Showing papers on "Microalloyed steel published in 2004"

Mechanisms and modeling of cleavage fracture in simulated heat-affected zone microstructures of a high-strength low alloy steel

Abstract: The effect of the welding cycle on the fracture toughness properties of high-strength low alloy (HSLA) steels is examined by means of thermal simulation of heat-affected zone (HAZ) microstructures. Tensile tests on notched bars and fracture toughness tests at various temperatures are performed together with fracture surface observations and cross-sectional analyses. The influence of martensite-austenite (M-A) constituents and of “crystallographic” bainite packets on cleavage fracture micromechanisms is, thus, evidenced as a function of temperature. Three weakest-link probabilistic models (the “Master-curve” (MC) approach, the Beremin model, and a “double-barrier” (DB) model) are applied to account for the ductile-to-brittle transition (DBT) fracture toughness curve. Some analogy, but also differences, are found between the MC approach and the Beremin model. The DB model, having nonfitted, physically based scatter parameters, is applied to the martensite-containing HAZ microstructures and gives promising results.

Charpy-impact-toughness prediction using an “effective” grain size for thermomechanically controlled rolled microalloyed steels

Abstract: Thermomechanically controlled rolling of steel plate can involve substantial straining in the intercritical temperature region, which may result in the final ferrite grains not fully recrystallizing, and, hence, the presence of low-angle grain boundaries. It is shown in this article that a Nb-microalloyed thermomechanically controlled rolled (TMCR) steel can contain a high proportion of low-angle grain boundaries (the extent depending on the thermomechanically controlled rolling schedule) and that during toughness testing, the crack front ignores boundaries with less than a 12 deg misorientation. Thus, the average microstructural unit experienced by the crack front (i.e., the cleavage facet) is significantly larger than the average metallographic, two-dimensional grain size. Consequently, use of the metallographic grain size gives a poor prediction of the impact transition temperature (ITT) and fracture stress for these steels. It is also shown that the micromechanism of crack initiation and propagation involves grain-boundary carbides and groups of closely aligned grains that act as single “effective” grains.

The effects of vanadium, niobium, titanium and zirconium on the microstructure and mechanical properties of thin slab cast steels

Abstract: The evolution of precipitation and microstructure during a simulation of the thin slab direct rolling process, in six vanadium based, low carbon, steels with V, V-N, V-Ti-N, V-Nb, V-Nb-Ti and V-Zr additions was studied by optical microscopy, analytical transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDAX) and parallel electron energy loss spectroscopy (PEELS). Tensile properties and Charpy vee-notch toughness of the final strip were also determined. The effects of microalloying additions and processing conditions, including equalisation temperature (1 200°C, 1 100°C and 1 050°C) and end water cool temperature, on the austenite and ferrite grain sizes, as well as the type and composition of the precipitates, were determined. The relationship between the microstructure and the properties in the steels was also ascertained.

Niobium Carbide Precipitation in Microalloyed Steel

Abstract: The precipitation of niobium carbo-nitrides in the austenite phase, interphase and ferrite phase of microalloyed steel was assessed by a critical literature review and a round table discussion. This work analyses the contribution of niobium carbide precipitates formed in ferrite in the precipitation hardening of commercially hot rolled strip. Thermodynamics and kinetics of niobium carbo-nitride precipitation as well as the effect of deformation and temperature on the precipitation kinetics are discussed in various examples to determine the amount of niobium in solid solution that will be available for precipitation hardening after thermomechanical rolling in the austenite phase and successive phase transformation.

Kinetics of Metadynamic Recrystallization in Microalloyed Hypereutectoid Steels

Abstract: The isothermal kinetics of the recrystallization processes of vanadium microalloyed high carbon steels has been measured and modelled. The overall softening data were obtained by double hit compression tests performed at temperatures between 900 to 1 050°C, strain rates of 0.01 to 1 s -1 , and inter-pass times of 0.1 to 30 s. The recrystallization behavior above and below and the critical strain for dynamic recrystallization was investigated. The results show that there is a transition strain region between where both static and metadynamic recrystallization take place during the inter-pass time. The results also revealed that V and Si have a strong solute drag effect, on the kinetics of metadynamic recrystallization. A kinetic model is proposed which takes the V and Si concentrations into account.

Modeling of Austenite Grain Size Distribution in Nb Microalloyed Steels Processed by Thin Slab Casting and Direct Rolling (TSDR) Route

Abstract: A mathematical model has been developed to predict the austenite microstructure evolution of Nb microalloyed steels during “Thin slab casting” and “Hot direct rolling” (TSDR) processing. The model is based on empirical equations specifically derived for the microstructural and processing features typical in these new technologies. Its main novelty is that it works with austenite grain size distributions instead of the typical mean values as used in conventional models to represent the microstructure. This fact is particularly important in working with as-cast austenite due to the wide range of grain sizes present in this microstructure. In the model the different softening and hardening mechanisms that can operate during hot working in austenite are considered: static, dynamic and metadynamic recrystallization, grain growth after recrystallization and Nb(C, N) strain induced precipitation. The model uses the initial austenite grain size distribution as input and provides the size distribution of recrystallized and unrecrystallized grains at the entry of any rolling pass. A validation of the model has been carried out in the laboratory by multipass torsion tests. The model is capable of predicting any heterogeneities that may appear in the final microstructure after this kind of processing and that are not well predicted by using conventional models based on mean values. Additionally, it can calculate the deformation history, in terms of the strain accumulated in the austenite, and stress behavior, in terms of the mean flow stress (MFS) corresponding to each deformation pass.

The static and metadynamic recrystallization behaviour of an X60 Nb microalloyed steel

Abstract: 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.

New Interpretation of the Effect of Hydrogen on the Ion Distributions and Structure of Passive Films on Microalloyed Steel

Abstract: It has been reported that hydrogen decreases the stability of the passive films on iron and stainless steel and promotes their localized corrosion. However, the mechanism of hydrogen-promoted corrosion is still not clear. Potentiostatic, electrical impedance spectroscopy measurements, and Mott-Schottky analysis were used to investigate the effect of hydrogen on the properties of the passive film on X70 microalloyed steel. In the range of passive potentials, hydrogen retards the passive film formation and decreases its stability. Impedance spectra of passive film on X70 steel show that hydrogen decreases the resistance towards charge transfer and ion diffusion and increases the capacitance of the passive film. Mott-Schottky analyses show that hydrogen increases the capacitance and donor density, and decreases the flatband potential and the space-charge layer thickness of the passive film. A new explanation is proposed for the effects of hydrogen on the properties of the passive films. According to this explanation, hydrogen creates an additional electric repulsion on Fe 2+ or Fe 3+ and oxygen vacancies, and enhances the diffusion rate of cations and anion vacancies. At steady state, hydrogen will not only decrease the concentrations of O 2- and Fe 3+ , but also increase the concentration of Fe 2+ in the passive films, resulting in changes in the structure and a decrease in the thickness of the passive films.

Austenite Grain Coarsening Under the Influence of Niobium Carbonitrides

Abstract: This work investigates grain growth behaviour under the influence of pinning carbonitrides in a niobium microalloyed steel. The effect of temperature and heating rate on the grain size is studied. The grain coarsening temperature is determined as a function of the heating rate. It is found that unpinning by precipitates occurs around 40-70 K below the temperature of complete dissolution of carbonitrides. It has been found that experimental results are best described assuming a random distribution of carbonitrides in the matrix and niobium volume diffusion as the rate controlling process for the coarsening of precipitates. Austenite grain growth is explained theoretically taking as a basis the model proposed by Zener, which has been adapted for non-equilibrium kinetics, taking into account the experimental evidence that during a continuous heating, the amount of microalloyed element in solid solution is altered and different from that predicted by the solubility product.

Effect of carbonitride particles formed in austenite on the strength of microalloyed steels

Abstract: Precipitation strengthening from two different carbonitride populations was investigated in a niobium and vanadium microalloyed hot-rolled steel. Interphase precipitation as well as carbonitride particles formed in austenite were found to contribute to precipitation strengthening. After normalizing, austenite precipitation strengthening remained effective while the strengthening contribution of interphase precipitation was strongly reduced.

The Kinetics of Static Recrystallization in Microalloyed Hypereutectoid Steels

Abstract: Compression tests were conducted in order to study the static recrystallization kinetics of hot deformed austenite in hypereutectoid steels containing 1 % carbon with different levels of vanadium and silicon. Tests were performed over a temperature range of 875 to 1 100°C using strain rates of 0.01, 0.1 and 1 s -1 . Graphs of the recrystallized fraction versus time were used to quantify the kinetics of the strain-induced precipitation and generate the precipitation temperature time diagrams for the three steels. A kinetic model for static recrystallization is proposed which takes the V and Si concentrations into account.

Hydrogen-involved tensile and cyclic deformation behavior of low-alloy pressure vessel steel

Abstract: The temperature- and strain-rate-dependent tensile behavior of hydrogen-charged low-alloy pressure vessel steel ASTM A508 C1.3 has been investigated. The fatigue crack initiation and propagation behavior of the steel in high-temperature water environments has also been evaluated. It was found that hydrogen played significant roles in both tensile and cyclic deformation processes, especially in the temperature and strain-rate region of dynamic strain aging (DSA). The presence of hydrogen resulted in a distinct softening in tensile strength and a certain loss in tensile ductility in the DSA region. Remarkable degradation in fatigue crack initiation and propagation resistance in high-temperature water environments was observed in the DSA strain-rate region. Typical hydrogen-induced cracking features also appeared on the corresponding fatigue fracture surfaces. The interactions between hydrogen and DSA in tensile and cyclic deformation processes are discussed as well as their combined effects on the environmentally assisted cracking (EAC) mechanism of pressure vessel steels in high-temperature water environments.

Effect of duplex ferrite grain size distribution on local fracture stresses of Nb-microalloyed steels

Abstract: Two commercial thermomechanically controlled rolled (TMCR) microalloyed steel plates have been used to investigate the relationship between the duplex ferrite grain size distribution and local fracture stresses. Statistical analyses of the grain size distributions were performed for the fine and coarse ferrite grains in the two steel plates. Microhardness values were measured for each grain size region and it was found that the fine grain areas have significantly higher microhardness values than the coarse grain areas. Tensile and blunt-notch slow bend tests were carried out over a range of temperatures on samples from the two commercial TMCR steel plates. The local fracture stress (σF) values were calculated and the results show that the σF values are almost independent of temperature. The presence of a mixed grain size distribution results in significant scatter in the local fracture stresses of the steels. The distribution of fracture stress values can be correlated to the coarse grain size distribution in the steels examined.

Failure mode analysis and a mechanism for hot-ductility improvement in the Nb-microalloyed steel

Abstract: Loss of hot ductility at the straightening stage of the continuous casting of high-strength low-alloy steel is attributed to different microalloying elements, in particular, Nb. However, such elements are essential for the desired mechanical characteristics of the final product. Since the chemistry cannot be altered to alleviate the problem, thermomechanical processing was studied in order to improve the hot ductility. Two Nb-microalloyed steels, one also containing B, were examined. The thermal history occurring in the continuous casting process was taken into account as well. First, it was noticed that the steel with B has a higher hot ductility than the other after being subjected to in-situ melting followed by the thermal schedule. Grain boundary sliding was recognized as the failure mechanism. Then, the effect of deformation applied in the vicinity of the δ→γ transformation, while the thermal schedule was being executed, was investigated. Such deformation appeared to improve the hot ductility remarkably. Finally, the mechanism of such improvement in the hot ductility was elaborated.

Effect of Silicon on the Interaction between Recrystallization and Precipitationin Niobium Microalloyed Steels

Abstract: The effect of Si addition on the interaction between recrystallization and precipitation was investigated in terms of the no-recrystallization temperature (T nr ) on three microalloyed steels containing about 0.035 mass% Nb. The T nr was measured using torsion testing over the Si concentration range from 0.01 to 0.48 mass%. It was observed that the T nr increased with Si level, but appeared to saturate at long interpass times. In addition, high strains reduced the influence of Si on the T nr . This behaviour is attributed to the acceleration of Nb(C, N) precipitation by the addition of Si.

Effect of Heat Treatment on Microstructures and Tensile Properties of Ultrafine Grained C-Mn Steel Containing 0.34 mass% V

Abstract: An ultrafine grained (UFG) C-Mn steel containing a relatively large amount of vanadium was fabricated by equal channel angular pressing (ECAP) and its microstructures and tensile properties were examined. This investigation was aimed at demonstrating the effect of precipitation stage of vanadium precipitates in the course of the material processing on the tensile properties of the ultrafine grained C-Mn steel, especially strength and strain hardening capability. For this purpose, the two different heat treatments were carried out on the present steel: (a) conventional normalization for vanadium precipitation before ECAP, and (b) isothermal transformation for vanadium precipitation during ECAP and subsequent annealing. The results showed that the second heat treatment was more effective on improving the thermal stability and the overall tensile properties of the steel by better uniform distribution of nano-sized vanadium precipitates which yielded an extensive interaction with lattice dislocations inside ultrafine ferrite grains. In addition, in this report, the feasibility enhancing the strain hardening capability of the UFG C-Mn steel was explored by comparing the microstructure and stress-strain curve of the steel prepared by the second heat treatment with those of the UFG C-Mn steel without vanadium.

Effect of texture and microstructure on resistance to cracking of high-strength hot-rolled Nb-Ti microalloyed steels

Abstract: Significant texture gradient in the through-thickness direction was observed in high-strength hot-rolled 560 and 770 MPa Nb-Ti microalloyed steels, characterized by polygonal ferrite and ferrite bainite microstructures, respectively. {113}〈110〉 was the most intense deformation texture in the two high-strength grades of Nb-Ti steels and was dominant in the midthickness region compared to 10 and 25 pct depth below the surface. The recrystallization texture of austenite, {100}〈001〉, transformed into {100}〈011〉 component in the ferrite and indicated an increase in the intensity with increase in depth for the Nb-Ti microalloyed steels. The {100}〈011〉 texture has a detrimental effect on the edge formabiity of steels. However, the midthickness plane contained considerable intensity of desired texture, {332}〈113〉, which is expected to offset the undesirable {100}〈011〉 texture resulting in superior edge formability and impact toughness of Nb-Ti steels, consistent with experimental observations.

Effect of Manganese Partitioning on Transformation Induced Plasticity Characteristics in Microalloyed Dual Phase Steels

Abstract: The manganese contents of the dual phase steels with transformation induced plasticity characteristics are reasonably high. Efforts have been made to study the role of manganese in making stable austenite. The effect of the varying holding time at different intercritical annealing temperatures on the formation of manganese enriched austenite by a competitive process have been studied. The effect of manganese partitioned austenite so produced on the TRIP behaviour of the steel has also been studied through processing structure property correlation. It is seen that manganese enrichment in retained austenite depends on the time and temperature of intercritical annealing.

Validation of the New Regression Model for the Static Recrystallisation of Hot-Deformed Austenite in Special Steels

Abstract: A linear regression model consisting of the weighted sums of certain alloying elements has recently been developed to predict the activation energy (Qrex) and kinetics of static recrystallisation (SRX) for hot-deformed austenite based on stress relaxation test results for over 40 different carbon steels. The validity of the model has been further assessed here by determining the Qrex and the kinetics of SRX of certain high-Nb bearing steels, extra-low and low carbon Nb-Mo bainitic and high-Si dual phase and TRIP steels, and Nb-Ti grades with the varying N content. The validity of the model is shown to be fairly good for the Nb-Ti, Nb-Mo and Cr-Mo grades. The approach of maximum effective concentration of Nb and Si and the weight factor for Cr enable reasonable fit for DP, TRIP and Nb-Cr steels, as well. Possible influences of C and N on Qrex and the kinetics of SRX were checked, but none was observed in microalloyed steels.

Microstructure – property relationships in thermomechanically processed microalloyed medium carbon steels

Abstract: Hot forged plates of two microalloyed grades of 1541 steel have been produced by thermomechanical (TMP) forging schedules. The yield strength and Charpy impact toughness are both increased by TMP treatment, compared to a conventional forging schedule. The strength increase is due to reductions in the ferrite grain size and pearlite colony size. There is also significant precipitation strengthening in the Ti, V microalloyed steel (1541+ Ti, V). The lowering of the impact transition temperature and the increase in Charpy energy produced by the TMP treatments are attributed to the decreases in ferrite grain size and pearlite colony size. In addition, for Nb microalloyed steel, the elongated microstructure and intragranular ferrite produced by TMP forging make the effective pearlite colony size very small in the direction of crack propagation.

Evaluation of corrosion resistance of microalloyed reinforcing steel

Abstract: The corrosion resistance of three microalloyed steels and two conventional reinforcing steels in concrete was evaluated. The microalloyed steels contain concentrations of chromium, copper, and phosphorus that, while low, are significantly higher than used in conventional reinforcing steel. Two of the microalloyed steels contain amounts of phosphorus that exceed the amounts allowed in ASTM specifications (ASTM A 615), while the other microalloyed steel has normal amounts of phosphorus. One of the conventional steels and the three microalloyed steels are heat treated by the Thermex process, which includes quenching and tempering of the steel immediately after rolling, while the other conventional steel is hot-rolled. The study was undertaken because earlier tests on similar steels indicated that the Thermex-treated, microalloyed steel corrodes at only one-half the rate of conventional reinforcing steel. The relative corrosion rate dropped to one-tenth if both steels were epoxy-coated. In the current study, the reinforcing steels were tested using two rapid evaluation tests, the corrosion potential and corrosion macrocell tests, and three bench-scale tests, the Southern Exposure, cracked beam, and ASTM G 109 tests. The corrosion potential, corrosion rate, and mat-to-mat resistance are used to evaluate the steel. Tension and bending tests were performed to evaluate the effect of the microalloying and heat treatment on the mechanical properties of the reinforcing steel. Results show that the corrosion potential of the five steels is approximately the same, indicating that they have a similar tendency to corrode. The results from the rapid macrocell test showed that the five steels had similar corrosion rates, with no improved behavior for the microalloyed steels. The microalloyed steel with regular phosphorus content (CRT) exhibited consistently lower corrosion losses than conventional steel in the bench-scale tests. Although CRT appears to be much more corrosion resistant than conventional steel in the G 109 tests (64% less total corrosion loss after 70 weeks), its overall performance does not show such an advantage. In the cracked beam test after 70 weeks, it had only 4% less corrosion loss than conventional steel, which indicates that in cracked concrete the two steels behave in a similar manner. In the Southern Exposure test, CRT steel had an 11% lower corrosion loss than conventional steel after the same period. This improved behavior is not enough to use the steel without an epoxy coating or to justify continued research on the steel as a superior epoxy-coated material. The mechanical properties of the microalloyed steels were similar to those of conventional steel, indicating that the increased phosphorus content did not affect the mechanical properties.