Mechanical properties and microstructure of steel pipes for gas bottles

In this paper mechanical and microstructural results of laboratory heat tre atment (quenching and tempering) of seven various heats made of low-alloyed manganese steels are shown. The effect of vanadium and nitrogen contents o n the yield/tensile strength ratio of pipes in quenched and tempered state was analysed. The linear relationship between the total amount of vanadium and nitrogen contents and the yield/tensile strength ratio was found. The o btained results are suitable for various heats of following chemical compos ition: 0.32-0.35 wt.% C, 1.13-1.37 wt.% Mn, 0.15-0.22 wt.% V, and 0.0070-0. 0127 wt.% N. Quenching the pipes in oil from 850 degreesC, tempering them a t 650 degreesC and cooling them in air, the required mechanical properties axe obtained. In order to obtain the yield/tensile strength ratio below 0.9 5, the total amount of vanadium and nitrogen should be lower than 2200 ppm.