Experimental study on transverse cracking in micro-alloyed steels

微合金钢的表面横向裂纹问题一直是困扰钢铁工业的关键问题之一，它影响着从连铸到板坯运输、储存和热轧的各个生产环节。这可能会导致生产线出现故障，最终产品出现缺陷，甚至导致昂贵且耗时的板坯修复。高含量Ti、Nb和V的微合金钢比其他钢更容易开裂，因为它们在基体的晶界上产生有害的碳化物和氮化物形式的析出物的能力很高。这些析出物被认为是钢加工过程中热塑性低的主要原因之一，因此是开裂的原因。这些类型的钢在600 ° C至1000 ° C之间的温度范围内在连续铸造中的板坯弯曲/伸直期间对裂纹形成最敏感。另一个可能导致裂纹萌生的效应是在铁素体形成后稍长的温度范围内的板坯变形，其中大部分变形集中在少量新转变的铁素体上，并且在矫直过程中甚至在随后的板坯运输过程中在施加少量变形的情况下可能产生裂纹。微合金化Ti对热塑性的影响仍然是复杂的且不清楚的。Ti对TiN的形成、尺寸和基体分布以及不同冷却和应变条件下Ti：N化学计量比的影响将通过一个综合研究计划进行研究，该计划结合了变形速率计"DIL 805 T"的物理模拟和MatCalc计算的模拟。研究了不同化学成分的不同牌号钢在连铸车间凝固、弯曲和矫直过程中冷却、应变速率和铁素体-铁素体相变等工艺条件对Nb和Ti微合金钢热塑性的影响。确定铁素体起始温度的膨胀试验应与MatCalc计算进行比较。MatCalc还将在通过Scheil-Guliver模拟模型计算凝固过程中的初生析出物和研究凝固后冷却过程中的析出动力学方面发挥重要作用。利用光学显微镜、扫描电子显微镜和X射线能谱等显微组织分析手段，研究微合金元素和工艺条件对凝固过程中初生析出物的类型、位置、该项目的主要目的是绘制所考虑钢材的延展性图，显示在何种条件下加工此类钢材最有利于避免/最大限度地减少钢板的横向表面裂纹。

Transverse surface cracking of micro-alloyed steels is still one of the most critical problems in steel industry affecting different production areas starting from continuous casting, passing by slab transportation to storage, and ending by hot rolling. It might lead to disturbances at the production lines, defect at the final products, or even an expensive and time consuming slab repair.Micro-alloyed steels with high amounts of Ti, Nb and V are more prone to cracking than other steels due to their high ability to produce detrimental precipitates in the form of carbides and nitrides on the grain boundaries of the matrix. These precipitates are believed to be one of the main reasons of low hot ductility during steel processing and consequently being responsible for cracking. These types of steels are most sensitive to crack formation during the slab bending/unbending in continuous casting within a temperature range between 600 °C to 1000 °C. One more effect that could lead to crack initiation, is the slab deformation within a temperature range slightly after ferrite formation, where most of the deformation is concentrated on a small amount of freshly transformed ferrite and might produce cracks under a small amount of applied deformation during straightening or even later on during transportation of the slabs.The effect of micro-alloying Ti on hot ductility is still complex and not clear. Ti effect in respect with TiN formation, size and matrix distribution, and Ti:N stoichiometric ratio under different conditions of cooling and straining, is to be studied through an integrated research program combining both physical simulation by deformation dilatometer ‘DIL 805 T’ and simulation by MatCalc calculations. Different grades with different chemical compositions are to be studied aiming to understand the effect of processing conditions like cooling, straining rates and austenite-ferrite transformation on the hot ductility of Nb- and Ti-micro-alloyed steels during solidification, bending and straightening in continuous casting shop. Dilatation tests for determining the ferrite start temperature are to be compared to MatCalc calculations. MatCalc will also play an important role in calculating the primary precipitates during solidification through Scheil-Guliver simulation modelling and studying the precipitation kinetics during cooling after solidification.Means of microstructure analysis by light optical microscope, scanning electron microscope and energy dispersive X-ray spectroscopy will be used to study the effect of micro-alloying elements and process condition on the type, position, size and distribution of the formed precipitates and compare it to the MatCalc simulation results.The main objective of the project is to draw a ductility map for the considered steels, showing under which conditions it would be most beneficial in processing such types of steels to avoid/minimize transverse surface cracking of steel slabs.