High-strength large-size structural member and forming manufacturing method thereof

Abstract

The application relates to a high-strength large-size structural member and a forming manufacturing method thereof, and belongs to the technical field of high-strength structural member forming manufacturing. The forming manufacturing method is as follows: a steel plate is heated to an austenitizing temperature and is kept warm; after the keeping warm, the steel plate is discharged from the furnace and is cooled to a pressing temperature; the steel plate is pressed and formed by using a press; the mold is removed after keeping pressure for a certain time; the formed member is cooled to room temperature, and the high-strength large-size structural member is prepared. By controlling the heating treatment process and the pressing process of temperature control forming, and matching the steel plate with a specific component system, a martensite structure with high tensile strength and good low-temperature toughness is obtained; the tensile strength is above 1700 MPa, and the impact toughness at-40 DEG C is above 35 J.

Description

Technical Field

[0001] This invention relates to the field of high-strength structural component molding and manufacturing technology, and in particular to a high-strength large-size structural component and its molding and manufacturing method. Background Technology

[0002] The molding and manufacturing of structural components for mechanical equipment is a crucial process in production. As the requirements for load-bearing capacity and safety of mechanical equipment continue to increase, the strength and thickness of vehicle body structural components are constantly improving, and their shapes and structures are becoming increasingly complex.

[0003] Currently, there are two main routes for structural component molding:

[0004] (1) Cold forming: The raw steel plate is stamped and bent using a stamping press or bending machine. This process is suitable for parts with generally low strength, usually not exceeding 1000MPa, with thinner formed parts and simpler forming structures, making it impossible to form complex parts.

[0005] (2) Hot forming: The raw steel plate is heated to complete austenitization, transferred to a mold for rapid pressing and cooling, thereby achieving integrated, high-strength forming of complex structures. However, this process is usually used for forming thin steel plates with a diameter of less than 3mm. The pressing temperature is usually above 750℃. On the one hand, it requires large-tonnage presses and large-size water-cooled molds, and also limits the structure of the parts. On the other hand, the resulting structural parts have poor plasticity and low-temperature toughness, which cannot meet the requirements for use under harsh conditions.

[0006] It is evident that current cold forming and hot forming processes are insufficient to meet the manufacturing requirements of structural components that are high-strength, large-sized, thick, and complex in shape. Summary of the Invention

[0007] Based on the above analysis, the present invention provides a high-strength, large-size structural component and its molding and manufacturing method to solve the problems of insufficient strength and poor low-temperature toughness of existing structural components.

[0008] On one hand, the present invention provides a method for molding and manufacturing high-strength, large-size structural components, comprising the following steps:

[0009] S1: Heat the steel plate to the austenitizing temperature and hold it at that temperature;

[0010] S2: After heat preservation, the steel plate is taken out of the furnace and cooled to the pressing temperature;

[0011] S3: The mold is formed by pressing with a press and then removed after holding the pressure for a certain period of time.

[0012] S4: The molded part is cooled to room temperature to obtain a high-strength, large-size structural part;

[0013] The chemical composition of the structural component, by mass percentage, includes: C: 0.28-0.35%, Si: 0.60-1.00%, Mn: 1.5-2.0%, Cr: 1.8-2.5%, P≤0.010%, S≤0.003%, Mo: 0.35-0.45%, V: 0.10-0.20%; B: 0.001-0.004%, with the remainder being Fe and unavoidable impurities.

[0014] Furthermore, in step S1, the heating temperature is the austenite transformation completion temperature (A). C3 +60-120℃, and the heat preservation time is calculated according to the thickness of the steel plate at 1.5-5min / mm.

[0015] Furthermore, in step S2, the cooling rate is greater than the critical cooling rate of the steel plate.

[0016] Furthermore, in step S3, the pressing temperature is the martensitic transformation start temperature (Ms) + 50-200℃, and the pressing pressure is 5-10MPa.

[0017] Furthermore, in step S4, after cooling to room temperature, tempering is performed at a temperature controlled at 150-200℃ for 1-4 hours.

[0018] Furthermore, the tensile strength of the structural component is above 1700MPa, and the impact toughness at -40℃ is above 35J.

[0019] Furthermore, the microstructure of the structural component is entirely martensitic.

[0020] Furthermore, the critical cooling rate of the steel plate is 2-10℃ / s.

[0021] Furthermore, the chemical composition of the structural component, by mass percentage, includes: C: 0.30-0.34%, Si: 0.75-0.90%, Mn: 1.6-2.0%, Cr: 1.80-2.30%, P≤0.010%, S≤0.003%, Mo: 0.40-0.45%, V: 0.15-0.20%; B: 0.002-0.004%, with the remainder being Fe and unavoidable impurities.

[0022] This invention provides a high-strength, large-size structural component, which is prepared by the molding and manufacturing method described in this invention.

[0023] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0024] 1. This invention provides a method for forming and manufacturing structural components, which involves first heating the steel plate and then performing a temperature-controlled pressing process, thereby reducing the requirements for mold cooling capacity and press pressure during the forming process. Furthermore, by controlling the heating process and the temperature-controlled pressing process, the microstructure within the structural component is altered, simultaneously improving its tensile strength and low-temperature toughness; the tensile strength is above 1700 MPa, and the impact toughness at -40℃ is above 35 J.

[0025] 2. This invention designs the composition of structural components. By matching and controlling the content of elements, the incubation period of pearlite transformation and bainite transformation can be delayed under the action of elements such as Mn, Cr, and Mo. This reduces the critical cooling rate of martensite transformation, making the critical cooling rate of the steel plate 2-10℃ / s. This allows for obtaining a fully martensitic structure at a slower cooling rate, thereby improving the strength and low-temperature toughness of the resulting structure and meeting the requirements of use under higher conditions.

[0026] In this invention, the above-described technical solutions can be combined with each other to achieve more preferred combinations. Other features and advantages of this invention will be set forth in the following description, and some advantages may become apparent from the description or be learned by practicing the invention. The objects and other advantages of this invention can be realized and obtained from what is particularly pointed out in the description and drawings. Attached Figure Description

[0027] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts.

[0028] Figure 1 The metallographic photographs of the structural components obtained for comparison are shown.

[0029] Figure 2 The image shows the metallographic structure of the structural component obtained in the example. Detailed Implementation

[0030] The preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, which constitute a part of the present invention and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not intended to limit the scope of the present invention.

[0031] The molding and manufacturing of structural components for mechanical equipment is a crucial process in production. As the safety requirements of vehicle equipment continue to increase, the strength and thickness of vehicle body structural components are constantly improving, and their shapes and structures are becoming increasingly complex.

[0032] Currently, there are two main methods for forming structural components: cold forming and hot forming. Cold forming is used to produce structural components with a strength not exceeding 1000 MPa, and which are simple in structure and relatively thin. Hot forming is typically used for forming thin steel sheets with a thickness of less than 3 mm. The pressing temperature is usually above 750℃, which not only requires large equipment investment but also results in parts with insufficient plasticity and toughness.

[0033] To obtain structural components with higher strength and low-temperature toughness, this invention provides a method for molding and manufacturing high-strength, large-size structural components, comprising the following steps:

[0034] S1: Heat the steel plate to the austenitizing temperature and hold it at that temperature;

[0035] S2: After heat preservation, the steel plate is taken out of the furnace and cooled to the pressing temperature;

[0036] S3: The mold is formed by pressing with a press and then removed after holding the pressure for a certain period of time.

[0037] S4: The molded part is cooled to room temperature to obtain a high-strength, large-size structural part;

[0038] The chemical composition of the structural component, by mass percentage, includes: C: 0.28-0.35%, Si: 0.60-1.00%, Mn: 1.5-2.0%, Cr: 1.8-2.5%, P≤0.010%, S≤0.003%, Mo: 0.35-0.45%, V: 0.10-0.20%; B: 0.001-0.004%, with the remainder being Fe and unavoidable impurities.

[0039] Compared with existing technologies, this invention integrates the heat treatment process of hot forming and the pressing process of temperature-controlled forming. It utilizes the high-temperature softening effect of heat treatment to improve the formability of the steel sheet, and then achieves efficient forming through the temperature-controlled forming process. Pressing is performed at a lower temperature, thereby reducing the requirements for mold cooling capacity and press pressure during the forming process, and ultimately reducing the overall manufacturing cost of the parts. Furthermore, by controlling the heat treatment process and the temperature-controlled pressing process, the microstructure within the structural component is altered, simultaneously improving the tensile strength and low-temperature toughness of the component. The tensile strength of the structural component is above 1700 MPa, and the impact toughness at -40℃ is above 35 J.

[0040] Specifically, in step S1, the heating temperature is the austenite transformation completion temperature (A). C3 The temperature should be above 60-120℃, and the holding time should be calculated based on the thickness of the steel plate at 1.5-5 min / mm.

[0041] Specifically, in this invention, the austenite transformation completion temperature (A) C3 The temperature range is 770-800℃.

[0042] Specifically, in step S2, the cooling rate is greater than the critical cooling rate of the steel plate.

[0043] It should be noted that the austenitization process of the steel plate is a key prerequisite for achieving high strength in the structural components of this invention. This process enables recrystallization of the steel plate's microstructure, thereby contributing to grain refinement. The heating temperature needs to be controlled to be 60-120°C above the austenite transformation completion temperature, preferably 70-90°C. However, if the heating temperature is set too low, the austenitization time will be prolonged, increasing the grain coarsening coefficient and thus reducing strength; conversely, if the heating temperature is too high, it may directly lead to grain coarsening. Both of these factors will adversely affect the performance of large-size structural components.

[0044] Furthermore, attention must be paid to the cooling rate after austenitization, i.e., the cooling process in step S2, as it also has a significant impact on the microstructure and properties of large-sized structural components. A reasonable cooling rate helps to obtain an ideal microstructure, further improving the strength and toughness of large-sized structural components.

[0045] During the cooling process in step S2, the cooling method can be water spraying, air blowing, or natural cooling. However, the cooling rate should be controlled to be greater than the critical cooling rate of the steel plate to ensure that no ferrite, pearlite, or bainite phase transformation occurs before the steel is pressed into shape. Testing has shown that the critical cooling rate of the steel plate provided by this invention is 2-10℃ / s, such as 3℃ / s, 5℃ / s, 7℃ / s, or 10℃ / s, which is far lower than the 80-150℃ / s of existing steel grades, such as 22MnB5 and 30CrNiMoNb.

[0046] Specifically, in step S3, the pressing temperature is 50-200°C above the martensitic transformation start temperature (Ms), such as 50°C, 66°C, 90°C, 120°C, 150°C, 178°C, or 200°C; the pressing pressure is 5-10 MPa, such as 5 MPa, 6 MPa, 7 MPa, 8 MPa, 9 MPa, or 10 MPa.

[0047] Specifically, the martensitic transformation initiation temperature (Ms) in this invention is 280-350℃.

[0048] It should be noted that in step S3, the heated and cooled steel plate needs to be placed in a mold, and then pressed into shape using a press. The pressing temperature is 50-200℃ higher than the martensitic transformation temperature of the steel plate. If the pressing temperature is too high, the mold will overheat, resulting in a reduced cooling rate; if the pressing temperature is too low, the steel plate will have excessive strength and insufficient forming performance. The pressing pressure is crucial for ensuring the cooling rate of the steel plate. As the pressing pressure increases, the cooling rate of the part increases. However, excessive pressure places higher demands on the press's capacity, thus increasing costs. Therefore, the pressing pressure is controlled at 5-10 MPa.

[0049] Specifically, in step S4, after cooling to room temperature, tempering can be performed. The tempering temperature is controlled at 150-200℃, and the tempering time is 1-4 hours.

[0050] It should be noted that in this invention, because the mold has a low cooling rate, the residual heat of the structural component can be used for self-tempering during the slow cooling process after demolding, thereby releasing internal stress and reducing the subsequent aging deformation of the structural component.

[0051] Specifically, the tensile strength of the formed structural components is above 1700MPa, and the impact toughness at -40℃ is above 35J.

[0052] Preferably, the thickness of the structural components ranges from 3 to 15 mm, and the plate width can reach 2 m depending on the press capacity. 2 above.

[0053] It should be noted that in this invention, the synergistic effect between elements can delay the incubation period of pearlite and bainite transformations, thereby reducing the critical cooling rate of martensitic transformation and achieving a fully martensitic microstructure at a slower cooling rate. Furthermore, through the synergistic effect in the preparation method, it is possible to fabricate structural components with large thicknesses and large dimensions, such as thicknesses of 3mm, 5mm, 8mm, 10mm, or 15mm, and dimensions of 2m. 2 4m 2 or 5m 2 .

[0054] The chemical composition of the structural component described in this invention, by mass percentage, includes: C: 0.28-0.35%, Si: 0.60-1.00%, Mn: 1.5-2.0%, Cr: 1.8-2.5%, P≤0.010%, S≤0.003%, Mo: 0.35-0.45%, V: 0.10-0.20%; B: 0.001-0.004%, with the remainder being Fe and unavoidable impurities.

[0055] Compared with the prior art, the material composition provided by the present invention adopts a high hardenability design, the core of which lies in the addition of Mn, Cr and Mo elements, which can delay the incubation period of pearlite transformation and bainite transformation, thereby reducing the critical cooling rate of martensite transformation, achieving a full martensitic structure at a slower cooling rate, and thus improving the strength and low-temperature toughness of the obtained structure to meet the requirements of use under higher conditions.

[0056] The functions and proportions of each element in this invention are based on the following:

[0057] Carbon (C): A major factor affecting the strength of steel plates, it can also combine with elements such as V and Mo to form carbides, improving the properties of steel. Considering the strength grade and ductility requirements of the invented steel, the C content is controlled between 0.28% and 0.35%.

[0058] Si: A common solid solution element in steel, it can inhibit the precipitation of M3C type carbides and improve the steel's resistance to tempering softening. However, excessive amounts can easily cause surface decarburization, so it should be controlled at 0.60-1.00%.

[0059] Mn and Cr: Their main function is to improve the hardenability of steel. Their functions are similar, but excessive Mn and Cr will increase the welding difficulty of steel plates. Considering the characteristics of the invention process, high strength structure can still be obtained at low cooling rate. The designed Mn content is 1.5-2.0% and the Cr content is 1.8-2.5%.

[0060] Mo can effectively improve the hardenability of steel plates, increase precipitation strengthening, and inhibit the growth of precipitates, but excessive amounts can affect the weldability of steel. Therefore, the Mo content in steel plates should be controlled between 0.35% and 0.45%.

[0061] P and S: Harmful elements in steel that can form harmful inclusions and segregate at grain boundaries, severely reducing the plasticity and toughness of steel plates. Therefore, they should be eliminated as much as possible, with P ≤ 0.010% and S ≤ 0.003%.

[0062] B: It can significantly improve the hardenability of steel plates and purify grain boundaries. The effect is not obvious when the content is below 0.001%, and the increase in effect is not significant when it is above 0.004%. Therefore, the B content should be controlled within the range of 0.001-0.004%.

[0063] V (Volume) is a strong carbide-nitride forming element. It can form fine, dispersed MC-type carbides or nitrides, resulting in precipitation strengthening and refining austenite grains, thus improving the strength and toughness of steel plates. Adding too little V has little effect, while adding too much can easily lead to the formation of large-sized nitrides, affecting the toughness of the steel plate. Therefore, the V content should be controlled at 0.10-0.20%.

[0064] Preferably, the chemical composition of the structural component, by mass percentage, includes: C: 0.30-0.34%, Si: 0.75-0.90%, Mn: 1.6-2.0%, Cr: 1.80-2.30%, P≤0.010%, S≤0.003%, Mo: 0.40-0.45%, V: 0.15-0.20%; B: 0.002-0.004%, with the remainder being Fe and unavoidable impurities.

[0065] This invention designs the composition of structural components. By combining elements and controlling their content, the incubation period of pearlite and bainite transformation can be delayed under the influence of elements such as Mn, Cr, and Mo. This reduces the critical cooling rate of martensite transformation, making the critical cooling rate of the steel plate 2-10℃ / s, resulting in a structural component consisting entirely of martensite.

[0066] To more clearly describe the present invention, the following embodiments and comparative examples are provided for further illustration.

[0067] Preparation Example

[0068] Steel plate preparation: The steel was smelted in a vacuum induction furnace, with 150kg round ingots cast. After forging and rolling, 8mm thick hot-rolled steel plates were obtained. The compositions of the experimental and control steels are shown in Table 1. Experimental steels 1-3# represent the composition system suitable for this method. The control steels selected were currently used hot-formed 22MnB5 steel plates and high-strength 30CrNiMoNb steel plates.

[0069] Table 1 Chemical composition of the test steel and the control steel

[0070]

[0071] In Table 1, "--" indicates that the content is 0 or it is not present.

[0072] The austenite transformation completion temperature A of the test steel and the control steel was determined. C3 The onset temperature of austenite transformation, A C1 The martensite transformation begins at Ms, and the temperature at which 90% of the volume of austenite transforms into martensite is M. 90％ And critical cooling rate. As can be seen from the data in the table, the critical cooling rate of the test steel is 2-10℃ / s, which is significantly lower than that of the control steel.

[0073] Table 2 Characteristic parameters of the test steel and the comparison steel

[0074]

[0075] Example

[0076] Manufacturing methods for structural components:

[0077] S1: Heat the steel plate to above the austenitizing temperature and hold it at that temperature; the steel plate is heated in a box furnace, and the austenite transformation completion temperature (A) is determined based on the values ​​of the test steel and the control steel. C3 The heating temperature is set to 870℃ (A). C3 (+60-120℃), heat preservation time is 30min;

[0078] S2: After heat preservation, the steel plate is taken out of the furnace and cooled to a specific temperature (i.e., pressing temperature) in flowing air;

[0079] S3: Press molding is performed using a press, with Ms+(50-200)℃ as the mold pressing temperature and the mold pressure set to 5-10MPa. After holding the pressure for a certain time, the mold is removed and air-cooled.

[0080] S4: After the molded part is cooled to room temperature, it is tempered at 180℃ for 2 hours to obtain a high-strength, large-size structural part.

[0081] Examples and comparisons are shown in Table 3.

[0082] Table 3. Parameter Comparison between Examples and Comparative Examples

[0083]

[0084] In Table 3, 1-1 represents the first group of test steel 1, and 1-2 represents the second group of test steel 1.

[0085] Performance testing

[0086] The above embodiments and comparative examples were subjected to performance tests, mainly including tensile strength, impact toughness at -40℃ and microstructure analysis. The test results are shown in Table 4.

[0087] Table 4 Test Results

[0088]

[0089]

[0090] As can be seen from the embodiments and comparative examples of the present invention and Table 4, the structural components made using the method of the embodiments of the present invention and the experimental steel have tensile strengths of over 1700 MPa and low-temperature impact toughness of over 35 J at -40℃, with the microstructure being entirely martensite.

[0091] Reference Figure 1 and Figure 2 In the comparative example, when the comparative steel is used in conjunction with the forming method or forming temperature of the present invention, the resulting structural component has lower tensile strength and insufficient low-temperature impact toughness. Ultimately, the microstructure consists of multiple phases such as ferrite, pearlite, bainite and martensite.

[0092] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for molding and manufacturing a high-strength, large-size structural component, characterized in that, Includes the following steps: S1: Heat the steel plate to the austenitizing temperature and hold it at that temperature. The heating temperature is 70-90℃ above the austenite transformation completion temperature, and the austenite transformation completion temperature is 770-800℃. S2: After heat preservation, the steel plate is taken out of the furnace and cooled to the pressing temperature; the cooling rate is greater than the critical cooling rate of the steel plate; the critical cooling rate of the steel plate is 2-10℃ / s; S3: The molding process is carried out by pressing with a press, and the mold is removed after holding the pressure for a certain period of time; the pressing temperature is the martensite initiation temperature +150-200℃, and the martensite initiation temperature is 280-350℃; the pressing pressure is 5-10MPa. S4: The molded part is cooled to room temperature to obtain a high-strength, large-size structural part; the tensile strength of the structural part is above 1700MPa, and the impact toughness at -40℃ is above 35J; the microstructure of the structural part is entirely martensite. The chemical composition of the structural component, by mass percentage, includes: C: 0.28-0.35%, Si: 0.60-1.00%, Mn: 1.5-1.95%, Cr: 1.8-2.5%, P≤0.010%, S≤0.003%, Mo: 0.35-0.45%, V: 0.10-0.20%; B: 0.001-0.004%, with the remainder being Fe and unavoidable impurities.

2. The method for molding and manufacturing high-strength, large-size structural components according to claim 1, characterized in that, In step S1, the heat preservation time is calculated based on the steel plate thickness at 1.5-5 min / mm.

3. The method for molding and manufacturing high-strength, large-size structural components according to claim 1, characterized in that, In step S4, after cooling to room temperature, tempering is performed at a temperature of 150-200℃ for 1-4 hours.

4. The method for molding and manufacturing high-strength, large-size structural components according to claim 1, characterized in that, The critical cooling rate of the steel plate is 3-10℃ / s.

5. The method for molding and manufacturing high-strength, large-size structural components according to claim 1, characterized in that, The chemical composition of the structural component, by mass percentage, includes: C: 0.30-0.34%, Si: 0.75-0.90%, Mn: 1.6-1.95%, Cr: 1.80-2.30%, P≤0.010%, S≤0.003%, Mo: 0.40-0.45%, V: 0.15-0.20%; B: 0.002-0.004%, with the remainder being Fe and unavoidable impurities.

6. A high-strength, large-size structural component, characterized in that, It is prepared by the molding and manufacturing method according to any one of claims 1-5.

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