Study of Impact Toughness Anisotropy and Tendency Towards Lamination of Strenx 650 MC and Strenx 700 MC Steels After Thermomechanical Rolling

Resumo

Mechanical properties are investigated for tensile and impact bending in longitudinal and transverse Charpy and Menage specimens of Strenx 650 MC and Strenx 700 MC steels, produced by thermomechanical rolling technology. A marked effect is established for impact specimen cutting direction on critical brittleness temperatures T₅₀ (50% of the ductile component in a fracture) and T₃₄ (KCV = 34 J/cm²). For transverse specimens of rolled product 6 and 10 mm thick values of T₅₀ and T₃₄ are higher by 25–30 °C than for longitudinal specimens. Impact strength anisotropy coefficient for the studied steels varies in the range of 3.2–12.2. During tests on transverse specimens of steel Strenx 650 MC 6 mm thick an anomalous character (a sharp inflexion in the line for changes of KCV-B and KCU-B correlation) is detected: on the linear segment extending 2.4 mm for Charpy specimens and 2.8 mm on Menage specimens a low level of work for ductile crack propagation is revealed (A_r = 1.2 J/cm²), then work for crack propagation increases by more than an order of magnitude (121 and 570 J/cm²). A marked effect is demonstrated for coarse carbonitride-type inclusions based on titanium, and oxides based on Al₂O₃ on formation of large pits, contributing to formation of laminations and an increased tendency towards Menage and Charpy impact strength anisotropy.