Effect of rolling process on microstructure and properties of low-temperature steel LT-FH32

To meet the construction requirements of liquid ammonia transport ships, this study adopted a low-carbon Nb-Ti-Al composite microalloyed composition design combined with controlled rolling and controlled cooling (CRCC)processes to develop the low-temperature steel plate LT-FH32. Through tensile tests, low-temperature impact tests, metallographic observation and scanning electron microscopy analysis, systematic investigations were conducted on the variation of the microstructure and properties of the test steel with rolling process. The results indicate that with decreasing finish rolling temperature, the microstructure of the test steel transitions from a multiphase structure consisting of polygonal ferrite+quasi-polygonal ferrite+acicular ferrite to a dual-phase structure of polygonal ferrite+bainite. The volume fraction of the soft phase, polygonal ferrite increases from 37% to 63% and the hardness difference between soft and hard phases rises from 68HV0.01 to 119HV0.01. With further application of post-rolling relaxation process, the microstructure evolves into a dual-phase structure of polygonal ferrite+large-sized lath or blocky M/A (Martensite/Austenite) constituents. The volume fraction of polygonal ferrite continues to increase to 89% and the hardness difference between soft and hard phases further increases to 379HV0.01. This microstructural evolution leads to a decrease in yield strength, initial increase followed by decrease in tensile strength and a continuous reduction in the yield-to-tensile ratio. The crack initiation energy first increases then decreases while the crack propagation energy declines continuously, and the ductile-brittle transition temperature rises. When the finish rolling temperature is 770 ℃ and direct water cooling is applied after rolling, the test steel exhibits moderate strength margins with yield strength of 406 MPa and tensile strength of 519 MPa ensuring good safety performance. The yield-to-tensile ratio is 0.78, and the low-temperature toughness reserve is sufficient with ductile-brittle transition temperature below -80 ℃, thus achieving the optimal comprehensive performance.