Reducing NG Wavy Defects in Slit Coil Processing Using the DMAIC Approach: A Case Study in Steel Manufacturing

Abstract

NG wavy defects in slit coil products represent a recurring quality problem that disrupts downstream manufacturing processes and generates customer complaints. This study aims to identify the root causes of NG wavy defects and formulate corrective actions to improve process reliability in slit coil production. A descriptive qualitative approach was employed using field observations, document analysis, semi-structured interviews, and data triangulation involving customer representatives and personnel from the purchasing, production, and quality control functions. The analysis was conducted using the Plan, Do, Check, Act (PDCA) framework, supported by Fishbone Diagram and 5W+1H analysis. The findings revealed that NG wavy defects were repeatedly identified at the seventh slit coil position (B7). Triangulation of customer complaints, purchasing records, production history, and quality control inspections confirmed that the defect originated during the slitting process. Further investigation identified a non-standard part finger setup on the mini slitter machine as the primary root cause, resulting in surface waviness. Based on these findings, corrective actions were proposed, including machine setup standardization, operator training, and strengthened inspection procedures. The study highlights the importance of systematic process control in preventing defects and improving slit coil production quality.