Change in Properties of Pipe Steel with Two-Phase Structure During Low-Temperature Heating and Subsequent Plastic Deformation

Results are given for evaluation of the effect of longitudinal deformation on the change of the mechanical properties and steel properties with a two-phase pipe metal structure previously given low-temperature heat treatment simulating application of an anticorrosion coating. A reduction in steel deformation capacity to the minimum permissible level is evaluated by a criterion δ_un ≥ 3% and σ_y(R_t0.5)/σ_u ≤ 0.96–0.98. As a result of testing plates subjected to longitudinal tension with plastic deformation ε_p = 1.5–7.0%, it is shown that exhaustion of steel deformation capacity is predicted in the transverse direction with preliminary deformation ε_p ≈ 5%, and in the longitudinal direction with ε_p ≈ 4–5%. SEM and TEM methods are used to study the effect of steel main structural characteristics whose presence facilitates retention of capacity for steel strain hardening after heating (200–220°C, 5 min): fine islands of twinned martensite, fine cementite particles and abundant precipitation of nanosize particles of Nb and V carbonitrides. During branch pipe bending deformation (ε_p ≤ 3.0%) in a two-phase steel structure (main components are ferrite and lath martensite) includes transformation of residual austenite from the MA-component in twinned martensite.