Unique properties of new steels microalloyed with vanadium and nitrogen

The influence of microalloying with nitrogen, vanadium, and other nitride-forming elements on the mechanical, technological, and functional properties of structural steel is investigated in laboratory and industrial conditions. Thanks to decrease in austenite grain size, dispersional hardening by nitride nanophase, and the formation of perfect fine substructure, a unique combination of properties is obtained: increase in strength (by 30–50%) and plasticity; resistance to fatigue, brittle, and corrosional failure; and seismic strength and fire resistance. Control principles for the properties of steels of different strength in rolling are developed. In this approach, quenching is unnecessary, and the resulting steel may be used in place of traditional nickel steel.     

Carbonitride precipitation in niobium/vanadium microalloyed steels

A detailed study of carbonitride precipitation in niobium/vanadium microalloyed steels is presented. A thermodynamic model is developed to predict the austenite/carbonitride equilibrium in the Fe−Nb-V-C-N system, using published solubility data and the Hillert/Staffansson model for stoichiometric phases. The model can be used to estimate equilibrium austenite and carbonitride compositions, and the amounts of each phase, as a function of steel composition and temperature. The model also provides a method to estimate the carbonitride solution temperatures for different steel compositions. Actual carbonitride precipitation behavior in austenite is then examined in two experimental 0.03Nb steels containing 0.05V and 0.20V, respectively. Samples were solution treated, rolled at 954°C (20 pct or 50 pct), held isothermally for times up to 10,000 seconds at 843°C, 954°C, or 1066°C, and brine quenched. The process of carbonitride precipitation in deformed austenite is followed by analytical electron microscopy (AEM) of carbon extraction replicas. Precipitates are observed at prior-austenite grain boundaries, and also within the grains (presumably at substructure introduced by the rolling deformation). Analysis of the grain-boundary and matrix precipitate compositions by AEM indicates that the grain-boundary precipitates are consistently richer in vanadium than the matrix precipitates, although compositional trends with holding time and temperature are similar for the two types of precipitates. The compositions of both the grain-boundary and matrix precipitates are not significantly influenced by the rolling reduction or the holding time at temperature. As predicted by the thermodynamic model, the precipitates become more vanadium-rich as the vanadium level in the steel is increased and as the temperature is reduced. The agreement between the measured and predicted precipitate compositions is quite good for the grain-boundary precipitates, although the matrix precipitates are consistently more niobium-rich than predicted by the model.     

Carbonitride precipitation in niobium/vanadium microalloyed steels

A detailed study of carbonitride precipitation in niobium/vanadium microalloyed steels is presented. A thermodynamic model is developed to predict the austenite/carbonitride equilibrium in the Fe-Nb-V-C-N system, using published solubility data and the Hillert/Staffansson model for stoichiometric phases. The model can be used to estimate equilibrium austenite and carbonitride compositions, and the amounts of each phase, as a function of steel composition and temperature. The model also provides a method to estimate the carbonitride solution temperatures for different steel compositions. Actual carbonitride precipitation behavior in austenite is then examined in two experimental 0.03Nb steels containing 0.05V and 0.20V, respectively. Samples were solution treated, rolled at 954 °C (20 pct or 50 pct), held isothermally for times up to 10,000 seconds at 843 °C, 954 °C, or 1066 °C, and brine quenched. The process of carbonitride precipitation in deformed austenite is followed by analytical electron microscopy (AEM) of carbon extraction replicas. Precipitates are. observed at prior-austenite grain boundaries, and also within the grains (presumably at substructure introduced by the rolling deformation). Analysis of the grain-boundary and matrix precipitate compositions by AEM indicates that the grain-boundary precipitates are consistently richer in vanadium than the matrix precipitates, although compositional trends with holding time and temperature are similar for the two types of precipitates. The compositions of both the grain-boundary and matrix precipitates are not significantly influenced by the rolling reduction or the holding time at temperature. As predicted by the thermodynamic model, the precipitates become more vanadium-rich as the vanadium level in the steel is increased and as the temperature is reduced. The agreement between the measured and predicted precipitate compositions is quite good for the grain-boundary precipitates, although the matrix precipitates are consistently more niobium-rich than predicted by the model.     

Precipitation and fine structure in medium-carbon vanadium and vanadium/niobium microalloyed steels

Hot-rolled and continuously cooled, medium-carbon microalloyed steels containing 0.2 or 0.4 pct C with vanadium (0.15 pct) or vanadium (0.15 pct) plus niobium (0.04 pct) additions were investigated with light and transmission electron microscopy. Energy dispersive spectroscopy in a scanning transmission electron microscope was conducted on precipitates of the 0.4 pct C steel with vanadium and niobium additions. The vanadium steels contained fine interphase precipitates within ferrite, pearlite nodules devoid of interphase precipitates, and fine ferritic transformation twins. The vanadium plus niobium steels contained large Nb-rich precipitates, precipitates which formed in cellular arrays on deformed austenite substructure and contained about equal amounts of niobium and vanadium, and V-rich interphase precipitates. Transformation twins in the ferrite and interphase precipitates in the pearlitic ferrite were not observed in either of the steels containing both microalloying elements. Consistent with the effect of higher C concentrations on driving the microalloying precipitation reactions, substructure precipitation was much more frequently observed in the 0.4C-V-Nb steel than in the 0.2C-V-Nb steel, both in the ferritic and pearlitic regions of the microstructure. Also, superposition of interphase and substructure precipitation was more frequently observed in the high-C-V-Nb steel than in the similar low-C steel.     

Thermomechanical processing of microalloyed powder forged steels and a cast vanadium steel

The effects of controlled rolling on transformation behavior of two powder forged (P/F) microalloyed vanadium steels and a cast microalloyed vanadium steel were investigated. Rolling was carried out in the austenitic range below the recrystallization temperature. Equiaxed grain structures were produced in specimens subjected to different reductions and different cooling rates. The ferrite grain size decreased with increasing deformation and cooling rate. Ferrite nucleated on second phase particles, deformation bands, and on elongated prior austenite grain boundaries; consequently a high fractional ferrite refinement was achieved. Deformation raised the ferrite transformation start temperature while the time to transformation from the roll finish temperature decreased. Cooling rates in the cast steel were higher than in P/F steels for all four cooling media used, and the transformation start temperatures of cast steels were lower than that of P/F steel. Intragranular ferrite nucleation, which played a vital role in grain refinement, increased with cooling rate. Fully bainitic microstructures were formed at higher cooling rates in the cast steel. In the P/F steels inclusions and incompletely closed pores served as sites for ferrite nucleation, often forming a ‘secondary’ ferrite. The rolling schedule reduced the size of large pores and particle surface inclusions and removed interconnected porosity in the P/F steels. Formerly Postgraduate Researcher in the Department of Metallurgy and Materials Science, UMIST/University of Manchester, United Kingdom     

Role of carbon and nitrogen content on microstructural homogeneity in thin slab direct rolled microalloyed steels

Abstract

This paper analyses the effect of carbon and nitrogen content on the austenite microstructural homogeneity before transformation in the thin slab direct rolling of Nb and Nb–V microalloyed steels. The study was made with the help of a microstructural hot working model adapted to the metallurgical peculiarities associated with thin slab direct rolling. The results show that an increase in carbon content from 0·04 to 0·09% in 0·05%Nb microalloyed steels requires a significant increase in the initial rolling temperature in order to avoid the presence of isolated as cast austenite grains prior to transformation. Similarly, an increase in nitrogen content from 30 to 120 ppm does not imply changes as drastic as in the case of carbon. In both situations the changes required in the rolling temperature can be explained by the interaction between post-dynamic softening mechanisms and strain induced precipitation kinetics. In this context, the incidence of different final gauge thicknesses on microstructural homogeneity is evaluated.     

New post forging treatment of medium carbon microalloyed steels

In drop forging of parts for the transport industry the classical quench and tempering (QT) of alloyed steels is nowadays substituted by direct continuous cooling (CC) of microalloyed steels with elimination of quench cracking and expensive straightening and stress relieving cycles. Nevertheless, there are some limitations on strength and toughness achievable by this technique. On two commercial medium carbon (0.3%C) steels microalloyed with vanadium or vanadium and titanium, modified forging parameters and a new two-step cooling (TSC) strategy combined with an additional annealing were applied. Some increase in manufacturing costs, when compared to CC, can be justified by a significant increase in strength and ductility. The improvements attainable through such a modified treatment give evidence of the large potential of multi-phase ferrite-containing microstructures as a substitute of a tempered martensite.     

Titanium microalloyed steels

Compared with niobium and vanadium, titanium has been regarded as a relative minor element in microalloyed (MA) steels. More recently, titanium compounds in MA steels have been recognised as having a wider role than just involved in austenite grain refinement. A brief history is followed by considering the physical state of titanium and its compounds characterized in MA steels. Their solubility in iron and the morphology of the precipitates they form, lead to their functions in controlling mechanical and toughness properties of MA steels often involving the multiple alloying with niobium, vanadium, carbon and nitrogen. Titanium has become an important element in the development of linepipe steels, which can develop bainite/acicular ferrite (AF) microstructures. The influence of Ti on nucleation of AF is an active research area, particularly in welding of MA steels. Finally, the influence of titanium on hot ductility, continuous casting and thin slab direct-charging processes is discussed.     

含Nb低碳Si-Mn系TRIP钢的连续冷却转变

利用Gleeble-1500热应力/应变模拟机研究了Nb对低碳Si-Mn系TRIP钢连续冷却转变的影响,测定了含nb和不含Nb两种低碳Si-Mn系TRIP钢的连续冷却转变(CCT)曲线,结果表明:Nb的加入使得静态CCT曲线上移;动态CCT曲线下移.试验结果可为TRIP钢的TMCP(Thermo-Mechnical Control Processing)工艺提供理论依据.     

C和N含量对V微合金非调质钢静态再结晶量的影响

使用Gleeble-1500热/力模拟机,对四种V-N微合金非调质钢和一种非V-N微合金化对比钢进行了静态再结晶实验研究.研究结果表明:当钢中C质量分数为0.33%时,V-N微合金钢的静态再结晶要比未V-N微合金化的对比钢有明显滞后,尤以820~880℃温度范围内最为明显,因此钢中V析出物对道次间再结晶过程影响很大.进一步研究表明,V-N微合金非调质钢道次间静态再结晶量受C含量的影响并不呈简单线性关系:在760~880℃温度范围内,道次间静态再结晶量在钢中C质量分数为0.33%时均为极大值,而940℃下所有五种实验钢均完成了静态再结晶;钢中V析出物对道次间静态再结晶的影响机制相当复杂,与其析出时机关系很大.在此C含量下且V和Ti量均近似相同的V-N微合金实验钢中,发现当N质量分数从140×10^-6增加到210×10^-6时,该温度范围内道次间静态再结晶量下降14%~19%,N含量增加有明显抑制道次间静态再结晶的作用.     

增氮对钒微合金钢组织和性能的影响

 通过对不同钒、氮质量分数的试验钢进行热模拟压缩试验和实验室轧制试验,用OM、SEM和TEM分析试验钢的显微组织,研究增氮对钒微合金钢组织和性能的影响。结果表明,普通钒微合金钢为板条贝氏体＋粒状贝氏体组织,增加氮质量分数,可促进晶内铁素体相变,得到针状铁素体组织,使M/A组织细化且弥散分布,改善韧性;而增加钒质量分数,可以增加析出强化作用,提高强度,但组织形态无明显变化,不能提高韧性。增氮钢中的钒在奥氏体内以VN析出,低氮钢内的钒在铁素体内以VC的形式析出,奥氏体-铁素体、VC-铁素体和VN-铁素体的平面点阵错配度分别为6.72%、3.89% 和 1.55%,在奥氏体内析出的VN可以作为铁素体的优先形核位置,促进晶内铁素体相变。     

Hydrogen in microalloyed steels

To understand and control hydrogen induced cracking and stress corrosion cracking, the processes of hydrogen absorption, diffusion and trapping are of interest. Fundamentals of these processes are described and of the determination of permeation coefficient, diffusivity and solubility of H in iron and steels, using the electrochemical double cell. With this method trapping parameters are also obtained, i.e. numbers of traps and binding energies. Extended studies were conducted on hydrogen in ternary alloys Fe‐Me‐C or N (Me = Ti, Zr, V, Nb, Mo) and in pipeline steels. Flat traps with binding energies around ?19kJ/mol H can be discerned from deep traps with binding energies around ?57kJ/mol H. As shown by constant extension rate tests with the pipeline steels, only the mobile hydrogen in ideal solution and in the flat traps is involved in hydrogen induced stress corrosion cracking, not the hydrogen tied up in deep traps.     

A review of the development and application of microalloyed medium carbon steels

Microalloyed medium-carbon steels with ferrite-pearlite microstructure were developed in the FRG in early 1972, with the primary aim of saving the cost of heat treatment. A steel with roughly 0.47% C, 0.75% Mn, 0.060% S and 0.1 % V was first used for crankshafts in cars manufactured by one of the largest European automobile companies. The effect of microalloying elements such as vanadium and niobium (niobium instead of columbium is used in this paper) in these steels and their dependence on the cooling rate from drop-forging temperatures is reviewed. Although niobium is more effective than vanadium, it leads to problems while manufacturing these steels with ~0.47% C, due to the high solution temperature of the niobium precipitates, so that preference has been given to vanadium. Further development work carried out to improve the ductility of these steels is reported. Steel compositions, which could make these steels applicable for various automobile and other engineering components, are presented.     

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 microalloy...     

Low-cycle fatigue properties of microalloyed medium carbon precipitation hardening steels in comparison to quenched and tempered steels

Monotonic and cyclic stress strain curves and strain fatigue-life curves of a normalized carbon steel Cf 53 N, two quenched and tempered steels Ck 45 QT, 34 CrMoS 4 QT and three microalloyed medium-carbon precipitation-hardening steels 27 MnSiVS 6 + Ti BY, 38 MnSiVS 5 BY and 44 MnSiVS 6 + Ti BY, have been evaluated. Similar strain hardening was observed in the monotonic tensile tests whereas different hardening or softening was found under cyclic loading conditions. QT steels reveal pronounced cyclic softening over the entire strain range investigated, the ferritic pearlitic steels show only a slight decrease in the cyclic proof stress and cyclic hardening at larger strains. Strain fatigue-life curves result in a common scatterband of all steels investigated with the microalloyed steels 27 MnSiVS 6 + Ti BY and 44 MnSiVS 6 + Ti BY lying at the upper limit. Crack initiation probability of the microalloyed medium-carbon precipitation-hardening steels in the low-cycle fatigue range is equivalent or lower than for the normalized carbon steel and the QT-steels.     

钒微合金化低碳钢高温变形动态再结晶

利用热模拟压缩试验测定了不同钒含量的钒微合金化低碳钢在900～1000℃温度区间和0.1～1s-1变形速率范围内的真应力-真应变曲线.对曲线的分析表明:随钢中钒含量的增加,低碳钢的动态再结晶开始时间延长,变形奥氏体的动态再结晶名义激活能提高.实验钢薄膜试样的TEM观察表明,钢中的微量钒以固溶态存在于奥氏体中,微量的固溶钒对奥氏体动态再结晶起到抑制作用.     

Influence of titanium and carbon contents on the hydrogen trapping of microalloyed steels

The presence of traps for hydrogen atoms influences the diffusivity and solubility of hydrogen itself in steels. In the present work, permeation measurements were carried out on hot-rolled microalloyed steels with different C and Ti contents in order to evaluate the number of irreversible and reversible traps. The number per unit volume of irreversible traps was correlated to calculated volume fraction of Ti(C,N) precipitates. These results, combined with microstructural investigations by transmission electron microscopy (TEM), showed that the largest number of irreversible traps was associated with steels having the largest volume fraction of fine and coherent Ti(C,N) precipitates. The reversible traps were associated with free Ti atoms, dislocations, and ferritic grain boundaries. Theoretical calculations confirmed the hydrogen binding energy of Ti free atoms (−27.1 kJ/mol).     

The effect of nitrogen on precipitation and transformation kinetics in vanadium steels

The isothermal decomposition of austenite has been studied in a series of vanadium steels containing varying amounts of carbon and nitrogen, (in approximately stoichio-metric proportions), in the temperature range 700 to 850°C. In the basic alloy, Fe-0.27V–0.05C (composition in wt pct), below 810°C the austenite to polygonal ferrite trans-formation is accompanied by interphase precipitation of vanadium carbide, the finer dis-persions being associated with the lower transformation temperatures. However, below 760°C there is an additional precipitation reaction where dislocation precipitation of vanadium carbide predominates; this is shown to occur in association with Widmanstätten ferrite. Above 810° C, a proeutectoid ferrite reaction results, the ferrite being void of precipitates; evidence is provided to show that partitioning of vanadium from ferrite to austenite occurs during the transformation. In the two steels containing nitrogen, namely Fe-0.26V-0.022N-0.020C and Fe-0.29V-0.032 N the basic interphase precipitation re-action is unchanged, but the resultant precipitate dispersions are finer at a given trans-formation temperature. The temperature range over which interphase precipitation oc-curs is expanded by the presence of nitrogen, since the Widmanstätten start tempera-ture is depressed and the proeutectoid ferrite reaction is inhibited. Precipitation in austenite prior to transformation and twin formation during transformation are both en-couraged by the presence of nitrogen.