Effect of Changing Load Path on Yielding and Flow Behavior of Sheet Steels.

Abstract

The plastic flow of four sheet steels in the as-received condition and after prestraining in uniaxial, biaxial or plane-strain conditions was characterized by tension tests at 15(DEGREES) intervals to the rolling direction, biaxial tests and plane-strain compression tests. Flow stresses of the four steels obtained by uniaxial through-thickness compression tests were about 5% lower at a strain of 0.1 than those obtained by bulge tests. Through-thickness compression flow stresses of aluminum-killed steel ((')R = 1.72, (DELTA)R = 0.64) and dual-phase steel ((')R = 1.00, (DELTA)R =0.40) were lower than predicted by Hill's 1948 theory and higher than those by Hosford's theory, while those of normalized steel ((')R = 1.00, (DELTA)R = 0.07) and microalloyed steel ((')R = 1.00, (DELTA)R = -0.038) were higher than predicted by either theory. Plane-strain compression flow curves of the four steels agreed well with predictions from Hosford's theory. Hill's 1979 theory best fit biaxial flow curves of the four steels, because the exponents m of the theory were calculated from the biaxial flow curves, while predictions from this theory with these m-values led to large disparities in plane-strain paths for all four steels. Strain-path changes from uniaxial tension in one direction to uniaxial tension in a different direction or from plane-strain or biaxial tension to uniaxial tension in any direction led to large decreases in uniform strain in subsequent uniaxial tension. This behavior is thought to result from interaction of dislocations produced in particular slip planes during prestraining with dislocations in other slip planes active in subsequent deformation along the new strain path. Aging at 150(DEGREES)C after a uniaxial prestraining of 0.084 caused a large decrease in uniform strain of the aluminum-killed steel in the coaxial direction and significant increases in uniform strain in noncoaxial directions, compared with the values before aging. Yield strengths and overall work-hardening rates were increased and total strains to failure were decreased in all directions by the aging.