BD2 alloy ductile iron roller and preparation process thereof

Abstract

The invention relates to a BD2 alloy ductile iron roller and a preparation process thereof, and relates to the technical field of rollers. Pre-casting a stepped hole pattern; and carrying out precise temperature return spray quenching treatment. Through the cooperative process of stepped hole pattern precasting and precise temperature returning spray quenching, the problems that a BD2 deep groove roller is prone to cracking and uneven in hardness are solved from the two aspects of the physical structure and the thermodynamics. Through a further microalloying improvement scheme, the strengthening thought that a traditional BD2 alloy seriously depends on expensive nickel elements is broken through, trace aluminum and nitrogen elements are accurately introduced on the basis of reducing the nickel content, and the high chemical affinity of the trace aluminum and nitrogen elements in molten iron and the core-spun yarn synchronous feeding technology are utilized; and in-situ synthesis and uniform dispersion distribution of the high-melting-point AlN nanoparticles are realized.

Description

Technical Field

[0001] This application relates to the field of rolling mill technology, specifically to a BD2 alloy ductile iron rolling mill and its manufacturing process. Background Technology

[0002] BD2 alloy ductile iron (bainitic ductile iron) is widely used in the finishing stands of various types of steel rolling mills, bar mills, and high-speed wire rod mills due to its excellent wear resistance, good thermal fatigue resistance, and high strength and toughness. Especially in the production of heavy H-beams, rails, and large channel steel, the rolls not only have to withstand enormous rolling loads but also cope with severe alternating thermal shocks. Therefore, obtaining high-performance BD2 alloy ductile iron rolls with a dense matrix, uniform hardness, and controllable internal stress is crucial for ensuring the surface quality of the rolled material and improving the mill's operating rate.

[0003] However, existing deep-groove roll production technologies still face numerous insurmountable process bottlenecks. Traditional manufacturing processes typically employ high-nickel formulations (Ni content usually between 1.5% and 2.0% or even higher) to ensure hardenability, coupled with a heat treatment method of "continuous spray cooling to room temperature followed by tempering." However, for deep-groove rolls, the significant difference in geometry between the groove bottom and the roll surface leads to severe heat dissipation lag during continuous cooling. While the surface layer has undergone martensitic or bainitic transformation accompanied by volume expansion, the groove root remains in a high-temperature plastic state or undergoes pearlitic transformation. This asynchronous temperature gradient and phase transformation on the cross-section easily induces enormous tensile stress at the stress-concentrated groove bottom, resulting in "hot cracking" or "delayed fracture." Furthermore, traditional processes struggle to address the large hardness difference (i.e., "soft bottom") across the deep-groove roll cross-section, and the need to maintain performance necessitates the extensive use of expensive nickel, leading to high production costs and limiting the further enhancement of the market competitiveness of this type of roll.

[0004] Therefore, it is particularly urgent to develop a preparation process that can solve the problem of asynchronous stress concentration and microstructure transformation in deep grooves, while reducing the cost of precious alloys. Summary of the Invention

[0005] This application provides a BD2 alloy ductile iron roll and its manufacturing process, which solves the problem of asynchronous stress concentration and microstructure transformation in deep grooves, while reducing the cost of precious alloys.

[0006] Firstly, the manufacturing process of a BD2 alloy ductile iron roll provided in this application adopts the following technical solution: A manufacturing process for BD2 alloy ductile iron rolls includes the following steps: S1. Melting and Casting: The molten iron is melted according to the target composition ratio. The chemical composition of the BD2 alloy ductile iron rolls includes the following percentages by mass: C 3.2-3.6%, Si 1.4-1.8%, Mn 0.4-0.7%, Cr 0.4-0.8%, Ni 1.2-1.7%, Mo 0.2-0.5%, with the balance being Fe and unavoidable impurities. During the tapping process, spheroidization is performed using core wire feeding technology, and double inoculation is carried out with in-flow ferrosilicon and zirconium silicon particles, followed by casting. S2, stepped hole pre-casting: In the casting mold design stage, according to the hole characteristics of the finished roll, the inner cavity of the mold is designed as a non-cylindrical structure with stepped holes, so that the billet has pre-made stepped grooves. S3, Precision temperature-return spray quenching treatment: S31. Heat the billet to 920±10℃ and hold it at that temperature to austenitize the matrix; S32. After exiting the furnace, a spray cooling device is used for spray quenching. S33. Monitor the surface temperature of the rolls and the bottom of the trough in real time. When the temperature at the bottom of the trough drops to the 500℃ reheat point, stop the spray quenching immediately. S34. The rolls are quickly transferred into a low-temperature furnace with the temperature set at 560°C for tempering treatment, and the microstructure transformation is controlled by the temperature difference between the residual heat and the furnace temperature. S35. After tempering, the furnace is cooled to below 300°C before being removed and air-cooled.

[0007] By adopting the above technical solution and precisely controlling the proportions of C, Si, Mn, Cr, Ni, and Mo during the smelting stage, a highly hardenable alloy matrix was constructed. The combination of Ni and Cr improved the stability of austenite and ensured uniform deep microstructure. Combined with core wire feeding spheroidization and dual inoculation with ferrosilicon and zirconium silicon, the nucleation effect of Zr was used to refine the size of graphite spheres and improve their roundness, reducing matrix cracking.

[0008] The core of the process lies in the synergy between "stepped hole pre-casting" and "precise reheat spray quenching". The pre-cast stepped hole directly reduces the machining allowance, and more importantly, it makes the original solidification structure of the billet conform to the final stress direction. By monitoring the temperature at the bottom of the groove to 500℃ and stopping the spray quenching and quickly switching to 560℃ tempering, the "temperature difference coupling effect" of the residual heat in the roll core and the external furnace temperature is utilized. In the range of 500-560℃, the supercooled austenite undergoes bainite transformation and carbon atom energy diffusion and distribution, which effectively alleviates the stress concentration and crack tendency caused by the slow cooling rate in the deep groove part in the traditional cooling to room temperature and then tempering process. This makes the roll matrix have both high strength and good toughness, and solves the technical problems of uneven hardness and easy cracking of deep groove steel rolls.

[0009] Optionally, the chemical composition of the BD2 alloy ductile iron roll comprises, by mass percentage: C 3.3-3.5%, Si 1.5-1.7%, Mn 0.5-0.6%, Cr 0.5-0.6%, Ni 1.2-1.3%, Mo 0.3-0.4%, Al 0.04-0.08%, N 0.008-0.012%, with the balance being Fe and unavoidable impurities.

[0010] By adopting the above technical solution and adjusting through microalloying, the Ni content is reduced to 1.2-1.3%, and trace amounts of Al and N are introduced. Utilizing the strong chemical affinity between Al and N in molten iron, high-melting-point, high-hardness AlN nanoparticles are generated in situ. These in-situ generated AlN particles exert a strong grain boundary pinning effect during austenitization, inhibiting grain growth and achieving ultra-fine austenite grains. Simultaneously, AlN particles, as a dispersed strengthening phase, compensate for the potential loss of solid solution strengthening due to the reduced Ni content. This compositional optimization not only reduces the cost of expensive Ni elements but, more importantly, significantly improves the yield strength and thermal fatigue resistance of the roll matrix after high-temperature tempering through a dual mechanism of grain refinement and dispersion strengthening, providing a more stable microstructure for subsequent low-temperature furnace loading processes.

[0011] Optionally, the spheroidizing and inoculation treatment in step S1 is specifically carried out by feeding aluminum-containing cored wire and silicon-magnesium alloy cored wire simultaneously when the molten iron is tapped from the furnace; the nitrogen element is introduced by adding nitrogen-containing ferromanganese in the later stage of smelting or by bottom blowing nitrogen.

[0012] By employing the aforementioned technical solution, simultaneously feeding aluminum-core wire and silicon-magnesium alloy cored wire, along with the addition of nitrogen-containing ferromanganese or bottom-blown nitrogen gas during the later stages of smelting, this specific feeding method ensures the precise dissolution and distribution of Al in the molten iron, preventing premature oxidation and burn-off of aluminum at high temperatures. Al atoms and pre-dissolved N atoms enrich and react rapidly at the solidification front, ensuring that the generated AlN particles are small and uniformly dispersed at the matrix grain boundaries, rather than agglomerating into large inclusions. This in-situ synthesis control method maximizes the heterogeneous nucleation efficiency and grain boundary pinning ability of AlN particles, resulting in a denser final roll structure and improved impact toughness and wear resistance.

[0013] Optionally, a temperature equalization and holding step is included between steps S33 and S34: After stopping the spray quenching, the roll is placed in a heat preservation cover or buffer pit and held for 15-20 minutes to make the temperature of the roll surface and the root of the deep groove tend to be consistent within the range of 500±10℃ by using the residual heat of the roll core.

[0014] By adopting the above technical solution, a uniform temperature retention step is added after the quenching is stopped. Utilizing the enormous enthalpy of the roll's core to conduct heat outwards, the temperature gradient between the surface, the root of the deep groove, and the core is forcibly flattened within the relatively adiabatic environment of the insulation cover or buffer pit, making the overall temperature uniform at 500±10℃. This process eliminates the intense thermal stress field formed during the quenching stage, ensuring that the matrix structure is in a uniformly cooled austenitic metastable state before being transferred to the tempering furnace. This avoids uncontrollable early phase transformation or thermal shock cracking in the deep groove area during furnace heating due to excessive temperature differences, creating the necessary thermodynamic conditions for precise reverse tempering phase transformation control.

[0015] Optionally, step S34 is specifically a reverse tempering process: the rolls, which have undergone uniform temperature retention and whose overall temperature has been stabilized at 500±10℃, are quickly transferred into a low-temperature furnace that has been preheated to 560±10℃; in the initial stage after entering the furnace, the furnace circulating fan is turned on and heated at full power, so that the surface and shallow layer of the rolls are reversely heated from 500±10℃ to 560±10℃ within 30±5 minutes.

[0016] By employing the aforementioned technical solution, a unique "reverse heating phase transformation" process is constructed by rapidly transferring the rolls stabilized at 500℃ into a 560℃ furnace and heating them at full power. During this process, the supercooled austenite transforms into bainite under the stimulation of heating, accompanied by volume expansion. This expansion effect effectively counteracts the volume shrinkage tendency left by the initial cooling. More importantly, the "phase transformation-induced plasticity" mechanism present in the material during the phase transformation process is utilized. At the instant of the phase transformation at 500-560℃, the matrix undergoes microscopic plastic rheology, automatically releasing and healing the highly concentrated stress at complex geometric structures such as the root of the deep groove. This process cleverly combines phase transformation strengthening with stress relaxation, solving the problem of BD2 type deep groove rolls being prone to heat treatment cracks due to their complex structure, and obtaining a defect-free, high-strength, and high-toughness multiphase microstructure.

[0017] Optionally, the spray quenching cooling in step S32 adopts the differential temperature stop method: during the spray quenching process, when the surface temperature of the protruding part of the roll body is monitored to drop to 600-650℃, the nozzles targeting the protruding part of the roll body are closed first, while the nozzles targeting the bottom of the deep groove continue to spray for 30-60 seconds before stopping the spraying.

[0018] By adopting the above technical solution and employing the differential temperature stopping method, nozzles on the high-heat-dissipating raised parts are shut off in advance based on the geometric characteristics and heat dissipation rate differences of different parts of the roll, while nozzles on the slow-heat-dissipating bottom of the groove continue to spray. This is an active intervention method based on the principle of heat transfer. This operation breaks the trend of increasing temperature difference under natural cooling, artificially pre-balancing the temperature field between the deep groove and the roll body, preventing grain coarsening or delayed microstructure transformation caused by the deep groove being at high temperatures for a long time. It ensures that the potential energy of the entire roll cross section, especially the stress-concentrated bottom of the groove, reaches the predetermined return temperature point synchronously with other parts, improving the hardness and cross-sectional uniformity of the final product.

[0019] Secondly, this application provides a BD2 alloy ductile iron roll, wherein the matrix structure of the roll is a multiphase structure of lower bainite and a small amount of retained austenite, and AlN particles with a size of 10-50nm are dispersed at the matrix grain boundaries; the hardness difference between the bottom of the deep groove of the roll and the surface of the roll body is ≤3 HSD, and there are no thermal stress cracks at the bottom of the groove.

[0020] By adopting the above technical solution, the BD2 alloy ductile iron roll possesses a unique multiphase structure of lower bainite and a small amount of retained austenite, with 10-50nm AlN particles dispersed at the grain boundaries. This microstructure endows the roll with an excellent balance of strength and toughness: the lower bainite provides high hardness and wear resistance as a skeleton, the retained austenite acts as a tough phase to absorb impact energy, and the nanoscale AlN particles refine the grains and hinder dislocation movement through a strong pinning effect, further enhancing strength. Macroscopically, this manifests as a hardness difference of ≤3 HSD between the bottom of the deep groove and the roll surface, with no thermal stress cracks at the bottom of the groove. This means that the roll can withstand extremely high rolling loads during service without premature fracture or spalling, extending the service life of deep groove steel rolling and ensuring high-precision forming of the rolled material.

[0021] In summary, this application includes at least one of the following beneficial technical effects: 1. By combining the processes of "stepped die pre-casting" and "precision return temperature spray quenching", the problems of easy cracking and uneven hardness of BD2 deep groove rolls are solved from both physical structure and thermodynamic aspects. This process first utilizes pre-cast die to significantly reduce machining allowance and align the solidification structure with the direction of stress. Then, it uses the large enthalpy of the billet core as an internal heat source to construct a "reverse heating phase transformation" process that stops quenching at 500°C and rapidly transitions to a furnace temperature of 560°C. Within this specific temperature difference coupling range, the volume expansion effect accompanying the transformation of supercooled austenite to bainite not only effectively offsets the initial cooling shrinkage but, more importantly, stimulates phase transformation in the matrix, inducing plastic rheology. This microscopic automatic plastic deformation can release and heal the highly concentrated stress at the root of the deep groove in real time, thus avoiding thermal shock cracking caused by drastic temperature differences in the traditional process of cooling to room temperature and then tempering. Combined with the differential temperature stopping quenching method based on heat dissipation characteristics, the temperature field of the complex cross-section is artificially balanced, ensuring that the bottom of the deep groove can obtain a dense lower bainite multiphase structure consistent with the surface, achieving a perfect unity of high strength and toughness and stress-free defects. 2. Further microalloying improvements have broken through the traditional BD2 alloy strengthening approach that heavily relies on expensive nickel. By precisely introducing trace amounts of aluminum and nitrogen while reducing nickel content, and utilizing their strong chemical affinity in molten iron, along with synchronous cored wire feeding technology, in-situ synthesis and uniform dispersion of high-melting-point AlN nanoparticles were achieved. These in-situ generated AlN particles exerted a strong grain boundary pinning effect during austenitization and high-temperature tempering, acting like "nails" to firmly hinder grain boundary migration, effectively suppressing austenite grain growth and achieving ultra-fine grains. This nanoparticle-based fine-grain strengthening and dispersion strengthening mechanism not only compensates for the potential loss of solid solution strengthening due to reduced nickel content, significantly reducing raw material costs, but also endows the roll matrix with excellent thermal fatigue resistance and yield strength under high-temperature service conditions, fundamentally improving the wear life and fracture resistance of the rolls during high-load rolling processes. Detailed Implementation

[0022] Example 1 A BD2 alloy ductile iron roll is prepared by the following steps: S1. Smelting and Casting: The molten iron is smelted according to the target composition ratio. The chemical composition (mass percentage) of the BD2 alloy ductile iron rolls is: C 3.4%, Si 1.6%, Mn 0.55%, Cr 0.55%, Ni 1.25%, Mo 0.35%, Al 0.06%, N 0.010%, with the balance being Fe and unavoidable impurities. During the tapping process, core wire feeding technology is used for spheroidization treatment, combined with in-flow ferrosilicon and zirconium silicon particles for double inoculation, and bottom-blown nitrogen is used. S2, stepped hole pre-casting: In the casting mold design stage, according to the hole characteristics of the finished roll, the mold adopts a non-cylindrical structure with stepped hole, so that the billet has pre-made stepped grooves. S3, Precision temperature-return spray quenching treatment: S31. Heat the billet to 920℃ and hold it at that temperature to austenitize the matrix; S32. After exiting the furnace, spray cooling is carried out by spraying until the roller body reaches 620℃ / bottom of the trough and spraying continues for 60 seconds (stop when the temperature difference is too great). S33. Monitor the surface temperature of the roll and the bottom of the groove in real time. When the temperature of the bottom of the groove drops to the 500℃ return point, stop the spray quenching immediately. After stopping the spray quenching, place the roll in the heat preservation cover or buffer pit for 20 minutes. S34. The rolls are quickly transferred into a low-temperature furnace with the temperature set at 560°C for tempering treatment, and the microstructure transformation is controlled by the temperature difference between the residual heat and the furnace temperature. S35. After tempering, the furnace is cooled to below 300°C before being removed and air-cooled.

[0023] Example 2 A BD2 alloy ductile iron roll differs from Example 1 in that the chemical composition of the BD2 alloy ductile iron roll is as follows (mass percentage): C 3.3%, Si 1.5%, Mn 0.5%, Cr 0.5%, Ni 1.2%, Mo 0.3%, Al 0.04%, N 0.008%, with the balance being Fe and unavoidable impurities.

[0024] Example 3 A BD2 alloy ductile iron roll differs from Example 1 in that the chemical composition of the BD2 alloy ductile iron roll is as follows (mass percentage): C 3.5%, Si 1.7%, Mn 0.6%, Cr 0.6%, Ni 1.3%, Mo 0.4%, Al 0.08%, N 0.012%, with the balance being Fe and unavoidable impurities.

[0025] Example 4 A BD2 alloy ductile iron roll differs from Example 1 in that the BD2 alloy ductile iron roll has no Al / N added to its chemical composition and increases the Ni mass percentage to 1.5%.

[0026] Example 5 A BD2 alloy ductile iron roll differs from Example 1 in that it contains 3.6% C and 1.8% Si.

[0027] Example 6 A BD2 alloy ductile iron roll differs from Example 1 in that it contains 0.4% Mn and 0.4% Cr.

[0028] Example 7 A BD2 alloy ductile iron roll differs from Example 1 in that it contains 0.05% Al and 0.009% N.

[0029] Comparative Example 1 A BD2 alloy ductile iron roll differs from Example 1 in that it is spray-quenched to room temperature in steps S32 and S33.

[0030] Comparative Example 2 A BD2 alloy ductile iron roll differs from Example 1 in that the mold is a standard cylinder without pre-made steps; subsequent machining with a large amount of cutting is required to create the die shape.

[0031] Comparative Example 3 A BD2 alloy ductile iron roll differs from Example 1 in that the BD2 alloy ductile iron roll has no Al / N added in its chemical composition, and the mass percentage of Ni is increased to 1.8% and the mass percentage of Cr is increased to 0.8%.

[0032] Comparative Example 4 A BD2 alloy ductile iron roll differs from Example 1 in that aluminum ingots are directly fed into the furnace for melting, rather than being fed with cored wire, and the nitrogen content is not specifically controlled.

[0033] Comparative Example 5 A BD2 alloy ductile iron roll differs from Example 1 in that it is fully open and closed during spray quenching, and is directly fed into the furnace after stopping at 500°C, without any temperature uniformity retention.

[0034] Comparative Example 6 A BD2 alloy ductile iron roll differs from Example 1 in that it contains 0.8% Ni.

[0035] Comparative Example 7 A BD2 alloy ductile iron roll differs from Example 1 in that it contains 0.15% Al.

[0036] Comparative Example 8 A BD2 alloy ductile iron roll differs from Example 1 in that it contains 3.8% C.

[0037] Comparative Example 9 A BD2 alloy ductile iron roll differs from Example 1 in that it contains 1.2% Cr.

[0038] Detection example Flaw detection: In accordance with GB / T 13313-2008 "Shore Hardness Test Method for Rolls" and GB / T 7233.1-2009 "Ultrasonic Testing of Cast Steel Parts - Part 1: General Purpose Cast Steel Parts", ultrasonic non-destructive testing is carried out on the bottom of the deep groove and the variable cross-section of the roll to detect the presence of hot cracks or internal defects. Surface hardness: The surface hardness of the working layer of the roll body was measured using a Shore hardness tester in accordance with GB / T 13313-2008 "Shore Hardness Test Method for Rolls". Hardness uniformity: Cut along the radial direction of the roll and measure the Shore hardness of the raised surface of the roll body and the bottom of the deep groove respectively, and calculate the difference between the two (ΔHSD = roll body hardness - groove bottom hardness). Tensile properties: According to GB / T 228.1-2021 "Metallic materials - Tensile testing - Part 1: Test at room temperature", tensile specimens were cut from the same location (20 mm below the bottom of the deep groove) in the roll and the examples / comparative examples, and the tensile strength (Rm) was determined. Impact toughness: According to GB / T 229-2020 "Metallic materials Charpy pendulum impact test method", unnotched specimens were cut from the bottom of the deep groove of the roll for impact test, and the impact absorbed energy (Ak) was measured. The average value of three specimens was taken. The specific test results are shown in Table 1.

[0039] Table 1

[0040] Performance test data from Examples 1-3 and Comparative Example 1 show that Example 1, employing the "500℃ reheat quenching + 560℃ reverse tempering" process, exhibits no cracks in the deep groove region and its impact toughness is significantly higher than that of Comparative Example 1, which uses the traditional "cold-through tempering" process. This indicates that the process in Example 1 effectively releases tensile stress in the deep groove region by utilizing the volume expansion and TRIP effect during the austenite-bainite transformation, avoiding the large thermal stress cracks caused by the temperature difference in Comparative Example 1 due to cooling to room temperature, thus improving the safety and toughness of the roll. Furthermore, the hardness uniformity of Example 1 is superior to that of Comparative Example 1, demonstrating that the reheat process ensures sufficient transformation of the microstructure at the bottom of the groove.

[0041] The performance test data from Examples 1-4 and Comparative Example 3 show that, under the premise of using the new process, Example 1, which introduces Al-N microalloying and has a Ni content of only 1.25%, has a tensile strength that even exceeds that of the traditional high-alloy Example 3 with a Ni content as high as 1.8%, and is far superior to Example 4 without Al-N. This indicates that the in-situ generated nano-AlN particles play a key role in grain refinement and dispersion strengthening, successfully compensating for the solid solution strengthening loss caused by reducing the Ni content, and achieving high performance at low cost.

[0042] The performance test data from Examples 1-3 and Comparative Example 2 show that although the hardness levels of both are similar, the impact toughness of Example 1 is significantly higher than that of Comparative Example 2. This indicates that the "stepped die pre-casting" ensures that the metal flow lines of the roll are parallel to the die surface and are not cut off by machining, thus preserving better structural continuity and impact resistance in the deep groove stress area.

[0043] The performance test data from Examples 1-3 and Comparative Example 4 show that the incorrect direct aluminum addition method (Comparative Example 4) leads to severe inclusion defects and a sharp decline in various mechanical properties, which proves the importance of "core wire feeding" for the formation of dispersed fine AlN.

[0044] The performance test data from Examples 1-3 and Comparative Example 5 show that Comparative Example 5, which lacks the uniform temperature retention and differential temperature stopping steps, did not crack directly, but its hardness uniformity was poor. This indicates that auxiliary temperature control measures play an indispensable role in balancing the temperature field of complex cross-sections and ensuring that the bottom of the deep groove obtains an ideal bainitic structure.

[0045] Performance test data from Examples 1, 5 (high carbon silicon boundary), and 6 (low manganese chromium boundary) show that, within the chemical composition range defined in this application, the performance of the rolls, although fluctuating, remains at an excellent level through the synergy of microalloying and the process.

[0046] The performance test data from Example 1 and Comparative Example 6 show that when the nickel content drops to 0.8% (below the lower limit of 1.2% in this application), the performance of the rolls drops drastically. The surface hardness of Comparative Example 6 is only 47 HSD, with extremely poor hardness uniformity, and the tensile strength drops to 610 MPa. Controlling the nickel content to 1.2-1.7% is a necessary physical basis for achieving the performance of deep groove rolls.

[0047] The performance test data from Example 1 and Comparative Example 7 show that aluminum forms nano-AlN particles, which play a pinning and strengthening role. However, if the aluminum exceeds the standard, the excess aluminum will react with oxygen to form coarse alumina inclusions. These inclusions not only fail to strengthen the matrix but also become micro-crack sources that cleave the matrix.

[0048] The data from Comparative Example 8 shows that excessive carbon content leads to a decrease in tensile strength. This is because excessive carbon causes graphite to float and coarse, severely disrupting the continuity of the matrix. This indicates that an upper limit of 3.6% for C content is the guarantee of strength.

[0049] The data from Comparative Example 9 show that while excessive chromium content results in an extremely high surface hardness of 57 HSD, it also causes a dangerously low impact toughness of 4.5 J, exhibiting significant brittleness. This is because excessive chromium forms a network of carbides at the grain boundaries, which can easily lead to roll breakage during service.

[0050] Please note that the technical features of the above embodiments can be combined arbitrarily. For the sake of brevity, not all possible combinations of the technical features in the above embodiments have been described. However, as long as the combination of these technical features does not contradict each other, it should be considered within the scope of this specification. The above embodiments only illustrate several implementation methods of this application, and their descriptions are relatively specific and detailed, but they should not be construed as limiting the scope of the invention patent. It should be pointed out that for those skilled in the art, several modifications and improvements can be made without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this patent application should be determined by the appended claims.

Claims

1. A manufacturing process for BD2 alloy ductile iron rolls, characterized in that, Includes the following steps: S1. Melting and Casting: The molten iron is melted according to the target composition ratio. The chemical composition of the BD2 alloy ductile iron rolls includes the following percentages by mass: C 3.2-3.6%, Si 1.4-1.8%, Mn 0.4-0.7%, Cr 0.4-0.8%, Ni 1.2-1.7%, Mo 0.2-0.5%, with the balance being Fe and unavoidable impurities. During the tapping process, spheroidization is performed using core wire feeding technology, and double inoculation is carried out with in-flow ferrosilicon and zirconium silicon particles, followed by casting. S2, stepped hole pre-casting: In the casting mold design stage, according to the hole characteristics of the finished roll, the inner cavity of the mold is designed as a non-cylindrical structure with stepped holes, so that the billet has pre-made stepped grooves. S3, Precision temperature-return spray quenching treatment: S31. Heat the billet to 920±10℃ and hold it at that temperature to austenitize the matrix; S32. After exiting the furnace, a spray cooling device is used for spray quenching. S33. Monitor the surface temperature of the rolls and the bottom of the trough in real time. When the temperature at the bottom of the trough drops to the 500℃ reheat point, stop the spray quenching immediately. S34. The rolls are quickly transferred into a low-temperature furnace with the temperature set at 560°C for tempering treatment, and the microstructure transformation is controlled by the temperature difference between the residual heat and the furnace temperature. S35. After tempering, the furnace is cooled to below 300°C before being removed and air-cooled.

2. The manufacturing process of a BD2 alloy ductile iron roll according to claim 1, characterized in that, The chemical composition of the BD2 alloy ductile iron rolls comprises, by mass percentage: C 3.3-3.5%, Si 1.5-1.7%, Mn 0.5-0.6%, Cr 0.5-0.6%, Ni 1.2-1.3%, Mo 0.3-0.4%, Al 0.04-0.08%, N 0.008-0.012%, with the balance being Fe and unavoidable impurities.

3. The manufacturing process of a BD2 alloy ductile iron roll according to claim 2, characterized in that, The spheroidizing and inoculation treatment in step S1 is specifically carried out by feeding aluminum-containing cored wire and silicon-magnesium alloy cored wire simultaneously when the molten iron is tapped from the furnace; the nitrogen element is introduced by adding nitrogen-containing ferromanganese in the later stage of smelting or by bottom blowing nitrogen.

4. The manufacturing process of a BD2 alloy ductile iron roll according to claim 1, characterized in that, Between steps S33 and S34, there is also a temperature equalization and holding step: After stopping the spray quenching, the roll is placed in a heat preservation cover or buffer pit and held for 15-20 minutes, using the residual heat of the roll core to make the temperature of the roll surface and the root of the deep groove tend to be consistent within the range of 500±10℃.

5. The manufacturing process of a BD2 alloy ductile iron roll according to claim 1, characterized in that, Step S34 is specifically a reverse excitation tempering process: the rolls, which have undergone uniform temperature retention and whose overall temperature has been stabilized at 500±10℃, are quickly transferred into a low-temperature furnace that has been preheated to 560±10℃; in the initial stage after entering the furnace, the furnace circulation fan is turned on and heated at full power, so that the surface and shallow layer of the rolls are reversely heated from 500±10℃ to 560±10℃ within 30±5 minutes.

6. The manufacturing process of a BD2 alloy ductile iron roll according to claim 1, characterized in that, The spray quenching cooling in step S32 adopts the differential temperature stop method: during the spray quenching process, when the surface temperature of the protruding part of the roll body is monitored to drop to 600-650℃, the nozzles targeting the protruding part of the roll body are closed first, while the nozzles targeting the bottom of the deep groove continue to spray for 30-60 seconds before stopping the spraying.

7. The BD2 alloy ductile iron roll prepared by the preparation process according to any one of claims 1-6, characterized in that, The matrix structure of the roll is a multiphase structure of lower bainite and a small amount of retained austenite, and AlN particles with a size of 10-50nm are dispersed at the grain boundaries of the matrix; the hardness difference between the bottom of the deep groove of the roll and the surface of the roll body is ≤3 HSD, and there are no thermal stress cracks at the bottom of the groove.

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