a cold rolled sheet of the commercially available steel. From these figures, it can be seen that when the coiling temperature after the hot rolling and quenching is not higher than 650°C, there is particularly a considerably improved corrosion resistance for the extreme-low CSO steel. Thickness of hot rolled sheet of not more than 1.5 mm and draft of skin pass rolling of not more than 20%; In general, stainless steel sheets having a thickness of not more than 1.5 mm are produced by subjecting the hot rolled sheet to a cold rolling. Of course, cold rolled stainless steel sheets can be produced by applying the invention to the above process, but attempts have recently been made to produce stainless steel sheets having a thickness of not more than 1.5 mm by so-called hot rolling-annealing - pickling steps with the omission of the cold rolling step in accordance with the increase of capacity of the hot rolling mill and the reduction in slab thickness. If the steel sheet is produced using such steps according to the conventional technique, there is the problem that the surface chapping is still retained after the pickling and this lowers the corrosion resistance as compared to the conventional cold rolled sheet.On the other hand, the process according to the invention develops a remarkable effect when the steel sheet is produced using the above steps, particularly when the skin pass rolling is carried out at a draft of not more than 20% for a hot rolled sheet having a thickness of not more than 1.5 mm. That is, the thickness of the hot rolled sheet is restricted to not more than 1.5 mm and the draft of the skin pass rolling is restricted to not more than 20%, preferably 1-15%. In this way, it is possible to produce stainless steel corresponding to the conventional bright-finished cold rolled sheet using the above steps. Work roll diameter of not less than 250 mm in the cold rolling installation and total draft of more than 20% through the work rolls; In general, stainless steel cold rolled sheets are produced by cold rolling with rolls having a diameter of not more than 100 mm, but the productivity is very low as compared with tandem rolling mills using a large-size roll as are usually used in the rolling of general-purpose steel. Consequently, there has recently been an increase in subjecting stainless steel to cold rolling using a tandem rolling mill. However, when using a tandem rolling mill, there is the problem that surface defects are apt to be caused by the unevenness of the surface before the cold rolling which lowers the corrosion resistance.The process of the invention develops a remarkable effect in the above step, particularly when cold rolling is carried out at a total draft of more than 20% through work rolls having a diameter of not less than 250 mm. Thus the work roll diameter in the cold rolling installation should be restricted to not less than 250 mm and the total draft through the work rolls should be restricted to more than 20%. After such a cold rolling, annealing pickling or bright annealing may be conducted according to the usual manner.When carrying out the process of the invention, the production conditions, other than those in the above steps, are not particularly critical, and may be within the usual ranges. For example, it is favorable that the heating temperature of the slab is 1000-1300°C, and the annealing temperature is 700-1300°C, and the pickling step involves immersion in mixed acid (nitric acid and hydrofluoric acid) after immersion in sulfuric acid. Further, it is preferable to conduct a passivating treatment after the pickling in order to improve the corrosion resistance further.The chemical composition of stainless steel will be described below. C: not more than 0.010wt%, S: not more than 0.0050wt%, O: not more than 0.0050wt%; These elements lower not only the corrosion resistance of stainless steel but also the hot workability, so that it is desired to reduce the amounts of these elements. Particularly, when C, S and O are included in amounts of more than 0.0100 wt%, more than 0.0050 wt% and 0.0050 wt%, respectively, the corrosion resistance is considerably degraded, and good corrosion resistance cannot be obtained even if the stainless steel is produced under the conditions according to the invention process. Therefore, the amounts of these elements are restricted to C: not more than 0.0100 wt%, S: not more than 0.0050 wt% and O: not more than 0.0050 wt%, preferably C: not more than 0.0030 wt%, S: not more than 0.0020 wt% and O: not more than 0.0040 wt%. Si: not more than 3 wt%; Si is an element effective for increasing the strength of steel, improvement of oxidation resistance, reduction of the oxygen amount in steel and stabilisation of ferrite phase. However, when the Si amount exceeds 3 wt%, the unevenness after annealing-pickling increases due to the increase of surface defects in the hot rolling and degradation of corrosion resistance is caused by factors other than the Cr-removed layer, so that the Si amount is restricted to not more than 3 wt%. Moreover, the above effect appears in an amount of not less than 0.05 wt% and becomes clear in an amount of not less than 0.1 wt%. Mn: not more than 5 wt% (ferritic), Mn: not more than 20 wt% (austenitic, dual-phase); Mn is an element effective for the increase of strength and improvement of hot workability in ferritic stainless steel. When Mn is included in an amount of more than 5 wt%, the unevenness after annealing-pickling increases due to the increase of surface defects in the hot rolling and degradation of corrosion resistance is caused by factors other than the Cr-removed layer, so that the amount is restricted to not more than 5 wt%. Moreover, the effect of Mn appears in an amount of not less than 0.05 wt% in the ferritic stainless steel.Further, Mn is an element effective for not only the increase of strength and improvement of hot workability but also the stabilisation of the austenite phase in austenitic stainless steel or dual-phase stainless steel. When Mn is included in an amount of more than 20 wt%, the unevenness after annealing-pickling increases due to the increase of surface defects in the hot rolling and degradation of corrosion resistance is caused by factors other than the Cr-removed layer as above, so that the amount is restricted to not more than 20 wt%. Moreover, the effect of Mn appears in an amount of not less than 0.10 wt% in the austenitic stainless steel or dual-phase stainless steel. Cr: 9-50 wt%; Cr is an element for the improvement of corrosion resistance, but does not provide an improvement at an amount of less than 9 wt%. On the other hand, when Cr is included in an amount of more than 50 wt%, the unevenness after annealing - pickling increases due to the increase of surface defects in the hot rolling and degradation of corrosion resistance is caused by factors other than the Cr-removed layer, so that the amount is restricted to not more than 50 wt%.Moreover, it is preferable that the amount is 12-30 wt% from a viewpoint of the corrosion resistance and productivity. Ni: less than 5 wt% (ferritic), 5-20 wt% (austenitic, dual-phase); Ni is an element effective for improving workability, oxidation resistance and toughness in ferritic stainless steel, so that it may be included in an amount of not less than about 0.1 wt%. However, when it is included in an amount of not less than 5 wt%, martensite phase is formed and the steel becomes considerably brittle, so that the amount is restricted to less than 5 wt%.Further, Ni is an element required for not only the improvement of workability, corrosion resistance and toughness but also for the stabilisation of the austenite phase in austenitic stainless steel and dual-phase stainless steel. When the Ni amount is less than 5 wt%, the effect is not obtained, while when it exceeds 20 wt%, the unevenness after annealing-pickling increases due to the increase of surface defects in the hot rolling and degradation of corrosion resistance is caused by factors other than the Cr-removed layer, so that the amount is restricted to not more than 20 wt%. N: not more than 0.2000 wt% (austenitic, dual-phase); N is an element effective for the increase of strength and improvement of corrosion resistance in steel and for the stabilisation of the austenite phase in austenitic stainless steel and dual phase stainless steel. When it is included in an amount of more than 0.2000 wt%, the unevenness after annealing - pickling increases due to the increase of surface defects in the hot rolling and degradation of corrosion resistance is caused by factors other than the Cr-removed layer, so that the amount is restricted to not more than 0.2000 wt%. Moreover, the above effect appears in an amount of not less than about 0.01 wt%. Also, the N amount in ferritic stainless steel is desirably not more than 0.02 wt%.In the invention, one or more elements selected from Ti: 0.01-1.0 wt%, Nb: 0.01-1.0 wt%, V: 0.01-1.0 wt%, Zr: 0.01-1.0 wt%, Ta: 0.01-1.0 wt%, Co: 0.1-5 wt%, Cu: 0.1-5 wt%, Mo: 0.1-5 wt%, W.: 0.1-5 wt%, Al: 0.01-1.0 wt%, Ca: 0.0003-0.0100 wt% and B: 0.0003-0.0100 wt% may further be included into the above ferritic stainless steel, austenitic stainless steel and dual-phase stainless steel. The reason for these limitations will be described below. 1 ○ Ti: 0.01-1.0 wt%, Nb: 0.01-1.0 wt%, V: 0.01-1.0 wt%, Zr: 0.01-1.O wt%, Ta: 0.01-1.O wt%; These elements are added to fix C, N in steel to provide good mechanical properties. This effect is obtained with Ti: not less than 0.01 wt%, Nb: not less than 0.01 wt%, V: not less than 0.01 wt%, Zr: not less than 0.01 wt%, Ta: not less than 0.01 wt%. When the amounts of these elements are too large, the unevenness after annealing - pickling increases due to the increase of surface defects in the steel-making and hot rolling and degradation of corrosion resistance is caused by factors other than the Cr-removed layer, so that the amounts are restricted to Ti: not more than 1.0 wt%, Nb: not more than 1.0 wt%, V: not more than 1.0 wt%, Zr: not more than 1.0 wt%, Ta: not more than 1.0 wt%. Preferably, they are Ti: 0.01-0.6 wt%, Nb: 0.01-0.6 wt%, V: 0.01-0.6 wt%, Zr: 0.01-0.6 wt%, Ta: 0.01-0.6 wt%.Moreover, each element in this element group has a function and effect substantially common to those of the following element groups, so that substantially the same function and effect are developed even in a combination of the other elements when using one of these elements. Therefore, elements in each group will be described together in the following explanation. 2 ○ Co: 0.1-5 wt%, Cu: 0.1-5 wt%; These elements have an effect of improving the workability and toughness in ferritic stainless steel and have an effect of stabilising the austenite phase to control the formation of strain induced martensite or the like and improving the workability in the austenitic stainless steel and dual-phase stainless steel. These effects are obtained with Co: not more than 0.1 wt%, Cu: not less than 0.1 wt% in any stainless steels. However, when the amounts of these alloying elements are too large, the unevenness after annealing - pickling increases due to the increase of surface defects in the hot rolling and degradation of corrosion resistance is caused by factors other than the Cr-removed layer, so that the amounts are restricted to Co: not more than 5 wt%, Cu: not more than 5 wt%. 3 ○ Mo: 0.1-5 wt%, W: 0.1-5 wt%; These elements have an effect of improving the corrosion resistance of stainless steel. This effect is obtained with Mo: not less than 0.1 wt%, W: not less than 0.1 wt%. However, when the amounts of these alloying elements are too large, the unevenness after annealing - pickling increases due to the increase of surface defects in the hot rolling and degradation of corrosion resistance is caused by factors other than the Cr-removed layer, so that the amounts are restricted to Mo: not more than 5 wt%, W: not more than 5 wt%. 4 ○ A1: 0.005-5.0 wt%; A1 has an effect for improving not only the oxidation resistance of steel but also the strength. This effect is obtained in an amount of not less than 0.005 wt%. However, when the Al amount is too large, the unevenness after annealing - pickling increases due to the increase of surface defects in the steel-making and hot rolling and degradation of corrosion resistance is caused by factors other than the Cr-removed layer, so that the amount is restricted to not more than 5.0 wt%. 5 ○ Ca: 0.0003-0.0100 wt%; Ca has an effect of controlling the form of inclusion in steel and the strength to improve the mechanical properties and toughness. This effect is obtained in an amount of not less than 0.0003 wt%. However, when the addition amount is too large, the unevenness after annealing - pickling increases due to the increase of surface defects in the steel-making and hot rolling and degradation of corrosion resistance is caused by factors other than the Cr-removed layer, so that the amount is restricted to not more than 0.0100 wt%. 6 ○ B: 0.0003-0.0100 wt%; B has an effect of causing segregation in grain boundary to increase the strength of the grain boundary and improve secondary work brittleness. This effect is obtained with an amount of not less than 0.0003 wt%. However, when the addition amount is too large, the unevenness after annealing - pickling increases due to the increase of surface defects in the steel-making and hot rolling and degradation of corrosion resistance is caused by factors other than the Cr-removed layer, so that the amount is restricted to not more than 0.0100 wt%. Particularly, the other components are not necessarily restricted, but it is desirable that P is not more than 0.05 wt%. As the above selective addition elements in the invention, it is effective to use elements in each group of 1 ○ - 6 ○ alone or to use a combination of 2 or more elements selected from the groups of 1 ○ - 6 ○. BRIEF DESCRIPTION OF DRAWINGS Fig. 1 is a graph showing the relation between draft below 830°C and the rust generating area ratio in SUS 304 stainless steel. Fig. 2 is a graph showing the relation between draft below 830°C and the rust generating area ratio in SUS 430 stainless steel. Fig. 3 is a graph showing the relation between cooling rate after the completion of hot rolling and the rust generating area ratio in SUS 304 stainless steel. Fig. 4 is a graph showing the relation between cooling rate after the completion of hot rolling and the rust generating area ratio in SUS 430 stainless steel. Fig. 5 is a graph showing the relation between coiling temperature and the rust generating area ratio in SUS 304 stainless steel. Fig. 6 is a graph showing the relation between coiling temperature and the rust generating area ratio in SUS 430 stainless steel. Each of the stainless steels having the chemical compositions shown in Tables 1 to 4 (in the column headed "kind of steel" in each Table, F is ferritic, A is austenitic and D is dual-phase) was melted in a convertor, subjected to degassing by VOD process and adjustment of slight components, and continuously cast into a slab of 200 mm in thickness. Then, the slab was reheated at 1200°C for 2 hours, rough-rolled to a thickness of 10-20 mm, and further continuously finish rolled to obtain a hot rolled sheet having a thickness of 0.9-4 mm. This hot rolling step was carried out under various conditions of draft below 830°C, finish temperature of hot rolling, cooling rate and coiling temperature. After the hot rolling, the hot rolled sheets No. 1-49, 90, 92 and 94-98 were subjected to a continuous annealing in which they were heated at 1150°C in a butane burning atmosphere for 1 minute and cooled to room temperature with water, and the hot rolled sheets No. 50-56, No. 72, 80, 81 and 93 were subjected to a continuous annealing in which they were heated at 1000°C in a butane burning atmosphere for 1 minute and cooled to room temperature with water, and the hot rolled sheets No. 57-71, 73-79, 82-89, 91, 95 and 99-101 were subjected to a batch annealing in which they were heated at 850°C in an atmosphere of H 2 gas: 5% and the remainder: N 2 gas having a dew point of -30°C for 5 hours and gradually cooled to room temperature. Thereafter, the annealed sheets were subjected to a mechanical preliminary descaling with shot blast, immersed in an aqueous solution of 80°C containing H 2 SO 4 : 200 g/l (0.2 g/cm 3 ) for 10 seconds and then immersed in an aqueous solution of 60°C containing HF: 25 g/l (0.025 g/cm 3 ) and HNO 3 : 150 g/l (0.150 g/cm 3 ) for 10 seconds and washed with water to complete pickling and descaling. Test specimens of 1 ○ as-hot-rolled, 2 ○ subjected to 10% skin pass rolling or 3 ○ further subjected to cold rolling were each made from the above hot rolled sheets and then subjected to a test for corrosion resistance. Moreover, the test specimen 2 ○ was made from only the hot rolled sheets having a thickness of not more than 1.5 mm. Further, the test specimen 3 ○ was made by the following method. That is, the hot rolled sheets were subjected to a cold rolling at various drafts in a tandem rolling mill comprising rolls of 250 mm in diameter. Then, the cold rolled sheets No. 1-32, 66, 68, 70, 72-74 were subjected to an annealing in which they were heated at 1150°C in a butane gas burning atmosphere for 10 seconds and cooled in air to room temperature. Thereafter, they were subjected to an electrolysis in an aqueous solution of 80°C neutral salt containing Na 2 SO 4 : 200 g/l at a current density: 10 A/dm 2 for 40 seconds so as to dissolve the steel sheet at anode, immersed in an aqueous solution of 60°C containing HF: 25 g/l (0.025 g/cm 3 ), HNO 3 : 55 g/l (0.055 g/cm 3 ) for 10 seconds, and subjected to an electrolysis in an aqueous solution containing HNO 3 : 100 g/l (0.100 g/cm 3 ) at a current density: 10 A/dm 2 to passivate the steel sheet. The cold rolled sheets No. 33-65, 67, 69, 71, 75-77 were subjected to a bright annealing by heating at 900°C in an ammonia decomposed gas for 10 seconds. Tables 5 - 8 show not only the thickness of the hot rolled sheet but also the draft below 830°C, the finish temperature of the hot rolling, the cooling rate, the coiling temperature and the draft of cold rolling through work rolls having a diameter of 250 mm. The corrosion resistance was examined with respect to the test specimens made by the above method. That is, a CCT test involving spraying an aqueous solution at 35°C containing NaCl: 5% for 4 hours, drying for 2 hours and holding in a wet atmosphere for 2 hours as one cycle was conducted, and the degree of rust generation after 2 days was compared. The results are also shown in Tables 5-8. Sheets No. 1-89 according to the invention process exhibit good corrosion resistance because the rust generating area ratio was not more than 5% in all of hot rolled sheets, hot rolled-skin pass rolled sheets and cold rolled sheets. On the contrary, the rust generating area ratio exceeds 5% in sheets No. 90, 91, 93 wherein the draft below 830°C was less than 30%, sheets No. 92, 93 wherein the cooling rate was less than 25°C/sec, sheets No. 93, 94, 95 wherein the coiling temperature exceeded 650°C and sheets No. 96-101 wherein the production conditions were within the ranges defined in the invention but the C, S, O amounts were too high, so that these sheets were poor in corrosion resistance. INDUSTRIAL APPLICABILITY As mentioned above, the starting material steel containing C: not more than 0.100 wt%, S: not more than 0.0050 wt% and O: not more than 0.0050 wt% is hot rolled at a draft below 830°C of not less than 30%, cooled at a cooling rate of not less than 25°C/sec and coiled below 650°C, whereby the growth of Cr-removed layer in the annealing, which has become a problem in stainless steels having extreme-low amounts of C, S and O, can be controlled and the surface chapping of the steel sheet in subsequent pickling can be prevented. Consequently, it is possible to considerably improve the corrosion resistance of the extreme-low C, S, O stainless steel sheet, and particularly this effect becomes large when the sheet is finished by skin pass rolling after hot rolling - annealing - pickling, or when cold rolling is conducted through large size rolls. Furthermore, according to the invention, the surface defects can considerably be reduced, so that there are provided cold rolled sheets having a beautiful surface and a good gloss.

Claims: A process for the production of a stainless steel sheet having excellent corrosion resistance, which process comprises providing a ferritic stainless steel consisting of C in an amount of not more than 0.01 wt%, S in an amount of not more than 0.005 wt%, O in an amount of not more than 0.005 wt%, Si in an amount of not more than 3 wt%, Mn in an amount of not more than 5 wt%, Cr in an amount of from 9 to 50 wt%, less than 5 wt% of Ni, and optionally one or more elements selected from the group consisting of Ti: 0.01-1.0 wt%, Nb: 0.01-1.0 wt%, V: 0.01-1.0 wt%, Zr: 0.01-1.0 wt%, Ta: 0.01-1.0 wt%, Co: 0.1-5 wt%, Cu: 0.1-5 wt%, Mo: 0.1-5 wt%, W: 0.1-5 wt%, Al: 0.005-5.0 wt%, Ca: 0.0003-0.01 wt% and B: 0.0003-not more than 0.01 wt%, with the remainder being Fe and inevitable impurities; subjecting the steel to hot rolling at a draft below 830°C of not less than 30%; cooling the resultant hot rolled sheet at a cooling rate of not less than 25°C/sec; coiling the sheet at a temperature of not higher than 650°C; and thereafter subjecting the sheet to annealing and pickling. A process according to claim 1 wherein the hot rolling at a draft below 830°C of not less than 30% is effected to a thickness of not more than 1.5 mm; and the annealed and pickled sheet is subjected to skin pass rolling at a draft of not more than 20%. A process according to claim 1 wherein the annealed and pickled sheet is subjected to cold rolling at a total draft of more than 20% in a cold rolling installation provided with work rolls having a roll diameter of not less than 250 mm. A process for the production of a stainless steel sheet having excellent corrosion resistance, which process comprises providing an austenitic stainless steel or dual-phase stainless sheet consisting of C in an amount of not more than 0.01 wt%, S in an amount of not more than 0.005 wt%, O in an amount of not more than 0.005 wt%, Si in an amount of not more than 3 wt%, Mn in an amount of not more than 20 wt%, Cr in an amount of 9-50 wt%, Ni in an amount of 5-20 wt%, N in an amount of not more than 0.2 wt%, and optionally one or more elements selected from the group consisting of Ti: 0.01-1.0 wt%, Nb: 0.01-1.0 wt%, V: 0.01-1.0 wt%, Zr: 0.01-1.0 wt%, Ta: 0.01-1.0 wt%, Co: 0.1-5 wt%, Cu: 0.1-5 wt%, Mo: 0.1-5 wt%, W: 0.1-5 wt%, Al: 0.005-5.0 wt%, Ca: 0.0003-0.01 wt% and B: 0.0003-not more than 0.01 wt%, with the remainder being Fe and inevitable impurities; subjecting the steel to hot rolling at a draft below 830°C of not less than 30%; cooling the resultant hot rolled sheet at a cooling rate of not less than 25°C/sec; coiling the sheet at a temperature of not higher than 650°C; and thereafter subjecting the sheet to annealing and pickling. A process according to claim 4 wherein the hot rolling at a draft below 830°C of not less than 30% is effected to a thickness of not more than 1.5 mm; and the annealed and pickled sheet is subjected to skin pass rolling at a draft of not more than 20%. A process according to claim 4 wherein the annealed and pickled sheet is subjected to cold rolling at a total draft of more than 20% in a cold rolling installation provided with work rolls having a roll diameter of not less than 250 mm.

Inventors: FUJISAWA MITSUYUKI, FUKUDA KUNIO, KAWABATA YOSHIKAZU, SATOH SUSUMU, FUJISAWA, MITSUYUKI, FUKUDA, KUNIO, KAWABATA, YOSHIKAZU, SATOH, SUSUMU, FUJISAWA, MITSUYUKI, KAWASAKI STEEL CORPORATION, FUKUDA, KUNIO, KAWASAKI STEEL CORPORATION, KAWABATA, YOSHIKAZU, KAWASAKI STEEL CORPORATION, SATOH, SUSUMU, KAWASAKI STEEL CORPORATION
Applicants: KAWASAKI STEEL CO, KAWASAKI STEEL CORPORATION
Application year: 1995
Main classification: C21D8
IPC codes: C21D6, C21D8