【作者】 刘凯；

【导师】 程树森；

【作者基本信息】 北京科技大学 ， 冶金工程， 2023， 博士

【摘要】 S50C钢由于碳含量较高,易发生偏析。CSP(Compact Strip Production)工艺热履历独特,具有拉速高,冷却强度大,精轧前奥氏体晶粒粗大,热轧单道次变形量大,应变速率高的特点。本文基于酒钢CSP工艺,对S50C钢的元素偏析与组织演变进行了研究。酒钢CSP工艺下连铸过程未配备电磁搅拌设备,热轧前未配备粗轧机,板坯出均热炉后直接进入6架次精轧机组。研究发现,在钢种相同时,CSP工艺下生产的板坯凝固组织细密,元素分布均匀,具有较高的内部质量;与传统工艺相比,CSP工艺下的成品热轧板具有强度高,延伸率偏低的特点。酒钢利用CSP工艺生产的S50C钢成品热轧板作为冷轧基料时,出现了冷轧断带的现象。为了改善产品性能,本文对S50C钢在CSP工艺下的组织演变进行了研究,探索了 CSP工艺下S50C钢的元素偏析和组织演变的机理,在此基础上总结了 CSP工艺下S50C钢微观组织演变的规律,并对工艺进行了优化,最终解决了冷轧断带的问题。本文的主要研究内容和结果如下:首先,对酒钢CSP工艺下不同过热度和液芯压下量生产的S50C板坯进行了试验研究。低倍酸洗试验和金属原位分析仪(OPA-100)测试结果表明,酒钢CSP工艺下连铸过程取过热度为43℃,液芯压下量为20 mm时,板坯内部质量最高。根据S50C钢凝固特点,冶炼时C元素取上限,Mn元素取上限,Si元素取下限,S元素取下限,可以缩小包晶反应区,减少凝固收缩从而提高板坯内部质量。对S50C成品热轧板力学性能测试表明,4.0 mm、3.5 mm、3.0 mm厚度下的热轧板屈服强度在480～500 MPa,抗拉强度在720～800 MPa,均高于传统工艺下生产的成分相近的钢种,但是其延伸率仅有约20%。然后,对CSP工艺下S50C板坯的凝固组织和元素偏析规律进行了研究。CSP工艺下S50C钢板坯的凝固组织细密,一次枝晶间距(λp)和二次枝晶间距(λs)的范围分别为:80～240μm和30～110μm。由于CSP工艺下凝固组织细化,渗透率(Kp)降低,仅为0.1～2.3 μm2。将传热模型与凝固生长模型结合,建立了 CSP工艺下S50C钢种λp和λs的预测模型,并利用实测数据验证了模型的准确性,实现了微观组织演变与工艺参数的结合。在建立λs预测模型的基础上,推导了糊状区参数(Amush)的表达式。在此基础上,通过建立CSP工艺下S50C钢种在漏斗型结晶器内流动、传热、凝固物理数学模型,利用Fluent软件数值模拟了 CSP工艺下漏斗型结晶器内钢液的流动、传热与凝固现象。通过数值模拟,一方面对所建立的λs和Amush模型进行了验证,另一方面优化了模拟漏斗型结晶器内流动、传热与凝固现象的参数。确定Amush取值在108～109 kg·m-3·s-1之间时,能更准确的揭示漏斗型结晶器内的流动与凝固现象。利用优化后的Amush对结晶器出口处的坯壳厚度进行了预测,并与经验公式的计算值进行了对比,二者最大差值不超过1 mm,由此进一步验证了模型的准确性。元素偏析的研究发现,CSP工艺下元素偏析指数变化范围比较小。C元素中心偏析的形成主要是由于凝固收缩和鼓肚形成的负压,驱动枝晶间富集溶质的钢液流动造成的。对成分和鼓肚进行控制后,S50C板坯中C元素的偏析指数仅在1.0～1.1之间波动。利用超高温共聚焦显微镜(Ultra-High-Temperature Confocal Scanning Laser Microscope)对高温下S50C钢凝固组织的演变过程进行了原位观察,结合扫描电镜和Matlab图像处理揭示了高温下凝固组织的演变规律。研究发现,高温下S50C钢凝固组织的演变可以由枝晶间C元素的扩散来进行表征,其扩散过程可以通过Fick第二定律描述,由此建立了 C元素在枝晶间扩散的反问题数学模型。利用数值方法计算得到CSP工艺下S50C钢在1300℃下,C元素的综合扩散系数为:D=15 μm2/s。最后,为了解决CSP工艺下S50C成品热轧板作为冷轧基料时,出现的冷轧断带问题,对CSP工艺下,热轧S50C钢的过程中微观组织演变进行了研究。试验研究发现,酒钢CSP工艺下,S50C钢板坯热轧前的原始奥氏体尺寸在50～475 μm之间。EBSD的结果显示,成品热轧板的晶粒尺寸在3.6～4.8 μm之间,并且晶粒中有大量的小角度晶界,表明热轧过程中再结晶不充分,存在大量的亚结构。S50C热轧板(R1:优化前热轧板编号)力学性能检测表明,其抗拉强度为850 MPa,屈服强度为595 MPa,而延伸率仅为19.5%,在作为冷轧基料时,出现了断带现象。利用Gleeble-3500热模拟机,对不同变形条件下的组织演变进行了模拟试验。为了模拟CSP工艺下粗大奥氏体轧制的特点,利用超高温共聚焦显微镜对不同温度下奥氏体的演变进行了观察。结果表明,板坯的奥氏体化温度为1300℃,保温3 min时,基本达到了模拟CSP工艺粗大奥氏体轧制特点的要求。对热模拟试验下试样组织的分析表明,CSP工艺下S50C钢组织细化主要包括两个方面:一方面是在高温低应变速率下发生再结晶造成的。另一方面,由于CSP工艺热轧过程中,单道次的变形量大,应变速率高,在低温变形时,晶粒内部会储存较高的畸变能,使动态再结晶驱动力增加,晶粒发生动态再结晶,但是由于温度低,晶粒长大困难,变形明显,尺寸不均匀;因此,在进入层流冷却前,变形奥氏体晶粒内部存在大量的亚结构和缺陷,为后续的冷却相变提供了更多的形核点,使形核率大大提高,相变过程得到强化,从而使组织细化。基于以上结果,提出将卷曲温度提高至730℃的方法,来提高成品热轧板中铁素体含量和珠光体片层间距,从而改善其力学性能。参数优化后的热轧板(R2)组织中,铁素体含量提高至约25%,珠光片层间距增加至0.84μm;力学性能测试表明,R2抗拉强度降低至约700 MPa,屈服强度降低至约400 MPa,而延伸率升高,达到了约30%,热轧板的力学性能得到了优化,从而解决了冷轧断带的问题,实现了酒钢CSP工艺下S50C钢的稳定生产。更多还原

【Abstract】 As the relatively high carbon content in S50C steel,segregation usually tends to occur.The CSP(Compact Strip Production)process is characterized with high casting speed,high cooling strength,coarsened austenite grains before hot rolling,large deformation during a single hot rolling pass,and high strain rate.Compared with conventional strip production process,the extraordinary thermo-mechanical history of CSP is advantageous for the homogenization of chemical composition and microstructure.This work studies the element segregation and evolution of micro structure for S50C steel during the CSP process of Jiuquan Iron and Steel Corporation(JISCO).Under the CSP process of JISCO,the continuous casting process is not equipped with electromagnetic stirring equipment,and a rough rolling mill is not equipped before hot rolling.After the slab is discharged from the soaking furnace,it directly enters the 6 finishing rolling.It is found that the solidified structure of S50C steel by CSP is fine and the distribution of element is uniform indicating a high inner quality of the S50C slabs.However,the hot rolled strip of S50C steel produced by CSP exhibits high mechanical properties and low elongation leading to the breakage of cold rolled sheet when supplied with the hot rolled strip.To optimize the properties of hot rolled strip and avoid the breakage during cold rolling,studies of element segregation and microstructure evolution during CSP process for S50C steel are conducted in present work.Base on the study,the mechanism of element segregation and evolution of microstructure is explored.The main results and innovation are summarized as follows:Firstly,industrial research of S50C steel produced by CSP process under different superheat and mechanical reduction was carried out.The results of pickling test and analysis by OPA-100 show that to obtain a high inner quality of S50C slab,it is reasonable to control superheat of the S50C steel at 43℃ and reduction with 20mm during CSP process.In order to control the carbon segregation during solidification,chemical composition should be realized by taking the upper limit of carbon and manganese,the lower limit of silicon and sulphur according to the designed chemical composition.The mechanical property test of S50C hot rolled strip shows that the yield strength of the strip with the thickness of 4.0mm,3.5mm and 3.0mm is 480～500 MPa,and the tensile strength is 720～800 MPa,which are higher than the steel grades with similar composition produced under the conventional process,but its elongation is only about 20%.Secondly,the solidified structure and element segregation of S50C slab in CSP process were studied.The solidified structure of slabs produced by CSP process is fine,and the primary dendrite spacing(λp)and the secondary dendrite spacing(λs)range in 80～240μm and 30～110 μm,respectively.As the solidified structure is refined under CSP process,the permeability(Kp)is reduced,which is only 0.1～2.3 μm2。The predicted model for λp and λs under CSP process are developed by combining the heat transfer model with the solidification growth model and verified by measured data,which realized the combination of microstructure evolution and CSP process parameters.The mushy zone coefficient A mush was characterized based on the predicted model of λs,and by analyzing the solidification and fluid flow behavior in funnel-shaped mold zone with Fluent software,the predicted model was verified and the value of Amush was suggested in 108～109 kg·m-3·s-1.Through optimized Amush,the thickness of solidified shell was predicted and compared with the value calculated by the empirical formula,the difference between which is less than 1 mm,therefore the model was furtherly validated.The analysis of element segregation shows that,the distribution of elements in CSP process is relatively uniform on the whole.The formation of central segregation of element C is mainly caused by negative pressure formed by solidification shrinkage and bulging,which drives the flow of liquid steel with solute enriched between dendrites.After controlling the composition and bulging,the segregation index of carbon ranges only between 1.0～1.1.The evolution process of solidified structure of S50C steel produced by CSP at high-temperature was observed in situ by using the ultra-high-temperature confocal scanning laser microscope combined with SEM and Matlab picture processing.It is found that the evolution of solidification structure of S50C steel at high-temperature can be characterized by the diffusion of carbon in the interdendritic space,and its diffusion process can be described by Fick’s second law.With the help of SEM,Matlab processing and inverse problem method,the integrative diffusion coefficient of carbon in S50C steel produced by CSP during solidified structure evolution is determined with D=15 μm2/s by numerical calculation.Finally,aiming at settling the breakage problem during cold rolling,the microstructure evolution during hot rolling under CSP process was investigated.According to the experimental results,the original austenite size of S50C steel slab by CSP before hot rolling is 50～475 μm.The results of EBSD show that the grain size of hot-rolled strip is 3.6～4.8 μm,and there are a large number of low angle grain boundaries in the grains,indicating there are lots of defects and substructures in the deformed austenite grains.The mechanical property test of S50C by CSP hot rolled strip(R1)shows that its tensile strength is 850 MPa,yield strength is 595 MPa,and elongation is only 19.5%leading to the cold rolled sheet breakage.The microstructure evolution under different deformation conditions was simulated by Gleeble-3500 thermal simulator.In order to simulate the coarsened austenite rolling process in CSP,the evolution of austenite at different temperatures was observed by means of ultrahigh temperature confocal microscope.The results show that when austenitized at 1300℃ for 3 min,the size of austenite basically could meet the CSP simulating requirements.The analysis of sample micro structure under thermal simulation test shows that the refinement of coarsened austenite under CSP process is caused by austenite recrystallization and enhanced phase transformation:on the one hand,at high temperature and low strain rate,dynamic recrystallization occurs.On the other hand,at low temperature and high strain rate,due to the high distortion energy,the driving force of dynamic recrystallization increases,and the grains undergo dynamic recrystallization insufficiently.However,due to the low temperature,it is difficult for grains to grow,and the deformation is obvious,and the size is uneven.Besides,under low temperature and high strain rate,the internal substructure and defects of grains increase,which provides more nucleation sites resulting in enhancement of phase transformation during the subsequent cooling,so that the nucleation ratio is greatly improved and the structure is refined.Based on the above results,the parameters for hot rolling of R1 were optimized by adjusting the coiling temperature to 730℃(R2),and the results show that the ferrite content is increased to about 25%,and the adjacent cementite is increased to 0.84 μm.Correspondingly,the mechanical property test of R2 indicates that the tensile strength and yield strength reduced to700 MPa and 400 MPa,respectively,and the elongation is increased to about 30%.As the mechanical properties of the hot rolled strip are optimized,the cold rolled sheet breakage were avoided effectively.更多还原