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

Abstract

The invention relates to a high-strength large-size structural part and a forming and manufacturing method thereof, and belongs to the technical field of forming and manufacturing of high-strength structural parts. The forming manufacturing method comprises the following steps: heating a steel plate to austenitizing temperature for heat preservation; the steel plate is discharged out of the furnace after heat preservation and cooled to the pressing temperature; pressing and forming by adopting a press, keeping the pressure for a certain time and then moving out of the mold; and cooling a formed part to room temperature to obtain the high-strength large-size structural part. A martensite structure with high tensile strength and good low-temperature toughness is obtained by controlling a heating treatment procedure and a pressing procedure of temperature-controlled forming and matching with a steel plate of a specific component system; the tensile strength is 1700 MPa or above, and the impact toughness at-40 DEG C is 35 J or above.

Description

Technical Field

[0001] The present invention relates to the technical field of high-strength structural component forming and manufacturing, and in particular to a high-strength large-size structural component and a forming and manufacturing method thereof. Background Art

[0002] The molding and manufacturing of structural parts for mechanical equipment is an important process in production. As the requirements for load-bearing capacity and safety of mechanical equipment continue to increase, the strength of body structural parts continues to increase, the thickness gradually increases, and the shape and structure become more and more complex.

[0003] At present, there are two main routes for structural parts forming:

[0004] (1) Cold forming: Use a stamping machine or a bending machine to punch and bend the raw steel plate. The strength of the parts suitable for this process is generally low, usually not exceeding 1000MPa, the thickness of the formed parts is relatively thin, and the forming structure is relatively simple, which cannot achieve the forming of complex parts.

[0005] (2) Hot forming: The raw steel plate is heated to complete austenitization, transferred to the mold for rapid pressing and cooling, thereby achieving integrated, highly strengthened forming of complex structures. However, this process is usually used for forming thin steel plates below 3 mm, and the pressing temperature is usually above 750°C. On the one hand, it not only requires a large-tonnage press and a large-size water-cooled mold, but also has some restrictions on the structure of the parts. On the other hand, the plasticity and low-temperature toughness of the resulting structural parts are poor, and cannot meet the requirements for use under harsh conditions.

[0006] It can be seen that for high-strength, large-size, thick and complex-shaped structural parts, the current cold forming and hot forming processes are difficult to meet the manufacturing requirements of the parts. Summary of the invention

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

[0008] In one aspect, the present invention provides a method for forming and manufacturing a high-strength large-size structural part, comprising the following steps:

[0009] S1: heating the steel plate to austenitizing temperature and keeping it warm;

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

[0011] S3: Use a press to press and mold, and then remove the mold 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 part, calculated 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%, and the rest is Fe and unavoidable impurities.

[0014] Furthermore, in step S1, the heating temperature is the austenite transformation end temperature (A C3 )+60-120℃, the holding time is calculated based on the thickness of the steel plate at 1.5-5min / mm.

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

[0016] Further, in step S3, the pressing temperature is the martensite start transformation temperature (Ms) + 50-200°C, and the pressing pressure is 5-10 MPa.

[0017] Further, in step S4, after cooling to room temperature, tempering treatment is performed, the temperature is controlled at 150-200° C., and the tempering time is 1-4 hours.

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

[0019] Furthermore, the structure of the structural part is entirely martensite.

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

[0021] Furthermore, the chemical composition of the structural part, in terms of 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%, and the rest is Fe and unavoidable impurities.

[0022] The present invention provides a high-strength large-size structural part, which is prepared by the molding manufacturing method of the present invention.

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

[0024] 1. The present invention provides a method for forming and manufacturing a structural part, wherein the steel plate is first heated and then subjected to a temperature-controlled forming pressing process, thereby reducing the requirements for mold cooling capacity and press pressure during the forming process. In addition, by controlling the heating process and the temperature-controlled forming pressing process, the structure inside the structural part is changed, and the tensile strength and low-temperature toughness of the structural part are improved; the tensile strength is above 1700MPa, and the -40℃ impact toughness is above 35J.

[0025] 2. The present invention designs the composition of the structural parts. By matching the elements and controlling the content, under the action of elements such as Mn, Cr, and Mo, the gestation period of pearlite transformation and bainite transformation can be delayed, thereby reducing the critical cooling rate of martensite transformation, so that the critical cooling rate of the steel plate is 2-10°C / s, and a full martensitic structure can be obtained at a slower cooling rate, thereby improving the strength and low-temperature toughness of the obtained structure to meet the use under higher conditions.

[0026] In the present invention, the above-mentioned technical solutions can also be combined with each other to achieve more preferred combination solutions. Other features and advantages of the present invention will be described in the subsequent description, and some advantages can become obvious from the description, or can be understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained through the contents particularly pointed out in the description and the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The drawings are only for the purpose of illustrating particular embodiments and are not to be considered limiting of the present invention. Like reference symbols denote like components throughout the drawings.

[0028] Figure 1 This is a metallographic photograph of the structural part obtained in the comparative example;

[0029] Figure 2 The following is a metallographic photograph of the structural part obtained in the embodiment. DETAILED DESCRIPTION

[0030] The preferred embodiments of the present invention will be described in detail below in conjunction with the accompanying drawings, wherein the accompanying drawings 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 used to limit the scope of the present invention.

[0031] The molding and manufacturing of structural parts for mechanical equipment is an important process in production. As the safety requirements of vehicle equipment continue to increase, the strength of body structural parts continues to increase, the thickness gradually increases, and the shape and structure become more and more complex.

[0032] At present, there are two main methods for forming structural parts, one is cold forming and the other is hot forming. The cold forming method is used to prepare structural parts with a strength not exceeding 1000MPa, a simple structure and a small thickness. The hot forming method is usually used for forming thin-gauge steel plates with a thickness of less than 3mm. The pressing temperature is usually above 750℃. Not only does it require a large investment in equipment, but the plasticity and toughness of the formed parts are insufficient.

[0033] In order to obtain a structural part with higher strength and low-temperature toughness, the present invention provides a molding and manufacturing method for a high-strength large-size structural part, comprising the following steps:

[0034] S1: heating the steel plate to austenitizing temperature and keeping it warm;

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

[0036] S3: Use a press to press and mold, and then remove the mold 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 part, calculated 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%, and the rest is Fe and unavoidable impurities.

[0039] Compared with the prior art, the present invention combines the heat treatment process of hot forming and the pressing process of temperature-controlled forming, improves the forming performance of the steel plate by using the high-temperature softening of heat treatment, and then realizes efficient forming through the temperature-controlled forming process, and performs press forming at a lower temperature, thereby reducing the requirements for mold cooling capacity and press pressure during the forming process, thereby reducing the comprehensive manufacturing cost of the parts. In addition, by controlling the heat treatment process and the pressing process of temperature-controlled forming, the organization inside the structural part is changed, and the tensile strength and low-temperature toughness of the structural part are improved at the same time. The tensile strength of the structural part is above 1700MPa, and the -40℃ impact toughness is above 35J.

[0040] Specifically, in step S1, the heating temperature is the austenite transformation end temperature (A C3 ) above 60-120℃, the insulation time is calculated according to the thickness of the steel plate at 1.5-5min / mm.

[0041] Specifically, in the present invention, the austenite transformation end temperature (A C3 ) 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 the key prerequisite for the high strength of the structural parts in the present invention. Through this process, the recrystallization of the microstructure of the steel plate can be achieved, which in turn helps to refine the grain structure. It is necessary to control the heating temperature to be higher than the austenite transformation end temperature of 60-120°C, preferably 70-90°C. However, if the heating temperature is set too low, the time required for austenitization will be extended, the coefficient of occurrence of grain coarsening will increase, and the strength will be reduced; on the contrary, if the heating temperature is too high, it may directly lead to grain coarsening, both of which will have an adverse effect on the performance of large-size structural parts.

[0044] In addition, attention should be paid to the cooling rate after austenitization, that is, the cooling process in step S2, which also has an important influence on the microstructure and performance of large-size structural parts. A reasonable cooling rate helps to obtain an ideal microstructure morphology and further improve the strength and toughness of large-size structural parts.

[0045] During the cooling process of step S2, the cooling method can be water spraying, air blowing or natural cooling, but the cooling rate should be controlled to be greater than the critical cooling rate of the steel plate, and it is necessary to ensure that no ferrite, pearlite or bainite phase transformation occurs before the steel plate is taken out of the furnace and pressed into shape. After testing, the critical cooling rate of the steel plate provided by the present invention is 2-10℃ / s, such as 3℃ / s, 5℃ / s, 7℃ / s or 10℃ / s, which is much 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 martensite start transformation temperature (Ms), such as 50°C, 66°C, 90°C, 120°C, 150, 178°C or 200°C; the pressing pressure is 5-10MPa, such as 5MPa, 6MPa, 7MPa, 8MPa, 9MPa or 10MPa.

[0047] Specifically, in the present invention, the martensite start transformation temperature (Ms) is 280-350°C.

[0048] It should be noted that in step S3, the heated and cooled steel plate needs to be placed in a mold, and then a press is used to press and form the steel plate. The pressing temperature is 50-200°C higher than the martensite transformation temperature of the steel plate. If the pressing temperature is too high, the mold temperature will rise seriously, resulting in a decrease in the cooling rate. If the pressing temperature is too low, the strength of the steel plate will be too high and the forming performance will be insufficient. The pressing pressure is the guarantee of the cooling rate of the steel plate. As the pressing pressure increases, the cooling rate of the parts increases. However, excessive pressure places higher requirements on the press capacity, thereby increasing costs. Therefore, the pressing pressure is controlled at 5-10MPa.

[0049] Specifically, in step S4, after cooling to room temperature, tempering treatment may be performed, the temperature of the tempering treatment is controlled at 150-200° C., and the tempering time is 1-4 hours.

[0050] It should be noted that in the present invention, due to the low cooling rate of the mold, the residual temperature of the structural part can be used for self-tempering during the slow cooling process after demolding to release the internal stress and reduce the subsequent aging deformation of the structural part.

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

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

[0053] It should be noted that in the present invention, through the mutual coordination between the elements, the incubation period of pearlite transformation and bainite transformation can be delayed, thereby reducing the critical cooling rate of martensite transformation, and achieving full martensite structure at a slower cooling rate; and through the coordination in the preparation method, large-thickness and large-size structural parts can be prepared, such as 3mm, 5mm, 8mm, 10mm or 15mm in thickness; 2m in size 2 、4m 2 or 5m 2 .

[0054] The chemical composition of the structural part of the present invention, calculated 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%, and the rest is Fe and unavoidable impurities.

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

[0056] The functions and proportions of the elements of the present invention are as follows:

[0057] C: The main factor affecting the strength of steel plates. It can also combine with V, Mo and other elements to form carbides to improve the performance of steel. Considering the strength grade and plasticity requirements of the invention steel, the C content is controlled at 0.28-0.35%.

[0058] Si: A common solid solution element in steel. It can inhibit the precipitation of M3C carbides and improve the steel's resistance to temper softening. However, too high a content can easily cause surface decarburization, so it is 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 too high Mn and Cr will increase the difficulty of welding the steel plate. Considering the characteristics of the invented process, high-strength structure can still be obtained at a low cooling rate. The designed Mn content is 1.5-2.0% and the Cr content is 1.8-2.5%.

[0060] Mo: It can effectively improve the hardenability of steel plates, increase precipitation strengthening effects and inhibit the growth of precipitated phases, but too much will affect the welding performance of steel. Therefore, the Mo content in steel plates is controlled at 0.35-0.45%.

[0061] P and S: harmful elements in steel, which can form harmful inclusions and segregate at grain boundaries, seriously reducing the plastic toughness of the steel plate. Therefore, they should be eliminated as much as possible, controlling P≤0.010% and S≤0.003%.

[0062] B: It can significantly improve the hardenability of steel plates and purify grain boundaries. When the content is less than 0.001%, the effect is not obvious, and when it is higher than 0.004%, the effect is not obvious. Therefore, the B content should be controlled within the range of 0.001-0.004%.

[0063] V: It is a strong carbonitride forming element, which can form fine dispersed MC-type carbon or nitride, obtain precipitation strengthening and refine austenite grains, and improve the strength and toughness of steel plates. Adding less will not have obvious effect, while adding more will easily form large-sized nitrides, affecting the toughness of steel plates. Therefore, the V element content is controlled to be 0.10-0.20%.

[0064] Preferably, the chemical composition of the structural part, in terms of 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%, and the rest is Fe and unavoidable impurities.

[0065] The present invention designs the composition of the structural parts. By matching the elements and controlling the content, under the action of elements such as Mn, Cr, and Mo, the gestation period of pearlite transformation and bainite transformation can be delayed, thereby reducing the critical cooling rate of martensite transformation, so that the critical cooling rate of the steel plate is 2-10°C / s, so that the structural parts are all martensitic structures.

[0066] In order to more clearly describe the present invention, it is further illustrated by the following examples and comparative examples.

[0067] Preparation Example

[0068] Preparation of steel plate: The steel plate was smelted in a vacuum induction furnace, and the casting ingot was 150kg round ingot. After forging and rolling, 8mm thick hot-rolled steel plates were obtained. The compositions of the test steel and the comparison steel are shown in Table 1. Test steels 1-3# are composition systems suitable for this method, and the comparison steels are selected as 22MnB5 steel plates currently used for hot forming and high-strength 30CrNiMoNb steel plates.

[0069] Table 1 Chemical composition of test steel and comparison steel

[0070]

[0071] * In Table 1, “--” means that the content is 0 or not contained.

[0072] Detect the austenite transformation end temperature A of the test steel and the reference steel C3 , the starting temperature of austenite transformation is A C1 , the martensite transformation temperature Ms, the temperature at which 90% of the volume of austenite is transformed into martensite M 90％ And critical cooling rate. From the data in the table, it can be seen that 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 test steel and comparison steel

[0074]

[0075] Example

[0076] Forming and manufacturing method of structural parts:

[0077] S1: The steel plate is heated to above the austenitizing temperature and kept warm; the steel plate is heated in a box furnace, and the austenite transformation end temperature values ​​(A C3 ), set the heating temperature to 870℃(A C3 +60-120℃), the holding 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: Use a press to press and form, use Ms+(50-200)℃ as the mold pressing temperature, set the mold pressure to 5-10MPa, hold the pressure for a certain period of time, then remove from the mold and air cool;

[0080] S4: The molded part is cooled to room temperature and then subjected to tempering treatment at 180°C for 2 hours to obtain a high-strength large-size structural part.

[0081] The embodiments and comparative examples are shown in Table 3.

[0082] Table 3 Parameter comparison of the embodiment and the comparative example

[0083]

[0084] * In Table 3, 1-1 indicates the first group of test steel 1, and 1-2 indicates 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°C and structure tests. The test results are shown in Table 4.

[0087] Table 4 Test results

[0088]

[0089]

[0090] Combining the embodiments of the present invention and the comparative examples and Table 4, it can be seen that the tensile strength of the structural parts manufactured by the method of the embodiments of the present invention and the experimental steel reaches more than 1700 MPa, while the low-temperature impact toughness at -40°C reaches more than 35 J, and the structure is all martensite.

[0091] Reference Figure 1 and Figure 2In the comparative example, when the comparative steel is used in combination with the forming method or forming temperature of the present invention, the tensile strength of the obtained structural parts is low and the low-temperature impact toughness is insufficient. Finally, the structure exists in complex phases such as ferrite, pearlite, bainite and martensite.

[0092] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by any technician familiar with the technical field within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

Claims

1. A molding and manufacturing method for high-strength large-size structural parts, characterized in that: The following steps are involved: S1: heating the steel plate to austenitizing temperature and keeping it warm; S2: After heat preservation, the steel plate is taken out of the furnace and cooled to the pressing temperature; S3: Use a press to press and mold, and then remove the mold after holding the pressure for a certain period of time; S4: The molded part is cooled to room temperature to obtain a high-strength large-size structural part; The chemical composition of the structural part, calculated 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%, the rest are Fe and unavoidable impurities.

2. The molding and manufacturing method of high-strength large-size structural parts according to claim 1 is characterized in that: In step S1, the heating temperature is the austenite transformation end temperature + 60-120°C, and the holding time is calculated according to the thickness of the steel plate at 1.5-5 min / mm.

3. The molding and manufacturing method of high-strength large-size structural parts according to claim 1 is characterized in that: In step S2, the cooling rate is greater than the critical cooling rate of the steel plate.

4. The molding and manufacturing method of high-strength large-size structural parts according to claim 1 is characterized in that: In step S3, the pressing temperature is the martensite start transformation temperature + 50-200°C, and the pressing pressure is 5-10 MPa.

5. The molding and manufacturing method of high-strength large-size structural parts according to claim 1 is characterized in that: In step S4, after cooling to room temperature, tempering treatment is performed, the temperature is controlled at 150-200° C., and the tempering time is 1-4 hours.

6. The molding and manufacturing method of high-strength large-size structural parts according to claim 1 is characterized in that: The tensile strength of the structural component is above 1700 MPa, and the -40°C impact toughness is above 35 J.

7. The molding and manufacturing method of a high-strength large-size structural part according to any one of claims 1 to 5, characterized in that: The structure of the structural part is entirely martensite.

8. The molding and manufacturing method of high-strength large-size structural parts according to claim 1 is characterized in that: The critical cooling rate of the steel plate is 2-10°C / s.

9. The molding and manufacturing method of high-strength large-size structural parts according to claim 1 is characterized in that: The chemical composition of the structural part, calculated 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%, and the rest is Fe and unavoidable impurities.

10. A high-strength large-size structural part, characterized in that: The method is prepared by the molding manufacturing method according to any one of claims 1 to 9.

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