BAINITE STEEL MATERIAL OF LITTLE SCATTER OF QUALITY AND METHOD OF MANUFACTURING THE SAME

Abstract: This invention provides a bainite steel material, such as a steel plate of a large thickness, a steel strip, a shape or a steel bar which have little scatter of quality in the direction of the thickness thereof and among different types of steel materials, wherein the bainite steel material comprises a composition containing not less than 0.001 wt.% and less than 0.030 wt.% of C, not more than 0.60 wt.% of Si, 1.00-3.00 wt.% of Mn, 0.005-0.20 wt.% of Nb, 0.0003-0.0050 wt.% of B, not more than 0.100 wt.% of Al and not less than 90 wt.% of bainite structure.

Description: TECHNICAL FIELD This invention relates to steel materials having a thickness of not less than 30 mm such as plates, sheets, sections, rods and the like for use in fields of buildings, marine structures, pipes, shipbuildings, storing tanks, public works, construction machines and the like, and more particularly to steel materials having a less scattering of properties and a method of producing the same. BACKGROUND ART As mentioned above, thick steel materials represented by thick plates are used in various fields for improving properties such as high strength, high toughness and the like. Recently, it is demanded that these properties are uniform in the thickness direction of the steel material and the scattering of these properties between the steel materials is less. For example, it is reported in pages 11 to 21 of "Tetsu-to-Hagane", Vol. 74(1988), No. 6 that the destruction of high-rise buildings is designed to be controlled by absorbing vibration energy through the deformation of the building against big earthquake with the heightening of the building. Concretely, the framework of the building is fallen down at a given form in the occurrence of earthquake and hence the deconstruction of the building is controlled by the plasticization of the framework. That is, assuming that the framework of the building indicates a behavior aimed by the designer in the occurrence of earthquake, it is required that the proof ratio of the steel material used in pillar, beam and the like of the building is completely grasped by the designer. Therefore, it is inevitable that the steel materials such as plates, H-sections and the like for use in the pillar, beam and the like are homogeneous, and also the scattering of strength in the steel material becomes a great problem. Since the steel materials used in the buildings and shipbuildings are required to have high tension and toughness, these materials are usually produced by controlled rolling and controlled cooling process or so-called TMCP process. However, if thick steel materials are produced by the TMCP process, the cooling rate differs in the thickness direction or between the steel materials to change the texture, and hence the scattering of properties is caused in the thickness direction of the resulting steel material or between the steel materials. The scattering of properties is come out in the thickness direction of the thick steel plate, between web and flange of H-section steel because of ununiform cooling between the web and the flange, between the lots of the steel material or the like. In JP-A-4-224623, it is proposed that the texture in the thickness direction is changed into a mixed texture of ferrite and bainite by adding Nb and making the cooling rate after the rolling to not less than 3°C/s and rendering an upper limit of the cooling stop temperature into 500°C, whereby the strength of a center portion in the thickness direction is increased to small the difference of hardness in the thickness direction small. In this case, however, the cooling rate should strictly be controlled to not less than 3°C/s even in the center portion, so that if the cooling rate distribution is formed in the thickness direction, the scattering of properties is immediately caused. As a result, it is required to strictly control the production, so that the above proposal is unsuitable for the production in industrial scale. In JP-A-62-130212, it is proposed to improve low-temperature toughness by ensuring strength through precipitation strengthening of Cu and by cooling to 300-700°C at a cooling rate of not less than 0.5°C/s after the hot rolling and holding at a temperature zone of 500-650°C for a constant time and then cooling to room temperature. However, this technique aims at the improvement of low-temperature toughness, so that it is difficult to satisfy the uniformity of the properties required in recent structural steels or the like by controlling the scattering of properties at various forms as mentioned above. US 4,210,445 discloses a niobium-containing weldable structural steel having good weldability, wherein the amount of martensite islands in the weld heat affected zone are restricted to not greater than 15% in terms of the area fraction. EP 0080809 discloses a method of making wrought high tension steel having superior low temperature toughness. JP 55100924 discloses a high toughness bainite high tension steel for line pipe in cold regions. DISCLOSURE OF INVENTION It is, therefore, an object of the invention to provide steel materials solving the above problems or having a less scattering of properties in the thickness direction or between the steel materials without restriction in the production step as well as a method of producing these steel materials. Now, the scattering of properties in the thick steel materials, typically thick steel plate results from the fact that the variation of texture is caused by a large change of cooling rate in thickness direction ranging from a surface of a steel material to a center portion thereof or by a change of cooling rate based on the scattering of production conditions. In order to avoid the variation of texture, it is important to provide a homogeneous texture over a wide range of the cooling rate. Therefore, the inventors have made studies by going back to the starting point with respect to a technique of obtaining a homogeneous texture even if the production conditions are changed and found that steel materials having a less scattering of properties and a constant texture in the thickness direction irrespectively of the change of cooling rate are obtained by newly designing a chemical composition of the steel material. That is, the chemical composition completely eliminating the change of strength due to the change of carbide precipitation form based on the change of cooling rate is realized by adding proper amounts of Mn, Nb lowering Ar 3 point for rendering the texture into a single phase of bainite without depending upon the cooling rate, adding B lowering grain boundary energy of old austenite grain boundary for precipitating no ferrite even at a low cooling rate and further restricting C content to control the precipitation of carbide in bainite. According to this chemical composition, the texture is rendered into the single phase of bainite through usual production steps without depending upon the rolling conditions and cooling conditions, whereby the scattering of strength and toughness is suppressed to a minimum level. According to a first aspect of the present invention there is provided a method of producing a bainite steel material having less scattering of properties in a hot rolling by providing a starting steel material having a chemical composition consisting of C: not less than 0.001 wt%, but not more than 0.018 wt%, Si: not more than 0.60 wt%, Mn: 1.00 -3.00 wt%, Nb: 0.005 - 0.20 wt%, B: 0.0003 - 0.0050 wt%, Al: not more than 0.100 wt%, and optionally containing any of Cu: 0.7 - 2.0 wt%, Ti: 0.005 - 0.20 wt%, V: 0.005 - 0.20 wt%, not more than 0.02 wt% of at least one of REM and Ca, and one or more of Ni: not more than 2.0 wt%, Cr: not more than 0.5 wt%, Mo: not more than 0.5 wt%, W: not more than 0.5 wt% and Zr: not more than 0.5 wt%, the balance being Fe apart from impurities; heating the starting steel material to a temperature of AC 3 - 1350°C in the hot rolling of the starting steel material to obtain austenite formation, rolling the steel material in an austenite unrecrystallization temperature region of not lower than 800°C, and then a precipitation treatment is carried out by acceleration cooling at a cooling rate of 0.1 to 80°C/s to a given temperature of not lower than 500°C but lower than 800°C and carrying out a precipitation treatment by isothermally holding at the setting temperature or cooling at a cooling rate of not more than 1°C per second in this temperature region of not lower than 500°C but lower than 800°C for not less than 30 seconds, and thereafter a cooling is conducted, thereby producing a bainite steel material having not less than 90% bainite texture. The precipitation treatment may be carried out by reheating to and holding at a temperature region of not lower than 500°C but lower than 800°C. Furthermore, the above thick steel material can be produced by using a starting steel material having various compositions according to the chemical composition defined above and using the claimed method of production. A production method wherein the starting steel material is heated to a temperature of Ac 3 - 1350°C in the hot rolling of the starting steel material, and the rolling is terminated at an austenite unrecrystallization temperature region of not lower than 800°C and then a precipitation treatment is carried out by acceleration cooling at a cooling rate of 0.1 - 80°C/s to a given temperature of not lower than 500°C but lower than 800°C which is a precipitation treating temperature region and isothermally holding at the temperature region of not lower than 500°C but lower than 800°C or cooling at a cooling rate of not more than 1°C/s in this temperature region for not less than 30 seconds and thereafter the cooling is conducted. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a graph showing a relation between C content and scattering of strength in a thick steel plate; Fig. 2 is a graph showing a relation between cooling rate and strength in a thick steel plate; Fig. 3 is a graph showing a relation between rolling reduction at an unrecrystallization region and fracture appearance transition temperature in the resulting steel plate; Fig. 4 is diagrammatic views of a test specimen used in a test for evaluating a resistance to stress corrosion cracking through sulfide and a testing apparatus therefor; and Fig. 5 is a graph showing a relation between C content and resistance to stress corrosion cracking through sulfide in a thick steel plate. BEST MODE FOR CARRYING OUT THE INVENTION The reason on the limitation of the chemical composition in the steel materials according to the invention will be described below. C: not less than 0.001 wt% but not more than 0.018 wt% C is necessary to be not less than 0.001 wt% in order to provide a single phase of bainite irrespectively of the cooling rate. On the other hand, when it is not less than 0.030 wt%, carbide precipitates in an inner portion of bainite texture or a lath boundary and the precipitation form of carbide changes as the cooling rate varies, so that it is difficult to obtain a constant strength over a wide range of the cooling rate. In this connection, the difference of hardness between maximum value and minimum value in the thickness direction (change of hardness) is investigated with respect to thick steel plates having a thickness of 80 mm when C content is varied in the chemical composition according to the invention. Moreover, the chemical composition other than C comprises Si: 0.02 wt%, Mn: 1.6 wt%, Nb: 0.020 wt%, B: 0.0018 wt% and Al: 0.03 wt%. As seen from the results shown in Fig. 1, when C content is less than 0.001 wt%, the change of hardness exceeds Hv: 20 and the scattering of strength becomes conspicuous. Therefore, the C content is limited to not less than 0.001 wt%. Moreover, when the upper limit of C is not more than 0.018 wt%, the excellent resistance to stress corrosion cracking through sulfide can be provided in addition to the homogeneous texture. That is, the inclusion of hydrogen sulfide in crude oil or natural gas is frequent in pipeline of transferring petroleum or natural gas, a tank for storing LPG and the like. In this case, the surface of the steel plate is corroded in an atmosphere of hydrogen sulfide and atomic hydrogen penetrated from the corroded surface into steel locally enriches in steel, so that the susceptibility to cracking becomes higher. At the same time, stress corrosion cracking is created at the enriched region of atomic hydrogen by stress produced in a circumferential direction of a pipeline during the transportation of crude oil or natural gas to bring about the breakage of the steel material. Therefore, it is important to prevent the peculiar stress corrosion cracking under a sulfide environment or so-called stress corrosion cracking through sulfide. When the C content is restricted to not more than 0.018 wt%, transformation strain due to shearing transformation is solved to prevent the enrichment of atomic hydrogen penetrated into steel under sulfide corroding environment, while the rise of strength is attained by precipitation strengthening of Cu, whereby a very excellent resistance to stress corrosion cracking through sulfide can be obtained even under an environment of a high hydrogen sulfide concentration while providing the strength and low-temperature toughness equal to or more than those of the conventional steel material. Si: not more than 0.60 wt% When Si amount exceeds 0.60 wt%, the toughness at weld portion is degraded, so that it is restricted to not more than 0.60 wt%. Moreover, the amount added is preferable to be not less than 0.02 wt% in order to conduct deoxidation and ensure the strength. Mn: 1.00-3.00 wt% The amount of Mn is required to be not less than 1.0 wt%, preferably not less than 1.50 wt% in order to provide a single phase of bainite, particularly make the volume ratio on bainite texture to not less than 90%. However, when it exceeds 3.00 wt%, the hardening through welding becomes considerably higher and the toughness at a heat affected zone (HAZ) is degraded. Therefore, it is restricted to a range of 1.50-3.00 wt%. Nb: 0.005-0.20 wt% Nb has an effect of lowering Ar 3 to widen the range of forming bainite toward a side of low cooling rate and is required to stably provide the bainite texture. Furthermore, it contributes to the precipitation strengthening and is effective to improve the toughness. In order to expect these effects, it is necessary to be not less than 0.005 wt%. On the other hand, when it exceeds 0.20 wt%, the effect of improving the toughness is saturated and it becomes disadvantage from economical reasons. Therefore, the upper limit is 0.20 wt%. B: 0.0003-0.0050 wt% B is required to be not less than 0.0003 wt% in order to provide the single phase of bainite< while when it exceeds 0.0050 wt%, BN is precipitated to degrade the weldability, so that it is restricted to a range of 0.0003-0.0050 wt%. Al: not more than 0.100 wt% When the amount of Al exceeds 0.100 wt%, the weldability is damaged, so that it is restricted to not more than 0.100 wt%. Moreover, it is favorable to add in an amount of not less than 0.010 wt% for the deoxidation. The invention lies in that the homogeneous texture, concretely texture containing not less than 90% of bainite is obtained by adjusting the basic chemical composition to the above component ranges without hardly depending upon the production conditions, particularly cooling rate. This is clear from experimental results shown in Fig. 2. That is, Fig. 2 shows measurement results of tensile strength on steel sheets obtained by varying the cooling rate within a range of 0.1-50°C/s in the production step of the component-adjusted steel according to the invention (invention example) and the conventional steel used as a building material (conventional example). As seen from this figure, a constant strength is obtained by the component adjustment according to the invention without depending upon the cooling rate. Particularly, the scattering value of Y.S and T.S becomes less over a wide range of the cooling rate which has never been anticipated in the conventional technique. This is based on the contribution of restriction of C content and proper addition of Mn and Nb and further B as mentioned above. Therefore, the strength does not change in accordance with the cooling rate even if the cooling rate is changed in the thickness direction of the thick steel plate, and hence the thick steel plate having a less scattering of properties in the thickness direction is obtained. In this case, the invention example has a chemical composition comprising C: 0.007 wt%, Si: 0.02 wt%, Mn: 1.55 wt%, Nb: 0.024 wt%, B: 0.0018 wt% and Al: 0.032 wt% and the remainder being iron and inevitable impurities, while the conventional example has a chemical composition comprising C: 0.14 wt%, Si: 0.4 wt%, Mn: 1.31 wt%, Al: 0.024 wt%, Nb: 0.015 wt% and Ti: 0.013 wt%. Moreover, many thick steel plates having a thickness of 15 mm were produced by varying the cooling rate at the same production step and then the tensile strength was measured with respect to test specimens taken out from these thick steel plates. In the invention, the levels of strength and toughness can freely be controlled by adding given components to the above basic chemical composition. In this case, the previously formed homogeneous texture is hardly influenced by the addition of new components, so that thick steel plates having a less scattering of properties and high strength and/or high toughness are easily obtained. In order to improve the strength, Cu: 0.7-2.0 wt% and further Ti: 0.005-0.20 wt% and/or V: 0.005-0.20 wt% can be added as a precipitation strengthening component. Moreover, in case of adding these precipitation strengthening components, the strengthening may be more attained by subjecting to a precipitation strengthening treatment as mentioned below. Cu: 0.7-2.0 wt% Cu is added for attaining the precipitation strengthening and solid-solution strengthening. When the amount exceeds 2.0 wt%, the toughness is rapidly degraded, while when it is less than 0.7 wt%, the precipitation strengthening effect is less, so that it is restricted to a range of 0.7-2.0 wt%. Ti: 0.005-0.20 wt% Ti is required to be not less than 0.005 wt% for lowering Ar 3 to contribute to the formation of bainite texture and forming TiN to improve the toughness of weld portion and attaining the precipitation strengthening, while when it exceeds 0.20 wt%, the toughness is degraded, so that it is restricted to a range of 0.005-0.20 wt%. V: 0.005-0.20 wt% V is added in an amount of not less than 0.005 wt% for the precipitation strengthening. While, when it exceeds 0.20 wt%, the effect is saturated, so that the upper limit is 0.20 wt%. Further, one or more of Ni: not more than 2.0 wt%, Cr: not more than 0.5 wt%, Mo: not more than 0.5 wt%, W: not more than 0.5 wt% and Zr: not more than 0.5 wt% can be added in order to more improve the strength. Moreover, these components develop the effect even at slight amounts, so that the lower limit thereof may properly be set. Ni: not more than 2.0 wt% Ni improves the strength and toughness and is effective to prevent Cu cracking in the rolling if Cu is added. However, it is expensive and the effect is saturated at an excessive addition. Therefore, it is added in an amount of not more than 2.0 wt%. Moreover, when it is less than 0.05 wt%, the effect is insufficient, so that the addition amount is favorable to be not less than 0.05 wt%. Cr: not more than 0.5 wt% Cr gas an effect of raising the strength, but when the addition amount exceeds 0.5 wt%, the toughness of weld portion is degraded, so that the amount is restricted to not more than 0.5 wt%. Moreover, the lower limit is favorable to be 0.05 wt%. Mo: not more than 0.5 wt% Mo has an effect of raising the strength at room temperature and higher temperatures, but when it exceeds 0.5 wt%, the weldability is degraded, so that the addition amount is restricted to not more than 0.5 wt%. Moreover, the lower limit is favorable to be 0.05 wt% because when it is less than 0.05 wt%, the effect of raising the strength is insufficient. W: not more than 0.5 wt% W has an effect of raising the strength at higher temperatures, but is expensive. When it exceeds 0.5 wt%, the toughness is degraded, so that the addition amount is restricted to not more than 0.5 wt%. Moreover, the addition amount is favorable to be not less than 0.05 wt% because when it is less than 0.05 wt%, the effect of raising the strength is insufficient. Zr: not more than 0.5 wt% Zr has not only the effect of raising the strength but also an effect of improving the resistance to plated cracking when the steel material is subjected to, for example, zinc plating. When it is added so as to exceed 0.5 wt%, the toughness of weld portion is degraded, so that it is restricted to not more than 0.5 wt%. Moreover, the lower limit is favorable to be 0.05 wt%. And also, not more than 0.02 wt% of at least one of REM and Ca may be added for improving the toughness of HAZ. REM forms oxysulfide to suppress the growth of austenite grains and improve the toughness of HAZ. When it is added so as to exceed 0.02 wt%, the cleanness of steel is damaged, so that it is restricted to not more than 0.02 wt%. Moreover, when the addition amount is less than 0.001 wt%, the effect of improving the toughness of HAZ is insufficient, so that it is favorable to be not less than 0.001 wt%. Ca is effective to improve not only the toughness of HAZ but also the properties in the thickness direction through form control of sulfide in steel. However, when it is added so as to exceed 0.02 wt%, the amount of non-metallic inclusion is increased to create internal defect, so that it is restricted to not more than 0.02 wt%. Moreover, the addition amount is favorable to be not less than 0.0005 wt% because when it is less than 0.0005 wt%, the above effects are insufficient. In the steel plates the homogeneous texture is obtained by adjusting to the basic chemical composition as mentioned above, so that it is not necessary to strictly control the production conditions. For instance, it is recommended that a slab of steel having the above adjusted basic chemical composition is heated at a temperature of Ac 3 - 1350°C, and finish-rolled at a temperature of not lower than 800°C and thereafter cooled. That is, when the heating temperature is lower than Ac 3 , austenite can not completely be formed and the homogenization is insufficient, while when it exceeds 1350°C, the surface oxidation becomes violent, so that it is favorable to conduct the heating in a temperature region of Ac 3 - 1350°C. When the finish rolling temperature is lower than 800°C, the rolling efficiency lowers, so that it is preferable to be not lower than 800°C. Then, the cooling after the rolling is not strictly controlled as usual, so that it is possible to conduct either air cooling and acceleration cooling. The cooling is preferable to be carried out within a range of 0.5-80°C/s. When the cooling is carried out at a cooling rate exceeding 80°C/s, bainite·lath space becomes dense and the strength rises in dependence on the cooling rate, while when it is less than 0.5°C/s, ferrite is formed and it is difficult to provide the single phase of bainite. Even in the production method, the levels of the strength and toughness can freely be controlled by adding various treating steps likewise the case of adding components as mentioned above. At first, in the rolling step after the heating to a temperature of Ac 3 - 1350°C, the improvement of the toughness is attempted by rolling in an unrecrystallization temperature region of austenite above 800°C. That is, the rolling at the unrecrystallization region of austenite has an effect of finely dividing bainite texture through the introduction of work dislocation to improve the toughness. In this connection, the results investigated on a relation between rolling reduction and fracture appearance transition temperature in the unrecrystallization region are shown in Fig. 3, from which the reduction of not less than 30% is recommended because the effect of improving the toughness at a rolling reduction of not less than 30% becomes conspicuous. Moreover, the finish temperature in the experiment shown in Fig. 3 is 900°C, while the chemical composition of the steel plate used in the experiment comprises C: 0.007 wt%, Si: 0.02 wt%, Mn: 1.55 wt%, Al: 0.32 wt%, Nb: 0.024 wt% and B: 0.0018 wt% and the remainder being iron and inevitable impurities. On the other hand, the upper limit of the rolling reduction in the unrecrystallization region is not defined, but the reduction of not less than 95% may be disadvantageous in the operation from a viewpoint of rolling load. When Cu: 0.7-2.0 wt% and further Ti: 0.005-0.20 wt% and/or V: 0.005-0.20 wt% are added as a precipitation strengthening component, the acceleration cooling is carried out up to a precipitation treating temperature region of not lower than 500°C but lower than 800°C at a cooling rate of 0.1-80°C/s after the rolling and then the precipitation treatment is carried out by isothermally holding at the setting temperature for not less than 30 seconds or cooling at a cooling rate of not more than 1°C/s in the temperature region for not less than 30 seconds, which is effective to improve the strength. That is, when the rate cooling from the completion of the rolling to the precipitation treating temperature is less than 0.1°C/s, ferrite is formed in the bainite texture, while when it exceeds 80°C/s, the bainite·lath space becomes dense and the strength rises in dependence on the cooling rate and the bainite single phase is not formed due to the formation of ferrite at less than 0.1°C/s, so that the cooling rate is within a range of 0.1-80°C/s. After the acceleration cooling, the precipitation treatment is conducted by the isothermal holding within a temperature region of not lower than 500°C but lower than 800°C for not less than 30 seconds or the cooling in this temperature region at a cooling rate of not more than 1°C/s for not less than 30 seconds, whereby one or more of Cu, Ti(CN) and V(CN) and further Nb(CN) are precipitated to attempt the rise of strength. Furthermore, the homogenization of the texture is made by this precipitation treatment and the scattering of properties in the thickness direction is mitigated. When the temperature of the precipitation treatment is not lower than 800°C, the precipitation hardly occurs because the precipitating components are maintained at a dissolved state, so that it is necessary to conduct the precipitation treatment at a temperature of lower than 800°C for attaining sufficient precipitation. On the other hand, when it is lower than 500°C, the precipitation reaction hardly occurs, so that the temperature region is restricted to a range of not lower than 500°C but lower than 800°C. Further, the reason why the holding time is not less than 30 seconds is due to the fact that when it is less than 30 seconds, the sufficient precipitation strengthening can not be attained. On the other hand, the precipitation strengthening is attained even when the cooling is carried out in this temperature region at a cooling rate of not more than 1°C/s for not less than 30 seconds, but when the cooling rate exceeds 1°C/s, the sufficient precipitation strengthening is not obtained. Moreover, the cooling rate of not more than 1°C/s is desirable for attaining the sufficient precipitation strengthening. The above precipitation treatment may be conducted after the cooling followed to the rolling. That is, the steel plate may be reheated to a temperature region of not lower than 500°C but lower than 800°C and held at this temperature region after the cooling. When the C content in the steel slab is restricted to not more than 0.018 wt% for obtaining the excellent resistance to stress corrosion cracking through sulfide as mentioned above, the holding time or cooling time at the temperature region of not lower than 500°C but lower than 800°C is particularly favorable to be not less than 300 seconds. The precipitation treatment simultaneously solves surface defect of bainite grain succeeding rolling strain at not higher than 950°C and surface defect produced in the shearing transformation, so that the enrichment of atomic hydrogen penetrated into steel under a corrosion environment through sulfide is prevented and the resistance to stress corrosion cracking through sulfide is improved. [Example 1] A slab of steel having an adjusted chemical composition as shown in Table 1 is heated to 1150°C, rolled at a finish rolling temperature of 800°C so as to have a total rolling reduction of 74% and thereafter subjected to an acceleration cooling (cooling rate: 7°C/s) to produce a thick steel plate having a thickness of 80 mm. The tensile test and Charpy impact test are made with respect to the resulting thick steel plates to examine mechanical properties, while the hardness of the steel plate at section is measured from an outer surface thereof at a pitch of 2 mm to examine a hardness distribution in a thickness direction for evaluating the scattering of strength in the thickness direction. Further, in order to evaluate the toughness of HAZ, the steel plate is subjected to a heat cycle of heating to 1350°C and cooling from 800°C to 500°C for 300 seconds (corresponding to heat hysteresis of HAZ when the welding is carried out at a heat input of 500 kJ/cm), from which a specimen for Charpy impact test is taken out to measure a Charpy absorption energy at 0°C. The measured results are shown in Table 2, from which it is apparent that the thick steel plates has a tensile strength of not less than 400 MPa, and the scattering of hardness in the thickness direction for homogenizing the texture is very small as compared with that of the comparative example, and the difference of hardness between maximum value and minimum value is in 20 as Hv. Moreover, the volume ratio of bainite texture is measured by point counting process from an optical microphotograph of 400 magnification. Steel No. Change of hardness (*) Y.S (MPa) T.S (MPa) vTrs of base metal (°C) HAZ-vEo (J) Volume ratio of bainite (%) Remarks 1 65 424 494 -100 347 55 Comparative Example 2 11 446 497 -96 330 100 Invention Example 3 6 464 510 -95 320 100 Invention Example 4 17 407 507 -98 335 100 Invention Example 5 21 471 520 -95 293 100 Comparative Example 6 13 492 602 -53 46 93 Comparative Example 7 35 328 407 -86 330 34 Comparative Example 8 22 627 652 -23 32 100 Comparative Example 9 46 370 430 -74 330 7 Comparative Example 10 12 501 534 -97 284 100 Comparative Example 11 32 330 414 -110 301 12 Comparative Example 12 24 426 509 -48 30 100 Comparative Example 13 14 465 528 -20 22 100 Comparative Example 14 18 424 517 -103 120 100 Invention Example 15 11 457 547 -93 306 100 Invention Example 16 13 468 511 -101 284 100 Invention Example 17 20 433 535 -84 301 100 Invention Example 18 15 477 540 -89 320 100 Invention Example 19 17 431 533 -114 315 100 Invention Example 20 8 426 510 -82 282 100 Invention Example 21 13 420 515 -64 340 100 Invention Example 22 13 437 507 -94 304 100 Invention Example 23 16 451 511 -79 338 100 Invention Example 24 11 431 517 -96 308 100 Invention Example 25 10 474 538 -95 280 100 Invention Example [Example 2] A slab of steel having an adjusted chemical composition as shown in Table 3 is subjected to a treatment under various conditions as shown in Table 4, whereby a thick steel plate having a thickness of 80 mm is produced. As to the thus obtained thick steel plates, the tensile test and Charpy impact test are carried out in the same manner as in Example 1 to examine the mechanical properties, and also the scattering of strength in the thickness direction is examined. The measured results are shown in Table 4, from which it is apparent that the thick steel plates has a tensile strength of not less than 400 MPa, and the scattering of hardness in the thickness direction for homogenizing the texture is very small as compared with that of the comparative example. Furthermore, it is clear that the rise of strength is realized by adding precipitation strengthening elements and subjecting to precipitation strengthening treatment as compared with the invention examples containing no precipitation strengthening element as shown in Table 2. As to thick steel plates having a chemical composition as shown in Table 5, the resistance to stress corrosion cracking through sulfide is evaluated after the slab is heated to 1150°C, rolled at a reduction of 50% up to 800°C, subjected to a reheating precipitation treatment at 550°C for 40 minutes and then cooled in air. That is, a test specimen shown in Fig. 4(a) is taken out from a central portion of the thick steel plate in the thickness direction, and stress is applied to this test specimen in an apparatus shown in Fig. 4(b), which is immersed in an NACE solution (5% NaCl + 0.5% CH 3 COOH + saturated H 2 S) for 720 hours. The applied stress corresponds to 40-120% of 0.5% proof stress of the steel plate used in the tensile test. The resistance to stress corrosion cracking through sulfide is evaluated by a ratio of applied stress causing no breakage after the immersion of 720 hours to 0.5% proof stress. Moreover, the larger the numerical evaluation value, the better the resistance to stress corrosion cracking through sulfide. The evaluation results are also shown in Table 5, from which it is apparent that the steel sheets restricting C content to not more than 0.018 wt% are excellent in the resistance to stress corrosion cracking through sulfide. Examples 3 to 6 are comparative examples.

Claims: A method of producing a bainite steel material having less scattering of properties in a hot rolling by providing a starting steel material having a chemical composition consisting of C: not less than 0.001 wt%, but not more than 0.018 wt%, Si: not more than 0.60 wt%, Mn: 1.00 -3.00 wt%, Nb: 0.005 - 0.20 wt%, B: 0.0003 - 0.0050 wt%, Al: not more than 0.100 wt%, and optionally containing any of Cu: 0.7 - 2.0 wt%, Ti: 0.005 - 0.20 wt%, V: 0.005 - 0.20 wt%, not more than 0.02 wt% of at least one of REM and Ca, and one or more of Ni: not more than 2.0 wt%, Cr: not more than 0.5 wt%, Mo: not more than 0.5 wt%, W: not more than 0.5 wt% and Zr: not more than 0.5 wt%, the balance being Fe apart from impurities; heating the starting steel material to a temperature of AC 3 - 1350°C in the hot rolling of the starting steel material to obtain austenite formation, rolling the steel material in an austenite unrecrystallization temperature region of not lower than 800°C, and then a precipitation treatment is carried out by acceleration cooling at a cooling rate of 0.1 to 80°C/s to a given temperature of not lower than 500°C but lower than 800°C and carrying out a precipitation treatment by isothermally holding at the setting temperature or cooling at a cooling rate of not more than 1°C per second in this temperature region of not lower than 500°C but lower than 800°C for not less than 30 seconds, and thereafter a cooling is conducted, thereby producing a bainite steel material having not less than 90% bainite texture. A method of producing a bainite steel, material as claimed in claim 1, wherein the precipitation treatment is carried out by reheating to and holding at a temperature region of not lower than 500°C but lower than 800°C. A method of producing a bainite steel material as claimed in claim 1 or 2, whereby one or more of Cu, Ti(CN) and V(CN) and further Nb(CN) are precipitated to obtain a rise of strength. A method of producing a bainite steel material as claimed in any preceding claim wherein the bainite steel material contains 0.7 to 2.0 wt% copper. A method of producing a bainite steel material as claimed in any preceding claim, wherein the bainite steel material contains a single phase of bainite.

Inventors: AMANO KENITI, HAYASHI TOHRU, ITAKURA NORITSUGU, KAWABATA FUMIMARU, KOSEKI TOMOYA, OKATSU MITSUHIRO, OOI KENJI, OTA HIROKI, AMANO, KENITI, HAYASHI, TOHRU, ITAKURA, NORITSUGU, KAWABATA, FUMIMARU, KOSEKI, TOMOYA, OKATSU, MITSUHIRO, OOI, KENJI, OTA, HIROKI, AMANO, Keniti Kawasaki Steel Corporation, HAYASHI, Tohru Kawasaki Steel Corporation, ITAKURA, Noritsugu Kawasaki Steel Corporation, KAWABATA, Fumimaru Kawasaki Steel Corporation, KOSEKI, Tomoya Kawasaki Steel Corporation, OKATSU, Mitsuhiro Kawasaki Steel Corporation, OOI, Kenji Kawasaki Steel Corporation, OTA, Hiroki Kawasaki Steel Corporation
Applicants: KAWASAKI STEEL CO, KAWASAKI STEEL CORPORATION
Application year: 1995
Main classification: C22C38
IPC codes: C22C38, C21D8, C21D6