Correlation of Strain Accommodation Factor with the State of Microstructural Components in a Multiphase Steel

Abstract

A mathematical approach to strain accommodation was employed to address the deformation behavior and microstructural evolutions of a two-phase steel during intercritical deformation. For this purpose, uniaxial compression tests were conducted at 640–960°C at strain rates of 10⁻³ to 10⁻¹ s⁻¹ up to the strain of 0.6. Constitutive equations were used to assign a strain accommodation factor for austenite and ferrite at each deformation condition. Then, a correlation between the strain accommodation factor, volume fractions of ferrite and austenite and dynamic microstructural evolutions was established. The electron backscatter diffraction and optical observations indicated that ferrite is completely responsible for strain accommodation at 760°C when the microstructure is occupied by 70% ferrite. The strain accommodation factor of ferrite decreased sharply with increasing deformation temperature and increased slightly with increasing strain rate. At 800°C, when the phase fractions of austenite and ferrite were close to equal, ferrite seemed to have bore 72% of the total strain in average. Under such conditions, the strain accommodation factor of ferrite showed considerable strain rate sensitivity due to proper load transferring from austenite toward ferrite and effective strain accommodation in ferrite. However, when the deformation temperature was increased only by 40°C to 840°C and the austenite fraction exceeded 60%, the strain bearing phase changed from ferrite to austenite, which was seen as coarse subgrains in ferrite. The strain accommodation transition point is believed to lie between 800 and 840°C, where the volume fraction of ferrite was less than 40%.