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
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • 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
• • 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
  • B21B1/02—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 heavy work, e.g. ingots, slabs, blooms, or billets, in which the cross-sectional form is unimportant ; Rolling combined with forging or pressing
  • B21B1/026—Rolling
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21B—ROLLING OF METAL
  • B21B3/00—Rolling materials of special alloys so far as the composition of the alloy requires or permits special rolling methods or sequences ; Rolling of aluminium, copper, zinc or other non-ferrous metals
• • 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
  • 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
  • C22C38/002—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
  • C22C38/02—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
• • 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
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
  • C23C2/06—Zinc or cadmium or alloys based thereon
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
  • Y10T428/00—Stock material or miscellaneous articles
  • Y10T428/12—All metal or with adjacent metals
  • Y10T428/12493—Composite; i.e., plural, adjacent, spatially distinct metal components [e.g., layers, joint, etc.]
  • Y10T428/12771—Transition metal-base component
  • Y10T428/12785—Group IIB metal-base component
  • Y10T428/12792—Zn-base component
  • Y10T428/12799—Next to Fe-base component [e.g., galvanized]

Definitions

• the present invention relates to a steel sheet useful for warm forming at a forming temperature range of 400 ° C. or more and 700 ° C. or less, and has a tensile strength (TS) of 780 MPa or more at room temperature, even under severe processing conditions in the forming temperature range.
• the present invention relates to a high-strength steel sheet for warm forming that has extremely good ductility that can be accommodated and has a small change in mechanical properties before and after warm forming and a method for producing the same.
• Patent Document 1 discloses that a steel sheet is heated to an austenite region, and a forming process is started using a mold at a temperature equal to or higher than the Ac 3 transformation point.
• a technique has been proposed in which the steel sheet is quenched by heating and hardened by martensitic transformation to give the steel sheet hardenability after hot forming and excellent impact properties.
• Patent Document 2 discloses that the structure contains 10% or more by volume% of a bainite phase having a high content of solute C and a high dislocation density, and the total of the pearlite phase and the martensite phase is 10% or less by volume.
• a steel sheet for warm forming having a structure in which the balance is a ferrite phase has been proposed.
• the steel sheet before forming is heated to lower the steel sheet strength and improve the ductility, and the forming process is performed in a state where the deformation resistance of the steel sheet is small and the shape freezing property is improved. Therefore, according to warm forming, the occurrence of springback can be suppressed and the load on the mold is also reduced. Further, since the ductility is improved when the steel plate is heated, it can be formed into a complicated shape. Since the tensile strength and ductility before and after warm forming do not decrease, the impact absorbing ability of the formed member is not impaired. In addition, since the above effect can be obtained by heating at a temperature lower than that of the technique of Patent Document 1, it is advantageous in terms of energy cost.
• the steel sheet structure is a structure containing a bainite phase that is hard and poor in ductility.
• the steel plate strength is increased by strain aging, the ductility is further lowered, and there are problems of cracking and die damage during warm forming.
• the present invention advantageously solves the above-mentioned problems of the prior art, is excellent in workability (formability) during warm forming, can be applied to severe warm forming conditions, and has little material change due to heat. Therefore, an object is to provide a high-strength steel sheet suitable for warm-forming with a small decrease in strength and ductility after warm-forming, a method for producing the same, and a method for using the high-strength steel sheet.
• the present inventors diligently studied various factors affecting the warm formability (before heating, during heating, ductility after heating, strength, etc.) of a high-strength steel sheet.
• the yield stress in a predetermined heating temperature range is 80% or less of the yield stress at room temperature.
• the deformation resistance is lowered and the ductility is increased in the warm forming temperature region, and the steel sheet can be formed into a complicated shape, which is excellent. It has been found that it exhibits warm formability. Moreover, it was discovered that such a steel plate is also excellent in shape freezing property.
• the yield stress and the total elongation after heating to the heating temperature range and giving a strain of 20% or less and then cooling to room temperature are 70% or more of the yield stress and the total elongation at room temperature before the heating. It was found that the strength and ductility required for automobile members can be secured even after warm forming.
• the present inventors examined a steel sheet structure and a steel sheet composition for obtaining a steel sheet having the above characteristics.
• the inventors pay attention to a ferrite phase that has excellent ductility and little material change due to heat, and the steel sheet structure before warm forming, during warm forming, and after warm forming is substantially made of a single ferrite. I came up with a phase.
• the present inventors in the case of a substantially ferritic single-phase steel sheet that is likely to activate the movement of dislocations in the ferrite phase by heating, deformation resistance when heated to a warm forming temperature of 400 ° C. or more It has been found that since the ductility is improved while lowering, the warm formability is improved, the shape freezing property is improved, and excellent ductility is exhibited even after warm forming.
• the present inventors have studied means for increasing the strength of a steel sheet that is substantially a ferrite single phase. Strain age hardening with solute C and N generated during warm forming results in insufficient ductility of the steel sheet during warm forming and after warm forming even if the steel sheet can be strengthened after warm forming. . In addition, high strength by strengthening by grain refinement is not suitable for a material for warm forming because grains grow during heating.
• the present inventors have come to consider using precipitation strengthening by dispersing fine carbides.
• the present inventors have made it possible to improve the warm formability and the strength and ductility after warm forming by using a fine Ti carbide, or even a V carbide, Mo in a substantially single-phase ferrite matrix. It has been found that it is good to increase the strength of the steel sheet by precipitating carbide and W carbide. According to the study by the present inventors, these carbides are not coarsened in a warm forming temperature range (heating temperature range) of 700 ° C. or lower, and a finely precipitated state is maintained even after warm forming. That is, it has been found that a steel sheet having excellent strength can be obtained even after warm forming by precipitating these carbides in a substantially single-phase ferrite matrix.
• the present inventors adjust the content of Ti that is a carbide forming element, or further the content of Ti, V, Mo, W within an appropriate range, and the Ti content relative to the C content, or Furthermore, it has been found that adjusting the contents of Ti, V, Mo, and W within an appropriate range is important in making the steel sheet have the desired structure described above. Furthermore, when manufacturing a steel sheet having the above-described desired structure, it is important to adjust the cooling and winding conditions after hot rolling within an appropriate range, particularly in suppressing the coarsening of the carbide. I found out.
• the present invention has been completed based on the above findings, and the gist thereof is as follows.
• the tensile strength at room temperature is 780 MPa or more
• the yield stress in the heating temperature range from 400 ° C. to 700 ° C. is 80% or less of the yield stress at room temperature
• the total elongation in the heating temperature range is all at room temperature.
• Elongation is 1.1 times or more
• yield stress after cooling to the room temperature after heating to the heating temperature range is 20% or less
• the total elongation after heating to the heating temperature range and giving a strain of 20% or less and then cooling from the heating temperature to room temperature is 70% or more of the total elongation at room temperature before the heating.
• a high-strength steel sheet for warm forming comprising a matrix having a ferrite phase area ratio of 95% or more, and having a structure in which carbides having an average particle size of 10 nm or less are precipitated in the matrix.
• the composition further contains one or more of V%: 0.5% or less, Mo: 0.5% or less, W: 1.0% or less in mass%.
• V% 0.5% or less
• Mo 0.5% or less
• W 1.0% or less in mass%.
• the high strength steel sheet for warm forming according to any one of the above [1] to [5] is heated to a heating temperature range of 400 ° C. or more and 700 ° C. or less to give a strain of 20% or less.
• the composition further contains one or more of V%: 0.5% or less, Mo: 0.5% or less, W: 1.0% or less in mass%.
• a high-strength steel sheet having a tensile strength of 780 MPa or more and excellent in warm formability capable of warm-forming a member having a complicated shape with a small press load can be obtained.
• the high-strength steel sheet of the present invention is excellent in warm formability, and since the decrease in strength and ductility after warm forming is small, it can be used for automobile members and the like that require shock absorption at the time of collision. Is preferred.
• the high-strength steel sheet of the present invention has a structure in which the material change due to heat is small, the steel sheet characteristics hardly change even when subjected to a thermal history such as plating. Therefore, it is applicable also to the member which needs a plating process from a corrosion-resistant viewpoint, and there exists a remarkable effect on an industry.
• the high-strength steel sheet for warm forming according to the present invention is intended for a steel sheet having a tensile strength at room temperature of 780 MPa or more.
• room temperature means 22 ⁇ 5 ° C.
• the high strength steel sheet for warm forming according to the present invention has a tensile strength at room temperature of 780 MPa or more, a yield stress in a heating temperature range of 400 ° C. or more and 700 ° C.
• the total elongation in the temperature range is 1.1 times or more of the total elongation at room temperature
• the yield stress after cooling to the room temperature after heating to the heating temperature range and giving a strain of 20% or less is 70% or more of the yield stress at room temperature, and after applying a strain of 20% or less by heating to the heating temperature range, the total elongation after cooling from the heating temperature to room temperature is the total elongation at room temperature before the heating 70% or more.
• a steel sheet characteristic in a heating temperature range of 400 ° C. or more and 700 ° C. or less is defined assuming a warm forming temperature of 400 ° C. or more and 700 ° C. or less.
• a steel sheet with a tensile strength of 780 MPa or more at room temperature if the yield stress in the heating temperature range of 400 ° C to 700 ° C exceeds 80% of the yield stress at room temperature, the deformation resistance of the steel plate is sufficient during warm forming. Cannot be reduced. For this reason, it is necessary to increase the press load at the time of warm forming, which causes a problem of a decrease in mold life.
• the press machine main body when applying a large press load, the press machine main body is inevitably large, but when the press machine main body is large, it takes time to transport the steel sheet heated to the warm forming temperature to the press machine, The steel sheet temperature is lowered, making it difficult to perform warm forming at a desired temperature. Furthermore, since the shape freezing property is not sufficiently improved, the merit of the warm forming cannot be expressed.
• the total elongation in the heating temperature range of 400 ° C. or more and 700 ° C. or less is less than 1.1 times the total elongation at room temperature.
• the improvement effect is insufficient. For this reason, defects such as cracks occur during molding, which is a problem.
• the steel sheet strength after warm forming may decrease due to heating of the steel sheet.
• the ductility of the steel sheet after warm forming may decrease due to the strain aging or work hardening.
• a strain of about 1 to 10% is introduced into the steel plate as an equivalent plastic strain. Therefore, in the present invention, assuming a warm forming in which a strain of up to 20% is introduced in a temperature range of 400 ° C. or higher and 700 ° C. or lower, heating to a heating temperature range of 400 ° C. or higher and 700 ° C. or lower is 20% or lower.
• the yield stress and the total elongation of the steel sheet after cooling from the heating temperature to room temperature are defined. From the viewpoint of maintaining ductility before and after warm forming, it is desirable to apply a strain of 15% or less.
• strain applied by heating in a heating temperature range of 400 ° C. or more and 700 ° C. or less means equivalent plastic strain ( ⁇ ).
• ⁇ equivalent plastic strain
• the yield stress and total elongation after warm forming are less than 70% of the yield stress and total elongation at room temperature (before warm forming) before heating.
• strength and total elongation of the steel plate after warm forming will become inadequate.
• the yield stress and the total elongation of the steel sheet after cooling from the heating temperature to room temperature are subjected to thermoforming. 70% or more of the previous yield stress and total elongation at room temperature.
• the composition of the steel sheet is mass%, C: 0.03% or more and 0.14% or less, Si: 0.3% or less, Mn: more than 0.60%, 1.8% or less, P: 0.03% or less. , S: 0.005% or less, Al: 0.1% or less, N: 0.005% or less, Ti: 0.25% or less, with the balance being Fe and inevitable impurities, satisfying the following formulas (1) and (2)
• the steel sheet structure has a matrix having a ferrite grain size of 1 ⁇ m or more and a ferrite phase area ratio of 95% or more, and a structure in which carbides having an average grain size of 10 nm or less are precipitated in the matrix; It is preferable to do.
• the matrix of the steel sheet is substantially a ferrite single phase.
• the steel sheet matrix before heating to the warm forming temperature is substantially a ferrite single phase, it is heated to a heating temperature range (warm forming temperature) of 400 ° C. or more and 700 ° C. or less. Even so, the matrix of the steel sheet remains substantially in the ferrite single phase.
• the ductility increases, and the total elongation in the heating temperature range of 400 ° C. or more and 700 ° C. or less can be 1.1 times or more of the total elongation at room temperature.
• a steel sheet having the above composition when warm forming is performed in a temperature range of 400 ° C. or more and 700 ° C. or less, forming is performed while recovering dislocations, so that there is almost no decrease in ductility during warm forming. . Moreover, even if it cools to room temperature after warm forming, a structure change does not arise, Therefore The matrix of a steel plate is maintained with the ferrite single phase substantially, and shows the excellent ductility. Therefore, if the matrix of the steel sheet (before warm forming) is substantially a ferrite single phase, it is heated to a heating temperature range of 400 ° C. or more and 700 ° C. or less to give a strain of 20% or less.
• the total elongation of the steel sheet after cooling to room temperature can be 70% or more of the total elongation at room temperature before heat forming (before warm forming).
• the yield stress of the steel plate in the heating temperature range of 400 ° C. or more and 700 ° C. or less is 80% or less of the yield stress of the steel plate at room temperature.
• the ferrite particle size is preferably 1 ⁇ m or more. If the ferrite grain size is less than 1 ⁇ m, grain growth tends to occur during warm forming, and the material stability of the steel sheet after warm forming decreases. However, if the ferrite grain size becomes excessively large, it may be difficult to obtain a desired steel sheet strength due to a decrease in the amount of fine grain strengthening. Therefore, the ferrite particle size is preferably 15 ⁇ m or less. More preferably, it is 1 ⁇ m or more and 12 ⁇ m or less.
• the matrix of the steel sheet is made of a ferrite single phase.
• a bainite phase and a martensite phase which are hard phases
• the hardness difference between these hard phases and the ferrite phase is large, which may cause a decrease in warm formability.
• the ferrite single phase is substantially 95% or more of the area of the ferrite phase relative to the total area of the matrix, it is during warm forming and after warm forming. Sufficient ductility can be imparted to the steel sheet, and material change due to heat can be suppressed.
• examples of the metal structure other than the ferrite phase include cementite, pearlite, bainite phase, martensite phase, retained austenite phase, etc., and the total of these may be 5% or less in terms of the area ratio relative to the entire structure. Is acceptable.
• the matrix of the steel sheet before warm forming is substantially a ferrite single phase
• the ductility (total elongation) of the steel sheet can be sufficiently ensured during and after warm forming.
• fine carbides specifically Ti carbides, or further V carbides, Mo carbides, and W carbides are precipitated in a substantially single ferrite matrix to increase the strength of the steel sheet.
• the average particle diameter of the carbide exceeds 10 nm, the steel sheet cannot have a desired strength (tensile strength: 780 MPa or more). Therefore, the average particle diameter of the carbide is set to 10 nm or less. Preferably it is 7 nm or less.
• the carbide contained in the steel sheet is usually coarsened with heating and the precipitation strengthening ability is lowered.
• the heating temperature is 700 ° C. or less, it does not become coarse, and the average particle size is Maintained below 10 nm. That is, a steel sheet containing carbide (Ti carbide, or further V carbide, Mo carbide, W carbide) having an average particle diameter of 10 nm or less in a matrix of a ferrite single phase is heated to a heating temperature range of 400 ° C. to 700 ° C.
• a steel sheet structure containing carbide with an average particle size of 10 nm or less in a ferrite single phase matrix is applied to a heating temperature range of 400 ° C. or higher and 700 ° C. or lower, a maximum strain of 20% is applied.
• the yield stress of the steel sheet after cooling from the heating temperature to room temperature can be 70% or more of the yield stress at room temperature before heat forming (before warm forming).
• C 0.03% or more and 0.14% or less
• C is an essential element for forming Ti, or further carbides of V, Mo, and W and finely dispersing in steel to increase the strength of the steel sheet.
• the C content exceeds 0.14%, the toughness is significantly deteriorated, and a steel sheet having a good shock absorption capacity (for example, TS ⁇ El. TS: tensile strength, El: total elongation) is obtained. It cannot be obtained. Therefore, the C content is preferably 0.03% or more and 0.14% or less. More preferably, it is 0.04% or more and 0.13% or less.
• Si 0.3% or less
• Si is a solid solution strengthening element, which inhibits strength reduction in the heating temperature range and reduces warm formability. Therefore, Si is preferably reduced as much as possible, but up to 0.3% is acceptable. Therefore, the Si content is preferably 0.3% or less, and more preferably 0.1% or less.
• Mn More than 0.60% and 1.8% or less Mn is an element that contributes to strengthening by lowering the transformation point of steel and making it easy to obtain fine precipitates. Therefore, Mn is preferably contained in an amount exceeding 0.60%, and more preferably 0.8% or more. However, if the Mn content exceeds 1.8%, the workability of the steel sheet is remarkably deteriorated. Therefore, the Mn content is preferably 1.8% or less. Moreover, it is more preferable to set it as 1.5% or less.
• P 0.030% or less
• P is an element that has a very high solid-solution strengthening ability and prevents a reduction in steel sheet strength during warm forming. Furthermore, P segregates at the grain boundaries, and is also an element that decreases ductility during and after warm forming. Therefore, P is preferably reduced as much as possible, and is preferably 0.030% or less.
• S 0.005% or less
• S is a harmful element that exists as an inclusion in steel, and is an element that combines with Mn to form sulfides and lowers the ductility in the warm state. Therefore, S is preferably reduced as much as possible, and is preferably 0.005% or less.
• Al 0.1% or less
• Al is an element that acts as a deoxidizer, and in order to obtain such an effect, it is desirable to contain 0.02% or more.
• Al is an element that forms an oxide and lowers the ductility. If the Al content exceeds 0.1%, the influence of the ductility drop due to inclusions cannot be ignored, so the Al content is preferably 0.1% or less. Moreover, it is more preferable to set it as 0.07% or less.
• N 0.005% or less N is combined with Ti and V in the steelmaking stage to form coarse nitrides, so that the steel sheet strength is significantly reduced. Therefore, N is preferably reduced as much as possible, and is preferably 0.005% or less.
• Ti 0.25% or less
• Ti is an element that contributes to strengthening of steel sheets by forming carbides with C.
• Ti is an element that contributes to strengthening of the steel sheet by forming a carbide with C.
• the Ti content is preferably set to 0.01% or more.
• the Ti content is preferably 0.13% or more, and more preferably 0.15% or more, when the steel sheet strength is 780 MPa or more.
• the Ti content is preferably 0.25% or less. More preferably, it is 0.20% or less.
• V 0.5% or less
• Mo 0.5% or less
• W 1.0% or less
• V 0.5% or less
• Mo 0.5% or less
• W 1.0% or less
• V, Mo and W are elements that contribute to the strengthening of the steel sheet by forming carbides like Ti. Therefore, it can be contained arbitrarily when further strengthening of the steel sheet is required.
• the V content is 0.01% or more
• the Mo content is 0.01%
• the W content is It is preferable to set it as 0.01% or more.
• the V content is preferably 0.5% or less, and more preferably 0.35% or less.
• the Mo content and W content are preferably 0.5% or less and 1.0% or less, respectively, and more preferably 0.4% or less and 0.9% or less, respectively.
• the Ti content relative to the C content, or the V, Mo, and W contents are controlled.
• ([C] / 12) / ([Ti] / 48 + [V] / 51 + [Mo] / 96 + [W] / 184) is less than 0.8, the carbide constituent elements do not sufficiently precipitate as carbide.
• a steel sheet having a tensile strength at room temperature of 780 MPa or more cannot be obtained.
• the balance other than the above components is Fe and inevitable impurities.
• Inevitable impurities include elements not defined by the present invention, such as O (oxygen), Cu, Cr, Ni, Co, etc., and these contents are acceptable if the total content is 0.5% or less.
• the heating temperature is up to 700 ° C.
• the heat treatment affects the material. Will not affect. Therefore, it is possible to apply a plating treatment to the steel sheet and to provide a plating layer such as an electroplating layer, an electroless plating layer, a hot dipping layer, or the like on the surface thereof.
• the alloy component of a plating layer is not specifically limited, Zinc plating, alloying zinc plating, etc. are applicable.
• the steel sheet of the present invention exhibits excellent warm formability when subjected to a tensile equivalent strain of 20% or less in a heating temperature range of 400 ° C. or more and 700 ° C. or less, and after warm forming.
• the high-strength steel sheet for warm forming of the present invention melts molten steel having the above composition into a steel slab, and heats the steel slab to 1100 ° C. or higher and 1350 ° C. or lower, and then finish rolling temperature (hot rolling is performed).
• the method for melting steel is not particularly limited.
• steel having a desired component composition is manufactured by performing secondary refining in a vacuum degassing furnace after melting in a converter or electric furnace.
• the steel slab is formed by a conventionally known casting method, but it is preferably performed by a continuous casting method from the viewpoint of productivity and quality.
• the steel slab is heated according to the method of the present invention and then hot rolled.
• Heating temperature of steel slab 1100 ° C or higher and 1350 ° C or lower
• the heating temperature of the steel slab is set to 1100 ° C or higher and 1350 ° C or lower. Preferably they are 1150 degreeC or more and 1300 degrees C or less.
• the steel slab after casting is holding the said heating temperature (1100 degreeC or more and 1350 degrees C or less), you may carry out direct-rolling without heating a steel slab.
• the rough rolling conditions are not particularly limited.
• Finishing rolling temperature 820 ° C or higher If the finishing rolling temperature is lower than 820 ° C, the ferrite grains will be expanded, and a mixed grain structure with significantly different individual ferrite grain sizes will result. . Moreover, in order to obtain a structure having a ferrite grain size of 1 ⁇ m or more, it is necessary to prevent an excessive number of nucleation sites in the ferrite transformation, and the number of nucleation sites is closely related to the strain energy accumulated in the steel sheet during rolling. Here, if the finish rolling temperature is less than 820 ° C., accumulation of excessive strain energy cannot be prevented, and it becomes difficult to obtain a structure having a ferrite grain size of 1 ⁇ m or more. Therefore, the finish rolling temperature is 820 ° C. or higher. Preferably it is 860 ° C or more.
• the longer the time maintained at a high temperature the more likely the coarsening of the carbide by strain-induced precipitation proceeds. . Therefore, it is necessary to rapidly cool after finish rolling, and in order to suppress the coarsening of carbides, it is necessary to cool the temperature range from 820 ° C. or higher to the coiling temperature at an average cooling rate of 30 ° C./s or higher. Desirably, it is 50 ° C./s or more.
• Winding temperature 550 ° C. or more and 680 ° C. or less
• carbides precipitated in the steel sheet become insufficient, and the steel sheet strength decreases.
• the coiling temperature exceeds 680 ° C., the precipitated carbide is coarsened, so that the steel sheet strength is lowered. Therefore, the coiling temperature is set to 550 ° C or higher and 680 ° C or lower. Preferably they are 575 degreeC or more and 660 degreeC or less.
• the steel sheet obtained as described above may be plated to form a plating layer such as a hot dip galvanized layer or an alloyed hot dip galvanized layer on the steel sheet surface.
• the plating layer can be formed by a conventionally known adhesion method.
• the plating layer can be formed by immersing and pulling up a steel plate in a plating bath.
• the plating adhesion amount (plating layer thickness) varies depending on the immersion temperature and time of the plating bath, and the pulling speed, but the plating layer thickness is preferably 4 ⁇ m or more, and more preferably 6 ⁇ m or more.
• the alloying treatment for forming the alloyed hot-dip galvanized layer can be performed in a furnace capable of heating the surface of the steel sheet such as a gas furnace after the plating treatment.
• Steel plate No. 2 was plated without passing through a continuous hot-dip galvanizing line and the one formed with an alloyed hot-dip galvanized layer (test pieces No. b to e described in Table 3 to be described later). No layer was formed (test pieces No. f to h described in Table 3 to be described later).
• Samples are taken from the obtained hot-rolled steel sheet, tensile test, structure observation and precipitate observation, hole expansion test in the warm forming temperature range, tensile strength at room temperature, yield stress in the warm forming temperature range and After introducing the strain shown in Table 3 (maximum strain of 15%) in the total elongation and warm forming temperature range, the yield stress and total elongation after cooling to room temperature were determined. Further, a test piece is taken from the obtained hot rolled steel sheet, and the ferrite grain size before heating to the warm forming temperature range, the area ratio of the ferrite phase, the average particle size of the carbide, and the holes in the warm forming temperature range The expansion rate was calculated.
• the test method is as follows.
• test piece was collected in the same manner as described above, and subjected to a tensile test under the same conditions as in the high temperature tensile test. After introducing strains shown in Table 3 at each temperature, room temperature (22 ( ⁇ 5 ° C). Each test piece thus obtained was subjected to a tensile test at room temperature, and an average yield stress (YS-3), tensile strength (TS-3), and total elongation (El-3) were determined.
• YS-3 average yield stress
• TS-3 tensile strength
• El-3 total elongation
• test piece is heated to the temperature shown in Table 3 using an electric furnace so that the test piece temperature can be stably obtained within ⁇ 3 ° C. of the test temperature. After becoming, it was kept for 15 minutes.
• the average particle size of the carbide is 100 samples or more (100-300) by preparing a sample from the center of the thickness of the obtained hot-rolled steel sheet using a thin film method and observing it with a transmission electron microscope (magnification: 120,000 times). It was determined by the average of the particle size of the carbide particles. In calculating the carbide particle diameter, coarse cementite and nitride larger than micro order, that is, larger than 1 ⁇ m are not included.
• the steel plates of the invention examples all have a tensile strength (TS-1) at room temperature of 780 MPa or more.
• TS-1 tensile strength
• the yield stress (YS-2) when heated to a temperature range of 400 ° C to 700 ° C is 80% or less of the yield stress (YS-1) at room temperature, and the temperature range is 400 ° C to 700 ° C.
• the total elongation (El-2) when heated is at least 1.1 times the total elongation at room temperature (El-1).
• the steel sheets of the examples of the present invention all had a yield stress (YS-3) and total elongation (El-3) of room temperature when cooled to room temperature after giving a strain of 20% or less in the above heating temperature range.
• the yield stress (YS-1) and total elongation (El-1) were 70% or more (before strain introduction).
• the steel sheets of the inventive examples all had good warm formability.
• the steel plate of the comparative example (test piece No. f, h, i, j, k, l, m, n, t, u, v, w, x), that is, tensile strength at room temperature (TS-1) Yield stress (YS-2) or total elongation (El-2) when heated to a temperature range of 400 ° C to 700 ° C, and when cooled to room temperature after applying a strain of 20% or less in the above heating temperature range Any of the steel sheets whose yield stress (YS-3) or total elongation (El-3) was outside the scope of the present invention was poor in warm formability.
• TS-1 Yield stress
• El-2 total elongation
• the yield stress (YS-3) after cooling to room temperature is the yield stress (YS) at room temperature before heating.
• -1) is 70% or more of the total elongation (El-3) after cooling to room temperature is 70% or more of the total elongation (El-1) at room temperature before heating. The result was not satisfactory.
• test piece No. f which is a comparative example, has a high temperature tensile test temperature (heating temperature) of over 700 ° C, so that an austenite phase is generated during heating, and the carbide becomes coarse and machine after heating. The mechanical characteristics were significantly degraded.
• test piece No. h which is a comparative example, since the strain was applied too much, dislocations could not be recovered during heating, and the ductility was lowered when cooled to room temperature after heating.
• Test piece No. i which is a comparative example, has a low slab heating temperature, and test piece No. j has a low finish rolling temperature, so the tensile strength (TS-1) at room temperature did not reach 780 MPa. .
• Specimen No.k, l, m which is a comparative example, has a long average time after being subjected to finish rolling, or because the average cooling rate and coiling temperature are outside the scope of the present invention.
• the diameter exceeded 10 nm. Therefore, the tensile strength (TS-1) at room temperature did not reach 780 MPa.
• Test piece No. n which is a comparative example, had a low coiling temperature, so that sufficient carbide was not obtained, and the tensile strength (TS-1) at room temperature did not reach 780 MPa.
• carbides do not precipitate and contain a large amount of solute C, the solute C precipitates during strain aging during heating and prevents stress reduction and ductility increase during heating, and ductility when cooled to room temperature after heating. Declined.
• Specimen No.t which is a comparative example, does not satisfy the formula (2), and the balance of the contents of C, Ti, V, W, and Mo constituting the carbide is not appropriate, so the tensile strength at room temperature (TS -1) did not reach 780MPa.
• Test piece No. u which is a comparative example, had a low Mn content, so that carbide was precipitated and coarsened at a high temperature, so that the tensile strength (TS-1) at room temperature did not reach 780 MPa.
• Test piece No. v which is a comparative example, did not satisfy the formula (1), so the amount of precipitated carbide was insufficient, and the tensile strength (TS-1) at room temperature did not reach 780 MPa.
• Specimen No. w which is a comparative example, does not satisfy the formula (2) and has a large C content not involved with carbides. Therefore, strain aging occurs during heating during warm forming, and the heating temperature range (warm forming) The yield stress (YS-2) in the temperature range) was high, and the total elongation (El-2) in the heating temperature range (warm forming temperature range) was insufficient, making it unsuitable for warm forming.
• Test piece No. x which is a comparative example, has a large content of W, so that the ferrite transformation is delayed and the area ratio of the ferrite phase is small. For this reason, deterioration in mechanical properties at room temperature after heating is observed.
• the steel sheets of the present invention all have a tensile strength (TS-1) at room temperature of 780 MPa or more, and a yield stress (YS-2) when heated in a heating temperature range of 400 ° C to 700 ° C. ) Is less than 80% of the yield stress (YS-1) at room temperature, and the total elongation (El-2) when heated in a heating temperature range of 400 ° C to 700 ° C is the total elongation at room temperature (El-1)
• the yield stress (YS-3) and total elongation (El-3) when cooled to room temperature after applying a strain of 20% or less in the above heating temperature range are room temperature (El-3).
• the yield stress (YS-1) and total elongation (El-1) were 70% or more, respectively (before strain introduction).
• the steel sheet structure and the present invention example having the steel sheet composition described above as a preferable structure and composition are maintained in the heating temperature range of 400 ° C. or more and 700 ° C. or less, and a substantial ferrite single phase structure is maintained, and the state of carbides in the steel plate Does not change so as to affect the material of the steel plate. Therefore, after heating to a heating temperature range (warm forming temperature range) and performing warm forming, the cooling rate when cooling to room temperature has no effect on the material of the steel sheet after warm forming. Therefore, the high-strength steel sheet for warm forming according to the present invention can also be applied to warm forming equipment attached with a rapid cooling device for rapidly cooling the steel sheet after warm forming. Of course, the high-strength steel sheet for warm forming of the present invention can also be applied to warm forming equipment not accompanied by the quenching apparatus as described above.

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