【作者】 王宇；

【导师】 李花兵；

【作者基本信息】 东北大学 ， 冶金工程， 2023， 硕士

【摘要】 随着加压冶金技术的发展和对氮元素合金化作用机理研究的不断深入,国内外相继开发了一系列的高强度高氮不锈钢例如Cronidur30,M303,P2000,P900等。已经广泛应用于军用大型燃气轮机的护环、装甲和涡轮发动机的主轴承等领域。目前研究表明,凝固压力的增加可以从提高氮元素在钢中的溶解度,强化氮合金化的效果方面抑制氮气泡的析出,从而消除铸锭中氮气孔缺陷。并且加压可以使得氮元素的分布变得更为均匀,进一步提高了高氮钢的性能。但是对于加压如何影响氮气泡在钢液中的运动行为却没有得到很好的研究。对于加压下钢铁凝固方面,现有研究结果表明,当凝固压力在MPa级变化时,枝晶组织形貌会得到显著的转变,但是加压如何改变枝晶组织的形貌尚未得到具体研究。因此,本文针对以上问题做了以下研究:1.利用Ansys Fluent软件,基于VOF模型,模拟了加压下氮气泡在钢液中的运动行为,分析了压力对气泡形变,分裂和运动速度的影响,研究结果表明:(1)氮气泡在钢液上升过程中在射流和涡流的共同作用下,氮气泡在上升过程中分裂出三种不同类型的气泡,分别是:主氮气泡,子氮气泡和离散氮气泡。(2)氮气泡的运动速度和其大小成正比,并且氮气泡的运动速度与容器形貌有着明显关联,当容器的锥度大小范围为0.04～0.05之间时,氮气泡在钢液中运动时间最短。(3)当凝固压力从0.1MPa增加到2MPa,由于氮气泡内表面和外表面之间的压差增加,主氮气泡和子氮气泡尺寸降低。此外随着凝固压力的增加,氮气泡下表面产生的射流增强,有利于分裂形成子氮气泡。主氮气泡的最大运动速度随着尺寸的降低而降低,进而导致其对钢液的扰动程度降低,从而抑制了主氮气泡在运动末端离散氮气泡的形成。2.基于相场法建立了加压下Fe-0.9N合金凝固过程中的枝晶生长模型,并利用热力学软件Thermo-Calc和铸造模拟软件ProCAST获取了凝固压力对模型中相关参数的影响。利用Matlab 2021软件编译了压力下合金枝晶生长的相关程序。研究了凝固压力对铸锭心部等轴晶和边部柱状晶生长的影响,研究结果表明:(1)对于等轴晶生长而言,随着凝固压力的增加,二次枝晶间距逐渐降低。造成这种现象的主要原因是,加压导致合金液相线温度增加,进而导致体系自由能增加。此外,由凝固压力引起的等轴晶区温度梯度和形核率的变化并不是引起其二次枝晶减小的主要原因,但是形核率的改变会显著改变等轴晶的形貌。(2)对于柱状晶区而言,一次枝晶间距随着凝固压力的增加而降低。与等轴晶区不同的是增加凝固压力从提高液相线温度和温度梯度这两个方面共同影响了柱状晶组织生长。更多还原

【Abstract】 With the development of pressure metallurgy technology and the deepening research on the mechanism of nitrogen alloying,a series of high strength and high nitrogen stainless steel,such as Cronidur30,M303,P2000,P900,have been developed at home and abroad.It has been widely used in the preparation of ring and armor of large military gas turbine and the main bearing of turbine engine.The present research shows that the increase of solidification pressure can improve the solubility of nitrogen in steel,strengthen the effect of nitrogen alloying,inhibit the precipitation of nitrogen bubbles,eliminate the formation of nitrogen porosity defects in the ingot,make the distribution of nitrogen elements become more uniform,and further improve the performance of high nitrogen steel.However,the effect of increasing solidification pressure on the motion behavior of nitrogen bubble in molten steel has not been well studied.For the solidification of steel under pressure,the existing research results show that when the solidification pressure changes at MPa level,the dendrite microstructure morphology will be significantly changed,but how the increase of solidification pressure changes the dendrite microstructure morphology has not been specifically studied.Therefore,this paper has done the following research on the above problems:1.Using Ansys Fluent software and based on the VOF model,the motion behavior of nitrogen bubble in liquid steel under pressure was simulated,and the influence of pressure on bubble deformation,splitting and motion velocity was analyzed.The following conclusions were obtained:(1)Nitrogen bubble in the rising process of molten steel under the joint action of jet and eddy current,the nitrogen bubble splits into three different types of bubbles during the rising process,which are:main nitrogen bubble,daughter nitrogen bubble and discrete nitrogen bubble.(2)The velocity of the nitrogen bubble is proportional to its size,and the velocity of the nitrogen bubble is obviously related to the morphology of the container.When the size of the container taper ranges from 0.04 to 0.05,the motion time of the nitrogen bubble in the liquid steel is the shortest.(3)When the solidification pressure increases from 0.1 MPa to 2MPa,the size of the main nitrogen bubble and the sub-nitrogen bubble decreases due to the increased pressure difference between the inner and outer surfaces of the nitrogen bubble.In addition,with the increase of solidification pressure,the jet generated on the lower surface of the nitrogen bubble is enhanced,which is conducive to the formation of daughter nitrogen bubble.In addition,the decrease of the size of the main nitrogen bubble leads to the decrease of its maximum motion velocity,which leads to the decrease of its disturbance degree to the liquid steel,thus inhibiting the formation of discrete nitrogen bubble at the moving end of the main nitrogen bubble.2.The dendrite growth model of Fe-0.9N alloy during solidification under pressure was established based on the phase field method,and the influence of solidification pressure on the relevant parameters in the model was obtained by Thermo-Calc,a thermodynamic software,and Procast,a casting simulation software.Matlab 2021 software is used to compile the relevant program of alloy dendrite growth under pressure.The effect of solidification pressure on the growth of equiaxed crystal and cylindrical crystal at the core of ingot was studied.The results show that:(1)For equiaaxial crystal growth,the secondary dendrite spacing decreases gradually with the increase of solidification pressure.The main reason for this phenomenon is that the liquidus temperature increases with the increase of solidification pressure,which leads to the increase of system free energy.Moreover,the change of equiaemic grain temperature gradient and nucleation rate caused by solidification pressure is not the main reason for secondary dendrite reduction.However,the morphology of equiaxed crystals can be significantly changed with the change of nucleation rate.(2)For columnar region,the primary dendrite spacing decreases with increasing solidification pressure.Different from equiaxed crystal region,increasing solidification pressure can affect the growth of columnar crystal structure by increasing liquidus temperature and temperature gradient.更多还原