JP2008261011A - Method for producing high-strength steel material having yield stress of 470 MPa or more and tensile strength of 570 MPa or more excellent in toughness of weld heat-affected zone - Google Patents

Abstract

The present invention provides a method for producing a high-strength steel material of YS 470 MPa or higher and TS 570 MPa or higher, which is excellent in toughness of a weld heat affected zone.
SOLUTION: In mass%, C: 0.005% or more, less than 0.030%, Si <0.05%, Mn: 1.5-2.5%, Nb: 0.02-0.08% Ti: 0.005-0.030%, B: 0.0005-0.0020%, Al: 0.001-0.01%, N: 0.001-0.007%, One or two of Mo and W are each contained in an amount of 0.05 to 0.5%, P _CM ≦ 0.20, satisfy the relationship of Ti / 47.9 ≧ N / 14.0, and the balance iron and A steel slab composed of inevitable impurities is heated to 1000 to 1250 ° C., the cumulative reduction at 920 to 1020 ° C. is less than 60%, and the cumulative reduction at Ar ₃ points or more and 920 ° C. or less is 30% or more. After completion of rolling, accelerated cooling at 1 ° C./second or more is performed to a temperature range of 300 ° C. or more and less than 550 ° C., and then allowed to cool. And wherein the door.
[Selection figure] None

Description

  The present invention relates to a method for producing a high-strength steel material having a yield stress of 470 MPa or more and a tensile strength of 570 MPa or more, which is excellent in toughness of a weld heat affected zone, which is used in various fields such as architecture, shipbuilding, bridges, and civil engineering.

  In recent years, with the increase in the size of steel structures such as ships and buildings, the strength of steel materials used has been increasing. By using high-strength steel material, the amount of steel material used can be reduced, so that advantages such as expansion of the space in the structure and reduction in weight can be obtained.

  Conventionally, when manufacturing high-strength steel materials, in order to stably obtain strength and toughness in actual machine manufacturing, it is common to manufacture by a process that combines hot rolling, accelerated cooling, and tempering heat treatment. It is. For example, Patent Documents 1, 2, and 3 disclose a technique in which a steel sheet is hot-rolled, then quenched online, and further subjected to tempering heat treatment offline. However, the off-line tempering heat treatment generally causes an increase in manufacturing time, and a reduction in productivity becomes a problem.

  Therefore, various technological developments have been made so far for the purpose of improving the productivity of high-strength steel materials. For example, Patent Documents 4 and 5 show a technique for shortening the heat treatment time and improving productivity by using an induction heating method in a heating furnace for performing a tempering heat treatment.

In addition, in order to improve productivity, a manufacturing process that omits the tempering heat treatment itself has been developed, and a method for manufacturing a high-strength steel sheet while hot rolling is being developed. For example, in Patent Document 7, a steel containing 0.10 to 0.20% C and 0.01 to 0.20% Si is heated to 1200 to 1300 ° C and then hot at a finishing temperature of 950 ° C or higher. A manufacturing method for rolling is disclosed. In Patent Document 8, steel containing 0.10 to 0.20% C and 0.03 to 0.60% Si is heated to 1100 to 1250 ° C, and then Ar ₁ +10 to Ar ₃ -10 ° C. A technology for hot rolling is disclosed so that the cumulative rolling reduction in the temperature range of 16 to 30% and the finish rolling temperature is (Ar ₁ +30) ± 20 ° C. In addition, in Patent Document 9, after heating steel containing 0.03% to 0.20% C and 0.10% to 0.60% Si, the total reduction amount at 800 ° C. or lower is 5 to 15 mm. In addition, a technique is disclosed in which hot rolling is performed so that the finish rolling temperature is not more than the Ar ₃ point.

Furthermore, a method has been developed in which accelerated cooling is performed online after hot rolling of a steel sheet, and a high-strength steel sheet is produced while being quenched. For example, in Patent Documents 10 and 11, a steel containing 0.001 to 0.03% of C and 0.60% or less of Si is heated to Ac ₃ point to 1350 ° C., and then 800 ° C. or more and Ar ₃ point or more. A technique of performing hot rolling and then accelerated cooling is disclosed. Patent Document 12 discloses a technique in which steel containing 0.005% to 0.03% of C is heated to 1100 to 1350 ° C., hot-rolled at an Ar ₃ point or higher, and then accelerated cooling is performed. ing. Furthermore, in Patent Document 13, steel containing 0.005 to 0.030% C and 0.05 to 0.50% Si is heated to 1000 to 1300 ° C, and then the cumulative reduction in a temperature range of 950 degrees or less. A technique is disclosed in which hot rolling is performed with a rate of 30% or more and a rolling end temperature of 750 ° C. or more, and then accelerated cooling to 450 ° C. or less. Patent Document 14 discloses that steel containing 0.03 to 0.07% C and 0.1 to 0.6% Si is not less than the temperature determined from the contents of Nb, Ti, C, and N. Heat to less than 1020 ° C and heat up to a cumulative reduction of 15% or less, a range of 920 ° C or less, or a range of 860 ° C or more in the range of less than 1020 ° C or more than 920 ° C. Interim rolling is performed, and accelerated cooling starts at 800 ° C. or higher at a cooling rate of 2 to 30 ° C./second, and ends at 700 ° C. or lower and 600 ° C. or higher, and then at a cooling rate of 0.4 ° C./second or lower. Techniques for cooling are disclosed.

JP 52-081014 A JP-A-63-033521 Japanese Patent Laid-Open No. 02-205627 JP 2002-317227 A Japanese Patent Laid-Open No. 2003-082412 Japanese Patent No. 2776174 JP-A-10-219390 Japanese Patent Laid-Open No. 08-188823 Japanese Patent Laid-Open No. 05-171271 Japanese Patent Laid-Open No. 08-144019 JP-A-11-269602 JP 2003-147477 A JP 2004-232056 A Japanese Patent Laid-Open No. 2005-126819

  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 and 5 are advantageous in that productivity can be improved regardless of the strength range because online tempering is performed by induction heating. A very large capital investment is required for introduction.

  Moreover, in patent document 6, in order to ensure intensity | strength, in order for C amount to be 0.01 to 0.20% by a regulation, and also in an Example, it is 0.04 to 0.18%, and since there is much C amount It is considered that the strength of the steel material largely fluctuates due to changes in the cooling rate of accelerated cooling and the stop temperature. Moreover, the stop temperature of accelerated cooling is also limited to a narrow range of 450 to 540 ° C., and there is a problem that it is difficult to apply to a manufacturing process in an actual machine.

  Moreover, in patent document 7, since it manufactures with hot rolling as it is, it is advantageous at the point of manufacture stability in an actual machine, However, C amount is prescribed | regulated as 0.10% or more, and the large heat input welding was applied. The problem is that the toughness of the weld heat-affected zone during welding becomes severe, and if tempering heat treatment is not performed, the yield stress (YS) is low due to the island-like MA (Martensite-Austenite constituent) generated in the base material. There is.

Also, in Patent Documents 8 and 9, since it is produced as hot-rolled, it is advantageous in terms of production stability in an actual machine, but it is finish rolling in a two-phase region of Ar ₃ points or less, and In Patent Document 8, the amount of C is specified to be 0.03% or more, and in Patent Document 9, the amount of C is specified to be 0.10% or more. Since the Ar ₃ point is lowered, the temperature of the steel plate before finish rolling is performed. The time to wait for the decrease becomes longer, and the problem remains in terms of productivity improvement.

  Patent Document 10 discloses a method for producing a steel of 400 MPa class or higher having a high yield point in a component system of extremely low C and extremely low Si. 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, as far as the examples are concerned, the amount of Al added is 0.023% or more. According to the study by the present inventors, the addition of a large amount of Al promotes the formation of island-shaped MA in the base material. In particular, it is understood that the steel material of 570 MPa class is not optimized for manufacturing with accelerated cooling.

  Moreover, in patent document 11, the manufacturing method of the steel of 400 MPa grade or more which has a high yield point with the component system of ultra-low C and ultra-low Si is disclosed. 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.

  Patent Documents 12 and 13 specify that Si is 0.05% or more and Al is 0.01% or more. In the as-quenched manufacturing method, the production amount of island-like MA increases and YS decreases. There is a problem.

  Patent Document 14 discloses a technique for suppressing fluctuations in tensile strength (TS) with respect to the stop temperature of accelerated cooling by positively using Nb precipitation strengthening, but strongly strengthens Nb precipitation strengthening. In order to utilize it, the stop temperature of accelerated cooling is limited to a narrow range of 600 ° C. or more and 700 ° C. or less.

  Then, this invention aims at providing the manufacturing method of the high-strength steel materials of yield stress 470MPa or more excellent in the toughness of a welding heat-affected zone and tensile strength 570MPa or more which can solve the said problem advantageously. To do.

  The present inventors are a non-tempered manufacturing method in which tempering heat treatment is not performed after accelerated cooling of a steel sheet, and YS has a sufficiently high stability even in consideration of inevitable variations in various manufacturing conditions in an actual machine. Research and development were repeated through experiments and analysis on a method for producing a high-strength steel material having a high yield ratio of 500 MPa or more and TS of 570 MPa or more and 720 MPa or less. As a result, in the non-refined manufacturing method with accelerated cooling, the effects of various alloying elements and their combinations on YS and TS are extracted and clarified, compared with the conventional non-refined manufacturing method of high-strength steel, We have gained knowledge to significantly improve manufacturing stability in actual machines.

  In general, in a method for producing a high-strength steel material of 570 MPa class or higher in a non-tempered steel material, the alloy elements are designed with the aim of improving the hardenability of the steel. However, in the manufacturing method in which accelerated cooling is stopped halfway, there is a problem that alloy elements that improve hardenability tend to promote the formation of island-like MA in the base material, thereby reducing YS. Further, the indiscriminate addition of alloying elements that enhance the hardenability also raises the problem of increasing the dependence on the stop temperature of accelerated cooling of TS.

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

  Conventionally, as a method for reducing the dependence of the cooling rate and stop temperature of accelerated cooling of TS and YS, as disclosed in Patent Document 11, an extremely low C component system of 0.03% or less is adopted, Techniques have been proposed for making the steel structure a bainite single phase. By adopting this extremely low C component system, the present inventors have combined various alloy elements with respect to conditions for stably producing a high strength steel material of 570 MPa class while ensuring a high YS of 500 MPa or more in particular. The effects of the rolling and hot rolling conditions were repeatedly examined in detail.

  In order to secure a TS level of 570 MPa or higher in an extremely low C component system, it is necessary to positively add various alloy elements such as Si, Mn, Mo, Nb, Ni, Cu, Cr, Ti or B. . As a result of investigating in detail the effects of various alloy elements and their combinations on TS and YS through experiments, the present inventors have added these various alloy elements indiscriminately, so that even in an extremely low C component system, It has been found that the influence of the cooling rate of accelerated cooling and the stop temperature on TS is increased. For example, even in the case of extremely low C, the dependence of the accelerated cooling stop temperature of TS becomes large when Mn is 2.5% or Ni is 1% as in the examples. There is a problem in terms of the stability of actual machine manufacturing.

  In particular, the present inventors adopt a combination of C less than 0.030%, Nb 0.02% or more, B 0.0005% or more, and Mo 0.05% or more or W 0.05% or more, so that the accelerated cooling condition is Experiments have found that the effect on TS can be minimized. 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 thickness direction of the steel sheet can be remarkably reduced to 5% or less.

  In addition, Mn, Cr, and Ni that have increased the dependence on the accelerated cooling conditions of TS when added alone are also less than C0.030%, Nb 0.02% or more, B0.0005% or more, and Mo0.05% or more. Or, when used in combination with a combination of W 0.05% or more, the dependence of accelerated cooling conditions on TS becomes smaller than previously assumed, and the production stability of high-strength steel that is the object of the present invention is not impaired. I understood.

  Furthermore, in order to realize a highly stable manufacturing method of high-strength steel materials with YS of 470 MPa or more and TS of 570 MPa or more, which is the object of the present invention, it is not enough to pursue TS as described above. Therefore, 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 is applied. Quality production methods are often studied as methods for producing high strength steels with low yield ratios. In order to realize a high-productivity and high-stability manufacturing method for high-strength steel materials of YS 470 MPa or higher, which is the object of the present invention, the inventors have various alloy elements, hot rolling conditions, and accelerated cooling conditions. Experiments were also repeatedly extracted for effects that affect As a result, the aforementioned component system of less than C0.030%, Nb 0.02% or more, B0.0005% or more, and Mo0.05% or more or W0.05% or more is also used for the purpose of securing YS of 470 MPa or more. Although it is very advantageous, in order to reduce the amount of island MA, it is effective to suppress the addition amount of Si and Al that are actively added from the viewpoint of securing strength and toughness in many steel materials. I found out that there was.

  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 effect of the above-described TS on the cooling conditions is suppressed, so that the hardness of the weld heat-affected zone becomes a very uniform bainite structure and the MA amount is reduced in order to prevent YS reduction of the base material. This is presumed to be 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 0.030%, Si: less than 0.05%, Mn: 1.5% or more, 2.5% or less, P: 0.03% or less , S: 0.01% or less, Nb: 0.02% or more, 0.08% or less, Ti: 0.005% or more, 0.030% or less, B: 0.0005% or more, 0.0020% or less Al: 0.001% or more, 0.01% or less, N: 0.001% or more, 0.007% or less, Mo: 0.05% or more, 0.5% or less, W: 0.05% or more, and containing one or two of 0.5% or less, and a weld crack sensitivity index P _CM represented by the following formula 1 is 0.20 or less, represented by the following formula 2 A steel slab having a component composition that satisfies the relationship between the amount of Ti and N, with the balance being Fe and inevitable impurities, is 1000 ° C or higher and 1250 ° C or lower. When heated below and then rolled, rolling is performed so that the cumulative rolling reduction at 1020 ° C. or less and above 920 ° C. is less than 60%, and the cumulative rolling reduction at 920 ° C. or less and above Ar ₃ point is 30% or more. After the completion of rolling, the yield stress is 470 MPa, which is excellent in toughness of the heat affected zone of welding, characterized by performing accelerated cooling at a cooling rate of 1 ° C./second or more to a temperature range of less than 550 ° C. and 300 ° C. or more and then allowing to cool. A method for producing a high-strength steel material having a tensile strength of 570 MPa or more.
In addition, [] in Formula 1 and Formula 2 represents the addition amount of each alloy element in the mass%, and it is the same after that.
Equation _{1: P CM = [C]} + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Cr] / 20 + [Mo] / 15 + [V] / 10 + 5 [B]
Formula 2: [B] /10.8≧ [N] /14.0- [Ti] /47.9
(2) Further, in mass%, Cr: 0.01% or more, less than 0.3%, Cu: 0.01% or more, 1.0% or less, Ni: 0.01% or more, 1.0% or less The method for producing a high-strength steel material having a yield stress of 470 MPa or more and a tensile strength of 570 MPa or more, which is excellent in toughness of the weld heat-affected zone as described in (1) above, comprising one or more of the above.
(3) 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 0.00. It has excellent toughness of the weld heat affected zone according to the above (1) or (2), characterized by containing at least one of 010% and REM: 0.001 to 0.010% A method for producing a high-strength steel material having a yield stress of 470 MPa or more and a tensile strength of 570 MPa or more.

  According to the present invention, it is possible to obtain a high-strength steel sheet having a tensile strength of 570 MPa class, which is excellent in toughness, by an economical component system with less alloying elements and a highly tempered manufacturing method with high productivity. Further, the present invention is a manufacturing method capable of stably ensuring quality at an unprecedented level even with respect to various unavoidable fluctuations in manufacturing conditions in an actual machine, and its industrial contribution is extremely large.

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

  First, in the present invention, as described above, in producing a high strength steel of 570 MPa class, the structural strengthening and precipitation strengthening of the steel material are effectively utilized to cope with a large change in production conditions assumed in an actual machine. However, it is realized that it can be stably manufactured, and is characterized by limiting combinations of various alloy elements and their addition amounts according to 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 precipitation strengthening is used even in a non-tempered manufacturing method in which tempering heat treatment is not performed, Nb and Ti that are the fastest in ferrite and bainite are used as precipitation strengthening elements. 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 the present invention, in order to improve the production stability, the component range of C is limited to be lower than that of Patent Document 7, and therefore, precipitation of Nb and Ti in austenite is suppressed. As a result, the limit of the cumulative reduction amount above 920 ° C. and below 1020 ° C. is defined as less than 60%.

As for rolling at 920 ° C. or lower, it is effective from the viewpoint of securing YS to perform in a temperature range higher than the Ar ₃ point and to make the rolling reduction as large as possible. In general, in a manufacturing method that omits the tempering heat treatment, YS tends to be low instead of the TS level. To compensate for this, a reduction of 30% or more is required.

  With respect to accelerated cooling after the end of rolling, a cooling rate of 1 ° C./second or more is required in order to obtain an effect of increasing the strength by bainite transformation.

  When the accelerated cooling stop temperature is below 300 ° C., the martensite fraction in the structure increases at a quarter position in the thickness direction of the steel material, resulting in an increase in the density of movable dislocations in the steel material and yield strength. Therefore, the stop temperature of accelerated cooling is specified to be 300 ° C. or higher. Further, when the accelerated cooling stop temperature is 550 ° C. or more, a large amount of island-like MA is generated in the steel material, and particularly YS significantly decreases at a position of 1/2 in the thickness direction of the steel material. The stop temperature is specified to be 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. However, in the present invention, the dependency of the strength of the steel material on the stop temperature of accelerated cooling is reduced, and the weld HAZ toughness is reduced. In order to improve and suppress coarse precipitation of Nb in austenite, it is necessary to suppress the addition amount to less than 0.030%. Note that if the C addition amount is suppressed to less than 0.005%, the precipitation amount of carbides formed with other alloy elements such as Nb and Ti decreases, or the amount of solid solution C in austenite decreases extremely. Addition of 0.005% or more is necessary because the structural strengthening during accelerated cooling is significantly reduced and the strength is lowered.

  Si is an element effective for increasing the strength. In the present invention, however, the generation of residual MA that lowers YS is promoted in the region where the accelerated cooling stop temperature is 500 to 600 ° C., and the thermal effect during welding In the part, in order to generate 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.

  Mn is an element effective for increasing the strength, and 1.5% or more is added, but if added over 2.5%, the toughness of the weld heat-affected zone decreases, so the added amount is 2.5% or less. Limited to.

  P is limited to 0.03% or less in order to reduce toughness by adding a large amount. The inevitable impurity level is desirable.

  S is limited to 0.01% or less in order to reduce toughness by adding a large amount. The inevitable impurity level is desirable.

  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, but addition exceeding 0.08% remarkably reduces the toughness of the weld heat affected zone, so it is limited to 0.08% or less.

  Al is an element effective for deoxidation and refinement of the austenite grain size before accelerated cooling, etc., and 0.001% or more is added. In the present invention, as in the case of Si, the base material is added. In order to promote the formation of island-shaped MA that lowers YS, and in the heat-affected zone during welding, to promote the formation of island-shaped MA that lowers toughness, the amount of addition is limited to 0.01% or less. To do.

  N is an element that combines with Nb and Ti and is effective for refinement of austenite grains and precipitation strengthening in ferrite or bainite, so 0.0001% or more is added. Since the N amount is increased and the toughness of the base metal and the weld heat affected zone is lowered, the addition amount is limited to 0.007% or less.

Ti is an important element for obtaining precipitation strengthening even in a non-tempered manufacturing method due to rapid precipitation in ferrite or bainite, and contributes to refinement of the structure. In order to combine with N or O to improve toughness, addition of 0.005% or more is necessary. However, if it exceeds 0.02%, the toughness of the weld heat affected zone is remarkably lowered, so it is limited to 0.030% 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 2: [B] /10.8≧ [N] /14.0- [Ti] /47.9

  B is extremely effective in increasing the strength by strengthening the structure in the component system and rolling / cooling method of the present invention, and suppresses the formation of ferrite formed on the prior austenite grain boundaries in the weld heat affected zone, and Nb, Ti. In order to remarkably improve the toughness through refinement of the precipitates, etc., 0.0005% or more is added. However, excessive B exceeding 0.0020% deteriorates the toughness of the weld heat-affected zone, so the upper limit is limited to 0.0020%.

  Mo is effective in increasing the strength by strengthening the structure. In addition, in the component system of the present invention, the addition of 0.05% or more significantly reduces the dependence of the base metal strength on the stop temperature of the accelerated cooling. In addition, the dependence of the strength of the base metal on the hot rolling temperature and the dependence of the base strength on the acceleration cooling start waiting time after hot rolling are significantly reduced. An effect is obtained. In order to obtain these effects, addition of 0.05% or more is necessary, but if added over 0.5%, the toughness of the weld heat affected zone is lowered, so it is limited to 0.5% or less.

  W is an element effective in increasing the strength, and, like Mo, has the effect of reducing the stop temperature dependency of the base material strength on the accelerated cooling by adding, and further, the hot rolling temperature of the base material strength. There is an effect that the manufacturing stability in the actual machine is drastically improved, such as the effect of reducing the dependency of the base metal strength and the acceleration cooling start waiting time after hot rolling. In order to obtain these effects, addition of 0.05% or more is necessary, but if added over 0.5%, the toughness of the weld heat affected zone decreases, so the content is limited to 0.5% or less.

  Cr is an effective element 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 and rolling / cooling method of the present invention, the addition of a large amount of Cr promotes the formation of island-like MA in the base material, so that the improvement effect of YS remarkably slows down, and the toughness of the weld heat affected zone In the case where Cr is added, the range is 0.01% or more and less than 0.3%.

  Cu is an element effective for increasing the strength, and in order to obtain a clear increase in strength, 0.01% or more of addition is necessary. However, addition of 1.0% or more lowers the toughness of the weld heat affected zone. Therefore, when Cu is added, the range is 0.01% or more and 1.0% 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 present invention, when it is excessively added exceeding 1.0%, the dependency of the strength on the accelerated cooling stop temperature increases, and the change in the hardness distribution in the thickness direction occurs inside the steel plate. There is a problem that the TS at the 1/4 position in the thickness direction exceeds 720 MPa and the toughness of the weld heat affected zone is significantly reduced. Therefore, when adding Ni, it is made into the range of 0.01% or more and 1.0% or less.

  Zr, Ca, Mg, Hf, and REM can be added to improve deoxidation and toughness. Addition of 0.001% or more is necessary to obtain these effects, 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. Further, by suppressing the level weld crack sensitivity index P _CM also: 0.20% shown in the equation 1, has a range of weld crack is prevented.

  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 appropriately rolled, forged, or cut to produce a steel slab having a thickness of 80 to 500 mm, and the slab is subjected to hot rolling, accelerated cooling, and tempering heat treatment under the conditions shown in Table 2. To obtain a thick steel plate having a thickness of 12 to 50 mm.

  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 470 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 (3)

% By mass
C: 0.005% or more, less than 0.030%,
Si: less than 0.05%,
Mn: 1.5% or more, 2.5% or less,
P: 0.03% or less,
S: 0.01% or less,
Nb: 0.02% or more, 0.08% or less,
Ti: 0.005% or more, 0.030% or less,
B: 0.0005% or more, 0.0020% or less,
Al: 0.001% or more, 0.01% or less,
N: 0.001% or more and 0.007% or less, and
Mo: 0.05% or more, 0.5% or less,
W: 0.05% or more, and containing one or two of 0.5% or less, and a weld crack sensitivity index P _CM represented by the following formula 1 is 0.20 or less, by the following formula 2 A steel slab having a component composition composed of Fe and unavoidable impurities, satisfying the relationship between the amount of Ti and N expressed, is heated to 1000 ° C. or higher and 1250 ° C. or lower, and then rolled to 1020 ° C. or lower, Rolling is performed so that the cumulative rolling reduction at over 920 ° C. is less than 60%, 920 ° C. or less, and the cumulative rolling reduction at Ar ₃ points is 30% or more, and after rolling, the temperature is less than 550 ° C. and 300 ° C. or more. A method for producing a high-strength steel material having a yield stress of 470 MPa or more and a tensile strength of 570 MPa or more, which is excellent in toughness of a weld heat-affected zone, which is subjected to accelerated cooling at a cooling rate of 1 ° C./second or more to a region and then left to cool.
In addition, [] in Formula 1 and Formula 2 represents the addition amount of each alloy element in the mass%, and it is the same after that.
Equation _{1: P CM = [C]} + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Cr] / 20 + [Mo] / 15 + [V] / 10 + 5 [B]
Formula 2: [B] /10.8≧ [N] /14.0- [Ti] /47.9 Furthermore, in mass%,
Cr: 0.01% or more, less than 0.3%,
Cu: 0.01% or more, 1.0% or less,
Ni: 0.01% or more, 1.0% or less of 1 type or 2 types or more are contained, The yield stress excellent in the toughness of the weld heat affected zone according to claim 1, wherein the tensile stress is 470 MPa or more A method for producing a high-strength steel material having a strength 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%
The method for producing a high-strength steel material having a yield stress of 470 MPa or more and a tensile strength of 570 MPa or more, which is excellent in toughness of the weld heat-affected zone according to claim 1, comprising one or more of .