不同钢种的冷却和终轧温度

During hot working, the deformation termination temperature significantly affects the microstructure of steel. The higher the deformation termination temperature, the stronger the tendency for grain coarsening and growth, resulting in larger austenite grains. Therefore, the deformation termination temperature during processing should be minimized as much as possible, though generally not below Ar3. Grain refinement and improved product quality can thus be achieved by controlling rolling and cooling processes.

For low-carbon steels, the deformation termination temperature should be maintained near 800°C, but not lower than 750°C. For high-carbon steels, to prevent the formation of network cementite, the deformation termination temperature during processing should be controlled around 850°C. If this temperature is properly coordinated with rapid cooling after rolling, the precipitation of proeutectoid cementite can be effectively suppressed, avoiding or minimizing the formation of network cementite. Even if some forms, it will be fine and thin, easily removable without requiring additional processing steps.

In hypereutectoid carbon and alloy steels, excess cementite (carbide) forms a network along grain boundaries after rolling. Steel containing such cementite networks exhibits reduced cold workability and increased susceptibility to quenching cracks. To eliminate these networks, complex heat treatments are required, which are not always effective. Therefore, conditions must be created so that no cementite network forms during rolling. This can be achieved through finishing rolling at relatively low temperatures and rapid cooling after rolling. For example, in GCr15 steel, water-cooling devices are installed before the finishing mill to reduce the temperature prior to final rolling, followed by rapid cooling—using compressed air blowing and then transferring into a slow-cooling pit. However, slow cooling after rolling leads to coarse ferrite grains, reduces yield strength, and increases brittle transition temperature. Cooling rate depends on the cross-sectional size of the steel; larger sections are difficult to cool rapidly. Thus, for the same grade of steel, larger cross-sections typically exhibit inferior mechanical properties. In foreign practices, round bars are usually air-cooled after rolling, which is directly related to the low gas content in their steel. Online water-quenching would offer better results, although this technique is generally applied only to bars smaller than Ф75 mm. Water-quenching systems often result in long cooling lines and increased investment costs. Nevertheless, rapid cooling helps reduce secondary banding. However, when austenite grains are coarse—especially in steels with high manganese content—rapid cooling may lead to Widmanstätten ferrite formation. Therefore, rapid cooling after rolling must be combined with a low deformation termination temperature. When the deformation termination temperature is low and austenite grains are fine, even rapid cooling will not induce Widmanstätten ferrite. After rolling on medium-sized mills, alloy structural steels with diameters under 60 mm are cooled in air, while those above 60 mm are cooled in unheated slow-cooling pits. The time required for cooling in the pit to reach 100–150°C should be no less than 30 hours. Bearing steels have a tendency to develop white spots, so they should be slowly cooled after rolling or subjected to specified heat treatment procedures. During loading, the temperature should not be lower than 700°C, and billets should remain in the pit until the average temperature drops to no more than 100–200°C over approximately 72 hours. Even with relatively low final rolling temperatures, slow cooling may still result in cementite network formation. Cementite networks do not form when cooling begins below 650°C. Therefore, to avoid network formation, each bar should be individually cooled as quickly as possible directly to below 650°C. The cooling rate required to obtain bearing steel free from 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 required cooling rate can be reduced accordingly. Controlling the appropriate final finishing temperature (near Ac3) and applying suitable compression rates (approximately 40%) in the finishing mill can achieve ideal microstructures and optimal mechanical properties for low- and medium-carbon steels, as well as alloy steels such as spring steel and bearing steel. To this end, a water-cooling chamber is installed before the last two stands of the rod rolling mill, and a homogenizing section is placed prior to the finishing mill to ensure uniform internal and external temperatures after rapid cooling. After rolling, steel products can be cooled using several methods: 1. In air; 2. In materials with low thermal conductivity; 3. In insulation chambers; 4. In insulated pits without heating equipment; 5. In preheated insulated pits or furnaces; 6. In insulated pits or furnaces equipped with heating devices; 7. In water. The following are representative controlled rolling and controlled cooling methods used for specific steel grades: 1. Bearing steel and spring steel require finishing at low temperatures, followed by holding and slow cooling. To prevent the precipitation of network carbides, bearing steel should be rapidly cooled immediately after rolling, then slowly cooled. The final rolling temperature for bearing steel must be strictly controlled between 800–850°C to break up network carbides. If the final rolling temperature exceeds 900°C, water spraying is applied to quickly cool the steel to 600–650°C (to prevent further precipitation of network carbides), followed by slow cooling. For this purpose, a water-cooling chamber is installed before the finishing mill to control the incoming billet temperature. 2. Quenched and tempered steel (a dual treatment of quenching and high-temperature tempering) has a microstructure of tempered sorbite. This type of steel exhibits both high ultimate and yield strength, as well as sufficient ductility and toughness, thus offering excellent overall mechanical properties. Quenched and tempered steel is primarily used for critical components requiring high strength and subjected to impact or alternating loads, such as connecting rods and shafts. According to the continuous rolling product plan: 225,000 tons of high-quality carbon structural steel and 225,000 tons of alloy structural steel, accounting for 90% of total production. Precise temperature control over such large volumes of steel provides a significant advantage in enhancing competitiveness. 3. High-quality carbon structural steel and alloy structural steel both belong to hypo-eutectoid steels. The quenching temperature for hypo-eutectoid steels is 30–50°C above AC3. For round bars with diameters less than 40 mm, a cooling water tank is installed before the finishing mill to refine grain size and obtain martensitic microstructure after quenching. Subsequently, high-temperature tempering is performed—tempering involves heating the quenched steel below A1 to transform it into a stable tempered microstructure.

For larger-diameter round bars, online temperature control is implemented by manufacturers such as ABS LUNA in Udine, Italy. This facility produces round bars ranging from ∮20 to ∮100 mm, including 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 of ∮20 to ∮90 mm. Given current product positioning at Shijiazhuang Steel (Shi Gang), as customer demands evolve, providing automotive-grade steel and expanding into new markets has become essential. Delivering ideal microstructures and optimal mechanical properties gives a competitive edge. When designing cooling processes, water-cooling tanks should be installed before and after the finishing mill, primarily targeting round bars under 40 mm in diameter for effective online temperature control.

Installing a water-cooling tank after the finishing mill is considered by foreign experts to have limited effectiveness for large-diameter round bars. While it may remove oxide scale and improve surface quality, it offers little benefit in grain refinement and may instead lead to non-uniform grain sizes within the bar. Implementing online temperature control would inevitably extend the rolling line and increase investment. There are few reference cases regarding the length of post-finishing mill water-cooling tanks; only the ABS LUNA plant in Italy provides an example, with a 55-meter-long system.

Considering long-term development and quality requirements, online temperature control should be adopted. Installing a water-cooling tank after the finishing mill—at least—can effectively remove oxide scale and enhance surface quality. Heating, final rolling, and cooling schedules for various steel grades are shown in Table 1:

I. Controlled Rolling  
1. Theory of Controlled Rolling  
During hot rolling, by properly controlling heating, deformation, and temperature regimes, solid-state phase transformations can be combined with thermoplastic deformation to achieve fine-grained microstructures, thereby enhancing the overall mechanical properties of steel. For low-carbon and low-alloy steels, controlled rolling mainly refines deformed austenite grains through precise control of process parameters. During transformation from austenite to ferrite and pearlite, fine ferrite grains and relatively fine pearlite colonies form, improving strength, toughness, and weldability. For high-carbon and hypereutectoid steels, controlled-temperature rolling refines deformed austenite grains and performs final rolling near the austenite transformation point.

2. Hot-Rolling Followed by Controlled Cooling (Hot-Mechanical Rolling)  
Currently, hot-mechanical rolling is applied to round bars under ∮40 mm, primarily low-carbon and low-alloy steels, aiming to refine ferrite grains. Final rolling is conducted at 750–790°C, with water cooling applied before and after finishing passes. However, for larger-diameter bars, water cooling after rolling leads to uneven temperatures between surface and core, increasing the risk of fine surface cracks. During recrystallization, differences in grain size between core and surface result in non-uniform microstructure across the cross-section of the bar.

3. Normalizing Rolling  
Normalizing rolling can be used for round bars of ∮40 to ∮80 mm. The total deformation during the last four passes should be 50–60%. After entering the finishing mill, the material undergoes isothermal holding, with final rolling temperature maintained at 800–850°C, followed by rapid cooling.

4. Temperature-Controlled Rolling  
Final rolling temperature ranges from 850–900°C, with controlled cooling after rolling to improve surface quality. For high-carbon steels, this method produces finer pearlite colonies; for hypereutectoid steels, it reduces the formation of network carbides. 2. For producing round bars with diameters ranging from ∮50 to ∮80 using steel grades such as 20#, 45#, 20CrMo, 20CrMnTi, 40Cr, and 40MnB, Shigang Steel can adopt the normalized rolling process. However, before entering the finishing mill train, isothermal treatment is required, which increases the process distance, reduces production capacity, and necessitates greater deformation in the final passes. To ensure higher product accuracy and uniform deformation across the cross-section of the rolled piece, it is advisable to add a sizing mill, increasing capital investment. For bar sizes above ∮80, controlled-temperature rolling must be employed. For spring flat bar production, hot-rolled processing can be used, with final rolling performed within the dual-phase region of ferrite and austenite. This refines the austenite grain size during deformation, and through the phase transformation of austenite into ferrite and pearlite, produces fine ferrite grains and relatively fine pearlite nodules, thereby enhancing both strength and toughness of the steel. However, water quenching is required before and after the finishing mill, increasing investment and extending the process length in the rolling area. For bearing steel, controlled-temperature rolling must be applied throughout to prevent the precipitation of network carbides and improve surface quality. From the perspectives of investment and process layout, Shigang Steel adopts controlled-temperature rolling, lowering the initial rolling temperature, precisely controlling the final rolling temperature, and managing post-rolling cooling to achieve excellent surface quality and favorable internal microstructure.