The important microstructural features of microalloyed steel, i.e. grain refinement and precipitation strengthening, are achieved by the precipitation of microalloy-species in austenite and during or after the transformation to ferrite. The effectiveness of any precipitation reaction depends on the degree of dispersion and particle size. Since the titanium nitride is more stable and shows less tendency to coalesce than any other microalloy-nitride or carbide, an enhanced nitrogen content will maximise the ratio between particle volume fraction and particle size and hence maximise grain refinement and precipitation strengthening.
To understand the effect of the Ti/N ratio and cooling rate on the precipitation behavior and austenite grain growth characteristics during cooling, I have done the remelting experiment. The remelting was done in the high-frequency induction furnace and the cooling rate was controlled by PID controller. The composition of the used alloys is as follows ; The alloy A contains 0.018wt.%Ti-0.0082wt.%N and the Ti/N ratio is 2.20 (hypostoichiometric with respect to Ti). The alloy B contains 0.017wt.%Ti-0.0044wt.%N and the Ti/N ratio is 3.86 (hyperstoichiometic with respect to Ti).
Cooling was continued to room temperature by air cooling after the controlled cooling or interrupted at various temperatures (1350℃, 1200℃, 1050℃) in the austenite phase field by quenching. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were used to investigate the distribution and size of particle in processed ingots. And also, optical microscopy was used to observe the austenite grain structure and the proeutectoid ferrite grain structure.
When the controlled cooling rate is sufficiently fast, the low Ti/N-ratio specimen has the higher volume fraction of fine nitrides than the high Ti/N-ratio specimen. It is more effective to inhibit the movement of the austenite grain boundary by pinning so that the austenite grain size of specimen A is smaller than that of specimen B. Probably the enhanced N content in hypostoichiometric ratio will be more effective to increase GCT (Grain Coarsening Temperature) at reheating.
In specimens air-cooled after controlled cooling, as the controlled cooling rate is decreased, the inclusion size and the tendency of the intragranular ferrite nucleation at the nonmetallic inclusion increase.
In specimens quenched at various temperatures during controlled cooling, as the cooling rate and Ti/N ratio is decreased, the inclusion size is increased.
From the result of the compositional analysis of the inclusion, the inclusion is composed of single phase oxide or complex oxide compound(Al-Si-Ti-Mn oxide), manganese sulfide(MnS), and titanium nitride(TiN). It is observed that the particles containing MnS or TiN offer the effective site for intragranular ferrite nucleation. This oxide particles that form during solidification can act as nucleus for MnS precipitation. It is also observed that TiN precipitetes at the complex particle of the oxide and sulfide.
Therefore, it is believed that the inclusion size and MnS(or+TiN) formed at the inclusion(oxide compound) have relation to the intragranular-ferrite nucleation.
