The chemical composition of the steel with the transformation temperatures is shown in Table 1. The steel has 0.17%C, which is good for weldability and formability of the steel. The steel has 1.72% Mn which decreases the A1 temperature that results in enlarged austenite fraction during austenitization. The steel has 0.02%Nb, 0.04% Ti and 0.06% Al, all of which ensures that the steel maintains a fine grained structure, during deformation and thermal treatments. The steel has 1.35% Si which shifts the tempered martensite embrittlement region to higher temperature range during tempering, which enhances the low temperature range where high strength and ductility can be retained.
Table 1
Chemical composition (wt. %) and critical temperatures (oC) of the steel studied.
C
|
Mn
|
Si
|
Nb
|
Ti
|
N
|
Al
|
S
|
P
|
AC1
|
AC3
|
Bs
|
Ms
|
0.17
|
1.72
|
1.35
|
0.02
|
0.04
|
0.005
|
0.06
|
0.003
|
0.016
|
731
|
875
|
557
|
417
|
The pseudobinary phase diagram of the steel, as evaluted by Thermocalc shows the phase distribution as a function of increasing carbin content. The given steel on hating to 920 oC forms austente completely, while hetaing the steel to 810 oC results in 47% ferrite and 53% austenite as per phase diagram.
3.1. Microstructure
The optical microstructure of the steel hardened from 920oC with 20 min holding and after tempering typically at 250oC, 450oC and 650oC is shown in Fig. 3 along with their corresponding SEM micrographs. The microstructure shows lath martensitic in nature with high dislocation density in the as-quenched condition. Using Koinstein-Marbuerger equation the estimated fraction of martensite is 98.4% with retained austenite content < 1.6%.
fQTm = 1–exp (-1.1 x 10− 2 (Ms-Quench Temperature))…………………………(1)
Tempering up to 250oC still retains the lath microstructure and the tempering reaction tends to precipitate fine epsilon carbides that cannot be resolved in optical or SEM microstructures [14]. Beyond 250 oC, the retained austenite decomposes in low alloy steels. In high Si steels, the carbide precipitation may be delayed, which benefits the strength and ductility [1–5]. Tempering at 450 oC, enable greater carbide precipitation and recovery of the structure with laths broadening. Thin streaks of carbide decorate at lath boundaries. Tempering above 500 oC, leads to the coarsening of the carbides and the carbon free equilibrium ferrite becomes coarsened as shown in the optical and the SEM microstructures in Fig. 3.
The optical and SEM microstructures of the steel intercritically austenitized at 810oC for 20 min holding followed by quenching and tempering at 250, 450 and 650 oC as shown in Fig. 4. Theoretically, intercritical austenitization at 810 oC gives 47% ferrite content with austenite based on Fig. 2. The measured free ferrite content was 49% from the microstructure. Tempering the intercritically austenitized steel samples tempers the martensite fraction formed on hardening. The microstructure of the steel tempered till 250 oC shows intercritical ferrite with tempered martensite with same morphology as that of as quenched samples. Tempering at 450 oC precipitates fine carbide along the lath boundaries. Tempering at 650 oC shows that the carbides are coarsened. During the intercritical treatment, there is about 49% carbide free ferrite. As the carbon solubility in ferrite during inter critical heat treatment is lower, the equilibrium austenite is enriched with the carbon rejected by the ferrite. This carbon rich austenite forms a martensite which is stronger than the martensite without intercritical ferrite formation. Thus, a composite effect of hard and soft phases is built in the microstructure. Apart from this, during tempering, the carbides formed in the inter critically treated sample is likely to precipitate more in the martensite zones compared to the fully austenitized condition. Hence, the carbides in the tempered martensite microstructure, is likely to be more dense in the intercritically austenitized steel.
3.2. Mechanical properties
The mechanical properties such as yield strength, ultimate tensile strength, total elongation of the fully austenitized and intercritically austenitized steel with increase in tempering temperature is shown in Fig. 5. The presence of the softer intercritical ferrite in the matrix has softened the steel than in the samples in fully austenitized condition. The strength value decreases with increasing tempering temperature. The loss in strength is very moderate till 350 oC. The ductility values are reasonably high till 300 oC. Hence, this condition can be exploited for ultrahigh strength application such as in automotive structures with light weighting opportunity. The tensile strength and the yield strength is higher for the fully austenite steel with maximum martensite content than the steels with softer intercritical ferrite. The ductility values pass through a minimum, at about 450 oC, as shown in Fig. 5. This temperature corresponds to the tempered martensite embrittlement. This sort of embrittlement usually takes place in the conventional hardened and tempered low alloy steel at about 350 oC [3]. The presence of high Si content in the steel shifts the tempered martensite embrittlement to the higher tempering temperature of 450 oC [3]. Hence, the lowering of ductility is due to the embrittlement. Tempering the steel beyond 450 oC shows monotonic increase in ductility in the steel while the strength rapidly decreases. The strength values at 500 to 600 oC correspond to 1100 to 1000 MPa tensile strength with ductility 15 to 20% in fully austenitized steel. At higher tempering temperatures, the tensile strength of the fully austenitized steel and the intercritically annealed steel are very close above 550 oC tempering temperature although the yield strength of the fully austenitized and tempered steel is higher. Superior ductility of the intercritically austenitized steel is found at all tempering temperatures. The yield ratio and tensile toughness of the steels are shown in Fig. 6. Higher yield ratio is found in the fully austenitized followed by water quenched steel compared to the inter critically annealed followed by water quenching the steel due to the presence of large amount of hard martensite phase. The yield ratio increased with tempering temperature in both the steels and gets saturated at higher tempering temperatures, greater than 500oC. The tensile toughness is greater than 20GPa% up to 300oC tempering in both the steels and then decreased up to 450oC and then although increased, remain below 20 GPa.% level.
TRIP Steel subjected to DP steel heat treatment (intercritical annealing following water quenching) forms ferrite matrix with martensite and retained austenite. This is the condition when the present steel is water quenched before tempering. A mild heat treatment of tempering less than 300oC give excellent strength and ductility compared to DH steel. The property range extends with tensile toughness beyond 20 GPa%.
The work hardening behaviour of the steels were assessed from the true stress-true strain diagram in Fig. 7, using the Hollomon equation (σ = k εn) as shown in Fig. 8. The slope in this figure is the n value and it is seen that the n-value initially is nearly same for all conditions. The work hardening initially proceeds in the ferrite matrix where the dislocations accumulate with increase in strain. At higher strains, the dislocations move to tempered martensite zones rich in carbides, where dislocation move with difficulty.The work hardening characteristics have further been analysed using the modified Crussard-Jaoul (CJ) model, where the plastic strain during work hardening is related to the true stress as follows [15],
εp = ε0 + Cσm ……………..(2)
Where εp = actual plastic strain
ε0 = initial true strain
σ = true stress
m= (1/n)
C = material constant
On logarithmic scale the equation becomes,
Ln (dσ/dεp) = (1 − m) lnσ – ln Cm ……………..(3)
The plots so obtained from the above equation for austenitization at 810 and 920 oC are shown in Fig. 9. The change in slope is indicative of the change in deformation mechanism. In the initial stages dislocations are generated, move and accumulate in the softer ferrite phase. When the stress exceeds a critical level, the deformation takes place in the harder phases of tempered carbides. The rate of strain hardening decreases with increasing tempering temperatures. At similar tempering temperature, the strain hardening rate decreases for intercritically treated steel compared to the fully austenitized steel. From the Eq. (2) The (1-m) is a measure of the slopes in Fig. 7. When the value (1/m) is high, higher strain hardening takes place. It is seen that higher strain hardening capability is realized in the Table 2.
Table 2
The Work hardening characteristics at 920 oC and 810 oC.
|
Tempering
Temperature oC →
|
WQ
|
150
|
250
|
350
|
450
|
500
|
550
|
600
|
650
|
Austenitized at 920 oC
|
Stage 1
|
(1-m)
|
-2.04
|
-8.78
|
-2.28
|
-5.39
|
-4.44
|
5.17
|
--
|
5.17
|
-2.55
|
m
|
3.04
|
9.78
|
3.28
|
6.39
|
5.44
|
-4.17
|
--
|
-4.17
|
3.55
|
1/m
|
0.33
|
0.10
|
0.31
|
0.16
|
0.18
|
-0.24
|
--
|
-0.24
|
0.28
|
Stage 2
|
(1-m)
|
1.63
|
-1.04
|
-17.68
|
-0.82
|
-11.76
|
-16.98
|
--
|
-1.08
|
-19.82
|
m
|
-0.63
|
2.04
|
18.68
|
1.82
|
12.76
|
17.98
|
--
|
2.08
|
20.82
|
1/m
|
-1.58
|
0.49
|
0.05
|
0.55
|
0.08
|
0.06
|
--
|
0.48
|
0.05
|
Austenitized at 810 oC
|
Stage 1
|
(1-m)
|
-2.04
|
-3.91
|
-0.30
|
-2.81
|
-1.35
|
--
|
-0.40
|
--
|
-2.26
|
m
|
3.04
|
4.91
|
1.30
|
3.81
|
2.35
|
--
|
1.40
|
--
|
3.26
|
1/m
|
0.33
|
0.20
|
0.77
|
0.26
|
0.43
|
--
|
0.71
|
--
|
0.31
|
Stage 2
|
(1-m)
|
-0.06
|
-13.24
|
-5.81
|
-0.01
|
-1.72
|
--
|
-9.93
|
--
|
-14.42
|
m
|
1.06
|
14.24
|
6.81
|
1.01
|
2.72
|
--
|
10.93
|
--
|
15.42
|
1/m
|
0.94
|
0.07
|
0.15
|
0.99
|
0.37
|
--
|
0.09
|
--
|
0.06
|
3.3. Magnetic Properties
The change in coercivity, remanence, maximum induction of the steels with increase in tempering temperature is shown in Fig. 10. The coercivity decreased while maximum induction marginally increased with increase in tempering temperature while the remanence decreased up to 250oC and then increased. The as quenched steel from above A3 is having very high volume fraction of martensite along with high density of dislocations with lot of compressive residual stress. Hence the magnetic domain movement is restricted strongly leading to the higher value of coercivity of the steel in as-quenched condition. In the contrary the steel quenched after inter critical holding having almost 50% ferrite helps in easy movement of magnetic domain wall leading to relatively lower coercivity. With increase in tempering temperature the coercivity decrease in the fully austenized and quenched steel sharply due to the reduction in the residual stresses whereas in the inter critically austenized steel showed a slow decrease in the coercivity. Although at low temperature tempering fine epsilon carbide restricts the magnetic domains could lead to increase in coercivity, due to the large relaxation in the residual stress for the tempering of martensites overwhelming the pinning by epsilon carbides and showed a decrease in trend with tempering. The maximum induction and remanence are higher in the intercritically austenized steel due to the presence of higher volume fraction of ferrite. The decrease in remanence up to 250oC is probably due to the precipitation of the finer epsilon carbides after which the coarsening of the carbides and increase in the inter carbide distance leads to the rapid increase in the remanence of the steels. The decrease in coercivity is also due to the coarsening of carbides and increase in the inter carbide distance which lead to decrease in the pinning density for the magnetic domain wall motion.
As coercivity is increased with the strong pinning of the magnetic domain wall by density of dislocations or strain field of dislocation, carbides, secondary phases which are also restricts the dislocation movement leading to increase in tensile strength of the steels, correlation is expected between the two due to the common pinning points. Correlation of coercivity with the tensile strength of the steel is shown in Fig. 11. A strong correlation is found between the coercivity with the tensile strength of the steels (Equation-4) indicating that precise evaluation of the tensile strength can be made by monitoring the coercivity of the steels through a non-destructive magnetic evaluation device.
UTS (MPa) = 103.68*Hc (A/m) + 155.2………………………..(4)
R2 = 0.88