Elsevier

Materials Science and Engineering: A

Volume 735, 26 September 2018, Pages 295-301
Materials Science and Engineering: A

Effect of grain size on stretch-flangeability of twinning-induced plasticity steels

https://doi.org/10.1016/j.msea.2018.08.052Get rights and content

Abstract

The effect of grain size on stretch-flangeability was investigated to determine its influence on stretch-flangeability of high strength steels. To avoid other effects of microstructure, single-phase twinning-induced plasticity (TWIP) steels were selected for the investigation. To control the grain size of two types of TWIP steels, 1) the initial specimen was annealed at 1100 ℃ to increase its grain size, or 2) subjected to high-pressure torsion then annealed at 650 ℃ to reduce the grain size. The microstructural features were analyzed using the electron backscatter diffraction. The stretch-flangeability of TWIP steels with various grain sizes was evaluated using a hole-expansion test. It was found that the hole-expansion ratio follows the Hall-Petch correlation as does fracture toughness. To improve the stretch-flangeability of high strength steels, microstructural features should be designed to increase their fracture toughness.

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.

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