Precipitation in the austenite of microalloyed low carbon steel

Abstract The morphology and chemical compositions of Ti–rich cube and lath, Nb–rich, Al–rich and sulphide particles, were investigated on reheating Nb–Ti microalloyed, low carbon steels. A commercial steel containing 0.065% C, 0.037% Al, 0.026% Nb, 0.014% Ti and 0.005% N, and as–cast steel containing 0.096% C, 0.027% Nb, 0.007% Ti and 0.0035% N, were reheated between 850 and 1400C, and either quenched or cooled slowly. The precipitation in a few HDR specimens was also examined. The HDR specimens contained 0.065% C, 0.027% Nb, 0.005% N and 0.005–0.021% Ti, and had been hot direct rolled above 1100 degC. In the commercial steel, laths were richer in Ti than cubes and both were richer in Ti than predicted by a model of the solution thermodynamics. The composition of cubes approached that of laths on reheating at high temperatures. The cast steel contained only Nb–rich particles, including cubes, which dissolved as Ti–rich cubes and laths nucleated during reheating. Ti-rich cubes first appeared on reheating the cast steel at 1050 degC. Fine, Ti–rich oxides precipitated on reheating the cast steel at 1400 degC. MnS coprecipitated on the oxides, and these complex particles nucleated acicular ferrite during the transformation of austenite. Coring was observed in Ti–rich cubes and laths, and depended on the cooling rate after reheating. In the quenched specimens, the edges of the particles were richer in Ti than the cores. Slow cooling after reheating resulted in the cores being slightly richer in Ti than the edges. Sulphide particles and Nb–rich carbonitrides precipitated on existing Ti–rich cubes and laths in a variety of morphologies during slow cooling from reheating temperatures. MnS particles were often coated by CuS shells. Large growths of Nb-rich particles nucleated on cubes and laths at around 950C during slow cooling from 1325 degC. The growths were also observed in specimens reheated between 1000 and 1200 degC and rolled at 950 degC. The premature loss of microalloy precipitants from solution is expected to affect subsequent microstructure development. AlN particles precipitated with carbonitride particles on reheating the commercial steel between 850 and 950 degC. The precipitation of AlN particles depended mainly on the amount of free nitrogen in solution. In the specimens hot direct rolled at temperatures up to 1200 degC, fine, strain-induced, Nb–rich particles were present. This indicated the difficulty of precipitating Ti–rich particles in segregated austenite.