Effect of deformation on ferrite nucleation and growth in a plain carbon and two microalloyed steels
- PDF / 1,033,607 Bytes
- 12 Pages / 590.28 x 785 pts Page_size
- 29 Downloads / 224 Views
I.
INTRODUCTION
THE essence
of the controlled rolling of H S L A steels is to produce a fine ferrite grain size. This is normally achieved by finish rolling in the no-recrystallization temperature range. Still further refinement of the ferrite grain size can be produced by applying the last few passes at temperatures below the Ar3, i.e., in the austenite-plusferrite intercritical range. The formation of polygonal ferrite from deformed austenite is controlled by both nucleation and growth. However, the nucleation rate is more effective than the growth rate for controlling the ferrite grain size. Therefore, it is of considerable interest to characterize the effect of deformation on the nucleation process. The present study was undertaken with the further objective of determining the effect of the microalloying elements Mo, Nb, and V on ferrite nucleation and growth in a plain C and two microalloyed steels. The experiments were first carried out under dynamic conditions in order to study the effect of strain and strain rate on the kinetics of nucleation and growth during deformation. A second set was carried out under static conditions with different prestrains and prestraining strain rates. The approach of DeHoff ~ was used to determine quantitatively the nucleation and growth rates. This method is based on the assumption of a general two-parameter particle size distribution and relies on the estimation of these two parameters from three counting measurements carried out on metallographic sections of the structure. These counting measurements are used to determine (1) the total surface area, (2) the volume fraction, and (3) the number of ferrite "particles" observed per unit area.
E. ESSADIQI, formerly with the Department of Metallurgical Engineering, McGill University, is with the Industrial Materials Research Institute, 75 boul. de Mortagne, Boucherville, PQ J4B 6Y4. J.J. JONAS, CSIRA/NSERC Professor of Steel Processing, is with the Department of Metallurgical Engineering, McGill University, 3450 University Street, Montreal, PQ H3A 2A7, Canada. Manuscript submitted April 22, 1988. METALLURGICAL TRANSACTIONS A
II. M A T E R I A L S A N D EXPERIMENTAL PROCEDURE The chemical compositions of the three steels studied are listed in Table I. The procedure used to prepare and deform the specimens has been described in an earlier paper.J2] After straining and quenching, the test samples were sectioned parallel to the deformation axis and polished for optical examination. Two different etchants were used: the polished samples were first etched with a 3 pct nital solution for 10 to 15 seconds in order to reveal the ferrite grain boundaries; they were further etched with a solution consisting of 10 g of Na2S205 in 100 ml of H 2 0 for 5 to 10 seconds [31 in order to enhance the contrast between the ferrite and martensite. The volume fraction of ferrite was assessed using standard point counting procedures. [4]
Ferrite Grain Size Distribution The particle (ferrite) size distributions were determined from simple counting me
Data Loading...
