In the steel production process, the surface quality of steel plates is a key indicator for evaluating product grades. Among them, star-shaped cracks are a common and difficult-to-tackle surface defect. Their morphology is complex, presenting as clusters or incomplete loop polygons, with varying depths, and distributed in a band along the rolling direction. They not only affect the appearance of the product but also directly threaten the mechanical properties and service life of the steel plate. Minor star-shaped cracks can be salvaged through grinding, while severe ones directly lead to product scrapping, causing huge economic losses to enterprises.
Tong Yunfeng, an expert from Fujian Sansteel Group Co., Ltd., conducted in-depth research on the causes of star-shaped cracks in steel plates through systematic physical and chemical tests. This article will, based on his analysis results, detail the three main causes, microscopic features, and targeted improvement measures of star-shaped cracks, aiming to help industry colleagues quickly diagnose the root cause of the problem and optimize the production process.
I. Three Major Origins and Characteristic Identification of Star-shaped Cracks
Research has found that star-shaped cracks on the surface of steel plates mainly originate from three types of original defects in the cast billet: subsurface bubbles, intergranular cracks, and enrichment of low-melting-point elements. The cracks caused by these three reasons exhibit distinct characteristics at both macroscopic and microscopic levels.
Subsurface bubble rupture in the cast billet
Formation mechanism: Bubbles within a 10mm range beneath the surface of the cast billet will be flattened and elongated during subsequent rolling, gradually approaching the surface and eventually rupturing to form star-shaped cracks. Smaller bubbles may heal during rolling, but larger ones are difficult to eliminate.
Macroscopic features: After the sample was subjected to hot acid washing and observed, most of the cracks were closed, presenting as water-drop-shaped or irregular closed loops, with slight collapse inside the closed loops. Some cracks were hook-shaped, and the inner side of the hooks also had collapse.
Microscopic features (metallographic examination):
The crack tip is rounded, the defect interior is clean, and there are no oxidation spots around it.
After nitric acid alcohol etching, it can be seen that the microstructure at the defect area is ferrite + pearlite, and the grain sizes on both sides of the microstructure are obviously uneven. This is due to the fact that the surface defect cracked during rolling, resulting in uneven local deformation.
Key diagnostic points: No oxidation at the defect site, and the grain sizes on both sides are not uniform.
2. Primary intergranular cracks in the cast billet
Formation mechanism: During the solidification or cooling process of the cast billet, microcracks are generated at the grain boundaries due to stress or other factors. These initial cracks are then extended during subsequent rolling.
Macroscopic features: After hot pickling, most of the surface cracks are closed, but the opening directions are inconsistent, presenting a distinct polygonal feature.
Microscopic features (metallographic examination):
There are a large number of secondary oxidation particles in the matrix around the crack.
After corrosion, it was found that there was obvious decarburization at the crack location.
Energy dispersive spectroscopy (EDS) analysis shows that these secondary oxidation particles are mainly oxides containing manganese and silicon.
The key diagnostic points are: there are secondary oxidation particles around the crack and decarburization phenomenon. The formation of secondary oxidation particles requires a long-term high-temperature oxidation environment, which proves that they originated from the heating furnace stage rather than the brief rolling process. This, in turn, confirms that the crack already existed before entering the furnace (i.e., on the cast billet).
3. Enrichment of low-melting-point elements (such as Cu)
Formation mechanism: It mainly results from two mechanisms: Firstly, selective oxidation. When the steel billet is heated, elements like Fe, Si, and Mn are preferentially oxidized, while Cu, which is less reactive, remains and accumulates in the matrix beneath the iron oxide scale. Secondly, Cu atoms have high free energy at high temperatures and tend to diffuse and accumulate at the austenite grain boundaries. The low-melting-point phases enriched at the grain boundaries crack under the rolling stress, forming star-shaped cracks similar to “chapping”.
Macroscopic features: The polygonal characteristics of the cracks are most prominent, with a morphology similar to cracked dry soil (chapping), and generally no hooked features.
Microscopic features (metallographic examination):
The crack is relatively shallow and extends inward. A large number of secondary oxidation particles are present in the matrix around the crack and at the extension site.
After corrosion, the microstructure at the crack head is bainite + ferrite + a small amount of pearlite, and there is also a decarburization phenomenon.
Energy dispersive spectroscopy (EDS) clearly identified the enrichment of the low-melting-point element Cu at the crack site, which extended and distributed along the crack direction.
Key diagnostic points: Cu element enrichment detected at the crack site + macroscopic morphology of chapping + secondary oxidation particles.
II. Targeted Improvement Measures and Process Optimization
Based on the above three different causes, the steel plant should implement “tailored” improvement strategies:
1. For star-shaped cracks caused by subcutaneous bubbles
Control the total amount of gases in the molten steel: Strengthen deoxidation operations to reduce the gas content in the molten steel and prevent the formation of bubbles due to gas precipitation as the temperature drops during continuous casting.
Optimize the protective slag process: Check and ensure the dryness of the protective slag, select a more adaptable protective slag, and prevent external gases (such as water vapor) from entering the surface layer of the cast billet, which may cause sub-surface bubbles or pinholes.
2. For star-shaped cracks caused by primary intergranular cracks
Trace and optimize the continuous casting process: Determine the specific process steps where intergranular cracks occur (such as mold cooling, secondary cooling ratio, straightening process, etc.), and optimize and adjust the corresponding continuous casting process parameters to eliminate the crack sources in the cast billet itself.
3. Regarding star-shaped cracks caused by the enrichment of low-melting-point elements
Strict control of residual elements: Starting from raw materials, strictly control the content of low-melting-point residual elements such as Cu, Sn, and Pb in steel.
Optimize the heating process: In the high-temperature heating zone above 1100℃, adjust the air-fuel ratio to ensure that the furnace atmosphere is reducing or weakly reducing, thereby reducing the degree of selective oxidation of the steel matrix.
Surface protection: Research and apply high-temperature anti-oxidation coatings to prevent direct contact between the steel billet and the oxidizing atmosphere.
Enhance equipment quality: Improve continuous casting technology and the quality of the copper plate coating of the mold, reducing the probability of abnormal Cu element infiltration into the steel matrix due to coating damage.
III. Conclusion
The star-shaped cracks on the surface of the steel plate are not caused by a single factor, but rather are an extension of the quality defects of the cast billet during the rolling process. Through systematic macroscopic observation, metallographic examination, and energy spectrum analysis, the root cause of the cracks can be clearly identified:
- No oxidation, uneven grains → Subsurface bubbles
- The presence of secondary oxidation particles and decarburization → intergranular cracks in the cast billet.
- Cu enrichment and cracked texture → Enrichment of low-melting-point elements
Mastering these key discriminative features can help steel mills quickly and accurately identify the problem areas on the production site, thereby taking the most direct and effective process improvement measures, reducing the scrap rate, and enhancing the overall quality level and market competitiveness of steel plate products.