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
  • C22C38/18—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
• • 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
• • 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
  • C21D1/02—Hardening articles or materials formed by forging or rolling, with no further heating beyond that required for the formation
• • 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
  • C21D1/84—Controlled slow cooling
• • 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
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
• • 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
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/005—Heat treatment of ferrous alloys containing Mn
• • 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
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/008—Heat treatment of ferrous alloys containing Si
• • 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
  • C21D7/00—Modifying the physical properties of iron or steel by deformation
  • C21D7/13—Modifying the physical properties of iron or steel by deformation by hot working
• • 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 by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • 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
  • 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 by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties 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 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
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
  • C21D8/0421—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the working steps
  • C21D8/0426—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 by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
  • C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
  • C21D8/0463—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing 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
  • C22C38/12—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
  • C22C38/14—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
  • B21B2001/225—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 by 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
  • 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
  • C21D2211/003—Cementite
• • 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
  • C22C38/40—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
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium

Definitions

• the present invention is a high-strength hot-rolled steel sheet having a tensile strength of 980 MPa or more, which is suitable as a material for structural parts of automobiles (structural parts ⁇ of ⁇ automobile), frameworks, truck frames, steel pipes, etc. About.
• high-strength hot-rolled steel sheets having a predetermined strength are increasing year by year as materials for automobile parts and steel pipe materials.
• a high-strength hot-rolled steel sheet having a tensile strength of 980 MPa or more is highly expected as a material that can dramatically improve the fuel consumption of automobiles or a material that can significantly reduce the construction cost of pipelines.
• a high-strength hot-rolled steel sheet with a tensile strength of 780 MPa or more excellent in toughness is obtained by making the main phase fine bainite and reducing the hardness distribution in the thickness direction. It is going to be done.
• Patent Document 2 C: 0.05 to 0.18% by mass, Si: 0.10 to 0.60%, Mn: 0.90 to 2.0%, P: 0.025% or less (excluding 0%), S: 0.015% or less (0 %), Al: 0.001 to 0.1%, N: 0.002 to 0.01%, and the remaining iron and unavoidable impurities are heated to 950 ° C or higher and 1250 ° C or lower, and rolling is started at 820 ° C. After the rolling is completed, cool to 600 to 700 ° C at a cooling rate of 20 ° C / s or more, hold the temperature in this temperature range for 10 to 200 seconds and / or slowly cool, and then cool to 5 ° C / s or more.
• the ferrite ratio is 70 to 90% in the space ratio of the whole structure.
• Ferrite: martensite or mixed phase of martensite and austenite: 3 to 15%, balance: bainite (including the case of 0%) and a method for producing a steel sheet in which the average crystal grain size of the ferrite is 20 ⁇ m or less Has been proposed.
• the tensile strength is 490 N / mm 2 or more by making the metal structure a fine crystal grain ferrite and a structure containing martensite or a mixed phase of martensite and austenite. Therefore, it is said that a high toughness steel material having a yield ratio of 70% or less and a low yield ratio can be obtained.
• Patent Document 3 C: 0.02 to 0.25% by mass, Si: 1.0% or less, Mn: 0.3 to 2.3%, P: 0.03% or less, S: 0.03% or less, Al: 0.1% or less, Nb: 0.03
• the steel material containing ⁇ 0.25%, Ti: 0.001 ⁇ 0.10% and satisfying (Ti + Nb / 2) / C ⁇ 4 is hot-rolled, and after the hot rolling finish, The first cooling that is accelerated cooling until the surface temperature is below the Ar 3 transformation point and below the Ms point at an average cooling rate of 20 ° C / s or higher and less than martensite formation critical cooling rate.
• the coil After the second cooling, which is quenched until the sheet thickness center reaches 350 ° C or more and less than 600 ° C, the coil is wound in a coil shape at a coiling temperature of 350 ° C or more and less than 600 ° C.
• Thick, high-tensile heat that sequentially performs third cooling that is held or retained for 30 minutes or more in the temperature range of 350 to 600 ° C at least in the thickness direction 1/4 to 3/4 in the coil thickness direction.
• Method of manufacturing a steel sheet have been proposed. According to the technique proposed in Patent Document 3, the structure of the hot-rolled steel sheet is changed to a bainite phase or bainitic ferrite phase, and the amount of grain boundary cementite is adjusted to a specific value or less. By doing so, it is said that a high-strength ERW steel pipe material with excellent low-temperature toughness of X65 grade or higher can be obtained.
• a method for producing a high-strength hot-rolled steel sheet is described, in which the coil is cooled to a coiling temperature of 300 to 500 ° C. at an average cooling rate of 100 ° C./s or more and wound at the coiling temperature.
• a structure composed of a single phase of bainite phase having an average particle diameter of 5 ⁇ m or less, preferably more than 3.0 to 5.0 ⁇ m, and having a solid solution Ti of 0.02% or more, TS has a high strength of 780 MPa or more.
• a bainite phase having an area ratio of 90% or more and a second phase other than the bainite phase may be used, and the average particle size of the second phase may be 3 ⁇ m or less.
• Patent Document 5 in mass%, C: 0.01 to 0.08%, Si: 0.30 to 1.50%, Mn: 0.50 to 2.50%, P: 0.03% or less, S: 0.005% or less, and Ti: 0.01 to 0.20% , Nb: 0.01 to 0.04% of one or two of slabs composed of iron and inevitable impurities, hot rolled at an Ar 3 transformation temperature of 950 ° C after hot rolling, then 20 ° C Of high-strength hot-rolled steel sheet that is cooled to 650-800 ° C at a cooling rate of at least / s and then air-cooled for 2 to 15s, then cooled to 350-600 ° C at a cooling rate of at least 20 ° C / s. A method is described.
• one or two of Ca and REM may be contained in an amount of 0.0005 to 0.01%.
• Patent Document 6 describes a high-strength thin steel sheet excellent in hole expansibility and ductility.
• the high-strength thin steel sheet described in Patent Document 3 is in mass%, C: 0.01 to 0.20%, Si: 1.50% or less, Al: 1.5% or less, Mn: 0.5 to 3.5%, P: 0.2% or less, S : 0.0005 to 0.009%, N: 0.009% or less, Mg: 0.0006 to 0.01%, O: 0.005% or less, and Ti: 0.01 to 0.20%, Nb: 0.01 to 0.10%.
• the metal structure of the steel is a ferrite main phase structure, but when the tensile strength is 980 MPa class, the toughness of the ferrite phase may be significantly reduced.
• the low-temperature toughness is improved by controlling the amount of grain boundary cementite, but the hot-rolled steel sheet strength is insufficient, and as shown in the examples, the maximum is shown.
• Tensile strength about 800 MPa.
• it is necessary to increase the C content With the increase in grain size, it became difficult to control the grain boundary cementite, and there were cases where excellent toughness could not be secured stably.
• the present invention advantageously solves the above-mentioned problems of the prior art, has a high strength of tensile strength: 980 MPa or more, and has a better toughness, particularly a high-strength hot-rolled steel sheet having a thickness of 4 mm or more and 15 mm or less, and It aims at providing the manufacturing method.
• the target strength is the tensile strength TS: 780 MPa or more, but if the C content is increased, the tensile strength TS: 980 MPa or more can be ensured. it can. However, if the C content is increased to further increase the strength, it becomes difficult to control the amount of precipitation of Ti carbide, and 0.02% or more of the solid solution Ti required to improve hole expansion workability can be stabilized. There was a problem that it was difficult to remain.
• the steel sheet structure is a mixed structure of ferrite and bainite in which the ratio of ferrite having a grain size of 2 ⁇ m or more is 80% or more, and the obtained steel sheet strength is at most about 976 MPa, and the tensile strength is high.
• TS It is difficult to achieve a further increase in strength of 980 MPa or higher, and even if a tensile strength of TS: 980 MPa or higher is obtained, the toughness of the ferrite phase is significantly reduced, ensuring excellent hole expansion workability. There was a problem that I could not.
• the present invention provides a high-strength hot-rolled steel sheet having a further excellent hole expansion workability and a method for producing the same while solving such problems of the prior art and maintaining a high strength of tensile strength: 980 MPa or more.
• the high-strength hot-rolled steel sheet targeted by the present invention is a thin steel sheet having a thickness of 2 to 4 mm.
• the present invention has been completed after further studies based on such findings. That is, the gist configuration of the present invention is as follows. [1] By mass%, C: 0.05% to 0.18%, Si: 1.0% or less, Mn: 1.0% to 3.5%, P: 0.04% or less, S: 0.006% or less, Al: 0.10% or less, N : 0.008% or less, Ti: 0.05% or more and 0.20% or less, V: containing more than 0.1% and 0.3% or less, with the balance consisting of Fe and inevitable impurities, with a bainite phase exceeding 85% in area ratio
• a main phase one or more of ferrite phase, martensite phase and retained austenite phase is a second phase, and the second phase includes an area ratio of 0% to less than 15% in total,
• a high-strength hot-rolled steel sheet having a structure having an average lath interval of laths of 400 nm or less, an average major axis length of the laths of 5.0 ⁇ m
• the present inventors conducted further research, and in order to improve the hole expansion workability and further the local ductility while maintaining a high strength of tensile strength TS: 980 MPa or more, C Adjust the content balance of Si, Ti and V, further optimize the production conditions, adjust the cementite to 0.8% by mass and the average particle size of cementite to 150nm or less, and widen the space between cementites. I found out that this is important.
• the gist of the present invention is as follows. [5] By mass%, C: more than 0.1% and 0.2% or less, Si: 1.0% or less, Mn: 1.5 to 2.5%, P: 0.05% or less, S: 0.005% or less, Al: 0.10% or less, N: 0.007 %, Ti: 0.07 to 0.2%, V: more than 0.1% and 0.3% or less, the composition consisting of the balance Fe and inevitable impurities, and the main phase is a bainite phase with an area ratio of 90% or more , Cementite having a structure composed of one or more selected from the martensite phase, austenite phase, and ferrite phase, the balance other than the main phase being 10% or less in area ratio, and dispersed in the structure Is a high-strength hot-rolled steel sheet excellent in hole-expanding workability with a mass% of 0.8% or less, an average particle size of 150 nm or less, and
• the steel material is heated and subjected to hot rolling consisting of rough rolling and finish rolling, cooling is performed in two stages of first stage cooling and second stage cooling, and then a rolled hot rolled steel sheet is obtained.
• the steel material is, by mass%, C: more than 0.1% and 0.2% or less, Si: 1.0% or less, Mn: 1.5 to 2.5%, P: 0.05% or less, S: 0.005% or less, Al: 0.10% or less , N: 0.007% or less, Ti: 0.07 to 0.2%, V: more than 0.1% and 0.3% or less, and a steel material having a composition composed of the balance Fe and inevitable impurities, and the heating causes the steel material to be 1200 ° C
• the above-described heating process wherein the finish rolling is a finish rolling finish temperature: 850 to 950 ° C., and the first stage cooling starts cooling within 1.5 s after finishing the finish rolling.
• a high-strength hot-rolled steel sheet having a tensile strength of 980 MPa or more and excellent toughness can be obtained. Therefore, when the present invention is applied to a structural part, a skeleton, a truck frame, or the like of an automobile, the weight of the vehicle body can be reduced while ensuring the safety of the automobile, and the environmental load can be reduced.
• a welded steel pipe made of the hot-rolled steel sheet of the present invention as a transport pipe instead of a UOE steel pipe made of a thick steel plate, productivity is improved and further cost reduction is possible.
• the present invention can stably produce a hot-rolled steel sheet with improved toughness while maintaining a high strength of 980 MPa or more, and is extremely useful industrially.
• the hot-rolled steel sheet of the present invention can be used as a material for automobile undercarriage parts, structural parts, skeletons, truck frames, etc., while ensuring the safety of the automobile while reducing the weight of the vehicle body and reducing the environmental load. There is also an effect that it becomes possible to do.
• % showing the following component composition shall mean the mass% unless there is particular notice.
• C 0.05% or more and 0.18% or less C improves the strength of steel and promotes the formation of bainite. Therefore, in the present invention, the C content needs to be 0.05% or more. On the other hand, if the C content exceeds 0.18%, formation control of bainite becomes difficult, the formation of hard martensite increases, and the toughness of the hot-rolled steel sheet decreases. Therefore, the C content is 0.05% or more and 0.18% or less. Preferably, it is 0.08% or more and 0.17% or less, more preferably more than 0.10% and 0.16% or less. In addition, when the amount of Mn is 2.5% or more and 3.5% or less, the preferable amount of C is 0.06% or more and 0.15% or less.
• Si 1.0% or less
• Si is an element that suppresses coarse oxides and cementite that inhibit toughness and contributes to solute strengthening, but if the content exceeds 1.0%, The surface properties are significantly deteriorated, resulting in deterioration of chemical conversion treatability and corrosion resistance. Therefore, the Si content is 1.0% or less. Preferably they are 0.4% or more and 0.8% or less.
• Mn 1.0% or more and 3.5% or less
• Mn is an element that contributes to increasing the strength of the steel by solid solution and promotes the formation of bainite through the improvement of hardenability.
• the Mn content needs to be 1.0% or more.
• the Mn content is 1.0% or more and 3.5% or less.
• they are 1.5% or more and 3.0% or less, More preferably, they are 1.8% or more and 2.5% or less.
• P 0.04% or less
• P is an element that dissolves and contributes to increasing the strength of steel, but segregates at grain boundaries, especially prior-austenite grain boundaries, and lowers low-temperature toughness and workability. It is also an element that invites. For this reason, it is preferable to reduce the P content as much as possible, but a content of up to 0.04% is acceptable. Therefore, the P content is 0.04% or less. However, since an effect commensurate with the increase in refining costs cannot be obtained even if the P content is excessively reduced, the P content is preferably 0.003% or more and 0.03% or less, and 0.005% or more and 0.02% or less. Is more preferable.
• S 0.006% or less S combines with Ti and Mn to form coarse sulfides, and deteriorates the workability of hot-rolled steel sheets. Therefore, it is preferable to reduce the S content as much as possible, but a content of up to 0.006% is acceptable. Therefore, the S content is 0.006% or less. However, since an effect commensurate with the increase in refining costs cannot be obtained even if the S content is excessively reduced, the S content is preferably 0.0003% or more and 0.004% or less, and 0.0005% or more and 0.002% or less. Is more preferable.
• Al acts as a deoxidizer and is an effective element for improving the cleanliness of steel.
• the Al content is 0.10% or less.
• it is 0.005% or more and 0.08% or less. More preferably, it is 0.01% or more and 0.05% or less.
• N 0.008% or less N is precipitated as a nitride by combining with a nitride-forming element and contributes to refinement of crystal grains.
• N tends to bond to Ti at a high temperature to form coarse nitrides, thereby reducing the toughness of the hot-rolled steel sheet.
• N content shall be 0.008% or less.
• it is 0.001% or more and 0.006% or less. More preferably, it is 0.002% or more and 0.005% or less.
• Ti 0.05% or more and 0.20% or less Ti is one of the most important elements in the present invention. Ti contributes to increasing the strength of steel by forming carbonitrides to refine crystal grains and by precipitation strengthening. Ti also forms many fine (Ti, V) C clusters at low temperatures of 300 ° C to 450 ° C, reducing the amount of cementite in the steel and improving the toughness of hot-rolled steel sheets. Let In order to exhibit such an effect, the Ti content needs to be 0.05% or more. On the other hand, when the Ti content exceeds 0.20% and becomes excessive, the above-described effects are saturated, and coarse precipitates are increased, resulting in a decrease in toughness of the hot-rolled steel sheet. Therefore, the Ti content is limited to a range of 0.05% or more and 0.20% or less. Preferably they are 0.08% or more and 0.15% or less.
• V more than 0.1% and 0.3% or less V is one of the most important elements in the present invention.
• V contributes to increasing the strength of steel by forming carbonitrides to refine crystal grains and by precipitation strengthening. V also improves hardenability and contributes to the formation and refinement of the bainite phase.
• V forms many fine (Ti, V) C clusters at a low temperature of 300 ° C. or higher and 450 ° C. or lower, reduces the amount of cementite in the steel, and improves the toughness of the hot rolled steel sheet.
• the V content needs to exceed 0.1%.
• the V content exceeds 0.3% and becomes excessive, the above-described effects are saturated, resulting in high costs. Therefore, the V content is limited to a range of more than 0.1% and 0.3% or less. Preferably it is 0.15% or more and 0.25% or less.
• the above are the basic components of the hot-rolled steel sheet of the present invention.
• the hot-rolled steel sheet of the present invention is, for example, for the purpose of improving toughness and increasing strength, Nb: 0.005% or more and 0.4% or less, B: 0.0002 %: 0.005% or more, Cu: 0.005% or more and 0.2% or less, Ni: 0.005% or more and 0.2% or less, Cr: 0.005% or more and 0.4% or less, Mo: 0.005% or more and 0.4% or less Two or more kinds can be contained.
• Nb 0.005% or more and 0.4% or less
• Nb is an element that contributes to increasing the strength of steel through the formation of carbonitrides.
• the Nb content is preferably 0.005% or more.
• the Nb content exceeds 0.4%, the deformation resistance increases, so the hot rolling force increases during the production of hot-rolled steel sheets, increasing the burden on the rolling mill. After that, the rolling operation itself may be difficult.
• the Nb content exceeds 0.4%, coarse precipitates are formed and the toughness of the hot-rolled steel sheet tends to decrease. Therefore, the Nb content is preferably 0.005% or more and 0.4% or less. In addition, More preferably, it is 0.01% or more and 0.3% or less, More preferably, it is 0.02% or more and 0.2% or less.
• B 0.0002% or more and 0.0020% or less
• B is an element that segregates at austenite grain boundaries and suppresses the formation and growth of ferrite.
• B is also an element that improves the hardenability and contributes to the formation and refinement of the bainite phase.
• the B content is preferably 0.0002% or more. However, if the B content exceeds 0.0020%, the formation of martensite phase is promoted, so that the toughness of the hot-rolled steel sheet may be significantly reduced. Therefore, when it contains B, it is preferable to make the content into 0.0002% or more and 0.0020% or less. Further, it is more preferably 0.0004% or more and 0.0012% or less.
• Cu 0.005% or more and 0.2% or less
• Cu is an element that contributes to increasing the strength of steel by solid solution. Further, Cu has an effect of improving hardenability, and is also an element that contributes to refinement of the bainite phase by reducing the bainite transformation temperature.
• the Cu content is preferably set to 0.005% or more. However, if the content exceeds 0.2%, the surface properties of the hot-rolled steel sheet are deteriorated. Accordingly, the Cu content is preferably 0.005% or more and 0.2% or less. More preferably, it is 0.01% or more and 0.15% or less.
• Ni 0.005% or more and 0.2% or less
• Ni is an element that contributes to increasing the strength of steel by solid solution. Moreover, Ni has the effect
• the Ni content is preferably 0.005% or more. However, when the Ni content exceeds 0.2%, a martensite phase is likely to be generated, and the toughness of the hot-rolled steel sheet may be significantly reduced. Therefore, the Ni content is preferably 0.005% or more and 0.2% or less. More preferably, it is 0.01% or more and 0.15% or less.
• the Cr content is preferably 0.005% or more and 0.4% or less.
• the Cr content is preferably 0.005% or more and 0.4% or less. More preferably, it is 0.01% or more and 0.2% or less.
• Mo 0.005% or more and 0.4% or less Mo promotes the formation of a bainite phase through improvement of hardenability, and contributes to improvement of toughness and high strength of the hot-rolled steel sheet.
• the Mo content is preferably 0.005% or more.
• the Mo content is preferably 0.005% or more and 0.4% or less. More preferably, it is 0.01% or more and 0.2% or less.
• the hot-rolled steel sheet of the present invention can contain one or two selected from Ca: 0.0002% to 0.01% and REM: 0.0002% to 0.01% as necessary.
• Ca 0.0002% or more and 0.01% or less Ca controls the shape of sulfide inclusions and is effective in improving the bending workability and toughness of hot-rolled steel sheets.
• the Ca content is preferably 0.0002% or more.
• the Ca content is preferably 0.0002% or more and 0.01% or less. Further, it is more preferably 0.0004% or more and 0.005% or less.
• REM 0.0002% or more and 0.01% or less REM, like Ca, controls the shape of sulfide inclusions and improves the adverse effects of sulfide inclusions on the bending workability and toughness of hot-rolled steel sheets.
• the REM content is preferably 0.0002% or more.
• the balance other than the above is Fe and inevitable impurities.
• Inevitable impurities include Sb, Sn, Zn, etc., and these contents are acceptable if Sb: 0.01% or less, Sn: 0.1% or less, Zn: 0.01% or less.
• the hot-rolled steel sheet of the present invention has a bainite phase with an area ratio of more than 85% as a main phase, and one or more of a ferrite phase, a martensite phase and a retained austenite phase as a second phase. It has a structure in which phases are included in a total area ratio of 0% to less than 15%, an average lath interval of laths of the bainite phase is 400 nm or less, and an average major axis length of the laths is 5.0 ⁇ m or less.
• the fraction of bainite phase more than 85% in area ratio
• the hot-rolled steel sheet of the present invention has a bainite phase excellent in strength-toughness balance as a main phase.
• the fraction of bainite phase is more than 85% in area ratio.
• it is 87% or more, more preferably 90% or more.
• the fraction of the bainite phase is 100% in terms of area ratio and a bainite single phase structure is obtained.
• the fraction of one or more of the ferrite phase, martensite phase and retained austenite phase is the main phase
• a structure other than the bainite phase one or more of a ferrite phase, a martensite phase, and a retained austenite phase may be contained as the second phase.
• the structure is preferably a bainite single-phase structure.
• the fraction of the second phase is a total area ratio of 0% or more and less than 15%. Preferably it is 13% or less, More preferably, it is 11% or less.
• the average lath interval of laths in the bainite phase is set to 400 nm or less. Preferably it is 350 nm or less.
• the average major axis length of the lath of the bainite phase is set to 5.0 ⁇ m or less. Preferably it is 4.0 micrometers or less.
• the average lath spacing of the bainite phase lath is 100 nm or more, and the average major axis length of the bainite phase lath is 1.0 ⁇ m or more.
• a high-strength hot-rolled steel sheet having a tensile strength TS of 980 MPa or more and having the toughness required as a material for steel pipes such as materials for automobile parts and line pipes is obtained. can get.
• the thickness of the hot-rolled steel sheet of the present invention is not particularly limited, but is preferably about 4 mm to 15 mm.
• the steel material having the above composition is heated to 1200 ° C. or more, rough rolling, the cumulative rolling reduction in the temperature range of 1000 ° C. or less is 50% or more, and the finish rolling finish temperature is 820 ° C. or more and 930 ° C. or less.
• hot rolling consisting of finish rolling, start cooling within 4.0 s, cool at an average cooling rate of 20 ° C / s or higher, and wind at a winding temperature of 300 ° C to 450 ° C
• start cooling within 4.0 s
• cool at an average cooling rate of 20 ° C / s or higher cool at an average cooling rate of 20 ° C / s or higher
• wind at a winding temperature of 300 ° C to 450 ° C
• the manufacturing method of the steel material is not particularly limited, and any conventional method in which the molten steel having the above-described composition is melted in a converter or the like and is made into a steel material such as a slab by a casting method such as continuous casting. Is also applicable. An ingot-making method and an inblooming method may also be used.
• electromagnetic-stirring EMS
• IBSR intentional-bulging-soft-reduction-casting
• Heating temperature of steel material 1200 ° C or higher
• steel materials such as slabs
• most of carbonitride-forming elements such as Ti are present as coarse carbonitrides.
• the presence of coarse and uneven precipitates causes deterioration of various properties (for example, strength, toughness, hole expansion workability, etc.) of the hot-rolled steel sheet. Therefore, the steel material before hot rolling is heated to dissolve coarse precipitates.
• the heating temperature of the steel material needs to be 1200 ° C. or higher.
• the heating temperature of the steel material is preferably 1350 ° C. or lower. More preferably, it is 1220 ° C. or higher and 1300 ° C. or lower.
• the steel material is heated to a heating temperature of 1200 ° C or higher and held for a predetermined time, but if the holding time exceeds 4800 seconds, the amount of scale generation increases, resulting in scale biting in the subsequent hot rolling process. ) And the like tend to occur, and the surface quality of the hot-rolled steel sheet tends to deteriorate. Therefore, the holding time of the steel material in the temperature range of 1200 ° C. or higher is preferably 4800 seconds or less. More preferably, it is 4000 seconds or less.
• the steel material is subjected to hot rolling consisting of rough rolling and finish rolling.
• Rough rolling is not particularly limited as long as a desired sheet bar dimension can be secured.
• finish rolling is performed. Note that descaling is preferably performed before finish rolling or during rolling between stands.
• finish rolling the cumulative rolling reduction in the temperature range of 1000 ° C. or less is set to 50% or more, and the finish rolling finish temperature is set to 820 ° C. or more and 930 ° C. or less.
• Cumulative rolling reduction in the temperature range below 1000 ° C: 50% or more In order to refine the lath of the bainite phase, the rolling reduction in a relatively low temperature range was increased and the rolled crystal grains were expanded in the rolling direction. It is necessary to use crystal grains (crystal grains with a high elongation rate).
• the cumulative rolling reduction at 1000 ° C. or less is less than 50%, it becomes difficult to secure a bainite having a desired lath structure (average lath interval: 400 nm or less, average major axis length: 5.0 ⁇ m or less), and hot-rolled steel sheet The toughness of the steel decreases. Therefore, the cumulative rolling reduction at 1000 ° C. or less is set to 50% or more. Preferably it is 60% or more.
• the cumulative rolling reduction in the temperature range of 1000 ° C. or lower becomes excessively high, the crystal grains are excessively stretched in the rolling direction and ferrite is likely to be formed, so that a bainite having a desired lath structure is secured. May still be difficult.
• the cumulative rolling reduction in the temperature range of 1000 ° C. or lower is preferably 80% or lower.
• Finish rolling finish temperature 820 ° C or more and 930 ° C or less If the finish rolling finish temperature is less than 820 ° C, the rolling is performed at the two-phase temperature range of ferrite + austenite, so the processed structure remains after rolling and the toughness of the hot rolled steel sheet descend. On the other hand, when the finish rolling finish temperature is higher than 930 ° C., austenite grains grow and the bainite phase of the hot-rolled steel sheet obtained after cooling becomes coarse. As a result, it becomes difficult to secure a desired structure, and the toughness of the hot-rolled steel sheet decreases. Therefore, the finish rolling end temperature is set to 820 ° C. or higher and 930 ° C. or lower. Preferably they are 840 degreeC or more and 920 degrees C or less. Here, the finish rolling end temperature represents the surface temperature of the plate.
• Start of forced cooling within 4.0s after finishing rolling
• start forced cooling within 4.0s, preferably immediately, stop cooling at coiling temperature, and coil take.
• the time from the finish rolling to the start of forced cooling is longer than 4.0 s, the austenite grains become coarse and the bainite phase becomes coarse.
• the austenite grains become coarse, the hardenability of the steel sheet is increased, and a martensite phase is easily generated.
• the forced cooling start time is limited to 4.0 s after finishing rolling.
• Average cooling rate 20 ° C./s or more If the average cooling rate from the finish rolling finish temperature to the coiling temperature is less than 20 ° C./s, a bainite phase having a desired area ratio cannot be obtained. Therefore, the average cooling rate is set to 20 ° C./s or more. Preferably it is 30 ° C./s or more. The upper limit of the average cooling rate is not particularly specified, but if the average cooling rate is too high, the surface temperature is too low and martensite is easily generated on the steel sheet surface, so the average cooling rate is 60 ° C / s or less. It is preferable to do. In addition, let the said average cooling rate be an average cooling rate in the surface of a steel plate.
• Winding temperature 300 ° C. or higher and 450 ° C. or lower
• the winding temperature is lower than 300 ° C.
• a hard martensite phase or residual austenite phase is formed in the structure inside the steel sheet.
• the hot rolled steel sheet cannot be made to have a desired structure, and desired toughness cannot be ensured.
• the coiling temperature exceeds 450 ° C.
• ferrite and pearlite increase in the structure inside the steel sheet.
• the lath interval of the bainite phase increases, the toughness of the hot-rolled steel sheet is significantly reduced.
• the coiling temperature is in the range of 300 ° C to 450 ° C.
• they are 330 degreeC or more and 430 degrees C or less.
• the hot-rolled steel sheet may be subjected to temper rolling in accordance with a conventional method, and pickling is performed to remove the scale formed on the surface. May be.
• plating treatment galvanization process
• hot dip galvanizing hot dip galvanizing
• electrogalvanizing chemical conversion (treatment) treatment
• Molten steel having the composition shown in Table 1 was melted in a converter and made into a slab (steel material) by a continuous casting method.
• EMS electromagnetic stirring
• these steel materials are heated under the conditions shown in Table 2, hot-rolled consisting of rough rolling and finish rolling under the conditions shown in Table 2, and cooled under the conditions shown in Table 2 after finishing rolling.
• the steel sheet was wound at the winding temperature shown in Table 2 to obtain a hot-rolled steel sheet having the thickness shown in Table 2.
• Test specimens were collected from the obtained hot-rolled steel sheet and subjected to structure observation, tensile test, and Charpy impact test.
• the tissue observation method and various test methods were as follows.
• SEM scanning electron microscope
• Lath spacing of lath of bainite phase A specimen of size: 10 mm x 15 mm was taken from a hot-rolled steel sheet, and a transmission electron microscope (transmission) at a thickness 1/4 position and a thickness 1/2 position (thickness center position) A thin film sample for electron microscope (TEM) observation was made, and 10 fields of view were taken at each position at a magnification of 30000 times using a TEM.
• Each length was measured, and an average value of the lengths of the obtained line segments was defined as an average lath interval.
• a Charpy impact test was conducted, and a Charpy impact value (vE- 50 ) at a temperature of -50 ° C was measured to evaluate toughness.
• vE- 50 Charpy impact value
• a test piece was prepared with double-sided grinding with a plate thickness of 5 mm, and for hot-rolled steel sheets with a plate thickness of 5 mm or less, a test piece was prepared with the original thickness, Subjected to Charpy impact test.
• the measured vE -50 value was 40 J or more, the toughness was evaluated as good.
• the hot-rolled steel sheet of the inventive example is a hot-rolled steel sheet having both desired strength (TS: 980 MPa or more) and excellent toughness (vE- 50 value: 40 J or more).
• the hot-rolled steel sheet of the invention example has the desired strength and excellent toughness at both the thickness 1/4 position and the thickness 1/2 position (the thickness center position). This is a hot-rolled steel sheet with good characteristics.
• the hot-rolled steel sheet of the comparative example which is out of the scope of the present invention does not ensure a predetermined strength or does not ensure sufficient toughness.
• C more than 0.1% and 0.2% or less C is an element having an action of promoting the formation of bainite, increasing the strength of steel, and promoting the formation of bainite, and is one of the important elements in the present invention.
• the C content needs to exceed 0.1%.
• C since C combines with Fe to form cementite, the excessive C content increases the number of cementite, narrows the gap between the cementites that are the origin of voids, reduces local ductility, Spreading processability decreases. Further, if C is contained in excess of 0.2%, weldability is lowered. For these reasons, C is limited to the range of more than 0.1% and 0.2% or less.
• the content is 0.12 to 0.17%.
• Si 1.0% or less
• Si is an element having an effect of suppressing the formation of coarse cementite as a solid solution and contributing to an increase in steel strength, and is one of the important elements in the present invention.
• Si contributes to the improvement of local ductility and hole expansion workability by widening the interval of cementite, which is the origin of voids, particularly through the action of suppressing the formation of coarse cementite. In order to acquire such an effect, it is desirable to contain 0.1% or more. On the other hand, if the content exceeds 1.0%, the surface properties of the steel sheet are remarkably deteriorated, and chemical conversion treatment properties and corrosion resistance are reduced. For this reason, Si was limited to 1.0% or less. It is preferably 0.5 to 0.9%.
• Mn 1.5-2.5%
• Mn is an element that contributes to increasing the strength of the steel by forming a solid solution, and further promotes the formation of a bainite phase through improvement of hardenability.
• the content of 1.5% or more is required.
• the content exceeds 2.5% the central segregation becomes remarkable, and the appearances of punched surface of the steel sheet are lowered, and the hole expanding workability is lowered.
• the amount of Mn was limited to the range of 1.5 to 2.5%. In addition, Preferably it is 1.7 to 2.2% of range.
• P 0.05% or less P dissolves and contributes to increasing the strength of the steel, but segregates at grain boundaries, particularly prior austenite grain boundaries, and lowers low temperature toughness and workability. For this reason, it is preferable to reduce P as much as possible, but inclusion up to 0.05% is acceptable. Therefore, P is limited to 0.05% or less. In addition, Preferably it is 0.03% or less, More preferably, it is 0.02% or less.
• S 0.005% or less S combines with Ti and Mn to form coarse sulfides, thereby reducing workability. For this reason, it is preferable to reduce S as much as possible, but the content up to 0.005% is acceptable. For these reasons, S is limited to 0.005% or less. In addition, Preferably it is 0.003% or less, More preferably, it is 0.001% or less.
• Al 0.10% or less
• Al is an element that acts as a deoxidizer and contributes effectively to improving the cleanliness of steel. In order to acquire such an effect, it is desirable to contain 0.005% or more.
• an excessive content exceeding 0.10% causes an increase in oxide inclusions, which causes generation of flaws and decreases the workability of the steel sheet. For this reason, Al was limited to 0.10% or less.
• the content is 0.01 to 0.05%.
• N 0.007% or less
• N is an element that combines with a nitride-forming element and precipitates as a nitride, contributing to refinement of crystal grains.
• 0.007% is permissible.
• N is limited to 0.007% or less.
• Ti forms carbonitrides, refines the crystal grains, and contributes to increasing the strength of the steel by precipitation strengthening.
• Ti has the effect of reducing the amount of cementite in steel by forming many fine (Ti, V) C clusters in the temperature range of about 300 to 500 ° C (coiling temperature).
• Ti, V fine coarse
• Ti is 0.1 to 0.15%.
• V more than 0.1% and less than 0.3% V forms carbonitrides and refines the crystal grains, and contributes to increasing the strength of the steel by precipitation strengthening. It is an element that contributes to chemical conversion.
• V has the effect of reducing the amount of cementite in steel by forming many fine (Ti, V) C clusters in the temperature range of about 300 to 500 ° C (coiling temperature).
• Ti, V coarse fine
• the content of more than 0.1% is required.
• an excessive content exceeding 0.3% lowers the ductility and causes an increase in production cost. For this reason, V is limited to a range of more than 0.1% and 0.3% or less.
• the content is preferably 0.13 to 0.27%, more preferably 0.15 to 0.25%.
• Nb 0.005 to 0.1%
• B 0.0002 to 0.002%
• Cu 0.005 to 0.3
• % Ni: 0.005-0.3%
• Cr 0.005-0.3%
• Mo One or more selected from 0.005-0.3%
• Ca 0.0003-0.01%
• REM 0.0003- One or two selected from 0.01%
• Nb 0.005-0.1%
• B 0.0002-0.002%
• Cu 0.005-0.3%
• Ni 0.005-0.3%
• Cr 0.005-0.3%
• Mo 0.005-0.3%
• Two or more types Nb, B, Cu, Ni, Cr, and Mo are elements that contribute to an increase in the strength of the steel sheet, and can be selected as necessary to contain one or more types.
• Nb is an element that contributes to increasing the strength of steel through the formation of carbonitrides. In order to exhibit such an effect, it is preferable to contain 0.005% or more. On the other hand, if the content exceeds 0.1%, the deformation resistance increases, the rolling load of hot rolling increases, the burden on the rolling mill becomes too large, making the rolling operation itself difficult, and forming coarse precipitates. In addition, workability is reduced. For this reason, when contained, Nb is preferably limited to a range of 0.005 to 0.1%. More preferably, the content is 0.01 to 0.05%, and still more preferably 0.02 to 0.04%.
• B is an element that segregates at the austenite grain boundaries, suppresses the formation and growth of ferrite, improves the hardenability, contributes to the formation and refinement of the bainite phase, and increases the strength of the steel. In order to exhibit such an effect, it is preferable to contain 0.0002% or more, but if it exceeds 0.002%, the workability is remarkably lowered. Therefore, when contained, B is preferably limited to the range of 0.0002 to 0.002%. More preferably, the content is 0.0005 to 0.0015%.
• Cu is an element that has the effect of increasing the strength of steel by solid solution and improving the hardenability. Cu particularly lowers the bainite transformation temperature and contributes to refinement of the bainite phase. In order to acquire such an effect, it is preferable to contain 0.005% or more, but inclusion exceeding 0.3% causes the surface quality (surface quality) to fall. Therefore, when contained, Cu is preferably limited to a range of 0.005 to 0.3%. More preferably, the content is 0.01 to 0.2%.
• Ni is an element having a function of increasing the strength of steel by solid solution and improving the hardenability and facilitating the formation of a bainite phase. In order to acquire such an effect, it is preferable to contain 0.005% or more, but when it contains exceeding 0.3%, it will become easy to produce
• Cr is an element that forms carbides and contributes to increasing the strength of steel. In order to exhibit such an effect, it is preferable to contain 0.005% or more. On the other hand, an excessive content exceeding 0.3% lowers the corrosion resistance of the steel sheet. Therefore, when contained, Cr is preferably limited to a range of 0.005 to 0.3%. More preferably, the content is 0.01 to 0.2%.
• Mo is an element that has the effect of improving hardenability, facilitating the formation of a bainite phase, and increasing the strength of steel. In order to acquire such an effect, it is preferable to contain 0.005% or more, but when it contains exceeding 0.3%, it will become easy to produce
• Ca 0.0003 to 0.01%
• REM 0.0003 to 0.01%. Both Ca and REM improve the hole expansion workability through shape control of inclusions. It is an element which contributes to, and can be selected as necessary and can contain one or two kinds.
• Ca is an element that controls the shape of sulfide inclusions and contributes effectively to the improvement of hole expansion workability. In order to express this effect, the content of 0.0003% or more is required. On the other hand, an excessive content exceeding 0.01% increases the amount of inclusions and causes frequent surface defects. Therefore, when it is contained, Ca is preferably limited to a range of 0.0003 to 0.01%.
• REM like Ca
• REM is an element that controls the shape of sulfide inclusions, improves the adverse effects of sulfide inclusions on hole expansion processability, and contributes to improvement of hole expansion processability.
• the content of 0.0003% or more is required.
• an excessive content exceeding 0.01% increases the amount of inclusions, deteriorates the cleanliness of the steel, and decreases the hole expansion workability. Therefore, when contained, REM is preferably limited to a range of 0.0003 to 0.01%.
• the balance other than the above components is composed of Fe and inevitable impurities.
• Inevitable impurities include O (oxygen): 0.005% or less, W: 0.1% or less, Ta: 0.1% or less, Co: 0.1% or less, Sb: 0.1% or less, Sn: 0.1% or less, Zr: 0.1 % Or less is acceptable.
• the main phase is the bainite phase.
• the “main phase” here refers to a phase having an area ratio of 90% or more. If the phase other than the bainite phase is the main phase, the desired high strength and good hole expansion workability cannot be secured stably. Therefore, the main phase was a bainite phase having an area ratio of 90% or more. In addition, Preferably it is 92% or more, More preferably, it is 95% or more.
• the balance other than the bainite phase which is the main phase, is composed of one or more selected from a martensite phase, an austenite phase (residual austenite phase), and a ferrite phase.
• the remaining phases other than the main phase are 10% or less in total (including 0%) in terms of area ratio. If the remaining phase other than the bainite phase exceeds 10%, the desired high strength and good hole expansion workability cannot be secured stably. In particular, when the martensite phase increases, the desired good hole expansion workability cannot be secured stably.
• the present hot-rolled steel sheet has the above-described structure and exhibits a structure in which cementite is dispersed in the structure.
• Cementite exists mainly in a dispersed state in the bainite phase, but may exist in a phase other than bainite or at the boundary between phases.
• the cementite dispersed in the structure is 0.8% or less by mass% and the average particle size is 150 nm or less.
• cementite When cementite is dispersed in a large amount exceeding 0.8% by mass in the structure, the number of dispersed cementite increases, voids originating from cementite are easily connected during processing, local ductility is reduced, and holes are reduced. Spreading processability decreases. For this reason, cementite was limited to 0.8% or less by mass. In addition, Preferably it is 0.6% or less. More preferably, it is 0.5% or less.
• the average particle size of cementite exceeds 150 nm and becomes coarse, coarse voids starting from cementite are likely to occur during processing, and hole expansion workability is reduced. For this reason, the average particle diameter of cementite was limited to 150 nm or less. In addition, Preferably it is 130 nm or less, More preferably, it is 110 nm or less.
• the steel material is heated and subjected to hot rolling consisting of rough rolling and finish rolling, then subjected to cooling consisting of two stages of first stage cooling and second stage cooling, and then through a winding process, A hot-rolled steel sheet is used.
• the manufacturing method of the steel material that is the starting material is to melt the molten steel having the above-described composition by a conventional melting method such as a converter, and by a conventional casting method such as a continuous casting method, Any conventional manufacturing method can be applied, and there is no particular limitation. It should be noted that there is no problem even if the ingot-bundling method is used.
• electromagnetic stirring EMS
• IBSR light pressure casting
• EMS electromagnetic stirring
• IBSR light pressure casting
• EMS electromagnetic stirring
• equiaxed crystals can be formed in the center portion of the plate thickness, and segregation can be reduced.
• light pressure casting is performed, segregation at the central portion of the plate thickness can be reduced by preventing the flow of molten steel in the unsolidified portion of the continuous cast slab.
• the obtained steel material is heated to a heating temperature of 1200 ° C or higher.
• Heating temperature 1200 ° C or higher
• the steel material used in the present invention contains carbonitride-forming elements such as Ti, but these carbonitride-forming elements are mostly coarse carbonitrides (precipitates). Exist as.
• a carbonitride-forming element such as Ti is present as coarse precipitates, the amount of fine precipitates contributing to precipitation strengthening is reduced. For this reason, steel plate strength falls.
• the heating temperature was limited to 1200 ° C. or higher.
• the temperature is preferably 1220 ° C to 1350 ° C.
• the heated steel material is subjected to hot rolling consisting of rough rolling and finish rolling.
• finish rolling is performed at a finish rolling finish temperature of 850 to 950 ° C. Needless to say, descaling is performed before finish rolling or during rolling between finish rolling stands.
• Finishing rolling finish temperature 850-950 ° C
• the finish rolling finish temperature is less than 850 ° C.
• the finish rolling becomes a ferrite + austenite two-phase region rolling, and the processed structure remains after rolling, so that the hole expanding workability is lowered.
• the finish rolling finish temperature is higher than 950 ° C.
• austenite grains grow, and the bainite phase of the hot-rolled sheet obtained after cooling becomes coarse. For this reason, hole expansion workability falls.
• the finish rolling finish temperature was limited to the range of 850 to 950 ° C.
• the temperature is preferably 870 to 930 ° C.
• finishing rolling finish temperature is the surface temperature.
• cooling is performed in two stages: first stage cooling and second stage cooling.
• the cooling is started within 1.5 s, preferably immediately, and cooled to the first stage cooling stop temperature of 500 to 600 ° C. at an average cooling rate of 20 to 80 ° C./s. To do.
• the cooling start time of the first stage cooling is longer than 1.5 s, the austenite grains become coarse and the bainite phase becomes coarse. Further, when the austenite grains are coarse, the hardenability of the steel sheet is increased, the martensite phase is easily generated, and the desired excellent hole expanding workability cannot be ensured. For this reason, the cooling start time of the first stage cooling is limited to 1.5 s or less after finishing rolling.
• the average cooling rate of the first stage cooling is less than 20 ° C./s and the cooling is slow, the formation of ferrite or coarse bainite is promoted, and the desired high strength or hole expansion workability cannot be secured.
• the average cooling rate of the first stage cooling was limited to the range of 20 to 80 ° C./s.
• the temperature is preferably 25 to 60 ° C./s.
• the first stage cooling stop temperature is less than 500 ° C, it will enter the transition boiling region and the steel plate temperature will vary widely, the structure will become uneven, and the desired excellent hole expansion workability can be secured. Disappear.
• the first stage cooling stop temperature is higher than 600 ° C., the ferrite transformation is promoted and the desired high strength cannot be ensured. For this reason, the first stage cooling stop temperature was limited to 500 to 600 ° C.
• the temperature is preferably 520 to 580 ° C.
• the cooling start time of the second stage cooling is longer than 3 s, ferrite transformation starts and the desired high strength cannot be secured. For this reason, the cooling start time of the second stage cooling is limited to within 3 s after the end of the first stage cooling.
• the average cooling rate of the second stage cooling was limited to 90 ° C./s or more.
• the upper limit of the average cooling rate of the second stage cooling is not particularly limited, but the upper limit is about 250 ° C./s in relation to the thickness of the plate to be cooled and the capacity of the cooling equipment. It is preferably 100 to 200 ° C./s.
• the second stage cooling stop temperature is less than 330 ° C.
• a hard martensite phase or residual austenite phase is formed in the steel sheet structure, and a desired structure cannot be secured, resulting in a decrease in hole expansion workability.
• the temperature is higher than 470 ° C.
• the ferrite phase and martensite phase increase in the steel sheet structure, the desired structure cannot be secured, and the hole expansion workability is remarkably lowered.
• the first stage cooling stop temperature was limited to 330 to 470 ° C.
• the temperature is preferably 350 to 450 ° C.
• the second stage cooling stop temperature is taken as the coiling temperature, and the coil is wound into a coil shape to obtain a hot rolled steel sheet (hot rolled steel strip).
• the above temperature means the surface temperature of the steel plate.
• the hot-rolled steel sheet may be further subjected to temper rolling according to a conventional method. Moreover, the obtained hot-rolled steel sheet may be pickled to remove the scale formed on the surface. Or after pickling, you may give plating processing, such as hot dip galvanization and electrogalvanization, and chemical conversion treatment.
• Molten steel having the composition shown in Table 5 was melted in a converter and made into a slab (steel material) by a continuous casting method.
• electromagnetic stirring (EMS) was performed for the segregation reduction treatment of the components other than the hot-rolled steel plate No. 1 'of steel A2 in Tables 5 to 7B described later.
• these steel materials are heated under the conditions shown in Tables 6A and B, subjected to hot rolling consisting of rough rolling and finish rolling under the conditions shown in Tables 6A and B.
• Tables 6A and B are performed.
• the steel sheet was cooled at the winding temperature shown in Table 2, and the hot-rolled steel sheet having the thickness shown in Tables 6A and B was obtained. In some hot-rolled steel sheets, the cooling is one-step cooling.
• Test specimens were collected from the obtained hot-rolled steel sheet and subjected to structure observation, tensile test, and hole expansion test.
• the test method was as follows. (1) Microstructure observation A specimen for microstructural observation was collected from the obtained hot-rolled steel sheet, the cross section of the plate thickness parallel to the rolling direction was polished, and the microstructure was revealed with a corrosive solution (3% nital solution). Tissues were observed at 4 positions using a scanning electron microscope (SEM), tissues were imaged at 3 fields of view (magnification: 3000 times), and tissue fractions (area ratios) of each phase were identified by tissue identification and image analysis Was calculated.
• SEM scanning electron microscope
• a replica test piece (size: 10 mm x 15 mm) is taken from the 1/4 position of the thickness of the obtained hot-rolled steel sheet, and a replica film is prepared by a two-stage replica method.
• Collect cementite observe the collected cementite using a transmission electron microscope (TEM), photograph 5 fields of view (magnification: 50000 times), determine the particle size of each cementite, and average the relevant It was set as the average particle diameter of the cementite of a steel plate.
• TEM transmission electron microscope
• magnification: 50000 times determine the particle size of each cementite, and average the relevant It was set as the average particle diameter of the cementite of a steel plate.
• the average value of the major axis length and the minor axis length was used as the particle size of the cementite.
• a test piece for extracting electrolytic residue (size: t ⁇ 50 ⁇ 100 mm) was collected from the obtained hot rolled steel sheet, and 10% AA electrolyte (10 vol% acetylacetone-1 mass% tetramethylammonium chloride (tetramethylammonium chloride) chloride) / methanol (constant-current electrolysis) at a current density of 20 mA / cm 2 over the entire thickness of the test piece.
• the obtained electrolytic solution (electrolyte) was filtered, and the electrolytic residue remaining on the filter paper was analyzed using an inductively-coupled plasma spectrophotometric analyzers to measure the amount of Fe in the electrolytic residue.
• Fe 3 C (mass%) (1.0716 ⁇ [quantitative Fe (g)]) / [electrolytic weight (g)] ⁇ 100
• the amount of precipitated cementite was calculated.
• the atomic weight of Fe is 55.85 (g / mol), and the atomic weight of C is 12.01 (g / mol).
• the electrolysis weight was calculated
• test piece (GL: 50mm) was sampled from the obtained hot-rolled steel sheet so that the tensile direction was perpendicular to the rolling direction, and a tensile test was conducted according to JIS Z 2241.
• the yield strength (yield point) YP, the tensile strength TS, and the elongation El were determined.
• a test piece for hole expansion test (size: t x 100 x 100 mm) was taken from the obtained hot rolled steel sheet, and punch holes were punched out with a 10mm ⁇ punch in the center of the test piece with a clearance of 25.0%. Thereafter, a 60 ° conical punch is inserted into the punch hole so as to push it up from the punching direction, and the hole diameter dmm when the crack penetrates the plate thickness is obtained. Was calculated.
• the clearance is a ratio (%) to the plate thickness.
• ⁇ obtained in a hole expansion test performed on a punch hole punched with a clearance of 12.5% was obtained in a hole expansion test performed on a punch hole punched with a clearance of 60% or more and clearance of 25.0%.
• ⁇ 40% or more, the hole expansion workability was evaluated as good.
• All examples of the present invention are high strength hot-rolled steel sheets having high tensile strength: 980 MPa or more and excellent hole expansion workability.
• the desired tensile strength is not ensured or the hole expansion workability is lowered.

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• 2014
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