Steel material

Abstract

To provide a steel material that has high strength and excellent hot workability, and also has excellent SSC resistance and pitting corrosion resistance even under a CO2 environment.SOLUTION: A steel material according to the present disclosure comprises, in terms of mass%, C: 0.08% or less, Si: 0.60% or less, Mn: 1.0 to 2.5%, P: 0.05% or less, S: 0.005% or less, Cu: 0.05-0.50%, Ni: 0.05-0.50%, Cr: 0.25-0.50%, Mo: 0.02-0.50%, Ti: 0.001 to 0.050%, Al: 0.005 to 0.100%, Ca: 0.0005 to 0.0100%, a total of Sn and Sb: 0.001 to 0.100%, N: 0.0150% or less, O: 0.0050% or less, and the balance: Fe and impurities. CE defined by formula (1) described in the specification, is 0.45 or less. The yield strength is 448 MPa or more. The standard deviation of the grain size number of prior austenite grains in the microstructure is 0.80 or less.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present disclosure relates to steel materials, and more particularly, to steel materials suitable for use in a CO2 environment.

Background Art

[0002] A system that is installed on the ground, the seabed surface, etc. and transports natural gas, crude oil, etc. is called a pipeline. A pipeline laid on the seabed is composed of a plurality of steel pipes (line pipes). A pipeline laid on the seabed further receives a high pressure from the production fluid passing through the inside of the pipeline. The pipeline further receives repeated strain due to waves and seawater pressure from the outside. Therefore, high strength is required for the steel pipes (line pipes) that make up the pipeline.

[0003] In addition, natural gas, crude oil, etc. produced in recent years may contain wet hydrogen sulfide (H2S) gas. Therefore, line pipes that transport production fluids containing H2S gas are required to have not only high strength but also sulfide stress cracking resistance (Sulfide Stress Cracking resistance: hereinafter referred to as SSC resistance). Thus, steel materials that are high-strength and have excellent SSC resistance have been demanded, assuming application to line pipes.

[0004] To date, techniques for increasing the strength and SSC resistance of steel materials for line pipes have been proposed in Japanese Patent Application Laid-Open No. 2017-179482 (Patent Document 1) and International Publication No. 2018 / 181564 (Patent Document 2).

[0005] The steel material disclosed in Patent Document 1 is an electric resistance welded steel pipe for line pipes, and in terms of mass%, C: 0.01 to 0.1%, Si: 0.01 to 0.4%, Mn: 0.5 to 2%, P: 0.03% or less, S: 0.001% or less, Al: 0.01 to 0.05%, N: 0.003 to 0.008%, Nb: 0.01 to 0.05%, Ti: 0.005 to 0.02%, Ni: 0 to 0.2%, Mo: 0 to 0.2%, Ca: 0 to 0.0050%, and the balance consists of Fe and impurities, and it has a yield strength of 450 to 600 MPa and a tensile strength of 535 to 765 MPa. The maximum Vickers hardness of the inner surface layer of this steel material is 248 HV or less, and it is 5 HV or more smaller than the maximum Vickers hardness of the outer surface layer. Patent Document 1 discloses that this steel material has excellent SSC resistance even though it has high strength.

[0006] The steel material disclosed in Patent Document 2 is a high-strength steel plate for sour line pipes, and in terms of mass%, C: 0.02 to 0.08%, Si: 0.01 to 0.50%, Mn: 0.50 to 1.80%, P: 0.001 to 0.015%, S: 0.0002 to 0.0015%, Al: 0.01 to 0.08%, Ca: 0.0005 to 0.005%, and the balance consists of Fe and impurities, and CP (= 4.46C + 2.37Mn / 6 + (1.74Cu + 1.7Ni) / 15 + (1.18Cr + 1.95Mo + 1.74V) / 5 + 22.36P) is 1.00 or less. The steel structure at 0.5 mm below the steel plate surface of this steel material is a bainite structure with a dislocation density of 0.5×10 14 ~7.0×10 14 (m -2 ) and the difference ΔHV between the average value of the Vickers hardness at 0.5 mm below the steel plate surface and the average value of the Vickers hardness at the center of the steel plate thickness is 25 HV or less, and it has a tensile strength of 520 MPa or more. Patent Document 2 discloses that this steel material is excellent in SSC resistance and also excellent in hardness uniformity in the plate thickness direction.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

[0008] By the way, in recent years, the increase in the concentration of carbon dioxide (CO2) on the ground has become a global problem. Therefore, efforts have been made to suppress CO2 emissions. Among such efforts to suppress CO2 emissions, in particular, CCUS has been attracting attention. CCUS is an abbreviation for Carbon dioxide Capture, Utilization and Storage. That is, CCUS includes three technologies: CO2 recovery, utilization, and storage. Among these, as a technology for storing CO2, a technology that recovers CO2 emitted from industrial facilities such as power plants and factories and injects and stores CO2 into depleted oil wells has been attracting attention.

[0009] Therefore, steel materials assumed to be applied to line pipes are required to have excellent corrosion resistance even in an environment containing not only H2S gas but also CO2 gas (hereinafter, an environment containing H2S gas and CO2 gas is referred to as a "CO2 environment"). More specifically, in a CO2 environment, steel materials having SSC resistance and corrosion resistance against pitting corrosion and / or crevice corrosion (hereinafter referred to as "pitting corrosion resistance") have been required.

[0010] In addition, steel materials assumed to be applied to such line pipes may be subjected to hot working such as hot rolling and hot extrusion during manufacturing. Therefore, such steel materials are required to have excellent hot workability in addition to high strength and excellent corrosion resistance.

[0011] On the other hand, in Patent Documents 1 and 2 above, the SSC resistance and pitting corrosion resistance in a CO2 environment have not been studied at all. Further, in Patent Documents 1 and 2 above, the hot workability of steel materials has not been studied at all.

[0012] The object of the present disclosure is to provide a steel material having high strength, excellent hot workability, and excellent SSC resistance and pitting corrosion resistance even in a CO2 environment.

Means for Solving the Problems

[0013] The steel material according to the present disclosure is by mass%, C: 0.08% or less, Si: 0.60% or less, Mn: 1.0 - 2.5%, P: 0.05% or less, S: 0.005% or less, Cu: 0.05 - 0.50%, Ni: 0.05 - 0.50%, Cr: 0.25 - 0.50%, Mo: 0.02 - 0.50%, Ti: 0.001 - 0.050%, Al: 0.005 - 0.100%, Ca: 0.0005 - 0.0100%, Sn: 0 - 0.100%, Sb: 0 - 0.100%, Total of Sn and Sb: 0.001 - 0.100%, N: 0.0150% or less, O: 0.0050% or less, V: 0 - 0.10%, Nb: 0 - 0.05%, B: 0 - 0.0005%, and the balance consists of Fe and impurities, CE defined by formula (1) is 0.45 or less, the yield strength is 448 MPa or more, in the microstructure, the standard deviation of the crystal grain size number of prior austenite grains is 0.80 or less. CE = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 (1) Here, in formula (1), the content of the corresponding element is substituted for the element symbol in units of mass%. When the corresponding element is not contained, "0" is substituted for the element symbol. [Effect of the Invention]

[0014] The steel material according to the present disclosure has high strength and excellent hot workability, and also has excellent SSC resistance and pitting corrosion resistance even in a CO2 environment. [Brief Description of the Drawings]

[0015]

Figure 1

[0016] First, the present inventors considered obtaining a steel material having a yield strength of 448 MPa or more as high strength. Next, the present inventors examined from the viewpoint of chemical composition a steel material having a yield strength of 448 MPa or more, excellent hot workability, and excellent SSC resistance and pitting corrosion resistance even in a CO2 environment. As a result, the present inventors found that tin (Sn) and antimony (Sb), which have not been focused on so far, may enhance the SSC resistance and pitting corrosion resistance in a CO2 environment.

[0017] On the other hand, as a result of the study by the present inventors, it became clear that if the contents of Sn and Sb are too high, the SSC resistance and pitting corrosion resistance of the steel material increase, while the hot workability of the steel material decreases. Therefore, the present inventors further examined in detail the contents of Sn and Sb that can sufficiently enhance the SSC resistance and pitting corrosion resistance of the steel material while maintaining the hot workability of the steel material. As a result, it became clear that if the total content of Sn and Sb is 0.001 to 0.100%, there is a possibility of obtaining a yield strength of 448 MPa or more, excellent hot workability, and excellent SSC resistance and pitting corrosion resistance even in a CO2 environment.

[0018] More specifically, the inventors considered that if the steel material consists of C: 0.08% or less, Si: 0.60% or less, Mn: 1.0 - 2.5%, P: 0.05% or less, S: 0.005% or less, Cu: 0.05 - 0.50%, Ni: 0.05 - 0.50%, Cr: 0.25 - 0.50%, Mo: 0.02 - 0.50%, Ti: 0.001 - 0.050%, Al: 0.005 - 0.100%, Ca: 0.0005 - 0.0100%, Sn: 0 - 0.100%, Sb: 0 - 0.100%, total of Sn and Sb: 0.001 - 0.100%, N: 0.0150% or less, O: 0.0050% or less, V: 0 - 0.10%, Nb: 0 - 0.05%, B: 0 - 0.0005%, and the balance: Fe and impurities, it may have a yield strength of 448 MPa or more, excellent hot workability, and excellent SSC resistance and pitting corrosion resistance even in a CO2 environment.

[0019] As a result of further studies by the inventors, on the premise of having the above chemical composition, it was found that if the CE defined by formula (1) is 0.45 or less, the hot workability of the steel material can be stably improved. CE = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 (1) Here, in the element symbols in formula (1), the content of the corresponding element is substituted in units of mass%. When the corresponding element is not contained, "0" is substituted for the element symbol.

[0020] CE is the carbon equivalent. If CE is too high, the strength may become too high and the hot workability of the steel material may decrease. On the other hand, if it has the above chemical composition and CE is 0.45 or less, the hot workability of the steel material can be stably improved. Therefore, the steel material according to this embodiment has CE of 0.45 or less on the premise of having the above chemical composition.

[0021] On the other hand, even for a steel material having the above chemical composition and a CE of 0.45 or less, when it has a yield strength of 448 MPa or more, excellent SSC resistance and pitting corrosion resistance may not be obtained in a CO2 environment. Therefore, for a steel material having the above chemical composition and a CE of 0.45 or less, focusing on the microstructure of the steel material, a method for enhancing SSC resistance and pitting corrosion resistance in a CO2 environment was examined in detail.

[0022] As a result of detailed examinations by the present inventors, it was revealed that in a steel material having the above chemical composition, a CE of 0.45 or less, and a yield strength of 448 MPa or more, the standard deviation σ of the crystal grain size number in the microstructure affects the SSC resistance and pitting corrosion resistance of the steel material in a CO2 environment. This point will be specifically described with reference to the drawings. Fig. 1 is a diagram showing the relationship between the standard deviation σ of the crystal grain size number of prior austenite grains, the number of pitting corrosion (pieces) which is an index of pitting corrosion resistance, and SSC resistance in this example. In this specification, prior austenite grains are also referred to as "prior γ grains".

[0023] Fig. 1 was created using the value of the standard deviation σ of the crystal grain size number, the number of pitting corrosion (pieces) which is an index of pitting corrosion resistance, and the results of the SSC resistance test for an example in which the configurations other than the standard deviation σ of the crystal grain size number among the examples described later satisfy the conditions of this embodiment and have excellent hot workability. Here, "○" in Fig. 1 means a steel material having excellent SSC resistance in the SSC resistance test. On the other hand, "●" in Fig. 1 means a steel material that did not have excellent SSC resistance in the SSC resistance test.

[0024] Referring to FIG. 1, for a steel material having the above chemical composition, with CE being 0.45 or less and a yield strength of 448 MPa or more, if the standard deviation σ of the crystal grain size number of the prior γ grains is 0.80 or less, the number of pitting corrosion, which is an index of pitting corrosion resistance, is 10 or less, and it can be confirmed that it has excellent pitting corrosion resistance. In this case, it can further be confirmed that it has excellent SSC resistance in the SSC resistance test. Therefore, the steel material according to the present embodiment has the above chemical composition, CE is 0.45 or less, has a yield strength of 448 MPa or more, and further, the standard deviation σ of the crystal grain size number of the prior γ grains is 0.80 or less. As a result, the steel material according to the present embodiment has high strength, excellent hot workability, and excellent SSC resistance and pitting corrosion resistance even in a CO2 environment.

[0025] Regarding the reason why a steel material having the above chemical composition, with CE being 0.45 or less and a yield strength of 448 MPa or more, and having a standard deviation σ of the crystal grain size number of the prior γ grains of 0.80 or less has excellent SSC resistance and pitting corrosion resistance even in a CO2 environment, the details have not been clarified. However, it has been proven by the examples described later that a steel material having the above chemical composition, with CE being 0.45 or less, having a yield strength of 448 MPa or more, and further satisfying a standard deviation σ of the crystal grain size number of the prior γ grains of 0.80 or less has high strength, excellent hot workability, and excellent SSC resistance and pitting corrosion resistance even in a CO2 environment.

[0026] The gist of the steel material according to the present embodiment completed based on the above findings is as follows.

[0027] [1] By mass, C: 0.08% or less, Si: 0.60% or less, Mn: 1.0 - 2.5%, P: 0.05% or less, S: 0.005% or less, Cu: 0.05 - 0.50%, Ni: 0.05 - 0.50%, Cr: 0.25 - 0.50%, Mo: 0.02 - 0.50%, Ti: 0.001 to 0.050%, Al: 0.005 to 0.100%, Ca: 0.0005 to 0.0100%, Sn: 0 to 0.100%, Sb: 0 to 0.100%, Total of Sn and Sb: 0.001 to 0.100%, N: 0.0150% or less, O: 0.0050% or less, V: 0 to 0.10%, Nb: 0 to 0.05%, B: 0 to 0.0005%, and The balance consists of Fe and impurities, CE defined by formula (1) is 0.45 or less, The yield strength is 448 MPa or more, In the microstructure, the standard deviation of the crystal grain size number of prior austenite grains is 0.80 or less, Steel material. CE = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 (1) Here, for the element symbols in formula (1), the content of the corresponding element is substituted in units of mass%. When the corresponding element is not contained, "0" is substituted for that element symbol.

[0028] [2] The steel material according to [1], V: 0.01 to 0.10%, Nb: 0.01 to 0.05%, and Contains one or more elements selected from the group consisting of B: 0.0001 to 0.0005%, Steel material.

[0029] [3] The steel material according to [1] or [2], The steel material is a seamless steel pipe, Steel material.

[0030] Note that the shape of the steel material according to this embodiment is not particularly limited. The steel material according to this embodiment may be a steel pipe, a round steel (solid material), or a steel plate. Note that the round steel means a bar steel having a circular cross-section perpendicular to the axial direction. Further, the steel pipe may be a seamless steel pipe or a welded steel pipe.

[0031] Hereinafter, the steel material according to this embodiment will be described in detail. In the following description, the SSC resistance and the pitting corrosion resistance are collectively referred to as "corrosion resistance".

[0032] [Chemical composition] The chemical composition of the steel material according to this embodiment contains the following elements. "% " regarding the elements means mass% unless otherwise specified.

[0033] C: 0.08% or less Carbon (C) is inevitably contained. That is, the lower limit of the C content is more than 0%. C increases the hardenability of the steel material and increases the strength of the steel material. On the other hand, if the C content is too high, even if the contents of other elements are within the range of this embodiment, the strength of the steel material becomes too high and the corrosion resistance of the steel material decreases. Therefore, the C content is 0.08% or less. The preferable lower limit of the C content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferable upper limit of the C content is 0.07%, more preferably 0.06%.

[0034] Si: 0.60% or less Silicon (Si) is inevitably contained. That is, the lower limit of the Si content is more than 0%. Si deoxidizes steel. On the other hand, if the Si content is too high, even if the contents of other elements are within the range of this embodiment, the hot workability of the steel material decreases. Therefore, the Si content is 0.60% or less. The preferable upper limit of the Si content is 0.58%, more preferably 0.55%, and even more preferably 0.45%. The preferable lower limit of the Si content for obtaining the above effects more effectively is 0.01%, more preferably 0.02%, and even more preferably 0.04%.

[0035] Mn: 1.0 - 2.5% Manganese (Mn) enhances the hardenability of the steel material and increases the strength of the steel material. Mn further enhances the hot workability of the steel material. If the Mn content is too low, even if the contents of other elements are within the scope of this embodiment, the above effects cannot be sufficiently obtained. On the other hand, if the Mn content is too high, even if the contents of other elements are within the scope of this embodiment, the hardenability of the steel material becomes too high and the toughness of the steel material decreases. Therefore, the Mn content is 1.0 - 2.5%. The preferable lower limit of the Mn content is 1.1%, more preferably 1.2%, and even more preferably 1.3%. The preferable upper limit of the Mn content is 2.3%, more preferably 2.0%, and even more preferably 1.9%.

[0036] P: 0.05% or less Phosphorus (P) is an impurity that is inevitably contained. That is, the lower limit of the P content is more than 0%. If the P content is too high, even if the contents of other elements are within the scope of this embodiment, P segregates at the grain boundaries and the corrosion resistance of the steel material decreases. Therefore, the P content is 0.05% or less. The preferable upper limit of the P content is 0.04%, more preferably 0.03%, and even more preferably 0.02%. The lower the P content, the better. However, an extreme reduction in the P content significantly increases the manufacturing cost. Therefore, considering industrial production, the preferable lower limit of the P content is 0.01%.

[0037] S: 0.005% or less Sulfur (S) is an impurity that is inevitably contained. That is, the lower limit of the S content is more than 0%. If the S content is too high, even if the contents of other elements are within the scope of this embodiment, coarse sulfides are formed in the steel material and the corrosion resistance of the steel material decreases. Therefore, the S content is 0.005% or less. The preferable upper limit of the S content is 0.004%, more preferably 0.003%. The lower the S content, the better. However, an extreme reduction in the S content significantly increases the manufacturing cost. Therefore, considering industrial production, the preferable lower limit of the S content is 0.001%.

[0038] Cu: 0.05 - 0.50% Copper (Cu) forms a film on the surface of the steel material together with Ni, enhancing the corrosion resistance of the steel material. If the Cu content is too low, even if the contents of other elements are within the scope of this embodiment, the above effects cannot be fully obtained. On the other hand, if the Cu content is too high, even if the contents of other elements are within the scope of this embodiment, the hardenability of the steel material becomes too high and the toughness of the steel material decreases. Therefore, the Cu content is 0.05 - 0.50%. The preferable lower limit of the Cu content is 0.06%, more preferably 0.07%, and even more preferably 0.08%. The preferable upper limit of the Cu content is 0.48%, more preferably 0.45%, and even more preferably 0.40%.

[0039] Ni: 0.05 - 0.50% Nickel (Ni) forms a film on the surface of the steel material together with Cu, enhancing the corrosion resistance of the steel material. If the Ni content is too low, even if the contents of other elements are within the scope of this embodiment, the above effects cannot be fully obtained. On the other hand, if the Ni content is too high, even if the contents of other elements are within the scope of this embodiment, the hardenability of the steel material becomes too high and the toughness of the steel material decreases. Therefore, the Ni content is 0.05 - 0.50%. The preferable lower limit of the Ni content is 0.06%, more preferably 0.07%, and even more preferably 0.08%. The preferable upper limit of the Ni content is 0.48%, more preferably 0.46%, and even more preferably 0.45%.

[0040] Cr: 0.25 - 0.50% Chromium (Cr) enhances the hardenability of the steel material and the corrosion resistance of the steel material. If the Cr content is too low, even if the contents of other elements are within the scope of this embodiment, the above effects cannot be fully obtained. On the other hand, if the Cr content is too high, even if the contents of other elements are within the scope of this embodiment, the hardenability of the steel material becomes too high and the toughness of the steel material decreases. Therefore, the Cr content is 0.25 - 0.50%. The preferable lower limit of the Cr content is 0.26%, more preferably 0.27%. The preferable upper limit of the Cr content is 0.48%, more preferably 0.46%, and even more preferably 0.45%.

[0041] Mo: 0.02 - 0.50% Molybdenum (Mo) enhances the hardenability of steel and improves the corrosion resistance of steel. If the Mo content is too low, even if the contents of other elements are within the scope of this embodiment, the above effects cannot be fully obtained. On the other hand, if the Mo content is too high, even if the contents of other elements are within the scope of this embodiment, the toughness of the steel may decrease. Therefore, the Mo content is 0.02 - 0.50%. The preferable lower limit of the Mo content is 0.03%, more preferably 0.05%, and even more preferably 0.08%. The preferable upper limit of the Mo content is 0.48%, more preferably 0.45%, and even more preferably 0.40%.

[0042] Ti: 0.001 - 0.050% Titanium (Ti) forms nitrides to refine crystal grains and increase the strength of steel. If the Ti content is too low, even if the contents of other elements are within the scope of this embodiment, the above effects cannot be fully obtained. On the other hand, if the Ti content is too high, even if the contents of other elements are within the scope of this embodiment, coarse nitrides are formed and the hot workability of the steel decreases. Therefore, the Ti content is 0.001 - 0.050%. The preferable lower limit of the Ti content is 0.002%, more preferably 0.003%, and even more preferably 0.004%. The preferable upper limit of the Ti content is 0.045%, more preferably 0.040%, even more preferably 0.035%, and even more preferably 0.032%.

[0043] Al: 0.005 - 0.100% Aluminum (Al) deoxidizes steel. If the Al content is too low, even if the contents of other elements are within the scope of this embodiment, the above effects cannot be fully obtained. On the other hand, if the Al content is too high, even if the contents of other elements are within the scope of this embodiment, coarse oxide inclusions are formed, and the hot workability and corrosion resistance of the steel material deteriorate. Therefore, the Al content is 0.005 to 0.100%. The preferable lower limit of the Al content is 0.010%, more preferably 0.015%, and even more preferably 0.020%. The preferable upper limit of the Al content is 0.090%, more preferably 0.080%, and even more preferably 0.070%. The "Al" content referred to in this specification means the content of "acid-soluble Al", that is, "sol.Al".

[0044] Ca: 0.0005 to 0.0100% Calcium (Ca) renders S in the steel material harmless as sulfide and enhances the hot workability of the steel material. If the Ca content is too low, even if the contents of other elements are within the scope of this embodiment, the above effects cannot be fully obtained. On the other hand, if the Ca content is too high, even if the contents of other elements are within the scope of this embodiment, coarse inclusions are formed, and the corrosion resistance of the steel material deteriorates. Therefore, the Ca content is 0.0005 to 0.0100%. The preferable lower limit of the Ca content is 0.0006%, more preferably 0.0008%, and even more preferably 0.0009%. The preferable upper limit of the Ca content is 0.0090%, more preferably 0.0080%.

[0045] Sn: 0 to 0.100% Sb: 0 to 0.100% Total of Sn and Sb: 0.001 to 0.100%

[0046] Tin (Sn) enhances the corrosion resistance of the steel material. As described later, Sn may not be contained, and the lower limit of the Sn content is 0%. On the other hand, if the Sn content is too high, even if the contents of other elements are within the range of this embodiment, the hot workability of the steel material will decrease. Therefore, the Sn content is 0 to 0.100%. The preferable lower limit of the Sn content is more than 0%, more preferably 0.001%, and even more preferably 0.003%. The preferable upper limit of the Sn content is 0.098%, more preferably 0.095%, and even more preferably 0.090%.

[0047] Antimony (Sb) enhances the corrosion resistance of the steel material. As described later, Sb may not be contained, and the lower limit of the Sb content is 0%. On the other hand, if the Sb content is too high, even if the contents of other elements are within the range of this embodiment, Sb segregates at the grain boundaries, and the hot workability of the steel material will decrease. Therefore, the Sb content is 0 to 0.100%. The preferable lower limit of the Sb content is more than 0%, more preferably 0.001%, and even more preferably 0.003%. The preferable upper limit of the Sb content is 0.098%, more preferably 0.095%, and even more preferably 0.090%.

[0048] In the steel material according to this embodiment, Sn and Sb may contain only one of them, and the content of the other may be 0%. Sn and Sb may further contain both. As described above, both Sn and Sb enhance the corrosion resistance of the steel material. If the total of the Sn content and the Sb content is too low, even if the contents of other elements are within the range of this embodiment, the above effects cannot be sufficiently obtained. On the other hand, if the total of the Sn content and the Sb content is too high, even if the contents of other elements are within the range of this embodiment, the hot workability of the steel material will decrease. Therefore, the total of the Sn content and the Sb content is 0.001 to 0.100%. The preferable lower limit of the total of the Sn content and the Sb content is 0.002%, more preferably 0.003%, and even more preferably 0.004%. The preferable upper limit of the total of the Sn content and the Sb content is 0.098%, more preferably 0.095%, and even more preferably 0.090%.

[0049] N: Below 0.0150% Nitrogen (N) is inevitably contained. That is, the lower limit of the N content is more than 0%. N combines with Ti to form Ti nitride, which refines the crystal grains. On the other hand, if the N content is too high, even if the contents of other elements are within the range of this embodiment, the Ti nitride coarsens and the corrosion resistance of the steel material decreases. Therefore, the N content is 0.0150% or less. The preferable upper limit of the N content is 0.0140%, more preferably 0.0135%, and even more preferably 0.0120%. The preferable lower limit of the N content for more effectively obtaining the above effects is 0.0010%, more preferably 0.0020%, and even more preferably 0.0030%.

[0050] O: 0.0050% or less Oxygen (O) is an impurity inevitably contained. That is, the lower limit of the O content is more than 0%. O forms oxides, which reduce the hot workability and corrosion resistance of the steel material. Therefore, if the O content is too high, even if the contents of other elements are within the range of this embodiment, the hot workability and corrosion resistance of the steel material decrease. Therefore, the O content is 0.0050% or less. The preferable upper limit of the O content is 0.0048%, more preferably 0.0045%, and even more preferably 0.0040%. It is preferable that the O content is as low as possible. However, an extreme reduction in the O content increases the manufacturing cost. Therefore, considering industrial production, the preferable lower limit of the O content is 0.0001%, more preferably 0.0003%, and even more preferably 0.0005%.

[0051] The remainder of the chemical composition of the steel material according to this embodiment consists of Fe and impurities. Here, the impurities in the chemical composition refer to those mixed from ores, scraps, or manufacturing environments as raw materials during the industrial production of the steel material, and are those allowed within a range that does not adversely affect the steel material according to this embodiment.

[0052] [Optional element] The chemical composition of the steel material according to this embodiment may further contain at least one element selected from the group consisting of V, Nb, and B in place of a part of Fe. These elements are all optional elements and increase the strength of the steel material.

[0053] V: 0 to 0.10% Vanadium (V) is an optional element and may not be contained. That is, the V content may be 0%. When contained, V forms carbides or the like and increases the strength of the steel material. Even if a little V is contained, the above effects can be obtained to some extent. However, if the V content is too high, even if the contents of other elements are within the range of this embodiment, carbides or the like are excessively formed and the corrosion resistance of the steel material decreases. Therefore, the V content is 0 to 0.10%. The preferable lower limit of the V content is more than 0%, more preferably 0.01%, and even more preferably 0.02%. The preferable upper limit of the V content is 0.09%, more preferably 0.08%, and even more preferably 0.07%.

[0054] Nb: 0 to 0.05% Niobium (Nb) is an optional element and may not be contained. That is, the Nb content may be 0%. When contained, Nb forms carbides or the like and increases the strength of the steel material. Even if a little Nb is contained, the above effects can be obtained to some extent. However, if the Nb content is too high, even if the contents of other elements are within the range of this embodiment, carbides or the like are excessively formed and the corrosion resistance of the steel material decreases. Therefore, the Nb content is 0 to 0.05%. The preferable lower limit of the Nb content is more than 0%, more preferably 0.01%, and even more preferably 0.02%. The preferable upper limit of the Nb content is 0.04%.

[0055] B: 0 to 0.0005% Boron (B) is an optional element and may not be contained. That is, the B content may be 0%. When contained, B enhances the hardenability of the steel material and increases the strength of the steel material. Even if a small amount of B is contained, the above effects can be obtained to a certain extent. On the other hand, if the B content is too high, even if the contents of other elements are within the range of this embodiment, coarse nitrides are formed and the corrosion resistance of the steel material decreases. Therefore, the B content is 0 to 0.0005%. The preferable lower limit of the B content is more than 0%, more preferably 0.0001%. The preferable upper limit of the B content is 0.0004%, more preferably 0.0003%.

[0056] [CE] The steel material according to this embodiment has the above chemical composition and the CE defined by the following formula (1) is 0.45 or less. CE = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 (1) Here, for the element symbols in formula (1), the contents of the corresponding elements are substituted in units of mass%. When the corresponding element is not contained, "0" is substituted for the element symbol.

[0057] CE is the carbon equivalent and is an index of hot workability. If CE is too high, the strength may become too high and the hot workability of the steel material may decrease. On the other hand, if it has the above chemical composition and CE is 0.45 or less, the hot workability of the steel material can be stably enhanced. Therefore, the steel material according to this embodiment has CE of 0.45 or less on the premise of having the above chemical composition.

[0058] The preferable upper limit of CE is 0.44, more preferably 0.43. The lower limit of CE is not particularly limited, for example, it is 0.24. The lower limit of CE may be 0.25, may be 0.26, or may be 0.28. Note that CE is obtained by rounding the third decimal place of the obtained numerical value.

[0059] [Yield strength] The yield strength of the steel material according to this embodiment is 448 MPa or more (65 ksi or more). The upper limit of the yield strength of the steel material according to this embodiment is not particularly limited. For example, it is 586 MPa (85 ksi). That is, the yield strength of the steel material according to this embodiment may be 448 to 586 MPa. The preferable lower limit of the yield strength is 450 MPa, more preferably 455 MPa, and even more preferably 460 MPa. The upper limit of the yield strength may be 552 MPa (80 ksi) or 517 MPa (75 ksi).

[0060] In this embodiment, the tensile strength and yield ratio of the steel material are not particularly limited. The tensile strength of the steel material is, for example, 531 to 689 MPa. The yield ratio of the steel material is, for example, 0.75 or more. If the yield ratio of the steel material is 0.75 or more, the corrosion resistance of the steel material can be stably enhanced. Therefore, in this embodiment, the yield ratio is preferably 0.75 to 1.00. Note that the yield ratio (%) can be obtained as the ratio of the yield strength to the tensile strength.

[0061] In this embodiment, the yield strength, tensile strength, and yield ratio of the steel material are determined by the following method. Specifically, a tensile test is performed by a method compliant with JIS Z 2241 (2011). A test piece is fabricated from the steel material according to this embodiment. When the steel material is a steel plate, a tensile test piece is fabricated from the center of the plate thickness. In this case, the longitudinal direction of the tensile test piece is parallel to the rolling direction of the steel plate. When the steel material is a steel pipe, a tensile test piece or an arc-shaped test piece is fabricated from the center of the wall thickness. In this case, the longitudinal direction of the tensile test piece or the arc-shaped test piece is parallel to the pipe axis direction of the steel pipe. When the steel material is a round bar, a tensile test piece is fabricated from the R / 2 position. In this specification, the R / 2 position of the round bar means the central position of the radius R in a cross section perpendicular to the axial direction of the round bar. In this case, the longitudinal direction of the tensile test piece is parallel to the axial direction of the round bar.

[0062] For the tensile test piece, a No. 4 test piece (round bar test piece) specified in JIS Z 2241 (2011) is used. Also, depending on the thickness of the steel material, any of the No. 12A, No. 12B, or No. 12C test pieces (arc-shaped test pieces) specified in JIS Z 2241 (2011) may be used as the tensile test piece. Using the produced tensile test piece, a tensile test is carried out at room temperature (25°C) in the air, and the obtained 0.2% offset yield strength is defined as the yield strength (MPa). Also, the maximum stress during uniform elongation is defined as the tensile strength (MPa). The ratio of the yield strength to the tensile strength is defined as the yield ratio. Note that the yield strength (MPa) is obtained by rounding off the first decimal place of the obtained value. The tensile strength (MPa) is obtained by rounding off the first decimal place of the obtained value. The yield ratio is obtained by rounding off the third decimal place of the obtained value.

[0063] [Standard deviation σ of the crystal grain size number of prior austenite grains] The steel material according to this embodiment has the above-described chemical composition, CE is 0.45 or less, and further, the standard deviation σ of the crystal grain size number of prior austenite grains (prior γ grains) is 0.80 or less. As a result, even if the steel material according to this embodiment has a yield strength of 448 MPa or more, it has excellent hot workability, excellent SSC resistance and pitting corrosion resistance in a CO2 environment.

[0064] Here, when the standard deviation σ of the crystal grain size number of prior γ grains is large, regions where coarse prior γ grains (coarse grains) are unevenly distributed and regions where fine prior γ grains (fine grains) are unevenly distributed are generated in the steel material. As a result, in a CO2 environment, there is a possibility that a local decrease in SSC resistance and / or pitting corrosion resistance is likely to occur in the region where coarse grains are unevenly distributed. Therefore, for the steel material according to this embodiment, the standard deviation σ of the crystal grain size number of prior γ grains is 0.80 or less.

[0065] In this embodiment, the preferable upper limit of the standard deviation σ of the crystal grain size number of the prior γ grains is 0.79, more preferably 0.78, and even more preferably 0.77. In the steel material according to this embodiment, the standard deviation σ of the crystal grain size number of the prior γ grains is preferably small. That is, the lower limit of the standard deviation σ of the crystal grain size number of the prior austenite grains may be 0.00, 0.05, 0.10, or 0.15.

[0066] In addition, in the steel material according to this embodiment, the crystal grain size number of the prior γ grains only needs to have a standard deviation σ of 0.80 or less, and is not particularly limited. In this embodiment, the crystal grain size number of the prior γ grains is, for example, 0.0 or more. The lower limit of the crystal grain size number of the prior γ grains may be 0.5, 1.0, or 1.5. The upper limit of the crystal grain size number of the prior γ grains may be 9.0, 8.0, 7.0, 6.0, or 5.0.

[0067] In the steel material according to this embodiment, the standard deviation σ of the crystal grain size number of the prior γ grains can be obtained by the following method. Specifically, a test piece for microstructure observation is prepared from the steel material according to this embodiment. When the steel material is a steel plate, the test piece is prepared from the center of the plate thickness. When the steel material is a steel pipe, the test piece is prepared from the center of the wall thickness. When the steel material is round steel, the test piece is prepared from the R / 2 position. The size of the test piece only needs to obtain an observation surface as described later and is not particularly limited.

[0068] After polishing the observation surface of the prepared test piece to a mirror surface, it is immersed in a solution obtained by mixing an appropriate amount of surfactant into a saturated aqueous picric acid solution for about 60 seconds to reveal the prior γ grain boundaries by etching. Ten arbitrary fields are specified from the observation surface, and observation is carried out with an optical microscope to generate a photographic image. The magnification in the microscope observation can be appropriately set according to the crystal grain size. Specifically, in the microscope observation, for example, the magnification is set so that 50 or more crystal grains are included in the field of view.

[0069] For each field of view, perform image analysis on the obtained photographic image and measure the grain size number in accordance with ASTM E112 (2021). That is, obtain one grain size number for each observation field of view. Calculate the standard deviation of the obtained 10 grain size numbers and define it as the standard deviation σ of the grain size number of the prior austenite grains. Note that the standard deviation σ of the grain size number of the prior austenite grains is obtained by rounding the third decimal place of the obtained value.

[0070] [Hot workability] The steel material according to the present embodiment has the above-described chemical composition, CE is 0.45 or less, and further, the standard deviation σ of the grain size number of the prior austenite grains (prior γ grains) is 0.80 or less. As a result, even if the steel material according to the present embodiment has a yield strength of 448 MPa or more, it has excellent hot workability and excellent SSC resistance and pitting corrosion resistance in a CO2 environment. In the present embodiment, having excellent hot workability is defined as follows.

[0071] Perform a hot workability test (Gleeble test) on the steel material according to the present embodiment. Specifically, prepare a test piece for the Gleeble test from the steel material according to the present embodiment. When the steel material is a steel plate, prepare the test piece from the center of the plate thickness. In this case, the longitudinal direction of the test piece is parallel to the rolling direction of the steel plate. When the steel material is a steel pipe, prepare the test piece from the center of the wall thickness. In this case, the longitudinal direction of the test piece is parallel to the pipe axis direction of the steel pipe. When the steel material is a round bar, prepare the test piece from the R / 2 position. In this case, the longitudinal direction of the test piece is parallel to the axis direction of the round bar. The size of the test piece is, for example, a round bar test piece with a diameter of 10 mm and a length of 130 mm.

[0072] The tensile temperatures for the Gleeble test are five points: 1200°C, 1150°C, 1100°C, 1050°C, and 950°C. After heating the prepared test piece to 1230°C or higher, cool it to the tensile temperature at a rate of 100°C / second and hold it at the tensile temperature for 30 seconds. For the test piece held for 30 seconds, a strain rate of 10s -1Perform a tensile test to break the test piece. Note that different test pieces are used for each tensile temperature. Determine the reduction value (%) from the broken test piece. If the reduction values obtained at all tensile temperatures exceed 75%, it is evaluated as having excellent hot workability.

[0073] [SSC resistance] The steel material according to the present embodiment has the above-described chemical composition, CE is 0.45 or less, and the standard deviation σ of the crystal grain size number of the prior γ grains is 0.80 or less. As a result, even if the steel material according to the present embodiment has a yield strength of 448 MPa or more, it has excellent hot workability, excellent SSC resistance and pitting corrosion resistance in a CO2 environment. In the present embodiment, having excellent SSC resistance in a CO2 environment is defined as follows.

[0074] Perform a four-point bending test on the steel material according to the present embodiment. Specifically, prepare a test piece for the four-point bending test from the steel material according to the present embodiment. When the steel material is a steel plate, prepare the test piece from the center of the plate thickness. In this case, the longitudinal direction of the test piece is parallel to the rolling direction of the steel plate. When the steel material is a steel pipe, prepare the test piece from the center of the wall thickness. In this case, the longitudinal direction of the test piece is parallel to the pipe axis direction of the steel pipe. When the steel material is round steel, prepare the test piece from the R / 2 position. In this case, the longitudinal direction of the test piece is parallel to the axis direction of the round steel. The size of the test piece is, for example, 75 mm in length, 10 mm in width, and 2 mm in thickness.

[0075] In accordance with NACE TM0316 (2016), apply a stress corresponding to 100% of the actual yield stress to the test piece by four-point bending. Enclose the test piece with the test jig in an autoclave. Inject a 5.0 mass% aqueous sodium chloride solution into the autoclave so that the test piece is immersed. Pressurize and seal a mixed gas of H2S and CO2 into the autoclave to saturate the test solution to form a test bath. At this time, the total pressure of the mixed gas is 130 bar, and the partial pressure of H2S gas in the mixed gas is 0.1 bar. After sealing the autoclave, maintain the test bath at 50 °C and immerse the test piece for 720 hours while stirring the test bath.

[0076] After 720 hours have elapsed, observe the surface of the test piece with a magnifying glass having a magnification of 10 times to confirm the presence or absence of cracks. If the occurrence of cracks is suspected based on the observation with the magnifying glass, further cut out the portion where the occurrence of cracks is suspected in the test piece length direction, and observe the cross section with an optical microscope at a magnification of 100 times to confirm the presence or absence of cracks. In the present embodiment, if no cracks are confirmed as a result of the four-point bending test under the above conditions, it is evaluated that the steel has excellent SSC resistance even in a CO2 environment.

[0077] [Pitting corrosion resistance] The steel material according to the present embodiment has the above-described chemical composition, CE is 0.45 or less, and the standard deviation σ of the crystal grain size number of the old γ grains is 0.80 or less. As a result, even if the steel material according to the present embodiment has a yield strength of 448 MPa or more, it has excellent hot workability, excellent SSC resistance and pitting corrosion resistance in a CO2 environment. In the present embodiment, having excellent pitting corrosion resistance in a CO2 environment is defined as follows.

[0078] A pitting corrosion resistance test is performed on the steel material according to the present embodiment. Specifically, a test piece for a four-point bending test is produced from the steel material according to the present embodiment. When the steel material is a steel plate, the test piece is produced from the center of the plate thickness. When the steel material is a steel pipe, the test piece is produced from the center of the wall thickness. When the steel material is a round steel, the test piece is produced from the R / 2 position. The size of the test piece is, for example, 30 mm in length, 30 mm in width, and 3 mm in thickness.

[0079] The test piece is enclosed in an autoclave. A 5.0 mass% aqueous sodium chloride solution is injected into the autoclave. Note that the test piece is not immersed in the test solution (test bath), but is held in the gas phase region of the autoclave. A mixed gas of H2S and CO2 is pressure-sealed in the autoclave to saturate the test solution to obtain a test bath. At this time, the total pressure of the mixed gas is 130 bar, and the partial pressure of H2S gas in the mixed gas is 0.1 bar. After sealing the autoclave, the test bath is maintained at 50°C, and the test piece is held for 720 hours while stirring the test bath.

[0080] Among the test pieces after 720 hours, observe the surface with a length of 30 mm and a width of 30 mm to confirm the presence or absence of pitting corrosion. Pitting corrosion is specified as having a depth of 50 μm or more and an equivalent circle diameter of 1 μm or more. Specifically, observe the surface of the above-mentioned test piece with a magnifying glass with a magnification of 10 times to confirm the presence or absence of pitting corrosion. When pitting corrosion is confirmed, count the number of pitting corrosion. In this embodiment, as a result of the pitting corrosion resistance test under the above conditions, when the number of pitting corrosion is 10 or less, it is evaluated as having excellent pitting corrosion resistance even in a CO2 environment.

[0081] [Shape of steel material] As described above, the shape of the steel material according to this embodiment is not particularly limited. Preferably, the steel material according to this embodiment is a seamless steel pipe. When the steel material according to this embodiment is a seamless steel pipe, even if the wall thickness is 5 mm or more, it has a yield strength of 448 MPa or more, excellent hot workability, and excellent SSC resistance and pitting corrosion resistance in a CO2 environment.

[0082] [Microstructure] Preferably, the microstructure of the steel material according to this embodiment mainly consists of tempered bainite. More specifically, the microstructure of the steel material according to this embodiment has a volume ratio of tempered bainite of 90% or more. The remainder of the microstructure is, for example, ferrite or pearlite. In addition, the microstructure of the steel material according to this embodiment may contain a small amount of precipitates, inclusions, etc. in addition to tempered bainite, ferrite, and pearlite. However, in the microstructure of the steel material according to this embodiment, the volume ratio of precipitates, inclusions, etc. is negligibly small compared to tempered bainite, ferrite, and pearlite.

[0083] In addition, when the volume ratio of tempered bainite is obtained by observation, it can be obtained by the following method. First, prepare a test piece for microstructure observation from the steel material according to this embodiment. When the steel material is a steel plate, prepare a test piece with a surface including the rolling direction and the plate thickness direction from the center of the plate thickness as the observation surface. When the steel material is a steel pipe, cut out a test piece with a surface including the pipe axis direction and the pipe diameter direction from the center of the wall thickness as the observation surface. When the steel material is a round steel, cut out a test piece with a surface including the R / 2 position at the center and the axial direction as the observation surface.

[0084] After polishing the observation surface of the test piece to a mirror finish, immerse it in a nital etching solution for about 10 seconds to reveal the structure by etching. Observe the etched observation surface with a scanning electron microscope (SEM) in secondary electron images for 10 fields of view. The field of view area is, for example, 0.01 mm 2 (at a magnification of 1000 times). In each field of view, identify tempered bainite from the contrast. Determine the area ratio of the identified tempered bainite. The method for determining the area ratio is not particularly limited and may be a well-known method. For example, the area ratio of tempered bainite can be determined by image analysis. In this embodiment, the arithmetic mean value of the area ratios of tempered bainite obtained in all fields of view is defined as the volume ratio of tempered bainite.

[0085] [Manufacturing method] The manufacturing method of the steel material according to this embodiment will be described. Hereinafter, as an example of the steel material according to this embodiment, the manufacturing method of seamless steel pipe will be described. The manufacturing method of seamless steel pipe includes a step of preparing a base pipe (preparation step) and a step of performing quenching and tempering on the base pipe to obtain a seamless steel pipe (quenching step and tempering step). Note that the manufacturing method according to this embodiment is not limited to the manufacturing method described below. Each step will be described in detail below.

[0086] [Preparation step] In the preparation step, prepare an intermediate steel material having the above chemical composition. The manufacturing method of the intermediate steel material is not particularly limited as long as the intermediate steel material has the above chemical composition. The intermediate steel material referred to here is a plate-shaped steel material when the final product is a steel plate, and a base pipe when the final product is a steel pipe.

[0087] The preparation step may include a step of preparing a raw material (raw material preparation step) and a step of hot working the raw material to manufacture an intermediate steel material (hot working step). Hereinafter, the case including the raw material preparation step and the hot working step will be described in detail.

[0088] [Raw material preparation step] In the material preparation process, a material is manufactured using molten steel having the above-described chemical composition. The method for manufacturing the material is not particularly limited and may be a well-known method. Specifically, a slab (slab, bloom, or billet) may be manufactured by continuous casting using molten steel. An ingot may be manufactured by an ingot-making method using molten steel. If necessary, a billet may be manufactured by block rolling a slab, bloom, or ingot. A material (slab, bloom, or billet) is manufactured through the above steps.

[0089] [Hot working process] In the hot working process, the prepared material is hot worked to manufacture an intermediate steel product. When the steel product is a seamless steel pipe, the intermediate steel product corresponds to a plain pipe. First, the billet is heated in a heating furnace. Hot working is performed on the billet extracted from the heating furnace to manufacture a plain pipe (seamless steel pipe).

[0090] In this embodiment, it is preferable to set the conditions for heating the billet as follows. Heating temperature T: 1150 - 1300 °C Holding time t: 30 - 500 minutes Furthermore, the heating temperature T (°C) and the holding time t (minutes) satisfy the following formula (A). 30000 ≤ (273 + T) × (20 + Log(t / 60)) ≤ 32000 (A)

[0091] Here, the heating temperature T of the billet means the temperature (°C) of the heat treatment furnace when heating the billet. The holding time t in the heating of the billet means the time (minutes) held at the heating temperature T. If the heating temperature T is too high, the austenite grains may coarsen. On the other hand, if the heating temperature T is too low, the heating of the billet may be insufficient, and the load on the hot working equipment may be too high. Also, if the holding time t when heating the billet is too short, the heating of the billet may be insufficient, and the load on the hot working equipment may be too high. On the other hand, even if the holding time t is too long, the heating effect saturates.

[0092] In this embodiment, it is further preferable to satisfy the above formula (A). Here, LMP is defined as LMP = (273 + T) × (20 + Log(t / 60)). If LMP is too low, the heating of the billet may be insufficient, and temperature unevenness may occur in the material during hot working. In this case, the variation of the prior γ grains becomes large, and the standard deviation σ of the crystal grain size number of the prior γ grains becomes large. On the other hand, if LMP is too high, the Ostwald growth of the pinning particles typified by Ti nitride is promoted, and unevenness may occur in the size and distribution of the pinning particles. In this case, the variation of the prior γ grains becomes large, and the standard deviation σ of the crystal grain size number of the prior γ grains becomes large. Therefore, in the hot working process according to this embodiment, it is preferable that LMP during heating of the billet is 30000 to 32000.

[0093] Hot working is performed on the material extracted from the heating furnace to produce an intermediate steel material. The method of hot working is not particularly limited. However, when the intermediate steel material is a plain tube, it is preferable to perform the Mannesmann method to produce the plain tube. In this case, the round billet is pierced and rolled by a piercing mill. When piercing and rolling, the piercing ratio is not particularly limited. For example, it is 1.0 to 4.0. The pierced and rolled round billet is further hot rolled into a plain tube by a mandrel mill, a reducer, a sizing mill, etc.

[0094] When manufacturing a seamless tube by implementing the Mannesmann process, it is preferable that the time from the perforation rolling to the hot rolling (elongation rolling) by a mandrel mill be 30 to 180 seconds. In this specification, the time from the perforation rolling to the start of the elongation rolling is also referred to as the "perforation-elongation holding time". By having a certain perforation-elongation holding time, austenite grains tend to grow during the holding, and the size of the austenite grains tends to be uniform. Therefore, in the manufactured steel material, the standard deviation σ of the crystal grain size number of the prior γ grains can be reduced. On the other hand, if the perforation-elongation holding time is too long, the temperature of the intermediate steel material may decrease, and hot workability may not be obtained. In this case, furthermore, the austenite grains may grow too much, and the standard deviation σ of the crystal grain size number of the prior γ grains may rather increase. Therefore, in the hot working process according to this embodiment, it is preferable that the perforation-elongation holding time be 30 to 180 seconds.

[0095] The intermediate steel material produced by hot working may be air-cooled (As-Rolled), may be quenched directly after hot working without being cooled to room temperature, or may be quenched after reheating (reheating) after hot working. Hereinafter, the quenching process will be described in detail.

[0096] [Quenching Process] In the quenching process, quenching is performed on the prepared intermediate steel material. In this specification, "quenching" means rapidly cooling an intermediate steel material at or above the A3 point. The preferable quenching temperature is 850 to 1000 °C. If the quenching temperature is too high, the prior γ grains may coarsen, and corrosion resistance may not be obtained. Therefore, the quenching temperature is preferably 850 to 1000 °C.

[0097] The quenching method is, for example, continuously cooling the intermediate steel material from the quenching start temperature and continuously reducing the surface temperature of the intermediate steel material. The method of the continuous cooling treatment is not particularly limited and may be a well-known method. The method of the continuous cooling treatment is, for example, a method of immersing the intermediate steel material in a water tank for cooling or a method of accelerating the cooling of the intermediate steel material by shower water cooling or mist cooling. Hereinafter, the tempering process will be described in detail.

[0098] [Tempering Process] In the tempering process, tempering is performed on the intermediate steel material on which the above-mentioned quenching has been carried out. In this specification, "tempering" means reheating the intermediate steel material after quenching to a temperature below point A and holding it. Here, the tempering temperature corresponds to the temperature of the furnace when reheating and holding the intermediate steel material after quenching. The tempering time means the time from when the temperature of the intermediate steel material reaches a predetermined tempering temperature until it is extracted from the heat treatment furnace. c1 The tempering temperature is appropriately adjusted according to the chemical composition of the steel material and the yield strength to be obtained. That is, for the intermediate steel material having the chemical composition of this embodiment, the tempering temperature is adjusted so that the yield strength of the steel material is adjusted to 448 MPa or more. In the tempering process according to this embodiment, the preferable tempering temperature is 500 to 700°C. Also, in the tempering process of this embodiment, the tempering time is preferably 5 to 240 minutes.

[0099] The steel material according to this embodiment can be manufactured by the above manufacturing method. However, as described above, the above manufacturing method is an example, and it may be manufactured by other manufacturing methods. Hereinafter, the present invention will be described more specifically by examples.

[0100] Molten steel having the chemical compositions shown in Table 1A and Table 1B was produced. Note that "-" in Table 1A and Table 1B means that the content of each element is at the impurity level. Specifically, the V content and Nb content of Test No. 1, the Cu content and Ni content of Test No. 22 mean that they were 0% after rounding off the third decimal place. The Sb content of Test No. 1 and the Sn content of Test No. 2 mean that they were 0% after rounding off the fourth decimal place. The B content of Test No. 1 means that it was 0% after rounding off the fifth decimal place. Also, the element contents of each test number and CE (=C+Mn / 6+(Cu+Ni) / 15+(Cr+Mo+V) / 5) obtained from the above formula (1) are shown in Table 1B.

Examples

[0101]

[0102] ​

Table 1A

[0103]

Table 1B

[0104] Using the molten steel of each test number, round billets were produced by the continuous casting method. The round billets of each test number were heated using a heating furnace to perform hot working. The heating temperature T (°C), the holding time t (minutes), and the LMP obtained from the above formula (A) (= (273 + T) × (20 + Log(t / 60))) during the heating before hot working were as shown in Table 2.

[0105]

Table 2

[0106] Piercing rolling and stretching rolling were performed on the heated round billets. At this time, the time from the end of piercing rolling to the start of stretching rolling (piercing - stretching holding time) is shown in Table 2. In Table 2, "A (Appropriate)" in the piercing - stretching holding time column means that the piercing - stretching holding time was 30 to 180 seconds. In Table 2, "S (Short)" in the piercing - stretching holding time column means that the piercing - stretching holding time was less than 30 seconds. In Table 2, "L (Long)" in the piercing - stretching holding time column means that the piercing - stretching holding time exceeded 180 seconds. Also, the wall thickness of the manufactured intermediate steel material (plain pipe) is shown in Table 2.

[0107] Quenching and tempering were performed on the manufactured plain pipes. Specifically, for the plain pipes of each test number, after holding at 850 to 1000 °C for 5 to 90 minutes, quenching by water cooling was performed. Further, tempering was performed on the quenched plain pipes of each test number by holding at the temperature (°C) and for the time (minutes) shown in Table 2. Through the above manufacturing process, seamless steel pipes of each test number were obtained.

[0108] [Evaluation Test] For the seamless pipes of each test number after the above tempering, the following crystal grain size number measurement test, tensile test, Gleeble test, four-point bending test, and pitting corrosion resistance test were carried out.

[0109] [Crystal Grain Size Number Measurement Test] For the seamless pipes of each test number, a crystal grain size number measurement test was carried out to obtain the standard deviation σ of the crystal grain size number of the prior γ grains. Specifically, for the test pieces prepared by the above method, microscopic observation was carried out by the above method. Image analysis was carried out on the photographic images obtained by microscopic observation, and the crystal grain size number was measured in accordance with ASTM E112 (2021). For each test number, the crystal grain size numbers obtained in 10 fields of view are shown in Table 3A. The average value of the crystal grain size numbers obtained from the 10 obtained crystal grain size numbers and the standard deviation σ are shown in Table 3A.

[0110]

Table 3A

[0111] [Tensile Test] For the seamless pipes of each test number, a tensile test was carried out by a method conforming to JIS Z 2241 (2011). Specifically, from the central part of the wall thickness of the seamless pipes of each test number, a No. 4 test piece specified in JIS Z 2241 (2011) was prepared as a tensile test piece. The longitudinal direction of the tensile test piece was the pipe axis direction of the steel pipe. Using the prepared tensile test piece, a tensile test conforming to JIS Z 2241 (2011) was carried out at room temperature (25°C) in the air, and the obtained 0.2% offset yield strength was defined as the yield strength (MPa). The maximum stress during the uniform elongation obtained by the same tensile test was defined as the tensile strength (MPa). The ratio of the yield strength to the tensile strength was defined as the yield ratio. For the seamless pipes of each test number, the obtained yield strength (MPa) is shown as "YS (MPa)", the tensile strength is shown as "TS (MPa)", and the yield ratio is shown as "YR" in Table 3B.

[0112]

Table 3B

[0113] [Greeble test] For seamless pipes with each test number, a Greeble test was carried out to evaluate the hot workability. Specifically, five test pieces were prepared by the above-mentioned method and heated to the heating temperature T (°C) of the pre-hot working heating described in Table 2. Then, at each tensile temperature (1200 °C, 1150 °C, 1100 °C, 1050 °C, 950 °C), a tensile test was carried out by the above-mentioned method to break the test pieces. By the above method, for the test pieces with each test number, the reduction values were obtained for each tensile temperature. When the reduction values exceeded 75% at all tensile temperatures, it was judged to have excellent hot workability (「E」(Excellent) in Table 3B). On the other hand, when the reduction value was 75% or less at at least one tensile temperature, it was judged not to have excellent hot workability (「NA」(Not Acceptable) in Table 3B).

[0114] [Four-point bending test] For seamless pipes with each test number except test numbers 16 - 18 and 28, four-point bending was carried out to evaluate the SSC resistance. Specifically, for the test pieces prepared by the above-mentioned method, in accordance with NACE TM0316 (2016), a stress corresponding to 100% of the actual yield stress was applied to the test pieces by four-point bending. The test pieces were immersed for 720 hours while stirring the test bath under the above-mentioned conditions. For the surface of the test pieces after 720 hours, the presence or absence of cracks was confirmed by the above-mentioned method. When no cracks were confirmed, it was judged to have excellent SSC resistance even in a CO2 environment (「E」(Excellent) in Table 3B). On the other hand, when cracks were confirmed, it was judged not to have excellent SSC resistance in a CO2 environment (「NA」(Not Acceptable) in Table 3B). For test numbers 16 - 18 and 28, since excellent hot workability could not be obtained, the four-point bending test was not carried out (「-」 in Table 3B).

[0115] [Pitting corrosion resistance test] For seamless pipes of each test number except test numbers 16 to 18 and 28, a pitting corrosion resistance test was carried out to evaluate the pitting corrosion resistance. Specifically, the test pieces prepared by the above method were held in an autoclave for 720 hours under the above conditions. For the surface of the test pieces after 720 hours, the presence or absence of pitting corrosion was confirmed by the above method. For the test pieces of each test number, the number of pitting corrosion was counted to obtain the number of pitting corrosion (pieces). The obtained number of pitting corrosion (pieces) is shown in Table 3B. Note that for test numbers 16 to 18 and 28, since excellent hot workability could not be obtained, the pitting corrosion resistance test was not carried out ("-" in Table 3B).

[0116] [Evaluation Results] Referring to Table 1A, Table 1B, Table 2, Table 3A, and Table 3B, the seamless pipes of test numbers 1 to 15 had appropriate chemical compositions and a CE of 0.45 or less. These seamless pipes further had a yield strength of 448 MPa or more and a standard deviation of the crystal grain size number of old γ grains of 0.80 or less. As a result, these seamless pipes were judged to have excellent hot workability in the Gleeble test. These seamless pipes were further judged to have excellent SSC resistance even in a CO2 environment in the four-point bending test. These seamless pipes further had 10 or fewer pitting corrosion numbers and were judged to have excellent pitting corrosion resistance even in a CO2 environment.

[0117] On the other hand, for the seamless pipes of test numbers 16 to 18, the total content of Sn and Sb was too high. As a result, these seamless pipes were judged not to have excellent hot workability in the Gleeble test.

[0118] For the seamless pipe of test number 19, the total content of Sn and Sb was too low. As a result, this seamless pipe was judged not to have excellent SSC resistance in a CO2 environment in the four-point bending test. This seamless pipe further had more than 10 pitting corrosion numbers and was judged not to have excellent pitting corrosion resistance in a CO2 environment.

[0119] The seamless steel pipe of Test No. 20 had too low a Cu content. As a result, this seamless steel pipe was judged not to have excellent SSC resistance in a CO2 environment in the four-point bending test. Furthermore, this seamless steel pipe had more than 10 pitting corrosion pits and was judged not to have excellent pitting corrosion resistance in a CO2 environment.

[0120] The seamless steel pipe of Test No. 21 had too low a Ni content. As a result, this seamless steel pipe was judged not to have excellent SSC resistance in a CO2 environment in the four-point bending test. Furthermore, this seamless steel pipe had more than 10 pitting corrosion pits and was judged not to have excellent pitting corrosion resistance in a CO2 environment.

[0121] The seamless steel pipe of Test No. 22 had too low a Cu content and too low a Ni content. As a result, this seamless steel pipe was judged not to have excellent SSC resistance in a CO2 environment in the four-point bending test. Furthermore, this seamless steel pipe had more than 10 pitting corrosion pits and was judged not to have excellent pitting corrosion resistance in a CO2 environment.

[0122] The seamless steel pipe of Test No. 23 had too low a Mo content. As a result, this seamless steel pipe was judged not to have excellent SSC resistance in a CO2 environment in the four-point bending test. Furthermore, this seamless steel pipe had more than 10 pitting corrosion pits and was judged not to have excellent pitting corrosion resistance in a CO2 environment.

[0123] The seamless steel pipes of Test Nos. 24 and 25 had an LMP of less than 30000 during the heating before hot working. As a result, the standard deviation of the crystal grain size number of the old γ grains in these seamless steel pipes exceeded 0.80. As a result, these seamless steel pipes were judged not to have excellent SSC resistance in a CO2 environment in the four-point bending test. Furthermore, these seamless steel pipes had more than 10 pitting corrosion pits and were judged not to have excellent pitting corrosion resistance in a CO2 environment.

[0124] For the seamless pipe of Test No. 26, during the heating before hot working, the LMP exceeded 32000. As a result, for this seamless pipe, the standard deviation of the crystal grain size number of the old γ grains exceeded 0.80. As a result, it was determined that this seamless pipe did not have excellent SSC resistance in a CO2 environment in the four-point bending test. Furthermore, for this seamless pipe, the number of pitting corrosion holes exceeded 10, and it was determined that it did not have excellent pitting corrosion resistance in a CO2 environment.

[0125] For the seamless pipe of Test No. 27, the piercing - elongation holding time was too short. As a result, for this seamless pipe, the standard deviation of the crystal grain size number of the old γ grains exceeded 0.80. As a result, it was determined that this seamless pipe did not have excellent SSC resistance in a CO2 environment in the four-point bending test. Furthermore, for this seamless pipe, the number of pitting corrosion holes exceeded 10, and it was determined that it did not have excellent pitting corrosion resistance in a CO2 environment.

[0126] For the seamless pipe of Test No. 28, the piercing - elongation holding time was too long. As a result, for this seamless pipe, the standard deviation of the crystal grain size number of the old γ grains exceeded 0.80. As a result, it was determined that this seamless pipe did not have excellent hot workability in the Gleeble test.

[0127] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and the above-described embodiments can be appropriately modified and implemented without departing from the spirit thereof.

Claims

1. By mass percentage, C: 0.08% or less, Si: 0.60% or less, Mn: 1.0 - 2.5%, P: 0.05% or less, S: 0.005% or less, Cu: 0.05 - 0.50%, Ni: 0.05 - 0.50%, Cr: 0.25 - 0.50%, Mo: 0.02 - 0.50%, Ti: 0.001 - 0.050%, Al: 0.005 - 0.100%, Ca: 0.0005 - 0.0100%, Sn: 0 - 0.100%, Sb: 0 - 0.100%, Total of Sn and Sb: 0.001 - 0.100%, N: 0.0150% or less, O: 0.0050% or less, V: 0 - 0.10%, Nb: 0 - 0.05%, B: 0 - 0.0005%, and, The balance consists of Fe and impurities, CE defined by formula (1) is 0.45 or less, The yield strength is 448 MPa or more, In the microstructure, the standard deviation of the crystal grain size number of prior austenite grains is 0.80 or less, Steel material. CE = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 (1) Here, in the element symbols in formula (1), the content of the corresponding element is substituted in units of mass%. When the corresponding element is not contained, "0" is substituted for the element symbol.

2. The steel material according to Claim 1, wherein V: 0.01 - 0.10%, Nb: 0.01 - 0.05%, and, Contains one or more elements selected from the group consisting of B: 0.0001 - 0.0005%, Steel material.

3. The steel material according to Claim 1 or Claim 2, wherein The steel material is seamless steel pipe, Steel material.

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