Medium-carbon alloy steel for large industrial bearing and production method of medium-carbon alloy steel

Abstract

The invention relates to medium-carbon alloy steel for a large industrial bearing and a production method of the medium-carbon alloy steel, and belongs to the field of iron-based alloy. The round steel comprises the following chemical components: 0.44 to 0.50 percent of C, 0.15 to 0.35 percent of Si, 0.60 to 0.90 percent of Mn, 1.0 to 1.30 percent of Cr, 0.5 to 0.9 percent of Ni, 0.25 to 0.40 percent of Mo, 0.05 to 0.10 percent of V, less than or equal to 0.20 percent of Cu, less than or equal to 0.05 percent of Al, less than or equal to 0.0008 percent of Ca, less than or equal to 0.008 percent of S, less than or equal to 0.015 percent of P, less than or equal to 0.0020 percent of Ti, less than or equal to 0.0012 percent of O, less than or equal to 0.03 percent of As, less than or equal to 0.03 percent of Sn, less than or equal to 0.005 percent of Sb, less than or equal to 0.002 percent of Pb and the balance of Fe and inevitable impurities. The production process comprises the steps of molten iron pretreatment, converter smelting, LF refining, RH refining, CCM continuous casting round billet, annealing and finishing. Forging with the compression ratio of 6 or above is adopted, and the grain size is larger than or equal to 6 grades. And on the basis of meeting the quality requirement of the steel for the medium-carbon bearing, the production cost is lower, and the steel has very strong market competitiveness.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of iron-based alloy, more particularly relates to bearing steel and its production method. BACKGROUND

[0002] Medium-carbon alloy steel for large industrial bearings is the most critical component in industrial transmission equipment that dynamically bears pressure and rolling contact fatigue, and its design life is more than 20 years. After long-term development, international bearing steel has formed a series covering high-carbon chromium bearing steel, carburizing bearing steel, medium-carbon carbon bearing steel, stainless bearing steel, and high-temperature bearing steel. The related production technology of bearing steel has also become mature, and all kinds of bearing steels are manufactured by professional production lines. However, due to the increasingly fierce competition in large industries in recent years, the arrival of the era of flat prices means that the industry competes with power sources such as photovoltaic and ultra-high voltage power plants that already have flat price capabilities, and the entire industrial chain must reduce costs. As a material and component production enterprise at the front end of the industrial chain, using traditional production processes not only has high production costs, but also has a long production cycle. Therefore, by optimizing the production method and alloy composition to reduce production costs, and realizing mass production automation, production efficiency is significantly improved, and the comprehensive cost of large industrial bearing production is reduced. The emergence of medium-carbon alloy steel for large industrial bearings meets the huge market demand with the characteristics of low speed, large size, and long life.

[0003] Domestic and foreign production of steel for large industrial bearings adopts electroslag or mold casting process. Steel produced by mold casting process has the advantages of high uniformity and high density, but it is difficult to ensure the high purity requirement of large industrial bearings. In order to improve the purity of steel, foreign countries also use electroslag remelting process to produce steel for large industrial bearings. The steel produced by this process has the advantages of small and uniform distribution of non-metallic inclusion particles, but also has obvious disadvantages such as low production efficiency, low production capacity, high energy consumption, and high production cost. Therefore, the market competitiveness of steel produced by electroslag remelting process is very low. With the development of vacuum degassing and continuous casting technology, domestic and foreign countries also gradually use vacuum degassing + continuous casting process to produce steel for large industrial bearings. This process can greatly improve production efficiency and production capacity, greatly reduce energy consumption and production cost, and vacuum degassing can significantly reduce the oxygen content of steel, reduce the size of non-metallic inclusions, and improve the purity index. Therefore, vacuum degassing continuous casting material has more competitive advantage in the market, but compared with traditional mold casting and electroslag remelting process, the continuous casting process is difficult to ensure the requirement of high uniformity. The quality bottleneck of large industrial bearing steel restricts the application of high-end large industrial bearings in new energy equipment and other fields. SUMMARY

[0004] The present application is based on the current large industrial bearing production technology, through the optimization design of the key parameters of continuous casting process, further reduce the oxygen content and harmful element content in steel, reduce the content and size of non-metallic inclusions, improve the purity; through the improvement of continuous casting process, improve the uniformity of carbide and organization uniformity, so as to meet the quality requirements of large industrial bearing steel, and make the product have strong market competitiveness.

[0005] The technical scheme adopted by the present application is: a medium carbon alloy steel for large industrial bearing, in order to meet the uniformity of the steel for large industrial bearing, the steel satisfies: the round billet is not detected for grain size, the user adopts a compression ratio of 6 or more for forging, heat treatment, and detection according to ISO 643 standard, and the grain size is greater than or equal to 6 levels.

[0006] In order to meet the long fatigue life of the steel for large industrial bearing, the steel has high purity, and the steel has higher level in micro non-metallic inclusions, macro non-metallic inclusions and organization uniformity.

[0007] Micro non-metallic inclusions include A and C plastic inclusions, B and D brittle inclusions, plastic inclusions generally have high ductility, and are not easy to produce fatigue cracks, so the inclusions have little harm to the fatigue life of the bearing; brittle inclusions generally have high hardness, and are easy to cause stress concentration in the running process of the bearing, leading to fatigue cracks caused by separation from the matrix, affecting the fatigue life of the bearing, and the larger the size of the inclusions, the greater the harm to the fatigue life. The present application requires that the micro non-metallic inclusions are tested according to GB / T 10561 A method, and the specific requirements for micro non-metallic inclusions rating are shown in Table 1 below.

[0008] Table 1

[0009] Macro non-metallic inclusions can easily cause serious stress concentration in the use process of large industrial bearing, greatly reducing the service life of the bearing. The present application requires that the macro non-metallic inclusions are tested according to the water immersion high frequency detection method of SEP 1927 (water immersion ultrasonic detection method of pure steel bar of forging and rolling steel), and the detection is carried out with 5 level sensitivity, and the total volume is greater than or equal to 5dm 3 , the length of macro inclusions should be less than or equal to 5mm, and the density of inclusions in each furnace should be less than or equal to 16mm / dm 3 .

[0010] Since the organization uniformity and organization density of the steel have a great influence on the fatigue life, the present application requires that the steel is rated by ASTM E381 for steel macrostructure, the center porosity is less than or equal to 1.5 level, the center crack is less than or equal to 1.0 level, the shrinkage is less than or equal to 1.0 level, and there is no intermediate crack, subsurface crack and subsurface bubble.

[0011] The technical scheme adopted by the present application to solve the above problems is as follows: a medium-carbon alloy steel for large industrial bearings, C: 0.44-0.50%, Si: 0.15-0.35%, Mn: 0.60-0.90%, Cr: 1.0-1.30%, Ni: 0.5-0.9%, Mo: 0.25-0.40%, V: 0.05-0.10%, Cu≤0.20%, Al≤0.05%, Ca≤0.0008%, S≤0.008%, P≤0.015%, Ti≤0.0020%, O≤0.0012%, As≤0.03%, Sn≤0.03%, Sb≤0.005%, Pb≤0.002%, and the balance being Fe and unavoidable impurities. The specific chemical element design of the present application is as follows: 1) Determination of S content S mainly exists in the form of MnS non-metallic inclusions in steel, although such inclusions generally belong to plastic inclusions, and the influence degree on the fatigue life of the steel is much lower than that of brittle inclusions, but MnS inclusions still have a certain damaging effect on the uniformity of the structure. MnS is generally in the form of long strip, but it can also form a blocky morphology with "corners", which can have a serious harmful effect on the fatigue life. However, a small amount of S element in the steel can improve the cutting performance of the steel, so S≤0.008% is determined.

[0012] 2) Determination of P content P element is a common harmful element in steel, which can easily cause element segregation during continuous casting, reducing the uniformity of the structure. If the P content in the steel is too high, the plasticity and toughness of the steel can be significantly reduced, and the cold brittleness of the steel can be increased. P element is dissolved in the steel, which can easily cause grain coarsening, so P≤0.015% is determined. 3) Determination of Ti content Ti can easily combine with N element or C element to form TiN or Ti(C, N) type non-metallic inclusions during the solidification of molten steel, which are usually blocky inclusions with "corners". Such inclusions have high hardness, seriously affecting the uniformity of the structure, and can easily cause stress concentration at the corners during operation, seriously reducing the fatigue life of the bearing. Therefore, Ti≤0.0020% is required in the present application. 4) Determination of O content Oxygen naturally enters the steel during the steelmaking process and remains in the steel in the later stage. Oxygen mainly exists in the form of non-metallic inclusions such as SiO2 and Al2O3 in the steel, especially Al2O3 inclusions, which seriously affect the fatigue life of the bearing. A large number of fatigue life tests have shown that the reduction of oxygen content is beneficial to improving the purity of the steel, especially the reduction of the content and size of oxide brittle inclusions. Therefore, the oxygen content is required to be≤0.0012% in the present application.

[0013] 5) Determination of the content of As, Sn, Sb, Pb As, Sn, Sb, Pb and other trace elements, all belong to low melting point non-ferrous metals, and are harmful elements in steel. In the solidification process of molten steel, these harmful elements are easily enriched near the grain boundaries, reducing the hot plasticity of the steel and causing surface cracking of the continuous casting billet. Therefore, the content of these elements in the present application is determined as As≤0.03%, Sn≤0.03%, Sb≤0.005%, and Pb≤0.002%.

[0014] Another object of the present application is to provide a production method of medium-carbon alloy bearing steel for large industrial bearings. The blank is smelted by vacuum degassing and continuous casting. The specific production process flow is hot metal pretreatment, converter smelting, LF refining, RH refining, CCM continuous casting round billet, annealing, and finishing. The main production process features are as follows: (1) Molten steel smelting: the smelting raw materials must pass through KR hot metal pretreatment, converter smelting, LF refining, and RH furnace vacuum degassing in sequence.

[0015] Firstly, the hot metal must be subjected to KR hot metal pretreatment to preliminarily reduce the content of harmful elements S and P and obtain clean hot metal. The hot metal pretreatment can also reduce the addition amount of subsequent smelting slagging agent, shorten the smelting time, and improve the production efficiency. Secondly, primary smelting is carried out in the converter to reduce the content of C, convert the hot metal into molten steel, adjust the content of Si and Mn elements, further remove the content of P and S elements, add clean scrap steel, strictly control the content of residual elements in the scrap steel, strictly control the content of residual harmful elements in the molten steel, control the C content at the end of the tapping process at about 0.25%, add Al iron for pre-deoxidization according to the end C%, control the Al content in the refining furnace at ≤0.05%, control the end P at ≤0.020% or below, and control the tapping temperature at 1600°C or above. The converter or electric furnace tapping adopts techniques such as slag blocking with a slag blocking plug and post-furnace slagging to solve the problem of high content of harmful elements As, Sn, Pb, Sb, etc. in bearing steel. The content of each element is accurately controlled in the LF refining furnace, low-Ti and low-Ca alloys and high-quality refractory materials are selected, the Ti content in the molten steel is strictly controlled to be ≤0.0020wt%, high-performance refining synthetic slag is used in the LF process, the synthetic slag is a CaO-Al2O3-SiO2 ternary slag system, and argon stirring is adopted. On the one hand, it can accelerate the transfer of substances between the molten steel and the refining slag, which is beneficial to the desulfurization and deoxidization reactions. On the other hand, argon blowing can make Al2O3 non-metallic inclusions float and be removed. High-quality refractory materials are used to control the amount of MgO in the molten steel, maintain long-time LF refining time, and make the inclusions float and be removed. After LF refining, the molten steel must be vacuum degassed in the RH furnace, and the vacuum degree in the furnace must be ensured to be sufficient and the circulating treatment time must be sufficient to further remove harmful gases and non-metallic inclusions in the molten steel and improve the purity of the molten steel, the maximum vacuum degree is ≤1.5 mbar, the high vacuum time is ≥15 min, and after the vacuum is broken, argon soft blowing stirring is used, and the soft blowing time is ≥25 min, on the one hand to ensure vacuum degassing, and on the other hand to fully ensure that the inclusions float up and are removed; (2) Continuous casting: large cross-sectional size round continuous casting billets are used, the continuous casting billet specification is φ600 mm and above, the steel material is ensured to have a large compression ratio in the subsequent forging process, so as to ensure the density of the material; before the continuous casting process, a tundish covering agent is added in the tundish, the thickness is ≥100 mm, the molten steel is prevented from being oxidized before continuous casting, and protective casting is carried out by adding a protective slag in the whole continuous casting process, so that the molten steel is not secondarily oxidized and polluted; low superheat pouring is used in the continuous casting process, and the superheat is controlled to be ≤25℃, which can reduce the columnar crystal zone and expand the range of equiaxed crystal zone, not only can reduce the grain size, but also can effectively reduce the composition segregation of the continuous casting billet, and significantly improve the uniformity of the continuous casting billet organization; the pouring speed is 0.25-0.35 min / m, and the liquid level fluctuation is stable ≤5 mm; the tundish induction heating, light pressing and electromagnetic stirring technology are used, and the center porosity and shrinkage hole and other macroscopic defects are effectively improved; (3) Finishing: the continuous casting round billet is hoisted into the slow cooling pit after flame cutting, and after the slow cooling pit is covered with a heat preservation cover, the continuous casting round billet is slowly cooled to 200℃±30℃, then the cover is opened, and then the continuous casting round billet is put into an annealing furnace for annealing, the heating speed is ≤60℃ / hour, the temperature is raised to 700℃±10℃, the temperature is kept for 10-15 hours, and then the temperature is lowered to 300℃ at a cooling speed of ≤40℃ / hour, and the continuous casting round billet is taken out of the furnace and is cooled.

[0016] The medium-carbon alloy steel for large industrial bearings produced by the application meets the following index requirements: (1) The grain size of the steel material is rated according to ISO 643, and the grain size is ≥6 levels.

[0017] (2) The micro-inclusions are tested according to the GB / T 10561 A method, and meet the following requirements: A class fine system ≤1.5, A class coarse system ≤1.0, B class fine system ≤1.0, B class coarse system ≤0.5, C class fine system = 0, C class coarse system = 0, D class fine system ≤1.0, D class coarse system ≤0.5, and DS class ≤1.5.

[0018] (3) The macroscopic defects are tested according to the SEP 1927 water immersion high-frequency flaw detection method, and the total volume of the detection is ≥5 dm 3 , and the macroscopic inclusions meet the requirements that the length of a single defect is ≤5 mm, and the inclusions detection density of each furnace should be ≤16 mm / dm3.

[0019] (4) Macrostructure of the steel is rated according to ASTM E381, meeting the following requirements: center porosity: ≤1.5 grade, center crack: ≤1.0 grade, shrinkage: ≤1.0 grade, no intermediate crack, subsurface crack and subsurface bubble. DETAILED DESCRIPTION

[0020] The application will be further described in detail below with reference to the examples, which are exemplary and intended to explain the application, but cannot be understood as limiting the application.

[0021] The manufacturing process of the medium-carbon alloy steel for large industrial bearing of the examples is as follows: hot metal pretreatment, converter smelting, LF refining, RH refining, CCM continuous casting round billet, annealing and finishing.

[0022] Specifically, before smelting, the hot metal must be pretreated by KR; the molten steel is smelted by converter or electric furnace, clean scrap steel and high-quality raw and auxiliary materials are used, the tapping endpoint C is controlled at about 0.25%, the endpoint P is controlled below ≤0.020%, and the tapping temperature is above 1600℃; low-Ti, low-Ca alloy, high-quality refractory material and high-performance refining synthetic slag are selected in LF refining; sufficient vacuum degree and sufficient circulation treatment time are maintained in vacuum degassing process; protective casting is used in the whole continuous casting process, and intermediate ladle induction heating, light pressing down and electromagnetic stirring technology are used, low superheat casting is used, and the superheat is controlled at ≤25℃; the continuous casting billet should be slowly cooled in the pit, and the slow cooling time is not less than 60 hours; then, the continuous casting round billet is put into the annealing furnace for annealing, the heating speed is ≤60℃ / hour, the temperature is raised to 700℃±10℃, the temperature is kept for 10-15 hours, then the temperature is lowered to 300℃ at a lowering speed of ≤40℃ / hour, and the billet is discharged and cooled.

[0023] The chemical components (wt%) of the medium-carbon alloy steel for large industrial bearing of the examples and (as a comparison) the foreign imported material currently used in the market are shown in Tables 2 and 3.

[0024] Table 2

[0025] Table 3

[0026] From the comparison of the chemical components, it can be seen that the content of harmful elements such as P and S in the examples of the application is much lower than that of the foreign imported material. The content of As, Sn and Sb elements in the examples is lower than that of the imported material. The control of Ti content and O content is better than that of the imported material.

[0027] Table 4 Non-metallic inclusions of the steel of the examples

[0028] From the non-metallic inclusion inspection results, due to the strict control of S content, the number and size control of A coarse and A fine sulfides in the application are far superior to the control level of imported materials from abroad. Due to the strict control of oxygen content and the lower control of Al content in the smelting process, the control of B type, D type and DS type inclusions is superior to imported materials from abroad. The steel material in the application fully meets the high purity requirement of medium carbon alloy steel for large industrial bearings in the purity index.

[0029] Table 5 Water immersion high frequency flaw detection data of steel materials of each embodiment

[0030] Water immersion high frequency flaw detection is carried out according to SEP 1927 method, 5 level sensitivity detection is adopted, and the total volume of detection is 6dm 3 Due to the low S and low Al control in the smelting process, the oxygen content is ensured by adopting vacuum degassing process, and the macroscopic defects are not found in each embodiment of the application by adopting the large cross-section size continuous casting round billet with the same forging ratio, while the imported materials from abroad are found to have macroscopic defects.

[0031] Table 6 Macroscopic data of steel materials of each embodiment

[0032] The macroscopic index of the application and imported materials from abroad is equivalent according to ASTM E381 for testing the macrostructure of steel materials.

[0033] Table 7 Test data of grain size of steel materials of each embodiment

[0034] From the grain size, due to the accurate control of chemical composition, the grain of the application is finer and more uniform than that of imported materials.

[0035] In summary, the medium carbon alloy steel for large industrial bearings produced by the process of vacuum degassing + continuous casting in the application effectively removes harmful non-metallic inclusions through the processes of hot metal pretreatment, high-quality raw and auxiliary materials, high-performance precise smelting slag, vacuum degassing and protective pouring, thereby ensuring high purity of the steel material. By adopting the large cross-section size continuous casting round billet, high purity and high organizational uniformity of the steel material are finally obtained, thereby replacing imported materials from abroad, and the production efficiency, production cost and product quality stability are more competitive.

[0036] In addition to the above embodiments, the application also includes other embodiments, and any technical solutions formed by equivalent transformation or equivalent replacement shall fall within the protection scope of the claims of the application.

Claims

1. A medium carbon alloy steel for bearings, characterized in that: The elemental composition of the steel, by mass percentage, is as follows: C: 0.44–0.50%, Si: 0.15–0.35%, Mn: 0.60–0.90%, Cr: 1.0–1.30%, Ni: 0.5–0.9%, Mo: 0.25–0.40%, V: 0.05–0.10%, Cu≤0.20%, Al≤0.05%, Ca≤0.0008%, S≤0.008%, P≤0.015%, Ti≤0.0020%, O≤0.0012%, As≤0.03%, Sn≤0.03%, Sb≤0.005%, Pb≤0.002%, with the balance being Fe and unavoidable impurities. The grain size is rated according to ISO 643, with a grain size ≥ 6.

2. The medium carbon alloy steel for bearings according to claim 1, characterized in that: Microscopic inclusions are tested according to GB / T 10561A and meet the following requirements: Class A fine inclusions ≤ 1.5, Class A coarse inclusions ≤ 1.0, Class B fine inclusions ≤ 1.0, Class B coarse inclusions ≤ 0.5, Class C fine inclusions = 0, Class C coarse inclusions = 0, Class D fine inclusions ≤ 1.0, Class D coarse inclusions ≤ 0.5, and Class DS inclusions ≤ 1.

5. Macroscopic defects were inspected according to the SEP 1927 water immersion high-frequency flaw detection method, using a sensitivity level of 5, with a total inspection volume ≥5dm. 3 Macroscopic inclusions must meet the following requirements: individual defect length ≤ 5mm, and inclusion detection density per furnace ≤ 16mm / dm. 3 The low-magnification microstructure of the steel is rated according to ASTM E381, meeting the following requirements: central porosity: ≤1.5 grade, central crack: ≤1.0 grade, shrinkage cavity: ≤1.0 grade, and no intermediate cracks, subcutaneous cracks, or subcutaneous bubbles.

3. A method for producing the medium carbon alloy steel for bearings as described in claim 1, characterized in that: The process flow is steel smelting - CCM continuous casting of round billets - annealing - finishing - forging. The specific implementation methods of the main production processes are as follows: Steelmaking: The raw materials for steelmaking pass through KR hot metal pretreatment, converter smelting, LF refining, and RH furnace vacuum degassing in sequence. First, KR hot metal pretreatment removes harmful elements S and P. Second, primary refining is carried out in the converter, mainly to reduce the C content and convert the hot metal into steel. At the same time, the Si and Mn content is adjusted to the design value to further remove the P and S content. Clean scrap steel is added as raw material. The final C content during the steelmaking process is controlled at 0.25%. Based on the final C%, Al iron is added for pre-deoxidation to control the Al content in the refining furnace below 0.05% and the final P content below 0.020%. The steelmaking temperature is above 1600℃. Slag is blocked by slag plugs at the converter tapping and slag is removed after the furnace. During the LF refining stage, the content of alloying elements is precisely adjusted to the design value, and the Ti content in the molten steel is controlled to be ≤0.0020wt%. A refining synthesis slag is used in the LF refining stage, employing a CaO-Al2O3-SiO2 ternary slag system, and argon gas stirring is employed. Refractory materials are selected, the amount of MgO in the molten steel is controlled, and a long LF refining time (≥1 hour) is maintained to ensure that inclusions are fully floated and removed. After LF refining, the molten steel undergoes vacuum degassing in an RH furnace, with a maximum vacuum degree ≤1.5 mbar and a high vacuum time ≥15 min. After degassing, argon gas is used for soft blowing and stirring for ≥25 min to allow inclusions to float and be removed. Continuous casting: Round continuous casting billets are used, with billet specifications of φ600mm and above. Before the continuous casting process, a tundish covering agent with a thickness of ≥100mm is added to the tundish. Protective slag is added throughout the continuous casting process to prevent secondary oxidation of the molten steel. The continuous casting process adopts low superheat, with a pouring superheat of ≤25℃, a pouring speed of 0.25-0.35min / m, and a liquid level fluctuation of ≤5mm. The continuous casting process uses tundish induction heating, light reduction, and electromagnetic stirring technology to improve low-magnification defects such as central porosity and shrinkage cavities. Finishing: The continuously cast round billet is cut by flame and hoisted into the slow cooling pit. After being covered with an insulation cover, it is slowly cooled to 200℃±30℃. Then the cover is opened and the continuously cast round billet is put into the annealing furnace for annealing. The heating rate is ≤60℃ / hour, the temperature is raised to 700℃±10℃, and held for 10-15 hours. Then the temperature is lowered to below 300℃ at a cooling rate of ≤40℃ / hour before being taken out of the furnace and stacked for cooling.

4. The method for producing medium carbon alloy steel for bearings according to claim 3, characterized in that: The continuous casting round billet is forged into round steel using a forging process with a compression ratio ≥6.

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