**2. Experimental details**

The present micro-alloyed steel are provided by the Steel Authority of India Ltd. (SAIL), the chemical composition was predicted by using Thermo Jarrell Ash spark emission spectroscope as displayed in **Table 1**. Present steel is familiar by the name of SAILMA steel.

Samples were taken from the transverse direction of a 18 mm thick hot rolled plate. Samples were treated for isothermal physical simulation by the thermomechanical simulator Gleeble®3800. Resistance heating was provided by K-type (Chromel-Alumel) thermocouples to the samples by a machine working based on conductive heat transfer model. A Linear Variable Displacement Transducer, LVDT dilatometer (quartz based) was fitted above the sample (where the thermocouple was welded at mid position) to acquire diametric strain changes during thermal cycles. External water quenching was arranged on both side of the hollow region of sample to freeze the microstructure and achieve the desire cooling rates as per ASTM isothermal treatments. Desired cooling rate in programmed and actual thermal cycles was achieved in less than 1 s between the solutionizing temperature and the isothermal temperatures.

For isothermal treatment simulation, the samples were first heated by 5°C/s to the solutionizing temperature 1200°C for 300 s and followed by 200°C/s down to 14 isothermal temperatures between 750 and 100°C with a temperature gap of 50°C. The samples were kept at these isothermal temperatures until completion of phase transformation estimated through dilation strain changes. Subsequently, the sample was water quenched to room temperature by external water quenching arrangements. Details of sample design and isothermal treatment cycle for one isothermal temperature 550°C are shown in **Figure 1**.

Isothermal simulated samples were partitioned cross-sectionally from the position of thermocouples for microstructural examination. One piece of partitioned surface of sample was ground and polished according to standard technique of metallography and later etched by a 2% Nital etchant. Optical micrographs of base material and simulated IT samples were characterized by using optical microscope Leica DMI 5000 M equipped with digital imaging facility. The grain size of ferrite was measured by linear intercept method using Image J software. Average grain size of 10 readings is reported in the present work.


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**3. Results and discussion**

**Figure 1.**

**3.1 Microstructural evolution**

*Isothermal Transformation Behavior and Microstructural Evolution of Micro-Alloyed Steel*

The optical micrographs for base metal and isothermally simulated steel samples are shown in **Figures 2** and **3**. The base metal micrograph consists of pro-eutectoid ferrite and pearlite as shown by arrow in **Figure 2a**. The pearlite structure is found to be in banded form in the direction of rolling [6]. The average grain size of pro-eutectoid ferrite is found to be in range of 10 ± 3 μm. The average grain size of ferrite is found to be decrease with decreasing isothermal temperature as shown in **Figure 4**. At 750°C, the banded structure of pearlite is completely dissolved and new structure is formed which consists of widmanstätten ferrite (WF), pearlite and very small amount of martensite (**Figure 2b**). Rest bright phase fraction reveals as the pro-eutectoid ferrite. It is worthy noticed that the material has not undergone any transformation due to fast cooling rate more than 175°C/s to 750°C. The austenite grain boundaries are completely illuminated through ferrite

*Schematic diagrams: (a) sample design and (b) isothermal treatment cycle for isothermal temperature 550°C.*

*DOI: http://dx.doi.org/10.5772/intechopen.85900*

**Table 1.**

*The chemical composition of micro-alloyed steel under investigation, in wt%.*

*Isothermal Transformation Behavior and Microstructural Evolution of Micro-Alloyed Steel DOI: http://dx.doi.org/10.5772/intechopen.85900*

**Figure 1.** *Schematic diagrams: (a) sample design and (b) isothermal treatment cycle for isothermal temperature 550°C.*
