Hot stamping steel sheet having Al-Zn-Mg-Si plating layer and hot stamping method thereof

Abstract

The present invention discloses a hot stamping steel sheet having an Al-Zn-Mg-Si coating layer and a hot stamping method thereof. Before hot stamping, the initial structure of the Al-Zn-Mg-Si coating layer is composed of a Zn-rich phase, an Al-rich phase, an MgZn2 phase, an MgSi2 phase and an Fe-Al-Si alloy layer. After hot stamping, when the coating layer is held for a holding time t≦3 min, the coating layer is composed of upper and lower two layers, the structure of the first layer is composed of an Al-rich phase, a Zn-rich phase and an MgZn2 phase, and Z The total volume percentage of the two phases, the n-rich phase and the MgZn2 phase, is 20%-80%, the second layer is composed of FeAl3 phase and Zn-rich phase, and the volume percentage of the Zn-rich phase does not exceed 5%. When the holding time t>3min, the coating layer is composed of a single-layer alloy layer, the main composition of the alloy layer is FeAl3 phase, the Zn-rich phase and the MgZn2 phase exist between the FeAl3 phase, and the total volume percentage of the two phases, the Zn-rich phase and the MgZn2 phase, is 5%-50%. The present invention also relates to a hot stamping method. After hot stamping, the coating layer of the steel sheet of the present invention has good corrosion resistance and can avoid cracking of the substrate caused by liquid metal embrittlement.

Description

[Technical field]

[0001] The present invention relates to the field of hot forming technology, and in particular to a hot stamping steel sheet having an Al-Zn-Mg-Si plating layer and a hot stamping method thereof. [Background technology]

[0002] Automotive lightweight technology is one of the key technologies for adapting to the trend of modern automobile safety, energy saving and environmental protection. In the stamping forming process, thinning and high strength are the dual factors that deteriorate the formability, which not only makes the car body parts more likely to crack during forming, but also makes it more likely to cause excessive springback, which affects the subsequent car body assembly. Achieving high strength of the final parts by heat treatment is one of the methods, and hot stamping forming technology is the combination of heat treatment and high temperature forming to achieve high strength of parts, which can adequately solve the contradiction between high strength and cold forming.

[0003] Parts manufactured by thermoforming technology have the advantages of ultra-high strength, high forming accuracy, no springback and so on. More and more auto body parts adopt thermoforming technology, such as A-pillar, B-pillar, car bumper, anti-crash beam, door anti-crash beam, etc. In the traditional hot stamping parts without plating layer, decarburization and oxidation peeling occur on the surface of the stamping steel sheet during the heating process. In order to avoid the oxidation and decarburization of the surface of the hot stamping steel sheet and make the hot stamping steel sheet have heat resistance and corrosion resistance, a plating technology suitable for hot stamping steel sheet has been developed. At present, the plating layer of hot stamping mainly includes aluminum silicon (Al-10Si) plating layer, hot-dip pure zinc (GI) plating layer, alloyed zinc iron (GA) plating layer, electroplated zinc nickel (Zn-10Ni) plating layer, etc.

[0004] In the process of direct hot stamping, zinc-based plated hot stamping steels that can provide cathodic corrosion protection can cause microcracks (10um~100um) in the matrix of the steel sheet due to the action of liquid metal embrittlement (LME), and even macroscopic cracks that extend through the entire thickness direction of the sheet, which has hindered the application and development of zinc-based hot stamping plated steel sheets. The currently widely used aluminum / silicon plated layer cannot provide cathodic corrosion protection, so in order to further improve the corrosion resistance of the plated layer, the zinc plated material has also evolved from pure zinc to zinc alloys. Since Inland Steel proposed three patent applications GB1125965A, US3505043A, and US3505042A, which claimed that the corrosion resistance can be further improved by adding appropriate amounts of Al and Mg to Zn, people have carried out various developments and researches on such Zn-Al-Mg plated steel sheets, mainly focusing on further improving the corrosion resistance or facilitating production and reducing production costs by mixing various other additive elements or restricting the manufacturing process parameters.

[0005] In addition, CN100334250 proposes a Zn-Al-Mg-Si alloy-plated steel material with excellent corrosion resistance and its manufacturing method, mainly by controlling the content of Mg and Si added to the Zn-Al-based plating layer and controlling the amount and precipitation form of MgSi2 phase, which has the effect of improving corrosion resistance, not only can the problem of edge creep resistance of the cut surface after painting be solved, but also an alloy plating layer with particularly excellent performance can be provided. Patent CN103805930B adds Mg and / or Cr to a Galvalume plating layer (Zn-55Al) and studies the distribution of Cr in the interface alloy layer to provide a hot-dip Zn-Al-Mg-Cr-plated steel sheet with excellent workability and corrosion resistance. However, the above existing technology is only a technology for the Zn-Al-Mg-based plating layer used in cold-formed steel sheets, and there has been no research on hot-stamped Al-Zn-Mg-Si-plated steel sheet and its hot-stamping method. Summary of the Invention [Problem to be solved by the invention]

[0006] The present invention aims to provide a hot stamping steel sheet having an Al-Zn-Mg-Si coating layer and a hot stamping method thereof. The hot stamping part manufactured by the method of the present invention has excellent corrosion resistance and can effectively avoid substrate cracking caused by local stress and liquid metal embrittlement (LME) during hot stamping of the Al-Zn-Mg-Si coating sheet. [Means for solving the problem]

[0007] In order to achieve the above invention objectives, the technical solutions provided by the present invention are as follows:

[0008] A hot stamping steel sheet having an Al-Zn-Mg-Si plating layer, comprising a substrate and an Al-Zn-Mg-Si plating layer plated on the substrate, the Al-Zn-Mg-Si plating layer being composed of two structures in the thickness direction, namely, a surface oxide layer and an alloy layer located below the surface layer, the thickness of the alloy layer being 15 to 45 um, the alloy layer being composed of upper and lower two-layer structures, the structure of the first layer being composed of an Al-rich phase, a Zn-rich phase, an MgZn2 phase and an FeAl3 phase, and the structure of the first layer being composed of a mixture of two phases, the Zn-rich phase and the FeAl3 phase. a structure of the second layer consisting of an FeAl3 phase, an MgZn2 phase and a Zn-rich phase, and the volume percentages of the MgZn2 phase and the Zn-rich phase do not exceed 5%; or the alloy layer consists of a single-layer alloy layer, the main composition of the single-layer alloy layer is FeAl3 phase, the Zn-rich phase and the MgZn2 phase exist between the FeAl3 phase, and the total volume percentage of the two phases, the Zn-rich phase and the MgZn2 phase, is 5% to 50%.

[0009] In some embodiments, the alloy layer is composed of an upper and lower two-layer structure, the structure of the first layer being composed of an Al-rich phase, a Zn-rich phase, an MgZn2 phase, and an FeAl3 phase, and the total volume percentage of the Zn-rich phase, the MgZn2 phase, and the FeAl3 phase is 50% to 100%, and the structure of the second layer being composed of an FeAl3 phase, an MgZn2 phase, and a Zn-rich phase, and the volume percentage of the MgZn2 phase and the Zn-rich phase does not exceed 5%.

[0010] In some embodiments, the alloy layer is a single-layer alloy layer, the single-layer alloy layer is mainly composed of an FeAl3 phase, a Zn-rich phase and an MgZn2 phase are present between the FeAl3 phases, and the total volume percentage of the two phases, the Zn-rich phase and the MgZn2 phase, is 15% to 40%.

[0011] In some embodiments, the present invention provides a hot stamping steel sheet having an Al-Zn-Mg-Si plating layer, comprising a substrate and an Al-Zn-Mg-Si plating layer plated on the substrate, in which after hot stamping, the Al-Zn-Mg-Si plating layer has two structures in a thickness direction, namely, a surface oxide layer and a lower alloy layer, and the thickness of the alloy layer is 15 to 45 μm, and the structure of the alloy layer has the following relationship with a holding time t during an austenitization process:

[0012] (1) In the case where the holding time is t≦3 min, the alloy layer of the Al-Zn-Mg-Si plating layer is composed of two layers, an upper layer and an lower layer, the structure of the first layer is composed of an Al-rich phase, a Zn-rich phase, and an FeAl3 phase, and the total volume percentage of the Zn-rich phase and the FeAl3 phase is 20% to 80%, and the second layer is composed of an FeAl3 phase and a Zn-rich phase, and the volume percentage of the Zn-rich phase does not exceed 5%.

[0013] (2) In the case where the holding time t>3 min, the alloy layer of the Al-Zn-Mg-Si plating layer is composed of a single-layer alloy layer, the main composition of the single-layer alloy layer is FeAl3 phase, a Zn-rich phase and an MgZn2 phase exist between the FeAl3 phase, and the total volume percentage of the two phases, the Zn-rich phase and the MgZn2 phase, is 5% to 50%.

[0014] In the hot stamping steel sheet having an Al-Zn-Mg-Si coating layer of the present invention, before hot stamping, the initial structure of the Al-Zn-Mg-Si coating layer of the steel sheet comprises a Zn-rich phase, an Al-rich phase, an FeAl3 phase, an MgSi2 phase and an Fe-Al-Si alloy layer, and the thickness of the Al-Zn-Mg-Si coating layer is 8-30 μm.

[0015] In the hot stamping steel sheet having an Al-Zn-Mg-Si plating layer of the present invention, after hot stamping, an oxide layer on the surface of the plating layer of the steel sheet contains (Zn, Al, Mg, Si) oxides, and the thickness of the oxide layer of the Al-Zn-Mg-Si plating layer is within 3 um, for example, 0.5 to 3 um.

[0016] In the hot stamping steel sheet having an Al-Zn-Mg-Si coating layer of the present invention, the mass percentages of the chemical components of the Al-Zn-Mg-Si coating layer are Al: 45-65%, Mg: 0.2-5%, Si: 0.1-3%, and the remainder are Zn and other unavoidable impurities.

[0017] In the hot stamping steel sheet having an Al-Zn-Mg-Si plating layer of the present invention, the mass percentages of the chemical components of the Al-Zn-Mg-Si plating layer are Al: 45-65%, Mg: 0.2-5%, Si: 0.5-3%, and the remainder are Zn and other unavoidable impurities.

[0018] In the hot stamping steel sheet having an Al-Zn-Mg-Si plating layer of the present invention, the compositional components of the base material include, by mass percentage, C: 0.1% to 0.5%, Si: 0.05% to 2.0%, Mn: 0.5% to 3.0%, P: ≦0.1%, S: ≦0.05%, Al: 0.01 to 0.05%, N: 0.01% or less, and further include at least one of Nb: 0.01% to 0.1%, V: 0.01% to 1.0%, Mo: 0.01% to 1.0%, Ti: 0.01% to 0.1%, Cr: 0.01% to 1.0%, Ni: 0.01% to 1.0%, and B: 0.001% to 0.01%, with the remainder being Fe and unavoidable impurities.

[0019] In some embodiments, in the hot stamping steel sheet having an Al-Zn-Mg-Si plating layer according to the present invention, the composition of the base material includes, by mass percentage, C: 0.1% to 0.3%, Si: 0.05 to 0.3%, Mn: 0.8 to 1.5%, P: ≦0.01%, S: ≦0.0005%, Al: 0.01% to 0.05%, N: 0.01% or less, and further includes at least one of Nb: 0.01% to 0.1%, V: 0.01% to 1.0%, Mo: 0.01% to 1.0%, Ti: 0.01% to 0.1%, Cr: 0.1 to 0.4%, Ni: 0.01% to 1.0%, and B: 0.001% to 0.01%, with the remainder being Fe and unavoidable impurities.

[0020] In some embodiments, in the hot stamping steel sheet having an Al-Zn-Mg-Si plating layer according to the present invention, the composition of the base material includes, in mass percentages, C: 0.1% to 0.3%, Si: 0.05 to 0.3%, Mn: 0.8 to 1.5%, P: ≦0.01%, S: ≦0.0005%, Al: 0.01 to 0.05%, N: 0.01% or less, Ti: 0.01% to 0.1%, Cr: 0.1 to 0.4%, and B: 0.001% to 0.01%, with the remainder being Fe and unavoidable impurities.

[0021] The design principles of each chemical element in the steel plate according to the present invention are as follows. C: 0.1% to 0.5% The C content is the most important factor that determines the mechanical properties of a steel sheet, such as strength and hardness, and as the C content increases, the strength and hardness of the steel sheet increase, the plasticity and toughness of the steel sheet decrease, and the low-temperature brittleness tendency and aging tendency of the steel sheet increase. In addition, the C content also has a large effect on the mechanical properties immediately after quenching, so in the present invention, the upper limit of the C content is specified to be 0.5%, preferably 0.35%, so that the steel sheet has a certain plasticity and toughness, and the lower limit is specified to be 0.1% so that the steel sheet has a certain strength. In some embodiments, the C content is 0.1 to 0.3%.

[0022] Silicon: 0.05% to 2.0% Si is a substitutional solid solution element, and can promote the concentration of C in austenite, increase the stability of austenite, improve the strength of the steel sheet, and improve its toughness to a certain extent. Therefore, in the present invention, the lower limit of the Si content is specified as 0.05%. However, if the Si content is high, the probability of occurrence of a surface defect called red scale during hot rolling increases, and the rolling force increases, so that the ductility of the hot-rolled steel sheet deteriorates. Therefore, in the present invention, the upper limit of the Si content is specified as 1.0%. In order to ensure the galvanizability of the steel sheet, the upper limit of Si is preferably 1.0%. In some embodiments, the Si content is 0.05 to 0.5%, and in some embodiments, the Si content is 0.05 to 0.3%.

[0023] Mn: 0.5% to 3.0% Mn is an element that improves the hardenability of steel sheet, expands the austenite phase region, and effectively ensures the strength of the steel sheet after quenching. At the same time, Mn, as an element that expands the austenite phase region, can reduce the temperature of Ac3 and Ac1, delay the pearlite phase transformation, and thereby reduce the heating temperature of hot stamping. Ac3 refers to the actual temperature of phase transformation when carbon steel is heated, and Ac1 refers to the temperature at which austenite begins to form when steel is heated. If the Mn content is less than 0.5%, the ability to improve the hardenability of the steel sheet is insufficient. On the other hand, if the Mn content exceeds 3%, segregation occurs, and the performance uniformity of the base steel sheet and hot stamped parts is reduced. In some embodiments, the Mn content is 0.5 to 2.0%. In some embodiments, the Mn content is 0.8 to 1.5%.

[0024] P:≦0.1% P in steel significantly reduces the plasticity and toughness of steel, and causes the phenomenon of "low temperature brittleness" especially at low temperatures, so it must be strictly controlled and limited to less than 0.1%. In some embodiments, P:≦0.01%.

[0025] S:≦0.05% The S content is kept at a low level. When FeS is produced from S, it causes a problem of hot brittleness, so the S content is limited to 0.05% or less. Although there is no lower limit, for the same reason, the lower the S content, the more preferable it is. In some embodiments, S: ≦0.0005%.

[0026] N:≦0.01% If the N content exceeds 0.01%, nitrides of AlN are formed during hot rolling, which reduces the punchability and hardenability of the steel sheet. Therefore, the lower the N content, the better, and it is set to ≦0.01%. In some embodiments, N: ≦0.005%.

[0027] Al: 0.01% to 0.05% Al has a deoxidizing effect. When the Sol.Al (effective Al) content is less than 0.01%, the effect of adding is not clear, and when the Sol.Al content is more than 0.05%, the deoxidizing effect is saturated and the cost increases. Therefore, in the present invention, the Al content is limited to 0.01% to 0.05%.

[0028] Nb: 0.01% to 0.1% Nb is an important micro-alloy element in steel, and adding a small amount of Nb to steel can refine the grains of steel through diffusion precipitation of the mass points (size less than 5 nm) of carbon and nitrides and Nb solid solution when the carbon equivalent is low, greatly improving the strength and toughness of steel, especially low temperature toughness, while at the same time giving the steel good cold bending properties and weldability. Therefore, the present invention limits the Nb content to 0.01% to 0.1%, which can effectively refine the original austenite grains of the steel base material for the purpose of improving the toughness and cold bending properties of parts after hot forming.

[0029] V: 0.01% to 1.0% V is an element that refines the structure by forming carbides. When steel plate is heated to the Ac3 point or higher, the fine V carbides inhibit recrystallization and the formation of crystal grains, refine the austenite grains, and improve toughness. When the V content is less than 0.01%, the effect of adding is not clear, and when the V content is more than 1.0%, the effect of adding is saturated and the cost increases.

[0030] Mo: 0.01% to 1.0% Mo, like V, is an austenite refiner. When the Mo content is less than 0.01%, the effect of addition is not obvious, and when the Mo content is more than 1.0%, the effect of addition is saturated and the cost increases.

[0031] B: 0.001% to 0.01% B is an element with strong grain boundary segregation in steel, which can reduce the grain boundary energy of austenite and inhibit the formation of proeutectoid ferrite crystal nuclei, and has three features in improving the hardenability of steel, that is, its ability to improve hardenability is very strong, and the addition of a very small amount of B element is equivalent to the addition of various other valuable alloy elements, and B element is different from general alloy elements whose effect of improving hardenability increases as the content in steel increases, and has an optimal content for improving hardenability, and is very small. Therefore, the content of B element in the present invention is limited to 0.001% to 0.01%.

[0032] In the present invention, the chemical composition of the steel sheet substrate may contain at least one of Ti: 0.01% to 0.1%, Cr: 0.01% to 1.0%, and Ni: 0.01% to 1.0%.

[0033] Ti: 0.01% to 0.1% Ti is an element that can be effectively used because it forms nitrides and stably fulfills the role of B mentioned above. For this reason, the addition of 0.01% or more is necessary, but adding too much will result in excess nitrides, which will lead to a deterioration in toughness, so the upper limit is set at 0.10%.

[0034] Cr: 0.01%~1.0% Cr can increase the hardenability of steel and has the effect of secondary hardening. Chromium and iron form a continuous solid solution, which reduces the austenite phase region, and chromium can reduce the carbon concentration of pearlite and the final solubility of carbon in austenite. Chromium can also improve the oxidation resistance and corrosion resistance of steel. When the Cr content is <0.01%, the effect of addition is not obvious, and when the Cr content is >1.0%, the effect of addition is saturated and the cost increases. In some embodiments, the Cr content is 0.1-0.5%, for example, 0.1-0.4%.

[0035] Ni: 0.01% to 1.0% Ni can expand the austenite region of steel and is a major alloying element for forming and stabilizing austenite. Nickel also strengthens ferrite, refines and increases pearlite, and improves the strength of steel. The effect of adding nickel is significant at 0.01% or more, but since it is an expensive element, it is limited to 1.0% or less.

[0036] The present invention also relates to a hot stamping method applied to hot stamped steel sheet with Al-Zn-Mg-Si coating layer, in which the steel sheet is austenitized within a limited process range, and then hot stamping and in-die quenching are performed, so that the hot stamped parts have good corrosion resistance, and at the same time, the substrate crack caused by local stress and liquid metal embrittlement during the hot forming process can be avoided. The operation steps of the hot stamping method are as follows:

[0037] (1) Heating: A steel sheet having an Al-Zn-Mg-Si coating layer is conveyed into a heating furnace, and the steel sheet is heated to a temperature T higher than Ac3 at a heating rate V of 5°C / s or more and 1000°C / s or less. Ac3 The range of the heating temperature T must satisfy the following relational expression:

[0038]

number

[0039] where [Al] is the Al content in the coating layer (%), [Zn] is the Zn content in the coating layer (%), and V is the temperature rise rate during the heating process, in °C / s.

[0040] (2) Temperature holding: The austenitic steel plate is held, and the holding time t is set to satisfy the following relationship:

[0041]

number

[0042] Here, [Al] is the Al content in the plating layer (%), and [Zn] is the Zn content in the plating layer (%), and the unit is min.

[0043] (3) Hot stamping and in-mold quenching: The heated steel sheet is quickly transferred to the hot stamping die for stamping and quenching. After the hot stamping is completed, a blank is formed. The blank is first quench-hardened and cooled in the die, and then cooled to room temperature in the die, or removed from the die and cooled to room temperature.

[0044] In the hot stamping method applied to the hot stamping steel sheet having an Al-Zn-Mg-Si plating layer of the present invention, the heating rate V in the step (1) is greater than 5° C. / s and less than 1000° C. / s.

[0045] In the hot stamping method applied to the hot stamping steel sheet having an Al-Zn-Mg-Si plating layer of the present invention, the heating rate V in the step (1) is 5 to 200° C. / s. In some embodiments, the heating rate V is 5 to 50° C. / s. In some embodiments, the heating rate V is 10 to 50° C. / s.

[0046] In the hot stamping method applied to the hot stamping steel sheet having an Al-Zn-Mg-Si plating layer of the present invention, in the step (1), the austenitizing temperature of the steel sheet is controlled to a temperature not higher than 930°C and higher than Ac3.

[0047] In the hot stamping method of the present invention which is applied to a hot stamping steel sheet having an Al-Zn-Mg-Si plating layer, in the step (2), the holding time of the steel sheet during austenitization is controlled to be between 1 minute and 7 minutes.

[0048] In the hot stamping method applied to the hot stamped steel sheet having an Al-Zn-Mg-Si plating layer of the present invention, the holding time is further controlled to be more than 1 minute and less than 3 minutes as step (2), that is, in an optimized embodiment, the holding time is controlled to be more than 1 minute and less than 3 minutes.

[0049] In the hot stamping method applied to the hot stamped steel sheet having an Al-Zn-Mg-Si plating layer of the present invention, in the step (3), the stamping pressure of the hot stamping is 300 to 1000 tons, and the pressure holding time is 3 to 15 seconds.

[0050] In some embodiments, the hot stamping method applied to the hot stamped steel sheet having an Al-Zn-Mg-Si plating layer according to the present invention is used for manufacturing the hot stamped steel sheet having an Al-Zn-Mg-Si plating layer described in any of the embodiments of the present specification.

[0051] In some embodiments, the hot stamping method applicable to the hot stamped steel sheet having an Al-Zn-Mg-Si plating layer according to the present specification also includes a step of producing the hot stamped steel sheet having the Al-Zn-Mg-Si plating layer using a method having the following steps:

[0052] 1) Smelting raw materials for steel plate manufacturing. 2) Continuous casting is used to produce billets.

[0053] 3) Hot rolling: After the billet is heated to 1100-1250℃, the rolling is controlled so that the rolling start temperature is 950-1150℃, the final rolling temperature is 750-900℃, and the thickness of the hot-rolled plate is 20mm or less.

[0054] 4) After rolling, the steel is coiled at 500 to 850°C and cooled to room temperature, after which the structure becomes ferrite and pearlite.

[0055] 5) Pickling is performed to remove scale generated during hot rolling. 6) Cold rolling: the steel coil is cold rolled to a thickness of 2.0mm or less, and the cold rolling reduction is ≧35%.

[0056] 7) Continuous annealing and hot-dip galvanizing: The obtained hard-rolled strip is recoiled, washed, heated to a soaking temperature of 780-850°C, and held for 30-200s, during which the heating rate is 1-20°C / s, and the atmosphere in the heating and holding sections is N2-H2 mixed gas, during which the volume content of H2 is 0.5-20%, and the dew point of the annealing atmosphere is -40-10°C. Then, the steel sheet is immersed in a zinc pot for hot-dip galvanizing, and the temperature range of the plating solution is 565-605°C. After coming out of the plating solution, the steel sheet is cooled stepwise to obtain a plated steel sheet having an Al-Zn-Mg-Si plating layer on the substrate.

[0057] Preferably, the stepwise cooling includes controlling the cooling rate of the steel sheet to 15-25°C / s within the range from leaving the plating solution to 480°C, controlling the cooling rate of the steel sheet to 40-60°C / s within the range of 480-280°C, cooling to 280°C or below, and placing the steel sheet in a water quenching tank to cool to room temperature.

[0058] Based on the above technical solutions, the hot stamping steel sheet with Al-Zn-Mg-Si plating layer and the hot stamping method of the present invention have been put into practice, and have achieved the following beneficial effects:

[0059] 1. The present invention provides a hot stamping forming method for hot stamping steel sheet with Al-Zn-Mg-Si coating layer, which allows hot stamped parts to have good corrosion resistance within a limited process range. During the heating process, Al-Fe diffusion occurs preferentially at first, and at this time, the Zn-rich phase (η-Zn) and MgZn2 phase in the original coating layer are concentrated in 1 / 3 to 1 / 2 of the surface layer in the thickness direction of the coating layer. The surface layer is composed of Zn-rich phase (η-Zn), MgZn2 phase and Al-rich phase, and the interface layer is composed of AlFe3 phase. In a corrosive medium, the potential of the Zn-rich phase or MgZn2 phase in the surface layer is lower, so that it can form a primary battery with the interface alloy layer or matrix, and thus play the protective function of a sacrificial anode against the matrix.

[0060] 2. The present invention provides a hot stamping method for hot stamping steel sheet having an Al-Zn-Mg-Si coating layer, which can avoid substrate cracking caused by liquid metal embrittlement (LME) within the process range defined by the present invention and ensure that the hot stamped coating layer has good corrosion resistance.

[0061] 3. In the hot stamping steel sheet with Al-Zn-Mg-Si coating layer according to the present invention, Al-Fe diffusion takes precedence during the heating process, at which time the Zn-rich phase (η-Zn) and MgZn2 phase in the original coating layer are concentrated in 1 / 3 to 1 / 2 of the surface layer in the thickness direction of the coating layer. If the heating temperature is too high, after the Al-Fe is fully diffused, the driving force of Zn-Fe diffusion will cause the Zn-rich phase in the coating layer to diffuse into the grain boundaries of the AlFe3 phase. During the molding process, the Zn-rich phase with a low melting point is still liquid, so the binding force of the AlFe3 phase will be reduced under the action of stress, causing macroscopic cracks due to liquid metal embrittlement. Therefore, the heating temperature should be set to 1.03T. Ac3 Limited to +2×V×[Al] / [Zn] or less.

[0062] 4. The hot stamping steel sheet having an Al-Zn-Mg-Si coating layer according to the present invention has a detailed limit on the holding time of the hot stamping steel sheet, and if the holding time is less than 1 min, the steel sheet cannot be fully austenitized, and if the holding time is too long, the coating layer will similarly change to a single-layer alloy layer structure in which the (Al, Fe) phase and the Zn-rich phase coexist, causing macroscopic cracks due to liquid metal embrittlement (LME). Therefore, the holding time is limited to 2.7×[Al] / [Zn] or less. [Brief description of the drawings]

[0063] [Figure 1] 1 is a microstructure of Example 1 of the present invention before hot stamping. [Diagram 2] 1 is a microstructure of Example 1 of the present invention after hot stamping. [Diagram 3] 1 is a microstructure of Example 2 of the present invention after hot stamping. [Figure 4] 1 is the microstructure of Example 5 of the present invention after hot stamping. [Diagram 5] 1 is a microstructure of Example 6 of the present invention after hot stamping. [Figure 6] 1 is a cross-sectional view of Example 8 of the present invention after hot stamping. [Figure 7] 1 shows the LME characteristics of Comparative Example 4 of the present invention after hot stamping and the cross-sectional morphology of the plating layer at a high magnification. [Figure 8] 8 shows a comparative example of the present invention at a high magnification of position A in FIG. 7. [Figure 9] 8 shows a comparative example of the present invention at a high magnification of position B in FIG. 7. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0064] In order to make the objectives, technical solutions and advantages of the present invention clearer, the following describes the embodiments of the present invention in more detail with reference to the accompanying drawings.

[0065] The present invention relates to a hot stamp steel sheet having an Al-Zn-Mg-Si coating layer, which includes a substrate and an Al-Zn-Mg-Si coating layer coated on the substrate. Before hot stamping, the initial structure of the Al-Zn-Mg-Si coating layer is composed of a Zn-rich phase, an Al-rich phase, an FeAl3 phase, an MgSi2 phase, and an Fe-Al-Si alloy layer, and the thickness of the Al-Zn-Mg-Si coating layer is 8-30 um. After hot stamping, the structure of the coating layer is composed of two structures in the thickness direction, that is, an oxide layer at the surface and an alloy layer at the bottom of the surface, and the thickness of the alloy layer is 15-45 um, and the structure of this alloy layer has the following relationship with the holding time t during the austenitization process:

[0066] (1) In the case where the holding time is t≦3 min, the alloy layer of the Al-Zn-Mg-Si plating layer consists of two layers, an upper one and an lower one, the structure of the first layer consists of an Al-rich phase, a Zn-rich phase, and an FeAl3 phase, and the total volume percentage of the Zn-rich phase and the FeAl3 phase to the entire structure is 20% to 80%, and the second layer consists of an FeAl3 phase and a Zn-rich phase, and the volume percentage of the Zn-rich phase to the entire structure does not exceed 5%.

[0067] (2) In the case where the holding time t>3 min, the alloy layer of the Al-Zn-Mg-Si plating layer is composed of a single alloy layer, the main composition of the single alloy layer is FeAl3 phase, a Zn-rich phase and an MgZn2 phase are present between the FeAl3 phase, and the total volume percentage of the two phases, the Zn-rich phase and the MgZn2 phase, to the entire structure is 5% to 50%.

[0068] In the hot stamping steel sheet having an Al-Zn-Mg-Si coating layer of the present invention, the mass percentages of the chemical components of the Al-Zn-Mg-Si coating layer are Al: 45-65%, Mg: 0.2-5%, Si: 0.1-3%, and the remainder are Zn and other unavoidable impurities.

[0069] In the hot stamping steel sheet having an Al-Zn-Mg-Si coating layer of the present invention, after hot stamping, the oxide layer on the surface of the coating layer of the steel sheet contains (Zn, Al, Mg, Si) oxides, and the thickness of the oxide layer of the coating layer is within 3 um. The (Zn, Al, Mg, Si) oxides are a mixture of zinc oxide, aluminum oxide, magnesium oxide, and silicon oxide.

[0070] In the hot stamping steel sheet having an Al-Zn-Mg-Si plating layer of the present invention, the compositional components of the base material include, by mass percentage, C: 0.1% to 0.5%, Si: 0.05% to 2.0%, Mn: 0.5% to 3.0%, P: ≦0.1%, S: ≦0.05%, Al: 0.01 to 0.05%, N: 0.01% or less, and further include at least one of Nb: 0.01% to 0.1%, V: 0.01% to 1.0%, Mo: 0.01% to 1.0%, Ti: 0.01% to 0.1%, Cr: 0.01% to 1.0%, Ni: 0.01% to 1.0%, and B: 0.001% to 0.01%, with the remainder being Fe and unavoidable impurities.

[0071] The manufacturing process of the hot stamping steel sheet having the Al-Zn-Mg-Si plating layer for the above hot stamping forming is as follows:

[0072] 1) Smelting raw materials for steel plate manufacturing. 2) Continuous casting is used to produce billets.

[0073] 3) Hot rolling: After the billet is heated to 1100-1250℃, the rolling is controlled so that the rolling start temperature is 950-1150℃, the final rolling temperature is 750-900℃, and the thickness of the hot-rolled plate is 20mm or less.

[0074] 4) After rolling, the steel is coiled at 500 to 850°C and cooled to room temperature, after which the structure becomes ferrite and pearlite.

[0075] 5) Pickling is performed to remove scale generated during hot rolling. 6) Cold rolling: the steel coil is cold rolled to a thickness of 2.0mm or less, and the cold rolling reduction is ≧35%.

[0076] 7) Continuous annealing and hot-dip galvanizing: The obtained roll-hardened strip is recoiled, washed, and heated to a soaking temperature of 780-850°C and held for 30-200s, during which the heating rate is 1-20°C / s. The atmosphere in the heating and holding sections is N2-H2 mixed gas, during which the H2 volume content is 0.5-20%, and the dew point of the annealing atmosphere is -40-10°C. Then, the steel sheet is quenched and immersed in a zinc pot for hot-dip galvanizing. The temperature range of the galvanizing solution is 565-605°C. After coming out of the galvanizing solution, the steel sheet is cooled stepwise to obtain a galvanized steel sheet having an Al-Zn-Mg-Si coating layer on the substrate.

[0077] In the manufacturing process of the above hot stamped steel sheet, the temperature of the plating solution is controlled to 565-605°C in step 7) because if the temperature of the plating solution is lower than 565°C, the fluidity of the plating solution in the zinc pot decreases, making it difficult to control the thickness of the plating layer, and as a result, it becomes difficult to ensure the uniformity of the plating layer, and the uniformity is further deteriorated, especially when the thickness of the plating layer is thin. If the temperature of the zinc pot is higher than 605°C, the dissolution of the steel sheet in the zinc pot and the oxidation of the plating solution on the surface of the zinc pot become intense, and the bottom slag and surface slag of the plating solution increase during the hot-dip plating process. In addition, if the temperature of the plating solution is higher than 605°C, the evaporation of Zn in the hot-dip plating equipment such as grates becomes intense, and defects such as zinc ash on the steel sheet surface increase.

[0078] In particular, the present invention clearly defines different phase compositions of the coating layer structure of the plated steel sheet after hot stamping with different holding times. This is because the inventors have found, through extensive research, that the composition of each phase in the coating layer structure has the greatest relationship with the holding time during the austenitization process, and that the composition of different phases in the coating layer after hot stamping has a significant effect on the subsequent corrosion resistance. Therefore, based on many experiments, a hot stamping method that produces an optimal corrosion-resistant structure in accordance with the subsequent hot stamping method is also defined.

[0079] The present invention also relates to a hot stamping method applied to hot stamped steel sheet having an Al-Zn-Mg-Si coating layer, in which the steel sheet is austenitized within a limited process range, and then hot stamping forming and in-die quenching are performed to make the hot stamped part have good corrosion resistance, and the hot stamping method includes an austenitizing step, a hot stamping step and an in-die quenching step; Austenitizing stage: The steel plate used as the base material is transported to a heating furnace with a temperature higher than Ac3, where the steel plate is fully austenitized and kept at a temperature. Hot stamping and in-mold quenching stage: The heated steel sheet is quickly transferred to the hot stamping die for stamping and quenching. After the hot stamping is completed, a blank is formed, which is first quench-hardened and cooled in the die, and then cooled to room temperature in the die, or removed from the die and cooled to room temperature.

[0080] The reason why the heating temperature and heating time of the hot stamping steel sheet in the previous austenitizing stage are specifically limited is that the inventors have found through intensive research that the heating temperature and holding time during the austenitizing process affect the structural composition of the coating layer after heating, thereby affecting the corrosion resistance of the material. If the heating temperature is lower than the Ac3 temperature of the steel sheet, the steel sheet cannot enter the austenite single phase region, and cannot complete the martensitic transformation in the subsequent quench hardening process. During the heating process, Al-Fe diffusion occurs first preferentially, and at this time, the Zn-rich phase (η-Zn) and MgZn2 phase in the original coating layer are concentrated in 1 / 3 to 1 / 2 of the surface layer in the thickness direction of the coating layer. In the two-layer alloy phase structure in which the surface layer is composed of the Zn-rich phase (η-Zn), MgZn2 phase, and Al-rich phase, and the interface layer is composed of the AlFe3 phase, when in a corrosive medium, the potential of the Zn-rich phase or MgZn2 phase in the surface layer is lower, so that it can form a primary battery with the interface alloy layer or matrix, and thus perform the protective function of a sacrificial anode against the matrix. If the heating temperature is too high, after the Al-Fe has sufficiently diffused, the Zn-rich phase in the coating layer will diffuse into the grain boundaries of the AlFe3 phase due to the driving force of Zn-Fe diffusion, and the corrosion resistance of the coating layer will be significantly reduced. Therefore, in the present invention, the heating temperature is limited to a temperature not higher than 930°C and higher than Ac3.

[0081] In the previous austenitizing stage, the holding time during austenitizing of the hot stamping steel sheet is controlled to 1 to 7 minutes. The holding time during the austenitizing process is specifically limited, and the basic principle is the same as the selection of the heating temperature mentioned above. If the holding time is less than 1 minute, the steel sheet cannot be fully austenitized, and if the holding time is too long, the coating layer will similarly change to a single-layer alloy layer structure in which the AlFe3 phase and the Zn-rich phase coexist, resulting in a decrease in corrosion resistance. Therefore, it is recommended to limit the holding time to within 7 minutes, and further control the holding time to within 3 minutes.

[0082] In order to avoid substrate cracking caused by local stress and liquid metal embrittlement (LME) during the hot forming process of Al-Zn-Mg-Si plated steel sheet, the present invention has intensively studied a hot stamping method applicable to hot stamping steel sheet having an Al-Zn-Mg-Si plating layer, and aims to avoid substrate cracking by heating the steel sheet at a limited heating temperature and holding time, and then performing hot stamping and in-die quenching after completion of the heating. The operation steps of the hot stamping method are as follows: (1) Heating: A steel sheet having an Al-Zn-Mg-Si coating layer is conveyed into a heating furnace, and the steel sheet is heated to a temperature T higher than Ac3 at a heating rate V of 5°C / s or more and 1000°C / s or less. Ac3 There are many options for the heating method, which belong to the conventional technology. The range of the heating temperature T must satisfy the following relationship:

[0083]

number

[0084] where [Al] is the Al content in the coating layer (%), [Zn] is the Zn content in the coating layer (%), and V is the temperature rise rate during the heating process, in °C / s.

[0085] (2) Temperature holding: The austenitic steel plate is held, and the holding time t is set to satisfy the following relationship:

[0086]

number

[0087] Here, [Al] is the Al content in the plating layer (%), and [Zn] is the Zn content in the plating layer (%), and the unit is min.

[0088] (3) Hot stamping and in-mold quenching: The heated steel sheet is quickly transferred to the hot stamping die for stamping and quenching. After the hot stamping is completed, a blank is formed, which is first quenched and cooled in the die, and then cooled to room temperature in the die, or removed from the die and cooled to room temperature. The stamping pressure of the hot stamping in this step is set to 300-1000 tons, and the pressure holding time is set to 3-15 seconds.

[0089] Hereinafter, the hot stamping steel sheet having an Al-Zn-Mg-Si plating layer with excellent corrosion resistance and the hot stamping method thereof according to the present invention will be further explained in conjunction with the accompanying drawings of the specification and specific examples.

[0090] The base materials of the hot stamping steel sheets used in the following Examples 1 to 13 and Comparative Examples 1 to 5 have the following elemental compositions (mass %). TIFF2025504801000006.tif17170

[0091] The remainder consists of Fe and unavoidable impurities. The manufacturing process of these hot stamping steel sheets is as follows:

[0092] (1) Smelt raw materials for steel plate manufacturing according to the above composition. (2) Continuously cast the billet.

[0093] (3) Hot rolling: After the billet is heated to 1250°C, the rolling is controlled so that the rolling start temperature is 1050°C, the final rolling temperature is 880°C, and the thickness of the hot-rolled plate is 20 mm or less.

[0094] (4) After rolling, the sheet is coiled at 550°C and cooled to room temperature, and the structure becomes ferrite and pearlite.

[0095] (5) Pickling is performed to remove scale generated during the hot rolling process. (6) Cold rolling: the steel coil is cold rolled to a thickness of 2.0 mm or less, and the cold rolling reduction is ≧35%.

[0096] (7) Continuous annealing and hot dip plating. The above steel sheet is further processed to form a plating layer and used as a hot stamping steel sheet. The process is as follows:

[0097] 1) Substrate pretreatment: A 1.2 mm thick hard-rolled plate is used as the substrate. After degreasing, it is held at 780°C for 120s. The atmosphere in the heating and holding areas is a mixture of N2-5% H2, and the dew point of the annealing atmosphere is -40°C.

[0098] 2) The substrate was immersed in a plating solution for hot-dip plating. The temperature of the plating solution was 565 to 605° C. The details of the mixing ratio of the chemical components of the plating solution used in each example are shown in Table 1.

[0099] 3) After 3 seconds of hot dip plating, the steel sheet is taken out of the plating solution, and the thickness of the plating layer is controlled by the strength of the air knife purge. Then, stepwise cooling is performed, and the cooling rate of the steel sheet is controlled at 15-25°C / s from the time of taking out of the plating solution to 480°C, and the cooling rate of the steel sheet is controlled at 40-60°C / s from the time of taking out of the plating solution to 280°C or less. Then, the steel sheet is put into a water quenching tank to cool to room temperature.

[0100] (8) The Al-Zn-Mg-Si plated steel sheets having different component ratios produced by the above process are blanked, and the blanked steel sheets are transported to a heating furnace having a temperature higher than the austenitizing temperature of the steel sheets (i.e., the Ac3 temperature).

[0101] (9) The plated steel sheet is quickly transferred to a mold, hot stamped, and then quench-hardened.

[0102] The heating temperature and holding time for the hot stamping process are shown in Tables 1 and 2, respectively. Examples 1-7 and Comparative Examples 1-3 Comparative Example 1 in Table 1 was an AlSi-plated steel sheet, Comparative Example 2 was a pure zinc-plated hot stamping steel sheet, and Comparative Example 3 was a steel sheet without a plating layer. Each was heated to 930°C and held for 3 minutes, then transferred to a mold to be subjected to hot stamping and quench hardening.

[0103] Samples of Examples 1-7 and Comparative Examples 1-3 were sampled, and the surface structure of the plating layer was analyzed using a Zeiss scanning electron microscope, and the phase structure ratio was analyzed using EBSD. Electrochemical tests were performed with a Gamry Reference 600 potentiostat to measure the self-corrosion potential of each sample. The reference electrode was a saturated calomel electrode, the auxiliary electrode was a platinum electrode, and the sample was the working electrode. The test area was 1 cm. 2 The test was performed in a 3.5 wt% NaCl solution at room temperature. Before the test, the working electrode was fixed in the electrolyte and immersed for 30 min to obtain a stable open circuit potential. The obtained potentiodynamic polarization curve was Tafel fitted using Gamry Echem Analyst software attached to the potentiostat to obtain the self-corrosion potential of the plating layer.

[0104] FIG. 1 shows the microstructure of the plated steel sheet of Example 1 of the present invention before hot stamping. As shown in FIG. 1, the microstructure of the plated layer before hot stamping consists of a Zn-rich phase, an Al-rich phase, an Mg-Zn alloy phase, an Mg-Si alloy phase, and an Fe-Al-Si alloy layer. FIG. 2 shows the microstructure of the plated steel sheet of Example 2 of the present invention after heating at 930°C and holding for 3 minutes. As shown in FIG. 2, the plated layer consists of two layers: a first layer consisting of 80% η-Zn+MgZn2 and 20% AlFe3, and a second layer consisting of 2% η-Zn+MgZn2 and 98% AlFe3. FIG. 3 shows the microstructure of the plated steel sheet of Example 3 of the present invention after heating at 930°C and holding for 7 minutes. As shown in FIG. 3, it can be seen that as the holding time increases, the plated layer changes from two layers to a single layer consisting of 20% η-Zn+MgZn2 and 80% AlFe3. FIG. 4 shows the microstructure of the plated steel sheet of Example 6 of the present invention after heating at 900°C for 3 minutes. As shown in FIG. 4, the plated layer is composed of two layers: a first layer composed of 43% η-Zn+MgZn2, 37% Al-rich phase, and 20% AlFe3 phase, and a second layer composed of 100% AlFe3. FIG. 5 shows the microstructure of the plated steel sheet of Example 7 of the present invention after heating at 900°C for 7 minutes. As shown in FIG. 5, the plated layer also changes from a two-layer structure to a 35% η-Zn+MgZn2 phase and 65% AlFe3 phase. The phase ratios of the microstructures in the above-mentioned FIG. 1 to FIG. 5 were all measured using EBSD.

[0105] [Table 1]

[0106] According to Table 1, it can be seen that the self-corrosion potential of the hot stamping steel sheets of Comparative Examples 1-3 is higher than that of the Examples, compared with Examples 1-7 described in the present invention. In particular, compared with the substrate potential without the plating layer of Comparative Example 3 (-531 mV), the potential of the conventional AlSi plating layer (-505 mV) is higher, so that when in a corrosive medium, the substrate is corroded faster than the AlSi plating layer, and the protective effect of the plating layer is lost. The pure zinc-plated hot stamping steel sheet of Comparative Example 2 has a potential of -772 mV, which is lower than that of the substrate without the plating layer, so that it can effectively form a primary battery and play a protective role as a sacrificial anode. However, compared with Examples 1-7 of the present invention, its potential is still relatively high, so that the plating layer sample of the Example of the present invention has a better effect of protecting the substrate from corrosion after hot stamping. In particular, it should be noted that the corrosion potentials are lowest when the austenitizing hold times are shorter, such as in Example 1 (-1117 mV), Example 3 (-1097 mV), Example 4 (-1013 mV), Example 5 (-1105 mV), and Example 6 (-1145 mV), and therefore a short hold temperature is recommended.

[0107] Examples 8-13 and Comparative Examples 4-5 Based on the research of the present invention, it is found that the contact between molten zinc and austenite should be avoided as much as possible during the forming step, and the specific steps of the hot stamping forming process of the present invention are as follows:

[0108] (1) Heating: The Al-Zn-Mg-Si plated steel sheet is transported into a heating furnace, and the steel sheet or steel strip is heated to a temperature T higher than Ac3 at a heating rate V of 5°C / s or more and 1000°C / s or less. Ac3 The range of the heating temperature T must satisfy the following relational expression:

[0109]

number

[0110] where [Al] is the Al content in the coating layer (%), [Zn] is the Zn content in the coating layer (%), and V is the temperature rise rate during the heating process, in °C / s.

[0111] (2) Temperature holding: The austenitic steel plate is held, and the holding time t is set to satisfy the following relationship:

[0112]

number

[0113] Here, [Al] is the Al content in the plating layer (%), and [Zn] is the Zn content in the plating layer (%), and the unit is min.

[0114] (3) Hot stamping and in-mold quenching: The heated steel sheet is quickly transferred to the hot stamping die for stamping and quenching. After the hot stamping is completed, the blank is quench-hardened and cooled in the die, and then cooled to room temperature in the die, or removed from the die and cooled to room temperature.

[0115] Samples of Examples 8-13 and Comparative Examples 4-5 were sampled, and the cross-sectional structure of the coating layer was analyzed using a Zeiss scanning electron microscope to examine the LME cracks in the substrate. FIG. 6 shows the cross-sectional metal structure of the coated steel sheet of Example 8 of the present invention after hot stamping, and it can be seen that there are no LME cracks in the substrate. FIG. 7 shows the cross-sectional metal structure of the coated steel sheet of Comparative Example 4 of the present invention after hot stamping, and it can be seen that there are LME cracks with a depth of about 700 μm at the substrate position. FIG. 8 shows the morphology at position A in FIG. 7 at a high magnification, and it can be seen that the Zn-rich phase is diffused into the substrate. FIG. 9 shows the morphology at position B in FIG. 7 at a high magnification, and it can be seen that the structure of the coating layer is clearly different from that of the Examples (FIG. 6). In the Examples, the coating layer has a two-layer structure, and the one close to the matrix is ​​the high-melting AlFe3 phase, and the Zn-rich phase is concentrated in the surface layer of the coating layer, while in the Comparative Examples, the low-melting Zn-rich phase is aggregated at the grain boundaries of the FeAl3 phase. The main reason is that the Ac3 temperature of the corresponding design substrate in the comparative example is 825°C, the heating rate of the heating furnace is V = 5°C / s, and according to the [Al] / [Zn] ratio of the plating solution components in Table 2, the holding temperature of Examples 8-13 is T Ac3 <T<1.03T Ac3 It can be seen that the range of the heating temperature and the holding time was +2×V×[Al] / [Zn] and 1≦t<2.7×[Al] / [Zn]. Since the heating temperature and holding time of the comparative example exceeded the ranges specified in the present invention, severe LME cracking occurred during the hot stamping process.

[0116] [Table 2]

[0117] The hot stamping process described in the present invention can avoid matrix cracking caused by liquid metal embrittlement (hereinafter referred to as LME) after stamping of hot stamped steel sheet having an Al-Zn-Mg-Si plating layer, and is of great significance for the development of Al-Zn-Mg-Si plated hot stamping steel.

[0118] Although the present invention has been illustrated and described with reference to several preferred embodiments of the present invention, those skilled in the art should understand that the above contents are combined with specific embodiments to further explain the present invention in detail, and the specific embodiments of the present invention cannot be assumed to be limited only to these descriptions. Those skilled in the art can make various changes in form and details, including making some simple inferences or substitutions, without departing from the spirit and scope of the present invention.

Claims

1. A hot stamping steel sheet having an Al-Zn-Mg-Si plating layer, comprising a substrate and an Al-Zn-Mg-Si plating layer plated on the substrate, the Al-Zn-Mg-Si plating layer having two structures in a thickness direction, namely, a surface oxide layer and an alloy layer located below the surface layer, the thickness of the alloy layer being 15 to 45 um, The alloy layer is composed of two layers, upper and lower. The first layer is composed of an Al-rich phase, a Zn-rich phase, and an MgZn 2 phase and FeAl 3 phase, and Zn-rich phase and FeAl 3 The total volume percentage of the two phases is 20% to 80%, and the structure of the second phase is FeAl 3 Phase, MgZn 2 phase and Zn-rich phase, and 2 The volume percentage of the Zn-rich phase does not exceed 5%, or The alloy layer is a single-layer alloy layer, and the main composition of the single-layer alloy layer is FeAl 3 phase, FeAl 3 Between the phases are Zn-rich phase and MgZn 2 phase exists, and Zn-rich phase and MgZn 2 The hot stamping steel sheet having an Al-Zn-Mg-Si plating layer, wherein the total volume percentage of the two phases is 5% to 50%.

2. Before hot stamping, the initial structure of the Al-Zn-Mg-Si coating layer of the hot stamped steel sheet is Zn-rich phase, Al-rich phase, FeAl 3 Phase, MgSi 2 2. The hot stamping steel sheet having an Al-Zn-Mg-Si plating layer according to claim 1, characterized in that the Al-Zn-Mg-Si plating layer comprises a Fe-Al-Si alloy layer and a Fe-Al-Si phase, and the thickness of the Al-Zn-Mg-Si plating layer is 8-30 um.

3. 2. The hot stamping steel sheet having an Al-Zn-Mg-Si plating layer according to claim 1, wherein the oxide layer includes Zn oxide, Al oxide, Mg oxide, and Si oxide, and the thickness of the oxide layer is within 3 um.

4. The alloy layer is composed of two layers, upper and lower. The first layer is composed of an Al-rich phase, a Zn-rich phase, and an MgZn 2 phase and FeAl 3 phase, and Zn-rich phase, MgZn 2 phase and FeAl 3 The total volume percentage of the three phases is 50% to 100%, and the structure of the second layer is FeAl 3 Phase, MgZn 2 phase and Zn-rich phase, and 2 The hot stamping steel sheet having an Al-Zn-Mg-Si plating layer according to claim 1, characterized in that the volume percentage of the Al-Zn-Mg-Si phase and the Zn-rich phase does not exceed 5%.

5. The alloy layer is a single-layer alloy layer, and the main composition of the single-layer alloy layer is FeAl 3 phase, FeAl 3 Between the phases are Zn-rich phase and MgZn 2 phase exists, and Zn-rich phase and MgZn 2 The hot stamping steel sheet having an Al-Zn-Mg-Si plating layer according to any one of claims 1 to 3, characterized in that the total volume percentage of the two phases is 15% to 40%.

6. The Zn-rich phase and MgZn 2 The hot stamping steel sheet having an Al-Zn-Mg-Si plating layer according to any one of claims 1 to 5, characterized in that the phase is concentrated in 1 / 3 to 1 / 2 of the thickness of the plating layer.

7. The hot stamping steel sheet having an Al-Zn-Mg-Si plating layer according to any one of claims 1 to 6, characterized in that the Al-Zn-Mg-Si plating layer has chemical components with percentages by mass of Al: 45-65%, Mg: 0.2-5%, Si: 0.1-3%, and the remainder being Zn and other unavoidable impurities, preferably the Al-Zn-Mg-Si plating layer has chemical components with percentages by mass of Al: 45-65%, Mg: 0.2-5%, Si: 0.5-3%, and the remainder being Zn and other unavoidable impurities.

8. 8. The hot stamping steel sheet having an Al-Zn-Mg-Si plating layer according to claim 1, wherein a composition of the base material includes, by mass percentage, C: 0.1% to 0.5%, Si: 0.05% to 2.0%, Mn: 0.5% to 3.0%, P: ≦0.1%, S: ≦0.05%, Al: 0.01 to 0.05%, N: 0.01% or less, and further includes at least one of Nb: 0.01% to 0.1%, V: 0.01% to 1.0%, Mo: 0.01% to 1.0%, Ti: 0.01% to 0.1%, Cr: 0.01% to 1.0%, Ni: 0.01% to 1.0%, and B: 0.001% to 0.01%, with the remainder being Fe and unavoidable impurities.

9. 9. The hot stamping steel sheet having an Al-Zn-Mg-Si plating layer according to claim 8, wherein a composition of the base material includes, by mass percentage, C: 0.1% to 0.3%, Si: 0.05 to 0.3%, Mn: 0.8 to 1.5%, P: ≦0.01%, S: ≦0.0005%, Al: 0.01 to 0.05%, N: 0.01% or less, and further includes at least one of Nb: 0.01% to 0.1%, V: 0.01% to 1.0%, Mo: 0.01% to 1.0%, Ti: 0.01% to 0.1%, Cr: 0.1 to 0.4%, Ni: 0.01% to 1.0%, and B: 0.001% to 0.01%, with the remainder being Fe and unavoidable impurities.

10. 9. The hot stamping steel sheet having an Al-Zn-Mg-Si plating layer according to claim 8, wherein a composition of the substrate includes, in mass percentages, C: 0.1% to 0.3%, Si: 0.05 to 0.3%, Mn: 0.8 to 1.5%, P: ≦0.01%, S: ≦0.0005%, Al: 0.01 to 0.05%, N: 0.01% or less, Ti: 0.01% to 0.1%, Cr: 0.1 to 0.4%, and B: 0.001% to 0.01%, with the remainder being Fe and unavoidable impurities.

11. A hot stamping method used for hot stamping steel sheet having an Al-Zn-Mg-Si plating layer, the operation steps of the hot stamping method are as follows: (1) Heating: The steel sheet having an Al-Zn-Mg-Si coating layer is transported into a heating furnace and heated to a temperature T higher than Ac3 at a heating rate V of 5°C / s to 1000°C / s. Ac3 The range of the heating temperature T must satisfy the following relational expression: [0010] where [Al] is the Al content in the coating layer (%), [Zn] is the Zn content in the coating layer (%), and V is the temperature rise rate during the heating process (units: °C / s). (2) Temperature holding: The austenitic steel plate is held, and the holding time t is set to satisfy the following relationship: [0025] where [Al] is the Al content in the coating layer (%), [Zn] is the Zn content in the coating layer (%), and the unit is min. (3) Hot stamping and in-mold quenching: a method for hot stamping a hot stamped steel sheet with an Al-Zn-Mg-Si coating layer, characterized in that the heated steel sheet is transferred to a hot stamping die for stamping and quenching, and when the hot stamping is completed, a blank is formed, which is first quench-hardened and cooled in the die, and then cooled to room temperature in the die, or removed from the die and cooled to room temperature.

12. In step (1), the austenitizing temperature of the hot stamping steel sheet is controlled to a temperature not higher than 930° C. and higher than Ac3; and / or In step (2), the holding time of the hot stamped steel sheet during austenitization is controlled to 1 minute to 7 minutes, or the holding time is controlled to be greater than 1 minute and less than 3 minutes; and / or 12. The method for hot stamping a hot stamped steel sheet having an Al-Zn-Mg-Si plating layer as claimed in claim 11, characterized in that in step (3), the pressure holding time of hot stamping is 3-15 seconds, and the stamping pressure is 300-1000 tons.

13. 13. The hot stamping method for the hot stamping steel sheet with an Al-Zn-Mg-Si plating layer as claimed in claim 12, characterized in that the mass percentages of chemical components of the Al-Zn-Mg-Si plating layer are Al: 45-65%, Mg: 0.2-5%, Si: 0.5-3%, and the rest are Zn and other unavoidable impurities.

14. The composition of the base material of the hot stamping steel sheet includes, in mass percentage, C: 0.1% to 0.5%, Si: 0.05% to 2.0%, Mn: 0.5% to 3.0%, P: ≦0.1%, S: ≦0.05%, Al: 0.01 to 0.05%, N: 0.01% or less, and further includes Nb: 0.01% to 0.1%, V: 0.01% to 1.0%, Mo: 0.01% to 1.0%, Ti: 0.01% to 0.1%, Cr: 0.01%. The composition of the base material preferably includes, in mass percentage, at least one of C: 0.1% to 0.3%, Si: 0.05 to 0.3%, Mn: 0.8 to 1.5%, P: ≦0.01%, S: ≦0.0005%, Al: 0.01 to 0.05%, N: 0.01% or less, and further includes N. The composition of the base material is preferably at least one of b: 0.01% to 0.1%, V: 0.01% to 1.0%, Mo: 0.01% to 1.0%, Ti: 0.01% to 0.1%, Cr: 0.1 to 0.4%, Ni: 0.01% to 1.0%, and B: 0.001% to 0.01%, with the remainder being Fe and unavoidable impurities. More preferably, the composition of the base material is, in mass percentage, C: 0.1% to 0.3%, Si: 0.05 to 0.3%, M 12. The hot stamping method for a hot stamping steel sheet having an Al-Zn-Mg-Si plating layer according to claim 11, characterized in that the content of N is 0.8-1.5%, P is ≦0.01%, S is ≦0.0005%, Al is 0.01-0.05%, N is 0.01% or less, Ti is 0.01%-0.1%, Cr is 0.1-0.4%, and B is 0.001%-0.01%, with the remainder being Fe and unavoidable impurities.

15. The hot stamping steel sheet having the Al-Zn-Mg-Si plating layer is 1) Smelting raw materials for steel plate production; 2) Continuous casting to produce billets; 3) Hot rolling: After the billet is heated to 1100-1250 ° C, the rolling start temperature is 950-1150 ° C, the final rolling temperature is 750-900 ° C, and the rolling is controlled so that the thickness of the hot-rolled plate is 20 mm or less; 4) After rolling, the steel is coiled at 500-850°C and cooled to room temperature, after which the structure becomes ferrite and pearlite. 5) Pickling is performed to remove scale generated during hot rolling. 6) Cold rolling: cold rolling the steel coil to a thickness of 2.0mm or less, and the cold rolling reduction is ≥ 35%; 7) Continuous annealing and hot-dip galvanizing: The hard-rolled strip is recoiled, washed, and soaked at 780-850°C for 30-200s, during which the heating rate is 1-20°C / s. The atmosphere in the heating and holding areas is N 2 -H 2 A mixture of gases is used, of which H 2 the volume content of Al-Zn-Mg-Si is 0.5-20%, the dew point of the annealing atmosphere is -40-10°C, and then the steel sheet is immersed in a zinc pot for hot dip galvanization, the temperature range of the plating solution is 565-605°C, and after coming out of the plating solution, the steel sheet is cooled stepwise to obtain a plated steel sheet having an Al-Zn-Mg-Si plating layer on a substrate, The hot stamping method for a hot stamping steel sheet having an Al-Zn-Mg-Si coating layer according to claim 11, characterized in that the stepwise cooling preferably includes controlling the cooling rate of the steel sheet to 15-25°C / s in a range from leaving the coating solution to 480°C, controlling the cooling rate of the steel sheet to 40-60°C / s in a range from 480-280°C, cooling to 280°C or lower, and placing the steel sheet in a water quenching tank to cool to room temperature.

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