Recent Trends in Producing Ultrafine Grained Steels

Abstract

Ultrafine grained steels with grain sizes below about 1 μm offer the prospect of high strength and high toughness with traditional steel compositions. These materials are currently the subject of extensive research efforts worldwide. Alloy design is one of the first considered issues, while designing new steel with targeted mechanical properties. However, the alloying content of steel does not fully determine the mechanical properties, but manufacturing procedure, hot rolling and cooling parameters, heat treatment parameters etc. are also of vital importance. For instance, same steel with different processing conditions can exhibit rather large variations in properties. To be precise, chemical composition with the processing parameters determines the microstructure, which in turn determines the properties of the steel. Steel is defined as an iron alloy containing C, Mn and Si that are generally used as alloying elements in steel. Micro-alloying elements such as Nb, Ti V, and B, are considered to be effective, causing strengthening as well as microstructural refinement in small quantities below 0.1 wt% (therefore these are called micro-alloy elements) and are quite generally used in ultrafine grain steel. Substitution alloying elements, such as Mo, Ni, Cr and Cu are alloyed to suppress phase transformation temperatures, i.e. for reaching certain level of strengthening, since the strength of steel structures strongly depends on the phase transformation temperature. Accordingly, the alloy design of ultrafine grains steels with different structures generally relies on: i) carbon levels, ii) sufficient alloying to obtain the desired transformation temperature and iii) micro-alloying technology in conjunction with Thermo Mechanical Controlled Processes (TMCP). Also, both advanced thermo-mechanical processes and severe plastic deformation strategies are used to produce ultrafine grained steels. Both approaches are suited to produce submicron grain structures with attractive mechanical properties. This overview describes the various techniques to fabricate ultrafine grained steels.