Phase Composition and Residual Stresses in the Surface Layers of VNS9-Sh TRIP Steel

Abstract

The phase composition of thin-sheet (0.3 mm thick) austenitic–martensitic VNS9-Sh TRIP steel is studied by X-ray diffraction, and the residual stresses in the steel are determined. This cold-rolled steel is a gradient material with a high content of strain-induced martensite in the near-surface layers. The austenite content in the surface layer is shown to increase from 47% (as-delivered state) to 85% after removing a 10‑μm-thick surface layer. The positive volume effect of the martensitic transformation during cold rolling leads to the relaxation of high tensile stresses up to 850 MPa in the martensite phase and to the formation of compressive stresses up to 950 MPa in austenite in the surface layer. The removal of a 10-μm-thick surface layer brings about a substantial change in the stressed state; in this case, residual tensile stresses prevail in both the phases: 400–800 MPa in martensite and 70–280 MPa in austenite. A technique is proposed for estimating the residual stresses via measuring the lattice parameters for various reflections; it makes it possible to separate the contributions of residual stresses and the solid solution composition with allowance for the elastic anisotropy of the crystal lattice.