EP3299486A1

EP3299486A1 - Thick steel sheet and welded joint

Info

Publication number: EP3299486A1
Application number: EP16799856.6A
Authority: EP
Inventors: Makoto Kawamori; Fumio Yuse; Hidenori Nako; Yoshitomi Okazaki; Akira Ibano; Junichiro Kinugasa
Original assignee: Kobe Steel Ltd
Current assignee: Kobe Steel Ltd
Priority date: 2015-05-22
Filing date: 2016-05-16
Publication date: 2018-03-28
Also published as: CN107614724A; EP3299486A4; JP2016216819A; KR20170138505A

Abstract

The purpose of the present invention is to provide a thick steel sheet having excellent sour resistance. The present invention is a thick steel sheet which contains, in mass%, 0.01-0.12% of C, 0.02-0.50% of Si, 0.6-2.0% of Mn, more than 0% but 0.030% or less of P, more than 0% but 0.004% or less of S, 0.010-0.080% of Al, 0.10-1.50% of Cr, 0.002-0.050% of Nb, 0.0002-0.05% of REM, 0.0003-0.01% of Zr, 0.0003-0.006% of Ca, more than 0% but 0.010% or less of N, and more than 0% but 0.0040% or less of O, with the balance made up of iron and unavoidable impurities. With respect to the composition of the inclusions contained in the steel and having widths of 1 µm or more, the Zr amount in the inclusions is within 1-40%, the REM amount in the inclusions is within 5-50%, the Al amount in the inclusions is within 3-30% and the Ca amount in the inclusions is within 5-60%.

Description

TECHNICAL FIELD

The present invention relates to a thick steel plate and a welded joint, and more specifically relates to a thick steel plate suitable as a material steel plate of a structural component for energy field such as a line pipe and a marine structure, and a welded joint using the thick steel plate.

BACKGROUND ART

In recent years, accompanying increase of the global energy demand, development and practical application of various energy including renewable energy have been in progress. On the other hand, oil, natural gas and coal which are fossil fuel occupy a major part of energy resources, how to produce, transport and store the fossil energy safely and efficiently is also an important issue in securing energy, and particularly, a highly functional steel material for energy field becomes indispensable in production, transportation and the like of the fossil energy.
With respect to this steel material for energy field, when the function thereof cannot be exerted and an accident occurs once, the damage becomes enormous, and therefore high safety is required.
Steel for a pipe line is one of the steel material for energy field and is used for transportation of oil and natural gas, and not only the mechanical properties such as strength and toughness as a structural component but also resistance to oil and natural gas passing through the pipe is required for the steel. In recent years, in oil fields and gas fields of oil and natural gas, the quality of oil and gas produced has deteriorated and a large amount of H₂S has been mixed, and sour resistance represented by hydrogen-induced cracking resistance, that is, HIC resistance, has been strongly required in addition to the specification of the past.
Also, from the standpoint of reduction in cost during transportation and execution, reduction in thickness of a pipe is required in the steel for a line pipe. To achieve this, it is necessary to improve strength of a steel material, but, the improvement of strength of the steel material also has the disadvantage of deteriorating hydrogen-induced cracking resistance. Particularly, the T-cross weld joint that receives two kinds of thermal histories of seam welding in working a thick steel plate into a pipe and girth welding in joining pipes with each other is subjected to complicated thermal histories such as rapid heating and rapid cooling, and therefore, the strength, namely hardness, increases and a cracking called sulfide stress corrosion cracking is liable to occur in the welding heat-affected zone: HAZ. The sulfide stress corrosion cracking is hereinafter also referred to as SSCC. Therefore, to realize high strength line pipe steel, the SSCC resistance in the T-cross weld joint is also one of problems.
Related arts for the purpose of achieving the HIC resistance of a base plate or the SSCC resistance of the T-cross weld joint include a technique described in Patent Literature 1, and the like. Patent Literature 1 describes the technique that a block bainite structure which is considered to be harmful to the HIC resistance is reduced, and uniform upper bainite or acicular ferrite structure is developed, thereby X70 grade high strength thick steel plate of API standard could be obtained while securing the HIC resistance of a base plate.
On the other hand, Patent Literature 2 describes a technique that precipitation strengthening by fine Nb and V carbonitride is utilized and high strength of 56 kgf/mm² or more of tensile strength can be achieved. However, this Patent Literature does not describe the HIC resistance of a base plate, and only HAZ in seam welding is taken into consideration with respect to the SSCC resistance. Additionally, the test conditions described in the Examples are that dipping time in a solution simulating the sour environment, namely the environment containing many H₂S, is 21 days, and are therefore not sufficiently severe conditions.
Also, in Patent Literature 3, such a composition as suppressing increase of the hardness which is deemed to deteriorate the SSCC resistance of the T-cross weld joint is described. However, in the technique described in this literature, the SSCC resistance itself is not evaluated, and the HIC resistance of the base plate is not described.

CITATION LIST

Patent Literature

Patent Literature 1: JP-A-S61-165207
Patent Literature 2: JP-A-H1-96329
Patent Literature 3: JP-A-2005-186162

SUMMARY OF INVENTION

Technical Problem

The present invention has been made to overcome the above conventional problems, and its object is to provide a thick steel plate with excellent sour resistance, particularly HIC resistance. Further, other objects of the present invention are to provide a thick steel plate that can achieve a welded joint with excellent SSCC resistance of the T-cross weld joint and a welded joint with excellent SSCC resistance of the T-cross weld joint.

Solution of Problem

The thick steel plate of the present invention contains, in terms of mass %, C: 0.01 to 0.12%, Si: 0.02 to 0.50%, Mn: 0.6 to 2.0%, P: more than 0% and 0.030% or less, S: more than 0% and 0.004% or less, Al: 0.010 to 0.080%, Cr: 0.10 to 1.50%, Nb: 0.002 to 0.050%, REM: 0.0002 to 0.05%, Zr: 0.0003 to 0.01%, Ca: 0.0003 to 0.006%, N: more than 0% and 0.010% or less, and O: more than 0% and 0.0040% or less, with the remainder being iron and inevitable impurities, in which the steel includes an inclusion having a width of 1 µm or more, in which the inclusion has a composition satisfying that Zr amount in the inclusion is 1 to 40%, REM amount therein is 5 to 50%, Al amount therein is 3 to 30%, and Ca amount therein is 5 to 60%.
Also, in the thick steel plate of the present invention, S amount in the inclusion is preferably more than 0% and 20% or less.
Also, in the thick steel plate of the present invention, [Cr]/[Nb] is preferably 10 or more, provided that [ ] in the formula indicates mass %.
Also, the thick steel plate of the present invention preferably further contains, in terms of mass %, one kind or two or more kinds of Mg: more than 0% and 0.005% or less, Ti: 0.003 to 0.030%, Ni: 0.01 to 1.50%, Cu: 0.01 to 1.50%, Mo: 0.01 to 1.50%, V: 0.003 to 0.08%, and B: 0.0002 to 0.0032%, in which [Cr]+[Mo]+[Ni]+[Cu] is 2.1 or less, provided that [ ] in the formula indicates mass %.
Also, the thick steel plate of the present invention preferably contains, in terms of mass %, Ni: 0.01 to 1.50%, in which 0.25×[Cr]+[Ni] is 0.10 to 1.50, provided that [ ] in the formula indicates mass %.
Also, a welded joint of the present invention includes any one of the above thick steel plates of the present invention and a girth weld metal.
The welded joint of the present invention preferably has an immersion potential difference ΔE between the thick steel plate and the girth weld metal, obtained by the following formula, being 25 mV or less. $ΔE = Immersion potential (mV) after 1 hour of girth weld metal - immersion potential (mV) after 1 hour of thick steel plate$

Advantageous Effects of Invention

The effect obtained by a representative invention of the inventions disclosed in the present invention is briefly described below.
According to one embodiment of the present invention, a thick steel plate with excellent sour resistance can be provided. Also, a welded joint with excellent SSCC resistance of T-cross weld joint can be provided by using the thick steel plate of the present invention.

DESCRIPTION OF EMBODIMENTS

In order to achieve the object of the present invention, the present inventors repeated intensive researches and studies from the viewpoint of controlling inclusions in steel in addition to a component composition of a steel, which becomes the basis in exerting properties of a thick steel plate. As a result, it has been found that a thick steel plate with excellent sour resistance is obtained by holding coarse inclusions with 1 µm or more width in a predetermined component composition, and the present invention has been completed. The inclusions in the present invention mean coarse precipitates formed during melting and solidification, and specifically mean particles by oxides, carbides, sulfides, nitrides, and the like of alloy components in steel.
As a result of study from the viewpoint of HIC resistance of a base plate, it is assumed that, when hydrogen intrudes into steel in a sour environment, inclusions such as MnS which are coarse and have a coefficient of thermal expansion larger than that of steel form coarse voids around them, therefore the hydrogen that has intruded is accumulated concentrically in these voids, and cracking, namely hydrogen-induced cracking, is generated and propagates in steel by a pressure generated by vaporization of the hydrogen. Therefore, it was confirmed that HIC resistance of steel can be improved and secured by converting the coarse inclusions of 1 µm or more which become a cause of the hydrogen-induced cracking from inclusions having a coefficient of thermal expansion larger than that of steel into inclusions having a coefficient of thermal expansion smaller than that of steel, and by making such a inclusions. As the inclusions having a coefficient of thermal expansion smaller than that of steel, specifically oxides of Zr, Al and REM and the like are effective.
On the other hand, as a result of study from the viewpoint of SSCC resistance of the T-cross weld joint, it was found that, in the T-cross weld joint, when hardness rapidly changes from the vicinity of a weld metal to a base plate, the SSCC resistance decreases. This is considered to be due to that rapid stress concentration is generated in the region by the hardness change. It is considered that the reason for the rapid hardness change from the vicinity of the weld metal to a base plate is because hard martensite is formed in the vicinity of the weld metal, whereas soft ferrite is formed in a region apart to a base plate side from the weld metal in a certain extent.
In order to suppress this rapid hardness change, investigations have been made from both sides of reduction of soft ferrite and reduction of hard martensite. As a result, it has been confirmed that with respect to the reduction of soft ferrite, the formation of ferrite can be suppressed by adding alloy elements to a thick steel plate and improving quenchability.
With respect to the reduction of hard martensite, it has been confirmed that intragranular bainite transformation is accelerated by dispersing many inclusions becoming the origin of transformation, and additionally Nb segregation into grain boundary that increases nucleation driving force in the grain boundary is reduced by making [Cr]/[Nb] 10 or more, thereby accelerating bainite formation from the grain boundary, and as a result, the formed amount of martensite in the vicinity of girth weld metal can be reduced. In the formula, [ ] indicates mass %.
It has been also confirmed that strength of a base plate can be improved by securing quenchability by adding Cr and Nb.
In addition to the component composition, the microstructure and the composition of inclusions of the thick steel plate according to the present invention, the weld metal used in the T-cross weld joint will be explained in detail below including the reason of the determination. Below, all of % which is an expression unit of the composition means mass %.

(Component Composition of Thick Steel Plate)

[C: 0.01 to 0.12%]

C is an indispensable element for securing the strength of the thick steel plate, and should be contained in an amount of 0.01% or more. It is preferably 0.02% or more, and more preferably 0.03% or more. However, when the C amount is excessive, island martensite is liable to be formed in the base plate, this becomes the origin of hydrogen-induced cracking, and HIC resistance of the base plate deteriorates. Therefore, the C amount should be 0.12% or less. It is preferably 0.10% or less, and more preferably 0.08% or less.

[Si: 0.02 to 0.50%]

Si is effective in deoxidation. In order to obtain such an effect, Si amount is 0.02% or more. It is preferably 0.04% or more, and more preferably 0.06% or more. However, when the Si amount is excessive, island martensite is liable to be formed in the base plate, this becomes the origin of hydrogen-induced cracking, and HIC resistance of the base plate deteriorates. Therefore, the Si amount should be suppressed to 0.50% or less. It is preferably 0.45% or less, and more preferably 0.35% or less.

[Mn: 0.6 to 2.0%]

Mn is an indispensable element for securing the strength of the thick steel plate, and should be contained in an amount of 0.6% or more. It is preferably 0.8% or more, and more preferably 1.0% or more. However, when the Mn amount is excessive, MnS is formed, and HIC resistance deteriorates. Therefore, the upper limit of the Mn amount is 2.0%. It is preferably 1.9% or less, and more preferably 1.8% or less.

[P: more than 0% and 0.030% or less]

P is an element inevitably included in a steel material. When its content exceeds 0.030%, the HIC resistance and the SSCC resistance are adversely affected. Therefore, in the present invention, the P amount is suppressed to 0.030% or less. It is preferably 0.020% or less, and more preferably 0.010% or less.

[S: more than 0% and 0.004% or less]

When S is excessive, a large amount of MnS is formed and the HIC resistance remarkably deteriorates. Therefore, in the present invention, the upper limit of the S amount is 0.004%. It is preferably 0.003% or less, more preferably 0.0025% or less, and still more preferably 0.0020% or less.

[Al: 0.010 to 0.080%]

Al is effective in reducing voids against the matrix phase of steel by decreasing a coefficient of thermal expansion of inclusions, and improving the HIC resistance. Also, inclusions containing an appropriate amount of Al promote the formation of intragranular bainite, and therefore good SSCC resistance is obtained. In order to exert the effect, it should be contained in an amount of at least 0.010% or more. The Al amount is preferably 0.020% or more, and more preferably 0.025% or more. However, when the Al amount is excessive, Al oxide is formed in a cluster shape and becomes the origin of the hydrogen-induced cracking. Therefore, the Al amount should be 0.080% or less. The Al amount is preferably 0.060% or less, and more preferably 0.050% or less.

[Cr: 0.10 to 1.50%]

Cr is an indispensable element for securing strength. Also, it contributes to the improvement of SSCC resistance by suppressing soft ferrite in the T-cross weld joint. In order to exert such an effect, it should be contained in an amount of at least 0.10% or more. The Cr amount is preferably 0.15% or more, more preferably 0.17% or more, and still more preferably 0.20% or more. However, when the Cr amount is excessive, the amount of hard martensite in the T-cross weld joint increases, and the SSCC resistance deteriorates. Therefore, it is 1.50% or less. The Cr amount is preferably 1.00% or less, and more preferably 0.80% or less.

[Nb: 0.002 to 0.050%]

Nb is an indispensable element for securing strength, and further contributes to the improvement of the SSCC resistance by suppressing soft ferrite in the T-cross weld joint. In order to exert such an effect, it should be contained in an amount of at least 0.002% or more. The Nb amount is preferably 0.005% or more, and more preferably 0.010% or more. However, when the Nb amount is excessive, the amount of hard martensite in the T-cross weld joint increases, and the SSCC resistance deteriorates. Therefore, it is 0.050% or less. The Nb amount is preferably 0.033% or less, and more preferably 0.030% or less.

[REM: 0.0002 to 0.05%]

REM (Rare Earth Metal) is effective in reducing voids against the matrix phase of steel by reducing a coefficient of thermal expansion of inclusions, and securing the HIC resistance. The inclusions containing an appropriate amount of REM accelerate the formation of intragranular bainite, and therefore good SSCC resistance can be obtained. In order to exert such an effect, REM should be contained in an amount of 0.0002% or more. The REM amount is preferably 0.0005% or more, and more preferably 0.0010% or more. On the other hand, when REM is excessively contained, solute REM segregates at the grain boundaries, thereby decreasing strength of the grain boundary and deteriorating the SSCC resistance. Therefore, the upper limit of the REM amount is 0.05%. From the viewpoint of suppressing blockage of an immersion nozzle during casting and improving the productivity, it is preferably 0.03% or less, more preferably 0.01% or less, and still more preferably 0.005% or less. In the present invention, the REM means 15 elements from La to Lu in the periodic table, namely the lanthanoid elements, Sc and Y.

[Zr: 0.0003 to 0.01%]

Zr reduces voids against the matrix phase of steel by reducing a coefficient of thermal expansion of inclusions, and improves the HIC resistance. Inclusions containing an appropriate amount of Zr accelerate the formation of intragranular bainite, and as a result, good SSCC resistance can be obtained. In order to exert such an effect, Zr should be contained in an amount of 0.0003% or more. The Zr amount is preferably 0.0005% or more, and more preferably 0.0010% or more. On the other hand, when Zr is excessively contained, solute Zr in molten steel increases, crystallizes so as to surround oxides/sulfides during casting, and deteriorates the HIC resistance. Therefore, the upper limit of the Zr amount is 0.01%. The Zr amount is preferably 0.007% or less, and more preferably 0.005% or less.

[Ca: 0.0003 to 0.006%]

Ca has an action of forming CaS to fix S and reducing the amount of MnS formed, thereby improving the SSCC resistance. Also, inclusions containing an appropriate amount of Ca accelerate the formation of intragranular bainite, and therefore good SSCC resistance can be obtained. In order to exert such an effect, Ca should be contained in an amount of 0.0003% or more. The Ca amount is preferably 0.0005% or more, and more preferably 0.0010% or more. On the other hand, when Ca is excessively contained, CaS is formed excessively, is agglomerated, and deteriorates the HIC resistance. Therefore, the upper limit of the Ca amount is 0.006%. The Ca amount is preferably 0.005% or less, and more preferably 0.004% or less.

[N: more than 0% and 0.010% or less]

N is inevitable impurities, and segregates at the grain boundaries, thereby decreasing strength of the grain boundaries and deteriorating the SSCC resistance. Therefore, the upper limit of the N amount is 0.010%. The N amount is preferably 0.008% or less, and more preferably 0.006% or less.

[O: more than 0% and 0.0040% or less]

O (oxygen) is an element which forms inclusions. Coarse oxides are excessively formed by the excessive addition thereof, and hydrogen-induced cracking is generated from those as the origin. Therefore, the upper limit of the O amount is 0.0040%. The O amount is preferably 0.0030% or less, and more preferably 0.0020% or less.

[[Cr]/[Nb] is 10 or more]

The thick steel plate of the present invention is that the component composition described before is satisfied, and in addition to this, [Cr]/[Nb] is preferably 10 or more. In the formula, [ ] indicates mass %. When the thick steel plate satisfies this condition, Nb segregation into grain boundaries which increases nucleation driving force in grain boundaries is reduced in the T-cross weld joint, and the formation of bainite from the grain boundaries is accelerated. As a result, the amount of martensite formed in the vicinity of weld metal is decreased and the SSCC resistance is improved. Therefore, [Cr]/[Nb] is preferably 10 or more. It is more preferably 12 or more, and still more preferably 15 or more.
Although not an essential element in the thick steel plate of the present invention, in the case where Ni is contained as a component composition, 0.25×[Cr]+[Ni] can be adjusted so as to be 0.10 to 1.50, and the SSCC resistance can also be improved by satisfying this condition. [ ] in the formula indicates mass %. In the case where the condition that 0.25×[Cr]+[Ni] is 0.10 to 1.50 is satisfied, it is not always necessary to satisfy the condition that [Cr]/[Nb] is 10 or more. The detail is described hereinafter.
The component composition of the steel material of the thick steel plate of the present invention is described as above, and the remainder is iron and inevitable impurities. Also, by further containing, in addition to the elements described above, at least one kind or two or more kinds selected from the group consisting of Mg, Ti, Ni, Cu, Mo, V, and B in the amount described below, the HIC resistance, the SSCC resistance and the like can be improved. Those elements will be explained below.

[Mg: more than 0% and 0.005% or less]

Mg has an action of forming MgS and finely dispersing the sulfide, thereby improving the SSCC resistance of the base plate. However, when Mg is contained in an amount exceeding 0.005%, the effect saturates. Therefore, it is preferred that the upper limit of the Mg amount is 0.005%. It is more preferably 0.004% or less, and still more preferably 0.003% or less.

[Ti: 0.003 to 0.030%]

Ti is an element contributing to the improvement of the strength of the thick steel plate by precipitation strengthening. In order to exert this action, it is preferred to be contained in an amount of 0.003% or more. It is more preferably 0.004% or more, and still more preferably 0.005% or more. On the other hand, when the Ti content is excessive, the amount of hard martensite in the T-cross weld joint increases, thereby deteriorating the SSCC resistance. Therefore, it is preferred to be 0.030% or less. It is more preferably 0.025% or less, and more preferably 0.020% or less.

[Ni: 0.01 to 1.50%]

Ni is an element contributing to the improvement of strength of the thick steel plate. In order to exert the action, it is preferred to be contained in an amount of 0.01% or more. It is more preferably 0.05% or more, and still more preferably 0.10% or more. On the other hand, when the Ni content is excessive, the amount of hard martensite in the T-cross weld joint increases, and the SSCC resistance is deteriorated. Therefore, it is preferred to be 1.50% or less. It is more preferably 1.00% or less, and still more preferably 0.50% or less.

[Cu: 0.01 to 1.50%]

Cu is an element contributing to the improvement of strength of the thick steel plate. In order to exert this action, it is preferred to be contained in an amount of 0.01% or more. It is more preferably 0.05% or more, and still more preferably 0.10% or more. On the other hand, when the Cu content is excessive, the amount of hard martensite in the T-cross weld joint is increased, and the SSCC resistance is deteriorated. Therefore, it is preferred to be 1.50% or less. It is more preferably 1.00% or less, and still more preferably 0.50% or less.

[Mo: 0.01 to 1.50%]

Mo is an element contributing to the improvement of strength of the thick steel plate. In order to exert this action, it is preferred to be contained in an amount of 0.01% or more. It is more preferably 0.05% or more, and still more preferably 0.10% or more. On the other hand, when the Mo content is excessive, the amount of hard martensite in the T-cross weld joint is increased, and the SSCC resistance is deteriorated. Therefore, it is preferred to be 1.50% or less. It is more preferably 1.00% or less, and still more preferably 0.50% or less.

[V: 0.003 to 0.08%]

V is an element contributing to the improvement of strength of the thick steel plate. In order to exert this action, it is preferred to be contained in an amount of 0.003% or more. It is more preferably 0.005% or more, and still more preferably 0.010% or more. On the other hand, when the V content is excessive, the amount of hard martensite in the T-cross weld joint is increased, and the SSCC resistance is deteriorated. Therefore, it is preferred to be 0.08% or less. It is more preferably 0.07% or less, and still more preferably 0.05% or less.

[B: 0.0002 to 0.0032%]

B is an element contributing to the improvement of strength of the thick steel plate. In order to exert this action, it is preferred to be contained in an amount of 0.0002% or more. It is more preferably 0.0005% or more, and still more preferably 0.0010% or more. On the other hand, when the B content is excessive, the amount of hard martensite in the T-cross weld joint is increased, and the SSCC resistance is deteriorated. Therefore, it is preferred to be 0.0032% or less. It is more preferably 0.0030% or less, and still more preferably 0.0025% or less.

[[Cr]+[Mo]+[Ni]+[Cu] is 2.1 or less]

In the thick steel plate of the present invention, [Cr]+[Mo]+[Ni]+[Cu] is preferably 2.1 or less, in addition to satisfying the component composition described before. [ ] in the formula indicates mass %. When the amount of those elements added exceeds 2.1, the amount of hard martensite in the T-cross weld joint is increased, and the SSCC resistance is deteriorated. Therefore, it is preferred that [Cr]+[Mo]+[Ni]+[Cu] is 2.1 or less. It is more preferably 1.9 or less, and still more preferably 1.7 or less.

[Immersion potential difference ΔE (immersion potential of girth weld metal - immersion potential of thick steel plate) is 25 mV or less]

In the case where potential difference between a base plate and a girth weld metal is large in the T-cross weld joint, selective corrosion of the base plate and intrusion of hydrogen in the girth weld metal are accelerated by dissimilar metal contact effect, and as a result, the SSCC resistance is deteriorated. Particularly, stable and uniform sulfide coating is not formed within 1 hour from the immersion, and hydrogen intrusion due to the potential difference between the girth weld metal and the thick steel plate becomes remarkable. Therefore, it is preferred that the immersion potential difference ΔE of the thick steel plate and girth weld metal after 1 hour when immersed in solutions, obtained by the following formula, is 25 mV or less. It is more preferably 20 mV or less, and still more preferably 15 mV or less. $ΔE = Immersion potential (mV) after 1 hour of girth weld metal - immersion potential (mV) after 1 hour of thick steel plate$
Electrode potential appearing when a metal is immersed in a solution is sometimes defined as corrosion potential or mixed potential, but this is defined as "immersion potential" in the present invention.

[0.25×[Cr]+[Ni] is 0.10 to 1.50]

In the case where potential difference between a base plate and a girth weld metal is large in the T-cross weld joint, intrusion of hydrogen in the base plate or the girth weld metal is accelerated by dissimilar metal contact effect, and as a result, the SSCC resistance is deteriorated. For this reason, in the thick steel plate of the present invention, 0.25×[Cr]+[Ni] is preferably 0.10 to 1.50, in addition to satisfying the component composition described before, particularly the condition that Ni content is 0.10 to 1.50%. [ ] in the formula indicates mass %. The addition of these elements improves potential of the steel plate as a base plate and suppresses hydrogen intrusion in the T-cross weld joint by dissimilar metal contact effect, thereby contributing to the improvement of the SSCC resistance in the T-cross weld joint.
Therefore, the value obtained from 0.25×[Cr]+[Ni] is preferably 0.10 or more, more preferably 0.15 or more, and still more preferably 0.20 or more. On the other hand, when the value obtained from 0.25×[Cr]+[Ni] is excessive, the potential of the steel plate is greatly increased than the potential of the weld metal, selective corrosion of the weld metal by galvanic corrosion makes progress, and the SSCC resistance deteriorates. Therefore, the upper limit of the value obtained from 0.25×[Cr]+[Ni] is 1.50. The upper limit is more preferably 1.00 or less, and still more preferably 0.70 or less.

(Weld metal)

It is preferred that the metal used in girth welding has the following component composition for securing strength and toughness of the weld metal and improving corrosion resistance. Specifically, it is preferred to contain, in terms of mass %, C: 0.02 to 0.10%, Si: 0.10 to 0.60%, Mn: 0.90 to 2.50%, and Ni: 0.20 to 1.00%. It is allowed to further contain P: 0.015% or less, S: 0.010% or less, Cu: 1.0% or less, Mo: 1.0% or less, Nb: 0.5% or less, V: 0.3% or less, Ti: 0.05% or less, and Al: 0.1% or less as components other than the above. It is desirable that components other than those are iron and inevitable impurities. The reasons for limiting the component composition of the weld metal are described below.

[C: 0.02 to 0.10%]

C is an element necessary in securing strength of the weld metal. When the C content is less than 0.02%, predetermined strength is not obtained. However, when the C content is excessive, grain boundary carbide is coarsened, leading to deterioration of toughness. Therefore, it is 0.10% or less.

[Si: 0.10 to 0.60%]

Si is an element necessary in securing strength the weld metal. When the Si content is less than 0.10%, predetermined strength is not obtained. However, excessive Si content leads to deterioration of toughness. Therefore, it is 0.60% or less.

[Mn: 0.90 to 2.50%]

Mn is an element necessary in securing the balance between strength and toughness of the weld metal. In order to obtain this effect, the Mn content should be 0.90% or more. However, when the Mn content is too large, segregation is promoted, leading to deterioration of toughness. Therefore, it should be 2.50% or less.

[Ni: 0.20 to 1.00%]

Ni exerts the effect of increasing the potential of the weld metal and improving corrosion resistance. Further, it is an element effective in securing strength by increasing quenchability, and improving low temperature toughness. In order to obtain this effect, the Ni content should be 0.20% or more. On the other hand, when the Ni content is excessive, high temperature cracking is likely induced, and further the potential of the weld metal is increased excessively, leading to the generation of selective corrosion of the base plate. Therefore, the upper limit is 1.00% or less.

(Composition of Inclusions in Thick Steel Plate)

[Composition of Inclusions with 1 µm or more Width Contained in Steel]

[Zr amount is 1 to 40%]

In the present invention, Zr in the inclusions with 1 µm or more width is mainly present as an oxide. The Zr oxide has a coefficient of thermal expansion smaller than that of steel. Therefore, when the Zr amount in the inclusions is secured, voids against the surrounding steel matrix phase can be reduced, thereby improving the HIC resistance. Also, the oxide containing an appropriate amount of Zr accelerates the formation of intragranular bainite, and as a result, good SSCC resistance can be obtained. In order to exert the effect, the Zr content in the inclusions is 1 to 40%. When the Zr amount is less than 1% or exceeds 40%, the HIC resistance of the base plate or the SSCC resistance of the T-cross weld joint becomes insufficient.

[REM amount is 5 to 50%]

In the present invention, the REM in the inclusions with 1 µm or more width is present as an oxide, oxysulfide and the like. Of those, REM oxide has a coefficient of thermal expansion smaller than that of steel. Therefore, when the REM amount in the inclusions is secured, voids against the surrounding steel matrix phase can be reduced, thereby improving the HIC resistance. When present as oxysulfide, S is fixed, and the formation of a sulfide such as MnS adversely affecting the HIC resistance can be suppressed. Furthermore, those REM inclusions accelerate the formation of intragranular bainite, and as a result, good SSCC resistance can be obtained. In order to exert the effect, the REM amount in the inclusions is 5 to 50%. When the REM amount is less than 5% or exceeds 50%, the HIC resistance of the base plate or the SSCC resistance of the T-cross weld joint becomes insufficient.

[Al amount is 3 to 30%]

In the present invention, Al in the inclusions with a width of 1 µm or more is mainly present as an Al oxide. The Al oxide has a coefficient of thermal expansion smaller than that of steel. Therefore, when the Zr amount in the inclusions is secured, voids against the surrounding steel matrix phase can be reduced, and this is effective to improve the HIC resistance. Also, the oxide containing an appropriate amount of Al accelerates the formation of intragranular bainite, and as a result, good SSCC resistance can be obtained. In order to exert the effect, the Al amount in the inclusions is 3 to 30%. When the Al amount is less than 3% or exceeds 30%, the HIC resistance of the base plate or the SSCC resistance of the T-cross weld joint becomes insufficient.

[Ca amount is 5 to 60%]

In the present invention, Ca in the inclusions with 1 µm or more width contributes to the formation of the fine microstructure of steel in the T-cross weld joint during welding, and accelerates the formation of intragranular bainite structure originated from the inclusions. By this, the microstructure of steel in the T-cross weld joint after welding becomes fine, and good SSCC resistance can be obtained. In order to exert the effect, the Ca amount in the inclusions is 5 to 60%. When the Ca amount is less than 5% or exceeds 60%, the SSCC resistance in the T-cross weld joint cannot be improved.

[S amount is more than 0% and 20% or less]

S amount in the inclusions with 1 µm or more width can be reduced by limiting S content in a steel plate and the content of alloy components such as Zr and REM which refine and disperse sulfur inclusions, to the component composition described above, and additionally controlling the composition of the inclusions as described above. When the component composition and the composition of the inclusions are not appropriately controlled, the S amount in the inclusions exceeds 20%, coarse sulfide becomes excessive, and as a result, the HIC resistance of the base plate or the SSCC resistance of the T-cross weld joint becomes insufficient. On the other hand, in a steel plate having the S amount in the inclusions controlled to 20% or less, good HIC resistance and SSCC resistance are obtained. The S amount in the inclusions is better as being smaller. However, when it is 0%, it is considered that S cannot entirely be fixed by the inclusions and the HIC resistance of the base plate or the SSCC resistance of the T-cross weld joint becomes insufficient.
The total number of the inclusions is not particularly limited so long as the effect of the present invention is not remarkably impaired, but it is preferred that about 500 to 5,000 /cm² are dispersed in a steel plate. When it is less than 500 /cm², it is considered that the origin of intragranular bainite becomes insufficient, sufficient effect of formation of fine microstructure is not obtained, and as a result, the SSCC resistance deteriorates. On the other hand, when it exceeds 5,000 /cm², the inclusions act as the origin of fracture, and there is a possibility that both HIC resistance and SSCC resistance deteriorate.

(Manufacturing Method)

Next, a method for manufacturing the thick steel plate of the present invention will be explained in detail below.

[Molten Steel Processing Step]

In order to obtain the thick steel plate of the present invention having the microstructure described above, it is necessary in the molten steel processing step that (A) a desulfurizing step of making S 0.004% or less by using a slag that satisfies Fe: 0.1 to 10%, (B) a deoxidizing step of making a dissolved oxygen concentration "Of" of molten steel 10 or less in terms of a ratio relative to S concentration of the molten steel (Of/S), and (C) a step of adding Al, Zr, REM, and Ca in the order of Al, Zr, REM, and Ca, or adding Al, adding Zr and REM simultaneously and then adding Ca in this order, are included in this order; Ca is added 4 minutes or more later after adding REM; the time after adding Ca until completion of solidification is 200 minutes or less; and a cooling time at t/4 position of a slab of 1,300°C to 1,200°C in casting is 460 seconds or less. Also, the cooling time at t/4 position of the slab of 1,500°C to 1,450°C in casting is 300 seconds or less. The t above indicates a plate thickness. Each step will be explained below in order.

(A) Desulfurizing Step

In order to secure the HIC resistance, reduction of coarse sulfides is important, and in order to achieve this, it is important to control the S amount. In a converter or an electric furnace, with respect to molten steel melted such that a molten steel temperature becomes 1,550°C or higher, a slag satisfying Fe: 0.1 to 10% is used and S is set to 0.004% or less. By increasing Fe concentration in the slag, REM and Zr added after desulfurization and deoxidizing can form oxides preferentially without being dissolved in the molten steel. In order to obtain this effect, the Fe concentration in the slag is 0.1% or more. The Fe concentration in the slag is preferably 0.5% or more, and more preferably 1.0% or more. On the other hand, when the Fe concentration in the slag exceeds 10%, oxides are formed excessively, and the oxides become the origin of hydrogen-induced cracking. Therefore, the Fe concentration in the slag is 10% or less. It is preferably 8% or less, and more preferably 5% or less. Also, in adding Ca, by sufficiently executing desulfurization in the slag and suppressing S to 0.004% or less, CaS can be prevented from being formed excessively when Ca is added after adding REM, the composition of inclusions can be prevented from deviating from a predetermined range, and as a result, the HIC resistance and the SSCC resistance can be secured.
In order to achieve S: 0.004% or less, the CaO concentration in the slag is 10% or more. By the addition of Ca, CaO in the slag reacts with dissolved S in the molten steel and changes into CaS, thereby reduction of S in the molten steel, namely desulfurization, can be executed sufficiently. Also, at this time, if the CaO concentration in the slag is 10% or more, it is possible to adjust S to 0.004% or less. The CaO concentration in the slag is preferably 15% or more, and more preferably 20% or more. On the other hand, when CaO in the slag is excessive, desulfurization becomes difficult, and therefore the upper limit is approximately 80%.

(B) Deoxidizing Step

In order to improve the SSCC resistance, it is important to control oxides, and it is vital to control O amount in order to achieve this. In this step, because S amount that is influential for the HIC resistance slightly increases, namely so-called S-return takes place, it is important to control the O amount and the S amount simultaneously. In this step, prior to the addition of REM as described below, the dissolved oxygen concentration "Of" of the molten steel is 10 or less in terms of the ratio relative to the S concentration of the molten steel (Of/S). REM forms, when added to the molten steel, oxides at the same time of forming the sulfides thereof. When the above Of/S exceeds 10, the major portion of the REM added forms oxides, and the composition of the inclusions deviates from the predetermined range. As a result, the HIC resistance and the SSCC resistance deteriorate. Therefore, in the present invention, Of/S is 10 or less as described above. The Of/S is preferably 5 or less, more preferably 3.5 or less, and still more preferably 2 or less. Also, the lower limit of the Of/S is approximately 0.1. Adjusting Of/S to 10 or less as described above can be achieved by executing at least one deoxidation of deoxidation by an RH degassing apparatus and deoxidation by feeding deoxidizing elements such as Mn, Si and Ti.

(C) Adding Step of Al, Zr and REM

With respect to addition of Al, Zr and REM to the molten steel, Al is first added, and Zr and REM are then added. In this regard, when deoxidizing capacity of Al, and Zr and REM is compared, because the deoxidizing power of Zr and REM is stronger than that of Al, if Zr and REM are added prior to Al, the Al amount in inclusions cannot be made to be a desired value. Therefore, Al should be added prior to Zr and REM.
In the case of adding Ca, considering the desulfurizing powder and the deoxidizing power of each adding element described below, either method of adding Al first, then adding Zr, next adding REM, and adding Ca lastly; or adding Al first, then adding Zr and REM at the same time, and adding Ca lastly is to be employed. However, in either case, the time after adding REM until adding Ca is 4 minutes or more.
The reason of the above will be explained. First, when the desulfurizing capacity of REM and Ca is compared, the desulfurizing powder of REM is weaker than that of Ca, therefore, if Ca is added before adding REM, a large amount of CaS is formed, the composition of the inclusions deviates from the predetermined range, and as a result, the HIC resistance and the SSCC resistance are deteriorated. For this reason, REM should be added before adding Ca, and therefore, the adding order of Al, Zr, REM, and Ca should be Al→(Zr and REM)→Ca. Also, in order to control the composition of the inclusions to the predetermined range, the time after adding REM until adding Ca should be 4 min or more. The time after adding REM until adding Ca is preferably 5 min or more, and more preferably 8 min or more. From the viewpoint of the productivity, the upper limit of the time after adding REM until adding Ca is approximately 60 min.
Next, when the deoxidizing capacity of Zr, REM and Ca is compared, it is generally considered that the deoxidizing power of Ca is strongest and the deoxidizing power is in the order of Ca>REM>Zr. Thus, Zr is lowest. Therefore, in order to contain Zr in the inclusions, namely in order to form ZrO₂ as oxide-based inclusions, Zr should be added prior to adding Ca and REM whose deoxidizing power is stronger than that of Zr. For this reason, the adding order of Al, Zr, REM, and Ca should be Al→Zr→REM→Ca. However, because the deoxidizing capacity of REM is smaller as compared with Ca, even if REM is added simultaneously with Zr, it is possible to contain Zr in the inclusions. Therefore, the adding order of those may also be Al→(Zr and REM)→Ca.
With respect to the adding amount of each component above, the steel plate having each desired element amount only has to be obtained, and such a method can be cited for example to add Zr so as to become 3 to 100 ppm in terms of the concentration in the molten steel, to add REM thereafter or simultaneously so as to become 2 to 500 ppm in terms of the concentration in the molten steel, and, after 4 min or more elapses thereafter, to add Ca so as to become 3 to 60 ppm in terms of the concentration in the molten steel.

[Casting Step]

Casting is started quickly, for example, in 80 min or less after Ca is added as described above, and casting is executed so that the time after adding Ca until completion of solidification is 200 min or less. The reason of this is as follows. Because Ca is an element that is high in both desulfurization capacity and deoxidizing capacity, the composition of oxides and sulfides is liable to convert to stable CaS and CaO with the lapse of time after adding Ca, and the composition of inclusions cannot be made to fall within the predetermined range. Therefore, in the present invention, the time after adding Ca until completion of solidification is 200 min or less. It is preferably 180 min or less, and more preferably 160 min or less. The lower limit of the time is approximately 4 min from the viewpoint of homogenizing Ca.
When the cooling time at t/4 position of 1,500 to 1,450°C slab in casting exceeds 300 sec, complex formation of the oxide-based secondary inclusions on the inclusions is promoted, and the composition of the inclusions deviates from the predetermined range. As result, the SSCC resistance is deteriorated.
Furthermore, it is important that the cooling time at t/4 position of 1,300°C to 1,200°C slab in casting is 270 to 460 sec. When the cooling time exceeds the upper limit, complex formation of the sulfide-based secondary inclusions on the inclusions is mainly promoted, and the composition of the inclusions deviates from the predetermined range. As a result, the SSCC resistance is deteriorated. On the other hand, when the cooling time is lower than the lower limit, the cooling load significantly increases, which is not preferable practically.

[Steps of Rolling and Thereafter]

After the solidification described above, hot rolling is executed according to an ordinary method, and the thick steel plate can be manufactured. Also, by using the steel plate, a steel pipe for a line pipe can be manufactured by a method generally executed. Although the steps of rolling and thereafter are not particularly limited, it is preferable, for example, to heat a cast slab to 1,100°C or above to execute hot rolling with the compression reduction of 40% or more at a recrystallization temperature region, followed by accelerated cooling in a cooling rate of 10 to 20°C/s from 780°C. The conditioning thereafter is not necessary.

[Welded Joint]

In the present invention, a welded joint using the thick steel plate of the present invention is provided. The welded joint includes the thick steel plate and a girth weld metal, and is obtained by welding the edge of the thick steel plate with a girth weld metal. The welded joint of the present invention preferably has immersion potential difference ΔE between the thick steel plate and the girth weld metal, obtained by the following formula, being 25 mV or less. $ΔE = Immersion potential (mV) after 1 hour of girth weld metal - immersion potential (mV) after 1 hour of thick steel plate$
As described above, in the case where potential difference between a base plate and a girth weld metal is large in the T-cross weld joint, selective corrosion of the base plate and intrusion of hydrogen in the girth weld metal are accelerated by dissimilar metal contact effect, and as a result, the SSCC resistance is deteriorated. By using the thick steel plate of the present invention, the immersion potential difference ΔE is 25 mV or less, and the deterioration of the SSCC resistance in the weld joint can be suppressed. The immersion potential difference ΔE is more preferably 20 mV or less, and still more preferably 15 mV or less.
The weld metals described above are preferably used as the metal to be used in the girth welding.
A welding method for forming the welded joint is not particularly limited, and can be performed by conventional methods. Examples thereof include arc welding, laser welding and electron beam welding.

Examples

The present invention is descried in more detail below by reference to Examples. However, the present invention is not limited by the following Examples and can be performed by adding appropriate modifications in a range capable of conforming to the gist of the present invention, and those are included in the technical scope of the present invention.

(Examples 1 to 33 and Comparative Examples 1 to 19)

The molten steel refined in a 240t converter by an ordinary method was subjected to processing such as desulfurizing, deoxidizing, composition regulating, and inclusion controlling by using an LF furnace, various kinds of molten steel having the steel composition and the composition of the inclusions in steel shown in Tables 1 to 4 were prepared as slabs by the continuous casting method, those were subjected to hot rolling and then accelerated cooling, and thick steel plates with 40 mm thickness and 3,500 mm width were manufactured. By using the thick steel plates obtained and girth weld metals, welded joints described after were manufactured. Tables 5 and 6 show the main process conditions in the molten steel processing, continuous casting and accelerated cooling described above. Tables 7 and 8 show the various properties of each steel plate thus obtained. The analyzing method for the composition of the inclusions shown in Tables 3 and 4, and the measuring method and the evaluating method of each property of Tables 7 and 8 will be explained below.

[Analysis of Composition of Inclusions]

Analysis of the composition of the inclusions was executed as follows. The cross section in the plate thickness direction of the as-rolled material was observed focusing the plate thickness central part by using EPMA-8705 manufactured by Shimadzu Corporation. In detail, 3 cross sections were observed with 400 observation magnification and approximately 50 mm² field of view, and the component composition at the inclusion central part was quantitatively analyzed by wavelength dispersion spectrometry of the characteristic X-ray for the inclusions with 1 µm or more width. The observation field was the range of 7 mm in the plate thickness direction and 7 mm in the plate width direction such that the plate thickness center part became the central. The elements of the analyzing object were Al, Mn, Si, Mg, Ca, Ti, Zr, S, REM, and Nb. The REM used herein means La, Ce, Nd, Dy, and Y. The relationship between the X-ray strength and the element concentration of each element was obtained beforehand as an analytical curve by using known substances, and the element concentration of the inclusions was then quantitatively determined from the X-ray strength obtained from the inclusions and the analytical curve described above. Furthermore, the average value of the content of each element described above of the inclusions with 1 µm or more width in 3 cross sections described above was obtained, and was defined as the composition of inclusions.

[Measurement and Evaluation of Yield Strength YS of Base Plate]

No. 4 specimen of JIS Z2241 was taken in parallel with C direction from the t/4 position of each thick steel plate, the tensile test was executed by the method described in JIS Z2241 to measure the yield strength YS, and the strength of each thick steel plate was confirmed.

[Test and Evaluation of HIC Resistance]

The test and evaluation were executed according to the method defined in NACE standard TM0248-2003. Specifically, a specimen was dipped in 25°C 5 mass % NaCl+0.5 mass % CH₃COOH aqueous solution saturated with 1 atm hydrogen sulfide for 96 hours. With respect to evaluation of the HIC test, each specimen was cut at 10 mm pitch in the longitudinal direction, the cut surface was polished, all cross sections were observed with 100 magnifications by using an optical microscope, and the presence or absence of the cracking with 1 mm or more of the cracking length of HIC was confirmed.

[Test and Evaluation of SSCC Resistance of T-Cross Weld Joint]

In order to simulate the seam welding, the thick steel plate having the steel composition shown in Tables 1 and 2 was worked to have an X groove of 75°, and welding was executed by 2-pass submerged arc welding, thereby manufacturing the pipe. The heat input during welding was first pass: 3.7 kJ/mm and second pass: 5.4 kJ/mm. Also, in order to simulate the girth welding in joining the pipes with each other, 1 pass bead-on-plate welding by gas shield arc welding was executed so as to orthogonally cross the seam weld line by reference to "Practical application to UOE steel pipe excellent in SSCC resistance, Matsuyama et al., Welding Technology, September 1988, p. 58", thereby manufacturing the welded joint. The heat input during welding was 1.0 kJ/mm. Lincolnweld LA-81 (manufactured by Lincoln) was used as the welding metal in seam welding, and MX-A55Ni1 (manufactured by Kobe Steel, Ltd.) was used as the welding metal in girth welding.
The surface of the weld part of the pipe joint body after welding was subjected to grinding, and the excess metal portion of the bead welding was removed. From right below the bead weld part of this pipe joint body, a specimen of 115L×15W×5t was taken so that the longitudinal direction became parallel with the bead weld line. By using this specimen, the SSCC resistance evaluation test with 4-point bending test piece was executed based on ASTM G39, NACE TM0177-2005, Method B. Deflection equivalent to 388 MPa and 437 MPa of the load stress was applied, and immersion was performed for 720 hours in the NACE solution A: 5 mass % NaCl-0.5 mass % CH₃COOH, saturated with 1 atm hydrogen sulfide. The presence or absence of cracking on the surface of the specimen was executed by optical microscope observation with 10 magnifications.

[Evaluation of Immersion Potential Difference ΔE (Immersion Potential of Girth Weld Metal - Immersion Potential of Thick Steel Plate)]

Each of corrosion test specimens (20 mm vertical and horizontal × 2 mm thick) collected from a part of the thick steel plates having the steel compositions shown in Tables 1 and 2 and the above-described girth weld metals was wet polished with SiC #600 abrasive paper, and then subjected to ultrasonic cleaning. A lead was attached to the specimen with spot welding. The portion other than the test surface of the thick steel plate or the girth weld metal was covered with an epoxy resin. The specimen was dipped in NACE solution A (5 mass % NaCl-0.5 mass % CH₃COOH) saturated with 1 atm hydrogen sulfide. The potential after 1 hour from the dipping was measured. Saturated calomel electrode was used as a reference electrode, and the value obtained by subtracting immersion potential of the thick steel plate from immersion potential of the girth weld metal was calculated as the immersion potential difference ΔE.
As is apparent from the comparison of each property of the Examples in Table 7 and the Comparative Examples in Table 8 showing those results, the thick steel plates of the Examples satisfying the component composition and the composition of the coarse inclusions with 1 µm or more width in steel, defined in the present invention achieve high mechanical strength, and in addition to this, cracking by the HIC test is not generated, and the HIC resistance was excellent. Also, it was confirmed in the Examples that cracking was not generated in the SSCC test in which the deflection equivalent to 388 MPa of the load stress was applied. Furthermore, it was confirmed in Examples 4 to 33 that cracking is not generated in the SSCC resistance evaluation test in which the deflection equivalent to 438 MPa of the load stress was applied, and excellent SSCC resistance is achieved.

On the other hand, in the thick steel plates of the Comparative Examples that do not satisfy the component composition or the composition of coarse inclusions, defined in the present invention, the generation of cracking was confirmed in the HIC resistance test or the SSCC resistance test in which the deflection equivalent to 388 MPa of the load stress was applied.

[Table 1]

	Component composition (mass %)														Cr/ Nb	Ni+Cu+ Cr+Mo	Ni+ 0.25Cr
	C	Si	Mn	P	S	Al	Cr	Nb	REM	Zr	Ca	N	O	Others	Cr/ Nb	Ni+Cu+ Cr+Mo	Ni+ 0.25Cr
Ex. 1	0.05	0.40	0.92	0.007	0.0014	0.025	0.21	0.028	0.0040	0.0033	0.0022	0.0034	0.0019		7.5	0.2	0.05
Ex. 2	0.08	0.16	0.88	0.007	0.0011	0.021	0.18	0.024	0.0024	0.0032	0.0043	0.0044	0.0018		7.5	0.2	0.05
Ex. 3	0.06	0.21	1.53	0.006	0.0013	0.022	0.25	0.027	0.0021	0.0036	0.0028	0.0041	0.0019	Ti:0.014	9.3	0.3	0.06
Ex. 4	0.04	0.17	1.61	0.006	0.0017	0.027	0.46	0.017	0.0029	0.0019	0.0025	0.0044	0.0016		27.1	0.5	0.12
Ex. 5	0.03	0.31	1.26	0.007	0.0016	0.026	0.25	0.016	0.0034	0.0022	0.0024	0.0046	0.0017		15.6	0.3	0.06
Ex. 6	0.05	0.47	1.47	0.006	0.0013	0.021	0.45	0.020	0.0029	0.0020	0.0025	0.0047	0.0021		22.5	0.5	0.11
Ex. 7	0.04	0.32	1.33	0.007	0.0022	0.026	0.32	0.030	0.0038	0.0010	0.0026	0.0029	0.0017		10.7	0.3	0.08
Ex. 8	0.03	0.33	1.28	0.007	0.0014	0.022	0.44	0.028	0.0251	0.0029	0.0027	0.0048	0.0018	Ti:0.014	15.7	0.4	0.11
Ex. 9	0.05	0.19	1.74	0.007	0.0012	0.022	0.43	0.004	0.0027	0.0028	0.0027	0.0027	0.0030	Ti:0.018, Ni:0.61	107.5	1.0	0.72
Ex. 10	0.01	0.08	1.20	0.008	0.0016	0.024	0.51	0.015	0.0024	0.0037	0.0027	0.0046	0.0020	Mo:0.06	34.0	0.6	0.13
Ex. 11	0.04	0.03	1.65	0.007	0.0013	0.025	1.24	0.022	0.0022	0.0055	0.0027	0.0040	0.0037	Ni:0.35, Cu:0.11	56.4	1.7	0.66
Ex. 12	0.11	0.15	1.39	0.007	0.0015	0.023	0.77	0.021	0.0018	0.0017	0.0026	0.0044	0.0018	Ti:0.003	36.7	0.8	0.19
Ex. 13	0.04	0.41	0.66	0.024	0.0019	0.022	0.61	0.027	0.0105	0.0019	0.0019	0.0043	0.0019	Ti:0.023, Ni:0.81	22.6	1.4	0.96
Ex. 14	0.05	0.32	1.91	0.007	0.0021	0.029	0.87	0.031	0.0023	0.0041	0.0029	0.0038	0.0021	Ti:0.027, B:0.0004	28.1	0.9	0.22
Ex. 15	0.06	0.20	1.88	0.003	0.0013	0.022	1.43	0.022	0.0004	0.0022	0.0030	0.0049	0.0022	Mg:0.0016, V:0.004	65.0	1.4	0.36
Ex. 16	0.09	0.28	1.73	0.003	0.0011	0.021	0.29	0.025	0.0019	0.0004	0.0022	0.0051	0.0029	Ti:0.014, Mo:0.44	11.6	0.7	0.07
Ex. 17	0.04	0.20	1.53	0.007	0.0010	0.020	0.46	0.034	0.0007	0.0065	0.0026	0.0067	0.0017	Ti:0.014	13.5	0.5	0.12
Ex. 18	0.02	0.19	1.82	0.005	0.0008	0.067	0.30	0.008	0.0021	0.0009	0.0036	0.0048	0.0016	Ni:0.06	37.5	0.4	0.14
Ex. 19	0.05	0.14	1.20	0.007	0.0033	0.022	0.48	0.011	0.0020	0.0041	0.0054	0.0047	0.0027	Ti:0.016, Cu:0.06	43.6	0.5	0.12
Ex. 20	0.04	0.18	0.84	0.012	0.0035	0.021	0.43	0.021	0.0014	0.0086	0.0004	0.0044	0.0011	Ti:0.014, B:0.0031	20.5	0.4	0.11
Ex. 21	0.05	0.17	1.66	0.007	0.0012	0.022	0.50	0.017	0.0035	0.0028	0.0046	0.0095	0.0023	Cu:1.16	29.4	1.7	0.13
Ex. 22	0.10	0.35	1.30	0.005	0.0016	0.023	0.51	0.012	0.0031	0.0020	0.0038	0.0049	0.0013	Ti:0.014, Ni:0.30, Cu:0.63	42.5	1.4	0.43
Ex. 23	0.03	0.27	0.94	0.006	0.0012	0.021	0.72	0.015	0.0063	0.0025	0.0020	0.0041	0.0009	Ni:0.28, Mo:0.07	48.0	1.1	0.46
Ex. 24	0.07	0.41	1.71	0.018	0.0013	0.043	0.96	0.020	0.0059	0.0017	0.0008	0.0043	0.0021	Ti:0.022, B:0.0018	48.0	1.0	0.24
Ex. 25	0.05	0.33	1.53	0.007	0.0014	0.012	0.70	0.021	0.0070	0.0019	0.0022	0.0042	0.0017	Mo:1.19	33.3	1.9	0.18
Ex. 26	0.05	0.40	1.55	0.007	0.0028	0.051	0.51	0.018	0.0023	0.0032	0.0019	0.0044	0.0014	Mg:0.0032, Ni:1.24	28.3	1.8	1.37
Ex. 27	0.07	0.06	1.22	0.007	0.0019	0.024	0.33	0.009	0.0028	0.0033	0.0024	0.0078	0.0013	V:0.022, B:0.0013	36.7	0.3	0.08
Ex. 28	0.09	0.41	1.22	0.005	0.0010	0.024	0.47	0.007	0.0380	0.0036	0.0020	0.0033	0.0013	V:0.071	67.1	0.5	0.12
Ex. 29	0.03	0.16	1.55	0.007	0.0008	0.019	0.41	0.018	0.0021	0.0029	0.0015	0.0026	0.0018	Ni:0.25	22.8	0.7	0.35
Ex. 30	0.03	0.15	1.50	0.005	0.0009	0.022	0.28	0.029	0.0022	0.0030	0.0018	0.0029	0.0019	Ni:0.30	9.7	0.6	0.37
Ex. 31	0.08	0.20	1.50	0.006	0.0018	0.025	0.17	0.011	0.0049	0.0033	0.0039	0.0040	0.0020		15.5	0.2	0.04
Ex. 32	0.03	0.16	1.54	0.005	0.0008	0.021	0.41	0.045	0.0030	0.0010	0.0017	0.0041	0.0016	Ti:0.012, Ni:0.24, Cu:0.25, Mo:0.10	9.1	1.0	0.34
Ex. 33	0.06	0.30	1.03	0.005	0.0009	0.021	0.20	0.030	0.0026	0.0012	0.0013	0.0051	0.0015	Ti:0.013, Ni:0.25, Cu:0.26	6.7	0.7	0.30

[Table 2]

	Component composition (mass %)														Cr/ Nb	Ni+Cu+ Cr+Mo	Ni+ 0.25Cr
	C	Si	Mn	P	S	Al	Cr	Nb	REM	Zr	Ca	N	O	Others	Cr/ Nb	Ni+Cu+ Cr+Mo	Ni+ 0.25Cr
Com. Ex. 1	0.008	0.52	1.61	0.007	0.0019	0.024	0.29	0.010	0.0051	0.0028	0.0027	0.0056	0.0018		29.0	0.3	0.07
Com. Ex. 2	0.13	0.28	1.49	0.007	0.0020	0.021	0.52	0.020	0.0020	0.0031	0.0021	0.0041	0.0016		26.0	0.5	0.13
Com. Ex. 3	0.06	0.40	0.56	0.032	0.0045	0.029	0.66	0.020	0.0028	0.0020	0.0020	0.0040	0.0015		33.0	0.7	0.17
Com. Ex. 4	0.05	0.33	2.06	0.007	0.0015	0.021	0.63	0.028	0.0509	0.0029	0.0025	0.0046	0.0021		22.5	0.6	0.16
Com. Ex. 5	0.04	0.19	1.88	0.007	0.0042	0.025	0.41	0.021	0.0044	0.0017	0.0022	0.0044	0.0022		19.5	0.4	0.10
Com. Ex. 6	0.05	0.28	1.27	0.008	0.0013	0.008	0.47	0.027	0.0010	0.0015	0.0021	0.0045	0.0017		17.4	0.5	0.12
Com. Ex. 7	0.06	0.22	1.71	0.005	0.0011	0.081	0.24	0.019	0.0023	0.0033	0.0026	0.0045	0.0026	Ni:1.53	12.6	1.8	1.59
Com. Ex. 8	0.09	0.08	1.32	0.009	0.0021	0.032	0.16	0.013	0.0024	0.0105	0.0020	0.0045	0.0017		12.3	0.2	0.04
Com. Ex. 9	0.07	0.12	1.33	0.005	0.0020	0.030	1.58	0.025	0.0071	0.0016	0.0027	0.0045	0.0010		63.2	1.6	0.40
Com. Ex. 10	0.05	0.15	1.31	0.007	0.0009	0.023	0.82	0.001	0.0001	0.0019	0.0019	0.0045	0.0021		820.0	0.8	0.21
Com. Ex. 11	0.08	0.18	1.51	0.005	0.0011	0.024	0.61	0.037	0.0031	0.0020	0.0026	0.0041	0.0022		16.5	0.6	0.15
Com. Ex. 12	0.07	0.22	1.44	0.007	0.0010	0.021	0.47	0.014	0.0024	0.0002	0.0020	0.0049	0.0017	V:0.081	33.6	0.5	0.12
Com. Ex. 13	0.03	0.31	1.73	0.008	0.0010	0.022	0.29	0.016	0.0021	0.0026	0.0002	0.0039	0.0018		18.1	0.3	0.07
Com. Ex. 14	0.09	0.27	1.10	0.007	0.0018	0.025	0.45	0.018	0.0046	0.0023	0.0064	0.0040	0.0019	Ti:0.032	25.0	0.5	0.11
Com. Ex. 15	0.10	0.33	1.05	0.007	0.0016	0.021	0.51	0.018	0.0022	0.0033	0.0036	0.0112	0.0042		28.3	0.5	0.13
Com. Ex. 16	0.06	0.40	1.61	0.009	0.0011	0.037	0.86	0.021	0.0035	0.0019	0.0022	0.0044	0.0000	Mg:0.0052	41.0	0.9	0.22
Com. Ex. 17	0.05	0.36	1.81	0.007	0.0015	0.020	0.71	0.014	0.0031	0.0031	0.0031	0.0044	0.0018	Ni:0.77, Cu:0.51, Mo:0.23	50.7	2.2	0.95
Com. Ex. 18	0.07	0.31	1.44	0.008	0.0012	0.021	0.21	0.016	0.0050	0.0024	0.0032	0.0051	0.0019	Cu:1.61, B:0.0035	13.1	1.8	0.05
Com. Ex. 19	0.07	0.09	0.95	0.005	0.0013	0.029	0.24	0.022	0.0044	0.0020	0.0032	0.0037	0.0015	Mo:1.60	10.9	1.8	0.06

[Table 3]

	Composition of coarse inclusions in steel (mass %)
	Zr amount	REM amount	Al amount	S amount	Ca amount
Ex. 1	14	22	13	11	27
Ex. 2	10	12	10	8	55
Ex. 3	12	22	9	14	28
Ex. 4	11	20	10	13	38
Ex. 5	12	20	12	14	29
Ex. 6	5	20	15	13	42
Ex. 7	10	19	14	16	29
Ex. 8	7	38	8	18	21
Ex. 9	11	18	13	15	30
Ex. 10	10	19	9	18	30
Ex. 11	25	26	12	14	7
Ex. 12	12	23	14	10	25
Ex. 13	7	33	15	12	24
Ex. 14	8	29	11	10	35
Ex. 15	11	6	14	10	48
Ex. 16	8	30	15	12	24
Ex. 17	31	11	8	8	12
Ex. 18	13	8	30	5	33
Ex. 19	8	10	5	11	58
Ex. 20	36	8	9	17	6
Ex. 21	14	21	11	15	31
Ex. 22	15	22	19	13	21
Ex. 23	10	45	10	10	17
Ex. 24	10	33	24	11	16
Ex. 25	12	40	8	5	28
Ex. 26	15	17	25	15	20
Ex. 27	18	7	3	11	51
Ex. 28	8	46	10	12	20
Ex. 29	15	20	14	13	22
Ex. 30	16	19	16	14	20
Ex. 31	12	29	15	12	24
Ex. 32	15	10	15	10	18
Ex. 33	14	15	19	15	22

[Table 4]

	Composition of coarse inclusions in steel (mass %)
	Zr amount	REM amount	Al amount	S amount	Ca amount
Com. Ex. 1	25	4	2	13	50
Com. Ex. 2	18	26	12	10	29
Com. Ex. 3	7	9	7	21	7
Com. Ex. 4	7	48	11	8	17
Com. Ex. 5	8	8	8	21	51
Com. Ex. 6	19	27	4	11	34
Com. Ex. 7	8	20	28	15	21
Com. Ex. 8	42	8	7	7	12
Com. Ex. 9	17	30	12	9	21
Com. Ex. 10	11	3	32	15	31
Com. Ex. 11	20	4	17	14	42
Com. Ex. 12	0.4	22	18	24	20
Com. Ex. 13	18	41	15	15	4
Com. Ex. 14	3	12	11	6	63
Com. Ex. 15	18	35	20	10	2
Com. Ex. 16	10	52	7	3	7
Com. Ex. 17	21	20	14	8	20
Com. Ex. 18	20	31	4	13	12
Com. Ex. 19	2	29	25	6	24

[Table 5]

	Fe concentration in slag (%)	CaO concentration in slag (%)	Of/S ratio	Addition order of Al, Zr, REM and Ca	Time from REM addition until Ca addition (min)	Time from Ca addition until completion of solidification (min)	Casting time of 1,500 to 1,450°C (s)	Casting time of 1,300 to 1,200°C (s)
Ex. 1	1.5	40	0.4	Al→(Zr, REM)→Ca	10	140	268	280
Ex. 2	1.5	40	0.4	Al→(Zr, REM)→Ca	9	140	251	300
Ex. 3	1.5	40	0.4	Al→(Zr, REM)→Ca	10	145	252	280
Ex. 4	1.4	41	0.4	Al→(Zr, REM)→Ca	16	150	240	310
Ex. 5	1.5	40	0.5	Al→(Zr, REM)→Ca	10	150	242	300
Ex. 6	1.5	42	0.4	Al→(Zr, REM)→Ca	13	200	277	310
Ex. 7	1.5	44	0.4	Al→(Zr, REM)→Ca	10	140	224	310
Ex. 8	2.0	41	0.4	Al→(Zr, REM)→Ca	20	150	185	320
Ex. 9	1.5	40	0.3	Al→Zr→REM→Ca	10	150	249	320
Ex. 10	1.5	40	0.4	Al→(Zr, REM)→Ca	10	145	204	300
Ex. 11	1.4	41	0.4	Al→(Zr, REM)→Ca	11	140	291	280
Ex. 12	1.5	39	0.4	Al→(Zr, REM)→Ca	10	145	281	340
Ex. 13	1.5	40	0.4	Al→(Zr, REM)→Ca	25	190	270	320
Ex. 14	1.5	40	0.9	Al→(Zr, REM)→Ca	10	160	271	330
Ex. 15	4.5	39	0.4	Al→Zr→REM→Ca	10	135	246	300
Ex. 16	1.5	42	1.5	Al→(Zr, REM)→Ca	12	140	280	330
Ex. 17	1.5	38	4	Al→(Zr, REM)→Ca	8	155	279	300
Ex. 18	1.8	40	0.4	Al→Zr→REM→Ca	5	140	262	310
Ex. 19	1.5	70	0.4	Al→(Zr, REM)→Ca	10	140	255	290
Ex. 20	1.5	12	0.4	Al→(Zr, REM)→Ca	11	145	230	270
Ex. 21	9.5	44	0.4	Al→(Zr, REM)→Ca	10	145	282	320
Ex. 22	1.5	41	0.4	Al→(Zr, REM)→Ca	10	150	255	320
Ex. 23	3.1	40	8	Al→(Zr, REM)→Ca	11	140	216	300
Ex. 24	2.4	37	0.4	Al→Zr→REM→Ca	10	150	242	440
Ex. 25	7.5	40	0.4	Al→(Zr, REM)→Ca	10	140	267	310
Ex. 26	1.1	40	0.4	Al→(Zr, REM)→Ca	10	145	277	400
Ex. 27	0.2	40	0.4	Al→(Zr, REM)→Ca	9	150	248	310
Ex. 28	1.5	38	0.4	Al→(Zr, REM)→Ca	10	150	261	340
Ex. 29	1.4	41	0.4	Al→(Zr, REM)→Ca	10	150	249	320
Ex. 30	1.5	40	0.4	Al→(Zr, REM)→Ca	10	150	242	310
Ex. 31	1.3	42	0.4	Al→(Zr, REM)→Ca	10	150	250	320
Ex. 32	1.5	41	0.6	Al→(Zr, REM)→Ca	10	150	250	320
Ex. 33	1.6	40	0.9	Al→(Zr, REM)→Ca	10	150	250	320

[Table 6]

	Fe concentration in slag (%)	CaO concentration in slag (%)	Of/S ratio	Addition order of Al, Zr, REM and Ca	Time from REM addition until Ca addition (min)	Time from Ca addition until completion of solidification (min)	Casting time of 1,500 to 1,450°C (s)	Casting time of 1,300 to 1,200°C (s)
Com. Ex. 1	0.08	39	0.4	Al→(Zr, REM)→Ca	10	140	244	320
Com. Ex. 2	1.5	40	0.4	Al→(Zr, REM)→Ca	10	150	250	310
Com. Ex. 3	1.5	8	0.4	Al→Zr→REM→Ca	9	145	270	320
Com. Ex. 4	1.4	45	0.4	Al→(Zr, REM)→Ca	10	145	280	310
Com. Ex. 5	1.5	82	0.3	Al→(Zr, REM)→Ca	10	145	232	320
Com. Ex. 6	1.4	42	0.4	Al→Zr→REM→Ca	12	140	279	310
Com. Ex. 7	1.5	41	0.4	Al→(Zr, REM)→Ca	13	145	261	310
Com. Ex. 8	1.5	48	0.4	Al→(Zr, REM)→Ca	10	145	255	280
Com. Ex. 9	1.6	40	0.4	Al→(Zr, REM)→Ca	10	140	263	330
Com. Ex. 10	2.3	42	0.3	Al→(Zr, REM)→Ca	10	145	221	320
Com. Ex. 11	1.8	41	0.4	Al→(Zr, REM)→Ca	3	140	275	310
Com. Ex.12	1.4	47	0.3	Al→(Zr, REM)→Ca	10	140	270	520
Com. Ex. 13	1.4	43	0.4	Al→Zr→REM→Ca	10	150	269	310
Com. Ex. 14	1.5	42	0.4	Al→(Zr, REM)→Ca	14	210	215	310
Com. Ex. 15	1.6	46	0.4	Al→(Zr, REM)→Ca	10	140	312	300
Com. Ex. 16	1.8	45	12	Al→(Zr, REM)→Ca	9	140	232	320
Com. Ex. 17	1.2	43	0.4	Al→(Zr, REM)→Ca	9	145	249	320
Com. Ex. 18	1.4	43	0.4	(Zr, REM)→Al→Ca	9	140	273	310
Com. Ex. 19	1.3	43	0.4	REM→Al→Ca→Zr	10	140	255	300

[Table 7]

	Properties
	YS (MPa)	HIC Test	SSCC Test (388 MPa)	SSCC Test (438 MPa)	Immersion potential difference ΔE between thick steel plate and weld metal (mV)
Ex. 1	504	No cracking	No cracking	Cracked	34
Ex. 2	490	No cracking	No cracking	Cracked	-
Ex. 3	534	No cracking	No cracking	Cracked	28
Ex. 4	513	No cracking	No cracking	No cracking	21
Ex. 5	510	No cracking	No cracking	No cracking	-
Ex. 6	505	No cracking	No cracking	No cracking	22
Ex. 7	519	No cracking	No cracking	No cracking	-
Ex. 8	539	No cracking	No cracking	No cracking	-
Ex. 9	492	No cracking	No cracking	No cracking	-
Ex. 10	488	No cracking	No cracking	No cracking	-
Ex. 11	578	No cracking	No cracking	No cracking	-
Ex. 12	594	No cracking	No cracking	No cracking	-
Ex. 13	492	No cracking	No cracking	No cracking	-
Ex. 14	605	No cracking	No cracking	No cracking	-
Ex. 15	613	No cracking	No cracking	No cracking	-
Ex. 16	571	No cracking	No cracking	No cracking	-
Ex. 17	588	No cracking	No cracking	No cracking	-
Ex. 18	505	No cracking	No cracking	No cracking	-
Ex. 19	550	No cracking	No cracking	No cracking	-
Ex. 20	593	No cracking	No cracking	No cracking	-
Ex. 21	615	No cracking	No cracking	No cracking	-
Ex. 22	580	No cracking	No cracking	No cracking	-
Ex. 23	577	No cracking	No cracking	No cracking	-
Ex. 24	565	No cracking	No cracking	No cracking	-
Ex. 25	609	No cracking	No cracking	No cracking	-
Ex. 26	610	No cracking	No cracking	No cracking	-
Ex. 27	552	No cracking	No cracking	No cracking	-
Ex. 28	622	No cracking	No cracking	No cracking	-
Ex. 29	562	No cracking	No cracking	No cracking	-
Ex. 30	520	No cracking	No cracking	No cracking	3
Ex. 31	490	No cracking	No cracking	No cracking	30
Ex. 32	500	No cracking	No cracking	No cracking	2
Ex. 33	490	No cracking	No cracking	No cracking	4

"-" means that the measurement was not performed.

[Table 8]

	Properties
	YS (MPa)	HIC Test	SSCC Test (388 MPa)	SSCC Test (438 MPa)	Immersion potential difference ΔE between thick steel plate and weld metal (mV)
Com. Ex. 1	472	Cracked	Cracked	-	-
Com. Ex. 2	608	Cracked	Cracked	-	-
Com. Ex. 3	451	Cracked	Cracked	-	-
Com. Ex. 4	655	Cracked	Cracked	-	-
Com. Ex. 5	521	Cracked	Cracked	-	-
Com. Ex. 6	514	Cracked	Cracked	-	-
Com. Ex. 7	631	Cracked	Cracked	-	-
Com. Ex. 8	480	Cracked	Cracked	-	31
Com. Ex. 9	623	Cracked	Cracked	-	-
Com. Ex. 10	462	Cracked	Cracked	-	-
Com. Ex. 11	595	Cracked	Cracked	-	-
Com. Ex. 12	647	Cracked	Cracked	-	-
Com. Ex. 13	520	Cracked	Cracked	-	-
Com. Ex. 14	653	Cracked	Cracked	-	-
Com. Ex. 15	599	Cracked	Cracked	-	-
Com. Ex. 16	513	Cracked	Cracked	-	-
Com. Ex. 17	643	No cracking	Cracked	-	-
Com. Ex. 18	631	Cracked	Cracked	-	29
Com. Ex. 19	612	Cracked	Cracked	-	28

"-" in the SSCC test means that the test was not performed.
"-" in the immersion potential difference ΔE means that the measurement was not performed.

While the present invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope of the present invention. The present application is based on a Japanese patent application (Patent Application No. 2015-104617) filed on May 22, 2015 and a Japanese patent application (Patent Application No. 2016-064064) filed on March 28, 2016 , the content thereof being incorporated herein by reference.

Claims

A thick steel plate comprising, in terms of mass %, C: 0.01 to 0.12%, Si: 0.02 to 0.50%, Mn: 0.6 to 2.0%, P: more than 0% and 0.030% or less, S: more than 0% and 0.004% or less, Al: 0.010 to 0.080%, Cr: 0.10 to 1.50%, Nb: 0.002 to 0.050%, REM: 0.0002 to 0.05%, Zr: 0.0003 to 0.01%, Ca: 0.0003 to 0.006%, N: more than 0% and 0.010% or less, and O: more than 0% and 0.0040% or less, with the remainder being iron and inevitable impurities,
wherein the steel comprises an inclusion having a width of 1 µm or more, wherein the inclusion has a composition satisfying that Zr amount in the inclusion is 1 to 40%, REM amount therein is 5 to 50%, Al amount therein is 3 to 30%, and Ca amount therein is 5 to 60%.
The thick steel plate according to Claim 1, wherein S amount in the inclusion is more than 0% and 20% or less.
The thick steel plate according to Claim 1, wherein [Cr]/[Nb] is 10 or more, wherein [ ] in the formula indicates mass %.
The thick steel plate according to Claim 1, further comprising, in terms of mass %, one kind or two or more kinds of Mg: more than 0% and 0.005% or less, Ti: 0.003 to 0.030%, Ni: 0.01 to 1.50%, Cu: 0.01 to 1.50%, Mo: 0.01 to 1.50%, V: 0.003 to 0.08%, and B: 0.0002 to 0.0032%,
wherein [Cr]+[Mo]+[Ni]+[Cu] is 2.1 or less,
wherein [ ] in the formula indicates mass %.
The thick steel plate according to Claim 4, comprising, in terms of mass %, Ni: 0.01 to 1.50%,
wherein 0.25×[Cr]+[Ni] is 0.10 to 1.50,
wherein [ ] in the formula indicates mass %.
A welded joint, comprising the thick steel plate according to any one of Claims 1 to 5 and a girth weld metal.
The welded joint according to Claim 6, having an immersion potential difference ΔE between the thick steel plate and the girth weld metal, obtained by the following formula, being 25 mV or less: $ΔE = immersion potential (mV) after 1 hour of girth weld metal - immersion potential (mV) after 1 hour of thick steel plate.$