CN109641250B

CN109641250B - Outer layer material for hot rolling roll and composite roll for hot rolling

Info

Publication number: CN109641250B
Application number: CN201780050783.6A
Authority: CN
Inventors: 岩田直道; 铃木健史; 松村祥一
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2016-09-07
Filing date: 2017-06-28
Publication date: 2020-11-03
Anticipated expiration: 2037-06-28
Also published as: JPWO2018047444A1; JP6292362B1; KR20190029716A; EP3479915A4; EP3479915A1; WO2018047444A1; CN109641250A; KR102234330B1; TWI655295B; BR112019003360A2; TW201812028A; EP3479915B1

Abstract

The invention aims to provide a hot-rolling roll outer layer material and a hot-rolling composite roll, which ensure wear resistance, reduce pit-like defects on the surface of the roll and have excellent surface roughness resistance. The outer layer material of the hot rolling roll is characterized by having the following composition: contains, in mass%, C: 2.0-3.0%, Si: 0.2 to 1.0%, Mn: 0.2-1.0%, Cr: 4.0 to 7.0%, Mo: 3.0-6.5%, V: 5.0 to 7.5%, Nb: 0.5 to 3.0%, Ni: 0.05 to 3.0%, Co: 0.2-5.0%, W: 0.5 to 5.0%, wherein the contents of C, Cr, Mo, V, Nb, Ni and W satisfy the following formula (1), the balance being Fe and unavoidable impurities, 85% or more of the matrix structure is a tempered martensite and/or bainite structure, and the minor axis of the tempered martensite or bainite is 0.5 to 3.0 μm. 0.05 ≦ (% C-% (% V × 0.177-% Nb × 0.129-% Cr × 0.099-% Mo × 0.063-% W × 0.033) + (% Ni) ≦ 4.0 (1). Wherein,% C,% V,% Nb,% Cr,% Mo,% W,% Ni are the contents (mass%) of the respective elements.

Description

Outer layer material for hot rolling roll and composite roll for hot rolling

Technical Field

The present invention relates to a hot-rolling clad roll, and more particularly to a hot-rolling roll outer layer material and a hot-rolling clad roll suitable for use in a hot-rolling finishing mill for a steel sheet.

Background

In recent years, with the progress of hot rolling technology of steel sheets, the use environment of rolls has become severer, and the production amount of steel sheets having a large rolling load, such as high-strength steel sheets and thin-walled products, has also increased. Therefore, in the work rolls for rolling, surface roughening and chipping defects (chipping defects) due to fatigue of the rolled surface are often generated, and higher levels of surface roughening resistance and chipping resistance than in the past are strongly required. In hot rolling, high-speed steel rolls are often used, which are added with V in an amount of several% to form a large amount of hard carbides, thereby improving wear resistance.

As an outer layer material of such a high-speed steel roll, for example, patent document 1 proposes a roll outer layer material for rolling containing C: 1.5-3.5%, Ni: 5.5% or less, Cr: 5.5 to 12.0%, Mo: 2.0-8.0%, V: 3.0-10.0%, Nb: 0.5 to 7.0%, and Nb and V are contained so that the contents of Nb, V and C satisfy a specific relationship and the ratio of Nb to V falls within a specific range. Thus, even if the centrifugal casting method is applied, the segregation of hard carbide in the outer layer material is suppressed, and the rolling roll outer layer material having excellent wear resistance and crack resistance is obtained. Patent document 2 proposes a roll cover material for rolling, which contains C: 1.5-3.5%, Cr: 5.5 to 12.0%, Mo: 2.0-8.0%, V: 3.0-10.0%, Nb: 0.5 to 7.0%, and Nb and V are contained so that the contents of Nb, V and C satisfy a specific relationship and the ratio of Nb to V falls within a specific range. Thus, even if the centrifugal casting method is applied, the segregation of hard carbide in the outer layer material is suppressed, the wear resistance and the crack resistance are improved, and a large contribution is made to the improvement of the productivity of hot rolling.

On the other hand, from the viewpoint of improving the quality of hot rolled products and improving productivity, the use environment of hot rolling rolls is becoming severer, and the continuous rolling amount of steel sheets is increasing. In addition, the requirements for the surface quality of hot rolled products are becoming more stringent. Therefore, suppression of fatigue damage of the roller surface such as surface roughening becomes a greater problem than suppression of wear. In order to solve such a problem, patent document 3 proposes a centrifugal casting composite roll including C: 2.2-2.6%, Cr: 5.0-8.0%, Mo: 4.4-6.0%, V: 5.3 to 7.0%, Nb: 0.6 to 1.3%, and the contents of C, Mo, V and Nb are adjusted so that Mo + V, C-0.24V-0.13Nb is in a specific range, thereby making the fatigue resistance of the roll surface layer under the hot rolling environment excellent. Patent document 4 proposes a roll cover material for rolling, which contains C: 1.3-2.2%, Si: 0.3-1.2%, Mn: 0.1-1.5%, Cr: 2.0-9.0%, Mo: 9.0% or less, V: 4.0 to 15.0%, and W: 20.0% or less, Ni: 5.0% or less, Co: 10.0% or less, and the balance substantially Fe and inevitable impurities, wherein the size of carbide dispersed in the structure is within a specific range. In patent document 4, pit defects can be reduced by reducing the amount of eutectic carbides that easily form coarse carbides.

Documents of the prior art

Patent document

Patent document 1: japanese laid-open patent publication No. H04-365836

Patent document 2: japanese laid-open patent publication No. H05-1350

Patent document 3: japanese laid-open patent publication No. 2009-221573

Patent document 4: japanese patent No. 3962838

Disclosure of Invention

Problems to be solved by the invention

However, the recent rolling technology has been advancing at a remarkable rate toward the improvement of quality and grade of rolled steel sheets. At the same time, since the cost reduction is also strictly demanded for the rolling, the use environment of the roll is becoming severer. Therefore, in the conventional design of the roller material focusing only on the carbide, the generation of the crater-like defects may not be reduced.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a hot-rolling roll outer layer material and a hot-rolling clad roll which have excellent surface roughness resistance, while ensuring wear resistance and reducing crater defects on the roll surface.

Means for solving the problems

The inventors of the present application examined the pits formed on the surface of the hot-rolling rollThe generation site of the defect-like was examined in detail. As a result, it was clarified that the pit-like defects are caused by eutectic carbide (mainly M)₂C is, M₆C is, M₇C₃Is and M₂₃C₆Carbide) propagates in the matrix structure and chips peel off (chip away) into pits. Therefore, in order to reduce the crater defects, it is considered effective to reduce the propagation speed of cracks in the matrix structure, in addition to focusing on the kind and size of carbides as in the conventional art, and the present invention has been completed. That is, various factors affecting the hot rolling fatigue resistance and the matrix structure size of the roll outer layer material have been studied, and as a result, there has been obtained a conventionally unknown finding that the fatigue resistance during hot rolling is significantly improved by adjusting the composition ranges of the respective elements and adjusting the contents of the respective elements so that the respective elements satisfy specific relationships. Further, it is also found that fatigue resistance during hot rolling is further remarkably improved by controlling the matrix structure size.

First, the experimental results that are the basis of the present invention will be explained. A molten metal having the following composition was melted in a high-frequency induction furnace, and a ring-shaped roll material (outer diameter: 250 mm. phi., width: 65mm, wall thickness: 55mm) corresponding to a roll outer layer material was cast by a centrifugal casting method, the composition being: set as Si: 0.1 to 1.5%, Mn: 0.1 to 1.5%, C is in the range of 1.6 to 3.5%, Cr is in the range of 3.5 to 9.0%, Mo is in the range of 2.1 to 7.0%, V is in the range of 4.1 to 8.5%, Nb is in the range of 0.3 to 4.6%, Ni is in the range of 0.02 to 3.6%, Co is in the range of 0.3 to 8.0%, W is in the range of 0.2 to 8.0%, and the balance is Fe and unavoidable impurities. The casting temperature was 1450 ℃ to 1530 ℃, and the centrifugal force was set so that the outer peripheral portion of the ring roll material became 180G in terms of the multiple of gravity. After casting, quenching and tempering are performed to make the hardness HS 78-86. The quenching treatment is a treatment of heating to a heating temperature of 1070 ℃ and then air-cooling. In addition, the tempering treatment is carried out 2 or 3 times at a temperature of 530 to 570 ℃ depending on the composition so that the retained austenite amount is less than 10% (by volume%).

A hot rolling fatigue test piece (outer diameter: 60 mm. phi., wall thickness: 10mm) was collected from the obtained ring-shaped roll material, and a hot rolling fatigue test showing that the fatigue resistance of the hot rolling work roll in actual production can be evaluated with good reproducibility was carried out in accordance with Japanese patent application laid-open No. 2010-101752. In the fatigue test piece, notches (depth t: 1.2mm, circumferential length L: 0.8mm) as shown in FIG. 1 were introduced into 2 positions of the outer peripheral surface by an electric discharge machining (wire cutting) method using a wire rod (wire) of 0.2mm in diameter. In addition, the end of the rotating surface of the fatigue test piece was chamfered at 1.2C.

As shown in fig. 1, the hot rolling fatigue test was performed by a double-disc rolling-sliding-rolling mode (rolling-sliding mode) of a test piece having a notch (hot rolling fatigue test piece) and a heated counterpart. That is, as shown in FIG. 1, a test piece (hot-rolled fatigue test piece) 1 was rotated at 700rpm while being water-cooled with cooling water 2, and a mating piece (material: S45C, outer diameter: 190 mm. phi., width: 15mm)4 heated to 800 ℃ by a high-frequency induction heating coil 3 was pressed against the rotating test piece 1 with a load of 980N while being rotated at a slip ratio of 9%. The hot-rolled fatigue test piece 1 was rotated until the two notches 5 introduced into the hot-rolled fatigue test piece 1 were broken, the number of rotation times until the respective notches were broken was obtained, and the average value thereof was defined as the hot-rolled fatigue life. Further, the case where the hot rolling fatigue life exceeds 350 thousand times was evaluated as significantly excellent.

Further, the obtained ring-shaped roll material was subjected to structure observation. For the structure observation, a 10 × 10 × 5mm (5mm is the thickness direction of the ring) structure observation test piece was collected at an arbitrary position inside the ring-shaped roller material 10mm from the outer surface, and the 10 × 10mm surface was mirror-polished, etched with a nital solution (5 vol% nitric acid + ethanol) for about 10 seconds, and then the structure was observed using an optical microscope.

In order to measure the minor axis (minor axis length) of tempered martensite or bainite, measurement was performed by sampling at an arbitrary position within 10mm from the outer surface of the obtained ring roll materialTest pieces (5 mm. times.10 mm. times.5 mm) were mirror-polished on a surface of 5 mm. times.10 mm, and EBSD measurement was carried out. Under the conditions of 15kV of accelerating voltage and 0.1 μm of stepping size, the voltage is 10000 μm²The above regions were measured by Electron Back Scattering Diffraction (EBSD). Boundary lines are drawn at positions differing in azimuth from adjacent measurement points by 15 ° or more, and as shown in fig. 2, the region surrounded by the boundary lines is defined as one crystal, and the short diameters of 20 crystals having a long diameter of 5 μm or more are measured on the measurement surface, and the average value thereof is calculated.

As for the obtained results, the relationship between the hot rolling fatigue life and (% C-% > V × 0.177-% > Nb × 0.129-% > Cr × 0.099-% > Mo × 0.063-% > W × 0.033) + (% Ni) is shown in fig. 3, and the relationship between the hot rolling fatigue life and the short diameter of tempered martensite or bainite is shown in fig. 4.

As is clear from fig. 3, when (% C-%) V × 0.177-% > Nb × 0.129-% > Cr × 0.099- > Mo × 0.063-% W × 0.033) + (% Ni) is 0.05 or more and 4.0 or less, the hot rolling fatigue life is significantly improved. Wherein V, Cr, Mo, Nb, and W are elements which easily form carbides, and (% C-% V × 0.177-% Nb × 0.129-% Cr × 0.099-% Mo × 0.063-% W × 0.033) represents the amount of carbon dissolved in the matrix. Therefore, (% C-% > V.times.0.177-% Nb. times.0.129-% Cr. times.0.099-% Mo. times.0.063-% W. times.0.033) + (% Ni) is the sum of the amount of carbon and the amount of Ni dissolved in the matrix, and by adjusting this value to an appropriate range, the propagation speed of cracks in the matrix becomes slow, and a roll outer layer material excellent in hot rolling fatigue life can be obtained. Further, by satisfying the above composition ranges and controlling the crystal size of tempered martensite or bainite in the matrix structure within the range shown in fig. 4, the hot rolling fatigue life can be remarkably improved.

The present invention has been completed based on the above findings, and the gist thereof is as follows.

[1] The outer layer material of the hot-rolling roll is characterized by comprising the following components: contains, in mass%, C: 2.0-3.0%, Si: 0.2 to 1.0%, Mn: 0.2-1.0%, Cr: 4.0 to 7.0%, Mo: 3.0-6.5%, V: 5.0 to 7.5%, Nb: 0.5 to 3.0%, Ni: 0.05 to 3.0%, Co: 0.2-5.0%, W: 0.5 to 5.0%, and the contents of C, Cr, Mo, V, Nb, Ni and W satisfy the following formula (1), and the balance being Fe and unavoidable impurities,

in the outer layer material of the hot rolling roller, more than 85% of the matrix structure is tempered martensite and/or bainite structure, and the minor diameter of the tempered martensite or bainite is 0.5-3.0 μm.

0.05≤(％C-％V×0.177-％Nb×0.129-％Cr×0.099-％Mo×0.063-％W×0.033)+(％Ni)≤4.0 (1)

Wherein,% C,% V,% Nb,% Cr,% Mo,% W,% Ni are the contents (mass%) of the respective elements.

[2] A composite roll for hot rolling, which is formed by fusing and integrating an outer layer and an inner layer, characterized in that the outer layer is formed of the outer layer material of the roll for hot rolling of [1 ].

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, a hot-rolling roll outer layer material and a hot-rolling composite roll, in which the propagation speed of cracks is significantly reduced, can be produced. As a result, surface damage due to hot rolling, such as surface roughening and chipping, can be suppressed, and the effects of extending the continuous rolling distance and improving the roll life can be achieved.

Drawings

Pharynx 1 fig. 1 is an explanatory view schematically showing the configuration of a testing machine used in a thermal rolling fatigue test, a thermal rolling fatigue test piece (fatigue test piece), and the shape and size of a notch introduced to the outer peripheral surface of the thermal rolling fatigue test piece (fatigue test piece).

Pharynx 2 fig. 2 is a graph showing the results of measurement of the outer layer material of the hot-rolling roll according to the embodiment of the present invention by EBSD.

FIG. 3 is a graph showing the relationship between the hot rolling fatigue life and (% C-% > V.times.0.177-% Nb.times.0.129-% Cr.times.0.099-% Mo.times.0.063-% W.times.0.033) + (% Ni) in the hot rolling fatigue test.

FIG. 4 is a graph showing the relationship between the hot rolling fatigue life and the minor axis of tempered martensite or bainite in the hot rolling fatigue test.

Detailed Description

The outer layer material for a hot-rolling roll of the present invention can be produced by a known casting method such as a centrifugal casting method or a continuous casting cladding method, and can be used as it is as a ring roll or a sleeve roll (sleeve roll). The composite roll for hot rolling according to the present invention is composed of an outer layer and an inner layer welded and integrated with the outer layer. An intermediate layer may be disposed between the outer layer and the inner layer. That is, instead of the inner layer fusion-bonded to the outer layer, an intermediate layer fusion-bonded to the outer layer and an inner layer fusion-bonded to the intermediate layer may be formed. In the present invention, the compositions of the inner layer and the intermediate layer are not particularly limited, and it is preferable that the inner layer is spheroidal graphite cast iron (nodular cast iron) or forged steel, and the intermediate layer is C: 1.5 to 3.0 mass% of a high carbon material.

First, the reason for defining the composition of the outer layer (outer layer material) of the hot-rolling clad roll of the present invention will be described. Hereinafter, unless otherwise specified, the mass% is simply referred to as%.

C：2.0～3.0％

C has the following functions: the hard carbide is formed by bonding with the carbide-forming element, thereby improving the wear resistance of the roll outer layer material. The amount of eutectic carbides varies depending on the C content. Eutectic carbides affect rolling service characteristics. Therefore, when the C content is less than 2.0%, the amount of eutectic carbides is insufficient, the friction force during rolling increases, rolling becomes unstable, and the amount of C dissolved in the matrix structure is low, so that the hot rolling fatigue resistance is lowered. On the other hand, if C is contained in an amount exceeding 3.0%, the carbide becomes coarse, the amount of eutectic carbide increases excessively, the roll outer layer material becomes hard and brittle, the generation and growth of fatigue cracks are accelerated, and the fatigue resistance is lowered. Therefore, C is limited to a range of 2.0 to 3.0%. The content is preferably 2.1 to 2.8%.

Si：0.2～1.0％

Si is an element that acts as a deoxidizer and improves castability of molten metal. In order to obtain such an effect, Si must be contained by 0.2% or more. On the other hand, even if Si is contained in an amount exceeding 1.0%, the effect is saturated and an effect commensurate with the content cannot be expected, which is economically disadvantageous, and the matrix structure may be embrittled. Therefore, Si is limited to 0.2 to 1.0%. Preferably, the concentration is 0.3 to 0.7%.

Mn：0.2～1.0％

Mn is an element having an effect of fixing S to MnS to make S harmless and an effect of improving hardenability by partially dissolving it in a matrix structure. In order to obtain such an effect, 0.2% or more of Mn must be contained. On the other hand, even if Mn is contained in an amount exceeding 1.0%, the effect is saturated, and the effect commensurate with the content cannot be expected, and the material may be embrittled. Therefore, Mn is limited to 0.2 to 1.0%. Preferably, the concentration is 0.3 to 0.8%.

Cr：4.0～7.0％

Cr is an element having the following effects: which forms eutectic carbides mainly by bonding with C to improve wear resistance, and reduces friction with the steel sheet during rolling to reduce surface damage of the rolls and stabilize rolling. In order to obtain such an effect, 4.0% or more of Cr must be contained. On the other hand, when Cr is contained in an amount exceeding 7.0%, coarse eutectic carbides increase, and fatigue resistance is lowered. Therefore, Cr is limited to a range of 4.0 to 7.0%. Preferably, the concentration is 4.3 to 6.5%.

Mo：3.0～6.5％

Mo is an element that forms hard carbide by bonding with C to improve wear resistance. Further, Mo is dissolved in solid solution in hard MC type carbides to which V, Nb, and C are bonded to strengthen the carbides, and also dissolved in eutectic carbides to increase fracture resistance (fracture resistance) of the carbides. Through such an action, Mo improves the wear resistance and fatigue resistance of the roll outer layer material. In order to obtain such an effect, 3.0% or more of Mo must be contained. On the other hand, when more than 6.5% of Mo is contained, brittle and hard carbides mainly composed of Mo are generated, and the hot rolling fatigue resistance is lowered, and the fatigue resistance is lowered. Therefore, Mo is limited to a range of 3.0 to 6.5%. Preferably, the concentration is 3.5 to 6.0%.

V：5.0～7.5％

V is an important element for achieving both wear resistance and fatigue resistance of the roller in the present invention. V is the following element: this improves wear resistance by forming extremely hard carbide (MC type carbide), and also effectively acts to divide eutectic carbide and disperse the carbide into crystals, thereby improving rolling fatigue resistance and remarkably improving fatigue resistance of the roll outer layer material. Such an effect becomes remarkable by containing 5.0% or more of V. On the other hand, when V is contained in an amount exceeding 7.5%, the MC type carbide is coarsened, and thus various properties of the rolling roll become unstable. Therefore, V is limited to a range of 5.0 to 7.5%. Preferably, the concentration is 5.2 to 7.0%.

Nb：0.5～3.0％

Nb is dissolved in the MC type carbide to strengthen the MC type carbide, and improves the wear resistance, particularly the fatigue resistance, through the effect of increasing the fracture resistance of the MC type carbide. By dissolving Nb in carbide together with Mo, the wear resistance and fatigue resistance are remarkably improved. In addition, Nb is an element that promotes the splitting of eutectic carbides and suppresses the fracture of eutectic carbides, thereby improving the fatigue resistance of the roll outer layer material. In addition, Nb also has an effect of suppressing segregation during centrifugal casting of MC type carbide. Such an effect becomes remarkable by containing 0.5% or more of Nb. On the other hand, if the content exceeds 3.0%, the growth of MC type carbide in the molten metal is promoted, and the hot rolling fatigue resistance is deteriorated. Therefore, Nb is limited to a range of 0.5 to 3.0%. Preferably, the concentration is 0.8 to 1.5%.

Ni：0.05～3.0％

Ni is an element that is dissolved in the matrix, lowers the transformation temperature of austenite during heat treatment, and improves the hardenability of the matrix. In order to obtain such an effect, 0.05% or more of Ni must be contained. On the other hand, when Ni is contained in an amount exceeding 3.0%, the austenite transformation temperature becomes too low and the hardenability is improved, so that austenite is likely to remain after the heat treatment. If austenite remains, cracks or the like occur during hot rolling, and the hot rolling fatigue resistance is reduced. Therefore, Ni is limited to the range of 0.05 to 3.0%. In view of ease of operation, in which the crystal size of the matrix structure can be made fine even if the cooling rate in the heat treatment is slow, it is preferably 0.2 to 3.0%.

Co：0.2～5.0％

Co is an element having the following effects: solid-dissolved in the matrix, thereby strengthening the matrix, particularly at high temperatures, resulting in improved fatigue resistance. In order to obtain such an effect, 0.2% or more of Co must be contained. On the other hand, even if Co is contained in an amount exceeding 5.0%, the effect is saturated, and an effect commensurate with the content cannot be expected, which is economically disadvantageous. Therefore, Co is limited to the range of 0.2 to 5.0%. Preferably, the concentration is 0.5 to 3.0%.

W：0.5～5.0％

W is an element having an action of strengthening the matrix, particularly at high temperatures, by dissolving W in the matrix, thereby improving fatigue resistance, and M is formed₂C or M₆The C-based carbide improves the wear resistance. In order to obtain such an effect, it is necessary to contain 0.5% or more of W. On the other hand, when W is contained in an amount exceeding 5.0%, not only the effect is saturated but also coarse M is formed₂C or M₆C-based carbide lowers the hot rolling fatigue resistance. Therefore, W is limited to the range of 0.5 to 5.0%. The content is preferably 1.0 to 3.5%.

In the present invention, C, Cr, Mo, V, Nb, Ni, and W are contained in the above ranges, and each element is contained so as to satisfy the following formula (1).

By adjusting (% C-% > V × 0.177-% > Nb × 0.129-% > Cr × 0.099-% Mo × 0.063-% W × 0.033) + (% Ni) so as to satisfy the above formula (1), the number of damaged turns is significantly increased, and the hot rolling contact fatigue resistance is significantly improved. (% C-% V × 0.177-% Nb × 0.129-% Cr × 0.099-% Mo × 0.063-% W × 0.033) + (% Ni) is an important factor as a driving force for improving the hot rolling fatigue resistance, and when the driving force is out of the range of the above formula (1), the hot rolling fatigue resistance is deteriorated. V, Cr, Mo, Nb, and W are elements that easily form carbides, and (% C-% > V × 0.177-% Nb × 0.129-% Cr × 0.099-% Mo × 0.063-% W × 0.033) represents the amount of carbon dissolved in the matrix. Therefore, (% C-% > V.times.0.177-% Nb. times.0.129-% Cr. times.0.099-% Mo. times.0.063-% W. times.0.033) + (% Ni) is the sum of the amount of carbon and the amount of Ni dissolved in the matrix, and by adjusting this value to an appropriate range, the propagation speed of cracks in the matrix becomes slow, and a roll outer layer material excellent in hot rolling fatigue life can be obtained. Therefore, in the present invention, (% C-%) V × 0.177-% > Nb × 0.129-% > Cr × 0.099-% Mo × 0.063-% > W × 0.033) + (% Ni) is adjusted so as to satisfy the above formula (1).

The balance other than the above components is Fe and inevitable impurities.

In the present invention, it is preferable that 85% or more of the matrix structure is a tempered martensite and/or bainite structure, and the minor axis of the tempered martensite or bainite is 0.5 to 3.0 μm. Since the hot rolling fatigue resistance is reduced when the fraction of retained austenite and pearlite structure is large, the matrix structure preferably contains tempered martensite and/or bainite structure in an amount of 85% or more, and more preferably 90% or more from the viewpoint of hot rolling fatigue resistance. The balance may be retained austenite and/or pearlite. The number of repetitions of the step of heating to 500 to 570 ℃ and holding and then cooling may be controlled so that at least 85% of the matrix structure is tempered martensite and/or bainite.

In addition, in a component system in which the minor axis of tempered martensite or bainite is less than 0.5 μm, the transformation temperature is too low, and even if tempering is repeated, it is difficult to reduce the retained austenite amount, and cracks due to the retained austenite may be generated in hot rolling, and the hot rolling fatigue resistance may be reduced. Further, when the minor axis of tempered martensite or bainite exceeds 3.0 μm, the crack propagation speed of the matrix structure becomes high, and the hot rolling fatigue resistance is lowered. Therefore, the minor axis of tempered martensite or bainite in the matrix structure is preferably limited to a range of 0.5 to 3.0 μm. From the viewpoint of thermal rolling fatigue resistance, it is preferably in the range of 0.5 to 2.0. mu.m. In order to obtain the short diameter, the composition and cooling rate may be controlled so that the phase transition temperature of the matrix is in the range of 200 to 400 ℃.

Next, a preferred method for producing the hot-rolling clad roll of the present invention will be described.

In the present invention, the method for producing the roll shell material is preferably produced by a known casting method such as a centrifugal casting method or a continuous casting and cladding method. Needless to say, the present invention is not limited to these methods.

When casting a roll outer layer material by a centrifugal casting method, first, a molten metal composed of the roll outer layer material is cast into a rotating mold having an inner surface coated with a refractory mainly made of zircon or the like in a thickness of 1 to 5mm so as to have a predetermined thickness, and centrifugal casting is performed. Among them, the rotational speed of the mold is preferably set so that the multiple of gravity applied to the outer surface of the roll is in the range of 120 to 220G. In the case of forming the intermediate layer, it is preferable to perform centrifugal casting by casting a molten metal having the composition of the intermediate layer while rotating the mold during or after the solidification of the roll outer layer material. Preferably, after the outer layer or the intermediate layer is completely solidified, the rotation of the mold is stopped and the mold is raised, and then the inner layer material is subjected to stationary casting to produce the composite roll. The inner surface side of the roll outer layer material is thereby remelted to form a composite roll in which the outer layer is integrated by welding with the inner layer, or the outer layer is integrated by welding with the intermediate layer, or the intermediate layer is integrated by welding with the inner layer.

As the inner layer to be still cast, nodular cast iron, vermicular cast iron (CV cast iron), or the like having excellent castability and mechanical properties is preferably used. The outer layer and the inner layer of the centrifugal casting roll are integrally welded, so that about 1-8% of the components of the outer layer material are mixed into the inner layer. When carbide-forming elements such as Cr and V contained in the outer layer material are mixed in the inner layer, the inner layer is weakened. Therefore, the mixing ratio of the outer layer component into the inner layer is preferably suppressed to less than 6%.

In the case of forming the intermediate layer, graphite steel, high-carbon steel, hypoeutectic cast iron, or the like is preferably used as the intermediate layer material. The intermediate layer and the outer layer are integrally welded in the same manner, and the outer layer component is mixed into the intermediate layer in the range of 10-95%. From the viewpoint of suppressing the mixing amount of the outer layer component into the inner layer, it is important to reduce the mixing amount of the outer layer component into the intermediate layer as much as possible.

The hot-rolling composite roll of the present invention is preferably subjected to heat treatment after casting. The heat treatment is preferably performed 2 or more times by heating to 950 to 1100 ℃ and air-cooling or air-cooling (air blast cooling), and further heating to 500 to 570 ℃ and holding, and then cooling. In this case, the aforementioned preferable minor axis size can be obtained by adjusting the cooling rate in accordance with the composition so that the phase transition temperature is in the range of 200 to 400 ℃. The amount of tempered martensite and/or bainite in the matrix structure varies depending on the number of repetitions of the step of heating to 500 to 570 ℃ and holding and then cooling, and therefore the number of repetitions may be set so that 85% or more of the matrix structure is tempered martensite and/or bainite.

The hardness of the hot-rolling clad roll of the present invention is preferably 79 to 88HS (shore hardness), and more preferably 80 to 86 HS. If the hardness is less than 80HS, the wear resistance is deteriorated, whereas if the hardness exceeds 86HS, it is difficult to remove cracks formed on the surface of the hot rolling roll during hot rolling by polishing. The heat treatment temperature and the heat treatment time after casting are preferably adjusted so that such hardness can be stably ensured.

Examples

The molten metal having the composition of the material for the outer layer of the roll shown in Table 1 was obtained by melting in a high-frequency induction furnace, and the ring-shaped test material (ring-shaped roll; outer diameter: 250 mm. phi., width: 65mm, wall thickness: 55mm) was produced by a centrifugal casting method. The casting temperature is 1450 to 1530 ℃, and the centrifugal force is set so that the outer peripheral portion of the ring roll material is 180G in terms of the multiple of gravity. After casting, the steel is quenched by reheating to a quenching temperature of 1070 ℃ and then air-cooling, and the tempering is performed 2 or 3 times at a temperature of 530 to 570 ℃ depending on the composition so that the retained austenite amount is less than 10% (by volume), and the hardness is adjusted to 78 to 86 HS. From the obtained annular test material, a hardness test piece, a thermal rolling fatigue test piece, and an EBSD measurement test piece were collected, and a hardness test, a thermal rolling fatigue test, and a structure observation test were performed.

[ Table 1]

The hardness test piece thus obtained was measured for Vickers hardness HV50 using a Vickers hardness tester (test force: 50kgf (490N)) in accordance with JIS Z2244, and converted to Shore HS in accordance with the JIS conversion table. The measurement points were set to 10 points each, and the highest value and the lowest value were removed to calculate an average value as the hardness of the test material.

The hot rolling fatigue test method is as follows. A hot rolling fatigue test piece (outer diameter: 60 mm. phi., wall thickness: 10mm, chamfered) was collected from the obtained ring-shaped test material. In the hot rolling fatigue test piece, notches (depth t: 1.2mm, circumferential length L: 0.8mm) as shown in FIG. 1 were introduced into two portions (positions 180 degrees apart) of the outer peripheral surface by an electrical discharge machining (wire cutting) method using a wire rod of 0.20mm φ. As shown in fig. 1, the thermal rolling fatigue test was performed by a double-disk sliding rotation manner of the test piece and the counterpart. A test piece 1 was rotated at 700rpm while being water-cooled with cooling water 2, and a mating piece (material: S45C, outer diameter: 190 mm. phi., width: 15mm, C1 chamfer) 4 heated to 800 ℃ by a high-frequency induction heating coil 3 was pressed against the rotating test piece 1 with a load of 980N and rotated at a slip ratio of 9%. Then, the hot rolling fatigue test piece 1 was rotated until the two notches 5 introduced into the hot rolling fatigue test piece 1 were broken, the number of rotation rotations until the respective notches were broken was obtained, and the average value thereof was used as the hot rolling fatigue life. Further, the case where the hot rolling fatigue life exceeds 350 thousand times was evaluated as significantly excellent.

For the structure observation, a 10 × 10 × 5mm (5mm is the thickness direction of the ring) structure observation test piece was collected at an arbitrary position inside the ring-shaped roller material 10mm from the outer surface, and the 10 × 10mm surface was mirror-polished, etched with a nital solution (5 vol% nitric acid + ethanol) for about 10 seconds, and then the structure was observed using an optical microscope.

The minor axis (minor axis length) of tempered martensite or bainite was determined by collecting EBSD measurement test pieces (5mm × 10mm × 5mm) from arbitrary positions inside the ring roll material obtained at a distance of 10mm from the outer surface, mirror-polishing the surface of 5mm × 10mm, and EBSD measurement. Under the conditions of 15kV of accelerating voltage and 0.1 μm of stepping size, the voltage is 10000 μm²EBSD measurements were performed in the above regions. Using the obtained data, as shown in fig. 2, a boundary line was drawn at a portion having an azimuth difference of 15 ° or more from an adjacent measurement point, and a region surrounded by the boundary line was defined as one crystal, and the short diameters of 20 crystals having a long diameter of 10 μm or more were measured on the measurement surface, and the average value thereof was calculated.

The obtained results are shown in table 2.

[ Table 2]

Numbering	Minor diameter (mum)	Hardness (HS)	Hot rolling fatigue life (thousands times)	Remarks for note
					1	2.8	80	387	Examples of the invention
2	2.4	82	467	Examples of the invention
					3	2.0	80	501	Examples of the invention
4	2.5	78	481	Examples of the invention
					5	2.3	85	463	Examples of the invention
6	2.9	86	398	Examples of the invention
					7	2.4	80	432	Examples of the invention
8	1.7	81	556	Examples of the invention
					9	1.0	83	587	Examples of the invention
10	0.5	79	643	Examples of the invention
					11	2.1	78	497	Examples of the invention
12	1.4	80	569	Examples of the invention
					13	2.0	80	521	Examples of the invention
14	0.7	83	596	Examples of the invention
					15	2.3	85	496	Examples of the invention
16	2.8	86	223	Comparative example
					17	1.2	84	301	Comparative example
18	2.2	82	298	Comparative example
					19	2.7	80	190	Comparative example
20	2.6	81	187	Comparative example
					21	2.9	81	176	Comparative example
22	0.5	83	332	Comparative example
					23	2.8	84	153	Comparative example
24	1.9	86	206	Comparative example
					25	0.6	80	321	Comparative example
26	0.5	79	336	Comparative example
					27	3.6	78	123	Comparative example
28	3.8	78	146	Comparative example
					29	0.5	80	314	Comparative example

In the inventive example, the hot rolling fatigue life was significantly increased, showing excellent hot rolling fatigue life of more than 350 thousand times. In all the examples of the present invention, it was confirmed that 85% or more of the matrix structure was a tempered martensite and/or bainite structure as a result of the structure observation.

Therefore, according to the present invention, a hot-rolling composite roll in which the propagation speed of cracks is significantly reduced can be manufactured. As a result, surface damage due to hot rolling, such as surface roughening and chipping, can be suppressed, and therefore, the effects of extending the continuous rolling distance and improving the roll life can be achieved.

Description of the reference numerals

1 test piece (Hot rolling fatigue test piece)

2 Cooling Water

3 high frequency induction heating coil

4 pairing sheet

5 gap

Claims

1. The outer layer material of the hot-rolling roll is characterized by comprising the following components: contains in mass%

C：2.0～3.0％、

Si：0.2～1.0％、

Mn：0.2～1.0％、

Cr：4.0～7.0％、

Mo：3.0～6.5％、

V：5.0～7.5％、

Nb：0.5～3.0％、

Ni：0.05～3.0％、

Co：0.2～5.0％、

W：0.5～5.0％，

And the contents of C, Cr, Mo, V, Nb, Ni, and W satisfy the following formula (1), and the balance is Fe and unavoidable impurities,

in the outer layer material of the hot rolling roller, more than 85% of matrix structure is tempered martensite structure, the short diameter of the tempered martensite is 0.5-3.0 μm,

0.05≤(％C－％V×0.177－％Nb×0.129－％Cr×0.099－％Mo×0.063－％W×0.033)+(％Ni)≤4.0 (1)

wherein,% C,% V,% Nb,% Cr,% Mo,% W,% Ni are the contents of the respective elements in mass%.

2. The outer layer material of the hot-rolling roll is characterized by comprising the following components: contains in mass%

C：2.0～3.0％、

Si：0.2～1.0％、

Mn：0.2～1.0％、

Cr：4.0～7.0％、

Mo：3.0～6.5％、

V：5.0～7.5％、

Nb：0.5～3.0％、

Ni：0.05～3.0％、

Co：0.2～5.0％、

W：0.5～5.0％，

in the outer layer material of the hot rolling roller, more than 85% of the matrix structure is tempered martensite and bainite structures, the minor diameter of the tempered martensite or bainite is 0.5-3.0 μm,

3. The outer layer material of the hot-rolling roll is characterized by comprising the following components: contains in mass%

C：2.0～3.0％、

Si：0.2～1.0％、

Mn：0.2～1.0％、

Cr：4.0～7.0％、

Mo：3.0～6.5％、

V：5.0～7.5％、

Nb：0.5～3.0％、

Ni：0.05～3.0％、

Co：0.2～5.0％、

W：0.5～5.0％，

in the outer layer material of the hot rolling roller, more than 85% of the matrix structure is a bainite structure, the minor diameter of the bainite is 0.5-3.0 μm,

4. A composite roll for hot rolling comprising an outer layer and an inner layer welded and integrated together, wherein the outer layer is formed of the outer layer material for a hot rolling roll according to any one of claims 1 to 3.