TEM Investigation on the Interphase Precipitation of Nanometer-sized Carbides in Advanced Ultra High-Strength Steels
Date Issued
2011
Date
2011
Author(s)
Yen, Hung-Wei
Abstract
To reduce fuel consumption and CO2 emission in automobiles, the development of advanced high-strength steels (AHSS) has been the bull’s eye in recent years. The ultra high-strength hot-rolled steel strips have been developed with tensile strength of ~780MPa and excellent formability by JFE and China Steel Co. The strong ferrite in the steels has been achieved by nanometer-sized carbides which contribute about 300MPa to the total strength. These tiny carbides nucleate on moving γ/α interface during austenite-to-ferrite transformation. It has been well known as interphase precipitation. This steel strip is considered to be avatar of interphase precipitation in low-carbon steels. For the purpose to explore a complete scope for the mechanism of interphase precipitation, TEM techniques have been developed and discussed to characterize the nanometer-sized interphase-precipitated MX carbides in this study.
This study initially utilized Moiré fringes in high resolution TEM (HRTEM) to characterize the crystal structure and orientation relationship (OR) of nanometer-sized carbide. It was found that TiC carbides in ferrite will transit from single variant of Baker-Nutting OR to multi variants of Nishiyama-Wessermann OR during isothermal holding at 755oC. HRTEM associated with NanoProbe EDS provided the evidence to suggestthat (Ti, Mo)C carbide is a NaCl structured MX-type carbide. The distribution of Mo has been revealed by high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM). Besides, this study provided the observation condition: hu+kv+lw = 0 to measure the sheet spacing. By estimating the sample thickness from electron energy loss spectrum (ELLS), the interparticle spacing in sheet could be calculated. The orientation of sheet plane has been identified by analyzing the convergent beam Kikuchi diffraction patterns.
Using the developed TEM techniques, experiments and investigations were conducted in steels with different chemical compositions under different heat treated conditions. The planar sheets of carbides have been analyzed and found to be oriented close to ferrite planes {211}, {210} and {111}; transmission electron microscopy results provide strong evidence to suggest that the development of interphase-precipitated carbides can be associated with the growth of incoherent ferrite/austenite interface by the ledge mechanism. The sheet spacing corresponding to the ledge height can be predicted by Bhadeshia’s formula. And the variation of interparticle spacing in sheet is related to both the moving speed of ledges and carbide nucleation rate.
Based on the new mechanism of interphase precipitation, the ultra high-strength hot-rolled steel strips have been developed in lab-level. The tensile strength of the strips can exceed 700 MPa and the total elongation can be over 20%. With measured microstructural parameters from TEM, an anisotropy-related Orowan equation was applied to estimate contribution of nanometer-sized carbides to the yield strength; the value is higher than 200 MPa.
Furthermore, atom probe tomography (APT) has been used to study the thermal stability of interphase-precipitated (Ti, V)C complex carbides in atomic scale. It is found that the clusters of Ti, V and C can be classified into two groups: (1) tiny clusters with 2 to 30 atoms and (2) coarse clusters with 31 to 350 atoms. It is proposed that the tiny clusters with 2 to 30 atoms in the ferritic matrix retards the diffusion rates of carbide forming elements so that the coarsening rate of carbides could be suppressed. Besides, the density of coarse clusters with above 30 atoms is higher than the density of carbides estimated from TEM by one order. The distribution of clusters is also sheeted distribution and it seems that the clustering occurs during austenite-to-ferrite phase transformation. Since the discovery of interphase precipitation in 1964, the results of this work bring about a whole new perspective to the theory of interphase precipitation.
Subjects
advanced ultra high-strength steel
nanometer-sized carbide
interphase precipitation
transmission electron microscopy
3D atom probe tomography
Type
thesis
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