Mechanical properties and retained austenite in intercritically heat-treated bainite-transformed steel and their variati
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I.
INTRODUCTION
IT
is well known that dual-phase steels til have high strength and ductility combinations for steel sheets applied to press forming. Further improvements in strength and ductility can be achieved by transformationinduced plasticity (TRIP) of retained austenite. t2.3,4] Transformation-induced plasticity was the phenomenon first found in steel with large amounts of Ni and Cr by Zackay et al.t51 However, large additions of costly alloying elements and the accompanying troublesome process make it difficult to employ this phenomenon in manufacturing cost-effective steel sheets for automotive use. It is also well known that a significant amount of retained austenite can be obtained in steel containing about 2 pct Si by cooling to room temperature through the two-stage bainite transformation regime. [6-91 Many studies 1~~ have been conducted in order to obtain a better understanding of the essentials of bainite transformation, and recently, the latter has been used to try to improve the toughness of steelf115-19] Reports on the application of this phenomenon to improve the formability of sheet steels were rare; t2~ however, the authors t21'221 found that in steel sheets with comparatively low carbon and silicon contents, larger amounts of austenite could be retained, and formability was improved when rapid cooling was performed not from a single austenite range but from the intercritical range. The treatment, which is compatible with existing continuous annealing lines, produces about 20 pct retained austenite susceptible to TRIP, and cold-rolled steel sheets with a
YASUHARU SAKUMA, Visiting Scientist, Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, CO, is with the Sheet and Coil Laboratory, R&D-II, Nippon Steel Corporation. OSAMU MATSUMURA and HIROSHI TAKECI-H are with the Sheet and Coil Laboratory, R&D-II, Nippon Steel Corporation, Sagamihara 229, Japan. Manuscript submitted January 3, 1989. METALLURGICALTRANSACTIONSA
tensile strength of about 980 MPa and a total elongation larger than 30 pct can be obtained. Attempts to understand the role of Si in this process t23j have already been tried, but the details involving the influence of Mn t241are still ambiguous. Also, the feature of intercritical annealing and the effects of the chemistry and heat-treatment condition on the resulting mechanical properties have not been investigated sufficiently. This paper intends to clear up the noticeable phenomena mentioned above using 0.2 pct C steel with different levels of Si and Mn contents.
II.
EXPERIMENTAL PROCEDURE
One-hundred-kilogram vacuum-melted ingots with a chemistry and estimated transformation temperatures Act and Ac3,1251 given in Table I, were prepared. Slabbed materials 30-mm thick were reheated to 1200 ~ for 1 hour and rolled in five passes to 4.5 mm by finishing at 900 ~ The hot-rolled sheets were air-cooled to room temperature, so the microstructure was composed of ferrite and pearlite. After surface grinding to 3.5 mm, the sheets were cold-rolled wi
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