Development of Nb-bearing heavy steel plates with ultra high strength and excellent toughness

Large engineering machinery is widely used all over the world and the market is still growing. Therefore, the demand for high strength and ultra-high strength steel plates with good toughness is increasing. To satisfy this need, Baosteel has developed Nb-bearing high strength heavy plates with high toughness and good weldability by direct quenching (DQ) or direct quenching and tempering (DQ+T) processes. With Nb/Nb+Mo microalloying, heavy plates with yield strengths from 690 to 1150 MPa were developed. The microstructures of these steel plates were composed of bainitic ferrite plates with 0.1~1.5 µm thickness and martensite-austenite constituent decorating the boundaries of the bainitic ferrite plates. Spherical cementite precipitates were observed at the bainitic ferrite boundaries after the tempering process. It was found by instrumented impact experiments that the steel plates exhibited good crack arrest properties at -40 °C. Steel plates with yield strengths of 690/890/1150 MPa, for which the C contents are less than 0.11% and carbon equivalents are less than 0.59%, have excellent weldability. These steels are commercially produced and already widely used in the mining industry. (AU) Copyright © 2018 Companhia Brasileira de Metalurgia e Mineração (CBMM) All rights reserved