Static strain aging behavior of ultra low carbon bake hardening steel.

摘要

Static strain aging in a ULC BH steel was studied bythe measurement of the changes in upper yield stress due to agingin specimens prestrained up to 10%. The aging, in general, doesnot go beyond the Cottrell hardening stage in the temperature andtime range studied. A small increase in the yield stress after thefirst stage of aging occurs, however, at prestrains <1% and higheraging temperatures (>140 deg C). This might be due to theformation of low temperature carbides or the clustering of soluteatoms. TEM studies on thin foils did not reveal the presence of anyprecipitate of that type in either the matrix or at thedislocations. The maximum increase in the yield strength due tothe atmosphere formation was found to be independent of thedegree of prestrain within the range of prestrains studied. It wasobserved that the atmosphere density at saturation did not varywith increasing dislocation density for the range of prestrainsstudied. However, the amount of prestrain has a significantinfluence on the spread of the aging stage from Cottrellatmosphere formation to precipitation. In the initial stage, theaging obeys the Cotrell's t exp 2/3 law. The activation energy forsegregation of C to the dislocations was found to be 18.3plus/minus0.5 kcal/mol which is in good agreement with the valuesreported in literature. The reduced carbon activity or the amountof prestrain did not reflect any influence on the activation energyin the present steel.