JP6631353B2 - Sour-resistant steel plate and sour-resistant steel pipe - Google Patents

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, and the like, and a sour-resistant steel pipe.

  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. Sour-resistant steel sheets used in line pipes are required not only to have excellent hydrogen-induced cracking resistance (HIC resistance), but also to have high strength and thickening from the viewpoint of improving transport efficiency. In some cases. 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 increase the purity and purity of the steel by limiting impurities such as S and O, and to control the form of sulfide-based inclusions by adding Ca. Further, a method has been proposed in which the microstructure of the center segregation portion is improved by accelerated cooling, and in particular, the generation of a hardened structure is suppressed to improve the HIC resistance (see, for example, Patent Document 1). Furthermore, there is a method of preventing the center segregated portion from being left uncompressed and improving the 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 carbonitrides) 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 HIC resistance. This tendency is more remarkable as the strength becomes higher. For a high-strength material such as API 5L X80 grade, the adverse effect of the coarse Nb precipitate on the stability of the HIC resistance cannot be ignored. On the other hand, when the slab is heated to a considerably 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. Causes 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). However, the strength of the steel sheet of Patent Document 3 is API 5L X65 grade or X70 grade, and is not a high strength material such as X80 grade.

  On the other hand, some of the present inventors have proposed a method for producing an API 5L X65 grade 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 describes (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 technology that realizes the refinement of the γ structure by rolling of (4), and (4) securing of HIC resistance and enhancement of transformation by accelerated cooling after rolling.

  In addition, some of the present inventors have proposed a method of improving the BDWTT characteristics of a sour-resistant steel sheet by increasing the amount of separation generated on a BDWTT fracture surface (for example, see Patent Document 5). ). This is a technique in which, in controlled rolling, rolling is performed with a large rolling reduction in each pass and a large cumulative rolling reduction to develop a texture of γ, thereby developing a texture of bainite after transformation and generating separation. Specifically, in a temperature range of Ar3 or more to Ar3 + 100 ° C. or less, rolling is performed in which the number of passes is 60% or more, the rolling reduction of one pass is 15% or more, and the cumulative rolling reduction is 80% or more. By applying accelerated cooling to steel, bainite with a developed texture can be obtained, and this is a technique for producing a steel sheet with a large amount of separation. In this technique, Nb plays an extremely important role in developing a γ texture, and it is necessary to add 0.01% or more of Nb. In the example of Patent Document 5, this technique is applied to a steel plate having a strength of API 5L X60 or more and X70 or less and a plate thickness of 30 mm or less.

  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 6). ). 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-A-8-85841 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 manufacturing 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 rolling end temperature decrease 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 becomes insufficiently fine. In addition, in the case of a high-strength material such as API 5L X80 grade, BDWTT is used in a low-temperature environment such as -20 ° C. (Battelle Drop Weight Tear Test) It was difficult to achieve the characteristics stably. Here, the BDWTT characteristic is a brittle crack propagation stopping characteristic which 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 is excellent in the HIC resistance of API 5L X80 class having a plate thickness of 31 mm or more and 50 mm or less, which is 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 secure the BDWTT characteristics of the base material of the steel plate and the Charpy impact characteristics of the welding heat affected zone (HAZ) at a low temperature of −20 ° C. or lower in a sour resistant steel plate.

  The present inventors studied applying a new technique for suppressing the growth of γ grains at the time of slab heating, which is the initial stage of TMCP, and remarkably reducing the γ grain size during heating as compared with the conventional technique. Then, from the viewpoint of the HIC resistance, the present inventors made 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 the extremely low temperature heating condition of 900 to 1050 ° C., extremely strong γ-grain growth suppressing effect is exhibited by TiN which precipitates compositely with fine Mg-containing oxide as a nucleus and TiN which solely precipitates in ground iron. I understood. Furthermore, in order to improve the BDWTT characteristics, it was found that it is effective not only to refine the crystal grains but also to stretch in the rolling direction to promote the generation of separation.

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% to 0.100%, Mn: 1.00% to 1.80%, S: 0.0001% to 0.0010%, 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, 0.0030% or less, O: 0.0010% or more, 0.0030% or less, Si: 0.30% or less, P: 0.015% or less %, Nb: limited to 0.004% or less, the balance being composed of Fe and unavoidable impurities, satisfying the following expression (1), and bainite at a position 1/4 of the sheet thickness from the surface in the sheet thickness direction. Has an area ratio of 80% or more, and has a flat circle equivalent diameter in effective crystal grains. Value is not more 25μm or less, the average value of the aspect ratio in the effective crystal grain is sour steel is 4 or more.
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 is 31 mm or more and 50 m or less, the yield stress is 555 MPa or more and 705 MPa or less, and the tensile strength is 625 MPa or more and 825 MPa or less. In the effective area of the BDWTT fracture surface where the ductile fracture ratio is 85% or more, the maximum value of the separation index evaluated as the value obtained by dividing the total length (mm) of the separation length by the effective area (mm ² ) is obtained. It may be 0.02 mm ^-1 or more.

Further, the sour resistant steel pipe according to one embodiment of the present invention includes:
(5) A steel pipe comprising a base material and a welded part, wherein the base material is a sour resistant steel pipe comprising the sour resistant steel sheet according to any one of the above (1) to (4).

  Advantageous Effects of Invention According to the present invention, HIC resistance of 31 mm or more and 50 mm or less, having a strength of the American Petroleum Institute (API) standard X80 grade and having excellent BDWTT characteristics at a low temperature of -20 ° C or less. It is possible to provide excellent sour resistant steel sheets. 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 provides (a) Ti-Mg addition, (b) a sour-resistant component (Ca addition, ultra-low S, Nb-free addition), (c) Al reduction, and (d) finely stretched in the rolling direction. It has the feature of bainite-based structure, and is based on the finding that a more remarkable gamma-grain growth inhibitory effect is exhibited as compared with the conventional art. In the present invention, the Ostwald growth of TiN is reduced when heated to 900 to 1050 ° C., that is, particularly at (e) cryogenic heating, by reducing the solute S and solute Al present in γ. As a result, the finely dispersed state of TiN is maintained, and as a result, an extremely strong effect of inhibiting 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, thereby suppressing the 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 (e), 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 fine metal structure even if the sheet thickness exceeds 30 mm, and can stably achieve the BDWTT characteristics at a low temperature of −20 ° C. or lower.

At this time, the area ratio of bainite at a position 1/4 of the plate thickness from the surface in the plate thickness direction (hereinafter, sometimes referred to as a 1/4 position of the plate thickness) is 80% or more, and is equivalent to a circle in the effective crystal grains. The average value of the diameter is 25 μm or less. Further, the average value of the aspect ratio in the effective crystal grains is 4 or more, and the maximum value of the separation index in the BDWTT fracture surface where the ductile fracture ratio is 85% or more is 0.02 mm ^-1 or more.

  Here, the effective crystal grain size is, for example, a size of a region in which the angle difference between crystal directions 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 the size of a region in the needle-like crystal bundle where the crystal directions are substantially aligned. Further, the aspect ratio of the effective crystal grains represents the extent to which the effective crystal grains are stretched in the rolling direction, and is a value obtained by dividing the maximum diameter in the rolling direction by the maximum diameter in the sheet thickness direction.

Further, the separation index (mm ^-1 ) is a value obtained by dividing the total sum (mm) of the separation length by the effective area (mm ² ) in the effective area of the BDWTT fracture surface. It has been conventionally known that the generation of separation is effective in addition to the refinement of effective crystal grains in order to improve the BDWTT characteristics. However, in the present invention, Nb is not added, and API 5L X80 grade is used. High strength and a plate thickness exceeding 30 mm are characterized in that both the refinement of effective crystal grains and the increase in the amount of separation occur.

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

(C: 0.020% or more, 0.100% 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 a high strength of X80 class. Preferably, the C content is 0.030% or more, more preferably 0.035% or more, and further 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. Therefore, the upper limit is 0.100%. Preferably, the C content is 0.095% or less, more preferably 0.090% or less.

(Si: 0.30% 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 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 the 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 and 1.80% or less)
Mn is an element that contributes to strengthening of the steel by increasing the hardenability, and the Mn content is set to 1.00% or more in order to obtain X80-class high strength. 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 intensifies the center segregation of the slab and remarkably degrades the HIC resistance, so the upper limit is 1.80%. The Mn content is preferably set to 1.75% or less, more preferably 1.70% or less, and further preferably 1.45% or less.

(P: 0.015% or less)
P is an impurity, which strengthens the center segregation of the slab and remarkably degrades the HIC resistance. Therefore, the P content is limited to 0.015% or less. Since the lower the P, the higher the HIC resistance, the lower limit is not particularly defined, but the P content 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 that forms MnS that is stretched by rolling, which is harmful to the HIC resistance, and it is necessary to limit the amount of S 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 of slabs and HAZs by a pinning effect. The lower limit of the Ti content is set to 0.005% in order to exert 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 TiN particles become coarse, a sufficient pinning effect cannot be obtained, and the toughness of the base material and the HAZ 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 secure the HIC resistance. If the Nb content exceeds 0.004%, the Nb carbonitride left undissolved in the central segregation portion when the slab is heated degrades the HIC resistance. Therefore, it is necessary to limit the amount of Nb to 0.004%. In the present invention, from the viewpoint of securing 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% to prevent the undissolved Nb carbonitride. It is preferable to reduce it below. More preferably, the Nb content is 0.002% or less. Nb is also harmful to HAZ toughness, so that substantially not containing Nb has the effect of increasing HAZ toughness.

(Al: 0.001% or more, 0.020% or less)
Al is an important element to be controlled in 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 0.020%, the amount of solid solution Al during slab heating 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 ensure 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 the growth of γ grains during slab heating. Mg forms a micron-sized coarse oxide and suppresses the deposition 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 γ grain growth 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 remarkably 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 in steel more than necessary, 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 for pinning the γ-grain growth of slabs and HAZs. 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 HAZ deteriorates, and 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, when 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-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 of 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 improve 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 weldability and 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, 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 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 the center from segregating 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 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 the 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, excessive addition impairs economic efficiency and may cause deterioration of the toughness and weldability of the base material and HAZ, so the upper limit of the W content 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, since excessive addition impairs economic efficiency and may be unfavorable for weldability, 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 be deteriorated by 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, further preferably 0.0015% or less.

(REM: 0.004% or less)
REM means a rare earth element such as La, Ce or Nd. REM combines with S in preference to Mn similarly to Ca, 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 binds to S in preference to Mn like Ca and REM, forms sulfides and oxysulfides, suppresses generation of stretched MnS, and enhances HIC resistance. 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, the balance between S and O can be optimized by treating REM as 3.5 times the effect of Ca and Zr as 2.3 times the effect of Ca. Therefore, it is desirable 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 improve 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 according to the present invention requires that the area ratio of bainite at a 1/4 thickness position be 80% or more. Bainite is lath-, plate-, or lump-type ferrite containing carbide (one or more of residual austenite and M-A), or carbide (residual austenite, (One or more of MA). Here, MA is a hybrid of martensite and austenite. Martensite is a structure in which carbides are not precipitated between lath-like ferrites, between plate-like ferrites, or within lath-like ferrites or within plate-like ferrites. Retained austenite is austenite that remains without transformation of austenite formed at high temperatures. That is, in the present invention, bainite is not limited to so-called upper bainite, lower bainite and a mixed structure thereof, but also a ferrite having a high dislocation density, and in some cases, a microstructure including cementite therein is treated as bainite. For example, bainite also includes a microstructure conventionally called bainitic ferrite or acicular ferrite. In the present invention, the remaining microstructure obtained by subtracting ferrite having a low dislocation density and pearlite is treated as bainite, and its area ratio is 80% or more.

  If the bainite area ratio is less than 80%, the API 5L X80 class strength and HIC resistance cannot be simultaneously achieved. The remainder of bainite consists of one or two of ferrite and pearlite, hardly generates martensite, and its area ratio can be ignored. In the case of such a mixed structure, the effective crystal grains have a stronger correlation with the toughness than the crystal grains observed by an optical microscope, and it is desirable to refine the effective crystal grains in order to improve the toughness.

Further, in the sour resistant steel sheet of the present invention, it is necessary that the average value of the aspect ratio of the effective crystal grains at the position of 1/4 of the sheet thickness is 4 or more. The average value of the aspect ratios of the effective crystal grains is an index of the development of texture that is correlated with the amount of separation. Therefore, if the average value of the aspect ratio of the effective crystal grains is less than 4, and the development of the texture is insufficient, the BDWTT characteristics cannot be stably achieved at a low temperature of −20 ° C. or lower. Specifically, in the case of a steel plate having a strength of API 5L X80 class and a plate thickness of 31 mm or more and 50 mm or less, which is the object of the present invention, the maximum separation index at the BDWTT fracture surface where the ductile fracture ratio is 85% or more. The value will be less than 0.02 mm ^-1 .

  Further, the sour resistant steel sheet of the present invention is suitable as a material for a sour resistant steel pipe. The sour resistant steel pipe of the present invention is a steel pipe composed of a base material and a welded part, and is a sour resistant steel pipe whose base material is composed of the sour resistant steel sheet according to the present invention. The sour resistant steel pipe of the present invention is a welded steel pipe formed by using the steel sheet according to the present invention as a raw material by a UOE process or a roll bending method.

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 280 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 a 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, if the heating temperature is high, the heated γ grains may be coarsened, and the microstructure may not be sufficiently refined to deteriorate the low-temperature toughness. More preferably, the heating temperature is 1000 ° C. or lower.

  In the subsequent hot rolling, in the γ low temperature range, rolling is performed with a large cumulative rolling reduction in a limited temperature range, whereby the microstructure is sufficiently refined, the texture is sufficiently developed, and the amount of separation generated And very good low-temperature toughness can be obtained even with high strength. Therefore, in the hot rolling, the cumulative rolling reduction is preferably 80% or more in the temperature range of Ar3 or more to Ar3 + 50 ° C., and the rolling reduction per pass is 15% or more when the number of passes is 80% or more. Is preferred. If the cumulative rolling reduction in the temperature range of Ar3 or more to Ar3 + 50 ° C. is less than 80%, or the ratio of the number of passes at which the rolling reduction per pass is 15% or more is less than 80%, the microstructure after transformation is reduced. The microstructure is not refined, the texture is not developed, the amount of separation is reduced, and good low-temperature toughness may not be obtained.

  It is preferable that hot rolling is performed so that the plate thickness becomes 31 mm or more and 50 mm or less, and the rolling is completed when Ar 3 or more. If the end temperature of the hot rolling is lower than Ar3, C may be concentrated in the central segregation portion 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 the hot rolling, it is preferable to perform accelerated cooling from Ar3 or more. The accelerated cooling improves the HIC resistance by improving the microstructure of the central segregation portion, 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./second or more and 50 ° C./second or less, and it is preferable that the accelerated cooling be completed within a range of 550 ° C. or less and 300 ° C. or more, followed by air cooling.

  When 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 is sometimes 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 widely applied to applications requiring HIC resistance, such as sour resistant line pipes, sour resistant pressure vessels and sour resistant structural steel pipes.

Examples of the present invention will be described below, but 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 a 300 mm thick slab having the chemical components shown in Tables 1 and 2 was produced by continuous casting.

  After the obtained slab was cooled to room temperature, it was heated to 980 to 1030 ° C. and hot-rolled. At this time, in the temperature range of Ar3 or more to Ar3 + 50 ° C., the cumulative rolling reduction was 80 to 90%, and the rolling reduction per pass was 15% or more for 80% or more of the number of passes. Further, the rolling end temperature is from Ar3 or more to Ar3 + 20 ° C or less, and after hot rolling, accelerated cooling of 5 to 35 ° C / sec is applied from the range of Ar3 or more to Ar3 + 20 ° C and within the range of 350 to 450 ° C. The accelerated cooling was stopped, followed by air cooling. However, the symbol B22 in Table 2 indicates that the cumulative rolling reduction in the temperature range from Ar3 or more to Ar3 + 50 ° C. in the above manufacturing conditions was 70%. The symbol B23 in Table 2 indicates that the stop temperature of the accelerated cooling was 560 ° C. in the above manufacturing conditions.

With the longitudinal direction being the width direction perpendicular to the rolling direction, a full-thickness test piece conforming to the API 5L standard was sampled from the obtained steel sheet, and a tensile test was performed at room temperature according to the API standard 2000. In addition, 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.]). At this time, in the effective area of the BDWTT fracture surface where the ductile fracture ratio is 85% or more, the value obtained by dividing the total (mm) of the separation length by the effective area (mm ² ) is defined as the separation index (mm ⁻¹ ). did.

The area ratio of bainite and the grain size and aspect ratio of effective crystal grains at the 1/4 plate thickness position were measured by using EBSD over a 0.02 mm ² or more area in the cross section of the micro test piece at that position. In observation of the microstructure with an optical microscope, it is sometimes difficult to distinguish between bainite and ferrite. Therefore, the area ratio of ferrite was measured by the Kernel Average Misorientation (KAM) method in EBSD.

  The KAM method refers to six adjacent hexagonal pixels (first approximation) in the measurement data, further 12 out of them (second approximation), and further 18 out of them (second approximation). (Third approximation) is a method of averaging the azimuth differences between pixels and calculating the value as the value of the center pixel for each pixel. In the present invention, the area ratio of a pixel whose azimuth difference between adjacent pixels is calculated to be 1 degree or less by the third approximation is defined as the area ratio of ferrite.

  On the other hand, the area ratio of pearlite was measured by nital corrosion in the region where the EBSD was measured, and observed by an optical microscope. Then, in the EBSD measurement region, the remainder obtained by subtracting the ferrite area ratio and the pearlite area ratio was defined as the bainite area ratio. In the EBSD measurement, a region where the crystal orientation difference was within 3 degrees was regarded as an effective crystal grain, and the average value of the circle equivalent diameter was determined as the effective crystal grain size. Further, a value obtained by dividing the maximum diameter of the effective crystal grains in the rolling direction by the maximum diameter in the sheet thickness direction was defined as an aspect ratio, and the average value was determined.

The HIC test of the base material was performed 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 the UO steel pipe were subjected to submerged arc welding to produce a welded joint. In addition, the welding metal of the 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 collected based on the joint portion of the welded joint, subjected to a 2 mm V notch so that the ratio of the weld metal to HAZ was 50:50, and subjected to three tests at −20 ° 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, the steel of the present invention satisfies API 5L X80 class (yield strength YS: 555 MPa or more, 705 MPa or less, tensile strength TS: 625 MPa or more, 825 MPa or less) in a steel sheet having a thickness of 31 to 50 mm. The strength of the material and the transition temperature of the ductile fracture surface in the drop-weight tear test are as low as −20 ° C. or less, and have good BDWTT characteristics. The bainite area ratio is 80% or more, the effective crystal grain size is 25 μm or less, and the aspect ratio is 4 or more. 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 -20 ° C.

  On the other hand, as shown in Table 2, since the chemical composition of the comparative steels (symbols B1 to B21) was out of the range of the present invention, as shown in Table 4, the mechanical properties of the base material, the HIC resistance, The HAZ toughness of the part may be inferior. Symbols B22 and B23 indicate that the chemical components are within the scope of the present invention, but as described above, the production conditions outside the preferred range were adopted. Therefore, as shown in Table 4, the mechanical properties and HIC resistance of the base material were as shown in Table 4. In some cases, the HAZ toughness of the welded portion is inferior.

  Symbol B1 has too low C amount, and symbol B4 has insufficient strength because Mn amount is too low. The reference B2 has an excessively high C content, so that the HIC resistance deteriorates and the HAZ toughness also tends to deteriorate. The symbol B3 indicates that the HAZ toughness is deteriorated because the amount of Si is too high. The symbol B5 has a deteriorated 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 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 25 μm and the BDWTT characteristics and the HAZ toughness are deteriorated. Symbol B9 indicates that the effective crystal grain size exceeds 25 μm because the amount of Al is too high, and the BDWTT characteristic is deteriorated. The reference B11 has an effective crystal grain size exceeding 25 μm because the Ti content is too high, and the BDWTT characteristics and the HAZ toughness are deteriorated.

  In the case of symbol B12, the HIC resistance deteriorates because the Nb content is too high, 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 symbol B15 indicates that the Mg content is too low, the effective crystal grain size exceeds 25 μm, and the BDWTT characteristics and the HAZ toughness are deteriorated.

  Symbol B16 has an effective crystal grain size exceeding 25 μm because the N content is too low, and the BDWTT characteristics and the HAZ toughness are deteriorated. The symbol B17 has a tendency that the HIC resistance is deteriorated because the N amount is too high, and the HAZ toughness is deteriorated. Since the O content is too low, the effective crystal grain size of B18 exceeds 15 μm and becomes coarse, and the BDWTT characteristics 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 equation (1), which is an index of sulfide form control, is too low, so that the HIC resistance is degraded. Symbol 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.

  Symbol B22 is an example in which the cumulative rolling reduction in the temperature range of Ar3 or higher to Ar3 + 50 ° C. is low, and the aspect ratio of the effective crystal grains is too low, so that the separation index is low and the BDWTT characteristics are degraded. Symbol B23 is an example in which the stop temperature of the accelerated cooling is high, and the bainite area ratio is too low, so that the strength is insufficient and the HIC resistance is deteriorated.

  Further, the steel pipes according to the present invention shown in Tables 1 and 3 are formed by U-press and O-press, and a one-pass inner surface and a one-pass outer surface are subjected to submerged arc welding at the joint, and the steel pipe is expanded by a UOE process. Was manufactured. Then, as a result of evaluating the above characteristics, it was confirmed that all the steel pipes had almost the same performance as in Table 3.

  The present invention relates to a steel pipe such as a line pipe having excellent HIC resistance and strength of API 5L X80 class, and having a significantly lower low-temperature toughness than before, and a steel sheet suitable for a material of the steel pipe. Yes, in the steel industry, it is desirable to apply to thick plate mills. Further, the steel pipe according to the present invention has a base material and a welded portion, and uses the steel plate according to the present invention as a base material, and is preferably manufactured by a UOE process or a roll bending method.

Claims (4)

In mass%,
C: 0.020% or more, 0.100% or less,
Mn: 1.00% or more and 1.80% 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 is composed of Fe and unavoidable impurities, and the contents of Ca, O, and S satisfy the following expression (1), and are 1/1 of the plate thickness from the surface in the plate thickness direction. area ratio of bainite in the 4 position is not less than 80%, and 25μm or less the average value of the circle-phase duty diameter in the effective crystal grain size state, and are the average value of the aspect ratio of 4 or more in the effective crystal grains,
The board thickness is 31mm or more and 50mm or less,
Yield stress is 555MPa or more, 705MPa or less,
Tensile strength is 625 MPa or more and 825 MPa or less,
In the effective area of the BDWTT fracture surface where the ductile fracture ratio is 85% or more, the maximum value of the separation index evaluated as a value obtained by dividing the total (mm) of the separation length by the effective area (mm ² ) is 0.02 mm. ^-1 or more
Sour steel sheet, characterized in der Rukoto.
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,
3. The sour-resistant steel sheet according to claim 1, wherein the steel sheet contains one or both of Zr and 0.005% or less, and satisfies the following expression (2) instead of the expression (1). 4.
1.0 ≦ [(Ca + 3.5 × REM + 2.3 × Zr) × (1-124 × O)] / (1.25 × S) ≦ 8.0 (2) A sour-resistant steel pipe comprising a base material and a welded part, wherein the base material is made of the sour-resistant steel plate according to any one of claims 1 to 3 .