Role of Structure and Microstructure in the Enhancement of Strength and Fracture Resistance of Ultra-High Strength Hot Rolled Steels

摘要

There is presently a strong interest in using as-hot rolled steels in automotive and mechanical applications in place of the quenched and tempered (Q&T) steels. This is because of the inherent metallurgical disadvantages associated with the latter; such as retained austenite, residual stresses, quench-cracking and distortion. Additionally hot rolled steels are likely to be cost effective and energy efficient, in that the additional processing through the conventional heat treatment system is eliminated. LTV Steel developed ultra-high strength hot rolled microalloyed steels with a minimum yield strength of 758 MPa (110 ksi).Generally, these steels have 0.05 wt. percent C and 1.5wt. percent Mn. They use microalloying concepts to attain a tine-grained ferrite-bainite microstructure that possesses ultra-high strength and good toughness. The mechanical behavior of the newly developed steels in terms of toughness, fatigue and formability is comparable to or better than the equivalent strength Q&T steels. These steels are used in truck bodies, car brackets, crane booms and structural tubings. They are also used as a viable substitute for lower strength 550 MPa (80 ksi) HSLA steel, with weight savings proportional to increased strength.