Cooling and final rolling temperatures for different steel grades

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During hot working, the finish-straining temperature has a significant effect on the steel’s microstructure. The higher this temperature is, the greater the tendency for grain coarsening; consequently, the resulting austenite grains become larger. Therefore, it is desirable to minimize the finish-straining temperature during processing. However, it generally should not be set too low. In other words, grain refinement and product quality improvement are achieved by controlling both rolling and cooling processes. The deformation termination temperature for low-carbon steel should be maintained at around 800°C; at a minimum, it should not fall below 750°C. For high-carbon steel, in order to prevent the formation of networked cementite, the deformation termination temperature during shaping should be controlled at around 850°C. If this deformation termination temperature is combined effectively with rapid cooling after rolling, it is possible to completely suppress the precipitation of pro-eutectoid cementite, thereby avoiding the formation of networked cementite; and if such cementite does form, it will be thin and easy to remove, without the need for additional processing steps. After rolling of hypereutectoid carbon steels and alloy steels, the excess cementite forms a network of cementite (carbide) distributed along the grain boundaries. Steels with a cementite network reduce their cold deformation capacity and increase the tendency to develop quenching cracks. Steel requires complex heat treatments to eliminate the cementite network; however, these heat treatments do not always achieve the desired results. Therefore, conditions must be created such that no cementite networks form in the steel after rolling. Final rolling at low temperatures and relatively rapid cooling after steel rolling can achieve the purpose. For example, in the case of GCr15, in order to reduce the temperature before final rolling, a water cooling system is installed ahead of the finishing rolling mill; rapid cooling takes place after rolling, and compressed air can be used followed by cooling in a slow-cooling pit. Post-rolling slow cooling results in coarse ferrite grains, along with a decrease in yield point and an increase in the brittle transition temperature. The cooling rate is related to the cross-sectional size of the steel; it is difficult to achieve rapid cooling for steel with a large cross-section. Therefore, for steel of the same grade, the mechanical properties of steel with a larger cross-section are somewhat lower. Overseas, round steel is typically air-cooled after rolling; this is directly related to the low gas content in foreign steels. Online through-water cooling yields better results. The specifications of round steel subjected to through-water cooling are all below Ф75mm. However, applying this technology often results in excessively long water cooling lines, thereby increasing investment costs. Nevertheless, rapid cooling helps to reduce the formation of secondary banded structures. On the other hand, when austenite grains are coarse—especially in steels with a high manganese content—rapid cooling may lead to the formation of Widmanstätten ferrite. Therefore, rapid post-rolling cooling must be combined with a low deformation termination temperature. When the deformation termination temperature is low and the austenite grains are relatively fine, even rapid cooling will not result in the formation of Widmanstätten ferrite. After being rolled on medium-sized rolling mills, alloy structural steel with a diameter of 60 mm or less is air-cooled in piles, while that with a diameter greater than 60 mm is cooled in unheated slow-cooling pits. The time required for the steel to cool down to 100–150°C in the pit should be no less than 30 hours. Ball bearing steel has a tendency to develop white spots; therefore, needle bearing steel should be cooled slowly after rolling, or heat-treated according to specified procedures. The temperature during loading should be no lower than 700°C, and the billet should be left in the pit until its temperature drops to between 100–200°C, over a period of 72 hours on average. Even when the final rolling temperature is low, slow cooling of the steel still results in the formation of cementite networks within it. During slow cooling, no cementite network is formed when the temperature is below 650°C; therefore, to avoid the formation of such a network, each rod of steel is cooled separately after rolling, so as to be cooled as quickly as possible directly to a temperature below 650°C. The cooling rate required to ensure a ball bearing steel free of cementite networks depends on the final rolling temperature; when the final rolling temperature is between 900–950°C, the cooling rate must be at least 45–50°C/min. As the final rolling temperature decreases, the cooling rate can be reduced. Maintaining an appropriate final finish-rolling temperature (a temperature in the vicinity of this value) and employing a suitable reduction rate (about 40%) in the finish-rolling mill enables low- and medium-carbon steels, as well as medium-alloy steels such as alloy steels, spring steels, and bearing steels, to achieve the desired metallographic structure and optimal mechanical properties. To this end, a water cooling box is installed before the last two stands of the bar finish-rolling mill; furthermore, a temperature equalization section is placed prior to the bar finish-rolling mill to ensure uniform temperature throughout both the interior and exterior of the rolled products after rapid cooling. After rolling, steel can be cooled using the following methods: 1. In air. 2. In materials with low thermal conductivity. 3. In the incubator. 4. In an insulated pit without heating equipment. 5. Preheat and insulate the pits and furnaces. 6. In insulated pits and furnaces equipped with heating devices. 7. In water. The following are the typical methods employed for controlled rolling and cooling of representative steel grades: 1. Bearing steel and spring steel: It is required to complete finish rolling at low temperatures; after rolling, slow cooling under heat preservation is necessary. For bearing steel, in order to prevent the precipitation of network-like carbides, rapid cooling followed by slow cooling is carried out after rolling. The final rolling temperature of bearing steel is strictly controlled at 800–850°C to facilitate the fragmentation of network-like carbides. When the final rolling temperature exceeds 900°C, water can be sprayed onto the steel to rapidly cool it to 600–650°C (to prevent further precipitation of network-like carbides), after which it is cooled slowly. To this end, a cooling water tank is installed in front of the finish rolling mill to control the temperature of the rolled strips entering the mill. 2. Quenched and tempered steel (treated with both quenching and high-temperature tempering). The microstructure of quenched and tempered steel is tempered sorbite; such steel possesses high strength and yield limits, as well as sufficient ductility and toughness, thereby having excellent overall mechanical properties. Steel that has undergone quenching and tempering is mainly used for high-strength components subject to impact or alternating loads, such as connecting rods and shafts. Based on the product portfolio for continuous rolling: high-quality carbon structural steel: 225,000 tons; alloy structural steel: 225,000 tons. These two types account for 90% of the total production volume. Effective temperature control over such large quantities of steel provides a competitive advantage. 3. High-quality carbon structural steels and alloy structural steels – Online temperature control for round bars of larger dimensions. Manufacturer: ABS LUNA plant in Udine, Italy. Production specifications: Round bars with diameters ranging from ∮20 to ∮100 mm. Types of steel include: carbon steel, surface-hardened steel, quenched and tempered steel, microalloyed steel, bearing steel, spring steel, and stainless steel. Online temperature control is applied to round bars with diameters from ∮20 to ∮90 mm. Given the current positioning issues of Shigang’s products, and as the customers in the steel industry change, it has become necessary to supply steel for use in automobiles and to move towards higher-end markets. Providing customers with an ideal microstructure and the best mechanical properties gives a competitive advantage. When considering cooling systems, water cooling boxes should be installed before and after the finishing rolling mills, especially for round bars smaller than 40 mm, in order to achieve online temperature control. A water cooling box is installed after the finish rolling mill; abroad, it is believed that for large-diameter round bars, this setup can only help remove scale and improve surface quality, with little effect on grain refinement. In fact, it may lead to uneven grain sizes within the bars. Implementing online temperature control will undoubtedly lengthen the rolling line and increase costs. There are few manufacturers that provide references regarding the length of water cooling boxes to be installed after the finishing mill; only the Italian company ABSLUNA offers one with a length of 55 meters. From the perspective of long-term development and quality requirements, online temperature control should be considered. Installing a water cooling box after the finishing mill can at least remove scale and improve the surface quality. The heating, final rolling, and cooling procedures for various types of steel are shown in Table 1: I. Controlled rolling 1. Theory of controlled rolling During the hot rolling process, by properly controlling the heating, deformation, and temperature parameters, solid-state phase transformation is combined with thermoplastic deformation, thereby achieving a fine-grained structure that confers excellent overall mechanical properties on the steel. For low-carbon steel and low-alloy steel, the controlled rolling process aims to refine the deformed austenite grains by controlling various process parameters. Through the phase transformation of austenite into ferrite and pearlite, fine ferrite grains and relatively small pearlite aggregates are formed, thereby improving the strength, toughness, and weldability of the steel ； For high-carbon steel and hypereutectoid steel, temperature-controlled rolling is employed to refine the grains of deformed austenite, with finishing rolling carried out near the austenite transformation point. 2. Hot mechanical rolling: Currently, the specifications for hot mechanically rolled round steel are limited to diameters of ∮40 or less. The steel used is mainly low-carbon steel and low-alloy steel. The primary purpose of this process is to refine the ferrite grains. The final rolling is carried out at temperatures between 750°C and 790°C; water cooling is applied both before and after the finish rolling. For round steels of larger sizes, after water cooling, the temperature difference between the surface and the core is significant; as a result, fine cracks tend to form on the surface after rolling ； During recrystallization after rolling, the grain sizes at the core and the surface differ, resulting in a non-uniform microstructure in the cross-section of the rod. 3. Normalizing rolling: For round bars with a diameter of ∮40–∮80, normalizing rolling can be employed. The total deformation amount in the last four passes should be 50–60%. Isostatic treatment is carried out before entering the finishing mill; the final rolling temperature is 800°C–850°C, followed by rapid cooling after rolling. 4. Temperature-controlled rolling: The final rolling temperature is 850°C–900°C, with controlled cooling after rolling to improve surface quality. For high-carbon steel, relatively fine pearlite spheres can be obtained ； For hypereutectoid steels, it can mitigate the precipitation of network carbides. II. Shigang employs a rolling process to produce round bars with diameters ranging from ∮50 to ∮80 using steel grades such as 20#, 45#, 20CrMo, 20CrMnTi, 40Cr, and 40MnB. The normalizing rolling process can be utilized for this purpose; however, isothermal treatment is required prior to feeding the material into the finish rolling mill. This adds to the overall processing steps and reduces production output. Moreover, the degree of deformation increases in the final four passes. To ensure higher product precision and uniform deformation across the cross-section of the rolled pieces, it is advisable to install a sizing mill, which entails additional investment costs. For bar sizes above ∮80, temperature-controlled rolling must be employed ； For the production of spring flat steel, a hot mechanical rolling process can be employed; final rolling is carried out in the ferritic-austenitic two-phase region to refine the deformed austenite grains. Through the phase transformation from austenite to ferrite and pearlite, fine ferrite grains and relatively small pearlite aggregates are formed, thereby enhancing the strength and toughness of the steel. However, water cooling is required both before and after finish rolling, which increases investment costs and extends the overall process length within the rolling area ； For bearing steel, temperature-controlled rolling must be employed throughout to prevent the precipitation of network-like carbides and to improve surface quality. From the perspectives of investment and process location, Shigang employs a temperature-controlled rolling process: lowering the start-rolling temperature, controlling the finish-rolling temperature, and implementing controlled cooling after rolling, in order to achieve good surface quality and relatively favorable internal microstructure.