Effect of Grain Size on Mechanical Properties of Dual Phase Steels Composed of Ferrite and Martensite

Abstract

It is well-known that dual phase (DP) steels composed of ferrite and martensite have good ductility and plasticity as well as high strength. Due to their excellent mechanical properties, DP steels are widely used in the industrial field. The mechanical properties of DP steels strongly depend on several factors such as fraction, distribution and grain size of each phase. In this study, the grain size effect on mechanical properties of DP steels was investigated. In order to obtain DP structures with different grain sizes, intercritical heat treatment in ferrite + austenite two-phase region was carried out for ferrite-pearlite structures having coarse and fine ferrite grain sizes. These ferrite-pearlite structures with coarse and fine grains were fabricated by two types of heat treatments; austenitizing heat treatment and repetitive heat treatment. Ferrite grain sizes of the specimens heat-treated by austenitizing and repetitive heat treatment were 47.5 µm (coarse grain) and 4.5 µm (fine grain), respectively. The ferrite grain sizes in the final DP structures fabricated from the coarse-grained and fine-grained ferrite-pearlite structures were 58.3 µm and 4.1µm, respectively. The mechanical behavior of the DP structures with different grain sizes was evaluated by an uniaxial tensile test at room temperature. The local strain distribution in the specimens during tensile test was obtained by a digital image correlation (DIC) technique. Results of the tensile test showed that the fine-grained DP structure had higher strength and larger elongation than the coarse-grained DP structure. It was found by the DIC analysis that the fine-grained DP structure showed homogeneous deformation compared with the coarse-grained DP structure.

Effect of the Initial Microstructure and Thermal Path on the Final Microstructure and Bendability of a High Strength Ferrite-martensite Dual Phase Steel

“…Generally, strength is mainly affected by grain size and martensite volume fraction for certain chemical composition in DP steel. 3,4) Therefore, it can be concluded that the bendability of DP steel is determined by the combined structural parameters, such as grain size, volume fraction as well as spatial distribution of martensite.…”

“…The effects of chemical composition, initial microstructure and processing parameters on the mechanical properties of DP steel have been addressed in many experimental and computational studies. [1][2][3][4] It is well known that the final microstructure of DP steels is determined by the initial microstructure and the processing parameters of the heating and intercritical dwelling stage. [5][6][7][8][9][10] Two interacting metallurgical phenomena, ferrite recrystallization and austenite formation take place during the heating process.…”

Unlocking the Mechanisms behind Austenite Formation of Cold‐Rolled Ferrite–Martensite Dual‐Phase Steels: The Role of Ferrite Recrystallization

“…These microstructural factors are dependent on the chemical composition and the processing parameters in the intercritical annealing, and on the initial hot-rolled microstructure due to the microstructural heritage. [1][2][3][4][5][6][7][8] Among these, austenite formation has been extensively studied, including the influence of initial microstructures, [9][10][11][12] heating parameters, [2,6,13] the underlying mechanisms such as elements partitioning, and interface characteristics accompanied by the α/γ interface movement. [14][15][16][17] It is known that ferrite recrystallization also occurs during the intercritical annealing process, and these two metallurgical phenomena usually have an interaction effect [18,19] especially under a relatively high heating rate.…”

Characterization of local deformation and fracture behavior in ferrite + martensite dual-phase steels having different grain sizes

“…Dual-phase (DP) steels consisting of soft ferrite phase and hard martensite phase are widely used steels due to their good mechanical balance between strength and ductility. Over several decades, many studies of DP steels have been achieving much better mechanical performance by changing microstructural factors such as phase distribution, grain size, phase strength (hardness) and phase fraction [1][2][3]. Recently, it has been found that the grain refinement for DP steels is a promising strategy to simultaneously improve strength and ductility IOP Publishing doi:10.1088/1757-899X/1249/1/012041 2 (especially post-uniform elongation) [2,4].…”

“…Over several decades, many studies of DP steels have been achieving much better mechanical performance by changing microstructural factors such as phase distribution, grain size, phase strength (hardness) and phase fraction [1][2][3]. Recently, it has been found that the grain refinement for DP steels is a promising strategy to simultaneously improve strength and ductility IOP Publishing doi:10.1088/1757-899X/1249/1/012041 2 (especially post-uniform elongation) [2,4]. However, the underlying reason for the enhancement of post-uniform elongation realized by grain refinement is still not fully understood.…”