CN115210396A - Steel pipe and steel plate - Google Patents

Abstract

A steel pipe having a base material portion and a welded portion, the base material portion having a predetermined chemical composition, and the metal structure of the base material portion ranging from the surface to a depth of 1 mm, that is, the surface layer portion is selected from polygonal iron. One or more kinds of element body, granular bainite, acicular ferrite, and bainite are constituted, the highest hardness in the surface layer part of the base material part is 250HV or less, the yield stress is 415-630MPa, and the stress The proportional limit in the strain curve is above 90% of the yield stress.

Description

Steel pipe and steel plate

technical field

The present invention relates to steel pipes and steel plates. In particular, the present invention relates to welded steel pipes for line pipes and steel sheets suitable as raw materials thereof.

Background technique

A system installed on the ground, on the bottom of the sea, etc. and transporting oil and gas is called a line pipe. The steel pipes for pipelines constituting such pipelines are called line pipes. Among large-diameter line pipes having a pipe diameter of 508 mm or more that constitute long-distance pipelines, straight seam arc welded steel pipes (hereinafter referred to as arc welded steel pipes, welded steel pipes, or steel pipes) are widely used. Here, a straight seam arc welded steel pipe is a steel pipe produced by forming a thick steel plate into a cylindrical open pipe, and welding a butt portion (seam portion) by an arc welding method such as submerged arc welding. Depending on the forming method, it is also sometimes referred to as UOE steel pipe and JCOE steel pipe.

In recent years, the construction of pipelines has been expanded to areas with harsh environments such as cold areas and acid environments. Here, the acid environment means an oxidized humid hydrogen sulfide environment containing corrosive gas H ₂ S. It is known that hydrogen induced cracking (HIC) sometimes occurs if linepipe is exposed to an acid environment. On the other hand, sulfide stress cracking (SSC) may occur in oil country tubular goods having a strength higher than that of line pipes. However, even in a line pipe, if the partial pressure of hydrogen sulfide becomes high or the stress becomes high, SSC may occur. As described above, in addition to HIC resistance properties, SSC resistance properties are also required for line pipes used in severe acid environments (acid-resistant line pipes).

In Patent Document 1 and Non-Patent Document 2, based on the knowledge that the hardness affects the acid resistance, a welded steel pipe or a steel sheet for the steel pipe having excellent acid resistance with the hardness of the base metal part and the welded part being 220 Hv or less has been proposed.

In addition, Patent Document 2 proposes a high-strength steel sheet for acid-resistant line pipe in which, in mass %, a CP value (=4.46[%C]+2.37[%Mn]), which is an index representing the hardness of the center segregation portion /6+(1.74[%Cu]+1.7[%Ni])/15+(1.18[%Cr]+1.95[%Mo]+1.74[%V])/5+22.36[%P]) is 1.0 or less , the steel structure is a bainite structure, the variation ΔHV of the hardness in the thickness direction is 30 or less, and the variation ΔHV of the hardness in the width direction is 30 or less.

Patent Document 3 proposes a high-strength steel sheet for acid-resistant line pipe excellent in material uniformity within the steel sheet, wherein the metallographic structure is a bainite structure, the variation in hardness in the sheet thickness direction is ΔHv ₁₀ 25 or less, and the sheet width is ΔHv 10 25 or less. The deviation of the hardness in the direction is ΔHv ₁₀ 25 or less, and the maximum hardness of the surface layer portion of the steel sheet is Hv ₁₀ 220 or less.

Furthermore, Patent Document 4 proposes a quenched and tempered steel sheet excellent in resistance to hydrogen-induced cracking, wherein the metallographic structure in the range of 1 mm in the thickness direction from the surface of the steel sheet is selected from tempered martensite, tempered One or two types of fire bainite, in the metallographic structure within the range of ±1mm in the plate thickness direction from the center of the plate thickness, selected from tempered martensite and tempered bainite The main phase composed of one or two kinds of the main phase is 80% or more in area ratio, and the remainder other than the main phase is composed of one or more kinds selected from ferrite, pearlite, cementite, and retained austenite. In addition, the hardness at a position 1 mm from the surface of the steel plate in the thickness direction is 250 HV or less in Vickers hardness, and the difference in hardness between a position 1 mm from the surface of the steel plate and the center of the plate thickness is 60 HV or less in Vickers hardness.

The steel sheets of Patent Documents 1 to 4 and Non-Patent Document 2 satisfy acid resistance in an environment where the partial pressure of hydrogen sulfide is 0.1 MPa (1 bar) or less and the load stress is 90% or less of the yield stress. However, recently, the use environment of oil country tubular goods and line pipes has become more severe, and the required level of acid resistance of welded steel pipes for line pipes has become higher.

Conventionally, acid resistance in an environment with a hydrogen sulfide partial pressure of 0.1 MPa (1 bar) or less has been required, but recently, a material design that can withstand a high-pressure hydrogen sulfide environment exceeding 0.1 MPa has been required. Furthermore, conventionally, the load stress was 90% or less of the yield stress, but recently, a material design that can withstand a high-pressure hydrogen sulfide environment under a load stress exceeding 90% of the yield stress is required.

According to the study of the present inventors, the steel sheets of Patent Documents 1 to 4 and the steel sheet of Non-Patent Document 2 have insufficient acid resistance in an environment where the hydrogen sulfide partial pressure exceeds 0.1 MPa (1 bar) and exceeds 90% of the yield stress.

In order to solve such a problem, Patent Document 5 discloses a 30 steel having HIC resistance equal to or higher than that of conventional steel, a yield strength of 350 MPa or more, and containing hydrogen sulfide whose partial pressure exceeds 0.1 MPa. A steel pipe with excellent SSC resistance that does not cause cracks even under a stress of 90% or more of the yield strength in an environment of ℃ or lower.

However, Patent Document 5 shows that the load stress in the sulfide stress corrosion cracking test is excellent in SSC resistance of 90% of the yield stress, but does not show that the load stress exceeds 90% of the yield stress.

prior art literature

Patent Literature

Patent Document 1: Japanese Patent Laid-Open No. 2011-017048

Patent Document 2: Japanese Patent Application Laid-Open No. 2012-077331

Patent Document 3: Japanese Patent Laid-Open No. 2013-139630

Patent Document 4: Japanese Patent Laid-Open No. 2014-218707

Patent Document 5: Japanese Patent No. 6369658

Non-patent literature

Non-Patent Document 1: Nippon Steel & Sumitomo Metal Technical Report No. 397 (2013), p. 17-22

Non-Patent Document 2: JFE Technical Report No. 9 (August 2005), p. 19-24

SUMMARY OF THE INVENTION

As described above, the use environment of recent line pipes has become more severe, and the level of demand for acid resistance of welded steel pipes for line pipes has become more advanced. Therefore, an object of the present invention is to provide a welded steel pipe, particularly a straight seam arc welded steel pipe, and a steel sheet (particularly a thick steel sheet) serving as a raw material thereof, which can be used in a severe high-pressure hydrogen sulfide environment and is excellent in acid resistance.

More specifically, the object of the present invention is to provide HIC resistance equivalent to or higher than that of conventional steels, a yield stress of 350 MPa or more, and under an environment of 30° C. or less containing hydrogen sulfide exceeding 0.1 MPa, even if the load exceeds A steel pipe having excellent SSC resistance, and a steel sheet serving as a raw material thereof, are excellent in SSC resistance, which does not cause cracks even when the stress is 90% of the yield stress, specifically, the stress exceeds 95% of the yield stress.

The present invention has been made in order to solve the above-mentioned problems, and has the following steel pipes and steel sheets as the gist.

(1) A steel pipe according to an aspect of the present invention is a steel pipe having a base material portion and a welded portion, and the chemical composition of the base material portion includes, in mass %, C: 0.030 to 0.100%, Si: 0.50% or less, Mn : 0.80 to 1.60%, P: 0.020% or less, S: 0.0030% or less, Al: 0.060% or less, Ti: 0.001 to 0.030%, Nb: 0.006 to 0.100%, N: 0.0010 to 0.0080%, Ca: 0.0005 to 0.0050 %, O: 0.0050% or less, Cr: 0 to 1.00%, Mo: 0 to 0.50%, Ni: 0 to 1.00%, Cu: 0 to 1.00%, V: 0 to 0.10%, Mg: 0 to 0.0100%, REM: 0 to 0.0100%, the balance is Fe and impurities, ESSP represented by the following formula (i) is 1.5 to 3.0, Ceq represented by the following formula (ii) is 0.20 to 0.50, and the base material portion has The range from the surface to the depth of 1 mm, that is, the metallographic structure of the surface layer portion is composed of one or more selected from the group consisting of polygonal ferrite, granular bainite, acicular ferrite, and bainite. The highest hardness in the surface layer portion is 250 HV or less, the yield stress is 415 to 630 MPa, and the proportional limit in the stress-strain curve is 90% or more of the aforementioned yield stress.

ESSP=Ca×(1-124×O)/(1.25×S)...(i)

Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5...(ii)

Here, the symbol of each element in the formula represents the content (mass %) of each element contained in the steel, and is set to zero when not contained.

(2) In the steel pipe described in (1) above, in the metal structure of the surface layer portion of the base material portion, the total area ratio of granular bainite, acicular ferrite, and bainite may exceed 80%.

(3) The steel sheet according to the above (1) or (2), wherein the chemical composition of the base material portion may contain a chemical composition selected from the group consisting of Cr: 0.10 to 1.00%, Mo: 0.03 to 0.50%, and Ni: 0.10 to 0.10% by mass. One or more of 1.00%, Cu: 0.10-1.00%, V: 0.005-0.10%, Mg: 0.001-0.0100%, and REM: 0.001-0.0100%.

(4) The steel pipe according to any one of the above (1) to (3), wherein the chemical composition of the base metal portion includes Nb: 0.01 to 0.04% in mass %, and the welded portion includes a welding heat-affected zone and The weld metal part, the metal structure of the surface layer part in the welding heat-affected zone contains one or more kinds selected from bainite and acicular ferrite, and the highest hardness of the surface layer part in the welding heat-affected zone is Below 250HV, the angle of the weld toe portion on the inner side of the steel pipe is in the range of 130 to 180°.

(5) The steel pipe according to any one of (1) to (4) above, wherein the base material portion may have a thickness of 10 to 40 mm and a pipe diameter of 508 mm or more.

(6) A steel sheet according to another aspect of the present invention is used for the base material portion of the steel pipe according to any one of (1) to (5).

According to the above-mentioned aspect of the present invention, it is possible to provide a steel pipe having excellent SSC resistance, which does not cause cracks even when a stress exceeding 90% of the yield stress is applied in an environment of 30° C. or lower containing hydrogen sulfide exceeding 0.1 MPa, and can A steel sheet used as its raw material.

Moreover, according to the preferable aspect of this invention, the steel pipe which has the weld part excellent in acid resistance which can be used in a severe high pressure hydrogen sulfide environment can be provided.

Description of drawings

FIG. 1 is a schematic diagram for explaining the angle of the weld toe portion of the steel pipe according to the present embodiment.

FIG. 2 is a schematic diagram showing a portion where a sample is cut out from the steel pipe according to the present embodiment.

Detailed ways

In order to study a method for solving the above-mentioned problems, the present inventors conducted a test in a high-pressure hydrogen sulfide environment exceeding 0.1 MPa (for example, in a H ₂ S saturated solution containing 5% common salt and acetic acid) and a load stress exceeding 90% The cross-section, structure, etc. of the base metal part and the welded part of the cracked steel pipe were observed. Furthermore, the stress-strain curve of the steel pipe was also investigated. As a result, the following findings were obtained.

(a) In order to improve the acid resistance in a high-pressure hydrogen sulfide environment exceeding 0.1 MPa or more, it is necessary to control not only the HIC resistance but also the SSC resistance. HIC occurs in the center segregation part which exists in the vicinity of the center part in the thickness direction of a steel pipe. On the other hand, SSC depends on the structure and hardness in the range of 1 mm (surface layer part) from the surface of the steel pipe, which has not been considered in the past.

(b) When the metal structure of the surface layer portion is mainly composed of one or more kinds selected from the group consisting of polygonal ferrite, granular bainite, acicular ferrite, and bainite, the highest hardness is If it is 250HV or less, the acid resistance is improved. In addition, when the total area ratio of one or more kinds selected from granular bainite, acicular ferrite, and bainite exceeds 80%, the SSC property is further improved.

(c) When controlling the structure of the surface layer portion as described above, it is important to strictly control the cooling mode in addition to controlling the carbon equivalent Ceq to 0.20 to 0.50.

(d) When the manufacturing method of the hot-rolled steel sheet premised on coiling is applied, the cooling rate after the accelerated cooling is stopped is slower than the cooling rate of the standing cooling. In this case, although the variation in hardness is reduced, the structure and/or hardness of the surface layer portion described above cannot be obtained. Therefore, in order to obtain the above-mentioned structure and hardness of the surface layer portion, it is necessary to manufacture through a thick plate process.

(e) By appropriately controlling the hardness of the weld heat-affected zone and the shape of the weld toe (see FIG. 1 ), the stress concentration at the weld toe is alleviated, thereby improving the SSC resistance of the weld.

The present invention has been accomplished based on the above findings.

Hereinafter, the steel pipe according to one embodiment of the present invention (the steel pipe according to the present embodiment) and the steel sheet for the steel pipe (the steel sheet according to the present embodiment) will be described.

The steel pipe according to the present embodiment is a welded steel pipe having a base material portion and a welded portion. The base material portion has a cylindrical shape, and the welded portion extends in a direction parallel to the axial direction of the steel pipe. The welded portion includes a weld metal portion, which is a metal portion that melts and solidifies during welding, and a welding heat-affected zone, which is a portion of the metal that is not melted during welding but due to heat input and heat from welding. Subsequent cooling results in a region where the tissue and the like have changed.

In addition, the steel sheet according to the present embodiment is used for the base material portion of the above-mentioned steel pipe. That is, as will be described later, the above-mentioned steel pipe can be obtained by forming the above-mentioned steel sheet into a cylindrical shape, butt-butting and welding both ends of the above-mentioned steel sheet. Therefore, the chemical composition, metallographic structure, and mechanical properties of the steel sheet are the same as those of the base material portion of the steel pipe. Therefore, the following description about the base material portion of the steel pipe according to the present embodiment is also applicable to the steel sheet according to the present embodiment.

1. Chemical composition

The reason for the limitation of each element is as follows. In the following description, "%" about content means "mass %".

1-1. Chemical composition of the base metal part (steel plate) of the steel pipe

The chemical composition of the base material portion of the steel pipe according to the present embodiment (the steel sheet according to the present embodiment) will be described.

C: 0.030～0.100%

C is an element which improves the strength of steel. When the C content is less than 0.030%, the strength improvement effect cannot be sufficiently obtained. Therefore, the C content is made 0.030% or more. Preferably it is 0.035% or more.

On the other hand, when the C content exceeds 0.100%, the hardness of the surface layer portion becomes high, and SSC tends to occur. In addition, carbides are formed, and HIC is likely to occur. Therefore, the C content is made 0.100% or less. The C content is preferably 0.070% or less, and more preferably 0.060% or less, in order to ensure more excellent SSC resistance and HIC resistance and to suppress the decrease in weldability and toughness.

Si: 0.50% or less

When the Si content exceeds 0.50%, the toughness of the welded portion decreases. Therefore, the Si content is made 0.50% or less. It is preferably 0.35% or less, and more preferably 0.30% or less. The lower limit of the Si content includes 0%.

On the other hand, Si is unavoidably mixed from the steel raw material and/or in the steelmaking process, so 0.01% is the substantial lower limit of the Si content in practical steel. In addition, Si may be added for deoxidation, and in this case, the lower limit of the Si content can be made 0.10%.

Mn: 0.80～1.60%

Mn is an element which improves the strength and toughness of steel. When the Mn content is less than 0.80%, these effects cannot be sufficiently obtained. Therefore, the Mn content is made 0.80% or more. The Mn content is preferably 0.90% or more, and more preferably 1.00% or more.

On the other hand, when the Mn content exceeds 1.60%, the acid resistance decreases. Therefore, the Mn content is made 1.60% or less. Preferably it is 1.50% or less.

P: 0.020% or less

P is an element inevitably contained as an impurity. When the P content exceeds 0.020%, the HIC resistance decreases, and the toughness of the welded portion decreases. Therefore, the P content is made 0.020% or less. It is preferably 0.015% or less, and more preferably 0.010% or less. The P content is preferably small, and the lower limit includes 0%. However, if the P content is reduced to less than 0.001%, the manufacturing cost will be greatly increased. Therefore, in practical steel, 0.001% is the substantial lower limit of the P content.

S: 0.0030% or less

S is an element inevitably contained as an impurity. In addition, S is an element which forms MnS extending in the rolling direction during hot rolling and reduces HIC resistance. When the S content exceeds 0.0030%, the HIC resistance is remarkably lowered, so the S content is made 0.0030% or less. It is preferably 0.0020% or less, and more preferably 0.0010% or less. The lower limit includes 0%. However, when the S content is reduced to less than 0.0001%, the manufacturing cost will be greatly increased, so 0.0001% is a substantial lower limit in a practical steel sheet.

Al: 0.060% or less

When the Al content exceeds 0.060%, clusters in which Al oxides are accumulated are formed, and the HIC resistance decreases. Therefore, the Al content is made 0.060% or less. It is preferably 0.050% or less, more preferably 0.035% or less, still more preferably 0.030% or less. The Al content is preferably small, and the lower limit of the Al content includes 0%.

On the other hand, Al is inevitably mixed in from the steel raw material and/or in the steelmaking process, so 0.001% is the substantial lower limit of the Al content in practical steel. In addition, Al may be added for deoxidation, and in this case, the lower limit of the Al content may be set to 0.010%.

Ti: 0.001～0.030%

Ti is an element that forms carbonitrides and contributes to the refinement of crystal grains. When the Ti content is less than 0.001%, this effect cannot be sufficiently obtained. Therefore, the Ti content is made 0.001% or more. It is preferably 0.008% or more, and more preferably 0.010% or more.

On the other hand, when the Ti content exceeds 0.030%, carbonitrides are excessively generated, and the HIC resistance and toughness decrease. Therefore, the Ti content is made 0.030% or less. It is preferably 0.025% or less, and more preferably 0.020% or less.

Nb: 0.006～0.100%

Nb is an element that forms carbides and/or nitrides and contributes to increasing strength. When the Nb content is less than 0.006%, these effects cannot be sufficiently obtained. Therefore, the Nb content is made 0.006% or more. It is preferably 0.008% or more, and more preferably 0.010% or more. In particular, when securing the hardness of the welding heat-affected zone, the Nb content is preferably 0.010% or more, more preferably 0.015% or more, and still more preferably 0.017% or more.

On the other hand, when the Nb content exceeds 0.100%, the carbonitride of Nb accumulates in the central segregation portion, and the HIC resistance decreases. Therefore, the Nb content is made 0.100% or less. It is preferably 0.080% or less, and more preferably 0.060% or less.

In addition, in order to improve the toughness of the welded portion (weld heat affected zone and weld metal portion), the Nb content is preferably 0.040% or less, more preferably 0.035% or less, and still more preferably 0.033% or less.

N: 0.0010～0.0080%

N is an element that combines with Ti and/or Nb to form nitrides and contributes to the refinement of the austenite grain size during heating. When the N content is less than 0.0010%, the above effects cannot be sufficiently obtained. Therefore, the N content is made 0.0010% or more. Preferably it is 0.0020% or more.

On the other hand, when the N content exceeds 0.0080%, the nitrides of Ti and/or Nb will accumulate and the HIC resistance will decrease. Therefore, the N content is made 0.0080% or less. It is preferably 0.0060% or less, and more preferably 0.0050% or less.

Ca: 0.0005～0.0050%

Ca is an element which suppresses the formation of MnS elongated in the rolling direction by forming CaS in steel, and contributes to the improvement of HIC resistance as a result. If the Ca content is less than 0.0005%, the above-mentioned effects cannot be sufficiently obtained. Therefore, the Ca content is made 0.0005% or more. It is preferably 0.0010% or more, and more preferably 0.0015% or more.

On the other hand, when the Ca content exceeds 0.0050%, oxides accumulate and the HIC resistance decreases. Therefore, the Ca content is made 0.0050% or less. It is preferably 0.0045% or less, and more preferably 0.0040% or less.

O: 0.0050% or less

O is an element that inevitably remains. When the O content exceeds 0.0050%, oxides are formed and the HIC resistance decreases. Therefore, the O content is made 0.0050% or less. From the viewpoint of securing the toughness of the steel sheet and the toughness of the welded portion, it is preferably 0.0040% or less, and more preferably 0.0030% or less. The smaller the O content, the more preferable it is, and it may be 0%. However, when O is reduced to less than 0.0001%, the manufacturing cost increases significantly. Therefore, the O content may be 0.0001% or more. From the viewpoint of production cost, it is preferably 0.0005% or more.

Cr: 0～1.00%

Mo: 0～0.50%

Ni: 0～1.00%

Cu: 0～1.00%

V: 0～0.10%

Cr, Mo, Ni, Cu, and V are elements that improve the hardenability of steel. Therefore, one or more kinds selected from these elements may be contained as necessary.

In order to obtain the above effects, it is preferable to contain at least one selected from the group consisting of Cr: 0.10% or more, Mo: 0.03% or more, Ni: 0.10% or more, Cu: 0.10% or more, and V: 0.005% or more.

On the other hand, when the content of Cr, Ni and Cu exceeds 1.00%, the Mo content exceeds 0.50%, or the V content exceeds 0.10%, the hardness increases and the acid resistance decreases. Therefore, the contents of Cr, Ni, and Cu are all 1.00% or less, the Mo content is 0.50% or less, and the V content is 0.10% or less. Preferably, Cr: 0.50% or less, Mo: 0.40% or less, Ni: 0.50% or less, Cu: 0.50% or less, and V: 0.06% or less.

Mg: 0～0.0100%

REM: 0～0.0100%

Mg and REM are elements that control the sulfide morphology. In order to obtain the above-mentioned effects, it is preferable to contain one or two kinds selected from the group consisting of Mg: 0.001% or more and REM: 0.001% or more.

On the other hand, when the contents of Mg and REM exceed 0.0100%, respectively, the sulfides become coarse and their effects cannot be exhibited. Therefore, the contents of Mg and REM are both set to 0.0100% or less. Preferably it is 0.0050% or less.

Here, REM is a rare earth element, and is a general term for 16 elements in total of Sc and lanthanoids, and the REM content means the total content of these elements.

In the above chemical composition, the balance is Fe and impurities. Here, the term “impurities” refers to components mixed in by raw materials such as ores and scraps, and various factors in the production process during industrial steel production, and refers to components allowed within a range that does not adversely affect the present invention.

When Sb, Sn, Co, As, Pb, Bi, H, W, Zr, Ta, B, Nd, Y, Hf, and Re are contained as impurities, the respective contents are preferably controlled within the ranges described later.

Sb: 0.10% or less

Sn: 0.10% or less

Co: 0.10% or less

As: 0.10% or less

Pb: 0.005% or less

Bi: 0.005% or less

H: 0.0005% or less

Sb, Sn, Co, As, Pb, Bi, and H may be mixed from the steel raw material as impurities or unavoidable mixed elements, but as long as they are within the above-mentioned ranges, the characteristics of the steel pipe according to the present embodiment are not impaired. Therefore, these elements are preferably limited to the above-mentioned ranges.

W, Zr, Ta, B, Nd, Y, Hf and Re: 0.10% or less in total

These elements may be mixed from the steel raw material as impurities or unavoidable mixed elements, but as long as they are within the above-mentioned ranges, the characteristics of the steel pipe according to the present embodiment are not impaired. Therefore, the total content of these elements is limited to 0.10% or less.

The chemical composition of the base material part needs to satisfy predetermined conditions for the values of ESSP and Ceq calculated from the content of the components, as shown below, except that the content of each element is within the above-mentioned range.

ESSP: 1.5～3.0

ESSP is a value indicating whether or not there is an effective amount of Ca in an amount commensurate with the S content on the premise that the remaining Ca (effective Ca) after deducting Ca bound to oxygen is bound to S at an atomic weight ratio. (i) expression. In the steel pipe according to the present embodiment, in order to ensure HIC resistance equal to or higher than that of conventional steel, it is necessary to set the value of ESSP in the range of 1.5 to 3.0.

ESSP=Ca×(1-124×O)/(1.25×S)...(i)

Here, the symbol of each element in the formula represents the content (mass %) of each element contained in the steel, and is set to zero when not contained.

In order to secure HIC resistance, it is effective to suppress the generation of MnS extending in the rolling direction. In addition, in order to suppress the generation of MnS extending in the rolling direction, it is an effective method to reduce the S content and add Ca to form CaS and fix S. On the other hand, Ca has a stronger oxygen affinity than S, so in order to form a necessary amount of CaS, the O content needs to be reduced.

When ESSP is less than 1.5, the Ca content is insufficient with respect to the O content and the S content, and MnS is generated. MnS stretched during rolling causes deterioration of HIC resistance, so ESSP is set to 1.5 or more. It is preferably 1.6 or more, and more preferably 1.7 or more.

On the other hand, when the Ca content becomes excessive, a large amount of cluster inclusions is generated, and there is a concern that the control of the morphology of MnS is hindered. When the O content and the S content are reduced, the formation of cluster inclusions can be suppressed, but when the ESSP exceeds 3.0, the manufacturing cost for reducing the O content and the S content is significantly increased. Therefore, ESSP is set to 3.0 or less. It is preferably 2.8 or less, and more preferably 2.6 or less.

When the value of ESSP is in the range of 1.5 to 3.0, the effective Ca amount is adjusted to be equal to or more than the minimum amount necessary for controlling the morphology of MnS and equal to or less than the critical amount that does not generate cluster inclusions, so that excellent HIC resistance can be obtained. characteristic.

Ceq: 0.20～0.50

Ceq means a value of carbon equivalent that is an index of hardenability, and is represented by the following formula (ii). In the base material portion of the steel pipe according to the present embodiment, as described later, in order to obtain at the surface layer portion at least one selected from the group consisting of polygonal ferrite, granular bainite, acicular ferrite, and bainite It is preferable to obtain a metallographic structure containing more than 80% in total of one or more selected from granular bainite, acicular ferrite, and bainite, and it is necessary to appropriately control the hardenability of the steel. Therefore, the value of Ceq needs to be 0.20 to 0.50.

Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5...(ii)

Here, the symbol of each element in the formula represents the content (mass %) of each element contained in the steel, and is set to zero when not contained.

When Ceq is less than 0.20, a tensile strength of 530 MPa or more cannot be obtained. Therefore, Ceq is set to 0.20 or more. Preferably it is 0.25 or more. On the other hand, when Ceq exceeds 0.50, the surface hardness of a welded part will become high, and acid resistance will fall. Therefore, Ceq is set to 0.50 or less. Preferably it is 0.45 or less.

1-2. Chemical composition of welded part

The welding heat-affected zone is a portion where the base metal portion is not melted even by welding. Therefore, its chemical composition is the same as that of the base material part, and the reason for limitation is also the same.

On the other hand, the chemical composition of the weld metal portion in the welded portion is not particularly limited. However, in order to increase the strength of the weld metal portion to a level equal to or greater than the strength of the base metal portion, it is preferable to set the chemical composition of the weld metal portion to the following range.

That is, the chemical composition of the weld metal portion in the welded portion is preferably C: 0.02 to 0.20%, Si: 0.01 to 1.00%, Mn: 0.1 to 2.0%, P: 0.015% or less, S: 0.0050 in terms of mass %. % or less, Cu: 1.0% or less, Ni: 1.0% or less, Mo: 1.0% or less, Cr: 0.1% or less, Nb: 0.5% or less, V: 0.3% or less, Ti: 0.05% or less, Al: 0.005 to 0.100 %, O: 0.010 to 0.070%, Cr: 0 to 1.00%, Ni: 0 to 1.00%, Cu: 0 to 1.00%, Mo: 0 to 0.50%, V: 0 to 0.10%, Mg: 0 to 0.01% , REM: 0 to 0.01%, balance: Fe and impurities.

The chemical composition of the weld metal portion is determined by the inflow ratio of the base metal and the welding material during welding. As the solder material, a commercially available material may be used, and for example, Y-D, Y-DM, Y-DMH wire, and flux of NF5000B or NF2000 can be used. In addition, in order to control the composition range in the said weld metal part, it is preferable to adjust welding conditions to the range mentioned later.

2. Metal organization

2-1. Metal structure of base metal

Next, the metallographic structure of the base material portion (steel plate) of the steel pipe will be described.

The metallographic structure in the surface layer portion of the base material portion is a structure composed of at least one selected from the group consisting of polygonal ferrite, granular bainite, acicular ferrite, and bainite. In this embodiment, the surface layer portion means a range up to 1.0 mm from the surface of the base material portion.

In the steel pipe according to the present embodiment, in order to suppress the maximum hardness of the surface layer part of the base metal part to 250HV or less, and to ensure required strength and excellent acid resistance, the metal structure in the surface layer part is made of polygonal iron. A structure composed of at least one of element body, granular bainite, acicular ferrite, and bainite. Preferably, the total area ratio of one or more kinds selected from granular bainite, acicular ferrite, and bainite exceeds 80%. When the above-mentioned total area ratio exceeds 80%, the strength and acid resistance are further improved. More preferably, it is 85% or more.

The measurement of the area ratio of each structure is obtained by observing, with a scanning electron microscope (SEM), a metal structure developed by etching with a mixed solution of 3% nitric acid and 97% ethanol or the like. The structure of the surface layer portion may be measured by taking a position 0.5 mm from the surface of the steel sheet as a representative.

The metallographic structure of the surface layer part in the base metal part refers to the metallographic structure of the base metal part which is not affected by welding. In the steel pipe according to the present embodiment, it refers to the metal of the surface layer portion at positions of 90°, 180°, and 270° in the circumferential direction of the steel pipe from the butted portion (the seam portion, which corresponds to the end portion in the width direction of the steel sheet). organization, etc. The above-mentioned positions correspond to the metal structure of the surface layer portion at the positions of 1/4, 1/2, and 3/4 in the width direction of the steel sheet in the steel sheet.

In the present embodiment, polygonal ferrite is a structure observed as a massive structure without coarse precipitates such as coarse cementite and MA in grains, and acicular ferrite is prior austenite The grain boundaries of the body are unclear, and a structure of needle-like ferrite (without carbides and austenite-martensite components) is generated in the grains with random (random) crystallographic orientation.

On the other hand, the so-called worked ferrite refers to ferrite that has been worked, and grains flattened in the rolling direction are observed in optical microscope and SEM observation. Flattening means that the aspect ratio (the ratio of the ferrite length in the rolling direction to the ferrite length in the sheet thickness direction) is 2.0 or more. In addition, the so-called pearlite is a structure in which ferrite and cementite are layered, and a structure in which cementite forming a layer is broken in the middle of pearlite is quasi-pearlite (suspected pearlite: Suspected パーライト).

With regard to retained austenite, the structure that appeared white using the corrected Lepera liquid was determined as retained austenite.

Granular bainite is formed at an intermediate transformation temperature between acicular ferrite and bainite, and has intermediate texture characteristics. It is a structure in which the prior austenite grain boundary can be partially seen, and the following two parts are mixed. The two parts are a thick lath structure in the grain, and the lath is scattered in the lath and between the laths. Parts of carbides and austenite-martensite constituents, and parts of acicular or amorphous ferrite with unclear prior austenite grain boundaries.

Bainite and martensite are structures with well-defined prior austenite grain boundaries and well-developed intragranular lath structures. Bainite and martensite cannot be easily distinguished by SEM observation. However, in this embodiment, the prior austenite grain boundary is clear, the lath structure with fine grains is developed, and the hardness is 250Hv or more. It is martensite, and a structure with a well-defined prior austenite grain boundary, a fine lath structure within the grain, and a hardness of less than 250 Hv is regarded as bainite. Whether the hardness is 250Hv or more or less than 250Hv is determined by measuring the target structure at 10 points using a micro-Vickers hardness tester with a load of 100gf, and determining whether the maximum value is 250Hv or less than 250Hv. All the structures are tempered during reheating and heat treatment of steel pipes, but there is no special distinction between tempering and not.

In the steel pipe according to the present embodiment, the structure other than the surface layer portion is not particularly limited. However, when the structure of the surface layer portion is controlled as described above by the production method described later, it is preferable that the structure other than the surface layer portion, for example, the structure of the thickness center portion (the thickness center portion of the steel sheet) does not contain processed ferrite It is mainly composed of body, pearlite (including quasi-pearlite), martensitic, acicular ferrite and bainite, and the highest hardness is below 250Hv.

2-2. Metal structure of welding heat affected zone

In the steel pipe according to the present embodiment, in order to form a similar metal structure in the entire steel pipe, the metal structure of the surface layer portion in the welded heat-affected zone preferably contains at least one selected from the group consisting of bainite and acicular ferrite. . In addition, the metal structure of the surface layer portion in the welding heat-affected zone is preferably a uniform structure, that is, a structure composed of bainite and/or acicular ferrite.

The weld metal portion preferably has a structure composed of acicular ferrite.

In order to make the welding heat-affected zone into the above-mentioned metal structure, the following conditions are desirable as welding conditions. For example, as the solder material, Y-D, Y-DM, Y-DMH wire, and flux of NF5000B or NF2000 are preferably used. In addition, it is preferable to perform inner surface welding and outer surface welding, and it is preferable to perform submerged arc welding using 3 electrodes on the inner surface and 4 electrodes on the outer surface. It is preferable to perform welding within the range of 2.0 kJ/mm to 10 kJ/mm depending on the thickness of the plate as for the heat energy at the time of welding.

Regarding the metal structure of the welded heat-affected zone, a sample including the weld metal portion was cut out from the welded portion of the steel pipe, and a sample for microstructure observation was produced. Then, observation was performed in the same manner as the base material portion.

3. Mechanical properties

Next, the mechanical properties of the steel pipe will be described.

3-1. Mechanical properties of base material

The highest hardness of the surface layer: 250HV or less

SSC occurs due to microscopic flaws or microcracks on the surface of the steel sheet, and therefore the metallographic structure and hardness of the surface layer portion, which is the source of occurrence of microscopic flaws and microcracks, are important.

In the steel pipe according to the present embodiment, in order to ensure excellent SSC resistance, the maximum hardness of the surface layer portion of the base material portion is 250HV or less after controlling the metal structure of the surface layer portion of the base material portion as described above. The maximum hardness of the surface layer portion is preferably 245 HV or less, and more preferably 240 HV or less.

The measurement of the highest hardness of the surface layer part was performed by the following method. First, from positions of 90°, 180°, and 270° from the welded portion in the circumferential direction of the steel pipe, specimens having an axial length of 20 mm and a circumferential length of 20 mm were prepared by mechanical cutting. In the case of a steel sheet, samples having a length of 20 mm and a width of 20 mm were taken from positions 1/4, 1/2, and 3/4 in the width direction of the steel sheet from the ends in the width direction.

Next, the above-mentioned sample was ground by mechanical grinding. Using a Vickers hardness tester (test force: 100 gf), the ground samples were measured at 10 points in the thickness direction at 0.1 mm intervals in the thickness direction, starting at 0.1 mm from the surface, and at 1 mm intervals in the width direction for the same depth By measuring 10 points, a total of 100 points were measured.

Furthermore, as a result of the above measurement, if two or more measurement points exceeding 250HV did not appear continuously in the plate thickness direction, it was determined that the maximum hardness of the surface layer portion was 250HV or less.

In the base material portion of the steel pipe, a high value (abnormal value) may locally appear due to inclusions or the like. However, since inclusions do not cause cracks, SSC resistance can be secured even if such an abnormal value occurs. On the other hand, when two or more measurement points exceeding 250HV continuously exist in the plate thickness direction, the SSC resistance is not caused by inclusions, and therefore is not allowed.

Therefore, in the present invention, even if there is one measurement point exceeding 250Hv, if two or more points do not appear continuously in the plate thickness direction, the point is regarded as an abnormal point and is not used, and the next highest value is taken as Highest hardness. On the other hand, when two or more measurement points exceeding 250 Hv existed continuously in the plate thickness direction, the hardness was made the highest hardness.

Proportional limit: more than 90% of yield stress

The present inventors have studied the SSC resistance in a more severe environment. As a result, it was found that SSC does not occur even when the load stress exceeds 90% (eg, 95%) of the yield stress when the proportional limit in the stress-strain curve is 90% or more of the yield stress.

When the proportional limit is less than 90% of the yield stress, in the case where the load stress in the sulfide stress corrosion cracking test is 90% of the actual yield stress, plastic deformation occurs and dislocations multiply. As a result, the intruded hydrogen during the sulfide stress corrosion test is trapped by the multiplying dislocations, and the amount of hydrogen increases, so that cracks occur. On the other hand, if the proportional limit is 90% or more of the yield stress, plastic deformation does not occur even if the yield stress exceeds 90%. Therefore, the proliferation of dislocations does not increase, and hydrogen does not accumulate there. Also, cracking can be prevented as a result.

As described above, since the proportional limit is 90% or more of the yield stress, the base material portion of the steel pipe according to the present embodiment (the steel sheet according to the present embodiment) is even in a solution environment containing 5% of common salt and acetic acid at 30°C or lower. Loads exceeding 90% of the yield stress will not produce sulfide stress cracks. The proportional limit is more preferably 95% or more of the yield stress.

In this embodiment, the proportional limit is measured by the following procedure.

First, in accordance with API5L, a round bar tensile test specimen was prepared in a direction (C direction) perpendicular to the longitudinal direction of the steel pipe, and a tensile test was performed. The tensile test was performed under stroke control (tensile speed: 1 mm/min), the test force and displacement were measured at 0.05 second intervals, and the stress and strain at each measurement time were determined based on these. Then, the yield stress (YS) was obtained from the obtained stress-strain curve. As YS, when the yield point was not clearly confirmed, the conditional yield strength σ _0.2 was used.

Then, taking into account the measurement error, smoothing processing of the values of stress and strain is performed. Specifically, for each measurement time, an average value of the measurement time±2.50 seconds was calculated, and this value was used as the result at each measurement time. For example, as the value of stress and strain at 2.50 seconds, the average value of 101 measured values during the period of 0 to 5.00 seconds is employed.

Next, the inclination of the linear portion of the stress-strain curve after the smoothing process was performed was obtained. The slope of the linear portion was calculated by the least squares method using the value of the period during which the stress changed from 0.2 YS to 0.4 YS as a representative value.

Next, the slope of the stress-strain curve at each measurement time was calculated. Specifically, for each measurement time, the inclination was calculated by the least squares method from the value in the period of ±0.50 seconds of the measurement time. For example, the slope of the stress-strain curve at 60.00 seconds is calculated by the least squares method using 21 measured values during the period of 59.50 to 60.50 seconds.

Then, the value of the previous stress at which the slope of the stress-strain curve continues to be lower than 0.95 times the slope of the straight line portion is taken as the proportional limit. Even if the inclination of the stress-strain curve was once lower than 0.95 times the inclination of the above-mentioned straight line part on the way due to the influence of measurement errors, and again exceeded 0.95 times the inclination of the straight part, this value was not adopted.

Yield stress: above 415MPa

Tensile strength: above 530MPa

In order to secure the required strength in the steel pipe according to the present embodiment, the yield stress of the base material portion of the steel pipe according to the present embodiment is set to 415 MPa or more. Preferably it is 430 MPa or more. Regarding the upper limit of the yield stress, about 630 MPa prescribed by X70 of API5L is a substantial upper limit in terms of workability. In terms of workability, the yield stress is preferably 600 MPa or less.

In addition, in order to secure the required strength in the steel pipe according to the present embodiment, the tensile strength of the base material portion of the steel pipe according to the present embodiment is preferably 530 MPa or more. More preferably, it is 550 MPa or more. The upper limit of the tensile stress is not particularly limited, but from the viewpoint of workability, 690 MPa specified by X70 of API5L is a substantial upper limit. From the viewpoint of workability, it is preferably 650 MPa or less.

3-2. Mechanical properties of welded parts

Maximum hardness of the surface layer in the welding heat-affected zone: 250Hv or less

In the steel pipe according to the present embodiment, in order to ensure good SSC resistance, it is preferable that the maximum hardness of the surface layer portion in the welded heat-affected zone is 250 HV or less. The maximum hardness of the surface layer portion is more preferably 245 HV or less, and further preferably 240 HV or less.

On the other hand, in order to obtain the strength of X60 or more of the API standard, it is preferable to set the maximum hardness of the surface layer part in the welding heat-affected zone to 150 HV or more. The maximum hardness of the surface layer portion is more preferably 160 HV or more, and still more preferably 170 HV or more.

The highest hardness of the surface layer part in the welding heat-affected zone refers to the highest hardness measured in a region from the surface to a depth of 0.9 mm in the wall thickness direction. Regarding the highest hardness of the surface layer part in the welding heat-affected zone, a sample as shown in FIG. 2 was cut out, and the distance from the surface of the weld toe part (the boundary between the weld metal part and the base metal part) to the base metal part was The positions of 0.3 mm, 0.6 mm, and 0.9 mm were measured at 40 points at a pitch of 0.5 mm, so that a total of 120 points were measured, and the highest hardness was measured.

As a result of the above measurement, if two or more measurement points less than 150HV or more than 250HV did not appear continuously in the wall thickness direction, it was determined that the maximum hardness of the surface layer portion in the welding heat-affected zone was 150 to 250HV. The measurement of the hardness in this way is based on the same reason as the highest hardness of the surface layer part in the above-mentioned base material part.

4. Dimensions

Plate thickness: 10～40mm

Pipe diameter: 508mm (20 inches) or more

In the case of a steel pipe for drilling or a steel pipe for line pipe of oil, natural gas, etc., the plate thickness is preferably 10 to 40 mm, and the pipe diameter (outer diameter) is preferably 508 mm or more. The upper limit of the pipe diameter is not particularly limited, but 1422.4 mm (56 inches) or less is a substantial upper limit.

5. Angle of welding toe

In the steel pipe according to the present embodiment, in order to improve the SSC resistance of the welded portion, it is preferable to control the angle of the weld toe portion of the seam welded portion. In the present embodiment, the angle of the weld toe portion refers to an angle as shown in FIG. 1 . That is, the angle of the weld toe portion is the angle of the excess height front end portion of the weld metal portion, that is, the angle formed by the tangential direction of the weld metal and the surface of the base metal portion. Also known as the so-called flank angle.

In order to suppress SSC, the angle of the weld toe portion on the inner side of the steel pipe is preferably in the range of 130° to 180°. When the angle of the weld toe portion is less than 130° and is a sharper angle, strain is accumulated in the weld heat-affected zone, the penetration of hydrogen is promoted, and cracks are easily generated. In FIG. 1 , it is described that only the lower left angle is measured, but in the present embodiment, the left and right angles are measured, and the smaller angle is used as the angle of the weld toe (toe angle).

5. Manufacturing method

A preferred manufacturing method of the steel pipe according to the present embodiment and the steel sheet used as its raw material will be described.

The steel pipe according to the present embodiment can obtain the effects as long as it has the above-mentioned structure regardless of the manufacturing method, but it is preferable to use the following manufacturing method, for example, since it can be obtained stably.

The steel sheet according to the present embodiment can be obtained by a manufacturing method including the following steps.

(A) a hot rolling step, in which a steel slab having the above-mentioned predetermined chemical composition is heated to 1000 to 1250° C. for hot rolling, and the hot rolling is completed at a temperature of Ar ₃ or higher;

(B) The first cooling step of performing multi-stage accelerated cooling, in which the steel sheet after the hot rolling step is subjected to water cooling three or more times from a temperature of Ar ₃ or higher Water cooling such that the maximum attainable temperature due to reheating after water cooling exceeds 500°C; and

(C) After that, the second cooling step of cooling to a temperature of 500° C. or lower at an average cooling rate of 0.2° C./s or higher.

The steel pipe according to the present embodiment can be obtained by performing the following steps in addition to the steps (A) to (C).

(D) forming process of forming the above-mentioned steel sheet into a cylindrical shape;

(E) a welding process of butt joint and welding of both ends of the tubular steel sheet; and

(F) A heat treatment step in which the steel pipe obtained by welding is subjected to heat treatment under the conditions of a temperature range of 100 to 300° C. and a holding time of 1 minute or more.

About each process, preferable conditions are demonstrated.

(Hot rolling process)

A slab produced by casting molten steel having the same chemical composition as the base material portion of the steel pipe according to the present embodiment is heated to 1000 to 1250° C. for hot rolling. Casting of molten steel before hot rolling and production of a billet may be carried out in accordance with conventional methods.

During the rolling of the slab, when the heating temperature is lower than 1000°C, the deformation resistance does not decrease and the load on the rolling mill increases, so the heating temperature is set to 1000°C or higher. Preferably it is 1100 degreeC or more. On the other hand, when the heating temperature exceeds 1250°C, the crystal grains of the steel slab become coarse and the strength and toughness decrease, so the heating temperature is made 1250°C or lower. It is preferably 1210°C or lower.

The heated slab is hot-rolled at a temperature range of Ar ₃ or higher to obtain a steel sheet, and the hot-rolling is completed at a temperature of Ar ₃ or higher. When the hot-rolling temperature is lower than the Ar ₃ point, worked ferrite is formed in the structure of the steel sheet, and the strength is lowered. Therefore, the hot-rolling temperature is set to be equal to or higher than the Ar ₃ point.

(1st cooling process)

The hot-rolled steel sheet is subjected to accelerated cooling from a temperature of Ar ₃ or higher. At this time, multi-stage accelerated cooling is performed, and in the above-mentioned multi-stage accelerated cooling, the water cooling is performed twice or more in terms of surface temperature. The stop temperature is 500°C or less, and the maximum temperature reached by reheating after water cooling is stopped. Water cooling such as over 500°C. It is preferable to carry out 3 or more times.

It is important to increase the temperature difference between the surface and the interior in order to make the maximum attained temperature due to reheating exceed 500°C. The temperature difference between the surface and the inside can be adjusted by changing the water density and impact pressure in the water cooling.

When the maximum temperature reached by reheating is 500°C or lower, the hardness of the steel sheet, particularly the maximum hardness of the surface layer portion from the surface to a depth of 1 mm cannot be made 250HV or lower. In addition, even when the number of times of reheating exceeding 500° C. is less than 2 times, the maximum hardness of the surface layer portion cannot be made 250 HV or less. Therefore, accelerated cooling is performed so that the reheating of the maximum reached temperature to a temperature exceeding 500° C. is three or more times.

For the reason of not generating a hard phase, it is preferable that each water cooling cooling stop temperature in the multi-stage cooling is set to a temperature exceeding the Ms point.

In addition, when the water cooling stop temperature before reheating exceeds 500°C, a predetermined structure cannot be obtained, so the water cooling stop temperature is made 500°C or lower. The water cooling stop temperature is preferably 500°C or lower.

By performing reheating three or more times, the maximum hardness HVmax of the surface layer portion of the steel sheet from the surface to a depth of 1 mm is reduced to 250 HV or less. The number of times of reheating is the number of times until the maximum hardness HVmax of the surface layer portion becomes 250 HV or less, and therefore it is not necessary to specify the upper limit of the number of times of reheating.

(2nd cooling process)

In the first cooling step, after completion of three or more water cooling and reheating, cooling is performed to a temperature of 500° C. or lower at an average cooling rate of 0.2° C./s or higher. If the cooling rate is slowed by finishing cooling at a temperature exceeding 500°C, coiling, etc., and the average cooling rate until 500°C or lower is less than 0.2°C/s, the variation in hardness becomes small, but the above-mentioned cannot be obtained. The texture and/or hardness of the superficial portion.

(forming process and welding process)

The forming of the steel sheet into the steel pipe according to the present embodiment is not limited to a specific forming method. For example, warm working can also be used, but cold working is preferable from the viewpoint of dimensional accuracy.

After the steel sheet is formed into a cylindrical shape, both ends of the steel sheet are butted and arc welded (seam welding). Arc welding is not limited to specific welding, but submerged arc welding is preferable. In addition, the welding conditions may be performed under known conditions. For example, it is preferable to perform welding in a range of 2.0 to 10 kJ/mm in line energy according to the thickness of the plate using three electrodes or four electrodes. In order to make the welding heat-affected zone into the above-mentioned metal structure, for example, as the welding material, Y-D, Y-DM, Y-DMH wire, and flux of NF5000B or NF2000 are preferably used. In addition, it is preferable to perform inner surface welding and outer surface welding, and it is preferable to perform submerged arc welding using 3 electrodes on the inner surface and 4 electrodes on the outer surface.

(heat treatment process)

After that (after pipe making), the steel pipe is heat-treated under the conditions of a temperature range of 100 to 300° C. and a holding time of 1 minute or more. The upper limit is not particularly limited, but is, for example, 60 minutes or less.

(Other process)

Furthermore, in order to prevent the formation of a structure detrimental to acid resistance (ferrite and pearlite with an area ratio exceeding 20%) in the welded portion, a seam heat treatment in which the welded portion is heated to Ac ₁ point or less and then tempered may be performed. This heat treatment may be performed immediately after seam welding.

Since the base material portion of the steel pipe according to the present embodiment is not subjected to heat treatment at a temperature exceeding the Ac ₁ point, the metal structure of the base material portion is the same as that of the steel sheet according to the present embodiment. Therefore, in the steel pipe according to the present embodiment, in addition to the HIC resistance equal to or higher than that of the conventional steel, all of the base material portion and the welded portion have excellent SSC resistance.

Hereinafter, the present invention will be described in more detail by way of examples, but the present invention is not limited to these examples.

Example

The molten steel having the chemical compositions shown in Tables 1-1 and 1-2 was continuously cast to manufacture a steel slab with a thickness of 240 mm, and the manufacturing conditions shown in Tables 2-1 to 2-3 (heating temperature, finish rolling temperature, the highest attained temperature by reheating after the first water cooling stop in the multi-stage cooling, and the number of times of reheating exceeding 500° C.) to manufacture a steel sheet. In Table 2-1 to Table 2-3, in the column of water cooling stop temperature, OK indicates that the water cooling stop temperature is below 500°C after each water cooling of multi-stage accelerated cooling, and NG indicates that the cooling stop temperature exceeds 500°C example of the situation.

A round bar tensile test specimen was prepared from the obtained steel sheet according to API 5L, and the tensile strength was measured. In addition, the highest hardness of the surface layer portion from the surface to a depth of 1 mm was measured, and the metallographic structure was observed by SEM. In addition, for reference, the tissue at a position 5 mm from the surface and a tissue at a position (1/2 part) at 1/2 of the plate thickness from the surface were also observed.

Regarding the highest hardness of the surface layer portion, first, a steel sheet of 300 mm square was cut out by gas cutting at positions of 1/4, 1/2 and 3/4 of the width direction of the steel sheet from the end portion in the width direction of the steel sheet, The center of the steel plate was mechanically cut to obtain a block sample with a length of 20 mm and a width of 20 mm, which was ground by mechanical grinding. Using a Vickers hardness tester (load of 100 g) for this block sample, starting from a position with a depth of 0.1 mm from the surface of the steel sheet, 10 points were measured at intervals of 0.1 mm in the plate thickness direction, and the same depth was measured at 1 mm in the width direction. By measuring 20 points at intervals, a total of 200 points were measured, and the highest hardness was obtained. At this time, even if there is one measurement point exceeding 250HV, if two or more points do not appear continuously in the plate thickness direction, this point is regarded as an abnormal point, and is not adopted, and the next highest value is regarded as the highest hardness. On the other hand, when two or more measurement points exceeding 250HV continuously existed in the plate thickness direction, the highest value was made the highest hardness.

Regarding the metal structure, a sample obtained by grinding a sample prepared in such a way that the position of 0.5 mm from the surface (in the surface layer portion), 5 mm from the surface, and 1/2 of the plate thickness from the surface can be observed , immersed in a mixed solution of 3% nitric acid and 97% ethanol for several seconds to several tens of seconds to corrode, the metal structure is visualized, and it is observed by SEM, and, for bainite and martensite, according to the micro Vickers hardness sort. The results are shown in Tables 3-1 to 3-3. In the observation of the metal structure, a correction Lepera solution was also used as needed.

After that, each steel sheet was cold-worked into a cylindrical shape, and the both ends of the cylindrical steel sheet were butted together, and the process was carried out under the condition that the linear energy was in the range of 2.0 kJ/mm to 10 kJ/mm with three electrodes or four electrodes according to the thickness of the sheet. Submerged arc welding (SAW), to manufacture the steel pipe.

As soldering materials, Y-D, Y-DM, Y-D wire and NF-5000B flux are used on the inner side, Y-DM, Y-DMH, Y-DM, Y-DM wire is used on the outer side, and NF-5000 is used as the flux . The welding conditions were as follows: 3 electrodes on the inner surface and 4 electrodes on the outer surface, and the radiation energy during welding was adjusted in the range of 2.0 kJ/mm to 10 kJ/mm according to the plate thickness.

About the obtained steel pipe, the base material part was heat-processed under the conditions shown to Table 2-1 - Table 2-3 with respect to some steel plates. In addition, with respect to some steel pipes (Test No. 58), the heat treatment of heating to 400° C. to Ac ₁ point was performed on the welded portion.

Each of the obtained steel pipes was mechanically cut from positions at 90°, 180°, and 270° from the welded portion in the circumferential direction of the steel pipe, to obtain samples with an axial length of 20 mm and a circumferential length of 20 mm. Then, using this sample, the highest hardness of the surface layer portion of the steel pipe was obtained by the same method as described above. Since it can be considered that the metal structure after the pipe is made into a steel pipe is the same as the metal structure of the steel plate, the above-mentioned measurement results were used as they are.

In addition, as evaluation of SSC resistance, round bar samples were prepared from the obtained steel pipes according to API5L, and yield stress and tensile strength were measured.

Furthermore, a 4-point bending specimen with a width of 15 mm, a length of 115 mm, and a thickness of 5 mm was prepared from the inner surface of the base material portion of the steel pipe with the remaining inner surface, and was investigated in accordance with NACE TM 0316-2016 at various hydrogen sulfide partial pressures. , Whether cracks occur in the solution environment of pH 3.5. The load stress in the 4-point bending test was set to 90% and 95% of the actual yield stress.

Furthermore, as the evaluation of the HIC resistance, a hydrogen-induced cracking test (hereinafter, referred to as "HIC test") was implemented. The HIC test was conducted based on NACE™ 0284 2016. Specifically, a sample of 100 mm in length and 20 mm in width having a curvature along the inner surface prepared from the base material portion was placed in solution A (Solution A) (5 mass % NaCl+0.5 mass % glacial acetic acid aqueous solution) ) in a test solution saturated with 100% H ₂ S gas for 96 hours. Then, the area ratio (CAR) where cracks occurred was measured for the surface layer portion and the central portion. If the CAR is 5% or less, it is judged that the HIC resistance is excellent.

In addition, based on the result of the round bar tensile test, the proportional limit of each steel sheet was calculated by the above-mentioned method. These results are collectively shown in Table 4-1 to Table 4-3.

Test Nos. 1 to 22 and 60 to 65 (the steel pipes of the present invention) had HIC resistance properties equal to or higher than conventional steel pipes, and were excellent in SSC resistance.

The chemical composition of the weld metal portion was obtained from the above-mentioned steel pipe No. 1. As a result, the chemical composition of the weld metal was C: 0.07%, Si: 0.41%, Mn: 1.45%, P: 0.010%, S: 0.0030%, Cu: 0.04%, Ni: 0.12%, Cr: 0.16%, Mo: 0.24%, Nb: 0.02, Ti: 0.02%, Al: 0.02%, O: 0.045%, the balance of Fe and impurities.

(shape of weld toe)

For the obtained steel pipe, the angle of the front end portion of the excess height of the weld metal portion, that is, the angle formed by the tangential direction of the weld metal on both sides and the surface of the base metal portion was obtained, and the smaller angle was defined as the weld toe portion. Angle.

(SSC resistance)

In addition, as the evaluation of SSC resistance, a 4-point bend of width 15 mm, length 115 mm, and thickness 5 mm was prepared from the inner surface of the steel pipe with the remaining inner surface so that the weld toe was arranged in the longitudinal center of the sample. For the samples, the presence or absence of cracks was investigated in a solution environment with various hydrogen sulfide partial pressures and pH 3.5 in accordance with NACE TM 0316-2016. The load stress in the 4-point bending test was set to 90% and 95% of the actual yield stress.

(The highest hardness of the surface layer of the welding heat-affected zone)

The hardness of the surface layer portion in the welding heat-affected zone was measured. The hardness was measured in the surface layer portion from the surface to the depth position of 1.0 mm or 0.9 mm from the center portion in the circumferential direction and the longitudinal direction of the steel pipe. The method of cutting out the test piece for the hardness test of the welded heat-affected zone is as described above.

Specifically, in the measurement of the hardness of the welding heat-affected zone, from the weld toe (the boundary between the weld metal portion and the base metal portion) to the base metal portion side, at positions 0.3 mm, 0.6 mm, and 0.9 mm from the surface at 0.5 mm The mm pitch was measured at 40 points, so that a total of 120 points were measured, and the maximum hardness was calculated.

In addition, the metal structure in the surface layer portion of the welding heat-affected zone was also observed, and the area ratio was also measured. The metal structure of the surface layer portion is a metal structure at a depth of 0.5 mm from the surface in the thickness direction. The results are collectively shown in Table 5.

table 5

B: Bainite

Test Nos. 2, 2', 11, and 11' were excellent in SSC resistance including the welded part. On the other hand, in Test No. 2" and 11", SSC occurred from the weld toe.

Industrial Availability

According to the present invention, it is possible to provide a yield stress of 350 MPa or more and an excellent SSC resistance that does not cause cracks even when a stress exceeding 90% of the yield stress is applied in an environment of 30° C. or lower containing hydrogen sulfide exceeding 0.1 MPa. Steel pipes with properties, and steel sheets that can be used as their raw materials. Specifically, the steel pipe according to the present invention is suitable for a steel pipe used in a high-pressure hydrogen sulfide environment, such as a steel pipe for drilling of oil and natural gas, or a steel pipe for transportation.

Description of reference numerals

1 Weld metal part

2 Base material department

3 Angle of weld toe

4 Welding heat affected zone

5 Sample cut-out

Claims (6)

1. A steel pipe having a base material part and a welded part, The chemical composition of the base material part is contained in mass % C: 0.030 to 0.100%, Si: 0.50% or less, Mn: 0.80 to 1.60%, P: 0.020% or less, S: 0.0030% or less, Al: 0.060% or less, Ti: 0.001 to 0.030%, Nb: 0.006 to 0.100%, N: 0.0010 to 0.0080%, Ca: 0.0005 to 0.0050%, O: 0.0050% or less, Cr: 0 to 1.00%, Mo: 0 to 0.50%, Ni: 0 to 1.00%, Cu: 0 to 1.00%, V: 0 to 0.10%, Mg: 0 to 0.0100%, REM: 0 to 0.0100%, The balance is Fe and impurities, ESSP represented by the following formula (i) is 1.5 to 3.0, Ceq represented by the following formula (ii) is 0.20 to 0.50, The metal structure of the surface layer portion in the range from the surface to the depth of 1 mm of the base material portion is composed of one or more selected from polygonal ferrite, granular bainite, acicular ferrite, and bainite, The maximum hardness in the surface layer portion of the base material portion is 250 HV or less, the yield stress is 415-630 MPa, and the proportional limit in the stress-strain curve is 90% or more of the aforementioned yield stress, ESSP=Ca×(1-124×O)/(1.25×S) …(i) Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5 …(ii) Here, the symbol of each element in the formula represents the content in mass % of each element contained in the steel, and if it is not contained, it is recorded as zero. 2. The steel pipe according to claim 1, In the metal structure of the surface layer portion of the base material portion, the total area ratio of granular bainite, acicular ferrite, and bainite exceeds 80%. 3. The steel pipe according to claim 1 or 2, The chemical composition of the base material portion contains in mass % selected from Cr: 0.10 to 1.00%, Mo: 0.03 to 0.50%, Ni: 0.10 to 1.00%, Cu: 0.10 to 1.00%, V: 0.005 to 0.10%, Mg: 0.001 to 0.0100%, and REM: 0.001～0.0100% 1 or more of them. 4. The steel pipe according to any one of claims 1 to 3, The chemical composition of the base material portion includes Nb in mass %: 0.01 to 0.04%, The welding part includes a welding heat-affected zone and a weld metal part, The metal structure of the surface layer portion in the welding heat-affected zone includes at least one selected from the group consisting of bainite and acicular ferrite, The maximum hardness of the surface layer in the welding heat-affected zone is below 250HV, The angle of the weld toe on the inner side of the steel pipe is in the range of 130 to 180°. 5. The steel pipe according to any one of claims 1 to 4, The thickness of the base material portion is 10 to 40 mm, and the pipe diameter is 508 mm or more. 6 . A steel sheet used for the base material portion of the steel pipe according to claim 1 . 7 .

Images