A steel for torsion beam and a method for manufacturing the same

Abstract

The application relates to a torsion beam steel and a preparation method thereof, and the chemical components of the torsion beam steel include the following contents: the content of C is 0.05-0.08%, the content of Si is <=0.15%, the content of Mn is 1.2-1.7%, the content of P is <=0.015%, the content of S is <=0.003%, the content of Nb is 0.01-0.05%, the content of Ti is 0.03-0.08%, the content of Cr is 0.1%-0.4%, the content of Mo is 0.1-0.25%, the content of V is 0.01-0.04%, and the content of Ca is 0.0005-0.005%; and the following conditions are met: [Ti] / [Mo] <=0.6, and [Cr]+[Mo] = 0.2-0.5%; the microstructure of the torsion beam steel includes ferrite and bainite. The torsion beam steel has the advantages that the tensile strength is >=650 MPa, the hole expansion rate lambda is >=85%, and the elongation A80 is >=24%.

Description

Technical Field

[0001] This application relates to the field of steel preparation technology, and in particular to a steel for torsion beams and a method for preparing the same. Background Technology

[0002] Torsion beams are an important component of passenger vehicle chassis structures. Due to their simple structure, space-saving design, low manufacturing cost, and easy maintenance, they are particularly suitable for use in compact passenger vehicles and new energy vehicles.

[0003] To achieve the goals of lightweighting and improved service performance, torsion beam design has evolved from U / V torsion beams to tubular torsion beams. Simultaneously, to improve production efficiency and shorten production processes, high-frequency resistance welded tubes for the torsion beam crossbeams, combined with a process without subsequent annealing, are commonly used. However, it is difficult to obtain high-strength torsion beam steel with high formability. Summary of the Invention

[0004] This application provides a steel for torsion beams and a method for preparing the same, in order to solve the following technical problem: how to improve the formability of steel for torsion beams.

[0005] In a first aspect, this application provides a steel for torsion beams, the chemical composition of which includes:

[0006] The compounds are C, Si, Mn, P, S, Nb, Ti, Cr, Mo, V, Ca, and Fe; wherein, by mass fraction, the content of C is 0.05–0.08%, the content of Si is ≤0.15%, the content of Mn is 1.2–1.7%, the content of P is ≤0.015%, the content of S is ≤0.003%, the content of Nb is 0.01–0.05%, the content of Ti is 0.03–0.08%, the content of Cr is 0.1%–0.4%, the content of Mo is 0.1%–0.25%, the content of V is 0.01%–0.04%, and the content of Ca is 0.0005%–0.005%.

[0007] And it satisfies [Ti] / [Mo]≤0.6, [Cr]+[Mo]=0.2%~0.5%,

[0008] In the formula, [Ti] represents the mass fraction of Ti, [Mo] represents the mass fraction of Mo, and [Cr] represents the mass fraction of Cr;

[0009] The microstructure of the steel used in the torsion beam comprises ferrite and bainite, wherein the bainite includes granular bainite and lath bainite, wherein, in terms of area fraction,

[0010] The content of ferrite is ≥50%, the content of granular bainite is 20% to 35%, and the content of lath bainite is ≤10%.

[0011] Optionally, the C content is 0.072%–0.076%, the Si content is 0.08%–0.12%, the Mn content is 1.22%–1.65%, the Nb content is 0.0251%–0.0252%, the Ti content is 0.042%, the Cr content is 0.23%–0.37%, the Mo content is 0.11%–0.12%, the V content is 0.012%–0.028%, and the Ca content is 0.002%.

[0012] Optionally, the welding sensitivity coefficient Pcm ≤ 0.18%.

[0013] Optionally, the equivalent grain size in the microstructure is 2.5 μm to 5 μm.

[0014] Optionally, the steel used for the torsion beam meets the following properties: tensile strength ≥ 650 MPa, hole expansion ratio λ ≥ 85%, elongation A80 ≥ 24%, and surface quality grade FB.

[0015] 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:

[0016] The molten iron is pretreated, then smelted, calcium treated, and continuously cast to obtain a slab.

[0017] The slab is heated and rolled, then cooled and coiled to obtain steel for torsion beams; wherein the coiling temperature is 520℃~600℃.

[0018] Optionally, the winding temperature is 560℃~580℃.

[0019] Optionally, the heating temperature is 1230℃~1280℃.

[0020] Optionally, the rolling process includes roughing and finishing, wherein the final temperature of the roughing is 1050℃~1100℃ and the final rolling temperature of the finishing is 830℃~900℃.

[0021] Optionally, the method further includes:

[0022] The steel used for the torsion beam is flattened and then pickled to obtain the finished steel product for the torsion beam; wherein...

[0023] The elongation rate of the flattened strip is 1% to 3%, and the running speed of the pickled strip is 60m / min to 100m / min.

[0024] The technical solutions provided in this application have the following advantages compared with the prior art:

[0025] The torsion beam steel provided in this application embodiment has the following chemical composition: While ensuring strength, the C content guarantees the material's formability and weldability; the Mn content effectively strengthens through solid solution and grain refinement; the S and P contents are at low levels, reducing material brittleness and improving formability; the Si content strengthens through solid solution, inhibits carbide precipitation, and delays pearlite formation; the Ni and V contents refine the grains and provide precipitation strengthening; the Ti content acts as a nitrogen fixation agent, combining with free nitrogen atoms in the steel to form TiN. The TiC and Ti(C,N) formed during high-strength steel welding further refine the grains, and the precipitates also inhibit cementite precipitation during phase transformation, promoting carbon formation in the two-phase region after rolling cooling. The ferrite-bainite ratio enhances strength and inhibits weld softening; Cr content delays pearlite transformation and expands the bainite transformation zone; Mo content improves hardenability and controls the ferrite-bainite microstructure. Furthermore, it forms dispersed and stable precipitation during post-rolling cooling, improving strength and weldability; Ni and V refine grain size and promote precipitation strengthening; Ca content reduces oxide inclusions and purifies the steel; limiting the [Ti] / [Mo] ratio allows Mo to combine with Ti to form fine, dispersed precipitation; and limiting the [Cr]+[Mo] ratio achieves high hardenability and precipitation strengthening. By comprehensively designing the content of these chemical elements and controlling the ideal microstructure of ferrite + bainite, the grain size of the microstructure is refined, thereby improving the formability of the torsion beam steel. Attached Figure Description

[0026] 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.

[0027] 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.

[0028] Figure 1 Microstructure diagram of steel for a torsion beam provided according to some embodiments of this application;

[0029] Figure 2 This is a schematic flowchart illustrating a method for preparing steel for a torsion beam according to some embodiments of this application. Detailed Implementation

[0030] 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.

[0031] 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.

[0032] In this application, the terms "including" or "comprising" mean "including but not limited to".

[0033] 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.

[0034] This application provides a steel for torsion beams, the chemical composition of which includes:

[0035] C, Si, Mn, P, S, Nb, Ti, Cr, Mo, V, Ca, and Fe; wherein, by mass fraction,

[0036] The content of C is 0.05-0.08%, the content of Si is ≤0.15%, the content of Mn is 1.2-1.7%, the content of P is ≤0.015%, the content of S is ≤0.003%, the content of Nb is 0.01-0.05%, the content of Ti is 0.03-0.08%, the content of Cr is 0.1%-0.4%, the content of Mo is 0.1-0.25%, the content of V is 0.01-0.04%, and the content of Ca is 0.0005-0.005%.

[0037] And it satisfies [Ti] / [Mo]≤0.6, [Cr]+[Mo]=0.2%~0.5%,

[0038] In the formula, [Ti] represents the mass fraction of Ti, [Mo] represents the mass fraction of Mo, and [Cr] represents the mass fraction of Cr;

[0039] The microstructure of the steel used in the torsion beam includes ferrite and bainite, wherein the bainite includes granular bainite and lath bainite. Figure 1 Here is a microstructure diagram of a torsion beam steel provided according to some embodiments of this application; please refer to Figure 1 Of which, in terms of area fraction,

[0040] The content of ferrite is ≥50%, the content of granular bainite is 20% to 35%, and the content of lath bainite is ≤10%.

[0041] In some embodiments, the content of C is 0.072% to 0.076%, the content of Si is 0.08% to 0.12%, the content of Mn is 1.22% to 1.65%, the content of Nb is 0.0251% to 0.0252%, the content of Ti is 0.042%, the content of Cr is 0.23% to 0.37%, the content of Mo is 0.11% to 0.12%, the content of V is 0.012% to 0.028%, and the content of Ca is 0.002%.

[0042] Carbon (C) is an austenitic element that improves the stability of austenite. The carbon content largely determines the strength of the steel plate. In this embodiment, C not only enhances strength through solid solution strengthening and precipitation strengthening by forming microalloyed precipitates, but it is also a crucial indicator affecting the carbon equivalent. Excessive C content leads to both excessively high material strength and excessively high carbon equivalent, affecting the material's formability and weldability. Conversely, insufficient C content makes it difficult to form enough microalloyed precipitates to ensure material strength. For example, the C content can be 0.05%, 0.06%, 0.07%, 0.08%, 0.09%, etc.

[0043] Si has a certain solid solution strengthening effect in steel. Si delays pearlite formation by inhibiting carbide precipitation. When the mass fraction of silicon is too high, it will cause red iron scale to form on the surface of the steel after rolling, which will deteriorate the surface quality after pickling and is not conducive to pipe making and forming. For example, the above-mentioned Si content can be 0.15%, 0.14%, 0.13%, 0.12%, etc.

[0044] Manganese (Mn) is an austenite stabilizing element that improves the hardenability of steel, delays the pearlite transformation, lowers the bainite transformation temperature, refines the substructure of the steel, and provides a certain degree of solid solution strengthening. When the mass fraction of Mn is too high, it leads to decreased plasticity and easily forms segregation and MnS inclusions, thus affecting the performance stability and fatigue properties of pipe making and slitting. When the mass fraction of Mn is too low, it is difficult to effectively exert solid solution strengthening and grain refinement strengthening effects, resulting in insufficient strength. For example, the Mn content mentioned above can be 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, etc.

[0045] Phosphorus (P) in steel is generally dissolved in ferrite, exhibiting strong solid solution strengthening properties to increase strength but reduce toughness. However, excessively high P content can lead to phosphorus agglomeration, resulting in decreased grain boundary strength and affecting both strength and weldability. For example, the P content could be 0.015%, 0.013%, 0.011%, or 0.009%.

[0046] The sulfur content and sulfide morphology are the main factors affecting formability and fatigue performance. The higher the amount of sulfides and the larger the size of the sulfide inclusions, the more detrimental it is to plasticity and the worse the fatigue performance. For example, the sulfur content can be 0.003%, 0.0025%, 0.002%, etc.

[0047] Nitrogen (Nb) is an important precipitation and grain-refining element. During rolling, it strongly inhibits the recrystallization of deformed austenite, increases the recrystallization temperature, and forms micro-carbonitride precipitates during cooling, improving strength and toughness. This positively impacts formability and fatigue performance. However, excessively high Nb content leads to decreased plasticity in the steel, while excessively low Nb content fails to effectively refine grains and promote precipitation strengthening. For example, the Nb content can be 0.01%, 0.02%, 0.03%, 0.04%, or 0.05%.

[0048] Ti is an important precipitation strengthening and grain refinement strengthening element. In the embodiments of this application, Ti plays a nitrogen-fixing role, combining with free nitrogen atoms in the steel to form TiN. On the other hand, a small amount of titanium forms TiC and Ti(C,N) during the welding process of high-strength steel, which can refine the grains. At the same time, the precipitates can also inhibit the precipitation of cementite during the phase transformation, and promote the transformation of carbon elements into austenite in the two-phase region after rolling and cooling, thereby improving strength and inhibiting weld softening. When the mass fraction of Ti is too large, large-sized TiN will be formed, which will worsen fatigue performance and lead to a narrow process window, resulting in unsuitable performance. When the mass fraction of Ti is too low, it is difficult to achieve the effects of grain refinement and precipitation strengthening. For example, the above-mentioned Ti content can be 0.03%, 0.04%, 0.05%, 0.06%, 0.07%, etc.

[0049] Cr (Cr) delays the pearlite phase transformation, expands the bainite transformation region, improves strength, and widens the processing window. Typically, a Cr content greater than 0.1% is required to significantly delay pearlite formation, achieving reasonable control of the ferrite-bainite volume fraction. In the embodiments of this application, Cr can replace some of the functions of Si without causing surface deterioration. Excessive chromium mass fraction leads to excessive bainite content, insufficient formability, and a tendency to form segregation, worsening fatigue performance. Conversely, insufficient chromium mass fraction fails to improve hardenability and delay pearlite formation. For example, the Cr content can be 0.1%, 0.2%, 0.3%, 0.4%, etc.

[0050] Mo can improve the hardenability of materials, achieve ferrite-bainite microstructure control, and form dispersed and stable precipitation during post-rolling cooling, thereby improving strength and weldability. However, excessively high Mo content leads to increased costs; conversely, excessively low Mo content fails to effectively improve hardenability and precipitation strengthening. For example, the Mo content can be 0.1%, 0.2%, or 0.25%.

[0051] V enhances strength and improves material fatigue properties through precipitation strengthening and microstructure refinement. However, excessively high V content can lead to aggregated carbides, reducing strength and deteriorating formability; conversely, excessively low V content is insufficient to provide precipitation strengthening and improve weldability. For example, the V content can be 0.01%, 0.02%, 0.03%, 0.04%, etc.

[0052] Ca is an important element for improving fatigue performance in the embodiments of this application. Calcium treatment can reduce oxide inclusions in steel, purify the steel, and control the morphology and quantity of inclusions, which is beneficial to improving the material's formability and fatigue performance. When the mass fraction of calcium is too high, it will lead to an increase in inclusions and deteriorate the performance. When the mass fraction of calcium is too low, it is difficult to achieve the effect of calcium treatment on modifying inclusions. For example, the content of Ca can be 0.005%, 0.007%, 0.009%, 0.0005%, 0.0007%, 0.0009%, 0.001%, 0.002%, 0.003%, 0.004%, etc.

[0053] Limiting the [Ti] / [Mo] ratio can further promote smaller precipitate sizes, improve microstructure uniformity, and enhance fatigue performance. For example, the [Ti] / [Mo] ratio could be 0.6, 0.5, 0.4, 0.3, etc.

[0054] The [Cr]+[Mo] content is limited, as Cr and Mo work together to achieve high hardenability and precipitation strengthening. If the sum of these mass percentages is too high, the steel's plasticity will be too poor, making it difficult to meet the bainitic-ferrite microstructure requirements; if the sum is too low, the material strength will be too low to meet the requirements. For example, the [Cr]+[Mo] content can be 0.2%, 0.3%, 0.4%, 0.5%, etc.

[0055] In some embodiments, the content of C can be 0.072% to 0.076%, the content of Si can be 0.08% to 0.12%, the content of Mn can be 1.22% to 1.65%, the content of Nb can be 0.0251% to 0.0252%, the content of Ti can be 0.042%, the content of Cr can be 0.23% to 0.37%, the content of Mo can be 0.11% to 0.12%, the content of V can be 0.012% to 0.028%, and the content of Ca can be 0.002%.

[0056] Figure 1 Here is a microstructure diagram of a torsion beam steel provided according to some embodiments of this application; please refer to Figure 1 By limiting the area fractions of ferrite, lath bainite, and granular bainite, not only can the tensile strength requirement of 650 MPa be achieved, but the requirement of high formability can also be met. For example, the ferrite content is 50%, 52%, 54%, 56%, 58%, 60%, etc., the granular bainite content is 20%, 25%, 30%, 35%, etc., and the lath bainite content is 10%, 9%, 8%, 7%, etc.

[0057] In some embodiments, the equivalent grain size in the microstructure is 2.5 μm to 5 μm.

[0058] In this embodiment, the microstructure of the steel used for the torsion beam consists of fine-grained ferrite and bainite, with an equivalent grain size of 2.5 μm to 5 μm. The bainite microstructure is uniform and has good formability. During bench testing, due to its uniform structure, stress concentration and crack initiation are less likely to occur in the steel, thus ensuring good fatigue performance. For example, the equivalent grain size in the above microstructure can be 2.5 μm, 3.0 μm, 3.5 μm, 4.0 μm, 4.5 μm, 5 μm, etc. Definition of equivalent grain size (d): Where A is the grain area.

[0059] In some embodiments, the welding sensitivity coefficient Pcm ≤ 0.18%.

[0060] In this embodiment, the weld sensitivity coefficient Pcm = ([C] + [Si] / 30 + [Mn] / 20 + [Mo] / 15 + [Cr] / 20 + [V] / 10) ≤ 0.18%. The smaller this coefficient, the better the weldability of the steel. A weld sensitivity coefficient ≤ 0.18% indicates excellent weldability. For example, the weld sensitivity coefficient Pcm can be 0.18%, 0.17%, 0.16%, etc.

[0061] In some embodiments, the steel used for the torsion beam meets the following properties: tensile strength ≥ 650 MPa, hole expansion ratio λ ≥ 85%, elongation A80 ≥ 24%, and surface quality grade FB.

[0062] In the embodiments of this application, the steel used for the torsion beam meets the following properties: tensile strength ≥ 650 MPa, hole expansion ratio λ ≥ 85%, elongation A80 ≥ 24%, and surface quality is FB grade (good or above).

[0063] Figure 2 This is a schematic flowchart illustrating a method for preparing steel for a torsion beam according to some embodiments of this application; please refer to [link / reference]. Figure 2 This application provides a method for preparing steel for a torsion beam according to any embodiment of the first aspect, the method comprising:

[0064] S1. The molten iron is pretreated, then smelted, calcium treated and continuously cast to obtain a slab.

[0065] In the embodiments of this application, the above-mentioned smelting includes converter smelting, LF refining, and RH refining. The above-mentioned calcium treatment can be performed with a wire feed of 450m to 550m per heat, which achieves the effect of fine-tuning the composition of molten steel and modifying inclusions. If the wire feed is too low, oxide inclusions cannot be effectively removed and pure steel cannot be achieved; if the wire feed is too high, it is difficult to obtain the target composition. Because the calcium treatment of molten steel after refining improves the cleanliness of molten steel, the casting speed is controlled at 1.0 to 1.2m / min and the superheat is 40 to 60°C during the continuous casting process. The full protective casting with high superheat and low casting speed improves the castability of molten steel, ensures the cleanliness of molten steel, and ensures the uniformity of the billet structure.

[0066] S2. The slab is heated and rolled, then cooled and coiled to obtain steel for torsion beams; wherein the coiling temperature is 520℃~600℃.

[0067] In some embodiments, the winding temperature is 560°C to 580°C.

[0068] In this embodiment, the cooling process employs concentrated pre-cooling to the winding temperature. Rapid cooling to this temperature range promotes the nucleation of the ferrite phase while avoiding the formation of pearlite at excessively high temperatures and the formation of martensite or other defects at excessively low temperatures. For example, the winding temperature can be 520°C, 540°C, 560°C, 580°C, 600°C, etc.; further, the winding temperature can be 560°C, 565°C, 570°C, 580°C, etc.

[0069] In some embodiments, the rolling process includes roughing and finishing, wherein the final temperature of the roughing is 1050°C to 1100°C and the final temperature of the finishing is 830°C to 900°C.

[0070] In this embodiment, if the final temperature of the rough rolling is too high, acicular ferrite is easily formed, resulting in poor plasticity; while if the final temperature of the rough rolling is too low, elongated ferrite grains are easily formed or enter the γ+α two-phase region. Therefore, the final temperature of the rough rolling is limited to the aforementioned values. Within the range of the final rolling temperature of the finishing rolling, the required volume fraction of ferrite in the material microstructure can be achieved, resulting in good performance matching. For example, the final temperature of the rough rolling can be 1050℃, 1060℃, 1070℃, 1080℃, 1090℃, 1100℃, etc., and the final rolling temperature of the finishing rolling can be 830℃, 840℃, 850℃, 860℃, 870℃, 880℃, 890℃, 900℃, etc.

[0071] In some embodiments, the heating temperature is 1230°C to 1280°C.

[0072] In this embodiment, limiting the heating temperature allows the alloy to fully remelt while avoiding excessive heat that could lead to severe burn-off and oxidation. For example, the heating temperature could be 1230°C, 1240°C, 1250°C, 1260°C, 1270°C, 1280°C, etc.

[0073] In some embodiments, the method further includes:

[0074] S3. The steel for the torsion beam is flattened and then pickled to obtain the finished steel for the torsion beam; wherein...

[0075] The elongation rate of the flattened strip is 1% to 3%, and the running speed of the pickled strip is 60m / min to 100m / min.

[0076] In this embodiment, the hot-rolled coil is first cooled to room temperature and then leveled, with a limited elongation rate for leveling to control the sheet shape and ensure the plasticity of the steel sheet. The running speed of the pickling strip is limited to meet the FB-grade surface quality requirements of the strip. For example, the elongation rate of the leveling can be 1%, 2%, 3%, etc., and the running speed of the pickling strip can be 60m / min, 70m / min, 80m / min, 90m / min, 100m / min, etc.

[0077] The preparation method of the steel for torsion beams is based on the chemical composition of the steel for torsion beams described above. The specific steps for preparing the chemical composition of the steel for torsion beams can be referred to in the above embodiments. Since the preparation method of the steel for torsion beams adopts some or all of the technical solutions of the above embodiments, it has at least all the beneficial effects brought about by the technical solutions of the above embodiments, which will not be elaborated here.

[0078] One or more technical solutions provided in the embodiments of this application have at least the following technical effects or advantages:

[0079] 1. By designing the chemical composition of the steel and controlling the alloy composition, the C content is kept low while ensuring strength, so as to ensure the formability and weldability of the material. By micro-Ti design and the addition of Cr and Mo elements, the dispersion precipitation strengthening effect can be improved while inhibiting the formation of large-sized TiN. By controlling the Mn content, solid solution strengthening and grain refinement strengthening can be effectively achieved. By controlling the S and P element content to a low level, the brittleness of the material is reduced and the formability is improved. By adding V, Mo and other elements, the weldability and thermal stability of the material are improved, and it is easy to obtain excellent bench fatigue performance of the parts.

[0080] 2. Because calcium treatment of molten steel after smelting improves the cleanliness of the molten steel, and the preparation method of this application embodiment is adopted, the obtained microstructure is ferrite and bainite, with good uniformity and small grain size, which can meet the forming and welding process in the high-frequency welding tube manufacturing process. It will not cause cracking in tube manufacturing or forming due to local coarse structure or poor surface quality. At the same time, it improves the level of inclusion control and ensures that the parts have excellent fatigue life.

[0081] 3. Through improvements in the chemical composition and rolling process of the steel plate, the microstructure of the finished strip steel is ferrite and bainite, including granular bainite and lath bainite. The yield strength is ≥580MPa, the tensile strength is ≥650MPa, the elongation after fracture A80 is ≥24%, the hole expansion rate is ≥85%, and it has excellent surface quality and weldability.

[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 national standards. If there is no corresponding national standard, then general international standards, conventional conditions, or conditions recommended by the manufacturer are followed.

[0083] This application provides a torsion beam steel. Please refer to Table 1 for the chemical composition of the torsion beam steel, Table 2 for the microstructure of the torsion beam steel, Table 3 for the preparation process parameters of the torsion beam steel, and Table 4 for the properties of the torsion beam steel.

[0084] Table 1. Chemical composition (wt%) of steel used in torsion beams, the remainder being Fe and unavoidable impurities.

[0085] Example 1 0.069 0.12 1.52 0.0449 0.056 0.23 0.17 Example 2 0.065 0.11 1.58 0.0374 0.072 0.28 0.12 Example 3 0.076 0.08 1.22 0.0251 0.042 0.37 0.11 Example 4 0.072 0.12 1.65 0.0252 0.042 0.23 0.12 Comparative Example 1 0.032 0.4 1.42 0.035 0.124 0.62 0.0043 Comparative Example 2 0.12 0.2 2.01 0 0.004 0.61 0.14 Comparative Example 3 0.076 0.11 1.66 0.046 0.061 0.45 0.36 serial number V Ca P S [Ti] / [Mo] [Cr]+[Mo] Pcm Example 1 0.039 0.003 0.0108 0.001 0.33 0.4 0.176 Example 2 0.036 0.003 0.0102 0.001 0.6 0.4 0.1737 Example 3 0.028 0.002 0.0102 0.001 0.38 0.48 0.168 Example 4 0.022 0.002 0.0102 0.001 0.35 0.35 0.179 Comparative Example 1 0 - 0.0102 0.001 28.833 0.6243 0.147 Comparative Example 2 0.02 0.002 0.0108 0.001 0.028 0.75 0.269 Comparative Example 3 0.022 0.002 0.0102 0.001 0.174 0.81 0.211

[0086] Table 2 Microstructure of steel for torsion beams

[0087]

[0088]

[0089] Table 3. Manufacturing process parameters for steel used in torsion beams

[0090] Example 1 have 1250 1060 890 530 Example 2 have 1250 1070 900 600 Example 3 have 1250 1020 860 580 Example 4 have 1260 1050 880 560 Comparative Example 1 none 1250 1060 840 420 Comparative Example 2 have 1260 1100 880 400 Comparative Example 3 have 1255 1080 880 550

[0091] Table 4 Properties of steel for torsion beams

[0092] Example 1 623MPa 693MPa 26％ 88％ 315MPa Example 2 592MPa 656MPa 27.5％ 86％ 345Mpa Example 3 640MPa 692MPa 24％ 93％ 330MPa Example 4 664MPa 727MPa 24％ 90％ 330MPa Comparative Example 1 654MPa 750MPa 18.5％ 68％ 330MPa Comparative Example 2 743MPa 824MPa 16％ 45％ 345MPa Comparative Example 3 732MPa 789MPa 16％ 62％ 315MPa

[0093] In summary, the torsion beam steel provided in this application has good formability, especially outstanding hole expansion performance.

[0094] 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 torsion beams, characterized in that, The chemical composition of the steel used for the torsion beam is as follows: By mass fraction, the content of C is 0.072%–0.076%, the content of Si is 0.08%–0.12%, the content of Mn is 1.22%–1.65%, the content of P is ≤0.015%, the content of S is ≤0.003%, the content of Nb is 0.0251%–0.0252%, the content of Ti is 0.042%, the content of Cr is 0.23%–0.37%, the content of Mo is 0.11%–0.12%, the content of V is 0.012%–0.028%, the content of Ca is 0.002%, and the remainder is Fe and unavoidable impurities; And it satisfies [Ti] / [Mo]≤0.6, [Cr]+[Mo]=0.2~0.5%. In the formula, [Ti] represents the mass fraction of Ti, [Mo] represents the mass fraction of Mo, and [Cr] represents the mass fraction of Cr; The microstructure of the steel used in the torsion beam comprises ferrite and bainite, wherein the bainite includes granular bainite and lath bainite, wherein, in terms of area fraction, The ferrite content is ≥50%, the granular bainite content is 20%~35%, the lath bainite content is ≤10%, and the equivalent grain size in the microstructure is 2.5μm~5μm; The steel used for the torsion beam meets the following properties: tensile strength ≥ 650 MPa, hole expansion rate λ ≥ 85%, elongation A80 ≥ 24%, surface quality is FB grade, and welding sensitivity coefficient Pcm ≤ 0.18%.

2. A method for preparing steel for a torsion beam as described in claim 1, characterized in that, The method includes: The molten iron is pretreated, then smelted, calcium treated, and continuously cast to obtain a slab. The slab is heated and rolled, then cooled and coiled to obtain steel for torsion beams; wherein... The winding temperature is 520℃~600℃.

3. The method according to claim 2, characterized in that, The winding temperature is 560℃~580℃.

4. The method according to claim 2, characterized in that, The heating temperature is 1230℃~1280℃.

5. The method according to claim 2, characterized in that, The rolling process includes roughing and finishing, wherein the final temperature of the roughing is 1050℃~1100℃ and the final temperature of the finishing is 830℃~900℃.

6. The method according to claim 2, characterized in that, The method further includes: The steel used for the torsion beam is flattened and then pickled to obtain the finished steel product for the torsion beam; wherein... The elongation rate of the flattened strip is 1% to 3%, and the running speed of the pickled strip is 60m / min to 100m / min.

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