【作者】 刘旭峰；

【导师】 翟启杰； 张捷宇；

【作者基本信息】 上海大学 ， 钢铁冶金， 2006， 博士

【摘要】 利用CSP工艺生产不锈钢可极大地提高产品的市场竞争力，而目前国内利用CSP工艺生产不锈钢尚属空白，国外相关研究也鲜见报导。本文根据不锈钢的生产技术需求和薄板坯连铸技术的发展实际，首次采用数值模拟和离线实验相结合的方法，探讨了应用CSP技术生产不锈钢的可能性。首先，在详细推导了流热耦合湍流有限差分方程的基础上，借助商业软件包ANSYS-CFX4，并通过二次代码开发，采用焓热法并结合了流体流动与热量传递的相互影响，建立了CSP连铸过程的三维流热耦合模型。在此基础上首次模拟研究了CSP连铸机生产1Cr18Ni9Ti不锈钢的结晶器内和二次冷却段的流场和温度场，探讨了应用CSP技术生产不锈钢过程中钢液的流动和凝固行为。目前计算整个连铸凝固过程一般采用有效导热模型，尽管在校正后，可给出铸坯的表面温度和坯壳的厚度，但是通过扩大有效导热系数来考虑流动对传热和凝固的影响，不能客观地反映铸坯内部的流体流动、传热和凝固之间的相互作用；而新近发展的耦合求解模型又很少考虑二冷中的凝固过程及流动行为对最终凝固过程影响。本文模型主要特点有：(a)用于建立模型的数据取自于工厂的实际参数，使模型更为准确和可靠；(b)在计算可能的情况下，最小程度的采用简化条件，如：拟合了CSP漏斗型结晶器的复杂壁面方程，采用了近似的侧开孔水口造型，以及函数变化的结晶器热流密度和二冷区换热系数；(c)计算域延伸至整个二冷喷水区域；(d)考虑了液相区和两相区的流体流动对凝固的影响。其次，在国内尚不具备直接研究CSP生产的不锈钢条件下，利用实验室离线实验研究了不锈钢在不同冷却速度下的凝固特点及组织和性能变化，并在此基础上讨论了CSP薄板坯连铸条件下不锈钢的凝固特性和组织特点。最后，在系统的对比研究CSP生产线在LCR工艺和非LCR工艺条件下SPA-H钢铸坯和成品板的室温组织及性能基础上，首次探讨了LCR技术对CSP不锈钢铸坯质量的影响。在上述工作的基础上，从技术角度对CSP工艺生产不锈钢形成以下认识：(1)数值模拟研究表明，凝固坯壳形成对钢液在CSP薄板坯连铸结晶器内流动的影响较大，钢液在结晶器内形成明显的上下涡旋，涡旋形态与中厚板类似，但下涡旋区受到水口注流压缩变小，上涡旋区也相应扩大。整个流场涡旋基本控制在结晶器内，出结晶器后流速趋于平均，呈现活塞流流态。对应钢水射流区域的发展，水口下方由于钢液过热存在一个高温区。随着钢液在结晶器内腔的冲击深入和强烈对流，过热也逐渐扩散而消失。在铸坯凝固末端呈现锯齿形温度分布，伴随二冷段的延伸，这种非均匀性在二冷区逐渐消失。另外，在靠近熔池液面和窄面交接处发现了存在二次涡现象，该结果在同类研究工作中还不多见，其冶金特征和影响有待进一步深入研究。(2)与碳钢的CSP连铸过程相比，由于不锈钢液的粘度较大，在同拉速下不锈钢的CSP连铸过程中水口流出的流股要弱于碳钢，向上的速度相对变小，钢液表面波动减缓；由于不锈钢液的导热性较差，在同过热度条件下不锈钢的液相穴的深度要大于碳钢，铸坯内部的温降过程相对缓慢，难以形核，熔体中温度梯度大，因此，不锈钢的CSP铸坯组织中柱状晶区较为发达。(3)凝固坯壳厚度随拉速或过热度的增加而减小，随二次冷却水量的增加而增加，低拉速的情况下过热度对坯壳厚度影响较大。水口浸入深度对坯壳厚度的影响不大，但对整个流场的形态影响较为显著。在对坯壳厚度影响因素中，拉速是主要影响因素，其次是二冷水量的大小，过热度的影响较小。薄板坯连铸的结晶器出口处坯壳很薄，应尽量采取低过热度浇注。二冷水量与拉速要相匹配，在保证一定的结晶器出口坯壳厚度和不产生裂纹的情况下可适当提高拉速，并相应增加二冷水量。冶金长度随二冷水量的增加变短，随拉速的增加或过热度的增加变长，但过热度对冶金长度的影响较小，拉速对冶金长度的影响则是很明显的。(4)离线实验研究表明，不锈钢凝固组织和性能与冷却条件有密切关系。1Cr18Ni9Ti钢凝固组织随冷却速度的增加而细化，强度也随之升高，在冷速为10℃／s左右时，抗拉强度达到990MPa左右，延伸率达到33％。冷速继续增加，晶粒细化和强度提高趋缓。不锈钢的凝固形态比较复杂，在凝固和冷却过程中存在多个区间的脆性温度区。在高温脆性区铸坯表层受拉应力作用，内部受压应力作用；在中温脆性区正好相反。在CSP的不锈钢铸坯结构中，应主要以较细密的柱状晶构成，而较难有中心等轴晶区的出现。且由于CSP薄板坯的冷却强度远远大于传统的板坯，其原始铸态组织晶粒比传统板坯更细、更均匀，为最终成品板材组织的细化创造了条件。另外，因为拉坯速度快，减少了在脆性区间和γ＋δ两相区的停留时间，因此降低了不锈钢铸坯的裂纹产生的可能性。(5)运用定量金相和概率论知识，在对晶粒形状作出合理假设的基础上，针对CSP薄板坯成品板材凝固组织，提出了柱状晶粒三维尺寸的表征表达式，实测并计算表明，在CSP工艺下，SPA-H钢板材晶粒轴长平均尺寸为9μm左右，长度平均尺寸为12μm左右，各方向上晶粒表征尺寸以5～15μm为主，约占全部晶粒的90％。其成品板材性能优越，强度达到525MPa左右，延伸率接近33％。LCR工艺与非LCR工艺相比较，铸坯表面组织和内部组织有较明显的差异；在两种工艺下，铸坯和板材的性能都有一定差异。但因为液芯压下只有一道次且压下量较小，经过6道次热轧后两种工艺下的成品板组织和性能差别不明显。CSP工艺下铸坯的宏观偏析很少，主要是C、P元素偏析，偏析比在1.1以内，而经LCR工艺后偏析程度更小。磁粉探测铸坯有表面裂纹，裂纹较浅，数量很少；超声波检测铸坯中的各种缺陷和夹杂以中心对称分布，铸坯中心出现不规则坑状缺陷，主要是中心疏松；夹杂物以铝酸盐为主，钙处理后都呈球形，多数小于5μm，不会对钢材质量产生较大影响。可以预见通过合理采用LCR技术，能够改善不锈钢内部结晶组织，消除或者减少不锈钢在薄板坯连铸过程中出现的多数缺陷，最终提高不锈钢产品质量。上述实验和数值模拟研究结果表明，通过制定合理的连铸生产制度和相关工艺参数，在国内应用CSP工艺生产不锈钢是可行可为的。更多还原

【Abstract】 To produce stainless steel using CSP process may greatly increase the market competition ability of steel casting products. But in our country, the testing procedure and experienced data for CSP procedure were almost nothing. There was also little report about this in international journal. Based on the demand of techniques of the stainless steels production and the status of thin slab continuous casting development in this paper, according to numerical simulation with off-line experiment, the feasibility of production of stainless steels applying CSP process were first investigated.Firstly, on the basis of detailed derivation of the fluid flow and heat transfer coupled finite-difference turbulent equation, a three-dimensional coupled model has been formulated, using the commercial code ANSYS-CFX4 with secondary development, to first describe the fluid flow and heat transfer in the funnel mold and secondary cooling segment of CSP stainless steel casting process. In this model, the latent heat release is treated by the enthalpy method and the each effect of fluid flow and the convection heat transfer of molten steel is effectively considered. Traditionally, the effective thermal conductivity model is used to simulate the surface temperature distribution and solidification shell growth, but the effect of fluid flow on the convection heat transfer of molten steel is artificially considered with the effective conductivity coefficient, which is somewhat too simple to describe the flow and thermal field and the interaction of transport phenomena. Very little detailed information is available on solidification process of spray cooling zone and the effect of transport characteristics on the final solidification in new developed coupled fluid flow and heat transfer model. The model in this paper features are as follows: a) A large amount of data from the plant manufacture practice are used to establish and verify the model, which make the model more accuracy and trusty. b) Supposed to meet the demand of calculation, we simplify the condition with the minimum degree, for example: the complex geometrical surface of funnel shape mold is fit by the equations, and a analogous submerged entrance nozzle (SEN) structure with a two big exit ports design is using, and functionally heat flux in mould and functionally heat convection coefficient in secondary cooling segment are applied. c) The calculation domain of the model is extended to spray cooling zone of CSP casting. d) The effect of fluid flow in liquid and mushy zone on solidification process has been considered.Secondly, on condition that it is no capable of directly studying CSP stainless steel casting process, the solidification microstructure and properties of the stainless steels under variation cooling conditions are studied by the off-line experiment. The solidification and continuous casting characteristics of the stainless steels by CSP technology are investigated;Finally, a systemic comparative study has been made for the qualities of the SPA-H slabs and steel strips produced by LCR and Non-LCR in CSP. The microstructure, performance as well as the defects and inclusions during CSP are studied. The reasons for grain refinement and composition segregation in CSP process are discussed. In addition, the influences of CSP stainless steels product’s quality by LCR technology are analyzed.According to above these study work, for stainless steel manufacture by CSP technology, we can draw these conclusions:(1) The fluid field character in CSP mold is similar to the one in the conventional slab continuous caster mold, which forms two violent circumfluence fields in vertical direction. The distinction lies in that the lower recirculation zone is compressed, while the upper recirculation zone is expanded accordingly. These recirculation zones are all under control in the mold cavity, when leaving the mold exit, the steel fluid enveloped in the solidification shell has the character of piston flow. Temperature distribution is consistent with flow field. There exists a high temperature region due to the superheat of steel under the exit ports of nozzle. With the violent convection and the diffusion of turbulence, the superheat is removed in the impinging jet zone from the port of SEN to the narrow face of the mold. The serration shape of the isotherms occurs at the top of liquid core, the unevenness of solidification shell can disappear gradually at the solidification end. In addition, both in water model and in numerical model, the secondary vortex phenomenon are discovered at the juncture of the narrow face and meniscus level. It was also little report about this in such studies. The formation condition of the secondary vortex, metallurgy characteristic and the influences need further research.(2) Comparison with carbon steel CSP casting technology, with the same casting speed, the stainless steel strand flow of casting SEN was weaker than that of carbon steel due to the larger viscosity, which brings to a smaller upward flow speed and a slower fluctuating of melt surface; With the same superheat temperature, the slab liquid core temperature decreases slowly, and the liquid core length of stainless steel is longer than that of carbon steel due to the lower material thermal conductivity, which lead to a higher temperature gradient and difficultly nucleate. Therefore, columnar crystals zone were broader in CSP stainless steel slab. (3) The solidifying shell thickness is getting thinner with casting speed and superheat increasing. However, it is getting thicker with secondary cooling segment water amount increasing. The variety of SEN submergence depth is not obvious with solidifying shell thickness, while it is significant with flow field. The primary reason for the influenceable factor of solidifying shell thickness is casting speed. Cooling segment water amount is of secondary importance and superheat is something nonsignificant. Casting speed must match uo to cooling segment water amount. On condition that assure solidifying shell adequate thickness at mold exit and not bring out the crack of thin-slab, we can increase casting speed to some extent and increase secondary cooling segment water amount in proportion. In addition, increasing superheat makes casting process unhindered, but high inlet temperature also let the quality of the slab down. The length of liquid core is getting longer as casting speed increased or superheat increased or secondary cooling segment water amount decreased. The variety of superheat is no longer apparent, while casting speed has an obvious influence for the length on liquid core.(4) A close relationship exists between the solidification microstructure and properties of the stainless steels and cooling conditions. Refined solidification microstructures are obtained with increasing cooling rate in 1Cr18Ni9Ti steel, and so as the tensile strength. The respective tensile strength and elongation are 990MPa and 33% at a cooling rate of 10°C/s. Further increasing the cooling rate leads to a reduction in the extent of grain refinement and tensile strength. The solidified morphologies of stainless steels are relatively complex, which have several embrittlement temperature regions during cooling process. In high-temperature embrittlement region, the surface of the slab is under tensile stress and the center of the slab is under compressive stress, while the situation is just the opposite in moderate temperature embrittlement region. The structure of stainless steel gotten by CSP technics is mainly the slim columnar dendrite and rarely has center equiaxed dendrite. Because the cooling rate of CSP is much greater than the conventional slab continuous casting, the structure with CSP process is refined and distributed uniformly. This is good for the formation of fine structure of the final strip. Furthermore, when the casting speed is faster, the time in the embrittlemet temperature ranges andγ+δdouble phrase are decreased, and thus, the brittle of stainless steel is reduced. At the same time, the opportunity of formation of crack is decreased.(5) On the basis of assumption to crystal grain shape, the token expression of three-dimension size of columnar is established along with crystal grain in microstructure of the CSP finished strip according to quantitative metallurgical analysis and probability theory. And quantitative description about three-dimension growth feature of crystal grain in microstructure of the CSP finished strip is established. The average size of axis length and length are about 9μm and 12μm, respectively. The grain size ranges in every dimension is about 5-15μm, which is about 90% of the overall grains. The tensile strength and elongation of strip reaches about 525MPa and 33%, respectively. In comparison with Non-LCR process, the surface microstructure of the slab differs remarkably from the interiors in LCR process. Moreover, there are some differences in properties of slab and strip in these two processes. In the CSP production, the microstructure of steel strip consists of refined grains with excellent properties. Due to a single pass and small reduction, there is no significant difference in microstructures and properties between finished steel strips after six-pass hot rolling. In terms of the CSP process, few macro-segregations in slab, which are mainly carbon and phosphors segregations, are observed, whereas the extent of segregation in CSP process is much smaller. The result of Magnetic Powder Test (MPT) indicates that the a few shallow cracks appear on the surface of casting billets. Ultrasonic Test (UT) results show that the defects and inclusions are distributed centrosymmetrically, and loose are present in hole-shape in the center of slabs. Moreover, the inclusions mainly consist of spherical aluminates with majority of less than 5μm after calcium treatment, which has little influence on the steel quality. It is anticipated that if LCR technology could be reasonably adopting, it would improve the solidification structure, eliminate or decrease the forming defects in casting process; and finally improve CSP stainless steels product’s quality.According to above experimental and simulated results, it is showed that, with the reasonable continuous casting production institutions and relative process parameters, the application of CSP process is necessary and feasible for stainless steel productin of steel plane in our country.更多还原