薄带连铸AISI304不锈钢中铁素体行为

为了确定薄带连铸AISI304不锈钢凝固过程中残留铁素体的生成及转变行为,采用彩色金相、电解侵蚀、电子背散射衍射分析技术及X射线衍射分析等研究手段对双辊薄带连铸AISI304不锈钢凝固组织及残留铁素体特征进行了研究.结果表明AISI304不锈钢薄带的凝固组织由表层胞状晶区、中间柱状晶区和中心等轴晶区三部分组成.薄带表层胞状晶区内残留铁素体呈棒状,柱状晶区的残留铁素体形态为鱼骨状,中心等轴晶区的残留铁素体呈弯曲的树枝状;薄带的表层胞状晶区残留铁素体的质量分数为4.6%～6.6%,柱状晶区内的残留铁素体质量分数为3.6%～3.7%,中心等轴晶区内的残留铁素体质量分数为11.27%～11.34%;残留铁素体沿着厚度方向呈现\     

双辊连铸304不锈钢2.0 mm薄带中铁素体的形态与分布

采用化学侵蚀、彩色金相显示和电子探针(EPMA)分析研究了304奥氏体不锈钢(%:0.08C、1.37Mn、0.62Si、17.77Cr、9.09Ni)2.0 mm连铸薄带内残留铁素体的形态和分布。结果表明,在薄带表层铁素体呈棒状,其间距≤20μm;在薄带柱状树枝晶区,铁素体位于一次枝晶臂和二次枝晶臂中心,铁素体二次枝晶间距≤10μm;在中心等轴晶区,残留铁素体形貌和分布与等轴晶区半固态形成机理有关。先凝固的固相颗粒内部铁素体为弯曲树枝状和网状形貌,固相颗粒间隙铁素体呈岛状形貌。     

奥氏体不锈钢凝固组织中残留铁素体特征研究

 采用化学侵蚀、彩色金相和电子探针（EPMA）等方法对AISI304奥氏体不锈钢连铸方坯凝固组织及其残留铁素体的形貌、化学组成等特征进行了分析,并采用DICTRA软件对铁素体向奥氏体的扩散转变进行了模拟研究,结果表明AISI304不锈钢铸坯表层树枝晶区的二次枝晶间距为12~20μm,富Cr贫Ni的残留铁素体呈骨骼状分布;铸坯中心等轴晶区的残留铁素体为蠕虫状分布,中心等轴晶区的残留铁素体富Cr贫Ni的同时富Si贫Mn,且其富Cr贫Ni的程度比柱状晶区的残留铁素体轻。钢液成分和先析铁素体向奥氏体扩散转变时间、距离是影响残留铁素体化学组成的主要因素。     

冷却速率对奥氏体不锈钢凝固过程影响的原位观察

通过采用激光共聚焦扫描显微镜对AISI304奥氏体不锈钢的凝固过程进行了原位动态观察研究.发现当冷却速率为0.05℃·s^-1时,奥氏体不锈钢以胞状晶方式凝固,其凝固模式为FA模式,即δ铁素体相先从液相中形核并长大,γ相在1 448.9℃时通过与液相发生包晶反应(L+δ→γ)在δ铁素体相界形成,当温度降到1 431.3℃时液相消失,δ铁素体相通过固态相变转变为γ相,富Cr贫Ni的残留铁素体位于胞状晶之间.当冷却速率为3.0℃·s^-1时,奥氏体不锈钢以枝晶方式生长,冷却到1346.4℃时包晶反应在液相与δ铁素体相界之间进行,其残留铁素体位于枝晶干,与冷却速率为0.05℃·s^-1时相比,其残留铁素体的数量增多,残留铁素体富Cr贫Ni的程度减轻.     

Grain structures in gas tungsten-arc welds of austenitic stainless steels with ferrite primary phase

The grain structures were investigated in full penetration gas tungsten-arc (GTA) welds in sheets of 304 and 321 austenitic stainless steels for a range of welding conditions. In type 321 steel welds, fine equiaxed ferrite dendrites were observed in the ferrite phase. The equiaxed structure was ascribed to heterogeneous nucleation of ferrite on Ti-rich cuboidal inclusions present in this steel, since these inclusions were observed at the origin of equiaxed dendrites. In type 304 welds, the ferrite grains were columnar, except in less complete penetration specimens, where a few coarse equiaxed dendrites appeared to originate from the weld surface. The secondary austenitic grain structure was columnar in both steels. In type 304 steel, the columnar austenitic grain structure did not necessarily correspond to the primary ferrite grains. In type 321 steel, the secondary austenite was columnar despite the equiaxed structure of the primary ferrite. Factors which affect the columnar-to-equiaxed transition (CET) are discussed. The failure to form equiaxed austenitic grains in type 321 steel is ascribed to austenite growing across the space between ferrite grains instead of renucleating on the primary equiaxed ferrite.     

双辊连铸不锈钢薄带凝固组织特点

 通过金相观察分析了同径双辊薄带连铸机上生产的奥氏体不锈钢薄带的凝固组织,结果表明：铸带凝固组织包括2个柱状晶区和1个等轴晶区,其等轴晶呈近球形或蔷薇形。与传统连铸板坯相比,其柱状晶区一次及二次枝晶的间距较小,等轴晶粒内部为非枝晶结构,其尺寸大约是连铸坯等轴晶的1/10,凝固组织更致密。     

微观组织参数及工艺条件对430不锈钢凝固显微结构的影响

在多元合金CAFE模型的基础上,分析了微观组织参数（形核密度、高斯分解参数、Gibbs-Thomson系数等）与430不锈钢凝固过程中晶粒形貌的复杂关系,以及过热度与冷却强度等工艺参数对凝固组织的影响.研究发现,晶粒尺寸和柱状晶向等轴晶转变不仅与体最大形核过冷度有关,也受体形核密度的影响.高斯分解参数和Gibbs-Thomson系数增大时,一次枝晶间距减小,等轴晶范围增大;但当它们增加至一定范围后,其对显微结构的影响逐渐变得不明显.过热度或冷却强度增大时,等轴晶范围减小,但一次枝晶间距的变化不明显.     

含氮不锈钢凝固模式及显微组织研究

研究了四种不同N含量的18Mn18Cr N不锈钢的凝固模式、显微组织和元素分布.结果表明：N含量影响18Mn18Cr N合金系的凝固模式和显微组织.氮的质量分数由0.07%增加至0.72%时,实验钢的凝固模式由F模式转变为A模式,显微组织由铁素体和奥氏体魏氏两相组织转变为铁素体和奥氏体两相组织以及单相奥氏体组织.N含量影响奥氏体相形貌,随N含量增加,奥氏体由板条状、针状转变为枝晶间和等轴状.枝晶间和等轴状奥氏体晶粒中存在褶皱形貌,且随着氮含量增加,褶皱数量增多.褶皱的产生与凝固过程中奥氏体相内部Fe、Mn、Cr元素的偏析有关,且该凝固偏析被保留至室温组织中.     

Investigation on the cold-strip breakage of SUS445J2 ultra-pure ferritic stainless steel during rolling process

In this work,the cause of the cold-strip breakage of SUS445J2 ultra-pure ferritic stainless steel was investigated using optical microscopy,energy dispersive spectroscopy,and electron backscatter diffraction technique.Results show that during cold rolling,the breakage of the stainless steel strip was mainly caused by massive niobium-rich(Nb-rich)carbides present in the center of the strip.Furthermore,cracks initiated at these carbides in the central layer.The massive carbide precipitates resulted from the segregation and enrichment of Nb,titanium(Ti),and carbon(C)in the central layer during the subsequent solidification stage of SUS445J2 stainless steel.Additionally,the banded structure and coarse ferrite grains in the central layer reduced the plasticity of the material,causing cracks to propagate along these grains and finally leading to cold-strip breakage.By reducing the solidification cooling rate of the billet,increasing the pouring temperature,discretizing Nb,Ti,and C enrichment via electromagnetic stirring,and forming equiaxed crystals in the central layer,the precipitation of harmful massive Nb-rich carbides in the central layer could be effectively avoided.     

Numerical simulation of solidification structure during electroslag remelting casting of ZG06Cr13Ni4Mo ingot based on CAFE and moving boundary method

A comprehensive mathematical model of the solidification structure during the process of electroslag remelting casting (ESRC) of low carbon martensite stainless steel ZG06Cr13Ni4Mo has been established. The change of metal pool profile and grain growth and the microstructure evolution process from the beginning to the steady stage of the ESRC process were investigated by using the moving boundary method and the coupled technology CAFE method (cellular automaton – finite element method). The transition from equiaxed grains at the lateral wall of the mould to columnar grains has been revealed. In addition, casting of this steel has been carried out and the microstructure of the ingot obtained after grinding and acid leaching. According to the comparison of the metal pool profile, morphology and growth direction of the dendrite and the secondary dendrite arm spacing (SDAS) between the experimental results and the simulation results, the validity of the model has been demonstrated, which can provide a favourable theoretical foundation to optimise the process parameters for the control of solidification structure of ESRC of low carbon martensite stainless steel ZG06Cr13Ni4Mo.     

自蔓延离心法制备不锈钢内衬复合钢管的显微结构

用扫描电镜和透射电镜研究了自蔓延离心法制备的不锈钢内衬(Cr,Ni分别为13%~14%和14~16%)复合钢管不锈钢层的结晶特点和组织结构.研究表明,不锈钢熔体主要以柱状晶形式结晶,柱状晶由奥氏体组成,柱状晶间有一薄层铁素体,铁素体区有AlNi金属间化合物析出相,在奥氏体和铁素体相界的铁素体区一侧有σ相析出.     

振动激发金属液原位形核的物理模拟

为了提高铸坯的等轴晶率、细化凝固组织,以30%氯化铵水溶液和铁素体不锈钢为研究对象,分别进行了物理模拟和浇注实验.结果发现:当一种带有冷却结构和高频振动的晶核发射器棒体插入氯化铵溶液时,在棒体表面将迅速形成大量的细小晶粒.这些晶粒在振动作用下被连续不断地弹射至溶液中,成为凝固过程中等轴晶的形核核心;晶核发射器的冷却强度越大、振动频率越高,则形成的晶粒数量越多且粒径越小;铁素体不锈钢液经振动激发形核处理后,凝固组织中的等轴晶率超过了80%.     

304奥氏体不锈钢亚快速凝固组织演化和形成机理

通过感应炉熔化的304钢(/%:0.053C、0.55Si、1.50Mn、0.030P、0.002S、17.02Cr、8.01 Ni、0.50Cu、0.08Mo)直接浇铸在水冷铜模上得到厚7 mm直径25 mm的圆形试样,研究了Cr、当量/Ni当量和1.5～1 000℃/s的冷却速率对奥氏体不锈钢铸态凝固组织形态和分布的影响。结果表明,随冷却速率增加至75～90℃/s,该钢的凝固模式由FA(铁素体-奥氏体)模式向AF(奥氏体-铁素体)模式转变,初生相由枝晶铁素体转变成枝晶奥氏体,但冷却为～1 000℃/s时,观察到块状铁素体组织,并且枝晶状奥氏体转变成胞状奥氏体。     

Microstructure and forming characteristics of spray-formed M42 high speed steel

M42 high speed steel(HSS),a high grade alloy,was produced using the spray forming technology.Optical metallography(OM),X-ray diffraction(XRD) and scanning electron microscope(SEM) were used to investigate the microstructures of the as-sprayed M42 HSS and the as-atomized powders.It is found that as-sprayed M42 HSS is composed of martensite,retained austenite and carbides.The grain size,as well as the morphology,size and distribution of the carbides have been greatly improved compared with those of the as-cast M42 steel(a traditional process).The size of the powder obtained at a high cold speed was ranged from 50 μm to 100 μm in diameter.Its representative microstructures include dendrite,dendritic fragments,a mixture of dendrites,equiaxed grains and dendritic fragments,and equiaxed grains.The rapid solidification in the spray forming is a key factor to cause the refinement of the M42 HSS.     

节铬型铁素体不锈钢连铸板坯凝固过程热模拟

连铸板坯凝固传热主要在厚度方向进行,这造成了连铸坯大部分区域由侧面向中心凝固,因此可以近似地用非稳态定向凝固进行热模拟。利用自主研发的水平式连铸坯枝晶生长热模拟装置研究了新型节铬铁素体不锈钢连铸坯凝固组织,以期在工业生产前预测连铸工艺对其凝固组织的影响。热模拟试样热端温度采用连铸坯心部冷却曲线进行控制,并通过调节冷却水流量控制热模拟试样冷端的冷却强度,从而实现由冷端向热端的非稳态定向凝固。实验发现过热度和冷却强度对热模拟试样的等轴晶率及其平均晶粒尺寸影响不显著,但大的冷却强度会导致柱状晶长度增加。     

稀土对铁素体不锈钢凝固组织的改善

报道了稀土对1C r17(430)铁素体不锈钢凝固组织的影响,如稀土细化柱状晶,扩大了中心等轴晶区域,并且初步分析了稀土元素改善凝固组织的作用机理。     

Solidification Structure Refining of 430 Ferrite Stainless Steel With TiN Nucleation

The thermodynamics of TiN precipitation in liquid steel of 430 ferrite stainless steel has been calculated to find out the condition of TiN precipitation during the initial solidification stage. The difference in the solidification structure of 430 ferrite stainless steel has been discussed through comparative tests of vacuum induction furnace melting with different contents of Ti. It has been found that the equiaxed grain proportion can be increased from 20% to 69% as the content of Ti from 0.1% up to 0. 4%. The size of the TiN particles precipitated is 1-3 μm and the number of TiN particles is about （200- 300）/μm^2. It is found that the effect of using TiN to refine the solidification structure has been confirmed under the strict process condition used for 430 ferrite stainless steel.     

镁处理对430铁素体不锈钢夹杂物形成和凝固组织的影响

试验研究了0.000 5%～0.001 2%Mg对60 kg真空感应炉熔炼的430铁素体不锈钢(/%:0.04C、0.25～0.32Si、0.28～0.38Mn、16.5～16.9Cr)夹杂物形成和凝固组织的影响。结果表明,430钢液中添加镁合金后,钢中形成平均粒径更小,数量密度更大的含MgO复合夹杂物;镁合金的加入可以改善430钢的凝固组织,且浇铸温度越低,改善效果越明显,在1 580℃浇铸时,等轴晶率由常规钢的30.8%提高至镁处理钢的88.5%,相应等轴晶尺寸由1 741.6μm降至945.3μm。含MgO夹杂物与δ相二维错配度极小,可作为430钢有效异质形核剂,促进等轴晶的形成,抑制柱状晶的生长,细化凝固组织。     

Solidification modeling and solid-state transformations in high-energy density stainless steel welds

The solidification and solid-state transformations which occur during the high-energy density (HED) welding of austenitic stainless steel were studied. Comparisons were made between structures observed in gas tungsten arc (GTA) welds and those of electron beam (EB) and laser welds using Fe-Ni-Cr ternary alloys with Cr/Ni ratios ranging from 1.5 to 1.85. Weld solidification and microsegregation was modeled using a finite difference analysis and compared with experimental results. These calculations were also used to help interpret the origin of the observed microstructures. Calculations showed that little solid-state diffusion occurs during the solidification and cooling of primary austenite solidified welds, whereas structures which solidify as ferrite may become almost completely homogenized as a result of diffusion. A change in solidification mode from primary austenite to primary ferrite was found to occur at higher Cr/Ni ratios with the HED welds than with GTA welds and is attributed to dendrite tip undercooling. A nearly segregation-free, single-phase austenite structure which appears to be unique to the rapid solidification velocities and cooling rates of HED welds was also observed. It is suggested that this structure is a product of ferrite solidification which transforms to austenitevia a massive transformation.     

Effect of M-EMS on the solidification structure of a steel billet

Effects of mold electromagnetic stirring (M-EMS) on the solidification structure of 45# steel billet were investigated by examination of interdendritic corrosion The results show that the primary and secondary dendrite arm spacings increase from the edge of the billet to the center and decrease obviously with increasing electromagnetic torque, which will be beneficial to refine the solidification structure and enlarge the equiaxed crystal zone. The ratio of equiaxed crystal increases by 15.9% with the electromagnetic torque increasing from 230 to to the central area, where the cooling rates are similar at different torques. The closer to the central area, the less the influence of M-EMS on the cooling rate is. The ratio of the primary to secondary dendrite arm spacing is approximately 2.0, namely, λ1 ≈2λ2, and is constant irrespective of the stirring intensity and position of the billet. Original position analysis (OPA) results indicate that the center segregation of the billet is greatly improved, and the more uniform and compact solidification structure will be obtained with the increase of stirring intensity.