【作者】 刘江；

【导师】 文光华；

【作者基本信息】 重庆大学 ， 冶金工程， 2017， 博士

【摘要】 本文以攀钢和宝钢生产的亚共析微合金钢为研究对象,通过高温共聚焦激光扫描显微镜(CLSM)模拟了连铸过程中钢液在结晶器内的初始凝固,奥氏体向铁素体相变以及铸坯表层在二冷段的热循环过程。对微合金钢凝固过程中铁素体对原始奥氏体的影响、奥氏体向铁素体相变以及铁素体对双相变细化原始奥氏体晶粒的影响等方面展开了深入地研究;同时,针对热拉伸实验通过热塑性损失来表征微合金钢连铸坯表面横裂纹形成倾向的不足,建立了表征铸坯横裂纹敏感性的高温组织模型,提出了微合金钢铸坯表面横裂纹敏感性判据。论文主要研究特点及研究结果概括如下:(1)通过CLSM动态原位观察了微合金钢初始凝固冷却过程中原始奥氏体晶粒长大过程,分析了冷却速度对?枝晶臂间距的影响,建立了奥氏体晶界迁移率与冷却速度的关系,并在此基础上对现有奥氏体生长模型进行了修正。研究表明:当冷却速度小于4℃/s时,?枝晶臂间距随冷却速度的增加急剧变小,而大于4℃/s时,?枝晶臂间距随着冷却速度的增加变化平缓;晶界迁移率随冷却速度的增大而减小,与冷却速度呈指数关系;修正后的奥氏体生长模型实现了大冷却速度下原始奥氏体晶粒尺寸的预测,其预测误差在10%以内。(2)对铸坯冷却过程中奥氏体向铁素体相变进行了模拟研究,探讨了不同冷却速度和冷却温度对先共析铁素体体积分数以及形貌的影响,并利用电子背散射衍射研究了先共析铁素体的形核生长取向以及形貌晶体学。结果表明:铁素体体积分数随着冷却速度的增大呈现先增大后减小的趋势,当冷却速度为4℃/s时,铁素体体积分数最高可达80.17%;铁素体体积分数随着冷却温度的降低逐渐增大,冷却温度控制在640℃时,先共析铁素体体积分数接近理论转变量;当冷却速度为1℃/s时,膜状先共析铁素体沿奥氏体晶界析出,铁素体晶粒生长取向与晶界的取向差角处于较低水平(<7o),铁素体晶粒尺寸粗大(200?m),原始奥氏体晶界处小角度晶界密度较低。当冷却速度为5℃/s时,沿奥氏体晶界的膜状铁素体消失,铁素体晶粒生长取向与晶界的取向差角处于较高水平,铁素体晶粒尺寸细小且均匀(<100?m),原始奥氏体晶界处小角度晶界密度较高。(3)利用CLSM模拟了微合金钢连铸双相变工艺中铸坯表层热历程,探讨了先共析铁素体、二次相变回热条件以及第二相析出物对双相变细化原始奥氏体晶粒的影响,并在此基础上对现有双相变工艺参数进行了优化。研究结果如下:二次相变后奥氏体晶粒尺寸随着先共析铁素体体积分数的增加而变小,当铁素体体积分数达到80%时,原始奥氏体晶粒可从1.20mm细化至0.47mm;当冷却速度为1℃/s时,膜状先共析铁素体晶粒生长取向与晶界[1 1 0]平面的取向差小于15o,有利于铸坯回热过程中新奥氏体晶粒的形核;二次相变过程中,回热温度以及回热速度决定了新奥氏体晶粒的尺寸,回热温度较高时,应当提高回热速度以避免奥氏体晶粒之间相互吞并而造成晶粒的粗大,而回热温度较低时,则应降低回热速度以确保新奥氏体晶粒形核后长大形成稳定晶粒;当回热温度为1000℃时,固溶的微合金元素会大量析出以抑制新奥氏体晶粒的长大,促进了原始奥氏体晶粒的细化;针对实验钢种,采用优化的双相变工艺(冷却条件:以5℃/s冷却至800℃,然后以1℃/s冷却至640℃;回热条件:以3℃/s回热至1000℃),可以将奥氏体晶粒平均尺寸细化至0.3mm以下,将现有双相变工艺中原始奥氏体晶粒尺寸的细化率提高了31%。(4)通过对铸坯表层高温组织特征(原始奥氏体晶粒尺寸、膜状先共析铁素体体积分数以及第二相析出物体积分数)的分析,建立了高温组织模型,在此基础上提出了微合金钢铸坯表面横裂纹敏感性判据。研究表明:当原始奥氏体晶粒尺寸可通过铸态组织表征时,铸坯横裂纹敏感性决定于原始奥氏体当量直径(DγP),铸坯表面横裂纹产生的临界DγP尺寸为0.69mm;当原始奥氏体晶粒尺寸不能通过铸态组织表征时,采用铸坯室温组织,即铁素体平均晶粒尺寸(DF)、铁素体混晶指数(△E)、膜状先共析铁素体体积分数(fMF)以及第二相析出物分布来判断横裂纹敏感性。当DF<35μm,ΔE<1.0,fM<3%,析出物弥散分布,裂纹指数趋近于0,铸坯表面无横裂纹;当35μm<DF<50μm,1<ΔE<2,3%<fM<5%,析出物链状分布,裂纹指数小于35,裂纹敏感性弱;当DF>50μm,ΔE>3,fM>5%,析出物链状分布,裂纹指数大于50,裂纹敏感性强。更多还原

【Abstract】 This paper focused on the refining of the prior austenite grain in the surface of micro-alloyed slab produced by Pansteel and Baosteel.The process of initial solidification,the transformation of austenite to ferrite,and the thermal cycle of slab surface during the second cooling section of continuous casting were simulated by using confocal scanning laser microscope(CSLM).The effect of ferrite on prior austenite grain during solidification,the transformation of austenite from ferrite,and the refinement mechanism of ferrite austenite grain during double phase transformation process for the micro-alloyed steel were deeply investigated.In addition,considering the limitations of the tensile test in characterizing the formation tendency of the surface transverse crack for the micro-alloyed steel,an improved model was established to estimate the susceptibility of transverse cracking on the slab corner of the micro-alloyed steel based on the features of high-temperature microstructure.Furthermore,a criterion was proposed to evaluate the sensitivity of the transverse crack on slab surface of the micro-alloyed steel.The main results and conclusions of this work are listed as follows:(1)The solidification cooling process was simulated by CLSM,and the growth behavior of austenite was investigated.To characterize the initial austenite grain diameter,the dendrite spacing was measured by in situ observation.The variation of austenite grain diameter with cooling rate is consistent with the austenite initial grain size.Based on the analysis of austenite grain growth,the relationship between the grain boundary migration rate and the cooling rate was established.When the cooling rate is higher than 2 oC /s,the grain boundary mobility has an exponential relationship with the cooling rate.Based on the above results,the existing austenite growth model was modified.The prior austenite grain size is predicted by the large cooling rate,and the prediction error is less than 10%,which makes up the deficiency of the original model.(2)The transformation of austenite to ferrite during the cooling process of slab was simulated.The volume fraction and the morphology of proeutectoid ferrite were analyzed at different cooling conditions.With the increase of cooling rate,the volume fraction of ferrite increased firstly and then decreased.When the cooling rate is 4oC/s,the volume fraction of ferrite is up to 80.17%.The temperature during cooling process is an important factor to control the volume fraction of ferrite.The ferrite volume fraction increases with the decrease of cooling temperature.When the cooling temperature is controlled at 640℃,the proeutectoid ferrite volume fraction is close to the theoretical amount of change.The proeutectoid ferrite was studied by electron backscatter diffraction.The cooling rate can change the ferrite nucleation and grain boundary misorientation angle,which leads to the precipitation of proeutectoid ferrite formed different morphology.The proeutectoid ferrite iron with different morphology will lead to the original austenite ferrite grain boundary near the size and density of small angle grain boundaries.It is the fundamental reason of the initiation and propagation of micro cracks.(3)The influencing factors of austenite grain refinement were studied by simulating the thermal process of double phase transformation of microalloyed steel.In addition to the proeutectoid ferrite as the two phase transition source,the morphology of austenite nucleation also have different effects.The ferrite grain orientation is less than that of the grain boundary [1 1 0] plane,which is beneficial to the nucleation of austenite grain in the process of slab heating.In the double phase transition process,the final size of the newly formed austenite grain depends on the temperature and the rate of heat recovery determine,and the essence of the rate control is to control the heating time.When the heating temperature is high,the heat recovery time should be reduced so as to avoid the grain coarsening.On the other hand,when the heat recovery temperature is low,the heating time should be prolonged to avoid the accumulation of the unstable austenite.In addition,the distribution of the second phase precipitates in steel not only inhibits the ferrite to austenite phase transformation,but also plays a role in pinning the new austenite grain growth in the heat recovery process.Based on the analysis of the influencing factors of austenite grain refinement,the existing dual phase change process parameters were optimized.According to the experimental steel,using double phase change process optimization(cooling conditions: 5oC/s cooling to 900 oC,then 1 oC/s cooling to 640 oC;heat condition: 3 oC/s to heat to 1000 oC),the average grain size of austenite can be refined to below 0.3mm.For the prior double phase change working art,refinement rate of the austenite grain is increased by 31%.(4)According to the analysis of the microstructure of billet surface temperature stiffness,the concept of the original austenite equivalent diameter(DγP)was proposed,Considering that the film like proeutectoid ferrite and the precipitation of micro alloying elements weakens the original austenite grain boundaries after the correction of the original austenite grain size,its essence is in the calculation of casting billet surface under temperature tissue stiffness.When the original austenite grain size can be measured,the sensitivity criterion of the transverse crack on the surface of the micro alloyed steel slab can be determined by the size of DγP.The critical DγP diameter on the slab surface is 0.69 mm.When the austenite grain size cannot be obtained,the casting microstructure,ferrite grain size(DF),ferrite mischcrystal index((35)E)and film like proeutectoid ferrite volume fraction(fMF)can be used to determine the transverse crack sensitivity.When DF<35?m,(35)E <1.0,fMF <3%,precipitation distributed evenly,and the crack index is close to 0.In this case,the transverse cracks are difficult to form on the slab surface.When 35?m <DF<50?m,1<(35)E <2,3%< fMF<5% E,chain distribution of the precipitates is observed,and the crack index is less than 35.In this case,the crack sensitivity is weak.When DF >50 ?m;(35)E >3,fMF >5%,chain distribution of the precipitates is observed,and the crack index is higher than 50,meanwhile,the transverse crack sensitivity of slab surface is strong.更多还原