A scale-free 1800MPa grade cold-rolled QP medium manganese steel and its preparation method

Abstract

The present invention relates to a descaling-free 1800MPa grade cold-rolled QP medium manganese steel, the chemical composition of which is, by mass percentage, the following: C 0.20% to 0.25%, Mn 7% to 9%, Cr 3% to 5%, Si 0.6% to 1%, Al 0.2% to 0.8%, with the remainder being Fe and unavoidable impurity elements. The advantages of the present invention are: under QP process conditions, the blank after heat treatment is directly water quenched to room temperature, then reheated to 170 to 200°C and held for 15 to 20 minutes, without the need for multiple temperature and cooling rate controls; the tensile strength after heat treatment exceeds 1800MPa, and the elongation A50 is greater than 10%; by adding relatively high contents of Cr and Si elements and a small amount of Al element, the high-temperature oxidation resistance of the QP medium manganese steel is improved; no special gas shielding is required during the QP annealing process; a dense, non-detachable oxide layer is formed on the surface of the cold-rolled slab, and no subsequent descaling process is required.

Description

Technical Field

[0001] The present invention relates to a metal material and a manufacturing method thereof, in particular to a scale-free 1800MPa grade cold-rolled QP medium manganese steel and a preparation method thereof. Background Art

[0002] Advanced high-strength steel (AHSS) boasts higher strength and stiffness than traditional steel, enabling lighter structural designs while providing comparable or even greater safety. Consequently, it is widely used in the automotive, aerospace, and construction sectors. In automotive steel research and development, AHSS significantly reduces vehicle weight, thereby lowering energy consumption and improving fuel economy. In the automotive industry, the use of high-strength steel can effectively improve a vehicle's overall fuel efficiency, thereby reducing demand for oil resources and lowering carbon emissions from oil extraction and transportation.

[0003] QP steel, a third-generation advanced high-strength steel, boasts excellent strength, ductility, and weldability, garnering widespread attention and application. However, its relatively complex heat treatment process, particularly during the annealing process for cold-rolled QP steel, requires a protective atmosphere to prevent the formation of scale. Otherwise, descaling is necessary, placing high demands on process conditions and equipment, increasing the cost and difficulty of QP steel development. Furthermore, the demand for higher strength grades for high-strength automotive steels poses challenges to the development of ultra-high-strength QP steel.

[0004] Patent publication number CN 114381655 A proposes a high-strength-ductility cold-rolled QP steel, its annealing process, and manufacturing method. Through rational chemical composition design and a multi-pass annealing process, this steel achieves excellent strength and ductility. However, this QP steel incorporates multiple annealing steps and requires strict cooling rate control to ensure martensite formation, increasing both implementation difficulty and cost.

[0005] Patent publication number CN 115386703 A proposes a process for improving the pretreatment quality of cold-rolled, quenched, ductile steel for automotive coating applications. This method utilizes two-phase annealing and three-stage heating, with specific temperatures and heating rates for each stage, as well as weak nitrogen humidification at specific locations within the furnace. This method addresses the rusting issue commonly encountered in the pretreatment of cold-rolled, quenched, and ductile steel for automotive coating applications. This method requires a three-stage QP heating process and a protective atmosphere, making it a relatively complex process. Summary of the Invention

[0006] The present invention aims to provide a scale-free 1800MPa grade cold-rolled QP medium manganese steel and a preparation method thereof. After QP annealing process treatment, the tensile strength is ≥1800MPa and the elongation A50 is ≥10%. The QP medium manganese steel is designed with high Mn and Cr. During the QP heat treatment process, no multi-stage controlled cooling rate annealing process and atmosphere protection treatment are required. The process is simple and easy to implement.

[0007] To achieve the above object, the present invention is implemented through the following technical solutions:

[0008] The invention discloses a scale-free 1800MPa grade cold-rolled QP medium manganese steel, the chemical composition of which is as follows by mass percentage: C 0.20%-0.25%, Mn 7%-9%, Cr 3%-5%, Si 0.6%-1%, Al 0.2%-0.8%, and the remainder is Fe and unavoidable impurity elements.

[0009] The functions of alloying elements are:

[0010] C: effectively expands the austenite phase region, increases the volume fraction and stability of austenite in medium manganese steel; reduces the A3 point temperature of the steel grade and improves hardenability; excessively high C content will deteriorate welding performance. The carbon content selected in the present invention is in the range of 0.2% to 0.25%.

[0011] Mn: It can expand the austenite phase area, reduce the A3 point temperature of the steel, improve the hardenability of the steel; and increase the volume fraction and stability of the organizational austenite.

[0012] Cr: significantly increases the ductile-brittle transition temperature of medium manganese steel and improves the hardenability of medium manganese steel; significantly improves the steel's antioxidant effect and increases the steel's corrosion resistance; when used in combination with Al and Si elements, it can obtain excellent high-temperature oxidation resistance. The present invention controls the Cr content to 3% to 5%.

[0013] Al: As a ferrite stabilizing element, it can increase the Ms point temperature and effectively improve the high-temperature oxidation resistance of medium manganese steel; the Al content range selected by the present invention is 0.2% to 0.8%, and adding too much Al element to medium manganese steel is not conducive to the increase of austenite content.

[0014] Si: improves the high-temperature oxidation resistance of steel, inhibits the formation of carbides, and is beneficial to increasing the C content in retained austenite. Therefore, the present invention controls the Si content to 0.6% to 1%.

[0015] A method for preparing scale-free 1800MPa grade cold-rolled QP medium manganese steel, comprising smelting, continuous casting, hot rolling, coiling, pickling, cold rolling, and annealing, specifically comprising the following steps:

[0016] S1. The continuous casting billet is placed in a heating furnace and heated to 1100-1250°C, kept at this temperature for 2-3 hours, and then rolled on a rolling mill in multiple passes. The hot rolling finishing temperature is 850-900°C. After rolling, the billet is cooled to coiling temperature of 620-650°C and then air-cooled to room temperature.

[0017] S2, pickling and cold rolling the hot rolled plate with a cold rolling reduction of 50%;

[0018] S3. Heat the cold-rolled sheet in a heating furnace to 770-820°C and keep it warm for 3-5 minutes;

[0019] S4. The blank after heat treatment is directly water quenched to room temperature, and then heated to 170-200°C and kept warm for 15-20 minutes.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] 1. Under the QP process conditions, the billet is directly quenched to room temperature after heat treatment, and then heated to 170-200℃ and kept warm for 15-20 minutes. There is no need to control the temperature and cooling rate multiple times. After heat treatment, the tensile strength exceeds 1800MPa and the elongation A50 is greater than 10%;

[0022] 2. By adding higher contents of Cr, Si and a small amount of Al, the high-temperature oxidation resistance of QP medium-manganese steel is improved. No special gas protection is required during the QP annealing process. A dense and non-detachable oxide layer is formed on the surface of the cold-rolled slab, and no subsequent descaling process is required. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is a flow chart of the warm forming process.

[0024] Figure 2 This is a diagram of the mechanical properties of QP medium manganese steel and the surface state of the tensile specimen.

[0025] Figure 3 This is the microstructure of QP medium manganese steel. DETAILED DESCRIPTION

[0026] The present invention will be described in detail below with reference to the accompanying drawings, but it should be noted that the implementation of the present invention is not limited to the following embodiments.

[0027] The following examples are implemented under the premise of the technical solution of the present invention, and provide detailed implementation methods and specific operating processes, but the scope of protection of the present invention is not limited to the following examples. The methods used in the following examples are conventional methods unless otherwise specified.

[0028] [Example 1]

[0029] A scale-free 1800MPa grade cold-rolled QP medium manganese steel is characterized in that its chemical composition by mass percentage is: C 0.2%, Mn 8%, Cr 3%, Si 0.6%, Al 0.2%, and the remainder is Fe and unavoidable impurity elements.

[0030] A method for preparing scale-free 1800MPa grade cold-rolled QP medium manganese steel, smelting ingots that meet the composition requirements; the ingots are hot-charged and sent to a heating furnace, the temperature of the ingots out of the heating furnace is 1200℃, and the holding time after reaching the temperature in the furnace is 120min; after being taken out of the furnace, they are hot-rolled, the starting rolling temperature is 1000℃, the final rolling temperature is 850℃, and after the final rolling, they are cooled and coiled, the coiling temperature is 650℃, and a 3.2mm thick hot-rolled strip is obtained. The hot-rolled strip is pickled and then cold-rolled, with a deformation of 50% and a rolling time of 1.6mm. The cold-rolled thin strip is subjected to a complete austenitizing temperature treatment, the holding temperature is 780℃, the holding time is 5min, and it is water quenched to room temperature. After quenching, it is kept at 180℃ to complete the partitioning process. Standard tensile specimens are cut on the steel strip and mechanical properties are tested. The tensile specimens and mechanical properties results are shown in the table. Figure 2 The mechanical properties of the manufactured products were tested in accordance with GB / T228.1-2010, "Tensile Tests on Metallic Materials - Part 1: Room-Temperature Test Methods," using a gauge length of 50mm. QP medium-manganese steel boasts a tensile strength exceeding 1800MPa and an A50 elongation greater than 10%. Its microstructure is composed of martensite and retained austenite, achieving an excellent balance between strength and plasticity. Furthermore, by incorporating the high-temperature oxidation-resistant elements Cr, Si, and Al into its composition, the surface oxide layer remains dense and resists shedding even after heat treatment without atmosphere protection.

[0031] The alloy compositions of the substrates of different embodiments are shown in Table 1; the production process parameters of different embodiments are shown in Table 2.

[0032] Table 1 Alloy composition of substrates in different embodiments (wt%)

[0033] C Mn Cr Si Al Example 1 0.22 8 3 0.6 0.2 Example 2 0.23 8 3 0.6 0.2 Example 3 0.25 8 3 0.6 0.2 Example 4 0.25 7 3 0.6 0.2 Example 5 0.25 7 4 0.6 0.2

[0034] Table 2 Production process parameters of different embodiments

[0035]

[0036] Table 1 lists the chemical compositions of the scale-free 1800 MPa grade cold-rolled QP medium manganese steels of Examples 1-5.

[0037] Table 2 lists the specific process parameters of the scale-free 1800MPa grade cold-rolled QP medium manganese steel with high strength and plasticity in Examples 1-5.

[0038] The mechanical properties of manganese steel in different embodiments QP are shown in Table 3.

[0039] Table 3 Mechanical properties of QP medium manganese steel in different embodiments

[0040]

[0041] Table 3 lists the mechanical properties of the scale-free 1800MPa grade cold-rolled QP medium manganese steel and the mechanical properties of the high-strength-ductility cold-rolled QP steel of Examples 1-5.

[0042] Figure 1 This is the heat treatment process route for the scale-free 1800MPa grade cold-rolled QP medium manganese steel with high mechanical properties and high strength and plasticity of cold-rolled QP steel in Example 1.

[0043] Figure 2 This is the engineering stress-strain curve of the scale-free 1800MPa grade cold-rolled QP medium manganese steel with high mechanical properties and high strength and plasticity of cold-rolled QP steel in Example 1.

[0044] Figure 3 The scanning microstructure, EBSD phase diagram and inverse pole figure of the scale-free 1800MPa grade cold-rolled QP medium manganese steel with mechanical properties and high strength and plasticity of the cold-rolled QP steel in Example 1.

[0045] Under the QP process conditions of the present invention, the blank after heat treatment is directly water quenched to room temperature, and then heated to 170-200°C and kept warm for 15-20 minutes, without the need for multiple temperature and cooling rate controls. After heat treatment, the tensile strength exceeds 1800 MPa and the elongation A50 is greater than 10%. By adding relatively high contents of Cr and Si elements and a small amount of Al element, the high-temperature oxidation resistance of the QP medium-manganese steel is improved. No special gas protection is required during the QP annealing process, and a dense, non-detachable oxide layer is formed on the surface of the cold-rolled slab, eliminating the need for a subsequent descaling process.

Claims

1. A scale-free 1800MPa grade cold-rolled QP medium manganese steel, characterized in that: Its chemical composition by mass percentage is: C 0.20% to 0.25%, Mn 7% to 9%, Cr 3% to 5%, Si 0.6% to 1%, Al 0.2% to 0.8%, and the rest is Fe and unavoidable impurity elements; The method for preparing the scale-free 1800MPa grade cold-rolled QP medium manganese steel comprises smelting, continuous casting, hot rolling, coiling, pickling, cold rolling, and annealing, specifically comprising the following steps: S1. The continuous casting billet is placed in a heating furnace and heated to 1100-1250°C, kept at this temperature for 2-3 hours, and then rolled on a rolling mill in multiple passes. The hot rolling finishing temperature is 850-900°C. After rolling, the billet is cooled to coiling temperature of 620-650°C and then air-cooled to room temperature. S2, pickling and cold rolling the hot rolled plate with a cold rolling reduction of 50%; S3. Heat the cold-rolled sheet in a heating furnace to 770-820°C and keep it warm for 3-5 minutes; S4. The blank after heat treatment is directly water quenched to room temperature, and then heated to 170-200°C and kept warm for 15-20 minutes.

2. The method for preparing a scale-free 1800MPa grade cold-rolled QP medium manganese steel according to claim 1, characterized in that: It includes smelting, continuous casting, hot rolling, coiling, pickling, cold rolling and annealing, and specifically includes the following steps: S1. The continuous casting billet is placed in a heating furnace and heated to 1100-1250°C, kept at this temperature for 2-3 hours, and then rolled on a rolling mill in multiple passes. The hot rolling finishing temperature is 850-900°C. After rolling, the billet is cooled to coiling temperature of 620-650°C and then air-cooled to room temperature. S2, pickling and cold rolling the hot rolled plate with a cold rolling reduction of 50%; S3. Heat the cold-rolled sheet in a heating furnace to 770-820°C and keep it warm for 3-5 minutes; S4. The blank after heat treatment is directly water quenched to room temperature, and then heated to 170-200°C and kept warm for 15-20 minutes.

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