Influence of cooling rate on the microstructure and retained austenite in an intercritically annealed vanadium containin
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I.
INTRODUCTION
THE recently
developed "dual phase" steels ~-s exhibit far superior strength/ductility combinations compared to the high strength, low alloy (HSLA) steels of identical composition. Dual phase steels are "natural composites" produced by the exploitation of solid-state phase transformations and typically consist of a strong martensite phase finely dispersed in a soft ferrite matrix. However, many steels described as "dual phase" actually deviate substantially from a strictly ferrite-martensite microstructure and have been reported to contain as much as 8 vol pet of fine (submicron), blocky retained austenite. 6,7 Although the emphasis has centered initially on the ferrite and martensite, the importance of retained austenite on the mechanical properties as well as its characterization are the focus of some recent studies. 6'7'8 One of the important processing variables for the production of dual phase steels is the cooling rate following intercritical annealing. (See, for example, the conference proceedings listed in References 3 and 9.) Rashid 3 showed the strength-ductility combination to be optimum around 8 ~ per second (for 2.5 mm section) and decrease with either faster or slower cooling rates. The purpose of the present study was to characterize in detail, by transmission electron microscopy, the microstructure of an intercritically annealed vanadium containing dual phase steel (see Table 1 for composition) but which had been cooled at different rates. Two samples were obtained: one was air cooled (6 ~ per second) and the other oil quenched (14 ~ per second) following a four minute intercritical anneal at
NARASIMHA-RAO V. BANGARU (formerly known as B. V. N. Rao), formerly with General Motors Research Laboratories, Warren, MI, is now Staff Engineer, Exxon Research and Engineering Company, P.O. Box 101, Building SP-240, Florham Park, NJ 07932. ANIL K. SACHDEV is Staff Research Engineer in the Metallurgy Department, General Motors Research Laboratories, Warren, MI 48090. Manuscript submitted February 11, 1981. METALLURGICALTRANSACTIONS A
1450 ~ (788 ~ Stress strain curves of samples of these two steels are shown in Figure 1.
II.
EXPERIMENTAL PROCEDURE
Transmission electron microscopy specimens were obtained from heat treated tensile blanks. About 2.5 cm long pieces were cut from the gage sections of the 2 mm thick x 12.8 mm wide tensile blanks. These sections were surface ground to 0.625 mm with flood cooling, and material was removed equally from both sides, about 0.012 mm at a time. Chemical thinning in a solution containing 5 pct HF in H202 (30 pct concentration) was used to reduce the thickness to 0.125 mm. Final electropolishing was done in a Fischione twin jet apparatus at room temperature using a chromic-acetic acid solution. Thin foils so obtained were examined in a JEOL JEM 200C transmission/scanning transmission electron microscope equipped with a Kevex energy dispersive X-ray analyzer at an accelerating voltage of 200 KV. Microchemical analysis was performed in the STEM mode of op
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