PROCESS FOR MANUFACTURING COLD-ROLLED STEEL HAVING EXCELLENT PRESS MOLDABILITY

Abstract: A process for manufacturing cold-rolled steel with excellent press moldability, which comprises subjecting a super-low-carbon steel piece having 0.005 wt% or less C, 1.2 wt % or less Si, 0.05 to 1.00 wt % Mn, 0.150 wt % or less P, and 0.002 to 0.150 wt % of at least one of Nb, Cr, Ti, Al, B, and W and having a heat history of low temperature soaking treatment at 800 to 1,100° C to hot rolling, cold rolling, and recrystallization annealing in a conventional manner. This rolled steel has improved overhang moldability, deep drawa­ bility, and aging resistance.

Description: This invention belongs to the technical field concerned with manufacturing cold rolled steel sheets having excellent press-formability. In general, cold rolled steel sheets for press forming which are used for the outer plates of automobiles, gasoline tanks and the like are required to have excellent stretch formability, deep-drawability and aging resistance. The lower the yield strength (YS) and the higher the elongation (EL) and the work hardening exponent (n value) of a steel sheet, the better is the stretch formability. The deep-drawability of a steel sheet is almost dominated by the Lankford value (r value). The higher the r value, the higher the limit of the deep-drawability. On the other hand, it is known that when C and N in a solid solution state remain in a steel sheet, the problem called "stretcher strain" occurs upon working during press forming due to aging at room temperature. This necessitates the sheet having aging resistance, which is ordinarily evaluated by the aging index (Al). This index is represented by the difference between the yield strength at 7.5% preliminary strain and the yield strength after a heat treatment of 100°C for 30 minutes. Steel sheets for use in press working are required to have an Al value of not more than 3 kg/mm². There have been heretofore proposed many processes for manufacturing cold rolled steel sheets which have excellent stretch formability, deep-drawability, and aging resistance. For instance, there is a method involving box annealing a low-carbon aluminium-killed steel having a carbon content of about 0.04% by weight (the amounts of the steel ingredients are hereinafter referred to briefly as "%"); and there is a method involving box annealing or continuously annealing a steel sheet in which a carbonitride-forming element such as Ti, Nb or the like is added to an extremely low-carbon steel having a carbon content of not more than 0.01 %. However, these conventional processes have the common feature that the temperature (hereinafter referred to as the "soaking temperature") at which the steel slabs are uniformly heated prior to the hot rolling is extremely hot viz. near 1,200°C. The reason why the soaking temperature is so high is as follows: In the case of the low-carbon aluminium-killed steels, it is necessary to completely solid-solve AlN when soaking the steel slabs in order to obtain a high r value by the action of AlN precipitated during box annealing after the cold rolling. In the case of the extremely low-carbon steel containing added Ti or Nb, since the Ar₃ transformation point at which the austenite phase is transformed into the ferrite phase is extremely high (viz. near 900°C), the hot roll-finishing temperature (FDT) must be high so as to avoid deterioration of the material properties which would occur if the hot rolling was carried out at a temperature lower than the Ar₃ transformation temperature. Not only is a huge amount of energy required for heating the steel slab at the high temperature of about 1,200°C, but also the higher soaking temperature decreases the yield of the steel slab due to surface oxidation and further promotes internal oxidization in the vicinity of the surface of the steel slab. Thus such a method has the drawback that problems such as surface defects, surface hardening and the like frequently occur. As mentioned above, the heating of the steel slab at high temperature leads to not only the consumption of much energy but also to surface defects and therefore it is strongly desired to provide a process of manufacturing cold rolled steel sheets which involves a lower soaking temperature for the steel slab and also gives excellent press-formability. There have been proposed several processes for manufacturing cold rolled steel in which the soaking is carried out at a low temperature of not higher than 1,200°C, followed by hot rolling, for instance, Japanese Patent Laid Open Application No. Sho 49-129,622 (Japanese Patent Application No. Sho 48-43,856), Japanese Patent Laid Open Application No. Sho 51 -59,008 (Japanese Patent Application No. Sho 49-132,622) and Japanese Patent Laid Open Application No. Sho 55-58,333 (Japanese Patent Application No. Sho 53-129,071). However, in each case, in order to ensure that the hot roll-finishing temperature is not lower than the Ar₃ transformation point, the soaking temperature must be actually not lower than 1,100°C and in the very recent Japanese Patent Laid Open Application No. Sho 57-13,123 (Japanese Patent Application No. Sho 55-84,696), the soaking temperature of the steel slab is 1, 100―1,250°C. However, in a low-temperature soaking process in which the lower limit is 1,100°C, the above described effects for saving energy and avoiding a decrease in the yield occur only to an extremely small degree and the material properties of the cold rolled steel sheets are not sufficiently improved as is described hereinafter. In addition to the above, Japanese Patent Laid Open Application No. Sho 53-64,616 (Japanese Patent Application No. Sho 51-140,532) discloses a process of manufacturing a steel sheet having an r value of 1.17 - 1.20 in which a rimmed steel slab having a C content of 0.05 - 0.11% is soaked at 980 - 1,050°C, and finished at a temperature of 710 - 750°C. Japanese Patent Laid Open Application No. Sho 56-15,882 (Japanese Patent Application No. Sho 55-60,713) discloses a process of manufacturing a steel sheet having an r value of 1.1 in which a steel slab having a C content of 0.03% and an Al content of 0.05% is soaked at 950°C and finished at a temperature of 750°C. However, they both result in the manufacture of steel sheets having an r value as low as not more than 1.2 which are essentially different from the deep-drawing steel sheet aimed at by the invention. An object of the invention is to provide a process of manufacturing a cold rolled steel sheet having excellent press-formability which overcomes the above described drawbacks in the prior art for producing cold rolled steel sheets for press working, and enables the soaking treatment to be carried out at a temperature which is far lower than that of the above prior art. EP-A-0 041 354 discloses the production of non-aging cold rolled steel sheets having good formability by limiting the ratio of Nb and C in the steel to a specific range and by controlling the process so that the steel is hot rolled at a total reduction of not less than 90%, subjected to finishing rolling at a rolling speed of not less than 40 m/min and a finishing temperature of not less than 830°C, coiled at a temperature of 680 to 800°C, cold rolled, and subjected to continuous annealing at 700 to 900°C for 10 seconds to 5 minutes. Essentially the invention of this reference involves controlling the precipitation of Nb carbide and nitride whilst subjecting the steel to the aforementioned rolling, coiling and annealing conditions. In a specific example, the steel used contains 0.004% C, 0.010% Si, 0.07% Mn, 0.010 % P, 0.035% sol Al, and 0.036% Nb and a slab of this steel is soaked at a temperature of 1080°C for 35 minutes before being rolled, coiled and annealed in accordance with the above specified conditions. However, there is no suggestion in this reference of the importance of the soaking temperature in the control of precipitation. According to the present invention there is provided a process of manufacturing a cold rolled steel sheet having excellent press-formability by subjecting a steel slab to soaking at elevated temperature followed by hot rolling, cold rolling and recrystallization annealing wherein the soaking is effected at a temperature of from 800°C to 1050°C and the composition of the slab consists of not more than 0.005% by weight of C, not more than 1.20% by weight of Si, 0.05 to 1.0% by weight of Mn, not more than 0.150% by weight of P, and at least one element selected from the group consisting of Nb, Cr, Ti, Al, B and W in a total amount of 0.002 - 0.150% by weight, with the remainder being Fe and incidental impurities. For a better understanding of the invention and to show how the same may be carried out, reference will now be made, by way of example, to the accompanying drawing, in which Figs. 1(A), (B), (C), and (D) are correlation views showing the influence of various soaking temperatures for steel slabs on the aging index (AI), r value, elongation (El), and yield strength (YS), respectively as determined in fundamental experiments carried out with a view to accomplishing the present invention. First the fundamental experiments carried out by the inventors will be explained. Two kinds of steel slabs having the compositions shown in Table 1 were prepared by continuously casting molten iron obtained through a bottom-blown converter and an RH degassing furnace. After the above two kinds of steel slabs have been left to cool at room temperature, they were then soaked in a soaking pit. The soaking temperature was varied over a range of 750―1,250°C, and the soaked steel slabs were hot rolled by means of a rougher consisting of 4 row rolls and passed to a hot finisher consisting of 7 row rolls at two hot roll-finishing temperatures (FDT) of about 900°C and about 710°C, and coiled as steel strips having a thickness of 3.2 mm at a constant temperature of about 500°C. The hot rolled steel strips were pickled and cold rolled into cold rolled sheets having a thickness of 0.8 mm and then maintained at a temperature of 800°C through continuous annealing and skin-pass rolled finally at a reduction rate of 0.6% to obtain test samples. The influences upon the material properties of the test samples due to the differences in the soaking temperatures of the steel slabs are shown in Figs. 1 (A), (B), (C), and (D). In the measurements of the material properties of the test samples, the tensile strength and the aging index (Al) were determined respectively using a tensile test piece in accordance with JIS Z 22015 and a test piece taken in the rolling direction, and the r value, the elongation and the yield strength were expressed by the average value in three directions, i.e., the rolling direction, and directions at 40° and 90° to the rolling direction. As seen from the measured results in Fig. 1, in the case of test steel sample No. 2 having a carbon content of 0.0061% as shown in Table 1, there is substantially no correlation between the soaking temperature within a temperature range of 1,000―1,250°C and the material properties of the cold rolled-annealed sheet, and the r value of the low FDT steel is low. On the other hand, it was found that the properties of the test steel sample No. 1 having a C content of 0.0022% strongly depends upon the soaking temperature of the steel slab. More specifically, when the results in the case of a hot roll-finishing temperature (FDT) of 900°C (represented by the mark "o") are considered, as the soaking temperature is lowered from 1,250°C to 1,100°C and then to 1,000°C, the elongation and the r value increases and the aging index (Al) and the yield strength (YS) becomes lower and this indicates that the press formability is conspicuously improved. Further, when the measured results at the hot roll-finishing temperature (FDT) of 710°C (expressed by the mark "·") are considered, the material properties in the case where the soaking temperature is higher than 1,100°C, are fairly inferior to those in the case where the soaking temperature is 900°C. However, when the soaking temperature of the steel slab is not higher than 1,100°C, the material properties become as excellent as those when the hot roll-finishing temperature is 900°C. However, when the soaking temperature is as low as less than 800°C, it is apparent that the material properties are rapidly deteriorated. This is an extremely important discovery. In the conventional process of manufacturing cold rolled steel sheets for press forming, it has been common knowledge that the hot roll-finishing should not be effected at a temperature less than the Ar₃ transformation point at which the steel is transformed from the γ -phase to α-phase, because such heat treatment causes a remarkable deterioration of the material properties. However, the Ar₃ transformation point of the test steel No. 1 used in the above test by the inventors is about 830°C, and therefore the above test results are completely contrary to the conventional common knowledge. The phenomenon observed in test steel No. 1 in the experimental results shown in Fig. 1 is caused by setting the soaking temperature of the steel slab to a far lower range than that of the conventional processes. For this reason, according to the invention, the soaking temperature of the steel slab for the hot rolling is limited to a range of 800 to 1050°C. Based on the results of this fundamental experiment, the inventors have repeated the same experiment for confirming the effect of soaking the steel slab at low temperature with respect to a variety of steel slabs having compositions different to that of test steel No. 1 and have confirmed that the effect of the low temperature soaking is more improved by limiting the steel components as follows and that cold rolled steel sheets having excellent formability can be obtained. C: not more than 0.005%. As can be seen from the properties of test steel No. 2 having C of 0.0061% shown in Fig. 1, the effect obtained by the low-temperature soaking disappears if the carbon content exceeds 0.005%. Thus, the carbon content is limited to not more than 0.005% and preferably to not more than 0.004%. Si: not more than 1.20% Si is an element which is effective for strengthening the steel. However, if it exceeds 1.2%, the hardness is conspicuously increased, the elongation decreases and the yield strength is raised. Thus, it is limited to not more than 1.20%. Mn: 0.05 - 1.00% At least 0.05% of Mn is required to prevent red shortness due to S, but if it exceeds 1.00%, it damages the ductility of the steel in a similar way to Si. Thus, the content of Mn is limited to a range of 0.05 - 1.00%. P: not more than 0.150% P has a high ability to strengthen the steel due to the formation of a solid solution and is an element having activity for increasing the strength. However if it exceeds 0.150%, it brings about conspicuous deterioration of the spot weldability. Thus, the content of P is limited to not more than 0.150%. Nb, Cr, Ti, Al, B and W: Total amount of at least one of these elements being about 0.002 - 0.150%. These elements are important in the invention. The function and effects of these elements are considered as follows: (1) Any of these elements is a carbide, nitride or sulfide-forming element and when the steel slab is soaked at 800 1050°C according to the invention, the formation of these precipitates has an extremely effective influence on the press-formability of the final product. (2) Apart from the effect based on the formation of the above precipitates, these elements behave similarly in view of the extremely great influence they have upon the formation of micro-crystal grains and the improvement in texture when soaking the steel slab in the solid solution state. These additive elements have been heretofore widely used for improving the properties of steel materials, but it has been considered that their effects vary depending upon their addition amounts and their combined addition with other elements. Their effects also depend greatly upon the chemical composition of the base steel. However, it has been found that these additive elements serve very effectively to improve the formability of cold rolled steel sheets which have been subjected to a soaking treatment at a low temperature of 800 to less than 1080°C only in the case of an extremely low-carbon steel having a carbon content of not more than 0.005%, and that the functional effect is substantially equivalent in any of these elements. Therefore, these elements may be added alone or in a combination of two or more elements. If the total addition amount is less than 0.002%, no effect is observed, while if it exceeds 0.150%, the effect is not increased in proportion to the increased amount and the ductility is adversely affected due to the hardening of the solid solution. Thus the total addition amount is limited to a range of 0.002―0.150%. The optimum addition amount and combination of these elements differ slightly depending upon the elements. Particularly, in the case where Al is present in addition to Nb and/or W, the Al content should be within a range of 0.005―0.08%. In the case where Nb and/or W are present, the total amount of Nb and/or W should preferably be in the range of 0.002―0.020%. When at least two elements of Cr, Ti, B and Al are selected, the total amount thereof is optionally in a range of 0.002 - 0.090%. The reason for the limitation of the consituents of the steel according to the invention has been explained above. The balance consists of iron and incidental impurities besides the above constituents. Explanation will now be made with respect to the steps involved in producing cold rolled steel sheets having the above described composition in accordance with the present invention. The steel making process is not particularly limited but the combination of a converter and a degassing furnace is more effective in order to suppress the carbon content to not more than 0.005%. The process of manufacturing the steel slab may be a conventional slabbing technique that is an ingot making-blooming method or a continuous casting method. With respect to the heating of the steel slab, it is important to soak it at a temperature range of 800 to 1050°C. If the soaking can be carried out within this temperature range, the method and apparatus for heating the slab are not limited and the temperature of the steel slab prior to the soaking is arbitrary. Accordingly, the steel slab may be one completely cooled to room temperature or one having a temperature higher than room temperature so that it is merely necessary to reheat the slab to the temperature range of 800 to 1050°C to effect the soaking. The soaking time is not particularly limited and if the entire steel slab is heated to the soaking temperature of 800 to 1050°C, the object can be attained. However, the soaking time is preferred to be from 10 minutes to one hour. In the case of a steel slab manufactured by continuous casting, when the temperature of the steel slab is not lower than 800°C, it is not necessary to cool and reheat it. Rather it is merely necessary to keep the temperature in a range of 800 to less than 1080°C or to gradually cool the slab to this temperature range. Therefore, no particular heating furnace is necessary in the case of a steel slab obtained by continuous casting, and it is possible to attain satisfactory effects merely by regulating the cooling speed. In the hot rolling of the thus soaked steel slab, no adverse effect on the material properties of the final cold rolled steel sheet occurs so long as the rolling conditions, such as rolling speed, rolling reduction, distribution of reduction in rolling, roll finishing temperature, coiling temperature and the like are within the usual ranges. However, if the finishing temperature in the hot rolling is too low, the deformation resistance becomes high and this makes the rolling difficult, so that it is preferable for the finishing temperature to be higher than 550°C. Further, since the surface oxidized layer formed on the hot rolled steel strip after the finish rolling and before coiling highly influences the surface profile of the final cold rolled steel sheet, the finishing temperature is preferred to be as low as possible. Therefore, the finishing temperature is preferably 550―850°C. It is particularly preferred for the finishing temperature to be not greater than the Ar₃ transformation point. Since steel containing an element or elements other than Nb and W has very low deformation resistance in the ferrite region, the finishing temperature may be lower than that of a steel to which Nb or W is added, and in this case the preferred temperature is 550―680°C. On the other hand, the temperature for coiling the hot rolled steel sheet is preferably in the range of 400―600°C, because as said temperature is lower, the pickling ability is improved so the pickling cost is reduced and a good surface profile can be ensured. Thus the coiling temperature is preferred to be 400―600°C. The reduction in the cold rolling is preferred to be 50―95%. The recrystallization annealing may be carried out by an process of box annealing using a bell furnace or continuous annealing of the rapid heating type. However, continuous annealing is more preferable in view of the productivity and the uniformity of the material quality. The annealing temperature is preferably in the range of 650-850°C. The cooling speed after the soaking, or the presence or absence of an over aging treatment in the case of continuous annealing, have no substantial influence upon the present invention. In order to correct the profile of the cold rolled steel sheet after annealing, a tempering rolling may be additionally carried out using a reduction rate of not more than 1.5% through a skinpass. The following Example ilustrates the invention. Example With respect to the compositions A to L shown in Table 2 satisfying the requirements of the invention, molten iron was produced by means of a bottom-blow converter and an RH degassing furnace and then continuously cast or ingot-made and then bloomed to produce a steel slab. Steel slabs C to L thus obtained were subjected to soaking treatments at a temperature range of 850―1050°C as shown in Table 3. The temperatures of the steel slabs prior to the soaking were different and varied between 20°C and 870°C as shown in this Table. The thus soaked steel slabs were hot rolled at a hot roll-finishing temperature of 620―850°C, and a hot roll-coiling temperature of 320―550°C to obtain hot rolled sheets each having a thickness of 2.8―3.2 mm. Then, the hot rolled sheets were cold rolled to form cold rolled sheets each having the thickness of 0.8 mm and, as indicated in Table 3, they were subjected to re-crystallization annealing in a continuous annealing furnace at a uniform temperature of 760―800°C. All the annealed test sample sheets were treated by a skinpass to obtain the final products. The average properties of the final product in the rolling direction, and in directions at 45° and 90° to the rolling direction are shown in Table 4. As can be seen from the property values of the materials shown in Table 4, the tensile strengths and the test sample steels G, I, and K show values of not less than 35 kg/mm². The other samples have values of not more than 32 kg/mm². All the sample steels have low yield strength and high elongation, r value and n value. They all have an aging index (AI) of not more than 3 kg/mm². This indicates that all samples C - L are cold rolled steel sheets having excellent stretch formability, deep-drawability and aging resistance. The steel slabs shown in the above Example are ones having a thickness of about 10 - 250 mm and produced by the ingot making-blooming method or a continuous casting method. However the invention is obviously applicable to a sheet bar having a thickness of 20 - 60 mm produced directly from molten steel through a sheet bar caster. Thus, when the sheet bar is subjected to the hot rolling, it is merely necessary to uniformly heat the bar within a temperature range of 800 to 1050°C or to keep the temperature at said temperature range. Further, the cold rolled steel sheets according to the invention can be used effectively as raw materials for manufacturing all sorts of surface treated steel sheets such as continuous hot-dip galvanized steel sheets by an in-line annealing system. According to the invention, a cold rolled steel sheet having excellent stretch formability, deep-drawability and aging resistance can be manufactured merely by effecting the soaking treatment at a temperature range of 800 to 1050°C when hot rolling a steel slab in which at least one of Nb, Cr, Ti, Al, B and W has been added in a total amount of 0.002 - 0.15% to an extremely low carbon steel having a carbon content of 0.005% or less without being influenced by the subsequent hot rolling and cold rolling conditions and the annealing conditions. As mentioned above, the temperature range for the soaking treatment according to the invention is low temperature range which is contrary to conventional common knowledge, and therefore not only can a huge amount of energy consumption be saved to a large extent but also the yield and the properties of the surface and of the interior of the product can be largely improved due to the reduction in the amount of surface oxidation.

Claims: 1. A process of manufacturing a cold rolled steel sheet having excellent press-formability by subjecting a steel slab to soaking at elevated temperature followed by hot rolling, cold rolling and recrystallization annealing wherein the soaking is effected at a temperatute of from 800°C to 1050°C and the composition of the slab consists of not more than 0.005% by weight of C, not mote than 1.20% by weight of Si, 0.05 to 1.0% by weight of Mn, not more than 0.150% by weight of P, and at least one element selected from the group consisting of Nb, Cr, Ti, Al, B and W in a total amount of 0.002 - 0.150% by weight, with the remainder being Fe and incidental impurities. 2. A process as claimed in claim 1, wherein the hot rolling is effected at a finishing temperature of from 550°C to the Ar₃ transformation point and the hot rolled steel is coiled at a coiling temperature of not higher than 600°C before being cold rolled and continuously annealed. 3. A process according to claim 1 or 2 wherein the carbon content of the composition is not more than 0.004% by weight and the composition contains from 0.005 to 0.080% by weight of Al, and at least one of Nb and W in a total amount of from 0.002 to 0.020% by weight. 4. A process according to claim 1 or 2 wherein the carbon content of the composition is not more than 0.004% by weight, the composition contains at least one element selected from the group consisting of Cr, Ti, Al and B in a total amount of from 0.002 to 0.090% by weight, and the hot rolling is effected at a finishing temperature of from 550 to 680°C.

Inventors: HASHIMOTO, OSAMU, IRIE, TOSHIO, SATOH, SUSUMU
Applicants: KAWASAKI STEEL CORPORATION
Application year: 1983
Main classification: C21D8
IPC codes: C21D8, C22C38, C21D9