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Strain rate sensitivity is an important material property in the formability of sheet metal. In this study, strain rate sensitivity is evaluated for several different grades of steel. Strain rate sensitivity varies from 0.01 to 0.022 for the steels tested. It was found that formable steels such as IF and AKDQ steels have both high n-value (strain hardening) and m-value (strain rate sensitivity). Positive strain rate sensitivity results in a significant increase in the yield strength and tensile strength at higher strain rates. The n-value decreases with strain rate for all of the steels. The total elongation decreases slightly with strain rate for the lower strength steels but is constant or even increases slightly with strain rate for high strength steels. For a typical AKDQ steel, the increase in yield strength can be as high as 43% for an increase in strain rate from 0.002 /s to 2.0 /s.

The effects of key alloy and process parameters on the mechanical properties of high strength, highly formable low carbon sheet steels have been determined by a combination of laboratory processing, Taguchi Design of Experiments method, and multiple linear regression analysis. Factors studied include C, Mn, P, hot mill coiling temperature, in-line annealing temperature, and annealing line speed. Tensile strength levels were varied from 40 to 50 kg/mm2 with elongation of 30 to 40%. Annealing conditions were varied from partial to full recrystallization in both the single and α + γ two phase region. Effects of the chemistry and process parameters on the mechanical properties will be discussed, with particular emphasis on conditions required to produce a hot dipped galvannealed product meeting 45 kg/mm2 tensile strength and 35% elongation minimum requirements.

In order to increase the strength and formability of batch annealed sheet steels, design of experiment method was used to set up an experimental matrix with five factors, including C, Mn, P in steels, coiling, and batch annealing temperatures, at two levels. Effects of these factors were analyzed using analysis of variance and linear regression methods for cold spot and hot spot, respectively. Linear regression results showed that higher alloying element contents and coiling temperature will increase strength and deteriorate elongation, which is opposite to the effect of annealing temperature. Analysis of variance showed similar results to those of linear regression, except the effect of C on elongation and effect of coiling temperature on tensile strength and elongation are negligible for cold spot. For hot spot, effect of coiling temperature on tensile strength is small.

The high strain rate behavior of three types of low carbon steel has been investigated in the strain rate regime of .006-100 sec-1. These types included hot and/or cold rolled (i) rimmed steels, (ii) aluminum killed steels and (iii) high strength low alloy steels with various combinations of cold rolling and aging treatments. The tensile properties reported at four strain rates in the above range are yield strength, tensile strength, uniform and total elongation, strain hardening exponent and strength coefficient. Strain rate hardening exponent at several strain levels is also reported. New experimental techniques are discussed. The results are presented in tabular form and are discussed with representative graphs. In general, strength increases and ductility decreases with increasing strain rate.