A high-impact energy anchor rod steel

Abstract

The present application relates to the technical field of metallurgy, and particularly relates to a high impact energy anchor rod steel, high-purity fine-grain billets are obtained through rare earth boron composite micro-alloying and pulse magnetic oscillation homogenization, radial non-uniform rheology is initiated by surface layer gradient precooling and nanometer precipitation is induced, and then precise heat treatment is supplemented, which covers dual-phase zone inoculation, lower bainite isothermal transformation and quenching carbon distribution, finally, a complex phase structure of synergistically toughened tempered martensite, lower bainite and thin film residual austenite is constructed. The present application significantly improves the impact energy and hardness of the anchor rod steel through a series of measures, such as vacuum or inert gas protection in the raw material smelting stage, micro-alloying treatment, pulse magnetic oscillation and forced cooling in the semi-solid paste zone after ingot solidification or remelting, specific regulation and control in the process of hot rolling deformation and post-rolling cooling, and surface modification process.

Description

Technical Field

[0001] This invention relates to the field of metallurgical technology, and in particular to a high-impact anchor steel. Background Technology

[0002] High-impact anchor steel is a special type of steel developed specifically for underground engineering support, possessing high strength and high impact absorption performance. It is also a core structural material in deep geotechnical engineering support systems, primarily used in coal mine underground roadway support, traffic tunnel surrounding rock reinforcement, deep mining, and slope stabilization. As my country's mineral resources and underground space development continue to extend deeper, the geological environment of underground engineering projects is becoming increasingly complex, with frequent occurrences of high ground stress, strong disturbances, and dynamic geological disasters such as rock bursts and rock pressure. The surrounding rock of engineering projects will bear significant dynamic impact loads, placing higher demands on the impact resistance of support components. As the core load-bearing component of active support in underground engineering, the material properties of anchors directly determine the reliability of surrounding rock support and the safety of engineering construction. High-impact anchor steel, combining structural strength and impact toughness, is gradually becoming the mainstream development direction for support materials. This type of steel, relying on refined alloy composition design, controlled rolling and cooling and heat treatment processes, and microstructure control technology, achieves a synergistic match between high strength and high impact energy. It can effectively absorb impact deformation energy under complex stress environments and maintain the overall stability of the support structure.

[0003] In existing technologies, alloying or heat treatment is used to introduce toughness to improve impact energy, but this often sacrifices some strength; conversely, increasing strength can lead to a decrease in toughness, creating an inverse relationship. Inclusions, segregation, and microstructure inhomogeneity inherent in steel can become crack initiation points under impact loads, especially under high strain rate dynamic loading, easily leading to brittle fracture. In complex stress and temperature fluctuation environments, the microstructure of steel is prone to aging changes, causing impact toughness to decrease over service time, affecting the long-term reliability of support systems. Furthermore, in humid and corrosive environments such as underground coal mines, steel is prone to corrosion, reducing mechanical properties and service life; under the combined action of tensile stress and corrosive media, there is a risk of stress corrosion cracking, especially in environments containing Cl. - In corrosive ionic environments, service safety is significantly reduced. In addition, existing products are not well adapted to extreme temperatures (such as high temperatures in deep wells or low temperatures in cold regions). They are prone to ductile-brittle transition in low-temperature environments, resulting in a sharp decrease in impact toughness and an increased risk of fracture. Summary of the Invention

[0004] To address the problems mentioned in the background section, this invention provides a high-impact-energy anchor steel.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: The present invention discloses a high impact energy anchor steel, which is prepared by a specific preparation method. The specific preparation steps are as follows: Step S1: Select raw materials containing, by mass percentage, C, 0.18%–0.20%, Si, 0.80%–1.20%, Mn, 1.80%–2.20%, Cr, 0.50%–0.80%, Mo, 0.25%–0.35%, V, 0.08%–0.12%, Nb, 0.02%–0.04%, Ti, 0.015%–0.025%, with the balance being Fe and unavoidable impurities. Melt the above raw materials under vacuum or inert gas protection, wherein the inert gas is argon or nitrogen, the melting temperature is controlled at 1550℃–1650℃, and the melting time is 60–90 min. During the smelting process, a micro-alloying treatment containing rare earth elements is carried out, and then the molten steel is cast into ingots with ingot dimensions of Φ150~200mm×800~1200mm and a casting temperature of 1480℃~1520℃.

[0006] The aforementioned microalloying treatment specifically involves adding a nickel-magnesium alloy with a pure magnesium content of 0.0010%–0.0030%, a boron-iron alloy with a pure boron content of 0.0015%–0.0030%, and a mixed rare-earth ferrosilicon alloy with a total rare-earth content of 0.015%–0.030%. Specifically, the nickel-magnesium alloy is a Ni-Mg50 type alloy with a magnesium content of 48%–52% and a nickel content of 48%–52%; the boron-iron alloy is an Fe-B18 type alloy with a boron content of 17%–19%, the balance being Fe and unavoidable impurities; and the mixed rare-earth ferrosilicon alloy is a Re-Si75 type alloy with a total rare-earth content of 23%–27% and a silicon content of 73%–77%. The rare-earth elements include Ce and La, with a Ce to La mass ratio of 2:1 to 3:1.

[0007] Step S2: The ingot obtained in Step S1 is subjected to high-temperature homogenization treatment at a temperature of 1150℃~1250℃ for 2~4h, with a cooling rate of 50~80℃ / h. Subsequently, pulsed magnetostrictive oscillation is applied to the semi-solid paste region after solidification or remelting of the ingot, while forced cooling is performed simultaneously to obtain a homogeneous solidified structure with a high central equiaxed crystal ratio. Specifically, the temperature of the semi-solid paste region is 1300℃~1350℃, and the central equiaxed crystal ratio is ≥85%. The parameters for pulsed magnetostrictive oscillation are set as follows: frequency 100~200Hz, peak current 150~250A, treatment time 60~180s, magnetic field strength 0.1~0.3T, and oscillation waveform is sinusoidal. Forced cooling is achieved using water spray cooling with a water volume of 0.2~0.4L / kg and a cooling rate of 15~25℃ / s.

[0008] Step S3: The billet treated in Step S2 is pre-cooled at the surface, causing the temperature of the surface layer (15-20 mm deep) to rapidly drop below the Ac3 temperature, while the core temperature remains at least 50°C above the Ac3 temperature. Specifically, the Ac3 temperature is 830°C-860°C, the pre-cooled surface temperature is 780°C-820°C, and the core temperature is 910°C-950°C. After surface pre-cooling, hot rolling deformation is performed. The flow stress difference generated by the temperature gradient between the core and the surface induces a non-uniform strain distribution, where the flow stress difference is controlled between 50-100 MPa, and the non-uniform strain distribution difference is 0.05-0.10. Hot rolling deformation includes roughing rolling under the above temperature gradient and finishing rolling in the non-recrystallized austenite region. The roughing rolling temperature is 1050℃~1150℃, the deformation per pass is 15%~25%, and there are 3~5 passes. The temperature range of the non-recrystallized austenite region is 820℃~950℃, the finishing rolling temperature is 820℃~860℃, the finishing rolling passes are 2~3, and the total deformation is 30%~40%.

[0009] After hot rolling, a controlled post-rolling cooling regime was implemented to induce strain-induced precipitation of nanoscale carbonitrides. The specific cooling regime was as follows: First, ultra-rapid cooling to 700℃–750℃ was performed at a rate of 30–50℃ / s using high-pressure water mist as the cooling medium. Subsequently, slow cooling was carried out at a rate of 0.5℃ / s–2.0℃ / s to 600℃–700℃, followed by holding at this temperature for 20–60 seconds. Slow cooling was achieved through furnace cooling or air cooling, using a soaking furnace for holding. This cooling regime promoted the dispersed precipitation of V and Nb carbonitrides at a size of 3nm–10nm, with a precipitate number density of 10. 14 ~10 15 pcs / cm 3 .

[0010] Optionally, after step S3 and before subsequent step S4, step S3a can be added: laser shock pretreatment. The surface of the rod is subjected to laser shock strengthening treatment. The laser shock parameters are set as follows: laser power 1000–2000 W, spot diameter 2–5 mm, number of shocks 1–2, and scanning speed 5–10 mm / s. This treatment forms a gradient deformation layer with a high-density dislocation substructure and a residual compressive stress field on the surface of the rod. The thickness of the gradient deformation layer is 50–100 μm, and the surface residual compressive stress is -300 to -500 MPa.

[0011] Step S4: The rolled material (or the bar material after step S3a) is subjected to precision heat treatment including low-temperature austenitization, medium-temperature salt bath inoculation in the dual-phase region, isothermal transformation of lower bainite, cryogenic treatment, and low-temperature tempering carbon distribution, and finally obtains a multiphase structure composed of tempered martensite, lower bainite and stable retained austenite, wherein the volume percentage of each phase in the multiphase structure is: tempered martensite 50% to 70%, lower bainite 20% to 40%, and stable retained austenite 5% to 15%.

[0012] The specific steps of the above precision heat treatment are as follows: S41, Low-temperature austenitization: First, hold at a first temperature 20℃~50℃ above the Ac3 temperature, the first temperature being 880℃~910℃, and the holding time being 30~60min; then raise the temperature to a second temperature 50℃~100℃ higher than the first temperature and hold for a short time, the second temperature being 930℃~1010℃, and the holding time being 10~20min, with a heating rate of 10~15℃ / min.

[0013] S42. Medium-temperature salt bath incubation in the two-phase region: The material after step austenitization is quenched into a first salt bath at 400℃~480℃. The first salt bath is a mixture of potassium nitrate and sodium nitrite in a mass ratio of 1:1. The temperature fluctuation of the salt bath is ≤±5℃, and the isothermal residence time is 10~30s.

[0014] S43, Lower Bainite Isothermal Transformation: The material is transferred to a second salt bath at 300℃~340℃. The second salt bath is a mixed salt of sodium nitrite, sodium nitrate and potassium nitrate with a mass ratio of 2:3:5. The temperature fluctuation of the salt bath is ≤±3℃, and the isothermal holding time is 5~15min.

[0015] S44. Cryogenic Treatment: The material is subjected to cryogenic treatment at temperatures below -80℃. The cryogenic treatment temperature is -80℃ to -120℃, the holding time is 2 to 4 hours, the cooling medium is liquid nitrogen, and the cooling rate is 10 to 15℃ / min.

[0016] S45, Low-temperature tempering carbon distribution: Temper the material at 200℃~300℃, with tempering temperature fluctuation ≤±5℃. The tempering equipment is a box-type tempering furnace, and the tempering time is 30~120min.

[0017] Following step S4, additional processing steps can be added as needed, specifically as follows: Optionally, step S5, surface induction tempering, can be added: The surface of the bar is rapidly induction heated and quenched, so that the tempering temperature of the surface layer (1-3 mm deep) is higher than that of the core, thereby forming a radial hardness gradient that gradually decreases from the surface to the interior. The induction tempering parameters are set as follows: induction frequency 10-20 kHz, heating time 5-10 s, surface heating temperature 350-400℃, core temperature maintained at 200-300℃, and hardness gradient of 50-100 HV / mm. This step is applicable to anchor steel prepared in step S1 or pretreated in step S3a.

[0018] Optionally, steps S6, low-temperature dehydrogenation aging, and S7, ultrasonic surface rolling nano-aging are added: Step S6 involves holding the product at 120℃~180℃ for 4~12h, using a pit-type aging furnace for low-temperature dehydrogenation aging, with a heating rate of 5~10℃ / min, followed by cooling to room temperature with the furnace after the holding period; Step S7 involves ultrasonic rolling treatment of the finished product surface, with the ultrasonic rolling parameters set as follows: rolling pressure 500~1000N, rolling speed 10~20mm / s, and ultrasonic frequency 20~40kHz. This treatment forms a surface nanocrystalline layer, further increases the surface residual compressive stress, and reduces the surface roughness. The thickness of the surface nanocrystalline layer is 20~50nm, the surface residual compressive stress is increased to -500~-700MPa, and the surface roughness Ra≤0.8μm.

[0019] The high impact-energy anchor steel of the present invention has a microstructure comprising tempered martensite, lower bainite, and a volume fraction of 5% to 15% thin-film retained austenite, wherein the thickness of the thin-film retained austenite is 5 to 15 nm and it is distributed at the grain boundaries of martensite and bainite; at the same time, V and Nb carbonitride precipitates with a size of 3 nm to 10 nm are dispersed in the microstructure, and the precipitates are mainly V(C,N) and Nb(C,N) with a mass ratio of 3:1 to 4:1.

[0020] The beneficial effects of this invention are as follows: 1. This invention employs vacuum or inert gas protection during the raw material smelting stage, combined with microalloying treatment containing rare earth elements. Simultaneously, pulsed magnetostrictive oscillation and forced cooling are applied to the semi-solid paste-like region after ingot solidification or remelting. The pulsed magnetostrictive oscillation can break primary dendrites and increase nucleation cores through electromagnetic force, while forced cooling can effectively suppress solute atom segregation and metallurgical defects such as porosity and shrinkage cavities. Rare earth elements and microalloying elements such as magnesium and boron can also achieve grain boundary purification and inclusion refinement modification, ultimately obtaining a homogeneous solidified structure with high central equiaxed crystal ratio, no obvious columnar crystals, and low compositional segregation. This homogeneous structure eliminates the weak core area and stress concentration source from the root, avoids brittle fracture caused by uneven structure, and significantly improves the overall plasticity and impact toughness uniformity of the billet, laying a solid homogeneous matrix for subsequent hot rolling and heat treatment structure control.

[0021] 2. During hot rolling deformation and post-rolling cooling, this invention creates a significant temperature gradient between the core and surface of the billet by pre-cooling the surface layer. This temperature gradient generates a difference in rheological stress, inducing a non-uniform strain distribution. Final rolling is completed in the non-recrystallized austenite region. Simultaneously, a specific post-rolling cooling regime is employed: ultra-rapid cooling to 700℃~750℃, followed by slow cooling at a rate of 0.5℃ / s~2.0℃ / s to 600℃~700℃ and holding at that temperature. This control method can induce the formation of nanoscale carbonitriding with dimensions of 3nm~10nm for elements such as V and Nb. The nanoscale carbonitrides, as coherent or semi-coherent precipitates, can effectively pin austenite grain boundaries to inhibit grain growth and achieve fine grain strengthening, and can also pin dislocations to hinder dislocation slip and achieve precipitation strengthening. Furthermore, the nanoscale precipitates do not cleave the metal matrix, avoiding grain boundary embrittlement caused by coarse precipitates. Ultimately, without sacrificing the toughness of the matrix, the yield strength and tensile strength of the anchor steel are significantly improved, meeting the high load-bearing requirements of downhole support. At the same time, the fine grain structure can also effectively absorb impact energy, providing important structural support for the high impact energy of the anchor steel.

[0022] 3. This invention utilizes surface modification processes such as laser shock pretreatment, surface induction tempering, and ultrasonic surface rolling nanostructuring to form a gradient deformation layer, nanocrystalline layer, and residual compressive stress field with a high-density dislocation substructure on the surface of the bar stock. Laser shock and ultrasonic rolling refine the surface grains, improving surface hardness and wear resistance. Surface induction tempering creates a radial hardness gradient with a hard surface and a tough core, preventing high-hardness embrittlement of the surface layer. The residual compressive stress field on the surface can also offset tensile stress during service. Ultrasonic rolling can simultaneously eliminate surface micro-defects and stress concentration sources. The construction of these surface gradient microstructures significantly inhibits the initiation and propagation of surface fatigue cracks and stress corrosion cracks, effectively improving the fatigue resistance, wear resistance, and downhole service life of the anchor steel, further ensuring the structural integrity of the anchor steel under impact loads. Detailed Implementation

[0023] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and 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.

[0024] Example 1

[0025] A method for preparing high impact energy anchor steel includes the following steps: S1, by mass percentage, raw materials containing C 0.18%, Si 0.80%, Mn 1.80%, Cr 0.50%, Mo 0.25%, V 0.08%, Nb 0.02%, Ti 0.015%, with the balance being Fe and unavoidable impurities are melted under vacuum protection and subjected to micro-alloying treatment containing rare earth elements. The micro-alloying treatment includes adding a nickel-magnesium alloy with a pure magnesium content of 0.0010%, a boron-iron alloy with a pure boron content of 0.0015%, and a mixed rare earth ferrosilicon alloy with a total rare earth content of 0.015%, and then casting it into an ingot.

[0026] S2. The ingot is subjected to high-temperature homogenization treatment, and then pulsed magnetostrictive oscillation is applied to its solidified semi-solid paste region while forced cooling is performed to obtain a homogeneous solidified structure with high central equiaxed crystal ratio; the parameters of the pulsed magnetostrictive oscillation are: frequency 100Hz, peak current 150A, and treatment time 60s; the specific water volume of the forced cooling is 0.2L / kg.

[0027] S3. The billet treated in S2 is pre-cooled at the surface to form a significant temperature gradient between the core and the surface. The surface pre-cooling is to rapidly reduce the temperature within a 15mm depth range of the billet surface to below the Ac3 temperature, while the core temperature remains 50°C above the Ac3 temperature. Subsequently, hot rolling deformation is performed, utilizing the rheological stress difference generated by the temperature gradient to induce a non-uniform strain distribution. Final rolling is completed in the non-recrystallized austenite region. The hot rolling deformation includes the initial rolling under the temperature gradient and the finishing rolling at a final rolling temperature of 820°C in the non-recrystallized austenite region. The post-rolling cooling regime is controlled to induce strain-induced precipitation of nanoscale carbonitrides. The controlled post-rolling cooling regime includes: first, ultra-rapid cooling to 700°C, followed by slow cooling at a rate of 0.5°C / s to the 600°C range and holding at that temperature for 20s to promote the dispersion precipitation of V and Nb carbonitrides at a size of 3nm.

[0028] S4. The rolled material is subjected to precision heat treatment in sequence, including low-temperature austenitization, medium-temperature salt bath inoculation in the dual-phase region, isothermal transformation of lower bainite, cryogenic treatment, and low-temperature tempering carbon distribution, to obtain a multiphase structure composed of tempered martensite, lower bainite and stable retained austenite; the precision heat treatment specifically includes: S41, low-temperature austenitization: first, holding at a first temperature 20°C above Ac3 temperature, and then raising the temperature to a second temperature 50°C above the first temperature for a short time.

[0029] S42, Medium-temperature salt bath dual-phase incubation: Quench into the first salt bath at 400℃ and hold isothermally for 10 seconds.

[0030] S43, Lower Bainite Isothermal Transformation: Transfer to a second salt bath at 300℃ and hold isothermally for 5 minutes.

[0031] S44. Cryogenic treatment: Cryogenic treatment is carried out at temperatures below -80℃.

[0032] S45, Low-temperature tempering carbon distribution: Temper at 200℃ for 30 minutes.

[0033] Example 2

[0034] A method for preparing high impact energy anchor steel includes the following steps: S1, by mass percentage, raw materials containing C, 0.19%, Si, 1.00%, Mn, 2.00%, Cr, 0.65%, Mo, 0.30%, V, 0.10%, Nb, 0.03%, Ti, and the balance being Fe and unavoidable impurities are melted under argon (inert gas) protection and subjected to microalloying treatment containing rare earth elements. The microalloying treatment includes adding a nickel-magnesium alloy with a pure magnesium content of 0.0020%, a boron-iron alloy with a pure boron content of 0.0022%, and a mixed rare earth ferrosilicon alloy with a total rare earth content of 0.022%, and then casting it into an ingot.

[0035] S2. The ingot is subjected to high-temperature homogenization treatment, and then pulsed magnetostrictive oscillation is applied to the semi-solid paste region after remelting, while forced cooling is performed to obtain a homogeneous solidified structure with high central equiaxed crystal ratio; the parameters of the pulsed magnetostrictive oscillation are: frequency 150Hz, peak current 200A, and treatment time 120s; the specific water volume of the forced cooling is 0.3L / kg.

[0036] S3. The billet treated in S2 is pre-cooled at the surface to form a significant temperature gradient between the core and the surface. The surface pre-cooling is to rapidly reduce the temperature within a 17mm depth range of the billet surface to below the Ac3 temperature, while the core temperature remains 65°C above the Ac3 temperature. Then, hot rolling deformation is performed, using the rheological stress difference generated by the temperature gradient to induce a non-uniform strain distribution. The final rolling is completed in the non-recrystallized austenite region. The hot rolling deformation includes the initial rolling under the temperature gradient and the finishing rolling at a final rolling temperature of 840°C in the non-recrystallized austenite region. The post-rolling cooling regime is controlled to induce strain-induced precipitation of nanoscale carbonitrides. The controlled post-rolling cooling regime includes: first, ultra-rapid cooling to 725°C, followed by slow cooling at a cooling rate of 1.2°C / s to the 650°C range and holding for 40s to promote the dispersion precipitation of V and Nb carbonitrides at a size of 6nm.

[0037] S3a. Laser shock pretreatment: Laser shock strengthening treatment is applied to the surface of the bar to form a gradient deformation layer with a high-density dislocation substructure and a residual compressive stress field on the surface.

[0038] S4. The rolled material treated by S3a is subjected to precision heat treatment including low-temperature austenitization, medium-temperature salt bath inoculation in the dual-phase region, isothermal transformation of lower bainite, cryogenic treatment, and low-temperature tempering carbon distribution to obtain a multiphase structure composed of tempered martensite, lower bainite and stable retained austenite; the precision heat treatment specifically includes: S41, low-temperature austenitization: first, holding at a first temperature 35°C above the Ac3 temperature, and then raising the temperature to a second temperature 75°C above the first temperature and holding for a short time.

[0039] S42, Medium-temperature salt bath dual-phase incubation: Quench into the first salt bath at 440℃ and hold isothermally for 20 seconds.

[0040] S43, Lower Bainite Isothermal Transformation: Transfer to a second salt bath at 320℃ and hold isothermally for 10 minutes.

[0041] S44. Cryogenic treatment: Cryogenic treatment is carried out at temperatures below -80℃.

[0042] S45, Low-temperature tempering carbon distribution: Temper at 250℃ for 75 minutes.

[0043] S5. Surface Induction Tempering: The surface of the bar is rapidly induction heated and quenched, so that the tempering temperature of the surface 2mm deep area is higher than that of the core, thereby forming a hardness gradient that gradually decreases from the surface to the inside in the radial direction.

[0044] Example 3

[0045] A method for preparing high impact energy anchor steel includes the following steps: S1, by mass percentage, raw materials containing C 0.20%, Si 1.20%, Mn 2.20%, Cr 0.80%, Mo 0.35%, V 0.12%, Nb 0.04%, Ti 0.025%, with the balance being Fe and unavoidable impurities are smelted under nitrogen (inert gas) protection and subjected to microalloying treatment containing rare earth elements. The microalloying treatment includes adding a nickel-magnesium alloy with a pure magnesium content of 0.0030%, a boron-iron alloy with a pure boron content of 0.0030%, and a mixed rare earth ferrosilicon alloy with a total rare earth content of 0.030%, and then casting it into an ingot.

[0046] S2. The ingot is subjected to high-temperature homogenization treatment, and then pulsed magnetostrictive oscillation is applied to its solidified semi-solid paste region while forced cooling is performed to obtain a homogeneous solidified structure with high central equiaxed crystal ratio; the parameters of the pulsed magnetostrictive oscillation are: frequency 200Hz, peak current 250A, and treatment time 180s; the specific water volume of the forced cooling is 0.4L / kg.

[0047] S3. The billet treated in S2 is pre-cooled at the surface to form a significant temperature gradient between the core and the surface. The surface pre-cooling is to rapidly reduce the temperature within a 20mm depth range of the billet surface to below the Ac3 temperature, while the core temperature remains 70°C above the Ac3 temperature. Then, hot rolling deformation is performed, using the rheological stress difference generated by the temperature gradient to induce a non-uniform strain distribution. The final rolling is completed in the non-recrystallized austenite region. The hot rolling deformation includes the initial rolling under the temperature gradient and the finishing rolling at a final rolling temperature of 860°C in the non-recrystallized austenite region. The post-rolling cooling regime is controlled to induce strain-induced precipitation of nanoscale carbonitrides. The controlled post-rolling cooling regime includes: first, ultra-rapid cooling to 750°C, followed by slow cooling at a cooling rate of 2.0°C / s to the 700°C range and holding at that temperature for 60s to promote the dispersion precipitation of V and Nb carbonitrides at a size of 10nm.

[0048] S4. The rolled material is subjected to precision heat treatment in sequence, including low-temperature austenitization, medium-temperature salt bath inoculation in the dual-phase region, isothermal transformation of lower bainite, cryogenic treatment, and low-temperature tempering carbon distribution, to obtain a multiphase structure composed of tempered martensite, lower bainite and stable retained austenite; the precision heat treatment specifically includes: S41, low-temperature austenitization: first, holding at a first temperature 50°C above Ac3 temperature, and then raising the temperature to a second temperature 100°C above the first temperature for a short time.

[0049] S42, Medium-temperature salt bath dual-phase incubation: Quench into the first salt bath at 480℃ and hold isothermally for 30 seconds.

[0050] S43, Lower Bainite Isothermal Transformation: Transfer to a second salt bath at 340℃ and hold isothermally for 15 minutes.

[0051] S44. Cryogenic treatment: Cryogenic treatment is carried out at temperatures below -80℃.

[0052] S45, Low-temperature tempering carbon distribution: Temper at 300℃ for 120 minutes.

[0053] S6, Low-temperature dehydrogenation aging: Hold at 180℃ for 12 hours; and S7, Ultrasonic surface rolling nano-sizing: Perform ultrasonic rolling treatment on the surface of the finished product to form a surface nanocrystalline layer, further improve the surface residual compressive stress and reduce the surface roughness.

[0054] Example 4

[0055] A method for preparing high impact energy anchor steel includes the following steps: S1, by mass percentage, raw materials containing C, 0.185%, Si, 0.90%, Mn, 1.90%, Cr, 0.60%, Mo, 0.28%, V, 0.09%, Nb, 0.025%, Ti, and the balance being Fe and unavoidable impurities are melted under argon (inert gas) protection and subjected to microalloying treatment containing rare earth elements. The microalloying treatment includes adding a nickel-magnesium alloy with a pure magnesium content of 0.0015%, a boron-iron alloy with a pure boron content of 0.0018%, and a mixed rare earth ferrosilicon alloy with a total rare earth content of 0.018%, and then casting it into an ingot.

[0056] S2. The ingot is subjected to high-temperature homogenization treatment, and then pulsed magnetostrictive oscillation is applied to its solidified semi-solid paste region while forced cooling is performed to obtain a homogeneous solidified structure with high central equiaxed crystal ratio; the parameters of the pulsed magnetostrictive oscillation are: frequency 120Hz, peak current 180A, and treatment time 90s; the specific water volume of the forced cooling is 0.25L / kg.

[0057] S3. The billet treated in S2 is pre-cooled at the surface to form a significant temperature gradient between the core and the surface. The surface pre-cooling is to rapidly reduce the temperature within a 16mm depth range of the billet surface to below the Ac3 temperature, while the core temperature remains 55°C above the Ac3 temperature. Subsequently, hot rolling deformation is performed, utilizing the rheological stress difference generated by the temperature gradient to induce a non-uniform strain distribution. Final rolling is completed in the non-recrystallized austenite region. The hot rolling deformation includes initial rolling under the temperature gradient and finishing rolling at a final rolling temperature of 830°C in the non-recrystallized austenite region. The post-rolling cooling regime is controlled to induce strain-induced precipitation of nanoscale carbonitrides. The controlled post-rolling cooling regime includes: first, ultra-rapid cooling to 710°C, followed by slow cooling at a rate of 0.8°C / s to the 620°C range and holding for 30s to promote the dispersion precipitation of V and Nb carbonitrides at a size of 5nm.

[0058] S3a. Laser shock pretreatment: Laser shock strengthening treatment is applied to the surface of the bar to form a gradient deformation layer with a high-density dislocation substructure and a residual compressive stress field on the surface.

[0059] S4. The rolled material treated by S3a is subjected to precision heat treatment including low-temperature austenitization, medium-temperature salt bath inoculation in the dual-phase region, isothermal transformation of lower bainite, cryogenic treatment, and low-temperature tempering carbon distribution to obtain a multiphase structure composed of tempered martensite, lower bainite and stable retained austenite; the precision heat treatment specifically includes: S41, low-temperature austenitization: first holding at a first temperature 25°C above the Ac3 temperature, and then raising the temperature to a second temperature 60°C above the first temperature and holding for a short time.

[0060] S42, Medium-temperature salt bath dual-phase incubation: Quench into the first salt bath at 420℃ and hold isothermally for 15 seconds.

[0061] S43, Lower Bainite Isothermal Transformation: Transfer to a second salt bath at 310℃ and hold isothermally for 8 minutes.

[0062] S44. Cryogenic treatment: Cryogenic treatment is carried out at temperatures below -80℃.

[0063] S45, Low-temperature tempering carbon distribution: Temper at 220℃ for 50 minutes.

[0064] S6, Low-temperature dehydrogenation aging: Hold at 150℃ for 8 hours; and S7, Ultrasonic surface rolling nano-sizing: Perform ultrasonic rolling treatment on the surface of the finished product to form a surface nanocrystalline layer, further improve the surface residual compressive stress and reduce the surface roughness.

[0065] Comparative Example 1

[0066] The difference between this comparative example and Example 1 is that in step S1, the microalloying treatment omits the mixed rare earth silicon-iron alloy, and only adds nickel-magnesium alloy with a pure magnesium content of 0.0010% and boron-iron with a pure boron content of 0.0015%. The other raw material composition and parameters of each step are completely consistent with Example 1.

[0067] Comparative Example 2

[0068] The difference between this comparative example and Example 2 is that in step S1, the Mo element in the raw material is replaced with the W element, and the mass percentage of the W element after replacement is 0.25% to 0.35%. The other raw material composition, microalloying treatment and parameters of each step are completely consistent with those of Example 2.

[0069] Comparative Example 3

[0070] The difference between this comparative example and Example 3 is that in step S1, the Ti element in the raw materials is removed, while the other raw material composition, microalloying treatment and parameters of each step are completely consistent with Example 3.

[0071] Comparative Example 4

[0072] The difference between this comparative example and Example 4 is that in step S1, the nickel-magnesium alloy is removed in the microalloying treatment, and only ferroboron with a pure boron content of 0.0018% and a mixed rare earth ferrosilicon alloy with a total rare earth content of 0.018% are added. The other raw material composition and parameters of each step are completely consistent with those of Example 4.

[0073] (a) Impact energy test.

[0074] Turn on the impact testing machine and preheat for 30 min ± 2 min, maintaining the ambient temperature at 25℃ ± 2℃. Start the calibration program and calibrate the instrument's zero point using a 100J ± 1J standard impact block, performing three consecutive calibrations. The calibration error should be ≤ ± 1J. Testing can only begin after confirming the instrument is operating normally. Take out each group of parallel samples sequentially and, according to their numbers, place them into the sample holder of the testing machine, ensuring the sample notch faces the impact hammer. The clamping depth should be 10mm ± 0.1mm, ensuring a secure clamping without looseness; the sample should not shift when gently pushed by hand. Before testing each group of samples, recheck the clamping position to avoid clamping deviations. Start the testing machine. After the impact hammer strikes the sample, immediately record the impact energy value (unit: J) of each sample, accurate to 0.1J. Test each group of three parallel samples independently, with a testing interval of 1 min ± 0.1 min to prevent sample detachment or abnormal fracture locations during testing. The fracture location must be within ± 1mm ​​of the notch; otherwise, the test is considered invalid. After testing, calculate the average impact energy of each group of samples, accurate to 0.1J.

[0075] (ii) Hardness test.

[0076] Turn on the Brinell hardness tester and preheat for 15 min ± 2 min at an ambient temperature of 25℃ ± 2℃. Calibrate the instrument using a 200 HBW ± 2 HBW standard hardness block, performing three consecutive calibrations with a calibration error ≤ ± 2 HBW to ensure the instrument's measurement accuracy meets requirements. Place the hardness test sample on the hardness tester's worktable and adjust the sample position so that the test surface is perpendicular to the indenter, with a perpendicularity error ≤ 0.5°. Apply a load of 3000 N ± 5 N, hold the load for 10 s ± 0.1 s, and then unload it at a uniform speed. Record the hardness value (unit: HBW), accurate to 1 HBW. Test three points on each sample at different locations, with a spacing of not less than 2.0 mm ± 0.1 mm between test points, and ensure the test points are not close to the sample edge, with a distance ≥ 1.5 mm from the edge. Calculate the average hardness of each sample, accurate to 0.1 HBW. Then calculate the average hardness of each group of three parallel samples, accurate to 0.1 HBW.

[0077] The results are shown in Table 1.

[0078] Table 1. Experimental results of different embodiments and comparative examples

[0079]

[0080] As shown in Table 1, the average impact energy and average hardness of Examples 1-4 are generally higher than those of Comparative Examples 1-4, which indicates that the high impact energy anchor steel preparation method provided by the present invention has significant advantages in improving the performance of anchor steel.

[0081] The average impact energy of Example 1 was 34.8 J, while that of Comparative Example 1 was 28.4 J. The impact energy of Example 1 was significantly higher than that of Comparative Example 1. Comparative Example 1 omitted the mixed rare-earth ferrosilicon alloy in step S1's microalloying treatment. In this invention, rare earth elements, along with microalloying elements such as magnesium and boron, can achieve grain boundary purification and inclusion refinement modification, ultimately obtaining a homogeneous solidified structure with high central equiaxed crystal ratio, no obvious columnar crystals, and low compositional segregation. This homogeneous structure eliminates the weak core area and stress concentration source at the root, avoiding brittle fracture caused by uneven structure, and significantly improving the overall plasticity and impact toughness uniformity of the billet. Comparative Example 1 lacked rare earth elements, failing to fully utilize the effects of grain boundary purification and inclusion refinement modification, resulting in poorer structure uniformity, reduced impact toughness, and consequently, lower impact energy.

[0082] The average impact energy of Example 2 was 38.3 J, while that of Comparative Example 2 was 31.3 J. The impact energy of Example 2 was higher than that of Comparative Example 2. In Comparative Example 2, Mo was replaced with W in the raw materials. In this invention, Mo works synergistically with other elements during subsequent heat treatment and other processes, contributing to the formation of a suitable microstructure and improving material performance. However, W and Mo have different properties, and replacing W may not produce the same synergistic effect as Mo, affecting the material's microstructure and properties, leading to a decrease in impact energy.

[0083] The average impact energy of Example 3 was 39.2 J, while that of Comparative Example 3 was 30.6 J. The impact energy of Example 3 was higher than that of Comparative Example 3. Comparative Example 3 omitted Ti from its raw materials. In this invention, Ti may participate in the formation of specific compounds or interact with other elements, affecting the microstructure and properties of the material. The absence of Ti may alter the microstructure of the material, leading to a decrease in impact toughness and impact energy.

[0084] The average impact energy of Example 4 was 36.5 J, while that of Comparative Example 4 was 32.1 J. The impact energy of Example 4 was higher than that of Comparative Example 4. In Comparative Example 4, the nickel-magnesium alloy was omitted during the microalloying treatment in step S1. In this invention, magnesium works synergistically with rare earth elements and boron to achieve grain boundary purification and inclusion refinement, improving the material's microstructure uniformity and impact toughness. Without magnesium, this synergistic effect is weakened, the material's microstructure uniformity deteriorates, and the impact energy decreases.

[0085] The average hardness of Example 1 was 286.7 HBW, while that of Comparative Example 1 was 274.7 HBW, indicating that Example 1 had a higher hardness than Comparative Example 1. This was also because Comparative Example 1 lacked rare earth elements, which prevented it from fully utilizing the grain boundary purification and inclusion refinement effects, resulting in poorer microstructure uniformity and a less effective hardness improvement than Example 1. In this invention, a homogeneous microstructure and a suitable microstructure formed during subsequent heat treatment contribute to improving the material's hardness, while the lack of rare earth elements affected this process.

[0086] The average hardness of Example 2 was 292.3 HBW, while that of Comparative Example 2 was 281.7 HBW. The hardness of Example 2 was higher than that of Comparative Example 2. In Comparative Example 2, Mo was replaced by W. The microstructure formed by Mo in the material has a positive impact on hardness, while the replacement of W may not form the same microstructure, resulting in a less effective hardness improvement than in Example 2.

[0087] The average hardness of Example 3 was 295.7 HBW, while that of Comparative Example 3 was 279.7 HBW. The hardness of Example 3 was higher than that of Comparative Example 3. Comparative Example 3 lacked Ti, which may participate in the formation of the reinforcing phase or affect the microstructure of the material, thus contributing to its hardness. The lack of Ti weakened the reinforcing effect and reduced the hardness of the material.

[0088] The average hardness of Example 4 was 288.7 HBW, while that of Comparative Example 4 was 283.2 HBW. The hardness of Example 4 was higher than that of Comparative Example 4. Comparative Example 4 lacked a nickel-magnesium alloy, which affected the role of magnesium in the material. It could not fully exert its synergistic effect with other elements to improve the hardness of the material, resulting in a lower hardness than Example 4.

[0089] In summary, this invention significantly improves the impact energy and hardness of anchor bolt steel through a series of measures, including using vacuum or inert gas protection and microalloying treatment during the raw material smelting stage, applying pulsed magnetostrictive oscillation and forced cooling to the semi-solid paste region after ingot solidification or remelting, specific control during hot rolling deformation and post-rolling cooling, and surface modification processes.

[0090] In the description of this specification, the reference to terms such as "embodiment," "various embodiments," etc., indicates that a specific feature, structure, material, or characteristic described in connection with that embodiment or preparation example is included in at least one embodiment of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments.

[0091] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A high-impact anchor bolt steel, characterized in that, It is prepared by a method including the following steps: S1. By mass percentage, raw materials containing C, 0.18%–0.20%, Si, 0.80%–1.20%, Mn, 1.80%–2.20%, Cr, 0.50%–0.80%, Mo, 0.25%–0.35%, V, 0.08%–0.12%, Nb, 0.02%–0.04%, Ti, 0.015%–0.025%, with the balance being Fe and unavoidable impurities, are melted under vacuum or inert gas protection and subjected to microalloying treatment containing rare earth elements, and then cast into ingots; S2. The ingot is subjected to high-temperature homogenization treatment, and then pulsed magnetostrictive oscillation is applied to the semi-solid paste region after solidification or remelting, while forced cooling is performed to obtain a homogeneous solidified structure with high central equiaxed crystal ratio. S3. The billet treated by S2 is pre-cooled on the surface to form a significant temperature gradient between the core and the surface. Then, hot rolling deformation is carried out. The rheological stress difference generated by this temperature gradient is used to induce a non-uniform strain distribution. The final rolling is completed in the non-recrystallized austenite region, and the post-rolling cooling regime is controlled to induce strain-induced nanoscale carbonitride precipitation. S4. The rolled material is subjected to precision heat treatment in sequence, including low-temperature austenitization, medium-temperature salt bath inoculation in the dual-phase region, isothermal transformation of lower bainite, cryogenic treatment, and low-temperature tempering carbon distribution, to obtain a multiphase structure composed of tempered martensite, lower bainite and stable retained austenite.

2. The high impact energy anchor steel according to claim 1, characterized in that, In step S1, the microalloying treatment includes adding a nickel-magnesium alloy with a pure magnesium content of 0.0010% to 0.0030%, a boron-iron alloy with a pure boron content of 0.0015% to 0.0030%, and a mixed rare earth ferrosilicon alloy with a total rare earth content of 0.015% to 0.030%.

3. The high impact energy anchor steel according to claim 1, characterized in that, In step S2, the parameters of the pulsed magnetostrictive oscillation are: frequency 100-200Hz, peak current 150-250A, and processing time 60-180s; the specific water volume of the forced cooling is 0.2-0.4L / kg.

4. The high impact energy anchor steel according to claim 1, characterized in that, In step S3, the surface precooling is to rapidly reduce the temperature of the surface layer of the billet to below the Ac3 temperature within a depth range of 15-20 mm, while the core temperature remains at least 50°C above the Ac3 temperature; the hot rolling deformation includes the initial rolling under the temperature gradient, and the finishing rolling in the non-recrystallized austenite region at a final rolling temperature of 820°C-860°C.

5. The high impact energy anchor steel according to claim 1, characterized in that, In step S3, the controlled post-rolling cooling regime includes: firstly, ultra-rapid cooling to 700℃~750℃ after rolling, followed by slow cooling to the 600℃~700℃ range at a cooling rate of 0.5℃ / s~2.0℃ / s and holding at that temperature for 20~60s, so as to promote the dispersion precipitation of V and Nb carbonitrides at a size of 3nm~10nm.

6. The high impact energy anchor steel according to claim 1, characterized in that, In step S4, the precision heat treatment specifically includes: S41, Low-temperature austenitization: First, hold at a first temperature 20°C to 50°C above Ac3, then raise the temperature to a second temperature 50°C to 100°C above the first temperature and hold for a short time. S42, Medium-temperature salt bath dual-phase incubation: Quench into the first salt bath at 400℃~480℃ and hold isothermally for 10~30s; S43, Lower Bainite Isothermal Transformation: Transfer to a second salt bath at 300℃~340℃ and hold isothermally for 5~15min; S44. Cryogenic treatment: Cryogenic treatment is carried out at temperatures below -80℃; S45, Low-temperature tempering carbon distribution: Temper at 200℃~300℃ for 30~120min.

7. The high impact energy anchor steel according to claim 1, characterized in that, After step S3 and before step S4, step S3a, laser shock pretreatment, is also included: laser shock strengthening treatment is performed on the surface of the bar to form a gradient deformation layer with a high-density dislocation substructure and a residual compressive stress field on the surface.

8. The high impact energy anchor steel according to claim 1 or 7, characterized in that, After step S4, step S5, surface induction tempering, is also included: the surface of the bar is rapidly induction heated and quenched, so that the tempering temperature of the surface layer 1-3mm deep area is higher than that of the core, thereby forming a hardness gradient that gradually decreases from the surface to the inside in the radial direction.

9. The high impact energy anchor steel according to claim 1, characterized in that, After step S4, the process also includes step S6, low-temperature dehydrogenation aging: holding at 120℃~180℃ for 4~12h; and step S7, ultrasonic surface rolling nano-forming: ultrasonic rolling treatment is performed on the surface of the finished product to form a surface nanocrystalline layer, further improve the surface residual compressive stress and reduce the surface roughness.

10. The high impact energy anchor steel according to claim 1, characterized in that, Its microstructure includes tempered martensite, lower bainite, and thin-film retained austenite with a volume fraction of 5% to 15%, and V and Nb carbonitride precipitates with a size of 3 nm to 10 nm are dispersed in the microstructure.

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