Showing papers on "Microalloyed steel published in 1997"

Modeling the Flow Behavior of a Medium Carbon Microalloyed Steel under Hot Working Conditions

Abstract: The constitutive equations for the flow behavior of a commercial 0.34 pct C-1.5 pct Mn-0.7 pct Si-0.083 pct V-0.018 pct Ti microalloyed steel were determined. For this purpose, uniaxial hot compression tests were carried out over a wide range of strain rates (10−4 to 10 s−1) and temperatures (1123 to 1423 K). In combination with models developed in the literature, the experimental results permit the flow stress of the present steel to be predicted within ± 5 pct. It is shown that the classical constitutive equations must be modified to take the grain size into account, particularly when the latter is below 30 µm.

Precipitation behavior in ultra-low-carbon steels containing titanium and niobium

Abstract: This work revealed the basic mechanism for the stabilization of carbon in ultra-low-carbon (ULC) steels that contain moderate S (0.004 to 0.010 wt pct), adequate Ti (0.060 to 0.080), and low Mn (≤0.20). During cooling through the austenitic region to the ferritic, the initially formed sulfide particles (TiS) undergo an in situ transformation into carbosulfides (H-Ti4C2S2) by absorbing C and Ti. The transformation from TiS to H may be considered as a hybrid of shear and diffusion, i.e., faulted Ti8S9 (9R)+10[Ti]+9[C] → 4 1/2Ti4C2S2 (H). At low temperature (≤930 °C), the stabilization process continues through epitaxial growth of carbides on H phase, i.e., [M]+x[C]+H → epitaxial MCx (on H). This mechanism differs from the traditional view of stabilization, where the carbon is removed from solution by the formation of free-standing or independently nucleated H and/or MCN precipitates. While these two forms of carbon stabilization are now well known, this article presents a method of predicting which mechanism of stabilization will be operative in a given steel based on its bulk composition. Implications bearing upon new ULC steel design, considering the role of S, will be discussed.

Retained austenite characteristics in thermomechanically processed Si-Mn transformation-induced plasticity steels

Abstract: It is well known that a significant amount of retained austenite can be obtained in steels containing high additions (>1 pct) of Si, where bainite is the predominant microconstituent. Furthermore, retained austenite with optimum characteristics (volume fraction, composition, morphology, size, and distribution), when present in ferrite plus bainite microstructures, can potentially increase strength and ductility, such that formability and final properties are greatly improved. These beneficial properties can be obtained largely by transformation-induced plasticity (TRIP). In this work, the effect of a microalloy addition (0.035 pct Nb) in a 0.22 pct C-1.55 pct Si-1.55 pct Mn TRIP steel was investigated. Niobium was added to enable the steel to be processed by a variety of thermomechanical processing (TMP) routes, thus allowing the effects of prior austenite grain size, austenite recrystallization temperature, Nb in austenite solid solution, and Nb as a precipitate to be studied. The results, which were compared with those of the same steel without Nb, indicate that the retained austenite volume fraction is strongly influenced by both prior austenite grain size and the state of Nb in austenite. Promoting Nb(CN) precipitation by the change in TMP conditions resulted in a decrease in the V RA . These findings are rationalized by considering the effects of changes in the TMP conditions on the subsequent transformation characteristics of the parent austenite.

Dispersion hardening by niobium carbonitride precipitation in ferrite

Abstract: The formation offine carbonitride distributions in an experimental, low carbon, high niobium microalloyed steel transformed at various cooling rates was accompanied by TEM observations, tensile tes...

Effect of the Ti/N ratio on the hardenability and mechanical properties of a quenched-and-tempered C-Mn-B steel

Abstract: Ten experimental 0.18 pct C-1.2 pct Mn- 0.002 pct B steels with various Ti/N ratios were evaluated in this study. The hardenability of these steels was first determined using Jominy tests. Slab sections were then rolled to produce 12.5-mm-thick plates, and subsequently quenched and tempered for mechanical property evaluation. The volume fraction of coarse (greater than 1 µm) TiN particles was measured in all steels using quantitative metallographic techniques. Scanning transmission electron microscopy was used to investigate fine precipitates, and scanning electron microscopy was used to examine the fracture surface of Charpy specimens. The results show that a complete boron (B) hardenability effect is obtained with Ti/N ratios ≥2.9, a value slightly below the stoichiometric Ti/N ratio of 3.4. Any excess Ti, above that which combines with N, provides an additional increase in hardenability on quenching (effect of Ti in solution) and an increase in strength on tempering (Ti (C,N) precipitation). Steels with a higher (Ti)(N) product develop a higher volume fraction of coarse TiN particles during solidification. These coarse TiN particles result in reduced toughness levels of the heat-treated plates evaluated in the present study.

Determination of the energetic parameters controlling cleavage fracture initiation in steels

Abstract: The process of brittle fracture in steels can be divided into three distinct steps: (1) initiation of a microcrack in a brittle particle, (2) propagation of the microcrack into the surrounding matrix and, finally, (3) crack progression through the matrix. Depending on microstructure, temperature and loading rate, the critical step which controls cleavage fracture is subject to change. In this work the behaviour of different microalloyed steels is considered and the energies γ pm and γ mm , which define the stress necessary for the microcrack to surmount steps 2 and 3 have been experimentally determined. While the γ pm value remains constant around 7 J/m 2 , it has been observed that γ mm is dependent on temperature. At -196°C the value is lower than 50 J/m 2 and at room temperature it is higher than 200 J/m 2 . This increase in the matrix-matrix energy with temperature increases the probability of microcracks, generated in particles, arresting at grain boundaries. This is the reason why refinement of grain size has an important effect in improving the fracture toughness at room temperature.

Quantitative study of carbonitride precipitation in niobium and titanium microalloyed hot strip steel

Abstract: A commercial microalloyed steel was examined by optical and transmission electron microscopy in order to determine the origin and the amount of carbonitride precipitation which had formed as the result of industrial processing on a hot strip mill. Almost one-half of the total microalloy addition (0.06Ti and 0.02 Nb) had remained in the form of eutectic or undissolved particles after soaking. Intragranular nucleation of fine carbonitride particles was observed to have occurred only in austenite. No additional carbonitride particles had formed in ferrite during coiling and almost one-half of the microalloying elements dissolved during soaking had remained in solution at the end of the industrial rolling process. According to well established structure-propertyrelationships, a precipitation strengthening potential of 60-80 MN m−2 could be assigned to thecarbonitride particles whichhad nucleated in austenite.

Abnormal grain growth of austenite in a V-Nb microalloyed steel

Abstract: An investigation was made of the grain-growth kinetics of the V-Nb microalloyed steel austenite phase at temperatures in the range 1173–1373 K, relevant for the understanding of the steel thermomechanical processing The fine precipitation of vanadium- and niobium-rich particles plays an important role in helping the alloy to maintain a fine grain structure, which was characterized using optical microscopy, scanning electron microscopy and transmission electron microscopy It was found that at temperatures where the fine precipitate particles are stable, the grain-growth process is slow and the mode of grain growth is normal At higher temperatures where the precipitates are prone to dissolve, the abnormal grain-growth mode dominates All the observations were comprised in a time-temperature map of the grain-growth modes The map was built up by employing extensive line-intercept measurements of grain sizes at different temperatures and annealing times, followed by numerical calculations yielding curves of grain-growth rate as a function of grain size at different time intervals The present experimental observations were found consistent with the results of calculations and predictions of the earlier theoretical work

Effect of Boron Addition on the Microstructure of Hot-deformed Ti-added Interstitial Free Steel

Abstract: Effect of boron (B) addition and hot-deformation on microstructure of ferrite was studied using Ti-added interstitial free (IF) steels containing different amount of B. It was clarified that bainitic ferrite having typically lath or plate morphology with high dislocation density can be obtained also in plain IF steels by rapid cooling from austenite region. B addition largely affected the austenite-ferrite transformation and the amount of bainitic ferrite increased with increasing B content. Hot-deformation of austenite enhanced the transformation to polygonal or quasi-polygonal ferrite, resulting in decrease of bainitic ferrite. Interrupting compression tests indicated that enhanced transformation by hot-deformation is mainly due to grain refinement of austenite by recrystallization after hot-deformation. The cooling rate after hot-deformation produced the reversed effect of B on grain size of polygonal or quasi-polygonal ferrite. Ferrite grain size slightly decreased with increasing B content in the case of relatively large cooling rate. When the materials were cooled slowly after hot-deformation, on the other hand, the ferrite grain size increased with increasing B content. The coarse ferrite in B-bearing steels had fairly irregular shape of grain boundaries and inhomogeneous grain size, which suggested the contribution of somewhat discontinuous grain growth. This coarsening of ferrite would be responsible for the previously reported fact that B addition lowers Lankford value of cold-rolled and annealed IF steel sheets.

Weldability and toughness assessment of Ti-microalloyed offshore steel

Abstract: The present study has been carried out to investigate the coarse-grained heat-affected zone (CGHAZ) microstructure and crack tip opening displacement (CTOD) toughness of grade StE 355 Ti-microalloyed offshore steels. Three parent plates (40-mm thick) were studied, two of which had Ti microalloying with either Nb + V or Nb also present. As a third steel, conventional StE 355 steel without Ti addition was welded for comparison purposes. Multipass tandem submerged arc weld (SAW) and manual metal arc weld (SMAW) welds were produced. Different heat-affected zone (HAZ) microstructures were simulated to ascertain the detrimental effect of welding on toughness. All HAZ microstructures were examined using optical and electron microscopy. It can be concluded that Ti addition with appropriate steel processing, which disperses fine TiN precipitates uniformly, with a fine balance of other microalloying elements and with a Ti/N weight ratio of about 2.2, is beneficial for HAZ properties of StE 355 grade steel.

Determination of precipitation–time–temperature (PTT) diagrams for Nb, Ti or V micro-alloyed steels

Abstract: A method is described, developed in CENIM-CSIC, for studying the kinetics of strain induced precipitation in micro-alloyed steels. Using torsion tests, the statically recrystallized fraction has been determined for three Nb, V and Ti micro-alloyed steels at different temperatures and strains. When precipitation starts, the recrystallized fraction deviates from Avrami’s equation, making it possible to identify the moment at which precipitation starts (Ps) and finishes (Pf). In this way precipitation–time–temperature(PTT) curves can be drawn, showing the precipitation kinetics in graph form.

Orientation dependence of microfracture behavior in a dual-phase high-strength low-alloy steel

Abstract: In selecting the processing conditions and evaluating the reliability of structural materials, microscopic observations and identification of the fracture mechanisms in local cracking behavior are required. An important instance in the failure of the local brittle zone (LBZ) in the welding zone. The LBZ, which is very brittle, is the coarse-grained heat-affected zone near the fusion line, a zone known to be critical to the fracture toughness of welded parts. Thus, maintaining stable fracture resistance by predicting the microfracture behavior is important when using high-strength low-alloy (HSLA) steels in offshore structural steel welds. Depending on the thermal cycles involved during welding, the ferrite/martensite structure can have various morphologies of martensite particles, for example, fibrous and blocky martensite. In summary, in situ SEM fracture tests reveal that in the L-oriented IQ DCB specimen, a microcrack tends to propagate relatively uniformly throughout the ferrite and well-distributed fine fibrous martensite, yielding good elongation with high strength level. Also, the IQ structure in the T orientation shows similar microfracture behavior. On the other hand, in the SQ structure, where blocky-type martensite is mixed with ferrite, strain is localized into shear bands mostly in the ferrite region, and a local microcrack propagates along the strain-localizedmore » band formed in the ferrite, resulting in the SQ structure in the T orientation, where the ferrite-martensite bands are parallel to the notch direction, the martensite cannot act as an efficient barrier to microcrack advance, and thus the tensile ductility is decreased.« less

Recrystallization-induced Precipitation Interaction in a Medium Carbon Vanadium Microalloyed Steel

Abstract: Using torsion tests, a study has been made of Recrystallization-Precipitation (R-P) interaction in a vanadium microalloyed steel for two strains (0.20 and 0.35). When strain induced precipitation starts, the recrystallized fraction deviates from Avrami's equation, giving rise to the formation of a plateau on the curves which represent the recrystallized fraction against time. This makes it possible to know the moment at which precipitation starts (Ps) and the moment at which it finishes (Pf). After the end of precipitation, recrystallization continues to progress in accordance with Avrami's law. Recrystallization-Precipitation-Time-Temperature (RPTT) diagrams have been drawn, superposing on them the lines corresponding to different recrystallized fractions. This makes it possible to represent in graph form the Recrystallization-Precipitation interaction and the determination of the static recrystallization critical temperature (SRCT). It is demonstrated that during the interval of time in which precipitation occurs (Pf - Ps), recrystallization does not advance. The work which has been carried out establishes new aspects in the phenomenon of R-P interaction.

Influence of AlN Precipitation on Thermodynamic Parameters in C-Al-V-N Microalloyed Steels

Abstract: An estimation of the equilibrium compositions of the austenite and carbonitride phases, as well as the mole fraction of each phase in C-Al-V-N microalloyed steels at different austenitising temperatures was made by calculations based on experimental data. Further, a comparison of the mole fraction was made from two thermodynamic models due to Adrian and Rios, with and without considering aluminium in the steels. The results indicate that both models produce very similar results and can be used to calculate the equilibrium parameters and predict the solution temperature of carbonitrides and aluminium nitride in the range 800–1300°C for an alloy system contained up to three microalloying elements and aluminium. Both models predict that most of the carbon remains in solution at the calculated temperature. When AlN precipitation is included in the calculation, it is seen that the mole fraction of the carbonitrides fP and the atomic fraction of carbon in the interstitial lattice of the carbonitrides fC decreases, while the atomic fraction of nitrogen in the interstitial lattice of carbonitride fN, increases. The effect of aluminium on these equilibrium parameters depends on the chemical composition of the steel. Increasing the contents of Al, N, C, and V together in the experimental steels has a more significant influence on these equilibrium parameters than changing only the contents of Al; Al and V or Al and N.

Effect of test variables on apparent activation energy for hot working and critical recrystallization temperatures of V-microalloyed steel

Abstract: The effect of test variables on the apparent activation energy for hot working of a medium carbon V-microalloyed steel, Q Hw , and the critical recrystallization temperature, T nr , have been studied by means of isothermal continuous and anisothermal multipass torsion test. The Q HW is found to be temperature dependent. Above the T nr , the Q U HW is close to that of austenite self diffusion, QSD, irrespective of the type of test or test variables. Below the T nr , the Q L HW is not only considerably higher, but that based on multipass flow curve exibits a strong interpass time dependence. With increasing interpass time from 1.8 to 10 s the Q L HW and the T nr increase and decrease respectively. The former is assumed to be controlled by solute drag, while the latter both by solute drag and precipitation pinning. Beyond 10 s, the two parameters exhibit the same trend, and the precipitation pinning is assumed to be the only recrystallization inhibiting mechanism.

Influence of strain rate on the microstructure of a thermo-mechanically processed medium carbon microalloyed steel

Abstract: The microstructure of a medium carbon microalloyed steel has been studied in the as-cast condition as well as after high temperature torsion deformation. The range of deformation parameters of the tests covered the conditions used in the forming process of a real part. The influence of thermomechanical treatment conditions (true strain, true strain rate and temperature) on the final austenite grain size has been studied. The evolution of the austenite grain size is analyzed under conditions of static and dynamic recrystallization. It was found that the strain rate can be as important as the strain to influence the final microstructure and therefore the mechanical properties of the final product.

Microstructural evolution during thermomechanical processing of a Ti-Nb interstitial-free steel just below the Ar 3 temperature

Abstract: Laboratory thermomechanical processing (TMP) experiments have been carried out to study the austenite transformation characteristics, precipitation behavior, and recrystallization of deformed ferrite for an interstitial-free (IF) steel in the temperature range just below Ar 3. For cooling rates in the range 0.1 °C s−1 to 130 °C s−1, austenite transforms to either polygonal ferrite (PF) or massive ferrite (MF). The transformation temperatures vary systematically with cooling rate and austenite condition. There is indirect evidence that the transformation rates for both PF and MF are decreased by the presence of substitutional solute atoms and precipitate particles. When unstable austenite is deformed at 850 °C, it transforms to an extremely fine strain-induced MF. Under conditions of high supersaturation of Ti, Nb, and S, (Ti,Nb) x S y precipitates form at 850 °C as coprecipitates on pre-existing (Ti,Nb)N particles and as discrete precipitates within PF grains. Pre-existing intragranular (Ti,Nb) x S y precipitates retard recrystallization and grain coarsening of PF deformed at 850 °C and result in a stable, recovered subgrain structure. The results are relevant to the design of TMP schedules for warm rolling of IF steels.

Influences of Cu in Steel on the Formation of Alloy Layer in Hot-dip Galvanizing

Abstract: The influence of Cu in steel on hot-dip galvanizing of hot rolled steel was studied. We investigated the formation of the alloy layer formed at the overlay-substrate interface. On Cu-Ti-added steel, the formation of δ 1 phase was promoted, while Fe 2 Al 5 was the main phase on the usual Ti-added steel. We concluded that this phenomenon was caused by the dissolution of Cu into the Zn bath that had deposited on the surface during pickling, and suggested that the dissolution of Cu should decrease the activity of Al and make Fe 2 Al 5 less stable than that of Ti-added steel, which caused the easy precipitation of δ 1 .

Wire stock suitable for reinforcement

Abstract: This wire stock is made of a microalloyed steel having a carbon content of between 0.2% and 0.6% by weight and furthermore containing at least one alloying element selected from the group consisting of vanadium, molybdenum and chromium, in a proportion, by weight, of at least 0.05% and at most 0.5% of said alloying element or of the combination of said alloying elements. Such wire stocks are manufactured in order to produce, by deformation and heat treatment, a ready-to-use wire which is used, for example, to reinforce articles made of plastics or made of rubber, especially tire covers, plies, belts, hoses.

Materials database with microstructural information

Abstract: In material science and technology, computerized systems have been created with information on many types of materials and properties. Many properties, such as mechanical properties, are strongly related to the chemical composition, heat treatment history, and microstructure. Unfortunately, microstructural parameters are represented to a very limited extent in existing databases and then restricted only to a few parameters such as the phase fraction or average grain size. For detailed analysis of property data, this is a serious drawback. Without detailed information about the microstructure, a more complete understanding of properties can never be developed. The aim of the present work is to introduce a representation of microstructures both in qualitative and quantitative form suitable for a material property database. A system with microstructural description and microstructural images of various engineering materials and their treatment data has been developed. Materials in the database are welded simulated stainless steels, sintered high-strength steels, ferritic-pearlitic steels, aluminum-alloy-based composite, induction-hardened microalloyed steel, and deformation-dilatometry-tested microalloyed steel. The database covers materials specification, metallographic characteristics, and microstructural images. Later, property data will be added. Both qualitative morphology descriptions of microstructure and the definitions of quantitative parameters are included. Modem automatic image analysis of original structures as well as of database-collected images was used to characterize microstructures. Contents, sizes, and shapes of grains, particles, inclusions, voids, packages, bands, agglomerates, and phases were measured. Examples of database applications are given.

Sealer for slide ring of running gear of crawler

Abstract: In a slide ring seal for the running gear of crawler track vehicles and construction machines with ferrous metal slide rings, the slide rings are nitrided or carbonitrided. Pref. the slide rings consist of (a) unalloyed or low alloy cast iron of GG, GGV, GGG, GTS or GTW type; (b) unhardened steel, microalloyed steel or chromium steel; or (c) unalloyed or low alloy sintered ferrous metal. Pref. the ferrous metal additionally contains up to 5 wt.% Cr, up to 0.3 wt.% N and/or up to 3 wt.% Al. Alternatively, the slide rings may consist of alloyed or high alloy stellites, chill cast irons, steels or sintered materials.

Diagrama de Recristalización-Precipitación-Tiempo-Temperatura (RPTT) de un acero microaleado al vanadio

Abstract: A method developed in the CENIM is described which allows to study the interaction recrystallization-induced precipitation by the deformation of vanadium microalloyed steel. By means of torsion test and applying the back extrapolation method, has been determined the recristallized fraction to diferent temperatures. When the precipitation begins, the recristallized fraction is separated of the Avrami's equation and this allows to know the instant the precipitation begins (Ps) and the instant the precipitation finishes (Pf). Thereby can be traced the Recrystallization- Precipitation-Time-Temperature (PTT) diagrams, which graphically show the interaction Recrystallization-Precipitation and simultaneously allows the determination of the static recrystallization critical temperature (SCRT). The mentioned temperature represents the limit between the two phases, before and after of the precipitation.