Dynamic Recrystallization Behaviour of Nb-Ti Microalloyed Steels

The dynamic recrystallization （DRX） behavior of Nb-Ti microalloyed steels was investigated by isothermal single compression tests in the temperature range of 900-1 150 ℃ at constant strain rates of 0.1-5 s^-1. DRX was retarded effectively at low temperature due to the onset of dynamic precipitation of Nb and Ti carbonitrides, resulting in higher values of the peak strain. An expression was developed for the activation energy of deformation as a function of the contents of Nb and Ti in solution as well as other alloying elements. A new value of corrective factor was determined and applied to quantify the retardation produced by increase in the amount of Nb and Ti dissolved at the reheating temperature. The ratio of critical strain to peak strain decreases with increasing equivalent Nb content. In addition, the effects of Ti content and deformation conditions on DRX kinetics and steady state grain size were determined. Finally, the kinetics of dynamic precipitation was determined and effect of dynamic precipitation on the onset of DRX was clarified based on the comparison between precipitate pinning force and recrystallization driving force.

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.

Effect of Ti content on the yield strength of Nb-Ti microalloyed steels

The hot rolling experiment investigates into the relationship between the microstructures and the mechanical properties of Nb-Ti microalloyed steels with various Ti contents. The results indicate that the effect of the bainite fraction of Nb-Ti microalloyed steels on the yield strength of the steels is not related to Ti content, while the slope of the Hall- Petch relationship decreases with the increase of Ti content. Accordingly, the Misra model for the yield strength of Nb-Ti microalloyed steels is modified, and the factors which cause the change in the slope of Hall-Petch relationship are discussed.

Influence of cooling rates on the precipitation behavior and microstructure of Nb-Ti microalloyed steel

Influence of different cooling rates on the microstructure and the precipitation behavior of Nb-Ti microalloyed steel was investigated by CSLM, OM, SEM and EDS. The results show that the precipitation process of carbonitrides can be in-situ observed by CSLM, and with the increase of the cooling rate, the distribution of precipitates changes from along the austenitic grain boundaries to within the grains. With the increase of the cooling rate, the proeutectoid ferritic film becomes smaller and smaller and then disappears, and the original austenitic grains become finer and finer. In order to obtain non-film like proeutectoid ferrites or non-chain like precipitates along the austenitic grain boundaries and finer austenitic grains,the cooling rate should be at least 5℃/s.

Effect of Strain Rate on the Ferrite Grain Refinement in a Low Carbon Nb-Ti Microalloyed Steel during Low Temperature Deformation

Grain refinement is one of the effective methods to develop new generation low carbon microalloyed steels possessing excellent combination of mechanical properties. The microstructural evolution and ferrite grain refinement at the deformation temperature of 865℃, above Ar3, with different strain rates were investigated using single pass isothermal hot compression experiments for a low carbon Nb-Ti microalloyed steel. The physical processes that occurred during deformation were discussed by observing the optical microstructure and analyzing the true stress-true strain responses. At strain rates of 0.001 and 0.01s^-1, there is no evidence of work hardening behavior during hot deformation and strain-induced transformation （SIT） leads to dynamic flow softening in flow curves. Optical microscopy observation shows that ultrafine and equiaxed ferrite with grain sizes of 2μm can be obtained by applying deformation with strain rate of 0.1 s^-1 due to SIT just after deformation. Furthermore, increasing the strain rate from 0.001 to 0.1 s^-1 reduces both the grain size of the equiaxed ferrite and the amount of deformed ferrite.

Composition Optimization of Nb-Ti Microalloyed High Strength Steel

Four Nb-Ti microalloyed steels were refined and rolled to study the composition optimization of Nb-Ti microalloyed steels. The effects of Nb and Ti on the microstructures, precipitates and properties of Nb-Ti microalloyed steel were investigated. The results showed that an increase in Ti content resulted in the appearance of many fine precipitates leading to a strong precipitation strengthening effect. Hence, the yield strength increased. Besides, the increased strength by the combined increase of Nb and Ti was similar to that observed for the increase in Ti content alone. This increase in strength was attributed widely to the increase in the Ti content alone rather than Nb. Moreover, the increase in Nb content beyond 0.036 wt% exerted no significant effect on the strength of Ti-Nb microalloyed steels, in which more Ti could be added to further improve the strength of steels.

Review of Sc microalloying effects in Al-Cu alloys

Artificially controlling the solid-state precipitation in aluminum (Al) alloys is an efficient way to achieve well-performed properties,and the microalloying strategy is the most frequently adopted method for such a purpose.In this paper,recent advances in lengthscale-dependent scandium (Sc) microalloying effects in Al-Cu model alloys are reviewed.In coarse-grained Al-Cu alloys,the Sc-aided Cu/Sc/vacancies complexes that act as heterogeneous nuclei and Sc segregation at the θ′-Al_(2)Cu/matrix interface that reduces interfacial energy contribute significantly to θ′precipitation.By grain size refinement to the fine/ultrafine-grained scale,the strongly bonded Cu/Sc/vacancies complexes inhibit Cu and vacancy diffusing toward grain boundaries,promoting the desired intragranular θ′precipitation.At nanocrystalline scale,the applied high strain producing high-density vacancies results in the formation of a large quantity of (Cu Sc,vacancy)-rich atomic complexes with high thermal stability,outstandingly improving the strength/ductility synergy and preventing the intractable low-temperature precipitation.This review recommends the use of microalloying technology to modify the precipitation behaviors toward better combined mechanical properties and thermal stability in Al alloys.

Development in oxide metallurgy for improving the weldability of high -strength low-alloy steel-Combined deoxidizers and microalloying elements

The mechanisms of oxide metallurgy include inducing the formation of intragranular acicular ferrite(IAF)using micron-sized inclusions and restricting the growth of prior austenite grains(PAGs)by nanosized particles during welding.The chaotically oriented IAF and refined PAGs inhibit crack initiation and propagation in the steel,resulting in high impact toughness.This work summarizes the com-bined effect of deoxidizers and alloying elements,with the aim to provide a new perspective for the research and practice related to im-proving the impact toughness of the heat affected zone(HAZ)during the high heat input welding.Ti complex deoxidation with other strong deoxidants,such as Mg,Ca,Zr,and rare earth metals(REMs),can improve the toughness of the heat-affected zone(HAZ)by re-fining PAGs or increasing IAF contents.However,it is difficult to identify the specific phase responsible for IAF nucleation because ef-fective inclusions formed by complex deoxidation are usually multiphase.Increasing alloying elements,such as C,Si,Al,Nb,or Cr,con-tents can impair HAZ toughness.A high C content typically increases the number of coarse carbides and decreases the potency of IAF formation.Si,Cr,or Al addition leads to the formation of undesirable microstructures.Nb reduces the high-temperature stability of the precipitates.Mo,V,and B can enhance HAZ toughness.Mo-containing precipitates present good thermal stability.VN or V(C,N)is ef-fective in promoting IAF nucleation due to its good coherent crystallographic relationship with ferrite.The formation of the B-depleted zone around the inclusion promotes IAF formation.The interactions between alloying elements are complex,and the effect of adding dif-ferent alloying elements remains to be evaluated.In the future,the interactions between various alloying elements and their effects on ox-ide metallurgy,as well as the calculation of the nucleation effects of effective inclusions using first principles calculations will become the focus of oxide metallurgy.

Influence of Finishing Cooling Temperature and Holding Time on Nanometer-size Carbide of Nb-Ti Microalloyed Steel

The nanometer-size carbides formed in ferrite matrix of Nb-Ti microalloyed steel at different finishing cooling temperatures and holding time have been investigated. The characteristics of nanometer-size carbides in ferrite were observed by transmission electron microscopy, and mechanical properties of ferrite were detected by a nano-hardness tester. The results showed that interphase precipitation and diffusion precipitation were observed at different finishing cooling temperatures, and the interphase precipitation was planar and curved. Sheet spacing of inter-phase precipitation increased with the increase of finishing cooling temperature and changed a little when holding for 50--1000 s. Interphase precipitation shows higher nano-hardness at 640℃ compared with diffusion precipitation at 600℃, and the contribution of interphase precipitation to the mechanical properties of ferrite was larger than that of diffusion precipitation.

Determination of Optimal Recrystallization Annealing Temperature of Nb-Ti Microalloyed High Strength IF Steel

The Nb-Ti microalloyed high strength IF steel sheet was used to study the effect of annealing temperature on the microstructures,mechanical properties and textures.The experimental results show that experimental steel is incomplete recrystallization at 750℃ annealing,but complete recrystallization from 780℃ to 870℃ under experimental conditions.When the annealing temperature was increased,the yield strength and tensile strength would gradually reduce,the plastic strain ratio and yield point elongation would gradually increase.The yield strength,tensile strength,elongation,the plastic strain ratio and the strain hardening exponent were approximate 300MPa,410MPa,36.5%,1.7 and 0.22 respectively under annealing temperature 810℃ to 840℃.When the annealing temperature was increased,the α-textures and γ-textures were gradually weakened,and the α-textures have a trend to {111} texture.Therefore,the suggestion of the optimal recrystallization annealing temperature is about 810℃ to 840℃ in industrial production.

变形工艺参数对Nb-Ti微合金钢再结晶的影响

通过热压缩实验研究了应变量及加热温度对Nb-Ti微合金钢的再结晶行为及变形后相变产物的影响。结果表明:当加热温度为1100℃时,奥氏体晶粒尺寸不均匀,导致压缩过程中各晶粒的应变累积不均匀,当总应变量为0.380时即发生了动态再结晶;奥氏体加热温度为1050和1000℃时,动态再结晶的临界应变量分别为0.668和0.552;当变形温度为850℃,应变量超过0.552时,试样在道次间隔发生静态再结晶;奥氏体化1100℃保温后压缩样品的冷却相变显微组织为贝氏体和准多边形铁素体,而1050℃和1000℃奥氏体化保温后压缩样品冷却相变后显微组织均为多边形铁素体,1000℃奥氏体化保温后压缩样品冷却相变后多边形铁素体晶粒尺寸更细小(2~5μm)。

Effects of Cooling Rates on Microstructures and Mechanical Properties of Nb-Ti Microalloyed Steel

The mechanical properties and microstructure of as-cast Nb-Ti microalloyed steel at different temperatures and cooling rates are investigated in this paper.TheⅢbrittle zone(700-900℃) is revealed.The reduction of the hot ductility is due to the film-like proeutectoid ferrite or the Nb(C,N) precipitates along the austenitic grain boundaries.In the temperature range of 850-1000℃,with the increase of the cooling rate,the hot ductility decreases.However,in the range of 650-850℃,the appearance of large volume fractions of ferrite on austenite grain boundaries minimizes the effect of cooling rate on hot ductility.When the cooling rate is 10℃/s, austenite transforms more quickly to ferrite and at a lower temperature a larger amount of ferrite nucleates and precipitates in the grain,which leads to a sharper improvement in the hot ductility at 650℃.

Recrystallization and Precipitation Behavior during the Hot Deformation of Nb-Ti Microalloyed Steels

Constant strain rate compression tests at the temperature 900～1100 ℃ and the strain rate 10 -3 ～20 s -1 and the interpass time 300～1000 s of double stage interrupted tests were carried out for a microalloyed steel containing 0.024%Nb, 0.016%Ti, 1.39%Mn and 0.15%C, to investigate its dynamic (DRX), static (SRX) and metadynamic recrystallization (MDRX) and precipitation behavior. Softening curves were determined by the offset method and back extrapolation method. It was found that the SRX and MDRX process have different Avrami exponent n and apparent activation energy Q . For the former n=1, Q =359 kJ/mol, while for the later n = 0.57 , Q =259 kJ/mol. Influences of temperature, prestrain and strain rate on the kinetics of SRX process were discussed and a mathematical model was furtherly constructed. Finally, the precipitation time temperature (PTT) curve was determined by the stress relaxation tests, and the influence of precipitation on the SRX process was also discussed. Direct microstructural evidence for precipitation during and after deformation was obtained by transmission electron microscopy.

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.

Remarkably slow corrosion rate of high-purity Mg microalloyed with 0.05wt% Sc

We report that the corrosion resistance of Mg is significantly improved by adding 0.05wt%Sc.Corrosion rates evaluated from weight loss values after room-temperature immersion in 0.6 M NaCl solution for two weeks were 0.27 and 4.0 mm y^(-1)for the high-purity Mg samples with and without microalloyed 0.05wt%Sc,respectively.The beneficial effect of Sc microalloying on the corrosion resistance of Mg is discussed in connection with Sc-induced microstructural modifications.

Effect of Ti and Ce Microalloy on Microstructure and Properties of Al-Si-Cu-Zr-Sr Cast Aluminum Alloy

The effect of Ti and Ce microalloying on the mechanical properties of Al-9Si-3.5Cu-0.2Zr-0.1Sr cast aluminum alloy was investigated,and it was hoped that the cast aluminum alloy with excellent comprehensive properties could be obtained.On the basis of Zr-Sr microalloyed cast aluminum alloy(Al-9Si-3.5Cu-0.2Zr-0.1Sr),the effects of 0.2Zr-0.1Sr-0.16Ti ternary microalloying and 0.2Zr-0.1Sr-0.16Ti-0.1Ce quaternary microalloying on the microstructure and properties of the alloy were investigated.The experimental results show that compared with Zr-Sr microalloying,Zr-Sr-Ti microalloying and Zr-Sr-Ti-Ce microalloying can effectively refine the microstructure,improve the modification effect of Si phase,and promote the improvement of Al_(2)Cu phase,thus improving the properties.The higher the degree of microalloying,the hardness is gradually increasing,but the electrical conductivity is gradually decreasing.Zr-Sr-Ti microalloying can increase the tensile strength of the alloy to 400.07 MPa and the elongation to 9.5%.Zr-Sr-Ti-Ce microalloying do not continue to improve the properties of the alloy,and the tensile strength and elongation after fracture decrease to a certain extent due to the addition of Ce.Therefore,the best comprehensive properties can be obtained by ZrSr-Ti microalloying(Al-9Si-3.5Cu-0.2Zr-0.1Sr-0.16Ti).

Effects of Rare-earth and Microalloying Elements on the Microstructure Characteristics of Hypereutectoid Rails

We performed thermal simulation experiments of double-pass deformation of hypereutectoid rails with different microalloying elements at a cooling rate of 1℃/s and deformation of 80%to explore the influence of rare-earth and microalloying elements on the structure of hypereutectoid rails and optimize the composition design of hypereutectoid rails.Scanning electron microscopy,transmission electron microscopy,X-ray diffraction,and other characterization techniques were employed to quantitatively analyzed the effects of different microalloying elements,including rare-earth elements,on pearlite lamellar spacing,cementite characteristics,and dislocation density.It was found that the lamellar spacing was reduced by adding various microalloying elements.Cementite lamellar thickness decreased with the refinement of pearlite lamellar spacing while the cementite content per unit volume increased.Local cementite spheroidization,dispersed in the ferrite matrix in granular form and thus playing the role of dispersion strengthening,was observed upon adding cerium(Ce).The contributions of dislocation density to the alloy strength of four steel sheet samples with and without the addition of nickel,Ce,and Ce–copper(Cu)composite were 26,27,32,and 37 MPa,respectively,indicating that the Ce–Cu composite had the highest dislocation strengthening effect.The Ce–Cu composite has played a meaningful role in the cementite characteristics and dislocation strengthening,which provides a theoretical basis for optimizing the composition design of hypereutectoid rails in actual production conditions.

Enhancing corrosion resistance of ZK60 magnesium alloys via Ca microalloying: The impact of nanoscale precipitates

Enhancing corrosion resistance of Mg-Zn alloys with high strength and low cost was critical for broadening their large-scale practical applications. Here we prepared solutionized, peak-and over-aged ZK60 alloys with and without microalloying Ca(0.26 wt.%) to explore the effects of nanoscale precipitates on their corrosion behavior in detail via experimental analyses and theoretical calculations. The results suggested the peak-aged ZK60 alloy with Ca addition showed improved corrosion resistance in comparison with the alloys without Ca,owing to the contribution of Ca on the refinement of precipitates and increase in their number density. Although the precipitates and Mg matrix formed micro-galvanic couples leading to dissolution, the fine and dense precipitates could generate “in-situ pinning” effect on the corrosion products, forming a spider-web-like structure and improving the corrosion inhibition ability accordingly. The pinning effect was closely related to the size and number density of precipitates. This study provided important insight into the design and development of advanced corrosion resistant Mg alloys.

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.

Microstructure and mechanical properties of an Nb-microalloyed EH47 steel

The loading capacity of ultralarge container ships has reached 24000 TEUs so far,and to ensure the safe operation of these ships,the maxmium thickness of crack arrest steel used in the upper deck areas reaches 100 mm,and crack arrest toughness(K_(ca))needs to be>8000 N/mm^(3)/2.The EH47 steel was employed to study the effect of Nb on the phase transformation of supercooled austenite in the continuous cooling process after rolling and the effect of Nb on microstructure and mechanical properties of the crack arrest steel plate.It was found that the addition of 0.02%Nb can inhibit the ferrite transformation,improve the steel plate strength,and reduce the strength inhomogeneity in the thickness direction.Industrial production of 100 mm-thick EH47 was carried out based on the function of Nb in EH47 steel,and the test results reveal that high-strength EH47 shipbuilding plates with high toughness,excellent fracture and crack arrest toughness,and good welding properties can be produced using Nb-microalloyed composition design and the thermal mechanical control process(TMCP);furthermore,the value of the crack arrest toughness reached 9450.7 N/mm^(3/2) at-10℃.