The effect of industrial composition variations on the transformation behavior of ultralow-carbon steels
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I. INTRODUCTION
ULTRALOW-carbon (ULC) steels have developed into a feasible alternative to higher-carbon high-strength steels in a variety of applications. The concept for these steels, also known as ULC bainitic steels,[1,2] was initially proposed by McEvily and co-workers more than 30 years ago.[3] In an attempt to prevent the formation of carbides, which have a deleterious effect on fracture toughness, they minimized the amount of carbon in the alloy (current ULC alloys typically have less than about 0.03 wt pct carbon[2]). Alloying additions of a few percentages of nickel and molybdenum and up to 2 pct of manganese were then added to compensate for this lack of carbon. This combination of elements contributes to the potential high strength, high hardenability, good low-temperature toughness, and good weldability of these alloys.[1,3] However, the interactions between these elements and their effects on the microstructure are complex and can vary considerably with composition.[1] The low concentrations of many of the alloying elements, particularly carbon, in these ULC steels begin to approach the level of the compositional variations that can be expected from production-sized heats (Table I).[4] To determine the potential impact of these production variations on the microstructures and properties of ULC steels, two steels were fabricated, with compositions representing the potential rich and lean compositional extremes that could be expected from a production-sized heat. These steels are variants of CTC-03, a ULC steel developed by the National Center for Excellence in Metalworking Technology (NCEMT), which is operated by Concurrent Technologies Corporation (Johnstown, PA), for use as a consumable in welding highstrength steels. The two compositional variants were, therefore, designated CTC-03R and CTC-03L for the rich and lean variants, respectively. Deviations from the nominal CTC-03 composition were determined both from historical data on compositional variations of similar ULC steels provided to ESAB (Florence, SC) (a weld-consumable manufacturer) and from compositional tolerances quoted by four steel vendors.[4,5] These deviations were adjusted slightly to R.W. FONDA and G. SPANOS, Metallurgists, are with the Naval Research Laboratory, Washington, DC 20375-5000. Manuscript submitted March 13, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
fulfill empirical criteria developed to ensure high strength and weldability.[4,6] This article expands upon an earlier report[7] to detail the effect of these potential compositional variations on the transformation behavior, microstructure, and microhardness of the CTC-03 ULC steel.
II. EXPERIMENTAL PROCEDURE The two ULC alloys used in this study were obtained from ESAB in the form of 4.7-mm-diameter rods. Sections of these rods were encapsulated in quartz under a 1/3 atm of argon and homogenized for 3 days at 1200 ⬚C to remove any chemical inhomogeneities. Samples measuring 3.0 mm in diameter by 10-mm long were machined from these rods and then heated in a heating
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