The effect of the substitution of silicon by aluminium on the mechanical properties and the microstructure af cold rolled C-Mn-Si TRI P steels was investigated for different continuous annealing cycles,The mechanical properties were evaluated using tensile testing, It wasseen that the Al alloyed steel had very good mechanical properties with an improved formability comparedto the conventional C-Mn-Si TRIP steel. The strain hardening behaviour was studied in detail. All the investigated specimens showed a very high n value but their strain dependence was different.For the conventional C-Mn-Si TRIP steel the maximum n value was reached at low strain, while the Al substituted TRIP steel showed a gradual increase of the n value. The latter effect resulted in a larger uniform elongation for the C-Mn-Al-Si TRIP steel.Investigation of the microstructure using color etching and scanning electron mic!oscopy revealed that the Al alloyed steel composition contained a larger amount of bainite with a finer structure than the C-M n-Si TRIP steel, Furthermore, XRDmeasurementsshowed that the Al evolution of the M~temperature corresponds to an isothermal holding at 400'C, which results in a phase distribution as shown in Fig. 4on

The influence of the stress state on the plastic deformation of CMnSi, CMnSi(Nb), and CMnAlSi transformation induced plasticity (TRIP)-aided steel has been analyzed. Imposing hydrostatic pressures up to 800 MPa during tensile deformation made it possible to change the stress state of the tensile testing specimens. It was found that the ratio of normal to shear stresses has a pronounced effect on the evolution of the microstructure, the austenite volume fraction change during straining, and the fracture surface morphology. The CMnAlSi TRIP steel, which has the largest uniform elongation and the smallest equivalent strain at fracture in the absence of the hydrostatic pressure, had a more pronounced improvement of all plastic characteristics at increasing hydrostatic pressure. An increased austenite stabilization, brought about by the high hydrostatic pressure, was clearly observed. The austenite stabilization results in a decrease of 20 ЊC to 25 ЊC of M s for an increase of 100 MPa of the hydrostatic pressure. The implications of the observations could be far-reaching for new sheet forming technologies, such as hydroforming, as the full transformation potential is available for crashsensitive structural parts by avoiding the formation of the martensite during forming operations.

The texture of two transformation induced plasticity steels has been studied by means of crystallographic orientation mapping. Texture measurements were carried out on ferrite, bainite, and austenite. The polygonal ferrite and the bainite texture, both bcc, could be distinguished based on the image quality parameter of the electron backscattering diffraction measurement. Both bcc textures were very similar, the main difference being the more pronounced n111m a || ND (ND~normal direction) and n110m a || RD (RD~rolling direction) fibre textures in the polygonal ferrite. The fcc texture was a strong c deformation texture, characterised by the b fibre. The presence of the a fibre confirmed the strong n110m c || ND direction, which was previously detected by means of X ray diffraction (XRD). The measured fcc and bcc textures were used to calculate orientation distribution function transformations according the Bain, Kurdjumov -Sachs, and Nishiyama -Wasserman orientation relationships. The predicted cube component {001} c n100m c , which was missing in the measured texture, of c ret indicates a variant selection for the cAa B transformation. In addition it was shown that crystallographic orientation mapping could be used to make reliable phase fraction determinations, which were previously based on the light optical microscopy of colour etched specimens. This also proves that XRD determination of c ret is flawed owing to the strong texturing of all phases present in the microstructure.MST/4939

The processing of cold rolled intercritically annealed steels which exhibit transformation induced plasticity (TRIP) requires the precise determination of equilibrium parameters, the kinetics of the intercritical g phase formation and the kinetics of the isothermal bainitic transformation. In addition, the aging phenomena associated with the bainitic transformation must be taken into account. A detailed analysis of the kinetics of the combination of both transformations carried out in succession has hitherto not been reported. The present contribution proposes an empirical method to study these transformations in low C intercritically annealed TRIP steels: a standard CMnSi TRIP steel and a CMnAlSi TRIP steel in which part of the Si is replaced by Al. The latter TRIP steel is more likely to be used in continuous galvanising lines. Dilatometry was used to determine the soaking time necessary to obtain the equilibrium phase distribution during the intercritical annealing. Furthermore, the transformation kinetics and the evolution of the C content in the retained austenite and the bainitic ferrite were evaluated. The results show clearly that the kinetics of the intercritical austenite decomposition during the bainitic transformation cannot be ® tted to a single transformation mechanism owing to the formation of carbides. It is shown that during intercritical annealing local equilibrium conditions are achieved at the phase boundaries for the substitutional solutes. This results in an inhomogeneous g phase composition, which was observed in the Al alloyed TRIP steels. In addition, it was found that whereas equilibrium thermodynamic calculations can be used to predict phase boundaries reliably, the C content of the retained austenite was much larger than the calculated C content based on the free energy of ferrite~free energy of austenite condition.MST/5298

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Complex phase (CP) steels have very high ultimate tensile strengths, resulting from the use of specific alloying elements, which improve the hardenability but cause difficulties when applying a zinc coating by means of continuous hot‐dip galvanizing. The galvanizability of a cold rolled 1000MPa complex phase steel was investigated by monitoring the surface chemistry before dipping and evaluating the quality of the zinc coatings applied by a laboratory hot‐dipping simulator. Two steel compositions with different Cr levels were used. The influence of the most important production parameters, the annealing temperature and the dew point of the annealing atmosphere, was investigated. Both steel compositions were galvanizable, but both the surface appearance and zinc coating adhesion were improved when low Cr contents were used. At a low dew point of ‐30°C, Cr, Mn and Si segregated to the surface and the presence of Mn2SiO4 could be demonstrated. At high dew point of +10°C, less oxides were present at the steel surface. There was no effect of the annealing temperature on the coatability.

Key factors influencing the mechanical properties of low alloy intercritically annealed sheet steel containing high C retained austenite are discussed. Ferrous microstructures containing retained austenite are fundamentally different from most current automotive sheet steel products both in terms of their asproduced properties and application related performance (formability, bake hardening and high strain rate deformation). The properties are mainly determined by the C distribution and the latter is influenced by the composition, in particular the Si, Al and P contents, the main processing parameters (the intercritical and isothermal bainitic transformation temperatures and times) and the type of deformation. The properties of the retained austenite islands control the static strain hardening, the bake hardening and the high strain rate dependence of the transformation induced plasticity (TRIP) effect. The short term technical solution of fundamental questions related to low alloy intercritically annealed sheet steel containing high C retained austenite which are the influence of the distribution of substitutional solutes, the C distribution and the strain induced transformation of these steels are reviewed Résumé -On discute des facteurs-clés influençant les propriétés mécaniques de tôle d'acier faiblement allié, à recuit intercritique, contenant de l'austénite résiduelle à haute teneur en carbone. Les microstructures de fer contenant de l'austénite résiduelle sont fondamentalement différentes des produits les plus communs de tôle d'acier pour automobiles, tant en termes de leurs propriétés à l'état brut que du rendement relié à l'application, i.e. formabilité, durcissement par cuisson et vitesse de déformation élevée. Les propriétés sont déterminées principalement par la distribution de C et cette dernière est influencée par la composition, en particulier la teneur en Si, Al et P, par les principaux paramètres de traitement, i.e. par les températures et les durées de transformation intercritique et bainitique isotherme ainsi que par le type de déformation. Les propriétés des îles d'austénite résiduelle contrôlent l'écrouissage statique, le durcissement par cuisson et la dépendance de la vitesse de déformation élevée de l'effet TRIP. On examine la solution technique à court terme des questions fondamentales reliées à la tôle d'acier faiblement allié à recuit intercritique, contenant de l'austénite résiduelle à haute teneur en C, i.e. l'influence de la distribution des solutés de substitution, la distribution du C et la transformation induite par la déformation de ces aciers.

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