US20110126944A1 - Thick-walled high-strength hot rolled steel sheet with excellent low-temperature toughness and method for producing same - Google Patents

Thick-walled high-strength hot rolled steel sheet with excellent low-temperature toughness and method for producing same Download PDF

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US20110126944A1
US20110126944A1 US13/056,791 US200913056791A US2011126944A1 US 20110126944 A1 US20110126944 A1 US 20110126944A1 US 200913056791 A US200913056791 A US 200913056791A US 2011126944 A1 US2011126944 A1 US 2011126944A1
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steel sheet
thickness direction
temperature
steel
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Chikara Kami
Hiroshi Nakata
Kinya Nakagawa
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JFE Steel Corp
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JFE Steel Corp
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Priority claimed from JP2008198314A external-priority patent/JP5401863B2/ja
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    • 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
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    • C21D1/18—Hardening; Quenching with or without subsequent tempering
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • 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
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    • C21D2211/00—Microstructure comprising significant phases
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

Definitions

  • This disclosure relates to a thick-walled high-strength hot rolled steel sheet suitable as a material for high strength electric resistance welded steel pipes and high strength spiral steel pipes used for transport pipes through which crude oil, natural gas, and the like are transported and which are required to have high toughness, and relates to a method for producing the steel sheet.
  • the disclosure relates to improvement in low-temperature toughness.
  • steel sheet includes steel plates and steel strips.
  • high-strength hot rolled steel sheet” used here indicates a hot rolled steel sheet with a high tensile strength (TS) of 510 MPa or more.
  • TS tensile strength
  • Thick-walled steel sheet indicates a steel sheet with a thickness of 11 mm or more.
  • transport pipes used for transporting crude oil and natural gas that contain hydrogen sulfide are required to have excellent sour gas resistance, such as hydrogen induced cracking resistance (HIC resistance) and stress corrosion cracking resistance, in addition to the characteristics, for example, high strength and high toughness.
  • HIC resistance hydrogen induced cracking resistance
  • stress corrosion cracking resistance in addition to the characteristics, for example, high strength and high toughness.
  • Japanese Unexamined Patent Application Publication No. 08-319538 discloses a method for producing a low yield ratio and high strength hot rolled steel sheet having excellent toughness, the method including the steps of hot-rolling steel that contains, on a mass percent, 0.005% to less than 0.030% C, 0.0002% to 0.0100% B, one or both elements selected from 0.20% or less Ti and 0.25% or less Nb in amounts such that (Ti+Nb/2)/C is 4 or more, and Si, Mn, P, S, Al, and N in appropriate amounts, cooling the steel at a cooling rate of 5 to 20° C./s, coiling the steel at a temperature in the range of higher than 550° C. to 700° C.
  • the microstructure is composed of ferrite and/or bainitic ferrite, and the amount of solid solution carbon in grains is in the range of 1.0 to 4.0 ppm.
  • the technique described in JP '538 seems to provide a low yield ratio and high strength hot rolled steel sheet having excellent toughness, weldability, and sour gas resistance without causing the nonuniformity of the material in the thickness direction and longitudinal direction.
  • the amount of solid solution carbon in crystal grains is 1.0 to 4.0 ppm.
  • heat input during girth welding is disadvantageously liable to cause grain growth. That is, coarse grains are formed in a welded heat affected zone. This is liable to cause a deterioration in the toughness in the welded heat affected zone of a girth welded portion.
  • Japanese Unexamined Patent Application Publication No. 09-296216 discloses a method for producing a high-strength steel sheet having excellent hydrogen induced cracking resistance, the method including terminating hot rolling of a steel slab at a temperature of Ar 3 +100° C. or higher, the steel slab containing, on a mass percent, 0.01%-0.12% C, 0.5% or less Si, 0.5%-1.8% Mn, 0.010%-0.030% Ti, 0.01%-0.05% Nb, and 0.0005%-0.0050% Ca to satisfy a carbon equivalent of 0.40 or less and a Ca/0 of 1.5 to 2.0; performing air cooling for 1 to 20 seconds; cooling the steel sheet from the Ar 3 point or higher to 550° C. to 650° C.
  • JP '216 seems to provide a steel sheet for a transport pipe specified by API X60 to X70 grade, the steel sheet having hydrogen induced cracking resistance.
  • a desired cooling time is not ensured. To ensure desired properties, further improvement in cooling capacity is disadvantageously needed.
  • Japanese Unexamined Patent Application Publication No. 2008-056962 discloses a method for producing a thick high-strength steel plate for a transport pipe having excellent hydrogen induced cracking resistance, the method including heating steel containing, on a mass percent, 0.03%-0.06% C, 0.01%-0.5% Si, 0.8%-1.5% Mn, 0.0015% or less S, 0.08% or less Al, 0.001%-0.005% Ca, and 0.0030% or less O, Ca, S, and O satisfying a specific relationship; performing accelerated cooling at a cooling rate of 5° C./s or more from the Ar 3 transformation point to 400° C.
  • JP '962 seems to provide a steel plate in which the fraction of a second phase in the metal microstructure is 3% or less and in which the difference in hardness between a surface layer and the middle position of the steel plate in the thickness direction is 40 points or less in terms of Vickers hardness, the thick steel plate having excellent hydrogen induced cracking resistance.
  • the reheating step is needed, making the production process complex. Furthermore, it is necessary to install a reheating apparatus and so forth.
  • Japanese Unexamined Patent Application Publication No. 2001-240936 discloses a method for producing a thick high-strength steel plate having a coarse-grained ferrite layer on each of the upper and lower surfaces, the method including performing rolling at a cumulative rolling reduction of 2% or more and a temperature of Ac 1 -50° C. or lower in a cooling step after hot rolling a cast slab containing, on a mass percent, 0.01%-0.3% C, 0.6% or less Si, 0.2%-2.0% Mn, 0.06% or less Al, 0.005%-0.035% Ti, and 0.001%-0.006% N; heating the steel sheet to a temperature exceeding Ac 1 and less than Ac 3 ; and allowing the steel sheet to cool.
  • JP '936 seems to contribute to improvement in the SCC sensitivity, weather resistance, and corrosion resistance of a steel material, and to the suppression of the degradation of the material after cold forming.
  • the reheating step is needed, making the production process complex. Furthermore, it is necessary to install a reheating apparatus and so forth.
  • Japanese Unexamined Patent Application Publication No. 2001-207220 discloses a method for producing a hot rolled steel sheet for a high-strength electric resistance welded steel pipe, the method including heating a steel slab containing, on a mass percent, C, Si, Mn, and N in an appropriate amount, Si and Mn in such a manner that Mn/Si satisfies 5 to 8, and 0.01%-0.1% Nb; performing rough rolling under conditions in which the reduction rate of first rolling at 1100° C. or higher is 15% to 30%, the total reduction rate at 1000° C.
  • a steel sheet produced by the technique described in JP '220 seems to be formed into a high-strength electric resistance welded steel pipe having a fine microstructure of a surface layer of the steel sheet and excellent low-temperature toughness, in particular, excellent DWTT characteristics, without adding an expensive alloy element or performing heat treatment of the entire steel pipe.
  • a desired cooling time is not ensured. To ensure desired properties, further improvement in cooling capacity is disadvantageously needed.
  • Japanese Unexamined Patent Application Publication No. 2004-315957 discloses method for producing a hot rolled steel strip for high-strength electric resistance welded steel pipe having excellent low-temperature toughness and excellent weldability, the method including heating a steel slab containing, on a mass percent, C, Si, Mn, Al, and N in appropriate amounts, 0.001%-0.1% Nb, 0.001%-0.1% V, and 0.001%-0.1% Ti, and one or two or more of Cu, Ni, and Mo, the steel slab having a Pcm value of 0.17 or less; terminating finish rolling under conditions in which the surface temperature is (Ar 3 -50° C.) or higher; thereafter rapidly cooling the steel sheet; coiling the steel sheet at 700° C. or lower; and performing slow cooling.
  • a hot rolled steel sheet in the related art varies widely in material properties at points in the longitudinal direction and width direction of the sheet, in many cases.
  • excellent CTOD characteristics indicates that a critical opening displacement (CTOD value) is 0.30 mm or more when a CTOD test is performed at a test temperature of ⁇ 10° C. in conformity with the regulation of ASTM E 1290.
  • excellent DWTT characteristics indicates that in the case where a DWTT test is performed in conformity with the regulation of ASTM E 436, the lowest temperature (DWTT temperature) when the percent shear fracture is 85% is ⁇ 35° C. or lower.
  • ferrite serving as a main phase indicates that a microstructure serving as a main phase is hard low-temperature transformation ferrite, i.e., indicates bainitic ferrite or bainite, excluding soft high-temperature transformation ferrite (granular polygonal ferrite).
  • the term “ferrite serving as a main phase” indicates hard low-temperature transformation ferrite (bainitic ferrite, bainite, or a mixed phase thereof), unless otherwise specified.
  • the second phase indicates perlite, martensite, a martensite-austenite constituent (MA) (also referred to as island martensite), or a mixed phase thereof.
  • MA martensite-austenite constituent
  • the temperature used in the finish rolling is indicated by a temperature of the surface. Values of the temperature at the middle position of the steel sheet in the thickness direction in the accelerated cooling, the cooling rate, and the coiling temperature are determined using heat transfer calculation or the like from surface temperatures measured.
  • the steel sheet has only small nonuniformity of the material in the longitudinal direction and the width direction of the sheet, i.e., the steel sheet has excellent uniformity of the material.
  • the steel sheet has excellent dimensional accuracy.
  • the steel sheet has excellent pipe formability and excellent dimensional accuracy.
  • FIG. 1 is a graph illustrating the relationship between ⁇ D and ⁇ V that affect DWTT.
  • FIG. 2 is a graph illustrating the relationship among ⁇ D, ⁇ V, and the cooling stop temperature of accelerated cooling.
  • FIG. 3 is a graph illustrating the relationship among ⁇ D, ⁇ V, and the coiling temperature.
  • FIG. 4A is a graph illustrating the effect of the mill scale on the tensile strength of a surface layer.
  • FIG. 4B is a graph illustrating the effect of the mill scale on the elongation of a surface layer.
  • FIG. 5 is a graph illustrating the effect of the carbon equivalent Ceq on ⁇ HV.
  • FIG. 6 is a graph illustrating the effect of the average cooling rate on ⁇ HV at a position 1 mm from a surface of a steel sheet in the thickness direction (at a carbon equivalent Ceq of 0.37%).
  • FIG. 7 is a graph illustrating the effect of the coiling temperature on the relationship between the minimum lath spacing and the carbon equivalent Ceq.
  • a slab containing, on a mass percent basis, 0.037% C-0.20% Si-1.59% Mn-0.016% P-0.0023% S-0.041% Al-0.061% Nb-0.013% Ti-balance Fe was used as a steel material, provided that (Ti+Nb/2)/C was 1.18.
  • the steel material having the foregoing composition was heated to 1230° C. and subjected to hot rolling at a finish rolling start temperature of 980° C. and a finish rolling end temperature of 800° C. to form hot rolled steel sheets having a thickness of 14.5 mm.
  • the hot rolled steel sheets were subjected to accelerated cooling to various cooling stop temperatures at a cooling rate of 18° C./s in a temperature region in which a temperature at each middle position in the thickness direction exceeded 750° C., followed by coiling at various coiling temperatures (temperature at each middle position in the thickness direction) to form hot rolled steel sheets (steel strips).
  • Test specimens were taken from the resulting hot rolled steel sheet.
  • the microstructures and the DWTT characteristics were investigated.
  • the average grain size ( ⁇ m) of ferrite serving as a main phase and the fraction (percent by volume) of a second phase were determined at a position (surface layer portion) 1 mm from a surface of each steel sheet in the thickness direction and the middle position (middle portion in the thickness direction) of each steel sheet in the thickness direction.
  • the difference ⁇ D between the average grain size of ferrite serving as the main phase at the position (surface layer portion) 1 mm from the surface of each steel sheet and the average grain size of ferrite serving as the main phase at the middle position (middle portion in the thickness direction) of the steel sheet in the thickness direction were calculated from the resulting measurement values.
  • the difference ⁇ V between the fraction of the second phase at the position (surface layer portion) 1 mm from the surface of each steel sheet and the fraction of the second phase at the middle position (middle portion in the thickness direction) of the steel sheet in the thickness direction were calculated from the resulting measurement values.
  • the second phase is composed of, for example, pearlite, martensite, or a martensite-austenite constituent (MA) (also referred to as “island martensite”).
  • microstructures and the DWTT characteristics were investigated as in (1) Microstructure Observation and (4) DWTT Test in Example 1 described below.
  • FIG. 1 demonstrates that the “excellent DWTT characteristics,” in which the DWTT is ⁇ 35° C. or lower, are reliably maintained at a ⁇ D of 2 ⁇ m or less and a ⁇ V of 2% or less.
  • FIG. 2 illustrates the relationship among ⁇ D, ⁇ V, and the cooling stop temperature.
  • FIG. 3 illustrates the relationship among ⁇ D, ⁇ V, and the coiling temperature.
  • FIGS. 2 and 3 demonstrate that to achieve a ⁇ D of 2 ⁇ m or less and a ⁇ V of 2% or less, the cooling stop temperature and the coiling temperature for the steel used need to be adjusted to 620° C. or lower and 647° C. or lower, respectively.
  • the cooling stop temperature and the coiling temperature required to achieve a ⁇ D of 2 ⁇ m or less and a ⁇ V of 2% or less are determined, mainly depending on the alloy element content and the cooling rate after the completion of the hot rolling, which affect the bainitic transformation start temperature. That is, to achieve a ⁇ D of 2 ⁇ m or less and a ⁇ V of 2% or less, it is important that the cooling stop temperature at the middle position of the steel sheet in the thickness direction is set to BFS or lower, BFS being defined by the expression:
  • BFS0 BFS0
  • a slab containing, on a mass percent basis, 0.053% C-0.20% Si-1.60% Mn-0.012% P-0.0026% S-0.035% Al-0.061% Nb-0.013% Ti-0.0032% N-balance Fe was used as a steel material, provided that (Ti+Nb/2)/C was 0.82.
  • the steel material having the composition described above was heated to 1200° C. and subjected to hot rolling including rough rolling and finish rolling to form hot rolled steel sheets (steel strips).
  • scale removal treatment was performed with a rough scale breaker (RSB) before the rough rolling.
  • finish rolling scale removal treatment was performed with a finish scale breaker (FSB) before the finish rolling, and hot rolling was performed at various finish entry temperatures (FETs) and finish delivery temperatures (FDTs), thereby forming 15.6-mm-thick hot rolled steel sheets with different thicknesses of mill scale.
  • FETs finish entry temperatures
  • FDTs finish delivery temperatures
  • the hot rolled steel sheets were subjected to accelerated cooling to a cooling stop temperature of 540° C. at a cooling rate of 50° C./s in a temperature region in which a temperature at the middle position of each steel sheet in the thickness direction was 750° C. or lower, followed by coiling at a coiling temperature of 520° C.
  • FIGS. 4A and 4B illustrate the relationship between the tensile properties (tensile strength TS and elongation El) and the thickness ( ⁇ m) of mill scale on the basis of the results. Note that the tensile properties and the thickness of mill scale were measured as in (2) Tensile Test and the measurement of the thickness of mill scale in (1) Microstructure Observation in Example 2 described below.
  • FIGS. 4A and 4B show that a thickness of the mill scale of 5 to 30 ⁇ m results in only small changes in the tensile properties (TS and El) of the surface layer. From the results, we discovered that the adjustment of the thickness of the mill scale in an appropriate range reduces variations in the tensile properties of the surface layer and the nonuniformity of the material of the steel sheet in the longitudinal direction and the width direction, thereby further improving the uniformity of the material.
  • the carbon equivalent Ceq was calculated using the expression:
  • the steel material having the foregoing composition was heated to 1200° C. and subjected to hot rolling at a finish rolling start temperature of 1010° C. and a finish rolling end temperature of 810° C. to form hot rolled steel sheets having a thickness of 25.4 mm. After the completion of the hot rolling, the hot rolled steel sheets were subjected to accelerated cooling to a cooling stop temperature of 470° C. to 490° C.
  • Vickers hardness HV 1mm at the position 1 mm from the surface of each steel sheet in the thickness direction and Vickers hardness HV 1/2t at the middle position of each steel sheet in the thickness direction were measured with a Vickers hardness tester (load: 10 kgf) in a cross section orthogonal to the direction of the hot rolling.
  • FIG. 5 illustrates the relationship between ⁇ HV and the carbon equivalent Ceq on the basis of the results when the accelerated cooling operations were performed at average cooling rates of 80° C./s and 200° C./s at the positions 1 mm from the surfaces of the steel sheets in the thickness direction. Note that ⁇ HV was measured as in (2) Tensile Test in Example 3 described below.
  • FIG. 5 shows that when ⁇ HV is 50 points, the Ceq values are 0.40% at an average cooling rate of 80° C./s and 0.37% at 200° C./s. To achieve a ⁇ HV of 50 points or less, the results demonstrate that if Ceq exceeds 0.37%, the average cooling rate at the position 1 mm from the surface of the steel sheet in the thickness direction needs to be 200° C./s or less.
  • the hot rolled steel sheets were subjected to cooling operations at average cooling rates of 10 to 350° C./s at the position 1 mm from the surface of each steel sheet in the thickness direction.
  • Test specimens for the measurement of hardness were taken from the resulting hot rolled steel sheets.
  • Vickers hardness HV 1mm at the position 1 mm from the surface of each steel sheet in the thickness direction and Vickers hardness HV 1/2t at the middle position of each steel sheet in the thickness direction were measured in a cross section orthogonal to the direction of the hot rolling.
  • FIG. 6 illustrates the relationship ⁇ HV and the average cooling rate at the position 1 mm from the surface of the steel sheet in the thickness direction on the basis of the results.
  • FIG. 6 shows that the cooling rate at the position 1 mm from the surface of the steel sheet in the thickness direction needs to be 200° C./s or less to achieve a ⁇ HV of 50 points or less.
  • the carbon equivalent Ceq was calculated using the expression:
  • the steel material having the foregoing composition was heated to 1210° C. and subjected to hot rolling at a finish rolling start temperature of 1000° C. and a finish rolling end temperature of 800° C. to form hot rolled steel sheets having a thickness of 25.4 mm.
  • the hot rolled steel sheets were subjected to accelerated cooling to a cooling stop temperature of 200° C. to 500° C. at the middle position of each steel sheet in the thickness direction at a cooling rate of 34° C./s at the middle position of each steel sheet in the thickness direction and an average cooling rate of 300° C./s at the position 1 mm from the surface of each steel sheet in the thickness direction, followed by coiling at two coiling temperatures of lower than 300° C. and 300° C.
  • test specimens for microstructure observation were taken from the resulting hot rolled steel sheets.
  • FIG. 7 illustrates the relationship between the minimum lath spacing and the carbon equivalent Ceq on the basis of the results.
  • FIG. 7 shows that a coiling temperature CT of 300° C. or higher allows the minimum lath spacing in the bainite phase, the bainitic ferrite phase, or the tempered martensitic phase at the position 1 mm from the surface of the steel sheet in the thickness direction to be 0.1 ⁇ m or more, regardless of the carbon equivalent Ceq.
  • the resulting steel sheet is subjected to cooling on the hot run table to a cooling stop temperature of 300° C. to BFS at the middle position of the steel sheet in the thickness direction and then coiling at a coiling temperature of 300° C. or higher at the middle position of the steel sheet in the thickness direction to promote self-annealing, thereby achieving a minimum lath spacing of 0.1 ⁇ m or more in the bainite phase (including bainitic ferrite phase) or the tempered martensitic phase at the position 1 mm from the surface of the steel sheet in the thickness direction.
  • the C is an element having the effect of increasing the strength of steel.
  • the C content needs to be 0.02% or more.
  • An excessively high C content exceeding 0.08% causes an increase in the fraction of a second phase, such as pearlite, thereby deteriorating the toughness of the base metal and the toughness of a welded heat affected zone.
  • the C content is limited to 0.02% to 0.08%.
  • the C content is preferably in the range of 0.04% to 0.06%.
  • Si has the effect of enhancing solid-solution strengthening and improving hardenability to increase the strength of steel. The effect is observed at a Si content of 0.01% or more. Furthermore, Si has the effect of allowing the C content in a ⁇ phase (austenite phase) to be increased during the y (austenite) to a (ferrite) transformation to promote the formation of the martensitic phase serving as a second phase. This results in an increase in ⁇ D, deteriorating the toughness of the steel sheet. Moreover, Si forms a Si-containing oxide during electric resistance welding, thereby deteriorating the quality of a welded portion and the toughness of a welded heat affected zone. From such a viewpoint, while Si is preferably minimized, a Si content of 0.50% is acceptable. Thus, the Si content is limited to 0.01% to 0.50%. The Si content is preferably 0.40% or less.
  • Si forms low-melting-point manganese silicate.
  • the oxide is easily ejected from a welded portion.
  • the Si content may be 0.10% to 0.30%.
  • Mn has the effect of improving hardenability and thereby increasing the strength of a steel sheet. Furthermore, Mn forms MnS to fix S, thereby preventing the grain boundary segregation of S and suppressing the cracking of a slab (steel material). To provide the effect, the Mn content needs to be 0.5% or more.
  • a Mn content exceeding 1.8% results in the promotion of solidification segregation during slab casting, a high Mn content portion left in a steel sheet, and the increase of the occurrence of separation.
  • heating to a temperature exceeding 1300° C. is needed.
  • the implementation of such heat treatment in an industrial scale is impractical.
  • the Mn content is limited to 0.5% to 1.8%.
  • the Mn content is preferably in the range of 0.9% to 1.7%.
  • P is inevitably contained as an impurity in steel and has the effect of increasing the strength of steel.
  • an excessively high P content exceeding 0.025% leads to a deterioration reduction in weldability.
  • the P content is limited to 0.025% or less.
  • the P content is preferably 0.015% or less.
  • S is inevitably contained as an impurity in steel.
  • a S content exceeding 0.005% causes slab cracking and the formation of coarse MnS in a hot rolled steel sheet, thereby deteriorating the ductility.
  • the S content is limited to 0.005% or less.
  • the S content is preferably 0.004% or less.
  • Al is an element that functions as a deoxidant. To provide the effect, an Al content of 0.005% or more is preferred. Meanwhile, an Al content exceeding 0.10% leads to significant deterioration in the cleanliness of a welded portion during electric resistance welding. Thus, the Al content is limited to 0.005% to 0.10%.
  • the Al content is preferably 0.08% or less.
  • Nb is an element having the effect of suppressing the recrystallization and an increase in the size of austenite grains. Nb permits hot finish rolling to be performed in a temperature range in which austenite is not recrystallized. Even if the Nb content is low, Nb has the effect of increasing the strength of a hot rolled steel sheet by the fine precipitation of carbonitride, without impairing weldability. To provide the effect, the Nb content needs to be 0.01% or more. Meanwhile, an excessively high Nb content exceeding 0.10% results in an increase in rolling load during hot finish rolling, making it difficult to perform hot rolling in some cases. Thus, the Nb content is limited to 0.01% to 0.10%. The Nb content is preferably in the range of 0.03% to 0.09%.
  • Ti has the effect of preventing the cracking of slab (steel material) by forming a nitride to fix N. Furthermore, the strength of a steel sheet is increased by the fine precipitation of carbide. The effect is significant in a Ti content of 0.001% or more. However, a Ti content exceeding 0.05% results in a marked increase in yield point due to precipitation strengthening. Thus, the Ti content is limited to 0.001% to 0.05%. The Ti content is preferably in the range of 0.005% to 0.035%.
  • Nb, Ti, and C are contained in amounts described above, and the proportions of Nb, Ti, and C are adjusted in such a manner that the expression (1):
  • Nb and Ti are elements that have a strong tendency to form carbide. It is assumed that in the case of a low C content, most of C is formed into carbide, thereby markedly reducing the amount of solid solution carbon in ferrite grains. However, the marked reduction in the amount of solid solution carbon in ferrite grains adversely affects girth weldability in pipeline construction.
  • the reason for this is as follows: in the case where a steel pipe produced from a steel sheet in which the amount of solid solution carbon in ferrite grains is markedly reduced is used as a transport pipe and where girth weld is performed, significant grain growth is observed in a welded heat affected zone of a girth welded portion, so that the toughness of the welded heat affected zone of the girth welded portion can be deteriorated.
  • the proportions of Nb, Ti, and C are adjusted to satisfy expression (1).
  • This permits the amount of solid solution carbon in ferrite grains to be 10 ppm or more, thereby preventing the deterioration in the toughness of the welded heat affected zone of the girth welded portion.
  • the left-hand side of expression (1) is preferably 3 or less.
  • the foregoing components are basic components.
  • one or two or more elements selected from 0.01% to 0.10% V, 0.01% to 0.50% Mo, 0.01% to 1.0% Cr, 0.01% to 0.50% Cu, and 0.01% to 0.50% Ni may be contained as additional elements, and/or 0.0005% to 0.005% Ca may be contained.
  • One or Two or More Elements Selected From 0.01% to 0.10% V, 0.01% to 0.50% Mo, 0.01% to 1.0% Cr, 0.01% to 0.50% Cu, and 0.01% to 0.50% Ni
  • V, Mo, Cr, Cu, and Ni are each element that improves hardenability and increase the strength of a steel sheet.
  • One or two or more selected therefrom may be contained, as needed.
  • V is an element that has the effect of improving hardenability and increasing the strength of a steel sheet by the formation of carbonitride.
  • the V content is preferably 0.01% or more. Meanwhile, an excessively high V content exceeding 0.10% results in a deterioration in weldability.
  • the V content is preferably limited to 0.01% to 0.10%. More preferably, the V content is in the range of 0.03% to 0.08%.
  • Mo is an element that has the effect of improving hardenability and increasing the strength of a steel sheet by the formation of carbonitride.
  • the Mo content is preferably 0.01% or more. Meanwhile, an excessively high Mo content exceeding 0.50% results in a deterioration in weldability.
  • the Mo content is preferably limited to 0.01% to 0.50%. More preferably, the Mo content is in the range of 0.05% to 0.30%.
  • the Cr is an element that has the effect of improving hardenability and increasing the strength of a steel sheet.
  • the Cr content is preferably 0.01% or more. Meanwhile, an excessively high Cr content exceeding 1.0% is more liable to cause the formation of weld defects during electric resistance welding.
  • the Cr content is preferably limited to 0.01% to 1.0%. More preferably, the Cr content is in the range of 0.01% to 0.80%.
  • the Cu is an element that has the effect of improving hardenability and increasing the strength of a steel sheet by solid-solution strengthening or precipitation strengthening.
  • the Cu content is preferably 0.01% or more.
  • a Cu content exceeding 0.50% results in a deterioration in hot workability.
  • the Cu content is preferably limited to 0.01% to 0.50%. More preferably, the Cu content is in the range of 0.10% to 0.40%.
  • Ni is an element that has the effect of improving hardenability, increasing the strength of steel, and improving the roughness of a steel sheet.
  • the Ni content is preferably 0.01% or more. Even if the Ni content exceeds 0.50%, the effect is saturated. Hence, an effect comparable to the Ni content is not provided, which is disadvantageous in cost.
  • the Ni content is preferably limited to 0.01% to 0.50%. More preferably, the Ni content is in the range of 0.10% to 0.45%.
  • Ca is an element that has the effect of fixing S in the form of CaS, spheroidizing sulfide inclusions to control the forms of inclusions, and reducing the lattice strain of the base metal around the inclusions to reduce the ability to trap hydrogen.
  • a significant effect is provided in a Ca content of 0.0005% or more.
  • a Ca content exceeding 0.005% leads to an increase in the CaO content, thereby deteriorating corrosion resistance and toughness.
  • the Ca content is preferably limited to 0.0005% to 0.005%. More preferably, the Ca content is in the range of 0.0009% to 0.003%.
  • the balance other than the component described above is Fe and incidental impurities.
  • incidental impurities 0.005% or less N, 0.005% or less O, 0.003% or less Mg, and 0.005% or less Sn are acceptable.
  • N is inevitably contained in steel.
  • An excessively high N content often causes the cracking of a steel material (slab) during casting.
  • the N content is preferably limited to 0.005% or less. More preferably, the N content is 0.004% or less.
  • O is present in steel in the form of various oxides, causing a deterioration in hot workability, corrosion resistance, toughness, and the like.
  • the O content is preferably minimized, an O content of 0.005% or less is acceptable.
  • An extreme reduction in the O content leads to an increase in refining cost.
  • the O content is preferably limited to 0.005% or less.
  • Mg has the effect of forming oxide and sulfide and suppressing the formation of coarse MnS.
  • a Mg content exceeding 0.003% often causes the formation of clusters of Mg oxide and Mg sulfide, thereby deteriorating toughness.
  • Mg is preferably limited to 0.003% or less.
  • Sn is incorporated from scrap used as a raw material for steelmaking Sn is an element that is likely to be segregated in grain boundaries.
  • a high Sn content exceeding 0.005% results in a reduction in the strength of grain boundaries, thereby deteriorating the toughness.
  • the Sn content is preferably limited to 0.005% or less.
  • the thick-walled high-strength hot rolled steel sheet has the composition described above and a microstructure in which the difference ⁇ D between the average grain size ( ⁇ m) of a ferrite phase serving as a main phase at the position 1 mm from the surface of the steel sheet in the thickness direction and the average grain size ( ⁇ m) of the ferrite phase serving as the main phase at the middle position of the steel sheet in the thickness direction is 2 ⁇ m or less and in which the difference ⁇ V between the fraction (percent by volume) of a second phase at the position 1 mm from the surface of the steel sheet in the thickness direction and the fraction (percent by volume) of the second phase at the middle position of the steel sheet in the thickness direction is 2% or less.
  • ferrite which is the main phase of the hot rolled steel sheet, includes bainite, low-temperature transformation products, such as bainitic ferrite, and mixtures thereof.
  • the second phase include pearlite, martensite, a martensite-austenite constituent (MA), and mixed phases thereof.
  • the low-temperature toughness of the thick-walled high-strength hot rolled steel sheet is significantly improved and, in particular, the DWTT characteristics and the CTOD characteristics using full-thickness test specimens are significantly improved.
  • the DWTT is higher than ⁇ 35° C. to degrade the DWTT characteristics, deteriorating the low-temperature toughness.
  • the microstructure is limited to a microstructure in which the difference ⁇ D between the average grain size ( ⁇ m) of the ferrite phase serving as the main phase at the position 1 mm from the surface of the steel sheet in the thickness direction and the average grain size ( ⁇ m) of the ferrite phase serving as the main phase at the middle position of the steel sheet in the thickness direction is 2 ⁇ m or less and in which the difference ⁇ V between the fraction (percent by volume) of the second phase at the position 1 mm from the surface of the steel sheet in the thickness direction and the fraction (percent by volume) of the second phase at the middle position of the steel sheet in the thickness direction is 2% or less.
  • the difference ⁇ D* between the average grain size ( ⁇ m) of the ferrite phase serving as the main phase at the position 1 mm from the surface of the steel sheet in the thickness direction and the average grain size ( ⁇ m) of the ferrite phase serving as the main phase at a position away from the surface of the steel sheet in the thickness direction by 1 ⁇ 4 of the thickness is 2 ⁇ m or less
  • the difference ⁇ V* between the fraction (%) of the second phase at the position 1 mm from the surface of the steel sheet in the thickness direction and the fraction (%) of the second phase at the position away from the surface of the steel sheet in the thickness direction by 1 ⁇ 4 of the thickness is 2% or less
  • the thick-walled high-strength hot rolled steel sheet preferably has uniform mill scale having a thickness of 3 to 30 ⁇ m on a surface of the steel sheet.
  • the heat transfer coefficient is reduced compared with the case of a larger thickness, leading to a reduction in tensile strength as illustrated in FIG. 4A .
  • the mill scale partially has a thickness of less than 3 ⁇ m, uneven cooling occurs to cause a local reduction in strength.
  • the heat transfer coefficient is increased compared with the case of a smaller thickness, leading to an increase in tensile strength as illustrated in FIG. 4A .
  • the thickness of the mill scale formed on the surface is limited to 3 to 30 ⁇ m. In the case where the thickness of the mill scale formed on the surface is adjusted within this range, variations in strength and ductility at positions in the steel sheet are reduced, thereby improving the uniformity of the material at the positions in the steel sheet.
  • the hot rolled steel sheet has the foregoing composition, the foregoing microstructure, and a hardness distribution in which the difference ⁇ HV between the Vickers hardness HV 1mm at the position 1 mm from the surface of the steel sheet in the thickness direction and the Vickers hardness HV 1/2t at the middle position of the steel sheet in the thickness direction is 50 points or less.
  • a ⁇ HV exceeding 50 points is liable to cause a local increase in strength, thereby deteriorating the pipe formability and deteriorating the circularity a pipe.
  • the difference ⁇ HV between HV 1mm and HV 1/2t is limited to 50 points or less.
  • the hot rolled steel sheet has the foregoing composition, the foregoing microstructure, and the microstructure in which the minimum lath spacing of the bainite phase (including bainitic ferrite phase) or the tempered martensitic phase is 0.1 ⁇ m or more at the position 1 mm from the surface of the steel sheet in the thickness direction.
  • the hot rolled steel sheet having the structure has excellent pipe formability.
  • molten steel having the foregoing composition is made by a common method with a converter or the like and formed into a steel material, such as a slab, by a common casting method, such as a continuous casting process.
  • a common casting method such as a continuous casting process.
  • this disclosure is not limited to the method.
  • the steel material having the composition is heated and subjected to hot rolling.
  • the hot rolling includes rough rolling that forms the steel material into a sheet bar and finish rolling that forms the sheet bar into a hot rolled steel sheet.
  • the heating temperature of the steel material may be a temperature at which the steel material can be rolled into a hot rolled steel sheet. While the heating temperature need not be particularly limited, the heating temperature is preferably in the range of 1100° C. to 1300° C. A heating temperature of less than 1100° C. results in a high resistance to distortion, increasing the rolling load to cause an excessively high load on a rolling mill. A heating temperature exceeding 1300° C. results in coarse crystal grains, deteriorating the low-temperature toughness, increasing the amount of scale formed, and reducing the yield. Thus, the heating temperature during the hot rolling is preferably in the range of 1100° C. to 1300° C.
  • the heated steel material is subjected to rough rolling into a sheet bar.
  • the conditions of the rough rolling are not particularly limited as long as a sheet bar having desired dimensions is formed. From the viewpoint of ensuring low-temperature toughness, the rolling end temperature of the rough rolling is preferably 1050° C. or lower.
  • the steel material is subjected to scale removal treatment, in which primary scale formed on the surface of the steel material by heating is removed with a rough scale breaker (RSB) for a roughing mill, before the rough rolling.
  • the scale removal treatment may be repeatedly performed in the course of the rough rolling in addition to before the rough rolling.
  • it is preferred that an excessive use of the scale breaker is avoided.
  • the resulting sheet bar is then subjected to finish rolling.
  • the finish rolling start temperature is preferably adjusted by subjecting the sheet bar to accelerated cooling before the finish rolling or to, for example, oscillation on a table. This permits a reduction rate (effective reduction rate) in a finishing mill to be increased in a temperature region effective in improving the toughness.
  • a temperature used in the finish rolling is indicated by a temperature of the surface.
  • the finish entry temperature is set in the range of 800° C. to 1050° C.
  • the finish delivery temperature is set in the range of 750° C. to 950° C.
  • a finish delivery temperature (FDT) of less than 800° C. a portion in the vicinity of the surface is excessively cooled, so that the portion can have a temperature of less than the Ar 3 transformation point, thereby leading to a nonuniform microstructure in the thickness direction to deteriorate the toughness.
  • An FET exceeding 1050° C. can cause the formation of secondary scale in the finishing mill, making it difficult to adjust the thickness of the mill scale in a desired appropriate range.
  • the portion in the vicinity of the surface can have a temperature of less than the Ar 3 transformation point, thereby leading to a nonuniform microstructure in the thickness direction to deteriorate the toughness.
  • An FDT exceeding 950° C. results in the formation of secondary scale in the finishing mill, making it difficult to adjust the thickness of the mill scale in a desired appropriate range.
  • the finish entry temperature is preferably adjusted by subjecting the sheet bar to accelerated cooling before the finish rolling or to, for example, oscillation on the table. This permits a reduction rate in a finishing mill to be increased in a temperature region effective in improving the toughness.
  • the steel material is subjected to scale removal treatment, in which secondary scale formed on the sheet bar is removed with a finish scale breaker (FSB) for the finishing mill, before the finish rolling.
  • the scale removal treatment may be repeatedly performed by cooling between stands of the finishing mill in addition to before the finish rolling.
  • the sheet bar preferably has a temperature of 800° C. to 1050° C. during the scale removal treatment. To adjust the thickness of mill scale of the product (hot rolled steel sheet) in an appropriate range, it is preferred that an excessive use of the scale breaker is avoided.
  • the scale removal treatment can also adjust the finish entry temperature.
  • the effective reduction rate is preferably set to 20% or more from the viewpoint of improving the toughness.
  • the term “effective reduction rate” indicates the total amount of rolling reduction (%) at temperatures of 950° C. or less.
  • the effective reduction rate at the middle position of the steel sheet in the thickness direction preferably satisfies 20% or more.
  • the accelerated cooling is preferably performed to a cooling stop temperature of BFS or lower at an average cooling rate of 10° C./s or more at the middle position of the steel sheet in the thickness direction.
  • the average cooling rate is defined as an average cooling rate in the temperature range of 750° C. to 650° C.
  • a cooling rate of less than 10° C./s is liable to cause the formation of high-temperature transformation ferrite (polygonal ferrite).
  • the fraction of the second phase is increased at the middle position of the steel sheet in the thickness direction, failing to the desired microstructure described above.
  • the accelerated cooling after the completion of the hot rolling is preferably performed at an average cooling rate of 10° C./s or more at the middle position of the steel sheet in the thickness direction. More preferably, the average cooling rate is set to 20° C./s or more.
  • the upper limit of the cooling rate is determined, depending on the ability of a cooling apparatus used.
  • the upper limit is preferably lower than a martensite-forming cooling rate, which is a cooling rate without a deterioration in the shape of the steel sheet, for example, camber.
  • the cooling rate can be achieved with a water cooler using, for example, a flat nozzle, a rod-like nozzle, or a circular-tube nozzle.
  • Values of the temperature at the middle position of the steel sheet in the thickness direction, the cooling rate, the coiling temperature, and the like are determined using heat transfer calculation or the like.
  • the cooling stop temperature in the accelerated cooling is preferably BFS or lower at the middle position of the steel sheet in the thickness direction. More preferably, the cooling stop temperature is (BFS ⁇ 20° C.) or lower.
  • BFS is defined by the expression (2):
  • the hot rolled steel sheet is coiled at a coiling temperature of BFS0 or lower at the middle position of the steel sheet in the thickness direction. More preferably, the coiling temperature is (BFS0 ⁇ 20° C.) or lower.
  • BFS0 is defined by the expression (3):
  • a cooling stop temperature in the accelerated cooling of BFS or lower and a coiling temperature of BFS0 or lower result in a ⁇ D of 2 ⁇ m or less and a ⁇ V of 2% or less, providing the extremely uniform microstructure in the thickness direction. This ensured excellent DWTT characteristics and excellent CTOD characteristics, providing the thick-walled high-strength hot rolled steel sheet having significantly improved low-temperature toughness.
  • the coiled hot rolled steel sheet is preferably cooled to room temperature at a cooling rate of 20 to 60° C./hr at the middle portion of the coil (the middle portion of the coil in the longitudinal direction).
  • a cooling rate of less than 20° C./hr can lead to a deterioration in toughness due to the progress of crystal grain growth.
  • a cooling rate exceeding 60° C./hr is liable to cause an increase in the difference in temperature between the middle portion of the coil and the outer and inner portions of the coil, thereby deteriorating the shape of the coil.
  • Test specimens were taken from the resulting hot rolled steel sheets. Microstructure observation, a tensile test, an impact test, a DWTT test, and a CTOD test were conducted. The electric resistance welded steel pipes were also subjected to the DWTT test and the CTOD test. Methods of the tests were described below.
  • Test specimens for microstructure observation were taken from the hot rolled steel sheets. Cross sections in the rolling direction were polished and etched. Each test specimen was observed in two or more fields of view using an optical microscope (magnification: 1000 ⁇ ) or a scanning electron microscope (magnification: 1000 ⁇ ). Images of each test specimen were taken.
  • the average grain size of a ferrite phase serving as a main phase (indicates hard low-temperature transformation ferrite and includes bainitic ferrite, bainite, and a mixed phase thereof) and the fraction (percent by volume) of a second phase (pearlite, martensite, a martensite-austenite constituent (MA), and a mixed phase thereof) other than the ferrite phase serving as the main phase were measured with an image analysis system.
  • Observation positions were set to a position 1 mm from a surface of each steel sheet in the thickness direction and the middle position of each steel sheet in the thickness direction.
  • the average grain size of the ferrite phase serving as the main phase was determined by an intercept method.
  • a nominal grain size was defined as the average grain size at the position.
  • Plate-like test specimens (width of parallel portion: 25 mm, gage length: 50 mm) were taken from the resulting hot rolled steel sheets in such a manner that a direction (c direction) orthogonal to a rolling direction was a tensile test direction.
  • a tensile test was performed at room temperature in conformity with the regulation of ASTM E8M-04, and the tensile strength TS was determined.
  • V-notch test specimens were taken from the middle positions of the resulting hot rolled steel sheets in the thickness direction in such a manner that the direction (c direction) orthogonal to the rolling direction was a longitudinal direction.
  • the Charpy impact test was performed in conformity with the regulation of JIS Z 2242. Absorbed energy (J) at a test temperature of ⁇ 80° C. was determined. Three test specimens were used. The arithmetic mean of the resulting absorbed energy values was determined and defined as vE 80 (J), which was the absorbed energy of the steel sheet. In the case where vE 80 was 300 J or more, the steel sheet was evaluated to have “satisfactory toughness.”
  • DWTT test specimens (dimensions: thickness ⁇ 3 in. wide ⁇ 12 in. long) were taken from the resulting hot rolled steel sheets in such a manner that the direction (c direction) orthogonal to the rolling direction was a longitudinal direction.
  • a DWTT test was performed in conformity with the regulation of ASTM E 436. The lowest temperature (DWTT) when the percent shear fracture was 85% was determined. In the case where DWTT was ⁇ 35° C. or lower, the steel sheet was evaluated to have “excellent DWTT characteristics.”
  • DWTT test specimens were also taken from base metal of the electric resistance welded steel pipes and tested in the same way as the steel sheets.
  • CTOD test specimens (dimensions: thickness ⁇ width (2 ⁇ thickness) ⁇ length (10 ⁇ thickness)) were taken from the resulting hot rolled steel sheets in such a manner that the direction (c direction) orthogonal to the rolling direction was a longitudinal direction.
  • a CTOD test was performed in conformity with the regulation of ASTM E 1290 at a test temperature of ⁇ 10° C.
  • a critical opening displacement (CTOD value) at ⁇ 10° C. was determined.
  • a test load was applied by three-point bending.
  • a displacement gage was attached to a notched portion, and the critical opening displacement (CTOD value) was measured. In the case where the CTOD value was 0.30 mm or more, the steel sheet was evaluated to have “excellent CTOD characteristics.”
  • CTOD test specimens were also taken from the electric resistance welded steel pipes in such a manner that a direction orthogonal to the direction of tube axis was the longitudinal direction of the test specimens. Notches were made in base metal and seam portions. The test specimens were tested in the same way as the steel sheets.
  • the hot rolled steel sheet has an appropriate microstructure, a high tensile strength TS of 521 MPa or more, and excellent low-temperature toughness, in which vE 80 is 300 J or more, the CTOD value is 0.30 mm or more, and DWTT is ⁇ 35° C. or lower.
  • the hot rolled steel sheet has excellent CTOD characteristics and excellent DWTT characteristics.
  • the CTOD value is 0.30 mm or more, and DWTT is ⁇ 20° C. or lower. That is, the steel pipes have excellent low-temperature toughness.
  • vE 80 is less than 300 J
  • the CTOD value is less than 0.30 mm
  • DWTT exceeds ⁇ 35° C. That is, the steel sheets have deteriorated low-temperature toughness.
  • a comparative example (steel sheet 5) in which the cooling rate after the completion of the hot rolling is lower than our range, the difference ⁇ V of the fractions of the second phase exceeds 2%, so that the steel sheet has deteriorated low-temperature toughness.
  • ⁇ D exceeds 2 ⁇ m, so that the steel sheet has deteriorated low-temperature toughness.
  • Test specimens were taken from the resulting hot rolled steel sheets. Microstructure observation, a tensile test, an impact test, a DWTT test, and a CTOD test were conducted. The electric resistance welded steel pipes were also subjected to the DWTT test and the CTOD test. Methods of the tests were described below.
  • Test specimens for microstructure observation were taken from the hot rolled steel sheets. Cross sections in the rolling direction were polished and etched. Each test specimen was observed in two or more fields of view using an optical microscope (magnification: 1000 ⁇ ) or a scanning electron microscope (magnification: 1000 ⁇ ). Images of each test specimen were taken.
  • the average grain size of a ferrite phase serving as a main phase (indicates hard low-temperature transformation ferrite and includes bainitic ferrite, bainite, and a mixed phase thereof) and the fraction (percent by volume) of a second phase (pearlite, martensite, a martensite-austenite constituent (MA), and a mixed phase thereof) other than the ferrite phase serving as the main phase were measured with an image analysis system.
  • Observation positions were set to a position 1 mm from a surface of each steel sheet in the thickness direction and the middle position of each steel sheet in the thickness direction.
  • the average grain size of the ferrite phase serving as the main phase was determined by an intercept method.
  • a nominal grain size was defined as the average grain size at the position.
  • Test specimens for the measurement of the thickness of mill scale were taken from points (four points at intervals of 40 m in the longitudinal direction) of each of the resulting hot rolled steel sheets in the longitudinal direction and points (four points at intervals of 0.4 m in the width direction) in the width direction. Cross sections in the rolling direction were polished.
  • the mill scale thicknesses were measured with the optical microscope or the scanning electron microscope.
  • the average mill scale thickness ts which is the average value of the resulting mill scale thicknesses, and the difference ⁇ ts between the maximum value and the minimum value of the mill scale thicknesses at the points were calculated.
  • Plate-like test specimens (width of parallel portion: 25 mm, gage length: 50 mm) were taken from points (four points at intervals of 40 m in the longitudinal direction) of each of the resulting hot rolled steel sheets in the longitudinal direction and points (four points at intervals of 0.4 m in the width direction) in the width direction in such a manner that a direction (c direction) orthogonal to a rolling direction was the longitudinal direction.
  • a tensile test was performed at room temperature in conformity with the regulation of ASTM E8M-04, and the tensile strength TS was determined. The difference between the minimum value and the maximum value of the values of the tensile strength TS at the points was determined and defined as variations ⁇ TS. The variations in tensile strength at the points of each steel sheet were evaluated. In the case where ⁇ TS was 35 MPa or lower, the steel sheet was evaluated to be uniform.
  • V-notch test specimens were taken from points (four points at intervals of 40 m in the longitudinal direction) of each of the resulting hot rolled steel sheets in the longitudinal direction and points (four points at intervals of 0.4 m in the width direction) in the width direction, the points being located at the middle positions of the resulting hot rolled steel sheets in the thickness direction, in such a manner that the direction (c direction) orthogonal to the rolling direction was the longitudinal direction.
  • the Charpy impact test was performed in conformity with the regulation of JIS Z 2242. Absorbed energy (J) at a test temperature of ⁇ 80° C. was determined. Three test specimens were used.
  • vE 80 J
  • vE 80 the absorbed energy of the steel sheet.
  • vE 80 300 J or more
  • the steel sheet was evaluated to have “satisfactory toughness.”
  • the difference between the minimum value and the maximum value of the values of vE 80 at the points was determined and defined as variations ⁇ vE 80 .
  • the variations in toughness at the points of each steel sheet were evaluated. In the case where ⁇ vE 80 was 45 J or less, the steel sheet was evaluated to be uniform.
  • DWTT test specimens (dimensions: thickness ⁇ 3 in. wide ⁇ 12 in. long) were taken from the resulting hot rolled steel sheets in such a manner that the direction (c direction) orthogonal to the rolling direction was a longitudinal direction.
  • a DWTT test was performed in conformity with the regulation of ASTM E 436. The lowest temperature (DWTT) when the percent shear fracture was 85% was determined. In the case where DWTT was ⁇ 35° C. or lower, the steel sheet was evaluated to have “excellent DWTT characteristics.”
  • DWTT test specimens were also taken from base metal of the electric resistance welded steel pipes and tested in the same way as the steel sheets.
  • CTOD test specimens (dimensions: thickness t ⁇ width (2 ⁇ t) ⁇ length (10 ⁇ t)) were taken from the resulting hot rolled steel sheets in such a manner that the direction (c direction) orthogonal to the rolling direction was a longitudinal direction.
  • a CTOD test was performed in conformity with the regulation of ASTM E 1290 at a test temperature of ⁇ 10° C.
  • a critical opening displacement (CTOD value) at ⁇ 10° C. was determined.
  • a test load was applied by three-point bending.
  • a displacement gage was attached to a notched portion, and the critical opening displacement (CTOD value) was measured. In the case where the CTOD value was 0.30 mm or more, the steel sheet was evaluated to have “excellent CTOD characteristics.”
  • CTOD test specimens were also taken from the electric resistance welded steel pipes in such a manner that a direction orthogonal to the direction of tube axis was the longitudinal direction of the test specimens. Notches were made in base metal and seam portions. The test specimens were tested in the same way as the steel sheets.
  • the hot rolled steel sheet has mill scale with an appropriate thickness, an appropriate microstructure, a high tensile strength TS of 510 MPa or more, and excellent low-temperature toughness, in which vE 80 is 300 J or more, the CTOD value is 0.30 mm or more, and DWTT is ⁇ 35° C. or lower. Furthermore, the hot rolled steel sheet has only small nonuniformity of the material in the longitudinal direction and width direction of the sheet and has a uniform material. In particular, the hot rolled steel sheet has excellent CTOD characteristics and excellent DWTT characteristics.
  • the CTOD value is 0.30 mm or more, and DWTT is ⁇ 20° C. or lower. That is, the steel pipes have excellent low-temperature toughness.
  • vE 80 is less than 300 J
  • the CTOD value is less than 0.30 mm
  • DWTT exceeds ⁇ 35° C. That is, the steel sheets have deteriorated low-temperature toughness.
  • the mill scale thicknesses vary widely. The nonuniformity of the material is increased in the longitudinal direction and the width direction of each sheet.
  • the difference ⁇ V of the fractions of the second phase exceeds 2%, so that the steel sheet has deteriorated low-temperature toughness.
  • the average thickness of the mill scale exceeds 30 ⁇ m, and there are variations in mill scale thickness. ⁇ D exceeds 2 ⁇ m, so that the steel sheet has deteriorated low-temperature toughness. In addition, the tensile strength ⁇ TS varies widely.
  • the average thickness of the mill scale is less than 3 ⁇ m, and ⁇ V exceeds 2%, so that the steel sheet has deteriorated low-temperature toughness.
  • the average thickness of the mill scale exceeds 30 ⁇ m, the mill scale thicknesses vary widely, and the tensile strength ⁇ TS varies widely.
  • the average thickness of the mill scale exceeds 30 ⁇ m, the mill scale thicknesses vary widely, and the tensile strength ⁇ TS varies widely.
  • ⁇ D exceeds 2 ⁇ m, and ⁇ V exceeds 2%, so that the steel sheet has deteriorated low-temperature toughness.
  • ⁇ D exceeds 2 ⁇ m, so that the steel sheet has deteriorated low-temperature toughness.
  • the base metal and the seam portions have deteriorated low-temperature toughness.
  • Test specimens were taken from the resulting hot rolled steel sheets. Microstructure observation, a hardness test, a tensile test, an impact test, a DWTT test, and a CTOD test were conducted. The electric resistance welded steel pipes were also subjected to the DWTT test and the CTOD test. Methods of the tests were described below.
  • Test specimens for microstructure observation were taken from the hot rolled steel sheets. Cross sections in the rolling direction were polished and etched. Each test specimen was observed in two or more fields of view using an optical microscope (magnification: 1000 ⁇ ) or a scanning electron microscope (magnification: 2000 ⁇ ). Images of each test specimen were taken.
  • the average grain size of a ferrite phase serving as a main phase (indicates hard low-temperature transformation ferrite and includes bainitic ferrite, bainite, and a mixed phase thereof) and the fraction (percent by volume) of a second phase (pearlite, martensite, a martensite-austenite constituent (MA), and a mixed phase thereof) other than the ferrite phase serving as the main phase were measured with an image analysis system.
  • Observation positions were set to a position 1 mm from a surface of each steel sheet in the thickness direction and the middle position of each steel sheet in the thickness direction.
  • the average grain size of the ferrite phase serving as the main phase was determined by measuring areas of ferrite grains, calculating the diameters of the equivalent circles from the areas, and determining the arithmetic mean of the diameters of the equivalent circles of the ferrite grains.
  • Test specimens for microstructure observation were taken from the hot rolled steel sheets.
  • Hardness HV in each cross section in the rolling direction was measured with a Vickers hardness tester (test load: 98 N (load: 10 kgf)).
  • Measurement positions were set to the positions 1 mm from the surfaces of the steel sheets in the thickness direction and the middle positions of the steel sheets in the thickness direction. The hardness measurement was performed at three points in each position. The arithmetic mean of the measurement results were determined and defined as the hardness at each position.
  • Plate-like test specimens (width of parallel portion: 25 mm, gage length: 50 mm) were taken from the resulting hot rolled steel sheets in such a manner that a direction (c direction) orthogonal to a rolling direction was the longitudinal direction.
  • a tensile test was performed at room temperature in conformity with the regulation of ASTM E8M-04, and the tensile strength TS was determined.
  • V-notch test specimens were taken from the middle positions of the resulting hot rolled steel sheets in the thickness direction in such a manner that the direction (c direction) orthogonal to the rolling direction was a longitudinal direction.
  • the Charpy impact test was performed in conformity with the regulation of JIS Z 2242. Absorbed energy (J) at a test temperature of ⁇ 80° C. was determined. Three test specimens were used. The arithmetic mean of the resulting absorbed energy values was determined and defined as vE 80 (J), which was the absorbed energy of the steel sheet. In the case where vE 80 was 200 J or more, the steel sheet was evaluated to have “satisfactory toughness.”
  • DWTT test specimens (dimensions: thickness ⁇ 3 in. wide ⁇ 12 in. long) were taken from the resulting hot rolled steel sheets in such a manner that the direction (c direction) orthogonal to the rolling direction was a longitudinal direction.
  • a DWTT test was performed in conformity with the regulation of ASTM E 436. The lowest temperature (DWTT) when the percent shear fracture was 85% was determined. In the case where DWTT was ⁇ 35° C. or lower, the steel sheet was evaluated to have “excellent DWTT characteristics.”
  • DWTT test specimens were also taken from base metal of the electric resistance welded steel pipes and tested in the same way as the steel sheets.
  • CTOD test specimens (dimensions: thickness ⁇ width (2 ⁇ thickness) ⁇ length (10 ⁇ thickness)) were taken from the resulting hot rolled steel sheets in such a manner that the direction (c direction) orthogonal to the rolling direction was a longitudinal direction.
  • a CTOD test was performed in conformity with the regulation of ASTM E 1290 at a test temperature of ⁇ 10° C.
  • a critical opening displacement (CTOD value) at ⁇ 10° C. was determined.
  • a test load was applied by three-point bending.
  • a displacement gage was attached to a notched portion, and the critical opening displacement (CTOD value) was measured. In the case where the CTOD value was 0.30 mm or more, the steel sheet was evaluated to have “excellent CTOD characteristics.”
  • CTOD test specimens were also taken from the electric resistance welded steel pipes in such a manner that a direction orthogonal to the direction of tube axis was the longitudinal direction of the test specimens. Notches were made in base metal and seam portions. The test specimens were tested in the same way as the steel sheets.
  • Table 9 shows the results. The circularity of each of the resulting electric resistance welded steel pipes was measured.
  • each of the steel pipes was measured at a cross section orthogonal to the longitudinal direction of the steel pipe.
  • the circularity of the cross section of the pipe was determined using the following expression:
  • Circularity (%) ⁇ (maximum outer diameter ⁇ minimum outer diameter)/(nominal diameter) ⁇ 100.
  • the hot rolled steel sheet has, in the thickness direction, an appropriate microstructure, an appropriate difference in hardness, a high tensile strength TS of 521 MPa or more, and excellent low-temperature toughness, in which vE 80 is 200 J or more, the CTOD value is 0.30 mm or more, and DWTT is ⁇ 35° C. or lower.
  • the hot rolled steel sheet has excellent CTOD characteristics and excellent DWTT characteristics.
  • the CTOD value is 0.30 mm or more, and DWTT is ⁇ 20° C. or lower. That is, the steel pipes have excellent low-temperature toughness.
  • the circularity of each of the electric resistance welded steel pipes made from the hot rolled steel sheets of the examples is less than 0.90%, which is satisfactory.
  • vE 80 is less than 200 J
  • the CTOD value is less than 0.30 mm
  • DWTT exceeds ⁇ 35° C.
  • ⁇ HV exceeds 50 points.
  • the circularity is 0.90% or more, which is degraded.
  • the difference ⁇ V of the fractions of the second phase exceeds 2%, so that the steel sheet has deteriorated low-temperature toughness.
  • the CTOD value at the seam portion of the electric resistance welded steel pipe is less than 0.30 mm, so that the pipe has deteriorated low-temperature toughness.
  • the cooling rate in the accelerated cooling at the position 1 mm from the surface of the steel sheet in the thickness direction is higher than our range because of the carbon equivalent Ceq and in which ⁇ HV exceeds 50 points, which is outside our range, the circularity is deteriorated to be 0.90%.
  • Test specimens were taken from the resulting hot rolled steel sheets. Microstructure observation, a tensile test, an impact test, a DWTT test, and a CTOD test were conducted. The electric resistance welded steel pipes were also subjected to the DWTT test and the CTOD test. Methods of the tests were described below.
  • Test specimens for microstructure observation were taken from the hot rolled steel sheets. Cross sections in the rolling direction were polished and etched. Each test specimen was observed in two or more fields of view using an optical microscope (magnification: 1000 ⁇ ) or a scanning electron microscope (magnification: 2000 ⁇ ). Images of each test specimen were taken.
  • the average grain size of a ferrite phase serving as a main phase (indicates hard low-temperature transformation ferrite and includes bainitic ferrite and bainite) and the fraction (percent by volume) of a second phase (pearlite, martensite, a martensite-austenite constituent (MA), and a mixed phase thereof) other than the ferrite phase serving as the main phase were measured with an image analysis system.
  • Observation positions were set to a position 1 mm from a surface of each steel sheet in the thickness direction and the middle position of each steel sheet in the thickness direction.
  • the average grain size of the ferrite phase serving as the main phase was determined by measuring areas of ferrite grains, calculating the diameters of the equivalent circles from the areas, and determining the arithmetic mean of the diameters of the equivalent circles of the ferrite grains.
  • Thin film specimens were taken from positions 1 mm from surfaces of the steel sheets in the thickness direction. Each thin film specimen was observed in three or more fields of view with a transmission electron microscope (magnification: 50,000 ⁇ ). Images of each thin film specimen were taken. The lath spacing of bainite (including bainitic ferrite) or tempered martensite was measured. Among the resulting lath spacing values, the minimum lath spacing value was determined.
  • Plate-like test specimens (width of parallel portion: 25 mm, gage length: 50 mm) were taken from the resulting hot rolled steel sheets in such a manner that a direction (c direction) orthogonal to a rolling direction was a longitudinal direction.
  • a tensile test was performed at room temperature in conformity with the regulation of ASTM E8M-04, and the tensile strength TS was determined.
  • V-notch test specimens were taken from the middle positions of the resulting hot rolled steel sheets in the thickness direction in such a manner that the direction (c direction) orthogonal to the rolling direction was a longitudinal direction.
  • the Charpy impact test was performed in conformity with the regulation of JIS Z 2242. Absorbed energy (J) at a test temperature of ⁇ 80° C. was determined. Three test specimens were used. The arithmetic mean of the resulting absorbed energy values was determined and defined as vE 80 (J), which was the absorbed energy of the steel sheet. In the case where vE 80 was 250 J or more, the steel sheet was evaluated to have “satisfactory toughness.”
  • DWTT test specimens (dimensions: thickness ⁇ 3 in. wide ⁇ 12 in. long) were taken from the resulting hot rolled steel sheets in such a manner that the direction (c direction) orthogonal to the rolling direction was a longitudinal direction.
  • a DWTT test was performed in conformity with the regulation of ASTM E 436. The lowest temperature (DWTT) when the percent shear fracture was 85% was determined. In the case where DWTT was ⁇ 50° C. or lower, the steel sheet was evaluated to have “excellent DWTT characteristics.”
  • DWTT test specimens were also taken from base metal of the electric resistance welded steel pipes and tested in the same way as the steel sheets.
  • CTOD test specimens (dimensions: thickness ⁇ width (2 ⁇ thickness) ⁇ length (10 ⁇ thickness)) were taken from the resulting hot rolled steel sheets in such a manner that the direction (c direction) orthogonal to the rolling direction was a longitudinal direction.
  • a CTOD test was performed in conformity with the regulation of ASTM E 1290 at a test temperature of ⁇ 10° C.
  • a critical opening displacement (CTOD value) at ⁇ 10° C. was determined.
  • a test load was applied by three-point bending.
  • a displacement gage was attached to a notched portion, and the critical opening displacement (CTOD value) was measured. In the case where the CTOD value was 0.30 mm or more, the steel sheet was evaluated to have “excellent CTOD characteristics.”
  • CTOD test specimens were also taken from the electric resistance welded steel pipes in such a manner that a direction orthogonal to the direction of tube axis was the longitudinal direction of the test specimens. Notches were made in base metal and seam portions. The test specimens were tested in the same way as the steel sheets.
  • Table 12 shows the results.
  • the circularity of each of the resulting electric resistance welded steel pipes was investigated.
  • the outer diameter of each of the steel pipes was measured at a cross section orthogonal to the axial direction of the steel pipe. According to JIS B 0182, the circularity was determined using ⁇ (maximum outer diameter ⁇ minimum outer diameter)/(nominal diameter) ⁇ 100(%).
  • the hot rolled steel sheet has, in the thickness direction, an appropriate microstructure, a high tensile strength TS of 510 MPa or more, and excellent low-temperature toughness, in which vE 80 is 250 J or more, the CTOD value is 0.30 mm or more, and DWTT is ⁇ 50° C. or lower.
  • the hot rolled steel sheet has excellent CTOD characteristics and excellent DWTT characteristics.
  • the CTOD value is 0.30 mm or more, and DWTT is ⁇ 40° C. or lower. That is, the steel pipes have excellent low-temperature toughness.
  • vE 80 is less than 250 J
  • the CTOD value is less than 0.30 mm
  • DWTT exceeds ⁇ 50° C.
  • the circularity of the pipe is degraded.
  • the difference ⁇ V of the fractions of the second phase exceeds 2%, so that the steel sheet has deteriorated low-temperature toughness.
  • the steel sheet has only small nonuniformity of the material in the longitudinal direction and the width direction of the sheet, i.e., the steel sheet has excellent uniformity of the material.
  • the steel sheet has excellent dimensional accuracy.
  • the steel sheet has excellent pipe formability and excellent dimensional accuracy.
  • Example 2 A 0.2 0.1 0.3 581 367 ⁇ 80 1.02 0.79 ⁇ 60 1.09 0.98
  • Example 2 A 0.1 0.2 0.3 577 365 ⁇ 65 0.98 0.78 ⁇ 45 0.97 0.89
  • Example 3 A 0.1 0.2 0.08 583 367 ⁇ 65 0.68 0.94 ⁇ 40 0.66 0.5
  • Comparative Example 4 B 0.2 0.1 0.3 570 327 ⁇ 75 0.77 0.72 ⁇ 50 0.96 0.77
  • Example 5 B 0.2 0.2 0.2 574 310 ⁇ 70 0.82 0.79 ⁇ 45 1.06 0.88
  • Example 6 B 2.0 3.8 —*** 584 78 ⁇ 20 0.32 0.86 5 1.03 1.12
  • Comparative Example 7 C 0.1 0.1 0.2 636 278 ⁇ 70 0.93 0.79 ⁇ 45 0.86 0.83
  • Example 8 D 0.1 0.2 0.3 674 295 ⁇ 70 0.85 0.83 ⁇ 45 1.

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Publication number Priority date Publication date Assignee Title
US20140072824A1 (en) * 2012-09-10 2014-03-13 Siemens Vai Metals Technologies Gmbh Method for producing microalloyed tubular steel in combined casting-rolling installation and microalloyed tubular steel
US20140216609A1 (en) * 2011-06-30 2014-08-07 Jfe Steel Corporation High strength hot-rolled steel sheet for welded steel line pipe having excellent souring resistance, and method for producing same (as amended)
US9200342B2 (en) 2010-06-30 2015-12-01 Nippon Steel & Sumitomo Metal Corporation Hot-rolled steel sheet and manufacturing method thereof
US9573432B2 (en) 2013-10-01 2017-02-21 Hendrickson Usa, L.L.C. Leaf spring and method of manufacture thereof having sections with different levels of through hardness
CN108154530A (zh) * 2017-11-30 2018-06-12 华中科技大学 一种分布式计算有向图围长的方法
US20180237892A1 (en) * 2012-03-23 2018-08-23 Salzgitter Flachstahl Gmbh Non-scaling heat-treatable steel and method for producing a non-scaling component from said steel
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Families Citing this family (31)

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Publication number Priority date Publication date Assignee Title
CN102301026B (zh) * 2009-01-30 2014-11-05 杰富意钢铁株式会社 低温韧性优良的厚壁高强度热轧钢板及其制造方法
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JP6047947B2 (ja) * 2011-06-30 2016-12-21 Ｊｆｅスチール株式会社 耐サワー性に優れたラインパイプ用厚肉高強度継目無鋼管およびその製造方法
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RU2516213C1 (ru) * 2012-12-05 2014-05-20 Открытое акционерное общество "Магнитогорский металлургический комбинат" Способ получения металлоизделия с заданным структурным состоянием
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DE102014017274A1 (de) * 2014-11-18 2016-05-19 Salzgitter Flachstahl Gmbh Höchstfester lufthärtender Mehrphasenstahl mit hervorragenden Verarbeitungseigenschaften und Verfahren zur Herstellung eines Bandes aus diesem Stahl
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CN111074156A (zh) * 2019-12-26 2020-04-28 舞阳钢铁有限责任公司 一种具备优良低温韧性的超高强度钢板及其生产方法
CN114107825A (zh) * 2021-12-02 2022-03-01 河北普阳钢铁有限公司 一种低碳当量含钛q420md钢板及其制备方法

Citations (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5853503A (en) * 1995-08-31 1998-12-29 Kawasaki Steel Corporation Hot rolled steel sheets and method of producing the same
US6695932B2 (en) * 2000-05-31 2004-02-24 Jfe Steel Corporation Cold-rolled steel sheet having excellent strain aging hardening properties and method for producing the same
US6702904B2 (en) * 2000-02-29 2004-03-09 Jfe Steel Corporation High tensile cold-rolled steel sheet having excellent strain aging hardening properties
US20050183798A1 (en) * 2004-02-24 2005-08-25 Jfe Steel Corporation, A Corporation Of Japan Hot-rolled steel sheet for high-strength electric-resistance welded pipe having sour-gas resistance and exellent weld toughness, and method for manufacturing the same
US7101445B2 (en) * 2000-05-26 2006-09-05 Jfe Steel Corporation Cold rolled steel sheet and galvanized steel sheet having strain age hardenability and method of producing the same
JP2006299415A (ja) * 2005-03-24 2006-11-02 Jfe Steel Kk 低温靭性に優れた低降伏比電縫鋼管用熱延鋼板の製造方法
US7501030B2 (en) * 2003-03-27 2009-03-10 Jfe Steel Corporation Hot-rolled steel strip for high strength electric resistance welding pipe and manufacturing method thereof

Family Cites Families (17)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS6425916A (en) * 1987-07-21 1989-01-27 Nippon Steel Corp Manufacture of high-strength steel for electric resistance welded tube excellent in toughness at low temperature
JPH021719A (ja) 1988-03-09 1990-01-08 Shinko Kagaku Kogyo Kk ポリウレタン系粘着性組成物
JPH0421719A (ja) * 1990-05-14 1992-01-24 Sumitomo Metal Ind Ltd 電縫管用鋼板の製造方法
JP3390596B2 (ja) 1995-03-23 2003-03-24 川崎製鉄株式会社 靱性に優れる低降伏比高強度熱延鋼板およびその製造方法
JP3214353B2 (ja) 1996-05-08 2001-10-02 住友金属工業株式会社 耐水素誘起割れ性に優れた高強度鋼板の製造方法
DZ2535A1 (fr) * 1997-06-20 2003-01-08 Exxon Production Research Co Procédé perfectionné pour la liquéfaction de gaz naturel.
ES2264572T3 (es) * 1997-07-28 2007-01-01 Exxonmobil Upstream Research Company Aceros soldables ultrarresistentes con una tenacidad excelente a temperaturas ultrabajas.
JPH1180833A (ja) * 1997-09-05 1999-03-26 Nkk Corp 耐ｈｉｃ性に優れた高強度ラインパイプ用鋼板の製造方法
JP3546726B2 (ja) * 1998-12-02 2004-07-28 Ｊｆｅスチール株式会社 耐ｈｉｃ性に優れた高強度厚鋼板の製造方法
JP4277405B2 (ja) 2000-01-26 2009-06-10 Ｊｆｅスチール株式会社 低温靱性および溶接性に優れた高強度電縫鋼管用熱延鋼板の製造方法
JP4264177B2 (ja) 2000-03-01 2009-05-13 新日本製鐵株式会社 表層に粗粒フェライト層を有する鋼材の製造方法
JP4299435B2 (ja) * 2000-04-05 2009-07-22 新日本製鐵株式会社 熱延鋼板の製造法
EP1325967A4 (fr) 2001-07-13 2005-02-23 Jfe Steel Corp Tube d'acier a resistance elevee, superieure a celle de la norme api x6
JP3968011B2 (ja) * 2002-05-27 2007-08-29 新日本製鐵株式会社 低温靱性および溶接熱影響部靱性に優れた高強度鋼とその製造方法および高強度鋼管の製造方法
JP4375087B2 (ja) * 2004-03-31 2009-12-02 Ｊｆｅスチール株式会社 材質均質性の優れた高強度高靭性熱延鋼帯及びその製造方法
JP4940882B2 (ja) * 2005-10-18 2012-05-30 Ｊｆｅスチール株式会社 厚手高強度熱延鋼板およびその製造方法
JP5098256B2 (ja) 2006-08-30 2012-12-12 Ｊｆｅスチール株式会社 耐水素誘起割れ性能に優れたバウシンガー効果による降伏応力低下が小さい高強度ラインパイプ用鋼板およびその製造方法

Patent Citations (10)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5853503A (en) * 1995-08-31 1998-12-29 Kawasaki Steel Corporation Hot rolled steel sheets and method of producing the same
US6702904B2 (en) * 2000-02-29 2004-03-09 Jfe Steel Corporation High tensile cold-rolled steel sheet having excellent strain aging hardening properties
US6899771B2 (en) * 2000-02-29 2005-05-31 Jfe Steel Corporation High tensile strength cold rolled steel sheet having excellent strain age hardening characteristics and the production thereof
US6902632B2 (en) * 2000-02-29 2005-06-07 Jfe Steel Corporation High tensile strength cold rolled steel sheet having excellent strain age hardening characteristics and the production thereof
US7101445B2 (en) * 2000-05-26 2006-09-05 Jfe Steel Corporation Cold rolled steel sheet and galvanized steel sheet having strain age hardenability and method of producing the same
US6695932B2 (en) * 2000-05-31 2004-02-24 Jfe Steel Corporation Cold-rolled steel sheet having excellent strain aging hardening properties and method for producing the same
US7501030B2 (en) * 2003-03-27 2009-03-10 Jfe Steel Corporation Hot-rolled steel strip for high strength electric resistance welding pipe and manufacturing method thereof
US20050183798A1 (en) * 2004-02-24 2005-08-25 Jfe Steel Corporation, A Corporation Of Japan Hot-rolled steel sheet for high-strength electric-resistance welded pipe having sour-gas resistance and exellent weld toughness, and method for manufacturing the same
US7879287B2 (en) * 2004-02-24 2011-02-01 Jfe Steel Corporation Hot-rolled steel sheet for high-strength electric-resistance welded pipe having sour-gas resistance and excellent weld toughness, and method for manufacturing the same
JP2006299415A (ja) * 2005-03-24 2006-11-02 Jfe Steel Kk 低温靭性に優れた低降伏比電縫鋼管用熱延鋼板の製造方法

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
B. Mishra. "Steelmaking Practices and Their Influence on Properties." ASM Handbook, ASM International, 2002. *

Cited By (13)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US9200342B2 (en) 2010-06-30 2015-12-01 Nippon Steel & Sumitomo Metal Corporation Hot-rolled steel sheet and manufacturing method thereof
US20140216609A1 (en) * 2011-06-30 2014-08-07 Jfe Steel Corporation High strength hot-rolled steel sheet for welded steel line pipe having excellent souring resistance, and method for producing same (as amended)
US9540717B2 (en) * 2011-06-30 2017-01-10 Jfe Steel Corporation High strength hot-rolled steel sheet for welded steel line pipe having excellent souring resistance, and method for producing same
US20180237892A1 (en) * 2012-03-23 2018-08-23 Salzgitter Flachstahl Gmbh Non-scaling heat-treatable steel and method for producing a non-scaling component from said steel
US10822681B2 (en) * 2012-03-23 2020-11-03 Salzgitter Flachstahl Gmbh Non-scaling heat-treatable steel and method for producing a non-scaling component from said steel
US9144839B2 (en) * 2012-09-10 2015-09-29 Primetals Technologies Austria GmbH Method for producing microalloyed tubular steel in combined casting-rolling installation and microalloyed tubular steel
US20140072824A1 (en) * 2012-09-10 2014-03-13 Siemens Vai Metals Technologies Gmbh Method for producing microalloyed tubular steel in combined casting-rolling installation and microalloyed tubular steel
US10689735B2 (en) * 2012-12-27 2020-06-23 Posco High strength steel sheet having excellent cryogenic temperature toughness and low yield ratio properties, and method for manufacturing same
US10287661B2 (en) 2013-04-04 2019-05-14 Jfe Steel Corporation Hot-rolled steel sheet and method for producing the same
US9890440B2 (en) 2013-10-01 2018-02-13 Hendrickson Usa, L.L.C. Leaf spring and method of manufacture thereof having sections with different levels of through hardness
US9573432B2 (en) 2013-10-01 2017-02-21 Hendrickson Usa, L.L.C. Leaf spring and method of manufacture thereof having sections with different levels of through hardness
US11041223B2 (en) * 2014-12-25 2021-06-22 Jfe Steel Corporation High strength thick-walled electric-resistance-welded steel pipe for deep-well conductor casing, method for manufacturing the same, and high strength thick-walled conductor casing for deep wells
CN108154530A (zh) * 2017-11-30 2018-06-12 华中科技大学 一种分布式计算有向图围长的方法

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US9493865B2 (en) 2016-11-15
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US20150176110A1 (en) 2015-06-25

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