【作者】 李云峰；

【导师】 文光华；

【作者基本信息】 重庆大学 ， 冶金工程， 2014， 博士

【摘要】 目前，高强度微合金钢的研发、生产和应用已成为衡量一个企业和国家钢铁工业发展水平的重要标志。然而，在微合钢板坯连铸过程中，铸坯表面横向开裂一直是国内外钢厂急待解决的一大难题。铸坯在弯曲或矫直操作时采取避开第三脆性区温度区间的方法是目前降低铸坯表面横裂纹发生率的主要措施。然而，铸坯角部由于受二维传热的影响使其表层温度不能完全避开第三脆性区温度区间，角部横裂纹问题依旧没有得到很好的解决。从横裂纹的形成机理来看，粗大的原始奥氏体晶粒是导致裂纹沿晶界形成和扩展的重要因素。若能在连铸过程中实现原始奥氏体晶粒的细化，则可盼减少角部横裂纹的发生。本文以攀钢和宝钢生产的微合金钢铸坯为研究对象，采用高温共聚焦激光扫描显微镜模拟连铸过程中钢液在结晶器内的初始凝固过程，以及铸坯表层在二冷段的热循环过程。针对现有文献报道中对微合金钢连铸过程中奥氏体长大行为、双相变细化奥氏体晶粒机制和横裂纹敏感性表征等方面的不足，展开了深入地研究；并在实验室模拟研究的基础上，利用攀钢板坯弧形连铸机进行双相变细化原始奥氏体晶粒的工业试验，对铸机二冷段参数和工艺参数控制对奥氏体晶粒细化的影响进行了分析。本文主要研究特点及研究结果可简要概括如下：①通过高温共聚焦激光扫描显微镜模拟微合金钢凝固冷却过程，对奥氏体长大行为进行研究，建立了奥氏体开始长大温度与碳当量之间的量化关系：在碳当量CP≤0.18%范围内，奥氏体开始长大温度与碳当量呈指数函数关系。通过分析微合元素对奥氏体长大行为的影响认为连铸过程中微合金元素在原始奥氏体晶界的钉扎作用主要体现在TiN的析出上，并采用误差分析方法建立了表征钉扎作用的影响因子(K)，其与钛元素的析出量(PT i)之间呈二次函数关系。利用上述研究结果对现有模型进行修正后模型对奥氏体晶粒尺寸的计算误差降低5%。②通过模拟铸坯表层双相变热循环过程，深入研究了双相变工艺对原始奥氏体晶粒的细化机制。详细阐述了在冷却阶段的一次相变过程中“γ/α”转变率和转变区域的分布，以及在回热阶段的二次相变过程中回热条件的控制对奥氏体晶粒细化结果的影响，从晶粒细化机制上弥补了现有文献的不足。在实验钢种热物性参数条件下，当冷却速度控制在5℃/s，冷却温度为640℃，回热温度为1100℃，回热速度为3℃/s时，可使粗大的原始奥氏体晶粒（晶粒尺寸1.2mm）细化至0.47mm，且粒度分布较为均匀，混晶指数仅为0.5。③采用攀钢板坯弧形连铸机进工业试验，对不同二冷条件下的铸坯表层组织演变进行了分析，得出了铸坯表层组织随着表面深度的延伸存在三个组织演变区：接近铸坯表面的强冷转变区、由于热循环而发生双相变转变的双相变区和随后的缓冷转变区。通过工艺优化扩大双相变区，使铸坯在矫直时得到分布均匀、细小的奥氏体晶粒将是减轻表面横裂纹敏感性的关键。工业试验中，在控制冷却模式下，由于获得了较宽的双相变区（表面深度3.5-8mm），组织分布均匀、细小，铸坯角部裂纹敏感性得到了降低，裂纹指数仅为0.4。虽然通过对铸坯出结晶后二冷工艺的优化能使铸坯表层组织发生双相变转变，从而使原始奥氏体晶粒得到细化降低铸坯角部横裂纹敏感性，但双相变工艺对钢种热物性参数（Ar3和Ac3相变温度）、铸机二冷段参数及浇铸断面都有一定的适用性要求。未来研究中，若能从结晶器出口到矫直点间的传热机制上对现有铸机二冷参数进行改造，改善冷却与回热之间的矛盾，则可扩大钢种热塑性参数和断面尺寸的适用范围。④通过对现场铸坯角部组织的分析，建立了能够表征铸坯在弯曲或矫直时的高温组织特征的铸态组织参数：铁素体平均晶粒尺寸(DF)、铁素体混晶指数(E)和膜状先共析铁素体体积分数(fMF)。三个参数实质上是反应了铸坯在弯曲或矫直时的原始奥氏体晶粒尺寸和粒度分布。铸坯表面横裂纹指数与DF、 E和fMF三者呈正相关关系。通过对铸坯表层组织刚度分析，提出了原始奥氏体当量直径(DγP)的概念。DγP实质上是在刚度计算时，在考虑了膜状先共析铁素体和微合金元素析出物弱化了原始奥氏体晶界后，对原始奥氏体晶粒粒径的修正。通过分析DγP与铸坯表面横裂纹指数的关系，建立了基于铸态组织参数的微合金钢铸坯表面横裂纹敏感性判据，弥补了传统热拉伸实验中对铸坯表面横裂纹敏感性表征方面的不足。更多还原

【Abstract】 The research，production and application of high strength micro-alloyed steel hasbecome an important symbol for measuring the level of industrial development for thenational steel industry. However, the transverse cracking on the slab surface is a majorproblem encountered during the continuous casting of microalloyed steel. In thetraditional continuous casting process, making slab surface temperature avoid the rangeof the third brittle zone during bending or straightening operation is not enough toprevent cracking. Because the slab corner is affected by the two-dimensional heattransfer from both narrow and wide sides, it is inevitable for the corner temperature toreach the third brittle temperature range. According the formation mechanism oftransverse cracking，the prior austenite grain size is an important factor affecting thesurface transverse cracking; and refinement of the prior austenite grain size will be aeffective measures to reduce the cracking susceptibility.In this paper, accoring to the above idear of refining the prior austenite grain in themicro-alloyed slab surface layer which from the Pansteel and Baosteel, the process ofinitial solidification and thermal cycle was simulated by confocal scanning lasermicroscope (CSLM). The behavior of austenite grain growth (for low carbonmicroalloyed steel) during the initial solidification, austenite grain refinementmechanism during double phase transformation process and method of assessing slabtransverse cracking susceptibility were investigated. Based on the laboratory researchresults, the slab arc continuous caster fron Pansteel was used for the casting of lowcarbon microalloyed steel to analye the effect of caster parameter and technologicalparameter control on the refinement of prior austenite grains. The main work is listed inthe followings:①Based on the investigation of behavior of austenite grain growth, it shows thatthe starting temperature for austenite grain growth (Tγ) can be measured through in situobservation using CLSM. For low carbon micro-alloyed steel (CP≤0.18%), Tγis mainlyrelated to the CPunder the certain cooling rate, and shows the exponential functionrelation to the later; the pinning effect of the precipitates on the grain boundaries can bereflected by the amount of titanium precipitates PTiwhich is related to the solubilityproduct of TiN. The revision factor K, used for describing the effect of micro-alloyed elements on austenite grain size, has a quadratic function relation with PTi. Both the Tγand K were used to revise the C.Bernhard model. The results demonstrated that thecalculated value by the revision model closely corresponds to the measured values; andthe average relative error of the Bernhard’s model can be reduced by5%after revising.②Austenite grain refinement mechanism was analyzed via simulating doublephase transformation process. The introduction explains the effect of “γ/α”ratio anddistribution and reheating control on the grian refinement, to solve the inefficiency ofcurrent literatures. For the experiment steel, when the cooling rate, cooling temperature,reheating temperature and reheating rate are5℃/s,640℃,1100℃,3℃/s, respectively.The coarse prior austenite grain(1.2mm) can be refined as fine grains(0.47mm) witheven distribution, the mixed grains index is only0.5.③The results of continuous casting test show that there are three transformationregions for microstructure evolution at different distance to the slab subsurface: strongcooling transformation region near the slab surface, double phase transformation regionin which double phase transformation occurs due to the thermal cycles, and thesubsequent mild cooling transformation region. The key to reduce slab crackingsusceptibility is to obtain widely-distributed double phase transformation region in slabsubsurface. Under control cooling pattern, not only is film-like ferrite eliminated, in thewidely-distributed DPT region, but homogeneous and fine microstructure is obtained,and the cracking susceptibility is decreased with a crack index of0.4. The refinement ofaustenite grain is suitable to the thermal physical property of steel (Ar3and Ac3temprature), secondary cooling segment parameters of slab caster and slab sections. Inthe future, if the secondary cooling segment parameters can be improved accdoding toheat transfer principle inorder to solve the contradiction between the cooling andreheating, the cracking susceptibility will be reduced via double phase transformation.④Features of high-temperature microstructure (HTM) can be demonstrated by theparameters of as-cast microstructure (ACM): Ferrite grain size (DF), ferrite mixed grainindex (E), film-like ferrite precipitation fraction (fMF). The three parameters essentiallyreflect the austenitinte grain size and distribution in HTM. The transverse crackingsusceptibility can be reflected by the DF, fMFand E in the ACM, and shows the positivecorrelation with the parameters. Based on the stiffness ansalysis of salb surface,Equivalent diameter of austenite grain was put forward (DγP). DγPis a correction to theprior austenite grain diameter during the calculation of stiffness; and the effect of film-like territe and precipitates microalloyed elements on the austenite grain boundaryare included in the DγP. The DΓp, used to assessing the transverse cracking susceptibility,can be calculated by the parameters of as-cast microstructure. It makes up for theinadequacy of the hot tensile test for determination of cracking susceptibility.更多还原