A hot forming method and a steel component

Abstract

The present invention relates to a hot forming method and a steel component. The hot forming method includes the following steps: S1. Process the steel into a size suitable for stamping, heat it, and keep it warm for 10 to 15 minutes; S2. Heat the first part of the mold, keep the second part at room temperature, transfer the workpiece to the mold, and inject cooling water into the mold for cooling while stamping and forming; S3. Interrupt the supply of cooling water, keep the pressure of the workpiece in the mold for cooling, control the cooling rate of the first part, so that the corresponding area of the workpiece also cools down at a certain rate; S4. Stop heating the first part, and let the workpiece cool naturally to room temperature. The area of the steel component corresponding to the first part is a high toughness area, and the area of the steel component corresponding to the second part is a high strength area. Compared with the prior art, the present invention provides a hot forming method. By controlling the cooling rate of specific areas of the mold, the corresponding parts of the steel are cooled, kept under pressure, and cooled at different rates after forming, forming a high strength area and a high toughness area.

Description

Technical Field

[0001] The present invention relates to the technical field of heat treatment of steel materials, and in particular to a hot forming method and a steel component. Background Art

[0002] High-strength steel has been widely used in the manufacture of modern armored vehicle protective structures. In order to meet the needs of armored vehicles for high survivability, high mobility and light weight, countries have attached importance to the research of high-performance high-strength steel, and are in urgent need of high-strength steel materials with high strength, high toughness and high hardness.

[0003] Protection against kinetic projectiles is one of the most difficult technical problems in current tank armor protection. In order to resist the invasion of kinetic projectiles, high-strength steel components need to have extremely high strength and sufficient toughness to fully absorb the impact energy of kinetic projectiles. However, strength and toughness are often opposite in physical properties: the higher the strength, the lower the toughness.

[0004] At present, the forming process of high-strength steel combines the processes of high-temperature quenching and tempering. Normally, the steel plate is heated to 700-900°C, at which time the cubic iron crystals transform from the ferrite structure at room temperature to the austenite structure at high temperature. Subsequently, the austenite structure is transformed into a high-strength martensite structure by quenching. Although the hard-brittle martensite structure ensures the ultra-high strength of the formed parts, the high-strength steel components after hot forming generally have the problem of low plasticity. In order to solve the "hard and brittle" problem of high-strength steel components, a secondary heat treatment process-tempering is required. Tempering can enhance the toughness of the component and reduce its sensitivity to fatigue cracks, but it also significantly reduces the strength of the component.

[0005] This results in the strength and toughness of steel being in opposition in physical properties: the higher the strength, the lower the toughness. While high-strength steel provides high-strength performance, its excessive strength also brings difficulties to subsequent processing. Summary of the invention

[0006] In view of this, it is necessary to provide a hot forming method and a steel component to solve the technical problem in the prior art that it is difficult to balance the strength and toughness of steel.

[0007] The invention provides a hot forming method, which comprises the following steps: S1, processing steel into a size suitable for stamping and heating it to 910-950°C, and keeping the temperature for 10-15 minutes; S2, heating the first part of the mold to 290-310°C, keeping the second part of the mold at room temperature, transferring the workpiece processed in step S1 to the mold, injecting cooling water into the mold for cooling during stamping, so that the workpiece is cooled to 290-310°C within 5 seconds, and during this process, the temperature of the first part is kept at 290-310°C; S3, interrupting the supply of cooling water, so that the workpiece is cooled under pressure in the mold, and by controlling the cooling rate of the first part, the area of ​​the workpiece corresponding to the first part is also cooled at a certain rate for a certain time; S4, stopping heating the first part, and the workpiece is naturally cooled to room temperature.

[0008] Furthermore, in step S1, a protective gas is used when heating and keeping the steel warm to prevent oxidation and decarburization of the steel.

[0009] Furthermore, in step S2, the time for transferring the workpiece to the mold is 3 to 7 seconds, and the temperature of the workpiece after the transfer is greater than 550°C.

[0010] Furthermore, in step S2, the water quenching cooling rate of the workpiece is 70-100°C / s.

[0011] Furthermore, in step S2, the punching speed is 50-100 mm / s, and the friction coefficient is 0.1-0.4.

[0012] Further, in step S3, the temperature drop rate of the area corresponding to the first part of the workpiece is controlled to be 0.5-1°C / s.

[0013] Furthermore, in step S3, the pressure-maintaining cooling time is 5-10 seconds.

[0014] Further, in step S3, the temperature of the area corresponding to the first part of the workpiece is controlled to be maintained at 290-310°C, and the workpiece is maintained under pressure in the mold for 5-10 seconds.

[0015] Furthermore, the first part has a heater, and the second part has a cooling water pipeline.

[0016] The present invention provides a steel component, which is processed by the above-mentioned hot forming method. The area of ​​the steel component corresponding to the first part is a high-toughness area, and the area of ​​the steel component corresponding to the second part is a high-strength area. The elongation of the high-toughness area is higher than that of the high-strength area, and the yield strength of the high-strength area is higher than that of the high-toughness area.

[0017] Compared with the prior art, the present invention provides a hot forming method, which controls the temperature of different areas of the mold so that the corresponding parts of the steel are cooled at different rates. The parts of the steel component that require strength are quickly cooled to form a high-strength zone, and the parts that require toughness are slowly cooled to form a high-toughness zone. The interior of the high-toughness zone is a complex phase structure of martensite and lower bainite. After forming and quenching, no secondary heat treatment is performed, but only an isothermal pressure holding process is added. This not only avoids the reduction in strength of the formed component caused by tempering, but also ensures that specific areas of the component have higher toughness.

[0018] The above description is only an overview of the technical solution of the present invention. In order to more clearly understand the technical means of the present invention and implement it according to the contents of the specification, the preferred embodiments of the present invention are described in detail as follows with reference to the accompanying drawings. The specific implementation of the present invention is given in detail by the following embodiments and their accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The drawings described herein are used to provide a further understanding of the present invention and constitute a part of this application. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the drawings:

[0020] FIG. 1 is a flow chart of a preferred embodiment of the thermoforming method provided by the present invention. DETAILED DESCRIPTION

[0021] The preferred embodiments of the present invention are described in detail below in conjunction with the accompanying drawings, wherein the accompanying drawings constitute a part of this application 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.

[0022] See also Figure 1 The present invention provides a hot forming method. The hot forming method comprises the following steps: S1, processing the steel into a size suitable for stamping and heating it to 910-950°C for 10-15 minutes to ensure that the interior of the workpiece is completely transformed into an austenite structure.

[0023] S2, heat the first part of the mold to 290-310°C, keep the second part of the mold at room temperature, transfer the workpiece processed in step S1 to the mold, inject a large amount of cooling water into the mold to cool the workpiece during stamping, and cool the workpiece to 290-310°C within 5s. During this process, the temperature of the first part is always maintained at 290-310°C. That is, when cooling down, it is necessary to increase the heating amount of the first part in time to provide additional heat to offset the cooling effect of the cooling water, so that the temperature of the first part can be maintained at 290-310°C throughout the process.

[0024] S3, interrupting the cooling water supply to cool the workpiece under pressure in the mold, and controlling the cooling rate of the first part so that the area of ​​the workpiece corresponding to the first part is also cooled at a certain rate for a certain period of time.

[0025] The temperature of the second part is low, and the area of ​​the workpiece corresponding to the second part cools down quickly, while the temperature of the first part is high, and the area of ​​the workpiece corresponding to the first part cools down slowly. In step S2, the stamping causes a certain deformation of the workpiece. By controlling the cooling rate of the first part of the mold and maintaining the stamping pressure, the corresponding area of ​​the workpiece is kept under a certain temperature and pressure for a period of time, which promotes the formation of lower bainite, especially the area with a larger deformation will produce more lower bainite.

[0026] S4. The heating of the first part stops and the workpiece cools naturally to room temperature.

[0027] By controlling the temperature of the first part of the mold, the cooling rate of the workpiece area corresponding to the first part is indirectly controlled. After cooling, the interior is composed of a composite structure of martensite and lower bainite, of which the volume content of martensite is greater than 95%, the volume content of lower bainite is 4.5-4.9%, and the rest is residual austenite. This internal structure is reflected in the performance of lower strength but higher toughness. The second part of the mold is at room temperature, and the cooling rate of the steel corresponding to the second part is faster, generally 50℃ / s. After cooling, the interior is mainly composed of martensite. This internal structure is reflected in the performance of higher strength but lower toughness.

[0028] This hot forming method controls the temperature of different areas of the mold so that the corresponding parts of the steel are cooled at different rates. The parts of the steel component that need strength are quickly cooled to form a high-strength area, and the parts that need toughness are slowly cooled to form a high-toughness area. The interior of the high-toughness area is a complex phase structure of martensite and lower bainite. After forming and quenching, no secondary heat treatment is performed, but only an isothermal pressure holding process is added. This not only avoids the reduction in strength of the formed component caused by tempering, but also ensures that specific areas of the component have higher toughness.

[0029] It is easy to understand that there can be multiple high-strength zones and high-toughness zones in a steel component. By controlling the temperature and cooling rate separately, the yield strength and toughness between each high-strength zone can be different, and the same is true between each high-toughness zone.

[0030] In step S1 of some embodiments, the steel material is high-strength steel, i.e., grade Q460 steel. When using steel plate processing, the thickness of the steel plate is 1-5 mm, and it is cut into a size suitable for stamping by laser. In other embodiments, the following steel materials can also be used, and the chemical composition mass percentage is: C: 0.46%-0.48%; Mn: 0.65%-0.76%; Si: 0.9%-1.1%; Cr: 0.4%-0.46%; Ni: 1.9%-2.1%; Mo: 0.2%-0.24%; V: 0.05%-0.08%; P: ≤0.005%; Ti: ≤0.02%; S: ≤0.003% and the balance of Fe.

[0031] In step S1 of some embodiments, a protective gas is used when heating and keeping the steel warm to prevent oxidation and decarburization of the steel. The protective gas may be nitrogen or a rare gas.

[0032] In some embodiments, the first part of the mold has a heater and the second part has a cooling water pipe. The heater can generate heat by connecting to an external power source, and the heating power can be controlled externally, so as to control the temperature of the corresponding area as required. In other embodiments, the first part of the mold has a heater, and both the first part and the second part have cooling water pipes, so that cooling water also flows into the first part, and the area of ​​the workpiece corresponding to the first part can also be quickly cooled to 290~310℃.

[0033] In step S2 of some embodiments, the time for transferring the workpiece to the first mold is 3-7 seconds, and the temperature of the workpiece after the transfer is greater than 550° C. The water quenching cooling rate of the workpiece is 70-100° C. / s.

[0034] In step S2 of some embodiments, the punching speed is 50-100 mm / s, the friction coefficient is 0.1-0.4, and the pressure holding cooling time is 5-10 s.

[0035] In step S3 of some embodiments, the temperature of the area corresponding to the first part of the workpiece is controlled to be maintained at 290-310°C, and the workpiece is kept under pressure in the mold for 5-10 seconds. The deformation of the workpiece at about 300°C reaches more than 10%. Through multiple tests, the applicant found that lower bainite will be generated by keeping the pressure for more than 5 seconds. Therefore, partial bainite can be generated by keeping the pressure for 5-10 seconds.

[0036] In step S3 of some other embodiments, since it is difficult to maintain the temperature within the range of 290-310° C., the area of ​​the workpiece corresponding to the first part can also be controlled to slowly cool at a rate of 0.5-1° C. / s. The pressure holding cooling time is 5-10s.

[0037] In step S3 of some embodiments, during the pressure-maintaining cooling process of the workpiece, the pressure is not less than 500,000 N (ie, a force of 50 tons).

[0038] The present invention also provides a steel component, which is processed by the above-mentioned hot forming method. The area of ​​the steel component corresponding to the first part is a high-toughness area, and the area of ​​the steel component corresponding to the second part is a high-strength area. The elongation of the high-toughness area is higher than that of the high-strength area, and the yield strength of the high-strength area is higher than that of the high-toughness area.

[0039] It is easy to understand that the part of the steel component that is mainly used to bear load or impact and provide hardness or strength is the high-strength area, while the part that bears less load or impact but needs to be cut, bent, and shaped in subsequent processing is the high-toughness area. The tensile strength of the steel component made in the above way can reach 2000-2500MPa, and the elongation can reach 9-11%.

[0040] Example 1

[0041] (1) Select a high-strength steel plate with a thickness of 1.5 mm and cut it into a suitable stamping profile on a laser cutting machine. Heat the cut high-strength steel plate to 930°C in a vacuum heating furnace and keep it warm for 15 minutes so that the inside of the plate is completely transformed into austenite.

[0042] (3) Stamping and isothermal pressure holding: Heat different parts of the mold to different temperatures. Parts with plasticity requirements are kept at 310°C, and parts with high strength requirements are kept at room temperature. Then the fully austenitized high-temperature and high-strength steel sheet is quickly transferred from the vacuum heating furnace to the hot stamping water-cooled mold. While completing the stamping, a large amount of cooling water is injected into the mold to cool the high-temperature sheet to about 310°C within 5 seconds. Monitor the sheet temperature so that different parts of the formed component are kept at different cooling rates. The heated parts of the mold continue to be kept at a cooling rate of 0.5-1°C / S for 9 seconds, and the normal temperature mold parts are kept at a cooling rate of 50°C / s for 9 seconds.

[0043] After the above process steps, the formed component samples were taken out and the results were obtained by selecting 4 measuring points (measurement points 1 and 2 were 310℃ slow cooling and pressure maintenance, and measurement points 3 and 4 were room temperature mold pressure maintenance):

[0044] Serial number Yield / MPa Tensile strength / MPa Elongation Impact toughness / J·cm2 Measuring point 1 1450 2360 11.4% 36.3 Measuring point 2 1455 2362 11.2% 36.2 Measuring point 3 1660 2484 6.9% 35.0 Measuring point 4 1658 2489 6.7% 35.1

[0045] Example 2

[0046] (1) Select a high-strength steel plate with a thickness of 4 mm and cut it into a suitable stamping profile on a laser cutting machine. Heat the cut high-strength steel plate to 930°C in a vacuum heating furnace and keep it warm for 15 minutes so that the inside of the plate is completely transformed into austenite.

[0047] (3) Stamping and isothermal pressure holding: Heat different parts of the mold to different temperatures. Parts with plasticity requirements are kept at 300°C, and parts with high strength requirements are kept at room temperature. Then the fully austenitized high-temperature and high-strength steel sheet is quickly transferred from the vacuum heating furnace to the hot stamping water-cooled mold. While completing the stamping, a large amount of cooling water is injected into the mold to cool the high-temperature sheet to about 300°C within 5 seconds. Monitor the sheet temperature so that different parts of the formed component are kept at different cooling rates. The heated parts of the mold continue to be kept at a cooling rate of 0.5-1°C / S for 8 seconds, and the normal temperature mold parts are kept at a cooling rate of 50°C / s for 8 seconds.

[0048] After the above process steps, the formed component samples were taken out and the results of the measurements at 4 measuring points were as follows (measurement points 5 and 6 were 300℃ slow cooling and pressure maintenance, and measurement points 7 and 8 were room temperature mold pressure maintenance):

[0049] Serial number Yield / MPa Tensile strength / MPa Elongation Impact toughness / J·cm2 Measuring point 5 1610 2461 9.8% 24.3 Measuring point 6 1612 2464 9.6% 24.4 Measuring point 7 1746 2580 5.7% 23.9 Measuring point 8 1742 2576 5.9% 23.8

[0050] Compared with the prior art, the present invention provides a hot forming method, which controls the temperature of different areas of the mold so that the corresponding parts of the steel are cooled at different rates. The parts of the steel component that require strength are quickly cooled to form a high-strength zone, and the parts that require toughness are slowly cooled to form a high-toughness zone. The interior of the high-toughness zone is a complex phase structure of martensite and lower bainite. After forming and quenching, no secondary heat treatment is performed, but only an isothermal pressure holding process is added. This not only avoids the reduction in strength of the formed component caused by tempering, but also ensures that specific areas of the component have higher toughness.

[0051] 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 hot forming method, characterized in that: It includes the following steps: S1. Process the steel into a size suitable for stamping and heat it to 910-950℃, keeping it warm for 10-15 minutes; S2, heating the first part of the mold to 290-310° C., keeping the second part of the mold at room temperature, transferring the workpiece processed in step S1 to the mold, injecting cooling water into the mold to cool the workpiece during stamping, and cooling the workpiece to 290-310° C. within 5 seconds. During this process, the temperature of the first part is kept at 290-310° C.; S3, interrupting the supply of cooling water to cool the workpiece under pressure in the mold, and controlling the cooling rate of the first part so that the area of ​​the workpiece corresponding to the first part is also cooled at a certain rate for a certain time; controlling the cooling rate of the area of ​​the workpiece corresponding to the first part to be 0.5-1°C / s, or controlling the temperature of the area of ​​the workpiece corresponding to the first part to be maintained at 290-310°C, and the workpiece is kept under pressure in the mold for 5-10s; S4: The first part stops heating, and the workpiece cools naturally to room temperature.

2. The hot forming method according to claim 1, characterized in that: In step S1, a protective gas is used when heating and keeping the steel warm to prevent oxidation and decarburization of the steel.

3. The hot forming method according to claim 1, characterized in that: In step S2, the time for transferring the workpiece to the mold is 3 to 7 seconds, and the temperature of the workpiece after the transfer is completed is greater than 550°C.

4. The hot forming method according to claim 3, characterized in that: In step S2, the workpiece is water quenched at a cooling rate of 70-100°C / s.

5. The hot forming method according to claim 1, characterized in that: In step S2, the punching speed is 50-100 mm / s, and the friction coefficient is 0.1-0.

4.

6. The hot forming method according to claim 1, characterized in that: In step S3, the pressure-maintaining cooling time is 5-10 seconds.

7. The hot forming method according to claim 1, characterized in that: The first portion has a heater, and the second portion has a cooling water conduit.

8. A steel component, characterized in that: It is processed by the hot forming method described in any one of claims 1 to 7, the area of ​​the steel component corresponding to the first part is a high-toughness area, the area of ​​the steel component corresponding to the second part is a high-strength area, the elongation of the high-toughness area is higher than that of the high-strength area, and the yield strength of the high-strength area is higher than that of the high-toughness area.

Images