JP2007302978A - Method for manufacturing high-strength steel of tensile strength of 780 mpa class having excellent toughness of weld heat affected zone - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing high-strength steel of tensile strength of 780 MPa class having excellent toughness of a weld heat affected zone with unexperienced low alloy component and at high productivity. <P>SOLUTION: In themethod for manufacturing the high-strength steel of tensile strength of 780 MPa class having excellent toughness of the weld heat affected zone, a steel slab having the composition consisting of, by mass, ≥0.005% and <0.030% C, <0.05% Si, 1.0-2.5% Mn, 0.02-0.08% Nb, 0.001-0.10% Al, and 0.0001 to ≤0.01% N with the weld cracking parameter PCM being ≤0.25, and the balance Fe with inevitable impurities is heated to 1,020-1,300°C, and rolled so that the cumulative draft in the temperature range of ≤1,020°C and >920°C is >60%, and the cumulative draft below Ar<SB>3</SB>point is 30-95%, and cooled after the rolling. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for producing a high strength steel material having a tensile strength of 780 MPa class and excellent in toughness of a weld heat affected zone used in various fields such as construction, 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 manufacturing, it is common to manufacture by a method of performing tempering heat treatment after quenching by accelerated cooling. For example, Patent Document 1 discloses an invention in which a steel plate is rolled, quenched online, and then subjected to tempering heat treatment offline. Patent Document 2 discloses an invention in which a steel sheet is rolled, quenched, and then subjected to tempering heat treatment online.

In addition, in order to improve productivity, manufacturing processes that omit the tempering heat treatment itself have been developed. For example, Patent Document 3 discloses an invention in which, in a method for producing a high-strength steel material, hot rolling is finished at an Ar ₃ point or higher and 800 ° C. or lower, and then accelerated cooling is performed to a temperature range of 450 ° C. or higher and 540 ° C. or lower. Is disclosed. Further, in Patent Document 4, in a method for producing a high-strength steel material, rolling is performed so that the cumulative rolling reduction is 15% or less in a range of less than 1020 ° C. and more than 920 ° C., and 920 ° C. or less and 860 ° C. or more. An invention is disclosed in which rolling is performed so that the cumulative reduction ratio is 20% or more and 50% or less in a range, and then accelerated cooling is performed from 800 ° C or more, and accelerated cooling is stopped in a range of 700 ° C or less and 600 ° C or more. ing. Patent Documents 5 and 6 disclose inventions in which hot rolling is finished at an Ar ₃ point or higher and then cooling is performed at an average cooling rate of 20 ° C./second. Patent Document 7 discloses an invention in which, in a method for producing a high-strength steel material, rolling is performed at a cumulative reduction ratio of 30% or more in a range of 1000 ° C. or higher and 1250 ° C., and finish rolling is performed at an Ar ₃ point or higher. ing.

Furthermore, not only tempering heat treatment but also accelerated cooling is omitted, and a method for producing a high-strength steel material as it is in hot rolling has been developed. For example, Patent Document 8 discloses an invention in which hot rolling is finished at over 600 ° C. and below 700 ° C. in a method for producing a high-strength steel material. Patent Document 9 discloses an invention in which hot rolling is finished at 800 ° C. or higher in a method for producing a high-strength steel material. Patent Document 10, a method for producing a high-strength steel, Ar ₁ + 10 ° C. or more, Ar ₃ -10 ° C. cumulative rolling reduction of 16% or more below performs the following rolling 30%, more Ar ₁ + 10 ° C. As described above, an invention in which finish rolling is performed in a range of Ar ₁ + 50 ° C. or lower and then air cooling is disclosed. Patent Document 11 discloses an invention in which hot rolling is finished at 950 ° C. or higher in a method for producing a high-strength steel material. Patent Document 12 discloses an invention in which 50% or more of the total reduction is hot-rolled in the austenite partial recrystallization temperature region and then cooled in a method for producing a high-strength steel material.

Japanese Patent Laid-Open No. 05-171272 JP 2002-317227 A Japanese Patent No. 2776174 Japanese Patent Laid-Open No. 2005-126819 JP 2004-052063 A JP 2004-084019 A JP 2003-147477 A Japanese Patent Publication No. 62-001457 Japanese Patent Laid-Open No. 08-144019 Japanese Patent Laid-Open No. 08-188823 JP-A-10-219390 JP 2005-226158 A

However, the invention described in Patent Document 1 requires an off-line tempering heat treatment in the manufacturing process of the steel sheet, and there is a problem in that a reduction in productivity is inevitable. In the invention described in Patent Document 2, online tempering heat treatment is realized by rapid heating by induction heating, which is advantageous in that productivity can be improved regardless of the strength range. There is a problem that a very large capital investment is required to introduce the induction heating furnace. Further, in the invention described in Patent Document 3, according to the examples, it is necessary to add 0.10% of C in order to obtain the strength of 80k class, and therefore it is considered that the toughness of the weld heat affected zone decreases. It is done. In addition, since the amount of C is large, it is expected that the strength of the steel sheet will fluctuate greatly due to fluctuations in the cooling rate of accelerated cooling and the stop temperature, and it is considered difficult to apply to actual manufacturing processes. Further, in the invention described in Patent Document 4, the C amount is limited to 0.03% or more and 0.07% or less in order to suppress a change in strength due to a change in the cooling rate or stop temperature of accelerated cooling, When accelerated cooling is used, it is possible to manufacture with high productivity. However, when accelerated cooling is not used in the corresponding component system, it is necessary to lower the hot rolling finishing temperature, and there is a problem that productivity decreases. is there. Further, in the invention described in Patent Document 5, the alloying elements are reduced by limiting the C content to 0.03% or less, and it is considered advantageous in the toughness of the weld heat affected zone, but 20 ° C./second or more. Targeted for steel plates with a plate thickness of 20 mm or less that can be accelerated by the average cooling rate of the above, and with thicker plates and on-line accelerated cooling, the shape deteriorates, so accelerated cooling is actually manufactured. This process is not applicable to thin materials with a plate thickness of about 10 mm or less that are considered to be difficult. Moreover, in the invention described in Patent Document 6, it is specified that accelerated cooling after hot rolling is performed at an average cooling rate of 20 ° C./second or more. Further, in the invention described in Patent Document 7, the C content is specified to be 0.05% or more, and it seems that the toughness of the weld heat affected zone is lowered. Further, in the invention described in Patent Document 8, the C content is specified to be less than 0.03%, and it seems that the weld heat-affected zone has excellent toughness, but the alloy addition amount is low, and hot rolling is performed. Since it may be completed at 800 ° C. or higher, the strength of 780 MPa class is not obtained. Further, in the invention described in Patent Document 9, the C content is specified to be 0.10% or more and 0.20% or less, and since the C content is large, the toughness of the weld heat affected zone is reduced, Mo The amount is 0.10% or more and 0.80% or less, B is added 0.0005% or more and 0.0015% or less, and the hardenability is improved more than necessary, and the Ar ₃ point is low. Therefore, there is a problem that the time for waiting for the temperature of the steel sheet to fall to the two-phase region temperature becomes long, and the productivity is lowered more than necessary. Moreover, in invention of patent document 10, C amount is prescribed | regulated to 0.10% or more and 0.20% or less, and it seems that the toughness of a welding heat affected zone falls. Moreover, it stipulates that the hot rolling is finished at 950 ° C. or higher. Therefore, when the tempering heat treatment is not performed and the hot rolling is left as it is, there is a problem that YS becomes lower than necessary. Further, in the invention described in Patent Document 11, cooling is performed after hot rolling, but according to the embodiment, all cooling is performed at a cooling rate that cannot be achieved unless accelerated cooling is used. It is considered that sufficient strength cannot be obtained by air cooling. Further, in the invention described in Patent Document 12, to ensure strength, the Ni amount is specified to be 0.40% or more, the Cr amount is set to 0.30% or more, and Mo is set to 0.10% or more, respectively. There is a problem that the alloy cost is high.

  Accordingly, the present invention advantageously solves the above-described problems, and provides a method for producing a high strength steel material having a tensile strength of 780 MPa class, which has an unprecedented low alloy component and high productivity and is excellent in the toughness of the weld heat affected zone. It is intended to provide.

  The inventors of the present invention have made extensive research and development through experiments and analyzes on a manufacturing method that can secure high productivity by air-cooling after hot rolling with respect to a manufacturing method of a high strength steel material having a tensile strength of 780 MPa. As a result, the factors governing the YS and TS of the steel were clarified, and knowledge for producing a high-strength steel material with higher productivity and lower alloy components than conventional ones was obtained.

That is, in producing a high strength steel material having a tensile strength of 780 MPa or more, in order to simultaneously achieve high YS of 650 MPa or more as hot rolling, ferrite or bainite by controlled rolling at a temperature around Ar ₃ point is used. In addition to strengthening the fine structure, strengthening the work by rolling the structure after transformation such as ferrite, bainite, and pearlite at an Ar ₃ point or less is more advantageous in terms of cost in actual production.

When manufacturing high-strength steel by a hot-rolled manufacturing process that does not use accelerated cooling, a large amount of alloying elements that enhance hardenability are obtained in order to obtain structural strengthening by bainite-martensite transformation even at a cooling rate of air cooling. It has been carried out to add a large amount of alloying elements for utilizing precipitation strengthening or solid solution strengthening. With these guidelines, it is possible to achieve a high TS, but when steel with such a high alloy composition is hot-rolled, YS is reduced because a large amount of island-like MA is produced in the manufactured steel. Therefore, the yield ratio is lower than necessary. Therefore, it is necessary to add more alloy elements than necessary to secure YS. In addition, since the Ar ₃ transformation point decreases with the addition of a large amount of alloying elements that enhance hardenability, the effective strength securing method in the hot rolling process, which is strengthening by grain refinement and two-phase rolling, is used. In doing so, the time to wait for the temperature drop of the steel slabs becomes longer, which is also a problem in terms of productivity. In addition, there is a problem that the acoustic anisotropy of the steel material increases as the rolling temperature decreases.

Further, as noted above, the when problems such as the loss of the upper yield point by islands MA, the study of the present inventors, the higher Ar ₃ point is higher component system, the following _three points Ar two- It has been found that the recovery of the upper yield point during phase rolling is large.

Based on the above findings, the present inventors have studied in detail a component system for producing a high-strength steel material of 780 MPa class or higher with an unprecedented low alloy system and high productivity. As a result, in order to produce a high strength steel material with a tensile strength of 780 MPa class in the process of hot rolling without using accelerated cooling, the addition of elements that improve hardenability, unlike the conventional high strength steel component design In order to increase the Ar ₃ point and suppress the formation of island-like MA, and to increase the amount of strengthening below the Ar ₃ point, the rate of work-induced precipitation in ferrite and bainite It was concluded that it is effective to actively use a fast alloying element.

In accordance with such knowledge, the present inventors, in a high strength steel material having a tensile strength of 780 MPa, have a component composition for increasing the temperature of the Ar ₃ point to a level not assumed by conventional high strength steel materials, Extensive examination of specific requirements for satisfying necessary properties such as necessary conditions, and not only achieving strength of tensile strength of 780 MPa class, but also excellent welding with low alloy components and high productivity that have never been seen before It came to make this invention which can acquire the toughness of a heat affected zone.

The gist of the present invention is as follows.
(1) By mass%, C: 0.005% or more, less than 0.030%, Si: less than 0.05%, Mn: 1.0% or more, 2.5% or less, Nb: 0.02% or more 0.08% or less, Al: 0.001% or more, 0.10% or less, N: 0.0001% or more, 0.01% or less, and a weld cracking sensitivity index P represented by the following formula 1 A steel slab having a component composition consisting of _CM of 0.25 or less and the balance 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 and more than 920 ° C. The welding heat-affected zone is characterized in that the cumulative rolling reduction in the range is less than 60%, the cumulative rolling reduction at less than Ar ₃ point is 30% or more and 95% or less, and cooling is performed after the end of rolling. A method for producing a high strength steel material having a tensile strength of 780 MPa which is excellent in toughness.
Equation _{1: P CM = [C]} + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Cr] / 20 + [Mo] / 15 + [V] / 10 + 5 [B]
In addition, [] in Formula 1 represents the addition amount of each alloy element in mass%.
(2) Further, by mass, Cr: 0.01% or more, 2.0% or less, Ti: 0.001% or more, 0.05% or less, Cu: 0.01% or more, 2.0% or less , Mo: 0.01% or more, 1.0% or less, V: 0.001% or more, 0.050% or less, W: 0.01% or more, 3.0% or less The method for producing a high strength steel material having a tensile strength of 780 MPa class, which is excellent in toughness of the weld heat affected zone as described in (1) above.
(3) Further, in mass%, Zr: 0.001 to 0.010, Ca: 0.001 to 0.010, Mg: 0.001 to 0.010, Hf: 0.001 to 0.010, REM : One or more of 0.001 to 0.010 is contained, and the tensile strength of 780 MPa class excellent in the toughness of the weld heat affected zone according to (1) or (2) above Manufacturing method of high strength steel.
(4) Further, the tempering heat treatment is performed at a temperature of Ac ₁ point or less, and the tensile strength excellent in the toughness of the weld heat affected zone according to any one of (1) to (3) above A method for producing a high strength steel material of 780 MPa.

  According to the present invention, it is possible to obtain a high-strength steel sheet having a tensile strength of 780 MPa, which is excellent in toughness of the heat affected zone, by an economical component system with less alloy elements and a highly productive non-tempered manufacturing method. The industrial effects are immeasurable.

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

First, in the present invention, in order to secure YS and TS of high-strength steel of 780 MPa class, the purpose is to actively use two-phase rolling, strengthening by work-induced precipitation, and productivity For the purpose of improvement, it is characterized in that the Ar ₃ point is increased more than before, and the component composition and rolling conditions are realized.

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

  In the present invention, the steel slab is heated to 1020 ° C. or higher and 1300 ° C. or lower and then rolled. If kept for a long time in austenite of less than 1020 ° C., Nb and Ti will coarsely precipitate and increase the loss that does not contribute to the increase in strength, so the lower limit was made 1020 ° C. On the other hand, considering the use and the component system of the steel of the present invention, it is not necessary to heat at a temperature exceeding 1300 ° C, and heating at a temperature exceeding 1300 ° C unnecessarily promotes oxidation of the surface of the steel material. The temperature was 1300 ° C.

In the present invention, Nb, which is the fastest precipitation in ferrite and bainite, is used as a precipitation strengthening element in order to maximize the use of work-induced precipitation by rolling at an Ar ₃ point or less.
Precipitation of Nb in ferrite, bainite, or austenite is promoted by hot rolling. However, when precipitation of Nb occurs during rough rolling in high-temperature austenite, this precipitate is rapidly coarsened, resulting in useless precipitation that does not contribute to an increase in strength after manufacturing the steel sheet. It is made clear by the described invention. 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, the component range of C is limited to be lower than that of the invention described in Patent Document 4 in order to reduce the hardenability and improve the toughness of the weld heat affected zone. Therefore, in the present invention, the solid solution limit temperature in austenite determined from the solubility product of Nb-C is lowered, and as a result, rolling above 920 ° C. and below 1020 ° C. is allowed, but this is not allowed. The upper limit of the cumulative reduction amount is less than 60%.

Regarding the reduction in the temperature range below Ar ₃ point, the strength increases as the reduction amount increases, but if the cumulative reduction amount is less than 30%, YS of 650 MPa or more cannot be obtained, and if the reduction amount exceeds 95%, strengthening is achieved. Because of saturation, the cumulative reduction amount is specified as 30% or more and 95% or less.

In the present invention, the components are designed so that the processing strengthening by rolling at the Ar ₃ point or less can be greatly obtained. Therefore, after the rolling, it may be allowed to cool in the air, but further accelerated cooling is performed thereafter. However, it does not detract from the spirit of the present invention.

  At this time, since the C content is low, the precipitation strengthening of Nb or Ti is used, and the amount of island-like MA produced is small, the steel according to the present invention is YS and TS in a wide range of water cooling stop 400 to 650 ° C. Stable results are obtained. However, the accelerated cooling stop temperature needs to be set to 650 ° C. or lower in order to obtain an increase in strength.

Furthermore, after hot rolling, after cooling in the air or after accelerated cooling, the strength can be further increased and stabilized by performing a tempering heat treatment process. The temperature of this tempering heat treatment needs to be limited to Ac ₁ point or less from the viewpoint of securing stable strength.

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

C needs to be added to less than 0.030% in order to increase the temperature of the Ar ₃ point, improve the weld zone HAZ toughness, and suppress coarse precipitation of Nb in austenite. If the C addition amount is suppressed to less than 0.005%, the precipitation amount of carbide formed with other alloy elements such as Nb is reduced and the strength is insufficient, so addition of 0.005% or more is necessary. .

  In order to promote the formation of island-like MA and lower YS, the amount of Si needs to be limited to less than 0.05%. More preferably, it is not added positively, but an inevitable impurity level is set.

Mn is an element effective for increasing the strength, and 1.0% or more is added to secure TS. However, since it is desirable to suppress the addition amount because it lowers the Ar ₃ point, 2.5% or less Limited.

Nb is effective for obtaining work-induced precipitation below the Ar ₃ point because it precipitates rapidly in ferrite or bainite, and also contributes to the refinement of the structure. In order to obtain these effects, addition of 0.02% or more is necessary. However, if over 0.08% is added, the weld HAZ toughness is remarkably lowered and strengthening becomes dull. Limit to 08%.

Al is an element effective for deoxidation and refinement of the austenite grain size before accelerated cooling, and also has an effect of increasing the temperature of the Ar ₃ point, so it is necessary to add 0.001% or more. However, if added over 0.10%, the ductility and toughness are greatly reduced due to the formation of coarse oxides, so the content is made 0.001 to 0.10%.

  N is an element effective for refining austenite grains and strengthening precipitation in ferrite or bainite by combining with Al or Nb, so 0.0001% or more is added. Since the N amount is increased and the toughness is deteriorated, the upper limit is limited to 0.01%.

  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, addition of more than 2.0% lowers the weld zone HAZ toughness, so when adding Cr, the range is 0.01% or more and 2.0% or less.

Since Ti precipitates quickly in ferrite or bainite, it is effective for obtaining work-induced precipitation at an Ar ₃ point or lower, greatly contributes to strengthening the refinement of the structure, and further stabilizes ferrite. It is an element and contributes to increasing the temperature of the Ar ₃ point. In order to exert these effects, addition of 0.001% or more is necessary, but addition of more than 0.05% significantly reduces the toughness of the weld heat affected zone and saturates the strengthening. When adding Ti, the range is from 0.001% to 0.05%.

  Cu is an element effective in increasing the strength because it does not significantly increase the hardenability at the cooling rate of air cooling, and also brings about precipitation strengthening and solid solution strengthening. In order to exhibit these effects, addition of 0.01% or more is necessary. However, addition of over 2.0% decreases the toughness of the weld heat affected zone. % Or more and 2.0% or less.

  Mo is effective in increasing the strength by strengthening the structure, and in order to obtain a clear increase in strength, addition of 0.01% or more is necessary. However, if added over 1.0%, residual MA is generated. Since there are problems such as lowering YS and significantly reducing the toughness of the weld heat affected zone, when adding Mo, the range is 0.01% or more and 1.0% or less.

  V is effective for increasing the strength by precipitation strengthening, and 0.001% or more is necessary to obtain a clear increase in strength, but if added over 0.050%, the weld HAZ toughness is decreased. Therefore, when adding V, it is limited to 0.001% or more and 0.050% or less.

  W is an element effective for increasing the strength by solid solution strengthening and structure strengthening, and in order to obtain this effect, addition of 0.01% or more is required, but if added over 3.0%, the cost is increased. Therefore, when W is added, the range is 0.01% or more and 3.0% 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 made into the range of 0.001% or more and 0.010% or less.

  In addition, since the component system according to the present invention avoids the addition of elements that contribute to hardenability including C as much as possible, the toughness of the thermal influence during welding also shows very good results.

In the present invention, by setting the steel components as described above, it is possible to sufficiently raise the Ar ₃ point temperature and balance the precipitation strengthening amount.

Further, weld cracking sensitivity index P _CM is also suppressed to a level of 0.25% or less, and has a range of weld crack is prevented.
Equation _{1: P CM = [C]} + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Cr] / 20 + [Mo] / 15 + [V] / 10 + 5 [B]
In addition, [] in Formula 1 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 blank in Table 1 indicates that the component is not added. The steel slab is appropriately rolled, forged, or cut to produce a slab having a thickness of 60 to 500 mm. The slab is subjected to hot rolling, accelerated cooling, and tempering heat treatment under the conditions shown in Table 2. The steel plate was 6 to 50 mm thick.

Although the Ar ₃ point temperature varies slightly depending on the rolling method, it can be almost determined only by the steel component within the range of the present embodiment. The Ar ₃ point temperatures shown in Table 2 are calculated based on the following equation calibrated from experimental data and literature.
_{Ar 3 = 910-310 × [C]} -80 × [Mn] -15 [Nb] -50 × [Ni] -80 × [Mo] -15 × [Cr] -20 × [Cu] -15 × [W ] −1050 × [B] + 24.6 × [Si] + 700 × [P] + 60 × [Ti] + 190 × [V] + 40 × [Al]
In addition, [] in a formula represents the addition amount of each alloy element in the mass%.

  Table 2 shows the results of YS and TS measured for these thick steel plates by performing a tensile test in accordance with JIS Z 2241. As the tensile test piece, a No. 13B or No. 10 test piece based on JIS Z 2201 was used. In addition, Table 2 shows the results of an oblique y-type weld cracking test based on JIS Z 3158 for these thick steel plates. Moreover, about these thick steel plates, the 2 mmV notch test piece or subsize 2mmV notch based on JIS Z2202 which gave the thermal cycle equivalent to the heat affected zone 1mm position (HAZ1) at the time of submerged arc welding of heat input 10kJ / mm Table 2 shows the results of the Charpy test performed at -5 ° C using the test pieces. In addition, the target value of each characteristic is that YS is 650 MPa, TS is 780 MPa, weld heat affected zone toughness is absorption energy 47J or more, and the weld cracking test does not crack.

  From the results in 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 such as YS, TS, weld cracking, and weld heat affected zone toughness. On the other hand, it can be seen that the comparative steel deviating from the scope of the steel of the present invention has at least one or more basic characteristics such as YS, TS, weld cracking, and weld heat-affected toughness.

The results of Tables 1 and 2 are plotted in FIG. 1 with the amount of reduction at Ar ₃ or less plotted on the horizontal axis and YS and TS plotted on the vertical axis. In FIG. 1, the steel of the present invention is plotted with a circle and the comparative steel is plotted with a triangle, but the steels of the present invention all satisfy the target strength range. In the case of the comparative steel, the strength range is not satisfied, or the strength range is satisfied, but the toughness of the weld heat affected zone is poor or weld cracking occurs.

Cumulative rolling reduction and YS below Ar ₃ point of the steel according to an embodiment of the present invention, is a diagram showing the relationship between TS.

Claims (4)

% By mass
C: 0.005% or more, less than 0.030%,
Si: less than 0.05%,
Mn: 1.0% or more, 2.5% or less,
Nb: 0.02% or more, 0.08% or less,
Al: 0.001% or more, 0.10% or less,
N: 0.0001% or more, containing 0.01% or less, the welding crack sensitivity index P _CM represented by the following formula 1 is 0.25 or less, the component composition and the balance being Fe and unavoidable impurities When the steel slab is heated to 1020 ° C. or higher and 1300 ° C. or lower and then rolled, the cumulative rolling reduction in the range of 1020 ° C. or lower and over 920 ° C. is less than 60%, and the cumulative rolling reduction is less than the Ar ₃ point. 30% or more and 95% or less, and cooling after the end of rolling. A method for producing a high strength steel material having a tensile strength of 780 MPa class excellent in toughness of a weld heat affected zone.
Equation _{1: P CM = [C]} + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Cr] / 20 + [Mo] / 15 + [V] / 10 + 5 [B]
In addition, [] in Formula 1 represents the addition amount of each alloy element in mass%. Furthermore, in mass%,
Cr: 0.01% or more, 2.0% or less,
Ti: 0.001% or more, 0.05% or less,
Cu: 0.01% or more, 2.0% or less,
Mo: 0.01% or more, 1.0% or less,
V: 0.001% or more, 0.050% or less,
The tensile strength of 780 MPa class excellent in toughness of the weld heat-affected zone according to claim 1, characterized by containing one or more of W: 0.01% or more and 3.0% or less. Manufacturing method of high strength steel. Furthermore, in mass%,
Zr: 0.001 to 0.010,
Ca: 0.001 to 0.010,
Mg: 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 tensile strength of 780 MPa class excellent in toughness of the weld heat-affected zone according to claim 1, comprising one or more of the above. Furthermore, the tempering heat treatment is performed at a temperature of Ac ₁ point or less, and the tensile strength 780 MPa class high strength steel material excellent in toughness of the weld heat affected zone according to any one of claims 1 to 3. Production method.

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