重轨钢中MnS析出热力学和动力学分析

 采用无水有机溶液电解法分离提取重轨钢中的MnS夹杂物,采用扫描电镜观察铸坯内和钢轨中MnS夹杂物的三维形貌,并结合能谱仪分析其成分。铸坯被轧制成钢轨后,相应的MnS夹杂物都沿着轧制方向被轧制成长条状。基于热力学和动力学模型,分析重轨钢中MnS夹杂物析出行为以及在钢液凝固过程中锰元素和硫元素偏析的程度。热力学分析表明,MnS夹杂物在凝固末期凝固分数为0.94时开始析出,其析出量由初始[w([Mn])]和初始[w([S])]决定,且在凝固过程受到冷却速率的影响,对比发现,热力学的计算析出结果与Thermo-Calc和FactSage6.4的计算结果有较好的一致性;动力学分析表明,在钢液凝固过程增加冷却速率,凝固析出的MnS颗粒尺寸将减小。通过调整钢中[w([Mn])]和[w([S])]以及改变冷却速率,可以控制MnS的析出时机和形态,减小其对钢性能的有害影响。     

MnS precipitation in association with manganese silicate inclusions in Si/Mn deoxidized steel

When manganese silicate inclusions were formed during cooling from 1600 °C, manganese and sulfur contents in the manganese silicate inclusions were much lower than their equilibrium values within the steel matrix, i.e., the steel matrix was supersaturated with Mn and S against the inclusions. The formation of a Mn-depleted zone around an inclusion and the precipitation of a MnS phase on the inclusion were greatly affected by the thermal history of the steel. Slow cooling helped the formation of both the Mn-depleted zone and the MnS phase on the inclusion, but fast cooling suppressed it. Subsequent isothermal holding at 1200 °C diminished the existing Mn-depleted zone in slow-cooled steel, but created a Mn-depleted zone for fast-cooled steel. The mass transfer within an inclusion was sluggish, and the formation of a MnS phase is due to the local saturation of Mn and S at the outer part of an inclusion. It was suggested that the major factors affecting the formation of the Mn-depleted zone and the MnS phase are the cooling rate, isothermal holding, and the sulfide capacity of the inclusion.     

Strain-induced nucleation of MnS in electrical steels

The nucleation of MnS was investigated during the creep of electrical steels. Precipitation start(P _s) times were measured in the temperature range from 800 °C to 1100 °C. Direct evidence regarding the locations of the nucleation sites was obtained by means of electron microscopy. The results show that both dislocations and grain boundaries act as nucleation sites for such strain-induced precipitation. The experimental data were analyzed using classical nucleation theory, on the basis of which it is demonstrated that nucleation at grain boundaries is dominant at the higher testing temperatures. TheP _s values in this temperature range are determined by the corresponding nucleation rate. As the temperature is decreased, however, nucleation on dislocations becomes more important. This is due to the additional driving force contributed by deformation-induced vacancies, as well as because the higher dislocation densities at the lower temperatures provide a higher density of potential nucleation sites. In addition, the influence of the growth of these particles following nucleation is considered in the analysis pertaining to theP _s curves.     

A model for interphase precipitation in V-microalloyed structural steels

A model for interphase precipitation, with a predictive capacity, is presented. This article deals with its application to V-microalloyed steels. The model rests on an analysis of the growth of the V-depleted zone ahead of a sheet of V(C,N) particles and the simultaneous advance of the γ/α interface in which it was nucleated. It is shown that volume diffusion of V cannot explain the observed intersheet spacings and that a faster diffusion process is required. It is postulated that the γ/α boundary will bow out some time after a sheet of V(C,N) particles has formed in it. Part of the V in the γ will then be fed to V(C,N) particles in the sheet by boundary diffusion as the γ transforms to α. The V content at the front will, thus, be lower than the initial content in the austenite. However, the reduction will be less the further the interface has moved away from the sheet of V(C,N) particles. At a sufficient distance, the V content is again high enough to allow new V(C,N) particles to nucleate, and a new sheet of particles will form. Between the two sheets, there will be a ledge (or superledge) that will advance along the first sheet. The height of the ledge will, thus, be determined by the distance in which V(C,N) particles can again be nucleated. The model exhibits reasonably good agreement with observed values of intersheet spacing, with its temperature dependence and transition from interphase to general precipitation, and with its dependence on C, V, and N content. It also provides physically sound explanations of these dependencies.     

Al脱氧钢中氧化物对MnS析出的影响

使用10 kg真空感应炉Al脱氧冶炼较高S含量超低氧高强度钢,钢中T[O]降到0.0010%,S的质量分数为0.0190%.采用ASPEX explorer全自动扫描电镜对钢中非金属夹杂物进行检测,发现98%非金属夹杂物都是弥散分布的MnS和MnS+Al₂O₃复合夹杂物.MnS夹杂物棱角分明,从形貌特征来看应属于第Ⅲ类硫化物.MnS+Al₂O₃复合夹杂物以Al₂O₃为核心,外层包裹MnS,其数量约占9%~32%;作为核心的Al₂O₃平均直径为1.5μm.其生成过程可描述为:凝固过程中,小尺寸Al₂O₃被推至固液两相区,而选分结晶作用使得钢中的Mn和S在凝固前沿富集,并以Al₂O₃作为异质形核质点析出MnS夹杂物.对凝固过程中Al₂O₃的推动和捕获行为进行了相关计算.计算结果表明:直径小于4μm的Al₂O₃可被推动,并作为MnS的异质形核质点.     

A thermal processing technique for TRIP steels

An investigation was made to determine the effect of thermal cycling between martensite and reverted austenite on the structure and mechanical properties of a Fe-24 pct Ni-4 pct Mo-0.3 pct C TRIP steel. Measurements of hardness, tensile properties, X-ray line broadening, and metallographic structure indicate that repetitive cycling both strengthens the austenite and raises theM _D from below room temperature to above room temperature. It is shown that cyclical thermal processing produces mechanical properties essentially comparable with thermomechanical processing in this particular TRIP steel.     

MnS在固态低硫钢中析出的实验室研究

采用高温激光共聚焦显微镜对低硫钢试样抛光表面的MnS夹杂物析出进行了观察,利用高温激光共聚焦观察结果进行了控温轧制M nS析出试验,用SEM-EDS和ASPEX分析了析出物的成分.结果表明,MnS在表面的首次析出发生在1067℃并伴随有表面变形,在859.7℃当界面能随表面变形释放后停止,MnS的第二次析出发生在790...     

低碳钢碳氮析出物的热力学计算

钢中的碳氮析出物通过细晶强化和析出强化方式对钢的力学性能有非常重要的作用.基于规则溶液的双亚点阵模型(其中一个为金属亚点阵,另一个为间隙原子亚点阵)建立了碳氮化钛、氮化铝以及硫化锰的热力学计算模型用以研究析出物的析出开始温度、给定温度的奥氏体成分,并将这一结果应用于CSP过程.经计算得Ti (C_xN_1-x),MnS和AlN的析出温度分别是1200℃,1440℃和1010℃,最大的体积分数分别为2.315×10^-5,4.18×10^-4和3.1×10^-4.对比发现热力学的计算结果与Thermo-Calc的计算结果和有关文献的实验数据有较好的一致性.     

A mass transport theory for interphase precipitation with application to vanadium steels

A mass transport theory is developed for interphase precipitation reactions in binary and pseudobinary systems where constant concentrations of solute are maintained at the interphase boundary. The theory presents exact solutions to the problem of diffusion-controlled growth involving the coupled motion of two boundaries. It is generally applicable to interphase pre-cipitation in both solid-solid and solid-liquid systems. Aspects of the theory are discussed with reference to the interphase precipitation reaction in model iron-carbon-vanadium alloys.     

High temperature creep damage in steels and superalloys

The nucleation and growth of cavities was examined in steel bicrystals (Fe-3%-Si, X 8 CrNiNb 16 13) and in the ODS superalloy Inconel MA 754 (Inconel MA 754 (78% Ni; 20% Cr; 0.5% Ti; 0.3% Al; 0.6% Y₂O₃). Cavity density distributions were measured on metallographic sections and on cleaved grain boundaries as a function of time, strain, temperature and stress. Nucleation and growth laws were obtained by evaluating the distributions with appropriate models. For the fcc and bcc bicrystals, it was found that cavities nucleated continuously at sulfide and carbide particles during creep. They grew by grain boundary diffusion. But the growth rate was delayed with increasing creep strain due to cavities which nucleated in the surroundings of existing cavities. For the ODS alloy, however, many round cavities preexisted on quasi-boundaries consisting of the aggregate of coarse oxide and carbide particles. They grew initially by diffusion, but with increasing creep time (cavity size), the growth mechanism switched from growth controlled by grain boundary diffusion to growth controlled by power law creep. Implications for life predictions are discussed.     

低碳贝氏体钢中的时效析出行为

测定了Cu-Nb-Cr-Mo系低碳贝氏体钢的时效硬度变化行为,在Gleeble-1500热模拟试验机上模拟了各钢种的蠕变行为,利用金相显微镜、TEM研究了不同钢种的微观组织与沉淀析出.结果表明,含铜钢在不同温度时效时会发生明显的时效硬化效应,在不同温度的蠕变曲线上会出现平台,平台现象是由于蠕变过程中出现了第二相析出,平台出现意味着开始了第二相析出,平台结束则析出过程结束.     

Optimization and verification of creep equations for heat resistant steels

A direct creep curve assessment of multi-heat creep data delivers condensed creep curve bands of a steel type for a limited number of stress groups. The mean creep curves of these stress groups are the basis for an examination and if necessary optimization of a creep equation which was originally based on a time temperature parameter assessment. This procedure is demonstrated for creep equations of steel types 10 CrMo 9 10 and X 21 CrMoV 12 1. The creep equations are available in the form of a subroutine Kara FW which serves as a user subroutine for finite element programmes. The scatter band covered by such a creep equation can be described by strain factors for initial plastic strain and for creep strain. A critical strain was developed to indicate creep rupture or local crack initiation. This critical strain is the uniform elongation which appears at rupture. It can be determined by finite element analyses of the specimens tested. Further, the optimized creep equations are verified by the recalculation of different service type tests using the programme Abaqus and the subroutine Kara FW.     

The prediction of precipitation strengthening in microalloyed steels

The effectiveness of interphase precipitation strengthening in microalloyed steels depends on the temperature dependence of the solubility of the precipitation phase in austenite and on the temperature utilized in soaking. Using an approximate method of calculating the solubility of microalloying elements in the presence of both carbon and nitrogen, a precipitation strengthening potential parameter was developed. On relating this parameter to the chemical compositions and thermal histories of microalloyed steels, it was determined that the interphase precipitation strengthening determined in this and other studies increases linearly with the strengthening potential parameter. On the basis of the linear dependence of precipitation strengthening on the precipitation potential and other observations, it appears that interphase precipitation strengthening is not due to the Orowan looping mechanism but rather to interactions of gliding dislocations with the strain fields of coherent precipitates.     

The kinetics of carbide precipitation in silicon-aluminum steels

The kinetics of carbide precipitation in a fully processed 2.3 wt Pct silicon, 0.66 wt Pct aluminum electrical steel with carbon contents of 0.005 to 0.016 wt Pct were investigated over the temperature range from 150 to 760 °C and times from 30 seconds to 240 hours. The size, morphology, and distribution of the carbide phases, as functions of aging time and temperature, were determined by optical and transmission electron microscopy. The 1.5T core loss was also evaluated and correlated with the changes in precipitation. Distinct C curves were observed for the formation of grain-boundary cementite at temperatures above 350 °C and a transition carbide ({100} _α habit plane) at temperatures below 350 °C. Grain-boundary cementite had a relatively small effect on core loss. The large increases in core loss that accompanied transition carbide precipitation peaked at specific aging temperatures depending on the carbon content of the steel. Once a transition carbide dispersion was initially established at a given aging temperature, particle coarsening and core loss changes were generally insensitive to aging time. The influence of a combined addition of silicon and aluminum on the solubility of cementite and the transition carbide in iron was estimated and discussed. This paper is based on a presentation made at the symposium “Physical Metallurgy of Electrical Steels” held at the 1985 annual AIME meeting in New York on February 24–28, 1985, under the auspices of the TMS Ferrous Metallurgy Committee.     

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.