Stress relaxation behavior of low carbon structural steels

The stress relaxation usually appears on the extended tensile specimen during the fixed displacement. This is caused by the room temperature creep straining. In the present work, the creep straining behaviors at room temperature were examined by both of the tensile strain holding and the tensile stress holding. The settled stress after stress relaxation under a fixed strain was evaluated in several plastic straining levels, and the stress-strain curve after stress relaxation was compared with the stress-strain curve obtained under the conventional strain rate. The degradations of the flow stress after stress relaxation were evaluated for the seven kinds of the structural steels with the wide strength range from 197MPa to 826MPa. The effect of the metallurgical strengthening systems on the stress relaxation behaviors was also discussed. The degradation ratio in the flow stress due to the room temperature creep deformation was 30% in the case of interstitial free steel, and 8% in the 780MPa strength class steel. However, the strengthening systems such as a solution hardening, a precipitation hardening and a strain hardening did not give an apparent influence on the stress relaxation resistance.