Showing papers on "Microalloyed steel published in 2010"

Nonisothermal Austenite Grain Growth Kinetics in a Microalloyed X80 Linepipe Steel

Abstract: Nonisothermal austenite grain growth kinetics under the influence of several combinations of Nb, Ti, and Mo containing complex precipitates has been studied in a microalloyed linepipe steel. The goal of this study is the development of a grain growth model to predict the austenite grain size in the weld heat affected zone (HAZ). Electron microscopy investigations of the as-received steel proved the presence of Ti-rich, Nb-rich, and Mo-rich precipitates. The steel has then been subjected to austenitizing heat treatments to selected peak temperatures at various heating rates that are typical for thermal cycles in the HAZ. Thermal cycles have a strong effect on the final austenite grain size. Using a mean field approach, a model is proposed for the dissolution of Nb-rich precipitates. This model has been coupled to a Zener-type austenite grain growth model in the presence of pinning particles. This coupling leads to accurate prediction of the austenite grain size along the nonisothermal heating path simulating selected thermal profiles of the HAZ.

Determination of critical conditions for dynamic recrystallization of a microalloyed steel

Abstract: A low carbon Nb–Ti microalloyed steel was subjected to hot torsion testing over the range of temperatures from 900 to 1100 °C and strain rates from 0.01 to 1 s −1 to characterize its hot deformation behavior. The initiation and evolution of dynamic recrystallization were investigated by analyzing of hot flow curves. Two important dynamic recrystallization parameters, the critical strain and the point of maximum dynamic softening, derived from strain hardening rate- stress curves. These parameters then were used to predict the dynamic recrystallized fraction. The results showed that the critical stress and strain increase with decreasing deformation temperature and increasing strain rate. The hot deformation activation energy of the steel investigated in the present work is 375 kJ/mol, and the expression for steady state flow stress is σ SS = 0.07 Z 0.14 = 0.07 ⋅ e ˙ exp 375,000 R T 0.14 The volume fraction of dynamic recrystallization as a function of processing variables was established. It was found that the model used for predicting the kinetic of dynamic recrystallization is in good agreement with the data directly acquired from experimental flow curves.

Deformation behaviour of spot-welded high strength steels for automotive applications

Abstract: Numerical simulation of component and assembly behaviour under different loading conditions is a main tool for safety design in automobile body shell mass production. Knowledge of local material behaviour is fundamental to such simulation tests. As a contribution to the verification of simulation results, the local deformation properties of spot-welded similar and dissimilar material joints in shear tension tests were investigated in this study for a TRIP steel (HCT690T) and a micro-alloyed steel (HX340LAD). For this reason, the local strain distribution was calculated by the digital image correlation technique (DIC). On the basis of the hardness values and microstructure of the spot welds, the differences in local strain between the selected material combinations are discussed. Additionally, the retained austenite content in the TRIP steel was analysed to explain the local strain values. Results obtained in this study regarding similar material welds suggest significant lower local strain values of the TRIP steel HCT690T compared to HX340LAD. One reason could be the decrease of retained austenite in the welded area. Furthermore, it has been ascertained that the local strain in dissimilar material welds decreases for each component compared with the corresponding similar material weld.

Kinetics of AlN precipitation in microalloyed steel

Abstract: In this work, the thermodynamic information on AlN formation in steel and experimental data on AlN precipitation kinetics are reviewed. A revised expression for the Gibbs energy of AlN is presented with special emphasis on microalloyed steel. Using the software package MatCalc, computer simulations of AlN precipitation are performed and compared with independent experimental results from the literature. A new model for grain boundary precipitation is employed, which takes into account fast short-circuit diffusion along grain boundaries as well as slower bulk diffusion inside the grain, together with the classical treatment for randomly distributed precipitates with spherical diffusion fields. It is demonstrated that the precipitation of AlN can be modelled in a consistent way if precipitation at grain boundaries and dislocations is taken into account, dependent on chemical composition, grain size and annealing temperature. It is also demonstrated that, for consistent simulations, the influence of volumetric misfit stress must be taken into account for homogeneous precipitation of AlN in the bulk and heterogeneous precipitation at dislocations.

Carbide characterization in a Nb-microalloyed advanced ultrahigh strength steel after quenching-partitioning-tempering process

Abstract: Based on the observations of scanning electron microscopy and transmission electron microscopy, four kinds of carbides were identified in a Nb-microalloyed steel after quenching–partitioning–tempering treatment. In addition to transitional epsilon carbide that usually forms in silicon-free carbon steel, other three types of niobium carbides (NbC) formed at various treatment stages respectively. They are incoherent NbC inclusion that nucleated at solidification mainly, fine NbC that nucleated in lath martensite at tempering stage and regular polygonal NbC that nucleated in austenite before quenching. Their formation mechanisms on steel were discussed briefly based on thermodynamics.

Effect of strain rate on the microstructural development through continuous dynamic recrystallization in a microalloyed steel

Abstract: Continuous dynamic recrystallization (CDRX) is one of the possible mechanisms for the grain refinement in metals and alloys. In the present research, intercritical warm torsion testing under different strain rates was conducted on a low carbon microalloyed steel. The effect of deformation strain rate on the microstructural refinement during CDRX was investigated. Microstructural characteristics and deformation behavior of investigated steel were discussed by analyzing of warm flow curves, optical microscope and electron back-scattering diffraction (EBSD). The results show that the peak and steady-state stresses are influenced by increasing of strain rate. The effect of CDRX on grain refinement increases with increasing strain rate, so that the volume fraction of CDRXed grains increases and grain sizes decrease. It is also observed that at high strain rate subgrain sizes decrease significantly. In contrast, at lower strain rate the CDRXed grains become larger. It is realized that deformation under high strain rate allows little time for CDRX to occur. In addition, under high strain rate deformation the time for grain growth is diminished and the coalescence of CDRXed grains is inhibited.

Effect of welding heat input on microstructure and mechanical properties of simulated HAZ in Cu containing microalloyed steel

Abstract: The influence of weld thermal simulation on ICGC HAZ microstructure and mechanical properties of Cu containing Nb-Ti-microalloyed steel has been investigated. Low heat input of 0.7 kJ/mm (simulated fast cooling of Δt 8/5 = 5 s) and high heat input of 4.5 kJ/mm (simulated slow cooling of Δt 8/5 = 61 s) were used to generate double-pass thermal cycles with peak temperatures of 1350 and 800 °C, respectively. The microstructure after high heat input mainly consisted of polygonal and quasi-polygonal ferrite (QF) grains with certain amount of acicular ferrite, whereas, after the low heat input, microstructure mainly consisted of lath or elongated bainite–ferrite, QF and M–A constituents. The size of ferrite grains decreased and volume of M/A constituents increased with fast cooling rate. The precipitation characteristics were found to be similar in both cooling rates. However, the precipitation of Cu-related phases was promoted by slow cooling rate. By fast cooling rate, the investigated steel exhibited an increase in hardness from 187HV to 197HV. Consequently higher yield strength with considerable loss in the (−10 °C) CTOD fracture toughness (δfast cooling = 0.86 mm and δslow cooling = 1.12 mm) were demonstrated.

Ultrahigh strength and low yield ratio of niobium-microalloyed 900 MPa pipeline steel with nano/ultrafine bainitic lath

Abstract: An ultra-low carbon niobium-microalloyed steel with yield strength of ∼900 MPa has been processed on a pilot-plant scale. The microstructure of the steel is primarily characterized by lower bainite and acicular ferrite, with small fraction of lath-martensite and martensite–austenite (MA) constituents. Bainite is present as fine domains. A combination of niobium and titanium precipitates was observed at the grain boundaries and in the interior of the grains and includes irregular (∼40–150 nm of (Nb, Ti)(C, N)) and fine cuboidal/spherical particles of NbC (∼30–50 nm). It was observed that accelerated cooling inhibited the precipitation of Nb and Ti carbides. The Charpy impact toughness at −20 °C was 200 J and tensile elongation was 15% with the yield ratio of less than 0.84. The good matching of high strength and low yield ratio was realized by two-stage thermo-mechanical rolling combined with fast cooling.

Study on the ferrite grain refinement during intercritical deformation of a microalloyed steel

Abstract: In the present research, the ferrite grain refinement during intercritical deformation of a low carbon microalloyed steel within the two phase (α + γ) region was investigated using hot torsion testing. The physical processes that occurred during intercritical deformation were discussed by observing the optical microstructure and analyzing the flow curve responses. The shape of the flow curve suggests that the certain dynamic softening mechanisms take place during deformation. Dynamic softening mechanisms compensate for the hardening effect of deformation and gradually keep balance with it. This flow softening is the result of deformation-induced ferrite transformation and continuous dynamic recrystallization of ferrite. Strain increasing promotes both of the softening mechanisms. Consequently, ultrafine ferrite grains continuously nucleate not only at grain boundaries but also inside austenite and pre-eutectoid ferrite. As a result, ultrafine ferrite with average grain size of ∼1.8 μm achieved. It is concluded that with strain increasing, in addition to deformation-induced ferrite transformation, continuous dynamic recrystallization of ferrite contributes to the further ferrite grain refinement.

Effect of microstructure and grain size on the fracture toughness of a micro-alloyed steel

Abstract: It has been reported that the high temperature austenitisation heat treatments have resulted in improving the fracture toughness of several structural steels. This is contrary to the conventional understanding of the relationship between larger prior austenite grain sizes, associated with high austenitising temperature, and fracture toughness. This anomalous increase in fracture toughness has some important practical significance to welded joints where the material adjacent to fusion zone experiences high austenitising temperatures resulting in a coarse grain zone. This investigation has been undertaken to verify the effects of microstructure and grain size on fracture toughness in a typical pressure vessel steel that is extensively welded during fabrications. A range of simulated weld heat affected zone microstructures were produced by austenitising the specimens at various temperatures followed by quenching at three different cooling rates. The trends of variations in properties, such as, fracture toughness are explained in terms of grain size, and microstructural analyses. It was observed that the specimens provided with low temperature austenitisation followed by air-quench possess a better fracture toughness than the base metal.

The Ferrite Transformation in Hot Deformed 0.036% Nb Austenite at Temperatures Above the Ae3

Abstract: A 0.036% Nb microalloyed steel was deformed in torsion over the temperature range 816–896°C in a 2%H2–Ar gas atmosphere. Strains of 0.5–5.0 were applied at strain rates of 0.04 and 0.4 s−1. The experimental parameters were varied in order to study the effects of strain, strain rate and temperature on the formation of ferrite by dynamic transformation (DT) at temperatures above the Ae3. The critical strain for ferrite formation by DT was 0.5 and the volume fraction formed increased with strain and slightly with strain rate. It was also observed that the applied strain has a far greater influence on the transformation than the time. Average ferrite grain sizes of 2 to 3.5 μm were produced, the size increasing with the transformation temperature and decreasing strain rate. By comparison with the behavior of plain C steels, it is evident that the addition of niobium slows the reverse transformation to a considerable degree. Two stages were detected in the reverse transformation: i) in stage I, observed during the initial 200 s of isothermal holding, the deformation-induced ferrite was fairly stable; ii) in stage II, observed after 200 s of holding, the reverse transformation began to take place, going to completion in about 400 s. The results of these experiments support the view that it is the stored energy of the ‘inhomogeneously’ distributed dislocation (i.e. those in shear bands and sub-boundaries) that provides the driving force for such “non-equilibrium” transformation.

Mean-field model for the growth and coarsening of stoichiometric precipitates at grain boundaries

Abstract: In this paper, a model for growth and coarsening of precipitates at grain boundaries is developed. The concept takes into account that the evolution of grain boundary precipitates involves fast short-circuit diffusion along grain boundaries as well as slow bulk diffusion of atoms from the grain interior to the grain boundaries. The mathematical formalism is based on a meanfield approximation, utilizing the thermodynamic extremal principle. The model is applied to the precipitation of aluminum nitrides in microalloyed steel in austenite, where precipitation occurs predominately at the austenite grain boundaries. It is shown that the kinetics of precipitation predicted by the proposed model differs significantly from that calculated for randomly distributed precipitates with spherical diffusion fields. Good agreement of the numerical solution is found with experimental observations as well as theoretical treatment of precipitate coarsening.

Characterization of the austenite recrystallization by comparing double deformation and stress relaxation tests

Abstract: A high amount of deformation below the non-recrystallization temperature (T-nr) is a common industrial practice to achieve a good combination of toughness and strength in microalloyed steels. To combine the industrially relevant optimum combination of high productivity and product quality, an accurate knowledge of T-nr and the recrystallization kinetics is required. Although a lot of literature data is available on the recrystallization behaviour of microalloyed steels, correlations are often difficult to be made due to the effect of different experimental setups, types of analysis and test schedules that are used to obtain this data. Although this would significantly improve the knowledge about these steels, so far, no systematic comparison has been presented in literature to correlate the different techniques and methods. In this study, different hot rolling simulation techniques, testing schedules and types of analysis were used to determine the recrystallization kinetics of a microalloyed steel. On the one hand, good agreement was found between the results from different test equipment for the double deformations tests. On the other hand, stress relaxation tests showed accelerated kinetics and appeared to be less effective.

Causes of transverse corner cracks in microalloyed steel in vertical bending continuous slab casters

Abstract: Transverse corner cracks have been frequently observed and are extremely difficult to prevent in some microalloyed steels produced on some vertical bending type continuous slab casters. The cracks are usually found on the fixed (outer) side corner of the slabs through inspection of the acid pickled surface and macroscopic examination. In the present study, the slab surface microstructure was investigated, and the results show that the causes of the crack formation are chain-like precipitations and film-like proeutectoid ferrite in the austenite grain boundaries when the slabs are bent. Initially, when the temperature of the slabs dropped after solidification, the Nb, V or Ti carbides and/or nitrides precipitated in chain-like way, then pinned onto the austenite grain boundaries. This process hindered grain boundary slippage and lowered the force on the grain boundaries. Second, due to the stress mismatch between the matrix and the fine precipitates during bending operations, the chain-like precipi...

Computational Analysis of Precipitation during Continuous Casting of Microalloyed Steel

Abstract: In this paper, the kinetics of TiN, V(C,N)) and AlN precipitation in microalloyed steel during continuous casting is investigated experimentally and theoretically. The precipitate phase fraction, mean radius, number density and composition are simulated with the thermo-kinetic software MatCalc and compared with experimental results obtained from transmission electron microscopy analysis. A new methodology for modelling precipitation in cast steel is proposed, which consists of two parts: First, a Scheil – Gulliver simulation, which is carried out to obtain information on the amount of microsegregation during solidification. Then, based on this information, two precipitation kinetics simulations are performed: One with the chemical composition representative for the solute-poor core of the secondary dendrite arms, the other with the composition of the residual liquid at a fraction of 5%, corresponding to the segregated solute-rich interdendritic regions. The results of the computer simulations using the new methodology are in good agreement with experimental observation.

Study of Grain-Growth Kinetics in Delta-Ferrite and Austenite with Application to Thin-Slab Cast Direct-Rolling Microalloyed Steels

Abstract: The high-temperature grain-growth kinetics in delta-ferrite and austenite is investigated. The delta-ferrite growth kinetics was observed directly on a model alloy that contained 2.5 wt pct aluminum in order to stabilize delta-ferrite down to room temperature. The gamma grain-growth kinetics was by etching the former austenite grain boundaries in a precipitate-free variant of APIX60 steel. At high temperatures and in the absence of precipitation, the growth kinetics in both delta-ferrite and austenite appeared to follow a simple parabolic growth law. The findings are applied to the problem of grain-size control during the process of thin-slab casting direct rolling (TSCDR).

The Present and the Future of Line Pipe Steels for Petroleum Industry

Abstract: The growth of petroleum distribution systems has always been closely associated with the development of microalloyed steel grades having high strength, toughness, and weldability. In many ways, the research in microalloyed steel drew impetus from the huge demands of such steels from the petroleum pipeline projects. The article reviews the evolution and trends in steel for petroleum pipelines, the manufacturing processes, metallurgy and quality management aspects, research, and frontiers, followed by the corresponding Indian scenario. The first specification for steel for pipelines introduced by the American Petroleum Institute (API) in 1948 was X 42 with yield strength of 42 ksi or 290 MPa. The strength levels underwent large scale changes over the decades, and at present X 80 is commonplace, while X 100 and X 120 are in sight. The advantage of the relatively recent high temperature process (HTP) for producing higher grades of line pipe steel under the Indian context is discussed. The necessity of incorpo...

Effects of V and Nb on static recrystallisation of austenite and precipitate size in microalloyed steels

Abstract: Recrystallisation and precipitation in two microalloyed steels, one with Nb and the other with V, have been studied. The Nb-steel displayed two plateaus on the curves of recrystallised fraction against time. The difference between activation energies allow to predict the efficiency of different precipitates to strengthen the austenite during hot rolling. RPTT diagrams showed the interaction between both phenomena. It is found that NbCN particles nucleate and grow faster than VCN, but the latter are smaller.

Microstructure and Transformation Characteristics of Acicular Ferrite in High Niobium-Bearing Microalloyed Steel

Abstract: The transformation behavior and microstructural characteristics of a low carbon high niobium-bearing microalloyed pipeline steel were investigated by deformation dilatometry and microstructure observation. The continuous cooling transformation curves of the test steel were constructed. The results showed that high niobium content and deformation enhanced the formation of acicular ferrite; the microstructures changed from polygonal ferrite, quasi-polygonal ferrite to acicular ferrite with increasing cooling rates from 0.5 to 50 °C/s and was dominated by acicular ferrite in a broadened cooling rate higher than 5 °C/s. The chaotic microstructure consisted of non-equiaxed ferrite and interwoven ferrite laths with high density dislocations and subunits. The results of isothermal holding treatment showed that acicular ferrite microstructure was formed at 550 — 600 °C and consisted of highly misoriented plate packets having internal low angle boundaries. With increasing the holding time or temperature, some low misorientation boundaries changed to high misorientation owing to the movement of dislocations and coarsening of grain.

In situ observation and investigation of effect of cooling rate on slab surface microstructure evolution in microalloyed steel

Abstract: In the process of continuous casting, the cooling rate is the key factor affecting and deciding the slab surface microstructure. In this study, confocal laser scanning microscopy was used for in situ observation of, and research on, slab surface microstructure evolution under different cooling rates. It was found that the slab microstructure was uniform, and there was no obvious filmlike proeutectoid ferrite or chainlike microalloyed element precipitation in the prior austenite grain boundary under the cooling rate of 3–6°C s−1. The results of hot tensile experiments showed that this microstructure had a higher hot ductility, which contributed to the virtual disappearance of the brittle trough and reduction of the steel's cracking susceptibility.

Effect of the strain reversal on austenite–ferrite phase transformation in a Nb-microalloyed steel

Abstract: Multipass torsion tests were performed on an Nb-microalloyed steel at decreasing temperatures so as to analyze the effect of strain reversal – in the strain accumulation region – on the γ → α transformation kinetics. Strain reversal involved a significant delay in the start of the transformation as well as a microstructural transient compared to monotonic strain conditions. However, after several passes with no reversal performed within the inter-critical temperature region, both types of tests led to similar microstructures.

Phase field simulation of austenite grain growth in the HAZ of microalloyed linepipe steel

Abstract: Phase field modelling is used to simulate austenite grain growth in the heat affected zone (HAZ) of an X80 linepipe steel The HAZ experiences a very steep temperature gradient during welding which restricts grain growth In addition to this phenomenon known as thermal pinning, austenite grain growth is affected by pinning due to precipitates and their potential dissolution Grain growth has first been simulated for bulk samples subjected to rapid heating conditions to replicate thermal cycles at various positions in the HAZ Effective grain boundary mobilities are introduced that are consistent with strong pinning at lower temperatures and weak pinning at higher temperatures These two temperature regimes are separated by the estimated dissolution temperature of fine NbC precipitates These mobility relationships are then used to predict austenite grain growth in the HAZ using typical time–temperature profiles

Influence of Nb on microstructure and property of low-carbon Mn-series air-cooled bainitic steel

Abstract: The effect of Nb on the microstructure evolution of low-carbon Mn-series air cooled bainitic steels has been studied by using thermomechanical simulation machine, SEM, and TEM observation. The results show that the amount of ferrites is relatively higher in the steel without Nb than that of Nb-bearing steel under same deformation conditions, and the ferrites in Nb-bearing steel are more finer. The mechanical properties of the Nb-bearing steel are higher than those of the steel without Nb at the same finishing rolling temperature (FRT), and the toughness of the Nb-bearing steel is about 100 J higher than that of the steel without Nb at the finishing rolling temperature 750 °C. The toughness of Nb microalloyed steel will be improved more effectively than that of the steel without Nb with the decrease of FRT.

Properties and homogeneity of 550-MPa grade TMCP steel for ship hull

Abstract: Ultra low carbon steels by the thermal mechanical control process (TMCP) with less Ni, Cr, and Mo contents have been developed for 550 MPa grade heavy gauge ship hulls and offshore structures. The relationships among microstructures, process, and properties of the studied steel have been investigated. A series of accurate control technologies have been developed for this kind of steel. Cu microalloying and TMCP+relaxation precipitation control (RPC)+accelerated cooling process were employed to optimize the mechanical properties and ensure the homogeneity of the 80-mm thick plate. The microstructures of thin plates slightly changed from surface to center, but the microstructures of the heavy gauge plate (80 mm) changed notably. Adopting the simple composition, it can meet the requirement of thin plates by adopting a few microalloys. As for thick plates (80 mm), a little higher Cu and Ni contents should be adopted. These steels can meet the needs without tempering. By these ways, the properties of the steels can be optimized, and the cost can be decreased notably.

Concurrent Precipitation of AlN and VN in Microalloyed Steel

Abstract: The precipitation kinetics of AlN and VN in microalloyed steel is investigated numerically with the thermo-kinetic software MatCalc. To mimic the heterogeneous precipitation of AlN along austenite grain boundaries, a new model is utilized, which takes into account the fast short circuit diffusion along grain boundaries and sluggish bulk diffusion inside the grains. It is demonstrated that the precipitation of VN inside the grains can be more rapid than the precipitation of AlN at the grain boundaries. However, the thermodynamically more stable AlN can overgrow and dissolve the existing VN again. The computed results are compared to experimental results from literature and good agreement is observed.