In the hot working process, the finishing temperature greatly influences the microstructure of the steel. Higher finishing temperatures increase the tendency for grains to coalescence and grow, resulting in coarser austenitic grains.
Therefore, it is crucial to minimize the finishing temperature during production, typically not falling below the Ar3 point. This can be achieved through controlled rolling and cooling methods to refine grain size and improve product quality.
For low carbon steels, the finishing temperature must be maintained close to 800°C, and must not fall below 750°C.
In the case of high carbon steels, to avoid the formation of a cementite network, the finishing temperature during production must be controlled at around 850°C.
Combining this with rapid cooling after rolling can effectively suppress the precipitation of proeutectoid cementite, preventing the formation of a cementite network, or at least ensuring that it is fine and easily sloughable without additional processing steps.
In hypereutectoid carbon steels and alloy steels, excess cementite forms a network along grain boundaries after rolling. Steels with a cementite network have reduced cold forming capacity and a greater tendency to crack during quenching.
To eliminate this network, complex heat treatments are necessary, which are not always effective.
Therefore, conditions must be created to prevent the formation of a cementite network after rolling. Low temperature finishing and rapid cooling after lamination can achieve this goal.
For example, GCr15 steel is cooled with water before the finishing mill to lower the temperature before final rolling. Rapid post-rolling cooling is achieved by compressed air blasting, followed by slow cooling in a well.
Slow cooling after rolling results in coarse ferritic grains, a lower yield point, and an increased brittle transition temperature. The cooling rate depends on the cross-sectional size of the steel; Larger sections are more difficult to cool quickly and therefore generally have lower mechanical properties.
In foreign countries, round steels are generally air-cooled after rolling due to the lower gas content in their steel. The use of online water cooling can be more effective, but is limited to round steels with diameters below 75 mm. Although rapid cooling helps reduce banding, in steels with high manganese content and coarse austenitic grains, it can lead to the formation of Widmanstätten ferrite.
Therefore, rapid post-lamination cooling must be combined with a low finishing temperature. When the austenitic grain size is small, even rapid cooling will not lead to the formation of Widmanstätten ferrite.
For alloyed structural steels rolled in medium-sized rolling mills, those with diameters below 60 mm are cooled in air stacks, while those with diameters greater than 60 mm are cooled in unheated pits. The steel must cool in the pit to 100-150°C for at least 30 hours.
Bearing steel is prone to white spotting, so it must be cooled slowly after rolling or heat treated as specified. During charging, the temperature must not be lower than 700°C. The billets are placed in the pit until the temperature does not exceed 100-200°C, on average 72 hours.
Even at lower finishing temperatures, slow cooling can lead to the formation of a cementite network in the steel.
To avoid this, each bar must be cooled individually as quickly as possible to below 650°C.
The cooling rate for bearing steels without cementite mesh depends on the final rolling temperature; at 900-950°C the rate should be at least 45-50°C/min, which can be reduced as the finishing temperature decreases.
Controlling the appropriate final finishing temperature (near the Ac3 point) and combining it with a suitable reduction rate (about 40%) can achieve ideal metallurgical structures and ideal mechanical properties in low- and medium-carbon steels as well as in -alloy, spring steels and bearing steels.
To achieve this, water cooling boxes are installed before the last two benches of the bar finishing mill. To ensure uniform internal and external temperatures in rapidly cooled rolled parts, a temperature equalization section is installed before the finishing rolling group.
Post-rolling steel cooling methods include:
- Up in the air.
- In materials with low thermal conductivity.
- In insulating boxes.
- In unheated insulating wells.
- In pits and preheated insulating ovens.
- In heated insulating pits and ovens.
- In the water.
Representative steel grades and their controlled rolling and cooling methods include:
1. Steels for bearings and springs
These require finishing at low temperatures, followed by isolated slow cooling. To prevent precipitation of lattice carbides, bearing steels are cooled quickly after rolling and then cooled slowly.
The finishing temperature of bearing steel is strictly controlled between 800-850°C to help break down network carbides.
When the finishing temperature exceeds 900°C, the steel can be sprayed with water to rapidly cool to 600-650°C (to avoid further precipitation of lattice carbides) and then slowly cooled. Cooling water boxes are installed before the finishing mill to control the temperature of parts entering the mill.
2. Quenched and tempered steels
These steels have a tempered sorbite structure and are used in high-resistance, impact or alternating load parts, such as connecting rods and axles. They offer comprehensive high mechanical performance due to their high strength and yield limits as well as sufficient ductility and toughness.
The production scheme includes 225,000 tons of quality carbon structural steel and 225,000 tons of alloy structural steel, representing 90% of total production. Controlling the temperature of such a large volume of steel provides a competitive advantage.
3. Quality carbon structural steels and alloy structural steels
Both are hypoeutectoid steels with a quenching temperature of about 30-50°C above AC3. For round steels smaller than 40 mm in diameter, cooling water boxes are installed before the finishing mill to refine the grain size and obtain a post-quenching martensitic structure.
The steels are then tempered at high temperatures below A1 to transition to a stable tempered structure. For larger diameter round steels, on-line temperature control is practiced by manufacturers such as the ABS LUNA factory in Udine, Italy, which produces round steels with a diameter of 20 to 100 mm, including carbon steel, surface-hardened steel, tempered steel and tempered steel, micro alloy steel, bearing steel, spring steel and stainless steel. They control the temperature of steels from 20 to 90 mm in diameter online.
Given Shigang's (Shi Steel) product positioning and the growing needs of steel users, supplying automotive steels and moving into high-quality markets has become essential.
Offering ideal metallurgical structures and ideal mechanical properties provides a competitive advantage. When considering cooling regimes, water cooling boxes should be installed before and after the finishing mill, particularly targeting round steels smaller than 40mm, for online temperature control.
The installation of water cooling boxes after the finishing mill is debated internationally. For large diameter round steels, it is considered to only remove scale and improve surface quality, having little effect on grain refinement and potentially causing irregular internal grain size.
Online temperature control would undoubtedly prolong the lamination line and increase investment. The length of the water cooling box to be installed after finishing machining is not widely recommended, with the ABS LUNA factory in Italy being a reference, having a 55 meter long box.
Considering long-term development and quality requirements, online temperature control should be considered. Initial installation of water cooling boxes after finishing can at least remove scale and improve surface quality. The heating, final rolling and cooling regimes for various steels are detailed in Table 1.
Table 1: Heating, finishing rolling and cooling temperatures of various types of steel.
| 45# | Heating temperature ℃ | 1050——————1180 |
| Cooling method | Air cooling | |
| Finishing Temperature (°C) | ≥850℃ | |
| 40cr | Heating temperature ℃ | 1050——————1180 |
| Cooling method | Air cooling | |
| Finishing Temperature (°C) | ≥850℃ | |
| 20MnV, 40MnB, 20CrMo | Heating temperature ℃ | 1050——————1180 |
| Cooling method | Stack Cooling | |
| Finishing Temperature (°C) | ≥850℃ | |
| GCr15 | Heating temperature ℃ | 1050——————1100 |
| Cooling method | Well cooling, inlet temperature ≥ 600°C | |
| Finishing Temperature (°C) | ≥850℃ | |
| 20CrMnTi | Heating temperature ℃ | 1050——————1120 |
| Cooling method | For diameters below 85mm, Stack Cooling; for diameters 85mm and above, well cooling, inlet temperature ≥600℃ | |
| Finishing Temperature (°C) | ≥850℃ | |
| 45Mn2, 27SiMn | Heating temperature ℃ | 1050——————1180 |
| Cooling method | Well cooling, inlet temperature ≥ 400°C | |
| Finishing Temperature (°C) | ≥850℃ | |
| 60Si2Mn | Heating temperature ℃ | 1030——————1120 |
| Cooling method | Well cooling, inlet temperature ≥400℃ | |
| Finishing Temperature (°C) | ≥850℃ |
Controlled Bearing
Controlled Bearing Theory
In the hot rolling process, reasonable control of metal heating, deformation and temperature regimes enables the combination of solid-state phase transformation and thermoplastic deformation to obtain fine-grained structures, thereby improving the comprehensive mechanical properties of steel.
For low-carbon and low-alloy steels, controlled rolling mainly refines the deformed austenitic grains, leading to fine ferritic grains and smaller pearlite colonies after the transformation of austenite to ferrite and pearlite. This increases the strength, toughness and weldability of the steel.
For high-carbon and hypereutectoid steels, temperature-controlled rolling refines the deformed austenitic grains and terminates the rolling near the austenite transformation point.
1. Thermomechanical Lamination
Currently, thermomechanical rolling of round steels is limited to diameters below 40mm, mainly in low carbon and low alloy steels, aiming at refining ferritic grains. The finishing rolling temperature is between 750°C and 790°C.
Before and after finishing lamination, water cooling is required. For larger diameter round steels, uneven temperatures between the surface and core after water quenching can cause surface microcracks and inconsistent grain sizes between the core and surface during recrystallization, leading to uneven structural integrity across the entire cross section. .
2. Standardized Lamination
For round steels between 40mm and 80mm, standard rolling is used, with the last four passes totaling 50-60% deformation. The steel is balanced before entering the finishing mill, with a finishing temperature of 800°C to 850°C, followed by rapid cooling.
3. Temperature-controlled lamination
The finishing temperature ranges from 850°C to 900°C, followed by controlled cooling to improve surface quality.
For high-carbon steels, this process produces finer pearlite colonies; for hypereutectoid steels, it reduces the precipitation of network carbides.
Shi Steel's rolling processes
For steel grades such as 20#, 45#, 20CrMo, 20CrMnTi, 40Cr, 40MnB, producing round steels from 50mm to 80mm in diameter, standard rolling is used.
However, balancing is required before finish machining, increasing process distance and reducing production. The deformation in the last four passes is increased to ensure greater product precision and uniform deformation across the entire cross section, suggesting the addition of a calibration mill, which increases investment. For diameters above 80mm, temperature-controlled rolling is required.
For the production of flat steel for springs, thermomechanical rolling is used. Finishing in the two-phase ferrite-austenite zone refines the deformed austenitic grains, resulting in fine ferritic grains and smaller pearlite colonies, thereby increasing the strength and toughness of the steel.
However, this requires water cooling before and after finishing rolling, increasing investment and enlarging the rolling zone.
For bearing steel, temperature-controlled rolling is necessary to prevent precipitation of network carbides and improve surface quality.
Considering the investment and process location, Shi Steel employs temperature-controlled rolling, reducing the initial rolling temperature, controlling the finishing temperature, and implementing controlled cooling after rolling to obtain good surface quality and internal structure.
Table 2: Rolling process for different steel types and specifications
| Steel | Note | Lamination process |
| 20#、45#、20CrMo、20CrMnTi、40Cr、40MnB | ∮50——∮80 | Standardized Lamination; Temperature-controlled lamination |
| ∮80——∮150 | Temperature-controlled lamination | |
| GCr15 | ∮50——∮95 | Temperature-controlled lamination |
| 60Si2Mn | 14mm—20mm×165mm—160mm | Thermomechanical Lamination (with water cooling before and after finishing the lamination); Temperature-controlled lamination |
