JP6948556B2 - Manufacturing method of composite roll for hot rolling made by centrifugal casting - Google Patents

Description

The present invention relates to a centrifugal casting hot rolling composite roll having a roll outer layer and a shaft core portion, and particularly to a centrifugal casting hot rolling composite roll suitable for a work roll for hot rolling of a steel sheet.

In the work roll used for hot rolling of a steel sheet, the surface of the roll outer layer (hereinafter sometimes abbreviated as "outer layer") wears as the rolling amount increases. Generally, in the vicinity of the width ends (edges) on both sides of the steel sheet to be rolled, the temperature of the steel sheet is lower than that in the center of the steel sheet, so the rolling conditions become harsh, and the outer layer surface of the roll that the width ends of the steel sheet come into contact with is other. It is known that the outer layer wears more locally than the surface portion of the outer layer. If such local wear becomes large, it contributes to the shape defect of the rolled material and causes deterioration of product quality. Therefore, when the roll is subjected to rolling again, it is necessary to polish it flat until the locally worn portion disappears during the roll polishing, and there is a problem that the larger the local wear, the larger the loss of the roll outer layer material.

As a conventional technique, local wear of the outer layer is performed so that the central portion of the outer layer facing the central portion of the width of the rolled steel sheet and the outer layer end portions facing the vicinity of the width ends on both sides of the steel sheet proceed on average. In order to reduce the amount, a composite roll for rolling in which the outer layer forming the central portion of the outer layer and the end portion of the outer layer are made of different materials has been proposed.

For example, in Patent Document 1, in order to give a component change in the roll axis direction, a material forming a plate ear portion (near the width ends on both sides of the rolled steel plate) of the surface layer portion of the roll cylinder and a central portion of the roll cylinder are formed. The metal cylindrical consumable electrode made of a composite material having a material to be rolled is redissolved by electroslag and welded around the columnar roll core material, and the selvage portion of the surface layer of the roll body has higher wear resistance than the center part of the roll body. A rolling roll made of a metal material having a slag is disclosed. However, this has a problem that the manufacturing method by electroslag dissolution is not simple and the manufacturing cost is high. In addition, the electroslag melting method has problems such as cracking due to heat shrinkage after casting and difficulty in inoculation to crystallize graphite sufficiently uniformly. Therefore, graphite used for finishing rolls for hot rolling is used. It is not suitable for producing the outer layer containing it.

Patent Document 2 describes in% by weight, C: 2.5 to 3.7%, Si: 0.2 to 2.2%, Mn: 0.2 to 1.5%, P: 0.1% or less, S: 0.08% or less, Ni: 0.8 to 4.5%, Cr. A cast iron roll having a composition of 0.5 to 5.0%, Mo: 0.2 to 1.5%, and the balance substantially consisting of Fe. Edge resistant edge made of graphite cast iron at the center of the roll body and white cast iron at the end of the roll body. A rolling roll having excellent wear resistance is disclosed. This cast iron roll is manufactured by a centrifugal casting method, and graphite cast iron is used by using a mold in which the central part of the roll body is made of graphite cast iron and the part corresponding to the white cast iron at the end of the body is made of a sand mold and a mold, respectively. , It is stated that white cast iron will appear. However, it is necessary to use a special mold in which a sand mold is formed in the center of the roll body and a mold is formed in the end of the body, which causes a problem of high manufacturing cost. Since the cooling rate is different, the structure size of the outer layer is different accordingly, which makes it difficult to manufacture a steel sheet of uniform quality. Furthermore, since the joining timing when the inner layer is cast and joined and integrated after the outer layer is solidified differs in the axial direction, it is extremely difficult to obtain a sound joining of the outer and inner layers over the entire axial direction. Moreover, since the outer layer does not contain a large amount of hard carbides such as MC carbides, the wear resistance of the roll was insufficient.

A method for centrifugal casting of the outer layer of a rolling roll is described in, for example, Patent Document 3. FIG. 3 shows a cross-sectional view of a main part of the horizontal centrifugal casting apparatus of Document 3, in which the centrifugal casting die 41 is rotatably supported by a rotary roller 46, and the outer layer molten metal is rotatably supported from the funnel 43 to the pouring nozzle 45. It is cast into the end inside the mold via. For the shaft core, a centrifugal casting mold having an outer layer after solidification is erected, and upper and lower molds for forming the shaft core are connected to both ends to form a stationary mold, and the shaft is formed inside the mold. This is a method of casting a molten core material to obtain a composite roll in which the outer layer and the shaft core are integrated.

Japanese Patent Application Laid-Open No. 62-54509 Japanese Unexamined Patent Publication No. 63-174706 Japanese Unexamined Patent Publication No. 11-264051

An object of the present invention is to localize the outer layer so that the central portion of the outer layer facing the central width of the rolled steel sheet and the end of the outer layer facing the vicinity of the width ends on both sides of the steel sheet are worn evenly. It is an object of the present invention to provide a composite roll for hot rolling by centrifugal casting, which has an outer layer which can reduce wear and has excellent wear resistance, and which is particularly suitable for a work roll for hot rolling of a steel sheet, and a method for producing the same.

The composite roll for hot rolling made by centrifugal casting of the present invention has C: 2.6 to 3.8%, Si: 0.1 to 3.0%, Mn: 0.3 to 2.0%, Ni: 2.3 to 5.5%, Cr: 0.5 to 0.5 on a mass basis. A structure containing 2.5%, Mo: 0.2 to 3.0%, V: 0.2 to 3.8%, Nb: 0.4 to 6.8%, the balance consisting of Fe and unavoidable impurities, and 0.3 to 10% graphite phase on an area basis. The C content at positions 100 mm apart from both end faces of the outer layer in the axial direction is the C content at the center of the axial length of the outer layer. The Nb content at positions axially separated from both end faces of the outer layer by 0.05 to 0.3% by mass is 0.5 to 3.0 mass with respect to the central Nb content of the axial length of the outer layer. It is characterized by a large percentage.

In the composite roll for hot rolling made by centrifugal casting of the present invention, the outer layer is further based on mass, W: 0.01 to 3.0%, Ti: 0.01 to 0.5%, Al: 0.01 to 0.5%, Zr: 0.01 to 0.5%, It is preferable to contain any one or more of B: 0.001 to 0.5% and Co: 0.1 to 5.0%.

The method for producing a composite roll for hot rolling made by centrifugal casting of the present invention is based on mass in a rotating centrifugal casting mold, C: 2.8 to 3.8%, Si: 0.1 to 3.0%, Mn: 0.3 to 2.0. %, Ni: 2.3-5.5%, Cr: 0.5-2.5%, Mo: 0.2-3.0%, V: 0.3-4.0%, Nb: 0.5-4.0%, and the balance is an outer layer consisting of Fe and unavoidable impurities. After the molten metal for centrifugal casting is dropped into the center of the centrifugal casting mold in the axial direction for centrifugal casting, the molten metal of ductile cast iron for forming the shaft core is poured into the centrifugal casting mold. It is characterized by.

In the method for producing a composite roll for hot rolling by centrifugal casting of the present invention, the molten metal for the outer layer is further based on mass, W: 0.01 to 3.0%, Ti: 0.01 to 0.5%, Al: 0.01 to 0.5%, Zr. It is preferable to contain any one or more of: 0.01 to 0.5%, B: 0.001 to 0.5%, and Co: 0.1 to 5.0%.

In the composite roll for hot rolling made by centrifugal casting of the present invention, the central part of the outer layer facing the central part of the width of the steel sheet to be rolled and the end part of the outer layer facing the vicinity of the width end on both sides of the steel sheet are worn on average. It has an outer layer having excellent wear resistance as well as being able to reduce local wear of the outer layer so that Since the outer layer wears evenly in the roll axis direction, when the roll is subjected to rolling again, the polishing until the locally worn part disappears during roll polishing is suppressed with a small loss, and the loss of the outer layer material is suppressed. Can be made smaller.

It is a schematic cross-sectional view which shows the composite roll for hot rolling of this invention. It is the schematic which shows the manufacturing method of the composite roll for hot rolling made by centrifugal casting of this invention. It is sectional drawing of the main part of the horizontal centrifugal casting apparatus which shows the conventional centrifugal casting method.

Although embodiments of the present invention will be described in detail below, the present invention is not limited thereto, and various modifications may be made without departing from the technical idea of the present invention. Unless otherwise specified, the term "%" simply means "mass%".

[1] Centrifugal casting composite roll for hot rolling FIG. 1 shows a composite roll for hot rolling 1 composed of an outer layer 2 formed by a centrifugal casting method and a shaft core portion 3 welded and integrated with the outer layer 2. .. The shaft core portion 3 made of ductile cast iron has a trunk core portion 4 welded to the outer layer 2 and shaft portions 5 and 6 integrally extending from both ends of the barrel core portion 4. The outer layer 2 has an outer layer central portion 21 facing the width center portion of the rolled steel sheet, and an outer layer end portion 24 facing the vicinity of the width end portion of the rolled steel sheet near one outer layer end surface 22 of the outer layer 2. Near the other outer layer end face 23, there is an outer layer end 25 facing the vicinity of the width end of the rolled steel sheet. In the present invention, positions 27 and 28 100 mm from the axial end face of the outer layer exist at the outer layer end portions 24 and 25. Since the temperature of the width end portion of the rolled steel sheet is lower than that of the central portion and the deformation resistance is high, the rolling load on the roll at the outer layer end portion is also high, and the roll wear is likely to proceed. There are various width dimensions of the steel sheet, and the position of the width end of the steel sheet also differs accordingly, but the width end of the steel sheet is often around 100 mm from the end of the outer layer of the roll, and the wear resistance at this position is improved. Improving the wear compared to the central part reduces the wear at the width end and is effective in averaging the wear in the roll axial direction. Further, in the present invention, the central 26 of the axial length of the outer layer refers to a position obtained by equally dividing the axial length of the outer layer.

(A) Outer layer The composition of outer layer 2 will be described below. (1) Composition (i) Essential composition (a) C: 2.6 to 3.8% by mass C combines with V, Nb, Cr, Mo and W to form hard carbides, which contributes to the improvement of wear resistance. In addition, graphitization-promoting elements such as Si and Ni crystallize as graphite in the structure, thereby imparting seizure resistance to the outer layer and improving the toughness of the outer layer. If C is less than 2.6% by mass, not only the crystallization of graphite is insufficient, but also the amount of crystallization of hard carbides is too small to impart sufficient wear resistance to the outer layer.

On the other hand, when C exceeds 3.8% by mass, graphite becomes excessive and the shape becomes string-like, and the strength of the outer layer decreases. In addition, the amount of carbides crystallized becomes excessive, the toughness of the outer layer decreases, and the crack resistance decreases, so that cracks due to rolling become deeper and roll loss increases. The lower limit of the C content is preferably 2.7% by mass, more preferably 2.8% by mass. The upper limit of the C content is preferably 3.7% by mass.

(b) Si: 0.1 to 3.0% by mass Si reduces oxide defects by deoxidizing the molten metal and has the effect of promoting the crystallization of graphite, which contributes to seizure resistance and suppression of crack growth. If Si is less than 0.1% by mass, the deoxidizing action of the molten metal is insufficient, and the action of graphite crystallization is also small. On the other hand, when Si exceeds 3.0% by mass, the alloy matrix becomes brittle and the toughness of the outer layer decreases. The lower limit of the Si content is preferably 0.5% by mass. The upper limit of the Si content is preferably 2.9% by mass.

(c) Mn: 0.3 to 2.0% by mass Mn has an action of fixing the impurity S as MnS in addition to the deoxidizing action of the molten metal. If Mn is less than 0.3% by mass, their effects are insufficient. On the other hand, even if Mn exceeds 2.0% by mass, no further effect can be obtained. The lower limit of the Mn content is preferably 0.4% by mass. The upper limit of the Mn content is preferably 1.9% by mass.

(d) Ni: 2.3 to 5.5% by mass Ni has the effect of crystallizing graphite and contributes to seizure resistance. Ni also has the effect of improving the hardenability of the matrix structure. If Ni is less than 2.3% by mass, the effect cannot be sufficiently obtained. On the other hand, when Ni exceeds 5.5% by mass, austenite becomes too stable and it becomes difficult to transform into bainite or martensite. The lower limit of the Ni content is preferably 2.4% by mass. The upper limit of the Ni content is preferably 4.9% by mass.

(e) Cr: 0.5 to 2.5% by mass Cr is an element effective for improving hardenability, maintaining hardness by making the base bainite or martensite, and maintaining wear resistance. If Cr is less than 0.5% by mass, the effect of addition is insufficient. On the other hand, when Cr exceeds 2.5% by mass, it not only inhibits the crystallization of graphite but also forms coarse eutectic carbides, which lowers the toughness of the matrix structure. The lower limit of the Cr content is preferably 0.6% by mass. The upper limit of the Cr content is preferably 2.4% by mass.

(f) Mo: 0.2 to 3.0% by mass Mo combines with C to form hard Mo carbides, increasing the hardness of the outer layer and improving the hardenability of the matrix. If Mo is less than 0.2% by mass, those effects are insufficient. On the other hand, when Mo exceeds 3.0% by mass, the toughness of the outer layer deteriorates and the tendency toward white pig iron becomes stronger, which hinders the crystallization of graphite.
The lower limit of the Mo content is preferably 0.3% by mass. The upper limit of the Mo content is preferably 2.9% by mass.

(g) V: 0.2 to 3.8% by mass V is an element that combines with C to form hard MC carbide. If V is less than 0.2% by mass, the amount of MC carbide crystallized is insufficient. On the other hand, when V exceeds 3.8% by mass, MC carbide having a light specific gravity is concentrated inside the outer layer due to the centrifugal force during centrifugal casting, and not only the radial segregation of MC carbide becomes remarkable, but also MC carbide becomes coarse. The alloy structure becomes rough and the skin becomes rough during rolling. The lower limit of the V content is preferably 1.0% by mass, more preferably 1.5% by mass. The upper limit of the V content is preferably 3.7% by mass.

(h) Nb: 0.4 to 6.8% by mass Nb combines with C to form MC carbide. Nb is dissolved in MC carbide by compound addition with V to strengthen MC carbide and improve the wear resistance of the outer layer. Since the difference between the NbC-based MC carbide and the molten metal density is smaller than that of the VC-based MC carbide, segregation of the MC carbide is reduced. If Nb is less than 0.4% by mass, these effects are insufficient. When Nb is less than 0.4% by mass, the Nb content at positions 100 mm apart from both end faces of the outer layer in the axial direction is 0.5% by mass or more more than the amount of Nb at the center of the axial length of the outer layer. It becomes difficult to do. On the other hand, when Nb exceeds 6.8% by mass, MC carbides aggregate and it becomes difficult to obtain a healthy outer layer. The lower limit of the Nb content is preferably 0.5% by mass. The upper limit of the Nb content is preferably 6.7% by mass.

(ii) Arbitrary Composition The outer layer of the composite roll for hot rolling made by centrifugal casting of the present invention may contain at least one of the following elements in addition to the above-mentioned essential composition requirements.

(a) W: 0.01 to 3.0% by mass W combines with C to form hard _{carbides of M 2} C, which contributes to the improvement of wear resistance of the outer layer. It also has the effect of dissolving in MC carbide as a solid solution to increase its specific gravity and reduce segregation. However, when W exceeds 3.0% by mass, the specific gravity of the molten metal becomes heavy, so that carbide segregation is likely to occur. Therefore, when W is added, its preferable content is 3.0% by mass or less. The lower limit of the W content is more preferably 0.02% by mass. The upper limit of the W content is more preferably 2.9% by mass.

(b) Ti: 0.01 to 0.5% by mass Ti combines with N and O, which are graphitizing inhibitors, to form oxides or nitrides. Oxides or nitrides are suspended in the molten metal to form nuclei, which miniaturize and homogenize MC carbides. However, when Ti exceeds 0.5% by mass, the viscosity of the molten metal increases, and casting defects are likely to occur. Therefore, when Ti is added, its preferable content is 0.5% by mass or less. On the other hand, if Ti is less than 0.01% by mass, the effect of addition is insufficient. The lower limit of the Ti content is more preferably 0.02% by mass. The upper limit of the Ti content is more preferably 0.4% by mass.

(c) Al: 0.01 to 0.5% by mass Al combines with N and O, which are graphitizing inhibitors, to form oxides or nitrides, which are suspended in the molten metal to form nuclei, which form MC carbides. Crystallize finely and uniformly. However, if Al exceeds 0.5% by mass, the outer layer becomes brittle, leading to deterioration of mechanical properties. Therefore, when Al is added, its preferable content is 0.5% by mass or less. On the other hand, if the Al content is less than 0.01% by mass, the effect of addition is insufficient. The lower limit of the Al content is more preferably 0.02% by mass. The upper limit of the Al content is more preferably 0.4% by mass.

(d) Zr: 0.01 to 0.5% by mass Zr combines with C to form MC carbides, improving the wear resistance of the outer layer. Moreover, since the Zr oxide generated in the molten metal acts as crystal nuclei, the solidified structure becomes fine. It also increases the specific density of MC carbides to prevent segregation. However, if Zr exceeds 0.5% by mass, inclusions are formed, which is not preferable. Therefore, when Zr is added, its content is preferably 0.5% by mass or less. On the other hand, if Zr is less than 0.01% by mass, the effect of addition is insufficient. The lower limit of the Zr content is more preferably 0.02% by mass. The upper limit of the Zr content is more preferably 0.4% by mass.

(e) B: 0.001 to 0.5% by mass B has an action of refining carbides. In addition, a small amount of B contributes to the crystallization of graphite. However, when B exceeds 0.5% by mass, the whitening effect becomes strong and graphite becomes difficult to crystallize. Therefore, when B is added, its content is preferably 0.5% by mass or less. On the other hand, if B is less than 0.001% by mass, the effect of addition is insufficient. The lower limit of the B content is more preferably 0.002% by mass. The upper limit of the B content is more preferably 0.4% by mass.

(f) Co: 0.1 to 5.0% by mass Co is an element effective for strengthening the matrix structure. In addition, Co facilitates the crystallization of graphite. However, when Co exceeds 5.0% by mass, the toughness of the outer layer decreases. Therefore, when Co is added, its content is preferably 5.0% by mass or less. On the other hand, if Co is less than 0.1% by mass, the effect of addition is insufficient. The lower limit of the Co content is more preferably 0.2% by mass. The upper limit of the Co content is more preferably 4.9% by mass.

(iii) Impurities The rest of the outer layer composition consists substantially of Fe and unavoidable impurities. Of the unavoidable impurities, P and S cause deterioration of mechanical properties, so it is preferable to reduce them as much as possible. Specifically, the content of P is preferably 0.1% by mass or less, and the content of S is preferably 0.1% by mass or less. As other unavoidable impurities, elements such as Cu, Sb, Te, and Ce may be 0.7% by mass or less in total.

(2) Composition distribution In the composite roll for hot rolling made by centrifugal casting of the present invention, the C content at positions 100 mm apart from both end faces of the outer layer in the axial direction is the C content at the center of the axial length of the outer layer. The Nb content, which is 0.05 to 0.3% by mass more than the content and is 100 mm away from both end faces of the outer layer in the axial direction, is 0.5 to 0.5 to the Nb content at the center of the axial length of the outer layer. 3.0% more. That is, the C content and the Nb content of the outer layer are higher at the axial end than at the axial center. Although the reason for this has not been clarified, it was obtained by casting a molten metal having a specific composition by a specific centrifugal casting method described in the method for producing a composite roll for centrifugal casting and rolling of the present invention, which will be described later. I think it is.

With such a configuration, the amount of MC carbide at the axial end of the outer layer increases from the axial center of the outer layer, and the width of the outer layer center facing the center of the width of the rolled steel sheet and the width of both sides of the steel sheet. The wear progresses evenly with the outer layer end portion facing the vicinity of the end portion, and the local wear of the outer layer can be reduced.

The C content at positions 100 mm apart from both end faces of the outer layer in the axial direction is not more than 0.05% by mass with respect to the C content at the center of the axial length of the outer layer, or in the axial direction from both end faces of the outer layer. If the Nb content at positions 100 mm apart from each other is not more than 0.5% by mass with respect to the central Nb content of the axial length of the outer layer, the local wear at the end of the outer layer tends to be large, and the local area is likely to increase. The effect of reducing wear is insufficient.

If the C content at positions 100 mm apart from both end faces of the outer layer in the axial direction is more than 0.3% by mass with respect to the C content at the center of the axial length of the outer layer, or from both end faces of the outer layer to the shaft. If the Nb content at positions 100 mm apart in each direction is more than 3.0% by mass with respect to the Nb content at the center of the axial length of the outer layer, the outer layer end portion becomes too hard as compared with the outer layer central portion. The central portion of the outer layer and the end portion of the outer layer are less likely to be worn on average, and the effect of reducing local wear is insufficient.

The C content at the center of the axial length of the outer layer is preferably 3.6% by mass or less. If the C content at the center of the axial length of the outer layer exceeds 3.6% by mass, the C content at positions 100 mm apart from both end faces of the outer layer in the axial direction may exceed 3.8% by mass in the axial direction. This is because the toughness of the end portion is reduced, which may lead to a decrease in crack resistance. Further, it is preferable that Nb at the center of the axial length of the outer layer is 4% by mass or less. If the Nb content at the center of the axial length of the outer layer exceeds 4% by mass, the Nb content at positions 100 mm apart from both end faces of the outer layer in the axial direction may exceed 6.8% by mass in the axial direction. This is because MC carbides may aggregate at the edges, making it difficult to obtain a healthy outer layer.

(3) Structure The central part of the outer layer, which is the area mainly used for rolling, is described. The structure of the outer layer (central part of the outer layer) of the composite roll for hot rolling made by centrifugal casting of the present invention has a matrix, graphite, MC carbide, cementite, MC carbide and carbide other than cementite (M ₂ C, etc.). The structure of the outer layer of the composite roll for centrifugal casting and rolling of the present invention has a graphite phase of 0.3 to 10 area%. The outer layer structure has 3 to 20 area% of MC carbide. The outer base structure is preferably composed substantially of martensite, bainite or pearlite. The outer matrix structure preferably has an additional 15-45 area% cementite phase.

(a) Area ratio of graphite phase: 0.3 to 10% The area ratio of the graphite phase (graphite particles) crystallized in the outer layer structure is 0.3 to 10%. If the area ratio of the graphite phase is less than 0.3%, the effect of improving the seizure resistance of the outer layer is insufficient. On the other hand, when the graphite phase exceeds 10 area%, the mechanical properties of the outer layer deteriorate. The area ratio of the graphite phase is preferably 0.5 to 8%, more preferably 1 to 7%.

(b) Area ratio of MC carbide: 2 to 15% If the area ratio of MC carbide crystallized in the outer layer structure is less than 2%, the outer layer may not have sufficient wear resistance. Moreover, it is difficult to increase the area ratio of MC carbide to more than 15% due to the coexistence relationship with graphite. The area ratio of MC carbide is more preferably 2.2% or more, further preferably 2.5% or more. Further, the MC carbide is more preferably 12% or less, further preferably 10% or less, from the viewpoint of setting the area ratio of graphite to 0.3 to 10%. Further, 7% or less is more preferable.

Further, in order to obtain the effect of reducing local wear, the area ratio of MC carbides at positions 100 mm apart from both end faces of the outer layer in the axial direction is set with respect to the area ratio of MC carbides at the center of the axial length of the outer layer. , 0.5 to 3% larger is preferable in order to obtain the effect of reducing local wear.

(4) Characteristics (a) Wear resistance The wear resistance of the outer layer is _{obtained by hard carbides such as MC and M 2} C and a hard matrix structure. In particular, MC carbides consisting of V, Nb, etc. are very hard. The hard matrix structure is obtained by elements such as Mo and W.

(B) Shaft core The composition of ductile cast iron that forms the shaft core is C: 2.3 to 3.6%, Si: 1.5 to 3.5%, Mn: 0.2 to 2.0%, Ni: 0.3 to 2.0%, Cr on a mass basis. : 0.05 to 1.0%, Mo: 0.05 to 1.0%, Mg: 0.01 to 0.08%, and V: 0.05 to 1.0%, preferably composed of the balance Fe and unavoidable impurities. In addition to the above essential elements, Nb: 0.7% or less and W: 0.7% or less may be contained. In ductile cast iron, the iron base is mainly ferrite and pearlite, and the others mainly contain graphite and a trace amount of cementite.

(C) Roll size / application The size of the composite roll for hot rolling made by centrifugal casting of the present invention is not particularly limited, but a preferable example is an outer layer having an outer diameter of 400 to 1300 mm and an outer layer length of 1400 to 6000. In mm, the thickness of the outer layer used for rolling (effective rolling diameter) is 30 to 200 mm. The composite roll for hot rolling made by centrifugal casting of the present invention is suitable for a work roll for hot rolling of a steel sheet.

[2] Method for Manufacturing Composite Roll for Centrifugal Casting A method for manufacturing a composite roll for hot rolling for centrifugal casting of the present invention will be described with reference to FIG. The method for producing a composite roll for hot rolling made by centrifugal casting of the present invention is based on (a) mass-based inside a cylindrical mold 31 for centrifugal casting which is rotationally supported by a rotating roller 36 so as to apply a desired centrifugal force. C: 2.8-3.8%, Si: 0.1-3.0%, Mn: 0.3-2.0%, Ni: 2.3-5.5%, Cr: 0.5-2.5%, Mo: 0.2-3.0%, V: 0.3-4.0% , Nb: 0.5 to 4.0%, the balance is Fe and unavoidable impurities. The molten metal 32 for the outer layer is poured into the funnel 33, and from the end of the pouring nozzle 35, the shaft in the centrifugal casting mold. After dropping to the center of the direction and centrifugal casting, (b) after the outer layer is solidified, a cylindrical mold having an outer layer is erected, and upper and lower molds are provided at the upper and lower ends to form a static casting mold. It is manufactured by (c) casting a molten metal for a shaft core into a hollow portion (cavity) composed of the upper mold, the cylindrical mold having the outer layer, and the lower mold. Centrifugal casting is performed by horizontal or inclined centrifugal casting. A mold in which a cylindrical mold for forming an outer layer and an upper mold and a lower mold for forming a shaft core portion are integrally provided in advance may be used as a static casting mold. Here, the axial center portion in the centrifugal casting mold means a position within ± 300 mm in the axial direction from the center 34 of the internal dimension axial length of the centrifugal casting mold. The center 26 of the axial length of the outer layer of the composite roll of the present invention faces the axial center portion in the centrifugal casting die.

(1) Casting of molten metal for outer layer When casting molten metal for outer layer into a centrifugal casting die, in the case of horizontal or inclined centrifugal casting, the cast molten metal is axially forward and backward from the position where the molten metal falls. It is divided into both directions on the side and dispersed in the axial direction in the centrifugal casting die. The transferred molten metal solidifies by removing heat from the centrifugal casting die. By dropping the molten metal for the outer layer having the composition of the method for producing a composite roll for hot rolling by centrifugal casting of the present invention into the axial center in the centrifugal casting mold and performing centrifugal casting, from both end faces of the outer layer. The C content at positions 100 mm apart in the axial direction was 0.05 to 0.3 mass% more than the C content at the center of the axial length of the outer layer, and each was 100 mm away from both end faces of the outer layer in the axial direction. It is considered that a composite roll in which the Nb content at the position is 0.5 to 3.0% by mass higher than the Nb content at the center of the axial length of the outer layer can be obtained. Usually, the molten metal for the outer layer is cast into the centrifugal casting die from the ladle through the funnel 33, the pouring nozzle 35, etc., or from the tundish through the pouring nozzle 35, etc.

(2) Heat treatment It is desirable to perform tempering treatment at 400 to 550 ° C at least once after casting the shaft core of the composite roll.

The present invention will be described in detail with reference to the following examples, but the present invention is not limited thereto.

The molten metal having the composition (% by mass) shown in Table 1 is cast into a cylindrical mold for centrifugal casting made of ductile cast iron having an inner diameter of 400 mm and a length of 1500 mm, which rotates at high speed under the casting conditions shown in Table 2, and is an example. The outer layers of Nos. 1 and 2 and Comparative Examples 1 and 2 were centrifugally cast. For centrifugal casting, a long pouring nozzle 35 was used to adjust the axial drop position of the cylindrical mold for centrifugal casting of molten metal.

After the hollow outer layer has solidified, the rotation of the cylindrical mold for centrifugal casting is stopped, and an upper mold (length 1000 mm) and a lower mold (length 1000 mm) are provided at the upper and lower ends of the cylindrical mold, respectively. A mold for stationary casting was constructed. C: 3.1%, Si: 2.5%, Mn: 0.6%, P: 0.03% or less, Ni: 0.6%, Cr: Ductile cast iron molten metal for the shaft core, which contains 0.1%, Mo: 0.1%, V: 0.1%, Mg: 0.07% and has a composition of Fe and unavoidable impurities in the balance, is cast to keep the shaft core static. It was cast. The casting temperature of the molten ductile cast iron for the shaft core was 1450 ° C.

After the solidification of the shaft core was completed, the mold for static casting was disassembled, the obtained composite roll was taken out, and tempering was performed at 500 ° C. for 10 hours. As a result of inspection by ultrasonic flaw detection, it was confirmed that there were no defects in the obtained joint between the shaft core and the outer layer, and that both were welded soundly. The average thickness of the obtained outer layer was 96 mm. Then, machining was performed to obtain composite rolls of Examples 1 and 2 and Comparative Examples 1 and 2 having an outer layer having an outer diameter of 370 mm and an outer layer having a length of 1200 mm. During machining, the machining allowance from both end faces of the axial length of the outer layer was made uniform.

At a depth of 25 mm from the outer layer surface of the test piece cut out from the outer layer of the obtained composite rolls of Examples 1 and 2 and Comparative Examples 1 and 2 from a position 100 mm away from the center and end faces of the axial length of the outer layer. The composition of the outer layer was analyzed. The results are shown in Table 3.

In addition, these test pieces were mirror-polished and etched, and optical micrographs were taken. From the optical micrographs, the area ratios of graphite particles and MC carbides were determined using image analysis software. The results are shown in Table 4.

In the composite rolls of Examples 1 and 2, the C content at positions 100 mm apart from both end faces of the outer layer in the axial direction is 0.05 to 0.3% by mass more than the C content at the center of the axial length of the outer layer. The Nb content at positions 100 mm apart from both end faces of the outer layer in the axial direction was 0.5 to 3.0% by mass higher than the Nb content at the center of the axial length of the outer layer. On the other hand, in the composite rolls of Comparative Examples 1 and 2, the C content at positions 100 mm apart from both end faces of the outer layer in the axial direction is 0.05% by mass with respect to the C content at the center of the axial length of the outer layer. The Nb content at positions 100 mm apart from both end faces of the outer layer in the axial direction was less than 0.5% by mass with respect to the Nb content at the center of the axial length of the outer layer.

As described above, since the composite rolls of Examples 1 and 2 have a difference in Nb content between the outer layer end portion and the outer layer central portion, the outer layer central portion facing the width central portion of the rolled steel sheet and both sides of the steel sheet. Wear progresses on average with the outer layer end facing the width end, that is, the outer layer end facing the width end on both sides of the rolled steel sheet wears significantly more locally than the outer layer center. It can be expected to reduce the local wear of the outer layer. On the other hand, the composite rolls of Comparative Example 1 and Comparative Example 2 have the same Nb content at the outer layer end portion and the outer layer central portion, with respect to the outer layer central portion facing the width central portion of the rolled steel sheet. , The amount of wear of the outer layer end portions facing the vicinity of the width ends on both sides of the steel sheet increases, and local wear of the outer layer occurs, so that the loss of the outer layer may increase.

1 ... Composite roll for rolling, 2 ... Outer layer, 3 ... Shaft core, 4 ... Body core, 5 ... Shaft, 6 ... Shaft, 21 ... Outer layer Central part, 22 ... one outer layer end face, 23 ... the other outer layer end face, 24 ... outer layer end part, 25 ... outer layer end part 26 ... central 27 of the axial length of the outer layer, 28 ・ ・ ・ Positions 100 mm apart from both end faces of the outer layer in the axial direction 31, 41 ・ ・ ・ Centrifugal casting mold, 32 ・ ・ ・ Molten metal, 33, 43 ・ ・ ・ Lute, 34 ・ ・ ・ Inside the mold Dimensional axis length center, 35, 45 ... pouring nozzle 36, 46 ... rotating roller

Claims (2)

In a rotating centrifugal casting mold, C: 2.8 to 3.8%, Si: 0.1 to 3.0%, Mn: 0.3 to 2.0%, Ni: 2.3 to 5.5%, Cr: 0.5 to 2.5%, Mo A molten metal for the outer layer containing 0.2 to 3.0%, V: 0.3 to 4.0%, Nb: 0.5 to 4.0% and the balance consisting of Fe and unavoidable impurities is placed in the axial center of the centrifugal casting mold. A method for producing a composite roll for hot rolling by centrifugal casting, which comprises pouring molten duct iron for forming a shaft core into the centrifugal casting mold after dropping and centrifugal casting. In the method for producing a composite roll for hot rolling by centrifugal casting according to claim 1, the molten metal for the outer layer is further based on mass, W: 0.01 to 3.0%, Ti: 0.01 to 0.5%, Al: 0.01 to 0.5. A method for producing a composite roll for hot rolling by centrifugal casting, which comprises any one or more of%, Zr: 0.01 to 0.5%, B: 0.001 to 0.5% and Co: 0.1 to 5.0%.

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