Recrystallization and formation of austenite in deformed lath martensitic structure of low carbon steels
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I.
INTRODUCTION
W H E N austenite is transformed to a final structure such as ferrite, pearlite, or martensite, the initial austenitic condition, particularly the grain size, is important to develop the refined microstructure and excellent mechanical properties. It is well known that the behavior of austenite formation (and thus the austenite grain size) is strongly dependent on the starting structure of the steels.~-5 The behavior of austenite formation on heating low carbon steels having lath martensitic structure has received much attention from the point of view of grain refinement of austenite. 6-~~In these works, a virtually coincident conclusion has been obtained about the progress of nucleation and growth of austenite. The effect of alloying elements on the preferred nucleation sites of austenite formed from lath martensitic structure has been also studied, ll,12 However, there have been very few works about the effect of prior deformation on the formation of austenite from lath martensite in low carbon steels. There are some papers on the effect of deformation of lath martensite on the formation of austenite 13'~4 in Fe-Ni and Fe-Mn alloys in conjunction with the formation of microduplex structure by intercritical heat treatment. Recently, Enomoto and Furubayashi ~4 have shown that the formation of austenite in deformed Fe-Ni martensitic alloys proceeds
Table I. Steel A Steel B Steel C
in competition with the recrystallization process of lath martensite on heating in the two-phase (ferrite and austenite) region, and that the final duplex structure is markedly influenced by the sequence of the process of both reactions. However, the behavior of austenite formation from deformed lath martensite in low carbon steels has not been reported in detail so far. The purpose of the present investigation is to study the effects of deformation of lath martensite on the behaviors of recrystallization and austenite formation in low carbon steels mainly from the point of view of austenite grain refinement. In this study, the recrystallization of lath martensite, which influences the subsequent a---~y transformation, was first examined, and then the interrelationship between the recrystallization of lath martensite and the formation of austenite during austenitizing was studied.
II.
EXPERIMENTAL PROCEDURE
Three commercial steels containing about 0.2 pct C whose chemical compositions are shown in Table I were used. The steel A contains 1 pet Cr and 0.2 pct Mo, and steels B and C are plain carbon steels. Heat treatments of these steels are shown in Figure 1. Steels A and B were used to study the recrystallization behavior of as-quenched or
Chemical Compositions of Specimens (Mass Pct)
C
Si
Mn
P
S
Cu
Ni
Cr
Mo
A1
0.16 0.20 0.21
0.24 0.26 0.22
0.61 0.45 0.52
0.015 0.016 0.021
0.018 0.024 0.017
0.12 0.01 0.01
0.16 0.01 0.02
0.90 0.02 0.02
0.19 ---
-0.02 --
M. TOKIZANE is Professor, Department of Mechanical Engineering, Ritsumeikan University, Kyoto 603, Japan. N. MATSUMURA is Graduate Student, Ritsum
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