Method for producing axle housing steel, axle housing steel

Abstract

The application relates to a preparation method of axle housing steel, which comprises the following steps: providing molten steel, performing a continuous casting process on the molten steel to form a casting blank; performing heating, rough rolling and finish rolling on the casting blank to obtain a rolled steel plate, wherein the finish rolling temperature is 800-820 DEG C; cooling the rolled steel plate to 300-450 DEG C at a rate of 30-40 DEG C / s to obtain a steel plate to be coiled; coiling the steel plate to be coiled at 300-420 DEG C, and obtaining the axle housing steel after cooling, wherein the molten steel comprises 0.03%-0.06% of C, 0.05%-0.07% of Nb and 0.15-0.25% of Cr in percentage of the mass of the molten steel. By adopting a low-carbon+Cr, Mo and Nb component system and controlling the cooling rate of the rolled steel plate, superfine acicular ferrite and granular low-carbon bainite composite grain structures can be formed, the 700MPa high-strength performance of the steel plate with a thickness of more than 10 mm is ensured, and excellent cold forming performance and welding performance are achieved.

Description

Technical Field

[0001] This application relates to the steel industry, and in particular to bridge shell steel. Background Technology

[0002] With the continuous innovation of modern automobile manufacturing processes and the increasing demand for high-strength steel materials, especially the development and changes in the automotive axle housing manufacturing industry, higher demands are being placed on steel products used in the manufacture of automotive axle housings. As a crucial load-bearing component of the chassis system, one of the three major assemblies of commercial vehicles, the steel consumption of automotive axle housings accounts for approximately 4% of the total steel consumption in automobiles. Light-duty axle housings required for medium-duty trucks, light-duty trucks, mini-trucks, and small buses are generally produced using cold stamping processes, with an annual steel consumption of approximately 120,000 tons. Heavy-duty axle housings (10 tons or more) required for heavy-duty trucks and large buses are generally produced using hot stamping processes, with an annual steel consumption of approximately 200,000 tons. Cold-stamped welded axle housings are lightweight, have high material utilization, and low manufacturing costs, making them suitable for mass production. They are widely used in passenger cars, light vehicles, medium-duty vehicles, and heavy-duty vehicles, and cold stamping welding has become the main production method for drive axles. Automotive axle housings have high safety requirements, needing to meet stringent component fatigue performance standards. This is reflected in the steel plates used for axle housings, which require stable performance, resistance to low-temperature impact, and good weldability. However, existing high-strength bridge shell steels with a strength of 600MPa or higher all have technical challenges such as high carbon equivalent, high production cost, and poor cold forming performance, making them unsuitable for the lightweight requirements of higher-end automobiles. Summary of the Invention

[0003] This application provides a method for preparing bridge shell steel and bridge shell steel, in order to solve the technical problems of existing high-strength bridge shell steels with a strength of 600MPa or above having high carbon equivalent, high production cost, and poor cold forming performance.

[0004] In a first aspect, embodiments of this application provide a method for preparing bridge shell steel, the method comprising the following steps:

[0005] Molten steel is provided, and the molten steel is subjected to a continuous casting process to form a billet;

[0006] The billet is heated, rough rolled, and finish rolled to obtain rolled steel plate, wherein the final rolling temperature of the finish rolling is 800-820℃;

[0007] The rolled steel sheet is cooled to 300-450°C at a rate of 30-40°C / s to obtain the steel sheet to be coiled.

[0008] The steel plate to be coiled is wound at 300–420°C, and after cooling, the bridge shell steel is obtained.

[0009] The molten steel comprises, by mass percentage, 0.03% to 0.06% C, 0.05% to 0.07% Nb, and 0.15% to 0.25% Cr.

[0010] In some embodiments of this application, the molten steel comprises 0.033–0.055% C, 0.051–0.063% Nb, and 0.15–0.23% Cr.

[0011] In some embodiments of this application, the molten steel further includes at least one of Si, Mn, Al, Ti, and N.

[0012] In some embodiments of this application, the molten steel further includes 0.20% to 0.30% Si, 1.75% to 2.00% Mn, 0.015% to 0.050% Al, 0.008% to 0.020% Ti, and 0% to 0.005% N.

[0013] In some embodiments of this application, the molten steel comprises 0.021% to 0.027% Si, 1.79% to 1.90% Mn, 0.021% to 0.031% Al, 0.010% to 0.018% Ti, and 0.0021% to 0.0039% N.

[0014] In some embodiments of this application, the heating of the billet includes a homogenization and heat preservation stage, the homogenization temperature is 1200-1250°C, the total homogenization time is not less than 60 minutes, and the total heating time is not less than 140 minutes.

[0015] In some embodiments of this application, the roughing mill exit temperature is 1000–1050°C.

[0016] In some embodiments of this application, the finishing rolling temperature is 980–1020°C.

[0017] Secondly, embodiments of this application provide a bridge shell steel, wherein the bridge shell steel is a bridge shell steel prepared by the method described in any embodiment of the first aspect, and the bridge shell steel comprises, by mass percentage: 0.03% to 0.06% C, 0.05% to 0.07% Nb, 0.20% to 0.30% Si, 1.75% to 2.00% Mn, 0.015% to 0.050% Al, 0.008% to 0.020% Ti, and 0% to 0.005% N.

[0018] In some embodiments of this application, the bridge shell steel comprises: 0.033–0.055% C, 0.051–0.063% Nb, 0.15–0.23% Cr, 0.021%–0.027% Si, 1.79%–1.90% Mn, 0.021%–0.031% Al, 0.010%–0.018% Ti, and 0.0021%–0.0039% N.

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

[0020] The bridge shell steel preparation method provided in this application adopts a low carbon + Cr, Mo, Nb composition system and controls the cooling rate of the rolled steel plate to form an ultrafine acicular ferrite and granular low carbon bainite composite grain structure, which ensures the high strength of 700MPa for steel plates with a thickness of more than 10mm and has excellent cold forming and welding performance. Attached Figure Description

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

[0022] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0023] Figure 1 This is a metallographic diagram of a bridge shell steel provided in Embodiment 1 of this application. Detailed Implementation

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

[0025] Unless otherwise specified, the terminology used herein should be understood as having the meaning as commonly used in the art. Therefore, unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains. In case of any conflict, this specification shall prevail.

[0026] 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.

[0027] Existing high-strength bridge shell steels with a strength of 600MPa or higher all suffer from technical problems such as high carbon equivalent, high production cost, and poor cold forming performance.

[0028] The technical solution provided in this application is to solve the above-mentioned technical problems, and the general idea is as follows:

[0029] In a first aspect, embodiments of this application provide a method for preparing bridge shell steel, the method comprising the following steps:

[0030] S1: Provide molten steel and perform continuous casting process on the molten steel to form a billet;

[0031] S2: The billet is heated, rough rolled and finish rolled to obtain rolled steel plate, wherein the final rolling temperature of finish rolling is 800-820℃;

[0032] S3: Cool the rolled steel plate to 300-450°C at a rate of 30-40°C / s to obtain the steel plate to be coiled;

[0033] S4: The steel plate to be coiled is coiled at 300-420℃, and after cooling, the bridge shell steel is obtained.

[0034] The molten steel comprises, by mass percentage, 0.03% to 0.06% C, 0.05% to 0.07% Nb, and 0.15% to 0.25% Cr.

[0035] Carbon (C) plays a role in solid solution strengthening in steel. When the C content is low, during the heating, rolling, and cooling process, a small amount of carbon decomposes from austenite into ferrite and cementite phases, forming acicular or lath-shaped ferrite with excellent toughness and discontinuous carbides. Considering all factors, the suitable C content is 0.03–0.06%.

[0036] When the Nb content is low, Nb is dissolved in the austenite matrix and segregates at the grain boundaries, hindering the movement of new grain boundaries after austenite deformation and recrystallization, thus greatly refining the grains. During the cooling process after high-temperature deformation, the segregation of Nb atoms at the grain boundaries greatly hinders the nucleation of new phases at the grain boundaries, causing the proeutectoid ferrite formation region to shift to the right, resulting in a uniform bainitic structure over a wide range of cooling rates. Considering all factors, the suitable Nb content is 0.050–0.070%.

[0037] Cr can improve the hardenability of steel, giving it better overall properties after quenching and tempering. However, high Cr content can cause larger sparks during welding, which can worsen the welding quality. Considering all factors, a Cr weight percentage of 0.15% to 0.25% is recommended.

[0038] This application employs a rapid cooling rate of 30–40°C / s to directly cool to a laminar flow temperature of 300–450°C, and then controls the coiling temperature within the range of 300–420°C for coiling. This is to obtain a refined granular bainite structure during the ultra-rapid cooling stage, and to utilize the residual heat in the center of the thick steel plate during the laminar flow coiling process to achieve self-tempering treatment, precipitating a certain amount of fine acicular ferrite structure to improve plasticity.

[0039] This application, by adopting a low-carbon + Cr, Mo, Nb composition system and controlling the cooling rate of the rolled steel plate, can form an ultra-fine acicular ferrite and granular low-carbon bainite composite grain structure, ensuring the high strength of 700MPa for steel plates with a thickness of 10mm or more, and possessing excellent cold forming and welding properties.

[0040] In some embodiments of this application, the molten steel comprises 0.033–0.055% C, 0.051–0.063% Nb, and 0.15–0.23% Cr.

[0041] In some embodiments of this application, the molten steel further includes at least one of Si, Mn, Al, Ti, and N.

[0042] In some embodiments of this application, the molten steel further includes 0.20% to 0.30% Si, 1.75% to 2.00% Mn, 0.015% to 0.050% Al, 0.008% to 0.020% Ti, and 0% to 0.005% N.

[0043] In some embodiments of this application, the molten steel comprises 0.021% to 0.027% Si, 1.79% to 1.90% Mn, 0.021% to 0.031% Al, 0.010% to 0.018% Ti, and 0.0021% to 0.0039% N.

[0044] Si has a good deoxidizing effect on molten steel. When the Si content is high, there are more oxides on the surface of the steel plate. Taking all factors into consideration, the suitable Si content is 0.20-0.30%, preferably 0.021-0.027%.

[0045] The solid solution formed by manganese and iron can effectively improve the hardness and strength of steel. Simultaneously, Mn entering the cementite and replacing some iron atoms can improve the toughness of the steel. Taking all factors into consideration, the suitable Mn content is 1.75–2.0%, preferably 1.79–1.90%.

[0046] Al is a strong deoxidizing element, and the resulting AlN element can effectively refine grains. Deoxidation is insufficient when the content is below 0.01%, and it reduces the toughness of the weld heat-affected zone when it exceeds 0.070%. Considering all factors, the suitable Al content is 0.015–0.050%, preferably 0.021–0.031%.

[0047] During the solidification process of steel, Ti can combine with N to form TiN. The resulting carbonitrides precipitate in the matrix, pinning the grain boundaries of austenite and hindering austenite grain growth, thereby refining the austenite grains. This is beneficial for improving the strength of the weld after welding. Taking all factors into consideration, the suitable Ti content is 0.008-0.020%, preferably 0.010-0.018%.

[0048] Nitrogen dissolved in molten steel precipitates during solidification due to decreased solubility. It combines with Si and Al elements in the steel to form nitrides such as SiN and AlN. A small amount of nitrides can refine the steel's grain structure, while a large amount reduces the steel's plasticity and toughness. Considering all factors, N ≤ 0.005%, preferably 0.0021–0.0039%.

[0049] In some embodiments of this application, the heating of the cast billet includes a homogenization and heat preservation stage, wherein the homogenization temperature is 1200–1250°C, the total homogenization time is not less than 60 min, and the total heating time is not less than 140 min.

[0050] In some embodiments of this application, the heating of the billet includes a first heating section, a second heating section, and a heat soaking and heat preservation stage. The first heating section is a preheating section, where the billet temperature rises to above 900°C as the furnace temperature increases. The second heating section raises the temperature to 1240-1250°C, and the heat soaking and heat preservation stage is used for heat preservation.

[0051] The temperature of the billet is controlled at 1200-1250℃ to ensure complete solid solution and full austenitization of the alloying elements, and to control the original austenite grains to be relatively fine.

[0052] In some embodiments of this application, the roughing mill exit temperature is 1000–1050°C.

[0053] The roughing exit temperature is controlled at 1000-1050℃ in order to further refine the austenite grains by roughing in the recrystallization temperature range.

[0054] In some embodiments of this application, the finishing rolling temperature is 980–1020°C.

[0055] The initial rolling temperature of finishing mill is 980-1020℃, and the final rolling temperature is 800-820℃. This is to adapt to the rolling capacity of the finishing mill equipment and ensure uniform plastic deformation.

[0056] Secondly, embodiments of this application provide a bridge shell steel, wherein the bridge shell steel is a bridge shell steel prepared by the method described in any embodiment of the first aspect, and the bridge shell steel comprises, by mass percentage: 0.03% to 0.06% C, 0.05% to 0.07% Nb, 0.20% to 0.30% Si, 1.75% to 2.00% Mn, 0.015% to 0.050% Al, 0.008% to 0.020% Ti, and 0% to 0.005% N.

[0057] Those skilled in the art will understand that, in addition to the elements mentioned above, the bridge shell steel may also contain unavoidable impurity elements.

[0058] Those skilled in the art will understand that the bridge shell steel may also contain phosphorus (P) and sulfur (S). P in steel tends to segregate, reducing the steel's toughness and weldability, while sulfur tends to form plastic sulfides, causing delamination defects. Therefore, the lower the P and S content, the better. Considering all factors, P is generally controlled to ≤0.015%, and S to ≤0.003%.

[0059] In some embodiments of this application, the bridge shell steel comprises: 0.033–0.055% C, 0.051–0.063% Nb, 0.15–0.23% Cr, 0.021%–0.027% Si, 1.79%–1.90% Mn, 0.021%–0.031% Al, 0.010%–0.018% Ti, and 0.0021%–0.0039% N.

[0060] 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.

[0061] Example 1

[0062] The application provides a method for preparing bridge shell steel, the method comprising the following steps:

[0063] Molten steel is provided and subjected to a continuous casting process to form a billet. The elemental composition of the molten steel is the same as that of the final bridge shell steel.

[0064] The billet is heated, rough rolled, and finish rolled to obtain rolled steel plate. The exit temperature of rough rolling is shown in Table 2, the starting rolling temperature of finish rolling is shown in Table 2, and the final rolling temperature is shown in Table 2.

[0065] The rolled steel sheet is cooled to the cooled temperature shown in Table 2 at the cooling rate shown in Table 2 to obtain the steel sheet to be coiled.

[0066] The steel sheet to be wound is wound at the winding temperatures shown in Table 2, and after cooling, the bridge shell steel is obtained.

[0067] The heating of the billet, the temperature for homogenization is shown in Table 2, the total duration of the second heating stage and homogenization, i.e. the holding time at the temperature of the second heating stage and homogenization, is shown in Table 2, and the total heating time is shown in Table 2.

[0068] This embodiment also provides a bridge shell steel, the elemental composition of which is shown in Table 1, and the bridge shell steel is a steel plate with a thickness shown in Table 2.

[0069] Example 2

[0070] The differences between this embodiment and Embodiment 1 are shown in Tables 1 and 2.

[0071] Example 3

[0072] The differences between this embodiment and Embodiment 1 are shown in Tables 1 and 2.

[0073] Example 4

[0074] The differences between this embodiment and Embodiment 1 are shown in Tables 1 and 2.

[0075] Example 5

[0076] The differences between this embodiment and Embodiment 1 are shown in Tables 1 and 2.

[0077] Example 6

[0078] The differences between this embodiment and Embodiment 1 are shown in Tables 1 and 2.

[0079] Example 7

[0080] The differences between this embodiment and Embodiment 1 are shown in Tables 1 and 2.

[0081] Example 8

[0082] The differences between this embodiment and Embodiment 1 are shown in Tables 1 and 2.

[0083] Example 9

[0084] The differences between this embodiment and Embodiment 1 are shown in Tables 1 and 2.

[0085] Example 10

[0086] The differences between this embodiment and Embodiment 1 are shown in Tables 1 and 2.

[0087] Comparative Example 1

[0088] The differences between this embodiment and Embodiment 1 are shown in Tables 1 and 2.

[0089] Comparative Example 2

[0090] The differences between this embodiment and Embodiment 1 are shown in Tables 1 and 2.

[0091] Comparative Example 3

[0092] The differences between this embodiment and Embodiment 1 are shown in Tables 1 and 2.

[0093] Comparative Example 4

[0094] The differences between this embodiment and Embodiment 1 are shown in Tables 1 and 2.

[0095] Relevant experimental and effect data:

[0096] First, Table 1 is provided, which shows the elemental composition of the bridge shell steel obtained in Examples 1-10 and Comparative Examples 1-4.

[0097]

[0098] Table 1. Then, Table 2 is provided, which shows the process parameters of Examples 1 to 10 and Comparative Examples 1 to 4.

[0099]

[0100]

[0101] Table 2

[0102] According to the national standards GB / T228 and GB / T231, tests were conducted on Examples 1 to 10 and Comparative Examples 1 to 4, and the yield strength, tensile strength, elongation, and cold bending performance are shown in Table 3.

[0103]

[0104] Table 3

[0105] As shown in Table 3, the bridge shell steel obtained in Examples 1-10 has a yield strength ≥600MPa, tensile strength ≥700MPa, elongation A50 ≥24%, and acceptable cold bending performance D=a and 180°. The steel in Comparative Example 1 has a tensile strength below 700MPa, and its elongation and cold bending performance are poor, lower than those of the examples. The steel in Comparative Example 2 has a high carbon content, and its elongation is comparable to that of the examples, but its yield strength and cold bending performance are lower than those of the examples. The steel in Comparative Example 3 has relatively high strength, but its elongation and cold bending performance are poor, lower than those of the examples. The steel in Comparative Example 4 has a tensile strength above 800MPa, but the cost of precious alloys such as Mo and Nb is high, and its elongation and cold bending performance are lower than those of the examples.

[0106] Metallographic images of Example 1 were taken, see... Figure 1 .

[0107] The metallographic structure is a composite grain structure of acicular ferrite and granular low-carbon bainite, with the ferrite grain size reaching level 12 or above. This structure ensures the high strength of the steel while also possessing good impact toughness and plasticity.

[0108] 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 range referred to.

[0109] In this application, unless otherwise stated, directional terms such as "upper" and "lower" specifically refer to the drawing directions in the accompanying drawings. Furthermore, in the description of this application, the terms "comprising," "including," etc., mean "including but not limited to." Moreover, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element. 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. For associations involving three or more related objects described using "and / or", it indicates that any one of the three related objects can exist alone, or at least two of them can exist simultaneously. For example, for A, and / or B, and / or C, it can mean that any one of A, B, and C exists alone, or any two of them exist simultaneously, or all three of them exist simultaneously. In this document, "at least one" means one or more, and "more than one" 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 multiple 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 each be single or multiple.

[0110] 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 method for preparing bridge shell steel, characterized in that, The method includes the following steps: Molten steel is provided, and the molten steel is subjected to a continuous casting process to form a billet; The billet is heated, rough rolled, and finish rolled to obtain rolled steel plate, wherein the finishing rolling temperature is 800~820℃, the rough rolling exit temperature is 1000~1050℃, and the finishing rolling start temperature is 980~1020℃. The rolled steel sheet is cooled to 300-450°C at a rate of 30-40°C / s to obtain the steel sheet to be coiled. The steel plate to be coiled is wound at 300~420℃, and after cooling, the bridge shell steel is obtained. The molten steel comprises, by mass percentage, 0.033-0.055% C, 0.051-0.063% Nb, 0.15-0.23% Cr, 0.20-0.30% Si, 1.75-2.00% Mn, 0.015-0.050% Al, 0.008-0.020% Ti, 0%-0.005% N, 0.10% or 0.11% or 0.12% or 0.13% Mo, P≤0.015%, S≤0.003%, with the balance being Fe and unavoidable impurities. The metallographic structure of the bridge shell steel is a composite grain structure of acicular ferrite and granular low-carbon bainite, with the ferrite grain size reaching grade 12 or above. The yield strength of the bridge shell steel is ≥600MPa, the tensile strength is ≥700MPa, the elongation A50 is ≥24%, and the cold bending performance D=a and 180° is qualified.

2. The method for preparing bridge shell steel according to claim 1, characterized in that, The molten steel comprises 0.21%~0.27% Si, 1.79%~1.90% Mn, 0.021%~0.031% Al, 0.010%~0.018% Ti, and 0.0021%~0.0039% N.

3. The method for preparing bridge shell steel according to claim 1, characterized in that, The billet is heated, and the heating process includes a homogenization and heat preservation stage. The homogenization temperature is 1200~1250℃, the total homogenization time is not less than 60min, and the total heating time is not less than 140min.

4. A bridge shell steel, characterized in that, The bridge shell steel is prepared by the method described in any one of claims 1 to 3.

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