Low-temperature tempering method for hot stamping steel

Abstract

The invention belongs to the technical field of material heat treatment, and particularly relates to a low-temperature tempering method of hot stamping steel, which comprises the following steps: S1, heating and preserving heat of an annealed steel plate to completely austenitize the steel plate; s2, quenching the steel plate treated in the step S1; s3, the steel plate treated in the step S2 is subjected to first-step tempering; s4, machining the steel plate treated in the step S3; and S5, the steel plate treated in the step S4 is subjected to second-step tempering, and the hot stamping steel is obtained. Wherein the process parameters of the second-step tempering are determined by a baking varnish process, and the process parameters of the first-step tempering are obtained by calculating the theoretical optimal tempering process parameters and the process parameters of the second-step tempering. Compared with the prior art, the invention solves the problem that the tempering effect of the stoving varnish process cannot be fully utilized and better comprehensive mechanical properties cannot be achieved in the prior art. According to the scheme, by improving a low-temperature tempering strategy, the mechanical property in the service stage is improved, and meanwhile the cracking risk before paint baking is reduced.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of material heat treatment, and particularly relates to a low-temperature tempering method of hot stamping steel. BACKGROUND

[0002] With the development of the automobile industry, the requirements of the vehicle body for light weight and safety are gradually increasing, and the use of hot stamping steel is an important way to achieve the above two requirements. In the traditional hot stamping process, boron steel is heated to 30-50 DEG C above Ac3 for 3-5 minutes to achieve complete austenitization, and then quickly transferred to the mold for forming and quenching. The microstructure of the hot stamping part after quenching is mainly martensite, which has high strength and is widely used in automobile safety parts such as anti-collision, A column, B column, etc.

[0003] With the development of hot stamping technology, the number of steel grades suitable for hot stamping technology gradually increases. Among them, medium manganese steel is used as a representative, which shows brittle fracture characteristics after hot stamping, and needs to be additionally tempered at low temperature to eliminate brittleness and improve elongation, thereby achieving excellent comprehensive mechanical properties. At present, a processing method is proposed in the patent with publication number CN115058650A, which uses parameters of 150-250 DEG C / 5-60 min (170-180 DEG C / 20 min) to achieve the above-mentioned low-temperature tempering process. This tempering process actually takes advantage of the automobile baking process. For hot stamping steel that needs low-temperature tempering to eliminate brittleness, if the heat treatment of the baking process can be fully utilized to achieve low-temperature tempering, theoretically, no additional cost will be generated. However, this approach has two drawbacks: On the one hand, the baking process is at the end of the overall manufacturing process. The material that has completed heating, holding and quenching will still go through processes such as hot stamping, subsequent plastic deformation after hot stamping, storage and transportation to the automobile factory during the production of automobile parts. That is, there are processes such as punching, hole expansion and flanging between the hot stamping process and the baking process. The intermediate product of the steel plate that has not been tempered at low temperature to eliminate brittleness will show brittleness and increase the risk of cracking when these processes are performed; On the other hand, there is a long time interval between the hot stamping process and the baking process. During this period, the steel plate intermediate product remains in a brittle state, which increases the risk of hydrogen-induced cracking.

[0004] In addition, for steel materials of different hot stamping steel grades, it is difficult to meet the needs of all varieties to achieve the best tempering state by using the general baking process parameters, resulting in that the mechanical properties of some steel grades cannot fully reach the theoretical optimum by relying only on the tempering effect brought by the baking process.

[0005] CN115522021A discloses a heat treatment method for improving the hydrogen embrittlement resistance of coated hot stamping steel, which performs stepwise tempering after hot stamping to improve the plasticity and toughness of the steel; however, this scheme directly adjusts the steel material to the optimal state through the optimal tempering process after hot stamping, resulting in the complete waste of the tempering effect brought by the subsequent paint baking process (the paint baking process is to spray paint on the vehicle body in the automobile assembly plant, and then to perform heating and baking to shape and accelerate solidification). Moreover, the paint baking process (equivalent to re-tempering) in this case may cause the coarsening of carbides or the further diffusion of solid-solution carbon atoms, thereby causing slight softening (strength decrease).

[0006] CN116103472A discloses a heat treatment process for improving the low-temperature toughness of low-carbon high-strength martensitic stainless steel, which sequentially performs intermediate low-temperature tempering and tempering steps after quenching to change the initial element distribution state of the induced tempering brittleness precipitated phase (or element segregation) to avoid the precipitation of harmful precipitated phases; both are through the tempering process to heat treat the steel material to strengthen the comprehensive mechanical properties of the steel product; however, the heat treatment method proposed in this scheme is for stainless steel materials, and the tempering effect is to adjust the dislocation of the martensitic structure. For manganese steel in hot stamping, it is the process of partitioning C from martensite to austenite and eliminating interfacial residual stress. The two tempering processes to eliminate brittleness mechanisms are different. In addition, the tempering process proposed in this scheme is not in the context of automobile production and manufacturing, and does not need to consider the subsequent plastic deformation process after hot stamping. At the same time, this also means that there is no paint baking process in this scheme, which is not limited by the parameters of the paint baking process and does not need to consider the full use of the tempering effect brought by the paint baking process. Therefore, this scheme provides a heat treatment process, but based on different processing objects and application backgrounds, the existing objective conditions and target effects are obviously different, resulting in the fact that the heat treatment process is difficult to apply to the heat treatment of hot stamping steel in the automobile field.

[0007] Therefore, it is necessary to propose a heat treatment method for hot stamping steel that can fully utilize the tempering effect of the paint baking process to meet the comprehensive mechanical property requirements of hot stamping steel at a lower processing cost. SUMMARY

[0008] The purpose of the present application is to provide a low-temperature tempering method for hot stamping steel to solve at least one of the above problems, to solve the problem that the tempering effect of the paint baking process cannot be fully utilized in the prior art and to achieve better comprehensive mechanical properties (strength and plasticity product), or to ignore the high cracking risk existing in the mechanical processing process between hot stamping and paint baking by only using the paint baking process for tempering. This scheme improves the low-temperature tempering strategy to improve the mechanical properties during service while reducing the cracking risk before paint baking.

[0009] The purpose of the present application is achieved by the following technical scheme: A low-temperature tempering method of hot stamping steel, comprising the following steps: S1: heating and holding the annealed steel plate to completely austenitize the steel plate; S2: quenching the steel plate after step S1; S3: first tempering the steel plate after step S2; S4: machining the steel plate after step S3; S5: second tempering the steel plate after step S4 to obtain the hot stamping steel; wherein the process parameters of the second tempering are determined by a paint baking process, and the process parameters of the first tempering are calculated from the theoretical optimal tempering process parameters and the process parameters of the second tempering.

[0010] The specific calculation method of the process parameters of the first tempering is: changing the time and temperature of the first tempering under fixed process parameters of the second paint baking tempering (such as 180℃ / 20min), measuring the mechanical properties after the first and second tempering respectively, and establishing a data set. Based on the test results of the mechanical properties, in the process parameter range in which the first tempering can eliminate brittleness, the sample with the highest product of strength and ductility after the second tempering is selected, and the processing time and temperature of the first tempering are determined as the optimal process parameters of the first tempering.

[0011] Preferably, in step S1, the annealed steel plate is a steel plate sequentially subjected to smelting, hot forging, hot rolling, soft annealing, cold rolling and critical annealing.

[0012] Preferably, the ingot obtained by smelting comprises the following components by mass fraction: 0.13-0.18% of carbon, 6.0-9.0% of manganese and 0.2-0.3% of silicon, and the rest is Fe and inevitable impurities.

[0013] Preferably, one or more of the following are included: i) the hot forging is: heated to 1100-1250℃ for 1-2h, followed by hot forging treatment, and then cooled to room temperature; ii) the hot rolling is: first heated to 1100-1250℃ for 1-2h, then hot rolled several times at 900-1200℃, and then quenched to room temperature; iii) the soft annealing is: heated to 600-700℃ for 0.5-5h, and then cooled to room temperature; iv) the critical annealing is: heated to 600-700℃ for 10-20h, and then cooled to room temperature.

[0014] Preferably, in step S1, the heating and holding are as follows: Heating to 30-50℃ above Ac3 temperature at a heating rate of 8-15℃ / s by a vacuum heating furnace and holding for 3-5min.

[0015] Preferably, in step S2, the quenching is as follows: Quenching to room temperature by liquid assisted quenching or die quenching.

[0016] Preferably, in step S3, the first tempering is as follows: Heating to 150-220℃ and holding for 5-20min, and then cooling to room temperature.

[0017] Preferably, in step S4, the mechanical processing includes punching, hole expanding and flanging.

[0018] Preferably, in step S5, the second tempering is as follows: Heating to 170-180℃ and holding for 15-25min.

[0019] Preferably, the theoretical optimal tempering process parameters are as follows: The comprehensive mechanical properties of the hot stamping steel are optimal (the highest product of strength and ductility) after two-step tempering.

[0020] The working principle of the present application is as follows: The new hot stamping steel represented by medium-manganese steel for automobiles obtains a dual-phase structure of martensite + residual austenite after quenching; due to the fast quenching rate, there are high residual stress and high density of geometric necessary dislocations at the interface between the martensite and the residual austenite, which will cause stress-assisted phase transition under a lower applied stress, resulting in brittle fracture. During low-temperature tempering, the process of carbon partitioning from martensite to austenite occurs, which improves the stability of residual austenite and changes the fracture mode from brittle to ductile.

[0021] When the stability of residual austenite is moderate, TRIP effect can continuously occur in a wider strain range, and the best mechanical properties can be obtained. Therefore, the first low-temperature tempering parameters can be calculated by combining the best tempering parameters and the inherent baking paint heat treatment parameters in the automobile production process; thus, the first low-temperature tempering process is directly implemented in the hot stamping plant after hot stamping and before the next plastic deformation process, which on the one hand eliminates brittleness to facilitate the smooth progress of the subsequent plastic deformation process; on the other hand, after the second tempering combined with the subsequent baking paint, the best comprehensive mechanical properties can be achieved.

[0022] Compared with the prior art, the present application has the following beneficial effects: 1) The present application breaks through the dependence of traditional low-temperature tempering on fixed baking parameters, and for steel materials that have not reached the optimal tempering state after hot stamping, a first-step tempering process is introduced to further optimize the organizational state, significantly improving the comprehensive mechanical properties of the material under service conditions.

[0023] 2) By immediately implementing low-temperature tempering after hot stamping, the micro-brittleness generated during quenching is effectively eliminated, and the toughness of the material is improved, thereby significantly reducing the cracking risk during subsequent local large deformation processing (such as trimming, punching, etc.).

[0024] 3) The first-step low-temperature tempering can timely eliminate quenching stress and promote hydrogen diffusion and escape, reduce material brittleness, effectively inhibit the risk of hydrogen-induced delayed fracture caused by residual stress and hydrogen accumulation between hot stamping and baking processes, and improve the reliability and safety during the manufacturing process of parts. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 Stress-strain curves of the present application example 1 and comparative example 1, comparative example 2 after the first-step tempering; Figure 2 Stress-strain curves of the present application example 1 and comparative example 1, comparative example 2 after the second-step tempering; Figure 3 Surface morphology of the present application example 1 and comparative example 1 after the first-step tempering and die punching, wherein (a-b) is comparative example 1, and (c-d) is example 1. DETAILED DESCRIPTION

[0026] The present application will be described in detail below in conjunction with the drawings and specific examples. The present embodiment is implemented on the premise of the technical solution of the present application, and detailed implementation methods and specific operation processes are given, but the protection scope of the present application is not limited to the following examples.

[0027] In the following description, if not specifically stated, the reagents used are conventional commercially available products, and the methods used are well-known means in the art, and the remaining matters are well-known common sense.

[0028] The present application proposes a new type of low-temperature tempering method, which immediately adds a short-time low-temperature tempering after hot stamping (heating, holding and quenching) to eliminate brittleness and promote hydrogen diffusion and escape, and then completes the second-step tempering through baking at the automobile assembly plant, which can fully utilize the tempering effect of the baking process and improve the comprehensive mechanical properties during the service stage.

[0029] The specific treatment method of the hot stamping steel is as follows: Smelting: smelt certain C, Mn, Si, Fe elements according to the proportion by mass percentage, and pour the molten steel into ingots; wherein, the ingots after smelting include the following components by mass fraction: C: 0.13-0.18%, Mn: 6.0-9.0%, Si: 0.2-0.3%, and the rest is Fe and unavoidable impurities.

[0030] Hot forging: heat the ingot to 1100-1250℃ for 1-2h, then hot forging treatment and cooling to room temperature.

[0031] Hot rolling: heat the hot forged plate to 1100-1250℃ for 1-2h, hot rolling at 900-1200℃ for multiple times, and then quenching to room temperature.

[0032] Soft annealing: heat the hot rolled plate to 600-700℃ for 0.5h-5h, and then air cooling to room temperature.

[0033] Cold rolling: cold rolling the softened steel plate at room temperature.

[0034] Inter critical annealing: heat the cold rolled plate to 600-700℃ for 10-20h, and then air cooling to room temperature.

[0035] Heating and holding: using a vacuum heating furnace, heat the cold rolled annealed steel to 30-50℃ above Ac3 temperature at a heating rate of about 10℃ / s for 3-5min to achieve complete austenitization.

[0036] Transfer quenching: quickly transfer the steel plate to a forming die, and use liquid assisted quenching or die quenching to room temperature First step low temperature tempering: heat the quenched steel plate to 150-220℃ for a short time (5min-20min), and then air cooling to room temperature. After the first step low temperature tempering, the comprehensive mechanical properties of the material have not yet reached the best, but the brittleness is effectively reduced.

[0037] Hot stamping follow-up process: punch, expand, flange, etc. process the steel plate after the first step tempering, and then store and transport to the automobile assembly plant.

[0038] Second step low temperature tempering: at the automobile assembly plant, realize the second step low temperature tempering through the baking process (heat to 170-180℃, hold for about 20min). After the second step low temperature tempering, the comprehensive mechanical properties are further enhanced, the optimal strength and plastic product is achieved, and then used in the service stage.

[0039] The specific process parameters of the first low-temperature tempering are calculated from the theoretical optimal tempering process parameters and the second low-temperature tempering process parameters, and are specifically: changing the time and temperature of the first tempering under the fixed second baking tempering process parameters (such as 180℃ / 20min), measuring the mechanical properties after the first and second tempering, respectively, and establishing a data set. Based on the test results of the mechanical properties, within the process parameter range in which the brittleness has been eliminated in the first tempering, the sample with the highest strength and ductility product after the second tempering is selected, and the processing time and temperature of the first tempering are determined as the optimal process parameters of the first tempering.

[0040] Example 1 This example takes hot stamping manganese steel as a representative of hot stamping steel that needs low-temperature tempering, and the specific steps are: (1) According to the hot stamping manganese steel ratio of raw materials, the prepared raw materials are put into an electric arc furnace for high-temperature melting treatment to obtain a steel liquid. After homogenization, the steel liquid is subjected to casting, hot forging, hot rolling, softening annealing, cold rolling, and critical annealing to obtain a cold-rolled annealed manganese steel with a thickness of 1.5mm. According to the mass fraction, the composition of the manganese steel is: C: 0.16%; Mn: 7.2%; Si: 0.23%; and the rest is Fe and unavoidable impurities.

[0041] (2) The cold-rolled annealed sheet in step (1) is heated to 800℃ in a vacuum heating furnace for 4min, and then water quenched to room temperature.

[0042] (3) The quenched manganese steel in step (2) is heated to 180℃ in a vacuum heating furnace for 10min, and then air-cooled to room temperature to achieve the first low-temperature tempering.

[0043] It should be noted that for the hot stamping steel in Example 1, the test results show that the optimal strength and ductility product is obtained when the tempering parameters are 180℃ / 10min+180℃ / 20min. Therefore, the parameters of 180℃ / 10min adopted in step (3) are obtained by combining the optimal tempering parameters and the inherent process baking parameters in the automobile production process.

[0044] The specific process for the above-mentioned experimental test is as follows: the quenched manganese steel is heated to 100 / 180 / 300 / 400℃, respectively, and is kept at temperature for 10 / 20 / 30 / 40min (first tempering), and then air-cooled to room temperature, and then an additional step of 180℃ / 20min tempering process (second tempering, simulating the baking process) is added. The mechanical properties after the first and second tempering are measured, respectively. The results show that when the first tempering process is 180℃ / 10min, the brittleness has been eliminated after the first tempering, and the comprehensive mechanical properties (strength and ductility product) after the second tempering are optimal, so this group of process parameters is identified as the optimal tempering parameters.

[0045] (4) The medium manganese steel after short time tempering in step (3) is left to stand for 6 h in natural state to simulate the storage and transportation time between hot stamping and painting.

[0046] (5) The time after standing in step (4) is heated to 180℃ in vacuum heating furnace, and after holding for 20 min, it is air cooled to room temperature to simulate the painting process in actual production process, and to realize the second step tempering.

[0047] Comparative Example 1 The process of this comparative example is basically the same as that of Example 1, and the only difference is that: Step (3) is adjusted to: the quenched medium manganese steel is left to stand for 10 min at room temperature without first step low temperature tempering.

[0048] Comparative Example 2 The process of this comparative example is basically the same as that of Example 1, and the only difference is that: Step (3) is adjusted to: the quenched medium manganese steel is heated to 400℃ and held for 30 min, and then air cooled to room temperature.

[0049] The stress-strain curves of Example 1 and Comparative Examples 1 and 2 after the first step low temperature tempering are shown in Figure 1 , which reflects the mechanical properties after hot stamping and before painting. As can be seen from the figure, Comparative Example 1 presents a brittle state before tempering by painting, and no necking occurs, i.e. fracture occurs and the elongation is low, and the comprehensive performance is poor; while Example 1, which has experienced one step short time tempering, effectively eliminates brittleness, and an obvious necking segment appears on the stress-strain curve, reflecting the characteristics of ductile fracture; while Comparative Example 2 is over-tempered, and the tempering degree exceeds the optimal tempering range, although it also presents ductile fracture, but due to excessive softening of the matrix during tempering, the strength is greatly reduced, which damages the mechanical properties.

[0050] The stress-strain curves of Example 1 and Comparative Examples 1 and 2 after the second step low temperature tempering are shown in Figure 2 , which reflects the mechanical properties of the parts during the service stage after painting. After the second step tempering, Example 1 reaches the optimal tempering state, and has the optimal strength and ductility product, while Comparative Example 1 is slightly under-tempered, and has slightly higher strength and lower elongation, and slightly lower strength and ductility product, compared with Example 1. While Comparative Example 2 further increases the degree of over-tempering, and the strength and elongation decrease, and the comprehensive mechanical properties are poor.

[0051] The surface morphology of Example 1 and Comparative Example 1 after die punching after the first step low temperature tempering is shown in Figure 3The die piercing is one of the typical representatives of the local large deformation process between hot stamping and paint baking, and the specific parameters are as follows: hole diameter Φ 12 mm, plate thickness 1.5 mm, punching speed 16.5 mm / s, double-sided gap 0.16 mm, and punch roundness radius 0, that is, a conventional punch without fillet. The die piercing test is carried out at room temperature, and the piercing performance is judged by observing whether there is obvious crack on the surface of the sample after piercing. The die piercing results show that the piercing quality of example 1 is better, and the piercing surface is smooth and flat, while example 1 has obvious cracks around the hole after piercing, which will lead to material scrap. The piercing results prove that the new process is beneficial to reduce the risk of damage of the material in the local large deformation process between hot stamping and paint baking.

[0052] The delayed fracture time results of example 1 and comparative example 1 in the four-point bending immersion test after the first low-temperature tempering are shown in table 1. The four-point bending immersion test can reflect the hydrogen embrittlement sensitivity of the material, specifically, using the equivalent 0.9 times the yield stress of the lower pressure, the steel plate is immersed in 0.1 mol / L HCl, the solution is replaced every 48 h, and the time of delayed fracture is recorded, each state is repeated for 3 times, and 3 parallel samples are obtained (example 1-1 to example 1-3 and comparative example 1-1 to comparative example 1-3). As can be seen from the results, the average delayed cracking time of comparative example 1 is 29.7 h, which reflects its high hydrogen embrittlement sensitivity; while in example 1, except for one sample cracking at 167 h (still much higher than the fracture time of comparative example 1), the other two samples have not cracked within 200 h, which reflects the improvement of hydrogen embrittlement resistance, and it is proved that the new tempering process proposed in the scheme is beneficial to reduce the damage of the material caused by hydrogen embrittlement in the process of hot stamping and paint baking.

[0053] Table 1 Delayed fracture time results of example 1 and comparative example 1 in the four-point bending immersion test after the first low-temperature tempering Example 2 The process of this comparative example is basically the same as that of example 1, and the only difference is that: Step (3) is adjusted as follows: the quenched medium manganese steel is heated to 150℃ for 15 min, and then air cooled to room temperature. Its performance is close to the result of example 1, and is better than the performance of the product only subjected to the paint baking process.

[0054] Example 3 The process of this comparative example is basically the same as that of example 1, and the only difference is that: Step (3) is adjusted as follows: the quenched medium manganese steel is heated to 220℃ for 5 min, and then air cooled to room temperature. Its performance is close to the result of example 1, and is better than the performance of the product only subjected to the paint baking process.

[0055] Example 4 The procedure of this comparative example is basically the same as that of Example 1, except that: Step (5) is adjusted as follows: the quenched medium manganese steel is heated to 170°C for 20 min, and then air-cooled to room temperature. The performance is close to that of Example 1, and is superior to the performance of the product only subjected to the baking finish process.

[0056] Example 5 The procedure of this comparative example is basically the same as that of Example 1, except that: Step (3) is adjusted as follows: the quenched medium manganese steel is heated to 200°C for 10 min, and then air-cooled to room temperature. Step (5) is adjusted as follows: the quenched medium manganese steel is heated to 170°C for 22 min, and then air-cooled to room temperature. The performance is close to that of Example 1, and is superior to the performance of the product only subjected to the baking finish process.

[0057] Example 6 The procedure of this comparative example is basically the same as that of Example 1, except that: Step (3) is adjusted as follows: the quenched medium manganese steel is heated to 170°C for 18 min, and then air-cooled to room temperature. Step (5) is adjusted as follows: the quenched medium manganese steel is heated to 180°C for 17 min, and then air-cooled to room temperature. The performance is close to that of Example 1, and is superior to the performance of the product only subjected to the baking finish process.

[0058] The above-described comparative examples are for the purpose of facilitating the understanding and use of the invention by those of ordinary skill in the art. Those skilled in the art can easily make various modifications to these examples, and apply the general principles described herein to other examples without the need for creative efforts. Therefore, the present invention is not limited to the above-described examples, and improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the present invention should be within the scope of protection of the present invention.

Claims

1. A method of low temperature tempering of a hot stamped steel, characterized in that, The method comprises the following steps: S1: heating and holding the annealed steel plate to completely austenitize the steel plate; S2: quenching the steel plate after step S1; S3: first tempering the steel plate after step S2; S4: machining the steel plate after step S3; S5: second tempering the steel plate after step S4 to obtain a hot stamping steel; The process parameters of the second tempering are determined by a paint baking process, and the process parameters of the first tempering are calculated based on the process parameters of the second tempering and theoretical optimal tempering process parameters; The calculation method of the process parameters of the first tempering is as follows: changing the time and temperature of the first tempering under fixed process parameters of the second tempering, measuring the mechanical properties after the first and second tempering respectively, and establishing a data set; based on the test results of the mechanical properties, selecting the sample with the highest product of strength and ductility after the second tempering in the process parameter range in which the first tempering can eliminate brittleness, and determining the processing time and temperature of the first tempering as the optimal process parameters of the first tempering.

2. A method of low temperature tempering of a hot stamped steel according to claim 1, characterized in that, In step S1, the annealed steel plate is a steel plate sequentially subjected to smelting, hot forging, hot rolling, soft annealing, cold rolling and critical annealing.

3. A method of low temperature tempering of a hot stamped steel according to claim 2, characterized in that, The ingot obtained by smelting comprises the following components in mass fraction: 0.13-0.18% of carbon, 6.0-9.0% of manganese and 0.2-0.3% of silicon, and the rest is iron and inevitable impurities.

4. A method of low temperature tempering of a hot stamped steel according to claim 2, characterized in that, The method comprises one or more of the following: i) the hot forging is: heating to 1100-1250℃ for 1-2h, then hot forging treatment, and then cooling to room temperature; ii) the hot rolling is: first heating to 1100-1250℃ for 1-2h, then hot rolling several times at 900-1200℃, and then quenching to room temperature; iii) the soft annealing is: heating to 600-700℃ for 0.5-5h, and then cooling to room temperature; iv) the critical annealing is: heating to 600-700℃ for 10-20h, and then cooling to room temperature.

5. The method of low temperature tempering of a hot stamped steel according to claim 1, characterized in that, In step S1, the heating and holding is: heating to 30-50℃ above the Ac3 temperature at a heating rate of 8-15℃ / s by a vacuum heating furnace and holding for 3-5min.

6. A method of low temperature tempering of a hot stamped steel according to claim 1, characterized in that, In step S2, the quenching is: quenching to room temperature by liquid assisted quenching or die quenching.

7. The method of claim 1, wherein the hot stamped steel is a dual phase steel. In step S3, the first tempering is: heating to 150-220℃ and holding for 5-20min, and then cooling to room temperature.

8. The method of low temperature tempering of a hot stamped steel according to claim 1, characterized in that, In step S4, the machining includes punching, hole expanding and flanging.

9. The method of claim 1, wherein the hot stamped steel is a low carbon steel. In step S5, the second tempering is: heating to 170-180℃ and holding for 15-25min.

10. The method of low temperature tempering of a hot stamped steel according to claim 1, characterized in that, The theoretical optimal tempering process parameters are: making the product of strength and ductility of the hot stamping steel highest after two-step tempering.

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