Mechanical properties improvement of low 0.03 wt.% C steel through intercritical annealing

In this research, the improvement of mechanical properties, especially on the tensile strength and hardness, of low 0.03 wt.% C Steel through intercritical annealing process have been studied. The steel was intercritical annealed at the temperature between AC1 and AC3 to increase the strength and hardness. The temperature of AC1 and AC3 was determined around 737 and 895°C, respectively. After being austenitized (920°C) for 30 minutes and then briefly cooling to the intercritical annealing temperature for 15 minutes, the low carbon steel - contains 0.0302 wt.% C and 0.2183 wt.% Mn - was rapidly quench into the water at 30°C. The intercritical annealing temperature was varied to 750, 775, 800, 825, and 850°C. The mechanical properties of this steel were analysed and evaluated using different heat treatment temperatures. Each specimen's hardness, tensile, and microstructure tests were carried out to compare with the initial condition of low-carbon steel. The results showed that the hardness and tensile strength of the specimens increased in mechanical properties depending on the intercritical annealing temperature. After intercritical annealed, the microstructure transformed to ferrite and martensite phase. The increase in temperature increased the martensite fraction from 25.16 to 33.03%. Af, which directly improved the tensile strength. The highest strength (680 MPa for TS and 495 MPa for YS) and hardness (351.9 VHN) was obtained by the intercritical annealed at 800°C, due to the higher martensite fraction (31.35 %. Af) and the smaller grain size (7.69 μm), when compared to the sample underwent intercritical annealed at lower temperature (700°C and 725°C).