JP7136037B2 - Roll outer layer material for hot rolling and composite roll for hot rolling - Google Patents

Description

TECHNICAL FIELD The present invention relates to a hot rolling roll outer layer material and a hot rolling composite roll, and more particularly to a hot rolling roll outer layer material and a hot rolling composite roll suitable as work rolls for hot rolling mills for steel sheets. .

In recent years, with the progress in hot rolling technology for steel sheets, the environment in which rolls are used has become more severe, and the production of high-strength steel sheets, thin-walled steel sheets, and other steel sheets that require a large rolling load is increasing. Therefore, the quality level required for rolling work rolls is increasing, and high-performance rolling work rolls are required. Work rolls having an outer layer made of grainen cast iron or high-alloy grain cast iron, which are excellent in accident resistance, are used in the latter stage of the hot finishing rolling mill. A drawing accident can be mentioned as a rolling accident that occurs in the latter stand of a hot finishing mill. A drawing accident is a phenomenon in which the ends of a material to be rolled are folded and caught between rolls. When a drawing accident occurs, the material to be rolled is seized on the roll surface, and a large thermal and mechanical load is generated on the roll, and cracks may occur on the roll surface. Such cracks may have a depth of 1 mm or more, and it is necessary to remove the cracks by grinding the roll surface, which poses problems of increased work costs and shortened roll life. Therefore, even if a drawing accident occurs, there is a demand for a work roll for hot rolling in which cracks are less likely to occur and propagate.

As an outer layer material of such a work roll for hot rolling, for example, Patent Document 1 describes, in mass%, C: more than 3.0% and 4.0% or less, Si: 3.0% or less, Ni : 2.3-5.5%, Cr: 1.0-2.0%, V: 0.3-10.0%, Ti: 0.01-2.0%, the balance being Fe and impurities A rolling roll outer layer material has been proposed which is composed of elements, has graphite and MC-based carbide in the metal structure, and is characterized in that the graphite has a spheroidization rate of 0.5 or more. Patent Document 1 states that the graphite is finely spherical and dispersed uniformly in the metal structure to improve wear resistance, surface roughness resistance, and accident resistance.

In addition, in Patent Document 2, in mass%, C: 3.0 to 4.5%, Si: more than 0% and 2.0% or less, Mn: more than 0% and 1.5% or less, Ni : 3.0 to 5.0%, Cr: 1.4 to 4.0%, Mo: 0.1 to 1.5%, V: more than 0% and 3.0% or less, and the balance Fe and unavoidable impurity elements, C, Si, and Cr are 4.0% ≤ C + Si / 3 + Cr / 7.5 ≤ 5.5%, and the metal structure of the peripheral surface subjected to rolling of the outer layer is cementite A composite roll for rolling characterized by having an area ratio of 40% or more and less than 46% has been proposed. According to Patent Document 2, since a large amount of hard cementite is present, the composite roll for rolling is excellent in wear resistance and surface roughening resistance.

JP-A-2005-177808 JP 2018-75638 A

From the viewpoint of manufacturing high-grade steel sheets or improving hot rolling productivity, the rolling environment for hot rolling is becoming more severe year by year, and higher quality work rolls for hot rolling, such as improved wear resistance, are required. . In particular, there is a demand for hot rolling rolls with excellent accident resistance in which cracks are less likely to occur or propagate when a drawing accident is encountered. is not sufficient.

The present invention has been made in view of the above circumstances, and provides a roll outer layer material for hot rolling and a composite roll for hot rolling that have the same wear resistance as the conventional ones and are excellent in accident resistance. With the goal.

Hot rolling rolls with an outer layer made of grain cast iron or high-alloy grain cast iron contain graphite in the metal structure. Suppressing the occurrence and progression of On the other hand, if the area ratio of graphite is increased in order to improve accident resistance, since graphite is softer than carbides and matrixes, there is a problem that wear resistance decreases. In order to solve such problems, the present inventors conducted extensive studies, and as a result, the graphite area ratio S _t in the cross section (cross section perpendicular to the roll axis) of the roll outer layer material was calculated as the radial cross section (on the roll outer surface If it is larger than the graphite area ratio S _s in the parallel plane) (S _t > S _s ), the crack caused by the drawing accident does not deteriorate, and the accident resistance is improved. I obtained unprecedented knowledge.

The present invention has been completed based on the above findings, and the gist thereof is as follows.
[1] In mass%, C: 3.0 to 4.0%, Si: 0.4 to 3.5%, Cr: 1.0 to 2.5%, Ni: 3.0 to 5.5% , V: 0.1 to 2.5%, with the balance consisting of Fe and unavoidable impurities,
The metal structure has graphite, cementite, and MC type carbide, and the area ratio of graphite in the cross section perpendicular to the roll axis of the roll outer layer material is the area of graphite in the cross section parallel to the roll axis of the roll outer layer material A roll outer layer material for hot rolling characterized in that it is greater than the modulus.
[2] Furthermore, in mass %, Mn: 0.2 to 1.5%, Nb: 0.1 to 1.5%, Mo: 0.1 to 2.0%, W: 0.1 to 1.0%. 5%, Co: 0.1 to 2.0%, B: 0.003 to 0.050%, any one or more of the hot rolling according to [1] roll outer layer material.
[3] The hot rolling roll according to [1] or [2], wherein the content of C, Si, Ni, Cr, Mo, V, Nb and W satisfies the following formula (1) Outer layer material.
0.70≦([%C]+0.3×[%Si]+0.02×[%Ni])/([%Cr]+[%Mo]+[%V]+[%Nb]+[% W]) ≤ 1.80 (1)
Here, [%C], [%Si], [%Ni], [%Cr], [%Mo], [%V], [%Nb], [%W] are the contents of each element ( % by mass), and the content of elements not contained is set to 0.
[4] A hot rolling composite roll having a three-layer structure of an outer layer, an intermediate layer and an inner layer or a two-layer structure of an outer layer and an inner layer, wherein the outer layer is the heat according to any one of [1] to [3] A composite roll for hot rolling characterized by comprising a roll outer layer material for hot rolling.

According to the present invention, it is possible to obtain a hot rolling roll outer layer material and a hot rolling composite roll which have wear resistance equal to or higher than that of conventional rolls and which are remarkably excellent in accident resistance.

FIG. 1 is a schematic diagram showing the positions of sampled specimens for structure observation from the outer layer material of the hot rolling roll (sleeve roll), and FIG. BRIEF DESCRIPTION OF THE DRAWINGS It is a cross-sectional schematic diagram which shows a position, FIG.1(b) is a schematic diagram of the test piece for vertical section observation, FIG.1(c) is a schematic diagram of the test piece for parallel section observation. 2A and 2B are schematic diagrams showing the positions for collecting wear test pieces from the sleeve roll, FIG. 2A is a front view of the sleeve roll viewed from the axial direction, and FIG. 2B is FIG. 2A. 2(c) is a side view of the sleeve roll seen from direction B in FIG. 2(a). FIG. 3 is a diagram schematically showing the configuration of the testing machine used in the wear test and the shape of the test piece for the wear test (wear test piece). FIG. 4 is a diagram schematically showing the configuration of the testing machine used in the thermal shock test. 5A and 5B are schematic diagrams showing the positions of the thermal shock test specimens taken from the sleeve roll. FIG. 5A is a front view of the sleeve roll viewed from the axial direction, and FIG. ), and FIG. 5(c) is a right side view of the sleeve roll as seen from direction B in FIG. 5(a). 6A and 6B are schematic diagrams showing crack depth measurement points of a thermal shock test piece, FIG. 6A is a schematic diagram showing a cutting position of the thermal shock test piece, and FIG. It is a schematic diagram which shows the crack depth in the cut surface of a test piece. FIG. 7 is an optical microscope photograph showing the metal structure in the example, FIG. 7(a) is an image of the metal structure in a vertical section (a section perpendicular to the roll axis), and FIG. 7(b) is a parallel section ( It is an image of the metallographic structure of the cross section parallel to the roll axis).

The hot rolling roll outer layer material of the present invention is manufactured by a centrifugal casting method and can be used as a ring roll or a sleeve roll as it is, but the outer layer material of a hot rolling composite roll suitable for hot finish rolling applied as Further, the composite roll for hot rolling of the present invention comprises an outer layer and an inner layer welded and integrated with the outer layer. An intermediate layer may be arranged between the outer layer and the inner layer. That is, instead of the inner layer welded and integrated with the outer layer, an intermediate layer welded and integrated with the outer layer and an inner layer welded and integrated with the intermediate layer may be used. In the present invention, the composition of the inner layer and the intermediate layer is not particularly limited, but it is preferable that the inner layer is made of spheroidal graphite cast iron (ductile cast iron) and the intermediate layer is made of a high carbon material containing 1.5 to 3.0% by mass of C. .

First, the reasons for limiting the composition of the outer layer (outer layer material) of the composite roll for hot rolling of the present invention will be described. In addition, hereinafter, mass % is simply described as % unless otherwise specified.

C: 3.0-4.0%
C increases the hardness of the matrix (martensite and/or bainite) and combines with carbide-forming elements such as Fe, V, and Cr to form hard carbides, contributing to improved wear resistance. Furthermore, it is an element that crystallizes as graphite in the metal structure due to graphitization promoting elements such as Si and Ni, thereby improving accident resistance. If the C content is less than 3.0%, the amount of carbide and graphite is insufficient, resulting in deterioration of wear resistance and accident resistance. On the other hand, if the content exceeds 4.0%, coarsening of carbides is caused and the rough surface resistance is lowered. Therefore, C is limited to 3.0 to 4.0%. In addition, preferably, it is 3.0 to 3.6%.

Si: 0.4-3.5%
Si is an element that acts as a deoxidizing agent and improves castability of the molten metal. In addition, Si is a graphitization promoting element and has the effect of crystallizing C as graphite. In order to obtain such an effect, the content of 0.4% or more is required. On the other hand, when the content exceeds 3.5%, the matrix structure becomes embrittled. Therefore, Si is limited to 0.4 to 3.5%. The content is preferably 0.6 to 2.5%, more preferably 0.6 to 2.0%.

Cr: 1.0-2.5%
Cr combines with C to form mainly hard Cr _- based carbides ( _{M7C3 , M23C6} , etc.), and further improves the hardenability of the matrix to transform the matrix into martensite and/or bainite. It is an element that improves wear resistance. In order to obtain such an effect, the content of 1.0% or more is required. On the other hand, if the Cr content exceeds 2.5%, coarse Cr-based carbides increase, and cracks tend to develop, which is not preferable. Therefore, Cr is limited to 1.0 to 2.5%. In addition, preferably, it is 1.2 to 2.2%.

Ni: 3.0-5.5%
Ni is an element that dissolves in the matrix and lowers the transformation temperature of austenite during heat treatment to improve the hardenability of the matrix, and also promotes graphitization to increase the amount of graphite produced. If the content is less than 3.0%, these effects are insufficient, and if it exceeds 5.5%, the transformation temperature of austenite becomes too low, and austenite remains even after heat treatment, resulting in wear resistance. reduce sexuality. Therefore, the Ni content is limited to 3.0 to 5.5%. In addition, preferably, it is 3.5 to 4.8%.

V: 0.1-2.5%
V is an element that forms extremely hard V-based carbides (MC-type carbides) and improves wear resistance. Such an effect is obtained at a content of 0.1% or more. On the other hand, if the content exceeds 2.5%, the MC-type carbides become coarse and the amount of graphite produced is reduced, thus destabilizing various properties of the roll for rolling. Therefore, V is limited to 0.1 to 2.5%. In addition, preferably, it is 0.3 to 2.2%.

Furthermore, in the present invention, one or more of the following elements can be selectively contained.

Mn: 0.2-1.5%
Mn is an element that has the effect of fixing S as MnS and detoxifying S, and partly dissolving in the matrix structure, improving hardenability and strengthening the matrix (solution strengthening). . In order to obtain such an effect, the content of 0.2% or more is required. On the other hand, even if the content exceeds 1.5%, the effect is saturated and the effect commensurate with the content cannot be expected, and the material may become embrittled. Therefore, when Mn is contained, it is limited to 0.2 to 1.5%. Incidentally, preferably, it is 0.3 to 1.4%.

Nb: 0.1-1.5%
Nb is an element that dissolves in MC-type carbide to strengthen the MC-type carbide and improves wear resistance through the action of increasing the fracture resistance of MC-type carbide. In addition, Nb also has the effect of suppressing segregation during centrifugal casting of MC type carbide. Such an effect becomes remarkable at a content of 0.1% or more. On the other hand, if the content exceeds 1.5%, the growth of MC-type carbides in the molten metal is promoted, and the rough surface resistance is lowered. Therefore, when Nb is contained, it is limited to 0.1 to 1.5%. Incidentally, preferably, it is 0.2 to 1.2%.

Mo: 0.1-2.0%
Mo is an element that combines with C to form a solid solution in cementite and improves wear resistance. Mo also dissolves in hard MC-type carbides in which V, Nb, and C combine to strengthen the carbides. Mo improves the wear resistance of the roll outer layer material through such action. In order to obtain such an effect, the content of 0.1% or more is required. On the other hand, if the content exceeds 2.0%, coarse Mo-based carbides are formed, and the rough surface resistance is lowered. Therefore, when Mo is contained, it is limited to 0.1 to 2.0%. Incidentally, it is preferably 0.2 to 1.8%.

W: 0.1-1.5%
W is an element that dissolves in the matrix and strengthens the matrix to improve wear resistance, and dissolves in cementite to improve wear resistance. In order to obtain such an effect, the content of 0.1% or more is required. On the other hand, when the content exceeds 1.5%, not only the effect saturates, but also coarse M ₂ C or M ₆ C carbides are formed, reducing the roughening resistance. Therefore, when W is contained, it is limited to the range of 0.1 to 1.5%. Incidentally, it is preferably 0.2 to 1.2%.

Co: 0.1-2.0%
Co is an element that dissolves in the matrix and has the effect of strengthening the matrix and improving wear resistance. In order to obtain such an effect, the content of 0.1% or more is required. On the other hand, if the content exceeds 2.0%, the effect is saturated and the effect commensurate with the content cannot be expected, which is economically disadvantageous. Therefore, when Co is contained, it is limited to the range of 0.1 to 2.0%. Incidentally, it is preferably 0.2 to 1.5%.

B: 0.003-0.050%
B is an element that improves the hardenability of the matrix, transforms the matrix into martensite and/or bainite, and improves wear resistance. Such an effect is obtained at a content of 0.003% or more. On the other hand, a content exceeding 0.050% is not preferable because brittle borocarbides are formed. Therefore, when B is contained, it is limited to the range of 0.003 to 0.050%. In addition, preferably, it is 0.005 to 0.040%.

Furthermore, in the present invention, C, Si, Ni, Cr, Mo, V, Nb and W preferably satisfy the following formula (1).
0.70≦([%C]+0.3×[%Si]+0.02×[%Ni])/([%Cr]+[%Mo]+[%V]+[%Nb]+[% W]) ≤ 1.80 (1)
Here, [%C], [%Si], [%Ni], [%Cr], [%Mo], [%V], [%Nb], [%W] are the contents of each element ( % by mass), and the content of elements not contained is set to 0.

([% C] + 0.3 × [% Si] + 0.02 × [% Ni]) / ([% Cr] + [% Mo] + [% V] + [% Nb] + [% W]) If it is less than 0.70, the amount of the carbide-forming element is too large, making it difficult to produce graphite, and graphite elongated in the direction perpendicular to the outer surface of the roll cannot be obtained. On the other hand, ([% C] + 0.3 × [% Si] + 0.02 × [% Ni]) / ([% Cr] + [% Mo] + [% V] + [% Nb] + [% W] ) is greater than 1.80, the carbide-forming element is insufficient, resulting in a decrease in wear resistance. More preferably, 0.75 ≤ ([% C] + 0.3 × [% Si] + 0.02 × [% Ni]) / ([% Cr] + [% Mo] + [% V] + [% Nb ]+[% W])≦1.70.

The balance is Fe and unavoidable impurities. In addition, Al, S, P, Sb, Ti, Mg, Cu, etc. are mentioned as an unavoidable impurity. These are mixed from raw materials and refractories during melting. These unavoidable impurities are Al: 0.3% or less, S: 0.05% or less, P: 0.1% or less, Sb: 0.2% or less, Ti: 0.1% or less, Mg: 0 0.1% or less and Cu: 0.1% or less.

The hot rolling roll outer layer material of the present invention contains graphite, cementite and MC type carbide in its metal structure. The graphite present on the outer surface of the rolls suppresses or reduces the seizure of the material to be rolled onto the roll surface when a drawing accident occurs, thereby suppressing the occurrence of cracks. It is the most important factor that contributes to accident resistance by suppressing it. In addition, cementite and MC type carbide are important elements for ensuring wear resistance. The area ratio of graphite is preferably 1.0 to 7.0%, more preferably 1.2 to 6.5%. The area ratio of cementite is preferably 12.0 to 45.0%, more preferably 14.0 to 42.0%. The area ratio of MC type carbide is preferably 0.1 to 10.0%, more preferably 0.2 to 8.0%. The remainder of the metal structure includes martensite, bainite, pearlite, and retained austenite.

Furthermore, in the present invention, the area ratio S _t (%) of graphite in the cross section perpendicular to the roll axis of the roll outer layer material is the cross section parallel to the roll axis of the roll outer layer material (plane parallel to the roll outer surface) It is necessary that the area ratio S _s (%) of graphite in is greater than (S _t >S _s ). By satisfying S _t >S _s , the propagation of cracks caused by a throttle accident is suppressed, the cracks can be made shallow, and the accident resistance is remarkably improved. In order to obtain such an effect, S _t is preferably 0.3% or more larger than S _s (S _t −S _s ≧0.3%), more preferably 0.4% or more. Large (S _t −S _s ≧0.4%) is preferred. This relational expression of the graphite area ratio (S _t >S _s ) is preferably satisfied within a range of 20 mm in the radial direction from the position corresponding to the maximum diameter of the roll used for rolling.

Although the method for measuring the area ratio of graphite is not particularly limited, the area ratio of graphite was measured by the following method in the present invention. As shown in FIG. 1, a test piece for vertical section observation and a test piece for parallel plane observation were taken from a roll outer layer material having an outer diameter equal to the maximum diameter of the roll used for rolling. Metallographic photographs of five fields of view were taken at magnification. At this time, for the test piece for parallel section observation, arbitrary 5 points were photographed in the entire observation surface, and for the test piece for vertical section observation, arbitrary images were taken in an area within 1 mm in the radial direction from the outer surface of the roll in the observation surface. I took 5 shots. Based on the obtained metal structure photograph, the area ratio of graphite was measured by image analysis, and the area ratio of graphite obtained from the test piece for vertical cross section observation was calculated as the area of graphite in the cross section perpendicular to the roll axis of the outer layer material of the roll. S _t (%), the area ratio S _s (%) of graphite in the cross section parallel to the roll axis of the roll outer layer material (plane parallel to the outer surface of the roll), and the average value of S _t and S _s is calculated. and the area ratio of graphite in the test piece was obtained.

In addition, the area ratios of cementite and MC carbide were measured using a test piece corroded with 3% nital after the observation surface of the test piece for parallel section observation was mirror-polished. The area ratio of cementite is 100 times, and the metal structure photograph of arbitrary 5 fields of view is taken. The area ratio of cementite or MC type carbide in the test piece was calculated by calculating the average value. Calculation of MC carbide and cementite area ratio by image analysis was performed by the following method. The area ratio of MC carbides was calculated by measuring the area of a white region with an equivalent circle diameter of 10 μm or less by image analysis and dividing it by the area of the entire observation field. The cementite area ratio was calculated by measuring the area of the white region by image analysis and dividing it by the area of the entire observation field. Small white regions of 10 μm or less in the 500-fold metallographic photograph are MC carbides, and white regions in the 100-fold metallographic photograph are cementite.

Next, a method for manufacturing the hot rolling roll outer layer material and the hot rolling composite roll of the present invention will be described.

When casting the hot rolling roll outer layer material, first, a mold whose inner surface is coated with a refractory mainly made of zircon or the like with a thickness of 0.5 to 5 mm is rotated to obtain the above hot rolling roll outer layer material. Molten metal with the composition is poured to a predetermined thickness and cast by vertical (vertical axis of rotation), horizontal (horizontal axis of rotation) or inclined (diagonal axis of rotation) centrifugal casting. Centrifugal casting.

In the hot rolling roll outer layer material manufactured by the conventional method, the graphite has a nearly spherical shape, so S _t and S _s are almost the same, and the difference between them is 0≦|S _t −S _s |≦0.1. Therefore, in the present invention, the area ratio of graphite in the cross section of the roll outer layer material perpendicular to the roll axis is larger than the area ratio of graphite in the cross section of the roll outer layer material parallel to the roll axis (S _t >S _s ). In order to obtain the roll outer layer material, it is necessary to make the graphite elongated in the direction perpendicular to the roll outer surface. As described above, the outer layer of the hot rolling roll outer layer material or the hot rolling composite roll of the present invention may be of a vertical type (rotation axis is in the vertical direction), horizontal type (rotation axis is in the horizontal direction) or inclined type (rotation axis is in the horizontal direction). oblique direction) centrifugal casting method. In the present invention, it is necessary that the centrifugal force on the outer surface of the outer layer molten metal at this time is greater than 150 G in terms of gravity multiple GNo (gravity multiple GNo is the rotation speed of the centrifugal casting mold N (rpm), centrifugal casting GNo=N×N×D/1790000, where D (mm) is the inner diameter of the casting mold). When the centrifugal force is 150 G or less, the solidified structure (dendrite) does not grow large, so graphite elongated in the direction perpendicular to the outer surface of the roll cannot be obtained. Preferably it is 160G or more.

In addition to the above, the ratio R between the thickness of the outer layer molten metal and the thickness of the centrifugal casting mold (R = outer layer molten metal thickness / centrifugal casting mold thickness) must be 0.20 ≤ R ≤ 0.60. . Here, the thickness of the outer layer molten metal is the thickness of the sleeve when the outer layer molten metal is poured into the centrifugal casting mold and becomes hollow (sleeve shape), the weight and density of the outer layer molten metal, and the inner diameter of the centrifugal casting mold. It can be calculated from length. If R is less than 0.20, the outer layer melt solidifies faster than the growth rate of graphite, and graphite elongated in the direction perpendicular to the outer surface of the roll cannot be obtained. On the other hand, when R is larger than 0.60, the solidification of the outer layer molten metal is significantly delayed, so that solidification proceeds from the inner surface of the outer layer molten metal, and the quality inside the outer layer deteriorates. More preferably, 0.25≦R≦0.55.

When forming the intermediate layer, it is preferable to pour a molten metal having the composition of the intermediate layer while rotating the mold during or after solidifying the outer layer material, and perform centrifugal casting. After the outer layer or the intermediate layer has completely solidified, it is preferable to stop the rotation of the mold and erect the mold, and then statically cast the inner layer material to form a composite roll. As a result, the inner surface of the roll outer layer material is melted again to form a composite roll in which the outer layer and the inner layer, the outer layer and the intermediate layer, or the intermediate layer and the inner layer are welded and integrated.

In addition, it is preferable to use spheroidal graphite cast iron, worm-like graphite cast iron (CV cast iron), etc., which are excellent in castability and mechanical properties, for the inner layer to be statically cast. Since the outer layer and the inner layer of the centrifugally cast roll are integrally welded, about 1 to 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 into the inner layer, the inner layer becomes brittle. Therefore, it is preferable to suppress the mixing ratio of the outer layer components to the inner layer to less than 6%.

When forming the intermediate layer, it is preferable to use graphite steel, high-carbon steel, hypoeutectic cast iron, or the like as the material for the intermediate layer. The intermediate layer and the outer layer are similarly welded together, and the outer layer components are mixed into the intermediate layer in a range of 10 to 95%. From the viewpoint of suppressing the amount of the outer layer component mixed into the inner layer, it is important to reduce the amount of the outer layer component mixed into the intermediate layer as much as possible.

As described above, a composite roll for hot rolling having a three-layer structure of an outer layer, an intermediate layer and an inner layer or a two-layer structure of an outer layer and an inner layer can be obtained.

The hot rolling roll outer layer material and the hot rolling composite roll of the present invention are preferably subjected to heat treatment after casting. The heat treatment is preferably carried out by removing the roll from the mold after casting, heating and holding at 400 to 550° C., and then slowly cooling.

The hardness of the composite roll for hot rolling of the present invention is preferably 74-88 HS (Shore hardness), more preferably 76-86 HS. If the hardness is lower than 74HS, wear resistance deteriorates, and if the hardness exceeds 88HS, cracks caused by drawing accidents tend to propagate. Such hardness can be obtained by adjusting the heat treatment temperature.

A molten metal of 1400 ° C. obtained by melting the materials shown in Table 1 is supplied to the mold of a horizontal centrifugal casting machine under the casting conditions shown in Table 1, and a sleeve roll having an outer diameter of 250 mm and a depth of 70 mm (corresponding to the outer layer material for the rolling roll). was cast. In order to change the ratio R between the thickness of the outer layer molten metal and the thickness of the centrifugal casting mold (R = outer layer molten metal thickness / centrifugal casting mold thickness), molds with a thickness of 60 mm, 100 mm, 150 mm, and 200 mm were used, and a thickness of 200 mm was used. 0.10 ≤ R < 0.20 for the mold, 0.20 ≤ R < 0.30 for the mold with a thickness of 150 mm, 0.30 ≤ R < 0.50 for the mold with a thickness of 100 mm, 0.50 for the mold with a thickness of 60 mm Used for casting sleeve rolls with ≦R<0.80. Moreover, the thickness of the zircon-based refractory on the inner surface of the mold is 0.5 mm. After casting, the sleeve roll was heated at 410° C. for 10 hours and then slowly cooled to obtain various test pieces.

Test material No. shown in Table 1. 1 to 22 are examples of the present invention, test material Nos. 24 to 28 are comparative examples. In addition, test material No. No. 23 is a conventional example containing no V.

Structure observation was performed in order to evaluate the graphite area ratio. After cutting the outer surface of the sleeve roll by cutting to reduce the outer diameter from 250 mm to 240 mm, a test piece for observing a vertical section and a test piece for observing a parallel section as shown in FIG. taken from the position of The size of the test piece is 30 mm × 20 mm × t mm (30 mm is the circumferential direction of the sleeve roll, 20 mm is the depth (axis) direction of the sleeve roll, t mm is the radial direction of the sleeve roll, and the thickness of the outer layer melt and the centrifugal mold From the relationship of the thickness ratio R, t is in the range of 25 to 42 mm). After the observation surface of the test piece was mirror-polished, photographs of the metallographic structure were taken at 100 times the field of view at five locations. At this time, for the test piece for parallel section observation, select arbitrarily 5 points in the entire observation surface (30 mm × 20 mm surface), and for the test piece for vertical section observation, in the area within 1 mm in the radial direction from the outer surface of the roll Randomly selected 5 locations. Based on the obtained metal structure photograph, the area ratio of graphite was measured by image analysis, and the area ratio of graphite obtained from the test piece for vertical cross section observation was calculated as the area of graphite in the cross section perpendicular to the roll axis of the outer layer material of the roll. ratio S _t (%), and the area ratio S _s (%) of graphite in the cross section parallel to the roll axis of the roll outer layer material (plane parallel to the roll outer surface). In this case, samples having a difference S _t −S _s in area ratio of more than 0.1 were regarded as acceptable.

The area ratios of cementite and MC-type carbide were measured by the same method as the above-described measurement method.

Shore hardness (HS) was measured by the following method. Vickers hardness HV50 is measured with a Vickers hardness tester (test force: 50 kgf (490 N)) in accordance with the provisions of JIS Z 2244, and converted to Shore hardness HS using the JIS conversion table. did. In addition, the measurement position is an arbitrary position on the observation surface, 10 points are measured, the average value of 8 points is calculated by removing the highest value and the lowest value, and the hardness of the test material is obtained.

A wear test was performed to evaluate wear resistance. As shown in FIG. 2, an abrasion test piece (outer diameter 60 mm, wall thickness 10 mm, chamfered) was taken from a position 5 mm inside from the outer surface of the obtained sleeve roll. As shown in FIG. 3, the wear test was carried out by a two-disc sliding rolling method using a test piece and a mating piece. The test piece 1 was rotated at 700 rpm while cooling with cooling water 2, and the rotating test piece 1 was heated to 800 ° C. with a high-frequency induction heating coil 3 on the outer surface of the mating piece 4 (material: S45C, outer diameter: 190 mm, Width: 15 mm, C1 chamfering) were brought into contact with each other with a load of 980 N, and were rolled at a slip ratio of 9%. The wear test was performed for 300 minutes, and the test was performed by replacing the mating piece with a new one every 50 minutes. Based on the conventional example, the ratio of the wear amount of each test piece to the reference value was evaluated by the wear ratio (= (wear amount of the reference piece) / (wear amount of each test piece)), and the wear ratio was 0.97. It was determined that the wear resistance in the above cases was equal to or higher than that of the conventional example, and that the wear resistance was inferior in the case of less than 0.97.

In order to evaluate the accident resistance, a thermal shock test was performed using a drop weight friction thermal shock tester shown in FIG. As for the test piece, as shown in FIG. 5, a test piece of 30 mm×25 mm×20 mm was taken from a position 5 mm inside from the outer surface of the obtained sleeve roll (30 mm is the circumferential direction of the sleeve roll, 25 mm is the 20 mm in the depth direction of the roll is the radial direction of the sleeve roll). As shown in FIG. 4, the drop weight friction and thermal shock tester drops a weight 6 onto a rack 5 to rotate a pinion 7, and a mating piece 9 (material: mild steel ) is rubbed. Accident resistance was evaluated by the depth of cracks generated in the test piece 8 . Specifically, as shown in FIG. 6( a ), the center of the longitudinal direction of the place (indentation 10 ) where the mating piece 9 is rubbed against the surface of the test piece 8 (where L is the length of the indentation, 0.5 L position), the test piece 8 is cut perpendicular to the indentation 10 and the cut surface is mirror-polished, and then the metal structure of the cut surface is observed with an optical microscope as shown in FIG. was measured. When multiple cracks existed, the maximum depth among them was taken as the crack depth of the test material. Using the conventional example as a reference, the ratio of the crack depth of each test piece to the reference value is evaluated by the crack ratio (= (crack depth of each test piece) / (crack depth of the reference piece)), and the crack ratio is A value of 0.60 or less was determined to be excellent in accident resistance, and a value of greater than 0.60 was determined to be poor in accident resistance.

Comprehensive evaluation was performed based on the above evaluations of wear resistance and accident resistance. Abrasion resistance equal to or higher than the conventional one (abrasion ratio ≧0.97) and excellent accident resistance (crack ratio ≦0.60) was evaluated as ◯, and others were evaluated as x, and only ◯ was evaluated as acceptable.

It can be seen that the examples of the present invention have wear resistance equal to or higher than that of the conventional one, and also have excellent accident resistance. Moreover, test material No. is shown in FIG. An example of 10 tissue photographs is shown. FIG. 7(a) shows the metallographic structure of the observation specimen of the vertical section, and FIG. 7(b) shows the metallographic structure of the observation specimen of the parallel section. As described above, it can be seen that in the present invention example, graphite elongated in the direction perpendicular to the outer surface of the roll is produced, and there is a large difference in the graphite area ratio between the perpendicular cross section and the parallel cross section.

Therefore, according to the present invention, it is possible to manufacture a hot rolling roll outer layer material and a hot rolling composite roll that are excellent in wear resistance and accident resistance. As a result, there is also the effect that the surface quality of the material to be rolled can be remarkably improved and the productivity of hot rolling can be improved.

1 test piece (wear test piece)
2 cooling water 3 high-frequency induction heating coil 4 mating piece 5 rack 6 weight 7 pinion 8 test piece (thermal shock test piece)
9 mating piece 10 impression 11 crack L length of impression t depth of crack

Claims (4)

% by mass, C: 3.0 to 4.0%, Si: 0.4 to 3.5%, Cr: 1.0 to 2.5%, Ni: 3.0 to 5.5%, V: It contains 0.1 to 2.5%, and has a component composition consisting of the balance Fe and inevitable impurities,
In the radial direction from the outer surface of the roll outer layer material having the outer diameter of the maximum diameter of the roll used for rolling in the cross section perpendicular to the roll axis of the roll outer layer material, the metal structure having graphite, cementite, and MC type carbide The area ratio of graphite in the region inside 1 mm is larger than the area ratio of graphite on the outer surface of the outer surface of the roll outer layer material whose outer diameter is the maximum diameter of the roll used in rolling parallel to the roll axis of the roll outer layer material. A roll outer layer material for hot rolling characterized by: Furthermore, in mass% , Nb: 0.1 to 1.5% , W : 0.1 to 1.5%, Co: 0.1 to 2.0%, B: 0.003 to 0.050% 2. The hot rolling roll outer layer material according to claim 1, characterized by containing one or more of the above. 3. The hot rolling roll outer layer material according to claim 1 or 2, wherein the contents of C, Si, Ni, Cr, Mo, V, Nb and W satisfy the following formula (1).
0.70≦([%C]+0.3×[%Si]+0.02×[%Ni])/([%Cr]+[%Mo]+[%V]+[%Nb]+[% W]) ≤ 1.80 (1)
Here, [%C], [%Si], [%Ni], [%Cr], [%Mo], [%V], [%Nb], [%W] are the contents of each element ( % by mass), and the content of elements not contained is set to 0. A hot rolling composite roll having a three-layer structure of an outer layer, an intermediate layer and an inner layer or a two-layer structure of an outer layer and an inner layer, wherein the outer layer is the hot rolling roll outer layer according to any one of claims 1 to 3. A composite roll for hot rolling characterized by being made of a material.

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