# US 8,784,577 B2 · Thick high-tensile-strength hot-rolled steel sheet having excellent low-temperature toughness and manufacturing method thereof > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 8,784,577 B2 - Document ID: US8784577B2 - Status: Granted - Kind: B2 - Application number: 13/146,747 - Filed: 2010-01-29 - Published: 2011-11-24 - Granted: 2014-07-22 - Earliest claimed: 2009-01-30 - Assignees: Jfe Steel Corporation - Applicants: Chikara Kami; Hiroshi Nakata; Kinya Nakagawa - Inventors: Chikara Kami; Hiroshi Nakata; Kinya Nakagawa - Web page: https://patentgrep.com/patent/US8784577B2 ## CPC - C22C 38/58: with more than 1.5% by weight of manganese - C21D 8/02: during manufacturing of plates or strips (C21D8/12 takes precedence) - C21D 8/0263: {following hot rolling} - C21D 8/10: during manufacturing of tubular bodies - C21D 9/46: for sheet metals - C22C 38/001: {containing N} - C22C 38/002: {containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60} - C22C 38/02: containing silicon - C22C 38/04: containing manganese - C22C 38/06: containing aluminium - C22C 38/08: containing nickel {(C22C38/105 takes precedence)} - C22C 38/12: containing tungsten, tantalum, molybdenum, vanadium, or niobium - C22C 38/14: containing titanium or zirconium - C22C 38/16: containing copper - C22C 38/26: with niobium or tantalum - C22C 38/28: with titanium or zirconium - C22C 38/38: with more than 1.5% by weight of manganese - C22C 38/42: with copper - C22C 38/44: with molybdenum or tungsten - C22C 38/46: with vanadium - C22C 38/48: with niobium or tantalum - C22C 38/50: with titanium or zirconium - B21B 3/00: Rolling materials of special alloys so far as the composition of the alloy requires or permits special rolling methods or sequences {; Rolling of aluminium, copper, zinc or other non-ferrous metals}(altering special metallurgical properties of alloys, other than structure consolidation or mechanical properties resulting therefrom C21D, C22F) - C21D 2211/005: Ferrite - C21D 2211/008: Martensite - C22C: ALLOYS (treatment of alloys C21D, C22F) - C21D: MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING (surface treatment of metallic material involving at least one process provided for in class C23 and at least one process covered by this subclass C23F17/00; unidirectional solidification of eutectic materials or unidirectional demixing of eutectoid materials C30B) - B21B: ROLLING OF METAL (auxiliary operations used in connection with metal-working operations covered in B21, see B21C; bending by rolling B21D; manufacture of particular objects, e.g. screws, wheels, rings, barrels, balls, by rolling B21H; pressure welding by means of a rolling mill B23K20/04) ## Abstract A high-tensile-strength hot-rolled steel sheet is provided having a composition which contains 0.02 to 0.08% C, 0.01 to 0.10% Nb, 0.001 to 0.05% Ti and Fe and unavoidable impurities as a balance, wherein the steel sheet contains C, Ti and Nb in such a manner that (Ti+(Nb/2))/C<4 is satisfied, and the steel sheet has a structure where a primary phase of the structure at a position 1 mm away from a surface in a sheet thickness direction is one selected from a group consisting of a ferrite phase, tempered martensite and a mixture structure of a ferrite phase and tempered martensite, a primary phase of the structure at a sheet thickness center position is formed of a ferrite phase, and a difference ΔV between a structural fraction (volume %) of a secondary phase at the position 1 mm away from the surface in the sheet thickness direction and a structural fraction (volume %) of a secondary phase at the sheet thickness center position is 2% or less. ## Claims ### Claim 1 A high-tensile-strength hot-rolled steel sheet having a composition which contains by mass % 0.02 to 0.08% C, 0.01 to 0.50% Si, 0.5 to 1.8% Mn, 0.025% or less P, 0.005% or less S, 0.005 to 0.10% Al, 0.01 to 0.10% Nb, 0.001 to 0.05% Ti, and Fe and unavoidable impurities as a balance, wherein the steel sheet contains C, Ti and Nb in such a manner that a following formula (1) is satisfied, and the steel sheet has a structure where a primary phase of the structure at a position 1 mm away from a surface of the steel sheet in a sheet thickness direction is one selected from a group consisting of a ferrite phase and a mixture structure of a ferrite phase and tempered martensite, a primary phase of the structure at a sheet thickness center position is formed of a ferrite phase, a difference ΔV between a structural fraction (volume %) of a secondary phase at the position 1 mm away from the surface of the steel sheet in the sheet thickness direction and a structural fraction (volume %) of a secondary phase at the sheet thickness center position is 2% or less, and a difference ΔD between an average grain size of the ferrite phase at the position 1 mm away from the surface of the steel sheet in the sheet thickness direction and an average grain size of the ferrite phase at the sheet thickness center position is 2 μm or less; wherein (Ti+(Nb/2))/C<4; and Ti, Nb, C: contents of respective elements (mass %). ### Claim 2 (depends on 1) The high-tensile-strength hot-rolled steel sheet according to claim 1, wherein the structure at the position 1 mm away from the surface in the sheet thickness direction is a structure where the primary phase is formed of the ferrite phase. ### Claim 3 (depends on 2) The high-tensile-strength hot-rolled steel sheet according to claim 2, wherein the average grain size of the ferrite phase at the sheet thickness center position is 5 μm or less, the structural fraction (volume %) of the secondary phase is 2% or less, and a sheet thickness is more than 22 mm. ### Claim 4 (depends on 1) The high-tensile-strength hot-rolled steel sheet according to claim 1, wherein the high-tensile-strength hot-rolled steel sheet has the composition which further contains by mass % one or two kinds or more 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 in addition to the above-mentioned composition. ### Claim 5 (depends on 1) The high-tensile-strength hot-rolled steel sheet according to claim 1, wherein the high-tensile-strength hot-rolled steel sheet has the composition which further contains by mass % 0.0005 to 0.005% Ca in addition to the above-mentioned composition. ### Claim 6 (depends on 2) A method of manufacturing the high-tensile-strength hot-rolled steel sheet according to claim 2, wherein in manufacturing the hot-rolled steel sheet by heating a steel material having the composition according to claim 2 and by applying hot rolling constituted of rough rolling and finish rolling to the steel material, the accelerated cooling is constituted of primary accelerated cooling and secondary accelerated cooling, wherein the primary accelerated cooling is performed in such a manner that cooling in which an average cooling rate at the sheet thickness center position is 10° C./s or more and a cooling rate difference between an average cooling rate at a sheet thickness center position and an average cooling rate at a position 1 mm away from a surface in a sheet thickness direction is less than 80° C./s is performed until a primary cooling stop temperature by which a temperature at a position 1 mm away from the surface in the sheet thickness direction becomes a temperature in a temperature range of 650° C. or below and 500° C. or above is obtained, and the secondary accelerated cooling is performed in such a manner that cooling in which the average cooling rate at the sheet thickness center position is 10° C./s or more, and the cooling rate difference between the average cooling rate at the sheet thickness center position and the average cooling rate at the position 1 mm away from the surface in the sheet thickness direction is 80° C./s or more is performed until the temperature at the sheet thickness center position becomes a secondary cooling stop temperature of BFS which is defined by a following formula (2) or below, and a hot-rolled steel sheet is coiled at a coiling temperature of BFS0 which is defined by a following formula (3) or below as the temperature at the sheet thickness center position after the secondary accelerated cooling, wherein BFS(° C.)=770−300C−70Mn−70Cr−170Mo−40Cu−40Ni−1.5CR, BFS0(° C.)=770−300C−70Mn−70Cr−170Mo−40Cu−40Ni, C, Mn, Cr, Mo, Cu, Ni: contents of respective elements (mass %), and CR: cooling rate (° C./s). ### Claim 7 (depends on 6) The method of manufacturing the high-tensile-strength hot-rolled steel sheet according to claim 6, wherein air cooling is performed for 10 s or less between the primary accelerated cooling and the secondary accelerated cooling. ### Claim 8 (depends on 6) The method of manufacturing the high-tensile-strength hot-rolled steel sheet according to claim 6, wherein the accelerated cooling is performed at the average cooling rate of 10° C./s or more in the temperature range of 750 to 650° C. at the sheet thickness center position. ### Claim 9 (depends on 6) The method of manufacturing the high-tensile-strength hot-rolled steel sheet according to claim 6, wherein the difference between the cooling stop temperature at the position 1 mm away from the surface in the sheet thickness direction and the coiling temperature in the second accelerated cooling falls within 300° C. ### Claim 10 (depends on 6) The method of manufacturing the high-tensile-strength hot-rolled steel sheet according to claim 6, wherein the hot-rolled steel sheet has the composition which further contains by mass % one or two kinds or more 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 in addition to the composition. ### Claim 11 (depends on 6) The method of manufacturing the high-tensile-strength hot-rolled steel sheet according to claim 6, wherein the hot-rolled steel sheet has the composition which further contains by mass % 0.0005 to 0.005% Ca in addition to the composition. ### Claim 12 (depends on 3) A method of manufacturing the high-tensile-strength hot-rolled steel sheet having a sheet thickness exceeding 22 mm according to claim 3 and, wherein a hot-rolled steel sheet is manufactured by heating a steel material having the composition according to claim 3 and by applying hot rolling constituted of rough rolling and finish rolling to the steel material and, subsequently, accelerated cooling is applied to the hot-rolled steel sheet after completing the finish rolling at 10° C./s or more in terms of an average cooling rate at a sheet thickness center position until a cooling stop temperature of BFS defined by the following formula (2) or below is obtained, and in coiling the hot-rolled steel sheet at a coiling temperature of BFSO defined by a following formula (3) or below, a temperature of the hot-rolled steel sheet at the sheet thickness center position is adjusted in such a manner that a holding time through which a temperature of the hot-rolled steel sheet at the sheet thickness center position reaches a temperature (T−20° C.) from a temperature T(° C.) which is a temperature at the time of starting the accelerated cooling is set to 20 s or less, and a cooling time from the temperature T to the temperature of BFS at the sheet thickness center position is set to 30 s or less, wherein BFS(° C.)=770−300C−70Mn−70Cr−170Mo−40Cu−40Ni−1.5CR, BFS0(° C.)=770−300C−70Mn−70Cr−170Mo−40Cu−40Ni, C, Mn, Cr, Mo, Cu, Ni: contents of respective elements (mass %), and CR: cooling rate (° C./s). ### Claim 13 (depends on 12) The method of manufacturing the high-tensile-strength hot-rolled steel sheet according to claim 12, wherein the hot-rolled steel sheet has the composition which further contains by mass % one or two or more kinds 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 in addition to the above-mentioned composition. ### Claim 14 (depends on 12) The method of manufacturing the high-tensile-strength hot-rolled steel sheet according to claim 12, wherein the hot-rolled steel sheet has the composition which further contains by mass % 0.0005 to 0.005% Ca in addition to the composition. ### Claim 15 (depends on 1) The high-tensile-strength hot-rolled steel sheet according to claim 1, wherein the secondary phase is formed of any one of martensite and MA or the mixture of these phases. ### Claim 16 (depends on 1) The high-tensile-strength hot-rolled steel sheet according to claim 1, wherein ΔV is 0.1 to 2% and ΔD is 0.1 to 2 μm. ### Claim 17 (depends on 4) The high-tensile-strength hot-rolled steel sheet according to claim 4, wherein the high-tensile-strength hot-rolled steel sheet has the composition which further contains by mass % 0.0005 to 0.005% Ca in addition to the above-mentioned composition. ### Claim 18 (depends on 10) The method of manufacturing the high-tensile-strength hot-rolled steel sheet according to claim 10, wherein the hot-rolled steel sheet has the composition which further contains by mass % 0.0005 to 0.005% Ca in addition to the composition. ### Claim 19 (depends on 13) The method of manufacturing the high-tensile-strength hot-rolled steel sheet according to claim 13, wherein the hot-rolled steel sheet has the composition which further contains by mass % 0.0005 to 0.005% Ca in addition to the composition.