JP5212124B2 - Thick steel plate and manufacturing method thereof - Google Patents

Description

The present invention relates to a thick steel plate used for shipbuilding and the like and a method for manufacturing the same.

  As the hull size increases in recent years, steel plates for hulls are becoming extremely thick. It is required to apply high heat input welding to thick steel plates for ship hulls in order to complete welding in one pass for reasons such as improving work efficiency and reducing construction costs. In particular, in the case of a steel plate having a plate thickness exceeding 50 mm, the heat input may exceed 400 kJ / cm. On the other hand, there is an increasing demand for high strength steel plates for ship hulls, and high strength steel of 600 MPa or more is also being applied depending on the part.

  Various attempts have been made to apply a high strength extra heavy steel material of 600 MPa or more to high heat input welding. However, in the conventional research, it has been studied mainly to stabilize the toughness of the weld heat affected zone (hereinafter also referred to as “HAZ”). After high heat input welding, the actual structure has a structure of 600 MPa or more. It is not considered that the tensile strength can be secured. That is, the carbon equivalent is set low for improving the weld heat-affected zone toughness, but when the carbon equivalent is low, remarkable softening occurs in the HAZ outer layer during high heat input welding. For this reason, even if the base material strength is 600 MPa or more, 600 MPa cannot be secured as a structure after welding.

  Furthermore, in steel plates for ship hulls, in recent years, it has been required to ensure brittle crack propagation stop characteristics (arrest characteristics) at full thickness. However, the evaluation of toughness is based only on the test results at the 1/4 position of the plate thickness, and does not ensure the safety at the center of the plate thickness where the performance is required as a structure.

  Patent Document 1 proposes a thick high-strength steel plate excellent in high heat input welding heat-affected zone toughness. However, the steel sheet described in this document cannot stably obtain a high joint strength of 600 MPa or more.

  Patent Document 2 proposes a steel material excellent in high heat input welding characteristics with a tensile strength exceeding 570 MPa. However, in this document, mention is not made of securing toughness up to the center of the plate thickness of the steel sheet, and the joint toughness evaluation temperature is not specified, and the low temperature of the steel sheet for a hull of the steel sheet described in this document. Toughness is unknown.

  In patent document 3, the steel plate excellent in the welding heat affected zone toughness of the high heat input welding exceeding 400 kJ / cm is proposed. However, the steel sheet described in this document does not stably obtain a high joint strength of 600 MPa or more, and this document does not mention the toughness of the center part of the steel sheet.

JP 2005-307261 A JP 2002-285279 A JP 2002-256379 A

  The present invention stabilizes the low temperature toughness at the central portion of the steel sheet thickness in an ultra-thick steel sheet having a plate thickness of 50 mm or more and a tensile strength of 600 MPa or more, and performs high heat input welding of 400 kJ / cm or more. Even if applied, the object is to achieve both the weld heat-affected zone toughness at a low temperature and a high strength of 600 MPa or more.

  The inventors of the present invention have made extensive studies in order to achieve the above object, and have obtained the following knowledge.

  (A) In order to ensure the target steel strength and the toughness of the center portion of the plate thickness in the high heat input welded joint, it is not sufficient to specify the content of each alloy element, and C, Si, Mn, Cu, It is important to adjust the balance of the contents of Ni, Cr, Mo and B. That is, when the balance of the contents of these elements is poor, the formation of fine ferrite is suppressed in the high heat input welding heat-affected zone, becoming a bainite structure main body, and it is difficult to ensure the low temperature toughness of the welding heat-affected zone. Or, it is difficult to ensure the strength of the steel material. For this reason, the parameter | index for adjusting the balance of content of said element is required.

  (B) In order to ensure the strength of the steel material, it is necessary to make the hardness (usually evaluated by the plate thickness surface portion) a certain level or more. However, in the ultra-thick steel plate, it is difficult to make the crystal grain refinement in the central portion of the plate thickness during the rolling process. Therefore, if the steel plate has a coarse bainite single phase structure, the toughness in the central portion of the plate thickness is deteriorated. Further, in order to stabilize the toughness, it is necessary to disperse the fine ferrite structure. For this reason, it is necessary to evaluate the hardness in the plate | board thickness center part of a steel plate.

  (C) When the content of each alloy element is specified, the balance of the content of each element is adjusted, and the hardness of the center of the plate thickness is suppressed, remarkable softening occurs at the outer layer of the HAZ in the high heat input welded joint. As a result, it may be difficult to ensure the strength of the joint. In order to increase the joint strength of the high heat input weld joint to 600 MPa or more, it is necessary to control the hardness of the steel surface to 250 to 330. As a result, stable strength can be secured as the total thickness of the steel sheet due to the effects of the hardened layers on the front and back surfaces of the steel sheet, and further, the toughness of the steel sheet surface does not deteriorate.

  This invention is based on said knowledge, and makes a summary the manufacturing method of the thick steel plate shown to the following (A) and (B) and the thick steel plate shown to the following (C) and (D).

但し、上記（１）式中の各元素記号は、それぞれの元素の含有量（質量％）を意味する。
(A) In mass%,
C: 0.02~ 0.07%,
Si: 0.1 to 0.5%,
Mn: 1.0-2.0%,
P: 0.02% or less,
S: 0.01% or less,
Ni: 0.1 to 1.0%,
sol. Al: 0.005 to 0.08%,
Ti: 0.003 to 0.02%,
Nb: 0.005 to 0.03%,
N: 0.003 to 0.007%,
B: 0.0002 to 0.0020% and O: 0.003% or less,
further,
Cu: 0.1 to 0.6%,
Cr: 0.05-0.60%,
Containing one or more elements selected from Mo: 0.02-0.10% and V: 0.01-0.05%;
The balance is a thick steel plate made of iron and impurities,
The content ratio of Ti and N (Ti / N) is in the range of 1.0 to 3.0,
K value calculated | required from following (1) Formula is 150-250,
A thick steel plate having a Vickers hardness of 250 to 330 on the surface of the steel plate and a Vickers hardness of 230 or less in the center of the thickness of the steel plate.

However, each element symbol in the above formula (1) means the content (% by mass) of each element.

(B)% by mass, further Ca: 0.005% or less,
Mg: 0.005% or less and sol. Zr: The thick steel plate according to the above (1) containing at least one selected from 0.01% or less.

(C) A method for producing a thick steel sheet, comprising sequentially performing the following steps (1) to (3) on a slab having the chemical composition (A) or (B).
(1) Heating step for heating to a temperature of 1000 to 1180 ° C. (2) Rolling step of performing rolling at a reduction rate of 45% or higher at a temperature of 850 ° C. or lower and completing the rolling at a temperature of 720 ° C. or higher (3 ) Cooling process in which water cooling is started from a temperature of 680 ° C. or higher, the average cooling rate of the steel sheet surface between 800 and 500 ° C. is 30 ° C./second or higher, and the water cooling is stopped when the surface temperature is 350 ° C. or lower.

  (D) The method for producing a thick steel plate according to (C), wherein a step of reheating to a temperature of 450 ° C. or lower is further performed after the cooling step.

  According to the present invention, in an extra-thick steel plate having a thickness of 50 mm or more, the low temperature toughness at the center of the thickness of the steel material is stabilized, and even when high heat input welding of 400 kJ / cm or more is applied, It is possible to achieve both weld heat affected zone toughness and high strength of 600 MPa or more.

  Hereinafter, the thick steel plate according to the present invention will be described. “%” For each element means “mass%”.

C: 0.02-0.10%
C is an element that improves the strength of the steel material and causes the effect of refining the structure when adding Nb, V, or the like. These effects are not sufficient if the content is less than 0.02%. However, when the content of C is excessive, a hardened structure called MA is formed in the welded portion to deteriorate the weld heat affected zone toughness and adversely affect the toughness and weldability of the base material. Therefore, the content of C is set to 0.02 to 0.10% or less. The preferable lower limit of the C content is 0.03%, and the preferable upper limit is 0.08%.

Si: 0.1 to 0.5%
Si is an element effective for preliminary deoxidation of molten steel and an element effective for increasing the strength of the base metal. These effects are insufficient when the content is less than 0.1%. However, since Si does not dissolve in cementite, when its content is excessive, it prevents the untransformed austenite grains from being decomposed into ferrite grains and cementite, and promotes the formation of island martensite. Therefore, the Si content is 0.1 to 0.5%. The upper limit with preferable Si content is 0.3%.

Mn: 1.0-2.0%
Mn is an element necessary for ensuring the strength of the steel material and is an element effective as a deoxidizer. For this reason, the content of Mn needs to be 1.0% or more. However, an excessive content of Mn excessively increases the hardenability and degrades the weldability and the weld heat affected zone toughness. In particular, when the content of Mn exceeds 2.0%, center segregation becomes remarkable. Therefore, the Mn content is 1.0 to 2.0%. The minimum with preferable Mn content is 1.4%, and a preferable upper limit is 1.8%.

P: 0.02% or less P is an impurity element unavoidably contained in steel, promotes grain boundary segregation, and causes grain boundary cracking in the weld heat affected zone. The content is limited to 0.02% or less in order not to increase the base metal toughness, the toughness reduction of the weld metal part and the weld heat affected zone, and the center segregation of the slab. P is preferably limited to 0.015% or less.

S: 0.01% or less S is an impurity element inevitably contained in steel. When a large amount exists, a precipitate of MnS simple substance that becomes a welding crack starting point is generated. The content is limited to 0.01% or less in order not to increase the toughness of the base metal, the toughness of the weld metal part and the weld heat affected zone, and the center segregation of the slab. S is preferably limited to 0.005% or less.

Ni: 0.1 to 1.0%
Ni is an element having an effect of increasing the strength and toughness of the steel material and further increasing the toughness of the weld heat affected zone. However, if the content is less than 0.1%, those effects are not obtained, and if the content exceeds 1.0%, it is not possible to obtain an effect commensurate with the cost. For this reason, the Ni content is set to 0.1 to 1.0%. The minimum with preferable Ni content is 0.2%, and a preferable upper limit is 0.8%.

sol. Al: 0.005 to 0.08%
Al is an effective element for preliminary deoxidation of molten steel, but excessive content inhibits the decomposition of untransformed austenite grains into ferrite grains and cementite, and also promotes the formation of island martensite. Reduces the toughness of the heat affected zone. For this reason, the content of Al is set to 0.005 to 0.08%. The minimum with preferable Al content is 0.007%, and a preferable upper limit is 0.05%. The Al content of the present invention refers to acid-soluble Al (so-called “sol.Al”).

Ti: 0.003-0.02%
Ti has the effect | action which refines | miniaturizes a transformation structure | tissue while producing | generating a nitride and suppressing the coarsening of a crystal grain. However, when the content is less than 0.003%, the above-described effect is not exhibited, and when the content exceeds 0.02%, the base material toughness and the welded portion toughness are adversely affected. For this reason, content of Ti shall be 0.003-0.02%. The preferable lower limit of the Ti content is 0.006%, and the preferable upper limit is 0.015%.

Nb: 0.005 to 0.03%
Nb has an effect of suppressing recovery and recrystallization of non-recrystallized austenite grains that have been processed by rolling, and is effective in securing the base material toughness. However, when the content is less than 0.005%, the above-mentioned effect is not exhibited, and when the content exceeds 0.03%, the base material toughness and the weld zone toughness are adversely affected. For this reason, the Nb content is set to 0.005 to 0.03%. The minimum with preferable Nb content is 0.007%, and a preferable upper limit is 0.02%.

N: 0.003 to 0.007%
N contributes to the refinement of the structure by forming nitrides. In order to acquire this effect, it is necessary to make it contain 0.003% or more. However, when N is contained exceeding 0.007%, the toughness is deteriorated through the aggregation of nitrides. For this reason, content of N shall be 0.003-0.007%. The minimum with preferable N content is 0.004%, and a preferable upper limit is 0.006%.

B: 0.0002 to 0.0020%
B is an element effective for improving hardenability, and is important for ensuring the strength of the steel material. Furthermore, it is an element that can improve the toughness of the weld heat affected zone by suppressing the formation of coarse grain boundary ferrite in the weld heat affected zone. However, when the content is less than 0.0002%, the above-described effect cannot be exhibited, and when the content exceeds 0.0020%, the base material toughness and the welded portion toughness are adversely affected. For this reason, content of B shall be 0.0002 to 0.0020%. The minimum with preferable B content is 0.001%, and a preferable upper limit is 0.0018%.

O: 0.003% or less O (oxygen) is an impurity element inevitably contained in steel. When it is present in a large amount, the cleanliness deteriorates remarkably, and it becomes difficult to ensure practical toughness for the base metal, the weld metal part and the weld heat affected part. Therefore, the O content is limited to 0.003% or less. The O content is preferably limited to 0.002% or less.

  In the thick steel plate of the present invention, one or more elements selected from Cu, Cr, Mo and V are contained for the purpose of improving the strength of the steel plate. The reasons for limiting each element are described below.

Cu: 0.1 to 0.6%
Cu has an effect of increasing the strength of the steel sheet. However, when the content is increased, the sensitivity to hot cracking of the weld increases, and the welding work such as preheating becomes complicated. For this reason, the content is made 0.1 to 0.6%. Preferably it is 0.2%, and a preferable upper limit is 0.5%.

Cr: 0.05-0.60%,
Cr increases the hardenability of steel and is effective in securing strength. However, when its content is excessive, it tends to harden the weld metal part and weld heat affected zone to increase the weld cold cracking susceptibility. is there. For this reason, the content is made 0.05 to 0.60%. The minimum with preferable Cr content is 0.07%, and a preferable upper limit is 0.40%.

Mo: 0.02-0.10%
Mo increases the hardenability of steel and is effective in securing strength. However, when its content is excessive, it tends to harden the weld metal and weld heat affected zone to increase the weld cold cracking susceptibility. is there. For this reason, the content is made 0.02 to 0.10%. A preferable lower limit of the Mo content is 0.03%, and a preferable upper limit is 0.05%.

V: 0.01-0.05%
V is effective in securing the strength of the steel material by forming carbides and nitrides, but if its content is excessive, it adversely affects the base metal toughness and weld zone toughness. For this reason, the content is made 0.01 to 0.05%. The minimum with preferable V content is 0.02%.

  In addition, from the viewpoint of preventing toughness deterioration, it is preferable that the contents of Cu, Cr, Mo and V satisfy Cu / 20 + Cr / 20 + Mo / 15 + V ≦ 0.08%.

  The thick steel plate of the present invention contains each of the above elements, and the balance consists of iron and impurities. The impurity means an unavoidable component mixed from raw ore, scrap, etc., and is allowed within a range that does not adversely affect the present invention.

  However, even if the content of Ti and N is limited to the above range, if solid solution N or Ti that cannot be combined as TiN is excessive, stabilization of the toughness of the weld heat affected zone in high heat input welding is not possible. It may be sufficient to adjust the Ti and N content ratio (Ti / N).

Ratio of Ti and N content (Ti / N): 1.0 to 3.0
As described above, in order to stabilize the toughness of the weld heat affected zone, it is necessary to adjust the content ratio (Ti / N) of Ti and N. Here, when Ti / N is less than 1.0, solid solution N that cannot be combined as TiN increases, and the toughness of the weld heat affected zone is deteriorated. On the other hand, when Ti / N exceeds 3.0, Ti that cannot be combined as TiN forms coarse carbides and deteriorates the toughness of the weld heat affected zone. For this reason, the mass ratio of Ti / N is set to 1.0 to 3.0.

  In order to improve the low temperature toughness, the thick steel plate of the present invention can further contain one or more selected from Ca, Mg and Zr in addition to the above chemical composition.

  Ca, Mg, and Zr are elements that generate oxides and sulfides that serve as precipitation nuclei for intragranular ferrite. Moreover, these elements have the effect of controlling low-temperature toughness by controlling the form of sulfide. These effects become remarkable when Ca is 0.0005% or more, Mg is 0.0001% or more, and Zr is 0.0001% or more. On the other hand, if Ca exceeds 0.005%, Mg exceeds 0.005%, and Zr exceeds 0.01%, coarse inclusions or clusters may be generated to deteriorate the cleanliness of the steel. Therefore, the content when one or more selected from these elements is contained is 0.0005 to 0.005% for Ca, 0.0001 to 0.005% for Mg, and 0.0001 to 0 for Zr, respectively. 0.01% is preferable. The Zr content of the present invention refers to acid-soluble Zr (so-called “sol.Zr”). The total amount of Ca, Mg and Zr is preferably 0.015% or less.

但し、上記（１）式中の各元素記号は、それぞれの元素の含有量（質量％）を意味する。
Ｋ値は、鋼板の強度および靭性、ならびに、大入熱継手の強度および靭性を安定化させるために制御することが必要な指標である。このＫ値が１５０未満の場合は、鋼材の板厚中心部における靭性を維持しようとすると、強度を上昇させるのが困難となり、さらに鋼板表面における硬度を上昇させることができなくなる。一方、Ｋ値が２５０を超える場合には、鋼材の板厚中心部における靭性、鋼板表面における靭性、大入熱継手靭性の改善が難しい。従って、上記のＫ値は１５０〜２５０の範囲に調整することとする。 K value calculated | required from following (1) Formula: 150-250

However, each element symbol in the above formula (1) means the content (% by mass) of each element.
The K value is an index that needs to be controlled to stabilize the strength and toughness of the steel sheet and the strength and toughness of the large heat input joint. When the K value is less than 150, it is difficult to increase the strength and to increase the hardness on the surface of the steel sheet if it is attempted to maintain the toughness at the center portion of the steel sheet thickness. On the other hand, when the K value exceeds 250, it is difficult to improve the toughness at the center of the plate thickness of the steel material, the toughness at the surface of the steel plate, and the toughness of the large heat input joint. Therefore, the K value is adjusted to a range of 150 to 250.

Vickers hardness of steel plate surface: 250-330
For materials aimed at improving the toughness of large heat input welds, the hardenability of steel is set low. However, when high heat input welding is applied to such a material, softening occurs in the outer layer portion of the HAZ, resulting in a problem of reduced strength of the joint. This is a big problem in a thick steel plate having a high strength of a tensile strength of 600 MPa or more. Therefore, it is necessary to adjust the hardness of the steel sheet surface to 250 to 330 by Vickers hardness. Thereby, by restraining deformation on the front and back surfaces, sufficient joint strength can be ensured even in a high heat input welded joint, and the toughness of the steel sheet surface is not impaired. Therefore, the Vickers hardness of the steel plate surface is set to 250 to 330.

Vickers hardness at the center of the thickness of the steel sheet: 230 or less Increasing the hardness of the steel sheet surface causes a decrease in toughness. This problem becomes apparent especially in the central portion of the plate thickness of the steel plate. That is, it becomes difficult to improve toughness at the center of the plate thickness of the steel plate. For this reason, it is necessary to improve the deformability of a steel plate, and it is necessary to limit the hardness in the center part of the plate thickness of a steel plate. Accordingly, the Vickers hardness at the center of the thickness of the steel sheet is 230 or less. In addition, plate | board thickness center part means the board thickness 1 / 2t part of a steel plate.

Hereinafter, the manufacturing method of the thick steel plate which concerns on this invention is demonstrated.
Steelmaking process:
In the production of the thick steel plate of the present invention, the steelmaking process is not particularly limited, but it is preferable to produce a slab by a continuous casting method from the viewpoint of cost reduction. At this time, in order to control inclusions at the center position of the plate thickness, the temperature of the molten steel is not excessively increased in the continuous casting process, and the difference is controlled within 50 ° C. with respect to the solidification temperature determined from the molten steel composition. Is preferred. In addition to this, it is preferable to perform electromagnetic stirring immediately before solidification and reduction during solidification.

Heating process:
In a heating process, it is necessary to heat a slab to the temperature of 1000-1180 degreeC. If the heating temperature of the slab is less than 1000 ° C., sufficient strength cannot be obtained only by changing the production conditions in the subsequent rolling process. On the other hand, when the heating temperature of the slab exceeds 1180 ° C., the austenite grains cannot be kept fine and sized, and the austenite grains cannot be made fine and sized even in subsequent rolling. Accordingly, the heating temperature of the slab is set to 1000 to 1180 ° C.

Rolling process:
The heated slab is hot rolled. Hot rolling needs to be performed at a temperature of 850 ° C. or less and a rolling reduction of 45% or more. This is because a cell-like dislocation structure is formed and a fine bainite structure is formed by sufficiently securing the rolling reduction at an unrecrystallization temperature of 850 ° C. or less. Since it is necessary to form a fine bainite structure to achieve both strength and toughness, this rolling needs to be completed at 720 ° C. or higher. When the rolling start temperature and the finishing temperature are lower than 720 ° C., the precipitation of ferrite becomes remarkable and the fine bainite structure fraction decreases, so that the target strength and toughness cannot be satisfied. Therefore, in the rolling process, rolling at a rolling reduction of 45% or more is performed at a temperature of 850 ° C. or lower, and rolling is completed at a temperature of 720 ° C. or higher. The rolling completion temperature is preferably 750 ° C. or higher.

Cooling process:
After the rolling, water cooling is started from a temperature of 680 ° C. or higher, and an average cooling rate of the steel sheet surface up to 500 ° C. is set to 30 ° C./second or higher, and the water cooling is stopped when the surface temperature is 350 ° C. or lower. There is a need. This is for generating a hardened layer having excellent toughness on the surface of the steel sheet, and for securing a sufficient cooling rate at the center of the steel sheet thickness by heat conduction to stabilize the toughness. The cooling start temperature is preferably 730 ° C. or lower. The average cooling rate of the steel sheet surface from the start of water cooling to 500 ° C. is preferably 150 ° C./s.

Reheating process:
Thereafter, the thick steel plate after the cooling step can be further reheated to a temperature of 450 ° C. or lower, that is, tempered. This is to stabilize toughness by tempering treatment while minimizing the decrease in hardness of the hardened layer on the steel sheet surface. A preferable lower limit of the reheating temperature is 300 ° C, and a preferable upper limit is 400 ° C. The soaking time in the reheating step is preferably 60 minutes or longer.
The temperature in the heating process means the furnace atmosphere temperature, the temperature in the rolling process and the water cooling process means the steel sheet surface layer temperature, and the temperature in the reheating process means the furnace atmosphere temperature.

  First, a slab having a thickness of 300 mm having chemical components shown in Tables 1 and 2 was produced by a continuous casting method. Here, from the viewpoint of inclusion control at the center position of the plate thickness, in the continuous casting process, the temperature of the molten steel is not excessively increased, and the difference is within 50 ° C. with respect to the solidification temperature determined from the molten steel composition. In addition, electromagnetic stirring immediately before solidification and reduction during solidification were performed.

  A thick steel plate was produced from the obtained slab under the conditions shown in Table 3, and various performances were investigated under the following conditions. The results are shown in Table 4.

<Vickers hardness>
In accordance with JIS, a 10 × 10 mm 2 mm V notch test piece was taken from a direction parallel to the rolling direction (steel plate surface) and a thickness of 1/2 t (plate thickness center), and the characteristics at −60 ° C. were evaluated.

<Tensile test>
In accordance with JIS, a tensile test piece having a parallel portion of 14 mmφ was taken from the center of the plate thickness perpendicular to the rolling direction, and a tensile test was performed.

  Then, the welded joint was produced by the electrogas arc welding (EGW) which is 1 pass longitudinal welding about the steel plate processed into the 20 degreeV groove | channel. The welding material used was DWS-1LG manufactured by Kobe Steel.

<Joint toughness>
A test piece having a bond notch was taken from a position 2 mm below the weld joint and subjected to a test at −40 ° C.

<Fitting strength>
In accordance with JIS, a test piece having a total thickness of -25 mm and a parallel portion = welded metal width + 12 mm was taken from the welded joint and subjected to a tensile test.

  As shown in Table 4, in Comparative Examples 1 to 11 that do not satisfy the range of the chemical composition defined in the present invention, one of YS, TS and low temperature toughness of the base material, and low temperature toughness of the welded joint portion The above performance was inferior. Further, although satisfying the chemical composition defined in the present invention but not satisfying the production conditions, in Comparative Examples 13 to 17 where the hardness is outside the range defined in the present invention, the strength of the welded joint portion is less than 600 MPa. There wasn't.

  According to the present invention, in an extra-thick steel plate having a thickness of 50 mm or more, the low temperature toughness at the center of the thickness of the steel material is stabilized, and even when high heat input welding of 400 kJ / cm or more is applied, It is possible to achieve both weld heat affected zone toughness and high strength of 600 MPa or more.

Claims (4)

% By mass
C: 0.02~ 0.07%,
Si: 0.1 to 0.5%,
Mn: 1.0-2.0%,
P: 0.02% or less,
S: 0.01% or less,
Ni: 0.1 to 1.0%,
sol. Al: 0.005 to 0.08%,
Ti: 0.003 to 0.02%,
Nb: 0.005 to 0.03%,
N: 0.003 to 0.007%,
B: 0.0002 to 0.0020% and O: 0.003% or less,
further,
Cu: 0.1 to 0.6%,
Cr: 0.05-0.60%,
Containing one or more elements selected from Mo: 0.02-0.10% and V: 0.01-0.05%;
The balance is a thick steel plate made of iron and impurities,
The content ratio of Ti and N (Ti / N) is in the range of 1.0 to 3.0,
K value calculated | required from following (1) Formula is 150-250,
A thick steel plate having a Vickers hardness of 250 to 330 on the surface of the steel plate and a Vickers hardness of 230 or less in the thickness center of the steel plate.

However, each element symbol in the above formula (1) means the content (% by mass) of each element.
% By mass, further Ca: 0.005% or less,
Mg: 0.005% or less and sol. The thick steel plate according to claim 1, comprising one or more selected from Zr: 0.01% or less.
The manufacturing method of the thick steel plate characterized by performing the following process (1)-(3) sequentially on the slab which has the chemical composition of Claim 1 or 2.
(1) Heating step for heating to a temperature of 1000 to 1180 ° C. (2) Rolling step of performing rolling at a reduction rate of 45% or higher at a temperature of 850 ° C. or lower and completing the rolling at a temperature of 720 ° C. or higher (3 ) Cooling process in which water cooling is started from a temperature of 680 ° C. or higher, the average cooling rate of the steel sheet surface up to 500 ° C. is 30 ° C./second or higher, and water cooling is stopped when the surface temperature is 350 ° C. or lower.   The method for producing a thick steel plate according to claim 3, further comprising a step of reheating to a temperature of 450 ° C or lower after the cooling step.