440MPa-grade bake hardening steel and preparation method thereof

Abstract

The invention relates to 440MPa-grade bake hardening steel and a preparation method thereof, and belongs to the technical field of steel preparation. The chilled plate with the set chemical components is obtained; wherein the set chemical components comprise C, Mn, Si, P, S, Alt, Cr, B and Fe; the content of C is 0.045%-0.065% by mass, the content of Mn is 1.2%-1.4% by mass, the content of Cr is 0.45%-0.6% by mass, and the content of B is 0.001%-0.002% by mass; and the cold hard plate is subjected to continuous hot galvanizing production line treatment including continuous annealing, galvanizing, alloying and cooling, alloying process parameters are controlled, so that the hardness difference between ferrite and martensite of the structure of a galvanized plate obtained through galvanizing is reduced, and the 440 MPa-grade bake-hardened steel is obtained. The hole expansion rate lambda of the 440 MPa grade bake-hardening steel provided by the embodiment of the invention is 70% or above.

Description

Technical Field

[0001] The present application relates to the technical field of steel preparation, and in particular to a 440MPa grade bake hardening steel and a preparation method thereof. Background Art

[0002] The annual domestic production and sales of new energy vehicles have reached 10 million units respectively. The hidden door handles (inner flanges) and punching holes of the lower door guard plates of popular new energy vehicles require the flange height to be more than 7mm. On the other hand, bake hardened steel (BH steel) has a lower yield strength before stamping. After stamping or pre-stretching deformation, the yield strength of the material can be improved to a certain extent through baking paint temperature aging treatment, thereby achieving the purpose of unifying the strength, deep drawing performance and anti-sag performance of cold-rolled steel sheets for automobiles. It is the first choice for steel for automobile exterior panels.

[0003] When the strength of traditional BH steel products reaches 440MPa, the hole expansion performance of the material decreases rapidly, making it difficult to meet the current market requirement of a flange height of more than 7mm in the streamlined design concept of new energy vehicles, thus limiting the styling design and development of new energy vehicles. Summary of the invention

[0004] The present application provides a 440MPa grade bake hardening steel and a preparation method thereof to solve the following technical problem: how to improve the hole expansion rate of the 440MPa grade bake hardening steel.

[0005] In a first aspect, an embodiment of the present application provides a method for preparing a 440 MPa grade bake hardening steel, the method comprising:

[0006] A chilled plate with a set chemical composition is obtained; wherein the set chemical composition includes: C, Mn, Si, P, S, Alt, Cr, B, and Fe; in terms of mass fraction, the C content is 0.045% to 0.065%, the Mn content is 1.2% to 1.4%, the Cr content is 0.45% to 0.6%, and the B content is 0.001% to 0.002%;

[0007] The cold hardened plate is subjected to a continuous hot-dip galvanizing production line process including continuous annealing, galvanizing, alloying and cooling, and the process parameters of the alloying are controlled to reduce the hardness difference between ferrite and martensite in the structure of the galvanized plate obtained by the galvanizing, thereby obtaining a 440MPa grade bake hardening steel.

[0008] Optionally, the alloying process parameters include: a temperature of 545° C. to 555° C. and a time of 5 s to 10 s.

[0009] Optionally, in terms of mass fraction, the Si content is 0.1% to 0.2%, the P content is ≤0.015%, the S content is ≤0.005%, and the Alt content is 0.02% to 0.06%.

[0010] Optionally, the continuous annealing includes a first heating stage, a second heating stage, a soaking stage, and a cooling stage; wherein,

[0011] The terminal temperature of the first stage heating is 215°C to 225°C, the terminal temperature of the second stage heating is 775°C to 785°C, the temperature of the equalizing stage is 775°C to 785°C, and the terminal temperature of the cooling stage is 455°C to 465°C.

[0012] Optionally, the cooling includes first cooling and second cooling; wherein, the end point temperature of the first cooling is 295°C to 305°C, and the end point temperature of the second cooling is 245°C to 255°C.

[0013] Optionally, the step of obtaining a cold hardened plate having a set chemical composition comprises:

[0014] The ingot with the set chemical composition is sequentially hot rolled, coiled and cold rolled to obtain a chilled plate.

[0015] Optionally, the hot rolling includes heating and rolling, and the heating temperature is 1180° C. to 1220° C.; and / or,

[0016] The final rolling temperature of the rolling is 870°C to 910°C; and / or,

[0017] The coiling temperature is 660°C to 700°C; and / or,

[0018] The cold rolling reduction ratio is 70% to 80%.

[0019] Optionally, the 440 MPa grade bake hardening steel meets the following properties: yield strength is ≥260 MPa, tensile strength is ≥440 MPa, elongation is ≥28%, hole expansion rate is ≥70%, and BH value is ≥30 MPa.

[0020] Optionally, the metallographic structure of the 440 MPa grade bake hardening steel includes: ferrite and martensite; wherein the volume fraction of the martensite is 4% to 5%.

[0021] In a second aspect, an embodiment of the present application provides a 440 MPa grade bake-hardened steel prepared by the method described in any embodiment of the first aspect.

[0022] The above technical solution provided by the embodiment of the present application has the following advantages compared with the prior art:

[0023] The preparation method of the 440MPa grade bake-hardening steel provided in an embodiment of the present application comprises: obtaining a chilled plate with a set chemical composition; wherein the set chemical composition comprises: C, Mn, Si, P, S, Alt, Cr, B, and Fe; in terms of mass fraction, the C content is 0.045% to 0.065%, the Mn content is 1.2% to 1.4%, the Cr content is 0.45% to 0.6%, and the B content is 0.001% to 0.002%; subjecting the chilled plate to a continuous hot-dip galvanizing production line treatment including continuous annealing, galvanizing, alloying, and cooling, and controlling the process parameters of the alloying to reduce the hardness difference between ferrite and martensite of the galvanized plate obtained by galvanizing, so as to obtain a 440MPa grade bake-hardening steel. The chemical composition is set to include: C, Mn, Si, P, S, Alt, Cr, B, and Fe, and the C content is 0.045% to 0.065%, the Mn content is 1.2% to 1.4%, the Cr content is 0.45% to 0.6%, and the B content is 0.001% to 0.002%. The reasonable design of the C, Mn, Cr, and B contents can reduce the martensitic phase transformation temperature to 465°C in the subsequent continuous hot-dip galvanizing production line treatment, so that the steel produces a ferrite + martensite dual-phase structure with a time-limited strength of >4 40MPa, while balancing the strength, elongation and hole expansion rate of 440MPa grade bake hardening steel; continuous annealing of the cold hardened plate can regulate the organization of the cold hardened plate (forming a ferrite + martensite dual-phase organization) and its ratio, thereby ensuring the strength and elongation of the cold hardened plate after continuous annealing; alloying the galvanized plate obtained by galvanizing, and controlling the alloying process parameters, can make carbon diffuse into the ferrite, increase the solid solution carbon content in the ferrite, reduce the hardness difference between the ferrite and martensite of the galvanized plate, thereby increasing the hole expansion rate of the steel. Therefore, the hole expansion rate of the 440MPa grade bake hardening steel is improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the present application and, together with the description, serve to explain the principles of the present application.

[0025] In order to more clearly illustrate the technical solutions in the embodiments of the present application or 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.

[0026] Figure 1 A schematic flow chart of a method for preparing a 440MPa grade bake hardening steel provided in an embodiment of the present application;

[0027] Figure 2A microstructure diagram of a 440MPa grade bake hardening steel provided in an embodiment of the present application;

[0028] Figure 3 A microstructure diagram of a coating structure of a 440MPa grade bake-hardened steel provided in an embodiment of the present application;

[0029] Figure 4 A stress-strain curve of a 440MPa grade bake-hardened steel provided in an embodiment of the present application;

[0030] Figure 5 A schematic diagram of a sample hole expansion of a 440MPa grade bake-hardened steel provided in Example 1 of the present application. DETAILED DESCRIPTION

[0031] 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.

[0032] Various embodiments of the present application may be presented in the form of a range; it should be understood that the description in the form of a range is only for convenience and brevity, and should not be understood as a rigid limitation on the scope of the present application; therefore, the range description should be considered to have specifically disclosed all possible sub-ranges and single numerical values ​​within the range. For example, the range description from 1 to 6 should be considered to have 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., as well as single numbers within the range, such as 1, 2, 3, 4, 5 and 6, which apply regardless of the range. In addition, whenever a numerical range is indicated herein, it is meant to include any cited number (fractional or integer) within the indicated range.

[0033] In the present application, unless otherwise specified, the directional words used, such as "upper" and "lower", refer specifically to the directions of the drawings in the accompanying drawings. In addition, in the description of the specification of the present application, the terms "include", "comprise", etc. mean "including but not limited to". 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. In this article, "and / or" describes the association relationship of associated objects, indicating that three relationships may exist. For example, A and / or B can represent: the existence of A alone, the existence of A and B at the same time, and the existence of B alone. Among them, A and B can be singular or plural.

[0034] Unless otherwise specified, various raw materials, reagents, instruments and equipment used in this application can be purchased from the market or prepared by existing methods.

[0035] In a first aspect, the present application provides a method for preparing 440 MPa grade bake hardening steel. Figure 1 A schematic diagram of a process for preparing a 440MPa grade bake hardening steel provided in an embodiment of the present application; see Figure 1 , the method comprising:

[0036] S1. Obtaining a chilled plate with a set chemical composition; wherein the set chemical composition includes: C, Mn, Si, P, S, Alt, Cr, B, and Fe; in terms of mass fraction, the C content is 0.045% to 0.065%, the Mn content is 1.2% to 1.4%, the Cr content is 0.45% to 0.6%, and the B content is 0.001% to 0.002%;

[0037] Bake hardening steel (BH steel for short) is a kind of steel with both high strength and high formability. The strength of the final parts is obtained mainly through work hardening during processing and aging during the painting process. The preparation process starts with a cold hardened plate with a specific chemical composition. The cold hardened plate is a plate that has been preliminarily processed by the cold rolling process. Its internal structure has undergone a certain degree of deformation and adjustment, laying the foundation for subsequent processing.

[0038] The chemical composition of this steel grade is set to include C, Mn, Si, P, S, Alt, Cr, B and Fe. The mass fraction range of key elements is particularly pointed out, C is 0.045% to 0.065%, Mn is 1.2% to 1.4%, Cr is 0.45% to 0.6%, and B is 0.001% to 0.002%. By rationally designing the content of these elements, the martensitic phase transformation temperature in the subsequent continuous hot-dip galvanizing production line can be reduced to 465°C, the steel produces a ferrite + martensite dual-phase structure, and the aging strength is guaranteed to be greater than 440MPa, while achieving a balance between strength, elongation and hole expansion rate. Carbon (C) is an important element that affects the strength and phase transformation of steel. A certain content of carbon helps to form martensite and improve strength. Manganese (Mn) can reduce the critical cooling rate of steel, increase hardenability, and adjust the phase transformation temperature in combination with other elements. Chromium (Cr) can improve the strength, hardness and wear resistance of steel, and also affects phase transformation. Boron (B) can significantly improve the hardenability of steel in trace amounts and plays a key role in forming the required dual-phase structure.

[0039] C can be 0.045% to 0.065%, and carbon is the core element that affects the strength and phase transformation of steel. In the content range of 0.045%-0.065%, carbon plays a key role in the formation of martensite. Martensite is a supersaturated solid solution of carbon in α-Fe, and the solid solution strengthening effect of carbon significantly improves the strength of steel. During the quenching process, the carbon content determines the carbon content and morphology of martensite, which in turn affects the hardness and strength of martensite. At the same time, carbon also participates in various strengthening mechanisms in the subsequent aging process, which has an important influence on the aging strength, thereby achieving bake hardening steel to reach 440MPa level; Mn can be 1.2% to 1.4%, and Cr can be 0.45% to 0.6%, and the two synergistically promote the formation of martensite. Manganese reduces the critical cooling rate of steel, increases hardenability, and makes it easier for steel to form martensite during cooling. Chromium improves the strength, hardness and wear resistance of steel, and also affects the phase transformation process. It works together with manganese to change the stability of austenite, so that under appropriate cooling conditions, a sufficient amount of martensite can be generated, thereby cooperating with carbon to ensure that the bake-hardened steel reaches the strength requirement of 440MPa level; the content of B can be 0.001% to 0.002%, and trace amounts of boron significantly improve the hardenability of steel. Boron atoms are concentrated at the grain boundaries, inhibiting the nucleation of ferrite and promoting the formation of martensite, which indirectly plays a role in improving the strength of steel. The role of boron in strengthening the grain boundaries not only contributes to the strength, but more importantly, it ensures the elongation of bake-hardened steel. The strengthened grain boundaries can better coordinate the deformation between grains during the deformation of steel, reduce stress concentration at the grain boundaries, and thus improve the plastic deformation capacity of steel, that is, the elongation. And good elongation is an important basis for improving the hole expansion rate. During the hole expansion process, the steel needs to have sufficient plasticity to withstand deformation without breaking. Boron indirectly improves the hole expansion rate of the steel by increasing the elongation. For example, the C content may be 0.045%, 0.046%, 0.048%, 0.049%, 0.05%, 0.051%, 0.052%, 0.053%, 0.054%, 0.055%, 0.056%, 0.057%, 0.058%, 0.059%, 0.06%, 0.061%, 0.062%, 0.063%, 0.065, etc.; the Mn content may be 1.2%, 1.3%, 1.4%, etc.; the Cr content may be 0.45%, 0.066%, 0.067%, 0.068%, 0.070%, 0.071%, 0.072%, 0.073%, 0.074%, 0.075%, 0.076%, 0.077%, 0.078%, 0.079%, 0.080%, 0.081%, 0.082%, 0.083%, 0.084%, 0.085%, 0.086%, 0.087%, 0.088%, 0.089%, 0.090%, 0.091%, 0.092%, 0.093%, 0.094%, 0.095%, 0.096%, 0.097%, 0.098%, 0.099%, 0.0 The content of B can be 0.001%, 0.0011%, 0.0012%, 0.0013%, 0.0014%, 0.0015%, 0.0016%, 0.0017%, 0.0018%, 0.0019%, 0.002%, etc.

[0040] In some embodiments, the Si content is 0.1% to 0.2% by mass, and the P content is

[0041] ≤

[0042] 0.015%, the S content is ≤0.005%, and the Alt content is 0.02% to 0.06%.

[0043] In the embodiment of the present application, the content of Si is 0.1% to 0.2%. Silicon can dissolve in ferrite, produce a solid solution strengthening effect, and improve the strength and hardness of the steel. The appropriate amount of silicon content in this steel grade helps to improve the overall strength of the steel while ensuring a certain plasticity and toughness. It works together with other strengthening elements (such as C, Mn, etc.) to make the steel reach the strength requirement of 440MPa level. The content of P is ≤0.015%, and the content of S is ≤0.005%. P and S are impurity elements. Reducing the content of P and S avoids adverse effects on the mechanical properties of steel. The content of Alt is 0.02% to 0.06%. Aluminum mainly plays a deoxidation role in steel. At the same time, incompletely deoxidized aluminum can also form fine dispersed particles during the solidification process of steel, inhibiting the growth of austenite grains and refining grains. During the deformation process of fine-grained steel, due to its small grains and large grain boundary area, the grain boundary has a stronger barrier effect on the movement of dislocations, which makes the degree of dislocation accumulation at the grain boundary relatively small, thereby reducing the degree of stress concentration. During the hole expansion process, the stress distribution inside the material is more uniform, and it is not easy to generate local crack sources, which improves the hole expansion rate of steel. Illustratively, the Si content can be 0.1%, 0.11%, 0.12%, 0.13%, 0.14%, 0.15%, 0.16%, 0.17%, 0.18%, 0.19%, 0.2%, etc.; the P content can be 0.015%, 0.012%, 0.010%, 0.009%, etc.; the S content can be 0.005%, 0.004%, 0.003%, 0.002%, etc.; the Alt content can be 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, etc.

[0044] In some embodiments, obtaining a cold hardened plate having a set chemical composition comprises:

[0045] The ingot with the set chemical composition is sequentially hot rolled, coiled and cold rolled to obtain a chilled plate.

[0046] In some embodiments, the hot rolling includes heating and rolling, and the heating temperature is 1180° C. to 1220° C.; and / or,

[0047] The final rolling temperature of the rolling is 870°C to 910°C; and / or,

[0048] The coiling temperature is 660°C to 700°C; and / or,

[0049] The cold rolling reduction ratio is 70% to 80%.

[0050] In an embodiment of the present application, a billet with a set chemical composition is sequentially hot rolled, coiled and cold rolled to obtain a chilled plate. The heating temperature can be 1180°C to 1220°C, so that each phase in the billet is fully austenitized, and the plasticity of the steel is improved, the deformation resistance is reduced, and good conditions are created for subsequent rolling. At this temperature, the atomic activity is enhanced, which is conducive to the homogenization process, so that the chemical composition is more evenly distributed and the consistency of the steel performance is improved. Exemplarily, the heating temperature can be 1180, 1190, 1200, 1210, 1220, etc. The final rolling temperature of the rolling can be 870°C to 910°C, which can regulate the deformation and recrystallization behavior of the austenite grains and help to ensure the stability of the rolling process. Exemplarily, the final rolling temperature of the rolling can be 870°C, 880°C, 890°C, 900°C, 910°C, etc. The coiling temperature can be 660℃~700℃. A higher coiling temperature is conducive to the full nucleation and growth of ferrite, and may reduce the amount of martensite formed, thereby improving the plasticity of the steel, thus balancing the strength and plasticity of the steel to a certain extent. The appropriate coiling temperature helps to eliminate some of the residual stress generated during the hot rolling process and improve the stress distribution state inside the steel. Exemplarily, the coiling temperature can be 660℃, 670℃, 680℃, 690℃, 700℃, etc. The cold rolling reduction rate can be 70%~80%. The higher cold rolling reduction rate causes the steel to undergo strong plastic deformation during the cold rolling process, the grains are significantly elongated and broken, a large number of dislocations and substructures are formed, and the strength and hardness of the steel are greatly improved. At the same time, through cold rolling with a large deformation amount, the grains can be further refined, the organizational structure of the steel can be more dense and uniform, and a better organizational basis can be provided for subsequent heat treatment (such as continuous annealing). Exemplarily, the cold rolling reduction ratio can be 70%, 71%, 72%, 73%, 74%, 75%, 76%, 77%, 78%, 79%, 80%, etc.

[0051] S2. The cold hardened sheet is subjected to a continuous hot-dip galvanizing production line process including continuous annealing, galvanizing, alloying and cooling, and the process parameters of the alloying are controlled to reduce the hardness difference between ferrite and martensite in the structure of the galvanized sheet obtained by the galvanizing, so as to obtain a 440 MPa grade bake hardening steel.

[0052] Continuous annealing: Continuously anneal the cold hardened plate to adjust the organizational structure of the cold hardened plate to form a ferrite + martensite dual-phase organization and adjust the ratio of the two to ensure the strength and elongation of the cold hardened plate after continuous annealing. Galvanizing: Galvanizing is carried out after continuous annealing. The purpose of galvanizing is to provide good corrosion resistance for steel and form a zinc protective layer on the surface of the steel to prevent the steel from rusting. Alloying: Alloying the galvanized sheet obtained by galvanizing, and controlling the alloying process parameters. Through the alloying process, carbon diffuses into the ferrite, thereby increasing the content of solid solution carbon in the ferrite. Due to the large difference in hardness between ferrite and martensite, reducing the hardness difference between the two can effectively increase the hole expansion rate of steel. Cooling: Cooling after alloying is completed to maintain the favorable organizational structure and performance formed during the alloying process.

[0053] In some embodiments, the alloying process parameters include: a temperature of 545° C. to 555° C. and a time of 5 s to 10 s.

[0054] In an embodiment of the present application, the alloying temperature can be 545°C to 555°C, and the time can be 5s to 10s. In the temperature range of 545°C to 555°C, atoms have sufficient activity for diffusion movement. During the alloying process, the appropriate temperature promotes the diffusion of carbon elements from the martensite phase or other high-carbon regions to the ferrite phase. As the carbon element diffuses into the ferrite, the solid solution carbon content in the ferrite gradually increases. The increase in solid solution carbon in the ferrite will cause lattice distortion and produce a solid solution strengthening effect, thereby improving the strength and hardness of the ferrite. This strengthening mechanism helps to balance the strength difference between ferrite and martensite in the steel, so that the steel has better comprehensive mechanical properties as a whole. Alloying treatment for 5s to 10s at a temperature of 545°C to 555°C not only promotes the diffusion of carbon elements, but also makes the internal structure of the steel more uniform. The diffusion and redistribution of carbon elements reduce the performance inhomogeneity between martensite and ferrite due to differences in carbon content. At the same time, the thermal movement and diffusion of atoms during the alloying process help eliminate the internal stress concentration areas generated during the rolling and cooling process of the steel, and further improve the uniformity of the organizational structure. The hardness difference between ferrite and martensite is reduced, so that when the steel is deformed by force, the stress can be more evenly distributed between the ferrite and martensite phases, reducing the occurrence of stress concentration, thereby improving the plasticity and toughness of the steel and reducing the risk of crack initiation and expansion. Since the alloying treatment reduces the hardness difference between ferrite and martensite and improves the uniformity of the organizational structure, the internal stress distribution of the steel is more uniform during the hole expansion process, reducing the stress concentration caused by hardness differences and uneven organizational structure. Stress concentration is one of the important reasons for crack initiation. When the stress concentration exceeds the local strength of the material, cracks will occur in the stress concentration area. By reducing the stress concentration, the alloying treatment effectively reduces the initiation of cracks during the hole expansion process, thereby improving the hole expansion rate of the steel.

[0055] In some embodiments, the continuous annealing includes one-stage heating, two-stage heating, a soaking section and a cooling section; wherein the endpoint temperature of the one-stage heating is 215°C to 225°C, the endpoint temperature of the two-stage heating is 775°C to 785°C, the temperature of the soaking section is 775°C to 785°C, and the endpoint temperature of the cooling section is 455°C to 465°C.

[0056] In the embodiment of the present application, the end temperature of the first stage heating can be 215℃~225℃, and the dislocations inside the steel begin to gradually move and rearrange, preparing for the subsequent heating process. The end temperature of the second stage heating can be 775℃~785℃, and ferrite and cementite begin to gradually transform into austenite. Since the end temperature of the second stage heating is close to the critical temperature of austenitization, the steel can be fully austenitized at this temperature and avoid excessive growth of austenite grains, thereby ensuring the strength and toughness of the steel. The temperature of the soaking section can be 775℃~785℃, making the internal structure of the steel more uniform. During the second stage heating process, although the steel transforms to austenite as a whole, due to certain differences in the chemical composition, stress state, etc. of various parts inside the steel, the degree of austenitization may not be completely consistent. Through the treatment of the soaking section, the atoms inside the steel have enough time to diffuse and redistribute, so that the composition of the austenite is more uniform and the grain size is more consistent. The terminal temperature of the cooling section can be 455℃~465℃, which can promote the transformation of austenite to martensite, ensure the formation of ferrite + martensite dual-phase structure, and control the martensite ratio to 4~4.5%. A good balance of steel strength, plasticity and toughness is achieved. For example, the terminal temperature of the first heating stage can be 215℃, 217℃, 219℃, 220℃, 222℃, 224, 225, etc.; the terminal temperature of the second heating stage can be 775℃, 779℃, 780℃, 781℃, 783℃, 785℃, etc.; the temperature of the soaking section can be 775℃, 779℃, 780℃, 781℃, 783℃, 785℃, etc.; the terminal temperature of the cooling section can be 455℃, 457℃, 459℃, 460℃, 462℃, 464℃, 465℃, etc.

[0057] In some embodiments, the cooling includes a first cooling and a second cooling; wherein the end point temperature of the first cooling is 295°C to 305°C, and the end point temperature of the second cooling is 245°C to 255°C.

[0058] In the embodiment of the present application, cooling is performed after alloying is completed to maintain the favorable organizational structure and performance formed during the alloying process. The terminal temperature of the first cooling can be 295°C to 305°C, and the terminal temperature of the second cooling can be 245°C to 255°C, so as to fully suppress the growth of the steel grains and help maintain the plasticity and toughness of the steel. The alloying increases the hole expansion rate by reducing the hardness difference between ferrite and martensite. During the cooling process, the uniformity of this organizational structure and the good coordination between the phases can be maintained to maintain the hole expansion rate. Exemplarily, the terminal temperature of the first cooling can be 295°C, 297°C, 299°C, 301°C, 303°C, 305°C, etc.; the terminal temperature of the second cooling can be 245°C, 247°C, 249°C, 250°C, 251°C, 253°C, 255°C, etc.

[0059] Exemplarily, one implementation of step S2 includes: continuous annealing: the cold hard plate is placed in a non-oxygen atmosphere composed of H2 and N2 in a furnace from room temperature and is continuously heated to 220°C for 22 seconds, further heated to 780°C for 185 seconds, kept warm for 55 seconds, and then cooled to 460°C for 40 seconds; galvanizing; alloying: heated to 550°C for 7 seconds and kept warm for 8 seconds;

[0060] Cooling: Cool to 300℃ in 9 seconds; cool to 250℃ in 13 seconds, then water cool to room temperature.

[0061] In some embodiments, the 440 MPa grade bake hardening steel meets the following properties: yield strength ≥260 MPa, tensile strength ≥440 MPa, elongation ≥28%, hole expansion ratio ≥70%, and BH value ≥30 MPa.

[0062] In some embodiments, the metallographic structure of the 440 MPa grade bake hardening steel includes: ferrite and martensite; wherein the volume fraction of the martensite is 4% to 5%.

[0063] The method for preparing 440MPa grade bake hardening steel provided in the embodiment of the present application has the following advantages:

[0064] 1. Relationship between strength and organization: The ferrite + martensite dual-phase organization is formed through chemical composition design and continuous annealing. The martensite phase provides high strength, and the ferrite phase ensures a certain plasticity and toughness, so that the aging strength of the steel is greater than 440MPa, meeting the strength requirements.

[0065] 2. Improved hole expansion rate: During the alloying process, carbon diffuses into ferrite, reducing the hardness difference between ferrite and martensite. During hole expansion and other forming processes, due to the reduced hardness difference, the material deformation is more uniform, and it is not easy to generate stress concentration at the interface, which leads to crack initiation and expansion, thus improving the hole expansion rate.

[0066] In a second aspect, an embodiment of the present application provides a 440 MPa grade bake-hardened steel prepared by the method described in any embodiment of the first aspect.

[0067] The 440MPa grade bake-hardened steel is realized based on the preparation method of the above-mentioned 440MPa grade bake-hardened steel. The specific steps of the preparation method of the 440MPa grade bake-hardened steel can refer to the above-mentioned embodiment. Since the 440MPa grade bake-hardened steel adopts part or all of the technical solutions of the above-mentioned embodiments, it has at least all the beneficial effects brought by the technical solutions of the above-mentioned embodiments, which will not be repeated here one by one.

[0068] The present application will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are intended only to illustrate the present application and are not intended to limit the scope of the present application. The experimental methods for the unrecorded specific conditions in the following examples are usually measured according to national standards. If there is no corresponding national standard, then carry out according to general international standards, normal conditions or according to the conditions recommended by the manufacturer.

[0069] Example 1

[0070] The present application embodiment provides a method for preparing 440MPa grade bake hardening steel, comprising:

[0071] A chilled plate with a set chemical composition is obtained; (the ingot with the set chemical composition is sequentially subjected to hot rolling, coiling and cold rolling to obtain a chilled plate, the heating temperature of hot rolling is 1200°C, the final rolling temperature of hot rolling is 890°C, and the coiling temperature is 680°C); wherein the set chemical composition is: C, Mn, Si, P, S, Alt, Cr, B, and Fe and unavoidable impurities; in terms of mass fraction, the content of C is 0.045%, the content of Mn is 1.2%, the content of Si is 0.1%, the content of P is 0.008%, the content of S is 0.002%, the content of Alt is 0.02%, the content of Cr is 0.45%, and the content of B is 0.001%;

[0072] The cold hardened plate is subjected to a continuous hot-dip galvanizing production line treatment including continuous annealing, galvanizing, alloying and cooling, and the process parameters of the alloying are controlled to reduce the hardness difference between the ferrite and martensite of the galvanized plate obtained by galvanizing, so as to obtain a 440MPa grade bake hardening steel; wherein, continuous annealing: the cold hardened plate enters the furnace from room temperature in a non-oxygen atmosphere composed of H2 and N2, and is continuously heated to 220°C for 22 seconds (first stage heating), further heated to 780°C for 185 seconds (second stage heating), kept warm for 55s (heating section), and then cooled to 460°C for 40 seconds (cooling section); galvanizing; alloying: heating to 550°C for 7 seconds and kept warm for 8 seconds; cooling: cooling to 300°C for 9 seconds; cooling to 250°C for 13 seconds, and then water cooling to room temperature.

[0073] Example 2

[0074] Based on the disclosure of the embodiment, the difference between embodiment 2 and embodiment 1 is that: the content of C is 0.050%, the content of Mn is 1.3%, the content of Cr is 0.48%, and the content of B is 0.0015%;

[0075] Alloying: Heat to 545°C for 7 seconds and keep at this temperature for 7 seconds.

[0076] Example 3

[0077] Based on the disclosure of Example 1, Example 3 differs from Example 1 in that the content of C is 0.065%, the content of Mn is 1.4%, the content of Cr is 0.6%, and the content of B is 0.0019%.

[0078] Alloying: Heat to 555°C for 7 seconds and keep warm for 10 seconds.

[0079] Comparative Example 1

[0080] Based on the disclosure of Example 1, the difference between Comparative Example 1 and Example 1 is that the content of B is 0.003%.

[0081] Comparative Example 2

[0082] Based on the disclosure of Example 1, the difference between Comparative Example 2 and Example 1 is: Alloying: heating to 570° C. for 7 seconds and keeping warm for 8 seconds.

[0083] The 440 MPa grade bake hardening steels prepared in Examples 1 to 3 and Comparative Examples 1 to 2 were subjected to mechanical property and microstructural tests, and the results are shown in Table 1.

[0084] Table 1 Mechanical properties and microstructure of 440MPa grade bake hardening steel (BH440 product)

[0085]

[0086] As shown in Table 1, the elongation of the BH440 products prepared in Examples 1 to 3 can reach more than 30%, the hole expansion rate can reach more than 70%, and the flange height can reach more than 7 mm, which are all higher than those in Comparative Examples 1 to 2. Comparative Example 1 results: the product yield strength is too high, the elongation is less than 30%, the hole expansion rate is less than 50%, and the flange height is less than 7 mm. Comparative Example 2 results: the martensite ratio is too high, causing the material tensile strength to reach 500 MPa, affecting the product elongation below 30%, and the final hole expansion rate and flange height are both low.

[0087] Table 2 Hole expansion test results of 440MPa grade bake hardening steel of Example 1

[0088]

[0089] It can be seen from Table 2 that the hole expansion rate of the BH440 product prepared in Example 1 can reach more than 70% in three tests.

[0090] Figure 2 A microstructure diagram of a 440MPa grade bake hardening steel provided in an embodiment of the present application; Figure 3 A microstructure diagram of the coating structure of a 440MPa grade bake-hardened steel provided in the embodiment of the present application; see Figures 2-3 The microstructure (metallographic structure) is ferrite (a small amount of carbides are distributed on the grains) + martensite (4-5%), and the coating structure is δ phase and ζ phase. Figure 4 A stress-strain curve of a 440MPa grade bake-hardened steel provided in the embodiment of the present application; see Figure 4 , indicating that the tensile strength of the BH440 product prepared by the embodiment of the present application is greater than 440MPa, and the elongation reaches 30%. Figure 5 A schematic diagram of a sample hole expansion of a 440MPa grade bake hardening steel provided in Example 1 of the present application; see Figure 5 , indicating that under the condition of punching diameter 10mm, there is no crack on the edge of BH440 after hole expansion, and the hole expansion quality is normal.

[0091] One or more technical solutions in the embodiments of the present application also have at least the following technical effects or advantages:

[0092] (1) By optimizing and controlling the ratio of carbon, manganese, chromium and boron elements, the phase transition temperature of the product reaches 465°C. The product is heated to 780°C, kept at this temperature for 55 seconds, and then cooled to 460°C for 40 seconds to achieve the recrystallization and cooling process of ferrite + martensite structure (4-5%). The alloying process of heating to 550°C for 7 seconds and keeping at this temperature for 8 seconds increases the solid solution carbon content in the ferrite, while reducing the hardness difference between ferrite and martensite. The hole expansion rate (λ) of 440MPa grade hot-dip galvanized alloyed high-strength bake-hardening steel is stably controlled to be above 70%;

[0093] (2) By increasing the hole expansion rate of 440MPa grade bake-hardened steel, high-strength hot-dip galvanized alloyed automotive steel has the high strength, high ductility, and high dent resistance of traditional products while meeting the high flanging requirements of new energy vehicles and the body design trends of the new era.

[0094] The above description is only a specific implementation of the present application, so that those skilled in the art can understand or implement the present application. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to the embodiments shown herein, but will conform to the widest range consistent with the principles and novel features applied for herein.

Claims

1. A method for preparing 440MPa grade bake hardening steel, the method comprising: A chilled plate with a set chemical composition is obtained; wherein the set chemical composition includes: C, Mn, Si, P, S, Alt, Cr, B, and Fe; in terms of mass fraction, the C content is 0.045% to 0.065%, the Mn content is 1.2% to 1.4%, the Cr content is 0.45% to 0.6%, and the B content is 0.001% to 0.002%; The cold hardened plate is subjected to a continuous hot-dip galvanizing production line process including continuous annealing, galvanizing, alloying and cooling, and the process parameters of the alloying are controlled to reduce the hardness difference between ferrite and martensite in the structure of the galvanized plate obtained by the galvanizing, thereby obtaining a 440MPa grade bake hardening steel.

2. The method according to claim 1, characterized in that The alloying process parameters include: temperature of 545° C. to 555° C. and time of 5s to 10s.

3. The method according to claim 1, characterized in that In terms of mass fraction, the Si content is 0.1% to 0.2%, the P content is ≤0.015%, the S content is ≤0.005%, and the Alt content is 0.02% to 0.06%.

4. The method according to claim 1, characterized in that The continuous annealing includes a first stage heating, a second stage heating, a soaking stage and a cooling stage; wherein, The terminal temperature of the first stage heating is 215°C to 225°C, the terminal temperature of the second stage heating is 775°C to 785°C, the temperature of the equalizing stage is 775°C to 785°C, and the terminal temperature of the cooling stage is 455°C to 465°C.

5. The method according to claim 1, characterized in that The cooling includes a first cooling and a second cooling; wherein the end point temperature of the first cooling is 295°C to 305°C, and the end point temperature of the second cooling is 245°C to 255°C.

6. The method according to claim 1, characterized in that The method of obtaining a cold hardened plate with a set chemical composition comprises: The ingot with the set chemical composition is sequentially hot rolled, coiled and cold rolled to obtain a chilled plate.

7. The method according to claim 6, characterized in that The hot rolling includes heating and rolling, the heating temperature is 1180° C. to 1220° C.; and / or, The final rolling temperature of the rolling is 870°C to 910°C; and / or, The coiling temperature is 660°C to 700°C; and / or, The cold rolling reduction ratio is 70% to 80%.

8. The method according to claim 1, characterized in that The 440MPa grade bake hardening steel meets the following properties: yield strength ≥260MPa, tensile strength ≥440MPa, elongation ≥28%, hole expansion ratio ≥70%, and BH value ≥30MPa.

9. The method according to claim 1, characterized in that: The metallographic structure of the 440MPa grade bake hardening steel includes ferrite and martensite; wherein the volume fraction of the martensite is 4% to 5%.

10. A 440 MPa grade bake hardening steel prepared by the method according to any one of claims 1 to 9.

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