Process: Surface hardening and self-tempering of ribbed steel bars -- QTB process
The flexible production of product quality and performance control is one of the most representative technologies in the development trend of rolling technology. In order to give full play to the potential of materials and save resources, the products of different levels can be produced by using different process control during the rolling process for the same chemical composition of the steel. Surface hardening and self-tempering of ribbed steel bars. QTB technology is developed in order to adapt to this trend.
The principle of control
The control principle of the process is shown as shown in the diagram. In the quenching stage, when the rod leaves the final rolling mill, it is necessary for the cold hard layer to form a cold hard layer on the surface of the rod, and the cooling rate is higher than the martensite velocity to get the rough martensite. In the tempering stage, the heat is transferred from the core of the bar with high temperature to the quenched surface, so that the horse obtained in the first stage can be obtained. The body is self-tempered. The third stage is the semi isothermal phase transformation of the austenite in the core of the steel bar.
Due to the formation of tempered martensite structure on the surface of the bar, the yield strength of the martensite can be increased by about 150Mpa, which can replace the further processing that affects the toughness of the material. The product has high strength and ensures enough toughness. The quality of the product is better than that of hot rolled and air-cooled micro-alloyed steel and low alloy steel. Moreover, due to the low carbon equivalent, better welding performance can be obtained by using the QTB ribbed bar.
In this process, the final rolling temperature, quenching time and water flow are the key parameters of the quenching process, and the quenching process determines a specific tempering temperature. These factors determine the proportion of the tempering martensite ring area, and will ultimately determine the mechanical properties of the products, especially the mechanical Tensile properties of the product.
The economic benefits brought about by the development of QTB technology are very obvious. Replacing the alloy steel with low carbon steel can save the production cost of the steelmaking workshop by 18%. As instead of low alloy steel, the cost can be saved by 8%. If the billet is rolled, using low carbon steel instead of alloy steel, the production cost can be saved by 15%. If the low alloy steel is replaced, the production cost can be saved by 6%.