Temper-Aging of Continuously Annealed Low Carbon Dual Phase Steel
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I.
INTRODUCTION
RECENT developments in continuous annealing technology have made it possible to produce high strength, paint bake hardenable, dual phase sheet steels from plain low carbon steels. ~Because of their low hardenability, low carbon steels are normally water quenched after an intercritical anneal to insure a dual phase structure. The water quench results not only in the formation of martensite but also in a supersaturation of carbon in ferrite which, if not properly controlled, could impair the formability of the sheet steel due to quench and strain aging. Hence, water quenched dual phase steel is generally reheated to some appropriate temperature below the A1 temperature in order to control the carbon distribution.2The latter process has been termed overaging 2'3 or tempering ~'2~4-6by several different researchers but will be referred to as temper-aging in the present work. The term "temper-aging" is recommended because in reality both the tempering of martensite and quench, possibly strain, aging of ferrite (not always to the degree of being "overaged") is occurring in a dual phase steel during this low temperature reheating process. The temper-aging of dual phase steel is of considerable practical importance because it controls not only the mechanical properties but also the paint bake hardenability of the steel. Although the tempering of martensite 7'8 and quench aging9'~~and strain aging 1~'~2of ferrite are fairly well understood individually, their relative importance and their interactions during the temper-aging of dual phase steel have not been fully explored yet. Recently, some light has been shed on the temper-aging of dual phase steels (mostly high manganese steels) by the works of Rashid and Rao, 4 Davies, 5 and Speich and Miller. 6 But systematic study of this subject for the plain low carbon steel with particular emphasis on its application to the continuous annealing process has, to the author's knowledge, not been made. An PENG-HENG CHANG is Research Engineer with Inland Steel Company Research Laboratories, 3001 East Columbus Drive, East Chicago, IN 46312. Manuscript submitted January 7, 1983. METALLURGICAL TRANSACTIONS A
attempt is thus made to provide such information in the present investigation. The attention in this work has been focused on the carbide precipitation in both martensite and ferrite of the dual phase steel during the temper-aging cycle of a continuous annealing process.
II.
EXPERIMENTAL PROCEDURE
The materials investigated were secured from the Inland Steel, Indiana Harbor Works, as 2.54 mm (0.10 inch) thick hot rolled and pickled bands with 0.05 pct C, 0.33 pct Mn, 0.021 pct Si, 0.012 pct P, 0.017 pct S, 0.052 pct A1, and 0.006 pct N. The hot bands were cold reduced to 0.762 mm (0.030 inch) gauge using a laboratory cold rolling mill. Standard ASTM E8 sheet tensile specimens with a 50 mm (2 inch) by 12.5 mm (0.5 inch) gauge section were prepared from the cold rolled blanks with their tensile axis parallel to the rolling direction. All the tensile specimens were
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