An ultra-high strength steel for a control arm and a method of manufacturing the same

Abstract

The application provides a kind of ultra-high strength steel for control arm and a preparation method thereof, and belongs to the field of steel preparation.The chemical composition of the ultra-high strength steel includes, by mass fraction: C: 0.05%-0.12%, Si>0 and <0.3%, Mn: 1.0%-2.2%, P≤0.013%, S≤0.002%, Nb: 0.02%-0.06%, Ti: 0.03%-0.15%, Cr: 0.1%-1.0%, Mo: 0.05%-0.3%, N≤0.005%, V: 0.005%-0.1%, Cu: 0.05%-0.3%, B>0 and ≤0.003%, and the balance is Fe and unavoidable impurities.Thereby, the technical problem that the existing ultra-high strength steel is difficult to simultaneously satisfy high formability and high fatigue life is solved.

Description

Technical Field

[0001] This application relates to the field of steel preparation technology, and in particular to an ultra-high strength steel for control arms and its preparation method. Background Technology

[0002] The front suspension control arm, as an important component of the automotive suspension system, is a crucial load-bearing component in the car's suspension system. It is also a part that is difficult to stamp and form. It is mounted on the axle assembly and must be able to bounce and rebound under the impact of the road surface. It also transmits the longitudinal and lateral forces from the wheels to the subframe through rubber. The control arm has a significant impact on the safety of vehicle driving.

[0003] To meet the increasingly demanding requirements of control arms with complex forming and fatigue resistance, various ultra-high-strength steels have been developed and used. However, these steels still suffer from poor formability and unsatisfactory fatigue performance. Therefore, there is an urgent need to develop an ultra-high-strength steel that can simultaneously meet the requirements of high formability and high fatigue life. Summary of the Invention

[0004] This application provides an ultra-high strength steel for control arms and a method for preparing the same, in order to solve the technical problem that ultra-high strength steel in the prior art is difficult to simultaneously meet the requirements of high formability and high fatigue life.

[0005] In a first aspect, this application provides an ultra-high strength steel for a control arm, wherein the chemical composition of the ultra-high strength steel, by mass fraction, includes: C: 0.05%–0.12%, Si > 0 and < 0.3%, Mn: 1.0%–2.2%, P ≤ 0.013%, S ≤ 0.002%, Nb: 0.02%–0.06%, Ti: 0.03%–0.15%, Cr: 0.1%–1.0%, Mo: 0.05%–0.3%, N ≤ 0.005%, V: 0.005%–0.1%, Cu: 0.05%–0.3%, B > 0 and ≤ 0.003%, with the balance being Fe and unavoidable impurities.

[0006] Optionally, by volume fraction, the microstructure of the ultra-high strength steel includes: martensite or ferrite >0 and <3%, lower bainite: 15% to 30%, and the balance being granular bainite.

[0007] Optionally, the lower bainite and the granular bainite satisfy the following relationship: Hv(lower bainite) - Hv (粒状贝氏体) ≤60,

[0008] In the formula, Hv (下贝氏体) Hv represents the average hardness of lower bainite. (粒状贝氏体) This indicates the average hardness of granular bainite.

[0009] Optionally, Ti precipitates are distributed on the lower bainite and the granular bainite matrix, and the average diameter of the precipitates is ≤15nm.

[0010] Optionally, the ultra-high strength steel meets at least one of the following properties: yield strength ≥750MPa, tensile strength ≥950MPa, hole expansion ratio λ ≥25%, and elongation A ≥10%.

[0011] Secondly, this application provides a method for preparing ultra-high strength steel for control arms as described in any embodiment of the first aspect, the method comprising:

[0012] Molten steel with the aforementioned chemical composition is continuously cast to obtain a slab;

[0013] The slab is heated, rough-rolled, and finish-rolled to obtain a hot-rolled plate; and

[0014] The hot-rolled plate is cooled and coiled to obtain ultra-high strength steel.

[0015] Optionally, the final rolling temperature of the roughing roll is 1050℃~1100℃.

[0016] Optionally, the parameters of the finishing rolling include: a rolling speed of 5 m / s to 10 m / s, a final rolling temperature of Ar3 to (Ar3+80℃), a reduction rate of ≥40% in the non-recrystallized austenite region, and a lubricant flow rate of 80 mL / min to 120 mL / min in the non-recrystallized austenite region; the thickness of the hot-rolled plate is 1.8 mm to 5 mm.

[0017] Optionally, the cooling rate is ≥30℃ / s, and the termination temperature is 300℃~500℃.

[0018] Optionally, the continuous casting process parameters include: molten steel superheat ≤ 25℃, secondary cooling water volume ≥ 0.6L / kg, and final solidification reduction ≥ 3.5mm.

[0019] The technical solutions provided in this application have the following advantages compared with the prior art:

[0020] This application provides an ultra-high strength steel for control arms, with a rationally designed chemical composition. By employing a C-Mn-Cr-Nb-V-Mo-Cu-V composition system, the microstructure is controlled by granular bainite and lower bainite, with a small amount of martensite or ferrite, resulting in a product with both high strength and a high porosity. The prepared ultra-high strength steel has a yield strength ≥750MPa, tensile strength ≥950MPa, porosity λ ≥25%, and elongation A ≥10%. The product exhibits high formability and long fatigue life, making it suitable for complex shaped components such as control arms. This solves the technical problem in existing technologies where ultra-high strength steels cannot simultaneously achieve high formability and long fatigue life. Attached Figure Description

[0021] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0022] To more clearly illustrate the technical solutions in the embodiments of this application 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.

[0023] Figure 1 This is a schematic flowchart illustrating a method for preparing ultra-high strength steel for a control arm, as provided in an embodiment of this application. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0025] Various embodiments of this application may exist in the form of a range; it should be understood that the description in the form of a range is merely for convenience and brevity and should not be construed as a hard limitation on the scope of this application; therefore, it should be considered that the range description has specifically disclosed all possible sub-ranges and single numerical values ​​within that range. For example, it should be considered that the range description from 1 to 6 has specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., and single numbers within the range, such as 1, 2, 3, 4, 5, and 6, regardless of the range. Furthermore, whenever a numerical range is referred to herein, it means including any referenced number (fraction or integer) within the referred range.

[0026] Furthermore, in the description of this application, the terms "comprising," "including," etc., mean "including but not limited to." In this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. In this document, "and / or" describes the relationship between related objects, indicating that three relationships can exist; for example, A and / or B can represent: A alone, A and B simultaneously, or B alone. A and B can be singular or plural. In this document, "at least one" means one or more, and "more than" means two or more. "At least one," "at least one of the following," or similar expressions refer to any combination of these items, including any combination of single or plural items. For example, "at least one of a, b, or c" or "at least one of a, b, and c" can both mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be a single or multiple.

[0027] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this application can be purchased from the market or prepared by existing methods.

[0028] This application provides an ultra-high strength steel for control arms. The chemical composition of the ultra-high strength steel, by mass fraction, includes: C: 0.05%–0.12%, Si > 0 and < 0.3%, Mn: 1.0%–2.2%, P ≤ 0.013%, S ≤ 0.002%, Nb: 0.02%–0.06%, Ti: 0.03%–0.15%, Cr: 0.1%–1.0%, Mo: 0.05%–0.3%, N ≤ 0.005%, V: 0.005%–0.1%, Cu: 0.05%–0.3%, B > 0 and ≤ 0.003%, with the balance being Fe and unavoidable impurities.

[0029] In this embodiment, the positive effects of controlling the mass fraction of C to be between 0.05% and 0.12% are as follows: Within this range, C is the most important solid solution strengthening element in steel and the element that ensures the hardenability of austenite. Therefore, an appropriate C content can ensure that the steel obtains sufficient martensite during cooling to guarantee its strength. Simultaneously, C can form carbonitrides with microalloyed Nb and Ti elements during heat treatment, refining the grains and strengthening ferrite, thus improving the mechanical properties of the steel. When the mass fraction value exceeds the maximum value at the end of this range, the C content will be too high, resulting in excessively hard steel and affecting its flexibility. When the mass fraction value is less than the minimum value at the end of this range, the C content will be too low, failing to guarantee the strength of the steel. For example, the mass fraction of C can be 0.05%, 0.06%, 0.07%, 0.08%, 0.09%, 0.10%, 0.11%, 0.12%, etc.

[0030] The positive effects of controlling the Si mass fraction to be between 0 and 0.3% are as follows: Within this range, Si is a solid solution strengthening element for steel, and it also promotes the enrichment of C into austenite, improving austenite hardenability. Simultaneously, it purifies the ferrite phase and improves the elongation of the steel. When the mass fraction exceeds the maximum value at the end of this range, the austenite hardenability of the steel will be insufficient, and the presence of Fe2SiO4 on the hot-rolled surface will lead to a decrease in the surface quality of the pickled surface. For example, the Si mass fraction can be 0.05%, 0.10%, 0.15%, 0.20%, 0.25%, 0.29%, etc.

[0031] The positive effects of controlling the Mn mass fraction to be between 1.0% and 2.2% are as follows: Within this range, Mn is an important element for solid solution strengthening and austenite stabilization, playing a crucial role in enhancing the mechanical properties of steel. When the mass fraction exceeds the maximum value of this range, excessive Mn content can easily cause microstructure segregation, leading to cracking during steel forming and deteriorating the steel's mechanical properties. Furthermore, during annealing, the segregated precipitates can accumulate on the surface, causing surface cracking. Conversely, when the mass fraction is less than the minimum value of this range, insufficient Mn content will prevent effective solid solution strengthening and austenite stabilization. For example, the Mn mass fraction can be 1.0%, 1.2%, 1.4%, 1.6%, 1.8%, 2.0%, or 2.2%.

[0032] The positive effects of controlling the mass fraction of phosphorus (P) to ≤0.013% are as follows: Within this range, P inhibits carbide formation and ensures that the overall carbon equivalent of the steel remains within a suitable range. When the mass fraction exceeds the maximum value at the extreme end of this range, it leads to a decrease in grain boundary strength and deteriorates the material's mechanical properties. Examples of P mass fractions include 0.003%, 0.005%, 0.007%, 0.009%, 0.010%, 0.012%, and 0.013%.

[0033] The positive effects of controlling the S mass fraction to ≤0.002% are as follows: Within this range, S is a harmful element and will combine with Mn to produce MnS, thereby deteriorating the mechanical properties of the steel. When the mass fraction exceeds the maximum value at the end of this range, it will result in excessive S content. On the one hand, this requires increasing the amount of Ti, and on the other hand, it will lead to weaker mechanical properties of the steel, affecting the hole-expanding performance. For example, the S mass fraction can be 0.0005%, 0.0008%, 0.0010%, 0.0015%, 0.002%, etc.

[0034] The positive effects of controlling the Nb mass fraction to be between 0.02% and 0.06% are as follows: Within this range, Nb can combine with C or N to form nano-precipitates, thereby refining the grains and promoting precipitation strengthening. This significantly improves the microstructure and yield strength, while also refining the austenite grain size during heating, ultimately achieving hard phase dispersion, which positively impacts porosity. However, if the mass fraction exceeds the maximum value of this range, the elongation of the steel will decrease. Conversely, if the mass fraction is below the minimum value, insufficient Nb content will prevent the formation of enough strengthening precipitates, thus failing to achieve the grain refining and precipitation strengthening effects. Examples of Nb mass fractions include 0.02%, 0.03%, 0.04%, 0.05%, and 0.06%.

[0035] The positive effects of controlling the Ti mass fraction to be between 0.03% and 0.15% are as follows: Within this range, Ti can combine with C or N to form nano-precipitates, thereby refining the grains and promoting precipitation strengthening. This significantly improves the microstructure and yield strength, while also refining the austenite grain size during heating, ultimately achieving hard phase dispersion, which positively impacts porosity. However, if the mass fraction exceeds the maximum value of this range, the elongation of the steel will decrease. Conversely, if the mass fraction is below the minimum value, insufficient Ti content will prevent the formation of enough strengthening phases, thus failing to achieve the grain refining and precipitation strengthening effects. Examples of Ti mass fractions include 0.03%, 0.05%, 0.07%, 0.09%, 0.10%, 0.12%, 0.14%, and 0.15%.

[0036] The positive effects of controlling the Cr mass fraction to be between 0.1% and 1.0% include improved hardenability within this range. When the mass fraction exceeds the maximum value of this range, the adverse effects are high cost and decreased surface quality; when the mass fraction is less than the minimum value of this range, the adverse effect is insufficient hardenability. Examples of Cr mass fractions include 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.8%, and 1.0%.

[0037] The positive effects of controlling the Mo mass fraction to be between 0.05% and 0.3% include improved hardenability within this range. A higher Mo mass fraction (the maximum value at the end of the range) results in higher costs, while a lower Mo mass fraction (the minimum value at the end of the range) results in insufficient hardenability. Examples of Mo mass fractions include 0.05%, 0.10%, 0.15%, 0.20%, 0.25%, and 0.3%.

[0038] The positive effects of controlling the mass fraction of nitrogen (N) to ≤0.005% include: within this range, N forms nano-precipitates with Ti and Nb, which refines the grains and strengthens the steel, thus improving its strength. However, when the mass fraction exceeds the maximum value at the end of this range, excessive N content leads to increased precipitates and negatively impacts the steel's performance. Examples of N mass fractions include 0.001%, 0.002%, 0.003%, 0.004%, and 0.005%.

[0039] The positive effects of controlling the mass fraction of V to be between 0.005% and 0.1% are as follows: Within this range, smaller-sized strengthening phases are formed, thereby improving the strength of the steel and ensuring its excellent mechanical properties. When the mass fraction exceeds the maximum value of this range, the cost of the steel increases; when the mass fraction is less than the minimum value, the strengthening phase cannot precipitate effectively. Examples of V mass fractions include 0.005%, 0.010%, 0.03%, 0.05%, 0.07%, 0.09%, and 0.1%.

[0040] The positive effects of controlling the Cu mass fraction to be between 0.05% and 0.3% are as follows: Within this range, an appropriate amount of Cu can form fine, dispersed interphase precipitates or Cu atomic clusters, thereby improving the tensile strength of the product without significantly reducing the porosity. When the mass fraction exceeds the maximum value of this range, the cost of the steel will increase; conversely, when the mass fraction is less than the minimum value, the strengthening phase cannot precipitate effectively. Examples of Cu mass fractions include 0.05%, 0.10%, 0.15%, 0.20%, 0.25%, and 0.30%.

[0041] The positive effects of controlling the mass fraction of boron (B) to be >0 and ≤0.003% are as follows: Within this range, an appropriate amount of B can increase the binding force of grain boundaries and effectively inhibit the formation of pearlite and ferrite. When the mass fraction exceeds the maximum value at the end of this range, it indicates that the B content is too high, which will lead to the precipitation of borides and reduce the effect of B atoms. For example, the mass fraction of B can be 0.0005%, 0.001%, 0.0015%, 0.002%, 0.0025%, 0.003%, etc.

[0042] In some embodiments, the microstructure of the ultra-high strength steel, by volume fraction, comprises: martensite or ferrite >0 and <3%, lower bainite: 15% to 30%, and the balance being granular bainite.

[0043] In this embodiment, the matrix structure is controlled to be granular bainite + lower bainite, with a small amount of generated martensite or ferrite, resulting in a product with high strength and a high porosity. For example, the content of martensite or ferrite can be 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, 3%, etc., and the content of lower bainite can be 15%, 17%, 19%, 20%, 22%, 25%, 28%, 30%, etc.

[0044] In some embodiments, the lower bainite and the granular bainite satisfy the following relationship: Hv (下贝氏体) -Hv (粒状贝氏体) ≤60,

[0045] In the formula, Hv (下贝氏体) Hv represents the average hardness of lower bainite. (粒状贝氏体) This indicates the average hardness of granular bainite.

[0046] In this embodiment, the porosity is increased by controlling the hardness difference. For example, Hv (下贝氏体) -Hv (粒状贝氏体) The values ​​can be 30, 35, 40, 45, 50, 55, 60, etc.

[0047] In some embodiments, Ti precipitates are distributed on the lower bainite and the matrix of the granular bainite, the average diameter of the precipitates being ≤15 nm.

[0048] In this embodiment of the application, the strength of steel is improved by the precipitation of Ti. For example, the average diameter of the precipitate can be 8nm, 9nm, 10nm, 11nm, 12nm, 13nm, 14nm, 15nm, etc.

[0049] In some embodiments, the ultra-high strength steel satisfies at least one of the following properties: yield strength ≥ 750 MPa, tensile strength ≥ 950 MPa, hole expansion ratio λ ≥ 25%, and elongation A ≥ 10%.

[0050] In this embodiment, the ultra-high strength steel product possesses high formability and high fatigue life, and can be applied to complex shaped parts such as control arms. For example, the yield strength can be 750MPa, 760MPa, 770MPa, 780MPa, 790MPa, 800MPa, etc., the tensile strength can be 950MPa, 960MPa, 970MPa, 980MPa, 990MPa, 1000MPa, etc., the porosity λ can be 25%, 25.2%, 25.4%, 25.6%, 25.8%, 26.0%, 26.5%, etc., and the elongation A can be 10%, 10.2%, 10.5%, 10.8%, 11.0%, 11.5%, etc.

[0051] Figure 1 This is a schematic flowchart illustrating a method for preparing ultra-high strength steel for a control arm, as provided in an embodiment of this application.

[0052] Please see Figure 1 This application provides a method for preparing ultra-high strength steel for control arms, the method comprising:

[0053] S1. Continuously cast molten steel with the aforementioned chemical composition to obtain a slab;

[0054] In some embodiments, before step S1, the process further includes: pre-treating the molten iron, followed by converter smelting, LF refining, and RH refining to obtain molten steel with the aforementioned chemical composition.

[0055] In some embodiments, the process parameters of the continuous casting include: molten steel superheat ≤25℃, secondary cooling changed from weak cooling to strong cooling, secondary cooling specific water volume ≥0.6L / kg, and solidification end light reduction ≥3.5mm.

[0056] In the embodiments of this application, the positive effects of controlling the superheat of molten steel to ≤25°C are: rapid solidification of the molten steel prevents low-melting-point impurities from accumulating, thereby reducing component segregation. For example, the superheat of the molten steel can be 10°C, 15°C, 20°C, 25°C, etc.

[0057] The positive effects of changing the secondary cooling model from weak to strong cooling, with a secondary cooling specific water volume ≥ 0.6 L / kg, include: improved billet microstructure and reduced dendritic segregation. For example, the secondary cooling specific water volume can be 0.6 L / kg, 0.7 L / kg, 0.8 L / kg, 0.9 L / kg, 1.0 L / kg, 1.2 L / kg, etc.

[0058] The positive effects of controlling the solidification end reduction to ≥3.5mm include: improving the billet microstructure and reducing dendritic segregation. For example, the solidification end reduction can be 3.5mm, 4.0mm, 10mm, 15mm, 20mm, 25mm, etc.

[0059] S2. The slab is heated, rough-rolled, and finish-rolled to obtain a hot-rolled plate;

[0060] In some embodiments, the final rolling temperature of the roughing mill is 1050℃~1100℃; the rolling speed of the finishing mill is 5m / s~10m / s, and the final rolling temperature is Ar3~(Ar3+80℃); the thickness of the hot-rolled plate is 1.8mm~5mm.

[0061] In some embodiments, the finishing rolling temperature is Ar3 to (Ar3+50℃).

[0062] In the embodiments of this application, by controlling the rolling process parameters, the average hardness Hv of the bainite is made to satisfy Hv(lower bainite) - Hv(granular bainite) ≤ 60.

[0063] The positive effects of controlling the final rolling temperature of roughing mill to 1050℃~1100℃ include: controlling the size of deformed austenite and improving the uniformity of the microstructure. For example, the final rolling temperature of the roughing mill is 1050℃, 1060℃, 1070℃, 1080℃, 1090℃, 1100℃, etc.

[0064] For example, the final rolling temperature can be Ar3, Ar3+10℃, Ar3+30℃, Ar3+40℃, Ar3+60℃, Ar3+70℃, Ar3+80℃, etc.

[0065] Constant speed rolling is used in finish rolling, and the thickness of the hot-rolled plate is controlled within the range of 1.8–5 mm. The rolling speed is controlled within 5–10 m / s depending on the thickness, which can improve the uniformity of the microstructure. For example, the rolling speed of finish rolling can be 5 m / s, 6 m / s, 7 m / s, 8 m / s, 9 m / s, 10 m / s, etc.; the thickness of the hot-rolled plate can be 1.8 mm, 2 mm, 3 mm, 4 mm, 5 mm, etc.

[0066] In some embodiments, the finishing rolling includes rolling of the non-recrystallized austenite region, wherein the reduction rate of the non-recrystallized austenite region rolling is ≥40%.

[0067] The positive effects of controlling the reduction rate of rolling the non-recrystallized austenite region to ≥40% include: thinning the austenite layer, increasing the ferrite nucleation area, and ultimately refining the grains. For example, the reduction rate of rolling the non-recrystallized austenite region can be 40%, 42%, 44%, 46%, 48%, 50%, etc.

[0068] In some embodiments, during the rolling process in the non-recrystallized austenite region, rolling lubricant is introduced into the rolling mill, and the flow rate of the rolling lubricant is controlled to be 80 mL / min to 120 mL / min.

[0069] By applying rolling lubrication through the non-recrystallization zone of the rolling mill, the uniformity of the microstructure of the hot-rolled plate is ensured, thereby improving the porosity. For example, the flow rate of the rolling lubricant can be 80 mL / min, 85 mL / min, 90 mL / min, 95 mL / min, 100 mL / min, 110 mL / min, 120 mL / min, etc.

[0070] S3. Cool and coil the hot-rolled plate to obtain ultra-high strength steel.

[0071] In some embodiments, the cooling rate is ≥30°C / s, and the termination temperature is 300°C to 500°C.

[0072] The positive effects of controlling the cooling rate to ≥30℃ / s and the termination temperature to 300℃~500℃ include ensuring tissue uniformity. For example, the cooling rate can be 30℃ / s, 32℃ / s, 35℃ / s, 38℃ / s, 40℃ / s, 45℃ / s, etc., and the termination temperature can be 300℃, 350℃ / s, 380℃ / s, 400℃ / s, 420℃ / s, 450℃ / s, 480℃ / s, 500℃, etc.

[0073] In some embodiments, the cooling termination temperature is 350°C to 450°C.

[0074] The product prepared by the method of preparing ultra-high strength steel is the ultra-high strength steel mentioned above. The chemical composition and microstructure of the ultra-high strength steel prepared by the method of preparing ultra-high strength steel can be referred to the above embodiments. Since the method of preparing ultra-high strength steel adopts some or all of the technical solutions of the ultra-high strength steel embodiments, it has at least all the beneficial effects brought about by the technical solutions of the ultra-high strength steel embodiments, which will not be elaborated here.

[0075] The present application is further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the application. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to industry standards. If there is no corresponding industry standard, then common international standards, conventional conditions, or conditions recommended by the manufacturer are followed.

[0076] Example 1

[0077] This embodiment provides an ultra-high strength steel for control arms and its preparation method. Its chemical composition, by weight percentage, is as follows: C: 0.07%, Si: 0.2%, Mn: 1.2%, P: 0.008%, S: 0.0015%, Nb: 0.03%, Ti: 0.05%, Cr: 0.9%, Mo: 0.2%, N: 0.005%, V: 0.05%, Cu: 0.15%, B: 0.003%, with the remainder being Fe and unavoidable impurities.

[0078] The preparation method includes: pretreating molten iron, then continuously casting the molten steel that has undergone converter smelting, LF refining, and RH refining to obtain a slab. The superheat of the molten steel is controlled at 20℃, the secondary cooling model is changed from weak cooling to strong cooling, the specific water content is increased to 0.6L / kg, and the light reduction at the end of solidification is 4.5mm. The molten steel is then subjected to rough rolling and finish rolling to obtain a hot-rolled plate. Constant speed rolling is used during the finish rolling. The rough rolling termination temperature is controlled at 1080℃, the finish rolling final rolling temperature is controlled at Ar3+45℃, the reduction rate in the non-recrystallized austenite region is 55%, and the mill in the non-recrystallized region is lubricated with a flow rate controlled at 100mL / min. The hot-rolled plate is rapidly cooled to a coiling temperature of 400℃ to obtain the ultra-high strength steel product.

[0079] The microstructure of the finished product consists of granular bainite + lower bainite, and a small amount of martensite or ferrite, with martensite or ferrite content of 2%, lower bainite content of 20%, and the remainder being granular bainite. Hv (下贝氏体) -Hv (粒状贝氏体)=45, precipitates containing Ti are formed in the bainite, and the average diameter of the precipitates is 12nm. The strip has a yield strength of 780MPa, a tensile strength of 975MPa, a porosity λ = 28%, and an elongation A = 10%. The product has high formability and long fatigue life, and can be used in complex forming parts such as control arms.

[0080] Example 2

[0081] This embodiment provides an ultra-high strength steel for control arms and its preparation method. Its chemical composition, by weight percentage, is as follows: C: 0.05%, Si: 0.15%, Mn: 1.5%, P: 0.01%, S: 0.001%, Nb: 0.05%, Ti: 0.07%, Cr: 0.6%, Mo: 0.15%, N: 0.035%, V: 0.06%, Cu: 0.2%, B: 0.0002%, with the remainder being Fe and unavoidable impurities.

[0082] The preparation method includes: pretreating molten iron, then continuously casting the molten steel that has undergone converter smelting, LF refining, and RH refining to obtain a slab. The superheat of the molten steel is controlled at 16℃, the secondary cooling model is changed from weak cooling to strong cooling, the specific water content is increased to 0.8L / kg, and the light reduction at the end of solidification is 5mm. The molten steel is then subjected to rough rolling and finish rolling to obtain a hot-rolled plate. Constant speed rolling is used during the finish rolling. The rough rolling termination temperature is controlled at 1080℃, the finish rolling final rolling temperature is controlled at Ar3+30℃, the reduction rate in the non-recrystallized austenite region is 58%, and the mill in the non-recrystallized region is lubricated with a flow rate controlled at 110mL / min. The hot-rolled plate is rapidly cooled to a coiling temperature of 420℃.

[0083] The microstructure of the finished product consists of granular bainite + lower bainite, and a small amount of martensite or ferrite, with a martensite or ferrite content of 1%, lower bainite content of 25%, and the remainder being granular bainite. Hv (下贝氏体) -Hv (粒状贝氏体) =40, precipitates containing Ti are formed in the bainite, and the average diameter of the precipitates is 10nm. The strip has a yield strength of 795MPa, a tensile strength of 990MPa, a porosity λ = 26%, and an elongation A = 11%. The product has high formability and long fatigue life, and can be used in complex forming parts such as control arms.

[0084] Comparative Example 1

[0085] This comparative example provides an ultra-high strength steel for control arms and its preparation method. Its chemical composition, by weight percentage, is as follows: C: 0.13%, Si: 0.5%, Mn: 1.2%, P: 0.01%, S: 0.003%, Nb: 0.01%, Ti: 0.05%, Cr: 0.6%, Mo: 0.02%, N: 0.035%, V: 0.01%, Cu: 0.15%, B: 0.005%, with the remainder being Fe and unavoidable impurities.

[0086] The preparation method includes: pretreating molten iron, then continuously casting the molten steel that has undergone converter smelting, LF refining, and RH refining to obtain a slab. The superheat of the molten steel is controlled at 16℃, the secondary cooling model is changed from weak cooling to strong cooling, the specific water content is increased to 0.8L / kg, and the light reduction at the end of solidification is 5mm. The molten steel is then subjected to rough rolling and finish rolling to obtain a hot-rolled plate. Constant speed rolling is used during the finish rolling. The rough rolling termination temperature is controlled at 1110℃, and the final rolling temperature is controlled at Ar3-(Ar3+30℃). The reduction rate in the non-recrystallized austenite region is 58%, and the mill in the non-recrystallized region is lubricated with a flow rate controlled at 110mL / min. The hot-rolled plate is rapidly cooled to a coiling temperature of 330℃.

[0087] The microstructure of the finished product consists of granular bainite + lower bainite, and a small amount of martensite or ferrite, with martensite accounting for 8%, lower bainite for 35%, and the remainder being granular bainite. Hv (下贝氏体) -Hv (粒状贝氏体) =65, Ti-containing precipitates are formed in the bainite, with an average diameter of 10 nm. The strip has a yield strength of 825 MPa, a tensile strength of 1150 MPa, a porosity λ = 30%, and an elongation A = 9%. The product does not have high formability or high fatigue life.

[0088] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A type of ultra-high strength steel for control arms, characterized in that, The chemical composition of the ultra-high strength steel, by mass fraction, is as follows: C: 0.05%~0.12%, Si>0 and <0.3%, Mn: 1.0%~2.2%, P≤0.013%, S≤0.002%, Nb: 0.02%~0.06%, Ti: 0.03%~0.15%, Cr: 0.1%~1.0%, Mo: 0.05%~0.3%, N≤0.005%, V: 0.005%~0.1%, Cu: 0.05%~0.3%, B>0 and ≤0.003%, with the balance being Fe and unavoidable impurities; The microstructure of the ultra-high strength steel, by volume fraction, includes: martensite or ferrite >0 and <3%, lower bainite: 15%~30%, and the balance being granular bainite. The lower bainite and the granular bainite satisfy the following relationship: Hv 下贝氏体 -Hv 粒状贝氏体 ≤60, where Hv 下贝氏体 Hv represents the average hardness of lower bainite. 粒状贝氏体 This indicates the average hardness of granular bainite; Ti precipitates are distributed on the matrix of the lower bainite and the granular bainite, and the average diameter of the precipitates is ≤15nm.

2. The ultra-high strength steel according to claim 1, characterized in that, The ultra-high strength steel meets at least one of the following properties: yield strength ≥750MPa, tensile strength ≥950MPa, hole expansion ratio λ ≥25%, and elongation A ≥10%.

3. A method for preparing ultra-high strength steel for a control arm according to any one of claims 1-2, characterized in that, The method includes: Molten steel with the aforementioned chemical composition is continuously cast to obtain a slab; The slab is heated, rough-rolled, and finish-rolled to obtain a hot-rolled plate; and The hot-rolled plate is cooled and coiled to obtain ultra-high strength steel.

4. The method according to claim 3, characterized in that, The final rolling temperature of the roughing mill is 1050℃~1100℃.

5. The method according to claim 3, characterized in that, The parameters of the finishing rolling include: rolling speed of 5m / s to 10m / s, finishing rolling temperature of Ar3 to (Ar3+80℃), reduction rate of ≥40% in the non-recrystallized austenite region, and lubricant flow rate of 80mL / min to 120mL / min in the non-recrystallized austenite region; the thickness of the hot-rolled plate is 1.8mm to 5mm.

6. The method according to claim 3, characterized in that, The cooling rate is ≥30℃ / s, and the termination temperature is 300℃~500℃.

7. The method according to claim 3, characterized in that, The continuous casting process parameters include: molten steel superheat ≤ 25℃, secondary cooling water volume ≥ 0.6L / kg, and final solidification reduction ≥ 3.5mm.

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