Effect of tension on the formation of local voids and integral damages in DC04 steel sheets

The effect of tensile strains (e) on changes of the damage level (D) for steel DC04 (0.06 % C, up to 0.35 % Mn, up to 0.40 % Si, - 0.025 % S and P) sheets was studied. D was evaluated by electron microscopy and the change in Young’s modulus (E) of a conditionally intact sample (standard). Pre-recrystallization annealing results in the formation of a structure with a minimum D and increased E in the rolling direction (RD) and transverse direction (TD) with unchanged texture and anisotropy E (ETD > ERD). Tensile strain (TS) at the first stage (~ 5 %) leads to the formation of nucleation damage in the form of spherical pores up to 100 nm in size in the near-surface layer of steel, distributed over the volume and grain boundaries. ETD and ERD decrease to pre-annealing values. At a deformation of 10-20 %, new pores and chains of them are formed. E decreases and D increases uniformly. At deformations of more than 20 %, the pores in the chains show a tendency to merge (coagulate) with the formation of cracks in the form of plates, which are perpendicular to the direction of (TD). The decrease in the Young’s modulus in RD and TD and the increase in D are sharper. An analysis of texture changes confirms the predominant contribution of damage to the effect of a decrease in Young’s modulus under tensile deformation.