Effect of Boron on Spheroidizing Treatment of Medium Carbon Steels

摘要

Deep drawing and ironing process have been widely used for making seamless pressure vessel of medium carbon steels. These steels have been usually softened by spheroidizing treatment before deep drawing process to provide enough formability for cold forming. It is necessary to keep long periods of time for heat treatment to obtain enough formability, however longer heat treatment causes a substantial decrease in the strength due to the growth of carbides and induces decarburization at the surfaces of the steels and drives up the costs for the process. Therefore, it is essential to optimize the time for the heat treatment to achieve the good combination of formability and strength at the same time for industrial applications. The kinetic of sphcroidization is mostly affected by the initial microstructure. The development of spheroidized carbides occurs rapidly in the fine initial structures such as bainite and martensite. Since the boron greatly increases the hardenability of steels, fine carbides can be uniformly distributed in the microstructures at the initial stage in boron added steels. In this study, the effect of boron with an addition (from 0ppm to 80ppm) on the kinetics of spheroidization and the hardness reduction at the decarburization surfaces of steels for pressure vessel having medium carbon has been investigated. The hot rolled plates were spheroidized at 750°C for 8hrs to apply the manufacturing processes of CNG storage vessel and then air-cooled to room temperature. The spheroidized specimens were examined by OM and SEM, and mechanical tests were performed with high temperature and micro-vickers hardness testers. Initial microstructure of steel without boron consists of the quasi-polygonal ferrite and granular bainite, while the acicular ferrite and bainitic ferrite were formed in boron-bearing steels. A significant improvement in the kinetic of spheroidization was achieved by the addition of boron. Moreover, the decrease in hardness at the decarburization surface was effectively suppressed in boron added steels.