KR102792300B1 - Steel pipes and plates - Google Patents

Abstract

This steel pipe is a steel pipe having a parent material portion and a welded portion, wherein the parent material portion has a predetermined chemical composition, and a metal structure of a surface portion in a range from a surface of the parent material portion to a depth of 1 mm is composed of at least one selected from polygonal ferrite, granular bainite, acicular ferrite, and bainite, and the maximum hardness of the surface portion of the parent material portion is 250 HV or less, a yield stress is 415 to 630 MPa, and a proportional limit in a stress-strain curve is 90% or more of the yield stress.

Description

Steel pipes and plates

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 plates suitable as materials therefor.

A system installed on the ground or the seabed to transport oil or gas is called a pipeline. The steel pipes for pipelines that constitute such pipelines are called line pipes. For large-diameter line pipes with a 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 manufactured by forming a thick steel plate into a cylindrical open pipe and welding the buttress portion (core portion) using an arc welding method such as a submerged arc welding method. Depending on the forming method, it is also sometimes called a UOE steel pipe or a JCOE steel pipe.

In recent years, pipeline construction has expanded to harsh environments such as cold regions and sour environments. Here, a sour environment means an acidified, moist hydrogen sulfide environment containing _H2S , a corrosive gas. It is known that hydrogen-induced cracking (HIC) may occur when a line pipe is exposed to a sour environment. On the other hand, sulfide stress cracking (SSC) may occur in wellbore pipes that have higher strength than line pipes. However, even in line pipes, SSC may occur when the hydrogen sulfide partial pressure or stress increases. Thus, line pipes used in harsh sour environments (sour-resistant line pipes) are required to have SSC resistance in addition to HIC resistance.

Patent Document 1 and Non-Patent Document 2 propose a welded steel pipe with excellent sour resistance or a steel plate for the steel pipe, in which the hardness of the base material and welded portion is set to 220 Hv or less, based on the knowledge that hardness affects sour resistance.

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

Patent Document 3 proposes a high-strength steel plate for a sour line pipe having excellent material uniformity within the steel plate, in which the metal structure is a bainite structure, the variation in hardness in the plate thickness direction is ΔHv ₁₀ 25 or less, the variation in hardness in the plate width direction is ΔHv ₁₀ 25 or less, and the maximum hardness of the surface layer of the steel plate is Hv ₁₀ 220 or less.

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

In the steel plates of Patent Documents 1 to 4 and Non-Patent Document 2, the sour resistance is satisfied in an environment where the hydrogen sulfide partial pressure is 0.1 MPa (1 bar) or less and the load stress is 90% or less of the yield stress. However, the recent usage environment of oil wells or line pipes has become harsher, and the level of demand for the sour resistance of welded steel pipes for line pipes has become higher.

In the past, sour resistance was required in an environment where the partial pressure of hydrogen sulfide was 0.1 MPa (1 bar) or less, but recently, a material design that can withstand a high-pressure hydrogen sulfide environment exceeding 0.1 MPa has been required. In addition, while in the past, the load stress was 90% or less of the yield stress, recently, a material design that can withstand a high-pressure hydrogen sulfide environment under a load stress exceeding 90% of the yield stress has been required.

According to the review by the present inventors, the steel plates of Patent Documents 1 to 4 and the steel plates of Non-Patent Document 2 were not sufficient in terms of sour resistance in an environment where the hydrogen sulfide partial pressure exceeds 0.1 MPa (1 bar) and also exceeds 90% of the yield stress.

For such a task, Patent Document 5 discloses a steel pipe having an HIC resistance equal to or greater than that of conventional steel, a yield strength of 350 MPa or more, and excellent SSC resistance in which cracks do not occur even when a stress of 90% or more of the yield strength is applied in an environment of 30°C or less containing hydrogen sulfide having a hydrogen sulfide partial pressure exceeding 0.1 MPa.

However, in Patent Document 5, although it is shown that the SSC resistance is excellent when the load stress of the sulfide stress corrosion cracking test is 90% of the yield stress, it is not shown when the load stress exceeds 90% of the yield stress.

Japanese Patent Publication No. 2011-017048 Japanese Patent Publication No. 2012-077331 Japanese Patent Publication No. 2013-139630 Japanese Patent Publication No. 2014-218707 Japanese Patent No. 6369658

Shin-Nippon-Tetsu Sumikin Kiho No. 397 (2013), p.17-22 JFE Kibo No.9 (August 2005), p.19-24

As described above, the usage environment of recent line pipes has become harsher, and the level of demand for the sour resistance of welded steel pipes for line pipes has become more sophisticated. Therefore, the purpose of the present invention is to provide a welded steel pipe with excellent sour resistance that can be used in a harsh high-pressure hydrogen sulfide environment, particularly a straight seam arc welded steel pipe, and a steel plate (particularly a thick steel plate) as a material thereof.

More specifically, the purpose is to provide a steel pipe having HIC resistance equivalent to or higher than that of conventional steel, a yield stress of 350 MPa or more, and excellent SSC resistance in which cracks do not occur even when a stress exceeding 90% of the yield stress, specifically, a stress of 95% of the yield stress, is loaded in an environment of 30°C or lower containing hydrogen sulfide exceeding 0.1 MPa, and a steel sheet to be used as the material for the steel pipe.

The present invention has been made to solve the above problems, and is based on the following steel pipe and steel plate.

(1) A steel pipe according to one embodiment of the present invention is a steel pipe having a base material portion and a welded portion, wherein the chemical composition of the base material portion is, 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 remainder: Fe and impurities, and an ESSP represented by the following formula (i) is 1.5 to 3.0, a Ceq represented by the following formula (ii) is 0.20 to 0.50, and a metal structure of a surface layer in a range from the surface of the base material to a depth of 1 mm is composed of at least one selected from polygonal ferrite, granular bainite, acicular ferrite, and bainite, and a maximum hardness of the surface layer of the base material is 250 HV or less, a yield stress is 415 to 630 MPa, and a proportional limit in a stress-strain curve is 90% or more of the yield stress.

ESSP=Ca×(1-124×O)/(1.25×S)··· (i)

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

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

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

(3) The steel plate described in (1) or (2) above may contain, in mass%, at least one selected from the following chemical compositions of the parent material: 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 to 0.0100%.

(4) The steel pipe described in any one of (1) to (3) above has a chemical composition of the base material portion containing, in mass%, 0.01 to 0.04% Nb, the weld portion is composed of a weld heat-affected zone and a weld metal portion, the metal structure of a surface portion in the weld heat-affected zone contains at least one selected from bainite and acicular ferrite, the maximum hardness of the surface portion in the weld heat-affected zone is 250 HV or less, and the angle of the weld toe portion on the inside of the steel pipe is in a range of 130 to 180°.

(5) The steel pipe described in any of the above (1) to (4) may have a thickness of the parent material of 10 to 40 mm and a diameter of 508 mm or more.

(6) A steel plate according to another aspect of the present invention is used in the parent material portion of the steel pipe described in any one of (1) to (5).

According to the above aspect of the present invention, it is possible to provide a steel pipe having 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 less containing hydrogen sulfide exceeding 0.1 MPa, and a steel plate that can be used as a material for the steel pipe.

In addition, according to a preferred embodiment of the present invention, a steel pipe having a welded portion with excellent sour resistance that can be used in a harsh high-pressure hydrogen sulfide environment can be provided.

Fig. 1 is a schematic diagram for explaining the angle of the welded end of the steel pipe according to the present embodiment.
Fig. 2 is a schematic diagram showing a portion for cutting a test piece from a steel pipe according to the present embodiment.

In order to examine a method for solving the above problem, the inventors of the present invention observed the fracture surface, structure, etc. of the parent material and weld portion of a cracked steel pipe in a high-pressure hydrogen sulfide environment exceeding 0.1 MPa (for example, in a _H2S saturated solution containing 5% salt and acetic acid) and in a test in which the load stress exceeded 90%. In addition, 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 sour resistance in a high-pressure hydrogen sulfide environment exceeding 0.1 MPa, it is necessary to control not only the HIC resistance but also the SSC resistance. HIC occurs in the central segregation zone that exists near the center in the thickness direction of the steel pipe. On the other hand, SSC depends on the structure and hardness in the range of 1 mm (surface layer) from the surface of the steel pipe, which has not been considered in the past.

(b) When the metal structure of the surface layer is made into a structure mainly composed of at least one selected from polygonal ferrite, granular bainite, acicular ferrite, and bainite, and the maximum hardness is set to 250 HV or less, the sour resistance is improved. In addition, when the total area ratio of at least one selected from granular bainite, acicular ferrite, and bainite exceeds 80%, the SSC property is further improved.

(c) When controlling the surface layer structure as described above, it is important to strictly control the cooling pattern after controlling the carbon equivalent Ceq to 0.20 to 0.50.

(d) When a method for manufacturing hot-rolled steel sheets based on coiling is applied, the cooling rate after stopping accelerated cooling becomes slower than that of cooling alone. In this case, the variation in hardness is reduced, but the surface layer structure and/or hardness described above are not obtained. Therefore, in order to obtain the surface layer structure and hardness described above, 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 SSC resistance of the weld is improved by alleviating the stress concentration in the toe.

The present invention was made based on the above findings.

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

The steel pipe according to the present embodiment is a welded steel pipe having a parent material portion and a weld portion. The parent material portion is cylindrical, and the weld portion extends in a direction parallel to the axial direction of the steel pipe. The weld portion is composed of a weld metal portion, which is a metal portion that is melted and solidified during welding, and a weld heat-affected zone, which is a region where changes in the structure, etc. occur due to heat input by welding and subsequent cooling, although it is not melted during welding.

In addition, the steel plate according to the present embodiment is used as the parent material of the steel pipe. That is, as described below, the steel plate is formed into a cylindrical shape, and both ends of the steel plate are butt-welded to obtain the steel pipe. Therefore, the chemical composition, metal structure, and mechanical properties of the steel plate are the same as those of the parent material of the steel pipe. Therefore, the description of the parent material of the steel pipe according to the present embodiment below also applies to the steel plate according to the present embodiment.

1. Chemical composition

The reasons for the limitation of each element are as follows. In the explanation below, "%" for the content means "mass%".

1-1. Chemical composition of the parent material (steel plate) of the steel pipe

The chemical composition of the parent material of the steel pipe according to this embodiment (the steel plate according to this embodiment) is described.

C: 0.030 to 0.100%

C is an element that improves the strength of a steel. If the C content is less than 0.030%, the strength improvement effect is not sufficiently obtained. Therefore, the C content is set to 0.030% or more. Preferably, it is 0.035% or more.

On the other hand, if the C content exceeds 0.100%, the hardness of the surface layer increases, making it easy for SSC to occur. In addition, carbides are generated, making it easy for HIC to occur. Therefore, the C content is set to 0.100% or less. In order to secure better SSC resistance and HIC resistance, and to suppress deterioration of weldability and toughness, the C content is preferably 0.070% or less, and more preferably 0.060% or less.

Si: 0.50% or less

If the Si content exceeds 0.50%, the toughness of the weld deteriorates. Therefore, the Si content is set to 0.50% or less. Preferably, it is 0.35% or less, more preferably, 0.30% or less. The lower limit of the Si content includes 0%.

Meanwhile, since Si is inevitably mixed in from steel raw materials and/or during the steelmaking process, 0.01% is the practical 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 may be 0.10%.

Mn: 0.80 to 1.60%

Mn is an element that improves the strength and toughness of steel. If the Mn content is less than 0.80%, these effects are not sufficiently obtained. Therefore, the Mn content is set to 0.80% or more. The Mn content is preferably 0.90% or more, and more preferably 1.00% or more.

On the other hand, if the Mn content exceeds 1.60%, the sour resistance deteriorates. Therefore, the Mn content is set to 1.60% or less. Preferably, it is 1.50% or less.

P: 0.020% or less

P is an element that is inevitably contained as an impurity. If the P content exceeds 0.020%, the HIC resistance deteriorates and the toughness of the weld deteriorates. Therefore, the P content is set to 0.020% or less. It is preferably 0.015% or less, and more preferably 0.010% or less. The lower P content is preferable, and the lower limit includes 0%. However, if the P content is reduced to less than 0.001%, the manufacturing cost increases significantly, so in practical steel, 0.001% is the practical lower limit of the P content.

S: 0.0030% or less

S is an element that is inevitably contained as an impurity. In addition, S is an element that forms MnS that is elongated in the rolling direction during hot rolling, thereby lowering HIC resistance. When the S content exceeds 0.0030%, the HIC resistance is significantly lowered, so the S content is set to 0.0030% or less. Preferably, it is 0.0020% or less, more preferably 0.0010% or less. The lower limit includes 0%, but when the S content is reduced to less than 0.0001%, the manufacturing cost increases significantly, so 0.0001% is a practical lower limit on practical steel sheets.

Al: 0.060% or less

When the Al content exceeds 0.060%, clusters in which Al oxide is accumulated are generated, and the HIC resistance deteriorates. Therefore, the Al content is set to 0.060% or less. Preferably, it is 0.050% or less, more preferably 0.035% or less, and even more preferably 0.030% or less. The lower Al content is preferable, and the lower limit of the Al content includes 0%.

Meanwhile, since Al is inevitably mixed in from steel raw materials and/or during the steelmaking process, 0.001% is the practical 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 0.010%.

Ti: 0.001 to 0.030%

Ti is an element that contributes to the refinement of crystal grains by forming carbonitride. If the Ti content is less than 0.001%, this effect is not sufficiently obtained. Therefore, the Ti content is set to 0.001% or more. Preferably, it is 0.008% or more, more preferably, it is 0.010% or more.

On the other hand, if the Ti content exceeds 0.030%, carbon nitride is excessively generated, which reduces HIC resistance and toughness. Therefore, the Ti content is set to 0.030% or less. Preferably, it is 0.025% or less, more preferably, it is 0.020% or less.

Nb: 0.006 to 0.100%

Nb is an element that contributes to the improvement of strength by forming carbides and/or nitrides. If the Nb content is less than 0.006%, these effects are not sufficiently obtained. Therefore, the Nb content is set to 0.006% or more. Preferably, it is 0.008% or more, and more preferably, it is 0.010% or more. In particular, when securing the hardness of the weld 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, if the Nb content exceeds 0.100%, Nb carbonitrides accumulate in the central segregation region, thereby lowering the HIC resistance. Therefore, the Nb content is set to 0.100% or less. Preferably, it is 0.080% or less, and more preferably, it is 0.060% or less.

In addition, when improving the toughness of the weld (weld heat-affected zone and weld metal zone), the Nb content is preferably 0.040% or less, more preferably 0.035% or less, and even more preferably 0.033% or less.

N: 0.0010 to 0.0080%

N is an element that forms nitrides by combining with Ti and/or Nb and contributes to the refinement of austenite grain size during heating. If the N content is less than 0.0010%, the above effect is not sufficiently obtained. Therefore, the N content is set to 0.0010% or more. Preferably, it is 0.0020% or more.

On the other hand, if the N content exceeds 0.0080%, nitrides of Ti and/or Nb are accumulated, thereby lowering the HIC resistance. Therefore, the N content is set to 0.0080% or less. Preferably, it is 0.0060% or less, and more preferably, it is 0.0050% or less.

Ca: 0.0005 to 0.0050%

Ca is an element that suppresses the formation of MnS, which is elongated in the rolling direction, by forming CaS in the steel, and as a result, contributes to the improvement of HIC resistance. If the Ca content is less than 0.0005%, the above effect is not sufficiently obtained. Therefore, the Ca content is set to 0.0005% or more. Preferably, it is 0.0010% or more, and more preferably, it is 0.0015% or more.

On the other hand, if the Ca content exceeds 0.0050%, oxides accumulate and HIC resistance deteriorates. Therefore, the Ca content is set to 0.0050% or less. Preferably, it is 0.0045% or less, and more preferably, it is 0.0040% or less.

O: 0.0050% or less

O is an element that inevitably remains. If the O content exceeds 0.0050%, oxides are generated and HIC resistance deteriorates. Therefore, the O content is set to 0.0050% or less. In terms of securing the toughness of the steel plate and the toughness of the weld, 0.0040% or less is preferable, and 0.0030% or less is more preferable. A smaller O content is preferable, and may be 0%. However, if O is reduced to less than 0.0001%, the manufacturing cost increases significantly. Therefore, the O content may be 0.0001% or more. In terms of manufacturing cost, 0.0005% or more is preferable.

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%

Cr, Mo, Ni, Cu and V are elements that enhance the steel's ??nicking properties. Therefore, one or more of these elements may be included as needed.

In order to obtain the above effect, it is preferable to contain at least one selected from 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, if the contents of Cr, Ni and Cu each exceed 1.00%, or the Mo content exceeds 0.50%, or the V content exceeds 0.10%, the hardness increases and the sour resistance deteriorates. Therefore, the contents of Cr, Ni and Cu are all set to 1.00% or less, the Mo content is set to 0.50% or less, and the V content is set to 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 to 0.0100%

REM: 0 to 0.0100%

Mg and REM are elements that control the form of sulfide. In order to obtain the above effect, it is preferable to contain one or two kinds selected from Mg: 0.001% or more and REM: 0.001% or more.

On the other hand, if the contents of Mg and REM exceed 0.0100%, respectively, the sulfide becomes coarse and cannot exert its effect. Therefore, the contents of Mg and REM are both set to 0.0100% or less. Preferably, they are 0.0050% or less.

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

In the above chemical composition, the remainder is Fe and impurities. Here, "impurities" are components mixed in due to various factors in the raw materials such as ore and scrap and the manufacturing process when manufacturing steel industrially, and are permitted within a range that does not adversely affect the present invention.

In cases where Sb, Sn, Co, As, Pb, Bi, H, W, Zr, Ta, B, Nd, Y, Hf and Re are included as impurities, it is preferable to control each content within the range described below.

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

As for Sb, Sn, Co, As, Pb, Bi, and H, there are cases where they are mixed as impurities or unavoidable mixed elements from steel raw materials, but if they are within the above range, they do not impair the properties of the steel pipe according to the present embodiment. Therefore, it is preferable to limit these elements to the above range.

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

These elements may be mixed in as impurities or unavoidable mixed elements from steel raw materials, but if within the above range, they do not impair the properties of the steel pipe according to the present embodiment. Therefore, the total content of these elements is limited to 0.10% or less.

The chemical composition of the parent material must satisfy the conditions set forth below, in addition to the content of each element being within the above-mentioned range, and the values of ESSP and Ceq calculated from the content of the components.

ESSP: 1.5 to 3.0

ESSP is a value that serves as an indicator of whether an amount of available Ca equivalent to the S content exists, assuming that the remaining Ca (available Ca) after excluding Ca combined with oxygen combines with S in an atomic ratio, and is expressed by the following equation (i). In the steel pipe according to the present embodiment, in order to secure HIC resistance characteristics equivalent to or higher than those of conventional steel, it is necessary that the ESSP value be within the range of 1.5 to 3.0.

ESSP=Ca×(1-124×O)/(1.25×S)··· (i)

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

In order to secure the HIC properties, it is effective to suppress the formation of MnS that is elongated in the rolling direction. In addition, in order to suppress the formation of MnS that is elongated in the rolling direction, it is effective to reduce the S content, add Ca, form CaS, and fix S. On the other hand, since Ca has a stronger affinity for oxygen than S, it is necessary to reduce the O content in order to form the required amount of CaS.

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. Since MnS elongated by rolling causes deterioration of HIC resistance, ESSP is set to 1.5 or more. Preferably, it is 1.6 or more, and more preferably, it is 1.7 or more.

On the other hand, if the Ca content is excessive, there is concern that a large amount of cluster-like inclusions will be generated, which will hinder the shape control of MnS. If the O content and S content are reduced, the generation of cluster-like inclusions can be suppressed, but if the ESSP exceeds 3.0, the manufacturing cost for reducing the O content and S content increases significantly. Therefore, the ESSP is set to 3.0 or less. Preferably, it is 2.8 or less, and more preferably, it is 2.6 or less.

When the value of ESSP is within the range of 1.5 to 3.0, the effective amount of Ca is more than the minimum amount required for morphological control of MnS, and is also adjusted to be below the critical amount at which cluster-like inclusions are not formed, so excellent HIC resistance characteristics are obtained.

Ceq: 0.20 to 0.50

Ceq is a value that is an index of ??quenching, meaning carbon equivalent, and is expressed by the following formula (ii). In the base material portion of the steel pipe according to the present embodiment, as described later, in the surface portion, it is necessary to appropriately control the ??quenching property of the steel in order to obtain a metal structure comprising at least one selected from polygonal ferrite, granular bainite, acicular ferrite, and bainite, preferably a metal structure containing more than 80% in total of at least one selected from granular bainite, acicular ferrite, and bainite. 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)

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

If Ceq is less than 0.20, a tensile strength of 530 MPa or more is not obtained. Therefore, Ceq is set to 0.20 or more. Preferably, it is 0.25 or more. On the other hand, if Ceq exceeds 0.50, the surface hardness of the weld increases, and the sour resistance deteriorates. Therefore, Ceq is set to 0.50 or less. Preferably, it is 0.45 or less.

1-2. Chemical composition of welded joint

The weld heat-affected zone is the part of the base metal that is not melted by welding. Therefore, its chemical composition is the same as that of the base metal, and the reasons for its limitation are also the same.

Meanwhile, there is no particular limitation on the chemical composition of the weld metal portion in the welded portion. However, in order to increase the strength of the weld metal portion to the same degree or more than the strength of the base metal portion, it is preferable that the chemical composition of the weld metal portion be within the following range.

That is, the chemical composition of the weld metal in the weld is preferably, in mass%, 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% 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%, Mg: 0 to 0.01%, REM: 0 to 0.01%, and the remainder: Fe and impurities.

The chemical composition of the weld metal is determined by the inflow ratio of the base metal and the welding material during welding. As the welding material, commercially available materials can be used, and for example, Y-D, Y-DM, Y-DMH wires, and NF5000B or NF2000 flux can be used. In addition, in order to control the composition range of the weld metal, it is preferable to adjust the welding conditions to the range described below.

2. Metal organization

2-1. Metal structure of the parent material

Next, the metal structure of the parent material (steel plate) of the steel pipe is explained.

The metal structure in the surface layer of the parent material is a structure composed of at least one selected from polygonal ferrite, granular bainite, needle-shaped ferrite, and bainite. In the present embodiment, the surface layer means a range from the surface of the parent material to 1.0 mm.

In the steel pipe according to the present embodiment, in order to secure the required strength and excellent sour resistance by suppressing the maximum hardness of the surface layer of the base material to 250 HV or less, the metal structure in the surface layer is made of one or more selected from polygonal ferrite, granular bainite, acicular ferrite, and bainite. Preferably, the total area ratio of one or more selected from granular bainite, acicular ferrite, and bainite exceeds 80%. When the total area ratio exceeds 80%, the strength and sour resistance are further improved. More preferably, it is 85% or more.

The area ratio of each tissue is measured by observing the metal tissue etched with a mixed solution of 3% nitric acid and 97% ethanol, etc., using a scanning electron microscope (SEM). The surface tissue can be measured at a representative position of 0.5 mm from the surface of the steel plate.

The surface metal structure of the parent material refers to the metal structure of the parent material that is not affected by welding. In the steel pipe according to the present embodiment, it refers to the surface metal structure at positions 90°, 180°, and 270° in the circumferential direction of the steel pipe from the butting portion (corresponding to the core portion and the end portion in the width direction of the steel plate). In the case of the steel plate, the positions correspond to the surface metal structure at positions 1/4, 1/2, and 3/4 in the width direction of the steel plate.

In the present embodiment, polygonal ferrite is a structure observed as a lumpy structure that does not include coarse precipitates such as coarse cementite or MA within the grains, and acicular ferrite is a structure in which the prior austenite grain boundaries are unclear and acicular ferrite (no carbide or austenite-martensite hybrid exists) is formed in a random crystal orientation within the grains.

Meanwhile, processed ferrite is processed ferrite, and when observed under an optical microscope or SEM, flat particles are observed in the rolling direction. Flat means that the aspect ratio (ferrite length in the rolling direction to ferrite length in the plate thickness direction) is 2.0 or more. In addition, pearlite is a layered structure of ferrite and cementite, and among pearlite, a structure in which the cementite forming the layer is broken in the middle is pseudo-pearlite.

Retained austenite is determined by the white appearance of the modified reference solution.

Granular bainite is formed at a transformation temperature between that of acicular ferrite and bainite, and has intermediate structural characteristics. It is a structure in which the old austenite grain boundaries are partially visible, a coarse lath structure exists within the grains, and fine carbides and austenite-martensite hybrids are scattered within and between the laths, and the old austenite grain boundaries are unclear and acicular or irregular ferrite parts exist mixed.

Bainite and martensite are structures in which prior austenite grain boundaries are clear, and a fine lath structure is developed within the grains. Bainite and martensite cannot be easily distinguished by SEM observation, but in the present embodiment, a structure in which prior austenite grain boundaries are clear, a fine lath structure is developed within the grains, and a hardness of 250 Hv or more is referred to as martensite, and a structure in which prior austenite grain boundaries are clear, a fine lath structure is developed within the grains, and a hardness of less than 250 Hv is referred to as bainite. Whether the hardness is 250 Hv or more or less than 250 Hv is determined by measuring the target structure at 10 points with a micro Vickers tester at a load of 100 gf, and determining whether the maximum value is 250 Hv or less than 250 Hv. All tissues are tempered during heat treatment in steel pipes during reheating, but there is no special distinction made on the presence or absence of tempering.

In the steel pipe according to the present embodiment, there is no particular limitation on the structure other than the surface layer. However, when the structure of the surface layer is controlled as described above by the manufacturing method described later, the structure other than the surface layer, for example, the structure of the center of thickness (the center of the plate thickness of the steel plate), is a structure mainly composed of needle-like ferrite and bainite that does not include worked ferrite, pearlite (including pseudo-pearlite), or martensite, and it is preferable that the maximum hardness is 250 Hv or less.

2-2. Metal structure of the weld heat affected zone

In the steel pipe according to the present embodiment, in order to have a similar metal structure throughout the steel pipe, it is preferable that the metal structure of the surface layer in the welded heat-affected zone includes at least one selected from bainite and acicular ferrite. Furthermore, it is preferable that the metal structure of the surface layer in the welded heat-affected zone is a uniform structure, that is, a structure composed of bainite and/or acicular ferrite.

It is preferable that the weld metal part be a structure composed of needle-like ferrite.

In order to make the weld heat-affected zone have the above metal structure, the following conditions are preferable as welding conditions. For example, as welding materials, it is preferable to use Y-D, Y-DM, Y-DMH wire, and NF5000B or NF2000 flux. In addition, it is preferable to perform inner surface welding and outer surface welding, and it is preferable to perform submerged arc welding with three electrodes on the inner surface and four electrodes on the outer surface. It is preferable to perform welding with a heat input of 2.0 kJ/mm to 10 kJ/mm depending on the plate thickness.

The metal structure of the weld heat-affected zone is prepared by cutting a test piece including the weld metal from the weld of the steel pipe to produce a sample for microstructure observation. Then, it is observed in the same way as the base metal.

3. Machine characteristics

Next, the mechanical characteristics of the steel pipe are explained.

3-1. Mechanical characteristics of the parent material

Maximum hardness of the surface layer: 250HV or less

Since SSC occurs due to micro-scratches or micro-cracks on the surface of the steel plate, the metal structure and hardness of the surface layer, which is the source of the micro-scratches and micro-cracks, are important.

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

The measurement of the maximum hardness of the surface layer is performed by the following method. First, test pieces of 20 mm in axial length and 20 mm in circumferential length are taken by mechanical cutting from positions spaced 90°, 180°, and 270° apart from the welded portion in the circumferential direction of the steel pipe. In the case of steel plates, test pieces of 20 mm in length and 20 mm in width are taken from positions 1/4, 1/2, and 3/4 from the end in the width direction of the steel plate.

Next, the above test piece is polished by mechanical polishing. For the test piece after polishing, a Vickers hardness tester (test force: 100 gf) is used to measure 10 points at 0.1 mm intervals in the plate thickness direction, starting from 0.1 mm from the surface, and 10 points at 1 mm intervals in the width direction for the same depth, for a total of 100 points.

And, as a result of the above measurement, if two or more measurement points exceeding 250 HV do not appear consecutively in the plate thickness direction, the highest hardness of the surface layer is judged to be 250 HV or less.

In the parent material of the steel pipe, there are cases where high values (abnormal values) appear locally due to inclusions, etc. However, since inclusions do not cause cracks, even if such abnormal values appear, SSC resistance can be secured. On the other hand, if there are two or more consecutive measurement points exceeding 250 HV in the plate thickness direction, it is not allowed because it is not due to inclusions but because SSC resistance is reduced.

Therefore, in the present invention, even if there is one measurement point exceeding 250 HV, if two or more points do not appear consecutively in the plate thickness direction, that point is considered an abnormal point and is not adopted, and the next highest value is set as the highest hardness. On the other hand, if there are two or more measurement points exceeding 250 HV consecutively in the plate thickness direction, that hardness is set as the highest hardness.

Proportional limit: 90% or more of yield stress

The inventors of the present invention have investigated the SSC resistance under more stringent conditions. As a result, it was found that when the proportional limit in the stress-strain curve is 90% or more of the yield stress, SSC does not occur even when the load stress exceeds 90% (e.g., 95%) of the yield stress.

If the proportional limit is less than 90% of the yield stress, when the load stress in the sulfide stress corrosion cracking test is 90% of the actual yield stress, plastic deformation occurs and dislocations propagate. As a result, hydrogen that has invaded during the sulfide stress corrosion test is captured by the propagated dislocations, and since the amount of hydrogen increases, 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 propagated dislocations do not increase, and hydrogen does not accumulate there. As a result, cracking can be prevented.

As described above, since the proportional limit is 90% or more of the yield stress, the parent material of the steel pipe according to the present embodiment (the steel plate according to the present embodiment) does not cause sulfide stress cracking even when a stress exceeding 90% of the yield stress is applied in a solution environment containing 5% salt and acetic acid at 30°C or lower. It is more preferable that the proportional limit be 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 is collected perpendicular to the long side direction of the steel pipe (C direction), and a tensile test is performed. The tensile test is performed under stroke control (tensile speed: 1 mm/min), the test force and displacement are measured at 0.05 s intervals, and based on these, the stress and strain at each measurement time are obtained. Then, the yield stress (YS) is obtained from the obtained stress-strain curve. As YS, when the yield point is not clearly visible, 0.20% proof stress is adopted.

Thereafter, considering the measurement error, the values of stress and strain are smoothed. Specifically, for each measurement time, the average value of the measurement time ±2.50 s is calculated, and that value is used as the result for each measurement time. For example, as the values of stress and strain at 2.50 s, the average value of 101 measurement values for 0 to 5.00 s is adopted.

Next, the slope of the straight line portion of the stress-strain curve after smoothing is obtained. The slope of the straight line portion is calculated by the least squares method using the value when the stress increases from 0.2YS to 0.4YS as a representative value.

Subsequently, the slope of the stress-strain curve at each measurement time is calculated. Specifically, for each measurement time, the slope is calculated by the least-squares method from the values for the corresponding measurement time ±0.50 s. For example, the slope of the stress-strain curve at 60.00 s is calculated by the least-squares method using 21 measurement values for 59.50 to 60.50 s.

And, the value of the stress before the slope of the stress-strain curve continues to fall below 0.95 times the slope of the straight line is set as the proportional limit. Even if the slope of the stress-strain curve falls below 0.95 times the slope of the straight line once due to the influence of the measurement error, if it again exceeds 0.95 times the slope of the straight line, that value is not adopted.

Yield stress: 415 MPa or more

Tensile strength: 530 MPa or more

The yield stress of the parent material of the steel pipe according to the present embodiment is set to 415 MPa or more in order to secure the required strength of the steel pipe according to the present embodiment. Preferably, it is 430 MPa or more. The upper limit of the yield stress is, in terms of workability, about 630 MPa specified in X70 of API5L, which is a practical upper limit. In terms of workability, the yield stress is preferably 600 MPa or less.

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

3-2. Mechanical characteristics of welded joints

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

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

Meanwhile, in order to obtain a strength higher than X60 of the API standard, it is preferable that the maximum hardness of the surface layer in the welded heat-affected zone be 150 HV or higher. The maximum hardness of the surface layer is more preferably 160 HV or higher, and even more preferably 170 HV or higher.

The highest hardness of the surface layer in the weld heat-affected zone is the highest hardness measured in the area from the surface to a depth of 0.9 mm in the thickness direction. The highest hardness of the surface layer in the weld heat-affected zone is measured by cutting a sample as shown in Fig. 2, and measuring 40 points, a total of 120 points, at positions of 0.3 mm, 0.6 mm, and 0.9 mm from the surface on the base material side from the weld edge (the boundary between the weld metal part and the base material part) at 0.5 mm pitches.

As a result of the above measurement, if two or more measurement points less than 150 HV or more than 250 HV do not appear consecutively in the thickness direction, the highest hardness of the surface layer in the weld heat-affected zone is judged to be 150 to 250 HV. The reason for measuring the hardness in this way is the same as the highest hardness of the surface layer in the base material portion described above.

4. Dimensions

Plate thickness: 10 to 40mm

Diameter: 508mm (20 inches) or more

When used as steel pipes for excavation or line pipes for oil, natural gas, etc., the plate thickness is preferably 10 to 40 mm, and the diameter (outer diameter) is preferably 508 mm or more. There is no specific restriction on the upper limit of the diameter, but the practical upper limit is 1422.4 mm (56 inches) or less.

5. Angle of weld joint

In the steel pipe according to the present embodiment, in order to improve the SSC resistance of the weld, it is preferable to control the angle of the weld toe of the deep weld. In the present embodiment, the angle of the weld toe is an angle as shown in Fig. 1. That is, the angle of the weld toe is an angle of the female end of the weld metal, that is, an angle formed by the tangential direction of the weld metal and the surface of the base material. It can also be called the so-called flank angle.

In order to suppress SSC, it is preferable that the angle of the weld toe on the inside of the steel pipe be in the range of 130° to 180°. If the angle of the weld toe is less than 130° and is more acute, deformation accumulates in the weld heat-affected zone, hydrogen intrusion is promoted, and cracks are likely to occur. In Fig. 1, only the angle at the lower left is described to be measured, but in this embodiment, the angles on the left and right are measured, and the smaller angle is taken as the angle of the weld toe (toe angle).

5. Manufacturing method

A preferred method for manufacturing a steel pipe and a steel plate that is the material for the steel pipe according to the present embodiment is described.

The steel pipe according to the present embodiment can obtain the effect as long as it has the above-described configuration, regardless of the manufacturing method. However, for example, the following manufacturing method is preferable because it can be obtained stably.

The steel plate according to this embodiment is,

(A) A hot rolling process in which a steel sheet having the above-described chemical composition is heated to 1000 to 1250°C and subjected to hot rolling, and the hot rolling is terminated at a temperature of Ar ₃ point or higher;

(B) A first cooling process for performing multi-stage accelerated cooling, in which the steel sheet after the hot rolling process is cooled three or more times from a temperature of Ar ₃ point or higher, at a water cooling stop temperature of 500°C or lower, and further, at a maximum temperature reached by reheating exceeding 500°C after stopping water cooling;

(C) After that, a second cooling process is performed to cool to a temperature of 500℃ or lower at an average cooling rate of 0.2℃/s or higher.

Obtained by a manufacturing method including:

The steel pipe according to the present embodiment, in addition to the processes (A) to (C),

(D) A forming process for forming the above steel plate into a cylindrical shape,

(E) A welding process for welding both ends of a tubular steel plate together,

(F) A heat treatment process for a steel pipe obtained by welding, in which heat treatment is performed under conditions in which the temperature range is 100 to 300℃ and the holding time is 1 minute or longer.

is obtained by doing .

For each process, the desired conditions are described.

(Hot rolling process)

A steel billet manufactured by casting molten steel having the same chemical composition as the parent material of the steel pipe according to the present embodiment is heated to 1000 to 1250°C and provided for hot rolling. The casting of the molten steel and the manufacturing of the billet prior to hot rolling may be performed in accordance with commercial methods.

When rolling a steel billet, if the heating temperature is less than 1000℃, the deformation resistance does not decrease and the load of the rolling mill increases, so the heating temperature is set to 1000℃ or higher. Preferably, it is 1100℃ or higher. On the other hand, if the heating temperature exceeds 1250℃, the grains of the steel billet become coarser and the strength and toughness deteriorate, so the heating temperature is set to 1250℃ or lower. Preferably, it is 1210℃ or lower.

The heated steel billet is hot rolled in a temperature range of Ar ₃ point or higher to make a steel plate, and the hot rolling is terminated at Ar ₃ point or higher. If the hot rolling treatment temperature is lower than Ar ₃ point, wrought ferrite is generated in the steel plate structure, which reduces the strength. Therefore, the hot rolling treatment temperature is set to Ar ₃ point or higher.

(1st cooling process)

For a steel sheet that has completed hot rolling, accelerated cooling is initiated from a temperature of Ar ₃ point or higher. At that time, multi-stage accelerated cooling is performed twice or more, in which the water cooling stop temperature is 500°C or lower at the surface temperature, and further, the maximum temperature reached by reheating after stopping the water cooling exceeds 500°C. Preferably, it is performed three or more times.

In order to ensure that the maximum temperature reached by the heat exchanger exceeds 500℃, it is important to increase the temperature difference between the surface and the interior. The temperature difference between the surface and the interior can be adjusted by changing the water density and collision pressure in the water cooling.

If the maximum temperature reached by reheating is 500℃ or less, the hardness of the steel plate, especially the maximum hardness of the surface layer from the surface to a depth of 1mm, cannot be lower than 250HV. In addition, even if the number of reheating times exceeding 500℃ is less than 2, the maximum hardness of the surface layer cannot be lower than 250HV. Therefore, accelerated cooling is performed so that reheating at a temperature exceeding 500℃ is performed 3 times or more.

In multi-stage cooling, it is desirable to set the water cooling stop temperature to a temperature exceeding the Ms point because it does not form a hard phase.

In addition, if the water cooling stop temperature before heating exceeds 500℃, the desired tissue cannot be obtained, so the water cooling stop temperature is set to 500℃ or lower. Preferably, the water cooling stop temperature is set to 500℃ or lower.

By performing the re-rolling three or more times, the maximum hardness HVmax of the surface layer from the surface of the steel plate to a depth of 1 mm is reduced to 250 HV or less. Since the number of re-rollings is the number of times until the maximum hardness HVmax of the surface layer reaches 250 HV or less, there is no need to specify an upper limit on the number of re-rollings.

(2nd cooling process)

In the first cooling process, after three or more water cooling and reheating are completed, cooling is performed at an average cooling rate of 0.2°C/s or more to a temperature of 500°C or less. If cooling is completed at a temperature exceeding 500°C or the cooling rate is slowed down by performing coiling or the like, and the average cooling rate to 500°C or less is less than 0.2°C/s, the variation in hardness is reduced, but the structure and/or hardness of the surface layer described above are not obtained.

(Forming process and welding process)

The forming of the steel plate into a 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 in terms of dimensional accuracy.

After forming the steel plate into a tubular shape, both ends of the steel plate are butted together and arc-welded (seam welding). Arc welding is not limited to a specific welding method, but submerged arc welding is preferable. In addition, the welding conditions may be performed under known conditions. For example, it is preferable to weld in a range where the heat input is 2.0 to 10 kJ/mm depending on the plate thickness with three or four electrodes. In order to make the weld heat-affected zone have the above-described metal structure, it is preferable to use, for example, Y-D, Y-DM, Y-DMH wire, and NF5000B or NF2000 flux as the welding material. In addition, it is preferable to perform inner surface welding and outer surface welding, and it is preferable to perform submerged arc welding with three electrodes on the inner surface and four electrodes on the outer surface.

(heat treatment process)

Thereafter (after tempering), the steel pipe is heat treated under conditions where the temperature range is 100 to 300℃ and the holding time is 1 minute or longer. The upper limit is not particularly limited, but is, for example, 60 minutes or less.

(Other processes)

In addition, in order to prevent the formation of tissues harmful to sour resistance (ferrite and pearlite exceeding 20% in area ratio) in the weld, a core heat treatment may be performed by heating the weld to a point Ac ₁ or lower and tempering it. This heat treatment may be performed immediately after the core 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 Ac ₁ , the metal structure of the base material portion is the same as the metal structure of the steel plate according to the present embodiment. Therefore, the steel pipe according to the present embodiment has excellent SSC resistance in addition to HIC resistance equivalent to or higher than that of conventional steel, in both the base material portion and the welded portion.

Hereinafter, the present invention will be described more specifically by examples, but the present invention is not limited to these examples.

Example

Molten steel having the chemical compositions shown in Tables 1-1 and 1-2 was continuously cast to manufacture a steel slab having a thickness of 240 mm, and a steel plate was manufactured under the manufacturing conditions (heating temperature, finish rolling temperature, maximum temperature reached by reheating after the first water cooling stop during multi-stage cooling, number of reheatings exceeding 500°C) shown in Tables 2-1 to 2-3. In Tables 2-1 to 2-3, in the column for water cooling stop temperature, OK indicates an example in which the water cooling stop temperature was 500°C or lower after each water cooling of the multi-stage accelerated cooling, and NG indicates an example in which the cooling stop temperature exceeded 500°C.

[Table 1-1]

[Table 1-2]

[Table 2-1]

[Table 2-2]

[Table 2-3]

From the obtained steel plate, a round bar tensile test piece was collected in accordance with API5L, and the tensile strength was measured. In addition, the maximum hardness of the surface layer from the surface to a depth of 1 mm was measured, and the metal structure was observed with a SEM. In addition, for reference, the structure at a position of 5 mm from the surface and the structure at a position of 1/2 of the plate thickness from the surface (1/2 section) were also observed.

The highest hardness of the surface layer was first obtained by cutting a square steel plate with a side length of 300 mm from the positions of 1/4, 1/2, and 3/4 in the width direction from the end in the width direction of the steel plate by gas cutting, and then collecting a block test piece with a length of 20 mm and a width of 20 mm from the center of the cut steel plate by mechanical cutting and polishing by mechanical polishing. For this block test piece, a Vickers hardness meter (load 100 g) was used to measure 10 points at 0.1 mm intervals in the plate thickness direction, 20 points at 1 mm intervals in the width direction for the same depth, and a total of 200 points, to obtain the highest hardness. At this time, even if there was one measurement point exceeding 250 HV, if two or more points did not appear continuously in the plate thickness direction, that point was considered to be an abnormal point and was not adopted, and the next highest value was set as the highest hardness. Meanwhile, if there are two or more consecutive measurement points exceeding 250 HV in the plate thickness direction, the highest value was considered the highest hardness.

The metal structure was observed at positions 0.5 mm (surface layer) from the surface, 5 mm from the surface, and 1/2 the plate thickness from the surface. The polished test piece was immersed in a mixed solution of 3% nitric acid and 97% ethanol for several seconds to tens of seconds to etch the metal structure, which was then observed with a SEM. The bainite and martensite were classified by micro Vickers hardness. The results are shown in Tables 3-1 to 3-3. A modified Lepera solution was also used for the observation of the metal structure as necessary.

[Table 3-1]

[Table 3-2]

[Table 3-3]

Thereafter, each steel plate was cold worked into a cylindrical shape, and both ends of the cylindrical steel plate were placed against each other, and submerged arc welding (SAW) was performed under conditions where the heat input was in the range of 2.0 kJ/mm to 10 kJ/mm depending on the plate thickness using three or four electrodes to manufacture a steel pipe.

As welding materials, Y-D, Y-DM, Y-D wire and NF-5000B flux were used on the inner side, and Y-DM, Y-DMH, Y-DM, Y-DM, and NF-5000 flux were used on the outer side. The welding conditions were 3 electrodes on the inner side and 4 electrodes on the outer side, and the heat input during welding was adjusted in the range of 2.0 kJ/mm to 10 kJ/mm depending on the plate thickness.

For the obtained steel pipes, heat treatment was performed on some of the steel plates, on the base material, under the conditions shown in Tables 2-1 to 2-3. In addition, for some of the steel pipes (Test No. 58), heat treatment was performed on the welded portion by heating at 400°C to point Ac ₁ .

For each obtained steel pipe, test pieces of 20 mm in axial length and 20 mm in circumferential length were cut mechanically from positions spaced 90°, 180°, and 270° apart from the weld in the circumferential direction of the steel pipe. Then, using the test pieces, the maximum hardness of the surface layer of the steel pipe was obtained by the same method as above. Since the metal structure after being manufactured into a steel pipe is thought to be the same as the metal structure of the steel plate, the above measurement results were used as they are.

In addition, as an evaluation of the SSC resistance, round bar test specimens were collected from the obtained steel pipe in accordance with API 5L, and the yield stress and tensile strength were measured.

In addition, four-point bending test specimens measuring 15 mm in width, 115 mm in length, and 5 mm in thickness were collected from the inner surface of the parent material of the steel pipe in a form that leaves the inner surface intact, and the occurrence of cracks in various hydrogen sulfide partial pressures and solution environments of pH 3.5 was investigated in accordance with NACE TM 0316-2016. The load stress during the four-point bending test was set to 90% and 95% of the actual yield stress.

And, as an evaluation of HIC resistance, a hydrogen-induced cracking test (hereinafter referred to as “HIC test”) was conducted. The HIC test was conducted in accordance with NACE TM0284 2016. Specifically, a test piece having a length of 100 mm and a width of 20 mm and having a curvature along the inner surface, taken from the parent material, was immersed in a test solution saturated with 100% _H2S gas in Solution A (5 mass% NaCl + 0.5 mass% glacial acetic acid aqueous solution) for 96 hours. Thereafter, the area ratio (CAR) where cracks occurred was measured for the surface and the center. If the CAR was 5% or less, the HIC resistance was judged to be excellent.

In addition, based on the results of the tensile test of the circular bar, the proportional limit of each steel plate was calculated by the above-described method. The results are summarized and presented in Tables 4-1 to 4-3.

[Table 4-1]

[Table 4-2]

[Table 4-3]

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

The chemical composition of the weld metal was obtained from the above steel pipe No. 1. As a result, the chemical composition of the weld metal was as follows: 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%, with the remainder being Fe and impurities.

(Shape of weld joint)

For the obtained steel pipe, the angle of the female end of the weld metal portion, i.e., the angle formed by the tangent direction of the weld metal and the surface of the parent material on both sides, was obtained, and the smaller angle was taken as the angle of the weld end.

(My SSC)

In addition, as an evaluation of the SSC resistance, a four-point bending test specimen with a width of 15 mm, a length of 115 mm, and a thickness of 5 mm was collected from the inner surface of the steel pipe so that the inner surface was left intact and the weld toe was placed at the center of the long side of the test specimen, and the occurrence of cracks in various hydrogen sulfide partial pressures and solution environments of pH 3.5 was investigated in accordance with NACE TM 0316-2016. The load stress during the four-point bending test was set to 90% and 95% of the actual yield stress.

(Highest hardness of the surface layer of the weld heat affected zone)

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

Specifically, for the hardness measurement of the weld heat-affected zone, 40 points were measured at 0.5 mm pitches at positions 0.3 mm, 0.6 mm, and 0.9 mm from the surface on the base metal side from the weld zone (the boundary between the weld metal and the base metal), for a total of 120 points, and the maximum hardness was calculated.

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

[Table 5]

Tests No. 2, 2', 11, and 11' showed excellent SSC resistance, including welded portions. On the other hand, in Tests No. 2" and 11", SSC occurred from the welded portion.

According to the present invention, it is possible to provide a steel pipe having excellent SSC resistance in which cracks do not occur even when a stress exceeding 90% of the yield stress is applied in an environment of 30℃ or lower containing hydrogen sulfide exceeding 0.1 MPa and having a yield stress of 350 MPa or more, and a steel plate that can be used as a material for the steel pipe. 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 excavating petroleum, natural gas, or the like, or a steel pipe for transportation.

1: Welding metal part
2: Mother's Department
3: Angle of weld joint
4: Welding heat affected zone
5: Sample cutting section

Claims (11)

It is a steel pipe having a parent material and a welded part.
The chemical composition of the above parent material is, in mass%,
C: 0.030 to 0.100%,
Si: 0.01 to 0.50%,
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%,
Residue: Fe and impurities,
The ESSP expressed by the following formula (i) is 1.5 to 3.0,
Ceq, expressed by the following equation (ii), is 0.20 to 0.50,
The metal structure of the surface layer, which is a range from the surface of the above-mentioned parent material to a depth of 1 mm, is composed of at least one selected from polygonal ferrite, granular bainite, needle-shaped ferrite, and bainite.
The maximum hardness of the surface layer of the above-mentioned parent material is 250 HV or less,
The yield stress is 415 to 630 MPa,
The proportional limit in the stress-strain curve is 90% or more of the yield stress,
A steel pipe, wherein the total area ratio of granular bainite, acicular ferrite, and bainite in the metal structure of the surface layer of the above-mentioned parent material exceeds 80%.
ESSP=Ca×(1-124×O)/(1.25×S)··· (i)
Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5 ··· (ii)
However, each element symbol in the formula represents the content (mass%) of each element contained in the steel, and is set to zero if not contained. delete In the first paragraph,
The chemical composition of the above parent material is, in mass%,
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 to 0.0100%
A steel pipe containing at least one type selected from: delete In clause 1 or 3,
The chemical composition of the above-mentioned parent material comprises, in mass%, Nb: 0.01 to 0.04%,
The above welded portion is composed of a weld heat affected zone and a weld metal portion,
The metal structure of the surface layer in the above welding heat affected zone includes at least one selected from bainite and needle-shaped ferrite,
The maximum hardness of the surface layer in the above welding heat affected zone is 250 HV or less,
A steel pipe, wherein the angle of the weld joint on the inside of the steel pipe is in the range of 130 to 180°. In clause 1 or 3,
A steel pipe having a thickness of the above-mentioned parent material of 10 to 40 mm and a diameter of 508 mm or more. In paragraph 5,
A steel pipe having a thickness of the above-mentioned parent material of 10 to 40 mm and a diameter of 508 mm or more. A steel plate used in the parent material of the steel pipe described in Article 1 or 3. Steel plate used for the parent material of the steel pipe described in Article 5. Steel plate used for the parent material of the steel pipe described in Article 6. Steel plate used for the parent material of the steel pipe described in Article 7.

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