A heat treatment method for high-alumina cold-rolled medium-manganese steel with an elongation of 45%

Abstract

This invention relates to a heat treatment method for high-alumina cold-rolled medium-manganese steel with an elongation of 45%. Specifically, the heat treatment method involves improving the performance of the cold-rolled medium-manganese steel by using an Ac1 cyclic holding process, based on the cold rolling (CR) process. First, the material is heated to 800℃ (two-phase region) at a certain rate and held for 5 minutes. Then, the material is rapidly transferred to 680℃ (below Ac1) and held for 20 minutes. This process is repeated multiple times, and finally, the material is water-quenched (WQ) to room temperature. The Ac1 cyclic holding method of this invention yields medium-manganese steel with a tensile strength ≥930MPa, an elongation ≥45%, and a strength-ductility product ≥42GPa%, maintaining high strength while possessing high elongation, demonstrating promising application prospects.

Description

Technical Field

[0001] This invention relates to the field of advanced high-strength steel preparation technology, and in particular to a heat treatment method for high-alumina cold-rolled medium-manganese steel with an elongation of 45%. Background Technology

[0002] With the continuous advancement of science and technology and the booming development of the economy, the requirements for energy conservation, environmental protection and safety of automobiles are constantly increasing, and lightweighting of automobiles has become the theme of the development of the automotive industry today.

[0003] Currently, the performance indicators of the third-generation advanced high-strength steel are being designed, with the goal of combining ultra-high strength and good plasticity. Researchers have made many attempts to design Q&P steel in terms of process path and steel composition, mainly focusing on how to maximize the acquisition of retained austenite by controlling the process and how to compensate for the decrease in the volume fraction of martensite caused by the increase in the volume fraction of retained austenite, which leads to a decrease in the strength of the material matrix. Summary of the Invention

[0004] The key focus of this invention is to improve the overall performance of Q&P steel by using Ac1 cyclic heat preservation, obtaining more and more stable residual austenite without reducing the martensite content, thereby increasing its contribution to the TRIP effect in the steel and maximizing the improvement of the ductility and toughness of medium manganese steel. The goal is to obtain medium manganese steel with high strength, high elongation, high strength-ductility product, and significantly improved overall performance.

[0005] This invention designs a heat treatment method for high-alumina cold-rolled medium-manganese steel with an elongation of 45%. The chemical element mass percentage of the high-alumina cold-rolled medium-manganese steel used is C: 0.09%, Mn: 7.23%, Si: 1.99%, Al: 4.42%, Cu: 0.50%, Mo: 0.20%, Cr: 0.33%, Nb: 0.11%, B: 0.001%, with the balance being Fe and unavoidable impurities.

[0006] The high-alumina cold-rolled medium-manganese steel containing the above element ratios is processed through the following steps:

[0007] (1) The high-alumina cold-rolled medium-manganese steel is quickly placed in a muffle furnace at 800℃ (Above Ac1) and held for 5 minutes;

[0008] (2) Quickly transfer the high-alumina cold-rolled medium-manganese steel from the 800℃ muffle furnace to another muffle furnace at 680℃ (below Ac1), and hold for 20 minutes;

[0009] (3) Treat steps (1) and (2) as a loop, which is performed N times in total;

[0010] (4) Quench the sample after N cycles in water to room temperature.

[0011] The austenite transformation start temperature (Ac1) and austenite transformation end temperature (Ac3) of high-alumina cold-rolled medium-manganese steel without heat treatment were measured using a TA instrument DIL805 thermal expansion phase transformation meter. The results were: Ac1 = 728℃ and Ac3 = 1227℃. Before heat treatment, the high-alumina cold-rolled medium-manganese steel was soaked in cleaning alcohol for a period of time, then cleaned, and the surface was derusted and air-dried to make the surface smooth and avoid uneven heating during heat treatment.

[0012] In steps (1) and (2) of the heat treatment method for high-alumina cold-rolled medium-manganese steel with an elongation of 45%, the upper and lower temperatures of Ac1 are provided by two muffle furnaces respectively. The sample does not undergo any extra treatment in the middle and is directly and quickly transferred between the two heat treatment furnaces.

[0013] In step (4) of the heat treatment method for high-alumina cold-rolled medium-manganese steel with an elongation of 45%, the cooling rate is determined by the static CCT curve (expansion-temperature curve) measured by the thermal expansion instrument. The critical cooling rate of the martensitic phase transformation can be obtained through the static CCT curve.

[0014] The advantages of this invention are as follows: The high-alumina cold-rolled medium-manganese steel obtained by the heat treatment method of this invention has a microstructure of lath martensite, ferrite, and retained austenite, with a tensile strength of over 930 MPa, an elongation of over 45%, and a strength-ductility product ≥ 42 GPa•%, exhibiting excellent cold and hot working properties. The heat treatment method involves cyclic holding at around Ac1 temperature. After multiple cycles, the redistribution of carbon and manganese elements further improves the stability of the retained austenite. Austenite can nucleate at the original austenite grain boundaries, lath bundle boundaries, and lath block boundaries. When the cyclic treatment is repeated 4-5 times, the microstructure is significantly refined, and the grain orientation is improved, which facilitates the TRIP effect of retained austenite with different orientations. It also increases the uniformity of the internal microstructure, lattice distortion, and dislocation density, thereby increasing its chemical stability. In automotive applications, this can reduce weight, improve safety, lower production costs, and reduce energy consumption and emissions. Attached Figure Description

[0015] Figure 1 This is a process flow diagram of the present invention.

[0016] Figure 2 This is a metallographic photograph of Embodiment 1 of the present invention.

[0017] Figure 3 This is a graph showing the mechanical performance test results of Embodiment 1 of the present invention.

[0018] Figure 4 This is a metallographic photograph of Embodiment 2 of the present invention.

[0019] Figure 5 This is a graph showing the mechanical performance test results of Embodiment 2 of the present invention.

[0020] Figure 6 This is a metallographic photograph of Embodiment 3 of the present invention.

[0021] Figure 7 This is a graph showing the mechanical performance test results of Embodiment 3 of the present invention.

[0022] Figure 8 This is a metallographic photograph of Embodiment 4 of the present invention.

[0023] Figure 9 This is a graph showing the mechanical performance test results of Embodiment 4 of the present invention.

[0024] Figure 10 This is a metallographic photograph of Embodiment 5 of the present invention.

[0025] Figure 11 This is a graph showing the mechanical performance test results of Embodiment 5 of the present invention. Detailed Implementation

[0026] The specific implementation methods are described in detail below with reference to the accompanying drawings and embodiments.

[0027] Specific implementation

[0028] In the embodiments of the present invention, the metallographic specimens were photographed under an optical microscope to obtain microstructure images; the tensile specimens were prepared according to the ASTM E8 standard and were subjected to tensile tests at a tensile rate of 1 mm / min at room temperature using a WDW-100E electronic universal testing machine. The tensile strength, elongation after fracture, and strength-ductility product of each specimen were obtained by testing and calculation.

[0029] Furthermore, this invention relates to some technical terms related to the heat treatment of iron-carbon alloys. To facilitate understanding of this invention by those skilled in the art, the relevant terms are explained below. However, the content of these explanations does not necessarily constitute common knowledge in the field. Specifically, these include:

[0030] The term "martensite" refers to a supersaturated solid solution formed by carbon dissolving in α-Fe, which is formed after quenching austenite. The morphology of martensite after quenching varies depending on the carbon content in the austenite. Generally, when the carbon content in the austenite is ≤0.25%, lath-shaped martensite is formed after quenching; when the carbon content exceeds this, bamboo leaf or lenticular martensite is formed.

[0031] The term "ferrite" refers to an interstitial solid solution formed by carbon dissolving in α-Fe, which has a body-centered cubic cell structure.

[0032] The term "austenite" refers to an interstitial solid solution formed by carbon dissolving in γ-Fe, which has a face-centered cubic cell structure.

[0033] The term "Ac1," or "austenite transformation onset temperature," refers to the temperature at which ferrite begins to transform into austenite upon heating. Above this temperature, ferrite and austenite coexist in the steel, and the complete transformation from ferrite to austenite requires continuously increasing the temperature.

[0034] The term "Ac3," or "austenite transformation end temperature," refers to the temperature at which the transformation of ferrite to austenite ends during heating. Above this temperature, the transformation of ferrite to austenite in steel ends, and all ferrite is transformed into austenite.

[0035] Figure 1 The heat treatment process diagram of the present invention is shown in the figure. The high-alumina medium-manganese steel plate is held at a temperature above Ac1 (800°C) for 5 minutes and below Ac1 (680°C) for 20 minutes, which is considered as one cycle. This cycle is performed N times (N=0, 1, 2, 3, 4, 5), and then water quenched (WQ) to room temperature. Specific Implementation Example 1

[0037] The chemical composition of high-aluminum cold-rolled manganese steel by mass percentage is C: 0.09%, Mn: 7.23%, Si: 1.99%, Al: 4.42%, Cu: 0.50%, Mo: 0.20%, Cr: 0.33%, Nb: 0.11%, B: 0.001%, with the balance being Fe and unavoidable impurities;

[0038] (1) The high-alumina cold-rolled medium-manganese steel is quickly placed in a muffle furnace at 800℃ (Above Ac1) and held for 5 minutes;

[0039] (2) Quickly transfer the high-alumina cold-rolled medium-manganese steel from the 800℃ muffle furnace to another muffle furnace at 680℃ (below Ac1), and hold for 20 minutes;

[0040] (3) Repeat steps (1) and (2) once. After the temperature is maintained at 680℃, quench the sample in water to room temperature.

[0041] The microstructure of the high-alumina cold-rolled medium-manganese steel prepared in this embodiment under an optical microscope is as follows: Figure 2 As shown, the stress-strain curve is as follows: Figure 3 As shown, the microstructure consists of martensite, ferrite, and retained austenite. The elongation was measured to be 29%, the tensile strength to be 948 MPa, and the strength-ductility product to be 28 GPa. Specific Implementation Example 2

[0043] The chemical composition of high-aluminum cold-rolled manganese steel by mass percentage is C: 0.09%, Mn: 7.23%, Si: 1.99%, Al: 4.42%, Cu: 0.50%, Mo: 0.20%, Cr: 0.33%, Nb: 0.11%, B: 0.001%, with the balance being Fe and unavoidable impurities;

[0044] (1) The high-alumina cold-rolled medium-manganese steel is quickly placed in a muffle furnace at 800℃ (Above Ac1) and held for 5 minutes;

[0045] (2) Quickly transfer the high-alumina cold-rolled medium-manganese steel from the 800℃ muffle furnace to another muffle furnace at 680℃ (below Ac1), and hold for 20 minutes;

[0046] (3) Repeat steps (1) and (2) twice. After the second heat preservation at 680℃, quench the sample in water to room temperature.

[0047] The microstructure of the high-alumina cold-rolled medium-manganese steel prepared in this embodiment under an optical microscope is as follows: Figure 4 As shown, the stress-strain curve is as follows: Figure 5 As shown, the microstructure consists of martensite, ferrite, and retained austenite. The elongation was measured to be 39%, the tensile strength to be 996 MPa, and the strength-ductility product to be 39 GPa. Specific Implementation Example 3

[0049] The chemical composition of high-aluminum cold-rolled manganese steel by mass percentage is C: 0.09%, Mn: 7.23%, Si: 1.99%, Al: 4.42%, Cu: 0.50%, Mo: 0.20%, Cr: 0.33%, Nb: 0.11%, B: 0.001%, with the balance being Fe and unavoidable impurities;

[0050] (1) The high-alumina cold-rolled medium-manganese steel is quickly placed in a muffle furnace at 800℃ (Above Ac1) and held for 5 minutes;

[0051] (2) Quickly transfer the high-alumina cold-rolled medium-manganese steel from the 800℃ muffle furnace to another muffle furnace at 680℃ (below Ac1), and hold for 20 minutes;

[0052] (3) Repeat steps (1) and (2) three times. After the third heat treatment at 680℃, quench the sample in water to room temperature.

[0053] The microstructure of the high-alumina cold-rolled medium-manganese steel prepared in this embodiment under an optical microscope is as follows: Figure 6 As shown, the stress-strain curve is as follows: Figure 7 As shown, the microstructure consists of martensite, ferrite, and retained austenite. The elongation was measured to be 41%, the tensile strength to be 946 MPa, and the strength-ductility product to be 39 GPa. Specific Implementation Example 4

[0055] The chemical composition of high-aluminum cold-rolled manganese steel by mass percentage is C: 0.09%, Mn: 7.23%, Si: 1.99%, Al: 4.42%, Cu: 0.50%, Mo: 0.20%, Cr: 0.33%, Nb: 0.11%, B: 0.001%, with the balance being Fe and unavoidable impurities;

[0056] (1) The high-alumina cold-rolled medium-manganese steel is quickly placed in a muffle furnace at 800℃ (Above Ac1) and held for 5 minutes;

[0057] (2) Quickly transfer the high-alumina cold-rolled medium-manganese steel from the 800℃ muffle furnace to another muffle furnace at 680℃ (below Ac1), and hold for 20 minutes;

[0058] (3) Repeat steps (1) and (2) four times. After the fourth heat treatment at 680℃, quench the sample in water to room temperature.

[0059] The microstructure of the high-alumina cold-rolled medium-manganese steel prepared in this embodiment under an optical microscope is as follows: Figure 8 As shown, the stress-strain curve is as follows: Figure 9 As shown, the microstructure consists of martensite, ferrite, and retained austenite. The elongation was measured to be 41%, the tensile strength to be 972 MPa, and the strength-ductility product to be 40 GPa. Specific Implementation Example 5

[0061] The chemical composition of high-aluminum cold-rolled manganese steel by mass percentage is C: 0.09%, Mn: 7.23%, Si: 1.99%, Al: 4.42%, Cu: 0.50%, Mo: 0.20%, Cr: 0.33%, Nb: 0.11%, B: 0.001%, with the balance being Fe and unavoidable impurities;

[0062] (1) The high-alumina cold-rolled medium-manganese steel is quickly placed in a muffle furnace at 800℃ (Above Ac1) and held for 5 minutes;

[0063] (2) Quickly transfer the high-alumina cold-rolled medium-manganese steel from the 800℃ muffle furnace to another muffle furnace at 680℃ (below Ac1), and hold for 20 minutes;

[0064] (3) Repeat steps (1) and (2) five times. After the fifth heat treatment at 680℃, quench the sample in water to room temperature.

[0065] The microstructure of the high-alumina cold-rolled medium-manganese steel prepared in this embodiment under an optical microscope is as follows: Figure 10 As shown, the stress-strain curve is as follows: Figure 11As shown, the microstructure consists of martensite, ferrite, and retained austenite. The elongation was measured to be 45%, the tensile strength to be 931 MPa, and the strength-ductility product to be 42 GPa.

Claims

1. A heat treatment method of a high-aluminum cold-rolled medium-manganese steel having an elongation of 45%, characterized by, The high-aluminum cold-rolled medium manganese steel has the following chemical composition by mass percentage: C: 0.09%, Mn: 7.23%, Si: 1.99%, Al: 4.42%, Cu: 0.50%, Mo: 0.20%, Cr: 0.33%, Nb: 0.11%, B: 0.001%, and the balance of Fe and inevitable impurities; The high-aluminum cold-rolled medium manganese steel containing the above element ratio is treated by the following steps: (1) The high-aluminum cold-rolled medium manganese steel is quickly placed in a muffle furnace at 800 DEG C for 5 min; (2) The high-aluminum cold-rolled medium manganese steel in the 800 DEG C muffle furnace is quickly transferred to another muffle furnace at 680 DEG C for 20 min; (3) Steps (1) and (2) are regarded as a cycle, and the cycle is performed for 5 times in total; (4) The sample after 5 cycles is water quenched to room temperature.

Images