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Acta Metall Sin  2024, Vol. 60 Issue (1): 69-79    DOI: 10.11900/0412.1961.2022.00315
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Effect of Mn Heterogeneous Distribution on Microstructures and Mechanical Properties of Quenching and Partitioning Steels
ZHANG Chao1, XIONG Zhiping1,2(), YANG Dezhen1, CHENG Xingwang1,2
1 National Key Laboratory of Science and Technology on Materials under Shock and Impact, School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, China
2 Tangshan Research Institute, Beijing Institute of Technology, Tangshan 063000, China
Cite this article: 

ZHANG Chao, XIONG Zhiping, YANG Dezhen, CHENG Xingwang. Effect of Mn Heterogeneous Distribution on Microstructures and Mechanical Properties of Quenching and Partitioning Steels. Acta Metall Sin, 2024, 60(1): 69-79.

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Abstract  

The ever increasing demand for safe and lightweight steel has promoted the development of advanced high-strength steel (AHSS). Recently, many AHSSs have been developed through chemical heterogeneity, resulting in microstructure refinement and mechanical property optimization. Although many efforts emphasize the construction of Mn-heterogeneous high-temperature austenite (γ-Fe), the influence of Mn-heterogeneous distribution remains unclear. In this work, different austenitization times and temperatures are applied to Mn-partitioned pearlite, followed by the same quenching and partitioning process. The effect of Mn distribution in high-temperature austenite on the microstructural evolution and mechanical properties is systematically investigated. Results show that the Mn-heterogeneous high-temperature austenite can tailor the austenite-to-martensite transformation during quenching. The Mn-depleted austenite is then readily transformed into lath martensite, and the Mn-enriched austenite is mainly retained as film roughness (RA), both of which assemble the ghost pearlite. With an increase in austenitization time and temperature, the Mn atom diffusion from the Mn-enriched austenite (originated from cementite lamellae) to the Mn-depleted one (originated from ferrite lamellae) increases, leading to the decreased chemical heterogeneity in high-temperature austenite. Thus, the fraction of ghost pearlite decreases while the fraction and size of blocky RA and coarse lath martensite increase. A wider lath martensite lowers the strength of the yield or the elastic limit of steel. The increased fraction and size of blocky RA ensure an increased uniform elongation by transformation-induced plasticity effect, whereas the transformation product (i.e., fresh martensite) is detrimental to the post-uniform elongation. Meanwhile, because the fractions of RA and martensite hardly change with austenitization condition, the ultimate tensile strength (about 1700 MPa) and total elongation (about 20%) are relatively constant. Therefore, tuning the Mn distribution in high-temperature austenite provides an effective strategy to tailor yield strength and uniform elongation while maintaining large ultimate tensile strength and total elongation.

Key words:  heterogeneous microstructure      Mn partition      pearlite      retained austenite      high-temperature austenite     
Received:  24 June 2022     
ZTFLH:  TG142.1  
Fund: National Natural Science Foundation of China(52271004);National Natural Science Foundation of China(51901021);Science and Technology Innovation Project of Beijing Institute of Technology(2019CX01019)
Corresponding Authors:  XIONG Zhiping, associate professor, Tel: (010)68912490, E-mail: zpxiong@bit.edu.cn

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2022.00315     OR     https://www.ams.org.cn/EN/Y2024/V60/I1/69

Fig.1  Heat treatment schematic for partitioned pearlite-based quenching and partitioning (PPQ&P) process (WQ—water quench)
Fig.2  SEM (a) and TEM (b) images of partitioned pearlite obtained after holding at 590oC for 6 h following austenitization at 800oC for 600 s (Inset shows EDS line scanning, indicating Mn heterogeneity. UMn—Mn fraction at substitutional lattice site)
Fig.3  SEM images of microstructures after directly quenching following austenitization at different temperatures and time (M—martensite)
(a) 770oC for 10 s (b) 770oC for 90 s (c) 770oC for 1200 s (d) 790oC for 10 s
Fig.4  SEM images of microstructures of PPQ&P samples after austenitization at different temperatures and time (RA—retained austenite)
(a) 770oC for 10 s (b) 770oC for 90 s (c) 770oC for 1200 s (d) 790oC for 10 s
Fig.5  XRD spectra of PPQ&P samples after austenitization at different temperatures and time
Fig.6  Evolution of phase faction (a) and grain size (b) in PPQ&P samples with austenitization temperature and time (ECD—equivalent circle diameter)
Fig.7  Morphologies, Mn distributions along the red lines (insets at the bottom), SAED patterns of the green circles (insets at the top), and TKD phase map of RA in PPQ&P 770-10 sample
(a) ghost pearlite within the conventional martensite matrix (TM—tempered martensite)
(b) TKD phase map corresponding to the rectangle in Fig.7a
(c) blocky RA within the conventional martensite matrix
(d) film RA within the conventional martensite matrix
Fig.8  Engineering stress-engineering strain curves (a) and strain hardening exponent (b) of PPQ&P samples after austenitization at 770oC for different time (n—strain hardening exponent, ε—true strain)

Time

s

YS

MPa

UTS

MPa

TEL

%

UEL

%

TEL - UEL

%

101499 ± 91719 ± 620.7 ± 1.813.8 ± 1.06.9
901430 ± 41677 ± 2120.1 ± 1.415.0 ± 1.55.1
12001247 ± 121697 ± 221.8 ± 0.416.5 ± 0.25.3
Table 1  Tensile properties of PPQ&P samples after austenitization at 770oC for different time
Fig.9  SEM images of fracture surfaces after austenitization at 770oC for 10 s (a), 90 s (b), and 1200 s (c)
Fig.10  Diffusion distance of Mn when austenitization at 770℃ for different time (a) and schematics of microstructure evolution with austenitization during PPQ&P process (b) (tA—austenitization time)
Fig.11  Evolution of martensite fraction with quenching temperature deduced from the dilatation curves based on lever rule (a) and first order derivation of the transformation curves in Fig.9a (b) (QT—quenching temperature)
Fig.12  Microstructures of PPQ&P 770-10 sample when the uniaxial tension is interrupted at the strains of 0 (a), 6.9% (b), 13.8% (c), and about 20% (fractured) (d)(FM—fresh martensite. Inset in Fig.12d shows the FM transformed from ghost pearlite)
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