Study of the recrystallization and crystallographic texture evolution during final annealing of UNS S32304 Lean Duplex stainless steel

doi:10.1016/j.matchar.2017.05.025

Materials Characterization

Volume 130, August 2017, Pages 39-49

https://doi.org/10.1016/j.matchar.2017.05.025 Get rights and content

Highlights

•
The nucleation sites of the new grains in ferrite and austenite phases occur at HAGBs and phase boundaries.
•
The banded microstructure did not influence the recrystallization of austenite.
•
No change in texture of austenite is a result of grains nucleation with typical deformation texture and random orientations.
•
The austenite lamellae act as barriers to the growth of γ-fiber recrystallized grains.

Abstract

The banded microstructure effects on the texture and recrystallization evolution in UNS S32304 Lean Duplex stainless steel was investigated by Electron Backscatter Diffraction (EBSD), hardness and microhardness measurements. The annealed hot rolled band was cold rolled and annealed in the laboratory with interruptions at temperatures ranging from 900 °C to 1050 °C and according to industrial process conditions. The experimental results indicated that the nucleation during recrystallization occurred at high angle grain boundaries and phase boundaries. The austenite recrystallized independently of the ferrite and showed typical deformation texture. The inexistence of transition from α-fiber to γ-fiber and the prevalence of the recovery process in ferrite during annealing could be adequately explained by the presence of austenite lamellae acting as barriers to the growth of γ-fiber recrystallized grains.

Introduction

The development of duplex stainless steels (DSS) with the objective of reducing the elements cost in the chemical composition led the manufacturing of Lean duplex steel like UNS S32304. This steel has a lower content of molybdenum and nickel, higher yield limit and similar corrosion resistance in comparison to AISI 316L [1], [2].

The industrial processing of DSS becomes interesting due to the different behavior and interaction between the ferrite and austenite [3], [4]. During the cold rolling, different deformation mechanisms take place in the ferrite and austenite as well as a synergistic effect between the two phases [5], [6], [7], [8]. The recovery process of ferrite during annealing is favored due to the banded microstructure, the most favorable dislocations arrangement and the high rate of diffusion. The austenite remains nearly unrecovered until the beginning of recrystallization [4], [9], [10]. Humphreys and Hatherly [11] reported that the recrystallization of the phases in duplex alloys happens independently and in a way that is largely predictable from knowledge of the recrystallization of individual phases.

From a technological point of view, the knowledge of the microstructure and texture evolution is indispensable to optimize the mechanical properties [12]. In ferritic stainless steels, the intensity of the α-fiber decreases due to the strengthening of the γ-fiber during the recrystallization [13], [14], [15], [16]. In the case of AISI 304 austenitic stainless steel, Chowdhury et al. [17] have shown that the recrystallization causes the lower intensity of overall texture and the presence of new orientations as {236}<385> Brass Recrystallized, characteristic of recrystallized metals with face-centered cubic structure with low stacking fault energy. This orientation has an approximate 40°〈111〉 orientation relationship to {110}<112> Brass orientation (the main rolling texture component in low stacking fault energy metals) contributing to the oriented growth mechanism [18]. In contrast, the literature reported that the austenite texture after annealing of the DSS shows typical deformation texture and the γ-fiber is not fully developed in the ferrite [3], [9], [19], [20].

It is important to note that the crystallographic texture and microstructure evolution in duplex stainless steel during final annealing has been investigated; however few studies address the correlation between the duplex banded microstructure and crystallographic texture, recovery and recrystallization of both phases [3], [9], [19], [20], [21]. The aim of this work is to study the effect of banded microstructure on the textures and recrystallization evolution in ferrite and austenite during final annealing of UNS S32304 steel.

Section snippets

Materials and Methods

The annealed hot rolled band (as-received condition) of UNS S32304 Lean Duplex stainless steel with a thickness of 4.0 mm was cold rolled to 1.5 mm. The chemical composition is shown in Table 1.

The industrial annealing was simulated in the laboratory in a nitrogen atmosphere at 1050 °C and soaking time of 20 s. To investigate the microstructure and textures evolution in both phases, annealing was interrupted at 900 °C, 950 °C, 1000 °C and 1050 °C and the samples quenched in cold water.

The texture

Microstructure and Texture of Starting Material

Fig. 1 shows the orientation maps and the ODFs of ferrite and austenite from the surface and center regions for annealed hot rolled band.

The presence of recrystallized and recovered grains can be clearly noticed inside the ferrite and austenite bands (Fig. 1a–b). The textures of both phases were significantly different in the regions analyzed (Fig. 1a–d). In the surface region of the ferrite, the maximum intensity in ODF was associated with {011}<100> Goss component (f(g) = 12.9). In addition, a

Conclusions

The conclusions can be summarized as follows:

(i)
Ferrite and austenite show textures inhomogeneity throughout the thickness and partially recrystallized microstructures after hot rolling and annealing industrial processes.
(ii)
The nucleation sites of the new grains during recrystallization in ferrite and austenite occur at HAGBs and phase boundaries.
(iii)
The texture of grains with GOS < 1° in ferrite displays γ-fiber weakly developed. The austenite lamellae prevent that the γ-fiber recrystallized grains grow

Acknowledgements

The authors acknowledge the Instituto Federal do Espírito Santo, Aperam South America and CAPES. Research supported by LNNano - Brazilian Nanotechnology National Laboratory, CNPEM/MCTI.

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Study of the recrystallization and crystallographic texture evolution during final annealing of UNS S32304 Lean Duplex stainless steel

Highlights

Abstract

Introduction

Section snippets

Materials and Methods

Microstructure and Texture of Starting Material

Conclusions

Acknowledgements

Acta Mater.

Mater. Sci. Eng. A

J. Iron Steel Res. Int.

Mater. Charact.

Mater. Charact.

Mater. Sci. Eng. A

Mater. Charact.

Acta Mater.

J. Alloys Compd.

Mater. Sci. Eng. A

Mater. Sci. Eng. A

Acta Mater.

J. Iron Steel Res. Int.

Mater. Sci. Eng. A

Scr. Metall.

Mater. Sci. Eng. A

Mater. Sci. Eng. A