High performance low cost steels with ultrafine grained and multi-phased microstructure

Abstract

Ultrafine grained ferrite was obtained through tempering cold rolled martensite with an average grain size of 200-400 nm in a low carbon and a microalloyed steel. Thermal and mechanical stability of the two steels was studied. Due to the pinning effect of microalloyed precipitates on the movement of dislocations and grain boundaries, the recrystallization and grain growth rate were retarded, and the thermal stability of ultrafine grained microstructure was improved. The ultrafine grained ferritic steel was strengthened, but its strain hardening rate was reduced. It seems that the tiny carbide precipitates have no significant effect on work hardening rate. The ultrafine grained ferrite+martensite dual phase microstructure was obtained in the microalloyed steel through intercritically annealing cold rolled martensite. The resulting multiphase microstructure has a tensile strength higher than 1.0 GPa with a yield ratio lower than 0.7. Another type of multiphase microstructure with nanoscaled lath bainite+ retained austenite was obtained through an isothermal heat treatment in low temperature bainite transformation region in high carbon steel. The tensile strength was as high as 1.64 GPa with a yield ratio of 0.84. ultrafine grained steel, multiphase, high strength, yield ratio Under the situations of the global shortage of natural resources and the substantial fluctuation of energy cost, it is more important than ever to use advanced technology to realize energy and material saving, to reduce emission and to operate an environment friendly steel industry with sustainable development. In the field of steel rolling, it is necessary to produce new generations of steel products with high performance low cost (HPLC) by using the most economical technology in terms of processing optimization, product development and technology upgrading.One of the HPLC materials that have great commercial potential is the ultrafine grained steels. Ultrafine grained (UFG) materials and their production methods have received extensive attention from material scientists during the past decades. Most existing methods for producing UFG structure are based on severe plastic deformation (SPD), such as equal channel angular pressing (ECAP) [1,2] , accumulative roll-bonding (ARB) [3,4] and so on. Although these methods are capable of obtaining submicron or nano-scaled microstructure, the need of using special equipments to achieve the necessary huge accumulative strain makes them inadequate and uneconomic in mass production of UFG materials.Tsuji et al. [5][6][7] fabricated ultrafine grained steels through tempering cold rolled martensite. By martensite transformation, the martensite packets and blocks divided the prior austenite grain into several regions with high angle boundaries, effectively refining the microstructure [8] . And by cold rolling, the dislocation cells were generated with larger boundary misorientation, which evolved into ultrafine grains during the subsequent tem-

Ultrafine Grained Steels with Improved Ductility Fabricated by Repetitive Thermo‐Mechanical Processing

“…This is usually considered to be resulted from the low dislocation storage efficiency in the ultrafine grains. In some report,9 embedding precipitates into UFG microstructure was suggested an effective way to enhance ductility by increasing strain hardening, but there are still some disputes 10, 11. Wang et al12 fabricated a bimodal grained microstructure in Cu, with micrometer‐sized grains embedded inside ultrafine grains (<300 nm), where the ultrafine grains are responsible for high strength and the micrometer‐sized grains supply enough ductility.…”