COLD-ROLLED STEEL SHEETS AND A METHOD OF MANUFACTURING THE SAME

Abstract: A cold-rolled steel sheet for deep drawing having an improved bake hardenability is disclosed, which comprises 0.005-0.015 wt% of C, not more than 1.0 wt% of Si, not more than 1.0 wt% of Mn, not more than 0.15 wt% of P, 0.005-0.10 wt% of Al, not more than 0.003 wt% of S and not more than 0.004 wt% of N provided that S + N is not more than 0.005 wt%, and Ti satisfying 1 ≦ Ti* /C ≦ 20, in which Ti* (%) Ti(%)- 48 / 14 N(%)- 48 / 32 S(%). Such a cold-rolled steel sheet is obtained by continuously annealing the steel sheet after the cold rolling, provided that a resistence time over a temperature region above recrystallization temperature is within 300 seconds.

Description: This invention relates to cold-rolled steel sheets for deep drawing having an improved bake hardenability and a method of manufacturing the same. Recently there has been a strong demand to increase the strength of external automotive steel sheets to reduce the weight of automotive vehicles in order to improve fuel consumption. On the other hand, such steel sheets need to have a low yield strength, a high elongation, a high r-value and the like from the viewpoint of press formability. From the above conflicting requirements, therefore, the steel sheet needs to be soft and have a good workability during press forming and to exhibit the property of increasing its yield strength, or so-called bake hardenability, in the subsequent paint baking. As regards cold-rolled steel sheets having bake hardenability and the method of manufacturing the same, there are descriptions of Ti-containing steel in Japanese Patent laid open No. 53-114,717, Nb-containing steel in Japanese laid open No. 57-70,258, and Ti and Nb-containing steel in Japanese Patent laid open No. 59-31,827. In each case, the bake hardenability is imparted without deterioration of other properties by controlling the amounts of Ti and Nb added or the cooling rate during the annealing to properly adjust the amount of solute carbon in the steel. However, if it is intended to adjust the solute carbon by controlling the addition amounts of Ti or Nb, the properties of the steel sheet are considerably influenced by delicate changes of the amounts added. That is, when the addition amount of Ti or Nb is outside the predetermined range, the properties affecting formability such as elongation, r-value and the like are degraded or the bake hardenability is not obtained satisfactorily. Therefore, the exact control of the addition amount is considered to be significant in the production step. As to the amounts of each of S and N, Japanese Patent laid open No. 58-110,659 mentions that S is limited to a range of 0.001-0.020% by weight and N is limited to not more than 0.0035%, while Japanese Patent laid open No. 58-42,752 mentions that N is limited to not more than 0.0025%. However, the former is only to prevent the occurrence of surface defects by reducing the amounts of Ti and B, and the latter is only to improve the secondary workability and r-value. In a paper published on pages 155-171 of "Metallurgy of continuous-annealed sheet steel" (Proceedings of a symposium held at the AIME Annual Meeting, Dallas, Texas, February 1982), T. Irie et al report the results of a study of the influences of composition and temperature on the mechanical properties and cold-work embrittlement of steels containing from 0.004 to 0.010% carbon together with about 0.07% phosphorus and additions of titanium or niobium. The effects of varying the ratio of effective titanium (Ti*), where Ti*(%) = Ti(%)-(48/32)S(%)-(48/14)N(%), to carbon were studied in the range Ti* = 1.6 to 27.2, and it was found that to obtain both a good r -value and high BH required the steel to be annealed using a soaking temperature of at least 830°C. It is an object of the present invention to provide cold-rolled steel sheets for deep drawing having a high bake hardenability together with a high r-value without the need for annealing at high temperatures. To this end, the present inventors have made studies with respect to the relationship between the amount of S and N in Ti-containing steel with extremely low carbon and the properties of the steel, and have found that this object can be achieved with annealing at a temperature less than 830°C if the C content of the steel is maintained less than 0.004%, the amount of each of S and N and the total amount of S and N are limited to specified ranges, and the addition amount of Ti is restricted to a specified range in consideration of the S and N values. According to a first aspect of the invention, there is provided a cold-rooled steel sheet for deep drawing that exhibits an r -value of not less than 1.8 and a bake hardenability, BH of not less than 3.1 kgf/mm², which steel sheet has a composition consisting of 0.0005 to less than 0.004% by weight of C, not more than 1.0% by weight of Si, not more than 1.0% by weight of Mn, not more than 0.15% by weight of P, 0.005 to 0.100% by weight of Al, S in an amount of not more than 0.003% by weight, N in an amount of not more than 0.004% by weight provided that the value of S+N is not more than 0.005% by weight, not more than 0.05% by weight of Nb, not more than 0.0050% by weight of B, not more than 1.0% by weight of Cr, not more than 1.0% by weight of Cu, not more than 1.0% by weight of V, not more than 1.0% by weight of Zr, not more than 0.05% by weight of Sb, not more than 0.05% by weight of Ca, and Ti corresponding to Ti(wt%) represented by the following equation (1) when the effective Ti content, expressed by Ti* in the equation (1), satisfies the following inequality (2), with the balance being Fe and inevitable impurities: Ti* (wt%) = Ti (wt%) - 48 14 N(wt%) - 48 32 (wt%) 1 x C(wt%) ≦ Ti* (wt%) ≦ 20 x C(wt%) In a preferred embodiment of the invention, the effective Ti content (Ti*) is from 1 to less than 4 times the C content (wt%). According to a second aspect of the invention, there is provided a method of manufacturing a cold-rolled steel sheet for deep drawing and having an improved bake hardenability, which comprises the steps of: melting a steel having the composition set forth above, continuously casting the resulting molten steel to produce a cast slab; hot rolling the resulting cast slab; cold rolling the resulting hot-rolled sheet; and subjecting the resulting cold-rolled sheet to a continuous annealing in which the residence time above the recrystallization temperature, including heating it to, holding it at and cooling from a soaking temperature of less than 830°C, is within 300 seconds. In a preferred embodiment of the invention, the cast slab is heated at a heating temperature of not less than 1,150°C before the hot rolling step. For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example to the accompanying drawings, in which: Figs.1 and 2 are graphs showing the relationship between the amount of (S+N) in steel and the properties of the steel sheet, respectively; Fig.3 is a graph illustrating an outline for the measurement of bake hardenability; Fig.4 is a graph showing the influence on bake hardenability of residence time at a temperature above the recrystallisation temperature; and Fig.5 is a graph showing the relationship between the slab reheating temperature and the r-value. First, the invention will be described by reference to experimental results on which the invention is based. A slab of vacuum molten steel comprising 0.0015% of C, 0.1% of Mn, 0.04% of Al and variable amounts of N, S and Ti was hot rolled to a thickness of 3.5 mm and then cold rolled to a thickness of 0.8 mm in a laboratory. Then, the cold-rolled sheet was subjected to a heat treatment under such a heat cycle that the sheet was soaked at 800°C for 40 seconds and then temper rolled at a reduction of about 0.8%. With these sheets, the influence of the (S + N) amount on the bake hardenability (hereinafter abbreviated as BH), the r-value and the total elongation (hereinafter abbreviated as EI) was examined to obtain the results shown in Figs. 1 and 2. Moreover, BH was evaluated by measuring the increasing amount of the yield point when applying a preliminary strain of 2% and subjecting the sheet to an aging treatment corresponding to a baking at 170°C for 20 minutes as shown in Fig. 3. Each of the E I values and r -values was an average of the measured values obtained on three test pieces sampled at three angles of 0°, 45° and 90° with respect to the rolling direction as calculated according to the following equations: E I = EI₀ + EI₉₀ + 2EI₄₅ 4 r = r₀ + r₉₀ + 2r₄₅ 4 In Figs. 1 and 2, symbol ○ indicates the case where S ≦ 30 ppm, symbol  indicates the case where S = 40 ppm and variable amounts of N, and symbol ▴ indicates the case where N = 45 ppm and variable amounts of S. Moreover, Fig. 1 shows the data under the condition where 4 ≦ Ti*/C ≦ 20, while Fig. 2 particularly shows the data under the condition where 1 ≦ Ti*/C < 4. As seen from Fig. 1, when S ≦ 30 ppm, S+N ≦ 50 ppm and 4 ≦ TI*/C ≦ 20, BH of at least 2 kgf/mm² can be obtained and is enhanced without degrading the E I and r -values as the total amount of S + N becomes smaller. On the other hand, when S = 40 ppm or N = 45 ppm, even if S + N = 50 ppm, BH is 1.5 kgf/mm² at the most. Particularly, as seen from Fig. 2, when 1 ≦ Ti*/C < 4, S ≦ 30 ppm and S + N ≦ 50 ppm, BH of 5.5 kgf/mm² or more is obtained without degrading the E I and r -values. Although the reason why BH of at least 2 kgf/mm² is obtained as shown in Figs. 1 and 2 is not clear, it is considered to be due to the following facts. That is, Ti in steel forms precipitates of TiS and TiN by reaction with S and N before the formation of TiC. Therefore, in order to fix C as TiC, it is required to consider the ratio of the effective Ti amount, obtained by subtracting the amount of Ti bonded to S and N from the total Ti amount (Ti* = Ti - 48 32 S - 48 14 N), to the C amount. In this regard, Ti*/C = 4 by weight ratio means that the atomic ratio of Ti to C is 1:1, which is a measure for completely fixing C as TiC. Thus, when Ti*/C ≧ 4 under the equilibrium state, even if all of the C amount is precipitated as TiC, an excess amount of Ti still remains without producing solute C. The inventors have found from various studies and experiments that, since the precipitation of TiC is progressed by utilizing TiS and TiN as a precipitation site, it is difficult to precipitate TiC by reducing the TiS and TiN or the amounts of S and N. Therefore, even if 20≧Ti*/C≧4, solute C can be left under metastable condition, which contributes to the improvement of BH as shown in Fig. 1. On the other hand, when 1≦Ti*/C<4, a proper amount of solute C can be stably held, which contributes to the considerable increase of BH as shown in Fig. 2. According to the invention, the reason why the composition of the steel is limited to the above ranges is as follows. C : The C content is advantageously as low as possible for improving the properties of the steel. On the other hand, if the C content is less than 0.0005%, improved BH can not be obtained. Thus, the C content is restricted to a range of 0.005 to less than 0.004%. Si, Mn : Each of Si and Mn effectively contributes to increase the strength of the steel sheet without degradation of deep drawability. However, when Si and Mn are more than 1.0%, respectively, the elongation and drawability of the steel sheet are considerably degraded. Therefore, Si and Mn are restricted to not more than 1.0%, respectively. P: P is effective for increasing the strength of the steel sheet without degradation of deep drawability as in the case of Si and Mn. However, if P is more than 0.15%, the elongation and drawability of the steel sheet are considerably degraded. Therefore, P is restricted to not more than 0.15%. Al: Al is added in an amount of not less than 0.005% for deoxidation or the like. On the other hand, the addition of more than 0.100% of Al adversely affects the surface properties of the steel sheet. Thus, Al is restricted to a range of 0.005-0.100%. S, N: S and N are most important ingredients according to the invention. As is apparent from the aforementioned experimental results, S ≦ 0.003%, N ≦ 0.004% and S + N ≦ 0.005% are required to advantageously provide the improved bake hardenability. Ti: Ti is added for fixing S, N and C. In this case, when the effective Ti amount [Ti* (%) = TI (%) - 48 14 N (%) - 48 32 S (%)] is within a range of 1 to 20 times the C content, a bake hardenability of at least 2 kgf/mm² can be obtained with a high r-value. If Ti* is less than the C content (or the atomic ratio of Ti*/C is less than 0.25), solute C excessively remains in the steel, which is apt to cause yield elongation. On the other hand, the excess addition of Ti causes degradation of the surface properties of the steel sheet and becomes disadvantageous in view of the cost so the upper limit of Ti* is restricted to 20 times of C content. In a steel sheet of the above composition, at least one of Nb and B may be added to enhance the r-value and Eℓ without damaging the desired bake hardenability. However, when Nb is more than 0.05% and B is more than 0.0050%, the addition effect is saturated and the cost becomes disadvantageous, so that the upper limits of Nb and B are restricted to not more than 0.05% and not more than 0.0050%, respectively. Moreover, not more than 1.0% of each of Cr, Cu, V and Zr and not more than 0.05% of each of Sb and Ca may be added, if necessary, because they do not degrade BH and deep drawability. According to the invention, a cold-rolled steel sheet having the above composition is produced by forming molten steel, tapped from a converter or an electric furnace, into a slab by an ingot making-slabbing process or a continuous casting process, hot rolling and cold rolling the slab and continuously annealing the cold-rolled sheet with the total residence time above the recrystallization temperature, including heating it to, holding it at and cooling from a soaking temperature of less than 830°C, is within 300 seconds. In this connection, a slab of vacuum molten steel comprising 0.0020% of C, 0.1% of Mn, 0.04% of Aℓ, 0.026% of Ti, 0.0022% of S and 0.0019% of N (i.e. Ti*/C≅8.1) was hot rolled to a thickness of 3.5 mm and then cold rolled to a thickness of 0.8 mm in a laboratory. Moreover, the recrystallization temperature of the cold-rolled sheet was 660°C. In Fig. 4 there is shown the relationship between BH and the residence time, t (sec), at a temperature above the recrystallization temperature (T R ) when the above cold-rolled sheet is subjected to continuous annealing under such conditions that the heating and cooling rates are 10°C/sec, respectively and the soaking time is varied. As seen from Fig. 4, a high BH value can stably be obtained when the residence time at a temperature above the recrystallization temperature is within 300 seconds. This is considered to be due to the fact that the long-term annealing becomes disadvantageous for securing solute C because the precipitation of TiC progresses during the annealing. In the continuous annealing inclusive of heating and cooling, therefore, the residence time in the temperature region above the recrystallization temperature must be shortened and is within 300 seconds, preferably 100 seconds. Moreover, the relationship between the slab reheating temperature before the hot rolling and the r -value of the steel sheet after the continuous annealing was examined to obtain the results shown in Fig. 5. In the continuous annealing, the residence time in the temperature region above the recrystallization temperature (660°C) was 140 seconds and the soaking temperature was 800°C. As seen from Fig. 5, the r -value is considerably enhanced when the slab reheating temperature is not less than 1,150°C. This is considered to be due to the fact that, when the slab is reheated at higher temperature, the distribution and morphology of the composite precipitate of TiS and TiC in the hot-rolled sheet change to advantageously develop the recrystallization texture of {111} in the cold rolling and annealing. As a result of subsequent experiments, it has been confirmed that when the slab reheating temperature is not less than 1,150°C, steel sheets having a considerably high r -value with a high BH value can be obtained irrespective of the heat history of the slab to be heated, the hot rolling conditions and the coiling temperature. The cold-rolled steel sheets according to the invention have excellent phosphate treating properties, hot dipping properties and secondary workability and may be used as an original steel sheet for surface treatment such as electric zinc coating or the like. The following Examples illustrate the invention. Example 1 Steel materials having the chemical compositions as shown in Table 1 were each melted in a converter, subjected to a degassing treatment under vacuum, and then cast by a continuous casting apparatus to form a slab. This slab was hot rolled and cold rolled in the usual manner to form a cold-rolled steel sheet having a thickness of 0.8 mm, which was subjected to a continuous annealing (soaking conditions: 800°C, 30 seconds) and a temper rolling (reduction: 0.5-1%). The mechanical properties of the thus obtained products are shown in Table 2. The mechanical properties were all measured by using JIS No. 5 test pieces. Each of the YS , TS , E I and r -values is the average value ( x = X₀ + X₉₀ + 2 x₄₅ 4 ) of test results with respect to the rolling direction (x₀), 45° to the rolling direction (x₄₅), and 90° to the rolling direction (x₉₀). YEI, BH and the aging index Al (increment in yield point after aging under preliminary strain of 7.5% at 100°C for 30 minutes) are test results with respect to the test piece sampled parallel with the rolling direction. In the steel sheets according to the invention, r-values of not less than 1.9 and BH of not less than 3.2 kgf/mm² were obtained. However, with respect to Comparative Steel No. 6 in which the S content was outside of the range defined in the invention and Comparative Steel No. 7 in which the total amount of S + N was outside the range defined in the invention, BH was as low as 1.2 kgf/mm² and 0.8 kgf/mm², respectively. Further, with respect to Comparative Steel No. 8 in which the C content was in excess, the EI and r -values were deteriorated. Example 2 Each of steel materials (Nos. 14-17) having a chemical composition as shown in Table 3 were melted in a converter, subjected to a degassing treatment under vacuum and continuously cast to form a slab. The slab thus obtained was hot rolled and then cold rolled in the usual manner to form a cold-rolled steel sheet having a thickness of 0.8 mm, which was subjected to a continuous annealing (soaking conditions: 800°C, 30 seconds) and a temper rolling (reduction: 0.5-1%). The mechanical properties of the products thus obtained were examined in the same manner as in Example 1 to obtain the results shown in Table 4. In each of Steel Nos. 14-24 according to the invention, an r -value of not less than 1.8, a BH of not less than 3.1 kgf/mm² and YEI of not more than 0.2% were obtained. On the contrary, in each of Comparative Steel Nos. 25 and 26 in which the S and N content was outside the range defined in the invention, BH was extremely low. In Comparative Steel No. 27 in which the C content exceeded the upper limit, the BH property was excellent, but the E I and r -values were conspicuously deteriorated. All of Steel Nos. 14-24 according to the invention were 2 ≦ Al ≦ 5 kgf/mm². Example 3 Each of steel materials (Nos. 28-30) having a chemical composition as shown in Table 5 were melted in a converter, subjected to a degassing treatment under vacuum and continuously cast to form a slab. The thus obtained slab was heated at 1,100-1,220°C, hot rolled, and then cold rolled to form a cold-rolled steel sheet having a thickness of 0.8 mm, which was subjected to a continuous annealing. In the continuous annealing under such a cycle that the steel sheet was heated to 820°C and then cooled from this temperature, the residence time in a temperature region above the recrystallization temperature was varied. The mechanical properties and BH of the products thus obtained were examined to obtain the results shown in Table 6. As seen from Table 6, a high BH value was obtained with no problems in the mechanical properties when the residence time in the temperature region above the recrystallization temperature was within 300 seconds. In all products, Al was not less than 2 kgf/mm². By the way, the recrystallization temperature was 650°C, 720°C and 760°C in the cases of Steel No. 28, Steel No. 29 and Steel No. 30, respectively. Example 4 Each of steel materials A and B having a chemical composition as shown in Table 7 was melted in a converter, subjected to a degassing treatment under vacuum, and cast by a continuous casting apparatus to form a slab. The thus obtained slab was heated and soaked at 1,090-1,330°C for 3-4 hours and then hot rolled. In this case, the hot rolling finish temperature and the coiling temperature were 910-880°C and 510-600°C, respectively. After being pickled, the hot-rolled steel sheet was cold rolled to form a cold-rolled steel sheet having a thickness of 0.8 mm, which was then subjected to a continuous annealing. In the continuous annealing, the residence time in the temperature region above the recrystallization temperature was set in a range of 75-92 seconds, and the attained maximum temperature was 790-820°C. The properties of the steel sheets after the temper rolling at a reduction of 0.5-0.8% are shown in Table 8. By setting the slab reheating temperature at 1,210-1,330°C, a high BH value was ensured, and an r -value of 2.3 to 2.6 and AI of not less than 2 kgf/mm² were obtained. As mentioned above, according to the invention, a proper bake hardenability can be obtained together with deep drawability in a cold-roller sheet of extremely low carbon aluminum killed steel containing less than 0.004% carbon by restricting the S, N and S+N amounts in the steel to particular ranges and satisfying 1≦Ti*/C≦20 as the Ti amount. Particularly, the proper bake hardenability is advantageously ensured by continuous annealing under the specified recrystallization annealing conditions.

Claims: A cold-rolled steel sheet for deep drawing that exhibits an r -value of not less than 1.8 and a bake hardenability of not less than 3.1 kgf/mm², said sheet having a composition consisting of 0.0005 to less than 0.004% by weight of C, not more than 1.0% by weight of Si, not more than 1.0% by weight of Mn, not more than 0.15% by weight of P, 0.005 to 0.100% by weight of Al, S in an amount of not more than 0.003% by weight, N in an amount of not more than 0.004% by weight provided that the value of S+N is not more than 0.005% by weight, not more than 0.05% by weight of Nb, not more than 0.0050% by weight of B, not more than 1.0% by weight of Cr, not more than 1.0% by weight of Cu, not more than 1% by weight of V, not more than 1.0% by weight of Zr, not more than 0.05% by weight of Sb, not more than 0.05% by weight of Ca, and Ti corresponding to Ti(wt%) represented by the following equation (1) when the effective Ti content, expressed by Ti* in the equation (1), satisfies the following inequality (2), with the balance being Fe and inevitable impurities Ti* (wt%) = Ti (wt%) - 48 14 N(wt%) - 48 32 S(wt%) 1 x C(wt%) ≦ Ti* (wt%) ≦ 20 x C(wt%) The cold-rolled steel sheet as claimed in claim 1 wherein one or both of niobium and boron is present. The cold-rolled steel sheet as claimed in claim 1 or claim 2, wherein said effective Ti content, expressed by Ti*, is from 1 to less than 4 times said C content. A method of manufacturing a cold-rolled steel sheet for deep drawing that exhibits an r -value of not less than 1.8 and a bake hardenability BH of not less than 3.1 kgf/mm² which comprises the steps of: melting a steel having the composition set forth in any preceding claim, continuously casting the resulting molten steel to produce a cast slab; hot rolling the resulting cast slab; cold rolling the resulting hot-rolled sheet; and subjecting the resulting cold-rolled sheet to a continuous annealing in which the residence time above the recrystallization temperature, inclusive of heating to, holding at and cooling from, a soaking temperature of less than 830°C, is within 300 seconds. The method according to claim 4, wherein said cast slab is heated to a heating temperature of not less than 1,150°C before said hot rolling step.

Inventors: KUROSAWA, MITSUMASA TECHNICAL RESEARCH DIVISION, OBARA, TAKASHI TECHNICAL RESEARCH DIVISION, SATO, SUSUMU TECHNICAL RESEARCH DIVISION, SUZUKI, HIDEO TECHNICAL RESEARCH DIVISION, TSUNOYAMA, KOZO TECHNICAL RESEARCH DIVISION
Applicants: KAWASAKI STEEL CORPORATION
Application year: 1985
Main classification: C22C38
IPC codes: C22C38, C21D8