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Progress in on-line heat treatment of hot rolled steel

Date:2019-02-22    View:644      Tag:Progress in on-line heat treatment of hot rolled steel
 1 Online heat treatment technology:


  Steel heat treatment is a process that changes the structure and properties of solid steel materials through certain processes such as heating, heat preservation and cooling. Compared with other processing techniques, heat treatment generally does not change the shape of the workpiece and the overall chemical composition, but rather imparts or improves the performance of the workpiece by changing the microstructure inside the workpiece or changing the chemical composition of the surface of the workpiece. It is characterized by improved internal quality of the workpiece. Heat treatment is especially important for steel materials. This is because the steel material undergoes a very complicated phase change during heating and cooling. If the parameters of the heating, heat preservation and cooling processes in the heat treatment process can be controlled, the phase transformation of the steel can be controlled to a large extent. A large range of changes in the organization of the steel, giving it different properties. Heat treatment is an extremely important means for people to control steel materials, regulate the organization of steel materials, and tap the potential of steel materials. The basic steel heat treatment process has annealing, normalizing, quenching and tempering. Annealing is a heat treatment process in which steel is heated to a phase change or partial phase transition temperature, held for a period of time, and then slowly cooled. Normalizing is a heat treatment process in which steel is heated to a temperature above the complete phase change and then cooled in air for a certain period of time. Quenching is a heat treatment process in which steel is heated to a phase change or partial phase change temperature and then cooled for a period of time. Tempering is a heat treatment process in which the quenched steel is reheated to a certain temperature (below the phase transition temperature), held for a while, and then cooled. The above several process combinations can also obtain a composite heat treatment process, such as quenching steel parts, followed by high temperature tempering, which is called quenching and tempering. The above heat treatment method is carried out after the completion of the roll forming, and can also be said to be "offline" (away from the steel processing line). Since the forming process of the steel is usually heated to the austenite region, if the hot working process is terminated at a suitable temperature and subsequent heat preservation, cooling, tempering, etc., the process of combining the deformation with the heat treatment is usually It is called deformation heat treatment, and English is abbreviated as TMCP (ThermoMechanical Controlled Processing). Since this heat treatment is carried out on a steel processing line, it can also be referred to as "on-line" heat treatment. The deformation heat treatment is a comprehensive strengthening process combining deformation strengthening and phase transformation strengthening. It includes two processes of plastic deformation and solid phase transformation of metal materials, and organically combines the two, using the change of the microstructure of the metal material during the deformation process, affecting the phase transformation process and phase transformation products to obtain the desired Organization and performance. Deformation (on-line) heat treatment combines the formation of metal materials with the ultimate properties of the resulting materials, simplifying the production process, reducing energy consumption and equipment investment. Compared with ordinary heat treatment, the metal material after deformation heat treatment can achieve better mechanical properties of strength and toughness. For the rolling process of steel materials, this deformation heat treatment process is limited to the control of rolling and cooling processes for a long time, and basically does not involve the heating process such as online tempering, so it is called “controlled rolling and cooling”. Sometimes "controlled rolling and cooling" is also called "TMCP". It should be noted that TMCP is actually more extensive, and in addition to cooling, it also includes tempering and other heating processes.      


  2 Online heat treatment technology in the field of steel development


  The heat treatment process has evolved with the development of steel materials. Ordinary offline heat treatment has been around for thousands of years. The heat treatment of deformation has been rapidly developed in the last century. Controlled rolling and controlled cooling techniques are a representative example.


  Before the Second World War, only a few sporadic studies of cryogenically controlled rolling were carried out. In the second world war, the welded structure of the vessel suffered a brittle fracture accident, and people began to study the improvement of the low temperature toughness of the material. The main goal of the study at that time was to use controlled rolling instead of normalizing to improve the low temperature toughness of the material through grain refinement. By the 1960s, the driving force behind the development of controlled rolling technology came from the development of the energy industry, mainly the demand for pipeline steel. Focusing on the microalloying of Nb, controlled rolling technology was used to obtain steel for pipelines with excellent performance. In the 1970s, people gradually realized the importance of controlling cooling technology to improve the performance of materials. Various large companies have developed various types of accelerated cooling systems. Controlled rolling and controlled cooling have made great strides from theory to practice, and people have begun a new era of controlling rolling and controlling cooling. At this stage, phase transformation strengthening can be achieved with a certain alloy design and corresponding controlled cooling, thereby developing low carbon bainitic steel, acicular ferritic steel, ultra low carbon bainitic steel, and the like. In the future, with the increasing demand for lightweight vehicles, phase change strengthened steels such as duplex steel and TRIP steel for hot rolled automobiles have received much attention. People use reasonable cooling and cooling path control to achieve steel performance control and control cooling. New progress has been made. In the 1980s, Japanese steel mills developed on-line quenching equipment, and direct quenching technology was gradually applied to practice. With the application of cooling technology, a number of controlled cooling practical devices have been developed internationally, such as Japan's OLAC (Japan Steel Pipe), CLC (Nippon Steel), DAC (Sumitomo), etc., and European water pillow type, waterjet cooling, etc. The international conference “ACCELERATED COOLING OF STEELS” held in 1985 summarized this.


  By the turn of the century, there have been two new changes in online heat treatment. The first one is the ultra-rapid cooling device represented by UFC, Super-OLAC, etc., which provides a new means for the cooling process of online heat treatment. The new controlled rolling and controlled cooling technology using ultra-fast cooling and the conventional controlled rolling control cooling mentioned above have adopted different ideas in the control of fine crystal, precipitation, phase transformation, etc., and thus it is possible to give The material has better performance. The second is the emergence of online tempering and other heating processes, the "offline" heat treatment process is further "online", and TMCP technology has a new development. These two new changes provide an important means of adjustment for online heat treatment, which will inevitably promote the flexibility of online heat treatment. 
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