JP5037204B2 - Method for producing high-strength steel material having yield stress of 500 MPa or more and tensile strength of 570 MPa or more which is excellent in toughness of weld heat affected zone - Google Patents

Description

  The present invention relates to a method for producing a high-strength steel material having a yield stress of 500 MPa or more and a tensile strength of 570 MPa or more, which is excellent in toughness of a weld heat affected zone, and is used in various fields such as construction, shipbuilding, bridges, and civil engineering. As used herein, “570 MPa class” refers to a material having a tensile strength (TS) in the range of 570 MPa to 720 MPa.

  In recent years, the strength of steel materials used in steel structures such as ships and buildings has been increasing. By using high-strength steel materials, it becomes possible to reduce the amount of steel materials used, and it is possible to obtain merits such as expanding the space in the structure and reducing the weight.

  Conventionally, when manufacturing high-strength thick steel plates, in order to stably obtain strength and toughness in actual machine manufacturing, it is common to manufacture by a process that combines hot rolling, direct quenching, and tempering heat treatment. there were.

  For example, Patent Documents 1, 2, and 3 disclose a technique in which a steel sheet is hot-rolled, directly quenched online, and then tempered offline. However, the off-line tempering heat treatment generally causes an increase in manufacturing time, and a reduction in productivity becomes a problem. In order to meet the recent increase in demand for high-strength thick steel plates, a production method with higher productivity is desired.

  On the other hand, various technical developments have been made for the purpose of improving the productivity of high-strength thick steel plates. For example, Patent Documents 4, 5 and 6 show a technique for shortening the heating time of the tempering heat treatment and improving the productivity by using an induction heating method in a heating furnace for performing the tempering heat treatment.

  On the other hand, in order to improve productivity, a method of manufacturing by hot rolling without omitting the tempering heat treatment itself has been developed. For example, Patent Document 7 discloses a method of manufacturing a steel containing 0.03 to 0.20% C and 0.10 to 0.60% Si as hot-rolled. Patent Document 8 discloses a method for producing a steel containing 0.10 to 0.20% C and 0.03 to 0.60% Si as hot-rolled. Patent Document 9 discloses a method for producing a steel containing 0.06% or less of C and 0.1 to 0.6% of Si as hot-rolled.

Furthermore, a method for producing a high-strength thick steel plate while being directly quenched after hot rolling has been developed. For example, Patent Document 10 discloses a manufacturing method in which a steel containing 0.030% or less of C and 0.60% or less of Si is hot-rolled and then directly quenched to a temperature of 550 ° C. or more. Patent Document 11 discloses a manufacturing method in which steel containing 0.030% or less of C and 0.60% or less of Si is hot-rolled and then directly quenched to a temperature of 550 ° C. or higher. In addition, Patent Document 12 describes a production in which steel containing 0.04 to 0.09% C and 0.1 to 0.5% Si is hot-rolled and then directly quenched to a temperature range of 300 to 600 ° C. The method is shown. Patent Document 13 discloses a manufacturing method in which a steel containing 0.01 to 0.03% C and 0.05 to 1.0% Si is hot-rolled and then directly quenched. Patent Document 14 discloses a manufacturing method in which steel containing 0.01 to 0.06% of C is hot-rolled and then directly quenched. In addition, Patent Document 15 discloses a method in which steel containing 0.03 to 0.07% C and 0.1 to 0.6% Si is hot-rolled and then directly quenched to a temperature range of 600 to 700 ° C. The method is shown. Patent Document 16 discloses a manufacturing method in which steel containing 0.01 to 0.1% of C and 1.0% or less of Si is hot-rolled and then directly quenched. Patent Document 17 discloses a manufacturing method in which steel containing 0.05 to 0.18% C and 0.05 to 0.5% Si is hot-rolled and then directly quenched to a temperature of less than 620 ° C. Has been. Patent Document 18 discloses that after hot rolling a steel containing 0.03 to 0.18% C and 0.05 to 0.5% Si, (Ar ₃ -300) to (Ar ₃ -50) A production method is shown in which quenching is directly performed up to a temperature range of ° C.

JP 52-081014 A JP-A-63-033521 Japanese Patent Laid-Open No. 02-205627 JP 2002-317227 A JP 2003-82412 A JP-A-2005-226106 JP-A-5-171271 JP-A-8-188823 JP-A-2005-76056 JP-A-8-144019 JP-A-11-269602 JP 2001-64723 A JP 2004-143479 A JP 2004-300567 A Japanese Patent Laid-Open No. 2005-126819 JP-A-2005-350691 JP 2006-249469 A JP 2006-291348 A

  However, in the methods described in Patent Documents 1, 2, and 3, offline tempering heat treatment is necessary in the manufacturing process of the steel sheet, and thus a reduction in productivity is inevitable.

  The methods described in Patent Documents 4, 5 and 6 are advantageous in that productivity can be improved regardless of the strength range because online tempering is performed by induction heating. There is a problem that a very large capital investment is required to introduce a heating furnace.

Further, in Patent Document 7, C is defined as 0.03% or more and Si is defined as 0.10% or more. In the study by the present inventors, when manufacturing is performed in such a component system as hot rolling. It has been found that island MA (Martensite-Austenite constituent) that lowers the yield stress (YS) is produced in a large amount in steel. Further, the finishing temperature is defined as Ar ₃ or less, and it is necessary to wait for the temperature to drop on the rolling line, so the productivity is significantly reduced.

  In Patent Document 8, Si is specified to be 0.05% or more, and in the method of Patent Document 9, Si is specified to be 0.10% or more, and it is considered that YS decreases due to generation of island-shaped MA. Therefore, Patent Document 9 aims at producing a steel having a yield ratio of 80% or less.

  Patent Documents 10 and 11 disclose a method of producing a steel of 400 MPa class or higher having a high yield point while maintaining accelerated cooling in an extremely low C component system. However, as far as the examples are concerned, although it is possible to produce a steel material of 500 MPa class or less by a non-tempered production method that does not use tempering heat treatment, for a steel material of 570 MPa class or more intended by the present invention, An isothermal holding or tempering heat treatment for obtaining precipitation strengthening is added, and it cannot be said that productivity is high. Furthermore, in Patent Document 10, the amount of Al added is 0.023% or more as far as the examples are seen. According to the study by the present inventors, this large amount of Al is added to the island-shaped MA in the base material. In this respect, it can be seen that a steel material of 570 MPa class is not optimized in terms of production with accelerated cooling.

  In patent document 12, the manufacturing method which stops accelerated cooling at 300-600 degreeC is shown. However, when C is as high as 0.04 to 0.09% and Si is as high as 0.1 to 0.5%, island-like MA is generated and YS tends to decrease. In addition, since it is not specified that a precipitation strengthening element is used, it is not an extremely low C component system, and the manufacturing method becomes more dependent on the stop temperature of accelerated cooling of TS and YS of the steel material. Thus, it is considered difficult to stably keep the strength of the steel material in the range of 570 MPa.

  Patent Documents 13, 17 and 18 stipulate that Si is 0.05% or more, and causes an increase in the amount of island-shaped MA that lowers YS in steel materials that are still accelerated. Therefore, these manufacturing methods aim at steel materials having a yield ratio of 80% or less, and there is a problem that the amount of alloy addition increases in order to obtain high YS.

Patent Document 14 stipulates that addition of Cr is essential. However, according to the study by the present inventors, when Cr is added in a component system that is extremely low C, the bainite transformation start temperature and end time during accelerated cooling are determined. The difference in temperature increases, and as a result the yield ratio decreases, which is not desirable from the viewpoint of optimizing the component system and reducing costs. Another problem is that the addition of Cr greatly reduces the toughness of the heat-affected zone during welding.

  Patent Document 15 discloses a technique for suppressing fluctuations in steel strength with respect to the stop temperature of accelerated cooling by positively using Nb precipitation strengthening. However, in order to make strong use of Nb precipitation strengthening, acceleration is proposed. The cooling stop temperature is limited to a narrow range of 600 ° C. or more and 700 ° C. or less.

  In Patent Document 16, Si is 0.1% or more in the embodiment, which causes a problem that YS is reduced due to generation of island-like MA. Further, since the Ti amount is specified to be 0.03% or more, it is expected that the toughness of the heat-affected zone during welding is lowered.

  Then, this invention aims at providing the manufacturing method of the high strength steel materials of the yield stress 500MPa or more excellent in the toughness of a welding heat-affected zone, and the tensile strength 570MPa or more which can solve the said problem advantageously. To do.

  In the production method in which accelerated cooling is performed directly after hot rolling of a steel sheet, the present inventors do not perform tempering heat treatment, and are inevitably in hot rolling temperature and accelerated cooling stop temperature in an actual machine in a non-tempered production method. Research and development are repeated through experiments and analysis on the method of manufacturing high strength steel materials with high yield ratios such as YS of 500MPa or more, TS of 570MPa or more and 720MPa or less, with sufficiently high stability even if fluctuations are taken into account. It was.

  Conventionally, in the non-tempered manufacturing method of a high strength thick steel plate of 570 MPa class or higher, it is common to design the alloy elements with the aim of improving the hardenability of the steel. However, in the production method in which accelerated cooling is stopped halfway, alloy elements that enhance hardenability tend to promote the formation of island MA in the base material, particularly when accelerated cooling is stopped at 500 ° C. or higher. There is a problem that YS decreases. Furthermore, the indiscriminate addition of alloy elements that enhance the hardenability also raises the problem of increasing the dependency of the accelerated cooling of TS on the stop temperature.

  When these problems cannot be solved, in order to keep TS and YS within the target range when manufacturing a steel product of 570 MPa class or higher, the stop temperature range of accelerated cooling must be made very narrow. Problems arise and the stability in manufacturing the actual machine is lost.

  Conventionally, as a method for reducing the dependency of the cooling rate and the stop temperature of the accelerated cooling of TS in particular, an extremely low C component system of 0.03% or less has been adopted as disclosed in Patent Documents 10 and 11. A technique has been proposed in which the steel structure is a single phase of bainite. In order to utilize this extremely low C component system, the present inventors have further studied conditions for stably producing a high strength steel material of 570 MPa class while securing a high YS of 500 MPa or more. As a result, first, the present inventors have found that even in an extremely low C component system, the influence of the cooling rate and stop temperature of accelerated cooling on TS differs depending on the combination of alloy elements, and in particular C0.040% The effect of accelerated cooling conditions on TS can be minimized by adopting a combination of less than 1, less than Mn, less than 1.5%, Nb 0.02% or more, B0.0005% or more, and Mo 0.05% or more. I found it. As a result, it was also found that the difference between the TS position at the 1/4 position and the 1/2 position in the sheet thickness direction of the steel sheet can be remarkably reduced to 5% or less.

  Furthermore, in order to realize a highly stable manufacturing method of high-strength steel materials in which YS is 500 MPa or more and TS is 570 MPa or more and 720 MPa or less, which is the object of the present invention, it is not possible only by pursuing the above TS. It was sufficient, and it was necessary to repeatedly study the securing of YS stability.

  In general, steel materials manufactured by a non-tempered manufacturing method that stops accelerated cooling in the middle tend to have a lower YS for the TS level than steels to which tempering heat treatment has been applied. Quality production methods are often studied as methods for producing high strength steels with low yield ratios. Therefore, the present inventors are particularly required for forming a preferable bainite structure after investigating the influence of the structure factor inside the steel material that affects YS in an extremely low C non-heat treated steel material. Experiments were repeated and detailed studies were repeated regarding the combination of various alloy elements, hot rolling and accelerated cooling conditions.

  First, as one of the factors that lower YS in non-tempered steel materials, the presence of island-like MA structures in the steel materials is raised as pointed out conventionally. In response to this, the inventors of the present invention have adopted an extremely low C component system and further made extremely low Si and extremely low Al, so that the accelerated cooling stop temperature is within a range of 550 ° C. or less. It has been clarified that the formation of the shaped MA tissue can be suppressed to 1% or less in volume fraction. As a result, it has become possible to prevent YS from being lowered by island-like MA in the bainite single-phase structure. However, even in such a bainite single-phase structure having no island-like MA, YS is different depending on the component system. It has become clear that this will occur.

  The present inventors investigated in detail about the YS control factor in the bainite single phase structure which does not have this island-like MA by experiment and analysis. As a result, it was found that a large number of mobile dislocations were generated in the bainite single-phase steel due to the difference in local bainite transformation temperature inside the steel material. Specifically, from TEM observation and measurement with a microhardness meter, the bainite structure transformed at a high temperature and the bainite structure transformed at a relatively low temperature are caused by volume expansion during the bainite transformation. The strain remained as movable dislocations, and as a result, a result suggesting that the macroscopic YS of the steel material has decreased was obtained. That is, the inventors have found that it is important to reduce the difference between the start temperature and the end temperature of the bainite transformation in a bainite single-phase structure having no island MA. Furthermore, a result suggesting that the higher the bainite transformation temperature, the easier it is to improve YS due to the fixation of C and N atoms to movable dislocations.

  From the above, in the bainite single-phase structure having no island-like MA, the most ideal structure formation process for obtaining a high yield ratio is that the difference between the start temperature and the end temperature of the bainite transformation is small, and the transformation temperature. It was clarified that the whole would be hot even a little. In addition, by adopting the component system obtained from the above knowledge about TS and YS, the toughness of the heat affected zone at the time of performing large heat input welding corresponding to heat input of 10 kJ / mm is remarkably improved. There was found. This is because the bainite structure of the weld heat-affected zone is made uniform by the component system designed to suppress the influence of the cooling condition as the above-mentioned TS countermeasure and to uniformize the bainite formation temperature of the base material as the YS countermeasure. This is presumed to be due to the fact that reducing the amount of MA in order to prevent YS reduction of the base metal is also advantageous in terms of improving the toughness of the weld heat affected zone.

Furthermore, in the component system of the present invention, P _CM representing the weld cracking sensitivity has become a low level and 0.20 or less, and has a level as not to weld cracking problems at low temperature environment.

In this way, the present inventors have clarified the specific requirements for satisfying the required characteristics in the high-strength steel material of 570 MPa class by experiments, and have further intensively studied to make the present invention. The summary is as follows.
(1) By mass%, C: 0.005% or more, less than 0.040%, Si: less than 0.05%, Mn: 1.0% or more, less than 1.5%, P: 0.03% or less , S: 0.01% or less, Mo: 0.10% or more, 0.50% or less, Nb: 0.02% or more, 0.08% or less, Ti: 0.005% or more, 0.020% or less B: 0.0005% or more, 0.0030% or less, Al: 0.001% or more, 0.010% or less, N: 0.0010% or more, 0.0070% or less, A steel slab that satisfies the relationship between the amount of Ti and N expressed and the balance is composed of Fe and inevitable impurities is heated to 1020 ° C or higher and 1300 ° C or lower, and then rolled to 1020 ° C or lower. less than 920 ° C. cumulative reduction ratio in excess is 60%, 920 ° C. or less, in Ar ₃ point than Rolling is performed so that the rolling reduction ratio is 50% or more and 90% or less, and accelerated cooling is started at a cooling rate of 1 ° C./second or more within 60 seconds after the completion of rolling, in a temperature range of less than 550 ° C. and 300 ° C. A method for producing a high-strength steel material having a yield stress of 500 MPa or more and a tensile strength of 570 MPa or more, which is excellent in toughness of a weld heat affected zone, characterized by stopping accelerated cooling and then allowing to cool.
In addition, [] in Formula 1 represents the addition amount of each alloy element in mass%, and the same applies thereafter.
Formula 1: [B] /10.8> = [N] /14.0- [Ti] /47.9
(2) Further, the yield stress is 500 MPa or more, which is excellent in toughness of the weld heat-affected zone according to (1) above, characterized by containing, by mass%, W: 0.05% or more and 0.50% or less. A method for producing a high-strength steel material having a tensile strength of 570 MPa or more.
(3) Further, it is characterized by containing one or two of Cu: 0.01% or more and 0.50% or less, Ni: 0.01% or more and 0.50% or less in mass%. The method for producing a high strength steel material having a yield stress of 500 MPa or more and a tensile strength of 570 MPa or more, which is excellent in the toughness of the weld heat affected zone described in (1) or (2).
(4) Further, by mass%, Ca: 0.001 to 0.010%, Mg: 0.001 to 0.010%, Zr: 0.001 to 0.010%, Hf: 0.001 to 0.0. Welding heat influence according to any one of (1) to (3) above, characterized by containing one or more of 010% and REM: 0.001 to 0.010% A method for producing a high-strength steel material having a yield stress of 500 MPa or more and a tensile strength of 570 MPa or more, which is excellent in the toughness of the part.

  According to the present invention, it is possible to obtain a high-strength steel sheet having a tensile strength of 570 MPa, which is excellent in the toughness of the heat affected zone by an economical component system with less alloying elements and a highly productive non-tempered manufacturing method. It becomes. Further, the present invention is a manufacturing method capable of stably ensuring quality against fluctuations in manufacturing conditions in an actual machine, and its industrial contribution is extremely large.

  Below, the hot rolling conditions in this invention, accelerated cooling conditions, and the reason for limitation of a component composition are described.

  First, in the present invention, as described above, in producing a high strength steel of 570 MPa class, by adopting a component system based on extremely low C, the dependence on the accelerated cooling stop temperature of TS is increased. It is possible to reduce the island-like MA and improve the YS through the uniform and high temperature bainite transformation, so that it can be stably manufactured even with a large change in the manufacturing conditions assumed in the actual machine. It has been realized and is characterized in that the combinations of various alloy elements and their addition amounts are limited in accordance with the conditions of hot rolling and accelerated cooling.

  First, hot rolling conditions and accelerated cooling conditions in the present invention will be described.

  In the present invention, since the precipitation strengthening is used even in the non-tempered manufacturing method in which no tempering heat treatment is performed, Nb and Ti, which are the fastest precipitation in bainite, are used as a precipitation strengthening element for improving YS. The precipitation of Nb and Ti at this rolling stage is promoted by rolling strain. However, when precipitation of Nb and Ti occurs during rolling in high-temperature austenite, these precipitates are rapidly coarsened, resulting in useless precipitation that does not contribute to an increase in strength after steel sheet production. It is clarified by. Therefore, in order to minimize the loss due to coarse precipitation in the austenite, it is preferable not to perform rolling in a temperature range higher than 920 ° C. and not higher than 1020 ° C. as much as possible.

  However, in this invention, in order to improve manufacturing stability, the component range of C is restrict | limited low from patent document 15, and the precipitation of Nb and Ti in austenite is suppressed for that reason. As a result, the limit of the cumulative reduction amount above 920 ° C. and below 1020 ° C. is defined as less than 60%.

Regarding rolling at 920 ° C. or lower, it is performed in a temperature range higher than the Ar ₃ point, and by taking a reduction ratio as large as possible, the bainite transformation temperature is increased to reduce the movable dislocation density in the steel material and the structure. It is effective for YS improvement by making it finer.

  In general, in the manufacturing method that omits the tempering heat treatment, YS tends to be lower than the TS level, and in order to compensate for this, a reduction of 50% or more is required including the effect of improving YS by refining the structure. However, excessive reduction reduces the hardenability and lowers TS, so the upper limit is defined as 90% or less.

  With respect to accelerated cooling after the end of rolling, a cooling rate of at least 1 ° C./second or more is required to obtain a bainite single-phase structure. Note that the range of 5 to 50 ° C./second in which the fluctuation of TS is small is more desirable.

  When the accelerated cooling stop temperature is lower than 300 ° C., the movable dislocation in the steel material is remarkably increased and the YS is remarkably lowered. Therefore, the accelerated cooling stop temperature is defined as 300 ° C. or more. Further, when the accelerated cooling stop temperature is 550 ° C. or higher, island-like MA is likely to be generated in the steel material, and particularly YS significantly decreases at a position of 1/2 t in the thickness direction of the steel material. The temperature is defined as less than 550 ° C.

  The reasons for limiting the component composition in the present invention will be described below.

  C is an element indispensable for strengthening the structure of steel and is added in an amount of 0.005% or more. In the present invention, however, the dependency of the steel material TS on the accelerated cooling stop is reduced, and the island-like MA in the steel material is reduced. In order to improve YS by the reduction and to improve the toughness of the weld heat affected zone, it is necessary to suppress the addition amount to less than 0.040%. If the amount of C added is less than 0.005%, the amount of solid solution C in the austenite is extremely reduced, so that the structural strengthening during accelerated cooling is significantly reduced and the strength is lowered. Must be added.

  Si is an element effective for increasing the strength. In the present invention, however, the formation of island-like MA that lowers YS is promoted in the region where the stop temperature of accelerated cooling is 500 ° C. or more, and the thermal effect during welding Also in the structure of the part, in order to promote the generation of island-like MA that lowers toughness, when added, the amount needs to be suppressed to less than 0.05%. Desirably, it is not added positively, but it is set to an inevitable impurity level.

  In order to significantly reduce toughness, P is added in an amount of 0.03% or less, preferably at an unavoidable impurity level.

  In order to significantly reduce toughness, S is added in an amount of 0.01% or less, preferably at an inevitable impurity level.

  Mn is an element effective in increasing the strength, so 1.0% or more is added. However, if added in a large amount, the difference between the bainite transformation start temperature and the end temperature opens, and the yield ratio decreases slightly, and welding is performed. Since the toughness of the heat-affected zone slightly decreases, the addition amount is limited to less than 1.5%.

  Nb is a fast precipitation in ferrite or bainite, and is an important element for obtaining precipitation strengthening even in a non-tempered manufacturing method, and also contributes to refinement of the structure and strengthening of the structure. Addition of 0.02% or more is carried out. However, if over 0.08% is added, the toughness of the weld heat affected zone is remarkably lowered, so the content is limited to 0.08% or less.

  In the component system and rolling / cooling method of the present invention, B is extremely effective for suppressing the formation of ferrite from the austenite grain boundaries and making the structure a bainite single-phase structure. Addition of 0.0005% or more is carried out in order to suppress the formation of ferrite formed on the austenite grain boundaries and to significantly improve the toughness through refinement of precipitates such as Nb and Ti. However, excessive B exceeding 0.0030% deteriorates the toughness of the weld heat affected zone, so the upper limit is limited to 0.0030%.

  Mo can be used in combination with B in the component system of the present invention to increase the effect of suppressing the formation of ferrite from the austenite grain boundaries, and to reduce the amount of dissolved C in austenite by forming a compound with C. In order to reduce the dependence of the strength on the accelerated cooling stop temperature, 0.10% or more is added. However, if added over 0.50%, the toughness of the weld heat-affected zone decreases, so 0% is added. Limited to 50% or less.

  Al is an element effective for deoxidation and refinement of the austenite grain size before accelerated cooling, etc., and is added in an amount of 0.001% or more. In the present invention, similarly to Si, In order to promote the formation of island-shaped MA that lowers YS and promote the formation of island-shaped MA that lowers toughness, the amount of addition is limited to 0.010% or less.

  N is combined with Nb or Ti, and is an element effective for refining austenite grains and precipitation strengthening in bainite, so 0.0010% or more is added. Since the toughness is lowered, the upper limit is limited to 0.0070%.

Ti precipitates quickly in bainite, so it is an important element for obtaining precipitation strengthening in the non-tempered manufacturing method of the present invention, contributes to the refinement of the structure, and in the steel. In order to improve the toughness by combining with N and O, the addition of 0.005% or more is necessary. However, if over 0.020% is added, the toughness of the weld heat affected zone is remarkably lowered, so the content is limited to 0.020% or less. Furthermore, in the present invention, it is necessary to secure B that does not bond with N. For this reason, Ti is used for the purpose of bonding with N. It is necessary to limit the amount of addition.
Formula 1: [B] /10.8> = [N] /14.0- [Ti] /47.9

W is an important element in the present invention, and by adding together with B, the effect of suppressing the formation of ferrite from the austenite grain boundary is enhanced and effective in increasing the strength by strengthening the structure, in order to obtain a clear increase in strength. performs the addition of 0.05% or more in order to lower the toughness of the heat affected zone and the addition of more than 0.5%, when adding W 0.05% or more, 0.5% or less Limited to.

  Cu is an element effective for increasing the strength, and in order to obtain a clear increase in strength, addition of 0.01% or more is necessary. However, addition of 0.50% or more significantly reduces the bainite transformation temperature. In addition, in order to reduce the toughness of the heat affected zone, when adding Cu, the range is 0.01% or more and 0.50% or less.

  Ni is an element effective for increasing the strength, and in order to obtain a clear increase in strength, it is necessary to add 0.01% or more. However, in the component system of the present invention, excessive addition significantly lowers the bainite transformation end temperature and significantly reduces the toughness of the weld heat affected zone. Limit to.

  Regarding Ca, Mg, Zr, Hf, and REM, addition can be performed for deoxidation and improvement of toughness. In order to obtain these effects, addition of 0.001% or more is necessary, but the upper limit is limited to 0.010% due to cost problems. Therefore, when adding Zr, Ca, Mg, Hf, and REM, it is set as 0.001% or more and 0.010% or less.

In addition, the component system by this invention shows a very favorable result also about the toughness of a welding heat affected zone. Furthermore, and has a range and reduced to levels weld crack sensitivity index P _CM also: 0.20% represented by the following formula 2, which also weld cracking is prevented.
Equation _{2: P CM = [C]} + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Mo] / 15 + [V] / 10 + 5 [B]
In addition, [] in Formula 2 represents the addition amount of each alloy element in mass%.

  Molten steel having the composition shown in Table 1 was produced in a vacuum melting furnace and cast into an ingot shape. The steel slab is rolled or cut to produce a steel slab having a thickness of 80 to 500 mm. The slab is subjected to hot rolling, accelerated cooling, and tempering treatment under the conditions shown in Table 2 to obtain a thickness of 12 A thick steel plate of ˜50 mm was used.

  Table 2 shows the results of measuring the base material YS, base material TS, base material toughness, and weld heat affected zone toughness of these thick steel plates.

  Regarding the base materials YS and TS, the results measured by a tensile test in accordance with JIS Z 2241 are shown in Table 2. As the tensile test piece, a 1A full thickness tensile test piece or No. 4 round bar tensile test piece based on JIS Z 2201 was used.

  Regarding the base material toughness, the results of measurement at −5 ° C. by the method defined in JIS Z 2242 are shown in Table 2. As the impact test piece, a 2 mm V notch test piece based on JIS Z 2202 was used from the center of the thickness in the direction perpendicular to the rolling direction.

  Regarding the weld heat-affected zone toughness, the results measured at −5 ° C. by the method prescribed in JIS Z 2242 are shown in Table 2. As the impact test piece, a 2 mm V notch test piece based on JIS Z 2202 to which a thermal cycle corresponding to a heat affected zone 1 mm position (HAZ1) at the time of submerged arc welding with a heat input of 10 kJ / mm was used.

  The target values of each characteristic are YS of 500 MPa, TS of 570 MPa, the base material toughness and the weld heat affected zone toughness are both 100 J or more in absorbed energy, and the numerical values that do not satisfy the target value are underlined.

  From the results of Tables 1 and 2, it can be seen that the component composition and production method according to the method of the present invention all show good results for YS, TS, base metal toughness and weld heat affected zone toughness. On the other hand, it can be seen that the comparative steel deviating from the range of the steel of the present invention has at least one or more basic characteristics of YS, TS and weld heat affected zone toughness.

Claims (4)

% By mass
C: 0.005% or more, less than 0.040%,
Si: less than 0.05% Mn: 1.0% or more, less than 1.5%,
P: 0.03% or less,
S: 0.01% or less,
Mo: 0.10% or more, 0.50% or less,
Nb: 0.02% or more, 0.08% or less,
Ti: 0.005% or more, 0.020% or less,
B: 0.0005% or more, 0.0030% or less,
Al: 0.001% or more, 0.010% or less,
N: A steel slab containing 0.0010% or more and 0.0070% or less, satisfying the relationship between the amounts of Ti and N represented by the following formula 1, and the balance having a composition composed of Fe and inevitable impurities When heated to 1020 ° C or higher and 1300 ° C or lower and then rolled, the cumulative rolling reduction at 1020 ° C or lower and above 920 ° C is less than 60%, and the cumulative rolling reduction at 920 ° C or lower and above Ar ₃ point is 50%. Above, rolled to be 90% or less, started accelerated cooling at a cooling rate of 1 ° C / second or more within 60 seconds after the end of rolling, stopped accelerated cooling at a temperature range of less than 550 ° C and 300 ° C or more, A method for producing a high-strength steel material having a yield stress of 500 MPa or more and a tensile strength of 570 MPa or more that is excellent in toughness of the weld heat-affected zone.
In addition, [] in Formula 1 represents the addition amount of each alloy element in mass%, and the same applies thereafter.
Formula 1: [B] /10.8> = [N] /14.0- [Ti] /47.9 Furthermore, in mass%,
W: 0.05% or more and 0.50% or less of the high-strength steel material having a yield stress of 500 MPa or more and a tensile strength of 570 MPa or more excellent in the toughness of the weld heat affected zone according to claim 1 Production method. Furthermore, in mass%,
Cu: 0.01% or more, 0.50% or less,
Ni: 0.01% or more and 0.50% or less of 1 type or 2 types, The yield stress 500MPa or more tensile strength which is excellent in the toughness of the welding heat affected zone of Claim 1 or 2 characterized by the above-mentioned A method for producing a high-strength steel material having a thickness of 570 MPa or more. Furthermore, in mass%,
Ca: 0.001 to 0.010%,
Mg: 0.001 to 0.010%,
Zr: 0.001 to 0.010%,
Hf: 0.001 to 0.010%,
REM: 0.001 to 0.010%
A high yield strength of 500 MPa or more and a tensile strength of 570 MPa or more excellent in the toughness of the weld heat-affected zone according to any one of claims 1 to 3, characterized by containing at least one of A manufacturing method for high strength steel.