热轧结构钢氧化铁皮结构及其对酸洗质量的影响

Oxide scale structure of hot rolled structural steel and its effect on pickling quality

为研究热轧结构钢氧化铁皮厚度、结构及其对酸洗质量的影响,设计了“高温、低冷速”和“低温、高冷速”两种热轧工艺,并进行了热轧结构钢的生产制备。利用光学显微镜、扫描电镜分析了带钢宽度方向上氧化铁皮的厚度分布、结构特点和带钢酸洗后的表面形貌;基于不同热轧工艺下氧化铁皮厚度、结构差异,对带钢的酸洗效果以及酸洗色差缺陷的产生机理进行了探讨。结果表明,热轧结构钢氧化铁皮由外侧Fe_(3)O_(4)和内侧FeO的低温转变组织构成。由于热轧过程和卷取后温度场的不均匀分布,带钢边部氧化铁皮厚度小、结构致密,Fe_(3)O_(4)的含量比中部高;带钢中部氧化铁皮厚度大、结构疏松,FeO组织的低温转变比边部更充分。“高温、低冷速”热轧模式下氧化铁皮横向均匀性较差,边部、中部厚度相差4.7μm,其中Fe_(3)O_(4)层的厚度差为2.5μm。“低温、高冷速”热轧模式下氧化铁皮横向均匀性良好,不同部位氧化铁皮的厚度相差2.5μm,相比前者减少了46.81%,而Fe_(3)O_(4)层的厚度相差0.2μm,仅为前者的8.0%。氧化铁皮厚度、结构的横向差异是带钢酸洗色差缺陷的主要诱因。共析组织(α-Fe+Fe_(3)O_(4))疏松多孔,其酸洗过程以“剥离”的方式进行,酸洗效率更高。带钢优先完成酸洗的部位持续与酸液接触,在表面产生大量的“侵蚀裂纹”,形成宏观色差。改善氧化铁皮分布,控制氧化铁皮中Fe_(3)O_(4)、共析组织的比例是优化生产工艺、提升酸洗质量的关键。

To clarify the oxide scale thickness and structure of hot rolled structural steel and the effect on pickling quality,two hot rolling processes called“High Temperature and Low Cooling Speed(HT-LCS)”and“Low Temperature and High Cooling Speed(LT-HCS)”were designed in this work.The preparation of hot rolled structural steel was carried out accordingly.Oxide scale thickness and structure in the transverse direction of hot rolled strip under different hot rolling processes and its surface morphology after pickling were analyzed by OM and SEM.On the basis of oxide scale thickness and structure difference,the pickling effects and causes of color difference defect after pickling were discussed.The results show that the oxide scale for both designed hot rolling processes is composed of outer Fe_(3)O_(4) layer and low-temperature transformation structure of FeO located inside of oxide scale.Because of uneven distribution of temperature field during rolling process and after coiling,the thickness of oxide scale at the edge of strip is small and structure is compact,and the content of Fe_(3)O_(4) at the edge of strip is more than that in the middle.On the other side,oxide scale is thick and porous in the middle of strip,and the transformation of FeO at low temperature is more sufficient than in the edge area.The transverse uniformity of oxide scale,including oxide scale thickness and structure distribution,was poor under HT-LCS process.Specifically,the thickness difference of oxide scale between edge and middle area of strip was 4.7μm,and the thickness difference of Fe_(3)O_(4) was up to 2.5μm.However,the transverse uniformity of oxide scale was much better under LT-HCS process.The thickness difference of oxide scale between edge and middle area was 2.5μm,which was reduced by 46.81%compared with that of HT-LCS process,and the thickness difference of Fe_(3)O_(4) layer was 0.2μm,which was only 8.0%of the former.The transverse uneven distribution of thickness and structure for oxide scale are the main inducement to defect of“pickled dark strip”.Eutectoid structure(α-Fe+Fe_(3)O_(4))is loose and porous,and its pickling process is carried out in the way of“stripping”,which is more efficient in pickling.The parts of strip that are preferentially pickled will be continuously immersed in the acid solution,resulting in a large number of“erosion cracks”on the strip surface,and lead to a“color difference”finally.Optimizing the oxide scale distribution,adjusting the proportion of Fe_(3)O_(4) and eutectoid structure in oxide scale are important to optimize production process and improve the pickling quality.