Hyqst technology refers to high-yield quenching and self-tempering technology.
Self-tempering quenching is to heat all the workpieces to be processed, but only immerse the part that needs to be hardened (usually the working part) into the quenching liquid to cool, and take it out in the air immediately when the fire color of the unimmersed part disappears.
After quenching, the hardened part can be properly tempered by using the residual heat of the unhardened part. The tempering temperature is determined by the tempering color of the hardened part. After reaching the tempering temperature, the entire workpiece is put into the quenching liquid to cool, so as to terminate the self-tempering.
Self-tempering and quenching are often used to deal with simple tools (such as chisels, hammers, etc.) and rail joints that are subjected to impact. For example, in order to obtain the tempered sorbate structure of the rail head, the heated head can be immersed in water at 25~30C for 30~40s (the depth of immersion is 20~25mm), and then taken out. The waste heat contained in the rest of the rail can make the head self-tempering at 500~520°C, so as to meet the performance requirements.
Thermomechanical Rolling Technology of High-Speed Rod and Wire
Thermomechanical rolling
The thermomechanical rolling process has been successfully used in the production of plates for many years to produce marine corrosion-resistant steel structures, shipbuilding steel, bridge steel, and pipeline steel. The desired product attributes and performance focus on strength, impact toughness, ductility, and weldability. For long products, it has been used for some time, especially after the development of sizing unit equipment, it has become the follow-up finishing unit of the long product rolling mill. However, market demand for these products is growing slowly.
There are generally three basic temperature ranges for hot rolling in rod and wire mills: conventional rolling (CR), normalizing rolling (NR), and thermomechanical rolling (TMR). Although the layout of various rolling mills is ever-changing, the temperature of conventional rolling is higher than 950 °C, which is much higher than the phase transition temperature of Ac3, because the rolling mill equipment is limited. Normalizing rolling is to ensure that the rolled piece is in a stable austenite state range, which is usually about 60°C higher than the Ac3 transformation temperature. Thermomechanical rolling is just rolling in the austenite and sub-austenite temperature range, that is, close to the Ac3 transformation temperature.
Thermomechanical rolling leads to grain refinement. In conventional high-speed wire and bar rolling, although the finished rod and wire maintain a very high fine-grained structure, reaching ASTM standard 13 when the wire and bar leave the last rolling mill, it is very short before reaching the water cooling device. Within a short period of time, the refined grains grow up immediately. For most specifications of rods and wires, it takes less than 0.5 seconds for the grains to rapidly increase to grade 8. After passing through the water cooling device, although the surface layer has been quenched, the internal grains still continue to grow at high temperatures, the grains on the surface of the bar are refined, and the grains gradually increase inward. Air cooling or forced cooling is carried out on the Stelmo cooling line after spinning. After rolling in the conventional 10-stand no-twist finishing mill, the grain size obtained through the Steermo cooling line is generally in the range of 5-10.
The recrystallization mechanism is dynamic during rolling without twisting, that is, rapid dynamic recrystallization without an incubation period, and finally static grain growth occurs when the temperature drops. When deformation occurs, nucleation and growth occur simultaneously, and dynamic recrystallization occurs within the metal, as described by Sellers and Whiteman. In order to maintain dynamic recrystallization, certain conditions must be met, in ferritic materials, the strain must exceed 0.12 (ie 15% reduction rate).
In high-speed wire rod mills, this value can easily be reached and exceeded in the no-twist finishing block due to the accumulation of strain, ie there is not enough time to recover between passes. This is described by Neishi et al. If the critical strain is lower than the critical strain to a certain extent, such as 0.06-0.1, it will lead to the generation of mixed crystals, which is usually referred to as the growth of deformed grains. However, Morgan, which is now owned by Siemens, is aware of this problem. In order to avoid this phenomenon, they design the RSM racks very close to each other, and the time between two passes is very short, that is, less than 0.05 seconds, so that the strain can be accumulated and maintained, exceeding the critical 0.12 strain requirement. Distorted grains have not been recorded in this type of mill since the introduction of the RSM unit in 1993.
During RSM rolling, the dynamic recrystallization process occurs uniformly across the entire cross-section, and the most influential factor is the cooling rate. As the bar size increases, the core temperature is high, and the large grains continue to grow. If rolling at 850°C in the NTM unit and 800°C in the RSM unit, there is no need for water cooling before the laying head, and it can directly enter the Steermo cooling. The grain size of the bar produced by this process is basically the same from the surface to the core, and the temperature is low, the grain size is small, the performance of the product is uniform, and the ductility is good, but the tensile limit is slightly lower. However, the core grain size is much smaller as a result of thermomechanical rolling. In hypereutectoid steels, such as bearing steels, this results in a minimal distribution of iron carbides at grain boundaries, which often leads to the central segregation of carbon.
As the effect of dynamic recrystallization, TMR refines the grains of the final product, which has an important impact on the final performance of the product when combined with online water-through cooling and Stelmo-controlled cooling. For medium and low-carbon steel products, the subsequent spheroidizing treatment is very beneficial. Due to the intense grain refinement, the subsequent phase transformation to harder structures such as bainite and martensite is also affected by the initiation time and temperature of the phase transformation. In this way, thermomechanical rolling can be beneficial to the subsequent cold working, and it is also effective to reduce the tempering time.
High Strength Concrete Rebar
Concrete reinforcement occupies a very large proportion of long products, for example, the proportion of this product in China has reached about 20% of the total steel. In developing countries, due to the construction of various infrastructures, the demand for long products is continuously increasing. Even in developed countries, outdated infrastructures need to be replaced. Strong demand has spurred research on such steel products.
Different countries have different standards for steel bars. In many countries, the yield strength in the domestic market is only 335Mpa, or 400Mpa, while in some countries it is 500Mpa or higher. The use of high-strength steel bars is economical, as compared to lower-strength steel bars, and a significant saving in steel weight can be achieved.
(1) Cold-rolled deformation-cold-rolled deformation and formation of transverse ribs, or cold-drawn and hot-rolled ribbed steel bars.
(2) Strengthened with titanium, niobium, and vanadium microalloying elements.
(3) Quenching and tempering change the structure, forming a tempered martensite structure on the surface layer of the bar, and forming a fine grain structure in the core.
(4) Thermomechanical rolling creates a fine-grain structure throughout the cross-section.
Cold rolling deformation – cold rolling deformation at room temperature to form transverse rib steel bars, which is one of the oldest cold working strengthening methods. By means of cold rolling or cold drawing, the strength is increased, but the increase is limited, and there are other disadvantages.
Coiled wire cold rolling is a typical way to form transverse ribs and sufficient area reduction to cause work hardening. The cold-worked product can be coiled again or cut into long products for welding steel mesh, or other steel bar forms.
The coiled plain carbon steel rods and wires are drawn and extended to meet the specifications required by users.
The method of cold working deformation to improve strength is widely used all over the world, but it has limitations and disadvantages. From the performance point of view, the malleability of metals is greatly reduced, which limits the selection of users. In addition, such products have poor fire performance compared to other high-strength steel materials. One of the most important disadvantages of this production method is that the production efficiency is not high due to the low drawing speed, and it is necessary to leave the hot rolling production line for uncoiling and to draw again.
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Quenching and Tempering
The use of quenching and tempering to increase the strength of steel bars is the most widely used method. The high temperature after rolling is used for controlled cooling to obtain the required microstructure, and the required mechanical performance steel bars are directly produced on the rolling line.
The main principle of controlled cooling is to quickly reduce the surface layer of the rod below the martensitic transformation point and at the same time reduce the temperature of the core to a temperature close to the critical temperature. The grain growth rate increases exponentially above 800°C, and almost freezes at about 650°C. In order to prevent the dynamic recrystallization of grain freezing, the bar is rapidly cooled in the water cooling device, the rolled piece passes through the refrigerant in the water cooling device quickly, and the heat energy kept in the core is dissipated outward. Due to the huge temperature difference between the surface of the bar and the core, the surface is quickly reheated and tempered, while the temperature of the core is lower than the equilibrium temperature. Therefore, the design requires that the temperature of the core is lower than the subcritical temperature so that the surface structure will not recrystallize.
Tempering allows the surface layers of the bar to improve toughness while the core remains strong. From the metallographic photos, the outer layer has obvious tempered martensite bands, and the core structure is ferrite, variable pearlite, bainite, carbides, and retained austenite arranged in some gaps. The composition of variable pearlite is composed of coarse layered carbide spheres and spheroidized carbides in the pearlite field, and the entire organizational structure is very fine. The fine-grained nature of ferrite results in very high yield strength of the bar.
The combined microstructure of fine-grained core and self-tempered martensite on the surface produces higher yield strength, and the contribution of grain refinement is the most significant.