Heat treatment process for improving hydrogen embrittlement resistance of advanced high-strength steel for automobiles
By introducing a pre-martensite heat treatment process into the bainite structure and regulating the variant distribution characteristics, the problem of bainite steel being prone to hydrogen embrittlement is solved, and the material's high hydrogen embrittlement resistance and strength are improved.
Patent Information
- Application Number
- CN202510713540.2
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-05-30
- Publication Date
- 2025-09-26
AI Technical Summary
In the existing heat treatment process, bainitic steel is prone to hydrogen embrittlement, which affects the safety and performance of the material.
A heat treatment process of austenitizing followed by austempering below the starting point of martensitic transformation is adopted to introduce pre-martensite into the bainite structure. By regulating the distribution characteristics of the variants, hydrogen traps are increased and hydrogen-induced cracking is suppressed.
It effectively reduces the local accumulation of hydrogen in metal materials, significantly improves the resistance to hydrogen embrittlement, and enhances the strength and toughness of the material.
Smart Images
Figure CN120700249A_ABST
Abstract
Description
Technical Field
[0001] The present application relates to the field of heat treatment, and in particular to a heat treatment process for improving the hydrogen embrittlement resistance of advanced high-strength steel for automobiles. Background Art
[0002] As a readily available, green, low-carbon, and widely applicable secondary energy source, hydrogen's development and utilization are triggering a profound energy revolution. Hydrogen may become the key to overcoming the energy crisis and building a clean, low-carbon, safe, and efficient modern energy system. However, the development and application of hydrogen energy raises a new challenge: hydrogen embrittlement in metals. Hydrogen atoms in metals originate from two sources. One is the inevitable incorporation of hydrogen into the metal during production and processing, such as smelting, pickling, welding, and hot stamping. This can easily lead to delayed fracture under load during service. The other source is high hydrogen pressure, where hydrogen molecules adsorb on the metal surface, decompose into atoms, and then dissolve into the metal's interior. Driven by thermal activation and concentration gradients, hydrogen diffuses through the crystal lattice, becoming easily trapped by defects such as grain boundaries and dislocations that distort the lattice structure. When the local hydrogen concentration reaches a critical value, hydrogen-induced cracking occurs, causing the material to transition from ductile to brittle fracture, severely impacting its properties.
[0003] Due to its excellent comprehensive properties, bainitic steel is widely used in automobiles, rails, construction, pipeline transportation, etc. However, the occurrence of hydrogen embrittlement has a great impact on the safety of workpieces. Summary of the Invention
[0004] The main purpose of this application is to provide a heat treatment process to improve the hydrogen embrittlement resistance of advanced high-strength steel for automobiles, aiming to solve the problem of bainitic steel being prone to hydrogen embrittlement in existing heat treatment processes.
[0005] To achieve the above-mentioned objectives, the present application provides a heat treatment process for improving the hydrogen embrittlement resistance of advanced high-strength steel for automobiles, comprising: homogenizing a steel ingot to obtain a steel billet; wherein the homogenization temperature is 1100-1250°C and the time is 2-4 h; free forging the steel billet at a temperature of 1050-1250°C to obtain a cast billet; hot rolling the cast billet at a temperature of 850-900°C to obtain a hot-rolled steel plate; austenitizing the hot-rolled steel plate to obtain an austenitized steel plate, and placing the austenitized steel plate in a salt bath furnace for austenitizing; wherein, during the austenitizing process, the temperature is 900-1000°C and the holding time is 25-35 min; during the austempering process, the temperature is less than the Ms point and the time is 6300-8700s; and the austempering steel plate is cooled to obtain a heat-treated steel plate.
[0006] Optionally, the heat-treated steel plate includes pre-martensite and bainite, and the pre-martensite volume fraction is 35-50%; the tensile strain rate of the heat-treated steel plate is 10 -6 -10 -5 s -1 , the hydrogen embrittlement sensitivity index is 10-14.5%.
[0007] Optionally, before hot rolling the cast slab, the method further comprises keeping the cast slab at a temperature of 1050-1200° C. for 2-4 hours.
[0008] Optionally, the thickness of the cast strand is 45-55 mm.
[0009] Optionally, the hot rolled steel plate has a thickness of 8-12 mm.
[0010] Optionally, the heating rate during the austenitizing treatment is 4-6°C / s.
[0011] Optionally, the austenitizing treatment is carried out in a muffle furnace, and the time for transferring the austenitized steel sheet from the muffle furnace to the salt bath furnace does not exceed 3 seconds.
[0012] Optionally, cooling the austempered steel plate comprises placing the austempered steel plate in water for cooling, wherein the time for the austempered steel plate to be cooled from the salt bath furnace to the water does not exceed 3 s.
[0013] Compared with the prior art, the present invention has the following advantages: The heat treatment process for improving the hydrogen embrittlement resistance of advanced high-strength steel for automobiles of the present invention introduces pre-martensite into the bainite structure through a heat treatment process of austenitization followed by austempering below the starting point of the martensitic phase transformation. The pre-martensite is a softer phase than bainite and can act as a "buffer" to relieve strain concentration, inhibit hydrogen-induced cracking, and thus reduce the likelihood of hydrogen embrittlement. Austempering below the Ms point can generate a large number of small-angle v1-v4 variant pairs, while the v1-v3 (v5) content is also significantly increased. Since the v1-v3 (v5) variant pairs are large-angle (>45°) grain boundaries and have a high degree of mismatch, they contain a large number of defects and have a hydrogen trapping effect. Although the v1-v4 variant pairs are small-angle (<15°) grain boundaries, they are substructures and have a hydrogen trapping effect. By regulating the distribution characteristics of the variants to increase hydrogen traps, the degree of local hydrogen accumulation is effectively reduced, and the hydrogen embrittlement resistance of the metal material can be improved fundamentally. BRIEF DESCRIPTION OF THE DRAWINGS
[0014] Figure 1 A schematic flow chart of a heat treatment process for improving the hydrogen embrittlement resistance of advanced high-strength steel for automobiles in this application; Figure 2 This is a heat treatment process roadmap for an embodiment of the present application; Figure 3 This is a scanned image of a heat-treated steel plate according to an embodiment of the present application; Figure 4 This is a statistical diagram of the variant distribution of the embodiment of the present application; Figure 5 The engineering stress-strain curve of the embodiment of the present application; Figure 6 This is the heat treatment process roadmap for the comparative example of this application; Figure 7 This is a scanned image of a heat-treated steel plate of a comparative example of this application; Figure 8 This is a statistical diagram of the variant distribution of the comparative ratio of this application; Figure 9 This is the engineering stress-strain curve of the comparative example of this application.
[0015] The realization of the objectives, functional features and advantages of this application will be further explained in conjunction with embodiments and with reference to the accompanying drawings. DETAILED DESCRIPTION
[0016] To make the objectives, technical solutions, and advantages of this application more clear, the technical solutions of this application will be clearly and completely described below in conjunction with the accompanying drawings. Obviously, the embodiments described are only part of the embodiments of this application, not all of them. Based on the embodiments of this application, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of this application.
[0017] The embodiment of the present invention provides a heat treatment process for improving the hydrogen embrittlement resistance of advanced high-strength steel for automobiles, such as Figure 1 As shown, the specific steps include: Step S1, homogenizing the steel ingot to obtain a steel billet; wherein the homogenization temperature is 1100-1250° C. and the time is 2-4 hours; Step S2, free forging the steel billet at a temperature of 1050-1250° C. to obtain a billet with a thickness of 45-55 mm; Step S3, hot rolling the ingot at a temperature of 850-900° C. to obtain a hot-rolled steel plate with a thickness of 8-12 mm; Before hot rolling, the ingot is kept at a temperature of 1050-1200℃ for 2-4 hours; this can increase grain stability, reduce texture, and further improve organizational uniformity.
[0018] Step S4, placing the hot-rolled steel plate in a muffle furnace for austenitization treatment to obtain an austenitized steel plate, and taking out the austenitized steel plate and placing it in a salt bath furnace for austempering within 3 seconds; Among them, during the austenitization process, the heating rate is 4-6℃ / s, the temperature is 900-1000℃, and the holding time is 25-35min; during the isothermal quenching process, the temperature is lower than the Ms point, and the time is 6300-8700s. The long-term holding reduces the hardness of the pre-martensite and causes carbon partitioning, making it a soft phase compared to bainite, which can act as a "buffer" to adjust strain concentration. Since strain concentration will promote the aggregation of hydrogen atoms and promote crack initiation and expansion, adjusting the strain concentration can suppress hydrogen-induced cracking to a certain extent.
[0019] In step S5, after removing the austempered steel plate from the salt bath furnace, the plate is cooled in water within 3 seconds to obtain a heat-treated steel plate. The heat-treated steel plate includes pre-martensite and bainite. Based on the material composition of the steel plate, the pre-martensite volume fraction is calculated by the Koistinen-Marburger model to be 35-50%, as calculated by the following formula: ,
[0020] Among them, Ms is the martensitic transformation starting temperature of the organization, T is the quenching temperature, It is a parameter related to the composition. The tensile strain rate of the heat-treated steel plate is 10 -6 -10 -5 s -1 The hydrogen embrittlement sensitivity index is 10-14.5%. Before the hydrogen concentration reaches saturation, the longer the hydrogen charging time, the higher the hydrogen embrittlement sensitivity index. The data from repeated experiments with hydrogen charging for 2 hours show a range of 10-14.5%.
[0021] In this embodiment, a heat treatment process of austenitization followed by austempering below the martensitic transformation onset point introduces pre-martensite into the bainite structure. Pre-martensite is a softer phase than bainite and acts as a "buffer" to alleviate strain concentration and inhibit hydrogen-induced cracking. Furthermore, the introduction of pre-martensite into the bainite structure shifts the variant distribution from being dominated by v1-v2 to a coexistence of v1-v2 and v1-v4. This is because the conditions for v1-v2 formation require, first, a sufficiently high driving force and, second, carbon diffusion capacity. Above or near the Ms point, the transformation driving force is high and carbon easily diffuses, making the formation of v1-v2 variant pairs more likely. However, below the Ms point, despite the high transformation driving force, carbon diffusion is slow, making the formation of v1-v2 variant pairs difficult. Furthermore, due to the high strain, the excessive transformation driving force causes the austenite grains to rapidly transform, generating a large number of small-angle v1-v4 variant pairs and a significant increase in the v1-v3 (v5) content. Because the v1-v3 (v5) variant pairs are high-angle (>45°) grain boundaries, they exhibit a high degree of mismatch and, therefore, contain numerous defects, which trap hydrogen. While the v1-v4 variant pairs are low-angle (<15°) grain boundaries, they are substructures and thus also trap hydrogen. Therefore, this embodiment modulates the distribution characteristics of the variants to increase hydrogen traps, effectively reducing the degree of localized hydrogen accumulation and fundamentally improving the metal material's resistance to hydrogen embrittlement.
[0022] Example The raw materials are selected, and the raw materials include the following components in mass percentage: 0.19% C, 3.09% Mn, 1.46% Si, 0.005% V, 0.005% S, and the balance is Fe and inevitable impurities.
[0023] Step S1, vacuum melting the raw materials to obtain an ingot, and homogenizing the steel ingot in a muffle furnace at 1200° C. for 2 hours to obtain a steel billet; Step S2, free forging the steel billet at a temperature of 1100° C. to obtain a billet with a thickness of 45 mm; Step S3, keeping the ingot at 1150° C. for 2 hours, and hot rolling it to a thickness of 10 mm with a final rolling temperature of not less than 850° C., and then air cooling it to room temperature to form a hot-rolled steel plate; Step S4, as Figure 2 As shown, the hot-rolled steel plate was placed in a muffle furnace at 950°C, kept warm for 30 minutes, and then quickly taken out. It was then placed in a 320°C salt bath furnace within 3 seconds to austemper the steel plate for 7200 seconds. In step S5, after the austempered steel plate is taken out from the salt bath furnace, it is placed in water for cooling within 3 seconds to obtain a heat-treated steel plate.
[0024] Step S6, testing the steel plate after heat treatment, and the structure picture observed under the scanning electron microscope after corrosion after heat treatment is as follows: Figure 3 The gray structure with iron carbide particles is pre-martensite. The volume fraction of pre-martensite calculated by the Koistinen-Marburger model is 38.8%, and the rest of the structure is bainite. In addition, the heat-treated steel plate was cut into block samples for electron backscatter diffraction, sanded to 2000 mesh, mechanically polished, and then electrolytically polished for EBSD testing. The EBSD results were imported into the mtex toolbox for austenite parent phase grain reconstruction and variant analysis, where the variant type distribution statistics are as follows: Figure 4 As shown, v1-v2, v1-v4, and v1-v3 (v5) coexist. Since v1-v3 (v5) is a large-angle grain boundary (>45°) and v1-v4 is a substructure, both have a greater effect on hydrogen capture and can effectively improve the material's resistance to hydrogen embrittlement.
[0025] In order to evaluate the hydrogen embrittlement sensitivity of the material, slow strain rate tensile tests were performed on the material before and after electrochemical hydrogen charging. The strain rate was 10 -5 s -1 , the engineering stress-strain curves before and after hydrogen charging are obtained, such as Figure 5 As shown in the figure, the ratio of the difference in elongation after fracture between the material before and after hydrogen charging and the elongation after fracture of the non-hydrogen charging sample is defined as the hydrogen embrittlement sensitivity index, and the calculated hydrogen embrittlement sensitivity index is 14.5%.
[0026] Comparative Example: The mass percentage of the chemical composition of the target material used is 0.19% C, 3.09% Mn, 1.46% Si, 0.005% V, 0.005% S, and the balance is Fe and unavoidable impurities. The raw materials are vacuum melted to form ingots, and the ingots are homogenized at 1200°C for 2 hours using a muffle furnace to obtain steel billets; the steel billets are then free forged at 1100°C to a thickness of 45mm, and then the billets are kept at 1150°C for 2 hours and hot rolled to a thickness of 10mm. The final rolling temperature is not lower than 850°C, and then air-cooled to room temperature to form hot-rolled steel plates. The hot-rolled steel plates are placed in a muffle furnace at 950°C and kept warm for 30 minutes, then quickly taken out (≤3s) and placed in a 370°C salt bath furnace for isothermal 7200s, as shown in FIG. Figure 6 As shown, the plate is quickly taken out of the salt bath furnace (≤3s) and placed in water to quickly cool to room temperature. The microstructure image observed under a scanning electron microscope after heat treatment and corrosion is as follows: Figure 7, in which there is basically no pre-martensite, and most of it is bainite. The steel plate was cut into block samples for electron backscatter diffraction, sanded to 2000 mesh with sandpaper, mechanically polished, and then electrolytically polished for EBSD testing. The EBSD results were imported into the mtex toolbox for austenite parent phase grain reconstruction and variant analysis. The variant distribution statistics are shown in the figure below. Figure 8 As shown, v1-v2 is the main one.
[0027] In order to evaluate the hydrogen embrittlement sensitivity of the material, slow strain rate tensile tests were carried out on the tensile samples of the material before and after electrochemical hydrogen charging. -5 s -1 , the engineering stress-strain curves before and after hydrogen charging are obtained, such as Figure 9 As shown in the figure, the ratio of the difference in elongation after fracture of the material before and after hydrogen charging to the elongation after fracture of the non-hydrogen charged sample is defined as the hydrogen embrittlement sensitivity index, and the calculated hydrogen embrittlement sensitivity index is 24.4%.
[0028] Compared with the embodiment, the comparative example is austempered above the martensitic transformation starting point, Figure 5 and Figure 9 From the comparison of the engineering stress-strain curves, it can be seen that the introduction of pre-martensite in the embodiment significantly increases the material strength, while the hydrogen embrittlement sensitivity index decreases from 24.4% to 14.5%.
[0029] The above are only preferred embodiments of the present application and do not limit the patent scope of the present application. Any equivalent structure or equivalent process transformation made using the contents of the present application specification and drawings, or directly or indirectly applied in other related technical fields, are also included in the patent protection scope of the present application.
Claims
1. A heat treatment process for improving the hydrogen embrittlement resistance of advanced high-strength steel for automobiles, characterized in that: include: The steel ingot is homogenized to obtain a steel billet; wherein the homogenization temperature is 1100-1250° C. and the time is 2-4 hours; The steel billet is subjected to free forging at a temperature of 1050-1250°C to obtain a cast billet; Hot rolling the cast slab at a temperature of 850-900° C. to obtain a hot-rolled steel plate; austenitizing the hot-rolled steel plate to obtain an austenitized steel plate, and placing the austenitized steel plate in a salt bath furnace for austempering the steel plate; Wherein, during the austenitizing process, the temperature is 900-1000°C and the holding time is 25-35 min; during the isothermal quenching process, the temperature is lower than the Ms point and the holding time is 6300-8700s; The austempered steel plate is cooled to obtain a heat-treated steel plate.
2. The heat treatment process for improving the hydrogen embrittlement resistance of advanced high-strength steel for automobiles according to claim 1, characterized in that: The heat-treated steel plate comprises pre-martensite and bainite, wherein the volume fraction of the pre-martensite is 35-50%; the tensile strain rate of the heat-treated steel plate is 10 -6 -10 -5 s -1 , the hydrogen embrittlement sensitivity index is 10-14.5%.
3. The heat treatment process for improving the hydrogen embrittlement resistance of advanced high-strength steel for automobiles according to claim 1, characterized in that: Before hot rolling the cast slab, the method further comprises: The cast slab is kept at a temperature of 1050-1200° C. for 2-4 hours.
4. The heat treatment process for improving the hydrogen embrittlement resistance of advanced high-strength steel for automobiles according to claim 1, characterized in that: The thickness of the cast slab is 45-55 mm.
5. The heat treatment process for improving the hydrogen embrittlement resistance of advanced high-strength steel for automobiles according to claim 1, characterized in that: The thickness of the hot-rolled steel plate is 8-12 mm.
6. The heat treatment process for improving the hydrogen embrittlement resistance of advanced high-strength steel for automobiles according to claim 1, characterized in that: The heating rate during the austenitizing treatment is 4-6°C / s.
7. The heat treatment process for improving the hydrogen embrittlement resistance of advanced high-strength steel for automobiles according to claim 1, characterized in that: The austenitizing treatment is carried out in a muffle furnace, and the time for the austenitized steel plate to be transferred from the muffle furnace to the salt bath furnace does not exceed 3 seconds.
8. The heat treatment process for improving the hydrogen embrittlement resistance of advanced high-strength steel for automobiles according to claim 1, characterized in that: The step of cooling the austempered steel plate comprises: The austempered steel plate is cooled by placing it in water for cooling, and the time for the austempered steel plate to be cooled from the salt bath furnace to the water does not exceed 3 s.