A heat treatment repair process for hot-rolled support rolls

Abstract

This invention discloses a heat treatment repair process for hot-rolled support rolls, including annealing and a final heat treatment. The annealing process involves segmented, uniformly heated sections of the hot-rolled support roll to be repaired, followed by high-temperature annealing at 880–950°C. The treated hot-rolled support roll is then slowly cooled to 200°C and air-cooled after removal from the furnace. The final heat treatment involves quenching and tempering of the annealed hot-rolled support roll. This invention employs an annealing process, along with quenching and tempering as the final heat treatment, taking into account hardness, strength, and toughness to address the problems of microcracks, roll breakage, and insufficient hardness and wear resistance in hot-rolled support rolls with a working layer diameter Φ≤1530mm. It improves the mechanical structure and properties of the remaining working layer, enhancing its strength and hardness to meet usage requirements.

Description

Technical Field

[0001] This invention belongs to the field of hot-rolled support roll technology, and particularly relates to a heat treatment repair process for hot-rolled support rolls. Background Technology

[0002] Hot rolling mill rolls are essential mechanical equipment in industrial production, playing an irreplaceable role in improving the quality of rolled products and production efficiency. The support rolls used in hot strip mills are typically made of imported German composite cast Cr5 material or domestically forged Cr5 material. These support rolls are large in size, heavy in weight, and have complex and difficult manufacturing processes. They are also subject to heavy loads, high temperatures, and water cooling during use.

[0003] As hot rolling mills continue to develop towards higher speeds, larger sizes, and higher precision, the hardness of hot rolling mill work rolls has generally increased, placing increasingly higher demands on the corresponding support rolls. The Cr content in work roll materials has increased from 2 wt.% to 3 wt.%, 5 wt.%, and even towards 8 wt.% and 10 wt.%, while support roll materials have evolved from 9Cr2Mo to the Cr3 series support rolls represented by 70Cr3Mo. In recent years, to meet the higher performance requirements of high-speed steel and semi-high-speed steel work rolls for support rolls, new support roll materials with higher Cr content, such as Cr4 and Cr5, will be further developed. Correspondingly, the thickness of the hardened layer and the heat treatment process of support rolls also urgently need to be improved.

[0004] The chemical composition of existing hot-rolled support rolls is as follows: C: 0.48–0.58 wt%, Si: 0.4–0.7 wt%, Mn: 0.4–0.7 wt%, P < 0.015 wt%, S < 0.015 wt%, Cr: 4.5–5.5 wt%, Ni: 0.4–0.5 wt%, Mo: 0.45–0.6 wt%, V: 0.1–0.2 wt%, with the balance being iron and unavoidable impurities. During the later stages of use, some of these hot-rolled support rolls often experience a situation where the hardened layer of the roll body is not used to the required depth, causing the hardness to drop below the scrap hardness, failing to meet on-site usage requirements and rendering the support rolls unusable. Steel mills typically scrap these support rolls, but some of these scrapped support rolls still have 1 / 3 to 1 / 2 of the hardened layer remaining unused. Considering the difficulty in manufacturing support rolls, directly scrapping them would result in a waste of resources. Therefore, researching repair techniques to improve the performance of support rolls that still have a significant amount of unused hardened layers, so that the mechanical properties of the support rolls can once again meet the requirements for field use, is an important research direction to avoid wasting resources in steel plants.

[0005] Chinese patent application CN201110346472.9 discloses a "post-forging heat treatment method for forged steel support rolls for large-scale heavy plate rolling mills". Through full annealing, spheroidizing annealing, normalizing and tempering processes, the heavy plate support rolls after post-forging heat treatment can obtain pearlitic and sorbitic structures with good mechanical properties, and the roll neck hardness reaches 30-40HS. The mechanical properties meet the technical requirements, which prepares the microstructure for subsequent differential temperature heat treatment.

[0006] Chinese patent application CN201410817984.2 discloses a "local annealing method for large forged steel support rolls". After the local annealing method, the roll body of the large forged steel support roll is annealed while the hardness of the roll neck does not decrease. Thus, no pre-heat treatment is required before secondary quenching.

[0007] Chinese patent applications CN201510520737.0 and CN201510518592.0 disclose "a method for controlling the absence of soft strips at the end of the roll body of a large forged steel support roll after quenching" and "a method for controlling the absence of ring cracks at the end of the roll body of a large forged steel support roll after quenching". The large forged steel support rolls obtained by these two processes do not have ring cracks or soft strips at the end of the roll body.

[0008] Chinese patent application CN201510527500.5 discloses a "heat treatment method for large forged steel support rolls". Through overall heating and quenching and differential temperature tempering, the roll body and roll neck of large forged steel support rolls can meet the relevant requirements respectively, and the hardness at the transition between the roll body and the roll neck is slowly transitioned. This not only simplifies the process, shortens the production cycle, and improves production efficiency, but also reduces production costs.

[0009] As can be seen from the above technologies, the heat treatment process is a key factor affecting the mechanical properties of hot-rolled support rolls and needs to be precisely controlled according to the operating conditions. These technologies employ different methods to improve a particular aspect to meet the requirements of the operating conditions. However, some heat treatment processes are designed to be overly complex, increasing time and cost. Furthermore, none of these methods are suitable for the repair heat treatment conditions and requirements that improve the performance of hot-rolled support rolls with low hardness during use.

[0010] The effective working layer diameter of hot-rolled support rolls is Φ1630mm~Φ1440mm. Analysis of hot-rolled support rolls revealed that when the diameter is less than Φ1530mm, the roll body hardness decreases from 65~71HSD on the roll surface to 50~54HSD. This may be because during the original heat treatment of the roll, due to the larger roll diameter, the different cooling rates on the roll surface and inside the roll resulted in different microstructures. Therefore, as the roll diameter decreases, the yield stress that the roll can withstand decreases. Under the same rolling force, when the roll surface comes into contact with the high-temperature rolled material, the roll surface expands due to heat. When cooled by cooling water, it contracts. When the axial contraction is obstructed, microcracks are generated. At this time, the wear and machining amount increase significantly, and in severe cases, it can cause roll breakage.

[0011] Therefore, to repair and reuse idle hot-rolled support rolls with reduced hardness and reduce tooling costs, it is urgent to design a method to repair and improve the performance of hot-rolled support rolls, improve the mechanical structure and properties of the remaining working layer, increase its strength and hardness, and meet the usage requirements. Summary of the Invention

[0012] To address the aforementioned deficiencies in the existing technology, the purpose of this invention is to provide a heat treatment repair process for hot-rolled support rolls. This process employs annealing, quenching, and tempering as the final heat treatment, taking into account hardness, strength, and toughness. This addresses the problems of microcracks, roll breakage, and insufficient hardness and wear resistance in hot-rolled support rolls with a working layer diameter Φ≤1530mm. It also improves the mechanical structure and properties of the remaining working layer, enhancing its strength and hardness to meet usage requirements.

[0013] To achieve the above objectives, the present invention adopts the following technical solution:

[0014] This invention provides a heat treatment repair process for hot-rolled support rolls, comprising the following steps:

[0015] S1, Annealing treatment: The hot-rolled support roll to be repaired is subjected to segmented uniform temperature holding treatment by stepped heating, and then the temperature is raised to 880-950℃ for high-temperature annealing treatment. After that, the hot-rolled support roll is slowly cooled to 200℃ and then air-cooled after being taken out of the furnace.

[0016] S2, final heat treatment, involves quenching and tempering the hot-rolled support rolls after step S1.

[0017] Preferably, in step S1, before the annealing process, the hot-rolled support roll to be repaired is subjected to roll surface inspection, ultrasonic inspection and dual crystal flaw detection to ensure that no defect echo exceeding the standard is found in the hot-rolled support roll to be repaired within the sensitivity range Φ1.

[0018] Preferably, in step S1:

[0019] During the annealing process, the heating rate is 80–100 °C / h; and / or

[0020] The segmented temperature equalization and heat preservation treatment includes a first-stage temperature equalization and heat preservation and a second-stage temperature equalization and heat preservation; the temperature of the first-stage temperature equalization and heat preservation is 300±10℃, and the heat preservation time is 8~12h; the temperature of the second-stage temperature equalization and heat preservation is 600±10℃, and the heat preservation time is 8~12h.

[0021] Preferably, in step S1:

[0022] During the high-temperature annealing process, the holding time is 6–9 hours; and / or

[0023] During the slow cooling process, the cooling rate is 10-15℃ / h.

[0024] Preferably, in step S1, the surface hardness of the hot-rolled support roll after annealing is 30-35 HSD.

[0025] Preferably, in step S2, the quenching process includes the following steps:

[0026] S21, heat the hot-rolled support rolls processed in step S1 to 300±10℃ and keep them at the same temperature for 8~12h;

[0027] S22, heat the hot-rolled support rolls processed in step S21 to 600±10℃ and keep them at the same temperature for 8~12h;

[0028] S23. The hot-rolled support rolls processed in step S22 are rapidly heated to 990-1090°C and held for 6-8 hours. Then, they are cooled in the furnace to 880-990°C and held for 1-1.5 hours. After that, they are removed from the furnace and sprayed to cool to below 200°C within 30 minutes. Then, they are air-cooled and tempered in time.

[0029] Preferably, in the quenching process:

[0030] In step S21, the heating rate is 80–100 °C / h; and / or

[0031] In step S22, the heating rate is 80–100 °C / h; and / or

[0032] In step S23, the heating rate is 200-250℃ / h.

[0033] Preferably, in step S2, during the tempering process, the hot-rolled support roll that has undergone the quenching treatment is heated and held at a tempering temperature of 480-550°C for 18-24 hours, then slowly cooled to 200°C, and then air-cooled after being removed from the furnace.

[0034] Preferably, in step S2:

[0035] In the tempering process, the slow cooling rate is 10-15°C / h;

[0036] The tempering process is repeated 2 to 3 times.

[0037] Preferably, in step S2:

[0038] The surface hardness of the hot-rolled support roll after the final heat treatment is 65–75 HSD; and / or

[0039] The residual stress on the surface of the hot-rolled support roll after the final heat treatment is controlled within -300MPa.

[0040] The heat treatment repair process for hot-rolled support rolls provided by this invention has the following beneficial effects:

[0041] 1. The heat treatment repair process of the hot-rolled support roll of the present invention adopts annealing process as well as quenching and tempering as the final heat treatment, and takes into account hardness, strength and toughness to solve the problems of micro-cracks, roll breakage, insufficient hardness and wear resistance when the working layer diameter of the hot-rolled support roll is Φ≤1530mm, improves the mechanical structure and properties of the remaining working layer, and improves its strength and hardness to meet the use requirements.

[0042] 2. To address the problem that when the support roll reaches a diameter of Φ1530mm, the surface hardness of the roll body rapidly decreases locally, resulting in poor uniformity of surface hardness and failing to meet on-site usage requirements, the heat treatment repair process for hot-rolled support rolls in this invention achieves a comprehensive restoration and improvement of mechanical properties after the diameter of the hot-rolled support roll reaches Φ1530mm, effectively extending its service life. This solves the problems of roll shutdowns, roll replacements, and breakage at the end of the working layer due to insufficient hardness and wear resistance, thereby improving production efficiency and reducing roll consumption and production costs. Attached Figure Description

[0043] Other features, objects, and advantages of the present invention will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings:

[0044] Figure 1 This is a graph showing the annealing temperature curve of the heat treatment repair process for the hot-rolled support roll of the present invention.

[0045] Figure 2 This is a temperature curve diagram of the final heat treatment in the heat treatment repair process of the hot-rolled support roll of the present invention. Detailed Implementation

[0046] 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.

[0047] The present invention provides a heat treatment repair process for hot-rolled support rolls, which is mainly aimed at the performance repair of low-hardness hot-rolled support rolls after use, and solves the problems of micro-cracks, roll breakage, insufficient hardness and wear resistance when the diameter of the hot-rolled support roll is Φ≤1530mm.

[0048] The heat treatment repair process for hot-rolled support rolls provided by this invention specifically includes the following steps:

[0049] S1, Annealing treatment: The hot-rolled support roll to be repaired is subjected to segmented uniform temperature holding treatment by stepped heating, and then the temperature is raised to 880-950℃ for high-temperature annealing treatment. After that, the hot-rolled support roll is slowly cooled to 200℃ and then air-cooled after being taken out of the furnace.

[0050] Specifically, the process begins by selecting a suitable hot-rolled support roll for repair. The roll is then subjected to ultrasonic testing. If no defect echo exceeding the standard is detected within a sensitivity range of Φ2, bicrystalline flaw detection is performed. If no defect echo exceeding the standard is detected within a sensitivity range of Φ1, repair can proceed. These ultrasonic and bicrystalline flaw detection methods ensure that the repaired hot-rolled support roll is free of inclusions, deep fretting, and other defects. Next, the hot-rolled support roll undergoes annealing. This process has three main characteristics: first, it reduces the hardness of the working layer of the roll body, facilitating subsequent final heat treatment; second, it relieves stress, eliminating residual stress from heat treatment and machining; and third, it helps to disperse network carbides in the matrix structure, improving microstructure uniformity, adjusting the matrix mechanical properties, and fully utilizing the role of alloying elements. During the annealing process, the hot-rolled support roll to be repaired is first subjected to segmented uniform temperature holding treatment using stepped heating. Then, the temperature is further increased to 880–950℃ for high-temperature annealing. Afterward, the treated hot-rolled support roll is slowly cooled to 200℃ and then air-cooled after removal from the furnace. Specifically, combined with… Figure 1 As shown, the annealing process includes:

[0051] (1) Segmented temperature uniformity insulation treatment:

[0052] A single-stage uniform temperature insulation: heating at a heating rate of 80-100℃ / h, and then maintaining a uniform temperature for 8-12 hours at 300±10℃;

[0053] Two-stage uniform temperature insulation: The temperature is increased at a rate of 80-100℃ / h, and uniform temperature insulation is carried out at 600±10℃ for 8-12 hours.

[0054] (2) High temperature annealing treatment: After the segmented uniform temperature holding treatment, the hot-rolled support roll is heated to 880-950℃ at a heating rate of 80-100℃ / h for high temperature annealing, held for 6-9h, and then slowly cooled to 200℃ at a rate of 10-15℃ / h before being removed from the furnace and air-cooled.

[0055] In the above process, the heating rate for annealing was set to 80–100℃ / h, with two uniform temperature holding periods during the heating stage. This is because the roll cross-section is very large, and significant internal stress is easily generated during heating. To mitigate or relax stress concentration, stepped heating was adopted, using a "first-stage uniform temperature holding" and a "second-stage uniform temperature holding" to reduce the harmful effects of thermal stress. Therefore, when the annealing temperature is high, a slightly lower heating rate and a short uniform temperature holding period can ensure uniform heating of the roll and prevent roll deformation; thus, two uniform temperature holding periods of 8–12 hours were performed at 300±10℃ and 600±10℃.

[0056] Secondly, considering the high carbide content in the roll, the annealing process employs high-temperature annealing at 880–950℃ and a holding time of 6–9 hours to ensure sufficient carbide decomposition and matrix homogenization, preparing for the subsequent quenching and tempering. To prevent deformation and cracking of the support roll, furnace-controlled cooling is selected to avoid generating new stresses. This annealing process yields a matrix hardness of 30–35 HSD.

[0057] S2, final heat treatment, involves quenching and tempering the hot-rolled support rolls after step S1.

[0058] In this repair process, the following points need to be considered: First, the thickness of the working layer needs to be controlled to ensure the toughness of the core; second, the microstructure and mechanical properties of the working layer need to be controlled; and third, residual stress needs to be controlled. Therefore, during the final heat treatment, in order to ensure that the strength and hardness of the rolls meet the harsh conditions of high rolling force, large reduction, and high rolling temperature of the hot rolling roughing support rolls, the final microstructure is controlled to be a mixed microstructure of bainite and martensite, with a small amount of retained austenite and carbides, a hardness of 70±5HSD, and a hardness layer thickness of 50-70mm on each side, so as to ensure the hardness, wear resistance, and thermal crack resistance of the working layer while ensuring the toughness of the core.

[0059] To achieve the above objectives, the heating rate, holding time, and cooling method must be strictly controlled during the final heat treatment.

[0060] Combination Figure 2 The quenching process shown includes the following steps:

[0061] (S21) The hot-rolled support rolls treated in step S1 are heated to 300±10℃ at a heating rate of 80~100℃ / h and kept at the same temperature for 8~12h.

[0062] (S22) The hot-rolled support rolls treated in step S21 are heated to 600±10℃ at a heating rate of 80~100℃ / h and kept at the same temperature for 8~12h.

[0063] (S23) The hot-rolled support rolls processed in step S22 are rapidly heated to 990-1090°C at a heating rate of 200-250°C / h and held for 6-8 hours. Then, they are cooled in the furnace to 880-990°C and held for 1-1.5 hours. After that, they are removed from the furnace and sprayed to cool to below 200°C within a 30-minute cooling time. Then, they are air-cooled and tempered in time.

[0064] Combination Figure 2 As shown, the tempering process involves heating the quenched hot-rolled support rolls at a heating rate of 80–100℃ / h, holding them at a tempering temperature of 480–550℃ for 18–24 hours, and then slowly cooling them to 200℃ at a cooling rate of 10–15℃ / h before air cooling them out of the furnace. The tempering process is repeated 2–3 times.

[0065] The purpose of the aforementioned final heat treatment is to improve the bainitic + martensitic matrix structure, increase the spheroidization and dispersion of carbides, refine the microstructure, and enhance the strength, hardness, and wear resistance of the remaining working layer of the support roller. Experimental studies show that due to the large diameter of the support roller, slow heating is necessary, with the heating rate set between 80 and 100℃ / h. Simultaneously, two uniform temperature holding cycles are performed to ensure uniform heating of the working layer of the support roller during the heating process, guaranteeing a consistent temperature throughout the roller body and allowing the entire roller body to enter a plastic state. This also prevents grain growth and maintains the temperature difference between the roller surface and the roller core, ensuring that the toughness of the roller core is not affected. Therefore, in steps S21 and S22 of the quenching treatment, two uniform temperature holding cycles of 8–12 h are performed at 300℃ and 600℃, respectively.

[0066] In step S23, the roll is rapidly heated to 990–1090°C at a heating rate of 200–250°C / h, and then held at this high temperature for 6–8 hours. The rapid heating rate is used to create a temperature difference between the inside and outside of the roll body, allowing the roll surface to develop a certain depth of austenite layer while the core remains below the phase transformation point, thus achieving a differential temperature quenching effect. On one hand, controlling the heating and holding time controls the thickness of the heating layer, i.e., the working layer. During the holding time, the roll surface temperature rises to the required quenching temperature, ensuring complete austenitization of the working layer matrix and sufficient dissolution of carbides into the matrix, increasing the solubility of alloying elements and carbon in the matrix, while ensuring the core structure remains unaffected. On the other hand, the higher austenitization temperature provides a good foundation for subsequent tempering, improving the strength and tempering resistance of the support roll, as well as its thermal cycling stability. The test results show that if the holding time is less than 6 hours, the working layer temperature is uneven, the matrix is ​​not sufficiently austenitized, and the final microstructure requirements are not met. If the holding time is greater than 8 hours, the matrix grains in the working layer begin to grow, and the thickness of the quenching temperature layer and the working layer will be greater than 70 mm. The effective core size is small, the toughness is reduced, and the risk of roll breakage during rolling is increased.

[0067] Because the support roll is made of traditional Cr5 material with a large diameter, the strength and toughness of its remaining working layer must be achieved by controlling the matrix microstructure and mechanical properties. Therefore, in step S23, the treated hot-rolled support roll is cooled in the furnace to 880–990°C and held for 1–1.5 hours. Tests show that this process allows the hot-rolled support roll to achieve ideal working layer thickness, microstructure, and mechanical properties, and the internal temperature during quenching can be controlled to remain below the AC1 point (critical point), resulting in lower plastic deformation stress in the center of the roll body. This step prepares the matrix for the transformation from bainite to martensite, further improving the matrix's strength, toughness, and resistance to thermal cracking. Simultaneously, rapid cooling via spray quenching improves surface hardness and toughness, minimizing internal residual stress. Experimental results show that the quenching process needs to be controlled within 30 minutes to reduce the roller body temperature to below 200℃. Keeping the support roller in a rotating state during the quenching process can obtain a matrix structure of 20-25% bainite and martensite, while avoiding microcracks on the roller surface.

[0068] The tempering process employs medium-temperature tempering, controlling the tempering temperature between 480 and 550℃, and the tempering holding time within 18 to 24 hours. This is because the microstructure and properties of the support roll undergo significant changes during thermal fatigue. To achieve excellent thermal cycling stability, it is necessary to improve the roll's tempering resistance under cycling temperature conditions through heat treatment process design. Experiments show that the main alloying element Cr in this support roll forms M... 23 C6 type carbides, when tempered at temperatures above 500℃, can be transformed from Cr-saturated Fe3C or precipitated directly from the matrix. They are not prone to aggregation and growth, and precipitation increases tempering hardness and thermal stability. When the tempering temperature exceeds 550℃, the final hardness of the rolls is low; when the tempering temperature is below 480℃, the bainite content in the support rolls is unsatisfactory, resulting in insufficient strength and resistance to hot cracking. Therefore, through experimental optimization, a tempering temperature controlled between 480 and 550℃, and a tempering holding time controlled within 18 to 24 hours, can ensure that the hardness meets requirements while maintaining high strength and toughness. Tempering can be repeated 2 to 3 times.

[0069] This invention has achieved substantial progress and significant results. Before heat treatment, the surface hardness of the hot-rolled support roll at Φ1530mm was 42–50 HSD. After the heat treatment repair process of this invention, the hardness recovered to 70±5 HSD. In a further preferred embodiment, the hardness of the hot-rolled support roll after the heat treatment repair process recovered to 66–73 HSD, reaching the surface hardness level of a new roll. The residual stress on the roll surface was controlled within -300MPa (where positive and negative indicate compressive stress and tensile stress, respectively; here -300MPa represents a tensile stress of 300MPa, meaning the residual tensile stress on the roll surface was controlled between 0 and 300MPa). Experiments show that the repaired hot-rolled support roll achieved the effect of differential temperature quenching.

[0070] Before being used on the machine, the hot-rolled support rolls that have undergone the above treatment are machined using conventional turning methods to achieve the required roll surface dimensions and roughness.

[0071] The heat treatment repair process of the hot-rolled support roll of the present invention will be further described below with specific examples.

[0072] Example

[0073] In this embodiment, the hot-rolled support roll to be repaired is treated using the heat treatment repair process of the present invention as follows:

[0074] 1. Perform macroscopic inspection of the roll surface to be treated, ensuring there are no large cracks or peeling defects; test the hardness as the basis for the repair heat treatment process; use ultrasonic testing to check for serious casting defects or crack defects inside the cracks; if there are no major defects, proceed to the next step of repair heat treatment.

[0075] 2. Heat treatment repair process

[0076] (a) Annealing treatment

[0077] (1) Segmented uniform temperature heat preservation treatment: heating at a heating rate of 80℃ / h, and uniform temperature heat preservation for 8h at 300℃; heating at a heating rate of 80℃ / h, and uniform temperature heat preservation for 8h at 600℃.

[0078] (2) High temperature annealing: Anneal at 880℃ for 6 hours, then slowly cool to 200℃ at a rate of 10℃ / h, and air cool after removal from the furnace.

[0079] (II) Final heat treatment (quenching + tempering)

[0080] Quenching treatment:

[0081] (1) Heat at a heating rate of 80℃ / h and maintain a uniform temperature of 300℃ for 8 hours;

[0082] (2) Heat up at a rate of 80℃ / h and maintain the temperature at 600℃ for 8 hours.

[0083] (3) Heat up rapidly at a heating rate of 200℃ / h, hold at 990℃ for 6 hours before quenching, cool down to 880℃ with the furnace, hold for 1 hour, then spray quenching and cool down, control the cooling time to cool to below 200℃ within 30 minutes, air cool after taking it out of the furnace, and temper it in time.

[0084] Tempering treatment: Heat at a rate of 80℃ / h, temper at 480℃ and hold for 18h; then slowly cool to 200℃ at a rate of 10℃ / h, and air cool after removal from the furnace. The tempering is performed 3 times.

[0085] 3. Roll turning

[0086] The roller surface is machined using conventional turning methods to achieve the required roller surface dimensions and surface roughness for use on the machine.

[0087] 4. Testing of organizational and mechanical properties

[0088] The hardness of the roll surface was tested using a portable hardness tester, and the microstructure of the roll surface was examined using a metallographic coating method. The results showed that after the repair heat treatment, the hardness of the roll surface reached 66 HSD. The microstructure of the roll surface was a mixed microstructure of bainite and martensite, carbides and a small amount of retained austenite. The residual stress of the roll surface was controlled at -290 MPa, which was consistent with the microstructure and mechanical properties of the new roll, and all met the requirements of the rolling mill for the roll.

[0089] 5. Comparative test on the machine

[0090] The results of the on-machine test are compared in Table 1. As can be seen from Table 1, after the treatment of the technical solution of the present invention, the support rolls that were originally scrapped due to insufficient hardness can not only continue to be used, greatly recovering the scrap loss, but also have their single rolling tonnage increased due to the improved mechanical properties.

[0091] Table 1 Comparison of Computer Lab Results

[0092]

[0093]

[0094] Example 2

[0095] In this embodiment, the hot-rolled support roll to be repaired is treated using the heat treatment repair process of the present invention as follows:

[0096] 1. Perform macroscopic inspection of the roll surface to be treated, ensuring there are no large cracks or peeling defects; test the hardness as the basis for the repair heat treatment process; use ultrasonic testing to check for serious casting defects or crack defects inside the cracks; if there are no major defects, proceed to the next step of repair heat treatment.

[0097] 2. Heat treatment repair process

[0098] (a) Annealing treatment

[0099] (1) Segmented uniform temperature heat preservation treatment: heating at a heating rate of 90℃ / h, and uniform temperature heat preservation for 9h at 308℃; heating at a heating rate of 90℃ / h, and uniform temperature heat preservation for 10h at 608℃.

[0100] (2) High temperature annealing: Anneal at 900℃ for 8 hours, then slowly cool to 200℃ at a rate of 12℃ / h, and air cool after removal from the furnace.

[0101] (II) Final heat treatment (quenching + tempering)

[0102] Quenching treatment:

[0103] (1) Heat at a heating rate of 90℃ / h and maintain a uniform temperature for 10 hours at 300℃;

[0104] (2) Heat up at a rate of 90℃ / h and maintain the temperature at 600℃ for 10 hours.

[0105] (3) Heat up rapidly at a heating rate of 220℃ / h, hold at 1000℃ for 7 hours before quenching, cool down to 900℃ with the furnace, hold for 1.2 hours, then spray quenching and control the cooling time to cool to below 200℃ within 30 minutes, air cool after removal from the furnace, and temper in time.

[0106] Tempering treatment: Heat at a rate of 90℃ / h, temper at 500℃ and hold for 20h; then slowly cool to 200℃ at a rate of 12℃ / h, and air cool after removal from the furnace. Tempering is performed 3 times.

[0107] 3. Roll turning

[0108] The roller surface is machined using conventional turning methods to achieve the required roller surface dimensions and surface roughness for use on the machine.

[0109] 4. Testing of organizational and mechanical properties

[0110] The hardness of the roll surface was tested using a portable hardness tester, and the microstructure of the roll surface was examined using the metallographic coating method. The results showed that after the repair heat treatment, the hardness of the roll surface reached 70 HSD. The microstructure of the roll surface was a mixed microstructure of bainite and martensite, carbides and a small amount of retained austenite. The residual stress of the roll surface was controlled at -290 MPa, which was consistent with the microstructure and mechanical properties of the new roll, and all met the requirements of the rolling mill for the roll.

[0111] 5. Comparative test on the machine

[0112] The results of the on-machine tests are compared in Table 2. As can be seen from Table 2, after the processing of the technical solution of the present invention, the support rolls that were originally scrapped due to insufficient hardness can not only continue to be used, greatly recovering the scrap loss, but also have their single rolling tonnage increased due to the improved mechanical properties.

[0113] Table 2 Comparison of Computer Lab Results

[0114]

[0115] Example 3

[0116] In this embodiment, the hot-rolled support roll to be repaired is treated using the heat treatment repair process of the present invention as follows:

[0117] 1. Perform macroscopic inspection of the roll surface to be treated, ensuring there are no large cracks or peeling defects; test the hardness as the basis for the repair heat treatment process; use ultrasonic testing to check for serious casting defects or crack defects inside the cracks; if there are no major defects, proceed to the next step of repair heat treatment.

[0118] 2. Heat treatment repair process

[0119] (a) Annealing treatment

[0120] (1) Segmented uniform temperature heat preservation treatment: Heat at a heating rate of 100℃ / h, and perform uniform temperature heat preservation for 12h at 310℃; Heat at a heating rate of 100℃ / h, and perform uniform temperature heat preservation for 12h at 610℃.

[0121] (2) High temperature annealing: Anneal at 950℃ for 9 hours, then slowly cool to 200℃ at a rate of 15℃ / h, and air cool after removal from the furnace.

[0122] (II) Final heat treatment (quenching + tempering)

[0123] Quenching treatment:

[0124] (1) Heat at a heating rate of 100℃ / h and maintain a uniform temperature at 310℃ for 12 hours.

[0125] (2) Heat up at a rate of 100℃ / h and maintain the temperature at 610℃ for 12 hours.

[0126] (3) Heat up rapidly at a heating rate of 250℃ / h, hold at 1090℃ for 8 hours before quenching, cool down to 990℃ with the furnace, hold for 1.5 hours, then spray quenching and cool down, control the cooling time to cool to below 200℃ within 30 minutes, air cool after taking it out of the furnace, and temper it in time.

[0127] Tempering treatment: Heat at a rate of 100℃ / h, temper at 550℃ and hold for 24h; then slowly cool to 200℃ at a rate of 15℃ / h, and air cool after removal from the furnace. The tempering is performed 3 times.

[0128] 3. Roll turning

[0129] The roller surface is machined using conventional turning methods to achieve the required roller surface dimensions and surface roughness for use on the machine.

[0130] 4. Testing of organizational and mechanical properties

[0131] The hardness of the roll surface was tested using a portable hardness tester, and the microstructure of the roll surface was examined using a metallographic coating method. The results showed that after the repair heat treatment, the hardness of the roll surface reached 73 HSD. The microstructure of the roll surface was a mixed microstructure of bainite and martensite, carbides and a small amount of retained austenite. The residual stress of the roll surface was controlled at -295 MPa, which was consistent with the microstructure and mechanical properties of the new roll, and all met the requirements of the rolling mill for the roll.

[0132] 5. Comparative test on the machine

[0133] The results of the on-machine tests are compared in Table 3. As can be seen from Table 3, after the processing of the technical solution of the present invention, the support rolls that were originally scrapped due to insufficient hardness can not only continue to be used, greatly recovering the scrap loss, but also have their single rolling tonnage increased due to the improved mechanical properties.

[0134] Table 3 Comparison of Computer Lab Results

[0135]

[0136] 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 heat treatment repair process for a hot rolling backup roll, characterized by, Includes the following steps: S1, Annealing treatment: The hot-rolled support roll to be repaired is subjected to segmented uniform temperature holding treatment by stepped heating, and then the temperature is raised to 880-950℃ for high-temperature annealing treatment. After that, the hot-rolled support roll is slowly cooled to 200℃ and then air-cooled after being taken out of the furnace. Before the annealing process, the hot-rolled support roll to be repaired is subjected to roll surface inspection, ultrasonic testing and dual crystal flaw detection to ensure that no defect echo exceeding the standard is found in the hot-rolled support roll to be repaired within the sensitivity range of Φ1. The surface hardness of the hot-rolled support roll after annealing is 30-35 HSD; S2, final heat treatment, involves quenching and tempering the hot-rolled support rolls processed in step S1. The quenching process includes the following steps: S21, heat the hot-rolled support roll after step S1 to 300±10℃, keep it at the same temperature for 8~12h, and the heating rate is 80~100℃ / h. S22, heat the hot-rolled support rolls processed in step S21 to 600±10℃, keep them at the same temperature for 8~12h, and the heating rate is 80~100℃ / h. S23, the hot-rolled support rolls treated in step S22 are rapidly heated to 990–1090°C and held at that temperature for 6–8 hours, with a heating rate of 200–250°C / h. The sample is then cooled in the furnace to 880–990°C and held at that temperature for 1–1.5 hours. After that, it is removed from the furnace and spray-quenched to cool to below 200°C within 30 minutes. Finally, it is air-cooled and tempered promptly. In the tempering process, the hot-rolled support rolls that have undergone the quenching process are heated and held at a tempering temperature of 480-550°C for 18-24 hours, then slowly cooled to 200°C, and then air-cooled after being taken out of the furnace. The slow cooling rate is 10-15°C / h. The tempering process is repeated 2 to 3 times. The residual stress on the surface of the hot-rolled support roll after the final heat treatment is controlled within -300MPa.

2. The heat treatment repair process for a hot rolling backup roll according to claim 1, characterized by, In step S1: During the annealing process, the heating rate is 80–100 °C / h; and / or The segmented temperature equalization and heat preservation treatment includes a first-stage temperature equalization and heat preservation and a second-stage temperature equalization and heat preservation; the temperature of the first-stage temperature equalization and heat preservation is 300±10℃, and the heat preservation time is 8~12h; the temperature of the second-stage temperature equalization and heat preservation is 600±10℃, and the heat preservation time is 8~12h.

3. The heat treatment repair process for a hot rolling backup roll according to claim 1, characterized by, In step S1: During the high-temperature annealing process, the holding time is 6–9 hours; and / or During the slow cooling process, the cooling rate is 10-15℃ / h.

4. The heat treatment repair process of a hot-rolled backup roll according to any one of claims 1 to 3, characterized in that, In step S2: The surface hardness of the hot-rolled support roll after final heat treatment is 65-75 HSD.

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