Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21B—ROLLING OF METAL
  • B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
  • B21B1/22—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling plates, strips, bands or sheets of indefinite length
  • B21B1/24—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling plates, strips, bands or sheets of indefinite length in a continuous or semi-continuous process
  • B21B1/26—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling plates, strips, bands or sheets of indefinite length in a continuous or semi-continuous process by hot-rolling, e.g. Steckel hot mill
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/26—Methods of annealing
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0263—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/001—Austenite
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/002—Bainite
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/004—Dispersions; Precipitations

Definitions

• the present invention relates to a high-strength hot rolled steel sheet and a method for producing the same and more particularly to a high-strength hot rolled steel sheet suitable for a material of automotive parts and a method for producing the same.
• Patent Literature 1 no consideration is given to the performance of parts such as strength, toughness, etc. after the post-heating, and there is room for improvement.
• the steel sheet disclosed in Patent Literature 2 the increase in strength after heat treatment is significant, and the precipitation of fine carbides after heat treatment is significant. Therefore, one problem with this steel sheet is toughness after heat treatment, and no consideration is given to the blankability of the base sheet before the heat treatment, so that there is room for improvement.
• Patent Literature 3 attention is focused on the properties of the base sheet, and no consideration is given to the strength and toughness of the steel sheet after post-heating, so that there is room for improvement.
• the present invention has been made in view of the foregoing circumstances, and it is an object to provide a high-strength hot rolled steel sheet having good toughness and blankability and exhibiting high strength and good toughness after post-heating and to provide a method for producing the high-strength hot rolled steel sheet.
• the inventors have focused attention on the behavior of precipitation of Ti after post-heating of hot rolled steel sheets and have achieved the improvement in the properties of the steel sheets after post-heating by controlling the initial amounts of coarse Ti-containing precipitates and solute Ti before post-heating.
• a hot rolled steel sheet that has high strength, good blankability, and good toughness, exhibits properties close to those of the base sheet even after post-heating, and has good toughness and high strength after post-heating can be obtained by adjusting the chemical components, forming bainite as a main phase, adjusting the volume fraction of retained ⁇ to less than 3%, adjusting the ratio of the amount of solute Ti to the amount of Ti contained, i.e., (the amount of solute Ti / the total amount of Ti), to 0.30 or more and less than 0.80, and adjusting the amount of Ti present as precipitates having a diameter of 100 nm or more to 0.010 to 0.030% by mass.
• the invention has been completed.
• the high strength means a tensile strength (TS) of 780 MPa or more and less than 1320 MPa.
• the phrase "the toughness is good” means that, in a Charpy impact test using a test specimen cut from a hot rolled steel sheet (a base sheet or a hot rolled steel sheet subjected to post heating), the percent ductile fracture at -40°C is 50% or more.
• the thickness of the test specimen is 0.6 to 3.0 mm. When the thickness of the hot rolled steel sheet exceeds 3.0 mm, the front and back surfaces of a test specimen cut from the hot rolled steel sheet are ground, and the resulting test specimen is used for the Charpy impact test.
• the phrase "the strength after post-heating is high” means that the reduction in the strength of a hot rolled steel sheet after post-heating with respect to the strength of the hot rolled steel sheet before post-heating (the base sheet) is 40 or less in terms of Vickers hardness.
• the present invention can provide a high-strength hot rolled steel sheet having good toughness and blankability and exhibiting high strength and good toughness after post-heating and can provide a method for producing the high-strength hot rolled steel sheet.
• the high-strength hot rolled steel sheet obtained in the invention is suitable for a material of automotive parts and exhibits high strength and good toughness even after post-heating.
• the thickness is more preferably 6.0 mm or less.
• the width of the high-strength hot rolled steel sheet of the invention is preferably 500 mm or more and more preferably 700 mm or more.
• the width of the high-strength hot rolled steel sheet of the invention is preferably 1800 mm or less and more preferably 1400 mm or less.
• the high-strength hot rolled steel sheet of the invention has a specific chemical composition and a specific steel microstructure.
• the chemical composition and the steel microstructure will be described in this order.
• the high-strength hot rolled steel sheet of the invention has a chemical composition containing, in % by mass, C: 0.04 to 0.18%, Si :0.1 to 3.0%, Mn: 0.5 to 3.5%, P: 0.050% or less (excluding 0%), S: 0.010% or less (excluding 0%), Al: 1.5% or less (excluding 0%), N: 0.010% or less (excluding 0%), O: 0.003% or less (excluding 0%), and Ti: 0.040 to 0.150%, with the balance being Fe and incidental impurities.
• C is an element that increases the TS through the formation and strengthening of bainite, is bonded with Ti, N, etc. to form precipitates, and is thereby effective in preventing a reduction in the strength after post-heating. If the content of C is less than 0.04%, these effects are not obtained, and the TS of the steel sheet (base sheet) is not 780 MPa or more, or the steel sheet does not exhibit high strength after post-heating. If the content of C exceeds 0.18%, the reduction in toughness and blankability is significant, and the properties of the invention are not obtained. Therefore, the content of C is 0.04 to 0.18%.
• the content of C is preferably 0.05% or more.
• the content of C is preferably 0.16% or less and more preferably 0.11% or less.
• Si is an element effective in solid solution strengthening of the steel, in suppressing the formation of cementite in bainite, and in preventing a reduction in the strength after post-heating. To obtain these effects, the content of Si must be 0.1% or more. If the content of Si exceeds 3.0%, the amount of polygonal ferrite formed is excessively large, so that the steel microstructure in the invention is not obtained. Therefore, the content of Si is 0.1 to 3.0%.
• the content of Si is preferably 0.2% or more.
• the content of Si is preferably 2.0% or less and more preferably 1.5% or less.
• Mn is an element effective in suppressing the formation of ferrite to thereby allow the formation of bainite. If the content of Mn is less than 0.5%, this effect is not obtained sufficiently, and polygonal ferrite etc. are formed, so that the microstructure in the invention is not obtained. If the content of Mn exceeds 3.5%, the amount of martensite is large, and the toughness decreases significantly, so that the good toughness in the invention is not obtained. Therefore, the content of Mn is 0.5 to 3.5%.
• the content of Mn is preferably 1.0% or more.
• the content of Mn is preferably 2.7% or less.
• the allowable content of P is 0.050%. Therefore, the content of P is 0.050% or less.
• the content of P is preferably 0.030% or less. No particular limitation is imposed on the lower limit, and the content of P may be more than 0%. However, if the content of P is less than 0.001%, the production efficiency is low. Therefore, the content of P is preferably 0.001% or more.
• the allowable content of S is 0.010%. Therefore, the content of S is 0.010% or less.
• the content of S is preferably 0.0050% or less and more preferably 0.0020% or less. No particular limitation is imposed on the lower limit, and the content of S may be more than 0%. However, if the content of S is less than 0.0002%, the production efficiency is low. Therefore, the content of S is preferably 0.0002% or more.
• Al more than 0% and 1.5% or less
• Al serves as a deoxidizing agent, and it is preferable to add Al in a deoxidization step.
• the content of Al may be more than 0%. However, from the viewpoint of using Al as a deoxidizing agent, the content of Al is preferably 0.01% or more. If a large amount of Al is contained, a large amount of polygonal ferrite is formed, and the steel microstructure in the invention is not obtained. In the present invention, the allowable content of Al is 1.5%. Therefore, the content of Al is 1.5% or less.
• the content of Al is preferably 0.50% or less, more preferably 0.10% or less, and still more preferably 0.05% or less.
• N forms TiN and inhibits the precipitation of TiC, and it is therefore preferable to reduce the amount of N as much as possible.
• the allowable content of N is 0.010%. Therefore, the content of N is 0.010% or less.
• the content of N is preferably 0.007% or less. No particular limitation is imposed on the lower limit, and the content of N may be more than 0%. However, if the content of N is less than 0.0005%, the production efficiency is low. Therefore, the content of N is preferably 0.0005% or more.
• the allowable content of O is 0.003%. Therefore, the content of O is 0.003% or less.
• the content of O is preferably 0.002% or less. No particular limitation is imposed on the lower limit, and the content of O may be more than 0%. However, if the content of O is less than 0.0002%, the production efficiency is low. Therefore, the content of O is preferably 0.0002% or more.
• Ti is the most important element in the invention and is an element necessary to appropriately form precipitates such as TiC after post-heating to thereby obtain high strength and good toughness after post-heating. If the content of Ti is less than 0.040%, the above effect is not obtained sufficiently, and the strength after post-heating is not high. If the content of Ti exceeds 0.150%, the amount of precipitates after post-heating increases excessively, and the toughness after post-heating is not good. Therefore, the content of Ti is 0.040 to 0.150%. The content of Ti is preferably 0.050% or more. The content of Ti is preferably 0.120% or less.
• the above components are the basic components of the high-strength hot rolled steel sheet of the invention.
• the high-strength hot rolled steel sheet of the invention may have a chemical composition containing the above components with the balance being Fe and incidental impurities.
• the high-strength hot rolled steel sheet of the invention may further contain, in addition to the components described above, at least one or two or more elements selected from Cr: 0.005 to 2.0%, Cu: 0.005 to 0.5%, Ni: 0.005 to 2.0%, Mo: 0.005 to 1.0%, V: 0.005 to 0.5%, B: 0.0002 to 0.0050%, Ca: 0.0001 to 0.0050%, REM: 0.0001 to 0.0050%, Sb: 0.0010 to 0.10%, and Sn: 0.0010 to 0.50%.
• Cr is an element effective in the formation of bainite while the formation of ferrite is suppressed.
• the content of Cr is preferably 0.005% or more in order to obtain the above effect. If the content of Cr exceeds 2.0%, a significant reduction in corrosion resistance may occur. Therefore, when Cr is contained, the content of Cr is preferably 2.0% or less.
• the content of Cr is more preferably 0.1% or more.
• the content of Cr is more preferably 1.0% or less and still more preferably 0.8% or less.
• Cu is an element effective in stabilizing austenite and the formation of bainite.
• the content of Cu is preferably 0.005% or more in order to obtain the above effect. If the content of Cu exceeds 0.5%, the formation of Cu precipitates is significant, and this may cause deterioration in toughness. Therefore, when Cu is contained, the content of Cu is preferably 0.5% or less.
• the content of Cu is more preferably 0.05% or more.
• the content of Cu is more preferably 0.3% or less.
• Ni is an element effective in the formation of bainite while the formation of ferrite is suppressed.
• the content of Ni is preferably 0.005% or more in order to obtain the above effect. If the content of Ni exceeds 2.0%, large amounts of martensite and retained ⁇ are formed, and this may cause deterioration in toughness. Therefore, when Ni is contained, the content of Ni is preferably 2.0% or less.
• the content of Ni is more preferably 0.05% or more.
• the content of Ni is more preferably 0.8% or less and still more preferably 0.5% or less.
• Mo is an element effective in increasing the hardenability of the steel sheet and in the formation of bainite.
• the content of Mo is preferably 0.005% or more in order to obtain these effects. If the content of Mo exceeds 1.0%, the formation of Mo-based precipitates is significant, and this may cause deterioration in toughness. Therefore, when Mo is contained, the content of Mo is preferably 1.0% or less.
• the content of Mo is more preferably 0.05% or more.
• the content of Mo is more preferably 0.50% or less.
• V is an element effective in increasing the hardenability of the steel sheet and in the formation of bainite.
• the content of V is preferably 0.005% or more in order to obtain these effects. If the content of V exceeds 0.5%, a significantly large number of V-based precipitates are formed, and this may cause deterioration in toughness. Therefore, when V is contained, the content of V is preferably 0.5% or less.
• the content of V is more preferably 0.01% or more.
• the content of V is more preferably 0.1% or less.
• B is an element effective in increasing the hardenability of the steel sheet and in the formation of bainite.
• the content of B is preferably 0.0002% or more in order to obtain these effects. If the content of B exceeds 0.0050%, the amount of B-based compounds increases, and this may cause deterioration in toughness. Therefore, when B is contained, the content of B is preferably 0.0050% or less.
• the content of B is more preferably 0.0005% or more.
• the content of B is more preferably 0.0040% or less.
• Ca and REM are elements effective in improving the workability through shape control of inclusions.
• their contents are each preferably 0.0001% or more in order to obtain the above effect. If the contents of Ca and REM each exceed 0.0050%, the influence of the increase in the amount of inclusions is significant, and this may cause deterioration in toughness. Therefore, when Ca and REM are contained, the contents of Ca and REM are each preferably 0.0050% or less.
• the content of Ca is more preferably 0.0005% or more.
• the content of Ca is more preferably 0.0030% or less.
• the content of REM is more preferably 0.0005% or more.
• the content of REM is more preferably 0.0030% or less.
• the REM is a generic term for Sc, Y, and 15 elements from lanthanum (La) with an atomic number of 57 to lutetium (Lu) with an atomic number of 71.
• the content of REM mentioned here is the total content of these elements.
• Sb and Sn are each an element effective in suppressing surface reactions such as oxidation, denitrification, and deboronation to improve the surface properties of the steel sheet and in improving the toughness.
• the content of Sb and Sn are each preferably 0.0010% or more in order to obtain the above effects. If the content of Sb exceeds 0.10% or if the content of Sn exceeds 0.50%, the steel sheet is embrittled, and the toughness may deteriorate significantly. Therefore, when Sb is contained, the content of Sb is preferably 0.10% or less.
• Sn is contained, the content of Sn is preferably 0.50% or less.
• the content of Sb is more preferably 0.0050% or more.
• the content of Sb is more preferably 0.030% or less.
• the content of Sn is more preferably 0.0050% or more.
• the content of Sn is more preferably 0.050% or less.
• the steel sheet may contain, in addition to the chemical composition described above, in % by mass, one or two or more of Mg, As, W, Ta, Pb, Zr, Hf, Te, Bi, and Se in the range of a total amount of 0.3% or less.
• the content of each of these elements is limited to 0.03% or less.
• the high-strength hot rolled steel sheet of the invention has a steel microstructure including bainite as a main phase and further including retained ⁇ at a volume fraction or less than 3%.
• the microstructure includes bainite as a main phase. If ferrite, pearlite, retained ⁇ , etc. is a main phase, it is difficult to achieve high strength, good toughness, and good blankability simultaneously. If the main phase is martensite, toughness and blankability deteriorate, which is not preferred. Therefore, in the steel microstructure, the main phase is bainite.
• the bainite may be any of upper bainite, lower bainite, tempered bainite, and bainitic ferrite.
• the main phase is a phase whose area fraction is 50% or more.
• the area fraction of the main phase is preferably 55% or more and more preferably 65% or more.
• the area fraction of the main phase is preferably 95% or less.
• Retained austenite is a microstructure that causes deterioration in the toughness of the steel sheet and transforms to pearlite after post-heating, causing significant deterioration in strength and toughness. Therefore, it is preferable to reduce the amount of retained austenite as much as possible.
• the allowable volume fraction of retained ⁇ is less than 3%. Therefore, the volume fraction of the retained ⁇ is less than 3%.
• the volume fraction of the retained ⁇ is preferably less than 2% and more preferably less than 1%. No particular limitation is imposed on the lower limit of the volume fraction of the retained ⁇ , and the volume fraction of the retained ⁇ may be 0%.
• Phases other than bainite and retained ⁇ include one or two or more of ferrite, pearlite, and martensite.
• the total area fraction of the other phases is preferably 40% or less. No particular limitation is imposed on the lower limit of the area fraction of the other phases, and the total area fraction of the other phases is preferably 3% or more, more preferably 5% or more, and still more preferably 10% or more.
• Amount of solute Ti / total amount of Ti 0.30 or more and less than 0.80
• the ratio of the amount of solute Ti to the amount of Ti contained which is given by (the amount of solute Ti / the total amount of Ti), is less than 0.30, the amount of solute Ti that forms precipitates during post-heating is insufficient to compensate for the reduction in strength. In this case, the strength after post-heating may decrease, and fine precipitates may be formed, causing deterioration in toughness. If the ratio is 0.80% or more, the increase in strength due to precipitation after post-heating is significant, and the toughness after post-heating is not good. Therefore, the amount of solute Ti / the total amount of Ti is 0.30 or more and less than 0.80. The ratio is preferably 0.35 or more. The ratio is preferably 0.70 or less. The amount of solute Ti / the total amount of Ti can be determined by a method described in Examples.
• the amount of Ti present as precipitates having a diameter of 100 nm or more must be 0.010% by mass or more. If the amount of Ti exceeds 0.030% by mass, the reduction in toughness due to coarse precipitates is significant. Therefore, the amount of Ti present as precipitates having a diameter of 100 nm or more must be 0.030% by mass or less.
• the amount of Ti present as precipitates having a diameter of 100 nm or more is 0.010 to 0.030% by mass.
• the amount is preferably 0.013% by mass or more.
• the amount is preferably 0.027% by mass or less.
• the amount of Ti present as precipitates having a diameter of 100 nm or more can be determined by a method described in Examples.
• the high-strength hot rolled steel sheet of the invention is produced as follows.
• a slab having the chemical composition described above is heated in a temperature range of 1150 to 1300°C, held in this temperature range for 0.2 to 3.5 hours, and hot-rolled including rough rolling in a temperature range of 1080°C or higher at a total rolling reduction of 80 to 90% and subsequent finish rolling.
• the finish rolling is performed under the condition that the rolling reduction per pass at lower than or equal to T (°C) determined from a formula below is 25% or less.
• the steel sheet After completion of the finish rolling, the steel sheet is allowed to naturally cool for 1.0 s or longer, cooled in a temperature range down to 550°C at an average cooling rate of 50°C/s or more, and then coiled at a coiling temperature higher than or equal to Ms temperature (°C) and 550°C or lower.
• T ° C 800 + 1000 Ti
• [Ti] is the content (% by mass) of Ti.
• the total rolling reduction in the temperature range of 1080°C or higher is determined from the ratio of the thickness at 1080°C with respect to the thickness of the slab before the hot rolling.
• the rolling reduction per pass at T (°C) or lower is determined from the ratio of the thickness after a rolling pass to the thickness before the rolling pass at T (°C) or lower.
• the temperature described above is the temperature of the surface of the steel sheet at the widthwise center
• the average cooling rate and the cooling rate described above are the average cooling rate and the cooling rate, respectively, at the surface of the steel sheet at the widthwise center.
• the average cooling rate is [(cooling start temperature - cooling stop temperature) / cooling time from cooling start temperature to cooling stop temperature], unless otherwise specified.
• Heating temperature of slab 1150 to 1300°C
• the Ti-containing precipitates do not dissolve sufficiently.
• the value of the amount of solute Ti / the total amount of Ti does not fall within the range of 0.30 or more and less than 0.80, and the amount of Ti present as precipitates having a diameter of 100 nm or more does not fall within the range of 0.010 to 0.030% by mass. If the heating temperature of the slab exceeds 1300°C, the Ti-containing precipitates dissolve excessively.
• the heating temperature of the slab is 1150 to 1300°C.
• the heating temperature is preferably 1170°C or higher and more preferably 1185°C or higher.
• the heating temperature is preferably 1280°C or lower and more preferably 1265°C or lower.
• the holding time in the temperature range is 0.2 to 3.5 hours.
• the holding time is preferably 0.4 hours or longer.
• the holding time is preferably 2.5 hours or shorter.
• the hot rolling includes rough rolling in a temperature range of 1080°C or higher at a total rolling reduction of 80 to 90% and subsequent finish rolling.
• the total rolling reduction is less than 80%, the formation of the precipitates having a diameter of 100 nm or more is insufficient, and the amount of Ti present as precipitates having a diameter of 100 nm or more is less than 0.010% by mass. If the total rolling reduction exceeds 90%, the precipitates having a diameter of 100 nm or more are formed excessively, and the amount of Ti present as the precipitates having a diameter of 100 nm or more is more than 0.030% by mass. Therefore, the total rolling reduction in the temperature range of 1080°C or higher is 80 to 90%. The total rolling reduction is preferably 81% or more. The total rolling reduction is preferably 88% or less.
• [Ti] is the content (% by mass) of Ti.
• the natural cooling time after the finish rolling must be 1.0 s or longer.
• the natural cooling time is preferably 1.5 s or longer, more preferably 2.0 s or longer, and still more preferably 2.2 s or longer. No particular limitation is imposed on the upper limit of the natural cooling time. However, when the natal cooling time is 5.0 s or shorter, the subsequent hot rolling can be easily controlled. Therefore, the natural cooling time is preferably 5.0 s or shorter.
• the natural cooling means that the steel sheet is exposed to the atmosphere (air cooling) without active cooling (accelerated cooling) such as injection of water.
• the steel sheet After the natural cooling, the steel sheet is cooled in a temperature range down to 550°C at an average cooling rate of 50°C/s or more. If the average cooling rate to 550°C is less than 50°C/s, excessively large amounts of ferrite and Ti-containing precipitates are formed, and the phase structure and the precipitates in the invention are not obtained. Therefore, the average cooling rate in the temperature range from the cooling start temperature down to 550°C after the natural cooling is 50°C/s or more. The average cooling rate is preferably 70°C/s or more. No particular limitation is imposed on the upper limit of the average cooling rate. However, if the average cooling rate is 500°C/s or more, the shape of the steel sheet may deteriorate. Therefore, the average cooling rate is preferably less than 500°C/s and more preferably 300°C/s or less.
• Coiling temperature Ms temperature (°C) or higher and 550°C or lower
• the coiling temperature is the Ms temperature (°C) or higher and 550°C or lower.
• the coiling temperature is preferably higher than or equal to (Ms temperature + 20)°C.
• the coiling temperature is preferably 530°C or lower.
• the Ms temperature (°C) is the martensite start temperature and is determined, for example, by a Thermec Mastor-Z.
• Thermec Mastor-Z is used to determine the Ms temperature (°C), for example, a sample is heated to 1250°C, held at this temperature for 300 s, and cooled at a cooling rate of 100°C/s. Then the temperature at which the size of the sample undergoing contraction begins to increase can be determined as the Ms temperature (°C).
• the steel sheet is cooled in a temperature range down to 550°C at an average cooling rate of 50°C/s or more and then coiled. During this period, it is preferable that the cooling is stopped at a cooling stop temperature in the temperature range of 480 to 550°C and then the steel sheet is held at the cooling stop temperature ⁇ 20°C for 0.5 to 4.0 s. In this manner, the blankability can be further improved. By holding the steel sheet for 0.5 s or longer, Fe-containing precipitates can be formed, and the blankability can be further improved.
• the holding time at the cooling stop temperature ⁇ 20°C is preferably 0.5 to 4.0 s and more preferably 0.5 to 2.0 s.
• the high-strength hot rolled steel sheet of the invention exhibits high strength and good toughness after post-heating.
• the heating temperature of the post-heating may be 400°C or higher. No particular limitation is imposed on the upper limit of the heating temperature of the post-heating.
• the heating temperature of the post-heating is, for example, 1150°C or lower.
• the heating time is, for example, longer than 0 s.
• the heating time is, for example, 3600 s or shorter.
• Table 1 Steel material having chemical compositions shown in Table 1 was produced using converters and formed into slabs. Then the slabs were heated and hot-rolled under conditions shown in Table 2 to produce hot rolled steel sheets (base sheets). The hot rolled steel sheets obtained were used to perform microstructure observation, to analyze solute Ti, Ti-containing precipitates, and Fe-containing precipitates, and to evaluate tensile properties, hardness, blankability, and toughness according to test methods described below. In addition, the hot rolled steel sheets were subjected to post-heating as shown in Table 2, and the hot rolled steel sheets subjected to the post-heating were used to evaluate hardness, toughness, and delayed fracture resistance according to test methods described below.
• the temperature of the post-heating was 400°C or higher at which the improvement in the stretch flangeability was found, and the post-heating time was 3600 s or shorter from the viewpoint of productivity. "-" in Table 2 means that the corresponding treatment was not performed.
• the Ms temperature (°C) in Table 2 was determined by a test using a Thermec Mastor-Z.
• the area fraction of bainite is the ratio of the area of bainite in the observation area.
• the area fraction of bainite was determined as follows. A sample was cut from one of the obtained hot rolled steel sheets, and its thicknesswise cross section parallel to the rolling direction was polished and etched with 3% nital. Then, at a position 1/4 of the thickness, photographs were taken in three viewing areas using an SEM (scanning electron microscope) at a magnification of 1500X. The image data of the obtained secondary electron images was used to determine the area fractions of the microstructures using Image-Pro available from Media Cybernetics, and the average area fractions in the three viewing areas were used as the area fractions of the microstructures.
• a general classification method may be used, and the following method, for example, may be used for identification.
• black or dark gray regions containing carbides or martensite grains having linear boundaries are identified as bainite, and black, dark gray, gray, or light gray regions containing uniformly oriented carbides are identified as lower bainite.
• Martensite is observed as a black to light gray microstructure containing a plurality of regularly arranged carbides with different orientations or a white or light gray microstructure containing no carbides.
• Retained ⁇ is observed as white or light gray regions containing no carbide. Part of martensite and retained ⁇ cannot be distinguished from each other in some cases.
• the area fraction of retained ⁇ was determined by a method described later, and the area fraction of retained ⁇ was subtracted from the total area fraction of martensite and retained ⁇ determined from the SEM images to thereby determine the area fraction of martensite.
• fresh martensite, auto-tempered martensite, tempered martensite, etc. is formed depending on the degree of tempering, but the martensite may be any of them.
• the bainite may be any of upper bainite, lower bainite, tempered bainite, etc. but is more preferably upper bainite or tempered bainite. As the degree of tempering increases, the black contrast of the matrix in the microstructure image increases, and therefore the color of the matrix is merely a guide.
• the identification was made in a comprehensive manner based on the amount of carbides, the form of the microstructures, etc., and each of the microstructures including microstructures described later was classified into a microstructure having characteristics similar thereto.
• Carbides appear as white dots or lines.
• Microstructures other than those described above include the following microstructures. Ferrite is a black or dark gray microstructure containing no carbides and no substructures such as laths. Pearlite can be identified as a black and white lamellar or partially discontinuous lamellar microstructure. The amount of retained ⁇ is determined as follows.
• the microstructure forming the main phase with an area fraction of 50% or more was determined using the obtained area fractions of the microstructures, and the main phase and the other microstructures are shown in Table 3.
• B represents bainite
• ⁇ represents retained austenite
• O represents the other phases.
• the other phases include one or two or more of ferrite, pearlite, and martensite.
• a test specimen having a width of 30 mm and a length of 30 mm was cut from one of the obtained hot rolled steel sheets and subjected to constant-current electrolysis in a non-aqueous solvent-based electrolyte (10% AA-based electrolyte: 10 vol% acetylacetone-1 mass% tetramethylammonium chloride-methanol).
• the current density was set to 20 mA/cm 2 , and the amount of electrolysis was about 0.2 g.
• the electrolyte after electrolysis was used as an analysis solution, and the concentrations (% by mass) of Ti and Fe serving as a comparison element in the solution were measured by ICP mass spectrometry.
• the concentration ratio of Ti with respect to Fe was computed based on the obtained concentrations and multiplied by the content (% by mass) of Fe in the test specimen to thereby obtained the amount (% by mass) of solute Ti.
• the content (% by mass) of Fe in the test specimen was determined by subtracting the total content (% by mass) of components other than Fe from 100% by mass.
• the amount (% by mass) of solute Ti was used to compute the ratio of the amount (% by mass) of solute Ti to the amount of (% by mass) Ti contained.
• the test specimen with precipitates adhering to the surface was removed from the electrolyte and immersed in an aqueous sodium hexametaphosphate solution (500 mg/L) (hereinafter referred to as an aqueous SHMP solution). Then ultrasonic vibrations were applied to the test specimen, and the precipitates were thereby separated from the test specimen and extracted into the aqueous SHMP solution. Next, the aqueous SHMP solution containing the precipitates was filtered using a filter with a pore size of 100 nm, and then the precipitates collected on the 100 nm filter were decomposed using an acid. The decomposition solution was analyzed using an ICP emission spectrometer to measure the absolute value of Ti in the decomposition solution.
• the obtained absolute value of Ti was divided by the electrolyzed mass to obtain the amount of Ti (% by mass with respect to 100% by mass of all the components in the test specimen) contained in precipitates having a diameter of 100 nm or more.
• the obtained Ti amount (% by mass) was divided by the amount (% by mass) of Ti contained in the test specimen to obtain the amount (% by mass) of Ti present as Ti-containing precipitates with a diameter of 100 nm or more.
• the electrolyzed mass was determined by measuring the mass of the test specimen from which the precipitates had been removed and subtracting the determined mass from the mass of the test specimen before the electrolysis.
• a test specimen having a width of 30 mm and a length of 30 mm was cut from one of the obtained hot rolled steel sheets and subjected to constant-current electrolysis in a non-aqueous solvent-based electrolyte (10% AA-based electrolyte: 10 vol% acetylacetone-1 mass% tetramethylammonium chloride-methanol).
• the current density was set to 20 mA/cm 2 , and the amount of electrolysis was about 0.2 g.
• the test specimen with precipitates adhering to the surface was removed from the electrolyte and immersed in an aqueous SHMP solution.
• the aqueous SHMP solution containing the precipitates was filtered using a filter with a pore size of 100 nm, and then the precipitates collected on the 100 nm filter were decomposed using an acid.
• the decomposition solution was analyzed using an ICP emission spectrometer to measure the absolute value of Fe in the decomposition solution. The obtained absolute value of Fe was divided by the electrolyzed mass to obtain the amount of Fe (% by mass with respect to 100% by mass of all the components in the test specimen) contained in precipitates having a diameter of 100 nm or more.
• the obtained Fe amount (% by mass) was divided by the amount (% by mass) of Fe contained in the test specimen to obtain the amount (% by mass) of Fe present as Fe-containing precipitates with a diameter of 100 nm or more.
• the electrolyzed mass was determined by measuring the mass of the test specimen from which the precipitates had been removed and subtracting the determined mass from the mass of the test specimen before the electrolysis.
• One of the obtained hot rolled steel sheets was cut in a direction parallel to the rolling direction to obtain a JIS No. 5 tensile test piece (JIS Z 2241: 2011), and a tensile test was performed at a strain rate of 10 -3 /s according to the specifications of JIS Z 2241:2011 to determine the TS.
• a steel sheet with a TS of 780 MPa or more and less than 1320 MPa was rated pass.
• Samples were cut from the obtained hot rolled steel sheets and from the hot rolled steel sheets subjected to post-heating. A thicknesswise cross section of each sample parallel to the rolling direction was polished, and a Vickers hardness test with a load of 5 kg was performed at a position 1/4 of the thickness. The measurement was performed at 5 points, and the average (arithmetic mean) was used as the Vickers hardness of the steel sheet. When the difference in hardness ( ⁇ HV) before and after the post-heating was 40 or less, the strength after the post-heating was judged as good, and the steel sheet was rated pass.
• ⁇ HV difference in hardness
• a test specimen having a width of 50 mm and a length of 50 mm was cut from one of the obtained hot rolled steel sheets.
• a blanking punch with ⁇ 10 mm was used to perform blanking while the clearance was changed in the range of 5 to 30%.
• the blanking was performed three times at each of the different clearances to determine the clearance range in which no chipping and no cracking occurred on end faces in all the three operations.
• the test specimen was rated pass when the clearance range was 10% or more.
• Test specimens having a width of 10 mm and a length of 55 mm were cut from one of the obtained hot rolled steel sheets and from the hot rolled steel sheets subjected to post-heating, and Charpy impact test specimens having a V-notch with an included angle of 45°, a tip radius of 0.25 mm, and a depth of 2 mm were produced. Then the Charpy impact test was performed five times at -40°C according to JIS Z 2242: 2018 to evaluate the percent ductile fracture. When the average percent ductile fracture of the five measurements was 50% or more, the toughness was judged as good, and the steel sheet was rated pass. The sheet thickness was 2.9 mm, and the direction of the notch was parallel to the rolling direction.
• [Ti] is the content (% by mass) of Ti.
• [Table 3] Steel sheet No. Steel microstructure Properties Properties after post-heating Remarks V(B) (area fraction) V( ⁇ ) (volume fraction) V(O) (area fraction) Amount of solute Ti / total amount of Ti Amount of Ti present as precipitates equal to or larger than 100 nm (% by mass) Amount of Fe present as precipitates equal to or larger than 100 nm (% by mass) TS (MPa) Range of clearance for blanking (%) Percent ductile fracture (%) ⁇ HV Percent ductile fracture after post-heating (%) 1 92 1 7 0.63 0.022 0.130 983 10 100 10 95 Inventive Example 2 93 1 6 0.55 0.035 0.133 979 10 45 5 40 Comparative Example 3 86 1 13 0.28 0.045 0.134 959 5 40 22 30 Comparative Example 4 58 2 40 0.50 0.012 0.002 1258 15 85
• the TS was 780 MPa or more and less than 1320 MPa.
• the blankability and toughness were good, and the strength and toughness after post-heating were also good.
• at least one of the desired strength, the desired blankability, and the desired toughness was not obtained, or at least one of the desired strength and the desired toughness after post-heating was not obtained.
• a high-strength hot rolled steel sheet having a TS of 780 MPa or more and less than 1320 MPa, having good blankability and toughness, and exhibiting high strength and good toughness after post-heating can be obtained.
• the steel sheet of the invention can significantly contribute to improvement in crash safety of the automobiles and improvement in their fuel economy.

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