Ferrite Formation Dynamics and Microstructure Due to Inclusion Engineering in Low-Alloy Steels by Ti2O3 and TiN Addition

Microstructure control in steel according to inclusion engineering

Previous researches have been conducted on study of final microstructural characteristics of the formed intragranular ferrite and grain boundary ferrite. However, a better understanding on microstructural changes and phase deformation process at critical temperatures is needed in view of having a desirable balance between the formed intragranular ferrite and grain boundary ferrite with use of either oxide metallurgy or inclusion engineering.

In line with this, Dr. Wangzhong Mu and colleagues investigated the dynamic formation of intragranular ferrite and grain boundary ferrite in inclusion engineering steels with either titanium (III) oxide Ti₂O₃ and  titanium nitride TiN additions. The work which was published in the journal, Metallurgical and Materials Transactions B provided information on effects of the inclusions, cooling rate and prior austenite grain size on ferrites formation.

In their experiment, the steel specimens with the addition of titanium (III) oxide and titanium nitride were named alloy A and B respectively. The specimens were then placed in high temperature confocal laser scanning microscopy CLSM and heated to a temperature of 1400⁰C. Constant cooling rates of 3.6, 70 and 678⁰C/min were also observed. Specimens were at some specific temperatures in order to control the prior austenite grain size.

The experimental calculations of thermodynamic driving forces on misfit strain energy between inclusions and austenite/ferrite showed good agreement with previous studies as a method for calculating the grain boundary nucleation from grain boundaries was also defined.

Electron probe microanalysis on alloy A showed that the atomic ratio of oxygen to titanium was in the range of 1.55 and 1.69. Manganese sulfide inclusions and small amounts of Mn-Al-Si-O oxide were also randomly distributed on the corners of the titanium oxide core. Aluminum, manganese and sulfur were also present in the oxide phase.

For alloy B, elements such as silicon, aluminum, manganese, titanium and oxygen with presence of manganese sulfide were found in the core part of titanium nitride. The atomic ratio of nitrogen to titanium found between 0.8 and 1.3.

An increase in prior austenite grain size was observed for both alloys but alloy A had a greater grain size compared to alloy B. Area fraction of the intragranular ferrite was larger in alloy A compared to alloy B at a constant cooling rate and grain size. However, the intragranular ferrite fraction for both alloys increased with an increase in prior austenite grain size with the highest fraction found at a cooling rate of 70⁰C/min.

When specimens were visualized by confocal laser scanning microscopy showed that the starting temperature of grain boundary ferrite formation in steel was higher for smaller grain size while that of intragranular ferrite was not affected by the grain size. Starting temperature of grain boundary ferrite formation for alloy A was lower compared with alloy B but the starting temperature of intragranular ferrite formation remained the same for both alloys. Both starting temperatures of grain boundary ferrite and intragranular ferrite formation decreased with an increase in cooling rate.

This study provided profound additional information on the preferential formation of intragranular ferrite and grain boundary ferrite.