Method for improving high-temperature oxidation resistance of Cr-Si alloy hot stamping steel

Abstract

The invention relates to a method for improving the high-temperature oxidation resistance of Cr-Si alloy hot stamping steel, which comprises the following steps of: pretreating the Cr-Si alloy hot stamping steel, preserving the heat of the Cr-Si alloy hot stamping steel at the temperature of 650-780 DEG C for 8-48 hours until the proportion of Cr-rich carbides with the grain size of more than 120 nm in a Cr-Si alloy hot stamping steel matrix is more than 70% and the Cr content in the Cr-rich carbides is 30-50 wt.%; cooling to room temperature; polishing off an oxide layer and a decarburized layer formed on the surface; in the air environment, heat preservation is conducted after heating, at the moment, the Cr-rich carbide is dissolved, Cr diffuses from the interior of the base body to the surface, and a continuous and compact Cr2O3 layer is formed; and carrying out stamping forming, pressure maintaining and heat preservation to obtain the hot stamping steel. The problem of serious high-temperature oxidation in the austenitizing process is solved, and the production cost is reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of hot stamping, and in particular to a method for improving the high-temperature oxidation resistance of Cr-Si alloy hot stamping steel. Background Art

[0002] Lightweighting vehicles is a key development direction and a major initiative for countries around the world to achieve energy conservation, emission reduction, and consumption reduction. The use of ultra-high-strength steel hot-stamped components can reduce vehicle weight while ensuring vehicle strength and safety, making it a key approach to achieving vehicle lightweighting. The hot stamping process, leveraging the principles of thermoplastic metal forming, allows simultaneous quenching of sheet metal during forming, improving the material's formability and significantly expanding the application range of high-strength and ultra-high-strength steel in automotive parts. During the hot stamping process, the steel is heated to austenitization, held in the austenitization temperature range for a period of time, and then quickly transferred to the hot stamping die for forming and quenching. To achieve austenitization, the temperature is typically 900°C to 950°C, with a holding time of 3 to 10 minutes. Such high temperatures typically require the use of high-purity inert gas for protection during the heating process, and a highly sealed heating furnace is required to prevent severe high-temperature oxidation. However, in actual production, these high-purity inert gas and sealed heating furnaces are difficult to control and implement. Moreover, at high temperatures, the steel matrix and alloying elements react easily with oxygen, carbon dioxide, and moisture in the air to form thick oxide scales, resulting in severe oxidation. Therefore, this will lead to the problem of unstable surface quality of parts obtained during the actual production process. Currently, there are two main solutions to solve the problems of mold damage and reduced surface accuracy caused by severe oxidation of steel plates during hot stamping of hot stamped steel: one is to use coating technology, and the other is non-coating technology. However, the coating technology has a complex process; in the non-coating technology, a high-purity protective atmosphere needs to be introduced, and the unstable atmosphere in the furnace easily leads to severe oxidation, forming a loose and thick oxide scale, and the surface quality cannot be guaranteed to be stable.

[0003] Chinese patent CN117551852A discloses a method for improving the high-temperature oxidation resistance of Cr-Si alloy hot-formed steel. This method, for the first time, employs pre-oxidation in air to reduce oxide scale thickness, thereby achieving lightweighting. However, this method faces two practical challenges: first, its compatibility with existing industrial production; second, its limited range of applicable steel compositions limits its application. Regarding the first question, the existing process is to supply the steel plate in a cold-rolled state before austenitizing treatment at 900~950℃, which is convenient for subsequent processes (cold rolling and cutting). However, in order to obtain a dense pre-oxidation layer through pre-oxidation in CN117551852A, the technical means adopted are a pre-oxidation temperature of 700~820℃ and a time of 1~5min. The short time of 1~5min only realizes the hot rolling of the steel plate, and does not undergo a long annealing and holding step (annealing and holding function: softening the workpiece and improving residual stress, etc.). Its hardness is relatively high, which is not conducive to the subsequent cold rolling and cutting processes; however, if the steel plate in CN117551852A undergoes a long pre-oxidation treatment (or is understood as being combined with an annealing step) so that its holding time is much greater than 5min, then the pre-oxidation layer will be very thick and very loose and not dense, and it will not be possible to improve the oxidation resistance in the subsequent hot stamping at 900~950℃ to achieve austenitization, and the high-temperature oxidation resistance cannot be achieved through the surface oxide layer. Regarding the second question, the oxide layer produced on the surface in step 1 is mainly Fe2MnO4 and MnO2, which has high requirements on the Mn content in the alloy composition and a small range of steel composition, limiting its scope of use.

[0004] In summary, the main problems of the existing technology are reflected in the following aspects: the coating technology has a complex process; in the non-coating technology, the oxide scale is thick and the surface quality cannot be guaranteed to be stable; the technical means of pre-oxidation in the air still has room for improvement. Summary of the Invention

[0005] The purpose of the present invention is to solve the problems in the prior art that Cr-Si alloy hot stamping steel needs to be introduced into a high-purity protective atmosphere during the heating process, and the atmosphere in the furnace is unstable and easily causes severe oxidation to form a loose and thick oxide scale, which causes the oxide scale to fall off during the hot stamping process, damages the mold, and reduces the surface quality of the steel plate, as well as the problem of poor compatibility with existing industrial production.

[0006] In order to solve the above technical problems, the technical solutions adopted by the present invention are as follows: The present invention provides a method for improving the high-temperature oxidation resistance of Cr-Si alloy hot stamping steel, comprising the following steps: Step 1: Pre-treat the Cr-Si alloy hot stamping steel by keeping the Cr-Si alloy hot stamping steel at a temperature of 650°C to 780°C for 8 hours to 48 hours until the proportion of Cr-rich carbide grains with a size of more than 120 nm in the Cr-Si alloy hot stamping steel matrix is greater than 70%, and the Cr content in the Cr-rich carbide is concentrated in the range of 30 wt.% to 50 wt.%, and then take it out and cool it to room temperature in air; Step 2: Grind away the oxide layer and decarburized layer formed on the surface of the Cr-Si alloy hot stamping steel during the pretreatment process; Step 3: In an air environment, the Cr-Si alloy hot stamping steel obtained after grinding in step 2 is heated and then kept warm. At this time, the Cr-rich carbide dissolves, and Cr diffuses from the matrix to the surface to form a continuous and dense Cr2O3 layer; until the hot stamping steel is completely austenitized, it is then transferred to a stamping die for stamping and is kept in a pressure-holding state. Finally, it is placed in a heat treatment furnace for heat preservation to obtain hot stamping steel.

[0007] Furthermore, the Cr-Si alloy hot stamping steel contains the following alloy components, in terms of mass percentage: C: 0.15~0.35%, Mn: 0.8~3.2%, Si: 0.6~3.8%, Cr: 1.5~3.9%, Nb: 0.01~0.05%, S: <0.01%, P: <0.015%, Al: 0.01~0.05%, V: 0.01~0.05%, Ti: 0.01~0.03%, Cu: 0.05~0.15%, and the balance is Fe and other inevitable impurities.

[0008] Furthermore, the Cr-Si alloy hot stamping steel further comprises the following alloy components, calculated by mass percentage: Y: 0.1-0.3% or Ce: 0.1-0.5%.

[0009] Furthermore, step 1 includes steelmaking, continuous casting, hot rolling and pickling processes.

[0010] Furthermore, the microstructure of the steel plate obtained in step 1 is recrystallized ferrite and chromium-rich carbides.

[0011] Preferably, the heating temperature of the insulation after heating in step 3 is 900℃~950℃, and the insulation time is 3min~10min; the stamping die temperature in step 3 is 730℃~850℃; the holding time in step 3 is 9s~13s, the pressure is 3MPa~25MPa, and the cooling rate is 5℃ / s~50℃ / s; the insulation temperature in the heat treatment furnace in step 3 is 170℃~200℃, and the insulation time is 18min~25min.

[0012] Furthermore, the microstructure of the Cr-Si alloy hot stamping steel obtained in step 3 is martensite, residual carbides and retained austenite.

[0013] Furthermore, under the condition of no protective atmosphere, the Cr-Si alloy hot stamping steel obtained in step 3 has a yield strength ≥1200 MPa, a tensile strength ≥1400 MPa, a total elongation ≥6%, and a bending angle ≥65°.

[0014] Furthermore, in the absence of a protective atmosphere, the oxide layer thickness of the Cr-Si alloy hot stamping steel obtained in step 3 is ≤1.0 μm.

[0015] In summary, compared with the prior art, the present invention has the following beneficial effects: The present invention proposes for the first time that a pretreatment step generates chromium-rich carbides within the substrate, ensuring the size and length of the chromium-rich carbides, as well as the chromium content in the chromium-rich carbides, satisfies the requirements for the subsequent high-temperature austenitization hot stamping step. The chromium-rich carbides dissolve during the high-temperature austenitization hot stamping heating process, and the chromium in the chromium-rich carbides diffuses from the substrate to the outer surface, forming a continuous and dense Cr2O3 layer, thereby improving the high-temperature oxidation resistance of the Cr-Si alloy hot stamping steel. Specifically, the present invention preheats the Cr-Si alloy hot stamping steel at 650°C to 780°C for 8 to 48 hours. During the heat treatment, chromium-rich carbides are formed within the Cr-Si alloy steel substrate. As the pretreatment time increases, the chromium-rich carbides reach a size of 120 nm or more, and the chromium content in the chromium-rich carbides is primarily in the range of 30 wt.% to 50 wt.%. The oxide layer formed on the surface of the Cr-Si alloy steel during the pretreatment is then removed. When the pretreatment step satisfies the requirement that the size of Cr-rich carbides reaches above 120nm and the chromium content in the carbides is mainly in the range of 30wt.%~50wt.%, the Cr-rich carbides, as a reservoir of chromium, can continuously provide chromium for diffusion outward from the matrix, thereby maintaining the continuous growth of a continuous and dense Cr2O3 layer instead of forming defects and transforming into an oxide with poor protectiveness. This protective layer can effectively inhibit the mutual diffusion of iron and oxygen, thereby effectively improving the high-temperature oxidation resistance of Cr-Si alloy hot stamping steel.

[0016] 2. Compared to non-coating technologies, the present invention eliminates the need for nitrogen generation equipment and a sealed heating furnace during the hot stamping process, simplifying the equipment required for hot stamping and reducing the environmental requirements of Cr-Si alloy hot stamping steel. A thin oxide layer can still be obtained, achieving good surface quality. The hot stamping process is stable and virtually unaffected by the atmosphere. Compared to coating technologies, the present invention significantly simplifies the process and saves costs. Compared to pre-oxidation methods, the present invention eliminates the need for additional steps and can implement pretreatment within the existing annealing process, offering good compatibility with industrial production. Due to the long heat preservation time (8 to 48 hours) of the steel plate, the hardness is low, facilitating subsequent cold rolling and cutting processes. By controlling temperature and time, the present invention can produce Cr-rich carbides with improved oxidation resistance, thus extending the applicable composition range and providing more options for material designers, thereby expanding the scope of industrial applications.

[0017] 3. After pretreatment, the steel plate of the present invention can form an oxide layer (Cr2O3 layer) with a thickness of less than 1.0 μm within the hot stamping process window, even without a protective atmosphere. This continuous, dense Cr2O3 layer remains dense and intact throughout the hot stamping process, maintaining its integrity for a relatively long time. The oxide layer is also less susceptible to cracking and shedding during the hot stamping process, thus preventing damage to the mold. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 Flow chart of the method of the present invention; Figure 2 This is the energy spectrum of Cr-rich carbide after pretreatment in Example 1 of the present invention; Figure 3 1 is a grain size distribution histogram of Cr-rich carbides in Example 1 and Comparative Example 2; Figure 4 1 is a distribution diagram of the particle diameter of Cr-rich carbides and the corresponding Cr content in the carbides in Example 1 and Comparative Example 2; Figure 5 The microstructures after pretreatment are detected by field emission scanning electron microscopy, (a) is the microstructure after pretreatment of Example 1, (b) is the microstructure after pretreatment of Example 2, (c) is the microstructure of Comparative Example 1 without pretreatment, and (d) is the microstructure after pretreatment of Comparative Example 2; Figure 6 The macromorphology after hot stamping is shown in Figure 1. (a) is the macromorphology of Example 1, (b) is the macromorphology of Example 2, (c) is the macromorphology of Example 3, (d) is the macromorphology of Example 4, (e) is the macromorphology of Comparative Example 1, and (f) is the macromorphology of Comparative Example 2. Figure 7 Surface morphologies after hot stamping, (a) is the surface morphology after hot stamping of Example 2, (b) is the surface morphology after hot stamping of Comparative Example 2; Figure 8 are cross-sectional morphology images of the oxide layer, wherein a is a cross-sectional morphology image of the oxide layer of Example 2, and b is a morphology image magnified 10,000 times at the frame in a; Figure 9 are cross-sectional morphology images of the oxide layer, wherein a is a cross-sectional morphology image of the oxide layer of Comparative Example 1, and b is a morphology image magnified 4,000 times at the frame in a; Figure 10 2 is the element distribution diagram of the oxide layer of Example 2; Figure 11 : is the element distribution diagram of the oxide layer of comparative example 1; Figure 12 2 are cross-sectional microstructures after hot stamping, (a) is the cross-sectional microstructure of Example 2, (b) is the cross-sectional microstructure of Comparative Example 1, and (c) is the cross-sectional microstructure of Comparative Example 2. DETAILED DESCRIPTION

[0019] The present invention is described in more detail below with reference to the embodiments and accompanying drawings.

[0020] The heating furnace used in the examples is a muffle furnace; the hot stamping equipment is a hot stamping forming press; the equipment used to observe the cross-sectional morphology and elemental analysis of the oxide scale in the examples is a Zeiss Ultra 55 field emission scanning electron microscope; the composition, size, and content of the carbides in the pretreated matrix are analyzed using a Czech Talos F200X G2 high-resolution field emission transmission electron microscope.

[0021] The present invention proposes a method for improving the high temperature oxidation resistance of Cr-Si alloy hot stamping steel, such as Figure 1 As shown, the following steps are included: Step 1: Pretreatment: Pretreat the Cr-Si alloy hot stamping steel by keeping the Cr-Si alloy hot stamping steel at a temperature of 650°C to 780°C for 8 hours to 48 hours until the proportion of Cr-rich carbide grains with a size of more than 120nm in the Cr-Si alloy hot stamping steel matrix is greater than 70%, and the Cr content in the Cr-rich carbide is concentrated in the range of 30wt.% to 50wt.%, and then take it out and cool it to room temperature in air; Pretreatment parameters were determined based on the current hot-rolling coiling temperature (650°C) and annealing temperature (680°C) used in industrial production of high-strength steel, combined with compositional calculations and thermodynamic calculations. The formation and dissolution temperatures of the single Cr-rich carbide in this invention are 460°C and 780°C or higher, respectively. This means that single Cr-rich carbides can be formed by heat treatment above 460°C, but cannot form at temperatures above 780°C. Given the actual hot-rolling coiling temperature of 650°C in production, the pretreatment temperature in this invention is selected to be between 650°C and 780°C to ensure compatibility with existing processes, reduce costs, improve feasibility, and facilitate implementation and widespread adoption. Since 780°C is close to the critical value, a temperature between 650°C and 750°C is preferred to avoid inadequacies in the final product due to the critical value. During the heat treatment process at 650℃~780℃, chromium-rich carbides are formed. As the pretreatment time increases, the size of the Cr-rich carbides reaches above 120nm, and the chromium content in the carbides is mainly in the range of 30wt.%~50wt.%. The Cr-rich carbides serve as a reservoir of chromium, ensuring the production of a continuous (sustained) dense Cr2O3 layer during high-temperature austenitization.

[0022] The method of the present invention is more compatible with existing industrial production. Due to the long holding time in step 1, it is well compatible with current steel plate production. For example, in the annealing process, pretreatment can be performed simultaneously with the annealing process, thus eliminating the need for any additional steps and achieving a more cost-effective approach. It also facilitates subsequent cold rolling and cutting processes. By controlling temperature and time, the present invention can produce Cr-rich carbides with enhanced oxidation resistance. Therefore, the composition range of the applicable composition is wider, not limited by alloy composition, providing more options for material designers and thus expanding the scope of industrial application.

[0023] Step 2: Grind away the oxide layer and decarburized layer formed on the surface of the Cr-Si alloy hot stamping steel during the pretreatment process; This application is different from Chinese patent CN117551852A, which uses a thin oxide layer on the surface of the alloy to achieve high-temperature anti-oxidation effect during the high-temperature austenitizing hot stamping process. This application grinds off the thick oxide layer formed on the surface, and by limiting the insulation temperature to 650-780°C for 8h-48h in step 1, controls the proportion of Cr-rich carbide grains with a size of more than 120 nm in the alloy matrix to be greater than 70%, and the Cr content of the carbide is required to be concentrated in 30-50wt.%, thereby satisfying the requirement that the Cr-rich carbide serves as a chromium storage reservoir in the high-temperature austenitizing hot stamping process.

[0024] Step 3: In an air environment, the Cr-Si alloy hot stamping steel, obtained after polishing in Step 2, is heated and held at this temperature. This allows the Cr-rich carbides to dissolve, and Cr to diffuse from the matrix to the surface, forming a continuous, dense Cr2O3 layer. The hot stamping steel is completely austenitized, without the need for shielding gas in the furnace. The steel is then transferred to a stamping die for forming, with the die clamping temperature set between 730°C and 850°C. During the hot stamping process, the steel plate is hot stamped in a die equipped with an internal cooling system, maintaining a holding pressure for 9 to 13 seconds at a pressure of 3 MPa to 25 MPa and a cooling rate of 5°C / s to 50°C / s. The steel is then placed in a heat treatment furnace and held at this temperature for 18 to 25 minutes to obtain the hot stamping steel.

[0025] During the austenitization process, there is no need to introduce high-purity inert gas for protection. In this method, a large number of chromium-rich carbides with a size of more than 120nm are formed in the steel plate matrix after pretreatment. They dissolve during the subsequent hot stamping heating process. Since the Cr-rich carbides serve as a reservoir of chromium, they can continuously provide chromium for diffusion from the matrix to the outer surface, thereby promoting the formation of a continuous and dense Cr2O3 layer. At the same time, the dissolution of the carbides maintains the growth of the oxide layer, thereby hindering the mutual diffusion of iron and oxygen and improving the high-temperature oxidation resistance of the Cr-Si alloy hot stamping steel. This avoids the problem of introducing a high-purity protective atmosphere in the existing process, which easily causes severe oxidation and forms loose and thick oxide scales due to the unstable atmosphere in the furnace, resulting in the shedding of oxide scales during hot stamping, damaging the mold and reducing the surface quality of the steel plate.

[0026] The present invention no longer requires nitrogen generation equipment and a closed heating furnace during the hot stamping process, simplifies the equipment required for hot stamping, reduces the environmental requirements of Cr-Si alloy hot stamping steel, and still can obtain a thin oxide layer and good surface quality. The hot stamping process is stable and is hardly affected by the atmosphere.

[0027] The continuous dense Cr2O3 layer obtained by the present invention is dense and complete during the entire hot stamping process and can be maintained for a relatively long time. The oxide layer is not easy to crack or fall off during the hot stamping process, thereby not damaging the mold.

[0028] Example 1 The composition of the Cr-Si alloy hot stamping steel used is shown in Table 1: Table 1 Composition of Cr-Si alloy hot stamping steel in Example 1

[0029] Pickled hot rolled plates of the alloys with the composition shown in Table 1 were cut and processed into plates with the size of 160×200×1.9mm using a metal shearing machine. 2Small steel plate.

[0030] Step 1: Pretreatment: Place the pickled hot-rolled plate into a heating furnace at 670°C, keep it warm for 8 hours, then take it out and cool it to room temperature in the air.

[0031] Step 2: Surface treatment: Use a grinder to process the surface of the pretreated steel plate, grind off the oxide layer formed by the pretreatment, grind the steel plate from 1.9mm to 1.4mm, and remove the oxide layer and decarburization layer.

[0032] Step 3: Hot stamping: heat the polished steel plate to 950°C in a heating furnace without a protective atmosphere and heat it in air for 6 minutes to completely austenitize the steel plate. Then transfer it to a stamping die for stamping. The stamping die temperature is 800°C, and the pressure is maintained at 7MPa for cooling at a cooling rate of 40°C / s. Then, place it in a heat treatment furnace at 170°C and keep it warm for 20 minutes to obtain hot stamping steel.

[0033] Example 2 The composition of the Cr-Si alloy hot stamping steel used is shown in Table 2: Table 2 Composition of Cr-Si alloy hot stamping steel in Example 2

[0034] Pickled hot-rolled plates of the alloys whose compositions are shown in Table 2 were cut and processed into small steel plates with dimensions of 160 × 200 × 1.9 mm using a metal shearing machine.

[0035] Step 1: Pretreatment: Place the pickled hot-rolled plate into a heating furnace at 700°C, keep it warm for 16 hours, then take it out and cool it to room temperature in the air.

[0036] Step 2: Surface treatment: Use a grinder to process the surface of the pretreated steel plate, grind off the oxide layer formed by the pretreatment, grind the steel plate from 1.9mm to 1.4mm, and remove the oxide layer and decarburization layer.

[0037] Step 3: Hot stamping: heat the polished steel plate to 950°C in a heating furnace without a protective atmosphere and heat it in air for 10 minutes to completely austenitize the steel plate. Then transfer it to a stamping die for stamping. The stamping die temperature is 800°C, and the steel plate is cooled under 7MPa pressure at a cooling rate of 40°C / s. Then, the steel plate is placed in a heat treatment furnace at 170°C and kept warm for 20 minutes to obtain hot stamping steel.

[0038] Example 3 The composition of the Cr-Si alloy hot stamping steel used is shown in Table 3: Table 3 Composition of Cr-Si alloy hot stamping steel in Example 3

[0039] Pickled hot rolled plates of the alloys with the composition shown in Table 3 were cut and processed into plates with the size of 160×200×1.9mm using a metal shearing machine. 2 Small steel plate.

[0040] Step 1: Pretreatment: Place the pickled hot-rolled plate into a heating furnace at 750°C, keep it warm for 24 hours, then take it out and cool it to room temperature in the air.

[0041] Step 2: Surface treatment: Use a grinder to process the surface of the pretreated steel plate, grind off the oxide layer formed by the pretreatment, grind the steel plate from 1.9mm to 1.4mm, and remove the oxide layer and decarburization layer.

[0042] Step 3: Hot stamping: heat the polished steel plate to 950°C in a heating furnace without a protective atmosphere and heat it in air for 10 minutes to completely austenitize the steel plate. Then transfer it to a stamping die for stamping. The stamping die temperature is 800°C, and the steel plate is cooled under 7MPa pressure at a cooling rate of 40°C / s. Then, the steel plate is placed in a heat treatment furnace at 170°C and kept warm for 20 minutes to obtain hot stamping steel.

[0043] Example 4 The composition of the Cr-Si alloy hot stamping steel used is shown in Table 4: Table 4 Composition of Cr-Si alloy hot stamping steel in Example 4

[0044] Pickled hot rolled plates of the alloys with the composition shown in Table 4 were cut and processed into plates with the size of 160×200×1.9mm using a metal shearing machine. 2 Small steel plate.

[0045] Step 1: Pretreatment: Place the pickled hot-rolled plate into a heating furnace at 700°C, keep it warm for 48 hours, then take it out and cool it to room temperature in the air.

[0046] Step 2: Surface treatment: Use a grinder to process the surface of the pretreated steel plate, grind off the oxide layer formed by the pretreatment, grind the steel plate from 1.9mm to 1.4mm, and remove the oxide layer and decarburization layer.

[0047] Step 3: Hot stamping: heat the polished steel plate to 950°C in a heating furnace without a protective atmosphere and heat it in air for 10 minutes to completely austenitize the steel plate. Then transfer it to a stamping die for stamping. The stamping die temperature is 800°C, and the steel plate is cooled under 7MPa pressure at a cooling rate of 40°C / s. Then, the steel plate is placed in a heat treatment furnace at 170°C and kept warm for 20 minutes to obtain hot stamping steel.

[0048] Comparative Example 1 The difference between Comparative Example 1 and Example 1 is that the pretreatment in step 1 of Example 1 was not performed.

[0049] After the steelmaking, continuous casting, hot rolling and pickling process steps, the surface treatment is carried out directly. The pickled hot rolled plate of the alloy with the composition shown in Table 1 is cut and processed into the size of 160×200×1.9mm using a metal shearing machine. 2 Small steel plate.

[0050] Surface treatment: The surface of the pickled hot-rolled steel plate was treated using a grinder, and the steel plate was ground from 1.9 mm to 1.4 mm to make the surface state consistent with Examples 1 to 4.

[0051] Hot stamping: The untreated steel plate is heated to 950°C in a heating furnace without a protective atmosphere and heated in air for 10 minutes to completely austenitize the steel plate. The steel plate is then transferred to a stamping die for stamping. The stamping die temperature is 800°C, and the plate is cooled under 4 MPa pressure at a cooling rate of 40°C / s. The plate is then placed in a heat treatment furnace at 170°C and kept warm for 20 minutes to obtain hot stamping steel.

[0052] Comparative Example 2 The main difference between Comparative Example 1 and Example 1 is that in the pretreatment of step 1, the holding time is less than 8 h.

[0053] Pickled hot rolled plates of the alloys with the composition shown in Table 1 were cut and processed into plates with the size of 160×200×1.9mm using a metal shearing machine. 2 Small steel plate.

[0054] Step 1 Pretreatment: Place the pickled hot-rolled plate into a heating furnace at 700°C, keep it warm for 1 hour, then take it out and cool it to room temperature in air.

[0055] Surface treatment: Use a grinder to process the surface of the pretreated steel plate, grind off the oxide layer formed by the pretreatment, grind the steel plate from 1.9mm to 1.4mm, and remove the oxide layer and decarburization layer.

[0056] Hot stamping: The polished steel plate is heated to 950°C in a heating furnace without a protective atmosphere and heated in air for 10 minutes to completely austenitize the steel plate. The steel plate is then transferred to a stamping die for stamping. The stamping die temperature is 800°C, and the plate is cooled under 7 MPa pressure at a cooling rate of 40°C / s. The plate is then placed in a heat treatment furnace at 170°C and kept warm for 20 minutes to obtain hot stamping steel.

[0057] The detection experiment and data analysis are as follows: The steel plates in Example 1 and Comparative Example 2 were cut into small samples of 10 mm × 10 mm, and then the samples were mechanically polished to about 50 microns, and then electrolytically polished to prepare samples suitable for transmission electron microscopy. A high-resolution field emission transmission electron microscope equipped with an energy dispersive X-ray spectrometer was used to perform element point scanning on the sample in Example 1. The results are as follows: Figure 2 Then, the carbide sizes at different locations of Example 1 and Comparative Example 2 were measured using a transmission electron microscope and statistically analyzed as follows: Figure 3 As shown, the Cr content in the carbide is obtained by element point scanning, as shown Figure 4 shown.

[0058] Figures 2 to 4 The composition, content and size of the carbides after pretreatment in the embodiments of the present invention and the comparative examples are shown. Figure 2 It can be seen that the carbides after pretreatment are rich in Cr. Figure 3 and Figure 4 It can be seen that the size of the carbides in Example 1 is mainly distributed in the range of 120nm~240nm, and the proportion of those larger than 120nm is greater than 70%, and the Cr content in the carbides is mainly above 30wt.%, accounting for almost 100%. In Comparative Example 2, since the holding time is less than 8h, the size of the carbides is mainly distributed between 40nm~160nm, and the proportion of those larger than 120nm is less than 50%. And the Cr content in the carbides is distributed between 5wt.%~50wt.%, mainly below 30wt.%, and the proportion of those above 30wt.% is only 50%. The carbide size and Cr content are significantly smaller. In this state, the Cr content in the carbide is insufficient, which is not conducive to the formation of a continuous and dense Cr2O3 layer in the subsequent hot stamping process. It cannot solve the technical problem of high-temperature oxidation resistance.

[0059] The steel plates in Example 1, Example 2, Comparative Example 1 and Comparative Example 2 were cut into small samples of 10 mm × 10 mm, and then mechanically polished, and then etched with nitric alcohol to obtain metallographic samples. Finally, the microstructure was photographed using a field emission scanning electron microscope, as shown in FIG. Figure 5 shown.

[0060] from Figure 5 It can be seen that the microstructures of Examples 1 and 2 after pretreatment have a large amount of chromium-rich carbides, and Figure 5 (a) and Figure 5 (b) It can be seen that the chromium-rich carbides in the substrate of Example 2 are coarser than those in Example 1, indicating that the carbide size becomes larger with the extension of the pretreatment time. Figure 5As can be seen in (c), no carbides are formed in Comparative Example 1. This indicates that the pretreatment step of the present invention is the key to forming chromium-rich carbides, and the holding time of 8h to 48h at a temperature of 650°C to 780°C determines whether chromium-rich carbides are formed. Figure 5 As can be seen in (d), chromium-rich carbides can be formed in Comparative Example 2, but the amount is small and the distribution is uneven. This indicates that the holding time in the pretreatment step determines the amount of chromium-rich carbides formed. The Cr-rich carbide grain size of 120 nm or larger must account for more than 70%, and the Cr content in the carbides must be concentrated between 30 wt.% and 50 wt.%. This ensures the Cr content in the carbides and facilitates the subsequent formation of a continuous and dense Cr2O3 layer, thereby solving the technical problem of high-temperature oxidation resistance.

[0061] The steel plates of Examples 1 to 4 and Comparative Examples 1 and 2 were photographed using a dedicated camera to obtain the following images: Figure 6 Macro photo shown.

[0062] from Figure 6 (a) Figure 6 (b) Figure 6 (c) and Figure 6 From the macroscopic surface of (d), it can be seen that the surfaces of Examples 1 to 4 are golden oxide scales, which are relatively thin oxide layers. Figure 6 (e) and Figure 6 From the macroscopic surface of (f), it can be seen that the surfaces of Comparative Examples 1 and 2 are black oxide scales, which are relatively thick oxide layers.

[0063] The steel plates of Example 2 and Comparative Example 2 were cut into small samples of 10 mm × 10 mm, and then the surface morphology was photographed using a field emission scanning electron microscope. Figure 7 shown. Figure 7 The illustration in the upper right corner of (a) is a morphological image of a certain location on the surface after being magnified 10,000 times.

[0064] from Figure 7 (a) It can be seen that a dense oxide layer is formed on the surface of Example 2. Figure 7 (b) It can be seen that a loose and porous oxide layer is formed on the surface of Comparative Example 2. This indicates that the Cr content in the carbide is insufficient to form a dense oxide layer, and the technical problem of high-temperature oxidation resistance cannot be solved.

[0065] The steel plates of Example 2 and Comparative Example 1 were cut into small samples of 10 mm × 10 mm, and then their oxide layers were wrapped with aluminum foil for protection. They were then mechanically polished, and finally the morphologies were photographed using a field emission scanning electron microscope. Figure 8 and Figure 9 shown.

[0066] from Figure 8 and Figure 9It can be seen that the oxide layer thickness formed in Example 2 is 0.12 μm, while the oxide layer thickness in Comparative Example 1 is 5.36 μm. The chromium-rich carbides in Example 2 dissolve rapidly during the hot forming heating process, and the higher chromium content quickly forms a continuous chromium-rich oxide layer on the surface, which hinders the outward diffusion of iron ions. At the same time, the carbides with a higher chromium content continuously provide chromium to the continuous and dense chromium-rich layer, thereby keeping the continuous and dense Cr2O3 layer intact during the hot stamping heating process, thereby improving the high-temperature oxidation resistance of the hot stamping steel. In Comparative Example 1, there are no chromium-rich carbides. The continuous Cr2O3 layer initially formed during the hot stamping heating process cannot be maintained for a long time, and defects appear in the oxide layer, which is unable to continue to hinder the outward diffusion of iron ions. Therefore, a thicker oxide layer is formed on the surface of Comparative Example 1, and the oxidation resistance is poor.

[0067] The steel plates of Example 2 and Comparative Example 1 were cut into small samples of 10 mm × 10 mm, and then their oxide layers were wrapped with aluminum foil for protection. They were then mechanically polished, and finally the oxide layer cross-sections were scanned using a field emission scanning electron microscope. Figure 10 and 11 shown.

[0068] from Figure 10 It can be seen that in Example 2, there is a continuous chromium-rich layer in the oxide layer after hot stamping. Since the chromium-rich carbides in the steel plate matrix in Example 2 dissolve quickly, and the carbides with higher chromium content dissolve and continuously provide sufficient chromium to the outside, a continuous and dense chromium-rich layer, i.e., a continuous Cr2O3 layer, is formed. This layer lasts until the hot stamping is completed, thereby improving the oxidation resistance of the hot stamped steel. Figure 11 It can be seen that a thicker iron-rich layer and a discontinuous chromium-rich layer are formed in the oxide layer of Comparative Example 1 after hot stamping. This is because there is no chromium-rich carbide in Comparative Example 1, so the dense chromium-rich layer cannot be maintained for a long time during the hot stamping heating process, resulting in defects in the oxide layer. As a result, iron ions continue to diffuse outward to form a thicker iron-rich oxide layer with poor antioxidant performance.

[0069] The steel plates of Example 2 and Comparative Examples 1 and 2 were first cut into small samples of 10 mm × 10 mm, and then their oxide layers were wrapped with aluminum foil for protection. They were then mechanically polished and etched with nitric alcohol. Finally, the microstructures were photographed using a field emission scanning electron microscope. Figure 12 As shown. Figure 12It can be seen that the microstructure of Example 2 after hot stamping is martensite, residual carbides, and retained austenite, while the microstructure of Comparative Examples 1 and 2 after hot stamping is mainly martensite without obvious carbides. This indicates that the small amount of Cr-rich carbides initially formed in Comparative Example 2 have all dissolved near the interface between the oxide layer and the substrate, and are unable to continuously provide chromium to the outside, making it impossible for the externally formed Cr2O3 layer to be maintained during the hot stamping heating period. As a result, iron ions diffuse outward, forming a loose oxide layer with poor oxidation resistance.

[0070] In summary, the method of the present invention must meet two essential technical features to form a dense oxide layer during the high-temperature austenitization hot stamping step. First, a pretreatment step must ensure that the temperature is maintained at 650°C to 780°C for 8 to 48 hours, forming Cr-rich carbides that serve as a reservoir for chromium. Second, the Cr-rich carbides must have a grain size primarily above 120 nm and a Cr content concentrated between 30 wt.% and 50 wt.%. This chromium reservoir must contain a certain Cr-rich content to ultimately ensure a continuous (and persistent) dense Cr2O3 layer during high-temperature austenitization. If the Cr content in the Cr-rich carbides does not reach 30 wt.%, a dense Cr2O3 layer cannot be formed, and the high-temperature oxidation resistance cannot be improved. Meeting these two essential technical features allows the chromium-rich carbides formed during the hot stamping heating process to dissolve, allowing chromium to diffuse outward from the substrate, forming a continuous, dense Cr2O3 layer. Since Cr-rich carbides act as a reservoir of chromium, they can continuously provide chromium to diffuse outward from the matrix, thereby maintaining the continuous growth of a continuous and dense Cr2O3 layer instead of forming defects and converting into oxides with poor protective properties. This protective layer can effectively inhibit the mutual diffusion of iron and oxygen, thereby effectively improving the high-temperature oxidation resistance of Cr-Si alloy hot stamping steel.

[0071] After the steel plates of each embodiment and comparative example were processed according to the hot stamping process, the mechanical properties such as yield strength, tensile strength, and elongation were tested in accordance with national standards; the thickness of the oxide layer of each embodiment and comparative example was measured using a field emission scanning electron microscope. The specific test data are shown in Table 5.

[0072] Table 5 Test data of each embodiment and comparative example

[0073] As shown in Table 5, the steel plates in Examples 1 to 4 of the present invention achieved yield strength ≥1200 MPa, tensile strength ≥1400 MPa, total elongation ≥6.0%, and bending angle ≥65° after hot stamping, meeting national standards. This demonstrates that the mechanical properties of the pretreated steel plates after hot stamping still meet the performance requirements of high-strength automotive steel. Furthermore, the oxide layer thickness was reduced by 98.1% compared to Comparative Example 1 and by 95.8% compared to Comparative Example 2, demonstrating that this method can achieve lightweighting in automobiles.

[0074] In summary, the present invention provides a method for improving the high-temperature oxidation resistance of Cr-Si alloy hot stamping steel with low cost, simple process and easy industrialization. The plate for hot stamping is pretreated by a heat treatment process to form chromium-rich carbide, the size of the Cr-rich carbide reaches more than 120nm, and the chromium content in the carbide is mainly in the range of 30wt.%~50wt.%. At this time, the formed chromium-rich carbide will dissolve in the subsequent hot stamping heating process, and chromium will quickly diffuse to the steel surface to form a continuous and dense Cr2O3 layer, and the chromium-rich carbide with a higher content can continuously provide chromium to the outside, maintaining the density of the Cr2O3 oxide layer during the hot stamping time. The oxide layer can effectively hinder the mutual diffusion of iron and oxygen, thereby improving the high-temperature oxidation resistance of the Cr-Si alloy hot stamping steel, and an oxide layer of no more than 1μm is formed on the surface under air conditions.

[0075] The above description is only a preferred embodiment of the present invention, and the scope of protection of the present invention is not limited to the above embodiment. For those skilled in the art, any modification, equivalent replacement, improvement, etc. made without departing from the technical concept of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for improving the high temperature oxidation resistance of Cr-Si alloy hot stamping steel, characterized in that: The following steps are involved: Step 1: Pre-treat the Cr-Si alloy hot stamping steel by keeping the Cr-Si alloy hot stamping steel at a temperature of 650°C to 780°C for 8 hours to 48 hours until the proportion of Cr-rich carbide grains with a size of more than 120 nm in the Cr-Si alloy hot stamping steel matrix is greater than 70%, and the Cr content in the Cr-rich carbide is concentrated in the range of 30 wt.% to 50 wt.%, and then take it out and cool it to room temperature in air; Step 2: Grind away the oxide layer and decarburized layer formed on the surface of the Cr-Si alloy hot stamping steel during the pretreatment process; Step 3: In an air environment, the Cr-Si alloy hot stamping steel obtained after grinding in step 2 is heated and then kept warm. At this time, the Cr-rich carbide dissolves, and Cr diffuses from the matrix to the surface to form a continuous and dense Cr2O3 layer; until the hot stamping steel is completely austenitized, it is then transferred to a stamping die for stamping and is kept in a pressure-holding state. Finally, it is placed in a heat treatment furnace for heat preservation to obtain hot stamping steel.

2. The method for improving the high temperature oxidation resistance of Cr-Si alloy hot stamping steel according to claim 1, characterized in that: In terms of mass percentage, the Cr-Si alloy hot stamping steel contains the following alloy components: C: 0.15~0.35%, Mn: 0.8~3.2%, Si: 0.6~3.8%, Cr: 1.5~3.9%, Nb: 0.01~0.05%, S: <0.01%, P: <0.015%, Al: 0.01~0.05%, V: 0.01~0.05%, Ti: 0.01~0.03%, Cu: 0.05~0.15%, and the balance is Fe and other inevitable impurities.

3. The method for improving the high temperature oxidation resistance of Cr-Si alloy hot stamping steel according to claim 2, characterized in that: The Cr-Si alloy hot stamping steel further comprises the following alloy components in percentage by mass: Y: 0.1-0.3% or Ce: 0.1-0.5%.

4. The method for improving the high temperature oxidation resistance of Cr-Si alloy hot stamping steel according to claim 1, characterized in that: Step 1 includes steelmaking, continuous casting, hot rolling and pickling processes.

5. The method for improving the high temperature oxidation resistance of Cr-Si alloy hot stamping steel according to claim 1, characterized in that: The microstructure of the steel plate obtained in step 1 is recrystallized ferrite and chromium-rich carbides.

6. The method for improving the high temperature oxidation resistance of Cr-Si alloy hot stamping steel according to claim 1, characterized in that: The heating temperature of the insulation after heating in step 3 is 900℃~950℃, and the insulation time is 3min~10min; the stamping die temperature in step 3 is 730℃~850℃; the holding time in step 3 is 9s~13s, the pressure is 3MPa~25MPa, and the cooling rate is 5℃ / s~50℃ / s; the insulation temperature in the heat treatment furnace in step 3 is 170℃~200℃, and the insulation time is 18min~25min.

7. The method for improving the high temperature oxidation resistance of Cr-Si alloy hot stamping steel according to claim 1, characterized in that: The microstructure of the Cr-Si alloy hot stamping steel obtained in step 3 is martensite, residual carbides and retained austenite.

8. The method for improving the high temperature oxidation resistance of Cr-Si alloy hot stamping steel according to claim 1, characterized in that: Under the condition of no protective atmosphere, the Cr-Si alloy hot stamping steel obtained in step 3 has a yield strength of ≥1200 MPa, a tensile strength of ≥1400 MPa, a total elongation of ≥6%, and a bending angle of ≥65°.

9. The method for improving the high temperature oxidation resistance of Cr-Si alloy hot stamping steel according to claim 1, characterized in that: Under the condition of no protective atmosphere, the oxide layer thickness of the Cr-Si alloy hot stamping steel obtained in step 3 is ≤1.0 μm.

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