1.How to make a preliminary judgment?
Online Automated Inspection (Prevention and Monitoring)
Surface Quality Inspection System: Standard equipment on modern cold rolling lines, employing machine vision (line scan camera + special light source). The system can identify pits (manifested as characteristic dark spots) in real time, triggering alarms, marking locations, and assigning ratings in the control room. It is crucial for controlling batch defects.
Manual Inspection: Utilizing a stroboscope, the high-speed moving strip is "frozen" at the exit section for easy visual observation.
Offline Macroscopic Inspection (First Step, Key Analysis) Upon discovering a problematic roll, a detailed static inspection is required:
Illumination and Observation: Using a strong flashlight or LED light strip at an angle, pits will become visible due to different light reflections. Observe their distribution:
Periodic/Regular Distribution: Appearing at equal intervals along the rolling direction is a typical characteristic of roll surface damage (such as chipping or sticking). Measure the distance L between adjacent pits. If L = π × roll diameter, the problematic roll can be identified.
Random/clustered distribution: Often caused by foreign matter indentation (such as iron oxide scale, furnace roll nodules, refractory materials) or genetic defects in the raw materials.
Concentrated at the edges or ends: May be related to interlayer scratches or shear burrs.
Tactile examination: Feel whether the edges of the dent are sharp or rounded, and whether the bottom is flat.
Tape lamination method: A rapid evidence collection method. High-transparency special tape or soft silicone is pressed onto the dent; after removal, a negative mold is obtained.
2.How to quantitatively analyze three-dimensional morphology and dimensions?
Tools: White light interferometer/3D surface profilometer.
Function: To accurately measure the depth, width, volume, and edge angle of pits. Distinguishing between sharp "impact craters" and gentle "indentation craters" is crucial for determining their cause.
3.How to analyze microscopic morphology and composition?
Tools: Scanning electron microscope paired with energy dispersive spectroscopy (EDS).
Analysis Process and Diagnostic Logic:
Microscopic Morphology Observation: Observe the bottom and edges of the pits at high magnification to check for foreign matter residue, material tearing, melting marks, etc.
EDS Composition Analysis: This is the most crucial step. Perform micro-area compositional analysis on the bottom of the pits to directly pinpoint the contamination source based on elemental composition:
Detection of mainly Fe and O → Iron oxide scale indentation. Indicates incomplete hot rolling descaling or inadequate pickling.
Detection of Al, Si, Ca, Mg, etc. → Refractory material/slag indentation. Indicates steelmaking, continuous casting, or heating furnace processes.
Detection of Cu, Zn, Sn, etc. → Copper/non-ferrous metal contamination. Indicates wear or foreign matter contamination in guides, bearings, etc.
Detection of abnormally high C content → Rolling oil coking particles or organic matter indentation.
Composition completely identical to the matrix → Possibly caused by raw material bubbles or roll damage.
4.How to analyze a metallographic cross-section?
Method: Samples were cut perpendicular to the pit to prepare metallographic specimens.
Observation: The metal flow lines beneath the pit were examined for deformation, microcracks, abnormal matrix structure, and inclusions embedded at the bottom. This verifies the energy dispersive spectroscopy (EDS) results and assesses the potential impact of defects on material properties.
5.What are some practical action guidelines?
Step 1 (Rapid Response): Upon discovering a pit, immediately record its location and use a flashlight at an angle to preliminarily determine its distribution pattern. If it is periodic, immediately stop the line and inspect the corresponding roll.
Step 2 (Sampling and Analysis): For random pits, take samples and send them to the laboratory. The preferred and most effective analysis is SEM/EDS compositional analysis, which can provide a clear indication quickly.
Step 3 (Root Cause Rectification): Based on the process indicated in the test report (e.g., hot rolling, pickling, steelmaking), investigate specific process parameters or equipment status related to that process (e.g., descaling pressure, acid concentration, furnace roll condition, guide cleanliness, etc.).
Step 4 (Preventive Measures): Strengthen incoming material inspection; regularly inspect and grind rolls; maintain production line cleanliness; optimize key process parameters.