WO2015087940A1 - Steel plate with excellent sour resistance, haz toughness and haz hardness, and steep pipe for line pipe - Google Patents

Abstract

The present invention pertains to a steel plate which exhibits excellent sour resistance, excellent HAZ toughness and excellent HAZ hardness. This steel plate contains, in mass%, 0.02 to 0.20% of C, 0.02 to 0.50% of Si, 0.6 to 2.0% of Mn, more than 0 to 0.030% of P, more than 0 to 0.004% of S, 0.010 to 0.08% of Al, 0.001 to 0.01% of N, 0.002 to less than 0.05% of Nb, more than 0 to 0.0040% of O, 0.0002 to 0.05% of REM and 0.0003 to 0.020% of Zr with the balance being Fe and unavoidable impurities and satisfies the relationship: 10000×[Nb] + 31×Di -82 ≥0 [wherein Di=([C]/10)^0.5 × (1+0.7×[Si]) × (1+3.33×[Mn]) × (1+0.35×[Cu]) × (1+0.36×[Ni]) × (1+2.16×[Cr]) × (1+3×[Mo]) × (1+1.75×[V]) × 1.115.] The inclusions which are contained in the steel and which have widths of 1μm or more have a composition that comprises 1 to 40% of Zr, 5 to 50% of REM, 3 to 30% of Al, and more than 0 to less than 20% of S.

Description

Steel plate and line pipe steel pipe with excellent sour resistance, HAZ toughness and HAZ hardness

The present invention relates to a steel plate excellent in sour resistance, HAZ toughness and HAZ hardness suitable as a material steel plate for energy structural materials such as for line pipes and offshore structures, and for line pipes produced using the steel plates. It relates to steel pipes.

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 issue, and high-function steel for energy is indispensable especially 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 line pipe steel, which is used for the transportation of oil and natural gas (LNG), and not only the properties (strength and toughness) as structural materials. , Resistance to oil and natural gas passing through the pipe 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 crack resistance (resistance to resistance) There is a strong demand for sour resistance typified by HIC properties.

Steel plates such as steel for line pipes are used as welded structures. In general, the material weakened part of the welded structure is a heat affected zone (HAZ) near the welded part, and it is required to ensure the toughness of the part. On the other hand, from the viewpoint of the facility, there is always a demand for improved weldability. is there. That is, it has been demanded to ensure HAZ toughness and weldability.
In order to improve welding workability, for example, there is an increase in the heat input of the welding heat, but there is a problem that the HAZ toughness is significantly deteriorated under the high heat input welding conditions.

Patent document 1 etc. are mentioned as a prior art which has achieved this sour resistance and HAZ toughness ensuring.
That is, in Patent Document 1, the main component balance (Ceq value) and the precipitate control of 20 nm or less mainly composed of Ti, the sour resistance without HIC cracking in the strength class of the base material of X65 to X80, and 1400 ° C. × 1 Second, Tc = 50 seconds after thermal cycle, ˜−10 ° level of vTrs HAZ toughness is achieved. However, no particular mention is made to make the inclination of the HAZ hardness from coarse to fine to be small and smooth (reduction in variation in HAZ hardness).

On the other hand, Patent Document 2 is cited as an example in which the average value and the minimum value of the HAZ toughness are improved even under large heat input welding conditions by inclusion inclusion and inclusion composition control.
That is, Patent Document 2 is intended for thick steel plates for bridges and shipbuilding, and controls the oxide composition of Ti, Ca, Al, etc. and the amount of its trace components to increase the intragranular α production rate. Thus, the HAZ toughness at the level of vTrs-40 ° C. is achieved under conditions equivalent to high heat input welding of 1400 ° C. × 60 seconds and Tc = 400 seconds (corresponding to heat input 50 J / mm). Therefore, this document does not intend to improve the sour resistance required for line pipes, and is focused exclusively on the oxide control described above, in order to suppress coarse sulfides that deteriorate the sour resistance. Is not specially desulfurized. Although emphasis is placed on reducing the variation in HAZ toughness, there is no specific description regarding the variation in HAZ hardness from coarse to fine.

None of these conventional technologies simultaneously achieve the three aspects of sour resistance, HAZ toughness, and reduced variation in HAZ hardness.

In addition, the line pipe is manufactured by bending a thick steel plate for a line pipe into a tubular shape and welding both end edges. The line pipes thus manufactured are joined by further welding the pipes and used as an actual oil transportation line.

T-cross welds that receive two thermal histories, seam welding when processing thick steel plates into pipes and circumferential welding when joining pipes, receive complex thermal histories such as rapid heating and rapid cooling. Therefore, in HAZ, the strength (hardness) is increased, and cracks called sulfide stress corrosion cracking (SSCC) are likely to occur. Therefore, in steel for line pipes, in addition to the above-mentioned HIC resistance (sour resistance) of the base material, it is necessary to ensure the SSCC resistance of the T-cross weld.

Examples of conventional techniques considering improvement in SSCC resistance include the techniques described in Patent Document 3 and Patent Document 4. The technique described in Patent Document 3 is a technique for achieving high strength of a tensile strength of 56 kgf / mm ² or more by utilizing precipitation strengthening by fine Nb and V carbonitrides. However, the HIC resistance of the base material is not considered, and only the seam-welded HAZ is considered for the SSCC resistance. Further, the immersion time in a solution simulating a sour environment is 21 days, which is not a sufficiently severe test condition. Patent Document 4 discloses a component system that suppresses an increase in hardness that is supposed to degrade the SSCC resistance of a T-cross weld. However, the SSCC resistance itself is not evaluated, and the HIC resistance of the base material is not considered.

Japanese Unexamined Patent Publication No. 2009-52137 Japanese Unexamined Patent Publication No. 2010-168644 Japanese Unexamined Patent Publication No. 1-96329 Japanese Unexamined Patent Publication No. 2005-186162

Therefore, the object (problem) of the present invention is to satisfy the three factors of sour resistance, HAZ toughness, and HAZ hardness variation reduction at the same time, and for line pipes having high functional properties such as yield strength and tensile strength. It is providing the steel plate suitable for steel materials for energy, and the steel pipe for line pipes manufactured using this steel plate.

The present invention is weight percent,
C: 0.02 to 0.20%
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 0.08%,
N: 0.001 to 0.01%
Nb: 0.002% or more and less than 0.05%,
O: more than 0% and 0.0040% or less,
REM: 0.0002 to 0.05%, and Zr: 0.0003 to 0.020%
The balance is iron and inevitable impurities,
Formula 10000 × [Nb] + 31 × Di−82 ≧ 0
(However, Di = ([C] / 10) ^0.5 × (1 + 0.7 × [Si]) × (1 + 3.33 × [Mn]) × (1 + 0.35 × [Cu]) × (1 + 0.36) × [Ni]) × (1 + 2.16 × [Cr]) × (1 + 3 × [Mo]) × (1 + 1.75 × [V]) × 1.115)
The filling,
In the composition of inclusions having a width of 1 μm or more contained in steel,
The amount of Zr in the inclusion is 1 to 40%,
REM amount is 5-50%,
Al content is 3-30%,
A steel sheet excellent in sour resistance, HAZ toughness, and HAZ hardness with an S content of more than 0% and less than 20% is provided.

The steel plate
Ca: 0.0003 to 0.0060%, and Mg: 0.0003 to 0.005%
One or more elements selected from may be further included.

The steel plate
In the composition of the inclusions having a width of 1 μm or more contained in the steel,
The amount of Zr in the inclusion is 1 to 40%,
REM amount is 5-50%,
Al content is 3-30%,
Ca content 5-60%,
The S amount may be more than 0% and less than 20%.

The steel plate is
Ti: 0.003-0.03%,
B: 0.0002 to 0.005%,
V: 0.003-0.1%
Cu: 0.01 to 1.5%,
Ni: 0.01 to 3.5%,
Cr: 0.01 to 1.5%, and Mo: 0.01 to 1.5%,
One or more elements selected from may be further included.

The steel plate may be for a line pipe.

Moreover, this invention provides the steel pipe for line pipes manufactured using the said steel plate.

According to the present invention, the sour resistance, HAZ toughness, and HAZ hardness are excellent, having excellent HAZ toughness and HAZ hardness even under high heat input welding conditions, such as resistance to hydrogen-induced cracking. A steel plate that satisfies the three reductions at the same time, and that can be advantageously applied as an energy steel material for line pipes, marine structures, etc. with high functional properties with high yield strength and tensile strength, and using the steel plate A manufactured steel pipe for a line pipe can be provided.

In order to achieve the object of the present invention, the present inventor has earnestly studied, studied, and studied from the viewpoint of controlling inclusions in steel in addition to the composition of steel, which is fundamental for exhibiting the properties of steel sheets. As a result of superposition, by holding coarse inclusions having a width of 1 μm or more in a specific component composition, an excellent steel sheet that simultaneously satisfies all the characteristics of the sour resistance, HAZ toughness, and HAZ hardness can be obtained. Based on this finding, the present invention has been completed.

In consideration of sour resistance, when hydrogen enters the steel in the sour environment, inclusions such as MnS, which are coarse and have a higher thermal expansion coefficient than the steel, form coarse voids around the steel. It is presumed that the hydrogen accumulated in these voids intensively cracks in the steel at the pressure at which they vaporize, that is, hydrogen-induced cracking occurs and progresses. Therefore, by converting the 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, the resistance of the steel is increased. We thought that sourness could 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 improving HAZ toughness and reducing the hardness gradient from coarse grains to fine grains (reducing hardness variation), microstructure refinement is one of the methods to improve these characteristics. Focused on. Generally, when welding heat input is applied, the structure becomes coarse and the HAZ toughness deteriorates. Further, since the vicinity of the welded portion is exposed to various heat inputs according to the distance from the welded portion, the hardness can be distributed. In particular, the difference in hardness becomes large at the point where the transformation transitions from ferrite to bainite. On the other hand, it is possible to improve the dispersion of HAZ toughness and HAZ hardness by introducing inclusions that can promote intragranular transformation and refine the structure.

At the same time, the hardness gradient can be reduced by positively introducing intragranular needles α having a medium transformation point between ferrite having a high transformation point and bainite having a low transformation point. In order for the inclusions to exert such an effect, there are methods such as lowering the melting point and increasing the lattice matching with the steel matrix, specifically, a small amount of Zr, REM, Al, TiN, Ti-Ca oxides are considered effective.

However, considering these aspects of sour resistance, HAZ toughness, and HAZ hardness gradient reduction, effective inclusions are common as components, but as a specific composition (component ratio) Since they do not necessarily match, the present inventors have further studied and experimented with an emphasis on appropriately balancing these inclusion compositions together with the steel composition and structure. It has succeeded in finding the right range.

Furthermore, by setting the Ca concentration in the inclusions in a specific range amount, the intragranular needle-like α starting from the inclusions can be generated in the T-cross welded portion. It has also been found that the SSCC resistance of the weld is improved.

Hereinafter, the steel composition, structure, and inclusion composition of the steel sheet of the present invention will be described in detail, including the reasons for their definition. In addition,% which is a display unit of a composition means mass%. Here, in this specification, the percentage (mass%) based on mass is the same as the percentage (wt%) based on weight. In addition, regarding the content of each chemical component, “X% or less (not including 0%)” may be expressed as “over 0% and X% or less”.

(Steel composition)
[C: 0.02 to 0.20%]
C is an indispensable element for securing the hardenability of the HAZ part, and needs to be contained by 0.02% or more. The amount of C is preferably 0.03% or more, and more preferably 0.05% or more. If the amount of C is excessive, martensite (including MA = island martensite) is likely to be generated, and the HAZ toughness is deteriorated. Therefore, the C amount needs to be 0.20% or less. The amount of C is preferably 0.15% or less, more preferably 0.12% 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. The amount of Si is preferably 0.05% or more, and more preferably 0.15% or more. However, if the amount of Si is excessive, island martensite is easily formed and the HAZ toughness is deteriorated. Therefore, the amount of Si needs to be suppressed to 0.50% or less. The amount of Si is preferably 0.45% or less, more preferably 0.35% or less.

[Mn: 0.6 to 2.0%]
Mn is an element effective for ensuring the hardenability of the HAZ part, and is contained in an amount of 0.6% or more in the present invention. The amount of Mn is preferably 0.8% or more, and more preferably 1.0% or more. However, if the amount of Mn is too large, not only MnS is generated and the hydrogen-induced cracking resistance deteriorates but also the HAZ toughness deteriorates, so the upper limit of the Mn amount is made 2.0% or less. The amount of Mn is preferably 1.8% or less, more preferably 1.6% or less.

[P: 0.030% or less (excluding 0%)]
P is an element inevitably contained in the steel material. When the amount of P exceeds 0.030%, the HAZ toughness is significantly deteriorated and the hydrogen-induced crack resistance is also deteriorated. Therefore, in the present invention, the amount of P is suppressed to 0.030% or less. The amount of P is preferably 0.020% or less, more preferably 0.010% or less.

[S: 0.004% or less (excluding 0%)]
If S is too much, a large amount of MnS is generated and hydrogen-induced cracking resistance is remarkably deteriorated. Therefore, in the present invention, the upper limit of the amount of S is made 0.004%. The amount of S is preferably 0.003% or less, more preferably 0.0025% or less, and still more preferably 0.0020% or less. Thus, although it is desirable that it is less from the viewpoint of improving hydrogen-induced cracking resistance, it is difficult to make it less than 0.0001% industrially, so the lower limit of the amount of S is approximately 0.0001%.

[Al: 0.010 to 0.08%]
Al is effective in reducing the voids with the steel matrix by reducing the thermal expansion coefficient of inclusions and ensuring sour resistance. Moreover, it is effective for lowering the melting point of inclusions to increase the intragranular acicular α production rate, to ensure HAZ toughness, and to reduce the hardness gradient from coarse grains to fine grains. In order to exhibit this effect, Al needs to be 0.010% or more. The amount of Al is preferably 0.020% or more, more preferably 0.030% or more.
On the other hand, if Al is added prior to Zr and the Al content is too high, Al oxide is formed preferentially over Zr oxide and the Zr concentration in inclusions decreases, Al oxides form in clusters and become the starting point of hydrogen-induced cracking. Therefore, the Al amount needs to be 0.08% or less. The amount of Al is preferably 0.06% or less, and more preferably 0.05% or less.

[N: 0.001 to 0.01%]
N is an element that precipitates as TiN in the steel structure, suppresses coarsening of the austenite grains in the HAZ part, further promotes ferrite transformation, and improves the toughness of the HAZ part. In order to acquire this effect, it is necessary to contain N 0.001% or more. The N amount is preferably 0.003% or more, and more preferably 0.0040% or more. However, if the amount of N is too large, the HAZ toughness deteriorates due to the presence of solute N, so the amount of N needs to be 0.01% or less. The N amount is preferably 0.008% or less, and more preferably 0.0060% or less.

[Nb: 0.002 to 0.05% (not including 0.05%)]
Nb is an element effective for increasing the strength without degrading the weldability. In order to obtain this effect, the Nb amount needs to be 0.002% or more. The Nb amount is preferably 0.010% or more, more preferably 0.020% or more. However, when the Nb amount is 0.05% or more, the toughness of the HAZ deteriorates. Therefore, in the present invention, the Nb amount is less than 0.05%. The Nb amount is preferably 0.040% or less, more preferably 0.030% or less.

[O: 0.0040% or less (excluding 0%)]
O (oxygen) is preferably low from the viewpoint of improving cleanliness. When a large amount of O is contained, in addition to deterioration of toughness, HIC is generated starting from oxide, and resistance to hydrogen-induced cracking is deteriorated. . From this viewpoint, the amount of O needs to be 0.0040% or less, preferably 0.0030% or less, more preferably 0.0020% or less.

[REM: 0.0002 to 0.05%]
REM (rare earth element) is effective in reducing voids from the steel matrix by reducing the thermal expansion coefficient of inclusions and ensuring sour resistance. Moreover, it is effective for lowering the melting point of inclusions to increase the intragranular acicular α production rate, to ensure HAZ toughness, and to reduce the hardness gradient from coarse grains to fine grains. In order to exhibit such an effect, 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, the effect is saturated even if a large amount of REM is contained. Therefore, the upper limit of the REM amount is set to 0.05%. From the viewpoint of suppressing productivity of the immersion nozzle during casting, the REM content is preferably 0.03% or less, more preferably 0.010% or less, and further preferably 0.0050% or less. is there.
In the present invention, the REM means a lanthanoid element (15 elements from La to Lu), Sc (scandium), and Y8 (yttrium).

[Zr: 0.0003 to 0.020%]
Zr is effective in reducing voids with the steel matrix by reducing the coefficient of thermal expansion of inclusions and ensuring sour resistance. Moreover, it is effective for lowering the melting point of inclusions to increase the intragranular acicular α production rate, to ensure HAZ toughness, and to reduce the hardness gradient from coarse grains to fine grains. In order to make the Zr concentration in the inclusions 5% or more in order to remarkably improve the resistance to hydrogen-induced cracking, the amount of Zr needs to be 0.0003% or more. The Zr amount is preferably 0.0005% or more, more preferably 0.0010% or more, and still more preferably 0.0015% or more. On the other hand, when Zr is added excessively, the solid solution Zr in the molten steel increases and crystallizes so as to surround the acid / sulfide during casting, thereby degrading the HAZ toughness and the resistance to hydrogen-induced cracking. Therefore, the amount of Zr needs to be 0.020% or less. The amount of Zr is preferably 0.010% or less, more preferably 0.0070% or less, and still more preferably 0.0050% or less.

The component composition of the steel material of the steel sheet of the present invention is as described above, and the balance is iron and inevitable impurities. Further, in addition to the above elements, the HAZ toughness is obtained by adding one or more elements selected from the group consisting of Ca, Mg, Ti, B, V, Cu, Ni, Cr, and Mo in the following amounts. It is possible to improve the strength and the strength. Hereinafter, these elements will be described.

[Ca: 0.0003 to 0.0060%]
Ca has the effect of forming CaS and finely dispersing sulfides. In order to obtain this effect, the Ca content needs to be 0.0003% or more. The Ca content is preferably 0.0005% or more, and more preferably 0.0010% or more. On the other hand, when the amount of Ca exceeds 0.0060%, a large amount of CaS is formed and they aggregate to adversely affect the HAZ toughness and HIC characteristics. Therefore, in the present invention, the upper limit of the Ca amount is set to 0.0060%. The Ca content is preferably 0.0050% or less, more preferably 0.0040% or less.

[Mg: 0.0003 to 0.005%]
Mg has the effect of forming MgS and finely dispersing sulfides. In order to obtain this effect, it is preferable to contain 0.0003% or more of Mg. The amount of Mg is more preferably 0.001% or more. On the other hand, since the effect is saturated even if Mg is contained in excess of 0.005%, the upper limit of the amount of Mg is preferably 0.005%. The amount of Mg is more preferably 0.0030% or less.

[Ti: 0.003-0.03%]
Ti is an element necessary for improving the toughness of the HAZ part in order to prevent coarsening of austenite grains in the HAZ part during welding and promote ferrite transformation by precipitating as TiN in the steel. . In order to obtain such an effect, 0.003% or more of Ti is preferably contained. The amount of Ti is more preferably 0.005% or more, and still more preferably 0.010% or more. On the other hand, when the Ti content is excessive, solute Ti and TiC are precipitated and the HAZ toughness is deteriorated, so 0.03% or less is preferable. The amount of Ti is more preferably 0.02% or less.

[B: 0.0002 to 0.005%]
B improves the HAZ toughness in order to improve the hardenability. In order to acquire this effect, it is preferable to contain B 0.0002% or more. The amount of B is more preferably 0.0005% or more, and further preferably 0.0010% or more. However, if the B content is excessive, the HAZ toughness is deteriorated or the weldability is deteriorated. Therefore, the B content is preferably 0.005% or less. The amount of B is more preferably 0.004% or less, and still more preferably 0.003% or less.

[V: 0.003 to 0.1%]
V is an element effective for improving the strength. To obtain this effect, V is preferably contained in an amount of 0.003% or more. The amount of V is more preferably 0.010% or more. On the other hand, if the V content exceeds 0.1%, the weldability deteriorates. Therefore, the V amount is preferably 0.1% or less, and more preferably 0.08% or less.

[Cu: 0.01 to 1.5%]
Cu is an element effective for improving the hardenability and increasing the strength. In order to acquire this effect, it is preferable to contain 0.01% or more of Cu. The amount of Cu is more preferably 0.05% or more, and still more preferably 0.10% or more. However, if the Cu content exceeds 1.5%, the strength becomes too high and the toughness deteriorates. The amount of Cu is more preferably 1.0% or less, still more preferably 0.50% or less.

[Ni: 0.01 to 3.5%]
Ni is an element effective for improving the base material strength and the HAZ toughness. In order to obtain this effect, the Ni content is preferably 0.01% or more. The amount of Ni is more preferably 0.05% or more, and still more preferably 0.10% or more. However, if Ni is contained in a large amount, it becomes extremely expensive as a structural steel material. Therefore, the Ni content is preferably 1.5% or less from an economical viewpoint. The amount of Ni is more preferably 1.0% or less, and still more preferably 0.50% or less.

[Cr: 0.01 to 1.5%]
Cr is an element effective for improving the strength, and in order to obtain this effect, it is preferable to contain 0.01% or more. The amount of Cr is more preferably 0.05% or more, and still more preferably 0.10% or more. On the other hand, if the Cr content exceeds 1.5%, the HAZ toughness deteriorates. Therefore, the Cr content is preferably 1.5% or less. The amount of Cr is more preferably 1.0% or less, and still more preferably 0.50% or less.

[Mo: 0.01 to 1.5%]
Mo is an element effective for improving the base material strength. In order to obtain this effect, the Mo amount is preferably 0.01% or more. The amount of Mo is more preferably 0.05% or more, and still more preferably 0.10% or more. However, if the Mo amount exceeds 1.5%, the HAZ toughness and weldability deteriorate. Therefore, the Mo amount is preferably 1.5% or less, more preferably 1.0% or less, and still more preferably 0.50% or less.

[10000 × [Nb] + 31 × Di−82 ≧ 0 1 set]
(However, Di = ([C] / 10) ^0.5 × (1 + 0.7 × [Si]) × (1 + 3.33 × [Mn]) × (1 + 0.35 × [Cu]) × (1 + 0.36) × [Ni]) × (1 + 2.16 × [Cr]) × (1 + 3 × [Mo]) × (1 + 1.75 × [V]) × 1.115 (2 formulas)
This term defines the relationship between the Nb-Di balance, that is, the Nb amount and the Di value (hardenability: 2 formulas), and it is necessary to satisfy the above 1 formula. The composition in [] is mass%. The above two formulas relating to the hardenability Di value are described as Grossmann's formula (Trans. Metall. Soc. AIME, 150 (1942), p. 227).

By controlling the steel composition so that the Nb-Di balance satisfies the above equation (1), it is possible to secure a higher base metal strength (yield strength, tensile strength) by accelerated cooling, a small HAZ hardness gradient, and hydrogen resistance induction. It becomes possible to obtain a steel sheet having excellent cracking properties.
Although there are no particular problems with the characteristics defined in the present invention, the preferable range of the Nb-Di balance is [200 ≦ 10000 × [Nb] + 31 × Di ≦ 300] from the viewpoint of the tolerance of the base material property control. is there.

(Inclusion composition)
[Composition of inclusions with a width of 1 μm or more contained in steel]
[Zr content is 1-40%]
Since 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, which effectively functions to ensure sour resistance. In addition, the Zr oxide lowers the melting point of inclusions and increases the intragranular formation rate, and is effective in improving HAZ toughness and reducing variation in HAZ hardness. 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%, the sour resistance and / or HAZ toughness and reduction of HAZ hardness variation are insufficient. On the other hand, if the amount of Zr exceeds 40%, the lattice consistency between the inclusions and the steel matrix phase is lowered, so the intragranular α production rate is lowered, and the reduction in variation in HAZ toughness and HAZ hardness is lowered.

[REM amount is 5-50%]
Since the thermal expansion coefficient of REM oxide is smaller than that of steel, if the amount of REM in the inclusion is ensured, voids with the surrounding steel matrix can be reduced, and S can be fixed and finely dispersed. It functions effectively to ensure sourness. Further, the REM oxide reduces the melting point of inclusions to increase the intragranular formation rate, and is effective in improving HAZ toughness and reducing variation in HAZ hardness. In order to exert such an effect, the amount of REM in the inclusion is set to 5 to 50%. If the amount of REM is less than 5%, the sour resistance and / or HAZ toughness and the reduction of HAZ hardness variation are insufficient. On the other hand, if the amount of REM exceeds 50%, the lattice consistency between inclusions and the steel matrix decreases, so the intragranular α production rate decreases, the HAZ toughness decreases, and the HAZ hardness variation increases. .

[Al content is 3-30%]
Since the 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, and it effectively functions to ensure sour resistance. In addition, Al oxides are effective in reducing the melting point of inclusions to increase the intragranular formation rate, improving HAZ toughness, and reducing variation in HAZ hardness. In order to exert such an effect, the amount of Al in the inclusion is made 3 to 30%. If the Al content is less than 3%, the sour resistance and / or HAZ toughness and the reduction of the HAZ hardness variation are insufficient. On the other hand, if the Al content exceeds 30%, the lattice consistency between the inclusions and the steel matrix decreases, so the intragranular α production rate decreases, the HAZ toughness decreases, and the HAZ hardness variation increases. .

[S amount exceeds 0% and less than 20%]
Since coarse sulfide deteriorates sour resistance, it is necessary to reduce this. In order to reduce the adverse effect of coarse sulfides on sour resistance, it is effective to reduce the amount of S in steel by desulfurization and to finely disperse it. S can be fixed and finely dispersed by adding REM. This effect can be grasped indirectly by measuring the amount of S in the inclusion, and when the amount of S in the inclusion is more than 0% and less than 20%, the adverse effect on the sour resistance of the coarse sulfide is suppressed. be able to. When the amount of S is 0%, it indicates that S is not fixed and sour resistance is deteriorated. On the other hand, when the amount of S is 20% or more, although S can be fixed, coarse sulfides are easily formed and sour resistance is deteriorated.

[Ca content is 5-60%]
When the steel sheet of the present invention contains Ca, by setting the amount of Ca in the inclusions in a specific range, the intragranular needle-like α starting from the inclusions is generated in the T-cross welded portion. The SSCC resistance of the T-cross weld is improved by the effect of refining the structure. 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.

(Production method)
[Molten steel treatment process]
Next, the manufacturing method of the steel sheet of the present invention will be described in detail below.
In obtaining the steel sheet of the present invention, in the molten steel treatment process,
(A) a desulfurization step in which S is 0.004% or less using slag satisfying Fe: 0.1 to 10%;
(B) Deoxidation step in which the dissolved oxygen concentration Of of the molten steel is 10 or less in a ratio (Of / S) to the S concentration of the molten steel,
(C) A step of adding Zr, REM and Ca in the order of Zr, REM and Ca, or adding Zr and REM at the same time and then adding in the order of Ca (however, the time from REM addition to Ca addition is 4 minutes or more And)
In this order, and the time from the addition of Ca to the completion of solidification is within 200 minutes, and the cooling time at the slab t / 4 position (t: plate thickness) at 1300 ° C to 1200 ° C during casting is within 460 seconds. There is a need. Furthermore, it is preferable that the cooling time at the slab t / 4 position (t: plate thickness) at 1500 to 1450 ° C. during casting is 300 seconds or less because SSCC resistance can be improved.
The above steps will be described in order below.

(A) Desulfurization process In order to ensure sour 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 not only becomes a starting point of hydrogen-induced cracking, but also deteriorates the toughness of the base material and the weld heat affected zone. Therefore, the Fe concentration in the slag is set to 10% or less. The Fe concentration in the slag is preferably 8% or less, more preferably 5% or less. In addition, when Ca is added, it is possible to prevent a large amount of CaS from being formed when Ca is added after REM addition by sufficiently performing desulfurization with slag and suppressing S to 0.004% or less. Further, by preventing the composition of inclusions from deviating from a predetermined range, it is possible to ensure HIC resistance and SSCC resistance.

The following (a) and (b) are mentioned as means for making the S 0.004% or less.
Using (a) e.g. ladle desulfurization (such as LF), flow rate ^{5 Nm} 3 / h or more (preferably 10 Nm ³ / h or more, the upper limit of the flow rate approximately 300 Nm ³ / h) blowing the inert gas (such as Ar) And stirring for 3 minutes or more (preferably 10 minutes or more, more preferably 20 minutes or more, and the upper limit of the stirring time is about 200 minutes from the viewpoint of productivity).
(B) Moreover, when adding Ca, the CaO density | concentration in the said slag shall be 10% or more. From the addition of Ca, CaO in the slag reacts with the dissolved S in the molten steel and changes to CaS, so that S in the molten steel can be sufficiently reduced, that is, desulfurized. 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, since desulfurization becomes difficult even if there is too much CaO in the slag, the upper limit of the CaO concentration in the slag is about 80%.

(B) Deoxidation step In order to improve the HAZ toughness, oxide control is important. To achieve this, it is important to control the amount of O. In this process, since the so-called recovery S occurs that slightly increases the S amount that has an effect on the sour resistance, 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 a ratio (Of / S) to the S concentration of the molten steel. 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 2.0 or less. The lower limit value of Of / S is about 0.1. In order to reduce the Of / S to 10 or less, it can be achieved by deoxidation by an RH degassing apparatus and / or deoxidation by adding deoxidation elements such as Mn, Si, Ti and the like.

(C) Addition step of Al, Zr, REM (and Ca) 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 and (Zr, REM), the deoxidizing power of (Zr, REM) is stronger than that of Al. Therefore, when (Zr, REM) is added prior to Al, The amount of Al cannot be a desired value. Therefore, the order of addition needs to be Al → (Zr, REM).

In addition to these elements, when adding Ca further, considering the desulfurization power and deoxidation power of each additional element described below, Al is added first, then Zr is added, and then REM is added. Either Ca is added, and finally Ca is added, or Al is added first, then Zr and REM are added simultaneously, 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. Doing so will degrade the sour resistance. 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, REM) → Ca. Moreover, in order to control the range of inclusions to a predetermined range, it is necessary to leave a time from REM addition to Ca addition of 4 minutes or more. 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 a stronger deoxidizing power than Zr. I must. Therefore, the order of addition of Al, Zr, REM and Ca needs to be Al → Zr → REM → Ca. However, since REM has a lower deoxidizing capacity than Ca, it is possible to contain Zr in inclusions even if it is added simultaneously with Zr, so the order of addition is Al → (Zr, REM). → Ca may be used.

With respect to the amount of each element added, it is only necessary to obtain a steel sheet having a desired amount of each element. For example, Zr is added to a concentration of 3 to 100 ppm in the molten steel, and then 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 Ca addition, casting is started immediately (for example, within 80 minutes), and casting is performed so that the time from the addition of Ca to the completion of solidification is 200 minutes or less. The reason is as follows. That is, since Ca is an element having high desulfurization ability and deoxidation ability, the Ca concentration in inclusions tends to increase, and the composition of inclusions deviates from a predetermined range. Therefore, in the present invention, the time from the addition of Ca to the completion of solidification is set to be within 200 minutes. It is preferably within 180 minutes, more preferably within 160 minutes. In addition, the minimum of the said time will be about 4 minutes from a viewpoint of homogenizing Ca.

Also, it is important that the cooling time at 1300 ° C. to 1200 ° C. during casting is 270 to 460 sec. When the cooling time exceeds the upper limit, composite formation of mainly sulfide-based secondary inclusions on the inclusions is promoted, and the difference in HAZ hardness is caused by the inclusion composition deviating from a predetermined range. It deviates from the predetermined range. On the other hand, if the cooling time falls below the lower limit, the cooling load increases greatly, which is not preferable in practice.

Furthermore, by setting the cooling time at 1500 to 1450 ° C. during casting to 300 seconds or less, composite formation of oxide-based secondary inclusions on the inclusions is promoted, and it is more effective for the formation of acicular α. The inclusion composition is realized, and an improvement effect is also obtained in the SSCC resistance of the T-cross weld.

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

[Example]
The molten steel refined in a 240t converter by a conventional method is subjected to treatment (molten steel treatment) such as desulfurization, deoxidation, component adjustment and inclusion control using an LF furnace. Tables 1, 2, 9 (Invention Examples) and Tables Various molten steels having steel compositions shown in 3, 4 and 9 (comparative examples) and inclusions in steel are used as slabs by a continuous casting method, and these are hot-rolled and accelerated and cooled, and then a steel plate (thickness 40 mm, width 3500 mm) ( Thick steel plate). Tables 2 and 10 (invention examples) and Tables 4 and 10 (comparative examples) also show the composition of coarse inclusions in steel. Tables 5 and 11 (invention examples) and Tables 6 and 11 (comparative examples) show main process conditions in the molten steel treatment, continuous casting, and accelerated cooling. Tables 7 and 12 (invention examples) and Tables 8 and 12 (comparative examples) show various properties of the steel sheets thus obtained.
The method for analyzing the composition of inclusions shown in Tables 2, 4, and 10, the method for measuring (testing) each property in Tables 7, 8, and 12, and the method for evaluation 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 central portion of the plate thickness. Specifically, three cross-sections are observed at an observation magnification of 400 times and an observation field of view of about 50 mm ² (7 mm in the plate thickness direction and 7 mm in the plate width direction so that the center of the plate thickness is the center of the observation field) and the width is 1 μm For the above inclusions, the component composition at the center of the inclusion was quantitatively analyzed by wavelength dispersion spectroscopy of characteristic X-rays.
The analysis target elements were Al, Mn, Si, Mg, Ca, Ti, Zr, S, REM (La, Ce, Nd, Dy, Y), and Nb. 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 (composition of inclusions) of the content of each element of inclusions having a width of 1 μm or more in the three cross sections was determined.

[Measurement and evaluation of yield strength YS and tensile strength TS of base material]
Sample No. 4 of JIS Z2241 was sampled in parallel to the C direction from the t / 4 position (t: plate thickness) of each steel plate, and a tensile test was performed by the method described in ZIS Z2241, tensile strength TS, and yield strength. YS was measured. In this example, when YS is 415 MPa or more and TS is 520 MPa or more, it is evaluated that the base material strength is excellent (pass), and those with less than 415 MPa and less than 520 MPa are inferior in base material strength (fail), respectively. evaluated.

[Test and evaluation of base material HIC resistance]
Tested and evaluated according to the method specified in NACE standard TM0284-2003. Specifically, the test piece was immersed in a 25 ° C. (0.5% NaCl + 0.5% acetic acid) aqueous solution 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 times using an optical microscope, and the crack length of the HIC was 200 μm or more. The number of cracks and the number of cracks of 1 mm or more were measured. And in this invention, when the crack length of the said HIC does not have a crack of 1 mm or more, it evaluates that it is excellent in HIC resistance (pass), and the case where one or more cracks of 1 mm or more exist is made HIC resistance. Evaluated as inferior (failed).

[Test and evaluation of SSCC resistance of T-cross welds]
In order to simulate seam welding when a thick steel plate was processed into a pipe, the rolled plate was processed into a 75 ° X groove and welded by a two-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. The heat input during welding was 1.0 kJ / mm.

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. After applying a deflection corresponding to a load stress of 332 MPa and 374 MPa and immersing in NACE solution A (5 mass% NaCl-0.5 mass% CH ₃ COOH) saturated with 1 atm of hydrogen sulfide for 720 hours, it cracks on the surface of the specimen. Those that did not occur were evaluated as acceptable.

[Measurement and evaluation of HAZ toughness (brittle fracture surface ratio in C direction)]
A heat cycle test piece (12.5 t × 33 L × 55 W) was taken in parallel with the C direction from the t / 4 position (t: plate thickness) of each steel plate, and 1400 ° C. (maximum temperature) × 5 seconds (heat retention time). , Tc (cooling time of 800 to 500 ° C.) = 400 seconds thermal cycle. After that, two Charpy impact test pieces (V-notch test piece of JIS Z 2242) are collected from each of the heat cycle test pieces, and the impact test is performed by the method described in JIS Z2242 at each measurement temperature, and vTrs is obtained. Asked. A sample having vTrs of −10 ° C. or lower was evaluated as being excellent in HAZ toughness (pass), and a sample having vTrs exceeding −10 ° C. was evaluated as being inferior in HAZ toughness (failed). The thermal cycle test corresponds to a high heat input welding condition corresponding to heat input = 60 kJ / mm.

[Measurement of hardness difference between CG-HAZ and FG-HAZ]
A thermal cycle test piece (12.5 t × 33 L × 55 W) was taken in parallel to the C direction from the t / 4 position (t: plate thickness) of each steel plate, 1400 ° C. × 5 seconds, Tc = 40 seconds (CG-HAZ) Test) and 1100 ° C. × 5 seconds, Tc = 40 seconds (FG-HAZ test). After that, a steel piece corresponding to a Charpy impact test piece (a JIS Z2242 V-notch test piece) was taken from the CG-HAZ and FG-HAZ thermal cycle test pieces, and the hardness of the cross section in the C direction was N = 3 or more at 10 kg of Vickers. The average value was obtained by measurement, and the difference between the CG-HAZ hardness and the FG-HAZ hardness was calculated. And this hardness difference (CG-HAZ hardness-FG-HAZ hardness) is less than 45 when the hardness variation is low and the HAZ hardness is evaluated as good (pass). Was evaluated as inferior in HAZ hardness (failed).

As is apparent from the comparison of the characteristics of the inventive examples in Tables 7 and 12 and the comparative examples in Tables 8 and 12 showing the results of these examples, the steel composition (including Nb-Di balance) defined by the present invention and The steel sheet of the invention example satisfying the composition of coarse inclusions with a width of 1 μm or more in steel has high mechanical strength of yield strength (YS) of 415 MPa or more and tensile strength (TS) of 520 MPa or more, and HIC test No cracking due to cracking, excellent sour resistance, vTrs (CG) by impact test exceeding -10 ° C or less, excellent HAZ toughness even under high heat input welding conditions, and CG- It can be seen that the difference between the HAZ hardness and the FG-HAZ hardness is stable at around 30, and has excellent HAZ hardness with little variation. Inventive examples 26 to 32 in which the Ca content in the coarse inclusions having a width of 1 μm or more in the steel is in the range of 5 to 60% does not cause cracks in the SSCC test. On the other hand, the steel plate of the comparative example that does not satisfy the steel composition or the coarse inclusion composition specified by the present invention has almost the same characteristics as the invention example with respect to the mechanical strength, but the sour resistance And / or it turns out that it is remarkably inferior compared with the invention example in HAZ toughness and HAZ hardness.

Although the invention has been described in detail 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 December 11, 2013 (Japanese Patent Application No. 2013-256080), which is incorporated by reference in its entirety.

Claims (8)

1. % By mass
   C: 0.02 to 0.20%
   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 0.08%,
   N: 0.001 to 0.01%
   Nb: 0.002% or more and less than 0.05%,
   O: more than 0% and 0.0040% or less,
   REM: 0.0002 to 0.05%, and Zr: 0.0003 to 0.020%
   The balance is iron and inevitable impurities,
   Formula 10000 × [Nb] + 31 × Di−82 ≧ 0
   (However, Di = ([C] / 10) ^0.5 × (1 + 0.7 × [Si]) × (1 + 3.33 × [Mn]) × (1 + 0.35 × [Cu]) × (1 + 0.36) × [Ni]) × (1 + 2.16 × [Cr]) × (1 + 3 × [Mo]) × (1 + 1.75 × [V]) × 1.115)
   The filling,
   In the composition of inclusions having a width of 1 μm or more contained in steel,
   The amount of Zr in the inclusion is 1 to 40%,
   REM amount is 5-50%,
   Al content is 3-30%,
   Steel sheet excellent in sour resistance, HAZ toughness, and HAZ hardness, with an S content of more than 0% and less than 20%.
2. Furthermore, Ca: 0.0003 to 0.0060%, and Mg: 0.0003 to 0.005%
   The steel plate according to claim 1, comprising at least one element selected from the group consisting of:
3. In the composition of the inclusions having a width of 1 μm or more contained in the steel,
   The amount of Zr in the inclusion is 1 to 40%,
   REM amount is 5-50%,
   Al content is 3-30%,
   Ca content 5-60%,
   The steel sheet according to claim 2, wherein the amount of S is more than 0% and less than 20%.
4. Furthermore,
   Ti: 0.003-0.03%,
   B: 0.0002 to 0.005%,
   V: 0.003-0.1%
   Cu: 0.01 to 1.5%,
   Ni: 0.01 to 3.5%,
   Cr: 0.01 to 1.5%, and Mo: 0.01 to 1.5%,
   The steel sheet according to any one of claims 1 to 3, comprising at least one element selected from the group consisting of:
5. The steel sheet according to any one of claims 1 to 3, wherein the steel sheet is used for a line pipe.
6. The steel plate according to claim 4, which is for a line pipe.
7. A steel pipe for a line pipe manufactured using the steel sheet according to any one of claims 1 to 3.
8. A steel pipe for a line pipe manufactured using the steel sheet according to claim 4.