Longitudinal crack on slab surface at straightening stage during continuous casting using finite element method

Deformation behavior of slab at the straightening stage during continuous casting was simulated by the explicit dynamic finite element method,and the stress distribution along the width direction of the slab and its change regularity at slab center during continuous casting were obtained.The influence of distribution and change of stress on the propagation of longitudinal cracks on slab surface was discussed.The results show that the tensional stress appears on slab surface at the inner arc side and the compressive stress appears on slab surface at the outer arc side at stages 6-8 in straightening zone during continuous casting.Longitudinal cracks generally appear on slab top surface and do not appear on slab bottom surface,which are also observed in industry.

Strain in solidifying shell of continuous casting slabs

Two-dimension unsteady heat transfer model was applied to obtain the surfacetemperature and the shell thickness of continuous casting slabs during the process ofsolidification. On the basis of which, the mathematical model of strain at the interface of solidand liquid steel was set up. Through which, the strain in the solidifying shell under normal andabnormal operation conditions was gained. The results indicate that the strain is small under thenormal operation conditions and the internal crack never happens. However, when the variation of theroll gap is above 2 mm, the strain caused by which is greater than that caused by bulging.Furthermore, the total strain exceeds the critical one and the internal crack is the result. So itis of great importance to maintain the fine state of continuous casting machine to avoid theappearance of internal crack.

Evaluation of the performance of mold powder for continuous casting of plate slabs

The No. 3 slab caster,which mainly provides slabs to the 5000 mm plate mill at Baosteel, was put into production in December,2004. The size of the biggest slab produced by this caster is 2300 mm in width and 300 mm in thickness. The designed output of the caster is 2.3 Mt/a. Slab surface longitudinal crack defects,which were related to the heat flux of the mold, frequently occurred in the early stage of the startup of the caster. As mild cooling powder is beneficial to the uniformity of the shell of initial slabs ,the concentration of stress is reduced, and the longitudinal cracking on the surface is avoided. This study evaluates the performance of several kinds of powder, and the results show that mold powder of high basicity, high crystallization proportion and low heat flux is to the benefit of the reduction of the longitudinal cracks on the surface and the defects of slabs.

Preventing Occurrence of Transverse Corner Cracks on Nb, V and Ti Microalloying Steel CC Slabs

Based on their hot ductilities, Nb, V and Ti microalloying steels can be classified into two groups. The first group includes steels with lower carbon content (≤0.10%). Ductilities of steels of this group recover and rise with decreasing temperature when temperature lowers to below 825℃. Another group includes steels which contain more carbon (>0.12%) or contain more Nb and V. The low ductility temperature Region Ⅲ for steels of mis group extends to temperature as low as 725℃. The occurrence of the transverse corner cracks of the Nb, V and Ti microalloying steel CC slabs has be considerably decreased by stabilizing casting speed, increasing mold steel level automatic control ratio,enhancing caster segment radial alignment and adopting proper secondary cooling patterns to make slab corner temperature at straightening out off the low ductility temperature region.

A Model for Diagnosing the Formation of Internal Cracks in Continuously CastSlabs

The internal cracks in continuously cast slabs are attributed to the excessive tensile strain occurring at the solidifying frontduring the continuous casting process. Based on the understanding, a model for diagnosing the formation of the internal cracks was established, in which the strains at the solidifying front caused by' bulging, straightening or unbending, and roll misalignment were calculated and compared with a critical strain value to estimate whether the internal cracks form. Moreover, the established model was appliedto a real slab caster to reveal the distribution of the strains in casting direction and its effect on the internal cracks. It was proved that themodel was reliable and useful for optimizing the operation of continuous casting.

Formation mechanism of internal cracks in continuously cast slab

In order to make clear the formation mechanism of centerline cracks incontinuously cast slabs, the form, distribution and other characteristics of the cracks wereanalyzed. The final solidification point, surface temperature of the slabs and strain in solidifyingshell were investigated. The results were that: (1) Five relatively low temperature zones exist onslab surface below the three water spraying nozzles and near the two edges, respectively, whichcorresponds to the places of centerline cracks and triangle-zone cracks. (2) Centerline cracks andtriangle-zone cracks occur because of weak secondary cooling, uneven cooling along slab width, andlarge variation of roll gap. (3) After minimizing the variation of roll gap and applying the newsecondary cooling pattern, the occurring frequency of centerline and triangle-zone cracks minimizesto zero.

Correlation between longitudinal crack occurrence and mold heat transfer in continuous casting based on process data mining

The correlation between the longitudinal crack occurrence and integrated heat transfer of the mold with data mining methods was investigated.Firstly,three kinds of support vector machine models based on principal component analysis with different input features were established to explore the effect of integrated heat transfer on the accuracy of the prediction model for the longitudinal crack.The results show that the accuracy was improved while features including mean and standard deviation of integrated heat transfer were added.Then,the difference in integrated heat transfer between defect and normal samples under the same process parameters was quantitatively compared.Compared with normal samples,the temperature difference of cooling water for defect samples decreased by 0.65%,and the temperature difference fluctuation increased by 31.1%.Finally,the literature data were used to provide support for the quantitative correlation according to defect formation mechanism.A new criterion for the prediction of longitudinal crack and a discovering method for correlation between product quality and process parameters in the manufacturing industry have been provided.

Effect of Slab Subsurface Microstructure Evolution on Transverse Cracking of Microalloyed Steel during Continuous Casting

As to the continuous casting process of low carbon microalloyed steel, subsurface microstructure evolution plays an important role in the slab surface cracking. In order to study the effect of the slab subsurface microstructure evolution on the transverse cracking, three different secondary cooling patterns （i. e. , mild cooling, strong cooling and controlled cooling） were performed in the corresponding slab curved continuous caster. Based on the metallo- graphic results, three transformation regions were found to be formed with the evolution of microstructures at different depths in the slab subsurface. The three regions are strong cooling transformation （SCT） region, double phase transformation （DPT） region and mild cooling transformation （MCT） region, respectively. Meanwhile, it was also found that the crack index used for evaluating slab surface cracking susceptibility was decreased when the range of the DPT region was increased. This can be explained by the fact that the double phase transformation （austenite-ferrite-austenite） occurred resulting from thermal cycling in DPT region, which resulted in promoting the refinement of prior austenite grains and inhibiting the precipitation of film-like ferrite and chain-like precipitates. Under the con- trolled cooling pattern, the widely-distributed DPT region was formed in the range of 3.5--8.0 mm to the slab surface. And compared with other cooling patterns, the cracking susceptibility is lowest with a crack index of 0.4.

The optimization of mold fluxes used for casting the peritectic steel of heavy slabs at Baosteel

In order to solve the problem of the high surface longitudinal crack ratio of heavy peritectic steel slabs produced by the No. 3 continuous caster at Baosteel,the physical properties of the original mold flux and the optimized mold flux were compared in a comprehensive way by using analytical measures, such as a slag film heat-flow simulator, a thermowire molten flux crystallization tester and an X-ray diffractometer in the laboratory. The results reveal that one of the major reasons for the cracks is the poor heat transfer ability of the original mold flux. However, the optimized mold flux with a high basicity features a high crystallizing rate,low crystallization temperature and low heat-flow density. Therefore, the optimized mold flux is more suitable for casting peritectic steel by the heavy slab continuous caster. The test results show that the slabs produced by using the optimized mold flux had no surface longitudinal crack in four test casts, while the surface longitudinal crack ratio of the slabs produced by using the original mold flux was 5%. The optimized mold flux can effectively prevent slab surface longitudinal cracks from occurring.

Thermo-elasto-visco-plastic finite element analysis on formation and propagation of off-corner subsurface cracks in bloom continuous casting

The formation and propagation of the popular off-corner subsurface cracks in bloom continuous casting were investigated through thermo-mechanical analysis using three coupled thermo-mechanical models. A two-dimensional thermo-elasto-visco-plastic finite element model was developed to predict the mould gap evolution, temperature profiles and deformation behavior of the solidified shell in the mould region. Then, a three-dimensional model was adopted to calculate the shell growth, tempera- ture history and the development of stresses and strains of the shell in the following secondary cooling zones. Finally, another three-dimensional model was used to analyze the stress distributions in the straightening region, The results showed that the off-corner cracks in the shell originated from the mould owing to the tensile strain developed in the crack sensitive regions of the solidification front, and they could be driven deeper by the possible severe surface temperature rebound and the extensive tensile stress in the secondary cooling zone, especially upon the straightening operation of the bloom casting. It is revealed that more homogenous shell temperature and thickness can be obtained through optimization of mould corner radius, casting speed and secondary cooling scheme, which help to decrease stress and strain concentration and therefore prevent the initiation of the cracks.

Technology Development for Controlling Slab Transverse Corner Crack of Typical Micro-alloyed Steels

A mathematical model for simulating the fluid flow, heat transfer and solidification in the conventional mold and the chamfer mold, together with a finite element stress-strain model in the straightening process of both molds, were established for the typical niobium, vanadium, and titanium micro-alloyed steels. On the basis of both numerical analysis, the mold copper plate with an optimum chamfered shape was designed and applied in industrial tests. The predicted results from numerical simulation of fluid flow, heat transfer and solidification in the conven tional mold and the chamfer mold show that the increased chamfered angle leads to an approximately linear increase o[ the slab surface temperature, but it also causes strong flow near the slab corner. Very small chamfered length can lead to a significant increase of the temperature near the slab corner. However, with further increasing the chamfered length, the temperature of the slab corner increased slightly. The calculated results from the finite element analysis of stress-strain during the straightening process show that at the same slope width, the tangential strain on the slat） edges and corners is minimum when the chamfered angle is 30° and 45°, which is only 40° to 46° of rectangular slabs with the same cross-section area. At the same chamfered angle of 30°, when the chamfered length is controlled between 65-85 mm, the tangential strain on the part of the slab edges and corners is relatively smaller. Industrial test results show that the slab corner temperature at straightening segment increases about 100 ℃ by using chamfer mold compared to the conventional molds. The slab transverse corner cracks have been reduced more than 95° in comparison with those in the conventional mold.

Analysis on Surface Longitudinal Cracking of Q345D Slab

The surface longitudinal cracking of peritectic steel Q345 D slab was studied by means of metallographic microscopy and scanning electron microscopy. The results show that the surface longitudinal cracking is caused by the shell repeatedly tearing and healing under the action of stress, which is due to disruption of the steel matrix continuity with inclusion aggregation on the shell surface. The relative precautions of reducing the cracking are raised according to the practical situation.

氮元素对高钛含铌钢裂纹影响的定量研究

应用试验、热力学计算及统计,定量研究了氮(N)对高钛(Ti)含铌(Nb)钢连铸横裂纹的影响,提出N控定量标准。结果表明:高钛含铌钢的高温塑性与氮化物形成具有强相关性;凝固末期TiN的析出影响第一脆性区,第二脆性区与Nb(C,N)的析出有关,温度区间为从Nb(C,N)析出到其析出速率开始缓和的温度,第三脆性区与铁素体或AlN析出有关,温度区间为从铁素体或AlN析出到渗碳体开始析出的温度。高钛含铌钢的N控瓶颈在于TiN在凝固末期的析出,根据Ti、N积建立控N标准,Ti、N积为4×10-5~7×10-5(%•%)。

Real-Time Slab Quality Diagnosis and Analysis System Based on Solidification Mechanism

According to the two types of slab defects （surface crack and internal crack）, a real-time slab quality diag nosis and analysis system named CISDI_SQDS ONLINE R2011 was developed based on heat transfer and solidifica tion simulation, stress and strain calculation as well as metallurgical process analysis about continuous casting, com- bined with expert knowledge database for specific casting events. The respective crack formation indexes were intro duced as the theoretical reference data for slab quality level evaluation. Meanwhile, both the quality loss factors relat ed to casting metallurgical rules and the adjusting factors related to casting events were taken into full consideration, which was integrated organically with theoretical analysis and expert system. The system can be widely used as an important theoretical tool for prediction and control of slab quality in slab continuous casting process.

14Mn5Nb无缝钢管“鱼鳞”形外折分析与控制

某厂14Mn5Nb连铸圆坯(Φ330 mm)轧制成无缝钢管(Φ323.8 mm×8.3 mm)后表面出现大量“鱼鳞”状外折缺陷,影响成材率,成为亟须解决的问题。采用缺陷金相分析,“刻槽”试验,铸坯酸洗确定钢管表面“鱼鳞状”缺陷是由连铸圆坯表面横裂纹造成。采用扫描电镜检测,钢种热溯性分析以及现场数据统计,确定铸坯表面横裂纹产生原因为:Φ330 mm圆坯在当前工艺参数下铸坯进拉矫温度在900~950℃,处于第III脆性区转变温度940℃附近;表面存在较多Nb(C、N)析出物,增加了钢种的裂纹敏感性。通过调整钢种Ti含量至0.02%~0.03%,保护渣碱度由0.97提高至1.14,降低结晶器水量,调整二冷水量配比以确保进拉矫温度≥970℃等措施,有效避免了铸坯横裂纹和无缝钢管鱼鳞外折的产生。

冷轧双相钢DP590的连铸坯角部横裂纹研究

冷轧双相钢DP590连铸坯角部横裂纹给冷轧产品的质量带来很大困扰,导致其后续连退产品边部出现夹渣、翘皮或裂纹等缺陷。通过对连铸坯角部横裂纹部位进行观察,发现角部横裂纹发生在连铸坯的振谷部位,横裂纹向铸坯宽面和窄面有一定的延伸,发生横裂纹的振谷通常是不规则振痕,其形貌带有横向的凹陷特征。通过连铸坯的枝晶侵蚀试验发现,连铸坯发生角部横裂纹的部位,二次枝晶间距大,裂纹处的连铸坯表面冷却速率显著低于正常的连铸坯表面,这说明连铸坯坯壳冷却均匀性需要改善。并且裂纹附近有明显异常粗大的枝晶间距,通过电子探针检测发现这些异常粗大枝晶间存在硫、锰的富集现象,说明这是凝固过程中枝晶间发生断裂,富集锰硫溶质的钢液补充形成的。结合浇铸的结晶器热电偶测量温度趋势,判断该型保护渣控制传热能力以及渣膜的稳定性不足。通过优化保护渣和提高铸坯拉速,消除了连铸坯凹陷和角部横裂纹。

180 mm×180 mm AH32钢铸坯振痕特征对其角部横裂的影响

AH32钢连铸坯常用于制作球扁钢。某尺寸为180 mm×180 mm的AH32钢铸坯角部出现横向裂纹。为此,采用热酸洗、图像处理、扫描电子显微镜等研究了铸坯角部和中部振痕的不均匀度和深度对其角部横裂纹发生的影响。结果表明:铸坯角部振痕不均匀度小于14%时,角部横裂发生率均值为0.75%,大于14%时角部横裂发生率明显增大;铸坯中部振痕不均匀度小于44%时,角部横裂发生率小于0.50%,且铸坯角部振痕深度控制在500μm以内,角部横裂发生率能保持在0.25%以下,满足质量要求。

Q355B宽带钢表面开裂原因

某Q355B钢边部表面发生开裂现象。采用宏观观察、化学成分分析、金相检验、扫描电镜及能谱分析等方法分析了裂纹产生的原因。结果表明:钢板表面裂纹产生于连铸工艺环节,裂纹附近存在砷、铜等有害元素富集现象;砷、铜等有害元素形成了低熔点的富集相,使钢板的塑性变差,破坏了钢基体连续性,在拉应力的作用下,钢板表面发生开裂现象。

高强焊丝钢N2M2T连铸坯表面缺陷分析及工艺改进

针对某钢厂在冶炼N2M2T高强焊丝钢时,连铸坯表面出现的凹陷、角部深振痕裂纹及渣坑等表面缺陷,结合理论分析及现场实际生产情况,从保护渣理化性能、结晶器振动参数、一冷配水及钢中N含量等对铸坯表面产生的缺陷进行分析。结果表明,连铸坯产生的表面缺陷主要与保护渣理化指标、结晶器振动参数、钢中N含量有关,而与一冷配水关系不大。生产实践表明,通过将保护渣碱度从0.6提高到1.19,w[C]从13.51%降低到8.21%,粘度从0.666 Pa.s降低到0.156 Pa.s,结晶器振动参数偏斜率由0.15提高至0.20,振频公式由60+50 V调整为90+60V,钢中N含量控制在<50×10^(-6),连铸工序△N<7×10^(-6)等措施,N2M2T高强焊丝钢连铸坯的表面缺陷得到明显改善,铸坯合格率从85%提高到95%以上,为后续轧钢的稳定轧制提供了保证。

连铸板坯侧面横向凹陷的控制

Al含量较高的包晶亚包晶钢种具有最高的初生坯壳表面凹陷倾向。凹陷处传热效率下降,加剧了奥氏体晶粒粗化,同时使坯壳变得薄弱,增大了板坯表面横向凹陷裂纹的敏感性。针对河钢集团唐钢公司生产的连铸板坯侧面存在的横向凹陷微裂纹缺陷,分析其是由弯月面区域铸坯局部过冷、钢液溢流、初生坯壳凝固不均产生热应力以及弯月面附近渣圈和钢水静压力共同作用产生的。通过保持合理的结晶器倒锥度、优化结晶器和二次冷却均匀性及结晶器振动工艺,加强结晶器足辊段的支撑和冷却、合理控制结晶器内钢液流动、提高结晶器保护渣的黏度与液渣层厚度,保证结晶器保护渣黏度与铸坯拉速的乘积为0.15~0.35 Pa·s·m/min、保护渣液渣厚度10 mm以上,以及优化配炭工艺,连铸板坯侧面横向凹陷微裂纹缺陷明显减轻。通过批量试验并在大生产应用,连铸板坯侧面横向凹陷微裂纹得到良好控制。