A method for producing a zinc-based alloy plated hot press-formed steel

Abstract

The application relates to the field of steel smelting, in particular to a zinc-based alloy plated hot stamping forming steel and a preparation method thereof. The method comprises the following steps: smelting and continuous casting of a steel raw material to obtain a slab; thin slab continuous casting and rolling production line treatment of the slab to obtain a strip steel; pickling of the strip steel, then continuous annealing and hot dip galvanizing to obtain a zinc-based alloy plated steel coil; hot stamping forming of the plated steel coil to obtain the zinc-based alloy plated hot stamping forming steel; the zinc-based alloy plated hot stamping forming steel comprises a hot stamping forming base plate and a zinc-based alloy plating layer; the chemical components of the base plate comprise C, Si, Al, Mn, Cr, Mo, B, S, P, N, O, first additive elements, second additive elements, and the rest are Fe and inevitable inclusions; the chemical components of the plating layer comprise Al, Mg, Si, and the rest are Zn and inevitable impurities; the thin slab continuous casting and rolling production line treatment is introduced into the preparation of the hot stamping forming steel, so that the length of the whole production line process is effectively shortened.

Description

Technical Field

[0001] This application relates to the field of steel smelting, and more particularly to a zinc-based alloy coated hot stamping steel and its preparation method. Background Technology

[0002] With the development of the automotive industry, international and domestic requirements for vehicle safety, energy conservation, and emissions are becoming increasingly stringent, leading to higher demands for lightweight vehicle bodies. This has resulted in significant development of high-strength and ultra-high-strength steels. However, high-strength and ultra-high-strength steels require processing using hot stamping forming technology to ensure their excellent mechanical properties. Hot stamping forming technology utilizes the characteristics of increased plasticity and decreased forming resistance of steel sheets under high-temperature conditions. After heating the sheet material with low initial strength at high temperature, it is rapidly stamped and quenched in a mold with a cooling system to obtain ultra-high-strength parts. This technology can effectively solve problems such as easy cracking and severe springback in cold forming on traditional production lines.

[0003] Currently, most hot-stamped steel is coated using hot-dip galvanizing, with the substrate being pickled and rolled strip steel. The traditional production process for hot-stamped steel includes smelting, refining, billet casting, rough rolling, finish rolling, coiling, leveling, pickling, cold rolling, continuous hot-dip galvanizing annealing, and hot-stamping forming quenching. However, this process is characterized by long production time and high energy consumption. Therefore, shortening the production process of hot-stamped steel is a pressing technical problem that needs to be solved. Summary of the Invention

[0004] This application provides a zinc-based alloy coated hot-formed steel and its preparation method to solve the technical problem of excessively long hot-stamped steel sheet production process in the prior art.

[0005] In a first aspect, this application provides a method for preparing hot-stamped steel with a zinc-based alloy coating, the method comprising:

[0006] Steel raw materials are smelted and continuously cast to obtain slabs;

[0007] The slab is processed in a thin slab continuous casting and rolling production line to obtain strip steel;

[0008] The strip steel is pickled, then continuously annealed and hot-dip galvanized to obtain a zinc-based alloy coated steel coil.

[0009] The coated steel coil is hot-stamped to obtain zinc-based alloy coated hot-stamped steel.

[0010] The thin slab continuous casting and rolling production line process includes heat treatment, rolling, laminar flow cooling, coiling and cooling;

[0011] The rolling process includes descaling before rough rolling, rough rolling, electromagnetic induction heating, descaling before finishing rolling, and finishing rolling.

[0012] Optionally, the rolling mode of the thin slab continuous casting and rolling production line includes one or more of the following: single slab rolling mode, semi-automatic headless rolling mode, and fully automatic headless rolling mode; the descaling before finishing rolling includes descaling before double-row finishing rolling or descaling before single-row finishing rolling.

[0013] The thin slab continuous casting and rolling production line treatment and the pre-finishing descaling satisfy the following: if the thin slab continuous casting and rolling production line treatment adopts the single-slab rolling mode, then the pre-finishing descaling adopts double-row pre-finishing descaling, and the pressure of the double-row pre-finishing descaling is ≥30MPa.

[0014] If the thin slab continuous casting and rolling production line adopts the semi-automatic headless rolling mode or the fully automatic headless rolling mode, then the descaling before finishing rolling adopts single-row descaling before finishing rolling, and the pressure of single-row descaling before finishing rolling is ≥35MPa.

[0015] Optionally, the heat treatment includes heat treatment in a roller hearth tunnel homogenizer, wherein the final temperature of the heat treatment is 1100℃~1200℃.

[0016] Optionally, the winding temperature is 500℃~650℃;

[0017] The dephosphorization before rough rolling includes double-row dephosphorization before rough rolling or single-row dephosphorization before rough rolling, and the pressure of the dephosphorization before rough rolling is ≥30MPa;

[0018] The inlet temperature of the roughing mill is ≥1140℃, and the final rolling temperature of the roughing mill is 950℃~980℃;

[0019] The outlet temperature of the electromagnetic induction heating is 1050℃~1250℃;

[0020] The finishing rolling temperature is 830℃~880℃, and the total reduction rate of the finishing rolling is 65%~80%.

[0021] Optionally, the hot stamping process includes a heating stage;

[0022] The heating stage includes a first heating stage, a first heat preservation stage, a second heating stage, and a second heat preservation stage, wherein the final temperature of the first heating stage is 600℃~850℃.

[0023] The first heat preservation stage includes heat preservation at the end temperature of the first heating stage, and the duration of the first heat preservation stage is 1 min to 2 min.

[0024] The final temperature of the second heating stage is 850℃~1000℃;

[0025] The second heat preservation stage includes heat preservation at the end temperature of the second heating stage, and the duration of the second heat preservation stage is 1 min to 9 min.

[0026] Optionally, the hot stamping process further includes pre-cooling and hot stamping quenching. The pre-cooling includes pre-cooling at the end temperature of the second heating stage, the pre-cooling rate is ≥30℃ / s, and the end temperature of the pre-cooling is 550℃~650℃.

[0027] Optionally, the endpoint temperature of the continuous annealing is 720℃~800℃, and the temperature of the hot-dip galvanizing is 450℃~600℃; the hot-dip galvanizing includes a galvanizing heating section, a galvanizing soaking section, and an alloying treatment; the galvanizing heating section is carried out by pre-oxidation; the dew point temperature of the galvanizing heating section is -30℃~10℃; the H2 content of the galvanizing soaking section is 3%~15%; the temperature of the alloying treatment is 500℃~680℃; and the alloying treatment time is 10s~100s.

[0028] In a second aspect, this application provides a zinc-based alloy coated hot-stamped steel, which is prepared by the method described in the first aspect, and the zinc-based alloy coated hot-stamped steel includes a hot-stamped substrate and a zinc-based alloy coating.

[0029] The chemical composition of the hot-stamped substrate, by mass fraction, includes:

[0030] C: 0.18%–0.45%, Si ≤ 0.4%, Al ≤ 0.1%, Mn: 1.0%–5.0%, Cr: 0.01%–0.7%, Mo: 0.01%–0.7%, B: 0.001%–0.005%, S ≤ 0.005%, P ≤ 0.01%, N ≤ 0.008%, O ≤ 0.003%, first additive element: 0.01%–0.10%, second additive element ≤ 0.5%, the remainder being Fe and unavoidable inclusions;

[0031] The chemical composition of the zinc-based alloy coating includes: Al: 1%–20%, Mg: 0.01%–3%, Si: 2%–5%, with the remainder being Zn and unavoidable impurities;

[0032] The first added element includes one or more of Ti, Nb and V, and the second added element includes Ni and / or Cu.

[0033] Optionally, the first added element includes Ti, Nb, and V; the second added element includes Ni and Cu.

[0034] The chemical composition of the hot-stamped substrate satisfies:

[0035] 0.025%≤[Ti]+[Nb]+[V]≤0.25%;

[0036] and / or [Ni] + [Cu] ≤ 0.5%;

[0037] In the formula, [Ti] is the mass fraction of Ti, [Nb] is the mass fraction of Nb, [V] is the mass fraction of V, [Ni] is the mass fraction of N, and [Cu] is the mass fraction of Cu.

[0038] Optionally, the chemical composition of the hot-stamped substrate satisfies:

[0039] 0.15% ≤ [Cr] + [Mo] ≤ 1.0%

[0040] In the formula, [Cr] is the mass fraction of Cr, and Mo is the mass fraction of Mo.

[0041] The technical solutions provided in this application have the following advantages compared with the prior art:

[0042] This application provides a method for preparing zinc-based alloy coated hot stamping steel. By introducing a thin slab continuous casting and rolling production line into the zinc-based alloy coated hot stamping steel process, the continuous casting and rolling production line is used to process the continuously cast slab in sequence, including heat treatment, descaling before rough rolling, rough rolling, electromagnetic induction heating, descaling before finish rolling, finish rolling, laminar flow cooling, coiling, and cooling. This method differs from the traditional process sequence of rough rolling, finish rolling, coiling, and leveling. By introducing electromagnetic induction heating and placing descaling before rough rolling or finish rolling, the cumbersome descaling and heat preservation treatment between conventional rough rolling and finish rolling can be avoided. Furthermore, the use of laminar flow cooling before coiling and cooling after coiling can shorten the subsequent pickling process, thereby effectively shortening the length of the entire production line process. Attached Figure Description

[0043] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with the invention and, together with the description, serve to explain the principles of the invention.

[0044] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, those skilled in the art can obtain other drawings based on these drawings without creative effort.

[0045] Figure 1 A flowchart illustrating the method provided in the embodiments of this application;

[0046] Figure 2 A detailed flowchart illustrating the method provided in the embodiments of this application;

[0047] Figure 3 A schematic diagram of the metallographic structure of the zinc-based alloy coated steel coil provided in the embodiments of this application;

[0048] Figure 4 This is a schematic diagram of the metallographic structure of hot-stamped steel with zinc-based alloy coating provided in the embodiments of this application. Detailed Implementation

[0049] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0050] The inventive concept of this application is as follows: With the development of the steel industry, the short-process technology of continuous casting and rolling of thin slabs has been developed. The use of the short-process technology of continuous casting and rolling of thin slabs to prepare hot-formed steel has a cost advantage over the traditional production process.

[0051] Currently, the main coatings for hot-stamped steel include Al-Si coating, GI coating, GA coating, X-TEC coating, and Zn-Ni coating. Among them, zinc-based coatings for hot-stamped steel sheets not only prevent surface oxidation and decarburization during the heating process, eliminating the need for shot peening, but also provide sacrificial anode protection and improve corrosion resistance after painting, thus having broad application prospects.

[0052] However, due to the presence of liquefied zinc in hot-stamped steel sheets at temperatures above 900°C, the steel sheets will become brittle, a condition known as LME (Liquid Metal Embrittlement). Furthermore, parts made from hot-stamped steel sheets with zinc-based coatings are prone to surface cracks that propagate into the substrate during the stamping process above 700°C due to zinc liquefaction, leading to problems such as low steel sheet strength and affecting its usability. In addition, to prevent the Zn in the coating from evaporating, a narrow austenitization window needs to be controlled, further limiting the performance of the steel sheet.

[0053] Therefore, based on the adoption of the short-process technology of continuous casting and rolling of thin slabs, it is necessary to further explore how to avoid steel plate embrittlement.

[0054] In one embodiment of this application, a method for preparing hot-stamped steel with a zinc-based alloy coating is provided, the method comprising:

[0055] S1. Smelting and continuously casting steel raw materials to obtain slabs;

[0056] S2. The slab is processed in a thin slab continuous casting and rolling production line to obtain strip steel;

[0057] S3. The strip steel is pickled, then continuously annealed and hot-dip galvanized to obtain a zinc-based alloy coated steel coil;

[0058] S4. The coated steel coil is hot-stamped to obtain a zinc-based alloy coated hot-stamped steel with low cracking risk;

[0059] The thin slab continuous casting and rolling production line process includes heat treatment, rolling, laminar flow cooling, coiling and cooling;

[0060] The rolling process includes descaling before rough rolling, rough rolling, electromagnetic induction heating, descaling before finishing rolling, and finishing rolling.

[0061] In this embodiment of the application, by adopting a rolling method including descaling before rough rolling, rough rolling, electromagnetic induction heating, descaling before finishing rolling, and finishing rolling, the grains of the slab can be further refined, thereby ensuring the uniform distribution of grain boundaries in the slab to a certain extent and reducing the possibility of steel plate embrittlement.

[0062] In some optional embodiments, the rolling mode of the thin slab continuous casting and rolling production line includes one or more of the following: single slab rolling mode, semi-automatic headless rolling mode, and fully automatic headless rolling mode; the descaling before finishing rolling includes descaling before double-row finishing rolling or descaling before single-row finishing rolling.

[0063] The thin slab continuous casting and rolling production line treatment and the pre-finishing descaling satisfy the following: if the thin slab continuous casting and rolling production line treatment adopts the single-slab rolling mode, then the pre-finishing descaling adopts double-row pre-finishing descaling, and the pressure of the double-row pre-finishing descaling is ≥30MPa.

[0064] If the thin slab continuous casting and rolling production line adopts the semi-automatic headless rolling mode or the fully automatic headless rolling mode, then the descaling before finishing rolling adopts single-row descaling before finishing rolling, and the pressure of single-row descaling before finishing rolling is ≥35MPa.

[0065] In this embodiment of the application, by corresponding the rolling mode and the descaling mode before finishing rolling of the thin slab continuous casting and rolling production line, the positive effect is that the impurities and precipitates on the thin slab before finishing rolling are completely removed during the continuous casting and rolling production line process, so that the surface of the thin slab after continuous casting and rolling is free of defects.

[0066] In some alternative embodiments, the heat treatment includes heat treatment in a roller hearth tunnel homogenizer, wherein the final temperature of the heat treatment is 1100°C to 1200°C.

[0067] The limited heating treatment is carried out in a roller hearth tunnel soaking furnace. On the one hand, it can improve the edge and corner temperature of the steel plate, improve the temperature uniformity of the steel plate in the width direction, which is conducive to the control of the steel plate shape, improve the uniformity of the steel plate performance, and eliminate edge defects of the steel plate. On the other hand, it can provide a buffer time for changing the roller group during the heating process. Furthermore, it can realize multi-mode rolling, thereby effectively expanding the thickness range of the product to 0.9mm to 2.0mm.

[0068] In some optional embodiments, the roller-bottom tunnel homogenizer includes a fixed section and a moving section, wherein the length of the fixed section is 50m to 55m and the length of the moving section is 25m to 30m.

[0069] In this embodiment of the application, by limiting the lengths of the fixed section and the moving section of the roller hearth tunnel homogenizing furnace respectively, it is possible to effectively ensure that the thin slab continuous casting and rolling production line is in the short process stage.

[0070] In some optional embodiments, the winding temperature is 500°C to 650°C;

[0071] The dephosphorization before rough rolling includes double-row dephosphorization before rough rolling or single-row dephosphorization before rough rolling, and the pressure of the dephosphorization before rough rolling is ≥30MPa;

[0072] The inlet temperature of the roughing mill is ≥1140℃, and the final rolling temperature of the roughing mill is 950℃~980℃;

[0073] The outlet temperature of the electromagnetic induction heating is 1050℃~1250℃;

[0074] The finishing rolling temperature is 830℃~880℃, and the total reduction rate of the finishing rolling is 65%~80%.

[0075] In this embodiment of the application, the positive effect of the winding temperature being 500℃~650℃ is that within this temperature range, the metallographic structure of the rolled steel strip can be completely changed, resulting in a steel product with the expected metallographic structure.

[0076] The positive effect of using a descaling pressure of ≥30MPa before rough rolling is to ensure effective descaling, thereby avoiding defects in the final steel product.

[0077] The positive effect of using a final rolling temperature of 950℃~980℃ for rough rolling is that within this temperature range, the metallographic structure of the steel plate after rough rolling is initially rolled to be uniform, which facilitates the effect obtained in subsequent finish rolling.

[0078] The positive effect of electromagnetic induction heating with an outlet temperature of 1050℃~1250℃ is that within this temperature range, it can further ensure that the steel plate after rough rolling completes the transformation of the metallographic structure, thereby obtaining steel products with the expected mechanical properties.

[0079] The positive effect of a finishing rolling temperature of 830℃~880℃ is that within this temperature range, the finished steel plate can be guaranteed to meet the expected mechanical properties of the steel product.

[0080] The positive effect of a total reduction rate of 65% to 80% in finishing rolling is that it makes the metallographic structure of the steel uniformly distributed, thus ensuring the performance of the steel products.

[0081] In some optional embodiments, the heating stage includes a first heating stage, a first heat preservation stage, a second heating stage, and a second heat preservation stage, wherein the final temperature of the first heating stage is 600℃~850℃.

[0082] The first heat preservation stage includes heat preservation at the end temperature of the first heating stage, and the duration of the first heat preservation stage is 1 min to 2 min.

[0083] The final temperature of the second heating stage is 850℃~1000℃;

[0084] The second heat preservation stage includes heat preservation at the end temperature of the second heating stage, and the duration of the second heat preservation stage is 1 min to 9 min.

[0085] In the application embodiment, the positive effect of the end temperature of the first heating stage being 600℃~850℃ is that within this temperature range, the microstructure between the steel and the zinc-based coating is initially fused and undergoes a phase transformation, resulting in a firm initial bond between the steel and the coating.

[0086] The positive effect of the first heat preservation stage lasting 1 to 2 minutes is that within this time range, it can ensure the initial fusion between the structure of the steel and the zinc-based coating, thereby making the bond between the steel and the coating stronger.

[0087] The positive effect of setting the final temperature of the second heating stage to 850℃~1000℃ is that within this temperature range, the microstructures of the steel and the zinc-based coating are further fused and the phase transformation is completed.

[0088] The positive effect of the second heat preservation stage being 1 to 9 minutes is that within this time range, it can ensure complete fusion between the steel and the zinc-based coating, thereby making the bond between the steel and the coating strong.

[0089] In some optional embodiments, the hot stamping process further includes pre-cooling and hot stamping quenching, wherein the pre-cooling includes pre-cooling at the end temperature of the second heating stage, the pre-cooling rate is ≥30℃ / s, and the end temperature of the pre-cooling is 550℃~650℃.

[0090] In this embodiment of the application, the positive effect of a pre-cooling rate ≥30℃ / s is that within this cooling rate range, the microstructure between the steel and the coating after phase transformation tends to be stable, making the bond between the steel and the coating more stable.

[0091] The positive effect of pre-cooling to a final temperature of 550℃~650℃ is that within this temperature range, the effect of subsequent hot stamping quenching can be guaranteed.

[0092] In some optional embodiments, the endpoint temperature of the continuous annealing is 720℃~800℃, and the temperature of the hot-dip galvanizing is 450℃~600℃; the hot-dip galvanizing includes a galvanizing heating section, a galvanizing soaking section, and an alloying treatment; the galvanizing heating section is carried out by pre-oxidation; the dew point temperature of the galvanizing heating section is -30℃~10℃; the H2 content of the galvanizing soaking section is 3%~15%; the temperature of the alloying treatment is 500℃~680℃; and the alloying treatment time is 10s~100s.

[0093] In some alternative implementations, such as Figure 2 As shown, the process of smelting and continuously casting steel raw materials to obtain slabs specifically includes:

[0094] S101. The steel raw material is subjected to KR desulfurization treatment, converter smelting, LF refining, VD refining and continuous casting in sequence to obtain a slab; wherein the steel casting speed of the continuous casting is 3m / min to 6m / min and the thickness of the slab is 110mm to 125mm.

[0095] In this embodiment of the application, the positive effect of continuously casting steel at a casting speed of 3m / min to 6m / min is to ensure that a uniform casting billet is obtained.

[0096] The positive effect of having a slab thickness of 110mm to 125mm is to ensure the thickness requirements of the sheet metal for subsequent rolling and hot stamping.

[0097] In one embodiment of this application, a zinc-based alloy coated hot-stamped steel is provided, wherein the zinc-based alloy coated hot-stamped steel is prepared by the method described above, and the zinc-based alloy coated hot-stamped steel includes a hot-stamped substrate and a zinc-based alloy coating;

[0098] The chemical composition of the hot-stamped substrate, by mass fraction, includes:

[0099] C: 0.18%–0.45%, Si ≤ 0.4%, Al ≤ 0.1%, Mn: 1.0%–5.0%, Cr: 0.01%–0.7%, Mo: 0.01%–0.7%, B: 0.001%–0.005%, S ≤ 0.005%, P ≤ 0.01%, N ≤ 0.008%, O ≤ 0.003%, first additive element: 0.01%–0.10%, second additive element ≤ 0.5%, the remainder being Fe and unavoidable inclusions;

[0100] The chemical composition of the zinc-based alloy coating includes: Al: 1%–20%, Mg: 0.01%–3%, Si: 2%–5%, with the remainder being Zn and unavoidable impurities;

[0101] The first added element includes one or more of Ti, Nb and V, and the second added element includes Ni and / or Cu.

[0102] In this embodiment of the application, the positive effect of having a C mass fraction of 0.18% to 0.45% in the hot stamping substrate is that within this mass fraction range, C is the most effective and cheapest solid solution strengthening element, which can effectively guarantee the strength level of the hot stamping steel. At the same time, C is also an austenite stabilizing element, which can effectively stabilize austenite, thereby ensuring the uniformity of the microstructure of the hot stamping steel and guaranteeing its strength.

[0103] The positive effect of Si ≤ 0.4% is that it can effectively improve the hardenability and tempering resistance of steel within this mass fraction range; when the mass fraction value is greater than the extreme value of this range, it will lead to a decrease in the plating suitability of steel and affect the uniform distribution of the coating.

[0104] The positive effect of Al ≤ 0.1% is due to Al being...

[0105] The positive effect of a Mn mass fraction of 1.0% to 5.0% is that within this range, it can increase the austenite region and lower the austenitizing temperature, while improving the hardenability of the steel.

[0106] A Cr mass fraction of 0.01% to 0.7% has the positive effect of significantly increasing the hardenability of steel and reducing severe oxidation on the steel surface caused by high temperatures. However, when the mass fraction exceeds the maximum value of this range, the adverse effect is that excessive Cr content promotes bainite formation; therefore, it should not be too high.

[0107] The positive effect of a Mo mass fraction of 0.01% to 0.7% is that within this range, Mo can refine the grain structure of the steel, thereby improving its hardenability.

[0108] The positive effect of a B mass fraction of 0.001% to 0.005% is that within this mass fraction range, the steel can be guaranteed to have sufficient hardenability.

[0109] The positive effect of keeping S ≤ 0.005% is that within this mass fraction range, since S is a harmful element, excessive S will form MnS inclusions with Mn. Segregation at grain boundaries will worsen the toughness of steel, thereby reducing the toughness and plasticity of steel. At the same time, it will increase the sensitivity of hydrogen-induced delayed fracture of steel, leading to embrittlement of steel. Therefore, S needs to be controlled below 0.005%.

[0110] The positive effect of P ≤ 0.01% is that within this mass fraction range, P tends to form micro-segregation during the solidification of molten steel, and will agglomerate to the grain boundaries during the post-austenite heating stage, leading to a significant increase in the brittleness of the steel, which in turn increases the sensitivity of hydrogen-induced delayed fracture of the steel. Therefore, P needs to be controlled below 0.01%.

[0111] The positive effect of N≤0.008% is that within this mass fraction range, it can combine with Al, Ti, Nb and V to form compounds, thereby refining the grains and reducing the susceptibility of hydrogen-induced delayed fracture in steel. When the mass fraction value is greater than the extreme value of this range, the formed compounds will agglomerate at the grain boundaries, thereby reducing the grain boundary strength.

[0112] The positive effect of O ≤ 0.003% is that O is a harmful gas and affects the hydrogen-induced delayed fracture sensitivity of steel. It can also form coarse alumina inclusions with Al, which in turn worsens the toughness of steel. Therefore, it is necessary to control the O content to below 0.003%.

[0113] The positive effect of adding the first element at a mass fraction of 0.01% to 0.10% is that within this mass fraction range, it can ensure that it combines with N or C to form a compound, thereby using the compound to refine the grains and reduce the susceptibility of hydrogen-induced delayed fracture in steel.

[0114] The positive effect of adding ≤0.5% of the second element is that within this mass fraction range, it can effectively improve the hardenability of the steel.

[0115] In zinc-based alloy coatings, the positive effect of having an Al mass fraction of 1% to 20% is that within this range, the melting point of the coating can be increased and the risk of liquid metal embrittlement can be reduced.

[0116] The positive effects of a Mg mass fraction of 0.01% to 3% are that within this range, Mg not only increases the fluidity of the plating solution, but also forms a MgO film on the coating surface during hot forming. The formed MgO film can prevent external water vapor from reacting with Al in the coating to generate H. At the same time, the formed MgO film can also improve the corrosion resistance of hot-stamped steel. When the mass fraction is greater than the maximum value at the end of this range, the coating activity of the steel will increase sharply during the hot-dip galvanizing stage, resulting in a deterioration of the coating appearance. When the mass fraction is less than the minimum value at the end of this range, the steel will not be able to form a continuous and dense MgO film during hot stamping.

[0117] The positive effect of having a Si mass fraction of 2% to 5% is that within this range, the thickness of the Al-Fe alloy inhibition layer between the substrate and the coating can be controlled, thereby improving the toughness of the coating and preventing embrittlement and cracking.

[0118] In some alternative implementations, the first added element includes Ti, Nb, and V; the second added element includes Ni and Cu.

[0119] The chemical composition of the hot-stamped substrate satisfies:

[0120] 0.025%≤[Ti]+[Nb]+[V]≤0.25%;

[0121] and / or [Ni] + [Cu] ≤ 0.5%;

[0122] In the formula, [Ti] is the mass fraction of Ti, [Nb] is the mass fraction of Nb, [V] is the mass fraction of V, [Ni] is the mass fraction of N, and [Cu] is the mass fraction of Cu.

[0123] In the embodiments of this application, the positive effect of 0.025% ≤ [Ti] + [Nb] + [V] ≤ 0.25% is that within this mass fraction range, Nb, Ti and V can combine with C and N respectively to form precipitates, which are used to refine austenite grains. They can also act as H traps to capture H atoms, thereby improving the toughness of the steel.

[0124] The positive effect of [Ni]+[Cu]≤0.5% is that within this mass fraction range, it can guarantee the improvement of the hardenability of steel.

[0125] In some optional embodiments, the chemical composition of the hot-stamped substrate also satisfies:

[0126] 0.15% ≤ [Cr] + [Mo] ≤ 1.0%

[0127] In the formula, [Cr] is the mass fraction of Cr, and Mo is the mass fraction of Mo;

[0128] In the embodiments of this application, the positive effect of 0.15% ≤ [Cr] + [Mo] ≤ 1.0% is to further control the mass fraction relationship between Cr and Mo, which can not only ensure the corrosion resistance of the hot stamping substrate, but also improve the hardenability of the hot stamping substrate.

[0129] The positive effect of having a zinc-based alloy coating thickness of 3μm to 33μm is that within this thickness range, the coating thickness distribution can be ensured to be uniform, while also ensuring that the coating improves the corrosion resistance of the steel plate and alleviates the hydrogen embrittlement problem of the steel plate.

[0130] In some alternative embodiments, the metallographic structure of the zinc-based alloy coated hot-stamped steel, by volume fraction, comprises:

[0131] Martensite ≥ 90%, the remainder is ferrite.

[0132] In the embodiments of this application, the positive effect of martensite ≥90% is that within this volume fraction range, the strength and hardness of hot-stamped steel with zinc-based alloy coating can be guaranteed.

[0133] The chemical composition of the hot-stamped substrates of each embodiment and comparative example is shown in Table 1.

[0134] Table 1

[0135]

[0136]

[0137] The process parameters for the preparation stages of each embodiment and comparative example are shown in Table 2.

[0138] Table 2

[0139]

[0140] The dew point temperature and chemical composition of the coatings for each embodiment and comparative example are shown in Table 3.

[0141] Table 3

[0142]

[0143]

[0144] The hot stamping forming parameters for each embodiment and comparative example are shown in Table 4.

[0145] Table 4

[0146]

[0147]

[0148] Example 1

[0149] A method for preparing zinc-based alloy coated hot-stamped steel includes:

[0150] S101. The steel raw materials are subjected to KR desulfurization treatment, converter smelting, LF refining, VD refining and continuous casting in sequence to obtain slabs; wherein, the steel casting speed is 3.9m / min, and the continuous casting adopts dynamic light reduction and electromagnetic stirring rollers to reduce component segregation and banded structure.

[0151] S2. The slab is processed by a thin slab continuous casting and rolling production line to obtain strip steel, wherein the thickness of the strip steel is 1.6mm;

[0152] S3. The strip steel is pickled, then continuously annealed and hot-dip galvanized to obtain the following result: Figure 3 The zinc-based alloy coated steel coil shown is obtained by hot-dip galvanizing with the composition of the plating solution as shown in Table 3. The surface of the zinc-based alloy coated steel coil is slightly decarburized and the depth of the decarburized layer is controllable.

[0153] S4. The coated steel coil is hot-stamped to obtain the following: Figure 4 The zinc-based alloy coated hot-stamped steel shown;

[0154] The thin slab continuous casting and rolling production line process includes heat treatment, rolling, laminar flow cooling, coiling and cooling;

[0155] Rolling includes descaling before rough rolling, rough rolling, electromagnetic induction heating, descaling before finishing rolling, and finishing rolling.

[0156] The roughing process uses an irreversible three-pass rolling process, while the finishing process uses a five-pass rolling process. The slab after roughing is heated by an electromagnetic induction heating device to compensate for the temperature drop and reduce the rolling difficulty.

[0157] Dephosphorization before rough rolling includes dephosphorization before double-row rough rolling or dephosphorization before single-row rough rolling, and the pressure for dephosphorization before rough rolling is 38MPa.

[0158] The entry temperature of the roughing mill is 1180℃, and the final rolling temperature of the roughing mill is 960℃.

[0159] The outlet temperature of the electromagnetic induction heating is 1130℃;

[0160] The finishing rolling temperature is 830℃~880℃, and the total reduction rate of the rolling process is 98.6%.

[0161] The heat treatment includes heating in a roller-hearth tunnel homogenizer, with the final temperature of the heat treatment being 1150°C.

[0162] Hot stamping includes a heating stage, which comprises a first heating stage, a first holding stage, a second heating stage, and a second holding stage. During the first holding stage, some Fe elements diffuse into the coating, increasing the melting point and reducing the risk of LME (Liquid Metal Melting). After the zinc-based alloy coated steel coil undergoes the second holding stage, it is then hot stamped. The transfer time is controlled within 10 seconds. After hot stamping, the substrate's microstructure is fully martensitic.

[0163] Hot stamping also includes pre-cooling and hot stamping quenching. Pre-cooling includes pre-cooling at the end temperature of the second heating stage, with a pre-cooling rate ≥30℃ / s and an end temperature of 600℃.

[0164] The final temperature of continuous annealing is 750℃, and the temperature of hot-dip galvanizing is 500℃. Hot-dip galvanizing includes a galvanizing heating section, a galvanizing soaking section, and an alloying treatment. The galvanizing heating section is carried out by pre-oxidation, and the temperature of the alloying treatment is 600℃, and the alloying treatment time is 80s.

[0165] Comparative Example 1

[0166] Comparing Example 1 and Comparative Example 1, the differences between Example 1 and Comparative Example 1 are as follows:

[0167] The method includes: hot rolling the billet, ensuring that the roughing entry temperature is 1235℃, the roughing finishing temperature is 900℃, the coiling temperature is 620℃, and the total reduction rate of hot rolling is >98%, to obtain hot-rolled steel coils;

[0168] Pickling is performed on hot-rolled steel coils to remove the iron oxide scale generated during the hot rolling process;

[0169] The pickled steel coils were cold rolled with a cold rolling reduction of 60%, and the metallographic structure of the substrate after cold rolling was controlled to be ferrite and pearlite.

[0170] The cold-rolled substrate is hot-dip galvanized, wherein the dew point temperature of the heating section of the hot-dip galvanizing process is -20℃ to 30℃.

[0171] The hot-dip galvanized zinc-based alloy coated steel coil is hot-stamped, specifically including: heating the hot-dip galvanized zinc-based alloy coated steel coil at 930℃ for 5 minutes, and then quickly transferring it into a mold for hot stamping. After hot forming, the metallographic structure of the substrate is fully martensitic.

[0172] The mechanical properties of the hot-stamped steels obtained in each embodiment and comparative example are shown in Table 5.

[0173] Table 5

[0174]

[0175]

[0176] Detailed analysis in Table 5:

[0177] LME crack depth refers to the depth of cracks caused by the embrittlement of liquid metal in the prepared steel plate. The lower the LME crack depth, the lower the risk of cracking of the steel plate.

[0178] Tensile strength refers to the maximum stress that a prepared steel plate can withstand before it breaks. The greater the tensile strength, the greater the maximum stress that the steel plate can withstand before it breaks.

[0179] Elongation after fracture refers to the percentage by which the gauge length of a steel plate extends beyond its original gauge length after it breaks. The higher the elongation after fracture, the better the toughness of the steel plate.

[0180] Yield strength refers to the yield limit of a steel plate when it undergoes yielding, which is the stress that resists slight plastic deformation. The greater the yield strength, the higher the yield limit of the steel plate.

[0181] From the data in Examples 1-7, we can see that:

[0182] When the method of this application is adopted, by introducing the thin slab continuous casting and rolling production line into the hot stamping forming of zinc-based alloy coated steel, the process of heating treatment, descaling before rough rolling, rough rolling, electromagnetic induction heating, descaling before finish rolling, finish rolling, laminar flow cooling, coiling and cooling can be carried out in sequence, thereby effectively shortening the length of the entire production line process.

[0183] From the data in Comparative Examples 1-2, we can see that:

[0184] If the thin slab continuous casting and rolling production line of this application is not used, or if the parameters specified in this application are not used, the mechanical properties and LME crack depth of the obtained steel plate are both lower.

[0185] One or more technical solutions in the embodiments of this application have at least the following technical effects or advantages:

[0186] (1) The method provided in this application introduces the thin slab continuous casting and rolling production line into the hot stamping forming steel of zinc-based alloy coating, and sequentially performs heat treatment, descaling before rough rolling, rough rolling, electromagnetic induction heating, descaling before finishing rolling, finishing rolling, laminar flow cooling, coiling and cooling. The introduction of electromagnetic induction heating and the placement of descaling before rough rolling or finishing rolling can avoid the relatively cumbersome descaling and heat preservation treatment between conventional rough rolling and finishing rolling. The use of laminar flow cooling before coiling and cooling after coiling can shorten the subsequent pickling process, thereby effectively shortening the length of the entire production line process.

[0187] (2) The method provided in this application has a total length of only 285m to 288m for thin slab continuous casting and rolling production line, while the total length of thick slab hot continuous rolling production line in the prior art is 700m to 1000m, and even the total length of short-process CSP production line is 430m. Therefore, it can be seen that the total length of thin slab continuous casting and rolling production line is greatly shortened, and it has the advantages of short production line, short process, energy saving, emission reduction and cost reduction.

[0188] (3) The method provided in this application adopts a multi-mode thin slab continuous casting and rolling production line, so its energy consumption per ton of steel is significantly reduced compared with the existing production line, which is conducive to reducing greenhouse gas emissions of steel production enterprises.

[0189] (4) The hot-stamped steel provided in this application embodiment can control the austenite content by limiting the C content, and can effectively improve the hardenability and tempering resistance of the steel by limiting the Si content. Furthermore, limiting the Mn and Cr content can improve the hardenability of the steel and reduce severe surface oxidation caused by high temperatures. Moreover, due to the use of a thin slab continuous casting and rolling production line, the required Mn content can be lower than that of traditional production lines, further reducing production costs. Limiting the Mo content can further improve the hardenability of the steel. Furthermore, limiting the first additive element to include any one or more of Nb, Ti, and V allows the characteristics of Nb, Ti, and V forming precipitates with C and N to refine the austenite grains and act as H traps to capture H atoms, thereby improving the toughness of the substrate. Finally… Limiting the second added element to include Ni and / or Cu can improve the hardenability of steel and avoid hydrogen-induced delayed fracture. By limiting the Si content in the coating, the thickness of the Al-Fe alloy inhibition layer between the substrate and the coating can be effectively controlled, thereby improving the toughness of the coating. By limiting the Mg content in the coating, the MgO film formed on the coating surface can prevent water vapor from reacting with Al in the coating to generate H, thus avoiding hydrogen-induced delayed fracture of the coating. By limiting the Al content in the coating, not only can the melting point of the coating be increased, but the risk of embrittlement of liquid metal can also be reduced. By controlling the chemical composition of the hot-stamped substrate and the zinc-based alloy coating, the embrittlement problems of the substrate and the coating can be effectively avoided, thereby effectively avoiding the embrittlement and cracking problems of hot-stamped steel with zinc-based alloy coating.

[0190] (5) The hot stamping steel provided in this application provides hot-formed parts with good mechanical properties and corrosion resistance, excellent corrosion resistance, and good adhesion of the coating.

[0191] (6) The hot stamping steel provided in this application has a coating layer formed by adding elements such as Al, Mg, and Si to the alloy plating solution while coating the substrate with Zn base. This coating layer has good corrosion resistance after hot forming and can effectively prevent oxide scale production. In addition, it can make reasonable use of microalloying to reduce the risk of hydrogen embrittlement cracking.

[0192] It should be noted that, in this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0193] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0194] The above description is merely a specific embodiment of the present invention, enabling those skilled in the art to understand or implement the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the present invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A method for preparing hot-stamped steel with a zinc-based alloy coating, characterized in that, The method includes: Steel raw materials are smelted and continuously cast to obtain slabs; The slab is processed in a thin slab continuous casting and rolling production line to obtain strip steel; The strip steel is pickled, then continuously annealed and hot-dip galvanized to obtain a zinc-based alloy coated steel coil. The coated steel coil is hot-stamped to obtain zinc-based alloy coated hot-stamped steel. The thin slab continuous casting and rolling production line process includes heat treatment, rolling, laminar flow cooling, coiling and cooling; The rolling process includes descaling before rough rolling, rough rolling, electromagnetic induction heating, descaling before finish rolling, and finish rolling. The total length of the thin slab continuous casting and rolling production line is 285m to 288m. The hot stamping process includes a heating stage, pre-cooling, and hot stamping quenching. The heating stage includes a first heating stage, a first heat preservation stage, a second heating stage, and a second heat preservation stage; The final temperature of the first heating stage is 600℃~850℃, and the duration of the first heat preservation stage is 1min~2min; The final temperature of the second heating stage is 850℃~1000℃, and the duration of the second holding stage is 1min~9min; The precooling rate is ≥30℃ / s, and the final temperature of the precooling is 550℃~650℃; The chemical composition of the hot-stamped steel substrate, by mass fraction, includes: C: 0.18%–0.45%, Si ≤ 0.4%, Al ≤ 0.1%, Mn: 1.0%–5.0%, Cr: 0.01%–0.7%, Mo: 0.01%–0.7%, B: 0.001%–0.005%, S ≤ 0.005%, P ≤ 0.01%, N ≤ 0.008%, O ≤ 0.003%, a first additive element: 0.01%–0.10%, a second additive element ≤ 0.5%, and the remainder being Fe and unavoidable inclusions; the first additive element includes one or more of Ti, Nb, and V, and the second additive element includes Ni and / or Cu; The chemical composition of the zinc-based alloy coating includes: Al: 1%–20%, Mg: 0.01%–3%, Si: 2%–5%, with the remainder being Zn and unavoidable impurities.

2. The method according to claim 1, characterized in that, The rolling mode of the thin slab continuous casting and rolling production line includes one or more of the following: single slab rolling mode, semi-automatic headless rolling mode, and fully automatic headless rolling mode; the descaling before finishing rolling includes descaling before double-row finishing rolling or descaling before single-row finishing rolling. The thin slab continuous casting and rolling production line treatment and the pre-finishing descaling satisfy the following: if the thin slab continuous casting and rolling production line treatment adopts the single-slab rolling mode, then the pre-finishing descaling adopts double-row pre-finishing descaling, and the pressure of the double-row pre-finishing descaling is ≥30MPa. If the thin slab continuous casting and rolling production line adopts the semi-automatic headless rolling mode or the fully automatic headless rolling mode, then the descaling before finishing rolling adopts single-row descaling before finishing rolling, and the pressure of single-row descaling before finishing rolling is ≥35MPa.

3. The method according to claim 1, characterized in that, The heat treatment includes heat treatment using a roller-bottom tunnel homogenizing furnace, and the final temperature of the heat treatment is 1100℃~1200℃.

4. The method according to claim 1, characterized in that, The winding temperature is 500℃~650℃; The dephosphorization before rough rolling includes double-row dephosphorization before rough rolling or single-row dephosphorization before rough rolling, and the pressure of the dephosphorization before rough rolling is ≥30MPa; The inlet temperature of the roughing mill is ≥1140℃, and the final rolling temperature of the roughing mill is 950℃~980℃; The outlet temperature of the electromagnetic induction heating is 1050℃~1250℃; The finishing rolling temperature is 830℃~880℃, and the total reduction rate of the finishing rolling is 65%~80%.

5. The method according to claim 1, characterized in that, The endpoint temperature of the continuous annealing is 720℃~800℃, and the temperature of the hot-dip galvanizing is 450℃~600℃. The hot-dip galvanizing includes a galvanizing heating section, a galvanizing soaking section, and an alloying treatment. The galvanizing heating section is carried out by pre-oxidation. The dew point temperature of the galvanizing heating section is -30℃~10℃. The H2 content of the galvanizing soaking section is 3%~15%. The temperature of the alloying treatment is 500℃~680℃, and the alloying treatment time is 10s~100s.

6. A zinc-based alloy coated hot-stamped steel, characterized in that, The zinc-based alloy coated hot-stamped steel is prepared by the method according to any one of claims 1-5, wherein the zinc-based alloy coated hot-stamped steel comprises a hot-stamped substrate and a zinc-based alloy coating. The chemical composition of the hot-stamped substrate, by mass fraction, includes: C: 0.18%~0.45%, Si≤0.4%, Al≤0.1%, Mn: 1.0%~5.0%, Cr: 0.01%~0.7%, Mo: 0.01%~0.7%, B: 0.001%~0.005%, S≤0.005%, P≤0.01%, N≤0.008%, O≤0.003%, first additive element: 0.01%~0.10%, second additive element ≤0.5%, the remainder is Fe and unavoidable inclusions; The chemical composition of the zinc-based alloy coating includes: Al: 1%–20%, Mg: 0.01%–3%, Si: 2%–5%, with the remainder being Zn and unavoidable impurities; The first added element includes one or more of Ti, Nb and V, and the second added element includes Ni and / or Cu.

7. The hot-stamped steel according to claim 6, characterized in that, The first added element includes Ti, Nb, and V; the second added element includes Ni and Cu; The chemical composition of the hot-stamped substrate satisfies: 0.025%≤[Ti]+[Nb]+[V]≤0.25%; and / or [Ni] + [Cu] ≤ 0.5%; In the formula, [Ti] is the mass fraction of Ti, [Nb] is the mass fraction of Nb, [V] is the mass fraction of V, [Ni] is the mass fraction of Ni, and [Cu] is the mass fraction of Cu.

8. The hot-stamped steel according to claim 6, characterized in that, The chemical composition of the hot-stamped substrate also satisfies: 0.15%≤[Cr]+[Mo]≤1.0%, In the formula, [Cr] is the mass fraction of Cr, and [Mo] is the mass fraction of Mo.

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