2205双相不锈钢连铸凝固组织热模拟研究
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摘要
自1991年典型牌号2205（Fe-22.5Cr-5Ni-3Mo-0.17N）问世以来，双相不锈钢已广泛用于对耐蚀性要求较高的石油天然气管道、化工、造纸、建筑、海洋平台、能源热交换器、淡水处理等工业领域。但由于不锈钢导热性能差、钢水粘度大、凝固形态比较复杂和含多种合金元素，使得其在连铸生产中极易出现组织粗大、裂纹、堵塞水口等问题，浇铸过程中易形成夹杂物等生产缺陷，降低了其生产效益。
针对双相不锈钢新材料的特点以及连铸坯生产中遇到的问题，本文分别采用传统定向凝固、连铸坯热模拟装置以及异质核心技术，研究了2205双相不锈钢的凝固特性、连铸工艺参数对凝固组织的影响以及异质核心对凝固组织细化的作用。
根据相选择理论分析和定向凝固实验结果显示，2205双相不锈钢凝固模式为F型，先形成相为铁素体，奥氏体在铁素体基底上由固态相变析出，相变过程为：L→L+δ→δ→δ+γ。根据界面响应函数计算和定向凝固实验结果，得到2205双相不锈钢平-胞转换临界速率为V_(cs)=2μm/s，胞-枝转变速率为V_(cd)=5μm/s。生长速率为10~100μm/s时，固/液界面形貌为树枝状。当生长速率为500~1000μm/s时，界面形貌为细树枝状。
固/液界面处主要溶质元素Cr、Ni、Mo、Mn和Si的波普分析(WDS)表明，元素Cr为k>1的元素，其它元素Ni、Mo、Mn和Si则均为k<1。并且随着固/液界面推进速率的增加，奥氏体析出温度升高。在2205双相不锈钢中，奥氏体析出温度从2μm/s的1096℃增加至100μm/s的1275℃。
对比分析表明，连铸坯枝晶生长热模拟试样的宏观组织及微观组织与实际连铸坯相似，但模拟试样等轴晶区的晶粒比连铸坯大很多。模拟实验研究发现，调整过热度与冷却强度对2205双相不锈钢的柱状晶向等轴晶转变（CET）的影响较小。对不同实验参数下动态温度梯度的分析发现，由于双相不锈钢导热能力较差，实验参数调节对样品距水冷底面60mm以内的温度场影响较大；而样品60mm以上的温度场几乎没有影响，致使工艺参数的调节对CET的影响减弱。
热模拟实验中施加机械搅拌可有效提高双相不锈钢的等轴晶率、细化凝固组织。随着搅拌速率的增加，柱状晶区域长度从8.2cm降到了4.6cm，等轴晶区域的平均晶粒尺寸从9.5mm降到了1.2mm；同时搅拌速率增加，双相不锈钢晶粒内部的第二相也明显细化，奥氏体形貌由粗大的针状和魏氏组织转变为细碎的颗粒和岛状形貌。
通过对两相间元素分布分析，发现晶界上两相间的元素分配差别大于晶粒内部，但柱状晶区和等轴晶区内元素分布特性差别不大。
理论计算表明，凝固过程中由于偏析均能导致Ti、N、O产生过饱和，而且Ti_2O_3的过饱和度显著大于TiN的过饱和度，这有利于Ti_2O_3优先于TiN析出，而形成Ti_2O_3+TiN异质核心，使得形核核心大幅提高，从而使得等轴晶率显著提高。
利用添加适量的钛粉的模拟实验证实，利用Ti_2O_3/TiN异质核心作为异质形核核心是一种细化双相不锈钢凝固组织的有效手段，可明显提高铸坯的等轴晶率、细化凝固组织和第二相组织，并且在复合核心的基础上，较低的Ti含量就可获得较高的等轴晶组织，可有效避免含钛钢连铸过程中水口结瘤和表面夹杂等问题。
Since the typical Grade2205(Fe-22.5Cr-5Ni-3Mo-0.17N) was confirmed in1991, duplex stainless steel has been widely used for nearly20years, which is broadapplied in various industry fields requiring high corrosion resistance, such as oil andgas pipeline, chemical engineering, paper manufacturing, architecture，ocean platform，energy and heat exchanger，fresh water treatment and so on. However，due to the poorthermal conductivity，high liquid viscosity，complicated solidification microstructureand containing many alloy elements for stainless steel，several problems such ascoarse microstructure, crack and nozzle clogging easily tend to exist in continuouscasting process, resulting in the manufacturing defects, such as the formation ofinclusions, which reduces the production effectiveness.
According to the analysis on the characteristics of the DSS and the problemsexist in the production of the continuous casting slab, traditional directionalsolidification, the simulation equipment for continuous casting slab andheterogeneous nucleation technology are adopted in this article, studying thesolidification characteristic of2205DSS, the influence of the parameters of thecontinuous casting processing on solidification microstructure and the solidificationmicrostructure refinement of heterogeneous nucleation technology.
According to the formula of phase selection and the results of directionalsolidification experiment, we can conclude that the solidification mode of2205DSSis F type, prior forming phase is ferrite, on the basement of which forming theaustenite, and the phase transformation process is as follow: L→L+δ→δ→δ+γ.According to the LKT model and the results of the directional solidificationexperiment, the critic velocity for planar-cell transition of2205DSS is Vcs=2μm/s, thecritic velocity for cell-dendrite transition is Vcd=5μm/s. When the growth velocity is10~100μm/s, the interface morphology is typical dendrite; while the growth velocityis500~1000μm/s, the interface morphology is fine dendrite.
The WDS analysis on the main solutes such as Cr, Ni, Mo and Si on the S/Linterface shows that Cr is an element of K＞1，while the K value of other elementssuch as Ni，Mo，Mn and Si is below1. The growth temperature of austenite gradually increases with the increasing growth velocity. For2205DSS，increasing the growthvelocity from2μm/s to100μm/s，the growth temperature of austenite increases from1096℃to1270℃respectively.
Compared the simulated samples to the continuous casting slab， theirmacrostructure and microstructure can be demonstrated to be very similar，howeverthe grain size of simulated samples in equiaxed grain zone is much coarser than thatof continuous casting slab. Adjusting the superheat and cooling strength have littleinfluence on the CET transition. According to the analysis on the dynamictemperature gradient of the samples under different experiment parameters，we canfind that within the60mm distance from water-cooled side upward of the sample, thetemperature gradient changes a lot in response to the adjustment of the experimentparameters, while beyond that zone, the temperature gradient almost doesn’t change.This is due to the poor heat conductivity of DSS, the heat resistance of the unmeltedpart of the sample is much larger than that of heat-transfer of cooling-waterconvection, and the heat conductivity of the sample becomes the main heat resistance,thus alleviating the influence the adjusting of processing parameters bring to CETtransition.
Mechanical stirring can effectively increase the ratio of equiaxed grain in DSSand refine the solidification microstructure. Increasing the stirring rate，the length ofcolumn zone reduces from8.2cm to4.6cm and the mean grain size of equiaxed grainreduces from9.5mm to1.2mm. Meanwhile，with the refinement of the grains，themicrostructure of second phase in the DSS are also obviously refined. With therotation rate increasing, the morphology of austenite changes from coarse acicular andwidmanstatten structure to refined particles and islands structure. According to theanalysis on the distribution of the elements between the two phases, it is obvious thatthe distribution difference of the elements between the two phases on the grainboundary is larger than that in the grain, but the distribution state of the elements onthe grain boundary or in the grain of the column grain zone is similar to that ofequiaxed grain zone. In addition, stirring has no significant effects on the distributioncoefficient of the elements between the two phases.
The theoretical calculation shows that the Ti, N and O all form the segregation, resulting in the formation of the supersaturation degree during the solidificationprocess, and the supersaturation degree of Ti_2O_3is significantly larger than that of TiN,which helps that the precipitation of Ti_2O_3has priority to that of TiN; Due to the highmatching ability, high-bond supersaturation between the composite nuclei basementand TiN,it can give rise to the nucleation rate; High amount of nuclei can obviouslyincrease the critical temperature gradient G, leading to the tendency to attain highequiaxed crystal ratio.
According to the analysis on the observation of the precipitation in the simulatedsamples, it shows that the acting of Ti_2O_3-TiN composite nuclei as nuclei ofheterogeneous nucleation plays a very important role in the solidification structurerefinement of the DSS, which can be realized by the addition of proper amount of Tipowders, thus increasing the equiaxed crystal ratio of the casting slab, refining thesolidification and second phase structure. Moreover, on the basis of the compositenuclei, this technology adopts low Ti content to attain high equiaxed crystal ratio,which can effectively avoid the problems, like nozzle clogging and surface inclusions,existing in the continuous casting process of the steels containing Ti.
针对双相不锈钢新材料的特点以及连铸坯生产中遇到的问题，本文分别采用传统定向凝固、连铸坯热模拟装置以及异质核心技术，研究了2205双相不锈钢的凝固特性、连铸工艺参数对凝固组织的影响以及异质核心对凝固组织细化的作用。
根据相选择理论分析和定向凝固实验结果显示，2205双相不锈钢凝固模式为F型，先形成相为铁素体，奥氏体在铁素体基底上由固态相变析出，相变过程为：L→L+δ→δ→δ+γ。根据界面响应函数计算和定向凝固实验结果，得到2205双相不锈钢平-胞转换临界速率为V_(cs)=2μm/s，胞-枝转变速率为V_(cd)=5μm/s。生长速率为10~100μm/s时，固/液界面形貌为树枝状。当生长速率为500~1000μm/s时，界面形貌为细树枝状。
固/液界面处主要溶质元素Cr、Ni、Mo、Mn和Si的波普分析(WDS)表明，元素Cr为k>1的元素，其它元素Ni、Mo、Mn和Si则均为k<1。并且随着固/液界面推进速率的增加，奥氏体析出温度升高。在2205双相不锈钢中，奥氏体析出温度从2μm/s的1096℃增加至100μm/s的1275℃。
对比分析表明，连铸坯枝晶生长热模拟试样的宏观组织及微观组织与实际连铸坯相似，但模拟试样等轴晶区的晶粒比连铸坯大很多。模拟实验研究发现，调整过热度与冷却强度对2205双相不锈钢的柱状晶向等轴晶转变（CET）的影响较小。对不同实验参数下动态温度梯度的分析发现，由于双相不锈钢导热能力较差，实验参数调节对样品距水冷底面60mm以内的温度场影响较大；而样品60mm以上的温度场几乎没有影响，致使工艺参数的调节对CET的影响减弱。
热模拟实验中施加机械搅拌可有效提高双相不锈钢的等轴晶率、细化凝固组织。随着搅拌速率的增加，柱状晶区域长度从8.2cm降到了4.6cm，等轴晶区域的平均晶粒尺寸从9.5mm降到了1.2mm；同时搅拌速率增加，双相不锈钢晶粒内部的第二相也明显细化，奥氏体形貌由粗大的针状和魏氏组织转变为细碎的颗粒和岛状形貌。
通过对两相间元素分布分析，发现晶界上两相间的元素分配差别大于晶粒内部，但柱状晶区和等轴晶区内元素分布特性差别不大。
理论计算表明，凝固过程中由于偏析均能导致Ti、N、O产生过饱和，而且Ti_2O_3的过饱和度显著大于TiN的过饱和度，这有利于Ti_2O_3优先于TiN析出，而形成Ti_2O_3+TiN异质核心，使得形核核心大幅提高，从而使得等轴晶率显著提高。
利用添加适量的钛粉的模拟实验证实，利用Ti_2O_3/TiN异质核心作为异质形核核心是一种细化双相不锈钢凝固组织的有效手段，可明显提高铸坯的等轴晶率、细化凝固组织和第二相组织，并且在复合核心的基础上，较低的Ti含量就可获得较高的等轴晶组织，可有效避免含钛钢连铸过程中水口结瘤和表面夹杂等问题。
Since the typical Grade2205(Fe-22.5Cr-5Ni-3Mo-0.17N) was confirmed in1991, duplex stainless steel has been widely used for nearly20years, which is broadapplied in various industry fields requiring high corrosion resistance, such as oil andgas pipeline, chemical engineering, paper manufacturing, architecture，ocean platform，energy and heat exchanger，fresh water treatment and so on. However，due to the poorthermal conductivity，high liquid viscosity，complicated solidification microstructureand containing many alloy elements for stainless steel，several problems such ascoarse microstructure, crack and nozzle clogging easily tend to exist in continuouscasting process, resulting in the manufacturing defects, such as the formation ofinclusions, which reduces the production effectiveness.
According to the analysis on the characteristics of the DSS and the problemsexist in the production of the continuous casting slab, traditional directionalsolidification, the simulation equipment for continuous casting slab andheterogeneous nucleation technology are adopted in this article, studying thesolidification characteristic of2205DSS, the influence of the parameters of thecontinuous casting processing on solidification microstructure and the solidificationmicrostructure refinement of heterogeneous nucleation technology.
According to the formula of phase selection and the results of directionalsolidification experiment, we can conclude that the solidification mode of2205DSSis F type, prior forming phase is ferrite, on the basement of which forming theaustenite, and the phase transformation process is as follow: L→L+δ→δ→δ+γ.According to the LKT model and the results of the directional solidificationexperiment, the critic velocity for planar-cell transition of2205DSS is Vcs=2μm/s, thecritic velocity for cell-dendrite transition is Vcd=5μm/s. When the growth velocity is10~100μm/s, the interface morphology is typical dendrite; while the growth velocityis500~1000μm/s, the interface morphology is fine dendrite.
The WDS analysis on the main solutes such as Cr, Ni, Mo and Si on the S/Linterface shows that Cr is an element of K＞1，while the K value of other elementssuch as Ni，Mo，Mn and Si is below1. The growth temperature of austenite gradually increases with the increasing growth velocity. For2205DSS，increasing the growthvelocity from2μm/s to100μm/s，the growth temperature of austenite increases from1096℃to1270℃respectively.
Compared the simulated samples to the continuous casting slab， theirmacrostructure and microstructure can be demonstrated to be very similar，howeverthe grain size of simulated samples in equiaxed grain zone is much coarser than thatof continuous casting slab. Adjusting the superheat and cooling strength have littleinfluence on the CET transition. According to the analysis on the dynamictemperature gradient of the samples under different experiment parameters，we canfind that within the60mm distance from water-cooled side upward of the sample, thetemperature gradient changes a lot in response to the adjustment of the experimentparameters, while beyond that zone, the temperature gradient almost doesn’t change.This is due to the poor heat conductivity of DSS, the heat resistance of the unmeltedpart of the sample is much larger than that of heat-transfer of cooling-waterconvection, and the heat conductivity of the sample becomes the main heat resistance,thus alleviating the influence the adjusting of processing parameters bring to CETtransition.
Mechanical stirring can effectively increase the ratio of equiaxed grain in DSSand refine the solidification microstructure. Increasing the stirring rate，the length ofcolumn zone reduces from8.2cm to4.6cm and the mean grain size of equiaxed grainreduces from9.5mm to1.2mm. Meanwhile，with the refinement of the grains，themicrostructure of second phase in the DSS are also obviously refined. With therotation rate increasing, the morphology of austenite changes from coarse acicular andwidmanstatten structure to refined particles and islands structure. According to theanalysis on the distribution of the elements between the two phases, it is obvious thatthe distribution difference of the elements between the two phases on the grainboundary is larger than that in the grain, but the distribution state of the elements onthe grain boundary or in the grain of the column grain zone is similar to that ofequiaxed grain zone. In addition, stirring has no significant effects on the distributioncoefficient of the elements between the two phases.
The theoretical calculation shows that the Ti, N and O all form the segregation, resulting in the formation of the supersaturation degree during the solidificationprocess, and the supersaturation degree of Ti_2O_3is significantly larger than that of TiN,which helps that the precipitation of Ti_2O_3has priority to that of TiN; Due to the highmatching ability, high-bond supersaturation between the composite nuclei basementand TiN,it can give rise to the nucleation rate; High amount of nuclei can obviouslyincrease the critical temperature gradient G, leading to the tendency to attain highequiaxed crystal ratio.
According to the analysis on the observation of the precipitation in the simulatedsamples, it shows that the acting of Ti_2O_3-TiN composite nuclei as nuclei ofheterogeneous nucleation plays a very important role in the solidification structurerefinement of the DSS, which can be realized by the addition of proper amount of Tipowders, thus increasing the equiaxed crystal ratio of the casting slab, refining thesolidification and second phase structure. Moreover, on the basis of the compositenuclei, this technology adopts low Ti content to attain high equiaxed crystal ratio,which can effectively avoid the problems, like nozzle clogging and surface inclusions,existing in the continuous casting process of the steels containing Ti.
引文
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