A high-temperature, high-strength steel for friction stir welding heads and its preparation method

Abstract

This invention belongs to the field of friction stir welding technology, and relates to a high-temperature, high-strength steel for friction stir welding heads and its preparation method. The steel's mass percentage composition is as follows: C: 0.25%–0.40%, Mn: 0.15%–0.8%, Si: 0.05%–0.3%, Cr: 0.2%–1.0%, Mo: 1.0%–1.5%, V: 0.05%–0.2%, Nb: 0.005%–0.015%, S: ≤0.03%, P: ≤0.03%, with the balance being Fe and unavoidable impurity elements. Compared to traditional high-temperature steels, this invention reduces the content of V and Nb elements and adds Si element, reducing the impact of large-size primary carbides on the material's strength and toughness. While maintaining the high comprehensive mechanical properties of high-temperature steel, it also reduces the temperature requirements for high-temperature diffusion annealing and hot working, saving material and energy costs.

Description

Technical Field

[0001] This invention belongs to the field of friction stir welding technology, and particularly relates to a high-temperature, high-strength steel for friction stir welding heads and its preparation method. Background Technology

[0002] Due to their low price, low density, high specific strength, and good corrosion resistance, lightweight materials such as magnesium and aluminum alloys are highly favored by the market. Friction stir welding (FSW) is a new solid-state joining technology with significant advantages such as high quality, energy saving, and no pollution, and is especially suitable for welding low-melting-point alloys such as magnesium and aluminum. The stirring head is a key tool for realizing friction stir welding technology. The material performance not only affects the service life of the stirring head and the quality of the weld, but also, due to the harsh working conditions, the stirring head is one of the most easily worn consumables, which also has a significant impact on the cost of magnesium alloy and aluminum alloy welded parts.

[0003] Currently, both domestically and internationally, H13 hot-work die steel is commonly used as the material for stirring heads in friction stir welding using low-melting-point alloys such as magnesium and aluminum. However, despite its high alloy content, this material still has limitations in high-temperature strength and thermal stability. In fact, when the stirring head is subjected to high-temperature environments and continuous cyclic loads and severe high-temperature friction, H13 steel, due to its insufficient high-temperature strength, will quickly deform and wear, thus shortening its service life and affecting the welding quality. Therefore, it is necessary to find high-temperature, high-strength alloy materials that can replace H13 steel to prepare stirring heads to meet the higher requirements of advanced low-melting-point alloy welding technology. In recent years, some new high-temperature, high-strength alloy materials, such as cobalt-based, nickel-based, or tungsten-based alloy materials, have been widely researched and applied. Although these materials can maintain good performance under harsh working conditions, their extremely high price severely limits their application. Therefore, researching and preparing low-cost, high-temperature, high-strength friction stir welding materials is a very urgent need for the friction stir welding industry. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention proposes a high-temperature, high-strength steel for friction stir welding heads and its preparation method. The steel for friction stir welding heads of this invention contains multiple elements capable of forming MC carbides, and the ratio of each element to C is precisely designed. Based on this, this invention further proposes a method for preparing high-temperature, high-strength materials by controlling the heat treatment process.

[0005] The specific technical solution of the present invention is as follows:

[0006] A high-temperature, high-strength steel for friction stir welding heads has the following composition by mass percentage: C: 0.25%–0.40%, Mn: 0.15%–0.8%, Si: 0.05%–0.3%, Cr: 0.2%–1.0%, Mo: 1.0%–1.5%, V: 0.05%–0.195%, Nb: 0.001%–0.0095%, S: ≤0.03%, P: ≤0.03%, with the balance being Fe and unavoidable impurity elements.

[0007] Preferably, the steel used for the friction stir welding head has a room temperature hardness ≥44HRC; a tensile strength ≥1350MPa, a yield strength ≥1250MPa, an elongation ≥13%, and a reduction of area ≥22% at room temperature; and a tensile strength ≥420MPa, a yield strength ≥350MPa, an elongation ≥18%, and a reduction of area ≥60% at 700℃.

[0008] Preferably, the microstructure of the steel used for friction stir welding heads comprises parallel lath structures with a lath width of 0.15–1.5 μm; spherical nanoscale MC-type carbides, spherical Fe3C, strip-shaped nanoscale M7C3-type carbides, and irregularly shaped M are distributed on the boundaries of the lath structures. 23 C6 type carbides.

[0009] Preferably, the lath structure contains nanoscale rod-shaped carbides and diffusely distributed needle-shaped cementite carbides. The nanoscale rod-shaped carbides are MC-type nanocarbides of V, Mo, and Nb, with a NaCl structure. The length of the nanoscale rod-shaped MC carbides is 6±4 nm, and the cross-sectional size is 1.5±1 nm. The needle-shaped cementite is M3C-type carbides of Fe, V, Mo, and Cr, with a length of 25±10 nm and a cross-sectional size of 6±3 nm.

[0010] This invention also proposes a method for preparing steel for high-temperature, high-strength friction stir welding heads, comprising the following steps:

[0011] S1 The raw materials are weighed according to the mass percentage composition of the steel for high-temperature and high-strength friction stir welding heads, and the electrode rod is obtained by casting after melting, refining and vacuum degassing.

[0012] S2 pre-treats the electrode rod and then electroslag remelts it to obtain an electroslag ingot.

[0013] S3 uses a homogenization temperature of 1180℃-1250℃ to perform high-temperature homogenization treatment on electroslag ingots for 2-20 hours.

[0014] S4 involves cooling the steel ingot from the homogenization temperature to below 500°C in the furnace at a cooling rate of no more than 100°C / h, or forging or rolling the steel ingot into workpieces after reducing it from the homogenization temperature to the rolling or forging temperature.

[0015] S5 annealing process: The furnace is put into the furnace at a temperature below 500℃, heated to 840~880℃ at a heating rate of no more than 150℃ / h, held for 2~5 hours, and then cooled in the furnace at a rate of 20-50℃ / h to below 500℃ before being removed from the furnace and air-cooled.

[0016] S6 Flaw Detection and Correction: The surface cracks of the annealed workpiece need to be inspected and flawed, and the cracks found during inspection and flaw detection need to be repaired, ground or cut off.

[0017] S7 Quenching Process: After flaw detection and correction, the workpiece is placed in a high-temperature quenching furnace and heated to 950-1020℃. After holding at this temperature for 0.5-5 hours, it is quenched in water. The holding time is not less than 15min + 1min / mm × r mm or 15min + 1min / mm × 1 / 2 × H mm, where r is the material radius and H is the material thickness.

[0018] S8 tempering process: After the quenching process is completed, a first tempering or second tempering should be carried out immediately. The tempering temperature is 500-650℃ and the tempering time is 1.5-6 hours.

[0019] Preferably, the pretreatment in step S2 is: polishing the surface of the electrode rod and removing surface oxidation.

[0020] Preferably, the forging process in step S4 is as follows: holding at 1080-1140℃ for 1-6 hours, initial forging temperature of 1030-1100℃, final forging temperature ≥880℃, multiple upsetting and drawing, forging ratio of 2-9, and after forging, transferring to a medium-temperature furnace at 450-650℃ for holding for 1-6 hours to relieve stress, or sand embedding treatment;

[0021] Preferably, the rolling process in step S4 is as follows: holding at 1080-1140℃ for 1-6 hours, initial rolling temperature is 1050-1100℃, final rolling temperature is ≥880℃, and after rolling, it is transferred to a medium-temperature furnace at 450-650℃ for 1-6 hours to relieve stress, or sand is buried.

[0022] Preferably, if the weight of the steel ingot is less than or equal to 50 kg, steps S2 and S3 are not performed.

[0023] The beneficial effects of this invention compared to the prior art are as follows:

[0024] (1) The present invention provides a high-temperature and high-strength steel for friction stir welding heads. Compared with traditional high-temperature steel, it reduces the content of V and Nb elements, reduces the influence of large-size primary carbides on the strength and toughness of the material, and while maintaining the high comprehensive mechanical properties of high-temperature steel, it also reduces the temperature requirements for high-temperature diffusion annealing and hot working, saving material and energy costs.

[0025] (2) The high-temperature and high-strength steel for friction stir welding provided by the present invention contains 0.05-0.3% Si, which reduces the diffusion of C atoms and improves M3C and M 23 The thermal stability of carbides such as C6 and M7C3 hinders their growth under high-temperature conditions; fine carbides at high temperatures can further maintain the stability of the martensitic lath structure of steel in high-temperature working environments.

[0026] (3) In the method for preparing steel for high-temperature and high-strength friction stir welding head of the present invention, the quenching temperature is not lower than 950°C, so as to ensure the content of solid solution alloying elements in the steel matrix, improve the thermal stability of the matrix and increase the amount of nanoscale carbide precipitation during the tempering stage; at the same time, since the content of alloying elements (V, Nb) that hinder grain growth in the alloy is small, the quenching temperature of the material is not higher than 1020°C, which effectively ensures that the final grain size of the material is kept small.

[0027] (4) In the method for preparing steel for high-temperature and high-strength friction stir welding head of the present invention, the diffusion annealing temperature is higher than 1180℃, which can fully dissolve the coarse M2C and MC type primary carbides in the structure, reduce the cutting effect of large-sized carbides on the material structure, and eliminate the influence of coarse primary carbides on the toughness and strength of the material; at the same time, the diffusion annealing temperature of the material is lower than 1250℃, which effectively prevents the overheating phenomenon in the structure. Attached Figure Description

[0028] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0029] Figure 1 This is a process flow diagram of the preparation of a high-temperature, high-strength steel for friction stir welding heads in this invention;

[0030] Figure 2 This is a comparison of the room temperature tensile curves of a high-temperature, high-strength friction stir welding head steel of the present invention and a comparative material, H13 steel.

[0031] Figure 3 This is a comparison of the high-temperature, high-strength friction stir welding head steel of the present invention and the comparative material H13 steel at 700℃.

[0032] Figure 4 This is a TEM dark-field image of finely dispersed nanophases in the matrix of a high-temperature and high-strength friction stir welding head steel after quenching at 950℃ and tempering at 580℃.

[0033] Figure 5 The microstructure of a high-temperature and high-strength friction stir welding head steel in this invention is compared before and after heat preservation for 30 minutes. (a) Before heat preservation, (b) After heat preservation.

[0034] Figure 6 Comparison of the microstructure of H13 steel before and after holding at 700℃ for 30 minutes: (a) before holding, (b) after holding. Detailed Implementation

[0035] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0036] This invention provides a high-temperature, high-strength friction stir welding head steel (Nano-HT450) with the following chemical composition by mass percentage: C: 0.25%–0.40%, Mn: 0.15%–0.6%, Si: 0.05%–0.3%, Cr: 0.2%–1.0%, Mo: 1.0%–1.5%, V: 0.05%–0.195%, Nb: 0.001%–0.0095%, S: ≤0.03%, P: ≤0.03%, with the balance being Fe and unavoidable impurity elements. MC-type nanocarbides with a scale of 2–10 nm are beneficial second-phase structures in steel materials, enhancing the room temperature and high-temperature strength of the alloy. In traditional high-temperature steels, V and Nb contents are relatively high; excessively high V and Nb contents not only fail to promote further formation of MC-type nanocarbides but also significantly increase alloy costs. Furthermore, high-V and Nb content steels also generate a large number of coarse primary carbides and spherical MC-type carbides with a scale of 100–800 nm, whose strengthening effect is slightly weaker than that of MC-type nano carbides with a scale of 2–10 nm. Therefore, this invention reduces the impact of large-sized primary carbides on the strength and toughness of the material by appropriately reducing the V and Nb content. While maintaining the high comprehensive mechanical properties of high-temperature steel, it also lowers the high-temperature diffusion annealing and hot working temperatures, saving material and energy costs.

[0037] The table below compares the composition and cost of the steel of this invention with that of traditional hot work die steel H13.

[0038] Table 1. Typical chemical compositions (weight percentage, wt%) and alloy costs (cost per ton of alloying elements) of the invented steel and comparative steel.

[0039]

[0040]

[0041] Example 1

[0042] (1) Vacuum melting: The chemical composition and content used in this embodiment are shown in the table below. 15kg of steel nails are melted according to the following alloy element ratio. The cross-sectional diameter of the steel nails is 55-70mm. Since the steel ingot is small in weight, electroslag remelting and homogenization are not carried out.

[0043] Table 2 Chemical composition of the invented steel in Example 1

[0044] Alloy elements C Mn Cr Mo V Nb Mass fraction (wt.%) 0.25 0.3 0.5 1.0 0.1 0.001

[0045] (2) Forging and stress relief after forging: The smelted steel ingot is placed in a forging furnace and heated to 1120℃. After holding for 1 hour, it is forged to Φ20~22 in one forging. After forging, it is transferred to a 500℃ medium temperature furnace and held for 4 hours to relieve stress.

[0046] (3) Annealing: After stress relief, the forging bar is put into the furnace at a temperature below 500°C, heated to 850°C at a heating rate of no more than 150°C / h, held for 2 hours, and then cooled in the furnace at a rate of 30°C / h to below 500°C before being taken out and air-cooled.

[0047] (4) Flaw detection and surface defect repair: After forging and stress relief, the forged billet is inspected and flawed, and there is no damage to the surface or the interior.

[0048] (5) Quenching: After flaw detection and repair, the forging billet is placed in a high-temperature quenching furnace, heated to 980°C, held for 1 hour, and then quenched in water.

[0049] (6) Tempering: After the quenching process is completed, the bar is immediately tempered at a temperature of 580℃ and the tempering holding time is 2 hours.

[0050] Example 2

[0051] (1) Vacuum melting: The chemical composition and content used in this embodiment are shown in the table below. The alloy elements are melted according to the following ratio, and finally cast to obtain a 95kg shell-shaped steel ingot with a small end of Φ115 and a large end of Φ175.

[0052] Table 3 Chemical composition of the invented steel in Example 2 of this embodiment

[0053]

[0054] (2) Homogenization treatment: The steel ingots obtained by melting and casting are subjected to high-temperature homogenization treatment at a temperature of 1250℃ for 2 hours.

[0055] (3) The homogenized steel ingots were cooled in the furnace to 1140℃ at a cooling rate of 80℃ / h;

[0056] (4) Forging and stress relief after forging: After holding at 1140℃ for 1 hour, forging is carried out. The shell ingot shape is forged into Φ35~40 steel bar in one fire. After forging, it is transferred to a 500℃ medium temperature furnace for holding for 4 hours to relieve stress.

[0057] (5) Annealing: After stress relief, the forging billet is put into the furnace at a temperature below 500°C, heated to 850°C at a heating rate of no more than 150°C / h, held for 2 hours, and then cooled in the furnace at a rate of 30°C / h to below 500°C before being taken out and air-cooled.

[0058] (6) After inspection and flaw detection, no cracks or folds were found in the stress-relieved forging billet after forging. It can be quenched directly without any treatment.

[0059] (7) The forging billet is placed in a high-temperature quenching furnace and heated to 980°C. After holding at that temperature for 1 hour, it is then quenched in water.

[0060] (8) After the quenching process is completed, the bar is immediately tempered at a temperature of 580℃ and the tempering holding time is 2.5 hours.

[0061] Table 4 Comparison of room temperature and high temperature strength between the comparative steel and the invented steel

[0062]

[0063]

[0064] Note: The final heat treatment regime for the steel of this invention in Table 5 is: quenching at 980℃ and tempering at 580℃.

[0065] The final heat treatment process for H13 steel is: quenching at 1020℃ and tempering at 580℃.

[0066] All the raw materials listed in this invention, as well as the upper and lower limits and ranges of the raw materials and the upper and lower limits and ranges of the process parameters (such as temperature, time, etc.), can realize this invention. Examples are not listed one by one here.

[0067] The above description is only a partial embodiment of the present invention, and should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make several improvements and modifications without departing from the principle of the present invention, and these improvements and modifications are also considered to be within the scope of protection of the present invention.

Claims

1. A high-temperature high-strength steel for a friction stir welding tool head, characterized by, The mass percentage composition is as follows: C: 0.25%-0.40%, Mn: 0.15%-0.8%, Si: 0.05-0.3%, Cr: 0.2%-1.0%, Mo: 1.0%-1.5%, V: 0.05-0.195%, Nb: 0.001%-0.0095%, S: ≤0.03%, P: ≤0.03%, the balance being Fe and inevitable impurity elements; The steel for the friction stir welding head has a room temperature hardness of ≥44HRC, a tensile strength of ≥1350MPa at room temperature, a yield strength of ≥1250MPa, an elongation of ≥13%, and a reduction of area of ≥22%; a tensile strength of ≥420MPa at 700℃, a yield strength of ≥350MPa, an elongation of ≥18%, and a reduction of area of ≥60%; The microstructure of the steel for friction stir welding head comprises parallel martensite lath structure, the lath width is 0.15-1.5 μm; the lath structure boundary is distributed with spherical nanometer scale MC type carbide, spherical Fe3C, strip-shaped nanometer scale M7C3 type carbide and irregular M 23 C6 type carbide; The plate structure contains nanometer-scale rod-shaped carbides and dispersedly distributed needle-shaped cementite carbides, the nanometer-scale rod-shaped carbides are MC-type nanometer carbides of V, Mo and Nb, and have a NaCl structure; the nanometer-scale rod-shaped MC carbides have a length of 6±4 nm and a cross-sectional dimension of 1.5±1 nm; and the needle-shaped cementite is M3C-type carbide of Fe, V, Mo and Cr, has a length of 25±10 nm and a cross-sectional dimension of 6±3 nm.

2. A preparation method of the steel for the friction stir welding head according to claim 1, comprising the following steps: S1. Raw materials are weighed according to the mass percentage composition of the high-temperature and high-strength steel for the friction stir welding head, and an electrode rod is obtained by casting after melting, refining and vacuum degassing; S2. The electrode rod is pretreated, and an electroslag ingot is obtained by electroslag remelting; S3. The electroslag ingot is subjected to high-temperature homogenization treatment at a homogenization temperature of 1180-1250℃ for 2-20h; S4. The steel ingot is cooled to below 500℃ at a cooling rate of not more than 100℃ / h from the homogenization temperature, or the steel ingot is reduced to a rolling or forging temperature, and then is forged or rolled into a workpiece; S5. Annealing process: the workpiece is heated to 840-880℃ at a heating rate of not more than 150℃ / h after being put into a furnace at a furnace temperature of less than 500℃, and then is kept for 2-5h, and then is cooled to below 500℃ at a rate of 20-50℃ / h, and is taken out of the furnace and air-cooled; S6. Flaw detection and correction: the workpiece after annealing needs to be subjected to surface crack inspection and flaw detection, and cracks found in the inspection and flaw detection are polished or cut off and removed; S7. Quenching process: the workpiece after flaw detection and correction is put into a high-temperature quenching furnace and heated to 950-1020℃, and kept for 0.5-5h, and then is quenched after the keeping, and the keeping time is not less than 15min+1min / mm×r mm or 15min+1min / mm×1 / 2×Hmm, r is the radius of the material, and H is the thickness of the material; S8. Tempering process: after the quenching process, one-time tempering or two-time tempering should be immediately performed, and the tempering temperature is 500-650℃, and the tempering time is 1.5-6h; The pretreatment in step S2 is polishing the surface of the electrode rod and removing surface oxidation.

3. The preparation method according to claim 2, characterized in that, The forging process in step S4 is: 1080-1140 ℃ for 1-6 hours, initial forging temperature is 1030-1100 ℃, final forging temperature is ≥880 ℃, multiple upsetting and drawing, forging ratio is 2-9, after forging, it is transferred into a medium temperature furnace of 450-650 ℃ for 1-6 hours for stress relief, or sand embedding treatment.

4. The preparation method according to claim 3, characterized in that, The rolling process in step S4 is: 1080-1140 ℃ for 1-6 hours, initial rolling temperature is 1050-1100 ℃, final rolling temperature is ≥880 ℃, after rolling, it is transferred into a medium temperature furnace of 450-650 ℃ for 1-6 hours for stress relief, or sand embedding treatment.

5. The preparation method according to claim 4, characterized in that, If the weight of the ingot is less than or equal to 50 kg, steps S2 and S3 are not performed.

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