Effects of negative roll gap rolling and pulsed current annealing on microstructure and properties of Invar 36 alloy thin strip
Enzheng Wang, Liping Bian, Yao Zhao, Hongwei Ping, Tao Wang, Zhongkai Ren, Shuyong Jiang, Qingxue Huang
Abstract
This study proposes a novel processing route combining negative roll-gap cold rolling with pulsed current annealing for manufacturing high-performance Invar 36 alloy thin strips. The microstructure evolution and texture development were systematically investigated under various rolling reduction ratios and pulsed current annealing parameters. The results indicate a strong correlation between cold rolling reduction ratios and the pulsed current annealing process. Negative roll gap cold rolling leads to a significant increase in strength and a sharp decrease in ductility as the cold rolling reduction ratios rise. After 90 % cold rolling reduction, the processed specimen achieves enhanced mechanical properties with a tensile strength of 820 MPa and a hardness of 283 HV. This is attributed to the increased dislocation density, the formation of low-angle grain boundaries (LAGBs) and deformation twins, and the development of strong crystallographic textures including copper-type {112}<111>, brass-type {110}<112>, and S-type {123}<634>. Pulsed current annealing significantly enhances the ductility of rolled thin strips while concurrently reducing their strength. This improvement is attributed to dynamic recovery, recrystallization and the development of a strong cubic texture {100}<001>, driven by high-energy pulsed current induced thermal and non-thermal effects. High-strength-ductility Invar 36 alloys, including a 0.5 mm-thick thin strip with a tensile strength of 650 MPa, yield strength of 575 MPa, and fracture elongation of 5.9 %, as well as a 0.1 mm-thick ultra-thin strip with a tensile strength of 600 MPa, yield strength of 520 MPa, and fracture elongation of 5.8 %, were successfully fabricated using a combined approach of negative roll gap cold rolling with 50 % and 90 % reductions followed by pulsed current annealing for only 40 s. This approach provides a promising and efficient pathway for the rapid production of high-performance Invar 36 alloy thin strips.