Press-hardened 22MnB5 Steel Research Articles

Effect of heat treatment conditions on the fatigue resistance of press hardened 22MnB5 steel evaluated through rapid testing technique

Fatigue strength is considered as a crucial parameter for automotive applications subjected to cyclic loads during their long service life, as chassis parts. The high yield stress of press hardened steels poses them as good candidates for lightweight solutions with improved fatigue resistance. However, their high strength leads to an increase in notch sensitivity which can ruin the whole part’s integrity. This behaviour was observed in previous works on press hardened steels, where their high fatigue strength was significantly affected by the surface conditions and by heat treatment conditions. Nevertheless, press hardening steels are still good candidates to manufacture complex geometry parts reaching high performance.Aiming at increasing the existing knowledge on the fatigue behaviour of press hardened steels, this paper analyses the fatigue performance of boron steel (22MnB5) under different time austenitizing times. Fatigue resistance is evaluated using a novel rapid fatigue testing technique based on the stiffness evolution. The method permits a fast and reliable determination of the fatigue limit. Based on results obtained with this rapid testing method, the most suitable heat treatment to mitigate fatigue notch sensitivity and then achieving the best fatigue performance for chassis applications is discussed.

Effects of laser beam defocusing on high-strain-rate tensile behavior of press-hardened Zn-coated 22MnB5 steel welds

Zn-based coatings have emerged as viable alternatives to the conventional Al-Si-coating for 22MnB5 press-hardening steel. However, excessive weld concavity associated with laser beam welding of Zn-coated 22MnB5 imposes an adverse effect on dynamic tensile behavior of the welds after press-hardening. In the present study, laser beam defocusing is used to mitigate excessive weld concavity during laser welding of Zn-coated 22MnB5 steel. The high-speed digital image correlation (DIC) technique during high strain rate tensile testing confirmed that the modified laser weld concavity by beam defocusing can successfully shift the failure location from the fusion zone to the base material. Laser beam defocusing reverses the negative strain rate sensitivity of the welds which drastically improves the dynamic loading performance of Zn-coated 22MnB5 laser joints in automotive crash situations.

Influence of processing window on laser welding-brazing of Al to press-hardened 22MnB5 steel

Laser welding-brazing of AA6061 alloys and press-hardened 22MnB5 steel in lap configuration was carried out with Zn-Al15 filler metal. The processing window in different defocused distances was investigated and its influence on microstructure and mechanical properties of Al/steel dissimilar joints was explored. The results revealed that good appearances could be acquired in different processing windows with variation of defocused distances. The processing window was enlarged with increase of laser spot diameters, from laser power of 2.1–2.3 kW in defocused distance of +20 mm (laser spot diameter of 1.4 mm) to 2.8–3.6 kW in defocused distance of +40 mm (laser spot diameter of 2.4 mm). The thickness of interfacial intermetallic layer increased gradually from 0.47 to 1.86 μm for defocused distance of +20 mm to 3.76–11.03 μm of +40 mm. The newly formed phase at the interface at defocused distance of +20 mm was identified as Fe2(Al, Zn)5 and those were identified as the mixture of Fe2(Al, Zn)5 and FeZn10 when the defocused distances were +30 mm and +40 mm. The tensile-shear test of laser welded-brazed joints produced in various processing windows showed two types of fracture modes occurred. The interfacial failure appeared with the defocused distance less than +30 mm. The maximum fracture load reached 2793 N, representing 84.6% joint efficiency in regard to aluminum alloys. However, when the defocused distance increased to +40 mm, the maximum tensile-shear force decreased to 2242 N and failure occurred in the softening region of HAZ in aluminum alloy caused by effect of large laser irradiation. At larger defocused distance, the weak position of the dissimilar joints would transfer from the interface to the HAZ of the aluminum alloy, resulting in a bottleneck to improve mechanical property of joints.

Microstructure and mechanical properties of fiber laser welded QP980/press-hardened 22MnB5 steel joint

Fiber laser welding of QP980 steel and press-hardened 22MnB5 steel in butt configuration was performed in this study. The effect of heat input on microstructure evolution and mechanical properties of dissimilar joint was investigated. Laser power employed was varied from 1.6 to 2.4 kW while welding speed was kept constant at 2 m/min. The microstructure of fusion zone (FZ) was changed with the increasing laser power. Martensite and few bainite were observed when laser power ranged from 1.8 to 2.2 kW. Out of this range, the microstructure in FZ consisted of mixed structure of martensite and ferrite. Transformation from liquid phase to ferrite was promoted by Al-Si coating on the surface of press-hardened 22MnB5 steel. Heat affected zones (HAZs) of dissimilar joint were composed of martensite and bainite. Softened zone (SZ) appeared at the HAZ of 22MnB5 steel due to tempering behavior. The hardness of SZ and QP980 steel base metal declined with microstructure evolution. The fracture occurred at QP980 base metal when dissimilar joints were produced at laser power of 1.8 and 2.0 kW. When laser power reached 2.2 kW, the tensile samples failed at SZ owing to the increased softening degree. With further increasing laser power, the weakest region was FZ since ferrite declined the whole joint strength. A better Erichsen value was obtained at 2.0 kW (4.04 mm), which was balanced by more crash energy absorption and better deformation resistance.

Influence of Al–Si coating on microstructure and mechanical properties of fiber laser welded and then press-hardened 22MnB5 steel

The de-coated/coated 22MnB5 steel was laser welded and then press hardened in the present study, and the effect of Al–Si coating on microstructure and mechanical properties was comparatively investigated. The results showed that the fusion zone (FZ) of the de-coated joint was approximately composed of 90% (area fraction, the same as below) martensite and 10% (ferrite + bainite), while that of the coated joint contained about 40% martensite and 60% ferrite. The Al–Si coating dissolving in the FZ facilitated the formation of ferrite during the press hardening process. This mixed structure caused a low hardness of 300 HV in the FZ of the coated joint, and other regions composed of regular martensite showed hardness around 500 HV. The tensile strength of the de-coated joint was 1310 MPa, representing 89% joint efficiency relative to the base metal (BM). Ductile and brittle fracture characteristics were both observed. While only brittle fracture occurred for the coated joint showing poorer tensile properties. The digital image correlation (DIC) analysis revealed that the cracks initiated and finally cracked in the FZ for both joints. Both the break bending load and Erichsen value of the de-coated joint were also higher than those of the coated one. The martensite formed in the FZ of de-coated joint was uniform and contained high density of geometrically necessary dislocations (GNDs), which could lead to high strength and ductility due to the coordinated deformation among martensite grains. The large amount of ferrite in the FZ of coated joint led to larger grain misorientation and higher stress concentration.

Microstructural characteristics and mechanical properties of the Al–Si coating on press hardened 22MnB5 steel

The microstructural characteristics and mechanical properties of the phases in the substrate-coating interdiffusion layer of an Al–10%Si coated 22MnB5 press hardened steel (PHS) were studied by means of electron diffraction, chemical analysis, and nano-indentation. The interdiffusion layer of the Al–Si coated PHS, austenitized at 900 °C for 6.5 min, consisted of α-Fe(Al), Fe3Al, FeAl, Fe2Al5, and an Fe–Al–Si ternary intermetallic phase. The chemical analysis results indicated that the Fe3Al and Fe2Al5 phases were non-stoichiometric, and their composition ranges were not fully consistent with existing Fe–Al and Fe–Al–Si phase diagrams. These results partly explain the significant discrepancies between research groups in phase analysis of the interdiffusion layer. The observed Fe–Al or Fe–Al–Si phases were harder and more brittle compared to the martensitic matrix. With increasing austenitizing temperature and hold time, the thickness of the interdiffusion layer increased and the major constituent in the coating changed from Fe2Al5 to a multi-layered structure of FeAl, Fe3Al, and Fe(Al), leading to significant softening of the interdiffusion layer. Free bend testing was employed to investigate cracking and fracture behavior of the interdiffusion layer. The cracks formed in the thicker coating associated with the highest austenitizing temperature propagated deeper into the substrate material than in the case of lower austenitizing temperatures. Fracture of the interdiffusion layer involved transgranular cleavage and intergranular fracture features. The fracture surface appearance depended on both the phases present and the Al distribution in the coating. FeAl phase, known to be relatively “ductile”, was more susceptible to intergranular fracture than other phases (Fe2Al5, Fe3Al, and α-Fe(Al)).

Microstructure and dynamic tensile characteristics of dissimilar fiber laser welded advanced high strength steels

The desire for production of anti-crash weight-optimized modern vehicles has resulted in the development of tailor welded blanks (TWBs) of 3rd-generation advanced high strength steels (3G-AHSSs). Therefore, understanding crash response (dynamic loading: strain rate of 1–100 s−1) of TWBs is essential for deploying new classes of AHSSs. The present work investigates dissimilar fiber laser joints between press-hardened 22MnB5 steel - medium manganese transformation induced plasticity (MMn-TRIP), and TRIP1100 steels. Lack of mixing within the fusion zone has been evidenced in the MMn-TRIP and TRIP1100 sides of the joints through retention of considerable austenite within the structure and the non-uniform hardness distribution, respectively. It was found that the inter-critical heat-affected zone of the MMn-TRIP side of the joints with a large volume fraction of austenite is responsible for the initiation of Lüders bands (discontinuous yield). The detailed digital image correlation technique demonstrated that the rapid decomposition of martensite in the heat-affected zone of the 22MnB5, and TRIP1100 sides of the joints localizes the plastic flow. Although the strength of the dissimilar joints showed positive strain rate sensitivity, however, under the higher applied strain rates elongation to fracture shows a transition from positive to negative strain rate sensitivity as a consequence of the suppression of the TRIP effect.

Effect of galvanneal-coating evolution during press-hardening on laser welding of 22MnB5 steel

ABSTRACTUnderstanding phase evolution and growth kinetics of Zn-coating during press-hardening is essential in predicting welding behaviour of press-hardened 22MnB5 steel. The results showed that during press-hardening, the initial δ and Γ-Γ1 phases in the coating transform into α-Fe(Zn), Γ, and ZnO phases. As heat-treating continued at a constant temperature, the α-Fe(Zn) thickness grew and the overall Zn-content decreased. This resulted in the full-penetration weld window shifting towards higher heat-inputs. However, when heat-treating at higher temperatures, severe oxidation and Fe-Zn inter-diffusion led to the reduction of Zn-content in the evolved coating. It was found that higher heat-treatment times resulted in wider weld beads, thereby increasing joint strength.

Characterization of Static Performance and Failure of Resistance Spot Welds of High-Strength and Press-Hardened Steels

The static performances and failure characteristics of resistance spot welds (RSW) of steel sheets were studied in this work. Both similar and dissimilar joints of press-hardened steel grade 22MnB5 and as-rolled high-strength steel grade 1000 were examined. From metallographic analyses and micro-hardness measurements, it was shown that the microstructures of base metal (BM), heat affected zone (HAZ) and fusion zone of steel joints were noticeably different. In particular, the HAZ close to BM or subcritical HAZ (SCHAZ) of 22MnB5 steel exhibited greatly decreased hardness due to occurring tempered martensite, in which carbide precipitation was clearly observed. Furthermore, tensile shear test and cross-tension test of the welded steel sheets were carried out. The determined maximum loads, energy absorbability and corresponding failure mechanisms were analyzed and compared. The RSW samples of similar steel grade 1000 provided the highest load-carrying capacity and absorbed energy in both tests. Due to the large hardness difference and soft zone in weldment of dissimilar steel specimen, failure loads and static absorbed energies were deteriorated up to 17 and 68%, respectively. The observed fracture occurrences of all investigated joints were pullout failure mode, in which BM and HAZ were the crack initiation sites of grades 1000 steel and 22MnB5 steel, respectively. The SCHAZ of welded boron steel considerably caused the failure onset in tension shear test, but seemed to have negligible effect on cross-tension test.

Influence of austenitizing parameters on microstructure and mechanical properties of Al-Si coated press hardened steel

The influence of the austenitizing parameters on the microstructure and mechanical behavior of Al-Si coated press hardened 22MnB5 steel was evaluated in this study. Increasing the austenitizing temperature and hold time resulted in microstructural homogenization, an increase in the prior austenite grain size (PAGS), and increased thickness of the substrate-coating interdiffusion layer. The mechanical properties were evaluated using smooth-sided tensile testing, double edge-notch tensile testing, and free bend testing. Refinement of the PAGS was, in general, beneficial for the mechanical performance of the press hardened steel (PHS); i.e. strength, notch displacement, maximum bending load, and bend angle at maximum load all generally increased with decreasing PAGS. The stress and strain state experienced during each type of test influenced the sensitivity of mechanical and fracture behavior of the PHS to austenitizing conditions. Furthermore, the thickening of the substrate-coating interdiffusion layer, composed of brittle intermetallic phases, at higher austenitizing temperature negatively influenced bendability of the PHS. Ductile fracture was commonly observed in the steel matrix composed of a lath martensitic structure. However, quasi-cleavage fracture was observed for the martensitic matrix with the largest PAGS, which implies a loss of toughness, possibly because of the increased substructure size.

Microstructure based modeling of deformation and failure of spot-welded advanced high strength steels sheets

Microstructures and hardness of different weld zones of spot-welded high strength steel grade 1000 and press-hardened 22MnB5 steel were characterized. Tensile shear and cross tension test of welded specimens were conducted, in which load responses and fracture developments were identified. Then, samples with microstructures of each weld zone in all examined joints were generated by specific heat treatment processes. 2D representative volume elements (RVEs) for these microstructures were modelled. Flow stress curves of the individual phases were described by micromechanics based models under consideration of local chemical compositions and phase constituents. It was clearly shown that predicted overall stress-strain responses of all weld regions were well in accordance with the tensile test results of heat-treated samples. Furthermore, fracture criteria were determined for each weld zone by using samples having varying geometries for a wide range of stress triaxialities along with the digital image correlation technique. Finally, FE simulations of the tensile shear and cross tension tests of all investigated joints were carried out, in which the obtained flow stress curves and fracture criteria were defined for the individual weld regions. The experimental force-displacement curves up to fracture and observed failure occurrences could be correctly described by the simulation results.

Determining Material Data for Welding Simulation of Presshardened Steel

In automotive body-in-white production, presshardened 22MnB5 steel is the most widely used ultra-high-strength steel grade. Welding is the most important faying technique for this steel type, as other faying technologies often cannot deliver the same strength-to-cost ratio. In order to conduct precise numerical simulations of the welding process, flow stress curves and thermophysical properties from room temperature up to the melting point are required. Sheet metal parts made out of 22MnB5 are welded in a presshardened, that is, martensitic state. On the contrary, only flow stress curves for soft annealed or austenitized 22MnB5 are available in the literature. Available physical material data does not cover the required temperature range or is not available at all. This work provides experimentally determined hot-flow stress curves for rapid heating of 22MnB5 from the martensitic state. The data is complemented by a comprehensive set of thermophysical data of 22MnB5 between room temperature and melting. Materials simulation methods as well as a critical literature review were employed to obtain sound thermophysical data. A comparison of the numerically computed nugget growth curve in spot welding with experimental welding results ensures the validity of the hot-flow stress curves and thermophysical data presented.

Internal-friction analysis of dislocation–interstitial carbon interactions in press-hardened 22MnB5 steel

The dislocation–point defect interactions in press hardened 22MnB5 steel were characterized by the impulse internal friction technique. The analysis focused on the interaction between the interstitial carbon and the dislocation in the as-die quenched lath martensite and the effect of the paint-baking process on the carbon/dislocation interaction. The results showed that whereas the paint-baking affected the dislocation-enhanced Snoek peak, the Snoek-Kê-Köster peak was unaffected by the aging processes occurring during paint baking. A deformation carried out after the paint-baking resulted in the enhancement of the dislocation-enhanced Snoek peak.

Determination of hydrogen input in welded joints of press-hardened 22MnB5 steel

Over the last decades, press hardening or hot forming has become all pervasive for automotive body-in-white design. Press-hardened boron-microalloyed steels are widely used in modern body structure for high safety standard and lightweight achievement. Considering the welding of press-hardened components, some special considerations have to be taken into account compared to conventional deep-drawing steel grades. The welding process results in a significant hardness drop in the heat-affected zone (HAZ) which may lead to a change in failure mode or premature failure. Furthermore, the mechanical properties of the welded joints are influenced by the weld metal hydrogen content. Hydrogen pickup of press-hardened steel may result from the heat treatment or the welding process. Due to the martensitic microstructure and the high strength level, a critical hydrogen content may cause hydrogen embrittlement or hydrogen-assisted cold cracking (HACC). This paper contributes to the determination of the diffusible hydrogen content in resistance spot-welded and gas metal arc-welded joints of press-hardened boron-microalloyed steel. It also promotes the understanding of the effect of diffusible hydrogen on the mechanical properties of those joints. Hydrogen was deliberately introduced during welding by wetting of the sheet surface with hydrogenated fluids. The hydrogen content in the weld metal was quantified by thermal desorption mass spectrometry (TDMS) technique. Finally, the influence on the mechanical properties of the welded joints was determined based on a simple component test. A GMA-welded component was set under a constant static load and evaluated for delayed hydrogen-assisted cracking.

Effect of the cutting process on the fatigue behaviour of press hardened and high strength dual phase steels

Press hardened 22MnB5 steels are steady broadening their applications in vehicles since they allow to meet the increasing demands for weight reduction and safety standards. Press hardened parts have been typically applied as structural reinforcements where high rigidity and crash resistance are required. However, the need for further weight reductions in vehicles leads to potential applications in chassis areas, where parts must face up to cyclic loads. The fatigue behaviour of the press hardened 22MnB5 steel and the effect that post-forming processes (such as trimming or punching that usually follows press hardening) on the fatigue behaviour is scarcely known. Thus, the aim of this work is to analyse the fatigue behaviour of 22MnB5 press hardened steels cut using different strategies. The fatigue limit has been obtained in tensile samples of 22MnB5 with an Al–Si coating. Samples were cut by laser and shearing with two different clearance values. Results are compared to those obtained with a high drawing quality mild steel and a dual phase steel with a tensile strength of 1000MPa, DP1000. It is shown that the fatigue behaviour of press hardened 22MnB5 steels and DP1000 is governed by the defects introduced in cut edges, while the behaviour of the mild steel is almost independent of the cut edge quality. This finding indicates that high strength steels are markedly sensitive to pre-existent defects, such as burr, cracks at the cut edge or surface cracks, and they can be considered as low damage tolerant steels. On the contrary, mild steels are highly damage tolerant, i.e. the fatigue life is mainly independent of the initial defect size. In press hardened 22MnB5, even when cut edges have a high surface quality, the fatigue limit is still limited by the presence of cracks in the brittle Al–Si coating. Crack propagation due to the low damage tolerance capacity of the press hardened steel has been successfully rationalized through a fracture mechanics approach. Thus, if coated press hardened 22MnB5 steels are to be applied in vehicle components subjected to cyclic loads, they must be designed following fracture mechanics concepts to state the safe loading conditions for adequate fatigue resistance.