Ultrasonic evaluation of formability r in cold rolled and annealed auto grade steel sheets using Rayleigh waves

Abstract

An ultrasonic method developed for non-destructive evaluation of formability r in cold rolled and annealed steel sheets is reported. Times of flight T₀, T₄₅ and T₉₀ at 0, 45 and 90° to the rolling direction of sheets 0˙8–1˙2 mm in thickness were measured, from which empirical equations were developed by regression analysis. Predictions using averaged time T resulted in correlation coefficient R²=0˙74 with standard deviation σ=0˙095. An attempt to improve accuracy of prediction, using a relationship of the form r=f (T₀, T₄₅, T₉₀, T_bar, %C, %Mn, % P, % Ti, %Al, HB), where HB is the Brinell hardness, resulted in only minor improvement (R²=0˙812, σ=0˙08). However, a strong correlation (R²=0˙905, σ=0˙058) was obtained using a relationship of the form r=f (T₄₅). The standard deviation was further improved to σ=0˙045 using a relationship of the form r=f (T₄₅, HB).