Temperature Dependence of the Static and Dynamic Behaviour in a Quenching and Partitioning Processed Low-Si Steel

Abstract

Because of their excellent combination of strength and ductility, quenching and partitioning (Q & P) steels have a great chance of being added to the third generation of advanced high strength steels. The large ductility of Q & P steels arises from the presence of 10% to 15% of retained austenite which postpones necking due to the transformation induced plasticity (TRIP) effect. Moreover, Q & P steels show promising forming properties with favourable Lankford coefficients, while their planar anisotropy is low due to a weak texture. The stability of the metastable austenite is the key to obtain tailored properties for these steels. To become part of the newest generation of advanced high strength steels, Q & P steels have to preserve their mechanical properties at dynamic strain rates and over a wide range of temperatures. Therefore, in the present study, a low-Si Q & P steel was tested at temperatures from −40 • C to 80 • C and strain rates from 0.001 s −1 to 500 s −1 . Results show that the mechanical properties are well-preserved at the lowest temperatures. Indeed, at −40 • C and room temperature, no significant loss of the deformation capacity is observed even at dynamic strain rates. This is attributed to the presence of a large fraction of austenite that is so (thermally) stable that it does not transform in the absence of deformation. In addition, the high stability of the austenite decreases the elongation at high test temperatures (80 • C). The additional adiabatic heating in the dynamic tests causes the largest reduction of the uniform strain for the samples tested at 80 • C. Quantification of the retained austenite fraction in the samples after testing confirmed that, at the highest temperature and strain rate, the TRIP effect is suppressed.Metals 2020, 10, 509 2 of 22 twinning induced plasticity (TWIP) and austenitic steels. These steels have excellent properties, though their use is limited because they require large amounts of expensive alloying elements such as Ni, Mn and Cr [8][9][10][11]. In recent years, a third generation of AHSS is being developed. Steels which may claim to be part of the third generation are medium to high manganese steels [12][13][14][15], complex phase steels [16], steels produced via severe plastic deformation [17] or ultrafast thermal processing routes [18][19][20][21], and quenched and partitioned (Q & P) steels [22,23].Q & P steels first appeared in literature in 2003 and have gained research interest since then [23]. Q & P steels combine high strength and ductility because of their complex multi-phase microstructure. This microstructure is obtained by imposing a (two-step) thermal cycle to low carbon steel with Si/Al as most the important alloying elements. After initial austenisation, the material is quenched to the quenching temperature situated in between the martensite start (Ms) and martensite finish temperature (Mf). This creates a microstructure of martensite and austenite of equal carbon content. The fractions of both are determined by the quenching temperature ...

High Strain Rate Mechanical Behavior of Advanced High Strength Steels

“…1.27b. The suppression of TRIP effect due to increased strain rate was also reported in [197], where the RA transformation was retarded when the strain rate reached to 500 s −1 . However, such effect of strain rate on RA transformation was not observed in Q&P 980 steel studied by interrupted SHTB tests [198].…”

Advanced High-Strength Steels by Quenching and Partitioning

“…Vercruysse et al [8] studied the static and dynamic tensile properties of a Q&Ptreated Fe-0-2C-1.25Si-2.4Mn-0.023Al (wt.%) steel in the temperature range from −40 • C to 80 • C. The material showed excellent low-temperature tensile properties because of the high thermal stability of retained austenite, which could contribute to the transformationinduced plasticity (TRIP) effect during testing at low temperatures. However, uniform elongation significantly dropped at 80 • C due to less effective use of the TRIP effect at this temperature.…”

The effect of Nb on the high strain rate hydrogen embrittlement of Q&P steel

“…Although some research has been carried out on the effect of the stability of austenite on hydrogen susceptibility, it is still not completely understood. In earlier publications of the authors, it was reported that imposing a high strain rate, as would be the case in a car crash, affects the TRIP effect due to the adiabatic heating influencing the stability of austenite [4]. Therefore, the present research is focused on the effect of the addition of Nb on the HE susceptibility of a Q&P steel under static and dynamic loading conditions.…”