Lightweight thin-coating zinc-manganese-boron-plated high-strength steel material and heat treatment method

Abstract

The invention discloses a light-weight thin-coating zinc-manganese-boron-plated high-strength steel material and a heat treatment method, the steel material comprises the following components in percentage by weight: 0.22-0.24% of C, 0.2-0.3% of Si, 0.1-0.3% of Cr, 1.2-1.4% of Mn, 0.003-0.007% of B and the like, and the balance of Fe, the surface of the steel material is provided with a thin zinc-plated layer with the thickness of 20-30 m, the manganese content of the zinc-plated layer is 1.2%, and the zinc-plated layer is subjected to flash nickel plating pretreatment. The purpose of light weight of the steel material is achieved, the process is simple, and the cost is low. The method can be widely applied to the fields of automobile manufacturing and the like, and has remarkable social and economic benefits.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of galvanizing manganese-boron high-strength steel, in particular to a lightweight thin-coating galvanizing manganese-boron high-strength steel material and a heat treatment method, which is suitable for the field of automobile manufacturing and other fields that have demands for lightweight, high-strength and high-corrosion-resistance materials. BACKGROUND

[0002] With the increasing demand for lightweight and safety in the automobile and construction industries, high-strength steel is increasingly widely used. Among them, manganese-boron alloyed high-strength steel has become a research hotspot due to its high hardenability, good strength-plasticity ratio and relatively low cost. In order to improve the corrosion resistance, this kind of steel usually needs to be treated by hot-dip galvanizing.

[0003] In the prior art, in order to achieve high strength, methods such as increasing alloy content, complex multi-step heat treatment or conventional galvanizing treatment are usually used. However, the prior art has many defects: there is a contradiction between high alloying and poor galvanizing property, silicon elements are easy to accumulate on the surface of the steel plate to form an oxide film during annealing, which hinders the infiltration and reaction of the zinc liquid and the steel matrix, resulting in problems such as poor adhesion of the galvanized layer and plating leakage; there is a contradiction between thick plating layer and lightweight and formability, and the thick galvanized layer is contrary to the lightweight goal, and is easy to produce micro-cracks or peeling in the subsequent forming process; there is a contradiction between high strength and toughness and plasticity, and simply increasing the carbon content or using a full martensite structure will sacrifice the toughness and ductility of the material, affecting the processing performance; the heat treatment process is complex, the cost is high, the equipment and control precision are extremely high, and the production stability is difficult to guarantee; the zinc-based coating hot stamping steel is easy to produce zinc volatilization and liquid metal embrittlement during high-temperature heating, affecting the service performance of the parts.

[0004] The steel material with aluminum-silicon coating developed by Arcelor Mittal Steel Company and the galvanized hot forming material and indirect hot forming process developed by steel enterprises represented by Aosteel have solved some problems to a certain extent, but the zinc in the coating has a low melting point, which is easy to cause uneven distribution of the zinc layer during synchronous hot stamping at high temperature, and the zinc may penetrate into the steel matrix to produce a brittle phase, affecting the hardness stability of the steel and the safety performance of the automobile. Therefore, it is urgent to develop a galvanizing manganese-boron high-strength steel material and a heat treatment method that can solve the above technical problems and simultaneously realize high strength and toughness of the matrix, easy plating on the surface, ultra-thin and firmly adhered galvanized layer and lightweight. SUMMARY

[0005] The purpose of the present application is to overcome the deficiencies of the prior art, provide a lightweight thin coating galvanized manganese boron high-strength steel material and a heat treatment method, break through the key of the coating formula of galvanized manganese boron steel, solve the control technology of the diffusion of the galvanized layer to the hot-formed steel substrate, effectively improve the hardenability, strength, tempering stability, hot working performance, high-temperature oxidation resistance of the steel, while realizing the lightweight, high corrosion resistance and excellent comprehensive mechanical properties of the material, and meeting the use requirements of the automobile manufacturing industry and the like.

[0006] To achieve the above-mentioned purpose, the present application adopts the following technical solutions: A lightweight thin coating galvanized manganese boron high-strength steel material, by weight percentage, the components include: C 0.22-0.24%, Si 0.2-0.3%, Cr 0.1-0.3%, Mn 1.2-1.4%, B 0.003-0.007%, Al 0.02-0.06%, Nb≤0.002%, Ti 0.01-0.05%, S≤0.003%, P≤0.015%, and the balance is Fe; the surface of the steel material is provided with a thin galvanized layer, and the thickness of the zinc layer is 20-30µm.

[0007] Further, the manganese content in the galvanized layer is 1.2%, and the coating layer is also added with carbon elements, copper elements and other alloy elements.

[0008] The present application also provides a heat treatment method of the lightweight thin coating galvanized manganese boron high-strength steel material, including the following steps: (1) blanking: processing the steel material into a coil or a sheet and shearing according to the required size; (2) cold stamping forming: cold stamping forming the sheared steel material by using a die to obtain a steel workpiece; (3) preheating and temperature rising: heating the steel workpiece at a speed of 8-15℃ / s, and keeping the temperature below the melting point of zinc (420℃) to form an oxide layer for protecting the zinc layer; (4) heating: heating the preheated steel workpiece to above 700℃ at a speed of 15-30℃ / s; (5) austenitizing treatment: heating the steel workpiece to above AC3, keeping the temperature for 40-60s to complete the processes of austenite crystal nucleus formation, growth, residual cementite dissolution and composition homogenization; (6) quenching: controlling the quenching starting temperature to be between 800-900℃, and water quenching the steel workpiece to room temperature by using a spraying method to obtain a full martensite structure; and (7) shot blasting treatment: shot blasting the quenched steel workpiece.

[0009] Further, the temperature of the austenitizing treatment in step (5) is 830℃, and the holding time is 40-60s.

[0010] Further, the end point temperature of the preheating and temperature rising in step (3) is controlled to be between 380-420℃, and the holding time is 10-15s.

[0011] Further, the cooling water temperature for spray quenching in step (6) is 20-30 DEG C, and the spray pressure is 0.3-0.5 MPa.

[0012] Further, after heat treatment, the steel material has a tensile strength Rm of 1300-1700 MPa, a yield strength Rp of 1000-1300 MPa, and an elongation at break A80 / 50 of greater than or equal to 5%.

[0013] Further, after heat treatment, the steel material has a Vickers hardness HV of 416-516 HRA, a Rockwell hardness HRC of 43-50.5 HRA, a Rockwell hardness HRA of 71-76 HRA, a bending degree of greater than or equal to 43 DEG, and a contact resistance of less than or equal to 1.5 mΩ.

[0014] Further, the galvanized layer is pretreated by flash plating nickel technology, and a nanometer nickel layer is plated on the surface of the substrate before hot galvanizing, with a thickness of 500 nm.

[0015] Further, in step (5), the holding time for austenite composition homogenization is not less than 30 s, so as to ensure uniform distribution of the austenite composition.

[0016] The present application has the advantages that: through precise proportioning of chemical components and optimization of heat treatment process, the material after treatment has a full martensite structure, a tensile strength of 1300-1700 MPa, a yield strength of 1000-1300 MPa, an elongation at break A80 / 50 of greater than or equal to 5%, a Vickers hardness HV of 416-516 HRA, a Rockwell hardness HRC of 43-50.5 HRA, a bending degree of greater than or equal to 43 DEG, and a contact resistance of less than or equal to 1.5 mΩ. The material has high strength, good toughness and plasticity, and meets the forming requirements of complex structural parts and the safety requirements in service. The anti-hydrogen embrittlement test result shows that the material has no obvious embrittlement phenomenon after immersion in 0.1 mol / L hydrochloric acid for 300 h, and has excellent safety in service.

[0017] The material has excellent corrosion resistance, and the coating surface remains intact and has less rust after 72 h of neutral salt spray test; the coating is still intact after 20 days of salt spray test, and has excellent corrosion resistance. The corrosion degree of the material in 12 days of neutral salt spray test is much smaller than that of the material without nickel plating, effectively prolonging the service life of the material. Compared with the aluminum-silicon coating in the prior art (a large amount of red rust appears after 48 h), the corrosion resistance of the material of the present application is significantly improved.

[0018] Realize the lightweight goal: the material has super high strength, under the premise of guaranteeing the strength requirement of automobile body parts, the weight of parts can be greatly reduced by thinning the thickness of steel, realizing the lightweight goal of automobile body. The thin galvanized layer design avoids the weight increase caused by thick plating layer, further improves the lightweight effect, helps to reduce the energy consumption and emission of automobile.

[0019] Simple process, low cost: the use of Mn-B synergistic design to improve the hardenability makes the material use water quenching instead of expensive polymer quenching liquid, simplifies the heat treatment process and reduces the production cost; the heat treatment process steps are clear, the control parameters are clear, the production window is wide, the stability is easy to guarantee, and it is suitable for industrial large-scale production; the material composition does not use expensive alloy elements, and the harmful element content is low, so the steel smelting cost is relatively low. Compared with the early "two-step" hot stamping process, the production efficiency is high, the part precision is good, and the production cost is further reduced.

[0020] Solve many contradictions of prior art: solve the contradiction between high alloying and poor galvanizing, low silicon content design avoids the formation of silicon oxide film, ensures the effective combination of galvanized layer and matrix; solve the contradiction between thick plating layer and lightweight and formability, thin galvanized layer not only meets the corrosion resistance requirement, but also ensures good formability; solve the contradiction between high strength and toughness and plasticity, through fine grain austenitizing and low temperature tempering, fine lath martensite structure is obtained, which takes into account high strength and good toughness and plasticity; solve the problem of zinc evaporation and liquid metal embrittlement of zinc-based coating hot stamping steel, through flash nickel pretreatment and galvanized layer composition optimization, the melting point of the coating is improved, and the crack initiation and propagation are inhibited.

[0021] The social and economic benefits are remarkable: the research and development of the steel breaks the raw material import dependence of the world's first steel group, fills the domestic gap; helps the transformation and upgrading of automobile industry, drives the linkage development of new energy automobile lightweight upstream and downstream related industries, promotes the construction of ecological civilization and green circular low carbon development, and meets the requirements of national industrial policy; promotes the breakthrough of high strength steel key technology, effectively expands the use range of new energy vehicles, promotes the creation of higher value of domestic new energy vehicle brand, and improves the international competitiveness of China's new energy vehicle manufacturing.

[0022] In order to more clearly illustrate the structural features and effects of the present application, the present application will be described in detail below with reference to the drawings and specific embodiments. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 The schematic diagram for the galvanized layer of the present application using flash nickel pretreatment.

[0024] Figure 2 The Zn-Ni phase diagram mentioned in the present application.

[0025] Figure 3 This is the Zn-Fe phase diagram mentioned in this invention.

[0026] Figure 4 This is an electronic image of the coating before hot stamping mentioned in this invention.

[0027] Figure 5 , Figure 6 These are comparative electronic images of the plating layers after hot stamping mentioned in this invention (comparison with and without nickel plating process).

[0028] Figure 7 This is a depth distribution curve of the coating elements mentioned in this invention (diffusion law of Ni and Al elements).

[0029] Figure 8 This is a diagram showing the surface state of the coating after hot stamping mentioned in this invention.

[0030] Figure 9a and Figure 9b This is a diagram showing the surface state of the coating after hot stamping mentioned in this invention (the effect of temperature on appearance and composition).

[0031] Figure 10 , Figure 11 This is a comparison image of the surface state of the coating after the neutral salt spray test mentioned in this invention.

[0032] Figure 12 The images show the SEM and EDS spectra of the coating cross-section after the salt spray test mentioned in this invention.

[0033] Figure 13 This is a schematic diagram of the hydrogen embrittlement resistance experiment mentioned in this invention.

[0034] Figure 14 This is a schematic diagram of the mechanical performance test mentioned in this invention.

[0035] Figure 15 This is a schematic diagram of the hardness test mentioned in this invention.

[0036] Figure 16 , Figure 17 These are schematic diagrams of the bending performance and contact resistance tests mentioned in this invention.

[0037] Figure 18 This is the infrared thermal image mentioned in this invention.

[0038] Figures 19-24 The image shown is a cross-sectional microstructure diagram mentioned in this invention.

[0039] Figure 25 This is a comparison image of the surface state before and after shot blasting mentioned in this invention.

[0040] Figure 26 This is the potentiodynamic polarization curve mentioned in this invention. Detailed Implementation

[0041] The present invention will now be further described in conjunction with the accompanying drawings and relevant knowledge, and will be described clearly and completely. Obviously, the described applications are only some embodiments of the present invention, and not all embodiments.

[0042] The present invention discloses a lightweight, thin-coated zinc-manganese-boron high-strength steel material, the composition of which, by weight percentage, comprises: carbon C 0.22-0.24%, silicon Si 0.2-0.3%, chromium Cr 0.1-0.3%, manganese Mn 1.2-1.4%, boron B 0.003-0.007%, aluminum Al 0.02-0.06%, niobium Nb ≤0.002%, titanium Ti 0.01-0.05%, sulfur S ≤0.003%, phosphorus P ≤0.015%, and the balance being iron Fe.

[0043] The steel surface is coated with a thin zinc layer, 20µm-30µm thick, containing 1.2% manganese, and also containing carbon, copper, and other alloying elements. The zinc layer is pretreated using flash nickel plating technology, where a 500nm thick nano-metallic nickel layer is electroplated onto the substrate surface before hot-dip galvanizing.

[0044] The heat treatment method for lightweight thin-coated zinc-manganese-boron high-strength steel material of the present invention includes the following steps: blanking: the steel material with the above composition is processed into coils or sheets, and precisely cut according to the required size of automotive parts, etc., to ensure that the blanking size error is controlled within ±0.05mm.

[0045] Cold stamping: Custom molds are used to cold stamp the sheared steel. The mold accuracy is controlled within ±0.02mm. The stamping pressure is adjusted to 100-200MPa according to the thickness and shape of the steel to form a preliminary steel workpiece, reducing the deformation in subsequent hot working.

[0046] Preheating: Place the steel workpiece in the heating equipment and heat it at a rate of 8-15℃ / s. Monitor the workpiece temperature in real time during the heating process. When the temperature reaches 380-420℃, hold it at that temperature for 10-15 seconds. This temperature range is lower than the melting point of zinc (420℃), which allows a dense oxide layer to form on the surface of the galvanized layer, effectively protecting the galvanized layer from damage during subsequent heating.

[0047] Heating: Adjust the parameters of the heating equipment to rapidly heat the preheated steel workpiece to above 700℃ at a rate of 15-30℃ / s. This temperature range allows the pearlite in the steel to begin to transform into austenite. Rapid heating can reduce austenite grain growth and ensure subsequent microstructure refinement.

[0048] Austenitizing treatment: The steel workpiece is further heated to 830℃ (above AC3) and held for 40-60 seconds to complete the austenitizing process. This process consists of four stages: The first stage is the formation of austenite nuclei, where a large number of austenite nuclei are generated at the interface between the pearlite and ferrite phases; the second stage is austenite growth, where the austenite nuclei extend into the ferrite and cementite regions until the ferrite completely disappears; the third stage is the dissolution of residual cementite, where the cementite that was not completely dissolved during the holding process continues to dissolve into the austenite; the fourth stage is the homogenization of austenite composition, where carbon atoms diffuse fully to ensure a uniform distribution of the composition within the austenite. The holding time is not less than 30 seconds to ensure compositional uniformity.

[0049] Quenching: Controlling the natural cooling time of the steel workpiece after it exits the furnace ensures that the initial quenching temperature is between 800 and 900℃. Water quenching is performed using a spray method, with the cooling water temperature at 20-30℃ and the spray pressure at 0.3-0.5MPa, allowing the workpiece to cool rapidly to room temperature and obtain a fully martensitic structure. Water quenching provides rapid cooling and effectively inhibits the transformation of austenite into low-strength phases such as pearlite and bainite, ensuring high material strength.

[0050] Shot blasting: Shot blasting is performed on quenched steel workpieces to remove surface oxide scale and impurities, improve surface roughness, enhance the adhesion between the galvanized layer and the substrate, and improve the surface properties of the workpiece.

[0051] To ensure the feasibility and performance stability of the technical solution of this invention, the following testing methods were used for technical verification: CCT Curve Testing and Analysis: The thermal expansion method was used. The sample was heated to a certain temperature using a thermal simulation testing machine and held at that temperature for a period of time. Immediately afterward, the sample was cooled at different rates under controlled temperature. The change curve of the sample's expansion amount versus temperature was measured and recorded. The transformation point temperature was determined according to metallurgical industry standards YB / T5127-1993 (tangent method) and YB / T5128-1993 "Determination Method of Continuous Cooling Transformation Curve of Steel (Expansion Method)" and related standards. With temperature as the ordinate and time as the abscissa, the start and end points of phase transformations of the same properties were connected to form curves. The final microstructure and hardness values ​​of the regions enclosed by each curve were marked, resulting in the continuous cooling transformation curve of the steel. Finally, the continuous cooling curve of 22MnB5 steel containing B was determined.

[0052] Microstructural analysis: OM Metallographic Observation: Metallographic structures are observed using an inverted metallographic microscope. This equipment, in conjunction with professional image processing software, can observe and capture metallographic photographs at magnifications of 50, 100, 200, and 400 times. The observation surface of the metallographic sample is etched with a 4% nitric acid alcohol solution for 10–15 seconds (until the sample surface changes color), then quickly cleaned. Next, the sample surface is dehydrated using an appropriate amount of anhydrous ethanol, and finally dried. During the drying process, avoid scratches or water stains that may affect the observation results.

[0053] SEM and EDS observation: Scanning electron microscopy (SEM) observation was performed. The observation surfaces of the prepared samples were etched with a 4% nitric acid-alcohol solution for approximately 30 seconds (deeper etching facilitates SEM observation). When using SEM for tissue observation, an energy dispersive spectroscopy (EDS) instrument can be used simultaneously to perform point, line, and area scanning energy dispersive spectroscopy analysis on the selected tissue regions.

[0054] Tensile fracture observation: Take a suitable length (not exceeding 10 mm) of the broken tensile specimen along the fracture surface, ensuring that the bottom cut of the removed part is flat and smooth. Clean it with alcohol, blow it dry and store it, and then observe it with a scanning electron microscope.

[0055] TEM observation: Tissue observation was performed using a high-resolution transmission electron microscope. After punching and polishing the sample with a φ3mm metal disc cutter, a φ3mm diameter metal disc was obtained. The sample was placed in a magnetically driven double-jet electrolytic thinner. A double-jet electrolyte solution of 10% perchloric acid and 90% anhydrous ethanol was prepared. The temperature was set to -25 to -30℃, the voltage to 40V, and the current to 40mA. Double-jet was started, and once the sample was observed to be translucent, the double-jet process was stopped. The sample was then removed, cleaned with alcohol, and dried with filter paper for storage. To avoid residual electrolyte or unclean surfaces affecting the observation results, an ion thinner was used to clean the sample surface. The high voltage was set to 3–5kV, the rotation speed to 3–5rpm, the jet angle to 4–10°, and the time to 15min.

[0056] X-ray diffraction analysis: XRD experiments were conducted using an X-ray diffractometer with a copper target. During the test, the XRD scanning angle was set to 20–110° with a step size of 0.03°. The XRD patterns of the samples were analyzed using software based on the PDF-2004 PDF card database. Sample preparation: 10mm × 5mm × 3mm samples were prepared by wire cutting, ensuring the observation surface and its corresponding surface were flat. After degreasing, the samples were cleaned by immersion in alcohol, dried, and then the observation surface was treated using the same method as for metallographic observation, but without etching. Based on Bragg's formula 2dsinθ = nλ, X-ray diffraction patterns were obtained. The strong peaks were compared with materials in the PDF card database using software to determine the main phases.

[0057] Mechanical property testing: Microhardness Testing: Before testing, the sample undergoes simple treatment, removing stains, oil, and wire cutting lubricant, and then grinding away obvious scratches to ensure both the test surface and the bottom surface are smooth and flat. The test is performed on a Vickers microhardness tester with a load of 0.98 N and a holding time of 10 seconds, yielding a clear rhomboid indentation. The diagonal distance of the rhomboid is measured using a hairspring, and the microhardness data is recorded. Five data points are selected at different points during the test; the maximum and minimum values ​​are removed, and the average value is calculated as the final microhardness value.

[0058] Tensile testing: Following the national standard GB / T228-2002 "Metallic materials, tensile testing at room temperature," the dimensions of the tensile specimens are designed. Three to four specimens are used per group, with a tensile rate of 1 mm / min. The corresponding tensile strength is obtained, and the average of the effective values ​​(fracture surface within the original gauge length) is taken. Tensile testing can measure the yield strength (ReL or Rp0.2), tensile strength (Rm), elongation (A), and other mechanical properties of the material. For thin plate specimens, the dimensions are also designed according to GB / T228-2002, and the mechanical properties are measured. The average value is calculated after measuring three groups of data per group.

[0059] Hydrogen embrittlement resistance test: The stress loading method of Volkswagen PV1074 was adopted. The material was prepared at 900℃ for 5 minutes without baking. It was then soaked in 0.1mol / L hydrochloric acid for 300 hours, and the pH was adjusted every 24 hours to ensure that the pH was stable at 1-2.

[0060] Salt spray test: The neutral salt spray test method was used to conduct salt spray tests on samples treated at different heating temperatures (850℃, 900℃, 930℃, 950℃) and samples with and without nickel plating. The surface condition of the coating was recorded after different times (24h, 48h, 72h, 5 days, 12 days, 20 days) to evaluate corrosion resistance.

[0061] In this invention, carbon is the solid solution strengthening element with the best strengthening effect, playing a decisive role in achieving ultra-high strength. By precisely controlling the carbon content to 0.22-0.24% and employing a converter steelmaking + RH refining process during steelmaking, uniform carbon distribution is ensured. This content range guarantees the formation of a high-density dislocation martensite matrix after heat treatment, achieving a tensile strength of 1300-1700 MPa, while avoiding problems such as increased rolling deformation resistance, reduced plasticity, difficulty in cold working, and deterioration of welding performance caused by excessive carbon content. Compared with existing methods (C>0.2%, Si>0.3%), this invention provides more precise carbon content control, ensuring both strength and processing performance.

[0062] Silicon (Si): Silicon improves the hardenability of steel, reduces volume change during the transformation of austenite to martensite, and effectively controls the formation of quenching cracks. During low-temperature tempering, it hinders carbon diffusion, delays martensite decomposition and carbide aggregation and growth, significantly improving the tempering stability and strength of steel. By controlling the silicon content to 0.2–0.3% and using alloying addition, the uniform distribution of silicon in the steel matrix is ​​ensured. This content range effectively strengthens the steel while avoiding the formation of an oxide film on the surface during annealing due to excessive silicon content, which hinders the bonding between the zinc plating layer and the substrate. This resolves the contradiction between high alloying and poor zinc plating properties in existing technologies.

[0063] Manganese (Mn): Manganese plays a solid solution strengthening role, removing FeO from steel and improving its quality; it reacts with sulfides to form high-melting-point MnS, mitigating the harmful effects of sulfur and improving the hot working properties of steel; it can be infinitely dissolved in austenite, reducing the phase transformation driving force, shifting the C-curve to the right, improving the hardenability of steel, expanding the γ-phase region, lowering the Ms point of steel, and ensuring the formation of martensite at a suitable cooling rate. By controlling the manganese content to 1.2-1.4%, it is added synergistically with other alloying elements during the steelmaking process to ensure uniform distribution. This content range allows it to fully exert its solid solution strengthening and hardenability-enhancing effects, and its synergistic effect with boron ensures high strength and good hot working properties of the material.

[0064] Boron (B): Boron improves the hardenability of low-alloy steel, enhances heat resistance in austenitic steel, prevents the formation of pearlite and ferrite structures, and further improves hardenability and strength. By controlling the boron content to 0.003–0.007% and employing a micro-alloying process, uniform boron distribution is ensured. The synergistic effect of boron and manganese significantly improves the hardenability of steel, enabling the material to achieve a fully martensitic structure under water quenching conditions, eliminating the need for expensive polymer quenching fluids and reducing production costs. Simultaneously, titanium can fix nitrogen in the steel, preventing nitrogen from combining with boron and affecting hardenability, thus achieving a synergistic effect between elements.

[0065] Chromium (Cr): Chromium can reduce the driving force of phase transformation, decrease the nucleation and growth of carbides during phase transformation, and improve the hardenability of steel; at the same time, it can improve the tempering stability and high-temperature oxidation resistance of steel. By controlling the chromium content to 0.1%–0.3% and using alloying methods to ensure uniform distribution, this content range can exert its strengthening and oxidation-resistant effects while avoiding the deterioration of steel's machinability due to excessive chromium content.

[0066] Aluminum (Al): Aluminum acts as a deoxidizer in steel, eliminating the adverse effects of nitrogen and oxygen atoms on its properties. By controlling the aluminum content to 0.02–0.06%, it is added as a deoxidizer during the steelmaking process to ensure a certain amount of acid-soluble aluminum in the steel. This content range allows for full utilization of the deoxidizing effect while avoiding the formation of aluminum inclusions in the steel due to excessive aluminum content, which would affect the smelting and casting quality of the steel.

[0067] Titanium (Ti): Titanium is a strong carbon and nitrogen compound forming element, which can fix nitrogen in steel and avoid the adverse effects of nitrogen on steel properties; it also contributes to the hardenability of steel. By controlling the titanium content to 0.01~0.05% and using alloying methods, uniform distribution is ensured. This content range can both exert its role in fixing nitrogen and avoid the reduction of hardness and strength of martensite after quenching due to excessive titanium content combining with carbon.

[0068] Niobium (Nb): Niobium can refine grains and improve the strength and toughness of steel. By controlling the niobium content to ≤0.002% and adding it in trace amounts, the increased cost and deterioration of processing performance caused by excessive niobium content can be avoided.

[0069] Sulfur (S) and phosphorus (P): Sulfur and phosphorus are harmful elements in steel. Sulfur easily causes hot brittleness in steel, while phosphorus easily causes segregation in the center of the billet, increasing the brittleness of the steel. By controlling the sulfur content to ≤0.003% and the phosphorus content to ≤0.015%, and by using refining processes during steelmaking to remove harmful elements, the purity of the steel is ensured, and the toughness and processing performance of the steel are improved.

[0070] In this invention, a thin zinc coating (20µm-30µm) provides excellent corrosion resistance to the steel. It is prepared using a hot-dip galvanizing process, controlling the galvanizing temperature at 450-460℃ and the galvanizing time at 3-5 seconds to ensure uniform zinc layer thickness. This thin zinc coating design satisfies both corrosion resistance requirements and lightweight objectives, avoiding problems such as poor formability and easy peeling caused by thick coatings. It resolves the contradiction between thick coatings and lightweighting / formability in existing technologies.

[0071] Manganese is added to the zinc plating bath, with the manganese content controlled at 1.2%. A stirring device ensures uniform distribution of manganese in the zinc plating bath. Manganese improves the microstructure of the zinc plating layer, refining its structure, increasing its hardness and wear resistance, and enhancing the adhesion between the zinc plating layer and the steel substrate, thus improving corrosion resistance. Furthermore, manganese has a strong affinity for oxygen, and during heating, it accumulates on the surface to form MnO, which, together with Al2O3, forms a composite oxide layer, further improving the high-temperature oxidation resistance of the coating.

[0072] Adding alloying elements such as carbon and copper to the galvanized layer: Appropriate amounts of carbon, copper, and other alloying elements are added to the galvanizing bath. Through composition adjustment and stirring processes, the uniform distribution of each element is ensured. These alloying elements can increase the melting point and high-temperature stability of the galvanized layer, reduce zinc volatilization and liquid metal embrittlement during hot forming, and simultaneously improve the corrosion resistance and wear resistance of the galvanized layer.

[0073] Flash nickel plating pretreatment (500nm nano-sized nickel layer): A 500nm thick nano-sized nickel layer is prepared on the steel substrate surface using an electroplating process. The electroplating solution is a nickel sulfate system, and the electroplating current density is controlled at 2–5 A / dm², with an electroplating time of 10–15 min. The nickel layer can increase the melting point of the coating on the substrate surface, inhibiting the initiation and propagation of cracks during hot forming; it slows down the diffusion rate of iron to the zinc coating, resulting in more zinc-rich phases under the same heating conditions, thus improving the corrosion resistance of the zinc coating; at the same time, it enhances the adhesion between the zinc coating and the steel substrate, preventing the zinc coating from peeling off. After austenitizing heating, Ni diffuses to the coating surface and accumulates at a distance of about 3μm from the surface. As the heating temperature increases, the enrichment of Ni increases, further strengthening the barrier effect of the nickel layer.

[0074] In this invention, preheating (8-15°C / s, holding at 380-420°C for 10-15s): By controlling the heating rate and holding parameters, a dense oxide layer is formed on the surface of the galvanized layer. This oxide layer protects the galvanized layer during subsequent high-temperature heating, preventing oxidation and volatilization of the zinc layer, and ensuring the integrity and corrosion resistance of the galvanized layer. Controlling the heating rate to 8-15°C / s ensures sufficient oxide layer formation while avoiding premature oxidation and failure of the galvanized layer due to excessively slow heating.

[0075] Rapid heating (15-30℃ / s, above 700℃): By adopting a rapid heating method, the residence time of steel in the medium temperature zone is reduced, avoiding austenite grain growth and ensuring the formation of fine austenite grains during the subsequent austenitization process. This lays the foundation for obtaining fine lath martensite structure and improves the strength and toughness of the material.

[0076] Austenitizing treatment (830℃, 40–60s): Precise control of the austenitizing temperature and holding time ensures complete transformation of pearlite into austenite with uniform austenite composition. The austenitizing temperature of 830℃ refines the austenite grains, while the holding time of 40–60s ensures complete dissolution of residual cementite and homogenization of austenite composition, guaranteeing a uniform full-martensite microstructure after quenching.

[0077] Spray quenching (initial temperature 800-900℃, cooling water 20-30℃, spray pressure 0.3-0.5MPa): Controlling the initial quenching temperature at 800-900℃ avoids excessive natural cooling of the workpiece after removal from the furnace, preventing the temperature from falling below AC3 and ensuring that the quenched structure remains a single-phase austenitic structure. The cooling water temperature of 20-30℃ and the spray pressure of 0.3-0.5MPa ensure a sufficiently fast cooling rate, inhibiting the transformation of austenite to low-strength phases such as pearlite and bainite, promoting martensitic transformation, and obtaining a fully martensitic structure. Compared to polymer quenching fluids, water quenching is lower in cost, more environmentally friendly, and easier to maintain, while providing stable cooling and ensuring consistent material properties.

[0078] Shot blasting: Shot blasting removes oxide scale and impurities from the surface of the workpiece, improves surface roughness, enhances the adhesion between the zinc plating layer and the substrate, and generates residual compressive stress on the workpiece surface, thereby improving the fatigue strength of the material.

[0079] In this invention, carbon, manganese, and boron work synergistically. Carbon provides solid solution strengthening, while manganese and boron significantly improve the hardenability of steel. The combination of these three elements allows the material to achieve a fully martensitic structure under water quenching conditions, resulting in an ultra-high tensile strength of 1300-1700 MPa. Silicon and chromium work synergistically. Silicon improves the tempering stability of steel, while chromium improves the hardenability and high-temperature oxidation resistance. The combination of these two elements further enhances the strength and corrosion resistance of the material. Aluminum and titanium work synergistically. Aluminum acts as a deoxidizer, while titanium fixes nitrogen, jointly improving the purity and toughness of the steel. Strict control of harmful elements such as sulfur and phosphorus provides a favorable matrix environment for other beneficial elements to play their role.

[0080] Furthermore, the low silicon content (0.2–0.3%) in the steel substrate prevents silicon from accumulating on the surface and forming an oxide film, creating conditions for effective bonding between the galvanized layer and the substrate. The manganese elements in the steel substrate and the galvanized layer work synergistically to improve the compatibility and bonding strength between the galvanized layer and the substrate. The flash nickel pretreatment layer forms a good interfacial bond with the steel substrate and the galvanized layer. The nickel layer can prevent iron from diffusing into the galvanized layer, while the zinc layer can provide cathodic protection for the steel substrate. The synergistic effect of these three elements significantly improves the corrosion resistance of the material.

[0081] This invention achieves its goals through the synergy between chemical composition and heat treatment processes. The synergistic effect of manganese and boron elements in the steel matrix improves the hardenability of the steel, enabling the material to withstand water quenching without the need for a complex cooling system. Precise control of carbon content, combined with austenitizing temperature and quenching process, ensures the formation of a high-density dislocation martensite structure, achieving high strength. The combination of silicon and chromium elements with tempering processes (implied in subsequent natural or artificial tempering) improves the tempering stability of the material, ensuring stable performance during service. The grain-refining effect of aluminum and titanium elements, combined with rapid heating and appropriate austenitizing temperature, further refines the martensite structure, improving the strength and toughness of the material.

[0082] Synergy between the galvanized layer and the heat treatment process: The preheating process forms a protective oxide layer for the galvanized layer, preventing the volatilization and oxidation of zinc during subsequent high-temperature heating; the control of the austenitizing temperature (830℃) and holding time, in synergy with the flash nickel pretreatment layer, inhibits the occurrence of liquid metal embrittlement; the rapid cooling method of spray quenching reduces the diffusion reaction between the zinc layer and the substrate, avoids the generation of brittle phases, and ensures the integrity and adhesion of the galvanized layer; shot blasting works synergistically with the galvanized layer to improve the adhesion of the galvanized layer and prevent the galvanized layer from peeling off during subsequent forming processes.

[0083] Furthermore, blanking and cold stamping provide a well-shaped workpiece for subsequent heat treatment processes, reducing deformation during heat treatment; preheating provides protection for high-temperature heating and austenitization treatment, preventing damage to the galvanized layer; rapid heating combined with austenitization treatment ensures the formation of fine and uniform austenitic grains; austenitization treatment provides a uniform microstructure for the quenching process, ensuring a fully martensitic microstructure after quenching; shot blasting provides a good surface quality for the final product. Each step is interconnected and works synergistically to achieve the excellent properties of the material. Example 1, Steel Composition Preparation: The following raw materials were prepared by weight percentage: 0.23% carbon (C), 0.25% silicon (Si), 0.2% chromium (Cr), 1.3% manganese (Mn), 0.005% boron (B), 0.04% aluminum (Al), 0.001% niobium (Nb), 0.03% titanium (Ti), 0.002% sulfur (S), 0.01% phosphorus (P), with the balance being iron (Fe). The steel was smelted using a converter steelmaking + RH refining process, with the smelting temperature controlled at 1600–1650℃ and the refining time at 30–40 minutes to ensure uniform distribution of the elements, resulting in a steel billet.

[0084] Preparation of the galvanized layer: After rolling the steel billet into a steel plate, a 500 nm thick nano-metallic nickel layer was prepared on the surface of the steel plate using flash nickel plating technology. The electroplating solution was a nickel sulfate system, the current density was 3 A / dm², and the electroplating time was 12 min. Subsequently, hot-dip galvanizing was performed at a galvanizing temperature of 455℃ for 4 s, controlling the thickness of the galvanized layer to be 25 µm. The galvanized layer contained 1.2% manganese, and appropriate amounts of carbon, copper, and other alloying elements were added.

[0085] Heat treatment process: (1) Blanking: The galvanized steel plate is processed into sheet material and cut according to the required size. The size error is controlled within ±0.05mm. (2) Cold stamping: Custom molds are used for cold stamping. The stamping pressure is 150MPa and the mold accuracy is ±0.02mm to obtain steel workpieces. (3) Preheating: The steel workpieces are placed in the heating furnace and heated to 400℃ at a rate of 10℃ / s. The temperature is held for 12s to form an oxide layer that protects the zinc layer. (4) Heating: The heating furnace parameters are adjusted and the workpieces are heated to 750℃ at a rate of 20℃ / s. (5) Austenitization treatment: The temperature is continued to be heated to 830℃ and held for 50s to complete the austenite nucleus formation, growth, residual cementite dissolution and composition homogenization process. (6) Quenching: After the workpiece is taken out of the furnace, the natural cooling time is controlled to ensure that the quenching start temperature is 850℃. Water quenching is carried out by spraying, the cooling water temperature is 25℃, the spraying pressure is 0.4MPa, and the workpiece is cooled to room temperature. (7) Shot blasting: The quenched workpiece is shot blasted to remove the surface oxide scale and impurities.

[0086] Performance Testing: Performance tests were conducted on the treated workpiece, and the results are as follows: tensile strength Rm is 1550 MPa, yield strength Rp is 1150 MPa, elongation at break A80 / 50 is 6.2%, Vickers hardness HV is 468 HRA, Rockwell hardness HRC is 47.3 HRA, Rockwell hardness HRA is 73.5 HRA, flexural angle is 65°, and contact resistance is 0.85 mΩ. After a 72-hour neutral salt spray test, the coating surface showed minimal rust; after a 12-day neutral salt spray test, the corrosion was slight, demonstrating good corrosion resistance. Hydrogen Embrittlement Resistance: Using the Volkswagen PV1074 stress loading method, after immersion in 0.1 mol / L hydrochloric acid for 300 hours, no significant embrittlement was observed, meeting service requirements. Microstructure: Observation using OM, SEM, and TEM revealed a fine and uniform martensitic structure, a tight bond between the zinc coating and the substrate, and Ni enrichment at a distance of 3 μm from the coating surface. Example 2, Steel Composition Preparation: The following raw materials were prepared by weight percentage: 0.22% carbon (C), 0.2% silicon (Si), 0.1% chromium (Cr), 1.2% manganese (Mn), 0.003% boron (B), 0.02% aluminum (Al), 0.0005% niobium (Nb), 0.01% titanium (Ti), 0.001% sulfur (S), 0.008% phosphorus (P), with the balance being iron (Fe). The steel was smelted using a converter steelmaking + RH refining process at a smelting temperature of 1600℃ for 30 minutes to obtain a steel billet.

[0087] Preparation of galvanized layer: After the steel billet is rolled into steel plate, it is flash-plated with nickel pretreatment (current density 2A / dm², electroplating time 10min, nickel layer thickness 500nm), followed by hot-dip galvanizing (galvanizing temperature 450℃, galvanizing time 3s, zinc layer thickness 20µm). The galvanized layer contains 1.2% manganese and appropriate amounts of alloying elements such as carbon and copper are added.

[0088] Heat treatment process: (1) Blanking: Shear the sheet metal to the required size with an error of ±0.05mm. (2) Cold stamping: Stamping pressure of 100MPa, mold accuracy of ±0.02mm. (3) Preheating: Heating rate of 8℃ / s, heating to 380℃, holding for 10s. (4) Heating: Heating rate of 15℃ / s, heating to 700℃. (5) Austenitizing treatment: Heating to 830℃, holding for 40s. (6) Quenching: Quenching starting temperature of 800℃, cooling water temperature of 20℃, spraying pressure of 0.3MPa. (7) Shot blasting: Remove surface oxide scale and impurities.

[0089] Performance Testing: Tensile strength Rm is 1350 MPa, yield strength Rp is 1000 MPa, elongation at break (A80 / 50) is 5.5%, Vickers hardness HV is 425 HRA, Rockwell hardness HRC is 43.8 HRA, Rockwell hardness HRA is 71.2 HRA, flexural angle is 62°, and contact resistance is 0.78 mΩ. After a 72-hour salt spray test, the coating remained intact with minimal rust, demonstrating good corrosion resistance. Hydrogen embrittlement resistance: Tested according to Volkswagen PV1074 standard, no embrittlement was observed after 300 hours of hydrochloric acid immersion. Microstructure: XRD analysis showed the presence of an Al2O3 and MnO composite oxide layer, and TEM observation revealed fine martensitic laths with no gaps at the interface between the coating and the substrate.

[0090] Example 3, Steel Composition Preparation: The following raw materials were prepared by weight percentage: 0.24% carbon (C), 0.3% silicon (Si), 0.3% chromium (Cr), 1.4% manganese (Mn), 0.007% boron (B), 0.06% aluminum (Al), 0.002% niobium (Nb), 0.05% titanium (Ti), 0.003% sulfur (S), 0.015% phosphorus (P), with the balance being iron (Fe). Steel was produced by converter steelmaking followed by RH refining at a smelting temperature of 1650℃ for 40 minutes to obtain a steel billet.

[0091] Preparation of galvanized layer: After the steel billet is rolled into steel plate, it is flash-plated with nickel (current density 5A / dm², electroplating time 15min, nickel layer thickness 500nm), and then hot-dip galvanized (galvanizing temperature 460℃, galvanizing time 5s, zinc layer thickness 30µm). The galvanized layer contains 1.2% manganese and appropriate amounts of alloying elements such as carbon and copper are added.

[0092] Heat treatment process: (1) Blanking: Shear the sheet metal to the required size with an error of ±0.05mm. (2) Cold stamping: Stamping pressure 200MPa, mold accuracy ±0.02mm. (3) Preheating: Heating rate 15℃ / s, heating to 420℃, holding for 15s. (4) Heating: Heating rate 30℃ / s, heating to 800℃. (5) Austenitizing treatment: Heating to 830℃, holding for 60s. (6) Quenching: Quenching starting temperature 900℃, cooling water temperature 30℃, spraying pressure 0.5MPa. (7) Shot blasting: Remove surface oxide scale and impurities.

[0093] Performance Testing: Tensile strength Rm is 1680 MPa, yield strength Rp is 1280 MPa, elongation at break (A80 / 50) is 5.1%, Vickers hardness HV is 510 HRA, Rockwell hardness HRC is 50.2 HRA, Rockwell hardness HRA is 75.8 HRA, flexural angle is 63°, and contact resistance is 0.91 mΩ. After a 72-hour salt spray test, the coating showed minimal rust, and after a 20-day salt spray test, the coating remained intact, demonstrating excellent corrosion resistance. Hydrogen embrittlement resistance: Tested according to Volkswagen PV1074 standard, no embrittlement was observed after 300 hours of hydrochloric acid immersion, indicating good service safety. Microstructure: SEM observation showed no cracks in the coating, and EDS analysis showed that Ni was enriched at 3 μm on the coating surface, and the Fe diffusion rate was significantly reduced.

[0094] In Examples 1-3, the detection data refer to Figures 1-25 For specific details, please refer to Figures 1-3As shown, the zinc plating layer is pretreated using flash nickel plating technology. The zinc plating layer contains nickel and is designed with a multi-element alloy composition. The austenitizing temperature is controlled at 830℃ during hot forming. The liquid phase region, β phase region, and (Ni) solid solution phase region of the Zn-Ni alloy are defined, revealing the effect of nickel on increasing the melting point of the zinc plating layer. This invention prepares a 500nm nano-nickel layer through flash nickel plating. Utilizing the formation rules of the β phase (high melting point stable phase) in the phase diagram, the coating maintains structural stability at the 830℃ austenitizing temperature, avoiding the volatilization problem of pure zinc coatings at high temperatures. This solves the technical pain point of zinc volatilization during hot stamping of Zn-based coatings in the background technology. It shows that the liquid metal embrittlement critical temperature of the Zn-Ni alloy is higher than that of pure zinc. Combined with the austenitizing temperature of 830℃ in this invention (lower than the embrittlement critical temperature of 850℃ in the Zn-Fe phase diagram), the thickness of the 500nm nickel layer is determined by the correlation between Ni content and embrittlement sensitivity in the phase diagram, effectively preventing liquid zinc from penetrating into the austenite grain boundaries. Experimental results demonstrate that this design eliminates grain boundary embrittlement cracks in the coating after hot stamping, improving the plasticity and fatigue life of parts by over 30% compared to materials without nickel plating. Furthermore, it reveals the phase interface compatibility between the Zn-Ni alloy and the steel substrate, guiding the addition ratio of elements such as manganese and copper in the zinc plating layer. By utilizing the dissolution patterns of multi-element alloys in the phase diagram, a stable metallurgical bond is formed between the coating and the substrate. Tensile testing verifies that the coating adhesion is 40% higher than existing technologies, and no coating peeling occurs after cold bending, meeting the complex forming requirements of automotive parts. exist Figure 3 The original text clearly indicates the liquid metal embrittlement region (LMIEarea) and critical temperature of 850℃ for the Zn-Fe system. This invention sets the austenitizing temperature at 830℃, below this critical temperature, thus fundamentally preventing the formation of liquid zinc and its penetration into the austenite grain boundaries. Combined with the formation rules of α-Fe(Zn) solid solution in the phase diagram, this invention ensures that there is no excessive formation of brittle Fe-Zn intermetallic compounds at the interface between the coating and the substrate after hot stamping. The cold bending resistance of the parts reaches ≥60°, far exceeding the existing standard of 43°. The invention also shows the diffusion rate of Fe and Zn with temperature and alloy element content. By adding 1.2% manganese, this invention utilizes the inhibitory effect of manganese on Fe-Zn diffusion, ensuring that the zinc content in the coating remains above 40% after hot forming at 900℃. EDS line scanning verification showed that the diffusion depth of Fe into the coating was reduced by 50% compared to manganese-free materials, effectively avoiding the decrease in corrosion resistance caused by zinc deficiency in the coating. This resulted in no red rust formation after 20 days of salt spray testing and revealed the reaction limit of the Fe-Zn interface under thin coating conditions, guiding this invention to control the zinc layer thickness at 20-30 μm. This design ensures corrosion resistance through the cathodic protection effect of Zn in the phase diagram while avoiding the brittle cracking problem of thick coatings during the forming process. This reduces the weight of parts by 15-20% compared to existing thick-coated products, achieving the goal of automotive lightweighting.

[0095] ReferenceFigure 4 As shown, the process involves flash nickel plating pretreatment (500nm nano-sized nickel layer), a multi-layer coating structure (steel substrate-nickel layer-zinc layer), and control of uniform element distribution. SEM images show a uniform nickel layer thickness (500nm) with no gaps or pores at the interface with the steel substrate and zinc layer. TEM images confirm that the nickel layer has a dense nanocrystalline structure. This structure ensures that the nickel layer effectively blocks direct contact between Fe and Zn, solving the problem of poor adhesion between the coating and the substrate in the prior art. Peel tests verify that the coating adhesion reaches over 10N / mm, meeting the long-term service requirements of automotive parts. EDS elemental characteristic peaks (AlKa1, NiKa1, FeKa1, ZnKa1) show that there are no obvious impurities in the nickel layer, and the elements are uniformly distributed. The results demonstrate that the flash nickel plating process parameters (current density 2-5 A / dm², plating time 10-15 min) are reasonably designed, ensuring stable coating performance and preventing corrosion perforation caused by localized compositional inhomogeneity. This guarantees that the rust coverage rate on the coating surface is less than 5% after a 72-hour salt spray test. The dense nickel layer structure forms an effective barrier during hot forming, preventing zinc layer volatilization and oxidation. Combined with subsequent hot stamping images, the nickel layer ensures that the coating exhibits no cracking or peeling at 830℃, guaranteeing the integrity of the coating after heat treatment and laying a structural foundation for the synergistic improvement of the material's corrosion resistance and mechanical properties.

[0096] Reference Figure 5 , Figure 6 As shown, this invention employs a flash nickel plating pretreatment process, controlled austenitization temperature (830℃), and a design for the interface bonding between the plating layer and the substrate. Comparative images show that the unplated group (without-Ni) exhibits significant cracks and interface voids after heating at 900℃ and 950℃, while the nickel-plated group (with-Ni) has a complete, crack-free plating layer with a uniform α-Fe(Zn) phase distribution. This result demonstrates that the nickel layer effectively inhibits the initiation and propagation of cracks during hot stamping, solving the liquid metal embrittlement problem of Zn-based coatings during hot stamping in the prior art, thus increasing the fatigue life of parts by more than two times. The area of ​​the zinc-rich phase (bright area) in the nickel-plated group is significantly larger than that in the unplated group, and EDS analysis shows that the Fe content in the nickel-plated group is 60% lower than that in the unplated group. This effect stems from the nickel layer's blocking of Fe-Zn diffusion, allowing the plating layer to maintain a high zinc content at high temperatures, ensuring continuous and effective cathodic protection. In a 12-day salt spray test, the corrosion degree was only 1 / 5 of that of the unplated material. Even under ultra-high temperature conditions of 950℃, the nickel-plated group maintained the integrity of the coating, while the coating of the non-nickel-plated group was severely oxidized and peeled off. This result proves that the nickel plating process of the present invention expands the hot forming temperature window of the material to 800-950℃, which is wider than the prior art (800-850℃), significantly improves production stability, and reduces the difficulty of control in industrial production.

[0097] Reference Figure 7 ,Figure 8 As shown, this invention employs a flash nickel plating pretreatment process, adding aluminum alloy elements (0.02-0.06%) and controlling the austenitizing temperature gradient (800-950℃). The curves show that Ni diffuses to the coating surface with increasing temperature, forming an enrichment peak at 3μm from the surface, with the highest enrichment degree at 950℃. This enrichment phenomenon enables the nickel layer to form a denser barrier at high temperatures, effectively blocking the diffusion of Fe and Zn. Experiments verify that after heating to 900℃, the thickness of the brittle phase at the interface between the coating and the substrate is reduced by 70% compared to nickel-free materials, and the tensile strength of the parts remains above 1500MPa. Al elements enrich the surface and form an Al2O3 oxide layer, and the thickness increases with increasing temperature (approximately 1μm at 800℃ and approximately 2μm at 950℃). The oxide layer forms a composite anti-oxidation layer with MnO generated from Mn, resulting in no significant oxidation peeling even at 950℃. This solves the problem of poor high-temperature oxidation resistance in the prior art, improving the high-temperature oxidation resistance by three times compared to existing aluminum-silicon coatings. Furthermore, it shows that the enrichment and diffusion depth of Ni and Al elements reach an optimal balance at 830℃, ensuring both barrier function and oxidation resistance while avoiding energy waste caused by excessively high temperatures. This result provides data support for setting the austenitizing temperature to 830℃, reducing production energy consumption by 15% compared to the 900℃ process, while ensuring stable material properties.

[0098] Reference Figure 9a and Figure 9b As shown, this invention utilizes controlled austenitizing temperature (830℃), the addition of multi-element alloys (Mn, Al) to the coating, and a thin zinc plating layer design. Visual inspection photos show that the coating is light gray (normal appearance) at 900℃, turning brown and developing zinc oxide blistering after heating above 900℃. XRD patterns confirm a significant increase in the intensity of zinc oxide characteristic peaks above 900℃, indicating intensified oxidation of the coating. The chosen austenitizing temperature of 830℃ avoids excessive oxidation while ensuring complete transformation of pearlite to austenite, resulting in a coating appearance qualification rate of over 99% and a tensile strength of 1495 MPa. The presence of Al2O3 and MnO characteristic peaks in the XRD pattern proves the formation of a composite oxide layer on the coating surface. This oxide layer is dense and has strong adhesion, effectively blocking the penetration of corrosive media. After a 72-hour salt spray test, the amount of white rust on the coating is reduced by 80% compared to existing technologies, solving the problem of decreased corrosion resistance after high temperatures in the prior art. Even after heating to 830°C, the thin coating (20-30μm) remained intact without significant oxidation or weight loss. This result demonstrates that the thin coating design of this invention, under reasonable temperature control, can balance lightweight and corrosion resistance, reducing weight by 20% compared to thicker coating products (40-50μm) while maintaining comparable corrosion resistance, thus meeting the automotive industry's dual requirements for lightweighting and long service life.

[0099] Reference Figure 10 ,Figure 11 As shown, this invention utilizes a flash nickel plating pretreatment process, controlled austenitization temperature (830℃), and a multi-element alloy design for the coating. Comparison of different temperature groups shows that the 900℃ group exhibited the least white rust after salt spray testing, while the 950℃ group showed the most. This result demonstrates that an austenitization temperature of 830℃ (close to 900℃) can minimize coating oxidation damage while ensuring complete austenitization, resulting in a 40% reduction in rust after 72 hours of salt spray compared to the 850℃ process and a 60% reduction compared to the 950℃ process. A comparison between the nickel-plated and non-nickel-plated groups shows that after 12 days of salt spray testing, the non-nickel-plated group exhibited large-area corrosion pits, while the nickel-plated group only showed slight white rust in localized areas. This result proves that the barrier effect of the nickel layer significantly improves corrosion resistance, reducing the corrosion rate by 80% compared to the non-nickel-plated material, solving the problem of insufficient corrosion resistance of Zn-based coatings in the prior art, and extending the service life of parts to over 10 years. After 20 days of salt spray testing, the coating cross-section remained intact, with corrosion products concentrated only on the surface layer, and no perforation was observed in the coating after removing the corrosion products. EDS analysis showed that the zinc-rich phase corroded preferentially during the corrosion process, while Ni remained enriched, proving that the cathodic protection of the coating was continuously effective. This effect enables the material to adapt to harsh environments such as marine climates, and its corrosion resistance is an order of magnitude higher than that of existing aluminum-silicon coatings (48h red rust).

[0100] Reference Figure 12 As shown, this invention utilizes a thin zinc plating layer design, a flash nickel plating pretreatment process, and a synergistic design of a multi-element alloy (Zn, Ni, Mn) in the coating. SEM images show that the zinc-rich phase corrodes first during salt spraying, and EDS spectra confirm that Zn preferentially dissolves (Zn content from left to right 40.69% → Zn content in the middle 20.61%), while Ni remains stably enriched (Ni content 8.10% → Ni content 49.40%). This mechanism demonstrates that the coating plays a sacrificial anode cathodic protection role, preventing corrosion of the steel substrate and solving the problem of rapid substrate failure after coating corrosion in the prior art. After 20 days of salt spraying, the coating cross-section remains intact, with no delamination at the substrate interface, and the corrosion depth is only 5 μm, far less than the coating thickness (20-30 μm). This result proves that the thin coating design, under the synergistic effect of the multi-element alloy, can achieve long-term corrosion resistance, reducing the corrosion depth by 60% compared to existing thick coatings (40-50 μm) at the same corrosion time, while also reducing weight by 20%. EDS spectra show that Mn is uniformly distributed in the corrosion products, and its synergistic effect with Zn and Ni ensures a uniform corrosion process without localized pitting or perforation. This effect prevents sudden component failure caused by pitting corrosion, keeping the corrosion rate fluctuation range within ±5%, which is more stable than the ±15% of manganese-free materials, thus improving product service safety.

[0101] Reference Figure 13As shown, the stress loading method of Volkswagen PV1074 was adopted. The material was prepared at 900℃ for 5 min without baking, with 0.1 mol / L hydrochloric acid for 300 h. The pH was adjusted once every 24 h and the pH was stabilized at 1-2.

[0102] Reference Figure 14 As shown, this invention utilizes precise chemical composition (C 0.22-0.24%, Mn 1.2-1.4%, etc.), a fully martensitic microstructure formation process, and controlled fine-grain austenitization. Test data shows that the sample yield strength Rp0.2 = 1142 MPa and tensile strength Rm = 1495 MPa, far exceeding the existing steel tensile strength standard of 1000 MPa in the background technology, reaching the design target of 1300-1700 MPa. This result proves that the synergistic solid solution strengthening of elements such as C, Mn, and B and the martensitic phase transformation strengthening are effective, resolving the contradiction between high strength and processing performance in the prior art. The elongation after fracture A50 = 5.3%, higher than the design requirement of ≥5%, and the stress-strain curve shows a clear yield stage of the material with no obvious necking after fracture. This effect stems from the fine lath martensite structure formed by fine-grained austenitization (830℃), which allows the material to maintain good toughness while achieving high strength. Compared to existing fully martensitic steels (elongation 3-4%), its formability is improved by 30%, meeting the bending and hole-expanding requirements of complex structural components such as automotive A-pillars and bumpers. The mechanical property data of the three sets of samples fluctuated within ±3%, demonstrating stable chemical composition control and heat treatment processes, and a wide production window. This result solves the problems of complex heat treatment processes and large performance fluctuations in existing technologies, achieving a product qualification rate of over 98%, meeting the requirements of large-scale industrial production.

[0103] Figure 15As shown, this invention utilizes a fully martensitic microstructure formation process, synergistic chemical composition design (C, Mn, B, Cr, etc.), and controlled quenching cooling parameters. Test data shows a Rockwell hardness of HRC=46.5HRA (within the design range of 43-50.5HRA) and a Vickers hardness of HV=462HRA (within the design range of 416-516HRA). This hardness level gives the material excellent wear resistance, improving it by 25% compared to existing ordinary high-strength steel (HRC35-40), reducing wear on automotive parts during service and extending their service life. The hardness data fluctuation range of 32 measuring points is less than ±5%, proving that the martensitic microstructure is uniform, without compositional segregation or microstructure inhomogeneity. This effect stems from the compositional homogenization during austenitization (holding time ≥30s) and the uniform cooling during spray quenching (cooling water temperature 20-30℃, pressure 0.3-0.5MPa), avoiding failures caused by insufficient local hardness, ensuring consistent performance of parts under complex stress conditions, and improving vehicle driving safety. Hardness and tensile strength are positively correlated (HV462 corresponds to Rm1495MPa), proving that the material's strengthening mechanism is effective. This synergistic matching allows the material to meet high strength requirements while maintaining a reasonable hardness range, avoiding brittle fracture caused by excessive hardness, and reducing the fracture risk by 40% compared to existing ultra-high hardness steels (HRC55 and above).

[0104] Reference Figure 16 , Figure 17 As shown, this invention utilizes a fine-grained martensitic structure formation process, a thin galvanized layer design, and a shot blasting process. Bending test results show an average bending angle of 64°, exceeding the design requirement of >60°, and no cracks were observed after bending. This effect stems from the synergistic effect of the fine lath martensitic structure and the thin coating design, solving the problem of poor bending performance (≤43°) of existing high-strength steel. This allows the material to be used in complex bending structures of automotive bodies, expanding its application range. The average contact resistance is 0.82mΩ, far below the design limit of 1.5mΩ. This result proves that the surface roughness of the coating after shot blasting is moderate, and the thin galvanized layer does not affect the conductivity, meeting the automotive industry's contact resistance requirements for welded parts (≤1.5mΩ). Welding experiments verify that the strength of the welded joint reaches more than 95% of the base material strength, with no incomplete welds or burn-through, solving the problem of poor weldability of existing galvanized steel. Multiple sets of test data show fluctuations within ±3%, proving that the bending performance and contact resistance are stable and unaffected by production batches. This effect enables the material to be adapted for mass production of automotive parts, improving production efficiency by 20% and reducing production costs by 15% compared to the large performance fluctuations in existing technologies.

[0105] Reference Figure 18As shown, the infrared thermal image (A-pillar lower plate reinforcement Th: 1.4mm) demonstrates the temperature uniformity control achieved through the heat treatment heating process of this invention. The infrared thermal image shows a uniform temperature distribution in the heated area of ​​the workpiece, with no local overheating or underheating, ensuring that the 1.4mm thick A-pillar lower plate reinforcement undergoes synchronous phase transformation between the core and surface at an austenitizing temperature of 830℃, avoiding residual ferrite affecting strength. Combined with the core hardness testing requirements, this image provides a visual basis for hardness measurement point location, ensuring that the test results reflect the overall material performance. The uniformity of the temperature gradient in the image proves that the preheating (8-15℃ / s) and rapid heating (15-30℃ / s) process parameters are reasonably designed, ensuring uniform thermal stress distribution during the heating process and eliminating the risk of deformation or cracking. Production verification shows that the workpiece dimensional accuracy (±0.05mm) under this process achieves a 100% compliance rate, meeting the assembly requirements of automotive parts.

[0106] Reference Figures 19-24 As shown, this invention utilizes a multi-layered coating structure (steel substrate-nickel layer-zinc layer) to ensure process stability throughout the entire production cycle; the coating-substrate interface bonding state is also demonstrated. Microstructure images show that the interface between the coating (black area) and the steel substrate (light-colored area) is free of peeling and pores in the initial, middle, and final stages of production, and the nickel layer (transition layer) is uniform and continuous in thickness. This structure proves that the flash nickel plating pretreatment (500nm) and hot-dip galvanizing process are stable and controllable throughout the entire production cycle, ensuring consistent coating adhesion (peel test force ≥10N / mm) and solving the problem of coating adhesion fluctuations during mass production in existing technologies. The microstructure images from different production stages show no significant differences, indicating that the chemical composition control (C, Mn, B, etc.) and heat treatment processes (austenitization, quenching) parameters are stable throughout the entire cycle. EDS analysis shows that the Ni and Mn elements in the coating are uniformly distributed without component segregation, ensuring that the tensile strength fluctuation of the material is ≤3% (1495±45MPa) throughout the entire production cycle, meeting the stringent requirements of the automotive industry for consistent component performance. The consistency of the coating thickness (approximately 5-8μm) throughout the production cycle demonstrates that the thin coating (20-30μm) design has a wide process window, ensuring quality without frequent adjustments to galvanizing parameters. This design simultaneously achieves the goals of material corrosion resistance (no white rust after 72h salt spray) and lightweighting (15% weight reduction), while reducing production costs by 20% compared to thicker coating processes.

[0107] Reference Figure 25As shown, this invention utilizes shot blasting technology; controls the surface roughness of the coating; and improves subsequent welding and coating performance. Before shot blasting, the surface contains oxide scale and impurities (brown areas). After shot blasting, the surface roughness increases, forming a uniform micro-uneven structure. This structure increases the contact area between the zinc coating and the substrate, improving adhesion by 40%. No coating peeling occurs after a cold bending test (≥60°), solving the problem of easy coating peeling during forming in the prior art. The shot-blasted surface provides a good base for welding and coating. Welding experiments show that the strength of the welded joint after shot blasting reaches more than 95% of the base material, with no false welds. The coating adhesion test (cross-cut method) reaches level 0, meeting the corrosion resistance and appearance requirements of automotive bodies. This effect proves that shot blasting is a key link connecting heat treatment and subsequent processing, ensuring the overall performance of the material meets standards.

[0108] Reference Figure 26 As shown, this invention utilizes a synergistic coating of multi-element alloys (Zn, Ni, Mn, Al), a composite oxide layer (Al2O3, MnO), and a long-lasting corrosion resistance design. Polarization curves show that the material's self-corrosion potential is stable (max -312.5mV, min -500mV), and the corrosion current density is <1×10⁻⁶. -6 A / cm 2 This is far below the corrosion resistance requirements of the automotive industry (corrosion current density <5×10). - 6 A / cm 2 This data, from an electrochemical perspective, confirms that the Ni barrier effect of the coating, in synergy with the Al2O3-MnO composite oxide layer, enables the material to form a stable passivation film in corrosive environments, improving corrosion resistance by two times compared to existing galvanized steel. The potential in the curve shows no significant decrease after prolonged polarization, proving the continued effectiveness of the coating's cathodic protection and the absence of cracking in the composite oxide layer. Real-vehicle road tests demonstrate that car bodies using this material showed no red rust after 5 years of service in marine climates, meeting the requirement of a service life of over 10 years and solving the problem of high after-sales maintenance costs caused by insufficient corrosion resistance of existing galvanized steel.

[0109] Those skilled in the art should consider the specification as a whole. The technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art, all of which fall within the protection scope of this invention.

Claims

1. A lightweight, thin-coated zinc-manganese-boron high-strength steel material, characterized in that, By weight percentage, the composition includes: C 0.22-0.24%, Si 0.2-0.3%, Cr 0.1-0.3%, Mn 1.2-1.4%, B 0.003-0.007%, Al 0.02-0.06%, Nb≤0.002%, Ti 0.01-0.05%, S≤0.003%, P≤0.015%, with the balance being Fe; the steel surface is provided with a thin zinc plating layer with a zinc layer thickness of 20µm-30µm.

2. The lightweight thin-coated zinc-manganese-boron high-strength steel material according to claim 1, characterized in that, The zinc coating contains 1.2% manganese, and also contains carbon, copper and other alloying elements.

3. A heat treatment method for the lightweight thin-coated zinc-manganese-boron high-strength steel material as described in claim 1, characterized in that, Includes the following steps: (1) Process steel into coils or plates and cut them to the required size; (2) Use a mold to cold stamp the cut steel to obtain steel workpieces; (3) Heat the steel workpieces at a rate of 8-15℃ / s and keep them at a temperature below the melting point of zinc 420℃ to form an oxide layer that protects the zinc layer; (4) Heat the preheated steel workpieces to a temperature above 700℃ at a rate of 15-30℃ / s; (5) Heat the steel workpieces to a temperature above AC3 and keep them at a temperature of 40-60s to complete the formation, growth, dissolution of residual cementite and homogenization of composition of austenite nuclei; (6) Control the quenching start temperature between 800-900℃ and use a spray method to water quench the steel workpieces to room temperature to obtain a full martensitic structure; (7) Perform shot blasting on the quenched steel workpieces.

4. The heat treatment method according to claim 3, characterized in that, In step (5), the austenitizing treatment temperature is 830℃ and the holding time is 40-60s.

5. The heat treatment method according to claim 3, characterized in that, In step (3), the final temperature of the preheating is controlled between 380 and 420°C, and the holding time is 10 to 15 seconds.

6. The heat treatment method according to claim 3, characterized in that, In step (6), the cooling water temperature for spray quenching is 20-30℃ and the spray pressure is 0.3-0.5MPa.

7. The heat treatment method according to claim 3, characterized in that, In step (5), the holding time for homogenizing the austenite composition should not be less than 30 seconds to ensure that the austenite composition is evenly distributed.

8. The heat treatment method according to claim 3, characterized in that, After heat treatment, the steel has a tensile strength Rm of 1300-1700 MPa, a yield strength Rp of 1000-1300 MPa, and an elongation at break A80 / 50 ≥ 5%.

9. The heat treatment method according to claim 3, characterized in that, After heat treatment, the steel has a Vickers hardness (HV) of 416-516 HRA, a Rockwell hardness (HRC) of 43-50.5 HRA, a Rockwell hardness (HRA) of 71-76 HRA, a bending degree of ≥43°, and a contact resistance of ≤1.5 mΩ.

10. The heat treatment method according to claim 3, characterized in that, The zinc plating layer is pretreated using flash nickel plating technology, in which a nano-metallic nickel layer is electroplated on the substrate surface before hot-dip galvanizing, with a nickel layer thickness of 500nm.

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