Technical Key to the Development of ultralow Carbon Bainite Steel
The bainite steel has the advantages of low carbon, high strength, good forming and welding performance, long fatigue life, high toughness, low notch sensitivity and good wear resistance. Compared with the traditional quenching and tempering steel, the strength and ductility of bainite steel is higher and the notch sensitivity is lower. Therefore, it is an ideal material for automobiles and ships. In recent years, with the research of lath bainite, ultrafine lath bainite, boron added bainite andMartensite-bainite dual phase steel, bainitic steel has become one of the mainstream of automotive structural materials. However, the strength of bainite is generally lower than that of martensite and its application has been limited.
Therefore, in recent years, researchers have paid more attention to the research of high strength bainite steel. Through the optimization of quenching process parameters and the addition of trace elements, the ultra-low carbon bainite with high hardness and complete bainite structure has been obtained, and the application scope of bainite steel has been expanded. This article briefly introduces the research progress of ultra-low carbon bainite steel.
1.Optimization of quenching process parameters
The transformation from austenite to bainite occurs mostly on the continuous cooling curve around the Ms temperature. Therefore, the optimization of quenching process parameters is the basis for improving the strength of bainite steel. Through the research and test of many scholars at home and abroad, combined with the thermodynamic calculation and numerical simulation, the quenching parameters selection and quenching process optimization method of the present bainitic steel.
1.1 Quenching temperature selection
The quenching temperature of bainite steel is usually selected in a range of 250 ~ 150 ℃. If the quenching temperature is higher, bainite appears in a more massive granular state, resulting in a decrease in strength; while a lower quenching temperature will cause incomplete transformation of austenite and low strength. Studies have shown that the bainite quenching temperature should be selected close to the Ms temperature.
1.2 Quenching continuous cooling rate selection
The selection of quenching continuous cooling rate is an important part of quenching process optimization. It is found from the experiment that when the quenching temperature is lower than 180 ℃, the continuous cooling process is mainly affected by the heating temperature and environment temperature. When the quenching temperature is higher than 180 ℃, the continuous cooling rate should be greater than 10 ℃/s to obtain the relatively dense lath bainite.
2.Addition of trace elements
At present, many scholars have carried out research on trace element additions in bainite to increase the strength and ductility of bainite steel. Among them, trace element Ti can be used to adjust the transformation temperature, promote the transformation of austenite to ferrite, reduce the austenite grain size, thus improving the strength of bainite steel significantly. In addition, trace elements Cr and Si can also promote the formation of acicular bainite, reduce the quenching cooling rate range and obtain the high strength bainite steel.
In summary, the development of ultra-low carbon bainite steel pays attention to the reasonable selection of quenching process parameters and the research of trace elements addition. Through reasonable quenching process design, select the right quenching temperature and quenching cooling rate to obtain ultra-low carbon bainite steel with high strength and good mechanical properties. At the same time, trace elements are added to reduce the quenching cooling rate range and improve the strength of bainite steel. Through these two elements and measures, ultra-low carbon bainite steel with complete bainite structure and high strength has been obtained, and its application scope has been significantly expanded.
