Influence of Cold Forming and Heat Treatment on the Microstructure and Mechanical Properties of an Air‐Hardening Steel

Abstract

The use of air‐hardening steels in the manufacture of automobile body components shortens the corresponding process chain by means of eliminating the immersion or gas quenching operation since hardening occurs in still air. To choose the optimal forming and heat‐treatment parameters, the development of the microstructure needs to be considered as a consequence of the thermomechanical production processes. The object of this work is to investigate the effects of forming and the subsequent heat‐treating on the changes of the microstructural parameters for the air‐hardening steel LH800. For this purpose, specimens were processed under different forming and heat‐treating conditions and subsequently examined by means of SEM and mechanically tested using tensile tests. Based on this, an empirical equation was derived which describes the influence of both the martensite's bundle thickness and the original austenite's grain size on the yield stress. In this way, microstructural parameters can be identified which lead to good mechanical properties. The measured correlation can thus be used for modeling the LH800 steel's forming and heat‐treatment processes.

Formation and Properties of Mixed Ferritic–Martensitic Microstructures in the Air‐Hardening Steel LH800

“…Another approach to create a dual phase microstructure is the controlled cooling from austenitizing temperatures to room temperature by usage of, e.g., specified cooling rates or stepwise cooling. Here, the CCT diagram of this steel reveals the formation of a small amount of ferrite when cooling within 20 min from austenitizing temperatures to 700°C. This does not correspond to the here applied free air quenching conditions.…”

“…The change of the microstructure and the properties due to cold plastic deformation and subsequent air-hardening, as well as the interaction between the yield stress and microstructural parameters were studied in. [8] The tensile strength, achieved by a combined treatment with optimal parameters exceeded 1000 MPa. Even higher strength values of about 1100 MPa were induced using press-hardening.…”

“…Based on dilatometric tests, metallographic analysis and a series of hardness measurements the time–temperature‐austenitizing (TTA)‐austenite grain size diagram (see Figure ), the martensite transformation curve and the tempering diagram were obtained. The change of the microstructure and the properties due to cold plastic deformation and subsequent air‐hardening, as well as the interaction between the yield stress and microstructural parameters were studied in . The tensile strength, achieved by a combined treatment with optimal parameters exceeded 1000 MPa.…”

Evolution of Microstructure and Properties of Steel 22MnB5 due to Short Austenitization with Subsequent Quenching

“…In the above‐mentioned study, an influence of a relatively high tool temperature and a short austenitization on the properties of steel 22MnB5 were investigated. Nevertheless, as it has been reported by other researchers, lower temperature during austenitization is more preferable for achieving the finer prior austenite grain size and thus the improvement of mechanical properties …”

“…Nevertheless, as it has been reported by other researchers, lower temperature during austenitization is more preferable for achieving the finer prior austenite grain size and thus the improvement of mechanical properties. [15,18,19] In addition, previous studies demonstrated that the initial microstructure influences the a-g transformation and properties after cooling. Therefore, this influence should be also investigated for steel 22MnB5, which is available in a wide range of possible initial microstructures.…”