A spray formed cold roll work roll and its final heat treatment process

Abstract

The application discloses a final heat treatment process of a spray forming cold rolling work roll, and the process shortens the heat treatment time, saves energy, improves the hardness, red hardness and service life of the spray forming cold rolling work roll blank, and maintains sufficient toughness, so as to meet the harsh working conditions such as the warm rolling process of a super-high-strength steel production line with large rolling force.

Description

Technical Field

[0001] This invention belongs to the technical field of rolling mill roll manufacturing, and relates to a spray-formed cold rolling work roll and its final heat treatment process. Background Technology

[0002] As the mechanical properties of ultra-high strength steel products continue to improve, the yield strength of ultra-high strength steel strips is constantly increasing. During the preparation of ultra-high strength steel strips, the contact stress between the work rolls and the strip is relatively large, which makes it easy to slip during the rolling process. This can affect the quality of the strip, or even cause accidents such as strip deviation or steel piling. Therefore, the selection of work roll material and the corresponding roll system design are important factors affecting the efficiency of the rolling mill.

[0003] High-speed steel rolls have become a research and development hotspot both domestically and internationally due to their excellent wear resistance, accident resistance, and good surface roughness retention as work rolls for downstream stands. They represent the main development direction for future roll materials. High-speed steel cold-rolled rolls produced using the electroslag remelting-forging process contain a large number of carbide-forming elements, forming abundant MC, M2C, and M6C carbides, which improves the strength, wear resistance, and other performance properties of the cold-rolled rolls. However, with the significant increase in alloy content, the roll material exhibits extremely high deformation resistance, making the forging process risky and resulting in a low yield of forged high-speed steel cold-rolled rolls, thus increasing manufacturing costs. Furthermore, with the increase in roll size, the forging process may not completely break down coarse crystalline structures, leading to a large amount of network carbide structures in the rolls, which is extremely detrimental to the rolls' toughness and accident resistance. Cast high-speed steel rolls, due to their relatively coarse dendrite structure, are currently not suitable for the production of high-surface-quality cold-rolled products. Therefore, the development of high-speed steel and advanced manufacturing technologies suitable for various rolling requirements is urgently needed.

[0004] Chinese patent application CN201410414393.0 discloses a method for preparing spray-formed multi-gradient high-speed steel. The high-speed steel's chemical composition, by weight percentage, consists of the following components: C: 1.5-2.0%, Cr: 4.5-5.0%, Mo: 4.5-5.5%, W: 7.0-7.5%, V: 3.0-3.8%, Nb: 0.6-1.0%, Ti: 0.2-0.5%, Si: 0.5-1.0%, Mn: 0.1-0.5%, with the remainder being Fe and unavoidable impurities. The preparation method includes the following steps: batching → melting → spray forming → air cooling → annealing → hot forging → quenching and tempering. Using this method improves nozzle atomization efficiency and deposition efficiency, enabling the continuous preparation of multi-layer composite materials. The beneficial effects obtained are: low porosity of the deposited preform (2-4%), fine and uniform microstructure (average grain size 0.9-20μm), good interfacial bonding performance, and high material yield (72%-85%).

[0005] Chinese Patent Application No. CN201210434226.3 discloses a method for preparing cobalt-free high-speed steel by spray forming. The chemical composition of the high-speed steel, by weight percentage, consists of the following components: C 1.5-1.7%, Cr 4.4-4.8%, Mo 4.8-5.2%, W 7.0-7.4%, V 3.2-3.6%, Nb 0.6-1.0%, Ti 0.2-0.4%, Si 0.5-0.7%, Mn 0.1-0.3%, N 0.06-0.1%, S≤0.03%, P≤0.03%, with the remainder being Fe and unavoidable impurities. The preparation method includes the following steps: 1) batching process; 2) melting process; 3) spray forming; 4) air cooling process; 5) annealing process; 6) hot forging process; 7) quenching and tempering process.

[0006] Chinese Patent Application No. CN201310291138.7 discloses a cryogenic process for improving the lifespan of spray-formed high-speed steel end mills, relating to the field of cryogenic technology, specifically a cryogenic process for improving the lifespan of spray-formed high-speed steel end mills. The cryogenic process for improving the lifespan of spray-formed high-speed steel end mills is characterized by the following specific steps: preheating; vacuum quenching; cooling; first tempering; cryogenic treatment; second tempering. Compared with existing technologies, this process employs a novel combination of vacuum quenching, tempering, and cryogenic treatment for spray-formed high-speed steel end mill materials. After this cryogenic treatment, the wear resistance and hardness of the spray-formed high-speed steel end mill material are significantly improved, thereby enhancing the overall mechanical properties and service life of the end mill.

[0007] In summary, spray forming is a novel integrated process combining rapid solidification, semi-solid processing, and near-net-shape machining. Compared to traditional metallurgical processes, spray forming integrates alloy melting and forming into a single step, significantly reducing the possibility of material oxidation and resulting in finer grains. Therefore, spray forming technology (SF technology) can generally improve the performance of forging alloys and, in some cases, even replace powder metallurgy products. Furthermore, spray forming can be used to develop new alloys, metal matrix composites, in-situ reactive alloys, semi-solid materials, and composite metallurgical products. However, SF technology is a cutting-edge technology, currently only researched and applied in some cutting tools, hot-working dies, and cold-working dies; research on the application of SF technology in the field of roll manufacturing is in its early stages; the technology for preparing high-grade cold-rolled work roll blanks using the SF method is internationally leading and a first in China. Many key and challenging issues remain regarding the materials used in spray forming and the spray-formed cold-rolled work roll blanks obtained through the spray forming process.

[0008] Therefore, there is an urgent need to develop a material and final heat treatment method for spray-formed cold-rolled work rolls, so as to obtain spray-formed cold-rolled work roll blanks with excellent toughness, wear resistance and thermal crack resistance. This would improve the problems that commonly used electroslag remelting (ESR) smelting of high-speed steel work rolls in production lines often encounter during use, such as difficulty in machining the center hole, residual oxide and decarburized layer, the presence of mesh and large carbides in large-size forged high-speed steel materials, more serious carbide distribution in the core area, axial cracks or cracking, and roll body breakage. Summary of the Invention

[0009] In view of the above-mentioned defects in the existing technology, the purpose of this invention is to provide a spray-formed cold rolling work roll and its final heat treatment process. By performing differential temperature quenching and low-high temperature combined tempering on the spray-formed cold rolling work roll blank, the heat treatment time is shortened, energy is saved, the hardness, red hardness and service life of the spray-formed cold rolling work roll blank are improved, and sufficient toughness is maintained, thereby meeting the harsh working conditions such as the high rolling force of the warm rolling process in the production line of ultra-high strength steel such as non-oriented silicon steel.

[0010] To achieve the above objectives, the present invention adopts the following technical solution:

[0011] A first aspect of the present invention provides a spray-formed cold rolling work roll having a microstructure comprising bainite, martensite, carbides, and retained austenite; wherein the content of the bainite and martensite microstructure is 18-20%; and the content of the retained austenite is ≤3%.

[0012] Preferably, the tensile strength R of the spray-formed cold rolling work roll at room temperature is... m The impact energy is 798–830 MPa. KU2 It has a strength of 3-4 J and a hardness HSD of 90-95.

[0013] Preferably, the material of the spray-formed cold-rolled work roll blank comprises the following components by weight percentage: C: 0.82-1.07%, Si: 0.6-1.0%, Mn: 0.4-0.6%, P≤0.03%, S≤0.01%, Cr: 5.5-7.5%, W: 0.8-1.2%, Mo: 1.0-1.3%, V: 2.0-2.5%, N: 0.3-0.5%, Co: 1.2-2.0%, with the balance being Fe and other unavoidable impurities.

[0014] A second aspect of the present invention provides a final heat treatment process for spray-formed cold rolling work rolls as described in the first aspect of the present invention, wherein the spray-formed cold rolling work roll blank is subjected to differential temperature quenching and low-high temperature combined tempering to finally obtain the spray-formed cold rolling work roll.

[0015] Preferably, the spray-formed cold-rolled work roll blank is prepared by the following method:

[0016] Based on the material ratio of the spray-formed cold rolling work roll, the raw material is then subjected to spray forming, hot pressing sintering, forging, and post-forging annealing to finally obtain the spray-formed cold rolling work roll blank.

[0017] Preferably, the differential temperature quenching process is as follows:

[0018] (1) A single temperature equalization process is performed by heating the spray-formed cold-rolled work roll blank to 600±10℃ and performing a temperature equalization process for 8 to 10 minutes.

[0019] (2) Secondary temperature equalization treatment: The spray-formed cold rolling work roll blank after the first temperature equalization treatment is heated to 800±10℃ and subjected to temperature equalization treatment for 8 to 10 minutes.

[0020] (3) Quenching: The spray-formed cold-rolled work roll blank after the second uniform temperature treatment is heated to 1250-1300℃ and held for 10-15 minutes. Then it is cooled in the furnace to 1100-1200℃ and held for 45-60 minutes. After that, it is cooled to below 300℃ by blowing argon gas. It is then air-cooled to room temperature and subjected to the low-high temperature combined tempering treatment within 2-3 hours.

[0021] Preferably, in the differential temperature quenching process:

[0022] During the heating process in steps (1) and (2), the heating rate is 200–230 °C / h; and / or

[0023] In step (3), during the argon gas cooling process, the cooling rate is controlled to be 4-4.5℃ / s.

[0024] Preferably, the low-high temperature combined tempering process includes low-temperature tempering and high-temperature tempering;

[0025] The low-temperature tempering: The spray-formed cold-rolled work roll blank after the differential temperature quenching treatment is heated to 320-350°C and held for 2-4 hours, then cooled in the furnace to 200-210°C, and then cooled to room temperature after being taken out of the furnace.

[0026] The high-temperature tempering involves further heating the spray-formed cold-rolled work roll blank, which has undergone the low-temperature tempering treatment, to 540–580°C and holding it at that temperature for 2–4 hours. After that, it is cooled in the furnace to 200–210°C and then removed from the furnace and cooled to room temperature.

[0027] Preferably, during the low-temperature tempering and high-temperature tempering processes, the heating rate is 200–230 °C / h; and / or

[0028] In the aforementioned low-high temperature combined tempering process, the high-temperature tempering is repeated 2 to 3 times; and / or

[0029] During both the low-temperature tempering and high-temperature tempering processes, air cooling is used for the cooling process.

[0030] The design principle of the components in the spray-formed cold-rolled work roll blank provided by this invention is as follows: Based on high-speed steel, reduce Mo and W content, increase V content to maintain its secondary hardening ability, increase Cr content to improve hardenability, reduce total carbides, add appropriate amount of N to improve microstructure uniformity and increase steel toughness, adjust carbide type structure, and increase the proportion of VC type. Experimental results show that, although the alloy content is reduced in this design, the high-grade cold-rolled work roll blank still maintains the high wear resistance, high-temperature tempering secondary hardening ability, and high thermal stability (resistance to softening) of the original high-speed steel, while its toughness is 2 to 4 times higher.

[0031] The design features of the Co content are as follows: Based on the research on the influence of Co content on the secondary hardness of high-speed steel, the secondary hardness of the roll blank can be rapidly improved when the Co content is controlled within the range of 0-3%. When the Co content is less than 1.2%, the secondary hardening effect is not obvious. When the Co content is 3%, the secondary hardening is improved the most, but the corresponding production cost will increase significantly by 10-12%. Therefore, the most effective and optimal Co content is 1.2-2.0%, which ensures both the improvement of hardness and the cost-effectiveness.

[0032] The design features of the carbon content are as follows: Based on the chemical equilibrium carbon theory, increasing the carbon content can increase the carbon content in the austenite of high-speed steel during quenching and heating, and enhance the dispersion hardening effect during tempering, thereby improving the hardness at room temperature and high temperature. However, excessive carbon content will affect the properties of the steel itself. Based on the alloying principle, the equilibrium carbon calculation formula C = 0.033W + 0.063Mo + 0.060Cr + 0.2V is used to control the carbon content, and the final carbon content is determined to be 0.82-1.07%. When the carbon content is lower than 0.82%, the alloying is insufficient, which will reduce the hardness of the roll blank. When the carbon content is higher than 1.07%, the brittleness of the roll blank will increase, and both will ultimately reduce the service life.

[0033] The design features of the vanadium content are as follows: increasing the vanadium content enhances the grain refinement of the roll blank, thereby improving the high-temperature hardness, high-temperature strength, toughness, and wear resistance of the roll blank. When it melts into austenite at high temperature, it can increase the hardenability of the roll blank and prevent overheating sensitivity. The increase in vanadium content causes the formation of VC carbides in the roll blank, maximizing the role of vanadium and improving wear resistance. However, if the vanadium content is too high, the mechanical properties of the roll blank will be poor after tempering, and the grinding performance will be poor. After repeated experiments, it was finally determined that the vanadium content used in this invention is 2.0-2.5%.

[0034] The design features of the nitrogen (N) content are as follows: Utilizing the patented method, the N content is controlled below 0.5% to weaken the formation of large carbide particles and improve machinability. During the preparation process, vanadium forms VC (vanadium carbide) type carbides. Increasing V significantly increases the proportion of highly wear-resistant MC (M is the carbide-forming element) carbides in the steel, thereby significantly improving wear resistance. Experiments show that excessive N and V readily form VN at high temperatures, and after nucleation, VC grows on top, which is a significant factor in the eventual formation of numerous large carbide particles. Another factor is that excessive N leads to the formation of a large amount of brittle AlN (aluminum nitride inclusions) phase in the steel, which is extremely detrimental to the toughness of the roll blank. Therefore, controlling the N content to 0.3–0.5% can weaken the formation of large carbide particles and reduce the amount of brittle AlN inclusions, with minimal impact on the toughness of the roll blank. Simultaneously, controlling the VC particles also effectively improves machinability.

[0035] The final heat treatment process for spray-formed cold rolling work rolls provided by this invention has the following beneficial effects:

[0036] 1. The final heat treatment process of the spray-formed cold rolling work roll of the present invention shortens the heat treatment time, saves energy, improves the hardness, red hardness, and service life of the spray-formed cold rolling work roll blank by differential temperature quenching and low-high temperature combined tempering, and maintains sufficient toughness, thereby meeting the harsh working conditions such as the high rolling force of the warm rolling process in the production line of ultra-high strength steel such as non-oriented silicon steel.

[0037] 2. The final heat treatment process of the spray-formed cold rolling work roll of the present invention improves the problems that often occur in the production line due to the high carbon, alloy and carbide content of high-speed steel work rolls manufactured by electroslag remelting (ESR). These problems include difficulty in machining the center hole, residual oxide and decarburized layer, and the presence of mesh and large carbide in large-size forged high-speed steel materials, as well as more serious carbide distribution in the core area, axial cracks or cracks, leading to roll body breakage. This invention ensures safe and stable production and cost control.

[0038] 3. The final heat treatment process of the spray-formed cold rolling work roll of this invention features an innovative design of short-time differential temperature quenching and low-high temperature combined tempering. This ensures the working layer thickness and mechanical properties while greatly improving the strength, toughness, and thermal crack resistance of the matrix. It controls the content of bainite and retained austenite, ultimately obtaining a spray-formed cold rolling work roll blank with excellent microstructure and mechanical properties. This completely solves the problems that often occur in the use of work rolls made of ESR smelting and high-speed steel, such as the high carbon, alloy, and carbide content in the material, resulting in difficult machining of the center hole, residual oxide and decarburized layer, and the presence of network and large carbide in large-size forged high-speed steel materials, as well as more severe carbide distribution in the core area, leading to axial cracks or fractures and causing roll body breakage. This fills the gap in the domestic application of spray forming in the field of roll manufacturing and is in a leading position internationally.

[0039] 4. Addressing the shortcomings of conventional ESR high-speed steel roll production lines, which use electroslag remelting (ESR) to manufacture high-speed steel work rolls, such as uneven primary carbides, relatively insufficient wear resistance, and susceptibility to accidents, this invention employs spray forming and forging processes to produce spray-formed cold-rolled work roll blanks. Furthermore, it innovatively designs a differential temperature quenching process and a low-to-high temperature combined tempering process to obtain high-specification spray-formed cold-rolled work roll blanks with excellent toughness, wear resistance, and thermal crack resistance. This solves the rolling challenges of ultra-high-strength steel and has promising prospects for widespread application. Detailed Implementation

[0040] To better understand the above-mentioned technical solutions of the present invention, the technical solutions of the present invention will be further described below in conjunction with embodiments.

[0041] The present invention provides a spray-formed cold rolling work roll, the microstructure of which contains bainite, martensite, carbides and retained austenite; the content of bainite microstructure is 18-20%; and the content of retained austenite is ≤3%.

[0042] The tensile strength R of the spray-formed cold rolling work roll of this invention at room temperature m The impact energy is 798–830 MPa. KU2 It has a strength of 3-4J, a hardness HSD of 90-95, and wear resistance is 20% higher than that of conventional ESR forging rolls.

[0043] The material of the spray-formed cold rolling work roll comprises the following components by weight percentage: C: 0.82–1.07%, Si: 0.6–1.0%, Mn: 0.4–0.6%, P≤0.03%, S≤0.01%, Cr: 5.5–7.5%, W: 0.8–1.2%, Mo: 1.0–1.3%, V: 2.0–2.5%, N: 0.3–0.5%, Co: 1.2–2.0%, with the balance being Fe and other unavoidable impurities. In a further preferred embodiment, the spray-formed cold rolling work roll comprises the following components by weight percentage: C: 0.8%, Si: 0.6%, Mn: 0.5%, P ≤ 0.03%, S ≤ 0.01%, Cr: 7.5%, W: 1.2%, Mo: 1.3%, V: 2.5%, N: 0.3%, Co: 1.8%, with the balance being Fe and other unavoidable impurities.

[0044] This invention focuses on overcoming the aforementioned heat treatment process for spray-formed cold-rolled work roll blanks. The aim is to improve the performance of work rolls made of high-speed steel using electroslag remelting (ESR) on production lines. These work rolls often suffer from problems due to the high carbon, alloy, and carbide content in the material, leading to difficulties in machining the center hole, residual oxide and decarburized layers, and, in large-size forged high-speed steel, the presence of network and large carbide deposits, more severe carbide distribution in the core area, axial cracks, or fissures, resulting in roll body breakage. This research aims to ensure safe and stable production and cost control. Conventional high-speed steel heat treatment processes can achieve sufficiently high hardness and red hardness, but poor toughness. To achieve the above objectives, this invention first analyzes the operating conditions of cold-rolled work rolls used in ultra-high-strength steel rolling, establishes their mechanical properties, and then innovatively develops their heat treatment process. The heat treatment process of this invention can shorten heat treatment time, save energy, improve the hardness, red hardness, and service life of spray-formed cold-rolled work rolls, while maintaining sufficient toughness.

[0045] The final heat treatment process of the spray-formed cold rolling work roll of the present invention is as follows: the spray-formed cold rolling work roll blank is subjected to differential temperature quenching and low-high temperature combined tempering to finally obtain the spray-formed cold rolling work roll. The key to the present invention lies in the differential temperature quenching and low-high temperature combined tempering.

[0046] (I) Differential temperature quenching treatment

[0047] The final heat treatment process of this invention is characterized by ensuring that the strength and hardness of the rolls ultimately meet the harsh working conditions of high rolling force and warm rolling processes in ultra-high strength steel production lines such as non-oriented silicon steel. The final microstructure of the working layer is controlled to contain a small amount of bainite and martensite, with a hardness of 90-95 HSD and a thickness of 70-100 mm on each side. This ensures the hardness, wear resistance, and heat crack resistance of the working layer while also guaranteeing the toughness of the core.

[0048] To achieve the above objectives, it is necessary to strictly control the heating rate, holding time, and cooling method of the repair heat treatment process. Therefore, the differential temperature quenching process is as follows:

[0049] (1) First uniform temperature treatment: The spray-formed cold-rolled work roll blank is heated to 600±10℃ at a heating rate of 200~230℃ / h and subjected to uniform temperature treatment for 8~10min.

[0050] (2) Secondary temperature equalization treatment: The spray-formed cold rolling work roll blank after the first temperature equalization treatment is heated to 800±10℃ at a heating rate of 200~230℃ / h and subjected to temperature equalization treatment for 8~10min.

[0051] (3) Quenching: After the spray-formed cold-rolled work roll blank has undergone two homogenization treatments, it is heated to 1250-1300℃ and held for 10-15 minutes. Then it is cooled in the furnace to 1100-1200℃ and held for 45-60 minutes. After that, it is cooled by blowing argon gas and the cooling rate is controlled at 4.0-5.5℃ / s. It is cooled to below 300℃, removed from the furnace and air-cooled to room temperature. Then it is subjected to low-high temperature combined tempering treatment within 2-3 hours.

[0052] The purpose of the final heat treatment process is to obtain a bainitic and martensitic matrix structure, increase the dispersion and spheroidization of carbides, refine the structure, and improve the strength, hardness and wear resistance of the rolls. Experimental studies have shown that the heating rate should be set between 200 and 230°C / h, and two uniform temperature holding times should be performed at the same time. This can ensure that the working layer of the spray-formed cold rolling work roll blank is heated evenly during the heating process, and that the grains do not grow. At the same time, it can ensure the temperature difference between the roll surface and the roll core, and ensure that the toughness of the roll core is not affected. Therefore, in steps (1) and (2) of this invention, two uniform temperature holding times of 8 to 12 minutes are performed at 600°C and 800°C.

[0053] In step (3), the temperature is rapidly increased at a rate of 200–230 °C / h to 1250–1300 °C for the spray-formed cold-rolled work roll blank. This high temperature is then maintained for 10–15 minutes. The rapid heating is intended to create a temperature difference between the inside and outside of the roll body, allowing the surface to develop a certain depth of austenite while the core remains below the phase transformation point, thus achieving differential temperature quenching. On one hand, the heating layer is controlled by adjusting the heating and holding time, meaning the working layer thickness is controlled at 70–100 mm. During the holding time, the roll surface temperature rises to the required quenching temperature, ensuring complete austenitization of the working layer matrix. On the other hand, the short holding time at a high temperature allows for the full dissolution of more carbides into the matrix, increasing the solubility of matrix alloying elements and carbon, while ensuring the core structure remains unaffected. This also lays a good foundation for subsequent tempering, improving the strength and tempering resistance of the work roll, as well as its thermal cycling stability. The test results show that if the holding time is less than 10 min, the working layer temperature is uneven, the matrix is ​​not sufficiently austenitized, and the final microstructure requirements are not met. If the holding time is greater than 15 min, the matrix grains in the working layer begin to grow at higher temperatures. At the same time, the final quenching temperature layer and the working layer thickness will be greater than 100 mm, the effective core size will be smaller, the toughness will be reduced, and the risk of roll breakage during rolling will be increased.

[0054] The blank is then cooled in the furnace to 1100–1200℃ and held for 45–60 minutes for quenching. The spray-formed cold-rolled work roll contains a large amount of high-melting-point carbides, such as MC, M6C, M7C3, and M23C6. MC is a V carbide that precipitates during eutectic transformation or from austenite, with an initial solution temperature of 1100–1150℃. MC particles are fine and uniformly distributed. M6C is a W and Mo carbide that dissolves in austenite at 1050–1250℃. M6C is quite stable and does not easily aggregate, increasing the hardness and wear resistance of the die. M7C3 is a Cr carbide, a primary eutectic carbide or a secondary carbide precipitated from austenite. It can dissolve W, Mo, V, and other elements, increasing wear resistance and reducing the coefficient of friction. Secondary M7C3 dissolves into austenite at 980–1180℃; M23C6 is another Cr carbide that begins to dissolve at 1020–1050℃, but requires 1150–2000℃ to fully dissolve in austenite. Therefore, a higher quenching temperature of 1100–1200℃ is necessary to ensure that the carbides are fully dissolved into the matrix.

[0055] Extensive experiments have shown that using a higher quenching temperature of 1100–1200℃ can increase the austenitization temperature of alloying elements in the matrix, allowing more alloys to dissolve into the matrix, reducing the amount of twinned martensite and increasing the amount of dislocation martensite. Simultaneously, controlling the quenching cooling rate at 4–5.5℃ / s can yield a mixed microstructure of 18–20% bainite and martensite, avoiding the formation of pearlite and keeping the austenite content below 3%. Therefore, the strength, toughness, and thermal fatigue resistance of the matrix are improved, the hot crack initiation time is delayed, and the crack propagation rate is also reduced.

[0056] (II) Low-high temperature combined tempering treatment

[0057] After quenching, the cold-rolled work roll blank for spray forming must be tempered in a timely manner. The purpose of tempering is fourfold: 1) to completely eliminate quenching stress; 2) to promote the full decomposition of retained austenite; 3) to produce the best secondary hardening effect; and 4) to achieve the required comprehensive mechanical and performance properties.

[0058] The innovation of the tempering process in this invention is the use of a combination of low and high temperature tempering methods, specifically including low temperature tempering and high temperature tempering.

[0059] (1) Low temperature tempering: The spray-formed cold-rolled work roll blank after differential temperature quenching is heated to 320-350℃ at a heating rate of 200-230℃ / h and held at the tempering temperature for 2-4h. Then it is cooled to 200-210℃ in the furnace and cooled to room temperature after being taken out of the furnace.

[0060] (2) High temperature tempering: The spray-formed cold-rolled work roll blank that has been tempered at low temperature is heated to 540-580℃ at a heating rate of 200-230℃ / h and held at the tempering temperature for 2-4h. Then it is cooled to 200-210℃ in the furnace and cooled to room temperature after being taken out of the furnace. The high temperature tempering is repeated 2-3 times.

[0061] The novel process in this invention patent differs from ordinary methods in that the tempering regime after quenching is different, innovatively employing a combination of low- and high-temperature tempering. First, a low-temperature tempering at 320–350℃ is performed for 2–4 hours, followed by a high-temperature tempering at 540–580℃ for 2–4 hours, for a total of 2–3 high-temperature tempering cycles. Air cooling is used after both the low- and high-temperature tempering cycles.

[0062] Conventional tempering methods employ three high-temperature tempering processes. Due to the strong diffusion ability of alloying elements at high temperatures, carbides easily precipitate along the original austenite grain boundaries, and these precipitated carbides are also relatively large, causing grain boundary embrittlement and resulting in poor toughness of the steel. Simultaneously, the secondary hardening effect is not fully realized during high-temperature tempering, thus preventing the steel from reaching its maximum hardness and red hardness.

[0063] The low-high temperature combined tempering of this invention, by first tempering at around 320-350℃, causes carbides to precipitate uniformly along the martensite inertial planes, resulting in fine carbides. During low-temperature tempering, the diffusion ability of alloying elements is low, preventing the formation of large-sized carbides along the original austenite grain boundaries; therefore, the grain boundaries are relatively clean, improving the strength and toughness of the matrix. Subsequently, during high-temperature tempering at 540-580℃, the carbides precipitated during low-temperature tempering decompose and refine, while new alloy carbides precipitate. The core of these newly precipitated carbides is fine cementite; therefore, the newly precipitated alloy carbides do not agglomerate towards the grain boundaries but are uniformly distributed along the grain interior and grain boundaries. Multiple experimental results show that the finer the cementite precipitated during low-temperature tempering, the greater its dispersion and the more uniform its distribution; consequently, the finer the newly precipitated alloy carbides, the greater their dispersion and the more uniform their distribution.

[0064] Hardness testing after low-high temperature tempering shows that the new tempering process fully utilizes the secondary hardening effect while controlling the austenite content to within 3%, thus improving the hardness and red hardness of the steel, as well as its toughness.

[0065] The preparation process of the spray-formed cold-rolled work roll blank used in the above process is as follows:

[0066] The injection-formed cold-rolled work roll blank is prepared by proportioning the raw materials according to the material composition of the injection-formed cold-rolled work roll, and then obtaining the injection-formed cold-rolled work roll blank through injection forming, hot pressing sintering, forging, and post-forging annealing. The specific process of the final heat treatment of the injection-formed cold-rolled work roll blank is as follows:

[0067] (1) Spray forming process

[0068] The process parameters used for preparing the deposited billet are as follows:

[0069] Furnace power / kW: 18~22kW; Furnace power refers to the power of the furnace during the raw material melting process.

[0070] Alloy melt mass flow rate (kg / min): 3.5~4.5 kg / min;

[0071] Nebulizing gas: Nitrogen gas with a purity of ≥99%, such as 99.99% high-purity nitrogen gas;

[0072] Atomizing gas pressure / MPa: 0.8~1.2MPa;

[0073] Atomization angle (°): 30°~60°; Atomization angle is the angle at which the atomized gas is injected.

[0074] Liquid guide tube extension length / mm: 3~5mm;

[0075] The jetting temperature of the molten droplet is 820–880℃.

[0076] Preheating temperature of deposition substrate: 500~600℃;

[0077] Deposition substrate rotation speed (r / min): 9–12 r / min;

[0078] (2) Hot pressing sintering process

[0079] The deposited blanks prepared by the spray forming process have problems such as porosity and pinholes. If the diameter is forged, porosity is likely to form in the core, which will affect the overall strength of the blank. Therefore, the spray-formed deposited blanks are subjected to hot pressing sintering treatment to improve density and homogenize before forging.

[0080] The hot pressing sintering process is as follows:

[0081] (2.1) The deposited preform obtained by spray molding is heated to 600±10℃ at a heating rate of 200~230℃ / h and subjected to a homogenization treatment for 8~10min;

[0082] (2.2) Continue heating the deposited billet to 800±10℃ at a heating rate of 200~230℃ / h, and perform a homogenization treatment for 8~10min;

[0083] (2.3) Homogenization sintering: The deposited billet is further heated to 1450–1550℃ and held for 5–8 minutes to rapidly achieve a density of 90–93% or higher at a higher temperature. Then, it is cooled in the furnace to 1050–1250℃ and held for 4–6 hours. At this temperature, the atomic diffusion activation energy of the deposited billet is less than the grain boundary migration energy. The long holding time can increase the density of the deposited billet to 98–99% without grain growth. Afterward, it is cooled in the furnace to below 300℃, removed from the furnace and air-cooled to room temperature in preparation for subsequent forging.

[0084] Through the aforementioned hot-pressing sintering process that enhances density and homogenizes the material, the density of the deposited billet obtained by spray forming is increased from the original 85-88% to 98-99%. During this process, carbides are also decomposed, which helps solve problems such as ingot breakage, severe oxidation, flaw detection defects, M2C Leadslow decomposition and refinement, and difficulties in annealing the microstructure during forging.

[0085] (3) Forging process

[0086] Forging further breaks down carbides, improves the density, strength, and toughness of the roll blank, and optimizes its microstructure. In this process, the deposited billet, after hot pressing and sintering, is forged into a forged billet. The parameters involved in the forging process are as follows:

[0087] Forging ratio: 4-6;

[0088] Dependent variable: 70-75%;

[0089] Initial forging temperature / ℃: 1200~1250℃;

[0090] Final forging temperature / ℃: 1100~1150℃;

[0091] Die pressing speed (mm / s): 0.05~0.08mm / s;

[0092] Spiral feed / °: 20~30°;

[0093] Because the spray-formed cold-rolled work roll blank prepared by this invention has a relatively high alloy content, its deformation temperature range is relatively narrow. Based on the simulation calculation of the melting point of the new steel grade and forging experience, the high-temperature homogenization heating and initial forging temperature of this roll blank are designed to be 1200-1250℃, and the final temperature is designed to be 1100-1150℃.

[0094] During the forging process, a 30MN fast forging blank is used to eliminate the loose core structure of the forging material, effectively break down carbides, and increase the forging ratio. Experimental results show that the forging ratio of the deposited blank in this invention is suitable to be selected as 4 to 6.

[0095] In forging, large voids are caused by excessive pressure deformation during rapid forging, resulting in internal transverse cracks that do not extend to the surface and connect to the outside. In rotary forging, these cracks, instead of being welded together, are stretched open by the rotational torque, forming voids. This indicates that the forging method amplifies existing internal cracks (especially in the core) under the combined force of radial forging force and helical feed, leading to voids. Multiple experiments show that the optimal die pressing rate, rotational speed (20–30°), and strain (70–75%) for forging of the spray-formed deposited billet in this invention are optimal during the forging process.

[0096] The above innovative forging process enables a forging yield of 70%, which is 40% higher than the conventional maximum yield of 50%.

[0097] (4) Post-forging annealing process

[0098] The post-forging annealing process plays an irreplaceable role in eliminating residual stress after forging, promoting carbide spheroidization, and improving the stability of the microstructure and mechanical properties of the forging billet.

[0099] In the post-forging annealing process of this invention: after forging, the forged billet is slowly cooled to 600-700°C and then held at that temperature. After 10 hours, it is transferred to an annealing furnace for annealing, with the annealing temperature controlled at 760-810°C and held for 4-6 hours. During the above process, the microstructure is transformed into supercooled austenite-pearlite-bainite or martensite.

[0100] In this invention, the spray-formed deposited billet has excellent hardenability and a high Ms phase transformation point. If the slow cooling after forging is inadequate or the hot-transfer annealing is not timely, resulting in a low temperature, the head will experience greater structural stress and a longer longitudinal cracking extension, leading to a significant reduction in yield. To prevent this, this invention employs a slow cooling device with good heat preservation performance, with a heat preservation temperature designed to be 600–700°C. It also ensures that ignition and heating are carried out within 10 hours of the hot-transfer after forging, guaranteeing that the billet temperature does not drop below the Ms point before annealing.

[0101] When forging billets are annealed at a relatively high temperature, experimental studies show that when the initial furnace temperature for annealing is 600-700℃, the forging billets do not undergo bainite and martensite transformation, and the microstructure is entirely supercooled austenite. Since the supercooled austenite does not reach the effective pearlite transformation zone of 720-760℃, it does not undergo any changes. If the temperature drops below the Ms point, the supercooled austenite transforms into martensite from the outside to the inside. Due to the large cross-section, this causes significant microstructure transformation stress cracking. The cracking characteristics are crack opening and radial cracking to the center, resulting in the scrapping of the forging billet.

[0102] Therefore, the innovative design of this invention involves reheating the forging billet promptly after it enters the annealing furnace. Before the billet transforms into martensite or a small amount of bainite, the annealing temperature is raised to 760–810°C. This allows the supercooled austenite to pass through the effective pearlite transformation zone of 720–760°C for a sufficient time during the subsequent slow cooling process, ensuring that the austenite is fully transformed into pearlite. Although this process appears to be the same as low-temperature annealing (high-temperature tempering), it is essentially a phase transformation annealing. The difference is that the austenite is not formed by heating the billet above the Ac1 point in the annealing furnace, but by heating it during forging deformation.

[0103] The above processing yields a spray-formed cold-rolled work roll blank with a density ≥99% and a grain size ≤16μm. The gas content of the spray-formed cold-rolled work roll blank is: [H]≤2.0ppmm, [O]≤25ppmm, [N]≤40ppmm. The spray-formed cold-rolled work roll blank is free of internal defects such as white spots, internal cracks, shrinkage cavities, and non-metallic inclusions, and the surface is free of visually visible defects such as cracks, folds, scars, and inclusions. Ultrasonic testing is performed according to GB / T13314, and the quality level should meet the requirements of Grade A in Table A.2: no echo defects with an equivalent diameter greater than φ2 appear in the working layer of the roll body. Surface wave testing is performed according to GB / T 23904, with grass-like waves (noise waves) below 20% and no defect waves on the roll surface.

[0104] Therefore, this invention innovatively employs short-time differential temperature quenching and a low-high temperature tempering process to ensure the working layer thickness and mechanical properties while significantly improving the strength, toughness, and thermal crack resistance of the matrix. This results in controlled bainite and retained austenite content, ultimately producing a spray-formed work roll with excellent microstructure and mechanical properties. This invention completely solves the problems often encountered in ESR-smelted high-speed steel work rolls during use, such as difficult machining of the center hole, residual oxide and decarburized layers, and the presence of mesh-like and large carbide deposits in large-size forged high-speed steel materials, as well as more severe carbide distribution in the core area, leading to axial cracks or fractures and ultimately roll body breakage. It fills a gap in the domestic application of spray forming in roll manufacturing and is at the forefront internationally.

[0105] The microstructure and room temperature and high temperature mechanical properties of the spray-formed cold-rolled work roll prepared in this invention are compared with those of ESR forged high-speed steel work rolls produced by conventional heat treatment processes, as shown in Table 1:

[0106] Table 1 Comparison of microstructure and mechanical properties at room temperature and high temperature

[0107]

[0108] The final heat treatment process of the spray-formed cold rolling work roll of the present invention will be further described below with specific examples.

[0109] Example 1

[0110] In this embodiment, the raw materials are proportioned according to the material composition of the spray-formed cold rolling work roll, and then the spray-formed cold rolling work roll blank is obtained through spray forming, hot pressing sintering, forging, and post-forging annealing. The material of the spray-formed cold rolling work roll includes the following components by weight percentage: C: 0.8%, Si: 0.6%, Mn: 0.5%, P≤0.03%, S≤0.01%, Cr: 7.5%, W: 1.2%, Mo: 1.3%, V: 2.5%, N: 0.3%, Co: 1.8%, with the balance being Fe and other unavoidable impurities.

[0111] The final heat treatment process for the spray-formed cold rolling work roll in this embodiment is as follows:

[0112] (I) Differential temperature quenching process

[0113] (1) First uniform temperature treatment: the spray-formed cold rolling work roll blank is heated at a heating rate of 200℃ / h and then subjected to uniform temperature holding treatment at 600℃ for 8 minutes.

[0114] (2) Secondary temperature equalization treatment: The spray-formed cold rolling work roll blank is heated at a heating rate of 200℃ / h and then subjected to temperature equalization and heat preservation treatment at 800℃ for 8 minutes.

[0115] (3) Quenching: The spray-formed cold-rolled work roll blank is heated to 1250℃ for quenching and held for 15 minutes. Then it is cooled to 1200℃ in the furnace and held for 60 minutes. Then it is cooled by blowing argon gas and controlling the cooling rate to 5℃ / s. After cooling to below 300℃, it is taken out of the furnace and air-cooled to room temperature. Then it is tempered within 2 hours.

[0116] (II) Low-high temperature combined tempering process

[0117] Low-temperature tempering: The spray-formed cold-rolled work roll blank that has been quenched is heated at a heating rate of 200℃ / h, then held at a tempering temperature of 350℃ for 4h, cooled to 200℃ in the furnace, and then air-cooled to room temperature after being removed from the furnace.

[0118] High-temperature tempering: The spray-formed cold-rolled work roll blank, which has been tempered at low temperature, is heated at a heating rate of 200℃ / h, and then held at a tempering temperature of 560℃ for 4 hours. It is then cooled to 200℃ in the furnace and air-cooled to room temperature after being removed from the furnace. The high-temperature tempering process is repeated twice.

[0119] The spray-formed cold rolling work roll obtained by the innovative heat treatment process in this embodiment has a tensile strength of 820 MPa and an impact energy of A at room temperature. KU2 The J value is 3.5 and the HSD value is 95. Table 2 compares the machine test results of the spray-formed cold-rolled work roll prepared in this embodiment with those of the ESR forged high-speed steel work roll prepared by conventional heat treatment process.

[0120] Table 2 Comparison of On-Machine Test Results

[0121]

[0122] Example 2

[0123] In this embodiment, the raw materials are proportioned according to the material composition of the spray-formed cold rolling work roll, and then the spray-formed cold rolling work roll blank is obtained through spray forming, hot pressing sintering, forging, and post-forging annealing. The material of the spray-formed cold rolling work roll includes the following components by weight percentage: C: 0.8%, Si: 0.6%, Mn: 0.5%, P≤0.03%, S≤0.01%, Cr: 7.5%, W: 1.2%, Mo: 1.3%, V: 2.5%, N: 0.3%, Co: 1.8%, with the balance being Fe and other unavoidable impurities.

[0124] The final heat treatment process for the spray-formed cold rolling work roll in this embodiment is as follows:

[0125] (I) Differential temperature quenching process

[0126] (1) First uniform temperature treatment: The spray-formed cold rolling work roll blank is heated at a heating rate of 200℃ / h and then subjected to uniform temperature holding treatment at 605℃ for 8 minutes.

[0127] (2) Secondary temperature uniformization treatment: The spray-formed cold rolling work roll blank is heated at a heating rate of 200℃ / h and subjected to a temperature uniformization and heat preservation treatment at 805℃ for 8 minutes.

[0128] (3) Quenching: The spray-formed cold-rolled work roll blank is heated to 1280℃ for quenching and held for 12 minutes. Then it is cooled to 1180℃ in the furnace and held for 50 minutes. Then it is cooled by blowing argon gas and controlling the cooling rate to 4.5℃ / s. After cooling to below 300℃, it is taken out of the furnace and air-cooled to room temperature. Then it is tempered within 2.5 hours.

[0129] (II) Low-high temperature combined tempering process

[0130] Low-temperature tempering: The spray-formed cold-rolled work roll blank after quenching is heated at a heating rate of 200℃ / h, then held at a tempering temperature of 340℃ for 3h, cooled in the furnace to 205℃, and then air-cooled to room temperature after being removed from the furnace.

[0131] High-temperature tempering: The spray-formed cold-rolled work roll blank, which has been tempered at low temperature, is heated at a heating rate of 200℃ / h, and then held at a tempering temperature of 550℃ for 3h. It is then cooled to 200℃ in the furnace and air-cooled to room temperature after being removed from the furnace. The high-temperature tempering process is repeated twice.

[0132] The spray-formed cold rolling work roll obtained by the innovative heat treatment process in this embodiment has a tensile strength of 825 MPa and an impact energy of A at room temperature. KU2 The J value is 3.2 and the HSD value is 93. Table 3 compares the machine test results of the spray-formed cold-rolled work roll prepared in this embodiment with those of the ESR forged high-speed steel work roll prepared by conventional heat treatment process.

[0133] Table 3 Comparison of On-Machine Test Results

[0134]

[0135] Example 3

[0136] In this embodiment, the raw materials are proportioned according to the material composition of the spray-formed cold rolling work roll, and then the spray-formed cold rolling work roll blank is obtained through spray forming, hot pressing sintering, forging, and post-forging annealing. The material of the spray-formed cold rolling work roll includes the following components by weight percentage: C: 0.8%, Si: 0.6%, Mn: 0.5%, P≤0.03%, S≤0.01%, Cr: 7.5%, W: 1.2%, Mo: 1.3%, V: 2.5%, N: 0.3%, Co: 1.8%, with the balance being Fe and other unavoidable impurities.

[0137] The final heat treatment process for the spray-formed cold rolling work roll in this embodiment is as follows:

[0138] (I) Differential temperature quenching process

[0139] (1) First uniform temperature treatment: The spray-formed cold rolling work roll blank is heated at a heating rate of 200℃ / h and then subjected to uniform temperature holding treatment at 610℃ for 8 minutes.

[0140] (2) Secondary temperature equalization treatment: The spray-formed cold rolling work roll blank is heated at a heating rate of 200℃ / h and then subjected to temperature equalization and heat preservation treatment at 810℃ for 8 minutes.

[0141] (3) Quenching: The spray-formed cold-rolled work roll blank is heated to 1260℃ for quenching and held for 15 minutes. Then it is cooled to 1150℃ in the furnace and held for 45 minutes. Then it is cooled by blowing argon gas and the cooling rate is controlled at 4.8℃ / s. After cooling to below 300℃, it is taken out of the furnace and air-cooled to room temperature. Then it is tempered within 2 hours.

[0142] (II) Low-high temperature combined tempering process

[0143] Low-temperature tempering: The spray-formed cold-rolled work roll blank after quenching is heated at a heating rate of 210℃ / h, then held at a tempering temperature of 330℃ for 3.5h, cooled to 200℃ in the furnace, and then air-cooled to room temperature after being removed from the furnace.

[0144] High-temperature tempering: The spray-formed cold-rolled work roll blank, which has been tempered at low temperature, is heated at a heating rate of 200℃ / h, and then held at a tempering temperature of 570℃ for 2 hours. It is then cooled to 200℃ in the furnace and air-cooled to room temperature after being removed from the furnace. The high-temperature tempering process is repeated twice.

[0145] The spray-formed cold rolling work roll obtained by the innovative heat treatment process in this embodiment has a tensile strength of 815 MPa and an impact energy of A at room temperature. KU2 The J value is 3.1, and the HSD hardness is 92. Table 4 compares the machine test results of the spray-formed cold-rolled work roll prepared in this embodiment with those of the ESR forged high-speed steel work roll prepared by conventional heat treatment process.

[0146] Table 4 Comparison of On-Machine Test Results

[0147]

[0148] In summary, this invention addresses the needs of rolling and the problems existing in existing rolls by employing spray forming and forging processes to obtain high-quality cold-rolled work rolls. Through extensive experimentation, a differential temperature quenching process and a low-to-high temperature combined tempering process were innovatively designed to obtain high-specification cold-rolled work roll spray-formed forging blanks with excellent strength, toughness, wear resistance, and thermal crack resistance. This solves the rolling problem of ultra-high-strength steel and has good prospects for widespread application.

[0149] Those skilled in the art should recognize that the above embodiments are merely illustrative of the present invention and are not intended to limit the present invention. Any variations or modifications to the above embodiments that are within the spirit and essence of the present invention will fall within the scope of the claims of the present invention.

Claims

1. A spray-formed cold rolling work roll, characterized in that, Its microstructure consists of bainite, martensite, carbides, and retained austenite; the content of bainite and martensite is 18-20%; the content of retained austenite is ≤3%. The material of the spray-formed cold-rolled work roll blank includes the following components by weight percentage: C: 0.82-1.07%, Si: 0.6-1.0%, Mn: 0.4-0.6%, P≤0.03%, S≤0.01%, Cr: 5.5-7.5%, W: 0.8-1.2%, Mo: 1.0-1.3%, V: 2.0-2.5%, N: 0.3-0.5%, Co: 1.2-2.0%, with the balance being Fe and other unavoidable impurities.

2. The spray-formed cold rolling work roll according to claim 1, characterized in that, The tensile strength R of the spray-formed cold-rolled work roll blank at room temperature m The impact energy is 798–830 MPa. KU2 It has a strength of 3-4 J and a hardness HSD of 90-95.

3. A final heat treatment process for spray-formed cold rolling work rolls as described in any one of claims 1 to 2, characterized in that, The spray-formed cold-rolled work roll blank is subjected to differential temperature quenching and low-high temperature combined tempering to finally obtain the spray-formed cold-rolled work roll.

4. The final heat treatment process for the spray-formed cold rolling work roll according to claim 3, characterized in that, The spray-formed cold-rolled work roll blank is prepared by the following method: Based on the material ratio of the spray-formed cold rolling work roll, the raw material is then subjected to spray forming, hot pressing sintering, forging, and post-forging annealing to finally obtain the spray-formed cold rolling work roll blank.

5. The final heat treatment process for the spray-formed cold rolling work roll according to claim 4, characterized in that, The differential temperature quenching process is as follows: (1) A single temperature equalization process is performed by heating the spray-formed cold-rolled work roll blank to 600±10℃ and performing a temperature equalization process for 8 to 10 minutes. (2) Secondary temperature equalization treatment: The spray-formed cold rolling work roll blank after the first temperature equalization treatment is heated to 800±10℃ and subjected to temperature equalization treatment for 8 to 10 minutes. (3) Quenching: The spray-formed cold-rolled work roll blank after the second uniform temperature treatment is heated to 1250-1300℃ and held for 10-15 minutes. Then it is cooled in the furnace to 1100-1200℃ and held for 45-60 minutes. After that, it is cooled to below 300℃ by blowing argon gas. It is then air-cooled to room temperature and subjected to the low-high temperature combination tempering treatment within 2-3 hours.

6. The final heat treatment process for the spray-formed cold rolling work roll according to claim 5, characterized in that, In the differential temperature quenching process: During the heating process in steps (1) and (2), the heating rate is 200–230 °C / h; and / or In step (3), during the argon gas cooling process, the cooling rate is controlled to be 4-4.5℃ / s.

7. The final heat treatment process for the spray-formed cold rolling work roll according to claim 3, characterized in that, The low-high temperature combined tempering process includes low-temperature tempering and high-temperature tempering; The low-temperature tempering: The spray-formed cold-rolled work roll blank after the differential temperature quenching treatment is heated to 320-350°C and held for 2-4 hours, then cooled in the furnace to 200-210°C, and then cooled to room temperature after being taken out of the furnace. The high-temperature tempering involves further heating the spray-formed cold-rolled work roll blank, which has undergone the low-temperature tempering treatment, to 540–580°C and holding it at that temperature for 2–4 hours. After that, it is cooled in the furnace to 200–210°C and then removed from the furnace and cooled to room temperature.

8. The final heat treatment process for the spray-formed cold rolling work roll according to claim 7, characterized in that: During the low-temperature tempering and high-temperature tempering processes, the heating rate is 200–230 °C / h; and / or In the aforementioned low-high temperature combined tempering process, the high-temperature tempering is repeated 2 to 3 times; and / or During both the low-temperature tempering and high-temperature tempering processes, air cooling is used for the cooling process.

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