800 Mpa级含Ti高强钢铸坯断裂的原因

通过高温热塑性试验,借助光学显微镜、透射电镜对某钢厂生产的 800 MPa 级高强钢铸坯开裂原因进行了研究,并提出了改进措施.研究结果表明:铸坯开裂形式为"角部开裂";其主要原因是连铸后铸坯缓冷不充分,造成铸坯角部与其内部温度差异较大,在组织变化和热应力的双重作用下导致裂纹萌生,并进一步扩展为"角部开裂".连铸后采用扣罩缓冷措施可解决铸坯开裂问题.     

Effect of Cooling Rate on the Precipitation Behavior of Carbonitride in Microalloyed Steel Slab

In the continuous casting of the microalloyed steel, the slab surface transversal cracking could be prevented through the control of the slab surface microstructure, which correlates with the precipitation behavior of carbonitrides in the microalloyed steel. Therefore, the cooling rate is the key factor to determine the precipitation behavior of carbonitrides. This article used confocal laser scanning microscopy to study the effect of different cooling rates on the precipitation behavior of the carbonitrides in the microalloyed steel slab. When the cooling rate is less than 3 K·s⁻¹, the precipitates in the steel are coarse, growing out along the austenite grain boundaries, and form a chain-like distribution. These precipitates seriously reduced the hot ductility of slab. Quantitative study between the cooling rate and the precipitation behavior of carbonitrides in microalloyed steel also has been developed. The results of this study could be used to improve the understanding of the slab surface microstructure controlling to enhance the hot ductility of the slab and avoid the surface crack of the slab.     

CSP连轧过程中低碳钢的组织变化规律

对珠钢CSP生产现场同一低碳钢轧件的铸坯及不同道次变形后室温组织的研究表明：铸坯组织由细晶区（急冷层）和树枝晶区组成，随轧制道次增加，变形后轧件的室温组织细化，沿铁素体晶界分布的珠光体变得均匀，弥散，连轧前铸坯表面和心部的组织差异随着轧制道次增加逐渐减小，珠钢成品板组织细化的原因可以归结为大压下连轧工艺，钢中大量弥散析出的氧化物，硫化物和终轧后的层流冷却。     

基于表面粗糙度对连铸钢碳当量计算方法的判断

 碳当量是连铸生产实践中分析钢种表面纵裂纹敏感性最常用的方法。为了正确判断连铸过程中钢的表面纵裂纹敏感性,基于表面粗糙度试验对比分析了合金成分和冷却速率对钢的碳当量计算结果的影响。结果表明,合金成分和冷却速率的变化改变了钢种纵裂纹敏感时的碳当量范围。根据凝固过程中冷却速率对钢种纵裂纹敏感性的影响,碳当量计算方法可分为3类,第一类适用于平衡状态下分析碳当量的Yasumoto、BSSTC和Howe公式;第二类适用于传统连铸生产时分析碳当量的Wolf、Trico、Xia和Presoly公式,其中Presoly公式的结果最为可靠;第三类适用于薄板坯连铸生产时分析碳当量的SMS公式。     

冷却速度对含铌、钛微合金钢铸坯表层组织结构的影响

铸坯出结晶器后的冷却速度是影响和决定铸坯表层组织结构(0~5 mm)及第2相析出物分布的关键因素.在开发重熔凝固冷却实验装置的基础上,模拟铸坯在垂直段的凝固冷却条件,研究不同冷却速度对铸坯表层组织与第2相析出物分布的影响.研究表明,在0.8 m·min^-1拉速下,以5℃.s-1的冷却速度,使铸坯在出结晶器后表面温度下降到A₃温度以下,得到的表层组织均匀,晶界无膜状先共析铁素体,微合金元素析出物在晶内分布均匀,有利于提高铸坯热塑性及降低裂纹敏感性.     

Evolution of Surface Transversal Crack in Intermediate Slabs during Hot Rolling Deformation

In the continuous casting of steels,surface transversal cracks are often found.These defects may severely influence the final quality of the products.The evolution of transversal V-shaped cracks with different depth on the surface of a continuously cast steel slab during hot rolling was studied.The artificial V-shaped cracks were made on the surface.The rolling process parameters from an industrial rolling mill have been used as a reference.The specimens of rolled workpiece in intermediate slabs were obtained after different rolling passes.The morphology of surface crack and microstructure evolution in the rolling process were investigated by optical microscopy.The results show that the depth of surface transversal crack gradually decreased with the increase of rolling passes.The grain size of ferrite and pearlite on the sample surface also gradually reduced.The microstructures around cracks with the different depth are almost identical,without direct correlation with the initial crack depth.     

Surface microstructure control of microalloyed steel during slab casting

Lots of work has been done to investigate slab surface microstructure evolution during continuous casting in order to improve hot ductility and avoid transverse cracks.The slab surface microstructure after continuous casting was characterized by optical microscopy, and the precipitation behavior was investigated by transmission electron microscopy.At the same time, the mechanical properties of the slabs were measured using a Gleeble 1500D thermal simulator and the transformation temperatures were examined by means of a thermal dilatometer.The experimental results show that homogeneous microstructure without film-like ferrites and chain-like precipitates at grain boundary can be obtained through surface intensive cooling and transverse cracks do not occur on the slab surface.For the ex-perimental steel, fine ferrite can form at slab surface when the water flow rate is larger than 1560 L/min at vertical section.As the distance to surface increases, microstructure turned to ferrite and pearlite. Moreover, nano-size carbonitrides precipitated in the ferrite grain and the size was larger at the junc-tion of the dislocations.The mechanical experiment results show that the hot ductility of the sample deformed at 650℃ was better than that of the sample deformed at 750℃.The reason is that film-like ferrite formed at the grain boundary in the sample deformed at 750℃.Thus, the slab must be cooled quickly below A r3 to prevent the occurrence of film-like ferrite and transverse cracks on the slab surface during casting.     

微合金钢连铸坯角部裂纹控制技术研发及应用

微合金钢连铸过程频发铸坯角部裂纹缺陷是钢铁行业的共性技术难题。基于微合金钢铸坯角部裂纹组织结构与析出特征检测，以及铸坯在结晶器与二冷铸流内的凝固热/力学行为演变规律定量化模拟，开发形成了基于新型角部高效传热曲面结晶器和铸坯二冷高温区角部晶粒超细化控冷工艺与装备的微合金钢连铸坯角部裂纹控制技术。研究结果表明，传统板坯连铸工艺下，窄面直线型结晶器无法充分补偿坯壳收缩，致使厚保护渣膜与气隙在坯壳角部集中生成，大幅降低了结晶器中下部坯壳角部传热，引发微合金碳氮化物沿奥氏体晶界析出。传统二冷配水条件下，奥氏体晶界不可避免生成先共析铁素体膜低塑性组织。两者共同作用致使铸坯角部高温塑性不足而引发裂纹。通过开发新型曲面结晶器，坯壳角部于其内高效传热，凝固全程冷却速度大于5℃/s，弥散化了微合金碳氮化物高温析出。同时，基于窄面足辊超强冷新控冷结构，对铸坯角部实施γ→α→γ循环相变，铸坯角部晶粒显著超细化，高塑化控制了铸坯角部裂纹产生。     

唐钢薄板坯连铸高碳钢65Mn的质量控制

针对唐钢薄板坯连铸连轧线生产高碳钢65Mn出现的带钢表面翘皮和铸坯内部偏析问题,分析了缺陷产生的原因机理。带钢表面翘皮为铸坯边部在矫直过程中形成角裂轧制而成,铸坯内部质量问题主要影响因素为连铸二冷强度、软压下终点位置和钢中硫质量分数。通过调整LF脱硫工艺、优化连铸保护渣、提高二冷水强度、调整软压下终点等措施,有效控制了高碳钢65Mn带钢表面翘皮缺陷和铸坯内部偏析。     

微合金钢连铸坯表面裂纹敏感性预测模型

为了从凝固及相变特性角度解决微合金钢连铸坯表面裂纹问题,建立了与合金化相关联的初始凝固包晶反应度模型、奥氏体晶粒长大模型、铁素体转变量模型以及碳氮化物的析出模型。结合铸坯实际冷却条件,进一步建立了包晶反应度预测、初生奥氏体晶粒长大、铁素体转变、析出相析出等对铸坯表面裂纹敏感性的预测模型。针对某J55钢连铸板坯,奥氏体晶粒尺寸超过1 mm、铁素体析出量为10%、二次相析出量增加时,横裂纹敏感性最大。表面裂纹敏感性预测模型有助于实现基于成分微调和组织调控的微合金钢连铸、热装等生产过程表面裂纹控制技术。     

980 MPa级冷轧超高强度双相钢边裂原因分析与工艺优化

汽车用超高强度双相钢CR550/980DP冷轧边裂问题,严重影响热轧/冷轧工序界面生产顺行,易造成冷轧机架间及连退炉内断带事故,成为超高强度双相钢生产的难题。基于高温热塑性曲线和热轧动态CCT曲线,采用对显微组织、力学性能、裂纹扩展分析等手段明确冷轧边裂产生原因。试验结果分别指出,精轧阶段带钢横向温度分布不均匀、边部温降大,导致在第Ⅲ脆性区轧制;同时,受Nb作用再结晶温度提高,边部低温区为未再结晶区轧制;当应变量超过塑性极限、轧制力超过边部热强度时,形成热轧卷边裂。边部形成细小弥散的铁素体(F)和马氏体(M)两相组织,不协调应变将导致F/M相界面产生应力集中而形成裂纹;裂纹以微孔聚集方式进行扩展,形成热轧卷无边裂-冷轧边裂现象。通过投用边部加热器和优化初轧定宽量、精轧入口温度、精轧机架间冷却水、终轧温度、卷取温度等措施,实现热轧卷边部质量改善、解决边裂问题。     

980 MPa级冷轧超高强度双相钢边裂原因分析与工艺优化

汽车用超高强度双相钢CR550/980DP冷轧边裂问题,严重影响热轧/冷轧工序界面生产顺行,易造成冷轧机架间及连退炉内断带事故,成为超高强度双相钢生产的难题。基于高温热塑性曲线和热轧动态CCT曲线,采用对显微组织、力学性能、裂纹扩展分析等手段明确冷轧边裂产生原因。试验结果分别指出,精轧阶段带钢横向温度分布不均匀、边部温降大,导致在第Ⅲ脆性区轧制;同时,受Nb作用再结晶温度提高,边部低温区为未再结晶区轧制;当应变量超过塑性极限、轧制力超过边部热强度时,形成热轧卷边裂。边部形成细小弥散的铁素体(F)和马氏体(M)两相组织,不协调应变将导致F/M相界面产生应力集中而形成裂纹;裂纹以微孔聚集方式进行扩展,形成热轧卷无边裂-冷轧边裂现象。通过投用边部加热器和优化初轧定宽量、精轧入口温度、精轧机架间冷却水、终轧温度、卷取温度等措施,实现热轧卷边部质量改善、解决边裂问题。     

Formation of Edge Crack in 1.4%Si Non-oriented Electrical Steel during Hot Rolling简

Microstructure evolution, dynamic recrystallization, high temperature oxidation and hot ductility of 1.4 % Si non-oriented electrical steel sheets were investigated to reduce edge cracking. The causes of cracking were found to be coarse as-cast microstructure, grain boundary oxidation in reheating furnace, lack of dynamic recrystallization during hot rolling and increase of temperature, resulting in reduced hot ductility in strip edge region. Countermeas- ures against the edge crack are proposed accordingly. Lowering reheating temperature and reducing holding time re- duced oxidation and decarburization. Hot charging temperature was increased to decrease reheating temperature. And using an edger can refine microstructure in strip edge region. Finally, edge heater can be added to increase edge re- gion formability by inducing dynamic reerystallization and ductility by increasing temperature.     

连铸板坯中碳钢开浇用发热保护渣

连铸板坯开浇时的浇铸状况恶劣,开浇炉的铸坯表面缺陷发生率较高。使铸坯在结晶器内缓冷是防止铸坯纵裂的有效措施。针对通常使用的低碱度、低结晶温度的发热型开浇渣不能使铸坯在开浇阶段得到缓冷的缺点,开发了连铸板坯中碳钢开浇用发热保护渣,除进一步改善发热性能外,高碱度、高结晶率的开浇渣所形成的渣膜增大了结晶器与铸坯问的热阻,改善了铸坯在结晶器内的缓冷条件,开浇炉的第1、2块板坯的纵裂发生率明显下降。     

宽厚板典型表面缺陷及其控制

分析了包钢宽厚板出现的钢板星裂、表面横裂纹、边直裂纹等缺陷产生的原因。通过将微合金钢w(N)控制在40×10-6以下,动态调节连铸过程冷却水量以保证铸坯矫直温度保持在塑性区域范围内,合理选取连铸过程中的保护渣,提高铸坯出加热炉温度,减少轧机辊道冷却水对钢板下表面温降影响,减少上下表面温差以减轻由轧制过程中上下表面金属流动性差异造成的折叠程度等措施,钢板星裂、表面横裂纹、边直裂纹等缺陷得到了有效控制,提高了产线热装热送率及钢板的成材率。     

优化10PCuRE炼钢工艺消除铸坯表面纵裂纹

10PCuRE钢是一种C含量较低、P含量很高的稀土钢种,浇铸时难度大,板坯容易产生裂纹和夹杂等缺陷。针对10PCuRE连铸坯表面纵裂缝问题,对原10PCuRE钢冶炼工艺进行了优化,加强了出钢过程的钢液脱氧,并用合成渣技术和强化吹氩搅拌,以改善钢包中的渣—金反应;结晶器中使用专用稀土保护渣以在浇铸过程中有效地防止钢液的二次氧化,较好地润滑初始凝固坯壳和结晶器内模壁,降低表面热裂纹的倾向。应用优化后的冶炼工艺后,钢中氧含量和硫含量得到控制,稀土利用率提高,连铸坯表面纵裂缝消失,铸坯角部裂纹和内部裂纹问题大大改善,并且成品钢成分、晶粒组织、夹杂物的级别均得到了很好的改善。     

中厚板表面片状开裂原因分析及控制

针对中厚板表面片状开裂缺陷,采用光学显微镜及扫描电镜等设备对缺陷进行分析验证。结果表明,开裂的原因是扇形段辊子上铜制的轴承保持架破损压入铸坯,在轧制过程中造成开裂并扩散;而轴承破损主要是由于高温环境下设备冷却及润滑不良,通过更改轴承材质、改善轴承润滑油路、冷却水路等措施,片状开裂缺陷得到有效控制。     

Improvement of surface quality on peritectic steel slabs

Peritectic steel grades are very sensitive to microcracking along austenitic grain boundaries in continuous casting. Irsid and Aktiengesellschaft der Dillinger Hüttenwerke (DH) have combined laboratory studies and industrial trials to improve surface quality on these sensitive grades. Laboratory studies at Irsid confirmed the hypothesis that a very thin layer of ferrite along austenitic grain boundaries is detrimental for cracking and indicate that the risk of cracking decreases as soon as ferrite ratio is above 10 %. Dilatometric investigations demonstrate that there is a strong shift between thermodynamic equilibrium and beginning of γ→α phase transformation under casting conditions. Furthermore, at the slab surface, there is no cyclic transformation γ→α→γ induced by thermal cycling in front of spray nozzles and supporting rolls. DH performed trials with various cooling strategies on its new vertical caster No. 5. No cracks appear with intensive cooling whereas microcracks are present with soft cooling. These results are in agreement with laboratory studies. Intensive cooling is the standard condition at DH. With this process, microcracking is avoided for all slab formats.     

Q345GJC钢板裂纹原因分析及控制

取样分析了高层建筑结构钢Q345GJC钢板的裂纹原因,结果表明,Q345GJC钢板上的裂纹是由于铸坯上产生的皮下裂纹所致,而且微合金化元素添加量及种类越多,连铸坯的裂纹敏感性越强。采取措施后,有效控制了Q345GJC钢板的裂纹发生率。     

连铸坯枝晶臂间距检测方法的优化与应用实践

介绍了连铸坯凝固组织中树枝晶臂间距检测方法的优化试验及其应用效果。通过对连铸坯凝固冷却过程的分析及批量数据的采集,确定了测定连铸坯树枝晶臂间距的代表性区域,建立了一种科学测量枝晶臂间距的方法。将该方法应用于连铸圆坯生产检验中,能快速准确地找出铸坯枝晶臂间距随连铸二冷强度变化的规律,采取调整连铸二冷强度的措施,来控制一、二次枝晶臂间距大小,达到了减轻或消除铸坯开裂缺陷、提高连铸质量的目的。