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New bainitic steels for forgings

Keul C., Wirths V., Bleck W.

Archives of Civil and Mechanical Engineering

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2012

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Vol. 12, no. 2

119--125

EN

Steels with a bainitic microstructure offer great possibilities for highly stressed forged components. The variety of different bainitic morphologies requests for an aligned thermal treatment after forging in order to achieve the maximum performance. In dependence of the alloying concept and heat treatment bainite is composed of different microstructural components like the ferritic primary phase and the secondary phase, which consists of either carbides, martensite and/or austenite. Different combinations of mechanical properties can thereby be adjusted in these steels, dependent on the arrangement of the primary and secondary phase. Three steels have been investigated, which contain approximately 0.22% C, 1.5% Si, 1.5% Mn, 0.08% Mo, 0.003% B and 0.01 Ti. Their chromium contents vary between 0% for grade 1 and 1.3% for grades 2 and 3. The niobium content varies between 0% for grades 1 and 2 and 0.03% for grade 3. The Si addition is utilized to suppress the carbide formation, so a carbide free bainitic microstructure is expected to form, whereas Mo and B are employed in order to promote the bainite formation. The bainitic microstructure of these steels can be formed either after isothermal phase transformation or after continuous cooling. These two process routes lead to different results with regard to the mechanical properties, especially the Y/T-ratio. While after isothermal transformation the low chromium containing grade 1 exhibits a higher Y/T-ratio than grades 2 and 3, this fact is turned around for the case of the bainite formation after continuous cooling. In the latter case the high chromium containing steel exhibits both higher yield and tensile strength. These differences in mechanical properties can be correlated to characteristic features of the primary and secondary phases of the bainitic microstructures. The specific role of chromium is explained by its effect on the phase transformation kinetics.

2

Fast algorithms for phase transformations in dual phase steels on a hot strip mill run-out table (ROT)

Suwanpinij P., Prahl U., Bleck W., Kawalla R.

Archives of Civil and Mechanical Engineering

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2012

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Vol. 12, no. 3

305--311

EN

The hot rolling of dual phase steel, alloyed with 0.064% C, 0.05% Al, 0.48% Cr, 0.93% Mn, and 0.41% Si, was simulated by a deformation dilatometer. To model the ferrite transformation behaviour on the run-out table, the rate law approach proposed by Leblond and Devaux was employed. It models the ferrite transformation kinetics as a function of holding temperature and austenite conditioning, which is achieved by varying the austenite grain size and retained strain. Its transformation kinetics during cooling to the coiling temperature was investigated by varying the carbon content of the remaining austenite. The martensite transformation kinetics was also modelled by Leblond and Devaux's approach with a parameter coupled to the Koistinen–Marburger model. The modelling shows that this alloying concept allows a large process window for both temperature and austenite grain size. Finally, a selected set of processing parameters was chosen and transferred to a pilot rolling mill. The mechanical properties of the rolled sheets are satisfactory.

3

Numerical prediction of microstructure in high-strength ductile forging parts

Back A., Urban M., Keul C., Bleck W., Hirt G.

Computer Methods in Materials Science

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2010

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Vol. 10, No. 4

271-278

EN

The automotive industry has an ongoing request for lighter, stiffer and at the same time cheaper parts to maintain the economic and technical progress. Especially in case of safety relevant components a combination of high stiffness and sufficient ductility is required. Regarding these demands the main subject of this project was to improve the mechanical properties of forging steel alloys by employing a high-strength and ductile bainitic microstructure while maintaining a cost effective process chain for the high-stressed forged parts. Then again the need of a bainitic microstructure entails high experimental effort for identifying the process parameters and geometries that enable the target microstructure. Hence, the second aim in this project is to prove if by easy process simulation sufficient results for the prediction of microstructure can be provided. The implemented numerical approach is based on FEM simulations of the forging and cooling combined with deformation-cooling-time-temperature-transformation diagrams.

PL

Przemysł samochodowy charakteryzuje ciągły wzrost zapotrzebowania na lżejsze, bardziej wytrzymałe i przy tym tańsze części, aby możliwe było utrzymanie ekonomicznego i technologicznego postępu. Połączenie wysokiej wytrzymałości i plastyczności jest wymagane szczególnie w przypadku części mających wpływ na bezpieczeństwo. Aby spełnić te wymagania, celem niniejszej pracy jest poprawa własności mechanicznych stali poprzez zastosowanie wysokowytrzymałych, plastycznych stali o strukturze bainitycznej, przy utrzymaniu efektywnego ekonomicznie procesu wytwarzania dla stali wysokowytrzymałych. W tym celu opracowano nowy skład chemiczny stali i zaproponowano cykl wytwarzania tej stali. Dla zmniejszenia liczby kosztownych badań doświadczalnych do identyfikacji j parametrów procesu i kształtu przedkuwek, które umożliwiłyby uzyskanie docelowej mikrostruktury, opracowano numeryczny model przewidujący rozwój mikrostruktury stali w trakcie wytwarzania. Model numeryczny opiera się na symulacjach MES procesów kucia i chłodzenia połączonych z wykresami odkształcenie-chłodzenie-czas-temperatura-przemiana.

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Modelling the mechanical properties of multiphase steels

Thomser C., Prahl U., Vegter H., Bleck W.

Computer Methods in Materials Science

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2007

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Vol. 7, No. 1

42-46

EN

Modelling the mechanical properties of multiphase steels Corinna Thomser*, Ulrich Prahl*, Henk Vegter**, Wolfgang Bleck* *Institute of Ferrous Metallurgy, RWTH Aachen University, Germany **Corus RD&T, IJmuiden, Netherlands Due to economic, environmental and safety reasons the use of high strength steels for the automotive industry is increasing rapidly. For an optimal use of strength and formability of multiphase steels an accurate material model is required for forming simulations. At the moment the microstructure of multiphase steels, which is the most important factor influencing the strain hardening behaviour of multiphase steels, is not taken into account in FE simulations. In international projects like the ULSAB project, especially dual phase steels play an important role for the automotive industry. Their strain hardening behaviour is strongly influenced by the microstructure as is well known from several experimental investigations. Within this work an approach is presented which describes the microstructure evolution during intercritical annealing by thermodynamic calculations and predicts the strain hardening behaviour of dual phase steels by means of FE simulation of representative volume elements based on microstructural characterisations. After cold rolling, dual phase steels are intercritically annealed. The fractions and the carbon contents of austenite and ferrite depend on the annealing temperature and on the holding time. After a fast quenching, the austenite transforms to martensite, which yields a material of a soft ferrite matrix with strong martensite islands. For the determination of the phase fractions and the carbon partitioning between the two phases a DICTRA calculation was carried out, which considers thermodynamic and kinetic effects during intercritical annealing, thus taking into account that full equilibrium is not always reached. All other elements except of carbon are assumed to be uniformly distributed in both phases. The carbon content was used to calculate the strain hardening behaviour of ferrite and martensite based on dislocation theory models /1/-/2/ for different annealing temperatures. As well known from literature, the strain-hardening behaviour of martensite is mainly dependent on the carbon content, while for the prediction of the strain hardening behaviour of ferrite to the local chemical composition the grain size is needed additionally. A three-dimensional representative volume element is used to describe the interaction of ferrite and martensite in a dual phase steel during deformation in a FE simulation, done in Abaqus. The resulting strain hardening behaviour in simulation is in good agreement to the experimental determined strain hardening behaviour in tensile tests within the error range of the metallographical microstructural characterisation. Additionally to the strain hardening behaviour, the tensile strength and the uniform elongation can be determined in experiments and simulation by using the Considere criteria. References /1/ Rodriguez, R.; Gutierrez, I.: Proceeding of TMP ’04, B-Liege, 2004, p. 356-363 /2/ Rodriguez, R.; Gutierrez, I.: Materials Science Forum, Vols. 426-432, 2003, p. 4525-4530

PL

Z powodów ekonomicznych, środowiskowych oraz bezpieczeństwa gwałtownie wzrasta zapotrzebowanie na wytrzymałą stal dla przemysłu samochodowego. W celu otrzymania optymalnej kombinacji wytrzymałości i plastyczności dla wielofazowego, wymagany jest odpowiedni model materoiału, który następnie zostanie wykorzystany w symulacji. Obecnie mikrostruktura materiału wielofazowego nie jest brana pod uwagę, a w symulacjach procesów przeróbki plastycznej i elementów skończonych jest ona jednym z najważniejszych czynników wpływających na umocnienie materiału. Niniejsza praca przedstawia podejście opisujące rozwój mikrostruktury podczas etapu wyżarzania poprzez zastosowanie obliczeń modynamicznych oraz przewidujące efekt umacniania dwufazowej stali. Analizę prowadzono z zastosowaniem odpowiednich symulacji MES. Obliczone parametry wykorzystanezostaną w rzeczywistym procesie przeróbki.