Injection-molded cold-rolled work roll blank and method for producing same

Abstract

The application discloses a spray forming cold-rolled work roll blank and a preparation method thereof. By designing material components, the toughness of the base body is increased to meet the rolling requirement of ultra-high strength steel. Meanwhile, the spray forming technology is adopted, so that the prepared spray forming cold-rolled work roll blank has comprehensive improvement in normal temperature and high temperature mechanical properties, especially the high temperature toughness, wear resistance and heat crack resistance are greatly improved, so as to solve the peeling, uneven carbide and relatively insufficient wear resistance of the high-speed steel work roll prepared by ESR smelting and forging technology.

Description

Technical Field

[0001] This invention belongs to the field of spray forming technology and relates to a spray-formed cold rolling work roll blank and its preparation method. Background Technology

[0002] To meet the requirements of ultra-high strength steel products and processes, the rolls used to produce ultra-high strength steel must have excellent mechanical properties. Specifically, the surface of the roll must have extremely high hardness to meet the requirements of high temperature resistance, corrosion resistance, wear resistance, and fatigue resistance, while the core must have high strength and toughness to meet the requirements of torque bending moment, impact, and fatigue resistance.

[0003] With the development of mechanical properties of high-strength steel products, small-diameter continuous rolling mills or 20-roll mills are gradually being promoted. Therefore, new requirements are put forward for the mechanical properties of rolls. ESR high-speed steel roll production lines conventionally use electroslag remelting (ESR) to manufacture high-speed steel work rolls, which has the advantages of low cost and relatively low manufacturing difficulty. However, it has disadvantages such as uneven primary carbides, relatively insufficient wear resistance, and easy accidents, which can no longer meet the needs of rolling.

[0004] In recent years, the use of single-stand rolling mills for ultra-high strength steel has increased significantly. A notable feature is the use of side supports on the work rolls. These side supports enhance the stability of the work rolls on the mill stand, reducing lateral offset and enabling the mill to achieve higher rolling yields and higher torque on strips. This unit commonly uses high-speed steel work rolls smelted and forged using electroslag remelting (ESR), which offers advantages such as low cost and relatively easy manufacturing. However, due to the significant difficulty in rolling high-strength steel, these work rolls have experienced dozens of cracking and breakage incidents during use, causing considerable disruption to production. In particular, the high-speed steel work rolls are characterized by high carbon, high alloy content, high hardness, high brittleness, high wear resistance, and high carbide content. A series of problems have arisen during their use, such as: difficulty in machining the center hole, residual oxide and decarburized layers; the presence of mesh-like and large carbide deposits in large-size forged high-speed steel materials, with more severe carbide distribution in the core area; and axial cracks or fissures leading to roll breakage. Therefore, this type of unit urgently needs to develop work rolls that can improve wear resistance while ensuring accident resistance, as well as advanced manufacturing technologies.

[0005] Spray forming, a novel rapid solidification technology, involves atomizing molten metal or alloy into fine liquid particles under the impact of high-speed inert gases such as argon or nitrogen. These particles are then sprayed onto a metal depositor and welded together to rapidly form a high-density preform. Because the fine particles rapidly release heat during flight and remain in a semi-solid or supercooled liquid state during deposition, the shape and size of the preform can be precisely controlled by manipulating the substrate movement. Therefore, spray forming is a comprehensive 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.

[0006] Chinese patent application CN201610492979.8 discloses an aluminum alloy material that can replace QT600 rolls and its spray forming method. The main components, by weight percentage, are: cobalt hexafluoride anion [CoF6]3-: 0.005%-0.02%, manganese (Mn): ≤2%, cadmium (Cd): 0.05%-0.5%, copper (Cu): 4.2%-8.0% and Cu≥0.8Mn+4.05%; the average grain size of the alloy is <120 micrometers, and the balance is aluminum (Al).

[0007] Chinese patent application CN201711251633.X discloses a graphene-reinforced copper-based composite material and its spray molding method. The method involves melting industrial pure copper and a copper-silver alloy in a vacuum electromagnetic induction furnace to obtain molten copper. This molten copper is then injected into an inert gas spray atomizing device, where high-pressure inert gas atomizes the copper into small droplets. Simultaneously, graphene flakes are sprayed into the atomization chamber, adhering to the sinking copper droplets to form a liquid or semi-molten copper / graphene mixture. After cooling and casting into an ingot, the mixture is rolled to obtain the graphene-reinforced copper-based composite material. The preparation method of this invention is simple and feasible, the steps are easy to operate, and the prepared product possesses advantages such as uniform material composition, dense structure, high strength, good wear resistance, and good electrical and thermal conductivity.

[0008] 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%).

[0009] 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.

[0010] 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 specific steps of this cryogenic process are as follows: preheating; vacuum quenching; cooling; first tempering; cryogenic treatment; second tempering. Compared with existing technologies, this new process combines vacuum quenching, tempering, and cryogenic treatment on the spray-formed high-speed steel end mill material. After this cryogenic treatment, the wear resistance and hardness of the spray-formed high-speed steel end mill material are significantly improved, thereby increasing the overall mechanical properties and service life of the end mill.

[0011] Chinese Patent Application No. CN201010262931.0 discloses a method for preparing an electrically heated ceramic mold for precision spray molding. The method involves: designing and fabricating a prototype with an uneven surface; casting a silicone rubber template on the prototype; casting a wax template on the silicone rubber template; casting a ceramic template with an uneven surface on the wax template; and assembling the ceramic template and an electric heating wire furnace to form an electrically heated ceramic mold. The ceramic template is preheated from the bottom within an atomizing canister for spray molding to improve the density and surface quality of the precision spray-molded preform. The advantage of this invention is that it solves the design and manufacturing problems related to preheating the ceramic mold within an atomizing canister, allowing for more accurate temperature control of the ceramic mold and thus improving the density of the precision spray-molded preform.

[0012] In summary, the above technologies demonstrate that spray forming 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, spray forming technology (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 in the materials and processes of spray forming, requiring further research and development of technical solutions.

[0013] Therefore, there is an urgent need to develop a method for applying spray forming technology to the preparation process of high-grade cold-rolled work roll blanks, so as to develop a cold-rolled work roll blank with low cost, finer structure and better comprehensive mechanical properties. Summary of the Invention

[0014] To address the aforementioned deficiencies in existing technologies, the present invention aims to provide a spray-formed cold-rolled work roll blank and its preparation method. By designing the material composition, the strength and toughness of the matrix are increased to meet the rolling requirements of ultra-high strength steel. At the same time, the use of spray forming technology comprehensively improves the mechanical properties of the prepared spray-formed cold-rolled work roll blank at both room temperature and high temperature, especially significantly enhancing its high-temperature strength, toughness, wear resistance, and resistance to hot cracking. This solves the shortcomings of high-speed steel work rolls produced by ESR smelting and forging technologies, such as spalling, uneven carbide distribution, and relatively insufficient wear resistance.

[0015] To achieve the above objectives, the present invention adopts the following technical solution:

[0016] The first aspect of the present invention provides a spray-formed cold-rolled work roll blank, the material of which 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.

[0017] Preferably, the yield strength of the spray-formed cold-rolled work roll blank is ≥745MPa, the tensile strength is ≥975MPa, the density is ≥99%, and the grain size is ≤16μm.

[0018] The second aspect of the present invention provides a method for preparing a spray-formed cold-rolled work roll blank. The method comprises using the composition ratio of the spray-formed cold-rolled work roll blank material according to the first aspect of the present invention, and then obtaining the spray-formed cold-rolled work roll blank through spray forming, hot pressing sintering, forging and post-forging annealing treatment.

[0019] Preferably, in the spray molding process, the raw material is melted to obtain an alloy melt, which is atomized by atomizing gas through a liquid guide tube to form a molten droplet jet, and is directly sprayed onto the deposition substrate to form a deposition blank.

[0020] Preferably, during the spray forming process:

[0021] The mass flow rate of the alloy melt is 3.5–4.5 kg / min; and / or

[0022] The extension length of the liquid guide tube is 3-5 mm; and / or

[0023] The preheating temperature of the deposition substrate is 500–600°C; and / or

[0024] The rotational speed of the deposition substrate is 9–12 r / min; and / or

[0025] The atomizing gas is nitrogen with a purity ≥99%, the pressure of the atomizing gas is 0.8–1.2 MPa, and the atomization angle is 30–60°; and / or

[0026] The injection temperature of the molten droplet jet is 820–880°C.

[0027] Preferably, the hot pressing sintering process is as follows:

[0028] (1) A single temperature homogenization process is performed, in which the deposited blank obtained by spray molding is heated to 600±10℃ and subjected to a temperature homogenization process for 8 to 10 minutes.

[0029] (2) Secondary isothermal treatment: The deposited blank after the first isothermal treatment is heated to 800±10℃ and subjected to isothermal treatment for 8 to 10 minutes.

[0030] (3) Homogenization sintering: The deposited billet after the second homogenization treatment is heated to 1450-1550℃ and held for 5-8 minutes. Then it is cooled to 1050-1250℃ in the furnace and held for 4-6 hours. After that, it is cooled to below 300℃ in the furnace and then air-cooled to room temperature.

[0031] Preferably, in the hot pressing sintering process, the heating rate in steps (1) and (2) is 200-230℃ / h.

[0032] Preferably, during the forging process, the initial forging temperature is 1200–1250°C, and the final forging temperature is 1100–1150°C; and / or

[0033] During the forging process, the forging ratio is 4 to 6; and / or

[0034] During the forging process, the die pressing rate is 0.05–0.08 mm / s, the rotation rate is 20–30°, and the strain is 70–75%.

[0035] Preferably, in the post-forging annealing process, the forged billet obtained after forging is slowly cooled to 600-700°C and held at that temperature. Then, it is transferred to an annealing furnace for annealing within 10 hours, with the annealing temperature controlled at 760-810°C and held for 4-6 hours.

[0036] Preferably, the yield strength of the spray-formed cold-rolled work roll blank is ≥745MPa, the tensile strength is ≥975MPa, the density is ≥99%, and the grain size is ≤16μm.

[0037] The design principle of the components in the provided spray-formed cold-rolled work roll material 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 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.

[0038] 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.

[0039] 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.

[0040] 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%.

[0041] 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.

[0042] The spray-formed cold-rolled work roll blank and its preparation method provided by this invention have the following beneficial effects:

[0043] 1. The spray-formed cold-rolled work roll blank and its preparation method of the present invention increase the strength and toughness of the matrix by designing the material composition to meet the rolling requirements of ultra-high strength steel. At the same time, the spray-forming technology is adopted, which improves the mechanical properties of the prepared spray-formed cold-rolled work roll blank at both room temperature and high temperature. In particular, the high temperature strength and toughness, wear resistance and thermal crack resistance are greatly improved, so as to solve the shortcomings of high-speed steel work rolls made by ESR smelting and forging technology, such as spalling, uneven carbide and relatively insufficient wear resistance.

[0044] 2. The spray-formed cold-rolled work roll blank and its preparation method of the present invention, through material design, spray forming, hot pressing sintering, forging and post-forging heat treatment, thereby obtaining a spray-formed high-speed steel work roll blank with low cost, finer structure and better comprehensive mechanical properties.

[0045] 3. The spray-formed cold-rolled work roll blank and its preparation method of the present invention optimize the design of existing ultra-high strength steel rolling roll materials and innovatively design a spray-forming process to obtain spray-formed roll blanks. In order to further improve the density and homogenize the microstructure, a hot pressing sintering process is specially designed, laying a solid foundation for forging. The shortcomings of traditional forging and annealing processes are fully analyzed during the forging and post-forging heat treatment processes, and creative designs are made. Finally, a spray-formed forged roll blank with excellent microstructure and mechanical properties is obtained, filling the gap in the domestic application of spray forming in the field of roll manufacturing and placing it in a leading position internationally.

[0046] 4. In view of 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 easy accidents, the spray-formed cold-rolled work roll blank of the present invention has excellent strength, toughness, wear resistance and thermal crack resistance, which can solve the rolling problem of ultra-high strength steel and has good prospects for promotion and application. Detailed Implementation

[0047] 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.

[0048] The present invention provides a spray-formed cold-rolled work roll blank, the material of which 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.

[0049] The above-mentioned spray-formed cold-rolled work roll blank is prepared by the following method: the raw materials are proportioned according to the composition of the above-mentioned spray-formed cold-rolled work roll blank material, and then the spray-formed cold-rolled work roll blank is obtained by spray forming, hot pressing sintering, forging and post-forging annealing treatment.

[0050] The specific process of preparing the spray-formed cold-rolled work roll blank of the present invention is as follows:

[0051] (I) Spray forming process

[0052] The spray molding process of the present invention can be carried out on a silicon controlled rectifier medium frequency multi-functional supersonic gas atomization spray deposition equipment. The equipment mainly consists of a melting section, an atomization section, a deposition section, a gas supply section, a control system, and a powder collection system.

[0053] After the raw materials are proportioned according to the composition of the cold-rolled work roll blank material for spray forming, the raw materials are added to the heating furnace of the thyristor medium-frequency multi-functional supersonic gas atomization spray deposition equipment for melting. After the raw materials are melted, an alloy melt is obtained. The alloy melt is atomized into molten droplet jets under the action of atomizing gas through the liquid guide pipe, and directly sprayed onto the deposition substrate to form a deposition blank.

[0054] Before spraying begins, the deposition substrate needs to be preheated to prevent the molten liquid falling onto the substrate from undergoing rapid cooling and causing microstructural distortion. If the preheating is ineffective or uneven, large molten droplets or particles will fall onto the deposition substrate after spraying begins, while smaller particles will splash off the substrate upon impact. This results in large molten liquid particles landing on the substrate, leading to a grain size larger near the substrate compared to other areas.

[0055] The process parameters used in the preparation of the deposited preform are as follows:

[0056] Furnace power / kW: 18~22kW; Furnace power refers to the power of the furnace during the raw material melting process.

[0057] Alloy melt mass flow rate (kg / min): 3.5~4.5 kg / min;

[0058] Nebulizing gas: Nitrogen gas with a purity of ≥99%, such as 99.99% high-purity nitrogen gas;

[0059] Atomizing gas pressure / MPa: 0.8~1.2MPa;

[0060] Atomization angle (°): 30°~60°; Atomization angle is the angle at which the atomized gas is injected.

[0061] Liquid guide tube extension length / mm: 3~5mm;

[0062] The jetting temperature of the molten droplet is 820–880℃.

[0063] Preheating temperature of deposition substrate: 500~600℃;

[0064] Deposition substrate rotation speed (r / min): 9–12 r / min;

[0065] (II) Hot pressing sintering process

[0066] 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.

[0067] The hot pressing sintering process is as follows:

[0068] (1) A single temperature equalization process is performed, in which the deposited blank obtained by spray molding is heated to 600±10℃ at a heating rate of 200~230℃ / h and subjected to a temperature equalization process of 8~10min.

[0069] (2) Secondary isothermal treatment: The deposited blank after the first isothermal treatment is heated to 800±10℃ at a heating rate of 200~230℃ / h and subjected to isothermal treatment for 8~10min.

[0070] (3) Homogenization sintering: The deposited billet after the secondary homogenization treatment 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.

[0071] 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.

[0072] (III) Forging Process

[0073] 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:

[0074] Forging ratio: 4-6;

[0075] Dependent variable: 70-75%;

[0076] Initial forging temperature / ℃: 1200~1250℃;

[0077] Final forging temperature / ℃: 1100~1150℃;

[0078] Die pressing speed (mm / s): 0.05~0.08mm / s;

[0079] Spiral feed / °: 20~30°;

[0080] 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℃.

[0081] 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.

[0082] 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.

[0083] The above innovative forging process enables a forging yield of 70%, which is 40% higher than the conventional maximum yield of 50%.

[0084] (iv) Post-forging annealing process

[0085] 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.

[0086] 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.

[0087] 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.

[0088] 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.

[0089] 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.

[0090] Therefore, it not only saves energy but also avoids the disadvantages of martensitic forgings, such as poor toughness, low fracture strength, and large expansion coefficient, thus completely solving the problem of forging fracture during annealing.

[0091] The above-described method for preparing spray-formed cold-rolled work roll blanks ultimately yields spray-formed cold-rolled work roll blanks with significantly improved quality. These blanks have a yield strength ≥745MPa, a tensile strength ≥975MPa, a density ≥99% (in a further preferred embodiment, the density is 99.2-99.8%), and a grain size ≤16μm.

[0092] The gas content of the spray-formed cold-rolled work roll blank is: [H]≤2.0ppmm, [O]≤25ppmm, [N]≤40ppmm; the interior of the spray-formed cold-rolled work roll blank is free of 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, and the grass-like wave (noise wave) is less than 20%, and there are no defect waves on the roll surface.

[0093] The following section provides a further description of the spray-formed cold-rolled work roll blank and its preparation method, using specific examples.

[0094] Examples 1-3

[0095] The composition of the spray-formed cold-rolled work roll blank material in Examples 1-3 is shown in Table 1;

[0096] Table 1. Composition (wt%) of the material of spray-formed cold-rolled work roll blanks

[0097] C Si Mn Cr W Mo V N Co Fe Example 1 0.85 0.6 0.4 5.5 0.9 1.0 2.0 0.3 1.2 margin Example 2 0.9 0.8 0.5 6.0 1.0 1.1 2.3 0.4 1.5 margin Example 3 1.0 1.0 0.6 7.0 1.2 1.2 2.5 0.5 2.0 margin

[0098] (I) Spray forming process

[0099] After the raw materials are proportioned according to the composition of the cold-rolled work roll blank material for spray forming, the raw materials are added to the heating furnace of the thyristor medium-frequency multi-functional supersonic gas atomization spray deposition equipment for melting. After the raw materials are melted, an alloy melt is obtained. The alloy melt is atomized into molten droplet jets under the action of atomizing gas through the liquid guide pipe, and directly sprayed onto the deposition substrate to form a deposition blank.

[0100] The process parameters used in the preparation of the deposited preform are as follows:

[0101] Heating furnace power / kW: 20kW;

[0102] Alloy melt mass flow rate (kg / min): 4.0 kg / min;

[0103] Atomizing gas: 99.99% high-purity nitrogen;

[0104] Atomizing gas pressure / MPa: 1.0MPa;

[0105] Atomization angle (°): 50°;

[0106] Liquid delivery tube extension length / mm: 4mm;

[0107] The injection temperature of the molten droplet jet is 850℃.

[0108] Preheating temperature of the deposition substrate: 550℃;

[0109] Deposition substrate rotation speed (r / min): 10 r / min;

[0110] (II) Hot pressing sintering process

[0111] (1) A single temperature uniform treatment is performed by heating the deposited blank obtained by spray molding at a heating rate of 200℃ / h and then holding it at 600℃ for 10 minutes.

[0112] (2) Secondary temperature uniformization treatment: The deposited billet is heated at a heating rate of 200℃ / h and then subjected to a temperature uniformization and holding treatment at 800℃ for 10 minutes.

[0113] (3) Homogenization sintering: The deposited billet is heated to 1500℃ and held for 8 minutes to achieve a density of over 92% at a higher temperature. Then, it is cooled to 1250℃ in the furnace and held for 4-6 hours. Finally, it is cooled to below 300℃ in the furnace and then air-cooled to room temperature in preparation for forging.

[0114] (III) Forging Process

[0115] The parameters during the forging process are as follows:

[0116] Forging ratio: 5;

[0117] Dependent variable: 75%;

[0118] Initial forging temperature / °C: 1250°C;

[0119] Final forging temperature / °C: 1150°C;

[0120] Die pressing rate (mm / s): 0.06 mm / s;

[0121] Spiral feed / °: 230°;

[0122] (iv) Post-forging annealing process

[0123] The forging holding temperature was controlled at 650℃, and the annealing process was controlled at 800℃ for 6 hours.

[0124] After the above processing, the quality of the final spray-formed cold-rolled work roll blank is greatly improved, and its technical indicators are as follows:

[0125] Gas content: [H] 1.2 ppm, [O] 25 ppm, [N] 40 ppm;

[0126] The spray-formed cold-rolled work roll blank is free of defects such as white spots, internal cracks, shrinkage cavities, and non-metallic inclusions.

[0127] The surface of the spray-formed cold-rolled work roll blank is free of visually visible defects such as cracks, folds, scars, and inclusions;

[0128] Ultrasonic testing shall be performed in accordance with GB / T13314, and the quality level shall meet the requirements of Grade A in Table A.2 of Appendix A: no echo defects with an equivalent diameter greater than φ2 shall appear in the working layer of the roller body.

[0129] The surface wave of the spray-formed cold rolling work roll blank shall be tested in accordance with GB / T 23904. The grass-like wave (impurity wave) shall be less than 20% and there shall be no defect wave on the roll surface.

[0130] The performance parameters of the spray-formed cold-rolled work rolls prepared in Examples 1-3 are shown in Table 2;

[0131] Table 2 Performance parameters of spray-formed cold-rolled work roll blanks

[0132] Yield strength (MPa) Tensile strength (MPa) Density Grain size Example 1 745 976 99.5 15.6 Example 2 748 978 99.6 15.3 Example 3 750 980 99.8 15

[0133] In summary, this invention optimizes the design of existing ultra-high strength steel rolling mill roll materials and innovatively designs a spray forming process to obtain spray-formed roll blanks. To further improve density and homogenize the microstructure, a hot pressing sintering process is specially designed, laying a solid foundation for forging. During the forging and post-forging heat treatment processes, the shortcomings of traditional forging and annealing processes are fully analyzed, and creative designs are made. Finally, a spray-formed forged roll blank with excellent microstructure and mechanical properties is obtained, filling the gap in the domestic application of spray forming in the field of roll manufacturing and placing it in a leading position internationally.

[0134] 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-rolled work roll blank, characterized in that, Its material 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. The spray-formed cold-rolled work roll blank is prepared by the following method: according to the material composition ratio of the spray-formed cold-rolled work roll blank, the raw materials are then processed by spray forming, hot pressing sintering, forging, and post-forging annealing to obtain the spray-formed cold-rolled work roll blank. In the spray molding process, the raw material is melted to obtain an alloy melt, which is then atomized by atomizing gas through a liquid guide tube to form a molten droplet jet, and directly sprayed onto the deposition substrate to form a deposition blank. During the spray forming process: The mass flow rate of the alloy melt is 3.5–4.5 kg / min; The extension length of the liquid guide tube is 3-5 mm; The preheating temperature of the deposition substrate is 500–600°C; The rotational speed of the deposition substrate is 9–12 r / min; The atomizing gas is nitrogen with a purity of ≥99%, the pressure of the atomizing gas is 0.8~1.2MPa, and the atomization angle is 30~60°; The injection temperature of the molten droplet jet is 820–880°C; During the post-forging annealing process, the forged billet obtained after forging is slowly cooled to 600-700°C and held at that temperature. Then, it is transferred to an annealing furnace within 10 hours for annealing, with the annealing temperature controlled at 760-810°C and held for 4-6 hours.

2. The spray-formed cold-rolled work roll blank according to claim 1, characterized in that, The spray-formed cold-rolled work roll blank has a yield strength of ≥745MPa, a tensile strength of ≥975MPa, a density of ≥99%, and a grain size of ≤16μm.

3. A method for preparing a spray-formed cold-rolled work roll blank, characterized in that, The process includes the following steps: using the material composition ratio of the spray-formed cold-rolled work roll blank according to claim 1, and then obtaining the spray-formed cold-rolled work roll blank through spray forming, hot pressing sintering, forging, and post-forging annealing. In the spray molding process, the raw material is melted to obtain an alloy melt, which is then atomized by atomizing gas through a liquid guide tube to form a molten droplet jet, and directly sprayed onto the deposition substrate to form a deposition blank. During the spray forming process: The mass flow rate of the alloy melt is 3.5–4.5 kg / min; The extension length of the liquid guide tube is 3-5 mm; The preheating temperature of the deposition substrate is 500–600°C; The rotational speed of the deposition substrate is 9–12 r / min; The atomizing gas is nitrogen with a purity of ≥99%, the pressure of the atomizing gas is 0.8~1.2MPa, and the atomization angle is 30~60°; The injection temperature of the molten droplet jet is 820–880°C; During the post-forging annealing process, the forged billet obtained after forging is slowly cooled to 600-700°C and held at that temperature. Then, it is transferred to an annealing furnace within 10 hours for annealing, with the annealing temperature controlled at 760-810°C and held for 4-6 hours.

4. The method for preparing a spray-formed cold-rolled work roll blank according to claim 3, characterized in that, The hot pressing and sintering process is as follows: (1) A single temperature homogenization process is performed, in which the deposited blank obtained by spray molding is heated to 600±10℃ and subjected to a temperature homogenization process for 8 to 10 minutes. (2) Secondary isothermal treatment: The deposited blank after the first isothermal treatment is heated to 800±10℃ and subjected to isothermal treatment for 8 to 10 minutes. (3) Homogenization sintering: The deposited billet after the second homogenization treatment is heated to 1450-1550℃ and held for 5-8 minutes. Then it is cooled to 1050-1250℃ in the furnace and held for 4-6 hours. After that, it is cooled to below 300℃ in the furnace and then air-cooled to room temperature.

5. The method for preparing spray-formed cold-rolled work roll blank according to claim 4, characterized in that, In the hot pressing sintering process, the heating rate in steps (1) and (2) is 200-230℃ / h.

6. The method for preparing a spray-formed cold-rolled work roll blank according to claim 3, characterized in that, During the forging process, the initial forging temperature is 1200–1250℃, and the final forging temperature is 1100–1150℃; and / or During the forging process, the forging ratio is 4 to 6; and / or During the forging process, the die pressing rate is 0.05–0.08 mm / s, the rotation rate is 20–30°, and the strain is 70–75%.

7. The method for preparing spray-formed cold-rolled work roll blank according to any one of claims 3 to 6, characterized in that, The spray-formed cold-rolled work roll blank has a yield strength of ≥745MPa, a tensile strength of ≥975MPa, a density of ≥99%, and a grain size of ≤16μm.