A New Opportunity for the Design of Advanced High Strength Steels with Heterogeneous-Phase Microstructures via Rapid Thermal Processing

Abstract

In the following paper, the developments in Advanced High Strength Steels (AHSS) that are used in automotive industry are summarized. AHSS show good formability while maintaining high strength values. These properties can be achieved via grain refining, mixed ferritic microstructures and by leveraging phenomena such as Transformation Induced Plasticity and Twinning Induced Plasticity (TWIP). However, time and energy consuming thermal processes as well as expensive alloying additions should be employed in order to achieve these properties. The new era of AHSS requires combinations of greater strength -ductility via ultra-grain -refined ferrite, martensite, bainite and austenite at a cost significantly less than the previous grades. It is envisaged that microstructural evolution under the effect of ultra-fast heating and the subsequent quenching or air cooling may result to a new outlook for alloy design. The key aspect in this technological possibility is the effect of chemical heterogeneity in the initial microstructure. The inhomogeneous microstructure remains unchanged due to the rapid reheating as a result of the restriction of the time available for solid state diffusion. A promising example of such technology is the singlestepped reheating of steel which, although unconventional, has been observed to lead to complex microstructures of chemical heterogeneity.

Refinement of Banded Structure via Thermal Cycling and Its Effects on Mechanical Properties of Dual Phase Steel

“…Different techniques have been used for grain refinement of DP steels. The intercritical annealing of cold‐worked martensite or ultrafine grained (UFG) ferrite‐carbide aggregate, fast and ultrafast heating, ultrafast cooling (UFC), Deformation‐induced ferrite transformation (DIFT), and severe plastic deformation have been recently practiced. Most of these techniques are based on prior deformation and need rolling facilities or more complicated and expensive deformation routes.…”

Ultrafast Heating and Initial Microstructure Effect on Phase Transformation Evolution of a CrMo Steel

“…where c The Cr, Mn or Mo, as substitutional atoms, hinder the diffusion of C and are characterized from lower diffusion coefficients than this of C, which is an interstitial atom. These lower diffusion coefficients contribute to smaller velocities of the substitutional elements through the interface according to the Equations (3) and (4). The substitutional alloying elements have been redistributed into the matrix alleviating micro-segregation and augmenting grains' radius.…”

“…The application of ultrafast heat treatment or "flash annealing" (rapid heating followed by peak austenitization and water quenching) has been applied as a cost-efficient reheating process for designing steels with strength commensurate to that of advanced high strength steels (AHSS). This process is characterized by rapid thermal cycles with duration less than 10 s [1][2][3][4][5]. The thermal process is divided into three stages.…”