High-temperature wear-resistant steel for friction stir welding stirring head and preparation method thereof

Abstract

The present invention discloses a high-temperature wear-resistant steel for a friction stir welding stirring head and a preparation method thereof. The chemical composition of the steel is as follows by weight: C: 0.25% to 0.35%, Mn: 0.15% to 0.8%, Cr: 0.5% to 1.0%, Mo: 0.5% to 1.5%, V: 0.2% to 0.75%, Nb: 0.01% to 0.03%, S: ≤ 0.03%, P: ≤ 0.03%, and the balance is Fe and inevitable impurity elements. The present invention promotes the formation of a large amount of MC-type nanocarbides with excellent thermal stability, extremely small size, and dispersed distribution in the steel matrix by adding Mo, V, and Nb elements (MC-type nanocarbides-forming elements) in appropriate proportions and contents, thereby enabling the prepared high-temperature wear-resistant steel to obtain extremely high high-temperature strength, excellent friction and wear resistance, and very stable structure and performance under high temperature conditions.

Description

Technical Field

[0001] The present invention belongs to the technical field of friction stir welding, and in particular, relates to a high-temperature wear-resistant steel (Nano-HT500) for a friction stir welding stirring head and a preparation method thereof. Background Art

[0002] With the rapid development of my country's transportation, rail vehicle, and aerospace industries, lightweight materials such as magnesium and aluminum alloys are gaining popularity due to their low price, low density, high specific strength, and excellent corrosion resistance. Friction stir welding (FSW) is a new solid-state joining technology that offers advantages such as high quality, energy efficiency, and pollution-free operation. It offers unique advantages for welding low-melting-point alloys such as magnesium and aluminum. The stir head is a key tool in FSW. Its material selection not only affects its service life but also determines FSW's ability to weld thick plates and is crucial for weld quality. During the welding process, the stir head comes into direct contact with the workpiece, subjecting it to temperatures of 600-700°C, strong cyclic loads, and intense high-temperature friction. Under these harsh working conditions, insufficient high-temperature strength, high-temperature wear resistance, and thermal stability can easily lead to severe deformation, increased wear, and microstructural degradation in the stir head, shortening its service life and compromising the quality of the plate weld.

[0003] At present, among the mainstream wear-resistant steels at home and abroad, low-alloy wear-resistant steel has broad application prospects and huge development potential in high-stress and high-wear service scenarios due to its good comprehensive mechanical properties and advantages in price, processing, and forming. The Hardox series of martensitic wear-resistant steels, a typical foreign low-alloy wear-resistant steel, adopts high-temperature quenching and low-temperature tempering to obtain a supersaturated solid solution martensite structure of C and a small amount of residual austenite structure. Therefore, the material can achieve a very high hardness on the basis of good toughness, so that the material obtains high wear resistance. my country's wear-resistant steel design also adopts the design concept of high-temperature quenching and low-temperature tempering of low-carbon and low-alloy steel, and has developed domestic low-alloy wear-resistant steels such as the NM450 and NM500 series, and has achieved good results. However, the supersaturated solid solution martensite structure and residual austenite structure obtained by traditional low-carbon low-alloy wear-resistant steel after high-temperature quenching and low-temperature tempering are both in a thermodynamically metastable state and are extremely sensitive to temperature. When the operating temperature is higher than the tempering temperature, the performance will decay rapidly. Therefore, the structure and performance of traditional low-alloy wear-resistant steel cannot meet the high-temperature wear conditions of stir friction welding stirring heads.

[0004] Existing friction stir welding heads for low-melting-point alloys like magnesium and aluminum are often made from H13 hot-work die steel, a widely used hot-work die steel worldwide. Although this material has a high alloying density, its high-temperature strength and thermal stability are still insufficient. When the ambient temperature exceeds 600°C, the material exhibits a rapid and drastic decrease in high-temperature strength, making it difficult to meet the rigorous operating conditions of the stir head. Summary of the Invention

[0005] During the welding process, the stirring head is in direct contact with the workpiece and is subjected to temperatures of 600-700°C, strong cyclic loads, and intense high-temperature friction. Furthermore, as process requirements continue to increase, the operating temperature of the stirring head is constantly increasing, and the loads it bears are also increasing. Under these harsh working conditions, traditional materials such as H13 steel will easily suffer from severe deformation, increased wear, and microstructural degradation of the stirring head due to their insufficient high-temperature strength, high-temperature wear resistance, and thermal stability. This shortens the service life of the stirring head and reduces the quality of the plate weld.

[0006] The present invention aims to overcome the shortcomings of the above-mentioned prior art and provide a high-temperature wear-resistant steel for a friction stir welding head and a method for preparing the same. The high-temperature wear-resistant steel for a friction stir welding head produced by this method has excellent thermal stability, a high-temperature strength that is 1 / 3 to 1 / 2 greater than that of H13 steel, and high-temperature wear resistance that is far superior to H13 steel.

[0007] The traditional approach to improving the high-temperature strength, thermal stability, and wear performance of steel materials is to increase the alloying degree. It is believed that the higher the alloy carbide content in the material matrix, the better the material's high-temperature performance. However, blindly alloying or increasing the alloying degree will not only increase the material preparation cost, but may also aggravate the segregation of alloying elements, promote the precipitation and growth of the second phase under high temperature conditions, destroy the uniformity of the material's organization and performance, and reduce the material's thermal stability.

[0008] The present invention relates to a high-temperature wear-resistant steel for a friction stir welding stirring head and a preparation method thereof. The steel is prepared by rationally proportioning MC carbide-forming elements such as Mo, V, and Nb with the C element, supplemented by a rational combination of material preparation processes, so as to significantly increase the content of nanoscale MC carbides with extremely high thermal stability and dispersed distribution in a material matrix. The thermal stability of the MC carbides and their effect of hindering dislocations under high-temperature conditions are utilized to improve the high-temperature performance and high-temperature service life of the material.

[0009] The present invention aims to provide a high-temperature wear-resistant steel for a friction stir welding stirring head to solve the problems raised in the above background technology. To achieve the above object, the present invention provides the following technical solutions:

[0010] The invention discloses a high-temperature wear-resistant steel for a friction stir welding stirring head. The chemical composition of the steel is as follows: C: 0.25% to 0.35%, Mn: 0.15% to 0.8%, Cr: 0.5% to 1.0%, Mo: 0.5% to 1.5%, V: 0.2% to 0.75%, Nb: 0.01% to 0.03%, S: ≤0.03%, P: ≤0.03%, and the balance is Fe and unavoidable impurity elements.

[0011] Furthermore, ω(Mo) / ω(V)≥2 and the sum of the mass percentages of Mo and V is not less than 1.5%.

[0012] Furthermore, after heat treatment, its room temperature hardness is ≥44HRC; at room temperature, its tensile strength is ≥1350MPa, its yield strength is ≥1250MPa, its elongation is ≥13%, and its cross-sectional shrinkage is ≥22%; at 700°C, its tensile strength is ≥450MPa, its yield strength is ≥350MPa, its elongation is ≥18%, and its cross-sectional shrinkage is ≥60%.

[0013] Furthermore, the microstructure of the high temperature wear-resistant steel includes parallel lath structures with a lath width of 0.2 to 1 μm; spherical nano-scale MC carbides, strip-shaped nano-scale M7C3 carbides, and irregular morphology of M 23 C6 type carbide.

[0014] Furthermore, the lath structure contains nano-scale rod-shaped carbides, which are MC-type nano-carbides of V, Mo, and Nb and have a NaCl structure. The nano-scale rod-shaped carbides have a length of 10±5 nm and a cross-sectional size of 2.5±1.25 nm.

[0015] A method for preparing high-temperature wear-resistant steel for a friction stir welding stirring head, for preparing the high-temperature wear-resistant steel for a friction stir welding stirring head, comprising the following steps:

[0016] (1) Vacuum melting: The raw materials are prepared according to the required mass percentage of the components, placed in an electric furnace for melting, refining, vacuum degassing, and then cast into electrode rods;

[0017] (2) Electroslag remelting: The surface of the electrode rod is polished to remove surface oxidation and then electroslag remelted to obtain electroslag ingots. During electroslag remelting, ferrosilicon powder is used for deoxidation.

[0018] (3) Homogenization process: After the electroslag ingot is demoulded, it is subjected to high-temperature homogenization treatment. The homogenization temperature is 1200℃~1300℃ and the homogenization time is 2~20h.

[0019] (4) The steel ingot after homogenization treatment is cooled to below 500°C at a cooling rate of 80-100°C / h and then taken out of the furnace, or the steel ingot after homogenization treatment is cooled from the homogenization temperature to the corresponding rolling or forging temperature and then subjected to a forging or rolling process; wherein the forging process or rolling process is specifically:

[0020] Forging process: The forging process is 1100-1150℃ for 1-6 hours, the initial forging temperature is 1050-1100℃, and the final forging temperature is ≥880℃. It can be repeatedly uprooted and drawn, with a forging ratio of 2-9. After forging, it is transferred to a medium-temperature furnace at 450-650℃ for 1-6 hours for stress relief, or sand buried.

[0021] Rolling process: The rolling process is 1100-1150℃ for 1-6 hours, the starting rolling temperature is 1050-1100℃, the final rolling temperature is ≥880℃, and after rolling, it is transferred to a medium temperature furnace of 450-650℃ for 1-6 hours for stress relief, or buried in sand;

[0022] (5) Annealing process: put the steel into the furnace at a temperature below 500℃, heat it to 840-880℃ at a heating rate of no more than 150℃ / h, keep it at that temperature for 2-5 hours, and then cool it to below 500℃ at a rate of 20-50℃ / h.

[0023] (6) Flaw detection and correction: The annealed workpiece obtained in step (5) above needs to be inspected and tested for surface cracks, and the cracks found during inspection and testing should be trimmed, polished or cut off and removed;

[0024] (7) Quenching process: the workpiece after flaw detection and trimming in step (6) is placed in a high-temperature quenching furnace, heated to 950-1050°C, and then kept warm for 1-5 hours. After the heat preservation, it is quenched with water. The heat preservation time is not less than 15 min + 1 min / mm × r mm or 15 min + 1 min / mm × 1 / 2 × H mm, where r is the material radius and H is the material thickness;

[0025] (8) Tempering process: The above-mentioned quenched materials should be tempered immediately after completing the quenching process. The tempering temperature is selected in the range of 500-650℃ according to the hardness required for the finished product. Single tempering or double tempering can be used, and the temperature should be kept for 1.5-6 hours.

[0026] Furthermore, if the steel ingot is small (less than or equal to 50 kg) or the quality requirement is not high, the vacuum-melted steel ingot can be directly subjected to the homogenization process without the electroslag remelting process.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] The present invention provides a high-temperature wear-resistant steel for a friction stir welding stirring head, comprising the following percentages by mass: C: (0.25-0.35)%, Mn: (0.15-0.8)%, Cr: (0.5-1.0)%, Mo: (0.5-1.5)%, V: (0.2-0.75)%, Nb: (0.01-0.03)%, S: (≤0.03)%, P: (≤0.03)%, with the remainder being Fe and unavoidable impurity elements. In the steel of the present invention, ω(Mo) / ω(V) is ≥2, and the sum of the mass percentages of Mo and V is not less than 1.7%. The present invention promotes the formation of a large number of MC-type nanocarbides with excellent thermal stability, extremely small size and dispersed distribution in the inventive steel matrix by adding Mo, V and Nb elements (MC-type nanocarbide-forming elements) in appropriate proportions and contents, thereby enabling the inventive steel to obtain extremely high high-temperature strength, excellent anti-friction and wear performance, and very stable structure and performance under high-temperature conditions.

[0029] The present invention provides a high-temperature wear-resistant steel for a stir friction welding stirring head, the quenching temperature of which is not lower than 950°C. The purpose is to ensure the content of solid-solution alloy elements in the steel matrix, improve the thermal stability of the matrix, and increase the amount of nano-scale carbide precipitation in the tempering stage; at the same time, the quenching temperature of the material is not higher than 1050°C, the purpose of which is to ensure that the final grain size of the material remains small. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is a process flow chart for preparing a high-temperature wear-resistant steel for a friction stir welding stirring head according to the present invention;

[0031] Figure 2 This is a comparison chart of room temperature tensile curves of a high-temperature wear-resistant steel for a friction stir welding stirring head in the present invention and comparison materials NM500 and H13 steel;

[0032] Figure 3 A comparison chart of 700°C high-temperature tensile curves of a high-temperature wear-resistant steel for a friction stir welding stirring head according to the present invention and comparison materials NM500 and H13 steel;

[0033] Figure 4 The nano-phase is dispersed in the matrix of a high-temperature wear-resistant steel for a friction stir welding stirring head according to the present invention after being quenched at 980°C and tempered at 625°C.

[0034] Figure 5 The nano-phase morphology and distribution in the matrix of a high-temperature wear-resistant steel for a stir friction welding stirring head according to the present invention after being stretched at 700°C;

[0035] Figure 6 The nano-phase morphology and distribution in the matrix of a high-temperature wear-resistant steel for a stir friction welding stirring head of the present invention after being kept at 700°C for 20 hours;

[0036] Figure 7 Comparison of the microstructure of H13 steel before and after stretching at 700℃. DETAILED DESCRIPTION

[0037] Examples Comparative materials selected are traditional wear-resistant material NM500 steel and traditional hot working die steel H13 steel. Typical compositions of the comparative materials are shown in the following table.

[0038] Table 1 Comparative typical chemical composition of steel (weight percentage, wt%)

[0039] Brand C Cr Ni Mo V Si Mn B NM500 0.40 1.00 0.5 0.5 -- 0.25 1.2 0.005 H13 0.40 5.00 -- 1.50 1.00 1.00 0.30 --

[0040] Example 1

[0041] (1) Vacuum Melting: The chemical compositions and contents used in this embodiment are shown in the table below. 200 kg of the alloy was melted according to the following alloy element ratios and cast into electrode rods of Φ90 (unit: mm, the same below);

[0042] Table 2 Chemical composition of the invention steel in Example 1

[0043] alloying elements C Mn Cr Mo V Nb Mass fraction (wt.%) 0.25 0.15 0.5 1.2 0.3 0.015

[0044] (2) Electroslag remelting: The electrode rods obtained by casting are ground to remove surface oxidation and then electroslag remelted to obtain electroslag ingots. During the electroslag remelting, ferrosilicon powder is used for deoxidation to obtain Φ220 steel ingots;

[0045] (3) Homogenization treatment: After demoulding, the electroslag ingot is subjected to high-temperature homogenization treatment at a temperature of 1200°C for 6 hours;

[0046] (4) The homogenized ingot can be cooled to 1150°C at a cooling rate of 80°C / h;

[0047] (5) Forging and stress relief after forging: The steel ingot is kept at 1150℃ for 1 hour before forging. The first blank is cut into Φ100-120, and then it is returned to the furnace and kept at 1150℃ for 15 minutes before forging into Φ50-55 for the second time. After forging, it is transferred to a medium temperature furnace at 500℃ and kept at this temperature for 4 hours for stress relief.

[0048] (6) Annealing: After stress relief, the forging billet is placed in the furnace at a temperature below 500°C, heated to 850°C at a heating rate of no more than 150°C / h, and then kept at this temperature for 2 hours. It is then cooled with the furnace at a rate of 30°C / h to below 500°C and then taken out of the furnace for air cooling.

[0049] (6) Flaw detection and surface defect repair: After forging, stress relief forging blanks are inspected and flaw detected, and only a small amount of forging breakage is found, which can be removed by grinding with an angle grinder;

[0050] (7) Quenching: Place the forging blank after flaw detection and trimming into a high-temperature quenching furnace, heat it to 980°C, keep it at this temperature for 1 hour, and then quench it with water;

[0051] (8) Tempering: After the quenching process is completed, the bar is tempered immediately. The tempering temperature is several temperatures between 200-625℃, and the tempering holding time is 2 hours.

[0052] Example 2

[0053] (1) Vacuum melting: The chemical components and their contents used in this embodiment are shown in the following table. Smelting was performed according to the following alloy element ratios, and finally a 45 kg cannonball-shaped steel ingot with a small end of Φ80 and a large end of Φ155 was obtained by casting;

[0054] Table 3 Chemical composition of the invention steel in Example 2

[0055] alloying elements C Mn Cr Mo V Nb Mass fraction (wt.%) 0.32 0.3 1.0 1.5 0.6 0.03

[0056] (2) Homogenization treatment: The steel ingot obtained by melting and pouring is subjected to high-temperature homogenization treatment at a temperature of 1280°C for 2 hours;

[0057] (3) The homogenized ingot was furnace cooled at a cooling rate of 80°C / h to 1150°C;

[0058] (4) Forging and stress relief after forging: After being kept at 1150℃ for 1 hour, forging is carried out. The shell ingot is forged into Φ30-35 steel bars in one fire. After forging, it is transferred to a medium temperature furnace at 500℃ and kept for 4 hours to relieve stress;

[0059] (5) Annealing: After stress relief, the forging billet is placed in the furnace at a temperature below 500°C, heated to 850°C at a heating rate of no more than 150°C / h, and then kept at this temperature for 2 hours. It is then cooled with the furnace at a rate of 30°C / h to below 500°C and then taken out of the furnace for air cooling.

[0060] (6) After forging, stress relief forging blanks are inspected and tested for cracks and folds, and can be directly quenched without treatment;

[0061] (7) Place the forging blank into a high-temperature quenching furnace, heat it to 980°C, hold it for 1 hour, and then quench it with water;

[0062] (8) After the quenching process is completed, the bar is immediately tempered at a temperature between 200-625℃ and the tempering holding time is 2.5 hours.

[0063] Table 4 Hardness of the invention steel and the comparative steel after quenching at 980℃ and tempering at different temperatures for 2.5 hours

[0064]

[0065]

[0066] Table 5 Comparison of room temperature strength and high temperature strength between comparative steel and invention steel

[0067]

[0068] Note: The final heat treatment system of the steel of the present invention in Table 5 is: 980℃ quenching and 625℃ tempering;

[0069] Final heat treatment system of NM500 steel: 980℃ quenching and 200℃ tempering;

[0070] Final heat treatment system of H13 steel: quenching at 1020℃ and tempering at 580℃.

[0071] The raw materials listed in the present invention, as well as the upper and lower limits and interval values ​​of the raw materials, and the upper and lower limits and interval values ​​of the process parameters (such as temperature, time, etc.) can all realize the present invention, and the embodiments are not listed one by one here.

[0072] The above description is only a partial implementation method of the present invention, which certainly cannot be used to limit the scope of rights of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and changes can be made without departing from the principles of the present invention, and these improvements and changes are also considered to be within the scope of protection of the present invention.

Claims

1. A high temperature wear-resistant steel for a friction stir welding stirring head, characterized in that: Its chemical composition by mass percentage is: C: 0.25% to 0.35%, Mn: 0.15% to 0.8%, Cr: 0.5% to 1.0%, Mo: 0.5% to 1.5%, V: 0.2% to 0.75%, Nb: 0.01% to 0.03%, S: ≤ 0.03%, P: ≤ 0.03%, and the balance is Fe and unavoidable impurity elements; ω(Mo) / ω(V)≥2 and the sum of the mass percentages of Mo and V is not less than 1.5%; After heat treatment, its room temperature hardness is ≥44HRC; at room temperature, its tensile strength is ≥1350MPa, its yield strength is ≥1250MPa, its elongation is ≥13%, and its cross-sectional shrinkage is ≥22%; at 700℃, its tensile strength is ≥450MPa, its yield strength is ≥350MPa, its elongation is ≥18%, and its cross-sectional shrinkage is ≥60%; The microstructure of the high temperature wear-resistant steel includes parallel lath structures with a lath width of 0.2 to 1 μm; spherical nano-scale MC carbides, strip-shaped nano-scale M7C3 carbides, and irregular M 23 C6 type carbide; The lath structure contains nano-scale rod-shaped carbides, which are MC-type nano-carbides of V, Mo, and Nb and have a NaCl structure. The nano-scale rod-shaped carbides have a length of 10±5 nm and a cross-sectional size of 2.5±1.25 nm.

2. A method for preparing high-temperature wear-resistant steel for a friction stir welding stirring head, characterized in that: The method for preparing the high-temperature wear-resistant steel for the friction stir welding stirring head according to claim 1 comprises the following steps: (1) Vacuum melting: The raw materials are prepared according to the required mass percentage of the components, placed in an electric furnace for melting, refining, vacuum degassing, and then cast into electrode rods; (2) Electroslag remelting: The surface of the electrode rod is polished to remove surface oxidation and then electroslag remelted to obtain electroslag ingots. During electroslag remelting, ferrosilicon powder is used for deoxidation. (3) Homogenization process: After the electroslag ingot is demoulded, it is subjected to high-temperature homogenization treatment. The homogenization temperature is 1200℃~1300℃ and the homogenization time is 2~20h. (4) The steel ingot after homogenization treatment is cooled to below 500°C at a cooling rate of 80-100°C / h and then taken out of the furnace, or the steel ingot after homogenization treatment is cooled from the homogenization temperature to the corresponding rolling or forging temperature and then subjected to a forging or rolling process; wherein the forging process or rolling process is specifically: Forging process: The forging process is 1100-1150℃ for 1-6 hours, the initial forging temperature is 1050-1100℃, and the final forging temperature is ≥880℃. It can be repeatedly uprooted and drawn, with a forging ratio of 2-9. After forging, it is transferred to a medium-temperature furnace at 450-650℃ for 1-6 hours for stress relief, or sand buried. Rolling process: The rolling process is 1100-1150℃ for 1-6 hours, the starting rolling temperature is 1050-1100℃, the final rolling temperature is ≥880℃, and after rolling, it is transferred to a medium temperature furnace of 450-650℃ for 1-6 hours for stress relief, or buried in sand; (5) Annealing process: put the steel into the furnace at a temperature below 500℃, heat it to 840-880℃ at a heating rate of no more than 150℃ / h, keep it at that temperature for 2-5 hours, and then cool it to below 500℃ at a rate of 20-50℃ / h. (6) Flaw detection and correction: The annealed workpiece obtained in step (5) above needs to be inspected and tested for surface cracks, and the cracks found during inspection and testing should be trimmed, polished or cut off and removed; (7) Quenching process: the workpiece after flaw detection and trimming in step (6) is placed in a high-temperature quenching furnace, heated to 950-1050°C, and then kept warm for 1-5 hours. After the heat preservation, it is quenched with water. The heat preservation time is not less than 15 min + 1 min / mm × r mm or 15 min + 1 min / mm × 1 / 2 × H mm, where r is the material radius and H is the material thickness; (8) Tempering process: The above-mentioned quenched materials should be tempered immediately after completing the quenching process. The tempering temperature is selected in the range of 500-650℃ according to the hardness required for the finished product. Single tempering or double tempering can be used, and the temperature should be kept for 1.5-6 hours.

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