A steel for high-speed EMU axle box bearings and its production method

Abstract

The present invention relates to a steel for high-speed EMU axle box bearings and a production method thereof, belonging to the technical field of iron-based alloys. The chemical composition of the steel, by mass percentage, is as follows: C: 0.95 - 1.05%, Si: ≤0.10%, Mn: 0.25 - 0.35%, Cr: 1.35 - 1.60%, Mo: ≤0.10%, Ni: 0.20 - 0.25%, Al ≤0.05%, P ≤0.005%, S ≤0.0005%, Cu ≤0.20%, Ca ≤0.0010%, Ti ≤0.0010%, O ≤0.0005%, and the balance is Fe and inevitable impurities. The microscopic brittle inclusions meet the following requirements: B fine series ≤1.0 grade, B coarse series ≤0.5 grade, D fine series ≤1.0 grade, D coarse series ≤0.5 grade, DS series ≤1.0 grade; the carbide network in the structure ≤2.0 grade, the carbide banding ≤2.0 grade, and there is no carbide liquation and microscopic pores. The production process: primary melting and first refining → secondary secondary refining outside the furnace → tertiary secondary refining outside the furnace → vacuum degassing → continuous casting → continuous rolling.

Description

Technical Field

[0001] The present invention belongs to the field of metallurgical technology, and particularly relates to bearing steel and a corresponding production process. Background Art

[0002] In recent years, to meet the requirements of China's railway transportation, the operating speed of railways has been continuously increased, and EMUs with a speed of over 250 km / h are becoming the main models for passenger railway transportation. The running gear of high-speed EMUs, namely the bogie, is the core component to ensure the high-speed, safe and stable operation of the train, and the bearing is one of the key parts. High-speed railway bearings mainly refer to axle box bearings, traction motor bearings and gearbox bearings in high-speed railway locomotives and vehicles. The first requirement of railway transportation is safety, especially for passenger trains, and safe operation is of utmost importance.

[0003] The high-speed EMU axle box bearing has the characteristics of high rotational speed, large load, strong impact, long life and extremely high reliability. Moreover, due to the vast territory of China, the service conditions of high-speed railways are far more demanding and complex than those in foreign countries. The main manifestations are long continuous operation time, large changes in operating environment temperature, the axle box bearing needs to bear the weight of the entire vehicle body and the load, and also needs to bear the forces in all directions generated by the vehicle sway during operation, that is, in addition to bearing static and dynamic radial loads, it also bears non-constant axial loads, making it extremely difficult to manufacture. Therefore, the requirements for the raw material bearing steel for EMU bearings processed into bearings are extremely strict. The bearing steel for EMUs should have high and uniform hardness, high contact fatigue strength, and must also have appropriate toughness and a certain hardenability. The requirements for the uniformity of the chemical composition of the bearing steel, the content of non-metallic inclusions and the distribution of carbides are all very strict. At present, the steel for high-speed EMU axle box bearings generally adopts a process combining vacuum induction + electroslag remelting process to improve the purity of the steel, but its process is complex, production efficiency is low, cost is high, competitiveness is low, and it is difficult to achieve mass production. Summary of the Invention

[0004] Through a new composition design of the bearing steel, the present invention proposes a new type of bearing steel material different from the steel types listed in the current national standard GB / T 18254 (high-carbon chromium bearing steel) for manufacturing the bearing ring of high-speed EMU axle box bearings. In addition, through the innovative optimization of the high-efficiency modern steelmaking processes of vacuum degassing, continuous casting and rolling, the control of high purity and high uniformity of the high-speed EMU axle box bearing material is realized.

[0005] Based on the above innovations, the resulting bearing steel has strong market competitiveness while meeting the quality requirements of high-speed EMU axle box bearing steel.

[0006] The technical solution adopted by the present invention is as follows: a steel for high-speed EMU axle box bearings, the chemical composition of the steel is calculated by mass percentage as C: 0.95 - 1.05%, Si: ≤0.10%, Mn: 0.25 - 0.35%, Cr: 1.35 - 1.60%, Mo: ≤0.10%, Ni: 0.20 - 0.25%, Al ≤0.05%, P ≤0.005%, S ≤0.0005%, Cu ≤0.20%, Ca ≤0.0010%, Ti ≤0.0010%, O ≤0.0005%, and the balance is Fe and inevitable impurities.

[0007] The design basis of the main chemical composition of the steel of the present invention is as follows:

[0008] 1) Determination of C content

[0009] In high-carbon chromium bearing steel, the carbon content is generally about 1.0%, which is one of the most important elements to ensure the hardenability, hardness and wear resistance of bearing steel. However, if the carbon content is higher, it has little effect on the hardness, but it is easy to produce carbides in bearing steel, and it will also reduce the toughness of bearing steel. The C content of the present invention is the same as that of the traditional high-carbon chromium bearing steel GCr15, and the range is determined to be 0.95 - 1.05%.

[0010] 2) Determination of Si content

[0011] Silicon exists as a reducing agent and deoxidizer in the steelmaking process, so more than 0.15% of silicon is added to current high-carbon chromium bearing steels. For example, the silicon content of GCr15 specified in GB / T 18254 is 0.15 - 0.35%. However, in high-carbon bearing steel, silicon increases the overheating sensitivity, crack and decarburization tendency of the steel, and silicon also brings side effects to the cold workability of the steel, reducing the plasticity and toughness of the steel. To reduce the adverse effects of Si, the range of Si content in the present invention is determined to be ≤0.10%.

[0012] 3) Determination of Mn content

[0013] As a deoxidizing element in the steelmaking process, Mn can improve the hardenability of the steel. Mn can also fix the form of sulfur in the steel and form MnS and (Fe, Mn)S with less harm to the performance of the steel, reducing or inhibiting the production of FeS. Therefore, a small amount of manganese in the steel can improve the purity and performance of the steel. However, if the Mn content in the steel is too high, obvious temper brittleness will occur, and Mn has the effect of promoting grain growth. Therefore, it will lead to an increase in the overheating sensitivity and crack tendency of the steel, and a decrease in dimensional stability, which has an adverse impact on customer use. The range of Mn content in the present invention is determined to be 0.25 - 0.35%.

[0014] 4) Determination of Cr content

[0015] Cr is a carbide-forming element that can improve the hardenability, wear resistance, and corrosion resistance of steel. However, when the Cr content is too high, it combines with the carbon in the steel and is prone to form large carbide particles. These insoluble carbides reduce the toughness of the steel and shorten the bearing life. In this invention, the Cr content is designed to be in the range of 1.35 - 1.60%.

[0016] 5) Determination of Al content

[0017] Al is added as a deoxidizing element in steel. Besides reducing the dissolved oxygen in the molten steel, Al forms fine and dispersed aluminum nitride inclusions with N, which can refine the grain size. However, when the Al content is too high, large particle Al2O3 and other brittle inclusions are easily formed during the melting process of the molten steel, reducing the purity of the molten steel and affecting the service life of the finished product. In this invention, the Al content is determined to be ≤0.05%.

[0018] 6) Determination of Ni content

[0019] Ni can improve the strength of steel while maintaining good plasticity and toughness. Adding Ni to steel can significantly enhance the impact resistance of bearing steel. However, on the one hand, Ni is expensive, and on the other hand, when the Ni content in high-carbon steel is too high, the hardness is excessive, which is not conducive to subsequent processing. In this invention, the Ni content is determined to be in the range of 0.20 - 0.25%.

[0020] 7) Determination of Ca content

[0021] The Ca content will increase the number and size of dot-shaped oxides in the steel. At the same time, due to the high hardness and poor plasticity of dot-shaped oxides, they do not deform during the deformation of the steel and are prone to form voids at the interface, deteriorating the performance of the steel. In this invention, the Ca content is determined to be ≤0.001%.

[0022] 8) Determination of Ti content

[0023] The Ti element combines with the N element to form titanium nitride inclusions. Due to their high hardness and sharp angles, they are prone to cause stress concentration during the operation of the bearing, which has a greater impact on the bearing life. Therefore, it is determined that Ti ≤0.0010%

[0024] 9) Determination of O content

[0025] The oxygen content represents the total amount of oxide inclusions. Oxide brittle inclusions limit and affect the service life of the finished product. A large number of experiments show that reducing the oxygen content is significantly beneficial to improving the purity of the steel, especially reducing the content of oxide brittle inclusions in the steel grade. In this invention, the oxygen content is determined to be O ≤0.0005%.

[0026] 10) Determination of P and S contents

[0027] The P element causes element segregation during the solidification of steel. It dissolves in ferrite, distorts and coarsens the grains, and increases cold brittleness. Therefore, it is determined that P ≤ 0.005%. The S element easily causes hot brittleness in steel, reduces the ductility and toughness of steel, and the formed sulfides also destroy the continuity of steel. Therefore, it is determined that S ≤ 0.0005%.

[0028] Since the product is used for high-speed EMU bearings, according to its operating conditions, the requirements for bearing materials are extremely strict. Therefore, the control of harmful elements in steel is significantly stricter than that of traditional bearing steels. As mentioned above, it is particularly required that O ≤ 0.0005%, P ≤ 0.005%, and S ≤ 0.0005%. To meet the above special requirements, special processes are required during the smelting process.

[0029] Determination of the main inspection items and indicators of the present invention:

[0030] The present invention puts forward strict requirements for both micro-inclusions and macro-inclusions. Micro-inclusions include type A and type C plastic inclusions and type B and type D brittle inclusions. Since brittle inclusions are hard particles in steel, stress concentration occurs during the operation of the bearing. At the same time, during the deformation process of steel, they are prone to separate from the matrix and generate cracks, which further exacerbates the stress concentration and is likely to cause cracking. The larger the inclusion particles and the longer the length, the greater the harm; while plastic inclusions are soft particles in steel and are not prone to separate from the matrix during the deformation process of steel, so their harm is relatively small.

[0031] The present invention requires that micro brittle inclusions be fine: B fine series ≤ 1.0 grade, B coarse series ≤ 0.5 grade, D fine series ≤ 1.0 grade, D coarse series ≤ 0.5 grade, DS series ≤ 1.0 grade. The specific requirements are shown in Table 1 below. Macro defects significantly reduce the wear resistance of steel and cause serious stress concentration, which is likely to cause early failure during the use of the bearing. The present invention requires that macro defects be inspected by the SEP 1927 method for immersion high-frequency flaw detection in water, and the total ultrasonic defect index is not allowed to exceed 5 mm / dm 3 , and the maximum length of a single ultrasonic defect is not allowed to exceed 2 mm.

[0032] Table 1

[0033]

[0034] The uniformity and density of the macrostructure of steel have an impact on the life of the bearing. The present invention requires that the macrostructure of the steel be rated according to GB / T 1979 for the macrostructure of steel, and requires that the central porosity ≤ 1.0 grade, general porosity ≤ 1.0 grade, ingot type segregation ≤ 1.0 grade, central segregation ≤ 1.0 grade, and shrinkage cavities, cracks and subsurface bubbles are not allowed to appear.

[0035] To ensure the performance of the steel, the metallographic structure required by the present invention is as follows: Check the carbide network, banding, liquid segregation, and microvoids according to GB / T 18254-2016. Among them, the carbide network is ≤2.0 grade, the carbide banding is ≤2.0 grade, and carbide liquid segregation and microvoids are not allowed to exist.

[0036] Another object of the present application is to provide a production method for the steel used in high-speed train axle box bearings. One of the production keys of the present application is to control the contents of P, S, T, O, etc. in the steel. The traditional smelting method cannot meet such requirements. Accordingly, the present application designs the following production method:

[0037] The entire production process: Electric furnace or converter (primary melting) and first refining → secondary secondary refining outside the furnace → tertiary secondary refining outside the furnace → (VD or RH) vacuum degassing → continuous casting → continuous rolling → finishing → punching and warehousing. The main production steps are as follows:

[0038] Step 1 Primary melting and first refining: Prepare alloy raw materials and hot metal according to the designed element composition, load them into the primary melting furnace, energize and blow oxygen to assist melting. For P removal and Ti removal, it needs to be carried out in a strongly oxidizing range. Strong oxygen blowing can achieve the purpose of P removal and Ti removal. However, if strong oxygen blowing is used, the oxidability of the molten steel is too strong, which is also not conducive to subsequent S removal (S removal requires a low-oxidation precondition). Therefore, the present invention particularly adopts a method of multiple refining to cooperate with primary melting. Through strong oxygen blowing in primary melting, the P and Ti in the converter or electric furnace are controlled at very low contents. Different from the traditional smelting of bearing steel, no deoxidizer such as Al iron is added during tapping. Instead, a P-removing agent is added during tapping, and preferably a P-removing agent with the main components of lime + fluorite is used. Adding lime provides the alkalinity of the molten steel to continue P removal, and adding fluorite aims to improve the fluidity of the steel slag. Otherwise, due to the excessive addition of lime series, the steel slag is too viscous, which is not conducive to continuous P removal.

[0039] Then, transfer the ladle to the refining furnace for first refining, energize and heat (preferably for about 10 minutes) to promote the continuous P removal reaction of the molten steel. Through primary melting and first refining, control the P in the molten steel in the ladle ≤0.005% and Ti ≤0.0010%. However, although this operation reduces P and Ti to very low levels, the oxidability of the molten steel is too strong, and the purpose of low S and low O cannot be achieved. Therefore, after the first refining is completed, the ladle is removed from the refining station, and the strongly oxidizing slag (including the oxidation slag containing Ti and P) of the molten steel is skimmed to reduce the subsequent reabsorption of P by the molten steel. After the slag skimming is completed, re-add the slag used for bearing steel smelting (mainly lime), and transfer it to the refining furnace for secondary refining.

[0040] Step 2 Secondary refining:

[0041] During the whole process of secondary refining, argon is blown from the bottom to stir the molten steel. Slag materials including synthetic slag and lime are added on the surface of the molten steel to form slag, and fluorite is not allowed to be added. The slag materials cover the entire surface of the molten steel. Strong deoxidation is carried out on the molten steel during the process. As a deoxidation method: Al particles are added to the molten steel in multiple times for diffusion deoxidation, and at the same time, Al wire is fed into the molten steel for precipitation deoxidation. It should be particularly noted that since the product is low-silicon steel, it is necessary to avoid using Si-containing alloys for deoxidation as much as possible to prevent the Si content in the molten steel from exceeding the standard. During the secondary refining process, the O content can basically be reduced to the required control level, but the S content still cannot reach the extremely low level of ≤0.0005%. If further desulfurization is required, only the basicity of the molten steel can be increased, that is, lime is continuously added. However, after adding lime to a certain amount, the steel slag becomes too viscous, and fluorite cannot be added (which will bring inclusions), and there is no effect of continuing desulfurization. Therefore, the ladle is removed from the refining station, and the slag on the molten steel is skimmed again, and the slag with poor fluidity is skimmed off. After the slag skimming is completed, the special slag for bearing steel smelting is added again, and it is transferred to the refining furnace for tertiary refining.

[0042] Step 3 Tertiary refining:

[0043] The tertiary refining is basically the same as the secondary refining. The molten steel is continuously deoxidized and desulfurized, and at the same time, alloys are continuously proportioned during the tertiary refining process to make the alloy composition reach the technical requirements. After the tertiary refining, both S and O are reduced to the required ranges, and the chemical composition also meets the requirements. After the tertiary refining is completed, Al is controlled between 0.020 - 0.025%, and the ladle is transferred to the vacuum degassing station for vacuum degassing treatment.

[0044] Step 4 Vacuum degassing:

[0045] The molten steel is degassed under high vacuum conditions. Preferably: the vacuum degree ≤ 133 Pa, and the holding time of the vacuum degree ≥ 15 min. After the vacuum treatment, the Al content is basically controlled between 0.015 - 0.020%.

[0046] It is not allowed to add Ca and Ca-containing alloys after the vacuum degassing of traditional high-carbon bearing steel to avoid producing Ca-containing granular inclusions and reducing the bearing fatigue life. However, through research, when the O and S contents in the molten steel are extremely low, if the Ca wire is fed after the vacuum degassing and the wire feeding amount is appropriate, the inclusions in the molten steel can be modified, so that the original long-strip inclusions are dispersed into small-particle inclusions, and a CaS layer is wrapped outside the inclusions, which has plasticity and can deform with the matrix. Compared with the traditional non-deformable brittle inclusions, the bearing fatigue life can be significantly improved. After the wire feeding is completed, a covering agent is added to the surface of the molten steel to ensure that the molten steel does not contact the air. After the vacuum degassing is completed, soft argon blowing should be carried out to make the molten steel surface fluctuate slightly, so as to ensure that the inclusions in the molten steel continue to float up. The soft argon blowing time is preferably not less than 25 min.

[0047] Step 5 Continuous casting:

[0048] The molten steel is cast into billets by continuous casting. It is preferably to control the whole process of continuous casting to isolate the molten steel from air. The superheat of the molten steel is ≤20°C, the drawing speed is 0.50 - 0.80 m / min, and electromagnetic stirring in the mold, electromagnetic stirring at the end of the casting stream, and soft reduction are adopted during continuous casting to ensure the uniform composition of the molten steel.

[0049] Step 6 Heating, rolling, and finishing

[0050] Heating of the continuous casting billet: The continuous casting billet is directly hot-transported to the heating furnace. As a preferred heating method, the temperature in the preheating section is controlled at 700 - 850°C, the temperature in the heating section is controlled at 1100 - 1250°C, and the temperature in the soaking section is controlled at 1150 - 1250°C to ensure that the billet is heated evenly, thereby improving the carbide segregation of the steel, eliminating carbide liquid segregation. At the same time, to prevent the generation of microvoids and excessive decarburization due to too long heating time, the total heating time is controlled at 10 - 15 hours.

[0051] Rolling the continuous casting billet into finished products: After descaling with high-pressure water, the starting rolling temperature is controlled at 1000 - 1100°C to start rolling the steel in the single-phase austenite region, and then it is rolled through multiple horizontal and vertical rolling mills alternately. The deformation per pass during rolling is controlled at 12% - 18%. During the rolling process, the steel is cooled by water through the steel, and the cooling rate is controlled at 20°C - 30°C / s. The finishing rolling temperature is controlled at 800°C - 830°C. This finishing rolling temperature is within the temperature range of the two-phase region of austenite and carbide. Rolling within this temperature range enables the steel to undergo plastic processing in the two-phase region, making the pre-precipitated carbides and the non-recrystallized austenite simultaneously plastically deformed, refining the grains, reducing carbide aggregation, and improving the carbide segregation of the steel.

[0052] After the finishing rolling, the steel is cooled by water through the steel for the second time, and the cooling rate is 40°C - 50°C / s. This process of rapid cooling enables the steel to quickly jump out of the precipitation region of a large amount of secondary cementite, improving the carbide network. After the water-through cooling, the surface return red temperature of the steel is controlled at 570°C - 650°C. Then the steel is transferred to the cooling bed for normal cooling.

[0053] Finishing: 100% surface and internal flaw detection is carried out on the rolled steel to ensure the quality of the steel.

[0054] The main production process characteristics are as follows:

[0055] 1. Adopt a special smelting method of triple refining to reduce P, S, Ti, and O in the molten steel to extremely low levels.

[0056] 2. Through calcium treatment of bearing steel, the morphology of inclusions in the steel is modified. The brittle inclusions that affect the fatigue life of the bearing are transformed into plastic inclusions that can improve the fatigue life of the bearing, ultimately improving the product life.

[0057] The high-speed EMU axle box bearing steel produced by the present invention meets the following index requirements:

[0058] The microscopic inclusions are inspected according to the GB / T 10561 A method, with B fine series ≤ 1.0 grade, B coarse series ≤ 0.5 grade, D fine series ≤ 1.0 grade, D coarse series ≤ 0.5 grade, and DS series ≤ 1.0 grade; for the macroscopic defects, the SEP 1927 method is required for immersion high-frequency flaw detection inspection, and the total ultrasonic defect index is not allowed to exceed 5 mm / dm. 3 For the macrostructure, the GB / T 1979 is used to grade the macrostructure of the steel, with center porosity ≤ 1.0 grade, general porosity ≤ 1.0 grade, ingot type segregation ≤ 1.0 grade, center segregation ≤ 1.0 grade, and shrinkage cavities, cracks, and subcutaneous air bubbles are not allowed. The steel of the invention is inspected for carbide network, banding, liquid segregation, and microscopic porosity according to GB / T 18254-2016. Among them, the carbide network ≤ 2.0 grade, the carbide banding ≤ 2.0 grade, and carbide liquid segregation and microscopic porosity are not allowed. Detailed implementation manners

[0059] The following further describes the present invention in detail with reference to the embodiments. The embodiments are exemplary and are intended to explain the present invention, but should not be construed as limiting the present invention.

[0060] The chemical compositions (wt%) of the GCr15 produced by the vacuum induction + electroslag remelting process used in each embodiment of the present invention and (for comparison) in the current market are shown in Table 2.

[0061] Table 2

[0062] Example C Si Mn P S Cr Cu Ni Al The present invention 1 0.95 0.03 0.25 0.0029 0.0003 1.38 0.02 0.21 0.015 The present invention 2 0.97 0.09 0.28 0.0025 0.0004 1.45 0.02 0.24 0.018 The present invention 3 1.03 0.07 0.34 0.0023 0.0004 1.57 0.03 0.21 0.016 Comparative steel GCr15 0.97 0.26 0.35 0.018 0.003 1.45 0.03 0.02 0.01

[0063] Table 2

[0064] Example Mo As Sn Pb Nb Ca Ti O The present invention 1 0.02 0.003 0.0051 0.001 0.04 0.0001 0.0007 0.00038 The present invention 2 0.02 0.002 0.0048 0.001 0.04 0.0002 0.0007 0.00043 The present invention 3 0.03 0.002 0.0030 0.001 0.05 0.0002 0.0008 0.00040 Comparative steel GCr15 0.02 0.004 0.0057 0.001 0.001 0.0002 0.0007 0.00058

[0065] Tables 3-5 show the comparison of the inspection situations of each embodiment and the above-mentioned comparison steel in terms of non-metallic inclusions, carbides, microscopic porosity, and macrostructure.

[0066] Table 3 shows the detection situations of non-metallic inclusions of each embodiment and the comparison steel.

[0067]

[0068] Table 4 shows the carbide banding, carbide network, carbide liquid segregation, and microscopic porosity of each embodiment.

[0069] Network carbide Banded carbide Liquid segregation carbide Microscopic porosity Example 1 of the present invention 1.5 1.0 0 None Example 2 of the present invention 1.5 1.5 0 None Example 3 of the present invention 1.5 1.5 0 None Comparative steel 1.5 1.0 0 None

[0070] Table 5 shows the macrostructure data of each embodiment of the steel.

[0071] Central porosity General porosity Ingot segregation Central segregation Crack Shrinkage cavity Subcutaneous blowhole Example 1 of the present invention 0.5 0.5 0.5 0.5 None None None Example 2 of the present invention 0.5 0.5 0.5 0.5 None None None Example 3 of the present invention 0.5 0.5 0.5 0.5 None None None Comparative steel 0.5 0.5 0.5 0.5 None None None

[0072] The present invention will be further described in detail below in conjunction with embodiments.

[0073] The manufacturing process of the high-speed EMU axle box bearing steel in the embodiment of the present invention is to produce steel by using a top and bottom combined blown converter BOF (high-power electric arc furnace EAF) - ladle furnace LF - vacuum circulation degassing furnace RH (VD furnace) - large-section continuous casting CCM - hot delivery of continuous casting billets - heating and rolling into finished products - finishing process.

[0074] Specifically, high-quality molten iron, scrap steel and raw and auxiliary materials are selected during smelting, and high-quality deoxidizers and refractory materials are selected. During the production process of the electric furnace / converter, high-quality molten iron and high-quality alloys are proportioned according to the element composition described in the present invention, and the raw materials are charged into the electric furnace or converter, and oxygen is blown to assist melting. Then, the operations of dephosphorization, desulfurization and deoxidation are carried out according to the aforementioned method of three slag skimmings + three refinings to make the harmful elements meet the technical requirements. After the vacuum degassing is completed, according to the specific situation, 80 - 100 m of Ca-Si wire is fed respectively to realize the modification treatment of inclusions. The continuous casting superheat is controlled within ≤18°C, and the continuous casting drawing speed is controlled at 0.45 - 0.55 m / min. The heating time of the continuous casting billet in the walking beam reheating furnace is about 13 hours, and the actual temperature in the soaking section is controlled within the range of 1220 ± 10°C. During the rolling process, the final rolling temperature is controlled by water cooling, and the final rolling temperature is controlled at 800 - 830°C. After rolling, water cooling is continued to control the steel return red temperature, and the return red temperature is controlled within the range of 590 ± 20°C. The carbide network is improved, and then subsequent finishing and warehousing are carried out.

[0075] Combined with the inspection conditions in Tables 2 - 5:

[0076] From the perspective of composition, the main chemical composition difference between the present invention and the comparative steel is that there is a significant difference in the composition of Si and Ni, the elements that affect the plasticity and toughness of steel. The present invention reduces Si, an element that is unfavorable to the toughness and plasticity of steel, while increasing Ni, an element that is beneficial to the plasticity and toughness of steel, thereby improving the toughness and plasticity of the material, so that it can better meet the requirements of high-speed EMU axle box bearing steel. In addition, in terms of the control of harmful elements such as P, S, and O, the present invention is significantly better than the comparative steel. From the results of low-power, microscopic pores, and carbide liquid analysis tests, the present invention and the comparative steel have equivalent indicators. Judging from the inspection results of non-metallic inclusions and carbide bands and networks, the present invention is slightly inferior to the GCr15 comparison steel produced by vacuum induction + electroslag remelting. Although the vacuum induction and electroslag remelting process can significantly improve the purity of the material and improve the segregation of steel, its process is complicated, the production efficiency is low, and the cost is high. The present invention adopts vacuum degassing and continuous casting process for production, which has significant advantages in production efficiency and cost. Although the corresponding indicators are slightly worse than those of products produced by vacuum induction + electroslag remelting process, it can fully meet the requirements of high-speed EMU axle box bearing steel, and has a very large competitive advantage.

[0077] Based on the above analysis, the high-speed train axle box bearing steel in each embodiment of the present invention has the following differences compared with the current products:

[0078] 1. The main chemical composition design is different.

[0079] 2. The control of harmful elements is significantly better than that of the comparison steel.

[0080] 3. The production process is different. The vacuum degassing + continuous casting process is used to replace the current vacuum induction + electroslag remelting process. On the premise of fully meeting the requirements of steel, it improves production efficiency and steel yield rate, reduces production costs, and has better market competitiveness.

[0081] In summary, the present invention relates to a bearing steel for axle boxes of high-speed EMUs and a production method thereof. Through a new design of the chemical composition of the steel, a high-efficiency, high-capacity, and low-cost process route of vacuum degassing, continuous casting, and rolling is adopted, and key processes are optimized, studied, and controlled. Thus, the steel has a high purity and a good segregation control quality level, making it more competitive in production efficiency, production cost, and product quality stability.

Claims

1. A production method for steel used in the axle box bearings of high-speed EMUs, characterized in that: The chemical composition of the steel, by mass percentage, is C: 0.95 - 1.05%, Si: ≤0.10%, Mn: 0.25 - 0.35%, Cr: 1.35 - 1.60%, Mo: ≤0.10%, Ni: 0.20 - 0.25%, Al ≤0.05%, P ≤0.005%, S ≤0.0005%, Cu ≤0.20%, Ca ≤0.0010%, Ti ≤0.0010%, O ≤0.0005%, and the balance is Fe and unavoidable impurities; Production process: primary melting and first refining → secondary secondary refining outside the furnace → tertiary secondary refining outside the furnace → vacuum degassing → continuous casting → continuous rolling, and the main steps are as follows: Step 1 Primary melting and first refining: Prepare alloy raw materials and hot metal according to the designed elemental composition, load them into the primary melting furnace, energize and blow oxygen to assist melting, strongly blow oxygen to remove P and Ti, control P and Ti at very low levels, and add P-removing agents instead of deoxidizers when tapping the primary melt to continue removing P from the molten steel; then transfer the ladle to the refining furnace for the first refining, energize and heat to promote the continuous P-removing reaction of the molten steel, and control P ≤0.005% and Ti ≤0.0010% in the molten steel in the ladle through primary melting and the first refining. At this time, the oxidizability of the molten steel is extremely strong. After the first refining is completed, remove the ladle from the refining station, skim the strongly oxidizing slag from the molten steel. After skimming, add the slag for bearing steel smelting again and transfer it to the refining furnace to prepare for the second refining; Step 2 Secondary refining: During the whole process of secondary refining, the molten steel is stirred by bottom blowing argon. Slag materials including synthetic slag and lime are added on the surface of the molten steel to make slag, and fluorite is not allowed to be added. The slag materials cover the entire surface of the molten steel. The molten steel is strongly deoxidized during the process, and silicon alloy is avoided as a deoxidizer for deoxidation. During the secondary refining process, after the O content is reduced to the required control level, the S content generally has not reached the control level yet. The ladle is removed from the refining station, and the molten steel is skimmed again to remove the slag with poor fluidity. After skimming, add the special slag for bearing steel smelting again and transfer it to the refining furnace for the third refining; Step 3 Tertiary refining: The third refining continues to deoxidize and desulfurize the molten steel, and at the same time, the alloy is continuously proportioned during the third refining to make the alloy composition reach the designed range. After the third refining, after O reaches the control level, S is also reduced to the required range, and the chemical composition also meets the design requirements. After the third refining, Al is controlled between 0.020 - 0.025%. The ladle is transferred to the vacuum degassing station for vacuum degassing treatment; Step 4 Vacuum degassing: The molten steel is degassed under high vacuum conditions. After the vacuum treatment, the Al content is basically controlled between 0.015 - 0.020%. Under the condition of extremely low O and S contents in the molten steel, after vacuum degassing, a Ca wire is fed into the molten steel to modify the inclusions in the molten steel, so that the original long strip inclusions are dispersed into small particle inclusions, and a CaS layer is wrapped outside the inclusions. After wire feeding, a covering agent is added on the surface of the molten steel to isolate the molten steel from the air. After vacuum degassing, soft blowing argon makes the liquid level of the molten steel fluctuate slightly, and the inclusions in the molten steel continue to float up; Step 5 Continuous casting: The molten steel is cast into a steel billet by continuous casting process; Step 6: Heating and rolling Heating of continuous casting billet: The continuous casting billet is directly hot-transported to the heating furnace for heating, and the billet is fully and evenly heated, and then rolled into finished products; Rolling of continuous casting billet into finished products: After the heated continuous casting billet is descaled by high-pressure water, the starting rolling temperature is controlled at 1000 - 1100 °C, so that the steel starts to be rolled in the austenite single-phase region, and then is rolled alternately through multiple horizontal and vertical rolling mills. The deformation amount of each pass in the rolling process is controlled at 12% - 18%. During the rolling process, the steel is subjected to water-cooling through the water, and the cooling rate is controlled at 20 °C - 30 °C / s. The final rolling temperature is controlled at 800 °C - 830 °C, and this final rolling temperature is within the temperature range of the austenite and carbide two-phase region; After the final rolling, the steel is subjected to secondary water-cooling through the water, and the cooling rate is 40 °C - 50 °C / s, so that the steel quickly jumps out of the precipitation region of a large amount of secondary cementite. After the water-cooling through the water ends, the surface return red temperature of the steel is controlled at 570 °C - 650 °C, and then the steel is transferred to the cooling bed for normal cooling.

2. The method according to claim 1, characterized in that: In Step 1, a de-P agent mainly composed of lime and fluorite is added during the initial melting and tapping. Adding lime provides the alkalinity of the molten steel for continuous de-P, and adding fluorite improves the fluidity of the steel slag.

3. The method according to claim 1, characterized in that: In Step 1, after the first refining is completed, the molten steel is slagged off. After the slagging-off is completed, lime is added as one of the smelting slags, and the second refining is started; after the second refining is completed, the molten steel is slagged off, and lime is added again as one of the smelting slags after the slagging-off is completed.

4. The method according to claim 1, wherein: In Step 2, during the deoxidation process of the second refining, Al particles are added to the molten steel in multiple times for diffusion deoxidation, and at the same time, the method of feeding Al wire into the molten steel is used for precipitation deoxidation.

5. The method according to claim 4, characterized in that: In Step 4, the vacuum degree ≤ 133 Pa, the vacuum holding time ≥ 15 min, and the soft blowing argon time after vacuum degassing is not less than 25 min.

6. The method according to claim 4, wherein: In Step 5, the whole continuous casting process is controlled to isolate the molten steel and air. The superheat of the molten steel ≤ 20 °C, the drawing speed is 0.50 - 0.80 m / min. During the continuous casting process, mold electromagnetic stirring, electromagnetic stirring at the end of the casting stream, and soft reduction are adopted to ensure the uniform composition of the molten steel.

7. The method according to claim 1, wherein: In Step 6, the heating method of the continuous casting billet in the heating furnace: the temperature of the preheating section is controlled at 700 - 850 °C, the temperature of the heating section is controlled at 1100 - 1250 °C, the temperature of the soaking section is controlled at 1150 - 1250 °C, and the total heating time is controlled at 10 - 15 hours.

8. The method according to claim 1, characterized in that: The micro brittle inclusions of the steel satisfy: B fine series ≤ grade 1.0, B coarse series ≤ grade 0.5, D fine series ≤ grade 1.0, D coarse series ≤ grade 0.5, DS series ≤ grade 1.0; The macroscopic defects satisfy: inspected by water immersion high-frequency flaw detection using SEP 1927 method, the total ultrasonic defect index is not allowed to exceed 5 mm / dm 3 , and the maximum length of a single ultrasonic defect does not exceed 2 mm; The macrostructure satisfies: the macrostructure of the steel is rated according to GB / T 1979, center porosity ≤ grade 1.0, general porosity ≤ grade 1.0, ingot type segregation ≤ grade 1.0, center segregation ≤ grade 1.0, and no shrinkage cavity, crack or subsurface air bubble appears.

9. The method according to claim 1, characterized in that: Metallographic structure of steel: Check the carbide network, banding, liquid segregation and microvoids according to GB / T 18254-2016. Among them, the carbide network ≤ 2.0 grades, the carbide banding ≤ 2.0 grades, and there is no carbide liquid segregation and microvoids.

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