A method for hot stamping forming of high-strength steel parts

Abstract

The application discloses a high-strength steel part hot stamping forming method, and solves the problem of considering strength and plasticity of a steel plate part. The hot stamping forming method comprises the following steps: heating a plate to complete austenitization and rapid precooling; secondary heating and holding: the pre-cooled plate is heated and held again, the temperature is 500-800 DEG C, and the time is 30-180 s, so that the plate enters a diffusion phase change zone, part of the austenite is converted into ferrite or bainite (soft phase), at this time, the plate has good formability and small deformation resistance; the plate is rapidly transported to a die and formed and quenched, at this time, the remaining austenite is converted into martensite (hard phase), the strength of the part is increased; finally, the die is opened, and the part is taken out. The application adjusts and controls the temperature and time in the hot forming process, can meet the double indexes of the strength and plasticity of the part, controls the component proportion of the soft phase and the hard phase, and can realize the accurate control of the strength and elongation of the final part.

Description

Technical Field

[0001] The present invention relates to the technical field of steel plate hot forming, in particular to a hot stamping forming method for high-strength steel parts. Background Art

[0002] As national energy conservation and emission reduction targets continue to rise, the demand for lightweight vehicles is becoming more stringent. For every 10% reduction in vehicle weight, fuel consumption for gasoline-powered vehicles decreases by 6-8%, while for electric vehicles, electricity consumption decreases by approximately 5.5%. High-strength and ultra-high-strength steels are key approaches to achieving vehicle lightweighting. However, as material strength increases, steel sheet metal suffers from drawbacks such as difficulty forming complex components, significant springback, and severe die wear during room-temperature forming. To overcome these drawbacks, hot stamping technology has gradually developed and is being applied to the production of automotive parts.

[0003] The current hot forming process involves austenitizing steel plates at high temperatures, then rapidly transferring them to a mold equipped with a cooling system for press quenching. After an appropriate holding time, the plates are removed. During this process, the steel plates form a martensitic structure after quenching, thereby achieving high strength. After laser cutting, they are then used for automotive body structural parts. However, the high-strength formed parts obtained through this hot stamping process often have poor plasticity, with elongation generally not exceeding 8%. This results in insufficient energy absorption during a collision, and they will break and fail after even a small deformation, restricting the application of ultra-high-strength steel in the automotive industry. Some parts, such as B-pillar reinforcements and saddle floors, require both strength to ensure safety during service and high toughness to ensure good energy absorption, meaning both strength and plasticity are required.

[0004] Existing methods for producing high-strength and high-plasticity hot-formed parts primarily fall into two categories: improving materials and adjusting processes. Publication No. CN113106338A describes a method for preparing ultra-high-strength, high-plasticity hot-stamped steel. While maintaining the hot-forming process, this method improves the steel's hardenability and plasticity through material composition and microstructure design. This often requires the addition of large amounts of alloying elements to the matrix, resulting in unmanageable costs.

[0005] Publication No. CN102806259B discloses a hot forming method for improving the strength and plasticity of automotive parts, using secondary heating and low-temperature tempering after hot forming. Publication No. CN109517946B describes an integrated method for hot stamping and in-die quenching and partitioning of steel, and Publication No. CN107326163B describes a method for producing advanced high-strength steel by isothermal deformation in the bainite region combined with hot stamping. While these methods leverage the quenching and partitioning process to improve plasticity through metastable retained austenite, they fail to consider the precise control of strength and plasticity. Summary of the Invention

[0006] The purpose of the present invention is to overcome the shortcomings of the existing technology and provide a hot stamping forming method for high-strength steel parts, so as to solve the problem of balancing strength and plasticity of steel plate parts, achieve a combination of strength and elongation of the final component, and the formed parts have good comprehensive performance, reduce costs and improve efficiency.

[0007] The present invention provides a hot stamping method for high-strength steel parts, which is implemented by the following steps:

[0008] S1. Heating and austenitizing: heating and keeping the sheet material warm to austenitize the sheet material;

[0009] S2. Rapid precooling: precool the austenitized sheet; cooling rate ≥ 45℃ / s, cooling time 1~12s;

[0010] S3, secondary heating and heat preservation: keep the pre-cooled sheet material warm again;

[0011] S4, transfer: quickly transfer the sheet material that has been heated and kept warm twice to the mold;

[0012] S5. Forming quenching: The mold is closed, and the sheet is cooled after the mold is closed to complete the mold quenching.

[0013] In step S2, the pre-cooling conditions are: cooling rate ≥ 45°C / s, cooling time 1-12s. After rapid pre-cooling, the material is still in the austenite region. If the cooling rate is too fast or the cooling time is too long, the material will transform into martensite, which is too hard to be formed in this state.

[0014] In step S1, the heating temperature is 875-950°C, and the temperature is kept at 4-10 minutes. The blank sheet is determined according to the required forming parts, and the sheet is heated and kept warm to make the sheet completely austenitized. When the heating temperature is higher than 950°C, the austenite grains of the sheet are obviously coarsened, and coarse martensite laths are formed after quenching, and the elongation is poor. When the temperature is lower than 875°C, the austenitization time of the sheet is long, the austenite grains grow, and coarse lath martensite structure is formed after quenching, and the elongation decreases. When the holding time is less than 3 minutes, the austenitization temperature of the surface and core of the sheet is uneven, and a mixed martensite structure with grains is formed after quenching, and the performance is poor. When the holding time is higher than 10 minutes, the austenite grains of the sheet are coarsened, and the performance of the sheet is reduced after quenching.

[0015] In step S3, the holding temperature is 500-800°C for 30-180 seconds. The pre-cooled sheet is then heated again at 500-800°C for 30-180 seconds, allowing the sheet to enter the diffusion phase transformation zone, where part of the austenite transforms into ferrite or bainite (soft phase). This allows the sheet to have good formability and low deformation resistance.

[0016] In step S3, the heating furnace for secondary heating and holding is heated, or heated by a salt bath.

[0017] In step S4, the time for the transportation is ≤ 2s.

[0018] In step S5: the die closing pressure is ≥ 20MPa, the cooling rate of the plate after die closing is ≥ 2℃ / s, and the die press quenching is completed. At this time, the remaining austenite is transformed into martensite (hard phase), and the strength of the part is increased. Finally, the mold is opened, and the part is taken out.

[0019] By adjusting the isothermal transformation temperature and time, the mixing ratio of the soft phase and the hard phase in the structure can be adjusted, so that the strength and plasticity of the final part can be accurately controlled.

[0020] In S2, a cooling gas nozzle is provided. For a non-coated steel sheet, the cooling gas is nitrogen, and the cooling is carried out in the cooling cavity; for a coated steel sheet, the cooling gas is air, and the cooling is carried out in an air environment.

[0021] The steel sheet prepared by the preparation method also belongs to the protection scope of the present application.

[0022] The application of the steel sheet in the preparation of automobile parts also belongs to the protection scope of the present application.

[0023] After adopting the above technical scheme, the present application has the following advantages:

[0024] 1) The present application forms a soft phase of ferrite or bainite in the hot forming process, and the remaining austenite is transformed into a hard phase of martensite during forming quenching, which can meet the dual indicators of part strength and plasticity.

[0025] 2) The present application controls the composition ratio of the soft phase and the hard phase by adjusting the temperature and time in the process, which can realize accurate control of the strength and elongation of the final part.

[0026] 3) The present application adds or improves the pre-cooling and secondary heating and holding process on the existing hot stamping process line, which is easy to realize and reduces the cost. BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1 is a schematic diagram of the implementation process of the forming method of example 1 in the specific embodiment of the present application.

[0028] Figure 2 is a schematic diagram of the implementation process of the forming method of example 5 in the specific embodiment of the present application.

[0029] Figure 3 is a schematic diagram of the position A of the integrated movable first heating furnace in example 6 in the specific embodiment of the present application.

[0030] Figure 4It is a schematic diagram of position B of the integrated movable first heating furnace in Example 7 in a specific implementation manner of the present invention.

[0031] Figure 5 1 is a temperature-time curve diagram of ferrite formation in a specific embodiment of the present invention.

[0032] Figure 6 1 is a temperature-time curve diagram of bainite formation in a specific embodiment of the present invention.

[0033] Figure 7 It is a temperature-time curve diagram of a comparative example in a specific embodiment of the present invention. DETAILED DESCRIPTION

[0034] The present invention will be described in detail below with reference to the examples. The following examples will help those skilled in the art to further understand the present invention, but are not intended to limit the present invention in any form. It should be noted that those skilled in the art may make several adjustments and improvements without departing from the scope of the present invention. These all fall within the scope of protection of the present invention.

[0035] The following is a combination of specific embodiments and Figure 1-Figure 7 The present invention is further described.

[0036] The steel plate material used in the following examples is 22MnB5 and has a thickness of 1.6 mm.

[0037] Example 1

[0038] See also Figure 1 This embodiment provides a method for hot stamping a high-strength steel part, comprising the following steps:

[0039] Step 1: Heating and austenitizing: Place the sheet into a heating furnace at 900°C and keep it warm for 4 minutes to completely austenitize the sheet.

[0040] Step 2: Rapid pre-cooling: Take the sheet out of the heating furnace and blow air quickly in the air. The pre-cooling time is 2s; the cooling rate is 45℃ / s;

[0041] Step 3: Secondary heating and holding: The pre-cooled sheet is quickly transferred to another heating furnace, the heating temperature is set to 700°C, and the holding time is 30 seconds. At this time, part of the austenite in the sheet is transformed into ferrite;

[0042] Step 4: Transfer: The sheet material that has been heated and kept warm twice is quickly transferred to the mold. The transfer time is ≤ 2s, and the forming temperature is 695℃.

[0043] Step 5: Forming and quenching: The mold is closed and press-quenched. Finally, the mold is opened and the part is taken out. The mold closing pressure is ≥20MPa and the sheet cooling rate after mold closing is ≥2℃ / s. Finally, the mold is opened and the part is taken out.

[0044] See also Figure 5 After the initial sheet metal undergoes high-temperature austenitization, it partially transforms into ferrite during the secondary heating and holding process. During the forming and quenching process, the remaining austenite transforms into martensite, and some transforms into bainite, forming a mixed structure of soft and hard phases. Through these steps, a part with both strength and plasticity is obtained.

[0045] Example 2

[0046] The steps of this embodiment are the same as those of Example 1. The heating temperature in step 3 of Example 1 is changed to 600°C, and the holding time is 30s. In step 4, the forming start temperature is 595°C. At this time, compared with Example 1, more austenite is transformed into ferrite, the proportion of soft phase components in the final part is increased, and the plasticity is improved.

[0047] Example 3

[0048] The steps of this embodiment are the same as those of embodiment 1. In embodiment 1, the heating temperature in step 3 is changed to 500°C, and the holding time is 50s; in step 4, the forming starting temperature is 495°C.

[0049] See also Figure 6 After the initial sheet is austenitized at high temperature, it is partially transformed into lower bainite during the secondary heating and insulation process. The parts are formed under the mixed structure of austenite and bainite. The bainite is retained in the final part, and the remaining austenite is transformed into martensite during the forming and quenching process, finally achieving the dual indicators of strength and plasticity.

[0050] Example 4

[0051] The steps of this embodiment are the same as those of Example 3. The heating temperature in step 3 is 500°C and the holding time is 80s. At this time, the austenite is transformed into lower bainite, and the proportion of bainite components increases. Accordingly, the plasticity of the final part is improved compared with that of Example 3.

[0052] The following embodiments (Example 5, Example 6, Example 7) are specific implementation methods of the present invention using other different process equipment.

[0053] Example 5

[0054] See also Figure 2 , this embodiment includes the following steps:

[0055] Step 1: Heating and austenitizing: Place the sheet into a heating furnace at 900°C and keep it warm for 4 minutes to completely austenitize the sheet.

[0056] Step 2: Rapid pre-cooling: Take the sheet out of the heating furnace and blow air quickly in the air. The pre-cooling time is 2s; the cooling rate is 45℃ / s;

[0057] Step 3: Secondary heating and heat preservation: Take the sheet out of the heating furnace and immediately put it into the salt bath. The sheet is quickly cooled to the set temperature in the salt bath. The salt bath temperature is 700℃ and the heat preservation time is 30s. At this time, part of the austenite in the sheet is transformed into ferrite.

[0058] Step 4: Transfer: Quickly transfer the sheet from the salt bath to the mold; the transfer time is ≤ 2s, and the forming temperature is 695°C;

[0059] Step 5: Close the mold with a clamping pressure of ≥20MPa. After closing the mold, the sheet metal cooling rate should be ≥2℃ / s to complete the press quenching. Finally, open the mold and remove the part.

[0060] Through the above steps, parts with both strength and plasticity indicators are obtained.

[0061] The salt bath can achieve the effects of the rapid pre-cooling and secondary heating and heat preservation steps of the present invention, and the composition and proportion of the sheet material converted into the soft phase can be controlled by controlling the temperature and time of the salt bath.

[0062] Example 6

[0063] See also Figure 3 , this embodiment includes the following steps:

[0064] Step 1: Heating and austenitizing: The sheet is placed in the first heating furnace of the integrated movable heating furnace, heated to 900°C, and kept warm for 4 minutes to completely austenitize the sheet;

[0065] Step 2: Rapid pre-cooling: The sheet moves with the conveyor belt to the cooling channel and is cooled with nitrogen for 2 seconds.

[0066] Step 3: Secondary heating and insulation: The pre-cooled sheet is moved along the conveyor belt to the second heating furnace of the integrated movable heating furnace, see Figure 3 The first and second heating furnaces in the integrated movable heating furnace are located at position A. The heating temperature is set to 700°C and the holding time is 30 seconds. At this time, part of the austenite in the sheet is transformed into ferrite.

[0067] Step 4: Transfer: The sheet material that has been heated and insulated twice is quickly transferred to the mold; the transfer time is ≤ 2s, and the forming temperature is 695°C;

[0068] Step 5: Forming and quenching: The mold is closed and the mold is quenched. Finally, the mold is opened and the part is taken out.

[0069] Through the above steps, parts with both strength and plasticity indicators are obtained.

[0070] Example 7

[0071] See also Figure 4 The steps of this embodiment are the same as those of embodiment 6, except that the relative positions of the first heating furnace and the second heating furnace of the integrated movable heating furnace in embodiment 6 are increased. Figure 4 The first and second heating furnaces in the integrated movable heating furnace are located at position B. The sheet metal conveyor belt maintains a constant speed, allowing the sheet metal to be cooled to a lower temperature after a longer period of rapid pre-cooling. The second heating furnace's heating temperature is set at 600°C, the holding time is 30 seconds, and the forming start temperature is set at 595°C. Compared to Example 6, this results in a greater transformation of austenite into ferrite, resulting in improved plasticity in the final part.

[0072] Comparative Example 1

[0073] This comparative example is a traditional hot stamping process, comprising the following steps:

[0074] Step 1: Heating and austenitizing: Place the sheet into a heating furnace at 900°C and keep it warm for 4 minutes to completely austenitize the sheet.

[0075] Step 2: Transfer: The austenitized sheet is quickly transferred to the mold; the transfer time is ≤ 2s, and the forming temperature is 550°C;

[0076] Step 3: Forming quenching: The mold is closed and die quenched. Finally, the mold is opened and the part is taken out. The mold closing pressure is ≥20MPa and the cooling rate of the sheet metal after mold closing is ≥2℃ / s.

[0077] See also Figure 7 After the initial sheet is austenitized at high temperature, the austenite is completely transformed into martensite during the forming and quenching process, and finally a high-strength part is obtained.

[0078] Comparative Example 2

[0079] The difference between this comparative example and comparative example 1 is that in step 1, the heating temperature is 950°C.

[0080] Comparative Example 3

[0081] The difference between this comparative example and comparative example 1 is that in step 1, the heating temperature is 1000°C.

[0082] Performance Testing

[0083] Mechanical properties of the products prepared in each embodiment of the present invention and the comparative example were tested. Uniaxial tensile specimens were cut from the formed parts by wire cutting according to ASTM E8 / E8M standards. The width and thickness of the specimens were measured and recorded using a micrometer. A uniaxial tensile test was performed on a mechanical properties testing machine to test the tensile strength (MPa) and total elongation (%).

[0084] Table 1 shows the mechanical properties of the final parts under different process parameters in the embodiment.

[0085]

[0086]

[0087] According to the results in Table 1, the parts of the embodiment of the unified hot stamping method of the present invention meet the dual indicators of final part strength and plasticity, compared with the problem of poor plasticity of traditional hot stamping parts in the comparative example. In addition, by regulating the heating temperature and holding time in the process path, the tensile strength and total elongation of the parts can be accurately controlled.

[0088] The above describes the specific embodiments of the present invention. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art may make various variations or modifications within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. A hot stamping method for high-strength steel parts, characterized in that: The method comprises the following steps: S1. Heating and austenitizing: heating and keeping the sheet material warm to austenitize the sheet material; S2. Rapid precooling: Precool the austenitized sheet; the cooling rate is ≥45℃ / s, and the cooling time is 1~12s; the material after rapid precooling is still in the austenite zone; S3. Secondary heating and heat preservation: The pre-cooled sheet is heated again at a temperature of 500-800°C for 30-180 seconds. This allows the sheet to enter the diffusion phase transformation zone, where part of the austenite is transformed into ferrite or bainite. S4, transfer: quickly transfer the sheet material that has been heated and kept warm twice to the mold; S5. Forming quenching: The mold is closed, and the sheet is cooled after the mold is closed to complete the mold quenching.

2. The hot stamping method for high-strength steel parts according to claim 1, characterized in that: In step S1, the heating temperature is 875-950° C. and the temperature is kept for 4-10 minutes.

3. The hot stamping forming method of high-strength steel parts according to claim 1, characterized in that: In step S4, the transfer time is ≤ 2s.

4. The hot stamping method for high-strength steel parts according to claim 1, characterized in that: In step S3, the secondary heating and heat preservation is performed by heating in a heating furnace or by heating in a salt bath.

5. The hot stamping method for high-strength steel parts according to claim 1, characterized in that: In step S5, the mold clamping pressure is ≥20 MPa, and the sheet cooling rate after mold clamping is ≥2°C / s.

6. A steel plate produced by the method according to any one of claims 1 to 5.

7. Use of the steel plate according to claim 6 in the preparation of automobile parts.

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