US20150368736A1 - Hot-rolled steel sheet for high strength linepipe - Google Patents
Hot-rolled steel sheet for high strength linepipe Download PDFInfo
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- US20150368736A1 US20150368736A1 US14/763,403 US201414763403A US2015368736A1 US 20150368736 A1 US20150368736 A1 US 20150368736A1 US 201414763403 A US201414763403 A US 201414763403A US 2015368736 A1 US2015368736 A1 US 2015368736A1
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- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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- C21D2211/00—Microstructure comprising significant phases
- C21D2211/005—Ferrite
Definitions
- This application is directed to a hot-rolled steel sheet having hydrogen induced cracking resistance (hereinafter, called HIC resistance) and a strength of X52 or more in accordance with API (American Petroleum Institute) standards which can preferably be used as a material for an electric resistance welded steel pipe.
- the electric resistant welded steel pipe is used as a line pipe for transporting energy resources such as crude oil and a natural gas.
- This application also relates to a method for manufacturing the steel sheet.
- UOE steel pipes have been mainly used for linepipes to date from the viewpoint of transport efficiency because UOE steel pipes can be manufactured to have a large diameter and a large thickness
- high strength electric resistance welded steel pipes which are manufactured with a high productivity from less expensive material, hot-rolled steel sheets, in a coil shape (hot-rolled steel strips)
- Electric resistance welded steel pipes have an advantage in that they are superior to UOE steel pipes in terms of variation in wall thickness and roundness in addition to cost.
- the pipe production method for electric resistance welded steel pipes involves cold roll forming, it is characteristic that, when pipe production is performed, plastic strain given to the cold-rolled steel pipes is significantly large compared to that given to UOE steel pipes.
- HIC is a phenomenon in which hydrogen ions, which have been generated by a corrosion reaction, increase internal pressure by becoming hydrogen atoms at the surface of a steel sheet, by entering the steel, and by accumulating around inclusions such as MnS, around carbides having a large grain diameter such as NbC, and around a second hard phase so as to cause the steel material to eventually crack.
- inclusions such as MnS
- carbides having a large grain diameter such as NbC
- Patent Literature 1 discloses a method for improving HIC resistance in which inclusions, which become the origins of HIC, are rendered harmless by controlling the total contents of chemical elements which combine respectively with S (sulfur), O (oxygen), and N (nitrogen) to form inclusions to be 0.01% or less or by controlling the maximum diameter of inclusions to be 5 ⁇ m or less, and in which the hardness of a center segregation part is controlled to be Hv 330 or less.
- Patent Literature 2 discloses a method for decreasing the area ratio of HIC by decreasing the size of TiN grains, which become the origin of HIC. Specifically, the size of Al—Ca-based sulfides in molten steel is decreased by controlling a weight ratio CaO/Al 2 O 3 to be 1.2 to 1.5 as a result of controlling the contents of Al and Ca in order to control the grain diameter of Al—Ti—Ca-based compound inclusions, which are formed using the sulfides as nuclei, to be 30 ⁇ m or less.
- Patent Literature 3 discloses a method in which the formation of carbonitrides of Nb and Ti, which become the origins of HIC, is less likely to occur by controlling Nb concentration to be 0.06% or less and Ti concentration to be 0.025% or less in a region located at a distance in the thickness direction of 5% of the thickness from the central part in the thickness direction.
- Patent Literature 4 discloses a method for manufacturing a high strength linepipe excellent in terms of HIC resistance in which HIC resistance is improved by decreasing the degree of center segregation as a result of decreasing Mn content in steel and in which Cr and Mo, which are comparatively less likely to undergo center segregation, are utilized.
- an object of the disclosed embodiments is to provide an electric resistance welded steel pipe for a high strength linepipe excellent in terms of HIC resistance which can preferably be used for an electric resistance welded steel linepipe and with which, for example, a crack length ratio (herein after, called CLR) is 15% or less when HIC occurs after the linepipe is given 10% of plastic strain.
- CLR crack length ratio
- CLR crack length ratio
- Disclosed embodiments have been completed in order to decrease the hardness of a center segregation part and in order to achieve desired strength on the basis of the knowledge which has been obtained by conducting many experiments regarding the relationship between the hardness of a center segregation part and steel chemical composition and the relationship of constituent microstructures to HIC performance and manufacturing conditions.
- FIG. 1 illustrates the relationship between the hardness ratio of a center segregation part to a non-segregation part (the Vickers hardness of a center segregation part/the Vickers hardness of a non-segregation part) and a crack length ratio (CLR). As FIG. 1 indicates, it was found that a CLR is 15% or less in the case where the hardness ratio is 1.20 or less.
- FIG. 2 illustrates the relationship between the hardness ratio of a center segregation part to a non-segregation part and an SP value. From the results, it was found that it is necessary to control an SP value to be 1.90 or less in order to control the hardness ratio of a center segregation part to a non-segregation part to be less than 1.20.
- a hot-rolled steel sheet for a high strength linepipe excellent in terms of HIC resistance the steel sheet having a chemical composition containing, by massa, C: 0.02% or more and 0.06% or less, Si: 0.05% or more and 0.25% or less, Mn: 0.60% or more and 1.10% or less, P: 0.008% or less, S: 0.0010% or less, Nb: 0.010% or more and 0.060% or less, Ti: 0.001% or more and 0.020% or less, Mo: 0.05% or less, Cr: 0.05% or more and 0.50% or less, Al: 0.01% or more and 0.08% or less, Ca: 0.0005% or more and 0.0050% or less, O: 0.005% or less, one or more selected from among Cu: 0.50% or less, Ni: 0.50% or less, and V: 0.10% or less, and the balance being Fe and inevitable impurities, and a metallic structure composed of bainitic-ferrite, in which expression (1) below is satisfied, and in which the
- a method for manufacturing a hot-rolled steel sheet for a high strength linepipe excellent in terms of HIC resistance including heating a steel slab having the chemical composition according to item [1] or [2] at a temperature of 1100° C. or higher and 1300° C. or lower, performing rough rolling, thereafter performing finish rolling under condition that cumulative rolling reduction ratio is 20% or more in a temperature range of 930° C. or lower, performing accelerated cooling on the hot-rolled steel sheet to a temperature of 380° C. or higher and 600° C. or lower at an average cooling rate of 5° C./sec. or more and 100° C./sec. or less in terms of the temperature of the central part in the thickness direction, and coiling the cooled steel sheet into a coil shape.
- a hot-rolled steel sheet for an electric resistance welded steel linepipe that has an improved HIC resistance after formation of the electric resistance welded steel pipe, which has been subjected to large plastic strain and that can be used without any problem under a harsh environment equivalent to a NACE solution.
- the hot-rolled steel sheet manufactured according to embodiments can also be used for a spiral steel pipe for a linepipe.
- FIG. 1 is a diagram illustrating the relationship between (the hardness of a center segregation part/the hardness of a non-segregation part) and a crack length ratio (CLR) according to an embodiment
- FIG. 2 is a diagram illustrating the relationship between an SP value and (the hardness of a center segregation part/the hardness of a non-segregation part) according to an embodiment
- FIG. 3 is a diagram illustrating the positions where the hardness of a center segregation part and the hardness of a non-segregation part were determined according to an embodiment.
- the C is a chemical element which significantly contributes to an increase in the strength of steel, and such an effect is realized in the case where the C content is 0.02% or more, but, in the case where the C content is more than 0.06%, since a second phase such as a pearlite microstructure is likely to be formed, there is a deterioration in HIC resistance. Therefore, the C content is set to be in a range of 0.02% or more and 0.06% or less, or preferably in a range of 0.03% or more and 0.05% or less.
- Si 0.05% or more and 0.25% or less
- Si is a chemical element which is added for solute strengthening and added in order to decrease scale-off quantity when hot rolling is performed, and such an effect is realized in the case where the Si content is 0.05% or more, but, in the case where the Si content is more than 0.25%, since red scale excessively grows, cooling ununiformity occurs when hot rolling is performed, which results in a deterioration in the uniformity of aesthetic appearance and material properties. Therefore, the Si content is set to be in a range of 0.05% or more and 0.25% or less, or preferably 0.10% or more and 0.25% or less.
- Si be added so that the ratio Mn/Si be 4.0 or more and 12 or less.
- Mn 0.60% or more and 1.10% or less
- Mn is a chemical element which contributes to an improvement in strength and toughness as a result of decreasing the grain diameter of a steel microstructure, and such an effect is realized in the case where the Mn content is 0.60% or more.
- the Mn content is set to be in a range of 0.60% or more and 1.10% or less, or preferably in a range of 0.75% or more and 1.05% or less.
- the P content be as small as possible, however, a P content of 0.008% or less is acceptable. Moreover, since there is an increase in cost due to an increase in refining time in order to markedly decrease the P content, it is preferable that the P content be 0.002% or more.
- S is, like P, a chemical element which is inevitably contained in steel, and since S forms MnS in steel, it is preferable that the S content be as small as possible, however, a S content of 0.0010% or less is acceptable, or preferably 0.0006% or less.
- Nb 0.010% or more and 0.060% or less
- Nb is a chemical element which contributes to an increase in the strength of steel as a result of precipitating in the form of fine Nb carbonitrides in a coiling process when hot-rolled steel sheet is manufactured. Also, Nb is a chemical element which contributes to an improvement in the toughness of a weld zone as a result of inhibiting the growth of austenite grains when electric resistance welding is performed. Such effects are realized in the case where the Nb content is 0.010% or more. On the other hand, in the case where the Nb content is more than 0.060%, Nb carbonitrides having a large grain diameter, which become the origins of HIC, are more likely to be formed. Therefore, the Nb content is set to be in a range of 0.010% or more and 0.060% or less, or preferably in a range of 0.030% or more and 0.060% or less.
- Ti is a chemical element which is added in order to render N, which significantly deteriorates the toughness of steel, harmless by fixing N in the form of TiN. Such an effect is realized in the case where the Ti content is more than 0.001%.
- the Ti content is set to be in a range of 0.001% or more and 0.020% or less, or preferably in a range of 0.005% or more and 0.015% or less.
- Mo is a chemical element which is significantly effective for improving the toughness and strength of steel by improving hardenability, but, since Mo forms a martensite microstructure as a result of being concentrated in a center segregation part, there is a deterioration in HIC resistance. Therefore, it is preferable that the Mo content be as small as possible, however, a Mo content of 0.05% or less is acceptable. It is more preferable that the Mo content be 0.01% or less.
- the Cr is a chemical element which is effective for improving the toughness and strength of steel by improving hardenability, and such an effect is realized in the case where the Cr is added 0.05% or more, but, in the case where the Cr is added more than 0.50%, there is a significant deterioration in the toughness of a weld zone as a result of forming Cr oxides when electric resistance welding is performed.
- the Cr content is set to be in a range of 0.05% or more and 0.50% or less, or preferably in a range of 0.05% or more and 0.300 or less.
- Al 0.01% or more and 0.08% or less
- the Al content is set to be in a range of 0.01% or more and 0.08% or less, or preferably in a range of 0.01% or more and 0.05% or less.
- Ca is a chemical element which is effective for improving HIC resistance by controlling the shape of sulfide-based inclusions, and such an effect is realized in the case where the Ca content is 0.0005% or more.
- the Ca content is set to be in a range of 0.0005% or more and 0.0050% or less, or preferably in a range of 0.0010% or more and 0.0030% or less.
- oxygen content be as small as possible, however, an oxygen content of 0.005% or less is acceptable, or preferably 0.00350 or less.
- one or more selected from among Cu, Ni, and V may be further added in the amounts described below.
- Cu is a chemical element which contributes to an improvement in the toughness and strength of steel through an improvement in hardenability, and, since Cu is less likely to be concentrated in a center segregation part than Mn and Mo which have similar effect, Cu can increase the strength of steel without deteriorating HIC resistance. Therefore, Cu is added in accordance with the strength grade. Such an effect is realized in the case where the Cu content is 0.05% or more, but, in the case where the Cu content is more than 0.50%, the effect becomes saturated and there is an unnecessary increase in cost. Therefore, the Cu content is 0.50% or less, or preferably 0.40% or less.
- Ni is, like Cu, a chemical element which contributes to an improvement in the toughness and strength of steel through an improvement in hardenability, and, since Ni is less likely to be concentrated in a center segregation part than Mn and Mo which have a similar effect, Ni can increase the strength of steel without deteriorating HIC resistance. Therefore, Ni is added in accordance with the strength grade. Such an effect is realized in the case where the Ni content is 0.05% or more, but, in the case where the Ni content is more than 0.50%, the effect becomes saturated and there is an unnecessary increase in cost. Therefore, the Ni content is 0.50% or less, or preferably 0.40% or less.
- V 0.10% or less
- V is a chemical element which contributes to an increase in the strength of steel through solute strengthening and precipitation strengthening in the case where the V content is 0.005% or more, but, in the case where the V content is more than 0.10%, since there is an increase in the hardness of a center segregation part, there is a deterioration in HIC resistance. Therefore, the V content is set to be 0.10% or less, or preferably 0.080% or less.
- an SP value which is derived from the contents of various alloy chemical elements, satisfies expression (1) below.
- An SP value was formulated in order to estimate the hardness of a center segregation part of a hot-rolled steel sheet which is used as a raw material of an electric resistance welded steel pipe using the contents of various alloy chemical elements, and, since the chemical elements are markedly concentrated in a center segregation part in the case where the SP value is more than 1.90, the condition that the hardness ratio of a center segregation part to a non-segregation part is less than 1.20 is not satisfied.
- the hardness ratio of a center segregation part to a non-segregation part decreases with decreasing SP value, it is necessary to control the upper limit of the SP value to be, for example, 1.75 in the case where it is required that HIC resistance be further improved in order to achieve a CLR of 5% or less.
- an EC value which is described below satisfy expression (2) below in order to effectively render sulfide-based inclusions harmless by adding Ca.
- the EC value indicates whether the content of Ca, which is added in order to control the shape of sulfide-based inclusions, is sufficient to form CaS, and the Ca content is insufficient in the case where the EC value is less than 1.2, which results in MnS, which becomes the origin of HIC, being formed.
- the EC value is more than 4.0, since Ca-based oxides are formed in a large amount, there is a deterioration in HIC resistance due to a deterioration in the cleaning level of steel. Therefore, it is preferable that the EC value be in a range of 1.2 or more and 4.0 or less, or more preferably in a range of 1.4 or more and 3.6 or less.
- the remainder of the chemical elements other than constituents described above consists of Fe and inevitable impurities.
- other trace elements may be added as long as the effects of the disclosed embodiments are not decreased.
- the area fractions of the microstructures other than a bainitic-ferrite microstructure be as small as possible.
- the microstructures other than a bainitic-ferrite microstructure may be included to some extent.
- a microstructure having a total area fraction of the steel microstructures other than a bainitic-ferrite microstructure may be considered to be a single bainitic-ferrite microstructure and is included in embodiments.
- the metallic structure described above can be achieved by using steel having the chemical composition described above and the manufacturing method described below.
- the CLR is 151 or less in the case where the hardness ratio of a center segregation part to a non-segregation part (the Vickers hardness of a center segregation part/the Vickers hardness of a non-segregation part) is less than 1.20.
- the SP value of the steel chemical composition with which the ratio of the hardness of a center segregation part to the hardness of a non-segregation part becomes less than 1.20 is 1.90 or less.
- the hardness of a center segregation part and the hardness of a non-segregation part were, as FIG. 3 illustrates, respectively determined for 15 points each on a center segregation line and in a portion located at 200 ⁇ m from the center segregation line, and the arithmetic average values of the determined values were derived, where the center segregation line was exposed by performing etching using a 2%-nital solution for a duration of 30 seconds or more on a test piece for microstructure observation.
- a slab heating temperature is set to be 1100° C. or higher and 1300° C. or lower.
- the temperature is lower than 1100° C., since the temperature is not high enough for carbides, which are formed in steel when continuous casting is performed, to completely form solid solutions, the required strength cannot be achieved.
- the temperature is higher than 1300° C., since there is a marked increase in austenite grain diameter, there is a deterioration in toughness.
- this temperature refers to the temperature of the interior of the heating furnace, and the center of the slab is presumed to be heated to this temperature.
- finish rolling it is necessary that finish rolling be performed under the condition that cumulative rolling reduction ratio is 20% or more at a temperature of 930° C. or lower.
- the cumulative rolling reduction ratio is less than 20%, since there are an insufficient number of nucleation sites of a bainitic-ferrite microstructure, there is an excessive increase in the grain diameter of the microstructure, which results in a deterioration in toughness.
- the cumulative rolling reduction ratio is more than 80%, since the effect becomes saturated, and since a very high load is applied to a rolling mill, it is preferable that the upper limit of the cumulative rolling reduction ratio be 80% or less.
- the average cooling rate for the central part in the thickness direction of a steel sheet is set to be 5° C./sec. or more and 100° C./sec. or less.
- the cooling rate is less than 5° C./sec, the area fractions of a ferrite microstructure and/or a pearlite microstructure become 3% or more even if hardenability increasing chemical elements such as Cu, Ni, and Cr are added. Therefore, it is necessary that the cooling rate be 5° C./sec. or more.
- the cooling rate is more than 100° C./sec, the area fraction of a martensite microstructure becomes 3% or more.
- the cooling rate of the central part in the thickness direction of a steel sheet was calculated by deriving the temperature history of the central part in the thickness direction of the steel sheet by performing heat-transfer calculation using the cooling capacity (heat-transfer coefficient) of a run-out, which had been investigated in advance, and the surface temperature of the steel sheet, which had been determined using a radiation thermometer on the run-out.
- the cooling stop temperature is set to be in a range of 380° C. or higher and 600° C. or lower.
- the cooling stop temperature is higher than 600° C., since the area fraction of a ferrite microstructure and a pearlite microstructure becomes 3% or more, and since there is an increase in the diameter of precipitation strengthening grains such as Nb carbonitrides, there is a decrease in strength.
- the cooling stop temperature is lower than 380° C., since there is an improvement in the deformation resistance of a steel sheet, it is difficult to coil the steel sheet into a coil shape, and there is a decrease in strength due to precipitation strengthening grains such as Nb carbonitrides not being precipitated.
- steel grade G through K are comparative example steels having a chemical composition, SP value or the like which is out of the range according to disclosed embodiments.
- *1 SP Mn + Mo + 11.3 ⁇ C + 0.29 ⁇ (Cu + Ni) + 0.60 ⁇ Cr + 0.88 ⁇ V, where an atomic symbols in the equation respectively represent the contents (mass %) of the corresponding chemical elements.
- *2 EC [Ca]eff/(1.25 ⁇ S), where [Ca]eff is derived from Ca ⁇ (0.18 + 130 ⁇ Ca) ⁇ O, and where atomic symbols Ca, S, and O in the equations respectively represent the contents (mass %) of the corresponding chemical elements.
- test pieces which had been collected from the obtained hot-rolled steel sheet, and by performing microstructure observation, a tensile test, a Charpy impact test, hardness determination, and a HIC test, tensile properties, toughness, and HIC resistance were evaluated.
- a tensile test piece was collected from the obtained hot-rolled steel sheet so that the longitudinal direction was at a right angle to the rolling direction (C direction), and a tensile test was performed at room temperature in accordance with API-5L specification in order to determine yield strength YS (deformation stress for a nominal strain of 0.5%) and tensile strength TS.
- a v-notched test piece was collected from the central part in the thickness direction of the obtained hot-rolled steel sheet so that the longitudinal direction was at a right angle to the rolling direction (C direction), and absorbed energy and a percent brittle fracture were determined by performing a Charpy impact test at a temperature range of ⁇ 140° C. to 0° C. in accordance with JIS Z 2242 in order to determine a temperature (fracture transition temperature) at which a percent brittle fracture was 50%.
- three test pieces were used for one temperature in order to obtain the arithmetic averages of the determined absorbed energy and percent brittle fracture.
- the hardness of a center segregation part and the hardness of a non-segregation part were respectively determined for 15 points each on a segregation line and in a portion located at 200 ⁇ m from the segregation line, and the arithmetic average values of the determined values were derived, where the segregation line was exposed by performing etching using a 2%-nital solution for a duration of 30 seconds or more on a test piece for microstructure observation ( FIG. 3 ).
- the hardness was determined using a Vickers hardness meter with a testing force of 0.3 kgf.
- the hardness ratio was calculated by dividing the hardness of a segregation part by the hardness of a non-segregation part.
- HIC test piece having the thickness of the steel sheet, a width of 20 mm, and a length of 100 mm which was collected from the obtained hot-rolled steel sheet so that the longitudinal direction was the rolling direction of the steel sheet
- a HIC test was performed using an A solution in accordance with NACE TM 0284 in order to evaluate HIC resistance.
- 10 test pieces were used for one coil, and a compressive strain of 10% in a width direction was applied to the test pieces in advance in order to simulate plastic strain which is applied to a steel sheet when forming is performed in a process for manufacturing an electric resistance welded steel pipe. From the test results, in the case where CLR was 15% or less for all the test pieces for one coil, the coil was judged as satisfactory ( ⁇ ) in terms of HIC resistance. In the case where the CLR was more than 15% for one or more of the test pieces for one coil, the coil was judged as unsatisfactory (x) in terms of HIC resistance.
- the examples according to embodiments were all steel sheets having a high yield strength YS of 380 MPa or more, minimum required toughness for a linepipe as indicated by a vTrs of ⁇ 60° C. or lower, and excellent HIC resistance as indicated by a hardness ratio of less than 1.20.
- the comparative examples which were out of the range according to embodiments, did not achieve the desired properties for a hot-rolled steel sheet for a high strength electric resistance welded steel pipe excellent in terms of HIC resistance, because the desired toughness was not achieved, or because there was a deterioration in HIC resistance.
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PCT/JP2014/000320 WO2014115549A1 (ja) | 2013-01-24 | 2014-01-23 | 高強度ラインパイプ用熱延鋼板 |
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US (1) | US20150368736A1 (ja) |
EP (1) | EP2927339B1 (ja) |
JP (1) | JP5884202B2 (ja) |
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CN113406291A (zh) * | 2021-06-29 | 2021-09-17 | 西安热工研究院有限公司 | 一种风电塔用结构钢板的质量验证方法 |
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CN104404383A (zh) * | 2014-11-28 | 2015-03-11 | 钢铁研究总院 | 一种超低碳抗硫化氢腐蚀x80管线钢及制备方法 |
CN105132833B (zh) * | 2015-10-10 | 2017-12-08 | 武汉钢铁有限公司 | 一种经济型高强度海底管线钢及生产方法 |
EP3604592B1 (en) * | 2017-03-30 | 2022-03-23 | JFE Steel Corporation | High strength steel plate for sour-resistant line pipe, method for manufacturing same, and high strength steel pipe using high strength steel plate for sour-resistant line pipe |
CN107974613B (zh) * | 2017-11-23 | 2019-12-27 | 武汉钢铁有限公司 | 抗硫化物应力腐蚀开裂的x80级管线钢的生产方法 |
CN111270137A (zh) * | 2020-02-17 | 2020-06-12 | 本钢板材股份有限公司 | 一种抗酸腐蚀管线钢x52ms热轧卷板及其制备方法 |
KR102498135B1 (ko) * | 2020-12-18 | 2023-02-08 | 주식회사 포스코 | 황화물 응력부식 균열 저항성이 우수한 고강도 강재 및 이의 제조방법 |
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- 2014-01-23 US US14/763,403 patent/US20150368736A1/en not_active Abandoned
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JP5884202B2 (ja) | 2016-03-15 |
KR20150087424A (ko) | 2015-07-29 |
WO2014115549A1 (ja) | 2014-07-31 |
EP2927339A4 (en) | 2015-12-16 |
KR101718267B1 (ko) | 2017-03-20 |
EP2927339A1 (en) | 2015-10-07 |
CN104937125B (zh) | 2018-01-09 |
EP2927339B1 (en) | 2016-11-02 |
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CN104937125A (zh) | 2015-09-23 |
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