WO2016190150A1 - Thick steel sheet and welded joint - Google Patents

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

Thick steel plate and welded joint

The present invention relates to a thick steel plate and a welded joint, and more particularly to a thick steel plate suitable as a material steel plate for energy structural materials such as for line pipes and marine structures, and a welded joint using the thick steel plate. is there.

In recent years, with the increase in global energy demand, development and commercialization of various types of energy including renewable energy have been promoted. On the other hand, oil, natural gas, and coal, which are fossil fuels, occupy most of the energy resources, and how to secure, efficiently and efficiently produce, transport and store this fossil energy. This is an important problem, and high-function steel for energy is indispensable particularly in the production and transportation of the fossil energy.

This steel material for energy does not perform its function, and once an accident occurs, the damage is enormous, so high safety is required.

One of the energy steels is steel for line pipes, which is used for the transportation of oil and natural gas. This steel has not only characteristics such as strength and toughness as a structural material, but also the inside of the pipe. Resistance to passing oil and natural gas is required. In recent years, oil and gas wells of oil and natural gas have deteriorated in quality of oil and gas produced, and a lot of H ₂ S has been mixed. In addition to the specifications so far, hydrogen-induced cracking resistance, There has been a strong demand for sour resistance typified by HIC resistance.

Also, line pipe steel is required to be thinned from the viewpoint of cost reduction during transportation and construction. For that purpose, it is necessary to improve the strength of the steel material, but the improvement of the steel material strength has the disadvantage of deteriorating the hydrogen-induced cracking resistance. In particular, T-cross welds that receive two thermal histories, seam welding when processing thick steel plates into pipes, that is, seam welding, and circumferential welding when joining pipes, have a complex thermal history of rapid heating and rapid cooling. Therefore, in the weld heat affected zone: HAZ, the strength, that is, the hardness increases, and cracks called sulfide stress corrosion cracking are likely to occur. Hereinafter, the sulfide stress corrosion cracking is also referred to as SSCC: sulfide stress corrosion cracking. Therefore, in order to realize high-strength line pipe steel, the SSCC resistance of the T-cross welded part is also one of the problems.

As a conventional technique for achieving the HIC resistance of a base material or the SSCC resistance of a T-cross weld, there is a technique described in Patent Document 1. Patent Document 1 discloses an API that reduces the block-like bainite structure, which is considered to have poor HIC resistance, and develops a uniform upper bainite or acicular ferrite structure, while ensuring the HIC resistance of the base material. A technology for realizing a standard X70 grade high strength thick steel sheet is described.

On the other hand, Patent Document 2 describes a technology that can achieve high strength of 56 kgf / mm ² or more in tensile strength by utilizing precipitation strengthening by fine Nb and V carbonitrides. However, this document does not describe the HIC resistance of the base material, and only the seam-welded HAZ is considered for the SSCC resistance. In addition, the test conditions described in the examples are not sufficiently severe so that the immersion time in a solution simulating a sour environment, that is, an environment containing a large amount of H ₂ S, is 21 days.

Further, Patent Document 3 describes a component system that suppresses an increase in hardness, which is supposed to deteriorate the SSCC resistance of a T-cross weld. However, the technology described in this document does not evaluate SSCC resistance itself, and does not describe the HIC resistance of the base material.

Japanese Unexamined Patent Publication No. Sho 61-165207 Japanese Unexamined Patent Publication No. 1-96329 Japanese Unexamined Patent Publication No. 2005-186162

The present invention has been made as a solution to the above-mentioned conventional problems, and an object thereof is to provide a thick steel plate excellent in sour resistance, particularly HIC resistance. Another object of the present invention is to provide a thick steel plate capable of realizing a welded joint excellent in SSCC resistance of a T-cross welded part and a welded joint excellent in SSCC resistance of a T-cross welded part.

The thick steel plate of the present invention is in mass%, C: 0.01 to 0.12%, Si: 0.02 to 0.50%, Mn: 0.6 to 2.0%, P: more than 0% 0 0.03% or less, S: more than 0% to 0.004% or less, Al: 0.010 to 0.080%, Cr: 0.10 to 1.50%, Nb: 0.002 to 0.050%, REM : 0.0002-0.05%, Zr: 0.0003-0.01%, Ca: 0.0003-0.006%, N: more than 0% and not more than 0.010%, O: more than 0%. A thick steel plate containing 0040% or less, the balance being iron and inevitable impurities, and the composition of inclusions with a width of 1 μm or more contained in the steel, the Zr content in the inclusions is 1 to 40% The REM content is 5 to 50%, the Al content is 3 to 30%, and the Ca content is 5 to 60%.

Further, in the thick steel plate of the present invention, the amount of S in the inclusions is preferably more than 0% and 20% or less.

Further, the thick steel plate of the present invention preferably has [Cr] / [Nb] of 10 or more. However, in the above formula, [] indicates mass%.

Further, the thick steel plate of the present invention is further in terms of mass%, Mg: more than 0% and 0.005% or less, Ti: 0.003 to 0.030%, Ni: 0.01 to 1.50%, Cu: One or more of 0.01 to 1.50%, Mo: 0.01 to 1.50%, V: 0.003 to 0.08%, and B: 0.0002 to 0.0032% It is preferable that [Cr] + [Mo] + [Ni] + [Cu] is 2.1 or less. However, in the above formula, [] indicates mass%.

Further, the thick steel plate of the present invention contains, by mass%, Ni: 0.01 to 1.50%, and 0.25 × [Cr] + [Ni] is 0.10 to 1.50. preferable. However, in the above formula, [] indicates mass%.

Further, the welded joint of the present invention is characterized by including any of the above-described thick steel plates and peripheral weld metals.

In the welded joint of the present invention, the immersion potential difference ΔE between the thick steel plate and the circumferential weld metal, which is obtained by the following formula, is preferably 25 mV or less.
ΔE = Immersion potential after 1 hour of circumferential weld metal (mV) −Immersion potential after 1 hour of thick steel plate (mV)

The effects obtained by typical ones of the inventions disclosed in the present invention will be briefly described as follows.

According to an embodiment of the present invention, a thick steel plate having excellent sour resistance can be provided. Moreover, the welded joint excellent in the SSCC resistance of a T cross weld part can be provided using the thick steel plate of this invention.

In order to achieve the above-described problems of the present invention, the present inventors have earnestly studied from the viewpoint of inclusion control in steel, in addition to the composition of steel, which is fundamental for exhibiting the characteristics of thick steel plates. , Repeated examination. As a result, it was found that a thick steel plate excellent in sour resistance can be obtained by maintaining coarse inclusions having a width of 1 μm or more in a specific component composition, and the present invention has been completed. Here, the inclusions in the present invention mean coarse precipitate grains generated in molten steel or at the time of solidification, and specifically, particles due to oxides, carbides, sulfides, nitrides, etc. of alloy components in steel. means.

First, when considering the HIC resistance of the base material, when hydrogen penetrates into the steel in a sour environment, inclusions that are coarse and have a higher coefficient of thermal expansion than steel, such as MnS, have coarse voids around them. It is presumed that the hydrogen that has intruded intensively accumulates in the voids and cracks in the steel, that is, hydrogen-induced cracks are generated and propagated by the pressure at which they vaporize. Therefore, by converting coarse inclusions of 1 μm or more that cause hydrogen-induced cracking from inclusions having a higher thermal expansion coefficient than steel to inclusions having a lower thermal expansion coefficient than steel, It was confirmed that the HIC resistance can be improved and secured. Specifically, Zr, Al, and REM oxides are effective as inclusions having a smaller thermal expansion coefficient than steel.

On the other hand, from the viewpoint of the SSCC resistance of the T-cross welded portion, it was found that the SSCC resistance deteriorates when the hardness changes rapidly from the vicinity of the weld metal to the base metal in the T-cross welded portion. This is presumably because a sudden stress concentration occurs in the corresponding part due to a change in hardness. This sudden change in hardness from the vicinity of the weld metal to the base metal produces hard martensite in the vicinity of the weld metal, whereas soft ferrite forms at a location some distance from the weld metal to the base metal side. it is conceivable that.

In order to suppress this sudden change in hardness, we decided to study from both sides of soft ferrite and hard martensite. As a result, it was confirmed that the formation of ferrite can be suppressed by reducing the soft ferrite by adding an alloy element to the thick steel plate and improving the hardenability.

Further, for the reduction of hard martensite, the intergranular bainite transformation is promoted by dispersing a large number of inclusions that are the starting points of transformation, and the [Cr] / [Nb] is made 10 or more, so that the grain boundary It has been confirmed that Nb segregation to the grain boundaries that increase the nucleation driving force in the steel is reduced, bainite production from the grain boundaries is promoted, and as a result, the amount of martensite produced in the vicinity of the weld metal can be reduced. However, in the above formula, [] indicates mass%.

It was also confirmed that the strength of the base metal can be improved by adding Cr and Nb to ensure hardenability.

Hereinafter, in addition to the component composition, structure, and inclusion composition of the thick steel plate according to the present invention, the weld metal used for the T-cross welded portion will be described in detail including the reason for its definition. Hereinafter, “%” which is a unit for displaying the composition means “% by mass”.

(Component composition of thick steel plate)
[C: 0.01 to 0.12%]
C is an indispensable element for ensuring the strength of the thick steel plate and needs to be contained in an amount of 0.01% or more. Preferably it is 0.02% or more, More preferably, it is 0.03% or more. However, if the amount of C is excessive, island-like martensite is likely to be generated in the base material, which becomes the starting point of hydrogen-induced cracking and deteriorates the HIC resistance of the base material. Therefore, the C amount needs to be 0.12% or less. Preferably it is 0.10% or less, More preferably, it is 0.08% or less.

[Si: 0.02 to 0.50%]
Si is effective for deoxidation. In order to obtain these effects, the Si content is set to 0.02% or more. Preferably it is 0.04% or more, More preferably, it is 0.06% or more. However, if the amount of Si is excessive, island-like martensite is likely to be generated in the base material, which becomes the starting point of hydrogen-induced cracking and deteriorates the HIC resistance of the base material. Therefore, the amount of Si needs to be suppressed to 0.50% or less. Preferably it is 0.45% or less, More preferably, it is 0.35% or less.

[Mn: 0.6 to 2.0%]
Mn is an element indispensable for securing the strength of the thick steel plate, and it is necessary to contain 0.6% or more. Preferably it is 0.8% or more, More preferably, it is 1.0% or more. However, if the amount of Mn is excessive, MnS is generated and the HIC resistance deteriorates, so the upper limit of the amount of Mn is set to 2.0%. Preferably it is 1.9% or less, more preferably 1.8% or less.

[P: more than 0% and 0.030% or less]
P is an element inevitably contained in the steel material, and if its content exceeds 0.030%, it adversely affects HIC resistance and SSCC resistance. Therefore, in the present invention, the amount of P is suppressed to 0.030% or less. Preferably it is 0.020% or less, More preferably, it is 0.010% or less.

[S: more than 0% and 0.004% or less]
If the amount of S is too large, a large amount of MnS is generated and the HIC resistance is remarkably deteriorated. Therefore, in the present invention, the upper limit of the amount of S is made 0.004%. Preferably it is 0.003% or less, More preferably, it is 0.0025% or less, More preferably, it is 0.0020% or less.

[Al: 0.010 to 0.080%]
Al is effective in reducing voids with the steel matrix by reducing the thermal expansion coefficient of inclusions and improving the HIC resistance. Moreover, since the inclusion containing an appropriate amount of Al promotes the formation of intragranular bainite, good SSCC resistance can be obtained. In order to exhibit these effects, it is necessary to contain at least 0.010% or more. The amount of Al is preferably 0.020% or more, and more preferably 0.025% or more. However, if the amount of Al is excessive, Al oxide is generated in a cluster shape and becomes a starting point of hydrogen-induced cracking. Therefore, the Al amount needs to be 0.080% or less. The amount of Al is preferably 0.060% or less, more preferably 0.050% or less.

[Cr: 0.10 to 1.50%]
Cr is an element indispensable for ensuring the strength, and contributes to the improvement of SSCC resistance by suppressing soft ferrite in the T-cross weld. In order to exhibit these effects, it is necessary to contain at least 0.10% or more. The amount of Cr is preferably 0.15% or more, more preferably 0.17% or more, and further preferably 0.20% or more. However, if the amount of Cr is excessive, the hard martensite of the T-cross welded portion is increased and the SSCC resistance is lowered, so the content is made 1.50% or less. The amount of Cr is preferably 1.00% or less, and more preferably 0.80% or less.

[Nb: 0.002 to 0.050%]
Nb is an element indispensable for ensuring strength, and also contributes to improvement of SSCC resistance by suppressing soft ferrite in the T-cross weld. In order to exhibit these effects, it is necessary to contain at least 0.002% or more. The Nb amount is preferably 0.005% or more, and more preferably 0.010% or more. However, if the amount of Nb is excessive, the hard martensite of the T-cross welded portion is increased and the SSCC resistance is lowered, so the content is made 0.050% or less. The amount of Nb is preferably 0.033% or less, and more preferably 0.030% or less.

[REM: 0.0002 to 0.05%]
REM (rare earth element) is effective in reducing voids with the steel parent phase by reducing the thermal expansion coefficient of inclusions and ensuring HIC resistance. Moreover, since the inclusion containing an appropriate amount of REM promotes the formation of intragranular bainite, good SSCC resistance can be obtained. In order to exhibit these effects, it is necessary to contain REM 0.0002% or more. The amount of REM is preferably 0.0005% or more, more preferably 0.0010% or more. On the other hand, when REM is contained excessively, the solid solution REM segregates at the grain boundary, thereby lowering the grain boundary strength and degrading the SSCC resistance. Therefore, the upper limit of the REM amount is set to 0.05%. From the viewpoint of increasing productivity by suppressing the clogging of the immersion nozzle during casting, the content is preferably 0.03% or less, more preferably 0.01% or less, and still more preferably 0.005% or less. In the present invention, REM means 15 elements from La to Lu in the periodic table, that is, lanthanoid elements, and Sc and Y.

[Zr: 0.0003 to 0.01%]
Zr reduces voids with the steel matrix by reducing the thermal expansion coefficient of inclusions, and improves HIC resistance. Moreover, since the inclusion containing an appropriate amount of Zr promotes the formation of intragranular bainite, good SSCC resistance can be obtained. In order to exert these effects, it is necessary to contain 0.0003% or more of Zr. The amount of Zr is preferably 0.0005% or more, more preferably 0.0010% or more. On the other hand, when Zr is contained excessively, the solid solution Zr in the molten steel increases and crystallizes so as to surround the acid / sulfide during casting, thereby degrading the HIC resistance. Therefore, the upper limit of the Zr amount is set to 0.01%. The amount of Zr is preferably 0.007% or less, more preferably 0.005% or less.

[Ca: 0.0003 to 0.006%]
Ca improves the SSCC resistance by forming CaS, fixing S, and reducing the amount of MnS produced. Moreover, since the inclusion containing an appropriate amount of Ca promotes the formation of intragranular bainite, good SSCC resistance can be obtained. In order to exert these effects, it is necessary to contain 0.0003% or more of Ca. The Ca content is preferably 0.0005% or more, more preferably 0.0010% or more. On the other hand, when Ca is excessively contained, a large amount of CaS is formed, and they aggregate to deteriorate the HIC resistance. Therefore, the upper limit of the Ca content is set to 0.006%. The amount of Ca is preferably 0.005% or less, and more preferably 0.004% or less.

[N: more than 0% and 0.010% or less]
N is an unavoidable impurity, but segregates at the grain boundary to lower the grain boundary strength and degrade 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 that forms inclusions, and excessive addition generates a large amount of coarse oxide, which causes hydrogen-induced cracking. Therefore, the upper limit of the O amount is set to 0.0040%. The amount of O is preferably 0.0030% or less, and more preferably 0.0020% or less.

[[Cr] / [Nb] is 10 or more]
Further, the thick steel plate of the present invention preferably satisfies the above component composition and has [Cr] / [Nb] of 10 or more. However, in the formula, [] indicates mass%. When the thick steel plate satisfies this condition, Nb segregation to the grain boundary that increases the nucleation driving force at the grain boundary is reduced in the T-cross welded portion, and bainite production from the grain boundary is promoted. Thereby, the amount of martensite produced in the vicinity of the weld metal is reduced, and the SSCC resistance is improved. Therefore, [Cr] / [Nb] is preferably 10 or more. More preferably, it is 12 or more, More preferably, it is 15 or more.

Further, although it is not an essential element of the thick steel plate of the present invention, when Ni is included as a component composition, 0.25 × [Cr] + [Ni] is adjusted to be 0.10 to 1.50. SSCC resistance can also be improved by satisfying the conditions. However, in the formula, [] indicates mass%. When 0.25 × [Cr] + [Ni] satisfies the condition of 0.10 to 1.50, it is not always necessary to satisfy the condition that [Cr] / [Nb] is 10 or more. Details will be described later.

The component composition of the steel material of the steel plate of the present invention is as described above, and the balance is iron and inevitable impurities. Further, in addition to the above elements, by adding at least one or more selected from the group consisting of Mg, Ti, Ni, Cu, Mo, V and B in the following amounts, HIC resistance and resistance The SSCC property can be improved. Hereinafter, these elements will be described.

[Mg: more than 0% and 0.005% or less]
Mg has the effect of improving the SSCC resistance of the base material by forming MgS and finely dispersing sulfides. However, since the effect is saturated even if Mg is contained in excess of 0.005%, the upper limit of Mg content is preferably 0.005%. More preferably, it is 0.004% or less, More preferably, it is 0.003% or less.

[Ti: 0.003-0.030%]
Ti is an element that contributes to improving the strength of the thick steel plate by precipitation strengthening. In order to exhibit this effect, it is preferable to contain 0.003% or more. More preferably it is 0.004% or more, and still more preferably 0.005% or more. On the other hand, if the Ti content is excessive, the hard martensite in the T-cross welded portion is increased and the SSCC resistance is lowered. Therefore, the content is preferably 0.030% or less. More preferably, it is 0.025% or less, More preferably, it is 0.020% or less.

[Ni: 0.01 to 1.50%]
Ni is an element that contributes to improving the strength of the thick steel plate. In order to exhibit this effect, it is preferable to contain 0.01% or more. More preferably, it is 0.05% or more, More preferably, it is 0.10% or more. On the other hand, when the Ni content is excessive, the hard martensite in the T-cross welded portion is increased and the SSCC resistance is lowered. Therefore, the Ni content is preferably 1.50% or less. More preferably, it is 1.00% or less, More preferably, it is 0.50% or less.

[Cu: 0.01 to 1.50%]
Cu is an element that contributes to improving the strength of the thick steel plate. In order to exhibit this effect, it is preferable to contain 0.01% or more. More preferably, it is 0.05% or more, More preferably, it is 0.10% or more. On the other hand, if the Cu content is excessive, it causes an increase in hard martensite in the T-cross welded portion and lowers the SSCC resistance, so it is preferably made 1.50% or less. More preferably, it is 1.00% or less, More preferably, it is 0.50% or less.

[Mo: 0.01 to 1.50%]
Mo is an element that contributes to improving the strength of the thick steel plate. In order to exhibit this effect, it is preferable to contain 0.01% or more. More preferably, it is 0.05% or more, More preferably, it is 0.10% or more. On the other hand, if the Mo content is excessive, hard martensite in the T-cross welded portion is increased and SSCC resistance is lowered. Therefore, the Mo content is preferably 1.50% or less. More preferably, it is 1.00% or less, More preferably, it is 0.50% or less.

[V: 0.003-0.08%]
V is an element that contributes to improving the strength of the thick steel plate. In order to exhibit this effect, it is preferable to contain 0.003% or more. More preferably it is 0.005% or more, and still more preferably 0.010% or more. On the other hand, if the V content is excessive, the hard martensite in the T-cross welded portion is increased and the SSCC resistance is lowered. Therefore, the V content is preferably 0.08% or less. More preferably, it is 0.07% or less, More preferably, it is 0.05% or less.

[B: 0.0002 to 0.0032%]
B is an element that contributes to improving the strength of the thick steel plate. In order to exhibit this effect, it is preferable to contain 0.0002% or more. More preferably, it is 0.0005% or more, More preferably, it is 0.0010% or more. On the other hand, if the B content is excessive, the hard martensite in the T-cross welded portion is increased and the SSCC resistance is lowered, so 0.0032% or less is preferable. More preferably, it is 0.0030% or less, More preferably, it is 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 after satisfying the above component composition. However, in the formula, [] indicates mass%. When the added amount of these elements exceeds 2.1, the hard martensite in the T-cross weld is increased and the SSCC resistance is lowered. Therefore, it is preferable that [Cr] + [Mo] + [Ni] + [Cu] is 2.1 or less. More preferably, it is 1.9 or less, More preferably, it is 1.7 or less.

[Immersion potential difference ΔE (immersion potential of peripheral weld metal−immersion potential of thick steel plate) is 25 mV or less]
In a T-cross weld, when the potential difference between the base metal and the peripheral weld metal is large, selective corrosion of the base metal and hydrogen intrusion in the peripheral weld metal are promoted by the effect of dissimilar metal, and SSCC resistance is reduced. In particular, a stable and uniform sulfide film is not formed within one hour after immersion, and hydrogen penetration due to the potential difference between the peripheral weld metal and the thick steel plate becomes significant. Therefore, it is preferable that the immersion potential difference ΔE after 1 hour when immersed in the solution of the thick steel plate and the peripheral weld metal, obtained by the following formula, is 25 mV or less. More preferably, it is 20 mV or less, More preferably, it is 15 mV or less.
ΔE = Immersion potential after 1 hour of circumferential weld metal (mV) −Immersion potential after 1 hour of thick steel plate (mV)
The electrode potential that appears when a metal is immersed in a solution may be defined as a corrosion potential or a hybrid potential. In the present invention, this is referred to as an “immersion potential”.

[0.25 × [Cr] + [Ni] is 0.10 to 1.50]
When the potential difference between the base metal and the peripheral weld metal is large in the T-cross welded portion, hydrogen penetration into the base metal or the peripheral weld metal is promoted by the dissimilar metal contact effect, so that the SSCC resistance is lowered. Therefore, the thick steel plate of the present invention satisfies the above-described component composition, particularly the condition that the Ni content is 0.10 to 1.50%, and 0.25 × [Cr] + [Ni] is 0.10. It is preferably ˜1.50. However, in the formula, [] indicates mass%. The addition of these elements contributes to the improvement of the SSCC resistance of the T-cross welded portion by improving the potential of the steel plate as a base material and suppressing hydrogen intrusion at the T-cross welded portion due to the dissimilar metal contact effect.

Therefore, the value obtained from 0.25 × [Cr] + [Ni] is preferably 0.10 or more, more preferably 0.15 or more, and further preferably 0.20 or more. On the other hand, if the value obtained from 0.25 × [Cr] + [Ni] is excessive, the potential of the steel plate is significantly increased from the potential of the weld metal, and the selective corrosion of the weld metal by galvanic corrosion proceeds. SSCC resistance decreases. Therefore, the upper limit of the value obtained from 0.25 × [Cr] + [Ni] is set to 1.50. The upper limit is more preferably 1.00 or less, and still more preferably 0.70 or less.

(Welded metal)
The metal used for circumferential welding preferably has the following component composition in order to ensure the strength and toughness of the weld metal and improve corrosion resistance. That is, by mass, C: 0.02 to 0.10%, Si: 0.10 to 0.60%, Mn: 0.90 to 2.50%, Ni: 0.20 to 1.00% It is preferable to contain. In addition to the above components, 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: It is allowable to contain 0.3% or less, Ti: 0.05% or less, and Al: 0.1% or less. Components other than these are desirably iron and inevitable impurities. Hereinafter, the reason for limiting the component composition of the weld metal will be described.

[C: 0.02 to 0.10%]
C is an element necessary for ensuring the strength of the weld metal. When the C content is less than 0.02%, a predetermined strength cannot be obtained. However, if the C content is excessive, coarsening of the grain boundary carbides causes a reduction in toughness, so the content is made 0.10% or less.

[Si: 0.10 to 0.60%]
Si is an element necessary for ensuring the strength of the weld metal. When the Si content is less than 0.10%, a predetermined strength cannot be obtained. However, if the Si content is excessive, it causes a decrease in toughness, so the content is made 0.60% or less.

[Mn: 0.90 to 2.50%]
Mn is an element necessary for ensuring the balance between the strength and toughness of the weld metal. In order to obtain such an effect, the Mn content needs to be 0.90% or more. However, if the Mn content is too large, segregation is promoted and the toughness is reduced, so it is necessary to make it 2.50% or less.

[Ni: 0.20 to 1.00%]
Ni increases the electric potential of the weld metal and exhibits an effect of improving the corrosion resistance. It is also an element effective in enhancing hardenability to ensure strength and improving low temperature toughness. In order to obtain such an effect, the Ni content needs to be 0.20% or more. On the other hand, if the Ni content is excessive, hot cracking may occur, and the potential of the weld metal is excessively increased, causing selective corrosion of the base metal. 00% or less.

(Inclusion composition in thick steel plate)
[Composition of inclusions with a width of 1 μm or more contained in steel]
[Zr content is 1-40%]
In the present invention, Zr in inclusions having a width of 1 μm or more exists mainly as an oxide. Since this Zr oxide has a smaller coefficient of thermal expansion than steel, if the amount of Zr in the inclusion is ensured, voids with the surrounding steel matrix can be reduced and the HIC resistance is improved. Moreover, since the oxide containing an appropriate amount of Zr promotes the formation of intragranular bainite, it can obtain good SSCC resistance. In order to exert such an effect, the amount of Zr in the inclusion is set to 1 to 40%. If the amount of Zr is less than 1% or more than 40%, the HIC resistance of the base material or the SSCC resistance of the T-cross weld becomes insufficient.

[REM amount is 5-50%]
In the present invention, REM in inclusions having a width of 1 μm or more exists as oxides or oxysulfides. Among these, REM oxide has a smaller coefficient of thermal expansion than steel. Therefore, if the amount of REM in the inclusion is ensured, voids with the surrounding steel matrix can be reduced, and HIC resistance is improved. Moreover, when it exists as an oxysulfide, S can be fixed and the production | generation of the sulfide which has a bad influence on HIC resistance, such as MnS, can be suppressed. Furthermore, since these REM inclusions promote the formation of intragranular bainite, good SSCC resistance can be obtained. In order to exert such an effect, the amount of REM in the inclusion is set to 5 to 50%. When the REM amount is less than 5% or more than 50%, the HIC resistance of the base material or the SSCC resistance of the T-cross welded portion becomes insufficient.

[Al content is 3-30%]
In the present invention, Al in inclusions having a width of 1 μm or more exists mainly as Al oxide. Since this Al oxide has a smaller coefficient of thermal expansion than steel, if the amount of Zr in the inclusion is ensured, voids with the surrounding steel matrix can be reduced, which is effective in improving the HIC resistance. Moreover, since the oxide containing an appropriate amount of Al promotes the formation of intragranular bainite, good SSCC resistance can be obtained. In order to exert such an effect, the amount of Al in the inclusion is made 3 to 30%. When the Al content is less than 3% or more than 30%, the HIC resistance of the base material or the SSCC resistance of the T-cross welded portion becomes insufficient.

[Ca content is 5-60%]
In the present invention, Ca in inclusions having a width of 1 μm or more contributes to the refinement of the steel structure in the T-cross weld during welding and promotes the formation of an intragranular bainite structure starting from the inclusions. As a result, the steel structure of the T-cross welded portion after welding becomes fine, and good SSCC resistance can be obtained. In order to exert such an effect, the Ca content in the inclusion is set to 5 to 60%. If the Ca content is less than 5% or exceeds 60%, the SSCC resistance of the T-cross weld cannot be improved.

[S amount is over 0% and below 20%]
The amount of S in the inclusions having a width of 1 μm or more is limited to the above-described component composition with respect to the content of S in the steel sheet and the content of alloy components such as Zr and REM for refining and dispersing sulfide inclusions. Furthermore, it can be reduced by controlling the inclusion composition as described above. If the component composition and the inclusion composition are not properly controlled, the amount of S in the inclusion exceeds 20%, and the coarse sulfide becomes excessive. As a result, the HIC resistance or T-cross of the base material is increased. The SSCC resistance of the welded portion becomes insufficient. On the other hand, in a steel sheet in which the amount of S in the inclusion is controlled to 20% or less, good HIC resistance and SSCC resistance can be obtained. The smaller the amount of S in the inclusions, the better. However, in the case of 0%, S cannot be fixed by inclusions at all, and the HIC resistance of the base material or the SSCC resistance of the T-cross weld is insufficient. It is considered to be.

The total number of inclusions is not particularly limited as long as the effects of the present invention are not significantly impaired. However, it is preferable that the inclusions are dispersed in an amount of about 500 to 5,000 / cm ² in the steel sheet. If it is less than 500 pieces / cm ² , the starting point of intragranular bainite is insufficient, and it is considered that sufficient microstructure refining action cannot be obtained and SSCC resistance is lowered. On the other hand, if it exceeds 5000 / cm ² , it will act as a starting point of destruction, and both HIC resistance and SSCC resistance may deteriorate.

(Production method)
Next, the manufacturing method of the thick steel plate concerning this invention is demonstrated in detail below.

[Molten steel treatment process]
In obtaining the thick steel sheet of the present invention having the above structure, in the molten steel treatment step, (A) a desulfurization step in which S is 0.004% or less using slag satisfying Fe: 0.1 to 10%, (B) molten steel Deoxygenation step in which the dissolved oxygen concentration Of of the molten steel is 10 or less in the ratio of S to the molten steel, (C) Al, Zr, REM and Ca are added in the order of Al, Zr, REM and Ca. Or adding Al, then adding Zr and REM at the same time, and then adding Ca in this order, Ca being added at least 4 minutes after the addition of REM, and from Ca addition It is necessary that the time until solidification is completed within 200 minutes, and the cooling time at the slab t / 4 position at 1300 ° C. to 1200 ° C. during casting is within 460 seconds. Further, the cooling time at the slab t / 4 position of 1500 to 1450 ° C. during casting is set to be within 300 seconds. The above t represents the plate thickness. Each step will be described below in order.

(A) Desulfurization process In order to ensure HIC resistance, it is important to reduce coarse sulfides, and to achieve this, it is important to control the amount of S. For molten steel melted in a converter or electric furnace so that the molten steel temperature becomes 1550 ° C. or higher, slag satisfying Fe: 0.1 to 10% is used, and S is made 0.004% or lower. By increasing the Fe concentration in the slag, REM and Zr added after desulfurization and deoxidation can form oxides preferentially without dissolving in the molten steel. In order to obtain this effect, the Fe concentration in the slag is set to 0.1% or more. The Fe concentration in the slag is preferably 0.5% or more, more preferably 1.0% or more. On the other hand, when the Fe concentration in the slag exceeds 10%, a large amount of oxide is generated, and the oxide becomes a starting point of hydrogen-induced cracking. Therefore, the Fe concentration in the slag is set to 10% or less. Preferably it is 8% or less, More preferably, it is 5% or less. In addition, when Ca is added, by sufficiently performing desulfurization with slag to suppress S to 0.004% or less, a large amount of CaS is formed when Ca is added after REM addition, and the composition of inclusions Can be prevented from deviating from a predetermined range, and HIC resistance and SSCC resistance can be secured.

In order to make the S 0.004% or less, the CaO concentration in the slag may be 10% or more. By the addition of Ca, CaO in the slag reacts with the dissolved S in the molten steel and changes to CaS, thereby sufficiently reducing S in the molten steel, that is, desulfurization. At this time, if the CaO concentration in the slag is 10% or more, S can be made 0.004% or less. The CaO concentration in the slag is preferably 15% or more, more preferably 20% or more. On the other hand, even if there is too much CaO in the slag, desulfurization becomes difficult, so the upper limit is about 80%.

(B) Deoxidation process In order to improve the SSCC resistance, oxide control is important. To achieve this, it is important to control the amount of O. In this step, since the amount of S that has an influence on the HIC resistance slightly increases, so-called recovery S occurs, it is important to control the O amount and the S amount simultaneously. In this step, before adding REM described later, the dissolved oxygen concentration Of of the molten steel is set to 10 or less in the ratio of the S concentration of the molten steel: Of / S. When REM is added to molten steel, it forms its sulfide and at the same time forms an oxide. When the Of / S exceeds 10, most of the added REM forms an oxide, and the composition of inclusions deviates from a predetermined range. As a result, the HIC resistance and SSCC resistance deteriorate. Therefore, in the present invention, Of / S is set to 10 or less as described above. Of / S is preferably 5 or less, more preferably 3.5 or less, and even more preferably 2 or less. The lower limit value of Of / S is about 0.1. The Of / S can be reduced to 10 or less by performing at least one of deoxidation using an RH degassing apparatus and deoxidation using a deoxidizing element such as Mn, Si, or Ti. .

(C) Addition step of Al, Zr, REM In addition of Al, Zr, REM to the molten steel, Al is added first, and then Zr, REM is added. This is because when comparing the deoxidizing ability of Al with Zr and REM, the deoxidizing power of Zr and REM is stronger than that of Al. Therefore, when Zr and REM are added prior to Al, the amount of Al in the inclusions is reduced. Cannot be the desired value. Therefore, it is necessary to add Al before Zr and REM.

Furthermore, when adding Ca, considering the desulfurization power and deoxidation power of each additive element described below, Al is added first, then Zr is added, then REM is added, and finally Ca is added. Or Al is added first, then Zr and REM are added at the same time, and finally Ca is added. However, in any case, the time from REM addition to Ca addition is 4 minutes or more.

Explain why. First, when comparing the desulfurization ability of REM and Ca, since the desulfurization power of REM is weaker than that of Ca, if Ca is added before REM addition, a large amount of CaS is generated and the composition of inclusions deviates from the predetermined range. As a result, the HIC resistance and SSCC resistance are deteriorated. Therefore, it is necessary to add REM before adding Ca. Therefore, the order of addition of Al, Zr, REM, and Ca must be Al → Zr and REM → Ca. In order to control the composition of inclusions within a predetermined range, the time from REM addition to Ca addition needs to be 4 minutes or longer. The time from REM addition to Ca addition is preferably 5 minutes or more, more preferably 8 minutes or more. From the viewpoint of productivity, the upper limit of the time from REM addition to Ca addition is about 60 minutes.

Next, when comparing the deoxidizing ability of Zr, REM, and Ca, the deoxidizing power is generally strongest in Ca, Ca>REM> Zr, and Zr is the weakest. Therefore, in order to contain Zr in inclusions, that is, to form ZrO ₂ as oxide inclusions, Zr must be added prior to the addition of Ca or REM, which has stronger deoxidizing power than Zr. Don't be. Therefore, the order of addition of Al, Zr, REM and Ca needs to be Al → Zr → REM → Ca. However, since REM has a smaller deoxidizing ability than Ca, Zr can be contained in inclusions even if it is added simultaneously with Zr. Therefore, the order of addition is Al → Zr and REM → Ca. It is good.

With respect to the amount of each element added, it is sufficient that a steel plate having a desired amount of each element is obtained. For example, Zr is added to a concentration of 3 to 100 ppm in the molten steel, and thereafter or simultaneously, REM is added to the concentration in the molten steel. After 4 minutes or more have passed since the addition of 2 to 500 ppm, Ca is added to a concentration of 3 to 60 ppm in the molten steel.

[Casting process]
After the addition of Ca, for example, casting is quickly started within 80 minutes, and casting is performed so that the time from the addition of Ca to the completion of solidification is within 200 minutes. The reason is as follows. That is, since Ca is an element having high desulfurization ability and deoxidation ability, the composition of oxides and sulfides tends to become stable CaS and CaO with the passage of time after addition of Ca, and the composition of inclusions is within a predetermined range. And can not. Therefore, in the present invention, the time from the addition of Ca to the completion of solidification is set within 200 minutes. It is preferably within 180 minutes, more preferably within 160 minutes. The lower limit of the time is about 4 minutes from the viewpoint of homogenizing Ca.

In addition, when the cooling time at the 1500 to 1450 ° C. slab t / 4 position during casting exceeds 300 seconds, complex formation of oxide-based secondary inclusions on the inclusions is promoted, and the composition of the inclusions is predetermined. By deviating from the range, the SSCC resistance is deteriorated.

Also, it is important to set the cooling time at the 1300 ° C. to 1200 ° C. slab t / 4 position during casting to 270 to 460 seconds. If the cooling time exceeds the upper limit, composite formation of sulfide-based secondary inclusions on the inclusions is promoted, and the SSCC resistance is deteriorated by the inclusion composition deviating from a predetermined range. End up. On the other hand, if the cooling time falls below the lower limit, the cooling load increases greatly, which is not preferable in practice.

[Steps below rolling]
After the solidification, the steel plate can be manufactured by hot rolling according to a conventional method. Moreover, the steel pipe for line pipes can be manufactured by the method generally performed using this steel plate. The process after rolling is not particularly limited. For example, the cast slab is heated to 1100 ° C. or higher and hot-rolled at a reduction rate of 40% or more in the recrystallization temperature range, and this is performed at 780. Accelerated cooling is preferably performed at a cooling rate of 10 to 20 ° C./s from 0 ° C. Subsequent tempering is unnecessary.

[Welded joint]
In the present invention, a welded joint using the thick steel plate of the present invention is provided. The welded joint is made of a thick steel plate and a circumferential weld metal, and is obtained by welding the end of the thick steel plate with the circumferential weld metal. In the welded joint of the present invention, the immersion potential difference ΔE between the thick steel plate and the peripheral weld metal, which is obtained by the following formula, is preferably 25 mV or less.
ΔE = Immersion potential after 1 hour of circumferential weld metal (mV) −Immersion potential after 1 hour of thick steel plate (mV)

As described above, when the potential difference between the base metal and the peripheral weld metal is large in the T-cross weld, selective corrosion of the base metal and hydrogen intrusion in the peripheral weld metal are promoted by the effect of dissimilar metals. Decreases. By using the steel plate of the present invention, the immersion potential difference ΔE becomes 25 mV or less, and a decrease in SSCC resistance in the welded portion can be suppressed. The immersion potential difference ΔE is more preferably 20 mV or less, and further preferably 15 mV or less.

Also, as the metal used for circumferential welding, the above-described weld metal is preferably used.

The welding method for forming the welded joint is not particularly limited, and can be performed by a conventionally known method. Examples thereof include arc welding, laser welding, and electron beam welding.

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, and the present invention is implemented with appropriate modifications within a range that can meet the gist of the present invention. These are all included in the technical scope of the present invention.

(Examples 1 to 33, Comparative Examples 1 to 19)
Molten steel refined in a 240t converter by a conventional method is treated with desulfurization, deoxidation, component adjustment, inclusion control, etc. using an LF furnace, and steel compositions and inclusion compositions in steel are shown in Tables 1 to 4. Various molten steels having slabs as slabs by a continuous casting method were subjected to accelerated cooling after hot rolling to produce a thick steel plate having a thickness of 40 mm and a width of 3500 mm. The welded joint mentioned later was produced using the obtained thick steel plate and the circumference weld metal. Tables 5 and 6 show main process conditions in the molten steel treatment, continuous casting, and accelerated cooling. Tables 7 and 8 show various properties of the steel sheets thus obtained. A method for analyzing the composition of inclusions shown in Tables 3 and 4 and a method for measuring and evaluating each characteristic in Tables 7 and 8 will be described below.

[Analysis of the composition of inclusions]
The analysis of the composition of inclusions was performed as follows. That is, in the cross section in the plate thickness direction of the rolled material, it was observed with EPMA-8705 manufactured by Shimadzu Corporation centering on the plate thickness central portion. Specifically, three cross-sections are observed at an observation magnification of 400 × and an observation field of view of about 50 mm ² , and the component composition at the center of the inclusion is quantitatively analyzed by wavelength dispersion spectroscopy of characteristic X-rays for inclusions with a width of 1 μm or more. did. The observation visual field was set to a range of 7 mm in the plate thickness direction and 7 mm in the plate width direction with the plate thickness center portion being the center. The analysis target elements were Al, Mn, Si, Mg, Ca, Ti, Zr, S, REM, and Nb. REM shown here refers to La, Ce, Nd, Dy, and Y. The relationship between the X-ray intensity of each element and the element concentration is obtained in advance using a known substance as a calibration curve, and then the element concentration of the inclusion is determined from the X-ray intensity obtained from the inclusion and the calibration curve. did. And the average value of content of each said element of the inclusion whose width | variety in the said 3 cross section is 1 micrometer or more was calculated | required, and it was set as the composition of the inclusion.

[Measurement and evaluation of yield strength YS of base metal]
Sample No. 4 of JIS Z2241 was taken in parallel to the C direction from the t / 4 position of each thick steel plate, the tensile strength test was performed by the method described in JIS Z2241, and the yield strength YS was measured. It was confirmed.

[Test and evaluation of HIC resistance]
Tested and evaluated according to the method specified in NACE standard TM0284-2003. Specifically, the test piece was immersed in an aqueous solution of 5% by mass NaCl + 0.5% by mass CH ₃ COOH at 25 ° C. saturated with 1 atm of hydrogen sulfide for 96 hours. In the evaluation of the HIC test, the longitudinal direction of each test piece was cut at a pitch of 10 mm, the cut surface was polished, the entire cross section was observed at a magnification of 100 using an optical microscope, and the crack length of the HIC was 1 mm or more. The presence or absence of cracks was confirmed.

[Test and evaluation of SSCC resistance of T-cross welds]
In order to simulate seam welding, a thick steel plate having the steel composition shown in Tables 1 and 2 was processed into an X groove of 75 ° and welded by a 2-pass submerged arc welding method to produce a pipe. The heat input during welding was 1st pass: 3.7 kJ / mm, 2nd pass: 5.4 kJ / mm. In addition, in order to simulate circumferential welding when joining pipes, referring to "Practical application of UOE steel pipe with excellent SSCC resistance, Matsuyama et al., Welding Technology, September 1988, P.58", Seam One-pass bead-on-plate welding by gas shielded arc welding was performed so as to be orthogonal to the weld line, and a weld joint was created. The heat input during welding was 1.0 kJ / mm. Lincolnweld LA-81 (manufactured by Lincoln) was used as the weld metal during seam welding, and MX-A55Ni1 (manufactured by Kobe Steel) was used as the weld metal during circumferential welding.

The surface of the welded portion of the welded pipe assembly was subjected to a grinder process, and the excess portion of the bead weld was removed. A test piece of 115 L × 15 W × 5 t was sampled from just below the bead welded portion of this pipe assembly so that the longitudinal direction was parallel to the bead weld line. Using this test piece, an SSCC resistance evaluation test using a four-point bending test piece was performed based on ASTM G39, NACE TM0177-2005 B method. NACE solution A which gave a deflection corresponding to a load stress of 388 MPa and 437 MPa and was saturated with hydrogen sulfide of 1 atm: after immersion in 720 hours in 5% by mass NaCl-0.5% by mass CH ₃ COOH, Presence / absence was determined by observation with an optical microscope at a magnification of 10 times.

[Evaluation of immersion potential difference ΔE (immersion potential of peripheral weld metal−immersion potential of thick steel plate)]
Thick steel plates having the steel compositions shown in Tables 1 and 2 and a corrosion test piece (length and width 20 mm × thickness 2 mm) obtained by collecting a part of the above-mentioned circumferential weld metal were wet-polished with SiC # 600 abrasive paper and ultrasonically cleaned. Thereafter, the conductor was attached to the sample by spot welding. Except for the test surface of the thick steel plate or the peripheral weld metal, it was coated with an epoxy resin. The sample was immersed in a NACE solution A (5 mass% NaCl-0.5 mass% CH ₃ COOH) saturated with 1 atm of hydrogen sulfide, and the potential after 1 hour was measured. A saturated calomel electrode was used as the reference electrode, and a value obtained by subtracting the immersion potential of the thick steel plate from the immersion potential of the peripheral weld metal was calculated as the immersion potential difference ΔE.

As is clear from the comparison of the properties of the examples of Table 7 and the comparative examples of Table 8 showing these results, the composition of the components specified in the present invention and the composition of coarse inclusions with a width in steel of 1 μm or more are satisfied The thick steel plate of the example to be obtained had high mechanical strength, was not cracked by the HIC test, and was excellent in HIC resistance. Moreover, in the Example, it was confirmed that the crack did not generate | occur | produce in the SSCC test which gave the deflection | deviation equivalent to 388 MPa of load stress. Further, in Examples 4 to 33, it was confirmed that cracks did not occur even in the SSCC resistance evaluation test in which a deflection corresponding to a load stress of 438 MPa was given, and excellent SSCC resistance was obtained.

On the other hand, in the thick steel plate of the comparative example that does not satisfy the component composition or coarse inclusion composition specified by the present invention, the occurrence of cracks was confirmed in the HIC resistance test or the SSCC resistance test with a deflection corresponding to a load stress of 388 MPa. .

Although the present invention has been described in detail and with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention. This application is based on a Japanese patent application filed on May 22, 2015 (Japanese Patent Application No. 2015-104617) and a Japanese patent application filed on March 28, 2016 (Japanese Patent Application No. 2016-066404). Incorporated herein by reference.

Claims (7)

1. In 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: Over 0% to 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. 0% 05%, Zr: 0.0003 to 0.01%, Ca: 0.0003 to 0.006%, N: more than 0% to 0.010% or less, O: more than 0% to 0.0040% or less, The balance is a thick steel plate made of iron and inevitable impurities,
   In the composition of inclusions with a width of 1 μm or more contained in the steel, the Zr amount in the inclusions is 1 to 40%, the REM amount is 5 to 50%, the Al amount is 3 to 30%, and the Ca amount is 5 to 5%. A thick steel plate characterized by satisfying 60%.
2. The thick steel plate according to claim 1, wherein the amount of S in the inclusion is more than 0% and not more than 20%.
3. The thick steel plate according to claim 1, wherein [Cr] / [Nb] is 10 or more.
   However, in the above formula, [] indicates mass%.
4. Further, by mass, Mg: more than 0% and 0.005% or less, Ti: 0.003-0.030%, Ni: 0.01-1.50%, Cu: 0.01-1.50%, Mo: 0.01 to 1.50%, V: 0.003 to 0.08%, and B: 0.0002 to 0.0032%, or one or more,
   The thick steel plate according to claim 1, wherein [Cr] + [Mo] + [Ni] + [Cu] is 2.1 or less.
   However, in the above formula, [] indicates mass%.
5. Ni: 0.01 to 1.50% in mass%,
   5. The thick steel plate according to claim 4, wherein 0.25 × [Cr] + [Ni] is 0.10 to 1.50.
   However, in the above formula, [] indicates mass%.
6. A welded joint comprising the thick steel plate according to any one of claims 1 to 5 and a circumferential weld metal.
7. The welded joint according to claim 6, wherein an immersion potential difference ΔE between the thick steel plate and the circumferential weld metal, obtained by the following formula, is 25 mV or less.
   ΔE = Immersion potential after 1 hour of circumferential weld metal (mV) −Immersion potential after 1 hour of thick steel plate (mV)