15. - 17. 5. 2013, Brno, Czech Republic, EU

A STUDY OF MICROSTRUCTURE AND PHASE TRANSFORMATIONS OF MEDIUM CARBON

DUAL PHASE STEELS

Ersoy Erişir1, Oğuz Gürkan Bilir1

1Department of Metallurgical and Materials Engineering, Kocaeli University, 41380-Kocaeli-Turkey

Abstract

In this study, the suitability for producing dual phase steel from hot rolled medium carbon steel for mining

industry and the effect of annealing temperature on hardness and microstructure has been investigated. The

critical equilibrium temperatures and phase diagram were calculated using Thermo-Calc software for 0.37C-

0.87Mn steel. In experimental studies using dilatometer, microstructures containing ferrite and martensite

were produced by intermediate quenching method with annealing at different temperatures followed by water

quenching. Before the intermediate quenching, the specimens were first quenched to form martensite from

austenitizing temperature of 1100 °C. Fully martensitic samples were then annealed respectively at 725,

730, 740, 750 °C for 15 min and quenched. During the annealing, the austenite nucleates and grows at

former boundaries of the martensite plates resulting fibrous microstructure of martensite and ferrite.

Microstructural investigations were carried out using light microscope and scanning electron microscope to

investigate the effect of the increasing annealing temperature on the microstructure. Vickers test was applied

to specimens. The calculations and experimental results show that hot rolled medium carbon steels are

suitable to produce a dual phase microstructure. It was observed that increasing annealing temperature

increases the amount of martensite and hardness because martensite is the main phase controls the

hardness value of steel. It is observed that the A1 and A3 temperatures are very close and the percentage of

austenite is much more than conventionally produced dual phase steels. It is related to including more

carbon content than conventionally produced dual phase steel.

Keywords: dual phase steel, medium carbon steel, intermediate quenching, microstructure, phase

transformation

1. INTRODUCTION

Dual phase steels are developed with the need of lighter steels with the automotive industry's purpose of

energy saving and producing eco-friendly cars [1-3]. Definition of the term “dual phase” means the consisting

of martensite and ferrite phases together in microstructure. However, retained austenite, pearlite, bainite or

new ferrite phase may also exist depending on the cooling rate or the chemical composition of steel. The

main important mechanical properties of dual phase steels are high strength, high work-hardening potential,

low yield/tensile strength ratio and high machinability. Dual phase steels resembles the composite materials,

because of the synergistic effect of two phases, while martensite controls the strength of steel, ferrite is the

responsible for the formability properties [1-11]. Production of dual phase microstructure requires the water

quenching after the annealing at intercritical region which is between A1 and A3 temperatures. Initial

microstructures (before annealing) can be martensite, ferrite+pearlite or austenite and affect the final

morphologies and properties. Initial microstructure is annealed at an intercritical region named different

production methods which are respectively, intermediate quenching, intercritical annealing and step

quenching [1-3, 12]. Amount of the different phases in dual phase microstructure and their morphologies are

key parameters for controlling the mechanical properties. Increasing the annealing temperature provides

more martensite volume fractions so the controlling the phase distributions is possible with controlling the

annealing temperature [11-16].

15. - 17. 5. 2013, Brno, Czech Republic, EU

Since automotive industry requires good weldability, dual phase microstructure commonly applied to low

carbon steels but theoretically it is possible to apply all hypoeutoctoid steels. Mining support profiles are

produced using hot rolled medium carbon steels. The profiles are quenched and tempered and finally

connected mechanically using clamps and without weldability operation. Thus, dual phase concept can be

potentially extended to mining support profiles. In this study, the suitability of a dual phase microstructure for

a hot rolled medium carbon steel is investigated.

2. MATERIAL AND EXPERIMENTAL METHODS

The steel used in this study was a hot rolled 0.37C-0.87Mn steel. The detailed chemical composition of the

steel is given in Table 1. The calculations were made with the ThermoCalc software using the TCS Steel

Database TCFE6 [17, 18]. The aim of the calculations was the prediction of + phase range using isopleth

of Fe-C phase diagram. Following elements are considered in the calculations according to chemical

composition in Table 1; % 0.279 Si, % 0.865 Mn, % 0.04 Cr, % 0.047 Al and 100 ppm N.

Table 1 Chemical composition of investigated steel.

Chemical composition (%weight)

C Si Mn P S Cr Mo Ni V W Cu

0,368 0,279 0,865 0,0195 0,0063 0,0413 0,0168 0,0769 0,0021 0,0310 0,0597

Al Nb Ti B

0,0472 0,0033 0,0034 0,0012

.

The intermediate quenching experiments were carried out in a Bähr DIL805 plastodilatometer in order to

investigate the phase transformations in the investigated steel. The standard specimen size of 5 mm

diameter and 10 mm length was used for the dilatometer experiments. The annealing conditions applied to

the specimens are given in Fig. 1. The samples were annealed at 1100 ºC for 5 min and followed by gas

quenching with a cooling rate of 130 K/s to obtain fully martensitic initial microstructure. After martensitic

transformation, specimens were annealed respectively at 725, 730, 740 and 750°C for 15 min and finally gas

quenched again. The samples were metallographically prepared. After sample preparation Nital etching was

applied, light microscope (LM) and scanning electron microscope (SEM) used to investigate the effect of

annealing temperature on microstructure. Vickers test was applied to understand microstructural evolutions.

Fig. 1 Intermediate quenching heat treatment cycle of steel.

15. - 17. 5. 2013, Brno, Czech Republic, EU

3. RESULTS

According to the intermediate quenching method [6], the specimens are annealed in the intercritical

temperature range of ferrite and austenite phase region. ThermoCalc is used to determine the intercritical

temperature range between A1 and A3 temperatures. Fig. 2 is an isopleth diagram showing stability of

phases as a function of temperature and carbon content. The dashed line in diagram indicates the relevant

carbon amount for the steel composition used here. According to the thermodynamic calculations, liquid (L),

ferrite, austenite, AlN, cementite phases are in equilibrium. According to the results, AC3, AC1b and AC1e are

equal to 786 ºC, 721 ºC and 711 ºC, respectively.

Fig. 2 Calculated isopleth section by using Thermo-Calc at wt. % 0.279 Si, % 0.865 Mn, % 0.04 Cr, % 0.047

Al and 100 ppm N of Fe-C system.

Four different annealing temperatures (725, 730, 740, and 750 °C) were chosen for the intermediate

quenching experiments. According to the thermodynamical calculations, all four temperatures should be lie

in + phase region. After quenching from annealing temperatures, typical intermediate quenching

microstructure is obtained. In Fig. 3, the same area can comparatively be seen after the intermediate

quenching in LM and SEM micrographs. The annealing resulted in fibrous microstructure of martensite and

ferrite phases. In LM micrograph, white regions are ferrite (F), dark regions bainite/pearlite (B/P) and rest of

the microstructure is martensite (M). The same area on SEM investigation reveals also martensite,

bainite/pearlite and ferrite regions as indicated in Fig. 3b.

15. - 17. 5. 2013, Brno, Czech Republic, EU

a)

b)

Fig. 3 LM and SEM micrographs of steel annealed at 730°C, M: martensite, F: ferrite, B/P bainite/pearlite.

Initial microstructures before the intermediate quenching are fully martensitic as shown in Fig. 4a and b. The

intermediate quenching at 725-750 °C results in formation of microstructures in Fig. 4c-j. Raising the

annealing temperature increased the amount of martensite as it can be seen in Fig. 4. Increased amount of

martensite can also be seen from the hardness measurements. Table 2 shows the Vickers test results and

indicates that raising the annealing temperature results in increased hardness. It is obtained that ferrite

phase is mostly in Widmanstaetten formation and martensite islands are distributed in ferrite matrix. Besides

ferrite and martensite, a dark colored third phase are present in microstructures for all annealing

temperatures but not in the initial microstructure. It is concluded that this regions are formed due to

quenching and may be bainite and/or pearlite. Small amaount of Bainite and/or pearlite (B/P) can be

appeared in dual phase microstructure [11]. Annealing at lower temperatures produces more B/P fraction as

seen in Fig. 4. On the other hand, annealing temperature has no significant effect on dual phase

microstructure except the martensite amount. The lowest annealing temperature (725 ºC) is very close to the

calculated A3 temperature of steel. Therefore, complete transformation of martensite to austenite could not

be obtained.

15. - 17. 5. 2013, Brno, Czech Republic, EU

1100 ºC a)

b)

725 ºC c)

d)

730 ºC e)

f)

740 ºC g)

h)

750 ºC i)

j)

Fig. 4 SEM and LM microstructures of specimens a,b ) fully martensitic, quenching temperature, 1100 ºC

and c-d, e-f, g-h, i-j represents annealing at 725, 730, 740, 750 °C respectively.

15. - 17. 5. 2013, Brno, Czech Republic, EU

Table 2 Vickers test results.

Annealing temperature (ºC) 1100 725 730 740 750

Vickers hardness (HV) 565,48 268,49 270,12 270,19 337,10

4. DISCUSSION

Dual phase microstructure and phase transformations of 0.37C-0.87Mn steel were investigated after the

intermediate quenching experiments from different annealing temperatures. Main results can be summarized

as follows:

-Thermodynamical calculations are shown that + phase region can be existed at temperatures between

786 ºC and 721 ºC. Thus, annealing temperatures for the intermediate quenching experiments could be

chosen at this temperature range.

-Dual phase microstructure can be produced by intermediate quenching experiments using 0.38C-0.87Mn

steel. Micrographs of specimens after intermediate quenching experiments showed martensite islands in

matrix of ferrite phase. The amount of martensite volume fraction is increased with rising annealing

temperature. The Vickers hardness of specimens indicates also hardness increase due to the increase of

martensitic fraction.

- A third phase is observed in dual phase microstructure between ferrite and martensite phases. This phase

may be pearlite and/or bainite.

ACKNOWLEDGEMENT

The authors are grateful to Yapı-Tek Çelik Sanayi A.Ş., Kocaeli, Turkey for the supplement of the alloy and to Dr. U. Prahl for access to a precision dilatometer.

REFERENCES

[1] OLIVER, S., JONES, T.B., FOURLAIS, G. Dual Phase versus Trip Strip Steels: Microstructural Changes as a

Consequence of Quasi-Static and Dynamic Tensile Testing. Materials Characterization, 2007, Vol. 58, Nr. 4,

page 390–400.

[2] DEMIR, B., ERDOGAN, M. The Hardenability of Austenite with Different Alloy Content and Dispersion in Dual-

Phase Steels. Journal of Materials Processing Technology, 2008, Vol. 208, Issue: 1-3, page 75–84.

[3] SARWAR, M., PRIESTNER, R. Hardenability of Austenite in a Dual-Phase Steel, Journal of Materials

Engineering and Performance. 1999, Vol. 8, Nr. 3, 380-384.

[4] ERDOGAN, M., TEKELI, S. The Effect of Martensite Particle Size on Tensile Fracture of Surface Carburised AISI

8620 Steel with Dual Phase Core Microstructure. Materials and Design, 2002, Vol. 23, Nr. 7, page 597–604.

[5] BAYRAM, A., UGUZ, A., ULA, M. Effects of Microstructure and Notches on the Mechanical Properties of Dual-

Phase Steels. Materials Characterization, 1999, Vol. 43, No. 4, page 259–269.

[6] AHMAD, E., MANZOOR, T., ZIAI, M.M.A., HUSSAIN, N. Effect of Martensite Morphology on Tensile Deformation

of Dual-Phase Steel. Journal of Materials Engineering and Performance, 2011, Vol. 21, Nr. 3.

[7] TURKMEN, M., GUNDUZ, S., The Effect of the Martensite Morphology on Static Strain Ageing Behaviour of Dual

Phase Steels, 6th International Advanced Technologies Symposium Proceedings: 16-18.05.2011,Elazığ, Turkey,

page 159-163.

[8] KORZEKWA, D. A., MATLOCK, D. K., KRAUSS, G. Dislocation Substructure as a Function of Strain in a Dual-

Phase Steel. Metallurgical Transactions A, 1984, Vol. 15, Nr. 6, page 1221-1228.

[9] BAYRAM, A., UGUZ, A. Effect of Microstructure on the Wear Behaviour of a Dual Phase Steel.

Materialwissenschaft und Werkstofftechnik, 2001, Vol 32, No 3, page 249-252.

[10] MOHAVED, P., KOLAHGARA, S., MARASHIA, S. P. H., POURANVARI, M., PARVINA, N. The Effect of

Intercritical Heat Treatment Temperature on The Tensile Properties and Work Hardening Behavior of Ferrite–

Martensite Dual Phase Steel Sheets. Materials Science and Engineering A, 2009, Vol. 518, Nr. 1, page 1-6.

15. - 17. 5. 2013, Brno, Czech Republic, EU

[11] SPEICH, G. R. Dual Phase Steel. ASM Handbook Volume 1: Properties and Selection: Irons, Steels and High

Performance Alloys section, ASM International, 2005, page 697-707.

[12] KABAKCI, F., SALAMCI E. Influence of New Ferrite Content on Tensile Properties of Dual Phase Steel. 5th

International Advanced Technologies Symposium Proceedings: 13-15.05.2009, Karabük, Turkey, page 943-948.

[13] SUN, S., PUGH, M. Properties of Thermomechanically Processed Dual-phase Steels Containing Fibrous

Martensite. Materials Science and Engineering A, 2002, Vol. 335, Issue: 1-2, page 298–308.

[14] SUN, X., CHOI, K., S., LIU, W. N., KHALEEL, M. A. Predicting Failure Modes and Ductility of Dual Phase Steels

Using Plastic Strain Localization. International Journal of Plasticity, 2009, Vol. 25, Nr. 10, page 1888–1909.

[15] LIS, J., LIS, A. K., KOLAN, C. Processing and Properties of C–Mn Steel with Dual-Phase Microstructure. Journal

of Materials Processing Technology. 2005, Vol. 162, page 350–354.

[16] HAYAT, F., UZUN, H. Effect of Heat Treatment on Microstructure Mechanical Properties and Fracture Behaviour

of Ship and Dual Phase Steels. Journal of Iron and Steel Research, 2011, Vol. 18, Nr. 8, page 65-72.

[17] Thermo-Calc TCC Software User’s Guide, Version S, Stockholm Technology Park, Stockholm, 2008.

[18] Thermodynamic database TCFE6 - TCS Steels/Fe-alloys database (v.6.2) for Thermo-Calc, Thermo-Calc

Software AB, www.thermocalc.com.