JP7726164B2 - Hot rolling roll outer layer material, its manufacturing method, hot rolling composite roll and its manufacturing method - Google Patents

Description

The present invention relates to a hot rolling roll outer layer material, its manufacturing method, and a hot rolling composite roll and its manufacturing method, and in particular to a hot rolling roll outer layer material suitable as a work roll for a hot finishing stand for steel plate, its manufacturing method, and a hot rolling composite roll and its manufacturing method.

In recent years, advances in hot rolling technology for steel sheets have led to increasingly severe operating environments for rolls, and the production volume of steel sheets that require heavy rolling loads, such as high-strength steel sheets and thin-walled products, is also increasing. As a result, the quality level required of rolling work rolls is increasing, and high-performance rolls with excellent surface roughening resistance, wear resistance, fatigue resistance, and seizure resistance are required for work rolls for finishing rolling stands in hot rolling equipment.

The rear stands of hot finishing mills use work rolls with an outer layer made of grain-based cast iron or high-alloy grain-based cast iron, which has excellent seizure resistance. One rolling problem that can occur in the rear stands of hot finishing mills is drawing trouble. Drawing trouble occurs when the edge of the rolled material folds over and becomes caught between the rolls. When drawing trouble occurs, the rolled material seizes onto the roll surface, causing large thermal and mechanical loads on the roll, which can lead to cracks or chipping on the roll surface. These cracks can be 1 mm or more deep, requiring the roll surface to be ground to remove them, which increases work costs and shortens the roll's life. Therefore, there is a need for hot rolling work rolls that are less likely to develop or propagate cracks even when drawing trouble occurs.

As an outer layer material for such hot rolling work rolls, for example, Patent Document 1 proposes a rolling roll outer layer material containing, by mass, C: more than 3.0% but not more than 4.0%, 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 impurity elements, with graphite and MC-based carbides contained in the metal structure, and a graphite spheroidization rate of 0.5 or more. Patent Document 1 claims that by making the shape of the graphite into fine spheres and dispersing the graphite uniformly throughout the metal structure, wear resistance, surface roughening resistance, and accident resistance are improved.

Patent Document 2 proposes a composite roll for rolling containing, by mass, C: 3.0 to 4.5%, Si: more than 0% but not more than 2.0%, Mn: more than 0% but not more than 1.5%, Ni: 3.0 to 5.0%, Cr: 1.4 to 4.0%, Mo: 0.1 to 1.5%, V: more than 0% but not more than 3.0%, with the balance being Fe and unavoidable impurities, with C, Si, and Cr satisfying the relationship 4.0%≦C+Si/3+Cr/7.5≦5.5%, and the metal structure of the peripheral surface used for rolling the outer layer having an area ratio of cementite of 40% or more but less than 46%. Patent Document 2 claims that the presence of a large amount of hard cementite results in a composite roll for rolling with excellent wear resistance and surface roughening resistance.

Patent Document 3 proposes a composite roll for rolling, characterized in that the outer layer contains, by mass, 1-3% C, 0.3-3% Si, 0.1-3% Mn, 0.5-5% Ni, 1-7% Cr, 2.2-8% Mo, 4-7% V, 0.005-0.15% N, and 0.05-0.2% B, with the balance consisting of Fe and unavoidable impurities; the intermediate layer contains 0.025-0.15% B by mass, the B content of the intermediate layer is 45-80% of the B content of the outer layer, and the total content of carbide-forming elements in the intermediate layer is 45-90% of the total content of carbide-forming elements in the outer layer. Patent Document 3 lists MnS and carboborides as structures that contribute to galling resistance, and claims that these result in a composite roll for rolling with excellent galling resistance.

Japanese Patent Application Laid-Open No. 2005-177808 JP 2018-75638 A Patent No. 6973416

From the perspective of manufacturing high-grade steel sheets or improving productivity in hot rolling, the rolling environment for hot rolling is becoming more severe every year, and higher quality hot rolling work rolls are required. In particular, there is a demand for hot rolling rolls that have excellent wear resistance and fatigue resistance, as well as excellent seizure resistance so that the steel sheet does not easily seize when drawing problems are encountered. However, the rolling composite rolls manufactured in Patent Documents 1 and 2 produce graphite, so they cannot be said to have sufficient wear resistance. Furthermore, the rolling composite roll manufactured in Patent Document 3 contains a large amount of B, which may be mixed into the intermediate and inner layers, causing embrittlement of the intermediate and inner layers.

The present invention was made in consideration of the above circumstances, and aims to provide a hot rolling roll outer layer material and manufacturing method thereof, as well as a hot rolling composite roll and manufacturing method thereof, which have excellent wear resistance, fatigue resistance, and seizure resistance.

Hot rolling rolls having an outer layer made of grain-based cast iron or high-alloy grain-based cast iron contain graphite in their metal structure, which makes the rolled material less susceptible to seizure even when drawing troubles are encountered, thereby suppressing the occurrence and progression of cracks. Increasing the area ratio of graphite is an effective way to improve seizure resistance, but graphite is softer than carbides and the matrix, which reduces wear resistance. On the other hand, hot rolling rolls having an outer layer made of high-speed steel cast iron or high-alloy white cast iron contain a large amount of carbides in their metal structure and have excellent wear resistance, but their fatigue resistance and seizure resistance are inferior to those of grain-based cast iron. Therefore, the present inventors conducted extensive research into a roll outer layer material that is excellent in wear resistance, fatigue resistance, and seizure resistance. As a result, the inventors have obtained the unprecedented finding that by forming one or more types of carbides selected from M ₂ C type carbides, M ₆ C type carbides and MC type carbides and one or more types of carbides selected from M ₇ C ₃ type carbides and M ₂₃ C ₆ type carbides adjacent to each other, it is possible to obtain an outer layer material for a hot rolling roll that is excellent in wear resistance, fatigue resistance and seizure resistance.

The present invention has been completed based on the above findings, and the gist of the present invention is as follows.
[1] In mass%,
C: 1.5-2.3%,
Si: 0.3-2.0%,
Mn: 0.3-2.0%,
Cr: 3.5-7.0%,
Mo: 3.0-6.0%,
V: 3.0-5.0%,
Nb: 0.1 to 2.0%,
Al: 0.01-0.10%,
Ni: 0.02-2.00%,
N: 0.050% or less,
Contains
Or, furthermore,
Ti: 0.50% or less,
B: 0.090% or less,
Co: 1.0% or less,
W: 1.5% or less,
Zr: 0.50% or less,
Contains one or more selected from
The balance consists of Fe and unavoidable impurities,
The contents of Cr, Mo, W, V, and Nb have a composition that satisfies the following formulas (1) and (2):
and a carbide composite having at least one carbide selected from the following groups A and B is dispersed therein:
0.85≦%Cr/(%Mo+%W/2)≦1.15... (1)
(%Cr+%Mo+%W)/(%V+%Nb)≧2.2... (2)
Here, %Cr, %Mo, %W, %V, and %Nb are the contents (mass%) of each element, and elements that are not contained are set to 0.
Group A: M ₂ C type carbide, M ₆ C type carbide, MC type carbide Group B: M ₇ C ₃ type carbide, M ₂₃ C ₆ type carbide [2] The outer layer material for a hot rolling roll according to [1], characterized in that the carbide composites are present in an area ratio of 2.0% or more.
[3] A method for producing a hot rolling roll outer layer material according to [1] or [2], comprising casting a molten metal having the above-mentioned component composition using a non-axisymmetric centrifugal casting mold having a portion that is not point-symmetric about the rotation axis of the centrifugal casting mold in a cross section perpendicular to the rotation axis of the centrifugal casting mold.
[4] A composite roll for hot rolling having two or more layers, an outer layer and an inner layer, wherein the outer layer is made of the outer layer material for hot rolling roll according to [1] or [2].
[5] A method for producing a composite roll for hot rolling, characterized in that an outer layer material obtained by the method for producing an outer layer material for a hot rolling roll according to [3] above is used.

The present invention provides a hot rolling roll outer layer material with excellent wear resistance, fatigue resistance, and seizure resistance, contributing to improved roll life and rolling quality.

FIG. 1 is a schematic side view showing an example of a centrifugal casting mold that is symmetrical about the axis of rotation of the centrifugal casting mold. FIG. 2 is a schematic side view showing an example of a centrifugal casting mold that is asymmetric about the axis of rotation of the centrifugal casting mold. FIG. 3 is a diagram showing a schematic diagram of the position where a test piece for EBSD measurement is taken from a ring-shaped test material. FIG. 4 is a diagram showing a schematic configuration of a testing machine used in the drop weight frictional thermal shock test. FIG. 5 is a diagram showing the configuration of the testing machine used in the hot abrasion test and a test piece for the hot abrasion test (abrasion test piece). FIG. 6 is a diagram schematically showing the position at which a test piece for cross-section observation is taken from a test piece for a hot wear test (wear test piece).

First, we will explain the reasons for limiting the composition of the outer layer (outer layer material) of the hot rolling composite roll of the present invention. Note that, hereinafter, mass % will simply be referred to as % unless otherwise specified.

C: 1.5-2.3%
C dissolves in the matrix to increase its hardness and bonds with carbide-forming elements to form hard carbides, thereby improving the wear resistance of the roll outer layer material. If the C content is less than 1.5%, the amount of carbide is insufficient, resulting in reduced wear resistance. Therefore, the C content is set to 1.5% or more. The C content is preferably set to 1.6% or more. On the other hand, if the C content exceeds 2.3%, the carbides become coarse and the amount of eutectic carbide increases excessively, promoting the initiation and growth of fatigue cracks and reducing fatigue resistance due to the formation of deep heat cracks. Furthermore, the increased amount of carbide increases residual stress, which may lead to breakage of the roll during roll manufacture or use in rolling. Therefore, the C content is limited to 2.3% or less. Preferably, it is set to 2.2% or less.

Si: 0.3-2.0%
Si acts as a deoxidizer and is an element that improves the castability of molten metal. Furthermore, Si dissolves in the matrix and strengthens it. To achieve this effect, a content of 0.3% or more is required. The Si content is preferably 0.4% or more. On the other hand, if the Si content exceeds 2.0%, the effect saturates and no effect commensurate with the content can be expected, which is economically disadvantageous. Furthermore, the Si content may embrittle the matrix structure and deteriorate fatigue resistance. Therefore, the Si content is limited to 2.0% or less. Preferably, the Si content is 1.8% or less.

Mn: 0.3-2.0%
Mn is an element that fixes S as MnS, rendering it harmless, and also has the effect of improving hardenability by partially dissolving in the matrix structure. Furthermore, Mn dissolves in the matrix and strengthens it (solid-solution strengthening). To achieve this effect, a content of 0.3% or more is required. The Mn content is preferably 0.4% or more. On the other hand, if the Mn content exceeds 2.0%, the effect saturates, and the expected effect commensurate with the content cannot be expected, and the material may even become embrittled. Therefore, the Mn content is limited to 2.0% or less. Preferably, the Mn content is 1.7% or less.

Cr: 3.5-7.0%
Cr is an element that combines with C to form mainly eutectic carbides ( _{M7C3 type} carbides, _{M23C6 type} carbides) and has the effect of improving wear resistance. To achieve this effect, a Cr content of 3.5% or more is required. The Cr content is preferably 3.8% or more. On the other hand, a Cr content exceeding 7.0% increases the amount of coarse eutectic carbides, thereby reducing fatigue resistance. For this reason, the Cr content is limited to 7.0% or less. Preferably, it is 6.5% or less.

Mo: 3.0-6.0%
Mo is an element that combines with C to form hard carbides ( _M2C -type carbides, _M6C- type carbides) and improves wear resistance. Mo also dissolves in hard MC-type carbides, which are formed by combining V, Nb, and C, strengthening the carbides. It also dissolves in eutectic carbides, increasing the fracture resistance of these carbides. Through these actions, Mo improves the wear resistance of the roll outer layer material. To achieve this effect, a Mo content of 3.0% or more is required. The Mo content is preferably 3.5% or more. On the other hand, a Mo content exceeding 6.0% causes the formation of coarse eutectic carbides and reduces fatigue resistance. Therefore, the Mo content is limited to 6.0% or less. Preferably, it is 5.0% or less.

V: 3.0-5.0%
V is an element that provides both wear resistance and fatigue resistance as a roll. V is an element that forms extremely hard carbides (MC type carbides) and improves wear resistance. This effect becomes significant when the V content is 3.0% or more. Therefore, the V content is set to 3.0% or more. The V content is preferably 3.3% or more. On the other hand, a V content exceeding 5.0% coarsens the MC type carbides and reduces seizure resistance. Therefore, the V content is limited to 5.0% or less. Preferably, it is 4.7% or less.

Nb: 0.1-2.0%
Nb dissolves in MC carbides to strengthen them, increasing their fracture resistance and improving wear resistance. Nb also has the effect of suppressing segregation of MC carbides during centrifugal casting. This effect is significant when the Nb content is 0.1% or more. Therefore, the Nb content is set to 0.1% or more. The Nb content is preferably 0.2% or more. On the other hand, if the Nb content exceeds 2.0%, coarse MC carbides are formed, which deteriorates seizure resistance. Therefore, the Nb content is limited to 2.0% or less. Preferably, the Nb content is 1.8% or less.

Al: 0.01~0.10%
Al is an element that acts as a deoxidizer and has the effect of preventing internal defects such as porosity. This effect becomes significant when the Al content is 0.01% or more. Therefore, the Al content is set to 0.01% or more. The Al content is preferably 0.02% or more. On the other hand, if the Al content exceeds 0.10%, coarse Al-based oxides are formed, which reduces fatigue resistance. Therefore, the Al content is limited to 0.10% or less. Preferably, it is 0.09% or less.

Ni: 0.02-2.00%
Ni is an element that dissolves in the matrix, lowers the austenite transformation temperature during heat treatment, and improves the hardenability of the matrix. This effect is significant when the Ni content is 0.02% or more. The Ni content is preferably 0.05% or more. If the Ni content exceeds 2.00%, the austenite transformation temperature becomes too low, making it more likely that austenite will remain after heat treatment. If austenite remains, wear resistance will deteriorate. Therefore, the Ni content is limited to 2.00% or less. From the viewpoint of hardenability, the Ni content is preferably 1.80% or less.

N: 0.050% or less N is an element that is mixed in from the atmosphere during the raw materials and melting/casting processes. If the N content exceeds 0.050%, coarse nitrides are formed, reducing fatigue resistance. Therefore, the N content is limited to 0.050% or less, preferably 0.045% or less.

The balance is Fe and unavoidable impurities, such as S, P, Cu, Ca, Sb, Zr, and O. These impurities are mixed in from the raw materials or refractories during melting.
In addition to the above components, the steel may contain one or more of the following: Ti: 0.50% or less, B: 0.090% or less, Co: 1.0% or less, W: 1.5% or less, and Zr: 0.50% or less.

Ti: 0.50% or less Ti is an element that easily combines with oxygen in the molten metal to form oxides, and these oxides act as nuclei to form fine, uniform carbides in the matrix. Through this action, it contributes to improving wear resistance. This effect is significant at a content of 0.50% or less. Therefore, the Ti content is limited to 0.50% or less. Preferably, the Ti content is 0.40% or less. Furthermore, to obtain this effect, it is preferable that Ti be contained in an amount of 0.01% or more, and more preferably 0.02% or more.

B: 0.090% or less B is an element that dissolves in the matrix and improves the hardenability of the matrix. This effect is significant when the content is 0.090% or less. If the content exceeds 0.090%, borocarbides are formed, saturating the hardenability improvement effect and reducing fatigue resistance. Furthermore, if B is mixed into the intermediate layer or inner layer, it may embrittle the intermediate layer or inner layer. Therefore, the B content is limited to 0.090% or less. It is preferably 0.080% or less, and more preferably 0.070% or less. From the viewpoint of hardenability, the B content is preferably 0.001% or more, and more preferably 0.002% or more.

Co: 1.0% or less Co is an element that dissolves in the matrix and increases the hardness of the matrix, thereby improving wear resistance. This effect is significant at a content of 1.0% or less. If the content exceeds 1.0%, the effect saturates and is economically disadvantageous. Therefore, the Co content is limited to 1.0% or less, preferably 0.9% or less. To obtain this effect, Co is preferably contained at 0.1% or more, and more preferably 0.2% or more.

W: 1.5% or less W is an element that dissolves in the matrix, strengthens the matrix, and improves surface roughening resistance. It also forms _M2C or _M6C carbides, improving wear resistance. On the other hand, if it is contained in an amount exceeding 1.5%, not only does the effect saturate, but coarse _M2C or _M6C carbides are formed, causing significant fatigue wear and reducing wear resistance. For these reasons, the W content must be 1.5% or less. The W content is preferably 1.2% or less.

Zr: 0.50% or less Zr is an element that combines with C to form MC-type carbides, improving wear resistance. On the other hand, if the content exceeds 0.50%, not only does the effect saturate, but coarse MC-type carbides are formed, reducing fatigue resistance. For these reasons, the Zr content must be 0.50% or less. The Zr content is preferably 0.30% or less.

The composition of the outer layer material for hot rolling rolls of the present invention must satisfy the following formulas (1) and (2):
0.85≦%Cr/(%Mo+%W/2)≦1.15... (1)
(%Cr+%Mo+%W)/(%V+%Nb)≧2.2... (2)
Here, %Cr, %Mo, %W, %V, and %Nb are the contents (mass%) of each element, and elements that are not contained are set to 0.
By satisfying the above formulas (1) and (2), it is possible to obtain a hot rolling roll outer layer material excellent in wear resistance, fatigue resistance, and seizure resistance. Specifically, if (%Cr/(%Mo+%W/2)) is less than 0.85, the amount of _M2C type carbide increases, making it difficult to form _{M7C3 type} carbide, and therefore the carbide composite described below will not be formed. Therefore, it is necessary to make it 0.85 or more, preferably 0.88 or more. On the other hand, if (%Cr/(%Mo+%W/2)) is more than 1.15, the amount of _M7C3 type _carbide increases, making it difficult to form _M2C type carbide, and therefore the carbide composite described below will not be formed. Therefore, it is necessary to make it 1.15 or less, preferably 1.12 or less. Furthermore, if ((%Cr+%Mo+%W)/(%V+%Nb)) is less than 2.2, the amount of MC type carbides increases, making it difficult to form _{M7C3 type} carbides and _M2C type carbides, and the amount of carbide composites described below falls outside the preferred range, so it is necessary to make it 2.2 or more. It is preferably 2.3 or more. There is no particular upper limit, but it is preferably 3.8 or less.

Furthermore, the outer layer material for hot rolling rolls of the present invention must have dispersed in the matrix structure a carbide composite having at least one carbide selected from Group A ( _M2C type carbide, _{M6C type} carbide, MC type carbide) and at least one carbide selected from Group B ( _M7C3 type carbide, _{M23C6 type} carbide). Dispersion of the carbide composite can greatly improve seizure resistance. The presence of the carbide composite reduces the area of the matrix adjacent to the _M2C type carbide, _M6C type carbide, MC type carbide, _M7C3 type carbide, _{and M23C6 type} carbide, and the carbide composite is formed in a form similar to a single coarse carbide, making it possible to improve seizure resistance. When a single carbide is formed coarsely to improve seizure resistance, fatigue resistance decreases, but by forming a carbide composite, it is possible to achieve both seizure resistance and fatigue resistance. In particular, when the area ratio of the carbide composite is 2.0% or more, seizure resistance is significantly improved. Therefore, it is preferable that the area ratio of the carbide composite is 2.0% or more.

In the present invention, the carbide composite refers to a composite in which at least one carbide selected from Group A ( _M2C type carbide, _{M6C type} carbide, MC type carbide) and at least one carbide selected from Group B ( _M7C3 type carbide, _M23C6 type carbide) are adjacent to each _other . Although not particularly limited, the adjacent state of at least one carbide selected from Group A ( _M2C type carbide, _M6C type carbide, MC type carbide) and at least one carbide selected from Group B _{( M7C3 type} carbide, _M23C6 type carbide) refers to a state in which the two carbides are in contact with each other. However, even if they are not in contact, an effect equivalent to contact can be obtained as long as the shortest distance between at least one carbide selected from Group A ( _M2C type carbide, _M6C type carbide, MC type carbide) and at least one carbide selected from Group B ( _{M7C3 type} carbide, _{M23C6 type} carbide) is 2 μm or less. Furthermore, it is preferable that these carbides are not concentrated in a specific location but are evenly dispersed throughout the structure. Furthermore, the size of one carbide composite is preferably 10 μm or more and 150 μm or less in terms of a circle-equivalent diameter. In addition to the above-mentioned carbide composites, the outer layer material for hot rolling rolls of the present invention also contains carbides that do not form carbide composites (MC type carbides, _M2C type carbides, _M6C type carbides, _{M7C3 type} carbides, and _{M23C6 type} carbides are each dispersed singly).

Carbide composites can be determined based on the results of SEM/EBSD measurements. Here, evaluation can be performed using the method described below.

Measurement by the SEM/EBSD method is carried out on a test piece taken from any five positions in the circumferential direction of the outer layer material of a hot rolling roll, and after polishing the roll surface side, on an area of 600 × 600 ^μm2 at an acceleration voltage of ₁₅ kV, a magnification of 150, and a step _size of 0.5 μm. The EBSD pattern obtained by the EBSD measurement is analyzed using software for the EBSD measurement device (for example, _{OIM Data Collection manufactured by TSL Solutions Co., Ltd.) to determine which carbide the pattern best matches among MC type carbide, M2C type carbide, M6C type carbide} , _M7C3 type carbide, and M23C6 type carbide.
Using the obtained results, a structural image is obtained in which carbides belonging to group A ( _M2C type carbide, _M6C type carbide, MC type carbide) are colored red, carbides belonging to group B ( _{M7C3 type} carbide, _{M23C6 type} carbide) are colored blue, phases other than the above are colored black, and boundaries with an orientation difference of 15° or more from adjacent measurement data are colored white. The above-mentioned structural image processing is obtained by analyzing data obtained by measurement using the SEM/EBSD method using OIM Analysis manufactured by EDAX. Using this structural image, when a red region and a blue region are adjacent to each other with a white line as the boundary, the combined region of the red region and the blue region is determined to be a carbide composite.
The area ratio of the carbide composite can be calculated by measuring the area C (μm ² ) of the carbide composite extracted by the above method and dividing it by the area measured by the SEM/EBSD method (600 × 600 μm ² ), using the formula C/(600 × 600) × 100%, and the average area ratio of five points in the circumferential direction is taken as the area ratio of the test piece.

Furthermore, since the formation of coarse MC-type carbides makes them more susceptible to seizure, the size of the MC-type carbides is preferably 30 μm or less in equivalent circle diameter, and more preferably 25 μm or less.

In the hot rolling roll outer layer material of the present invention, it is preferable that the matrix (structure other than carbides) has an area ratio of 90% or more of bainite and/or tempered martensite. If the area ratio of bainite and/or tempered martensite is less than 90%, wear resistance and fatigue resistance will decrease. Examples of the remaining structure include austenite, ferrite, and pearlite.

Next, we will explain the manufacturing method for the hot rolling roll outer layer material and hot rolling composite roll of the present invention.

When casting a hot rolling roll outer layer material, the inner surface of the mold is first coated with a refractory material to a thickness of 1 to 5 mm, and the mold is then rotated at a predetermined rotation speed. A molten metal having the above-mentioned composition of the hot rolling roll outer layer material (simply referred to as the outer layer material molten metal) is poured into the mold to achieve the desired thickness, followed by centrifugal casting. Examples of the above-mentioned refractory material include refractories primarily composed of zircon.

When forming an intermediate layer, it is preferable to pour molten metal of the intermediate layer composition into the rotating mold while the outer layer material is solidifying or after it has completely solidified, and then perform centrifugal casting. After the outer or intermediate layer has completely solidified, it is preferable to stop the rotation of the mold, erect the mold, and then statically cast the inner layer material to form a composite roll. This remelts the inner surface of the roll outer layer material, resulting in a composite roll in which the outer and inner layers, or the outer and intermediate layers, or the intermediate and inner layers are welded together.

When forming an intermediate layer, it is preferable to use graphite steel, high-carbon steel, hypoeutectic cast iron, etc. as the intermediate layer material. The intermediate layer and outer layer are similarly welded together, and the molten outer layer material is mixed into the molten intermediate layer material in a range of 10 to 35%. From the perspective of minimizing the amount of outer layer components mixed into the inner layer, it is important to keep the amount of outer layer components mixed into the intermediate layer as low as possible, as long as it does not interfere with the welding of the outer layer and intermediate layer. The proportion of molten outer layer material mixed into the molten intermediate layer material can be calculated as the amount of outer layer material remelted by pouring the molten intermediate layer material relative to the total amount of molten outer layer material ((amount of outer layer material remelted by pouring the molten intermediate layer material) / (total amount of molten outer layer material) x 100%).

As a result of the above, a composite roll for hot rolling having two or more layers, i.e., an outer layer and an inner layer, can be obtained. In particular, it is preferable to obtain 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.

Fig. 1 is a schematic side view showing an example of a conventional centrifugal casting mold that is symmetrical about its axis of rotation, and Fig. 2 is a schematic side view showing an example of a centrifugal casting mold that is asymmetrical about its axis of rotation according to the present invention. In Fig. 1, 1 denotes an axisymmetric centrifugal casting mold, 2a denotes rollers, 3a denotes a rotation axis, and 5a denotes a molten metal feed pipe. In Fig. 2, 4 denotes an asymmetric centrifugal casting mold, 2b denotes rollers, 3b denotes a rotation axis, and 5b denotes a molten metal feed pipe.
In conventional centrifugal casting, as shown in Fig. 1, the molten metal for the outer layer material is poured from a molten metal supply pipe 5a into a centrifugal casting mold 1 that is symmetrical about its rotation axis 3a to form the outer layer. In the production of the outer layer material for a hot rolling roll of the present invention, as shown in Fig. 2, it is necessary to pour the molten metal for the outer layer material from a molten metal supply pipe 5b into a non-axisymmetric centrifugal casting mold 4 that is not symmetrical about its rotation axis 3b to form the roll outer layer material. When a non-axisymmetric centrifugal casting mold 4 that has portions that are not point-symmetrical about its rotation axis is rotated, the center of gravity of the centrifugal casting mold 4 does not pass through the rotation axis, causing vibrations in the centrifugal casting mold 4. Therefore, by pouring the molten outer layer material into the mold, vibrations can be imparted to the molten outer layer material during solidification, causing rearrangement of the solid, resulting in the formation of a carbide composite, which is a region in which one or _more carbides selected from MC, _M2C , and _M6C type carbides are adjacent to one or more carbides selected from _M7C3 and _{M23C6 type} carbides. Furthermore, the vibration of the mold refines the matrix structure, making it difficult for cracks to occur and progress even if drawing problems occur, and allowing for the production of an outer layer material for hot rolling rolls with excellent crack resistance. The presence or absence of mold vibration can be measured by measuring the distance between a laser distance meter (displacement meter) or the like and the outer surface of the mold, as will be described later.

In centrifugal casting, the centrifugal casting mold is rotated by rotating the rollers that come into contact with the mold, so the shape of the parts of the mold that come into contact with the rollers is important for rotating the mold. Therefore, even when a non-axisymmetric centrifugal casting mold 4 is used, the shape of the parts of the mold 4 that come into contact with the rollers 2b must be axisymmetric, and the shape of the parts of the mold 4 that do not come into contact with the rollers 2b must be non-axisymmetric.

The magnitude (amplitude) of vibrations generated in the centrifugal casting mold 4 can be measured by measuring the distance between the laser distance meter and the mold's outer surface using a laser distance meter (displacement meter) or similar device. To achieve the effects of the present invention, it is necessary to apply vibrations that displace the centrifugal casting mold 4 by 100 μm or more in the direction perpendicular to the rotation axis 3b. The shape of the centrifugal casting mold 4 is determined so that such vibrations occur. To accurately measure the vibration amplitude of the centrifugal casting mold 4 using a laser distance meter or similar device, the measurement position of the vibration amplitude must be symmetrical about the rotation axis 3b. For example, in Figure 1 or Figure 2, a laser distance meter is installed on an extension of roller 2a or 2b, and the distance between the laser distance meter and the mold's outer surface is measured. The difference between the maximum and minimum values of the measured distance data is taken as the vibration amplitude. Note that because the vibration amplitude value varies depending on the rotation speed of the centrifugal casting mold 4, it is preferable to measure the vibration amplitude while the mold is rotating at the rotation speed used for centrifugal casting. In order to accurately evaluate the amplitude of vibrations occurring in the centrifugal casting mold 4, it is preferable to set the sampling period to 0.1 seconds or less when measuring the distance between the laser distance meter and the mold outer surface. Furthermore, because the surface roughness of the mold outer surface may affect the vibration amplitude value, when measuring the vibration amplitude of a non-axisymmetric mold, it is necessary to machine the mold so that the surface roughness of the mold outer surface is the same as that of an axisymmetric mold.

The hot rolling composite roll of the present invention is preferably subjected to heat treatment (quenching and tempering) after casting. The heat treatment preferably includes a quenching process in which the roll is heated to 900-1100°C and then air-cooled or air-blast cooled, and a tempering process in which the roll is further heated and held at 450-570°C, followed by cooling (air-cooling, air-blast cooling, furnace cooling), performed two or more times. Alternatively, after casting, the roll may be heated and held at 400-520°C, followed by cooling (air-cooling, air-blast cooling, furnace cooling), without quenching, and then a tempering process in which the roll is heated and held at 400-520°C, followed by cooling (air-cooling, air-blast cooling, furnace cooling), performed two or more times.

The preferred hardness of the outer layer material surface in the hot rolling composite roll of the present invention is 75 to 86 HS (Shore hardness). If the hardness is lower than 75 HS, wear resistance is likely to deteriorate, so a hardness of 75 HS or higher is preferred. 76 HS or higher is even more preferred. Conversely, if the hardness exceeds 86 HS, cracks formed on the hot rolling roll surface during hot rolling become difficult to remove by grinding. A more preferred hardness is 85 HS or lower.

The hot rolling roll outer layer material of the present invention is produced by centrifugal casting and can be used as a ring roll or sleeve roll as is. However, it is also used as an outer layer material for hot rolling composite rolls suitable for hot finishing rolling stands. The hot rolling composite roll of the present invention comprises 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 an 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 intermediate layer is not particularly limited, but it is preferably a high-carbon material with 1.5 to 3.0 mass% C.

Molten roll outer layer materials shown in Table 1, Test Steel Nos. 1 to 27, were melted in a high-frequency induction furnace, and ring-shaped test specimen 6 (outer diameter 250 mm, inner diameter 170 mm, depth 70 mm) was produced by centrifugal casting. A non-axisymmetric centrifugal casting mold was used, in which the thickness of the mold was varied circumferentially by 1 to 5 mm. The inner diameter of the mold was 250 mm, and since the mold thickness varied circumferentially, the outer diameter of the mold varied circumferentially. The centrifugal casting mold was fixed to a disk-shaped steel material (outer diameter 350 mm) and connected to a motor via the disk-shaped steel material. The center of the disk-shaped steel material and the rotation axis were coincident. A laser rangefinder was installed approximately 10 mm away from the outer surface of the disk-shaped steel material, and the distance between the disk-shaped steel material and the laser rangefinder was measured at a sampling period of 0.1 s to measure the vibration amplitude of the centrifugal casting mold. The amplitude of vibration of the centrifugal casting mold measured with a laser distance meter before the molten metal for the outer layer material was 202 μm ((maximum value of the distance between the laser distance meter and the centrifugal casting mold measured with the laser distance meter) - (minimum value of the distance between the laser distance meter and the centrifugal casting mold measured with the laser distance meter)).
The casting temperature was 1420°C, and the centrifugal force on the outer surface of the ring-shaped test material was 120 G in gravity. After casting, the test material was quenched from 1000°C and tempered at 510°C three times. Hardness test specimens, EBSD measurement specimens, drop-weight frictional thermal shock test specimens, and hot abrasion test specimens were collected from the resulting ring-shaped test material, and hardness tests, EBSD measurements, drop-weight frictional thermal shock tests, and hot abrasion tests were performed. As a reference material (a conventional example and comparative steel), a ring-shaped test material was prepared using an axisymmetric centrifugal casting mold with the composition shown in Table 1. After casting, the ring-shaped test material was quenched from 950°C and tempered twice at 500°C. The vibration amplitude of the centrifugal casting mold, measured with a laser distance meter before the outer layer material molten metal was poured, was 58 μm.

The hardness test was performed as follows. A 20 x 20 x 10 mm hardness test piece was taken from a random position on the ring-shaped test material 6 (10 mm is the radial direction of the ring-shaped test material). The 20 x 20 mm surface of the obtained hardness test piece was used as the hardness measurement surface. Vickers hardness HV50 was measured using a Vickers hardness tester (test force: 50 kgf (490 N)) in accordance with JIS Z 2244, and the converted Shore hardness VHS (HS) was calculated using the conversion formula of JIS B 7731. Ten measurements were taken, and the highest and lowest values were removed to calculate the average of the eight points, which was used as the hardness of the test material.

The EBSD measurement was performed as follows. As shown in Figure 3, five EBSD measurement specimens 7 measuring 10 x 10 x 5 mm (5 mm is the radial direction of the ring-shaped test material) were taken from positions 10 mm inside from the outer surface of the ring-shaped test material 6. The 10 x 10 mm surface of the obtained test specimens was mirror-polished, and then EBSD measurement was performed on an area of 600 x 600 ^µm² at an acceleration voltage of 15 kV, a magnification of 150, and a step size of _0.5 µm. Prior to the EBSD measurement, OIM Data Collection (manufactured by TSL Solutions Co., Ltd.) was set up so that _MC -type carbides, _M2C -type carbides, M6C-type carbides, _{M7C3 -} type carbides, and _M23C6- type carbides would be measured. The obtained data was subjected to image processing using OIM Analysis manufactured by EDAX to obtain a structural image in which carbides belonging to group A ( _M2C type carbide, _M6C type carbide, MC type carbide) were colored red, carbides belonging to group _B ( _M7C3 type carbide, _{M23C6 type} carbide) were colored blue, phases other than the above were colored black, and boundaries with an orientation difference of 15° or more from adjacent measurement data were colored white. Using this structural image, when a red region and a blue region were adjacent to each other with a white line as the boundary, the combined region of the red region and the blue region was determined to be a carbide composite, and the area C ( ^μm2 ) of the carbide composite within an area of 600 × 600 ^μm2 was calculated. The area ratio of the carbide composite was calculated by dividing the area C by the measurement area (600 × 600 μm ² ) by the SEM/EBSD method, using the formula C/(600 × 600) × 100%, and the average area ratio of the five test pieces was taken as the area ratio of that test material.

The drop-weight frictional thermal shock test was performed as follows. Drop-weight frictional thermal shock test pieces 12 (30 x 20 x 20 mm) were collected from the resulting ring-shaped test pieces 6. As shown in Figure 4, the drop-weight frictional thermal shock test involved dropping a weight 8 (50 kg) from a height of 1 m onto a rack 9. The energy generated by this drop-weight frictional thermal shock test rotated a pinion 11 equipped with a mild steel mating piece 10, forcing the mating piece 10 into strong contact with the test piece 12. After the test, photographs of the test piece surface were taken, and the areas of adhesion and contact with the mating piece 10 were measured. The seizure area ratio (= (area of adhesion to mating piece 10) / (area of contact with mating piece 10) × 100%) was calculated to evaluate seizure resistance. A seizure area ratio of less than 45% was rated "Good," indicating that the seizure resistance was acceptable.

The hot abrasion test was carried out as follows: A hot abrasion test piece 13 (outer diameter 60 mm, thickness 10 mm) was taken from the obtained ring-shaped test piece 6.
As shown in Figure 5, the hot wear test was performed using a two-disk rolling/sliding method between a test piece 13 and a mating piece 16. The test piece 13 was rotated at 700 rpm while being cooled with cooling water 14, and a mating piece 16 (material: S45C, outer diameter: 190 mmφ, width: 15 mm, C1 chamfer) 16 heated to 800°C by a high-frequency induction heating coil 15 was brought into contact with the rotating test piece 13 under a load of 490 N, and the test pieces were rolled for 120 minutes at a sliding ratio of 9%. The wear volume of the test piece 13 after the test (calculated from the outer periphery 13A of the wear test piece before the test and the outer periphery 13B of the wear test piece after the test) was measured, and the ratio of the wear volume of each test piece to the reference value, No. 28 manufactured by the conventional technology, was calculated as the wear ratio (= (wear volume of the reference piece) / (wear volume of each test piece)) to evaluate the wear resistance. The abrasion resistance was evaluated as "good" when the abrasion ratio was 1.0 or more, and the abrasion resistance was judged to be acceptable.

Fatigue resistance was evaluated by a hot abrasion test. A hot abrasion test piece 13 (outer diameter 60 mm, thickness 10 mm) was taken from the resulting ring-shaped test piece 6. The hot abrasion test was conducted using a two-disk rolling and sliding method between the test piece 13 and a mating piece 16, as shown in Figure 5. The test piece 13 was rotated at 10 rpm while being cooled with cooling water 14. A mating piece 16 (material: S45C, outer diameter: 190 mm, width: 15 mm, C1 chamfer) heated to 1000°C by a high-frequency induction heating coil 15 was brought into contact with the rotating test piece 13 under a load of 980 N and rolled for 240 minutes at a sliding ratio of 5%. As shown in Figure 6, cross-sectional observation specimens 17 were taken from three random locations on the test specimen after the test. One of the cut surfaces of each cross-sectional observation specimen was mirror-polished and then observed under an optical microscope to measure the depth of cracks 18 formed on the surface of the hot abrasion test specimen 13 (the contact surface with the mating piece 16). Note that cracks 18 present at the end of the test specimen were caused by the shape of the test specimen rather than the material of the test specimen. Therefore, if part of or the entire crack 18 was contained within a 1 mm range on either side of the test specimen, the crack 18 was excluded from measurement. If the maximum crack depth was less than 1.2 mm, it was rated "Good" and the fatigue resistance was deemed to have passed.

The overall evaluation was "pass" if the wear resistance, fatigue resistance, and seizure resistance were all "good."

Table 2 shows that the inventive example has wear resistance equal to or greater than that of comparative example No. 28 of the prior art, while also having excellent fatigue resistance and seizure resistance.

Comparative Example No. 11 had a C content below the range of the present invention, so the hardness was reduced and the wear ratio did not reach the desired value.
Comparative Example No. 12 had a Si content below the range of the present invention, and therefore the wear ratio did not reach the desired value.
Comparative Example No. 13 had a Mn content below the range of the present invention, and therefore the wear ratio did not reach the desired value.
In Comparative Example No. 14, the V content was below the range of the present invention, which is thought to have reduced the amount of MC type carbides, resulting in the wear ratio not reaching the desired value.
In Comparative Example No. 15, the V content exceeded the range of the present invention, which is thought to have resulted in the formation of coarse MC type carbides, and the seizure area ratio did not reach the desired value.
In Comparative Example No. 16, the Cr content was below the range of the present invention, so the seizure resistance did not reach the desired value. Also, it is thought that the total amount of carbides was reduced, and the wear ratio did not reach the desired value.
Comparative Example No. 17 had a Cr content exceeding the range of the present invention, and therefore the maximum crack depth did not reach the desired value.
Comparative Example No. 18 had a Mo content below the range of the present invention, so the seizure resistance did not reach the desired value. Also, the amount of carbides was reduced, so the wear ratio did not reach the desired value.
Comparative Example No. 19 had a Mo content exceeding the range of the present invention, and therefore the maximum crack depth and the seizure area ratio did not reach the desired values.
Comparative Example No. 20 had a Ni content below the range of the present invention, and therefore had insufficient hardenability and did not achieve the desired wear ratio.
Comparative Example No. 21 had a Ni content exceeding the range of the present invention, resulting in a large amount of residual austenite, which reduced hardness and prevented the wear ratio from reaching the desired value. Furthermore, the reduced hardness is thought to have caused slight deformation of the surface of the drop-weight frictional thermal shock test specimen during the drop-weight frictional thermal shock test. As a result, the seizure area ratio also did not reach the desired value. Furthermore, the maximum crack depth also did not reach the desired value.
In Comparative Example No. 22, the Nb content was below the range of the present invention, so the wear ratio did not reach the desired value, and the maximum crack depth also did not reach the desired value.
In Comparative Example No. 23, the Nb content exceeded the range of the present invention, so coarse MC type carbides were formed, and the seizure area ratio did not reach the desired value. In addition, the maximum crack depth also did not reach the desired value.
Comparative Example No. 24 had a W content exceeding the range of the present invention, so the maximum crack depth and seizure resistance did not reach the desired values. Also, coarse _M2C and _M6C carbides were formed, so the wear ratio did not reach the desired value.
In Comparative Example No. 25, the value of formula (2) was below the range of the present invention, and therefore the seizure resistance did not reach the desired value.
Comparative Example No. 26 had a Ti content exceeding the range of the present invention, so coarse Ti-based carbides were formed, and the seizure resistance did not reach the desired value. In addition, the maximum crack depth also did not reach the desired value.
In Comparative Example No. 27, the value of formula (1) exceeded the range of the present invention, and therefore the seizure resistance did not reach the desired value.
In Comparative Example No. 28, the formulas (1) and (2) were outside the ranges of the present invention, and the seizure resistance did not reach the desired value. In addition, the maximum crack depth also did not reach the desired value.
Therefore, according to the present invention, it is possible to manufacture an outer layer material for a hot rolling roll and a composite roll for hot rolling that are excellent in wear resistance, fatigue resistance, and seizure resistance, which results in a significant improvement in the surface quality of the rolled material and an increase in the roll life.

1. Centrifugal casting mold (axisymmetric)
2a, 2b Rollers 3a, 3b Rotating shaft 4 Centrifugal casting mold (non-axisymmetric)
5a, 5b Molten metal supply pipe 6 Ring-shaped test piece 7 Test piece for EBSD measurement 8 Weight 9 Rack 10 Counterpart 11 Pinion 12 Drop-weight friction thermal shock test piece 13 Hot wear test piece (test piece)
13A Outer circumference of abrasion test piece before test 13B Outer circumference of abrasion test piece after test 14 Cooling water 15 High frequency induction heating coil 16 Counterpart 17 Test piece for cross section observation 18 Crack

Claims (5)

In mass%,
C: 1.5-2.3%,
Si: 0.3-2.0%,
Mn: 0.3-2.0%,
Cr: 3.5-7.0%,
Mo: 3.0 to 6.0%,
V: 3.0-5.0%,
Nb: 0.1 to 2.0%,
Al: 0.01-0.10%,
Ni: 0.02-2.00%,
N: 0.050% or less,
Contains
Or, furthermore,
Ti: 0.50% or less,
B: 0.090% or less,
Co: 1.0% or less,
W: 1.5% or less,
Zr: 0.50% or less,
Contains one or more selected from
The balance consists of Fe and unavoidable impurities,
The contents of Cr, Mo, W, V, and Nb have a composition that satisfies the following formulas (1) and (2):
The outer layer material for a hot rolling roll is characterized in that it has carbides selected from at least one of Group A and Group B below, in which the shortest distance between the carbides is 2 μm or less, and further has dispersed therein carbide composites each having a size of 10 μm or more and 150 μm or less in terms of a circle-equivalent diameter :
0.85≦%Cr/(%Mo+%W/2)≦1.15... (1)
(%Cr+%Mo+%W)/(%V+%Nb)≧2.2... (2)
Here, %Cr, %Mo, %W, %V, and %Nb are the contents (mass%) of each element, and elements that are not contained are set to 0.
Group A: M ₂ C type carbide, M ₆ C type carbide, MC type carbide Group B: M ₇ C ₃ type carbide, M ₂₃ C ₆ type carbide The outer layer material for hot rolling rolls according to claim 1, characterized in that the carbide composite has an area ratio of 2.0% or more. A method for producing a hot rolling roll outer layer material according to claim 1 or 2, comprising casting a molten metal having the above-described composition using a non-axisymmetric centrifugal casting mold having a portion that is not point-symmetric about the rotation axis of the centrifugal casting mold in a cross section perpendicular to the rotation axis of the centrifugal casting mold. A composite roll for hot rolling having two or more layers, an outer layer and an inner layer, wherein the outer layer is made of the outer layer material for hot rolling rolls described in claim 1 or 2. A method for manufacturing a composite roll for hot rolling, characterized by using an outer layer material obtained by the method for manufacturing an outer layer material for a hot rolling roll described in claim 3.