Precipitation of TiN and (Ti,Nb)(C,N) during Solidification, Cooling and Hot Direct Deformation

Abstract

The precipitation behaviour of TiN and (Ti,Nb)(C,N) in low carbon HSLA steels has been studied by simulation of the casting and hot direct rolling of thin slabs. At ferritic solidification TiN particles 0.5 ... 1.0 pm in size were formed only in regions of maximum segregation where the supersaturation was higher than 10fold. During cooling, in the range of delta ferrite fast homogeneization of the segregated Ti takes place, and no TiN nuclei were formed. In undeformed austenite some large particles precipitated their arrangement reflecting the solidification structure. In the 1200 to 1100 °C range particle sizes between 20 and 70 nm were reached. A high density of deformation induced particles smaller than 10 nm could be obtained at or below 1050 °C. The density of the particles increased and their size decreased with decreasing deformation temperature. After fast cooling, below 1000 °C also in undeformed material finely dispersed precipitates were found. In TiNb microalloyed steels, complex precipitates are formed their Nb:Ti ratio being low at high temperature and increasing with decreasing precipitation temperature. Separate nucleation of Nb(C,N) particles is observed deformation-induced below 1000 °C and without deformation below 950 °C. Based on these results a number of thin slab casting and direct rolling experiments have been carried out on the thin slab pilot facility of Thyssen Krupp Stahl AG at Dortmund. With steels of 0.046...0.086%C-0.06%Si-0.92...1.05%Mn-0.03%Al-0.016...0.020%Ti and partly 0.035% Nb 50 mm slabs were rolled to 10 mm plates which after air cooling had the following mechanical properties: R eL = 368... 394 MPa, R m = 442... 464 MPa; A 5 = 34...37.4 %; T tr (50%) = -38...-45 °C.