Achieving high strength and large elongation in a strip casting microalloyed steel by ageing treatment
Shuai Xu, Shuize Wang, Shaohua Li, Rui Cao, Hong‐Hui Wu, Guilin Wu, Junheng Gao, Qingxiao Feng, Hualong Li, Xinping Mao
Abstract
The microstructure and mechanical properties of a Nb–V microalloyed steel produced by strip casting in as-coiled and aged conditions were investigated using scanning electron microscopy, electron backscatter diffraction , X-ray diffraction, transmission electron microscopy , atom probe tomography and tensile tests . Tensile test results show that, after ageing at 650 °C for 5 min, accompanying an increase of yield strength from 698 ± 3.6 MPa to 758 ± 7.9 MPa, the elongation increases from 7.1 ± 0.4% to 12.6 ± 0.4% simultaneously. Microstructural analysis shows that both the as-coiled and aged specimens possess a microstructure predominantly comprising a mixture of acicular ferrite and bainitic ferrite . Hardness tests for specimens aged at different temperatures show very similar peak hardness values but higher ageing temperature requires less time for peak ageing. Ageing treatment leads to a decrease of dislocation density , which is responsible for the enhancement of elongation for aged specimens , while Nb–N-rich and Nb–V–N-rich clusters develop successively during ageing, which contributes to the increase of yielding strength after ageing. The strengthening contributions from solid solution , dislocations, grain boundaries and clusters were calculated, and the results indicated that dislocations and grain boundaries are the dominant strengthening mechanisms for the current strip casting steels.