Method for producing a cold rolled steel sheet

Abstract: A cold rolled steel sheet having excellent stretch­ ability, deep drawability and secondary workabiiity, and method for producing the same which comprises the steps of hot rolling a steel containing not more than 0.07%C by weight and P and N in the relation of P+5N ≦ 0.02% by weight at a temperature of 850°C or more, cold rolling the hot rolled steel strip at a re­ duction of not less than 50%, and subjecting the cold rolled steel strip to continuous annealing at a tempera­ ture between the recrystallization temperature and the A₃ point.

Description: The present invention relates to a cold rolled steel sheet having excellent cold rolling efficiency together with excellent press formability by means of continuous annealing, and to a method for producing the same. Since cold rolled steel sheet having a good press formability has been heretofore manufactured chiefly with an eye to the mechanical properties of the steel sheet, the chemical composition and processing conditions thereof have mainly been fixed parameters. Quite recently, the thickness of hot rolled steel strips has tended to increase in order to reduce the amount of energy required and to attain high productivity strip rolling. Thus, the development of such a cold rolled steel sheet having a sufficient rupture strength during cold rolling and the lower energy consumption required for cold rolling, together with good press formability, and of a method for producing the same are now in great demand. As a method for producing a deep drawing steel sheet using continuous annealing, it has been known to coil a hot rolled steel sheet at high temperature in a hot strip mill. For instance, the method of coiling a steel containing C≦0.06% by weight at 630°C or higher (Japan Examined Patent Application No. 1969/74) is known), and another method for coiling a steel containing 0.01-0.10% Mn by weight, less than 0.003% S by weight, less than 0.005% P by weight, less than 0.006% N by weight, and 0.01-0.06% Al by weight at 650°C or higher (Japan Laid-open Patent Application No. 35726/81) has also been proposed. The former relates to an improvement in the deep drawability of the steel sheet by a method which comprises coiling the hot rolled steel strip at a high temperature in order to coarsen the carbide of the hot rolled steel strip, and the P and N contents of the steel are on a level with common Al-killed steel. The latter is directed to an improvement in the deep drawability of the steel sheet by a method which comprises extremely lowering the Mn content and S content as well as the P content in addition to the high coiling temperature, but the N content is on a level with common Al-killed steel. According to US-A-4,040,873 a low yield point cold-reduced steel sheet having excellent properties for press-forming can be obtained by a full continuous annealing process when a low carbon steel is manufactured by: 1. chemical composition is substantially controlled at the steel making stage as follows, /0/≦0.02%, preferably 0.014%, depending upon additions of Si and Al, /Si/≦0.2%, preferably 0.1% to 0.02%, /Sol.Al/≦0.009%, preferably 0.005%, 2. at the hot-rolling stage after ordinary slabbing, a hot-rolled steel strip is coiled within the range of 650° to 800°C, and 3. at the continuous annealing stage after ordinary pickling-cold-reducing, a cold-reduced steel strip is subjected to a full continuous annealing including an over-aging treatment. However, in the above-mentioned methods, high temperature coiling is performed during the hot rolling step. Therefore, when the coil of steel is cooled, the cooling is non-uniform throughout. As a result, the uniformity of mechanical properties in the longitudinal direction as well as the width direction is lowered. Particularly, the quality of the top and bottom ends of the coil is so extremely impaired as to seriously reduce the yield of the steel product. In addition, a thick scale is produced by the high temperature coiling, so there is the disadvantage that descaling efficiency of the hot rolled steel strip is low. It is the object of the present invention to provide a method for producing a cold rolled steel sheet having excellent stretchability, deep drawability, and an eminent secondary workability which appears after the press working by a continuous process with high productivity, high yield, low energy consumption and high cold rolling reduction. This object is achieved with the present invention. The subject matter of the invention therefore is a method for producing a cold rolled steel sheet having an excellent press formability which comprises the steps of providing an aluminum killed steel containing not more than 0.07% C by weight, more than 0.10% to not more than 0.40% Mn by weight, 0.010-0.050% Al by weight, nitrogen and phosphorus, and optionally not more than 0.02% Si by weight, not more than 0.10% Cr by weight, not more than 0.10% Ti by weight, not more than 0.10% Nb by weight and not more than 0.0030% B by weight, the remainder being Fe and unavoidable impurities, hot rolling said steel at a temperature of not less than 850°C, cold rolling said hot rolled steel at a reduction of not less than 50%, and finally subjecting said cold rolled steel to a recrystallization continuous annealing treatment at a temperature between the recrystallization temperature and the A₃ point for a period of not longer than five minutes, characterized in that the said steel contains not more than 0.0025% N by weight, and not more than 0.010% P by weight, with the relation between P and N being P+5N≦0.0175%, and wherein the coiling temperature of the hot rolled coil is not higher than 650°C. In the accompanying drawings Figure 1 is a graphic view showing the relation between the P and N content of a low carbon Al-killed steel and the r value, and the elongation of the steel sheet; Fig. 2 is a graphic view indicating the relation between the P and N content of a low carbon Al-killed steel and rupture property during cold rolling efficiency of the steel; Fig. 3 is a graphic view showing the relation between the secondary workability and the P and N content of an extremely low carbon Al-killed steel; Fig. 4 is a graphic view indicating the relation between the elongation, and the r value and the P and N content of an extremely low carbon Al-killed steel; Fig. 5 is a graphic view showing an embodiment of the relation between the coiling temperature of the low carbon Al-killed steel and the r value of a steel sheet; and Fig. 6 is also a graphic view showing an embodiment of the relation between the cold rolling reduction and the r value of a low carbon Al-killed steel. The inventors of the present invention conducted extensive and detailed research on press formability of low carbon Al-killed steel produced by the continuous annealing process. As a result, the inventors have found that N and P have an extremely great influence on the deep drawability and stretchability. The inventors have proceeded with the research so far that they have achieved the present invention in which the Mn content is on the usual level (more than 0.10%), yet the high temperature coiling is no longer required. The present invention is characterized by: (a) Being different from the methods of prior art, high temperature coiling is unnecessary so that both productivity and yield are high; (b) Being different from the steel obtained by the methods of prior art, the high cold reduction can be easily achieved in the cold rolling step, and by the high cold reduction the deep drawability can be much improved; and (c) In addition, a cold rolled steel sheet of highest grade stretchability and deep drawability can be easily produced by reducing carbon content to not more than 0.005%. First, the chemical composition of the steel of this invention will be explained below. If the carbon content exceeds 0.07%, the steel will be hardened, and the cold rolling efficiency, one feature of the invention, will be lost too. The preferred range of C is not more than 0.05%. The most important requirement of the chemical composition which constitutes the invention is to specify a closely inseparable correlation of P and N. In accordance with the present invention, it is required to specify P≦0.010%, N≦0.0025% and satisfy the relation P+5N≦0.0175%. These requirements must be satisfied in order to improve both press formability and cold rolling efficiency simultaneously. This will be explained in more detail hereinbelow. It is indispensable to limit the contents of P and N. As an embodiment, Fig. 1 shows the relation between the contents of P and N and the r value, and elongation in connection with a steel containing 0.02-0.040% C, 0.10-0.25% Mn, and 0.02-0.04% Al; and Fig. 2 indicates the relation between the content of P and N and the cold rolling efficiency. The relationships are shown by contour lines of the average values obtained from a large number of experiments. Other processing conditions are as follows: As is clearly understood from Fig. 1, the r value (solid line) favorably correlated with deep drawability and elongation (broken line) is much improved if P is not more than 0.010% and N is not more than 0.0025% and the formula P+5N≦0.0175% is satisfied. Particularly, it is seen that a considerably marked effect is exhibited in the region where P is not more than 0.007% and N not more than 0.0020%. Furthermore, if N is not more than 0.0015%, the highest deep drawability is exhibited. The r value and the elongation are high despite a relatively low coiling temperature, such as 575-650°C. Fig. 2 shows the relation between P and N content and rupture property during cold rolling. Strip fracture was evaluated by the following test: A notch was made at the edge of each hot rolled sheet (total: 20 sheets) 4.0 mm thick, then it was cold rolled by a cold rolling mill in the laboratory scale at a reduction of 85% to a sheet 0.6 mm thick; and the total number of the thus cold rolled sheet was investigated as to whether sheet fracture occurred or not. Fig. 2 shows the number of fracture sheets. As shown in Fig. 2, the steel fracture in the cold rolling strip scarcely occurs in the region where P is not more than 0.010% and N is not more than 0.0025% and P+5N≦0.0175%. Further, as is shown in an embodiment of the invention hereinafter, the energy consumption required for cold rolling is less than that of the prior art. As described hereinafter, in the cold rolling step of the invention, a higher reduction than that of the prior art is preferred, hence this excellent cold rolling efficiency should be evaluated as of great significance in industry. Particularly as shown in Fig. 2 this is conspicuous where P+5N≦0.0175% and P is not more than 0.007% and N not more than 0.0020%. Accordingly, the contents of P and N were specified as above taking both press formability and cold rolling efficiency of the steel sheet into account. Mn in an amount of at least 0.10% is required in order to inhibit hot shortness due to S in the hot rolling process and to satisfy the commonly accepted requirement Mn/S≧10. On the other hand, however, if Mn exceeds 0.40%, Mn hardens the steel and lowers press formability. If more eminent deep drawability is required, not more than 0.30% Mn is preferred. Al of at least 0.010% is required in order to kill the steel and to fix N in the steel as AIN. On the other hand, if Al exceeds 0.05%, the steel sheet will be hardened. The cost will also be higher. The preferred range is 0.010-0.040% Al. In order to inhibit the hot shortness, S should be specified to satisfy Mn/S≧10 as is usual, and S is preferred to be not more than 0.015% from the viewpoint of cold workability. The chemical composition of the steel in accordance with the present invention has been described in the foregoing. In order to further improve the characteristics of the invention, such an element as B or Cr which forms carbide or nitride may be suitably added in the commonly accepted range. To further enhance the cold workability of the steel sheet, B may be added to the Al-killed steel, whereby much better workability and cold rolling efficiency can be achieved without any loss of the merit of the present invention. In case B is added, B/N≦1.5 is preferred. In addition, if Cr is added, it is preferred to be not more than 0.10% as usually done. In accordance with the present invention, a cold rolled steel sheet favored with a combination of highest stretchability, deep drawability and embrittlement after deep-drawing (referred to as secondary workability hereinafter), all of the highest degree can be produced by adding additional requirements, not more than 0.005% C and P≦4C specified between P and C. The upper limit of C has been specified as 0.005% in order to obtain stretchability and deep drawability of the highest degree. However, the mere reduction of the carbon content tends to bring about the secondary working crack after press forming. For instance, if the carbon content is reduced to not more than 0.005%, it is known that the secondary working crack will occur, although the degree of the press working is not great. It has been found from a large number of experimental results that to prevent the occurrence of secondary working crack notwithstanding such a severe press working operation as about 3.5 of the drawing ratio, it is most effective to specify P as not more than 0.010% while maintaining the relation P≦4C so as to reduce P as well as C. The decrease of P together with the decrease of C contributes to the improvement of deep drawability as well as stretchability. As described hereinafter, it is understood that the reduction of P only also contributes to the improvement of deep drawability and stretchability. Therefore, in accordance with the present invention, the decrease of C is accompanied by the simultaneous decrease of P, hence its advantageous effect is much greater than in the steel of prior art. In addition, to exhibit the characteristics of the invention to the utmost, it is preferred to specify C as not more than 0.004% and P≦3C. Thus, in the case of extremely low carbon steel, the limiting of P and N has very great significance. Fig. 3 shows an embodiment of the relation between the contents of P and N and the secondary workability in connection with a steel containing 0.003-0.004% C, 0.20-0.25% Mn, and 0.01-0.04% Al; and Fig. 4 shows the relation between the content of P and N and the r value, elongation. The relationships are shown by contour lines based on average values obtained from a large number of experiments. In addition, in Figs. 34, the upper limiting of P is indicated as 0.014% (P=4C) in terms of C≒ 0.0035%. Other processing conditions are as follows: By the way, the examination of secondary workability shown in Fig. 3 is conducted as follows: steel sheets are drawn to cups with various drawing ratios, each of which is subjected to expansion with a conical punch at a temperature of 0°C, and at this time an investigation is made as to whether brittle rupture occurred on the thus formed cups or not. The secondary workability is evaluated with the greatest drawing ratio where no brittle rupture occurs. The numeral of Fig. 3 shows the greatest drawing ratio where the secondary working crack will not occur, and it means that the greater the numeral the better the secondary workability. In Fig. 4, the solid line refers to the elongation, the broken line to the r value, and the numerals refer to the elongation and the r value, respectively. As is clearly seen in Figs. 3-4, P has an influence not only on the secondary workability but also on the elongation strongly correlated with the stretchability and the r value strongly correlated with the deep drawability. At the range of P≦0.010%, an improved effect of elongation becomes extremely great while, at the same time, the r value is much improved with the reduction of P. Moreover, with reference to N, a new fact that the secondary workability is improved with the reduction of N is also found, and in the range of N≦25PPM the r value is remarkably improved, and the elongation is improved, too. By the above-mentioned method, on extremely low carbon cold rolled steel sheet having more than 52% of the elongation, more than 1.6 of the r value, and more than 3.5 of the drawing ratio without the secondary working crack can be obtained. It is understood that the above characteristics can be much improved by further reducing the contents of P and N to the lower level, and besides, a cold rolled steel sheet favored with the stretchability, deep drawability, and secondary workability of the highest degree can be produced by limiting P≦0.007% and N≦0.0020%. The fundamental compositions of the extremely low carbon steel of this invention have been described, and in addition thereto, Ti, Nb and B can be added in a suitable amount. Ti, Nb or B combine with N and C, and the present invention aims at lower C and N, so that the characteristics of the invention can be enhanced by the addition of these elements. In the addition of the elements, one or more of Ti≦0.10%, Nb≦0.10%, and B≦0.0030% can be added. When the content of each element exceeds its upper limit, its effect is saturated and the sheet cost is also raised. The steel sheet containing the above chemical composition is produced in the following way. The molten steel is produced by the conventional steel making method and in the manufacture of extremely low carbon steel the molten steel is subjected to vacuum degassing treatment and then made into slabs by the conventional method. In the present invention, the finishing temperature of hot rolling should be a higher temperature than 850°C. If it is less than 850°C, the deep drawability will be lowered. The temperature for heating the steel slab is not essential in the present invention. Accordingly, it is preferred to heat at a temperature not more than 1200°C from the viewpoint of energy saving policy and obtaining better press formability as described hereinafter. Also, hot slabs obtained by the continuous casting or break-down mill may be directly hot rolled, or hot-charged into slab heating furnace. Preferable hot rolling conditions are as follows. The finishing entry temperature of the finishing tandem stands is preferred to be not higher than 1000°C, so that the reduction in lower temperature range can become large. For instance, the total reduction of the final two-pass is preferred to be 40% or more. The finishing temperature is preferred to be higher than the Ar₃ point (referred to as A₃ hereinafter), and thereafter strip is forcedly cooled as soon as possible after rolling at a cooling rate more than 30°C per second. With the above processing conditions, the characteristics of the present invention will be exceedingly exhibited. This advantageous effect is particularly great in the extremely low carbon steel. The slab heating temperature may be preferred to be not higher than 1100°C in order to make the finish entry temperature not higher than 1000°C. Referring to the coiling temperature of this invention, a high coiling temperature is not required, which is characteristically different from the prior art. The coiling temperature of a low carbon Al-killed steel is preferred to be higher than 575°C in this invention in order to ensure more than 1.4 of the r value required for a deep drawing quality. Fig. 5 shows the relation between the coiling temperature and the r value in connection with a steel containing 0.03% C, or 0.20% Mn, 0.007 P, 0.0015% N, and 0.030% Al. The annealing condition is 700°C × 1 minute+400°C×3 minutes (continuous annealing process). As shown in Fig. 5, the higher coiling temperature such as 700°C is not required as the prior art, and a steel sheet of good deep drawability can be obtained even when coiled at a temperature lower than 630°C. When the softer steel sheet is required, the coiling temperature may be higher than 630°C. Even in this case, as described in the following example, the present invention has a distinct advantage in that even at high coiling temperature (for instance, 750°C), the quality variation in the longitudinal direction and width direction of the coil is extremely small as compared with the prior art. In the case of the extremely low carbon Al-killed steel, the characteristics of the invention are not affected by the coiling temperature at all. Therefore, the coiling temperature is preferred to be 550-650°C from the viewpoint of pickling or descaling efficiency. The hot rolled coil is subsequently subjected to descaling and cold rolling. Cold rolling is carried out at a reduction of at least 50% as in the conventional method. However, it has been confirmed that the cold workability of the steel of this invention is much improved with a higher reduction of the cold rolling than the common steel of prior art. The results thereof are shown in Fig. 6. The chemical composition and the hot rolling conditions of the samples illustrated in Fig. 6 are shown in Table 1. The annealing condition is 750°×1 minute+400°C×3 minutes. As is clear in Fig. 6, the steel A of this invention has a high r value, and it is seen that the cold reduction where the r value reaches the peak is about 87%. When the cold reduction becomes more than 70%, more than 1.4 of an r value is obtained. Therefore the cold reduction is preferred to be more than 70% and not more than 90% in order to obtain a high r value. Most preferable range is 75-90%. On the other hand, however, comparative steels B, C and D have low r values, and the cold reduction where the r value reaches its peak is about 75%. This high cold rolled reduction and thereby high r value is one of the features of the present invention. Moreover, the steel of the invention has excellent cold rolling efficiency, so that there is no problem even if the cold reduction is increased to 70-90%. The recrystallization annealing is carried out at a temperature between recrystallization temperature and the A₃ point by the continuous annealing method and the strip is subsequently cooled, and if necessary subjected to an overageing. The method of this invention can be applied to any continuous annealing method. Under typical annealing conditions, the steel is subjected to recrystallization at a soaking temperature of 650-850°C for a period of not more than 5 minutes, then cooled, and subjected to overageing at a temperature of 200-450°C for a period of not more than 10 minutes. To improve the deep drawability much further, the soaking temperature is preferred to be higher than 700°C. In addition, the typical annealing conditions to be applied to the extremely low carbon Al-killed steel are as follows: the steel is subjected to recrystallization at a soaking temperature of 700-800°C for a period of not more than three minutes and is then cooled. In this case, the overageing treatment is not required, but it may be conducted at a temperature of 200-450°C for a period of less than 5 minutes. The steel strip thus annealed is subjected to temper rolling, if necessary, and now it is ready for further processing into a product. Since the steel manufactured in accordance with the method of the present invention can be subjected to any surface treatment with no loss of the features of the invention, it can be applied to any surface treatment, such as the manufacture of tinplate, galvanized sheets, turn sheets, etc. Example 1 The steels shown in Table 2 were produced in a converter; the molten steel was cast in a continuous casting mold to obtain a slab; the slab was reheated to a temperature of 1050-1200°C; the hot slab was hot rolled into a strip 4.0 mm thick under the hot rolling conditions listed in Table 2; the hot rolled strip was descaled and the descaled hot rolled strip was cold rolled to a strip 0.8 mm thick which was subjected to recrystallization annealing at 700°C for one minute by continuous annealing; then it was cooled and subjected to an overageing treatment at 400°C for 3 minutes; and was finally subjected to temper rolling at a reduction of 1.3% to obtain a finished product. Table 2 also shows the mechanical properties and the cold rolling efficiency of the cold rolling process in connection with the steel sheet produced by the above method. The cold rolling efficiency is shown by an energy consumption ratio of the average value as compared with the prior art (common low carbon Al-killed steel) for the cold rolling. The steel sheet fracture property was evaluated by the total number of fractures occurring in the examination test wherein a notch was made at the edge of every hot rolled sheet (total: 20 sheets), then it was cold rolled with the reduction of 85% by a laboratory cold rolling mill to a sheet 0.6 mm thick. The tensile test piece is No. 5 as specified by JIS, and the mechanical property was indicated by the average value of the whole length of the coil, and the difference of r value between r M (the center of the longitudinal direction of the coil) and r B (the tail end of the longitudinal direction of the coil) are also shown. It is seen that every steel listed within the scope of the present invention has a low yield point, a high elongation, a high r value, good press formability, and excellent cold rolling efficiency despite the coiling temperature of less than 630°C. Coils Ncs. E and F are the same except for the finishing hot rolling conditions. It is seen that the r value of the coil No. F wherein the finishing hot rolling entry temperature is lower than that of the coil No. E is higher. The comparative steel coil No. N whose coiling temperature was 750°C has a fairly good r value and elongation, but the difference in r value of ( r M- r B) is very large, so that the quality fluctuation in the longitudinal direction of the coil is considerable and therefore product yield is low. Example 2 The steels listed in Table 3 were produced in a converter; The molten steel was subjected to vacuum degassing to lower the carbon content to a predetermined level; and then was cast in a continuous casting mold to obtain a slab: The slab was reheated to a temperature of 1050-1200°C and hot rolled under the conditions indicated in Table 3: The hot rolled strip was cold rolled to 0.8 mm thickness and then was annealed and subjected to temper rolling at 1.5% reduction. The properties of the cold rolled sheet thus obtained are listed in Table 3. The tensile test piece was No. 5 specified by JIS; and the secondary workability is shown by the largest drawing ratio where no brittle rupture occurs in drawn cups with various drawing ratios under the conical expansion test at 0°C. Each of the extremely low carbon steel sheets produced within the scope of the claims of the present invention has not only an eminent elongation strongly correlated with the stretchability, but also an excellent r value strongly correlated with the deep drawability, and further, a distinguished secondary workability, hence it can be said that the steel sheet of the present invention has press formability of the highest degree. The present invention has been described in detail in the foregoing, and the present invention has the following distinguishing characteristics as compared with the prior art. a) A high temperature coiling operation is not required in the hot rolling process, and coiling at temperature of 650°C or lower is feasible. Therefore the pickling or descaling efficiency is good and a high yield is possible. Further, even in the case of using a high coiling temperature as in the prior art, the quality at the top and bottom of a coil is excellent, resulting in high yield; b) An energy saving due to the low slab reheating temperature is possible and also the low temperature heating process improves the cold workability; c) Unlike the steel of prior art, the cold rolling reduction is so easily raised that the prcductivity of the hot rolling process will be improved, energy saving is also possible at the same time, deep-drawability will be more improved; and d) By making an extremely low carbon steel, a cold rolled steel sheet favored with a combination of the highest degree stretchability, deep drawability and secondary workability can be manufactured.

Claims: Method for producing a cold rolled steel sheet having an excellent press formability which comprises the steps of providing an aluminum killed steel containing not more than 0.07% C by weight, more than 0.10% to not more than 0.40% Mn by weight, 0.010-0.050% Al by weight, nitrogen and phosphorus, and optionally not more than 0.02% Si by weight, not more than 0.10% Cr by weight, not more than 0.10% Ti by weight, not more than 0.10% Nb by weight and not more than 0.0030% B by weight, the remainder being Fe and unavoidable impurities, hot rolling said steel at a temperature of not less than 850°C, cold rolling said hot rolled steel at a reduction of not less than 50%, and finally subjecting said cold rolled steel to a recrystallization continuous annealing treatment at a temperature between the recrystallization temperature and the A₃ point for a period of not longer than five minutes, characterized in that the said steel contains not more than 0.0025% N by weight, and not more than 0.10% P by weight,. with the relation between P and N being P+5N≦0.0175%, and wherein the coiling temperature of the hot rolled coil ist not higher than 650°C. Method as claimed in Claim 1 in which said P is not more than 0.007% by weight and N is not more than 0.0020% by weight with the relation between P and N being P+5N≦0.0175% by weight. Method as claimed in Claim 1 in which said P is not more than 0.007% by weight, and N is not more than 0.0015% by weight. Method as claimed in Claim 1 in which said steel further contains B in an amount such that B/N≦1.5. Method as claimed in Claim 1 in which said C is not more than 0.005% by weight and said relation between said P and said C is P≦4C.

Inventors: KAWANO TSUYOSHI C O NIPPON STE, NAKAJIMA KOE C O NIPPON STEEL, SANAGI SHIROH C O NIPPON STEEL, KAWANO, TSUYOSHI C/O NIPPON STEEL CORPORATION, NAKAJIMA, KOE C/O NIPPON STEEL CORPORATION, SANAGI, SHIROH C/O NIPPON STEEL CORPORATION
Applicants: NIPPON STEEL CORP, NIPPON STEEL CORPORATION
Application year: 1982
Main classification: C21D8
IPC codes: C21D8, C22C38