Classifications

• • 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
  • 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
• • 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 concerns with steel sheets useful for warm press forming at a forming temperature range of 400°C to 700°C.
• the invention relates to a high-strength steel sheet for warm press forming which has a tensile strength (TS) at room temperature of not less than 780 MPa, which exhibits such a good ductility that the steel sheet can be worked even under severe forming conditions at the above forming temperature range, and which shows small changes in mechanical characteristics between before and after warm press forming, and to a method for manufacturing such steel sheets.
• TS tensile strength
• the automobile industry As a whole recently aims at improving the fuel efficiency of automobiles in order to reduce CO 2 emissions. Improvements in fuel efficiency can be attained most effectively by making automobiles lighter through reducing the thickness of parts to be used.
• the thinning of parts lowers the crashworthiness of automobiles and thus results in a decrease in safety.
• the weight reduction of automobile bodies entails that parts are reduced in thickness and are increased in strength. Because a lot of automobile parts are manufactured by forming steel sheets into desired shapes, however, higher strength of steel sheets being formed increases the probability of the occurrence of problems such as deterioration in shape fixability, overloads to molds, and the occurrence of cracks, necking and wrinkles.
• Patent Literature 1 proposes a technique in which a steel sheet is heated to an austenitic range, starts to be formed with a mold at a temperature of not less than the Ac 3 transformation point, and is quenched simultaneously with the forming by removing heat through the mold and is hardened by martensite transformation. This technique thus provides steel sheets exhibiting hardenability after hot press forming and excellent impact characteristics.
• Patent Literature 2 proposes a steel sheet for warm press forming which has a microstructure containing not less than 10% by volume of a bainite phase with a high solute carbon content and a high dislocation density, not more than 10% by volume of a total of a pearlite phase and a martensite phase, and the balance being a ferrite phase. It is described that when a steel sheet having this microstructure is subjected to warm press forming at temperatures of not less than 250°C, a large amount of strain aging hardening can be obtained during the forming as well as the subsequent cooling with the result that the warm press formed steel sheet exhibits markedly improved strength. Y. Funukawa et al.
• Steel sheets having a tensile strength at room temperature of not less than 780 MPa are very difficult to form into a desired shape by cold press forming because the steel sheets being formed still have high strength and low shape fixability to cause the occurrence of spring back. Further, such forming of steel sheets keeping high strength incurs a heavy load to the mold and shortens the life of the mold.
• the formed steel sheets exhibit poor ductility because the martensite phase which is hard and poor in ductility is utilized.
• forming of such steel sheets into a desired shape cannot produce automobile parts having high strength and excellent ductility.
• automobile parts are required to exhibit desired impact absorption performance in case of crash, automobile parts with insufficient ductility are problematic in that the impact absorption performance during crash is low.
• the technique proposed in Patent Literature 1 entails heating of steel sheets to an austenitic range during forming, mass production of automobile parts utilizing the technique has a concern that high energy costs are incurred in the forming step.
• warm press forming a steel sheet as a workpiece is heated before forming to lower the strength of the steel sheet and to increase the ductility so that the steel sheet is formed while deformation resistance is lowered and shape fixability is improved.
• warm press forming can suppress the occurrence of spring back and reduces the gall of the mold.
• the enhancement in ductility by heating allows steel sheets to be formed into complicated shapes. If tensile strength and ductility are not decreased after warm press forming, the impact absorption performance of formed parts is not deteriorated.
• warm press forming is advantageous also in terms of energy costs because the above effects are obtained by heating at a lower temperature than in the technique of Patent Literature 1.
• the microstructure of the steel sheet includes a bainite phase which is hard and poor in ductility.
• the strength of the steel sheet is increased by strain aging, and this further reduces the ductility and causes the problematic occurrence of cracks or mold damages during warm press forming.
• Patent Literatures 1 and 2 involve steel sheets including a martensite or bainite phase which is largely deteriorated in quality by heat. That is, when these steel sheets are subjected to coating treatments with heating such as hot-dip galvanization and galvannealing, the heat history due to such coating treatments causes a change in characteristics, for example, a decrease in the strength of the steel sheets.
• the present invention advantageously solves the above problems encountered in the art. It is an object of the invention to provide a high-strength steel sheet suited for warm press forming which is excellent in workability (formability) during warm press forming and is applicable to warm press forming even under severe conditions and which has a small change in quality by heat and thus ensures minor deteriorations in strength and ductility after warm press forming, as well as to provide a method for manufacturing such high-strength steel sheets and a method of use of such high-strength steel sheets.
• the present inventors carried out extensive studies on various factors that would affect the warm press formability (such as ductility and strength before, during and after heating) of high-strength steel sheets.
• the present inventors have found that as long as the yield stress at a prescribed heating temperature range (warm press forming temperature range) is not more than 80% of the yield stress at room temperature and the total elongation at the heating temperature range is not less than 1.1 times the total elongation at room temperature, even a high-strength steel sheet having a tensile strength at room temperature of not less than 780 MPa shows excellent warm press formability by exhibiting a lowered deformation resistance as well as an increased ductility at the warm press forming temperature range and can be formed into a complicated shape.
• the inventors have found that such steel sheets also exhibit excellent shape fixability. Furthermore, the inventors have found that strength and ductility required for automobile parts can be ensured even after warm press forming as long as steel sheets are such that the yield stress and the total elongation after the steel sheets are heated to the heating temperature range, subjected to a strain of not more than 20% and cooled to room temperature are respectively not less than 70% of the yield stress and the total elongation at room temperature before the heating.
• the present inventors then studied microstructures and chemical compositions that would allow steel sheets to exhibit the above characteristics.
• the present inventors focused on a ferrite phase having excellent ductility and a small change in quality by heat, and came up with a configuration in which the microstructure of a steel sheet is controlled to be substantially a ferrite single phase before, during and after warm press forming. Further, the present inventors have found that a steel sheet substantially composed of a ferrite single phase in which a dislocation movement in the ferrite phase is easily activated by heating achieves improvements in warm press formability and in shape fixability because such a steel sheet exhibits a lowered deformation resistance as well as an enhanced ductility when heated to a warm press forming temperature of not less than 400°C, and have further found that such a steel sheet exhibits excellent ductility even after warm press forming.
• the present inventors then arrived at the use of precipitation strengthening by the dispersion of fine carbides. Further, the present inventors have found that in order to improve warm press formability as well as strength and ductility after warm press forming, it is appropriate to increase the strength of steel sheets by precipitating fine titanium carbide or further vanadium carbide, molybdenum carbide and tungsten carbide in a matrix substantially composed of a ferrite single phase. According to the studies carried out by the present inventors, these carbides do not become coarse at a warm press forming temperature range (a heating temperature range) of not more than 700°C and remain finely precipitated even after warm press forming. That is, the present inventors have found that steel sheets exhibiting excellent strength even after warm press forming can be obtained by precipitating these carbides in a matrix substantially composed of a ferrite single phase.
• a warm press forming temperature range a heating temperature range
• the present inventors have found that in order to obtain the above desired microstructure of steel sheets, it is important to control the contents of the elements forming the carbides, namely, the content of titanium or the contents of titanium, vanadium, molybdenum and tungsten in appropriate ranges as well as to control the content of titanium or the contents of titanium, vanadium, molybdenum and tungsten relative to the content of carbon in an appropriate range. Furthermore, the present inventors have found that controlling the conditions in cooling and coiling after hot rolling in appropriate ranges is important in the production of steel sheets having the above desired microstructure, in particular, in order to suppress the coarsening of the carbides.
• the present invention has been completed based on the above findings and relates to a high-strength steel sheet according to claim 1, a method of working a high-strength steel sheet according to claim 4, a method for manufacturing high-strength steel sheets according to claim 5.
• Preferred embodiments of the steel sheet of the present invention are defined in claims 2 and 3. yield stress at room temperature before the heating, and the total elongation of the steel sheet after the steel sheet is heated to the heating temperature range, subjected to a strain of not more than 20% and cooled from the heating temperature to room temperature is not less than 70% of the total elongation at room temperature before the heating.
• high-strength steel sheets having excellent warm press formability can be obtained which have a tensile strength of not less than 780 MPa and can be warm press formed with a low press load into parts with complicated shapes.
• the high-strength steel sheets of the invention have minor decreases in strength and ductility after warm press forming, and are therefore suitable for applications such as automobile parts requiring impact absorption performance in case of crash.
• the high-strength steel sheets of the invention include a microstructure having a small change in quality by heat, and consequently the characteristics of the steel sheets are not substantially altered even when the steel sheets have a heat history due to treatments such as coating treatments.
• the inventive steel sheets may be also applicable to the manufacturing of parts required coating treatment from the viewpoint of corrosion resistance.
• the invention achieves marked industrial effects. Description of Embodiments
• High-strength steel sheets for warm press forming according to the invention are steel sheets having a tensile strength at room temperature of not less than 780 MPa.
• room temperature indicates 22 ⁇ 5°C.
• a high-strength steel sheet for warm press forming according to the invention is characterized in that the tensile strength at room temperature is not less than 780 MPa, the yield stress at a heating temperature range of 400°C to 700°C is not more than 80% of the yield stress at room temperature, the total elongation at the heating temperature range is not less than 1.1 times the total elongation at room temperature, the yield stress of the steel sheet after the steel sheet is heated to the heating temperature range, subjected to a strain of not more than 20% and then cooled from the heating temperature to room temperature is not less than 70% of the yield stress at room temperature before the heating, and the total elongation of the steel sheet after the steel sheet is heated to the heating temperature range, subjected to a strain of not more than 20% and then cooled from the heating temperature to room temperature is not less than 70% of the total elongation at room temperature before the heating.
• warm press forming at temperatures of 400°C to 700°C is assumed.
• the invention specifies characteristics of steel sheets at a heating temperature range of 400°C to 700°C.
• Warm press forming of a steel sheet often results in a decrease in the strength of the warm press formed steel sheet primarily due to heating of the steel sheet. Further, when a steel sheet is subjected to warm press forming, the ductility of the steel sheet after the warm press forming is sometimes lowered problematically due to the strain ag i ng or work hardening.
• the steel sheet In the warm press forming of a steel sheet into a (automobile) part, the steel sheet is usually strained about 1 to 10% in terms of equivalent plastic strain.
• the present invention assumes warm press forming at the temperature range of 400°C to 700°C with a strain of 20% at a maximum. That is, the present invention specifies the yield stress and the total elongation of a steel sheet after the steel sheet is heated to the heating temperature range of 400°C to 700°C, subjected to a strain of not more than 20% and then cooled from the heating temperature to room temperature. From the view point of maintaining ductility between before and after warm press forming, the strain applied is desirably not more than 15%.
• the "strain" applied to a steel sheet heated to the heating temperature range of 400°C to 700°C indicates an equivalent plastic strain ( ⁇ ) and is usually represented by the following equation as described in, for example, Non Patent Literature 1.
• ⁇ 2 3 ⁇ xx p 2 + ⁇ yy p 2 + ⁇ zz p 2 + 1 3 ⁇ xy p 2 + ⁇ yz p 2 + ⁇ zy p 2
• NPL 1 Husahito YOSHIDA, "Dansosei Rikigaku no Kiso (Basics of elastic plastic dynamics)", first edition, third printing, published by KYORITSU SHUPPAN CO., LTD., October 5, 1999, p. 155 .
• the present invention provides that the yield stress and the total elongation of a steel sheet after the steel sheet is heated to the heating temperature range of 400°C to 700°C, subjected to a strain of not more than 20% and then cooled from the heating temperature to room temperature are not less than 70% of the yield stress and the total elongation at room temperature before the thermal forming.
• the steel sheet have a chemical composition containing, in mass%, C: not less than 0.03% and not more than 0.14%, Si: not more than 0.3%, Mn: above 0.60% and not more than 1.8%, P: not more than 0.03%, S: not more than 0.005%, Al: not more than 0.1%, N: not more than 0.005%, and Ti: not more than 0.25%, the balance being Fe and inevitable impurities, and satisfying Expressions (1) and (2) below, as well as that the steel sheet include a microstructure which has a matrix having a ferrite grain diameter of not less than 1 ⁇ m and a ferrite phase area fraction of not less than 95% and in which a carbide having an average particle diameter of not more than 10 nm is precipitated in the matrix: Expressions Ti / 48 + V / 51 + Mo / 96 + W / 184 > 0.0031 0.8 ⁇ C / 12 / Ti / 48 + V /
• the matrix of a steel sheet be substantially a ferrite single phase.
• the matrix of the steel sheet before the steel sheet is heated to a warm press forming temperature is substantially a ferrite single phase
• the matrix of the steel sheet substantially remains a ferrite single phase even when the steel sheet is heated to the heating temperature range (warm press forming temperature range) of 400°C to 700°C.
• the ductility is increased as the steel sheet is heated so that the total elongation at the heating temperature range of 400°C to 700°C can be brought to not less than 1.1 times the total elongation at room temperature.
• configuring the matrix of a steel sheet (before warm press forming) to be substantially a ferrite single phase ensures that the total elongation of the steel sheet after the steel sheet is heated to the heating temperature range of 400°C to 700°C, subjected to a strain of not more than 20% and then cooled from the heating temperature to room temperature is not less than 70% of the total elongation at room temperature before the thermal forming (before the warm press forming).
• Heating the ferrite phase to not less than 400°C lowers the deformation resistance because a dislocation movement is activated with an increase in temperature, resulting in a decrease in the yield stress of the steel sheet.
• the yield stress of the steel sheet at the heating temperature range of 400°C to 700°C becomes not more than 80% of the yield stress of the steel sheet at room temperature.
• the ferrite grain diameter is preferably not less than 1 ⁇ m. If the ferrite grain diameter is less than 1 ⁇ m, grain growth easily occurs during warm press forming and the stability of the quality of the warm press formed steel sheet is deteriorated. If the ferrite grain diameter is excessively large, however, it may be sometimes difficult to obtain a desired strength of the steel sheet because the amount of grain refining strengthening is small. Thus, it is preferable that the ferrite grain diameter be not more than 15 ⁇ m, and more preferably not less than 1 ⁇ m and not more than 12 ⁇ m.
• the matrix of a steel sheet be a ferrite single phase. If hard phases such as bainite phase and martensite phase are mixed in the ferrite phase, warm press formability may be lowered because these hard phases and ferrite phase have a large difference in hardness. Even if the matrix is not a perfect ferrite single phase, however, the steel sheet can exhibit sufficient ductility during and after warm press forming and can be kept from a change in quality by heat as long as the matrix is substantially a ferrite single phase, that is, the area fraction of the ferrite phase is not less than 95% relative to the area of the entirety of the matrix.
• exemplary metallic microstructures other than the ferrite phase include cementite, pearlite, bainite phase, martensite phase and retained austenite phase.
• the presence of these phases is acceptable as long as the total area fraction thereof is not more than 5% relative to the entire microstructure.
• sufficient ductility (total elongation) of a steel sheet during and after warm press forming can be ensured by configuring the matrix of the steel sheet before the warm press forming to be substantially a ferrite single phase.
• the present invention aims at increasing the strength of the steel sheet by precipitating fine carbides, namely, titanium carbide or further vanadium carbide, molybdenum carbide and tungsten carbide in the matrix substantially composed of a ferrite single phase.
• the desired strength of the steel sheet tensile strength: not less than 780 MPa
• the average particle diameter of the carbides is specified to be not more than 10 nm, and preferably not more than 7 nm.
• Carbides present in a steel sheet are usually coarsened during heating and lower their precipitation strengthening performance.
• the above carbides titanium carbide or further vanadium carbide, molybdenum carbide and tungsten carbide
• having an average particle diameter of not more than 10 nm are not coarsened and maintain an average particle diameter of not more than 10 nm as long as the heating temperature is not more than 700°C.
• the steel sheet having a matrix which is substantially a ferrite single phase and which includes the carbides (titanium carbide or further vanadium carbide, molybdenum carbide and tungsten carbide) with an average particle diameter of not more than 10 nm is heated to the heating temperature range of 400°C to 700°C and warm press formed while a decrease in the strength of the steel sheet after the warm press forming is significantly suppressed because the coarsening of the carbides is suppressed.
• carbides titanium carbide or further vanadium carbide, molybdenum carbide and tungsten carbide
• the configuration in which the steel sheet has a microstructure having a matrix which is substantially a ferrite single phase and which includes the carbides with an average particle diameter of not more than 10 nm ensures that the yield stress of the steel sheet after the steel sheet is heated to the heating temperature range of 400°C to 700°C, subjected to a strain of up to 20% and then cooled from the heating temperature to room temperature is not less than 70% of the yield stress at room temperature before the thermal forming (before the warm press forming).
• this element is essential in order to increase the strength of steel sheets.
• the steel In order to obtain a steel sheet having a tensile strength of not less than 780 MPa, the steel preferably contains carbon in at least 0.03% or more.
• the C content exceeds 0.14%, toughness is markedly deteriorated and the steel sheet fails to exhibit good impact absorption performance (represented by, for example, TS ⁇ El wherein TS: tensile strength and El: total elongation).
• the C content is preferably not less than 0.03% and not more than 0.14%, and more preferably not less than 0.04% and not more than 0.13%.
• Silicon is a solid solution strengthening element and lowers warm press formability by inhibiting the decrease in strength at the heating temperature range. It is therefore preferable that silicon be reduced as much as possible. However, a Si content of up to 0.3% is acceptable. Thus, the Si content is preferably not more than 0.3%, and more preferably not more than 0.1%.
• Manganese is an element which contributes to strengthening by lowering the transformation point of steel and facilitating the occurrence of fine precipitates.
• the Mn content be in excess of 0.60%, and more preferably not less than 0.8%. If the Mn content exceeds 1.8%, however, the workability of steel sheets is markedly deteriorated.
• the Mn content is preferably not more than 1.8%, and more preferably not more than 1.5%.
• Phosphorus is an element which has very high solid solution strengthening performance and inhibits the decrease in the strength of steel sheets during warm press forming. Further, phosphorus is an element which segregates at grain boundaries to lower ductility during and after warm press forming. Thus, phosphorus is preferably reduced as much as possible, and the P content is preferably not more than 0.030%.
• Sulfur is a harmful element which is present as an inclusion in steel.
• this element bonds to manganese to form a sulfide and lowers ductility at warm temperatures.
• sulfur is preferably reduced as much as possible, and the S content is preferably not more than 0.005%.
• Aluminum is an element which acts as a deoxidizer.
• the Al content is preferably not less than 0.02%.
• aluminum lowers ductility by forming oxides. If the Al content exceeds 0.1%, the inclusions come to exert considerable adverse effects on ductility at warm temperatures.
• the Al content is preferably not more than 0.1%, and more preferably not more than 0.07%.
• nitrogen is preferably reduced as much as possible, and the N content is preferably not more than 0.005%.
• Titanium is an element which contributes to strengthening of steel sheets by forming a carbide with carbon. Titanium is an element which contributes to strengthening of steel sheets by forming a carbide with carbon. In order to obtain this effect, the Ti content is not less than 0.01%. In the case where vanadium, molybdenum and tungsten described later are not added, the Ti content is preferably not less than 0.13%, and more preferably not less than 0.15% in order to obtain a steel sheet strength of not less than 780 MPa. If the Ti content exceeds 0.25%, however, coarse TiC remains during the heating of a slab before hot rolling to cause the formation of microvoids. Thus, the Ti content is preferably not more than 0.25%, and more preferably not more than 0.20%.
• the steel may further contain one, or two or more of V: not more than 0.5%, Mo: not more than 0.5% and W: not more than 1.0% in addition to the basic chemical composition.
• V not more than 0.5%
• Mo not more than 0.5%
• W not more than 1.0%
• vanadium, molybdenum and tungsten are elements which contribute to strengthening of steel sheets by forming carbides.
• these elements may be optionally added in the case where a further increase in the strength of steel sheets is required.
• the V content be not less than 0.01%, the Mo content 0.01%, and the W content not less than 0.01%.
• any V content exceeding 0.5% causes the facilitated coarsening of the carbide.
• the carbide is coarsened at the heating temperature range of 400°C to 700°C and will hardly have an average particle diameter of not more than 10 nm after cooled to room temperature.
• the V content is preferably not more than 0.5%, and more preferably not more than 0.35%.
• the Mo content and the W content are preferably not more than 0.5% and not more than 1.0%, respectively, and more preferably not more than 0.4% and not more than 0.9%, respectively.
• [C], [Ti], [V], [Mo] and [W] are the contents (mass%) of the respective elements. In the case where [V], [Mo] and [W] are each less than 0.01% or the elements are absent, these contents are regarded as zero in the calculation using the above Expressions.
• the strength of the steel sheet is increased by precipitation strengthening in which carbides, specifically, titanium carbide or further vanadium carbide, molybdenum carbide and tungsten carbide, having an average particle diameter of not more than 10 nm are finely dispersed in the matrix.
• carbides specifically, titanium carbide or further vanadium carbide, molybdenum carbide and tungsten carbide, having an average particle diameter of not more than 10 nm are finely dispersed in the matrix.
• carbides specifically, titanium carbide or further vanadium carbide, molybdenum carbide and tungsten carbide, having an average particle diameter of not more than 10 nm are finely dispersed in the matrix.
• the steel sheet contains a large amount of solute carbon, strain aging occurs during warm press forming and the ductility of the steel sheet during and after the warm press forming is deteriorated. Further, the presence of hard and micrometer-order cementite in the steel sheet causes a decrease in the ductility of the steel sheet during and after warm press forming because microvoids are formed at the interface between the ferrite phase and the cementite during the warm press forming.
• a steel sheet with the above chemical composition in order for a steel sheet with the above chemical composition to have a tensile strength at room temperature of not less than 780 MPa, exhibit excellent ductility during warm press forming and achieve excellent strength and ductility after warm press forming, it is preferable that the fine carbides be actively precipitated in the steel sheet as well as that the amount of carbon which is not involved in the formation of carbides be controlled so as to reduce the amounts of solute carbon and cementite in the steel sheet to a minimum.
• the content of titanium or further the contents of vanadium, molybdenum and tungsten relative to the content of carbon are controlled.
• the balance after the deduction of the aforementioned elements is iron and inevitable impurities.
• the inevitable impurities include elements which are not specified in the present invention such as O (oxygen), Cu, Cr, Ni and Co. The presence of such elements is acceptable as long as the total content thereof is not more than 0.5%.
• the steel sheet having a matrix which is substantially a ferrite single phase and in which fine carbides are precipitated can be heat treated without suffering adverse effects on its quality by the heat treatment as long as the heating temperature is up to 700°C.
• the steel sheet can be subjected to a coating treatment to form, on its surface, a coating layer such as an electroplating layer, an electroless plating layer or a hot-dip plating layer.
• the alloy components forming the coating layers are not particularly limited, and zinc coatings and zinc alloy coatings may be used.
• the steel sheet of the invention can exhibit excellent warm press formability and can also exhibit excellent strength and ductility after the warm press forming when the steel sheet has been subjected to an equivalent tensile strain of not more than 20% at the heating temperature range of 400°C to 700°C.
• the high-strength steel sheet for warm press forming according to the invention is preferably made into a part such as an automobile part by being heated to the heating temperature range of 400°C to 700°C and being warm press formed through working which applies a strain of not more than 20%.
• the inventive high-strength steel sheet for warm press forming may be obtained by producing a molten steel having the aforementioned composition to made into a steel slab, heating the steel slab to a temperature of not less than 1100°C and not more than 1350°C, then hot rolling the steel slab to a steel sheet at a finishing temperature (the temperature of the steel sheet at the completion of the hot rolling) of not less than 820°C, starting cooling within 2 seconds after the hot rolling, cooling the steel sheet at an average cooling rate of not less than 30°C/s in the temperature range from a temperature of not less than 820°C to a coiling temperature, and coiling the steel sheet into a coil at a coiling temperature of not less than 550°C and not more than 680°C.
• the steel may be produced by melting by any method without limitation.
• a steel having the desired chemical composition may be produced by melting in a furnace such as a converter or an electric furnace, and by subsequent secondary refining in a vacuum degassing furnace.
• the molten steel is made into a steel slab by a known casting method, and preferably by a continuous casting method in view of productivity and quality. After being cast, the steel slab is heated and hot rolled in accordance with the inventive method.
• Temperature for heating steel slab not less than 1100°C and not more than 1350°C
• the temperature for heating the steel slab is specified to be not less than 1100°C and not more than 1350°C, and preferably not less than 1150°C and not more than 1300°C.
• the steel slab When the steel slab, that is after casting, has the above heating temperature (not less than 1100°C and not more than 1350°C), the steel slab may be directly rolled without being heated.
• the rough rolling In the practice of hot rolling of the steel slab by rough rolling and finish rolling, the rough rolling may be performed under any conditions without limitation.
• Finishing temperature not less than 820°C
• the finishing temperature is less than 820°C, elongation of ferrite grains occurs in the microstructure and further a mixed grain microstructure having ferrite grain diameters significantly different each other is generated, causing a marked decrease in the strength of steel sheets.
• the number of nucleation sites during ferrite transformation be not excessively large.
• the number of nucleation sites is closely related to the strain energy accumulated in the steel sheet during rolling. If the finishing temperature is less than 820°C, excessive accumulation of strain energy cannot be prevented and it becomes difficult to obtain a microstructure having a ferrite grain diameter of not less than 1 ⁇ m.
• the finishing temperature is specified to be not less than 820°C, and preferably not less than 860°C.
• Time from completion of hot rolling to initiation of cooling not more than 2 seconds
• Average cooling rate in temperature range from temperature of not less than 820°C to coiling temperature: not less than 30°C/s
• the coarsening of carbides generated by strain-induced precipitation proceeds easily as the steel is held at a high temperature for a longer time. It is therefore necessary that the steel be quenched after the finish rolling.
• the steel sheet needs to be cooled at an average cooling rate of not less than 30°C/s, and desirably not less than 50°C/s in the temperature range from a temperature of not less than 820°C to a coiling temperature.
• Coiling temperature not less than 550°C and not more than 680°C
• the coiling temperature is specified to be not less than 550°C and not more than 680°C, and preferably not less than 575°C and not more than 660°C.
• the characteristics of the steel sheet are not changed irrespective of whether the steel sheet has scales attached on its surface or the steel sheet has been descaled by pickling.
• the steel sheet obtained above may be subjected to a coating treatment to form, on the surface of the steel sheet, a coating layer such as a hot-dip galvanized layer or a galvannealed layer.
• the coating layer may be formed by a known coating method, for example, by dipping the steel sheet into a plating bath.
• the coating amount (the thickness of the coating layer) is variable depending on the temperature of the plating bath and the duration of soaking in the bath as well as the speed of lifting from the bath. It is preferable that the thickness of the coating layer be not less than 4 ⁇ m, and more preferably not less than 6 ⁇ m.
• An alloying treatment for forming a galvannealed layer may be carried out in a furnace capable of heating the surface of the steel sheet, such as a gas furnace, after the coating treatment.
• Test pieces were sampled from the obtained hot-rolled steel sheets and were subjected to a tensile test, microstructure observation, precipitate observation, and an enlarge test at a warm press forming temperature range to determine the tensile strength at room temperature, the yield stress and the total elongation at the warm press forming temperature range, and the yield stress and the total elongation after the test pieces had been subjected to a strain (up to 15% strain) described in Table 3 at the warm press forming temperature range and cooled to room temperature.
• test pieces were sampled from the obtained hot-rolled steel sheets and were analyzed to determine the ferrite grain diameter, the ferrite phase area fraction and the average particle diameter of carbides before the steels were heated to the warm press forming temperature range, as well as to determine the hole expanding ratio at the warm press forming temperature range. Testing methods were as described below.
• test pieces were sampled in the same manner as above and were subjected to a tensile test under the same conditions as those in the above elevated temperature tensile test to introduce a strain described in Table 3 at each of the temperatures; thereafter, the test pieces were cooled to room temperature (22 ⁇ 5°C) at a cooling rate described in Table 3. The resultant test pieces were tensile tested at room temperature to determine the average yield stress (YS-3), tensile strength (TS-3) and total elongation (El-3).
• test pieces were heated in an electric furnace to a temperature set out in Table 3 and were held for 15 minutes after the temperature of the test pieces became stable in the testing temperature ⁇ 3°C.
• Test pieces were sampled from the hot-rolled steel sheets. A central portion along the sheet thickness in a cross section (L-cross section) parallel to the rolling direction was etched with 5% Nital and the exposed microstructure was observed with a scanning electron microscope (SEM) at x400 magnification. Ten fields of view were photographed.
• SEM scanning electron microscope
• the (SEM) images of the microstructure obtained above were analyzed to separate the ferrite phase from other phases, and the area fraction of the ferrite phase relative to the observed fields of view was obtained. While the ferrite phase is characteristic in that corrosion marks are not observed in the grains and the grain boundaries are seen as smooth curves, grain boundaries observed as linear shape were counted as part of the ferrite phase.
• the ferrite grain diameter was measured by a linear intercept method in accordance with ASTM E112-10 with respect to the images of the microstructure obtained above.
• a sample was prepared by a thin-film method from a central portion along the sheet thickness of the hot-rolled steel sheet, and was observed with a transmission electron microscope (magnification: x120000), and the diameters of at least 100 particles (100 to 300 particles) of carbides were measured, the results being averaged.
• particles larger than the micrometer order namely, coarse cementite larger than 1 ⁇ m and nitrides were excluded.
• the enlarge test was carried out in accordance with standards by The Japan Iron and Steel Federation (T1001-1996).
• a 100 W ⁇ 100 L mm test piece was sampled from the hot-rolled steel sheet, and a 10 mm diameter hole was formed by punching in the center of the test piece with a clearance of 12%.
• the test piece was heated and soaked at 600°C in a heating furnace, and a cylindrical base as a punch was inserted into the hole of the test piece at 550 ⁇ 25°C.
• the hole in the test piece was enlarged until the hole expanding ratio calculated by Expression (3) below became 80%.
• each test piece was inspected for the presence or absence of a crack running through the edge face of the hole. Further, part of the test piece was cut after the test, and a central portion along the sheet thickness of the exposed cross section was subjected to a Vickers test.
• the testing load in the Vickers test was 1 kgf, and the hardness was measured with respect to 5 points.
• Warm press formability was evaluated to be good ( ⁇ ) when there was no crack running through the edge face of the hole and the Vickers hardness of the test piece was not less than 260 HV.
• Warm press formability was evaluated to be poor ( ⁇ ) when there was a crack running through the edge face of the hole or when the Vickers hardness of the test piece was less than 260 HV.
• the tensile strength at room temperature (TS-1) was not less than 780 MPa
• the yield stress of the steel sheet heated to the temperature range of 400°C to 700°C (YS-2) was not more than 80% of the yield stress at room temperature (YS-1)
• the total elongation of the steel sheet heated to the temperature range of 400°C to 700°C (El-2) was not less than 1.1 times the total elongation at room temperature (El-1).
• the yield stress (YS-3) and the total elongation (El-3) after the steel sheet was subjected to a strain of not more than 20% at the above heating temperature range and cooled to room temperature were each not less than 70% of the yield stress (YS-1) and the total elongation (El-1) at room temperature (before the introduction of the strain). Furthermore, all the steel sheets in Inventive Examples exhibited good warm press formability.
• the steel sheets in Comparative Examples that is, the steel sheets which fail to satisfy the inventive range in terms of any of the tensile strength at room temperature (TS-1), the yield stress (YS-2) or the total elongation (E1-2) of the steel sheet heated to the temperature range of 400°C to 700°C, and the yield stress (YS-3) or the total elongation (El-3) after the steel sheet was subjected to a strain of not more than 20% at the above heating temperature range and cooled to room temperature, exhibited poor warm press formability.
• the testing temperature (the heating temperature) in the elevated temperature tensile test for the test piece No. f in Comparative Example had exceeded 700°C, an austenite phase was formed and carbides became coarse during heating, resulting in a marked deterioration in mechanical characteristics after heating.
• the tensile strength at room temperature (TS-1) did not reach 780 MPa because of the low temperature for heating the slab and because of the low finishing temperature, respectively.
• the average particle diameter of carbides was above 10 nm because of the excessively long exposure to a high temperature after finish rolling or because the average cooling rate or the coiling temperature had been outside the inventive range. Consequently, the tensile strength at room temperature (TS-1) did not reach 780 MPa.
• the test piece No. w in Comparative Example failed to satisfy Expression (2) and contained a large amount of carbon which was not involved in the formation of carbides.
• Expression (2) contained a large amount of carbon which was not involved in the formation of carbides.
• strain aging occurred during heating for warm press forming, the yield stress at the heating temperature range (the warm press forming temperature range) (YS-2) was high, and the total elongation at the heating temperature range (the warm press forming temperature range) (El-2) was insufficient.
• the steel sheet was shown to be unsuited for warm press forming.
• the steel sheets corresponding to Inventive Examples were tensile tested in the same manner as described above (the elevated temperature tensile test and the tensile test after cooling to room temperature) to determine relations between mechanical characteristics (yield stress and total elongation) at the heating temperature range of 400 to 700°C as well as the mechanical characteristics after the steel sheets were subjected to a strain of not more than 20% at the heating temperature range and cooled to room temperature, and the mechanical characteristics at room temperature before heating.
• a tensile test was carried out at a testing temperature of 400°C or 650°C to determine the average yield stress (Y2-2) and total elongation (El-2); separately, test pieces were subjected to a tensile test at 400°C or 650°C in which a strain of not more than 20% described in Table 5 was applied to the test piece, and were thereafter cooled to room temperature at a cooling rate described in Table 5, and the resultant test pieces were tensile tested at room temperature to determine the average yield stress (YS-3) and total elongation (El-3). The results are described in Table 5.
• the tensile strength at room temperature (TS-1) was not less than 780 MPa
• the yield stress of the steel sheet heated to the heating temperature range of 400°C to 700°C (YS-2) was not more than 80% of the yield stress at room temperature (YS-1)
• the total elongation of the steel sheet heated to the heating temperature range of 400°C to 700°C (E1-2) was not less than 1.1 times the total elongation at room temperature (El-1)
• the yield stress (YS-3) and the total elongation (El-3) after the steel sheet was subjected to a strain of not more than 20% at the heating temperature range stated above and cooled to room temperature were each not less than 70% of the yield stress (YS-1) and the total elongation (El-1) at room temperature (before the introduction of the strain).
• the microstructures and the chemical compositions of the steel sheets were controlled to be the preferred microstructures and chemical compositions, the microstructures remain substantially a ferrite single phase at the heating temperature range of 400°C to 700°C, and the state of carbides in the steel sheets does not change at the heating temperature range to such an extent that the quality of the steel sheets is adversely affected.
• the steel sheets which have been heated to the heating temperature range (warm press forming temperature range) and subjected to warm press forming may be cooled to room temperature at any cooling rate without suffering any adverse effects on the quality of the steel sheets after warm press forming.
• inventive high-strength steel sheets for warm press forming can be applied to warm press forming in a facility fitted with a quenching apparatus which rapidly cools the steel sheets after warm press forming. It is needless to mention that the inventive high-strength steel sheets for warm press forming can also be applied to warm press forming in a facility which is not fitted with such a quenching apparatus.

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