Press Hardening Steel 22MnB5 Research Articles

Characterization of Phase Transformations During Graded Thermo-Mechanical Processing of Press-Hardening Sheet Steel 22MnB5

Safety-relevant components in automobiles require materials that combine high strength with sufficient residual ductility and high-energy absorption. A graded thermo-mechanical treatment of the press-hardening steel 22MnB5 with graded microstructure can provide a material with such properties. Different austenitization temperatures, cooling and forming conditions within a sheet part lead to the development of microstructures with mixed phase compositions. To determine the resulting phase contents in such graded processed parts, a large number of dilatometric tests are usually required. With a non-contact characterization method, it is possible to detect local phase transformations on an inhomogeneously treated flat steel specimen. For press-hardening steel after heat treatment and thermo-mechanical processing, correlations between austenitization temperature, hot deformation strain, microstructure, and hardness are established.

Process development for self-pierce riveting with solid formable rivet of boron steel in multi-material design

Abstract Mechanical joining techniques are important joining processes for the implementation of lightweight structures in automotive engineering and other branches of industry. However, existing processes such as self-pierce riveting with semi-tubular rivet (SPR-ST) or solid rivet (SPR-S) reach their limits in certain applications, e.g. joining press-hardened boron steel sheets in multi-material constructions of automotive bodies. A new method to join these materials reliably and with high quality is the self-pierce riveting with solid formable rivet (SPR-SF). This paper shows different problems during the mechanical joining of press-hardened steel 22MnB5 by conventional SPR-S and illustrates the numerical as well as the experimental development of the new rivet concept: the solid formable rivet. On the basis of validated simulation models for conventional SPR-S numerical sensitivity analyses are carried out to investigate the correlations between the rivet properties (geometry parameters, material) and important quality characteristics of the joint like interlock, punching force and upsetting force for the new SPR-SF joining process. With the knowledge of these studies, specific rivet geometries are determined and prototypes manufactured for joining the material combination EN AW-6016 T4 (t = 1.5 mm) into 22MnB5 (t = 1.2 mm) in good quality. So, the application potential of the new SPR-SF is shown.

Effect of niobium precipitation behavior on microstructure and hydrogen induced cracking of press hardening steel 22MnB5

The effect of different content of Nb on the microstructure, mechanical properties and hydrogen trapping of press hardening steel (PHS) 22MnB5 was investigated. The function of Nb addition on the microstructure was studied by SEM, TEM and XRD, etc. which indicated that the microstructure of the steels containing Nb are finer than that of steel without Nb, what's more, the lath martensite is narrower with the increase of Nb content. Certainly, the tensile strength and elongation of hot-rolled steels, hot stamping steels also were measured. To understand the influence of adsorbed hydrogen on the mechanical property of PHS, Slow Strain Rate Test (SSRT) of smooth specimen was conducted. The results of precipitation behavior and its impact on hydrogen induced cracking manifested that hydrogen embrittlement susceptibility decreased obviously with the addition of Nb content due to reduced hydrogen diffusivity.

Towards virtual deformation dilatometry for the design of hot stamping process

Abstract Hot stamping is a well-established process for the production of structural automotive parts with an excellent strength-to-weight ratio. The complexity of the thermo-mechanical history in hot stamping requires a precise virtual design of hot stamping processes. The thermal and mechanical characteristics of the most common press hardening steel 22MnB5 were investigated comprehensively and are well represented by numerical models. For new steel compositions and parts involving tailored tempering, numerous experiments, e.g., on a deformation dilatometer or thermo-mechanical simulator are needed to characterise the microstructure evolution and the resulting properties. If appropriate material models were available, new steel compositions and thermo-mechanical histories could be partly explored virtually by simulated deformation dilatometer tests. This paper presents a comparison of experimental and virtual deformation dilatometer tests for the hot stamping steel 22MnB5. The virtual dilatometer is implemented as finite element model in the software LS-Dyna based on the material model *MAT248. With this model, virtual and real dilatometer curves correspond well only for conditions of full martensitic hardening, while pre-strains cause larger differences between model and experiments. When the most sensitive model parameters are formulated as a function of pre-strain, all experimental dilatometer curves can be reproduced by the model. Such a data-driven extension of the model seems to be effective for creating a virtual dilatometer model of high accuracy.

Laser Welding of Coated Press-hardened Steel 22MnB5

The press-hardening process is widely used for steels that are used in the automotive industry. Using ultra-high-strength steels enables car manufacturers to build lighter, stronger, and safer vehicles at a reduced cost and generating lower CO2 emissions. In the study, laser welding properties of the coated hot stamped steel 22BMn5 were studied. A constant 900°C temperature was used to heat the steel plates, and two different furnace times were used in the press-hardening, being 300 and 740seconds. Some of the plates were shot blasted to see the influence of the partly removed oxide layer on the laser welding and quality. The welding set-up, welding, and testing of the weld specimens complied with the automotive testing code SEP 1220.

Removal of oxides and brittle coating constituents at the surface of coated hot-forming 22MnB5 steel for a laser welding process with aluminum alloys

Abstract The surface of a press-hardened steel 22MnB5 coated with Al-base (AlSi10Fe3) and Zn-base (ZnNi10) was conditioned by a pulse laser and by sandblasting to remove undesirable oxides and brittle phases. Oxides formed on coating surfaces counteract the wettability of welding filler during a welding or brazing process. Furthermore, welding and brazing joints of 22MnB5 coated with aluminum alloys failed along the brittle intermetallic phases in the coating under a low mechanical load. Treated 22MnB5 surfaces were analyzed microscopically, and the phase compositions were investigated by synchrotron diffraction measurements. It was found that brittle phases could be locally removed by laser ablation; however, high laser energies led to remelting and oxidation of the coating surface. In contrast, sandblasting homogenously removed oxides and brittle intermetallic phases. Surface-treated 22MnB5 steel sheets were joined to AA6016 aluminum sheets by laser welding, and the strength of the weldment was determined by tensile tests. The measured mechanical strength of the aluminum/steel joints was 210–230 MPa. Failure of the weldments under tensile loading occurred within the aluminum sheet, away from the steel surface/welding filler interface if brittle coating components and oxides were removed homogenously.

A multiscale perspective on the kinetics of solid state transformations with application to bainite formation

We give an excerpt of recent developments in the experimentally benchmarked modeling of bainite formation in the press hardening process. As the press hardening process poses a heavily multi-parameter dependent modeling challenge, we focus on three main branches which complement each other. We emphasise the combination of basic sharp interface and phase field models with pragmatically adapted multi phase field models and experimentally parametrized implementations of the Johnson-Mehl-Avrami model. In the basic thermodynamic modeling part, we review fundamental aspects of displacive and diffusional-displacive transformations to predict dominant transformation morphologies. These results provide a link to multi-phase-field implementations which allow to simulate isothermal bainitic transformations, supported by available material data from thermodynamic databases. Excellent agreement with experiments, e.g. scanning electron microscopy for the transformed bainite in the high-carbon steel 100Cr6 shows the value of these model implementations. The further connection to Johnson-Mehl-Avrami models offers to extend the understanding to transformation plasticity for the press hardening steel 22MnB5.

Solutions for Hydrogen-Induced Delayed Fracture in Hot Stamping

One of the main targets in automotive industry is to reduce the weight of vehicle as well as increase the safety. To accomplish this goal, press-hardening steel and hot stamping parts have been used in car body. However, the possibility of hydrogen-induced delayed fracture (HDF) of hot stamping parts exits, which will decrease the car’s passive safety. A solution has been presented to reduce the sensitivity of HDF and improve hydrogen-induced delayed fracture resistance (HDFR) by Niobium micro-alloying technology. Traditional press-hardening steel 22MnB5 and new steel 22MnBNb2, 22MnBNb5 and 22MnBNb7 were studied, and it is shown that the appropriate addition of Nb is beneficial to the improvement of the delayed fracture resistance of the hot stamping steel, which indicates that Niobium micro-alloying technology is an effective solution to the HDF in hot stamping steels.

Short-time heat treatment of press hardened steel for laser assisted clinching

Clinching of high strength steels is currently difficult due to their relative low ductility. Hence a new method has been developed to clinch these steel grades. This could be achieved by heating the sheets locally at the joint by a laser during the clinching process. The short-time tempering behaviour of the press hardened steel 22MnB5 has been investigated by dilatometry respectively calorimetry experiments. Different stages of the tempering process have been identified depending on heating rates up to 1000 K s−1. Characteristic tempering reactions were shifted to higher temperatures with increasing heating rate. Based on these results, suitable short-time heat treatment parameters have been selected and transferred to the clinching process. Thereby the press hardened steel 22MnB5 has been successfully clinched with laser assistance for the very first time. The laser assisted clinched joint has been characterised by metallographic analysis and hardness testing. It could be shown, that in the clinching joint only a slight strength loss occurs due to the introduced laser heat.

Softening of High-Strength Steel for Laser Assisted Clinching

The conventional clinching of steels is currently limited to tensile strength less than 800 N/mm2 and to elongation at fracture more than 14 %. To realise the clinching of high-strength steels, the sheet can be heated locally at the joint, to improve ductility. Thereby the material characteristics outside the joint should be maintained. This could be achieved by means of short-time laser heating. The short-time tempering behaviour of press hardened steel 22MnB5 has been analysed. The mechanical properties during a short-time heat treatment were investigated by thermo-mechanical analysis in a deformation dilatometer. Thereby laser-assisted clinching shall be established and an efficient form-closed and force-closed connection shall be produced. As a result, the press hardened steel 22MnB5 could be clinched by laser assistance for the very first time.