Key words: rolling, temperature control
In the hot working process, the deformation termination temperature has an important influence on the structure of the steel. The higher the deformation termination temperature, the stronger the tendency of grain aggregation and growth, and the coarser the obtained austenite grains will be. Therefore, the deformation termination temperature should be reduced as much as possible, but generally not lower than Ar3, that is, the grains are refined by controlled rolling and cooling to improve product quality.
The deformation termination temperature of low carbon steel should be controlled close to 800°C, and the minimum should not be lower than 750°C.
For high carbon steel, in order to prevent the formation of reticular cementite, the deformation termination temperature should be controlled at about 850 °C when it is finished. If the deformation termination temperature is well matched with the rapid cooling after rolling, it is possible to completely suppress the precipitation of proeutectoid cementite without reticular cementite, even if formed, it is thin and easy to eliminate , no additional steps are required.
After the hypereutectoid carbon steel and alloy steel are rolled, the excess cementite forms a cementite (carbide) network distributed along the grain boundaries. Steels with a cementite network reduce cold deformation capability and increase the tendency to develop quench cracks. In order to eliminate the cementite network, the steel needs to undergo complicated heat treatment, and the heat treatment cannot achieve the goal at any time. Therefore, it is necessary to create conditions such that no cementite network is formed in the steel after rolling. Fairly rapid cooling after finish rolling and steel rolling at low temperatures is achievable.
For example: GCr15 is to reduce the temperature before the final rolling. A water cooling device is installed in front of the finishing rolling unit. After rolling, it can be cooled quickly, and compressed air can be blown and then enter the slow cooling pit.
For example: GCr15 is to reduce the temperature before the final rolling. A water cooling device is installed in front of the finishing rolling unit. After rolling, it can be cooled quickly, and compressed air can be blown and then enter the slow cooling pit.
Slow cooling after rolling will produce coarse ferrite grains, while the yield point will decrease and the brittle transition temperature will increase. The cooling rate is related to the cross-sectional size of the steel. It is difficult to achieve rapid cooling for large cross-section steel. Therefore, for the same grade of steel, the mechanical properties of large cross-section steel are lower. Foreign round steel is usually air-cooled after rolling. This is directly related to the low gas content in foreign steel, and the effect of online water cooling will be better. The specifications of the water-cooled round steel are all below Ф75mm, and the water-cooled technology often results in too long water-cooled lines and increased investment. However, rapid cooling is beneficial to reduce the secondary banded structure, but when the austenite grains are coarse, especially in the case of high manganese content in steel, rapid cooling may form Widmanstatten ferrite. Therefore, the rapid cooling after rolling should be matched with the low deformation termination temperature. When the deformation termination temperature is low and the austenite grains are relatively small, even rapid cooling will not form Widmanstatten ferrite.
After the alloy structural steel is rolled on a medium-sized rolling mill, the steel with a diameter of less than 60 mm is cooled in the air, while the steel with a diameter of more than 60 mm is cooled in an unheated slow cooling pit. The time for the steel to cool to 100—-150°C in the pit should not be less than 30h.
Ball bearing steel tends to produce white spots, so the needle bearing steel should be cooled slowly after rolling, or heat treated according to the prescribed system, and the temperature should not be lower than 700°C when charging. The steel billet is placed in the pit until the temperature is not higher than 100—200 ° C for an average of 72 hours, even when the final rolling temperature is low, the steel is slowly cooled to produce a cementite network in the steel. The cementite network is not formed when the temperature is lower than 650°C during slow cooling. Therefore, in order to avoid the cementite network, each bar after steel rolling is cooled separately and directly cooled to a temperature below 650°C as quickly as possible. Guaranteed to get the ball bearing steel without cementite network The cooling rate depends on the final rolling deformation temperature. When the final rolling temperature is 900–950°C, the cooling rate must not be lower than 45–50°C/min, and the cooling rate can be reduced as the final rolling temperature decreases.
Control the appropriate final finishing temperature (temperature near Ac3). In the finishing mill and matched with a suitable compression rate (about 40%), low and medium carbon steels, alloy steels, spring steels, bearing steels and other medium alloy steels can be obtained to obtain ideal metallographic structure and best mechanical properties . For this reason, a water-cooled box is installed in front of the last two stands of the bar finishing mill, and an equal temperature section is set in front of the bar finishing mill to make the internal and external temperature of the rapidly cooled rolled piece uniform.
Steel rolling can be divided into the following cooling methods:
1. in the air.
2. materials with low thermal conductivity.
3. in an incubator.
4. In an insulation pit without heating equipment.
5. Preheat holding pit and oven.
6. There are heating equipment in the holding pit and furnace.
7. in the water.
Controlled rolling and controlled cooling of representative steel grades:
Bearing steel and spring steel
It is required to complete the finish rolling at low temperature, and heat preservation and slow cooling are required after rolling. In order to prevent the precipitation of network carbides, the bearing steel should be cooled quickly and then slowly after rolling. The final rolling temperature of bearing steel is strictly controlled at 800-850°C to facilitate the crushing of network carbides. When the final rolling temperature is higher than 900°C, the steel can be rapidly cooled to 600-650°C by spraying water (to prevent the continued precipitation of network carbides), and then slowly cooled. For this reason, a cooling water tank is installed in front of the finishing mill to control the temperature of the rolled piece entering the finishing mill.
Quenched and tempered steel (dual treatment of quenching + high temperature tempering)
The quenched and tempered steel structure is tempered sorbite. The quenched and tempered steel not only has high strength limit and yield limit, but also has sufficient ductility and toughness, so it has high comprehensive mechanical properties. Quenched and tempered steel is mainly used for high-strength and important workpieces subject to impact or alternating loads, such as connecting rods and shafts.
High-quality carbon structural steel and alloy structural steel
Both high-quality carbon structural steel and alloy structural steel belong to hypoeutectoid steel, and the quenching temperature of hypoeutectoid steel is 30-50°C above AC3. For round steel less than 40mm in size, a cooling water tank is installed in front of the finishing mill to refine the grain and obtain a martensitic structure after quenching. Then after high temperature tempering, tempering is to heat the quenched steel below A1 to transform it into a stable tempered structure.
On-line temperature control manufacturer for larger size round steel: ABS LUNA factory in Udine, Italy. Production specification: ∮20—-∮100mm round steel. The steel types are: carbon steel, surface hardened steel, quenched and tempered steel, microalloy steel, bearing steel, spring steel, stainless steel, online temperature control for ∮20—-∮90mm round steel.
According to the current product positioning problem of Shigang, with the changes in steel users, it has become inevitable to provide automotive steel and develop to the high-end market. To provide users with ideal metallographic structure and best mechanical properties has a competitive advantage. When considering the cooling system, water-cooled boxes should be installed before and after the finishing mill to control the online temperature mainly for round steel less than 40mm.
Considering long-term development and quality requirements, on-line temperature control should be considered, and a water cooling box should be added after the finishing mill to remove scale and improve surface quality. The heating, final rolling and cooling systems of various steels are shown in Table 1:
| steel 45# | Heating temperature °C | 1050——————1180 |
| cooling method | air cooling | |
| Finishing temperature ℃ | ≥850℃ | |
| steel 40Cr | Heating temperature °C | 1050——————1180 |
| cooling method | air cooling | |
| Finishing temperature ℃ | ≥850℃ | |
| steel 20MnV、40MnB、20CrMo | Heating temperature °C | 1050——————1180 |
| cooling method | Heap cold | |
| Finishing temperature ℃ | ≥850℃ | |
| steel GCr15 | Heating temperature °C | 1050——————1100 |
| cooling method | Pit cold, pit entry temperature ≥ 600°C | |
| Finishing temperature ℃ | ≥850℃ | |
| steel 20CrMnTi | Heating temperature °C | 1050——————1120 |
| cooling method | Heap cooling below ∮85mm, pit cooling above ∮85mm, pit entry temperature≥℃ | |
| Finishing temperature ℃ | ≥850℃ | |
| 45Mn2、27SiMn | Heating temperature °C | 1050——————1180 |
| cooling method | Pit cold, pit entry temperature ≥ 400°C | |
| Finishing temperature ℃ | ≥850℃ | |
| 60Si2Mn | Heating temperature °C | 1030——————1120 |
| cooling method | Pit cold, pit entry temperature ≥ 400°C | |
| Finishing temperature ℃ | ≥850℃ |
Controlled rolling
Controlled Rolling Theory
In the hot rolling process, through the reasonable control of the metal heating system, deformation system and temperature system, the solid phase transformation and thermoplastic deformation are combined to obtain fine grain structure, so that the steel has excellent comprehensive mechanical properties.
For mild steel and low alloy steel. Mainly by controlling the rolling process parameters, the deformed austenite grains are refined, and through the phase transformation from austenite to ferrite and pearlite, fine ferrite grains and relatively fine pearlite pellets are formed. So as to achieve the purpose of improving the strength, toughness and welding performance of steel.
For high-carbon steel and hypereutectoid steel, the temperature-controlled rolling technology is used to refine the deformed austenite grains, and the final rolling is close to the austenite transformation point.
Thermomechanical rolling
At present, the specification of thermomechanically rolled round steel is below ∮40, and it is mainly low-carbon steel and low-alloy steel, with the main purpose of refining ferrite grains. Finish rolling at 750°C-790°C, water cooling before and after finish rolling. Large-scale round steel, after water cooling, the temperature of the surface and core is uneven, and small cracks are easy to form on the surface after rolling; when recrystallization after rolling, the grain size of the core and the surface are different, resulting in uneven structure on the cross section of the bar .
Normalized rolling
∮40——∮80 round steel can be normalized rolling, and the total deformation in the last four passes should be 50—60%.
Temperature-controlled rolling
The final rolling temperature is 850°C-900°C, and the cooling is controlled after rolling to improve the surface quality. For high carbon steel, relatively fine pearlite pellets can be obtained; for hypereutectoid steel, the precipitation of network carbide can be reduced.
Stone steel adopts rolling process
20#, 45#, 20CrMo, 20CrMnTi, 40Cr, 40MnB and other steel types can be used to produce ∮50-∮80 round steel by normalized rolling process, but it needs to be isothermal before entering the finishing rolling unit, which increases the process distance and reduces the output . The amount of deformation in the last 4 passes is increased. In order to ensure higher product precision and uniform deformation on the cross-section of the rolled piece, it is best to add a sizing machine, which increases investment. Temperature-controlled rolling is required for specifications above ∮80. For the production of spring flat steel, the thermomechanical rolling process can be used to finish rolling in the dual-phase region of ferrite and austenite to refine the deformed austenite grains and undergo phase transformation from austenite to ferrite and pearlite , forming fine ferrite grains and relatively small pearlite pellets, so as to achieve the purpose of improving the strength and toughness of the steel, but water cooling is required before and after finishing rolling, which increases investment and lengthens the process distance of the rolling area; For bearing steel, temperature-controlled rolling is required to prevent the precipitation of network carbides and improve the surface quality.
From the perspective of investment and process location, Shigang adopts a temperature-controlled rolling process to reduce the starting temperature, control the final rolling temperature, and control the cooling after rolling to obtain good surface quality and relatively good internal structure.
Different steel grades and specifications rolling process
| Steel | Specification | rolling process |
| 20#、45#、20CrMo、20CrMnTi、40Cr、40MnB | ∮50——∮80 | normalized rolling; temperature controlled rolling |
| ∮80——∮150 | Temperature controlled rolling | |
| GCr15 | ∮50——∮95 | Temperature controlled rolling |
| 60Si2Mn | 14mm—20mm×165mm—160mm | Thermomechanical rolling, water cooling before and after finishing rolling; temperature controlled rolling |
