Complex phase steel for wheel spokes and preparation method thereof

Abstract

The present application relates to the field of steel for automobile parts, and in particular to a complex phase steel for wheel spokes and a preparation method thereof; the chemical composition of the complex phase steel comprises: C: 0.09% to 0.22%, Si: 0.05% to 0.20%, Mn: 1.20% to 1.80%, P≤0.015%, S≤0.005%, Alt: 0.02% to 0.50%, Cr: 0.25% to 0.60%, Nb: 0.010% to 0.060%, Ti: 0.015% to 0.03%, B: 0.00 18% to 0.0030%, and the rest is Fe and inevitable impurities; the method comprises: heating, rough rolling, finish rolling, cooling and coiling a slab containing the chemical composition as claimed in claim 1 to obtain a high-strength complex phase steel matrix for wheel spokes; deforming, drilling and heat treating the high-strength complex phase steel for wheel spokes to obtain a complex phase steel for wheel spokes with bosses in bolt holes; by controlling the chemical composition and the preparation method, a steel containing a metallographic structure including ferrite, bainite and martensite can be obtained, and the strength of the steel can be effectively improved.

Description

Technical Field

[0001] The present application relates to the field of steel materials for automobile parts, and in particular to a complex phase steel for wheel spokes and a preparation method thereof. Background Art

[0002] Wheels are important safety parts of a car. They bear the torque of the rotation and motion output of the gearbox, and convert the power output into tension and thrust of linear motion, which is then transmitted to the road through the tires. They are important components for realizing the car's forward, backward, braking, steering and other functions.

[0003] At present, most car spokes are made of low-strength steel plates. This is because complex deformation treatment is required during the preparation of the spokes. If high-strength materials are used, deformation treatment is difficult to perform. However, due to the low strength of such spokes, their bearing capacity is low and is insufficient to withstand the repeatedly changing driving force, braking force, steering force and ground impact force. In complex road conditions, especially the bolt holes on the spokes that connect to the hub, deformation, instability, fatigue fracture and other accidents often occur due to the local severe bending, wear and other loads, posing a huge traffic safety hazard.

[0004] Therefore, how to improve the strength of spoke steel is a technical problem that urgently needs to be solved. Summary of the invention

[0005] The present application provides a complex phase steel for wheel spokes and a preparation method thereof, so as to solve the technical problem of low strength of wheel spoke steel in the prior art.

[0006] In a first aspect, the present application provides a complex phase steel for wheel spokes, wherein the chemical composition of the complex phase steel comprises, by mass fraction:

[0007] C: 0.09% ~ 0.22%, Si: 0.05% ~ 0.20%, Mn: 1.20% ~ 1.80%, P ≤ 0.015%, S ≤ 0.005%, Alt: 0.02% ~ 0.50%, Cr: 0.25% ~ 0.60%, Nb: 0.010% ~ 0.060%, Ti: 0.015% ~ 0.03%, B: 0.0018% ~ 0.0030%, the rest is Fe and unavoidable impurities.

[0008] Optionally, the metallographic structure of the complex phase steel includes, by mass fraction, ferrite: 50% to 70%, bainite: 20% to 40% and martensite: 5% to 15%.

[0009] Optionally, the thickness of the complex phase steel is 4 mm to 22 mm.

[0010] In a second aspect, the present application provides a method for preparing the complex phase steel according to the first aspect, the method comprising:

[0011] Heating, rough rolling, finish rolling, cooling and coiling the slab containing the chemical composition of the first aspect to obtain a high-strength complex phase steel matrix for wheel spokes;

[0012] The high-strength complex phase steel for wheel spokes is deformed, holed and heat treated to obtain the complex phase steel for wheel spokes with bosses in bolt holes.

[0013] Optionally, the initial temperature of the finishing rolling is 1000° C. to 1140° C., the final rolling temperature of the finishing rolling is 840° C. to 920° C., and the number of rolling passes of the finishing rolling is 6 to 7 times.

[0014] Optionally, the initial temperature of the rough rolling is 1150° C. to 1250° C., the final rolling temperature of the rough rolling is 1020° C. to 1180° C., and the number of rolling passes of the rough rolling is 5 to 7 times.

[0015] Optionally, the terminal temperature of the heating is 1200° C. to 1280° C., and the heating time is 120 min to 240 min.

[0016] Optionally, the initial temperature of the cooling is 840°C to 920°C, and the terminal temperature of the cooling is 300°C to 550°C.

[0017] Optionally, the heat treatment includes hot working and cooling working, the peak temperature of the hot working is 1000°C to 1200°C, the end temperature of the cooling working is 20°C to 50°C, and the cooling rate of the cooling working is 50°C / s to 300°C / s.

[0018] Optionally, the heat treatment also includes tempering treatment, which includes tempering heating and tempering cooling. The end point temperature of the tempering heating is 220°C to 350°C, the end point temperature of the tempering cooling is 20°C to 50°C, and the cooling rate of the tempering cooling is 5°C / s to 10°C / s.

[0019] The above technical solution provided by the embodiment of the present application has the following advantages compared with the prior art:

[0020] The embodiment of the present application provides a complex phase steel for wheel spokes, which increases the tensile strength of the steel by limiting the addition of C and Si, then adds the Ti element for second phase particle precipitation strengthening, adds Al for solid solution strengthening, then adds the Nb element for fine grain strengthening precipitation strengthening and improving the toughness of the steel, and then adds B and Cr elements to improve the hardenability of the steel, so that the metallographic structure of the steel includes ferrite, bainite and martensite structures. The ferrite structure enables the wheel spoke steel to have good toughness, which is beneficial to improving the plastic deformation performance of the steel. Since bainite is a strengthening phase, it can have a strong grain boundary bonding strength with ferrite, thereby improving the strength of the steel. Since the martensite structure is harder, it is ensured that the wheel spoke has good strength. Further, by controlling the metallographic structure of the steel, the strength of the wheel spoke steel can be effectively improved. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0022] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative labor.

[0023] Figure 1 A schematic diagram of a process for a method provided in an embodiment of the present application;

[0024] Figure 2 A schematic diagram of the structure of a complex phase steel for a wheel spoke having a bolt hole with an external boss provided in an embodiment of the present application;

[0025] Figure 3 A schematic structural diagram of a complex phase steel for a wheel spoke having a bolt hole with a built-in boss provided in an embodiment of the present application. DETAILED DESCRIPTION

[0026] In order to make the purpose, technical solution and advantages of the embodiments of the present application clearer, the technical solution in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of this application.

[0027] In one embodiment of the present application, a complex phase steel for wheel spokes is provided. The chemical composition of the complex phase steel includes, by mass fraction:

[0028] C: 0.09% ~ 0.22%, Si: 0.05% ~ 0.20%, Mn: 1.20% ~ 1.80%, P ≤ 0.015%, S ≤ 0.005%, Alt: 0.02% ~ 0.50%, Cr: 0.25% ~ 0.60%, Nb: 0.010% ~ 0.060%, Ti: 0.015% ~ 0.03%, B: 0.0018% ~ 0.0030%, the rest is Fe and unavoidable impurities.

[0029] In the embodiment of the present application, the positive effect of the C mass fraction of 0.09% to 0.22% is due to the fact that C is the basic strengthening element of steel, and the level of C content largely determines the level of tensile strength of the steel plate, and is an important indicator affecting the carbon equivalent; when the mass fraction value is greater than the maximum value of the endpoint of the range, it will lead to high hardenability to a certain extent. During the hole making and subsequent welding with the rim, too much high-hardness martensite brittle structure will be produced, and cold cracks are prone to occur. During the service of the wheel, the cracks quickly extend along the direction of the martensite lath, causing the wheel to crack and fail, affecting service safety. When the mass fraction value is less than the minimum value of the endpoint of the range, the hardenability will be reduced to a certain extent, resulting in insufficient strength of the steel. During the steel production and hole making process, the target bainite and martensite structure ratio in the complex phase steel cannot be obtained.

[0030] The positive effect of Si mass fraction of 0.05% to 0.20% is that Si is a solid solution strengthening element of steel, which can improve the hardenability of steel and thus improve the strength of steel; when the mass fraction value is greater than the maximum value at the endpoint of the range, it will cause the Si content to be too high to a certain extent, which will easily make the steel plate brittle; when the mass fraction value is less than the minimum value at the endpoint of the range, it will cause to a certain extent that the combined deoxidation of Si+Mn cannot be carried out in the subsequent welding of the spoke and the rim, and it will also cause the oxygen content in the weld to increase, thereby reducing the fatigue performance of the weld.

[0031] The positive effect of Mn mass fraction of 1.20% to 1.80% is due to the fact that Mn can improve the hardenability of the strip, thereby refining the microstructure of the strip; when the mass fraction value is greater than the maximum value at the endpoint of this range, the segregation tendency of Mn in the steel plate will increase to a certain extent, and a "delamination" defect will be formed during the spoke spinning process; when the mass fraction value is less than the minimum value at the endpoint of this range, the hardenability of the steel plate will be reduced to a certain extent, especially in the center of the plate thickness, the target microstructure ratio of bainite and martensite cannot be obtained.

[0032] The positive effect of P≤0.015% is that within this mass fraction range, since P easily affects the formability of steel, the higher the content, the more unfavorable it is to fatigue performance.

[0033] The positive effect of S≤0.005% is that within this mass fraction range, since S easily affects the formability of steel, the higher the content, the more unfavorable it is to fatigue performance.

[0034] The positive effect of Alt's mass fraction of 0.02% to 0.50% is that within this mass fraction range, the hardness of the steel product can be improved due to the solid solution strengthening effect of Al; when the mass fraction value is greater than the maximum value of the endpoint of the range, it will exceed the solid solubility to a certain extent, forming Al precipitation, thereby reducing the toughness of the steel; when the mass fraction value is less than the minimum value of the endpoint of the range, it will be impossible to deeply deoxidize the steel to a certain extent, which will increase the oxygen content in the steel and reduce the plasticity of the steel.

[0035] The positive effect of a Cr mass fraction of 0.25% to 0.60% is that within this mass fraction range, Cr can improve the hardenability of steel, is beneficial to the phase transformation of martensite after hot working, and has a certain solid solution strengthening effect; when the mass fraction value is greater than the maximum value of the endpoint of the range, it will cause Cr to segregate into a strip-like distribution structure to a certain extent, produce a high-hardness layered martensite structure, reduce the uniformity of the structure performance, and be unfavorable for the spinning plastic forming of the spoke; when the mass fraction value is less than the minimum value of the endpoint of the range, it will be impossible to obtain the target bainite and martensite structure ratio to a certain extent, resulting in insufficient strength of the final steel product.

[0036] The positive effect of Nb mass fraction of 0.010% to 0.060% is that within this mass fraction range, since Nb can refine the grain size, it can play a role in grain refinement and precipitation strengthening, and at the same time can improve the toughness of the steel; when the mass fraction value is greater than the maximum value of the endpoint of this range, too much niobium carbide will precipitate to a certain extent. Since a large amount of C will be adsorbed during the forming process of niobium carbide, the stability of austenite will be reduced, and the target bainite and martensite structure ratio cannot be obtained. When the mass fraction value is less than the minimum value of the endpoint of this range, it will cause grain coarsening and strength reduction to a certain extent.

[0037] The positive effect of Ti's mass fraction of 0.015% to 0.03% is that within this mass fraction range, since Ti can fix nitrogen and produce a strong precipitation strengthening effect, the plastic deformation ability of the steel can be improved; when the mass fraction value is greater than the maximum value at the endpoint of this range, large TiN inclusions will be formed to a certain extent. The formed inclusions will separate from the matrix during spinning to form crack sources, thereby causing the steel to crack; when the mass fraction value is less than the minimum value at the endpoint of this range, it will cause grain coarsening to a certain extent, thereby reducing the strength of the steel.

[0038] The positive effect of the B mass fraction of 0.0018% to 0.0030% is that within this mass fraction range, since B can improve the hardenability of the steel, the target bainite and martensite structures will be obtained during the local heating and rapid cooling process of the complex phase steel for wheel spokes with bosses in bolt holes, thereby enabling the steel to have high local hardness; when the mass fraction value is greater than the maximum value of the endpoint of the range, it will cause excessive hardenability to a certain extent, resulting in a sharp increase in phase transformation stress and cracking of the steel; when the mass fraction value is less than the minimum value of the endpoint of the range, it will cause insufficient hardenability of the steel to a certain extent, thereby causing the internal hardness of the steel to be low and affecting the load-bearing performance.

[0039] In some optional embodiments, the metallographic structure of the complex phase steel includes, by mass fraction, ferrite: 50% to 70%, bainite: 20% to 40% and martensite: 5% to 15%.

[0040] In the embodiment of the present application, the positive effect of the ferrite volume fraction of 50% to 70% is that the ferrite structure makes the wheel spoke steel have good toughness, and the plastic deformation performance of the steel is good, which is beneficial to the stamping and spinning of the spoke; when the volume fraction value is greater than the maximum value of the endpoint of the range, the strength will be reduced to a certain extent and the load-bearing performance will be affected. When the volume fraction value is less than the minimum value of the endpoint of the range, the plasticity will be reduced to a certain extent, thereby causing the problem of forming cracking, and at the same time making it difficult to process the steel into a wheel of specified shape and size.

[0041] The positive effect of bainite volume fraction of 20% to 40% is due to the fact that bainite is a strengthening phase and has a strong grain boundary bonding strength with ferrite, which can improve the strength of steel. When the volume fraction value is greater than the maximum value at the endpoint of the range, it will cause the grain boundary assembly strength to be too high to a certain extent, thereby weakening the metal fluidity and causing the steel to crack during forming. When the volume fraction value is less than the minimum value at the endpoint of the range, it will cause the grain boundary bonding strength to decrease to a certain extent, thereby affecting the bearing performance.

[0042] The volume fraction of martensite is 5% to 15%. The positive effect is that the martensite structure is relatively hard, which can ensure that the wheel spokes made of steel have good strength and strong bearing capacity, so that they can withstand repeatedly changing driving force, braking force, steering tension and impact force of uneven ground to ensure safety; when the volume fraction value is greater than the maximum value of the endpoint of the range, it will lead to more brittle phases to a certain extent, and the fatigue crack growth rate of the steel will be accelerated, thereby reducing the fatigue life. When the volume fraction value is less than the minimum value of the endpoint of the range, it will lead to a certain degree of reduction in the strength of the steel, affecting the bearing performance of the wheel spokes made of the steel.

[0043] In some optional embodiments, the thickness of the complex phase steel is 4 mm to 22 mm.

[0044] In the embodiment of the present application, the positive effect of the thickness of the complex phase steel being 4 mm to 22 mm is that within this thickness range, the steel is suitable for passenger car wheel spokes and commercial car wheel spokes, and the thickness specification can match the chemical composition and correspond to the rolling and heat treatment processes, so that the target organization ratio and mechanical properties can be obtained; when the thickness value is greater than the maximum value of the endpoint of the range, the consistency of the organization and performance in the thickness direction cannot be guaranteed to a certain extent, and when the thickness value is less than the minimum value of the endpoint of the range, it will be difficult to obtain the shape and size of the boss on the spoke to a certain extent.

[0045] In one embodiment of the present application, Figure 1 As shown, a method for preparing complex phase steel for wheel spokes is provided, the method comprising:

[0046] S1. The slab containing the chemical composition is heated, rough rolled, finished rolled, cooled and coiled to obtain a high-strength complex phase steel matrix for wheel spokes;

[0047] S2. The high-strength complex phase steel for wheel spokes is deformed, pierced and heat treated to obtain complex phase steel for wheel spokes with bosses in bolt holes.

[0048] In some optional embodiments, the initial temperature of the finishing rolling is 1000° C. to 1140° C., the final rolling temperature of the finishing rolling is 840° C. to 920° C., and the number of rolling passes of the finishing rolling is 6 to 7 times.

[0049] In the embodiment of the present application, the initial temperature of the finishing rolling is 1000℃~1140℃, and the positive effect is that within this temperature range, a suitable rolling deformation resistance can be obtained, which is beneficial to improving the plate width hit rate; when the temperature value is greater than the maximum value of the endpoint of the range, it will cause decarburization and thickening of the oxide scale on the steel surface to a certain extent, and the surface strength and surface flatness cannot be guaranteed. When the temperature value is less than the minimum value of the endpoint of the range, it will cause the rolling deformation resistance of the steel to a certain extent, increase the load of the rolling mill sharply, and cause damage to the equipment.

[0050] The positive effect of the final rolling temperature of 840℃~920℃ is that within this temperature range, a wider ferrite temperature transformation range can be obtained, and equiaxed ferrite grains can be obtained, which is beneficial to the plastic forming of the spokes; when the temperature value is greater than the maximum value of the endpoint of the range, it will cause the ferrite grains to be coarse to a certain extent, thereby reducing the fine grain strengthening effect; when the temperature value is less than the minimum value of the endpoint of the range, it will cause the organization to be flattened to a certain extent, making the grains in the thickness direction inconsistent, causing fluctuations in steel performance.

[0051] The positive effect of 6 to 7 finishing passes is that through multi-pass, small deformation finishing rolling, the grain size of the steel plate in the thickness direction can be made uniform and the performance is stable. When the value of the rolling passes is greater than the maximum value of the endpoint of the range, it will cause the rolling temperature to decrease to a certain extent, resulting in fine grains on the surface of the steel and increase the load of the subsequent mill. When the value of the rolling passes is less than the minimum value of the endpoint of the range, it will cause a large deformation in a single finishing pass to a certain extent, and the edge quality of the steel cannot be guaranteed.

[0052] In some optional embodiments, the initial temperature of the rough rolling is 1150° C. to 1250° C., the final rolling temperature of the rough rolling is 1020° C. to 1180° C., and the number of rolling passes of the rough rolling is 5 to 7 times.

[0053] In the embodiment of the present application, the positive effect of the initial temperature of rough rolling being 1150°C to 1250°C is that within this temperature range, a good austenite matrix structure can be obtained, which is beneficial to controlled rolling and controlled cooling during the steel rolling process, and ferrite, bainite and martensite structures can be obtained in sequence; when the temperature value is greater than the maximum value of the endpoint of the range, it will cause coarse structure to a certain extent, and cause surface decarburization of the steel and thickening of the oxide scale; when the temperature value is less than the minimum value of the endpoint of the range, it will cause the temperature to drop too fast between rough rolling and finish rolling to a certain extent, which is not conducive to the fine control of the grains.

[0054] The positive effect of the final rolling temperature of rough rolling being 1020℃~1180℃ is that within this temperature range, the change of grains can be controlled and the surface iron oxide scale can be removed, thereby obtaining a reasonable finishing rolling process temperature window; when the temperature value is greater than the maximum value of the endpoint of this range, it will cause the finishing temperature to be too high to a certain extent, making the steel grains coarse and the oxide scale thick. When the temperature value is less than the minimum value of the endpoint of this range, it will cause the load of the rolling mill to increase to a certain extent and shorten the temperature window for equalization.

[0055] The positive effect of 5 to 7 rough rolling passes is that within the range of the rolling passes, the loose shrinkage defects in the core of the ingot can be rolled out, thereby obtaining a good core structure; when the value of the rolling pass is greater than the maximum value of the endpoint of the range, it will cause the rolling temperature to drop significantly to a certain extent, which is not conducive to the initial temperature control of the finishing rolling; when the value of the rolling pass is less than the minimum value of the endpoint of the range, it will cause the deformation rate of the slab to be too large to a certain extent, and bulging and warping will occur during the flattening process of the steel, which is not conducive to finishing rolling control.

[0056] In some optional embodiments, the terminal temperature of the heating is 1200° C. to 1280° C., and the heating time is 120 min to 240 min.

[0057] In the embodiment of the present application, the positive effect of the heating end point temperature of 1200°C to 1280°C is that within this temperature range, the slab can be sufficiently heated, thereby facilitating the structural phase change; when the temperature value is greater than the maximum value of the endpoint of the range, it will cause excessive coarsening of the grains, affecting the strength of the steel; when the temperature value is less than the minimum value of the endpoint of the range, the rolling force during the rolling process will be too large, affecting the operation of the rolling mill.

[0058] In some optional embodiments, the initial temperature of the cooling is 840°C to 920°C, and the terminal temperature of the cooling is 300°C to 550°C.

[0059] In the embodiment of the present application, the positive effect of the initial cooling temperature being 840°C to 920°C is that within this temperature range, the initial transformation of ferrite can be completed, thereby obtaining a suitable proportion of matrix structure and nano-scale precipitated particles; when the temperature value is greater than the maximum value of the endpoint of the range, it will cause coarse grains to a certain extent, resulting in reduced plasticity and strength of the steel; when the temperature value is less than the minimum value of the endpoint of the range, it will cause flattening of the structure to a certain extent, resulting in large differences in the transverse and longitudinal properties of the steel.

[0060] The positive effect of the cooling end temperature being 300°C to 550°C is that within this temperature range, the target ratio of bainite to martensite can be obtained; when the temperature value is greater than the maximum value at the endpoint of this range, the ratio of martensite to bainite will be reduced to a certain extent, thereby reducing the strength of the steel; when the temperature value is less than the minimum value at the endpoint of this range, the ratio of martensite to bainite will be increased to a certain extent, thereby reducing the plasticity of the steel.

[0061] In some optional embodiments, the heat treatment includes hot working and cooling working, the peak temperature of the hot working is 1000°C to 1200°C, the endpoint temperature of the cooling working is 20°C to 50°C, and the cooling rate of the cooling working is 50°C / s to 300°C / s.

[0062] In the embodiment of the present application, the peak temperature of the hot working is 1000℃~1200℃. The positive effect is that within this temperature range, the metallographic structure of the steel can be completely austenitized, and in the subsequent cooling process, the metallographic structure can obtain sufficient phase transformation, thereby reducing the proportion of unstable structure; when the temperature value is greater than the maximum value of the endpoint of the range, it will cause overheating of the structure to a certain extent, and at the same time cause intergranular oxidation, thereby reducing the toughness of the steel; when the temperature value is less than the minimum value of the endpoint of the range, it will cause incomplete tissue transformation to a certain extent, making the final metallographic structure uneven, affecting the performance stability of the wheel product.

[0063] The positive effect of the terminal temperature of the cooling process being 20℃~50℃ is that within this temperature range, martensite and bainite can be completely transformed, thereby obtaining a local strengthened structure with high hardness; when the temperature value is greater than the maximum value of the endpoint of this range, it will cause insufficient local strength of the steel to a certain extent. When the temperature value is less than the minimum value of the endpoint of this range, it will cause a large phase change stress to be generated in the production process to a certain extent, which will cause dimensional deviation of the steel. At the same time, the residual stress will also reduce the fatigue service performance of the steel.

[0064] The positive effect of the cooling rate of the cooling process being 50°C / s to 300°C / s is that within the cooling rate range, the cooling process time period can be controlled, thereby obtaining a controllable bainite and martensite microstructure ratio; when the cooling rate value is greater than the maximum value at the endpoint of the range, it will cause excessive phase transformation stress to a certain extent, thereby causing steel cracking; when the cooling rate value is less than the minimum value at the endpoint of the range, it will be impossible to obtain the target bainite and martensite microstructure ratio to a certain extent.

[0065] In some optional embodiments, the heat treatment also includes tempering treatment, and the tempering treatment includes tempering heating and tempering cooling. The end point temperature of the tempering heating is 220°C to 350°C, the end point temperature of the tempering cooling is 20°C to 50°C, and the cooling rate of the tempering cooling is 5°C / s to 10°C / s.

[0066] In the embodiment of the present application, the positive effect of the end point temperature of the tempering heating being 220°C to 350°C is that within this temperature range, the martensite structure can be stabilized, thereby obtaining a tempered martensite solid and ensuring the toughness of the steel; when the temperature value is greater than the maximum value of the endpoint of the range, the proportion of the martensite structure will be reduced to a certain extent, thereby reducing the strength of the steel; when the temperature value is less than the minimum value of the endpoint of the range, the final martensite structure will be cold, hard and brittle to a certain extent, making the steel prone to fatigue cracking.

[0067] The positive effect of the end point temperature of tempering cooling being 20℃~50℃ is that a balanced proportion of each phase structure can be obtained within this temperature range; when the temperature value is greater than the maximum value of the endpoint of this range, it will cause continuous decomposition of martensite to a certain extent, thereby reducing the strength of the steel; when the temperature value is less than the minimum value of the endpoint of this range, it will cause instability of martensite to a certain extent.

[0068] The positive effect of tempering cooling at a cooling rate of 5°C / s to 10°C / s is that within this cooling rate range, phase transformation stress can be released, thereby reducing the residual stress level; when the cooling rate is greater than the maximum value at the endpoint of this range, it will cause excessive residual stress to a certain extent, reducing the bearing capacity of the steel product; when the cooling rate is less than the minimum value at the endpoint of this range, it will cause excessive decomposition of martensite to a certain extent, thereby reducing the product strength of the steel.

[0069] In some optional embodiments, the hardness of the complex phase steel used for the wheel spokes with bosses in the bolt holes is ≥ 280 HV.

[0070] Example 1

[0071] A complex phase steel for wheel spokes, wherein the chemical composition of the complex phase steel includes, by mass fraction:

[0072] C: 0.09%, Si: 0.20%, Mn: 1.80%, P≤0.015%, S≤0.005%, Alt: 0.03%, Cr: 0.60%, Nb: 0.02%, Ti: 0.02%, B: 0.0030%, and the rest are Fe and unavoidable impurities.

[0073] The thickness of the complex phase steel is 14 mm.

[0074] A method for preparing complex phase steel for wheel spokes, comprising:

[0075] S1. The slab is heated, rough rolled, finished rolled, cooled and coiled to obtain a high-strength composite steel matrix for wheel spokes;

[0076] S2. The high-strength wheel spoke complex steel is deformed, pierced and heat treated to obtain the following Figure 2 The wheel spokes with external bosses and bolt holes are made of complex steel and Figure 3 The wheel spokes shown have built-in bosses in the bolt holes and are made of complex phase steel.

[0077] The initial temperature of the finishing rolling is 1140°C, the final rolling temperature of the finishing rolling is 920°C, and the number of rolling passes of the finishing rolling is 7 times.

[0078] The initial temperature of rough rolling is 1250°C, the final rolling temperature of rough rolling is 1180°C, and the rough rolling passes are 6 times.

[0079] The final temperature of heating is 1280°C and the heating time is 240 min.

[0080] The initial temperature of cooling was 920°C, and the final temperature of cooling was 550°C.

[0081] The heat treatment includes hot working and cooling working. The peak temperature of the hot working is 1000°C, the terminal temperature of the cooling working is 50°C, and the cooling rate of the cooling working is 300°C / s.

[0082] The heat treatment also includes tempering treatment, which includes tempering heating and tempering cooling. The end point temperature of the tempering heating is 220°C, the end point temperature of the tempering cooling is 20°C, and the cooling rate of the tempering cooling is 10°C / s.

[0083] Example 2

[0084] Comparing Example 2 with Example 1, the difference between Example 2 and Example 1 is:

[0085] In terms of mass fraction, the chemical composition of complex phase steel includes:

[0086] C: 0.14%, Si: 0.15%, Mn: 1.60%, P≤0.015%, S≤0.005%, Alt: 0.10%, Cr: 0.50%, Nb: 0.010%, Ti: 0.015%, B: 0.0028%, and the rest are Fe and unavoidable impurities.

[0087] Example 3

[0088] Comparing Example 3 with Example 1, the difference between Example 3 and Example 1 is:

[0089] In terms of mass fraction, the chemical composition of complex phase steel includes:

[0090] C: 0.18%, Si: 0.10%, Mn: 1.60%, P≤0.015%, S≤0.005%, Alt: 0.25%, Cr: 0.40%, Nb: 0.030%, Ti: 0.015%, B: 0.0025%, and the rest are Fe and unavoidable impurities.

[0091] Example 4

[0092] Comparing Example 4 with Example 1, the difference between Example 4 and Example 1 is:

[0093] In terms of mass fraction, the chemical composition of complex phase steel includes:

[0094] C: 0.20%, Si: 0.08%, Mn: 1.40%, P≤0.015%, S≤0.005%, Alt: 0.30%, Cr: 0.25%, Nb: 0.040%, Ti: 0.030%, B: 0.0020%, and the rest are Fe and unavoidable impurities.

[0095] Example 5

[0096] Comparing Example 5 with Example 1, the difference between Example 5 and Example 1 is:

[0097] In terms of mass fraction, the chemical composition of complex phase steel includes:

[0098] C: 0.22%, Si: 0.05%, Mn: 1.20%, P≤0.015%, S≤0.005%, Alt: 0.50%, Cr: 0.22%, Nb: 0.060%, Ti: 0.025%, B: 0.0018%, and the rest are Fe and unavoidable impurities.

[0099] Example 6

[0100] Comparing Example 6 with Example 1, the difference between Example 6 and Example 1 is:

[0101] The thickness of the complex phase steel is 4 mm.

[0102] The initial temperature of the finishing rolling is 1000°C, the final rolling temperature of the finishing rolling is 840°C, and the number of rolling passes of the finishing rolling is 6 times.

[0103] The initial temperature of the rough rolling is 1150°C, the final rolling temperature of the rough rolling is 1020°C, and the number of rolling passes of the rough rolling is 5 times.

[0104] The final temperature of heating is 1200°C and the heating time is 120 min.

[0105] The initial temperature of cooling was 840°C, and the final temperature of cooling was 300°C.

[0106] The heat treatment includes hot working and cooling working. The peak temperature of the hot working is 1000°C, the end temperature of the cooling working is 20°C, and the cooling rate of the cooling working is 50°C / s.

[0107] The heat treatment also includes tempering treatment, which includes tempering heating and tempering cooling. The end point temperature of the tempering heating is 350°C, the end point temperature of the tempering cooling is 50°C, and the cooling rate of the tempering cooling is 10°C / s.

[0108] Example 7

[0109] Comparing Example 7 with Example 1, the difference between Example 7 and Example 1 is:

[0110] The thickness of the complex phase steel is 22 mm.

[0111] The initial temperature of the finishing rolling is 1140°C, the final rolling temperature of the finishing rolling is 920°C, and the number of rolling passes of the finishing rolling is 7 times.

[0112] The initial temperature of the rough rolling is 1250°C, the final rolling temperature of the rough rolling is 1180°C, and the number of rolling passes of the rough rolling is 7 times.

[0113] The final temperature of heating is 1280°C and the heating time is 240 min.

[0114] The initial temperature of cooling was 920°C, and the final temperature of cooling was 550°C.

[0115] The heat treatment includes hot working and cooling working. The peak temperature of the hot working is 1200°C, the end temperature of the cooling working is 50°C, and the cooling rate of the cooling working is 300°C / s.

[0116] The heat treatment also includes tempering treatment, which includes tempering heating and tempering cooling. The end point temperature of the tempering heating is 350°C, the end point temperature of the tempering cooling is 50°C, and the cooling rate of the tempering cooling is 10°C / s.

[0117] Comparative Example 1

[0118] Comparative Example 1 and Example 1 are compared. The difference between Comparative Example 1 and Example 1 is:

[0119] In terms of mass fraction, the chemical composition of steel includes:

[0120] C: 0.08%, Si: 0.02%, Mn: 1.00%, P≤0.015%, S≤0.005%, Alt: 0.01%, Cr: 0.20%, Nb: 0.005%, Ti: 0.010%, B: 0.0010%, and the rest are Fe and unavoidable impurities.

[0121] Comparative Example 2

[0122] Comparing Comparative Example 2 with Example 1, the difference between Comparative Example 2 and Example 1 is:

[0123] In terms of mass fraction, the chemical composition of steel includes:

[0124] C: 0.25%, Si: 0.25%, Mn: 1.00%, P≤0.015%, S≤0.005%, Alt: 0.55%, Cr: 0.70%, Nb: 0.080%, Ti: 0.05%, B: 0.0040%, and the rest are Fe and unavoidable impurities.

[0125] Related experiments:

[0126] The properties of the steel substrate for wheel spokes and the complex phase steel for wheel spokes with bosses in bolt holes obtained in Examples 1-7 and Comparative Examples 1-2 were statistically analyzed and the results are shown in Table 1.

[0127] Test methods for related experiments:

[0128] Metallographic structure detection: The metallographic structure was detected on an OLYMPS laser confocal microscope.

[0129] Hardness: The Vickers hardness of each area was tested using a HVS-10Z Vickers hardness tester.

[0130] Tensile strength: The tensile properties of the samples were tested using a WE-100 tensile testing machine.

[0131] Table 1

[0132]

[0133]

[0134] Detailed analysis of Table 1:

[0135] Tensile strength refers to the maximum stress value that the prepared steel matrix can withstand before it breaks. The greater the tensile strength, the greater the maximum stress value that the steel plate can withstand before it breaks.

[0136] From the data of Examples 1-7, it can be seen that:

[0137] If the chemical composition of the complex phase steel for wheel spokes provided in the present application is adopted, and the corresponding preparation method is used, C and Si are added to improve the tensile strength of the steel, and then the Ti element is added for second phase particle precipitation strengthening, Al is added for solid solution strengthening, and then the Nb element is added for fine grain strengthening precipitation strengthening and improving the toughness of the steel, and then B and Cr elements are added to improve the hardenability of the steel, thereby comprehensively improving the strength of the steel.

[0138] From the data of Comparative Examples 1-2, it can be seen that:

[0139] If the chemical composition of the complex phase steel for wheel spokes defined in the present application is not adopted, and the process parameters in the corresponding preparation method are not matched, the hardness and tensile strength of the prepared steel will be lower or higher than those in the embodiment.

[0140] One or more technical solutions in the embodiments of the present application also have at least the following technical effects or advantages:

[0141] (1) The complex phase steel for wheel spokes provided in the embodiment of the present application improves the tensile strength of the steel by adding C and Si, then adds the Ti element for second phase particle precipitation strengthening, adds Al for solid solution strengthening, then adds the Nb element for fine grain strengthening precipitation strengthening and improving the toughness of the steel, and then adds B and Cr elements to improve the hardenability of the steel, thereby obtaining a steel containing ferrite, bainite and martensite, and effectively improving the strength of the steel.

[0142] (2) The complex phase steel for wheel spokes provided in the embodiments of the present application has a metallographic structure comprising ferrite with a volume fraction of 50% to 70%, bainite with a volume fraction of 20% to 40%, and martensite with a volume fraction of 5% to 15%, and has a yield strength of 600 MPa to 800 MPa and a tensile strength of 800 MPa to 1000 MPa.

[0143] (3) The complex phase steel for wheel spokes provided in the embodiments of the present application can not only meet the requirements for preparing complex deformation of wheel spokes, but also make the wheel spokes have good strength. At the same time, the complex phase steel for wheel spokes with bosses in the bolt holes provided has high hardness, stiffness and other properties, thereby improving the driving safety performance of the automobile.

[0144] (4) The method provided in the embodiment of the present application can effectively ensure that a metallographic structure including ferrite with a volume fraction of 50% to 70%, bainite with a volume fraction of 20% to 40%, and martensite with a volume fraction of 5% to 15% is obtained by controlling the process flow, and can also ensure that the prepared complex phase steel for wheel spokes with bosses in bolt holes has the characteristics of high local strength and stiffness, resistance to bending fatigue and fretting fatigue, and long fatigue life.

[0145] It should be noted that, in this article, relational terms such as "first" and "second" are only used 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. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of further restrictions, the elements defined by the sentence "comprise a ..." do not exclude the existence of other identical elements in the process, method, article or device including the elements.

[0146] The endpoints and any values ​​of the ranges disclosed in this article are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of each range, the endpoint values ​​of each range and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this article.

[0147] The foregoing is merely a specific embodiment of the present invention, which enables those skilled in the art to understand or implement the present invention. Various modifications to these embodiments will be 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 the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A method for preparing complex phase steel, characterized in that: The method comprises: The slab is heated, rough-rolled, finish-rolled, cooled and coiled to obtain a high-strength complex-phase steel matrix for wheel spokes. The chemical composition of the slab includes: C: 0.09%~0.22%, Si: 0.05%~0.20%, Mn: 1.20%~1.80%, P≤0.015%, S≤0.005%, Alt: 0.02%~0.50%, Cr: 0.25%~0.60%, Nb: 0.010%~0.060%, Ti: 0.015%~0.03%, B: 0.0018%~0.0030%, the rest is Fe and unavoidable impurities; The high-strength complex phase steel for wheel spokes is deformed, holed and heat treated to obtain complex phase steel for wheel spokes with bosses in bolt holes; Wherein, the terminal temperature of the heating is 1200°C to 1280°C, the heating time is 120min to 240min, the initial temperature of the cooling is 840°C to 920°C, the terminal temperature of the cooling is 300°C to 550°C, the heat treatment includes hot working and cooling working, the peak temperature of the hot working is 1000°C to 1200°C, the terminal temperature of the cooling working is 20°C to 50°C, the cooling rate of the cooling working is 50°C / s to 300°C / s, the heat treatment also includes tempering treatment, the tempering treatment includes tempering heating and tempering cooling, the terminal temperature of the tempering heating is 220°C to 350°C, the terminal temperature of the tempering cooling is 20°C to 50°C, and the cooling rate of the tempering cooling is 5°C / s to 10°C / s; In terms of mass fraction, the metallographic structure of the complex phase steel includes: ferrite: 50% to 70%, bainite: 20% to 40% and martensite: 5% to 15%, and the thickness of the complex phase steel is 4 mm to 22 mm.

2. The method according to claim 1, characterized in that: The initial temperature of the finishing rolling is 1000° C. to 1140° C., the final rolling temperature of the finishing rolling is 840° C. to 920° C., and the number of rolling passes of the finishing rolling is 6 to 7 times.

3. The method according to claim 1, characterized in that The initial temperature of the rough rolling is 1150° C. to 1250° C., the final rolling temperature of the rough rolling is 1020° C. to 1180° C., and the number of rolling passes of the rough rolling is 5 to 7 times.

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