Neutron diffraction analysis of retained austenite in Mn-Si TRIP steel during plastic deformation

O. Muránsky ¹, P. Lukáš ¹, P. Sittner ², J. Zrník ³, E.C. Oliver ⁵

¹Nuclear Physics Institute, 250 68 Řež near Prague, Czech Republic

²Institute of Physics, Na Slovánce 2, 182 21 Prague, Czech Republic

³COMTES FHT, Borská 47, 320 13 Plzeň, Czech Republic

⁵ISIS Facility, CCLRC Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QK, United Kingdom

TRIP (Transformation Induced Plasticity)-aided multiphase steels are promising structural materials with well balanced properties, combining high yield strength and excellent formability. Structural materials with such beneficial mechanical properties are required particularly by the automotive industry for the manufacture of light but strong components which also provide good energy absorption during dynamic loading. The combination of high strength and ductility is attributed to the transformation-induced plasticity effect resulting from the strain-induced martensitic transformation of the retained austenite in the ferrite-bainite microstructure.

Neutron diffraction was used to examine the TRIP effect in different thermomechanically treated steel samples. In order to evaluate the effect of the applied thermomechanical processing parameters on the multiphase structure development and subsequently on retained austenite stability, samples were studied in-situ during tensile loading at room temperature using the ENGIN-X diffractometer at the ISIS neutron source. Neutron diffraction has been found to be a convenient method for the characterization of transformation kinetics of strain-induced martensitic transformation in TRIP steels during mechanical loading. The evolution of transformation stresses with applied external load was determined in the retained austenite and ferrite phases. The relationship between yield strength of TRIP steel and volume fraction of retained austenite was found.

Keywords: TRIP steel, neutron diffraction, thermomechanical treatment