Steel for the intermediate shaft of an automotive constant velocity universal joint and its manufacturing method

Abstract

The present invention relates to a steel for the intermediate shaft of an automotive constant velocity universal joint and a manufacturing method thereof, belonging to the field of metallurgical technology. The chemical composition of the round steel in weight percentage is as follows: C: 0.34 - 0.41%, Si: 0.10 - 0.30%, Mn: 0.70 - 0.95%, Cr: 0.70 - 1.20%, P: ≤0.015%, S: ≤0.008%, Al: 0.010 - 0.030%, N: 0.0050 - 0.0080%, Cu: ≤0.08%, Ni: ≤0.05%, Mo: ≤0.05%, and the balance is Fe. The round steel has the advantages of good tissue uniformity, small hardness dispersion, no decarburization and rust on the surface, uniform stress distribution, and extremely small bending of the steel. The finally produced steel for the intermediate shaft of an automotive constant velocity universal joint has no decarburization and rust on the surface after quenching and tempering, and the whole cross-section is a uniform tempered sorbite structure. The hardness dispersion of the whole cross-section is ≤2HRC, and the austenite grain size of the whole cross-section is 7.0 - 9.0 grades.

Description

Technical Field

[0001] The present invention belongs to the technical field of round steel for shafts, and particularly relates to round steel for automotive shafts and a manufacturing method thereof. Background Art

[0002] The intermediate shaft of an automotive constant velocity universal joint plays a role in transmission and support during vehicle driving and needs to withstand alternating stresses for a long time. Therefore, it is required that the steel has a high fatigue life. By controlling no decarburization and rust on the steel surface; reducing the hardness dispersion across the section and improving the tissue uniformity of the material; refining the austenite grains of the steel and controlling the grade range of the grain size, the produced steel has the best uniformity, thereby enhancing the fatigue life of the material.

[0003] Patent Publication No. CN112981266A discloses a steel for a passenger car steering gear rack and a manufacturing method thereof, belonging to the technical field of iron-based alloys. The chemical composition of the steel by mass percentage is C: 0.34 - 0.41%, Si: 0.07 - 0.37%, Mn: 0.60 - 0.90%, Cr: 0.90 - 1.20%, P: ≤0.020%, S: 0.010 - 0.040%, Al: 0.010 - 0.050%, N: 0.005 - 0.020%, Cu: ≤0.10%, Ni: ≤0.05%, Mo: ≤0.05%, (Cu + Ni + Mo): ≤0.12%, and the balance is Fe and inevitable impurity elements. The production process is primary melting - refining - vacuum degassing - continuous casting - continuous rolling - quenching and tempering - skin pass - ultrasonic flaw detection. The steel of this application has a stable tensile strength and an excellent microstructure, and this material can be used to manufacture a passenger car steering gear rack and has an excellent service life. Summary of the Invention

[0004] To meet the requirements of the steel for the intermediate shaft of an automotive constant velocity universal joint, the present invention invents a steel for the intermediate shaft of an automotive constant velocity universal joint and a manufacturing method thereof. After quenching and tempering, the surface of the round steel has no decarburization and rust, the entire cross-section is a uniform tempered sorbite structure, the hardness dispersion across the section is ≤2HRC, and the austenite grain size of the entire cross-section is 7.0 - 9.0 grades.

[0005] The technical solution adopted by the present invention to solve the above problems is: a steel for the intermediate shaft of an automotive constant velocity universal joint, the chemical composition by weight percentage is C: 0.34 - 0.41%, Si: 0.10 - 0.30%, Mn: 0.70 - 0.95%, Cr: 0.70 - 1.20%, P: ≤0.015%, S: ≤0.008%, Al: 0.010 - 0.030%, N: 0.0050 - 0.0080%, Cu: ≤0.08%, Ni: ≤0.05%, Mo: ≤0.05%, and the balance is Fe and inevitable impurities.

[0006] The main functions and design bases corresponding to the chemical elements of the steel of the present invention are as follows:

[0007] C: Carbon is the most important element affecting the hardness of steel. Adding a certain amount of carbon to steel is beneficial to improving the strength of steel. However, too high a carbon content will aggravate the carbon segregation of steel, resulting in a larger hardness dispersion across the section of the steel after quenching and tempering. Therefore, the carbon content is selected in the range of 0.34 - 0.41%.

[0008] Si: During the steelmaking process, silicon is added as a reducing agent and deoxidizer, playing a certain role in reduction and deoxidation. The selection range of Si in the present invention is 0.10 - 0.30%.

[0009] Mn, Cr: During the steelmaking process, manganese and chromium elements are added to improve the strength of the steel after quenching and tempering.

[0010] In the present invention, the selection range of Mn is 0.70 - 0.95%, and the selection range of Cr is 0.70 - 1.20%.

[0011] P, S: Phosphorus and sulfur in steel are both harmful elements. Phosphorus increases the cold brittleness of steel, and sulfur increases the hot brittleness of steel. Therefore, it is necessary to strictly control the contents of phosphorus and sulfur in steel. The selection ranges of P and S contents in the present invention are P: ≤0.015%, S ≤0.008%.

[0012] Al, N: Aluminum is the most important deoxidizer in the steel of the present invention, and the nitrogen element can combine with the aluminum element to form AlN particles. Adding a small amount of aluminum to the steel and cooperating with a small amount of nitrogen, the formed AlN particles can be evenly distributed in the steel through reasonable rolling and cooling processes, finally playing the role of refining grains and stabilizing the austenite grain size grade. The selection ranges of Al and N contents in the present invention are Al: 0.010 - 0.030%, N: 0.0050 - 0.0080%.

[0013] Cu, Ni, Mo: They belong to the residual elements in steel. Strictly restricting these three elements can improve the stability of the hardenability of the steel of the present invention, making the structure of the steel more consistent and the hardness more stable after quenching and tempering.

[0014] The manufacturing method of the above-mentioned steel for the intermediate shaft of an automotive constant velocity universal joint includes

[0015] Billet heating

[0016] In the billet heating stage, the billet is heated to complete austenitization, and the AlN particles in the billet are completely dissolved.

[0017] Descaling rolling

[0018] After the slab completes the slab heating stage, the surface oxide layer is removed by high-pressure water descaling, and then three-stage rolling is carried out: rough rolling, intermediate rolling, and finish rolling. The rough rolling temperature > intermediate rolling temperature > finish rolling temperature is set, and the entire rolling is carried out in the austenite phase region. The difference between the rough rolling temperature and the intermediate rolling temperature is 40 - 60 °C, and the difference between the finish rolling temperature and the intermediate rolling temperature is 60 - 80 °C; Research data shows that AlN particles will continuously precipitate during the rolling process, and the number of AlN precipitations has a strong correlation with the rolling process temperature and the cooling temperature. By stably controlling the temperature and temperature difference during rough rolling, intermediate rolling, and finish rolling, a stable precipitation amount of AlN during the steel rolling process can be achieved.

[0019] Cooling

[0020] After the slab completes the rough rolling, intermediate rolling, and finish rolling, it becomes round steel (with a specification of φ10mm - φ45mm). The round steel is immediately heat-preserved and slowly cooled. The temperature drop rate during the slow cooling process is low, and the temperature drop rate difference between each steel bar is small, so as to achieve a stable precipitation amount of AlN during the steel cooling process. By controlling the rolling process temperature and reducing the cooling temperature drop rate difference, the austenite grain size of the entire cross-section of the produced round steel is 7.0 - 9.0 grades. After the slow cooling ends, the round steel structure is uniform ferrite and pearlite, and the uniform ferrite and pearlite structure is very beneficial for the next steel quenching and tempering.

[0021] Surface treatment

[0022] After the round steel completes the slow cooling, the defects on the steel surface including the oxide layer and cracks are removed using the turning process; The advantage of turning the round steel before quenching and tempering is that the round steel after turning has high temperature conductivity during the quenching and tempering process, the temperature transfer is more uniform, and the structure uniformity and hardness uniformity after quenching and tempering are good; Moreover, the stress distribution of the round steel after quenching and tempering is more uniform, and the steel bar is not easily bent. The uniform ferrite and pearlite original structure also further improves the uniformity of temperature conduction during the quenching and tempering process, and the structure uniformity and hardness uniformity of the round steel after quenching and tempering are more excellent.

[0023] Quenching and tempering

[0024] The round steel behind the car body is protected by nitrogen throughout the heating process of quenching and tempering, effectively preventing direct contact between the steel and air, so decarburization on the steel surface will not occur. The round steel is heated to the quenching temperature by an induction coil, and soft water is sprayed onto the surface of the round steel for quenching. After quenching, the temperature of the steel ≤ 30°C; after quenching, the round steel is heated to the tempering temperature by an induction coil, and soft water is sprayed again for cooling. After tempering and cooling, the temperature of the steel ≤ 200°C; throughout the quenching and tempering process, soft water from which soluble calcium and magnesium compounds have been removed is used to spray and cool the round steel. The advantage of using soft water compared to tap water during the spraying process is that the nozzles for soft water will not form scale, so the nozzles will not become blocked as a whole or partially blocked. The soft water sprayed from each nozzle during the spraying process has the same water pressure, thus ensuring the consistency of the cooling intensity at each point on the circumference of the steel during the quenching and tempering process.

[0025] Drying and oiling

[0026] The steel after the quenching and tempering process is immediately dried on the surface to prevent the formation of floating rust on the steel surface. And anti-rust treatment is carried out.

[0027] As a preferred method in an embodiment of the present application: in the billet heating stage, the steel billet is heated in a heating furnace to 1260 - 1280°C and held at this temperature range for more than 70 minutes. Relevant research shows that the dissolution rate of AlN particles is closely related to the contents of Al and N and the temperature. When the Al content in the steel is 0.010 - 0.030%, and the N content is 0.0050 - 0.0080%, the optimal dissolution temperature of AlN particles is 1260 - 1280°C. In this temperature range, AlN particles have the fastest dissolution rate.

[0028] As a preferred method in an embodiment of the present application: in the descaling rolling stage, the starting temperature of the controlled rolling, that is, the starting temperature of rough rolling, is controlled at 1100 - 1120°C.

[0029] As a preferred method in an embodiment of the present application: in the cooling stage, the round steel is slowly cooled in a heat preservation pit.

[0030] As a preferred method in an embodiment of the present application: in the quenching and tempering stage, the quenching temperature of the round steel is 880°C, the water pressure of the quenching soft water spray is 0.35 MPa, and the quenching soft water flow rate is 2 m 3 / min; the tempering temperature of the round steel is 650°C, the water pressure of the tempering soft water spray is 0.15 MPa, and the tempering soft water flow rate is 0.5 m 3 / min.

[0031] As a preferred method in an embodiment of the present application: in the drying and oiling stage, an electrostatic oiling machine is used to coat the surface of the round steel with anti-rust oil with a thickness of 700 - 800 μm.

[0032] Compared with the prior art, the advantages of the present invention are as follows:

[0033] (1) Medium carbon is adopted in the composition, and a small amount of aluminum and nitrogen elements are added. By reasonably controlling the rolling and cooling processes, the stability of the austenite grain size grade of the steel is controlled, thereby improving the fatigue life of the steel.

[0034] (2) Before quenching and tempering, the original structure of the round steel is uniform ferrite and pearlite. The round steel is treated by turning to remove defects such as the oxide layer and cracks on the surface of the steel. After turning, the round steel with uniform ferrite and pearlite in the elemental structure has high temperature conductivity during the quenching and tempering process, and the temperature transfer is more uniform. The structure uniformity and hardness uniformity after quenching and tempering are very good, and the stress distribution of the round steel after quenching and tempering is more uniform, and the steel bar is not easy to bend. At present, the prior art does not require the structure of the steel before quenching and tempering, and the structure uniformity is poor after natural cooling of the steel; and the method of first quenching and tempering and then turning to remove the surface oxide layer and crack defects is adopted. If steel with poor structure uniformity, a thick surface oxide layer, and uneven oxide layer thickness and distribution is used for quenching and tempering, the temperature conductivity of the steel is poor during the quenching and tempering process, and the uniformity of temperature transfer is also poor. In this way, soft spots are likely to form on the surface of the quenched and tempered steel, and untransformed original structure is likely to remain in the central area, resulting in poor structure uniformity of the entire steel, large hardness scatter in the entire cross-section of the steel, poor stress distribution uniformity in the entire steel, and serious bending phenomenon in the quenched and tempered steel.

[0035] (3) The round steel after turning is quenched and tempered by an induction line with full nitrogen protection to prevent decarburization on the surface of the steel. During the quenching and tempering process of the round steel, constant 15°C soft water is used for spray cooling, and the cooled steel is immediately put into a high-temperature dryer to dry its surface to avoid the formation of floating rust on the surface of the steel.

[0036] (4) The steel for the intermediate shaft of an automotive constant velocity universal joint produced according to the method of the present invention has the advantages of good structure uniformity, small hardness scatter, no decarburization and rust on the surface, uniform stress distribution, and extremely small bending of the steel. Finally, the steel for the intermediate shaft of an automotive constant velocity universal joint produced has no decarburization and rust on the surface after quenching and tempering, a uniform tempered sorbite structure throughout the cross-section, a hardness scatter of ≤2HRC throughout the cross-section, and an austenite grain size of 7.0 - 9.0 grades throughout the cross-section. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 It is the tempered sorbite (×500 times) of the round steel corresponding to Example 1 of the present invention;

[0038] Figure 2 It is the tempered sorbite (×500 times) of the round steel corresponding to Example 2 of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0039] The present invention will be further described in detail below in conjunction with embodiments. The embodiments are exemplary and are intended to explain the present invention, but should not be construed as limiting the present invention.

[0040] Embodiment 1 and Embodiment 2:

[0041] A method for manufacturing steel for the intermediate shaft of an automotive constant velocity universal joint involved in the two embodiments: The molten steel is cast into a continuous casting square billet with a specification of 200mm * 200mm. The specific chemical compositions of the continuous casting square billets in the two embodiments are shown in Table 1.

[0042] Table 1 (wt%)

[0043] C Si Mn Cr P S Al N Cu Ni Mo Example 1 0.35 0.18 0.73 0.78 0.011 0.003 0.015 0.0061 0.02 0.01 0.02 Example 2 0.40 0.27 0.92 1.17 0.010 0.002 0.026 0.0073 0.03 0.02 0.01

[0044] The continuous casting square billets (Embodiment 1 and Embodiment 2) are heated in a heating furnace to 1260 - 1280 °C and held at this temperature range for 76 min (Embodiment 1) and 73 min (Embodiment 2). After heating, the steel billets are first descaled on the surface by high-pressure water, and then subjected to three-stage rolling. The rough rolling temperature is 1155 (Embodiment 1) and 1168 (Embodiment 2); the medium rolling temperature is 1104 °C (Embodiment 1) and 1116 °C (Embodiment 2); the finish rolling temperature is 1031 °C (Embodiment 1) and 1042 °C (Embodiment 2). The rolled round steels with a diameter of φ21mm (Embodiment 1) and φ42mm (Embodiment 2) are immediately put into a heat preservation pit for slow cooling. The structure of the round steel after slow cooling is uniform ferrite and pearlite.

[0045] The hot-rolled round steels after slow cooling (Embodiment 1 and Embodiment 2) are first treated with a skin-pass mill to remove surface defects such as oxide layers and surface cracks. The steel after skin-pass treatment is quenched and tempered using an induction line with full nitrogen protection: The round steel is heated to 880 °C through an induction coil, and soft water with a temperature of 15 °C is sprayed onto the surface of the round steel for quenching. The water pressure of the quenching soft water spray is 0.35 MPa, and the flow rate of the quenching soft water is 2 m 3 / min. The temperature of the steel after quenching is 27 °C (Embodiment 1) and 25 °C (Embodiment 2); the quenched round steel is heated to 650 °C through an induction coil, and again sprayed and cooled with 15 °C soft water. The water pressure of the tempering soft water spray is 0.15 MPa, and the flow rate of the tempering soft water is 0.5 m 3 / min. The temperature of the steel after tempering and cooling is 168 °C (Embodiment 1) and 190 °C (Embodiment 2). The steel after the quenching and tempering process is immediately put into a high-temperature dryer to dry its surface, and then an electrostatic oiling machine is used to evenly coat the surface of the steel with a rust preventive oil with a thickness of 700 um - 800 um.

[0046] The specific values of the surface decarburized layer depth, the hardness scatter of the entire cross-section, and the austenite grain size grade evaluation of the entire cross-section of the round steels obtained in Embodiment 1 and Embodiment 2 are shown in Table 2.

[0047] Table 2

[0048]

[0049] The present invention is directed to the steel for the intermediate shaft of automotive constant velocity joints. Medium carbon is adopted in terms of composition, and a small amount of aluminum and nitrogen elements are added. By controlling the stability of the austenite grain size grade of the steel through reasonable rolling and cooling processes, the fatigue life of the steel is improved. Before quenching and tempering, the surface treatment is used to remove defects such as the oxide layer and cracks on the surface of the steel. After the surface treatment, the original structure is a round steel of uniform ferrite + pearlite. The temperature conductivity during the quenching and tempering process is high, and the temperature transfer is more uniform. The structure uniformity and hardness uniformity after quenching and tempering are very good, and the stress distribution of the round steel after quenching and tempering is more uniform, and the steel bar is not prone to bending. The round steel after surface treatment is quenched and tempered by an induction line with full nitrogen protection to prevent decarburization on the surface of the steel. During the quenching and tempering process of the round steel, constant 15°C soft water is used for spray cooling. The steel after cooling is immediately put into a high-temperature dryer to dry its surface to avoid the formation of floating rust on the surface of the steel. Through reasonable processes such as steelmaking, rolling, and quenching and tempering, a steel for the intermediate shaft of automotive constant velocity joints is successfully manufactured, filling the domestic blank.

Claims

1. A manufacturing method of steel for the intermediate shaft of automotive constant velocity universal joint, characterized in that: the chemical composition of the steel by weight percentage is C: 0.34 - 0.41%, Si: 0.10 - 0.30%, Mn: 0.70 - 0.95%, Cr: 0.70 - 1.20%, P: ≤0.015%, S: ≤0.008%, Al: 0.010 - 0.030%, N: 0.0050 - 0.0080%, Cu: ≤0.08%, Ni: ≤0.05%, Mo: ≤0.05%, and the balance is Fe and unavoidable impurities; the method includes: Billet heating In the billet heating stage, the billet is heated to complete austenitization, and the AlN particles in the billet are completely dissolved; Descaling rolling After the billet completes the billet heating stage, the surface oxide layer is removed by high-pressure water surface descaling, and then three-stage rolling is carried out: rough rolling, medium rolling and finish rolling, and the rough rolling temperature > medium rolling temperature > finish rolling temperature is set. The whole rolling process is carried out in the austenite phase region. The difference between the rough rolling temperature and the medium rolling temperature is 40 - 60°C, and the difference between the finish rolling temperature and the medium rolling temperature is 60 - 80°C to control the stable precipitation amount of AlN in the whole rolling process; Cooling After the billet completes the rough rolling, medium rolling and finish rolling, it becomes round steel. The round steel is immediately heat-insulated and slowly cooled. After the slow cooling ends, the structure of the round steel is uniform ferrite and pearlite; Surface treatment After the round steel completes slow cooling, the defects on the surface of the steel including the oxide layer and cracks are removed by the skin pass process; Quenching and tempering During the whole heating process of the round steel after skin pass treatment, nitrogen protection is adopted. The round steel is heated to the quenching temperature through an induction coil, and soft water is sprayed onto the surface of the round steel for quenching. The temperature of the steel after quenching ≤ 30°C; after quenching, the round steel is heated to the tempering temperature through an induction coil, and soft water is sprayed again for cooling. The temperature of the steel after tempering and cooling ≤ 200°C; after the round steel is quenched and tempered, the surface does not decarburize and rust, the whole cross-section is uniform tempered sorbite structure, the hardness dispersion of the whole cross-section ≤ 2HRC, and the austenite grain size of the whole cross-section is 7.0 - 9.0 grades Drying and oiling After the steel passes through the quenching and tempering process, the surface of the steel is immediately dried, and preferably oiled for rust prevention.

2. The manufacturing method of steel for the intermediate shaft of automotive constant velocity universal joint according to claim 1, characterized in that: In the billet heating stage, according to the contents of Al and N in the steel, the heating temperature of the billet is set to 1260 - 1280°C, and it is held at this temperature range for more than 70 minutes to completely dissolve the AlN particles.

3. The manufacturing method of steel for the intermediate shaft of automotive constant velocity universal joint according to claim 1, characterized in that: In the descaling rolling stage, the starting temperature of rolling is controlled, that is, the starting temperature of rough rolling is 1100 - 1120°C.

4. The manufacturing method of steel for the intermediate shaft of automotive constant velocity universal joint according to claim 1, characterized in that: In the cooling stage, the round steel is slowly cooled in a heat-insulating pit.

5. The manufacturing method of steel for the intermediate shaft of automotive constant velocity universal joint according to claim 1, characterized in that: In the quenching and tempering stage, the quenching temperature of the round steel is 880 °C, the water pressure of the quenching soft water spray is 0.35 MPa, and the flow rate of the quenching soft water is 2 m 3 / min; the tempering temperature of the round steel is 650 °C, the water pressure of the tempering soft water spray is 0.15 MPa, and the flow rate of the tempering soft water is 0.5 m 3 / min.

6. The manufacturing method of steel for the intermediate shaft of automotive constant velocity universal joint according to claim 1, characterized in that: In the drying and oiling stage, an electrostatic oiler is used to apply a rust preventive oil with a thickness of 700um - 800um on the surface of the round steel.

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