JP2013151742A - High toughness and high tensile strength steel and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a thick and high tensile strength steel sheet excellent in strength and toughness of a base material and also excellent in toughness of a weld heat-affected zone, and an advantageous method for producing the same.SOLUTION: A thick and high tensile strength steel sheet excellent in toughness of a base material and of a weld heat-affected zone upon multi-layer welding has a chemical component including a basic chemical component of 0.005-0.05% of C, 0.05-0.3% of Si, 0.5-5% of Mn, 0.015% or less of P, 0.005% or less of S, 3% or less of Cr, 5% or less of Ni, 0.005-0.02% of Ti, 0.02-0.04% of Al, 0.007% or less of N, 0.0003-0.003% of B, and 0.0005-0.0030% of O, with the balance comprising Fe and unavoidable impurities, and also satisfies relations: Ceq≥0.65% and Pc≥5.5, when Ceqand Pc are defined by formula (1) and formula (2). In the production method, hot rolling with cumulative draft of at least 50% is performed, and then quenching and tempering treatment is performed.

Description

  The present invention relates to a high-strength steel plate used for steel structures such as ships, marine structures, pressure vessels, and penstocks, and a method for producing the same. In particular, the present invention relates to a high-toughness, high-tensile steel sheet excellent in the toughness of the weld heat-affected zone during multi-layer welding and a method for producing the same.

Steel materials used in ships, offshore structures, and pressure vessels are finished into structures of a desired shape by welding. These steels are required to be excellent not only in the base material toughness but also in the toughness of the weld heat affected zone in order to ensure the safety of the structure.
For the above steel, welding of thick steel plates is performed by multi-layer welding, but in the heat affected zone, it is easy to generate a local embrittlement region because it receives a complex heat history. A significant decrease in toughness is a problem in the two-phase region heating section of austenite. This is because, in such a region, an upper bainite structure accompanied with the generation of island martensite is formed, and the toughness is lowered.

  As a countermeasure to the above problem, Patent Document 1 discloses that the oxides such as Ti are finely dispersed in the steel to prevent coarsening of the austenite grains in the bond portion, thereby finely magnifying the ferrite grains and improving the toughness of the weld heat affected zone. Techniques for improving are disclosed. However, this technology is intended for relatively low-strength steel materials with ferrite as the parent phase, and the weld heat-affected zone of higher-strength materials has a structure that does not contain ferrite. The effect of improving the toughness of the heat affected zone is difficult to obtain.

  Further, the two-phase region heating part, that is, the region where the region heated to the vicinity of the melting point at the first welding becomes the two-phase region of ferrite and austenite by reheating by the subsequent welding is most brittle. This is because carbon concentrates in the austenite region transformed by the subsequent reheating, and this portion forms an upper bainite structure accompanied by island-like martensite formation during cooling, and remarkably deteriorates toughness. As a countermeasure against a welding heat affected zone represented by a two-phase zone heating zone, Patent Document 2 describes that island martensite generation is suppressed by lowering C and lowering Si, and Cu is added. Discloses a technique for ensuring the strength of a base material.

  Patent Document 3 discloses a technique for reducing the C and improving the weld heat affected zone toughness and increasing the strength with Cu. This prior art uses Cu precipitation due to aging treatment to increase the strength, but since a large amount of Cu is added, hot ductility is lowered and productivity is remarkably hindered. Patent Document 4 describes a steel sheet having a thickness of 690 MPa or more that satisfies the condition of Pcm ≦ 0.23 with respect to a steel sheet having a thickness of 45 mm or less in the examples. In order to achieve this, it is difficult to ensure the strength because the component is low as Pcm ≦ 0.23. The technique disclosed in Patent Document 5 requires heat treatment for a long time before hot rolling, resulting in a decrease in manufacturability.

  Furthermore, in the prior art, the following problems to be solved remain. For example, in the technique using Ti oxide, there is a problem that uniform fine dispersion of oxide in steel is difficult. Furthermore, with the increase in the size of the structure, the steel material used is required to have higher strength and thickness. Unlike the techniques of Patent Documents 2 and 3, it is effective to add a large amount of alloy elements to increase the strength and thickness. However, the addition of alloy elements has a problem of lowering the toughness of the weld heat affected zone. The technology of Patent Document 4 is a technology applicable to steel pipes that use a relatively thin plate in the thick plate field, and it is possible to re-use up to 1000 ° C in order to refine island martensite generated during welding. It cannot be applied to applications such as offshore structures that require thick and highly productive steel materials such as heating.

JP 2001-323336 A Japanese Patent Laid-Open No. 5-186823 Japanese Patent Laid-Open No. 2001-335484 Japanese Patent No. 2777538 JP-A-2005-213534

  Therefore, the object of the present invention is to solve the above-mentioned problems that have not been solved so far, even in a thick high-strength steel plate that has to increase the amount of addition of alloy elements, and excellent in the strength and toughness of the base material, An object of the present invention is to provide a high-strength high-strength steel plate having a thickness of 75 mm or more, which is excellent in the toughness of the weld heat affected zone, and an advantageous production method thereof.

The inventors have improved the toughness of a thick high-strength steel sheet (base material) to which strength is ensured by adding an alloy element, and a weld heat-affected zone formed when the thick high-strength steel sheet is multilayer-welded. Various comparative studies were conducted on methods for improving the toughness of (HAZ).
as a result,
1) The toughness of the base metal with a yield strength of 630 MPa or more is greatly affected by the amount of oxide in the steel. 2) The toughness deterioration of the weld heat affected zone during multi-layer welding is caused by embrittlement formed in the two-phase zone heating zone. We found that this was due to the generation of the organization.

  In order to reduce the amount of oxide in the steel of 1), it is effective to reduce the amount of oxygen in the steel. When the amount of oxygen in the steel is 0.0030% by mass or less, the toughness of the base material is greatly improved. It became clear to do.

  In addition, as a result of intensive studies on the method of improving the toughness of the brittle structure in the weld heat affected zone during multi-layer welding in 2) above, it is not sufficient to simply reduce C as in the prior art. It is necessary to reduce the size (area) of each island-like martensite generated in the phase zone heating section, and as an achievement means, Mn, Ni, Cr, Mo, V are added in appropriate amounts, We found that reducing C is effective.

Based on the above findings, the present inventors have completed the present invention.
That is, the gist of the present invention is as follows [1] to [5].
[1] By mass%, C: 0.005 to 0.05%, Si: 0.05 to 0.3%, Mn: 0.5 to 5%, P: 0.015% or less, S: 0.00. 005% or less, Cr: 3% or less, Ni: 5% or less, Ti: 0.005-0.02%, Al: 0.02-0.04%, N: 0.007% or less, B: 0.00. 0003-0.003%, O: 0.0005-0.0030% for each defined by the following formulas (1) and (2): Ceq ^IIW ≧ 0.65%, Pc ≧ 5.5 And the balance has a chemical component consisting of Fe and inevitable impurities, and the average area of individual island martensite formed in the weld heat affected zone during multilayer welding is 3 μm ² or less Thickness and toughness excellent in the strength and toughness of the base metal and the toughness of the heat affected zone during multi-layer welding Force steel plate.
Ceq ^IIW = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 (%) (1)
Pc = 2Mn + 3Ni + Cr + Mo + V-12.5 × C (%) (2)
[Each element symbol in a formula shows content (mass%) of each element. ]
[2] The chemical component further includes, in mass%, Cu: more than 0.2% and less than 0.5%, Mo: 1% or less, V: 0.2% or less, Nb: 0.1% or less A thick high-tensile steel sheet excellent in the strength and toughness of the base material described in [1] and the toughness of the weld heat-affected zone during multi-layer welding.
[3] The chemical component further contains at least one or two of Ca: 0.0005 to 0.003% and REM: 0.0003 to 0.003% by mass%. A thick high-tensile steel sheet excellent in the strength and toughness of the base material according to [1] or [2] and the toughness of the weld heat affected zone during multilayer welding.
[4] The slab having the chemical component according to any one of [1] to [3] is heated to Ac ₃ points to 1200 ° C., and hot-rolled so that the cumulative reduction ratio is 50% or more, Next, the strength of the base material, the toughness and the multilayer, characterized in that it is directly quenched from the temperature of Ar ₃ or higher until the center of the plate thickness is 350 ° C. or lower and then tempered at a temperature of 450 ° C. to 650 ° C. A method for producing a thick high-strength steel sheet that is excellent in the toughness of the heat-affected zone during welding.
[5] The slab having the chemical component according to any one of [1] to [3] was heated to Ac ₃ point to 1200 ° C. and hot-rolled so that the cumulative reduction ratio was 50% or more. After standing to cool, then heating again to Ac ₃ point to 1050 ° C, then rapidly cooling from Ar ₃ point or higher until the center of thickness is 350 ° C or lower, then tempering at 450 ° C to 650 ° C A method for producing a thick-walled, high-tensile steel sheet having excellent strength and toughness of a base material and toughness of a weld heat-affected zone during multilayer welding.

  By using the present invention, the base material has a high tensile strength of 720 MPa or more and a yield strength of 630 MPa or more, excellent toughness of the base material, and further to the toughness of the heat affected zone during multi-layer welding. In addition, it is possible to provide a thick high-tensile steel plate having a thickness of 75 mm or more and an advantageous manufacturing method thereof.

Hereinafter, the present invention will be described in detail.
First, the reasons for limiting the chemical components of the steel of the present invention will be described. In addition, all content of each element in a chemical component is the mass%.

・ C: 0.005 to 0.05%
C is an indispensable element for obtaining the strength required for structural steel, but when added in a large amount, the amount of island martensite generated in the weld heat affected zone increases, and island martensite further increases. The upper limit was set to 0.05% because the C concentration in the steel was increased to increase the hardness of the island martensite and reduce the toughness. On the other hand, if the addition amount is less than 0.005%, sufficient strength cannot be obtained, and a large amount of alloying element is required, resulting in an increase in production cost. Therefore, the lower limit is made 0.005%. Preferably, it is 0.01% to 0.05%.

・ Si: 0.05-0.3%
Si acts as a deoxidizer, and in the present invention, it is necessary to add 0.05% or more in order to carry out moderate deoxidation. However, if it exceeds 0.3%, the toughness of the base material is remarkably lowered. The formation of island martensite is promoted in the weld heat affected zone, and the weld heat affected zone toughness is significantly reduced. For this reason, the range of Si was made into 0.05%-0.3%.

Mn: 0.5-5%
Mn needs to be added in an amount of 0.5% or more from the viewpoint of securing the strength of the base material. On the other hand, if it is added more than 5%, the hardenability is excessively increased and the toughness of the weld heat affected zone is remarkably lowered, so it is necessary to make it 5% or less.

-P: 0.015% or less When P is contained exceeding 0.015%, the toughness of the base metal and the weld heat affected zone is remarkably lowered, so that it is limited to 0.015% or less.
S: 0.005% or less If S exceeds 0.005%, the toughness of the base metal and the weld heat affected zone is significantly reduced, so the content is made 0.005% or less.

-Cr: 3% or less Cr is an element effective for increasing the strength of the base material, but if added in a large amount, the toughness is reduced, so 3% or less. Preferably, it is 0.1% to 2.7%.
Ni: 5% or less Ni is a beneficial element that improves the strength of the steel and the toughness of the weld heat affected zone. However, since it is more expensive than other alloy elements, the upper limit is made 5%. Preferably it is 0.8 to 5%.

Ti: 0.005-0.02%
Ti forms Ti nitride in steel and reduces the amount of dissolved nitrogen, thereby suppressing precipitation of BN and sufficiently solidifying B in steel to ensure hardenability. Furthermore, Ti nitride is a stable precipitate even in the austenite temperature range, and it is possible to effectively suppress the coarsening of austenite in the weld heat affected zone. Therefore, it is necessary to add 0.005% or more of Ti. On the other hand, if the content is more than 0.02%, Ti nitride becomes coarse and lowers the toughness of the base metal and the weld heat affected zone, so it is necessary to limit it to 0.02% or less.

Al: 0.02% to 0.04%
Al needs to contain 0.02% or more in order to fully deoxidize molten steel. On the other hand, when the content is more than 0.04%, the amount of Al dissolved in the base material increases, and the base material toughness is lowered. Therefore, it is necessary to limit the content to 0.04% or less.

N: 0.007% or less N, when dissolved in the base material, significantly reduces the base material toughness, and further forms coarse carbonitrides in the weld heat affected zone to reduce the toughness. It is necessary to limit it to less than%.

・ B: 0.0003 to 0.003%
B has the effect of suppressing the ferrite transformation from the grain boundary by segregating at the austenite grain boundary and increasing the strength by increasing the bainite fraction, and it is necessary to add 0.0003% or more. However, if added over 0.003%, it precipitates as carbonitride, lowers the hardenability and lowers the toughness, so the upper limit is made 0.003%. Preferably it is 0.0005 to 0.0020%.

・ O: 0.0005 to 0.0030%
When O (oxygen) is present in the form of an oxide in the steel, the toughness of the base metal may be improved by suppressing the grain growth of austenite. However, in a high-tensile steel sheet having a yield strength of 630 MPa or more, Is high, the oxide tends to break, and the upper limit must be 0.0030%. On the other hand, if the oxygen amount is less than 0.0005%, the operational load is large, so the lower limit is made 0.0005%.

In addition to the above essential elements, the high-tensile steel of the present invention can contain at least one or more of Cu, Mo, V, and Nb for the purpose of further enhancing strength and toughness.
Cu: more than 0.2% and less than 0.5% Cu can improve the strength of the steel without impairing the toughness, but if added over 0.5%, it causes cracking on the surface of the steel sheet during hot rolling. Less than 5%. Further, when the addition amount is 0.2% or less, a sufficient increase in strength cannot be obtained. Therefore, when it is added, addition exceeding 0.2% is necessary.

Mo: 1% or less Mo is an element effective for increasing the strength of the base material. However, if added in a large amount, it causes an increase in hardness due to precipitation of alloy carbides and lowers toughness, so the content is made 1% or less.
V: 0.2% or less V is effective in improving the strength and toughness of the base material, and is effective in reducing solid solution N by being precipitated as VN. Since toughness is reduced by precipitation of hard VC, the content is made 0.2% or less. Preferably, it is 0.1% or less.
Nb: 0.1% or less Nb is an element effective for strengthening steel, but addition exceeding 0.1% significantly reduces the toughness of the weld heat affected zone during multi-layer welding, so 0.1% or less And

In addition to the above chemical components, the high-tensile steel of the present invention can contain at least one or two of Ca and REM for the purpose of further improving the material.
・ Ca: 0.0005 to 0.003%
When Ca is added in an amount of 0.0005% or more, harmful O and N are fixed as oxides and sulfides, and the steel material is improved. However, even if added over 0.003%, the effect is saturated, so the content is made 0.003% or less.
・ REM: 0.0003 to 0.003%
REM refers to rare earth metals including Ce and La. When REM is added in an amount of 0.0003% or more, an oxide and sulfide are formed in the steel in the same manner as Ca, and there is an effect of improving the material, but the effect is saturated even if it is added more than 0.003%. Therefore, it is limited to 0.0003 to 0.003%.

・ Ceq ^IIW ≧ 0.65%
In the present invention, the amount of C in the base material is reduced for the purpose of improving the toughness of the heat affected zone (HAZ) during multi-layer welding, but on the other hand, addition of an alloy element is necessary to ensure strength. ^Yes , if the components are added so that Ceq ^IIW defined by the following formula (1) satisfies the relationship of Ceq ^IIW ≧ 0.65%, the tensile strength of the base material is 720 MPa or more, and the yield strength is 630 MPa or more. The strength of the base material can be sufficiently secured.
Ceq ^IIW = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 (%) (1)
In addition, each element symbol in a formula shows content (mass%) of each element.

・ Pc ≧ 5.5
The deterioration in toughness in the vicinity of the bond portion of the weld heat affected zone during multi-layer welding is due to the formation of an embrittled structure including island martensite formed in the two-phase zone heating section during multi-layer welding. In order to improve the toughness of this embrittled structure, it is not sufficient to simply reduce C as in the prior art, and furthermore, the size (area) of the individual island martensite formed is reduced. It is necessary to reduce the hardness of the island martensite to reduce the hardness difference from the matrix, and as means for achieving this, Mn, Ni, Cr, Mo, V are added in appropriate amounts, and C is added. It is effective to reduce it, and it is necessary that Pc defined by the following formula (2) satisfies the relationship of Pc ≧ 5.5.
Pc = 2Mn + 3Ni + Cr + Mo + V-12.5 × C (%) (2)
In addition, each element symbol in a formula shows content (mass%) of each element.

Here, the meaning of the relationship of Pc ≧ 5.5 is as follows.
Since Mn and Ni are austenite stabilizing elements, by increasing the content of these elements, the increase in the concentration of C dissolved in austenite is suppressed, and further, carbide forming elements such as Cr, Mo, and V are added. The size of each of the island-like martensites generated in the weld heat affected zone can be refined by suppressing the precipitation of the precipitated carbides by the addition and concentration in the austenite.
Then, an appropriate amount of Mn, Ni, Cr, Mo, V is added so as to satisfy the relationship of Pc ≧ 5.5, and by reducing C, the average area of individual island martensite is 3 μm ² or less. At the same time, the hardness of the island-like martensite is reduced, and the hardness difference from the matrix structure can be reduced. As a result, the island martensite is less likely to be a starting point of fracture, and the toughness of the welded portion can be significantly improved.

In the present invention, the average area of the individual island martensite formed in the weld heat affected zone is the average value of the area of the individual island martensite observed in the cross section of the weld heat affected zone.
Specifically, for example, after the cross section of the weld heat affected zone is etched in two steps to reveal island martensite, the vicinity of the bond portion heated to the two-phase region is scanned with a scanning electron microscope (SEM). The average area of each island-like martensite can be measured by taking 10 fields of view at a magnification of 3000 and analyzing the images.

Next, the manufacturing process of the present invention will be described.
-Slab heating temperature: Ac ₃ points to 1200 ° C
A slab is produced from the molten steel having the above chemical components by a normal casting method such as a continuous casting method or an ingot-making method to obtain a rolled material.
The heating temperature of the slab needs to be Ac ₃ or more in order to sufficiently dissolve the additive element and reduce the rolling load. However, when the heating temperature exceeds 1200 ° C., austenite grains become coarse and sufficient toughness cannot be obtained. For this reason, the heating temperature of a slab shall be in the range of Ac _3- point-1200 degreeC.

In addition, Ac ₃ points | pieces are calculated from the following (3) Formula.
_{Ac 3 = 937.2-476.5C + 56Si-19.7Mn} -16.3Cu-
26.6Ni-4.9Cr + 38.1Mo + 124.8V + 136.3Ti-
19.1 Nb + 198.4Al + 3315B (° C.) (3)
In addition, each element symbol in (3) Formula shows content (mass%) of each element.

-Cumulative rolling reduction of hot rolling: 50% or more The slab heated to the above temperature range is sufficiently processed to the center of the plate thickness to refine the structure and improve strength and toughness. It is necessary to hot-roll so that it may become 50% or more.

・ Heat treatment after hot rolling (quenching): quenching until the center of the plate thickness is 350 ° C. or less Rapid quenching is performed as a heat treatment after hot rolling. In the present invention, either of the following 1) or 2) Cool down.
1) After hot rolling, quenching is performed directly from the quenching temperature of ₃ or more points of Ar until the center of the plate thickness is 350 ° C. or less (this process is referred to as “DQ-T”).
2) Allow to cool after hot rolling, reheat between Ac ₃ points to 1050 ° C, and perform quenching from the quenching temperature of Ar ₃ points or more until the center of the plate thickness is 350 ° C or less (this treatment "RQ-T").
Here, the reason why the rapid cooling is performed until the central portion of the plate thickness is 350 ° C. or less is to quench the entire steel.
In addition, in 2), the reheating temperature is set to 1050 ° C. or less because reheating at a high temperature exceeding 1050 ° C. significantly reduces the base material strength and toughness due to coarsening of austenite grains.

Ar ₃ points are calculated from the following equation (4).
Ar ₃ = 910-273C-74Mn-56Ni-16Cr-9Mo-5Cu (° C.)
... (4)
[Each element symbol in the formula (4) indicates the content (% by mass) of each element. ]

The temperature at the center of the plate thickness is obtained by simulation calculation or the like from the plate thickness, surface temperature, cooling conditions, and the like. For example, the plate thickness center temperature is obtained by calculating the temperature distribution in the plate thickness direction using the difference method.
The quenching method is generally water cooling industrially, but since it is desirable that the cooling rate be as fast as possible, the cooling method may be other than water cooling, for example, gas cooling.

-Tempering temperature: 450 ° C to 650 ° C
After the rapid cooling, tempering is performed at 450 ° C. to 650 ° C. in both cases 1) and 2) of the heat treatment.
After quenching, tempering at 450 ° C. to 650 ° C. is less effective for removing residual stress at temperatures lower than 450 ° C., while various carbonitrides precipitate at temperatures higher than 650 ° C. This is because it becomes coarse and the strength and toughness are greatly reduced.

In addition, industrially, it may be repeatedly quenched for the purpose of toughening the steel, but may be repeatedly quenched in the present invention, but is heated to Ac ₃ point to 1050 ° C. in the final quenching. Thereafter, it is necessary to rapidly cool the sheet thickness center portion to 350 ° C. or lower, and then to temper at 450 ° C. to 650 ° C.

  As described above, in the production of the high-strength steel sheet of the present invention, it is important to perform quenching and tempering, and this heat treatment can produce steel having excellent strength and toughness.

Using a slab adjusted to various chemical components (steel numbers 1 to 22) shown in Table 1 as a raw material, the slab is heated to 1050 to 1200 ° C. and subjected to hot rolling with a cumulative reduction ratio of 50 to 70%, and A thick steel plate (sample Nos. 1-27) having a plate thickness of 75-155 mm was manufactured under the manufacturing conditions shown in Table 2.
Here, steel No. in Table 1 1 to 14 are steels of the present invention that satisfy the conditions of the chemical components of the present invention. 15-22 are comparative steels that deviate from the conditions of the chemical components of the present invention.
The steel plate thus obtained was subjected to a tensile test and a Charpy test.
In the tensile test, a JIS No. 4 tensile test piece was taken in the rolling direction from a position of 1/4 of the thickness of each steel plate, and the tensile strength (TS) and the yield strength (YP) were measured.
In the Charpy test, a V-notch test piece was sampled in the rolling direction from a position of ¼ of the thickness of each steel plate, and the absorbed energy (vE- ₆₀ ) at −60 ° C. was obtained as an average value of three times. The toughness of was evaluated.

In addition, X-groove (groove angle 45 °) processing is performed on the steel plates taken from each steel plate, and a submerged arc welding with a heat input of 50 kJ / cm is performed to produce a multi-layer welded joint. V-notch specimens are taken in the rolling direction, the bond part is taken as the notch position, the absorbed energy at -60 ° C. (vE- ₆₀ ) is obtained as an average of three times, and the toughness of the weld heat affected zone (HAZ) is determined. evaluated.

The average area of the individual island martensite formed in the heat affected zone is the bond that is heated in a two-phase region after the cross section of the weld heat affected zone is etched in two steps to reveal the island martensite. The vicinity of the part was photographed with 10 fields of view at a magnification of 3000 using a scanning electron microscope (SEM), and image analysis was performed to measure the area of each island martensite, and the average value was obtained.

The above production conditions and test results are shown in Table 2.
From these results, the steel materials of the present invention examples (sample Nos. 1 to 15) all have a base material strength of TS (tensile strength) ≧ 720 MPa, YP (yield strength) ≧ 630 MPa, and a base material toughness of vE ₋₆₀ ≧ 120 J. It can be seen that both the strength and toughness of the base material are excellent. Furthermore, in the welding heat affected zone (HAZ), vE- ₆₀ ≧ 70 J, and it can be seen that the weld heat affected zone also has good toughness.

On the other hand, sample no. In Comparative Examples 16 to 23 (steel numbers 15 to 22), the base material has a tensile strength of TS <720 MPa or a yield strength of YP <630 MPa, or the base material has a toughness of vE- ₆₀ <120 J, or welding heat. In the affected zone (HAZ), vE- ₆₀ <70 J, and it is recognized that any one or more characteristics of the strength, toughness of the base metal and the toughness in the weld heat affected zone are inferior.

In addition, sample No. In Comparative Examples 24-27, the steel numbers are 8, 10, 13, and 13 and the chemical components are within the scope of the present invention, but the manufacturing conditions are outside the scope of the present invention, and the base material characteristics are low. It is recognized that at least one of the characteristics of the strength and toughness of the base material is inferior.

Claims (5)

In mass%, C: 0.005 to 0.05%, Si: 0.05 to 0.3%, Mn: 0.5 to 5%, P: 0.015% or less, S: 0.005% or less Cr: 3% or less, Ni: 5% or less, Ti: 0.005-0.02%, Al: 0.02-0.04%, N: 0.007% or less, B: 0.0003-0 0.003% and O: 0.0005 to 0.0030% satisfy the relations of Ceq ^IIW ≧ 0.65% and Pc ≧ 5.5 for each of the formulas (1) and (2) defined below. And the remainder has a chemical component composed of Fe and inevitable impurities, and the average area of individual island martensite formed in the weld heat affected zone during multilayer welding is 3 μm ² or less. Thick high-strength steel with excellent strength and toughness of the base metal and toughness of the heat affected zone during multi-layer welding .
Ceq ^IIW = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 (%) (1)
Pc = 2Mn + 3Ni + Cr + Mo + V-12.5 × C (%) (2)
[Each element symbol in a formula shows content (mass%) of each element. ]   Further, the chemical component is in mass%, Cu: more than 0.2% and less than 0.5%, Mo: 1% or less, V: 0.2% or less, Nb: 0.1% or less The thick high-tensile steel sheet excellent in the strength, toughness of the base material and the toughness of the weld heat-affected zone at the time of multilayer welding, comprising at least one kind or two or more kinds.   The chemical component further contains at least one or two of Ca: 0.0005 to 0.003% and REM: 0.0003 to 0.003% by mass%, A thick high-tensile steel sheet excellent in strength and toughness of the base material according to claim 1 or 2 and toughness of a weld heat affected zone during multilayer welding. The slab having the chemical component according to any one of claims 1 to 3 is heated to Ac ₃ point to 1200 ° C, hot-rolled so that the cumulative rolling reduction is 50% or more, and then Ar ₃ From the temperature above the point, quenching directly until the center of the plate thickness reaches 350 ° C. or less, and then tempering at a temperature of 450 ° C. to 650 ° C., strength of the base material, toughness and multi-layer welding A method for producing a thick high-strength steel sheet with excellent toughness of the heat affected zone. The slab having the chemical component according to any one of claims 1 to 3 is heated to Ac ₃ point to 1200 ° C, and hot-rolled so that the cumulative reduction ratio is 50% or more, and then allowed to cool. Then, after again heating to Ac ₃ point to 1050 ° C., then rapidly cooling from the temperature of Ar ₃ point or more until the center of thickness is 350 ° C. or less, and then tempering at 450 ° C. to 650 ° C. A method for producing a thick-walled high-tensile steel sheet having excellent strength and toughness of a base material and toughness of a weld heat-affected zone during multilayer welding.