Effect of grain size on stretch-flangeability of twinning-induced plasticity steels
Introduction
Nowadays, development of advanced high strength steels (AHSS) or ultra-high strength steels (UHSS) for lightweight vehicles has become more and more important due to the increasing trends of saving environment and strict safety regulations around the world. Third-generation steels (e.g., quenching-and-partitioning steels; medium Mn steels) [1], [2], [3], [4] with tensile properties superior to conventional AHSS (e.g., dual-phase (DP) steels; transformation induced plasticity (TRIP) steels; twinning induced plasticity (TWIP) steels) are being developed [5], [6], [7], [8], [9], [10]. Although the developed AHSS or UHSS have excellent tensile properties, these steels are inferior in other ways, such as high susceptibility to hydrogen embrittlement [11], [12], [13], inferior bendability [14], and poor stretch-flangeability [14], [15]. From among these, stretch-flangeability is an essential requirement to allow steel sheets to be formed successfully into automotive parts [16].
Microstructures of steels affect their macroscale characteristics, and knowledge of this relationship is used to guide development of steels that have desired properties. However, few systematic studies have considered how microstructures affect the stretch-flangeability of AHSS or UHSS. For this reason, it is difficult to set the direction of developing AHSS or UHSS having superior stretch-flangeability. Recently, the present authors and Casellas et al. independently discovered that the key factor governing stretch-flangeability is fracture toughness [17], [18], [19]. Therefore, if the effects of microstructural characteristics on fracture toughness are the same as on stretch-flangeability, then quantification of how microstructural characteristics affect fracture toughness may provide insight into ways to obtain superior stretch-flangeability. In other words, it is necessary to verify if the microstructural characteristics affecting fracture toughness have the same effect on stretch-flangeability.
Typical microstructural features that affect fracture toughness are grain size [20], [21], [22], morphology and distribution of secondary phases [23], [24], [25], [26], and inhomogeneity of plastic deformation caused by hardness difference between neighboring phases [27], [28]. In this study, we controlled the grain size as a single parameter and investigated its effect on stretch-flangeability because grain size is a greatly effective and easy-to-control parameter compared with other microstructural features that affect fracture toughness. Results showed that stretch-flangeability of a steel can be improved by controlling its microstructural features. The results will guide the development of AHSS or UHSS that have excellent stretch-flangeability.
Section snippets
Materials and grain size control
Single-phase TWIP steels were selected as an examination material; this choice avoided microstructural effects other than grain size (e.g., the secondary phase fraction, secondary phase morphology, and secondary phase distribution in the matrix.) on stretch-flangeability of AHSS. Two types of commercial TWIP steels, a hot-rolled (HR) TWIP steel and a cold-rolled (CR) TWIP steel, both manufactured by POSCO (Republic of Korea) were used. The initial state of HR TWIP and CR TWIP were named HR
Microstructural features and stretch-flangeability
The HR TWIP-initial specimen had Vickers hardness (HV) value = 263 ± 6, and the CR TWIP-initial specimen had HV value = 349 ± 9. The HV values of the HPT-processed TWIP specimens are shown in Fig. 1. The HR TWIP-initial and CR TWIP-initial have different initial HV values because the initial states (e.g., grain size and recrystallization fraction) are different. After the HPT process under 6 GPa, 1 RPM, 10 turns at ambient temperature condition, grain refinement of the two TWIP steels are
Conclusions
In this study, the effect of grain size on stretch-flangeability of TWIP steels was investigated. The grain sizes and stretch-flangeability of two kinds of TWIP steel with various grain size were analyzed to clarify the Hall-Petch relationship between grain size and stretch-flangeability. The following conclusions were obtained.
- 1.
HPT and post-process heat treatment can obtain a fully-recrystallized UFG TWIP specimen. Heat treatment of initial materials at 1100 ℃ can obtain CG TWIP specimens.
- 2.
Acknowledgment
This study was supported by Brain Korea 21 PLUS project for Center for Creative Industrial Materials (F16SN25D1706). Also, this work was supported by the National Research Foundation of Korea (NRF) grant funded by the Ministry of Science, ICT and Future Planning (MSIP) of Korea (NRF-2017R1A2A1A17069427).
Data availability
The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.
References (44)
- et al.
Quenching and partitioning (Q&P) processing of fully austenitic stainless steels
Acta Mater.
(2017) - et al.
Concurrent enhancement of ductility and toughness in an ultrahigh strength lean alloy steel treated by bainite-based quenching-partitioning-tempering process
Mater. Sci. Eng. A
(2017) - et al.
Advanced high strength steel (AHSS) development through chemical patterning of austenite
Scr. Mater.
(2018) - et al.
The significance of multi-step partitioning: processing-structure-property relationship in governing high strength-high ductility combination in medium-manganese steels
Acta Mater.
(2017) - et al.
Development of a high strength and ductile Nb-bearing dual phase steel by cold-rolling and intercritical annealing of the ferrite-martensite microstructures
Mater. Sci. Eng. A
(2016) - et al.
Mechanical properties and austenite stability in hot-rolled 0.2C–1.6/3.2Al–6Mn–Fe TRIP steel
Mater. Sci. Eng. A
(2015) - et al.
High manganese austenitic twinning induced plasticity steels: a review of the microstructure properties relationships
Curr. Opin. Solid St. M.
(2011) - et al.
Development of hot rolled steel sheet with 600 MPa UTS for automotive wheel application
Mater. Sci. Eng. A
(2012) - et al.
Correlation between fracture toughness and stretch-flangeability of advanced high strength steels
Mater. Lett.
(2016) - et al.
Ductile fracture toughness of polycrystalline armco iron of varying grain size
Acta Metall. Mater.
(1991)
Effect of grain refinement to 1 μm on strength and toughness of dual-phase steels
Mater. Sci. Eng. A
Effects of alloying elements on microstructure and fracture properties of cast high speed steel rolls: part II. Fracture behavior
Mater. Sci. Eng. A
Deformation and fracture mechanisms in fine- and ultrafine-grained ferrite/martensite dual-phase steels and the effect of aging
Acta Mater.
Recrystallization kinetics and microstructure evolution during annealing of a cold-rolled Fe–Mn–C alloy
Acta Mater.
Dislocation density-based finite element analysis of large strain deformation behavior of copper under high-pressure torsion
Acta Mater.
An investigation of hardness homogeneity throughout disks processed by high-pressure torsion
Acta Mater.
Using high-pressure torsion for metal processing: fundamentals and applications
Prog. Mater. Sci.
Experimental parameters influencing grain refinement and microstructural evolution during high-pressure torsion
Acta Mater.
Strain gradient plasticity modelling of high-pressure torsion
J. Mech. Phys. Solids
Strengthening mechanisms in nanostructured high-purity aluminium deformed to high strain and annealed
Acta Mater.
Grain size effects in aluminum processed by severe plastic deformation
Mater. Sci. Eng. A
Dynamic recrystallization of Ni-base alloys—Experimental results and comparisons with simulations
Mater. Sci. Eng. A
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