Effects of vanadium and processing parameters on the structures and properties of a direct-quenched low-carbon Mo-B stee
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I.
INTRODUCTION
S O M E steel producers have eliminated a heat treatment step for rolled products (e.g., plates and rails) by installing in-line water cooling units, thereby permitting the quenching of these products immediately after hot deformation (herein referred to as "direct quenching"). In conventionally off-line quenched-and-tempered steels (herein referred to as "reheat-quenched'), the austenite composition and grain size, which play important roles in determining the ultimate structure and properties, are controlled largely by the austenitizing temperature. In contrast, quenching immediately after hot deformation allows for some control of the "condition" of the austenite prior to transformation. For example, generating martensite from deformed austenite by rolling below the austenite recrystallization temperature may provide some of the property improvements traditionally associated with "ausforming. "*tl,2] *Ausforming refers to deformation below the Ae3 of metastable austenite prior to martensitic transformation. Direct quenching, in the present context, refers to deformation above the Ae3 of stable austenite prior to transformation. Hence, deformation temperatures for direct quenching would generally be higher than those for ausforming. Direct quenching and ausforming have also been referred to as "hightemperature thermomechanical treatment" and "low-temperature thermomechanical treatment," respectively.[~]
The early investigations of direct quenching conducted by the British Iron and Steel Research Association t4-7j examined effects of composition and various processing parameters on the mechanical properties of carbon and low-alloy steels. Subsequent investigations elsewhere have also considered plain carbon and low-alloy steels, I8,9] K.A. TAYLOR, Research Engineer, and S.S. HANSEN, Supervisor, are with the Research Department, Bethlehem Steel Corporation, Bethlehem, PA 18016. Manuscript submitted February 20, 1991. METALLURGICAL TRANSACTIONS A
Nb/V-microalloyed HSLA steels, [l~ nickel steels for cryogenic applications, t13] Ni-Cr-Mo steels, t14,15~ and Fe-Mn-Ni-C "model" alloys.[~6] However, many of these previous investigations have dealt with steels of relatively low hardenability; hence, relationships between composition or austenite grain morphology and mechanical properties are difficult to establish because the transformation behavior of many of these steels is influenced significantly by processing parameters. The present investigation focuses largely on establishing relationships between the condition of the austenite prior to quenching and the resultant strength/toughness balance in a series of low-carbon, martensitic steels based on a 0.1C-1.4Mn-0.5Mo-B (wt pct) composition. These steels were processed in the laboratory in simulation of (1) production rolling and direct quenching and (2) conventional processing (i.e., reheat quenching) involving a postrolling, off-line austenitizing treatment. In addition, the potential of vanadium (which already plays an important role in the metall
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