Effect of Intercritical Heat Treatment on Mechanical Properties of Plain Carbon Dual Phase Steel

Mechanical properties of DP (Dual Phase) steels are greatly influenced by the microstructural features such as grain size, morphology and martensite volume fraction (V m %). These microstructural features can be altered by changing the soaking time and temperature within intercritical zone. Present study aims to study the effect of intercritical annealing temperature and soaking time on V m % and its effects on mechanical properties of plain low carbon steel grade (AISI 1020) steel having ferrite-martensitemicrostructure. Nine DP steel specimens with various amount of martensite were produced via intercritical heat treatment. Mechanical properties including TS (Tensile Strength), hardness and toughness were characterized and co-related with martensite volume fraction. It was found that increasing the intercritical annealing temperature and soaking time increases the V m %. The optimum TS and hardness were found at 64V m % and then decrease with further increase in V m %. The toughness was found to have linear relationship with V m %.


INTRODUCTION
actual distribution between these two phases allows them low yield stress, a smooth flow stress curve, better plasticity and formability with a high strain hardening coefficient [5]. Processing to produce the DP steels is usually done by intercritical annealing from room temperature to two-phase (α + χ) region [6]. This involves rapidly cooling from a suitable temperature between upper and lower critical temperatures within the intercritical zone. During cooling the austenite phase being metastable transforms to martensite but ferrite remains unchanged resulting in the formation DP micro-structure which consist of ferrite and martensite structure instead of the conventional ferrite-pearlite microstructure [7]. D P steels are kind of advanced strength steel mostly used by automobile industry due to their special mechanical properties such as continuous yielding, high TS, high work hardening rate as well as good ductility [1][2][3][4]. As a result, the automotive industry is continuously exploring methods of enhancing their ductility and strength simultaneously to develop superior grades of such steels [2]. These properties are result of special microstructure designed to produce in DP steel which consists of soft matrix of ferrite in which islands of hard martensite are embedded [3,4]. It is like composite material where martensite is strengthening element and ferrite matrix assures good formability. The The mechanical properties of DP steels are directly related to the synergistic effect of two phases present in these steels in which martensite controls the strength of the steel while ferrite is responsible for formability properties [7][8][9]. Numerous studies have reported that the mechanical properties of ferrite-martensite DP steels have been quite variable depending on ferrite and martensite volume fractions in microstructures. Movahed et. al. [1] and Ebrahimian and Ghasemi, [10] have reported a linear variation of yield strength with martensite volume fraction and a non-linear one for TS consisting a peak hardening value around 50% martensite in a low carbon ferritemartensite DP steel. Speich and Miller [11] have suggested a completely linear dependence between hardness and martensite volume fraction in a low carbon low alloy ferritemartensite DP steel.
Chen and Cheng [8] found that martensite fraction increases the tensile strength up to a certain limit and then decreasing significantly making a non-linear relationship. Fallahi [9] showed that for DP steel consisting of 35-40% of fiber martensite and 4 µm grain size gives the optimum tensile and impact properties whereas Bag et. al. [12] showed that of finely dispersed ferrite and martensite phases at 50% proportion gives the optimum combination of high ductility, strength, and toughness.
Pierman et. al. [13] demonstrated that TS results are not in line with expectations based on the mixture law in DP steels containing10-70 vol.% martensite. The observations of various researchers [1,3,5-6,8-10,12-13,] suggest that the mechanical behavior of ferrite-martensite DP microstructure cannot be predicted by the general rule of mixture law. These findings are still controversial and no agreement has yet been developed Although many investigations have been made to characterize the microstructure-property relationship of micro alloyed low carbon DP steels, literature pertaining to formation of ferrite-martensite DP microstructure and its effect on mechanical properties of plain low carbon steels has not been well studied.
Present study therefore, aims to investigate the effect of intercritical annealing temperature and soaking time on martensite volume fraction and mechanical properties of AISI 1020 plain carbon steel.

MATERIALS AND MEHTOD
Chemical composition of a plain carbon steel grade (AISI 1020) used in present study is given in

Microstructure
The optical and SEM micrographs of as-received steel specimen are shown in Fig Table 3. Phase analysis indicated that V m % increases with increasing temperature and time within intercritical region. The amount of martensite formed in each specimen is in agreement with levers rule, according to which increasing the temperature within ferriteaustenite region increases the amount of austenite which will then transforms into martensite. Increase in grain size was also observed for the specimens soaked for 3hrs due to sufficient time available for grain growth.
Also at high magnification, plate type martensite was found in the specimens intercritically heat treated at relatively lower temperatures i.e. 775and 800 o C as shown in Fig. 11(a-b) whereas, lath type martensite was found in the specimens intercritically heat treated at 825 o C as shown in Fig. 11(b).

Mechanical Properties
The changes encountered in the mechanical properties due to variation of microstructure are summarized in Table 3.

Tensile Strength
TS values are listed in  (1) The TS generally increased due to higher volume fraction of martensite formed in DP steel specimens.
On contrary, martensite carbon content itself decreased with increasing its volume fraction.
As it is well known that strength of martensite phase mainly depends on its carbon content.
As carbon content decreases the martensite deform plastically at much lower stress [1,8].

Effect of Intercritical Heat Treatment on Mechanical Properties of Plain Carbon Dual Phase Steel
Also it is widely acknowledged in the literature [15][16], that the morphology of martensite changes from plate to lath type with a decrease in martensite carbon content.

TABLE 3. SUMMARY OF MECHANICAL PROPERTIES
To provide a reasonable explanation for significant variation of TS of DP specimens the phase transformation occurs during intercritical heat treatment has to be taken into consideration. The austenite to martensite phase transformation upon quenching from intercritical annealing temperature is accompanied by 4% volume change theoretically. Actual volume change depends upon the amount of carbon trapped within martensitebct (body centered tetragonal) crystal lattice [15,18]. Another reason for the significant change in mechanical properties of DP specimens is the tetragonality of bctlattice of martensite which depends upon its carbon content. The degree of tetragonality (usually measured by c/a ratio) given in Equation (1)  c/a = 1.005 + 0.045 (C m ) (1) Where c/a is Tetragonality ratio, C m isCarbon content of martensite, Cs is Carbon content of steel,V m is martensite phase volume (%).
In present case the tetragonality ratio of martensite formed in DP specimens was determined by using the empirical Equations (1-2). To calculate percent of carbon in martensite (C m ) percent V m was used in Equation (2).
Thereafter, tetragonality ratio was determined using the Equation (1). Using the Equations (1-2), the value of tetragonality ratio calculated from each microstructure at all intercritical heat treatment conditions is given in Table   4. This difference in axial ratios corresponds to higher

Hardness
Hardness values of as-received and intercritically heat treated specimens are listed in Table 3. Fig. 13 shows the effect of intercritical annealing temperature and soaking time on hardness of DP specimens. Hardness and strength are interrelated in a way that increasing the hardness generally improves the strength. Therefore, hardness result shows the similar trend compared to TS. The hardness of as-received specimen was found to be lower than hardness of DP specimens. The higher hardness is due to formation of harder phase i.e. martensite in DP specimen. Fig. 13 also shows that hardness first increases and then decreases with increasing soaking time at a particular temperature. This suggests that V m % is not only the microstructural feature that controls the properties of dual phase steels but the effect of carbon content of martensite must also be taken into account as previously discussed for TS results.

Toughness
Toughness values of as-received and intercritically heat treated specimens are listed in Table 3. It can be noted from Table 3 the toughness of as-received specimen is higher than DP specimens annealed at range of intercritical temperatures. The higher toughness of as-received specimen is due to higher ductility and ability of pearlite phaseto co-deform with ferrite compared to the martensite phase present in DP specimens. From Fig. 14 it can be seen that toughness of DP specimens increases with increasing temperature and time during intercritical annealing. At low martensite fraction the ductility of martensite phase is low due to its higher carbon content, while ductility significantly improves with high V m % owing to decrease in its carbon content. At high V m % the flow stress of martensite decreases and its ability to codeform with ferrite increases. Due to increase in V m % and improvement in ductility of martensite phase, the maximum toughness of 135 Joules was recorded for DP specimen (C3) corresponding to 83% V m . However, this trend is opposite with observations of researcher's Movahed et. al. [1] and Bag et. al. [12] that fine distribution of martensite content upto 60-78% results in sharp drop in impact energy measurements.

CONCLUSION
A range of specimens having DP microstructures were produced by subjecting AISI 1020 steel to intercritical heat treatment. Characterization of mechanical properties was done by means of hardness, impact and tensile test.
Based on experimental results following conclusions can be drawn.  (iii) Decrease in carbon content softens the martensite and significantly improves its plastic formability.
(iv) The hardness and TS was found to increase with increasing V m % with optimum value of 1277 MPa and 41 HRC. A further increase in Vm% was found to decrease both TS and hardness.
(v) The toughness of DP specimens was lower compared to as-received and increase with increasing V m %.