Abstract
In this paper mechanical and microstructural results of laboratory heat treatment (quenching and tempering) of seven various heats made of low-alloyed manganese steels are shown. The effect of vanadium and nitrogen contents on 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 obtained results are suitable for various heats of following chemical composition: 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 °C, tempering them at 650 °C and cooling them in air, the required mechanical properties are obtained. In order to obtain the yield/tensile strength ratio below 0.95, the total amount of vanadium and nitrogen should be lower than 2200 ppm.

Key words
low-alloy steel, mechanical properties, micro-alloying, microstructure, vanadium, nitrogen, precipitation

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