A method for improving the fatigue life of bearing steel using a hot working process

Abstract

The present application belongs to the technical field of steel processing, and particularly relates to a method for improving fatigue life of bearing steel by using a hot working process. After hot rolling or other deformation processing of the steel material is completed, the steel material is placed into a closed heating furnace, and the steel material is subjected to 1100-1200 DEG C hot working treatment. At the same time, argon is filled into the heating furnace, so that the argon pressure in the heating furnace reaches 900-1100 atm. After 250-280 min, the pressure is released and the heating is stopped, and the steel material is cooled with the furnace. Then the steel material cannot be subjected to further deformation processing, but can be subjected to non-deformation processing. By using the hot working process, the microscopic pores between the inclusions and the steel matrix caused by the hot rolling or other deformation processing of the steel material can be eliminated, so that the fatigue life of the bearing steel is greatly improved.

Description

Technical Field

[0001] This invention belongs to the field of steelmaking process technology, and specifically relates to a method for improving the fatigue life of bearing steel using hot working processes. Background Technology

[0002] Taking bearings, gears, and some automotive transmission systems as examples, improving the lifespan of steel is a perpetual pursuit for specialty steels. The main methods typically involve reducing or miniaturizing inclusions. A search reveals that many researchers, both domestically and internationally, have conducted studies on extending the lifespan of steel, but these differ significantly from the findings of this patent.

[0003] The patent "CN201911033033.5 A method for improving the contact fatigue life of high-carbon bearing steel" describes the following process steps and control parameters: High-carbon bearing steel is carburized to obtain a carburized layer greater than 0.2 mm on the surface. The carburizing temperature is 880–980℃, the carburizing time is 3–10 h, gas carburizing is used, the carburizing carbon potential is 1.5–3.0%, the vacuum carburizing furnace pressure is 700–900 Pa, the vacuum carburizing gas flow rate is 1500–2000 L / h, and the steel is air-cooled at 2–4 bar after carburizing. The high-carbon bearing steel is quenched within 4 h after carburizing, with a heating temperature of 880–950℃ and a holding time of 1–4 h, using water quenching, gas quenching, or oil cooling, and then cooled to room temperature. After carburizing and quenching, the steel is tempered at 120–220℃ for 1–4 h and then air-cooled. The advantages are: significantly improving the contact fatigue life and wear resistance of high-carbon bearing steel, increasing the contact fatigue life of bearing steel or bearings by up to 10 times or more, thus giving high-carbon bearing steel bearings a long service life and high reliability. The fatigue life of bearing steel is improved through optimized heat treatment processes.

[0004] Patent CN202110964076.6, "A Double-Refined High-Strength, High-Toughness, Long-Life Medium-High Carbon Bearing Steel and its Preparation Method," describes a bearing steel with the following chemical composition: 0.64–0.94 wt% C, 1.20–1.80 wt% Cr, ≤0.65 wt% Si, ≤0.65 wt% Mn, ≤0.30 wt% Ni, ≤0.25 wt% Cu, ≤0.15 wt% Mo, ≤0.15 wt% Nb, ≤0.15 wt% V, and ≤0.15 wt% Zr. The total amount of Nb, V, Mo, and Zr added is required to be 0.10% ≤ Nb + V + Mo + Zr ≤ 0.30%, with the balance being iron and unavoidable impurities. Its advantages include refined microstructure, high toughness, and long fatigue life, meeting the microstructure and performance requirements of high-end equipment for high impact, high speed, long life, high reliability, and low cost bearing steel. Long-life bearing steel was obtained by optimizing the steel composition and improving the steel microstructure.

[0005] The patent “CN202211067714.5 A quenching heat treatment method for improving the homogeneity of microstructure and fatigue life of high carbon chromium bearing steel” describes a method for (1) heating a spheroidized or cold-worked high carbon chromium bearing steel workpiece to a temperature plateau for oscillation or uniform temperature at a heating rate. The oscillation temperature or temperature plateau temperature T, time t-1, and vibration amplitude ΔT are determined based on the comprehensive parameters of the phase transformation point A-(c1), the Brinell hardness value H of the high carbon chromium bearing steel workpiece before quenching heat treatment, and the spheroidization microstructure rating C. The parameters are: D-(max) and D-(min), where D-(max) and D-(min) are the maximum and minimum cross-sectional dimensions of the inner and outer rings of the bearing, respectively; K-1 and K-2 are empirical values, taken as 20~120℃·mm~2 / m. (2) After the high carbon chromium bearing steel workpiece is uniformly heated or shaken and kept warm, it is heated to 60-100℃ above Ac-1 for complete austenitization. The holding time is calculated. Among them, K-3 and K-4 are empirical values, respectively 50-350℃·mm-2 / min and 20-60min / mm. After the holding is completed, the high carbon chromium bearing steel workpiece is immediately taken out of the furnace and placed into the quenching medium. The time gap between taking out of the furnace and entering the quenching medium is controlled within t-3≤5-10min. The cooling method is carried out according to the original quenching process of high carbon chromium bearing steel, that is: continuous quenching is adopted, and the quenching oil temperature is 60-100℃; or isothermal salt bath quenching is adopted, and the quenching temperature is 180-350℃.

[0006] All of the above are significantly different from the control method of this patent.

[0007] Therefore, for the purpose of using heat treatment processes, this invention provides a method to improve the fatigue life of bearing steel by using heat treatment processes, which replaces the traditional method of only optimizing the size of inclusions or improving the heat treatment process, and achieves the goal of extending the service life of steel. Summary of the Invention

[0008] The purpose of this invention is to develop a method for improving the fatigue life of bearing steel using hot working technology. This method, compared with traditional methods that only control oxygen content and composition, can obtain bearing steel with a high fatigue life.

[0009] The steel grade mentioned belongs to the steel grade with fatigue life requirements, such as ultra-high cleanliness bearing steel products;

[0010] A method for improving the fatigue life of bearing steel using hot working technology, the method comprising the following key points:

[0011] (1) The steel composition requirements are C: 0.95%~1.05%, Si: 0.15%~0.35%, Mn: 0.25%~0.45%, P: ≤0.025%, S: ≤0.02%, Cr: 1.40%~1.65%, Al: ≤0.05%, with the remainder being impurities and Fe.

[0012] (2) After the steel has been hot rolled or otherwise deformed, it is placed in a closed heating furnace. This process is to ensure that the steel has been deformed and is ready for subsequent hot working. Once the subsequent hot working is completed, it cannot be deformed again.

[0013] (3) The steel is subjected to hot working treatment at 1100-1200℃. At the same time, argon gas is introduced into the heating furnace so that the argon gas pressure in the heating furnace reaches 900-1100 atm. After maintaining this pressure for 250-280 minutes, the pressure is released and heating is stopped. The steel is cooled with the furnace.

[0014] (4) The steel cannot be further deformed afterward, but it can be processed without deformation, such as turning and grinding.

[0015] This process is designed for applications where steel must be hot-rolled or otherwise deformed before it can be used, and where no further deformation processing is permitted after this process, but non-deformable processing such as turning and grinding is allowed.

[0016] The fatigue life of steel is largely related to inclusions within the steel, and the conclusion that inclusions are detrimental to the fatigue life of the steel matrix has been widely verified. However, research has revealed that the fundamental reason for the harm caused by inclusions to the steel matrix is ​​not the direct stress concentration caused by the inclusions, but rather the inconsistency in the deformation capacity of the inclusions and the steel matrix during deformation. This results in voids at the steel-inclusion interface after deformation. The stress concentration generated by these voids exceeds the yield strength of the steel matrix, leading to tearing of the steel matrix and the formation of microcracks. These cracks then propagate and cause damage during subsequent operation. Therefore, controlling the elimination of voids at the steel matrix-inclusion interface after deformation processing can significantly improve the fatigue life of the steel. Thus, before implementing this patented technology, the steel must undergo deformation processing; after implementing this patented technology, the steel must not undergo further deformation processing to prevent the re-formation of voids at the steel matrix-inclusion interface.

[0017] This paper addresses the requirement of hot-working steel at 1100–1200℃, simultaneously filling the furnace with argon gas to a pressure of 900–1100 atm, maintaining this pressure for 250–280 minutes, then depressurizing and stopping heating, allowing the steel to cool with the furnace. 1100–1200℃ is the ideal heating temperature for bearing steel, under which all overall properties (hardness, strength, etc.) of the steel meet deformation requirements. Temperatures below 1100℃ increase the hardness and strength of the steel, hindering subsequent pressure deformation and impeding the sealing of pores at the steel matrix-inclusion interface. Temperatures above 1200℃, while further reducing strength and hardness, also lead to further thermal deformation, making subsequent material processing difficult and increasing the requirements for the furnace materials, thus raising production costs. The purpose of filling the heating furnace with argon is to increase the furnace pressure. Filling with nitrogen would cause nitrogen buildup on the steel substrate surface, damaging the steel's composition and surface properties; filling with oxygen would lead to oxidation and other harmful effects on the steel substrate surface. Therefore, an inert gas like argon is needed to increase the furnace pressure without affecting the steel substrate. Maintaining a furnace pressure of 900–1100 atm for 250–280 minutes aims to use this pressure and time to seal the pores at the steel substrate-inclusion interface. Insufficient pressure and time will affect the sealing effect; excessive pressure and time will place excessive demands on the furnace body and increase production costs.

[0018] Through production practice, the method of this invention has been verified to improve the thrust rolling fatigue test of steel. 10 Lifespan is ≥6.5E+07. Attached image description:

[0019] Figure 1 In Example 1, the steel matrix-inclusion interface has no void morphology.

[0020] Figure 2 Void morphology at the steel matrix-inclusion interface in Comparative Example 1.

[0021] Figure 3 The void morphology at the steel matrix-inclusion interface in Comparative Example 2.

[0022] Figure 4 Void morphology at the steel matrix-inclusion interface in Comparative Example 3.

[0023] Figure 5 Void morphology at the steel matrix-inclusion interface in Comparative Example 4.

[0024] Figure 6 Void morphology at the steel matrix-inclusion interface in Comparative Example 5. Detailed Implementation

[0025] The experiment was conducted using a 120t converter-120t refining furnace-RH vacuum furnace-300 cubic meter continuous casting machine to roll φ60mm rolled steel. The corresponding composition of the rolled steel was: C: 0.978%, Si: 0.238%, Mn: 0.325%, Cr: 1.465%, P: 0.012%, S: 0.001%, Al: 0.0195%, Ca: 0.00028%, Ti: 0.0017%, O: 0.00061%.

[0026] The hot-rolled material is cut into 6 segments. If heat treatment is to be performed, the steel is placed in a heating furnace and heated at a rate of 6.0℃ / min until the set temperature is reached.

[0027] Example 1

[0028] The cut steel sections were placed in a sealed heating furnace, with a target temperature of 1150℃ and an argon pressure of 1000 atm. After maintaining this temperature for 260 minutes, the pressure was released and heating was stopped, allowing the steel to cool with the furnace. No further deformation processing was performed on the steel after this process. A section was cut for analysis of the steel matrix-inclusion interface morphology (morphology as shown in...). Figure 1 Then cut 10 pieces for fatigue testing.

[0029] Comparative Example 1 (no heat treatment performed)

[0030] The cut steel sections are not subjected to further deformation processing. A section is cut off for steel matrix-inclusion interface morphology inspection (morphology as shown in...). Figure 2 Then cut 10 pieces for fatigue testing.

[0031] Comparative Example 2 (Heat treatment was performed, followed by deformation processing).

[0032] The cut steel sections were placed in a sealed heating furnace, with a target temperature of 1150℃ and an argon pressure of 1000 atm. After maintaining this temperature for 260 minutes, the pressure was released and heating was stopped, allowing the steel to cool with the furnace. Following this, the steel underwent hot forging deformation processing, ultimately being hot-forged into φ60mm round bars. A section was cut for steel matrix-inclusion interface morphology inspection (morphology as shown in...). Figure 3 Then cut 10 pieces for fatigue testing.

[0033] Comparative Example 3 (Lowering the heat treatment temperature)

[0034] The cut steel sections were placed in a sealed heating furnace, with a target temperature of 850℃ and an argon pressure of 1000 atm. After maintaining this temperature for 260 minutes, the pressure was released and heating was stopped, allowing the steel to cool with the furnace. No further deformation processing was performed on the steel after this process. A section was cut for analysis of the steel matrix-inclusion interface morphology (morphology as shown in...). Figure 4 Then cut 10 pieces for fatigue testing.

[0035] Comparative Example 4 (Reducing Heat Treatment Pressure)

[0036] The cut steel sections were placed in a sealed heating furnace, with a target temperature of 1150℃ and an argon pressure of 700 atm. After maintaining this temperature for 260 minutes, the pressure was released and heating was stopped, allowing the steel to cool with the furnace. No further deformation processing was performed on the steel after this process. A section was cut for analysis of the steel matrix-inclusion interface morphology (morphology as shown in...). Figure 5 Then cut 10 pieces for fatigue testing.

[0037] Comparative Example 5 (Shortening heat treatment time)

[0038] The cut steel sections were placed in a sealed heating furnace, with a target temperature of 1150℃ and an argon pressure of 1000 atm. After maintaining this temperature for 180 minutes, the pressure was released and heating was stopped, allowing the steel to cool with the furnace. No further deformation processing was performed on the steel after this process. A section was cut for analysis of the steel matrix-inclusion interface morphology (morphology as shown in...). Figure 6 Then cut 10 pieces for fatigue testing.

[0039] The method for detecting the interface morphology of the steel matrix and inclusions was as follows: The steel was cut into samples approximately 1 cm thick using a saw in a cold state with water cooling. After cold polishing, the samples were magnified 100x using an optical microscope, and a total of 4000 mm was examined. 2 The interface conditions between all inclusions ≥13mm and the steel matrix were recorded, and the number of pores at the interface was shown in Table 1 below. It can be seen that after applying the hot working process, the number of pores at the steel matrix-inclusion interface is 0. However, without the hot working process, or with a decrease in the temperature, pressure, and time of the process, the number of pores increases. Furthermore, if the molten steel is subjected to deformation processing again after the hot working process, the number of pores will also increase again.

[0040] The fatigue testing method involved cutting a 1cm thick φ60mm cylinder for heat treatment. After heating to 850℃ and holding for 25 minutes, the cylinder was oil-quenched, then tempered at 150℃ for 100 minutes. In a cold state, the steel was water-cooled and cut into 5.5mm thick discs using a sawing machine. These discs were then water-cooled and cut into rings with an outer diameter of 55mm and an inner diameter of 20mm using a wire cutting machine. Further cold-state fine grinding and polishing were then performed. A thrust-type rolling fatigue testing machine (model: TLP-1, manufactured by Luoyang Bearing) was used to test the fatigue life of each steel sample. The rotation frequency was 1800cpm, the number of steel balls was 3, the ball diameter was 9.525mm, the maximum pressure was 3.95GPa, and the stop condition was 2.0E+08; the test was stopped if fatigue was not observed. The results are shown in Table 1 below. It can be seen that the fatigue life of the steel significantly increased after heat treatment, meeting the L... 10The requirement of ≥6.5E+07 means that if heat treatment is not implemented, or if the temperature, pressure, and time of heat treatment decrease, it will lead to L 10 The decrease in fatigue life is consistent with the increase in the number of pores.

[0041] Table 1. Number of voids and fatigue life results at the steel matrix-inclusion interface.

[0042] serial number Number of pores at the steel matrix-inclusion interface <![CDATA[L 10 Fatigue life Example 1 0 6.7E+07~2.0E+08 Comparative Example 1 15 3.6E+05~6.9E+06 Comparative Example 2 13 4.8E+06~1.6E+07 Comparative Example 3 8 1.8E+07~5.8E+07 Comparative Example 4 7 2.5E+07~5.7E+07 Comparative Example 5 5 2.8E+07~6.0E+07

[0043] Unless otherwise specified, the raw materials and equipment used in this invention are all commonly used in the art; the methods used in this invention, unless otherwise specified, are all conventional methods in the art. The above descriptions are merely preferred embodiments of the present invention and are not intended to limit the invention. Any modifications made to the above embodiments based on the technical essence of the present invention are included within the protection scope of the present invention.

Claims

1. A method for improving the fatigue life of bearing steel using hot working technology, characterized in that, The process includes the following steps: After hot rolling or other deformation processing, the steel is placed in a closed heating furnace and subjected to hot working at 1100-1200℃. At the same time, argon gas is introduced into the heating furnace to make the argon gas pressure in the heating furnace reach 900-1100 atm. After maintaining this pressure for 250-280 minutes, the pressure is released and heating is stopped. The steel cools down with the furnace. The treated steel cannot be deformed again.

2. The method for improving the fatigue life of bearing steel using hot working technology according to claim 1, characterized in that, The bearing steel has the following composition: C: 0.95%–1.05%, Si: 0.15%–0.35%, Mn: 0.25%–0.45%, P: ≤0.025%, S: ≤0.02%, Cr: 1.40%–1.65%, Al: ≤0.05%, with the remainder being impurities and Fe.

3. The method for improving the fatigue life of bearing steel using hot working technology according to claim 1, characterized in that, The heating rate for hot working of steel is 4.8℃ / min to 7.2℃ / min.

4. The method for improving the fatigue life of bearing steel using hot working technology according to claim 1, characterized in that, If the temperature is below 1100-1200℃, the pressure inside the furnace can be <900 atm; once the temperature inside the furnace reaches 1100-1200℃, the pressure inside the furnace must meet 900-1100 atm.

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