Showing papers on "Microalloyed steel published in 2001"

Precipitation behavior and its effect on strengthening of an HSLA-Nb/ti steel

Abstract: The precipitation behavior of a commercial high-strength low-alloy (HSLA) steel microalloyed with 0.086 wt pct Nb and 0.047 wt pct Ti has been investigated using transmission electron microscopy (TEM) and mechanical testing. The emphasis of this study is to compare an industrially hot-rolled steel and samples from a laboratory hot torsion machine simulation. From TEM observations, the Ti and Nb containing precipitates could be grouped according to their size and shape. The precipitates in order of size were found to be cubic TiN particles with sizes in the range of 1 µm, grain boundary precipitates with diameters of approximately 10 nm, and very fine spherical or needle-shaped precipitates with sizes on the order of 1 nm. The needlelike precipitates were found on dislocations in ferrite and constituted the dominant population in terms of density. Thus, they appear to be responsible for the precipitation strengthening observed in this steel. Aging tests were carried out at 650°C to evaluate the precipitate strengthening kinetics in detail. The strengthening mechanisms can be described with a nonlinear superposition of dislocation and precipitation hardening. The mechanical properties of torsion-simulated material and as-coiled industrial material are similar; however, there are some microstructural differences that can be attributed to the somewhat different processing routes in the laboratory as compared to hot strip rolling.

Development of Steel Plates by Intensive Use of TMCP and Direct Quenching Processes

Abstract: Historical progress of Thermo-Mechanical Processing (TMP) in steels during a last half century is briefly reviewed in the first part of this paper. On-line accelerated cooling process in a wide plate mill was developed by a Japanese steel mill in the end of 1970's, and its equipment was installed into major steel plate mills in Japan and Europe in 1980's. The combination of controlled rolling and accelerated cooling called as TMCP provided a powerful means for microstructural control of hot rolled steel plates. Subsequently, direct quenching (DQ)-tempering process was industrialized, where direct quenching after finish-rolling was performed at the highest cooling rate available in the accelerated cooling equipment. Since then, various new steel plates have been developed by intensive use of TMCP and direct quenching process particularly in Japan. In the latter part of this paper, newly developed steel plates by use of TMCP or DQ process during last two decades were summarized based on classification of line pipe steel, weldable HSLA steel plates used for shipbuilding, offshore construction, building, bridge or penstock, and austenitic stainless steel plate. Various strengthening mechanisms such as grain refinement, precipitation hardening or strengthening due to transformed microstructures have been utilized in development of numerous TMCP and DQ steel plates.

The effect of vanadium and niobium on the properties and microstructure of the intercritically reheated coarse grained heat affected zone in low carbon microalloyed steels

Abstract: Four steels, C-Mn-0.05V, C-Mn-0.11V, C-Mn and C-Mn-0.03Nb, all essentially boron-free were subjected to processing to simulate the microstructure of a coarse grained heat affected zone (GC HAZ) and an intercritically reheated coarse grained HAZ (IC GC HAZ). This involved reheating to 1 350°C, rapid cooling (Dt8/524 s) to room temperature and then reheating to either 750°C or 800°C. The toughness of the simulated GC HAZ and IC GC HAZ was assessed using both Charpy and CTOD tests and the hardness of both zones was also measured. A detailed assessment of the size and area fraction of martensite-austenite (MA) phase in the IC GC HAZ in the steels was obtained from a combination of Scanning Electron Microscopy (SEM) and Image Analysis of the resultant SEM micrographs. In addition, the distribution of the M-A phase was examined by observing 250 fields at a magnification of 2 500 times in the SEM for each of the steels. It is clear that the alloying addition has a significant effect on the amount and size of the M-A phase. The addition of 0.05% V to the C-Mn steel resulted in the lowest IC GC HAZ Charpy 50J impact transition temperature and the 0.1 mm CTOD transition temperature. The corresponding size and area fraction of the M-A phase were the smallest of the four steels. Raising the level of vanadium to 0.11% caused a deterioration in IC GC HAZ toughness, which was reflected in a greater area fraction of M-A phase, larger mean and maximum sizes of M-A particles and significantly more fields containing M-A phase. The addition of 0.03%Nb produced poorer IC GC HAZ toughness data than C-Mn-V and C-Mn steels and this was related to the large size and area fraction of M-A phase quantified in the Nb steel. The presence of M-A phase is considered to be the dominant factor in determining the toughness of IC GC HAZ.

The Dynamic, Static and Metadynamic Recrystallization of a Nb-microalloyed Steel

Abstract: Using hot torsion tests, the dynamic (DRX), static (SRX) and metadynamic (MDRX) recrystallization characteristics of a Nb-microalloyed steel were studied. The torsion tests were carried out at temperatures in the range 850 to 1 050°C with strain rates ranging from 0.5 to 5/sec. At the higher temperatures, the Nb remained in solution, while precipitation was underway in the lower temperature range. The results indicate that Nb precipitation has little influence on the value of the critical strain (e c ) for dynamic recrystallization. The peak strain/Zener-Hollomon parameter equation is derived and the effect of austenite grain size on the peak strain is considered. The times for 50 % recrystallization for static and metadynamic recrystallization were established by means of interrupted torsion tests and are compared. The rate of metadynamic recrystallization increases with strain rate and temperature and is observed to be independent of the pass strain; this contrasts sharply with the observations for static recrystallization. Finally, an example is given of an industrial rolling process in which DRX/MDRX can play important roles. Here, the occurrence of dynamic/ metadynamic recrystallization causes the load to drop or else to increase less rapidly than in the case of pure strain accumulation in the absence of SRX.

Effects of Alloying Elements on the Corrosion of Steel in Industrial Atmospheres

Abstract: From 1934 to 1938, Bethlehem Steel Corporation initiated a series of outdoor corrosion tests involving several hundred low-alloy steel compositions. These tests were conducted in the indus...

Improved Model for Static Recrystallization Kinetics of Hot Deformed Austenite in Low Alloy and Nb/V Microalloyed Steels

Abstract: Using torsion tests a improved model has been constructed to predict the static recrystallization kinetics of deformed austenite in low alloy and microalloyed steels. The model quantifies the influence of the most common elements (C, Si, Mn, Mo) in low alloy steels and the typical elements (V, Nb) in microalloyed steels, when they are in solution. Activation energy (Q) is the parameter sensitive to the content and nature of each alloying element, and an expression for Q is shown as a function of the percentage of each one. Nb is the element that contributes most to increasing the value of Q, and thus that which most delays recrystallization kinetics. C is seen to be the only alloying element that contributes to lowering the value of Q, and thus to accelerating recrystallization kinetics. Extrapolation of the expression of Q to pure iron in the austenitic phase gives a value of 148 637 J mol-1, which is similar to other values found in the literature for the grain boundary self-diffusion energy of pure Feγ . Static recrystallization kinetics follow Avrami's law and expressions are given for the parameter t0.5 and the exponent n.

Influence of Tempering on the Microstructure and Mechanical Properties of HSLA-100 Steel Plates

Abstract: The influence of tempering on the microstructure and mechanical properties of HSLA-100 steel (with C-0.04, Mn-0.87, Cu-1.77, Cr-0.58, Mo-0.57, Ni-3.54, and Nb-.038 pct) has been studied. The plate samples were tempered from 300 °C to 700 °C for 1 hour after austenitizing and water quenching. The transmission electron microscopy (TEM) studies of the as-quenched steel revealed a predominantly lath martensite structure along with fine precipitates of Cu and Nb(C, N). A very small amount of retained austenite could be seen in the lath boundaries in the quenched condition. Profuse precipitation of Cu could be noticed on tempering at 450 °C, which enhanced the strength of the steel significantly (yield strength (YS)—1168 MPa, and ultimate tensile strength (UTS)—1219 MPa), though at the cost of its notch toughness, which dropped to 37 and 14 J at 25 °C and −85 °C, respectively. The precipitates became considerably coarsened and elongated on tempering at 650 °C, resulting in a phenomenal rise in impact toughness (Charpy V-notch (CVN) of 196 and 149 J, respectively, at 25 °C and −85 °C) at the expense of YS and UTS. The best combination of strength and toughness has been obtained on tempering at 600 °C for 1 hour (YS-1015 MPa and UTS-1068 MPa, with 88 J at −85 °C).

Interaction between recrystallization and precipitation during multipass rolling in a low carbon niobium microalloyed steel

Abstract: The interaction between recrystallization and precipitation processes during multipass deformation has been analysed in terms of the non recrystallization temperature (T nr ). Multipass torsion tests performed at continuous cooling conditions have been used for thermomechanical simulation. The effect of the pass-strain and the initial austenite microstructure on the T nr value have been investigated. Conventionally reheated austenite (T soak = 1 200-1 250°C) and as-cast austenite, the latter simulated in the laboratory by reheating at very high temperatures (T soak = 1 400°C), have been considered. It was observed that at low pass-strains the retardation on recrystallization was mainly due to solute drag effects, while increasing the strain, precipitation induced by deformation was allowed to occur during the interpass interval leading to a drastic reduction of the softening reached between passes at temperatures below the T nr . The increment of the reheating temperature led to higher T nr values, being this effect related to higher austenite supersaturation levels prior to deformation.

Processing of low carbon steel plate and hot strip—An overview

Abstract: Soaking temperature, drafting schedule, finish rolling and coiling temperatures all play important roles in processing of low carbon plate and strip. They control the kinetics of various physical and metallurgical processes, viz. austenitization, recrystallization and precipitation behaviour. The final transformed microstructures depend upon these processes and their interaction with each other. In view of increasing cost of input materials, new processing techniques such as recrystallized controlled rolling and warm rolling have been developed for production of plates and thinner hot bands with very good deep drawability respectively. Besides hybrid computer modelling is used for production of strip products with tailor made properties. Although there have been few reviews on low carbon microalloyed steels in the past the present one deals with new developments.

Influence of martensite content and morphology on the toughness and fatigue behavior of high-martensite dual-phase steels

Abstract: A series of high-martensite dual-phase (HMDP) steels exhibiting a 0.3 to 0.8 volume fraction of martensite (V m ), produced by intermediate quenching (IQ) of a vanadium and boron-containing microalloyed steel, have been studied for toughness and fatigue behavior to supplement the contents of a recent report by the present authors on the unusual tensile behavior of these steels. The studies included assessment of the quasi-static and dynamic fracture toughness and fatigue-crack growth (FCG) behavior of the developed steels. The experimental results show that the quasi-static fracturetoughness (K ICV ) increases with increasing V m in the range between V m =0.3 and 0.6 and then decreases, whereas the dynamic fracture-toughness parameters (K ID , K D , and J ID ) exhibit a significant increase in their magnitudes for steels containing 0.45 to 0.60 V m before achieving a saturation plateau. Both the quasi-static and dynamic fracture-toughness values exhibit the best range of toughnesses for specimens containing approximately equal amounts of precipitate-free ferrite and martensite in a refined microstructural state. The magnitudes of the fatigue threshold in HMDP steels, for V m between 0.55 and 0.60, appear to be superior to those of structural steels of a similar strength level. The Paris-law exponents (m) for the developed HMDP steels increase with increasing V m , with an attendant decrease in the pre-exponential factor (C).

Mathematical Modeling of the Recrystallization Kineticsof Nb Microalloyed Steels

Abstract: The recrystallization behavior of Nb microalloyed steels was studied using hot torsion testing with the aim of modeling the recrystallization processes taking place during hot rolling. Continuous and interrupted torsion tests were performed in the temperature range 850 to 1050°C at strain rates of 0.05 to 5/s on selected low carbon steels containing Cr, Mo, Nb, Ni and Ti. The kinetics of static and metadynamic recrystallization were characterized and appropriate expressions were formulated for the recrystallization kinetics. These are shown to depend on steel composition and the processing conditions. The rate of metadynamic recrystallization increases with strain rate and temperature and is observed to be independent of strain, in contrast to the observations for static recrystallization. By means of extrapolations to mill strain rates, it is shown that metadynamic recrystallization will always be more rapid than static recrystallization, even at the largest possible accumulated strains. These calculations support the view that the unexpected load drops occasionally observed in industrial mills (particularly in the final few passes) are probably due to strain accumulation leading to the initiation of dynamic recrystallization, followed by metadynamic recrystallization.

Strain-induced precipitation behaviour in hot rolled strip steel

Abstract: The strain-induced precipitation of Nb(C,N) into the austenite in a Nb-microalloyed steel was investigated both experimentally and using a predictive model. The precipitation of Nb(C,N) was measured indirectly from the hardness at room temperature after thermomechanical treatment. The predictive model combined the precipitation start model of Dutta and Sellars with the Avrami equation and the additivity principle to allow prediction of the volume fraction of Nb(C,N) precipitated. The effects of several thermomechanical schedules were studied. These were (i) the effect of isothermal hold temperature and duration; (ii) the effect of deformation temperature at high and low cooling rates; (iii) the effect of cooling rate prior to the austenite to ferrite transformation; and (iv) the effect of multiple pancaking deformations. The fit between the experimental data and calculated results was found to be good in all cases with the exception of the slow cooling rate results of schedule (ii). It was concluded that the model could, once calibrated, successfully predict the hardness and strength of thermomechanically processed Nb-microalloyed steels.

Static recrystallization kinetics in warm worked vanadium microalloyed steels

Abstract: The effect of vanadium on static recrystallization kinetics of vanadium microalloyed carbon steels after simulating warm working conditions has been determined using the stress relaxation method in plane strain compression tests. In the warm working regime, undissolved fine V(C,N) precipitates promote a fine austenite grain size during reheating and interact with the recrystallization process after working, leading to longer recrystallization times in comparison with plain C–Mn steels. The interaction between precipitates and recrystallization is different to that observed for hot working conditions, retarding the total recrystallization process and thus resulting in a lower value of the Avrami exponent and a longer t 0.5 time.

The effects of thermomechanical processing on the precipitation in an industrial dual-phase steel microalloyed with titanium

Abstract: Analytical transmission electron microscopy was employed to characterize the precipitation at each step of the fabrication process and thermomechanical treatment of an industrial dual-phase steel microalloyed with titanium. Theoretical thermodynamic calculations as well as experimental analysis showed that more than half of the titanium carbosulfide (Ti4C2S2) precipitates would dissolve during reheating at 1240 °C. Despite this dissolution at 1240 °C, the remaining titanium carbonitrides and carbosulfides were effective in pinning austenitic grain boundaries, keeping the austenitic grain size at around 40 µm (at 1240 °C). It is also shown that, during hot rolling, there exist three regions of titanium carbide precipitation. The first is defined by an increase of titanium carbide precipitation due to deformation. The second region is marked by the insignificant change in precipitation. The third region is indicated by another increase in precipitation due to the austenite-to-ferrite transformation. The experimental and theoretical results on the contribution of TiC precipitation to hardening of ferrite (Orowan mechanism) were in excellent agreement, showing that TiC precipitates have the most important effect on increasing the yield strength, overshadowing the austenitic grain-boundary pinning contributions by Ti(C,N) and Ti4C2S2 precipitates.

Acicular ferrite morphologies in a medium-carbon microalloyed steel

Abstract: The influence of time and isothermal transformation temperature on the morphology of acicular ferrite in a medium-carbon microalloyed steel has been studied using optical and transmission electron microscopy (TEM). This study has been carried out with the analysis of the microstructures obtained with one- and two-stage isothermal treatments at 400 °C and 450 °C, following austenitization at 1250 °C. The heat treatments were interrupted at different times to observe the evolution of the microstructure at each temperature. The results show that a decrease in the isothermal transformation temperature gives rise to the development of sheaves of parallel ferrite plates, similar to bainitic sheaves, but intragranularly nucleated. These replace the face-to-edge nucleation that dominates the transformation at higher temperatures. The TEM observations reveal that the plates correspond to upper acicular ferrite and the sheaves to lower acicular ferrite. In this last case, cementite precipitates are present at the ferrite unit interiors and between the different platelets.

Bainite transformation during continuous cooling of low carbon microalloyed steel

Abstract: The evolution of the final microstructure for a low carbon Nb–Ti microalloyed plate steel was studied during a simulation of thermomechanical processing for hot rolling following by accelerated cooling. The effects of austenite deformation below the non-recrystallisation temperature T NR, cooling rate, and interrupt temperature on the formation of conventional (intergranular) bainite (CB), acicular ferrite (intragranular) (AF), and martensite–austenite (MA) constituents were determined. With increases in austenite deformation and cooling rate, and decrease in the interrupt temperature, the final microstructure changed from a mixture of CB+MA through CB+AF+MA to a dual phase AF+MA.

Hot ductility and fracture mechanisms of a C-Mn-Nb-Al steel

Abstract: Hot-ductility tests of a C-Mn-Nb-Al steel were performed in a tensile machine at different strain rates of 1×10−4, 3×10−4, 1×10−3, and 3×10−3 s−1 and at temperatures of 650 °C, 710 °C, 770 °C, 840 °C, 900 °C, 960 °C, and 1020 °C, which are close to the continuous casting conditions of steel. Fracture surfaces were examined using a scanning electron microscope. It was found that low strain rates and coarse austenitic grains decrease hot ductility. At all test temperatures, when the strain rate decreases, the hot ductility also decreases because the void growth mechanism predominates over void nucleation, giving time for nucleated cracks to grow. This leads, finally, to the catastrophic failure. The minimum hot ductility was found at 900 °C for all strain rates, and the fracture was intergranular. Fractographic evidence showed that the voids formed during the deformation surrounded the austenite grains, indicating that the deformation was concentrated in ferrite bands located in the same places when the testing temperature was in the two-phase field.

Effects of Sulfur Content and Sulfide-forming Elements Addition on Impact Properties of Ferrite–Pearlitic Microalloyed Steels

Abstract: Longitudinal Charpy impact value of ferrite–pearlitic microalloyed steel was improved by large addition of S (0.05–0.1mass%), however, transverse Charpy impact value was rather deteriorated. This study focused on the sulfide shape, which have large effects on impact properties. The elements of Ca, Mg, Ti, and Zr were added to ferrite–pearlitic microalloyed steels with 0.1 mass% S for sulfide shape control, and their impact properties were compared. The addition of Ca or Mg to the steels improved the transverse Charpy impact value without changing the longitudinal Charpy impact value, while the addition of Ti or Zr to the steels resulted in deteriorating significantly both longitudinal and transverse Charpy impact values. The crack initiation energy of the Ca or Mg bearing steels improves by enlargement of the minute dimple area ratio in the ductile fracture surface because the elongation of sulfides in the hot forging process are controlled by the addition of Ca or Mg. On the other hand, the reason why the Charpy impact value of Ti or Zr bearing steels are deteriorated is that (Ti, V)C or plate type sulfides promote cleavage fracture and then raise their ductile-to-brittle transition temperature.

Modeling of Kinetics of Austenite-to-Allotriomorphic Ferrite Transformation in 0.37C-1.45Mn-0.11V Microalloyed Steel

Abstract: The present article is concerned with the theoretical and experimental study of the growth kinetics of allotriomorphic ferrite in medium carbon vanadium-titanium microalloyed steel. A theoretical model is presented in this work to calculate the evolution of austenite-to-allotriomorphic ferrite transformation with time at a very wide temperature range. At temperatures above eutectoid temperature, where allotriomorphic ferrite is the only austenite transformation product, the soft-impingement effect should be taken into account in the modeling. In that case, the Gilmour et al. analysis reliably predicts the progress of austenite-to-allotriomorphic ferrite transformation in this steel. By contrast, since pearlite acts as a carbon sink, the carbon enrichment of austenite due to the previous ferrite formation is avoided, and carbon concentration in austenite far from the α/γ interface remains the same as the overall carbon content of the steel. Hence, the soft-impingement effect should be neglected, and allotriomorphic ferrite is considered to grow under a parabolic law. Therefore, assumption of a semi-infinite extent austenite with constant boundary conditions is suitable for the kinetics of the isothermal decomposition of austenite. An excellent agreement (higher than 93 pct in R2) has been obtained between the experimental and predicted values of the volume fraction of ferrite in all of the ranges of temperature studied.

Model for static recrystallisation critical temperature in microalloyed steels

Abstract: By means of hot torsion tests, the static recrystallisation critical temperature (SRCT) has been determined for 18 microalloyed steels classified into two groups. In one group the metallic microalloying element is vanadium, and in the other it is niobium. In both groups the microalloying element, carbon, and nitrogen contents vary from one steel to another. Tests have been carried out at various strains and strain rates, and recrystallisation–precipitation–time–temperature (RPTT) diagrams have been drawn for each steel in each condition. The SRCT is the asymptote of strain induced precipitation start P s and end P f curves, and its determination has permitted the construction of a model that quantifies the effects of all the external variables implicit in hot working such as strain and strain rate, and the internal variables such as austenite grain size and chemical composition of the steel. Hence, the influence of each of these variables has been quantified, and the model's prediction, comparing ...

Effect of thermomechanical processing on the microstructure and mechanical properties of Nb–Ti microalloyed steel

Abstract: Based on analysis of austenite deformation behaviour during thermomechanical processing of Nb–Ti microalloyed steel, the rolling schedules were designed to produce (i) recrystallized austenite, (ii) unrecrystallized austenite, and (iii) ferrite-pearlite. The effects of austenite conditioning on the final ferrite-pearlite microstructure and mechanical properties of steel were investigated. To rationalise the variation in final ferrite grain size with different thermomechanical processing schedules, it is necessary to consider the ferrite grain growth in addition, to the density of ferrite nucleation sites. Mechanical properties were the means to evaluate the variation in austenite solutioning and deformation conditions introduced into individual applied rolling schedules. The benefit of tensile tests, especially yield strength and ductility values, in determining the optimum deformation schedule and coiling condition for given steel is demonstrated.

Microstructural, compositional, and microhardness variations across a gas-metal arc weldment made with an ultralow-carbon consumable

Abstract: An experimental gas-metal arc (GMA) weldment of HSLA-100 steel fabricated with an ultralowcarbon (ULC) consumable of interest for United States Navy applications, designated “ARC100,” was studied to determine the relationships among the microstructure, the solute redistributions at various positions across the weldment, and the local properties (microhardness). These relationships were investigated by a variety of techniques, including microhardness mapping, optical microscopy, transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS) (including compositional X-ray mapping), and parallel electron energy-loss spectroscopy (PEELS). The microconstituents observed in this weld include lath ferrite, degenerate ferrite, lath martensite, retained austenite, and oxide inclusions; no carbides or other solid-state precipitates are present within the weld metal. Microhardness mapping indicates an undermatched weld metal (lower hardness as compared to the base plate) in which the hardest regions are in the first and last top beads, the root passes, and between highly ferritic soft bands associated with the outer portion of each weld bead’s heat-affected zone (HAZ) (within the fusion zone). The majority of the gradient in the substitutional alloying elements (Ni, Cu, Mn, and Cr) occurs within a region of less than about 0.5 mm of the fusion boundary, but the composition still changes even well into the fusion zone. Appreciable segregation of Ni and Cu to solidification cell boundaries occurs, and there is appreciable enrichment of C, Ni, Cu, and Mn in thin films of interlath retained austenite. This ULC weld metal is softer than the base plate due to the preponderance of lath ferrite rather than lath martensite, even at the high cooling rates experienced in this low-heat-input weld. Alternatively, the strength of the weld metal is due to the presence of at least some untempered lath martensite and the fact that the majority of the ferrite is lath ferrite and not polygonal ferrite. The interlath retained austenite might enhance toughness, but might also serve as a source of hydrogen in solution, which could potentially contribute to hydrogen-assisted cracking.