Showing papers on "Microalloyed steel published in 2006"

Impact toughness and microstructure relationship in niobium- and vanadium-microalloyed steels processed with varied cooling rates to similar yield strength

Abstract: We describe here the relationship between microstructure and impact toughness behavior as a function of cooling rate for industrially processed Nb- and V-microalloyed steels of almost similar yield strength (∼60 ksi). Both Nb- and V-microalloyed steels exhibited increase in toughness with increase in cooling rates during processing. However, Nb-microalloyed steels were characterized by relatively higher toughness than the V-microalloyed steels under identical processing conditions. The microstructure of Nb- and V-microalloyed steels processed at conventional cooling rate, primarily consisted of polygonal ferrite–pearlite microconstituents, while Nb-microalloyed steels besides polygonal ferrite and pearlite contained significant fraction of degenerated pearlite. The microstructure of Nb- and V-microalloyed steels processed at relatively higher cooling rate contained degenerated pearlite and lath-type (acicular) ferrite in addition to the primary ferrite–pearlite constituents. The fraction of degenerated pearlite was higher in Nb-microalloyed steels than in the V-microalloyed steels. In both Nb- and V-microalloyed steels the precipitation characteristics were similar with precipitation occurring at grain boundaries, dislocations, and in the ferrite matrix. Fine-scale (∼5–10 nm) precipitation was observed in the ferrite matrix of both the steels. The selected area diffraction (SAD) pattern analysis revealed that these fine precipitates were MC type of niobium and vanadium carbides in the respective steels and followed Baker–Nutting orientation relationship with the ferrite matrix. The microstructural studies suggest that the increase in toughness of Nb-microalloyed steels is attributed to higher fraction of degenerated pearlite in the steel.

Effect of TiN inclusions on the impact toughness of low-carbon microalloyed steels

Abstract: Microalloying with various elements, including titanium, coupled with thermomechanically controlled processing, has become a major technology for the manufacture of high-quality steel plate. In this research, the influence of TiN inclusions on the impact toughness of low-carbon plate steels microalloyed with titanium, vanadium, and boron was investigated. The three experimental steels had Ti/N ratios of 2.44, 3.5, and 4.2, and all three had a granular bainite microstructure. However, Charpy V-notch testing showed that steel A had very high toughness at both room temperature and −20 °C, whereas steels B and C showed very low toughness at −20 °C and moderate toughness at room temperature. Scanning electron microscope fractography revealed that coarse TiN inclusions had acted as cleavage fracture initiation sites in steels B and C. The effect of Ti and N levels on TiN formation and growth is analyzed using alloy thermodynamics. It is shown that not only is the Ti/N ratio important, but also the product of total Ti and N plays a most important role in TiN formation and growth. It is concluded that the product of the total Ti and N contents should not be greater than the solubility product of TiN at the solidus temperature to prevent the precipitation of TiN particles before solidification. Furthermore, the ratio of Ti to N should also be maintained lower than the stoichiometric ratio of 3.42 to ensure a low coarsening rate for the TiN inclusions during soaking before rolling.

Microstructure of high strength niobium-containing pipeline steel

Abstract: The paper describes the microstructural constituents in a industrially processed Nb-microalloyed pipeline steel having yield strength of ∼620 MPa. The microstructure of base, heat affected zone (HAZ), and weld metal of the fabricated steel pipe was examined by optical and transmission electron microscopy. The microstructure of thermomechanically processed pipeline steel primarily consisted of non-equiaxed ferrite of mixed morphologies with small fraction of degenerated pearlite. The microstructure contained high dislocation density, sub-boundaries and dislocation substructures. The HAZ was characterized by a combination of fine and coarse grained polygonal ferrite structure with high density of dislocations and fine cementite particles. In the weld metal, the constituents of complex ferrite were low temperature transformation products formed during continuous cooling such as quasi-polygonal or massive ferrite, acicular ferrite, bainitic ferrite and dispersion of coarse and fine cementite particles in the ferrite matrix. The precipitates in the investigated pipeline steel were of duplex type containing either Nb and Ti or Ti and Mo, even though the steel contained low concentration of titanium. Precipitates of different morphology and size range were observed and include rectangular (∼100–130 nm), cuboidal/spherical (∼20–100 nm), fine (∼10–20 nm) and very fine (

High strength microalloyed CMn(V–Nb–Ti) and CMn(V–Nb) pipeline steels processed through CSP thin-slab technology: Microstructure, precipitation and mechanical properties

Abstract: Compact strip production (CSP) technology is an important upcoming processing route to produce low cost microalloyed high strength pipeline steels that meet the API standards. Hot strips of CMn(VNbTi) and CMn(VNb) steel grades with fine-grained ferrite–pearlite microstructure and small volume fraction of lower transformation product (non-polygonal ferrite: acicular ferrite/bainite) were produced using CSP technology with high strength and excellent low-temperature toughness up to −60 °C. For strip thicknesses between 6 and 12.5 mm, yield strength levels of up to 590 MPa and tensile strength levels up to 680 MPa were achieved. The CMn(VNb) steel exhibited outstanding notch-toughness in the range of 200 and 400 J/cm 2 , in spite of its higher yield strength (∼100 MPa or greater) over the CMn(VNbTi) steel. The precipitates present in CMn(VNbTi) and CMn(VNb) steels were characterized in terms of morphology, size and chemistry, and crystallography. The microalloying elements, Ti, Nb, and V form M 4 C 3 type of carbides in the ferrite matrix of both the steels. The multi-microalloying approaches of CMn(VNbTi) and CMn(VNb) results in the formation of duplex and triplex carbonitrides, respectively. The results of the development effort are described.

Continuous cooling transformation diagrams and properties of micro-alloyed TRIP steels

Abstract: Continuous cooling transformation (CCT) diagrams and properties of four kinds of low-silicon C–Mn–Si–Al transformation-induced plasticity (TRIP) steels with different carbon contents, with or without microalloy element Ti/V, as well as a reference TRIP steel containing 1.19 wt.% Si were studied. The CCT diagrams exhibited that as the carbon equivalent (CE) increased, it caused a shift of the ferrite forming and pearlite forming temperatures to the right side and the bainite forming and martensite forming to the lower temperatures of the diagram. The microstructural evolution obtained from the dilatometry samples revealed that the highest cooling rates produced fully martensitic microstructure in all cases except the reference TRIP steel. As the cooling rate decreased, more ferrite and bainite were formed. The increase of CE caused the increase of the amount of martesite in the microstructure. Tensile test and Erichsen test of the investigated steels showed an excellent mechanical strength and ductility combination, with tensile strength between 800 and 1000 MPa, total elongation of around 20%, and a quite good formability with a dome height of about 10 mm in all cases.

Microstructure and Precipitation Behavior of Nb, Ti Complex Microalloyed Steel Produced by Compact Strip Processing

Abstract: A comprehensive microstructure analyses were conducted for CSP processed Nb, Ti microalloyed steel, especially focusing on the precipitation behavior of the microalloying elements Nb and Ti. After coiling, the steel exhibits mainly a ferrite microstructure. The average ferrite grain size is 5.3 μm. The ferrite has a transitional morphology from polygonal ferrite to non-polygonal ferrite and is characterized by a moderate dislocation density of 2.47E+10/cm2. A high density of Nb, Ti complex star-like or cruciform shaped precipitates exist in the steel. They are Nb-rich and the average size is around 150 nm. About 49% Nb of the total in the steel is tied up in star-like precipitates, thus remarkably reducing the amount of Nb available for austenite conditioning, transformation temperature control and precipitation as small strengthening particles in ferrite. The main strengthening mechanisms found in the steel are the grain refinement and dislocation strengthening. Of the total yield strength, they represent contributions of 44% and 24%, respectively. There is a very little precipitation strengthening in the steel. It is thought that Nb, Ti complex star-like precipitate is prone to form in Ti-containing niobium microalloyed steel produced by compact strip processing.

The Effect of Boron on Hot Ductility of Nb-microalloyed Steels

Abstract: Two grades of the Nb-containing steel, one modified with B, were examined in order to study the effect of B on the hot ductility. Since the hot ductility loss of the Nb-containing steel during the continuous casting is manifested as surface cracking, the surface thermal schedule including in situ melting was taken into account. The results revealed that addition of B improves the hot ductility of austenite. Two mechanisms for such improvement are proposed that are not based on the presence of intragranular ferrite. These mechanisms concern with depletion of strengthening elements in the austenite lattice through faster kinetics of grain boundary precipitation and non-equilibrium segregation of precipitants.

Recrystallisation driving forces against pinning forces in hot rolling of Ti-microalloyed steels

Abstract: In this work the pinning forces exerted by TiN particles in the austenitic phase in four Ti-microalloyed steels have been determined and compared with the driving forces for recrystallisation determined in each rolling pass. The thermomechanical simulation has been carried out in the laboratory by means of torsion tests in a sequence of 20 passes with the final passes in a mixed austenitic/ferritic phase. The driving forces were found to be approximately two orders of magnitude higher than the pinning forces, which explains why the austenite in these steels barely experiences hardening during rolling and why the accumulated stress prior to the austenite → ferrite transformation is insufficient to refine the ferritic grain.

Influence of forging and cooling rate on microstructure and properties of medium carbon microalloy forging steel

Abstract: In view of their capacity to develop high strength following limited alloying and ease of processing medium carbon microalloyed (MA) steels are very cost-effective compared to quenched and tempered steels for the production of automotive components. To be able to substitute quenched and tempered steels, MA steels must be processed to similar strength levels and acceptable toughness [1]. The increased use of microalloyed forging steels in production applications should be supplemented with an increased understanding of not only the strengthening mechanisms that occur in these steels, but also the effects of the composition and forging parameters on these mechanisms. The size and percentage distribution of ferrite and pearlite within the microstructure play an important role on the final mechanical properties. Each of the microstructure variables is highly influenced by the composition of the microalloyed steels, the forging parameters utilized, and the post-forging cooling rate [2–4]. The aim of the present study is to investigate the influence of different cooling rates on structure and properties of MA steel processed through forging route. The chemical compositions of the steels used in this study are shown in Table I. The steels are medium carbon microalloyed steel with different vanadium and aluminum contents. Specimens obtained from steels 1 and 2 were heated at 1100 ◦C for 30 min and forging operation was carried out. Thirty-six percent deformation was applied by repeated strokes in temperature range of 1000–1100 ◦C. Then forged steel samples were cooled either in water, air, or sand. Room temperature tensile strength was measured by using an Instron machine at a crosshead speed of 1 mm/min. The pearlite grain size, volume fraction of ferrite, and pearlite were determined by using mean linear intercept (mli) method and point counting. Hardness measurement was also carried out using the Vickers hardness test. Fig. 1 shows the evaluation of the microstructure for both of the microalloyed steel under various cooling condition. Table II also shows volume fraction of ferrite and pearlite and mli grain sizes of pearlite in as-received,

Transformation twins in the weld HAZ of a low-carbon high-strength microalloyed steel

Abstract: Transformation twins were found in the weld heat-affected zone (HAZ) of a low-carbon microalloyed steel. The effect of weld thermal cycle and homogenisation of austenite with respect to dissolution of microalloy carbides on twinning is examined by applying a range of heat inputs as well as conducting simulated heat treatments with short and long annealing periods. Although twinning is more frequent in the martensitic coarse-grained HAZ when low values of heat input are applied, twinning can occur in the ferritic fine-grained HAZ when the peak temperature is not very high and cooling rates are low. It is also found that the annealing period during high-temperature holding of austenite does not significantly affect the volume fraction of twins, suggesting that inhomogeneities in the austenite cannot be a major cause of twinning. Weld effects, especially thermal stresses, are therefore concluded to be a major reason for twinning in the HAZ.

Microstructure Evolution in a Low Carbon Nb–Ti Microalloyed Steel

Abstract: The micro mechanical properties, stability and three-dimensional (3D) morphology of intragranular ferrite was studied in a low carbon microalloyed steel utilizing nano indentation, three-dimensional reconstruction techniques along with LEO1 450 scanning electronic microscopy. The mixed microstructures of intragranular ferrite, granular bainite and lath or plate-like bainite was obtained in a Nb–Ti microalloyed steel, which was water cooled immediately after relaxation for a fixed time as hot deformation ended. The elastic modulus and hardness increased in the sequence of intragranular ferrite, granular bainite and lath or plate-like bainite by approximately 15 GPa and 0.6 GPa, respectively. On the contrary, the tempering treatment of the specimens showed that the stability of the mixed microstructures decreased in the sequence of intragranular ferrite, granular bainite and lath or plate-like bainite. The results of stability, elastic modulus and hardness indicated that intragranular ferrite was formed prior to bainite transformation. The intragranular ferrite thus effectively sectioned the prior austenite into many small zones and thus the bainite transformed at lower temperatures was restricted in the small zones. It is likely that the formation of lath or plate-like intragranular ferrite prior to bainite transformation played an important role in the refinement of mixed microstructures.

Microstructural characteristic of low carbon microalloyed steels produced by thermo-mechanical controlled process

Abstract: The microstructural characteristic of the low carbon microalloyed steels produced by thermo-mechanical controlled processing was investigated by means of optical and transmission electron microscopies. Polygonal ferrite and acicular ferrite were found in OM; under TEM, acicular ferrite with high dislocation density, ultra-fine grain ferrite, layer of thin martensite film and precipitate phase were identified in 560MPa grade Ti-Nb and Ti-Nb-V microalloyed steels. An ultra-fine dispersion of precipitate phase was also found in Ti-Nb-V steel. These fine-scale microstructures exhibit excellent strength and fracture toughness, which is the main reason that TMCP is widely used in the production of high-strength low-alloy steels. (c) 2006 Elsevier B.V. All rights reserved.

Nitrocarburized microalloyed steel member

Abstract: A nitrocarburized microalloyed steel member consists of a microalloyed steel that includes a nitrocarburized layer on a surface, a cross-sectional structure of which steel except for the nitrocarburized layer includes a ferrite and pearlite structure. The microalloyed steel mainly consists of Fe and has a composition: C having a content of 0.30 mass % or more and 0.50 mass % or less; Si having a content of 0.05 mass % or more and 0.30 mass % or less; Mn having a content of 0.50 mass % or more and 1.00 mass % or less; S having a content of 0.03 mass % or more and 0.20 mass % or less; Cu having a content of 0.05 mass % or more and 0.60 mass % or less; Ni having a content of 0.02 mass % or more and 1.00 mass % or less; and Cr having a content of 0.05 mass % or more and 0.30 mass % or less. If the contents of the Cu, the Ni, and the Cr are represented by WCu, WNi, and WCr mass %, respectively, and composition parameters F1 and F2 are 185WCr+50WCu and 8+4WNi+1.5WCu-44WCr, respectively, then the composition parameters F1 and F2 satisfy F1>20 and F2>0.

Effects of vanadium and titanium on mechanical properties of low carbon as cast microalloyed steels

Abstract: Tensile, hardness and room temperature Charpy V notch impact tests were conducted to evaluate the variations in the mechanical properties of a low carbon cast steel containing vanadium and combinations of vanadium and titanium in the as cast condition. Tensile and hardness test results indicated that good combinations of strength and ductility can be achieved by microalloying additions. The effect of titanium on the yield strength and hardness, however, strongly depended on Ti/N ratio. Ti in hyperstoichiometric amounts increased the yield strength and hardness whereas in hypostoichiometric amounts, it did not have a considerable effect on those properties. Scanning electron microscopy and optical microscopy studies revealed that coarse TiN particles were responsible for this behaviour. On the other hand, microalloying additions significantly decreased the room temperature impact energy and led to the dominance of cleavage facets on the fracture surfaces. Although coarse TiN particles were identif...

The Influence of Reheating Temperature and Direct-cooling Rate after Forging on Microstructure and Mechanical Properties of V-microalloyed Steel 38MnSiVS5

Abstract: Wiht the aim of replacing quenched and tempered forging parts and reducing by this way costly and time consuming operations; an industrial forging procedure was developed to evaluate the influence of thermomechanical processing parameters on the microstructure and mechanical properties of vanadium microalloyed steel. In order to study the influence of reheating temperature, after determining the dissolving temperature of vanadium carbonitrides precipitates, samples were heating in temperature rang 1000 to 1200°C. After austenitization at 1100°C, the microalloyed steel billets were forged in a hydraulic press and then cooled with different cooling rates. The metallography and mechanical testing results indicated that by increasing the reheating temperature, the strength and toughness of V-microalloyed steel have not change significantly and so the temperature of 1200°C was selected for forging. By increasing cooling rate, both strength and toughness improve.

The application of stereological analysis in understanding differences in toughness of V- and Nb-microalloyed steels of similar yield strength

Abstract: The present paper describes an attempt to relate small variations in toughness of V- and Nb-microalloyed steels, characterized by almost similar yield strength and ferrite–pearlite microstructures, to stereological parameters. Stereological parameters derived from two-dimensional microstructure include mean intercept length (L¯α) of ferrite, ferrite grain boundary surface area per unit volume [SV]α, contiguity ratio (Cα) of ferrite grains, pearlite colony size, and pearlite interlamellar spacing. Stereological analysis was carried out as a function of depth from the surface using serial sectioning method. The analysis predicts that the toughness of the microalloyed steels is related to uniformity in the stereological parameters and the contiguity ratio. If the processing conditions such as cooling rate are increased, a convincingly higher toughness is obtained for Nb-containing steel in relation to V-microalloyed steels, at similar yield strength.

Transmission electron microscopy characterization of a Nb microalloyed steel for carburizing at high temperatures

Abstract: The carbide particles in AISI 5115 steel modified with added 0.038 wt.% Nb were characterized by means of transmission electron microscopy after simulation of carburizing heat treatments at high temperatures. The advantage of a high-temperature carburizing treatment over the standard 930 °C heat treatment is the significant reduction in the processing time. However, higher carburizing temperature leads to general and abnormal grain growth, which must be avoided by grain-pinning precipitates. The carburizing heat treatment simulations were performed at 950, 1000, and 1050 °C for 1 and 2 h in samples with two prior microstructural conditions: hot rolled and spheroidized. Characterization of the precipitates regarding composition, morphology, size, and distribution was performed using carbon extraction replicas and thin foil specimens. The results confirm the existence of complex Nb carbides. Particle size distribution curves, as log-normal functions, are shown to be extremely important in the interpretation of grain boundary pinning, rather than the mean particle size. The spheroidized samples showed a susceptibility to abnormal grain coarsening.

Modeling of microstructural evolution in microalloyed steel during hot forging process

Abstract: The microstructural evolution of microalloyed steel during hot forging process was investigated using physical simulation experiments. The dynamic recrystallized fraction was described by modifying Avrami's equation, the parameters of which were determined by single hit compression tests. Double hit compression tests were performed to model the equation describing the static recrystallized fraction, and the obtained predicted values were in good agreement with the measured values. Austenitic grain growth was modeled as: D i n c 5 = D 0 5 + 1.6 × 10 32 t ⋅ exp ( - 716870 RT ) using isothermal tests. Furthermore, an equation describing the dynamic recrystallized grain size was given as D dyn = 3771·Z −0.2 . The models of microstructural evolution could be applied to the numerical simulation of hot forging.

The determination of optimum forging conditions for the production of high strength-high impact toughness automotive parts

Abstract: The influences of various reheating and forging temperatures as well as cooling rates on the microstructure and mechanical properties, particularly impact energy, during the forging of a Nb-V microalloyed steel to be used for automotive safety parts were investigated. Increasing the prior austenite grain size increased the volume percent of acicular ferrite and reduced pearlite content in the microstructure even for very low post-forging cooling rates, resulting in improved toughness and tensile strength values. Increasing the cooling rate enhanced the acicular ferrite content, thereby increasing the impact energy properties. At lower reheating temperatures the yield strength and impact energy levels are determined by the percentage of pearlite present in the microstructure; while as the cooling rate is increased the amount of acicular ferrite and retained austenite are increased, improving the toughness and tensile strength of the forged part. This effect is more pronounced for the parts solutio...

Effects of Microalloying Elements on Mechanical Properties of Reinforcing Bars

Abstract: Reinforcing steel bars are an important material for construction. The strength level of rebar has been increased to as high as, for example, 345, 390 and 490 grade. In order to produce high-strength rebar, V microalloying technology has been successfully applied, while Nb application to reinforcing bars has been tried energetically recently. However, exact discussions about the effect of hot-rolling and cooling condition in Nb microalloyed steel on mechanical properties have not been reported exactly.This paper reports on the experimental results of the effects of reheating temperature and cooling rate after hot-rolling on the microstructure and mechanical properties of 0.25C–0.5Si–1.2Mn steel and 0.25C–0.5Si–1.2Mn–0.05Nb(+0.05V) steel (mass%).0.05% Nb and 0.05% Nb+0.05% V additions to 0.25C–0.5Si–1.2Mn steel (mass%) led to an effective increase in strength, especially at high reheating temperature. This strengthening is caused by precipitation hardening and grain refinement hardening. Accelerated cooling after hot-rolling to 700°C was effective in increasing strength and yield point elongation.

Effect of boron on the hot ductility of the Nb-microalloyed steel in austenite region

Abstract: Two grades of the Nb-microalloyed steel, one modified with B, were subjected toin situ melting and the thermal schedules experienced by the billet surface in the continuous casting process. The hot ductility was evaluated at various temperatures at the straightening stage of the process. It was found that addition of B improves the hot ductility in the austenite region. Such improvement could be due to fast precipitation at grain boundaries as well as depletion of precipitants and strengthening elements in the matrix.

Continuous Cooling Transformation Temperature and Microstructures of Microalloyed Hypereutectoid Steels

Abstract: The transformation behavior under continuous cooling conditions was investigated for four hypereutectoid steels of 1% carbon with different microalloying additions of vanadium and silicon. Continuous cooling compression testing of the hypereutectoid steels was employed to study the influence of processing conditions (re-heat temperature), microstructure (prior-austenite grain size) and chemical composition (vanadium and silicon) on the critical transformation temperature (Ar3). Overall, for the hypereutectoid steel compositions examined, the transformation temperatures were determined to be relatively stable, with a variation of roughly 15°C when the reheat temperature was changed from 1000 to 1200°C. The addition of microalloying elements such as vanadium and silicon was determined to increase the austenite-to-pearlite transformation start temperature of the hypereutectoid steels by about 10–30°C. These changes in the transformation behavior observed with decreasing re-heating temperature and microalloying additions were related to microstructural changes in the hypereutectoid steels, such as prior-austenite grain size refinement, carbide precipitation and grain boundary cementite fragmentation.

The Influence of Grinding Parameters on the Superficial Hardening Effect of 48MnV Microalloyed Steel

Abstract: The current surface strengthening process of microalloyed unquenched and tempered steel components is usually induction or laser quenching treatment. Subsequent to heat treatment, these structural parts are subjected to grinding, during which impairment of hardened materials can be caused by thermo-mechanical influence of the grinding process. This paper studies a new method of surface heat treatment by making use of grinding heat and stress to create favorable microstructures and promote high wear and fatigue resistance. This work outlines the influence of grinding parameters on the superficial hardening effect of 48MnV microalloyed steel. It was found that the thickness and hardness of the treated surface layer could be up to 1.6mm and HV750 respectively. The beneficial microstructure of the layer was created by an enhanced martensite transformation. It is highly possible that the method can be used to incorporate grinding and surface hardening into a single grinding operation to develop a cost-effective production method.