No abstract

Development of high strength or even ultra-high strength steels is mainly driven by the automotive industry which strives to reduce the weight of individual parts, fuel consumption, and CO2 emissions. Another important factor is to improve passenger safety. In order to achieve the required mechanical properties, it is necessary to use suitable heat treatment in addition to an appropriate alloying strategy. The main problem of these types of treatments is the isothermal holding step. For TRIP steels, the holding temperature lies in the field of bainitic transformation. These isothermal holds are economically demanding to perform in industrial conditions. Therefore new treatments without isothermal holds, which are possible to integrate directly into the production process, are searched. One way to produce high-strength sheet is the press-hardening technology. Physical simulation based on data from a real-world press-hardening process was tested on CMnSi TRIP steel. Mixed martensitic-bainitic structures with ferrite and retained austenite (RA) were obtained, having tensile strengths in excess of 1000 MPa.

Press-hardening is an intensively developing forming technology which is mainly used for the production of car body parts. Because it is a hot forming technology, small forming forces can be utilized and, due to the lower spring-back effect, more accurate products are achieved. In car bodies, materials with high energy absorption and a sufficient hardening coefficient are mainly used in impacted parts. One of these materials is TRIP multiphase steels with different chemical composition. In these steels, it is possible to achieve an ultimate strength up to 1000 MPa with the ductility of 20-30%. In order to achieve the desired properties, it is necessary to select a suitable heat treatment that allows to achieve a multiphase structure. Phase transformations and mechanical properties are influenced by the use of suitable alloying elements. Three low-alloy, multiphase TRIP steels with different chemical compositions with a carbon content of 0.2% were chosen for the experimental program. The first steel was alloyed only with manganese and silicon, in the second niobium was added, and in the third the influence of chromium on increase of hardenability and strength was investigated. Press-hardening was performed in a heated forming tool. To describe the effect of the cooling rate, the forming was carried out in a tool at room temperature and after preheating to 425°C. The influence of holding time in the tool at 425°C to support the formation of bainite and retained austenite stabilization was also investigated. Mixed ferritic-bainitic-martensitic structures with some retained austenite content were obtained.

Two oxide precipitation hardened (OPH) Fe-Al-O-based steels were prepared by mechanical alloying. In addition to the variant using yttria nano-precipitates to improve the mechanical properties, a variant using only alumina precipitates for strengthening was also prepared. Therefore, a more economically acceptable variant of these steels was developed. Hot consolidation is a significant production step for achieving the required mechanical properties. Hot consolidation was performed by either hot rolling or forging. The heat treatment was subsequently performed on both variants (0.85Fe–0.11Al–0.03Y2O3–0.01Y and 0.87Fe–0.11Al–0.02O2) of the alloys to support secondary recrystallization. The paper describes the influence of the consolidation method on grain size, material recrystallization, and mechanical properties. The difference in the consolidation method was reflected in the grain size after the heat treatment, where the material consolidated by hot rolling reached a grain size of almost 200 μm, while after forging the maximum grain size was around 30 μm. A higher ultimate tensile strength was achieved with forged states, both with and without the heat treatment.

Induction hardening is mainly used for treating parts in which high hardness, wear-resistance, and ductile core are required, such as spur gears, wind turbine rings, and bearings. Innovative induction hardening processes are also developed for complex automotive components, such as camshafts and crankshafts. In the process, steel is heated by passing high-frequency alternating current through a coil which is equivalent to the primary winding of a transformer. The material to be heat-treated is equivalent to a short-circuited secondary winding. The HDQT-R 30-12 device for incremental forming of round bars includes modules which can be used for heat-treating bars after rolling or even without rolling. Heating is provided by five induction coils at a faster rate than in a chamber furnace. In addition, there is less oxidation of the material surface. This paper presents findings related to microstructures and hardnesses in 42CrMo4, 13CrMo4, and S235JR steel grades after heating at different inductor coil power settings with subsequent hardening in a water bath.