• PDF / 1,294,188 Bytes
• 11 Pages / 612 x 792 pts (letter) Page_size
• 72 Downloads / 293 Views

DOWNLOAD

REPORT

---

INTRODUCTION

II. EXPERIMENTAL PROCEDURE

FERRITIC stainless steels are being introduced into a wide variety of applications due to their combination of weldability, corrosion resistance, thermal-fatigue performance, and cost. Automotive exhaust systems, for instance, have doubled in their life expectancy over the past 15 years due in large part to the replacement of carbon steel components with ferritic stainless grades.[1] With expanding use comes increased pressure to improve the formability of these alloys. While developments over the past 15 to 20 years in reducing interstitial content have drastically improved performance, ferritic stainless steels remain less formable when compared to low-carbon or austenitic stainless steels. This has driven recent research efforts toward metallurgical routes for optimizing formability.[2–5] A problem common to both carbon and ferritic stainless steel sheets is that recrystallization slows substantially during the final 5 to 10 pct of recrystallization. Substantial effort has been expended in exploring mechanisms for this “sluggish” recrystallization in carbon steels,[6–12] as the incorporation of a partially recrystallized microstructure results in a strong reduction in formability.[13,14] Various explanations have been forwarded to aid in explaining this phenomenon, most being based on the inhomogeneity of the cold-rolled microstructure.[7,8,10,12,15–19] It is generally agreed that the slow approach to final recrystallization is attributable to the low stored energy and rapid recovery in deformed grains with orientations near {001}
110.[11] Thus, while new grains may form and grow in grains with other orientations, it is observed that deformed grains with near {001}
110 orientations are the last to be removed from the microstructure. This sluggish behavior is an acute problem for ferritic stainless steels, given the sensitivity of the formability to “under-recrystallization.”[14] The present article describes the microstructure evolution during recrystallization of one commercial grade of titaniumstabilized ferritic stainless steel. This work focuses on the respective influence of the local microstructure on the kinetic slowdown at the end of this process. Particular attention will be paid to the role that fine Ti(C,N) precipitates have in controlling recrystallization.

Material from a coil of industrially hot-rolled, Ti-stabilized 12 pct Cr AISI 409 ferritic stainless steel sheet with the nominal composition given in Table I was used in this study. The as-received 2.7-mm-thick hot band had not undergone an industrial hot-band annealing treatment. The hot band was subsequently cold rolled to reductions of 25, 40, 50, 60, 75, and 90 pct using a laboratory rolling mill. The rolls were lubricated and the direction of material flow was reversed after each pass. Samples of cold-rolled material were cut and annealed in a controlled-atmosphere furnace. The material was initially heated at 20 °C/s and quenched at the end of annealing with nitrogen gas. Unless otherwise specified