CN115298340B - High-strength steel sheet for acid-proof pipeline, method for producing same, and high-strength steel pipe using high-strength steel sheet for acid-proof pipeline - Google Patents
High-strength steel sheet for acid-proof pipeline, method for producing same, and high-strength steel pipe using high-strength steel sheet for acid-proof pipeline Download PDFInfo
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- CN115298340B CN115298340B CN202180020748.6A CN202180020748A CN115298340B CN 115298340 B CN115298340 B CN 115298340B CN 202180020748 A CN202180020748 A CN 202180020748A CN 115298340 B CN115298340 B CN 115298340B
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- 229910000831 Steel Inorganic materials 0.000 title claims abstract description 225
- 239000010959 steel Substances 0.000 title claims abstract description 225
- 238000004519 manufacturing process Methods 0.000 title claims description 22
- SZVJSHCCFOBDDC-UHFFFAOYSA-N iron(II,III) oxide Inorganic materials O=[Fe]O[Fe]O[Fe]=O SZVJSHCCFOBDDC-UHFFFAOYSA-N 0.000 claims abstract description 24
- 239000002253 acid Substances 0.000 claims abstract description 16
- 239000012535 impurity Substances 0.000 claims abstract description 6
- 229910052748 manganese Inorganic materials 0.000 claims abstract description 5
- 229910052698 phosphorus Inorganic materials 0.000 claims abstract description 4
- 229910052757 nitrogen Inorganic materials 0.000 claims abstract description 3
- 229910052717 sulfur Inorganic materials 0.000 claims abstract description 3
- 238000001816 cooling Methods 0.000 claims description 90
- 238000005096 rolling process Methods 0.000 claims description 31
- 239000000203 mixture Substances 0.000 claims description 23
- 238000005098 hot rolling Methods 0.000 claims description 12
- 238000010438 heat treatment Methods 0.000 claims description 11
- 229910052720 vanadium Inorganic materials 0.000 claims description 6
- 229910052802 copper Inorganic materials 0.000 claims description 5
- 229910052758 niobium Inorganic materials 0.000 claims description 4
- 229910052719 titanium Inorganic materials 0.000 claims description 4
- RWSOTUBLDIXVET-UHFFFAOYSA-N Dihydrogen sulfide Chemical compound S RWSOTUBLDIXVET-UHFFFAOYSA-N 0.000 abstract description 22
- 229910000037 hydrogen sulfide Inorganic materials 0.000 abstract description 22
- 239000002344 surface layer Substances 0.000 description 25
- 238000012360 testing method Methods 0.000 description 21
- 238000000034 method Methods 0.000 description 19
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- 238000005336 cracking Methods 0.000 description 11
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- 239000000243 solution Substances 0.000 description 10
- 229910000859 α-Fe Inorganic materials 0.000 description 10
- 239000000463 material Substances 0.000 description 8
- VNWKTOKETHGBQD-UHFFFAOYSA-N methane Chemical compound C VNWKTOKETHGBQD-UHFFFAOYSA-N 0.000 description 8
- 238000003466 welding Methods 0.000 description 8
- 230000009466 transformation Effects 0.000 description 7
- CURLTUGMZLYLDI-UHFFFAOYSA-N Carbon dioxide Chemical compound O=C=O CURLTUGMZLYLDI-UHFFFAOYSA-N 0.000 description 6
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- 230000006872 improvement Effects 0.000 description 6
- 230000015572 biosynthetic process Effects 0.000 description 5
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- 238000011156 evaluation Methods 0.000 description 4
- 229910000734 martensite Inorganic materials 0.000 description 4
- 238000005259 measurement Methods 0.000 description 4
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- 230000008569 process Effects 0.000 description 4
- 239000006104 solid solution Substances 0.000 description 4
- UFHFLCQGNIYNRP-UHFFFAOYSA-N Hydrogen Chemical compound [H][H] UFHFLCQGNIYNRP-UHFFFAOYSA-N 0.000 description 3
- UCKMPCXJQFINFW-UHFFFAOYSA-N Sulphide Chemical compound [S-2] UCKMPCXJQFINFW-UHFFFAOYSA-N 0.000 description 3
- 238000005452 bending Methods 0.000 description 3
- 229910002092 carbon dioxide Inorganic materials 0.000 description 3
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- 238000007654 immersion Methods 0.000 description 3
- 238000007373 indentation Methods 0.000 description 3
- LIKBJVNGSGBSGK-UHFFFAOYSA-N iron(3+);oxygen(2-) Chemical compound [O-2].[O-2].[O-2].[Fe+3].[Fe+3] LIKBJVNGSGBSGK-UHFFFAOYSA-N 0.000 description 3
- 210000001503 joint Anatomy 0.000 description 3
- 229910001562 pearlite Inorganic materials 0.000 description 3
- 238000005728 strengthening Methods 0.000 description 3
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 3
- 238000002441 X-ray diffraction Methods 0.000 description 2
- 229910001566 austenite Inorganic materials 0.000 description 2
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- 229910052750 molybdenum Inorganic materials 0.000 description 2
- 229910052759 nickel Inorganic materials 0.000 description 2
- 239000003129 oil well Substances 0.000 description 2
- 230000003287 optical effect Effects 0.000 description 2
- 239000011028 pyrite Substances 0.000 description 2
- NIFIFKQPDTWWGU-UHFFFAOYSA-N pyrite Chemical compound [Fe+2].[S-][S-] NIFIFKQPDTWWGU-UHFFFAOYSA-N 0.000 description 2
- 229910052683 pyrite Inorganic materials 0.000 description 2
- 230000005855 radiation Effects 0.000 description 2
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- 229920006395 saturated elastomer Polymers 0.000 description 2
- 238000009864 tensile test Methods 0.000 description 2
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- LFQSCWFLJHTTHZ-UHFFFAOYSA-N Ethanol Chemical compound CCO LFQSCWFLJHTTHZ-UHFFFAOYSA-N 0.000 description 1
- YZCKVEUIGOORGS-UHFFFAOYSA-N Hydrogen atom Chemical compound [H] YZCKVEUIGOORGS-UHFFFAOYSA-N 0.000 description 1
- 229910002651 NO3 Inorganic materials 0.000 description 1
- NHNBFGGVMKEFGY-UHFFFAOYSA-N Nitrate Chemical compound [O-][N+]([O-])=O NHNBFGGVMKEFGY-UHFFFAOYSA-N 0.000 description 1
- 239000008186 active pharmaceutical agent Substances 0.000 description 1
- 229910045601 alloy Inorganic materials 0.000 description 1
- 239000000956 alloy Substances 0.000 description 1
- 238000004364 calculation method Methods 0.000 description 1
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- 238000005097 cold rolling Methods 0.000 description 1
- 230000000052 comparative effect Effects 0.000 description 1
- 238000009749 continuous casting Methods 0.000 description 1
- 238000007796 conventional method Methods 0.000 description 1
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- 239000004615 ingredient Substances 0.000 description 1
- UQSXHKLRYXJYBZ-UHFFFAOYSA-N iron oxide Inorganic materials [Fe]=O UQSXHKLRYXJYBZ-UHFFFAOYSA-N 0.000 description 1
- 239000010410 layer Substances 0.000 description 1
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- 239000002184 metal Substances 0.000 description 1
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- 230000009467 reduction Effects 0.000 description 1
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- 235000013619 trace mineral Nutrition 0.000 description 1
- 239000011573 trace mineral Substances 0.000 description 1
- 229910052726 zirconium Inorganic materials 0.000 description 1
Classifications
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/02—Hardening articles or materials formed by forging or rolling, with no further heating beyond that required for the formation
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
- C21D8/0226—Hot rolling
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
- C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
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- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0278—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips involving a particular surface treatment
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- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
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- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
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- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/002—Bainite
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/06—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires
- C21D8/065—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires of ferrous alloys
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/08—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes
Abstract
The present invention provides a high-strength steel sheet for acid-resistant pipelines, which has excellent HIC resistance, SSCC resistance in more severe corrosive environments, and SSCC resistance in environments with a low hydrogen sulfide partial pressure of less than 1 bar. The high-strength steel sheet for acid-proof pipeline of the present invention is characterized by comprising the following components: contains C in mass%: 0.020 to 0.080 percent, si:0.01 to 0.50 percent of Mn:0.50 to 1.80 percent of P: less than 0.015%, S: less than 0.0015%, al: 0.010-0.080%, N:0.0010 to 0.0080 percent, mo:0.01 to 0.50 percent of Ca:0.0005 to 0.0050%, the balance being Fe and unavoidable impurities, wherein the magnetite ratio in the scale existing on the surface of the steel sheet is 50% or more, the maximum Vickers hardness at 0.25mm below the surface of the steel sheet is 230HV or less, and the tensile strength is 520MPa or more.
Description
Technical Field
The present invention relates to a high strength steel sheet for an acid-resistant pipeline, which is suitable for pipelines in the fields of construction, marine structures, shipbuilding, civil engineering and construction industrial machinery and has excellent uniformity of material in the steel sheet, and a method for producing the same. The present invention also relates to a high-strength steel pipe using the high-strength steel sheet for acid-proof pipeline.
Background
In general, a pipeline is manufactured by forming a steel sheet manufactured by a heavy plate mill or a hot-rolling mill into a steel pipe by UOE forming, press bending forming, roll forming, or the like.
Among them, pipelines used for transportation of crude oil and natural gas containing hydrogen sulfide are required to have so-called acid resistance such as hydrogen induced cracking resistance (HIC (Hydrogen Induced Cracking) resistance) and sulfide stress corrosion cracking resistance (SSCC (Sulfide Stress Corrosion Cracking) resistance) in addition to strength, toughness, weldability, and the like. Among them, HIC is considered to be a problem in a pipeline having a relatively low strength level with respect to an oil well pipe because hydrogen ions generated by corrosion reaction are adsorbed on the surface of steel, enter the steel as atomic hydrogen, diffuse and accumulate around nonmetallic inclusions such as MnS and hard phase 2 structures in the steel, become molecular hydrogen, and crack due to the internal pressure thereof. On the other hand, it is known that SSCC is generated in a high-hardness region of a welded portion, and in general, it has not been considered as a problem in a high-strength seamless steel pipe for oil well or a pipeline having relatively low hardness. However, in recent years, the production environment of crude oil and natural gas has been becoming worse, and even in an environment where the hydrogen sulfide partial pressure is high or the pH is low, SSCC occurs in the base material portion of a pipeline, and it has been reported that it is important to control the hardness of the inner surface layer portion of a steel pipe and to improve the SSCC resistance in a more severe corrosive environment. In addition, in an environment where the hydrogen sulfide partial pressure is relatively low, micro cracks called cracks may occur, and SSCC may occur.
In general, when manufacturing high-strength steel sheet for pipeline, a so-called TMCP (Thermo-Mechanical Control Process) technique is applied in which rolling control and cooling control are combined. In order to increase the strength of the steel sheet by using this TMCP technique, it is effective to control the increase in cooling rate during cooling. However, when controlled cooling is performed at a high cooling rate, the surface layer portion of the steel sheet is rapidly cooled, and therefore the hardness of the surface layer portion is increased as compared with the inside of the steel sheet. Further, since the steel sheet is cured by working when being formed into a tubular shape, the hardness of the surface layer portion is further improved and the SSCC resistance is reduced.
In order to solve the above-described problems, patent documents 1 and 2 disclose methods for controlling cooling at a high cooling rate by reheating the surface after rolling and before the bainitic transformation of the surface layer portion is completed. Patent documents 3 and 4 disclose a method for manufacturing a steel sheet for a pipeline, in which the surface of a steel sheet after accelerated cooling is heated to a temperature higher than the internal temperature by using a high-frequency induction heating apparatus, thereby reducing the hardness of the surface layer portion.
On the other hand, when the thickness of the oxide scale on the surface of the steel sheet is uneven, the cooling rate of the steel sheet in the lower portion also varies during cooling. In contrast, patent documents 5 and 6 disclose methods for removing scale immediately before cooling a steel sheet, reducing uneven cooling caused by uneven thickness of the scale, and improving the shape of the steel sheet.
Prior art literature
Patent literature
Patent document 1: japanese patent No. 3951428
Patent document 2: japanese patent No. 3951429
Patent document 3: japanese patent laid-open No. 2002-327212
Patent document 4: japanese patent No. 3711896
Patent document 5: japanese patent laid-open No. 9-57327
Patent document 6: japanese patent No. 3796133
Disclosure of Invention
However, according to the studies by the present inventors, it has been found that the high-strength steel sheet obtained by the production methods described in the above patent documents 1 to 6 has room for improvement from the viewpoint of the SSCC resistance under more severe corrosive environments. The following reasons are considered.
In the manufacturing methods described in patent documents 1 to 4, the conditions for controlling cooling of the steel sheet have not been sufficiently optimized, and as a result, a local high-hardness portion is generated in the surface layer portion of the steel sheet.
In the methods described in patent documents 5 and 6, the surface texture defects caused by the indentation of the scale during hot straightening and the variation in the cooling stop temperature of the steel sheet are reduced by descaling, thereby improving the shape of the steel sheet. However, the descaling conditions are not optimized from the viewpoint of improving the SSCC resistance. In addition, there is no consideration of cooling conditions for reducing the hardness of the surface layer portion of the steel sheet.
In patent documents 1 to 6, the condition for avoiding micro cracking such as cracking in an environment where the hydrogen sulfide partial pressure is relatively low is not clarified.
In view of the above problems, an object of the present invention is to provide a high-strength steel sheet for acid-resistant pipelines, which is excellent in not only HIC resistance but also SSCC resistance in a more severe corrosive environment and SSCC resistance in an environment having a low hydrogen sulfide partial pressure of less than 1bar, and a method for producing the same. The present invention also provides a high-strength steel pipe using the high-strength steel sheet for acid-proof pipeline.
The present inventors have made intensive studies to solve the above problems, and have found that it is not sufficient to suppress only the hardness of the surface layer portion as in the prior art for further improving the SSCC resistance of a high-strength steel pipe. That is, in the conventional technique, even though the hardness of the surface layer portion is suppressed as a whole, a local high-hardness portion is generated in the surface layer portion as close as possible to the surface of the steel sheet, and SSCC is generated starting from the local high-hardness portion. Accordingly, the present inventors have repeatedly conducted a large number of experiments on the composition of the steel sheet, the properties of the scale existing on the surface of the steel sheet, and the manufacturing conditions of the steel sheet in order to obtain a high-strength steel sheet in which no local high hardness is present at the extremely surface layer portion, specifically, at a position 0.25mm below the surface of the steel sheet.
As a result, it was found that the formation of magnetite-based scale on the surface of a steel sheet with a predetermined composition is a necessary condition for obtaining a high-strength steel sheet having no local high hardness at a position of 0.25mm below the surface of the steel sheet. Further, it has been found that in order to form magnetite-based scale on the surface of a steel sheet, it is necessary to optimize the conditions for descaling performed in the hot rolling step and to set the cooling stop temperature at the time of controlled cooling within a predetermined range. Further, as a necessary condition concerning the production condition, it was found that it was necessary to strictly control the cooling rate at 0.25mm below the surface of the steel sheet, and the condition thereof was successfully established. The present invention is based on the above findings.
That is, the gist of the present invention is as follows.
[1] A high-strength steel sheet for acid-proof pipelines is characterized by comprising the following components: contains C in mass%: 0.020 to 0.080 percent, si:0.01 to 0.50 percent of Mn:0.50 to 1.80 percent of P: less than 0.015%, S: less than 0.0015%, al: 0.010-0.080%, N:0.0010 to 0.0080 percent, mo:0.01 to 0.50 percent of Ca:0.0005 to 0.0050%, the remainder being made up of Fe and unavoidable impurities,
the magnetite content in the scale existing on the surface of the steel sheet is 50% or more,
the maximum value of Vickers hardness at 0.25mm below the surface of the steel sheet is 230HV or less,
the tensile strength is 520MPa or more.
[2] The high-strength steel sheet for acid-resistant pipeline according to the above [1], wherein the above composition further contains, in mass%, a composition selected from the group consisting of Cu: less than 0.30%, ni:0.10% or less and Cr:0.50% or less of 1 or more.
[3] The high-strength steel sheet for acid-resistant piping according to the above [1] or [2], wherein the above composition further contains, in mass%, a composition selected from the group consisting of Nb: 0.005-0.1%, V:0.005 to 0.1 percent of Ti: 0.005-0.1%, zr:0.0005 to 0.02 percent of Mg: 0.0005-0.02% and REM:0.0005 to 0.02% of at least 1 kind.
[4] A method for producing a high-strength steel sheet for acid-proof pipeline, characterized by heating a steel sheet having the composition of any one of the above [1] to [3] to a temperature of 1000 to 1300 ℃,
thereafter, the steel sheet is hot-rolled to produce a steel sheet, and in this case, descaling is performed in a rolling pass having a discharge pressure of 10MPa or more for 50% or more of the number of passes of the hot rolling,
thereafter, the steel sheet was subjected to controlled cooling under the following conditions:
steel sheet surface temperature at the start of cooling: (Ar) 3 At a temperature of-10 ℃ or higher,
an average cooling rate from 750 ℃ to 550 ℃ with a steel plate thermometer at 0.25mm from the surface of the steel plate: 20-100 ℃/s,
average cooling rate from 750 ℃ to 550 ℃ with average steel plate temperature: 15 ℃/s or more, and
cooling stop temperature with a steel plate thermometer at 0.25mm below the steel plate surface: 250-550 ℃.
[5] A high-strength steel pipe using the high-strength steel sheet for acid-resistant pipeline as described in any one of [1] to [3 ].
The high-strength steel sheet for acid-resistant pipelines and the high-strength steel pipe using the high-strength steel sheet for acid-resistant pipelines of the present invention are excellent not only in HIC resistance but also in SSCC resistance under more severe corrosive environments and SSCC resistance under environments with a low hydrogen sulfide partial pressure of less than 1 bar. Further, according to the method for producing a high-strength steel sheet for acid-resistant piping of the present invention, it is possible to produce a high-strength steel sheet for acid-resistant piping which is excellent not only in HIC resistance, but also in SSCC resistance under more severe corrosive environments and in SSCC resistance under an environment of low hydrogen sulfide partial pressure of less than 1 bar.
Drawings
FIG. 1 is a schematic diagram for explaining a method of taking test pieces for evaluation of SSCC resistance in examples.
Detailed Description
Hereinafter, the high-strength steel sheet for acid-proof pipeline according to the present invention will be specifically described.
[ composition of ingredients ]
First, the composition of the components of the high-strength steel sheet of the present invention and the reasons for limiting the same will be described. The units expressed in% in the following description refer to% by mass unless otherwise specified.
C：0.020～0.080％
C contributes effectively to the improvement of strength, but if the content is less than 0.020%, sufficient strength cannot be ensured. Therefore, the C content is 0.020% or more, preferably 0.025% or more. On the other hand, if the C content exceeds 0.080%, the hardness of the surface layer portion and the center segregation portion increases during accelerated cooling, and therefore the SSCC resistance and HIC resistance deteriorate. In addition, toughness also deteriorates. Therefore, the C content is 0.080% or less, preferably 0.070% or less.
Si：0.01～0.50％
Si is added for deoxidation, and if the content is less than 0.01%, the deoxidizing effect is insufficient. Therefore, the Si content is 0.01% or more, preferably 0.05% or more. On the other hand, if the Si amount exceeds 0.50%, toughness and weldability are deteriorated. Therefore, the Si content is 0.50% or less, preferably 0.45% or less.
Mn：0.50～1.80％
Mn contributes effectively to improvement of strength and toughness, but if the content is less than 0.50%, the effect of addition is not sufficiently exhibited. Therefore, the Mn content is 0.50% or more, preferably 0.80% or more. On the other hand, if the Mn content exceeds 1.80%, the hardness of the surface layer portion and the center segregation portion increases during accelerated cooling, and therefore the SSCC resistance and HIC resistance deteriorate. In addition, weldability also deteriorates. Therefore, the Mn content is 1.80% or less, preferably 1.70% or less.
P: less than 0.015%
P is an unavoidable impurity element, and deteriorates weldability, and also improves hardness of the surface layer portion and the center segregation portion, thereby deteriorating SSCC resistance and HIC resistance. If the amount of P exceeds 0.015%, the tendency becomes remarkable, and therefore the amount of P is 0.015% or less, preferably 0.008% or less. The lower the P content, the better, but from the standpoint of refining cost, it is preferably 0.001% or more.
S: less than 0.0015%
S is an unavoidable impurity element, and is formed as an MnS inclusion in steel, thereby deteriorating HIC resistance. Therefore, the S content is 0.0015% or less, preferably 0.0010% or less. The lower the S content, the better, but from the standpoint of refining costs, it is preferably 0.0002% or more.
Al：0.010～0.080％
Al is added as a deoxidizer, but if the content is less than 0.010%, the effect is not sufficiently exhibited. Therefore, the Al content is 0.010% or more, preferably 0.015% or more. On the other hand, if the Al amount exceeds 0.080%, the cleanliness of the steel is lowered and the toughness is deteriorated. Therefore, the Al content is 0.080% or less, preferably 0.070% or less.
N：0.0010～0.0080％
N effectively contributes to improvement of strength, but if the content is less than 0.0010%, sufficient strength cannot be ensured. Therefore, the amount of N is 0.0010% or more, preferably 0.0015% or more. On the other hand, if the N amount exceeds 0.0080%, the hardness of the surface layer portion and the center segregation portion increases during accelerated cooling, and therefore the SSCC resistance and HIC resistance deteriorate. In addition, toughness also deteriorates. Therefore, the N content is 0.0080% or less, preferably 0.0070% or less.
Mo：0.01～0.50％
Mo is an element effective for improving toughness and strength, and effective for improving SSCC properties regardless of hydrogen sulfide partial pressure. The inventors of the present invention found that when a steel sheet containing Mo was subjected to the SSCC test, the surface of the steel sheet after the test was smoother than the surface of the steel sheet containing no Mo after the SSCC test. The mechanism is not yet clear, and this is considered to be related to the improvement of the SSCC resistance. In order to obtain this effect, the Mo amount must be 0.01% or more, preferably 0.10% or more. On the other hand, if the Mo amount exceeds 0.50%, hardenability becomes excessive, so that the hardness of the surface layer portion and the center segregation portion increases during accelerated cooling, and the SSCC resistance and HIC resistance deteriorate. In addition, weldability also deteriorates. Therefore, the Mo content is 0.50% or less, preferably 0.40% or less.
Ca：0.0005～0.0050％
Ca is an element effective for improving HIC resistance by controlling the morphology of sulfide-based inclusions, but if the content is less than 0.0005%, the effect is not sufficiently exhibited. Therefore, the Ca content is 0.0005% or more, preferably 0.0008% or more. On the other hand, when the Ca content exceeds 0.0050%, the above-mentioned effects are not only saturated, but the cleanliness of the steel is also lowered, and the HIC resistance is deteriorated. Therefore, the Ca content is 0.0050% or less, preferably 0.0045% or less.
In the present invention, in order to further improve the strength and toughness of the steel sheet, the composition of the components may optionally contain 1 or more selected from Cu, ni and Cr in the following ranges.
Cu: less than 0.30%
Cu is an element effective for improving toughness and strength, and the amount of Cu is preferably 0.05% or more to obtain this effect. However, when the Cu content exceeds 0.30%, micro cracks called cracks are easily generated in an environment of low hydrogen sulfide partial pressure of less than 1bar, and therefore, the SSCC resistance is deteriorated. Therefore, when Cu is added, the Cu content is 0.30% or less, preferably 0.25% or less.
Ni: less than 0.10%
Ni is an element effective for improving toughness and strength, and in order to obtain this effect, the Ni amount is preferably 0.01% or more. However, when the Ni content exceeds 0.10%, micro cracks called cracks are easily generated in an environment of low hydrogen sulfide partial pressure of less than 1bar, and therefore, SSCC resistance is deteriorated. Therefore, when Ni is added, the Ni content is 0.10% or less, preferably 0.05% or less.
Cr: less than 0.50%
Cr is an element effective for obtaining sufficient strength even when the content of C is low as in Mn, and the amount of Cr is preferably 0.05% or more for obtaining the effect. However, when the Cr content exceeds 0.50%, the hardenability is excessive, and therefore, the hardness of the surface layer portion and the center segregation portion is improved during accelerated cooling, and the SSCC resistance and HIC resistance are deteriorated. In addition, weldability also deteriorates. Therefore, when Cr is added, the Cr content is 0.50% or less, preferably 0.45% or less.
In the present invention, the component composition may further optionally contain 1 or more selected from Nb, V, ti, zr, mg and REM in the following ranges.
Selected from Nb: 0.005-0.1%, V:0.005 to 0.1 percent of Ti: 0.005-0.1%, zr:0.0005 to 0.02 percent of Mg: 0.0005-0.02% and REM: more than 1 of 0.0005 to 0.02 percent
Nb, V, and Ti are elements that can be arbitrarily added to improve strength and toughness of the steel sheet. The effect of each element is not fully reflected when the content is less than 0.005%. Therefore, when these elements are added, the content of each element is preferably 0.005% or more. On the other hand, if the content of these elements exceeds 0.1%, toughness of the welded portion deteriorates. Therefore, when these elements are added, the content of each element is preferably 0.1% or less.
Zr, mg, and REM are elements that can be added arbitrarily to improve toughness by grain refinement or cracking resistance by controlling inclusion properties. The effect is not sufficiently exhibited when the content of each element is less than 0.0005%. Therefore, when these elements are added, the content of each element is preferably 0.0005% or more. On the other hand, if the content of these elements exceeds 0.02%, the effect thereof is saturated. Therefore, when these elements are added, the content of each element is preferably 0.02% or less.
The present invention relates to a technique for improving the SSCC resistance of a high-strength steel pipe using a high-strength steel sheet for an acid-resistant pipeline, but it is needless to say that the HIC resistance must be satisfied at the same time as the acid resistance, and for example, the CP value obtained from the following formula (1) is preferably 1.00 or less. The element not added may be substituted with 0.
CP＝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]···(1)
Wherein, [%X ] represents the content (mass%) of X element in the steel.
Here, the CP value is a formula designed to estimate the material of the center segregation portion from the content of each alloy element, and the higher the CP value of the formula (1), the higher the component concentration of the center segregation portion, and the higher the hardness of the center segregation portion. Therefore, by setting the CP value obtained in the above formula (1) to 1.00 or less, the occurrence of cracking in the HIC test can be suppressed. Further, the lower the CP value, the lower the hardness of the center segregation portion, and when higher HIC resistance is required, the upper limit thereof may be 0.95.
The remainder other than the above elements is composed of Fe and unavoidable impurities. However, the content of other trace elements is not hindered as long as the effects of the present invention are not impaired. For example, O is an element inevitably contained in steel, and is allowed in the present invention as long as its content is 0.0050% or less, preferably 0.0040% or less.
[ Structure of Steel sheet ]
Next, the steel structure of the high-strength steel sheet for acid-proof pipeline according to the present invention will be described. In order to reduce the hardness of the surface layer portion, the steel structure of the surface layer portion is preferably a bainite phase. In particular, the maximum hardness of 0.25mm below the surface of the steel sheet is suppressed to a certain level or less, and in order to improve the SSCC resistance, it is preferable that the steel structure of 0.25mm below the surface of the steel sheet is a bainite phase. In order to achieve a high tensile strength of 520MPa or more, it is preferable that the entire steel structure of the steel sheet including the portions other than the surface layer portion is a bainitic phase. Specifically, the "portion other than the surface layer portion" is represented, and the microstructure in the center of the plate thickness may be a bainite phase.
Here, the bainitic phase includes a structure called bainitic ferrite or granular ferrite which contributes to transformation strengthening at the time of accelerated cooling or after accelerated cooling. In the bainitic phase, when different kinds of structures such as ferrite, martensite, pearlite, island-like martensite, and retained austenite are mixed, reduction in strength, deterioration in toughness, improvement in surface hardness, and the like occur, and therefore, the smaller the fraction of the structure other than the bainitic phase is, the better. However, in the case where the area fraction of the structure other than the bainite phase is sufficiently low, their influence is negligible, and thus a certain amount is allowable. Specifically, in the present invention, if the total of the steel structures (ferrite, martensite, pearlite, island martensite, retained austenite, etc.) other than bainite is less than 10% in terms of area fraction, no significant influence is exerted, and therefore, it is allowable, and more preferably, less than 5%.
[ oxide skin on the surface of Steel sheet ]
In the high-strength steel sheet of the present invention, from the viewpoint of further improving the SSCC resistance, it is critical to control the presence of scale on the surface of the steel sheet after coolingThe magnetite ratio of (2) is 50% or more. In general, oxide scale existing on the surface of a steel sheet after cooling is controlled by a method comprising the steps of pyrite (FeO), magnetite (Fe 3 O 4 ) Hematite (Fe) 2 O 3 ) The composition is formed. The inventors found that when the magnetite ratio was 50%, a local high hardness portion was formed at a position of 0.25mm below the surface of the steel sheet, and as a result, the maximum value of vickers hardness at a position of 0.25mm below the surface of the steel sheet exceeded 230HV. In other words, in order to set the maximum value of the vickers hardness at 0.25mm below the surface of the steel sheet to 230HV or less, it is necessary to set the magnetite ratio to at least 50% or more. The upper limit of the magnetite ratio is not particularly limited, and the magnetite ratio may be 100% or less or 95% or less.
[ hardness of polar surface layer portion ]
In the high-strength steel sheet of the present invention, it is important that the maximum value of the Vickers hardness (HV 0.5) at 0.25mm below the surface of the steel sheet is 230HV or less. By satisfying this condition, excellent SSCC resistance can be obtained in both a more severe corrosive environment and an environment with a low hydrogen sulfide partial pressure of less than 1 bar. When the maximum value of vickers hardness at 0.25mm below the surface of the steel sheet exceeds 230HV, a local high hardness portion exists on the surface layer of the steel sheet, and deterioration of SSCC resistance occurs starting from this portion. Here, the "maximum value of vickers hardness (HV 0.5) at 0.25mm below the surface of the steel sheet" is the maximum value of vickers hardness (HV 0.5) measured at 0.25mm below the surface of the steel sheet at 100 points at equal intervals in the width direction of the sheet in a cross section of the steel sheet perpendicular to the rolling direction. Here, HV0.5 is used instead of HV10 which is generally used, and since the indentation becomes smaller by measurement with HV0.5, hardness information at a position closer to the surface and hardness information more sensitive to the microstructure can be obtained. If measured at less than HV0.5, the indentation size is too small and the measurement unevenness increases. The reason why the evaluation is performed not by the average hardness but by the highest hardness is as follows. That is, if there is a locally hard position, the crack is likely to spread, so in order to examine crack progress sensitivity with high accuracy, it is more appropriate to evaluate based on the highest hardness at which the locally hard position can be detected.
[ tensile Strength ]
The high-strength steel sheet of the present invention has a tensile strength of 520MPa or more because it is a steel sheet for a steel pipe having a strength of X60 level or more of API 5L.
[ thickness of Steel sheet ]
The high-strength steel sheet of the present invention has a thickness of 14 to 39 mm.
[ method of production ]
Hereinafter, a method and conditions for producing the high-strength steel sheet for acid-proof pipeline will be specifically described. The production method of the present invention is to heat a steel sheet having the above-mentioned composition, then hot-roll the steel sheet to produce a steel sheet, and thereafter control-cool the steel sheet under predetermined conditions.
[ slab heating temperature ]
Slab heating temperature: 1000-1300 DEG C
If the slab heating temperature is less than 1000 ℃, the solid solution of carbide becomes insufficient, and the solid solution strengthening amount becomes small, so that the necessary strength is not obtained. Therefore, the slab heating temperature is 1000 ℃ or higher, preferably 1030 ℃ or higher. On the other hand, if the slab heating temperature exceeds 1300 ℃, the crystal grains become extremely coarse and the toughness deteriorates. Therefore, the slab heating temperature is 1300 ℃ or less, preferably 1250 ℃ or less. The temperature is the temperature in the heating furnace, and the slab is heated to the temperature at the center.
[ descaling ]
In the present invention, it is important to perform descaling at a discharge pressure of 10MPa or more in a rolling pass of 50% or more of the number of passes in hot rolling in the hot rolling step. The term "rolling pass" as used herein includes both a rough rolling pass and a finish rolling pass in the hot rolling step. Specifically, in a rolling pass of 50% or more of the number of rolling passes of hot rolling, descaling with a pressure of 10MPa or more is performed on the surface of a slab (sheet) at a position before the slab (sheet) is introduced into the rolling pass. The descaling condition is one of the requirements for suppressing the uneven formation of scale and controlling the magnetite ratio in the scale existing on the surface of the steel sheet to 50% or more after cooling. The "position before the slab is introduced into the rolling pass" is a position within 3m, preferably within 1.5m, of the position of the roll shaft of the rolling mill corresponding to the rolling pass in the longitudinal direction of the hot rolling line. The number of rough rolling passes is not particularly limited, and may be set arbitrarily within a general range, and is preferably 2 to 12, for example. The number of finish passes is also arbitrarily set in a general range, and is not particularly limited, but is preferably 5 to 15, for example. The descaling method may be performed according to a standard method, for example, by spraying high-pressure water onto the slab surface from a plurality of descaling nozzles arranged in the width direction of the hot rolling line. The conditions other than the discharge pressure (for example, the water amount, the distance between the nozzle and the slab, and the nozzle angle) may be general conditions.
If the discharge pressure is less than 10MPa, the oxide scale cannot be removed uniformly, and the hematite increases, so that the magnetite ratio cannot be set to 50% or more. Therefore, the discharge pressure is 10MPa or more, preferably 15MPa or more. The higher the discharge pressure is, the better, but the larger the equipment is, and therefore, 25MPa or less is preferable.
When the number of descaling is less than 50% of the number of pass, hematite increases, and as a result, the magnetite ratio cannot be made 50% or more. Therefore, the number of descaling is 50% or more, preferably 60% or more of the number of pass. The upper limit of the number of descaling is not particularly limited, and may be 100% of the number of pass, that is, descaling may be performed before all pass.
[ Rolling end temperature ]
In the hot rolling step, the lower the rolling end temperature is, the better, but on the other hand, the rolling efficiency is reduced, so that the rolling end temperature of the steel sheet surface temperature needs to be set in consideration of the required base material toughness and rolling efficiency. From the viewpoint of improving strength and HIC resistance, it is preferable that the rolling end temperature is Ar based on the steel sheet surface temperature 3 Above the phase transition point. Here, ar 3 The transformation point is the ferrite transformation start temperature during cooling, and can be obtained from the composition of steel according to the following formulaAnd (5) outputting. The surface temperature of the steel sheet may be measured by a radiation thermometer or the like.
Ar 3 (℃)＝910－310[％C]－80[％Mn]－20[％Cu]－15[％Cr]－55[％Ni]－80[％Mo]
Wherein, [%X ] represents the content (mass%) of X element in the steel.
[ control of Cooling Start temperature of Cooling ]
Cooling start temperature: the steel plate surface thermometer was (Ar) 3 Above-10℃)
If the surface temperature of the steel sheet at the start of cooling is low, the ferrite production amount before cooling is controlled to increase. In particular from less than (Ar) 3 At the start of cooling at a temperature of-10 c, ferrite exceeding 5% in terms of area fraction is generated, the strength decrease increases, and the HIC resistance deteriorates. Therefore, the steel sheet surface temperature at the start of cooling was (Ar) 3 -10 ℃ above. The surface temperature of the steel sheet at the start of cooling was equal to or lower than the rolling end temperature.
[ control cooling speed of Cooling ]
In order to achieve high strength and to set the maximum value of the Vickers hardness at 0.25mm below the surface of the steel sheet to 230HV or less, it is necessary to control the cooling rate at 0.25mm below the surface of the steel sheet.
An average cooling rate from 750 ℃ to 550 ℃ with a steel plate thermometer at 0.25mm below the steel plate surface: 20-100 ℃/s
It is important to minimize the generation of high-temperature phase transition by the average cooling rate of the steel plate thermometer from 750 to 550 c at 0.25mm below the surface of the steel plate, and the lower the cooling rate, the lower the maximum hardness can be. The temperature region from 750 ℃ to 550 ℃ is an important temperature region in bainite transformation, and thus it becomes important to control the cooling rate of the temperature region. When the average cooling rate exceeds 100 ℃/sec, the ratio of the low-temperature phase-change phase is high, and the maximum value of vickers hardness at 0.25mm below the surface of the steel sheet exceeds 230HV, so that the SSCC resistance after pipe making is deteriorated. Therefore, the average cooling rate is 100 ℃ per second or less, preferably 80 ℃ per second or less. When the average cooling rate is less than 20 ℃/sec, ferrite and pearlite are generated, and the strength is insufficient. Therefore, the average cooling rate is 20 ℃/sec or more.
It is preferable to increase the water density from the viewpoint of cooling at 550 ℃ or lower with a steel plate thermometer at 0.25mm below the steel plate surface, from the viewpoint of cooling in a stable nucleate boiling state. The cooling is performed in a stable nucleate boiling state, and in order to prevent the formation of local high hardness sites in the extremely surface layer portion of the steel sheet, the average cooling rate from 550 ℃ to the cooling stop temperature by a steel sheet thermometer at 0.25mm below the steel sheet surface is preferably 110 ℃/sec or more, more preferably 150 ℃/sec or more. In addition, from the viewpoint of more reliably suppressing the formation of the high hardness portion, the average cooling rate is preferably 200 ℃/sec or less.
Average cooling rate from 750 ℃ to 550 ℃ with an average steel plate thermometer: 15 ℃/s or more
If the average cooling rate from 750 ℃ to 550 ℃ is less than 15 ℃/sec by the average steel plate temperature meter, the fraction of phases other than the bainite phase increases, resulting in a decrease in strength and a deterioration in HIC resistance. Therefore, the average cooling rate by the steel plate average thermometer is 15 ℃/sec or more. From the standpoint of the difference in strength and hardness of the steel sheet, the average cooling rate by the average thermometer of the steel sheet is preferably 20 ℃/sec or more. The upper limit of the average cooling rate is not particularly limited, but in order to prevent excessive formation of the low-temperature phase-change product, the average cooling rate is preferably 80 ℃/sec or less.
The steel sheet temperature at 0.25mm below the surface of the steel sheet and the average temperature of the steel sheet cannot be directly measured physically, and the temperature distribution in the sheet thickness profile can be obtained in real time by differential calculation using a process computer, for example, based on the surface temperature at the start of cooling measured by a radiation thermometer and the surface temperature at the stop of cooling target. The temperature at 0.25mm below the steel plate surface in this temperature distribution is referred to as "steel plate temperature at 0.25mm below the steel plate surface" in the present specification, and the average value of the temperatures in the plate thickness direction in this temperature distribution is referred to as "steel plate average temperature" in the present specification.
[ Cooling stop temperature ]
Cooling stop temperature: the cooling stop temperature of 250 to 550 ℃ with a steel plate thermometer at 0.25mm below the steel plate surface is one of the requirements for controlling the magnetite ratio in the scale existing on the steel plate surface to 50% or more after cooling. If the cooling stop temperature exceeds 550 ℃, the bainite transformation is incomplete, and sufficient strength is not obtained. When the cooling stop temperature is less than 250 ℃, the amount of the pyrite increases, and as a result, the magnetite ratio cannot be made 50% or more. As a result, the maximum value of Vickers hardness at 0.25mm below the surface of the steel sheet exceeds 230HV, and therefore SSCC resistance after pipe making is deteriorated. In addition, the hardness of the center segregation portion also increases, and the HIC resistance also deteriorates. Therefore, the cooling stop temperature is 250 to 550 ℃ in terms of the steel plate temperature at 0.25mm below the steel plate surface.
[ high-Strength Steel tube ]
The high-strength steel sheet of the present invention can be formed into a tubular shape by press bending, roll forming, and UOE forming, and then welded to a butt joint to produce a high-strength steel pipe for acid-proof pipelines (UOE steel pipe, electric seam steel pipe, spiral steel pipe, etc.) suitable for transportation of crude oil and natural gas.
For example, UOE steel pipes are manufactured by chamfering the end portions of steel plates, forming the steel pipes by C stamping, U stamping, and O stamping, seam welding the butt portions by inner surface welding and outer surface welding, and if necessary, performing a pipe expanding process. The welding method may be any method as long as sufficient joining strength and joining toughness are obtained, and submerged arc welding is preferably used from the viewpoints of excellent welding quality and manufacturing efficiency. Further, after the steel plate is formed into a tubular shape by press bending, the butt joint is seam welded to obtain a steel pipe, and the steel pipe is expanded.
Examples
The steels having the compositions shown in table 1 were formed into slabs by continuous casting, heated to the temperatures shown in table 2, and hot-rolled at the rolling end temperatures shown in table 2 to obtain steel sheets having the sheet thicknesses shown in table 2. The hot rolling step consisted of 10 to 25 total of 2 to 12 rough rolling passes and 5 to 15 finish rolling passes, wherein the scale removal at the discharge pressure shown in Table 2 was performed in the rolling passes of the ratio shown in Table 2. Thereafter, the steel sheet was controlled cooled using a controlled cooling device of the water-cooled type under the conditions shown in table 2.
[ determination of tissue ]
The microstructure of the obtained steel sheet was observed by an optical microscope. Samples for observing a metal structure were collected from the center of the steel plate in the width direction. For these samples, a cross section parallel to the rolling longitudinal direction was mirror polished, and then subjected to nitrate alcohol etching. Then, photographs were randomly taken of 5 fields of view of the polished surface of each sample at a magnification in the range of 400 to 1000 times using an optical microscope, and the area fraction of each phase was calculated by image analysis processing. Table 3 shows the types of the structure at the position 0.25mm below the surface of the steel sheet and the structure at the center of the sheet thickness, and the area ratios of phases other than the bainite phase.
[ determination of magnetite ratio in oxide skin ]
Oxidized scale was collected from the surface of the obtained steel sheet. The scale is collected at 9 positions in total of the width center and the width direction both ends of the steel sheet at the front end, center and end in the length direction of the steel sheet, and the scale is collected at 0.5g or more at each position. The scale collected at each position was identified by X-ray diffraction (XRD: X-ray diffraction) and quantitatively analyzed (i.e., magnetite ratio was measured) by using a reference intensity ratio (RIR: reference Intensity Ratio) method. The average value of the magnetite ratios of the oxide scale at 9 positions is shown in table 3 as "magnetite ratio" in the present invention.
[ measurement of tensile Strength ]
The total thickness test piece in the direction perpendicular to the rolling direction was used as a tensile test piece for tensile test, and the yield strength and tensile strength were measured. The results are shown in Table 3.
[ measurement of Vickers hardness ]
The maximum hardness, average value and standard deviation σ of the cross section perpendicular to the rolling direction were determined by measuring the 100 point vickers hardness (HV 0.5) at a position 0.25mm below the steel plate surface according to JIS Z2244. The highest hardness, average value and 3 sigma value are shown in table 3.
[ evaluation of SSCC resistance ]
The SSCC resistance was evaluated by making a tube using a part of each of these steel sheets. The pipe is manufactured by chamfering the end of a steel plate, forming the steel plate into a steel pipe shape by C stamping, U stamping, and O stamping, then seam welding the butt joint between the inner surface and the outer surface by submerged arc welding, and performing a pipe expansion process. As shown in FIG. 1, after flattening the sample cut out from the obtained steel pipe, SSCC test pieces of 5X 15X 115mm were taken from the inner surface of the steel pipe. In this case, a test piece including both the welded portion and the base material was taken in addition to the test piece including only the base material without the welded portion. The inner surface of the surface to be inspected is directly provided with a black skin so as to remain the outermost layer. For the actual yield strength (0.5% ys) of the steel tubes loaded with the SSCC test pieces taken, 90% stress was measured using NACE specification TM0177 Solution a Solution at hydrogen sulfide partial pressure: 1bar was performed according to the 4-point bend SSCC test of the EFC16 specification. In addition, NACE Specification TM0177 Solution B Solution was used to divide hydrogen sulfide: 0.1 bar+carbon dioxide partial pressure: 0.9bar was performed according to the 4-point bend SSCC test of the EFC16 specification. NACE Specification TM0177 Solution A Solution was further used to divide hydrogen sulfide: 2 bar+carbon dioxide partial pressure: 3bar was performed according to the 4-point bend SSCC test of EFC16 specification. After 720 hours of immersion, the test piece including only the base material including no welded portion and the test piece including both the welded portion and the base material were judged to have good SSCC resistance when no cracking was observed, and the other test piece was judged to have failed when cracking was occurred. The results are shown in Table 3.
[ evaluation of HIC resistance ]
HIC resistance was measured using a NACE Specification TM0177 Solution A Solution at hydrogen sulfide partial pressure: 1bar was investigated by the HIC test for 96 hours of immersion. In addition, NACE Specification TM0177 Solution B Solution was used to divide hydrogen sulfide: 0.1 bar+carbon dioxide partial pressure: 0.9bar was investigated by the HIC test for 96 hours of immersion. Regarding the HIC resistance, the case where the cracking length ratio (CLR: crack Length Ratio) was 10% or less in the HIC test was judged to be excellent, the case where it was more than 10% and 15% or less was judged to be good, and the case where it was 15% more was judged to be insufficient. The results are shown in Table 3.
The object of the present invention is to provide a high strength steel sheet for acid-proof pipeline having tensile strength: 520MPa or more, a bainite structure of a microstructure at both a position of 0.25mm below the surface and a position of t/2, a maximum hardness of HV0.5 at 0.25mm below the surface of 230 or less, no cracks were observed in SSCC test, and a Crack Length Ratio (CLR) in HIC test was 15% or less.
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As shown in tables 2 and 3, nos. 1 to 6 and Nos. 30 to 31 are examples of the invention in which the composition of the components and the production conditions satisfy the appropriate ranges of the present invention. The tensile strength of the steel sheet is 520MPa or more, and the SSCC resistance and HIC resistance are good.
In contrast, the composition of the steel sheets of Nos. 7 to 18 is outside the scope of the present invention. Solid solution strengthening of No.7, no.9, no.12 was insufficient and low strength was achieved. No.8, no.10, no.11, no.13, no.15, no.18 have a maximum hardness of HV0.5 exceeding 230, and thus have poor SSCC resistance and HIC resistance. Since the steel sheet of No.14 does not contain Mo, the SSCC resistance is deteriorated under a very severe corrosive environment such as a hydrogen sulfide partial pressure of 2 Bar. Since the steel sheet of No.16 has an excessive Cu content, the SSCC resistance is deteriorated in an environment where the hydrogen sulfide partial pressure is low. Since the steel sheet of No.17 has an excessive Ni content, the SSCC resistance is deteriorated in an environment where the hydrogen sulfide partial pressure is low.
No.19 to No.29 are comparative examples in which the composition of the components was within the scope of the present invention, but the production conditions were outside the scope of the present invention. The slab of No.19 has a low heating temperature, and therefore, the homogenization of the microstructure and the solid solution of carbide are insufficient and have low strength. The descaling pressures of No.20 and No.28 were lower than 10MPa, so that scale unevenness was generated, the magnetite ratio was less than 50%, and the highest hardness of HV0.5 was over 230, so that the SSCC resistance and HIC resistance were poor. The ratio of the number of descaling in the rolling passes of Nos. 21 and 29 was less than 50%, so that the magnetite ratio was less than 50%, and the highest hardness of HV0.5 was over 230, so that the SSCC resistance and HIC resistance were poor. No.22 has a low cooling start temperature, and is a layered structure in which ferrite precipitates, and therefore has low strength and is deteriorated in HIC resistance. The controlled cooling condition of No.23 is outside the scope of the present invention, and the microstructure is a ferrite+bainite structure, and thus is low in strength and deteriorated in HIC resistance. The average cooling rate of 750 to 550 ℃ at 0.25mm below the surface of the steel sheet of No.24 exceeds 100 ℃/sec, so that the ratio of the low-temperature phase change phase becomes high, and the highest hardness of HV0.5 at 0.25mm below the surface of the steel sheet exceeds 230, so that the SSCC resistance is poor. No.25 has a low cooling stop temperature, a magnetite ratio of less than 50%, and a maximum hardness of HV0.5 exceeding 230, and therefore has poor SSCC resistance. The cooling stop temperature of No.26 is high, and the bainite transformation is incomplete, so that sufficient strength is not obtained. Note that, no.26 indicates that the cooling stop temperature was 560 ℃, and that no controlled cooling (accelerated cooling) was performed in a temperature range of 550 ℃ or less, and that the column "cooling rate of 550 ℃ or less (0.25 mm below the surface of the steel sheet)" in table 2 is left empty. The average cooling rate of 750 to 550 ℃ at 0.25mm below the surface of the steel sheet of No.27 exceeds 100 ℃/sec, and the cooling stop temperature is also low, so that the magnetite ratio is less than 50%, the highest hardness of HV0.5 exceeds 230, and SSCC resistance is poor.
Industrial applicability
According to the present invention, it is possible to provide a high-strength steel sheet for acid-resistant pipelines which is excellent not only in HIC resistance, but also in SSCC resistance in a more severe corrosive environment and in SSCC resistance in an environment having a low hydrogen sulfide partial pressure of less than 1 bar. Therefore, the steel pipe (e.g., a slit steel pipe, a spiral steel pipe, and a UOE steel pipe) manufactured by cold rolling the steel sheet can be suitably used for transportation of hydrogen sulfide-containing crude oil and natural gas, which require acid resistance.
Claims (3)
1. A method for producing a high-strength steel sheet for acid-proof pipelines, characterized by heating a steel sheet having a composition of: contains C in mass%: 0.020 to 0.080 percent, si:0.01 to 0.50 percent of Mn:0.50 to 1.80 percent of P: less than 0.015%, S: less than 0.0015%, al: 0.010-0.080%, N:0.0010 to 0.0080 percent, mo:0.01 to 0.50 percent of Ca:0.0005 to 0.0050%, the remainder being made up of Fe and unavoidable impurities,
thereafter, the steel sheet is hot-rolled to produce a steel sheet, and at this time, descaling is performed in a rolling pass having a discharge pressure of 10MPa or more for 50% or more of the number of passes of the hot rolling,
thereafter, the steel sheet was subjected to controlled cooling under the following conditions:
the steel sheet surface temperature at the start of cooling was (Ar 3 At a temperature of-10 ℃ or higher,
the average cooling speed from 750 ℃ to 550 ℃ is 20-100 ℃/s by a steel plate thermometer at 0.25mm below the surface of the steel plate,
an average cooling rate of 15 ℃/sec or more from 750 ℃ to 550 ℃ with an average steel plate temperature gauge, and
the cooling stop temperature of a steel plate thermometer at the position of 0.25mm below the surface of the steel plate is 250-550 ℃,
thus, a high-strength steel sheet for acid-resistant pipelines, which has a magnetite ratio of 50% or more in the scale existing on the surface of the steel sheet, a Vickers hardness of 230HV or less at 0.25mm below the surface of the steel sheet, and a tensile strength of 520MPa or more, is produced.
2. The method for producing a high-strength steel sheet for acid-proof pipeline as claimed in claim 1, wherein said composition further comprises, in mass%, a composition selected from the group consisting of Cu: less than 0.30%, ni:0.10% or less and Cr:0.50% or less of 1 or more.
3. The method for producing a high-strength steel sheet for acid-proof pipeline according to claim 1 or 2, wherein the composition of the components further contains, in mass%, a composition selected from the group consisting of Nb: 0.005-0.1%, V:0.005 to 0.1 percent of Ti: 0.005-0.1%, zr:0.0005 to 0.02 percent of Mg: 0.0005-0.02% and REM:0.0005 to 0.02% of at least 1 kind.
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| JP2020-056573 | 2020-03-26 | ||
| JP2020056573 | 2020-03-26 | ||
| PCT/JP2021/011238 WO2021193383A1 (en) | 2020-03-26 | 2021-03-18 | High-strength steel sheet for sour-resistant line pipe, manufacturing method thereof, and high-strength steel pipe made using high-strength steel sheet for sour-resistant line pipe |
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| EP (1) | EP4129510A1 (en) |
| JP (1) | JP7264269B2 (en) |
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| WO2023233734A1 (en) * | 2022-06-03 | 2023-12-07 | Ｊｆｅスチール株式会社 | High-strength steel sheet for sour gas equipment, and high-strength steel pipe using same |
| JP7332078B1 (en) | 2022-06-03 | 2023-08-23 | Ｊｆｅスチール株式会社 | High-strength steel plate for sour gas equipment and high-strength steel pipe using the same |
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| KR20220137970A (en) | 2022-10-12 |
| CN115298340A (en) | 2022-11-04 |
| JPWO2021193383A1 (en) | 2021-09-30 |
| WO2021193383A1 (en) | 2021-09-30 |
| BR112022019204A2 (en) | 2022-11-08 |
| EP4129510A1 (en) | 2023-02-08 |
| JP7264269B2 (en) | 2023-04-25 |
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