Research Papers

Effect of Tailoring Martensite Shape and Spatial Distribution on Tensile Deformation Characteristics of Dual Phase Steels

[+] Author and Article Information
B. Ravi Kumar

Materials Science and Evaluation (MSE),
CSIR-National Metallurgical Laboratory,
Jamshedpur 831007, India
e-mail: ravik@nmlindia.org

Vishal Singh

Mechanical Engineering Department,
Indus International University,
Una 174301, India
e-mail: vishalraizada57@gmail.com

Tarun Nanda

Mechanical Engineering Department,
Thapar University,
Patiala 147004, India
e-mail: tarunnanda@thapar.edu

Manashi Adhikary

Tata Steel Limited,
Jamshedpur 831001, India
e-mail: manashi.adhikary@tatasteel.com

Nimai Halder

Engineering Division,
CSIR-National Metallurgical Laboratory,
Jamshedpur 831007, India
e-mail: nimai@nmlindia.org

T. Venugopalan

Tata Steel Limited,
Jamshedpur 831001, India
e-mail: tvenugopalan@tatasteel.com

1Corresponding author.

Contributed by the Materials Division of ASME for publication in the JOURNAL OF ENGINEERING MATERIALS AND TECHNOLOGY. Manuscript received April 24, 2017; final manuscript received August 5, 2017; published online September 13, 2017. Assoc. Editor: Hareesh Tippur.

J. Eng. Mater. Technol 140(2), 021002 (Sep 13, 2017) (11 pages) Paper No: MATS-17-1114; doi: 10.1115/1.4037659 History: Received April 24, 2017; Revised August 05, 2017

The authors simulated the industrially used continuous annealing conditions to process dual phase (DP) steels by using a custom designed annealing simulator. Sixty-seven percentage of cold rolled steel sheets was subjected to different processing routes, including the conventional continuous annealing line (CAL), intercritical annealing (ICA), and thermal cycling (TC), to investigate the effect of change in volume fraction, shape, and spatial distribution of martensite on tensile deformation characteristics of DP steels. Annealing parameters were derived using commercial software, including thermo-calc, jmat-pro, and dictra. Through selection of appropriate process parameters, the authors found out possibilities of significantly altering the volume fraction, morphology, and grain size distribution of martensite phase. These constituent variations showed a strong influence on tensile properties of DP steels. It was observed that TC route modified the martensite morphology from the typical lath type to in-grain globular/oblong type and significantly reduced the martensite grain size. This route improved the strength–ductility combination from 590 MPa–33% (obtained through CAL route) to 660 MPa–30%. Finally, the underlying mechanisms of crack initiation/void formation, etc., in different DP microstructures were discussed.

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Fig. 1

(a) Result window of thermo-calc showing phase diagram for given steel chemistry, (b) result window of dictra simulation showing holding time required at annealing temperature of 750 °C, and (c) dictra predictions regarding holding time as a function of annealing temperature

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Fig. 2

(a) jmat-pro predictions showing austenite fraction and critical cooling rate for various annealing temperatures and (b) results window of jmat-pro showing continuous cooling transformation (CCT) diagram generated for steel at annealing temperatures of 800 °C

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Fig. 3

Custom designed annealing simulator for conducting annealing experiments

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Fig. 4

SEM micrograph of the starting material

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Fig. 5

(a) Annealing simulation of CAL process, (b) SEM micrograph, and (c) martensite grain size fraction distribution in the CAL processed specimen

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Fig. 6

Experimental simulation profiles at various annealing temperatures

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Fig. 7

SEM micrographs showing evolution of various microstructures at different annealing temperatures (F= ferrite and M= martensite)

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Fig. 8

Effect of annealing temperature on MVF and grain size distribution

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Fig. 9

Tensile properties of annealed specimens and their comparison with CAL processed specimen

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Fig. 10

Temperature profiles of TC simulations at 775 °C and 840 °C, respectively

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Fig. 11

SEM micrographs of specimens annealed by TC process at (a) 775 °C and (b) 840 °C showing presence of fine in-grain globular martensite phase. Arrows indicating globular martensite embedded within the ferrite grains.

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Fig. 12

Bright field TEM showing (a) globular/oblong in-grain martensite (indicated by arrows) and (b) SAD pattern of martensite (zone axis [011])

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Fig. 13

Martensite size and spatial distribution in the DP microstructures processed through TC routes

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Fig. 14

Tensile properties of specimens subjected to TC and their comparison with CAL processed specimen

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Fig. 15

Effect of annealing process routes on MVF and average ferrite grain size in the resulting DP microstructures

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Fig. 16

Effect of annealing process routes on UTS, percentage elongation, and yield to UTS ratio in the resulting DP microstructures

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Fig. 17

SEM micrographs showing microstructure near the tensile fracture region for intercritically annealed specimen at 825 °C showing (a) microcrack formation in a single martensite lath, (b) multiple interlath microcracks in a large martensite packet, and (c) SEM micrograph of thermal cycled specimen at 840 °C showing void nucleation near interface of in-grain globular type martensite and ferrite matrix (F= ferrite and M= martensite)




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