Abstract: This article briefly describes the casting and heat treatment process of high chromium cast iron rolls, and analyzes the problems that are easy to occur during use. High chromium cast iron rolls have been widely used in the front frame of hot-rolled wide-band steel finishing mills and some small bar and section steel finishing mills. It is widely loved by users for its good wear resistance and anti-“spotting” performance.
High chromium cast iron roll
High chromium cast iron rolls have been widely used as a conventional roll in hot strip mills, cold strip mills, wide and thick plate mills, small section steel mills and bar and wire mills, and have achieved good economic benefits to varying degrees. In order to improve the thermal conductivity of high-chromium cast iron rolls, Japan has also developed high-chromium cast iron rolls containing graphite in the working layer structure.
In the mid-1980s, some Chinese scientific research institutes and factories successively began to develop high-chromium cast iron rolls. Large-scale high-chromium cast iron rolls are produced by vertical and horizontal centrifugal casting methods, which are used as work rolls in the front section of hot strip hot rolling and finishing rolling, and the variety and application range are further expanded.
Production methods of high chromium cast iron rolls
At present, almost all high-chromium cast iron rolls are centrifugally cast, but the centrifuge has three forms: horizontal, vertical and inclined. Compared with previous production methods such as the “overflow method”, centrifugal casting can rapidly cool a small amount of high-chromium cast iron shell in order to obtain finer and dispersed carbide structures, and the production efficiency is further improved.
The core of the roll is usually made of high-strength ductile iron. Due to the high chromium content in the outer layer, the silicon content and nickel content in the core component should be appropriately increased compared with ordinary rolls, so as to reduce the carbide content in the core structure and enhance the core strength.
Usually, in order to prevent the chromium component with higher content in the outer layer from diffusing to the core when pouring the core. When the outer layer is poured, it is necessary to pour into the transition layer at an appropriate time. The molten iron in the transition layer can be made of medium chromium cast iron, semi-steel, gray cast iron and other materials. The pouring time, temperature and amount of molten iron should be strictly controlled.
Metallurgical properties of high chromium cast iron rolls
In Fe-Cr-C alloys, if the chromium content exceeds 15%, the cementite will become unstable, and it will be replaced by hexagonal carbide M7C3 with a complex structure. This kind of carbide is called chromium carbide, the main components are chromium and iron, and may contain small amounts of other alloying elements. The basic characteristic of the outer layer material of high chromium cast iron roll is that the microstructure eutectic carbide is dominated by (Cr, Fe) 7C3 type. Its microhardness is 1500-1800HV, while that of cementite is 1000-1200HV, which is why high chromium cast iron rolls have strong wear resistance. The main chemical composition (%) of high chromium cast iron roll is: C2.2~3.4, Cr10~25, Mo0.3~4, Ni0.3~3.0. The chromium-carbon ratio (Cr/C) determines the type of carbides in the outer structure of high-chromium cast iron, and the content of elements such as C, Cr, and Mo determines the amount of carbides. The role of Ni and Mo is to strengthen the matrix on the one hand, and to increase the hardenability of the matrix structure on the other hand.
Most of the research on Fe-Cr-C alloy system is based on the following Fe-Cr-C alloy phase diagram
Production process High chromium cast iron is generally smelted by induction furnace or electric arc furnace. The commonly used raw materials are pig iron, scrap steel, furnace charge, ferrochromium, ferromolybdenum, nickel, etc. The pig iron and ferrochrome used should pay attention to control the silicon content. The effect of various alloying elements in steel scrap on hardenability should remain stable. Ferrochromium is generally added at the later stage of smelting to prevent excessive chromium oxidation and burning. High chromium cast iron rolls are formed by centrifugal casting. The main advantage of this process is that the yield of molten iron and alloy is increased. It can effectively prevent the chromium in the outer layer from diffusing to the core, and ensure the strength and toughness of the roll core and roll neck. Excellent metallurgical bonding of two different materials can be achieved. Europe and the United States generally use centrifugal composite casting of two materials, while Japan uses centrifugal composite casting of three materials. The heat treatment of high chromium cast iron rolls usually includes high temperature austenitizing, normalizing and tempering. The role of heat treatment is to control the retained austenite content in the working layer and obtain the desired matrix structure. During the heat treatment process, attention should be paid to controlling the heating and cooling speed to prevent the roll from cracking. The typical metallographic structure of the outer layer of high chromium cast iron after heat treatment is shown in the figure.
High chrome roll
The (Cr, Fe)7C3 type carbides in the outer structure of high chromium cast iron rolls are different from the continuous distribution of M3C type eutectic carbides in ordinary white iron, and their main characteristics are high hardness and discontinuous distribution. Therefore, the strength and toughness of high chromium cast iron are higher than ordinary wear-resistant white cast iron and have high wear resistance. On the one hand, this is due to the fact that the working layer contains a large number of isolated high-hardness carbides. On the other hand, due to the high oxidation resistance of the working layer of high-chromium cast iron rolls, a dense oxide film is formed on the surface of the rolls during steel rolling, which plays a protective role. The high wear resistance and surface roughness resistance of high chromium cast iron rolls not only prolong their service life, but also improve the dimensional accuracy and surface quality of rolled products. For example, when used in the front section of the hot strip rolling mill, the high-chromium cast iron roll has the effect of resisting spot defects, so it can replace the unbounded (limited) chilled cast iron roll and semi-steel roll. The thermal conductivity of the working layer of the high chromium cast iron roll is low. In order to reduce the thermal stress on the working surface of the roll and prevent thermal cracking, sufficient water cooling must be carried out during use. In addition, the machining, grinding and roughening of high-chromium cast iron rolls (see roughening of the roll surface) are more difficult than ordinary cast iron rolls.
According to the different conditions of use, the number of carbides and the type of matrix structure in the working layer of the roll can be changed by adjusting the chemical composition and heat treatment process to obtain high-chromium cast iron rolls with different hardness levels. It can be roughly divided into two categories, namely, low-carbon and low-chromium anti-accident type and high-carbon and high-chromium anti-wear type. The chemical composition and application range of high chromium cast iron rolls with several common hardness levels are listed in the table.
High chromium cast steel roll
The roll is made of high chromium steel as the outer material of the roll, and generally uses ductile iron as the roll core and roll neck material, and is made by centrifugal composite casting. The primary carbides in the outer structure of high chromium cast steel rolls are mainly (Cr, Fe) 7 C3 type, and the main chemical components (%) are: C0.7～1.4, Cr6～15, Mo0.5～3.0, Ni0.3～2.0.
High chromium cast steel roll is another new type of high chromium roll developed after high chromium cast iron roll. In the early 1980s, Europe took the lead in trial-producing high-chromium cast steel rolls by centrifugal casting. So far, Europe, America and Japan have widely used high-chromium cast steel rolls as the work rolls of the hot-rolled strip roughing stand.
Production process High chromium cast steel is generally smelted in an electric arc furnace because of its high melting point. The raw materials used are similar to high chromium cast iron rolls. Core ductile iron requires special handling techniques to ensure good nodularization of graphite in large section castings. High chromium cast steel rolls are generally formed by centrifugal casting, and the advantages of centrifugal casting are similar to those of high chromium cast iron rolls.
In addition, high-chromium cast steel belongs to hypereutectoid chromium-molybdenum steel, which has a wide solidification range. Centrifugal casting can make molten steel solidify rapidly under the centrifugal force in the rotating mold, thereby eliminating casting defects, reducing segregation, and refining grains. . However, to obtain high-chromium cast steel rolls with good bonding layer quality and no casting defects requires scientific determination of those interrelated key process parameters and strict process control. High chromium cast steel is soft and machinable in the as-cast state and must be heat treated. The heat treatment process is generally normalizing or spray quenching and tempering. The microstructure of high chromium cast steel after heat treatment is: primary carbide (Cr, Fe) 7 C3 + tempered martensite + fine and dispersed secondary carbide + a small amount of retained austenite. After heat treatment, the outer layer of the roll has a higher circumferential compressive stress, which can improve the resistance of the roll to fatigue hot cracking.
Material characteristics of high chromium cast steel roll
(1) Due to the low carbon content of high-chromium cast steel, the number of primary carbides in the structure is reduced, generally less than 10%. Therefore, the yield strength and toughness of the working layer are higher than those of high-chromium cast iron, and the roughness resistance of the roll surface is improved. The thermal conductivity is high, so the thermal crack resistance and accident resistance performance are improved. Correspondingly, the water cooling requirements of high chromium cast steel rolls are not as strict as those of high chromium cast iron rolls. The biting property during rolling is improved, and the poor biting property is just a phenomenon that often occurs when high-chromium cast iron rolls are used in the hot-rolled strip roughing stand.
(2) Because the chromium-carbon ratio (Cr/C) of high-chromium cast steel and the chromium content in the matrix structure are higher than those of high-chromium cast iron. Therefore, its oxidation resistance is better, and the dense oxide film formed on the surface of the roll during use ensures its high wear resistance, prolongs the service life of the roll, and improves production efficiency.
(3) Both the outer layer of high chromium cast steel and the core of nodular cast iron have a higher elastic modulus, which is much smaller than the elastic deformation of the unbounded (limited) chilled cast iron roll when rolling steel. Some countries have tried high chromium cast steel rolls as work rolls for cold-rolled strip mills and hot-rolled strip finishing stands.