Coated steel and preparation method thereof, steel parts and preparation method thereof and application thereof

Abstract

The present application relates to a coated steel, the coated steel comprising: a steel substrate; a nickel-containing alloy layer disposed on the surface of the steel substrate; a zinc-rich layer disposed on the nickel-containing alloy layer, wherein, in terms of the mass percentage of the nickel-containing alloy layer, the nickel-containing alloy layer comprises 0.1% to 4% Al, 0.2% to 5% Ni, and the remainder is Fe and unavoidable impurities, Zn; in terms of the mass percentage of the zinc-rich layer, the zinc-rich layer comprises 0 to 10% Fe, 88% to 100% Zn, and the zinc-rich layer also comprises Al. The coated steel provided in the embodiment of the present application has a nickel-containing alloy layer between the zinc-rich layer and the steel substrate, and is not prone to cracking during hot forming. In addition, the melting point of the nickel-containing alloy layer is relatively high, reaching above 800°C, which is conducive to avoiding the risk of cracking of the matrix grain boundaries during the process of further heat treatment of the coated steel including hot forming due to the liquid zinc generated by the melting of zinc, thereby reducing the depth of crack propagation.

Description

Technical Field

[0001] The present application relates to steel products, and in particular to a coating technology for steel products. Background Art

[0002] In recent years, improving the passive safety of the car body, reducing the weight of the car body and achieving energy conservation and consumption reduction have always been the main development trends in the automotive industry. Among them, hot stamping is a common way to achieve high reinforcement of car body steel, improve passive safety and achieve lightweight at the same time. It achieves high strength of products by combining heat treatment and high-temperature forming and quenching. Hot stamping forming technology uses the easy-to-form and non-rebound characteristics at high temperatures and the quenching and cooling of the mold to solve the problems of easy cracking, wrinkling, and severe rebound in cold forming. Commonly used hot stamping products mainly include: front and rear door left and right anti-collision bars (beams), front and rear bumpers, A-pillar reinforcement plates, B-pillar reinforcement plates, C-pillar reinforcement plates, plate mid-channels, roof reinforcement beams and other safety structural parts. For some parts with special application requirements, the purpose is to prepare parts with a combination of high tensile strength, high plasticity and good corrosion resistance. There is a significant demand for this type of combination in the lightweight process in the new energy vehicle industry. This can be obtained in particular by using steel parts with high mechanical properties, the microstructure of the parts being martensite-bainite / ferrite or martensite-bainite structure.

[0003] During the heating process of traditional uncoated hot stamping parts, a large amount of surface oxide scale will be generated, which will reduce the service life of the mold. At the same time, the mold needs to be cleaned regularly, which reduces production efficiency. Pre-coating on the surface of hot stamping steel plates can avoid the generation of oxide scale and decarburization on the surface of hot stamping steel plates, and at the same time make the hot stamping steel plates corrosion-resistant. GI (Dip Galvanized Steel, hot-dip galvanized steel plate) coating technology is one of the main coatings currently used for hot stamping steel. Hot stamping steel plates using GI coating technology can not only prevent surface oxidation and decarburization during heating, but also provide sacrificial anode protection and improve the corrosion resistance after painting. The production methods of galvanized hot-formed steel mainly include direct hot forming and indirect hot forming. For direct hot forming process, in the direct hot forming of pressure-hardened steel with zinc coating, the Zn element partially liquefies during high-temperature forming and invades the high-temperature austenite grain boundary. The hot-dip galvanized steel plates are prone to microcracks of about 10 to 100 μm, or even macro cracks, which lead to the failure of parts. Summary of the invention

[0004] The embodiments of the present application provide a coated steel and a preparation method thereof, a steel part and a preparation method and application thereof, so as to solve the technical problem that hot-dip galvanized steel sheets are prone to cracks.

[0005] In a first aspect, an embodiment of the present application provides a coated steel, the coated steel comprising:

[0006] Steel substrate;

[0007] A nickel-containing alloy layer disposed on the surface of the steel substrate;

[0008] A zinc-rich layer is provided on the nickel-containing alloy layer,

[0009] Wherein, in terms of mass percentage of the nickel-containing alloy layer, the nickel-containing alloy layer comprises 0.1% to 4% Al, 0.2% to 5% Ni, 20% to 60% Fe, 0.1% to 1.5% Mn, and the remainder is Zn and unavoidable impurities;

[0010] Calculated by mass percentage of the zinc-rich layer, the zinc-rich layer includes 0-10% Fe, 88%-100% Zn, and 0-0.2% Al.

[0011] In some embodiments of the present application, the total thickness of the nickel-containing alloy layer and the zinc-rich layer is 5 to 20 μm; and / or,

[0012] The thickness of the nickel-containing alloy layer is 1 to 8 μm; and / or,

[0013] The thickness of the zinc-rich layer is 4 to 18 μm.

[0014] In some embodiments of the present application, the total content of Al in the nickel-containing alloy layer and the zinc-rich layer is 0.4% to 0.6% based on the total mass of the nickel-containing alloy layer and the zinc-rich layer.

[0015] In some embodiments of the present application, the steel matrix includes, in terms of mass percentage of the steel matrix, 0.05% to 0.4% C, 0.8% to 2.5% Mn, 0% to 0.4% Si, 0% to 0.5% Al, 0.1% to 0.3% Cr, 0% to 0.2% Mo, 0% to 0.4% Ni, 0.0001% to 0.005% B, and 0% to 0.01% N.

[0016] In some embodiments of the present application, the total mass of the three elements Cr, Mo, and Ni accounts for 0.11% to 0.45% of the mass of the steel matrix; and / or,

[0017] The percentage of the total mass of the two elements Al and Si to the mass of the steel matrix is ​​between 0.3% and 0.6%.

[0018] In some embodiments of the present application, the steel matrix further comprises 0.05% to 0.1% C and 1.5% to 2% Mn, based on the mass percentage of the steel matrix, or,

[0019] The steel matrix further comprises 0.15% to 0.25% C and 1.6% to 2.5% Mn, based on the mass percentage of the steel matrix, or

[0020] Calculated by mass percentage of the steel matrix, the steel matrix further includes 0.25% to 0.4% C and 0.8% to 1.5% Mn.

[0021] In some embodiments of the present application, the steel matrix further includes a combination of at least two of any two of Ti, Nb, and V, measured in terms of mass percentage of the steel matrix.

[0022] In some embodiments of the present application, when the steel matrix includes Ti, the mass of Ti accounts for a percentage of 0.01% to 0.1% of the mass of the steel matrix.

[0023] When the steel matrix includes Nb, the mass percentage of Nb to the mass percentage of the steel matrix is ​​between 0.01% and 0.06%,

[0024] When the steel matrix includes V, the mass of V accounts for a percentage of 0.01% to 0.10% of the mass of the steel matrix.

[0025] In some embodiments of the present application, the contents of the three elements Ti, Nb, and V satisfy the following relationship:

[0026] 0.05%≤N Ti +3N Nb +2N V <0.4%,

[0027] Among them, N Ti is the mass percentage of Ti in the steel matrix, N Nb is the mass percentage of Nb in the steel matrix, N V is the mass percentage of V in the steel matrix.

[0028] In some embodiments of the present application, the contents of the three elements Ti, Nb, and V also satisfy the following relationship:

[0029] N Ti / 2(N Nb +N V )≤1.

[0030] In a second aspect, an embodiment of the present application provides a method for preparing a coated steel, the method for preparing the coated steel comprising the following steps:

[0031] Providing a steel substrate;

[0032] Plating nickel on the surface of the steel substrate to form a nickel coating to obtain a nickel-plated substrate;

[0033] The nickel-plated substrate is annealed and galvanized to obtain the plated steel.

[0034] In some embodiments of the present application, the annealing temperature is 720-880°C.

[0035] In some embodiments of the present application, the nickel plating is performed by electroplating; and / or,

[0036] The nickel plating on the surface of the steel substrate is controlled to have a nickel plating amount of 300 to 1500 mg / side / m 2 .

[0037] In some embodiments of the present application, the galvanizing method is hot-dip galvanizing, and the zinc melt used for hot-dip galvanizing contains aluminum.

[0038] In some embodiments of the present application, the hot-dip coating temperature is 400-460° C.; and / or,

[0039] The duration of the hot dip coating is 3 to 20 seconds; and / or,

[0040] Calculated by mass percentage of the zinc melt, the aluminum content in the zinc melt is 0.25% to 0.35%.

[0041] In some embodiments of the present application, the steel substrate is the steel substrate described in any embodiment of the first aspect.

[0042] In a third aspect, an embodiment of the present application provides a steel part, wherein the steel part comprises:

[0043] Steel substrate;

[0044] A zinc-iron alloy layer disposed on the surface of the steel substrate;

[0045] An iron-zinc-nickel alloy layer is provided on the zinc-iron alloy layer,

[0046] The main phase of the zinc-iron alloy layer is α-Fe(Zn), and the main phase of the iron-zinc-nickel alloy layer is Γ-Fe(III) containing nickel. 3 Zn 10 .

[0047] In some embodiments of the present application, the zinc-iron alloy layer and the iron-zinc-nickel alloy layer include 30% to 70% Fe, 30% to 70% Zn, 0 to 0.5% Al, and 0.01% to 1% Ni, measured as a percentage of the total mass of the zinc-iron alloy layer and the iron-zinc-nickel alloy layer.

[0048] In some embodiments of the present application, the zinc-iron alloy layer comprises, by mass percentage, 15% to 65% Zn, 0 to 0.5% Al, and the remainder is Fe and unavoidable impurities; and / or,

[0049] Calculated by mass percentage of the iron-zinc-nickel alloy layer, the iron-zinc-nickel alloy layer includes 65% to 85% of Zn, 0.2% to 2% of Ni, and the remainder is Fe and unavoidable impurities.

[0050] In some embodiments of the present application, the steel part meets the following conditions: 0.01<p<1,

[0051] Herein, p is a value obtained by dividing the volume of the main phase of the iron-zinc-nickel alloy layer by the volume of the main phase of the zinc-iron alloy layer.

[0052] In some embodiments of the present application, among the grain boundaries of the α-Fe / Zn, the proportion of grain boundaries enriched with the Zn element is no more than 60%.

[0053] In some embodiments of the present application, the total thickness of the zinc-iron alloy layer and the iron-zinc-nickel alloy layer is 10 to 40 μm.

[0054] In some embodiments of the present application, the steel substrate is the steel substrate described in any embodiment of the first aspect.

[0055] In a fourth aspect, an embodiment of the present application provides a method for preparing a steel part, the method for preparing the steel part comprising the following steps:

[0056] Providing the coated steel as described in any embodiment of the first aspect, or the coated steel prepared by the method for preparing the coated steel as described in any embodiment of the second aspect;

[0057] After the coated steel is heated to a temperature above the Ac1 temperature of the steel substrate, the temperature of the coated steel is lowered to a hot forming temperature, and the coated steel is subjected to hot stamping treatment and quenching treatment to obtain the steel part.

[0058] In some embodiments of the present application, heating the coated steel to a temperature above the Ac1 temperature of the steel substrate comprises the following steps:

[0059] preheating the coated steel;

[0060] A first stage of austenitizing heating is performed on the coated steel;

[0061] The coated steel is subjected to austenitizing second stage heating,

[0062] The preheating temperature is not higher than the heating temperature of the second stage of austenitizing, and the heating temperature of the second stage of austenitizing is not higher than the heating temperature of the first stage of austenitizing.

[0063] In some embodiments of the present application, the thickness of the steel substrate is 0.7-1.4 mm, the preheating temperature is 600-860° C., the preheating time is 60s-200s, the heating temperature of the first stage of austenitization is 880-950° C., and the heating temperature of the second stage of austenitization is 850-900° C.; or,

[0064] The thickness of the steel substrate is 1.4-3 mm, the preheating temperature is 700-880° C., the time is 80s-220s, the heating temperature of the first stage of austenitization is 890-960° C., and the heating temperature of the second stage of austenitization is 840-890° C.

[0065] In some embodiments of the present application, the thickness of the steel substrate is 0.7-1 mm, the total time of the austenitizing first stage heating and the austenitizing second stage heating is 80-200 s, or,

[0066] The thickness of the steel substrate is 1 to 1.4 mm, the total time of the austenitizing first stage heating and the austenitizing second stage heating is 120 to 300 seconds, or,

[0067] The thickness of the steel substrate is 1.4 to 2 mm, the total time of the austenitizing first stage heating and the austenitizing second stage heating is 100 to 280 seconds, or,

[0068] The thickness of the steel substrate is 2-3 mm, and the total time of the austenitizing first stage heating and the austenitizing second stage heating is 160-360 s.

[0069] In some embodiments of the present application, among the preheating, the first stage austenitizing heating, and the second stage austenitizing heating, at least the preheating is performed in an atmosphere with an oxygen volume fraction of 2% to 20%.

[0070] In some embodiments of the present application, the thermoforming temperature is 500-740° C.; and / or,

[0071] The temperature of the coated steel is lowered to the hot forming temperature, and the duration of the cooling of the coated steel is 5 to 16 seconds.

[0072] In some embodiments of the present application, the steel substrate is the steel substrate described in any embodiment of the first aspect.

[0073] In a fifth aspect, an embodiment of the present application provides an application of a steel part, wherein the steel part is applied to hot stamping steel parts for automobiles.

[0074] In some embodiments of the present application, the automotive hot-stamped steel parts are any one of the front door left and right anti-collision bars, the front door left and right anti-collision bar beams, the rear door left and right anti-collision bars, the rear door left and right anti-collision bar beams, the front bumper, the rear bumper, the A-pillar reinforcement plate, the B-pillar reinforcement plate, the C-pillar reinforcement plate, the mid-plate channel, the roof reinforcement beam, the new energy vehicle battery shell, and the vehicle threshold safety structure.

[0075] The above technical solution provided by the embodiment of the present application has the following advantages compared with the prior art:

[0076] The coated steel provided in the embodiment of the present application has a nickel-containing alloy layer between the zinc-rich layer and the steel substrate, and is not prone to cracking during hot forming. In addition, the melting point of the nickel-containing alloy layer is relatively high, reaching above 800°C, which is conducive to avoiding the risk of liquid zinc-induced cracking of the substrate grain boundary during the forming process caused by the melting of zinc during further heat treatment including hot forming of the coated steel, thereby reducing the crack propagation depth. BRIEF DESCRIPTION OF THE DRAWINGS

[0077] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the present application and, together with the description, serve to explain the principles of the present application.

[0078] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative labor.

[0079] Figure 1 Element depth distribution diagram of the coated steel described in Example 3;

[0080] Figure 2 This is the electron probe analysis diagram of the steel part described in Example 3;

[0081] Figure 3 This is an electron probe analysis diagram of the steel part described in Comparative Example 3;

[0082] Figure 4 This is the phase diagram of zinc-iron alloy with different Ni addition amounts;

[0083] Figure 5 Scanning electron microscope images of thin slice samples obtained from the steel parts described in Example 3 and Comparative Example 2;

[0084] Figure 6This is a transmission electron microscope image of a thin slice sample made from the steel part described in Example 3. DETAILED DESCRIPTION

[0085] In order to make the purpose, technical solution and advantages of the embodiments of the present application clearer, the technical solution in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of this application.

[0086] Unless otherwise specified, the terms used herein should be understood as commonly used in the art. Therefore, unless otherwise defined, all technical and scientific terms used herein have the same meaning as those generally understood by those skilled in the art to which this application belongs. In the event of a conflict, this specification takes precedence.

[0087] Unless otherwise specified, various raw materials, reagents, instruments and equipment used in this application can be purchased from the market or prepared by existing methods.

[0088] At present, hot-dip galvanized steel sheets have the technical problem of being prone to cracks.

[0089] The technical solution provided by the embodiments of the present application is to solve the above technical problems, and the overall idea is as follows:

[0090] In a first aspect, an embodiment of the present application provides a coated steel, the coated steel comprising:

[0091] Steel substrate;

[0092] A nickel-containing alloy layer disposed on the surface of the steel substrate;

[0093] A zinc-rich layer is provided on the nickel-containing alloy layer,

[0094] Wherein, in terms of mass percentage of the nickel-containing alloy layer, the nickel-containing alloy layer comprises 0.1% to 4% Al, 0.2% to 5% Ni, 20% to 60% Fe, 0.1% to 1.5% Mn, and the remainder is Zn and unavoidable impurities;

[0095] Calculated by mass percentage of the zinc-rich layer, the zinc-rich layer includes 0-10% Fe, 88%-100% Zn, and 0-0.2% Al.

[0096] The coated steel provided in the embodiment of the present application has a nickel-containing alloy layer between the zinc-rich layer and the steel substrate, and is not prone to cracking during hot forming. In addition, the melting point of the nickel-containing alloy layer is relatively high, reaching above 800°C, which is conducive to avoiding the risk of liquid zinc-induced cracking of the substrate grain boundary during the forming process caused by the melting of zinc during further heat treatment including hot forming of the coated steel, thereby reducing the crack propagation depth.

[0097] The present application further theoretically analyzes the melting point of the nickel-containing alloy layer, and calculates the phase diagram under a specific composition using Pandat software. The obtained phase diagram is as follows: Figure 4 shown. Figure 4 It shows that when the addition of Ni element is below 2%, the melting point of the alloy of Zn:40%,Fe:60% is significantly improved.

[0098] In some embodiments of the present application, the total thickness of the nickel-containing alloy layer and the zinc-rich layer is 5 to 20 μm; and / or,

[0099] The thickness of the nickel-containing alloy layer is 1 to 8 μm; and / or,

[0100] The thickness of the zinc-rich layer is 4 to 18 μm.

[0101] As an example, the total thickness of the nickel-containing alloy layer and the zinc-rich layer may be 5 μm, 10 μm, 15 μm, or 20 μm.

[0102] As an example, the thickness of the nickel-containing alloy layer may be 1 μm, 2 μm, 4 μm, 6 μm, or 8 μm.

[0103] As an example, the thickness of the zinc-rich layer may be 4 μm, 8 μm, 10 μm, 14 μm, or 18 μm.

[0104] In some embodiments of the present application, the content of Al in the nickel-containing alloy layer and the zinc-rich layer is 0.4% to 0.6% based on the total mass of the nickel-containing alloy layer and the zinc-rich layer.

[0105] As an example, the Al content in the nickel-containing alloy layer and the zinc-rich layer may be 0.4%, 0.45%, 0.5%, 0.55%, or 0.6%.

[0106] In some embodiments of the present application, the steel matrix includes, in terms of mass percentage of the steel matrix, 0.05% to 0.4% C, 0.8% to 2.5% Mn, 0% to 0.4% Si, 0% to 0.5% Al, 0.1% to 0.3% Cr, 0% to 0.2% Mo, 0% to 0.4% Ni, 0.001% to 0.005% B, and 0% to 0.01% N.

[0107] It is easy to understand that, in addition to the above elements, the steel matrix may also include harmful elements such as P, S, O, etc., and their content should be reduced as much as possible. Usually, the P content is controlled below 0.02%, the S content is controlled below 0.005%, and the O content is controlled below 0.003%.

[0108] The above elements have the following functions:

[0109] C: C is the most effective and cheapest solid solution strengthening element, which can effectively ensure the strength of the steel matrix during hot stamping operations. However, too high a C content will not be conducive to the acquisition of toughness and plasticity, and too high a strength cannot effectively meet the effect of collision energy absorption. At the same time, too high a strength is not conducive to the welding of safety components with other parts; if the C content is too low, it is not conducive to the formation of sufficient strengthening effect to obtain a stable and sufficient content of high dislocation martensite, bainite and other strengthening phases.

[0110] Mn: Mn is used to increase the austenite region, lower the austenitizing temperature, and improve hardenability. Too high a Mn content will also affect the toughness of the steel matrix.

[0111] Cr: Cr can significantly increase the hardenability of the steel matrix and reduce surface oxidation, which is beneficial to obtaining a steel matrix with high surface quality. However, it is a strong carbide-forming element and promotes the formation of bainite, so its content should not be too high.

[0112] Mo: Molybdenum improves the hardenability of steel. Its effect is stronger than chromium, but slightly less than manganese. Molybdenum has a solid solution strengthening effect on ferrite, and also improves the stability of carbides, which is beneficial to control the size of precipitates, thereby improving the precipitation strengthening effect of steel and improving the hydrogen embrittlement resistance. However, when the content is high, the deformation resistance of steel to hot working increases, which is not conducive to hot forming.

[0113] Ni: Nickel can increase the strength of steel while maintaining good plasticity and toughness, lower the ductile-brittle transition temperature, and is beneficial for using parts at lower temperatures.

[0114] N: N combines with Al, Ti, Nb, V, etc. to form compounds, thereby refining the grains and reducing the sensitivity to hydrogen-induced delayed fracture, but it will also segregate at the grain boundaries and reduce the grain boundary strength. Therefore, the N content should not be too high.

[0115] Si: Si is a ferrite-forming element. When hot stamping steel is heated to the austenite zone (γ) and kept warm, Si atoms are dissolved in the ferrite. The dissolution of Si in ferrite increases the activity of C atoms, promotes the outward diffusion of C atoms in ferrite, and increases the carbon content in the surrounding austenite. As the carbon content in austenite increases, its stability also increases. Too high Si content will lead to increased brittleness;

[0116] Al: Al can improve its plasticity, increase the driving force of the transformation of austenite to bainite, accelerate the formation of bainite, significantly increase the activity of C atoms in ferrite, and inhibit the formation of cementite. Too high Al content will increase the difficulty of continuous casting.

[0117] In some embodiments of the present application, the total mass of the three elements Cr, Mo, and Ni accounts for 0.11% to 0.45% of the mass of the steel matrix; and / or,

[0118] The percentage of the total mass of the two elements Al and Si to the mass of the steel matrix is ​​between 0.3% and 0.6%.

[0119] In some embodiments of the present application, the steel matrix further comprises 0.05% to 0.1% C and 1.5% to 2% Mn, based on the mass percentage of the steel matrix, or,

[0120] The steel matrix further comprises 0.15% to 0.25% C and 1.6% to 2.5% Mn, based on the mass percentage of the steel matrix, or

[0121] Calculated by mass percentage of the steel matrix, the steel matrix further includes 0.25% to 0.4% C and 0.8% to 1.5% Mn.

[0122] In some embodiments of the present application, the steel matrix further includes a combination of at least two of any two of Ti, Nb, and V, measured in terms of mass percentage of the steel matrix.

[0123] It is easy to understand that Nb, Ti, V combine with C and N to form precipitates, which are mainly used to refine austenite grains and improve precipitation strengthening effects, thereby improving the toughness and hydrogen embrittlement resistance of parts.

[0124] In some embodiments of the present application, when the steel matrix includes Ti, the mass of Ti accounts for a percentage of 0.01% to 0.1% of the mass of the steel matrix.

[0125] When the steel matrix includes Nb, the mass percentage of Nb to the mass percentage of the steel matrix is ​​between 0.01% and 0.06%,

[0126] When the steel matrix includes V, the mass of V accounts for a percentage of 0.01% to 0.10% of the mass of the steel matrix.

[0127] In some embodiments of the present application, the contents of the three elements Ti, Nb, and V satisfy the following relationship:

[0128] 0.05%≤N Ti+3N Nb +2N V <0.4%,

[0129] Among them, N Ti is the mass percentage of Ti in the steel matrix, N Nb is the mass percentage of Nb in the steel matrix, N V is the mass percentage of V in the steel matrix.

[0130] It is easy to understand that Nb has the best grain refinement effect but is more expensive, V has a smaller precipitate size, and Ti is cheap. During the hot stamping process, a certain amount of NbC, VC, TiN, VC precipitates or (V, Ti, Nb) (C, N) composite carbides will precipitate in the steel matrix described in this application. Composite precipitation is beneficial to reducing the precipitate size and improving stability. The second phase particles effectively pin the austenite grains, which will refine the original austenite grains. For 0.05%≤N Ti +3N Nb +2N V Under the condition of <0.4%, the coefficient of Nb is the largest, followed by V, and the smallest, achieving the optimal ratio.

[0131] The inventor has found through in-depth research that when N Ti +3N Nb +2N V When the content is greater than 0.4%, the precipitate phase may be coarsened, which is not conducive to obtaining a nano-scale dispersed precipitate phase. In addition, when the content of (Ti+3Nb+2V) is less than 0.05%, the precipitation strengthening effect is not significantly improved, and it is insufficient to form sufficient precipitates to obtain precipitation strengthening and fine grain strengthening effects. In addition, the number of H traps formed by the precipitates is also insufficient, which is not conducive to the use of the product.

[0132] In some embodiments of the present application, the contents of the three elements Ti, Nb, and V also satisfy the following relationship:

[0133] N Ti / 2(N Nb +N V )≤1.

[0134] For N Ti / 2(N Nb +N V )≤1, N Ti / 2(N Nb +N V )>1, a large amount of Nb and V will adhere to the surface of coarse Ti precipitates, resulting in a decrease in the effect of Nb and V addition. Ti / 2(N Nb +N V )≤1.

[0135] In a second aspect, an embodiment of the present application provides a method for preparing a coated steel, the method for preparing the coated steel comprising the following steps:

[0136] S11: providing a steel substrate;

[0137] S12: plating nickel on the surface of the steel substrate to form a nickel coating to obtain a nickel-plated substrate;

[0138] S13: After annealing and galvanizing the nickel-plated substrate, the plated steel is obtained.

[0139] The anti-oxidation performance of nickel is much higher than that of the steel substrate. In the present application, nickel is pre-plated on the surface of the steel. During the galvanizing process, no oxide will appear on the surface of the nickel-plated substrate. The pre-plated nickel layer can also effectively inhibit the enrichment of Si and Mn elements to the surface during the galvanizing annealing process, avoiding direct contact between the steel substrate and the zinc liquid, and preventing elements such as silicon or manganese in the steel from entering the zinc liquid. During the galvanizing process, the pre-plated nickel layer will form a nickel-containing alloy layer on the surface of the steel substrate. The nickel-containing alloy layer specifically includes elements such as Fe, Ni, and Zn, which can effectively improve the bonding strength between the zinc coating and the steel substrate. At the same time, the melting point of the nickel-containing alloy layer is as high as 800°C or more, which is conducive to avoiding the risk of matrix grain boundary cracking during the liquid zinc induced forming process caused by the melting of zinc during further heat treatment of the coated steel including hot forming, thereby reducing the depth of crack propagation.

[0140] In some embodiments of the present application, the annealing temperature is 720-880°C.

[0141] As an example, the annealing temperature may be 720°C, 760°C, 800°C, 840°C, or 880°C.

[0142] In some embodiments of the present application, the nickel plating is performed by electroplating; and / or,

[0143] The nickel plating on the surface of the steel substrate is controlled to have a nickel plating amount of 300 to 1500 mg / side / m 2 .

[0144] In some embodiments of the present application, the galvanizing method is hot-dip galvanizing, and the zinc melt used for hot-dip galvanizing contains aluminum.

[0145] The zinc melt used for hot-dip plating contains aluminum, which will cause the coating on the surface of the coated steel to also contain aluminum. During further heat treatment of the coated steel including hot forming, AlO can be formed on the surface of the coated steel, thereby reducing the evaporation loss of the Zn element.

[0146] In some embodiments of the present application, the hot-dip coating temperature is 400-460° C.; and / or,

[0147] The duration of the hot dip coating is 3 to 20 seconds; and / or,

[0148] Calculated by mass percentage of the zinc melt, the aluminum content in the zinc melt is 0.25% to 0.35%.

[0149] As an example, the hot-dip plating temperature may be 400°C, 420°C, 440°C, or 460°C.

[0150] As an example, the hot dip coating time may be 3s, 6s, 10s, 15s, or 20s.

[0151] It is easy to understand that the temperature of the hot-dip plating, the duration of the hot-dip plating, and the aluminum content in the zinc melt will ultimately affect the aluminum content of the coating on the surface of the coated steel.

[0152] When the aluminum content is lower than 0.25%, excessive reaction of iron and zinc will affect the surface quality. At the same time, the mutual diffusion of iron and zinc during the heating process cannot be effectively suppressed, resulting in excessive iron oxide on the surface of the coated steel and too low Zn content in the coating, affecting the corrosion resistance. When the aluminum content is higher than 0.35%, the diffusion of iron and zinc is excessively suppressed, and the melting point of the coating on the surface of the coated steel cannot be increased, resulting in serious cracking of the matrix grain boundary during the liquid zinc induced forming process. At the same time, the aluminum oxide content increases, resulting in reduced surface adhesion, which is not conducive to subsequent coating and paint film processes.

[0153] In some embodiments of the present application, the steel substrate is the steel substrate described in any embodiment of the first aspect.

[0154] In a third aspect, an embodiment of the present application provides a steel part, wherein the steel part comprises:

[0155] Steel substrate;

[0156] A zinc-iron alloy layer disposed on the surface of the steel substrate;

[0157] An iron-zinc-nickel alloy layer is provided on the zinc-iron alloy layer,

[0158] The main phase of the zinc-iron alloy layer is α-Fe(Zn), and the main phase of the iron-zinc-nickel alloy layer is Γ-Fe(III) containing nickel. 3 Zn 10 .

[0159] In some embodiments of the present application, the zinc-iron alloy layer and the iron-zinc-nickel alloy layer include 30% to 70% Fe, 30% to 70% Zn, 0 to 0.5% Al, and 0.01% to 1% Ni, measured as a percentage of the total mass of the zinc-iron alloy layer and the iron-zinc-nickel alloy layer.

[0160] In some embodiments of the present application, the zinc-iron alloy layer comprises, by mass percentage, 15% to 65% Zn, 0 to 0.5% Al, and the remainder is Fe and unavoidable impurities; and / or,

[0161] Calculated by mass percentage of the iron-zinc-nickel alloy layer, the iron-zinc-nickel alloy layer includes 65% to 85% of Zn, 0.2% to 2% of Ni, and the remainder is Fe and unavoidable impurities.

[0162] In some embodiments of the present application, the steel part meets the following conditions: 0.01<p<1,

[0163] Herein, p is a value obtained by dividing the volume of the main phase of the iron-zinc-nickel alloy layer by the volume of the main phase of the zinc-iron alloy layer.

[0164] The specific value of p can be inferred from scanning electron microscopy observation or X-ray microanalysis results.

[0165] When p<0.01, the zinc content of the surface coating of the steel part is too low and the corrosion resistance is insufficient; when p>1, it may indicate that the coating steel is heated insufficiently in step S4, so p>1 may be accompanied by insufficient austenitization of the steel matrix.

[0166] As an example, the coating structure and the metal phase volume can be observed using a scanning electron microscope (SEM) or an electron probe X-ray microanalyser (EPMA).

[0167] In some embodiments of the present application, among the grain boundaries of the α-Fe / Zn, the proportion of grain boundaries enriched with the Zn element is no more than 60%.

[0168] The enrichment of Zn at the grain boundary may reduce the grain boundary strength. The preheating in step S41 can effectively reduce the Zn liquefaction problem and reduce the enrichment of Zn at the grain boundary.

[0169] The enrichment of Zn element in the grain boundary can be determined by transmission electron microscopy and EDS (Energy Dispersive Spectroscopy) scanning.

[0170] As an example, the element enrichment of the grain boundary can be determined by EDS (Energy Dispersive Spectroscopy) surface scanning.

[0171] In some embodiments of the present application, the total thickness of the zinc-iron alloy layer and the iron-zinc-nickel alloy layer is 10 to 40 μm.

[0172] As an example, the total thickness of the zinc-iron alloy layer and the iron-zinc-nickel alloy layer may be 10 μm, 20 μm, 30 μm, or 40 μm.

[0173] In some embodiments of the present application, the steel substrate is the steel substrate described in any embodiment of the first aspect.

[0174] In a fourth aspect, an embodiment of the present application provides a method for preparing a steel part, the method for preparing the steel part comprising the following steps:

[0175] S21: providing the coated steel described in any embodiment of the first aspect, or the coated steel prepared by the method for preparing the coated steel described in any embodiment of the second aspect;

[0176] S22: After heating the coated steel to a temperature above the Ac1 temperature of the steel substrate, lowering the temperature of the coated steel to a hot forming temperature, and performing hot stamping and quenching treatments on the coated steel to obtain the steel part.

[0177] In step S22, during the process of heating the coated steel to a temperature above the Ac1 temperature of the steel substrate and lowering the temperature of the coated steel to the hot forming temperature, an iron-zinc-nickel alloy layer is formed on the surface of the coated steel, and the main phase is Γ-Fe 3 Zn 10 alloy, Ni will combine with Zn and enrich in Γ-Fe 3 Zn 10 alloy, which will make Γ-Fe 3 Zn 10 The structure of the alloy is more stable, so that the iron-zinc-nickel alloy layer has excellent corrosion resistance, and ultimately the corrosion resistance of the steel part can be improved.

[0178] In addition, the process of heating the coated steel to above the Ac1 temperature of the steel substrate and reducing the temperature of the coated steel to the hot forming temperature is also a heat treatment process for the steel substrate itself. Through this heat treatment process, the heat treatment of the steel substrate and the heat treatment of the coating on the surface of the coated steel can be achieved simultaneously, simplifying the process flow.

[0179] In some embodiments of the present application, heating the coated steel to a temperature above the Ac1 temperature of the steel substrate comprises the following steps:

[0180] S221: preheating the coated steel;

[0181] S222: performing austenitization first stage heating on the coated steel;

[0182] S223: performing austenitization second stage heating on the coated steel,

[0183] The preheating temperature is not higher than the heating temperature of the second stage of austenitizing, and the heating temperature of the second stage of austenitizing is not higher than the heating temperature of the first stage of austenitizing.

[0184] The preheating in step S221 can diffuse enough elements such as iron and nickel to the coating on the surface of the coated steel at a temperature lower than the liquefaction temperature of zinc, thereby increasing its melting point. The heating temperature in the second stage of austenitization is not higher than the heating temperature in the first stage of austenitization. The higher first temperature range of austenitization is conducive to quickly completing austenitization, shortening the holding time, and saving energy; the lower second temperature range of austenitization is conducive to lowering the furnace temperature and shortening the time taken to reduce the temperature of the coated steel to the hot forming temperature in step S4.

[0185] In some embodiments of the present application, the thickness of the steel substrate is 0.7-1.4 mm, the preheating temperature is 600-860° C., the preheating time is 60s-200s, the heating temperature of the first stage of austenitization is 880-950° C., and the heating temperature of the second stage of austenitization is 850-900° C.; or,

[0186] The thickness of the steel substrate is 1.4-3 mm, the preheating temperature is 700-880° C., the time is 80s-220s, the heating temperature of the first stage of austenitization is 890-960° C., and the heating temperature of the second stage of austenitization is 840-890° C.

[0187] It is easy to understand that the present application sets different heating temperatures for steel substrates of different thicknesses so that the steel substrates can be fully austenitized.

[0188] In some embodiments of the present application, the thickness of the steel substrate is 0.7-1 mm, the total time of the austenitizing first stage heating and the austenitizing second stage heating is 80-200 s, or,

[0189] The thickness of the steel substrate is 1 to 1.4 mm, the total time of the austenitizing first stage heating and the austenitizing second stage heating is 120 to 300 seconds, or,

[0190] The thickness of the steel substrate is 1.4 to 2 mm, the total time of the austenitizing first stage heating and the austenitizing second stage heating is 100 to 280 seconds, or,

[0191] The thickness of the steel substrate is 2-3 mm, and the total time of the austenitizing first stage heating and the austenitizing second stage heating is 160-360 s.

[0192] It is easy to understand that, under the premise that different heating temperatures are set for steel substrates of different thicknesses, the present application further subdivides the thickness of the steel substrate and sets different heating times for different thicknesses so that the steel substrate can be fully austenitized.

[0193] In some embodiments of the present application, among the preheating, the first stage austenitizing heating, and the second stage austenitizing heating, at least the preheating is performed in an atmosphere with an oxygen volume fraction of 2% to 20%.

[0194] It is easy to understand that heat treatment in the above atmosphere will cause oxides such as ZnO and AlO to form on the surface of the coated steel, thereby reducing the evaporation loss of the Zn element.

[0195] As an example, the oxygen volume fraction of the atmosphere may be 2%, 4%, 8%, 15%, or 20%.

[0196] The beneficial effect of the oxygen volume fraction in the atmosphere being 2% to 20% is that oxides such as ZnO and AlO can be formed to reduce the evaporation loss of the Zn element, and the amount of the formed oxides will not be too much to bring additional surface treatment costs.

[0197] In some embodiments of the present application, the thermoforming temperature is 500-740° C.; and / or,

[0198] The temperature of the coated steel is lowered to the hot forming temperature, and the duration of the cooling of the coated steel is 5 to 16 seconds.

[0199] It is easy to understand that the hot forming process temperature of galvanized steel sheets is usually below 700° C. In the present application, due to the presence of high melting point metal Ni, the hot forming temperature can be extended to 740° C.

[0200] It is easy to understand that if the hot forming temperature is too low, the forming resistance of the steel matrix will increase, and even cause the steel part to crack; if the hot forming temperature is too high, there is a risk of Zn melting and inducing cracking of the matrix grain boundaries during the forming process.

[0201] As an example, the temperature of the coated steel is lowered to the hot forming temperature by at least one of jet cooling, water mist cooling, and aqueous solution cooling.

[0202] It is easy to understand that the beneficial effect of the duration of the cooling treatment of the coated steel being 5 to 16 seconds is that it can ensure that the temperature of the coated steel can be reduced to the hot forming temperature, and the cooling treatment time will not be too long to affect the efficiency.

[0203] In some embodiments of the present application, the steel substrate is the steel substrate described in any embodiment of the first aspect.

[0204] In a fifth aspect, an embodiment of the present application provides an application of a steel part, wherein the steel part is applied to hot stamping steel parts for automobiles.

[0205] In some embodiments of the present application, the automotive hot-stamped steel parts are any one of the front door left and right anti-collision bars, the front door left and right anti-collision bar beams, the rear door left and right anti-collision bars, the rear door left and right anti-collision bar beams, the front bumper, the rear bumper, the A-pillar reinforcement plate, the B-pillar reinforcement plate, the C-pillar reinforcement plate, the mid-plate channel, the roof reinforcement beam, the new energy vehicle battery shell, and the vehicle threshold safety structure.

[0206] The present application will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are intended only to illustrate the present application and are not intended to limit the scope of the present application. The experimental methods for which specific conditions are not specified in the following examples are usually measured according to industry standards. If there is no corresponding industry standard, then the conditions recommended by the manufacturer are followed.

[0207] Example 1

[0208] First, this embodiment provides a steel substrate having the composition shown in Table 1. The steel substrate is specifically a cold-hardened steel strip, which is prepared through smelting, casting, hot rolling, pickling and cold rolling processes.

[0209] In this embodiment, a steel part is prepared based on the steel substrate, and the specific preparation method of the steel part includes the following steps:

[0210] Sa: The steel substrate is subjected to continuous nickel plating pretreatment, specifically including alkaline solution degreasing, water washing, electrolytic degreasing, pickling, and water washing, to remove grease, residual iron and other impurities on the surface of the steel substrate, so that the surface of the strip is kept clean;

[0211] Sb: After the pretreatment is completed, nickel is electroplated on both sides of the steel substrate to obtain a nickel-plated substrate, wherein the weight of the electroplated nickel is shown in Table 2;

[0212] Sc: After the nickel-plated substrate is subjected to continuous annealing treatment, hot-dip plating is performed in a zinc melt, wherein the zinc melt contains aluminum, and after hot-dip plating, a nickel-containing alloy layer and a zinc-rich layer are formed on the surface of the steel substrate to obtain a coated steel, wherein the temperature and time of the continuous annealing, the temperature and time of the hot-dip plating, the aluminum content in the zinc melt, the thickness of the nickel-containing alloy layer and the zinc-rich layer, the content of each element in the nickel-containing alloy layer and the zinc-rich layer and the total content of aluminum are shown in Table 2, and the elemental composition of the nickel-containing alloy layer and the zinc-rich layer is shown in Table 3;

[0213] Sd: After preheating the plated steel, heating in the first stage of austenitization, and heating in the second stage of austenitization, the plated steel is subjected to hot stamping and quenching to obtain the steel part, which includes a steel substrate, a zinc-iron alloy layer formed on the surface of the steel substrate, and an iron-zinc-nickel alloy layer formed on the surface of the iron-zinc-nickel alloy layer, wherein the thickness of the steel substrate, the temperature and time of the preheating, the temperature and time of the first stage of austenitization heating, the temperature and time of the second stage of austenitization heating, the temperature of the hot stamping treatment, the thickness of the coating on the surface of the steel part, and the p value of the steel part (p is the value obtained by dividing the volume of the main phase of the iron-zinc-nickel alloy layer by the volume of the main phase of the zinc-iron alloy layer) are shown in Table 4, and the elemental compositions of the zinc-iron alloy layer and the iron-zinc-nickel alloy layer are shown in Table 5.

[0214] In addition, it should be noted that the sum of the contents of the elements in Table 1, Table 3 and Table 5 does not reach 100% because they contain other unavoidable impurities, such as Mn, Si and other elements diffused from the steel matrix.

[0215] Example 2

[0216] The differences between this embodiment and embodiment 1 are shown in Table 1, Table 2, Table 3, Table 4, and Table 5. Except for the data shown in Table 1, Table 2, Table 3, Table 4, and Table 5, the other operation modes and conditions of this embodiment and embodiment 1 are exactly the same.

[0217] Example 3

[0218] The differences between this embodiment and embodiment 1 are shown in Table 1, Table 2, Table 3, Table 4, and Table 5. Except for the data shown in Table 1, Table 2, Table 3, Table 4, and Table 5, the other operation modes and conditions of this embodiment and embodiment 1 are exactly the same.

[0219] In addition, the element depth distribution of the coated steel and steel parts obtained in this embodiment was tested and analyzed by GDS850A glow spectrometer, and the obtained element depth distribution diagram is as follows: Figure 1 .in Figure 1 (a) is the distribution of elements in the surface coating of the coated steel with depth, Figure 1 (b) is the distribution of elements in the surface coating of the steel part with depth. Figure 1 The elemental compositions of the zinc-iron alloy layer and the iron-zinc-nickel alloy layer can be calculated, and the specific results are shown in Table 5.

[0220] Example 4

[0221] The differences between this embodiment and embodiment 1 are shown in Table 1, Table 2, Table 3, Table 4, and Table 5. Except for the data shown in Table 1, Table 2, Table 3, Table 4, and Table 5, the other operation modes and conditions of this embodiment and embodiment 1 are exactly the same.

[0222] Example 5

[0223] The differences between this embodiment and embodiment 1 are shown in Table 1, Table 2, Table 3, Table 4, and Table 5. Except for the data shown in Table 1, Table 2, Table 3, Table 4, and Table 5, the other operation modes and conditions of this embodiment and embodiment 1 are exactly the same.

[0224] Example 6

[0225] The differences between this embodiment and embodiment 1 are shown in Table 1, Table 2, Table 3, Table 4, and Table 5. Except for the data shown in Table 1, Table 2, Table 3, Table 4, and Table 5, the other operation modes and conditions of this embodiment and embodiment 1 are exactly the same.

[0226] Comparative Example 1

[0227] The difference between this comparative example and Example 1 is shown in Table 1, Table 2, Table 3, Table 4, and Table 5. In addition to the data shown in Table 1, Table 2, Table 3, Table 4, and Table 5, the difference between this comparative example and Example 1 is that this comparative example does not include step Sb, but directly performs continuous annealing and hot dip coating in step Sc on the steel substrate subjected to step Sa.

[0228] Comparative Example 2

[0229] The difference between this comparative example and Example 1 is shown in Table 1, Table 2, Table 3, Table 4, and Table 5. In addition to the data shown in Table 1, Table 2, Table 3, Table 4, and Table 5, the difference between this comparative example and Example 1 is that this comparative example does not include step Sb, but directly performs continuous annealing and hot dip coating in step Sc on the steel substrate subjected to step Sa.

[0230] Comparative Example 3

[0231] The difference between this comparative example and Example 1 is shown in Table 1, Table 2, Table 3, Table 4, and Table 5. In addition to the data shown in Table 1, Table 2, Table 3, Table 4, and Table 5, the difference between this comparative example and Example 1 is that this comparative example does not include step Sb, but directly performs continuous annealing and hot dip coating in step Sc on the steel substrate subjected to step Sa.

[0232] Comparative Example 4

[0233] The difference between this comparative example and Example 1 is shown in Table 1, Table 2, Table 3, Table 4, and Table 5. In addition to the data shown in Table 1, Table 2, Table 3, Table 4, and Table 5, the difference between this comparative example and Example 1 is that this comparative example does not include step Sb, but directly performs continuous annealing and hot dip coating in step Sc on the steel substrate subjected to step Sa.

[0234] Table 1 Chemical composition of steel matrix in various embodiments and comparative examples

[0235]

[0236] Table 1

[0237]

[0238] Table 2 Process parameters and product parameters involved in step Sb and step Sc in each embodiment and comparative example

[0239]

[0240] Table 3 Elemental composition of nickel alloy layer and zinc-rich layer

[0241]

[0242] Table 4 Process parameters and product parameters involved in step Sd in each embodiment and comparative example

[0243]

[0244] Table 5

[0245]

[0246] Table 5

[0247]

[0248]

[0249] Related experiments and effect data:

[0250] The steel parts obtained in Examples 1 to 6 and Comparative Examples 1 to 3 were subjected to mechanical property tests to obtain yield strength, tensile strength and elongation at break data, and the test results are shown in Table 6; the martensite volume fraction, ferrite volume fraction and bainite volume fraction of the steel matrix in the steel parts obtained in Examples 1 to 6 and Comparative Examples 1 to 3 were analyzed by XRD (X-Ray Diffraction), and the analysis results are shown in Table 6; according to ISO 11997-1cycle B, whether the steel parts meet the corrosion resistance requirements of automobile factories is tested, and the results are shown in Table 6; the surface crack depths of the steel parts obtained in Examples 1 to 6 and Comparative Examples 1 to 3 are observed by SEM (Scanning Electron Microscope), and the results are shown in Table 6.

[0251] Table 6 Test and analysis results of steel parts obtained in Examples 11 to 6 and Comparative Examples 1 to 3

[0252]

[0253] Comprehensive analysis of Table 1, Table 4, and Table 6 leads to the following conclusions:

[0254] The steel matrix of Comparative Example 1 does not contain Nb and V elements, and various elements such as C and Cr do not meet the range required by the present application, which results in the yield strength of the steel parts of Comparative Example 1 being lower than 350 MPa.

[0255] Comparative Example 3 does not contain Nb and V elements, and the elongation of the obtained steel product is less than 4%.

[0256] Comparative Examples 1 to 4 do not contain nickel and cannot withstand high preheating temperature, austenite first stage heating temperature, austenite second stage heating temperature, and hot stamping temperature, resulting in zinc liquefaction in a high heat environment, inducing cracks in the steel matrix. The crack depths of Comparative Examples 1 to 4 are generally higher than those of the embodiments.

[0257] The Al content in the zinc melt of Comparative Examples 1, 2, and 3 is low, resulting in excessive reaction of iron and zinc. At the same time, the mutual diffusion of iron and zinc during the heating process cannot be effectively suppressed, resulting in excessive iron oxide on the surface of the coated steel and too low Zn content in the coating, which affects the corrosion resistance and cannot meet the corrosion resistance requirements.

[0258] The steel parts obtained in Example 3 and Comparative Example 3 were analyzed by EPMA (Electron Probe X-ray Micro-Analyzer), wherein the electron probe analysis diagram of the steel parts in Example 3 is as follows: Figure 2 The electron probe analysis diagram of the steel part described in Comparative Example 3 is as follows: Figure 3 .in, Figure 2 (a) is the backscattering image of the coating. Figure 2 (b) is the distribution diagram of Fe element content, Figure 2 (c) is the distribution diagram of Ni element content, Figure 2 (d) is the distribution diagram of Zn element content; Figure 3 (a) is the distribution diagram of Fe element content, Figure 3 (b) is the distribution diagram of Fe element content, Figure 3 (c) is the Zn element content distribution diagram. Figure 2 (a) Figure 3 In (a), the iron-zinc alloy phase α-Fe(Zn) is light gray, and the zinc-rich Γ-Fe3Zn10 phase is light white. Figure 2 The Ni element is enriched in the Γ phase, and the content of the element in each phase can be clearly seen from the element distribution diagram.

[0259] The steel parts obtained in Example 3 and Comparative Example 2 were processed into several thin slice samples with a thickness of about 5 to 30 nm and a length of 5 to 30 μm using a focused ion beam (FIB) processing device (Scios2 HiVac), and the coating structure of the thin slice samples was observed using a scanning electron microscope (SEM). The obtained scanning electron microscope pictures are as follows: Figure 5 ,in Figure 5 (a) is a scanning electron microscope image of the steel part of Example 3, Figure 5 (b) is a scanning electron microscope image of the steel part of Comparative Example 2. Figure 5 It shows that cracks exist in the coating of the steel part in Example 3, but the cracks do not further extend to the steel substrate; while the cracks in the coating of the steel part in Comparative Example 2 further extend to the steel substrate.

[0260] Take the thin slice sample of Example 3 and use a transmission electron microscope (TEM) (JEM-2100F manufactured by JEOL Ltd.) to randomly observe the grain boundaries of the α-Fe(Zn) solid solution adjacent to the interface. The element enrichment of the grain boundary is determined by EDS (Energy Dispersive Spectroscopy) surface scanning. The obtained transmission electron microscope picture is as follows: Figure 6 . Figure 6 This indicates that the α-Fe(Zn) adjacent to the matrix has Zn enrichment at the grain boundaries. By randomly observing the grain boundaries, the number of Zn-enriched grain boundaries is less than or equal to 60%.

[0261] Various embodiments of the present application may be presented in the form of a range; it should be understood that the description in the form of a range is only for convenience and brevity, and should not be understood as a rigid limitation on the scope of the present application; therefore, the range description should be considered to have specifically disclosed all possible sub-ranges and single numerical values ​​within the range. For example, the range description from 1 to 6 should be considered to have specifically disclosed sub-ranges, such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., as well as single numbers within the range, such as 1, 2, 3, 4, 5 and 6, which apply regardless of the range. In addition, whenever a numerical range is indicated herein, it is meant to include any cited number (fractional or integer) within the indicated range.

[0262] In the present application, in the absence of any contrary description, the directional words used, such as "upper" and "lower", are specifically the directions of the drawings in the accompanying drawings. In addition, in the description of the present application specification, the terms "including", "comprising", etc. refer to "including but not limited to". Moreover, the terms "including", "comprising" or any other variants thereof are intended to cover non-exclusive inclusion, so that the process, method, article or equipment including a series of elements not only includes those elements, but also includes other elements not explicitly listed, or also includes elements inherent to such process, method, article or equipment. In the absence of further restrictions, the elements defined by the sentence "including..." do not exclude the existence of other identical elements in the process, method, article or equipment including the elements. In this article, relational terms such as "first" and "second" are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply that there is any such actual relationship or order between these entities or operations. In this article, "and / or" describes the association relationship of associated objects, indicating that there can be three relationships, for example, A and / or B, which can represent: A exists alone, A and B exist at the same time, and B exists alone. For the association relationship of more than three associated objects described by "and / or", it means that any one of these three associated objects can exist alone, or any at least two of them exist at the same time. For example, for A, and / or B, and / or C, it can be represented that any one of A, B, and C exists alone, or any two of them exist at the same time, or three of them exist at the same time. In this article, "at least one" refers to one or more, and "multiple" refers to two or more. "At least one", "the following at least one (individual)" or similar expressions refer to any combination of these items, including any combination of single (individual) or plural (individual). For example, "at least one (individual) of a, b, or c", or "at least one (individual) of a, b, and c", can all represent: a, b, c, ab (i.e. a and b), ac, bc, or abc, wherein a, b, and c can be single or multiple, respectively.

[0263] The above description is only a specific implementation of the present application, so that those skilled in the art can understand or implement the present application. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to the embodiments shown herein, but will conform to the widest range consistent with the principles and novel features applied for herein.

Claims

1. A steel part, characterized in that: The steel parts include: Steel substrate; A zinc-iron alloy layer disposed on the surface of the steel substrate; An iron-zinc-nickel alloy layer is provided on the zinc-iron alloy layer, The main phase of the zinc-iron alloy layer is α-Fe / Zn, and the main phase of the iron-zinc-nickel alloy layer is Γ-Fe3Zn containing nickel. 10 ; Among the α-Fe / Zn grain boundaries, the number of grain boundaries enriched with Zn element accounts for no more than 60%, and the total thickness of the zinc-iron alloy layer and the iron-zinc-nickel alloy layer is 10 to 40 μm; Calculated by mass percentage of the zinc-iron alloy layer, the zinc-iron alloy layer comprises 15% to 65% Zn, 0 to 0.5% Al, and the remainder is Fe and unavoidable impurities; Calculated by mass percentage of the iron-zinc-nickel alloy layer, the iron-zinc-nickel alloy layer comprises 65% to 85% of Zn, 0.2% to 2% of Ni, and the remainder is Fe and unavoidable impurities; In terms of the mass percentage of the steel matrix, the steel matrix includes 0.05% to 0.4% C, 0.8% to 2.5% Mn, 0% to 0.4% Si, 0% to 0.5% Al, 0.1% to 0.3% Cr, 0% to 0.2% Mo, 0% to 0.4% Ni, 0.0001% to 0.005% B, 0% to 0.01% N, and a combination of at least two of any two of Ti, Nb, and V, and the contents of the three elements Ti, Nb, and V satisfy the following relationship: <h2 style=";text-align:left;direction:ltr">0.05%≤N<h2 style=";text-align:left;direction:ltr"> Ti <h2 style=";text-align:left;direction:ltr"> +3N<h2 style=";text-align:left;direction:ltr"> Nb <h2 style=";text-align:left;direction:ltr"> +2N<h2 style=";text-align:left;direction:ltr"> V <h2 style=";text-align:left;direction:ltr"> <0.4%, Among them, N Ti is the mass percentage of Ti in the steel matrix, N Nb is the mass percentage of Nb in the steel matrix, N V is the mass percentage of V in the steel matrix; The steel part meets the following conditions: 0.01<p<1, Wherein, p is the value obtained by dividing the volume of the main phase of the iron-zinc-nickel alloy layer by the volume of the main phase of the zinc-iron alloy layer; The method for preparing the steel part comprises the following steps: Provide coated steel; After heating the coated steel to a temperature above the Ac1 temperature of the steel substrate, lowering the temperature of the coated steel to a hot forming temperature, and performing hot stamping and quenching treatment on the coated steel to obtain the steel part; The step of heating the coated steel to a temperature above the Ac1 temperature of the steel substrate comprises the following steps: preheating the coated steel; A first stage of austenitizing heating is performed on the coated steel; A second stage of heating the coated steel for austenitization; The thickness of the steel substrate is 0.7-1.4 mm, the preheating temperature is 600-860° C., the preheating time is 60s-200s, the heating temperature of the first stage of austenitization is 880-950° C., and the heating temperature of the second stage of austenitization is 850-900° C.; or, The thickness of the steel substrate is 1.4-3 mm, the preheating temperature is 700-880° C., the time is 80s-220s, the heating temperature of the first stage of austenitization is 890-960° C., and the heating temperature of the second stage of austenitization is 840-890° C.; Among the preheating, the first stage austenitizing heating, and the second stage austenitizing heating, at least the preheating is carried out in an atmosphere with an oxygen volume fraction of 2% to 20%; The temperature of the coated steel is lowered to the hot forming temperature, and the duration of the cooling of the coated steel is 5 to 16 seconds.

2. The steel part according to claim 1, characterized in that The total mass of the three elements Cr, Mo and Ni accounts for 0.11% to 0.45% of the mass of the steel matrix; and / or, The percentage of the total mass of the two elements Al and Si to the mass of the steel matrix is ​​between 0.3% and 0.6%.

3. The steel part according to claim 2, characterized in that: The steel matrix further comprises 0.05% to 0.1% C and 1.5% to 2% Mn, based on the mass percentage of the steel matrix, or The steel matrix further comprises 0.15% to 0.25% C and 1.6% to 2.5% Mn, based on the mass percentage of the steel matrix, or, Calculated by mass percentage of the steel matrix, the steel matrix further includes 0.25% to 0.4% C and 0.8% to 1.5% Mn.

4. The steel part according to claim 1, characterized in that The contents of Ti, Nb and V also satisfy the following relationship: N Ti / 2(N Nb +N V )≤1。 5. A method for preparing a steel part according to any one of claims 1 to 4, characterized in that: The method for preparing the steel part comprises the following steps: Provide coated steel; After heating the coated steel to a temperature above the Ac1 temperature of the steel substrate, lowering the temperature of the coated steel to a hot forming temperature, and performing hot stamping and quenching treatment on the coated steel to obtain the steel part; The method for preparing the coated steel comprises the following steps: Providing a steel substrate; Plating nickel on the surface of the steel substrate to form a nickel coating to obtain a nickel-plated substrate; The nickel-plated substrate is annealed and galvanized to obtain the plated steel.

6. The method for preparing a steel part according to claim 5, characterized in that: The annealing temperature is 720-880°C.

7. The method for preparing a steel part according to claim 5, characterized in that: The nickel plating is performed by electroplating; and / or, The nickel plating on the surface of the steel substrate is controlled to have a nickel plating amount of 300 to 1500 mg / side / m 2 .

8. The method for preparing a steel part according to claim 5, characterized in that: The galvanizing method is hot-dip galvanizing, and the zinc melt used for hot-dip galvanizing contains aluminum.

9. The method for preparing a steel part according to claim 8, characterized in that: The hot-dip coating temperature is 400-460° C.; and / or, The duration of the hot dip coating is 3 to 20 seconds; and / or, Calculated by mass percentage of the zinc melt, the aluminum content in the zinc melt is 0.25% to 0.35%.

10. The method for preparing a steel part according to claim 5, characterized in that: The thickness of the steel substrate is 0.7 to 1 mm, the total time of the austenitizing first stage heating and the austenitizing second stage heating is 80 to 200 seconds, or, The thickness of the steel substrate is 1 to 1.4 mm, the total time of the austenitizing first stage heating and the austenitizing second stage heating is 120 to 300 seconds, or, The thickness of the steel substrate is 1.4 to 2 mm, the total time of the austenitizing first stage heating and the austenitizing second stage heating is 100 to 280 seconds, or, The thickness of the steel substrate is 2 to 3 mm, and the total time of the austenitizing first stage heating and the austenitizing second stage heating is 160 to 360 seconds.

11. The method for preparing a steel part according to claim 5, characterized in that: The thermoforming temperature is 500-740°C.

12. Use of a steel part according to any one of claims 1 to 4, characterized in that: The steel parts are applied to hot stamping steel parts for vehicles.

13. Use of the steel part according to claim 12, characterized in that: The automotive hot stamping steel parts are any one of the front door left and right anti-collision bars, front door left and right anti-collision bar beams, rear door left and right anti-collision bars, rear door left and right anti-collision bar beams, front bumper, rear bumper, A-pillar reinforcement plate, B-pillar reinforcement plate, C-pillar reinforcement plate, mid-plate channel, roof reinforcement beam, new energy vehicle battery shell, and vehicle threshold safety structural parts.

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