EP4186990A1

EP4186990A1 - Steel for ball-cage type universal joint retainer and production method therefor

Info

Publication number: EP4186990A1
Application number: EP21922314.6A
Authority: EP
Inventors: Min Chen; Yun Bai; Yuandong LUO; Xiaolin Wu; Qing Yin; Wenbin Li; Liukai HUA; Ye Liu
Original assignee: Jiangyin Xingcheng Special Steel Works Co Ltd
Current assignee: Jiangyin Xingcheng Special Steel Works Co Ltd
Priority date: 2021-01-28
Filing date: 2021-09-05
Publication date: 2023-05-31
Also published as: WO2022161180A1; CN112981237B; EP4186990A4; WO2022160720A1; CN112981237A; JP7766692B2; JP2024502743A

Abstract

The present invention belongs to the technical field of special steel smelting, and relates to a steel for a cage of a ball-cage universal joint and a production method therefor. The steel comprises the following chemical ingredients by wt%: 0.10-0.25% of C, 0.20-0.40% of Si, 0.40-0.65% of Mn, 0.40-0.70% of Cr, 0.0003-0.0025% of B, 0.010-0.035% of Ti, 0.30-0.45% of Mo, 0.0050-0.0100% of N, ≤0.015% of S, ≤0.025% of P, ≤0.25% of Ni, ≤0.30% of Cu, 0.015-0.035% of Al, ≤0.0010% of O, ≤ 0.04% of As, ≤ 0.03% of Sn, ≤0.005% of Sb, ≤0.002% of Pb, and the balance being Fe and unavoidable impurities. The microstructure of the steel is bainite, and the grain size of austenite G≥6. The production process comprises primary smelting of molten steel, refining of molten steel, vacuum degassing of molten steel, continuous casting, hot rolling and finishing. In the present application, the chemical ingredients are optimized, and the alloy costs are reduced. Furthermore, a hardenability similar to that of 20CrMnTi is achieved, the strength and toughness of the steel, manufactured by using the chemical ingredients combined with the production method, are better than the strength and toughness of 20CrMnTi, and the comprehensive performance meets the requirements of the steel for the cage of the ball-cage type universal joint.

Description

Technical Field

The invention relates to the technical field of special steel smelting, in particular relates to a steel for a ball-cage universal joint cage and a production method thereof.

Background Art

At present, a constant-velocity universal joint commonly used in cars is a ball-cage universal joint. The function of the ball-cage universal joint is to transmit the power of the engine from the transmission to two front wheels, so as to drive the car to go at high speed. It mainly comprises following main components like a spherical shell, a star sleeve, a supporting cage (ball cage), steel balls, etc. The constant-velocity universal joint has to transmit heavy driving torque and to bear heavy load, require high transmission precision, has a large demand in market, and at the same time forms a safety component in the car, therefore it has very high quality requirements for products.
The competition in modern automobile industry is becoming increasingly fierce, and there are higher requirements regarding power, operability, comfort and safety of automobiles. These requirements may be also coupled with the requirements regarding energy and environmental methods. Thus, when designing important functional parts of automobiles, it is necessary to comprehensively consider important indicators such as safety, functionality, economy and emission. Therefore, higher requirements are put forward for materials, wherein materials are required to be lighter on the premise of ensuring performance. For the materials used in automobile universal joints, the components play the role of transmission and support, and also suffer long-term actions of alternating load stresses, so the materials must have sufficient wear resistance, fatigue resistance and good toughness.
When a ball-cage universal joint is working, especially when its load is complex and it rotates at high speed, the cage has to bear a great amount of centrifugal force, shock and vibration, there is a great amount of sliding friction between the cage and the rolling balls, and thus generates a great amount of heat. The result of a combined action of force and heat leads to cage failures, and in severe cases it will cause burns and fractures of the cage. Therefore, the cage material is required to have good thermal conductivity, good wear resistance, small friction coefficient, low density, a certain combination of strength and toughness, good elasticity and stiffness, an expansion coefficient similar to that of rolling balls, and good processing performance. In addition, the cage is also subjected to chemical media, such as lubricants, lubricant additives, organic solvents and coolants, so it is also required to have certain corrosion resistances.
Currently, the cage of a ball-cage universal joint is usually made of 20CrMnTi. After carburizing, the steel grains are refined and uniform. On the surface the steel has good tensile and bending fatigue strength. In the core it has sufficient strength and toughness, which improves the wear resistance. However, its cost is high, and its strength and toughness do not have much extra amount.

Content of Invention

The steel of the present invention is designed on the basis of 20CrMnTi, by modifying the contents of alloy elements Mn and Cr, and by adding B and Mo elements. We further improve the production process, so as to achieve a hardenability similar to that of 20CrMnTi under the premise of optimized costs. Meanwhile, the strength and toughness of our product are better than those of 20CrMnTi. Finally our product meets the requirements of a steel for a cage of a ball-cage universal joint. Our product belongs to a carburized steel.
To be specific, the steel of the present application has a microstructure of bainite, with austenite grain size G≥6. Yield strength ≥850 MPa, tensile strength ≥1080 MPa, elongation ≥10%, Charpy impact energy at room temperature AK_U≥55 J. The hardenability of an end of the steel is evaluated according to the method of GB/T 225, wherein the result meets at J5 point: 35-42 HRC, at J9 point: 25-35 HRC, at J13 point: 20-30 HRC. The banded structures in steel are graded according to GB/T 13299, and the banded structures do not exceed grade 2.0. Non-metallic inclusions are graded according to the method A in GB/T 10561, wherein the result meets that Fine inclusions in Group A≤1.5, Thick inclusions in Group A≤1.0, Fine inclusions in Group B≤1.5, Thick inclusions in Group B≤0.5, Fine inclusions in Group C = 0, Thick inclusions in Group C = 0, Fine inclusions in Group D≤1.0, Thick inclusions in Group D≤0.5, Ds≤1.5. The low-magnification structure of steel is graded according to ASTM E381, which meets C≤grade 2.0, R≤grade 2.0, S≤grade 2.0.
The steel of the present application having the above-mentioned properties finally meets the requirements for the use as a steel for a cage of a ball-cage universal joint.
In order to solve the problem, the technical solution adopted in the present invention is: a steel for a cage of a ball-cage universal joint, characterized in that the chemical composition of the steel in wt% is C: 0.10-0.25%, Si: 0.20-0.40%, Mn: 0.40-0.65%, Cr: 0.40-0.70%, B: 0.0003-0.0025%, Ti: 0.010-0.035%, Mo: 0.30-0.45%, N: 0.0050-0.0100%, S≤0.015% , P≤0.025%, Ni≤0.25%, Cu≤0.30%, Al: 0.015-0.035%, O≤0.0010%, As≤0.04%, Sn≤0.03%, Sb≤0.005%, Pb≤0.002%, and the balance is Fe and unavoidable impurities.
The setting of the chemical composition of this application is based on the following:

1) Determining C content
C is necessary to ensure the wear resistance of a steel. Increasing the carbon content in steel will increase its martensitic transformation ability, thereby increasing its hardness and strength, thereby improving wear resistance. However, a too high content of C is detrimental to the toughness of steel. In addition, a too high content of C will also lead to serious central segregation of C, which will affect the core toughness of the steel. The present invention controls the content of C to be 0.10-0.25%.
2) Determining Si content
Si is a key element in the present invention. Si forms solid solution in the ferrite phase and has an effect of strongly strengthening the solid solution, which can significantly increase the strength of ferrite, but at the same time reduce the plasticity and toughness of ferrite. The range of Si content in the present invention is set to be 0.20-0.40%.
3) Determining Mn content
Mn is a deoxidizing element in the steelmaking process. Mn is an element effective in strengthening steel, and plays a role of strengthening the solid solution. Moreover, Mn can improve the hardenability of steel and improve the hot workability of steel. Mn can eliminate the influence of S (sulfur). In smelting, Mn and S can form MnS which has a high melting point, thereby weakening and eliminating the adverse effects of S. However, a high Mn content will reduce the toughness of steel. The Mn content of the present invention is controlled to be 0.40-0.65%.
4) Determining Cr content
Cr is an element that can form carbides and can improve the hardenability, the wear resistance and the corrosion resistance of steel. However, when the Cr content is too high, the hardness of the steel is too high, which is not conducive to customers' processing and use. Based on the above analysis, the range of Cr content in the present invention is set to be 0.40-0.70%.
5) Determining Al content
Al is added as a deoxidizing element in steel. In addition to Al reducing dissolved oxygen in molten steel, Al and N form dispersed fine aluminum nitride inclusions, so as to refine grains. However, when the Al content is too high, brittle inclusions such as large particles of Al₂O₃ are easily formed during the smelting process of molten steel, which reduces the purity of the molten steel and affects the service life of the final product. The range of Al content in the present invention is set to be 0.015-0.035%.
6) Determining B content
B can improve the hardenability of steel, and can also improve the high-temperature strength of steel, and can strengthen the grain boundary in steel. The range of B content in the present invention is set to be 0.0003-0.0025%.
7) Determining Mo content
Molybdenum can refine the grains of steel, improve hardenability and thermal strength, and maintain sufficient strength and creep resistance at high temperatures. Adding molybdenum to steel can improve mechanical properties, and can also inhibit the brittleness caused by tempering of alloy steel. However, molybdenum is an element that forms ferrite. When the molybdenum content is too high, the ferrite δ phase or other brittle phases are prone to appear, thereby reducing the toughness. In the present invention the range of Mo content is set to be 0.30-0.45%.
8) Determining Ti content
Titanium is a strong deoxidizer in steel. It can make the internal steel structure dense and refine the grains. However, Ti will form titanium carbonitride inclusions in steel, which are hard and angular, which seriously affect the fatigue life of the material. The range of Ti content in the present invention is set to be 0.01-0.035%.
9) Determining N content
Nitrogen can improve the strength, the low-temperature toughness and the weldability of steel, and increase ageing sensitivity. Adding an appropriate amount of aluminum to steel can generate stable AIN, which can suppress the formation and precipitation of Fe₄N. This can not only improve the ageing performance of the steel, but also prevent austenite grains from growing, and thus play a role of refining the grains. However, nitrogen and alloying elements in steel will form non-metallic inclusions as nitrides, and more importantly, nitrogen will reduce the effect of alloying elements. When the nitrogen content in the steel is high, the strength of steel increases and the impact toughness decreases. The N content of the present invention is set to be 0.0050-0.0100%
10) Determining O content
The content of oxygen represents the total amount of oxide inclusions. Brittle oxide inclusions will restrict the service life of a finished product. A large number of tests have shown that reducing the oxygen content is significantly beneficial to improving the purity of steel, especially in reducing the brittle oxide inclusions in steel. The range of oxygen content in the present invention is set to be ≤0.0010%.
11) Determining contents of P and S
P may cause segregation in steel during solidification, and P dissolves in ferrite, which distorts and coarsens grains, and increases cold brittleness. The range of P content in the present invention is set to be ≤0.025%. S causes hot brittleness of steel and reduces the ductility and toughness of steel, but S can improve the cutting performance of steel. The range of S content in the present invention is set to be ≤0.015%.
12) Determining contents of As, Sn, Sb and Pb
As, Sn, Sb, Pb and other trace elements are all non-ferrous metals with low melting points. Their presence in steel will cause soft spots on the surface and uneven hardness, so they are regarded as harmful elements in steel. The ranges of the contents of these elements in the present invention are set to be As≤0.04%, Sn≤0.03%, Sb≤0.005%, and Pb≤0.002%.

A production process of the above steel for the cage of ball-cage universal joint is as follows: electric furnace or converter (primary smelting) → LF refining → VD or RH vacuum degassing → continuous casting → rolling → finishing → packaging for storage.
The main production process is as follows:

(1) Smelting of molten steel:
The primary smelting uses high-quality molten iron, scrap steel and raw/auxiliary materials, and reduces the content of harmful elements in molten steel. Deoxidation should be enhanced during the smelting process, and the carbon content at the end point (the tapping) of the electric furnace or converter should be controlled. The carbon content at the end point (the tapping) should be controlled to be 0.05-0.15%. During the tapping process, Al iron should be added for pre-deoxidation, so as to create good conditions for subsequent deoxidation. After tapping, a slag removal technology is used to remove harmful steel slags.

When refining, new synthetic slags are added to the LF refining furnace, and at the same time the deoxidation is enhanced in the refining process. The refining process uses silicon carbide and aluminum to deoxidize, so that white slags are formed as soon as possible in the early stage of refining, and white slags are kept for more than 25 minutes. The aluminum content in the refining process is controlled to be 0.025%-0.045%, to ensure the deoxidation effect.
The steel of the present application belongs to such a steel type that is sensitive to cracks. Therefore, vacuum degassing should be enhanced. It is processed under high vacuum (below 133 Pa) for ≥15 min, so as to ensure that harmful gas H ≤2 ppm. After vacuum degassing, silicon-calcium wires should be fed, to change the inclusions. After vacuum degassing, soft blowing of argon should be performed for a long time, to ensure that inclusions are fully floating up, wherein the time of soft blowing of argon is ≥ 25 min.

(2) Continuous casting:

In the continuous casting process, anti-oxidation protection is carried out throughout the process (that is, the molten steel is isolated from the air), to reduce the number of inclusions in the steel. In addition, high-quality materials are used to reduce the pollution of foreign inclusions to molten steel and thus improve the control of the production process.
In the continuous casting process, electromagnetic stirring and soft reduction technology are used. By adjusting the pressure distribution of the soft reduction rollers, the center of the steel can be fully filled with molten steel when the molten steel solidifies, so as to avoid shrinkage cavities. At the same time, a mold electromagnetic stirring and a final electromagnetic stirring is enhanced, so that the solidification flow field of molten steel is changed, the internal structure of continuous casting molten steel is improved, and the segregation is reduced.
In the continuous casting process, low-superheat pouring is used, and the superheat of the continuous casting is controlled to be 10-30°C, which can effectively modify and reduce the segregation in the continuous casting billet. By the continuous casting, we obtain a continuous casting billet with a size of 300mm×300mm or larger, and the billet is consistent with the chemical composition of the target steel product; the continuous casting billet should be cooled slowly in a pit, so that the continuous casting billet can be prevented from cracking. The slow cooling lasts for no less than 24 hours. Thereafter, the continuous casting billet is sent to a step-by-step heating furnace for heating, and then rolled into the target product.

(3) Rolling:

Before rolling, the continuous casting billet is heated in the furnace: in a section of preheating, the temperature is controlled to be 600-850 °C, in a section of heating, the temperature is controlled to be 950-1100 °C, and in a section of soaking, the temperature is controlled to 1150-1200 °C. In order to ensure that the billet is fully and evenly heated, the total heating time is more than 240 minutes, and the time in the soaking section is more than 180 minutes. The start rolling temperature is controlled to be 950°C-1050°C, the final rolling temperature is controlled to be 800°C-900°C, and the entire rolling process is carried out in the austenite single-phase region. In order to realize the transformation from austenite to bainite after rolling, slow cooling should not be used in the process (from ending of final rolling to arriving at cooling bed), to prevent the appearance of coarse ferrite grains, which will reduce the strength and toughness of the steel. Nevertheless, it should not be cooled too quickly, so as to prevent the appearance of martensitic structure, which will also reduce the toughness of steel.
In the present invention, the cooling rate in the section from ending of final rolling to arriving at cooling bed is designed to be 10-15 °C/s. And at the same time, the rolling speed is relatively reduced, so as to control the passing time of steel in the section, so that the steel can be fully transformed into bainite structure. The temperature of the steel arriving at the cooling bed is finally controlled to be 600-650 °C. Since at this time the metallographic structure in steel has been basically completed the bainite transformation, the subsequent cooling on the cooling bed can be performed at a normal cooling rate, the cooling rate is 15-20 °C/min. Thereafter, the steel is removed off the production line. After straightening and flaw detection, a target bar product is obtained.
Compared with the prior art, the present invention has the following advantages.

(1) The steel of the present invention is designed on the basis of 20CrMnTi, by reducing the contents of alloy elements Mn and Cr, and harmful element Ti. Since the reduction of the contents of Mn and Cr elements will inevitably affect the hardenability of the steel, the invention simultaneously adds a trace amount of B element with a certain amount of alloying element Mo in the steel, so as to increase the hardenability, so that the hardenability of the steel of the invention is not worse than that of 20CrMnTi. In addition, by reducing the Ti element which is easy to form hard and non-deformable inclusions, and by adding N element which can also refine the grain, the grain size of the steel of the invention is equivalent to that of 20CrMnTi, and the purity of the steel is improved. At the same time, by controlling the transformation of the metallographic structure during the rolling process, it is ensured that the finished steel will form a bainite structure, so as to ensure that the strength and toughness of the steel of the invention are not lower than those of 20CrMnTi, and finally meet the requirements of steel for a cage of ball-cage universal joint.
(2) As mentioned above, the present invention enhances deoxidation and dehydrogenation during the smelting process, and at the same time uses high-quality raw materials to ensure high purity of steel. Continuous casting adopts a low-superheat pouring, and uses electromagnetic stirring and soft reduction control to control the steel segregation. The final product can effectively meet the requirements for steel for cages of ball-cage universal joints.

Description of Drawings

Fig. 1 is a picture of a typical metallographic structure of the embodiment of the present invention ×100;
Fig. 2 is a picture of a typical metallographic structure of the comparative example ×100.

Detailed Examples

The present invention will be described in further detail below in conjunction with the accompanying drawings. The embodiments are exemplary and intended to explain the present invention, but should not be understood as a limitation of the present invention.

The chemical composition (wt%) of the steel in each embodiment of the present invention is shown in Table 1 and Table 2, and is compared with the chemical composition of a comparative example of steel 20CrMnTi.

Table 1

	Example	C	Si	Mn	P	S	Cr	Cu	Ni	Al	Mo
Invention	1	0.15	0.29	0.56	0.015	0.004	0.6	0.02	0.02	0.027	0.34
Invention	2	0.17	0.30	0.57	0.016	0.002	0.59	0.04	0.02	0.023	0.38
Invention	3	0.19	0.30	0.60	0.015	0.001	0.62	0.02	0.03	0.025	0.35
Comparative Example		0.20	0.26	0.95	0.016	0.004	1.16	0.02	0.02	0.024	0.01

Table 2

	Example	B	As	Sn	Sb	Pb	N	Ti	O
Invention	1	0.0011	0.002	0.003	0.002	0.001	0.0071	0.0229	0.00092
Invention	2	0.0014	0.003	0.002	0.002	0.001	0.0080	0.025	0.00088
Invention	3	0.0013	0.002	0.002	0.001	0.002	0.0078	0.027	0.00085
Comparative Example		0.0001	0.005	0.002	0.001	0.002	0.0035	0.0515	0.0013

The data of the mechanical properties of the steels of each embodiment and comparative example are shown in the Table 3 for comparison.

Table 3

	Yield Strength Rel (MPa)	Tensile Strength Rm (MPa)	Elongation A₅ (%)	Charpy Impact Property AKu (at room temperature)
Invention	1078	1184	13%	104
Invention	1083	1195	12%	112
Invention	1068	1182	12%	105
Comparative Example 20CrMnTi	980	1160	12%	95

The data regarding hot-rolled metallographic structure, banded structure, grain size of the steels of each embodiment and comparative example are shown in Table 4.

Table 4

	Example	metallographic structure	grain size (G)	banded structure
Invention	1	Bainite	7	1.5
Invention	2	Bainite	7	2.0
Invention	3	Bainite	7	1.5
Comparative Example	20CrMnTi	Ferrite + Pearlite + Bainite	7	2.0

The typical structure of the steel in the examples of the present invention and that of the comparative example can be seen in Fig. 1 and Fig. 2 respectively. The structure shown in Fig. 1 is bainite, and the structure shown in Fig. 2 is ferrite + pearlite + bainite.
The comparison of the end hardenability properties of the steels of each example and comparative example can be seen in Table 5, wherein the unit is HRC. Table 5

J5 J9 J13

Example 1 of the invention 40.5 32 27

Example 2 of the invention 41.5 31.5 26

Example 3 of the invention 40 32.5 27.5

Comparative Example 20CrMnTi 41 31 26.5

The comparison of the inclusions of the steels of each example and comparative example can be seen in Table 6.

Table 6

	Ex am ple	Fine inclusions in Group A	Thick inclusions in Group A	Fine inclusions in Group B	Thick inclusions in Group B	Fine inclusions in Group C	Thick inclusions in Group C	Fine inclusions in Group D	Thick inclusions in Group D	Ds inclusi ons
Inve ntio n	1	0.5-1.0	0-0.5	0-0.5	0-0.5	0	0	0-0.5	0-0.5	0-1.0
Inve ntio n	2	0.5-1.0	0-0.5	0-0.5	0-0.5	0	0	0-0.5	0-0.5	0-1.0
Inve ntio n	3	0-1.0	0-0.5	0-0.5	0	0	0	0-1.0	0-0.5	0-0.5
Co mp arat ive Exa mpl e		0.5-1.0	0.5-1.0	0-0.5	0-0.5	0	0	0.5-1.5	0.5-1.0	1.0-2.0

The comparison of low-magnification data of the steels of each example and comparative example can be seen in Table 7. Table 7

C R S

Example 1 of the invention 1.0 1.0 1.0

Example 2 of the invention 1.0 1.0 1.0

Example 3 of the invention 1.0 1.0 1.0

Comparative Example 1.0 1.5 1.0
From the above test results, the impact and tensile properties of the present invention are better than those of 20CrMnTi in the comparative example, so the strength and toughness of the final product of the present application are better than those of 20CrMnTi. Some other performance indexes, including hardenability, grain size, inclusions, bands, low magnification, etc., of the present invention are close to those of the comparative steel. All performances of the present invention can meet the requirements of the steel used for the cage of ball-cage type universal joint.
A production method of the steel used for the cage of the ball-cage universal joint in the above-mentioned examples are described in detail below.
Production process: electric furnace or converter → LF refining → VD or RH vacuum degassing → continuous casting → rolling → finishing → packaging for storage.
In smelting, we use high-quality molten iron, scrap steel and raw/auxiliary materials, and use high-quality deoxidizer and refractory materials. In the production process of electric furnace/converter, the C content at an endpoint for tapping of the three embodiments is controlled within 0.05-0.15%, and the P content at the endpoint is controlled below 0.020%. Deoxidation is enhanced to control the tapping endpoint carbon of the electric furnace or converter. The carbon at the endpoint for tapping is controlled to be 0.05-0.15%. During the tapping process, Al iron is added for pre-deoxidation to create good conditions for subsequent deoxidation. After tapping, a slag removal technology is used to remove harmful steel slags.
When refining, new synthetic slags are add to the LF refining furnace, and at the same time the deoxidation is enhanced in the refining process. The refining process uses silicon carbide and aluminum to deoxidize, so that white slags are formed as soon as possible in the early stage of refining, and white slags are maintained for more than 25 minutes. The aluminum content in the refining process is controlled to be 0.025%-0.045%, to ensure the deoxidation effect.
The steel of the present application belongs to such a steel type that is sensitive to cracks. Therefore, vacuum degassing should be enhanced. It is processed under high vacuum (below 133 Pa) for ≥15 min, so as to ensure that harmful gas H ≤2 ppm. After vacuum degassing, silicon-calcium wires are fed, to change the inclusions. After vacuum degassing, soft blowing of argon is performed for a long time, to ensure that inclusions are fully floating up, wherein the time of soft blowing of argon is ≥ 25 min.
The superheat of the continuous casting is controlled to be 10-30°C, and the speed of continuous casting is 0.45-0.75 m/min. The continuous casting billet has a size of 300mm×300mm. The continuous casting billet is slowly cooled in a pit, and the slow cooling lasts for no less than 24 hours. After slow cooling, the continuous casting billet is sent to a step-by-step heating furnace for heating, and then rolled into the target product. The detailed rolling process is follows: in a section of preheating, the temperature is controlled to be 600-850 °C, in a section of heating, the temperature is controlled to be 950-1100 °C, and in a section of soaking, the temperature is controlled to 1150-1200 °C. In order to ensure that the billet is fully and evenly heated, the total heating time is more than 4 hours, and the time in the soaking section is more than 3 hours. The start rolling temperature is controlled to be 950°C-1050°C, the final rolling temperature is controlled to be 800°C-900°C. After the finial rolling is finished, the steel is controlled to cool at a cooling rate of 10-15 °C/min, which drives the austenite structures to fully transform into bainite. The temperature of the steel arriving at the cooling bed is controlled to be 600-650 °C. After straightening and flaw detection, a target bar product is obtained.

The rolling process parameters of each embodiment are shown in Table 8.

Table 8

No.	Heating Process		Rolling Process
No.	soaking temperature (°C)	total heating time (h)	start rolling temperature (°C)	final rolling temperature (°C)
1	1180	4h35min	1020	835
2	1164	4h50min	1028	820
3	1156	4h42min	1015	815

In addition to the above-mentioned embodiments, the present invention also includes other implementations. All technical solutions formed by equivalent transformation or equivalent replacement shall fall within the protection scope of the claims of the present invention.

Claims

Steel for a cage of a ball-cage universal joint, characterized in that the chemical composition of the steel in wt% is C: 0.10-0.25%, Si: 0.20-0.40%, Mn: 0.40-0.65%, Cr: 0.40-0.70%, B: 0.0003-0.0025%, Ti: 0.010-0.035%, Mo: 0.30-0.45%, N: 0.0050-0.0100%, S≤0.015% , P≤0.025%, Ni≤0.25%, Cu≤0.30%, Al: 0.015-0.035%, O≤0.0010%, As≤0.04%, Sn≤0.03%, Sb≤0.005%, Pb≤0.002%, and the balance is Fe and unavoidable impurities.
The steel for the cage of the ball-cage universal joint according to the claim 1, wherein the steel has a microstructure of bainite, with austenite grain size G≥6.
The steel for the cage of the ball-cage universal joint according to the claim 1, wherein the steel has yield strength ≥850 MPa, tensile strength ≥1080 MPa, elongation ≥10%, Charpy impact energy at room temperature AKu ≥55 J.
The steel for the cage of the ball-cage universal joint according to the claim 1, wherein
a hardenability of an end of the steel is evaluated according to the method of GB/T 225, wherein the hardenability meets at J5 point: 35-42 HRC, at J9 point: 25-35 HRC, at J13 point: 20-30 HRC;

banded structures in steel are graded according to GB/T 13299, and the banded structures do not exceed grade 2.0;

non-metallic inclusions are graded according to method A in GB/T 10561, wherein the inclusions meet that Fine inclusions in Group A≤1.5, Thick inclusions in Group A≤1.0, Fine inclusions in Group B≤1.5, Thick inclusions in Group B≤0.5, Fine inclusions in Group C = 0, Thick inclusions in Group C = 0, Fine inclusions in Group D≤1.0, Thick inclusions in Group D≤0.5, Ds≤1.5;

low-magnification structure of steel is graded according to ASTM E381, which meets C ≤ grade 2.0, R ≤ grade 2.0, S ≤ grade 2.0.
A production method of a steel for a cage of a ball-cage universal joint, wherein the process comprises: primary smelting of molten steel → LF refining of molten steel → VD or RH vacuum degassing of molten steel → continuous casting → rolling → finishing, wherein main producing processes require the following:
in smelting of the molten steel, deoxidation is enhanced, to control C content at an endpoint for tapping to be 0.05-0.15%; during the tapping process, Al iron is added for pre-deoxidation to create good conditions for subsequent deoxidation; after tapping, a slag removal technology is used to remove harmful steel slags;

in refining, synthetic slags are newly added to the LF refining furnace, and at the same time, deoxidation is enhanced in the refining process; silicon carbide and aluminum are used to deoxidize, so that white slags are formed as soon as possible in an early stage of refining, and white slags are maintained for more than 25 minutes; the aluminum content in the refining process is controlled to be 0.025%-0.045%, to ensure the deoxidation effect;

vacuum degassing is enhanced, to ensure that harmful gas H in the molten steel is ≤2 ppm; after vacuum degassing, silicon-calcium wires are fed, to change inclusions; after vacuum degassing, soft blowing of argon is performed, which agitates the molten steel and makes inclusions fully float up, wherein the soft blowing of argon lasts for ≥ 25 min;

a continuous casting billet obtained from a continuous casting process is slowly cooled in a pit, and the slow cooling lasts for no less than 24 hours;

before rolling, the continuous casting billet is sent to a furnace for heating, in a section of preheating, the temperature is set to be 600-850 °C, in a section of heating, the temperature is set to be 950-1100 °C, and in a section of soaking, the temperature is set to 1150-1200 °C, wherein a total heating time is more than 240 min, and a time in the soaking section is more than 180 min;

after being removed out of the furnace, the billet is ready for rolling; a start rolling temperature is set to be 950°C-1050°C, a final rolling temperature is set to be 800°C-900°C; the entire rolling process is carried out in austenite single-phase region; a cooling rate in a section from ending of final rolling to arriving at cooling bed is controlled to be 10-15 °C/s, and a time of steel passing the section is controlled, such a cooling process is to control the transformation of microstructure from austenite to bainite, so that the steel is fully transformed into bainite structure; a temperature of the steel arriving at a cooling bed is 600-650 °C, and the steel is further cooled on the cooling bed; after removing the steel off the the cool bed, straightening and finishing, a target bar product is obtained.
The production method of claim 5, wherein the primary smelting uses high-quality molten iron, scrap steel and raw/auxiliary materials, so as to reduce the content of harmful elements in the molten steel.
The production method of claim 5, wherein the vacuum degassing is processing the molten steel under a high vacuum below 133 Pa for ≥15 min.
The production method of claim 5, wherein in the continuous casting process, a mold electromagnetic stirring and a final electromagnetic stirring are used, and soft reduction technology is used, and a superheat of the continuous casting is controlled to be 10-30 °C.
The production method of claim 5, wherein a cooling rate of the steel on the cooling bed is 15-20 °C/min.