800mpa grade torsion beam steel and method of making

Abstract

The application provides a 800MPa-grade torsion beam steel and a preparation method, and belongs to the field of steel preparation.The chemical components of the torsion beam steel include:C:0.06%-0.09%, Si>0 and ≤0.6%, Mn:1.5%-2.1%, P≤0.015%, S≤0.003%, Nb:0.01%-0.05%, Ti:0.03%-0.07%, Cr:0.3%-0.7%, Al:0.3%-0.7%, N>0 and ≤0.005%;the internal microstructure of the torsion beam steel includes:≥85% of bainite, 5%-10% of residual austenite, and the rest of ferrite or martensite.Through reasonable design of the chemical components of the steel, the bainite structure containing residual austenite is obtained, the high strength and high forming performance of the raw material are realized, and the high strength and excellent processability of the torsion beam steel are simultaneously considered.

Description

Technical Field

[0001] This application relates to the field of steel preparation technology, and in particular to an 800MPa grade torsion beam steel and its preparation method. Background Technology

[0002] With advancements in forming technology, the torsion beam design has evolved from the traditional open torsion beam to a closed torsion beam structure. This not only inherits the advantages of the original design, such as its simple structure and small footprint, but also achieves an overall weight reduction of over 50% due to the application of high-strength steel and the absence of a stabilizer bar. Therefore, it has gained widespread popularity in compact cars, MPVs, and NEVs.

[0003] In the cold forming process of closed-loop torsion beams, the strip steel undergoes slitting, welding, stamping / hydraulic forming, shot peening, and / or stress-relief annealing. As the strength grade of the applied material increases, the processing difficulty also gradually increases. The introduction of stress and microcracks during processing can lead to a significant reduction in the overall bench life of the torsion beam component. Therefore, the development of steel for torsion beams must require materials that possess high strength while also considering the negative impacts of processing on the parts after strength enhancement, including more refined performance indicators such as material surface roughness and uniform elongation. Therefore, there is an urgent need to develop a torsion beam steel that can balance high strength and excellent machinability. Summary of the Invention

[0004] This application provides an 800MPa grade torsion beam steel and its preparation method to solve the following technical problem: how to simultaneously achieve both high strength and excellent machinability of torsion beam steel.

[0005] In a first aspect, this application provides an 800MPa grade torsion beam steel, wherein the chemical composition of the torsion beam steel, by mass fraction, comprises: C: 0.06%–0.09%, Si > 0 and ≤ 0.6%, Mn: 1.5%–2.1%, P ≤ 0.015%, S ≤ 0.003%, Nb: 0.01%–0.05%, Ti: 0.03%–0.07%, Cr: 0.3%–0.7%, Al: 0.3%–0.7%, N > 0 and ≤ 0.005%, with the balance being Fe and unavoidable impurities;

[0006] The internal microstructure of the steel used for the torsion beam, by volume fraction, comprises: bainite ≥ 85%, retained austenite 5% to 10%, and the balance being ferrite or martensite.

[0007] Optionally, the chemical components satisfy the following relationship:

[0008] [Si] / 2≤[Al]≤[Mn] / 3, [Cr]+[Mn] / 2≥1.0%

[0009] In the formula, [Si] represents the content of Si, [Al] represents the content of Al, [Mn] represents the content of Mn, and [Cr] represents the content of Cr.

[0010] Optionally, the chemical composition satisfies a carbon equivalent CE(IIW) ≤ 0.5%.

[0011] Optionally, the steel used for the torsion beam meets at least one of the following properties: longitudinal yield strength ≥600MPa, tensile strength ≥780MPa, elongation A80 ≥15%, uniform elongation ≥6%, and hole expansion rate ≥45%.

[0012] Secondly, this application provides a method for preparing steel for a torsion beam as described in any embodiment of the first aspect, the method comprising:

[0013] A slab having the aforementioned chemical composition is obtained;

[0014] The slab is heated, rough-rolled, and finish-rolled to obtain a hot-rolled plate.

[0015] The hot-rolled plate is subjected to laminar flow cooling and coiling to obtain a hot-rolled coil;

[0016] The hot-rolled coil is surface treated to obtain steel for torsion beams.

[0017] Optionally, the inclusion grade of the slab is <1.5; and / or, the heating temperature is 1240℃~1280℃, and the heating holding time is ≥80min.

[0018] Optionally, the RT2 temperature of the roughing mill is 1020℃~1080℃, the termination temperature of the finishing mill is 850℃~910℃, and the rolling speed of the roughing mill and the finishing mill is 5m / s~12m / s.

[0019] Optionally, the step of laminar flow cooling and coiling the hot-rolled sheet to obtain a hot-rolled coil includes:

[0020] The hot-rolled plate is air-cooled for 1 to 3 seconds.

[0021] The air-cooled hot-rolled plate is rapidly cooled to a set temperature and then wound up to obtain a hot-rolled coil; the set temperature is 430℃~480℃.

[0022] Optionally, the winding temperature is 400℃~450℃.

[0023] Optionally, the surface treatment of the hot-rolled coil to obtain steel for torsion beams includes:

[0024] The hot-rolled coil is leveled and pickled to obtain steel for torsion beams; the elongation of the leveling is 1% to 3%, and the pickling speed is 60m / min to 100m / min.

[0025] The technical solutions provided in this application have the following advantages compared with the prior art:

[0026] This application provides an 800MPa grade torsion beam steel. The chemical composition of the torsion beam steel, by mass fraction, includes: C: 0.06%–0.09%, Si > 0 and ≤ 0.6%, Mn: 1.5%–2.1%, P ≤ 0.015%, S ≤ 0.003%, Nb: 0.01%–0.05%, Ti: 0.03%–0.07%, Cr: 0.3%–0.7%, Al: 0.3%–0.7%, N > 0 and ≤ 0.005%, with the balance being Fe and unavoidable impurities. By volume fraction, the internal microstructure of the torsion beam steel includes: bainite ≥ 85%, retained austenite 5%–10%, with the balance being ferrite or martensite. By rationally designing the chemical composition of the steel, introducing Al and Cr content, and reducing Si and C content, a bainitic microstructure containing retained austenite is obtained, achieving high strength and high formability of the raw material, while also facilitating welding and high surface quality. Simultaneously, by introducing retained austenite into the bainitic matrix, the TRIP effect is utilized to improve the material's plasticity and work hardening ability. This achieves a balance between high strength and excellent machinability for torsion beam steel. Attached Figure Description

[0027] 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.

[0028] 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.

[0029] Figure 1 A schematic flowchart illustrating a method for preparing steel for a torsion beam, provided in an embodiment of this application;

[0030] Figure 2 This is an internal microstructure diagram of the steel used for torsion beams provided in Embodiment 2 of this application;

[0031] Figure 3 This is a microstructure diagram of the steel used for torsion beams provided in Embodiment 5 of this application. Detailed Implementation

[0032] 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.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] This application provides an 800MPa grade torsion beam steel. The chemical composition of the torsion beam steel, by mass fraction, includes: C: 0.06%–0.09%, Si > 0 and ≤ 0.6%, Mn: 1.5%–2.1%, P ≤ 0.015%, S ≤ 0.003%, Nb: 0.01%–0.05%, Ti: 0.03%–0.07%, Cr: 0.3%–0.7%, Al: 0.3%–0.7%, N > 0 and ≤ 0.005%, with the balance being Fe and unavoidable impurities.

[0037] The internal microstructure of the steel used for the torsion beam, by volume fraction, comprises: bainite ≥ 85%, retained austenite 5% to 10%, and the balance being ferrite or martensite.

[0038] The following describes the functions and limitations of the main components designed in this invention:

[0039] Carbon: Carbon is an austenitic element that improves the stability of austenite. The carbon content largely determines the strength of the steel plate. In this invention, carbon not only enhances strength through solid solution strengthening and precipitation strengthening by forming microalloyed precipitation, but also increases the retained austenite content, utilizing the TRIP effect to improve uniform elongation and tensile strength. Carbon is also a crucial indicator affecting carbon equivalent. A lower carbon content ensures good weldability, while an excessively low carbon content leads to insufficient strength and an inability to obtain sufficient retained austenite in the final microstructure. In this invention, the carbon content is controlled between 0.06% and 0.09%. For example, the C content is 0.06%, 0.07%, 0.08%, 0.085%, 0.09%, etc.

[0040] Silicon: Silicon has a certain solid solution strengthening effect in steel. It inhibits carbide precipitation and delays pearlite formation. It promotes carbon enrichment into austenite, increasing the volume fraction and stability of retained austenite, but it deteriorates the surface quality after pickling, which is detrimental to tube manufacturing process control and fatigue performance. This invention limits the Si content to ≤0.6%, and further controls it within the range of 0.2% to 0.6%. For example, the Si content is 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, etc.

[0041] Manganese: Manganese is an austenite stabilizing element, improving the hardenability of steel, delaying the pearlite transformation and lowering the bainite transformation temperature, refining the substructure of the steel, and providing a certain degree of solid solution strengthening. Excessive manganese content leads to decreased plasticity and easily forms segregation and MnS inclusions, thus affecting fatigue performance. Insufficient manganese content results in insufficient strength. Therefore, this invention uses an Mn content of 1.5%–2.1% to ensure strength and retained austenite volume fraction, achieving good formability while guaranteeing tensile strength. For example, the Mn content is 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2.0%, 2.1%, etc.

[0042] Phosphorus: In steel, phosphorus is generally dissolved in ferrite and has a strong solid solution strengthening effect, used to improve the strength of steel but reduce its toughness. However, excessive phosphorus content is detrimental to weldability and is a harmful element. Therefore, the phosphorus content should be minimized. In this invention, the phosphorus content is ≤0.015%. For example, the phosphorus content is 0.005%, 0.007%, 0.009%, 0.010%, 0.012%, 0.015%, etc.

[0043] Sulfur: The sulfur content and morphology of sulfides are the main factors affecting formability. The more numerous and larger the sulfides, the more detrimental they are to plasticity. Therefore, the actual control level of sulfur content in the product is not strictly limited in this invention. The sulfur content in this invention is ≤0.003%. For example, the sulfur content is 0.001%, 0.0015%, 0.002%, 0.0025%, 0.003%, etc.

[0044] Niobium: Niobium is an important precipitation and grain refiner. During rolling, it strongly inhibits the recrystallization of deformed austenite. Niobium can increase the recrystallization temperature, and during cooling, it forms tiny carbonitride precipitation, improving strength and toughness. In this invention, when the niobium content exceeds 0.05%, the precipitation strengthening effect essentially ceases to improve. Furthermore, due to the high cost of niobium, the niobium content is set in the range of 0.01% to 0.05%. For example, the Nb content is 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, etc.

[0045] Titanium: Titanium is one of the important precipitation strengthening and grain refinement strengthening elements. In this invention, titanium plays a nitrogen-fixing role, combining with free nitrogen atoms in steel to form TiN. On the other hand, a small amount of titanium, during the welding process of high-strength steel, forms TiC and Ti(C,N), which can refine the grains. Simultaneously, the precipitates can also promote the precipitation of cementite during the bulk phase transformation, and in the two-phase region after rolling cooling, promote the transformation of carbon elements into austenite, thereby improving strength and inhibiting weld softening. However, when the titanium content is too high, large-sized TiN will form, deteriorating fatigue performance. High titanium content can also lead to problems such as a narrow process window and unsuitable performance. Therefore, this invention controls the titanium content to 0.03–0.07%. For example, the Ti content is 0.03%, 0.04%, 0.05%, 0.06%, 0.07%, etc.

[0046] Chromium: Chromium is an important element in this invention. Chromium delays the pearlite phase transformation, expands the bainite transformation region, improves strength, and widens the processing window. Typically, a chromium content greater than 0.2% is required to significantly delay pearlite formation. In this invention, chromium can replace some of the functions of silicon without causing surface deterioration; therefore, the chromium content is controlled at 0.3%–0.7%. For example, the Cr content is 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, etc.

[0047] Aluminum: Aluminum is a deoxidizing element in steel, reducing oxide inclusions and purifying the steel, which is beneficial for improving the material's formability. It is also a ferrite-forming element, strongly inhibiting enrichment in austenite and bainite, thus improving austenite stability. However, excessive aluminum content can lead to Al2O3 inclusions, which can affect continuous casting production. Therefore, the aluminum content in this invention is controlled at 0.3%–0.7%. For example, the Al content is 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, etc.

[0048] Nitrogen: Nitrogen reacts with titanium during continuous casting to form TiN. Excessive nitrogen content in the steel leads to an increase in the size of the formed TiN, which must be limited. Therefore, in this invention, the nitrogen content is controlled to be less than or equal to 0.005%. For example, the N content is 0.001%, 0.002%, 0.003%, 0.004%, 0.005%, etc.

[0049] The internal microstructure of steel used in torsion beams consists of lath bainite and retained austenite, with bainite volume fraction ≥85%, retained austenite volume fraction between 5% and 10%, and the remainder being a small amount of ferrite or lath martensite. This microstructure is beneficial for improving material strength while ensuring formability. For example, the bainite content can be 85%, 86%, 87%, 88%, 89%, 90%, etc., and the retained austenite content can be 5%, 6%, 7%, 8%, 9%, 10%, etc.

[0050] In some embodiments, the chemical components satisfy the following relationship:

[0051] [Si] / 2≤[Al]≤[Mn] / 3, [Cr]+[Mn] / 2≥1.0%

[0052] In the formula, [Si] represents the content of Si, [Al] represents the content of Al, [Mn] represents the content of Mn, and [Cr] represents the content of Cr.

[0053] In the embodiments of this application, Si / 2 ≤ Al ≤ Mn / 3 is required. This is to ensure that the AC3 point temperature does not become excessively high, leading to rolling in the two-phase region, while simultaneously ensuring the stability of high-temperature austenite and maintaining a certain volume fraction of retained austenite in the microstructure at a given cooling rate. Meanwhile, Cr + Mn / 2 ≥ 1.0% is used to effectively delay the pearlite transformation, reduce the Ms point, and expand the bainite transformation phase region, which is beneficial for lath bainite formation and improves the material's formability.

[0054] In some embodiments, the chemical composition satisfies a carbon equivalent CE(IIW) ≤ 0.5%.

[0055] Carbon equivalent CE(IIW) = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5. This formula is a currently accepted empirical formula proposed by the International Welding Association. Substituting the percentage content of the chemical components into the formula allows for the prediction of the material's weldability. When the carbon equivalent CE(IIW) ≤ 0.5%, good weldability is guaranteed. For example, the carbon equivalent CE(IIW) value can be 0.4%, 0.42, 0.44, 0.46, 0.48, 0.50, etc.

[0056] In some embodiments, the steel used for the torsion beam satisfies at least one of the following properties: longitudinal yield strength ≥600MPa, tensile strength ≥780MPa, elongation A80 ≥15%, uniform elongation ≥6%, and hole expansion rate ≥45%.

[0057] Based on the component ratios and preparation methods provided in this application, the steel material for torsion beams needs to possess good formability, high uniform elongation and local forming ability, and good surface quality. Simultaneously, the material itself also possesses a certain degree of work hardening ability and excellent fatigue performance. These properties ensure the excellent performance of the material after it is manufactured into torsion beam parts. For example, the longitudinal yield strength can be 600MPa, 620MPa, 650MPa, 680MPa, 700MPa, 750MPa, 800MPa, etc.; the tensile strength can be 780MPa, 800MPa, 820MPa, 840MPa, 860MPa, 880MPa, 900MPa, etc.; the elongation A80 can be 15%, 16%, 17%, 18%, 19%, etc.; the uniform elongation can be 6%, 7%, 8%, 9%, 10%, 11%, etc.; and the hole expansion rate can be 45%, 47%, 49%, 50%, 52%, etc.

[0058] Figure 1 This is a schematic flowchart illustrating a method for preparing steel for a torsion beam, as provided in an embodiment of this application.

[0059] Please see Figure 1 This application provides a method for preparing steel for a torsion beam according to any embodiment of the first aspect, the method comprising:

[0060] S1. Obtain a slab having the aforementioned chemical composition;

[0061] In some embodiments, prior to step S1, the method further includes: pretreatment of molten iron, converter smelting, refining, and continuous casting.

[0062] In some embodiments, the inclusion grade of the slab is <1.5.

[0063] Based on production conditions, the level of center segregation and inclusions in the continuously cast billet is controlled by controlling superheat and light reduction processes, and the inclusion level is controlled to be less than 1.5.

[0064] S2. The slab is heated, rough-rolled, and finish-rolled to obtain a hot-rolled plate;

[0065] In some embodiments, the heating temperature is 1240℃~1280℃, and the heating holding time is ≥80min.

[0066] It should be noted that if the heating temperature or holding time is too low, the alloying elements cannot be fully dissolved, leading to increased rolling force and excessive internal and external temperature differences, resulting in finished product defects. If the heating temperature is too high, it will cause the billet to overheat, resulting in coarsened grains and a thicker iron oxide scale, which is not conducive to iron oxide scale descaling. By controlling the heating temperature to 1240℃~1280℃ and the holding time to ≥80min, the full dissolution of alloying elements can be ensured. For example, the heating temperature can be 1240℃, 1250℃, 1260℃, 1270℃, 1280℃, etc., and the holding time can be 80min, 90min, 100min, 110min, 120min, 150min, etc.

[0067] In some embodiments, the RT2 temperature of the roughing mill is 1020℃~1080℃, the termination temperature of the finishing mill is 850℃~910℃, and the rolling speed of the roughing mill and the finishing mill is 5m / s~12m / s.

[0068] It should be noted that RT2 temperature refers to the exit temperature of the R2 mill in the roughing mill. Controlling the RT2 temperature of the roughing mill to 1020℃~1080℃ ensures that the roughing stage is rolled in the recrystallization zone, forming equiaxed recrystallized austenite. For example, the RT2 temperature of this roughing mill is 1020℃, 1030℃, 1040℃, 1050℃, 1060℃, 1070℃, 1080℃, etc.

[0069] It should be noted that if the finishing rolling temperature is too high, acicular ferrite is easily formed, resulting in poor plasticity; while if the temperature is too low, elongated ferrite grains are easily formed or rolling in the γ+α two-phase region can easily lead to mixed grains. Controlling the finishing rolling temperature to 850℃~910℃ can ensure that the steel plate has a uniform and fine grain distribution, ensuring a balance between the material's strength and plasticity. For example, the finishing rolling temperature can be 850℃, 860℃, 870℃, 880℃, 890℃, 900℃, 910℃, etc.

[0070] During finishing rolling, constant-speed rolling is employed. The thickness of the hot-rolled plate is controlled within the range of 2.0–5.5 mm, and the rolling speed is controlled between 5–12 m / s depending on the thickness variation. The rolling speed is adjusted based on the finishing rolling termination temperature to ensure that the temperature is controlled within the aforementioned range. For example, the rolling speeds for roughing and finishing rolling can be 5 m / s, 6 m / s, 8 m / s, 10 m / s, 11 m / s, 12 m / s, etc.

[0071] S3. The hot-rolled plate is subjected to laminar flow cooling and coiling to obtain a hot-rolled coil;

[0072] In some embodiments, the step of laminar flow cooling and coiling the hot-rolled sheet to obtain a hot-rolled coil includes:

[0073] The hot-rolled plate is air-cooled for 1 to 3 seconds.

[0074] The air-cooled hot-rolled plate is rapidly cooled to a set temperature and then wound up to obtain a hot-rolled coil; the set temperature is 430℃~480℃.

[0075] It should be noted that controlling the air cooling time to 1-3 seconds allows numerous dislocations and deformation bands formed during the finishing rolling process to form a large number of fine dislocation cell structures during relaxation. These structures are conducive to strain precipitation, and these sites become preferential nucleation sites for phase deformation during subsequent cooling, promoting the formation of intragranular bainite. Controlling the final cooling temperature of laminar cooling, i.e., setting the temperature to 430℃-480℃, can form bainite structure. Too high a temperature easily leads to pearlite formation, while too low a temperature easily leads to martensite and other structures. For example, the air cooling time can be 1 second, 1.5 seconds, 2 seconds, 2.5 seconds, 3 seconds, etc., and the set temperature can be 430℃, 440℃, 450℃, 460℃, 470℃, 480℃, etc.

[0076] In some embodiments, the winding temperature is 400°C to 450°C.

[0077] S4. The hot-rolled coil is surface treated to obtain steel for torsion beams.

[0078] In some embodiments, the surface treatment of the hot-rolled coil to obtain steel for torsion beams includes:

[0079] The hot-rolled coil is leveled and pickled to obtain steel for torsion beams; the elongation of the leveling is 1% to 3%, and the pickling speed is 60m / min to 100m / min.

[0080] For example, the elongation rate of the smoothing can be 1%, 1.5%, 2%, 2.5%, 3%, etc., and the pickling speed can be 60m / min, 70m / min, 80m / min, 90m / min, 100m / min, etc.

[0081] The product prepared by the method for preparing torsion beam steel is the torsion beam steel described above. The chemical composition and microstructure of the torsion beam steel prepared by the method can be referred to the above embodiments. Since the method for preparing torsion beam steel adopts some or all of the technical solutions of the torsion beam steel embodiments, it has at least all the beneficial effects brought about by the technical solutions of the torsion beam steel embodiments, which will not be elaborated here.

[0082] 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.

[0083] In this embodiment of the application, molten iron is smelted to obtain the chemical composition of the steel for torsion beams as shown in Table 1.

[0084] Table 1. Chemical composition (wt%) of steel used in torsion beams, with the balance being impurities other than N, P, and S, and Fe.

[0085]

[0086]

[0087] Based on the above-mentioned chemical composition of the steel for torsion beams, this application provides a method for preparing the steel for torsion beams, the method comprising:

[0088] S1. After pretreatment of molten iron, it is smelted and refined in a converter to obtain molten steel with the above composition, and then continuously cast to obtain slabs.

[0089] S2. The slab is heated, rough-rolled, and finish-rolled to obtain a hot-rolled plate;

[0090] S3. The hot-rolled plate is subjected to laminar flow cooling and coiling to obtain a hot-rolled coil;

[0091] S4. The hot-rolled coil is leveled and pickled to obtain steel for torsion beams. Specifically, the process parameters for preparing the steel for torsion beams can be found in Table 2.

[0092] Table 2. Process parameters for the preparation of steel for torsion beams.

[0093]

[0094]

[0095] The mechanical properties of the steel used for torsion beams obtained in Examples 1-7 and Comparative Examples 1-3 were tested, and the results are shown in Table 3.

[0096] Table 3 Mechanical properties of steel used in torsion beams

[0097] Group Longitudinal yield strength / MPa Tensile strength / MPa Uniform elongation / % A80 elongation / % Average porosity / % Example 1 788 892 6 15.5 47 Example 2 754 850 7 18.5 47 Example 3 702 808 7.5 18 58 Example 4 630 825 7 16 51 Example 5 642 814 11.5 21.5 49 Example 6 722 833 8.5 18 49 Example 7 671 856 10.5 22 52 Comparative Example 1 746 826 5 12 46 Comparative Example 2 758 836 7 20 42 Comparative Example 3 763 846 7 19 54

[0098] As shown in Table 3, all embodiments of the present invention produce high-strength hot-rolled pickled steel sheets that meet the requirements of the invention. On the other hand, comparative examples outside the scope of the present invention do not achieve any one or more of the desired strength, elongation, and hole expansion rate.

[0099] Figure 2 This is an internal microstructure diagram of the steel used for torsion beams provided in Embodiment 2 of this application;

[0100] Depend on Figure 2 It can be seen that the steel used for the torsion beam in Example 2 is composed of granular bainite, lath bainite and retained austenite.

[0101] Figure 3 This is a microstructure diagram of the steel used for torsion beams provided in Embodiment 5 of this application.

[0102] Depend on Figure 3 It can be seen that the steel used for the torsion beam in Example 5 is composed of lath bainite and retained austenite.

[0103] Furthermore, one or more technical solutions in the embodiments of the present invention have at least the following technical effects or advantages:

[0104] (1) In this embodiment of the invention, retained austenite is introduced into the bainitic matrix through composition design and process control, and the TRIP effect is used to improve the plasticity and work hardening ability of the material.

[0105] (2) In this embodiment of the invention, the steel material for the torsion beam needs to have good formability, high uniform elongation and local forming ability, and good surface quality. Simultaneously, the material itself also possesses a certain degree of work hardening ability and excellent fatigue performance. The obtained torsion beam steel properties meet the following requirements: rolling direction yield strength ≥ 600 MPa, rolling direction tensile strength ≥ 780 MPa, A80 elongation ≥ 15%, uniform elongation ≥ 8%, and expansion rate ≥ 45%. These properties ensure that the material can possess excellent performance after being manufactured into torsion beam parts.

[0106] (3) In the embodiments of the present invention, compared with traditional steel containing retained austenitic material, traditional steel containing retained austenitic material usually adopts high C and high Si composition. The high C content is not conducive to the welding of subsequent materials. The iron oxide scale (2FeO-SiO2) generated by adding high Si content is difficult to remove, resulting in poor surface quality. The present invention reduces Si and C by introducing Al and Cr elements and post-rolling relaxation phase transformation technology to obtain bainitic structure containing retained austenitic material, thereby achieving high strength and high formability of raw materials.

[0107] (4) In this embodiment of the invention, the composition and process design are used to solve the problems of a significant decrease in the overall test bench life of the torsion beam parts caused by the introduction of stress and microcracks during the processing. It can take into account the negative impact of the processing on the parts after the strength is improved, including more refined performance indicators such as material surface roughness and uniform elongation.

[0108] 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 steel for 800MPa grade torsion beams, characterized in that, The chemical composition of the steel used for the torsion beam, by mass fraction, is as follows: C: 0.06%~0.09%, Si>0 and ≤0.6%, Mn: 1.5%~2.1%, P≤0.015%, S≤0.003%, Nb: 0.01%~0.05%, Ti: 0.03%~0.07%, Cr: 0.3%~0.7%, Al: 0.3%~0.7%, N>0 and ≤0.005%, with the balance being Fe and unavoidable impurities; The internal microstructure of the steel used for the torsion beam, by volume fraction, is: bainite ≥ 85%, retained austenite 5%~10%, and the balance being ferrite or martensite. The chemical components satisfy the following relationship: [Si] / 2≤[Al]≤[Mn] / 3, [Cr]+[Mn] / 2≥1.0%, carbon equivalent CE(IIW)≤0.5%; In the formula, [Si] represents the content of Si, [Al] represents the content of Al, [Mn] represents the content of Mn, and [Cr] represents the content of Cr.

2. The steel for torsion beams according to claim 1, characterized in that, The steel used for the torsion beam must meet at least one of the following properties: longitudinal yield strength ≥ 600 MPa, tensile strength ≥ 780 MPa, elongation A80 ≥ 15%, uniform elongation ≥ 6%, and hole expansion rate ≥ 45%.

3. A method for preparing steel for a torsion beam according to any one of claims 1 to 2, characterized in that, The method includes: A slab with the aforementioned chemical composition is obtained, wherein the inclusion grade of the slab is <1.

5. The slab is heated, rough-rolled, and finish-rolled to obtain a hot-rolled plate. The hot-rolled plate is air-cooled for 1 to 3 seconds. The air-cooled hot-rolled plate is rapidly cooled to a set temperature and then coiled to obtain a hot-rolled coil; the set temperature is 430℃~480℃, and the coiling temperature is 400℃~450℃. The hot-rolled coil is surface-treated to obtain steel for torsion beams; The heating temperature is 1240℃~1280℃, and the heating holding time is ≥80min; The RT2 temperature of the roughing mill is 1020℃~1080℃, the termination temperature of the finishing mill is 850℃~910℃, and the rolling speed of the roughing mill and the finishing mill is 5m / s~12m / s.

4. The method according to claim 3, characterized in that, The process of surface treating the hot-rolled coil to obtain steel for torsion beams includes: The hot-rolled coil is leveled and pickled to obtain steel for torsion beams; the elongation rate of the leveling is 1% to 3%, and the pickling speed is 60m / min to 100m / min.

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