High-speed steel is widely used in the production of hot rolling rolls due to its high hardness, good wear resistance and matching strength and toughness.
The manufacture of high-speed steel composite rolls adopts a centrifugal casting production process of three-layer composite three-time casting, which solves the technical problem of strength and chemical composition mismatch between the surface high-speed steel, the middle layer and the core ductile iron.
The chemical composition of the middle layer is between the core ductile iron and the surface high-speed steel. The layers are well bonded, and at the same time, the anti-graphitization elements in the surface high-speed steel are effectively blocked from diffusing into the core ductile iron. In order to make the high-speed steel composite roll obtain higher hardness, excellent wear resistance and good toughness, in addition to adding alloying elements to the surface high-speed steel, quenching and low-temperature tempering heat treatment should also be carried out. The structure after tempering is tempered martensite and residual austenite with a volume fraction of less than 5% and various carbides precipitated on the martensite matrix.
The working rolls of the finishing mill of the 1450 hot rolling production line of a steel plant use high-speed steel composite rolls with specifications of φ805mm×700mm. The rolls were put into use in August 2021. When rolling 3.5mm thick DC01 low-carbon steel in May 2022, a large area of
Analysis of the causes of spalling
High-speed steel composite rolls generally adopt a three-time pouring and three-time compounding centrifugal casting production process. This design idea solves the technical problem of low strength of the middle layer and the core ductile iron. The advanced composition design and effective modification treatment make the high-speed steel layer form a microstructure of MC, M6C, M2C and other types of high-hardness carbides precipitated on the high-hardness martensite matrix, which improves the wear resistance, high-temperature red hardness and thermal fatigue resistance of the roll.
However, in the casting process of high-speed steel composite rolls, the pouring time interval and pouring temperature of the middle layer are the key to process control. If the middle layer is poured too early, the high-speed steel layer poured first will be excessively dissolved, making the high-speed steel layer thin, failing to meet the process requirements, and reducing the service life; if the middle layer is poured too late, due to the small amount of molten steel, its temperature drops quickly, which is easy to cause poor interface bonding between the high-speed steel layer and the middle layer, and defects such as loose holes or slag inclusions. The best pouring time for the middle layer is when the temperature of the high-speed steel layer just reaches the solidus temperature. At this time, the heat of the middle layer steel liquid can melt the high-speed steel layer with a thickness of 10~20mm. At the same time, the antioxidant added after the high-speed steel pouring can be discharged to the inner surface of the middle layer, thereby ensuring the thickness of the high-speed steel layer and the good metallurgical bonding of the interface.
The large number of loose holes at the interface between the high-speed steel layer and the middle layer of the failed roller indicate that the roller was poured too late in the middle layer, resulting in poor bonding between the high-speed steel layer and the middle layer. Although the core ductile iron layer plays a filling and supporting role in the manufacturing process of the composite roller, if the content of harmful gases such as oxygen and nitrogen in the cast iron liquid is too high, the inclusions generated will float up and enrich at the interface between the core and the middle layer, affecting the metallurgical bonding between the middle layer and the core.
A 4.5 mm wide inclusion enrichment zone was found on the side of the ductile iron near the ductile iron at the interface between the ductile iron and the middle layer of the failed roller. The inclusion enrichment zone was formed by the floating of inclusions during the solidification of the ductile iron.
In summary, improper centrifugal casting process and low purity of core ductile iron liquid lead to a large number of loose holes and inclusions at the interface between the middle layer of the composite roller and the core ductile iron and high-speed steel layer, which greatly reduces the bonding strength at the interface;
Under the action of rolling stress, cracks preferentially initiate at the interface defects and extend along the loose holes and inclusions, while the presence of large-sized brittle carbides near the interface promotes rapid crack expansion, resulting in large-area peeling holes at the interface. When the hole area reaches the critical size, large-area peeling failure occurs on the surface of the composite roller.
Conclusion and Recommendations
Improper centrifugal casting process and low purity of core ductile iron liquid lead to the presence of lamellar loose holes and clustered non-metallic inclusions at the interface between the middle layer of the high-speed steel composite roll and the high-speed steel layer and the core cast iron, resulting in poor bonding at the interface; under the action of rolling force, cracks inside the roll initiate at these defects and extend along defects such as loose holes and inclusions, while the presence of large-sized brittle carbides near the interface promotes rapid crack expansion, ultimately leading to large-scale peeling of the roll surface.
It is recommended that roll manufacturers improve the casting process to reduce the solidification loose defects at the interface between the middle layer and the surface layer and the core; improve the purity of the core ductile iron liquid and reduce the content of inclusions; optimize the heat treatment process of the roll body, refine the carbide size in the high-speed steel layer and the middle layer, and improve the strength and toughness matching of the roll.
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