Strip-cast Steel Research Articles

The Effect of Direct Strip Casting on the Kinetics of Phase Transformation of a Dual Phase Steel

A dual phase steel has been produced directly from the liquid under conditions that simulate direct strip casting and thin slab casting. The kinetics of polygonal ferrite formation during the inter-critical anneal were quantified using the JMAK approach, and this revealed significantly retarded transformation kinetics in the strip cast samples compared to the commercial steel that was processed through the conventional hot rolling approach. The transformation rate in the strip cast samples were as much as three orders of magnitude slower compared to the commercial steel. It was found that the kinetics of the ferrite formation were retarded principally by the large prior austenite grain size in the strip cast samples, and this hypothesis was tested experimentally by both coarsening of the prior austenite grain size, and by refinement of the prior austenite grain size. However, even after grain size normalization, small differences in transformation kinetics between the direct strip cast and commercial steel specimens were observed. These differences were explained by investigation of MnS precipitation in the steels. It was found that the transformation rate is high when the solutes are in solid solution, and that the rate of transformation slows significantly when precipitation of nano-precipitates occurs.

The Effect of Molybdenum on Precipitation Behaviour in Austenite of Strip-Cast Steels Containing Niobium

Two low-C steels microalloyed with niobium (Nb) were fabricated by simulated strip casting, one with molybdenum (Mo) and the other without Mo. Both steels were heat treated to simulate coiling at 900 °C to investigate the effect of Mo on the precipitation behaviour in austenite in low-C strip-cast Nb steels. The mechanical properties results show that during the isothermal holding at 900 °C the hardness of both steels increases and reaches a peak after 3000 s and then decreased after 10,000 s. Additionally, the hardness of the Mo-containing steel is higher than that of the Mo-free steel in all heat-treated conditions. Thermo-Calc predictions suggest that MC-type carbides exist in equilibrium at 900 °C, which are confirmed by transmission electron microscopy (TEM). TEM examination shows that precipitates are formed after 1000 s of isothermal holding in both steels and the size of the particles is refined by the addition of Mo. Energy dispersive spectroscopy (EDS) and electron energy loss spectroscopy (EELS) reveal that the carbides are enriched in Nb and N. The presence of Mo is also observed in the particles in the Nb-Mo steel during isothermal holding at 900 °C. The concentration of Mo in the precipitates decreases with increasing particle size and isothermal holding time. The precipitates in the Nb-Mo steel provide significant strengthening increments of up to 140 MPa, higher than that in the Nb steel, ~96 MPa. A thermodynamic rationale is given, which explains that the enrichment of Mo in the precipitates reduces the interfacial energy between precipitates and matrix. This is likely to lower the energy barrier for their nucleation and also reduce the coarsening rate, thus leading to finer precipitates during isothermal holding at 900 °C.

The Microstructure, Antimicrobial Properties, and Corrosion Resistance of Cu‐Bearing Strip Cast Steel

The antimicrobial and corrosion properties of Cu‐bearing steels fabricated via simulated strip casting are examined. Two distinct microstructures are observed: those alloys with Cu concentrations below 5 wt% retain the Cu in solid solution, and those alloys with high Cu concentrations form a two‐phase structure containing an austenite matrix and large Cu‐rich precipitates. The alloys display antimicrobial properties against Escherichia coli, causing up to 65% of bacteria death upon direct contact for 24 h. Surface analysis by X‐ray photoelectron spectroscopy indicates that after exposure to bacteria, the surface passive film remains enriched in Cu and its oxides, and immersion studies confirm gradual release of Cu ions from the passive film in a wet environment. Corrosion testing of the Cu alloys demonstrates that when the copper is retained in solid solution, the corrosion behavior is not markedly different from the copper‐free reference alloy. However, in those higher‐concentration alloys that contain Cu‐rich precipitates, the anticorrosion performance of the steel is greatly deteriorated, with the corrosion mechanism changing from pitting corrosion to selective corrosion.

The effect of molybdenum on clustering and precipitation behaviour of strip-cast steels containing niobium

Two high-strength low-alloy (HSLA) steels containing Nb-carbonitrides were produced, one contained Mo and the other was Mo-free. The alloys were produced by simulated direct strip casting, and were fully bainitic in the as-cast condition. Isothermal ageing treatments were carried out to precipitate harden the alloy, and the strength was measured using a shear punch test. The dislocation density was measured with X-ray diffraction (XRD), and was found to be larger in the alloy containing Mo in all ageing conditions. Atom probe tomography (APT) showed the presence of solute clusters in the as-cast condition, and the addition of Mo increased both size and volume fraction of these clusters. The solute clusters provided significant strengthening increments of up to 112 MPa, and cluster strengthening was larger in the Mo-containing alloy. Precipitation of Nb-carbonitrides was observed after longer ageing times, which were refined by the addition of Mo. This was attributed to the higher dislocation density that increased the number of nucleation sites. Precipitate chemistry was similar for both alloys, and contrary to some literature reports, minimal Mo was observed to segregate to the precipitates. A thermodynamic rationale is presented which describes the reasons that Mo segregates to the Nb-carbide in some alloys but not in others, despite the alloy chemistries being relatively similar.

Computer simulation of microstructure evolution in strip-cast silicon steel with cube and fibre texture

ABSTRACTThin strip casting process which is already used for non-oriented grades of electrical steel, is relatively novel processing route for grain oriented electrical steel and to achieve the cube texture through suitable annealing. In the present investigation, a two-dimensional Monte Carlo model has been formulated to study texture evolution in strip-cast and annealed Fe–3.2%Si steel with a cube texture. Both random and non-random microstructures of Fe–3.2% silicon steel and corresponding Euler angles have been used as input parameters. Results obtained from the simulation characterised the evolving texture in terms of grain size distribution and misorientation distribution functions. It has been found that randomly textured microstructures follow power law growth kinetics with evolving grain size distribution similar to normal grain growth. Microstructures with random orientation texture evolve steady state misorientation distribution functions, while those with fibre texture narrows and shifts to low angles. The simulation results based on EBSD microstructure inputs have been compared and validated with microstructures obtained from computationally generated input microstructures as well as published literature for both random and non-random orientation texture and found to be in good agreement.

The Effect of Molybdenum on Clustering and Precipitation Behaviour of Strip-Cast Steels Containing Niobium

The Effect of Molybdenum on Clustering and Precipitation Behaviour of Strip-Cast Steels Containing Niobium

Effect of molybdenum on phase transformation and microstructural evolution of strip cast steels containing niobium

Molybdenum (Mo) is known to have a complex effect on phase transformations and precipitation in steels manufactured by conventional casting. The present work aims to examine the effect of Mo on phase transformations in Nb-containing steels produced by strip casting. Advanced experimental techniques have been utilised to simulate the strip casting process, and the microstructural features of the rapid solidification are retained for further study. Two cooling conditions from the austenite phase field were examined, isothermal holding and continuous cooling. It was found that at high cooling rates, the addition of Mo delayed the nucleation of bainite and lowered the bainite start temperature, but did not alter the bainite growth rate. The addition of Mo was also found to result in a slower transformation rate of polygonal ferrite under both isothermal and continuous cooling conditions. Thermodynamic simulations indicated that Mo did not affect the growth velocity of the polygonal ferrite, and quantitative metallography showed the nucleation density was significantly reduced by Mo addition. For the slowest continuous cooling rate, the addition of Mo completely inhibited pearlite formation, with bainitic ferrite forming instead. This has been suggested to be the result of the suppression of pearlite nucleation, rather than inhibition of growth.

Static recrystallisation of steels produced by direct strip casting – The effect of carbon and vanadium concentration

Six steel alloys containing a range of carbon and vanadium concentrations have been prepared by simulated strip casting. The alloys were cold rolled and annealed to examine the recrystallisation behaviour. The annealing treatment resulted in three processes occurring concurrently: recrystallisation; spheroidisation of cementite, and precipitation of vanadium-enriched nano-precipitates. These nano-precipitates had a much higher concentration of Fe than would be expected from traditional processing methods, and this increased their maximum attainable volume fraction. In the two alloys that did not contain second phase particles, recrystallisation showed typical reaction kinetics, and the recrystallised grain size continued to increase with increasing time at temperature. However, in those alloys with second phase particles the recrystallised grains showed unusual behaviour, rapidly reaching an upper limit to their size. Continued time at temperature was not accompanied by an increase in the grain size. This growth limit has been attributed to Zener pinning, with the limiting grain size being proportional to the Zener pinning pressure. It has been proposed that the delayed recrystallisation that is typically observed in strip cast steels is likely to be the result of nano-scale precipitation which is unique to rapidly cooled materials, such as those produced by strip casting.

Effect of coiling treatment on microstructural development and precipitate strengthening of a strip cast steel

The effect of a simulated coiling treatment on a strip cast Nb-containing steel has been investigated. A lath ferritic supersaturated microstructure was observed in the as-cast condition with no coiling. The microstructure remained lath like during coiling at high temperature (850 °C) and the formation of chemically complex Nb-rich precipitates containing C, N, Si and S was observed. Coiling at an intermediate temperature (700 °C) caused the formation of polygonal ferrite with a dendritic morphology due to chemical micro-segregation. The polygonal ferrite contained Nb(C,N) precipitates. The microstructure remained lath like at the lowest coiling temperature (600 °C). In the latter case the precipitation of Nb-rich clusters was observed, and atom probe tomography revealed them to be ∼85% Fe. Small angle neutron scattering and transmission electron microscopy were used to quantify precipitation kinetics during coiling and the mechanical properties were evaluated with a shear punch apparatus. A yield strength model was developed to describe the observed mechanical behaviour, and this showed that the two largest contributors to strength were the bainitic microstructure and the Nb-rich precipitates. Strategies to further strengthen these materials are suggested.

The effect of clustering on the mobility of dislocations during aging in Nb-microalloyed strip cast steels: In situ heating TEM observations

Cluster-strengthened Nb-microalloyed strip cast steels are of interest as clustering during aging leads to an enhancement in strength without compromising ductility, resulting in desirable mechanical properties. However, the precise strengthening mechanism is not well understood. Using in situ heating transmission electron microscopy, clustering was found to impede the movement of dislocations during aging. The attractive combination of ductility and strength was attributed to the effects of recovery and the restricted movement of dislocations through clustering.

A quantitative atom probe study of the Nb excess at prior austenite grain boundaries in a Nb microalloyed strip-cast steel

Most modern HSLA steels rely on the effect of Nb in steels to achieve the properties desired for a specific application. While the role of Nb in forming precipitates has been well characterized, its role in a solid solution is less well understood due to the difficulty of obtaining quantitative experimental data. In the current work, site-specific atom probe tomography was used to quantify the amount of Nb present at prior austenite grain boundaries in a commercial strip-cast steel, produced via the Castrip® process. This was compared to the amount of Nb found at ferrite–ferrite grain boundaries that had formed during the transformation from austenite to ferrite. With the interfacial excess Nb measured, thermodynamic calculations were carried out and compared to the change in transformation temperature obtained by dilatometry, with reference to a comparable Nb free, strip-cast steel.

Shaping the lens of the atom probe: Fabrication of site specific, oriented specimens and application to grain boundary analysis

The random sampling provided by classical atom probe sample preparation methods is one of the major factors limiting the types of problems that can be addressed using this powerful technique. A focused ion beam enables not only site-specific preparation, but can also be used to give the specimen, which acts as the lens in an atom probe experiment, a specific shape. In this paper we present a technique that uses low accelerating voltages (10 and 5 kV) in the focused ion beam (FIB) to reproducibly produce specimens with selected grain boundaries <100 nm from the tip at any desired orientation. These tips have a high rate of successfully running in the atom probe and no Ga contamination within the region of interest. This technique is applied to the analysis of grain boundaries in a high purity iron wire and a strip-cast steel. Lattice resolution is achieved around the boundary in certain areas. Reconstruction of these datasets reveals the distribution of light and heavy elements around the boundary. Issues surrounding the uneven distribution of certain solute elements as a result of field-induced diffusion are discussed.

Segregation in Twin Roll Strip Cast Steels and the Effect on Mechanical Properties

Strips of low carbon steels containing high contents of P and Cu have been produced by twin roll strip casting. The as-cast strips containing more than 0.1 mass% P exhibited obvious inverse surface segregation, which was difficult to be eliminated by the subsequent annealing treatments. After cold rolling and annealing treatment at 625°C for 30 min, the strip containing 0.1 mass% P exhibited similar plastic deformation and rupture behaviors with the strip containing 0.01% P, showing no obvious deteriorating effects of P on the ductility of as-cast strip. With increasing P content in steel, more cleavage cracks, micro voids and inter-granular rapture were observed in the fractured areas after tensile testing, which are believed to have been induced by the segregation of P in as cast strips. These results provide primary guidance for recycling dirty steel scraps.

Microstructure and Mechanical Properties of Strip Cast 1008 Steel after Simulated Coiling, Cold Rolling and Batch Annealing.

As-cast low carbon steel strips produced by twin roll strip casting process were treated by cold rolling and batch annealing. The microstructure and mechanical properties were compared to those of drawing and commercial quality strips. The mechanical properties of the as-cast strip are inferior to those of an industrial hot band due to the presence of Widmannstatten ferrite microstructure. After simulated coiling, cold rolling and batch annealing the ductility of the strip cast steel is lower than that of the industrial products, due to the hardening effect of small Cu-S precipitates and coarser iron carbides. However, for the strip cast steel processed according to a drawing quality (DQ) route, the mechanical properties are within the range of a commercial quality (CQ) product. Therefore, mechanical properties falling within the range of commercial quality steel can be obtained with strip cast steel without a hot rolling step if appropriate cold rolling and annealing treatment is used.