Deter mination of Hardness and Other Mechanical Properties of Thin-sheet Trip Steels by Indentation

Abstract

The chemical composition, microstructure and mechanical properties of thin-sheet (250 - 260 pm) TRIP steel VNS9-Sh obtained by cold rolling of a hot-rolled sheet with intermediate thermal treatment are studied. It is determined that the metal microstructure consists of strain-induced martensite and metastable austenite in work hardened condition. The value of optimum indentation load equal to 1 kgf (9.81 N) for the Vickers hardness test is determined proceeding from the average grain size (35 pm) and strip thickness. The indent covers several grains on the steel surface under this load and the ratio of the steel strip thickness to the indent depth is at least 20 thus excluding the impact of the substrate (specimen stage or other equipment) on the hardness value. To determine other mechanical properties an instrumented indentation by a ball diamond indenter with a radius of 0.2 mm is used. A technique for determination of the Young’s modulus, yield stress, and ultimate stress is proposed. The discrepancy between the values of the aforementioned mechanical characteristics determined by instrumented indentation and those determined in tensile tests range within ±7%.