Method for producing cold rolled steel sheets having a noticeably excellent formability

Abstract: Non-ageing cold rolled steel sheets having a very excel­ lent formability are produced by hot rolling a steel consisting of not more than 0.008% of C, not more than 0.20% of Si, 0.04-0.30% of Mn, not more than 0.03% of P, 0.01-0.10% of Aℓ, a content of Aℓ being more than N%x4, not more than 0.02% of S, not more than 0.01% of N, 0.01-0.07% of Nb, a content of Nb being C%x3-{C%x8 + 0.02}%, and the remainder being substantially Fe, at a total reduction of not less than 90%, a rolling speed of not less than 40 m/min in the finishing rolling and a finishing temperature of not lower than 830°C, coiling the hot rolled strip at a temperature of 600-800°C, cold rolling the coiled strip following to conventional manner to obtain a cold rolled strip having final gauge and then continuous annealing the cold rolled strip within a temperature range of 700-900°C for 10 seconds-5 minutes. It is possible to hot dip galvanize the thus treated strip continuously.

Description: The present invention relates to a method for producing non-ageing cold rolled steel sheets having a noticeably excellent formability. Outer panels and inner panels of automobiles are subjected to high press forming, so that non-ageing cold rolled steel sheets having a high r value and a large elongation have been used. In particular, fenders, quarter panels and oil pans have been formed from decarburized and denitrogenized steel sheets produced through open coil annealing and Ti killed extra low-carbon steel sheets. However, the former has high production costs, a large grain size, and low strength so that, upon press forming, skin roughness referred to as orange peel and wall fracture are apt to be caused. Furthermore, in the latter Ti killed steel sheets, Ti has a strong bonding force to not only C and N but also S and O, so that in order to ensure the non-ageing property, Ti must be added in an amount which is several times the stoichiometrically equivalent amount for C and N and the formed titanium sulfide and oxide constitute non-metallic inclusions and a large number of surface defects, referred to as sleevers, are formed. As an improved method, it has been proposed to fix C and N by Nb or Nb and Al, instead of Ti. In this way the ageing property and the r value are improved. For example, Met, Trans. 1972, vol. 3, pp. 2171 - 2182 discloses that when Mn and Al are compounded in sufficient amounts to fix S and N respectively in the steel, if Nb is contained in a sufficient amount to form NbC stoichiometrically with respect to C, that is 0.071% of Nb per 0.009% of C, non-ageing steel can be obtained by annealing at 700°C for 4 hours after cold rolling and if 0.177% of Nb per 0.014% of C is contained, even when annealing is effected at 870°C after cold rolling, no yield elongation occurs. United States Patent No. 3 761 324 discloses that when 0.068 - 0.25% of Nb is added to steel containing 0.002 - 0.02% of C and Mn in a sufficient amount equivalent to S, if Nb which is not bonded to C and N is present in an amount of more than 0.025%, the r value reaches more than 1.8. Examples in this patent show that non-ageing steel sheets having an r value of 1.78 - 2.10 and an alongation of 40 - 48% can be obtained by hot rolling and cold rolling a material containing 0.005 - 0.01% of C, about 0.006% of N, 0.015 0.020% of Al and 0.08 - 0.12% of Nb and then annealing the thus treated sheet at 700°C for 1 - 16 hours. Other than the above described methods, a plurality of methods wherein Nb alone is added without using Al have been proposed but any such method naturally needs a larger amount of Nb than the above described two methods in order to obtain non-ageing steel sheets and hence the formability is poor. Thus, the previously proposed Nb added steels are non-ageing cold rolled steel sheets having an r value of 1.6 - 2.1 and an elongation of 40 - 48% which are obtained by adding 0.07 - 0.18%, preferably 0.08 - 0.12% of Nb to extra low-carbon steel containing 0.005 0.02% of C. These steels have the following defects: (1) The steel sheets have an alongation of 40 - 48%, which is lower than the elongation of 50 - 54% of decarburized and denitrogenized steel sheets. (2) Nb is an expensive metal and the cost is necessarily increased by the addition of a large amount of Nb. An object of the present invention is to provide a method for producing non-ageing cold rolled steel sheets, in which the drawbacks of the previously known methods are obviated and improved. According to one aspect of the present invention there is provided a method for producing a cold rolled steel sheet by hot rolling a steel slab to form a hot rolled strip, coiling the hot rolled strip, cold rolling the coiled strip to a final gauge and annealing the cold rolled strip characterised in that (i) the steel slab has a composition consisting of not more than 0.008% of C, not more than 0.20% of Si, 0.04 - 0.30% of Mn, not more than 0.03% of P, 0.01 - 0.10% of Al provided that the Al content is more than 4 times the N content, not more than 0.02% of S, not more than 0.01% of N, and 0.01 - 0.07% of Nb provided that the Nb content is from C% x 3 to {C% x 8 + 0.02}% with the remainder being Fe and incidental impurities, (ii) the hot rolling is carried out at a total reduction of not less than 90%, a rolling speed of not less than 40m/min in the finishing rolling, and a finishing temperature of not lower than 830°C, (iii) the coiling of the hot rolled strip is carried out at a temperature of 600 - 800°C, and (iv) the annealing of the cold rolled strip is carried out by continuous annealing within a temperature range of 700 - 900°C for 10 seconds to 5 minutes whereby the cold rolled steel sheet has particularly good formability. In order to prolong the durable life of thin steel sheets used for the outer panels of automobiles, the demand for steel sheets on which a coating is applied has increased. For this coating, a variety of processes have been developed but in view of the production cost and the properties thereof, a continuous hot-dip galvanizing process is one of the most excellent processes. Since the outer panels and inner panels of automobiles are subjected to high press forming, non-ageing galvanized steel sheets having a high r value and a large elongation are necessary. The known methods for producing galvanized steel sheets to be used for such a purpose include (1 ) a method wherein cold rolled low-carbon steel sheets are plated by the usual step, that is continuous annealing-continuous plating and then subjected to over ageing treatment to obtain non-ageing steel sheets (Japanese patent application publication No. 74/72), and (2) a method wherein carbide-forming elements which do not retard the plating ability, such as Nb, are added to a steel material in adition to Ti, whereby non-ageing steel sheets are obtained (Japanese Patent Laid-Open Application No. 35,616/78). However these methods cannot provide satisfactory drawing properties, that is high elongation and high r value. Another object of the present invention is to provide a method for producing galvanized steel sheets having a very high r value and a high elongation, that is excellent deep drawing properties, and substantially non-ageing and excellent surface properties. According to another aspect of the present invention there is provided a method for producing a cold rolled steel sheet by hot rolling a steel slab to form a hot rolled strip, coiling the hot rolled strip, cold rolling the coiled strip to a final gauge and annealing the cold rolled strip characterised in that (i) the steel slab has a composition consisting of not more than 0.008% of C, not more than 0.20% of Si, 0.04 - 0.30% of Mn, not more than 0.03% of P, 0.01 - 0.10% of Al provided that the Al content is more than 4 times the N content, not more than 0.02% of S, not more than 0.01% of N, and 0.01 - 0.07% of Nb provided that the Nb content is from C% x 3 to {C% x 8 + 0.02}% with the remainder being Fe and incidental impurities, (ii) the hot rolling is carried out at a total reduction of not less than 90%, a rolling speed of not less than 40m/min in the finishing rolling, and a finishing temperature of not lower than 830°C, (iii) the coiling of the hot rolled strip is carried out at a temperature of 600 - 800°C, and (iv) the annealing of the cold rolled strip is carried out by continuous annealing within a temperature range of 700 - 900°C for 10 seconds to 5 minutes wherein the continuously annealed strip is subjected to continuous hot-dip galvanizing whereby a galvanized cold rolled steel sheet having particularly good formability is obtained. For a better understanding of the invention, reference will now be made to the accompanying drawings, wherein: Fig. 1 is a view showing the relationship between the A1 value and the r value and the parameter α for various steel sheets; Fig. 2 is a view showing the relationship between El(%) and the parameter β for the steel sheets; and Fig. 3 is a view showing the relationship between the r value and the coiling temperature of the steel sheets. The present invention will be explained in more detail with respect to the experimental data. Slabs of steel wherein the contents of C and Nb were varied as shown in the following Table 1 were hot rolled under the following conditions viz. a total reduction of 90%, a lowest rolling speed of 70 m/min in the finishing strip mill, and a finishing temperature of 870°C. The hot rolled sheets were then coiled at a temperature of 680°C and then cold rolled at a reduction of 80% to obtain cold rolled sheets having a final gauge and the cold rolled sheets were continuously annealed at 830° for 40 seconds. The relationship of the sheet properties (Al value, El value and r value) to the parameter α ≡ Nb(%)/C(%) and the parameter β ≡ Nb(%)-8C(%) is shown in Figs. 1 and 2. As can be seen from Fig. 1, when the parameter α is more than 3, the AI value, that is the ageing index, is less than 3 kg/mm² and the r value is more than 1.9 and completely non-ageing steel sheets having a high r value are obtained. Fig. 2 shows that the El value (elongation) varies in accordance with the parameter β and when β is not more than 0.02%, a satisfactorily high value is obtained. From this experiment, it can be concluded that the Nb content has to be more than 3 times the C(%) but β ≡ Nb(%)-8 x C(%), that is Nb(%) which is not bonded with C must be not more than 0.02%. Within the above described range, it is preferable in view of the desired balance of the property values of the sheet that the content of Nb is not more than 0.06% and also is within the range of 4 x C(%) to 8 x C(%) + 0.010% When C exceeds 0.008%, the r value and elongation are considerably lowered so that C must be not more than 0.008% and is preferred to be not more than 0.006%. Al must be added in an amount of not less than 0.01% in order to fix N as AlN and must be more than 4 times the N(%). Otherwise, N in the steel will be bonded with Nb in the steel and the amount of C which is not fixed with Nb will then be large and the AI value cannot be satisfactorily reduced. However, the addition of Al of more than 0.1% increases inclusions due to alumina clusters and becomes a cause of surface defects, so that such an addition should be avoided. When the content of N is higher, it is necessary to increase the content of Al and therefore when N is more than 0.01%, surface defects are increased owing to the increase of inclusions due to alumina clusters, so that N should be not more than 0.01%. The content of Mn may be that usually contained in cold rolled steel sheets and is 0.04 - 0.30%. Concerning Si, when the content is high, the ductility is deteriorated and the plating ability is considerably deteriorated, so that the content of Si must be not more than 0.20%. The content of other elements such as P, S, O and the like may be those usually contained in cold rolled steel sheets, as in the case of Si and Mn, and thus the content of P, S and O may be up to 0.030%, up to 0.020% and up to 0.008% respectively. The steel composition used in the present invention can be produced by any one of the conventional methods alone or in combination. However, C must be removed in the step for melting the steel and for this purpose, it is advantageous to carry out a vacuum decarburization treatment using the RH process, DH process and the like. Furthermore, it is advantageous to directly melt extra low-carbon steel by means of the pure oxygen bottom-blown converter process (Q-BOP process). In addition, a conventional ingot forming process or continuous casting process may be used. A slab of the steel may be produced by a continuous casting process or by a conventional slabbing process and is then subjected to a continuous hot rolling. According to the present invention, the reduction and the rolling speed in the continuous hot rolling must be limited. Concerning the reduction, the total reduction until the slab has passed through the roughing rollers and been delivered from the finishing rolling stand must be not less than 90%. The rolling speed of the finishing stand group should be at least 40 m/min and is preferred to be more than 80 m/min. When the above described conditions of reduction and rolling speed are satisfied, fine complex precipitates of, for example, less than 1,000 A and presumably consisting of Nb(C, N), AlN and MnS are very densely present and C stably exists in the steel around these precipitates, whereby substantially non-ageing steel sheets having an extremely good deep drawing property can be obtained. When the reduction is lower than 90% and the rolling speed is lower that 40 m/min, the above described phenomenon does not occur and non-ageing steel sheets having good deep drawing properties cannot be obtained. According to the present invention, the hot rolling finishing temperature must be not lower than 830°C. When the finishing temperature is lower than this, the r value and the elongation and ageing properties are deteriorated. In the present invention, the coiling temperature must be 600 - 800°C. Fig. 3 shows the relationship of the r value to the coiling temperature when a steel slab having a C content of 0.005% and a parameter α of 4.6 and a steel slab having a C content of 0.006% and a parameter α of 10.4 were hot rolled at a total reduction of 95%, a lowest rolling speed of 70 m/min and a finishing temperature of 870 - 900°C, and then coiled at various coiling temperatures, cold rolled at a reduction of about 80% and subjected to continuous annealing at 840°C for 40 seconds. Unless the coiling temperature is 600°C or more, the r value is low and a satisfactory deep drawing property cannot be ensured. If the coiling is effected at a temperature of higher than 750°C little increase in the r value tends to occur. When the coiling temperature exceeds 800°C, the formation of scales is increased, so that such a temperature should be avoided. A coiling temperature of 680 - 750°C is most preferable in view of the AI value, r value and El value. In order to obtain a coiling temperature within this temperature range, the water cooling after the finishing rolling may be reduced or omitted altogether. The thus obtained hot rolled coil is subjected to pickling following the conventional process to remove scale and then cold rolled, or cold rolled and then subject to pickling or polishing to remove scale. When the reduction upon cold rolling is less than 60%, the desired r value cannot be obtained. When the reduction exceeeds 90%, although the r value becomes higher, the anisotropy becomes larger so that in accordance with the present invention, the reduction in the cold rolling is preferred to be within a range of 75 - 85%. According to the present invention, the thus obtained cold rolled steel strip is further subjected to continuous annealing. The annealing temperature and time are properly selected depending upon the desired steel qualities within the range of 700 - 900°C and 10 seconds - 5 minutes. Within the temperature range of 700 - 900°C, the strength is lower at the higher temperature but the r value and elongation become higher. A soaking at 780 - 880°C for 30 - 120 seconds is particularly preferable. The cooling speed after the continuous annealing is not particularly limited but in the case of a composition where α (≡ Nb%/C%) is from 3 to 8, if a slow cooling of less than 10°C/sec is effected to near 700°C after the soaking, such a treatment is advantageous for improving the qualities of the sheet and particularly the ageing resistance. In the case where a continuous annealing furnace provided with an over ageing furnace is used, the addition of an over ageing treatment to the steels of the present invention does not give any adverse influence upon the steel quality. The steel sheets according to the present invention have an AI of from 1 to 3 kg/ mm² in the state subjected to the continuous annealing, and in normal useage, an AI within this range is hardly an ageing property and steels having such an AI can be regarded as substantially non-ageing. In this case, the tensile properties of the steels may involve more or less yield elongation but this can be overcome by temper rolling at a reduction of less than 2%. The production of hot-dip galvanized steel sheets may be carried out by heating a cold rolled steel sheet in the same manner as above described and then subjecting it to galvanizing. Alternatively the sheet may be subjected to a galvannealing process and, in this case, it is not necessary to particularly limit the cooling speed. In the steel sheets of the present invention, AI may be from 1 to 3 kg/mm² in the galvanized state but such an AI hardly represents an ageing property and hence such steels can be regarded as non-ageing. In this case, the steels may have a more or less yield elongation so that it is preferable to carry out a temper rolling at a reduction of less than 2% for correcting the shape. By hot-dip galvanizing, the r value and the elongation can be lowered by 0.1 - 0.2 and 1 - 3% respectively as compared with the case where no plating is effected. The present invention will be explained with respect to the following example illustrating the production of cold rolled steel sheets. Example (1) Production of steel. Formation of slab. Molten steels having the compositions shown in the following Table 2, I and II were obtained by means of a pure oxygen top-blown converter (LD converter) and an RH degassing step. The molten steel shown in Table 2, III was obtained using a pure oxygen bottom-blown converter (Q-BOP) and an RH degassing step. The degas treating time was 25 minutes for steel I, 23 minutes for Steel II and 35 minutes for Steel III. Nb and Al were added just before completing the degas treatment. Steels I and III were formed into slabs having a thickness of 220 mm by a slabbing process. Steel II was formed into a slab having the same thickness as described above by continuous casting. (2) Hot rolling. After the above described slabs were surface treated, the slabs formed from steels I and III were maintained at a uniform temperature of 1,080°C for 35 minutes and the slab formed from steel II was maintained at a temperature of 1,200°C for 30 minutes (the temperature was measured at the slab surface). Each slab was continuously rolled through 4 lines of roughers and 7 stands of a finishing mill to obtain a hot rolled steel strip having a thickness of 3.2 mm. The reduction when the steel strip was obtained from a sheet bar in the finishing rolling was 92% in the case of steels I and III and 93% in the case of steel II respectively. The rolling speeds (substantially corresponding to the speed of the strip at the exit of the roll) in the finishing mill were as follows. Steels I and III : First stand 98 m/min 7th stand 660 m/min Steel II : First stand 103 m/min 7th stand 945 m/min The finishing temperature was controlled at 890 - 920°C. The coiling temperature was 770°C in the case of steel I, 660°C in the case of steel II and 710°C in the case of steel III respectively. (3) Cold rolling. Annealing. The hot rolled steel strips were pickled and cold rolled to obtain cold rolled coils having a thickness of 0.7 mm (reduction: 78%) or 0.8 mm (reduction: 75%). The recrystallization annealing was carried out in a continuous annealing line under the following condition. Steel I : After soaking at 820 - 850°C for 30 seconds, the heated strip was cooled to 500°C at a cooling rate of about 45°C/sec and a temperature within the range of 500 - 350°C was maintained for 180 seconds. Steel II : After soaking at 800 - 830°C for 20 seconds, the heated strip was cooled to 700°C at a cooling rate of 1.5°C/sec and from 700°C to room temperature at a cooling rate of about 20°C/sec. Steel III : After soaking at 840 - 870°C for 40 seconds, the heated strip was cooled to room temperature at a cooling rate of about 25°C/sec. The annealed coils were subjected to skin pass rolling of 0.3 - 0.7% to obtain products, the mechanical properties of which are shown in the following Table 3. From surface inspection, all the products seemed to be equal to general Al killed steel and there was no problem in making practical use of them. Thus, it can be seen that the steel sheets of the present invention have excellent surface properties and are non-ageing cold rolled steel sheets. Now reference will be made to an example illustrating hot-dip galvanised steel sheets. Steels having compositions as shown in Table 2 were cold rolled through the same steps as in the production of the cold rolled steel sheets above to obtain cold rolled steel sheets having a thickness of 0.7 mm and 0.8 mm. The recrystallization annealing was carried out in a continuous hot-dip galvanizing line under the following condition. Steel I : Soaking at 830 - 860°C for 40 seconds. Steel II : Soaking at 780 - 820°C for 25 seconds. Steel III : Soaking at 860 - 880°C for 60 seconds. The cooling rate on introduction to a plating bath at about 460°C was 2 - 10°C/sec and the steel I was subjected to a galvannealing treatment at 580°C for 10 seconds after plating. Plated coils were subjected to skin pass rolling of 0.6 - 0.7% to obtain products, the mechanical properties and the plating ability of which are shown in the following Tables 4 and 5 respectively.

Claims: A method for producing a cold rolled steel sheet by hot rolling a steel slab to form a hot rolled strip, coiling the hot rolled strip, cold rolling the coiled strip to a final gauge and annealing the cold rolled strip characterised in that (i) the steel slab has a composition consisting of not more than 0.008% of C, not more than 0.20% of Si, 0.04 - 0.30% of Mn, not more than 0.03% of P, 0.01 - 0.10% of Al provided that the Al content is more than 4 times the N content, not more than 0.02% of S, not more than 0.01% of N, and 0.01 - 0.07% of Nb provided that the Nb content is from C% x 3 to {C% x 8 + 0.02}% with the remainder being Fe and incidental impurities, (ii) the hot rolling is carried out at a total reduction of not less than 90%, a rolling speed of not less than 40m/min in the finishing rolling, and a finishing temperature of not lower than 830°C, (iii) the coiling of the hot rolled strip is carried out at a temperature of 600 - 800°C, and (iv) the annealing of the cold rolled strip is carried out by continuous annealing within a temperature range of 700 - 900°C for 10 seconds to 5 minutes whereby the cold rolled steel sheet has particularly good formability. A method for producing a cold rolled steel sheet by hot rolling a steel slab to form a hot rolled strip, coiling the hot rolled strip, cold rolling the coiled strip to a final gauge and annealing the cold rolled strip characterised in that (i) the steel slab has a composition consisting of not more than 0.008% of C, not more than 0.20% of Si, 0.04 - 0.30% of Mn, not more than 0.03% of P, 0.01 - 0.10% of Al provided that the Al content is more than 4 times the N content, not more than 0.02% of S, not more than 0.01% of N, and 0.01 - 0.07% of Nb provided that the Nb content is from C% x 3 to {C% x 8 + 0.02}% with the remainder being Fe and incidental impurities, (ii) the hot rolling is carried out at a total reduction of not less than 90%, a rolling speed of not less than 40m/min in the finishing rolling, and a finishing temperature of not lower than 830°C, (iii) the coiling of the hot rolled strip is carried out at a temperature of 600 - 800°C, and (iv) the annealing of the cold rolled strip is carried out by continuous annealing within a temperature range of 700 - 900°C for 10 seconds to 5 minutes wherein the continuously annealed strip is subjected to continuous hot-dip galvanizing whereby a galvanized cold rolled steel sheet having particularly good formability is obtained. A method as claimed in claim 1 or 2, wherein the content of Nb is from 0.01 to 0.05% and is within a range of from 4 x C(%) to 8 x C(%) + 0.010%. A method as claimed in any one of the preceding claims wherein the content of C is not more than 0.006%. A method as claimed in any one of the preceding claims wherein the coiling temperature is 680 - 750°C. A method as claimed in any one of the preceding claims, wherein the cold rolling is effected at a reduction of 75 - 85%. A method as claimed in any one of the preceding claims, wherein the continuous annealing is effected by soaking the cold rolled strip at 780 - 880°C for 30 to 120 seconds. A method as claimed in any one of the preceding claims, wherein after the continuous annealing, the strip is subjected to a temper rolling at a reduction of less than 2%.

Inventors: HASHIMOTO OSAMU, IRIE TOSHIO, SATO SUSUMU, HASHIMOTO, OSAMU, IRIE, TOSHIO, SATO, SUSUMU
Applicants: KAWASAKI STEEL CO, KAWASAKI STEEL CORPORATION
Application year: 1981
Main classification: C21D9
IPC codes: C21D8