JP2017155290A - Sour resistant steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an API 5L X65 class sour resistant steel sheet having sheet thickness of 36.0 mm or more and securing HIC resistance and DWTT properties of a steel sheet at a low temperature of -30°C or less.SOLUTION: There is provided a sour resistant steel sheet containing, by mass%, Ca:0.0005 to 0.0030%, Zr:0.0005% to 0.03%, S:0.0001% to 0.0010%, O:0.0010% to 0.0030% with limitation of Nb:0.004% or less, satisfying the following formula (1) and having average of effective crystal grain diameter at a position of 1/2 of sheet thickness from a surface in a sheet thickness direction of 25 μm. 1.0≤(Ca-0.83×O)/(1.25×S)+(Zr-1.90×O)/(2.85×S)≤10.0 (1), where each mathematics is calculated as zero when (Ca-0.83×O) and (Zr-1.90×O) are negative value.SELECTED DRAWING: None

Description

  The present invention relates to a sour-resistant steel plate used in a sour environment, which is suitable for a material such as a sour-resistant steel pipe for transporting natural gas containing hydrogen sulfide, petroleum, or the like.

  In recent years, with the injection of seawater into crude oil and natural gas wells and the development of resources of inferior quality, the opportunity for steel to be exposed to sour environments where hydrogen sulfide is present has increased. When steel is used in a sour environment, generation of hydrogen induced cracking (HIC) may be a problem. Further, the sour steel plate used for the line pipe is required to have high strength and thickness not only from excellent hydrogen-induced cracking resistance (HIC resistance) but also from the viewpoint of improving transportation efficiency. Furthermore, the development of energy resources is becoming colder, and sour steel plates are also required to have low temperature toughness.

  In order to obtain a steel sheet having excellent HIC resistance, it is effective to increase the purity and cleanliness of the steel by limiting impurities such as S and O, and to control the form of sulfide inclusions by adding Ca. In addition, a method for improving the HIC resistance by improving the microstructure of the central segregation part by accelerated cooling, particularly by suppressing the formation of a hardened structure has been proposed (for example, see Patent Document 1). Furthermore, there is a method for improving the HIC resistance by preventing the center segregation part from remaining in the non-crimped area by reducing the center segregation due to light reduction during continuous casting and limiting the hydrogen content of the steel slab before hot rolling. It has been proposed (for example, see Patent Document 2).

  By the way, in steel added with Nb in order to achieve high strength, coarse Nb precipitates (Nb carbonitrides) that remain undissolved without heating when the slab is heated form clusters in the steel sheet, These may be the starting point and deteriorate the HIC resistance. On the other hand, when the slab is heated to a considerably high temperature in order to completely dissolve the Nb precipitates within a limited time during the slab heating, the austenite (γ) grains are coarsened and the energy cost is reduced. Incurs an increase. In order to solve such a problem, a thick steel plate in which the Nb amount is limited has been proposed (for example, see Patent Document 3).

  On the other hand, some of the present inventors have proposed a method of manufacturing a high-strength sour-resistant steel plate by applying accelerated cooling to steel not added with Nb (see, for example, Patent Document 4). This proposes a technique for producing a sour-resistant steel plate used in a low-temperature environment of −45 ° C. or less. Patent Document 4 describes (1) improvement of HIC resistance by limiting the amount of Nb, (2) suppression of coarsening of γ grains by lowering the heating temperature, and (3) a low temperature range in which the rolling reduction per pass is increased. This is a manufacturing technology that realizes the refinement of the γ structure by rolling (4) and the securing of HIC resistance and enhanced transformation by accelerated cooling after rolling.

  In addition, some of the present inventors use an oxide containing Ca and Zr as main components (hereinafter sometimes abbreviated as CaZr-based oxide) to reduce the equiaxed crystal ratio of the slab. The technique which raises is proposed (for example, refer patent document 5). This is a technique in which CaZr-based oxides are dispersed in molten steel before δ solidification, and these function as inoculation nuclei when the steel is solidified. Due to the dispersion of the inoculation nuclei, the equiaxed crystal ratio of the slab increases, and at the same time, the equiaxed crystal grain size becomes finer, and the adverse effects of casting defects such as segregation and porosity are reduced.

JP 2000-199029 A JP 2010-209460 A JP 2011-1607 A Japanese Patent Laid-Open No. 7-316652 JP 2008-127599 A

  Patent Document 3 discloses a thick sour-resistant steel plate having a DWTT (Drop Weight Tear Test) characteristic of −17 ° C. to −23 ° C. and a thickness of 35.5 mm satisfying the strength of API 5L X65. Here, the DWTT characteristic is a brittle crack propagation stop characteristic which is important as the low temperature toughness of the line pipe. Patent Document 4 discloses a thick sour-resistant steel plate having a thickness of 35 mm that has excellent toughness at a low temperature of −45 ° C. and satisfies the strength of API 5L X65.

  In Patent Document 4, in order to stabilize the HIC resistance, Nb is not added, the slab is heated at a low temperature, and the rolling end temperature is increased by about 20 to 30 ° C. from Ar3 (transformation start temperature during cooling). In addition, a sour-resistant steel plate manufacturing technique that starts accelerated cooling from Ar3 or higher has been proposed. However, among the measures to improve the HIC resistance, the addition of Nb and the higher end temperature of rolling reduce the effect of the thermo-mechanical control process (TMCP), resulting in the coarsening of the metal structure of the steel sheet. Invite.

  In particular, when the thickness of the steel sheet is 36.0 mm or more, the metal structure of the steel sheet is not sufficiently refined, and it is difficult to stably achieve the DWTT characteristics in a low temperature environment of −30 ° C. or lower. . In addition, the effect of the refinement of equiaxed crystal of the slab with CaZr-based oxide on the HIC resistance is unknown. For example, when the solidified structure is refined, the HIC resistance may deteriorate due to the propagation of microsegregation by cracks. Therefore, in order to apply the technique of patent document 5 to a sour-resistant steel plate, specific examination is required.

  In view of such circumstances, the present invention is used for line pipes for transporting energy resources such as natural gas containing hydrogen sulfide and petroleum, and has a plate thickness of 36.0 mm or more and 45.0 mm or less, and API 5L X65 class or more. An object of the present invention is to ensure DWTT characteristics of a base material of a steel sheet at a low temperature of −30 ° C. or lower in a sour-resistant steel sheet having excellent HIC resistance.

  The present inventors have studied to apply a technique for equiaxed crystallization of a slab by CaZr-based oxide and its refinement to a high-strength thick sour-resistant steel plate. As a result, the average value of the equiaxed grain size of the final solidified part (near the center part of the slab) is refined to 3 mm or less by the CaZr-based oxide, and if the Nb content is limited, the HIC resistance is not deteriorated. I understood it. When Nb is substantially not added, if the solidification segregation part formed between equiaxed crystals refined by the CaZr-based oxide is refined, C and Mn concentrated in the solidification segregation part are: Diffusion occurs under normal slab heating conditions, and generation of a local hardened structure is suppressed at the center segregation portion of the steel sheet.

  On the other hand, it was found that when Nb was added, the HIC resistance was deteriorated even if the equiaxed crystal was refined. This is because the solidification segregation of Nb is extremely large and the diffusion rate of Nb after solidification is small. That is, under normal slab heating conditions, particularly in the vicinity of the center part of the slab (center segregation part), the decrease in the Nb concentration in the solid segregation part is insufficient, and the hardened structure is locally present in the center segregation part of the steel sheet. HIC is easily generated and propagated, and the HIC resistance is deteriorated.

  In addition to making Nb substantially non-added and refining the cast structure with the CaZr-based oxide, and further finely dispersing the TiN particles, the effective crystal grain size of the steel sheet after hot rolling can be reduced. At the same time, it was found that the formation of a local hardened structure was suppressed at the center segregation portion of the steel sheet, and the toughness was remarkably improved. Since TiN particles preferentially precipitate in the solidified segregation part where Ti and N are concentrated during solidification cooling, when the solidification segregation part is refined by CaZr-based oxide, finer TiN particles are dispersed and the slab is heated. The pinning effect in γ grain growth is strengthened, and the effective crystal grain size in the steel sheet is refined. This pinning effect is effective in reducing the effective crystal grain size of HAZ, and contributes to the improvement of HAZ toughness.

  This invention is made | formed based on such knowledge, The summary is as follows.

The sour-resistant steel plate according to one aspect of the present invention is
(1) By mass%, C: 0.040% or more, 0.150% or less, Mn: 1.00% or more, 2.00% or less, S: 0.0001% or more, 0.0010% or less, Ca : 0.0005% or more, 0.0030% or less, Zr: 0.0005% or more, 0.030% or less, O: 0.0010% or more, 0.0030% or less, Al: 0.001% or more, 0 0.050% or less, Ti: 0.005% or more, 0.020% or less, N: 0.0015% or more, 0.0050% or less, Si: 0.40% or less, P: 0.015% Hereinafter, Nb is limited to 0.004% or less, the balance is Fe and inevitable impurities, the contents of Ca, Zr, O, and S satisfy the following formula (1), and the plate from the surface in the thickness direction Sour-resistant steel with an average effective grain size of 25 μm or less at half the thickness It is.
1.0 ≦ (Ca−0.83 × O) / (1.25 × S)
+ (Zr-1.90 × O) / (2.85 × S) ≦ 10.0 (1)
In the above equation (1), when (Ca−0.83 × O) and (Zr−1.90 × O) are negative values, each term is calculated as zero.

(2) Further, in the sour-resistant steel plate according to (1), further, by mass, Cu: 1.0% or less, Ni: 1.0% or less, Cr: 1.0% or less, Mo: 0 It may be limited to 5% or less, W: 0.5% or less, and Co: 0.5% or less.

(3) Moreover, in the sour-resistant steel plate described in (2) above, the mass may be further limited to V: 0.10% or less and B: 0.0030% or less.

(4) Further, in the sour-resistant steel plate according to any one of (1) to (3), the plate thickness is 36.0 mm or more and 45.0 mm or less, the yield stress is 448 MPa or more, and the tensile strength is 535 MPa or more. May be.

  According to the present invention, the plate thickness is 36.0 mm or more and 45.0 mm or less, having strength of American Petroleum Institute (API) standard X65 grade or more and having excellent DWTT characteristics at a low temperature of −30 ° C. or less. It is possible to provide a sour-resistant steel plate having excellent HIC resistance. According to the present invention, it is possible to rationally design a line pipe used for production and transportation of crude oil and natural gas in a low temperature sour environment. Therefore, the industrial contribution of the present invention is extremely remarkable.

  In the present invention, (a) composite addition of Zr and Ca (CaZr-based oxide formation), (b) no addition of Nb, (c) extremely low S, (d) composite addition of Ti and N (TiN particles) Of fine dispersion). Further, according to the present invention, the solidified equiaxed crystal is refined as compared with the conventional structure, so that the solidified segregation part is also refined, and the generation of a local hardened structure harmful to the HIC resistance is generated by the steel plate. This is based on the knowledge that sour resistance and toughness are improved in the central segregation portion of the steel. The central segregation part is the final solidification part when viewed macroscopically and is the center part of the slab thickness. When this center segregation portion is viewed microscopically, it is composed of solid equiaxed crystals. The solidified segregation part is between solidified equiaxed crystals, and the alloy elements are microscopically concentrated.

CaZr-based oxides effectively act as inoculum nuclei (solidification nuclei) in the production of slabs whose initial solidification phase is δ-Fe. The crystal structure of the CaZr-based oxide identified by electron beam diffraction is characterized in that all or part of each oxide particle has a perovskite structure. CaZrO ₃ has a perovskite structure and has a lower degree of lattice mismatch to δ-Fe and higher inoculation ability than TiN and Ce ₂ O ₃ .

  Due to the inoculation effect of the CaZr-based oxide, the equiaxed crystal ratio is increased, the solidified structure is refined, and the solidified equiaxed crystal is refined particularly in the central segregation portion of the slab. As a result, the solidified segregation portion becomes fine, and concentrated elements, particularly C and Mn, diffuse under normal slab heating conditions and the concentration decreases. And after hot rolling, in addition to the effective crystal grain size of the steel sheet becoming small, it becomes difficult to form a local hardened structure, especially in the central segregation part, and toughness and HIC resistance are improved.

  Thus, according to the above (a) to (d), the sour-resistant steel plate of the present invention has a refined metallographic structure even when the thickness is 36.0 mm or more, and the hardness of the center segregation part is also increased. It is suppressed, and it becomes possible to achieve DWTT characteristics and HIC resistance characteristics at a low temperature of −30 ° C. or lower. At this time, the average value of the effective crystal grain size at a position of ½ of the plate thickness in the plate thickness direction (hereinafter also referred to as a plate thickness ½ position) is 25 μm or less.

  Here, the effective crystal grain size is, for example, the size of a region in which the angle difference in the crystal direction is within 3 degrees, and can be measured by electron beam backscatter diffraction (EBSD). When the metal structure is ferrite, the crystal grain size is equivalent to the effective crystal grain size. On the other hand, in the case of acicular crystals such as bainite and martensite, the effective crystal grain size is the size of a region in which the crystal directions are substantially aligned in a bundle of acicular crystals.

  Hereinafter, a sour-resistant steel plate according to an embodiment of the present invention will be described.

(C: 0.040% or more, 0.150% or less)
C is an element that increases the strength of the steel, and the C content is 0.040% or more in order to obtain a high strength of X65 or more. Preferably, the C content is 0.050% or more, more preferably 0.060% or more, and still more preferably 0.070% or more. However, the increase in the amount of C strengthens the segregation of Mn and P in the center segregation of the slab and degrades the HIC resistance, so the upper limit is 0.150%. Preferably, the C content is 0.140% or less, more preferably 0.130% or less.

(Si: 0.40% or less)
Si may be contained in steel for deoxidation, but if the amount of Si is too large, weldability and HAZ toughness deteriorate, so it is limited to 0.40% or less. In the steel of the present invention, since deoxidation is possible with Al and Ti, the lower limit may be 0%, but 0.01% or more of Si can be contained. In consideration of HAZ toughness, it is desirable that the Si content be 0.30% or less. More preferably, the Si amount is 0.20% or less.

(Mn: 1.00% or more, 2.00% or less)
Mn is an element that enhances hardenability and contributes to strengthening of steel, and in order to obtain a high strength of X65 or higher, the amount of Mn is set to 1.00% or higher. Preferably, the Mn content is 1.05% or more, more preferably 1.10% or more, and still more preferably 1.15% or more. However, since the increase in the amount of Mn strengthens the center segregation of the slab and degrades the HIC resistance, the upper limit is 2.00%. Preferably, the Mn content is 1.90% or less, more preferably 1.80% or less, and still more preferably 1.70% or less.

(P: 0.015% or less)
P is an impurity and limits the amount of P to 0.015% or less in order to strengthen the center segregation of the slab and deteriorate the HIC resistance. The lower the P, the better the HIC resistance, so the lower limit is not particularly specified.

(S: 0.0001% or more, 0.0010% or less)
S is an element that forms MnS that is stretched by rolling, which is harmful to the HIC resistance, and the amount of S needs to be limited to 0.0010% or less. Preferably, the S content is 0.0008% or less, more preferably 0.0006% or less. Although reducing S is preferable from the viewpoint of the toughness of the base material and the HAZ, the amount of S is made 0.0001% or more from the viewpoint of manufacturing cost.

(Nb: 0.004% or less)
In the present invention, it is desirable that Nb is not substantially contained in order to ensure HIC resistance. When the amount of Nb exceeds 0.004%, the Nb carbonitride remaining undissolved at the center segregation part when the slab is heated deteriorates the HIC resistance. Therefore, the Nb content needs to be limited to 0.004%. In the present invention, from the viewpoint of securing DWTT characteristics, it is preferable to heat the slab at a low temperature such as 1100 ° C. or less, and in this case, the Nb content is 0.003% in order to prevent unmelted Nb carbonitride. It is preferable to reduce to the following. More preferably, the Nb amount is 0.002% or less. Since Nb is also harmful to the HAZ toughness, the fact that Nb is not substantially contained has the effect of increasing the HAZ toughness.

(Ca: 0.0005% or more, 0.0030% or less)
Ca is an important element that forms Zr and CaZr-based oxides. The CaZr-based oxide generated in the molten steel becomes inoculum nuclei (solidification nuclei), and refines the cast structure, particularly at the central segregation part. As a result, the diffusion of the concentrated element in the solidified segregation part is facilitated, and the formation of a hardened structure is suppressed particularly in the central segregation part of the steel sheet. Thus, Ca is an element that improves toughness and sour resistance, and in order to obtain an effect, the Ca content is set to 0.0005% or more. Preferably, the Ca content is 0.0007% or more, more preferably 0.0010% or more. However, if the Ca content exceeds 0.0030%, Ca-based inclusions increase and become the starting point of HIC and brittle fracture, so this is the upper limit. Preferably, the Ca content is 0.0025% or less, more preferably 0.0020% or less.

(Zr: 0.0005% or more, 0.03% or less)
Zr is an important element that forms a CaZr-based oxide when added together with Ca. Improvement of toughness and sour resistance by the CaZr-based oxide is as described above, and in order to obtain the effect, it is necessary to contain 0.0005% or more of Zr. However, if Zr is contained in an amount exceeding 0.03%, the Zr-based inclusions increase, which may become the starting point of HIC and brittle fracture, so this is the upper limit. Preferably, the Zr content is 0.02% or less, more preferably 0.01% or less.

(O: 0.0010% or more, 0.0030% or less)
O is an element that combines with Ca and Zr to form a CaZr-based oxide. In order to improve toughness and HIC resistance due to the effect of the above-described CaZr-based oxide, an O amount of 0.0010% or more is necessary. However, if O exceeds 0.0030%, the cleanliness of the steel decreases and the toughness of the base material and the HAZ deteriorates. The upper limit of the amount of O is 0.0030% in order to reduce oxide inclusions that are the starting point of HIC generation and to control sulfide morphology with Ca. The amount of O is preferably 0.0025% or less.

(1.0 ≦ (Ca−0.83 × O) / (1.25 × S)
+ (Zr-1.90 × O) / (2.85 × S) ≦ 10.0
In the sour-resistant steel sheet of the present invention, in order to ensure the HIC resistance, S is reduced as much as possible, Ca and Zr are added, and the generation of stretched MnS that is the origin of HIC generation is suppressed, and S is reduced to (Ca, Fix as Zr) S or (Ca, Zr) (O, S). At this time, the balance of S, Ca, Zr, and O is 1.0 ≦ (Ca−0.83 × O) / (1.25 × S) + (Zr−1.90 × O) / (2.85). If xS) is not satisfied, stretched MnS remains and HIC occurs. On the other hand, if (Ca−0.83 × O) / (1.25 × S) + (Zr−1.90 × O) / (2.85 × S) ≦ 10.0 is not satisfied, O or S On the other hand, since Ca and Zr are excessively contained, Ca-based inclusions and Zr-based inclusions increase, and HIC is generated.

Here, the term (Ca−0.83 × O) / (1.25 × S) + (Zr−1.90 × O) / (2.85 × S) indicates that CaZrO which is an inoculum nucleus by deoxidation. _This is an index for determining whether or not S is fixed by Ca and Zr remaining without being consumed in the deoxidation reaction, assuming that ₃ is generated. That is, when 1.0 ≦ (Ca−0.83 × O) / (1.25 × S) + (Zr−1.90 × O) / (2.85 × S) is satisfied, the deoxidation reaction is performed. With Ca and Zr remaining without being consumed, all S in the steel is precipitated as CaS and ZrS, and stretched MnS is not generated.

  Further, even if (Zr-1.90 × O) is a negative value, if it satisfies 1.0 ≦ (Ca−0.83 × O) / (1.25 × S), it is consumed by the deoxidation reaction. By the remaining Ca after subtracting Ca (Ca combined with O), almost all S in the steel is precipitated as CaS, and stretched MnS is not generated. Similarly, even if (Ca−0.83 × O) is a negative value, if 1.0 ≦ (Zr−1.90 × O) / (2.85 × S) is satisfied, deoxidation reaction may occur. By the remaining Zr after subtracting the consumed Zr (Zr combined with O), almost all S in the steel precipitates as ZrS, and stretched MnS does not form. Accordingly, when (Ca−0.83 × O) and (Zr−1.90 × O) are negative values, each term is set to zero.

  Thus, the term (Ca−0.83 × O) / (1.25 × S) + (Zr−1.90 × O) / (2.85 × S) is an index of sulfide morphology control. In order to secure the HIC characteristics, the following formula (1) needs to be satisfied. In order to enhance the HIC resistance, the lower limit of (Ca−0.83 × O) / (1.25 × S) + (Zr−1.90 × O) / (2.85 × S) is preferably 1. 5, more preferably 2.0, and the upper limit is preferably 9.5, more preferably 9.0.

1.0 ≦ (Ca−0.83 × O) / (1.25 × S)
+ (Zr-1.90 × O) / (2.85 × S) ≦ 10.0 (1)
In the above equation (1), when (Ca−0.83 × O) and (Zr−1.90 × O) are negative values, each term is calculated as zero.

(Al: 0.001% or more, 0.050% or less)
Al is a deoxidizing element, and the amount of Al needs to be 0.001% or more. Preferably, the Al content is 0.002% or more, more preferably 0.003% or more. However, if the Al content exceeds 0.050%, inclusions increase and the toughness is impaired, so this is the upper limit. Preferably, the Al content is 0.030% or less, more preferably 0.020% or less.

(Ti: 0.005% or more, 0.020% or less)
Ti is an element that forms TiN particles that suppress γ grain growth of slabs or HAZ by a pinning effect. In order to exhibit a sufficient γ grain growth suppressing effect by the pinning effect, the lower limit of the Ti amount is set to 0.005%. Preferably, the Ti amount is 0.007% or more. However, if the Ti content exceeds 0.020%, the toughness of the base material and HAZ deteriorates, and the surface quality of the cast slab deteriorates, so this is the upper limit. Preferably, the Ti content is 0.018% or less, more preferably 0.016% or less.

(N: 0.0015% or more, 0.0050% or less)
N is an element constituting TiN particles that pin the γ grain growth of slabs or HAZ. In order to exhibit a sufficient γ grain growth suppressing effect by the pinning effect, it is necessary to secure the minimum number of TiN particles by setting the lower limit of the N amount to 0.0015%. Preferably, the N content is 0.0020% or more, more preferably 0.0025% or more. On the other hand, if the N amount exceeds 0.0050%, the toughness of the base material and the HAZ deteriorates and the surface quality of the cast slab deteriorates, so this is the upper limit. Preferably, the N content is 0.0045% or less, more preferably 0.0040% or less.

  You may contain 1 type, or 2 or more types of Cu, Ni, Cr, Mo, W, Co, Ti, V, and B as needed. The Zr raw material used industrially may contain a trace amount of Hf. Hf, which is an element belonging to Zr, has the same action as Zr, and there is no particular problem even if a small amount of Hf is contained as an impurity in the steel.

(Cu: 1.0% or less)
Cu may be contained in an amount of 0.1% or more in order to improve the strength and toughness of the base material without adversely affecting weldability and HAZ toughness. However, excessive addition may cause Cu cracks during hot rolling, making production difficult, and may not be preferable for weldability, so the upper limit of the Cu content is preferably 1.0%. More preferably, the Cu amount is 0.5% or less, and further preferably 0.3% or less.

(Ni: 1.0% or less)
Ni may be contained in an amount of 0.1% or more in order to improve the strength and toughness of the base material without adversely affecting weldability and HAZ toughness. However, excessive addition impairs economic efficiency and may not be preferable for weldability, so the upper limit of Ni content is preferably 1.0%. More preferably, the Ni content is 0.8% or less, and further preferably 0.5% or less.

(Cr: 1.0% or less)
Cr may be contained in an amount of 0.1% or more in order to prevent center segregation in a continuous cast slab and improve the strength of the base material. However, since excessive addition may deteriorate the toughness and weldability of the base metal and the HAZ, the upper limit of Cr content is preferably 1.0%. More preferably, the Cr amount is 0.8% or less, and further preferably 0.5% or less.

(Mo: 0.5% or less)
Mo may contain 0.1% or more in order to improve both the strength and toughness of the base material. However, since excessive addition may cause deterioration of the toughness and weldability of the base material and HAZ, the upper limit of the Mo amount is preferably 0.5%. More preferably, the Mo amount is 0.3% or less.

(W: 0.5% or less)
W may contain 0.1% or more in order to improve both the strength and toughness of the base material. However, excessive addition impairs economic efficiency and may cause deterioration of the toughness and weldability of the base material and the HAZ, so the upper limit of the W amount is preferably 0.5%. More preferably, the W amount is 0.3% or less.

(Co: 0.5% or less)
Co may contain 0.1% or more in order to improve the strength and toughness of the base material without adversely affecting the weldability and the HAZ toughness. However, excessive addition impairs economic efficiency and may not be preferable for weldability, so the upper limit of Co content is preferably 0.5%. More preferably, the Co content is 0.3% or less.

(V: 0.10% or less)
V may be contained in an amount of 0.01% or more in order to increase the strength by precipitation hardening and improve the low temperature toughness by refining the microstructure. However, since excessive addition may cause deterioration of HAZ toughness and weldability, the upper limit of V content is preferably 0.10%. More preferably, the V amount is 0.08% or less, and further preferably 0.05% or less.

(B: 0.0030% or less)
B may be contained in an amount of 0.0003% or more in order to improve the hardenability and improve the strength and toughness of the base material and HAZ. However, since the HAZ toughness and weldability may deteriorate due to excessive addition, the upper limit of the B amount is preferably 0.0030%. More preferably, the B content is 0.0020% or less, and further preferably 0.0015% or less.

  The metal structure of the sour-resistant steel plate of the present invention is composed of one or more of ferrite, bainite, martensite, and pearlite. In the case of these mixed structures, the effective crystal grain size has a stronger correlation with toughness than the crystal grain size observed with an optical microscope. In order to improve toughness, it is desirable to reduce the effective crystal grain size, and the sour-resistant steel plate of the present invention has an average value of effective crystal grain size at a position half the plate thickness from the surface in the plate thickness direction. Is required to be 25 μm or less. When the effective crystal grain size exceeds 25 μm, the DWTT characteristics cannot be stably achieved at a low temperature of −30 ° C. or lower. Since it is preferable that the effective crystal grain size at the position of 1/2 the plate thickness is small, the lower limit is not specified, but it may be 5 μm or more from the viewpoint of manufacturing cost.

  Next, a method for manufacturing a sour-resistant steel plate according to this embodiment will be described.

It is preferable that a continuous cast slab having a thickness of 200 mm or more composed of the above-described chemical components is cooled to 400 ° C. or lower and then heated to 900 ° C. or higher and 1050 ° C. or lower to perform hot rolling.
When the slab is inserted into a heating furnace by hot charging without being cooled to 400 ° C. or lower, the coarse γ structure generated during casting remains after heating, and the structure may not be sufficiently refined and low temperature toughness may deteriorate.

  The heating temperature of the steel slab is preferably 900 ° C. or higher so that the rolling end temperature is Ar 3 or higher. On the other hand, when the heating temperature is high, the heated γ grains become coarse, the structure is not sufficiently refined, and the low temperature toughness may be deteriorated. More preferably, heating temperature shall be 1000 degrees C or less.

  In the subsequent hot rolling, a large number of rolling reductions with a large rolling reduction per pass are accumulated in the γ low temperature region, whereby the microstructure is sufficiently refined and very good low temperature toughness is obtained. Therefore, in hot rolling, the cumulative rolling reduction at 900 ° C. or less is preferably 60% or more, and the rolling reduction per pass is preferably 15% or more for 60% or more of the number of passes. If the cumulative rolling reduction at 900 ° C. or less is less than 60%, or the ratio of the number of passes at which the rolling reduction per pass is 15% or more is less than 60%, the microstructure after transformation becomes finer. Therefore, good low temperature toughness may not be obtained.

  It is preferable to perform hot rolling so that the plate thickness is 36.0 mm or more and 45.0 mm or less, and finish the rolling at Ar3 or more. If the end temperature of hot rolling is less than Ar3, C concentrates in the center segregation part with ferrite transformation, and a hardened structure is formed, which may deteriorate the HIC resistance. Also, in order to start accelerated cooling from a temperature of Ar3 or higher, it is preferable to end hot rolling at Ar3 or higher.

  After hot rolling, accelerated cooling is preferably performed from Ar3 or higher. Accelerated cooling improves the HIC resistance by improving the microstructure of the central segregation part, and also enables higher strength by transformation strengthening and higher toughness by grain refinement. The cooling rate of the accelerated cooling is preferably 3 ° C./second or more and 50 ° C./second or less, and it is preferable that the accelerated cooling is terminated within the range of 550 ° C. or less and 300 ° C. or more, and then air cooling is performed.

  When the cooling start temperature is less than Ar 3, the cooling rate is less than 3 ° C./second, or the cooling stop temperature exceeds 550 ° C., the hardened structure is caused by the concentration of C in the central segregation part due to ferrite transformation. As a result, the HIC resistance is deteriorated and the transformation strengthening is insufficient and the strength may be insufficient. On the other hand, if the cooling rate exceeds 50 ° C./second or the water cooling stop temperature is less than 300 ° C., a large amount of low-temperature transformation products may be generated, and the HIC resistance and low-temperature toughness may deteriorate.

Here, Ar3 is the transformation start temperature at the time of cooling calculated below, and is calculated using the chemical composition of steel.
Ar3 (° C.) = 868-396 × C + 24.6 × Si-68.1 × Mn
-36.1 × Ni-20.7 × Cu-24.8 × Cr
+ 29.1xMo
C, Si, Mn, Ni, Cu, Cr, and Mo in the above formula mean contents expressed in mass%.

  Note that tempering the steel sheet according to the present invention to a temperature of Ac1 (transformation start temperature during heating) or lower does not impair the properties of the steel of the present invention. The steel plate according to the present invention can be applied not only to a sour line pipe but also to a sour pressure vessel.

  Examples of the present invention will be described below. However, the following examples are examples of the present invention, and the present invention is not limited to the examples described below.

  Steel was melted by a converter and cast pieces having a thickness of 240 mm having chemical components shown in Tables 1 and 2 were produced by continuous casting. The obtained slab was cooled to room temperature and then heated to 980 to 1030 ° C. to perform hot rolling. At this time, the cumulative reduction rate of 900 ° C. or less was 75 to 80%, and the reduction rate per pass was 15% or more for 60% or more of the number of passes at that time. Further, the rolling end temperature is set to 770 to 790 ° C, and subsequently to hot rolling, accelerated cooling of 5 to 35 ° C / second is applied from the range of 750 to 770 ° C, and accelerated cooling is performed within the range of 330 to 430 ° C. Stopped and then air cooled. As is clear from Tables 1 and 2, the rolling end temperature and the start temperature of accelerated cooling are higher than Ar3 of steel.

  A full-thickness test piece based on the API5L standard was taken from the obtained steel sheet with the width direction perpendicular to the rolling direction as the longitudinal direction, and a tensile test was performed at room temperature based on the API standard 2000. Further, a DWTT test piece was collected, the surface on one side was cut, and the thickness was reduced to 3/4 inch, and a drop weight tear test was performed. The low temperature toughness was evaluated by the ductile fracture surface transition temperature (DWTT 85% SATT [° C.]).

The effective crystal grain size at the 1/2 position of the plate thickness was measured using EBSD in the cross section of the micro test piece at that portion. The crystal orientation was measured over an area of 0.02 mm ² or more by EBSD, the region where the crystal orientation difference was within 3 degrees was considered as the effective crystal grain, and the average value of the equivalent circle diameter was determined as the effective crystal grain size.

In addition, the HIC test of the base material was conducted using a NACE solution (5% NaCl + 0.5% acetic acid aqueous solution saturated with H ₂ S at 1 atm, pH 2.7) according to NACE TM0284, and the HIC area ratio CAR HIC length ratio CLR was measured.

  Furthermore, in order to evaluate the HAZ toughness, a submerged arc welding of one inner surface and one outer surface corresponding to the seam welded portion of the UO steel pipe was performed to produce a welded joint. In addition, the welding metal used the welding material of a low temperature specification so that the toughness of 100J or more may be acquired at -30 degreeC. A Charpy impact test piece was taken with reference to the joint of the welded joint, 2 mmV notch was applied so that the ratio of weld metal to HAZ was 50:50, and three tests were performed at -30 ° C to obtain the average value. The lowest value was measured. Tables 3 and 4 show the steel sheet thickness, mechanical properties, HIC resistance, and HAZ toughness.

  As shown in Table 3, the steel of the present invention is a base material satisfying API 5L X65 or more (yield strength YS: 448 MPa or more, tensile strength TS: 535 MPa or more) in a steel plate having a thickness of 36.0 to 45.0 mm. The strength and ductile fracture surface transition temperature in the drop weight tear test are as low as −30 ° C. or lower, and it has good DWTT characteristics. The effective crystal grain size is refined to 25 μm or less. At the same time, the steel of the present invention has excellent HIC resistance and HAZ toughness.

  On the other hand, as shown in Table 2, the chemical composition of the conventional steel is out of the scope of the present invention. Therefore, as shown in Table 4, the base material has poor mechanical properties, HIC resistance, and HAZ toughness of the weld. There is a problem.

  The code B1 has an insufficient C amount, and the code B4 has an insufficient Mn content because the Mn amount is too low. Since the amount of C of the code B2 is too high, the HIC resistance is deteriorated, and the HAZ toughness is also in a tendency to deteriorate. Since the amount of Si is too high, the sign B3 has degraded HAZ toughness. Since the amount of Mn is too high, the sign B5 has deteriorated HIC resistance.

  Since the amount of P of the code B6 is too high, the anti-HIC characteristics deteriorate and the HAZ toughness also tends to deteriorate. Since the amount of S in the code B7 is too high, the anti-HIC characteristics deteriorate and the DWTT characteristics tend to deteriorate. In B8, since the Al amount is too low, deoxidation is insufficient, the O amount is too high, the HIC resistance and HAZ toughness are deteriorated, and the DWTT property is also in a tendency to deteriorate. In the code B9, the amount of Al is too high, so that the HAZ toughness is deteriorated and the DWTT characteristic is also in a tendency to deteriorate. The code B10 has a Ti amount that is too low, and the code B17 has a N content that is too low, so that the DWTT characteristics and the HAZ toughness are deteriorated. The code B11 has an excessively high amount of Ti, and the code B18 has an excessively high amount of N, so that the HAZ toughness deteriorates and the DWTT characteristics also tend to deteriorate.

  Since the amount of Nb is too high for the code B12, the anti-HIC characteristics deteriorate and the HAZ toughness also tends to deteriorate. The code B13 has an excessively low Ca content, and the code B15 has an excessively low Zr content, so that the HIC resistance and the DWTT characteristics deteriorate, and the HAZ toughness tends to deteriorate. The code B14 has an excessively high amount of Ca, and the code B16 has an excessively high amount of Zr, so that the HIC resistance and HAZ toughness deteriorate, and the DWTT characteristic tends to deteriorate.

  Since the amount of O is too low, the BIC resistance and DWTT characteristics deteriorate, and the HAZ toughness also tends to deteriorate. Since the amount of O in the code B20 is too high, the HIC resistance and HAZ toughness deteriorate, and the DWTT characteristic also tends to deteriorate. Since the value of the formula (1) which is an index of sulfide form control is too low for the code B21 and the value of the formula (1) is too high for the code B22, the HIC resistance is deteriorated.

  The present invention relates to the manufacture of thick steel plates for line pipes having excellent HIC resistance and API 5L X65 strength and superior low temperature toughness than before. It is desirable to apply to.

Claims (4)

% By mass
C: 0.040% or more, 0.150% or less,
Mn: 1.00% or more, 2.00% or less,
S: 0.0001% or more, 0.0010% or less,
Ca: 0.0005% or more, 0.0030% or less,
Zr: 0.0005% or more, 0.030% or less,
O: 0.0010% or more, 0.0030% or less,
Al: 0.001% or more, 0.050% or less,
Ti: 0.005% or more, 0.020% or less,
N: 0.0015% or more and 0.0050% or less,
Si: 0.40% or less,
P: 0.015% or less,
Nb: limited to 0.004% or less, the balance is made of Fe and inevitable impurities, the contents of Ca, Zr, O, S satisfy the following formula (1), A sour-resistant steel plate, wherein an average value of effective crystal grain size at a position of 1/2 is 25 μm or less.
1.0 ≦ (Ca−0.83 × O) / (1.25 × S)
+ (Zr-1.90 × O) / (2.85 × S)
≦ 10.0 (1)
In the above equation (1), when (Ca−0.83 × O) and (Zr−1.90 × O) are negative values, each term is calculated as zero. Furthermore, in mass%,
Cu: 1.0% or less,
Ni: 1.0% or less,
Cr: 1.0% or less,
Mo: 0.5% or less,
W: 0.5% or less,
The sour-resistant steel plate according to claim 1, comprising Co: 0.5% or less of one kind or two or more kinds. Furthermore, in mass%,
V: 0.10% or less,
B: One or both of 0.0030% or less are contained, The sour-resistant steel plate of Claim 1 or 2 characterized by the above-mentioned. The plate thickness is 36.0 mm or more and 45.0 mm or less,
Yield stress is 448 MPa or more,
The sour-resistant steel plate according to any one of claims 1 to 3, wherein the tensile strength is 535 MPa or more.