Effect of alloy treatment and coiling temperature on microstructure and bending performance of ultra-high strength strip steel

Two different high strength B-containing microalloyed steel strips produced in industrial processing conditions, one treated with Ti and the other treated with Al, processed by controlled rolling, accelerated cooling and coiling in two different temperatures ranges [723 K to 733 K (450 °C to 460 °C)] and [633 K to 653 K (360 °C to 380 °C)] were subjected to bend testing. The Ti treated steel coiled at the higher temperature 733 K (460 °C) showed the best bending performance. The relatively softer (tensile strength of and even {112} in the sub-surface region as well as uniformity of through thickness texture of the rolled sheet improve the bendability. In the presence of crack initiators, like coarse and brittle TiN particles found in the Ti treated steel, a harder microstructure and the presence of Cube and Goss texture in the sub-surface layer, seen for the lower coiling temperature can cause local transgranular cleavage cracking. Finally the post-uniform elongation obtained from tensile testing and bendability follow a good correlation

Microstructure-Property Relationships of Novel Ultra-High-Strength Press Hardening Steels

The industrial significance of microalloyed martensitic steels manufactured via cold rolling, reaustenitization, and quenching has been typically recognized as low. However, it is currently believed that microalloying can improve the in-service properties of ultra-high-strength press hardening steels. In this work, five 34MnB5-based steels were designed to address the role of Ti and V when combined with Cr or Mo. Microstructure-property relationships were analyzed after die quenching and additional bake hardening (BH) heat treatment using advanced methods of microscopy, glow discharge optical emission spectroscopy, quasi-static tensile tests, and three-point bending tests. Results indicate that both Ti and V can provide grain size refinement through the formation of stabile nanosized precipitates. The BH treatment improved postuniform elongation values, indicating a trend of improved ductility. However, the expected improvements in bendability were clearly confirmed only for two V-microalloyed steels with the alloying concepts of 0.3Cr-0.15V-0.03Al-0.02Ti-0.0020B and 0.3Mo-0.15V-0.0060N (without Al-Ti-B additions) (wt pct). Thus, it was discovered that microalloying with V, when combined with either Cr or Mo, provides a promising combination of mechanical properties as far as the austenitization parameters are appropriately controlled.acceptedVersionPeer reviewe