3.1. Influence of Intercritical Quenching on the Structure of DP-A and DP-B Steels
Since the values of the mechanical characteristics of dual-phase steel depend on the structure [
1,
2,
3,
4,
5,
10,
11,
12,
19,
21], first, the influence of the technological parameters of the intercritical quenching (heating temperature and cooling medium) on the volume fraction of martensite (V
M) and the ferrite microhardness (HV0.01) was analyzed; the results obtained for the two dual-phase studied are presented in
Table 2 and
Table 3,
Figure 1 and
Figure 2, [
26,
28,
30].
Raising the T
Q temperature in the intercritical range (Ac
1–Ac
3) has caused an increase in the amount of austenite obtained by heating, a phase which, by quenching in water without mechanical agitation (W) and in water activated with ultrasounds (US59), has turned into martensite. Thus, the rise in heating temperature (T
Q) has led to an increase in the volume fraction of martensite (V
M) in structures of the DP-A and DP-B steels (
Table 2 and
Table 3,
Figure 1).
Also, for same heating temperatures, the use of ultrasounds in cooling has led to increasing the volume fraction of martensite (V
M),
Table 2 and
Table 3,
Figure 1; for example, in the DP-A steel samples, for T
Q = 760 °C, V
M increased with 3.55 percent (from 20.19% to 23.74%), and in the samples of DP-B steel, for T
Q = 820 °C, V
M increased with 4.08 percent (from 38.13% to 42.21%). The ultrasonic quenching medium eliminates one of the most dangerous phases of the cooling (calefaction), enhances heat exchange between product and cooling medium, and reduce the quenching deformations; on the other hand, the additional energy intake increases the volume fraction of martensite formed at quenching and reduces the amount of residual austenite [
3,
23,
24,
25,
26,
27,
28].
The mechanical properties of dual-phase steel (in particular those of strength) are also influenced by the carbon content of martensite (C
M), not only by the amount of this phase (V
M) [
2,
5,
10,
11,
12,
26,
31,
32]. The carbon content of martensite (C
M) can be determined by X-ray diffraction analysis, or it can be calculated with different equations, one of them being proposed by G.R. Speich and R.L. Miller [
2,
28,
33]:
In which: C
M is the carbon content of the martensite; C
o—the carbon content of the steel; C
F—the carbon content of the ferrite, (C
F = 0.002%); V
M—the volume fraction of martensite; ρ
F—the density of the ferrite; ρ
M—the density of the martensite, (ρ
F/ρ
M = 1.025) [
2,
28,
34]; using this equation, values of the carbon content of martensite (C
M) ranged from 0.431% for V
M = 20.19% (T
Q = 760 °C/W) and 0.213% for V
M = 40.71% (T
Q = 820 °C/US59), to the DP-A steel and from 0.458% for V
M = 22.10% (T
Q = 760 °C/W) and 0.239% for V
M = 42.21% (T
Q = 820 °C/US59), to the DP-B steel [
28].
The martensite obtained by quenching from 760 °C in water without mechanical agitation (W) was in the form of small islands, situated mainly at the boundaries of the ferrite grains (
Figure 3), most of them being located in regions which, in initial structure, was pearlite; this, by heating over the critical point Ac
1, was transformed into austenite, from which, through quenching, martensite resulted. This transformation mechanism led to a volume fraction of martensite (V
M) of approx. 14.70% in the case of DP-A steel, respectively 16.10% in the case of DP-B steel, percentages that constitute the equivalent of the amount of pearlite from the initial structures. The difference of martensite from 5.49% up to 20.19% for DP-A steel, respectively 6.0% up to 22.10% for DP-B steel (
Table 2 and
Table 3), resulted from austenite obtained by the allotropic transformation of the ferrite; the ultrasonic quenching medium increased the cooling rate, which led to an increase in the volume fraction of martensite (V
M) obtained in this way. Raising the heating temperature (T
Q) between 760 and 820 °C caused an increase in the amount of austenite that was formed by the allotropic transformation of the ferrite, which led to the rise in the volume fraction of martensite that resulted from quenching, both in water without mechanical agitation (W), and in water activated with ultrasounds (US59). At the same time, with the rising of the volume fraction of martensite in structures (and decreasing the amount of ferrite), an increase in the size of the martensite islands is observed (
Figure 3 and
Figure 4). Furthermore, a tendency of their connection and the formation of a network around the ferrite grains are marked [
26,
28].
The technological parameters of the intercritical quenching also influenced the ferrite microhardness (HV0.01); this has increased with both the raising of the heating temperature (T
Q) and the use of ultrasounds at cooling (
Table 2 and
Table 3,
Figure 2). In the initial structures (ferrite-pearlite structure), the values of ferrite microhardness were 164.18 HV0.01 for DP-A steel, respectively 167.82 HV0.01 for DP-B steel. The increase in the number of interstitial atoms (C, N) and density of dislocations in the crystal lattice of ferrite caused, most probably, the increase in the microhardness of this phase [
2,
26,
28,
35].
3.2. Influence of Intercritical Quenching on the Mechanical Properties of DP-A and DP-B Steels
The tensile tests applied to the intercritical quenched specimens led to the results in
Table 4 and
Table 5,
Figure 5,
Figure 6,
Figure 7,
Figure 8,
Figure 9 and
Figure 10; for the specimens with the initial structure (ferrite-pearlite structure), the tensile tests determined to the following average values of the ultimate tensile strength (R
m) and total elongation (A
5): R
m = 556.12 MPa and A
5 = 26.14% for DP-A steel, respectively R
m = 560.65 MPa and A
5 = 25.93% for DP-B steel [
28,
30]. The ferrite-martensite structures obtained by intercritical quenching ensured (for both studied alloys) higher ultimate tensile strengths and smaller total elongations than those determined in the specimens with ferrite-pearlite structures.
The increase in the volume fraction of martensite (V
M) and ferrite microhardness (HV0.01) due to the rise in quenching temperature (T
Q) has led to an increase of the ultimate tensile strength (R
m) and the decrease of the total elongation (A
5) of the two dual-phase steels studied (
Table 4 and
Table 5,
Figure 5 and
Figure 6).
Also, for same heating temperatures, cooling in water activated with ultrasounds (US59) has determined to increase the strength characteristics (R
m) and decrease deformability (A
5), compared to the values obtained at quenching in water without mechanical agitation (W); this effect was also generated by the increase in the volume fraction of martensite (V
M) and ferrite microhardness (HV0.01),
Table 4 and
Table 5,
Figure 5 and
Figure 6. For example, to the specimens made of DP-A steel, for T
Q = 760 °C, the ultimate tensile strength (R
m) increased with 27.54 MPa, from 631.32 MPa to 658.86 MPa, and the total elongation (A
5) decreased with 1.79 percent, from 24.46% to 22.67%; to the specimens made of DP-B steel, for T
Q = 800 °C, the ultimate tensile strength (R
m) increased with 16.30 MPa, from 698.62 MPa to 714.92 MPa, and the total elongation (A
5) decreased with 0.61 percent, from 19.29% to 18.68% [
28].
In dual-phase steels, the variation of the volume fraction of martensite (V
M) has two contradictory effects on the mechanical characteristics: on the one hand, for example, the strength properties increase with increasing the volume fraction of martensite (V
M), and on the other hand, the carbon content of martensite (C
M) decreases; and hence, its strength decreases with an increase in the volume fraction of martensite (V
M) [
5,
10,
11,
12,
28,
33,
34]. The variation of the mechanical characteristics of the two steels (R
m, A
5) depending on the volume fraction of martensite in the structure (V
M) is not linear (
Figure 7 and
Figure 8), being influenced by the carbon content of martensite (C
M). For V
M between 20.19% (C
M = 0.431%) and approx. 28% (C
M = 0.311%), to the DP-A steel and for V
M between 22.10% (C
M = 0.458%) and approx. 30% (C
M = 0.337%), to the DP-B steel, the variation of R
m and A
5 is more intense, than for V
M between 28% (C
M = 0.311%) and 40.71% (C
M = 0.213%), respectively between 30% (C
M = 0.337%) and 42.21% (C
M = 0.239%) [
28].
Unlike the nonlinear variation of the mechanical characteristics according to the volume fraction of martensite (V
M), their evolution (R
m, A
5) according to the ferrite microhardness (HV0.01) is almost linear (
Figure 9 and
Figure 10) [
28].
The data in
Table 2,
Table 3,
Table 4 and
Table 5 show (for both steels studied) that the additional energy intake assured by the ultrasounds with the frequency of 59 kHz determined obtaining of values for the volume fraction of martensite (V
M), ferrite microhardness (HV0.01), ultimate tensile strength (R
m), and total elongation (A
5) is very close to those obtained by quenching in water (W), but with heating at a higher T
Q temperature. For example, the results achieved by quenching T
Q = 760 °C/US59 are very close to those obtained by quenching T
Q = 780 °C/W, and the results from quenching T
Q = 780 °C/US59 are very close to those from quenching T
Q = 800 °C/W [
28].
The slightly higher carbon and manganese content of DP-B steel (compared to DP-A steel) determined (for both cooling mediums used) the obtaining of slightly higher values of the volume fraction of martensite in the structure and the ferrite microhardness, with effect on the ultimate tensile strength and total elongation (
Table 2,
Table 3,
Table 4 and
Table 5).
Comparing the results obtained for the DP
1.90Mn steel (
Table 6) with those determined for DP-A and DP-B steels (
Table 2,
Table 3,
Table 4 and
Table 5), significant differences were observed between the data sets, differences determined, in particular, by the very different manganese content of the two categories of steels [
3,
25,
27,
30].
In DP
1.9Mn steel, the position of the critical points A
c1 and A
c3 led to the obtaining, by intercritical heating, of a higher amount of austenite, which determined the formation, on quenching, of a higher volume fraction of martensite. Thus, for the temperature range 760–820 °C, the volume fraction of martensite in the structure was higher by percentages between 22.32 and 41.12%. Because of this, the values of ultimate tensile strength and total elongation at DP-A and DP-B steels were much different from those of DPS
1.90Mn steel (
Figure 11 and
Figure 12); the total elongation was higher with percentages between 9.28 and 6.98 and the ultimate tensile strength was lower with values between 325.68 and 433.03 MPa.