Production method for reducing surface stress cracks of steel for medium carbon hub bearing

Abstract

The invention relates to a production method for reducing surface stress cracks of steel for medium carbon hub bearings, and belongs to the field of metallurgy. Comprising the following steps: setting the heating temperature and the heat preservation time according to the temperature of the casting blank before entering a furnace; the casting blanks are cooled on a cooling bed, the cooling bed is provided with a heat preservation cover, the casting blanks are placed in the heat preservation cover to be slowly cooled before being fed into a furnace to be heated, a plurality of casting blanks are continuously conveyed, and the casting blanks need to be fed into the furnace to be heated within 10 min after being conveyed to a steel rolling rack. The periphery of the steel rolling rack can be protected by the heat insulation plates to achieve a heat insulation effect so as to reduce blank temperature difference. By means of the method, stress cracks on the surface of the casting blank are expected to be reduced, and therefore qualified medium-carbon bearing steel products can be produced and delivered stably and efficiently to meet the requirements of customers.

Description

Technical Field

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

[0002] With the rise of new energy vehicles, the demand for automotive steel has shrunk significantly. The contract orders of manufacturers such as engines and transmissions have decreased, and the quality competition and cost competition have become increasingly severe. Various users have continuously created new processing technologies to achieve the goal of continuous cost reduction. For example, the dimensional requirements for medium-carbon hub bearing products have become increasingly strict, and the machining allowance has decreased, which brings certain difficulties to the salvage of defective products on the production site. Therefore, improving product quality, reducing surface cracks of bars, while reducing the finishing processing cycle and reducing the cost of subsequent processing to achieve the goals of "high quality, low cost, fast production, and excellent performance" increases the competitiveness of products.

[0003] Based on the production requirements of the above-mentioned products and combined with the current production process path, the following difficulties mainly exist:

[0004] Currently, steelmaking production is continuous according to steel grades, but the conversion frequency of different specifications of the same steel grade in rolling is relatively high. Therefore, to ensure the production of the same specification, other varieties of materials will be interspersed in the production of medium-carbon steel. There are many uncertainties in the realization of key parameters of the heating system, and it is difficult to accurately control key parameters such as the temperature and time in the preheating section.

[0005] With the increase in the conversion frequency of production specifications, the small-batch production method affects whether the billets can be continuously charged into the furnace before rolling. Therefore, there must be a close connection between steelmaking and rolling, and the whereabouts of the continuous casting billets must be controlled in advance. Otherwise, the billets will cool for a long time on the charging table, resulting in a large temperature difference and easy formation of cracks.

[0006] During the cooling and crystallization process of continuous casting billets, various oxides and other inclusions in the steel after deoxidation continuously aggregate and are prone to precipitate at grain boundaries. At the same time, affected by the component Mn, the driving force for the nucleation of ferrite phase transformation increases, thus easily generating ferrite bands along the grain boundaries. These bands are also prone to crack defects under the influence of rolling stress during the subsequent rolling process. Summary of the Invention

[0007] Due to the changes in the production process and the influence of environmental temperature, the continuous casting billets of medium-carbon bearing steel are affected by temperature difference, resulting in tensile stress on the surface of the billets. The bonding ability at the grain boundaries is also relatively weak due to the influence of components. Therefore, cracks are easily generated on the surface of the billets. The present invention reduces the stress cracks on the surface of the billets by adjusting the pre-rolling heating process and controlling the cooling method of continuous casting billets, in order to stably and efficiently produce and deliver qualified bar products to meet the usage requirements.

[0008] The technical solution adopted by the present invention to solve the above problems is as follows: A production method for reducing surface stress cracks of medium-carbon hub bearing steel, including the furnace heating process of the continuous casting billet on the rolling mill bench: Select a suitable heating process according to the temperature detection before entering the furnace as follows. This process is used for billets with specifications of 300*300mm to 300*340mm.

[0009] Furnace inlet temperature °C Preheating section temperature °C Heating section temperature °C Soaking section temperature °C 350-550 750-900 1050-1200 1150-1200 500-650 800-950 1050-1200 1150-1200 Cold charging 650-850 1050-1200 1150-1200

[0010] The warm feeding temperature is between 350 - 550°C, and the heating time for each section is: soaking section ≥ 50 minutes, heating section ≥ 70 minutes, preheating section ≥ 70 minutes; ② The warm feeding temperature is between 500 - 650°C, and the heating time for each section is: soaking section ≥ 50 minutes, heating section ≥ 70 minutes, preheating section ≥ 50 minutes; ③ Cold charging process (room temperature): The heating time for each section is: soaking section ≥ 60 minutes, heating section ≥ 80 minutes, preheating section ≥ 80 minutes.

[0011] The continuous casting billet is provided with a heat preservation cover with strong heat preservation effect on the cooling bed, which can cover the cooling bed. After the continuous casting billet is placed on the cooling bed, slow cooling in the heat preservation cover or direct transportation is selected according to the actual situation of rolling. The steel billets are sent to the rolling mill bench one by one. Under normal circumstances, multiple continuous casting billets should not be transported simultaneously to prevent excessive temperature difference. After the continuous casting billet is transported to the rolling mill bench, it must enter the heating furnace within a short time (10 minutes). The surrounding of the rolling mill bench can be protected by heat insulation boards to achieve the effect of heat preservation and reduce the temperature difference of the billets.

[0012] Control and reduce the formation of ferrite bands on the surface or near the surface of the continuous casting billet, reduce the aggregation of oxides, and improve the bonding ability of the grain boundaries of the continuous casting billet. Therefore, in steelmaking, deoxidation control should be strengthened, and methods such as adding vibration method and chemical treatment (adding TI, Ni, Cr) are used to refine the grains, thereby strengthening the grain boundary bonding force.

[0013] Compared with the prior art, the advantages of the present invention are as follows: In order to reduce the processing cost of current mainstream products, improve product quality, and achieve rapid delivery to meet customer needs. Secondly, to improve the product grade, more and more customers pursue precision rolling products with higher dimensional accuracy, and customers stipulate that the material is not allowed to be locally ground. Therefore, once there are batch problems on the surface, the margin for subsequent salvage treatment is smaller, the size of defective products cannot meet the delivery requirements, and scrapping or remanufacturing has an adverse impact on production. Therefore, through the implementation of the above measures and the feedback of product quality, the product qualification rate can be increased by 10% - 15%, solving the problems of many defective products, long processing cycle, and high processing cost. The present invention reduces the generation of surface stress cracks of the continuous casting billet through the redesign and improvement of the cooling method of the continuous casting billet and the pre-rolling heating, and can stably produce high-quality products, quickly deliver, and meet the requirements of customer hub bearing steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 It is one of the metallographic diagrams of stress cracks of the existing continuous casting billets for medium-carbon bearing steel;

[0015] Figure 2 It is the second of the metallographic diagrams of stress cracks of the existing continuous casting billets for medium-carbon bearing steel;

[0016] Figure 3 It is the temperature schematic diagram of each position of the billet before the billet of the present invention enters the furnace. Detailed implementation manners

[0017] 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.

[0018] Based on the flaw detection data of the previous production history and the quality of the products feedback by users, the main problems of the products are stress cracks. The crack depth ranges from 0.1 to 0.7 mm. If the crack depth exceeds the standard, it is basically irreparable, and the cost of peeling and re-inspection needs to be increased; if the crack is within the flaw detection agreement range and reaches the customer, it will cause the return of the product or huge claims from users. We analyzed the continuous casting, heating process and flaw detection pass rate of the furnace numbers in the previous production history through big data, and formulated measures. Through continuous process exploration and process test practice, a new and stable production method was determined.

[0019] 1. The main component range of the steel for medium-carbon hub bearings and the specifications of the continuous casting billets. The chemical composition of the steel for medium-carbon hub bearings is wt[%]:

[0020] Main components C Si Mn Cr Ni Composition range 0.52-0.58 0.15-0.25 0.5-0.9 0.1-0.2 ＜0.25

[0021] The end face dimensions involved in the continuous casting billets are billets of 300*300 mm or 300*340 mm.

[0022] 2. Appropriate heating processes are mainly selected according to the temperature detection before entering the furnace. The temperature processes of the heating section and the soaking section remain unchanged on the original basis. Precise control is carried out for the temperature of the preheating section and the three-section time. The specific heating methods are as follows:

[0023] Furnace inlet temperature °C Preheating section (minutes) Heating section (minutes) Soaking section (minutes) Preheating section temperature °C Heating section temperature °C Soaking section temperature °C 350-550 ≥70 ≥70 ≥50 750-900 1050-1200 1150-1200 500-650 ≥50 ≥70 ≥50 800-950 1050-1200 1150-1200 Cold charging process ≥80 ≥80 ≥60 650-850 1050-1200 1150-1200

[0024] For the cold charging process, since the temperature of the preheating section is relatively high, other low-alloy steel continuous casting billets with medium and low carbon can be used to suppress the fire, or the method of empty steps can be selected to reduce the furnace temperature. If possible, the heating furnace can adopt the method of closing the burners in the preheating section and slowly heating through the remaining temperature in the heating section. The gas flow rate in the preheating section shall not exceed 5000 m 3 / h. When the furnace temperature in the preheating section exceeds the specified upper limit, the gas shall be immediately shut off.

[0025] 3. Pretreatment method of billets before entering the heating furnace: When continuous casting starts to produce medium-carbon hub bearings, the heat preservation cover is enabled. If there is a baking device, the surface preheating of the steel billets can be started to supplement the surface temperature loss of the cast billets, so that the internal and external temperature difference is small and the internal stress is reduced. At the connection between steelmaking and rolling, check whether the transfer machine is in normal use. After the continuous casting billets are transferred to the cooling bed, according to the actual situation of rolling, choose to slow cool in the heat preservation cover or directly through the transfer machine, and send the steel billets to the heating furnace one by one. Under normal circumstances, do not use fixtures to clamp and send multiple billets at the same time to prevent excessive temperature difference between the contact surface and the non-contact surface and the generation of crack sources due to stress concentration. After the rolling stock is loaded, it must enter the heating furnace within a short time. The lower the ambient temperature, the faster it should enter the furnace. The surrounding of the rolling bench can be protected by heat insulation boards to achieve the effect of heat preservation, so as to reduce the temperature difference of the billets. The heating furnace should monitor the detected temperature of the furnace in real time. When the temperature is low, contact the continuous casting process in time and take measures such as passing the billets through the heat preservation cover to increase the temperature of the cast billets. When there are many specifications to be converted, the person in charge of the rolling site should contact the steelmaking in time and do not arrange to transfer the billets to the rolling furnace roller table first. You can choose to pass the slow cooling cover or keep the billets in the pit for a longer time, and then send the billets in the pit to rolling when the conditions are met.

[0026] In summary, through the improvement and innovation of the continuous casting cooling process and heating process, and their application to the production process of the whole-process products, the various indicators of the rolled products are guaranteed, stable and efficient production is achieved, and the needs of users are met at the same time.

Claims

1. A production method for reducing surface stress cracks of medium carbon hub bearing steel, characterized in that: Including the heating process before the billet is rolled, the heating temperature and holding time are set according to the billet temperature before entering the furnace: the furnace temperature is 350-550℃, the preheating section temperature is 750-900℃, and the preheating section heating time is ≥70 minutes; the heating section temperature is 1050-1200℃, and the heating section heating time is ≥70; the soaking section temperature is 1150-1200℃, and the soaking section heating time is ≥50 minutes; the furnace temperature is 500-650℃, the preheating section temperature is 800-950℃, and the preheating section temperature is 1000-1200℃. Heating time in hot section ≥50 minutes; temperature in heating section 1050-1200℃, heating time in heating section ≥70; temperature in soaking section 1150-1200℃, heating time in soaking section ≥50 minutes; furnace entry temperature is room temperature, preheating section temperature 650-850℃, heating time in preheating section ≥80 minutes; heating section temperature 1050-1200℃, heating time in heating section ≥80; temperature in soaking section 1150-1200℃, heating time in soaking section ≥60 minutes.

2. The method according to claim 1, characterized in that: The method is suitable for casting billets with specifications of 300*300mm-300*340mm or round billets with equivalent cross-sectional sizes.

3. The method according to claim 1, characterized in that: The ingots are cooled on a cooling bed, which is equipped with a heat preservation cover. The ingots are placed in the heat preservation cover for slow cooling before entering the furnace for heating. Multiple ingots are continuously conveyed. After the ingots are conveyed to the rolling stand, they must be heated in the furnace within 10 minutes.

4. The method according to claim 3, characterized in that: Insulation plates are arranged around the rolling stand to provide insulation and protection for the ingots.

5. The method according to claim 1, characterized in that: The ingot of the medium carbon hub bearing steel is based on iron, and other elements include: C 0.52-0.58 wt%, Si 0.15-0.25 wt%, Mn 0.5-0.9 wt%, Cr 0.1-0.2 wt%, and Ni < 0.25%.

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