Evaluation of Mechanical Properties and Microstructures of Molybdenum and Niobium Microalloyed Thermomechanically Rolled High-Strength Press Hardening Steel
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This article studied the effect of molybdenum and niobium on the microstructures and mechanical properties of laboratory control rolled steels based on grade 22MnB5. The constructed continuous cooling transformation diagrams revealed that an addition of Mo significantly increased the hardenability. Especially in the case of austenite compressed below its recrystallization temperature, an Mo addition delayed ferrite and bainite formation, and it increased martensite hardness. Laboratory hot-rolling experiments with a finish rolling temperature of 850°C produced a flattened pancaked austenite. After hot rolling and direct quenching, samples were rapidly re-austenitized at 900°C followed by water quenching to simulate an actual press hardening process. Especially in the case of Nb-Mo steel, a strongly pancaked austenitic structure was achieved after hot rolling, which led to a fine, uniform prior austenite grain structure after re-austenitization and quenching. The Nb-Mo steel had a tensile strength > 1500 MPa and ~ 11% total elongation combined with good impact toughness, which can be considered excellent for this type of press hardening steel.
The use of ultra-high strength steels, such as transformation induced plasticity (TRIP), dual phase (DP), complex phase (CP) and press hardening steels (PHS), has substantially increased in automotive applications mainly because of increased passenger safety, weight reduction, improved part functionality and fuel efficiency.1 However, in the lifting, handling and transportation industries, for example, press hardening is not widely used, mainly because of the modest toughness properties. Similar difficulties are to be expected when increasing strength by using higher carbon contents.
Despite the development of the press hardening process itself, the most widely used steel in press hardening is still 22MnB5, which has remained practically unchanged since the 1970s. It was originally developed for use as a low-cost wear-resistant steel in agricultural equipment, when customer demands were totally different from those today.
The beneficial effect of microstructural refinement on the toughness of martensitic steels has been verified by many studies.2, 3, 4, 5 Nb microalloying can be beneficial for achieving a more finely grained structure because of its ability to control austenite grain size during reheating and refine grain size during deformation.v It is also known that Nb and Mo by themselves can improve the hardenability of the steel by supressing the ferrite formation by reducing the carbon diffusivity and exerting a strong drag force on the moving grain boundaries.7 For example, the effect of Mo on delaying the ferrite formation is approximately three and six times stronger than that of alloying elements, such as Mn and Ni.8 Mo and Nb alloying can also improve hardenability in boron alloyed steels by increasing the amount of soluble boron at the grain boundaries. This is caused by the suppression of Fe23(C,B)6 precipitates by Nb and Mo alloying.9
Also alloying with combined additions of Nb and Mo is known to have synergetic influences providing more benefits than the sum of the effects of Nb and Mo alone. For example, Mo by itself does not have any significant impact on static recrystallization (SRX) behavior during hot rolling, but a combination of Mo and Nb leads to a synergistic increase in the retardation of SRX.10 In this research, the target is to see the effect of Nb and Nb-Mo alloying on the mechanical properties of a 22MnB5-based composition.
Materials and Heat Treatment
The two materials used in the present investigation were medium-carbon (~ 0.22 wt.%) steels containing nominally equal amounts of manganese (~ 1.2 wt.%), silicon (~ 0.2 wt.%), chromium ~ 0.2 wt.%), niobium (~ 0.05 wt.%), titanium and boron and two levels of molybdenum: 0 wt.% and 0.16 wt.%. Each steel was named after the contents of Mo and Nb, producing markings of Nb steel and Nb-Mo steel. The steels were vacuum cast into approximately 70-kg slabs at the Tornio Research Centre of Outokumpu in Finland; 180 × 80 × 55-mm pieces of the castings were homogenized at 1200°C for 2 h and thermomechanically rolled to approximately 8-mm-thick plates using six passes with approximately 20% reduction in each pass. The temperature of the samples during rolling was monitored by thermocouples placed in holes drilled in the edges of the samples to the mid-width at mid-length. The finish rolling temperature (FRT) was 850°C, after which the rolled strips were direct quenched using water quenching. The hot rolled samples were re-austenitized at 900°C for 5-min holding time followed by water quenching [cooling rate (CR) ~ 80°C/s] to simulate an actual press hardening process.
Longitudinal tensile tests and Charpy-V impact tests were performed to evaluate the strength and low temperature toughness properties, respectively. Tensile tests were performed using flat specimens with 8 mm thickness according to the ISO 6892-1:2009 standard.11 Charpy-V impact tests were performed at various temperatures (2 specimens/temperature) according to the European standard EN 10 045-1:199012 to derive the Charpy V-transition curves using 5 × 10 × 55-mm specimens.
Microstructural characterization was executed using light microscopy (LM) and field-emission scanning electron microscopy combined with electron backscatter diffraction (FESEM-EBSD). The linear intercept method applied to laser scanning confocal microscopy (LSCM) images from direct-quenched specimens etched with saturated picral in a soap solution was used to determine the prior austenite grain sizes in two principal directions—the rolling direction (RD) and strip normal direction (ND)—at the quarter thickness of the specimen. Based on these measurements, average grain sizes in RD and ND directions and total reduction below the recrystallization temperature (Rtot) were calculated.
EBSD measurements and analyses were performed using the EDAX-OIM acquisition and analysis software. FESEM for the EBSD measurements was operated at 15 kV using a step size of 0.15 μm. Lath and effective grain sizes were determined as equivalent circle diameter (ECD) values with low- (2.5–15°) and high-angle boundary misorientations (15–62.8°), respectively.
Results and Discussion
Comparing CCT diagrams with deformation (Fig. 2c and d), it can be seen that the Mo addition had a more significant effect on the phase transformation kinetics. In the presence of Mo, the formation of ferrite was prevented when cooling at 10°C/s (403 HV10 versus 278 HV10) and the martensite fraction was increased, giving a hardness of 506 HV10 even at a cooling rate of 20°C/s. Deformation accelerated the formation of ferrite and bainite, leading to lower hardness values at the lower cooling rates. Mo addition delayed ferrite/bainite formation, thereby leading to a wider range of cooling rates able to form fully martensitic microstructures. Based on the CCT diagrams for deformed specimens, the Nb steel requires a cooling rate of approximately 30–40°C/s to produce a fully martensitic microstructure, whereas the Nb-Mo steel requires only ~ 20°C/s. Earlier studies have shown that soluble Nb has a stronger effect on lowering the transformation temperatures compared with precipitated Nb.13 The effect of Mo on lowering the transformation temperatures and producing higher hardness values in Nb-alloyed steel can be due to molybdenum’s ability to increase the solid solubility of Nb in the austenite, which produces lower transformation temperatures, increasing the amount of bainite and decreasing the amount of ferrite with lower cooling rates.13,16
Prior Austenite Grain Structure After Hot Rolling
Average austenite grain sizes in two principal directions after hot rolling and direct quenching (dRD, dND), corresponding austenite reduction percentage below the recrystallization temperature (Rtot) and average grain size after re-austenitization and quenching (dre-aust.)
Table I also presents the prior austenite grain sizes (dre-aust.) after rapid re-austenitization and water quenching to simulate the actual prior austenite grain sizes after press hardening. Both investigated steels had a relatively small prior austenite grain size after re-austenitization and quenching (7.4 μm and 6.9 μm); see Fig. 4b.
Also grain misorientation angle distributions were determined using the EBSD data, and the results are presented in Fig. 6f. Misorientation peaks at ~ 7.5°, 16°, 52.5° and 59° were detected, which are a result of the different variants of the Kurdjumov–Sachs orientation relationship. High peaks at ~ 7.5° (sub-block boundaries) and ~ 59° (packet or/and block boundaries) are characteristics of martensite.18,19 Based on misorientation distributions, no clear differences between the different steels were apparent, which confirms the conclusion from the other microstructural studies that the investigated steels comprised mainly martensite.
Inverse pole figures (IPF) and grain maps are presented in Fig. 6a and d. Comparing the grain maps in Fig. 6c and d shows that a clearly finer grain structure is achieved in the Nb-Mo steel, which confirms the grain size results from the EBSD analyses.
Mechanical Properties After Hot Rolling, Direct Quenching and Re-austenitization
The hardness profiles through the thickness of the tested materials are presented in Fig. 7a. Both steels had a robust microstructure through the thickness, producing hardness values in the range of 501–519 HV10, which correlates well with the measured tensile strength values.
Transition curves were constructed based on Charpy-V impact test results obtained in the temperature range of + 80°C to − 80°C (Fig. 7c). Both investigated steels gave similar impact toughness results, having 34 J/cm2 transition temperatures in the range of − 64°C to − 68°C. The Nb-Mo steel had slightly lower upper shelf energy due to its higher tensile strength, although at the same time the 34 J/cm2 transition temperature was slightly lower. Previous studies have shown that the impact transition temperature is controlled by the largest grains in the grain size distribution, where grain size refers to the ECD of grains with grain boundary misorientations > 15°, for example d90%. 20,21 By alloying with Nb-Mo, a pancaked austenite structure was achieved after hot rolling, which then produced a finer and more uniform prior austenite structure after re-austenitization, presumably due to the presence of more nucleation sites for austenite grains. This led to a finer martensitic structure with higher strength and better impact toughness.
Summary and Conclusion
Nb-Mo microalloying allows lower cooling rates to achieve a fully martensitic microstructure, which could mean an easier press hardening process and higher plate thicknesses. CCT diagrams show that Nb-Mo microalloying delayed the formation of ferrite and bainite compared with Nb microalloying alone. This was especially so in the case of deformed, unrecrystallized austenite, when cooling at 20°C/s resulted in a hardness > 500 HV10.
After reaustenitization and water quenching, EBSD analysis showed that a smaller and more uniform grain structure was achieved with Nb-Mo microalloying rather than Nb microalloying alone. Grain misorientation distributions were typical of fully martensitic microstructures with no difference between the two investigated steels.
More than 1500 MPa tensile strength and ~ 11% of total elongation were achieved in tensile testing with both investigated steels combined with 34 J/cm2 transition temperatures of ~ − 65°C after re-austenitization and water quenching. Improved mechanical properties were achieved in the case of Nb-Mo alloying because of refinement of the martensitic microstructure.
Open access funding provided by University of Oulu including Oulu University Hospital. The financial support of the SSAB Europe Oy is gratefully acknowledged.
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