Steel for automobile frame and manufacturing method therefor

Abstract

The present invention provides a steel for an automobile frame, the tensile strength of the steel for an automobile frame ≥ 850 MPa. The steel comprises the following chemical elements in mass percent: C: 0.05-0.18%, Si: 0.02-0.30%, Mn: 0.8-2.5%, Cr: 0.05-0.8%, Ti: 0.08-0.20%, and Al: 0.01-0.10%. The present invention reasonably designs an alloy content ratio, adds no or only a small amount of precious metal and, on the basis, manufactures the steel for an automobile frame by controlling finish rolling temperature and a laminar cooling rate in combination with high-temperature tempering, so as to obtain a microstructure of tempered martensite and ferrite, in which a TiC nano precipitated phase is dispersively distributed. While reducing alloy costs, the steel reaches required strength and exhibits high plasticity and cold bending performance.

Description

Steel for automobile beam and manufacturing method thereof Technical Field

[0001] The invention belongs to the field of steel production, and in particular relates to a method for manufacturing steel for automobile beams. Background Art

[0002] Against the backdrop of green, low-carbon development and energy conservation and weight reduction, commercial vehicles are experiencing an increasingly strong demand for high-strength steel. Improving the strength of steel can reduce the amount of steel used, thereby reducing the vehicle's deadweight. This can lower both the raw material costs of commercial vehicle manufacturers and the energy consumption during vehicle operation. In recent years, the strength of steel used in commercial vehicle mainframes has continued to rise, gradually increasing from a tensile strength of 510-610 MPa to 700-750 MPa, with demand for further increases to 850 MPa. As the primary load-bearing component of the vehicle, the mainframe of a commercial vehicle requires not only a high strength level but also good cold-forming properties and the ability to meet subsequent service safety requirements. Therefore, the steel used in automotive mainframes should possess excellent cold bending, crack arrest, and fatigue properties, while also maintaining a good plate shape.

[0003] At present, the domestic production of hot-rolled steel for automobile beams with a tensile strength of ≥850MPa is mainly based on conventional hot rolling or tempering heat treatment processes, and sometimes a bell furnace is used for annealing. Chinese patent application CN105543666A discloses a steel for automobile beams with a yield strength of 960MPa and a production method thereof. The application adopts a lower carbon and manganese content, fully utilizes the precipitation strengthening effect of V, Nb, and Ti, and adds a certain amount of Mo, Cr, and B to control the heat treatment organization state, thereby obtaining a fine and uniform tempered organization, so that the material has good strength and toughness, good welding performance and good low-temperature impact toughness, thereby improving the safety performance of the vehicle. However, the alloy cost of the steel plate of the patent application is relatively high, and the tempering heat treatment process further increases the manufacturing cost.

[0004] Chinese patent application CN106119702A discloses a 980MPa-grade hot-rolled high-strength, high-hole expansion steel and its manufacturing method. This patent application utilizes a conventional hot rolling process to produce the steel. The microstructure is granular bainite + martensite, with a yield strength ≥900MPa, a tensile strength ≥980MPa, an elongation ≥15%, and a hole expansion ratio ≥50%. This steel exhibits a good balance of strength and toughness, and is suitable for use in automotive chassis, beams, wheels, and other components requiring high-strength thinning and complex forming. However, this patent application requires the addition of a large number of alloying elements, resulting in a high alloy cost.

[0005] The ultra-high-strength automotive beam steels described in the aforementioned published patent applications suffer from high alloy and manufacturing costs. These typically require the addition of large quantities of precious alloying elements, significantly increasing alloy costs. Some are produced using offline quenching and tempering processes, driving up production costs and making it difficult to meet the low-cost demands of users. Furthermore, ultra-high-strength beam steels produced using quenching and tempering heat treatment processes typically exhibit poor cold bending properties, making them difficult to meet the cold forming requirements of commercial vehicle mainframe beams. Summary of the Invention

[0006] In order to solve the problem that the existing technology of steel alloys for automobile beams with tensile strength ≥850MPa is high in cost and generally has low cold bending performance.

[0007] The present invention provides a steel for an automobile beam. The tensile strength of the steel for the automobile beam is ≥850 MPa. The steel for the automobile beam contains the following chemical elements in percentage by mass: C: 0.05-0.18%, Si: 0.02-0.30%, Mn: 0.8-2.5%, Cr: 0.05-0.8%, Ti: 0.08-0.20%, and Al: 0.01-0.10%.

[0008] According to another specific embodiment of the present invention, the steel for automobile beams provided by the present invention is composed of the following chemical elements in mass percentage: C: 0.05-0.18%, Si: 0.02-0.30%, Mn: 0.8-2.5%, Cr: 0.05-0.8%, Ti: 0.08-0.20%, Al: 0.01-0.10%, Nb≤0.02%, and the balance is Fe and unavoidable impurities.

[0009] By adopting the above scheme, based on the reasonable design of alloy content ratio, no or only a small amount of precious gold is added, thereby reducing the alloy cost.

[0010] According to another specific embodiment of the present invention, among the inevitable impurities contained in the steel for automobile beams provided by the present invention, P, S, O, and N are controlled within the following mass percentages: P≤0.025%, S≤0.008%, O≤0.008%, and N≤0.005%.

[0011] By adopting the above scheme, the content of impurity elements is controlled to reduce the influence of impurities on the plasticity and toughness of steel.

[0012] According to another specific embodiment of the present invention, the chemical element composition of the steel for automobile beams provided by the present invention also satisfies: 2.5%≤10C+Mn+2.5Cr≤4.0%, Ti-3.5N≥0.10%, where each chemical element is substituted into the mass percentage of the corresponding chemical element.

[0013] In some embodiments, the chemical element composition of the steel for automobile beams provided by the present invention satisfies the following: 0.10%≤Ti-3.5N≤0.20%.

[0014] The above solution, by limiting the contents of C, Mn, and Cr, ensures that the high-strength steel possesses suitable hardenability, transforming the majority of austenite into martensite during cooling, while also preventing deterioration in impact toughness and cold bending properties caused by excessive martensite strength after tempering. By controlling Ti-3.5N to ≥ 0.10%, sufficient effective Ti is ensured in the steel. During hot rolling, this element can pin austenite grain boundaries and refine the austenite structure. During controlled cooling, TiC interphase precipitation occurs during the ferrite phase transformation, compensating for the lack of ferrite strength. During the subsequent tempering process, sufficient nanoscale TiC precipitates are generated to compensate for the strength loss caused by tempering softening of the martensite.

[0015] According to another specific embodiment of the present invention, the microstructure of the steel for automobile beams provided by the present invention includes tempered martensite and ferrite, and TiC nano-precipitates are dispersed therein; the volume percentage of ferrite is ≤20%; and the grain size of the TiC nano-precipitates is ≤10nm.

[0016] In some embodiments, in the microstructure of the steel for automobile beams provided by the present invention, the volume percentage of ferrite is 1 to 20%, for example, 2% to 19%.

[0017] By adopting the above scheme, the introduction of ferrite effectively improves the plasticity and cold bending properties. The TiC nano-precipitates are dispersed inside the ferrite, which effectively improves the strength of the ferrite and ensures that the strip can reach a strength level of ≥850MPa.

[0018] According to another specific embodiment of the present invention, in the steel for automobile beams provided by the present invention, the difference in micro-Vickers hardness between tempered martensite and ferrite is ≤100 HV.

[0019] In some embodiments, in the steel for automobile beams provided by the present invention, the difference between the micro-Vickers hardness of martensite and the micro-Vickers hardness of ferrite is 45-70 HV, for example, 50-70 HV.

[0020] In some embodiments, in the steel for automobile beams provided by the present invention, the ferrite micro-Vickers hardness is 245±11 HV, for example, 245±10 HV.

[0021] In some embodiments, in the steel for automobile beams provided by the present invention, the micro Vickers hardness of tempered martensite is 302±19 HV.

[0022] By adopting the above scheme, the strength difference between tempered martensite and ferrite is small, which is beneficial to improving the cold bending performance.

[0023] The automobile beam steel provided by the present invention has a yield strength of ≥780MPa, a tensile strength of ≥850MPa, and an elongation at break of ≥14%. Under cold bending conditions of d=2a, 90° and d=3a, 180°, the sample surface does not wrinkle or crack.

[0024] In some embodiments, the unevenness of the steel for automobile beams provided by the present invention is ≤10 mm, for example, 4-10 mm.

[0025] In some embodiments, the yield strength of the steel for automobile beams provided by the present invention is 780-910 MPa.

[0026] In some embodiments, the tensile strength of the steel for automobile beams provided by the present invention is 850-950 MPa.

[0027] In some embodiments, the elongation at break of the steel for automobile beams provided by the present invention is 14-19%.

[0028] The present invention also provides a method for manufacturing steel for automobile beams, the method comprising the following steps:

[0029] S1: Heat the slab to 1240-1300℃ and keep it warm for 30-90min;

[0030] S2: performing rough rolling and finish rolling on the slab, wherein the rough rolling outlet temperature is 1020-1080° C. and the finish rolling temperature is 830-890° C. to obtain a steel strip;

[0031] S3: Controlled cooling of the strip, i.e., specifying its cooling mode and cooling rate, with the final cooling temperature being less than or equal to 200°C;

[0032] S4: The strip enters the coiler for coiling;

[0033] S5: Tempering heat treatment of the strip steel at a temperature of 560-700°C.

[0034] The above scheme is based on the hot rolling-controlled cooling-high-temperature tempering process, especially controlling the finishing rolling temperature and laminar cooling rate, and then combining it with high-temperature tempering to obtain a tempered martensite + ferrite microstructure, in which TiC nano-precipitates with a size of less than 10nm are dispersed. The tempered martensite structure strengthening combined with TiC precipitation strengthening gives the strip high strength. The tempering heat treatment is carried out at a higher temperature to ensure the formation of sufficient nano-scale TiC precipitation phases during the tempering process, ensuring that the strip has sufficient strength. In addition, the higher tempering temperature can reduce internal stress, thereby improving the plate shape of the strip.

[0035] In some embodiments, in step S3 of the method for manufacturing steel for automobile beams provided by the present invention, laminar cooling is used for controlled cooling.

[0036] According to another specific embodiment of the present invention, in step S3 of the manufacturing method for automobile beam steel provided by the present invention, the strip is cooled in sections, and the cooling rate in the first stage is ≥50°C / s, the air cooling temperature is 600-700°C, and the time is 3-10s; the cooling rate in the second stage is ≥exp(5.8-2.53C-0.16Si-0.82Mn-0.95Cr)°C / s, where C, Si, Mn, and Cr respectively represent the mass percentages of the corresponding chemical components, the exp(x) function represents an exponential function with the natural constant e as the base, and exp(x) represents e to the power of x.

[0037] The purpose of controlling cooling by adopting the above scheme is to control the generation of a certain amount of ferrite, improve the elongation of the strip and control the strength. At the same time, it is also beneficial to the precipitation of TiC phases during the ferrite phase transformation, which can effectively improve the strength of ferrite and reduce the strength difference between tempered martensite and ferrite, which is beneficial to improving the cold bending performance.

[0038] According to another specific embodiment of the present invention, in step S5 of the method for manufacturing automobile beam steel provided by the present invention, a hot coil continuous heat treatment process can be used for tempering heat treatment, with a heating rate ≥10°C / s and a holding time of 60 to 300s.

[0039] By adopting the above scheme, induction heating and tempering are carried out through the hot coil continuous heat treatment device. The advantage of the fast heating speed of induction heating is utilized to carry out short-time high-temperature tempering. While TiC is fully precipitated, the growth of TiC precipitation phase is avoided, and the density and size of TiC precipitation phase are regulated, thereby obtaining a better strength-toughness match.

[0040] According to another specific embodiment of the present invention, in step S5 of the method for manufacturing automobile beam steel provided by the present invention, a single plate heat treatment process can be used for tempering heat treatment, and the holding time is 5-25 minutes.

[0041] Beneficial effects of the present invention:

[0042] The present invention rationally designs alloy content ratios, adding no or only a small amount of precious metal. Based on this, the steel for automotive beams is produced using a hot rolling-controlled cooling-high-temperature tempering process. Specifically, the finishing temperature and laminar cooling rate are controlled, and high-temperature tempering is combined to produce a tempered martensite + ferrite microstructure with dispersed TiC nano-precipitates less than 10 nm. The high-temperature tempered martensite exhibits relatively high plasticity, which is further enhanced by the plasticity enhancement provided by the ferrite. Furthermore, the high temperature used in the tempering heat treatment fully enhances the elongation of the steel strip and leverages the precipitation strengthening effect of the dissolved Ti element. This invention reduces the alloy cost of automotive beam steel with a tensile strength of 850 MPa or higher while also ensuring high plasticity and cold bending properties in the steel strip.

[0043] The automobile beam steel provided by the present invention has a yield strength of ≥780MPa, a tensile strength of ≥850MPa, and an elongation at break of ≥14%. Under cold bending conditions of d=2a, 90° and d=3a, 180°, the sample surface does not wrinkle or crack. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] FIG1 is a microstructure diagram of the automobile beam steel obtained in Example 1 of the present invention;

[0045] FIG2 is a microstructure diagram of the automobile beam steel obtained in Example 2 of the present invention;

[0046] FIG3 is a microstructure diagram of the automobile beam steel obtained in Example 9 of the present invention;

[0047] FIG4 is a microstructure diagram of the steel for automobile beams obtained in Example 10 of the present invention. DETAILED DESCRIPTION

[0048] In order to make the objectives, technical solutions and advantages of the present invention more clear, the embodiments of the present invention will be described in further detail below.

[0049] The present invention provides a steel for automobile beams. The tensile strength of the steel is ≥850 MPa. The steel for automobile beams contains the following chemical elements in percentage by mass: C: 0.05-0.18%, Si: 0.02-0.30%, Mn: 0.8-2.5%, Cr: 0.05-0.8%, Ti: 0.08-0.20%, and Al: 0.01-0.10%.

[0050] According to one specific embodiment of the present invention, the steel for automobile beams is composed of the following chemical elements in the following mass percentages: C: 0.05-0.18%, Si: 0.02-0.30%, Mn: 0.8-2.5%, Cr: 0.05-0.8%, Ti: 0.08-0.20%, Al: 0.01-0.10%, Nb≤0.02%, and the balance is Fe and unavoidable impurities.

[0051] The design principle of the chemical element content of the steel for automobile beams of the present invention is as follows:

[0052] Carbon (C): The C element has an important influence on the phase transformation process and products of steel. Increasing the C content can reduce the critical cooling rate for the formation of martensite, making it easier to form martensite under the same cooling conditions. The C element exists in the form of solid solution in martensite, which can significantly improve the strength of martensite, but a higher C content will generate a more brittle twin martensite structure, which is not conducive to the low-temperature impact toughness of steel. In the subsequent tempering process, a higher C content is likely to lead to the formation of coarse carbides and a decrease in plasticity and impact toughness. On the other hand, too low a C content is likely to generate a large amount of low-strength structures such as ferrite, which cannot meet the strength requirements of the present invention. Therefore, the C element content range of the present invention is 0.05-0.18%.

[0053] Silicon (Si): Si is a solid-solution element in steel and has a strong deoxidizing effect. High Si levels can easily cause surface scale to form during hot rolling and degrade the toughness and weldability of the martensite. Therefore, the Si content in this invention is within the range of 0.02-0.30%.

[0054] Manganese (Mn): Mn is an important alloying element in the present invention. The addition of Mn can inhibit the formation of proeutectoid ferrite and improve the hardenability of the steel. In addition, the addition of Mn can refine the microstructure of the steel, giving it a good balance of strength and toughness. However, excessive addition of Mn can easily lead to significant center segregation and the formation of high-carbon twinned martensitic structure in the corresponding position, which is not conducive to the toughness of martensitic high-strength steel. Therefore, the Mn content of the present invention ranges from 0.8-2.5%.

[0055] Chromium (Cr): Cr is a key alloying element in the present invention. Its addition significantly improves the hardenability of steel, facilitates the formation of a fully martensitic structure during quenching, and enhances the overall strength of the material. Furthermore, Cr forms carbide precipitates during tempering, providing some resistance to temper softening. Excessive Cr content can easily lead to decreased strength and toughness in the steel strip. Therefore, the Cr content in the present invention is in the range of 0.05-0.8%.

[0056] Titanium (Ti): Ti is a key microalloying element in this invention. It reacts with carbon and nitrogen to form fine dispersed phase particles, which pin austenite grain boundaries and refine the austenite structure during hot rolling. During the subsequent tempering process, Ti forms nanoscale TiC precipitates, which enhance the yield strength and tensile strength of the tempered martensite and significantly resist temper softening. To achieve the required strength, the Ti content in this invention is relatively high, ranging from 0.08% to 0.20%.

[0057] Aluminum (Al): Al acts as a deoxidizer for steel. Adding a small amount of Al can refine grains and improve impact toughness. However, excessive Al content can easily form alumina inclusions and promote the formation of proeutectoid ferrite, resulting in reduced strength. Therefore, the Al content in the present invention is within the range of 0.01-0.10%.

[0058] Niobium (Nb): Nb is an optional alloying element added in the present invention. A trace amount of Nb can significantly increase the recrystallization temperature of austenite in steel, significantly refine the austenite grains, and enhance the material's strength and toughness. Nb is a precious element, and its addition level must be controlled to control production costs. Therefore, the Nb content of the present invention does not exceed 0.02%. In some embodiments, when Nb is added, the Nb content of the present invention is 0.001-0.02%, for example, 0.005-0.02%.

[0059] The hot-rolled automobile beam steel with a tensile strength of 850 MPa or higher provided by the present invention is based on a rationally designed alloy content ratio, does not add precious metals or only adds a small amount of precious gold Nb, thereby reducing alloy costs.

[0060] According to one specific embodiment of the present invention, among the inevitable impurities, P, S, O, and N are controlled within the following mass percentages: P≤0.025%, S≤0.008%, O≤0.008%, and N≤0.005%.

[0061] Phosphorus (P), sulfur (S), oxygen (O), and nitrogen (N): P, S, O, and N are impurity elements in steel. When their contents are too high, they will significantly affect the plasticity and toughness of the steel. Therefore, the present invention controls their contents within the ranges of P≤0.025%, S≤0.008%, O≤0.008%, and N≤0.005%, respectively.

[0062] According to one specific embodiment of the present invention, the composition of chemical elements further satisfies: 2.5%≤10C+Mn+2.5Cr≤4.0%, Ti-3.5N≥0.1%, where each chemical element is substituted into the mass percentage of the corresponding chemical element.

[0063] By controlling the C, Mn, and Cr contents to meet the requirement of 2.5% ≤ 10C + Mn + 2.5Cr ≤ 4.0%, these C, Mn, and Cr content limits ensure that high-strength steel possesses suitable hardenability, enabling the majority of austenite to transform into martensite under the cooling conditions provided by conventional laminar cooling processes, while also preventing the deterioration of impact toughness and cold bending properties caused by excessive martensite strength. If the 10C + Mn + 2.5Cr content is less than 2.5%, hardenability is too low, resulting in excessive ferrite formation during cooling and relatively low strength. Above 4.0%, less ferrite is formed, and the martensite strength is too high, making it more susceptible to cold bending cracking.

[0064] By controlling the Ti content to meet Ti-3.5N ≥ 0.10%, sufficient effective Ti is ensured in the steel, enabling it to pin austenite grain boundaries and refine the austenite structure during hot rolling. During controlled cooling, TiC interphase precipitation forms during the ferrite phase transformation, compensating for the insufficient ferrite strength. During the subsequent tempering process, sufficient nano-TiC precipitation is ensured to compensate for the strength loss caused by tempering softening of the martensite, ultimately achieving a tensile strength of over 850 MPa. If Ti-3.5N is less than 0.10%, the precipitation strengthening effect of TiC is insufficient, resulting in relatively low strength after tempering, which cannot meet the strength requirements.

[0065] According to one specific embodiment of the present invention, the microstructure of the steel for automobile beams includes tempered martensite and ferrite, in which TiC nano-precipitates are dispersed; the volume percentage of ferrite is ≤20%; and the grain size of the TiC nano-precipitates is ≤10nm.

[0066] The automotive beam steel provided by the present invention has a microstructure of tempered martensite + ferrite, within which TiC nano-precipitates with a size of less than 10 nm are dispersed. Compared with a simple tempered martensite structure, the introduction of ferrite effectively improves plasticity and cold bending properties. Furthermore, the ferrite has a relatively fine grain size and is dispersed with TiC nano-precipitates with a size of less than 10 nm, including interphase and tempered precipitations. This effectively increases the strength of the ferrite, ensuring that the steel strip achieves the required strength level of the steel provided by the present invention.

[0067] According to one embodiment of the present invention, the difference in micro-Vickers hardness between tempered martensite and ferrite is ≤ 100 HV, indicating a small strength difference between the tempered martensite and ferrite, which is beneficial for improving cold bending performance. Furthermore, the difference in micro-Vickers hardness between the tempered martensite and ferrite is ≤ 70 HV.

[0068] The present invention also provides a method for manufacturing the above-mentioned steel for automobile beams. The process path is to obtain a slab through a steelmaking and continuous casting process, heat it in a heating furnace, and then coil it to obtain a hot-rolled strip after rough rolling, finish rolling, and laminar cooling, and then perform a tempering treatment, which specifically includes the following steps S1-S5.

[0069] S1: Steelmaking and refining are carried out in a converter or electric furnace, and slabs are obtained through a continuous casting process; the slabs are then heated to a specified temperature and subjected to insulation treatment.

[0070] During steelmaking, the alloy contains the following chemical elements by mass: C: 0.05-0.18%, Si: 0.02-0.30%, Mn: 0.8-2.5%, Cr: 0.05-0.8%, Ti: 0.08-0.20%, Al: 0.01-0.10%. Preferably, the alloy is composed of the following chemical elements by mass: C: 0.05-0.18%, Si: 0.02-0.30%, Mn: 0.8-2.5%, Cr: 0.05-0.8%, Ti: 0.08-0.20%, Al: 0.01-0.10%, Nb ≤ 0.02%, with the remainder being Fe and unavoidable impurities. Preferably, P, S, O, and N are controlled within the following mass percentages: P ≤ 0.025%, S ≤ 0.008%, O ≤ 0.008%, and N ≤ 0.005%. More preferably, the composition of chemical elements further satisfies: 2.5%≤10C+Mn+2.5Cr≤4.0%, Ti-3.5N≥0.10%, wherein each chemical element is substituted into the mass percentage of the corresponding chemical element.

[0071] According to one specific embodiment of the present invention, in order to ensure sufficient solid solution of Ti element, the slab is heated to a temperature of 1240-1300°C and the soaking time is 30-90 minutes, wherein the soaking time is calculated after the core of the slab reaches temperature.

[0072] S2: The heated slab is subjected to rough rolling and finish rolling to obtain a steel strip.

[0073] The descaling pressure should be sufficiently high during the rough rolling stage to achieve a good descaling effect. The rough rolling outlet temperature is an important indicator. If the rough rolling outlet temperature is too high, it will be detrimental to the refinement of austenite grains, resulting in deterioration of the impact toughness of the strip. If the rough rolling outlet temperature is too low, a large number of strain-induced precipitation TiC particles will be generated in the austenite. Due to the high precipitation temperature, the size of such TiC precipitates is too large, and the precipitation strengthening effect is very weak, which reduces the effective Ti content in the subsequent tempering process and leads to a decrease in the strength of the strip. Taking the above two factors into consideration, the rough rolling outlet temperature can be controlled at 1020-1080℃.

[0074] After rough rolling, the strip enters the finishing mill for finish rolling; a multi-stand continuous rolling process is adopted, and the appropriate final rolling temperature is selected according to the thickness specifications of the rolled strip. Compared with the general steel rolling process, the steel of the present invention requires a lower final rolling temperature. The purpose is to increase the rolling deformation energy, promote a small amount of ferrite transformation when the strip is segmented cooled on the layer cold roller, and promote the interphase precipitation of TiC during the ferrite phase transformation, significantly improve its precipitation strengthening effect, and compensate for the reduction in yield strength caused by the formation of a small amount of ferrite. On the other hand, in order to suppress the strain-induced precipitation of the TiC second phase in austenite, the final rolling temperature should not be too low. Taking the above two factors into consideration, the final rolling temperature is controlled at 830-890℃.

[0075] S3: Controlled cooling of the strip.

[0076] After finishing rolling, the strip enters the laminar cooling unit for controlled cooling of the strip, that is, the cooling mode is specified and the cooling rate is controlled to be stable, and the final cooling temperature is less than or equal to 200°C. The purpose of controlled cooling is to generate a certain amount of ferrite, improve the elongation of the strip and control the strength; it is also beneficial to the precipitation of TiC phases during the ferrite phase transformation, which can effectively improve the ferrite strength, reduce the strength difference between ferrite and martensite, and help improve the cold bending performance. If the final cooling temperature is too high, bainite and residual austenite are easily generated, resulting in a decrease in the strength of the strip, and the specified strength level cannot be achieved after tempering; the present invention controls the final cooling temperature to no more than 200°C, which is conducive to the formation of martensitic structure and ensures that the strength of the strip is at a high level.

[0077] According to one specific embodiment of the present invention, controlled cooling utilizes a staged cooling mode, with a first-stage cooling rate ≥50°C / s, an air cooling temperature of 600-700°C, and a cooling time of 3-10s; a second-stage cooling rate ≥exp(5.8-2.53C-0.16Si-0.82Mn-0.95Cr)°C / s. It should be noted that in the above calculation formula, C, Si, Mn, and Cr represent the mass percentages of the corresponding chemical components, respectively; the exp(x) function represents an exponential function with the natural constant e as its base, and exp(x) represents e raised to the power of x.

[0078] S4: After the laminar cooling process is completed, the strip enters the coiler for coiling. After the coil is formed, it should be avoided from being stacked together with other hot coils to avoid being baked and tempered, which will reduce the strength.

[0079] S5: Tempering the steel strip. Depending on the tempering production line, the steel strip can be tempered using processes such as continuous heat treatment of hot coils or single plate heat treatment.

[0080] In order to improve the toughness of the steel strip and reduce the risk of cold forming cracking, the existing method for preparing steel for automobile beams adopts a hot rolling low-temperature coiling process, and the coiling temperature is controlled at 200-400°C. However, laminar cooling in this temperature range is difficult to control, the coiling temperature fluctuates violently, and the yield rate is generally low, which limits its widespread application in large-scale industrial production. In order to improve the elongation of the steel strip and give full play to the precipitation strengthening effect of the solid solution Ti element, the present invention adopts a higher temperature for tempering heat treatment, and controls the tempering heat treatment temperature to 560-700°C. After the core of the steel strip reaches the tempering temperature, it begins to be kept warm, and the holding time is determined based on the tempering process. During the tempering process, the formation of a sufficient amount of nano-TiC precipitation phase is guaranteed to ensure that the steel strip has sufficient strength; and the higher tempering temperature can reduce internal stress, thereby improving the plate shape of the steel strip; further, the tempering temperature is preferably 600-700°C.

[0081] According to one specific embodiment of the present invention, tempering heat treatment is performed using a hot coil continuous heat treatment process, with a heating rate of ≥10°C / s and a holding time of 60-300s. Induction heating tempering is performed using a hot coil continuous heat treatment unit. Leveraging the rapid heating rate of induction heating, short-term high-temperature tempering allows for sufficient TiC precipitation while preventing the growth of TiC precipitates. This allows for the regulation of the density and size of TiC precipitates, resulting in an optimal balance of strength and toughness. Furthermore, shorter tempering times can significantly reduce carbon emissions and accelerate production.

[0082] According to another specific embodiment of the present invention, a single plate heat treatment process is adopted for tempering heat treatment, and the holding time is 5-25 minutes.

[0083] The production process provided by the present invention is based on the above-mentioned hot rolling-controlled cooling-high temperature tempering process, especially controlling the finishing rolling outlet temperature and laminar cooling rate, and then combining high temperature tempering to obtain a tempered martensite + ferrite microstructure. The tempered martensite still retains a relatively fine lath martensite structure, and TiC nano-precipitates less than 10nm are dispersed within it. The tempered martensite structure is strengthened and strengthened by TiC precipitation, resulting in a high strength strip. TiC interphase precipitation occurs during the ferrite formation process, and the ferrite grain size is relatively small, presenting a quasi-polygonal or long-axis polygonal morphology, and TiC nano-precipitates less than 10nm are dispersed within it. The martensite after high temperature tempering has relatively high plasticity, and the plasticity improvement brought by ferrite is combined, thereby ensuring that high plasticity and cold bending performance can be achieved under low alloy cost and required strength conditions. In addition, a higher temperature is used in the subsequent tempering heat treatment to fully improve the elongation of the strip and exert the precipitation strengthening effect of the solid solution Ti element.

[0084] After the above process, the yield strength of the steel for automobile beams is ≥780MPa, the tensile strength is ≥850MPa, the elongation at break is ≥14%, and under the cold bending conditions of d=2a, 90° and d=3a, 180°, no wrinkling or cracking occurs on the sample surface.

[0085] The manufacturing method of the automobile beam steel of the present invention will be further described below with reference to specific embodiments and the accompanying drawings.

[0086] Examples 1-18

[0087] Example 1-18 is a method for producing steel for automobile beams based on the above-mentioned design requirements and preparation process, wherein the mass percentage of each chemical element is shown in Table 1-1 and Table 1-2, and the balance is Fe and unavoidable impurities.

[0088] Table 1-1: Chemical composition content of Example 1-18

[0089] (Unit: wt%)

[0090] Note: In the above formulas 10C+Mn+2.5Cr and Ti-3.5N, the mass percentage of each chemical element is substituted into the corresponding chemical element.

[0091] Table 1-2: Chemical composition content of Example 1-18

[0092] (Unit: wt%)

[0093] Furthermore, a slab was obtained by converter smelting, refining and continuous casting processes, and after heating in a heating furnace, it was coiled to obtain a hot-rolled strip after rough rolling, finish rolling and controlled cooling, and then subjected to tempering heat treatment, wherein the controlled cooling adopted a segmented cooling mode, Examples 1-6 and Examples 13-18 adopted hot coil continuous heat treatment, and Examples 7-12 adopted a single plate heat treatment process for tempering heat treatment. The corresponding production process parameters are shown in Tables 2-4.

[0094] Table 2: Production process parameters of Examples 1-6

[0095] Table 3: Production process parameters of Examples 7-12

[0096] Table 4: Production process parameters of Examples 13-18

[0097] Comparative Examples 1-5

[0098] The specific weight percentages of the chemical elements in Comparative Examples 1-5 are shown in Tables 5-1 and 5-2, with the remainder being Fe and unavoidable impurities. The compositions of Comparative Examples 1-3 are within the scope of this application, and the composition of Comparative Example 1 is the same as that of Examples 1-2, and the compositions of Comparative Examples 2-3 are the same as those of Examples 3-4. The composition of Comparative Example 4 has a low Ti content and is outside the scope of this application. Comparative Example 5 does not contain Cr and is therefore outside the scope of this application.

[0099] Table 5-1: Chemical composition content of comparative examples 1-5

[0100] (Unit: wt%)

[0101] Note: In the above formulas 10C+Mn+2.5Cr and Ti-3.5N, the mass percentage of each chemical element is substituted into the corresponding chemical element.

[0102] Table 5-2: Chemical composition content of comparative examples 1-5

[0103] (Unit: wt%)

[0104] Furthermore, slabs were obtained through converter smelting, refining, and continuous casting. After heating in a heating furnace, they were subjected to rough rolling, finish rolling, and cooling before coiling to obtain hot-rolled strip. Comparative Example 1 and Comparative Examples 3-5 employed staged cooling with controlled cooling rates. Comparative Example 1 did not undergo tempering heat treatment, which is outside the scope of this application. Comparative Examples 2-5 employed a hot coil continuous heat treatment process for tempering heat treatment. Comparative Example 3 had a lower cooling rate. Comparative Examples 4-5 controlled the cooling rate in the second stage to be higher, resulting in rapid cooling. The corresponding production process parameters are shown in Table 6.

[0105] Table 6: Production process parameters of Comparative Examples 1-5

[0106] Microstructure and performance testing:

[0107] (1) Room temperature tensile properties

[0108] At room temperature, plate-shaped tensile specimens taken from the strip steel were tested in accordance with the national standard GB / T228.1-2010 "Room Temperature Tensile Test Method for Metallic Materials" to measure the yield strength, tensile strength, and elongation at break of the strip steel of each embodiment and the comparative example.

[0109] (2) Cold bending performance

[0110] Steel strip samples from various examples and comparative examples were tested in accordance with the national standard GB / T 2010, "Metallic Materials Bend Test Methods." The cold bending performance of the steel strips from Examples 1-18 and Comparative Examples 1-5 was measured under the conditions of a cold bending punch diameter of d = 2a, a bend angle of 90°, and a cold bending punch diameter of d = 3a, a bend angle of 180°. Acceptable cold bending performance refers to the absence of wrinkling or cracking on the sample surface after bending.

[0111] (3) Roughness

[0112] The strip steel samples of each embodiment and comparative example were tested in accordance with the national standard GB / T 709-2019 "Dimensions, shapes, weights and allowable deviations of hot-rolled steel plates and strips", and the flatness of each embodiment and comparative example was measured as a representative plate shape indicator.

[0113] (4) Microstructure

[0114] The volume percentage of each phase in the microstructure of the steel strip samples of each embodiment and comparative example was tested with reference to the national standard GB / T15749-2008 "Quantitative Metallographic Determination Method". The micro-Vickers hardness test of ferrite and martensite in the sample microstructure was performed with reference to the national standard GB / T 4340.1-2009 "Metallic Materials Vickers Hardness Test Part 1: Test Method" at a load of 0.01 kgF.

[0115] The microstructure and performance test results of Examples 1-18 are shown in Tables 7-9; the microstructure and performance test results of Comparative Examples 1-4 are shown in Table 10. The microstructure test results show that the microstructure of the steel strips of Examples 1-18 is composed of tempered martensite + ferrite, with dispersed TiC nanoprecipitates of less than 10 nm in size. The tempered martensite has a fine lath martensite structure. The ferrite grains are relatively small, exhibiting quasi-polygonal or long-axis polygonal morphologies.

[0116] Table 7: Microstructure and properties of the steel strips produced in Examples 1-6 of the present invention

[0117] Table 8: Microstructure and properties of the steel strips produced in Examples 7-12 of the present invention

[0118] Table 9: Microstructure and properties of the steel strips produced in Examples 13-18 of the present invention

[0119] Table 10: Microstructure and properties of the strip steel produced in Comparative Examples 1-5

[0120] It can be seen from Tables 4 to 9 that Examples 1-18 are based on a reasonable design of alloy content ratio, and a hot rolling-controlled cooling-high temperature tempering process to produce hot-rolled strip steel with high cold bending performance. The rough rolling outlet temperature is controlled to 1020-1080°C, the finishing rolling outlet temperature is 830-890°C, and segmented cooling is performed, followed by high temperature tempering at 560-700°C to obtain a microstructure of tempered martensite + ferrite; the ultra-high strength automobile beam steel with a yield strength ≥780MPa and a tensile strength ≥850MPa is produced by designing and producing low-cost alloys, and the elongation at break is ≥14%. Under the cold bending conditions of d=2a, 90° or d=3a, 180°, no wrinkling or cracking occurs on the sample surface, ensuring high plasticity and cold bending performance; and the unevenness of Examples 1-18 after cutting is ≤10mm, indicating that the ultra-high strength automobile beam steel provided by the present invention has good plate shape characteristics.

[0121] Among Examples 1-6, the process difference between Examples 1 and 2 lies in the different tempering processes, with Example 2 having a higher tempering temperature of 688°C. Similarly, the process difference between Examples 3 and 4 and between Examples 5 and 6 lies in the different tempering processes, with Examples 4 and 6 having higher tempering temperatures of 694°C and 674°C, respectively. Table 4 shows that Examples 2, 4, and 6 have lower roughness than Examples 1, 3, and 5, respectively, indicating that Examples 2, 4, and 6 have better plate shapes. Similarly, among Examples 7-12, the process difference between Examples 7 and 8, Examples 9 and 10, and Examples 11 and 12 lies in the different tempering processes, with Examples 7, 9, and 11 having higher tempering temperatures of 615°C, 618°C, and 608°C, respectively. Table 5 shows that Examples 7, 9, and 11 have lower roughness than Examples 8, 10, and 12, respectively, indicating that Examples 7, 9, and 11 have better plate shapes. The above results show that within the tempering heat treatment temperature range of the present invention, a relatively high tempering temperature is beneficial to improving the plate shape.

[0122] Furthermore, in Examples 1-18, the ferrite ratio is less than 20%, and the difference between the micro-Vickers hardness of martensite and the micro-Vickers hardness of ferrite is 45-70 HV; and the micro-Vickers hardness of ferrite is 245±11 HV, and the micro-Vickers hardness of tempered martensite is 302±19 HV, which indicates that the difference in the average micro-Vickers hardness of the two phases in the above examples is within 100 HV. In the present application, the ferrite Vickers hardness reaches 245±11 HV, indicating that the segmented control of cooling rate and tempering temperature control process of the present invention is conducive to the precipitation of TiC phases during the ferrite phase transformation, effectively improving the strength of ferrite, and controlling the strength difference between tempered martensite and ferrite to remain within a reasonable range, which is conducive to improving cold bending performance.

[0123] The microstructures obtained in Examples 1, 2, 9, and 10 are shown in Figures 1-4, respectively. As can be seen from the figures, the microstructures are tempered martensite + ferrite, with dispersed TiC nanoprecipitates of less than 10 nm in size, and the tempered martensite has a fine lath martensite structure. The ferrite grains are relatively small, exhibiting quasi-polygonal or long-axis polygonal morphologies.

[0124] Comparative Examples 1-5 cannot achieve the specified performance indicators because they do not meet the composition or process requirements. Among them, Comparative Example 1 did not undergo tempering heat treatment, resulting in a low elongation at break, unqualified cold bending performance, and high unevenness after cutting, indicating that the plate shape after cutting is poor. Comparative Example 2 did not control the cooling rate and cooling mode, resulting in a low elongation at break, unqualified cold bending performance, high unevenness after cutting, and poor plate shape after cutting. The cooling rate of Comparative Example 3 is low, wherein the cooling rate in the first stage is lower than 50℃ / s, and the cooling rate in the second stage is less than exp(5.8-2.53C-0.16Si-0.82Mn-0.95Cr)℃ / s; it was found that the amount of ferrite generated was too much, and the hardness of martensite was insufficient, resulting in its low strength. The Ti element content of Comparative Example 4 is low, does not meet the composition requirements, and makes Ti-3.5N <0.10%, resulting in low yield and tensile strength. Comparative Example 4 does not add Cr, so that 10C+Mn+2.5Cr is relatively low, less than 2.5%, which makes the hardenability too low, the martensite strength insufficient, and more ferrite is generated, resulting in low yield and tensile strengths.

[0125] The above describes the implementation manner of the present invention by means of specific specific embodiments. Those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. Although the description of the present invention will be introduced in conjunction with the preferred embodiment, this does not mean that the features of this invention are limited to this implementation manner. On the contrary, the purpose of introducing the invention in conjunction with the implementation manner is to cover other options or modifications that may be extended based on the claims of the present invention. In order to provide an in-depth understanding of the present invention, the following description will contain many specific details. The present invention can also be implemented without using these details. In addition, in order to avoid confusion or blurring the focus of the present invention, some specific details will be omitted in the description. It should be noted that the embodiments of the present invention and the features in the embodiments can be combined with each other unless there is a conflict.

[0126] Although the present invention has been illustrated and described with reference to certain preferred embodiments thereof, it should be understood by those skilled in the art that the above description is provided as a further detailed description of the present invention in conjunction with specific embodiments thereof, and that the specific implementation of the present invention is not limited to these descriptions. Those skilled in the art may make various changes in form and details, including simple deductions or substitutions, without departing from the spirit and scope of the present invention.

Claims

1. A steel for automobile beam, characterized in that: The tensile strength of the automobile beam steel is ≥850 MPa, and the automobile beam steel contains the following chemical elements in the following mass percentages: C: 0.05 to 0.18%, Si: 0.02 to 0.30%, Mn: 0.8 to 2.5%, Cr: 0.05 to 0.8%, Ti: 0.08 to 0.20%, Al: 0.01 to 0.10%.

2. The steel for automobile beams according to claim 1, characterized in that: The automobile beam steel is composed of the following chemical elements in the following mass percentages: C: 0.05~0.18%, Si: 0.02~0.30%, Mn: 0.8~2.5%, Cr: 0.05~0.8%, Ti: 0.08~0.20%, Al: 0.01~0.10%, Nb≤0.02%, and the balance is Fe and unavoidable impurities.

3. The steel for automobile beams according to claim 2, characterized in that: Among the inevitable impurities, P, S, O, and N are controlled within the following mass percentages: P≤0.025%, S≤0.008%, O≤0.008%, and N≤0.005%.

4. The steel for automobile beams according to claim 2, characterized in that: The composition of the chemical elements also satisfies: 2.5%≤10C+Mn+2.5Cr≤4.0%, Ti-3.5N≥0.10%, where each chemical element is substituted into the mass percentage of the corresponding chemical element.

5. The steel for automobile beams according to claim 1, characterized in that: The microstructure of the automobile beam steel includes tempered martensite and ferrite, and TiC nano-precipitates are dispersed therein.

6. The steel for automobile beams according to claim 5, characterized in that: The volume percentage of the ferrite is ≤20%; and the grain size of the TiC nano-precipitated phase is ≤10 nm.

7. The steel for automobile beams according to claim 5, characterized in that: The difference in micro Vickers hardness between the tempered martensite and the ferrite is ≤100 HV.

8. The steel for automobile beams according to claim 5, characterized in that: The difference in micro Vickers hardness between the tempered martensite and the ferrite is ≤70 HV.

9. The steel for automobile beams according to claim 5, characterized in that: The micro Vickers hardness of the ferrite is 245±11 HV, and the micro Vickers hardness of the tempered martensite is 302±19 HV.

10. The steel for automobile beams according to claim 5, characterized in that: The yield strength of the automobile beam steel is ≥780 MPa, the elongation at break is ≥14%, and under cold bending conditions of d=2a, 90° and d=3a, 180°, no wrinkling or cracking occurs on the sample surface.

11. The steel for automobile beams according to claim 10, characterized in that: The unevenness of the automobile beam steel is ≤10mm.

12. A method for manufacturing automobile beam steel according to claim 1, characterized in that: The method comprises the following steps: S1: Heat the slab to 1240-1300℃ and keep it warm for 30-90min; S2: performing rough rolling and finish rolling on the slab, wherein the rough rolling outlet temperature is 1020-1080° C. and the finish rolling temperature is 830-890° C. to obtain a steel strip; S3: Cooling the steel strip in a controlled manner, with the final cooling temperature being less than or equal to 200° C. S4: The strip enters the coiler for coiling; S5: performing a tempering heat treatment on the steel strip at a temperature of 560-700°C.

13. The method for manufacturing steel for automobile beams according to claim 12, wherein: In step S3, the strip is cooled in sections, with a cooling rate of ≥50°C / s, an air cooling temperature of 600-700°C, and a cooling time of 3-10s in the first stage; and a cooling rate of ≥exp(5.8-2.53C-0.16Si-0.82Mn-0.95Cr)°C / s in the second stage. In the calculation formula, C, Si, Mn, and Cr respectively represent the mass percentages of the corresponding chemical components, and the exp(x) function represents an exponential function with the natural constant e as the base, and exp(x) represents e to the power of x.

14. The method for manufacturing steel for automobile beams according to claim 12, wherein: In step S5, a hot coil continuous heat treatment process is adopted to perform tempering heat treatment, with a heating rate of ≥10°C / s and a holding time of 60 to 300s.

15. The method for manufacturing steel for automobile beams according to claim 12, wherein: In step S5, a single plate heat treatment process is used to perform tempering heat treatment, and the holding time is 5-25 minutes.

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