Warm forming processing method of ultrahigh-strength hot-rolled steel plate

Abstract

The invention discloses a warm forming machining method for an ultrahigh-strength hot-rolled steel plate, and belongs to the technical field of machining and application of the ultrahigh-strength hot-rolled steel plate. According to the machining method, the content of alloy elements in steel and the quality of a continuous casting billet should be controlled; the heating temperature, the finish rolling temperature and the coiling temperature in hot rolling are controlled; the temperature and the heat preservation time of softening and annealing heat treatment are controlled, preferably, inert gas or reducing gas is arranged in the furnace for protection, surface oxidation, decarburization and the like can be prevented, and the high surface requirement of the steel plate is met; and the temperature during part blank forming is controlled, it is guaranteed that steel burning is uniform and consistent during heating, and air cooling is conducted on the formed part. According to the part produced through the warm forming machining method, the yield strength reaches 950 MPa or above, the tensile strength reaches 1300-1550 MPa, the ductility reaches 6% or above, the toughness (AKv) is larger than or equal to 40 J at the low temperature of-40 DEG C, and the thickness specification coverage range of a steel plate is 1.5-20.0 mm; the strength value of the part is doubled, the weight of the automobile part can be greatly reduced, and meanwhile the material formability and safety are remarkably improved.

Description

Technical Field

[0001] The invention relates to a warm forming processing method for an ultra-high strength hot-rolled steel plate, and belongs to the technical field of application and processing of ultra-high strength hot-rolled steel plates. Background Art

[0002] Under the background of the rapid development of the global electric vehicle industry, the contradiction between the endurance of electric vehicles and the weight of the whole vehicle has become an important factor restricting the development of the electric vehicle industry. There may be various ways to solve this problem, but the most direct and effective way at present is to reduce the weight of the whole vehicle to provide space for large-capacity batteries, which requires the body of the electric vehicle to be made of higher-strength materials, which not only reduces the weight of the body, but also improves the safety of the vehicle.

[0003] As key components to ensure vehicle safety, the weight reduction technology of automobile pillars, anti-collision beams, wheels, etc. is the primary bottleneck that needs to be broken through. To this end, automobile designers propose to at least double the material strength of more than 70% of the body parts, so that the weight reduction of safety parts such as pillars, anti-collision beams, wheels, etc. will provide optimization space for battery life (large capacity).

[0004] The performance of automotive steel produced by existing technology (low strength and plasticity) can no longer meet the weight reduction requirements of electric vehicle parts. There is an urgent need for an ultra-high-strength steel material that can not only meet the vehicle's weight reduction requirements, but also meet its complex molding and safety requirements. Summary of the invention

[0005] The present invention aims to provide a warm forming processing method for ultra-high strength hot-rolled steel plates to realize the manufacture of ultra-high strength automotive parts. The required material thickness specifications cover a range of 1.5-20.0 mm and can be stably mass-produced.

[0006] In the present invention, in order to facilitate the pre-processing work before forming such as unrolling of steel coils and blanking of steel plates, the steel plates need to be softened and annealed. After softening, the yield strength of the steel plates is 400-600MPa, the tensile strength is 600-850MPa, and the elongation is 25-40%; then the steel plates are sheared into part blanks by blanking, the blanks are heated, and finally a special forming process (warm forming) is used to make the final yield strength of the parts ≥950MPa, the tensile strength reaches 1300-1550MPa, the elongation is ≥6%, and the toughness (AKv) is ≥40J at low temperature of -40°C, and the strength reaches more than twice that of the current highest strength material, achieving a doubling of the strength value, greatly reducing the weight of automotive parts, and at the same time greatly improving the formability and safety of the material.

[0007] The present invention provides a warm forming processing method for ultra-high strength hot-rolled steel plates, comprising the process of "steel plate production (continuous casting billet heating, rolling, heat treatment, performance testing) + parts forming (steel plate blanking and cutting, parts blank heating, stamping (warm forming), performance testing)".

[0008] In the above-mentioned warm forming processing method, the following factors should be focused on: ① The content of alloy elements in steel and the quality control of continuous casting billets, including the removal of non-metallic inclusions and center segregation; ② Control the hit rate of heating, final rolling and coiling temperature in hot rolling; ③ Accurately control the temperature and holding time during softening annealing heat treatment. If conditions permit, load the steel into a furnace protected by inert gas or reducing gas as much as possible, which can prevent surface oxidation and decarburization, which is beneficial to the high surface requirements of the steel plate; ④ When forming the blank of the part, focus on controlling the forming temperature and ensure that the steel is burned evenly during heating. After forming, the parts can be air-cooled.

[0009] The warm forming processing method provided by the present invention specifically includes the following contents: (1) To facilitate pretreatment before forming such as flattening of steel coils and blanking and shearing of steel plates, the steel plates need to be softened and annealed. After softening, the yield strength of the steel plates is 400-600MPa, the tensile strength is 600-850MPa, and the elongation is 25-40%; The specific process of softening annealing treatment is: uniform heating to 550-670℃, ensuring that the temperature difference in the furnace does not exceed 15℃ during the heating process; keeping warm for more than 2 hours, when the keeping warm time is up, change the cooling cover to cool to 60-150℃ and air cool out of the furnace; (2) Requirements are put forward for the control of chemical composition and hot rolling process during the production of hot-rolled steel plates. The control range of chemical composition is: C: 0.02-0.15wt%, Si≤0.25wt%, Mn: 2.0-15.0wt%, P≤0.012 wt%, S≤0.002wt%, Al: 0.02-0.07 wt%, [N]≤0.008wt%, [O]≤0.004wt%, [H]≤0.00015wt%, and trace alloying elements required for other properties. The rest are Fe and unavoidable impurities. (3) In addition to controlling the chemical composition during smelting, the focus is also on the temperature control of molten steel during the smelting process: converter tapping temperature ≥1645℃, LF refining furnace temperature 1630-1645℃, RH vacuum treatment temperature 1560-1575℃, continuous casting tundish temperature 1535-1550℃, and at the same time, the soft stirring time of molten steel ≥10 minutes and the calming time ≥5 minutes, so that the level of non-metallic inclusions in the steel (four categories A, B, C, and D) are all below level 0.5, and the center segregation is below level 0.5; (4) After the steel plate is cut into part blanks, the warm forming process is used. The specific requirements are as follows: the part blanks (to ensure high-quality surface, they should be placed in a furnace with a reducing atmosphere as much as possible) are heated to 650-890°C to ensure that they are burned through and the temperature of the blanks is uniform. The parts are formed in a mold (the mold does not need to be filled with water, oil or other fast cooling media). The parts are taken out and cooled in the air. The performance of the dissected parts should meet the following requirements: yield strength ≥950MPa, tensile strength reaches 1300-1550MPa, elongation ≥6%, and toughness (AKv) ≥40J at low temperature of -40°C.

[0010] Furthermore, in the hot rolling process, the heating temperature is controlled at 1230-1280°C, the final rolling temperature is controlled at 820-900°C, the coiling temperature is controlled at 250-520°C, and the rolling speed is set at 5.5-15.0m / s, so that the mechanical properties of the hot-rolled steel plate can reach: yield strength Rp 0.2 ≥950MPa, tensile strength R m It is 1050-1450MPa, and the elongation A≥8%.

[0011] Furthermore, the softening annealing treatment of steel plates requires that the coils (or plates) be carried out in a bell-type furnace or box-type furnace protected by inert gas or reducing gas. The specific process is: uniform heating to 550-670°C, and during the heating process, the temperature difference in the furnace (the inner and outer circles of the steel coil, or the upper and lower layers of the steel plate) is ensured not to exceed 15°C; the holding time is 2 hours (here 2 hours refers to the theoretical time required for the unit weight (1 kg) of steel plate to be heated to the target temperature, and the temperature is consistent and uniform. In actual production, the holding time should be converted accordingly based on factors such as furnace loading and heating power, until the temperature of the inner and outer circles of the steel coil, and the upper and lower layers of the steel plate stack are consistent and uniform) for more than 2 hours. When the holding time is up, replace the cooling hood to cool to 60-150°C and air cool out of the furnace.

[0012] The warm forming production method provided by the present invention produces automobile parts with a yield strength of ≥950MPa, a tensile strength of 1300-1550MPa, an elongation of ≥6%, a toughness (AKv) of ≥40J at a low temperature of -40°C, and a steel plate thickness specification covering a range of 1.5-20.0mm.

[0013] Beneficial effects of the present invention: (1) After soft annealing, the yield strength of the steel plate is 400-600MPa, the tensile strength is 600-850MPa, and the elongation is 25-40%. The purpose of setting soft annealing is to facilitate the production needs of the front processes such as flattening, blanking and shearing of steel coils; (2) Existing material forming technologies are divided into cold forming (normal ambient operating temperature, usually -10℃-30℃) and hot forming (temperature required for austenitization, usually 950-1250℃), while the warm forming temperature referred to in the present invention is 650-890℃. Since warm forming is 60-600℃ lower than the heating temperature of traditional hot forming steel, usually at least 200-300℃ lower, controlling this forming temperature can reduce energy consumption and extend furnace life during part processing; the cooling condition is air cooling, which reduces the control difficulty compared with traditional hot forming steel (water is added to the mold during cooling), thereby saving manufacturing costs and having greater promotion significance; (3) The technical solution proposed in the present invention can produce hot-rolled steel plates with a thickness specification range of 1.5-20.0 mm, a yield strength of ≥950 MPa after warm forming, a tensile strength of 1300-1550 MPa, an elongation of ≥6%, and a toughness (AKv) of ≥40 J at low temperature of -40°C on the existing hot rolling production line; the hot-rolled steel plates have excellent strength-toughness matching and are easy to weld. DETAILED DESCRIPTION

[0014] The specific implementation manner of the present invention is described in detail below in conjunction with the embodiments, but the specific implementation manner of the present invention is not limited to the following embodiments. Example 1

[0015] The production requirements of hot-rolled steel plates for automobile wheels with specifications of 2.7×1500×C mm are as follows: Ⅰ Smelting The chemical composition of the ingot (weight percentage, wt%) is: C: 0.10; Si: 0.05; Mn: 4.76; P: 0.010; S: 0.002; Al: 0.033; [N]: 0.0045wt%; [O]: 0.0021wt%, [H]: 0.00003wt%, and the rest are residual elements inevitably brought into the molten iron and scrap steel, the lower the content, the better, so that the non-metallic inclusions A, B, and C in the steel are all level 0, D is level 0.5, and the center segregation is level 0.5.

[0016] Ⅱ Rolling The continuous casting slab is heated in a walking beam heating furnace at a temperature of 1254°C. The heating ensures that the slab is burned through and the alloy elements are fully dissolved, but overburning and overheating must be prevented. The heating time is 204 minutes.

[0017] The final rolling temperature is 860°C, the coiling temperature is 490°C, and the rolling speed is set at 10.5 m / s.

[0018] Ⅲ Softening annealing treatment The steel coil is heated in a hydrogen protection hood furnace at a temperature of 660°C. The heating ensures that the temperature difference between the inner and outer rings of the steel coil does not exceed 10°C. After 8 hours of insulation time, the cooling hood is replaced to cool to 110°C and then air-cooled out of the furnace. Samples are taken at room temperature to test the performance.

[0019] It should be noted that theoretically, it takes 2 hours to heat a steel plate of unit weight (1 kg) to the target temperature with a consistent and uniform temperature. In actual production, the insulation time is calculated based on parameters such as furnace weight (30-90 tons) and heating power.

[0020] After softening annealing treatment, the performance test results are: yield strength is 554MPa, tensile strength is 694MPa, and elongation is 35%.

[0021] Ⅳ Warm Forming The part blanks are heated in a continuous electric induction furnace at 830°C and kept warm until they are burned through and the temperature is consistent. The blanks are transferred to the mold and the instantaneous temperature during stamping is 760°C. They are taken out and air-cooled to room temperature, and their performance is dissected and tested (randomly inspected).

[0022] The results of the anatomical performance test of the parts are: yield strength is 1192MPa, tensile strength is 1340MPa, and elongation is 12%. Example 2

[0023] The production requirements of hot-rolled steel plates for automobile anti-collision beams with specifications of 7.0×1680×C mm are as follows: Ⅰ Smelting The chemical composition of the ingot (weight percentage, wt%) is: C: 0.12; Si: 0.04; Mn: 6.12; P: 0.008; S: 0.001; Al: 0.045; [N]: 0.0033wt%; [O]: 0.0017wt%, [H]: 0.00002wt%, and the rest are residual elements inevitably brought into the molten iron and scrap steel, the lower the content, the better, so that the non-metallic inclusions A, B, C, and D in the steel are all level 0, and the central segregation is level 0.5.

[0024] Ⅱ Rolling The continuous casting slab is heated in a walking beam heating furnace at a temperature of 1265°C to ensure that the slab is burned through and the alloy elements are fully dissolved, but overburning and overheating must be prevented. The heating time is 198 minutes.

[0025] The final rolling temperature is 830°C, the coiling temperature is 340°C, and the rolling speed is set at 8.4 m / s.

[0026] Ⅲ Softening annealing treatment The steel coil is heated in a nitrogen-protected hood furnace at 570°C. The temperature difference between the inner and outer rings of the heated steel coil is ensured to be no more than 13°C. After 12 hours of insulation, the cooling hood is replaced to cool the coil to 70°C and then the coil is air-cooled. Samples are taken at room temperature to test the performance.

[0027] The performance test results are: yield strength is 488MPa, tensile strength is 749MPa, and elongation is 38%.

[0028] Ⅳ Warm Forming The part blanks are heated in a continuous electric induction furnace at 880°C and kept warm until they are burned through and the temperature is consistent. They are then put into the mold and the instantaneous temperature during stamping is 790°C. They are taken out and air-cooled to room temperature, and their performance is dissected and tested (randomly inspected).

[0029] The results of the anatomical performance test of the parts are: yield strength is 1341MPa, tensile strength is 1542MPa, and elongation is 10.2%.

Claims

1. A warm forming method for ultra-high strength hot-rolled steel plate, characterized in that Includes the following: (1) To facilitate pretreatment before forming, the steel plate needs to be softened and annealed. After softening, the yield strength of the steel plate is 400-600MPa, the tensile strength is 600-850MPa, and the elongation is 25-40%; (2) Requirements are put forward for the control of chemical composition and hot rolling process during the production of hot-rolled steel plates. The chemical composition is controlled within the following ranges: C: 0.02-0.15wt%, Si≤0.25wt%, Mn: 2.0-15.0wt%, P≤0.012 wt%, S≤0.002wt%, Al: 0.02-0.07wt%, [N]≤0.008wt%, [O]≤0.004wt%, [H]≤0.00015wt%, and the rest are Fe, trace alloying elements to be added, and unavoidable impurities; (3) Control the temperature of molten steel in the converter, LF refining furnace, RH vacuum treatment, and continuous casting ladle, and reasonably set the soft stirring time, calming time, and protective slag of the molten steel to ensure that the level of non-metallic inclusions in the steel is below 0.5 level and the center segregation is below 0.5 level; (4) After the steel plate is cut into part blanks, the warm forming process is followed. Specific requirements are as follows: the part blanks are heated to 650-890°C to ensure that they are burned through and the temperature of the blanks is uniform. The parts are formed in the mold and taken out to cool in the air.

2. The warm forming method of ultra-high strength hot-rolled steel plate according to claim 1, characterized in that: The softening annealing treatment of steel plates requires that the steel coil or steel plate be carried out in a bell-type furnace or box-type furnace protected by inert gas or reducing gas.

3. The warm forming method of ultra-high strength hot-rolled steel plate according to claim 2, characterized in that: The specific process of softening annealing treatment is: uniformly heat to 550-670℃, and ensure that the temperature difference in the furnace does not exceed 15℃ during the heating process; keep warm for more than 2 hours, and when the keeping warm time is up, replace the cooling cover to cool to 60-150℃ and air cool out of the furnace.

4. The warm forming method of ultra-high strength hot-rolled steel plate according to claim 1, characterized in that: In the hot rolling process, the heating temperature is controlled at 1230-1280℃, the final rolling temperature is controlled at 820-900℃, the coiling temperature is controlled at 250-520℃, and the rolling speed is set at 5.5-15.0m / s, so that the mechanical properties of the hot rolled steel plate reach: yield strength Rp 0.2 ≥950MPa, tensile strength R m It is 1050-1450MPa, and the elongation A≥8%.

5. The warm forming method of ultra-high strength hot-rolled steel plate according to claim 1, characterized in that: Temperature control of molten steel during smelting process: converter tapping temperature ≥1645℃, LF refining furnace temperature 1630-1645℃, RH vacuum treatment temperature 1560-1575℃, continuous casting tundish temperature 1535-1550℃, molten steel soft stirring time ≥10 minutes, calming time ≥5 minutes.

6. The warm forming method of ultra-high strength hot-rolled steel plate according to claim 1, characterized in that: After the warm forming process, the performance of the dissected parts is: yield strength ≥950MPa, tensile strength reaches 1300-1550MPa, elongation ≥6%, toughness ≥40J at low temperature of -40℃, and the steel plate thickness specification covers a range of 1.5-20.0mm.