Rolling and cooling process for Nb-containing medium-carbon hub bearing steel

Abstract

The invention discloses a rolling and cooling process for Nb-containing medium-carbon hub bearing steel, which relates to the technical field of steel production and sequentially comprises the following procedures of heating, rough rolling, continuous rolling, temperature holding, pre-finish rolling, finish rolling and cooling. Wherein in the heating process, the continuous casting billet comprises the following chemical components in percentage by mass: 0.53 to 0.58 percent of C, 0.20 to 0.35 percent of Si, 0.75 to 0.90 percent of Mn, less than or equal to 0.020 percent of P, less than or equal to 0.005 percent of S, 0.20 to 0.35 percent of Cr, 0.05 to 0.050 percent of Al, 0.10 to 0.20 percent of Nb and the balance of matrix Fe and inevitable impurities. According to the controlled rolling cooling process of the Nb-containing medium-carbon hub bearing steel, the actual hot rolling grain size of finished round steel is larger than or equal to 5 grades, the austenite grain size is larger than or equal to 8.0 grades, and the hot rolling state hardness is smaller than or equal to 235 HB.

Description

Technical Field

[0001] The invention relates to the technical field of steel production, in particular to a rolling cooling process for Nb-containing medium-carbon hub bearing steel. Background Art

[0002] The steel used for wheel hub bearings must have high strength and durability to withstand the heavy loads of vehicles and the impact of various road conditions. The development of special steel can significantly improve the fatigue resistance and wear resistance of bearings and extend their service life. The current rolling cooling process used for wheel hub bearing steel does not fully meet the performance requirements of the finished steel. Summary of the invention

[0003] The technical problem to be solved by the present invention is to overcome the disadvantages of the prior art and provide a rolling cooling process for a medium-carbon hub bearing steel containing Nb.

[0004] In order to solve the above technical problems, the technical solution of the present invention is as follows: A rolling and cooling process for a medium-carbon hub bearing steel containing Nb, comprising the following steps in sequence: heating, rough rolling, continuous rolling, waiting for temperature, pre-finishing rolling, final rolling and cooling; Wherein, in the heating process, the chemical composition and mass percentage of the continuous casting billet include: C: 0.53-0.58%, Si: 0.20-0.35%, Mn: 0.75-0.90%, P: ≤0.020%, S: ≤ 0.005%, Cr: 0.20-0.35%, Al: 0.05-0.050%, Nb: 0.10-0.20%, and the rest are matrix Fe and unavoidable impurities.

[0005] As a preferred solution of the rolling cooling process of the Nb-containing medium-carbon hub bearing steel of the present invention, the rough rolling process comprises: after the steel billet is removed from the furnace and descaled, the surface temperature of the rolled piece is 1110-1130°C, and the rolled piece enters the rough rolling mill for rough rolling.

[0006] As a preferred scheme of the rolling cooling process of the Nb-containing medium-carbon hub bearing steel of the present invention, wherein: in the rough rolling process, the rolling passes are adjusted from seven to nine, and the reduction amounts of the third and fifth passes are less than the original reduction amounts of the third and fifth passes.

[0007] As a preferred scheme of the rolling cooling process of the Nb-containing medium-carbon wheel hub bearing steel described in the present invention, the continuous rolling process includes: the billet surface temperature is 1050-1060°C and enters a ten-stand continuous rolling mill group, and the inlet linear speed of the first stand is 0.17-0.25m / s, and the outlet linear speed of the tenth stand is 1.2-1.5m / s.

[0008] As a preferred solution of the rolling cooling process of the Nb-containing medium-carbon hub bearing steel of the present invention, the temperature-keeping process includes: reducing the surface temperature of the rolled piece after continuous rolling to 820-850°C by air cooling.

[0009] As a preferred solution of the rolling cooling process of the Nb-containing medium-carbon hub bearing steel of the present invention, the pre-finishing rolling process comprises: sending the rolled piece into a four-stand pre-finishing rolling mill group for pre-finishing rolling when the surface temperature of the rolled piece is 810-830°C.

[0010] As a preferred solution of the rolling cooling process of the Nb-containing medium-carbon hub bearing steel of the present invention, the final rolling process includes: after weak water-penetrating cooling of the rolled piece, sending the rolled piece to the tension reducing mill for rolling.

[0011] As a preferred solution of the rolling cooling process of the Nb-containing medium-carbon hub bearing steel of the present invention, the inlet temperature of the tension reducing mill is 790-810°C.

[0012] As a preferred scheme of the rolling cooling process of the Nb-containing medium-carbon wheel hub bearing steel described in the present invention, the cooling process includes: cooling the surface temperature of the rolled piece to 710-720°C by water cooling, sending the rolled piece to a cooling bed covered with a heat-insulating cover, and exiting the heat-insulating cover after the surface temperature is cooled to less than or equal to 530°C.

[0013] The beneficial effects of the present invention are: (1) The controlled rolling and cooling process of the Nb-containing medium-carbon wheel hub bearing steel provided by the present invention has the following advantages: the actual grain size of the hot-rolled finished round steel is ≥5, the austenite grain size is ≥8.0, and the hot-rolled hardness is ≤235HB. When processed by the user, the oxidation and decarburization of the material during the high-temperature forging process can be reduced, and the machinability of the steel can be improved. During the induction quenching process, the fine grains can distribute stress and heat more evenly, thereby reducing the formation of cracks; they can hinder the expansion of cracks, thereby improving the impact resistance of parts; they can more effectively disperse and transfer stress, reducing stress concentration; they can improve the surface hardness while ensuring the depth and uniformity of the hardened layer, thereby improving the wear resistance and service life of the wheel hub bearing.

[0014] (2) The continuous casting billet of the present invention contains niobium, which can refine the grain structure of steel and improve the strength and toughness of the material. Fine-grained steel has higher yield strength and tensile strength while maintaining good toughness and can withstand high loads and impact loads. Niobium has advantages in the formation of carbides and nitrides. The uniform distribution of these particles significantly improves the wear resistance and fatigue strength of the material and extends the service life of the bearing. At the same time, the addition of niobium can improve the hardenability of steel, so that a more uniform martensitic structure is formed during the heat treatment process, improving hardness and wear resistance. Steel with good hardenability performs better in the heat treatment of large-sized parts and complex-shaped parts, ensuring overall performance consistency. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other accompanying drawings can be obtained based on these accompanying drawings without paying creative labor.

[0016] Figure 1 Schematic diagram of the actual grain size of the Nb-containing medium-carbon hub bearing steel prepared in Example 1; Figure 2 Schematic diagram of austenite grain size of the Nb-containing medium-carbon wheel hub bearing steel prepared in Example 1; Figure 3 Schematic diagram of austenite grain size of the Nb-containing medium-carbon wheel hub bearing steel prepared in Example 2; Figure 4 Schematic diagram of the austenite grain size of the Nb-containing medium-carbon wheel hub bearing steel prepared in Example 3. DETAILED DESCRIPTION

[0017] In order to make the content of the present invention more clearly understood, the present invention is further described in detail below according to specific implementation modes and in combination with the accompanying drawings.

[0018] The present application provides a rolling cooling process for a medium-carbon hub bearing steel containing Nb, which comprises the following steps in sequence: Process 1: Heating: heating the continuous casting billet in the furnace; Process 2: Rough rolling: The continuous casting slab is rolled into a nearly square rough rolling slab through a reversible rough rolling mill; Process 3: Continuous rolling: After the head and tail of the initial rolling billet are removed, it enters the continuous rolling unit for rolling; Process 4: Waiting for temperature: After leaving the continuous rolling mill, the rolled pieces wait for temperature on the transverse marshaling stand; Process 5: Pre-finishing rolling: After being heated, the rolled piece enters the pre-finishing rolling unit for continuous rolling; Process 6: Final rolling: The rolled piece is formed into round steel that meets the size and shape requirements through a tension reducing mill; Process seven: Cooling: After passing through the tension reducing unit, the round steel is water-cooled to a certain temperature range, enters the insulation cover on the cooling bed for slow cooling, and is taken off the production line after completion.

[0019] Specifically, in the above process 1, the composition of the continuous casting billet includes, by mass percentage, C: 0.53-0.58%, Si: 0.20-0.35%, Mn: 0.75-0.90%, P: ≤0.020%, S: ≤0.005%, Cr: 0.20-0.35%, Al: 0.05-0.050%, Nb: 0.10-0.20%, and the rest is matrix Fe and unavoidable impurities. The cross-sectional size of the continuous casting billet is 320×480mm, and the diameter of the finished round steel is 50-70mm.

[0020] In the above process 2, after the steel billet is removed from the furnace and descaled, the surface temperature of the rolled piece is 1110-1130°C, and it enters a 950mm diameter reversible rough rolling mill to be rolled into a nearly 220×220mm square.

[0021] According to the finite element calculation and verification of metal high-temperature rheology, in order to improve the frame segregation distribution, the reduction of the third and fifth passes was reduced, and the original conventional rolling was increased from seven passes to nine passes, so that the length-to-width ratio of the segregation frame was reduced from the original 1.35:1 to 1.1:1, which is beneficial to ensure the uniformity of the structure and hardness of the circumference of the ring after the finished round steel is processed / heat treated at the customer's site.

[0022] In the above process three, after rough rolling, the initial rolled billet is trimmed and sawed, with a surface temperature of 1050-1060°C, and enters a ten-stand continuous rolling unit, with an inlet line speed of 0.17-0.25m / s for the first stand and an outlet line speed of 1.2-1.5m / s for the tenth stand.

[0023] Because the carbide (NbC) and nitride (NbN) particles formed by niobium in steel can significantly inhibit recrystallization. These tiny particles will pin the grain boundaries and prevent the movement of dislocations, thereby increasing the recrystallization temperature. As a result, niobium microalloyed steel can still maintain a fine grain structure at a higher rolling temperature. The surface temperature of the rolled piece is controlled at 970-980℃ at the exit, and the rolled piece diameter is 110mm.

[0024] In the fourth process, the production rhythm is controlled, and the rolled pieces after continuous rolling are kept warm on the assembly rack, and the surface temperature of the rolled pieces is reduced to 820-850°C by air cooling.

[0025] Finite element calculations were performed using the verified cooling model. Under this surface temperature condition, the core temperature of the workpiece has entered the non-recrystallization zone, which helps to reduce the temperature gradient inside the workpiece. In addition, it can share the pressure of subsequent water cooling, avoid excessive temperature gradients in the workpiece caused by rapid cooling after final rolling, and thus reduce internal thermal stress and structural transformation stress.

[0026] In the above process 5, the heated rolled piece enters the four-stand pre-finishing rolling, at which time the surface temperature of the rolled piece is 810-830℃. Before the rolled piece enters the tension reducing mill for final rolling, its size and shape are preliminarily adjusted to ensure that the rolled piece can meet the required dimensional accuracy and shape requirements during the final rolling process. After the rolled piece is tested by the diameter gauge, the diameter and shape have reached a set of accuracy requirements in "GB / T 702- Size, shape, weight and allowable deviation of hot-rolled round steel and square steel".

[0027] In the above process six, weak water cooling is carried out through the water tank before entering the tension reducing unit, which can remove or reduce the scale and other defects on the surface of the steel and improve the surface quality of the final product. The inlet temperature of the tension reducing unit is 790-810℃. After rolling in the non-recrystallization zone, a large number of substructures such as dislocations and deformation bands are formed inside the rolled piece. After four-stand continuous rolling, the outlet linear speed of the finished round steel is 2.5-4m / s.

[0028] In the above process seven, according to the phase change point measured by the thermal expansion instrument and the dynamic CCT curve measured by the thermal simulation, the temperature range of the two-phase region between Ar3 and Ar1 of the medium-carbon hub bearing steel containing Nb is about 20 degrees under the cooling rate condition of 0.1-3℃ / s. Combined with the TTT curve calculated by the thermodynamic / kinetic software, it is determined that the round steel passes through the KOCKS final rolling water tank, the surface temperature is cooled to 710-720℃, and then enters the cooling bed covered with the insulation cover, moves forward slowly, and exits the insulation cover after the surface is cooled to ≤530℃, and then moves forward quickly → sawing → packaging → offline.

[0029] Because the substructures such as dislocations and deformation bands formed during final rolling increase the formation speed and content of the pro-eutectoid ferrite in this step; because the two-phase region is very narrow, in order to form ferrite as fully as possible, the temperature entering the heat preservation cover is controlled according to the nose temperature of the austenite→ferrite transformation calculated by TTT, and the internal structure of the round steel passes through the two-phase region under slow cooling conditions. Finally, a fine and uniform hot-rolled structure is obtained, which provides convenience for users' forging and heat treatment processing.

[0030] The following is further described by means of specific examples.

[0031] Embodiment 1: This embodiment provides a rolling cooling process for a medium-carbon hub bearing steel containing Nb, comprising the following steps: Heating: Among them, the mass percentage of medium carbon hub bearing steel containing Nb is: C: 0.55%, Si: 0.23%, Mn: 0.82%, P: 0.006%, S: 0.001%, Cr: 0.26%, Al: 0.020%, Nb: 0.07%; the cross-section of the continuous casting billet is 320×480mm; the diameter of the finished round steel is 50mm.

[0032] Rough rolling: After the continuous casting billet is removed from the furnace and descaled, it enters the rough rolling mill with a roller diameter of 950mm when the surface temperature is 1130℃. After 9 reciprocating rolling passes, it becomes a 220×220mm initial rolled billet.

[0033] Continuous rolling: After the initial billet is trimmed and sawed, the surface temperature drops to 1060℃ and enters the continuous rolling unit at a speed of 0.25m / s. The linear speed of the 10-stand outlet rolled pieces is 1.5m / s, the surface temperature is 980℃, and the diameter of the rolled piece is 111.06mm, which is close to the accuracy requirements of Group 2 in Table 1 of GB / T-702.

[0034] Waiting for temperature: After leaving the continuous rolling mill, the rolled pieces wait for temperature on the transverse grouping stand, and then move forward after the surface is air-cooled to 850℃.

[0035] Pre-finishing rolling: When the rolled piece enters pre-finishing rolling, the surface temperature is 830℃ and the exit size of the fourth rack is 73.5 mm, which has reached the precision requirements of Group 1 in Table 1 of GB / T-702.

[0036] Final rolling: After weak water penetration in the water tank, the surface temperature of the rolled piece is cooled to 810℃ and then enters the KOCKS unit for continuous rolling. The final rolling line speed is 4m / s. The finished round steel is tested by a diameter gauge and the diameter is 50.22mm, which not only meets the accuracy requirements of Group 1 in Table 1 of GB / T-702, but also meets the precision forging requirements of users.

[0037] Cooling: After final rolling of the KOCKS unit, the round steel is cooled by a water tank with a surface temperature of 720°C. It enters the insulation cover of the cooling bed and moves forward slowly. When it leaves the insulation cover, the surface temperature is 530°C. It then moves quickly and is collected offline.

[0038] The key test indicators of the round steel rolled according to this example are shown in Table 1, and the hardness is ≤235HB. The hot-rolled grain size is 6.0, see Figure 1 ; Austenite grain size 9.0, see Figure 2 .

[0039]

[0040] Table 1 Embodiment 2: This embodiment provides a rolling cooling process for a medium-carbon hub bearing steel containing Nb, comprising the following steps: Heating: Among them, the mass percentage of medium carbon wheel hub bearing steel containing Nb is: C: 0.53%, Si: 0.25%, Mn: 0.81%, P: 0.005%, S: 0.002%, Cr: 0.25%, Al: 0.021%, Nb: 0.09%; the cross-section of the continuous casting billet is 320×480mm; the diameter of the finished round steel is 60mm.

[0041] Rough rolling: After the continuous casting billet is removed from the furnace and descaled, it enters the rough rolling mill with a roller diameter of 950mm when the surface temperature is 1120℃. After 9 reciprocating rolling passes, it becomes a 220×220mm initial rolled billet.

[0042] Continuous rolling: After the initial billet is trimmed and sawed, the surface temperature drops to 1050℃ and enters the continuous rolling unit at a speed of 0.20m / s. The linear speed of the 10-stand outlet rolled pieces is 1.3m / s, the surface temperature is 973℃, and the diameter of the rolled piece is 111.15mm, which is close to the accuracy requirements of Group 2 in Table 1 of GB / T-702.

[0043] Waiting for temperature: After leaving the continuous rolling mill, the rolled pieces wait for temperature on the transverse grouping stand, and then move forward after the surface is air-cooled to 835℃.

[0044] Pre-finishing rolling: When the rolled piece enters pre-finishing rolling, the surface temperature is 820℃ and the exit size of the fourth rack is 73.6 mm, which has reached the precision requirements of Group 1 in Table 1 of GB / T-702.

[0045] Final rolling: After weak water penetration in the water tank, the surface temperature of the rolled piece is cooled to 802℃ and then enters the KOCKS continuous rolling. The final rolling line speed is 3.1m / s. The diameter of the finished round steel is 60.25mm after being tested by the diameter gauge, which not only meets the accuracy requirements of Group 1 in Table 1 of GB / T-702, but also meets the precision forging requirements of users.

[0046] Cooling: After KOCKS final rolling, the round steel is cooled by a water tank, the surface temperature is 715℃, and then it enters the insulation cover of the cooling bed and moves forward slowly. When it leaves the insulation cover, the surface temperature is 521℃, and then it moves quickly to be collected off the line; The key test indicators of the round steel rolled according to this example are shown in Table 2, and the hardness is ≤235HB. The hot-rolled grain size is 5.5; the austenite grain size is 8.5, see Figure 3 .

[0047]

[0048] Table 2 Embodiment 3: This embodiment provides a rolling cooling process for a medium-carbon hub bearing steel containing Nb, comprising the following steps: Heating: Among them, the mass percentage of medium carbon hub bearing steel containing Nb is: C: 0.57%, Si: 0.26%, Mn: 0.82%, P: 0.009%, S: 0.001%, Cr: 0.28%, Al: 0.023%, Nb: 0.08%; the cross-section of the continuous casting billet is 320×480mm; the diameter of the finished round steel is 70mm.

[0049] Rough rolling: After the continuous casting billet is removed from the furnace and descaled, it enters the rough rolling mill with a roller diameter of 950mm when the surface temperature is 1110℃. After 9 reciprocating rolling passes, it becomes a 220×220mm initial rolled billet.

[0050] Continuous rolling: After the initial billet is trimmed and sawed, the surface temperature drops to 1053℃ and enters the continuous rolling unit at a speed of 0.17m / s. The linear speed of the 10-stand outlet rolled pieces is 1.2m / s, the surface temperature is 970℃, and the diameter of the rolled piece is 111.21mm, which is close to the accuracy requirements of Group 2 in Table 1 of GB / T-702.

[0051] Waiting for temperature: After leaving the continuous rolling mill, the rolled pieces wait for temperature on the transverse grouping stand, and then move forward after the surface is air-cooled to 820℃.

[0052] Pre-finishing rolling: When the rolled piece enters pre-finishing rolling, the surface temperature is 810℃ and the exit size of the 4th stand is 96.08mm, which has reached the precision requirements of Group 1 in Table 1 of GB / T-702.

[0053] Final rolling: After weak water penetration in the water tank, the surface temperature of the rolled piece is cooled to 790℃ and then enters the KOCKS continuous rolling. The final rolling line speed is 2.5m / s. The diameter of the finished round steel is 70.31mm after being tested by the diameter gauge, which not only meets the accuracy requirements of Group 1 in Table 1 of GB / T-702, but also meets the precision forging requirements of users.

[0054] Cooling: After KOCKS final rolling, the round steel is cooled by a water tank, the surface temperature is 710℃, and then it enters the insulation cover of the cooling bed and moves forward slowly. When it leaves the insulation cover, the surface temperature is 512℃, and then it moves quickly to be collected off the line; The key test indicators of the round steel rolled according to this example are shown in Table 3, and the hardness is ≤235HB. The hot-rolled grain size is 5.5; the austenite grain size is 8.0, see Figure 4 .

[0055]

[0056] Table 3 Therefore, in the technical solution of the present application, the actual grain size of the hot-rolled finished round steel is ≥ level 5, the austenite grain size is ≥ level 8.0, and the hot-rolled hardness is ≤ 235HB. When processed by the user, the oxidation and decarburization of the material during high-temperature forging can be reduced, and the machinability of the steel can be improved. During the induction quenching process, the fine grains can distribute stress and heat more evenly, thereby reducing the formation of cracks; they can hinder the expansion of cracks, thereby improving the impact resistance of parts; they can more effectively disperse and transfer stress and reduce stress concentration; they can improve the surface hardness while ensuring the depth and uniformity of the hardened layer, thereby improving the wear resistance and service life of the wheel hub bearing.

[0057] In addition to the above embodiments, the present invention may also have other implementation modes; any technical solutions formed by equivalent replacement or equivalent transformation shall fall within the protection scope required by the present invention.

Claims

1. A rolling cooling process for a medium carbon hub bearing steel containing Nb, characterized in that: The process includes the following steps: heating, rough rolling, continuous rolling, temperature waiting, pre-finishing rolling, final rolling and cooling; Wherein, in the heating process, the chemical composition and mass percentage of the continuous casting billet include: C: 0.53-0.58%, Si: 0.20-0.35%, Mn: 0.75-0.90%, P: ≤0.020%, S: ≤ 0.005%, Cr: 0.20-0.35%, Al: 0.05-0.050%, Nb: 0.10-0.20%, and the rest are matrix Fe and unavoidable impurities.

2. The rolling cooling process of the Nb-containing medium-carbon wheel hub bearing steel according to claim 1, characterized in that: The rough rolling process includes: after the steel billet is taken out of the furnace and descaled, the surface temperature of the rolled piece is 1110-1130°C, and it enters the rough rolling mill for rough rolling.

3. The rolling cooling process of the Nb-containing medium-carbon wheel hub bearing steel according to claim 1, characterized in that: In the rough rolling process, the rolling passes are adjusted from seven to nine, and the reduction amounts of the third and fifth passes are smaller than those of the original third and fifth passes.

4. The rolling cooling process of the Nb-containing medium carbon wheel hub bearing steel according to claim 1, characterized in that: The continuous rolling process includes: the billet surface temperature is 1050-1060°C and enters a ten-stand continuous rolling mill group, and the entrance linear speed of the first stand is 0.17-0.25m / s, and the exit linear speed of the tenth stand is 1.2-1.5m / s.

5. The rolling cooling process of the Nb-containing medium-carbon wheel hub bearing steel according to claim 1, characterized in that: The temperature-waiting process includes: reducing the surface temperature of the rolled piece after continuous rolling to 820-850° C. by air cooling.

6. The rolling cooling process of the Nb-containing medium carbon wheel hub bearing steel according to claim 1, characterized in that: The pre-finishing rolling process includes: sending the rolled piece to a four-stand pre-finishing rolling mill group for pre-finishing rolling when the surface temperature of the rolled piece is 810-830°C.

7. The rolling cooling process of the Nb-containing medium carbon wheel hub bearing steel according to claim 1, characterized in that: The final rolling process includes: after weak water cooling of the rolled piece, sending the rolled piece to the tension reducing mill for rolling.

8. The rolling cooling process of the Nb-containing medium carbon hub bearing steel according to claim 7, characterized in that: The inlet temperature of the stretch reducing unit is 790-810°C.

9. The rolling cooling process of the Nb-containing medium carbon wheel hub bearing steel according to claim 1, characterized in that: The cooling process includes: cooling the surface temperature of the rolled piece to 710-720°C by water cooling, sending the rolled piece to a cooling bed covered with a heat preservation cover, and exiting the heat preservation cover after the surface temperature is cooled to less than or equal to 530°C.

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