Soft cold-rolled steel sheet and method for making the same

Abstract: A soft cold-rolled steel sheet contains: 0.06 wt.% or less C, 0.1 wt.% or less Si, 0.5 wt.% or less Mn, 0.03 wt.% or less P, 0.03 wt.% or less S, 0.006 wt.% or less N, 0.009 wt.% or less B and the balance being Fe. The method comprises the steps of: continuously casting a steel to produce a slab; hot-rolling the slab to form a hot-rolled steel sheet; cold-rolling the hot-rolled steel sheet to produce a cold-rolled steel sheet; and continuously annealing the cold-rolled steel sheet.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a soft cold-rolled steel sheet and a method for making the same. 2. Description of the Related Arts In conventional production of cold-rolled steel sheets for working which are produced by continuous annealing, high-temperature coiling has been performed in the hot rolling in order to prompt precipitation of AlN and coarsening of carbides and thus to achieve softening and high r-values. High-temperature coiling, however, causes an increased scale thickness at both ends of the coil by oxygen which is readily supplied, and thus causes deterioration of acid pickling characteristics. As a method for decreasing a coiling temperature using softening by boron addition, unexamined Japanese Patent Publication No. 2-263932 discloses a method for making a cold-rolled steel sheet for deep drawing, in which a boron containing steel having a specified Mn/S ratio is heated to 1,000 °C to 1,200 °C, coiled at 560 °C to 650 °C, and continuously annealed at a relatively high temperature of 730 °C to 880 °C. Various methods using excellent grain growth characteristics of boron containing steels have been proposed for achieving excellent workability by high-temperature continuous annealing after low-temperature coiling. For example, unexamined Japanese Patent Publication No. 7-3332 discloses a method for making a cold-rolled steel sheet for working which is characterized in that a boron containing steel sheet is coiled at 600 °C to 700 °C, and annealed at 740 °C to 930 °C. Unexamined Japanese Patent Publication No. 9-3550 discloses a method for making a cold-rolled steel sheet for working which is characterized in that a boron containing steel sheet is coiled at 630 °C to 720 °C and annealed at 800 °C to 880 °C. Also, unexamined Japanese Patent Publication No. 56-156720 discloses a method for making a cold-rolled steel sheet having excellent workability in which the relationship between B and N is specified and high-temperature annealing is performed after low-temperature coiling at 650 ° C or less. Among methods which specify the B/N ratio, added elements, and/or the heating temperature of the slab in order to achieve more excellent workability, unexamined Japanese Patent Publication No. 64-15327 discloses a method which specifies the heating temperature of the steel slab containing B in an amount of higher than the equivalent of N, that is, coiling at 550°C to 700°C and annealing at 750 ° C to 850 °C; and unexamined Japanese Patent Publication No. 61-266556 discloses a cold-rolled steel sheet having excellent press workability in which a steel containing 0.10 to 0.30% of Cr and having a B/N ratio in a specified range from 0.5 to 2.0 is coiled at 550 ° C to 700 ° C and annealed at approximately 800 °C. When a boron containing steel having excellent grain growth characteristics is annealed at a high temperature of 700 °C or more, a mixed grain texture will often form and thus surface quality will deteriorate during the working. In recent years, high-quality surface characteristics have been increasingly required. Deterioration of surface characteristics due to the mixed grain texture, which was out of consideration, is raising problems; however, the above-mentioned conventional technology do not teach a countermeasure against the decreased surface quality due to the mixed grain texture formed by annealing at 700 °C or more. As described above, there has not been a method for enhancing stability of the texture in a B containing steel during continuous annealing in order to prevent the formation of a mixed grain texture. Thin steel sheets used in automobiles and home electric products require high formability, and achievement of softening and a high r-value is in intensive progress. When such a thin steel sheet having high formability is made by continuous annealing using a low-carbon aluminum-killed steel, C and N must be fixed as coarse precipitates by high-temperature coiling in hot rolling. Since the ends of the coil in the longitudinal direction (the T section: the top section of the coil, and the B section: the tail section of the coil) and the ends in the width direction have high cooling rates by direct contact with air even in the high-temperature coiling, AlN does not sufficiently precipitate. Since the unprecipitated AlN finely precipitates in continuous annealing, the ends in the longitudinal and width directions are hardened compared with the central section of the coil, resulting in so-called coil end characteristics. The high-temperature coiling also causes decreased acid pickling characteristics due to an increased scale thickness. As a method for solving such coil end characteristics and acid pickling characteristics, unexamined Japanese Patent Publication No. 48-100314 discloses a method for reducing the coiling temperature by the addition of B which react with N to form coarse BN and thus suppress the formation of fine AlN. As described in unexamined Japanese Patent Publication No. 48-100314, improvement in the coil end characteristics is uniformly achieved by the addition of B, but a problem that the material quality varies arises. In the conventional technology, the steel is hardened with an increased O content in the steel, and the material quality may vary even at the same O content in some cases. In conventional production of cold-rolled steel sheets for working which are produced by continuous annealing, high-temperature coiling has been performed in the hot rolling in order to prompt precipitation of AlN and coarsening of carbides and thus to achieve softening and high r-values. High-temperature coiling, however, causes an increased scale thickness at both ends of the coil by oxygen which is readily supplied, and thus causes deterioration of acid pickling characteristics. Unexamined Japanese Patent Publication No. 48-100314 discloses a method for lowering the coiling temperature by fixing N with B as BN; however, application of this method to hot direct rolling does not cause effects by the lowered coiling temperature. In the heating furnace, a part of coarse MnS that precipitates in the slab is not solved. In contrast, in hot direct rolling, the rolling is performed in the state that MnS is entirely dissolved, hence fine MnS, which precipitates during the rolling, suppresses crystal grain growth. For the purpose of obtaining a soft material by hot direct rolling having substantially the same quality as that by the heating furnace, unexamined Japanese Patent Publication No. 7-242995 discloses a method for softening by controlling the S content to 0.004% or less so as to reduce the fine MnS content. Unexamined Japanese Patent Publication No. 9-3550 discloses a method for prompting coarsening of the precipitate, in which a continuously cast slab is subjected to rolling before cooling to the Ar 3 point or less so as to suppress the transformation of MnS, as nuclei of the precipitate, affected by the transformation of Fe before the rolling. When the S content is reduced to 0.004% or less by the method disclosed in unexamined Japanese Patent Publication No. 7-242995, desulfurization costs are significantly high and thus the use is limited to high class steel sheets. In the method disclosed in unexamined Japanese Patent Publication No. 9-3550, softening is not sufficiently performed and high-temperature annealing at 800 ° C or more is inevitable. As described above, a method enabling low-temperature coiling in the hot direct rolling is now not developed when a soft cold-rolled steel sheet is produced. SUMMARY OF THE INVENTION It is an object of the present invention to provide a soft cold-rolled steel sheet suitable for forming automobiles and home electric products, and a method for making the same. First, to attain the object, the present invention provides a soft cold-rolled steel sheet consisting of: 0,01 to 0.06 wt.% C, 0.1 wt.% or less Si, 0.5 wt.% or less Mn, 0.03 wt.% or less P, 0.03 wt.% or less S, 0.006 wt.% or less N, 0.009 wt.% or less B, stoichiometric ratio of B/N being 0.6 to 1.5, Al satisfying the following equation: sol. Al ≤ 0.035 ×( B/N × 0.6 ) ½ , optionally containing at least one element selected from the group consisting of 0.5 wt.% or less Cu, 0.5 wt.% or less Ni, 0.5 wt.% or less Cr, 0.5 wt.% or less Sn, 0.1 wt.% or less Ca, and 0.05 wt.% or less O, said at least one element being 2 wt.% or less the balance being Fe and inevitable impurities. The C content is preferably 0.01 to 0.03 wt.%. The N content is preferably 0.005 wt.% or less, more preferably 0.0035 wt.% or less. Secondly, the present invention provides a method for making a soft cold-rolled steel sheet as defined in claim 2. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a microscopic photograph of a cross-sectional texture of a B-containing steel in which coarse ferrite grains partly form by high-temperature annealing. FIG. 2 is a graph illustrating the relationship between the B/N ratio and the elongation (EL) and between the B/N ratio and the maximum grain size in Embodiment 1. FIG. 3 is a graph illustrating the relationship between the Al content and the elongation (EL) and between the Al content and the maximum grain size in Embodiment 1. DESCRIPTION OF THE EMBODIMENT Embodiment 1 A soft cold-rolled steel sheet of Embodiment 1 consists of: 0,01 to 0.06 wt.% C, 0.1 wt.% or less Si, 0.5 wt.% or less Mn, 0.03 wt.% or less P, 0.03 wt.% or less S, 0.006 wt.% or less N, 0.009 wt.% or less B, stoichiometric ratio of B/N being 0.6 to 1.5, Al satisfying the following equation: sol. Al ≤ 0.035 × ( B/N × 0.6 ) 1/2 , optionally further containing at least one element selected from the group consisting of 0.5 wt.% or less Cu, 0.5 wt.% or less Ni, 0.5 wt.% or less Cr, 0.5 wt.% or less Sn, 0.1 wt.% or less Ca, and 0.05 wt.% or less O, said at least one element being 2 wt.% or less, the balance being Fe and inevitable impurities. The C content is preferably 0.01 to 0.03 wt. %. The N content is preferably 0.005 wt.% or less, more preferably 0.0035 wt.% or less. It is preferable that the soft cold-rolled steel sheet. A method for making a soft cold-rolled steel sheet according to Embodiment 1 comprises the steps of: (a) providing a slab consisting of: 0.01 to 0.06 wt.% or less C, 0.1 wt.% or less Si, 0.5 wt.% or less Mn, 0.03 wt.% or less P, 0.03 wt.% or less S, 0.006 wt.% or less N, 0.009 wt.% or less B, stoichiometric ratio of B/N being 0.6 to 1.5, Al satisfying the following equation: Al ≤ 0.035 ×( B/N × 0.6 ) 1/2 the balance being Fe and inevitable impurities; (b) hot-rolling the slab at a finishing temperature of an Ar 3 point or more and at a coiling temperature of 650 ° C or less to produce a hot-rolled steel sheet; (c) cold-rolling the hot-rolled steel sheet to produce a cold-rolled steel sheet; and (d) continuously annealing the cold-rolled steel sheet at a heating rate of 1 °C/sec. or more and at an soaking temperature of 700 °C or more. The present inventors have repeated intensive study in order to achieve a boron-containing soft cold-rolled steel sheet having excellent texture stability during high-temperature annealing and a method for making the same, and results in the following knowledge. Since the boron-containing steel has excellent grain growth characteristics, high-temperature annealing readily causes a mixed grain texture. As an example is shown in Fig. 1, coarse ferrite grains partially form when a steel containing 0.015% of C, 0.023% of Al, 0.0007% of B, and 0.0020% of N, and having a B/N ratio of 0.45 is coiled at 600 °C and annealed at 800 °C. The present inventors have repeated intensive study on the reason of the formation of such a mixed grain texture during high-temperature annealing. As a result, they have discovered that high-temperature annealing in a state that dissolved N remains to some extent causes inhomogeneous precipitation of AlN and the local formation of coarse grains in boron-containing steel having excellent grain growth characteristics. It has also been discovered that in order to suppress the mixed grain texture, the B/N ratio is specified so as to reduce the dissolved N content in the hot-rolled steel sheet, and the Al content is reduced in cooperation with the B/N ratio based on the relationship represented by the following equation (1) : Al ≤ 0.35 × ( B/N × 0.6) ½ so as to delay the initiation of precipitation of AlN during annealing. Accordingly, it has been discovered that a soft cold-rolled steel sheet having excellent texture stability can be produced without inhibiting locally grain growth in the recrystallization process during high-temperature annealing. The experiments that conducted the knowledge will now described. Materials containing approximately 0.015% of C, approximately 0.20% of Mn, approximately 0.011% of P, approximately 0.008% of S, approximately 0.010% of Al, 0.0035% or less of B, and 0.0035% or less of N and having different B/N ratios were heated to 1,200 °C, finish-rolled at a temperature of the Ar 3 point or more, and coiled at 600 °C. After acid pickling and cold rolling, they were heated at a rate of 20 °C/sec. and annealed at 800 °C to prepare annealed sheets having a thickness of 1.2 mm. These were used for observation of the cross-sectional texture and for measurement of elongation (EL) using JIS No. 5 tensile test pieces. The results are shown in Fig. 2. Elongation slightly increases as the B/N ratio increases, and a softening effect is observed as conventionally described. At a B/N ratio of 0.2 or more, however, a significant softening effect is not observed. Nevertheless, the maximum grain size (the average of grain sizes of the top ten within a range of the thickness by 1 mm) significantly increases within a range of the B/N ratio of 0.2 to 0.6, and mixed grains form instead of the normal grain growth. When the B/N ratio is more than 1.5, elongation decreased due to the fine grain effect and the solid-solution strengthening caused by dissolved B. Next, materials containing approximately 0.015% of C, approximately 0.20% of Mn, approximately 0.011% of P, and approximately 0.008% of S, and having the B/N ratio of approximately 1 and different Al contents were heated to 1,200 °C, finish-rolled at a temperature of the Ar 3 point or more, and coiled at 600 ° C. After acid pickling and cold rolling, they were heated at a rate of 20 °C/sec. and annealed at 800 °C to prepare annealed plates having a thickness of 1.2 mm. These were used for observation of the cross-sectional texture and for measurement of elongation (EL) using JIS No. 5 tensile test pieces. The results are shown in Fig. 3. Although elongation moderately changes with a change in the Al content, the maximum grain size steeply increases for an Al content (0.027%) higher than that calculated by the equation (1) and thus the formation of a mixed grain texture is suggested. Based on the knowledge, the present inventors discovered a boron-containing soft cold-rolled steel sheet having excellent texture stability during high-temperature annealing and a method for making the same by controlling the B/N ratio and the Al content to given levels in the B-containing steel, and by optimizing the hot-rolling and annealing conditions. Bases of added components, limitation of the contents, and limitation of the production conditions will now be described. (1) Chemical Composition C ≤ 0.06% When more than 0.06% of C is added, large amounts of carbides precipitate, the r-value and elongation are decreased, and formability is inhibited. Thus, the upper limit is 0.06%. At less than 0.01%, the driving force for precipitation of carbides during overaging in the continuous annealing process is reduced, and overaging resistance deteriorates. Thus, the lower limit is preferably 0.01%. The C content is preferably 0.01 to 0.04 wt.%, more preferably 0.01 to 0.03 wt.%. Si ≤ 0.1% When Si is excessively added, the strength increases and the formability deteriorates. Thus, the content is 0.1% or less. Mn ≤ 0.5% It is preferable that the Mn content be 0.05% or more since it fixes S to form MnS, however, an excessive content causes hardening of the steel and deterioration of the formability. Thus, the upper limit is 0.5%. P ≤ 0.03% P is a solid-solution strengthening element, and a content of more than 0.03% causes hardening of the steel. Thus, the upper limit is 0.03%. S ≤ 0.03% Since S is an element inhibiting hot ductility and formability, it is fixed as MnS. Thus, it is preferable that the content be low. A content of higher than 0.03% causes an increased Mn content and decreased formability. Thus, the upper limit is 0.03%. N ≤ 0.006% N is fixed as BN; however, a large amount of BN causes decreased workability. Thus, the upper limit is 0.0035%. B ≤ 0.009% Although B is an element effective for softening, an excessive B content causes increased deformation resistance. Thus, the upper limit is 0.009%. B/N Ratio: 0.6 to 1.5 The B/N ratio is significantly important. At a B/N ratio of less than 0.6, a large amount of fine AlN precipitates, resulting in hardening of the steel, hence the lower limit of the B/N ratio is 0.6. At a B/N ratio of higher than 1.5, B in the steel forms, resulting in hardening of the steel, hence the upper limit of the B/N ratio is 1.5. sol. Al ≤ 0.035 × ( B/N × 0.6 ) 1/2 Since Al is used as a deoxidiser, it is contained in a certain amount; however, it affects the initiation time of precipitation of fine AlN during annealing in Embodiment 1. Thus, the content range is important. Although a large amount of Al has been added for the purpose of perfect fixing of N, the Al content must be reduced in Embodiment 1. The precipitation of AlN during annealing depends on the Al content and the dissolved N content. The precipitation of AlN is first initiated in un-recrystallized portions having a large driving force. When the dissolved N content is moderately low as in B-containing steel, N is consumed for precipitation of the un-recrystallized portions. Thus, it barely precipitates in the other portions, resulting in inhomogeneous precipitation. Although recrystallization and grain growth are suppressed in the portion in which AlN precipitates, the grain growth proceeds in the other portions. Since the resulting difference in the grain size is further prompted in the growing process, a mixed grain texture is formed. In contrast, the precipitation of AlN is delayed in the un-recrystallized portions by specifying the Al content as described in the equation (1), and thus the formation of the mixed grains is suppressed. In Embodiment 1, the steel sheet may contain 2% or less in total of at least one selected from the group consisting of 0.5% or less of Cu, 0.5% or less of Ni, 0.5% or less of Cr, 0.5% or less of Sn, 0.1% or less of Ca, and 0.05% or less of O. Since Cu, Ni, Cr, Sn, Ca and O do not inhibit the texture stability, these can be added in adequate amounts based on the same concept as general steels. That is, Cu, Ni, Cr, and Sn having the above contents prompt aggregation of carbides and improve aging resistance. Ca prompts aggregation of carbides when it is added in an amount within the rage. O is present as oxides in the steel, functions as nuclei for MnS and BN precipitation, and prompts the precipitation. By controlling the contents of the components as described above, a B-containing soft cold-rolled steel sheet having excellent texture stability during high-temperature annealing can be obtained. The steel sheet having such a characteristic can be produced by the following method. (2) Step of Producing Steel Sheet (Making Method) A steel having a composition within the above-described range was prepared by melting, and a slab prepared by continuous casting was finish-rolled at a temperature region of the Ar 3 point or higher and coiled at less than 650 °C. The coiled hot-rolled steel sheet was cold-rolled and continuously annealed at a heating rate of 1 °C/min. or more and at an soaking temperature of 700 °C or more. In the present invention, the temperatures of individual steps have important significance, and the effects in the present invention deteriorates if any one of these lacks. A. Finishing Temperature The finishing temperature is the Ar 3 point or more. A finishing temperature of less than the Ar 3 point causes the growth of the texture that causes a decreased r-value, hence the lower limit is the Ar 3 point. B. Coiling Temperature The upper limit of the coiling temperature is 650 °C in view of acid pickling characteristics; however, the shape of the coil is not stabilized at less than 200 ° C, hence it is preferred that the temperature be 200 °C or more. C. Heating Rate for Annealing In Embodiment 1, the heating rate is important. In Embodiment 1, the Al content and the B/N ratio are specified to delay the precipitation of AlN relative to recrystallization. At a heating rate of less than 1°C/sec., AlN readily precipitates, and AlN precipitates in the un-recrystallized portions before completion of the recrystallization and partially suppresses the recrystallization and crystal grain growth. Thus, the resulting texture includes mixed grains. Accordingly, the lower limit of the heating rate is 1 °C/sec, more preferably 10 °C/sec. D. Annealing Temperature Since softening is not sufficiently accomplished at an annealing temperature of less than 700 °C, the lower limit of the annealing temperature is 700 °C. Annealing at more than 900 °C causes the formation of a random texture during the cold rolling step, hence it is preferable that the temperature be 900 °C or less. Although the slab heating temperature is not specified, it is preferred that the temperature be 1,050 ° C or more in view of rolling load and the finishing temperature. Hot direct rolling without cooling the continuous cast slab may be also employed without trouble. The advantages in Embodiment 1 do not deteriorate when finish rolling is performed while heating and holding it after rough rolling. Continuous finish rolling of jointed rough bars after rough rolling will not cause problems. The advantages in Embodiment 1 do not deteriorate when using a thin slab. In the cold rolling after acid pickling, it is preferred that the reduction rate be 30 to 90% in view of workability and in particular deep drawability. Although the conditions for temper rolling are not limited, it is preferred that the reduction rate be 2% or less, since elongation significantly decreases at a reduction rate of more than 2%. In the composition control of the steel in accordance with Embodiment 1, either a converter or an electric furnace may be used. Example 1 Each steel containing chemical components shown in Table 1 was hot-rolled at a temperature of the Ar 3 point or more, and coiled at a coiling temperature shown in Table 2. After acid pickling and cold rolling, it was continuously annealed under the annealing conditions shown in Table 2, and then was subjected to temper rolling with a rolling reduction rate of 1.2% to form a sheet having a thickness of 0.7 mm (Examples in accordance with the present invention Nos. 1 to 4, 6 to 9, 11 to 14, 16 and 17, and Comparative Examples No. 5, 10 and 15). The texture stability was evaluated by texture observation measuring the maximum grain size (the average of top ten crystal grains among crystal grains lying within the range of the sheet thickness by 1 mm in the cross-sectional texture). The formability was evaluated by the tensile properties using a JIS #5 tensile testing piece. The results of the evaluation are also shown in Table 2. Table 2 demonstrates that Examples Nos. 1 to 4, 6 to 9, 11 to 14, 16 and 17 in accordance with the present invention have excellent texture stability and excellent formability. In contrast, Comparative Example No. 5 having a B/N ratio lower than the range of the present invention, No. 10 having an Al content larger than the range of the present invention, and No. 15 by an annealing temperature lower than the range of the present invention show inferior texture stability to that in Examples in accordance with the present invention. Accordingly, in accordance with, a steel sheet having a stabilized texture can be obtained even by a high-temperature annealing at 700 °C or more. Example 2 Each steel containing chemical components shown in Table 3, which had been just produced, was hot-rolled without cooling at a temperature of the Ar 3 point or higher. After acid pickling and cold rolling, it was continuously annealed at an annealing temperature shown in Table 4, and then subjected to temper rolling with a rolling reduction rate of 0.8% to form a sheet having a thickness of 1.6 mm. (Examples in accordance with Embodiment 1 Nos. 1 to 4, 6 to 9, 11 to 14, 16 and 17, and Comparative Examples Nos. 5, 10 and 15). The texture stability was evaluated by texture observation measuring the maximum grain size (the average of top ten crystal grains among crystal grains lying within the range of the sheet thickness by 1 mm in the cross-sectional texture). The formability was evaluated by the tensile properties using a JIS #5 tensile testing piece. The results of the evaluation are also shown in Table 4. Table 4 demonstrates that Examples Nos. 1 to 4, 6 to 9, 11 to 14, 16 and 17 in accordance with Embodiment 1 have excellent texture stability and excellent formability. In contrast, Comparative Example No. 5 having a B/N ratio higher than the range of the present invention, No. 10 having an Al content larger than the range of the present invention, and No. 15 by an annealing temperature lower than the range of the present invention show inferior texture stability to that in Examples in accordance with the present invention. Accordingly, in accordance with, a steel sheet having a stabilized texture can be obtained even by a high-temperature annealing at 700 °C or more.

Claims: A soft cold-rolled steel sheet consisting of: 0.01 to 0.06 wt.% C, 0.1 wt.% or less Si, 0.5 wt.% or less Mn, 0.03 wt.% or less P, 0.03 wt.% or less S, 0.006 wt.% or less N, 0.009 wt.% or less B, stoichiometric ratio of B/N being 0.6 to 1.5, Al satisfying the following equation: sol. Al ≤ 0.035 x (B/N x 0.6) ½ , optionally containing at least one element selected from the group consisting of 0.5 wt.% or less Cu, 0.5 wt.% or less Ni, 0.5 wt.% or less Cr, 0.5 wt.% or less Sn, 0.1 wt.% or less Ca, and 0.05 wt.% or less O, said at least one element being 2 wt.% or less, and the balance being Fe and inevitable impurities. The soft cold-rolled steel sheet of claim 1, wherein said C is 0.01 to 0.04 wt.%. The soft cold-rolled steel sheet of claim 2, wherein said C is 0.01 to 0.03 wt.%. The soft cold-rolled steel sheet of claim 1, wherein said N is 0.005 wt.% or less. The soft cold-rolled steel sheet of claim 4, wherein said N is 0.0035 wt.% or less. A method for making a soft cold-rolled steel sheet comprising the steps of: (a) providing a slab consisting of: 0.01 to 0.06 wt.% C, 0.1 wt.% or less Si, 0.5 wt.% or less Mn, 0.03 wt.% or less P, 0.03 wt.% or less S, 0.006 wt.% or less N, 0.009 wt.% or less B, stoichiometric ratio of B/N being 0.6 to 1.5, Al satisfying the following equation: sol. Al ≤ 0.035 x (B/N x 0.6) ½ , optionally containing at least one element selected from the group consisting of 0.5 wt.% or less Cu, 0.5 wt.% or less Ni, 0.5 wt.% or less Cr, 0.5 wt.% or less Sn, 0.1 wt.% or less Ca, and 0.05 wt.% or less O, said at least one element being 2 wt.% or less, and the balance being Fe and inevitable impurities; (b) hot-rolling the slab at a finishing temperature of an Ar 3 point or more and at a coiling temperature of 650°C or less to produce a hot-rolled steel sheet; (c) cold-rolling the hot-rolled steel sheet to produce a cold-rolled steel sheet; and (d) continuously annealing the cold-rolled steel sheet at a heating rate of 1 °C/sec or more and at an soaking temperature of 700°C or more.

Inventors: FUNAKAWA YOSHIMASA, INAZUMI TORU, MATSUI NAOKI, MITSUZUKA KENICHI, SAWADA HIROSHI, TANIAI JUN, FUNAKAWA, YOSHIMASA, INAZUMI, TORU, MATSUI, NAOKI, MITSUZUKA, KENICHI, SAWADA, HIROSHI, TANIAI, JUN
Applicants: NIPPON KOKAN KK, NKK CORPORATION
Application year: 1998
Main classification: C22C38
IPC codes: C22C38, C21D8