Method for controlling residual stress gradient of working layer of forged steel cold rolling work roll

Abstract

This invention discloses a method for controlling the residual stress gradient in the working layer of a cold-rolled forged steel work roll, comprising gradient induction hardening, gradient spray cooling, cryogenic treatment, and a stepped tempering process. Gradient induction hardening uses a variable frequency / variable power continuous induction heating method to heat the roll body, thereby constructing a smooth transition zone. Gradient spray cooling divides the spray area along the roll body axially into 3 to 7 independently controlled cooling zones; the spray pressure in each cooling zone gradually decreases from top to bottom. The stepped tempering process includes three stages of tempering and heat preservation. This invention achieves a smooth transition of radial cross-sectional stress by constructing a gradient transition layer during the quenching stage, precisely controlling the microstructure transformation during the cryogenic stage, and employing a multi-stage stress release process during the tempering stage, thereby reducing the overall stress level of the working layer while maintaining high hardness.

Description

Technical Field

[0001] This invention belongs to the technical field of cold rolling mill rolls for forged steel, and specifically relates to a method for controlling the residual stress gradient of the working layer of a cold rolling mill roll for forged steel. Background Technology

[0002] Cold rolling work rolls are subjected to extremely high cyclic rolling stress and contact stress during service. Their failure modes are mainly fatigue spalling and roll body fracture, which are closely related to the residual stress state inside the roll body. Secondly, when cold rolling ultra-high strength steel and high-grade non-oriented silicon steel, due to the large deformation resistance and thin plate thickness, strip breakage accidents are very likely to occur during the rolling process, causing thermal shock and stress concentration on the roll surface. Once surface cracks occur on the roll surface, due to the high stress level of the working layer of the work roll, the cracks can easily propagate rapidly and lead to spalling failure.

[0003] The ideal residual stress distribution characteristics are: the roller surface has beneficial residual compressive stress to improve the resistance to fatigue crack propagation; from the surface to the core, the stress value should smoothly transition from compressive stress to tensile stress, and the gradient should be as gentle as possible; the peak value of tensile stress in the core needs to be controlled within the material strength range.

[0004] Existing technologies typically achieve surface compressive stress through induction hardening, but they generally suffer from the following shortcomings: (1) The stress gradient is steep.

[0005] Traditional single-frequency induction heating often results in a narrow transition zone between the hardened layer and the core, with drastic stress changes on the radial cross section, forming a high-stress concentration zone in the transition zone, which becomes the source of spalling.

[0006] (2) Hardness and stress are contradictory.

[0007] The pursuit of high wear resistance and extremely high surface hardness often comes with excessively high martensitic phase transformation stress and thermal stress, resulting in an overall high stress level in the working layer and increased sensitivity to microcracks.

[0008] (3) The regulatory measures are limited.

[0009] The lack of systematic and coordinated control over the entire chain of quenching, cryogenic treatment, and tempering processes prevented the effective utilization of multi-stage tempering and microstructure evolution to relax stress.

[0010] In summary, there is an urgent need in this field for a residual stress control method that can optimize radial stress distribution, achieve a smooth stress transition, and reduce the overall stress level while ensuring the high hardness of the roll body. Summary of the Invention

[0011] The purpose of this invention is to solve the above-mentioned problems and provide a method for controlling the residual stress gradient of the working layer of cold-rolled forged steel. This method achieves a smooth transition of radial cross-sectional stress by constructing a gradient transition layer during the quenching stage, precisely controlling the microstructure transformation during the cryogenic stage, and adopting a multi-stage stress release process during the tempering stage, thereby reducing the overall stress level of the working layer while maintaining high hardness.

[0012] The technical solution to achieve the purpose of this invention is: a method for controlling the residual stress gradient of the working layer of a cold-rolled forged steel work roll, including gradient induction hardening, gradient spray cooling, cryogenic treatment, and stepped tempering process.

[0013] The gradient induction hardening process uses a variable frequency / variable power continuous induction heating method to heat the roll body, thereby creating a smooth transition zone. The heating process is divided into two stages: the first stage is low-frequency current + low power, and the second stage is high-frequency current + high power.

[0014] The frequency of the low-frequency current is 500-1000Hz; the frequency of the medium-high frequency current is 2000-4000Hz.

[0015] The low power is 150-250kW, and the high power is 300-400kW.

[0016] The first stage of low-frequency current and low power can deeply preheat the roller body, so that a certain depth (about 15-25mm) below the surface of the roller body reaches above the phase change point temperature, forming a deeper preheating layer and expanding the range of the heat-affected zone.

[0017] In the second stage, high-frequency current and high power can strengthen the surface of the roller body, rapidly raising the surface to the quenching temperature and obtaining fine cryptocrystalline martensite.

[0018] The gradient spray cooling divides the spray zone into 3 to 7 independently controlled cooling zones along the roller body axis; the water spray pressure in each cooling zone gradually decreases from top to bottom.

[0019] This gradient spray cooling method employs strong cooling in the initial stage of quenching, and gradually reduces the cooling intensity of the downstream cooling zone as the quenched layer expands inward, so that a stress buffer transition layer is formed between the hardened layer and the core.

[0020] The cryogenic treatment is a conventional method in the field, and the cryogenic treatment temperature is -80℃ to -150℃.

[0021] The stepped tempering process includes three stages of tempering and heat preservation.

[0022] The first stage of tempering involves holding at 100℃~130℃ for 6~10 hours. This stage primarily eliminates the instantaneous macroscopic stress generated during quenching, and simultaneously initiates the precipitation of ε-carbides.

[0023] The second stage involves heating to 120–150°C at an extremely slow rate (e.g., <10°C / h), holding at that temperature for 15–30 hours, and then air cooling. In this stage, the precipitation, aggregation, and rearrangement (recovery) of carbides induce creep relaxation of residual stress, significantly reducing the stress peak value inside the working layer (hardened layer).

[0024] The third stage involves reheating to 80℃~100℃ and holding for 3~8 hours, followed by slow cooling to room temperature in the furnace. This stage aims to eliminate the new thermal stress caused by the second stage of tempering, resulting in a more balanced stress distribution.

[0025] The positive effects of this invention are as follows: By constructing a gradient transition layer during the quenching stage, precisely controlling the microstructure transformation during the cryogenic stage, and employing a multi-stage stress release process during the tempering stage, this invention achieves a smooth transition of radial cross-sectional stress and reduces the overall stress level of the working layer while maintaining high hardness. Attached Figure Description

[0026] Figure 1 This is a comparison curve of the residual stress distribution of the radial section of the roll after treatment by the method of the present invention and the conventional method.

[0027] As shown in the figure, the stress curve of the method of the present invention (red line) transitions smoothly from surface compressive stress (-σ) to core tensile stress (+σ) without obvious inflection point, and the absolute value of stress in the working layer area (10-20mm below the surface) is lower than that of the traditional method (blue line). Detailed Implementation

[0028] (Example 1) This embodiment takes Cr5 series forged steel cold rolling work rolls as an example. Its chemical composition and weight percentage are as follows: carbon 0.80%, silicon 0.55%, manganese 0.35%, phosphorus 0.012%, sulfur 0.007%, chromium 5.00%, nickel 1.20%, molybdenum 1.30%, vanadium 0.30%, and the remainder is iron and unavoidable impurities.

[0029] The roller body specifications are Φ400×1850mm, the surface hardness of the roller body is required to be 93~97HSD, the hardened layer depth is required to be ≥20mm, and the scrap hardness is ≥90HSD.

[0030] The specific method for controlling the residual stress gradient of the working layer of the cold-rolled forged steel work roll in this embodiment is as follows: ① Place the heat-treated forged steel cold-rolled work roll on a quenching machine, keeping a gap of 5-15mm between the inductor and the roll surface. Then, preheat it with 800Hz frequency and 200kW power at a moving speed of 1.5mm / s. Next, switch to 3000Hz frequency and 350kW power for heating, and adjust the moving speed to 2.0mm / s.

[0031] ② After induction heating is completed, the water spray cooling system is started. Five independently controlled spray nozzles are set along the axial direction of the roll. The water spray pressures of the five spray nozzles are 0.4MPa, 0.3MPa, 0.2MPa, 0.15MPa and 0.10MPa respectively, thereby realizing gradient spray cooling.

[0032] ③ After quenching, once the roller body temperature drops to room temperature, immediately transfer it to a cryogenic chamber and keep it in a liquid nitrogen atmosphere at -100℃ for 3 hours. Then, take it out and air cool it to room temperature.

[0033] ④ Stepped tempering process, as detailed below: The forged steel cold-rolled work rolls are loaded into a tempering furnace and heated to 120°C at a rate of 15°C / h, and held at that temperature for 8 hours.

[0034] Then, the temperature is increased to 140℃ at an extremely slow rate of 6℃ / h and held for 20h to allow the working layer to be fully tempered, carbides to be fully precipitated, and dislocation density to be reduced, thereby effectively reducing the residual stress level of the working layer. Then, it is air-cooled.

[0035] Finally, heat to 90℃ and hold for 5 hours, then remove from the oven and air cool.

[0036] (Test example) Following the above process route, the rolls were dissected and inspected (using X-ray diffraction combined with peeling method). The results showed that: The surface hardness of the roller body is 95-96 HSD, and the residual compressive stress on the surface is -1000 MPa. [See...] Figure 1 】

[0037] From the surface inwards, the stress values ​​transition smoothly, with a compressive stress of -200MPa at a depth of 25mm and a stress approaching zero at a depth of 40mm, without any obvious stress abrupt changes.

[0038] (Comparative Example) The forged steel cold rolling work rolls are subjected to final heat treatment using traditional processes.

[0039] The results show that the stress remains basically unchanged between 15 and 35 mm, while there is a significant stress abrupt change step between 35 and 60 mm. In addition, the average stress level of the working layer (within 10 mm depth) is about 30% higher than that of the method in this application. The method in this application significantly improves the roll's resistance to accidents and service life.

Claims

1. A method for controlling the residual stress gradient in the working layer of a cold-rolled forged steel work roll, characterized in that... include: Gradient induction hardening, gradient spray cooling, cryogenic treatment, and stepped tempering processes.

2. The method for controlling the residual stress gradient of the working layer of forged steel cold rolling work rolls according to claim 1, characterized in that: The gradient induction hardening process uses a variable frequency / variable power continuous induction heating method to heat the roll body. The specific heating process is divided into two stages: the first stage is low-frequency current + low power, and the second stage is high-frequency current + high power.

3. The method for controlling the residual stress gradient of the working layer of forged steel cold rolling mill rolls according to claim 2, characterized in that: The frequency of the low-frequency current is 500-1000Hz; the frequency of the medium-high frequency current is 2000-4000Hz; the low power is 150-250kW; and the high power is 300-400kW.

4. The method for controlling the residual stress gradient of the working layer of forged steel cold rolling mill rolls according to claim 1, characterized in that: The gradient spray cooling divides the spray zone into 3 to 7 independently controlled cooling zones along the roller axis; the water spray pressure in each cooling zone gradually decreases from top to bottom.

5. The method for controlling the residual stress gradient of the working layer of forged steel cold rolling mill rolls according to claim 1, characterized in that: The cryogenic treatment temperature is -80℃ to -150℃.

6. The method for controlling the residual stress gradient of the working layer of forged steel cold rolling mill rolls according to claim 1, characterized in that: The stepped tempering process includes three stages of tempering and heat preservation; The first stage of tempering and heat preservation temperature is 100℃~130℃, and the heat preservation time is 6~10h; The second stage of tempering and heat preservation temperature is 120-150℃, and the heat preservation time is 15-30h; The third stage of tempering and heat preservation temperature is 80℃~100℃, and the heat preservation time is 3~8h.

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