Method for improving high-temperature oxidation resistance of cr-si alloy hot stamping steel

Abstract

The application relates to a method for improving the high-temperature oxidation resistance of Cr-Si alloy hot stamping steel, which comprises the following steps: pretreating the Cr-Si alloy hot stamping steel, keeping the Cr-Si alloy hot stamping steel at a temperature of 650 DEG C to 780 DEG C for 8h to 48h, so that the proportion of Cr-rich carbide grain sizes in the Cr-Si alloy hot stamping steel matrix which are larger than 120nm is greater than 70%, the Cr content in the Cr-rich carbide is concentrated in the range of 30wt.% to 50wt.%, and the hot stamping steel is cooled to room temperature; polishing off the oxidation layer and the decarburization layer formed on the surface; keeping the hot stamping steel at a temperature in an air environment, at this time, the Cr-rich carbide is dissolved, Cr diffuses from the matrix to the surface, a continuous and dense Cr2O3 layer is formed; until the hot stamping steel is completely austenitized, is punched and formed, is kept at a pressure, and is kept at a temperature to obtain the hot stamping steel. The application solves the serious high-temperature oxidation problem in the austenitizing process and reduces the production cost.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of hot stamping, and particularly relates to a method for improving high-temperature oxidation resistance of Cr-Si alloy hot stamping steel. BACKGROUND

[0002] Automobile lightening is an important development direction of automobiles and is also a major measure for realizing energy saving, emission reduction and consumption reduction in the world. Application of hot stamping components of super-high-strength steel can reduce the weight of the automobile while ensuring the strength and safety of the automobile body, and is an important way to realize automobile lightening. The hot stamping forming process is a process of using the principle of metal hot plastic forming to realize quenching heat treatment of the sheet during forming, improve the forming performance of the material, and greatly expand the application range of high-strength super-high-strength steel in automobile parts. In the hot stamping forming process, the hot stamping forming steel is heated to austenitization, kept at the austenite temperature interval for a period of time, and then quickly transferred to the hot stamping die for forming and quenching. In order to realize austenitization, the temperature is generally 900 DEG C to 950 DEG C, and the holding time is 3 min to 10 min. At such a high temperature, high-purity inert gas is generally introduced for protection during the heating process, and a high-tightness heating furnace is required to avoid serious high-temperature oxidation. However, in the actual production process, it is difficult to control and realize high-purity inert gas and high-tightness heating furnace. And at high temperature, the steel matrix and alloying elements are extremely easy to react with oxygen, carbon dioxide and moisture in the air to form thick oxide skin and cause serious oxidation. Therefore, this will cause the problem of unstable surface quality of the parts obtained in the actual production process. At present, there are mainly two methods to solve the problems of mold damage and surface precision reduction caused by serious oxidation of hot stamping steel during hot stamping: one is to use plating technology, and the other is to use non-plating technology. However, the plating technology has a complex process; in the non-plating technology, high-purity protective atmosphere needs to be introduced, and the unstable atmosphere in the furnace is easy to cause serious oxidation to form loose and thick oxide skin, and the surface quality cannot be guaranteed to be stable.

[0003] A method for improving the high-temperature oxidation resistance of Cr-Si alloy hot forming steel is disclosed in Chinese Patent CN117551852A, which first uses a pre-oxidation technique in air to achieve the technical effect of reducing the thickness of the oxide skin, thereby achieving lightweight. However, there are two problems in actual application. First, it has poor compatibility with existing industrial production. Second, its applicable steel composition range is small, limiting its scope of use. To solve the first problem, the existing process usually supplies cold-rolled plates before austenitizing at 900-950°C to facilitate subsequent processes (cold rolling and cutting). However, CN117551852A uses a pre-oxidation temperature of 700-820°C for 1-5 minutes to achieve a dense pre-oxidation layer. The short time of 1-5 minutes only achieves hot rolling of the steel plate without a long annealing holding step (annealing holding effect: softening the workpiece and improving residual stress, etc.), resulting in high hardness that is not conducive to subsequent cold rolling and cutting. However, if the steel plate in CN117551852A is subjected to a long pre-oxidation treatment (or understood as combined with the annealing process), the holding time is much longer than 5 minutes, resulting in a thick and loose pre-oxidation layer that cannot improve the oxidation resistance during the subsequent 900-950°C austenitizing hot stamping. The high-temperature oxidation resistance cannot be achieved through the surface oxidation layer. To solve the second problem, the oxidation layer produced in step 1 is mainly Fe2MnO4 and MnO2, which requires a high content of Mn in the alloy composition, limiting the use of the steel composition range.

[0004] In summary, the main problems of existing technologies are as follows: the plating technology is complex; in the non-plating technology, the oxide skin is thick, and the surface quality cannot be guaranteed; and the pre-oxidation technique in air still has room for improvement. SUMMARY

[0005] The present application aims to solve the problems of existing technologies, such as the need for high-purity protective atmosphere during heating of Cr-Si alloy hot stamping steel, the instability of the furnace atmosphere, the formation of loose and thick oxide skin, the damage to the mold during hot stamping, and the reduction of surface quality defects, as well as the poor compatibility with existing industrial production.

[0006] To solve the above technical problems, the technical solution adopted by the present application is as follows:

[0007] The present application proposes a method for improving the high-temperature oxidation resistance of Cr-Si alloy hot stamping steel, comprising the following steps:

[0008] Step 1: Pretreatment of Cr-Si alloy hot stamping steel, the Cr-Si alloy hot stamping steel is kept at a temperature of 650℃-780℃ for 8h-48h, so that the proportion of Cr-rich carbide grains in the Cr-Si alloy hot stamping steel substrate with a size of 120nm or more is greater than 70%, and the Cr content in the Cr-rich carbide is concentrated in 30wt.%-50wt.%, and then the Cr-Si alloy hot stamping steel is taken out and cooled to room temperature in air;

[0009] Step 2: Polishing off the oxide layer and decarburized layer formed on the surface of the Cr-Si alloy hot stamping steel during the pretreatment process;

[0010] Step 3: Heating and holding the Cr-Si alloy hot stamping steel obtained after polishing in step 2 in an air environment, at this time the Cr-rich carbide dissolves, Cr diffuses from the substrate to the surface, forming a continuous and dense Cr2O3 layer; until the hot stamping steel is completely austenitized, then transferred to a stamping die for stamping forming, and in a holding pressure state, finally placed in a heat treatment furnace for holding to obtain a hot stamping steel.

[0011] Further, the Cr-Si alloy hot stamping steel contains the following alloy components in 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 unavoidable impurities.

[0012] Further, the Cr-Si alloy hot stamping steel further contains the following alloy components in mass percentage: Y: 0.1-0.3% or Ce: 0.1-0.5%.

[0013] Further, the step 1 includes steelmaking, continuous casting, hot rolling and pickling processes.

[0014] Further, the steel plate obtained in step 1 has a microstructure of recrystallized ferrite and chromium-rich carbide.

[0015] Preferably, the heating temperature after heating and holding in step 3 is 900℃-950℃, and the holding time is 3min-10min; the die closing temperature for stamping in step 3 is 730℃-850℃; the time in the holding pressure state in step 3 is 9s-13s, the pressure is 3MPa-25MPa, and the cooling speed is 5℃ / s-50℃ / s; the holding temperature in the heat treatment furnace in step 3 is 170℃-200℃, and the holding time is 18min-25min.

[0016] Further, the Cr-Si alloy hot stamping steel obtained in step 3 has a structure of martensite, residual carbide and residual austenite.

[0017] Further, 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°.

[0018] Further, under the condition of no protective atmosphere, the Cr-Si alloy hot stamping steel obtained in step 3 has an oxide layer thickness of ≤1.0 μm.

[0019] In summary, compared with the prior art, the present application has the following beneficial effects:

[0020] The present application first proposes to produce chromium-rich carbide in the substrate by a pretreatment step, and to ensure the size length of the chromium-rich carbide and the chromium content in the chromium-rich carbide, so as to meet the requirements that the chromium-rich carbide acts as a repository of chromium during the subsequent high-temperature austenitization hot stamping step, the chromium-rich carbide dissolves during the high-temperature austenitization hot stamping forming heating process, the chromium in the chromium-rich carbide diffuses from the substrate to the surface, and a continuous and dense Cr2O3 layer is formed, thereby improving the high-temperature oxidation resistance of the Cr-Si alloy hot stamping steel. Specifically, the Cr-Si alloy hot stamping steel is pretreated at 650-780°C for 8-48h, and during the heat treatment process, chromium-rich carbide is formed in the Cr-Si alloy steel substrate, and as the pretreatment time is prolonged, the size of the Cr-rich carbide reaches 120 nm or more, and the chromium content in the chromium-rich carbide is mainly in the range of 30wt.% to 50wt.%. After that, the oxide layer formed on the surface of the Cr-Si alloy steel during the pretreatment process is removed. When the pretreatment step satisfies the size of the Cr-rich carbide reaching 120 nm or more, and the chromium content in the carbide is mainly in the range of 30wt.% to 50wt.%, the Cr-rich carbide acts as a repository of chromium, which can continuously provide chromium to diffuse outward from the substrate, thereby maintaining the continuous growth of the dense Cr2O3 layer, instead of forming defects and converting into oxides with poor protection. This protective layer can effectively inhibit the mutual diffusion of iron and oxygen, thereby effectively improving the high-temperature oxidation resistance of the Cr-Si alloy hot stamping steel.

[0021] 2. Compared to uncoated technologies, this invention eliminates the need for nitrogen generation equipment and sealed heating furnaces during hot stamping, simplifying the equipment required for hot stamping and reducing the environmental requirements for Cr-Si alloy hot-stamped steel. It still achieves a thin oxide layer and good surface quality, and the hot stamping process remains stable with minimal atmospheric influence. Compared to coating technologies, it significantly simplifies the process and saves costs. Compared to pre-oxidation methods, this invention requires no additional steps and can be pretreated within the existing annealing process, offering good industrial compatibility. The long holding time (8-48 hours) results in low hardness, facilitating subsequent cold rolling and cutting processes. By controlling temperature and time, this invention can obtain Cr-rich carbides with improved oxidation resistance, thus broadening the applicable composition range and providing material designers with more choices, thereby expanding its industrial applications.

[0022] 3. After pretreatment, the steel plate of the present invention can obtain an oxide layer (Cr2O3 layer) with a thickness of less than 1.0 μm within the hot stamping process window under conditions without a protective atmosphere. The obtained continuous and dense Cr2O3 layer remains dense and intact throughout the hot stamping process and can be maintained for a relatively long time. The oxide layer is not prone to cracking and peeling off during the hot stamping process, thus avoiding damage to the mold. Attached Figure Description

[0023] Figure 1 This is a flowchart of the method of the present invention;

[0024] Figure 2 This is the energy spectrum of the Cr-rich carbide after pretreatment in Example 1 of the present invention;

[0025] Figure 3 Histograms showing the grain size distribution of Cr-rich carbides in Example 1 and Comparative Example 2;

[0026] Figure 4 The distribution of particle diameter and Cr content in the Cr-rich carbides in Example 1 and Comparative Example 2 is shown in the figure.

[0027] Figure 5 The microstructures detected by field emission scanning electron microscopy after pretreatment are shown in (a) for example 1 after pretreatment, (b) for example 2 after pretreatment, (c) for comparative example 1 without pretreatment, and (d) for comparative example 2 after pretreatment.

[0028] Figure 6 The macroscopic morphology after hot stamping is shown in (a) for Example 1, (b) for Example 2, (c) for Example 3, (d) for Example 4, (e) for Comparative Example 1, and (f) for Comparative Example 2.

[0029] Figure 7 Surface topography after hot stamping, (a) is the surface topography after hot stamping of Example 2, (b) is the surface topography after hot stamping of Comparative Example 2;

[0030] Figure 8 Cross-section topography of oxide layer, wherein a is the cross-section topography of oxide layer of Example 2, b is the topography of 10,000 times magnification of the frame in a;

[0031] Figure 9 Cross-section topography of oxide layer, wherein a is the cross-section topography of oxide layer of Comparative Example 1, b is the topography of 4,000 times magnification of the frame in a;

[0032] Figure 10 Element distribution of oxide layer of Example 2;

[0033] Figure 11 Element distribution of oxide layer of Comparative Example 1;

[0034] Figure 12 Cross-section microstructure after hot stamping, (a) is the cross-section microstructure of Example 2, (b) is the cross-section microstructure of Comparative Example 1, (c) is the cross-section microstructure of Comparative Example 2. DETAILED DESCRIPTION

[0035] The application will be described in more detail below in conjunction with the embodiments and the accompanying drawings.

[0036] The heating furnace used in the embodiments is a muffle furnace; the hot stamping equipment is a hot stamping forming press; the equipment for observing the cross-section topography of the oxide scale and element analysis in the embodiments is an Ultra 55 field emission scanning electron microscope of Zeiss Company; the composition, size and content of the carbide in the substrate after pretreatment are analyzed by using a Talos F200X G2 high-resolution field emission transmission electron microscope of Czech Company.

[0037] The application provides a method for improving the high-temperature oxidation resistance of Cr-Si alloy hot stamping steel, as shown in Figure 1 The method comprises the following steps:

[0038] Step 1: pretreatment, the Cr-Si alloy hot stamping steel is pretreated, the Cr-Si alloy hot stamping steel is kept at a temperature of 650 DEG C to 780 DEG C for 8 h to 48 h, so that the proportion of Cr-rich carbide grains in the Cr-Si alloy hot stamping steel substrate with a size of more than 120 nm is greater than 70%, and the Cr content in the Cr-rich carbide is concentrated in 30 wt.% to 50 wt.%, and then taken out and cooled to room temperature in air;

[0039] The pre-treatment parameters are determined according to the hot rolling coiling temperature (650 DEG C) and the cover annealing temperature (680 DEG C) of the high-strength steel currently in industrial production as a reference, combined with the composition calculation rule and thermodynamic calculation. The formation temperature and the dissolution temperature of the single Cr-rich carbide in the application are above 460 DEG C and 780 DEG C respectively, that is, the single Cr-rich carbide can be obtained by heat treatment above 460 DEG C, and it cannot be formed above 780 DEG C. Combined with the hot rolling coiling temperature 650 DEG C in actual production, the pre-treatment temperature of the application is selected as 650 DEG C-780 DEG C, so as to be compatible with the existing process, save cost, improve the implementability, and be beneficial to implementation and popularization. Since 780 DEG C is close to the critical value, in order to avoid the influence of the critical value on the final product, 650 DEG C-750 DEG C can be preferably selected. The Cr-rich carbide is formed during the heat treatment of 650 DEG C-780 DEG C, and as the pre-treatment time is prolonged, the size of the Cr-rich carbide is ensured to be above 120 nm, and the chromium content in the carbide is mainly in the range of 30wt.%-50wt.%. The Cr-rich carbide is formed as a storage of chromium to ensure the formation of a continuous (continuous) dense Cr2O3 layer during high-temperature austenitization.

[0040] The method in the application is more in line with the existing industrial production. Since the holding time in step 1 is long, it can be well compatible with the current steel plate production, for example, the annealing process, the pre-treatment can be realized in the annealing process, therefore, it does not need to add any other process, and it is more in line with the low cost benefit. It is also beneficial to the later cold rolling and cutting process. The Cr-rich carbide with improved oxidation resistance can be obtained by controlling the temperature and time in the application, therefore, the applicable component range is wider, and it is not limited by the alloy component, providing more choices for material designers, so that the industrial application range is wider.

[0041] Step 2: polishing off the oxide layer and decarburized layer formed on the surface of the Cr-Si alloy hot stamping steel during the pre-treatment process;

[0042] The application is different from Chinese patent CN117551852A which utilizes the thin oxide layer on the surface of the alloy to realize the effect of high-temperature oxidation resistance in the high-temperature austenitization hot stamping process. The application polishes off the thick oxide layer formed on the surface, controls the proportion of the Cr-rich carbide grain size in the alloy matrix to be greater than 70% above 120 nm in step 1, and the Cr content of the carbide needs to be concentrated in 30-50wt.%, so as to meet the Cr-rich carbide as a storage of chromium in the high-temperature austenitization hot stamping process.

[0043] Step 3: After the Cr-Si alloy hot stamping steel obtained after polishing in step 2 is heated and kept, the Cr-rich carbide is dissolved, Cr diffuses from the matrix to the surface, and a continuous and dense Cr2O3 layer is formed; until the hot stamping steel is completely austenitized, the furnace does not need to be purged with protective gas; then transferred to the stamping die for stamping forming, the stamping die temperature is 730℃~850℃; the hot stamping forming treatment, the steel plate is hot stamped in the mold with internal cooling system and in the pressure holding state, the pressure holding time is 9s~13s, the pressure is 3MPa~25MPa, and the cooling speed is 5℃ / s~50℃ / s. Then put it into the heat treatment furnace to obtain the hot stamping steel. The temperature is 170℃~200℃, and the holding time is 18min~25min.

[0044] In the austenitizing process, high-purity inert gas is not needed to be purged for protection. In the present method, a large amount of Cr-rich carbide with a size of more than 120nm is formed in the steel plate matrix after pretreatment, which is dissolved during subsequent hot stamping forming heating. Since the Cr-rich carbide acts as a repository for chromium, it can continuously provide chromium for diffusion from the matrix to the surface, thereby promoting the formation of a continuous and dense Cr2O3 layer. At the same time, the growth of the oxidation layer is maintained due to the dissolution of the carbide, thereby hindering the mutual diffusion of iron and oxygen, improving the high-temperature oxidation resistance of the Cr-Si alloy hot stamping steel. Thus, the present method avoids the need for high-purity protective atmosphere in the existing process, and the unstable atmosphere in the furnace can easily cause severe oxidation to form a loose and thick oxide skin, resulting in problems such as damage to the mold and reduction of the surface quality of the steel plate during hot stamping.

[0045] The present method does not require a nitrogen making device and a sealed heating furnace during hot stamping, simplifies the equipment required for hot stamping forming, reduces the environmental requirements of the Cr-Si alloy hot stamping steel, and still obtains a thin oxidation layer and good surface quality, and the hot stamping process is stable and almost unaffected by the atmosphere.

[0046] The continuous and dense Cr2O3 layer obtained by the present method is dense and complete during the entire hot stamping process, can be maintained for a relatively long time, and the oxidation layer is not easy to crack and fall off during hot stamping, so as not to damage the mold.

[0047] Example 1

[0048] The composition of the Cr-Si alloy hot stamping steel used is shown in Table 1:

[0049] Table 1 Composition of Cr-Si alloy hot stamping steel in Example 1

[0050]

[0051] A pickled hot-rolled sheet of the alloy having the composition shown in Table 1 was cut. A small steel sheet having a size of 160 x 200 x 1.9 mm was processed using a metal shearing machine. 2

[0052] Step 1: Pre-treatment, the pickled hot-rolled sheet was put into a heating furnace at a temperature of 670°C, and after holding for 8 h, it was taken out and cooled to room temperature in air.

[0053] Step 2: Surface treatment, the surface of the pre-treated steel sheet was treated using a grinding machine, and the oxide layer formed during pre-treatment was polished off, and the steel sheet was polished from 1.9 mm to 1.4 mm, and the oxide layer and decarburized layer were removed.

[0054] Step 3: Hot stamping, the polished steel sheet was heated to 950°C, and there was no protective atmosphere in the heating furnace, and the steel sheet was heated in air for 6 min to completely austenitize the steel sheet, and then it was transferred to a stamping die to perform stamping forming, the stamping die temperature was 800°C, 7 MPa pressure holding cooling, the cooling rate was 40°C / s, and then it was put into a heat treatment furnace at 170°C for 20 min to obtain a hot stamped steel.

[0055] Example 2

[0056] The composition of the Cr-Si alloy hot stamped steel used is shown in Table 2:

[0057] Table 2 Composition of Cr-Si alloy hot stamped steel in Example 2

[0058]

[0059] A pickled hot-rolled sheet of the alloy having the composition shown in Table 2 was cut. A small steel sheet having a size of 160 x 200 x 1.9 mm was processed using a metal shearing machine.

[0060] Step 1: Pre-treatment, the pickled hot-rolled sheet was put into a heating furnace at a temperature of 700°C, and after holding for 16 h, it was taken out and cooled to room temperature in air.

[0061] Step 2: Surface treatment, the surface of the pre-treated steel sheet was treated using a grinding machine, and the oxide layer formed during pre-treatment was polished off, and the steel sheet was polished from 1.9 mm to 1.4 mm, and the oxide layer and decarburized layer were removed.

[0062] Step 3: Hot stamping, the polished steel sheet was heated to 950°C, and there was no protective atmosphere in the heating furnace, and the steel sheet was heated in air for 10 min to completely austenitize the steel sheet, and then it was transferred to a stamping die to perform stamping forming, the stamping die temperature was 800°C, 7 MPa pressure holding cooling, the cooling rate was 40°C / s, and then it was put into a heat treatment furnace at 170°C for 20 min to obtain a hot stamped steel.

[0063] Example 3 ​

[0064] The composition of the Cr-Si alloy hot stamping steel used is shown in Table 3:

[0065] Table 3 Composition of Cr-Si alloy hot stamping steel in Example 3

[0066]

[0067] Pickling hot rolled sheets of the alloy with the composition shown in Table 3 were cut. Small steel sheets with dimensions of 160 x 200 x 1.9 mm were processed using a metal shearing machine. 2

[0068] Step 1 : Pretreatment, the pickling hot rolled sheets were put into a heating furnace with a temperature of 750 °C, and after holding for 24 h, they were taken out and cooled to room temperature in air.

[0069] Step 2: Surface treatment, the surface of the pretreated steel sheets was treated using a grinding machine, and the oxide layer formed during pretreatment was polished off, the steel sheets were polished from 1.9 mm to 1.4 mm, and the oxide layer and decarburized layer were removed.

[0070] Step 3: Hot stamping, the polished steel sheets were heated to 950 °C, and there was no protective atmosphere in the heating furnace, and they were heated in air for 10 min to completely austenitize the steel sheets, and then they were transferred to a stamping die for stamping forming, the stamping die temperature was 800 °C, 7 MPa pressure holding cooling, the cooling rate was 40 °C / s, and then they were put into a heat treatment furnace at 170 °C for 20 min to obtain the hot stamping steel.

[0071] Example 4

[0072] The composition of the Cr-Si alloy hot stamping steel used is shown in Table 4:

[0073] Table 4 Composition of Cr-Si alloy hot stamping steel in Example 4

[0074]

[0075] Pickling hot rolled sheets of the alloy with the composition shown in Table 4 were cut. Small steel sheets with dimensions of 160 x 200 x 1.9 mm were processed using a metal shearing machine. 2

[0076] Step 1 : Pretreatment, the pickling hot rolled sheets were put into a heating furnace with a temperature of 700 °C, and after holding for 48 h, they were taken out and cooled to room temperature in air.

[0077] Step 2: Surface treatment, the surface of the pretreated steel sheets was treated using a grinding machine, and the oxide layer formed during pretreatment was polished off, the steel sheets were polished from 1.9 mm to 1.4 mm, and the oxide layer and decarburized layer were removed.

[0078] ​​Step 3: hot stamping, the ground steel plate is heated to 950℃, and there is no protective atmosphere in the heating furnace, heated in air for 10 min to make the steel plate completely austenitized, then transferred to the stamping die for stamping forming, the stamping die temperature is 800℃, 7 MPa pressure holding cooling, the cooling rate is 40℃ / s, then put into the heat treatment furnace at 170℃ for 20 min to obtain the hot stamped steel.

[0079] Comparative Example 1

[0080] The difference between Comparative Example 1 and Example 1 is that the steel plate is not pretreated by Step 1 of Example 1.

[0081] After the steelmaking, continuous casting, hot rolling and pickling process steps, the surface treatment is directly performed. The pickled hot-rolled plate of the alloy with the composition shown in Table 1 is cut. A small steel plate with a size of 160x200x1.9mm 2 is processed by a metal shearing machine.

[0082] Surface treatment: the surface of the pickled hot-rolled steel plate is treated by a grinding machine, and the steel plate is ground from 1.9mm to 1.4mm to make the surface state consistent with that of Examples 1 to 4.

[0083] Hot stamping: the steel plate is heated to 950℃, and there is no protective atmosphere in the heating furnace, heated in air for 10 min to make the steel plate completely austenitized, then transferred to the stamping die for stamping forming, the stamping die temperature is 800℃, 4 MPa pressure holding cooling, the cooling rate is 40℃ / s, then put into the heat treatment furnace at 170℃ for 20 min to obtain the hot stamped steel.

[0084] Comparative Example 2

[0085] The main difference between Comparative Example 1 and Example 1 is that the holding time in Step 1 pretreatment is less than 8h.

[0086] The pickled hot-rolled plate of the alloy with the composition shown in Table 1 is cut. A small steel plate with a size of 160x200x1.9mm 2 is processed by a metal shearing machine.

[0087] Step 1 pretreatment: the pickled hot-rolled plate is put into a heating furnace at a temperature of 700℃, and cooled to room temperature in air after holding for 1h.

[0088] Surface treatment: the surface of the pretreated steel plate is treated by a grinding machine, and the oxide layer formed by pretreatment is ground off, the steel plate is ground from 1.9mm to 1.4mm to remove the oxide layer and decarburized layer.

[0089] Hot stamping: the steel plate after polishing treatment is heated to 950℃, and there is no protective atmosphere in the heating furnace, heated in air for 10 min to make the steel plate fully austenitized, then transferred to the stamping die for stamping forming, the stamping die temperature is 800℃, 7MPa pressure holding cooling, the cooling rate is 40℃ / s, then put into the heat treatment furnace at 170℃ for 20min to obtain the hot stamping steel.

[0090] The detection experiment and data analysis are as follows:

[0091] The steel plates in Example 1 and Comparative Example 2 are cut into small samples of 10mmx10mm, and then the samples are mechanically polished to about 50 microns, and then prepared into samples suitable for transmission electron microscopy by electrolytic polishing. The sample in Example 1 is subjected to element point scanning by high-resolution field emission transmission electron microscopy equipped with an energy dispersive X-ray spectrometer, and the results are shown in Figure 2 . Then the carbide size at different positions of Example 1 and Comparative Example 2 is measured by transmission electron microscopy and counted as shown in Figure 3 , and the Cr content in the carbide is obtained by element point scanning as shown in Figure 4 .

[0092] Figures 2 to 4 The composition, content and size of the carbide after pretreatment of the embodiments and comparative examples of the application are shown in Figure 2 . It can be seen from Figure 3 and Figure 4 that the carbide after pretreatment is rich in Cr. It can be seen from Figure 3 and Figure 4 that the size of the carbide in Example 1 is mainly distributed in the range of 120nm-240nm, and the proportion of more than 120nm is more than 70%, and the Cr content in the carbide is mainly more than 30wt. % and the proportion is almost 100%. In Comparative Example 2, the holding time is less than 8h, and the size of the carbide is mainly distributed in the range of 40nm-160nm, and the proportion of more than 120nm is less than 50%. And the Cr content in the carbide is distributed between 5wt.% and 50wt.%, mainly below 30wt.%, and the proportion above 30wt.% is only 50%. The size of the carbide and the Cr content are obviously 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 forming process. It cannot solve the technical problem of high temperature oxidation resistance.

[0093] The steel plates in Example 1, Example 2, Comparative Example 1 and Comparative Example 2 are cut into small samples of 10mmx10mm, and then polished by mechanical polishing, then etched with nitric acid alcohol to obtain metallographic samples, and finally the microstructure is photographed by field emission scanning electron microscope, as shown in Figure 5 .

[0094] FromFigure 5 It can be seen that the microstructure of Examples 1 and 2 has a large amount of chromium-rich carbide after the pretreatment process, and the chromium-rich carbide is distributed uniformly in the matrix. Figure 5 (a) and Figure 5 (b) It can be seen that the chromium-rich carbide of the matrix in Example 2 is larger than that of Example 1, indicating that the size of the carbide becomes larger as the pretreatment time is prolonged. Figure 5 (c) It can be seen that Comparative Example 1 has no carbide. It indicates that the pretreatment step of the present application is the key to forming chromium-rich carbide, and the holding time at a temperature of 650°C to 780°C for 8h to 48h determines whether chromium-rich carbide is generated. Figure 5 (d) It can be seen that Comparative Example 2 can form chromium-rich carbide, but the amount is small and the distribution is uneven. It indicates that the holding time in the pretreatment step determines the amount of chromium-rich carbide generated, and must meet the condition that the proportion of chromium-rich carbide grain size above 120nm is greater than 70%, and the Cr content in the carbide is concentrated in 30wt.% to 50wt.%. Only in this way can the Cr content in the carbide be ensured, which is beneficial to the formation of a continuous and dense Cr2O3 layer in the later stage, so as to solve the technical problem of high-temperature oxidation resistance.

[0095] The steel plates of Examples 1 to 4, Comparative Examples 1 and 2 were photographed by using a special camera, so as to obtain the macroscopic photographs as shown in Figure 6 .

[0096] From the macroscopic surfaces of Figure 6 (a), Figure 6 (b), Figure 6 (c) and Figure 6 (d), it can be seen that the surfaces of Examples 1 to 4 are golden oxide scales, which are thin oxide layers; from Figure 6 (e) and Figure 6 (f), it can be seen that the surfaces of Comparative Examples 1 and 2 are black oxide scales, which are thick oxide layers.

[0097] The steel plates of Example 2 and Comparative Example 2 were cut into small samples of 10mm x 10mm, and then the surface topography was photographed by using a field emission scanning electron microscope, as shown in Figure 7 . Figure 7 The upper right corner of the insert in (a) is a topography diagram after local magnification of 10,000 times at a certain position on the surface.

[0098] Figure 7 From Figure 7 (a), it can be seen that a dense oxide layer is formed on the surface of Example 2, (b) can be seen that a loose and porous oxide layer is formed on the surface of Comparative Example 2. It indicates that the Cr content in the carbide is insufficient, which cannot form a dense oxide layer and cannot solve the technical problem of high-temperature oxidation resistance.

[0099] The steel plates of Example 2 and Comparative Example 1 were cut into small samples of 10 mm x 10 mm, then the oxide layers were wrapped with aluminum foil for protection, and then polished by mechanical polishing, and finally the morphology was photographed by field emission scanning electron microscopy, as shown in Figure 8 and Figure 9 .

[0100] As can be seen from Figure 8 and Figure 9 , the thickness of the oxide layer formed in Example 2 is 0.12 μm, and the thickness of the oxide layer of Comparative Example 1 is 5.36 μm. The chromium-rich carbides in Example 2 quickly dissolve during the heating process of hot forming, and the high chromium content quickly forms a continuous chromium-rich oxide layer on the surface, hindering the outward diffusion of iron ions. At the same time, the chromium-rich carbides with high chromium content continuously provide chromium for the continuous and dense chromium-rich layer, so that the continuous and dense Cr2O3 layer remains intact during the heating process of hot stamping forming, thereby improving the high-temperature oxidation resistance of the hot stamping forming steel. In Comparative Example 1, there are no chromium-rich carbides, and the initially formed continuous Cr2O3 layer cannot be maintained for a long time during the heating process of hot stamping forming, and defects appear in the oxide layer, so that the outward diffusion of iron ions is not hindered, and therefore a thick oxide layer is formed on the surface of Comparative Example 1, and the oxidation resistance is poor.

[0101] The steel plates of Example 2 and Comparative Example 1 were cut into small samples of 10 mm x 10 mm, then the oxide layers were wrapped with aluminum foil for protection, and then polished by mechanical polishing, and finally the cross-section of the oxide layer was scanned by field emission scanning electron microscopy, as shown in Figure 10 and 11 .

[0102] As can be seen from Figure 10 , there is a continuous chromium-rich layer in the oxide layer after hot stamping of Example 2. Because the chromium-rich carbides in the steel plate matrix of Example 2 quickly dissolve, and the chromium-rich carbides with high chromium content dissolve and continuously provide sufficient chromium outward, a continuous and dense chromium-rich layer, i.e. a continuous Cr2O3 layer, is formed, which continues until the end of hot stamping forming, thereby improving the oxidation resistance of the hot stamping forming steel. As can be seen from Figure 11 , a thick iron-rich layer and a discontinuous chromium-rich layer are formed in the oxide layer after hot stamping of Comparative Example 1. This is because there are no chromium-rich carbides in Comparative Example 1, so that the dense chromium-rich layer cannot be maintained for a long time during the heating process of hot stamping forming, resulting in defects in the oxide layer, so that iron ions continuously diffuse outward to form a thick iron-rich oxide layer, and the oxidation resistance is poor.

[0103] The steel plates of Example 2, Comparative Examples 1 and 2 were first cut into small samples of 10 mm x 10 mm, then the oxide layers were wrapped with aluminum foil for protection, followed by mechanical polishing and etching with nitric acid alcohol, and finally microstructure was photographed by field emission scanning electron microscopy, as shown in Figure 12 Figure 12 It can be seen that the microstructure of Example 2 after hot stamping forming is martensite and residual carbide and residual austenite, while the microstructure of Comparative Examples 1 and 2 after hot stamping forming is mainly martensite, without obvious carbide, which indicates that the small amount of Cr-rich carbide initially formed in Comparative Example 2 has been completely dissolved near the interface between the oxide layer and the substrate, and cannot continuously provide chromium outward, so that the Cr2O3 layer formed outside cannot be maintained during the heating of hot stamping, and thus iron ions diffuse outward to form a loose oxide layer, with poor oxidation resistance.

[0104] In summary, the dense oxide layer formed during the high-temperature austenitizing hot stamping step in the method of the present application needs to meet two necessary technical features, one is to form Cr-rich carbide by the pretreatment step at a temperature of 650°C to 780°C and for 8h to 48h, and the Cr-rich carbide serves as a chromium reservoir; the other is to ensure that the grain size of the Cr-rich carbide is mainly above 120 nm, and the Cr content in the Cr-rich carbide is concentrated in 30wt.% to 50wt.%, i.e. the chromium reservoir needs to meet a certain amount of Cr enrichment, in order to finally ensure the formation of a continuous (continuous) dense Cr2O3 layer during high-temperature austenitizing. If the Cr content in the Cr-rich carbide does not reach 30wt.%, a dense Cr2O3 layer cannot be formed, i.e. the high-temperature oxidation resistance cannot be improved. Meeting the above two necessary technical features, the Cr-rich carbide formed by pretreatment dissolves during the heating process of hot stamping forming, and chromium diffuses outward from the matrix to form a continuous and dense Cr2O3 layer. Since the Cr-rich carbide serves as a chromium reservoir, it can continuously provide chromium to diffuse outward from the matrix, thereby maintaining the continuous growth of the continuous and dense Cr2O3 layer, rather than forming defects and converting into poor protective oxides. This protective layer can effectively inhibit the mutual diffusion of iron and oxygen, thereby effectively improving the high-temperature oxidation resistance of the Cr-Si alloy hot stamping steel.

[0105] After the steel plates of each example and comparative example were treated according to the hot stamping forming process, the mechanical properties such as yield strength, tensile strength and elongation were detected according to the national standard; the thickness of the oxide layer of each example and comparative example was measured by field emission scanning electron microscopy, and the specific detection data are shown in Table 5.

[0106] Table 5: Detection data of each example and comparative example

[0107]

[0108] ​As can be seen from Table 5, the steel plates of the embodiments 1 to 4 of the present application have yield strength ≥ 1200 MPa, tensile strength ≥ 1400 MPa, total elongation ≥ 6.0%, and bending angle ≥ 65° after hot stamping forming, meeting the national standards. This shows that the mechanical properties of the steel plates after pretreatment after hot stamping forming can still meet the use performance of high-strength steel for automobiles. And the thickness of the oxidation layer is reduced by 98.1% compared with the comparative example 1 and by 95.8% compared with the comparative example 2, indicating that the method can achieve the light weight of automobiles.

[0109] In summary, the present application 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 hot stamping forming plate is pretreated by using a heat treatment process to form chromium-rich carbide, the size of the chromium-rich carbide is more than 120 nm, and the chromium content in the carbide is mainly in the range of 30wt.% to 50wt.%. At this time, the chromium-rich carbide formed will dissolve in the subsequent hot stamping heating process, and chromium will rapidly diffuse to the surface of the steel to form a continuous and dense Cr2O3 layer, and the high content of chromium-rich carbide can continuously provide chromium outward, maintaining the density of the Cr2O3 oxidation layer within the hot stamping time. The oxidation 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 forming an oxidation layer not more than 1 μm on the surface under air conditions.

[0110] The above only describes the preferred embodiments of the present application, and the protection scope of the present application is not limited to the above-described embodiments. Any modification, equivalent replacement, improvement, etc. made by those skilled in the art without departing from the technical concept of the present application should be included in the protection scope of the present application.

Claims

1. A method for improving the high temperature oxidation resistance of a Cr-Si alloy hot stamping steel, characterized in that, The method comprises the following steps: Step 1: Pre-treatment of the Cr-Si alloy hot stamping steel, the Cr-Si alloy hot stamping steel is kept at a temperature of 650-780 DEG C for 8-48 hours, the proportion of Cr-rich carbide grains with a size of more than 120 nm in the Cr-Si alloy hot stamping steel substrate is greater than 70%, and the Cr content in the Cr-rich carbide is concentrated in 30-50 wt.%, and then the Cr-Si alloy hot stamping steel is taken out and cooled to room temperature in air; the Cr-Si alloy hot stamping steel comprises the following alloy components in 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; Step 2: Polishing off the oxide layer and decarburized layer formed on the surface of the Cr-Si alloy hot stamping steel in the pre-treatment process; Step 3: Heating and keeping the Cr-Si alloy hot stamping steel obtained after polishing in step 2 in an air environment, at this time, the Cr-rich carbide is dissolved, Cr diffuses from the substrate to the surface, and a continuous and dense Cr2O3 layer is formed; until the hot stamping steel is completely austenitized, then transferred to a stamping die for stamping forming, and kept in a pressure-keeping state, and finally put into a heat treatment furnace for keeping.

2. The method for improving high-temperature oxidation resistance of Cr-Si alloy hot stamping steel according to claim 1, characterized in that, The Cr-Si alloy hot stamping steel further comprises the following alloy components in mass percentage: Y: 0.1-0.3% or Ce: 0.1-0.5%.

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

4. The method for improving high-temperature oxidation resistance of Cr-Si alloy hot stamping steel according to claim 1, characterized in that, The steel plate obtained in step 1 has a microstructure of recrystallized ferrite and chromium-rich carbide.

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

6. The method for improving high-temperature oxidation resistance of Cr-Si alloy hot stamping steel according to claim 1, characterized in that, The Cr-Si alloy hot stamping steel obtained in step 3 has a structure of martensite, residual carbide and residual austenite.

7. The method for improving 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 DEG.

8. The method for improving 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 thickness of the oxide layer of the Cr-Si alloy hot stamping steel obtained in step 3 is ≤1.0 μm.

Images