Low-cost steel for hub bearing of passenger vehicle and manufacturing method of low-cost steel

Abstract

The invention relates to low-cost steel for a hub bearing of a passenger vehicle, and belongs to the technical field of iron-based alloys. The steel comprises the following chemical components in percentage by mass: 0.62 to 0.72 percent of C, 0.40 to 0.70 percent of Si, 0.90 to 1.20 percent of Mn, less than or equal to 0.010 percent of P, 0.005 to 0.015 percent of S, 0.35 to 0.65 percent of Cr, less than or equal to 0.040 percent of Al, less than or equal to 0.0010 percent of Ti, 0.05 to 0.15 percent of V, less than or equal to 0.015 percent of N, less than or equal to 2.0 percent of Ca / O and the balance of Fe and inevitable impurity elements. The microstructure is uniform ferrite and pearlite, the surface hardness is larger than or equal to 65 HRC, the subcutaneous 4mm hardness is larger than or equal to 55 HRC, and the grain size is smaller than 7.0 level. The steel has high hardenability, purity, good wear resistance and good machinability, meanwhile, the normalizing procedure after hub bearing forging can be omitted, 55 # carbon structural steel materials can be completely replaced, and the use requirements of hub bearing products are met.

Description

Technical Field

[0001] The present invention belongs to the field of iron-based alloys, and particularly relates to a bearing steel and a manufacturing method thereof. Background Art

[0002] The hub bearing is one of the key components of an automobile. It is a component used at the axle of the automobile to bear the weight and provide precise guidance for the rotation of the hub. It bears both axial load and radial load, and is an important part of the load-bearing and rotation of the automobile. It has strict requirements for the hardenability, purity and wear resistance of the material.

[0003] Currently, most hub bearing product manufacturers use 55# carbon structural steel material. Due to the relatively low carbon content, the hardenability and wear resistance are less than satisfactory. At the same time, 55# is carbon structural steel and there are few grain-refining elements in the steel. After forging, the grains of the product are relatively coarse, and a normalizing process needs to be added for treatment, which greatly increases the energy consumption cost of product manufacturing. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a low-cost steel for passenger car hub bearings in view of the above-mentioned prior art. The steel has high hardenability, purity and good wear resistance, and has good machinability. At the same time, the normalizing process after forging of the hub bearing is omitted, and it can completely replace 55# carbon structural steel material to meet the use requirements of hub bearing products.

[0005] The present invention appropriately increases the contents of C, Si, Mn and adds a small amount of V and N elements, innovates the design of the chemical composition system, omits the post-forging normalizing treatment process, and invents a new low-cost steel for hub bearings and a manufacturing method thereof.

[0006] The technical solution adopted by the present invention to solve the above problems is as follows: A low-cost steel for passenger car hub bearings, the chemical composition of the steel by mass percentage is C: 0.62 - 0.72%, Si: 0.40 - 0.70%, Mn: 0.90 - 1.20%, P: ≤0.010%, S: 0.005 - 0.015%, Cr: 0.35 - 0.65%, Al: ≤0.040%, Ti: ≤0.0010%, V: 0.05 - 0.15%, N: ≤0.015%, Ca / O ≤ 2.0, and the balance is Fe and inevitable impurity elements.

[0007] Preferably, the chemical composition of the steel in the present invention is by mass percentage: C: 0.65 - 0.70%, Si: 0.60 - 0.70%, Mn: 1.00 - 1.10%, P: ≤0.008%, S: 0.005 - 0.010%, Cr: 0.45 - 0.55%, Al: 0.010 - 0.020%, Ti: ≤0.0008%, V: 0.05 - 0.09%, N: 0.009 - 0.012%, Ca / O ≤ 1.5, and the balance is Fe and inevitable impurity elements.

[0008] The chemical composition design of the low-cost steel for passenger car wheel hubs in the present invention is determined as follows:

[0009] C is the most basic and effective strengthening element in steel, which can improve the hardenability and wear resistance of the steel, and the use cost is the most economical. However, considering the need to balance the plastic and toughness properties of the steel, C is determined to be 0.62 - 0.72%, preferably C: 0.65 - 0.70%.

[0010] Si can be used as a deoxidizer and reducing agent during steelmaking. At the same time, Si has a strong solid solution strengthening effect in steel, which can significantly improve the elastic limit, yield strength and tensile strength of the steel, especially the yield strength. A high yield ratio is beneficial to the improvement of fatigue performance. Si is determined to be 0.40 - 0.70%, preferably Si: 0.60 - 0.70%.

[0011] Mn is an effective element for deoxidation and desulfurization in steel, which can improve the strength and hardness of the steel, significantly improve the hardenability and wear resistance of the steel, and improve the hot working performance of the steel; at the same time, Mn is the element with the strongest binding force with S in steel, and the formed MnS compound helps to improve the cutting performance of the steel. However, too high Mn content has an obvious temper brittleness phenomenon and will also promote grain growth. Mn is determined to be 0.90 - 1.20%, preferably Mn: 1.00 - 1.10%.

[0012] P is a harmful impurity in steel, which is a low melting point and easy segregation element, and its content in steel needs to be controlled. P is determined to be ≤0.010%, preferably P: ≤0.008%.

[0013] S is usually also a harmful impurity in steel, but when it reaches a certain content, it can significantly improve the cutting performance of the steel. While the strength of the steel in the present invention is increased, it will inevitably cause the deterioration of the cutting performance during the processing. Therefore, S element is added artificially. S is determined to be 0.005 - 0.015%, preferably S: 0.005 - 0.010%.

[0014] Cr can effectively improve the hardenability of steel to obtain the required strength and reduce the decarburization tendency on the surface during the heating process of steel. However, Cr is a carbide-forming element, and excessive content is likely to form large and insoluble carbides, which is not conducive to processing by users. It is determined that Cr: 0.35 - 0.65%, preferably Cr: 0.45 - 0.55%.

[0015] Al is an effective deoxidizer, and the formed AlN can refine the grains. When the content is low, the effect is not obvious; when the content is high, it is easy to form coarse inclusions and deteriorate the properties of steel. It is determined that Al: ≤0.040%, preferably Al: 0.010 - 0.020%

[0016] Ti has a strong binding force with N, and the formed TiN inclusions are irregular in shape and hard in texture, affecting the fatigue life of the product. It is determined that Ti: ≤0.0010%, preferably Ti: ≤0.0008%.

[0017] V has a high solubility in steel and is one of the most effective strengthening elements. By forming V(C, N) compounds and precipitating at the grain boundaries, it affects the structure and properties of steel, thereby refining the ferrite grains, increasing the strength and toughness of steel. Since V belongs to precious alloys, it is determined that V: 0.05 - 0.15%, preferably V: 0.05 - 0.09%.

[0018] N is a common element in steel, mainly strengthening the precipitation strengthening effect and refining the grains. Increasing the content of N in steel expands the precipitation range of carbonitrides and improves the effective action of microalloying elements, enabling the same mechanical properties to be obtained with less content of microalloying elements. However, considering that N is a gas element, excessive N will significantly reduce the toughness. It is determined that N: ≤0.015%, preferably N: 0.009 - 0.012%.

[0019] Ca / O ratio

[0020] A high Ca content in steel will significantly increase the number of large-sized dot-shaped oxides and deteriorate the properties of steel, especially the fatigue life. The same is true for the O content. In the present invention, while controlling the low Ca and O contents, the Ca / O ratio is controlled to further reduce the influence of Ca and O on the fatigue life of steel. This is also a prominent feature of the composition design of the present invention. It is determined that Ca / O ≤2.0, preferably Ca / O ≤1.5. Specific embodiments

[0021] The present invention will be further described in detail below 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.

[0022] Three furnaces of the steel of the present invention are manufactured using the chemical compositions shown in Table 1.

[0023] Table 1 Chemical Compositions of the Steel in Examples 1 - 3

[0024]

[0025] Table 2 Purity and Grain Size of Steel Obtained in Examples 1-3

[0026]

[0027] Table 3 Hardness of Steel Obtained in Examples 1-3

[0028]

[0029] Manufacture the low-cost steel for passenger car wheel hub bearings in Examples 1-3 according to the following production process

[0030] Smelting

[0031] Use scrap steel and hot metal as raw materials, with the hot metal ratio ≥ 85%. The hot metal is not desulfurized to minimize the addition of sulfur (sulfur) wire in the later stage and reduce production costs. The molten steel after the initial converter refining is subjected to secondary refining outside the furnace and then degassing treatment. The mass percentages of each chemical element are strictly controlled according to the standard requirements

[0032] Continuous casting

[0033] Use a 240mm×240mm square continuous caster for casting. Control the casting superheat temperature at 15-30°C. Adopt mold electromagnetic stirring with parameters of 2Hz / 180A and final electromagnetic stirring with parameters of 8Hz / A120 to improve the surface and internal quality of the billet

[0034] Heating

[0035] Use a walking beam reheating furnace to heat the billet. The temperature of the preheating section is controlled at 780-850°C, the temperature of the heating section is controlled at 960-1060°C, and the temperature of the soaking section is controlled at 1120-1180°C. The total heating time is 220 minutes or more to fully dissolve V into austenite. The time of the preheating section needs to be ≥ 90 minutes to ensure slow heating of the billet, uniform heating, and control the surface decarburized layer not greater than 0.25mm

[0036] Rolling

[0037] After the billet exits the heating furnace, use high-pressure water descaling. The rolling start temperature is 1000-1030°C, and then use 4 roughing mills + 6 intermediate mills + 4 pre-finishing mills + 5 finishing mills (KOCKS) for rolling. After roughing, wait for 2-3 minutes until the temperature reaches 860-880°C and then enter the intermediate mill. Control the final rolling temperature at ≤ 850°C to refine the grains; the rolling specification is φ55mm

[0038] Controlled cooling

[0039] After rolling, it is transferred to the cooling bed. Spray cooling is carried out in the starting section of the cooling bed. The stepping speed is controlled ≤ 600 rpm, and 3 fans under the cooling bed are fully opened to ensure that the temperature under the cooling bed ≤ 350 °C, so as to increase the cooling speed and further refine the grains.

[0040] As can be seen from the above table, the mechanical properties of the materials manufactured in Examples 1-3 all meet the requirements of the invention design, obtaining a fine-grained bearing steel with high hardenability, purity and good wear resistance, and the manufacturing process flow is simple, having high application value and advantages.

[0041] In addition to the above embodiments, the present invention also includes other implementation manners. Any technical solutions formed by equivalent transformation or equivalent substitution shall fall within the protection scope of the claims of the present invention.

Claims

1. A low-cost steel for passenger car wheel hub bearings, characterized by: The chemical composition mass percentage of the steel is C: 0.62-0.72%, Si: 0.40-0.70%, Mn: 0.90-1.20%, P: ≤0.010%, S: 0.005-0.015%, Cr: 0.35-0.65%, Al: ≤0.040%, Ti: ≤0.0010%, V: 0.05-0.15%, N: ≤0.015%, Ca / O≤2.0, and the balance is Fe and unavoidable impurity elements.

2. The steel according to claim 1, characterized in that: The microstructure is uniform ferrite + pearlite, with surface hardness ≥65HRC, subcutaneous 4mm hardness ≥55HRC, and grain size finer than grade 7.

0.

3. The steel according to claim 1, characterized in that: The chemical composition mass percentage of the steel is C: 0.65-0.70%, Si: 0.60-0.70%, Mn: 1.00-1.10%, P: ≤0.008%, S: 0.005-0.010%, Cr: 0.45-0.55%, Al: 0.010-0.020%, Ti: ≤0.0008%, V: 0.05-0.09%, N: 0.009-0.012%, Ca / O≤1.5, and the balance is Fe and unavoidable impurity elements.

4. A method for manufacturing the steel according to claim 1, characterized in that: include, Step 1, smelting: using scrap steel and molten iron as raw materials, the iron-to-metal ratio is ≥85%, the molten iron is not desulfurized, the raw materials are initially smelted in a converter, then refined outside the furnace, and then degassed, and the mass percentage of each chemical element is regulated according to the percentage content of the element; Step 2: Continuous casting: Use a continuous casting machine for casting, control the casting superheat at 15-30°C, and use electromagnetic stirring in the crystallizer and at the end to improve the surface and internal quality of the ingot; Step 3: Heating: heating the ingot to austenitize the structure and fully dissolve the elements in the austenite; Step 4, rolling: after the ingot is out of the heating furnace, it is descaled with high-pressure water, the rolling temperature is 1000-1030℃, and 4 initial rolling mills + 6 intermediate rolling mills + 4 pre-finishing rolling mills + 5 KOCKS finishing rolling mills are used for rolling. After the initial rolling, the temperature is kept at 860-880℃ for 2-3min before entering the intermediate rolling mill, and the final rolling temperature is controlled at ≤850℃ to refine the grains; Step 5: Controlled cooling: After rolling, the steel is placed on a cooling bed. The initial section of the cooling bed is spray-cooled. The stepping speed is controlled to be ≤600rpm, and the fan under the cooling bed is fully opened to ensure that the temperature of the lower cooling bed is ≤350℃. The cooling speed is increased to further refine the grains.

5. The method according to claim 4, characterized in that: Step 2: The crystallizer electromagnetic stirring process parameters are 2Hz / 180A, and the end electromagnetic stirring process parameters are 8Hz / A120.

6. The method according to claim 4, characterized in that: Step three, use a walking beam heating furnace to heat the ingot, the temperature of the preheating section is controlled at 780-850°C, the temperature of the heating section is controlled at 960-1060°C, the temperature of the soaking section is controlled at 1120-1180°C, and the total heating time is 220 minutes or more, so that V is fully dissolved in austenite, and the preheating section time must be ≥90 minutes to ensure that the ingot is slowly heated, fully and evenly heated, and the surface decarburization layer is controlled to be no larger than 0.25mm.

7. The method according to claim 4, characterized in that: Step 2 uses a 240mm×240mm square continuous casting machine, and step 4 rolls the steel into φ55mm round steel.

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