1. Introduction
Recent advancements in third-generation advanced high-strength steels (AHSS) have increasingly focused on bainite-containing steels due to their promising mechanical properties. Steels such as CP (complex-phase) steel [
1,
2], CFB (carbide-free bainitic) steel [
3,
4,
5], and Q&P (quenching and partitioning) steel [
6,
7] are known for their superior balance of strength, toughness, and ductility. The relationship between the microstructure and mechanical properties of TRIP (low-alloy transformation-induced plasticity) steels has become a prominent area of research, particularly concerning the role of retained austenite. Retained austenite provides sustained strain hardening under tensile strain, thereby enhancing the ductility of the steel [
8,
9].
The Q&P (quenching and partitioning) process, introduced by Professor John G. Speer, aims to optimize the balance of strength and ductility in low-alloy steels [
10,
11]. This process involves quenching to a temperature within the martensite-start (
Ms) and martensite-finish (
Mf) range, followed by a prescribed duration of isothermal holding. The resulting Q&P microstructure can be highly complex, and may contain carbon-depleted martensite, BF (bainitic ferrite), secondary martensite, and RA (retained austenite). The TRIP (transformation-induced plasticity) effect of retained austenite (RA) notably boosts the steel’s strength and ductility by improving its work-hardening ability [
12]. Additionally, the fine substructure and elevated dislocation density from martensite and bainite further enhance the steel’s strength.
The production of ultra-high-strength steel parts via hot stamping of quenchable steel sheets has been on the rise. Many automobile manufacturers are increasingly adopting hot stamping processes to produce various ultra-high-strength components, such as A/B-pillars and bumper beams, which reduce vehicle weight and enhance safety. However, steels like 22MnB5, after undergoing hot stamping (heating above the
Ac3 temperature, forming, followed by die quenching), typically achieve a tensile strength of around 1.5 GPa but exhibit low ductility, which can negatively impact vehicle safety [
13].
This study simulates the hot stamping process in combination with various quenching methods to investigate the effects of precipitation behavior on the yield strength of 1000 MPa grade low-alloyed lightweight bainitic steel. Additionally, this study aims to retain a significant proportion of austenite that can transform into martensite during a crash, thereby enhancing crashworthiness. The microstructure and mechanical properties, alongside precipitation behavior after different heat treatments are carried out and analyzed so that an optimal heat-treating process can be concluded. The calculations of four strengthening mechanisms devoted to yield strength were carried out. The results provide a guideline for optimizing hot stamping parameters and aim to establish a solid foundation for the subsequent development of high-quality lightweight vehicle manufacturing processes.
2. Experimental
Table 1 describes the chemical composition of typical GEN3 steel manufactured by U.S. Steel Corporation, Pittsburgh, PA, USA.
Dilatometer (DIL805A) (TA Instrument, New Castle, DE, USA) measurements were conducted to determine the critical transformation temperatures (
Ac1,
Ac3,
Ms) of GEN3 steel, where
Ac1 and
Ac3 are the starting and finishing temperatures of ferrite to austenite during the heating process, and
Ms represents the starting temperature of martensite formation during a fast-cooling process. Specimens, in the form of 4 × 1.2 × 10 mm billets, were heated from room temperature (RT) to 950 °C at a rate of 15 °C/s, maintained at 950 °C for 5 min, and then cooled to RT at a rate of 50 °C/s, as illustrated in
Figure 1.
From
Figure 1, the
Ac
1 and
Ac
3 temperatures were determined to be 738 °C and 890 °C, respectively, with an
Ms temperature of 355 °C. The
Bs temperature was calculated using a semi-empirical equation [
14]:
where
Bs represents the starting temperature of bainite formation, °C, and
w refers to the mass fraction of elements in steel, %. Thus,
Bs is determined at 552 °C based on Equation (1).
The as-annealed sample had been subject to the cold rolling and annealing process. The as-annealed specimen was marked as 1#. This sample was then heated to 900 °C, held for 5 min, and water-quenched, producing Sample 2# (the as-quenched sample). For the quench and temper (Q&T) process, Sample 2# was further tempered at 450 °C for 10 min, resulting in Sample 3#. Finally, the quench and partition (Q&P) process was carried out, where the steel was heated to 900 °C, held for 5 min, and then heated to 330 °C for 10 min in a salt bath before air cooling, producing Sample 4#. The procedures for Samples 2, 3, and 4 are shown in
Figure 2.
Microhardness tests were conducted with the Digital Micro Vickers Hardness Tester, Model No. HVS-1000 (Guangdong Micro Accuracy Co., Ltd., Dongguan, China). We also employed a CMT4105 electronic universal testing machine (MTS System Corporation, Shenzhen, China) to measure the yield strength and ultimate tensile strength. The test samples had a total length of 25 mm, with a thickness of 1.2 mm, a gauge length of 15 mm, and a width of 5 mm. Furthermore, the radius between the parallel section and the shoulder section was 2.5 mm. Sample microstructures were observed with a ZEISS Gemini 300 SEM (Zeiss, Oberkochen, Germany). The scanning electron microscope (SEM, Model No. JSM-7001F) with a Pegasus XM2 detector (JEOL, Akishima, Japan) was used to perform electron backscatter diffraction. The samples were subjected to electropolishing in an ethanol solution of 10% perchloric acid and 5% glycerol. Subsequently, the acquired data were analyzed using OIM (Orientation Imaging Microscopy) which was provided by EDAX Company, Boston, MA, USA.
Further investigation of precipitate morphology and distribution was conducted using a JEOL F200 high-resolution transmission electron microscope (HR-TEM) (JEOL, Akishima, Japan). TEM samples were prepared using both carbon extraction replicas and twin-jet electropolishing methods. Thermodynamic calculations were carried out with JMatPro 7.0 software (Sente Software Ltd., Surry, UK). Retained austenite (RA) content and dislocation density were determined using X-ray diffraction (XRD) with a Smartlab SE instrument (Rigaku, Tokyo, Japan).
The volume fraction of RA was calculated based on the following equation [
12]:
where
Vγ is the volume fraction of RA,
Iγ, and
Iα are the integral intensity of the {111}
γ austenite peak and the integral intensity of the {110}
α ferrite peaks, respectively.
aγ is the lattice parameter of austenite in Angstrom, which is calculated using the following equation based on the {111}
γ peak [
15].
where
λ, (
hkl), and
θhkl are the wavelength of the radiation, the three Miller indices of a plane, and the Bragg angle, respectively. To evaluate the stability of carbon in austenite, the following equation is introduced [
16]:
where
Cγ is carbon content in austenite, wt.%.
5. Conclusions
In this paper, various quenching processes were employed to simulate the hot stamping process and produce steel with varying levels of strength and ductility. We examined the structure–property characteristics of a 1000 MPa grade low-alloyed lightweight TRIP-assisted GEN3 steel with different matrix structures. The main conclusions are as follows:
(1) The retained austenite (RA) content in the as-annealed sample reaches up to 11.9%, resulting in a more gradual decrease in the work-hardening rate compared to other heat-treated samples.
(2) Although the stability of retained austenite (RA) improves after different quenching processes, both Q&P and Q&T samples still exhibit low work-hardening rates. This is primarily due to the uneven distribution of martensite laths with fine structures.
(3) GEN3 steel treated with the quench and partition (Q&P) process exhibits the highest dislocation density, indicating significant refinement of the substructure.
(4) Precipitation strengthening is the primary contributor to yield strength in the Q&P and Q&T processes due to the diffusion of carbon during the partition process and the formation of tempered martensite.