TENSILE PROPERTIES AND BENDABILITY OF B-BEARING MARTENSITIC STEELS

The tensile behavior and bendability of B-bearing martensitic steels with carbon 0.15 and 0.29% were investigated. After austenitized at 900 degrees C for 6 min and quenched in the closed tool, full martensitic microstructure was obtained in the steels with 1.5mm in thickness. It was confirmed that the tensile strength and bending angle of the martensitic steels were significantly dependent on the carbon content; however, their bendability was independent on the elongation. It was considered that the maximum bending angle increases with increasing yield ratio. The dislocation density of martensitic steels was determined using the XRD and TEM. The peak broadening analysis of XRD profiles quantifies the dislocation density of about 1.2-2.0 x 10(16) m(-2). TEM images support that 15MnB5 has a low dislocation density compared to 30MnB5. The lower number of solute carbon in 15MnB5 causes a decrease in the dislocation density during martensitic transformation. This thus results in the lower strength and higher bendability compared to 30MnB5.