A kind of phase transformation induced plasticity steel and its preparation method, automobile parts

Abstract

The application relates to a phase transformation induced plasticity steel and a preparation method and an automobile part thereof, the chemical components of the phase transformation induced plasticity steel comprising C, Si, Mn, Cu, P, S, N and Fe; wherein the content of the Si is 1.0%-2.0% in mass fraction, the content of the Cu is 0.5%-1.0% in mass fraction, the content of the C is 0.12%-0.17% in mass fraction, the content of the Mn is 1.3%-1.8% in mass fraction, the content of the P is <=0.01% in mass fraction, the content of the S is <=0.01% in mass fraction, and the content of the N is <=0.004% in mass fraction. The phase transformation induced plasticity steel provided by the application satisfies the following mechanical properties: the yield strength is 420-500 MPa, the tensile strength is 800-860 MPa, the elongation rate is 28%-35%, and the work hardening value is 0.18%-0.2%.

Description

Technical Field

[0001] This application relates to the field of high-strength steel preparation technology, and in particular to a phase transformation induced plasticity steel and its preparation method, and automotive parts. Background Technology

[0002] Phase transformation induced ductility steel is a high-strength steel with good plasticity and toughness. Due to its high strength, it can be used in automobile bodies and other applications to ensure safety.

[0003] Currently, the microstructure of traditional transformation-induced plasticity (CEPS) steel consists of a soft ferrite matrix, a hard second-phase bainite, and a small amount of retained austenite. This type of CEPS steel is prone to cracking during the hole-expanding forming process, reducing the yield. Therefore, while CEPS steel offers the advantage of high strength, it is difficult to meet users' requirements for processing performance. Summary of the Invention

[0004] This application provides a phase transformation induced ductile steel and its preparation method, as well as automotive parts, to solve the following technical problem: how to improve the processing performance of phase transformation induced ductile steel.

[0005] In a first aspect, this application provides a phase transformation induced ductility steel, the chemical composition of which includes:

[0006] C, Si, Mn, Cu, P, S, N, and Fe; where, by mass fraction,

[0007] The Si content is 1.0% to 2.0%, and the Cu content is 0.5% to 1.0%.

[0008] Optionally, the content of C is 0.12% to 0.17%, the content of Mn is 1.3% to 1.8%, the content of P is ≤0.01%, the content of S is ≤0.01%, and the content of N is ≤0.004%.

[0009] Optionally, the phase transformation-induced plasticity microstructure includes ferrite, bainite, and retained austenite; wherein, by volume fraction,

[0010] The ferrite content is 50%–60%, the bainite content is 30%–40%, and the retained austenite content is 10%–15%.

[0011] Optionally, the phase transformation induced ductility steel satisfies the following mechanical properties:

[0012] Yield strength 420MPa~500MPa, tensile strength 800MPa~860MPa, elongation 28%~35%, work hardening value 0.18%~0.2%.

[0013] Secondly, this application provides a method for preparing phase transformation-induced ductile steel according to any embodiment of the first aspect, the method comprising:

[0014] Obtain cold-hardened rolls;

[0015] The cold-hardened coil is subjected to heat treatment including preheating, heating, homogenization and cooling, followed by aging to obtain phase transformation induced plasticity steel; wherein the final temperature of the heating is 790℃~830℃.

[0016] Optionally, the heat equalization process parameters include: a temperature of 790℃~830℃ and a time of 60s~150s.

[0017] Optionally, the final temperature of the preheating is 210℃~230℃, and the preheating rate is 8℃ / s~12℃ / s.

[0018] Optionally, the cooling includes: a first cooling and a second cooling; wherein,

[0019] The first cooling rate is 2℃ / s to 6℃ / s, and the first cooling end temperature is 720℃ to 770℃; the second cooling rate is 30℃ / s to 40℃ / s, and the second cooling end temperature is 380℃ to 430℃; and / or,

[0020] The aging process parameters include: temperature of 380℃~430℃ and time of 60s~120s.

[0021] Optionally, obtaining the cold-hardened roll includes:

[0022] The slab is heated in a furnace, rolled, and coiled to obtain a hot-rolled coil;

[0023] The hot-rolled coil is then cold-rolled to obtain a cold-rolled hard coil; wherein...

[0024] The furnace heating temperature is 1150℃~1280℃; and / or,

[0025] The rolling process includes finishing rolling, wherein the finishing rolling temperature is 870℃~920℃; and / or,

[0026] The winding temperature is 550℃~620℃; and / or,

[0027] The reduction rate of the cold rolling is 50% to 60%.

[0028] Thirdly, this application provides an automotive part, the raw material of which includes the phase transformation induced plasticity steel described in any embodiment of the first aspect.

[0029] The technical solutions provided in this application have the following advantages compared with the prior art:

[0030] The phase transformation induced plasticity steel provided in this application embodiment has the following chemical composition: C, Si, Mn, Cu, P, S, N and Fe; wherein, by mass fraction, the content of Si is 1.0% to 2.0% and the content of Cu is 0.5% to 1.0%. With a Si content of 1.0%–2.0%, cementite precipitation in transformation-induced ductile steel can be suppressed. With a Cu content of 0.5%–1.0%, the strength and elastic limit of transformation-induced ductile steel can be improved, and austenite can be stabilized and its content increased. With the synergistic effect of Si content of 1.0%–2.0% and Cu content of 0.5%–1.0%, cementite precipitation and the formation of a thin film of retained austenite adjacent to bainite can be avoided, thus obtaining a high-strength steel with a mixed microstructure of ferrite matrix, thin film retained austenite, and bainite. This thin film retained austenite is different from the blocky retained austenite of traditional transformation-induced ductile steel and can undergo a gradual martensitic transformation, making the deformation stability of the steel more uniform. This allows the steel to have a relatively high work hardening rate under high strain in the later stage, enabling the TRIP effect to be fully utilized stably. At the same time, it improves the strength, elongation, and work hardening capacity of the steel, thereby improving the processing performance of transformation-induced ductile steel. Attached Figure Description

[0031] 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.

[0032] 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.

[0033] Figure 1 This is a schematic diagram illustrating the process of preparing a phase transformation induced ductile steel according to some embodiments of this application;

[0034] Figure 2 The image shows the metallographic structure of a phase transformation-induced ductile steel according to an embodiment of this application.

[0035] Figure 3 The image shows the metallographic structure of a phase transformation-induced ductile steel provided in accordance with the comparative examples of this application. Detailed Implementation

[0036] 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.

[0037] Various embodiments of this application may exist in the form of a range; it should be understood that the description in the form of a range is merely for convenience and brevity and should not be construed as a hard limitation on the scope of this application; therefore, it should be considered that the range description has specifically disclosed all possible sub-ranges and single numerical values ​​within that range. For example, it should be considered that the range description from 1 to 6 has specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., and single numbers within the range, such as 1, 2, 3, 4, 5, and 6, regardless of the range. Furthermore, whenever a numerical range is referred to herein, it means including any referenced number (fraction or integer) within the referred range.

[0038] 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, terms such as "comprising" and "including" mean "including but not limited to." In this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, 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 existing alone, A and B existing simultaneously, or B existing alone. Here, A and B can be singular or plural.

[0039] 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.

[0040] The inventors discovered that excessively high carbon equivalent in transformation-induced plasticity (CEPL) steel can negatively impact weldability and elongation. However, reducing the carbon content leads to insufficient austenite stability after two-phase treatment, failing to achieve the desired retained austenite content. Furthermore, during the refining process of scrap iron, elements such as Cu, Sn, and Cr are difficult to remove and remain entirely in the steel product; these elements are called residual elements. These residual elements have a certain impact on the mechanical properties and elongation of the steel. In particular, Cu in scrap iron has a lower oxidation potential energy than iron, making it difficult to remove during steelmaking. During high-temperature heating and rough rolling, it easily accumulates at the interface between the substrate and oxide layer and penetrates into grain boundaries, causing high-temperature brittleness. However, as an austenite stabilizing element, adding copper to advanced high-strength steel that requires the introduction of retained austenite is expected to retain more retained austenite, thereby improving the corresponding ductility. Therefore, the solid solution strengthening effect of Cu and the precipitation strengthening effect of ε-Cu precipitates can effectively enhance the strength of the soft-phase ferrite, and a uniform microstructure can be expected, thereby improving the yield strength and local formability of CEPL.

[0041] This application provides a phase transformation induced ductility steel, the chemical composition of which includes:

[0042] C, Si, Mn, Cu, P, S, N, and Fe; where, by mass fraction,

[0043] The Si content is 1.0% to 2.0%, and the Cu content is 0.5% to 1.0%.

[0044] In some embodiments, the content of C is 0.12% to 0.17%, the content of Mn is 1.3% to 1.8%, the content of P is ≤0.01%, the content of S is ≤0.01%, and the content of N is ≤0.004%.

[0045] In this embodiment, Si is an important element for suppressing cementite precipitation. If the Si content is too low, it is difficult to suppress cementite precipitation in the steel, resulting in a small amount of retained austenite and affecting the ductility of the steel; if the Si content is too high, it will worsen the steel's flanging performance. Adding Cu to the steel will increase the strength and elastic limit, and increase corrosion resistance. As an austenite stabilizing element, Cu can increase the austenite content by expanding the austenite domain. If the Cu content is too high, it will reduce the stability of retained austenite; if the Cu content is too low, it will not increase the retained austenite content. The synergistic effect of Si and Cu can prevent cementite precipitation and the formation of a thin film of retained austenite adjacent to bainite, thereby obtaining a high-strength steel with a mixed microstructure of ferrite matrix, thin film retained austenite, and bainite. This thin film retained austenite differs from the blocky retained austenite in traditional transformation-induced ductile steel; it can undergo a gradual martensitic transformation, resulting in more uniform deformation stability. This allows the steel to maintain a relatively high work hardening rate even under high strain in the later stages, ensuring the TRIP effect is fully and stably utilized, while simultaneously improving the steel's strength, elongation, and work hardening capacity. For example, the Si content is 1.0%, 1.2%, 1.4%, 1.6%, 1.8%, 2.0%, etc., and the Cu content is 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, etc.

[0046] In the embodiments of this application, carbon (C) is the most effective solid solution strengthening element, the most important element for ensuring the content of the hard phase in steel, and also an austenite stabilizing element, which is beneficial for the formation of retained austenite. If the C content is too low, it is difficult to ensure the content of the hard phase in the steel, and thus it is difficult to achieve the required strength; if the C content is too high, it will deteriorate the weldability of the steel. For example, the C content can be 0.12%, 0.13%, 0.14%, 0.15%, 0.16%, 0.17%, etc.

[0047] Mn is a solid solution strengthening element and an important element for stabilizing austenite. If the Mn content is too low, it is difficult to ensure the hard phase of the steel and achieve high strength; if the Mn content is too high, it will worsen the workability and weldability of the steel. In addition, when Mn combines with Si, the smaller the Si / Mn ratio, the easier it is to form banded structures, which deteriorates the ductility of the steel. Therefore, in order to increase the retained austenite fraction and improve its ductility, the Si / Mn value should be increased as much as possible to avoid the formation of banded structures. For example, the above-mentioned Mn contents are 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, etc.

[0048] P (phosphorus) can significantly reduce the plasticity and toughness of steel, therefore its content should be as low as possible. For example, the P content mentioned above can be 0.01%, 0.009%, 0.008%, 0.007%, etc.

[0049] Sulfur (S) is a harmful impurity element in steel, causing hot brittleness, reducing the ductility and toughness of the steel, and causing cracks during forging and rolling. For example, the S content mentioned above can be 0.01%, 0.009%, 0.008%, 0.007%, etc.

[0050] Like carbon (C), nitrogen (N) is also a solid-solution element. Increased N content in steel leads to decreased stamping performance. Furthermore, dissolved N is a major cause of aging in galvanized steel products, particularly affecting strain aging after leveling; therefore, N content should be kept as low as possible. For example, the N content could be 0.004%, 0.0035%, 0.003%, or 0.0038%.

[0051] In some embodiments, the phase transformation-induced plasticity microstructure includes ferrite, bainite, and retained austenite; wherein, by volume fraction,

[0052] The ferrite content is 50%–60%, the bainite content is 30%–40%, and the retained austenite content is 10%–15%.

[0053] In this embodiment, the ferrite phase is the matrix phase, and the retained austenite is in the form of a thin film, uniformly dispersed in the bainite matrix. It undergoes a gradual martensitic transformation, exhibiting relatively uniform deformation stability. This results in a relatively high work hardening rate even under high strain in the later stages, allowing the TRIP effect to be fully and stably utilized, delaying necking, and improving the elongation and work hardening capacity of the steel. In traditional transformation-induced ductile steel, the retained austenite is blocky. This retained austenite has poor stability, undergoing a rapid martensitic transformation in the initial strain stage. As deformation progresses, the work hardening rate decreases rapidly, and the elongation and yield strength also decrease. For example, the ferrite content can be 50%, 53%, 55%, 57%, 60%, etc.; the bainite content can be 30%, 32%, 34%, 36%, 38%, 40%, etc.; and the retained austenite content can be 10%, 11%, 12%, 13%, 14%, 15%, etc.

[0054] In some embodiments, the phase transformation-induced ductility steel satisfies the following mechanical properties:

[0055] Yield strength 420MPa~500MPa, tensile strength 800MPa~860MPa, elongation 28%~35%, work hardening value 0.18%~0.2%.

[0056] In the embodiments of this application, the phase transformation induced ductile steel possesses the aforementioned excellent mechanical properties: a yield strength of 420–500 MPa, a tensile strength of 800–860 MPa, an elongation of 28–35%, and a work hardening value of 0.18–0.2%. It exhibits good work hardening capability and can meet the special requirements of automotive parts for processing performance. The thickness of this phase transformation induced ductile steel can be 1.0 mm to 2.0 mm.

[0057] Figure 1 This is a schematic diagram illustrating the preparation process of a phase transformation-induced ductile steel according to some embodiments of this application; please refer to... Figure 1 This application provides a method for preparing phase transformation induced ductile steel according to any embodiment of the first aspect, the method comprising:

[0058] S1. Obtain a cold-hardened roll;

[0059] In some embodiments, obtaining the cold-hardened roll includes:

[0060] The slab is heated in a furnace, rolled, and coiled to obtain a hot-rolled coil;

[0061] The hot-rolled coil is then cold-rolled to obtain a cold-rolled hard coil; wherein...

[0062] The furnace heating temperature is 1150℃~1280℃; and / or,

[0063] The rolling process includes finishing rolling, wherein the finishing rolling temperature is 870℃~920℃; and / or,

[0064] The winding temperature is 550℃~620℃; and / or,

[0065] The reduction rate of the cold rolling is 50% to 60%.

[0066] In this embodiment, the furnace heating temperature is limited to ensure complete dissolution of the nitrogen carbides, thereby obtaining the desired steel strength and elongation. The finishing rolling end temperature is limited to prevent the formation of coarse ferrite during hot rolling, which could affect subsequent elongation. The coiling temperature is limited to ensure the smooth progress of subsequent cold rolling processes and to ensure uniform microstructure at the beginning and end of the hot-rolled sheet and along its width, thus avoiding performance fluctuations in subsequent cold-rolled sheets. The total cold rolling reduction is limited to ensure the smooth progress of the cold rolling process without achieving the target steel thickness. For example, the furnace heating temperature can be 1150℃, 1170℃, 1190℃, 1210℃, 1230℃, 1250℃, 1270℃, 1280℃, etc., the finishing rolling temperature can be 870℃, 880℃, 890℃, 900℃, 910℃, 920℃, etc., the coiling temperature can be 550℃, 560℃, 570℃, 580℃, 590℃, 600℃, 610℃, 620℃, etc., and the cold rolling reduction rate can be 50%, 52%, 545%, 56%, 58%, 60%, etc.

[0067] Before step S1, a converter smelting process is included to obtain molten steel. The final temperature of the converter is 1650-1670℃. During the tapping process, 200-800 kg of lime, 0-1000 kg of pre-melted slag, and 0-400 kg of fluorite are added to the molten steel to prevent oxidation. The slag runoff from the converter is ≤80 mm, and the tapping time is 4-9 minutes. The molten steel is then continuously cast to obtain the aforementioned slab.

[0068] S2. The cold-hardened coil is subjected to heat treatment including preheating, heating, homogenization and cooling in sequence, and then aged to obtain phase transformation induced plasticity steel; wherein the final temperature of the heating is 790℃~830℃.

[0069] In some embodiments, the final preheating temperature is 210°C to 230°C, and the preheating rate is 8°C.

[0070] / s~12℃ / s.

[0071] In this embodiment, preheating causes the cold-deformed ferrite generated during cold rolling to recover. Heating during heat treatment achieves recrystallization of the cold-rolled ferrite structure, with pearlite first transforming into austenite and then growing into ferrite. For example, the endpoint temperature of the heating can be 790°C, 800°C, 810°C, 820°C, 830°C, etc., the endpoint temperature of the preheating can be 210°C, 215°C, 220°C, 225°C, 230°C, etc., and the preheating rate can be 8°C / s, 9°C / s, 10°C / s, 11°C / s, 12°C / s, etc.

[0072] In some embodiments, the heat equalization process parameters include: a temperature of 790°C to 830°C and a time of 60s to 150s.

[0073] In this embodiment, homogenization can achieve full or partial austenitization, resulting in a greater amount of austenite. Simultaneously, it effectively controls austenite grain size and improves porosity. If the homogenization temperature is too high or the holding time is too long, it can lead to coarse austenite grains, affecting the grain size of the subsequent microstructure and deteriorating the steel's properties. Conversely, if the homogenization temperature is too low or the holding time is too short, it can result in an uneven initial microstructure, also affecting the subsequent steel properties. For example, the homogenization temperature can be 790℃, 800℃, 810℃, 820℃, 830℃, etc., and the homogenization time can be 60s, 70s, 80s, 90s, 100s, 110s, 120s, 130s, 140s, 150s, etc.

[0074] In some embodiments, the cooling includes: a first cooling and a second cooling; wherein,

[0075] The first cooling rate is 2℃ / s to 6℃ / s, and the first cooling end temperature is 720℃ to 770℃; the second cooling rate is 30℃ / s to 40℃ / s, and the second cooling end temperature is 380℃ to 430℃; and / or,

[0076] The aging process parameters include: temperature of 380℃~430℃ and time of 60s~120s.

[0077] In this embodiment, the first cooling process causes a portion of the austenite to transform into ferrite, with elements such as C and Mn accumulating within the austenite. The second cooling process causes a portion of the austenite to transform into the bainite matrix phase, providing the steel's strength. If the second cooling rate is too fast, it achieves excessively high strength but degrades elongation; conversely, if it is too slow, the required hard bainite content is not obtained, failing to meet the strength requirements of the transformation-induced ductility steel of this embodiment. For example, the first cooling rate can be 2℃ / s, 3℃ / s, 4℃ / s, 5℃ / s, 6℃ / s, etc., and the first cooling end temperature can be 720℃, 730℃, 740℃, 750℃, 760℃, 770℃, etc. The second cooling rate can be 30℃ / s, 33℃ / s, 35℃ / s, 38℃ / s, 40℃ / s, etc., and the second cooling end temperature can be 380℃, 390℃, 400℃, 410℃, 420℃, 430℃, etc. Aging treatment can yield a certain proportion of bainite and untransformed austenite. Insulation within the bainite transformation zone further promotes the aggregation of elements such as C and Mn into the austenite. If the aging temperature is too low, it will reduce the content of retained austenite and the carbon content within it, resulting in extremely high strength in the transformation-induced ductility steel, but at the cost of poor elongation, poor microstructure uniformity, and reduced porosity. Conversely, if the aging temperature is too high, it will also reduce the content of retained austenite and the carbon content within it, failing to achieve the required hard bainite content and thus the strength required for the transformation-induced ductility steel, resulting in poor microstructure uniformity and low porosity. Furthermore, if the aging time is too long, some carbides will precipitate, affecting and reducing the content of retained austenite and the carbon content within it. If the aging time is too short, elements such as C and Mn cannot further aggregate into the austenite, also reducing the content of retained austenite and the carbon content within it. For example, the aging temperature can be 380℃, 390℃, 400℃, 410℃, 420℃, 430℃, etc., and the aging time can be 60s, 70s, 80s, 90s, 100s, 110s, 120s, etc. After aging, it is cooled to 150℃~160℃ at a cooling rate of 10~20℃ / s; this process may transform some of the unstable austenite into martensite, which is beneficial to improving strength.

[0078] The preparation method of the phase transformation induced ductile steel is based on the above-mentioned phase transformation induced ductile steel. The specific chemical composition of the phase transformation induced ductile steel can be referred to the above embodiments. Since the preparation method of the phase transformation induced ductile steel adopts some or all of the technical solutions of the above embodiments, it has at least all the beneficial effects brought about by the technical solutions of the above embodiments, which will not be elaborated here.

[0079] Based on a general inventive concept, this application provides an automotive part, the raw material of which includes the phase transformation induced plasticity steel described in any embodiment of the first aspect.

[0080] The automotive part is based on the aforementioned phase transformation induced plasticity steel. The specific chemical composition of the phase transformation induced plasticity steel can be referred to in the above embodiments. Since the automotive part adopts some or all of the technical solutions of the above embodiments, it has at least all the beneficial effects brought about by the technical solutions of the above embodiments, which will not be elaborated here.

[0081] 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.

[0082] This application provides a phase transformation induced plasticity steel, the chemical composition of which is shown in Table 1.

[0083] Table 1. Chemical composition (wt%) of phase transformation-induced ductile steel, the remainder being Fe and unavoidable chemical components.

[0084] Example 1 0.12 1.5 1.8 0.008 0.005 0.5 0.003 Example 2 0.14 1.8 1.7 0.005 0.008 0.7 0.0032 Example 3 0.16 2.0 1.5 0.009 0.007 0.9 0.0028 Example 4 0.17 1.0 1.3 0.006 0.009 1 0.0035 Comparative Example 1 0.12 1.5 1.8 0.005 0.007 - 0.0032 Comparative Example 2 0.12 1.5 1.8 0.005 0.007 2 0.0032 Comparative Example 3 0.12 1.5 1.8 0.005 0.007 0.3 0.0032

[0085] Based on the chemical composition of the phase transformation induced ductile steel described above, this application provides a method for preparing phase transformation induced ductile steel. Please refer to Table 2 for the process parameters of cold-rolling preparation of phase transformation induced ductile steel, Table 2 for the process parameters of heat treatment of phase transformation induced ductile steel, and Table 4 for the process parameters of aging of phase transformation induced ductile steel.

[0086] Table 2. Process parameters for cold-rolling phase transformation induced ductility steel

[0087]

[0088] Table 3. Process parameters for heat treatment of phase transformation induced ductility steel

[0089]

[0090]

[0091] Table 4. Process parameters for aging of phase transformation-induced ductile steel

[0092]

[0093] The phase transformation induced plasticity steels provided in Examples 1-4 and Comparative Examples 1-3 of this application were subjected to microstructure analysis and mechanical property testing according to the national standard (GB / T 228.1-2010). The results are shown in Table 5.

[0094] Table 5 Mechanical properties of phase transformation-induced ductility steel

[0095]

[0096]

[0097] As can be seen from Table 5, the phase transformation induced plasticity steel provided in Examples 1-4 of this application has a yield strength of 420-500 MPa, a tensile strength of 800-860 MPa, an elongation of 28-35%, and a work hardening value of 0.18-0.2%, indicating good work hardening ability. Figure 2 This is a metallographic diagram of a phase transformation-induced ductility steel provided according to an embodiment of this application; please refer to [link / reference]. Figure 2 As can be seen, the microstructure of the phase transformation induced ductile steel provided in this application embodiment contains thin film-like residual austenite. Figure 3 Here is a metallographic diagram of a phase transformation-induced ductility steel provided as a comparative example in this application; please refer to [link / reference]. Figure 3 As can be seen, the microstructure of the phase transformation induced ductile steel provided in this application embodiment contains blocky retained austenite. However, the amount of Cu added in Comparative Examples 1-3 is outside the range of this application embodiment, and to some extent, the desired microstructure was not obtained, resulting in poorer mechanical properties compared to Examples 1-4.

[0098] 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 phase transformation induced plasticity steel, characterized in that, The chemical composition of the phase transformation-induced ductile steel is as follows: By mass fraction, the C content is 0.12%~0.17%, the Si content is 1.0%~2.0%, the Cu content is 0.6%~1.0%, the Mn content is 1.3%~1.8%, the P content is ≤0.01%, the S content is ≤0.01%, the N content is ≤0.004%, and the remainder is Fe and unavoidable impurities; The microstructure of the phase transformation induced ductile steel, by volume fraction, comprises 50%–60% ferrite, 30%–40% bainite, and 10%–15% thin-film retained austenite; The phase transformation induced plasticity steel meets the following mechanical properties: yield strength of 420MPa~500MPa, tensile strength of 800MPa~860MPa, elongation of 28%~35%, and work hardening value of 0.18%~0.2%.

2. A method for preparing phase transformation induced ductile steel according to claim 1, characterized in that, The method includes: Obtain cold-hardened rolls; The cold-rolled coil is subjected to heat treatment sequentially, including preheating, heating, homogenization, and cooling, followed by aging. A phase transformation induced plasticity steel is obtained; wherein the final heating temperature is 790℃~830℃; The heat equalization process parameters include: temperature of 790℃~830℃ and time of 60s~150s; The final temperature of the preheating is 210℃~230℃, and the preheating rate is 8℃ / s~12℃ / s.

3. The method of claim 2, wherein, The cooling includes: a first cooling and a second cooling. However; among them, The first cooling rate is 2℃ / s to 6℃ / s, and the end temperature of the first cooling is 720℃ to 770℃. The second cooling rate is 30℃ / s to 40℃ / s, and the end temperature of the second cooling is 380℃ to 430℃. And / or, The aging process parameters include: temperature of 380℃~430℃ and time of 60s~120s.

4. The method of claim 2, wherein, The process of obtaining the cold-hardened roll includes: The slab is heated in a furnace, rolled, and coiled to obtain a hot-rolled coil; The hot-rolled coil is then cold-rolled to obtain a cold-rolled hard coil; wherein... The furnace heating temperature is 1150℃~1280℃; and / or, The rolling process includes finishing rolling, wherein the finishing rolling temperature is 870℃~920℃; and / or, The winding temperature is 550℃~620℃; and / or, The reduction rate of the cold rolling is 50% to 60%.

5. An automotive part, characterized by The raw materials for the automotive parts include the phase transformation induced plasticity steel as described in claim 1.

Images