Carbon boron steels exhibit outstanding mechanical properties at a low price. The mechanical properties of carbon steel are maximized by the addition of micro additives such as titanium and boron which meet automotive industry needs. S355J2 and 27MnCrB5 rolled bars were hot forged at 1200°C with reduction in cross-sectional area of about 60%. The hot forged samples were air cooled. Dilatation measurements were carried out to determine the critical transformation temperatures. The mechanical properties were evaluated using tensile test. The effect of micro additives especially boron on continuous and discontinuous yielding behavior was studied. Microstructure investigation was carried out using optical microscope and SEM. New technique to determine the volume fraction of different phases was used. It depends on surface roughness deduced from optical or SEM micrographs to construct Abbott Firestone curve. Abbott Firestone curve technique results are reliable and very close to that Image j software results with a percentage of 95%.