Niobium in Microalloyed Steels Unchallenged for Its Main Applications

During the past five decades niobium has emerged as the most important microalloying element in high strength steels.The first theories to explain its role were presented in the seminal publications of Woodhead,Morrison and Gray.Today niobium microalloyed steels are unchallenged for their main applications.Some examples include linepipe for gas transportation,automotive steels used in body-in-white and structural parts,shipbuilding,steel towers and steel structures in civil construction that exhibit better fire resistance and seismic load performance.Modern technology for steelmaking helped emphasize the superior overall properties that can be attained with the use of niobium.Gas linepipe is the best example of cleaner steel with carbon levels below 0.05% manufactured economically.Modern niobium microalloyed steels present the optimal balance of strength,toughness,weldability and formability.This paper describes the evolution of the technology associated with niobium in microalloyed steels,technology that secured its status as the efficient solution to solving today's challenges:safety,energy efficiency and environmental concerns.The paper also presents evidence that enhances the case that there is a secure,stable,long-term supply of the ferroniobium used to manufacture these steels.

Effect of Ti/V ratio on thermodynamics and kinetics of MC in γ/α matrices of Ti-V microalloyed steels

Through the solubility product theory of the ternary secondary phase,classical nucleation theory,and Ostwald ripening theory,a model was established to describe the thermodynamics and kinetics of(Ti,V)C precipitates in austenite/ferrite(y/α)matrices.The model was used to calculate the volume fraction,precipitation-temperature-time(PTT)curve,and nucleation rate-temperature(NrT)curve of MC(M=Ti,V)precipitates in γ/α matrices in Ti-V microalloyed steels with various Ti/V ratios,which is verified by hardness tester,transmission electron microscopy and energy-dispersive X-ray spectroscopy.The calculations indicate that,by decreasing Ti/V ratio from Ti4V0 steel to Ti0V4 steel,the complete-dissolution temperature decreases monotonically from 1226 to 830℃,and the equilibrium volume fraction of MC pre-cipitated from austenite decreases from 0.333%to 0.091%at 900℃.Moreover,the maximum nucleation temperature of MC precipitated from α matrix decreases from 748 to 605℃and the fastest precipitation temperature decreases from 844 to 675℃as Ti/V ratio decreases.PTT and NrT diagrams of MC precipitated from α matrices in different Ti-V microalloyed steels all exhibit C-shaped and inverse C-shaped curves.In addition,both theoretical calculation and experimental results show that when tempered at 600℃for 100 h,Ti2V2 steel shows the largest hardness value of 312 HV among the three steels tested because it has a larger volume fraction(0.364%),a larger precipitate density(1689 μm-2),and the smallest average size(8.4 nm)of(Ti,V)C precipitates.The theoretical calculations are consistent with experimental results,which indicates that the thermodynamics and kinetics model for(Ti,V)C precipitates is reliable and accurate.

Isothermal transformation and precipitation behaviors of titanium microalloyed steels

The microstructure transformation and precipitation behavior of nano-carbides in Ti microalloyed steel during isothermal process were studied by a compression test on a Gleeble 3800 thermomechanical simulator and analyzed by optical microscopy,transmission electron microscopy and other methods.The results show thatγ→αphase transformation and TiC precipitation take place in Ti microalloyed steel during the isothermal process,and time–temperature–transformation curve and precipitation–time–temperature(PTT)curve are all of“C”-type.During the isothermal process,the interphase precipitation of TiC mostly occurs at the period of the phase transformation,and the random precipitation of TiC mostly occurs on the ferrite after the phase transformation.The increment in yield strength at the initial stage of isothermal transformation mainly comes from phase transformation strengthening.With the increase in isothermal time,the precipitation hardening effect becomes more important for nucleation and growth of titanium carbides and eventually reaches the maximum value at the precipitation finished point of the PTT curve.

Kinetics of austenite growth and bainite transformation during reheating and cooling treatments of high strength microalloyed steel produced by sub-rapid solidification

First,strip cast samples of high strength microalloyed steel with sub-rapid solidification characteristics were prepared by simulated strip casting technique.Next,the isothermal growth of austenite grain during the reheating treatment of strip casts was observed in situ through confocal laser scanning microscope(CLSM).The results indicated that the time exponent of grains growth suddenly rise when the isothermal temperature higher than 1000℃.And the activation energy for austenite grain growth were calculated to be 538.0 kJ/mol in the high temperature region(above 1000℃)and 693.2 kJ/mol in the low temperature region(below 1000℃),respectively.Then,the kinetics model of austenite isothermal growth was established,which can predict the austenite grain size during isothermal hold very well.Besides,high density of second phase particles with small size was found during the isothermal hold at the low temperature region,leading to the refinement of austenite grain.After isothermal hold at different temperature for 1800 s,the bainite transformation in microalloyed steel strip was also observed in situ during the continuous cooling process.And growth rates of bainite plates with different nucleation positions and different prior austenite grain size(PAGS)were calculated.It was indicated that the growth rate of the bainite plate is not only related to the nucleation position but also to the PAGS.

Effect of titanium microalloying on microstructure and mechanical properties of vanadium microalloyed steels for hot forging

Effect of titanium microalloying on the microstructure and mechanical properties of vanadium microalloyed steels for hot forging was studied.Titanium microalloying improves the mechanical properties mainly through refining the austenite grains.When the heating temperature is in the range of 1050–1300℃,the austenite grain diameter is decreased from 77–133 to 26–68μm by titanium microalloying.With the decrease in austenite grain diameter,the final microstructure is refined significantly,and the high misorientation boundaries are increased.After the steel is heated at 1200℃(the common hot forging temperature)and cooled slowly,titanium microalloying decreases the yield strength from 548.4 to 519.4 MPa,and the tensile strength decreases from 842.7 to 808.7 MPa.However,the elongation increases from 19.0%to 21.4%,and the impact energy increases from 9.8 to 38.2 J.V–Ti steel has a better combination of strength,plasticity and toughness than V steel.In addition,the nucleation of intragranular ferrite idiomorphs is promoted by titanium microalloying,which may have a beneficial effect on the property of steels with coarse microstructure caused by the critical deformation in hot die forging.

Friction and wear behavior and mechanism of low carbon microalloyed steel containing Nb

Dry sliding friction and wear test of Nb containing low carbon microalloyed steel was carried out at room temperature,and the effect of Nb on the wear behavior of the steel,as welll as the mechanism was studied.Scanning electron microscopy(SEM) and energy dispersive spectrometry(EDS) were employed to analyze the morphology and composition of the worn surface,and the structure evolution of the plastic deformation layer.The carbide content and type in the steel were analyzed by the electrolytic extraction device and X-ray diffraction(XRD).The experimental results demonstrate that the addition of 0.2% Nb can refine the grain and generate Nb C to improve the wear resistance of the steel.By enhancing the load and speed of wear experiment,the wear mechanism of the test steel with 0.2% Nb changes from slight oxidation wear to severe adhesion wear and oxidation wear.Compared with the load,the increase in the rotation speed exerts a greater influence on the wear of the test steel.

Effects of Quenching Process on Microstructure and Mechanical Properties of Low Carbon Nb-Ti Microalloyed Steel

The low carbon Nb-Ti microalloyed tested steel was prepared by the process of vacuum induction furnace smelting,forging and hot rolling.The new steel aims to meet the demand of high strength,high toughness and high plasticity for building facilities.The effects of quenching process on microstructure and mechanical properties of tested steel were investigated.The results showed that prior austenite grain size,phase type and precipitation behavior of(Nb,Ti)(C,N)play important roles in mechanical properties of the steel.Through modified appropriately,the model of austenite grain growth during heating and holding is d^(5.7778)=5.6478^(5.7778)+7.04×10^(22)t^(1.6136)exp(-427.15×10~3/(RT)).The grain growth activation energy is Q_g=427.15 kJ.During quenching,the microscopic structures are mainly martensite and lath bainite which contains lots of lath substructure and dislocations.The content of phases,fine and coarsening(Nb,Ti)(C,N)precipitated changes during different quenching temperatures and holding time.Finally compared with the hardness value,the best quenching process can be obtained that heating temperature and holding time are900℃and 50 mins,respectively.

Constitutive Modeling and Dynamic Recrystallization Mechanisms of an Ultralow-carbon Microalloyed Steel During Hot Compression Tests

The hot deformation behavior of an ultralow-carbon microalloyed steel was investigated using an MMS-200 thermal simulation test machine in a temperature range of 1073-1373 K and strain rate range of 0.01-10 s-1.The results show that the flow stress decreases with increasing deformation temperature or decreasing strain rate.The strain-compensated constitutive model based on the Arrhenius equation for this steel was established using the true stress-strain data obtained from a hot compression test.Furthermore,a new constitutive model based on the Z-parameter was proposed for this steel.The predictive ability of two constitutive models was compared with statistical measures.The results indicate the new constitutive model based on the Z-parameter can more accurately predict the flow stress of an ultralow-carbon microalloyed steel during hot deformation.The dynamic recrystallization(DRX)nucleation mechanism at different deformation temperatures was observed and analyzed by transmission electron microscopy(TEM),and strain-induced grain boundary migration was observed at 1373 K/0.01 s^-1.

Carbon Equivalent Fundamentals in Evaluating the Weldability of Microalloy and Low Alloy Steels

Understanding the weldability of steel in relation to the use of carbon equivalent is very necessary for the welding industry. The study was poised to unearth the fundamentals of carbon equivalent as applied in evaluating the weldability of steel. The study used a two-stage design approach to address the problem of carbon equivalence weldability of steel, thus, survey and experimental. Two different steels were tested to ascertain their chemical composition which could inform carbon equivalent calculation, and the results revealed microalloy and low alloy steels respectively. In subjecting the microalloy steel to carbon equivalent analyses of the AWS and IIW coefficients;revealed a value (CEV) = 0.11 each, suggesting that this microalloy steel has excellent weldability;no preheating is required. A successful welding operation on this steel does not depend on preheating. Also, the average results of the low alloy steel revealed a value (CEV) = 0.37 and 0.32 respectively, suggesting that this type of steel has very good weldability and may require to preheat. It is recommended that welders have a general idea about the weldability of steel with regard to carbon equivalent calculation. In addition, they should understand the chemical compositions of steels they are dealing with.

Feasibility of 0.02% Nb-Based Microalloyed Steel for the Application of One-Step Quenching and Partitioning Heat Treatment

To attain an enhanced combination of mechanical properties for low alloyed steel, the current study has been made to fulfill that growing need in the industry. Its results are introduced within this paper. One step Quenching and Partitioning (Q&P) heat treatment has been applied on Niobium-based microalloyed steel alloy with 0.2 %C, in the form of 2 mm thickness sheets. The target of this study is to investigate the viability of applying that significantly recommended, results-wise, heat treatment on the highly well-suited alloy steel samples, to achieve the main target of enhanced properties. A single temperature of 275°C was used as quenching and Partitioning temperature. Four Partitioning periods (30, 200, 500, and 1000 Seconds) were used for soaking at the same temperature. The results were analyzed in the light of microstructural investigation and mechanical testing. All applied cycles did not enhance the strength but moderately improved the ductility and toughness, mainly caused by the slightly high soaking temperature used. Niobium impact of grain refining was apparent through all cycles. The cycle of 500 Seconds Partitioning time obtained optimum values at that particular temperature. The 1000 Seconds Cycle obtained the worst combination of properties. A set of recommendations are set. More research is required at this point, where a lower Partitioning temperature is advised. In the light of the applied combination of parameters, the Partitioning period at such temperature is advised to be between 500 and 1000 Seconds. A high probability that periods closer to 500 than 1000 Seconds will produce better results. More research is needed between those two values of Partitioning time to precisely determine the optimum time at that temperature on that specific alloy.

High cycle fatigue behavior of titanium microalloyed high-strength beam steels

The realization of an ideal combination of mechanical and fatigue properties is prerequisites for practical application of titanium(Ti)microalloyed steel in automotive field.The fatigue behavior of four Ti microalloyed high-strength beam steels with different Ti contents was systematically studied.The results show that the content of microalloying element Ti has a significant effect on the fatigue properties,especially in the steel with a high Ti content.For the experimental Ti microalloyed steel,inclusion-induced crack initiation is the main fatigue failure mode.Different from general fatigue fracture mechanism in Ti-contained steel,no TiN,which is the most detrimental to fatigue behavior,was found in fatigue crack initiation area.However,the large-sized TiN and oxide complex inclusion with a core-shell structure is the dominant cause of fatigue fracture.Because of the intense-localized deformation at the interface between complex inclusion and matrix,the angular TiN in the outer shell has a serious deteriorating effect on the fatigue properties,which is consistent with the result of the Kernel average misorientation map.Besides,the modification effect of a small amount of MnS on large-sized inclusion is not obvious and has little effect on the fatigue behavior.For more practical guidance,the critical inclusion sizes of the experimental steels were also investigated by experimental extrapolation method.With the increasing tensile strength,the inclusion sensitivity of the experimental steels increases,leading to the small critical inclusion size.

Mechanism and control of nonuniform phase transformation of microalloyed dual-phase steel during cooling process after hot rolling

After cooling in the hot rolling process,the metallographic structure of microalloyed dual-phase steel is nonuniform along the rolling direction,while the thickness fluctuation of microalloyed dual-phase steel with a nonuniform metallographic structure will occur during cold rolling.The mechanism of nonuniform phase transformation of microalloyed dual-phase steels was studied during the cooling process after hot rolling,and the nonuniform phase transformation of microalloyed dual-phase steel was regulated during the cooling process after hot rolling through process optimization.First,the empirical equation of phase transformation temperature was measured by a dilatometer considering thermal expansion.Then,the phase field and temperature field of laminar cooling process were calculated to provide initial boundary conditions for the finite element model.After that,the coupling finite element model of the temperature phase transformation of the strip steel in coiling transportation process was established.The simulation results show that the different thermal contact conditions of the microalloyed dual-phase steel during coil transportation lead to uneven cooling of the coil,which leads to nonuniform transformation of the coil along the rolling direction.In addition,by prolonging the time interval from coiling to unloading,the phenomenon of nonuniform phase transformation of microalloyed dual-phase steel can be effectively controlled.The simulation results are applied to industrial production.The application results show that prolonging the time interval from coiling to unloading can effectively improve the nonuniform phase transformation of microalloyed dual-phase steel in the cooling process after hot rolling.

Stability of Ultra-fine Microstructures during Tempering

The stability of ultra-fine microstructure during tempering at 650℃ was investigated on a Nb-containing steel. The steel had undergone 5 passes controlled rolling, then was relaxed (air cooled) to 730 ℃ and cooled in water The evolution of microstructure was that, in early stage of tempering, no obvious change was detected by means of optical microscopy while dislocation cells were formed inside bainitic laths. With further tempering, bainitic laths started to coalesce in some regions. Finally, polygonal ferrite was formed while hardness decreased dramatically. Some samples taken from the same primary plate were reheated at 930 ℃for 0.5 h followed by quenching into water before tempering. Despite their lower original hardness, the reheated samples softened faster during tempering. Ferrite was quickly formed in the reheated samples. These results indicate that the evolution of microstructures towards equilibrium during tempering of the steel is mainly determined by whether dislocations are pinned rather than the dislocation density.

Evolution of Microalloyed Linepipe Steels With Particular Emphasis on the “Near Stoichiometry” Low Carbon,0.10 Percent Niobium “HTP” Concept

Microalloyed linepipe steels were first introduced in 1959 at strength levels around 52-60 ksi [1] [2].The steels were generally strengthened by niobium or vanadium used singly.As strengths levels increased to X-70 in the early 1970's niobium and vanadium were used in combination and controlled rolling was introduced on a broad basis.Higher strengths decrease tolerance for impurities and non-metallic inclusions which required improved steelmaking practices.Sulfur,carbon,and nitrogen were reduced and continuous casting was increasingly applied which required improved ladle refining and deoxidation practices.Niobium-molybdenum steels were introduced by IPSCO in 1972 [3] and by Italsider in 1974 [4].Steels with higher niobium contents were also introduced in the early 1970's but available steelmaking technologies limited the use of optimum niobium to carbon ratios,i.e.those approaching stoichiometry.Nevertheless the steels benefited from the effect of niobium in retarding austenite recrystallization at relatively high rolling temperatures and they developed attractive combinations of strength and toughness in old manufacturing facilities using simple (relaxed) rolling schedules.Fluctuations in the price of molybdenum and vanadium in the past decade and expanded application of API Grade X-80 linepipe,have led to more widespread use of niobium contents up to 0.11 percent,in combination with 0.02-0.03 percent carbon (near stoichiometry) thus maximizing the effects of solute niobium during rolling,in lowering transformation temperature,and thereby reducing reliance on other alloying,often expensive,elements.The history of these developments will be presented and applications of the concept will be summarized.

Dissolution of FeNb in liquid steel and best practices to increase niobium recovery during ladle refining

This paper presents a microstructural characterization of standard ferroniobium(FeNb) and a study of its dissolution in steel.Interrupted dissolution trials at different temperatures were carried out in a laboratory induction furnace.The microstructure at the interface between solid FeNb and frozen steel was analyzed and compared with the original FeNb particles.The results allowed clarifying both the FeNb dissolution mechanism and its incorporation kinetics at steelmaking temperatures.Experimental results from the dissolution of standard FeNb in liquid steel proved that the dissolution rates of FeNb particles were higher than those reported in the pertinent literature,where models assume that FeNb dissolves only by the diffusion of solid particles due to its high melting temperatures(over 1 600℃).Most recent FeNb phase diagrams and DTA analyses have shown that standard FeNb begins to melt down below 1 600℃.The experimental evidences in this work have led to the adoption of a new description of the dissolution mechanism whereby standard FeNb dissolves predominantly by the melting of its eutectic zones at 1500℃to 1510℃and,later on,by the melting of the primary laths between 1530℃and 1575℃,i.e.,below the steelmaking temperatures of around 1 600℃.Based on these results and on previous melting-shop experience,this paper recommends the best practices for the addition of FeNb during the microalloyed steels secondary refining phase.

Banded structure control of low carbon microalloyed steel based on oxide metallurgy

Banded structure is a common harmful microstructure for low carbon microalloyed steel,which seriously shortens the service life of processed parts.In order to study the effect of oxide metallurgy on improving banded structure,the Ti-Zr deoxidized low carbon microalloyed steel that can play the oxide metallurgical role of inclusion was chosen as the research object,and the inclusion characteristics,microstructure and transverse and longitudinal mechanical properties after hot rolling were analyzed.The results showed the inclusion number density increased in all experimental steels after hot rolling,and a large number of long strip inclusions with aspect ratio greater than 3 appeared along the rolling direction.In addition,after hot rolling,there were element segregation bands in the experimental steels,and granular bainite bands were formed in the element enrichment zone.However,the intragranular ferrite generated in the cooling process destroyed the continuity of granular bainite bands,so that the microstructure anisotropy indexes of experimental steels were small.The mechanical properties analysis showed that the anisotropy of performance was mainly reflected in plasticity and toughness in the experimental steels.Among them,the difference ratio of elongation,section shrinkage and impact energy of No.2 steel was 1.69%,3.87% and 1.69%,respectively,which were less than those of No.1 steel and No.3 steel.The anisotropy of microstructure and mechanical properties of No.2 steel that full played the role of oxide metallurgy were improved,and the banded structure control of low carbon microalloyed steel can be realized by oxide metallurgy technology.

Microstructural Evolution of Nb–V–Mo and V Containing TRIP-assisted Steels during Thermomechanical Processing

The microstructural evolution and precipitation behaviour of Nb–V–Mo and single V containing transformation induced plasticity assisted steels were investigated during thermomechanical processing. A plane strain compression testing machine was used to simulate the thermomechanical processing. Microstructures were characterised by optical microscopy, scanning-transmission electron microscopy and microanalysis, and X-ray diffraction analysis, and Vickers hardness was obtained from the deformed specimens. The resulting microstructure of both Nb–V–Mo and V steels at room temperature primarily consisted of an acicular/bainitic ferrite, retained austenite and martensite surrounded by allotriomorphic ferrite.The TEM analysis showed that a signi?cant number of Nb(V,Mo)(C,N) precipitates were formed in the microstructure down to the ?nishing stage in Nb–V–Mo steel(i.e. 830 °C). It was also found that the V(C,N)precipitation primarily occurred in both ferrite and deformed austenite below the ?nishing stage. The results suggested that Nb–Mo additions considerably increased the temperature stability of microalloy precipitates and controlled the microstructural evolution of austenite. However, the microalloy precipitation did not cause a signi?cant precipitation strengthening in both Nb–V–Mo and V steels at room temperature.

Modification for prediction model of austenite grain size at surface of microalloyed steel slabs based on in situ observation

The initial solidification process of microalloyed steels was simulated using a confocal scanning laser microscope,and the growth behavior of austenite grain was observed in situ.The method for measuring the initial austenite grain size was studied,and the M_(0)^(*)(the parameter to describe the grain boundary migration)values at different cooling rates were then calculated using the initial austenite grain size and the final grain size.Next,a newly modified model for predicting the austenite grain size was established by introducing the relationship between M_(0)^(*)and the cooling rate,and the value calculated from the modified model closely corresponds to the measured value,with average relative error being less than 5%.Further,the relationship between T^(γ)(the starting temperature for austenite grain growth)and equivalent carbon content C_(P)(C_(P)>0.18%)was obtained by in situ observation,and it was introduced into the modified model,which expanded the application scope of the model.Taking the continuous casting slab produced by a steel plant as the experimental object,the modified austenite grain size prediction model was used to predict the austenite grain size at different depths of oscillation mark on the surface of slab,and the predicted value was in good agreement with the actual measured value.

Research status and prospect of direct strip casting manufactured lowcarbon microalloyed steel

Direct strip casting(DSC)is one of the cutting-edge technologies for the steel industry in the twenty-first century.Under the background of carbon peak and carbon neutrality,DSC technology has a bright future of applications as it requires less production time and space with reduced energy consumption.Owing to its sub-rapid cooling rate during solidification and low reduction during hot rolling,DSC process exhibits a series of unique physical metallurgy characteristics.The process characteristics of DSC process and the microstructural evolution during the thermomechanical processing of low-carbon microalloyed steel are reviewed.The effects of hot rolling,cooling,coiling temperatures and microalloying elements on the microstructure and mechanical properties are then discussed.Finally,the future development orientations of DSC technology are suggested to fully utilize its unique features for the enhancement of its competitiveness and for the promotion of carbon neutrality of the steel industry.

Influence of Vanadium Content on Hot Deformation Behavior of Low-Carbon Boron Microalloyed Steel

Single-pass compression tests were performed to investigate the hot deformation behavior of low-carbon boron microalloyed steel containing three various vanadium contents at 900-1100℃and strain rate of 0.01-10 s-1 using the MMS-300 thermal mechanical simulator.The flow stress curves of investigated steels were obtained under the different deformation conditions,and the effects of the deformation temperature and strain rate on the flow stress were discussed.The characteristic points of flow stress were obtained from the stress dependence of strain hardening rate;the activation energy of investigated steels was determined by the regression analysis;the flow stress constitutive equations were developed;the effect of vanadium content on the flow stress and dynamic recrystallization(DRX)was investigated.The result showed that the flow stress and activation energy(3-6.5 kJ mol-1)of the steel containing 0.18 wt%V were significantly higher than those of the steels with0.042 wt%and 0.086 wt%V,which was related to the increase in solute drag and precipitation effects for higher vanadium content.DRX analysis showed that the addition of vanadium can delay the initiation and the rate of DRX.