New Developments in the Alloy Design of Micoralloyed and

New Developments in the Alloy Design of Microalloyed and Other Modern HSLA Steels
A.J. DeArdo

The structural steel industry has experienced a revolution during the last three decades. Prior to the mid-l960s, essentially only two types of steel were available for use in the transportation, energy and construction industries: low strength (250 ≤ YS ≤ 400 MPa) hot rolled or normalized plain carbon steels and high strength (YS ≥ 560 MPa) quenched and tempered (QT) low alloy steels. Since both the ferrite-pearlite microstructure in the as-rolled or normalized steels and the tempered martensite or bainite microstructure in the QT steels derive their strengths from their carbon contents, these steels have not always exhibited the desired levels of weldability, formability and resistance to brittle fracture. These shortcomings have rendered these traditional steels less cost-effective than they might be otherwise.

Thermomechanically treated microalloyed steel is now widely recognized and accepted as an alternative to the traditional steels. The controlled rolling and controlled cooling of these steels is practiced routinely around the world. The presence of the microalloying elements niobium, titanium and vanadium render these steels particularly amenable to thermo-mechanical processing. Furthermore, the low carbon contents exhibited by these steels cause marked improvement in their toughness and weldability.

Perhaps the most interesting and technologically important change that has occurred in these steels in the very recent past has been the shift away from ferrite-pearlite microstructures. The move to multi-phase microstructures, i.e. to mixtures of ferrjte, lower bainite, and auto-tempered martensite,when coupled with thermomechanically treated austenite, results in new steels which have properties that are often much superior to those exhibited by the older ferrite-pearlite steels. This paper will review recent advances made in the alloy design of these new multi-phase steels.

This paper was a plenary presentation at the 2nd International Conference on HSLA Steels held in 1990.

Publisher: TMS
Product Format: PDF
Pages: 21-31
Date Published: October 1, 1990
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