WO2020108593A1 - 一种耐腐蚀性优良的锌系镀层钢板或钢带的成形方法 - Google Patents

一种耐腐蚀性优良的锌系镀层钢板或钢带的成形方法 Download PDF

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WO2020108593A1
WO2020108593A1 PCT/CN2019/121859 CN2019121859W WO2020108593A1 WO 2020108593 A1 WO2020108593 A1 WO 2020108593A1 CN 2019121859 W CN2019121859 W CN 2019121859W WO 2020108593 A1 WO2020108593 A1 WO 2020108593A1
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zinc
forming
steel strip
coated steel
steel plate
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PCT/CN2019/121859
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English (en)
French (fr)
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毕文珍
洪继要
王利
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宝山钢铁股份有限公司
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Priority to US17/298,292 priority Critical patent/US20220023929A1/en
Publication of WO2020108593A1 publication Critical patent/WO2020108593A1/zh

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    • 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
    • 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
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B15/00—Layered products comprising a layer of metal
    • B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
    • B32B15/013—Layered products comprising a layer of metal all layers being exclusively metallic one layer being formed of an iron alloy or steel, another layer being formed of a metal other than iron or aluminium
    • 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
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C18/00—Alloys based on zinc
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C18/00—Alloys based on zinc
    • C22C18/04—Alloys based on zinc with aluminium as the next major constituent
    • 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
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
    • C—CHEMISTRY; METALLURGY
    • 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
    • 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
    • C23C2/34—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the shape of the material to be treated
    • C23C2/36—Elongated material
    • C23C2/40—Plates; Strips

Definitions

  • the invention relates to automobile steel, in particular to a method for forming a zinc-based coated steel plate or steel belt with excellent corrosion resistance, which is mainly used for manufacturing structural parts such as automobile B-pillars, door anti-collision beams, and bumpers.
  • the "lightweight" of automobiles can directly reduce emissions and reduce fuel consumption, which is the goal of the development of today's automobile manufacturing industry.
  • the application of high-strength steel and ultra-high-strength steel has become one of the mainstream development trends in the field of automobile manufacturing.
  • ultra-high-strength steel plates are prone to problems such as poor shape, high processing load and large springback during cold processing, which affects ultra-high strength
  • the use of steel plates, so hot stamping is an important way to achieve ultra-high strength steel parts.
  • the steel plate is heated to a temperature higher than the austenitizing temperature, and is kept at a certain temperature for a full austenitizing. After that, the heated steel sheet is transferred to a forming die and formed into a finished element in a one-step forming process and at the same time quenching is achieved by means of cooling of the die (the cooling rate of the die is greater than the critical cooling rate of the steel sheet).
  • the component is formed in a multi-step forming process to almost complete completion (generally 90% pre-forming). Then, the almost formed element is placed in a heating furnace and heated until it is fully austenitized and maintained for a period of time. Afterwards, the heated element is transferred to the forming mold of the final size of the element. Here, special attention needs to be paid to the thermal expansion of the pre-formed element. After the specific cooling mold ends, the pre-formed components are cooled in the mold at a cooling rate greater than the critical cooling rate to achieve hardening.
  • hot stamping coatings are mainly composed of aluminum-silicon coatings (Al ⁇ 10Si), hot-dip pure zinc (GI) coatings, alloyed zinc-iron (GA) coatings, and electroplated zinc-nickel (Zn ⁇ 10Ni) coatings.
  • the zinc-based coating hot stamping steel that can provide cathodic corrosion protection will cause micro cracks (10um to 100um) in the steel plate matrix due to the effect of LME, and may even extend to the entire macro direction of the thickness of the plate. Cracks, thus hindering the application and development of zinc-based hot stamping coated steel sheets.
  • Using the indirect hot stamping process in the subsequent hardening process after forming can reduce the number of cracks in the substrate, but it is also impossible to avoid cracks, and at the same time increases the production cost of parts. Because of this, to date, hot stamping steel for zinc-free coatings in Asia has been applied, and aluminum/silicon coatings are preferred. However, the aluminum/silicon coating cannot provide cathodic corrosion protection.
  • galvanized materials have also developed from pure zinc to zinc alloys. Since Inland Steel’s three patent applications for adding Al and Mg to Zn in an appropriate amount to further have good corrosion resistance: GB1125965A, US3505043A and US3505042A, various development studies have been conducted on this Zn-Al-Mg series coated steel plate The main work focuses on mixing various other added elements, or limiting production process parameters to further improve corrosion resistance or make it easier to manufacture and reduce production costs.
  • the purpose of the present invention is to provide a method for forming a zinc-coated steel sheet or steel strip with excellent corrosion resistance.
  • the zinc-coated steel sheet or steel strip has a tensile strength greater than 1450 MPa after hot stamping, preferably greater than or equal to 1500 MPa, while avoiding components Due to local stress and cracks in the substrate caused by LME, it is suitable for the manufacture of structural parts such as automobile B pillars, door collision beams, and bumpers.
  • a method for forming a zinc-based coated steel plate or steel strip with excellent corrosion resistance includes the following steps:
  • the zinc-coated steel sheet or steel strip is transported to the heating furnace, and the zinc-coated steel sheet or steel strip is completely austenitized and kept at a temperature of more than Ac3, preferably less than 1000°C for 1 to 7 minutes;
  • the zinc-based coated steel plate or steel strip includes a zinc-based coating on the substrate and at least one surface thereof;
  • the thin-coated steel plate or steel strip with a single-sided coating weight of ⁇ 50g/m 2 only needs air cooling before being transferred to the forming mold, preferably cooled to below 800°C;
  • Thick coated steel plate or steel strip with one-sided coating weight >50g/m 2 is pre-cooled by aerosol or water mist cooling before transfer to the forming mold, and the steel plate or steel strip is more than 30°C/s, such as 40 ⁇ 80°C /s cooling rate to below 700°C;
  • the zinc-coated steel sheet or steel strip is quickly transferred to the forming die for hot stamping, the transfer time is not more than 1 minute, and the hot stamping temperature is controlled at 400-800°C.
  • the matrix component of the zinc-based coated steel sheet or steel strip contains by mass: C: 0.1 to 0.8%, Si: 0.05 to 2.0%, Mn: 0.5 to 3.0%, P: 0.1% or less, S: 0.05 % Or less, Al: 0.1% or less, N: 0.01% or less, the balance Fe and inevitable impurities.
  • the base composition of the zinc-based coated steel sheet or steel strip further contains Cr: 0.01 to 1.0%, Mo: 0.01 to 1.0%, Ti: 0.2% or less, Nb: 0.01 to 0.08%, V: 0.01 to 1.0%, B: at least one of 0.001 to 0.08%.
  • the matrix composition of the zinc-based coated steel plate or steel strip further contains 0.05-0.2% Cu and 0.05-0.2% Ni by mass.
  • the zinc-based coated steel plate or steel strip is ultra-high-strength steel 22MnB5, and its elemental composition by mass is as follows: C, 0.22-0.25%; Mn, 1.2-1.4%; Si, 0.2-0.3%; P ⁇ 0.02%; S ⁇ 0.01%; B, 0.002 ⁇ 0.0035%; Al, 0.02 ⁇ 0.05%; Ti, 0.02 ⁇ 0.05%; Cr, 0.11 ⁇ 0.20%; Mo, 0.05 ⁇ 0.2%; Cu, 0.05 ⁇ 0.2 %; and Ni, 0.05 to 0.2%.
  • the chemical composition of the zinc-based coating layer contains Al 0.15% to 6.5%, Mg 0.2 to 7%, RE ⁇ 0.2%, Sr 0.002 to 0.2%, and the rest is Zn and inevitable impurities.
  • Mg/Al is greater than 1.0.
  • the zinc-based plating layer further contains at least one or two of Mn and Cr, the total amount of which is greater than 0.1%, preferably less than 5%.
  • composition design of the zinc-based coating of the present invention is the composition design of the zinc-based coating of the present invention:
  • the thin-coated steel plate or steel strip ( ⁇ 50g/m 2 single-sided) only needs air cooling, that is, the cooling rate is not more than 20°C/s, and the cooling to 650 ⁇ 800°C, preferably 650 ⁇ 750°C begins to form; and the thick-coated steel plate or Steel strips (>50g/m 2 single-sided) must be additionally cooled, such as aerosol cooling or water mist cooling, the purpose of which is to make the steel sheet hot stamped at a temperature below the melting point of the zinc layer.
  • the forming temperature range of the thick-coated steel plate or steel strip is lower than 780°C, and the present invention is preferably 400 to 650°C, and more preferably 450 to 650°C.
  • the holding time of hot stamping of the present invention is 3 to 15 seconds, and the pressing force is 300 to 1000 tons.
  • the steel plate or steel strip is cooled to below 200°C in the mold, such as 150 to 190°C, taken out of the mold and then cooled to room temperature to complete the martensite transformation.
  • the performance of the hot stamping parts obtained by the present invention is basically the same as that of the traditional hot stamping parts (tensile strength is greater than 1450MPa), but the present invention can improve the corrosion resistance of the steel plate.
  • the process can avoid substrate cracks caused by LME.
  • FIG. 1 is a cross-sectional metallography (no micro-cracks) of a zinc-based plated hot-formed steel sheet according to Example 3 of the present invention
  • Fig. 2 is a cross-sectional metallography (with cracks) of a comparative example.
  • the base materials of the zinc-coated steel sheets of each example and comparative example are ultra-high-strength steel 22MnB5.
  • the coating contains Al, Mg, Sr, Zn and inevitable impurities.
  • the Al content ranges from 0.15% to 6.5% and the Mg content The range is 0.2 to 7%, the Sr content range is 0.002 to 0.2%, the rest are Zn and inevitable impurities, and Mg/Al is greater than 1.0.
  • the plating layer is thin, as in Examples 1 to 3, and the mold is normally air-cooled before quenching, and no micro-cracks are generated.
  • the increase of the pre-cooling process can control the generation of micro-cracks, as in Examples 6 and 10. Therefore, as the thickness of the coating increases, to obtain qualified parts without micro-cracks, it is necessary to: 1) increase the soaking temperature of the furnace; 2) increase the austenitizing holding time in the furnace; 3) increase the pre-cooling process.
  • the method of the present invention applies different cooling methods before entering the mold for different plating thicknesses.
  • the hot stamping steel with zinc-based coating under conventional hot stamping will produce macro liquid metal brittleness (Liquid Metal Embryittlement LME) cracks or micro cracks (as shown in Figure 2).
  • macro liquid metal brittleness Liquid Metal Embryittlement LME
  • micro cracks as shown in Figure 2.
  • the hot stamping process described in the present invention can avoid the cracking of the base body of the hot stamped steel sheet of the zinc-based coating (see FIG. 1), and has better corrosion resistance, which is of great significance to the development of zinc-based hot stamping steel.

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  • Mechanical Engineering (AREA)
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Abstract

一种耐腐蚀性优良的锌系镀层钢板或钢带的成形方法，包括如下步骤：1)将钢板或钢带输送至高于Ac 3的加热炉中，将钢板或钢带完全奥氏体化并保温1～7分钟；2)钢板或钢带离开加热炉进入成形模具前根据镀层厚度不同，施加不同冷却方式：镀层单面重量≤50g/m 2，转移至成形模具之前只需空气冷却；镀层单面重量＞50g/m 2，转移至成形模具之前进行气雾或水雾预冷，将钢板以大于30℃/s的冷却速度冷却至700℃以下；3)将钢板或钢带快速转移至模具中进行热冲压成形，转移时间不超过1分钟，热冲压成形温度控制在400～800℃。钢板或钢带热冲压后抗拉强度大于1450MPa，同时避免元件中由于局部应力和LME导致的基板裂纹。

Description

一种耐腐蚀性优良的锌系镀层钢板或钢带的成形方法 技术领域
本发明涉及汽车用钢，特别涉及一种耐腐蚀性优良的锌系镀层钢板或钢带的成形方法，主要应用于制造汽车B柱、车门防撞梁、保险杠等结构件部件。
背景技术
汽车“轻量化”可直接减少排放，降低油耗，是当今汽车制造业发展的目标。高强钢和超高强度的应用成为汽车制造领域的主流发展趋势之一，但由于超高强度钢板在进行冷加工时易出现形状不良、加工成形载荷高、回弹量大等问题，影响超高强度钢板的使用，因此热冲压成形是实现超高强钢制件的一种重要方式。
现有的热冲压成形方法主要有两种，即直接热冲压和间接热冲压。直接热冲压方法中，钢板被加热至比奥氏体化温度更高的温度，并保温一定时间使钢板达到完全奥氏体化。之后，将加热的钢板转移至成形模具并且在其中以一步成形工艺成形为成品元件并同时借助于模具的冷却(模具的冷却速度大于钢板的临界冷却速度)实现淬硬。
间接热冲压方法中，首先以多步骤成形工艺，将元件成形至几乎完全完成(一般是预成形90％)。再将几乎已成形的元件放到加热炉中加热至完全奥氏体化并保温一段时间。之后将加热的元件转移到元件最终尺寸的成形模具中，这里需要特别注意的是需考虑到预先成形的元件的热膨胀。在具体的冷却模具结束后，预先成形的元件在模具中以大于临界冷却速度的冷速冷却从而达到硬化。
传统的无镀层热冲压件在加热过程中会引起冲压钢板表面脱碳和氧化起皮。为避免热冲压钢板表面的氧化和脱碳，使热冲压钢板具备耐高温性和耐腐蚀性，目前已开发出适合于热冲压钢用的镀层技术。目前热冲压镀层主要由铝硅镀层(Al～10Si)镀层、热镀纯锌(GI)镀层、合金化锌铁(GA)镀层和电镀锌镍(Zn～10Ni)镀层等。
而在直接热冲压过程中，能提供阴极腐蚀保护的锌基镀层热冲压用钢，由于 LME的作用会导致钢板基体中产生微裂纹(10um至100um)，甚至可延伸至整个板厚方向的宏观裂纹，因此阻碍了锌基热冲压镀层钢板的应用及发展。使用间接热冲压工艺在随后的成形后淬硬过程中，可减少基体裂纹的数量，但同样无法避免裂纹，同时提高了零件的生产成本。正因为如此，迄今为止，亚洲无锌基镀层的热冲压用钢应用，而偏爱使用铝/硅镀层。但铝/硅镀层无法提供阴极腐蚀保护，为进一步提高镀层的耐腐蚀性，镀锌材料也由纯锌向锌合金方向发展。自从Inland Steel关于在Zn中适量添加Al和Mg进一步具有良好的耐腐蚀性的三个专利申请：GB1125965A、US3505043A和US3505042A，一直以来人们对于这种Zn-Al-Mg系镀层钢板进行各种开发研究，主要工作集中为混合其它各种添加元素，或者限制生产工艺参数来进一步改善耐腐蚀性或者使之利于制造，降低生产成本。
发明内容
本发明的目的在于提供一种耐腐蚀性优良的锌系镀层钢板或钢带的成形方法，所述锌系镀层钢板或钢带热冲压后抗拉强度大于1450MPa，优选大于等于1500MPa，同时避免元件中由于局部应力和LME导致的基板裂纹，适用于制造汽车B柱、车门防撞梁、保险杠等结构件部件。
为达到上述目的，本发明的技术方案是：
一种耐腐蚀性优良的锌系镀层钢板或钢带的成形方法，其包括如下步骤：
1)将锌系镀层钢板或钢带输送至加热炉中，在高于Ac3、优选低于1000℃的温度下使锌系镀层钢板或钢带完全奥氏体化并保温1～7分钟；其中，所述锌系镀层钢板或钢带包括基体及其至少一个表面上的锌系镀层；
2)锌系镀层钢板或钢带离开加热炉进入成形模具前根据镀层厚度不同，施加不同的冷却方式：
镀层单面重量≤50g/m 2的薄镀层钢板或钢带转移至成形模具之前只需空气冷却，优选冷却至800℃以下；
镀层单面重量＞50g/m 2的厚镀层钢板或钢带转移至成形模具之前以气雾或水雾冷却方式进行预冷，将钢板或钢带以大于30℃/s、如40～80℃/s的冷却速度冷却至700℃以下；
3)冷却结束后，将锌系镀层钢板或钢带快速转移至成形模具中进行热冲压成 形，转移时间不超过1分钟，热冲压成形温度控制在400～800℃。
优选的，所述锌系镀层钢板或钢带的基体成分以质量计含有：C：0.1～0.8％、Si：0.05～2.0％、Mn：0.5～3.0％、P：0.1％以下、S：0.05％以下、Al：0.1％以下、N：0.01％以下，余量Fe和不可避免的杂质。
优选的，所述锌系镀层钢板或钢带的基体成分以质量计，还含有Cr：0.01～1.0％、Mo：0.01～1.0％、Ti：0.2％以下、Nb：0.01～0.08％、V：0.01～1.0％、B：0.001～0.08％中的至少一种。在一些实施方案中，所述锌系镀层钢板或钢带的基体成分以质量计，还含有0.05～0.2％的Cu和0.05～0.2％的Ni。
在一些实施方案中，所述锌系镀层钢板或钢带为超高强钢22MnB5，以质量计其元素组成如下：C，0.22～0.25％；Mn，1.2～1.4％；Si，0.2～0.3％；P≤0.02％；S≤0.01％；B，0.002～0.0035％；Al，0.02～0.05％；Ti，0.02～0.05％；Cr，0.11～0.20％；Mo，0.05～0.2％；Cu，0.05～0.2％；和Ni，0.05～0.2％。
优选的，所述锌系镀层的化学成分以质量计含有Al 0.15％～6.5％，Mg 0.2～7％，RE＜0.2％，Sr 0.002～0.2％，其余为Zn和不可避免的杂质，并且所述镀层的化学成分中Mg/Al大于1.0。
优选的，所述锌系镀层内还包含Mn和Cr中至少一种或两种，其总量大于0.1％，优选小于5％。
在本发明锌系镀层的成分设计中：
(1)当铝含量小于0.15％时，提高镀层耐蚀性的效果不显著，当铝含量高于6.5％时，焊接性能差。
(2)当Mg含量低于0.2％时，提高耐蚀性的效果不显著，当Mg含量高于7％时，镀液表面氧化严重，产品表面质量差。
(3)当Al/Mg大于1时，在同等Al含量下，限制了Mg元素的含量，阻碍了Mg元素对镀层耐蚀性作用的发挥，单纯提高Mg元素会使得镀液表面的氧化十分严重。本发明通过添加Sr和稀土元素抑制镀液表面的氧化，因此可以在同等Al含量下提高镀液中Mg元素的含量。既控制住了表面氧化的程度又发挥了Mg元素提高镀层耐蚀性的作用。
基于本发明的研究发现，在成形阶段必须尽可能避免熔化的锌与奥氏体接触，可通过降低模具开始冲压的温度(即成形开始温度)而尽可能避免这种接触。通过 尽可能避免熔化的锌与奥氏体接触，可避免热冲压成形过程中LME导致的基板裂纹。
本发明实施例中根据镀层厚度决定了钢板在加热保温后转移至模具前施加了不同的冷却方式。薄镀层钢板或钢带(≤50g/m 2单面)只需要空气冷却，即冷却速度不大于20℃/s，冷却至650～800℃、优选650～750℃开始成形；而厚镀层钢板或钢带(＞50g/m 2单面)则必须额外施加冷却，如气雾冷却或水雾冷却，其目的使钢板在低于锌层熔点的温度进行热冲压成形。厚镀层钢板或钢带的成形温度范围低于780℃，本发明优选400～650℃，更优选450～650℃。
本发明的热冲压成形的保压时间为3～15秒，冲压力为300～1000吨。热冲压成形完成后，钢板或钢带在模具中冷却至200℃以下，如150～190℃，从模具取出后冷却至室温，完成马氏体相变。
与现有技术相比，本发明得到的热冲压零件，性能与传统热冲压零件基本持平(抗拉强度大于1450MPa)，但本发明可提高钢板的耐腐蚀性能，且在热冲压成形后，本工艺可避免LME导致的基板裂纹。
附图说明
图1是本发明实施例3锌基镀层热成形钢板的截面金相(无微裂纹)；
图2是对比例的截面金相(有裂纹)。
具体实施方式
下面结合实施例和附图对本发明做进一步的说明。
本发明实施例参见表1。各实施例和对比例的锌系镀层钢板的基材均为超高强钢22MnB5，镀层含有Al、Mg、Sr、Zn和不可避免的杂质，其中，Al含量范围为0.15％～6.5％，Mg含量范围为0.2～7％，Sr含量范围为0.002～0.2％，其余为Zn和不可避免的杂质，并且Mg/Al大于1.0。
从表1的实施例和对比例结果可见，在镀层较薄，如实施例1～3，在模具淬硬前正常空冷，均不产生微裂纹。随着镀层厚度增加，在较高的加热温度和较长的保温时间下，增加预冷工序可控制微裂纹的产生，如实施例6和10。因此，随着镀层厚度增加，想得到没有微裂纹的合格零件，需要：1)提高加热炉均热温度；2) 提高加热炉中奥氏体化保温时间；3)增加预冷工序。
本发明方法针对不同镀层厚度，在进入模具成形前施加不同的冷却方式。
常规热冲压下的锌基镀层热冲压钢均会产生宏观液态金属脆性(Liquid Metal Embrittlement LME)裂纹或者微观裂纹(如图2所示)。
本发明所述的热冲压工艺可以避免锌基镀层的热冲压钢板冲压成形的基体裂纹(参见图1)，且具有更好的耐腐蚀性能，对锌基热冲压用钢的发展具有重要意义。
Figure PCTCN2019121859-appb-000001

Claims (10)

  1. 一种耐腐蚀性优良的锌系镀层钢板或钢带的成形方法，其特征是，其包括如下步骤：
    1)将锌系镀层钢板或钢带输送至加热炉中，在高于Ac 3的温度下使锌系镀层钢板或钢带完全奥氏体化并保温1～7分钟；其中，所述锌系镀层钢板或钢带包括基体及其至少一个表面上的锌系镀层；
    2)锌系镀层钢板或钢带离开加热炉进入成形模具前根据镀层厚度不同，施加不同的冷却方式：镀层单面重量≤50g/m 2的薄镀层钢板或钢带转移至成形模具之前只需空气冷却至800℃以下；镀层单面重量＞50g/m 2的厚镀层钢板或钢带转移至成形模具之前以气雾或水雾冷却方式进行预冷，将钢板或钢带以大于30℃/s的冷却速度冷却至700℃以下；
    3)冷却结束后，将锌系镀层钢板或钢带快速转移至成形模具中进行热冲压成形，转移时间不超过1分钟，热冲压成形温度控制在400～800℃。
  2. 如权利要求1所述的耐腐蚀性优良的锌系镀层钢板或钢带的成形方法，其特征是，所述锌系镀层钢板或钢带的基体成分以质量计，含有：C：0.1～0.8％、Si：0.05～2.0％、Mn：0.5～3.0％、P：0.1％以下、S：0.05％以下、Al：0.1％以下、N：0.01％以下，余量Fe和不可避免的杂质。
  3. 如权利要求2所述的耐腐蚀性优良的锌系镀层钢板或钢带的成形方法，其特征是，所述锌系镀层钢板或钢带的基体成分以质量计，还含有Cr：0.01～1.0％、Mo：0.01～1.0％、Ti：0.2％以下、Nb：0.01～0.08％、V：0.01～1.0％、B：0.005～0.08％中的至少一种。
  4. 如权利要求1所述的耐腐蚀性优良的锌系镀层钢板或钢带的成形方法，其特征是，所述锌系镀层钢板或钢带的基体为超高强钢22MnB5。
  5. 如权利要求1所述的耐腐蚀性优良的锌系镀层钢板或钢带的成形方法，其特征是，所述锌系镀层的化学成分以质量计含有Al 0.15％～6.5％，Mg 0.2～7％，RE＜0.2％，Sr 0.002～0.2％，其余为Zn和不可避免的杂质，并且所述镀层的化学成分中Mg/Al大于1.0。
  6. 如权利要求5所述的耐腐蚀性优良的锌系镀层钢板或钢带的成形方法，其 特征是，所述锌系镀层内还包含Mn和Cr中的至少一种或两种，其总量大于0.1％且低于5％。
  7. 如权利要求1所述的耐腐蚀性优良的锌系镀层钢板或钢带的成形方法，其特征是，所述薄镀层钢板或钢带转移至成形模具之前进行空气冷却，冷却至650～800℃后开始成形，冷却速度不大于20℃/s。
  8. 如权利要求1所述的耐腐蚀性优良的锌系镀层钢板或钢带的成形方法，其特征是，所述厚镀层钢板或钢带转移至成形模具之前进行气雾冷却或水雾冷却，冷却至400～650℃后开始成形，冷却速度大于30℃/s。
  9. 如权利要求1所述的耐腐蚀性优良的锌系镀层钢板或钢带的成形方法，其特征是，热冲压成形的保压时间为3～15秒，冲压力为300～1000吨。
  10. 如权利要求1所述的耐腐蚀性优良的锌系镀层钢板或钢带的成形方法，其特征是，所述方法还包括：热冲压成形完成后，使所述钢板或钢带在模具中冷却至200℃以下，然后从模具取出，冷却至室温。
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