JP4969275B2 - High tensile steel plate with excellent toughness of weld heat affected zone - Google Patents

Description

  The present invention relates to a steel sheet applied to a welded structure such as a bridge, a high-rise building, or a ship, and in particular, the toughness of a weld heat affected zone (hereinafter sometimes simply referred to as “HAZ”) after high heat input welding. It is related to the thick steel plate excellent in.

  In recent years, with the increase in size of the above various welded structures, it is inevitable to weld thick steel plates having a plate thickness of 50 mm or more. For this reason, in all fields, high heat input welding is aimed at for the purpose of improving welding construction efficiency and reducing construction costs.

  However, when high heat input welding is performed, the HAZ is heated to the high temperature austenite region and then gradually cooled. Therefore, it is caused by austenite grain growth during heating and generation of grain boundary ferrite from the austenite grain boundary during annealing. There is a problem that the structure of the HAZ becomes coarse and the toughness of the portion tends to deteriorate. For these reasons, it is desired to establish a technique for maintaining the toughness in HAZ (hereinafter sometimes referred to as “HAZ toughness”) at a high level even when high heat input welding is performed.

  A typical technique applied to ensure HAZ toughness during high heat input welding is a structure refinement technique by promoting intragranular transformation starting from inclusions such as oxides, sulfides or nitrides. In this technique, during the cooling after the end of welding, a fine transformation structure is developed by inclusions present in the grains, thereby suppressing the formation of coarse grain boundary ferrite, thereby ensuring the HAZ toughness.

  Among these techniques, for example, as a technique utilizing an oxide, Patent Document 1 discloses that a Ti-containing oxide in which MnS is complex-precipitated is finely dispersed by controlling an O concentration and a Ca concentration, and is used as a nucleus. A technique for promoting the intragranular ferrite transformation (that is, suppressing the formation of coarse grain boundary ferrite) has been proposed. Further, in Patent Document 2, a high strength steel for welding excellent in HAZ toughness is produced by generating a composite of Ti-containing oxide and MnS that becomes an intragranular ferrite nucleus in a system in which Ti and Mg are added in combination. Obtaining techniques are disclosed.

  As a technique utilizing sulfide, for example, Patent Document 3 discloses a high tensile strength for welding excellent in HAZ toughness by dispersing Mn sulfide containing 1 to 49 atomic% of Ca and promoting intragranular ferrite transformation. Techniques for obtaining steel have been proposed.

  Further, as a technique using nitride, for example, Patent Document 4 proposes a technique for improving HAZ toughness by using BN obtained by composite precipitation of TiN or the like as a nucleus of ferrite transformation.

In order for inclusions such as oxides, sulfides and nitrides to function as nuclei for intragranular transformation, a larger diameter is advantageous, and inclusions having a relatively small diameter do not contribute to intragranular transformation. Become. However, in the techniques proposed so far, it cannot be said that there are sufficient inclusions functioning as the nucleus of intragranular transformation, and the necessary HAZ toughness is not necessarily obtained.
Japanese Patent No. 3155104 Japanese Patent No. 3256118 JP 2003-321728 A JP-A-61-253344

  The present invention has been made paying attention to the circumstances as described above, and its purpose is to appropriately adjust the chemical composition and to change the form and number of inclusions such as oxides, sulfides or nitrides. An object of the present invention is to provide a high-strength thick steel plate capable of ensuring good HAZ toughness even in high heat input welding by appropriately controlling.

The high-tensile steel plate of the present invention that has achieved the above-mentioned object is C: 0.03 to 0.12% (meaning “mass%”, the same applies hereinafter), Si: 0.40% or less (0% Not included), Mn: 1.0 to 2.0%, P: 0.03% or less (not including 0%), S: 0.001 to 0.025%, Al: 0.05% or less (0 %), Ti: 0.005 to 0.10%, N: 0.0040 to 0.030% and O: 0.0005 to 0.010%, respectively, the balance being iron and inevitable impurities An oxide having a minimum curvature radius of 0.1 to 2.0 μm and an equivalent circle diameter of 0.2 to 5 μm. sulfides and inclusions consisting of one or more nitride, have a gist in that there more than 100 in the observation field of view 1 mm ² per Is shall.

  The “equivalent circle diameter” refers to the diameter of a circle that is assumed to have the same area by paying attention to the size of inclusions such as oxide, sulfide, or nitride. The inclusions targeted in the present invention include not only inclusions consisting of any one of oxides, sulfides, and nitrides, but also those in which two or more of these are complex precipitated.

In the high-tensile thick steel plate of the present invention, it is preferable that the relationship of the following formula (1) is satisfied. By satisfying these requirements, inclusions of the above-described form can be easily dispersed. it can.
0.05 ≦ 9 × [O] / ([S] + 10 × [N]) ≦ 1.20 (1)
However, [O], [S] and [N] indicate the contents (mass%) of O, S and N, respectively.

  In the high-tensile thick steel plate of the present invention, if necessary, (a) REM: 0.0001 to 0.05%, (b) Zr: 0.0001 to 0.05%, (c) Mg 0.0001 to 0 0.005% and / or Ca: 0.0001-0.005%, (d) Cu: 0.1-1.5%, Ni: 0.1-1.5%, Cr: 0.1-1. 1 or more types selected from the group consisting of 5% and Mo: 0.1-1.5%, (e) Nb: 0.01-0.06 and / or V: 0.01-0.1%, ( f) It is also useful to contain B: 0.001 to 0.005%, etc., and the inclusion of such an element will further improve the properties of the thick steel plate depending on the type.

  According to the present invention, the chemical composition of the steel sheet falls within an appropriate range, and the form of inclusions containing at least an oxide, sulfide or nitride, and the number of the inclusions are controlled appropriately, thereby allowing intragranular transformation. The number of inclusions contributing to the increase in the thickness of the steel sheet can be increased, and a high-tensile steel plate with improved HAZ toughness can be realized.

  In general, inclusions having a large diameter are advantageous for intragranular transformation starting from inclusions. Therefore, inclusions having a relatively small diameter do not contribute to intragranular transformation, and have sufficient HAZ toughness. Will not be obtained. Under such an idea, the inventors of the present invention are concerned with the relationship between the form of inclusions such as oxides, sulfides, and nitrides and intragranular transformation in order to achieve excellent HAZ toughness in high heat input welding. Considered from various angles.

  As a result, the minimum curvature radius of the concave portion is 0.1 to 2 among the inclusions in which many irregularities are formed on the surface by complex precipitation of secondary inclusions such as oxides, sulfides, and nitrides generated during casting. It was found that inclusions with a diameter of 0.0 μm and an equivalent circle diameter of 0.2 to 5 μm function effectively as the starting point of intragranular transformation. And what controlled so that these inclusions existed in predetermined amount discovered that the high-tensile steel plate for welding which exhibits the outstanding HAZ toughness was realizable, and completed this invention.

  In order to form the above irregularities on the surface of inclusions, secondary inclusions such as sulfides and nitrides that are complex-precipitated on the surface of inclusions such as oxides, sulfides and nitrides during the solidification process during casting. From such a viewpoint, the cooling time in the temperature range of 1450 to 1500 ° C. where solidification proceeds during casting may be controlled to 60 to 300 seconds. That is, by cooling the temperature range within a predetermined time, the complex precipitation form of secondary inclusions is controlled, and irregularities are effectively formed. If the cooling time is shorter than 60 seconds, a sufficient amount of composite precipitation cannot be secured, and if it is longer than 300 seconds, the composite precipitates so that secondary precipitates uniformly cover inclusions such as oxides, sulfides, and nitrides. And unevenness cannot be obtained.

In order to obtain a high-tensile thick steel plate having excellent HAZ toughness, the inclusions having the above-described form may be described as 100 pieces in the steel per 1 mm ² observation field (hereinafter referred to as “100 pieces / mm ² ”). ) It is necessary to exist above. The size of the inclusions targeted in the present invention is a circle equivalent diameter of 0.2 to 5.0 [mu] m, but if this size is smaller than 0.2 [mu] m, the necessary recesses are not sufficiently formed. On the other hand, if it exceeds 5.0 μm, it becomes the starting point of brittle fracture and decreases the HAZ toughness. In addition, when there are less than 100 inclusions / mm ² in the steel having irregularities, a sufficient intragranular transformation structure cannot be obtained, and the HAZ toughness is lowered.

  The reason why inclusions with recesses as described above function effectively in promoting intragranular transformation has not been fully elucidated, but it is probably thought that the ferrite transformation nucleation energy in the recesses is reduced. I was able to. From this point of view, the minimum radius of curvature of the recess is defined as 0.1 μm or more. However, when the minimum radius of curvature of the recess exceeds 2.0 μm, the effect of the recess is reduced because it is close to a smooth surface. In addition, the minimum curvature radius of a recessed part means the shape parameter measured by the below-mentioned method, and when this value is large, the form of an unevenness | corrugation will become flat. In addition, since the concave portion has a problem that the minimum radius of curvature becomes a problem because the smaller the radius of curvature promotes ferrite transformation, the present invention defines the minimum radius of curvature of the concave portion of the target inclusion. .

  The inclusions targeted in the present invention have the above-described form, but those that do not satisfy the above-described requirements are not directly related to the HAZ toughness. That is, even if there are many irregularities on the surface of the inclusions, those that do not satisfy the above requirements do not have a significant effect on improving the HAZ toughness, and therefore do not become inclusions targeted in the present invention.

  Next, the component composition in the steel plate of the present invention will be described. As described above, the steel sheet of the present invention cannot achieve excellent toughness in HAZ unless the chemical composition is within an appropriate range even if the form and number of inclusions satisfy the above requirements. . Therefore, in the thick steel plate of the present invention, in addition to the good distribution of inclusions, the amount of each chemical component must be within an appropriate range as described below. The reasons for limiting the ranges of these components are as follows.

[C: 0.03-0.12%]
C is an element indispensable for securing the strength of the steel sheet, and in order to exert such effects, it is necessary to contain 0.03% or more. Preferably it is 0.04% or more. However, if the C content exceeds 0.12%, a large number of island martensite phases (MA phases) are generated in the HAZ during welding, leading to deterioration of the toughness of the HAZ. Therefore, C must be suppressed to 0.12% or less (preferably 0.10% or less).

[Si: 0.40% or less (excluding 0%)]
Si is an element useful for securing the strength of a steel sheet by solid solution strengthening. However, if it is excessively contained, a large amount of island martensite phase (MA phase) is generated in the HAZ during welding, and the toughness of the HAZ deteriorates. Will be invited. From such a viewpoint, the Si content needs to be 0.40% or less, and is preferably suppressed to 0.35% or less.

[Mn: 1.0 to 2.0%]
Mn is an element useful for ensuring the strength of the steel sheet, and in order to effectively exhibit such effects, it is necessary to contain 1.0% or more. Preferably it is 1.4% or more. However, if the content exceeds 2.0% excessively, the strength of the HAZ increases excessively and the toughness deteriorates, so the Mn content is set to 2.0% or less. Preferably it is 1.8% or less.

[P: 0.03% or less (excluding 0%)]
P, which is an impurity element, easily causes grain boundary fracture, and adversely affects toughness. Therefore, the amount is preferably as small as possible. From the viewpoint of securing toughness, the P content needs to be suppressed to 0.03% or less, preferably 0.02% or less. However, industrially, it is difficult to make P in steel 0%.

[S: 0.001 to 0.025%]
S is an element indispensable for the formation of sulfides. In particular, unevenness on the surface of inclusions is caused by complex precipitation as sulfides on the surface of inclusions such as oxides, sulfides and nitrides during the solidification process during casting. It is effective in forming. If the S content is less than 0.001%, a composite precipitation amount sufficient to form the necessary irregularities cannot be obtained, so the lower limit was made 0.001%. On the other hand, if the S content exceeds 0.025%, sulfides are complex-precipitated so as to uniformly cover the inclusion surface, and the radius of curvature of the recesses increases, so that the effect of promoting intragranular transformation cannot be obtained. For these reasons, the upper limit of the S content was set to 0.025%. In addition, the minimum with preferable S content is 0.002%, and a preferable upper limit is 0.020%.

[Al: 0.050% or less (excluding 0%)]
Al is effective as a deoxidizing element, but if its content is excessive, coarse oxide inclusions are formed and the HAZ toughness is reduced, so the upper limit was made 0.050%. In addition, the upper limit with preferable Al content is 0.040%.

[Ti: 0.005 to 0.10%]
Ti is an element that forms nitrides or oxides and contributes to improving the toughness of HAZ. In order to exhibit such an effect effectively, it is necessary to contain Ti 0.005% or more, preferably 0.010% or more. However, if the Ti content is excessive, the nitride or oxide becomes coarse and deteriorates the toughness of the HAZ, so it should be suppressed to 0.10% or less. Preferably it is 0.080% or less.

[N: 0.0040 to 0.030%]
N is an element necessary for the formation of nitrides. In particular, the precipitates are unevenly formed on the surface of inclusions by being deposited as nitrides on the surface of inclusions such as oxides, sulfides and nitrides during the solidification process during casting. It is effective in forming. If the N content is less than 0.0040%, a composite precipitation amount sufficient to form the necessary unevenness cannot be obtained, so the lower limit was made 0.0040%. Further, when the N content exceeds 0.030% and excessive, nitride is complex-precipitated so as to uniformly cover the inclusion surface, and the radius of curvature of the concave portion increases, so that the effect of promoting intragranular transformation is obtained. It becomes impossible. For these reasons, the upper limit of the N content needs to be 0.030%. In addition, the minimum with preferable N content is 0.0050%, and a preferable upper limit is 0.025%.

[O: 0.0005 to 0.010%]
O is an element necessary for the generation of an oxide. If the O content is less than 0.0005%, a sufficient amount of oxide cannot be obtained, so the lower limit was made 0.0005%. Further, if the O content exceeds 0.010% and becomes excessive, the HAZ toughness is reduced due to the coarsening of the oxide, so the upper limit was made 0.010%. In addition, the minimum with preferable O content is 0.001%, and a preferable upper limit is 0.008%.

The contained elements specified in the present invention are as described above, and the balance is iron and unavoidable impurities, and elements brought in depending on the situation of raw materials, materials, manufacturing equipment, etc. (for example, Sn, As) , Pb, etc.) can be permitted. It is also effective to satisfy the following formula (1) within the range of the chemical component composition as described above. Furthermore, it is also effective to positively contain the following various elements as necessary, and the inclusion of such elements further improves the characteristics of the thick steel plate depending on the type.
0.05 ≦ 9 × [O] / ([S] + 10 × [N]) ≦ 1.20 (1)
However, [O], [S] and [N] indicate the contents (mass%) of O, S and N, respectively.

  The relationship of the above formula (1) is obtained based on experiments by selecting O, S, and N as elements that affect the generation of oxides, sulfides, nitrides, and the like. When the value of [9 × [O] / ([S] + 10 × [N]) (hereinafter sometimes referred to as “Z value”) is less than 0.05, sulfides and nitrides cause the inclusion surface to move. Since the composite precipitation is performed so as to cover uniformly, and the radius of curvature of the concave portion increases, it is difficult to obtain the effect of promoting intragranular transformation. When the Z value exceeds 1.20, it is difficult to obtain sufficient sulfide and nitride composite precipitates to form necessary irregularities on the surface of inclusions such as oxides, sulfides and nitrides. Become. A more preferable lower limit of the Z value is 0.20, and a more preferable upper limit is 1.0.

[REM: 0.0001 to 0.05%]
REM (rare earth element) is an element that promotes intragranular transformation by precipitating as an oxide or sulfide. If its content is less than 0.0001%, the effect of promoting intragranular transformation is sufficient. Since it was not obtained, the preferable lower limit was made 0.0001%. Moreover, when the preferable content of REM exceeds 0.05% and becomes excessive, the HAZ toughness is lowered due to the coarsening of oxides or sulfides, so the preferable upper limit was made 0.05%. A more preferable lower limit of the REM content is 0.0005%, and a more preferable upper limit is 0.04%. In the present invention, REM (rare earth element) means a lanthanoid element (15 elements from La to Lu), Sc (scandium) and Y (yttrium).

[Zr: 0.0001 to 0.05%]
Zr is an element effective for promoting intragranular transformation by precipitating as an oxide, and in order to exert such effects, it is preferably contained in an amount of 0.0001% or more. However, if the Zr content exceeds 0.05%, the HAZ toughness is reduced due to the coarsening of the oxide, so 0.05% or less is preferable. A more preferable lower limit of Zr is 0.0005%, and a more preferable upper limit is 0.04%.

[Mg: 0.0001 to 0.005% and / or Ca: 0.0001 to 0.005%]
Mg and Ca are both effective elements for fine dispersion of oxides, and if the content is less than 0.0001%, the effect of fine dispersion cannot be obtained sufficiently. 0001%. Moreover, since the effect of fine dispersion | distribution will fall by coarsening of an oxide when content exceeds 0.0050% in all, all made the upper limit 0.0050%. In addition, the more preferable minimum of these element content is 0.0005%, and a more preferable upper limit is 0.0030%.

[Selected from the group consisting of Cu: 0.1-1.5%, Ni: 0.1-1.5%, Cr: 0.1-1.5% and Mo: 0.1-1.5% One or more]
Cu, Ni, Cr, and Mo are all effective elements for increasing the strength of the steel sheet (base material), and in order to exert such effects, it is preferable that all of them be contained in an amount of 0.1% or more. However, if the content of these elements is excessive, the HAZ toughness is lowered, so that it is preferable that both be 1.5% or less. In addition, the more preferable lower limit of these elements is 0.2%, and the more preferable upper limit is 1.2%.

[Nb: 0.01 to 0.06 and / or V: 0.01 to 0.1%]
Nb and V are elements that precipitate as carbonitride and improve HAZ toughness by suppressing austenite grain coarsening. In order to exhibit such an effect, it is preferable to contain all 0.01% or more. However, if these contents are excessive, coarse carbonitrides are generated and the HAZ toughness is lowered, so that Nb is preferably 0.06% or less and V is 0.1% or less. A more preferable lower limit is 0.02% for both Nb and V, and a more preferable upper limit is 0.05% for Nb and 0.08% for V.

[B: 0.001 to 0.005%]
B is an element that improves the HAZ toughness by suppressing the formation of intergranular ferrite, and is preferably contained in an amount of 0.001% or more in order to exert its effect. However, if the B content becomes excessive and exceeds 0.005%, BN precipitates at the austenite grain boundary and causes a reduction in HAZ toughness, so 0.005% or less is preferable. A more preferable lower limit of the B content is 0.0015%, and a more preferable upper limit is 0.004%.

  The present invention relates to a thick steel plate. In this field, a thick steel plate generally refers to one having a plate thickness of 3.0 mm or more as defined by JIS. However, the thickness of the steel plate of the present invention is preferably 80 mm or more. That is, the steel plate of the present invention is a steel plate having a plate thickness of 80 mm or more, and exhibits good HAZ toughness even when high heat input welding is performed. However, the thickness of the steel sheet of the present invention is not limited to 80 mm or more, and does not exclude application to a steel sheet that is 50 mm or more or less.

  The steel plate of the present invention thus obtained can be used as a material for structures such as bridges, high-rise buildings, ships, etc., and deteriorates the toughness of the weld heat affected zone not only in small to medium heat input welding but also in large heat input welding. Can be prevented.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

[Example 1]
The steels having the compositions shown in Tables 1 and 2 below were melted using a vacuum melting furnace (150 kg) while changing the cooling time T (seconds) of 1450 to 1500 ° C. during which the solidification process during casting progressed, and the slab ( (Cross-sectional shape: 200 mm × 250 mm) After heating to 1100 ° C. and hot rolling so that the final rolling temperature was 880 ° C., a steel plate having a plate thickness of 80 mm was produced. The chemical component composition, cooling time T (seconds), and Z value of each thick steel plate are shown in Table 1 below. In Tables 1 and 2, REM was added in the form of a misch metal containing about 50% La and about 25% Ce. In Tables 1 and 2, “-” indicates that no element is added.

Test pieces for producing welded joints were collected from the obtained thick steel plates, and a Charpy impact test in the HAZ of each test piece was performed as follows. First, V-grooves were processed into each welded joint preparation test piece, and electrogas arc welding was performed at a heat input of 50 kJ / mm to obtain welded joints. From these welded joints, Charpy impact test pieces (No. 4 test piece of JIS Z 2201) in which notches were machined in the HAZ near the weld metal part were collected and evaluated for HAZ toughness. And based on JISZ2242, the Charpy impact test was done at _-40 degreeC and the absorbed energy (vE- ₄₀ ) was measured. At this time, the absorbed energy (vE _-40 ) was measured for the three test pieces, and the average value was obtained. And it evaluated that the value of vE- ₄₀ exceeding 180J was excellent in HAZ toughness.

After completion of the Charpy impact test, the HAZ in the center of the test piece, 2-6 mm away from the fracture surface, was measured for each thick steel plate using a field emission scanning electron microscope (FE-SEM) ["SUPRA35" (trade name ): Manufactured by Carl Zeiss Co., Ltd.] (of cross section observation), among inclusions having a large number of irregularities on the surface, the minimum radius of curvature of the concave portion is 0.1 to 2.0 μm, and the equivalent circle diameter Is the number density of inclusions (hereinafter referred to as “N _A (pieces / mm ² )”) consisting of one or more of oxides, sulfides and nitrides of 0.2 to 5.0 μm. It measured by the method of.

<Measurement method of inclusion number density N _A (pieces / mm ² )>
(I) First, the observation magnification of the FE-SEM was set to 5000 times, 20 observation fields corresponding to an area of 0.0024 mm ² were randomly selected, and images of each observation field were taken. At the same time, EDX (energy dispersive X-ray analysis) is performed on each inclusion particle included in each field of view, and an image of inclusion particles containing one or more of oxygen, nitrogen and sulfur as constituent elements Was taken at a magnification of 10,000. Among inclusions such as oxides, nitrides and sulfides present in the obtained 10,000 times image, those having an uneven surface shape were selected, and the concave curvature radius of the surface was measured by the following procedure (ii). .

(Ii) the contour of the inclusions in the images, inclusions recesses on both sides of the draw common tangent L ₁ on the convex portion (Fig. 1), 2-point contact (figure A, B) were obtained. Then, a tangent is drawn L ₂ of the common tangent parallel to the recess, to give the contact C, "Image-Pro Plus": by image analysis using a (software name Media Cybernetic, Inc.), these three contacts (A, A circle passing through B, C) was drawn, and the radius of the circle was defined as the curvature radius R of the concave portion of the inclusion surface.

(Iii) Among the recesses existing on the surface of each inclusion, inclusion particles whose minimum radius of curvature measured by the above method is 0.1 to 2.0 μm are selected, and the equivalent circle diameter is determined by image analysis. Asked. Among them, circle the number of equivalent diameter inclusions is 0.2 to 5.0 .mu.m, and counted by 20 field of view of the image at 5000 times, number density in terms of the observation field of view 1 mm ² _{N A} (pieces / mm ² )

The number density N _A obtained as described above are shown in the following Tables 3 and 4 together with the HAZ test results of the steel plate (the value of vE _-40).

From these results, it can be considered as follows. Intended thick steel plate satisfying the requirements specified in the present invention (Test Table 1,3 No.1~21) First, the number density N _A of inclusions of a given form becomes high with excellent HAZ toughness It can be seen that is achieved.

  On the other hand, it is understood that none of the thick steel plates lacking any of the requirements defined in the present invention (test Nos. 22 to 36 in Tables 2 and 4) have good HAZ toughness.

Specifically, Test No. In the case of 22 to 25, Z value [9 × [O] / ([S] + 10 × [N])], which is a preferable requirement, or a temperature range of 1450 to 1500 ° C. at which the solidification process during casting proceeds. cooling time T of, for not within the proper range, the number density N _a of the inclusions having a predetermined shape are 100 / mm ² without satisfied, not good HAZ toughness is obtained.

  Test No. In the case of No. 26, the Si content is excessive, and it is expected that the hard MA structure is increased, thereby reducing the HAZ toughness. Test No. In the samples of Nos. 27 and 28, the Al content and the O content are excessive, and it is expected that coarse inclusions are increased, and the HAZ toughness is reduced.

Test No. Those of 29 to 36, the content of any element, because is out of the appropriate range, the number density N _A of the inclusions having a predetermined shape has become less than 100 / mm ^2, HAZ Toughness is reduced.

Substituting a drawing micrograph for explaining a method of measuring the number density N _A of the inclusions.

Claims (8)

C: 0.03 to 0.12% (meaning “mass%”; the same applies hereinafter), Si: 0.40% or less (excluding 0%), Mn: 1.0 to 2.0%, P: 0.03% or less (not including 0%), S: 0.001 to 0.025%, Al: 0.05% or less (not including 0%), Ti: 0.005 to 0.10%, N: 0.0040 to 0.030% and O: 0.0005 to 0.010%, respectively, the balance is a thick steel plate made of iron and unavoidable impurities, and has inclusions with many irregularities on the surface Among them, the minimum curvature radius of the recess is 0.1 to 2.0 [mu] m, which is composed of one or more of oxide, sulfide and nitride, and has an equivalent circle diameter of 0.2 to 5 [mu] m However, a high-tensile thick steel plate excellent in toughness of the weld heat-affected zone, characterized in that there are 100 or more per 1 mm ² observation field. The high-tensile thick steel plate according to claim 1, which satisfies the relationship of the following formula (1).
0.05 ≦ 9 × [O] / ([S] + 10 × [N]) ≦ 1.20 (1)
However, [O], [S] and [N] indicate the contents (mass%) of O, S and N, respectively.   The high-tensile thick steel plate according to claim 1 or 2, further comprising REM: 0.0001 to 0.05%.   The high-tensile thick steel plate according to any one of claims 1 to 3, further comprising Zr: 0.0001 to 0.05%.   The high-tensile steel plate according to any one of claims 1 to 4, further comprising Mg 0.0001 to 0.005% and / or Ca: 0.0001 to 0.005%.   Further, selected from the group consisting of Cu: 0.1-1.5%, Ni: 0.1-1.5%, Cr: 0.1-1.5% and Mo: 0.1-1.5% The high-tensile thick steel plate according to any one of claims 1 to 5, wherein the high-tensile steel plate contains at least one selected from the above.   Furthermore, Nb: 0.01-0.06 and / or V: 0.01-0.1% is contained, The high-tensile steel plate in any one of Claims 1-6.   The high-tensile thick steel plate according to any one of claims 1 to 7, further comprising B: 0.001 to 0.005%.