Laboratory simulation of seamless tube piercing and rolling using dynamic recrystallization schedules
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I.
INTRODUCTION
THERMOMECHANICAL processing techniques have been applied extensively to control the final microstructure and, therefore, the mechanical properties of microalloyed plate and strip steels. The economic benefits resulting from the elimination of post-hot-rolling treatments are well known. These concepts are also applicable to other shapes, and such methods are gaining acceptance in the manufacture of rounds, bars, and forgings. I1'z'33 The present work is part of a continuing project t4'51concerning the application of microalloying methods to the production of seamless tubes. Rolling in the # 2 seamless tube mill of the Algoma Steel Corporation, Sault Ste. Marie, ON, Canada, is carried out in three main stages: piercing, retained mandrel mill rolling (the MPM stage), and stretch reducing (the SRM stage), t61 The rolling schedule employed in this process is similar to many other steel-rolling schedules in that it can be divided into a high-temperature or roughing stage (piercing and MPM) and a low-temperature or finishing stage (SRM). However, the seamless schedule is unusual in that an intermediate reheat furnace is employed between the MPM and SRM stages. Such an intermediate reheat can be used to produce grain refinement by cooling below the austenite-to-ferrite transformation temperature (A,O prior to reheating to the usual finish-rolling temperature of about 1000 ~ The specific rolling schedule depends primarily on the final tube size. Two extremes, corresponding to the largest (L) and smallest (S) sizes made at Algoma, were investigated in this work. The L size undergoes no SRM rolling (finishing), whereas a strain of about 1.6 is applied to the S size during finishing. The L schedule is typical of recrystallization-controlled rolling (RCR) and
relies in austenite grain refinement via repeated cycles of recrystallization, t71 By contrast, the S schedule, which involves relatively low temperatures and very short interpass times, leads to the initiation of dynamic recrystallization I81 and is thus based on dynamic recrystallization-controlled rolling (DRCR). Because recrystallization is a key component of both schedules, a conventional Nb microalloyed steel, in which recrystallization is heavily retarded, is unsuitable for this purpose. Instead, a series of Ti-V-N steels specifically designed for recrystallization-controlled rolling r4,5,9~was selected for this investigation. In these alloys, TiN precipitates prevent austenite grain growth during and after rolling, and subsequent V(CN) precipitation in the ferrite produces precipitation hardening, tgl II.
EXPERIMENTAL PROCEDURE
Industrial rolling schedules can be readily simulated in the laboratory by means of torsion testing. In such simulations, the key hot-working variables are the strain, strain rate, temperature, and interpass time. This method has been employed successfully for the design of reversing t1~ and strip mill t8'131 schedules.
A. Test Schedules The mill rolling variables for the S and L sizes were simplified into test schedule
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