WO2016131218A1 - 热冲压成形用钢板、热冲压成形工艺及热冲压成形构件 - Google Patents

热冲压成形用钢板、热冲压成形工艺及热冲压成形构件 Download PDF

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WO2016131218A1
WO2016131218A1 PCT/CN2015/079748 CN2015079748W WO2016131218A1 WO 2016131218 A1 WO2016131218 A1 WO 2016131218A1 CN 2015079748 W CN2015079748 W CN 2015079748W WO 2016131218 A1 WO2016131218 A1 WO 2016131218A1
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steel sheet
cooling
temperature
hot
forming
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PCT/CN2015/079748
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English (en)
French (fr)
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易红亮
杜鹏举
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重庆哈工易成形钢铁科技有限公司
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Priority to EP15882357.5A priority Critical patent/EP3260569B1/en
Priority to US15/551,325 priority patent/US10358690B2/en
Priority to ES15882357T priority patent/ES2837030T3/es
Priority to KR1020177024186A priority patent/KR101892661B1/ko
Priority to JP2017548470A priority patent/JP6475861B2/ja
Publication of WO2016131218A1 publication Critical patent/WO2016131218A1/zh

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    • 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
    • C21D9/48—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals deep-drawing sheets
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D—WORKING OR PROCESSING OF SHEET METAL OR METAL TUBES, RODS OR PROFILES WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D22/00—Shaping without cutting, by stamping, spinning, or deep-drawing
    • B21D22/02—Stamping using rigid devices or tools
    • B21D22/022—Stamping using rigid devices or tools by heating the blank or stamping associated with heat treatment
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D—WORKING OR PROCESSING OF SHEET METAL OR METAL TUBES, RODS OR PROFILES WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
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    • 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
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
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    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

Definitions

  • the present invention relates to a new steel sheet for hot stamping forming, a hot stamping forming process, and an ultra high strength and toughness forming member produced therefrom, and more particularly to a new steel sheet for hot stamping forming, which is subjected to hot stamping.
  • the forming process produces high strength and high toughness members for safety structural members and reinforcing members for automobiles, and other high strength and toughness members for automobiles.
  • thermoforming for manufacturing a vehicle component having a strength of 1000 MPa or more has been successfully developed and commercialized on a large scale.
  • the method is carried out by heating the steel sheet to an austenite region of 850-950 ° C and then placing it in a mold with a cooling system at a high temperature for forming at a temperature of only ⁇ 200 MPa.
  • the forming property is excellent, it can be formed into a complex component required for automobile design, and the resilience is small and the forming precision is high, and the steel plate is subjected to press hardening at the same time as the press forming, thereby obtaining a high martensite structure.
  • Strength forming member
  • US Pat. No. 6,296,805 B1 proposes a steel sheet for hot stamping forming coated with aluminum or aluminum-silicon alloy, which is in the base material during the hot stamping forming heating process.
  • the iron will diffuse to the aluminum coating layer and form an iron-aluminum alloy layer.
  • the austenitic heating temperature does not oxidize, and the steel sheet can be effectively protected from oxidation during the whole hot stamping forming process, and the coating layer is formed on the forming member.
  • the anti-corrosion performance of the service process has been improved, so it is widely used in commercial applications.
  • EP 1 143 029 proposes a method for producing a hot stamped steel member by using a zinc or zinc alloy coated with a hot rolled steel sheet to produce a hot stamped steel sheet, but the zinc plating layer has a lower melting point of about 780 ° C during the hot forming process. The evaporation of zinc and the melting of the zinc-iron coating occur, which leads to liquid-induced brittleness and reduces the strength of the hot-formed steel.
  • Patent CN103392022 proposes a hot stamping forming technology based on the quenching carbon partitioning process, which can obtain higher strength and elongation; however, it usually needs to control the cooling temperature in the range of 100-300 °C, and the temperature uniformity control of the parts is difficult. And the temperature control process is high in the heat treatment process, the production process is complicated, which is not conducive to the actual production of the hot stamped forming member; the austenitizing heat treatment temperature is high, which is not conducive to the hot stamping of the zinc plated plate, and the energy consumption high.
  • Patent CN101545071 proposes a new hot stamped steel sheet, which can reduce the austenitizing heating temperature by ⁇ 50 °C, which can reduce the manufacturing cost to a certain extent, but the toughness of hot stamped steel and conventional 22MnB5 There was no significant improvement in the comparison of hot stamping forming materials.
  • An alloy design and a press forming method which can reduce the hot stamping forming temperature are proposed in the patent CN102127675B.
  • the method comprises heating the material to 730-780 ° C under the condition of lowering the hot stamping forming temperature and pressing and cooling to 30-150 ° C below the Ms point (ie, usually cooling to 150-280 ° C), and further heating to 150-450 ° C.
  • the carbon is transferred from martensite to untransformed austenite to stabilize it to the final state.
  • the ductility of the material is improved based on the TRIP effect of retained austenite, but the material yields when the elongation exceeds 10%. The strength did not exceed 1150 MPa.
  • the method is required to cool the component to a specific temperature of 150-280 ° C and then increase the temperature to 150-450 ° C for heat preservation, which makes the component temperature accuracy and uniformity difficult to control, or requires a complicated production process to control it.
  • the quenching temperature is not conducive to the actual production of hot stamped forming members.
  • An object of the present invention is to provide a steel sheet for hot stamping forming, a hot stamping forming process and a forming member thereof, wherein the martensite transformation temperature of the steel sheet is low, thereby ensuring quenching at a lower temperature to obtain a member. Ultra high strength and toughness match.
  • the martensite transformation starting temperature point (Ms) of the material design is controlled to be 280 ° C or less, and the quenching temperature is usually set to be less than 150 ° below the martensite transformation starting temperature point.
  • the member after the press forming is directly cooled and cooled at 150 to 260 ° C (that is, usually cooled to 0 to 100 ° C), and then separately heated and insulated to ensure the super high strength and toughness matching of the stamping member, and the mechanical mechanical properties thereof are attained.
  • the tensile strength is above 1600MP
  • the yield strength is above 1200MPa
  • the elongation is more than 10%.
  • a steel sheet for hot stamping comprising, by weight percentage, 0.18 to 0.42% of C, 4 to 8.5% of Mn, and 0.8 to 3.0% of Si+A1. And a balance of Fe and unavoidable impurities, wherein the alloy composition of the steel sheet satisfies its
  • the actual measured value of the martensitic transformation start temperature after press forming is ⁇ 280 °C.
  • the smaller retained austenite fraction is not conducive to improving the ductility of the component, while the excessively high retained austenite volume fraction leads to a decrease in austenite stability, which causes the TRIP effect to occur earlier during tensile deformation or collision deformation. It is not conducive to improving the toughness of the component.
  • the present invention sets the quenching temperature of the formed member to a temperature in the range of 0 to 100 ° C in order to obtain a reasonable stability and a reasonable volume fraction.
  • the high-strength toughness member of the retained austenite, the steel alloy composition of the present invention is designed to satisfy the martensite start phase transition temperature point ⁇ 280 °C.
  • the steel sheet of the present invention is based on a high Mn design and has a Mn content of 4 to 8.5%, preferably 5 to 7.5%.
  • Manganese can reduce the initial temperature of martensite transformation.
  • the design of manganese and carbon in the steel of the present invention is designed to reduce the temperature at which the martensite begins to change to below 280 ° C to ensure the cooling conditions of the components after hot stamping.
  • the member can also retain a reasonable volume fraction of retained austenite to improve the mechanical properties of the component, for example, at room temperature or warm water quenching.
  • Manganese can reduce the austenitizing temperature of steel for hot stamping forming, so that the austenitizing heating temperature in the hot stamping process of galvanizing hot stamping can be lower than 780 °C, inhibiting liquefaction and severe oxidation of zinc, avoiding liquid state Zinc causes cracking and also saves energy due to the reduced austenitizing temperature. Since manganese has an excellent effect of inhibiting the transformation of austenite to ferrite, a high Mn content can improve the hardenability of steel.
  • the excessive manganese content that is, the content exceeding 8.5%, causes the material to form brittle martensite after quenching, thereby reducing the ductility of the steel sheet, so the upper limit of manganese is not too large, preferably 8.5%. . Applicants have found that setting the Mn content between 4 and 8.5% provides the best combination of high hardenability and high toughness.
  • the steel sheet further comprises at least one of the following components: 5% or less of Cr; 2.0% or less of Mo; 2.0% or less of W; 0.2% or less of Ti; and 0.2% or less of Nb; 0.2% or less of Zr; 0.2% or less of V; 2.0% or less of Cu and 4.0% or less of Ni; and 0.005% or less of B.
  • Applicants have found that by combining at least one of these components with the above-described basic components, the austenitizing temperature of the steel is lowered and the martensite start phase transition temperature point is lowered to below 280 ° C or the prior austenite is refined.
  • the grain size can further ensure the ultra-high strength and toughness matching of the stamping member, so that the mechanical properties reach a tensile strength of 1600MP or more, a yield strength of 1200 MPa or more, and an elongation of more than 10%.
  • the steel sheet comprises a hot rolled steel sheet, a cold rolled steel sheet, or a steel sheet with a coated layer.
  • the coated steel plate may be a zinc coated steel plate, which is in A hot-rolled steel sheet or a cold-rolled steel sheet on which a metal zinc layer is formed.
  • the zinc-coated steel sheet includes one selected from the group consisting of hot dip galvanizing (GI), galvannealing (GA), zinc plating, or zinc-iron plating (GE).
  • the steel sheet with a coating layer is a hot-rolled steel sheet or a cold-rolled steel sheet on which an aluminum-silicon layer is formed, or an organic-plated steel sheet or a steel sheet with other alloying plating layer.
  • a hot stamping forming process comprising the steps of: a) providing a steel sheet of any of the components described in the above first aspect or a preformed member thereof; b) The steel sheet or its preformed member is heated to 700 to 850 ° C; c) transferring the heated steel sheet or its preformed member into a mold for press forming to obtain a shaped member; and d) cooling the shaped member It is 150 to 260 ° C below the temperature at which the martensite transformation starts.
  • any cooling method can be used as long as the temperature of the shaped member can be lowered to 150 to 260 ° C below the martensite transformation starting temperature point, for example, by cooling in a mold or in the air. Cooling or cooling with water of 0 to 100 ° C, that is, there is no limitation on the cooling method.
  • the cooling temperature may preferably be room temperature or even lower.
  • the steel sheet heating temperature of the present invention is maintained at 700 to 850 ° C, and it is possible to ensure that the galvanized sheet can also be subjected to hot stamping and even indirect hot stamping.
  • the lower heating temperature can greatly save energy and reduce the equipment cost associated with high temperature heating.
  • the quenching temperature is greatly reduced as compared with the conventional temperature in the art (for example, 150-280 ° C in the above-mentioned patent CN102127675B), and can be controlled below 100 ° C, so that the cooling control method can be further improved.
  • the initial austenite content before tempering of the member can be controlled to 23% or less.
  • the tempering heat treatment step may be further performed, that is, heating to 160 to 450 ° C and then maintaining the temperature for 1 to 100,000 seconds, and then cooling to room temperature by any cooling method and condition to optimize the The microstructure and properties of the shaped member, the martensite portion of the phase transformation is re-transformed into austenite to increase the austenite fraction to not more than 32%, and then carbon can also occur from martensite to austenite.
  • the tempering heat treatment step may be performed after the shaped member subjected to the quenching step is left for a period of time, that is, the tempering heat treatment step does not have to be followed by quenching
  • the fire steps are carried out. It should be understood by those skilled in the art that since the QP (quenching partitioning) process in the prior art is required to control the quenching temperature to a temperature above 100 degrees, the forming member must be maintained at a temperature not lower than the quenching temperature. Immediately heating directly to a dispensing temperature above 250 °C, such operation is not conducive to process implementation and line layout.
  • the quenching temperature can be lowered to below 100 ° C in the present invention, for example, it can be controlled to room temperature or lower
  • the tempering heat treatment step of the present invention can be carried out without quenching, for example, at room temperature for any length of time.
  • the tempering heat treatment is then carried out, which is beneficial to the production line layout, process and production rhythm arrangement of the actual hot stamping forming industrial production.
  • the components after hot stamping can be tempered at any location, such as a heat treatment plant away from the hot stamping line, or a component transport process, or in a car assembly line.
  • a forming member prepared by the steel sheet of any one of the above-mentioned first aspects by any one of the above-described second aspects of the hot stamping forming process, wherein
  • the microstructure of the shaped member by volume after the above step d) comprises: 3% to 23% of retained austenite, less than 10% of ferrite, the remainder being martensite, or containing less than 2%. carbide.
  • the forming member may also be subjected to the tempering heat treatment after the above step d), in which case the microstructure of the forming member by volume includes: 7% to 32% of retained austenite, and 10% or less of ferrite The body, the remainder is martensite, or contains less than 2% of carbides, thereby obtaining a shaped member having a yield strength of ⁇ 1200 MPa, a tensile strength of ⁇ 1600 MPa, and a total elongation of ⁇ 10%.
  • the forming member can be used for at least one of an automotive safety structure, a reinforced structural member, and a high strength automotive structural member. More specifically, the forming member may be used for at least one of a B-pillar reinforcement, a bumper, and a door impact beam, a wheel spoke. Of course, the forming member can also be used in other land vehicles where lightweight high strength or high strength and high ductility members are required.
  • a heat treatment method for improving the toughness of a hot stamped forming member comprising: heating any one of the above steel sheets or a preformed member thereof to 700 to 850 ° C, Then, press forming is performed to obtain a formed member, wherein the steel sheet or its preformed member is maintained in this temperature range for a time of 1 second to 10000 seconds; the formed member is cooled to below the martensite transformation start temperature point 150 to 260 ° C, the cooling method includes cooling in the mold, air cooling, using water of 0 to 100 ° C, cooling rate of 0.1 to 1000 ° C / sec; reheating the formed member after cooling to a temperature range lower than or equal to Ac1 Performing a tempering heat treatment and maintaining the shaped member at this temperature range for 1 second to 100,000 seconds; Cool to room temperature by any cooling method and conditions.
  • the quenching temperature can be controlled at a temperature below 100 ° C (which can be achieved by hot water quenching), and the beneficial effects of uniform temperature, convenient and easy control are obtained, and heat energy can be saved, and the quenching with high temperature quenching can be reduced. Equipment costs.
  • the steel sheet of the invention has a low austenitizing temperature and a low quenching temperature, and can be less than 100 ° C, which is more favorable for temperature control, uniform temperature of components, consistency of structural properties and energy conservation.
  • the amount of austenite will increase significantly under the preferred condition, and the newly formed austenite will be obviously beneficial to improve the strong plasticity of the steel.
  • the steel of the invention obtains a higher yield strength, and its yield strength reaches 1200 MPa or more, and the high yield strength is an important index for improving the performance of the automobile safety structural member.
  • the steel sheet of the present invention obtains an ultra-high strength plastic product, a yield strength of 1200 MPa or more, and a tensile strength of 1600 MPa under the condition of achieving high hardenability of the steel sheet. Above, the elongation is 10% or more.
  • 1a and 1b are changes in the amount of retained austenite of the hot rolled sheet of the steel of the present invention
  • Figure 3 is a microstructure of an embodiment of the steel of the present invention after heat treatment according to the present invention
  • Figure 4 shows the microstructure of a typical slat distribution of the steel of the invention after heat treatment according to the invention.
  • the present invention provides a galvannealed direct hot stamped steel sheet and a formed member of the steel sheet, and provides a method of producing the formed member, and a heat treatment method for improving the toughness of the hot stamped forming member.
  • the forming member may have a yield strength of 1200 MPa or more and a tensile strength of 1600 MPa or more and an elongation of 10% or more.
  • the method of producing a shaped member has a low heating temperature and can greatly save energy.
  • the galvanized steel sheet can be used for direct hot stamping and maintains sufficient strength. Quenching to martensite transformation starting temperature when producing shaped members Below 150 degrees 260 ° C, it can be cooled by air cooling to room temperature or by quenching with warm water. The temperature is uniform and convenient and easy to control.
  • Carbon is the cheapest strengthening element and can strongly increase the strength of steel through gap solid solution. And the increase in carbon content will strongly reduce Ac3, thereby reducing the heating temperature and saving energy.
  • carbon can strongly reduce the martensite transformation start temperature, it must meet the requirements of martensite transformation starting temperature ⁇ 280 ° C and steel structure, and carbon is the most important interstitial solid solution strengthening element. For this reason, the lower limit of the carbon content is 0.18%.
  • the upper limit of carbon is set to 0.42%. A preferred value is from 0.22 to 0.38%.
  • Manganese is an important element in the present invention.
  • Manganese is a good deoxidizer and desulfurizer.
  • Manganese is an austenite stabilizing element that expands the austenite region and lowers the Ac3 temperature.
  • Manganese has an excellent effect of suppressing the transformation of austenite to ferrite and improving the hardenability of steel.
  • Chromium improves oxidation resistance and corrosion resistance and is an important alloying element in stainless steel. Chromium is a medium-strong carbide forming element, which not only improves the strength and hardness of steel through solid solution strengthening, but also has a slower diffusion rate in austenite and hinders the diffusion of carbon, thereby improving the austenite stability. Improve the hardenability of steel.
  • the percentage of manganese and chromium in the steel is determined according to the requirements for the martensitic transformation starting temperature in the alloy design and the carbon content in the steel.
  • One or both of manganese and chromium are added in combination.
  • the lower limit of manganese is limited to 4% to ensure the martensite transformation starting temperature ⁇ 280 ° C, while ensuring the material's full austenitizing temperature (Ac3) ⁇ 730 ° C to ensure its plating
  • the zinc plate can be hot stamped.
  • the addition of too high manganese causes brittle martensite to form after the material is quenched, so the upper limit of manganese is set to 8.5%.
  • the composite addition of chromium and manganese can further reduce the martensitic transformation starting temperature and the full austenitizing temperature of the material, and the ability to lower the martensite transformation starting temperature and the full austenitizing temperature is weaker than that of the manganese pair.
  • its cost is higher than manganese, so the upper limit is limited to 5%.
  • the preferable value of Mn is 4.5 to 7.5%, and Cr is preferable because the cost is high.
  • Both silicon and aluminum can inhibit the formation of carbides.
  • silicon and aluminum can inhibit the precipitation of carbides in the martensite, and the carbon is distributed into the retained austenite. Improve the austenite stability and increase the strong plastic product of steel.
  • Too little addition of Si and Al The precipitation of carbides during hot stamping cannot be sufficiently suppressed, so the lower limit of Si+Al is 0.8%.
  • too much Al will block the nozzle during continuous casting, increasing the difficulty of continuous casting, and Al will increase the martensitic transformation start temperature and full austenitizing temperature of the material, which does not conform to the structural control of the steel of the present invention.
  • the temperature is required, so the upper limit of Al is 1.5%.
  • High silicon content causes more impurities in the steel, the upper limit of Si is 2.5%, and the upper line of Si+Al is set to 3.0%.
  • a preferred value of Si is 0.8 to 2%, and a preferred value of Al is less than 0.5%.
  • phosphorus is a harmful element in steel, which increases the cold brittleness of steel, deteriorates weldability, reduces plasticity, and deteriorates cold bending performance. Sulfur is usually also a harmful element.
  • the steel is made to be hot brittle and reduce the ductility and weldability of the steel.
  • Nitrogen is an inevitable element found in steel. Nitrogen acts like carbon and contributes to baking hardening.
  • Mo and W improve the hardenability of steel and can effectively increase the strength of steel. Further, even in the case where the steel sheet is insufficiently cooled due to unstable contact with the mold during the high-temperature forming process, the steel can have a suitable strength due to the improved hardenability by Mo and W. In the case of more than 2.0%, no additional effect can be obtained, but the cost will increase. Since the high Mn design of the steel of the present invention has high hardenability, it is preferable to add Mo and W in order to reduce the cost.
  • Ti, Nb, Zr, and V refine the grain of the steel, increase the strength, and obtain good heat treatment characteristics. Too low a concentration of Ti, Nb, Zr, and V does not work, and more than 0.2% increases unnecessary cost. Since the steel of the present invention has a design of reasonable C and Mn, strength of more than 1600 MPa and good ductility can be obtained, and in order to reduce cost, it is preferable not to additionally add Ti, Nb, Zr, V.
  • Cu improves strength and toughness, especially atmospheric corrosion.
  • the content of Cu is more than 2.0%, the processability may be deteriorated, the hot rolling process may form a liquid phase to cause cracking, and a high Cu content causes an unnecessary cost increase.
  • Ni improves the strength of steel while maintaining good ductility and toughness.
  • a concentration of Ni greater than 4.0% increases the cost. Since the steel of the present invention is designed to have a reasonable C and Mn, a strength of more than 1600 MPa and a good ductility can be obtained, and in order to reduce the cost, it is preferable not to additionally add Cu or Ni.
  • B segregates at the austenite grain boundary, prevents the nucleation of ferrite, and can strongly improve the hardenability of steel. After heat treatment, the strength of the steel can be significantly increased. A B content higher than 0.005% does not significantly improve the function. Due to the high Mn design of the steel of the present invention, high hardenability has been obtained, and in order to reduce the cost, it is preferable not to additionally add B.
  • An object of the present invention is to produce a steel sheet having a yield strength of 1200 MPa or more, a tensile strength of 1600 MPa or more, and an elongation of 10% or more.
  • the steel sheets include hot rolled steel sheets, cold rolled steel sheets, and galvanized steel sheets.
  • the microstructure by volume before tempering treatment includes: 3% to 23% of retained austenite, less than 10% (including 0%) of ferrite, the rest being martensite, or including less than 2%. carbide. And the steel sheet can be directly hot stamped by galvanizing.
  • a method of manufacturing a molded article will be described below.
  • the steel sheet is subjected to press working, and the steel sheet is heated to 700 to 850 ° C before hot stamping, and then hot pressed, preferably 730 to 780 ° C.
  • the preform of the steel sheet it is heated to a temperature of 700 to 850 ° C after cold rolling, preferably 730 to 780 ° C.
  • the stamped steel sheet is then cooled by means of in-mold cooling or air cooling or other cooling methods to a temperature below the martensitic transformation starting point of 150 to 260 ° C, preferably between room temperature and 100 ° C.
  • the microstructure of the formed part by volume includes: 3% to 23% of retained austenite, 10% or less (including 0%) of ferrite, and the rest is martensite, or includes 2% or less. Carbide. Too much retained austenite will result in insufficient stability. If the amount of martensite is too high, the amount of retained austenite will decrease. The higher the formed carbide will reduce the carbon content in the austenite and cause instability. To the elongation required by the present invention. Deformation-induced ferrite may occur during hot forming, and the amount of ferrite to achieve the desired strength should not exceed 10%.
  • the press-formed product is tempered at a temperature of 160 to 450 ° C for 1 to 10,000 seconds, and then cooled to room temperature.
  • the microstructure by volume of the formed part after tempering at this time includes: 7% to 32% of retained austenite, 10% or less (including 0%) of ferrite, the remainder is martensite, or includes 2%.
  • the following carbides During the tempering process, the distribution of carbon from martensite to austenite occurs to stabilize the austenite, so that the member has a reasonable austenite volume fraction and stability in the steel in the final use state to obtain high strength. toughness. It should be particularly pointed out that according to the tempering heat treatment process of the present invention, the austenite volume percentage in the steel may be improved by more than 2% before tempering.
  • the alloy composition design of the steel in the present invention is required to satisfy the actual measured value of the martensitic transformation starting temperature of steel ⁇ 280 °C.
  • the addition of alloying elements will significantly reduce the austenitizing temperature of the steel.
  • the steel sheet or the preformed member is subjected to press forming after heating to 700 to 850 ° C, preferably 730 to 780 ° C, wherein the time during which the steel sheet is maintained in this temperature range is 1 second to 10000 seconds.
  • cooling methods include cooling in the mold, air cooling, hot water or cold water cooling, other cold However, the cooling rate is 0.1 to 1000 ° C / sec.
  • the member after the press forming cooling is reheated to a temperature range lower than or equal to Ac1, and the tempering heat treatment is performed, and the steel sheet is maintained in the temperature range, wherein the holding steel sheet is in the temperature range of 1 second to 10000 seconds. . It is then cooled to room temperature by any cooling method and conditions. If the holding time is less than 1 second, the carbon may not be sufficiently diffused into the retained austenite, and above 10000 seconds, the austenite may be excessively softened to lower the strength of the steel sheet, failing to meet the design requirements.
  • the partitioning of carbon from martensite to austenite occurs to stabilize the austenite and improve the toughness of the steel; preferably, the volume of retained austenite in the steel after low temperature tempering treatment
  • the percentage will increase significantly, increasing by more than 2% before tempering.
  • the newly formed austenite will significantly improve the plasticity of the steel and help to hinder the crack propagation, thereby greatly increasing the strong plastic product of the steel.
  • the experiment conducted with the steel sheet of the present invention is described below.
  • the ingots of the components identified in Table 1 were homogenized at 1200 ° C for 10 h, and then held at 1000 ° C to 1200 ° C for 1 h, followed by hot rolling to form hot rolled sheets.
  • the hot rolled sheet or the hot rolled pickled sheet can be kept at 600 to 700 ° C for 5 to 32 hours, and the simulated hood annealing is used to reduce the strength of the hot rolled sheet to facilitate cold rolling.
  • the hot-rolled pickling plate or the hot-rolled pickling annealed sheet is cold rolled to 1.5 mm.
  • the numbers IS1 to IS11 are steels of the present invention
  • CS1 to CS5 are comparative steels, and the components thereof are components described in the prior art.
  • the steel sheets of the above components were subjected to hot stamping using the process parameters shown in Table 2. Specifically, the steel sheet of the present invention or its preformed member is heated in an oven to 700 to 850 ° C (AT) for 10 minutes. It is then transferred to a mold for hot stamping and the shaped member is air cooled or otherwise cooled to below 100 ° C (QT). After a period of time, the treated shaped member is heated to 180 to 500 ° C (TT) for a period of time for tempering, and then cooled to room temperature. Further, the comparative steel sheets were subjected to forming and heat treatment in accordance with the parameters of the prior art hot stamping forming process in Table 3.
  • IS is the steel of the present invention
  • AT is the austenitizing temperature
  • TT is the tempering temperature
  • Ms is the martensite transformation starting temperature.
  • the equilibrium temperatures Ae1 and Ae3 in the table are calculated from the thermodynamic software Thermal-cal according to the composition of the steel.
  • Table 4 The number IS in Table 4 still indicates the steel of the present invention, and CS indicates the steel for comparison. Further, YS represents yield strength, TS represents tensile strength, TE represents elongation, HR is hot rolled steel, and CR is cold rolled steel. Further, the tensile specimens in Table 4 were ASTM standard specimens having a gauge length of 50 mm, and the strain rate of the tensile mechanical properties test was 5 ⁇ 10 -4 .
  • the steel sheet having the composition of the present invention can obtain a molded member excellent in strength and elongation comprehensive performance by the hot stamping forming process of the present invention.
  • the yield strength ⁇ 1200 MPa, the tensile strength ⁇ 1600 MPa, and the total elongation ⁇ 10% can be achieved.
  • the steel sheet having the prior art composition has a poor overall performance after the hot stamping forming process of the prior art, and the yield strength is less than 1200 MPa after the elongation exceeds 10%. Since the yield strength is an important parameter for measuring the performance of the automotive safety structural member, the steel sheet of the present invention obtains a comprehensive performance far superior to the prior art by the formed member obtained after the hot stamping forming process of the present invention.
  • analysis of the microstructure of the steel of the present invention shows that the microstructure by volume in the absence of tempering heat treatment includes: 3% to 23% of retained austenite, less than 10% of ferrite, and the rest is martensite. Or contain less than 2% of carbides.
  • the microstructure of the shaped member by volume includes: 7% to 32% of retained austenite, less than 10% of ferrite, the remainder being martensite, or containing less than 2%. carbide.
  • Figure 1a shows the tendency of the retained austenite amount of the hot rolled sheet of the steel of the present invention to vary with tempering time at the same temperature, i.e., 250 °C.
  • Figure 1b shows the tendency of the retained austenite amount of the hot rolled sheet of the steel of the present invention to vary with tempering time at the same temperature, i.e., 300 °C.
  • Fig. 2a shows the amount of change in retained austenite of the cold-rolled sheet of the steel of the present invention under different heat treatment processes at 250 °C.
  • Fig. 2b shows the amount of change in retained austenite of the cold-rolled sheet of the steel of the present invention under different heat treatment processes at 300 °C.
  • the content of retained austenite in the steel sheet of the present invention is generally on an increasing trend under different tempering processes.
  • a smaller residual austenite fraction is not conducive to improving the ductility of the component, while a higher residual austenite volume fraction causes austenite to form a coarse blocky structure, which occurs during tensile deformation or collision deformation.
  • the TRIP effect changes into a brittle martensitic mass structure with high hardness, which is not conducive to improving the ductility of the member. Therefore, the present invention controls the martensite to start the phase transition temperature point of 280 ° C or less, and the quenching temperature is 150 to 260 ° C below the martensite transformation starting temperature point, thereby ensuring a reasonable austenite volume fraction and slats. (or film) form.
  • Figure 3 shows the microstructure tempered at 300 °C for 5 minutes after austenitizing treatment, while Figure 4 shows the microstructure of a typical slat distribution.

Abstract

冲压成形用钢板、热冲压成形工艺及热冲压成形构件。所述热冲压成形用钢板包括０．１８～０．４２％的Ｃ、４～８．５％的Ｍｎ、０．８～３．０％的Ｓｉ＋Ａｌ以及余量的Ｆｅ和不可避免的杂质，其中所述钢板的合金成分满足其在热冲压成形后的马氏体相变开始温度实际测量值≤２８０℃。热冲压成形构件的制造方法包括将材料加热到７００～８５０℃后进行冲压成形后经冷却至马氏体相变开始温度点以下１５０～２６０℃，再将该已经冲压成形的构件加热至１６０～４５０℃保温１～１０００００秒的回火热处理后冷却至室温。成形构件的屈服强度≥１２００ＭＰａ，抗拉强度≥１６００ＭＰａ，且总延伸率≥１０％。

Description

热冲压成形用钢板、热冲压成形工艺及热冲压成形构件 技术领域
本发明涉及一种新的热冲压成形用钢板、热冲压成形工艺、及由其制得的超高强韧性成形构件，更具体而言，涉及一种新的热冲压成形用钢板，其通过热冲压成形工艺制造高强度高韧性的构件，用于汽车用安全结构件和增强构件、及其他汽车用高强韧构件。
背景技术
节能、安全、环保是当今世界汽车发展的主题，而汽车轻量化对节约资源、环境保护、促进汽车安全性的提高有重要作用。在减重的同时为保障安全性，高强度钢的使用就成为必然趋势。但通常钢铁材料的强度提高会导致成形性能降低，不容易成形为汽车设计所需要的形状复杂的构件；同时高强度钢成形回弹严重，导致其冲压成形构件尺寸精度控制困难；且高强度钢铁材料冷冲压成形过程模具磨损严重，导致其冲压成本提高。
为解决高强钢的冲压成形问题，一种称为热冲压成形或热成形、用来制造具有1000MPa或更高的强度的车辆构件的成形方法被成功开发并大规模商业化应用。该方法的实施步骤是：把钢板加热到850-950℃的奥氏体区，然后放进带有冷却系统的模具中在高温下进行冲压成形，该温度下材料仅有～200MPa的强度和高于40％的延伸率，成形性能极佳，可成形为汽车设计所需的复杂构件，且回弹小成形精度高，冲压成形的同时对钢板进行模压淬火，从而得到全马氏体组织的高强度成形构件。
裸钢在热成形过程中会发生氧化，这将影响钢的表面质量，并且会对模具造成影响。而常规钢板的锌镀层技术不能满足热冲压成形工艺条件，美国专利US 6296805 B1提出了一种涂镀铝或铝-硅合金的热冲压成形用钢板，在热冲压成形加热过程中基体材料中的铁会扩散至铝镀层并形成铁铝合金层，奥氏体化加热温度下该铁铝合金层不发生氧化，可在整个热冲压成形过程中有效保护钢板不发生氧化，且该镀层对成形构件服役过程的防腐性能有一定提高，因此被大批量商业应用。但与常规的锌镀层钢板相比，铝硅镀层不能提供电化学腐蚀保护。EP1143029中提出了一种使用锌或锌合金涂镀热轧钢板而制得的镀锌钢板来制造热冲压成形构件的方法，但是锌镀层的熔点较低大约在780℃左右，在热成形过程中会发生锌的蒸发和锌铁镀层的融化，这会导致液态诱导脆性，降低热成形钢的强度。
专利CN103392022中提出了一种基于淬火碳配分工艺提出的热冲压成形技术，能获得较高的强度和延伸率；但其通常需要控制冷却温度在100-300℃区间，零件温度均匀性控制较难，并且对热处理过程中温度的控制要求较高，生产工艺复杂，不利于热冲压成形构件的实际生产；其奥氏体化热处理温度较高，不利于锌镀层板的热冲压成形，且能量消耗高。
专利CN101545071中提出了一种新的热冲压成形钢板，将奥氏体化加热温度可降低～50℃，这在一定程度上可适当降低制造成本，但是其热冲压成形钢的强韧性与常规22MnB5热冲压成形材料对比没有显著提升。
专利CN102127675B中提出了一种可降低热冲压成形温度的合金设计及冲压成形方法。该方法包括在降低热冲压成形温度的条件下将材料加热至730～780℃并冲压冷却至Ms点以下30～150℃(即通常冷却到150～280℃)后，进一步加热至150～450℃保温1～5分钟，经碳从马氏体向未转变的奥氏体配分使其稳定至最终状态，基于残余奥氏体的TRIP效应提高材料延性，但其延伸率超过10％时的材料屈服强度均未超过1150MPa。该方法须将构件冷却到150～280℃的某一特定温度后再升高温度至150～450℃进行保温，造成构件温度精确度与均匀性难以控制，或需要复杂的生产工艺过程来控制其淬火温度，不利于热冲压成形构件的实际生产。
发明内容
本发明的一个目的在于提供一种热冲压成形用钢板、热冲压成形工艺及其成形构件，其钢板马氏体相变开始温度较低，从而能确保在较低温度下进行淬火，以便获得构件的超高强韧性匹配。在本发明的热冲压成形工艺中，将材料设计的马氏体相变开始温度点(Ms)控制为小于等于280℃，通常将淬火温度设定为马氏体相变开始温度点以下150～260℃，从而能通过例如空气中冷却或者0～100℃的冷水、温水或热水进行淬火，之后再另行进行加热保温，因此温度控制简单易行，温度均匀且精确，组织性能一致性好。本发明中将冲压成形后的构件直接冷Ms点以下150～260℃(即通常冷却至0～100℃)后再另行进行加热保温，可确保冲压构件的超高强韧性匹配，其机械力学性能达到抗拉强度1600MP以上，屈服强度达到1200MPa以上，同时延伸率达到10％以上。
根据本发明的第一方面，提供了一种用于热冲压成形的钢板，所述钢板以重量百分比计包括0.18～0.42％的C、4～8.5％的Mn、0.8～3.0％的Si+A1，以及余量的Fe和不可避免的杂质，其中所述钢板的合金成分满足其在热 冲压成形后的马氏体相变开始温度的实际测量值≤280℃。因较小的残余奥氏体分数不利于改善构件延性，而过高的残余奥氏体体积分数会导致奥氏体稳定性降低，使其在拉伸变形或碰撞变形过程中较早发生TRIP效应而不利于提高构件的强韧性，为获得合理稳定性及合理体积分数的残余奥氏体，须设计合理的马氏体开始相变温度和对应的淬火温度。为使构件可以通过例如空气冷却或者0～100℃的水进行淬火冷却，本发明将成形构件的淬火温度设定为0～100℃区间的某一温度，为获得含有合理稳定性及合理体积分数残余奥氏体的高强韧性构件，本发明设计钢板合金成分满足其马氏体开始相变温度点≤280℃。
本发明的钢板基于高Mn设计，Mn含量在4～8.5％之间，优选为5～7.5％。锰可降低马氏体相变起始温度，本发明钢种的锰与碳的配合设计旨在将材料马氏体开始相变温度点降低至280℃以下，以保证热冲压后构件的冷却条件能在设定为例如室温冷却或者温水淬火的情况下，构件亦能保留合理体积分数的残留奥氏体，以提高构件机械性能。锰可以降低热冲压成形用钢奥氏体化温度，使的镀锌热冲压成形用钢热冲压工艺过程中奥氏体化加热温度可以低于780℃，抑制锌的液化及严重氧化，避免液态锌致开裂，同时亦因降低的奥氏体化温度而节约能源。由于锰具有优良的抑制奥氏体向铁素体转变的作用，因此高Mn含量能提高钢的淬透性。然而申请人发现过高的锰含量，即含量超过8.5％后，会导致材料在淬火后形成脆性的ξ马氏体，从而降低钢板的延性，因此锰的上限不易过大，最好在8.5％。申请人发现将Mn含量设定在4～8.5％之间，能取得高淬透性和高强韧性的最佳组合。
根据本发明的一个优选实施例，所述钢板进一步包含以下成分中的至少一种：5％以下的Cr；2.0％以下的Mo；2.0以下％的W；0.2％以下的Ti；0.2％以下的Nb；0.2％以下的Zr；0.2％以下的V；2.0％以下的Cu及4.0％以下的Ni；0.005％以下的B。申请人发现，通过这些成分中至少一种与上述基本成分的组合，将降低钢的奥氏体化温度并进一步确保马氏体开始相变温度点降低至280℃以下或细化原奥氏体晶粒尺寸，从而能进一步确保冲压构件的超高强韧性匹配，使得机械力学性能达到抗拉强度1600MP以上，屈服强度达到1200MPa以上，同时延伸率达到10％以上。
根据本发明的一个优选实施例，所述钢板包括热轧钢板、冷轧钢板、或带有涂镀层的钢板。所述带有涂镀层的钢板可为锌涂镀钢板，它是在其 上形成金属锌层的热轧钢板或冷轧钢板。所述锌涂镀钢板包括选自热浸镀锌(GI)、镀锌退火(GA)、锌电镀或锌-铁电镀(GE)中的一种。所述带有涂镀层的钢板为在其上形成铝硅层的热轧钢板或冷轧钢板，或者有机镀层的钢板、或者带有其他合金化镀层的钢板。
根据本发明的第二方面，还提供了一种热冲压成形工艺，其包括以下步骤：a)提供上述第一方面中所述的任一种成分的钢板或其预成形的构件；b)将所述钢板或其预成形的构件加热到700～850℃；c)将加热后的钢板或其预成形的构件转移到模具中进行冲压成形以得到成形构件；以及d)将所述成形构件冷却至马氏体相变开始温度点以下150～260℃。本领域的技术人员应理解的是，只要能使成形构件的温度降低至马氏体相变开始温度点以下150～260℃，则可以使用任何冷却方法，例如经模具内冷却、或在空气中冷却、或用0～100℃的水进行冷却，即冷却方法没有任何限制。冷却温度可以优选为室温，甚至可以更低。本发明的钢板加热温度保持在700～850℃，能够确保镀锌板也能进行热冲压成形，甚至能进行间接热冲压成形。此外，该加热温度较低，可大大节约能源，亦降低与高温加热配套的设备成本。根据本发明的热冲压成形工艺，淬火温度与本领域的惯用温度(例如以上提及的专利CN102127675B中的150-280℃)相比大幅降低，能控制在100℃以下，使得冷却控制方法能够更灵活，例如用空气冷却或者用0～100℃的水进行(即热水淬火可实现)，这样能使水这种最廉价且最易控制的淬火介质得以应用在热冲压成形工艺中，从而获得温度均匀、方便易控的有益效果。此外还能节约热能，降低与高温淬火配套的设备成本。此外，通过本发明的热冲压成形工艺，能将构件回火前的初始奥氏体含量控制在23％以下。
根据本发明的一个优选实施例，在步骤d)之后还可以进行回火热处理步骤，即加热到160～450℃然后保温1～100000秒后再通过任意冷却方式和条件冷却至室温，以优化所述成形构件的组织和性能，实现已经相变的马氏体部分再重新相变为奥氏体以增加奥氏体分数至不超过32％，然后还能发生碳从马氏体到奥氏体中的配分(partition)以稳定奥氏体，从而获得屈服强度≥1200Mpa、抗拉强度≥1600MPa而且总延伸率≥10％的成形构件。
根据本发明的一个优选实施例，上述回火热处理步骤可以在经过淬火步骤的成形构件放置一段时间之后进行，即回火热处理步骤不必紧接着淬 火步骤进行。本领域的技术人员应该理解的是，由于现有技术中的QP(淬火配分)工艺须将淬火温度控制在100度以上某一温度，为保持构件温度不低于该淬火温度，须将成形构件立即直接加热至250℃以上的配分温度，这样的操作不利于工艺实施以及产线布置等。相比之下，由于本发明中淬火温度可降低到100℃以下，例如可控制为室温或更低，因此本发明的回火热处理步骤可不必紧接着淬火进行，例如可在室温下放置任意时长后再进行回火热处理，这有利于实际热冲压成形工业生产的产线布置、工艺与生产节奏安排等。此外，热冲压成形之后的构件可在任何场所进行回火热处理，例如远离热冲压生产线的热处理车间、或构件运输过程、或在汽车总装线等。
根据本发明的第三方面，还提供了一种成形构件，所述成形构件由上述第一方面的任一种成分的钢板经上述第二方面的任一种热冲压成形工艺制备而成，其中所述成形构件在经过上述步骤d)之后以体积计的微观组织包括：3％至23％的残余奥氏体，10％以下的铁素体，其余为马氏体，或包含2％以下的碳化物。此外，所述成形构件还可以受到上述步骤d)之后的回火热处理，此时所述成形构件以体积计的微观组织包括：7％至32％的残余奥氏体，10％以下的铁素体，其余为马氏体，或包含2％以下的碳化物，从而获得屈服强度≥1200Mpa、抗拉强度≥1600MPa而且总延伸率≥10％的成形构件。
根据本发明的一个优选实施例，所述成形构件可以用于汽车安全结构件、增强结构件、和高强韧汽车结构件中的至少一种。更具体而言，所述成形构件可以用于B柱增强件、保险杠、和车门防撞梁、车轮轮辐中的至少一种。当然，所述成形构件也可以用于其它所有陆用车辆中要求轻质的高强度或高强度加高延性的构件的场合。
根据本发明的第四方面，还提供了一种提高热冲压成形构件的强韧性的热处理方法，所述热处理方法包括：将上述任一种钢板或其预成形的构件加热至700～850℃，然后进行冲压成形以得到成形构件，其中将所述钢板或其预成形的构件保持在此温度范围的时间为1秒至10000秒；将所述成形构件冷却至马氏体相变开始温度点以下150～260℃，冷却方法包括模具中冷却、空冷、用0～100℃的水进行，冷却速率为0.1至1000℃/秒；将冷却之后的成形构件再加热至低于或者等于Ac1的温度范围进行回火热处理，并使所述成形构件保持在此温度范围达1秒至100000秒；以及再 通过任意冷却方式和条件冷却至室温。通过本发明的热处理方法，淬火温度能控制在100℃以下的某一温度(热水淬火可实现)，获得了温度均匀、方便易控的有益效果，此外还能节约热能，降低与高温淬火配套的设备成本。此外，还能实现已经相变的马氏体部分再重新相变为奥氏体以增加奥氏体分数，但通常不超过32％，然后还能发生碳配分以稳定奥氏体。
根据本发明的技术方案，至少能获得以下优点：
1.相对于现有技术，本发明的钢板的奥氏体化温度低，并且淬火温度低，可小于100℃，更利于温度的控制、构件温度均匀与组织性能一致性和节约能源。
2.基于成分设计，在回火碳配分的过程中，优选情况下奥氏体的量会明显的增加，新生成的奥氏体将明显有利于提高钢的强塑性。
3.相对于现有技术的直接淬火工艺，本发明钢获得了更高的屈服强度，其屈服强度达到1200MPa以上，而高屈服强度是提高汽车安全结构件性能的重要指标。
4.相对于常规热冲压成形用钢板，本发明的钢板在实现钢板高的淬透性的条件下，其热冲压成形构件获得了超高的强塑积，屈服强度1200MPa以上，抗拉强度1600MPa以上，延伸率10％以上。
附图说明
图1a和1b是本发明钢的热轧板残余奥氏体量的变化；
图2a和2b是本发明钢的冷轧板残余奥氏体量的变化；
图3是本发明钢的实施例的在根据本发明的热处理之后的微观结构；
图4示出了本发明钢在根据本发明的热处理之后的典型板条分布的微观组织。
具体实施方式
下面将参考实施例更详细的描述本发明。实施例旨在解释本发明的示例性的实施方案，且本发明不限于这些实施例。
本发明提供了一种可镀锌直接热冲压成形的钢板和所述钢板的成形构件，并提供了一种生产所述成形构件的方法，以及一种提高热冲压成形构件强韧性的热处理方法。所述成形构件可具有1200MPa以上屈服强度和1600MPa以上的抗拉强度以及10％以上的延伸率。所述生产成形构件的方法的加热温度低，可大大节约能源。所述镀锌钢板可以用于直接热冲压成形，并保持足够的强度。在生产成形构件时淬火至马氏体相变开始温 度点以下150～260℃，可通过空冷至室温或采用温水淬火的方式冷却，温度均匀，方便易控。
本发明所述钢的化学成分(以重量％计)限定的原因如下：
C：0.18％至0.42％
碳是最便宜的强化元素，可以通过间隙固溶强烈提高钢的强度。并且碳含量的升高会强烈降低Ac3，从而降低加热温度节约能源。虽然碳能强烈的降低马氏体相变开始温度，但是必须要符合合金设计时马氏体相变开始温度≤280℃和钢的组织的要求，且碳是最重要的间隙固溶强化元素，为此碳含量的下限为0.18％。但是过高的碳含量会导致钢的焊接性能不好，而且可能会引起板材强度过高而韧性下降。为此碳的上限设为0.42％。优选值为0.22～0.38％。
Mn：4％至8.5％，Cr：5％以下
锰是本发明中的重要元素。锰是良好的脱氧剂和脱硫剂。锰是奥氏体稳定元素，能扩大奥氏体区域，降低Ac3温度。锰具有优良的抑制奥氏体向铁素体转变而提高钢的淬透性的作用。铬能提高抗氧化性和耐腐蚀性，是不锈钢中的重要合金元素。铬是中强碳化物形成元素，其不仅能通过固溶强化提高钢的强度和硬度，而且其在奥氏体中的扩散速度较慢，并阻碍碳的扩散，因而提高了奥氏体稳定性，提高钢的淬透性。铬的含量提高能使淬火后的残余奥氏体量大幅增加。根据合金设计中关于马氏体相变开始温度的要求，以及钢中的碳含量，确定钢中锰和铬的百分含量。锰和铬这两种元素的其中一种或者复合添加。为了降低热处理时的加热温度，把锰的下限定为4％，以确保马氏体相变开始温度≤280℃，同时保证材料的全奥氏体化温度(Ac3)≤730℃以确保其镀锌板可以进行热冲压成形。过高的锰的添加，会导致材料淬火后形成脆性的ξ马氏体，因此锰的上限设为8.5％。铬与锰的复合添加，可进一步降低材料马氏体相变开始温度及全奥氏体化温度，其降低马氏体相变开始温度及全奥氏体化温度的能力与锰对比较弱，且其成本较锰高，因此限定其上限为5％。Mn的优选值为：4.5～7.5％，Cr因成本较高，优选值可不添加。
Si+Al：0.8％至3.0％
硅和铝都能抑制碳化物的形成，在钢淬火至室温后在低于Ac1温度范围保温时，硅和铝能够抑制马氏体中碳化物的析出，而使碳配分到残余奥氏体中，提高奥氏体稳定性，提高钢的强塑积。过少的Si、Al的添加， 不能充分抑制热冲压成形过程中碳化物的析出，因此Si+Al的下限为0.8％。工业生产时，过多的Al会在连铸时阻塞喷嘴，增加连铸的难度，且Al会提高材料马氏体相变开始温度及全奥氏体化温度，不符合本发明钢的组织控制温度要求，因此Al的上限定为1.5％。硅含量高会造成钢中的杂质较多，Si的上限定为2.5％，Si+Al的上线设定为3.0％。Si的优选值为0.8～2％，Al的优选值为小于0.5％。
P、S、N难以避免杂质
在一般情况下，磷是钢中的有害元素，会增加钢的冷脆性，使焊接性变坏，降低塑性，使冷弯性能变坏。硫通常情况下也是有害元素。使钢产生热脆性，降低钢的延性和焊接性能。氮是一种不可避免的存在于钢中的元素。氮与碳的作用类似，并有助于烘焙硬化。
Mo、W：2.0％以下
Mo、W提高钢的淬透性可有效提高钢的强度。此外，即使在由于在高温成形过程中与模具不稳定接触而使钢板冷却不充分的情况下，由于Mo和W带来的提高的淬透性，钢仍可有合适的强度。大于2.0％的情况下，也不能获得额外的效果，反而会增加成本。因本发明钢的高Mn设计，已具备较高的淬透性，为降低成本，优选可以不必另外添加Mo、W。
Ti、Nb、Zr、V：0.2％以下
Ti、Nb、Zr和V使钢的晶粒细化、强度升高并且获得良好的热处理特性。Ti、Nb、Zr和V的浓度过低则起不到作用，而大于0.2％则会增加不必要的成本。因本发明钢因合理的C和Mn的设计，可获得超过1600MPa的强度和较好的延性，为降低成本，优选可以不必另外添加Ti、Nb、Zr、V。
Cu：2.0％以下，Ni：4％以下
Cu能提高强度和韧性，特别是大气腐蚀性能。Cu的含量大于2.0％，则加工性可能劣化，热轧过程可形成液相导致开裂，且高的Cu含量导致不必要的成本增加。Ni能提高钢的强度，而又保持良好的塑性和韧性。Ni的浓度大于4.0％，则会增加成本。因本发明钢因合理的C和Mn的设计，可获得超过1600MPa的强度和较好的延性，为降低成本，优选可以不必另外添加Cu、Ni。
B：0.005％以下
B在奥氏体晶界偏析，阻止了铁素体的形核，可强烈提高钢的淬透性， 热处理后可显著提高钢的强度。B含量高于0.005％也不能明显提高起作用。因本发明钢的高Mn设计，已具备较高的淬透性，为降低成本，优选可以不必另外添加B。
本发明的一个目的是生产屈服强度1200MPa以上，抗拉强度1600MPa以上，以及10％以上延伸率的钢板。钢板包括热轧钢板、冷轧钢板和镀锌钢板。其在回火处理前以体积计的微观组织包括：3％至23％的残余奥氏体，10％以下(包含0％)的铁素体，其余为马氏体，或包括2％以下的碳化物。并且用所述钢板可以镀锌直接热冲压成形。
下面将描述成形件的制造方法。将所述钢板进行冲压加工，在热冲压前将所述钢板加热到700～850℃后进行热压，优选为730～780℃。对于所述钢板的预成形件，在冷冲压之后使之加热到700～850℃温度，优选为730～780℃。随后将冲压成形的钢板经模具内冷却或空冷或其他冷却方法冷却至马氏体相变开始温度点以下150～260℃，优选冷却至室温至100℃之间。此时所述成形件的以体积计的微观组织包括：3％至23％的残余奥氏体，10％以下(包含0％)的铁素体，其余为马氏体，或包括2％以下的碳化物。残余奥氏体太多会造成其稳定性不够，而马氏体量太高则残余奥氏体量减少，形成的碳化物较高则会降低奥氏体中碳含量造成其不稳定，达不到本发明要求的延伸率。在热成形过程中可能会发生形变诱导铁素体，为达到所要强度铁素体的量不应超过10％。
此后将冲压成形件在160～450℃温度范围内保温1～10000秒的回火处理后冷却至室温。此时回火后成形件的以体积计的微观组织包括：7％至32％的残余奥氏体，10％以下(包含0％)的铁素体，其余为马氏体，或包括2％以下的碳化物。回火处理过程中发生碳从马氏体到奥氏体中的配分以稳定奥氏体，使构件最终使用状态下在钢中具有合理的奥氏体体积分数和稳定性，以获得高的强韧性。须特别指出的是，根据本发明的回火热处理工艺，钢中的奥氏体体积百分数较回火前可能实现2％以上的提高。
本发明中钢的合金成分设计要求满足钢的马氏体相变开始温度的实际测量值≤280℃。合金元素的添加将明显降低钢的奥氏体化温度。钢板或预成形的构件加热至700～850℃后进行冲压成形，优选为730～780℃，其中所述保持钢板在此温度范围的时间为1秒至10000秒。将其冷却至马氏体相变开始温度点以下150～260℃，优选为冷却至100℃以下至室温或者更低温度，冷却方法包括模具中冷却、空冷、热水或冷水冷却、其他冷 却等，冷却速率为0.1至1000℃/秒。将其冲压成形冷却之后的构件再加热至低于或者等于Ac1的温度范围进行回火热处理，并使钢板保持在此温度范围，其中所述保持钢板在此温度范围的时间为1秒至10000秒。之后再以任意冷却方式和条件冷却至室温。如果保持时间低于1秒，则碳可能不能充分扩散到残余奥氏体中，而高于10000秒则可能使奥氏体过度软化降低钢板的强度，达不到设计要求。
回火处理过程中发生碳从马氏体到奥氏体中的配分以稳定奥氏体，提高钢的强韧性；优选情况下，其中在低温回火处理之后，钢中残余奥氏体的体积百分数会明显增加，较回火前增加2％以上，新生成的奥氏体将明显提高钢的塑性和有利于阻碍裂纹的扩展，从而大大提高钢的强塑积。
下面描述以本发明的钢板进行的实验。对如表1所确定的成分的钢锭在1200℃保温10h均质处理，之后在1000℃～1200℃之间保温1h后进行热轧以形成热轧板。可将热轧板或热轧酸洗板在600～700℃保温5～32h，模拟罩式退火以降低热轧板的强度有利于冷轧。再将热轧酸洗板或热轧酸洗退火板冷轧至1.5mm。表1中，编号IS1到IS11是本发明的钢，而CS1到CS5为对比用钢，其成分为现有技术中记载的成分。
表1钢的化学成分
Figure PCTCN2015079748-appb-000001
。
然后，对上述成分的钢板用如表2所示的工艺参数进行热冲压成形。具体而言，将本发明的钢板或其预成形的构件在炉子中加热至700～850℃(AT)保温10分钟。然后转移到模具中进行热冲压成形，并将成形构件以空冷或者其它方式冷却至100℃以下(QT)。一段时间后，再将处理后的成形构件加热至180～500℃(TT)保温一段时间进行回火处理，之后冷却至室温。另外，将对比用钢板按表3中的现有技术的热冲压成形工艺的参数进行成形和热处理。注意，表2和表3中，IS为本发明的钢，AT为奥氏体化温度，TT为回火温度，Ms为马氏体相变开始温度。表中的平衡温度Ae1和Ae3是根据钢的成分由热力学软件Thermal-cal计算得到。
表2
Figure PCTCN2015079748-appb-000002
表3
Figure PCTCN2015079748-appb-000003
在以上热处理成形和热处理工艺之后，分析不同钢及对应热处理工艺的常温力学性能，其结果如表4所示。表4中编号IS仍然表示本发明的钢，而CS表示对比用钢。另外，YS表示屈服强度，TS表示拉伸强度，TE表示延伸率，HR为热轧钢，CR为冷轧钢。此外，表4中拉伸试样为标距50mm的ASTM标准试样，拉伸力学性能测试的应变速率为5×10-4。
表4
Figure PCTCN2015079748-appb-000004
从表4的力学性能数据可知，具有本发明成分的钢板，通过本发明的热冲压成形工艺，能获得强度和延伸率综合性能特别优异的成形构件。具体而言，能实现屈服强度≥1200MPa，抗拉强度≥1600MPa，并且同时总延伸率≥10％。相比之下，具有现有技术成分的钢板，经过现有技术的热冲压成形工艺之后，获得的成形构件的综合性能较差，延伸率超过10％之后，屈服强度均低于1200MPa。因为屈服强度是衡量汽车安全结构件性能的重要参量，因此本发明的钢板经本发明的热冲压成形工艺以后获得的成形构件，获得了远超现有技术的综合性能。
此外，分析本发明钢的微观组织可知，在没有回火热处理时其以体积计的微观组织包括：3％至23％的残余奥氏体，10％以下的铁素体，其余为马氏体，或包含2％以下的碳化物。在进行回火热处理之后，所述成形构件以体积计的微观组织包括：7％至32％的残余奥氏体，10％以下的铁素体，其余为马氏体，或包含2％以下的碳化物。图1a示出了本发明钢的热轧板的残余奥氏体量在相同温度即250℃的情况下随不同回火时间的变化趋势。图1b示出了本发明钢的热轧板的残余奥氏体量在相同温度即300℃的情况下随不同回火时间的变化趋势。图2a示出了本发明钢的冷轧板在250℃不同热处理工艺下的残余奥氏体的变化量。图2b示出了本发明钢的冷轧板在300℃不同热处理工艺下的残余奥氏体的变化量。从这些图中可看出，在不同的回火工艺下，本发明的钢板中的残余奥氏体的含量总体呈上升趋势。
较小的残余奥氏体分数不利于改善构件的延性，而较高的残余奥氏体体积分数会导致奥氏体形成粗大的块状组织，而其会在拉伸变形或碰撞变形过程中发生TRIP效应而相变为高硬度的脆性马氏体块状组织，不利于提高构件的延性。因此本发明通过控制马氏体开始相变温度点小于等于280℃，且其淬火温度为马氏体相变开始温度点以下150～260℃，从而保证了合理的奥氏体体积分数及板条(或薄膜)状形态。图3示出了奥氏体化处理后在300℃回火5分钟的微观组织，而图4示出了典型板条分布的微观组织。
上述实施例为本发明的典型实施例。在不脱离本文所公开的发明构思的情况下，本领域的技术人员可对上述实施例做出各种修改，而不背离本发明的范围。

Claims (15)

  1. 一种用于热冲压成形的钢板，其特征在于，所述钢板以重量百分比计包括0.18～0.42％的C、4～8.5％的Mn、0.8～3.0％的Si+Al以及余量的Fe和不可避免的杂质，其中所述钢板的合金成分满足其在热冲压成形后的马氏体相变开始温度的实际测量值≤280℃。
  2. 如权利要求1所述的钢板，其特征在于，还可包括以下成分中的至少一种：
    5％以下的Cr；
    2.0％以下的Mo；
    2.0％以下的W；
    0.2％以下的Ti；
    0.2％以下的Nb；
    0.2％以下的Zr；
    0.2％以下的V；
    2.0％以下的Cu；
    4.0％以下的Ni；
    0.005％以下的B。
  3. 如权利要求1或2所述的钢板，其特征在于，所述钢板包括热轧钢板、冷轧钢板、或带有涂镀层的钢板。
  4. 如权利要求3所述的钢板，其特征在于，所述带有涂镀层的钢板为锌涂镀钢板，所述锌涂镀钢板是在其上形成金属锌层的热轧钢板或冷轧钢板，其中所述锌涂镀钢板包括选自热浸镀锌、镀锌退火、锌电镀或锌-铁电镀中的至少一种。
  5. 如权利要求3所述的钢板，其特征在于，所述带有涂镀层的钢板为在其上形成铝硅层的热轧钢板或冷轧钢板，或者有机镀层的钢板。
  6. 一种热冲压成形工艺，其特征在于，包括以下步骤：
    a)、提供如权利要求1至5中任一项所述的钢板或其预成形的构件；
    b)、将所述钢板或其预成形的构件加热到700～850℃；
    c)、将加热后的钢板或其预成形的构件转移到模具中进行冲压成形以得到成形构件；以及
    d)、通过任意冷却方式和冷却条件将所述成形构件冷却至马氏体相变 开始温度点以下150～260℃。
  7. 如权利要求6所述的热冲压成形工艺，其特征在于，所述冷却方式包括在模具内冷却、在空气中冷却、或用0～100℃的水进行冷却。
  8. 如权利要求6所述的热冲压成形工艺，其特征在于，在步骤d)之后再进行即加热到160～450℃然后保温1～100000秒后再通过任意冷却方式和条件冷却至室温的回火热处理步骤。
  9. 如权利要求6所述的热冲压成形工艺，其特征在于，在步骤d)之后再进行加热到160～450℃然后保温1～100000秒后再通过任意冷却方式和条件冷却至室温的回火热处理步骤，其中该回火热处理步骤在经过淬火步骤的成形构件放置一段时间之后进行。
  10. 一种成形构件，其特征在于，所述成形构件由权利要求1至5中任一项所述的钢板，经如权利要求6或7所述的热冲压成形工艺制备而成，其中所述成形构件以体积计的微观组织包括：3％至23％的残余奥氏体，10％以下的铁素体，其余为马氏体，或包含2％以下的碳化物。
  11. 如权利要求10所述的成形构件，其特征在于，所述成形构件还受到如权利要求8或9所述的回火热处理步骤，其中所述成形构件以体积计的微观组织包括：7％至32％的残余奥氏体，10％以下的铁素体，其余为马氏体，或包含2％以下的碳化物。
  12. 如权利要求10-11中任一项所述的成形构件，其特征在于，所述成形构件具有1200MPa以上屈服强度和1600MPa以上的抗拉强度、以及10％以上的延伸率。
  13. 如权利要求10-11中任一项所述的成形构件，其特征在于，所述成形构件用于陆用汽车的安全结构件、增强结构件、车轮构件、和高强韧汽车结构件中的至少一种。
  14. 如权利要求13所述的成形构件，其特征在于，所述成形构件用于B柱增强件、保险杠、车门防撞梁、和车轮轮辐中的至少一种。
  15. 一种提高热冲压成形构件的强韧性的热处理方法，所述热处理方法包括：
    将权利要求1至5中任一项所述的钢板或其预成形的构件加热至700～850℃，然后转移到模具中进行冲压成形以得到成形构件，其中将所述钢板或其预成形的构件保持在此温度范围的时间为1秒至10000秒；
    将所述成形构件冷却至马氏体相变开始温度点以下150～260℃，冷却 方法包括在模具中冷却、在空气中冷却、或用0～100℃的水进行冷却，冷却速率为0.1至1000℃/秒；
    将冷却之后的成形构件再加热至低于或者等于Ac1的温度范围进行回火热处理，并使所述成形构件保持在此温度范围达1秒至100000秒；以及
    再通过任意冷却方式和条件冷却至室温。
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