A method for processing a forged Cr-Ni-Mo steel roll blank

Abstract

The application discloses a forging Cr-Ni-Mo steel roll blank forging processing method, and relates to the technical field of forging processing. The forging Cr-Ni-Mo steel roll blank forging processing method adopts a high-purity smelting process with multiple processes in cooperation, strictly controls harmful residual elements, gases and inclusion indexes in the steel ingot, and is matched with a three-fire alternating large-pressing-upset upsetting and drawing forging process. Through the forging mode of wide flat anvil staggered anvil, high feed amount and high lap amount, the heart of the steel ingot is in a three-way compressive stress state all the time, the as-cast defects such as loose, shrinkage and micro-hole are fully forged, and the coarse dendrites are broken, so that the ultrasonic flaw detection of the roll blank is all up to the standard. The whole process of the segmented ladder temperature control post-forging heat treatment, special-shaped contour pretreatment and segmented quenching ladder quenching and tempering is matched, the uniform regulation and control of the hardness of the roll body are realized, the tensile strength and elongation of the finished product are greatly improved, and the hardness difference is controlled within 3HSD.

Description

Technical Field

[0001] This application belongs to the field of forging processing technology, and in particular relates to a method for processing Cr-Ni-Mo steel roll blanks for forging. Background Technology

[0002] Hot rolling work rolls are key components in the forming and processing of hot rolling production lines, directly determining the surface quality, dimensional accuracy, and overall production efficiency of rolled products such as sheet piles and heavy-duty long products. Currently, the industry commonly uses 60CrNiMo material for hot rolling work rolls, paired with a smooth cylindrical roll blank structure, which can meet the rolling requirements of ordinary plates after simple forging and tempering.

[0003] For the demanding rolling conditions of high-strength steel sheet piles and heavy-duty long products, rolls need to withstand greater contact loads and intense friction, requiring extremely high levels of roll hardness, hardness uniformity, wear resistance, and fatigue resistance. Traditional 60CrNiMo steel has a low carbon content and insufficient strength, wear resistance, and hardenability, making it prone to problems such as roll surface wear, uneven hardness, and localized crushing under heavy-duty rolling conditions, resulting in short service life and poor rolling stability. Therefore, the industry is gradually adopting the higher-performance 65CrNiMo material, which has a higher carbon content and offers higher strength, wear resistance, and hardenability, making it suitable for the needs of heavy-duty roll applications.

[0004] To meet the forming requirements of sheet piles, the 65CrNiMo hot-rolled work rolls adopt a slotted irregular structure. The roll body includes multiple sections of cylindrical and conical surfaces with varying diameters. Compared with the traditional smooth cylindrical roll blanks of 60CrNiMo, the structure is more complex, with more abrupt changes in cross-section and significant stress concentration, greatly increasing the difficulty of processing and heat treatment. However, the current industry's traditional forging process only uses two upsetting and two drawing processes, with the bilateral reduction amount being only 20% of the billet height. The billet compaction effect is poor, and it cannot effectively eliminate defects such as porosity, micropores, and segregation generated during the casting and forging process. Moreover, the forging anvil and turning process are crude, which easily leads to mixed grains and uneven microstructure. Summary of the Invention

[0005] To address some or all of the technical problems existing in the prior art, this application provides a method for processing Cr-Ni-Mo steel roll forgings.

[0006] This application provides a method for machining Cr-Ni-Mo steel roll forgings, which includes the following steps: Step S1, High-purity smelting and casting of ingots: Cr-Ni-Mo steel ingots are prepared by sequentially undergoing primary smelting in an electric arc furnace, refining in a ladle, vacuum degassing, argon soft stirring, and argon protection. Step S2, Alternating Upsetting and Drawing Forging: Forging is completed in three stages. In the first stage, the heated and held steel ingot is successively rounded, cut at the bottom, and pressed into jaws to complete the pre-forging billet preparation. In the second stage, after heating and holding the billet, it is upset and then drawn with a large reduction for the first time. In the third stage, after heating and holding the billet, it is upset and then drawn with a large reduction for the second time. The drawing is carried out using a wide flat anvil with a rounded transition. The drawing feed is not less than 90% of the anvil width, and an overlap is reserved between adjacent anvils. After each pressing pass, the billet is rotated 90°, and the anvil arrangement for the next pressing pass is staggered by half relative to the previous pass. During the forging process, cracks, pits, and fold defects on the surface of the billet are cleaned while hot. Step S3, Post-forging heat treatment: After the forged roll blank is air-cooled to a low temperature, it is put into the furnace and subjected to high-temperature normalizing and high-temperature tempering in sequence; Step S4, Pre-treatment of the contour before heat treatment: pre-machine the grooving of the roll blank, leave a machining allowance on one side of the finished size, and machine the corners of the roll body and the transition position of the groove bottom into large radius rounded corners; Step S5, stepped quenching and tempering heat treatment: The pretreated roll blank is quenched and tempered by a series of steps: multi-stage stepped heating, air cooling combined with liquid cooling for segmented quenching, segmented low temperature holding and homogenization after quenching, and stepped heating tempering, to obtain Cr-Ni-Mo steel roll blank forging.

[0007] Preferably, the Cr-Ni-Mo steel is 65CrNiMo steel; the content of each component in the steel ingot before forging is controlled as follows: C: 0.60~0.70, Si: 0.20~0.60, Mn: 0.70~1.00, P≤0.015, S≤0.015, Cr: 0.80~1.20, Ni: 1.50~2.00, Mo: 0.30~0.50, Cu≤0.20, Pb≤0.02, Sn≤0.02, As≤0.02, Sb≤0.02, Bi≤0.02, Zn≤0.02; the hydrogen content of the finished steel ingot is ≤1.0ppm, the oxygen content is ≤15ppm, and the nitrogen content is <70ppm; the coarse and fine grades of inclusions of types A, B, C, and D in the steel are all 0 to 1, and the grade of inclusions of type DS is ≤1.

[0008] Preferably, in step S1, during the initial refining stage of the electric arc furnace: the molten steel is kept boiling uniformly during the oxidation period and the boiling time exceeds 25 minutes; the tapping temperature is controlled at 1630-1660℃, the carbon content is not less than 0.15% and the phosphorus content is not more than 0.005% when tapping, and aluminum ingots and silicon manganese are added during the tapping process to complete pre-deoxidation. Ladle refining stage: Carbon powder and ferrosilicon powder are used for diffusion deoxidation, and argon gas is blown into the bottom of the ladle simultaneously to quickly prepare reducing slag to achieve deoxidation and desulfurization. The slag basicity, slag quantity and steel temperature are controlled, alloy materials are added to adjust the composition, and the temperature is raised to 1650-1660℃ before being transferred to vacuum treatment. Vacuum degassing stage: The molten steel boils uniformly in a vacuum environment for more than 25 minutes, relying on the carbon-oxygen reaction to remove oxygen, hydrogen and nitrogen from the steel and carry inclusions to the float; after the vacuum is completed, argon gas is continuously stirred for more than 20 minutes. Argon-protected casting stage: Argon gas is used to replace the entire casting system 30 minutes before casting, and argon gas is used to isolate and protect the casting process throughout; the surface temperature of the steel ingot after demolding is not lower than 700℃.

[0009] Preferably, in step S2: First heating: The steel ingot is heated to 1250±10℃ at a rate of ≤70℃ / h and held for 18h to complete rounding, bottom cutting and jaw pressing. Second heating: The billet is returned to the furnace and heated to 1250±10℃ and held for 30 hours. After upsetting, a wide flat anvil with a rounded transition is used to draw the billet to an octagon with a double-sided reduction of 30% of the billet height. Third heating: After cleaning the surface defects of the billet, return it to the furnace and heat it to 1250±10℃ for 25 hours. After upsetting, use a wide flat anvil with a rounded transition to draw it out to an octagon with a double-sided reduction of 30% of the billet height.

[0010] Preferably, in step S2, the overlap between adjacent anvils is 100-200 mm.

[0011] Preferably, in step S3, after the roll blank is forged, it is air-cooled to 200-300℃ and held for 10-20 hours. Then, it is heated to 640-660℃ at a step heating rate of ≤60℃ / h and held for 10-20 hours. After that, it is heated to 900-950℃ at a step heating rate of ≤60℃ / h and held for 30-40 hours for high-temperature normalizing. After normalizing, it is air-cooled to 200-300℃ and held for 10-20 hours. Then, it is heated to 640-660℃ at a step heating rate of ≤60℃ / h and held for 40-50 hours for high-temperature tempering. After tempering, it is cooled to below 200℃ in the furnace and then removed from the furnace to refine the grains and eliminate forging mixed grains.

[0012] Preferably, in step S4, a tempering processing allowance of 5mm to 15mm is reserved on one side, and the radius of the corner and groove bottom transition radius is 15mm to 25mm.

[0013] Preferably, step S5 involves tempering in a trolley furnace. The process is as follows: first, the temperature is held at 250–350℃ for 5–10 hours; then, the temperature is increased to 650–670℃ at a step heating rate of ≤40℃ / h and held for 5–10 hours; next, the temperature is increased to 800–850℃ at a heating rate of ≤50℃ / h and held for 15–20 hours, followed by segmented quenching using air cooling and liquid cooling; the temperature is then cooled to 250–350℃ and held for 5–10 hours; finally, the temperature is increased to 550–650℃ at a step heating rate of ≤40℃ / h and held for 35–50 hours for tempering; after tempering, the temperature is furnace cooled to 400℃, and then the furnace is removed and air-cooled to below 150℃.

[0014] The processing method for Cr-Ni-Mo steel roll forgings of this application has the following advantages and positive effects: (1) Adopt a high-purity smelting process with multi-process collaboration, strictly control the indicators of harmful residual elements, gases and inclusions in steel ingots; combine with a three-fire alternating high-pressure upsetting and drawing forging process, and through the forging method of wide flat anvil, large feed amount and high overlap amount, the core of the steel ingot is in a triaxial compressive stress state throughout the process, fully forging and combining casting defects such as porosity, shrinkage cavities and micropores, breaking coarse dendrites, avoiding the problem of mixed grains from the source, and ensuring that all ultrasonic flaw detection of roll blanks meets the standards.

[0015] (2) The complete process of segmented step temperature control forging heat treatment, irregular contour pretreatment, segmented quenching and step tempering is matched. Through multiple means such as large rounded corner transition, precise allowance reservation, slow step heating and cooling, and segmented uniform temperature cooling, heat treatment deformation and cracks are suppressed. At the same time, the uniform control of the hardness of the roll body is achieved, the tensile strength and elongation of the finished product are greatly improved, the hardness difference is controlled within 3HSD, and the toughness and wear resistance are matched with the heavy-load rolling conditions of steel sheet piles. Attached Figure Description

[0016] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only for further understanding of the embodiments of this application and constitute a part of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort. In the drawings: Figure 1 This is a heat treatment curve after forging in the processing method of Cr-Ni-Mo steel roll forging forgings of this application; Figure 2 This is a heat treatment curve diagram of the processing method for Cr-Ni-Mo steel roll blanks for forging in this application. Detailed Implementation

[0017] To make the objectives, technical solutions, and advantages of this application clearer, the technical solutions of this application will be clearly and completely described below in conjunction with specific embodiments and corresponding drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this application without creative effort are within the scope of protection of this application.

[0018] The method for machining Cr-Ni-Mo steel roll forgings according to this application includes the following steps in sequence: Step S1, High-purity smelting and casting of ingots: Cr-Ni-Mo steel ingots are prepared by sequentially undergoing primary smelting in an electric arc furnace, refining in a ladle, vacuum degassing, argon soft stirring, and argon protection. In the primary refining stage of the electric arc furnace: during the oxidation period, the molten steel is kept to boil uniformly and the boiling time is more than 25 minutes; the tapping temperature is controlled at 1630-1660℃, and the carbon content is not less than 0.15% and the phosphorus content is not more than 0.005% when tapping. Aluminum ingots and silicon manganese are added during the tapping process to complete pre-deoxidation. Ladle refining stage: Carbon powder and ferrosilicon powder are used for diffusion deoxidation, and argon gas is blown into the bottom of the ladle simultaneously to quickly prepare reducing slag to achieve deoxidation and desulfurization. The slag basicity, slag quantity and steel temperature are controlled, alloy materials are added to adjust the composition, and the temperature is raised to 1650-1660℃ before being transferred to vacuum treatment. Vacuum degassing stage: The molten steel boils uniformly in a vacuum environment for more than 25 minutes, relying on the carbon-oxygen reaction to remove oxygen, hydrogen and nitrogen from the steel and carry inclusions to the float; after the vacuum is completed, argon gas is continuously stirred for more than 20 minutes. Argon-protected casting stage: Argon gas is used to replace the entire casting system 30 minutes before casting, and argon gas is used to isolate and protect the casting process throughout; the surface temperature of the steel ingot after demolding is not lower than 700℃.

[0019] Among them, Cr-Ni-Mo steel is 65CrNiMo steel; the content of each component of the steel ingot before forging is controlled as follows: C: 0.60~0.70, Si: 0.20~0.60, Mn: 0.70~1.00, P≤0.015, S≤0.015, Cr: 0.80~1.20, Ni: 1.50~2.00, Mo: 0.30~0.50, Cu≤0.20, Pb≤0.02, Sn≤0.02, As≤0.02, Sb≤0.02, Bi≤0.02, Zn≤0.02; the hydrogen content of the finished steel ingot is ≤1.0ppm, oxygen content is ≤15ppm, and nitrogen content is <70ppm; the coarse and fine grades of inclusions of types A, B, C, and D in the steel are all 0 to 1, and the grade of inclusions of type DS is ≤1.

[0020] Step S2, Alternating Upsetting and Drawing Forging: Forging is completed in three passes. In the first pass, the heated and held steel ingot is successively rounded, cut at the bottom, and pressed into jaws to complete the pre-forging billet preparation. In the second pass, after heating and holding the billet, it is upset and then drawn with a large reduction for the first time. In the third pass, after heating and holding the billet, it is upset and then drawn with a large reduction for the second time. The drawing is carried out using a wide flat anvil with a rounded transition. The drawing feed is not less than 90% of the anvil width, and an overlap of 100-200mm is reserved between adjacent anvils. After each pass of pressing is completed, the billet is rotated 90°, and the anvils are staggered by half for the next pass. During the forging process, cracks, pits, and fold defects on the surface of the billet are cleaned while hot. First heating: The steel ingot is heated to 1250±10℃ at a rate of ≤70℃ / h and held for 18h to complete rounding, bottom cutting and jaw pressing. Second heating: The billet is returned to the furnace and heated to 1250±10℃ and held for 30 hours. After upsetting, a wide flat anvil with a rounded transition is used to draw the billet to an octagon with a double-sided reduction of 30% of the billet height. Third heating: After cleaning the surface defects of the billet, return it to the furnace and heat it to 1250±10℃ for 25 hours. After upsetting, use a wide flat anvil with a rounded transition to draw it out to an octagon with a double-sided reduction of 30% of the billet height.

[0021] Step S3, Post-forging heat treatment: After the forged roll blank is air-cooled to a low temperature, it is put into the furnace and subjected to high-temperature normalizing and high-temperature tempering in sequence; After forging, the roll blank is air-cooled to 200-300℃ and held for 10-20 hours. Then, it is heated to 640-660℃ at a step heating rate of ≤60℃ / h and held for 10-20 hours. After that, it is heated to 900-950℃ at a step heating rate of ≤60℃ / h and held for 30-40 hours for high-temperature normalizing. After normalizing, it is air-cooled to 200-300℃ and held for 10-20 hours. Then, it is heated to 640-660℃ at a step heating rate of ≤60℃ / h and held for 40-50 hours for high-temperature tempering. After tempering, it is cooled to below 200℃ in the furnace to refine the grains and eliminate forging mixed grains.

[0022] Step S4, Pre-processing of the contour before tempering: pre-process the grooving of the roll blank, leave a machining allowance on one side of the finished size, and process the corners of the roll body and the transition position of the groove bottom into large radius rounded corners; leave a tempering machining allowance of 5mm to 15mm on one side, and the radius of the rounded corners of the corners and the transition position of the groove bottom is 15mm to 25mm.

[0023] Step S5, stepped quenching and tempering heat treatment: The pretreated roll blank is quenched and tempered by a multi-stage stepped heating, air cooling combined with liquid cooling for segmented quenching, segmented low temperature holding and homogenization after quenching, and stepped heating tempering, to obtain Cr-Ni-Mo steel roll blank forging. The tempering process in the electric furnace is as follows: First, hold at 250-350℃ for 5-10 hours, then heat to 650-670℃ at a step heating rate of ≤40℃ / h and hold for 5-10 hours, then heat to 800-850℃ at a heating rate of ≤50℃ / h and hold for 15-20 hours, followed by segmented quenching using air cooling and liquid cooling; cool to 250-350℃ and hold for 5-10 hours, then heat to 550-650℃ at a step heating rate of ≤40℃ / h and hold for 35-50 hours for tempering; after tempering, furnace cool to 400℃, then remove from the furnace and air cool to below 150℃.

[0024] The present application will be further described in detail below with reference to the embodiments.

[0025] Example 1 In this embodiment, a slotted hot-rolled work roll blank is prepared using 65CrNiMo steel. The finished product dimensions are Ø1350mm × length 4150mm. A 50t grade steel ingot is used, with an ingot utilization rate of approximately 71%. The processing method includes the following steps in sequence: Step S1, High-purity smelting and casting of ingots: Steel ingots are prepared using the following process route: primary smelting in an 80-ton electric arc furnace → refining in a 120-ton ladle → vacuum degassing → argon soft stirring → argon protective injection.

[0026] In the initial smelting stage of the electric arc furnace: during the oxidation period, the molten steel is kept in a uniform boiling state for 30 minutes; the tapping temperature is controlled at 1630-1660℃, the carbon content of the tapped steel is not less than 0.15%, and the phosphorus content is not more than 0.005%. Aluminum ingots and silicon manganese are added during the tapping process to complete the pre-deoxidation.

[0027] In the ladle refining stage: carbon powder and ferrosilicon powder are used for diffusion deoxidation, simultaneously with argon blowing from the bottom of the ladle, to rapidly prepare reducing slag for deoxidation and desulfurization, and to maximize the flotation of non-metallic inclusions in the molten steel. Appropriate slag basicity, slag quantity, and molten steel temperature are maintained. Alloy materials are added according to process requirements, and the chemical composition is adjusted. Vacuum treatment is then performed at 1650~1660℃.

[0028] Controlling slag basicity, slag quantity, molten steel temperature, and alloying fine-tuning are all common and conventional technologies used in LF refining: high-alkali reducing slag is regulated by lime and fluxing slag, a protective slag layer is formed by quantitative slag production, temperature is controlled by electric arc heating, and the amount of alloy added is calculated by sampling and stirred evenly. This is a standardized purification and composition control method for alloy steel.

[0029] Vacuum degassing stage: The molten steel boils uniformly under vacuum for 30 minutes, and large boiling is strictly prohibited in the furnace. After high vacuum treatment, oxygen in the molten steel is removed by the carbon-oxygen reaction. The generated CO bubbles diffuse and float H, O, N and other gases and inclusions in the molten steel, thus purifying the steel. After vacuum treatment, the molten steel is allowed to stand and then gently stirred with argon gas for 25 minutes to further ensure that inclusions in the steel float to the surface.

[0030] Argon-protected casting stage: Argon gas is used to replace the casting system 30 minutes before casting. Argon gas is used to isolate and protect the casting process throughout to prevent oxidation of the molten steel. The steel ingot is demolded and removed according to the solidification and cooling process requirements. The surface temperature of the steel ingot is 720℃ after demolding. It is then loaded into an insulated car and transported to the forging workshop for heating and insulation.

[0031] In this embodiment, two qualified steel ingots were smelted. The results of the chemical composition, gas content, and residual element analysis of the steel ingots after demolding are shown in Table 1.

[0032] Table 1 Chemical composition of steel ingots (mass fraction, % ); excluding ppm

[0033] Two steel ingots produced by the above method were forged using the same process.

[0034] S2, Alternating upsetting and drawing forging: The forging process is completed in three stages.

[0035] 65CrNiMo belongs to the medium-high carbon Cr-Ni-Mo alloy structural steel system. The forging process plays a crucial role in "shaping, compacting defects, refining the microstructure, and optimizing streamlines," while post-forging heat treatment mainly aims to "eliminate stress, homogenize composition, adjust microstructure, and stabilize performance." Together, these processes can transform the inherent defects of the steel ingot into high-performance, usable parts. To meet the requirements of ultrasonic testing of the roll blank and obtain excellent internal quality, the forging process design must adhere to the following principles: compacting the core of the steel ingot, focusing on breaking down and refining coarse primary dendritic grains; forging away porosity, shrinkage cavities, and looseness to achieve microstructure densification; dispersing segregation, inclusions, and other inhomogeneous structures to promote compositional homogenization. After the roll blank is forged, effective post-forging heat treatment is necessary to fully eliminate thermal stress, reduce hardness for easier machining, improve chemical composition homogeneity, adjust and improve the inhomogeneous structure formed during forging, and refine the grains, thereby creating favorable internal microstructure conditions for ultrasonic testing and final heat treatment.

[0036] First heating pass: Hot steel ingots are fed into the furnace and heated to 1250±10℃ at a rate of ≤70℃ / h, then held for 18 hours. Using a 125MN press with an 850mm flat anvil on top and an 850mm V-shaped anvil on the bottom, the first heating pass involves rounding, cutting the bottom, and pressing the jaws. The manipulator jaws hold the riser end of the ingot, rounding the ingot body near the bottom by Ø1500mm, and cutting off the bottom conical portion of the ingot. The ingot is then turned around, and the manipulator jaws hold the rounded bottom portion, rounding the ingot body on the riser side by Ø1500mm. Along the riser line, the jaws are pressed about 100mm above the ingot body, and the jaws are rolled appropriately to ensure it can be placed in the upsetting tray for the next heating pass.

[0037] Second pass: The billet is returned to the furnace and heated to 1250±10℃ and held for 30 hours; using a 125MN press, with a Ø1000mm upsetting plate and a spherical upsetting cap, the billet is upset to a diameter of Ø2270mm; using a 1200mm wide flat anvil with a 100mm radius, the upset billet is lengthened to 1500mm on each side, with a reduction of 30% of the original billet height on both sides. The first anvil of each pass is fully pressed down, with the feed of each anvil being greater than 90% of the anvil width, and a 100-200mm overlap between anvils. After each pass, the anvils are rotated 90°, and the next pass is staggered by half the anvils to ensure effective coverage of the overall deformation of the forging.

[0038] Third upsetting: After hot cleaning of surface cracks, pits, and folds in the billet, it is returned to the furnace and heated to 1250±10℃ for 25 hours. Using a 125MN press, a Ø1000mm upsetting plate, and a spherical upsetting cap with a diameter of approximately Ø2300mm, a wide flat anvil of 1200mm in both the upper and lower sections is used with a flat anvil radius of R100mm. The billet after upsetting is then drawn with a large reduction, following the same process as after the first upsetting, until it is drawn to 1600mm in octagonal shape.

[0039] The synergistic effect of upsetting and drawing can achieve the following results: Bidirectional compaction, forging through the core: Upsetting can eliminate radial porosity, drawing can solve axial defects, and alternating deformation keeps the central area of ​​the steel ingot in a state of triaxial compressive stress, avoiding the "core tensile stress" generated by uniaxial deformation and greatly reducing the risk of cracking.

[0040] Reduce banded structure: Upsetting can disrupt dendrite orientation, and then elongation can prevent excessive enrichment of dendrite segregation in a single direction, thereby reducing the tendency of banded structure during subsequent heat treatment.

[0041] Uniform deformation and grain refinement: Repeated upsetting and drawing deformation promotes continuous dynamic recrystallization of grains, resulting in a uniform and fine recrystallized structure, which lays a good microstructure foundation for subsequent quenching and tempering treatment.

[0042] Steels like 65CrNiMo are extremely sensitive to segregation and porosity, so controlling the upsetting ratio and drawing ratio during forging is particularly crucial.

[0043] Step S3, Post-Forging Heat Treatment: The forged roll blank is air-cooled to 250°C and then placed in a furnace for high-temperature normalizing and high-temperature tempering. The heat treatment process curve is as follows: Figure 1 As shown.

[0044] The specific process is as follows: After forging the roll blank, air cool it to 200-300℃ and hold it for 10-20 hours. Then, heat it to 640-660℃ at a step heating rate of ≤60℃ / h and hold it for 10-20 hours. Next, heat it to 900-950℃ at a step heating rate of ≤60℃ / h and hold it for 30-40 hours for high-temperature normalizing. After normalizing, air cool it to 200-300℃ and hold it for 10-20 hours. Then, heat it to 640-660℃ at a step heating rate of ≤60℃ / h and hold it for 40-50 hours for high-temperature tempering. After tempering, cool it to below 200℃ in the furnace to refine the grains and eliminate forging mixed grains.

[0045] 65CrNiMo roll blanks belong to the stepped shaft category. Due to uneven deformation in each area, they are prone to phenomena such as mixed grains. In order to obtain a good internal microstructure, appropriate post-forging heat treatment is necessary. Through effective post-forging heat treatment, the goal of refining the grains and eliminating mixed grains in the roll blank can be achieved.

[0046] After air cooling following forging, 65CrNiMo roll blanks are loaded into the furnace for post-forging heat treatment. A high-temperature normalizing + tempering process is adopted. During the heat treatment process, the roll blank forging is fully undercooled and held at that temperature to ensure the core of the roll blank also cools below the lower bainite transformation temperature to complete the microstructure transformation. Then, the temperature is raised to austenitize, refining the grains through granular austenite formed at grain boundaries and granular austenite formed within the grains on the high-temperature side of the phase transformation range.

[0047] Step S4: Pre-treatment of the contour before tempering: After tempering, the microstructure of 65CrNiMo is tempered sorbite, composed of uniform and fine ferrite and dispersed granular carbides, exhibiting excellent comprehensive mechanical properties. The hardened layer depth of 65CrNiMo is approximately 40mm-60mm. Ensuring the hardness of the working surface after tempering and preventing deformation and cracking are core elements of the pre-tempering dimensional design. Therefore, the roll blank is pre-machined with grooving, and a 5mm-10mm tempering machining allowance is reserved on each side of the finished size. The corners of the roll body and the transition positions at the bottom of the groove are machined with large radius radii of R15mm-R25mm.

[0048] Step S5, Stepped tempering heat treatment: The tempering process is completed in a bogie furnace. The process curve for this heat treatment is as follows: Figure 2 As shown.

[0049] The specific process is as follows: The tempering process is completed in the trolley electric furnace as follows: First, it is held at 250-350℃ for 5-10 hours, then heated to 650-670℃ at a step heating rate of ≤40℃ / h and held for 5-10 hours, then heated to 800-850℃ at a heating rate of ≤50℃ / h and held for 15-20 hours, followed by segmented quenching using air cooling and liquid cooling; after cooling to 250-350℃ and holding for 5-10 hours, it is heated to 550-650℃ at a step heating rate of ≤40℃ / h and held for 35-50 hours for tempering; after tempering, it is furnace cooled to 400℃, and then removed from the furnace and air cooled to below 150℃.

[0050] Performance testing of finished products The results of the mechanical property analysis (longitudinal samples were taken from the riser end of the roll blank) are shown in Table 2.

[0051] Table 2

[0052] Both roll blanks exhibit significantly higher strength and plasticity than the design lower limit, demonstrating excellent strength-toughness matching.

[0053] Surface hardness testing: Repeated circumferential and radial tests were conducted at multiple points along the roll neck and multiple sections of the roll body. The hardness test results are shown in Table 3.

[0054] Table 3

[0055] The overall hardness range of roll blank 1 is 46.7~49.1HSD, with a range of 2.4HSD; the overall hardness range of roll blank 2 is 46.7~48.6HSD, with a range of 1.9HSD, both of which meet the hardness and uniformity indicators.

[0056] Ultrasonic flaw detection: Both roll blanks passed the flaw detection test. There were no equivalent defects larger than Ø2mm in the working layer of the roll blanks, no dense or continuous defects larger than Ø3mm in the core, no single defects larger than Ø4mm, and no bottom attenuation zone. The roll body has uniform hardness and excellent comprehensive mechanical properties, and all meet the technical requirements.

[0057] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application.

Claims

1. A method for machining Cr-Ni-Mo steel roll forgings, characterized in that, The steps are as follows: Step S1, High-purity smelting and casting of ingots: Cr-Ni-Mo steel ingots are prepared by sequentially undergoing primary smelting in an electric arc furnace, refining in a ladle, vacuum degassing, argon soft stirring, and argon protection. Step S2, Alternating Upsetting and Drawing Forging: Forging is completed in three stages. In the first stage, the heated and held steel ingot is successively rounded, cut at the bottom, and pressed into jaws to complete the pre-forging billet preparation. In the second stage, after heating and holding the billet, it is upset and then drawn with a large reduction for the first time. In the third stage, after heating and holding the billet, it is upset and then drawn with a large reduction for the second time. The drawing is carried out using a wide flat anvil with a rounded transition. The drawing feed is not less than 90% of the anvil width, and an overlap is reserved between adjacent anvils. After each pressing pass, the billet is rotated 90°, and the anvil arrangement for the next pressing pass is staggered by half relative to the previous pass. During the forging process, cracks, pits, and fold defects on the surface of the billet are cleaned while hot. Step S3, Post-forging heat treatment: After the forged roll blank is air-cooled to a low temperature, it is put into the furnace and subjected to high-temperature normalizing and high-temperature tempering in sequence; Step S4, Pre-treatment of the contour before heat treatment: pre-machine the grooving of the roll blank, leave a machining allowance on one side of the finished size, and machine the corners of the roll body and the transition position of the groove bottom into large radius rounded corners; Step S5, stepped quenching and tempering heat treatment: The pretreated roll blank is quenched and tempered by a series of steps: multi-stage stepped heating, air cooling combined with liquid cooling for segmented quenching, segmented low temperature holding and homogenization after quenching, and stepped heating tempering, to obtain Cr-Ni-Mo steel roll blank forging.

2. The method for processing Cr-Ni-Mo steel roll forgings according to claim 1, characterized in that, The Cr-Ni-Mo steel is 65CrNiMo steel; the content of each component in the steel ingot before forging is controlled as follows: C: 0.60~0.70, Si: 0.20~0.60, Mn: 0.70~1.00, P≤0.015, S≤0.015, Cr: 0.80~1.20, Ni: 1.50~2.00, Mo: 0.30~0.50, Cu≤0.20, Pb≤0.02, Sn≤0.02, As≤0.02, Sb≤0.02, Bi≤0.02, Zn≤0.02; the hydrogen content of the finished steel ingot is ≤1.0ppm, oxygen content is ≤15ppm, and nitrogen content is <70ppm; the coarse and fine grades of inclusions A, B, C, and D in the steel are all 0 to 1, and the grade of inclusions DS is ≤1.

3. The method for processing Cr-Ni-Mo steel roll forgings according to claim 1, characterized in that, In step S1, during the initial refining stage of the electric arc furnace: during the oxidation period, the molten steel is kept to boil uniformly and the boiling time exceeds 25 minutes; the tapping temperature is controlled at 1630-1660℃, and the carbon content is not less than 0.15% and the phosphorus content is not more than 0.005% when tapping. Aluminum ingots and silicon manganese are added during the tapping process to complete pre-deoxidation. Ladle refining stage: Carbon powder and ferrosilicon powder are used for diffusion deoxidation, and argon gas is blown into the bottom of the ladle simultaneously to quickly prepare reducing slag to achieve deoxidation and desulfurization. The slag basicity, slag quantity and steel temperature are controlled, alloy materials are added to adjust the composition, and the temperature is raised to 1650-1660℃ before being transferred to vacuum treatment. Vacuum degassing stage: The molten steel boils uniformly in a vacuum environment for more than 25 minutes, relying on the carbon-oxygen reaction to remove oxygen, hydrogen and nitrogen from the steel and carry inclusions to the float; after the vacuum is completed, argon gas is continuously stirred for more than 20 minutes. Argon-protected casting stage: Argon gas is used to replace the entire casting system 30 minutes before casting, and argon gas is used to isolate and protect the casting process throughout; the surface temperature of the steel ingot after demolding is not lower than 700℃.

4. The method for processing Cr-Ni-Mo steel roll forgings according to claim 1, characterized in that, In step S2: First heating: The steel ingot is heated to 1250±10℃ at a rate of ≤70℃ / h and held for 18h to complete rounding, bottom cutting and jaw pressing. Second heating: The billet is returned to the furnace and heated to 1250±10℃ and held for 30 hours. After upsetting, a wide flat anvil with a rounded transition is used to draw the billet to an octagon with a double-sided reduction of 30% of the billet height. Third heating: After cleaning the surface defects of the billet, return it to the furnace and heat it to 1250±10℃ for 25 hours. After upsetting, use a wide flat anvil with a rounded transition to draw it out to an octagon with a double-sided reduction of 30% of the billet height.

5. The method for processing Cr-Ni-Mo steel roll forgings according to claim 1, characterized in that, In step S2, the overlap between adjacent anvils is 100-200mm.

6. The method for processing Cr-Ni-Mo steel roll forgings according to claim 1, characterized in that, In step S3, after forging, the roll blank is air-cooled to 200-300℃ and held for 10-20 hours. Then, it is heated to 640-660℃ at a step heating rate of ≤60℃ / h and held for 10-20 hours. After that, it is heated to 900-950℃ at a step heating rate of ≤60℃ / h and held for 30-40 hours for high-temperature normalizing. After normalizing, it is air-cooled to 200-300℃ and held for 10-20 hours. Then, it is heated to 640-660℃ at a step heating rate of ≤60℃ / h and held for 40-50 hours for high-temperature tempering. After tempering, it is cooled to below 200℃ in the furnace and then removed from the furnace to refine the grains and eliminate forging mixed grains.

7. The method for processing Cr-Ni-Mo steel roll forgings according to claim 1, characterized in that, In step S4, a tempering allowance of 5mm to 15mm is reserved on one side, and the radius of the corner and groove bottom transition radius is 15mm to 25mm.

8. The method for processing Cr-Ni-Mo steel roll forgings according to claim 1, characterized in that, Step S5 involves tempering in a trolley furnace. The process is as follows: first, hold at 250–350℃ for 5–10 hours; then, heat to 650–670℃ at a step heating rate of ≤40℃ / h and hold for 5–10 hours; next, heat to 800–850℃ at a heating rate of ≤50℃ / h and hold for 15–20 hours, followed by segmented quenching using air cooling and liquid cooling; cool to 250–350℃ and hold for 5–10 hours; then, heat to 550–650℃ at a step heating rate of ≤40℃ / h and hold for 35–50 hours for tempering; after tempering, furnace cool to 400℃, then remove from the furnace and air cool to below 150℃.

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