CN115537677B - High-strength high-plasticity austenitic high-manganese steel with double-peak structure and production method thereof - Google Patents

Abstract

A kind of high-strength and high-plastic austenitic high-manganese steel with a bimodal structure. Its components and wt% are: C: 0.7~1.2%, Mn: 13.0~21.0%, Cr: 3.0~4.0%, Al: 1.0~1.5 %, Si: 0.05～0.3%, Cu: 0.1～0.5%, Nb: 0～0.2%, Mo: 0～0.5%, Ti: 0～0.3%, S≤0.015%, P≤0.005%; production method: Smelting and pouring into billet; heating the slab; hot rolling; annealing after cooling to room temperature; cold rolling to product thickness at room temperature; reverse phase change annealing; cooling to room temperature. Through reasonable element content and annealing process design, the present invention can greatly increase the austenite content without losing its stability, so that the TRIP and TWIP effects can occur in the structure during the deformation process; and its yield strength is: 750-950MPa. Tensile strength: 1100~1300MPa, elongation after fracture: 40~50%, thereby achieving simultaneous improvement in strength and plastic toughness.

Description

A kind of high-strength and high-plastic austenitic high-manganese steel with bimodal structure and production method

Technical field

The invention relates to a steel for engineering structural materials and a production method, and specifically belongs to a high-strength, high-plastic austenitic high manganese steel with a bimodal structure and a production method.

Background technique

Due to the high manganese content and carbon content of austenitic high manganese steel, the rolled structure of the steel is austenite and carbides. After heating and water toughening treatment at about 1200°C, most of the carbides are solid dissolved in austenite. In the body, the microstructure of steel presents single-phase austenite or austenite plus a small amount of carbides, so the steel has good plasticity and toughness, and the crack growth rate is low.

Bimodal structure refers to a microstructure in which the grain size is not uniform and the grain size distribution exhibits bimodal characteristics. By controlling the size of the grains, small nanocrystals are used to provide reinforcement, and larger nanocrystals or ultra-fine grains provide the ability to store dislocations, thereby achieving simultaneous improvement in strength and toughness.

At present, many strengthening methods cannot take into account strength and plasticity, such as phase change strengthening, work hardening, solid solution strengthening, etc. High manganese steel has undergone deformation processing such as cold forging, cold rolling, etc. Although the strength of high manganese steel has increased due to the deformation of martensitic strengthening phases or the formation of a large number of deformed structures to hinder dislocation movement, the plasticity has significantly decreased.

Searched:

The document of Chinese patent application number CN202010385048.4 discloses "A high-strength and high-strength plastic accumulation automobile steel and its preparation method". The chemical composition is: C: 0.15-0.60wt%, Mn: 3.0-6.0wt%, Ni : 0-2.0wt%, Al: 3.0-6.0wt%, Si: 0.15-2.0wt%, the balance is Fe and inevitable impurities; on this basis, add: Nb: 0.03-0.20wt%; Mo: 0.03-0.20wt%; V: 0.03-0.20wt%; Re: 0.001-0.05wt%, B: 0.001-0.05wt% of one or more micro-alloying elements; hot rolled at a temperature of 1100-1150°C Or forging, reverse phase transformation annealing at a temperature of 650-850°C is performed on forged and hot-rolled materials. This document uses dual-phase zone rolling and annealing to form a multiphase, multilayer, and metastable microstructure. The tensile strength of high manganese steel is 0.7-1.0GPa, and the product of tensile strength and elongation is ≥50GPa%. Hot-rolled high manganese steel has high strength and slightly poor plasticity. After reverse phase transformation annealing, the plasticity has improved. However, due to the high temperature during the rolling process and the annealing treatment, the crystal structure of high manganese steel has deteriorated. The grain refinement effect is not good, and the average grain size is too large, resulting in slightly lower strength (yield strength is 0.5-0.7GPa).

For refined grains, although it can greatly increase the strength, it can also keep the plasticity basically unchanged or decrease slightly. That is, although nanocrystalline and ultrafine-grained materials have high strength and toughness, their work hardening ability And the uniform elongation is significantly reduced. Especially when the average grain size drops below 100nm, the uniform elongation is significantly lower than that of the original material. Many nanocrystalline/ultrafine-grained materials have reached their fracture stress even in the elastic stage of the tensile deformation process, which severely limits Its application as a structural material.

Contents of the invention

The present invention is to overcome the shortcomings of the existing technology and provide a method that controls the grain size through reasonable component ratio and optimized deformation amount and annealing process, and finally obtains a yield strength of 750-950MPa, a tensile strength of 1100-1300MPa, and an elongation after fracture. 40-50% high-strength and high-plastic austenitic high-manganese steel with bimodal structure and production method.

Measures to achieve the above objectives:

A kind of high-strength and high-plastic austenitic high-manganese steel with a bimodal structure. Its components and weight percentages are: C: 0.7~1.2%, Mn: 13.0~21.0%, Cr: 3.0~4.0%, Al: 1.0~ 1.5%, Si: 0.05～0.3%, Cu: 0.1～0.5%, Nb: 0～0.2%, Mo: 0～0.5%, Ti: 0～0.3%, S≤0.015%, P≤0.005%, balance It is Fe and inevitable impurities; its metallographic structure is austenite with a volume ratio of 95 to 100% and martensite with a volume ratio of 5 to 0%; mechanical properties: yield strength: 750 to 950MPa, tensile strength: 1100 ~300MPa, elongation after break: 40~50%.

Preferably: the weight percentage content of C is 0.76-1.15%.

Preferably: the weight percentage content of Mn is 13.6-20.3%.

Preferably: the weight percentage content of Cr is between 3.15% and 3.85%.

A production method of high-strength, high-plasticity austenitic high-manganese steel with a bimodal structure, the steps of which are:

1) The thickness of the slab after smelting and pouring is controlled at 50~80mm;

2) Heating the slab, the heating temperature is controlled at 1100~1200℃, and kept at this temperature for 100~150min;

3) Carry out hot rolling, control the total rolling reduction rate to be no less than 90%, the opening rolling temperature to be no less than 1040°C, and the final rolling temperature to be between 900 and 960°C; control the thickness of the hot rolled plate to be between 5 and 8mm;

4) After natural cooling to room temperature, anneal, control the annealing temperature at 650-700°C, and keep it at this temperature for 10-30 minutes; and control the average grain size to be less than 4 μm;

5) Naturally cool to room temperature again and then cold-roll at room temperature to the product thickness, and control the total reduction rate at 85 to 92%;

6) Carry out reverse phase transformation annealing, heat the cold-rolled plate to 680~820℃, and keep it at this temperature for 80~300s; the metallographic structure after this annealing is: 95~100% austenite by volume. 0~5% martensite;

7) Carry out cooling at a cooling rate of 100 to 130°C/s and water-cool to room temperature.

Preferably: the annealing and heat preservation time after hot rolling is 10 to 23 minutes.

Preferably: the total reduction ratio of cold rolling is 87-92%.

Preferably: the heating temperature of the reverse phase change annealing is 700-800°C, and the holding time is 90-150 s.

The point is: when the thickness of the injection molding is not less than 100mm, the injection molding should be heated to 1100~1200℃ and kept at this temperature for 120~180min; then forged to a thickness of 50~80mm, and then normally cooled to room temperature. Carry out post-processing.

The functions and mechanisms of each raw material and main process in the present invention

C: C is an important solid solution strengthening element in high manganese steel and the most economical and effective strengthening element. If the C content is designed to be too low, good solid solution strengthening effects cannot be obtained; but if the C content is too high, it will cause excessive lattice distortion or the precipitation of large carbides on the grain boundaries, reducing the plasticity of the steel. Therefore, from the perspective of economy and comprehensive performance, the C percentage content control range in the present invention is 0.7-1.2%, and preferably the C content is 0.76-1.15%.

Si: Si plays the role of solid solution strengthening in high manganese steel. Because it can change the solubility of C in austenite, the effect of Si element on the mechanical properties of high manganese steel is relatively complex. The addition of Si element is beneficial to the formation of deformation twins during the deformation process of high manganese steel. However, when the Si content is high, it will affect the surface quality of high manganese steel hot-rolled plates and is not conducive to industrialization. Its content needs to be strictly controlled. Therefore, the Si content range is controlled between 0.05% and 0.3%.

Mn: Mn is the main alloy element in high manganese steel, which has the function of expanding the austenite phase area and stabilizing the austenite structure. When the C content in steel is constant, as the Mn content increases, its structure will gradually transform from pearlite to martensite and further to austenite, prompting the steel to form a single austenite structure at room temperature. In addition, the Mn element can affect the deformation mechanism of steel by affecting the stacking fault energy. As the Mn content increases, the deformation mechanism of austenitic steel will gradually change from the TRIP effect to the TWIP effect. Therefore, the content of Mn is controlled in the range of 13.0 to 21.0%, and preferably the content of Mn is in the range of 13.6 to 20.3%.

P: Since steel contains a large amount of Mn element, it will increase the segregation of P at the grain boundaries and weaken the grain boundaries, so the P content should be reduced as much as possible. Therefore, the P content range should be ≤0.005%.

S: Since steel contains a large amount of Mn element, S easily forms MnS in the steel, causing hot embrittlement, so the lower the S content, the better. Therefore, the content range of S should be ≤0.015%.

Al: The role of Al in high manganese steel is to increase stacking fault energy, inhibit the occurrence of martensitic phase transformation, and facilitate the formation of deformation twins, thereby improving strong plasticity. Studies have shown that when the Mn content is reduced and Al is added, the yield strength of TWIP steel increases but the tensile strength and elongation decrease. And the addition of Al element will make the deformation twins of TWIP steel more uniform after deformation and avoid stress concentration. However, excessive Al content will lead to a reduction in tensile strength and reduce the fluidity of molten steel during the production process, causing the pouring nozzle to be clogged. The Al content needs to be reasonably controlled. Therefore, the Al content range is controlled between 1.0 and 1.5%.

Cr: Cr is a stabilizing element, which is beneficial to improving the stability of room temperature austenite and is a carbide-forming element. When w(Cr):w(C) is greater than 3.5, the carbide consists of a network of (Fe, Cr ) ₃ C is transformed into island-like (Fe, Cr) ₇ C ₃ and (Fe, Cr) ₂₃ C ₆ , which enables high manganese steel to obtain high toughness. In addition, the addition of Cr element can effectively improve the corrosion resistance and oxidation resistance of high manganese steel. Therefore, the Cr content is controlled in the range of 3.0 to 4.0%, and preferably the Cr content is in the range of 3.15 to 3.85%.

Cu: As an austenite stabilizing element, Cu has a high solid solubility in austenite. Scrap steel usually contains Cu element, which reduces the requirements for raw materials. Cu-containing scrap steel can be used for smelting, which expands the scope of the smelting process. The Cu content can adjust the stacking fault energy of the material, thereby adjusting the deformation mechanism of the material and optimizing the strength and toughness of the material. After appropriate annealing process, the strength and toughness ratio of Cu alloyed Fe-C-Mn high manganese steel does not increase. The Fe-C-Mn high manganese steel with Cu has been greatly improved. Therefore, the Cu content range is controlled between 0.1 and 0.5%.

Nb: Nb is a microalloying element that can refine grains, affect phase change kinetics, and promote nucleation. Nb combines with C and N to form fine carbonitrides, which prevents grain growth and dislocation activation, and has the effect of significantly strengthening the matrix. Therefore, the Nb content range is controlled between 0 and 0.2%.

Mo: Mo is a medium-strong carbide forming element. In addition, Mo can further refine the dual-phase structure, improve hardenability and thermal strength properties, and maintain sufficient strength and creep resistance at high temperatures. In tool steel, the red hardness can be improved. Suppress the brittleness of alloy steel due to tempering. Therefore, the Mo content range is controlled between 0 and 0.5%.

Ti: Ti is a strong carbide forming element, which can play the role of precipitation strengthening and fine grain strengthening at the same time, and can significantly improve the tensile strength of steel. Therefore, the Ti content range is controlled between 0 and 0.06%.

The reason why the present invention controls the thickness of the slab between 50 and 80 mm is to achieve the total deformation rate and final product thickness required by the process.

The reason why the present invention controls the heating temperature of the slab at 1120-1200°C and maintains it at this temperature for 100-150 minutes is because there are certain casting or forging defects in the injection or forging billet, and some defects can be effectively eliminated at this temperature. Prepare for hot rolling. And depending on the alloy composition, heating to 1120~1200℃ can soften the material, improve the deformation capacity of the rolling mill, and achieve large reduction hot rolling. The holding time can only ensure that the core temperature of the billet reaches above 1080℃ when the holding time is 120~180min, but the holding time It should not be too long, as too long will lead to coarse grains of the material.

The reason why the present invention controls the total reduction rate of hot rolling to not be less than 90%, the opening rolling temperature to not be less than 1040°C, the final rolling temperature to be between 900 and 960°C, and the thickness of the hot rolled plate to be between 5 and 8mm is because it does not The total reduction rate of less than 90% is due to the effective refinement of grains, and the temperature control is due to the fact that only at this temperature can the material achieve a single large deformation rolling.

The reason why the annealing temperature in the present invention is 650-700°C and kept at this temperature for 10-30 minutes; and the average grain size is controlled to be less than 4 μm is because the residual stress after hot rolling is not conducive to the next step of cold rolling deformation, so annealing is required Processing, controlling the average grain size to less than 4 μm is to strictly control the average grain size throughout the entire process so that it will not be too large. The small average grain size is conducive to the formation of a bimodal structure after the completion of the later processes.

The reason why the present invention controls the total reduction rate of cold rolling between 85% and 92% is because a large total reduction rate can produce extremely strong work hardening and improve the strength of the material. Moreover, due to the large total reduction rate, cold rolling will produce a large number of dislocations, and the dislocations will tangle to form small dislocation cells. The next step of annealing can effectively reduce the average grain size, which is beneficial to the formation of doublets after subsequent annealing. peak organization.

The reason why the present invention controls the heating temperature of the reverse phase change annealed cold-rolled plate is 680-820°C, and maintains it at this temperature for 80-300s; the metallographic structure after annealing is: 95-100% austenite by volume percentage. body, 0 to 5% martensite, because annealing can eliminate the deformation structure after cold deformation, part of the dislocations and slip bands disappear, and part of the martensite reversely transforms into austenite, resulting in a size of 100 to 500 Nano-sized nearly defect-free equiaxed austenite grains (ultra-fine grains). At the same time, the annealing treatment performed within this temperature and time range will cause the original austenite grains to grow, forming austenite grains (fine grains) with a size of 1 to 3 microns and able to accommodate more dislocations, and the plasticity is obtained. At the same time, due to the small average grain size, it has excellent mechanical properties of high hardness and high strength. The martensite that may exist in the structure is produced due to cold deformation, and its content is affected by annealing conditions (temperature and time).

The reason why the present invention controls the cooling rate to be 100-130°C/s for water cooling to room temperature is because the annealing temperature duration must be strictly controlled. If the cooling rate is too slow, the austenite grains of high manganese steel will remain in a high temperature state for a long time. will grow up, the bimodal size effect of the grains in the structure will weaken and become a fine-grained structure with uniform size. However, the comprehensive strength and toughness of ordinary fine-grained high manganese steel is lower than that of bimodal high manganese steel. Therefore, a rapid cooling process is performed after the reverse phase change annealing process.

Compared with existing technology:

1) The present invention regulates the composition of alloys based on the level of stacking fault energy. It ensures the coordinated occurrence of TRIP and TWIP effects through precise elements such as Mn, C, and Cr, and adjusts the annealing process parameters after rolling to regulate the grain distribution size of the structure. and austenite stability.

2) Through reasonable element content and annealing process design, the present invention greatly increases the austenite content without losing its stability, so that the TRIP and TWIP effects of the structure can be coordinated during the deformation process. The austenitic high manganese steel prepared according to the above steps has bimodal structure characteristics, and its mechanical properties are: yield strength: 750~950MPa, tensile strength: 1100~1300MPa, elongation after fracture: 40~50%, thereby achieving strength and plasticity. Toughness is improved at the same time.

3) The present invention adopts a large reduction rolling-annealing process, and the bimodal effect of grain size distribution is significant. Fine grains can increase strength, and relatively coarse grains can accommodate more dislocations and increase the strain hardening rate. , has good strong plastic comprehensive mechanical properties.

Description of the drawings

Figure 1 is an EBSD diagram of high manganese steel of the present invention.

Detailed ways

The present invention is described in detail below:

Table 1 is a list of chemical components of various embodiments and comparative examples of the present invention;

Table 2 is a list of main process parameters for each embodiment of the present invention and comparative examples;

Table 3 is a list of performance testing conditions of various embodiments of the present invention and comparative examples.

Each embodiment of the present invention is produced according to the following steps

1) The thickness of the slab after smelting and pouring is controlled at 50~80mm;

2) Heating the slab, the heating temperature is controlled at 1100~1200℃, and kept at this temperature for 100~150min;

3) Carry out hot rolling, control the total rolling reduction rate to be no less than 90%, the opening rolling temperature to be no less than 1040°C, and the final rolling temperature to be between 900 and 960°C; control the thickness of the hot rolled plate to be between 5 and 8mm;

4) After natural cooling to room temperature, anneal, control the annealing temperature at 650-700°C, and keep it at this temperature for 10-30 minutes; and control the average grain size to be less than 4 μm;

5) Naturally cool to room temperature again and then cold-roll at room temperature to the product thickness, and control the total reduction rate at 85 to 92%;

6) Carry out reverse phase transformation annealing, heat the cold-rolled plate to 680~820℃, and keep it at this temperature for 80~300s; the metallographic structure after this annealing is: 95~100% austenite by volume. 0~5% martensite;

7) Carry out cooling at a cooling rate of 100 to 130°C/s and water-cool to room temperature.

Table 1 List of chemical components (wt%) of various embodiments and comparative examples of the present invention

Table 2 List of main process parameters of various embodiments and comparative examples of the present invention

Continued table 2

Table 3 List of mechanical property test results of various embodiments and comparative examples of the present invention

As can be seen from Table 3, the high manganese steel (Examples 1 to 10) produced according to the production method of the present invention has very high yield strength (762 to 923MPa), tensile strength (1154 to 1288MPa) and elongation after fracture. (42.7~48.5%), showing excellent strong plastic comprehensive mechanical properties.

The above embodiments can effectively control the grain size distribution by reasonably controlling the total reduction rate of cold rolling, annealing temperature and annealing time, especially controlling the total reduction rate of cold rolling at 87-92%, and the heating of reverse phase change annealing. When the temperature is controlled between 700 and 800°C and the holding time is controlled between 90 and 150 s, the bimodal structural characteristics of the obtained high manganese steel are most significant.

This specific implementation mode is only the best example and is not a restrictive implementation of the technical solution of the present invention.

Claims (5)

1. A high-strength and high-plastic austenitic high-manganese steel with a bimodal structure. Its components and weight percentages are: C: 0.7~1.19%, Mn: 13.0~15.84% or Mn: 20.15~21.0%, Cr: 3.0～4.0%, Al: 1.0～1.5%, Si: 0.05～0.23%, Cu: 0.1～0.5%, Nb: 0.11～0.2%, Mo: 0.05～0.5%, Ti: 0.02～0.3%, S≤0.015 %, P≤0.005%, the balance is Fe and inevitable impurities; its metallographic structure is austenite with a volume ratio of 95 to 100%, and martensite with a volume ratio of 5 to 0%; mechanical properties: Yield strength: 750~950MPa, tensile strength: 1100~1300MPa, elongation after break: 40~50%; production method: 1) The thickness of the slab after smelting and pouring is controlled at 50~80mm; 2) Heating the slab, the heating temperature is controlled at 1100~1200℃, and kept at this temperature for 100~150min; 3) Carry out hot rolling, control the total rolling reduction rate to no less than 90%, the opening rolling temperature to be no less than 1040°C, and the final rolling temperature to be between 900 and 960°C; control the thickness of the hot rolled plate to be between 5 and 8mm; 4) After natural cooling to room temperature, anneal, control the annealing temperature at 690-700°C, and keep it at this temperature for 10-30 minutes; and control the average grain size to be less than 4 μm; 5) Naturally cool to room temperature again and then cold-roll at room temperature to the product thickness, and control the total reduction rate at 85 to 92%; 6) Perform reverse phase transformation annealing, heat the cold-rolled plate to 730~820℃, and keep it at this temperature for 80~300s; the metallographic structure after this annealing is: 95~100% austenite by volume, 0~5% martensite; 7) Carry out cooling at a cooling rate of 100~130°C/s and water-cool to room temperature. 2. A method for producing high-strength, high-plasticity austenitic high-manganese steel with a bimodal structure as claimed in claim 1, the steps of which are: 1) The thickness of the slab after smelting and pouring is controlled at 50~80mm; 2) Heating the slab, the heating temperature is controlled at 1100~1200℃, and kept at this temperature for 100~150min; 3) Carry out hot rolling, control the total rolling reduction rate to no less than 90%, the opening rolling temperature to be no less than 1040°C, and the final rolling temperature to be between 900 and 960°C; control the thickness of the hot rolled plate to be between 5 and 8mm; 4) After natural cooling to room temperature, anneal, control the annealing temperature at 690-700°C, and keep it at this temperature for 10-30 minutes; and control the average grain size to be less than 4 μm; 5) Naturally cool to room temperature again and then cold-roll at room temperature to the product thickness, and control the total reduction rate at 85 to 92%; 6) Perform reverse phase transformation annealing, heat the cold-rolled plate to 730~820℃, and keep it at this temperature for 80~300s; the metallographic structure after this annealing is: 95~100% austenite by volume, 0~5% martensite; 7) Carry out cooling at a cooling rate of 100~130°C/s and water-cool to room temperature. 3. A method for producing high-strength, high-plastic austenitic high-manganese steel with a bimodal structure according to claim 2, characterized in that: the annealing and heat preservation time after hot rolling is between 10 and 23 minutes. 4. A method for producing high-strength, high-plastic austenitic high-manganese steel with a bimodal structure according to claim 2, characterized in that: the total rolling reduction rate of cold rolling is 87 to 92%. 5. A method for producing high-strength, high-plastic austenitic high-manganese steel with a bimodal structure according to claim 2, characterized in that: when the thickness of the injection molding is not less than 100mm, the injection molding should be heated to 1100 ~1200℃, and keep it at this temperature for 120~180min; then forge it to a thickness of 50~80mm, and then naturally cool it to room temperature and then carry out the post-process as usual.