Effect of phosphorus on the formation of retained austenite and mechanical properties in Si-containing low-carbon steel
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I.
INTRODUCTION
RECENTLY, high-strength, medium-carbon (about 0.4 wt pet) steel sheets characterized by a triple phase (ferrite, retained austenite, and bainite) microstructure have been developed by mtercritical' annealing, t12~ and heat-treating around 670 K followed by air cooling. ' Because these steels contained a large amount of retained austenite (more than 10 vol pet), a good combination of strength and ductility was obtained through the effect of TRIP (transformation induced plasticity), as shown by Zackay et al.i3] Both strength and ductility generally increase by increasing the amount of retained austenite, t~'2'4"5)Therefore, a large amount of retained austenite is desired in order to get high-strength and highformability steel sheet. It was reported that silicon, manganese, and nickel in steel had a beneficial contribution to the formation of retained austenite while silicon was the most effective element of these three elements, tSl It was also reported that about 2 pet Si in steel made a large contribution to the formation of retained austenite through the two-stage bainite transformation regime, t6'7'81 Furthermore, silicon tends to inhibit carbide precipitation; tg'~~ thus during bainite reaction, the austenite adjacent to ferrite can be enriched in carbon, becoming remarkably stable, tr) Some behavior of silicon and phosphorus in steel is similar in the sense that both elements are ferrite formers, carbide inhibitors, etc. [9-12]From this point of view, the present work was carried out to investigate whether the addition of phosphorus can affect the formation of retained austenite and mechanical properties even if a low-carbon steel was employed. H. C. CHEN, formerly Graduate Student, Kyushu University, is Metallurgical Engineer, R & D Department, China Steel Corporation, Kaohsiung, Taiwan, Republic of China. H. ERA, Research Associate, and M. SHIMIZU, Professor, are with the Department of Metallurgy, Faculty of Engineering, Kyushu University, Fukuoka, Japan. Manuscript submitted June 15, 1987. ME'FALLURGICAL TRANSACTIONS A
II.
EXPERIMENTAL
The steels used in this investigation were melted in an argon gas atmosphere and cast into iron molds. Ingot size was 150 • 60 • 30 mm. The chemical compositions of the steels are shown in Table I where the Ac~ and Ac3 temperatures, calculated from the empirical formula of Andrews, t~3j are also shown. The base composition of the Si-free steels (steels A, B, and C) was about 0.1 pct C and 2 pct Mn while phosphorus was changed from near zero to 0.28 pet. The base composition of 0.5 pet Si steels (steels D, E, and F) was about 0.15 pct C, 0.5 pct Si, and 1.5 pet Mn while phosphorus was changed up to 0.2 pet. As shown in Figure 1, the cast steels were soaked at 1523 K for 30 minutes, hot rolled to a thickness of 3.2 mm at 1223 K, and air cooled. The rolled sheets were aged at 773 K for 24 hours prior to cold rolling to simulate the coiling in a hot rolling process. They were further cold rolled by 75 pct to a thickness of 0.8 mm. Heat-treatments consisted of
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