WO2023134665A1 - 一种具有Al-Zn-Mg-Si镀层的热冲压钢板及其热冲压方法 - Google Patents

一种具有Al-Zn-Mg-Si镀层的热冲压钢板及其热冲压方法 Download PDF

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WO2023134665A1
WO2023134665A1 PCT/CN2023/071554 CN2023071554W WO2023134665A1 WO 2023134665 A1 WO2023134665 A1 WO 2023134665A1 CN 2023071554 W CN2023071554 W CN 2023071554W WO 2023134665 A1 WO2023134665 A1 WO 2023134665A1
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phase
hot
coating
steel sheet
layer
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French (fr)
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毕文珍
王凯
史良权
金鑫焱
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宝山钢铁股份有限公司
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    • C—CHEMISTRY; METALLURGY
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    • 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
    • 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
    • B—PERFORMING OPERATIONS; TRANSPORTING
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    • B21D22/00—Shaping without cutting, by stamping, spinning, or deep-drawing
    • B21D22/02—Stamping using rigid devices or tools
    • B—PERFORMING OPERATIONS; TRANSPORTING
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    • 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
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    • B21D22/022—Stamping using rigid devices or tools by heating the blank or stamping associated with heat treatment
    • B—PERFORMING OPERATIONS; TRANSPORTING
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    • B21D37/16—Heating or cooling
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    • 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
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Definitions

  • the invention relates to the technical field of hot forming, in particular to a hot stamping steel plate with Al-Zn-Mg-Si coating and a hot stamping method thereof.
  • Automobile lightweight technology is one of the key technologies to adapt to the trend of modern automobile safety, energy saving and environmental protection.
  • stamping forming process thinning and high strength are the dual factors that deteriorate the formability, which not only makes the body parts easy to crack during the forming process, but also prone to excessive springback, which affects the subsequent assembly of the body.
  • Realizing the high strength of the final part through heat treatment is one of the ways, and the hot stamping forming technology is a way to achieve high strength of the part through the combination of heat treatment and high temperature forming, which can better solve the problem of high strength and cold forming. contradiction problem.
  • thermoforming technology has the advantages of ultra-high strength, high forming precision, and no springback. More and more auto body parts adopt thermoforming technology, such as A-pillar, B-pillar, car bumper, anti-collision beam, door anti-collision beam and so on.
  • the traditional uncoated hot stamping parts will cause decarburization and oxidation peeling on the surface of the stamping steel plate during the heating process.
  • Coating technology suitable for hot stamping steel was developed.
  • the hot stamping coating mainly consists of aluminum-silicon coating (Al-10Si) coating, hot-dip pure zinc (GI) coating, alloyed zinc-iron (GA) coating and electroplated zinc-nickel (Zn-10Ni) coating, etc.
  • CN100334250 proposes a Zn-Al-Mg-Si alloy coated steel with excellent corrosion resistance and its manufacturing method, which mainly controls the content of Mg and Si added to the Zn-Al coating, and Controlling the precipitation amount and form of the MgSi 2 phase, which has the effect of improving corrosion resistance, can not only solve the problem of edge creep resistance of the cut surface after coating, but also provide an alloy coating with particularly excellent performance.
  • Patent CN103805930B adds Mg and/or Cr to the Galvalume coating (Zn-55Al), studies the distribution of Cr in the interface alloy layer, and provides a hot-dip Zn-Al-Mg-Cr coating with excellent processability and corrosion resistance steel plate.
  • the object of the present invention is to provide a hot stamping steel plate with Al-Zn-Mg-Si coating and a hot stamping method thereof.
  • the hot stamping part produced by the method of the invention has excellent corrosion resistance, and can effectively avoid substrate cracks caused by local stress and liquid metal embrittlement (LME) during the hot stamping forming process of Al-Zn-Mg-Si coated steel sheets.
  • LME liquid metal embrittlement
  • a hot-stamped steel sheet with an Al-Zn-Mg-Si coating includes a substrate and an Al-Zn-Mg-Si coating coated on the substrate, wherein the Al-Zn-Mg- The Si coating is composed of two structures in the thickness direction, namely the oxide layer on the surface and the alloy layer below the surface.
  • the thickness of the alloy layer is 15-45um, wherein: the alloy layer is composed of upper and lower layers.
  • the first layer structure is composed of Al-rich phase, Zn-rich phase, MgZn 2 phase and FeAl 3 phase, and the volume percentage of the sum of Zn-rich phase and FeAl 3 phase is between 20% and 80%;
  • the two-layer structure is composed of FeAl 3 phase, MgZn 2 phase and Zn-rich phase, and the volume percentage of MgZn 2 phase and Zn-rich phase is no more than 5%; or the alloy layer is composed of a single-layer alloy layer, and the The main composition of the single-layer alloy layer is FeAl 3 phase, there are Zn-rich phase and MgZn 2 phase between FeAl 3 phases, and the volume percentage of the sum of Zn-rich phase and MgZn 2 phase is 5% ⁇ 50%.
  • the alloy layer is composed of upper and lower two-layer structure
  • the first layer structure is composed of Al-rich phase, Zn-rich phase, MgZn 2 phase and FeAl 3 phase, and Zn-rich phase, MgZn 2 phase and The volume percentage of the sum of the three phases of FeAl 3 phase is between 50% and 100%
  • the second layer structure is composed of FeAl 3 phase, MgZn 2 phase and Zn-rich phase, and the MgZn 2 phase and Zn-rich phase account for The volume percentage is not more than 5%.
  • the alloy layer is composed of a single-layer alloy layer
  • the main composition of the single-layer alloy layer is FeAl 3 phase
  • the volume percentage of the sum of the Zn phase and the MgZn 2 phase is 15% to 40%.
  • the present invention provides a hot-stamped steel sheet with an Al-Zn-Mg-Si coating
  • the hot-stamped steel sheet includes a substrate and an Al-Zn-Mg-Si coating coated on the substrate, the Al -Zn-Mg-Si coating after hot stamping
  • the structure in the Al-Zn-Mg-Si coating consists of two structures in the thickness direction: an oxide layer on the surface and a lower alloy layer, and the thickness of the alloy layer is 15-45um
  • the structure of the alloy layer has the following relationship with the holding time t in the process of austenitization:
  • the alloy layer of the Al-Zn-Mg-Si coating consists of upper and lower layers
  • the first layer structure is composed of Al-rich phase, Zn-rich phase and FeAl 3 phase, and rich in
  • the volume percentage of the sum of the Zn phase and the FeAl 3 phase is between 20% and 80%
  • the second layer is composed of the FeAl 3 phase and the Zn-rich phase, and the volume percentage of the Zn-rich phase does not exceed 5%.
  • the alloy layer of described Al-Zn-Mg-Si plating layer is made up of single-layer alloy layer, and the main composition of described single-layer alloy layer is FeAl 3 phase, rich Zn phase and The MgZn 2 phase exists between the FeAl 3 phases, and the volume percentage of the sum of the Zn-rich phase and the MgZn 2 phase is 5% to 50%.
  • the initial structure of the Al-Zn-Mg-Si coating of the steel sheet before hot stamping is composed of Zn-rich phase, Al-rich phase, FeAl3 phase , MgSi 2 phase and Fe-Al-Si alloy layer, the thickness of the Al-Zn-Mg-Si coating is 8-30um.
  • the oxide layer on the coating surface of the steel sheet after hot stamping contains (Zn, Al, Mg, Si) oxides, and Al-Zn-
  • the thickness of the oxide layer of the Mg-Si coating is within 3um, such as 0.5-3um.
  • the chemical composition mass percentage of the Al-Zn-Mg-Si coating is: Al: 45-65%, Mg: 0.2-5% , Si: 0.1-3%, the balance is Zn and other unavoidable impurities.
  • the chemical composition mass percentage of the Al-Zn-Mg-Si coating is: Al: 45-65%, Mg: 0.2-5% , Si: 0.5-3%, the balance is Zn and other unavoidable impurities.
  • the components of the substrate include: C: 0.1%-0.5%; Si: 0.05%-2.0%; Mn: 0.5% ⁇ 3.0%; P: ⁇ 0.1%; S: ⁇ 0.05%; Al: 0.01 ⁇ 0.05%; N: below 0.01%; also includes Nb: 0.01% ⁇ 0.1%, V: 0.01% ⁇ 1.0%, Mo : 0.01% ⁇ 1.0%, Ti: 0.01% ⁇ 0.1%, Cr: 0.01% ⁇ 1.0%, Ni: 0.01% ⁇ 1.0%, B: at least one of 0.001% ⁇ 0.01%, the balance is Fe and not Avoid impurities.
  • the components of the substrate include: C: 0.1%-0.3%; Si: 0.05-0.3% ;Mn: 0.8 ⁇ 1.5%; P: ⁇ 0.01%; S: ⁇ 0.0005%; Al: 0.01 ⁇ 0.05%; , Mo: 0.01% to 1.0%, Ti: 0.01% to 0.1%, Cr: 0.1 to 0.4%, Ni: 0.01% to 1.0%, B: at least one of 0.001% to 0.01%, and the balance is Fe and unavoidable impurities.
  • the composition of the substrate is calculated by mass percentage: C: 0.1%-0.3%, Si: 0.05-0.3% , Mn: 0.8-1.5%, P: ⁇ 0.01%, S: ⁇ 0.0005%, Al: 0.01-0.05%, N: less than 0.01%, Ti: 0.01%-0.1%, Cr: 0.1-0.4%, and B : 0.001% to 0.01%, the balance being Fe and unavoidable impurities.
  • the C content is the most important factor determining the mechanical properties of the steel plate such as strength and hardness.
  • the strength and hardness of the steel plate increase with the increase of the C content
  • the plasticity and toughness of the steel plate decrease with the increase of the C content
  • the cold and brittleness of the steel plate Tendency and aging tendency increase with the increase of C content.
  • the C content also has a significant impact on the mechanical properties after quenching, so the present invention defines the upper limit of the C content as 0.5%, preferably 0.35%, to ensure that the steel plate has a certain plasticity and toughness; the lower limit is 0.1%, to ensure that the steel plate has a certain strength .
  • the content of C is 0.1-0.3%.
  • Si is a replacement solid-solution alloy element, which can promote the enrichment of C in austenite, increase the stability of austenite, improve the strength of the steel plate, and improve its toughness to a certain extent. Therefore, the present invention will increase the Si content
  • the lower limit is set at 0.05%, but when the Si content is large, the probability of surface defects that produce red scale during hot rolling increases, and the rolling force increases, resulting in deterioration of the ductility of the hot-rolled steel sheet. Therefore, the present invention uses Si
  • the upper limit of the content is specified as 1.0%. In order to ensure the plateability of the steel sheet, the upper limit of Si is preferably 1.0%. In some embodiments, the content of Si is 0.05-0.5%; in some embodiments, the content of Si is 0.05-0.3%.
  • Mn is an element that improves the hardenability of the steel plate, expands the austenite phase region, and effectively ensures the strength of the steel plate after quenching.
  • Mn can reduce the temperature of Ac3 and Ac1, and delay pearlite. Phase change, thereby reducing the hot stamping heating temperature.
  • Ac3 refers to the actual temperature of phase transformation when carbon steel is heated
  • Ac1 is the temperature at which austenite begins to form when steel is heated.
  • the content of Mn is 0.5-2.0%. In some embodiments, the content of Mn is 0.8-1.5%.
  • P in steel can significantly reduce the plasticity and toughness of steel, especially at low temperature, it will cause "cold brittleness", so it should be strictly controlled and limited to less than 0.1%. In some embodiments, P: ⁇ 0.01%.
  • the S content is maintained at a low level, because the hot brittleness problem caused by the formation of FeS from S is limited to 0.05% or less.
  • the lower limit is not defined because the lower the S content, the better for the same reason.
  • S ⁇ 0.0005%.
  • N ⁇ 0.005%.
  • Al has a deoxidizing effect.
  • the Sol.Al (effective Al) content is less than 0.01%, the addition effect is not obvious; when the Sol.Al content is greater than 0.05%, the deoxidation effect is saturated and the cost increases. Therefore, the present invention limits the Al content to 0.01%-0.05%.
  • Nb is an important microalloying element in steel. Adding a small amount of Nb in steel can ensure that the steel has a low carbon equivalent, through the dispersed precipitation of carbon and nitride points (size less than 5nm) and the solid solution of Nb. , refine the grain, greatly improve the strength and toughness of steel, especially the low temperature toughness, and make the steel have good cold bending performance and weldability. Therefore, the present invention limits the content of Nb to 0.01%-0.1%, which can effectively refine the original austenite grains of the steel substrate, and aims to improve the toughness and cold bending performance of the parts after hot forming.
  • V 0.01% to 1.0%
  • V is an element that refines the structure by forming carbides.
  • the fine carbides of V inhibit recrystallization and grain production, refine austenite grains, and improve toughness.
  • the addition effect is not obvious; when the V content is more than 1.0%, the addition effect is saturated and the cost increases.
  • Mo is an element for refining austenite.
  • the Mo content is less than 0.01%, the addition effect is not obvious; when the Mo content is more than 1.0%, the addition effect is saturated and the cost increases.
  • B is an element with severe grain boundary segregation in steel, which can reduce the grain boundary energy of austenite and inhibit the formation of proeutectoid ferrite nuclei. It has three characteristics for improving the hardenability of steel: improving hardenability The ability is very strong; a very small amount of B element is equivalent to the addition of other precious alloy elements; B element has the best content to improve hardenability and is very small, different from the effect of general alloy elements to improve hardenability. It increases with increasing content in steel. Therefore, the content of element B in the present invention is limited to 0.001%-0.01%.
  • the chemical composition of the steel plate substrate may also include at least one of the following: Ti: 0.01%-0.1%, Cr: 0.01%-1.0%, Ni: 0.01%-1.0%.
  • Ti plays the role of stably exerting the above-mentioned B through the formation of its nitride, so it is an element that can be effectively used. For this reason, the addition of 0.01% or more is necessary, but if too much is added, the nitrides will become excessive and the toughness will deteriorate, so the upper limit is made 0.10%.
  • Cr can increase the hardenability of steel and has secondary hardening effect. Chromium and iron form a continuous solid solution, reducing the austenite phase area, and chromium can also reduce the carbon concentration of pearlite and the limit solubility of carbon in austenite. Chromium can also improve the oxidation resistance and corrosion resistance of steel. When the Cr content is less than 0.01%, the addition effect is not obvious; when the Cr content is more than 1.0%, the addition effect is saturated and the cost increases. In some embodiments, the content of Cr is 0.1-0.5%, such as 0.1-0.4%.
  • Ni can expand the austenite region of steel and is the main alloying element to form and stabilize austenite. Nickel can also strengthen ferrite and refine and increase pearlite to improve the strength of steel. The effect of its addition is obvious at 0.01% or more, but since it is a high-priced element, it should be controlled at 1.0% or less.
  • the present invention also relates to a hot stamping method suitable for hot stamping steel sheets with Al-Zn-Mg-Si coatings, so that the steel sheets can be austenitized within a limited process range, and then hot stamping and in-mold quenching are performed. Make the parts after hot stamping have good corrosion resistance, and at the same time avoid substrate cracks caused by local stress and liquid metal brittleness during hot forming.
  • the operation steps of the hot stamping forming method are as follows:
  • Heating transfer the steel plate containing Al-Zn-Mg-Si coating to a heating furnace, and heat the steel plate to a temperature higher than Ac3 at a heating rate V greater than or equal to 5°C/s and less than or equal to 1000°C/s T Ac3 , the range of heating temperature T needs to satisfy the following relationship:
  • [Al] is the content % of Al in the coating
  • [Zn] is the content % of Zn in the coating
  • V is the heating rate in the heating process, and the unit is °C/s
  • [Al] is the content % of Al in the coating
  • [Zn] is the content % of Zn in the coating, and the unit is min
  • Hot stamping forming and in-mold quenching The heated steel plate is quickly moved to the hot stamping die for stamping and quenching. After the hot stamping is completed, a billet is formed. Cool to room temperature, or cool to room temperature after removing from the mold.
  • the heating rate V in the step (1) is greater than 5°C/s and less than 1000°C/s.
  • the temperature rise rate V in the step (1) is 5-200° C./s. In some embodiments, the heating rate V is 5-50°C/s. In some embodiments, the heating rate V is 10-50°C/s.
  • the austenitization temperature of the steel sheets in the step (1) is controlled at 930°C and below, and higher than Ac3 temperature.
  • the holding time of the steel sheet for austenitization is controlled within 1 minute to between 7 minutes.
  • the holding time is controlled to be greater than 1 minute and less than 3 minutes, also That is to say, in the optimization scheme, the holding time is controlled to be greater than 1 minute and less than 3 minutes.
  • the stamping force of hot stamping in the step (3) is 300 to 1000 tons, and the holding time is 3 to 10 tons. 15 seconds.
  • the hot stamping method that is applicable to the hot stamping steel plate with Al-Zn-Mg-Si coating described in the present invention is used for manufacturing any embodiment described herein has Al-Zn-Mg-Si coating hot stamped steel sheet.
  • the hot stamping method applicable to the hot stamped steel sheet with Al-Zn-Mg-Si coating described herein also includes preparing the described steel sheet with Al-Zn-Mg-Si coating by the method having the following steps Steps for hot stamping steel plate:
  • Hot rolling heating the billet to 1100-1250°C and then controlling the rolling, the starting rolling temperature is 950-1150°C, the final rolling temperature is 750-900°C, and the thickness of the hot-rolled plate is less than or equal to 20mm;
  • the segmental cooling includes: controlling the cooling rate of the steel plate to be 15-25°C/s in the range of leaving the plating solution to 480°C, and controlling the cooling rate of the steel plate to be 40-60°C in the range of 480-280°C. °C/s, cooling to below 280 °C, the steel plate enters the water quenching tank and cools to room temperature.
  • the hot-stamped steel sheet with Al-Zn-Mg-Si coating and the hot-stamping method of the present invention have been applied in practice, and have achieved the following beneficial effects:
  • the present invention provides a hot stamping forming method of a hot stamped steel sheet with an Al-Zn-Mg-Si coating.
  • the hot stamped parts have good corrosion resistance. Because the Al-Fe diffusion will first occur preferentially during the heating process, at this time the Zn-rich phase ( ⁇ -Zn) and MgZn phase in the original coating will concentrate in the surface layer of 1/3 to 1/2 of the thickness of the coating.
  • the surface layer is composed of Zn-rich phase ( ⁇ -Zn), MgZn 2 phase and Al-rich phase, and the interface layer is a double-layer alloy phase structure composed of AlFe 3 phase.
  • phase 2 When in a corrosive medium, due to the Zn-rich phase or MgZn phase of the surface layer The potential of phase 2 is lower, and it can form a primary battery with the interface alloy layer or the substrate, thereby serving as a sacrificial anode protection function for the substrate.
  • the present invention provides the hot stamping forming method of the hot stamping steel plate with Al-Zn-Mg-Si coating, within the process scope limited by the present invention, can avoid the substrate crack that causes because of Liquid Metal Embrittlement (Liquid Metal Embrittlement, LME) , It also makes the coating after hot stamping have good corrosion resistance.
  • Liquid Metal Embrittlement Liquid Metal Embrittlement, LME
  • the hot-stamped steel sheet with Al-Zn-Mg-Si coating of the present invention at first can preferentially take place Al-Fe diffusion in the heating process, at this moment the rich Zn phase ( ⁇ -Zn) and MgZn in the original coating
  • the 2 phases will concentrate in the surface layer 1/3 ⁇ 1/2 of the thickness of the coating, but if the heating temperature is too high, the Zn-rich phase in the coating will diffuse due to the driving force of Zn-Fe diffusion after the Al-Fe is fully diffused.
  • the heating temperature is limited to below 1.03T Ac3 +2 ⁇ V ⁇ [Al]/[Zn].
  • the holding time of the steel sheet used for hot stamping is limited in detail, if the holding time is less than 1min, the steel sheet cannot be fully austenitized , and the holding time is too long, the same coating will change into a single-layer alloy layer structure where the (Al, Fe) phase and the Zn-rich phase coexist, causing macroscopic cracks caused by Liquid Metal Embrittlement (LME), so the holding time It is limited to 2.7 ⁇ [Al]/[Zn] or less.
  • Fig. 1 is the microstructure of Example 1 of the present invention before hot stamping.
  • Fig. 2 is the microstructure of Example 1 of the present invention after hot stamping.
  • Fig. 3 is the microstructure of Example 2 of the present invention after hot stamping.
  • Fig. 4 is the microstructure of Example 5 of the present invention after hot stamping.
  • Fig. 5 is the microstructure of Example 6 of the present invention after hot stamping.
  • Fig. 6 is the cross-sectional appearance of Example 8 of the present invention after hot stamping.
  • Fig. 7 is the LME feature of Comparative Example 4 of the present invention after hot stamping and the cross-sectional morphology of the coating at high magnification.
  • Fig. 8 is a high-magnification image of position A in Fig. 7 of the comparative example of the present invention.
  • Fig. 9 is a high-magnification image of position B in Fig. 7 of the comparative example of the present invention.
  • the invention relates to a hot stamping steel plate with Al-Zn-Mg-Si coating, which comprises a base plate and an Al-Zn-Mg-Si coating coated on the base plate.
  • the initial structure of the Al-Zn-Mg-Si coating before hot stamping is composed of Zn-rich phase, Al-rich phase, FeAl 3 phase, MgSi 2 phase and Fe-Al-Si alloy layer, the Al-Zn-Mg -
  • the thickness of the Si coating is 8-30um.
  • the coating structure is composed of two structures in the thickness direction, namely: the oxide layer on the surface and the alloy layer on the lower part of the surface.
  • the thickness of the alloy layer is 15-45um.
  • the structure of the alloy layer is the same as
  • the holding time t in the austenitizing process has the following relationship:
  • the alloy layer of the Al-Zn-Mg-Si coating consists of upper and lower layers
  • the first layer structure is composed of Al-rich phase, Zn-rich phase and FeAl 3 phase, and rich in The sum of Zn phase and FeAl 3 phase accounts for 20% to 80% of the volume of all tissues
  • the second layer is composed of FeAl 3 phase and Zn-rich phase, and the Zn-rich phase accounts for no volume percentage of all tissues More than 5%.
  • the alloy layer of described Al-Zn-Mg-Si plating layer is made up of single-layer alloy layer, and the main composition of described single-layer alloy layer is FeAl 3 phase, rich Zn phase and The MgZn 2 phase exists between the FeAl 3 phases, and the volume percentage of the sum of the Zn-rich phase and the MgZn 2 phase in all structures is 5% to 50%.
  • the chemical composition mass percentage of the Al-Zn-Mg-Si coating is: Al: 45-65%, Mg: 0.2-5% , Si: 0.1-3%, the balance is Zn and other unavoidable impurities.
  • the oxide layer on the coating surface of the steel sheet after hot stamping comprises (Zn, Al, Mg, Si) oxide, and the oxide layer of the coating The thickness is within 3um.
  • (Zn, Al, Mg, Si) oxide refers to a mixture of zinc oxide, aluminum oxide, magnesium oxide and silicon oxide.
  • the components of the substrate include: C: 0.1%-0.5%; Si: 0.05%-2.0%; Mn: 0.5% ⁇ 3.0%; P: ⁇ 0.1%; S: ⁇ 0.05%; Al: 0.01 ⁇ 0.05%; N: below 0.01%; also includes Nb: 0.01% ⁇ 0.1%, V: 0.01% ⁇ 1.0%, Mo : 0.01% ⁇ 1.0%, Ti: 0.01% ⁇ 0.1%, Cr: 0.01% ⁇ 1.0%, Ni: 0.01% ⁇ 1.0%, B: at least one of 0.001% ⁇ 0.01%, the balance is Fe and not Avoid impurities.
  • Hot rolling heating the billet to 1100-1250°C and then controlling the rolling, the starting rolling temperature is 950-1150°C, the final rolling temperature is 750-900°C, and the thickness of the hot-rolled plate is less than or equal to 20mm;
  • the temperature of the plating solution is controlled at 565-605°C in step 7), because when the temperature of the plating solution is lower than 565°C, the fluidity of the plating solution in the zinc pot decreases, and the coating It becomes difficult to control the thickness of the coating, which makes it difficult to ensure the uniformity of the coating, especially when the thickness of the coating is thinner, the uniformity is worse.
  • the dissolution of the steel plate in the zinc pot and the oxidation of the plating solution on the surface of the zinc pot will intensify, which will lead to an increase in the bottom slag and surface slag of the plating solution during the hot-dip plating process; in addition, the plating solution If the temperature is higher than 605°C, it will also lead to the intensification of Zn evaporation in the hot-dip coating device such as the furnace grate, which will cause more defects such as zinc dust on the surface of the steel plate.
  • the present invention clearly defines the different phase compositions of the coating structure of the coated steel plate after hot stamping under different holding times. This is because the inventors have found through a lot of research that the composition of each phase in the structure of the coating has the greatest relationship with the holding time during the austenitization process, and the composition of different phases in the coating after hot stamping has a great influence on its subsequent corrosion resistance. Therefore, on the basis of a large number of experiments, combined with the subsequent hot stamping method, the hot stamping method that produces the optimal corrosion resistance structure is also defined.
  • the present invention also relates to a hot stamping method suitable for hot stamping steel sheets with Al-Zn-Mg-Si coatings, so that the steel sheets can be austenitized within a limited process range, and then hot stamping and in-mold quenching are performed. Make the parts after hot stamping have good corrosion resistance.
  • the hot stamping method includes austenitization, hot stamping and in-mold quenching:
  • Austenitization link the steel plate used as the substrate is transported to a heating furnace whose temperature is higher than Ac3, and the steel plate is completely austenitized and kept warm;
  • Hot stamping forming and in-mold quenching link The heated steel plate is quickly moved to the hot stamping die for stamping and quenching. After the hot stamping is completed, a blank is formed. The blank is first quenched and cooled in the mold, and then cooled in the mold to room temperature, or cool to room temperature after removing from the mold.
  • the heating temperature and heating time of the steel plate used for hot stamping were limited in detail. This is because the inventor found through a lot of research that the heating temperature and heat preservation Time affects the composition of the coating after heating, thereby affecting the corrosion resistance of the material. If the heating temperature is lower than the Ac3 temperature of the steel plate, the steel plate cannot enter the single-phase austenite zone, and the martensite transformation cannot be completed in the subsequent hardening process. During the heating process, Al-Fe diffusion will first occur preferentially. At this time, the Zn-rich phase ( ⁇ -Zn) and MgZn 2 phase in the original coating will concentrate in the surface layer of 1/3 to 1/2 of the coating thickness.
  • This surface layer is composed of Zn-rich phase ( ⁇ -Zn), MgZn 2 phase and Al-rich phase, and the interface layer is a double-layer alloy phase structure composed of AlFe 3 phase.
  • Zn-rich phase ⁇ -Zn
  • MgZn 2 phase MgZn 2 phase
  • Al-rich phase a double-layer alloy phase structure composed of AlFe 3 phase.
  • the heating temperature is limited below 930°C, but higher than the Ac3 temperature.
  • the holding time of the steel plate for hot stamping during austenitizing is controlled within 1 minute to 7 minutes.
  • the holding time in the austenitizing process is limited in detail, and the basic principle is the same as the selection of the above-mentioned heating temperature. If the holding time is less than 1min, the steel plate cannot be completely austenitized, and if the holding time is too long, the coating will also transform into a single-layer alloy layer structure in which AlFe 3 phase and Zn-rich phase coexist, and the corrosion resistance will be reduced. It is limited to within 7 minutes. Further, it is recommended to control the holding time within 3 minutes.
  • the hot stamping forming method of the hot stamping steel plate with Mg-Si coating is to make the steel plate heated at a limited heating temperature and holding time, then carry out hot stamping forming and in-mold quenching, which can avoid cracks in the substrate.
  • the hot stamping forming The operation steps of the method are as follows:
  • Heating transfer the steel plate containing Al-Zn-Mg-Si coating to a heating furnace, and heat the steel plate to a temperature T higher than Ac3 at a heating rate V greater than or equal to 5°C/s and less than or equal to 1000°C/s Ac3 , the heating method here can have multiple choices, which belongs to conventional technology, and the range of heating temperature T needs to satisfy the following relationship:
  • [Al] is the content % of Al in the coating
  • [Zn] is the content % of Zn in the coating
  • V is the heating rate in the heating process, and the unit is °C/s
  • [Al] is the content % of Al in the coating
  • [Zn] is the content % of Zn in the coating, and the unit is min
  • Hot stamping forming and in-mold quenching The heated steel plate is quickly moved to the hot stamping die for stamping and quenching. After the hot stamping is completed, a billet is formed. Cool to room temperature, or cool to room temperature after removing from the mold. In this step, the stamping force of the hot stamping is set at 300-1000 tons, and the holding time is set at 3-15 seconds.
  • a hot stamped steel sheet with Al-Zn-Mg-Si coating and its hot stamping method with excellent corrosion resistance and its hot stamping method according to the present invention will be further explained and described in conjunction with the accompanying drawings and specific examples.
  • the substrates of the hot stamped steel sheets used in the following Examples 1-13 and Comparative Examples 1-5 had the following elemental composition (mass %):
  • the remainder consists of Fe and unavoidable impurities.
  • Hot rolling the billet is heated to 1250°C and then controlled rolling, the starting rolling temperature is 1050°C, the final rolling temperature is 880°C, and the thickness of the hot-rolled plate is less than or equal to 20mm;
  • the above-mentioned steel sheet is further processed to form a plating layer to be used as a hot-stamped steel sheet.
  • the process is as follows:
  • Substrate pretreatment Use 1.2mm thick rolled hard board as the substrate. After degreasing treatment, keep it at 780°C for 120s. The atmosphere in the heating section and the holding section is N2-5 % H2 mixed gas. The dew point is -40°C.
  • the steel plate After dipping for 3 seconds, the steel plate leaves the plating solution, and the thickness of the coating is controlled by the blowing intensity of the air knife. Subsequent cooling is carried out, and the cooling rate of the steel plate is controlled to be 15-25 °C/s within the range of leaving the plating solution to 480 °C, and the cooling rate of the steel plate is controlled to be 40-60 °C/s within the range of 480-280 °C. After cooling to below 280°C, the steel plate enters the water quenching tank and cools to room temperature;
  • the coated steel plate is quickly transferred to a mold for hot stamping and hardening.
  • the heating temperature and holding time of the hot stamping treatment are shown in Tables 1 and 2, respectively.
  • Comparative Example 1 listed in Table 1 is an AlSi coated steel sheet
  • Comparative Example 2 is a pure zinc coated hot stamping steel sheet
  • Comparative Example 3 is an uncoated steel sheet. They were respectively heated to 930 ° C, kept for 3 minutes, transferred to a mold for hot stamping and formed. hardening.
  • Fig. 1 is the microstructure of the coated steel plate of Example 1 of the present invention before hot stamping, as shown in Fig. 1, the microstructure of the coating before hot stamping is by Zn-rich phase, Al-rich phase, Mg-Zn alloy phase, Mg-Si alloy phase phase and Fe-Al-Si alloy layer composition.
  • Fig. 2 is the microstructure of the coated steel plate of Example 2 of the present invention at a heating temperature of 930°C and a holding time of 3 minutes. As shown in Fig. 2, the coating is composed of two layers, and the first layer is 80% ⁇ -Zn+MgZn 2 and 20% AlFe 3 , the second layer is composed of 2% ⁇ -Zn+MgZn 2 and 98% AlFe 3 .
  • Fig. 3 is the microstructure of the coated steel plate in Example 3 of the present invention at a heating temperature of 930°C and a holding time of 7 minutes. As shown in Fig. 3, it can be found that as the holding time prolongs, the coating changes from two layers to a single layer. That is, 20% ⁇ -Zn+MgZn 2 and 80% AlFe 3 composition.
  • Fig. 4 is the microstructure of the coated steel plate of Example 6 of the present invention at a heating temperature of 900° C. and a holding time of 3 minutes. As shown in Fig.
  • the coating is composed of two layers, and the first layer is 43% ⁇ -Zn+MgZn 2 , 37% Al-rich phase and 20% AlFe 3 phase composition, the second layer is 100% AlFe 3 phase.
  • Fig. 5 is the 900 °C of heating temperature of coated steel plate of embodiment 7 of the present invention, and the microstructure after soaking time is 7 minutes is as shown in Fig. 5, and coating also transforms into 35% ⁇ -Zn+MgZn 2 phases and by two-layer structure 65% AlFe 3 phase.
  • the phase proportions of the microstructures in the above-mentioned Figures 1 to 5 are all measured by EBSD.
  • Table 1 The coating phase structure and surface potential of Examples 1-7 and Comparative Examples 1-3
  • the self-corrosion potential of the hot stamped steel sheet of Comparative Examples 1-3 is higher than that of Examples.
  • the substrate potential (-531mV) without coating in Comparative Example 3 it can be found that the potential (-505mV) of the traditional AlSi coating is higher, so once it is in a corrosive medium, the substrate will corrode earlier than the AlSi coating, Thus losing the protective effect of the coating.
  • Comparative Example 2 The hot stamped steel plate with pure zinc coating has a potential of -772mV lower than that of the uncoated substrate, which can effectively form a primary battery and thus play the role of sacrificial anode protection.
  • Example 1 compared with Examples 1-7 of the present invention, the potential is still relatively high, so the coating samples of the embodiments of the present invention have a better effect of protecting the substrate against corrosion after hot stamping. It should be noted that in the case of shorter austenitizing holding time, its corrosion potential is the lowest, as in Example 1 (-1117mV), Example 3 (-1097mV), and Example 4 (-1013mV). Example 5 (-1105mV), Example 6 (-1145mV), so a shorter soak temperature is recommended.
  • Heating transfer the Al-Zn-Mg-Si coated steel sheet to a heating furnace, and heat the steel sheet or strip to a temperature higher than Ac3 at a heating rate V greater than or equal to 5°C/s and less than or equal to 1000°C/s Temperature T Ac3 , the range of heating temperature T must satisfy the following relationship:
  • [Al] is the content % of Al in the coating
  • [Zn] is the content % of Zn in the coating
  • V is the heating rate in the heating process, and the unit is °C/s
  • [Al] is the content % of Al in the coating
  • [Zn] is the content % of Zn in the coating, and the unit is min
  • Hot stamping and in-mold quenching quickly move the heated steel plate to the hot stamping die for stamping and quenching. After the hot stamping is completed, the blank is hardened and cooled in the mold, and then cooled to room temperature in the mold , or cool to room temperature after removing from the mold.
  • Fig. 6 shows the cross-sectional metallographic structure with coating after hot stamping in Example 8 of the present invention, and it can be found that the substrate has no LME cracks.
  • Fig. 7 shows the cross-sectional metallographic structure with coating after hot stamping of Comparative Example 4 of the present invention, and it can be found that there is an LME crack with a depth of about 700um at the substrate position.
  • Fig. 8 is a high-magnification morphology of position A in Fig. 7, and it can be found that the Zn-rich phase diffuses to the substrate.
  • Fig. 6 shows the cross-sectional metallographic structure with coating after hot stamping in Example 8 of the present invention, and it can be found that the substrate has no LME cracks.
  • Fig. 7 shows the cross-sectional metallographic structure with coating after hot stamping of Comparative Example 4 of the present invention, and it can be found that there is an LME crack with a depth of about 700um at the substrate position.
  • Fig. 8 is a
  • the coating has a double-layer structure, and the AlFe 3 phase with a high melting point is close to the substrate, and the Zn-rich phase is concentrated in the surface layer of the coating; while in the comparative example, the Zn-rich phase with a low melting point is gathered in the FeAl 3 phase grain boundary.
  • the The holding temperature of -13 is in the range of T Ac3 ⁇ T ⁇ 1.03T Ac3 +2 ⁇ V ⁇ [Al]/[Zn], and the holding time is in the range of 1 ⁇ t ⁇ 2.7 ⁇ [Al]/[Zn].
  • the proportional heating temperature and holding time all exceed the range defined by the present invention, so severe LME cracks are produced in the hot stamping process.
  • ⁇ means no substrate without LME cracks
  • ⁇ means substrate with LME cracks
  • the hot stamping process of the present invention can avoid the substrate cracks caused by Liquid Metal Embrittlement (Liquid Metal Embrittlement, hereinafter marked as LME) after the hot stamping steel plate of Al-Zn-Mg-Si coating is stamped and formed, to Al-Zn-Mg -
  • LME Liquid Metal Embrittlement

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Abstract

本发明公开了一种具有Al-Zn-Mg-Si镀层的热冲压钢板及其热冲压方法，该Al-Zn-Mg-Si镀层在热冲压前的初始组织由富Zn相，富Al相，MgZn2相，MgSi2相以及Fe-Al-Si合金层组成，在热冲压后，镀层组织在保温时间t≦3min时，镀层由上下两层组成，第一层组织由富Al相，富Zn相和MgZn2相组成，且富Zn相与MgZn2相两相之和的体积百分比例在20％到80％之间；第二层由FeAl3相和富Zn相组成，且富Zn相体积百分比例不超过5％；在保温时间t>3min时，镀层由单层合金层组成，合金层的主要组成为FeAl3相，在FeAl3相之间存在有富Zn相与MgZn2相，且富Zn相和MgZn2相两相之和的体积百分比例为5％～50％；还涉及到热冲压方法。本发明的钢板在热冲压后镀层具有良好的耐腐蚀性能，还能够避免基板上因液态金属脆性导致的裂纹。

Description

一种具有Al-Zn-Mg-Si镀层的热冲压钢板及其热冲压方法 技术领域
本发明涉及热成形技术领域，特别涉及一种具有Al-Zn-Mg-Si镀层的热冲压钢板及其热冲压方法。
背景技术
汽车轻量化技术是适应现代汽车安全、节能、环保趋势的关键技术之一。对冲压成形工艺而言，减薄和高强是恶化成形性的双重因素，不仅使得车身零件在成形过程中容易开裂，而且容易产生过量回弹，影响车身后续的装配。通过热处理的方式实现最终零件的高强度是其中的一种途径，而热冲压成形技术是一种通过热处理和高温成形相结合的方式来实现零件高强度，能较好的解决高强度与冷成形的矛盾问题。
热成形技术生产的零件具有超高强度、成型精度高、无回弹等优点。越来越多的汽车车身零部件采用热成形技术，如A柱、B柱、汽车保险杠、防撞梁、车门防撞梁等。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，由于液态金属脆性(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。目前广泛应用的铝/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提出在Zn中适量添加Al和Mg能进一步提升耐腐蚀性的三个专利申请GB1125965A、US3505043A和US3505042A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。
另外，CN100334250提出了一种耐腐蚀性优良的Zn-Al-Mg-Si合金镀覆钢材及其制造方法，其主要是对添加到Zn-Al类镀层中的Mg和Si的含量进行控制，并对具有提高耐腐蚀性作用的MgSi 2相的析出量和析出形态进行控制，其不仅能解决在涂装后的切断面的耐边缘蠕 变性问题，还能提供性能特别优异的合金镀层。专利CN103805930B通过Galvalume镀层(Zn-55Al)中添加Mg和/或Cr，研究Cr在界面合金层中的分布，提供了加工性能和耐蚀性能都优异的热浸镀Zn-Al-Mg-Cr镀层钢板。但是以上的现有技术，都只是针对冷成型钢板所使用的Zn-Al-Mg系镀层，针对于热冲压的Al-Zn-Mg-Si镀层钢板及其热冲压方法目前尚没有这方面的研究。
发明内容
本发明的目的在于提供了一种具有Al-Zn-Mg-Si镀层的热冲压钢板及其热冲压方法。采用本发明方法制作的热冲压部件具有优良的耐腐蚀性能，能有效避免Al-Zn-Mg-Si镀层钢板热冲压成形过程中由于局部应力和液态金属脆性(LME)导致的基板裂纹。
为了达到上述发明目的，本发明提供的技术方案如下：
一种具有Al-Zn-Mg-Si镀层的热冲压钢板，该热冲压钢板包括基板和镀覆于基板上的Al-Zn-Mg-Si镀层，其特征在于，所述Al-Zn-Mg-Si镀层在厚度方向由两种结构组成，即表层的氧化层和位于表层下方的合金层，所述合金层的厚度为15～45um，其中：所述合金层由上、下两层组织组成，第一层组织由富Al相、富Zn相、MgZn 2相和FeAl 3相组成，且富Zn相与FeAl 3相两相之和所占的体积百分比例在20％到80％之间；第二层组织由FeAl 3相、MgZn 2相和富Zn相组成，且MgZn 2相和富Zn相所占的体积百分比例不超过5％；或所述合金层由单层合金层组成，所述的单层合金层的主要组成为FeAl 3相，在FeAl 3相之间存在有富Zn相与MgZn 2相，且富Zn相和MgZn 2相两相之和所占的体积百分比例为5％～50％。
在一些实施方案中，所述合金层由上、下两层组织组成，第一层组织由富Al相、富Zn相、MgZn 2相和FeAl 3相组成，且富Zn相、MgZn 2相与FeAl 3相三相之和所占的体积百分比例在50％到100％之间；第二层组织由FeAl 3相、MgZn 2相和富Zn相组成，且MgZn 2相和富Zn相所占的体积百分比例不超过5％。
在一些实施方案中，所述合金层由单层合金层组成，所述的单层合金层的主要组成为FeAl 3相，在FeAl 3相之间存在有富Zn相与MgZn 2相，且富Zn相和MgZn 2相两相之和所占的体积百分比例为15％～40％。
在一些实施方案中，本发明提供一种具有Al-Zn-Mg-Si镀层的热冲压钢板，该热冲压钢板包括基板和镀覆于基板上的Al-Zn-Mg-Si镀层，所述Al-Zn-Mg-Si镀层在热冲压后，所述Al-Zn-Mg-Si镀层中的组织在厚度方向由表层的氧化层和下部的合金层两种结构组成，所述合金层的厚度为15～45um，该合金层的组织与奥氏体化过程中的保温时间t呈如下关系：
(1)在保温时间t≦3min时，所述Al-Zn-Mg-Si镀层的合金层由上下两层组成，第一层组织由富Al相，富Zn相和FeAl 3相组成，且富Zn相与FeAl 3相两相之和所占的体积百分比 例在20％到80％之间；第二层由FeAl 3相和富Zn相组成，且富Zn相所占的体积百分比例不超过5％。
(2)在保温时间t>3min时，所述Al-Zn-Mg-Si镀层的合金层由单层合金层组成，所述的单层合金层的主要组成为FeAl 3相，富Zn相与MgZn 2相存在于FeAl 3相之间，且富Zn相和MgZn 2相两相之和所占的体积百分比例为5％～50％。
在本发明一种具有Al-Zn-Mg-Si镀层的热冲压钢板中，所述钢板在热冲压前Al-Zn-Mg-Si镀层的初始组织由富Zn相、富Al相、FeAl 3相、MgSi 2相以及Fe-Al-Si合金层组成，所述Al-Zn-Mg-Si镀层的厚度为8-30um。
在本发明一种具有Al-Zn-Mg-Si镀层的热冲压钢板中，所述钢板在热冲压后镀层表面的氧化层包含(Zn，Al，Mg，Si)氧化物，且Al-Zn-Mg-Si镀层的氧化层厚度在3um以内，如0.5～3um。
在本发明一种具有Al-Zn-Mg-Si镀层的热冲压钢板中，所述Al-Zn-Mg-Si镀层的化学成分质量百分比为：Al：45-65％，Mg：0.2-5％，Si：0.1-3％，余量为Zn和其它不可避免的杂质。
在本发明一种具有Al-Zn-Mg-Si镀层的热冲压钢板中，所述Al-Zn-Mg-Si镀层的化学成分质量百分比为：Al：45-65％，Mg：0.2-5％，Si：0.5-3％，余量为Zn和其它不可避免的杂质。
在本发明一种具有Al-Zn-Mg-Si镀层的热冲压钢板中，所述基板的组成成分按质量百分比计包含：C：0.1％～0.5％；Si：0.05％～2.0％；Mn：0.5％～3.0％；P：≤0.1％；S：≤0.05％；Al：0.01～0.05％；N：0.01％以下；还包括Nb：0.01％～0.1％，V：0.01％～1.0％，Mo：0.01％～1.0％，Ti：0.01％～0.1％，Cr：0.01％～1.0％，Ni：0.01％～1.0％，B：0.001％～0.01％中的至少一种，余量为Fe和不可避免的杂质。
在一些实施方案中，本发明所述具有Al-Zn-Mg-Si镀层的热冲压钢板中，其基板的组成成分按质量百分比计包含：C：0.1％～0.3％；Si：0.05～0.3％；Mn：0.8～1.5％；P：≤0.01％；S：≤0.0005％；Al：0.01～0.05％；N：0.01％以下；还包括Nb：0.01％～0.1％，V：0.01％～1.0％，Mo：0.01％～1.0％，Ti：0.01％～0.1％，Cr：0.1～0.4％，Ni：0.01％～1.0％，B：0.001％～0.01％中的至少一种，余量为Fe和不可避免的杂质。
在一些实施方案中，本发明所述具有Al-Zn-Mg-Si镀层的热冲压钢板中，其基板的组成成分按质量百分比计为：C：0.1％～0.3％，Si：0.05～0.3％，Mn：0.8～1.5％，P：≤0.01％，S：≤0.0005％，Al：0.01～0.05％，N：0.01％以下，Ti：0.01％～0.1％，Cr：0.1～0.4％，和B：0.001％～0.01％，余量为Fe和不可避免的杂质。
本发明所述的钢板中的各化学元素设计原理如下所述：
C：0.1％～0.5％
C含量是决定钢板强度、硬度等力学性能的最主要因素，钢板的强度、硬度随着C含量的升高而提高，钢板的塑性、韧性随着C含量的升高而降低，钢板的冷脆倾向性和时效倾向性随着C含量的升高而提高。而且C含量对刚才淬火后力学性能也有显著影响，因此本发明将C含量的上限规定为0.5％，优选为0.35％，保证钢板具有一定塑性和韧性；下限规定为0.1％，保证钢板具有一定强度。在一些实施方案中，C的含量为0.1～0.3％。
Si：0.05％～2.0％
Si是置换固溶合金元素，可以促进C在奥氏体中的富集，使得奥氏体稳定性增加，提高钢板的强度，并在一定程度上提高其韧性，因此，本发明将Si含量的下限规定为0.05％，但是当Si含量较大时，热轧时产生红色氧化铁皮的表面缺陷概率增大，且轧制力增大，导致热轧钢板的延展性劣化，因此，本发明将Si含量的上限规定为1.0％。为保证钢板的可镀性，Si的上限优选为1.0％。在一些实施方案中，Si的含量为0.05～0.5％；在一些实施方案中，Si的含量为0.05～0.3％。
Mn：0.5％～3.0％
Mn是提高钢板的淬透性，扩大奥氏体相区以及有效保证淬火后钢板强度的元素，同时Mn作为扩大奥氏体相区的元素，可以降低Ac3的温度和Ac1的温度，延缓珠光体相变，从而降低热冲压加热温度。Ac3是指碳钢加热时的相变实际温度，Ac1是钢加热时开始形成奥氏体的温度。Mn含量不足0.5％时，提高钢板的淬透性能力不足。另一方面，Mn含量超过3％时，会产生偏析，导致基体钢板和热冲压零件性能均匀性下降。在一些实施方案中，Mn的含量为0.5～2.0％。在一些实施方案中，Mn的含量为0.8～1.5％。
P：≤0.1％
P在钢中使钢的可塑性及韧性明显下降，特别是在低温下会导致“冷脆”现象，因此要严格控制，限制在0.1％以下。在一些实施方案中，P：≤0.01％。
S：≤0.05％
S含量维持在较低水平，因为由S形成FeS而引起热脆性问题，因而S含量限于0.05％以下。下限不定义，因为由于相同的原因S含量越低越好。在一些实施方案中，S：≤0.0005％。
N：≤0.01％
N含量超过0.01％时，在热轧时会形成AlN的氮化物，导致钢板的冲裁加工性能和淬透性降低。因此，N含量越低越好，设定在≤0.01％。在一些实施方案中，N：≤0.005％。
Al：0.01％～0.05％
Al具有脱氧的作用。当Sol.Al(有效Al)含量＜0.01％时，添加效果不明显；当Sol.Al含量＞0.05％时，脱氧作用饱和且成本增加。因此，本发明将Al含量限定在0.01％～0.05％。
Nb：0.01％～0.1％
Nb是钢中重要的微合金元素，在钢中添加微量的Nb可保证钢在碳当量较低的情况下，通过其碳、氮化物质点(尺寸小于5nm)的弥散析出及Nb的固溶，细化晶粒，极大地提高钢的强度、韧性，特别是低温韧性，同时使钢具有良好的冷弯性能和可焊性。因此，本发明将Nb的含量限定在0.01％～0.1％，可有效细化钢基板的原始奥氏体晶粒，目的在于提高热成形后零件的韧性和冷弯性能。
V：0.01％～1.0％
V是通过形成碳化物使组织微细化的元素。在将钢板加热至Ac3点以上时，微细的V的碳化物抑制再结晶及晶粒生产使奥氏体晶粒细化，改善韧性。当V含量＜0.01％时，添加效果不明显；当V含量＞1.0％时，添加效果饱和且成本增加。
Mo：0.01％～1.0％
Mo与V一样，是奥氏体细化的元素，当Mo含量＜0.01％时，添加效果不明显；当Mo含量＞1.0％时，添加效果饱和且成本增加。
B：0.001％～0.01％
B是钢中晶界偏聚剧烈的元素，能降低奥氏体的晶界能，抑制先共析铁素体晶核的形成，对于提高钢的淬透性具有三大特点：提高淬透性的能力很强；极少量的B元素相当于其他多种贵重合金元素的添加作用；B元素具有提高淬透性的最佳含量且十分微小，不同于一般合金元素提高淬透性的效果随其在钢中含量增加而增加。因此，本发明中B元素含量限定在0.001％～0.01％。
在本发明中，钢板基板的化学成分还可以包括以下至少一种：Ti：0.01％～0.1％，Cr：0.01％～1.0％，Ni：0.01％～1.0％。
Ti：0.01％～0.1％
Ti通过其氮化物的形成起到稳定地发挥前述B的作用，所以是可以有效地利用的元素。为此需要0.01％以上的添加，但是若过多地添加则氮化物变得过剩，会导致韧性的劣化因此上限规定为0.10％。
Cr：0.01％～1.0％
Cr能增加钢的淬透性并有二次硬化作用。铬与铁形成连续固溶体，缩小奥氏体相区域，铬，还可以使珠光体的碳浓度及奥氏体中碳的极限溶解度减少。铬又能提高钢的抗氧化性和耐腐蚀性。当Cr含量＜0.01％时，添加效果不明显；当Cr含量＞1.0％时，添加效果饱和且成本增加。在一些实施方案中，Cr的含量为0.1～0.5％，如0.1～0.4％。
Ni：0.01％～1.0％
Ni能扩大钢的奥氏体区，是形成和稳定奥氏体的主要合金元素，镍还可以强化铁素体并细化和增多珠光体，提高钢的强度。其添加效果在0.01％以上很明显，但由于其是高价元素，因此控制在1.0％以下。
本发明还涉及到一种适用于具有Al-Zn-Mg-Si镀层的热冲压钢板的热冲压方法，使钢板在限定的工艺范围内实现奥氏体化再进行热冲压成型与模内淬火，使热冲压后的部件具有良好的耐腐蚀性能，同时避免热成形过程中由于局部应力和液态金属脆性导致的基板裂纹。该热冲压成形方法的操作步骤如下：
(1)加热：将含有Al-Zn-Mg-Si镀层的钢板传送于加热炉中，以大于等于5℃/s且小于等于1000℃/s的升温速度V将钢板加热到高于Ac3的温度T Ac3，加热温度T的范围需满足以下关系式：
Figure PCTCN2023071554-appb-000001
其中，[Al]为镀层中Al的含量％，[Zn]为镀层中Zn的含量％，V为加热过程中的升温速度，单位为℃/s；
(2)保温：将奥氏体化的钢板进行保温，保温时间t的设定满足以下关系式：
Figure PCTCN2023071554-appb-000002
其中，[Al]为镀层中Al的含量％，[Zn]为镀层中Zn的含量％，单位为min；
(3)热冲压成形与模内淬火：将加热后的钢板快速移动到热冲压模具上冲压成型淬火，热冲压成型完成后形成坯料，该坯料先在模具中进行淬硬冷却，再在模具中冷却至室温，或从模具取出后冷却至室温。
在本发明一种适用于具有Al-Zn-Mg-Si镀层的热冲压钢板的热冲压方法中，所述步骤(1)中的升温速度V大于5℃/s小于1000℃/s。
在本发明一种适用于具有Al-Zn-Mg-Si镀层的热冲压钢板的热冲压方法中，所述步骤(1)中的升温速度V为5～200℃/s。在一些实施方案中，所述升温速度V为5～50℃/s。在一些实施方案中，所述升温速度V为10～50℃/s。
在本发明一种适用于具有Al-Zn-Mg-Si镀层的热冲压钢板的热冲压方法中，所述步骤(1)中钢板的奥氏体化温度控制在930℃及以下，且高于Ac3温度。
在本发明一种适用于具有Al-Zn-Mg-Si镀层的热冲压钢板的热冲压方法中，所述步骤(2)中，所述钢板在奥氏体化的保温时间控制在1分钟到7分钟之间。
在本发明一种适用于具有Al-Zn-Mg-Si镀层的热冲压钢板的热冲压方法中，作为步骤(2)进一步地，所述的保温时间控制在大于1分钟且小于3分钟，也就是说在优化方案中控制保温时间大于1分钟且小于3分钟。
在本发明一种适用于具有Al-Zn-Mg-Si镀层的热冲压钢板的热冲压方法中，所述步骤(3)中热冲压的冲压力为300～1000吨，保压时间为3～15秒。
在一些实施方案中，本发明所述的适用于具有Al-Zn-Mg-Si镀层的热冲压钢板的热冲压方法用于制造本文任一实施方案所述的具有Al-Zn-Mg-Si镀层的热冲压钢板。
在一些实施方案中，本文所述的适用于具有Al-Zn-Mg-Si镀层的热冲压钢板的热冲压方法还包括采用具有以下步骤的方法制备所述具有Al-Zn-Mg-Si镀层的热冲压钢板的步骤：
1)对生产钢板用的原料进行冶炼；
2)连铸生产铸坯；
3)热轧：将铸坯加热到1100～1250℃后控制轧制，开轧温度为950～1150℃，终轧温度为750～900℃，热轧板厚度小于等于20mm；
4)轧后在500～850℃进行卷取，冷却至室温后组织为铁素体和珠光体组织；
5)酸洗以去除热轧过程中产生的氧化铁皮；
6)冷轧：将钢卷冷轧至2.0mm以下的厚度，冷轧压下量≥35％；
7)连续退火、热镀：将获得的轧硬带钢开卷、清洗，加热至均热温度780～850℃，保温30～200s，其中，加热速率为1～20℃/s，加热段和保温段的气氛采用N 2-H 2混合气体，其中H 2的体积含量为0.5～20％，退火气氛的露点为-40～10℃；之后浸入锌锅中热浸镀，镀液的温度范围为565～605℃，钢板离开镀液后进行分段冷却，获得镀覆钢板，即在基板上镀覆有Al-Zn-Mg-Si镀层。
优选地，所述分段冷却包括：在离开镀液至480℃范围内，控制钢板的冷却速度为15～25℃/s，在480～280℃范围内，控制钢板的冷却速度为40～60℃/s，冷却到280℃以下，钢板进入水淬槽中冷却至室温。
基于上述技术方案，本发明的具有Al-Zn-Mg-Si镀层的热冲压钢板及热冲压方法经过了实践应用，取得了如下有益效果：
1.本发明提供具有Al-Zn-Mg-Si镀层的热冲压钢板的热冲压成型方法，在限定的工艺范围内，热冲压后的部件具有良好的耐腐蚀性能。因为在加热过程中首先会优先发生Al-Fe扩散，此时原始镀层中的富Zn相(η-Zn)和MgZn 2相会集中存在于镀层厚度的表层1/3～1/2处，这种表层由富Zn相(η-Zn)，MgZn 2相和富Al相组成，界面层由AlFe 3相组成的双层合金相结构，在处于腐蚀介质中时，由于表层的富Zn相或者MgZn 2相的电位更低，可以与界面合金层，或者基体形成原电池，从而对基体起到牺牲阳极保护的功能。
2.本发明提供具有Al-Zn-Mg-Si镀层的热冲压钢板的热冲压成型方法，在本发明限定的工艺范围内，能够避免由于液态金属脆性(Liquid Metal Embrittlement，LME)导致的基板裂纹，也使得热冲压后的镀层具有良好的耐腐蚀性能。
3.本发明所述的具有Al-Zn-Mg-Si镀层的热冲压钢板，在加热过程中首先会优先发生Al-Fe扩散，此时原始镀层中的富Zn相(η-Zn)和MgZn 2相会集中存在于镀层厚度的表层1/3～1/2处，但如果加热温度过高，Al-Fe充分扩散后由于Zn-Fe扩散的驱动力，镀层中的富Zn相则会扩散到AlFe 3相的晶界中，在模具成形过程中，由于低熔点的富Zn相仍为液态，在应力的作用下，会降低AlFe 3相的结合力，引起液态金属脆性导致的宏观裂纹，因此将加热温度限定在1.03T Ac3+2×V×[Al]/[Zn]以下。
4.本发明所述的具有Al-Zn-Mg-Si镀层的热冲压钢板，将热冲压成形用的钢板的保温时间做了详细限定，若保温时间低于1min，钢板无法完全奥氏体化，而保温时间过长，同样镀层会转变程(Al，Fe)相和富Zn相共存的单层合金层结构，引起液态金属脆性(Liquid Metal Embrittlement，LME)导致的宏观裂纹，因此将保温时间限定在2.7×[Al]/[Zn]以下。
附图说明
图1是本发明实施例1在热冲压之前的微观组织。
图2是本发明实施例1在热冲压之后的微观组织。
图3是本发明实施例2在热冲压之后的微观组织。
图4是本发明实施例5在热冲压之后的微观组织。
图5是本发明实施例6在热冲压之后的微观组织。
图6是本发明实施例8在热冲压之后的截面形貌。
图7是本发明对比例4在热冲压之后的LME特征以及镀层高倍下的截面形貌。
图8是本发明对比例图7中位置A的高倍形貌。
图9是本发明对比例图7中位置B的高倍形貌。
具体实施方式
为使本发明的目的、技术方案和优点更加清楚，下面将结合附图对本发明的实施方式作进一步地详细描述。
本发明涉及到一种具有Al-Zn-Mg-Si镀层的热冲压钢板，该钢板包括基板和镀覆于基板上的Al-Zn-Mg-Si镀层。在热冲压前所述Al-Zn-Mg-Si镀层的初始组织由富Zn相、富Al相、FeAl 3相、MgSi 2相以及Fe-Al-Si合金层组成，所述Al-Zn-Mg-Si镀层的厚度为8-30um。在热冲压后，所述镀层组织在厚度方向由两种结构组成，即：由表层的氧化层和表层下部的合金层组成，所述合金层的厚度为15～45um，该合金层的组织与奥氏体化过程中的保温时间t呈如下关系：
(1)在保温时间t≦3min时，所述Al-Zn-Mg-Si镀层的合金层由上下两层组成，第一层 组织由富Al相，富Zn相和FeAl 3相组成，且富Zn相与FeAl 3相两相之和占所有组织的体积百分比例在20％到80％之间；第二层由FeAl 3相和富Zn相组成，且富Zn相占所有组织体积百分比例不超过5％。
(2)在保温时间t>3min时，所述Al-Zn-Mg-Si镀层的合金层由单层合金层组成，所述的单层合金层的主要组成为FeAl 3相，富Zn相与MgZn 2相存在于FeAl 3相之间，且富Zn相和MgZn 2相两相之和在所有组织中所占的体积百分比例为5％～50％。
在本发明一种具有Al-Zn-Mg-Si镀层的热冲压钢板中，所述Al-Zn-Mg-Si镀层的化学成分质量百分比为：Al:45-65％，Mg：0.2-5％，Si:0.1-3％，余量为Zn和其它不可避免的杂质。
在本发明一种具有Al-Zn-Mg-Si镀层的热冲压钢板中，所述钢板在热冲压后镀层表面的氧化层包含(Zn，Al，Mg，Si)氧化物，且镀层的氧化层厚度在3um以内。(Zn，Al，Mg，Si)氧化物是指锌氧化物、铝氧化物、镁氧化物和硅氧化物的混合物。
在本发明一种具有Al-Zn-Mg-Si镀层的热冲压钢板中，所述基板的组成成分按质量百分比计包含：C：0.1％～0.5％；Si：0.05％～2.0％；Mn：0.5％～3.0％；P：≤0.1％；S：≤0.05％；Al：0.01～0.05％；N：0.01％以下；还包括Nb：0.01％～0.1％，V：0.01％～1.0％，Mo：0.01％～1.0％，Ti：0.01％～0.1％，Cr：0.01％～1.0％，Ni：0.01％～1.0％，B：0.001％～0.01％中的至少一种，余量为Fe和不可避免的杂质。
上述热冲压成形用的具有Al-Zn-Mg-Si镀层的热冲压钢板的生产过程如下：
1)对生产钢板用的原料进行冶炼；
2)连铸生产铸坯；
3)热轧：将铸坯加热到1100～1250℃后控制轧制，开轧温度为950～1150℃，终轧温度为750～900℃，热轧板厚度小于等于20mm；
4)轧后在500～850℃进行卷取，冷却至室温后组织为铁素体和珠光体组织；
5)酸洗以去除热轧过程中产生的氧化铁皮；
6)冷轧：将钢卷冷轧至2.0mm以下的厚度，冷轧压下量≥35％；
7)连续退火、热镀：将获得的轧硬带钢开卷、清洗，加热至均热温度780～850℃，保温30～200s，其中，加热速率为1～20℃/s，加热段和保温段的气氛采用N 2-H 2混合气体，其中H 2体积含量为0.5～20％，退火气氛的露点为-40～10℃；之后快冷浸入锌锅中热浸镀，镀液的温度范围为565-605℃，钢板离开镀液后进行分段冷却，获得镀覆钢板，即在基板上镀覆有Al-Zn-Mg-Si镀层。
在上述热冲压钢板的生产过程中，步骤7)中将镀液的温度控制在565-605℃，是因为当镀液的温度低于565℃时，镀液在锌锅内流动性降低，镀层的厚度控制变得困难，导致镀层的均匀性难以保证，特别是镀层厚度较薄时均匀性更差。当锌锅温度高于605℃时，钢板在 锌锅中的溶解以及锌锅表面镀液的氧化都将加剧，会导致热浸镀过程中镀液底渣、面渣都增多；此外，镀液温度高于605℃，也会导致热浸镀装置例如炉箅子内Zn的蒸发加剧，从而引起钢板表面锌灰等缺陷增多。
本发明特别地对镀层钢板在热冲压后的镀层组织在不同的保温时间下的不同相组成做了清晰界定。这是因为发明人在经过大量研究发现，镀层的组织中各相的组成与奥氏体化过程中的保温时间关系最大，且热冲压后镀层内不同相组成对其后续的耐腐蚀性能影响很大，因此在大量实验的基础上，结合后续的热冲压方法，对产生最优耐蚀性组织的热冲压方法也做了界定。
本发明还涉及到一种适用于具有Al-Zn-Mg-Si镀层的热冲压钢板的热冲压方法，使钢板在限定的工艺范围内实现奥氏体化再进行热冲压成型与模内淬火，使热冲压后的部件具有良好的耐腐蚀性能，该热冲压方法中包括奥氏体化环节和热冲压成形与模内淬火环节：
奥氏体化环节：将作为基板的钢板输送至温度高于Ac3的加热炉中，将钢板完全奥氏体化并保温；
热冲压成型与模内淬火环节：将加热后的钢板快速移动到热冲压模具上冲压成型淬火，热冲压成型完成后形成坯料，该坯料先在模具中进行淬硬冷却，再在模具中冷却至室温，或从模具取出后冷却至室温。
在前面的奥氏体化环节中，将热冲压成型用的钢板的加热温度和加热时间做了详细限定，这是因为发明人在通过大量研究发现，奥氏体化过程中的加热温度和保温时间影响了加热后镀层的组织组成，从而影响材料的耐腐蚀性能。如果加热温度低于钢板的Ac3温度以下，钢板无法进入单相奥氏体区，在后续的淬硬过程中无法完成马氏体转变。在加热过程中首先会优先发生Al-Fe扩散，此时原始镀层中的富Zn相(η-Zn)和MgZn 2相会集中存在于镀层厚度的表层1/3～1/2处。这种表层由富Zn相(η-Zn)，MgZn 2相和富Al相组成，界面层由AlFe 3相组成的双层合金相结构，在处于腐蚀介质中时，由于表层的富Zn相或者MgZn 2相的电位更低，可以与界面合金层，或者基体形成原电池，从而对基体起到牺牲阳极保护的功能。如果加热温度过高，Al-Fe充分扩散后由于Zn-Fe扩散的驱动力，镀层中的富Zn相则会扩散到AlFe 3相的晶界中，镀层的耐腐蚀性能大大降低，因此本发明将加热温度限定在930℃以下，但要高于Ac3温度。
在前面的奥氏体化环节中，将热冲压成型用的钢板在奥氏体化的保温时间控制在1分钟到7分钟。将奥氏体化过程中的保温时间做了详细限定，其基本原理与上述加热温度的选择相同。若保温时间低于1min，钢板无法完全奥氏体化，而保温时间过长，同样镀层会转变程AlFe 3相和富Zn相共存的单层合金层结构，耐腐蚀性能降低，因此将保温时间限定在7min以内，进一步的，推荐将保温时间控制在3分钟以内。
另外，为了避免Al-Zn-Mg-Si镀层钢板热成形过程中由于局部应力和液态金属脆性(Liquid Metal Embrittlement，缩写为LME)导致的基板裂纹，本发明深入研究了适用于具有Al-Zn-Mg-Si镀层的热冲压钢板的热冲压成形方法，目的是使钢板在限定的加热温度和保温时间下加热完成后，进行热冲压成形与模内淬火，可避免进而基板裂纹，该热冲压成形方法的操作步骤如下：
(1)加热：将含有Al-Zn-Mg-Si镀层的钢板传送于加热炉中，以大于等于5℃/s小于等于1000℃/s的升温速度V将钢板加热到高于Ac3的温度T Ac3，此处加热方式可以有多种选择，属于常规技术，加热温度T的范围需满足以下关系式：
Figure PCTCN2023071554-appb-000003
其中，[Al]为镀层中Al的含量％，[Zn]为镀层中Zn的含量％，V为加热过程中的升温速度，单位为℃/s；
(2)保温：将奥氏体化的钢板进行保温，保温时间t的设定满足以下关系式：
Figure PCTCN2023071554-appb-000004
其中，[Al]为镀层中Al的含量％，[Zn]为镀层中Zn的含量％，单位为min；
(3)热冲压成形与模内淬火：将加热后的钢板快速移动到热冲压模具上冲压成型淬火，热冲压成型完成后形成坯料，该坯料先在模具中进行淬硬冷却，再在模具中冷却至室温，或从模具取出后冷却至室温。本步骤中热冲压的冲压力为设定300～1000吨，保压时间设定为3～15秒。
下面将结合说明书附图和具体的实施例对本发明所述的一种耐腐蚀性能优良的具有Al-Zn-Mg-Si镀层的热冲压钢板及其热冲压方法做进一步的解释和说明。
下述实施例1-13和对比例1-5所用的热冲压钢板的基板具有以下元素组成(质量％)：
C Si Mn P S Al Cr Ti B N
0.22 0.19 1.14 0.006 0.0001 0.05 0.25 0.031 0.0025 0.0040
其余由Fe和不可避免的杂质组成。
这些热冲压钢板的生产过程如下：
(1)按上述成分对生产钢板用的原料进行冶炼；
(2)连铸生产铸坯；
(3)热轧：将铸坯加热到1250℃后控制轧制，开轧温度为1050℃，终轧温度为880℃，热轧板厚度小于等于20mm；
(4)轧后在550℃进行卷取，冷却至室温后组织为铁素体和珠光体组织；
(5)酸洗以去除热轧过程中产生的氧化铁皮；
(6)冷轧：将钢卷冷轧至2.0mm以下的厚度，冷轧压下量≥35％；
(7)连续退火、热镀；
上述钢板进一步处理以形成镀层，以作为热冲压钢板来使用。其过程如下：
1)基板预处理：采用1.2mm厚度的轧硬板作为基板，经脱脂处理后，在780℃下保温120s，加热段和保温段的气氛采用N 2-5％H 2混合气体，退火气氛的露点为-40℃。
2)将基板浸入镀液中热浸镀，所述镀液的温度为565～605℃，各实施例所采用的镀液的化学成分配比详见表1。
3)浸镀3秒钟后，钢板离开镀液，通过气刀吹扫强度控制镀层的厚度。随后进行分段冷却，在离开镀液至480℃范围内，控制钢板的冷却速度为15～25℃/s，在480～280℃范围内，控制钢板的冷却速度为40～60℃/s，冷却到280℃以下，钢板进入水淬槽中冷却至室温；
(8)将上述步骤生产的不同成分配比的Al-Zn-Mg-Si镀层钢板落料，并将落料片传送至高于钢板奥氏体化温度(即Ac3温度)的加热炉中；
(9)将镀层钢板快速转移至模具中进行热冲压成形并淬硬。
热冲压处理的加热温度和保温时间分别如表1和2所示。
实施例1-7和对比例1-3
表1中所列对比例1为AlSi镀层钢板，对比例2为纯锌镀层热冲压钢板，对比例3为无镀层钢板，分别加热至930℃，保温3min，转移到模具中进行热冲压成形并淬硬。
对实施例1-7和对比例1-3的样板取样，采用Zeiss扫描电子显微镜对镀层表面组织进行分析，并采用EBSD对相组织比例进行解析。在Gamry Reference 600恒电位仪上进行电化学试验测定各样板的自腐蚀电位。参比电极为饱和甘汞电极，辅助电极为铂电极，试样为工作电极，测试面积为1cm 2。试验在常温下的3.5wt％NaCl溶液中进行。试验前，先将工作电极固定在电解液中浸泡30min，以获得稳定的开路电位。获得的动电位极化曲线用恒电位仪配套的Gamry Echem Analyst软件进行Tafel拟合出镀层的自腐蚀电位。
图1为本发明实施例1镀层钢板在热冲压之前的微观组织，如图1所示，热冲压前镀层的微观组织由富Zn相，富Al相，Mg-Zn合金相，Mg-Si合金相以及Fe-Al-Si合金层组成。图2为本发明实施例2镀层钢板在加热温度930℃，保温时间为3分钟之后的微观组织，如图2所示，镀层由两层组成，第一层为80％的η-Zn+MgZn 2和20％的AlFe 3，第二层为2％的η-Zn+MgZn 2和98％的AlFe 3组成。图3为本发明实施例3镀层钢板在加热温度930℃，保温时间为7分钟之后的微观组织，如图3所示，可以发现随着保温时间的延长，镀层由两层转变成单层，即20％的η-Zn+MgZn 2和80％的AlFe 3组成。图4为本发明实施例6镀层钢板在 加热温度900℃，保温时间为3分钟之后的微观组织，如图4所示，镀层由两层组成，第一层为43％的η-Zn+MgZn 2，37％的富Al相和20％的AlFe 3相组成，第二层为100％的AlFe 3相。图5为本发明实施例7镀层钢板在加热温度900℃，保温时间为7分钟之后的微观组织如图5所示，镀层也由两层组织转变为35％的η-Zn+MgZn 2相和65％的AlFe 3相。上述图1至图5中微观组织的相比例均采用EBSD测定。
表1：实施例1-7和对比例1-3的镀层相结构与表面电位
Figure PCTCN2023071554-appb-000005
注：表中“-”表示含量为0。
根据表1可知，与本发明所列举的实施例1-7相比，对比例1-3热冲压钢板的自腐蚀电位比实施例高。特别是与对比例3无镀层的基板电位(-531mV)相比，可以发现传统的AlSi镀层的电位(-505mV)更高，因此一旦处于腐蚀介质中，基板会比AlSi镀层更早发生腐蚀，从而失去镀层的保护作用。对比例2纯锌镀层热冲压钢板，其电位-772mV比无镀层的基板低，可以有效形成原电池，从而起到牺牲阳极的保护作用。但是，与本发明的实施例1-7相比，其电位还是偏高，因此本发明的实施例镀层样板，在热冲压后具有更好的保护基板耐腐蚀效果。特别需要注意到是，在奥氏体化保温时间更短的情况下，其腐蚀电位最低，如实施例1(-1117mV)，实施例3(-1097mV)，实施例4(-1013mV)，实施例5(-1105mV)，实施例6(-1145mV)，因此推荐使用较短的保温温度。
实施例8-13和对比例4-5
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，本发明的热冲压成形工艺具体过程如下：
(1)加热：将Al-Zn-Mg-Si镀层钢板传送于加热炉中，以大于等于5℃/s且小于等于1000℃/s的升温速度V将钢板或钢带加热到高于Ac3的温度T Ac3，加热温度T的范围需满足以下关系式：
Figure PCTCN2023071554-appb-000006
其中，[Al]为镀层中Al的含量％，[Zn]为镀层中Zn的含量％，V为加热过程中的升温速度，单位为℃/s；
(2)保温：将奥氏体化的钢板进行保温，保温时间t的设定满足以下关系式：
Figure PCTCN2023071554-appb-000007
其中，[Al]为镀层中Al的含量％，[Zn]为镀层中Zn的含量％，单位为min；
(3)热冲压成形与模内淬火：将加热后的钢板快速移动到热冲压模具上冲压成形淬火，热冲压成形完成后，坯料在模具中进行淬硬冷却，然后可以在模具中冷却至室温，或从模具取出后冷却至室温。
对实施例8-13和对比例4-5的样板取样，采用Zeiss扫描电子显微镜对镀层截面组织进行分析基板的LME裂纹情况。图6所示为本发明实施例8热冲压后带镀层的截面金相组织，可以发现基板无LME裂纹。图7所示为本发明对比例4热冲压后带镀层的截面金相组织，可以发现基板位置有深度约700um的LME裂纹。图8为图7中的位置A的高倍形貌，可以发现富Zn相扩散至基板。图9为图7中位置B的高倍形貌，可以发现镀层的组织与实施例 (图6)明显不同。实施例中镀层为双层结构，靠近基体为熔点很高的AlFe 3相，富Zn相集中于镀层表层中；而对比例中低熔点的富Zn相聚集在FeAl 3相晶界。主要原因是对比例中对应设计基板的Ac3温度为825℃，加热炉的升温速度V＝5℃/s，根据表2中镀液成分的[Al]/[Zn]比，可以发现实施例8-13的保温温度在T Ac3<T<1.03T Ac3+2×V×[Al]/[Zn]范围内，保温时间在1≦t<2.7×[Al]/[Zn]范围内，而对比例的加热温度和保温时间均超过了本发明界定的范围，因此热冲压成形过程中产了严重的LME裂纹。
表2：实施例8-13和对比例4-5的基板LME情况对比
Figure PCTCN2023071554-appb-000008
注：○为无基板无LME裂纹，Δ为基板有LME裂纹。
本发明所述的热冲压工艺可以避免Al-Zn-Mg-Si镀层的热冲压钢板冲压成形后由于液态金属脆性(Liquid Metal Embrittlement，以下标记为LME)引起的基体裂纹，对Al-Zn-Mg-Si镀层热冲压用钢的发展具有重要意义。
虽然通过参照本发明的某些优选实施方式，已经对本发明进行了图示和描述，但本领域的普通技术人员应该明白，以上内容是结合具体的实施方式对本发明所作的进一步详细说明，不能认定本发明的具体实施只局限于这些说明。本领域技术人员可以在形式上和细节上对其作各种改变，包括做出若干简单推演或替换，而不偏离本发明的精神和范围。

Claims (15)

  1. 一种具有Al-Zn-Mg-Si镀层的热冲压钢板，该热冲压钢板包括基板和镀覆于基板上的Al-Zn-Mg-Si镀层，其特征在于，所述Al-Zn-Mg-Si镀层在厚度方向由两种结构组成，即表层的氧化层和位于表层下方的合金层，所述合金层的厚度为15～45um，其中：
    所述合金层由上、下两层组织组成，第一层组织由富Al相、富Zn相、MgZn 2相和FeAl 3相组成，且富Zn相与FeAl 3相两相之和所占的体积百分比例在20％到80％之间；第二层组织由FeAl 3相、MgZn 2相和富Zn相组成，且MgZn 2相和富Zn相所占的体积百分比例不超过5％；或
    所述合金层由单层合金层组成，所述的单层合金层的主要组成为FeAl 3相，在FeAl 3相之间存在有富Zn相与MgZn 2相，且富Zn相和MgZn 2相两相之和所占的体积百分比例为5％～50％。
  2. 根据权利要求1所述的一种具有Al-Zn-Mg-Si镀层的热冲压钢板，其特征在于，所述热冲压钢板在热冲压前Al-Zn-Mg-Si镀层的初始组织由富Zn相、富Al相、FeAl 3相、MgSi 2相以及Fe-Al-Si合金层组成，所述Al-Zn-Mg-Si镀层的厚度为8-30um。
  3. 根据权利要求1所述的一种具有Al-Zn-Mg-Si镀层的热冲压钢板，其特征在于，所述氧化层中包含Zn氧化物、Al氧化物、Mg氧化物和Si氧化物，且所述氧化层的厚度在3um以内。
  4. 根据权利要求1所述的一种具有Al-Zn-Mg-Si镀层的热冲压钢板，其特征在于，所述合金层由上、下两层组织组成，第一层组织由富Al相、富Zn相、MgZn 2相和FeAl 3相组成，且富Zn相、MgZn 2相与FeAl 3相三相之和所占的体积百分比例在50％到100％之间；第二层组织由FeAl 3相、MgZn 2相和富Zn相组成，且MgZn 2相和富Zn相所占的体积百分比例不超过5％。
  5. 根据权利要求1～3中任一项所述的一种具有Al-Zn-Mg-Si镀层的热冲压钢板，其特征在于，所述合金层由单层合金层组成，所述的单层合金层的主要组成为FeAl 3相，在FeAl 3相之间存在有富Zn相与MgZn 2相，且富Zn相和MgZn 2相两相之和所占的体积百分比例为15％～40％。
  6. 根据权利要求1～5中任一项所述的一种具有Al-Zn-Mg-Si镀层的热冲压钢板，其特征在于，所述富Zn相和MgZn 2相集中存在于所述镀层厚度的1/3～1/2处。
  7. 根据权利要求1～6中任一项所述的一种具有Al-Zn-Mg-Si镀层的热冲压钢板，其特征在于，所述Al-Zn-Mg-Si镀层的化学成分质量百分比为：Al：45-65％，Mg：0.2-5％，Si：0.1-3％，余量为Zn和其它不可避免的杂质；优选地，所述Al-Zn-Mg-Si镀层的化学成分质量百分比为：Al：45-65％，Mg：0.2-5％，Si：0.5-3％，余量为Zn和其它不可避免的杂质。
  8. 根据权利要求1～7中任一项所述的一种具有Al-Zn-Mg-Si镀层的热冲压钢板，其特征在于，所述基板的组成成分按质量百分比计包含：C：0.1％～0.5％；Si：0.05％～2.0％；Mn：0.5％～3.0％；P：≤0.1％；S：≤0.05％；Al：0.01～0.05％；N：0.01％以下；还包括Nb：0.01％～0.1％，V：0.01％～1.0％，Mo：0.01％～1.0％，Ti：0.01％～0.1％，Cr：0.01％～1.0％，Ni：0.01％～1.0％，B：0.001％～0.01％中的至少一种，余量为Fe和不可避免的杂质。
  9. 根据权利要求8所述的一种具有Al-Zn-Mg-Si镀层的热冲压钢板，其特征在于，所述基板的组成成分按质量百分比计包含：C：0.1％～0.3％；Si：0.05～0.3％；Mn：0.8～1.5％；P：≤0.01％；S：≤0.0005％；Al：0.01～0.05％；N：0.01％以下；还包括Nb：0.01％～0.1％，V：0.01％～1.0％，Mo：0.01％～1.0％，Ti：0.01％～0.1％，Cr：0.1～0.4％，Ni：0.01％～1.0％，B：0.001％～0.01％中的至少一种，余量为Fe和不可避免的杂质。
  10. 根据权利要求8所述的一种具有Al-Zn-Mg-Si镀层的热冲压钢板，其特征在于，所述基板的组成成分按质量百分比计为：C：0.1％～0.3％，Si：0.05～0.3％，Mn：0.8～1.5％，P：≤0.01％，S：≤0.0005％，Al：0.01～0.05％，N：0.01％以下，Ti：0.01％～0.1％，Cr：0.1～0.4％，和B：0.001％～0.01％，余量为Fe和不可避免的杂质。
  11. 一种用于具有Al-Zn-Mg-Si镀层的热冲压钢板的热冲压方法，其特征在于，该热冲压方法的操作步骤如下：
    (1)加热：将含有Al-Zn-Mg-Si镀层的钢板传送于加热炉中，以大于等于5℃/s且小于等于1000℃/s的升温速度V将钢板加热到高于Ac3的温度T Ac3，加热温度T的范围需满足以下关系式：
    Figure PCTCN2023071554-appb-100001
    其中，[Al]为镀层中Al的含量％，[Zn]为镀层中Zn的含量％，V为加热过程中的升温速度，单位为℃/s；
    (2)保温：将奥氏体化的钢板进行保温，保温时间t的设定满足以下关系式：
    Figure PCTCN2023071554-appb-100002
    其中，[Al]为镀层中Al的含量％，[Zn]为镀层中Zn的含量％，单位为min；
    (3)热冲压成形与模内淬火：将加热后的钢板移动到热冲压模具上冲压成型淬火，热冲压成型完成后形成坯料，该坯料先在模具中进行淬硬冷却，再在模具中冷却至室温，或从模具取出后冷却至室温。
  12. 根据权利要求11所述的具有Al-Zn-Mg-Si镀层的热冲压钢板的热冲压方法，其特征在于：
    在步骤(1)中，所述热冲压钢板的奥氏体化温度控制在930℃及以下，且高于Ac3温度；和/或
    在步骤(2)中，所述热冲压钢板在奥氏体化的保温时间控制在1分钟到7分钟，或所述的保温时间控制在大于1分钟且小于3分钟；和/或
    步骤(3)中热冲压保压时间为3～15秒，冲压力为300～1000吨。
  13. 根据权利要求11所述的具有Al-Zn-Mg-Si镀层的热冲压钢板的热冲压方法，其特征在于，所述Al-Zn-Mg-Si镀层的化学成分质量百分比为：Al：45-65％，Mg：0.2-5％，Si：0.5-3％，余量为Zn和其它不可避免的杂质。
  14. 根据权利要求11所述的具有Al-Zn-Mg-Si镀层的热冲压钢板的热冲压方法，其特征在于，所述热冲压钢板的基板的组成成分按质量百分比计包含：C：0.1％～0.5％；Si：0.05％～2.0％；Mn：0.5％～3.0％；P：≤0.1％；S：≤0.05％；Al：0.01～0.05％；N：0.01％以下；还包括Nb：0.01％～0.1％，V：0.01％～1.0％，Mo：0.01％～1.0％，Ti：0.01％～0.1％，Cr：0.01％～1.0％，Ni：0.01％～1.0％，B：0.001％～0.01％中的至少一种，余量为Fe和不可避免的杂质；优选地，所述基板的组成成分按质量百分比计包含：C：0.1％～0.3％；Si：0.05～0.3％；Mn：0.8～1.5％；P：≤0.01％；S：≤0.0005％；Al：0.01～0.05％；N：0.01％以下；还包括Nb：0.01％～0.1％，V：0.01％～1.0％，Mo：0.01％～1.0％，Ti：0.01％～0.1％，Cr：0.1～0.4％，Ni：0.01％～1.0％，B：0.001％～0.01％中的至少一种，余量为Fe和不可避免的杂质；更优选地，所述基板的组成成分按质量百分比计为：C：0.1％～0.3％，Si：0.05～0.3％，Mn：0.8～1.5％，P：≤0.01％，S：≤0.0005％，Al：0.01～0.05％，N：0.01％以下，Ti：0.01％～0.1％，Cr：0.1～0.4％，和B：0.001％～0.01％，余量为Fe和不可避免的杂质。
  15. 根据权利要求11所述的具有Al-Zn-Mg-Si镀层的热冲压钢板的热冲压方法，其特征在于，所述具有Al-Zn-Mg-Si镀层的热冲压钢板采用包括以下步骤的方法制备：
    1)对生产钢板用的原料进行冶炼；
    2)连铸生产铸坯；
    3)热轧：将铸坯加热到1100～1250℃后控制轧制，开轧温度为950～1150℃，终轧温度为750～900℃，热轧板厚度小于等于20mm；
    4)轧后在500～850℃进行卷取，冷却至室温后组织为铁素体和珠光体组织；
    5)酸洗以去除热轧过程中产生的氧化铁皮；
    6)冷轧：将钢卷冷轧至2.0mm以下的厚度，冷轧压下量≥35％；
    7)连续退火、热镀：将获得的轧硬带钢开卷、清洗，加热至均热温度780～850℃，保温30～200s，其中，加热速率为1～20℃/s，加热段和保温段的气氛采用N 2-H 2混合气体，其中H 2的体积含量为0.5～20％，退火气氛的露点为-40～10℃；之后浸入锌锅中热浸镀，镀液的温度范围为565～605℃，钢板离开镀液后进行分段冷却，获得镀覆钢板，即在基板上镀覆有Al-Zn-Mg-Si镀层；
    优选地，所述分段冷却包括：在离开镀液至480℃范围内，控制钢板的冷却速度为15～25℃/s，在480～280℃范围内，控制钢板的冷却速度为40～60℃/s，冷却到280℃以下，钢板进入水淬槽中冷却至室温。
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