JPH029882B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method for casting an outer shell of a composite roll used for intermediate or finish rolling of wire rods, steel bars, flat steel, etc. (Prior Art) Generally, a composite roll is composed of a roll outer shell that contacts the rolled material and a roll shaft that is fixed to the outer shell, and the fixing means is to weld the outer shell and the shaft together There are two methods: one method is to manufacture the outer shell as a cylindrical sleeve, and the other method is to mechanically connect the outer shell to a separately prepared shaft material by shrink fitting or the like. In composite rolls for intermediate and finish rolling of wire rods, steel bars, flat steel, etc., when the outer shell is a sleeve, the outer surface of the outer shell in contact with the rolled material is made of high-alloy chilled material or high-alloy grain material to ensure wear resistance. It is made of high-alloy hard cast iron material with crystallized high-hardness carbides, while the inner surface part that is fixed to the shaft material is a high-grade cast iron material with strong toughness to prevent cracking during casting and ensure strength during use. It is usually made of a soft cast iron material such as cast iron or ductile cast iron in which graphite crystallizes. In the case of welding rolls as well, the inner surface of the outer shell is formed of a soft cast iron material, and the shaft portion is cast from the same cast iron material on the inner surface. When the molten metal of the shaft member is poured into the inner surface of the outer shell, part of the inner surface of the outer shell is eroded and melted into the molten shaft member, but if the outer shell is made only of high-alloy hard cast iron, the outer shell This is because the high alloying components (carbide forming elements) mixed into the molten metal of the shaft member promote the formation of carbides in the shaft cast iron material, causing it to harden and deteriorate the toughness of the shaft member. The outer surface portion and the inner surface portion of the outer shell are usually
After the outer surface part is centrifugally cast, the inner surface part is centrifugally cast on the inner surface thereof, and both are welded and integrated. After casting, heat treatment (also referred to as tempering heat treatment) is performed to transform residual austenite in the base into a bainite or martensitic structure in order to ensure and improve the wear resistance of the outer surface portion. (Problem to be Solved by the Invention) However, as described above, the outer shell has an outer surface portion formed of a high-alloy hard cast iron material and an inner surface portion formed of a soft cast iron material. If centrifugal casting is attempted, two types of molten metals are required, making the casting process complicated and reducing productivity. Also,
Since the carbide of the conventional high-alloy hard cast iron material that forms the outer surface is mainly cementite, even if the base is hardened by heat treatment, the Hs is limited to 70 to 80°, making it difficult to improve the hardness of the outer surface. Furthermore, there is also a strong demand for improved wear resistance. The present invention was made in view of such problems,
It is an object of the present invention to provide a method for casting an outer shell of a composite roll in which a hard cast iron material layer with high hardness is easily integrally formed on the outer surface portion and a soft cast iron material layer is formed on the inner surface portion. (Means for Solving the Problems) The casting method of the present invention made to achieve the above object has the following chemical composition: C: 2.5 to 4.0% Ni: 3.5 to 5.0% Si: 1.0 to 2.5% Cr: 1.0 ~2.0% Mn: 0.5-1.5% Mo: 0.2-1.5% P: 0.1% or less W: 10-25% S: 0.1% or less by weight, with the remainder essentially consisting of Fe, is subjected to centrifugal force A soft cast iron material layer in which high-hardness W double carbide is crystallized in a matrix mainly composed of martensite and retained austenite on the outer surface of the outer shell in an as-cast state, and graphite crystallized on the inner surface of the outer shell. The structure of the invention is to integrally form the. (Function) The chemical composition of the molten cast iron used in the present invention is limited for the following reasons. The unit is weight %. C: 2.5-4.0% C, along with Si, is an important element that determines the ratio of carbides on the outer surface of the shell and graphite on the inner surface.If it is less than 2.5%, W, Cr, Mo,
Due to the small amount of Fe double carbide, high hardness cannot be obtained on the outer surface, resulting in poor wear resistance. On the other hand, if it exceeds 4.0%, the amount of carbides becomes excessive and the outer surface becomes brittle, which is not preferable. Si: 1.0-2.5% Si affects the crystallization and shape of graphite, and if it is less than 1.0%, the influence of W, Mo, and Cr, which have a strong tendency to form carbides, is large and contributes to the crystallization of graphite on the inner surface of the outer shell. do not. On the other hand, if it exceeds 2.5%, a large amount of graphite will crystallize on the inner surface, but on the other hand, the shape of the graphite will deteriorate,
On the contrary, it leads to a decrease in toughness. Mn: 0.5-1.5% Mn combines with S and has a deoxidizing effect and removes the harmful effects of S, but if it is less than 0.5%, it has almost no effect, while if it exceeds 1.5%, it has a negative effect on mechanical properties. Significant deterioration. P: 0.1% or less S: 0.1% or less P and S are preferably as small as possible in roll materials, and the upper limit is 0.1%, which does not cause any actual damage. Ni: 3.5 to 5.0% Ni is added in large amounts to increase the hardness of the matrix on the outer surface of the outer shell and to obtain martensite, lower bainite, or retained austenite structure in as-cast steel. That is, if the content is less than 3.5%, no significant increase in hardness of the base portion can be expected. However, if it exceeds 5.0%,
Retained austenite becomes excessive or stabilized, and transformation into martensite or bainite during subsequent heat treatment is suppressed, resulting in a decrease in hardness. Cr: 1.0~2.0% Cr preferentially enters carbides and forms double carbides with other alloying elements, increasing hardness.
A part of it is an effective element that dissolves in the matrix and improves hardenability. If it is less than 1.0%, this effect will be small; on the other hand, if it exceeds 2.0%, W or Mo will have a strong tendency to form carbides.
Further, since the C content is low, no further effect can be obtained by addition. Mo: 0.2-1.5% Mo increases the resistance to softening during quenching and tempering, and
It has the effect of promoting precipitation hardening, increases base hardness, and contributes to improved wear resistance. If Mo is less than 2.0%, this effect is almost absent, while if it exceeds 1.5%, retained austenite increases and stabilizes. W: 10-25% W combines with other alloying elements to crystallize W double carbide (e.g. (Fe, W) ₆ C) during solidification, and this double carbide has extremely high hardness compared to cementite. Therefore, W is an element that can contribute most to ensuring high hardness such as Hs 80 to 95° and also ensuring high wear resistance. If it is less than 10%, W is easily dissolved in austenite, which makes it difficult to crystallize W double carbide, and even if it crystallizes, the amount is small, making it impossible to ensure high hardness on the outer surface of the outer shell. %, on the other hand, the amount of carbide is too large, which is unsuitable because it also suppresses the crystallization of graphite on the inner surface of the outer shell. The high W alloy cast iron molten metal according to the present invention contains each of the above-mentioned components in each weight percent, and the remainder is substantially made of Fe, but some of the Fe is replaced with W, and V, V,
It is also possible to add alloying elements such as Ti and Nb, thereby further improving the wear resistance. When melting high W alloy cast iron, for example, ferrotungsten specified in JIS G 2306 can be used as a W additive, so expensive WC can be used.
There is no need to use , reducing manufacturing costs. The high W alloy cast iron molten metal is subjected to centrifugal force casting to become the outer shell of the composite roll. When the molten metal is centrifugally cast, a large amount of W with high specific gravity in the molten metal moves to the outer surface of the shell due to the action of centrifugal force, and becomes a large amount of highly hard W double carbide, which is more uniform than in the molten metal. crystallizes out. Furthermore, W carbide crystallized from the molten metal during the transition also migrates and concentrates on the outer surface side. On the other hand, the molten metal on the inner surface side of the outer shell has a reduced W content due to the migration of W or W double carbides to the outer surface side.
When all of the molten metal solidifies, the outer surface of the outer shell becomes a structure in which high-hardness W double carbide crystallizes in a matrix mainly composed of martensite and retained austenite, while the inner surface of the shell becomes As the W content decreases, there is almost no crystallization of W double carbides, and a tough soft cast iron structure with crystallization of graphite is formed, and both are formed integrally. When the outer shell cast according to the present invention is used as the outer shell of a welded composite roll, a strong soft cast iron material such as high-grade cast iron or ductile cast iron is cast on the inner surface of the outer shell to form the shaft. do it. On this occasion,
In the inner surface of the outer shell, the W content, which was high in the molten metal at the time of casting, has been reduced to a few percent, so when the molten metal of the shaft member is poured, a portion of the inner surface of the outer shell is eroded and damaged. Even if it melts into the molten metal of the shaft member, it does not promote the formation of carbides and does not cause deterioration of the toughness of the shaft member. Incidentally, when a composite sleeve is obtained by centrifugally casting an inner layer made of tough and soft cast iron on the inner surface of the outer shell, the same effect as that of the shaft of the welding roll can be obtained. When the above shell is used as a sleeve, it is centrifugally cast, and when it is used as the shell of a welded composite roll, after the shaft is cast and rough processed, residual austenite on the outer surface of the shell is transformed into martensite or bainite. Heat treatment (at 300-500℃) to
(retained for 10 to 30 hours) to make product sleeves and product rolls. By the above heat treatment, the outer surface of the outer shell has a hardness of 80 to 95 degrees Hs. By the way, in Japanese Patent Publication No. 52-15047, soft cast iron molten metal and WC powder are mixed, centrifugally cast, WC particles are separated by specific gravity and unevenly distributed on the outer periphery, and WC particles are mixed on the outer periphery. A method is disclosed for casting a cylindrical casting consisting of a hard layer bonded by soft cast iron, and a soft cast iron layer containing almost no WC particles in the center. However, casting requires WC powder and molten cast iron,
In addition, during the pouring period of centrifugal force casting, WC is added to the molten cast iron.
Since it is difficult to uniformly disperse and mix the particles, it is difficult to obtain a uniform high hardness layer over the entire outer periphery. This tendency is particularly remarkable for large-sized ones. Furthermore, since the WC particles at the outer periphery are simply embedded in the soft cast iron base, they are likely to chip.
On the other hand, in the present invention, only one type of molten metal is required for casting, and since the W double carbide on the outer surface is crystallized by the growth of solidification nuclei that are uniformly generated in the molten metal, there is no unevenness in hardness. This does not occur, and there is almost no possibility that chipping will occur. In addition, in Japanese Patent Application Laid-Open No. 53-25213, W is 0.5~
A technique has been disclosed in which a molten adamite material containing 3.5% is centrifugally cast to form a roll shell, but W is dissolved in solid austenite and precipitated as a carbide from the austenite as it cools. This is to lower the carbon content of austenite, and it does not coexist with the molten metal and crystallize as carbides in the molten metal. W double carbides do not crystallize on the outer surface of the shell due to the action of centrifugal force. Can not. Incidentally, the outer shell formed according to the above cited example is an adamite material (a white casting-like rolled material having eutectic cementite in the microstructure), and no crystallization of graphite is observed. In addition, in Japanese Patent Publication No. 50-14214, W is 0.5~
A roll outer shell material of white cast iron or spotted iron containing 2.0% is disclosed, but W in the outer shell material is also dissolved in the austenite as a solid solution or precipitated as a carbide from the austenite, as in the adamite material. However, it does not crystallize as a carbide from the molten metal, and high hardness W double carbide cannot be crystallized intensively on the outer periphery of the outer shell due to the action of centrifugal force. (Examples) Example 1 A sleeve with an outer diameter of 240φ x an inner diameter of 140φ x 100 (unit: mm) was manufactured by centrifugal casting. In this case, the casting thickness was 50 mm, the casting weight was 24 kg, and the ladle components of the molten metal were as shown in Table 1. In addition, the unit in the ingredient list is % by weight, and the remainder is substantially Fe. The same applies to other tables.

[Table] As a result of subjecting this sleeve to tempering heat treatment at 450°C x 10 hours, the outer surface hardness of the sleeve was Hs93.8°, and when the hardness change in the sleeve thickness direction was measured, the results were as shown in Figure 1. A significant change in hardness was obtained. The composition at position A, which is 10 mm from the outer surface of the sleeve, and position B, which is 10 mm from the inner surface, was investigated.
The results are shown in Table 2. From the same table, it can be seen that the W content in the inner surface portion is extremely small compared to that in the molten metal. Furthermore, when the cast iron structure on the inner surface was observed, it was found to be a gray cast iron structure.

[Table] On the other hand, for comparison, sleeves obtained by casting molten metal of the same size and having the same alloy composition except that it does not contain W as shown in Table 3 below were subjected to the same heat treatment as above. The results showed that the outer surface hardness was Hs71.2°.

[Table] Example 2 A 400φ×500 welded composite roll was manufactured. In this case, the outer shell was cast by centrifugal force casting with a shell thickness of 50 mm and a casting weight of 225 kg, while the shaft was cast by placing the mold in the outer shell by standing the mold upright and pouring it into the shell. The ladle components of the outer shell and shaft (high-grade cast iron) are as shown in Table 4 below.

[Table] This composite roll was heat treated at 420°C for 15 hours, resulting in a roll outer shell surface hardness of Hs94.2°. On the other hand, for comparison, the above composite rolls were obtained by casting the outer shell and shaft molten metal of the same size and having the same alloy composition except that the outer shell does not contain W as shown in Table 5 below. According to the result of applying the same heat treatment as , the surface hardness of the roll outer shell is
It was Hs73.0°.

[Table] Also, 10mm from the outer surface of the outer shell of both of the above composite rolls.
As a result of taking abrasion test pieces at the depth position and conducting an abrasion test using alumina powder as a lubricant, it was determined that the roll according to the present invention had a wear loss of 0.48 times that of the comparative roll. It has been found that such composite rolls have very good properties against rubbing wear. As a result, it is predicted that when the composite roll according to the present invention is actually used as a wire finish rolling roll, it will exhibit extremely high wear resistance compared to conventional high-alloy grain rolls and high-alloy chilled rolls. be done. In addition, according to the results of product analysis and investigation of the composite roll according to the present invention obtained in this example, the W content is 22.7% at a position 25 mm from the outer surface of the outer shell, and W content is 22.7% within 100 mm from the outer surface. Content 0.20%
It was confirmed that the diffusion of W into the interior was extremely small. (Effects of the Invention) As explained above, according to the method for casting the outer shell of a composite roll of the present invention, since a predetermined high W alloy cast iron molten metal containing W: 10 to 25% is centrifugally cast, centrifugal force is reduced. As a result, a large amount of highly hard W double carbide can be crystallized on the outer surface, while a soft cast iron material layer containing almost no double carbide and crystallized graphite with a low W content is created on the inner surface. can be formed integrally,
A shell with the desired properties required for the outer and inner surfaces can be cast very easily using a single molten metal. In addition, since the W double carbide on the outer surface has extremely high hardness compared to cementite, the retained austenite in the matrix on the outer surface is heat treated to martensite and bainite, thereby increasing the outer surface to Hs80~ It can be made into a high hardness layer of 95°. In addition, since the W double carbide on the outer surface is crystallized from the molten metal, it exists uniformly over the entire outer surface, has no uneven hardness, is less prone to chipping, and is not only wear resistant. It also has excellent roughness resistance.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram showing the hardness distribution of an embodiment of the outer shell of the composite roll according to the present invention.

Claims (1)

[Claims] 1 Chemical composition: C: 2.5-4.0% Ni: 3.5-5.0% Si: 1.0-2.5% Cr: 1.0-2.0% Mn: 0.5-1.5% Mo: 0.2-1.5% P: 0.1 % or less W: 10 to 25% S: Each weight% contains 0.1% or less, and the remainder is essentially Fe.The outer shell molten metal is centrifugally cast, and in the as-cast state, martensite remains mainly on the outer surface of the outer shell. Casting of the outer shell of a composite roll characterized by crystallizing high-hardness W double carbide in a matrix made of austenite and integrally forming a soft cast iron material layer with crystallized graphite on the inner surface of the outer shell. Method.