Papers by Author: Klaus Hulka

Abstract: Steels with bainitic microstructures show the capacity to fulfil the requirements of high strength and low temperature toughness necessary for plate steels in specialised industrial constructions. The introduction of steels with higher strength allows for weight reductions of steel constructions. This paper investigates the development of hot rolled structural plate steels through laboratory hot rolling simulations of thermo-mechanically controlled processes (TMCP). Specific alloying and microalloying along with an optimised TMCP process has allowed high tensile properties to be achieved in combination with high levels of toughness. Tensile strengths of up to 900 MPa have been achieved with Charpy V-notch toughness greater than 200J at –40°C. Elements such as molybdenum, niobium and boron have been added to low carbon steels to promote the formation of fully bainitic microstructures with much lightened chemical compositions. The presented concepts allow the production of steel grades above S500 up to S690.

Abstract: Hot- and cold-rolled steel sheets are the most important substrate materials for enamelling applications. Currently, industry is actively seeking ways to widen the area of application of hotrolled steel sheets for double-face enamelling. Another important task is search for new steel compositions that might substitute for currently used open-coil annealed, cold-rolled steel sheet for direct white enamelling applications. The purpose of this research project was the investigation of influence of chemical composition and processing conditions of several microalloyed steels on mechanical properties and fish-scale resistance of hot and cold rolled sheets. The results show that a high fish-scale resistance of the hot-rolled steel is achieved by creating a high volume fraction of fine precipitates (nitrides, carbides, carbonitrides, sulphides, etc.). By partial replacement of titanium with other carbide and nitride forming elements, such as niobium and/or boron, the suitability of cold-rolled IF steels for white direct-on enamelling can be remarkably improved without open-coil annealing. However, the processing conditions have to be tuned to ensure high fish-scale resistance without impairing formability.

Abstract: This article offers an overview on the ways and means of the application of Nb in quenched and tempered high-strength steel plates. Thereby, the outstanding role of Nb to control the austenite microstructure during rolling or heat treatment and to contribute to effective precipitation hardening is discussed. It is shown, that Nb is very effective to retard the transformation processes during quenching. For high-strength constructional steels with up to 1100 MPa yield strength and for wear resistant steels, the improvement in strength and toughness properties in Nb microalloyed steels as compared to Nb-free steels is pointed out. A remarkable effect is the improvement in toughness and brittle fracture resistance due to a very fine microstructure and finely dispersed Nb carbonitrides in a martensitic microstructure. Fields of application for the new Nb microalloyed high-strength structural steels are presented too.

Abstract: The design of base chemistry and optimization of rolling schedule are the two important factors that influence large strain accumulation in multi-pass rolling in order to obtain ultra-fine grain size by dynamic recrystallization. A base chemistry of 0.03C-0.003N-0.08Nb-0.015Ti-1.8Mn (all in weight %) of HTP steel design was chosen in order to control the time evolution of strain induced precipitation of NbC and the strain accumulation through precipitate interaction with recovery and recrystallization at short inter-pass times characteristic of strip rolling. Experimental data on the critical strain for static and dynamic recrystallisation for HTP steel are used in a quantitative model to predict strain accumulation pass by pass and to achieve grain refinement by dynamic recrystallisation through large strain accumulation. The model is used to optimize the time-temperature-deformation schedule to prevent static recrystallization during the inter-pass times and to target ultra-fine grain size through dynamic recrystallization by large strain accumulation. The model predictions are validated by simulation of strip rolling of HTP steel on the thermo-mechanical simulator (WUMSI) to obtain a uniform ultra-fine ferrite grain size of about 1.5 micrometer diameter in final ferrite microstructure.

Abstract: In the recent years several new high strength steel grades have been developed, which exhibit improved cold formability and thus are especially suitable for the car body. These multi-phase steels have to be processed via continuous annealing lines. The best combination of strength and uniform elongation is obtained with ferrite-bainite-retained austenite multiphase steel, which rely on transformation induced plasticity (TRIP effect) during the cold forming operation. The production route for TRIP steel, which relies on two isothermal heat treatments, is explained as well as the role of the major alloying elements. In order to optimize the property combinations, microalloying with niobium is a successful means. Niobium increases the strength of the steel with 15 MPa per 0.01 %Nb and simultaneously provides also a higher amount of stable austenite, thus guaranteeing both, increased strength and formability. The metallurgical background for providing these results is explained. Even though the amount of high strength steel in automobiles is continuously increasing, the application of TRIP steel is still limited. However, the good experience in trials and first successful applications make an increased usage of this steel type also realistic, despite its relatively high production costs and reduced weldability.