A hot-stamped steel sheet with an Al-Zn-Mg-Si coating and its hot-stamping method

Abstract

This invention discloses a hot-stamped steel sheet with an Al-Zn-Mg-Si coating and its hot-stamping method. The initial microstructure of the Al-Zn-Mg-Si coating before hot stamping consists of a Zn-rich phase, an Al-rich phase, a MgZn2 phase, a MgSi2 phase, and an Fe-Al-Si alloy layer. After hot stamping, when the holding time is t≦3min, the coating consists of two layers. The first layer consists of an Al-rich phase, a Zn-rich phase, and a MgZn2 phase, with the Zn-rich phase and the MgSi2 phase... The volume percentage of the sum of the two phases, gZn2 and MgZn2, is between 20% and 80%; the second layer consists of FeAl3 and Zn-rich phases, with the Zn-rich phase accounting for no more than 5% of the volume; when the holding time t > 3 min, the coating consists of a single alloy layer, the main component of which is FeAl3 phase, with Zn-rich and MgZn2 phases present between the FeAl3 phases, and the sum of the Zn-rich and MgZn2 phases accounting for 5% to 50% of the volume; a hot stamping method is also involved. The steel plate of this invention exhibits good corrosion resistance after hot stamping and can also prevent cracks on the substrate caused by the brittleness of liquid metal.

Description

Technical Field

[0001] This invention relates to the field of hot forming technology, and in particular to a hot-stamped steel sheet with an Al-Zn-Mg-Si coating and a hot-stamping method thereof. Background Technology

[0002] Lightweight automotive technology is one of the key technologies to adapt to the modern automotive trends of safety, energy conservation, and environmental protection. For stamping processes, thinning and high strength are dual factors that worsen formability, making body parts prone to cracking during forming and causing excessive springback, affecting subsequent assembly. Achieving high strength in the final part through heat treatment is one approach, while hot stamping technology combines heat treatment and high-temperature forming to achieve high strength, effectively resolving the conflict between high strength and cold forming.

[0003] Parts produced using hot forming technology have advantages such as ultra-high strength, high forming precision, and no springback. An increasing number of automotive body parts utilize hot forming technology, such as A-pillars, B-pillars, bumpers, anti-collision beams, and door anti-collision beams. Traditional uncoated hot-stamped parts suffer from decarburization and oxidation peeling on the surface of the stamped steel sheet during the heating process. To avoid oxidation and decarburization on the surface of hot-stamped steel sheets and to impart high-temperature resistance and corrosion resistance, coating technologies suitable for hot-stamped steel have been developed. Currently, hot-stamping coatings mainly include aluminum-silicon (Al-10Si) coatings, hot-dip pure zinc (GI) coatings, alloyed zinc-iron (GA) coatings, and electroplated zinc-nickel (Zn-10Ni) coatings.

[0004] In direct hot stamping, zinc-based coated hot-stamping steel, which provides cathodic corrosion protection, suffers from microcracks (10µm to 100µm) in the steel substrate due to liquid metal brittleness (LME), which can even extend to macroscopic cracks along the entire thickness of the sheet. This hinders the application and development of zinc-based hot-stamped coated steel sheets. Currently, widely used aluminum / silicon coatings cannot provide cathodic corrosion protection. To further improve the corrosion resistance of coatings, zinc plating materials are evolving from pure zinc to zinc alloys. Since Inland Steel filed three patent applications regarding the appropriate addition of Al and Mg to Zn to further enhance corrosion resistance: German patent GB1125965A, US3505043A, and US3505042A, research and development on this Zn-Al-Mg coated steel sheet has been ongoing. The main focus has been on mixing various other additives or limiting production process parameters to further improve corrosion resistance or facilitate manufacturing and reduce production costs.

[0005] In addition, patent CN100334250 in the prior art proposes a Zn-Al-Mg-Si alloy-coated steel with excellent corrosion resistance and its manufacturing method. This mainly involves controlling the content of Mg and Si added to the Zn-Al coating, and controlling the precipitation amount and morphology of the MgSi2 phase, which improves corrosion resistance. This not only solves the problem of edge creep resistance on the cut surface after coating, but also provides an alloy coating with particularly excellent performance. Patent CN103805930B, by adding Mg and / or Cr to the Galvalume coating (Zn-55Al), studies the distribution of Cr in the interfacial alloy layer, providing hot-dip Zn-Al-Mg-Cr coated steel sheets with excellent processing performance and corrosion resistance. However, the above-mentioned existing patent technologies only target Zn-Al-Mg coatings used in cold-formed steel sheets; there is currently no research on hot-stamped Al-Zn-Mg-Si coated steel sheets and their hot-stamping methods. Summary of the Invention

[0006] The purpose of this invention is to provide a hot-stamped steel sheet with an Al-Zn-Mg-Si coating and a hot-stamping method thereof. Hot-stamped parts manufactured using this method exhibit excellent corrosion resistance and can effectively avoid substrate cracking caused by localized stress and liquid metal brittleness (LME) during the hot-stamping process of the Al-Zn-Mg-Si coated steel sheet.

[0007] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0008] A hot-stamped steel sheet with an Al-Zn-Mg-Si coating, comprising a substrate and an Al-Zn-Mg-Si coating plated on the substrate, wherein after hot stamping, the Al-Zn-Mg-Si coating has a microstructure in the thickness direction consisting of a surface oxide layer and a lower alloy layer, the alloy layer having a thickness of 15–45 μm, and the microstructure of the alloy layer is related to the holding time t during the austenitization process as follows:

[0009] (1) When the heat preservation time t≦3min, the alloy layer of the Al-Zn-Mg-Si coating consists of two layers. The first layer is composed of Al-rich phase, Zn-rich phase and FeAl3 phase, and the total volume percentage of the two phases, Zn-rich phase and FeAl3 phase, is between 20% and 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%.

[0010] (2) When the heat preservation time t>3min, the alloy layer of the Al-Zn-Mg-Si coating is composed of a single alloy layer. The main component of the single alloy layer is FeAl3 phase, and Zn-rich phase and MgZn2 phase exist between the FeAl3 phase. The total volume percentage of the two phases, Zn-rich phase and MgZn2 phase, is 5% to 50%.

[0011] In a hot-stamped steel sheet with an Al-Zn-Mg-Si coating according to the present invention, the initial microstructure of the Al-Zn-Mg-Si coating before hot stamping consists of a Zn-rich phase, an Al-rich phase, a FeAl3 phase, a MgSi2 phase, and a Fe-Al-Si alloy layer, and the thickness of the Al-Zn-Mg-Si coating is 8-30 μm.

[0012] In a hot-stamped steel sheet with an Al-Zn-Mg-Si coating according to the present invention, the oxide layer on the surface of the coating after hot stamping of the steel sheet contains (Zn, Al, Mg, Si) oxides, and the oxide layer thickness of the Al-Zn-Mg-Si coating is less than 3 μm.

[0013] In a hot-stamped steel sheet with an Al-Zn-Mg-Si coating according to the present invention, the chemical composition of the Al-Zn-Mg-Si coating by mass percentage is: Al: 45-65%, Mg: 0.2-5%, Si: 0.1-3%, with the balance being Zn and other unavoidable impurities.

[0014] In a hot-stamped steel sheet with an Al-Zn-Mg-Si coating according to the present invention, the substrate comprises, 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: less than 0.01%; and further comprises 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 0.1%, Ni: 0.01% to 1.0%, and B: 0.001% to 0.01%, with the balance being Fe and unavoidable impurities.

[0015] The design principles of the chemical elements in the steel plate of the present invention are as follows:

[0016] C: 0.1%–0.5%

[0017] Carbon (C) content is the most important factor determining the strength, hardness, and other mechanical properties of steel plates. The strength and hardness of steel plates increase with increasing C content, while their plasticity and toughness decrease. The susceptibility to cold brittleness and aging increases with increasing C content. Furthermore, C content significantly affects the mechanical properties after quenching. Therefore, this invention specifies an upper limit of C content of 0.5%, preferably 0.35%, to ensure the steel plate possesses certain plasticity and toughness; and a lower limit of 0.1% to ensure the steel plate possesses certain strength.

[0018] Si: 0.05%~2.0%

[0019] Si is a substitutional solid solution alloying element that promotes the enrichment of C in austenite, increasing austenite stability, improving the strength of the steel sheet, and to some extent enhancing its toughness. Therefore, this invention specifies a lower limit of Si content of 0.05%. However, when the Si content is high, the probability of surface defects such as red iron oxide scale during hot rolling increases, and the rolling force increases, leading to a deterioration in the ductility of the hot-rolled steel sheet. Therefore, this invention specifies an upper limit of Si content of 1.0%. To ensure the plating applicability of the steel sheet, the upper limit of Si is preferably 1.0%.

[0020] Mn: 0.5%–3.0%

[0021] Manganese (Mn) is an element that improves the hardenability of steel sheets, expands the austenite phase region, and effectively ensures the strength of the steel sheet after quenching. As an element that expands the austenite phase region, Mn can lower the Ac3 and Ac1 temperatures, delaying the pearlite phase transformation and thus reducing the hot stamping heating temperature. Ac3 refers to the actual phase transformation temperature during carbon steel heating, and Ac1 is the temperature at which austenite begins to form during steel heating. When the Mn content is less than 0.5%, its ability to improve the hardenability of the steel sheet is insufficient. On the other hand, when the Mn content exceeds 3%, segregation will occur, leading to a decrease in the uniformity of properties of the base steel sheet and hot-stamped parts.

[0022] P: ≤0.1%

[0023] P in steel significantly reduces its plasticity and toughness, especially at low temperatures, leading to "cold brittleness." Therefore, it must be strictly controlled and limited to below 0.1%.

[0024] S: ≤0.05%

[0025] The sulfur (S) content is kept at a low level because the formation of FeS from S causes hot brittleness problems, and therefore the S content is limited to below 0.05%. A lower limit is not defined because, for the same reason, the lower the S content, the better.

[0026] N: ≤0.01%

[0027] When the nitrogen content exceeds 0.01%, AlN nitrides will form during hot rolling, leading to a decrease in the blanking performance and hardenability of the steel sheet. Therefore, the lower the nitrogen content, the better, ideally set at ≤0.01%.

[0028] Al: 0.01%–0.05%

[0029] Al has a deoxidizing effect. When the Sol.Al (available Al) content is <0.01%, the effect of adding it is not obvious; when the Sol.Al content is >0.05%, the deoxidizing effect is saturated and the cost increases. Therefore, this invention limits the Al content to 0.01% to 0.05%.

[0030] Nb: 0.01%~0.1%

[0031] Nitrogen (Nb) is an important microalloying element in steel. Adding trace amounts of Nb to steel can refine the grain size and significantly improve its strength and toughness, especially its low-temperature toughness, even with a low carbon equivalent, through the dispersed precipitation of its carbon and nitride particles (smaller than 5 nm) and the solid solution of Nb. This also gives the steel good cold bending properties and weldability. Therefore, this invention limits the Nb content to 0.01%–0.1%, which effectively refines the original austenite grains in the steel substrate, aiming to improve the toughness and cold bending performance of the hot-formed parts.

[0032] V: 0.01%~1.0%

[0033] V (V) is an element that refines the microstructure by forming carbides. When steel plates are heated above the Ac3 point, the fine V carbides inhibit recrystallization and grain formation, thus refining the austenite grains and improving toughness. When the V content is <0.01%, the effect of addition is not obvious; when the V content is >1.0%, the effect of addition is saturated and the cost increases.

[0034] Mo: 0.01%–1.0%

[0035] Like V, Mo is an element that refines austenite. When the Mo content is <0.01%, the effect of adding it is not obvious; when the Mo content is >1.0%, the effect of adding it is saturated and the cost increases.

[0036] B: 0.001%~0.01%

[0037] Boron (B) is an element that causes intense grain boundary segregation in steel. It can lower the grain boundary energy of austenite and inhibit the formation of proeutectoid ferrite nuclei. It has three main characteristics in improving the hardenability of steel: a strong ability to improve hardenability; the effect of adding even a very small amount of B is equivalent to adding many other precious alloying elements; and B has an optimal content for improving hardenability, and its concentration is extremely small, unlike the effect of most alloying elements which increases with their content in the steel. Therefore, in this invention, the B content is limited to 0.001% to 0.01%.

[0038] In this invention, the chemical composition of the steel plate substrate may further include at least one of the following: Ti: 0.01% to 0.1%, Cr: 0.01% to 0.1%, Ni: 0.01% to 1.0%.

[0039] Ti: 0.01%~0.1%

[0040] Ti plays a stable role in the aforementioned function of B through the formation of its nitrides, making it an element that can be effectively utilized. Therefore, an addition of at least 0.01% is required; however, excessive addition leads to an overabundance of nitrides, resulting in a deterioration in toughness. Therefore, the upper limit is specified as 0.10%.

[0041] Cr: 0.01%~1.0%

[0042] Chromium (Cr) increases the hardenability of steel and has a secondary hardening effect. It forms a continuous solid solution with iron, reducing the austenite phase region. Chromium also reduces the carbon concentration in pearlite and the limiting solubility of carbon in austenite. Furthermore, chromium improves the oxidation resistance and corrosion resistance of steel. When the Cr content is <0.01%, the effect is not significant; when the Cr content is >1.0%, the effect saturates and the cost increases.

[0043] Ni: 0.01%~1.0%

[0044] Ni can expand the austenite region of steel and is the main alloying element for forming and stabilizing austenite. Nickel can also strengthen ferrite and refine and increase pearlite, thereby improving the strength of steel. Its effect is significant above 0.01%, but because it is a high-valence element, it is controlled below 1.0%.

[0045] This invention also relates to a hot stamping method for hot-stamped steel sheets with Al-Zn-Mg-Si coatings. The method involves austenitizing the steel sheet within a defined process range before hot stamping and in-die quenching, resulting in hot-stamped parts with excellent corrosion resistance while avoiding substrate cracks caused by localized stress and liquid metal brittleness during hot forming. The operation steps of this hot stamping method are as follows:

[0046] (1) Heating: The steel plate containing the Al-Zn-Mg-Si coating is conveyed into a heating furnace and heated to a temperature higher than Ac3 at a heating rate greater than 5℃ / s and less than 1000℃ / s. The range of heating temperature T must satisfy the following relationship:

[0047]

[0048] Where [Al] represents the Al content in the coating (%), [Zn] represents the Zn content in the coating (%), and V represents the heating rate during the heating process, in °C.

[0049] (2) Heat preservation: The austenitized steel plate is heat-preserved, and the heat preservation time t is set to satisfy the following relationship:

[0050]

[0051] Wherein, [Al] represents the Al content in the coating (%), and [Zn] represents the Zn content in the coating (%), in min;

[0052] (3) Hot stamping and in-mold quenching: The heated steel plate is quickly moved to the hot stamping die for stamping and quenching. After hot stamping is completed, a blank is formed. The blank is first 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.

[0053] In the hot stamping method of the present invention applicable to hot stamping steel plates with Al-Zn-Mg-Si coating, the austenitizing temperature of the steel plate in step (1) is controlled at 930°C or below and higher than the Ac3 temperature.

[0054] In the hot stamping method of the present invention applicable to hot stamping steel plates with Al-Zn-Mg-Si coating, in step (2), the holding time of the steel plate during austenitization is controlled between 1 minute and 7 minutes.

[0055] In the hot stamping method of the present invention applicable to hot stamping steel plates with Al-Zn-Mg-Si coating, as a further step in step (2), the heat preservation time is controlled to be greater than 1 minute and less than 3 minutes, that is, in the optimized scheme, the heat preservation time is controlled to be greater than 1 minute and less than 3 minutes.

[0056] In the hot stamping method of the present invention applicable to hot stamping steel plates with Al-Zn-Mg-Si coating, the stamping pressure in step (3) is 300 to 1000 tons and the holding time is 3 to 15 seconds.

[0057] Based on the above technical solution, 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:

[0058] 1. This invention provides a hot stamping forming method for hot-stamped steel sheets with Al-Zn-Mg-Si coatings. Within a defined process range, the hot-stamped parts exhibit excellent corrosion resistance. Because Al-Fe diffusion preferentially occurs during heating, the Zn-rich phase (η-Zn) and MgZn2 phase in the original coating concentrate at the surface 1 / 3 to 1 / 2 of the coating thickness. This surface layer consists of Zn-rich phase (η-Zn), MgZn2 phase, and Al-rich phase, while the interface layer is a bilayer alloy phase structure composed of AlFe3 phase. When exposed to corrosive media, the Zn-rich phase or MgZn2 phase on the surface has a lower potential, allowing it to form a galvanic cell with the interface alloy layer or the substrate, thus providing sacrificial anodic protection for the substrate.

[0059] 2. The present invention provides a hot stamping forming method for hot stamping steel sheets with Al-Zn-Mg-Si coating. Within the process scope defined by the present invention, substrate cracks caused by liquid metal embrittlement (LME) can be avoided, and the coating after hot stamping also has good corrosion resistance.

[0060] 3. In the hot-stamped steel sheet with an Al-Zn-Mg-Si coating described in this invention, Al-Fe diffusion preferentially occurs during heating. At this time, the Zn-rich phase (η-Zn) and MgZn2 phase in the original coating will be concentrated in the surface 1 / 3 to 1 / 2 of the coating thickness. However, if the heating temperature is too high, after Al-Fe has fully diffused, due to the driving force of Zn-Fe diffusion, the Zn-rich phase in the coating will diffuse into the grain boundaries of the AlFe3 phase. During the mold forming process, since the low-melting-point Zn-rich phase is still in a liquid state, under the action of stress, it will reduce the bonding force of the AlFe3 phase, causing macroscopic cracks due to the brittleness of the liquid metal. Therefore, the heating temperature is limited to... the following.

[0061] 4. The hot-stamped steel sheet with an Al-Zn-Mg-Si coating described in this invention specifies a detailed limit on the holding time for hot-stamping. If the holding time is less than 1 minute, the steel sheet cannot be completely austenitized; if the holding time is too long, the coating will transform into a single-layer alloy structure with both Al and Fe phases and Zn-rich phases, causing macroscopic cracks due to liquid metal embrittlement (LME). Therefore, the holding time is limited to [specific time range]. the following. Attached Figure Description

[0062] Figure 1 This is the microstructure of Embodiment 1 of the present invention before hot stamping.

[0063] Figure 2 This is the microstructure of Embodiment 1 of the present invention after hot stamping.

[0064] Figure 3 This is the microstructure of Embodiment 2 of the present invention after hot stamping.

[0065] Figure 4 This is the microstructure of Embodiment 5 of the present invention after hot stamping.

[0066] Figure 5 This is the microstructure of Embodiment 6 of the present invention after hot stamping.

[0067] Figure 6 This is the cross-sectional morphology of Embodiment 8 of the present invention after hot stamping.

[0068] Figure 7 This is the LME characteristics and cross-sectional morphology of the coating under high magnification of Comparative Example 4 of the present invention after hot stamping.

[0069] Figure 8 This is a comparative example of the present invention. Figure 7 High magnification morphology of position A in the middle.

[0070] Figure 9 This is a comparative example of the present invention. Figure 7 High magnification morphology of position B in the middle. Detailed Implementation

[0071] To make the objectives, technical solutions, and advantages of the present invention clearer, the embodiments of the present invention will be described in further detail below with reference to the accompanying drawings.

[0072] This invention relates to a hot-stamped steel sheet with an Al-Zn-Mg-Si coating. The steel sheet includes a substrate and an Al-Zn-Mg-Si coating deposited on the substrate. Before hot stamping, the initial microstructure of the Al-Zn-Mg-Si coating consists of a Zn-rich phase, an Al-rich phase, a FeAl3 phase, a MgSi2 phase, and an Fe-Al-Si alloy layer. The thickness of the Al-Zn-Mg-Si coating is 8-30 μm. After hot stamping, the coating microstructure consists of two structures in the thickness direction: an oxide layer on the surface and an alloy layer beneath the surface. The thickness of the alloy layer is 15-45 μm. The microstructure of this alloy layer is related to the holding time t during the austenitization process as follows:

[0073] (1) When the heat preservation time t≦3min, the alloy layer of the Al-Zn-Mg-Si coating consists of two layers. The first layer consists of Al-rich phase, Zn-rich phase and FeAl3 phase, and the sum of Zn-rich phase and FeAl3 phase accounts for 20% to 80% of the total volume of the entire structure. The second layer consists of FeAl3 phase and Zn-rich phase, and the Zn-rich phase accounts for no more than 5% of the total volume of the entire structure.

[0074] (2) When the heat preservation time t>3min, the alloy layer of the Al-Zn-Mg-Si coating is composed of a single alloy layer. The main component of the single alloy layer is FeAl3 phase, and Zn-rich phase and MgZn2 phase exist between the FeAl3 phase. The sum of the Zn-rich phase and MgZn2 phase accounts for 5% to 50% of the total volume percentage of all structures.

[0075] In a hot-stamped steel sheet with an Al-Zn-Mg-Si coating according to the present invention, the chemical composition of the Al-Zn-Mg-Si coating by mass percentage is: Al: 45-65%, Mg: 0.2-5%, Si: 0.1-3%, with the balance being Zn and other unavoidable impurities.

[0076] In a hot-stamped steel sheet with an Al-Zn-Mg-Si coating according to the present invention, the oxide layer on the surface of the coating after hot stamping comprises (Zn, Al, Mg, Si) oxides, and the thickness of the oxide layer is less than 3 μm. (Zn, Al, Mg, Si) oxides refer to a mixture of zinc oxide, aluminum oxide, magnesium oxide, and silicon oxide.

[0077] In a hot-stamped steel sheet with an Al-Zn-Mg-Si coating according to the present invention, the substrate comprises, 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: less than 0.01%; and further comprises 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 0.1%, Ni: 0.01% to 1.0%, and B: 0.001% to 0.01%, with the balance being Fe and unavoidable impurities.

[0078] The production process of the hot-stamped steel sheet with Al-Zn-Mg-Si coating used for hot stamping is as follows:

[0079] 1) Smelting the raw materials used in the production of steel plates;

[0080] 2) Continuous casting produces billets;

[0081] 3) Hot rolling: After heating the billet to 1100~1250℃, the rolling is controlled. The initial rolling temperature is 950~1150℃, the final rolling temperature is 750~900℃, and the thickness of the hot-rolled plate is less than or equal to 20mm.

[0082] 4) After rolling, the coil is made at 500-850℃ and cooled to room temperature. The microstructure is composed of ferrite and pearlite.

[0083] 5) Pickling to remove iron oxide scale generated during hot rolling;

[0084] 6) Cold rolling: Cold rolling steel coils to a thickness of less than 2.0 mm, with a cold rolling reduction of ≥35%;

[0085] 7) Continuous annealing and hot-dip galvanizing: The obtained hardened strip steel is uncoiled, cleaned, heated to a uniform temperature of 780-850℃, and held for 30-200s. The heating rate is 1-20℃ / s. The atmosphere in the heating and holding sections is a N2-H2 mixture with an H2 content of 0.5-20%. The dew point of the annealing atmosphere is -40-10℃. Then, it is quickly cooled and immersed in a zinc bath for hot-dip galvanizing. The temperature range of the galvanizing solution is 565-605℃. After the steel plate leaves the galvanizing solution, it is cooled in sections to obtain a coated steel plate, i.e., an Al-Zn-Mg-Si coating is deposited on the substrate.

[0086] In the production process of the hot-stamped steel sheet described above, the temperature of the plating solution is controlled at 565-605℃ in step 7) because when the temperature of the plating solution is below 565℃, the fluidity of the plating solution in the zinc pot decreases, making it difficult to control the thickness of the coating and resulting in difficulty in ensuring the uniformity of the coating, especially when the coating thickness is thin. When the temperature of the zinc pot is above 605℃, the dissolution of the steel sheet in the zinc pot and the oxidation of the plating solution on the surface of the zinc pot will be aggravated, leading to an increase in bottom slag and surface slag during the hot-dip galvanizing process. In addition, a plating solution temperature above 605℃ will also lead to increased evaporation of Zn in the hot-dip galvanizing equipment, such as the grate, thereby causing an increase in defects such as zinc ash on the surface of the steel sheet.

[0087] This invention specifically defines the different phase compositions of the coating microstructure of coated steel sheets after hot stamping under different holding times. This is because the inventors discovered through extensive research that the composition of each phase in the coating microstructure is most closely related to the holding time during the austenitization process, and that the different phase compositions within the coating after hot stamping have a significant impact on its subsequent corrosion resistance. Therefore, based on numerous experiments and combined with subsequent hot stamping methods, the invention also defines the hot stamping method for producing the optimal corrosion-resistant microstructure.

[0088] This invention also relates to a hot stamping method for hot-stamped steel sheets with Al-Zn-Mg-Si coatings, which enables the steel sheet to achieve austenitization within a defined process range before hot stamping and in-die quenching, resulting in hot-stamped parts with good corrosion resistance. This hot stamping method includes an austenitization step and a hot stamping and in-die quenching step.

[0089] Austenitizing process: The steel plate used as the substrate is transported to a heating furnace at a temperature higher than Ac3 to fully austenitize the steel plate and hold it at that temperature.

[0090] Hot stamping and in-mold quenching process: The heated steel plate is quickly moved to the hot stamping die for stamping and quenching. After hot stamping is completed, a blank is formed. The blank is first 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.

[0091] In the preceding austenitizing process, the heating temperature and time for the steel sheet used in hot stamping were precisely limited. This is because the inventors discovered through extensive research that the heating temperature and holding time during austenitization affect the microstructure of the coating after heating, thus impacting the material's corrosion resistance. If the heating temperature is below the Ac3 temperature of the steel sheet, the steel sheet cannot enter the single-phase austenite region and cannot complete the martensitic transformation during subsequent hardening. During the heating process, Al-Fe diffusion preferentially occurs first. At this time, the Zn-rich phase (η-Zn) and MgZn2 phase in the original coating will be concentrated in the surface 1 / 3 to 1 / 2 of the coating thickness. This surface layer consists of a Zn-rich phase (η-Zn), MgZn2 phase, and Al-rich phase, while the interface layer consists of a two-layer alloy phase structure of AlFe3 phase. When in a corrosive medium, because the Zn-rich phase or MgZn2 phase on the surface has a lower potential, it can form a galvanic cell with the interface alloy layer or the substrate, thus providing sacrificial anodic protection for the substrate. If the heating temperature is too high, after Al-Fe has fully diffused, due to the driving force of Zn-Fe diffusion, the Zn-rich phase in the coating will diffuse into the grain boundaries of the AlFe3 phase, greatly reducing the corrosion resistance of the coating. Therefore, this invention limits the heating temperature to below 930℃, but above the Ac3 temperature.

[0092] In the preceding austenitizing process, the holding time for the hot-stamped steel sheet is controlled between 1 and 7 minutes. The holding time during austenitization is precisely limited, based on the same principle 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; if the holding time is too long, the coating will transform into a single-layer alloy structure with both AlFe3 and Zn-rich phases, reducing corrosion resistance. Therefore, the holding time is limited to within 7 minutes, and further, it is recommended to control the holding time to within 3 minutes.

[0093] Furthermore, to avoid substrate cracking caused by localized stress and liquid metal embrittlement (LME) during the hot forming process of Al-Zn-Mg-Si coated steel sheets, this invention has thoroughly investigated a hot stamping forming method suitable for hot-stamped steel sheets with Al-Zn-Mg-Si coatings. The aim is to ensure that after the steel sheet is heated at a limited heating temperature and holding time, hot stamping and in-die quenching are performed, thus preventing substrate cracking. The operation steps of this hot stamping forming method are as follows:

[0094] (1) Heating: The steel plate containing the Al-Zn-Mg-Si coating is conveyed into a heating furnace and heated to a temperature higher than Ac3 at a heating rate of greater than 5℃ / s and less than 1000℃ / s. There are many options for the heating method here, which is a conventional technology. The range of heating temperature T must meet the following relationship:

[0095]

[0096] Where [Al] represents the Al content in the coating (%), [Zn] represents the Zn content in the coating (%), and V represents the heating rate during the heating process, in °C.

[0097] (2) Heat preservation: The austenitized steel plate is heat-preserved, and the heat preservation time t is set to satisfy the following relationship:

[0098]

[0099] Wherein, [Al] represents the Al content in the coating (%), and [Zn] represents the Zn content in the coating (%), in min;

[0100] (3) Hot stamping and in-mold quenching: The heated steel plate is quickly moved to the hot stamping die for stamping and quenching. After hot stamping, a blank is formed. The blank is first 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. In this step, the stamping force is set to 300-1000 tons, and the holding time is set to 3-15 seconds.

[0101] Examples 1-7

[0102] The following will further explain and illustrate the hot-stamping steel sheet with Al-Zn-Mg-Si coating and its hot-stamping method with excellent corrosion resistance, as described in this invention, with reference to the accompanying drawings and specific embodiments.

[0103] According to the present invention, steel having the following alloy composition (wt%) is suitable for use in the present invention:

[0104] 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

[0105] The remainder consists of Fe and unavoidable impurities.

[0106] The production process of the above-mentioned hot-stamped steel sheet is as follows:

[0107] (1) Smelt the raw materials for producing steel plates according to the above composition;

[0108] (2) Continuous casting to produce billets;

[0109] (3) Hot rolling: The billet is heated to 1250℃ and then rolled under controlled conditions. The initial rolling temperature is 1050℃ and the final rolling temperature is 880℃. The thickness of the hot-rolled plate is less than or equal to 20mm.

[0110] (4) After rolling, the coil is made at 550°C and cooled to room temperature. The microstructure is ferrite and pearlite.

[0111] (5) Pickling to remove iron oxide scale generated during hot rolling;

[0112] (6) Cold rolling: Cold rolling the steel coil to a thickness of less than 2.0 mm, with a cold rolling reduction of ≥35%;

[0113] (7) Continuous annealing and hot-dip galvanizing;

[0114] The aforementioned steel sheet is further processed to form a coating for use as a hot-stamped steel sheet. The process is as follows:

[0115] 1) Substrate pretreatment: A 1.2mm thick rolled hard plate is used as the substrate. After degreasing, it is held at 780℃ for 120s. The atmosphere of the heating section and the holding section is a N2-5%H2 mixed gas. The dew point of the annealing atmosphere is -40℃.

[0116] 2) The substrate is immersed in the plating solution for hot-dip plating. The temperature of the plating solution is 565-605°C. The chemical composition ratio of the plating solution used in each embodiment is detailed in Table 1.

[0117] 3) After immersion in the plating solution for 3 seconds, the steel plate is removed from the plating solution, and the thickness of the coating is controlled by the intensity of air knife blowing. Then, segmented cooling is performed. In the range from leaving the plating solution to 480°C, the cooling rate of the steel plate is controlled at 15-25°C / s. In the range from 480°C to 280°C, the cooling rate of the steel plate is controlled at 40-60°C / s. When the temperature drops below 280°C, the steel plate is placed in a water quenching tank to cool to room temperature.

[0118] (8) The Al-Zn-Mg-Si coated steel sheets with different component ratios produced in the above steps are cut into blanks and the blanks are conveyed to a heating furnace that is higher than the austenitizing temperature of the steel sheet (i.e., Ac3 temperature).

[0119] (9) Rapidly transfer the coated steel sheet into a mold for hot stamping forming and hardening.

[0120] Examples A1 - A7 and Comparative Examples B1 - B3

[0121] In Comparative Example 1 listed in Table 1, it is an AlSi coated steel sheet, Comparative Example 2 is a hot stamping steel sheet with a pure zinc coating, and Comparative Example 3 is an uncoated steel sheet. They are respectively heated to 930 °C, held for 3 minutes, and then transferred to a mold for hot stamping forming and hardening.

[0122] Samples are taken from the templates of the above examples and comparative examples. The surface structure of the coating is analyzed using a Zeiss scanning electron microscope, and the phase structure ratio is analyzed using EBSD. Electrochemical tests are carried out on a Gamry Reference 600 potentiostat to measure the self - corrosion potential of each sample. The reference electrode is a saturated calomel electrode, the auxiliary electrode is a platinum electrode, the sample is the working electrode, and the test area is 1 cm2. The test is carried out in a 3.5 wt% NaCl solution at room temperature. Before the test, the working electrode is first fixed in the electrolyte and soaked for 30 minutes to obtain a stable open - circuit potential. The obtained potentiodynamic polarization curve is subjected to Tafel fitting using the Gamry Echem Analyst software equipped with the potentiostat to obtain the self - corrosion potential of the coating.

[0123] Figure 1 This is the microstructure of the coated steel sheet of Example 1 of the present invention before hot stamping, as Figure 1 shown. Before hot stamping, the microstructure of the coating consists of a Zn - rich phase, an Al - rich phase, a Mg - Zn alloy phase, a Mg - Si alloy phase, and an Fe - Al - Si alloy layer. Figure 2 This is the microstructure of the coated steel sheet of Example 2 of the present invention after heating at 930 °C for 3 minutes, as Figure 2 shown. The coating consists of two layers. The first layer is composed of 80% η - Zn + MgZn2 and 20% AlFe3, and the second layer is composed of 2% η - Zn + MgZn2 and 98% AlFe3. Figure 3 This is the microstructure of the coated steel sheet of Example 3 of the present invention after heating at 930 °C for 7 minutes, as Figure 3 shown. It can be found that as the holding time is prolonged, the coating changes from two layers to a single layer, that is, it is composed of 20% η - Zn + MgZn2 and 80% AlFe3. Figure 4 This is the microstructure of the coated steel sheet of Example 6 of the present invention after heating at 900 °C for 3 minutes, as Figure 4 shown. The coating consists of two layers. The first layer is composed of 43% η - Zn + MgZn2, 37% Al - rich phase, and 20% AlFe3 phase, and the second layer is composed of 100% AlFe3 phase. Figure 5The microstructure of the coated steel sheet in Example 7 of this invention after heating to 900°C and holding for 7 minutes is as follows: Figure 5 As shown, the coating also changed from a two-layer structure to 35% η-Zn+MgZn2 phase and 65% AlFe3 phase. The above... Figures 1 to 5 The phase ratios of the meso- and micro-structures were determined using EBSD.

[0124] Table 1. Coating phase structure and surface potential of Examples 1-7 and Comparative Examples 1-3

[0125]

[0126] Note: "-" in the table indicates a content of 0.

[0127] As shown in Table 1, the self-corrosion potential of the hot-stamped steel sheets in Comparative Examples B1-B3 is higher than that in Examples A1-A7 listed in this invention. In particular, compared to the potential of the uncoated substrate in Comparative Example B3 (-531mV), the potential of the traditional AlSi coating (-505mV) is higher. Therefore, once exposed to a corrosive medium, the substrate will corrode earlier than the AlSi coating, thus losing the protective effect of the coating. The hot-stamped steel sheet in Comparative Example B2 with a pure zinc coating has a potential of -772mV, which is lower than that of the uncoated substrate, effectively forming a galvanic cell and thus providing sacrificial anode protection. However, compared to Examples A1-A7 of this invention, its potential is still relatively high. Therefore, the coated samples of this invention have a better protective effect against corrosion after hot stamping. It is particularly important to note that the corrosion potential is lowest when the austenitizing holding time is shorter, as shown in Example 1 (-1117mV), Example 3 (-1097mV), Example 4 (-1013mV), Example 5 (-1105mV), and Example 6 (-1145mV). Therefore, it is recommended to use a shorter holding temperature.

[0128] Examples 8-13

[0129] Examples A8-A13 and Comparative Examples B4-B5.

[0130] Based on the research findings of this invention, it is essential to avoid contact between molten zinc and austenite as much as possible during the forming stage. The specific process of the hot stamping forming process of this invention is as follows:

[0131] (1) Heating: The Al-Zn-Mg-Si coated steel sheet is conveyed into a heating furnace and heated to a temperature higher than Ac3 at a heating rate greater than 5℃ / s and less than 1000℃ / s. The range of heating temperature T must satisfy the following relationship:

[0132]

[0133] Where [Al] represents the Al content in the coating (%), [Zn] represents the Zn content in the coating (%), and V represents the heating rate during the heating process, in °C.

[0134] (2) Heat preservation: The austenitized steel plate is heat-preserved, and the heat preservation time t is set to satisfy the following relationship:

[0135]

[0136] Wherein, [Al] represents the Al content in the coating (%), and [Zn] represents the Zn content in the coating (%), in min;

[0137] (3) Hot stamping and in-mold quenching: The heated steel plate is quickly moved to the hot stamping die for stamping and quenching. After hot stamping is completed, the billet is hardened and cooled in the die. Then it can be cooled to room temperature in the die or taken out of the die and cooled to room temperature.

[0138] Samples from the above embodiments and comparative examples were taken, and the cross-sectional microstructure of the coating was analyzed using a Zeiss scanning electron microscope to determine the LME cracking of the substrate. Figure 6 The image shows the cross-sectional metallographic structure of the substrate after hot stamping with a coating in Embodiment 8 of the present invention. It can be seen that there are no LME cracks in the substrate. Figure 7 The figure shown is the cross-sectional metallographic structure of the coated section after hot stamping in Comparative Example 4 of the present invention. It can be found that there is an LME crack with a depth of about 700 μm at the substrate location. Figure 8 Comparative examples of the present invention Figure 7 The high-magnification morphology at position A shows that the Zn-rich phase has diffused into the substrate. Figure 9 Comparative examples of the present invention Figure 7 The high-magnification morphology at position B reveals the microstructure of the coating, which is consistent with the example (). Figure 6 The differences are obvious. In the examples, the coating has a double-layer structure, with the high-melting-point AlFe3 phase near the substrate and the Zn-rich phase concentrated on the surface of the coating; while in the comparative examples, the low-melting-point Zn-rich phase is concentrated at the FeAl3 phase grain boundaries. The main reason is that the Ac3 temperature of the substrate corresponding to the design in the comparative examples of this invention is 825℃, and the heating rate of the furnace is V = 5℃ / s. According to the [Al] / [Zn] ratio of the plating solution composition in Table 2, it can be found that the holding temperature of examples A8-A13 is... Within the range, the heat preservation time is Within the range, the heating temperature and holding time of the comparative example both exceeded the range defined by this invention, thus resulting in severe LME cracks during the hot stamping process.

[0139] Table 2 Comparison of LME conditions of substrates in Examples 8-13 and Comparative Examples 4-5

[0140]

[0141]

[0142] Note: ○ indicates no substrate and no LME cracks, △ indicates substrate with LME cracks.

[0143] The hot stamping process described in this invention can avoid matrix cracks caused by liquid metal embrittlement (LME) after hot stamping of Al-Zn-Mg-Si coated steel sheets, which is of great significance to the development of hot stamping steel for Al-Zn-Mg-Si coatings.

[0144] While the present invention has been illustrated and described with reference to certain preferred embodiments, those skilled in the art should understand that the above description is a further detailed explanation of the invention in conjunction with specific embodiments, and should not be construed as limiting the specific implementation of the invention to these descriptions. Various changes in form and detail can be made by those skilled in the art, including several simple deductions or substitutions, without departing from the spirit and scope of the invention.

Claims

1. A hot-stamped steel sheet having an Al-Zn-Mg-Si coating, the hot-stamped steel sheet comprising a substrate and an Al-Zn-Mg-Si coating plated on the substrate, characterized in that, After hot stamping, the Al-Zn-Mg-Si coating consists of two structures in the thickness direction: an oxide layer on the surface and an alloy layer underneath. The thickness of the alloy layer is 15~45μm. The microstructure of this alloy layer is related to the holding time t during the austenitization process as follows: During the holding time... [Al] represents the Al content in the coating (%), and [Zn] represents the Zn content in the coating (%). The alloy layer of the Al-Zn-Mg-Si coating consists of a single alloy layer. The main component of the single alloy layer is the FeAl3 phase. Between the FeAl3 phase, there are Zn-rich phases and MgZn2 phases, and the total volume percentage of the two phases is 5% to 50%.

2. The hot-stamped steel sheet with an Al-Zn-Mg-Si coating according to claim 1, characterized in that, The initial microstructure of the Al-Zn-Mg-Si coating on the hot-stamped steel sheet before hot stamping consists of a Zn-rich phase, an Al-rich phase, a FeAl3 phase, a MgSi2 phase, and a Fe-Al-Si alloy layer. The thickness of the Al-Zn-Mg-Si coating is 8-30 μm.

3. The hot-stamped steel sheet with an Al-Zn-Mg-Si coating according to claim 1, characterized in that, The oxide layer on the surface of the Al-Zn-Mg-Si coating after hot stamping of the hot-stamped steel sheet contains Zn oxide, Al oxide, Mg oxide and Si oxide, and the thickness of the oxide layer of the Al-Zn-Mg-Si coating is less than 3 μm.

4. A hot-stamped steel sheet with an Al-Zn-Mg-Si coating according to claim 1, characterized in that, The chemical composition of the Al-Zn-Mg-Si coating is as follows (mass percentage): Al: 45-65%, Mg: 0.2-5%, Si: 0.1-3%, with the balance being Zn and other unavoidable impurities.

5. A hot-stamped steel sheet with an Al-Zn-Mg-Si coating according to claim 1, characterized in that, The substrate comprises, by mass percentage: 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 The content is less than 0.01%; it also includes at least one of Nb: 0.01%~0.1%, V: 0.01%~1.0%, Mo: 0.01%~1.0%, Ti: 0.01%~0.1%, Cr: 0.01%~0.1%, Ni: 0.01%~1.0%, and B: 0.001%~0.01%, with the balance being Fe and unavoidable impurities.

6. A hot stamping method suitable for hot-stamped steel sheets with Al-Zn-Mg-Si coatings, characterized in that, The operation steps of this hot stamping method are as follows: (1) Heating: The steel plate containing the Al-Zn-Mg-Si coating is conveyed into the heating furnace and heated to a temperature higher than Ac3 at a heating rate greater than 5℃ / s and less than 1000℃ / s. The range of heating temperature T must satisfy the following relationship: ， Where [Al] represents the Al content in the coating (%), [Zn] represents the Zn content in the coating (%), and V represents the heating rate during the heating process, in °C. (2) Insulation: The austenitized steel plate is insulated, and the insulation time t is set to satisfy the following relationship: ， Where [Al] represents the Al content in the coating (%), and [Zn] represents the Zn content in the coating (%), in min; (3) Hot stamping and in-mold quenching: The heated steel plate is quickly moved to the hot stamping die for stamping and quenching. After hot stamping is completed, a blank is formed. The blank is first hardened and cooled in the die, and then cooled to room temperature in the die, or taken out of the die and cooled to room temperature.

7. The hot stamping method for hot-stamped steel sheets with an Al-Zn-Mg-Si coating according to claim 6, characterized in that, In step (1), the austenitizing temperature of the hot-stamped steel plate is controlled at 930°C or below, and is higher than the Ac3 temperature.

8. The hot stamping method for hot-stamped steel sheets with an Al-Zn-Mg-Si coating according to claim 6, characterized in that, In step (2), the heat treatment time for austenitization of the hot-stamped steel plate is controlled between 3 and 7 minutes.

9. A hot stamping method for hot-stamped steel sheets with an Al-Zn-Mg-Si coating as described in claim 6, characterized in that, In step (3), the hot stamping holding time is 3 to 15 seconds and the stamping force is 300 to 1000 tons.

Images