Microstructure and texture evolution in dual-phase steels: Competition between recovery, recrystallization, and phase transformation

doi:10.1016/j.msea.2010.03.028

Materials Science and Engineering: A

Volume 527, Issues 16–17, 25 June 2010, Pages 4161-4168

https://doi.org/10.1016/j.msea.2010.03.028 Get rights and content

Abstract

The microstructure and texture evolution of dual-phase steel sheets with a cold reduction of about 50%, annealed at ferritic and intercritical temperatures, were analyzed by scanning electron microscopy and electron backscatter diffraction. The competition between recrystallization and phase transformation was of particular interest. The sheets were annealed in salt bath or were annealed in a MULTIPAS annealing simulator under variation of annealing temperature, annealing time, and heating rate. For low intercritical temperatures, recrystallization occurred before phase transformation. The sheets showed a similar through-thickness texture inhomogeneity with a plane-strain texture with strong α-fiber and weak γ-fiber as cold rolled sheets and a ferritic–martensitic band structure in the sheet center layers. An inverse correlation between the volume fractions of recrystallized ferrite and martensite was observed. This interdependence is attributed to a different phase transformation kinetics for recyrstallized and deformed ferrite and is discussed in terms of deformation strain energy, diffusion, and number of nucleation sites.

Introduction

Carbon–manganese dual-phase steels are known for their combination of high strength and good formability, and therefore, are prospective materials for the automotive industry to achieve low energy consumption through weight reduction [1], [2]. The results presented here are part of a series of analyses of microstructure evolution in hot rolled, cold rolled, and annealed sheets of dual-phase steels. One important step of thermal treatment of dual-phase steels is an intercritical annealing which converts the initial ferritic–pearlitic microstructure of cold rolled materials in a ferritic–martensitic microstructure after annealing. In the literature, microstructure analyses on dual-phase steels were published which covered the average texture evolution [3], [4], [5], [6], [7] and the morphology [8], [9], [10] of the constituents. The volume fractions of the constituents, their texture and morphologies are known to be determined by recovery, recrystallization, and phase transformation during intercritical annealing [1], [11], [12]. The competition between these mechanisms is determined by basic annealing parameters such as heating rate, intercritical annealing temperature, annealing time, cooling rate, and the final annealing temperature [2], [8], [12], [13]. However, there is still little understanding about the competition between recrystallization and phase transformation and their driving forces [1], [8], [11], [14].

Preliminary microstructure analyses of hot and cold rolled sheets of dual-phase steels yielded (i) a through-thickness texture inhomogeneity typical for BCC-steels [15] and (ii) a continuous through-thickness change of ferrite–pearlite spatial distribution from ferrite–pearlite band structure in the center to a heterogeneous distribution at the surface of the sheets. Therefore, in general the mechanical properties of the sheets before and after annealing are expected to be anisotropic and to depend on the through-thickness position. Annealing at high intercritical and particularly at austenitic temperatures yielded in the entire sheet a homogeneous texture, a heterogeneous distribution of the constituents, and thereby no dependence of the mechanical properties on the through-thickness position. For high annealing temperatures, the driving force for microstructure evolution was mainly attributed to a reduction in free enthalpy during phase transformation. The microstructure analyses of hot and cold rolled sheets and of sheets annealed at high intercritical and at austenitic temperatures will be presented elsewhere.

In this work, annealing was applied at ferritic and low intercritical temperatures. At these low annealing temperatures, recrystallization and phase transformation should depend on details of the microstructure in the different through-thickness positions, and therefore, inhomogeneities with respect to microstructure and mechanical properties are less likely to be eliminated by annealing. The aim of this work is to study the dependence of microstructure evolution on strain introduced by deformation, on texture, and on morphology and distribution of the constituents in dual-phase steels annealed at low intercritical temperatures.

Section snippets

Sample overview

A sample overview is given in Table 1. The starting material was hot rolled sheets with a thickness of 3.75 mm. Preliminary metallurgical measurements yielded volume fractions of about 65% ferrite and 35% pearlite. The sheets had a chemical composition of 0.147 wt.% C, 1.868 wt.% Mn, 0.403 wt.% Si, and smaller amounts of other alloying elements. The sheets were then industrially cold rolled to a thickness of 1.75 mm. Details about the chemical composition and the microstructure of the hot and cold

Hardness after annealing in the ferritic temperature range

The cold rolled sheets were annealed in salt bath using a fine temperature–time schedule with ferritic annealing temperatures between 620 °C and 695 °C (Table 1, samples 1). Only the hardness was measured on these samples. The hardness in dependence of annealing time is shown in Fig. 1 only for extreme annealing temperatures of 620 °C and 695 °C. As expected, (i) the hardness first decreased slightly due to recovery and then significantly due to recrystallization and (ii) the incubation and

Preservation of through-thickness texture inhomogeneity and band structure after annealing at low intercritical temperatures

The annealing experiments yielded recovery, recrystallization, and phase transformation. Recovery was attributed to a slight reduction of hardness at the beginning of annealing at ferritic temperatures (Fig. 1). Recrystallization was proven by (i) a strong reduction of hardness obtained by annealing at ferritic temperatures and at the beginning of intercritical annealing for low temperatures up to 740 °C (Fig. 1) and (ii) by the observation of new equiaxed ferrite grains with small orientation

Acknowledgements

The authors thank Dr. M. Masimov and Dr. B. Springub (Salzgitter Mannesmann Forschung GmbH, Eisenhüttenstrasse 99, 38239 Salzgitter, Germany) for scientific and technical support. Financial support by the German Ministry for Education and Research under the project “Bauteilbewertung auf der Basis integraler Werkstoffmodellierung entlang der Prozesskette” (grant no. 03X0501E) is gratefully acknowledged.

References (24)

R.K. Ray
Scripta Metallurgica
(1984)
R.K. Ray
Materials Science and Engineering
(1986)
D.K. Mondal et al.
Materials Science and Engineering A
(1992)
S.G. Chowdhury et al.
Materials Science and Engineering A
(2008)
B. Gardey et al.
Materials Science and Engineering A
(2005)
R.O. Rocha et al.
Materials Science and Engineering A
(2005)
Y.S. Zheng et al.
Materials Science and Engineering A
(1994)
J. Qu et al.
Materials Science and Engineering A
(2008)
S.J. Kim et al.
Materials and Design
(2009)
M.A.F. Oliveira et al.
Scripta Materialia
(2004)

E. Demir et al.

Acta Materialia

(2009)

M. Hölscher et al.

Acta Metallurgica

(1994)

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Microstructure and texture evolution in dual-phase steels: Competition between recovery, recrystallization, and phase transformation

Abstract

Introduction

Section snippets

Sample overview

Hardness after annealing in the ferritic temperature range

Preservation of through-thickness texture inhomogeneity and band structure after annealing at low intercritical temperatures

Acknowledgements

Scripta Metallurgica

Materials Science and Engineering

Materials Science and Engineering A

Materials Science and Engineering A

Materials Science and Engineering A

Materials Science and Engineering A

Materials Science and Engineering A

Materials Science and Engineering A

Materials and Design

Scripta Materialia

Acta Materialia

Acta Metallurgica