The effect of strain on interphase precipitation characteristics in a Ti-Mo steel

doi:10.1016/j.actamat.2019.03.022

Acta Materialia

Volume 170, 15 May 2019, Pages 75-86

https://doi.org/10.1016/j.actamat.2019.03.022 Get rights and content

Abstract

In the current study, the effect of deformation in the non-recrystallization region on the characteristics of interphase precipitation was extensively studied in a Ti-Mo steel. The present observation revealed a novel mechanism affecting the interphase precipitation characteristics during an austenite-to-ferrite transformation, as a result of the thermomechanical processing. The hot deformation primarily led to the fragmentation of an austenite grain interior through the formation of elongated dislocation walls, locally deviating the orientation within a grain. Relatively coarse strain-induced precipitates were initially formed on the dislocation walls upon cooling. During the isothermal austenite to ferrite transformation, the interphase precipitation took place on the advancing austenite-ferrite interface front. This was similar to the strain-free condition, though the alignment of planar rows was abruptly altered while encountering the microband walls due to local orientation change at either side of a given microband. In addition, the deformation reduced the planar spacing of interphase precipitates up to a given strain (i.e., ∼0.3), beyond which it became nearly constant. This was due to the saturation of dislocation density within austenite above a certain strain and/or the consumption of alloying elements through the strain-induced precipitation formed on the microbands prior to the austenite to ferrite transformation.

Graphical abstract

Introduction

High-strength low-alloy (HSLA) steels provide increased strength with good ductility making it possible to reduce the weight of components [1]. HSLA steels are generally produced by introducing different phases, such as martensite, bainite, and retained austenite into the microstructure [2,3]. However, multiphase steels possess poor stretch-flangeability [4], due to strain concentration on phase boundaries. Single phase ferrite microstructures with excellent stretch flangeability and low production cost can solve this problem, but it was generally believed that the strength level for this microstructure does not exceed 600 MPa [5]. Promising ways to improve the strength of ferrite without trading off ductility are to promote the formation of nano-precipitates by alloying with V, Nb, Ti and Mo [6] and/or grain refinement through thermomechanical processing.

JFE in Japan developed [7] a novel HSLA steel with a strength of 780 MPa and excellent stretch flangeability. In this type of steel, the ferrite matrix is strengthened by nano-sized Ti-Mo carbides formed through the interphase precipitation during austenite to ferrite phase transformation [[7], [8], [9]]. The precipitates nucleate readily on the migrating austenite-ferrite interface consuming carbon and destabilizing austenite ahead of the phase transformation front, which makes the interface advance to the next nucleation site [10]. These interphase precipitates are believed to be one of the most effective dispersion-strengthening particles due to their extremely fine size and high number density [7,11,12]. The mechanisms of interphase precipitation formation were intensively studied in different micro-alloyed steels [[13], [14], [15]]. It was shown that the strength of Ti-Mo alloyed steels could be enhanced by ∼300 MPa through the formation of clusters along with interphase precipitates [11,16].

There are some attempts to further enhance the mechanical properties of Ti-Mo steels through increasing the number density of precipitates via controlling the rate of austenite to ferrite phase transformation [8,15]. The rate of transformation is affected by phase transformation temperature [17], alloying content [17] and application of deformation [18]. In particular, Mo increases the hardenability of steel [[19], [20], [21]] and so will affect the austenite to ferrite transformation, even at the low level (e.g., 0.2 wt%) used in the Ti-Mo steel. Apart from alloying, the deformation applied in the austenite region can accelerate the rate of austenite to ferrite transformation, altering the size of precipitates and the spacing between precipitates rows [8,[22], [23], [24], [25]] and influence grain-boundary precipitation [26].

It has been found [8,24,25] that an increase in deformation decreases the interphase precipitate sheet spacing and in some cases changes precipitation from planar sheet to random. Despite previous work [[22], [23], [24], [25]] on the role of deformation on the interphase precipitation, it is not clear yet to what extent an increase in the hot deformation can alter the interphase precipitation characteristics (i.e., size and sheet spacing, composition and distribution).

The aim of the current study was to investigate the role of austenite hot deformation in the non-recrystallization region on the characteristics of interphase precipitation formed through isothermal austenite-to-ferrite transformation. The microstructure was thoroughly studied using advanced characterization techniques, namely electron back-scattered diffraction (EBSD), transmission electron microscopy (TEM) along with atom probe tomography (APT). The precipitates were characterised based on their size, distribution, shape, and their orientation relationship with the ferritic matrix.

Section snippets

Experimental

The composition of laboratory steel was Fe-0.04C-1.52Mn-0.2Si-0.03Al-0.22Mo-0.05Ti (wt. %). The as-received slab with an initial thickness of 40 mm was subjected to hot rolling at a temperature range of 1100–1200 °C through multi-pass deformations to obtain a final thickness of 13 mm. Afterwards, the material underwent homogenization treatment at a temperature of 1200 °C for 4 h under argon atmosphere. Cylindrical samples 15 mm long and 10 mm in diameter were machined, having their longitudinal

Results

The austenitisation of steel at 1200 °C for 180 s resulted in a prior austenite grain size of 72 ± 40 μm. The specimens subjected to hot compression at 890 °C revealed a consistent true strain-true stress behaviour at different strain levels (i.e., 0.3 to 1, Fig. 2). The flow curves were closely overlapped, suggesting that the sample deformed at a given strain can appropriately imitate the microstructural changes taking place during straining. The flow curves typically showed a progressive

Effect of strain on the ferrite grain characteristics

The current result clearly shows that the deformation at 890 °C significantly alters the ferrite microstructure characteristics (i.e. ferrite grain size and distribution). An increase in the strain in the non-recrystallised austenite region led to a progressive decrease in the average ferrite grain size with the strain from 42 ± 10 μm at a strain-free condition to 6 ± 2 μm at a strain of 1 (Fig. 3) mainly due to the increased number of nucleation sites for ferrite. The application of

Conclusions

In the present study, the effect of strain in the non-recrystallised region on the interphase precipitation characteristics during austenite to ferrite transformation was extensively studied in a Ti-Mo steel. The precipitates were characterised using advanced characterization techniques, namely TEM and APT, and classified based on their size, distribution, shape, and their OR with the ferritic matrix. From the current study the following conclusions can be made:

1.
Three types of precipitations

Acknowledgements

The authors would like to acknowledge the financial support of the Australian Research Council, Australia via DP150103062discovery grant. Deakin University's Advanced Characterization Facility is acknowledged for use of the EBSD, TEM and APT instruments.

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Full length articleThe effect of strain on interphase precipitation characteristics in a Ti-Mo steel

Abstract

Graphical abstract

Introduction

Section snippets

Experimental

Results

Effect of strain on the ferrite grain characteristics

Conclusions

Acknowledgements

Mater. Sci. Eng. A

Acta Mater.

Scr. Mater.

J. Alloy. Comp.

Acta Mater.

Acta Mater.

Acta Mater.

Mater. Sci. Eng. A

Mater. Sci. Eng. R

Mater. Sci. Eng. A

Mater. Sci. Eng. A

Mater. Des.

Acta Metall.

Surf. Sci.

Acta Mater.

Mater. Char.

Scr. Metall.

Acta Mater.

Acta Mater.

Acta Mater.

Acta Mater.

Precipitation behavior and its effect on strengthening of an HSLA Nb/Ti steel

Metall. Mater. Trans. A

Microstructure and mechanical properties of bainite containing martensite and retained austenite in low carbon HSLA steels

Mater. Trans., JIM

Full length article
The effect of strain on interphase precipitation characteristics in a Ti-Mo steel