The application of stereological analysis in understanding differences in toughness of V- and Nb-microalloyed steels of similar yield strength

doi:10.1016/j.msea.2006.02.022

Materials Science and Engineering: A

Volume 422, Issues 1–2, 25 April 2006, Pages 285-291

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Abstract

The present paper describes an attempt to relate small variations in toughness of V- and Nb-microalloyed steels, characterized by almost similar yield strength and ferrite–pearlite microstructures, to stereological parameters. Stereological parameters derived from two-dimensional microstructure include mean intercept length ( ${\bar{L}}_{α}$ ) of ferrite, ferrite grain boundary surface area per unit volume [S_V]_α, contiguity ratio (C_α) of ferrite grains, pearlite colony size, and pearlite interlamellar spacing. Stereological analysis was carried out as a function of depth from the surface using serial sectioning method. The analysis predicts that the toughness of the microalloyed steels is related to uniformity in the stereological parameters and the contiguity ratio. If the processing conditions such as cooling rate are increased, a convincingly higher toughness is obtained for Nb-containing steel in relation to V-microalloyed steels, at similar yield strength.

Section snippets

Background and introduction

It is now widely accepted that the microalloyed (MA) steels are cost-effective and exhibit superior properties such as fatigue strength, low distortion, and residual stress in comparison to the quenched and tempered (Q and T) steels. In general, to obtain good toughness, the carbon content of the microalloyed steels is maintained at ∼0.03–0.08 wt.% and manganese at ∼1–1.2 wt.% [1], [2], [3], [4]. The carbon and manganese primarily contribute to solid solution strengthening. Additional, strength

Materials

The chemical compositions of Nb- and V-microalloyed steels are presented in Table 1. In general, the thermomechanical processing schedule involved reheating of slabs followed by a series of successive roughing and finishing reductions to structural beams of size, W24 × 103. The designated size means that the nominal depth of the beam is 24 in. (i.e. when the beam is laying in the h-position with the web horizontal, the width is close to 24 in.). The 103 refers to the nominal weight in lbs/ft. The

Mechanical properties

The yield strength, tensile strength, and elongation for the V- and Nb-microalloyed steels are presented in Table 2. In general the tensile properties were similar for the steels. Also presented in Table 2 is the hardness data for the ferrite and pearlite microstructural constituents. The average ferrite hardness of V-microalloyed steel is greater than the Nb-microalloyed steel. The hardness of the ferrite phase in V-microalloyed steel is ∼19% higher than the Nb-microalloyed steel and is

Conclusions

Stereological analysis can be appropriately used to quantify microstructures of steel with small differences in mechanical properties, in particular, toughness. To understand the stereological parameters and relate them to mechanical properties, it is important to study their variation as a function of depth. Serial sectioning experiments indicated that V-microalloyed steel exhibited uniform ferrite volume fraction, ferrite grain boundary surface area per unit volume, and mean intercept length

Acknowledgement

R.R.T and R.D.K.M. gratefully acknowledge financial support from Reference Metals, Pittsburgh.

References (11)

D.K. Matlock et al.
J. Mater. Proc. Tech.
(2001)
I. Gonzalez-Bequest et al.
Steel Res.
(1997)
R.D.K. Misra et al.
Mater. Sci. Tech.
(2001)
S.W. Thompson et al.
Metall. Trans.
(1989)
R.D.K. Misra et al.
Metall. Trans.
(2003)

There are more references available in the full text version of this article.

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