Martensite in Steels : its Significance, Recent Developments and Trends

Martensite is generally known as a hard but brittle microstructure. This is only true for high carbon plate martensite. Recently developed steels with a lath martensite microstructure offer an excellent toughness at yield strength of 1000 MPa yield strength. A transformation into lath martensite by glide as invariant shear mechanism is only possible at a carbon content below 0,03 %. The source of both high strength and good toughness is the high dislocation density and the narrow lath width off less than 1 µm. By a thermomechanical treatment, that leads to a finer lath structure both strength and ductility can be improved to a yield strength of 1150 MPa and an elongation of 18 %. As, unlike high carbon plate martensite, the hardness of lath martensite is not achieved by the distortion of the tetragonal cell by carbon atoms, the hardness of lath martensite remains stable up - during an annealing treatment up to 600°C. This thermal stability of the lath martensit microstructure makes an additional increase of hardness by the precipitation of different types of intermetallic phases possible. The increase of the hardness from 300 HV to 600 HV by precipitation without volume changes and good cold deformability reveals many new application in manufacturing. In plate martensite too, comparatively high toughness values can be achieved, if carbon is replaced by nitrogen. The refining influence of nitrides on the austenite grain sizes and the precipitation of fine nitrides during the annealing process leads to impact values three times higher than those of comparable high carbon plate martensite

Austenite reconstruction via EBSD measurements: a tool to understand low Carbon martensite steel properties

The basic characterization of the austenite grain size and shape prior to quenching to martensite was already used in the past to optimize the mechanical properties and impact toughness of low Carbon martensitic steel. This basic characterization can typically be done by optical microscopy. To better understand the mechanisms that generate the different properties, however, a more detailed analysis is required. An algorithm has been developed to calculate the austenite orientation starting from the martensite orientation measured by EBSD. The method has been applied to explain the different properties of martensite after austenitization and quenching and of direct quenched martensite with different levels of accumulated strain. It is shown that a small austenite size is needed to improve the impact toughness, regardless of the process route. The strength is strongly depending on the dislocation density in the austenite

Experimental determination and numerical prediction of the dynamic forming limits of a press hardened steel

Material characteristics such as yield strength, failure strain, strain hardening and strain rate sensitivity parameter are affected by loading speed. Therefore, the strain rate dependency of materials for plasticity and failure behavior is taken into account in crash simulations. Moreover, a possibility for consideration of instability at multi-axial dynamic loadings in crash simulations is the use of dynamic forming limit curves (FLC). In this study, the dynamic FLC of the press hardened automotive steel Usibor 1500 (AlSi coated 22MnB5) is investigated. The experimental results are obtained from a unique high-speed Nakajima setup. Two models are used for the numerical prediction. One is the numerical algorithm CRACH as part of the modular material and failure model MF GenYld+CrachFEM 4.2. Furthermore, the extended modified maximum force criterion considering the strain rate effect is also used to predict the dynamic FLC. The comparison of the experimental and numerical results are presented and discussed.Peer reviewe

Iowa Cow-calf Production - Exploring Different Management Systems

This project was designed to identify costs, environmental impacts, and best practices from Iowa cow-calf operations based on three production systems. Twenty-eight producers from across the state partnered with the Iowa Beef Center at Iowa State University to assess emerging beef cow management technologies, detail benchmarks, and summarize production and environmental data. Ultimately, the goal of the project was to develop decision aids and educational tools to assist Iowa cow-calf producers across all production systems and improve sustainability of the cow-calf segment in Iowa.</p