Showing papers on "Microalloyed steel published in 2017"

Selective laser melting of HY100 steel: Process parameters, microstructure and mechanical properties

Abstract: Selective Laser Melting (SLM) of a high strength low alloy steel HY100 is considered in the present investigation. The current work describes (i) optimization of SLM process parameters for producing fully dense parts in HY100 steel and (ii) the effects of post-processing heat treatment on the microstructure and mechanical properties. Samples have been fabricated by SLM using different combinations of laser power, laser scan speed, and hatch spacing. Fully dense samples were achieved at an energy density of 65 J/mm3. Microstructures of the as-built and heat treated samples were investigated using optical and scanning electron microscopes, X-ray diffraction, and electron backscattered diffraction techniques. The as-built sample showed fully martensitic microstructure with alternate bands of untempered (hard) and auto-tempered (soft) regions. The as-built parts are unsuitable for direct application due to untempered, hard and brittle martensite microstructure. The as-built parts were subjected to post-processing heat treatments (“direct temper” and “quench and temper”). The direct tempered samples exhibited higher yield strength and ultimate strength than the quench and temper ones. Noticeable amounts of anisotropy with respect to the build orientation, especially in tensile elongation, were observed in the direct tempered samples due to in-homogenous microstructure. Quench and temper treatment of the parts resulted in recrystallized grains with uniform microstructure. The current investigation shows that quench and temper at 650 °C is an optimum post processing treatment for HY100 SLM parts as it manifests desired strength with good tensile elongation.

Effect of composition and thermo-mechanical processing schedule on the microstructure, precipitation and strengthening of Nb-microalloyed steel

Abstract: In order to study the influence of composition and thermo-mechanical processing schedule on the kinetics of austenite recrystallization, strain induced precipitation and final microstructural evolution in Nb-microalloyed steels, thermo-mechanical processing simulations have been carried out inside Gleeble® by varying the number of deformation passes (2-pass vs. 6-pass), deformation temperatures (1000–800 °C) and inter-pass times. Low-C high-Mn steel (LCHMn) has been found to offer finer ferrite grain size and finer Nb-precipitation which contributed to superior hardness to that steel, compared to high-C low-Mn steel (HCLMn). Among the deformation schedules applied, 6-pass schedule has been found to be superior over 2-pass schedule in terms of precipitation strengthening and hardness. This study also proposes a mathematical framework to explain the effect of composition and processing schedule in Nb-microalloyed steels following Dutta and Sellars approach on precipitation-recrystallization interaction.

Evolution of microstructure and crystallographic texture of microalloyed steel during warm rolling in dual phase region and their influence on mechanical properties

Abstract: High strength and high toughness steels can be developed by warm caliber rolling in ferrite region. However, high deformation resistance limits its application. In the present study, warm rolling was applied to plate rolling which is more suitable for industrial production to develop high strength and high toughness steels. To reduce deformation resistance, warm rolling was carried out in dual phase region. We elucidate here the evolution of microstructure and crystallographic texture and their influence on mechanical properties of microalloyed steel subjected to warm rolling. The study suggests that high strength and high toughness can also be obtained by warm rolling in the dual phase region. Elongated ultrafine microstructure and intense α-fiber texture component and γ-fiber texture component can be obtained through warm rolling. The main mechanism of microstructure evolution during warm rolling was dynamic recovery. Reducing warm rolling temperature can refine grain size, enhance α-fiber texture component and weaken γ-fiber texture component. Warm rolling can greatly enhance strength by ~64–158 MPa compared to the conventional controlled rolling (CR) process, and the warm-rolled plates had high elongation in spite of high strength. The toughness was improved because of grain refinement and delamination. Delamination can induce ductile fracture at low temperature, and delay the occurrence of brittle fracture such that high toughness is obtained in steel plates. The effect of warm rolling temperature and impact test temperature on delamination and impact property was elucidated.

Structure-properties relationship of ultra-fine grained V-microalloyed dual phase steels

Abstract: The effect of vanadium microalloying on ultra-high strength dual phase (DP) ferrite-martensite steel microstructure and properties was studied. It was found that the addition of 0.14 wt% V to a Fe-0.18C-1.5Mn-0.3Si-0.008N reference alloy introduced very significant ferrite grain size refinement in the cold rolled and annealed state. During continuous annealing the initial ferrite to austenite transformation kinetics were strongly retarded, however under slow cooling both pearlite and bainite transformations were suppressed indicating increased hardenability. After cold rolling and intercritical annealing at 750 ⁰C intense V(C,N) precipitates (mean radius 3.7 nm) were observed in the ferrite phase whereas precipitates were scarce in martensite (austenite) and much larger (mean radius 6.7 nm). Significant gains in YS, UTS and work hardening rate were observed at low martensite fractions due to a combination of selective precipitation strengthening and grain refinement of ferrite. However, at higher martensite fractions (> 45%) the YS, UTS and work hardening rate became lower than the reference, primarily due to softening of the martensite. The latter was attributed to the fixing of solute carbon by V(C,N). The net increase in tensile strength with martensite content of the vanadium alloy was ~ 4 MPa/%α’ compared to ~ 16 MPa/%α’ for the reference alloy. A recently developed size-sensitive mean field structure-properties model was extended to capture these microalloying effects. At iso-tensile strength both the fracture strain and hole expansion behaviour of the new microalloyed steel showed improved performance over the reference.

Evaluating Strengthening and Impact Toughness Mechanisms for Ferritic and Bainitic Microstructures in Nb, Nb-Mo and Ti-Mo Microalloyed Steels

Abstract: Low carbon microalloyed steels show interesting commercial possibilities by combining different “micro”-alloying elements when high strength and low temperature toughness properties are required. Depending on the elements chosen for the chemistry design, the mechanisms controlling the strengths and toughness may differ. In this paper, a detailed characterization of the microstructural features of three different microalloyed steels, Nb, Nb-Mo and Ti-Mo, is described using mainly the electron backscattered diffraction technique (EBSD) as well as transmission electron microscopy (TEM). The contribution of different strengthening mechanisms to yield strength and impact toughness is evaluated, and its relative weight is computed for different coiling temperatures. Grain refinement is shown to be the most effective mechanism for controlling both mechanical properties. As yield strength increases, the relative contribution of precipitation strengthening increases, and this factor is especially important in the Ti-Mo microalloyed steel where different combinations of interphase and random precipitation are detected depending on the coiling temperature. In addition to average grain size values, microstructural heterogeneity is considered in order to propose a new equation for predicting ductile–brittle transition temperature (DBTT). This equation considers the wide range of microstructures analyzed as well as the increase in the transition temperature related to precipitation strengthening.

The Influence of La and Ce Addition on Inclusion Modification in Cast Niobium Microalloyed Steels

Abstract: The main role of Rare Earth (RE) elements in the steelmaking industry is to affect the nature of inclusions (composition, geometry, size and volume fraction), which can potentially lead to the improvement of some mechanical properties such as the toughness in steels. In this study, different amounts of RE were added to a niobium microalloyed steel in as-cast condition to investigate its influence on: (i) type of inclusions and (ii) precipitation of niobium carbides. The characterization of the microstructure by optical, scanning and transmission electron microscopy shows that: (1) the addition of RE elements change the inclusion formation route during solidification; RE > 200 ppm promote formation of complex inclusions with a (La,Ce)(S,O) matrix instead of Al2O3-MnS inclusions; (2) the roundness of inclusions increases with RE, whereas more than 200 ppm addition would increase the area fraction and size of the inclusions; (3) it was found that the presence of MnS in the base and low RE-added steel provide nucleation sites for the precipitation of coarse niobium carbides and/or carbonitrides at the matrix–MnS interface. Thermodynamic calculations show that temperatures of the order of 1200 °C would be necessary to dissolve these coarse Nb-rich carbides so as to reprecipitate them as nanoparticles in the matrix.

Effects of Microstructure on CVN Impact Toughness in Thermomechanically Processed High Strength Microalloyed Steel

Abstract: Investigation on the correlation between microstructure and CVN impact toughness is of practical importance for the microstructure design of high strength microalloyed steels. In this work, three steels with characteristic microstructures were produced by cooling path control, i.e., steel A with granular bainite (GB), steel B with polygonal ferrite (PF) and martensite-austenite (M-A) constituent, and steel C with the mixture of bainitic ferrite (BF), acicular ferrite (AF), and M-A constituent. Under the same alloy composition and controlled rolling, similar ductile-to-brittle transition temperatures were obtained for the three steels. Steel A achieved the highest upper shelf energy (USE), while large variation of impact absorbed energy has been observed in the ductile-to-brittle transition region. With apparently large-sized PF and M-A constituent, steel B shows the lowest USE and delamination phenomenon in the ductile-to-brittle transition region. Steel C exhibits an extended upper shelf region, intermediate USE, and the fastest decrease of impact absorbed energy in the ductile-to-brittle transition region. The detailed CVN impact behavior is studied and then linked to the microstructural features.

Significant influence of carbon and niobium on the precipitation behavior and microstructural evolution and their consequent impact on mechanical properties in microalloyed steels

Abstract: Carbon and niobium (Nb) play an important role in influencing the ultimate microstructure and mechanical properties. In this regard, we elucidate here the impact of carbon and Nb on the microstructural evolution and precipitation behavior during continuous cooling of industrially processed microalloyed steels with varying carbon and Nb-content. The microstructure and precipitation evolution was studied via electron microscopy and related to the outcome of thermodynamic simulation. The increase of carbon content in steel increased the precipitation temperature of (Nb, Ti)(C, N), which led to relatively larger size (Nb, Ti)(C, N) precipitates. Furthermore, high carbon content contributed to stabilization of austenite and delayed the transformation of ferrite and bainite, such that martensite/austenite content (M/A) was obtained. The M/A islands in high carbon-containing steel contributed to highest strength and intermediate elongation. The high degree of (Nb, Ti)(C, N) precipitation in steel contributed to refinement of prior austenite grain size and strain accumulation, which increased ferrite and bainite start transformation temperature, resulting in higher volume fraction of polygonal ferrite. Polygonal ferrite in steel with high Nb-content was responsible for relatively low strength in comparison with steels with higher carbon or intermediate carbon-Nb contents. Granular bainite and lath bainite in steel with intermediate C and Nb-contents was characterized by best combination of strength and elongation. The outcomes of the thermodynamic simulations were consistent with the experimentally observed microstructure.

Assessing the recovery and recrystallization kinetics of cold rolled microalloyed steel through coercive field measurements

Abstract: Annealing after cold rolling brings about the activation of recovery and recrystallization in microalloyed steels. The importance of recovery has been most often associated with its effects on the recrystallization kinetics. However, recovery has gained particular importance as an alternative heat treatment under the name “back annealing” or “recovery annealing”. In the present work, various annealing treatments were applied to a Nb-microalloyed steel in the range of temperatures and times where recrystallization is not complete. As a consequence, a large set of tensile strength-ductility pairs was obtained, even for conditions in which recrystallization was avoided. Through non-destructive magnetic coercive field measurements, recovery and partial recrystallization were monitored for each annealing treatment. Magnetic softening is significantly greater than mechanical softening. The variation in recovery in terms of temperature and time is highly affected by the presence of Nb in solution in the hot band (before cold rolling). At low recovery annealing temperatures, 350–450 °C, Nb solute drag on dislocations is the main mechanism that controls recovery, whereas at 550 °C, Nb strain induced precipitation leads to a recovery plateau in terms of coercive field.

The Effect of the Sintering Temperature and Addition of Niobium and Vanadium on the Microstructure and Mechanical Properties of Microalloyed PM Steels

Abstract: In this work, the effect of the sintering temperature on the microstructure and mechanical properties of Nb-V added powder metallurgy (PM) steels was investigated. The microstructure and mechanical properties of the Nb-V added PM microalloyed steel were examined by optical microscopy, scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), optical emission spectrometer (OES), tensile and hardness tests. Results indicated that the optimal sintering temperature was 1350 °C and the addition of 0.1%, 0.15% or 0.2% of Nb-V increases the yield strength (YS), ultimate tensile strength (UTS) and hardness of the PM sintered steels. 0.2 wt % Nb-V added PM steel showed the highest values in yield strength (YS), ultimate tensile strength (UTS) and the highest hardness. Elongation also tends to improve with adding Nb-V content. In addition, Nb-V limited grain growth during austenitization.

Effect of lanthanum on the precipitation and dissolution of NbC in microalloyed steels

Abstract: The effect of lanthanum on the precipitation and dissolution of NbC in microalloyed steel was investigated using a combination method of thermo-mechanical experiment and the first-principle calculations. The interaction of niobium with lanthanum is attractive in the 2nn to 6nn shell, and that of carbon with lanthanum is relatively strongly attractive in 4nn(B) and 5nn coordination shells. These attractive interactions lead to an increase in the Nb and C solubility in fcc Fe, and a decrease in the chemical potential of these two solutes, thereby suppressing the formation of NbC. The strain-induced precipitation intensity was characterized using field emission scanning electron microscope (FE-SEM) and inductively coupled plasma optical emission spectrometry (ICP-OES). The experimental results reveal that after deformation NbC precipitated more in La free steel than in La addition steel. And since the relatively higher dissolution rate of NbC in La addition steel, the Zener pinning effect decreased slightly faster, leading to a higher coarsening rate of austenite grain during isothermally heating at 1200 °C.

Effect of Coiling Temperature on the Microstructure and Mechanical Properties of Hot-Rolled Ti–Nb Microalloyed Ultra High Strength Steel

Abstract: A novel low carbon Ti–Nb microalloyed hot rolled steel with minimum yield strength of 700 MPa and good balance of stretch-flangeability and impact toughness has been developed by controlled thermo-mechanical processing following thin slab direct rolling route. In the present work, the effects of two coiling temperatures on the resulting microstructure, micro-texture and mechanical properties on this Ti–Nb microalloyed steel have been studied. It is observed that increase in coiling temperature from 520 to 580 °C significantly affects the mechanical properties. Higher dislocation density and increased precipitation along with slightly smaller grain size is observed for 580 °C coiling temperature resulting in about 50 MPa increase in yield and tensile strengths as compared to 520 °C coiling temperature.

Microstructure and Mechanical Properties of V-Nb Microalloyed Ultrafine-Grained Dual-Phase Steels Processed Through Severe Cold Rolling and Intercritical Annealing

Abstract: Ultrafine-grained (UFG) dual-phase (DP) steel was produced by severe cold rolling (true strain of 2.4) and intercritical annealing of a low carbon V-Nb microalloyed steel in a temperature range of 1003 K to 1033 K (730 °C to 760 °C) for 2 minutes, and water quenching. The microstructure of UFG DP steels consisted of polygonal ferrite matrix with homogeneously distributed martensite islands (both of size <1 µm) and a small fraction of the inter lath films of retained austenite. The UFG DP steel produced through intercritical annealing at 1013 K (740 °C) has good combination of strength (1295 MPa) and ductility (uniform elongation, 13 pct). The nanoscale V- and Nb-based carbides/carbonitrides and spheroidized cementite particles have played a crucial role in achieving UFG DP microstructure and in improving the strength and work hardening. Analysis of work hardening behavior of the UFG DP steels through modified Crussard–Jaoul analysis showed a continuously varying work hardening rate response which could be approximated by 2 or 3 linear regimes. The transmission electron microscopy analysis on post tensile-tested samples indicated that these regimes are possibly related to the work hardening of ferrite, lath, and twin martensite, respectively.

Effect of vanadium on dynamic continuous cooling transformation behavior of medium-carbon forging steels

Abstract: Dynamic continuous cooling transformation (CCT) behavior of medium-carbon forging steels microalloyed with different V contents up to 0.29% was investigated by means of dilatometric measurement, microstructural observation and hardness measurement. The results showed that the dynamic CCT diagrams were similar and the main difference was that the fields of the transformation products were shifted to the right side of the diagrams with the increase of V content, and this effect was more noticeable with an addition of 0.29% V. The A c1 and A c3 temperatures showed increasing trends with increasing V content, while the critical cooling rates decreased with increasing V content. The increase of V content resulted in significant increase of hardness and this tendency was enhanced with increasing cooling rate until the formation of acicular ferrite (AF). A promising approach of remarkably improving the toughness of ferritic-pearlitic medium-carbon forging steels with suitable V addition and the introduction of AF without notable penalty on its strength level was suggested.

Effect of Annealing Treatments on the Microstructure and Texture Development in API 5L X60 Microalloyed Pipeline Steel

Abstract: Effect of annealing treatments at 600, 800, 1000 and 1200 °C on the microstructure, texture, grain boundary characteristic and recrystallization fraction of Nb-microalloyed X60 steel is evaluated by using x-ray diffraction and EBSD techniques. The results indicate that bimodal as-received microstructure is changed to a homogeneous equiaxed grain structure above annealing at 1000 °C. Macro-texture investigations depict that increasing annealing temperature results in considerable variation of texture intensity, especially at 1200 °C. Maximum intensity corresponds to {001}〈310〉, Goss, copper texture components as well as near γ-fiber at 1200 °C. Recrystallization analysis shows that volume fraction of recrystallization noticeably is increased by annealing temperature at 1200 °C. Recrystallized grains are mainly oriented along γ-fiber, especially close to {111}〈112〉 texture component. Moreover, coincidence site lattice (CSL) analysis shows that the effect of annealing temperature on the volume fraction of Σ3 boundary is negligible.

Characterization of HAZ of API X70 Microalloyed Steel Welded by Cold-Wire Tandem Submerged Arc Welding

Abstract: High-strength low-carbon microalloyed steels may be adversely affected by the high-heat input and thermal cycle that they experience during tandem submerged arc welding. The heat-affected zone (HAZ), particularly the coarse-grained heat-affected zone (CGHAZ), i.e., the region adjacent to the fusion line, has been known to show lower fracture toughness compared with the rest of the steel. The deterioration in toughness of the CGHAZ is attributed to the formation of martensite-austenite (M-A) constituents, local brittle zones, and large prior austenite grains (PAG). In the present work, the influence of the addition of a cold wire at various wire feed rates in cold-wire tandem submerged arc welding, a recently developed welding process for pipeline manufacturing, on the microstructure and mechanical properties of the HAZ of a microalloyed steel has been studied. The cold wire moderates the heat input of welding by consuming the heat of the trail electrode. Macrostructural analysis showed a decrease in the CGHAZ size by addition of a cold wire. Microstructural evaluation, using both tint etching optical microscopy and scanning electron microscopy, indicated the formation of finer PAGs and less fraction of M-A constituents with refined morphology within the CGHAZ when the cold wire was fed at 25.4 cm/min. This resulted in an improvement in the HAZ impact fracture toughness. These improvements are attributed to lower actual heat introduced to the weldment and lower peak temperature in the CGHAZ by cold-wire addition. However, a faster feed rate of the cold wire at 76.2 cm/min adversely affected the toughness due to the formation of slender M-A constituents caused by the relatively faster cooling rate in the CGHAZ.

Metadynamic recrystallization of Nb–V microalloyed steel during hot deformation

Abstract: The metadynamic recrystallization (MDRX) behavior of a Nb–V microalloyed nonquenched and tempered steel was investigated by isothermal hot compression tests on Gleeble-1500 thermal-mechanical simulator. Compression tests were performed using double hit schedules at temperatures of 1273–1423 K, strain rates of 0.01–5 s−1, initial grain sizes of 92–149 µm and an inter-pass time of 0.5–10 s. The experimental results show that MDRX softening fraction increases with the increasing of deformation temperature, strain rate, and inter-pass time, while it decreases with the increasing of initial grain size. Based on the experimental results, the MDRX softening fraction kinetic model and recrystallized grain size model of the tested steel was established. Besides, using the above mathematic models, a finite element model was built to simulate the MDRX process of the tested steel. The simulation results show good agreement with the experimental ones, which indicates that finite element method is an effective approach to analyze the MDRX behavior and the established that mathematic models of the tested steel are reliable and accurate.

Effects of Zr, Ti, and Al Additions on Nonmetallic Inclusions and Impact Toughness of Cast Low-Alloy Steel

Abstract: A microalloying of the low-carbon and low-alloy cast steel was conducted with Zr, Ti, and Al that were added to the steel in four combinations. After heat treatment, the samples were tested for impact toughness at room temperature using the Charpy method. The highest values of impact toughness were obtained in the group treated with Zr, while Zr-Ti and Zr-Ti-Al groups showed moderate toughness values; the lowest values were observed in the Zr-Al group. Difference among the treatment groups was observed in the fracture mechanisms, morphology, and area distribution of the inclusions. High toughness values achieved in the trials treated with zirconium corresponded with smooth ductile fracture. The metal treated with a combination of zirconium and titanium had a relatively small area occupied by inclusions, but its toughness was also moderate. Lowest impact toughness values corresponded with the larger area occupied by the inclusions in the trials treated with aluminum. Also, a connection between the solubility product [Al][N] and impact toughness was established. The study also provides a qualitative description and quantitative analysis of the nonmetallic inclusions formation as a result of microalloying treatment. The precipitation sequence of the inclusions was described based on the thermochemical calculations for the nonmetallic compounds discovered in the experimental steel. A description of the size distribution, morphology, and composition was conducted for the oxides, nitrides, sulfides, and multiphase particles.

Investigation of Austenitization in Low Carbon Microalloyed Steel During Continuous Heating

Abstract: Dilatation associated with the formation of austenite from ferrite–pearlite was calculated from equilibrium phase fraction and composition. Linear thermal expansion coefficient of ferrite required for the calculation was determined by iteration of dilatation data. A good match was obtained between the calculated and experimental dilatation curves. The calculated dilatation data were used to identify the stages of austenite formation: pearlite dissolution followed by ferrite to austenite transformation which is gradual at first before becoming rapid.

Effect of Austenite Deformation on the Microstructure Evolution and Grain Refinement Under Accelerated Cooling Conditions

Abstract: Although there has been much research regarding the effect of austenite deformation on accelerated cooled microstructures in microalloyed steels, there is still a lack of accurate data on boundary densities and effective grain sizes. Previous results observed from optical micrographs are not accurate enough, because, for displacive transformation products, a substantial part of the boundaries have disorientation angles below 15 deg. Therefore, in this research, a niobium microalloyed steel was used and electron backscattering diffraction mappings were performed on all of the transformed microstructures to obtain accurate results on boundary densities and grain refinement. It was found that with strain rising from 0 to 0.5, a transition from bainitic ferrite to acicular ferrite occurs and the effective grain size reduces from 5.7 to 3.1 μm. When further increasing strain from 0.5 to 0.7, dynamic recrystallization was triggered and postdynamic softening occurred during the accelerated cooling, leading to an inhomogeneous and coarse transformed microstructure. In the entire strain range, the density changes of boundaries with different disorientation angles are distinct, due to different boundary formation mechanisms. Finally, the controversial influence of austenite deformation on effective grain size of low-temperature transformation products was argued to be related to the differences in transformation conditions and final microstructures.