Cooling and final rolling temperatures for different steel grades

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During hot processing, the deformation termination temperature has a significant impact on the microstructure of steel. The higher the deformation termination temperature, the greater the tendency for grains to aggregate and grow, resulting in larger austenite grains. Therefore, it is necessary to minimize the deformation termination temperature during processing; however, it should generally not be lower than Ar3. Grain refinement and improved product quality are achieved by using controlled rolling and controlled cooling methods. 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 hypereutectoid carbon steels and alloy steels, excess cementite forms a network of cementites (carbides) distributed along the grain boundaries. Steel with a cementite network reduces its cold deformability and increases the tendency to develop quench cracks. Steel requires complex heat treatment to eliminate the cementite network, and heat treatment does not always achieve the desired result. Therefore, it is necessary to create conditions such that no cementite network is formed in the steel after rolling. Final rolling at low temperatures and relatively rapid cooling after steel rolling can achieve the goal. 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 slow cooling in a cooling pit. Slow cooling after rolling results in large ferrite grains, along with a decrease in yield strength 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 when the manganese content in the steel is high—rapid cooling may lead to the formation of Widmanstätten ferrite. Therefore, rapid cooling after rolling must be combined with a low deformation termination temperature; at lower deformation termination temperatures where 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 stacks, 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, after rolling, needle roller bearing steel should be cooled slowly or subjected to heat treatment according to specified procedures. The temperature during charging should not be lower than 700°C. The steel billets must be left in pits for an average of 72 hours until their temperature drops to no more than 100–200°C. Even when the final rolling temperature is relatively low, slow cooling of the steel afterward still results in the formation of a cementite network 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 obtain 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 no less than 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 around Ac3), along with 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 attain ideal metallographic structures 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. In the preheating holding pit and furnace. 6. In insulated pits and furnaces equipped with heating devices. 7. In water. The following are the representative methods for controlled rolling and controlled cooling of various steel grades: 1. For bearing steel and spring steel, finish rolling at low temperatures, and after rolling, slow cooling is required; to prevent the formation of networked carbides in bearing steel, rapid cooling is followed by slow cooling 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. Quenched and tempered steel is mainly used for important components that require high strength and must withstand impact or alternating loads, such as connecting rods and shafts. Based on the outline of continuous rolling products: high-quality carbon structural steel: 225,000 tons, alloy structural steel: 225,000 tons. These account for 90% of the total production volume; maintaining temperature control over such a large amount of steel provides an advantage in terms of competitiveness. 3. High-quality carbon structural steel and alloy structural steel: Both high-quality carbon structural steel and alloy structural steel belong to the category of hypoeutectoid steels. The quenching temperature for such steels is 30–50°C above AC3. For round bars with a diameter of less than 40 mm, cooling tanks are installed before the finishing rolling mill; this helps to refine the grain structure, resulting in a martensitic structure after quenching. Then, high-temperature tempering is performed. Tempering involves heating the quenched steel below the A1 temperature, thereby causing it to transform into a stable tempered structure. Manufacturer providing online temperature control for larger-sized round bars: ABS LUNA plant in Udine, Italy. Production specifications: round bars with diameters ranging from ∮20 to ∮100 mm. Steel types 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. By providing customers with an ideal microstructure and the best mechanical properties, a competitive advantage can be achieved. 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. Overseas, it is believed that for large-sized round bars, this device can only remove scale and improve surface quality; it has little effect on grain refinement. On the contrary, it may lead to uneven grain sizes within the round bars. Implementing online temperature control would undoubtedly lengthen the rolling line and increase investment 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 conditions, solid-state phase transformations can be combined with thermoplastic deformation. This results in a fine-grained microstructure, thereby endowing the steel with excellent overall mechanical properties. 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 steels and hypereutectoid steels, temperature-controlled rolling technology is employed to refine the deformed austenite grains, with final rolling performed 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 steel, the precipitation of reticular carbides can be reduced.