22MnB5 Steel Research Articles

Investigation of the Microstructural Evolution during Hot Stamping of a Carburized Complex Phase Steel by Laser-Ultrasonics.

Prior carburization of semi-finished steel sheets is a new process variant in hot stamping to manufacture parts with tailored properties. Compared to conventional hot stamping processes, a complex phase typed steel alloy is used instead of 22MnB5. Yet recent investigations focused on final mechanical properties rather than microstructural mechanisms cause an increase in strength. Thus, the influence of additional carburization on the microstructural evolution during hot stamping of a complex phase steel CP-W®800 is investigated within this work. The phase transformation behavior, as well as the grain growth during austenitization, is evaluated by in-situ measurements employing a laser-ultrasound sensor. The results are correlated with additional hardness measurements in as-quenched condition and supplementary micrographs. The experiments reveal that the carburization process significantly improves the hardenability of the CP-W®800. However, even at quenching rates of 70 K/s no fully martensitic microstructure was achievable. Still, the resulting hardness of the carburized samples might exceed the fully martensitic hardness of 22MnB5 derived from literature. Furthermore, the carburization process has no adverse effect on the fine grain stability of the complex phase steel. This makes it more robust in terms of grain size than the conventional hot stamping steel 22MnB5.

Depth profiling approach to evaluate the influence of hot stamping on the local electrochemical behaviour and galvanic series of hot-dip Al-Si coating on 22MnB5 steel

The influence of hot-stamping process on the corrosion properties of Al-Si coating on 22MnB5 steel was locally evaluated using a depth profiling approach combining GDOES and electrochemical micro cell. The results highlight the complexity of the system and the high dependence of the local galvanic series on the hot-stamping process. Due to iron diffusion from the steel substrate into the coating, the Al-rich layers presented active behaviour, whereas the initial coating condition showed a passive behaviour. Conversely, Fe/Si-rich sublayers acted as protective barrier for the steel substrate, as they show passive behaviour with nobler corrosion potentials and lower current densities.

Design Study for Automatic Production Line of a Sub-Assemblies of New Generation Car Body Structures Compliant with the “Industry 4.0” Concept

A design study of automatic line-to-production of a new generation of car body structures compliant with the Industry 4.0 concept is described in this paper. The line is based on the hot-stamping technology of components of a car body structure from 22MnB5 steel sheets. Additional modules of the designed production line are: laser-trimming station, station to completion (kitting-up), and spot-welding station of the subassemblies. Technical requirements to be complied with by such line and scheme of exchange of information between modules of the line were defined. The conclusions were formulated.

Effect of coating thickness on the in-situ reflectance and surface roughness of Al-Si coated 22MnB5 steel

A major challenge inherent to hot stamping aluminized 22MnB5 blanks involves the liquefaction of the protective Al-Si coating, followed by its transformation into an intermetallic Al-Fe-Si layer. The intermediate liquid state contaminates the furnace rollers and causes the blanks to migrate within the furnace, while the evolving surface topography of the intermetallic coating impacts the radiative properties of the blanks as well as the subsequent quenching rate. This study investigates how the heating profile and coating thickness impact the solidification process and surface roughness using in-situ laser-based reflectance measurements on blanks with two coating thicknesses. Temporally resolved reflectance measurements demonstrate that the duration over which the coating is liquid is approximately halved for blanks with the thinner coating. Moreover, in-situ light scattering measurements show that blanks with the thinner coating reach a higher maximum surface roughness of approximately 2.75 μm compared to 2.65 μm for blanks with the thicker coating. These results are also connected to ex-situ scanning electron microscope images and energy-dispersive X-ray spectroscopy measurements, which depict the growth of iron-rich intermetallic compounds Al4.5FeSi, Al7Fe2Si, Al5Fe2, and Al2Fe during heating.-

Determining Forming Limit Diagrams Using Sub-Sized Specimen Geometry and Comparing FLD Evaluation Methods

Sheet metal forming boundaries are established using the forming limit diagram (FLD). The Nakajima and Marciniak tests, which are based on stretching a material using a punch, are the most commonly used methods for determining the FLD or fracture forming limit diagram (FFLD). The results are usually evaluated by calculating local strain, strain rates, specimen thickness reduction or fracture strain. When the amount of experimental material is insufficient, miniaturization of the testing specimens may be a solution. However, the interchangeability of the results for standard and miniaturized specimens has not been proven yet. In this study, the Nakajima tests were performed using standard and sub-sized specimens made of manganese–boron steel 22MnB5, commonly used in the automotive industry. Afterwards, four FLD/FFLD evaluation methods were applied and compared. The miniaturized specimens yielded higher strain values, which was explained by the varied ratio of material thickness/punch diameter and different bending conditions. The highest compliance of the results was recorded for the standard and miniaturized FFLD.

High temperature tribological behaviour of additively manufactured tool material for applications in press hardening

In hot sheet metal forming, minimising material transfer and ensuring homogenous cooling of the workpiece are important for quality and productivity reasons. The development of additive manufacturing (AM) opens up possibilities for novel tooling concepts by tailoring the tool material (new chemistries, microstructure, and size/distribution of hard phases) and producing conformal cooling channels. Implementing surface functionality for improved tribological performance is another advantage of AM. Research pertaining to high temperature tribological behaviour of AM tool materials is, however, very limited.The aim of this work is to study the high temperature friction and wear behaviour of AM produced maraging steel and compare this to a conventional hot-work tool steel. The AM maraging steel was post-machined to different surface conditions (milled, ground and shot blasted).The tribological behaviour was evaluated using a hot-strip drawing tribometer and the counter surface was an Al–Si coated 22MnB5 steel. The temperatures of the Al–Si coated steel strips were 600 and 700 °C during the tribological tests.The results have shown that the friction behaviour of the maraging steel and the hot-work tool steel at 600 °C was stable and very similar. At 700 °C, the maraging steel showed more unstable friction and early failure of the tests due to high friction compared to the hot forming tool steel. This was associated with increased material transfer and embedment of FeAlSi intermetallics from the workpiece surface.The maraging steel experienced material removal and development of transfer layers composed of debris from both surfaces in contact. Comparable wear behaviour was observed at both temperatures, but its severity increased with temperature. The milled and ground surfaces showed similar wear mechanisms including ploughing and delamination of material, as well as embedment of FeAlSi particles. The shot blasted surface showed less build-up of transferred material but instead more deformation and folding of asperities leading to entrapment of FeAlSi particles in the near surface region.

Hot Stamping Process Simulation of 22MnB5 Steel Based on Identified Heat Boundary Condition

Finite element (FE) simulation is the powerful tool for an analysis to thermo-mechanical coupling process and helps the manufacturer in understanding the effect of the various parameters in the hot stamping process better. The simulation results are strongly dependent on the accuracy of boundary condition. In this paper, the heat boundary condition is firstly obtained based on an inverse analysis method. The characteristics of heat transfer in the interface between the sheet and the die was also analyzed. Then, the numerical simulation to the whole hot stamping process was made with simulation software by applying the identified heat boundary condition. The good agreement between the calculated and experimental temperatures demonstrates that the inverse analysis method is a feasible and effective tool for identifying the sheet-die heat boundary condition.

Study on the constitutive equation of hot forming of boron steel sheet

The high temperature constitutive equation of boron steel sheet can describe the relationship of the thermodynamic parameters on the hot forming process of boron steel sheet. It can reflect the relationship between flow stress and strain, the temperature and strain rate, and is indispensable in the forming simulation of hot stamping forming process. In this paper, around the ultra-high-strength boron steel sheet 22MnB5, conduct thermodynamic tensile test on it. And the high-temperature constitutive equation of the high-strength steel sheet under the high-temperature austenite state is established according to the deformation theory. Finally, it is confirmed that the improved Norton-Hoff constitutive equation fitted with the experimental results is in good agreement with the flow stress relationship of boron steel 22MnB5.

Optimizing weld morphology and mechanical properties of laser welded Al-Si coated 22MnB5 by surface application of colloidal graphite

In the automotive industry, the demand for reduced vehicle weight, improved safety and enhanced crashworthiness qualities continues to rise which introduces the need for parts with tailored properties. This demand can be met by employing tailor-welded blanks which are made of two or more types of press-hardened steels that are laser welded together and then hot-stamped to produce highly optimized parts for use in vehicle body-in-white applications. The most commonly used press-hardened steel for high strength applications is the Al-Si coated 22MnB5 grade steel that has a strength of around 1500 MPa in the hot-stamped condition. Although the Al-Si coating offers excellent barrier protection against oxidation and decarburization during hot-stamping, the Al-Si coating proves to be problematic during laser welding as it melts and mixes into the weld pool increasing the Al-content in the fusion zone, which promotes the formation of ferrite during the heat-treatment stage of hot-stamping, resulting in a dual-phase fusion zone microstructure that consists of ferrite grains embedded in a martensitic matrix. The presence of the softer ferrite phase has been shown to be the principal reason for premature failure of hot-stamped laser-welded joints. Currently, this issue is resolved by an additional, and relatively costly, manufacturing step that involves the laser ablation of the Al-Si coating prior to welding. The present work shows that this step can be eliminated from the manufacturing process by welding Al-Si coated 22MnB5 steel through a colloidal graphite coating which results in the in-situ ablation of the Al-Si coating during welding. This changes the alloying chemistry of the fusion zone which affects the resulting morphology by preventing ferrite formation which improves the mechanical properties of the weld. Failure in these welded samples was successfully shifted from the fusion zone, where it normally occurs, to the base material.

Relationship between mechanical properties in 42SiCr and 42SiMn medium-carbon steels and austempering temperatures

Sheet metal forming boundaries are established using the forming limit diagram (FLD). The Nakajima and Marciniak tests, which are based on stretching a material using a punch, are the most commonly used methods for determining the FLD or fracture forming limit diagram (FFLD). The results are usually evaluated by calculating local strain, strain rates, specimen thickness reduction or fracture strain. When the amount of experimental material is insufficient, miniaturization of the testing specimens may be a solution. However, the interchangeability of the results for standard and miniaturized specimens has not been proven yet. In this study, the Nakajima tests were performed using standard and sub-sized specimens made of manganese–boron steel 22MnB5, commonly used in the automotive industry. Afterwards, four FLD/FFLD evaluation methods were applied and compared. The miniaturized specimens yielded higher strain values, which was explained by the varied ratio of material thickness/punch diameter and different bending conditions. The highest compliance of the results was recorded for the standard and miniaturized FFLD

Determination and predication of formability on 22MnB5 steel under hot stamping conditions using Gleeble

ABSTRACT Hot stamping process is a thermo-mechanical process in which blank is heated to austenitization temperature and then subsequent forming and quenching are done simultaneously. Due to complex process cycle in hot stamping conditions, like high temperature and cooling etc., determination of formability is difficult. Therefore, in this work a series of experiment was performed on thermomechanical simulator (Gleeble-3800) at different temperatures (750oC, 800 oC, 850 oC) and strain rates (0.01/s, 0.1/s, 1/s) to obtain flow behavior and formability of 22MnB5 Steel. Due to limitation of the uniaxial deformation in Gleeble-3800, modified tensile specimen was used to obtain plain strain conditions. Thickness gradient-based necking criterion was validated for high temperature forming at different strain rates. This criterion was further used to determine the formability of 22MnB5 steel. Thickness distribution and thickness gradient for modified specimen and simple tensile specimen is observed under various conditions. It was found that with increase in temperature and strain rate formability of 22MnB5 steel increased as well as uniform deformation occurs. Thickness gradient necking criteria indicates that at point of necking thickness gradient ratio falls below the critical value of 0.92. .

Influence of Thermal Modification on Al-Si Coating of Hot-Stamped 22MnB5 Steel: Microstructure, Phase Transformation, and Mechanical Properties

The hot-stamped steel with ultrahigh strength is a promising material for the fabrication of automotive components. However, the coating on the sheet surface leads to a softening problem in the welded joint. Instead of the costly coating removal process, heat treatment is an economical and effective method for the diffusion process, which can decrease the Al concentration in the coating. In this study, a preheating treatment was carried out on Al-Si-coated 22MnB5 hot-stamped steels for the homogeneity of Al, followed by laser welding and hot stamping. The effects of the preheating on the microstructure and mechanical properties of the laser-welded joints were investigated. With the preheating treatment, the Al-Si coating transformed into an Fe-Al intermetallic compound and the difference in Al content between the coating and substrate was reduced. The Al content in the weld of the specimen with the preheating treatment was reduced, compared with that without the preheating treatment. The amount of δ-ferrite in the weld after laser welding was reduced largely. The distribution of long-bland-like segregation was changed to a fine and uniform distribution. With the preheating treatment, the tensile strength of the welded joint was significantly improved and comparable to that of the decoated joint. In conclusion, the preheating treatment before the welding is an effective method to suppress the formation of δ-ferrite and improve the mechanical properties of the welded joint.

Role of Vanadium Carbide in Hydrogen Embrittlement of Press-Hardened Steels: Strategy From 1500 to 2000 MPa

This work investigated hydrogen trapping and hydrogen embrittlement (HE) in two press-hardened steels, 22MnB5 for 1,500 MPa grade and 34MnB5V for 2000 MPa grade, respectively. Superior to the 22MnB5 steel, the newly developed 34MnB5V steel has an ultimate tensile strength of over 2000 MPa without sacrificing ductility due to the formation of vanadium carbides (VCs). Simulated press hardening was applied to two steels, and hydrogen was induced by cathodic charging. Susceptibility to HE was validated by slow strain-rate tensile test. When hydrogen content was high, the 34MnB5V steel fractured in elastic regime. However, when hydrogen content was 0.8–1.0 ppmw, the 34MnB5V steel bore much higher stress than the 22MnB5 steel before fracture. The behavior of hydrogen trapping was investigated by thermal desorption analyses. Although the two steels trapped similar amounts of hydrogen after cathodic charging, their trapping mechanisms and effective trapping sites were different. In summary, a finer prior austenite grain size due to the pinning effect of VCs can also improve the toughness of 34MnB5V steel. Moreover, trapping hydrogen by grain boundary suppresses the occurrence of hydrogen-enhanced local plasticity. Microstructural refinement enhanced by VCs improves the resistance to HE in 34MnB5V steel. Importantly, the correlation between hydrogen trapping by VCs and improvement of HE is not significant. Hence, this work presents the challenge in designing irreversible trapping sites in future press-hardened steels.

Characteristics and mechanisms of hydrogen-induced quasi-cleavage fracture of lath martensitic steel

This study presents an in-depth characterization of the microstructures, crystallographic orientations, and dislocation characteristics beneath hydrogen-induced quasi-cleavage fracture features of an as-quenched, lath martensitic (α') 22MnB5 steel. The fracture surfaces of gaseous hydrogen-embrittled martensitic specimens were analyzed by a combination of multiple analytical tools: scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), and transmission Kikuchi diffraction (TKD). The dominant fracture mode in the hydrogen-affected zones was quasi-cleavage fracture, which involved significant plasticity, evidenced by plastic zones near tear ridges and a high density of dislocations beneath the quasi-cleavage facets. The martensite constituent sizes, variant orientations, and boundaries influenced the quasi-cleavage surface morphologies. Fractography revealed the occurrence of {100}α'-type cleavage across martensitic laths, developing relatively “flat” quasi-cleavage surfaces, in addition to {110}α'-type cracking likely along lath and block boundaries, and fracture along non-cleavage planes. The likelihood of the formation of the relatively “flat” quasi-cleavage surfaces increased with increasing martensitic constituent sizes. Substantial dislocation bands formed on intersecting {112}α' slip planes within a martensite block below the hydrogen-induced cleavage fractures on {100}α' planes. River markings on the quasi-cleavage surfaces were found to originate from the complex, hierarchical lath martensitic microstructure. Steps and ridges on the quasi-cleavage facets generally linked with the various martensitic boundaries, suggesting that they were produced as a result of crack deviations at those boundaries. The fracture paths and martensite quasi-cleavage mechanisms are discussed in the context of the role of hydrogen and Cottrell cleavage model.

Thermomechanical analysis on the frictional contact behavior of a high-strength steel 22MnB5–die steel H13 tribopair at 800 °C by experiment and finite-element simulation

High-strength boron steels are widely used in manufacturing the auto bodies and parts of light-weight vehicles, but the high rates of surface scratches and die wear have consistently occurred during hot stamping for these steels. For an in-depth understanding of the tribological characteristics at this interface, the frictional contact behavior and thermomechanical mechanisms of boron steel 22MnB5 against die steel H13 at 800 °C were studied through experiments and finite-element simulations. The coefficient of friction and worn surface topography were investigated by pin-on-disk sliding tests. A three-dimensional thermomechanical finite-element model of a friction pair was established to explore the interfacial dynamic variations. Experimental and simulation results show that severe elastic–plastic deformation occurred on the worn surface of the boron steel, whereas an increase in the load decreased the coefficient of friction within a certain range because the growth rate of shear force was slower than that of the normal force. When the finite-element model was changed from the gradual loading stage to the initial sliding stage, the tangential friction force further increased the plastic deformation on the surface of boron steel. The scratches and furrows were mainly caused by the compression and shear from asperities of the rough surface, as confirmed by the high-frictional-stress regions concentrated on the peaks and flanks of asperities. During the high-temperature and high-pressure experiments, the plasticized and softened surface materials of the boron steel adhered to the die surface readily, resulting in peeling and delamination.

Comparison of dry and lubricated wear behaviour of 22MnB5 uncoated ultra high strength steel

Objective of this work is to effectively enhance the working life of boron steel (22MnB5) having application in agricultural area, where the most common problem is wear. Boron steel also known as medium carbon steel. It has carbon content ranging from 0.19 to 0.26 by weight percentage. Specimens were made ready and wear tests were conducted using pin-on-disc tribometer. SEM and XRD analysis techniques were used to study the microstructure and phase morphology of worn out surfaces. Specimens were tested both in wet and dry conditions using lubricating oil. Wear tests were carried out at 3 different load conditions in 7 cycles. Results have shown that under lubricated conditions even at high loads, wear rate is less and thus volume loss is also negligible. This study is expected to provide the insights into the wear behaviour of 22MnB5 uncoated ultra high strength steel (UUHSS) and contribute to the development of more wear resistant materials for agricultural applications.

Tests of Technological Conditions of the Resistance Welding and Adhesive Bonding of Coated Steel Sheets

The article presents the effect of welding technological parameters and adhesive bonding conditions (surface processing, overlap dimensions and the thickness of the adhesive layer) on the formation of hybrid (welded-adhesive bonded) joints of steel sheets provided with protective coatings. The tests involved the joining of sheets made of steel HC340LA. The sheets made of steel HC340LA (provided with a ZE50/50 Granocoat ZE two-layer organic coating) were in the as-received state and after cleaning with isopropyl alcohol. The tests also involved sheets made of steel 22MnB5 provided with an AlSi coating. The sheets made of steel 22MnB5 were subjected to heat treatment, cleaning with isopropyl alcohol, roughening and etching in similar configurations. The adhesive bonding process was performed applying one-component epoxy adhesive (used in the automotive industry).

State of Strain and Development of Microstructure of 22MnB5 Steel and Al-Si Coating During Deep Drawing of Automotive B Pillar

State of Strain and Development of Microstructure of 22MnB5 Steel and Al-Si Coating During Deep Drawing of Automotive B Pillar

Three dimensional numerical analysis of heat transfer during spray quenching of 22MnB5 steel with a single nozzle

In this paper, a three dimensional Computational Fluid Dynamics Model (CFD) model was generated for spray cooling of straight parts by using a single point nozzle. The numerical simulations of spray quenching were performed to investigate the cooling rate of a 22MnB5 hot steel blank commonly used material in the automotive industry. Experiments are carried out to examine infrared thermograph of hot steel blank surfaces during spray cooling process by using infrared thermal camera. Time dependent situations of surface temperatures, surface heat fluxes and cooling areas are investigated numerically. The 3D model with some assumptions is presented to improve the solution of numeric simulation and to meet the most appropriate acceptable results for spray cooling process. The developed 3D model will be used to investigate the heat transfer analysis of hot stamped parts during hybrid quenching to optimize the process parameters. The comparison of the numerical and experimental results showed that the presented approach can be effectively used to evaluate heat transfer during spray quenching with a single nozzle. With this research, it was clearly seen that the estimated cooling rates and the temperatures were in good agreement with the experimental cooling temperature data. By using the numerical results and experimental data obtained in this study, multi-nozzle spray apparatus which will be used to obtain different cooling rates on the parts by changing spray parameters such as spray height, spray angle, mass flow rate, the distance between nozzles and part etc. was developed for hybrid quenching process.

Evaluation of 22MnB5 Steel Austenitization Sub-Models for Simulating the Heating Phase of Hot Stamping

The heating phase of hot stamping is the most time-and energy-intensive part of the process, and therefore offers significant potential to improve process efficiency. This requires a robust thermometallurgical model that predicts the blank heating profile and the austenitization progress inside a roller hearth furnace. In this paper, an IM austenitization model for 22MnB5 steel is derived using dilatometry data and evaluated against a JMAK type model using temperature profiles of coupons heated within a laboratory-scale muffle furnace and blanks heated within a roller hearth furnace. The austenitization rates predicted by the two models are then compared to the inferred austenitization processes from those temperature profiles. The result shows that the IM model has potential to capture the trend of austenitization process, especially the rapid pearlite dissolution at the beginning of the transformation, whereas the JMAK type model is too restrictive and fails in this regard.