On the Effect of Q&P Processing on the Stretch-flange-formability of 0.2C Ultra-high Strength Steel Sheets

Abstract

Quenching and Partitioning (Q&P) has been proposed as a novel heat treatment to produce cold rolled sheets with excellent strength and sufficient formability for cold stamping. The impact of Q&P processing on microstructure and tensile properties has been extensively studied in contrast to the lower attention devoted to its effect on stretch-flange-formability. In this study, the stretch-flange-formability of Q&P microstructures is investigated by means of hole expansion tests carried out on punched holes. The balance between tensile properties and hole expansion ratios (HER) is discussed and compared to three model microstructures: biphasic dual-phase (DP), single phase quenched & tempered (Q&T) and quenched & austempered (QAT). It is shown that Q&P microstructures exhibit a better combination of tensile ductility and stretch-flange-formability than fully martensitic (excellent HER but poor tensile ductility) and austempered microstructures (good ductility but poor HER). The study of the impact of the Q&P parameters demonstrates that stretch-flange-formability is further promoted by choosing low quench temperatures and long partitioning times. The hole expansion properties are linked to the hardness gradients in the microstructure, evaluated by nanohardness mapping. The narrower nanohardness distribution in the Q&P microstructure leads to better hole expansion ratios compared to bainitic microstructures obtained by austempering, where the presence of hard M/A blocks is unavoidable.

Influence of the Quenching and Partitioning Process on the Transformation Kinetics and Hardness in a Lean Medium Manganese TRIP Steel

“…The RA can be stabilized by the combination of the C and Mn enrichment, ensuring an optimum strain-induced γ to α' transformation (TRIP-effect). Moreover, the presence of the hard C-depleted martensitic matrix ensures the superior performance of these steels in the forming operations such as sheet cutting, bending and hole expansion, since the ability to resist high local strains is linked to the hardness gradients in the microstructure [19].…”

Simultaneous Improvement of Toughness and Fatigue Life in a Typical Ultrahigh Strength Steel by a New Deep Cryogenic Treatment Process

“…This process is similar to the Q&P process. 42,43) During partitioning process, C atoms diffuse from martensite to austenite, increasing the carbon content of austenite. As a result, the stability of austenite is increased, and austenite can exist stably even at room temperature.…”

Tensile Properties and Stretch-Flangeability of TRIP Steels Produced by Quenching and Partitioning (Q&P) Process with Different Fractions of Constituent Phases

“…3,6) The HER test representing stretch-flangeability measures the change in diameter of the punched hole, which is expanded by a conical punch until a through-thickness failure occurs. The stretch-flangeability of DP (Dual Phase) [7][8][9][10] and TRIP-assisted steels [11][12][13][14] have been reported to be determined by the volume fraction, morphology, and the hardness difference of constituting phases. In this work, the influence of microstructure on tensile properties and stretch-flangeability of TRIP steels with tensile strengths higher than 1.2 GPa has been investigated.…”