On the Factors Influencing the High Stretch-Flangeability of a Low-Density 1180 MPa Fe-Mn-Al-C-Nb δ-QP Steel

doi:10.1007/s40195-024-01688-y

Abstract

Abstract:

Low-density δ-quenching and partitioning (δ-QP) steels with excellent strength and ductility have been recently developed. However, there are still rare reports on the formability of δ-QP steels, which are critical for satisfying the manufacture of structural parts during the application in automotive industry. In the present work, an 1180 MPa Fe-Mn-Al-C-Nb δ-QP steel with a high ductility was adopted for the stretch-flangeability study. The δ-QP steel was developed by separated quenching and partitioning processes. A good hole expansion ratio (HER) of 34.9±0.9% was obtained in the quenched steel, but it has been further increased to 52.2% by the tempering treatment. The improved stretch-flangeability was attributed to the enhanced austenite stability and deformation uniformity. On the one hand, the stability of austenite was increased by carbon partitioning during tempering, which reduced crack possibility via the suppression of the fresh martensite formation. On the other hand, the tempering treatment released the internal stress caused by martensitic transformation and reduced the difference in strength among different phases, resulting in an increase in the resistance to crack initiation and propagation.

Key words: Hole expansion ratio, Low-density δ-quenching and partitioning (δ-QP) steel, Stretch-flangeability, Austenite stability, Deformation uniformity

Dan-Dan Cui, Peng Chen, Peng-Fei Wang, Xiao-Wu Li. On the Factors Influencing the High Stretch-Flangeability of a Low-Density 1180 MPa Fe-Mn-Al-C-Nb δ-QP Steel[J]. Acta Metallurgica Sinica (English Letters), 2024, 37(8): 1291-1300.

Figures/Tables 10

Fig. 1 Dimension of the hole expanding specimen a, and the schematic diagram before b and after c hole expansion test

Fig. 2 SEM images of the microstructures in the samples DQ a and QP b

Fig. 3 X-ray diffraction patterns a and tensile engineering stress-strain curves b of the samples DQ and QP

Table 1 Experimental date of uniaxial tensile and hole expansion properties of the samples QP, DQ and referenced QP1180

Samples	YS (MPa)	UTS (MPa)	TE (%)	PSE (GPa%)	HER
DQ	509.8±10.9	1303.1±6.7	19.9±0.3	26.0	34.9±0.9
QP	598.5±6.8	1181.7±14.0	26.8±0.8	31.7	52.2±0.2
QP1180 [15]	1015	1165	13.2	15.4	28.0

Fig. 4 Measuring positions of hole expanding samples a-c (unit: mm); d volume fraction of austenite as a function of distance from the fracture based on the XRD results; e average grain size of RA as a function of distance from the fracture based on the EBSD results; f band contrast and phase distribution maps at different measuring positions. Note that the green color represents austenite

Fig. 5 Macroscale simulation of the HET process for the samples DQ a and QP b

Fig. 6 Microstructures near the fracture of the samples DQ a and QP b after HET. Note that F and MA represent ferrite and martensite/austenite

Fig. 7 KAM images of the sample DQ at different measuring positions: a-e total, a'-e' bcc, and a''-e'' austenite

Fig. 8 KAM images of the sample QP at different measuring positions: a-e total, a'-e' bcc, and a''-e'' austenite

Fig. 9 Variation of KAM value with the distance from the hole of the samples DQ a and QP b after HET

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