低合金铜时效强化钢奥氏体化过程的动力学模型

通过高温金相试验,研究了一种船用低合金铜时效强化钢在不同加热温度和保温时间下的奥氏体晶粒长大行为和尺寸分布规律。结果表明:随着加热温度的升高,奥氏体晶粒尺寸逐渐增大,并且在不同的温度区间,奥氏体晶粒具有不同的长大速度。随着保温时间的延长,奥氏体晶粒也逐渐长大,但加热温度越高,奥氏体晶粒长大速度越快。各加热温度及保温时间下奥氏体晶粒尺寸呈对数正态分布,且随着加热温度升高或保温时间延长,对数正态分布曲线峰值横坐标右移,峰值频率下降。通过对试验数据进行回归分析,建立了适用于本钢种的奥氏体晶粒长大的动力学模型,模型计算值与试验值吻合较好,平均相对误差小于5%,所建立的模型具有较高的精准性和可靠性。     

Austenite Grain Structures in Ti- and Nb-Containing High-Strength Low-Alloy Steel During Slab Reheating

Austenite-grain growth was investigated in a couple of microalloyed steels, one containing Ti and the other containing Nb, Ti, and V, using different reheating temperatures between 1273 K and 1523 K (1000 °C and 1250 °C). Nature and distribution of microalloy precipitates were quantitatively analyzed before and after reheating. Interdendritic segregation (or microsegregation) during casting can result in an inhomogeneous distribution of microalloy precipitates in the as-cast slabs, which can create austenite grain size variation (even grain size bimodality) after reheating. Ti addition reduced the grain size variation; however, it could not eliminate the grain size bimodality in Nb-containing steel, due to the differential pinning effect of Nb precipitates. A model was proposed for the prediction of austenite grain size variation in reheated steel by combining different models on microsegregation during solidification, thermodynamic stability, and dissolution of microalloy precipitates and austenite grain growth during reheating.     

Effects of second-phase particles on coarsening of austenite in 0.15 Pct carbon steels

The effects of second-phase particles formed by the addition of vanadium, nitrogen, and aluminum on the austenite grain coarsening behavior of 0.15 pct carbon steels were studied. The oxidation and etching technique has been adopted to reveal the prior austenite grain boundaries. The specimens were austenitized at intervals of 50°C within the range of 900°C to 1150°C under high vacuum (<10⁻⁴ torr) for half an hour, toward the end of which they were oxidized for about one minute by introducing oxygen at about 250 mm Hg to reveal the grain boundaries, and then quenched into iced water. The variation of prior austenite grain size with temperature in these steels indicates that vanadium carbonitride, V(C, N), is much more effective in austenite grain refinement than vanadium carbide, VC, at all temperatures. The effect of vanadium carbonitride in austenite grain refinement is more or less the same as that of aluminum nitride. AlN, at temepratures below 1000°C, but this effect of vanadium carbonitride in austenite grain refinement decreases with increasing temperature. Above 1000°C, aluminum nitride is a much better grain refiner than vanadium carbonitride. The presence of the V (C, N) and AlN particles in the same steel causes moderate grain growth of austenite. MD. Mohar Ali Bepari, formerly with the Department of Metallurgy, The University of Sheffield, Sheffield, England, is Associate Professor of Department of Metallurgical Engineering, Bangladesh University of Engineering and Technology, Dhaka, Bangladesh.     

25Cr2Mo1VA钢奥氏体晶粒长大规律探讨

 为了研究不同加热温度和保温时间下25Cr2Mo1VA钢奥氏体晶粒长大规律,利用金相显微镜、TEM、EDS以及截距法分析了不同状态下奥氏体晶粒大小、形貌,析出相类型及其大小分布等。结果表明,当加热温度在900 ℃以下,试验钢的奥氏体晶粒长大缓慢且细小;随着温度加热至950 ℃,奥氏体晶粒出现了“混晶”现象,再加热超过1 000 ℃后,奥氏体晶粒异常长大。随保温时间的延长,奥氏体晶粒也会长大,但保温时间对奥氏体晶粒尺寸的影响没有加热温度显著。试验钢中出现近似球形状的析出相,即为富钒的非计量化合物M₈C₇析出相。根据Anelli改进模型和回归分析,建立了试验钢奥氏体晶粒长大的热力学模型,并结合析出相粒子对奥氏体晶粒钉扎作用,探讨了25Cr2Mo1VA钢的奥氏体晶粒长大规律及成因。     

奥氏体化温度对奥氏体晶粒度及第二相固溶的影响

利用光学显微镜等研究了 SA5 1 6Gr60N 容器钢在不同奥氏体化温度下奥氏体晶粒尺寸的长大规律以及微合金元素 Nb、Ti、V 的固溶规律.研究结果表明,随着奥氏体化温度的升高,微合金元素 Nb、Ti、V 的固溶量逐渐增加;990~1 050 ℃时,原始奥氏体晶粒尺寸增加缓慢,晶粒细小均匀;1 070 ℃时晶粒出现异常长大现象,随后部分奥氏体晶粒急剧长大,不均匀性越来越明显;1 1 70~1 210 ℃时,奥氏体晶粒尺寸均匀化.     

Austenite Grain Growth Behavior of X80 Pipeline Steel in Heating Process

Through changing soaking temperature and soaking time,austenite grain growth behavior of X80 pipeline steel under different heating conditions was studied. Relationships of average grain size to soaking temperature and time were obtained respectively. The results show that the prior austenite grains grow with the increase of soaking temperature and time. When soaking temperature is lower than 1180℃,austenite grain size and growth rate are small; when it higher than 1200℃,austenite grains grow rapidly and abnormal grain growth appears. For soaking at 1180℃,austenite grain growth rate is initially high and then decreases when soaking time exceeds 1h.     

中锰马氏体NM500钢奥氏体晶粒长大行为

 中锰马氏体耐磨钢是一种新型的低成本高性能耐磨钢,揭示钢中奥氏体晶粒长大行为,并建立精确的预测模型,对其组织和性能的调控至关重要。利用Gleeble-3500型热模拟试验机、金相显微镜和透射电子显微镜等设备,系统研究了中锰马氏体NM500钢在不同加热温度和保温时间下的奥氏体晶粒长大行为,探讨了微合金第二相对奥氏体晶粒长大行为的影响。研究结果表明,加热温度对试验钢中奥氏体晶粒长大的影响明显大于保温时间,且试验钢中奥氏体晶粒长大行为受基体中V(C,N)粒子析出行为的影响,其可分为两个阶段。当加热温度小于950 ℃时,试验钢中存在大量未溶的纳米级球状和短棒状V(C,N)粒子,能够有效地钉扎奥氏体晶界,奥氏体晶粒长大缓慢;但当加热温度不低于950 ℃时,试验钢中V(C,N)粒子大量溶解和粗化。其中,加热温度为950 ℃、保温时间为60 min时,试验钢中V(C,N)粒子的体积分数仅为0.041%,平均粒径增大至45.78 nm。其对奥氏体晶粒的钉扎作用显著减低,且随着温度升高,原子扩散速度加快,奥氏体晶粒快速长大。基于Beck模型,建立了试验钢中奥氏体晶粒等温长大动力学模型,计算得到低温及高温阶段试验钢中奥氏体晶粒长大表观激活能分别为66.561 kg/mol和170.416 kJ/mol,且奥氏体晶粒的理论计算值与实测结果吻合较好。     

Effect of magnesium addition in low-carbon steel part 1: behaviour of austenite grain growth

To reveal the effect of Mg addition on the austenite grain growth in low-carbon steel, the steels containing different Mg contents were refined with a vacuum induction furnace. First, the steels were subjected to the temperature range of 1000–1300°C for a holding time of 30?min. Moreover, using a confocal scanning laser microscope, the growth of austenite grains was investigated under isothermal holding conditions (1400°C), and the γ–α phase transformation was also identified after the samples were subjected to a cooling rate of 5°C?s^?1. It reveals that the grain growth is inhibited by Mg addition after increasing the temperature to 1300°C. The kinetic equations of austenite grain growth were further established by regression analysis based on the experimental results. Furthermore, a significant increase in the proportion of intra-granular ferrite takes place in 0.0026%Mg-added steel at the initial stage of γ?→?α with a cooling rate of 5°C?s^?1. This is mainly attributed to the plenty of Mg-containing inclusions, which are demonstrated to be effective nuclei for acicular ferrite, being in the Mg-added steel.     

Ferrite recrystallization and austenite formation in cold-rolled intercritically annealed steel

The recrystallization of ferrite and austenite formation during intercritical annealing were studied in a 0.08C-1.45Mn-0.21Si steel by light and transmission electron microscopy. Normalized specimens were cold rolled 25 and 50 pct and annealed between 650 °C and 760 °C. Recrystallization of the 50 pct deformed ferrite was complete within 30 seconds at 760 °C. Austenite formation initiated concurrently with the ferrite recrystallization and continued beyond complete recrystallization of the ferrite matrix. The recrystallization of the deformed ferrite and the spheroidization of the cementite in the deformed pearlite strongly influence the formation and distribution of austenite produced by intercritical annealing. Austenite forms first at the grain boundaries of unrecrystallized and elongated ferrite grains and the spheroidized cementite colonies associated with ferrite grain boundaries. Spheroidized cementite particles dispersed within recrystallized ferrite grains by deformation and annealing phenomena were the sites for later austenite formation.     

Structure formation in steel 45 for railroad axles at high temperatures

The growth kinetics of austenite grains in steel 45 is studied. The influence of the holding time of railroad-axle blanks at different temperatures is found to be nonmonotonic. Above 1150°C, with prolonged holding (>5 h), significant austenite grain growth is observed in the steel, with considerable variation in the final grain size.     

35CrMo钢控温模锻加热过程中奥氏体晶粒长大行为

将35CrMo钢试样在不同的加热温度和保温时间下进行等温奥氏体化处理,采用正较实验法研究加热温度与保温时间对奥氏体平均晶粒尺寸的影响,并对奥氏体晶粒长大行为进行研究。结果表明:当保温时间一定时,奥氏体晶粒尺寸随加热温度升高而增大,奥氏体晶粒的粗化温度为950℃;当加热温度一定时,奥氏体晶粒尺寸随保温时间延长而增大,保温初期晶粒快速长大,随保温时间延长,晶粒长大速率放缓。综合考虑加热温度、保温时间和初始奥氏体晶粒尺寸的影响,推导出35CrMo钢奥氏体晶粒长大模型,用该模型计算的晶粒尺寸与实验结果基本吻合。     

Structural changes and kinetics in the gaseous reduction of hematite

The mechanism of the gaseous reduction of hematite grains to magnetite was studied. Grav-imetric measurements were carried out for the reduction of Carol Lake hematite pellets and grains in CO-CO₂ atmospheres over the temperature range 500 to 1100°C. The pore size distribution in the reduced magnetite was measured by mercury porosimetry. Partially reduced grains were examined by optical microscopy. At temperatures below 800°C, the reduction of a hematite grain to magnetite occurred at a well-defined shrinking-core inter-face. The average pore size in magnetite formed at 600°C was found to be 0.03 μm. An es-timate of the rate of CO diffusion through pores of this size indicated that the reaction rate at 600°C was controlled by a step near the hematite-magnetite interface. At temperatures above 800°C, the reaction mechanism became altered due to the preferential growth of magnetite along a single direction in each hematite grain. The reduction rate decreased with an increase in temperature, and no microporosity was present in magnetite formed at 1000°C and above. It was postulated that the reaction rate was controlled by the rate of formation of fresh nuclei and by their rate of subsequent growth. Formerly Professor of Applied Metallurgy, Imperial College     

Austenite Grain Coarsening Behaviour in a Medium Carbon Si‐Cr Spring Steel with and without Vanadium

The austenite grain coarsening behaviour in a medium carbon Si‐Cr spring steel with and without vanadium was investigated by the thermal etching method. This method is efficient when the austenitization temperature is not lower than 900 °C. The average grain sizes on the surfaces of the samples determined by thermal etching vary little from those in the bulk as revealed by chemical etching. The austenite grain coarsening behaviour of the steel with vanadium can be classified in three temperature regimes. Below a critical temperature the vanadium addition is effective to impede grain growth and fine grains are observed. The austenite grain size was significantly smaller than in the steel without vanadium. In a medium temperature regime the steel with vanadium exhibits a bimodal grain size distribution. Above the dissolution temperature of vanadium carbides normal grain growth is observed. An equation was set up to predict the grain coarsening behaviour in the steel without vanadium. The results show how the austenite grain size, which is very important for the toughness and ductility of the spring steels, can be controlled by microalloying with vanadium.     

大型盾构机用轴承钢奥氏体晶粒长大模型

 利用箱式电阻炉研究了加热温度为900,950,1 000,1 050,1 100,1 150 ℃,保温时间为10,30,60,90 min时大型盾构机用GCr15SiMn轴承钢的奥氏体晶粒长大规律,利用截线法统计奥氏体晶粒尺寸。试验结果表明,随着加热温度提高和保温时间延长,奥氏体晶粒尺寸和长大速率逐渐增大,加热温度的提高比保温时间的延长对奥氏体晶粒长大速率影响更大,奥氏体晶粒迅速长大的加热温度为1 000 ℃,保温时间为60 min。在已有晶粒长大模型的基础上,通过对试验数据进行线性回归,得到了描述GCr15SiMn钢奥氏体晶粒长大规律的数学模型。     

Modelling the microstructural evolution during hot compression of low carbon steel

Compression tests have been performed on low carbon cylindrical specimens in the temperature range of 900–1100°C in a thermomechanical simulator at a strain rate of 10 s^?1. True stress/true strain and load-displacement curves have been characterised over a strain of 0 to 0.8 at above temperatures. The specimens were helium quenched after an incremental true strain of 0.2 for microstructural study. From the experimental data, flow stress of the material at high temperatures has been determined as a function of Zener-Hollomon parameter. The flow stress equation was employed in a coupled finite element flow formulation model to compute the load for various incremental displacements. The predicted results of load-displacement and change in specimen geometry during compression showed good agreement with the measured values. The predicted rise in temperature due to deformation was of the order of 52 to 34°C in the temperature range of 900 to 1100°C at a strain rate of 10 s^?1. The prior austenite grain size has been measured in the specimen compressed up to a strain of 0.6 at 1100°C and compared with the predicted austenite grain size employing the microstructural model. Metallographic study showed an equiaxed recrystallized grains network in most of the region at the center of the specimen with average grain size of 43 μm. A coarse deformed grain structure with few recrystallized grains at the intersection boundary of austenite grains was observed at the top surface and bulge surface with an average grain size of 74 and 84 μm, respectively. The model predicted the evidence of fully dynamically recrystallised grains at the center of the specimen with a grain size of 42 μm. The predicted grain size at the top and bulge surface has been calculated as 90 and 106 μm, respectively.     

Effect of RE oxide on growth dynamics of primary austenite grain in hardfacing layer of medium-high carbon steel

The flux cored wires with different rare earth oxide additions for hardfacing the workpieces of medium-high carbon steel were developed. The microstructure of the hardfacing layer was observed using the optical microscopy. The average dimension of primary austenite grains in hardfacing layer was measured by image analyzer. The primary austenite grain growth activation energy and index were calculated according to Sellars’s mode and Beck formula, respectively. Moreover, the effect of rare earth oxide on the growth dynamics of primary austenite grain was analyzed, and then discussed with the misfit theory. The experimental results showed that, by adding rare earth oxide, the average dimension of primary austenite grains in hardfacing layer of medium-high carbon steel decreased, and it was the smallest when the addition of rare earth oxide was 5.17 wt.%. Meanwhile, at this rare earth oxide addition, the primary austenite grain growth activating energy in hardfacing layer was the largest, while its index was the smallest. The calculated results indicated that the primary austenite grain could be refined because LaAlO3 as heterogeneous nuclei of γ-Fe was the most effective.     

碳锰钢压缩过程中非均匀应变与再结晶之间关系的研究

采用有限元方法模拟了热模拟试验的变形过程,分析了热模拟变形过程中的非均匀应变对奥氏体动态再结晶及晶粒尺寸的影响.结果表明,在等效应变最大的区域,奥氏体动态再结晶并非最完全,而剪应变对动态再结晶的影响则较大,在剪应变最大的区域,再结晶最完全,晶粒最细小.在试验所设定的最大变形量为62%的变形条件下,等效应变对晶粒细化的影响存在一个临界值,当等效应变大于0.96时,不完全动态再结晶区域的奥氏体晶粒得不到进一步细化,而随着剪应变的增加,奥氏体晶粒不断细化,可见剪应变对奥氏体晶粒尺寸的影响更大.因此,用等效应变等于实际应变处的晶粒尺寸来考察实际晶粒尺寸的方法,存在着不合理性.     

Influence of annealing treatment on the formation of nano/submicron grain size AISI 301 Austenitic stainless steels

Nano/submicron austenitic stainless steels have attracted increasing attention over the past few years due to fine structural control for tailoring engineering properties. At the nano/submicron grain scales, grain boundary strengthening can be significant, while ductility remains attractive. To achieve a nano/submicron grain size, metastable austenitic stainless steels are heavily cold-worked, and annealed to convert the deformation-induced martensite formed during cold rolling into austenite. The amount of reverted austenite is a function of annealing temperature. In this work, an AISI 301 metastable austenitic stainless steel is 90 pct cold-rolled and subsequently annealed at temperatures varying from 600 °C to 900 °C for a dwelling time of 30 minutes. The effects of annealing on the microstructure, average austenite grain size, martensite-to-austenite ratio, and carbide formation are determined. Analysis of the as-cold-rolled microstructure reveals that a 90 pct cold reduction produces a combination of lath type and dislocation cell-type martensitic structure. For the annealed samples, the average austenite grain size increases from 0.28 μm at 600 °C to 5.85 μm at 900 °C. On the other hand, the amount of reverted austenite exhibits a maximum at 750 °C, where austenite grains with an average grain size of 1.7 μm compose approximately 95 pct of the microstructure. Annealing temperatures above 750 °C show an increase in the amount of martensite. Upon annealing, (Fe, Cr, Mo)₂₃C₆ carbides form within the grains and at the grain boundaries.     

热轧Nb微合金化TRIP钢高温区变形过程中Nb的析出行为

通过Gleeble热模拟实验研究了含0.038%Nb(质量分数)的热轧TRIP钢在高温奥氏体区的热加工工艺,借助光学显微镜、扫描电镜和透射电镜分析了组织演变和Nb的析出行为,并利用电感耦合等离子体发射光谱仪定量分析了Nb的固溶/析出程度.在1250℃奥氏体化5 min后添加Nb有70%固溶于奥氏体.在1000℃以上的奥氏体再结晶区变形过程中Nb的析出量仅占总固溶量的3%,不能有效抑制静态再结晶,奥氏体晶粒得到明显细化.在900℃的奥氏体未再结晶区变形前析出Nb量已达到总固溶量的9%,再结晶被抑制而获得拉长状奥氏体.奥氏体未再结晶区变形可促进铁素体转变并细化铁素体晶粒.再结晶奥氏体或形变奥氏体状态下冷却至650℃时分别有占总添加量的48%和40%的Nb仍以固溶态存在.     

Microstructural evolution of a nanocrystalline Ti−47Al−3Cr alloy during annealing in the α+γ-phase field

Prealloyed, gas-atomized (GA) Ti−47Al−3Cr alloy powder, containing about 70 pct of the α₂ (Ti₃Al) phase and 30 pct of the γ (TiAl) phase, was fully amorphized by mechanical alloying. The amorphous phase was stable during heating to 600°C, but decomposed at higher temperatures, with an exothermic reaction peak at 624°C as the material transformed to a mixture of α₂ and γ and then to a fully γ structure at 722°C. A nanocrystalline compact with a mean grain size of 42 nm was obtained by hot isostatic pressing (HIP'ing) of the amorphous powder at 725°C. Isothermal annealing experiments were conducted in the two-phase α+γ field, at 1200°C, using holding times of 5, 10, 25, and 35 hours, followed by air cooling. The X-ray diffractometry and analytical transmission electron microscopy investigations carried out on annealed and air-cooled specimens revealed only the presence of the γ grains, which coarsened on annealing. Initially, the grains grew, followed by a saturation stage after annealing for 25 hours, with a saturation grain size of about 1 μm. This grain growth and saturation behavior can be described with a normal grain growth mechanism in which a permanent pinning force is taken into account. Twins formed in the γ grains as a result of annealing and air cooling and exhibited a common twinning plane of (111) with the matrix phase. The minimum γ grain size in which twinning occurred in the annealed specimens was determined to be 0.25 μm, which suggests that twinning is energetically unfavorable in the nanometer-sized grains. N. SRISUKHUMBOWORNCHAI, formerly Master's Student, IMAP, University of Idaho