A high-strength and high-plasticity quenching-partitioning steel based on Mn chemical inhomogeneity and its preparation method

Abstract

The present invention relates to a high-strength and high-plastic quenching-partitioning steel based on Mn chemical inhomogeneity and a preparation method thereof, which belongs to the field of third-generation advanced high-strength steel; its chemical composition mass percentage is: C: 0.2-0.4%, Mn: 2-5%, Si: 1-3%, P: ≤ 0.005%, S: ≤ 0.005%, and the balance is Fe and unavoidable impurities. By optimizing process parameters such as critical annealing temperature and time, it is ensured that a reasonable volume fraction of poor / rich Mn zone and Mn content are obtained; and by utilizing rapid heating and extremely short austenitization and heat preservation, the diffusion of Mn element is suppressed, the uneven distribution of Mn is retained, and a high-temperature austenite consisting of polygonal Mn-poor austenite and polygonal Mn-rich austenite is formed. Due to the uneven distribution of Mn elements, the martensitic phase transformation temperature range is increased, and martensitic lath refinement is achieved using a lower quenching temperature. The present invention achieves martensitic lath refinement while retaining more residual austenite, and the yield strength of the material is significantly improved.

Description

Technical Field

[0001] The present invention belongs to the field of third-generation advanced high-strength steel, and specifically relates to a high-strength and high-plasticity quenching-partitioning steel based on Mn chemical inhomogeneity and a preparation method thereof. Background Art

[0002] Quench-partitioned steel is a third-generation advanced high-strength steel. Its typical room-temperature microstructure is lath martensite and retained austenite. Martensite acts as a matrix to ensure material strength, while retained austenite produces a transformation-induced plasticity (TRIP) effect during deformation, giving the material excellent deformation capability. Ultimately, quench-partitioned steel exhibits an excellent combination of strength and plasticity, attracting widespread attention in the industry.

[0003] At present, there are two main approaches to improve the mechanical properties of quenching-partitioning steel: 1. Refine the martensite laths and increase the yield strength of the material through significant grain refinement; 2. Increase the volume fraction and stability of retained austenite and increase the tensile strength and elongation of the material through the TRIP effect. For example, in order to explore the relationship between the structure and mechanical properties of quenching-partitioning steel under the same quenching temperature conditions, the prior art uses a lower quenching temperature. In this method, due to the partitioning effect of the martensite laths, the martensite laths generated later are finer, and the yield strength of the material is improved. The prior art discloses the use of a step-by-step multiple quenching-partitioning process, which not only refines the martensite and austenite structures, but also makes the carbon element diffuse more fully, increases the stability of the retained austenite, and ultimately obtains mechanical properties with higher tensile strength and improved elongation. The prior art also discloses a heat treatment process for medium-carbon silicon-manganese low-alloy steel based on carbon partitioning and two-step austempering. This method adopts a two-step austempering process. On the one hand, it eliminates the massive untransformed residual austenite, which plays a role in regulating the morphology and refining the structure. On the other hand, the more effective carbon partitioning retains more residual austenite at room temperature, ultimately achieving higher tensile strength and elongation.

[0004] It is worth noting that despite extensive research on quenching-partitioning process design, the following deficiencies remain: 1. In conventional low-alloy quenching-partitioning steels, due to the low content of austenite-stabilizing elements and the high martensite transformation end temperature, higher quenching temperatures are typically used to retain some untransformed austenite upon cooling to the quenching temperature. This results in larger martensite laths and lower yield strength. 2. In conventional low-alloy quenching-partitioning steels, the method of reducing the volume fraction of massive retained austenite is used to increase the stability of retained austenite. Although this method can avoid stress concentration in the early stages of deformation and improve the overall elongation of the material, it also reduces the uniform elongation of the material. Summary of the Invention

[0005] Technical issues to be solved:

[0006] To overcome the shortcomings of the prior art, the present invention provides a high-strength, high-plasticity quench-partitioned steel based on Mn chemical heterogeneity and a preparation method thereof. The steel aims to achieve martensite lath refinement through Mn elemental partitioning, increase the volume fraction and stability of massive retained austenite, and ultimately improve the mechanical properties of the quench-partitioned steel. The present invention utilizes the uneven distribution of Mn to expand the martensite phase transition temperature range of the quench-partitioned steel, achieving martensite lath refinement, improving the stability of retained austenite, and increasing the volume fraction of massive retained austenite. This is of great significance for expanding the microstructural design and mechanical property control window of the quench-partitioned steel and broadening the industrial application prospects of the quench-partitioned steel.

[0007] The technical solution of the present invention is: a high-strength and high-plastic quenching-partitioning steel based on Mn chemical inhomogeneity, whose chemical composition mass percentage is: C: 0.2-0.4%, Mn: 2-5%, Si: 1-3%, P: ≤0.005%, S: ≤0.005%, and the balance is Fe and inevitable impurities.

[0008] A preparation method of high-strength and high-plasticity quenching-partitioning steel based on Mn chemical inhomogeneity, comprising the following steps:

[0009] Step 1: Smelting to obtain a cast ingot according to the chemical composition mass percentage of claim 1, and pre-treating the cast ingot to form a hot-rolled martensitic steel plate;

[0010] Step 2: The hot-rolled martensitic steel plate is heated to the two-phase region temperature and kept at this temperature for a long time to make the Mn element unevenly distributed in the ferrite and austenite phases, and then water quenched to room temperature;

[0011] Step 3: Rapidly heat the critical annealed steel plate to A C3 Above temperature and briefly hold the temperature, the uneven distribution of Mn is inherited to the high temperature austenite through reverse austenite transformation;

[0012] Step 4: Cool the austenitic single-phase steel plate to a temperature between the martensitic transformation start temperature and the martensitic transformation end temperature, then heat it to the partitioning temperature and hold it for a period of time, and finally cool it to room temperature to obtain a quenching-partitioning steel with uneven Mn distribution.

[0013] A further technical solution of the present invention is: the pretreatment method in step 1 is to keep the smelted ingot at 1200°C for 2 hours for homogenization treatment, then forge it into a 40-60 mm thick ingot at 1100°C, keep the ingot at 1150°C for 2 hours, and then hot roll it to 10 mm, wherein the rolling termination temperature is not lower than 850-900°C, and finally cool it to room temperature to form a hot-rolled martensitic steel plate.

[0014] A further technical solution of the present invention is: in step 2, the insulation temperature of the two-phase region is 650-760° C., and the insulation time is 6-40 hours.

[0015] A further technical solution of the present invention is: in step 3, the heating rate is ≥30°C / s, the heating temperature is 850-950°C, and the holding time is 1-30s.

[0016] A further technical solution of the present invention is: in step 4, the quenching temperature is 20-280° C., the holding temperature is 200-500° C., and the holding time is 100-1000s.

[0017] The invention discloses an application of high-strength and high-plasticity quenching-partitioning steel based on Mn chemical inhomogeneity as high-strength automobile steel, with a yield strength of 1300-1500 MPa and an elongation of 12% to 16.7%.

[0018] Beneficial effects

[0019] The beneficial effects of the present invention are as follows: the present invention controls the proportion of the Mn-poor / rich regions and the Mn content by optimizing the critical annealing process parameters, retains the uneven distribution of Mn in the initial structure to the high-temperature austenite by rapid heating and short-time austenitization, and utilizes the uneven distribution of Mn elements to achieve the control of the martensitic phase transformation process, while refining the martensitic laths and increasing the volume fraction of massive retained austenite. First, in step (2), the process parameters such as the critical annealing temperature and time are optimized, and the ratio of ferrite to austenite and the partitioning behavior of Mn between the two phases during the critical annealing process are regulated to ensure that a reasonable volume fraction of the Mn-poor / rich regions and Mn content are obtained. Subsequently, in step (3), the diffusion of the Mn element is suppressed by rapid heating and extremely short-time austenitization and heat preservation, and the uneven distribution of Mn is retained, forming a high-temperature austenite composed of polygonal Mn-poor austenite and polygonal Mn-rich austenite. In step (4), due to the uneven distribution of Mn elements, the martensite phase transformation temperature range is increased, and the martensite laths are refined by using a lower quenching temperature. On the other hand, the austenite is more stable due to the enrichment of Mn. Only a small part of the retained austenite is transformed into martensite at the quenching temperature. Subsequently, after carbon partitioning, a C-rich and Mn-rich blocky retained austenite is formed at room temperature. The specific advantages are as follows:

[0020] (1) The martensite lath is refined while retaining more retained austenite, and the yield strength of the material is significantly improved. C1 With A C3Long-term heat preservation between the two phases allows Mn to partition from ferrite to austenite, taking advantage of the different chemical potentials of Mn in ferrite and austenite. This results in a heterogeneous structure composed of Mn-poor ferrite and Mn-rich austenite. Subsequently, rapid heating and a short austenitization period form a high-temperature austenite composed of polygonal Mn-poor austenite and polygonal Mn-rich austenite. During quenching, martensitic transformation occurs in stages. The Mn-poor austenite, with its lower stability, undergoes phase transformation first. However, due to the obstruction of the Mn-rich austenite, the growth of its martensite laths is suppressed, resulting in the refinement of the martensite laths. Furthermore, the lower quenching temperature further refines the structure. Ultimately, an excellent yield strength of 1300-1500 MPa is achieved at room temperature.

[0021] (2) While ensuring the stability of retained austenite, the volume fraction of massive retained austenite is increased, and the uniform elongation of the material is improved. Because the Mn-poor region undergoes martensite transformation first, resulting in volume expansion, compressive stress is generated on the surrounding Mn-rich austenite. At the same time, the high Mn content of the austenite in the Mn-rich region enhances its stability. At the quenching temperature, only a small portion of the Mn-rich austenite transforms into martensite. Subsequently, after carbon partitioning, most of the Mn-rich austenite remains as massive retained austenite at room temperature. Ultimately, excellent mechanical properties with a uniform elongation of 12% to 16.7% are achieved at room temperature. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is a heat treatment process diagram of high-strength and high-plasticity quenching-partitioning steel based on Mn chemical inhomogeneity of the present invention.

[0023] Figure 2 The two-phase phase diagram obtained by backscattered electron diffraction (EBSD) of Example 1 and the measurement results of the manganese content in the lined area in the phase diagram by scanning electron microscope energy dispersive spectroscopy (SEM-EDS) are shown.

[0024] Figure 3 The two-phase phase diagram of conventional quenching-partitioning steel 1 was obtained by backscattered electron diffraction (EBSD), and the manganese content in the lined area of ​​the phase diagram was determined by scanning electron microscope energy dispersive spectroscopy (SEM-EDS).

[0025] Figure 4 This is a SEM image of the microstructure of Example 1.

[0026] Figure 5 This is a SEM image of the microstructure of conventional quenching and partitioning steel 1.

[0027] Figure 6 1 is the room temperature tensile engineering stress-strain curve of Example 1 and conventional quenching-partitioning steel 1.

[0028] Figure 72 are room temperature tensile engineering stress-strain curves of Example 2 and conventional quenching-partitioning steel 2. DETAILED DESCRIPTION

[0029] The embodiments described below with reference to the accompanying drawings are exemplary and are intended to explain the present invention, but should not be construed as limiting the present invention.

[0030] In response to the problems existing in the prior art such as large martensite laths resulting in low material yield strength; or the method of reducing the volume fraction of blocky retained austenite to increase the stability of retained austenite, which leads to a decrease in the uniform elongation of the material while improving the total elongation of the material, the present invention designs a high-strength and high-plasticity quenching-partitioning steel based on Mn chemical inhomogeneity and a preparation method thereof, which can achieve the refinement of martensite laths while retaining more retained austenite, and significantly improve the yield strength of the material.

[0031] This embodiment provides a high-strength and high-plastic quenching-partitioning steel based on Mn chemical inhomogeneity, and its chemical composition by mass percentage is: C: 0.2-0.4%, Mn: 2-5%, Si: 1-3%, P: ≤ 0.005%, S: ≤ 0.005%, and the balance is Fe and unavoidable impurities.

[0032] This embodiment provides a method for preparing high-strength and high-plasticity quenching-partitioning steel based on Mn chemical inhomogeneity, comprising the following steps:

[0033] (1) Smelting is performed according to the mass percentage of the chemical composition to obtain a casting billet, and the billet is kept at 1200°C for 2 hours for homogenization treatment, and then forged at 1100°C into a casting billet with a thickness of 40-60 mm, and the billet is kept at 1150°C for 2 hours, and then hot rolled to 10 mm, wherein the rolling termination temperature is not lower than 850-900°C, and finally cooled to room temperature to form a hot-rolled martensitic steel plate.

[0034] (2) The hot-rolled steel plate is heated to the two-phase region temperature and kept warm for a long time to make the Mn element unevenly distributed in the ferrite and austenite phases, and then water quenched to room temperature, wherein the two-phase region holding temperature is 650-760℃ and the holding time is 6-40h.

[0035] (3) Rapidly heat the critical annealed steel plate to A C3 The above temperature is briefly held to inherit the uneven distribution of Mn into the high-temperature austenite through reverse austenite transformation, wherein the heating rate is ≥30℃ / s, the heating temperature is 850-950℃, and the holding time is 1-30s.

[0036] (4) The austenitic single-phase steel plate is cooled to a temperature between the start temperature of the martensitic phase transformation and the end temperature of the martensitic phase transformation, then heated to the partitioning temperature and kept warm for a period of time, and finally cooled to room temperature, wherein the quenching temperature is 20-280°C, the holding temperature is 200-500°C, and the holding time is 100-1000s.

[0037] Combine Figure 1 A method for preparing high-strength and high-plasticity quenching-partitioning steel based on Mn chemical inhomogeneity utilizes long-term intercritical annealing in the two-phase region to partition Mn. This inhomogeneity is then transferred to the austenite through a reverse austenite transformation via rapid heating and short-term holding, affecting the subsequent cooling and decomposition of the austenite. Ultimately, a quenching-partitioning steel with an inhomogeneous Mn distribution is obtained at room temperature. This provides a new preparation strategy for a new generation of high-strength and high-plasticity quenching-partitioning steel.

[0038] The present method is further described below with reference to embodiments and accompanying drawings.

[0039] Example 1

[0040] A preparation method of high-strength and high-plasticity quenching-partitioning steel based on Mn chemical inhomogeneity, wherein the alloy composition by mass percentage is: 0.38% C, 2.98% Mn, 1.49% Si, and the rest is Fe.

[0041] The preparation process of the high-strength and high-plasticity quenching-partitioning steel based on Mn chemical inhomogeneity is as follows:

[0042] (1) Smelting the ingot according to the selected chemical composition, holding it at 1200°C for 2 hours for homogenization, then forging it at 1100°C into a 40-60 mm thick ingot, holding it at 1150°C for 2 hours, and then hot rolling it to 10 mm, wherein the rolling termination temperature is not less than 850-900°C, and finally cooling it to room temperature to form a hot-rolled martensitic steel plate;

[0043] (2) The steel plate was sealed in a vacuum quartz tube and heated to 720°C at 1°C / s using a muffle furnace, kept at this temperature for 24 h, and then quenched with water to room temperature;

[0044] (3) Using a thermal expansion instrument, the critically annealed steel plate was heated to 910°C at a rate of 30°C / s and kept at this temperature for 5s;

[0045] (4) The sample was cooled from 910°C to 120°C at a rate of 10°C / s, then heated to 400°C at a rate of 20°C / s and kept at that temperature for 300s, and finally cooled to room temperature at a rate of 10°C / s; a quenching-partitioning steel with an uneven Mn distribution was obtained.

[0046] Tissue: The final room temperature tissue treated in Example 1 is as follows Figure 2As shown, it contains martensite (gray area) and retained austenite (white area), wherein the volume fraction of martensite is 73%, the volume fraction of retained austenite is 27%, and the Mn element shows an obviously uneven distribution in Example 1. Figure 4 As shown in FIG, the retained austenite in Example 1 is mainly in block form. The room temperature structure of conventional quenching-partitioning steel 1 is as follows Figure 3 As shown in FIG1 , it contains martensite (gray area) and retained austenite (white area), wherein the volume fraction of martensite is 75%, the volume fraction of retained austenite is 25%, and the Mn element is uniformly distributed in the conventional quenching-partitioning steel 1. Figure 5 As shown in the figure, the retained austenite of conventional quenching-partitioning steel 1 is mainly in the form of a film.

[0047] Mechanical properties test results (such as Figure 6 As shown in the figure, the conventional quenching-partitioning steel 1 having the same nominal composition and similar retained austenite volume fraction as Example 1 has a yield strength of 1097 MPa, a tensile strength of 1614 MPa, and a uniform elongation of 14.5%. In Example 1, the yield strength of the quenching-partitioning steel with uneven Mn distribution is increased to 1395 MPa, the tensile strength is increased to 1665 MPa, and the uniform elongation is increased to 16.4%.

[0048] Example 2

[0049] A preparation method of high-strength and high-plasticity quenching-partitioning steel based on Mn chemical inhomogeneity, wherein the alloy composition by mass percentage is: 0.38% C, 2.98% Mn, 1.49% Si, and the rest is Fe.

[0050] The preparation process of the high-strength and high-plasticity quenching-partitioning steel based on Mn chemical inhomogeneity is as follows:

[0051] (1) Smelting the ingot according to the selected chemical composition, holding it at 1200°C for 2 hours for homogenization, then forging it at 1100°C into a 40-60 mm thick ingot, holding it at 1150°C for 2 hours, and then hot rolling it to 10 mm, wherein the rolling termination temperature is not less than 850-900°C, and finally cooling it to room temperature to form a hot-rolled martensitic steel plate;

[0052] (2) The steel plate was sealed in a vacuum quartz tube and heated to 720°C at 1°C / s using a muffle furnace, kept at this temperature for 24 h, and then quenched with water to room temperature;

[0053] (3) Using a thermal expansion instrument, the critically annealed steel plate was heated to 910°C at a rate of 30°C / s and kept at this temperature for 5s;

[0054] (4) The sample was cooled from 910°C to 100°C at a rate of 10°C / s, then heated to 400°C at a rate of 20°C / s and kept at that temperature for 300s, and finally cooled to room temperature at a rate of 10°C / s; a quenching and partitioning steel with an uneven Mn distribution was obtained.

[0055] Mechanical properties test results (such as Figure 7 (As shown): Conventional quenching and partitioning steel 2 with the same nominal composition and similar retained austenite volume fraction as Example 2 has a yield strength of 1175 MPa, a tensile strength of 1541 MPa, and a uniform elongation of 12.0%; while in Example 2, the yield strength of the quenching and partitioning steel with uneven Mn distribution is increased to 1446 MPa, the tensile strength is increased to 1663 MPa, and the uniform elongation is increased to 12.6%.

[0056] In this embodiment, the high-strength and high-plasticity quenching-partitioning steel based on Mn chemical inhomogeneity as claimed in claim 1 is used as high-strength automotive steel, and its yield strength is 1300-1500 MPa and its elongation is 12% to 16.7%.

[0057] Although the embodiments of the present invention have been shown and described above, it will be understood that the above embodiments are illustrative and are not to be construed as limitations on the present invention. A person skilled in the art may change, modify, replace and modify the above embodiments within the scope of the present invention without departing from the principles and purpose of the present invention.

Claims

1. A high-strength and high-plasticity quenching-partitioning steel based on Mn chemical inhomogeneity, characterized by: Its chemical composition by mass percentage is: C: 0.2-0.4%, Mn: 2-5%, Si: 1-3%, P: ≤0.005%, S: ≤0.005%, the balance is Fe and unavoidable impurities; The method for preparing high-strength and high-plasticity quenching-partitioning steel based on Mn chemical inhomogeneity comprises the following specific steps: Step 1: Smelting to obtain a cast ingot according to the mass percentage of the chemical components, and pre-treating the cast ingot to form a hot-rolled martensitic steel plate; Step 2: The hot-rolled martensitic steel plate is heated to the two-phase region temperature and held at this temperature for a long time to make the Mn element unevenly distributed in the ferrite and austenite phases, and then water quenched to room temperature; the two-phase region holding temperature is 650-760 ° C, and the holding time is 6-40 hours; Step 3: Rapidly heat the critical annealed steel plate to A C3 The above temperature is briefly held to transfer the uneven distribution of Mn to the high-temperature austenite through reverse austenite transformation; wherein the heating rate is ≥30 °C / s, the heating temperature is 850-950 °C, and the holding time is 1-30 s; Step 4: Cool the austenitic single-phase steel plate to a temperature between the martensitic transformation start temperature and the martensitic transformation end temperature, then heat it to the partitioning temperature and hold it for a period of time, wherein the quenching temperature is 20-280°C, the holding temperature is 200-500°C, and the holding time is 100-1000s; finally, cool it to room temperature to obtain a quenched-partitioned steel with uneven Mn distribution.

2. The method for preparing high-strength and high-plasticity quenching-partitioning steel based on Mn chemical inhomogeneity according to claim 1, characterized in that: The pretreatment method in step 1 is to keep the smelted ingot at 1200°C for 2 hours for homogenization treatment, then forge it into a 40-60 mm thick ingot at 1100°C, keep the ingot at 1150°C for 2 hours, and then hot roll it to 10 mm, wherein the rolling termination temperature is not lower than 850-900°C, and finally cool it to room temperature to form a hot-rolled martensitic steel plate.

3. Use of the high-strength and high-plasticity quenching-partitioning steel based on Mn chemical inhomogeneity according to claim 1 as high-strength automotive steel, characterized in that: Its yield strength is 1300-1500 MPa and its elongation is 12% to 16.7%.

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