Carbonitride Precipitations in Low Carbon Ti- microalloyed Steels Produced by CSP Process

摘要

The microstructure and precipitation behavior of the carbonitride in low carbon Ti-microalloyed steels produced by CSP process was studied by means of TEM and various techniques. The steels are consist of (mass%) 0.04～0.07%C, Si: ≤ 0.6%, Mn: ≤ 0.6%^ P:< 0.12%, S: ≤ 0.01% and 0.06～0.14%Ti. The results showed that TiN precipitated at higher temperature and Ti-carbonitride formed in the austenite region are observed in the steels. Besides, interphase precipitation of titanium carbonitrides can take place in the steels with Ti content more than 0.06% (mass). For the steels containing Ti lower than 0.10%, the interphase precipitation could only observed in the slabs with 50 mm in thickness. But it is not found in hot strips (6mm in thickness) of the same steel. However, large amount of carbonitrides formed through interphase precipitation in the hot strips with 0.12% Ti. Many tiny particles distributed within the grains dispersively also exist in the strips. These precipitates formed in the ferrite region. The formation mechanism and conditions of the interphase precipitation in titanium microalloyed low carbon steels produced by the CSP process is discussed. One of the dominating factors for the interphase precipitation of carbonitride or carbides is supersaturation of Ti in the solid solution. And another is the cooling rate of the steel across the temperature region at which the γ→α transformation takes place. Obvious strengthening of the steels can be obtained by the dispersive Ti-carbonitrides or the interphase precipitates.