Evolution of strain-induced precipitates in a model austenitic Fe-30Ni-Nb steel and their effect on the flow behaviour

The present work investigated the evolution of strain-induced NbC precipitates in a model austenitic Fe-30Ni-Nb steel deformed at 925 °C to a strain of 0.2 during post-deformation holding between 3 and 1000 s and their effect on the reloading flow stress. The precipitate particles preferentially nucleated on the nodes of the periodic dislocation networks constituting microband walls. Holding for 10 s resulted in the formation of fine, largely coherent NbC particles with a mean diameter of ∼5 nm, which displayed a cube-on-cube orientation relationship with austenite and caused the maximum increase in the reloading steady-state flow stress. A further increase in the holding time from 30 to 1000 s led to the formation of semi-coherent, gradually coarser and more widely spaced particles with a mean diameter of 8 nm and above, which led to a gradual decrease in the reloading steady-state flow stress. The holding time increase resulted in progressive disintegration of the dislocation substructure and dislocation annihilation through static recovery processes, which was also reflected by the measured softening fractions. The precipitate particle shape changed during post-deformation annealing from elliptical to faceted octahedral and subsequently to tetra-kai-decahedral. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The static and metadynamic recrystallization behaviour of an X60 Nb Microalloyed Steel

A rapid method has been developed to determine recrystallization kinetics of Nb microalloyed steels by interrupted hot torsion test. The softening behaviour was achieved as a function of different processing parameters. The method clearly identified three regions, where the strain dependency of the recrystallization rate varied. Firstly, at large strains the rate of recrystallization was not a function of strain; this is generally ascribed to metadynamic recrystallization. At lower strains the time to 50% recrystallization showed a power low relationship with strain, characteristic of static recrystallization. A further break point exists on the time for 50% softening curve when strain induced precipitation occurs in the material. The onset of strain induced precipitation was at strains below the strain to the peak stress at temperatures below 900°C. The experimental results were used to estimate the time for 50% softening and to anticipate the onset of the strain induced precipitation for the alloy of this study. Grain refinement of the recrystallized austenite continued to strains significantly beyond the peak stress and beyond the static to metadynamic recrystallization rate transition.

The evolution of microbands and their interaction with NbC precipitates during hot deformation of a Fe-30Ni-Nb model austenitic steel

The present work has investigated the evolution of microbands (MBs) and their interaction with strain-induced NbC precipitates during uniaxial compression of a model austenitic Fe-30Ni-Nb steel at 925 °C. The (1 1 0) fibre grains, both without and with copious amounts of precipitates, contained up to large strains crystallographic MBs aligned close to the highly stressed {1 1 1} slip planes having large Schmid factors. The MBs thus maintained their crystallographic character during straining, through continuously rearranging themselves, and did not follow the macroscopically imposed rigid body rotation. During double-pass deformation, fine NbC particles formed at short inter-pass holding remained strongly pinned at small reloading strains and appeared to be dragged by rearranging MB walls. With increasing reloading strain, the fine precipitates became progressively released from the above walls. During reloading after increased holding time, the coarsened particles tended with their increased size to become increasingly detached from the MB walls already at a small strain. The precipitate-free MB wall segments rearranged during straining to maintain their crystallographic alignment, while the detached precipitates followed the sample shape change and rotated towards the compression plane. The MB wall rearrangement generally occurred through cooperative migration of the corresponding dislocation networks.

Tenacidade ao impacto e resistência a fadiga de aço API 5L X70 modificado ao Nb

O transporte de gás e derivados de petróleo é realizado pelo uso de tubulações, denominadas de oleodutos ou gasodutos, que necessitam de elevados níveis de resistência mecânica e corrosão, aliadas a uma boa tenacidade à fratura e resistência à fadiga. A adição de elementos de liga nesses aços, Ti, V e Nb entre outros, é realizada para o atendimento destes níveis de resistência após o processamento termomecânico das chapas para fabricação destes dutos, utilizando-se a norma API 5L do American Petroleum Institute, API, para a classificação destes aços. A adição de elementos de liga em associação com o processamento termomecânico visa o refino de grão da microestrutura austenítica, o qual é transferido para a estrutura ferrítica resultante. O Brasil é o detentor das maiores reservas mundiais de nióbio, que tem sido apresentado como refinador da microestrutura mais eficiente que outros elementos, como o V e Ti. Neste trabalho dois aços, denominados Normal e Alto Nb foram estudados. A norma API propõe que a soma das concentrações de Nióbio, Vanádio e Titânio devem ser menores que 0,15% no aço. As concentrações no aço contendo mais alto Nb é de 0,107%, contra 0,082% do aço de composição normal, ou seja, ambos atendem o valor especificado pela norma API. Entretanto, os aços são destinados ao uso em dutovias pela PETROBRÁS que impõe limites nos elementos microligantes para os aços aplicados em dutovias. Deste modo estudos foram desenvolvidos para verificar se os parâmetros de resistência à tração, ductilidade, tenacidade ao impacto e resistência à propagação de trinca por fadiga, estariam em acordo com a norma API 5L grau X70 e com os resultados que outros pesquisadores têm encontrado para aços dessa classe. Ainda, como para a formação de uma dutovia os tubos são unidos uns aos outros por processo de soldagem (circunferencial), o estudo de fadiga foi estendido para as regiões da solda e zona termicamente afetada (ZTA). Como conclusão final observa-se que o aço API 5L X70 com Nb modificado, produzido conforme processo desenvolvido pela ArcelorMittal - Tubarão, apresenta os parâmetros de resistência e ductilidade em tração, resistência ao impacto e resistência a propagação de trinca em fadiga (PTF) similar aos aços API 5L X70 com teores de Nb = 0,06 % peso e aqueles da literatura com teores de Nb+Ti+V < 0,15% peso. O metal base, metal de solda e zona termicamente afetada apresentaram curvas da/dN x ΔK similares, com os parâmetros do material C e m, da equação de Paris, respectivamente na faixa de 3,3 - 4,2 e 1.3x10-10 - 5.0x10-10 [(mm/ciclo)/(MPa.m1/2)m].

Effect of microstructure on the stability of retained austenite in transformation-induced-plasticity steels

Two Fe-0.2C-1.55Mn-1.5Si (in wt pet) steels, with and without the addition of 0.039Nb (in wt pet), were studied using laboratory rolling-mill simulations of controlled thermomechanical processing. The microstructures of all samples were characterized by optical metallography, X-ray diffraction (XRD), and transmission electron microscopy (TEM). The microstructural behavior of phases under applied strain was studied using a heat-tinting technique. Despite the similarity in the microstructures of the two steels (equal amounts of polygonal ferrite, carbide-free bainite, and retained austenite), the mechanical properties were different. The mechanical properties of these transformation-induced-plasticity (TRIP) steels depended not only on the individual behavior of all these phases, but also on the interaction between the phases during deformation. The polygonal ferrite and bainite of the C-Mn-Si steel contributed to the elongation more than these phases in the C-Mn-Si-Nb-steel. The stability of retained austenite depends on its location within the microstructure, the morphology of the bainite, and its interaction with other phases during straining. Granular bainite was the bainite morphology that provided the optimum stability of the retained austenite.

Effect of alloying elements on the microstructure property relationship in thermomechanically processed C-Mn-Si TRIP steels

The effect of additions of Nb, Al and Mo to Fe-C-Mn-Si TRIP steel on the final microstructure and mechanical properties after simulated&nbsp; thermomechanical processing (TMP) has been studied. The laboratory simulations of discontinuous cooling during TMP were performed using a hot rolling mill. All samples were characterised using optical microscopy and image analysis. The volume fraction of retained austenite was ascertained using a heat tinting technique and X-ray diffraction measurements. Room temperature mechanical properties were determined by a tensile test. From this a comprehensive understanding of the structural aspect of the bainite transformation in these types of TRIP steels has been developed. The&nbsp; results have shown that the final microstructures of thermomechanically processed TRIP steels comprise 50 % of polygonal ferrite, 7 - 12 % of retained austenite, non-carbide bainitic structure and martensite. All steels exhibited a good combination of ultimate tensile strength and total elongation. The microstructure-property examination revealed the relationship between the composition of TRIP steels and their mechanical properties. It has been shown that the addition of Mo to the C-Si-Mn-Nb TRIP steel increases the ultimate tensile strength up to 1020 MPa. The stability of the retained austenite of the Nb-Mo steel was degraded, which led to a decrease in the elongation (24 %). The results have demonstrated that the addition of Al to C-Si-Mn-Nb steel leads to a good combination of strength (∼ 940 MPa) and elongation (∼ 30 %) due to the formation of refined acicular ferrite and granular bainite structure with ∼7 - 8 % of stable retained austenite. Furthermore, it has been found that the addition of Al increases the volume fraction of bainitic ferrite laths. The investigations have shown an interesting result that, in the Nb-Mo-Al steel, Al has a more pronounced effect on the microstructure in comparison with Mo. It has been found that the bainitic structure of the Nb-Mo-Al steel appears to be more granular than in the Nb-Mo steel. Moreover, the volume fraction of the retained austenite increased (12 %) with decreasing bainitic ferrite content. The results have demonstrated that this steel has the best mechanical properties (1100 MPa and 28 % elongation). It has been concluded that the combined effect of Nb, Mo, and Al addition on the dispersion of the bainite, martensite and retained austenite in the ferrite matrix and the morphology of these phases is different than effect of Nb, Mo and Al, separately.

Effect of alloying elements on the microstructure-property relationship in thermomechanically processed C-Mn-Si TRIP steels

The effect of additions of Nb, Al and Mo to Fe-C-Mn-Si TRIP steel on the final microstructure and mechanical properties after simulated thermomechanical processing (TMP) has been studied. The laboratory simulations of discontinuous cooling during TMP were performed using a hot rolling mill. All samples were characterised using optical microscopy and image analysis. The volume fraction of retained austenite was ascertained using a heat tinting technique and X-ray diffraction measurements. Room temperature mechanical properties were determined by a tensile test. From this a comprehensive understanding of the structural aspect of the bainite transformation in these types of TRIP steels has been developed. The results have shown that the final microstructures of thermomechanically processed TRIP steels comprise 50 % of polygonal ferrite, 7 - 12 % of retained austenite, non-carbide bainitic structure and martensite. All steels exhibited a good combination of ultimate tensile strength and total elongation. The microstructure-property examination revealed the relationship between the composition of TRIP steels and their mechanical properties. It has been shown that the addition of Mo to the C-Si-Mn-Nb TRIP steel increases the ultimate tensile strength up to 1020 MPa. The stability of the retained austenite of the Nb-Mo steel was degraded, which led to a decrease in the elongation (24 %). The results have demonstrated that the addition of Al to C-Si-Mn-Nb steel leads to a good combination of strength (∼ 940 MPa) and elongation (∼ 30 %) due to the formation of refined acicular ferrite and granular bainite structure with ∼7 - 8 % of stable retained austenite. Furthermore, it has been found that the addition of Al increases the volume fraction of bainitic ferrite laths. The investigations have shown an interesting result that, in the Nb-Mo-Al steel, Al has a more pronounced effect on the microstructure in comparison with Mo. It has been found that the bainitic structure of the Nb-Mo-Al steel appears to be more granular than in the Nb-Mo steel. Moreover, the volume fraction of the retained austenite increased (12 %) with decreasing bainitic ferrite content. The results have demonstrated that this steel has the best mechanical properties (1100 MPa and 28 % elongation). It has been concluded that the combined effect of Nb, Mo, and Al addition on the dispersion of the bainite, martensite and retained austenite in the ferrite matrix and the morphology of these phases is different than effect of Nb, Mo and Al, separately.

Heat affected zone microstructures and their influence on toughness in two microalloyed HSLA steels

Microstructures and Charpy impact properties have been examined in two microalloyed steels following heat treatments to simulate weld heat affected zone (HAZ) structures over a range of heat input conditions, characterised by the cooling time from 800 to 500°C (Δt8/5). The base materials were low carbon structural steel plates microalloyed with vanadium and nitrogen (V-N) and niobium (Nb), respectively. The toughnesses of the HAZs displayed remarkably different behaviours as shown by their impact transition temperatures. For the V-N steel, the toughness improved with increasingly rapid cooling (low heat input conditions) whereas the Nb steel showed an opposite trend. Some of this behaviour could be explained by the presence of coarse ferrite grains in the slowly cooled V-N steel. However, other conditions where all the structures were bainitic and rather similar in optical micrographs gave widely different toughness values. The recently developed method of five dimensional boundary analysis based on electron backscattering diffraction has been applied to these cases for the first time. This showed that the lath boundaries in the bainite were predominantly on {1 1 0} planes of the ferrite and that the average spacing of these boundaries varied depending on steel composition and cooling rate. Since {1 1 0} is also the slip plane in ferrite, it is considered that close spacing between the lath boundaries inhibits general plasticity at stress concentrations and favours initiation of fracture. The differences between the two steels are believed to be due to their transformation behaviours on cooling where precipitation of vanadium nitride in austenite accelerates ferrite formation and raises the temperature of the phase transformation in V-N steels.

Avaliação da resistência ao desgaste de aços inoxidáveis endurecíveis por precipitação nitretados

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

The effect of Nb, Mo, and A1 additions on the transformation behaviour and mechanical properties of thermomechanically processed C-Si-Mn trip steel

The effect of additions of Nb, A1 and Mo to Fe-C-Mn-Si TRIP steels on the final microstructure and mechanical properties after simulated thermomechanical processing (TMP) has been studied. Laboratory simulations of continuous cooling during TMP were performed using a quench deformation dilatometer, while laboratory simulations of discontinuous cooling during TMP were performed using a hot rolling mill. From this a comprehensive understanding of the structural and kinetic aspects of the bainite transformation in these types of TRIP steels has been developed. All samples were characterised using optical microscopy and XRD. The relationships between the morphology of bainitic structure, volume fraction, stability of RA and mechanical properties were investigated.

The effect of simulated thermomechanical processing on the transformation behavior and microstructure of a low-carbon Mo-Nb linepipe steel

The present work investigates the transformation behavior of a low-carbon Mo-Nb linepipe steel and the corresponding transformation product microstructures using deformation dilatometry. The continuous cooling transformation (CCT) diagrams have been constructed for both the fully recrystallized austenite and that deformed in uniaxial compression at 1148 K (875 °C) to a strain of 0.5 for cooling rates ranging from 0.1 to about 100 K/s. The obtained microstructures have been studied in detail using electron backscattered diffraction complemented by transmission electron microscopy. Heavy deformation of the parent austenite has caused a significant expansion of the polygonal ferrite transformation field in the CCT diagram, as well as a shift in the non-equilibrium ferrite transformation fields toward higher cooling rates. Furthermore, the austenite deformation has resulted in a pronounced refinement in both the effective grain (sheaf/packet) size and substructure unit size of the non-equilibrium ferrite microstructures. The optimum microstructure expected to display an excellent balance between strength and toughness is a mix of quasi-polygonal ferrite and granular bainite (often termed “acicular ferrite”) produced from the heavily deformed austenite within a processing window covering the cooling rates from about 10 to about 100 K/s.

A Novel Mo and Nb Microalloyed Medium Mn TRIP Steel with Maximal Ultimate Strength and Moderate Ductility

The multi-phase, metastable, and multi-scale (M3) constitution of a novel transformation-induced plasticity (TRIP) steel (Fe-0.17C-6.5Mn-1.1Al-0.22Mo-0.05Nb, wt&nbsp;pct) was designed through thermodynamic calculations combined with experimental analysis. In this study, Mo and Nb microalloying was used to control the fraction of retained austenite and its mechanical stability during tensile deformation and to improve the yield strength. Thermodynamic calculations were developed to determine the critical annealing temperature, at which a large fraction of retained austenite (~38&nbsp;pct) would be obtained through the effects of solute enrichment. The experimental observation was in good agreement with the predicted results. According to the critical annealing temperature, such an ultrafine (<200&nbsp;nm) M3, microstructure with optimum mechanical stability was successfully achieved. The results of this work demonstrated the superior performance with improved yield strength of 1020 to 1140&nbsp;MPa and excellent ductility (>30&nbsp;pct), as compared with other TRIP steels. Both angle-selective backscatter and electron backscatter diffraction techniques were employed to interpret the transformation from the deformed martensitic laths to the ultrafine austenite and ferrite duplex structure.

Orientation dependence of the deformation microstructure in a Fe-30Ni-Nb model austenitic steel subjected to hot uniaxial compression

The present work was aimed at a detailed investigation of the orientation dependence of the microstructure characteristics in a Fe-30Ni-Nb austenitic model steel subjected to hot uniaxial compression at 1198 K (925 °C) at a strain rate of 1 s−1 to several strain levels up to 1.0. The quantification of the substructure evolution as a function of strain was performed for the stable 〈011〉 oriented grains. Other grain orientations were also investigated in detail at a strain of 0.2. The 〈110〉 oriented grains contained self-screening arrays of “microbands” (MBs) aligned with high Schmid factor {111} slip planes. The MB crystallographic alignment was largely maintained up to a strain of 1.0, which suggests that the corresponding boundaries kept continuously rearranging themselves during straining and did not follow the sample shape change. The mean MB spacing decreased and misorientation angle increased with strain towards saturation, indicating the operation of the “repolygonization” dynamic recovery mechanism. The non-〈011〉 oriented grains displayed a strong tendency to split during deformation into deformation bands having alternating orientations and being mutually rotated by large angles. The bands were separated by transition regions comprising arrays of closely spaced, extended sub-boundaries collectively accommodating large misorientations across very small distances.

FeNbP in ultra-low carbon Nb-added steel containing high P

Precipitation of FeTiP is reported to occur in Ti-added IF steels containing high P during thermomechanical processing. An ultra-low carbon (ULC) Nb-added steel ingot containing a higher P content (< 0.8 wt-%) was produced via aluminothermic reduction of Fe2O3 followed by double electron beam melting (EBM). FeNbP coarse precipitates were observed in the as-cast microstructure. After soaking at 1050C for 1 h, the plate was hot rolled from 31 mm down to 7 mm in thickness (total reduction of 77%). During cold rolling of these hot bands we observed embrittlement. We believe that this embrittlement can be attributed to the presence of the FeNbP precipitates. Light optical and scanning electron microscopy (SEM/EDS) were used to characterize the microstructure of this ULC steel. (C) 2000 Elsevier B.V. All rights reserved.