Structure of continuously cooled low-carbon vanadium steels
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I.
INTRODUCTION
VANADIUM has been added to steel for about 80 years as a means of increasing the yield strength. Its effect as a secondary hardening element is well known, and the structure of quenched and tempered mild steels containing about 1 wt pet vanadium has been reported in some detail. E1,2,3jMore recently, the precipitation of vanadium carbide in isothermally transformed alloys has been studied.[4-al The structures show another form of discontinuous precipitation in which particles are precipitated in sheets parallel to a former austenite/ferrite interphase boundary. In addition to these aspects, the effect of continuous cooling from the austenitizing temperature on the structure of vanadium steels has been examined by Tanino et al. 191They found that the hardness of the alloy reached a maximum value for cooling rates of 60 ~ to 80 ~ min. Dispersed dislocation loops and elongated dislocation dipoles, carbide particles both randomly dispersed and lined up in rows, and micro- and macrotwins were observed in foils prepared from the continuously cooled MD. MOHAR ALI BEPARI, formerly with the Department of Metallurgy, The University of Sheffield, Sheffield, England, is Professor, Department of Metallurgical Engineering, Bangladesh University of Engineering and Technology, Dhaka, Bangladesh. Manuscript submitted December 20, 1988. METALLURGICALTRANSACTIONSA
material. I91 Although recently Niltawach t~~ did some work on vanadium-containing steels continuously cooled from austenitizing temperature, no systematic (detailed) study of these structures has been reported prior to the present work.
II.
EXPERIMENTAL PROCEDURE
The alloys were made in an air-induction furnace, and the analyses are given in Table I. All melts were teemed at around 1600 ~ giving sound ingots of 40 kg each. All ingots were extruded to 16-mm-diameter bar at 1170 ~ using a glass lubricant. The tensile specimens of each alloy from the extruded bar were solution-treated at a temperature chosen corresponding to a common austenite grain size of 35/xm, t11'12'131 as shown in Table II, for half an hour and allowed to cool to about 450 ~ at four different cooling rates: 120 ~ 36 ~ 12 ~ and 3.6 ~ Below 450 ~ each specimen was allowed to cool down to ambient temperature at its natural cooling rate. A 5 kW radio frequency generator with automatic control system (programmable) was used for this purpose. The heat-treated tensile specimens were then tested to obtain data on tensile properties which have been reported elsewhere, tl4J VOLUME 21A, NOVEMBER 1990--2839
Table I.
Chemical Analysis of the Steels
Steel Chemical Analysis, Wt Pct Number C Mn V N 2 0.15 1.42 0.22 0.005 3 0.15 1.52 0.20 0.020 4 0.15 1.57 0.20 0.024 5 0.15 1.42 -0.020 The residuals are similar for each steel, and no element is present in an amount greater than 0.02 pct.
The structure was primarily studied by optical microscopy. The Ar~ and Ar3 transformation temperatures at the fast cooling rate of 120 ~ were determined by differential dilatometry and are shown in Table III.
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