A heat treatment method for improving contact fatigue strength of hot rolling support rolls

Abstract

The present invention discloses a heat treatment method for improving the contact fatigue strength of a support roller, comprising the following steps: 1) preparing steel raw materials according to the chemical components and weight percentages in the Cr5 support roller material, and obtaining a steel ingot according to a smelting process production process; 2) obtaining a roller blank from the steel ingot according to a forging process production process; 3) heat treating the roller blank to obtain a working layer with a high fatigue life; 4) processing and testing the roller blank. The present invention solves the problem of fatigue block shedding and peeling caused by insufficient contact fatigue performance of forged steel support rollers in hot rolling lines for rolling stainless steel, silicon steel, etc., and improves the service life and service cycle of the support rollers.

Description

Technical Field

[0001] The invention relates to the field of manufacturing rollers for steel rolling mills in the metallurgical equipment industry, and is a heat treatment method for improving the contact fatigue strength of support rollers. Background Art

[0002] Stainless steel is widely used in furniture, decoration and other fields, and the demand for silicon steel in new energy vehicles is growing rapidly. Therefore, hot-rolled stainless steel and silicon steel are developing in the direction of high efficiency and thin plates and strips in order to improve product quality and reduce manufacturing costs.

[0003] The rolling force of stainless steel and silicon steel hot rolling lines is large, and this working condition has high requirements for support rollers. Not only does it require good strength and toughness of the rollers, but it also requires good contact fatigue strength of the roller body working layer. When used in pairs with high-hardness, high-carbide content working rolls, it must not fail early due to block dropping, peeling, etc. In general, hot rolling lines use Cr5 forged steel support rollers, and rolling lines with a high proportion of stainless steel and silicon steel thin strips do not have targeted designs or use different support rollers. However, with the increase in the proportion of stainless steel and silicon steel thin strips, the number of early failures of support rollers has increased. The fundamental reason is that the wear resistance and contact fatigue strength of the support rollers are insufficient.

[0004] In addition, since the net weight of hot-rolled support rollers is usually about 40-50 tons, the steel ingots used to manufacture support rollers must reach more than 60 tons. It is very difficult to improve the hardness and fatigue strength of the working layer by increasing the content of alloy elements. The higher the material and the more serious the segregation, the more difficult it is to control the uniformity, and the forging is prone to cracking and the yield rate is low. Therefore, the material of the support roller is still Cr5 or below. In this context, various manufacturers are innovating various process technologies to improve product uniformity and performance.

[0005] Therefore, how to improve the hardness, wear resistance and contact fatigue strength of hot rolling support rollers, especially to develop hot rolling support rollers suitable for stainless steel and silicon steel thin strip rolling products to reduce or avoid early failure of support rollers, has become a technical problem that needs to be urgently solved in this field. Summary of the invention

[0006] The present invention provides a heat treatment method for improving the contact fatigue strength of hot rolling support rollers, which solves the problem of insufficient contact fatigue strength of conventional forged steel support rollers in hot rolling stainless steel and silicon steel thin plate and strip rolling lines, and improves the service cycle and service life of the support rollers.

[0007] Specifically, the present invention provides a heat treatment method for improving the contact fatigue strength of hot rolling support rollers, comprising the following steps:

[0008] ①Preparatory heat treatment: normalize the roll blank, heat it to 780-850℃, keep it warm for 10-20h, then slowly cool it to 680-720℃, keep it warm for 30-40h, and then furnace cool it to obtain sufficient spheroidization effect;

[0009] ② Rough machining, quenching and tempering, semi-finishing;

[0010] ③ Preheating: Preheat the semi-finished roll blank in a trolley furnace at a temperature range of 250-400°C for 20-30 hours; transfer the preheated roll blank to a differential temperature quenching device, heat the roll blank to 950-1050°C at a rate of 10-15°C / min, and keep it for 10-20 minutes to ensure rapid austenitization within 100mm of the working layer and fine austenite grains;

[0011] ④Quenching: The heated roll blank is transferred to a quenching device with a continuously controllable cooling speed for quenching;

[0012] ⑤ Tempering: The quenched roller blank is tempered in stages, and the first stage tempering temperature range is 350-450℃; after tempering, it is air-cooled to room temperature and then the second stage tempering is carried out, and the tempering temperature range is 450-550℃; after tempering, it is air-cooled to room temperature and then the third stage tempering is carried out, and the tempering temperature range is 450-550℃; after tempering, it is furnace-cooled to ≤350℃ and air-cooled to room temperature.

[0013] A further improvement of the present invention is that the differential temperature quenching device controls the temperature distribution within the outer layer 100mm of the support roller body to be close to the adjacent Figure 1 The curve.

[0014] A further improvement of the present invention is that the continuous controllable cooling described in step ④ is to control the quenching cooling speed by controlling the water flow rate, with rapid cooling for 15-20 minutes at the beginning, and the water flow rate is controlled to 150-200 cubic meters / hour per meter of roller length, and then the flow rate is reduced to 75-125 cubic meters / hour per meter of roller length, so that an equal hardness layer is obtained within 45-75mm of the working layer (the absolute value of the hardness difference is ≤2HS).

[0015] A further improvement of the present invention is that the time of the first, second and third stage tempering in step ⑤ is calculated according to t1 = roller body diameter φ / 100mm*1.5h, t2 = roller body diameter φ / 100mm*4h, t3 = roller body diameter φ / 100mm*6h respectively.

[0016] A further improvement of the present invention is that the roller blank is made by using a steel ingot in accordance with a forging process.

[0017] Preferably, before the forging process, the steel ingot is heated to 1200-1300°C for 20-30h; then it is transferred to an 80MN hydraulic press for forging, and the steel ingot is made into a forging blank through upsetting and drawing forging. The forging blank is subjected to post-forging heat treatment and then turned to obtain a fine-grained roller blank.

[0018] Preferably, the steel ingot is prepared by preparing steel raw materials according to the chemical components and weight percentages of the Cr5 support roller material, and is produced according to the smelting process production process, wherein the chemical components and weight percentages of the Cr5 support roller material are: C 0.45% ~ 0.75%, Si 0.40% ~ 0.80%, Mn 0.30% ~ 0.80%, Cr 4.50% ~ 5.50%, Ni 0.20% ~ 0.60%, Mo 0.40% ~ 0.80%, V 0.10% ~ 0.50%, P ≤ 0.020%, S ≤ 0.010%; the rest are Fe and unavoidable impurities. Further preferred is: C 0.45% ~ 0.60%, Si 0.40% ~ 0.45%, Mn 0.40% ~ 0.45%, Cr 4.80% ~ 5.20%, Ni 0.30% ~ 0.50%, Mo 0.40% ~ 0.60%, V 0.10 ~ 0.20%, P ≤ 0.020%; S ≤ 0.010%; the rest is Fe and unavoidable impurities.

[0019] Preferably, the smelting process of the steel ingot is as follows:

[0020] A. The steel raw materials are smelted in an electric arc furnace and a LF refining furnace, and the chemical composition and weight percentage of the molten steel in the refining furnace are tested. After the test is qualified, vacuum degassing is performed;

[0021] B. In a vacuum environment, molten iron is poured into an ingot mold to form an ingot.

[0022] The hot-rolled support roller treated by the present invention was inspected using a metallographic microscope. The metallographic structure within 45-75 mm of the roller surface was tempered martensite + dispersed fine-grained carbide, the microhardness was ≥680HV, and the absolute value of the hardness difference was ≤20HV. A contact fatigue test was carried out on a sample of the working layer of the roller using a rapid fatigue testing machine, and the service life was increased by more than 30%. The residual stress on the roller surface was -(200-350)MPa.

[0023] Due to the adoption of the above technical solution, the technical progress achieved by the present invention is:

[0024] 1. The present invention improves the conventional Cr5 support roller heat treatment method to obtain a forged steel support roller with higher contact fatigue strength. The working layer has good wear resistance, the contact fatigue strength is increased by 30%, and the residual stress is stable, so that the comprehensive use effect of the support roller is improved during the entire service life, the steel rolling production efficiency is improved, and the consumption of the support roller is reduced.

[0025] 2. The present invention is improved on the basis of conventional Cr5 support roller heat treatment. Spherical pearlite structure and a large number of granular carbides are obtained through preliminary heat treatment. The spherical structure and carbides can hinder the growth of austenite grains at high temperature, so that a high quenching temperature and fine grains can be selected during the subsequent performance heat treatment heating. Rapid heating austenitization creates good conditions for the temperature gradient distribution of the working layer. More alloy elements are dissolved in the high-temperature austenite, and a martensitic structure with a depth range of 45-75mm can be obtained during quenching. Through the improvement of the tempering process and the use of multiple tempering processes in stages, the residual stress of the roller body is effectively controlled, the residual stress is reasonable, and the working layer of the roller body obtains higher microhardness and fine carbides with dispersion strengthening. During the use of the support roller, the increase in microhardness can improve the wear mechanism of the roller and effectively protect the matrix. The matrix can relieve the higher internal stress borne by the carbide area through plastic deformation, and play a supporting and protective role on the carbide, effectively preventing crack propagation, thereby improving the contact fatigue strength of the support roller.

[0026] 3. The Cr5 support roller in the present invention has good uniformity of structure, and its microhardness is 20% higher than that of conventional Cr5 forged steel support roller. The contact fatigue strength of the Cr5 support roller in the present invention exceeds 2.2 million revolutions under experimental conditions, and the comprehensive service life is 30% higher than that of conventional Cr5 forged steel support roller. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is a temperature distribution curve. DETAILED DESCRIPTION

[0028] Examples 1-4

[0029] The chemical components and their weight percentages of the support rollers of Examples 1-4 of the present invention are shown in the table below.

[0030] Table 1 Main chemical components and their weight percentages of the support rollers of Examples 1 to 4

[0031]

[0032] The heat treatment method for improving the contact fatigue strength of the support roller according to embodiments 1-4 of the present invention comprises the following steps:

[0033] 1) Steel raw materials are prepared according to the chemical components and weight percentages of the Cr5 support roller material, and steel ingots are obtained according to the smelting process production process; wherein, the specific steps of the steel ingot production process are as follows: A. The steel raw materials are smelted in an electric arc furnace and an LF refining furnace, and the chemical components and weight percentages of the molten steel in the refining furnace are tested, and vacuum degassing is performed after the test is qualified; B. Molten iron is injected into an ingot mold in a vacuum environment to form a steel ingot.

[0034] 2) The steel ingot is produced into a roller blank according to the forging process production process.

[0035] 3) Perform preliminary heat treatment on the roll blank; the specific process of the preliminary heat treatment is as follows: heat the forging blank to 900-1000°C, normalize it, then transfer it to a furnace and heat it to 780-850°C, keep it warm for 10-20 hours, then slowly cool it to 680-720°C, keep it warm for 30-40 hours, and then cool it in the furnace.

[0036] 4) Rough machining, tempering treatment, and semi-finishing.

[0037] 5) The specific steps of performance heat treatment are as follows:

[0038] ① Preheating: Preheat the semi-finished roll blank in a trolley furnace at a preheating temperature range of 250-400°C for 20-30 hours;

[0039] The preheated roll blank is transferred to the differential temperature quenching device, and the temperature distribution within the outer layer of the support roll body is controlled by step-heating at a speed of 10-15°C / min, and the roll blank is quickly heated to 950-1050°C, and the holding time is 10-20 minutes;

[0040] The heated roll blank is transferred to a quenching device with a continuously controllable cooling speed for quenching. The quenching cooling speed is controlled by controlling the water flow rate. At the beginning, the rapid cooling is 15-20 minutes, and the water flow rate is controlled at 150-200 cubic meters / hour per meter of roll length. After that, the flow rate is reduced and controlled to 75-125 cubic meters / hour per meter of roll length.

[0041] ② The quenched roller blank is tempered in stages. The tempering temperature range of the first stage is 350-450℃, and the tempering time is according to t1=roller diameter φ / 100mm*1.5h; after tempering, air cool to room temperature and then carry out the second stage tempering, the tempering temperature range is 450-550℃, the tempering time is according to t2=roller diameter φ / 100mm*4h, after tempering, air cool to room temperature and then carry out the third stage tempering, the tempering temperature range is 450-550℃, the tempering time is according to t3=roller diameter φ / 100mm*6h, after tempering, cool to ≤350℃ and air cool to room temperature.

[0042] The specific process parameters of Examples 1 to 4 are shown in the following table.

[0043] Table 2 Process parameters of Examples 1 to 4

[0044]

[0045] Comparison Example

[0046] Steps 1) to 3) of this comparative example are the same as those of Example 1, except for step 4) and step 5).

[0047] 4) Preliminary heat treatment of the roll blank: The control roll blank was not subjected to normalizing treatment.

[0048] 5) The specific steps of performance heat treatment are as follows:

[0049] ① Preheating: Preheat the semi-finished roller blank in a trolley furnace, the preheating temperature range is 250-400℃, and the temperature is kept for 20-30h; transfer the preheated roller blank to the differential temperature quenching device, heat the roller body by conventional heating for 40min, and generate a large amount of heat by spraying flames through natural gas flame nozzles. The heat mainly heats the roller body surface by thermal radiation, and is transferred from the surface to the inner layer by heat conduction. The inner layer temperature is increased by long-term heat preservation to achieve austenitization. The uniformity and control accuracy of heating temperature are poor, because the temperature is controlled by the nozzle flow during the heating process, and the coordination of different nozzles is difficult. On the other hand, the heating process can only monitor the furnace temperature, and the furnace temperature is indirectly controlled by production experience. The heated roller blank is transferred to the quenching device for continuous large-volume cooling and quenching. The water flow is controlled at 120-160 cubic meters / hour per meter of roller length, and the continuous cooling is 1-1.5 hours.

[0050] ② After the quenched roller blank is tempered according to the first and second stage parameters of Example 1 (without the third stage tempering), the furnace is cooled to ≤350°C and air-cooled to room temperature.

[0051] Experimental example

[0052] The hot rolling support rollers treated by Examples 1 to 4 of the present invention and the control example were subjected to metallographic microscope inspection, microhardness test and contact fatigue test, respectively. The results are as follows:

[0053] 1) Metallographic microscope inspection results

[0054] The metallographic structure of the roller body surface within 45-75 mm of Examples 1-4 is tempered martensite + dispersed fine carbide particles, while the metallographic structure of the roller body surface within 45-60 mm of the control example is tempered martensite + dispersed fine carbide particles, and a certain amount of tempered troostite appears within 60-75 mm.

[0055] 2) Microhardness test results

[0056] The microhardness of the roller surface of Examples 1-4 at 45-75 mm is ≥680 HV, and the absolute value of the hardness difference is ≤20 HV, while the microhardness of the roller surface of the control example at 45-75 mm is ≥620 HV (see Table 3).

[0057] Table 3 Microhardness test results of Examples 1 to 4 and Control Examples

[0058]

[0059]

[0060] 3) Contact fatigue test results

[0061] The contact fatigue test was conducted on the working layer samples of the roller body using a rapid fatigue testing machine. The service life of Examples 1-4 was increased by more than 30% compared with the control example. The residual stress on the roller body surface of Examples 1-4 was -(200-350)MPa, which was basically equivalent to the residual stress on the roller surface of the control example. (See Table 4)

[0062] Table 4 Contact fatigue strength test results of Examples 1 to 4 and Control Examples

[0063] Example / Comparative Example Example 1 Example 2 Example 3 Example 4 Comparison Example Fatigue strength / 10,000 revolutions 225 242 228 235 170

[0064] It can be seen that compared with conventional heat treatment methods, the hot-rolled support roller treated by the method of the present invention has achieved significant technical progress in metallographic structure, microhardness, contact fatigue strength and other aspects.

Claims

1. A heat treatment method for improving the contact fatigue strength of hot rolling support rolls, characterized in that: The steps include: ① Preliminary heat treatment: normalizing the roller blank, heating it to 780-850°C, keeping it warm for 10-20h, slowly cooling it to 680-720°C, keeping it warm for 30-40h, and then furnace cooling it; the roller blank is made by making a steel ingot according to a forging process production process, and the steel ingot is made by preparing steel raw materials according to the chemical components and weight percentages in the Cr5 support roller material, and making it according to the smelting process production process, wherein the chemical components and weight percentages in the Cr5 support roller material are: C 0.45%-0.75%, Si 0.40%-0.80%, Mn 0.30%-0.80%, Cr 4.50%-5.50%, Ni 0.20%-0.60%, Mo 0.40%-0.80%, V 0.10%-0.50%, P ≤ 0.020%, S ≤ 0.010%; the rest are Fe and unavoidable impurities; ② Rough machining, quenching and tempering, semi-finishing; ③ Preheating: Preheat the semi-finished roll blank in a trolley furnace at a temperature range of 250-400°C for 20-30 hours; transfer the preheated roll blank to a differential temperature quenching device, heat the roll blank to 950-1050°C at a rate of 10-15°C / min, and keep it warm for 10-20 minutes; ④ Quenching: The heated roller blank is transferred to a quenching device with a continuously controllable cooling speed for quenching; the continuously controllable cooling is to control the quenching cooling speed by controlling the water flow rate. At the beginning, the rapid cooling is 15-20 minutes, and the water flow rate is controlled to 150-200 cubic meters / hour per meter of roller length. After that, the flow rate is reduced and the flow rate is controlled to 75-125 cubic meters / hour per meter of roller length, so that an equal hardness layer is obtained within 45-75mm of the working layer; ⑤ Tempering: The quenched roller blank is tempered in stages, and the first stage tempering temperature range is 350-450℃; after tempering, it is air-cooled to room temperature and then the second stage tempering is carried out, and the tempering temperature range is 450-550℃; after tempering, it is air-cooled to room temperature and then the third stage tempering is carried out, and the tempering temperature range is 450-550℃; after tempering, it is furnace-cooled to ≤350℃ and air-cooled to room temperature. The time for the first, second and third stage tempering is calculated according to t1=roller diameter φ / 100mm*1.5h, t2=roller diameter φ / 100mm*4h, and t3=roller diameter φ / 100mm*6h respectively.

2. The heat treatment method according to claim 1, wherein before the forging process, the steel ingot is heated to 1200-1300°C for 20-30 hours; then it is transferred to an 80MN hydraulic press for forging, and the steel ingot is made into a forging blank through upsetting and drawing forging, and the forging blank is subjected to post-forging heat treatment and then turned to obtain a fine-grained roller blank.

3. The heat treatment method according to claim 1, wherein the smelting process of the steel ingot is as follows: A. The steel raw materials are smelted in an electric arc furnace and a LF refining furnace, and the chemical composition and weight percentage of the molten steel in the refining furnace are tested. After the test is qualified, vacuum degassing is performed; B. In a vacuum environment, molten iron is poured into an ingot mold to form an ingot.

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