A low yield strength ratio, high ductility, 500MPa grade automotive chassis steel and its production method

Abstract

This invention relates to the field of steel production technology, specifically to a low yield strength ratio, high plasticity 500MPa grade automotive chassis steel and its production method. The production method includes smelting, continuous casting, hot rolling, cooling, coiling, slow cooling, leveling, pickling, oiling, and packaging after performance testing and surface inspection to obtain the finished product. The steel has a yield strength of 330~360MPa, a tensile strength of 500~550MPa, a yield strength ratio ≤0.71, and an elongation A... 50 ≥40%. This invention uses only C and Mn-strengthened low-cost composition design, significantly reducing alloy costs; in the hot rolling process, a large reduction rate in rough rolling is combined with low-temperature finishing rolling, and high-temperature coiling and slow cooling in stacking are used to achieve uniform microstructure, stable performance, low yield strength ratio, and increased safety; this steel grade only undergoes hot rolling leveling, without pickling leveling, and the leveling rolling force is strictly controlled to avoid work hardening after leveling, which reduces the plasticity of the steel.

Description

Technical Field

[0001] This invention relates to the field of steel production technology, specifically to a low yield strength ratio, high plasticity 500MPa grade automotive chassis steel and its production method. Background Technology

[0002] With the development of lightweighting in automobiles and the continuous upgrading of the automotive industry, OEMs have increasingly higher requirements for the steel used in automotive structures. For some complex automotive chassis structural components, such as the rear body of the left and right front longitudinal beams, the rear reinforcement plates of the left and right front longitudinal beams, and the upper and lower plates of the rear subframe, the requirements for the plastic deformation capacity of materials are extremely strict, requiring steel with both high strength and high plasticity. Moreover, traditional automotive structural steel has a high yield strength ratio, which makes it prone to defects such as stamping cracks and stamping necking thinning in areas of large deformation, resulting in poor adaptability. On the other hand, the lower the yield strength ratio of the steel, the greater its reliability when subjected to stress exceeding the yield point, and the higher the safety of the steel structure.

[0003] Patent CN102719755A describes a high-strength, high-formability hot-rolled pickled steel sheet for automotive structures and its production method. However, this patent has a drawback: the coiling temperature is set within the Ar1 phase transformation range, which can lead to austenite residue during production. Combined with the relatively slow cooling rate, the austenite residue continues to grow at high temperatures, easily causing abnormal grain growth in the steel coil, thus affecting the steel's properties and hindering practical applications. Furthermore, the addition of Cr and Ti alloying elements to this hot-rolled pickled steel sheet increases costs; additionally, its high leveling elongation during pickling (1%~2%) can easily cause work hardening of the steel sheet, affecting the material's plastic deformation capacity.

[0004] Patent CN109440005A describes a SAPH440 grain-refining steel and its production method. However, a drawback of this patent is that the elongation of this steel is only between 35% and 37%, making it unsuitable for automotive structural parts with complex forming requirements and extremely high plasticity. Furthermore, while the use of boron (B) for strengthening increases the steel's strength, it also increases its brittleness, affecting its plastic deformation capacity.

[0005] Patent CN111979478A describes a thin-gauge SAPH440 strip steel and its production method. The strip steel in this patent has a thickness of 1.6~2.5mm and uses Ti and B elements for strengthening. Not only is the alloy cost high, but the B element also increases the brittleness of the steel, resulting in a decrease in plasticity. The elongation of this steel is 32%~40%, which is not suitable for automotive structural parts with complex forming and extremely high plasticity requirements.

[0006] It is evident that most existing technologies use the addition of alloying elements such as Nb, Ti, Cr, and B to increase the strength of automotive structural steel. This not only results in high production costs, but also leads to a significant decrease in elongation as the strength of the steel increases. Furthermore, under traditional rolling processes, once the tensile strength of the steel coil exceeds 500 MPa, the elongation is mostly below 40%, resulting in a high yield strength ratio. Summary of the Invention

[0007] To address the technical problem that existing materials cannot simultaneously meet the requirements of high strength, high plasticity, and safety for some automotive chassis steels, this invention provides a low yield strength ratio, high plasticity 500MPa grade automotive chassis steel and its production method.

[0008] In a first aspect, the present invention provides a method for producing low yield strength ratio, high ductility, 500MPa grade automotive chassis steel, comprising:

[0009] (1) Smelting and continuous casting: By weight percentage, the composition of molten steel is: C 0.09%~0.12%, Si≤0.03%, Mn 1.55%~1.95%, P≤0.010%, S≤0.004%, Alt 0.050%~0.065%, with the remainder being Fe and unavoidable impurities; C and Mn elements play a role in solid solution strengthening, Ti element easily reacts with N, leading to unstable strength, Nb element alloy is expensive and easily increases yield strength ratio, and Si element affects the surface quality of steel coil.

[0010] (2) Hot rolling: The billet is sent into the heating furnace. The billet exit temperature is 1190~1230℃ and the time in the furnace is ≥160min. After exiting the furnace, the descaling is started in all passes. The descaling pressure is ≥25Mpa to ensure the descaling effect. The roughing rolling adopts a large reduction rate. The reduction rate ranges from 26% to 40% per pass. The exit temperature of the last pass of the roughing rolling is 960~1000℃. The exit temperature of the finishing rolling is controlled at 760~800℃.

[0011] (3) Cooling and winding: Laminar flow cooling mode is adopted; winding temperature is 660~700℃;

[0012] (4) Slow cooling and leveling: Stacking and slow cooling are carried out in the slow cooling area for ≥48 hours to make the grains uniform; after slow cooling to room temperature, hot rolling is used for leveling, with leveling rolling force ≤2500KN and leveling elongation ≤0.8%; surface wavy and tower-shaped defects are repaired to avoid leveling causing work hardening and affecting plastic deformation.

[0013] (5) Pickling and oiling;

[0014] (6) Conduct performance testing and surface inspection. If the product passes the inspection, package it to obtain the finished product.

[0015] Furthermore, in step (1), the residual element control requirements for the billet are: Nb≤0.005%, Ti≤0.005%, V≤0.005%, Mo≤0.05%, Cu≤0.05%, Cr≤0.05%, Ni≤0.05%, B≤0.0006%; the harmful gas control requirements are: O≤0.0030%, N≤0.0040%, H≤0.0002%; to reduce the impact of residual elements and harmful gases on the billet performance.

[0016] Furthermore, in step (1), LF single-stage refining is used in smelting, and the soft blowing time is not less than 12 minutes, which effectively reduces the presence of inclusions; the LF treatment cycle is ≥25 minutes.

[0017] Furthermore, in step (1), continuous casting is carried out under full protection. The superheat of the molten steel in the tundish is 20~35℃, the argon flow rate is 2~10L / min, and the casting speed is controlled at 0.9~1.4m / min. Constant casting speed is required, with a casting speed fluctuation of ±0.1m / min. The molten steel surface fluctuation and superheat are kept stable to obtain a steel billet with stable quality and a billet thickness of 230mm.

[0018] Furthermore, in step (2), the rough rolling is performed in 7 passes, and the reduction rate of the last 4 passes is between 25% and 36%, and the thickness of the intermediate billet is controlled between 30 and 50 mm.

[0019] Furthermore, in step (2), the intermediate billet is not temperature controlled from the roughing mill to the finishing mill inlet and directly enters the finishing mill. The finishing mill uses 7 passes and the steel coil thickness is 1.5~8mm.

[0020] Furthermore, in step (3), the first two stages of precise laminar flow cooling mode are adopted, and each group ensures that 1 to 2 cooling water pipes are opened, and the cooling rate is controlled at 8 to 16℃ / s.

[0021] Furthermore, in step (4), the slow-cooling area is surrounded by an insulated wall, which is specifically used to place the insulated products, and the slow-cooling effect is good.

[0022] Furthermore, in step (5), the concentration of free acid used for pickling is controlled at 50~180g / L; the amount of oil applied is ensured to be 3800~4500mg / m² on both sides. 2 .

[0023] Secondly, the present invention also provides a low yield strength to high plasticity 500MPa grade automotive chassis steel manufactured using the above-mentioned production method, with a yield strength of 330~360MPa, a tensile strength of 500~550MPa, a yield strength to high plasticity ratio ≤0.71, and an elongation A. 50≥40%; the thickness of the steel coil is 1.5~8mm, and the width of the steel coil is 800~1900mm; the microstructure is ferrite + pearlite, with ferrite accounting for 75%~85% and pearlite accounting for 15%~25%, and the average grain size is ≤10μm.

[0024] The beneficial effects of this invention are as follows:

[0025] This invention provides a low yield strength ratio, high plasticity 500MPa grade automotive chassis steel and its production method. It employs a low-cost composition design with only C and Mn reinforcement, eliminating the need for additional alloying elements such as silicon, chromium, niobium, vanadium, and titanium, significantly reducing alloy costs. Its hot rolling process utilizes a combination of high reduction in rough rolling and low-temperature finishing rolling to fully refine the steel grains. The steel coils are homogenized through high-temperature coiling and slow cooling in stacks, resulting in stable performance, a low yield strength ratio, and increased safety. Controlled Si content and low-temperature rolling reduce the formation of iron oxide scale, resulting in good surface quality. This steel only undergoes hot rolling leveling, eliminating the need for pickling leveling, and the leveling rolling force is strictly controlled to avoid work hardening after leveling, which reduces the steel's plasticity. This low yield strength ratio, high plasticity automotive chassis steel exhibits good formability, strong plastic deformation capacity, low production cost, stable performance, and good surface quality, making it suitable for various complex automotive chassis structural components and possessing broad market prospects. Attached Figure Description

[0026] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0027] Figure 1 These are typical metallographic images of the 1.8mm steel coil in Example 1.

[0028] Figure 2 These are typical metallographic images of the 4.0mm steel coil in Example 2.

[0029] Figure 3 These are typical metallographic images of the 8.0mm steel coil in Example 3. Detailed Implementation

[0030] To enable those skilled in the art to better understand the technical solutions of this invention, the technical solutions of the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of this invention.

[0031] Example 1

[0032] The production method of a 1.8mm thick, low yield strength ratio, high plasticity, 500MPa grade automotive chassis steel is as follows:

[0033] (1) Smelting and continuous casting: The composition of molten steel by weight percentage is: C 0.10%, Si 0.02%, Mn 1.61%, P 0.007%, S 0.002%, Alt 0.055%, with the remainder being Fe and unavoidable impurities; residual element control: Nb 0.001%, Ti 0.001%, V 0.001%, Mo 0.002%, Cu 0.012%, Cr 0.021%, Ni 0.007%, B 0.0002%; harmful gas control: O 0.0020%, N 0.0029%, H 0.00012%, to reduce the impact of residual elements and harmful gases on the performance of the steel coil. LF single-stage refining is used, with a soft blowing time of 12 min and an LF treatment cycle of 25 min. During continuous casting, the entire process is protected during pouring. The tundish superheat is 25°C, the argon flow rate is 6L / min, the casting speed is controlled at 1.1m / min, and the casting speed is constant. The casting speed fluctuation in this pour is 0m / min. This maintains the stability of the molten steel surface fluctuation and superheat, resulting in a steel billet with stable quality and a thickness of 230mm.

[0034] (2) Hot rolling: The billet is fed into the heating furnace. The billet exit temperature is 1208℃ and the furnace time is 183min. After exiting the furnace, the descaling is started in all passes. The descaling pressure is 28MPa to ensure the descaling effect. The roughing rolling adopts a large reduction rate, with a reduction rate range of 26%~40% per pass. There are 6 passes in the roughing rolling. The reduction rates of the last 4 passes are 27%, 29%, 33%, and 35%, respectively. The thickness of the intermediate billet obtained by roughing rolling is 36mm. The exit temperature of the last pass of roughing rolling is 972℃. The intermediate billet does not have temperature control from the entry of the roughing rolling mill to the finishing mill and directly enters the finishing mill. The finishing rolling adopts 7 passes. The exit temperature of the finishing mill is controlled at 778℃, and the thickness of the steel coil is 1.5mm. Under this rolling process, the steel grains are fully refined.

[0035] (3) Cooling and winding: The first two sections (a total of 8 groups) adopt a precise laminar flow cooling mode, with 1 to 2 cooling water pipes opened in each group, and the cooling rate controlled at 10℃ / s; the winding temperature is 682℃.

[0036] (4) Slow cooling and leveling: The slow cooling area is surrounded by an insulation wall and is specifically used to place insulation products. The products are stacked and slow cooled in the slow cooling area for 56 hours to make the grains uniform. After slow cooling to room temperature, hot rolling is used to level the surface and repair defects such as wavy and tower-shaped defects. The leveling rolling force is controlled at 1900KN and the leveling elongation rate is 0.6% to avoid the leveling process hardening from affecting plastic deformation.

[0037] (5) Pickling and oiling: The free acid concentration for pickling is controlled at 70g / L~160g / L; the oiling amount is controlled at 4000~4300mg / m² on both sides. 2 .

[0038] (6) Conduct performance testing, surface inspection, and other tests. Mechanical properties are tested by sampling from the beginning, middle, and end of the coil. The test results are shown in Table 1. Typical metallographic images of the steel coil are shown below. Figure 1 As shown; the test results are qualified, and the product is packaged to obtain the finished product.

[0039] Table 1. Mechanical property test results of steel coils in Example 1

[0040]

[0041] Example 2

[0042] The production method of a 4.0mm thick, low yield strength ratio, high plasticity, 500MPa grade automotive chassis steel is as follows:

[0043] (1) Smelting and continuous casting: The composition of molten steel by weight percentage is: C 0.11%, Si 0.01%, Mn 1.73%, P 0.006%, S 0.002%, Alt 0.058%, with the remainder being Fe and unavoidable impurities; residual element control: Nb 0.002%, Ti 0.001%, V 0%, Mo 0.002%, Cu 0.010%, Cr 0.018%, Ni 0.006%, B 0.0002%; harmful gas control: O 0.0018%, N 0.0022%, H 0.00014%, to reduce the impact of residual elements and harmful gases on the performance of the steel coil. LF single-stage refining is used, with a soft blowing time of 14 min and an LF treatment cycle of 30 min. During continuous casting, the entire process is protected during pouring. The tundish superheat is 26℃, the argon blowing flow rate is 7L / min, and the casting speed is controlled at 1.0~1.1m / min. The casting speed is constant, with a fluctuation of 0.1m / min in this pour. This maintains the stability of the molten steel surface fluctuation and superheat, resulting in a steel billet with stable quality and a thickness of 230mm.

[0044] (2) Hot rolling: The billet is fed into the heating furnace. The billet exit temperature is 1221℃ and the furnace time is 196min. After exiting the furnace, the descaling is started in all passes. The descaling pressure is 28MPa to ensure the descaling effect. The roughing rolling adopts a large reduction rate, with a reduction rate range of 26%~40% per pass. There are 6 passes in the roughing rolling. The reduction rates of the last 4 passes are 26%, 29%, 34%, and 36%, respectively. The thickness of the intermediate billet obtained by roughing rolling is 42mm. The exit temperature of the last pass of roughing rolling is 986℃. The intermediate billet does not have temperature control from the entry of the roughing rolling mill to the finishing mill and directly enters the finishing mill. The finishing rolling adopts 7 passes. The exit temperature of the finishing mill is controlled at 783℃, and the thickness of the steel coil is 4.0mm. Under this rolling process, the steel grains are fully refined.

[0045] (3) Cooling and winding: The first two sections (a total of 8 groups) adopt a precise laminar flow cooling mode, with 1 to 2 cooling water pipes opened in each group, and the cooling rate controlled at 12℃ / s; the winding temperature is 668℃.

[0046] (4) Slow cooling and leveling: The slow cooling area is surrounded by an insulation wall and is specifically used to place insulation products. The products are stacked and slow cooled in the slow cooling area for 60 hours to make the grains uniform. After slow cooling to room temperature, hot rolling is used to level the surface and repair defects such as wavy and tower-shaped defects. The leveling rolling force is controlled at 2000KN and the leveling elongation rate is 0.7% to avoid the leveling process hardening from affecting plastic deformation.

[0047] (5) Pickling and oiling: The free acid concentration for pickling is controlled at 70g / L~163g / L; the oiling amount is controlled at 4100~4400mg / m² on both sides. 2 .

[0048] (6) Conduct performance testing, surface inspection, and other tests. Mechanical properties are tested by sampling from the beginning, middle, and end of the coil. The test results are shown in Table 2. Typical metallographic images of the steel coil are shown below. Figure 2 As shown; the test results are qualified, and the product is packaged to obtain the finished product.

[0049] Table 2. Mechanical property test results of steel coils in Example 2

[0050]

[0051] Example 3

[0052] The production method of a 500MPa grade automotive chassis steel with a thickness of 8.0mm, low yield strength ratio, and high plasticity is as follows:

[0053] (1) Smelting and continuous casting: By weight percentage, the composition of molten steel is C 0.11%, Si 0.009%, Mn 1.80%, P 0.005%, S 0.003%, Alt 0.060%, with the remainder being Fe and unavoidable impurities; residual element control: Nb 0%, Ti 0.002%, V 0.002%, Mo 0.004%, Cu 0.008%, Cr 0.028%, Ni 0.005%, B 0.0004%, to reduce the impact of residual elements and harmful gases on the performance of the steel coil. LF single-stage refining is used, with a soft blowing time of 16 min and an LF treatment cycle of 35 min. During continuous casting, the entire process is protected during pouring. The tundish superheat is 30°C, the argon flow rate is 7L / min, the casting speed is controlled at 1.2m / min, and the casting speed is constant. The casting speed fluctuation in this pour is 0m / min, which keeps the molten steel level fluctuation and superheat stable, resulting in a steel billet with stable quality and a thickness of 230mm.

[0054] (2) Hot rolling: The billet is fed into the heating furnace. The billet exit temperature is 1213℃ and the furnace time is 203min. After exiting the furnace, the descaling is started in all passes. The descaling pressure is 28MPa to ensure the descaling effect. The roughing rolling adopts a large reduction rate, with a reduction rate range of 26%~40%. There are 6 passes in the roughing rolling. The reduction rates of the last 4 passes are 26%, 29%, 32%, and 33%, respectively. The thickness of the intermediate billet obtained by roughing rolling is 48mm. The exit temperature of the last pass of roughing rolling is 991℃. The intermediate billet does not have temperature control from the entry of the roughing rolling mill to the finishing mill and directly enters the finishing mill. The finishing rolling adopts 7 passes. The exit temperature of the finishing mill is controlled at 791℃, and the thickness of the steel coil is 8.0mm. Under this rolling process, the steel grains are fully refined.

[0055] (3) Cooling and winding: The first two sections (a total of 8 groups) adopt a precise laminar flow cooling mode, with 1 to 2 cooling water pipes opened in each group, and the cooling rate controlled at 11℃ / s; the winding temperature is 678℃.

[0056] (4) Slow cooling and leveling: The slow cooling area is surrounded by an insulation wall and is specifically used to place insulation products. The products are stacked and slow cooled in the slow cooling area for 60 hours to make the grains uniform. After slow cooling to room temperature, hot rolling is used to level the surface and repair defects such as wavy and tower-shaped defects. The leveling rolling force is controlled at 1900KN and the leveling elongation rate is 0.5% to avoid the leveling process hardening from affecting plastic deformation.

[0057] (5) Pickling and oiling: The free acid concentration for pickling is controlled at 80g / L~176g / L; the oiling amount is controlled at 4200~4400mg / m² on both sides. 2 .

[0058] (6) Conduct performance testing, surface inspection, and other tests. Mechanical properties are tested by sampling from the beginning, middle, and end of the coil. The test results are shown in Table 3. Typical metallographic images of the steel coil are shown below. Figure 3 As shown; the test results are qualified, and the product is packaged to obtain the finished product.

[0059] Table 3. Mechanical property test results of steel coils in Example 3

[0060]

[0061] Although the present invention has been described in detail with reference to the accompanying drawings and preferred embodiments, the present invention is not limited thereto. Various equivalent modifications or substitutions can be made to the embodiments of the present invention by those skilled in the art without departing from the spirit and essence of the invention, and such modifications or substitutions should all be within the scope of the present invention. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should also be covered within the protection scope of the present invention.

Claims

1. A method for producing low yield strength ratio, high ductility, 500MPa grade automotive chassis steel, characterized in that, include: (1) Smelting and continuous casting: The composition of molten steel by weight percentage is: C 0.09%~0.12%, Si≤0.03%, Mn1.55%~1.95%, P≤0.010%, S≤0.004%, Alt 0.050%~0.065%, with the remainder being Fe and unavoidable impurities; (2) Hot rolling: The billet is sent into the heating furnace. The billet exit temperature is 1190~1230℃ and the time in the furnace is ≥160min. After exiting the furnace, the descaling is started in all passes with a descaling pressure ≥25Mpa. The roughing rolling adopts a large reduction rate, with a reduction rate range of 26%~40% per pass. The exit temperature of the last pass of the roughing rolling is 960~1000℃. The exit temperature of the finishing rolling is controlled at 760~800℃. (3) Cooling and winding: Laminar flow cooling mode is adopted; winding temperature is 660~700℃; (4) Slow cooling and leveling: Stacking and slow cooling are carried out in the slow cooling zone for ≥48 hours to homogenize the grains; after slow cooling to room temperature, hot rolling is used for leveling, with leveling rolling force ≤2500KN and leveling elongation ≤0.8%; (5) Pickling and oiling; (6) Conduct performance testing and surface inspection. If the product passes the inspection, package it to obtain the finished product. The yield strength of the low yield-to-tensile ratio, high ductility 500MPa grade automotive chassis steel is 330~360MPa, tensile strength is 500~550MPa, yield-to-tensile ratio is ≤0.71, and elongation is A. 50 ≥40%.

2. The production method as described in claim 1, characterized in that, In step (1), the residual element control requirements for the steel billet are: Nb≤0.005%, Ti≤0.005%, V≤0.005%, Mo≤0.05%, Cu≤0.05%, Cr≤0.05%, Ni≤0.05%, B≤0.0006%; the harmful gas control requirements are: O≤0.0030%, N≤0.0040%, H≤0.0002%.

3. The production method as described in claim 1, characterized in that, In step (1), LF single-stage refining is used for smelting, the soft blowing time is not less than 12 min, and the LF treatment cycle is ≥25 min.

4. The production method as described in claim 1, characterized in that, In step (1), continuous casting is carried out under full protection. The superheat of the molten steel in the tundish is 20~35℃, the argon blowing flow rate is 2~10L / min, and the casting is carried out at a constant speed. The casting speed is controlled at 0.9~1.4m / min, and the casting speed fluctuation is ±0.1m / min. The molten steel surface fluctuation and superheat are kept stable, so as to obtain a steel billet with stable quality and a billet thickness of 230mm.

5. The production method as described in claim 1, characterized in that, In step (2), the rough rolling is performed in 7 passes, and the reduction rate of the last 4 passes is between 25% and 36%, and the thickness of the intermediate billet is controlled between 30 and 50 mm.

6. The production method as described in claim 1, characterized in that, In step (2), the intermediate billet is not temperature controlled at the entrance of the finishing mill from the roughing mill and directly enters the finishing mill. The finishing mill is rolled in 7 passes to obtain a steel coil with a thickness of 1.5~8mm.

7. The production method as described in claim 1, characterized in that, In step (3), the first two stages of precise laminar flow cooling mode are adopted, with 1 to 2 cooling water pipes opened in each group, and the cooling rate is controlled at 8 to 16℃ / s.

8. The production method as described in claim 1, characterized in that, In step (4), the slow-cooling area is surrounded by an insulated wall.

9. The production method as described in claim 1, characterized in that, In step (5), the concentration of free acid used for pickling is controlled at 50~180g / L; the amount of oil applied is 3800~4500mg / m² on both sides. 2 .

10. A low yield strength ratio, high plasticity 500MPa grade automotive chassis steel manufactured using the production method described in any one of claims 1-9, characterized in that, The thickness of the steel coil is 1.5~8mm, and the width of the steel coil is 800~1900mm; the microstructure is ferrite + pearlite, with ferrite accounting for 75%~85% and pearlite accounting for 15%~25%, and the average grain size is ≤10μm.

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