The boron hardenability effect in thermomechanically processed, direct-quenched 0.2 Pct C steels
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INTRODUCTION
B O R O N is often used for economically increasing the hardenability of steels. When present at low levels (5 to 30 ppm B is typically used in conventionally quenchedand-tempered steels) and adequately protected from nitrogen, i.e., from forming boron nitride, boron can typically increase the hardenability of low-alloy steels by a factor of 2 to 3. ~ The behavior of boron in austenitized-and-quenched (herein referred to as "reheatquenched") steels has been studied extensively, t2J and it is clear that boron segregates to the austenite grain boundaries and increases hardenability by suppressing the nucleation of ferrite. The steel industry has devoted considerable effort toward combining processing steps as a means of reducing costs and increasing overall efficiency. For example, some producers have eliminated a heat-treatment step for rolled products (heat-treated plates and rails) by installing inline water-quenching units, thereby permitting the quenching of these products immediately after hot working (herein referred to as "direct quenching"). From a metallurgical point of view, the capability of combining hot working with water quenching raises a host of issues, including whether or not boron-treated martensitic or bainitic steels can be processed reliably in such units. In contrast to a conventional off-line isothermal austenitizing treatment (reheat quenching), the austenite cools continuously and generally undergoes repeated deformation and recrystallization during hot working prior to direct quenching. Since the segregation of the boron to the austenite grain boundaries requires diffusion and is thus time-dependent, there is some question as to whether the boron hardenability effect will always be operative in a thermomechanically processed steel. Recent investigations of thermomechanically processed low-carbon steels containing about 10 ppm B t3,4~ found that after high-temperature deformation and rapid K.A. TAYLOR, Research Engineer, and S.S. HANSEN, Supervisor, are with the Research Department, Bethlehem Steel Corporation, Bethlehem, PA 18016. Manuscript submitted August 9, 1989. METALLURGICAL TRANSACTIONS A
subsequent recrystallization of the austenite, the boron segregated to the austenite grain boundaries very rapidly and a "normal" boron hardenability increment was obtained. However, when the deformation temperature was lowered so that recrystallization did not occur, the hardenability was generally found to be lower. This was attributed either to a lack of sufficient boron to neutralize the ferrite nucleation sites within the deformation substructure or, in some cases, to strain-induced precipitation of borocarbides (which presumably deplete the austenite grain boundaries of elemental boron). The present investigation was undertaken to further define the behavior of boron in thermomechanically processed steels. Considering that the notch toughness of direct-quenched martensitic steels can generally be improved by conducting deformation below the austenite recrystallization tempe
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