Large-size annealing-softening-free bearing hot-rolled round steel and manufacturing method thereof

Abstract

The invention belongs to the technical field of hot-rolled round steel for bearings, and particularly relates to large-size annealing-softening-free hot-rolled round steel for bearings and a manufacturing method of the large-size annealing-softening-free hot-rolled round steel for the bearings, and the steel comprises, by weight, 0.95%-1.05% of C, 0.15%-0.75% of Si, 0.20%-1.25% of Mn, smaller than or equal to 0.025% of P, smaller than or equal to 0.020% of S, 1.30%-1.95% of Cr, 0.1%-0.4% of Mo, 0.2%-0.8% of Co, 0.05%-0.2% of Ni, 0.002%-0.005% of B, smaller than or equal to 0.25% of Cu, smaller than or equal to 0.001% of Ca, smaller than or equal to 0.005% of Ti, smaller than or equal to 0.05% of The round steel has the advantages that the round steel has excellent mechanical property and lower section hardness, softening treatment is not needed, and the requirement of direct sawing blanking is met.

Description

Technical Field

[0001] The present invention belongs to the technical field of hot-rolled round steel for bearings, and particularly relates to a large-size non-annealed softened bearing hot-rolled round steel and a manufacturing method thereof. Background Art

[0002] Large-size high-carbon chromium bearing round steel is mainly used in the production of medium and large-sized bearing rings for supporting major equipment such as metallurgy, mining, and engineering machinery. In the first step of bearing ring production by domestic major bearing manufacturers, it is necessary to cut and blank the bearing steel raw material. In order to ensure accurate dimensions, many enterprises choose sawing for blanking. Therefore, it is required that the raw material has a lower hardness, which is convenient for blanking and does not easily wear the band saw blade. When bearing production enterprises select large-size bearing steel round steel, they require that the bearing round steel products of the steel mill be delivered in a softened and annealed state, and the hardness of the round steel is lower than 245 HBW. The softened annealing process is to heat the large-size bearing hot-rolled round steel in the hot-rolled state to 800-900 °C, hold for a period of time, and cool slowly, so as to release the internal stress of the round steel and achieve the purpose of softening. After blanking, most of the production processes of bearing rings are forging, that is, the raw material bearing round steel cut into sections is hot-formed by ring rolling after being heated at high temperature.

[0003] At present, the hardness of most large-size high-carbon chromium hot-rolled bearing round steel on the market is higher than 300 HBW, which cannot meet the requirements of sawing. It is necessary to undergo softened annealing or spheroidizing annealing to reduce the hardness of the round steel to meet the requirements of band saw blanking by bearing production enterprises. However, processes such as softened annealing or spheroidizing annealing reduce production efficiency and increase production costs.

[0004] In the prior art, the patent publication number CN106636591A discloses a controlled rolling and controlled cooling method for large-sized high-carbon chromium bearing steel. By controlling heating and the rolling reduction ratio and cooperating with stepped water cooling, the carbide grade and product performance of large-sized bearing steel are controlled. However, the method for controlling the microstructure and hardness is not mentioned, and it is impossible to meet the sawing and blanking requirements directly without the softening or spheroidizing annealing process. The patent publication number CN112792126A discloses a hot rolling method for large-reduction-ratio bearing steel. By controlling heating, mill arrangement, reduction ratio, and water cooling, the hot rolling method for bearing steel bars is controlled. The main purpose is to control the quantity, distribution, and morphological structure of carbides in the steel, and the grain size, microstructure control, and hardness control of the bearing steel are not mentioned. The patent publication number CN106086353A discloses a rolling method for controlling the precipitation of network carbide in large-section GCr15 bearing steel. By controlling heating, rolling, and multi-pass water cooling to control the cooling rate, the purpose of controlling the precipitation of network carbide is achieved, and the control method for replacing heat treatment is not mentioned. The patent publication number CN107058692A discloses an on-line rapid spheroidizing annealing method for GCr15 bearing steel after hot rolling. The GCr15 bearing steel is subjected to deformation induction at a lower temperature, and through multi-pass small deformations, carbides are precipitated and spheroidizing annealing is carried out within the spheroidizing temperature range. It is a spheroidizing treatment method. The softening effect cannot be obtained, and the sawing and blanking requirements of users cannot be met directly. Summary of the Invention

[0005] To overcome the deficiencies of the prior art, the purpose of the present invention is to provide a large-sized annealing-free softening hot-rolled bearing round steel and its manufacturing method. Based on the hot-rolled bearing round steel, the alloy element design is optimized to improve the plasticity, toughness, and fatigue resistance of the round steel. The controlled rolling and controlled cooling processes are adjusted to improve the mechanical properties of the round steel, and it has a relatively uniform and fine microstructure and a low and uniform cross-sectional hardness. Without subsequent softening treatment, it can meet the direct sawing and blanking requirements.

[0006] To achieve the above purpose, the present invention is realized through the following technical solutions:

[0007] A large-sized anneal-free softening bearing hot-rolled round steel, the chemical components in the steel are by weight percentage: C: 0.95% - 1.05%, Si: 0.15% - 0.75%, Mn: 0.20% - 1.25%, P ≤ 0.025%, S ≤ 0.020%, Cr: 1.30% - 1.95%, Mo: 0.1% - 0.4%, Co: 0.2% - 0.8%, Ni: 0.05% - 0.2%, B: 0.002% - 0.005%, Cu ≤ 0.25%, Ca ≤ 0.001%, Ti ≤ 0.005%, Al ≤ 0.05%, As ≤ 0.04%, Pb ≤ 0.002%, and the balance is Fe and unavoidable impurities.

[0008] The specifications of the round steel are

[0009] The cross-sectional hardness of the round steel is 220 - 250 HBW, and the grain size is 6 - 7 grades.

[0010] The selection of the addition amounts (by weight percentage) of the above elements and their function descriptions are as follows:

[0011] C forms the compound cementite with iron. As the carbon content in the steel increases, the amount of cementite increases, and the hardness and strength of the steel also increase, while the plasticity and toughness decrease. The C addition amount in the steel bars of the present invention is limited to 0.95% - 1.05%.

[0012] Si is added to the steel as a deoxidizer, which can increase the strength and hardness of the steel. Because when the Si content is high, its effect is similar to that when the C content is high, and excessive content reduces the toughness of the steel. The Si addition amount is limited to 0.15% - 0.75%.

[0013] Mn is also added to the steel as a deoxidizer, which can improve the strength and wear resistance of the steel. Excessive addition can reduce the plasticity and toughness of the steel. Therefore, the content should be appropriate. In the present invention, the Mn content is set at 0.20% - 1.25%.

[0014] P and S are harmful elements, which can reduce the plasticity and toughness of the steel. Therefore, the less the content, the better.

[0015] Cr can significantly improve the antioxidant effect of the steel, and can also significantly increase the hardenability of the steel, increasing the tendency of temper brittleness of the steel. The Cr content in the present invention is limited to 1.30% - 1.95%.

[0016] Mo can improve the hardenability of the steel, prevent temper brittleness, and can also prevent the pitting corrosion tendency of the steel. However, considering economy, the Mo content in the present invention is limited to 0.05% - 0.4%.

[0017] Co can form a continuous solid solution with iron, strengthen the matrix of the steel, significantly improve the hot strength of the steel, promote grain refinement, improve the plasticity, toughness and impact resistance of the steel, have good corrosion resistance, extend the service life of the steel, and the Co content in the present invention is limited to 0.2% - 0.8%.

[0018] Ni can improve the plasticity and toughness of the steel and its fatigue performance, but excessive addition will increase the tendency of white spot defects in the steel. The Ni content in the present invention is limited to 0.05% - 0.2%.

[0019] B has extremely strong ability to improve hardenability, can promote grain refinement, thereby improving the toughness, impact performance and thermal stability of bearing steel, and preventing the steel from generating over - sized grains at high temperatures. Element B can also improve the wear resistance of the steel and its machining performance. Only a small amount of B needs to be added to save a large amount of precious alloying elements. The B content in the present invention is limited to 0.002% - 0.005%.

[0020] Cu, Ca, Ti, Al, As, Pb, etc. all produce different types of inclusions, reducing the purity of the steel quality, and are all fatigue crack sources during the use of bearings. Therefore, the less the content, the better.

[0021] A manufacturing method for large - sized non - annealed softening bearing hot - rolled round steel includes a heating process, rough rolling, finish rolling, controlled cooling, air cooling on the cooling bed, and slow cooling in a slow - cooling pit. The specific preparation process is as follows:

[0022] Heating

[0023] Heated by a heating furnace, the temperature of the pre - heating section is 400 - 800 °C, the temperature of the first heating section is 800 - 1100 °C, the temperature of the second heating section is 1180 - 1250 °C, the temperature of the soaking section is 1180 - 1240 °C, the total heating time is 6 - 12 h, and the total heating time of the second heating section and the soaking section is 4 - 8 h;

[0024] Rough rolling

[0025] The starting rolling temperature is 1150 - 1200 °C, the high - pressure water descaling pressure is 20 - 25 MPa, the number of rolling passes is 7 - 13 passes, the odd - numbered passes are large - reduction deformation passes, and the reduction rate is 23% - 30%; the total compression rate of the continuous casting billet is 70% - 80%; before finish rolling after rough rolling, the rough - rolled billet is cooled by water spray, and the cooling water volume is 100 - 300 m 3 / h, so that the temperature of the billet drops by 100 - 200 °C;

[0026] Finish rolling

[0027] The temperature entering the finish rolling is 900 - 950 °C. After continuous rolling through a 4 - 8 pass horizontal and vertical alternating rolling mill, round steel of the target specification is obtained; the reduction ratio decreases successively, and the reduction ratio of the last pass does not exceed 5%; after finish rolling, the round steel is cooled by water spray, and the cooling water volume is 200 - 400 m 3 / h, causing the temperature of the billet to drop by 100 - 150 °C;

[0028] Air cooling

[0029] The finish - rolled round steel is cut according to the fixed - length, sent to the cooling bed for air cooling. The temperature of entering the cooling bed is controlled at 700 - 800 °C, and the air - cooling time on the cooling bed is controlled at 30 - 60 min;

[0030] Slow cooling

[0031] After leaving the cooling bed, the round steel is stacked in a slow - cooling pit for slow cooling. The temperature of entering the slow - cooling pit is 450 - 600 °C, the heating temperature of the slow - cooling pit is 450 - 600 °C, the heating time is 24 - 72 h, and it is cooled with the slow - cooling pit to below 200 °C and then air - cooled to room temperature after leaving the pit.

[0032] In step 1), the continuous - casting billet is (260 - 340 mm)×(360 - 430 mm).

[0033] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0034] 1. In the composition design of the present invention, Mo and B are added to improve hardenability, and Co and Ni are added to refine grains and improve the plasticity, toughness and fatigue resistance of the steel, enabling the large - specification bearing round steel to have excellent mechanical properties and low cross - section hardness, without the need for subsequent softening treatment, meeting the requirements of direct sawing for blanking.

[0035] 2. The present invention selects an excellent reduction - ratio rolling process for rolling, improves the internal quality and macro - defect of the large - specification bearing round steel, ensures the compactness of the internal structure, and makes the carbide distribution finer and more uniform.

[0036] 3. Through reasonable, simple and feasible means of controlled cooling, air cooling and slow cooling, the present invention improves the mechanical properties of the large - specification bearing round steel, obtains a fine and uniform structure, can achieve the purpose of direct sawing for blanking without annealing, and the cross - section hardness meets the processing requirements.

[0037] 4. The present invention optimizes alloying elements, combines the strengthening effect of metal elements in steel, and obtains a large - specification softened bearing steel hot - rolled round steel that can be directly sawed for blanking without annealing through optimizing the controlled rolling and controlled cooling process. The present invention improves the comprehensive performance of the hot - rolled bearing round steel, replaces the softened - treatment bearing steel, and reduces the comprehensive cost. Description of the Drawings

[0038] Figure 1It is the metallographic micrograph of Embodiment 1 of the present invention. Detailed Embodiments

[0039] The present invention will be described in detail below, but it should be noted that the implementation of the present invention is not limited to the following embodiments.

[0040] A large-sized hot-rolled bearing round steel without annealing softening, the chemical composition in the steel is calculated by weight percentage as follows: C: 0.95% - 1.05%, Si: 0.15% - 0.75%, Mn: 0.20% - 1.25%, P ≤ 0.025%, S ≤ 0.020%, Cr: 1.30% - 1.95%, Mo: 0.1% - 0.4%, Co: 0.2% - 0.8%, Ni: 0.05% - 0.2%, B: 0.002% - 0.005%, Cu ≤ 0.25%, Ca ≤ 0.001%, Ti ≤ 0.005%, Al ≤ 0.05%, As ≤ 0.04%, Pb ≤ 0.002%, and the balance is Fe and unavoidable impurities. The specification of the round steel is See Figure 1 , the hardness of the entire cross-section of the round steel is 220 - 250 HBW, and the grain size is 6 - 7 levels. When sawing, the damage to the saw blade for ordinary sawing blanking is very small, and it can be directly sawed for blanking. While the hardness of ordinary bearing steel after hot rolling is above 320 HBW, and it must go through the softening annealing process before it can be sawed for blanking.

[0041] A preparation method of a large-sized hot-rolled bearing round steel without annealing softening, comprising the following steps:

[0042] 1) Heating: The continuous casting billet (with dimensions of (260 - 340 mm) × (360 - 430 mm)) is heated in a heating furnace, the preheating section temperature is 400 - 800 °C, the first heating section temperature is 800 - 1100 °C, the second heating section temperature is 1180 - 1250 °C, the soaking section temperature is 1180 - 1240 °C, the total heating time is 6 - 12 h, and the total heating time of the second heating section and the soaking section is 4 - 8 h; this continuous casting billet. The control of the heating temperature and heating time of the heating furnace is beneficial to the diffusion of primary carbides in the bearing steel continuous casting billet, making it uniform, improving the carbide uniformity, preventing the generation of carbide liquation in the bearing round steel, reducing the carbide banding level, and being able to well improve low-magnification defects such as central porosity and segregation during the rough rolling process.

[0043] 2) Rough rolling: The starting rolling temperature is 1150 - 1200 °C, the high-pressure water descaling pressure is 20 - 25 MPa, the number of rolling passes is 7 - 13 passes, the odd-numbered passes are large reduction deformation passes, and the reduction ratio is 23% - 30%. This reduction ratio distribution method is beneficial to improving the central porosity and central segregation level of the continuous casting billet. The total reduction ratio of the continuous casting square billet is 70% - 80%; between the rough rolling mill and the finish rolling mill, a water mist cooling area is set up to spray water mist on the billet after rough rolling, and the cooling water volume is 100 - 300 m 3 / h, so that the billet temperature drops by 100 - 200 °C;

[0044] 3) Finish rolling: The entry temperature for finish rolling is 900 - 950 °C, and it is continuously rolled through a 4 - 8 pass horizontal and vertical alternating rolling mill to obtain round steel with a specification of The reduction ratio decreases successively, and the reduction ratio of the last pass does not exceed 5%. Through multi-pass continuous rolling, the structure of the round steel is made dense and the specification dimensions are precise. A water mist cooling area is set at the outlet of the finish rolling mill to spray water mist on the round steel after finish rolling, and the cooling water volume is 200 - 400 m 3 / h, so that the billet temperature drops by 100 - 150 °C; The water mist cooling at this stage can effectively inhibit the precipitation of carbide networks inside the round steel and prevent the formation of quenched structures on the surface of the round steel. Adopting post-rolling fog cooling can avoid the precipitation of network carbides.

[0045] 5) Air cooling: Cut the round steel after finish rolling according to the fixed length, put it on the cooling bed for air cooling, the temperature on the cooling bed is controlled at 700 - 800 °C, and the air cooling on the cooling bed can make the temperature of the round steel uniform, fully diffuse the remaining temperature inside the round steel to the entire cross-section, and complete the phase transformation on the cooling bed to form a strong, tough and uniform internal structure. The air cooling time on the cooling bed is controlled at 30 - 60 min.

[0046] 6) Slow cooling: After getting off the cooling bed, stack the round steel into a slow cooling pit for slow cooling. The temperature when entering the slow cooling pit is 450 - 600 °C, the heating temperature of the slow cooling pit is 450 - 600 °C, the heating time is 24 - 72 h, cool it down to below 200 °C with the slow cooling pit, and then air cool it to room temperature after getting out of the pit. Using a heatable slow cooling pit can eliminate the thermal stress of the steel, improve the non-uniform tissue state, reduce the hardness of the round steel, and eliminate the white spots in the steel at the same time.

[0047] See Figure 1 , the following is the production process corresponding to each chemical component in the examples and comparative examples of the present invention. Among them, Table 1 shows the specific components of the comparative examples and examples, Table 2 shows the hot rolling production controlled rolling and controlled cooling processes of the comparative examples and examples, and Table 3 shows the mechanical property test results of the comparative examples and examples.

[0048] Table 1 Chemical composition (wt%) of the hot-rolled round steel in the comparative examples and examples

[0049]

[0050]

[0051] Table 2 Conditions of controlled rolling and controlled cooling process

[0052]

[0053] Table 3 Results of mechanical properties and metallographic structure

[0054]

[0055]

[0056] The present invention optimizes alloying elements, combines the strengthening effect of metal elements in steel, and through optimizing the controlled rolling and controlled cooling process, enables large-sized bearing round steel to have excellent mechanical properties and low cross-sectional hardness, without the need for subsequent softening treatment, meeting the requirements of direct sawing for blanking.

Claims

1. A large-size annealing-free softened hot-rolled round steel for bearings, characterized in that: The chemical composition of the steel by weight percentage is: C: 0.95% ~ 1.05%, Si: 0.15% ~ 0.75%, Mn: 0.20% ~ 1.25%, P ≤ 0.025%, S ≤ 0.020%, Cr: 1.30% ~ 1.95%, Mo: 0.1% ~ 0.4%, Co: 0.2% ~ 0.8%, Ni: 0.05% ~ 0.2%, B: 0.002% ~ 0.005%, Cu ≤ 0.25%, Ca ≤ 0.001%, Ti ≤ 0.005%, Al ≤ 0.05%, As ≤ 0.04%, Pb ≤ 0.002%, and the balance is Fe and unavoidable impurities.

2. The large-size annealing-free softened hot-rolled round steel for bearings according to claim 1 is characterized in that: The round steel specifications are 3. The large-size annealing-free softened hot-rolled round steel for bearings according to claim 1 is characterized in that: The cross-sectional hardness of the round steel is 220-250 HBW, and the grain size is 6-7 levels.

4. A method for preparing large-size annealing-free softened hot-rolled round steel for bearings according to any one of claims 1 to 3, characterized in that: It includes heating process, rough rolling, finishing rolling, controlled cooling, air cooling on cooling bed, slow cooling in slow cooling pit, among which: heating Heating furnace heating, preheating section temperature is 400 ~ 800 ℃, the first heating section temperature is 800 ~ 1100 ℃, the second heating section temperature is 1180 ~ 1250 ℃, the soaking section temperature is 1180 ~ 1240 ℃, the total heating time is 6 ~ 12h, the total heating time of the second heating section and the soaking section is 4 ~ 8h; Rough rolling The rolling temperature is 1150-1200℃, the high-pressure water dephosphorization pressure is 20-25MPa, the rolling passes are 7-13, the odd-numbered passes are large-reduction deformation passes, and the reduction rate is 23%-30%; the total compression rate of the continuous casting billet is 70%-80%; the rough-rolled billet is sprayed with water mist after rough rolling and before fine rolling, and the cooling water volume is 100-300m 3 / h, the temperature of the steel billet drops by 100-200℃; Finishing The finishing rolling temperature is 900-950℃, and the round steel of target specification is obtained after 4-8 passes of horizontal and vertical alternating rolling mills; the compression rate decreases successively, and the compression rate of the last pass does not exceed 5%; after finishing rolling, the round steel is sprayed with water mist, and the cooling water volume is 200-400m 3 / h, the temperature of the steel billet drops by 100-150℃; air cooling The round steel after fine rolling is cut into fixed lengths and air-cooled on a cooling bed. The temperature of the cooling bed is controlled at 700-800°C and the air-cooling time of the cooling bed is controlled at 30-60 minutes. Slow cooling After getting off the cooling bed, the round steel is piled into the slow cooling pit for slow cooling. The temperature entering the slow cooling pit is 450-600℃, the heating temperature of the slow cooling pit is 450-600℃, the heating time is 24-72h, and as the slow cooling pit cools to below 200℃, it is air-cooled to room temperature after leaving the pit.

5. The method for preparing large-size annealing-free softened hot-rolled round steel for bearings according to claim 4, characterized in that: The continuous casting billet in step 1) is (260-340 mm)×(360-430 mm).

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