EFFECT OF ALLOYING ELEMENT Al ON HARDENABILITITY AND MECHANICAL PROPERTIES OF MICRO-B TREATED ULTRA-HEAVY PLATE STEELS

Utilizing Jominy end quenching test, chemical phase analysis and thermo- dynamical calculation, study of the effect of alloying elements on hardenability and mechanical properties of a B-bearing ultra-heavy plate steel was carried out. The results showed that small amount of Ti addition could form TiN for its much higher bonding ability than B, fixing N element and thus making B free. Normal Al content failed to prevent BN from precipitating due to the weaker competition for N than B. However, when Al content was increased as high as 0.07%, the competition of Al for N was distinctly improved, making solid-solution B increased. For proper chemical combination of B and N- fixing element, hardenability was increased and accordingly both microstructure and mechanical properties were improved so that the quantity and size of martensite/austenite (M/A) islands and granular bainite were decreased markedly, and low-temperature impact toughness and tensile properties were improved by a large degree. The calculation was in a good accord with experimental results.