JP6665515B2 - Sour-resistant steel plate - Google Patents

Description

  TECHNICAL FIELD 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, oil and the like.

  In recent years, with the injection of seawater into crude oil / natural gas wells and the development of inferior resources, there is an increasing number of opportunities for steel materials to be exposed to sour environments where hydrogen sulfide is present. When a steel material is used in a sour environment, the occurrence of Hydrogen Induced Cracking (HIC) may be a problem. Further, the sour-resistant steel plate used for the line pipe is required to have not only excellent hydrogen-induced cracking resistance (HIC resistance) but also high strength and thickening from the viewpoint of improving transport efficiency. Furthermore, the development of energy resources in colder regions is progressing, and sour-resistant steel sheets are also required to have low-temperature toughness.

  In order to obtain a steel sheet having excellent HIC resistance, it is effective to purify and purify steel by limiting impurities such as S and O, and to control the form of sulfide-based inclusions by adding Ca. Further, there has been proposed a method for improving the HIC resistance by improving the microstructure of the center segregated portion by accelerated cooling, particularly by suppressing the formation of a hardened structure (see, for example, Patent Document 1). Furthermore, there is a method of preventing center unseparated portions from being left uncompressed and improving HIC resistance by reducing center segregation due to light reduction during continuous casting and limiting the amount of hydrogen in the steel slab before hot rolling. It has been proposed (for example, see Patent Document 2).

  By the way, in steel to which Nb is added in order to achieve high strength, coarse Nb precipitates (Nb carbonitride) which are not dissolved but remain dissolved when the slab is heated form clusters in the steel sheet, These may serve as starting points to degrade the HIC resistance. On the other hand, when the slab is heated to a fairly high temperature in order to completely dissolve the Nb precipitate in solid solution within a limited time when the slab is heated, coarsening of austenite (γ) grains and reduction of energy cost are required. Cause an increase. To solve such a problem, a thick steel plate in which the amount of Nb 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 sheet by applying accelerated cooling to steel to which Nb is not added (for example, see Patent Document 4). This proposes a technique for manufacturing a sour-resistant steel sheet used in a low-temperature environment of -45 ° C or lower. Patent Document 4 discloses (1) improvement in HIC resistance by limiting the amount of Nb, (2) suppression of coarsening of γ grains by lowering the heating temperature, and (3) in a low-temperature region where the rolling reduction per pass is increased. This is a manufacturing technique for realizing the refinement of the γ-structure by the rolling of (4) and the securing of the HIC resistance and the enhancement of the transformation by accelerated cooling after the rolling.

  In addition, some of the present inventors have proposed a technique of remarkably suppressing the growth of γ grains during slab heating by adding Mg to Ti-added steel (for example, see Patent Document 5). ). In this technique, the heating temperature of the slab is as high as 1150 to 1300 ° C., but the average γ particle size is 100 μm or less. However, in order to further reduce the γ-grain size during slab heating, it is necessary to apply a new γ-grain growth suppression technology in addition to lowering the heating temperature.

JP-A-2000-199029 JP 2010-209460 A JP 2011-1607 A JP-A-7-316652 JP 2001-254139 A

  In Patent Document 4, in order to stabilize the HIC resistance, the slab is heated at a low temperature without adding Nb, and the rolling end temperature is raised by about 20 to 30 ° C. higher than Ar 3 (transformation start temperature at the time of cooling). A technique for producing a sour-resistant steel sheet 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 increase in the end-of-rolling temperature reduce the effect of the thermo-mechanical control process (TMCP) and increase the metal structure of the steel sheet. Invite. In particular, when the thickness of the steel sheet exceeds 30 mm, the metal structure of the steel sheet is not sufficiently refined, and it is difficult to stably achieve the BDWTT (Battelle Drop Weight Tear Test) characteristics in a low temperature environment such as -50 ° C. It was difficult. Here, the BDWTT characteristic is a brittle crack propagation stopping characteristic that is important as the low-temperature toughness of the line pipe.

  Furthermore, the thick sour resistant steel sheet excellent in low-temperature toughness, which is the subject of the present invention, needs to have Charpy impact properties of the weld heat affected zone in addition to the BDWTT properties of the base material of the steel sheet. In view of such circumstances, the present invention provides an API 5L X65 class or higher HIC resistance of 31 mm or more and 45 mm or less used for line pipes for transporting energy resources such as natural gas containing hydrogen sulfide and petroleum. It is an object of the present invention to simultaneously ensure the BDWTT characteristics of the base material of a steel plate and the Charpy impact characteristics of a welding heat affected zone (HAZ) at a low temperature of -50 ° C or less in an excellent sour resistant steel plate.

  The present inventors have studied a method of applying a new γ-grain growth suppression technique at the time of casting slab heating, which is the initial stage of TMCP, to significantly reduce the γ-grain size at the time of heating as compared with the conventional technique. In view of the HIC resistance, the present inventors set Ca addition and extremely low S, and when Nb was not added, in order to further enhance the pinning effect by Ti-Mg addition, the Al addition amount was 0.02% or less. Found that it is necessary to limit to. In particular, under extremely low temperature heating conditions of 900 to 1050 ° C., extremely strong γ-grain growth suppressing effect is exhibited by TiN that precipitates compositely with fine Mg-containing oxides as nuclei and TiN that solely precipitates in ground iron. I understood.

The present invention has been made based on such knowledge, and the gist is as follows.
The sour resistant steel sheet according to one embodiment of the present invention,
(1) In mass%, C: 0.020% or more and 0.060% or less, Mn: 1.00% or more and 1.60% or less, S: 0.0001% or more and 0.0010% or less, Al : 0.001% or more, 0.020% or less, Ti: 0.005% or more, 0.020% or less, Ca: 0.0005% or more, 0.0030% or less, Mg: 0.0003% or more, 0 0.0030% or less, N: 0.0015% or more, 0.0050% or less, O: 0.0010% or more, 0.0030% or less, Si: 0.30% or less, P: 0.015 %, Nb: limited to 0.004% or less, the balance being composed of Fe and unavoidable impurities, satisfying the following expression (1), and effective at a position 1/4 of the sheet thickness from the surface in the sheet thickness direction. This is a sour-resistant steel sheet having an average crystal grain size of 15 μm or less.
1.0 ≦ [Ca × (1-124 × O)] / (1.25 × S) ≦ 8.0 (1)

(2) Further, in the sour resistant steel sheet according to (1), further, in mass%, Cu: 1.0% or less, Ni: 1.0% or less, Cr: 1.0% or less, Mo: 0 0.5% or less, W: 0.5% or less, Co: 0.5% or less, V: 0.10% or less, B: 0.0030% or less.

(3) Further, in the sour resistant steel sheet according to the above (2), the mass% is further limited to REM: 0.004% or less, Zr: 0.005% or less, and the above formula (1) is replaced. May satisfy the following expression (2).
1.0 ≦ [(Ca + 3.5 × REM + 2.3 × Zr) × (1-124 × O)] / (1.25 × S) ≦ 8.0 (2)

(4) Further, in the sour resistant steel sheet according to any one of the above (1) to (3), the sheet thickness may be 31 mm or more and 45 mm or less, the yield stress may be 448 MPa or more, and the tensile strength may be 535 MPa or more. .

  ADVANTAGE OF THE INVENTION According to this invention, it has the strength of more than American Petroleum Institute (API) standard X65 grade, and has the excellent BDWTT property at low temperature of -50 degreeC or less, the board thickness of 31 mm or more and 45 mm or less, and the HIC resistance. It is possible to provide an excellent sour resistant steel sheet. 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 present invention has a remarkable industrial contribution.

  The present invention has the characteristics of (a) Ti-Mg addition, (b) Sour-resistant component (Ca addition, extremely low S content, no Nb addition), and (c) reduction of Al. This is based on the finding that a remarkable γ grain growth inhibitory effect is exhibited. In the present invention, Ostwald growth of TiN is reduced when heated to 900 to 1050 ° C., that is, particularly at (d) cryogenic heating, by reducing the solute S and solute Al present in γ. As a result, the fine dispersion state of TiN is maintained, and as a result, an extremely strong effect of suppressing the growth of γ grains is exhibited.

  This is considered to be because the solid solution C segregating at the interface (non-coherent interface) between TiN and γ increases by reducing the solid solution S and the solid solution Al, and suppresses Ostwald growth of TiN. Since the amount of solute C segregated at the interface of γ increases as the temperature decreases, the effect of suppressing the Ostwald growth of TiN (the effect of suppressing TiN Ostwald growth) becomes more pronounced as the heating temperature is lower. Therefore, according to the above (a) to (d), in the sour resistant steel sheet of the present invention, the composite precipitation TiN having the core of the ultrafine Mg-containing oxide as the pinning particles and the TiN solely precipitated in the ground iron are fine. There are many. The sour-resistant steel sheet of the present invention has a finer metal structure even if the thickness exceeds 30 mm, and can stably achieve the BDWTT characteristics at a low temperature of -50 ° C or lower.

  At this time, the average value of the effective crystal grain size at a position １ ／ of the plate thickness from the surface in the plate thickness direction (hereinafter, sometimes referred to as a 厚 position of the plate thickness) is 15 μm or less. Here, the effective crystal grain size is, for example, a size of a region where the angle difference in the crystal direction is within 3 degrees, and can be measured by an electron beam backscatter diffraction (EBSD). When the metal structure is ferrite, the crystal grain size is equal to the effective crystal grain size. On the other hand, in the case of a needle-like crystal such as bainite or martensite, the effective crystal grain size is a dimension of a region in the needle-like crystal bundle where the directions of the crystals are substantially aligned.

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

(C: 0.020% or more, 0.060% or less)
C is an element for increasing the strength of steel, and the C content is made 0.020% or more in order to obtain high strength of X65 or more. Preferably, the C content is 0.030% or more, more preferably 0.035% or more, and still more preferably 0.040% or more. However, an increase in the amount of C intensifies the segregation of Mn and P in the center segregation of the slab and significantly degrades the HIC resistance, so the upper limit is 0.060%. Preferably, the C content is 0.055% or less, more preferably 0.050% or less.

(Si: 0.30% or less)
Si may be contained in steel for deoxidation, but if the amount of Si is too large, the weldability and the HAZ toughness deteriorate, so the content is limited to 0.30% or less. In the steel of the present invention, since deoxidation is possible with Al, Ti, and Mg, 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 amount of Si be 0.15% or less. More preferably, the Si content is 0.10% or less.

(Mn: 1.00% or more, 1.60% or less)
Mn is an element that enhances the hardenability and contributes to strengthening of the steel. In order to obtain a high strength of X65 or more, the Mn content is set to 1.00% or more. Preferably, the Mn content is 1.05% or more, more preferably 1.10% or more, and still more preferably 1.15% or more. However, an increase in the amount of Mn strengthens the center segregation of the slab and remarkably degrades the HIC resistance. Therefore, the upper limit is 1.60%. Preferably, the Mn content is 1.55% or less, more preferably 1.50% or less, and still more preferably 1.45% or less.

(P: 0.015% or less)
P is an impurity, which strengthens the central segregation of the slab and significantly degrades the HIC resistance. Therefore, the P content is limited to 0.015% or less. The lower the content of P, the better the HIC resistance. Therefore, the lower limit is not particularly specified, but the content of P is preferably 0.001% or more from the viewpoint of manufacturing cost.

(S: 0.0001% or more, 0.0010% or less)
S is an element which forms MnS which is harmful to the HIC resistance and is stretched by rolling, and the S content 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 set to 0.0001% or more from the viewpoint of manufacturing cost.

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

(Nb: 0.004% or less)
In the present invention, it is desirable that Nb is not substantially contained in order to ensure the HIC resistance. If the Nb content exceeds 0.004%, Nb carbonitride left undissolved in the central segregation portion when the slab is heated deteriorates the HIC resistance. Therefore, it is necessary to limit the amount of Nb to 0.004%. In the present invention, from the viewpoint of ensuring the BDWTT characteristics, it is preferable to heat the slab to a low temperature of, for example, 1100 ° C. or less. In this case, the Nb content is set to 0.003% in order to prevent undissolved Nb carbonitride. It is preferable to reduce it below. More preferably, the Nb content is 0.002% or less. Since Nb is also harmful to HAZ toughness, the fact that Nb is not substantially contained has the effect of increasing HAZ toughness.

(Al: 0.001% or more, 0.020% or less)
Al is an important element to be controlled to an appropriate range in the present invention. Since Al combines with Mg and O to form an ultrafine Mg-Al-based oxide of 0.01 to 0.1 μm serving as a precipitation nucleus of TiN, the amount of Al needs to be 0.001% or more. is there. Preferably, the Al content is 0.002% or more, more preferably 0.003% or more. However, if the amount of Al exceeds 0.02%, the amount of solid solution Al at the time of heating the slab increases, and the TiN Ostwald growth suppression effect, which is a feature of the present invention, is reduced. Therefore, this is the upper limit. Preferably, the Al content is 0.018% or less, more preferably 0.016% or less.

(Ca: 0.0005% or more, 0.0030% or less)
Ca is an important element added to form CaS or Ca (O, S) which is difficult to be stretched by rolling, to control the form of the sulfide, and to secure the HIC resistance. The amount of Ca is set to 0.0005% or more in order to prevent the generation of MnS which is a starting point of HIC due to elongation by rolling. 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 amount is 0.0025% or less, more preferably 0.0020% or less.

(Mg: 0.0003% or more, 0.0030% or less)
Mg is an important element that combines with Al and O to form a fine oxide of 0.01 to 0.1 μm. The fine Mg-based oxide functions as a precipitation nucleus of TiN, and finely disperses a composite form of TiN that strongly pins γ-grain growth during slab heating. Mg forms a micron-sized coarse oxide and suppresses precipitation of TiN on the coarse oxide. As a result, the tendency of finer TiN to precipitate on the base iron than that of steel containing no Mg is increased, and the growth of γ grains during slab heating is effectively pinned. As described above, Mg forms a fine oxide and a coarse oxide, directly or indirectly promotes fine dispersion of TiN, and significantly enhances the ability of TiN particles to suppress γ-grain growth during slab heating. . In addition, such an effect of suppressing the growth of γ grains is also effective for refining the HAZ structure, and has an effect of increasing the HAZ toughness.
In order to exert such an effect, the amount of Mg of 0.0003% or more is necessary, and this is the lower limit. Preferably, the Mg content is 0.0005% or more, more preferably 0.0008% or more, and still more preferably 0.0010% or more. On the other hand, even if the content of Mg exceeds 0.0030%, the fine dispersion effect of TiN saturates, so that more Mg does not have any metallurgical effect. Since Mg is a very active element having a high vapor pressure and a strong oxidizing power, it is not preferable to include Mg more than necessary in steel because it increases the production cost. Therefore, the upper limit of the amount of Mg is 0.0030%. Preferably, the Mg content is 0.0025% or less, more preferably 0.0020% or less.

(N: 0.0015% or more, 0.0050% or less)
N is an element constituting TiN particles that pin the γ grain growth. In order to exhibit a sufficient effect of suppressing the growth of γ grains during slab heating, it is necessary to secure the minimum number of TiN particles by setting the lower limit of the amount of N 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 content 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.

(O: 0.0010% or more, 0.0030% or less)
O is an element that combines with a deoxidizing element such as Mg or Al to form a fine oxide of 0.01 to 0.1 μm or a coarse oxide of several μm. In order to directly or indirectly generate an Mg-based oxide that contributes to fine dispersion of TiN, an O amount of 0.0010% or more is required. 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 O content is 0.0030% in order to reduce oxide-based inclusions, which are the starting points of HIC, and to control the sulfide form by Ca.

(1.0 ≦ [Ca × (1-124 × O)] / (1.25 × S) ≦ 8.0)
In the sour resistant steel sheet of the present invention, in order to secure HIC resistance, S is reduced as much as possible, and then Ca is added to suppress the generation of stretched MnS, which is a starting point of HIC, to reduce S to CaS or Ca ( O, S). At this time, if the balance between S, Ca and O does not satisfy 1.0 ≦ [Ca × (1-124 × O)] / (1.25 × S), stretched MnS remains and HIC occurs. . On the other hand, when [Ca × (1-124 × O)] / (1.25 × S) ≦ 8.0 is not satisfied, Ca-based inclusions increase and HIC occurs. Therefore, it is necessary to satisfy the following expression (1).
1.0 ≦ [Ca × (1-124 × O)] / (1.25 × S) ≦ 8.0 (1)
In order to increase the HIC resistance, the lower limit of [Ca × (1-124 × O)] / (1.25 × S) is preferably 1.5, more preferably 2.0, and the upper limit is preferably 7.5. , More preferably 7.0.

  If necessary, one or more of Cu, Ni, Cr, Mo, W, Co, V, B, REM, and Zr may be contained.

(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 the weldability and the HAZ toughness. However, excessive addition may cause production of Cu cracks during hot rolling, making production difficult or unfavorable in weldability. Therefore, the upper limit of the Cu content is preferably 1.0%. More preferably, the amount of Cu 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 the weldability and the HAZ toughness. However, excessive addition impairs economic efficiency and may be unfavorable for weldability. Therefore, the upper limit of the Ni content is preferably 1.0%. More preferably, the Ni content is set to 0.8% or less, 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 segregation of the center in the continuous cast slab and improve the strength of the base material. However, since excessive addition may deteriorate the toughness and weldability of the base material and HAZ, the upper limit of the Cr content is preferably 1.0%. More preferably, the amount of Cr is 0.8% or less, 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 the 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 content is preferably 0.5%. More preferably, the Mo content is 0.3% or less.

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

(Co: 0.5% or less)
Co 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 be unfavorable for weldability. Therefore, the upper limit of the 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 making the microstructure finer. However, since excessive addition may cause deterioration of HAZ toughness and weldability, the upper limit of the V content is preferably 0.10%. More preferably, the V content is 0.08% or less, further preferably 0.05% or less.

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

(REM: 0.004% or less)
REM means a rare earth element such as La, Ce or Nd. REM, like Ca, binds to S in preference to Mn, forms sulfides and oxysulfides, suppresses the generation of stretched MnS, and contains 0.0001% or more to increase the HIC resistance. You may. However, if REM is added in excess of 0.004%, REM-based inclusions increase and may become a starting point of HIC or brittle fracture. Therefore, the upper limit of the REM content is 0.004%. Is preferred.

(Zr: 0.005% or less)
Zr combines with S in preference to Mn like Ca and REM, forms sulfides and oxysulfides, suppresses the generation of stretched MnS, and increases the HIC resistance by 0.0001% or more. You may make it contain. However, when Zr is added in excess of 0.005%, the amount of Zr-based inclusions increases and may become a starting point of HIC or brittle fracture. Therefore, the upper limit of the amount of Zr is preferably 0.005%. .

When REM and Zr are contained, it is possible to optimize the balance between S and O by treating REM as 3.5 times of Ca and Zr as 2.3 times of Ca. Therefore, it is preferable to adjust the contents of REM and Zr so as to satisfy the following expression (2) instead of the above expression. In the case where one of REM and Zr is not contained, it is sufficient to calculate them as 0. In order to increase the HIC resistance, the lower limit of the following formula (2) is preferably 1.5, more preferably 2.0, and the upper limit is preferably 7.5, more preferably 7.0.
1.0 ≦ [(Ca + 3.5 × REM + 2.3 × Zr) × (1-124 × O)] / (1.25 × S) ≦ 8.0 (2)

  The metal structure of the sour-resistant steel sheet of the present invention comprises 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 by an optical microscope. In order to improve toughness, it is desirable to reduce the effective crystal grain size. The sour resistant steel sheet of the present invention has an effective crystal grain size of 15 μm or less at a position 1/4 of the sheet thickness from the surface in the sheet thickness direction. It is necessary to be. If the effective crystal grain size exceeds 15 μm, the BDWTT characteristics cannot be stably achieved at a low temperature of −50 ° C. or less. Since the smaller the effective crystal grain size at the 1/4 thickness position is, the lower limit is not specified, but may be 3 μm or more from the viewpoint of manufacturing cost.

Next, a method for manufacturing the sour resistant steel sheet according to the 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 less, and then heated to 900 ° C. to 1050 ° C. to perform hot rolling.
If the slab is inserted into the heating furnace by hot charging without cooling it to 400 ° C. or lower, the coarse γ structure generated during casting may remain after heating, and the structure may not be sufficiently refined to lower the low-temperature toughness.

  The heating temperature of the slab is preferably 900 ° C. or higher in order to set the rolling end temperature to Ar 3 or higher. On the other hand, when the heating temperature is high, the heated γ grains are coarsened, and the microstructure may not be sufficiently refined, so that the low-temperature toughness may be deteriorated. More preferably, the heating temperature is 1000 ° C. or lower.

  In the subsequent hot rolling, the microstructure is sufficiently refined by accumulating a large number of reductions with a large reduction ratio per pass in the γ low temperature range, and very good low temperature toughness can be 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 when the number of passes is 60% or more. 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. And good low-temperature toughness may not be obtained.

  It is preferable to perform hot rolling so that the plate thickness becomes 31 mm or more and 45 mm or less, and finish the rolling at Ar3 or more. If the end temperature of the hot rolling is lower than Ar3, C may be concentrated in the central segregation part due to the ferrite transformation, a hardened structure may be formed, and the HIC resistance may deteriorate. Further, in order to start the accelerated cooling from a temperature of Ar3 or higher, it is preferable that the hot rolling is terminated at Ar3 or higher.

  After hot rolling, it is preferable to perform accelerated cooling from Ar3 or more. Accelerated cooling improves the microstructure of the center segregation part to improve the HIC resistance, and also enables high strength by transformation strengthening and high toughness by grain refinement. The cooling rate of the accelerated cooling is preferably 3 ° C./sec or more and 50 ° C./sec or less, and it is preferable that the accelerated cooling be completed within the range of 550 ° C. or less and 300 ° C. or more, and then air-cooled.

  If the cooling start temperature is lower than Ar3, the cooling rate is lower than 3 ° C./sec, or the cooling stop temperature is higher than 550 ° C., the hardened structure is formed by enrichment of C in the central segregation part due to ferrite transformation. When formed, the HIC resistance is degraded, and the transformation may be insufficiently strengthened due to insufficient reinforcement. On the other hand, if the cooling rate exceeds 50 ° C./sec or the water cooling stop temperature is less than 300 ° C., a large amount of low-temperature transformation products are generated, and the HIC resistance and low-temperature toughness may deteriorate.

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

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

Examples of the present invention will be described below, but the following examples are merely 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 chemical composition shown in Tables 1 and 2 and having a thickness of 240 mm were produced by continuous casting.

  After cooling the obtained slab to room temperature, it was heated to 980 to 1030 ° C. and hot-rolled. At this time, the cumulative rolling reduction at 900 ° C. or less was 75 to 80%, and the rolling reduction per pass was 60% or more and the rolling reduction per pass was 15% or more. Further, the rolling end temperature is set to 800 to 830 ° C., and after hot rolling, accelerated cooling of 5 to 35 ° C./sec is applied from a range of 790 to 810 ° C., and accelerated cooling is performed within a range of 350 to 450 ° 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.

  From the obtained steel sheet, a test piece having a thickness in a width direction perpendicular to the rolling direction as a longitudinal direction was sampled in accordance with the API 5L standard, and a tensile test was performed at room temperature in accordance with the API standard 2000. Further, a BDWTT test piece was sampled, and one side surface was cut to reduce the thickness to 3/4 inch, and a drop tear test was performed. The low-temperature toughness was evaluated by a ductile fracture surface transition temperature (BDWTT 85% SATT [° C.]).

The effective crystal grain size at the 1/4 position of the plate thickness was measured by using EBSD on the cross section of the micro test piece at that position. The crystal orientation was measured over an area of 0.02 mm ² or more by EBSD, and a region having a crystal orientation difference of 3 ° or less was regarded as an effective crystal grain, and the average value of equivalent circle diameters was determined as an effective crystal grain size.

The HIC test of the base material was carried out 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 And the HIC length ratio CLR were measured.

Further, in order to evaluate the HAZ toughness, one-pass inner surface and one-pass outer surface corresponding to a seam weld of a UO steel pipe were subjected to submerged arc welding to produce a welded joint. In addition, the welding metal of low temperature specification was used so that the toughness of 100 J or more could be obtained at -50 degreeC. A Charpy impact test specimen was sampled based on the joint portion of the welded joint, a 2 mm V notch was made so that the ratio of the weld metal to HAZ was 50:50, and three tests were performed at −50 ° C. The lowest value was measured.
Tables 3 and 4 show the thickness, mechanical properties, HIC resistance, and HAZ toughness of the welded portion of the steel sheet.

  As shown in Table 3, in the steel of the present invention, the strength of the 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 sheet having a thickness of 31 to 45 mm, The transition temperature of the ductile fracture surface in the heavy tear test is as low as −50 ° C. or less, and has good BDWTT characteristics. The effective crystal grain size is reduced to 15 μm or less. At the same time, the steel according to the invention has excellent HIC resistance. Further, the steel of the present invention has excellent HAZ toughness at -50 ° C.

  On the other hand, as shown in Table 2, since the chemical composition of the conventional steel is out of the range of the present invention, as shown in Table 4, the mechanical properties of the base material, the HIC resistance, and the HAZ toughness of the welded portion are inferior. Or surface flaws may occur.

  Symbol B1 has too low C amount, and symbol B4 has insufficient strength because Mn amount is too low. Reference B2 has an excessively high C content, so that the HIC resistance is deteriorated and the HAZ toughness tends to be deteriorated. The symbol B3 has deteriorated HAZ toughness because the amount of Si is too high. Symbol B5 degrades the HIC resistance because the Mn content is too high.

  Reference B6 has an excessively high P content, so that the HIC resistance is deteriorated and the HAZ toughness also tends to be deteriorated. The symbol B7 has a deteriorated HIC resistance because the amount of S is too high. The symbol B8 has an excessively low Al content, and the symbol B10 has an excessively low Ti content, so that the effective crystal grain size becomes larger than 15 μm and the BDWTT characteristics and the HAZ toughness are deteriorated. The symbol B9 indicates that the effective crystal grain size exceeds 15 μm because the Al content is too high, and the BDWTT characteristic is deteriorated. The reference B11 had an effective crystal grain size exceeding 15 μm because the Ti content was too high, and the BDWTT characteristics and the HAZ toughness were deteriorated.

  In the case of symbol B12, the Nb content is too high, so that the HIC resistance deteriorates and the HAZ toughness also tends to deteriorate. The symbol B13 has deteriorated HIC resistance because the amount of Ca is too low. The symbol B14 has deteriorated HIC resistance and HAZ toughness because the Ca content is too high. The reference B15 has an effective crystal grain size exceeding 15 μm because the Mg content is too low, and the BDWTT characteristic and the HAZ toughness are deteriorated.

  Reference B16 has an effective crystal grain size exceeding 15 μm because the N content is too low, and the BDWTT characteristics and the HAZ toughness are deteriorated. The symbol B17 had a tendency that the HIC resistance was degraded due to the excessively high N content, the HAZ toughness was degraded, and the occurrence of flaws on the slab surface was also observed. In the case of symbol B18, the O content is too low, the effective crystal grain size exceeds 15 μm, and the BWTT property and the HAZ toughness are deteriorated. The symbol B19 has an O content that is too high, so that the HIC resistance and the HAZ toughness are deteriorated.

  Reference numeral B20 indicates that the value of the expression (1), which is an index of sulfide form control, is too low, so that the HIC resistance is deteriorated. Reference numeral B21 indicates that the value of equation (1), which is an index of sulfide form control, is too high, so that the HIC resistance is degraded.

  The present invention relates to the production of a linepipe steel sheet having excellent HIC resistance and strength of API 5L X65 or more and having much lower low-temperature toughness than before. It is desirable to apply.

Claims (3)

In mass%,
C: 0.020% or more, 0.060% or less,
Mn: 1.00% or more, 1.60% or less,
S: 0.0001% or more, 0.0010% or less,
Al: 0.001% or more, 0.020% or less,
Ti: 0.005% or more, 0.020% or less,
Ca: 0.0005% or more, 0.0030% or less,
Mg: 0.0003% or more, 0.0030% or less,
N: 0.0015% or more, 0.0050% or less,
O: contains 0.0010% or more and 0.0030% or less,
Si: 0.30% or less,
P: 0.015% or less,
Nb: limited to 0.004% or less, the balance consists of Fe and unavoidable impurities, the content of Ca, O, and S satisfies the following formula (1), and 1/1 of the sheet thickness from the surface in the sheet thickness direction. the average value of the effective crystal grain size in 4 positions Ri der less 15 [mu] m,
The board thickness is 31mm or more and 45mm or less,
The yield stress is 448MPa or more,
Tensile strength, sour steel sheet, characterized in der Rukoto than 535MPa.
1.0 ≦ [Ca × (1-124 × O)] / (1.25 × S) ≦ 8.0 (1) 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,
Co: 0.5% or less,
V: 0.10% or less,
B: The sour-resistant steel sheet according to claim 1, comprising one or more of 0.0030% or less. Furthermore, in mass%,
REM: 0.004% or less,
The sour-resistant steel sheet according to claim 1, further comprising one or both of Zr: 0.005% or less and satisfying the following equation (2) instead of the equation (1).
1.0 ≦ [(Ca + 3.5 × REM + 2.3 × Zr) × (1-124 × O)] / (1.25 × S) ≦ 8.0 (2)