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The micro alloyed steels(MAsteels) developed to reduce the manufacturing cost of automotive parts, have reasonable mechanical properties, but their lower toughness compared to conventional quenched and tempered steels(QT steels) has limited their application to the automotive components. To understand the fracture behavior of MA steel, the simple mechanical properties were investigated and three kinds of fracture related tests; three point bend test, compact specimen fracture toughness test and fatigue test were performed. From the results of three point bend tests, absorbed energy for both materials, except for V-notched specimen of MA steels, was independent of strain rate with each notch geometry. And, MA steels were more sensitive to notch geometry than QT steels. Therefore, in practical application, components need to be designed to have low restrained notch geometry.

Full scale developments of microalloyed steel forgings (0.4C-V modified) for connecting rods and wheel hubs are discussed in this presentation. Mechanical properties were evaluated in terms of various processing parameters and the resulted microstructures. Tensile strength obtained in MA steel by air cooling was equal to or greater than that produced in the quenched and tempered steel. Generally, increasing austenitizing temperature (900 C - 1300 C ) and cooling rate (box cooling< air cooling< fan cooling ) resulted in increased tensile strength. Impact toughness did not show the significant dependence of cooling rate, while decreasing austenitizing temperature enhanced impact toughness. The impact toughness of the microalloyed steel forgings obtained from optimum process showed poorer performance than that of quenched and tempered steel forgings but satisfied the specification made from the design view point.