# US 9,752,216 B2 · High-strength hot rolled steel sheet with excellent bendability and low-temperature toughness, and method for manufacturing the same > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 9,752,216 B2 - Document ID: US9752216B2 - Status: Granted - Kind: B2 - Application number: 14/352,880 - Filed: 2012-10-31 - Published: 2014-09-11 - Granted: 2017-09-05 - Earliest claimed: 2011-11-01 - Assignees: Jfe Steel Corporation - Applicants: JFE Steel Corporation - Inventors: Chikara Kami; Kazuhiko Yamazaki - Web page: https://patentgrep.com/patent/US9752216B2 ## CPC - C22C 38/32: with boron - C21D 1/20: Isothermal quenching, e.g. bainitic hardening - C21D 8/02: during manufacturing of plates or strips (C21D8/12 takes precedence) - C21D 8/0263: {following hot rolling} - 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 38/22: with molybdenum or tungsten - C22C 38/24: with vanadium - C22C 38/26: with niobium or tantalum - C22C 38/28: with titanium or zirconium - C21D 2211/002: Bainite - 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) ## Abstract A high-strength hot rolled steel sheet with excellent bendability and low-temperature toughness includes a chemical composition including, in mass %, C: 0.08 to 0.25%, Si: 0.01 to 1.0%, Mn: 0.8 to 2.1%, P: not more than 0.025%, S: not more than 0.005% and Al: 0.005 to 0.10%, the balance including Fe and inevitable impurities, and a microstructure having a bainite phase and/or a tempered martensite phase as a main phase, the average grain diameter of prior austenite grains being not more than 20 μm as measured with respect to a cross section parallel to a rolling direction and not more than 15 μm as measured with respect to a cross section perpendicular to the rolling direction. ## Claims ### Claim 1 A high-strength hot rolled steel sheet with excellent bendability and low-temperature toughness comprising a chemical composition including, in mass %, C: 0.08 to 0.25%, Si: 0.01 to 1.0%, Mn: 0.8 to 2.1%, P: not more than 0.025%, S: not more than 0.005% and Al: 0.005 to 0.10%, the balance comprising Fe and inevitable impurities, and a microstructure having a bainite phase and/or a tempered martensite phase as a main phase, the average grain diameter of prior austenite grains being not more than 20 μm as measured with respect to a cross section parallel to a rolling direction and not more than 15 μm as measured with respect to a cross section perpendicular to the rolling direction, wherein the microstructure has an X-ray plane intensity {223}<252> of not more than 5.0. ### Claim 2 (depends on 1) The high-strength hot rolled steel sheet according to claim 1, wherein the prior austenite grains have a ratio of an average length in the rolling direction relative to an average length in a direction perpendicular to the rolling direction, (average length in rolling direction)/(average length in direction perpendicular to rolling direction), of not more than 10. ### Claim 3 (depends on 1) The high-strength hot rolled steel sheet according to claim 1, wherein the chemical composition further includes, in mass %, 0.0001 to 0.0050%. ### Claim 4 (depends on 1) The high-strength hot rolled steel sheet according to claim 1, wherein the chemical composition further includes, in mass %, at least one selected from the group consisting of Nb: 0.001 to 0.05%, Ti: 0.001 to 0.05%, Mo: 0.001 to 1.0%, Cr: 0.01 to 1.0%, V: 0.001 to 0.10%, Cu: 0.01 to 0.50% and 0.01 to 0.50%. ### Claim 5 (depends on 1) The high-strength hot rolled steel sheet according to claim 1, wherein the chemical composition further includes, in mass %, Ca: 0.0005 to 0.005%. ### Claim 6 (depends on 2) The high-strength hot rolled steel sheet according to claim 2, wherein the chemical composition further includes, in mass %, B: 0.0001 to 0.0050%. ### Claim 7 (depends on 2) The high-strength hot rolled steel sheet according to claim 2, wherein the chemical composition further includes, in mass %, at least one selected from the group consisting of Nb: 0.001 to 0.05%, Ti: 0.001 to 0.05%, Mo: 0.001 to 1.0%, Cr: 0.01 to 1.0%, V: 0.001 to 0.10%, Cu: 0.01 to 0.50% and Ni: 0.01 to 0.50%. ### Claim 8 (depends on 3) The high-strength hot rolled steel sheet according to claim 3, wherein the chemical composition further includes, in mass %, at least one selected from the group consisting of Nb: 0.001 to 0.05%, Ti: 0.001 to 0.05%, Mo: 0.001 to 1.0%, Cr: 0.01 to 1.0%, V: 0.001 to 0.10%, Cu: 0.01 to 0.50% and Ni: 0.01 to 0.50%. ### Claim 9 (depends on 1) The high-strength hot rolled steel sheet according to claim 1, wherein the yield strength is not less than 960 MPa. ### Claim 10 A method of manufacturing high-strength hot rolled steel sheets with excellent bendability and low-temperature toughness, comprising: subjecting a steel to a series of sequential steps including a heating step of heating the steel, a hot rolling step of subjecting the heated steel to hot rolling including rough rolling and finish rolling, a cooling step, and a coiling step, thereby producing a hot rolled steel sheet, wherein the steel has a chemical composition including, in mass %, C: 0.08 to 0.25%, Si: 0.01 to 1.0%, Mn: 0.8 to 2.1%, P: not more than 0.025%, S: not more than 0.005% and Al: 0.005 to 0.10%, the balance comprising Fe and inevitable impurities, and wherein the heating step is a step in which the steel is heated to a temperature of 1100 to 1250° C., the rough rolling in the hot rolling step is rolling of the steel heated in the heating step into a sheet bar, and the finish rolling in the hot rolling step is rolling of the sheet bar such that a cumulative reduction ratio in a partially recrystallized austenite region and a non-recrystallized austenite region divided by the cumulative reduction ratio in the recrystallized austenite region becomes 0 to 0.2, the cooling step includes a cooling treatment in which cooling is initiated immediately after completion of the finish rolling and the steel sheet is cooled to a cooling termination temperature that is not more than (Ms transformation temperature+150° C.) within 30 seconds from initiation of the cooling, the average cooling rate in a temperature range of 750° C. to 500° C. being not less than a critical cooling rate for occurrence of martensite formation, and a holding treatment in which after the cooling treatment is terminated, the steel sheet is held at a temperature of the cooling termination temperature±100° C. for 5 to 60 seconds, and the coiling step is a step in which the steel sheet is coiled into a coil at a coiling temperature of (cooling termination temperature±100° C.). ### Claim 11 (depends on 10) The method according to claim 10, wherein the chemical composition further includes, in mass %, B: 0.0001 to 0.0050%. ### Claim 12 (depends on 10) The method according to claim 10, wherein the chemical composition further includes, in mass %, at least one selected from the group consisting of Nb: 0.001 to 0.05%, Ti: 0.001 to 0.05%, Mo: 0.001 to 1.0%, Cr: 0.01 to 1.0%, V: 0.001 to 0.10%, Cu: 0.01 to 0.50% and Ni: 0.01 to 0.50%. ### Claim 13 (depends on 10) The method according to claim 10, wherein the chemical composition further includes, in mass %, Ca: 0.0005 to 0.005%. ### Claim 14 (depends on 11) The method according to claim 11, wherein the chemical composition further includes, in mass %, at least one selected from the group consisting of Nb: 0.001 to 0.05%, Ti: 0.001 to 0.05%, Mo: 0.001 to 1.0%, Cr: 0.01 to 1.0%, V: 0.001 to 0.10%, Cu: 0.01 to 0.50% and Ni: 0.01 to 0.50%. ### Claim 15 (depends on 11) The method according to claim 11, wherein the chemical composition further includes, in mass %, Ca: 0.0005 to 0.005%. ### Claim 16 (depends on 12) The method according to claim 12, wherein the chemical composition further includes, in mass %, Ca: 0.0005 to 0.005%.