Steel with a nominal composition of 0.25C–1.5Si–3Mn–0.023Al (mass %) was subjected to Quenching and Partitioning (Q&P) with varying parameters (quenching temperature, partitioning temperature and partitioning time) resulting in formation of multi-phase microstructure, which was thoroughly studied using X-ray (XRD) and Electron Backscatter Diffraction (EBSD). Mechanical properties of the Q&P steel were measured by tensile tests. Plastic deformation of Q&P steel at micro-scale was investigated by
The current trends in the automotive industry have been mainly focused on increasing the crashworthiness properties of vehicles, while decreasing fuel consumption and gas emissions at the same time. For this purpose, the steel industry is continuously presenting innovative solutions to the automotive industry, since this material has the ability to adapt to the changing requirements. In particular, the past few decades have witnessed a significant research effort directed towards the development of Advanced High-Strength Steel (AHSS) grades, as they provide an opportunity for the development of cost-effective and light-weight parts with improved safety and optimized environmental performance for automotive applications (Bhadeshia,
Speer
Steel with a nominal composition of 0.25C–1.5Si–3Mn–0.023Al (mass %), produced in a laboratory vacuum induction furnace was studied. After casting, the steel slabs were hot rolled to a final thickness of 2.5 mm, accelerated cooled by water jets to 600 °C and transferred to a furnace for coiling simulations at 560 °C. The hot rolled plates were pickled and cold rolled to a thickness of 1 mm imposing a total reduction in thickness of 60%. The obtained strips were cut perpendicular to the rolling direction and subsequently subjected to Q&P heat treatment cycles in the thermo-mechanical simulator Gleeble™ 3500. The specimens were heated to 850 °C for full austenitization, quenched to varying Quenching Temperatures (QT) at quenching rate of 20 °C s−1 in order to obtain microstructures consisting of martensite and austenite. Then the samples were reheated with a heating rate of 10 °C s−1 and kept isothermally at different Partitioning Temperatures (PT) for varying Partitioning times (Pt). During this stage, carbon diffusion occurs from supersaturated martensite into untransformed austenite that stabilizes austenite during further final quenching to room temperature with a rate of 20 °C s−1. Data on the Q&P parameters applied to the studied steel grade are presented in
Q&P processing parameters applied to the studied steel grade
Specimen | Quenching Temperature |
Partitioning Temperature |
Partitioning time |
---|---|---|---|
224-350-500 | 224 | 350 | 500 |
244-300-500 | 244 | 300 | 500 |
244-350-500 | 244 | 350 | 500 |
244-400-100 | 244 | 400 | 100 |
244-400-500 | 244 | 400 | 500 |
244-400-1000 | 244 | 400 | 1000 |
264-350-500 | 264 | 350 | 500 |
Specimens for microstructural characterization were ground and polished with a final polishing step of 1 µm diamond paste using classical metallographic techniques. For scanning electron microscopy characterization, specimens were etched with 2 vol.% HNO3 in ethanol (nital 2%) solution for 5 seconds at room temperature (22 °C). Examination of the microstructure was performed using a Scanning Electron Microscope (SEM) EVO MA15 operating at an accelerating voltage of 20 kV. Specimens for EBSD analysis were prepared using standard metallographic technique with final polishing with OP-U for 20 minutes. Orientation Imaging Microscopy (OIM) studies were performed using a FEI Quanta™ 450 FEG-SEM equipped with a Hikari detector controlled by the EDAX-TSL OIM-Data Collection (version 6.2®) software. The data were acquired at an accelerating voltage of 20 kV, a working distance of 16 mm, a tilt angle of 70° and a step size of 40 nm. The orientation data were post-processed and analysed with TSL-OIM Analysis 6.2© software. The post-processing procedure included appropriate “clean-up” steps and after that removing of the point with Confidence Index (CI) lower than 0.1 as dubious.
The volume fractions of RA and its average carbon content at room temperature were measured by XRD experiments performed on a Siemens Kristalloflex D5000 diffractometer equipped with a Mo-Kα source operating at 40 kV and 40 mA. A 2Θ-range of 25° to 45° was scanned using a step size of 0.01°, dwell-time of 2 seconds and a rotation speed of 15 rpm. The data were post-processed by subtracting the background radiation and Kα2 influence. The volume fractions of RA were determined by the Cullity formula (Cullity,
where aγ is the austenite lattice parameter in nm and XC, XMn and XAl are the concentrations of carbon, manganese and aluminium in austenite in wt.%. Volume fractions of Tempered Martensite (TM) and Untempered Martensite (UM) were estimated from the OIM images using the free ImageJ software (Schneider
Miniature tensile specimens having a gauge length of 4 mm, a gauge width of 1 mm and a thickness of 1 mm were machined from the Q&P processed strips.
The Digital Image Correlation (DIC) technique was utilized to determine local in-plane plastic strain distribution after tensile deformation of the Q&P steels. SEM images of the specimens before and after each deformation step were uploaded into the Vic-2D 2009 Digital Image Correlation software for full-field strain analysis and generation of plastic deformation maps.
OIM phase maps with bcc in blue and fcc in yellow superimposed with the image quality maps of the studied samples are shown in
OIM pictures of: (a) the sample quenched at 224 °C (224-350-500), (b) the sample quenched at 244 °C (244-350-500) and (c) the sample quenched at 264 °C for 500 s (264-350-500); all of them were partitioned at 350 °C for 500 s. (d) The sample partitioned at 300 °C for 500 s (244-300-500), (e) the sample partitioned at 350 °C for 500 s (244-350-500) and (f) the sample partitioned at 400 °C for 500 s (244-400-500); all of them were quenched at 244 °C. (g) The sample partitioned at 400 °C for 100 s (244-400-100), (h) the sample partitioned at 400 °C for 500 s (244-400-500) and (i) the sample partitioned at 400 °C for 1000 s (244-400-1000); all of them were quenched at 244 °C. Austenite is in yellow, tempered martensite in blue and untempered martensite in dark blue.
Average grain size (equivalent circle diameter) for the different microconstituents of the Q&P steel
Specimen | Tempered Martensite (TM) | Retained Austenite (RA) | ||
---|---|---|---|---|
|
|
|||
(µm) | (µm) | |||
224-350-500 | 2.7 | ±0.3 | 0.5 | ±0.1 |
244-300-500 | 2.3 | ±0.1 | 0.4 | ±0.1 |
244-350-500 | 2.2 | ±0.4 | 0.6 | ±0.1 |
244-400-100 | 2.2 | ±0.2 | 0.5 | ±0.1 |
244-400-500 | 2.2 | ±0.3 | 0.7 | ±0.1 |
244-400-1000 | 2.4 | ±0.1 | 0.7 | ±0.2 |
264-350-500 | 2.5 | ±0.3 | 0.5 | ±0.1 |
Volume fraction and carbon content (wt.%) of retained austenite and volume fraction of untempered and tempered martensite in the studied steel
Specimen | Retained Austenite (RA) | Untempered Martensite (UM) | Tempered Martensite (TM) | |
---|---|---|---|---|
|
|
|
||
(%) | %C (wt.) | (%) | (%) | |
224-350-500 | 14.2 | 0.91 | 13.8 | 72.0 |
244-300-500 | 14.7 | 1.05 | 17.7 | 67.6 |
244-350-500 | 14.4 | 0.99 | 14.1 | 71.5 |
244-400-100 | 18.3 | 0.84 | 9.9 | 71.8 |
244-400-500 | 17.9 | 1.03 | 12.2 | 69.9 |
244-400-1000 | 20.2 | 1.01 | 10.5 | 69.3 |
264-350-500 | 13.5 | 1.11 | 15.8 | 70.7 |
Mechanical properties measured on miniature samples of the studied steel grade (0.2% proof stress σ
Specimen |
|
|
|
|
|
---|---|---|---|---|---|
|
|
|
|
||
(MPa) | (MPa) | (%) | (%) | ||
224-350-500 | 900 | 1357 | 10 | 22 | 0.19 |
244-300-500 | 721 | 1419 | 10 | 21 | 0.25 |
244-350-500 | 803 | 1471 | 11 | 21 | 0.26 |
244-400-100 | 621 | 1462 | 17 | 26 | 0.24 |
244-400-500 | 821 | 1267 | 16 | 28 | 0.19 |
244-400-1000 | 681 | 1275 | 16 | 29 | 0.20 |
264-350-500 | 761 | 1354 | 13 | 24 | 0.24 |
The EBSD phase maps of the three samples quenched at different temperatures (224-350-500, 244-350-500 and 264-350-500) are shown in
Various RA morphologies are present in the microstructure. In the 244-350-500 sample mostly blocky RA can be found, whereas in the 224-350-500 and 246-350-500 samples the prevalent type is the interlath lamellar one, but some blocky RA is also observed. As it was already reported in literature (Tirumalasetty
A hypothetical explanation for these observations can be offered: the fraction of martensite that undergoes carbon partitioning to the neighbouring austenite during the partitioning step is determined to a large extent by the QT. The stabilized RA fraction is less dependent on higher QTs due to kinetic effects related to lower carbon diffusion in austenite creating pile-ups close to the grain boundaries resulting in local stabilization of austenite. Thus, a QT closely below the Ms temperature results in a small fraction of martensite, so the carbon available for partitioning might not be sufficient for the stabilization of the austenite. This indeed leads to a high fraction of relatively unstable austenite that transforms to martensite in the final quench. On the other hand, lower QT results in the formation of a higher fraction of martensite in the first quench. Therefore, a higher fraction of austenite is consumed during the initial quench, that will not be available for carbon enrichment during the partitioning step (Speer
Engineering stress – engineering strain curves from tensile testing of the: (a) 224-350-500, 244-350-500 and 264-350-500 samples; (b) 244-300-500, 244-350-500 and 244-400-500 samples; (c) 244-400-100, 244-400-500 and 244-400-1000 samples.
RA presenting film-like or interlath lamellar morphologies have better transformation stability compared with a blocky type, which tends to transform to martensite under a small strain and contributes little to the TRIP effect (Speer
Slightly inferior 0.2% proof stress (σ
In order to determine the effect of PT, the target QT was set to 244 °C, followed by isothermal holding during 500 seconds at different PT (300 °C, 350 °C and 400 °C). The RA average size is enlarged with increasing PT (0.4±0.1 µm, 0.6±0.1 µm and 0.7±0.1 µm, respectively). However, the PT does not have influence on the TM average size (
Engineering stress - engineering strain curves in
The influence of Pt on microstructure and mechanical properties of a Q&P steel was also investigated in this work. Samples were partitioned at 400 °C for 100, 500 and 1000 seconds (244-400-100, 244-400-500 and 244-400-1000, respectively) (
Orientation distribution functions (ODFs) in φ2 = 45°, 65° and 90° sections for austenite in sample 244-300-500 (a) and sample 244-350-500 (b) (RC = rotated cube, C = copper, RG = rotated goss, B = brass, and G = goss).
Orientation distribution functions (ODFs) in φ2 = 45° for: martensite in sample 244-300-500 (a) and sample 244-350-500 (b).
Distribution of local plastic strain on surface on: the 244-400-100 sample (a), the 244-400-500 sample (b) and the 244-400-1000 sample (c) after tensile deformation to a global plastic strain of 10%. Histograms of plastic strain distribution for all samples (d).
It was reported in
Steel with a nominal composition of 0.25C–1.5Si–3Mn–0.023Al (mass %) was subjected to “Quenching and Partitioning” (Q&P) treatment with varying Quenching Temperature (QT), Partitioning Temperature (PT) and Partitioning time (Pt). Multi-phase microstructure consisting of different fractions of Tempered Martensite (TM), Untempered Martensite (UM) and Retained Austenite (RA) was obtained. The effect of Q&P parameters on the microstructure, texture, tensile mechanical properties and plastic deformation at the micro-scale was thoroughly studied. It was shown that: The QT does not affect significantly the size and volume fraction of microconstituents, though the morphology of RA strongly depends on QT. Interlath lamellar RA prevails in the microstructure of the samples quenched at 224 °C and 260 °C and has high stability against transformation under strain, whereas blocky type RA dominates in the microstructure of the sample quenched at 244 °C. The latter has low stability against transformation under strain which results in higher ultimate tensile strength. The PT has a strong effect on size and volume fraction of microconstituents and mechanical properties of the Q&P steel. The RA average size increases with increasing of the PT, and the highest RA volume fraction is reached after partitioning at 400 °C due to effective control of carbon diffusion by PT. However, high PT reduces mechanical strength and strain hardening exponent of the material, though its ductility is enhanced. The influence of Pt at 400 °C on the size and volume fraction of microconstituents is ambiguous. The RA average size tends to grow with increasing Pt up to 500 seconds followed by its saturation, whereas RA volume fraction is not sensitive to Pt below 500 seconds and increases after partitioning for 1000 seconds. The material shows the highest ultimate tensile strength and strain hardening exponent at the shortest Pt followed by their significant reduction and slight gain in ductility with further increase of the Pt. There is no significant influence of Q&P parameters on the texture of RA. The main observed components include brass {011} 〈112〉, Goss {110} 〈001〉, rotated Goss {110} 〈110〉, and copper {211} 〈111〉. The weak rotated cube {001} 〈110〉 is only present in the sample 244-350-500. RA with the latter orientation has higher stability, resulting in the highest ultimate tensile strength for this Q&P condition. The Q&P parameters have no effect on crystallographic texture of martensite. It is similar to the texture frequently observed after double α There is strong partitioning of plastic strain between phases during plastic deformation of the Q&P steel. Variations of local plastic strain strongly depend on their local microstructure.
The authors would like to acknowledge financial support by the Research Fund for Coal and Steel via NewQP project (Grant Agreement N° RFSR-CT-2011-00017). DDK acknowledges partial funding by the governmental institute for the Promotion of Innovation by Science and Technology in Flanders, Belgium (IWT). IS acknowledges gratefully the Spanish Ministry of Economy and Competitiveness for financial support through the Ramon y Cajal Fellowship.