Tensile Properties of Chromium-bearing Extra Low Carbon Steel Sheets

Abstract

The influence of chromium on tensile properties of cold-rolled and annealed extra low carbon steel sheets was investigated. Tensile strength increased with an increase in chromium content whereas elongation decreased. Yield strength showed the minimum at chromium content of 5%. The yield strength in each steel changed with ferrite grain diameter according to Hall–Petch relationship. The coefficient in Hall–Petch equation linearly decreased with an increase in chromium content. Since ferrite region hot-rolling and generation of α′ phase did not affect the coefficient, the change can be attributed to reduction of solute carbon at grain boundaries by chromium carbide precipitation. The slope of the decrease in the coefficient per 1 mass% chromium was 0.02 MPa·m^1/2. The friction term in Hall–Petch equation showed the minimum at chromium content of 5% like the yield strength. The change in the friction term is the reason why 5% Cr steel inhibited the minimum yield strength. Aging at 150°C made the friction terms of low chromium steels up and friction term became to show linear relationship for chromium content. This result indicated that the lowest yield strength which 5% Cr steel showed is attributed to the retardation of aging by chromium during cooling after the annealing. It is concluded that solute chromium exhibited solid-solution strengthening. The increase in solid-solution strengthening of yield strength is about 5.6 MPa per 1 mass% chromium.