Effect of Heat-treatment Process and Nb Microalloying on Microstructure and Properties of 9Ni Steel

Abstract

To meet the use needs of LNG fuel tanks and storage tanks， the key process control of steel-making and rolling of 9Ni steel were carried， and the influence of two-phase area heat-treatment processes and Nb microalloying on micro⁃ structure and properties of 9Ni steel was studied. The results show that with the increase of intercritical quenching （IQ） temperature， the Yield stress（YS）， tensile strength （TS） and the ratio between YS and TS （Y/T） decreases firstly， then increases. Subsequently as the tempering temperature increases， the YS and TS gradually decrease， while the elongation increases； When the tempering temperature is 600 ℃， the YS and TS reach their lowest values， and the elongation （EL） reaches its peak. The microstructure after intercritical quenching presents distribution of large and small grains， which contributes to the reduction of Y/T ratio of steel plate. The microstructure of Nb micro-alloyed 9Ni low-temperature steel plate is mainly composed of tempered sorbite structure and 3% -8% reverse austenite. The addition of 0. 015% Nb in⁃ creases the average YS and TS of 9Ni low-temperature steel by about 50 MPa and 40 MPa， respectively. The impact ab⁃ sorption energy increases by about 40 J tested at -196 ℃.

Keywords

• lng； nb micro-alloyed；9ni steel； tempering sorbite