JP2007302977A - Method for manufacturing high-strength steel of tensile strength of 570 mpa class having excellent toughness of weld heat affected zone - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturinga high-strength steel of tensile strength of 570 MPa class having excellent toughness of a weld heat affected zone with unexperienced low alloy component and at high productivity. <P>SOLUTION: In the method for manufacturing the high-strength steel of tensile strength of 570 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, 0.001-0.105% Si, 0.01-2.0% Mn, 0.001-0.10% Nb, 0.001-0.10% Al, and 0.0001 to 0.010% 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 15-95%, 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 570 MPa class which is excellent in the toughness of a weld heat affected zone 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 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 sheet is rolled, reheated and quenched offline, and further subjected to tempering heat treatment. Patent Documents 3, 4 and 5 disclose inventions in which a steel sheet is 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, so that a decrease in productivity becomes a problem. For this reason, development aimed at further improving productivity is being carried out. For example, Patent Document 5 discloses an invention in which a heat treatment time is shortened by using an induction heating method in a heating furnace for performing a tempering heat treatment.

In addition, in order to improve productivity, manufacturing processes that omit the tempering heat treatment itself have been developed. For example, Patent Document 6 discloses an invention in which rolling is performed at 750 ° C. or higher and accelerated cooling is performed to a temperature of 450 ° C. or lower in a method for manufacturing a high-strength steel material. Further, in Patent Document 7, in a method for producing a high-strength steel material, rolling is performed so that the cumulative reduction ratio 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 Document 8 discloses an invention in which, in a method for producing a high-strength steel material, rolling is performed at an Ar ₃ point or higher, then accelerated cooling is performed, and accelerated cooling is stopped in a temperature range of 580 to 450 ° C. . Further, in Patent Document 9, in a method for producing a high-strength steel material, rolling is performed at 16% or more and 30% or less in the range of Ar ₃ −10 to Ar ₁ + 10 ° C., Ar ₁ point +50 or less, Ar ₁ point An invention is disclosed in which finish rolling is performed in a range of + 10 ° C. or higher and then air cooling is performed. Patent Document 10 discloses an invention in which rolling is performed in a temperature range of 800 ° C. or higher and then cooled in a method for producing a high-strength steel material. Furthermore, Patent Document 11 discloses an invention in which, in a method for producing a high-strength steel material, the total reduction amount at 800 ° C. or less is set to 5 to 15 mm, and the rolling is finished at an Ar ₃ point or less.

JP-A-01-149923 JP 52-081014 A JP-A-63-033521 Japanese Patent Laid-Open No. 02-205627 JP 2002-317227 A JP 2004-232056 A Japanese Patent Laid-Open No. 2005-126819 Japanese Patent No. 2776174 Japanese Patent Laid-Open No. 08-188823 JP-A-11-269602 Japanese Patent Laid-Open No. 05-171271

However, in the inventions disclosed in the above Patent Documents 1 to 4, offline tempering heat treatment is necessary in the manufacturing process of the steel sheet, and there is a problem in that a decrease in productivity is inevitable. The invention described in Patent Document 5 is advantageous in that productivity can be improved regardless of the strength range in order to perform online rapid tempering by induction heating. There is a problem that a very large capital investment is required to introduce the system. Further, in the invention described in Patent Document 6, the C content is limited to 0.03% or less in order to suppress the change in strength due to the variation in the cooling rate of the accelerated cooling and the stop temperature, so as to supplement the hardenability. In addition, Mn is 1.5% or more, Mo is 0.2% or less, and B is 0.0003% or more. As a result, there is a problem that the alloy cost is increased. Further, in the invention described in Patent Document 7, 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 of the accelerated cooling or the stop temperature. 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. In addition, in the invention described in Patent Document 8, in order to ensure strength, the C amount is 0.01 to 0.20% by definition, and 0.04 to 0.18% in the examples. Due to the large amount, this component system has a problem that the strength of the steel material is not necessarily stabilized due to the cold speed of the accelerated cooling and the fluctuation of the stop temperature, and is difficult to apply to the actual manufacturing process. In addition, in the invention described in Patent Document 9, the alloy addition amount is large, such as defining C amount to be 0.10% or more, Mo to be 0.10% or more, and B to be 0.0005% or more in order to ensure strength. Therefore, there are a problem that the alloy cost is high, a problem of weld cracking, and a problem that the toughness of the heat-affected zone during welding is lowered. Moreover, in the invention described in Patent Document 10, since finish rolling is performed at Ar ₃ points or more, there is a problem that it is necessary to add an excessive amount of alloy elements more than necessary in order to secure YS and TS. Further, in the invention described in Patent Document 11, an alloy such as C amount of 0.03 to 0.20%, Si amount of 0.10 to 0.60%, Mn of 0.90 to 2.50%, etc. Since there are many addition amounts, there exists a problem in a weld cracking property, and also the problem that the toughness of the heat affected zone at the time of welding falls.

  Therefore, 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 570 MPa class that 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 conducted extensive research and development through experiments and analyzes on a method for producing a high strength steel material having a tensile strength of 570 MPa class, which can ensure high productivity by air cooling after hot rolling. As a result, in the process of manufacturing high-strength steel materials as they are hot-rolled, the factors governing YS and TS of steel are clarified, and high-strength steel materials are manufactured with low productivity and higher productivity than before. The knowledge to do was obtained.

That is, in producing high strength steel materials with a tensile strength of 570 MPa or higher, in order to ensure a high YS of 450 MPa or higher in hot rolling, the strength by refining the structure by controlled rolling immediately above the Ar ₃ point Rather than the increase, the work strengthening by rolling the transformed structure such as ferrite, bainite, pearlite or the like below the Ar ₃ point is more effective in actual production.

In general, in the production of high strength steel materials having a tensile strength of 570 MPa class or higher, the alloy elements are designed with the aim of improving the hardenability of the steel. However, many alloying elements that improve hardenability reduce the Ar ₃ point. Therefore, when using the processing strengthening by rolling at Ar ₃ or less, there is a problem that the time for waiting for the temperature of the steel slab to decrease is increased, resulting in a decrease in productivity, and the rolling temperature. Along with the decrease, there is a problem that the acoustic anisotropy at the time of ultrasonic flaw detection increases.

Further, as a result of studies by the present inventors, when the Ar ₃ point is lowered, the amount of precipitation strengthening due to work-induced precipitation of alloy elements is reduced, so that the amount of work strengthening due to rolling below the Ar ₃ point is reduced. It turns out. Furthermore, in the case of a component system having a low Ar ₃ point, the upper yield point tends to disappear. However, in the case of such a component system, even if rolling is performed largely below the Ar ₃ point, the inventors have studied. It is known that the upper yield point is difficult to recover. Therefore, the hot rolling conditions for ensuring high YS become severe.

In addition, it is known that addition of a large amount of alloying elements that enhance hardenability increases the amount of island-shaped MA produced in the structure, which leads to a decrease in YS. Therefore, in order to produce high-strength steel materials with a tensile strength of 570 MPa or higher with a low alloy system and high productivity that are not conventional, unlike conventional high-strength steel component designs, elements that improve hardenability are added. In order to increase the Ar ₃ point while avoiding it as much as possible, to suppress the formation of island MA, and to increase the amount of strengthening below the Ar ₃ point, the rate of work-induced precipitation in ferrite and bainite is It was concluded that positive utilization of fast alloying elements is very effective.

In accordance with such knowledge, the present inventors, in a high-strength steel material having a tensile strength of 570 MPa, have a component composition for increasing the temperature of the Ar ₃ point to a level that is not assumed by conventional high-strength steel materials, Extensive examination of specific requirements for satisfying necessary properties such as necessary conditions, not only achieving a tensile strength of 570MPa class, but also good welding with unprecedented low alloy components and high productivity 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: 0.001% or more, 0.10% or less, Mn: 0.01% or more, 2.0% or less, Nb : 0.001% or more, 0.10% or less, Al: 0.001% or more, 0.10% or less, N: 0.0001% or more, 0.010% or less, represented by the following formula 1 that weld crack sensitivity index P _CM is 0.25 or less, and the Ar ₃ point is 780 ° C. or higher, the steel slab having the component composition and the balance being Fe and unavoidable impurities, 1020 ° C. or more, the 1300 ° C. or less When heating and then rolling, the cumulative rolling reduction in the range of 1020 ° C. or lower and over 920 ° C. is set to less than 60%, and the cumulative rolling reduction at less than Ar ₃ point is set to 15% or more and 95% or less. , Toughness of heat affected zone of welding, characterized by cooling after rolling Excellent tensile strength of 570MPa grade high strength method of manufacturing the steel.
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%. The following formula 2 is also the same.
(2) Further, by mass, Cr: 0.01% or more, 3.0% or less, Ti: 0.001% or more, 0.010% or less, Cu: 0.01% or more, 1.0% or less Ni: 0.01% or more, 2.0% or less, Mo: 0.01% or more, 0.50% or less, V: 0.001% or more, 0.050% or less, W: 0.01% or more The method for producing a high-strength steel material having a tensile strength of 570 MPa class excellent in toughness of the weld heat-affected zone as described in (1) above, comprising one or more of 3.0% or less .
(3) Furthermore, it contains B: 0.0001% or more and 0.0050% or less by mass% so as to satisfy the relationship of the following formula 2. The above (1) or (2) A method for producing a high-strength steel material having a tensile strength of 570 MPa that is excellent in the toughness of the weld heat affected zone.
Formula 2: [B] /10.8≦ [N] /14.0- [Ti] /47.9
(4) Further, by 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 : It is excellent in the toughness of the weld heat affected zone according to any one of the above (1) to (3), characterized by containing one or more of 0.001 to 0.010. A method for producing a high strength steel material having a tensile strength of 570 MPa.
(5) 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 (4) above 570 MPa class high strength steel manufacturing method.

  According to the present invention, it becomes possible to obtain a high-strength steel sheet having a tensile strength of 570 MPa class, which is excellent in the 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, as described above, in order to secure YS and TS of a high strength steel having a tensile strength of 570 MPa, the purpose is to actively use two-phase rolling, For the purpose of strengthening by work-induced precipitation, reduction of acoustic anisotropy, and improvement of productivity, it is characterized by realizing a higher Ar ₃ point and a lower alloy composition than conventional ones together with rolling conditions.

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

In the present invention, the Ar ₃ point is defined as 780 ° C. or higher. In the present invention, it is one of the characteristics to use rolling at an Ar ₃ point or less. If the Ar ₃ point is low, the time for waiting for the temperature of the steel material to fall below the Ar ₃ point becomes long, and the productivity is hindered. It will be. When the Ar ₃ point is 780 ° C. or higher, rolling with good productivity can be performed without causing such a problem.

  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 or Ti that precipitates most rapidly 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. The precipitation of Nb and Ti at this rolling stage is promoted by rolling strain. However, when precipitation of Nb and Ti occurs during rough rolling in high-temperature austenite, these precipitates rapidly become coarse, resulting in useless precipitation that does not contribute to an increase in strength after steel sheet production. This is clarified by the invention described in Item 7. 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 7 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%.

Next, regarding the level of the Ar ₃ point, which is the point of the present invention, and the magnitude of work-induced precipitation below the Ar ₃ point, these are strongly influenced by the hot rolling pass schedule.

Ar ₃ point becomes higher as the amount of rolling strain increases, with the upper limit being Ae ₃ point. Therefore, in order to increase the amount of strain just above Ar ₃ point, it is desirable to increase the cumulative reduction ratio at just above the Ar ₃ point, it is preferable to perform rolling at least 10%, which is not always necessary condition is not.

Regarding the reduction in the temperature range below Ar ₃ point, the strength increases as the amount of reduction increases, but if the cumulative reduction amount is less than 15%, the strength cannot be sufficiently increased, and if it exceeds 95%, Since the strengthening is saturated, the cumulative reduction amount is defined as 15% 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, it is possible to cool in the air after rolling, but further accelerated cooling is performed thereafter. It is possible to further increase the strength.

  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.

Further, the strength can be further increased and stabilized by performing a tempering heat treatment after rolling, after cooling in the air, or after performing accelerated cooling. 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 suppresses the addition amount to less than 0.030% in order to increase the temperature of the Ar ₃ point, to improve the weld HAZ (heat affected zone) toughness, and to suppress coarse precipitation of Nb in austenite. There is a need. If the C addition amount is suppressed to less than 0.005%, the strength decreases due to a decrease in the precipitation amount of carbide formed with other alloy elements such as Nb and Ti. is required.

  Si is an element effective for increasing the strength, and is added in an amount of 0.001% or more. In particular, in order to reduce the amount of residual MA that lowers YS, it is necessary to limit it to 0.10% or less.

Mn is an element effective for increasing the strength and is added in an amount of 0.01% or more. However, since it is desirable to suppress the amount of addition because it lowers the Ar ₃ point, it is limited to 2.0% or less.

Nb is effective for obtaining work-induced precipitation at an Ar ₃ point or less because precipitation in ferrite or bainite is fast, and also contributes to refinement of the structure. The above addition is performed. However, if the addition exceeds 0.10%, the weld zone HAZ toughness is remarkably lowered, so the content is limited to 0.10% or less.

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 precipitation strengthening in ferrite or bainite by combining with Al or Nb, so 0.0001% or more is added. Since the amount of N is increased and the toughness is deteriorated, the upper limit is limited to 0.010%.

  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 3.0% lowers the weld zone HAZ toughness, so when adding Cr, the range is 0.01% or more and 3.0% or less.

Since Ti precipitates quickly in ferrite or bainite, it is effective for obtaining work-induced precipitation below the Ar ₃ point, 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, 0.001% or more must be added. However, if over 0.010% is added, the toughness of the weld heat affected zone is remarkably reduced. The range is 0.001% or more and 0.010% or less.

  Cu is an element effective in increasing the strength because it does not significantly increase the hardenability at the air-cooled cooling speed and also brings about precipitation strengthening and solid solution strengthening. In order to exert these effects, addition of 0.01% or more is necessary, but addition of more than 1.0% lowers the toughness of the weld heat affected zone, so when adding Cu, 0.01% % Or more and 1.0% or less.

  Ni has the effect of increasing strength and toughness, and in order to obtain these effects, it is necessary to add 0.01% or more, but the addition exceeding 2.0% increases the cost. When added, the content is in the range of 0.01% to 2.0%.

  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 required. However, if added over 0.50%, 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 0.50% 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.

B suppresses the generation of ferrite formed on the prior austenite grain boundaries of the weld heat affected zone when performing high heat input welding with a heat input exceeding 5 kJ / mm on the steel material, and BN precipitates in the grains of the prior austenite By forming an object and promoting the formation of ferrite to lower the strength, there is an effect of improving the weld zone toughness. In order to obtain these effects, addition of 0.0001% or more is necessary, but excessive B exceeding 0.0050% deteriorates toughness, so the upper limit is limited to 0.0050%. Further, when B is dissolved in austenite at the time of manufacturing the steel material, the Ar ₃ point is lowered even at the cooling rate of air cooling. Therefore, for B, the upper limit is limited to 0.0050% and the following formula: It is necessary to limit the addition amount as shown in FIG. Therefore, when adding B, it is made into the range with which 0.0001% or more and 0.0050% or less and Formula 2 are satisfy | filled.
Formula 2: [B] /10.8≦ [N] /14.0- [Ti] /47.9

  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, 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 heat-affected zone during welding also shows very good results. Further, the formula is kept to weld cracking sensitivity index P _CM also: 0.25% level shown in 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 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 450 MPa, TS is 570 MPa, the weld heat affected zone toughness is an absorption energy of 100 J or more, and the weld crack test does not crack.

  From the results of Tables 1 and 2, it can be seen that the component composition and the production method according to the method of the present invention all show good results such as YS, TS and weld cracks. 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 and weld split.

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 (5)

% By mass
C: 0.005% or more, less than 0.030%,
Si: 0.001% or more, 0.10% or less,
Mn: 0.01% or more, 2.0% or less,
Nb: 0.001% or more, 0.10% or less,
Al: 0.001% or more, 0.10% or less,
N: 0.0001% or more, containing 0.010% or less, the welding crack sensitivity index P _CM represented by the following formula 1 is not less than 0.25, and the Ar ₃ point is 780 ° C. or higher, the balance When a steel slab having a composition composed of Fe and inevitable impurities 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 set to less than 60%. , Characterized in that the cumulative rolling reduction at less than Ar ₃ point is 15% or more and 95% or less, and is cooled after the end of rolling, and has a tensile strength of 570 MPa class which is excellent in the toughness of the weld heat affected zone. Steel manufacturing method.
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%. The following formula 2 is similar. Furthermore, in mass%,
Cr: 0.01% or more, 3.0% or less,
Ti: 0.001% or more, 0.010% or less,
Cu: 0.01% or more, 1.0% or less,
Ni: 0.01% or more, 2.0% or less,
Mo: 0.01% or more, 0.50% or less,
V: 0.001% or more, 0.050% or less,
The tensile strength of 570 MPa, which is 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%,
B: 0.0001% or more and 0.0050% or less is contained so as to satisfy the relationship of the following formula 2. Tensile strength excellent in toughness of weld heat affected zone according to claim 1 or 2 570 MPa class high strength steel manufacturing method.
Formula 2: [B] /10.8≦ [N] /14.0- [Ti] /47.9 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 manufacturing method of the tensile strength 570 MPa class high strength steel materials excellent in the toughness of the welding heat affected zone of any one of Claim 1 thru | or 3 characterized by including 1 type or 2 types or more of these . Furthermore, tempering heat treatment is performed at a temperature of Ac ₁ point or less, and the tensile strength 570 MPa class high strength steel material excellent in toughness of the weld heat affected zone according to any one of claims 1 to 4. Production method.

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