Intercritical annealing as a means of improving impact properties of plate steel
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I.
INTRODUCTION
C O N S I D E R A B L E use has been made in recent years of intercritical annealing to improve strength and formability of sheet steel. Dual-phase steels achieve improved properties by the introduction of a controlled amount of martensite which increases the strength of the steel and by an increase in the work hardening rate which improves formability. The dual-phase structure in steel sheets is usually obtained by intercritical annealing and cooling at such a rate that the austenite transforms to martensite. Steel composition and intercritical annealing temperature and cooling rate have to be strictly controlled so that the micro-partitioned austenite can transform to martensite rather than pearlite. Similar microstructures can be produced in plate steel without intercritical annealing by control-rolling and alloying additions to allow the formation of a controlled amount of martensite. This increases the yield strength without a significant influence on impact behavior. Intercritical annealing for plate material as a means of improving properties is not normally considered due to the long heat-treatment times required. Furthermore, formability is not generally a prime requirement. However, one area which offers possibilities for such treatment is for thick plate where cooling rates can often be so low that coarse microstructures and inferior impact performance result. The present work was initiated on the basis of the observation that for normalized micro-alloyed steels, the impact properties of thick plate often seem to be controlled more by the thickness of the grain boundary carbides than by the grain size. t~l Whereas slow cooling coarsens the grain boundary carbides, grain size is only marginally affected, B. MINTZ, Lecturer in Materials Science, and J. KOLAHI-AVAL, Research Student, are with the Department of Mechanical Engineering, The City University, London EC1 VOHB, United Kingdom. Manuscript submitted April 13, 1987.
Table A.
Steel A Plain C-Mn B C-Mn-A1 C C-Mn-Nb-A1
C 0.11 0.10 0.12
METALLURGICAL TRANSACTIONS A
Mn 0.56 0.59 1.40
Si 0.01 0.01 0.30
particularly for HSLA steels containing Nb and A1. The deterioration in impact behavior that occurs in C-Mn-Nb-A1 steels as the plate gage increases has been mainly ascribed to the presence of coarser carbides at the grain boundaries.[IJ Attention has therefore been concentrated into means of refining the carbides. One of the most effective methods of refining grain boundary carbides is to increase the Mn content of a steel, although the mechanism by which Mn achieves carbide refinement is not clear. It was postulated in the present investigation that heating into the two-phase (c~ + y) region to partition the Mn to the t~/y boundaries and into the y might lead on transformation to fine carbides and good impact behavior, even with relatively slow cooling rates to room temperature. This seemed a very attractive method for improving the properties of thick plate and the idea was further encouraged by the work of Leslie et al. t2] who h
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