Strengthening mechanisms in thermomechanically processed NbTi-microalloyed steel

The effect of deformation temperature on microstructure and mechanical properties was investigated for thermomechanically processed NbTi-microalloyed steel with ferrite-pearlite microstructure. With a decrease in the finish deformation temperature at 1348 K to 1098 K (1075 °C to 825 °C) temperature range, the ambient temperature yield stress did not vary significantly, work hardening rate decreased, ultimate tensile strength decreased, and elongation to failure increased. These variations in mechanical properties were correlated to the variations in microstructural parameters (such as ferrite grain size, solid solution concentrations, precipitate number density and dislocation density). Calculations based on the measured microstructural parameters suggested the grain refinement, solid solution strengthening, precipitation strengthening, and work hardening contributed up to 32 pct, up to 48 pct, up to 25 pct, and less than 3 pct to the yield stress, respectively. With a decrease in the finish deformation temperature, both the grain size strengthening and solid solution strengthening increased, the precipitation strengthening decreased, and the work hardening contribution did not vary significantly

Molecular orientation of individual LCP particles in injection-moulded PPS/LCP blends

Polarized light microscopy was used to investigate the presence of preferred molecular orientation in the LCP phase of PPS/LCP blends after injection moulding. Normal birefringence effects appeared to be complicated by artifacts due to sample preparation and by the complex nature of polarized light transmission through a multicomponent sample. It was found, however, that, during low-temperature cutting of optically transparent thin sections on a standard microtome, individual LCP particles could be separated from the PPS matrix, and their birefringence analyzed separately. Preferred orientation was detected only in LCP fibrils which dominated in skin regions, but not in droplet-shaped particles which had formed in core regions. Quantitative measurements indicated that the molecular orientation of the fibrils increased linearly with their length-to-diameter aspect ratios which ranged from 15 to 50. Even for the highest aspect ratios, however, the degree of orientation was always less than that which could easily be introduced into pure LCP thin-film samples by manual shearing

Carbonitride Precipitation In Microalloyed Steel V-nb Thin Slabs Thermomecanically Treated [precipitação De Carbonitretos Em Placas Finas De Aço Microligado V-nb Submetidas A Tratamento Termomecânico]

Transmission electron microscopy was used in order to investigate the precipitation of fine carbonitride particles in two thin slabs of vanadium and niobium microalloyed steels during thermomechanical processing. The slabs were solidified in a thin slab casting simulator and subsequently hot rolled from 42 to 12 mm thickness on a laboratory-scale rolling mill. The rolling schedule included 3 roughing passes and 5 finishing passes (with two different temperature regimes for finish rolling), followed by a simulated coiling stage in a tunnel furnace and finally air cooling. TEM results showed interphase precipitation and fine precipitation in austenite whose contributions to yield strength were calculated from the Orowan-Ashby model and compared to an empirical equation which accounts for strengthening effects from the steel's chemical composition and ferrite grain size. The amount of precipitation strengthening was estimated to have reached 130 MPa in one of the slabs, and 146 and 106 MPa, respectively, in the other slab, depending upon the finish rolling temperatures.31173126Honeycombe, R.W.K., (1981) Aços Microestrutura e Propriedades, , Lisboa: Fundação Calouste GulbenkianPriestner, R., Zhou, C., Simulation of microstructural evolution in Nb-Ti micro alloyed steel during hot direct rolling (1995) Ironmaking and Steelmaking, 22 (4), pp. 326-332Kaspar, R., Flüß, P., Laboratory simulation of the direct rolling of steel (1991) Steel Research, 62 (11), pp. 501-506Hansen, S.S., Vander Sande, J.B., Cohen, M., Niobium carbonitride precipitation and austenite recrystallization in hot-rolled micro alloyed steels (1980) Metallurgical Transactions A, 11, pp. 387-402. , MarPalmiere, E.J., Garcia, C.I., DeArdo, A.J., Compositional and microstructural changes which attend reheating and grain coarsening in steels containing Nb (1994) Metallurgical and Materials Transactions A, 25, pp. 277-286. , FebBowden, J.W., Samuel, F.H., Jonas, J.J., Effect of interpass time on austenite grain refinement by means of dynamic recrystallization of austenite (1991) Metallurgical Transactions A, 22, pp. 2947-2957. , DecKaspar, R., Peters, A., Efficient steel processing by disciplined hot forming and microalloying (1998) Steel Research, 69 (4-5), pp. 128-135Kestenbach, H.-J., Gallego, J., On dispersion hardening of microalloyed hot strip steels by carbonitride precipitation in austenite (2001) Scripta Materialia, 44, pp. 791-796Pereloma, E.V., Crawford, B.R., Hodgson, P.D., Strain-induced precipitation behaviour in hot rolled strip steel (2001) Materials Science & Engineering A, 299, pp. 27-37Kestenbach, H.-J., Dispersion hardening by niobium carbonitride precipitation in ferrite (1997) Material Science and Technology, 13, pp. 731-739. , SepItman, A., Cardoso, K.R., Kestenbach, H.-J., Quantitative study of carbonitride precipitation in niobium and titanium micro alloyed hot strip steel (1997) Materials Science and Technology, 13, pp. 47-55. , JanCampos, S.S., Morales, E.V., Kestenbach, H.-J., On strengthening Mechanisms in Commercial Nb-Ti Hot Strip Steels (2001) Metallurgical and Materials Transactions A, 32 A, pp. 1245-1248. , MayPark, J.S., Ajmal, M., Priestner, R., Tensile Properties of Simulated Thin Slab Cast and Direct Rolled Low-carbon Steel Microalloyed with Nb, V and Ti (2000) ISIJ International, 40 (4), pp. 380-385Gentile, F.C., (1999) Desenvolvimento de Protótipo para Simulação do Processo de Lingotamento Continuo de Placas, p. 91. , Campinas: UNICAMP-FEM. Dissertação (Mestrado)Mori, T., (1968) Tetsu to Hagane, 54, p. 763Costae Silva, A.L., Mei, P.R., (1988) Aços e Ligas Especiais. 2 a ed. Sumaré, pp. 213-214. , SP: Eletrometal S. A Metais EspeciaisBoratto, F., Barbosa, R., Yue, S., Jonas, J.J., Effect of chemical composition on the critical temperatures of microalloyed steels (1988) THERMEC-88, pp. 383-390. , The Iron and Steel Institute of Japan, Tokyo, JapanPickering, F.B., (1978) Physical Metallurgy and the Design of Steels, , London: Applied Science Publishers LtdOuchi, C., Sampei, T., Kozasu, I., The effect of hot-rolling condition and chemical-composition on the onset temperature of gamma-alpha-transformation after hot rolling (1982) Trans. ISIJ, 22, pp. 214-222Gladman, T., (1997) The Physical Metallurgy of Microalloyed Steels, pp. 47-56. , The Institute of Materials, LondonDutta, B., Palmiere, E.J., Sellars, C.M., Modelling the Kinetics of strain induced precipitation in Nb microalloyed steels (2001) Acta Materialia, 49, pp. 785-79