Martensite Transformation and Its Control in DP,TRIP and TWIP Steels

Designing of alloy concept and process for DP,TRIP and TWIP steels stressing at martensite transformation are analyzed.For DP steel,austenite volume percent and its carbon content at different intercritical temperatures are calculated as well as the tensile strength of the steel,which meet well with the experimental result.The condition for dissolution of carbide is discussed by experiments and predicted by kinetic estimation.Several sample TRIP steels are prepared and their concentration profiles are calculated showing different diffusion characteristics of elements.Calculation also shows carbon enrichment is successful in this stage through the quick diffusion of carbon from ferrite to austenie.In order to maintain the austenite stability or to prevent precipitation of cementite,minimum cooling rate from the intercritical zone to over aging stage is obtained through kinetic simulation.Bainite transformation is estimated,which indicates the carbon rerichment from ferrite of bainite structure to austenite in this stage is also successful.Thermal HCP martensite transformation and the strain induced martensite transformation in TWIP steel is introduced.Relationship between transformation and mechanical properties in the steel is also mentioned.     

High-rate carburizing in a glow- discharge methane plasma

Carburizing at high temperature (1040°C) in a glow-discharge methane plasma signifi-cantly reduces carburizing time. Sufficient carbon to produce a 1.0 mm case on conven-tional carburizing steels can be introduced in 10 min at methane pressures in the range of 1.3 to 2.7 kPa (10 to 20 Torr). To reduce the carbon concentration at the surface to an acceptable level (007E1.0 wt pct) the plasma carburizing stage is followed by a short (007E30 min) diffusion step. To aid in optimizing the division of time between the carburizing and diffusion stages, and in separating the effects of plasma carburizing and high-temperature diffusion, a computer program was written to model the diffusion equation for appropriate initial and boundary conditions. Both model and experimental results show that a diffusion time/carburizing time ratio of at least 3: 1 is required. The diffusion model indicates that the exceptionally high carburizing rates observed arise from the rapid infusion of carbon into the surface from the plasma, during the carburizing stage and not from anomalously high diffusion rates. Exceptionally uniform cases on surfaces of irregular geometry are achieved, and significant amounts of natural gas are saved owing to both the reduced car-burizing time and the low pressures employed.     

Nb微合金化对齿轮钢高温渗碳奥氏体晶粒度的影响

摘要：通过Nb微合金化提高渗碳温度是当前发展高端齿轮钢的重要思路。以20Cr钢为基准成分,通过实验室熔炼、锻造以及977~1134℃范围内高温伪渗碳实验,研究了0.02%、0.04%、0.06%、0.08%等不同Nb质量分数下渗碳后的奥氏体晶粒结构。在此基础上,依据热力学计算及析出颗粒熟化模型,对AlN、Nb(C,N)颗粒的钉扎强度进行估算并与晶粒尺寸建立联系,得到了适用于含Al、Nb齿轮钢的奥氏体晶粒度控制预测模型。最后,依据此模型分析了Nb含量对20Cr钢渗碳温度的影响,并基于高温渗碳目标提出了Nb微合金化的成分建议。     

高温渗碳齿轮钢铝氮含量对奥氏体尺寸的影响

轨道交通用高端齿轮钢往往要求长时间高温渗碳处理以提高其表面硬度与耐磨性,利用合适的铝、氮含量实现AlN粒子对奥氏体晶界的有效钉扎对保证齿轮的晶粒度、力学性能与尺寸精度至关重要。在通常的渗碳温度下,AlN已经发生了部分固溶,为了保证高温渗碳后奥氏体晶粒细小,齿轮钢中的酸溶铝质量分数一般需要保持在0.02%～0.055%以保证析出足量细小的AlN第二相粒子来钉扎晶界,且氮质量分数要求为0.01%～0.016%。这一元素含量范围较广,因此有必要研究钢在高温渗碳时所需要的恰当铝氮积与铝氮比,也就是钢中w(Al)与w(N)的乘积和比值的取值范围,还需要研究AlN粒子对于奥氏体的钉扎作用。针对不同含铝含氮轨道交通用齿轮钢进行了伪渗碳试验与AlN第二相粒子Ostwald熟化和Gladman钉扎模型计算研究,揭示了奥氏体晶粒不均匀性因子Z与加热温度T的定量关系式。研究了含铝含氮齿轮钢高温保温过程奥氏体晶粒半径R_A的变化规律,以及不同铝氮积和铝氮比对奥氏体晶粒生长的影响。结果表明,加热温度T在1 173～1 273 K范围内,此类微合金高强钢的奥氏体晶粒长大不均匀性因子服从线性规律...     

Effect of Niobium on Isothermal Transformation of Austenite to Ferrite in HSLA Low-Carbon Steel

Using thermomechanical simulation experiment,the kinetics of the isothermal transformation of austenite to ferrite in two HSLA low-carbon steels containing different amounts of niobium was investigated under the conditions of both deformation and undeformation.The results of optical microstructure observation and quantitative metallography analysis showed that the kinetics of the isothermal transformation of austenite to ferrite in lower niobium steel with and without deformation suggests a stage mechanism,wherein there exists a linear relationship between the logarithms of holding time and ferrite volume fraction according to Avrami equation,whereas the isothermal transformation of austenite to ferrite in high niobium steel proceeds via a two stage mechanism according to micrographs,wherein,the nucleation rate of ferrite in the initial stage of transformation is low,and in the second stage,the rate of transformation is high and the transformation of residual austenite to ferrite is rapidly complete.Using carbon extraction replica TEM,niobium carbide precipitation for different holding time was investigated and the results suggested that NbC precipitation and the presence of solute niobium would influence the transformation of austenite to ferrite.The mechanism of the effect of niobium on the isothermal transformation was discussed.     

Development of microalloyed steels for high temperature carburizing

The potential for use of microalloy additions to suppress abnormal austenite grain growth and produce steels with enhanced bending fatigue resistance after high temperature vacuum carburizing was investigated in a series of Ti-modified SAE 8620 steels with w(niobium) additions up to 0.1%.Results are considered from a series of papers at the Advanced Steel Processing and Products Research Center on the effects of Nb content,heating rate, rolling history,and processing temperature on the evolution of austenite grain structures in carburizing steels. Emphasis is placed on understanding the effects of alloying and processing on each stage in the annealing process including the as received laboratory rolled conditions,during the onset of carburizing after annealing at different heating rates,and after annealing for various times at carburizing temperatures up to 1 100℃.Heating rate to the carburizing temperature was shown to be an influential variable and suppression of abnormal grain growth was dependent on the development of a critical distribution of fine NbC precipitates,stable at the austenitizing temperature.The importance to industrial carburizing practice of heating rate effects on precipitates and austenite grain size evolution are discussed and correlated to selected data on fatigue performance.     

Carbon diffusion and phase transformations during gas carburizing of high-alloyed stainless steels: Experimental study and theoretical modeling

Gas carburizing of high-alloyed stainless steels increases surface hardness, as well as the overall mechanical characteristics of the surface. The growth of chromium-rich carbides during carbon transfer into the steel causes precipitation hardening in the surface, but decreases the chromium content in solid solution. In order to maintain a good corrosion resistance in the carburized layer, the stainless steel composition and the carburizing process need to be optimized. To limit the experimental work, a methodology using software for modeling the thermodynamic and kinetic properties in order to simulate carbon diffusion and phase transformations during gas carburizing is presented. Thermodynamic calculations are initially used to find the optimum parameters (T, carbon wt pct, etc.) in order to maintain the highest Cr and Mo contents in the austenitic solid solution. In a second step, kinetic calculations using the diffusion-controlled transformations (DICTRA) software are used to predict how the amount of the different phases varies and how the carbon profile in the steel changes as a function of time during the process. Experimental carbon profiles were determined using a wavelength-dispersive spectrometer for electron-probe microanalysis (WDS-EPMA), while carbide compositions were measured by energy-dispersive spectroscopy_X (EDS_X) analyses. A good agreement between calculated and experimental values was observed for the Fe-13Cr-5Co-3Ni-2Mo-0.07C and the Fe-12Cr-2Ni-2Mo-0.12C (wt pct) martensitic stainless steels at 955 °C and 980 °C.     

钛铌微合金化齿轮钢的奥氏体晶粒长大研究

 采用热模拟渗碳方法研究了Ti、Ti-Nb微合金化的20CrMnTi和20CrMnTiNb渗碳齿轮钢在930~1200℃的奥氏体晶粒长大规律。结果表明,添加0. 038%(质量分数,下同)的钛和0. 048%的铌的20CrMnTiNb钢中含有铌和钛的析出相,其粒子间距为0. 361μm;而含0. 054%的钛的20CrMnTi钢中仅含有较大尺寸的TiN析出相,粒子间距为0. 471μm,前者奥氏体晶粒粗化倾向明显低于后者。20CrMnTiNb钢经1000℃奥氏体化10h后奥氏体晶粒长大不明显,且无混晶现象,适合高温渗碳工艺。     

水电站用低碳贝氏体钢07MnCrMoVR的组织性能研究

摘要：采用不同的宽展比对水电站用低碳贝氏体钢07MnCrMoVR进行了轧制,对回火前后试验钢的微观组织形貌进行了观察,并对力学性能进行了检验,同时利用EDS能谱分析了回火过程中碳化物析出行为。结果表明：采用较小的宽展比能提高粗轧纵轧阶段的单道次压下率以及变形区系数,有效地破碎奥氏体再结晶晶粒,轧制后获得细小的粒状贝氏体组织,高温回火后析出大量的渗碳体和合金碳化物均匀弥散地分布在贝氏体铁素体基体上。随着回火温度的提高,试验钢强度性能呈现先升高再降低的现象,伸长率和低温冲击韧性持续升高。     

Formation of Austenite During Intercritical Annealing of Dual-Phase Steels

The formation of austenite during intercritical annealing at temperatures between 740 and 900 °C was studied in a series of 1.5 pct manganese steels containing 0.06 to 0.20 pct carbon and with a ferrite-pearlite starting microstructure, typical of most dual-phase steels. Austenite formation was separated into three stages: (1) very rapid growth of austenite into pearlite until pearlite dissolution is complete; (2) slower growth of austenite into ferrite at a rate that is controlled by carbon diffusion in austenite at high temperatures (~85O °C), and by manganese diffusion in ferrite (or along grain boundaries) at low temperatures (~750 °C); and (3) very slow final equilibration of ferrite and austenite at a rate that is controlled by manganese diffusion in austenite. Diffusion models for the various steps were analyzed and compared with experimental results.     

含碳球团熔融渗碳机理

使用CaO-SiO_2-Al_2O_3合成渣与磁铁矿、石墨粉制备含碳球团,考察了球团中渣相成分、渣量以及碳含量对铁水渗碳量的影响,并探讨了含碳球团内铁的熔融渗碳行为和机理。结果表明,熔融渗碳分为熔融还原渗碳以及铁熔体聚合过程渗碳2个阶段,前一阶段熔渣中Fe2+含量与铁的渗碳量处于动态平衡过程,主要受熔渣化学成分的影响;后一阶段的渗碳过程决定铁的物理渗碳极限,主要与铁、碳颗粒的接触条件有关,且熔渣中Fe2+的还原极限与渣相碱度和SiO_2绝对含量有关,铁的熔融渗碳量(质量分数)可以达到4%左右,满足铁锍分离要求。     

Carbon paraequilibrium in austenitic stainless steel

Carburization of austenitic stainless steels under paraequilibrium conditions—i.e., at (low) temperatures where there is essentially no substitutional diffusion—leads to a family of steels with remarkable properties: enhanced hardness, resulting in improved wear behavior, enhanced fatigue, and corrosion resistance, and with essentially no loss in ductility. These enhanced properties arise from an enormous carbon solubility, which, absent carbide formation, is orders of magnitude greater than the equilibrium solubility. Using interaction parameters from the latest CALPHAD assessment of the Fe-Cr-Ni-carbon system, the authors have calculated the equilibrium and paraequilibrium carbon solubility in a model Fe-18Cr-12 Ni (wt pct) austenitic steel (essentially a model 316L composition), as well as the carbon solubility in this austenite when paraequilibrium carbide formation occurs (i.e., when carbides form in a partitionless manner). For temperatures in the range 725 to 750 K, the calculations predict a paraequilibrium carbon solubility of ～5.5 at. pct. Carburization of 316L stainless steel at these temperatures, however, results in significantly higher concentrations of carbon in solid solution—up to 12 at. pct. Much better agreement with experimental data is obtained by calculating the paraequilibrium carbon solubility using Wagner interaction parameters, taken from the most comprehensive experimental study of this system. The discrepancy between the two predicted solubilities arises because the CALPHAD Cr-carbon interaction parameters are not sufficiently exothermic at the low temperatures used for paraequilibrium carburization. After multiple paraequilibrium carburization cycles, carbide formation can occur. The carbides that form under these conditions do so in a near-partitionless manner (there is modest Ni rejection to the austenite/carbide interface) and have an unusual stoichiometry: M₅C₂ (the Hägg or η carbide).     

Pearlite phase transformation in Si and V steel

Systematic research has been undertaken on the effects of single and combined additions of vanadium and silicon on the phase transformation and microstructure of pearlitic steels. Both alloy additions were found to result in the formation of nonlamellar products in the vicinity of austenite grain boundaries in hypereutectoid compositions (0.77 to 0.95 wt pct C). The products comprise discrete initial cementite particles and grain boundary ferrite, which is embedded with interphase precipitates of vanadium carbide. As the carbon content is increased further (up to 1.05 wt pct), the amount of grain boundary ferrite gradually decreases without any dramatic change in the morphology of the initial cementite particles. No continuous embrittling grain boundary cementite network was formed. The aspect ratios of the grain boundary cementite particles were decreased from 60:1 to 25:1 by the addition of the alloy elements. A compre-hensive model has been suggested to explain these effects. Other effects of these alloy elements on the microstructure of pearlitic steels have also been examined. For given austenitization conditions, an increase in carbon and vanadium content produced a decrease in austenite grain size. Silicon was found to increase the rate of interphase precipitation of vanadium carbides. Formerly Graduate Student, Department of Materials, Oxford University Formerly University Lecturer, Department of Materials, Oxford University     

热轧板翘皮缺陷的渗碳行为和组织特征

为了提高SPHC热轧带钢的表面质量,针对异常翘皮缺陷的成因展开研究.利用光学显微镜、扫描电镜、能谱仪和硬度仪分析了缺陷处与基体的组织、成分和硬度的差异.结果表明,翘皮缺陷主要是由连铸坯中存在大量的夹杂物所致,同时这些夹杂物碳含量较高,在板坯加热过程中对周围基体起到"渗碳"作用,渗碳层厚度可达5 727.2 μm.带钢横...     

粉末冶金铁基零件的热处理

本文介绍了高密度的粉末热锻零件,中等密度的复压复烧零件以及表面密度高而心部具有一定孔隙的粉末冶金滚压零件的热处理工艺。着重提出了用不均匀奥氏体渗碳处理来实现高浓度渗碳的方法。试验表明,对含铬的铁基结构零件,其渗层表面含碳量可达2%以上,碳化物量可达50%以上;复压复烧零件经不均匀奥氏体渗碳处理后,其耐磨性比20号钢氰化处理的零件提高2.22倍。     

In-situ observation on austenite grain growth of a Nb microalloyed high-carbon steel

It was reported in previous studies that the growth of austenite was inhibited by the pinning effect of Nb containing precipitates and the solute dragging effect of solute Nb. The effect of Nb on austenite grain growth of high carbon steel was investigated by laser scanning confocal microscope (LSCM). Microstructure evolution during heating process of the tested steel was observed by in situ observation. The results show that even without the pinning effect of Nb containing precipitates (at high temperatures), Nb can hinder the growth of austenite grains due to the solute dragging effect of Nb. Two models were used to fit the austenite grain growth process, and the Beck growth models of Nb microalloyed high carbon steels at different heating temperatures were established. The austenite grain growth kinetics model considering the influence of heating temperature and holding time can accurately predict the austenite grain growth process of Nb microalloyed high carbon steels.     

Nb微合金化高碳钢奥氏体晶粒长大原位观察

摘要：以往研究表明Nb析出相钉扎和固溶Nb溶质拖曳作用共同阻碍奥氏体晶粒长大。采用高温共聚焦显微镜研究了Nb对一种高碳含Nb钢奥氏体晶粒长大的影响,对含Nb钢加热过程组织演变进行原位观察。结果表明,Nb在没有钉扎作用下（即高温条件下）仍能起到阻碍奥氏体晶粒长大的作用,该阻碍效果主要是固溶Nb的溶质拖曳作用引起的。采用2种模型对奥氏体晶粒长大行为进行拟合,给出了不同加热温度下Nb微合金化高碳钢的Beck长大方程,同时考虑到加热温度和保温时间的共同影响,根据原位观察结果得到实验钢的奥氏体晶粒长大动力学模型,该模型能够较准确地预测Nb微合金化高碳钢奥氏体晶粒长大行为。     

Austenite stabilization from direct cementite conversion in low-alloy steels

Tr ansformation i nduced p lasticity (TRIP) effects associated with austenite dispersions in low alloy Fe-Mn-Si steels can be enhanced by austenite stabilisation. Austenite which forms during conventional intercritical annealing does not possess the required stability in order to exhibit TRIP effects. In this work, thermodynamic calculations indicated that it is feasible to form austenite by a cementite to austenite conversion which occurs under paraequilibrium conditions, i.e with partition of carbon but with no partition of substitutional alloying elements. In this way the austenite inherits the manganese content of cementite and is chemically stabilised. A treatment consisting of a two-step annealing has been examined. In the first step, soft annealing, an Mn-enriched cementite dispersion in ferrite is formed. In the second step, intercritical annealing, austenite nucleates on the cementite particles, which are consumed to form austenite. It was experimentally determined that this austenite has been enriched in manganese and carbon and, therefore, is stabilised. The conversion reaction is followed by the conventional austenite nucleation at ferrite grain boundaries. This austenite is lean in manganese and is not stable. The net effect of the two-step annealing treatment is a significant austenite stabilisation relative to simple intercritical annealing, indicating a potential for enhanced TRIP effects in this class of steels.     

Modeling Growth and Dissolution Kinetics of Grain-Boundary Cementite in Cyclic Carburizing

In vacuum carburizing of steels, short-time carburizing is usually followed by a diffusion period to eliminate the filmlike cementite (θ _GB ) grown on the austenite (γ) grain boundary surface. In order to obtain the θ _GB amount during the process, the conventional model estimates the amount of cementite (θ) with the equilibrium fractions for local C contents within a framework of the finite difference method (FDM), which overestimates the amount of θ _GB observed after several minutes of carburizing. In our newly developed model, a parabolic law is assumed for the growth of θ _GB and the rate controlling process is considered to be Si diffusion rejected from θ under the isoactivity condition. In contrast, the rate constant for the dissolution of θ _GB is considered to be controlled by Cr diffusion of θ. Both rate coefficients (α) were validated using multicomponent diffusion simulation for the moving velocity of the γ/θ interface. A one-dimensional (1-D) FDM program calculates an increment of θ _GB for all grid points by the updated diffusivities and local equilibrium using coupled CALPHAD software. Predictions of the carbon (C) profile and volume fraction of cementite represent the experimental analysis much better than the existing models, especially for both short-time carburization and the cyclic procedure of carburization and diffusion processes.     

Precipitation of TiC in thermally embrittled maraging steels

It is shown that maraging steels can be embrittled by the precipitation of TiC during slow cooling and/or intermediate annealing in the austenite temperature range. An important aspect in this embrittlement is the occurrence of lamellar precipitation of TiC at the austenite grain boundaries, generating a cellular structure of large fern leaf-like carbides. Within the austenite grains a nonuniform distribution of irregularly plate-shaped TiC particles are formed with (100) austenite habit orientation. Quenching to martensite, prior to any intermediate anneals, changes the carbide distribution upon subsequent annealing treatments into a fine dispersion of TiC particles. The embrittlement resulting from the various isothermal annealing treatments in the austenite temperature region could all be directly related to the carbide distribution in the prior austenite grain boundary region.