热轧圆钢Q345E表面裂纹的研究与控制

采用连铸圆坯冷送轧制Q345E圆钢,发现表面存在较多裂纹,裂纹长度为20 ~80 mm,呈离散型分布。经检测,裂纹两边发现较严重的脱碳层,裂纹根部存在较多高温氧化质点,未发现非金属夹杂物。理论研究表明,初生奥氏体晶界析出网状铁素体膜是导致钢材断面收缩率降低的根本原因,而Q345E中的Nb含量在特定的温度段降低了钢材的塑性,更加剧了钢材的开裂倾向性。当预热段温度较高(850~900 ℃)时,铸坯快速升温,表面局部发生α→γ的转变,体积收缩不均匀,产生拉应力。当拉应力超过钢材所能承受的极限时,产生表面裂纹。通过对铸坯的预热缓解了内外温度差,有效解决了Q345E热轧圆钢的表面裂纹问题。

Research and control of surface crack of hot rolled round steel Q345E

It was found that there were many cracks on the surface of Q345E round steel rolled by continuous casting round billet. The crack length was 20-80 mm, showing a discrete distribution. After inspection, a serious decarburization layer was found on both sides of the crack, and there were many high-temperature oxidation particles at the crack root, and no non-metallic inclusions were found. The theoretical study showed that the precipitation of network ferrite film at the primary austenite grain boundary was the fundamental reason for the reduction of section shrinkage of steel, and the Nb content in Q345E reduced the plasticity of steel at a specific temperature range, and increased the cracking tendency of steel. When the temperature of the preheating section was higher (850-900 ℃), the slab temperature rose rapidly, α→γ transformation occured locally on the surface, and the volume shrinkage was uneven, resulting in tensile stress. When the tensile stress exceeded the limit that the steel can bear, the surface crack occured. By preheating the billet, the temperature difference between inside and outside was alleviated, and the surface crack problem of Q345E hot-rolled round steel was effectively solved.