JP2024075137A - Roll outer layer material for hot rolling and composite roll for hot rolling - Google Patents

Abstract

To provide a roll outer layer material for hot rolling and a composite roll for hot rolling, both having excellent wear resistance, slip resistance and roughening resistance.SOLUTION: A roll outer layer material for hot rolling has a component composition containing, by mass, 1.1 to 2.6% C, 0.15 to 2.50% Si, 0.15 to 2.50% Mn, 0.1 to 6.0% Ni, 1.5 to 10.0% Cr, 3.5 to 12.5% Mo, 2.5 to 7.5% V, 0.1 to 6.0% W, 0.01 to 0.04% P and 0.001 to 0.015% S, the content of Si, Mn, Ni, Cr, Mo, V and W satisfying a specific formula, the content of C, Cr, Mo, V and W satisfying a specific formula, and the balance Fe with inevitable impurities, in which a carbide having a particle diameter of 1.0 μm or larger is present with an area ratio of 7.5 to 18.0%, a eutectic cell size is 65 to 100 μm, and a Shore hardness at 600°C is ≥44.0 HS and ≤52.0 HS.SELECTED DRAWING: None

Description

The present invention relates to a hot rolling roll outer layer material and a hot rolling composite roll that have excellent abrasion resistance, slip resistance, and surface roughening resistance, and in particular to a hot rolling roll outer layer material and a hot rolling composite roll that are suitable for rough rolling of steel plates.

In recent years, the demand for high-quality steel sheets has been increasing, and accordingly, there is a demand for improving the hot rolling technology of steel sheets. Therefore, there is a strong demand for improving the properties of hot rolling rolls used in hot rolling equipment, specifically, wear resistance, seizure resistance, slip resistance, surface roughening resistance, etc. In order to improve wear resistance, HiCr cast steel rolls in which Cr-based M ₇ C ₃ carbides are introduced into the structure, and high-speed steel rolls based on high-speed steel, a type of tool steel, contain carbide-forming elements such as V, Cr, Mo, and W, and introduce a large amount of hard carbides such as V-based MC carbides, Mo, W-based M ₂ C carbides, and Cr-based M ₇ C ₃ carbides (M indicates a metal element that forms carbides) into the structure are used. However, when a large amount of carbide is introduced to improve wear resistance, the roll surface roughness is reduced due to high hardness and good wear resistance, and the friction coefficient during rolling is reduced, making slippage more likely to occur and making the surface roughness more likely to occur due to the drop-off of coarse carbide particles that occurs when a large amount of carbide is introduced.

Various techniques have been disclosed to solve these problems. For example, Patent Document 1 proposes a composite roll for hot rolling that contains C: 0.8-4.0%, Si: 0.2-2.0%, Mn: 0.2-2.0%, Cr: 3.0-15%, V: 3.0-15%, one or two of Mo and W: ≧2%, and Mo+0.5W: ≧6.1%, or further contains one or more of the following: Ni: 0.2-5%, Co: 0.5-10%, Nb: 0.50-5.0%, one or more of Al, Ti, Zr: ≦0.5%, the metal structure of the outer layer material has an area ratio of 5-30% carbides, and the distribution of each carbide is such that the average gap between adjacent carbides is 20 μm or less. This allows the appropriate amount of granular carbide to be finely dispersed with small gaps between adjacent carbides, resulting in a composite roll for hot rolling with improved slip resistance and seizure resistance.

Patent Document 2 also describes a hot rolling roll containing C: 1.0-2.6%, Si: 1.2% or less, Mn: 1.2% or less, Ni: 3.0% or less, Cr: 1.5-6.0%, Mo and W as Mo+0.5W 1.5-5.0%, V: 6.0-12.0%, Co: 5.0% or less, and at least one of Ti: 2.0% or less and Nb: 2.0% or less, with the balance being Fe and unavoidable impurities. The MC type carbides have an area ratio Sr(%) of carbides with an equivalent particle size √Sc of 1 μm or more of the iron-based alloy composition and are dispersed so that the average intergranular spacing Lc(μm) is 30 μm or less, thereby improving wear resistance, surface roughening resistance, and seizure resistance.

Patent Document 3 describes a steel sheet containing, in mass %, C: 0.90 to 1.40%, Si: 0.50 to 1.50%, Mn: 0.50 to 1.50%, Ni: 0.5 to 2.0%, Cr: 9.0 to 16.0%, Mo: 1.00 to 3.00%, Al: 0.010 to 0.030%, and at least one of V: 0.05 to 0.50%, Ti: 0.05 to 0.50%, and Nb: 0.02 to 0.20%, and containing the following formula (1): A work roll for a hot rolling rough rolling stand has been proposed, which is characterized in that an outer shell layer made of cast steel having a molten metal composition consisting of the balance Fe and unavoidable impurities and a shaft core material made of ductile cast iron cast inside the outer shell layer are integrated through an intermediate layer. By adding appropriate amounts of C and Cr and controlling the amount of _{M7C3 carbide} , a work roll for a hot rolling rough rolling stand that can achieve both wear resistance and slip resistance is obtained.

JP 2004-255457 A JP 2005-28453 A JP 2020-63485 A

However, as demand for high-quality steel sheets increases and hot rolling technology for steel sheets improves, the properties required for hot rolling rolls are becoming increasingly strict, with particular demand for improved wear resistance. If rolls are designed to meet the wear resistance requirement, they will be more susceptible to slippage and roughening, resulting in an increased frequency of roll troubles. The conventional hot rolling rolls described in Patent Documents 1 to 3 are insufficient in either wear resistance, slippage resistance, or roughening resistance.

The present invention aims to provide a hot rolling roll outer layer material and a hot rolling composite roll that solve the above problems and have excellent abrasion resistance, slip resistance, and surface roughening resistance.

The present inventors have investigated in detail the relationship between the base of a hot rolling roll, carbides in the structure, hardness, wear amount, friction coefficient, chemical components (composition), and the structure of the roll material and the scale adhering to the roll. As a result, they have found that the wear resistance, slip resistance, and surface roughening resistance can be improved by optimizing the chemical components, casting method, and heat treatment conditions so that the amount and type of carbides, hardness, and eutectic cell size are within specific ranges.
The present invention has been completed based on these findings and further investigations. That is, the gist of the present invention is as follows.
[1] In mass%,
C: 1.1 to 2.6%,
Si: 0.15 to 2.50%,
Mn: 0.15 to 2.50%,
Ni: 0.1 to 6.0%,
Cr: 1.5 to 10.0%,
Mo: 3.5 to 12.5%,
V: 2.5 to 7.5%,
W: 0.1 to 6.0%,
P: 0.01 to 0.04%,
S: 0.001 to 0.015%;
The contents of Si, Mn, Ni, Cr, Mo, V, and W satisfy the following formula (1), the contents of C, Cr, Mo, V, and W satisfy the following formula (2), and the balance is Fe and unavoidable impurities,
A roll outer layer material for hot rolling, characterized in that carbides having a grain size of 1.0 μm or more are present in an area ratio of 7.5 to 18.0%, the eutectic cell size is 65 to 100 μm, and the Shore hardness at 600° C. is 44.0 HS or more and 52.0 HS or less.
50.0≦([%Cr]×[%Mo]×[%V]×[%W])/([%Si]×[%Mn]×[%Ni])≦150.0 ... (1)
0.90≦[%C]×((0.177×[%V])/(0.099×[%Cr]+0.063×[%Mo]+0.033×[%W]))≦3.00 ... (2)
In formulas (1) and (2), [%C], [%Si], [%Mn], [%Ni], [%Cr], [%Mo], [%V], and [%W] are the contents (mass%) of each element.
[2] A composite roll for hot rolling having two layers, an outer layer and an inner layer, or three layers, an outer layer, an intermediate layer and an inner layer, wherein the outer layer is, in mass%,
C: 1.1 to 2.6%,
Si: 0.15 to 2.50%,
Mn: 0.15 to 2.50%,
Ni: 0.1 to 6.0%,
Cr: 1.5 to 10.0%,
Mo: 3.5 to 12.5%,
V: 2.5 to 7.5%,
W: 0.1 to 6.0%,
P: 0.01 to 0.04%,
S: 0.001 to 0.015%;
The contents of Si, Mn, Ni, Cr, Mo, V, and W satisfy the following formula (1), the contents of C, Cr, Mo, V, and W satisfy the following formula (2), and the balance is Fe and unavoidable impurities,
A composite roll for hot rolling, characterized in that carbides having a grain size of 1.0 μm or more are present in an area ratio of 7.5 to 18.0%, the eutectic cell size is 65 to 100 μm, and the Shore hardness at 600° C. is 44.0 HS or more and 52.0 HS or less.
50.0≦([%Cr]×[%Mo]×[%V]×[%W])/([%Si]×[%Mn]×[%Ni])≦150.0 ... (1)
0.90≦[%C]×((0.177×[%V])/(0.099×[%Cr]+0.063×[%Mo]+0.033×[%W]))≦3.00 ... (2)
In formulas (1) and (2), [%C], [%Si], [%Mn], [%Ni], [%Cr], [%Mo], [%V], and [%W] are the contents (mass%) of each element.

The present invention provides a hot rolling roll outer layer material and a hot rolling composite roll that are excellent in wear resistance, slip resistance, and surface roughening resistance. As a result, the life of the hot rolling roll is improved, and the productivity of hot rolled steel sheets is improved accordingly.

FIG. 2 is an explanatory diagram showing a schematic configuration of a testing machine used in a hot rolling wear test.

The reasons for limiting the composition of the outer layer material (hereinafter simply referred to as the outer layer) of the hot rolling roll of the present invention will be explained. In the following, mass% will be simply written as % unless otherwise specified.

C: 1.1 to 2.6%
C combines with V, Cr, Mo, W, etc. to form hard carbides, which contribute to improving wear resistance. It also has the effect of improving the hardness of the matrix by solid solution strengthening. If C is less than 1.1%, the amount of carbides is insufficient, and excellent wear resistance cannot be obtained. In addition, the hardness is low, so plastic flow occurs, and the friction coefficient increases due to the effects of roughening caused by plastic flow. On the other hand, if C exceeds 2.6%, carbides are generated excessively, and the roughening resistance decreases. In addition, if C exceeds 2.6% and the hardness increases, the friction coefficient decreases and slip occurs. Therefore, the C content is limited to 1.1% or more and 2.6% or less. The C content is preferably 1.3% or more, and more preferably 1.5% or more. In addition, the C content is preferably 2.3% or less, and more preferably 2.0% or less.

Si: 0.15 to 2.50%
Silicon acts as a deoxidizer in the molten metal, improving the fluidity of the molten metal and preventing casting defects. If the Si content is less than 0.15%, the deoxidizing effect is insufficient, and if the Si content exceeds 2.50%, the deoxidizing effect is saturated. Therefore, the Si content is limited to 0.15% or more and 2.50% or less. The Si content is preferably 0.25% or more, and more preferably 0.60% or more. The Si content is preferably 2.00% or less, and more preferably 1.50% or less.

Mn: 0.15 to 2.50%
Mn has an effect of deoxidizing the molten metal and fixing S, which has a detrimental effect on the product, as MnS. If the Mn content is less than 0.15%, the effect of fixing S as MnS is insufficient. On the other hand, if the Mn content exceeds 2.50%, the effect is saturated. Therefore, the Mn content is limited to 0.15% or more and 2.50% or less. The Mn content is preferably 0.25% or more, and more preferably 0.35% or more. In addition, the Mn content is preferably 2.00% or less, and more preferably 1.50% or less.

Ni: 0.1 to 6.0%
Ni has the effect of improving the hardenability of the matrix and improving the hardness of the matrix. If the Ni content is less than 0.1%, the effect of improving the matrix hardness is insufficient. On the other hand, if the Ni content exceeds 6.0%, austenite is likely to remain, so the hardness decreases. Therefore, the Ni content is limited to 0.1% or more and 6.0% or less. The Ni content is preferably 1.0% or more, and more preferably 1.5% or more. In addition, the Ni content is preferably 5.0% or less, and more preferably 4.0% or less.

Cr: 1.5 to 10.0%
Cr is a carbide-forming element and combines with C to form _M7C3 carbide. Since _M7C3 carbide is a hard carbide, it has the effect of improving wear resistance. If the Cr content is less than 1.5%, the amount _{of M7C3 carbide is} insufficient, and wear resistance is reduced. On the other hand, if the _Cr content exceeds 10.0%, coarse _{M7C3 carbide} is generated, and wear resistance is deteriorated. Therefore, the Cr content is limited to 1.5% or more and 10.0% or less. The Cr content is preferably 2.5% or more, and more preferably 3.5% or more. The Cr content is preferably 8.0% or less, and more preferably 6.0% or less.

Mo: 3.5 to 12.5%
Mo is a carbide-forming element and combines with C to form _M2C carbide. _M2C carbide is harder than _{M7C3 carbide} , and therefore has the effect of further improving wear resistance. When the Mo content is less than 3.5%, the amount of _M2C carbide is insufficient, and the effect of improving wear resistance is insufficient. On the other hand, when the Mo content exceeds 12.5%, coarse _M2C carbide is generated, which deteriorates wear resistance and reduces toughness. Therefore, the Mo content is limited to 3.5% or more and 12.5% or less. The Mo content is preferably 4.0% or more, and more preferably 4.5% or more. The Mo content is preferably 10.5% or less, and more preferably 8.0% or less.

V: 2.5-7.5%
V is a carbide-forming element and combines with C to form MC carbide. MC carbide has a Vickers hardness Hv value of about 2800, is one of the hardest carbides, and has the effect of improving wear resistance. If the V content is less than 2.5%, the amount of MC carbide is insufficient, and the effect of improving wear resistance is insufficient. On the other hand, if the V content exceeds 7.5%, the VC carbide, which has a lower specific gravity than the molten iron, is concentrated inside the outer layer due to the centrifugal force during centrifugal casting, and segregation occurs. Therefore, the V content is limited to 2.5% or more and 7.5% or less. The V content is preferably 3.0% or more, and more preferably 4.0% or more. The V content is preferably 7.0% or less, and more preferably 6.5% or less.

W: 0.1 to 6.0%
W is a carbide-forming element, and combines with C to form hard carbides such as hard M ₂ C, which increases the hardness of the outer layer and improves wear resistance. If the W content is less than 0.1%, the effect is insufficient and wear resistance is deteriorated. On the other hand, if the W content exceeds 6.0%, coarse M ₂ C carbides are formed, and wear resistance is deteriorated. Therefore, the W content is limited to 0.1% or more and 6.0% or less. The W content is preferably 0.5% or more, and more preferably 1.0% or more. The W content is preferably 5.0% or less, and more preferably 4.5% or less.

P: 0.01 to 0.04%
It has been thought that P is mixed in during the roll manufacturing process and reduces mechanical properties, but as a result of intensive research by the present inventors, it has been clarified that the inclusion of a small amount of P has the effect of improving hardness and wear resistance. If the P content is less than 0.01%, the effect of improving hardness and wear resistance is insufficient, while if the P content exceeds 0.04%, the mechanical properties deteriorate. Therefore, the P content is limited to 0.01% or more and 0.04% or less. The P content is preferably 0.02% or more. Moreover, the P content is preferably 0.03% or less.

S: 0.001 to 0.015%
S is usually considered a harmful element in iron-based alloys and is limited to a certain amount or less, but within that range, MnS has a lubricant effect. On the other hand, if the content is too high, the material becomes brittle. Therefore, the S content is limited to 0.001% or more and 0.015% or less. The S content is preferably 0.002% or more. Also, the S content is preferably 0.010% or less.

The present invention is also characterized in that the contents of C, Si, Mn, Ni, Cr, Mo, V, and W are within the above-mentioned ranges, and further that the following formula (1) and formula (2) are satisfied:
50.0≦([%Cr]×[%Mo]×[%V]×[%W])/([%Si]×[%Mn]×[%Ni])≦150.0 ... (1)
0.90≦[%C]×((0.177×[%V])/(0.099×[%Cr]+0.063×[%Mo]+0.033×[%W]))≦3.00 ... (2)
In formulas (1) and (2), [%C], [%Si], [%Mn], [%Ni], [%Cr], [%Mo], [%V], and [%W] are the contents (mass%) of each element.

Regarding ([%Cr] x [%Mo] x [%V] x [%W]) / ([%Si] x [%Mn] x [%Ni]) in formula (1), this parameter indicates the ratio of the product of the contents of each carbide-forming element (V, Cr, Mo, W) to the product of the contents of each non-carbide-forming element (Si, Mn, Ni). By adjusting to satisfy formula (1), the amount of carbide generated and the eutectic cell size are optimized, and the wear resistance, slip resistance, and surface roughness resistance are improved. If the value of ([%Cr] x [%Mo] x [%V] x [%W]) / ([%Si] x [%Mn] x [%Ni]) is less than 50.0, the amount of carbide generated is insufficient and sufficient wear resistance cannot be obtained. In addition, the eutectic cell size becomes large, and the friction coefficient during rolling cannot be maintained at a high value, and sufficient slip resistance cannot be obtained. On the other hand, if it exceeds 150, a large amount of coarse carbides are generated, and sufficient wear resistance and surface roughness resistance cannot be obtained. In addition, the eutectic cell size becomes small, which increases the friction coefficient during rolling and makes the steel material more susceptible to seizure. Therefore, the value of ([%Cr] x [%Mo] x [%V] x [%W])/([%Si] x [%Mn] x [%Ni]) is limited to 50 or more and 150 or less. More preferably, it is 70 or more and 130 or less.

Regarding [%C] × ((0.177 × [%V]) / (0.099 × [%Cr] + 0.063 × [%Mo] + 0.033 × [%W])) in the formula (2), this parameter represents the ratio of the amount of MC carbide to (the amount of _M2C carbide + the amount of _{M7C3 carbide} ), and by adjusting it to satisfy the formula (2), the ratio of the amount of each carbide is optimized, and it is possible to improve the wear resistance and the surface roughening resistance, and to obtain a value of the friction coefficient during rolling that does not cause seizure or slippage. If the value of [%C] × ((0.177 × [%V]) / (0.099 × [%Cr] + 0.063 × [%Mo] + 0.033 × [%W])) is less than 0.90, the amount of hard MC carbide is small, and sufficient wear resistance cannot be obtained. In addition, the amount of MC carbide is reduced, and the coefficient of friction during rolling cannot be maintained at a high value, and sufficient slip resistance cannot be obtained. On the other hand, when it exceeds 3.00, the ratio of the amount of MC carbide increases. This is a fine granular carbide, and the number of protrusions on the roll surface increases, which increases the coefficient of friction during rolling and makes it easier for seizure to occur. Therefore, the value of [%C] x ((0.177 x [%V]) / (0.099 x [%Cr] + 0.063 x [%Mo] + 0.033 x [%W])) is limited to 0.90 or more and 3.00 or less. More preferably, it is 1.20 or more and 2.50 or less.

Balance: Fe and inevitable impurities The balance other than the above components consists of Fe and inevitable impurities.

Next, we will explain the reasons for limiting the structure of the outer layer material for hot rolling rolls of the present invention.

The hot rolling roll outer layer material of the present invention is characterized in that it has a component composition in the above-mentioned range, carbides with a grain size of 1.0 μm or more are present at an area ratio of 7.5 to 18.0%, and the eutectic cell size is 65 to 100 μm. The carbides are MC carbides, M ₂ C carbides, and M ₇ C ₃ carbides crystallized during solidification, and since the precipitated carbides present in the base are less than 1.0 μm, the carbides are limited to carbides of 1.0 μm or more. Here, the base is preferably martensite or bainite. In order to obtain an appropriate ratio of each carbide amount, it has been found that the wear resistance, slip resistance, and surface roughening resistance are improved by limiting the structure to one having a composition in the above-mentioned range, carbides with a grain size of 1 μm or more are present at an area ratio of 7.5 to 18.0%, and the eutectic cell size is 65 to 100 μm. The eutectic cell size is the average value of the size of the eutectic cells measured by the method described below. Although various studies have been conducted on hot rolling rolls, the relationship between hot rolling rolls and scale has hardly been studied. As a result of intensive studies by the inventors, it was found that scale adheres to the base of the roll, scale does not adhere to the carbide, and unevenness due to the presence or absence of scale adhesion exists on the roll surface. It has been thought that it is impossible to simultaneously satisfy all of the wear resistance, slip resistance, and surface roughness resistance, but it is possible to maintain a high friction coefficient and improve slip resistance by optimizing the carbide area ratio and the ratio of each carbide type, and controlling the unevenness interval on the roll surface. In addition, it is thought that the peeling of the black scale, which is one of the causes of surface roughness, is caused by the black scale on the base inside the eutectic cell being subjected to stress in the roll circumferential direction by the rotation of the roll, causing the black scale to peel off. Therefore, it is thought that the peeling of the black scale can be suppressed and the surface roughness resistance is improved by setting the eutectic cell size in an appropriate range.

If the eutectic cell size is less than 65 μm, the surface will be too uneven, resulting in a high coefficient of friction and seizure. On the other hand, if it exceeds 100 μm, the surface will be too uneven, resulting in a failure to maintain a sufficient coefficient of friction and causing slippage. Therefore, the eutectic cell size is limited to 65 μm or more and 100 μm or less. The eutectic cell size is preferably 70 μm or more, and more preferably 75 μm or more. The eutectic cell size is also preferably 95 μm or less, and more preferably 90 μm or less.

In addition to carbides, the structure (base structure) may contain martensite or bainite in an area ratio of 82.0 to 92.5%.

The method for observing the tissue is as follows.
First, the obtained outer layer material is mirror-polished, corroded with nital solution, and then the structure is observed with a digital microscope. The photograph is taken in a field where 200 or more eutectic cells can be confirmed within the field of view. In addition, an image analysis tool (ImageJ) is used to perform binarization processing of the photograph at a measurement magnification of 200 times. Since there is a difference in brightness between the base structure and the carbide in the photograph, the base structure and the carbide can be classified and their areas can be obtained by performing binarization processing. Five photographs of each sample are taken, and the average value of the area ratio of the carbide is calculated. The diameter of the carbide is measured from the digital microscope image taken at 200 times, and the diameter of the carbide is taken as the grain size of the carbide. In addition, when the shape of the carbide is an ellipse, a perpendicular bisector is drawn to the longest line segment in the carbide, and the length between the two points where the perpendicular bisector intersects with the grain boundary of the carbide is taken as the grain size of the carbide.

The outer layer material thus obtained is mirror-polished and then etched with nital solution, after which the structure is observed at a magnification of 100x using a digital microscope. Three fields of view are photographed for each sample, and a total of six straight lines are drawn for each image obtained. As shown in the following formula (3), the eutectic cell size is calculated from the number of intersections between the lines and carbides with a grain size of more than 5.0 μm divided by the length of the lines. The eutectic cell size of each sample was obtained from the average value of the three fields of view. The eutectic cell size is determined by the above method, taking advantage of the fact that carbides with a grain size of more than 5.0 μm exist at the boundaries of eutectic cells.
((L1/N1)+(L2/N2)+(L3/N3)+(L4/N4)+(L5/N5)+(L6/N6))/6 ... (3)
Here, Li is the length of the i-th straight line, and Ni is the number of intersections between the i-th straight line and carbides having a grain size exceeding 5.0 μm.

The hardness of the outer layer material for hot rolling rolls of the present invention is 44.0 HS or more and 52.0 HS or less in Shore hardness at 600°C. It is preferable that the Shore hardness at 20°C is 74.0 HS or more and 86.0 HS or less.

In addition, the roll surface temperature during hot rolling is around 600°C, and if the Shore hardness at 600°C is less than 44.0HS, plastic flow occurs and the steel material is likely to seize onto the roll surface. On the other hand, if the hardness exceeds 52.0HS, the roll hardness is too high and slippage is likely to occur during rolling. Such hardness can be reliably ensured by heat treating a roll having the composition of the present invention so that the tempering parameter P, described below, is in the range of 10,000 to 20,000.

To measure the Shore hardness at 20°C and at 600°C, first use a Vickers hardness tester (test force: 1 kgf) to measure the Vickers hardness HV at five points each at 20°C and 600°C, and then calculate the average value.

First, the Vickers hardness measurement at 20°C is performed using a Nikon QM-2 (simultaneous heating of indenter and test piece) testing machine, a diamond indenter is used, the test atmosphere is an argon gas atmosphere, and the load holding time is 10 seconds. After that, the temperature is raised to 600°C at a heating rate of 20°C/min, and the Vickers hardness at 600°C is measured under the same test conditions as at 20°C. The Vickers hardness measurement at 600°C complies with JIS Z2252 "High temperature Vickers hardness test method".
The obtained Vickers hardness is converted into Shore hardness using the calculation formula of JIS B 7731.

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

In the present invention, the method for producing the roll outer layer material is not particularly limited, and centrifugal casting and continuous build-up casting are preferred, but from the viewpoint of production costs, centrifugal casting is more preferred. When centrifugal casting is used, first, a molten metal having the above-mentioned composition of the hot rolling roll outer layer material (simply referred to as the molten metal of the outer layer material) is poured into a rotating mold whose inner surface is covered with a refractory material mainly made of zircon to a thickness of 1 to 5 mm, so as to obtain a predetermined thickness, and then centrifugal casting is performed.

When the roll outer layer material is cast by centrifugal casting, the hot rolling composite roll of the present invention has a centrifugally cast outer layer and an inner layer fused 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 fused and integrated with the outer layer, the hot rolling composite roll may have three layers: an intermediate layer fused and integrated with the outer layer, and an inner layer fused and integrated with the intermediate layer. The inner layer is preferably manufactured by static casting.

For the statically cast inner layer, it is preferable to use spheroidal graphite cast iron, caterpillar graphite cast iron (CV cast iron), etc., which have excellent castability and mechanical properties. In a centrifugally cast roll, the outer layer and inner layer are welded together, and the components of the outer layer material are mixed into the inner layer. If carbide-forming elements such as Cr and V contained in the outer layer material are mixed into the inner layer, it weakens the inner layer. For this reason, it is preferable to keep the mixing rate of the outer layer components as low as possible.

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 the outer layer are integrated by welding, and the components of the outer layer material are mixed into the intermediate layer. In order to suppress the mixing rate of the outer layer material components into the inner layer, it is preferable to suppress the mixing rate of the outer layer material into the intermediate layer as much as possible.

In order to obtain a target structure in the outer layer (outer layer material of hot rolling roll) constituting the hot rolling composite roll in which carbides having a grain size of 1.0 μm or more are present at an area ratio of 7.5 to 18.0% and the eutectic cell size is 65 to 100 μm, the die temperature is set to 250° C. or less and the casting temperature is set to 1350 to 1550° C. In addition, it is preferable that the heat treatment is a quenching treatment in which the material is heated to 900 to 1100° C. and air-cooled or air-blast cooled, and further a tempering treatment in which the material is heated and held and then cooled two or more times so that the tempering parameter P described in the following formula (4) is within the range of 10000 to 20000. At this time, the quenching temperature, tempering parameter, and number of tempering times can be changed within the ranges described depending on the components to obtain the above-mentioned structure.
P = T (log (t) + A) ... (4)
Here, T is the tempering temperature (K), t is the tempering time (h), and A is a constant (A=20 is used in the present invention).

As a result of the above, a composite roll for hot rolling can be obtained that has three layers, an outer layer, an intermediate layer, and an inner layer, or two layers, an outer layer and an inner layer.

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

The test materials No. 1 to 6 of the present invention and No. 7 to 16 of the comparative examples, with the chemical composition of the outer layer material of the hot rolling roll shown in Table 1 (the balance is Fe and unavoidable impurities), were heated to 1450 to 1550°C, melted, and cast into a Y-shaped keel block mold (rectangular parallelepiped part: thickness 35 mm, width 230 mm, height 120 mm). After cooling, the ingot was removed and quenched at 900 to 1100°C, and then tempered three times by heating and holding and then cooling so that the tempering parameter P was in the range of 10,000 to 20,000. Then, structure observation, hardness measurement, and hot rolling wear test were performed. In this example, the test pieces for structure observation, hardness measurement, and hot rolling wear were taken from the center of the wall thickness, but they may be taken from any position in the outer layer material.

The Vickers hardness HV at 20°C and 600°C of each sample cut from the ingots of the present invention and comparative examples was measured at five points using a Vickers hardness tester (test force: 1 kgf), and the average value was calculated.

For the Vickers hardness measurement at 20°C, a Nikon QM-2 (simultaneous heating of indenter and test piece) testing machine was used, a diamond indenter was used, the test atmosphere was argon gas, and the load holding time was 10 seconds. After that, the temperature was raised to 600°C at a heating rate of 20°C/min, and the Vickers hardness was measured at 600°C under the same test conditions as at 20°C. The Vickers hardness measurement at 600°C was performed in accordance with JIS Z2252 "High temperature Vickers hardness test method". The obtained Vickers hardness was converted to Shore hardness using the formula of JIS B 7731.

The hot rolling wear test method was as follows. Hot rolling wear test pieces (outer diameter 60 mmφ, width 10 mm, with C1 chamfer) were taken from the ingots of each of the present invention examples and each of the comparative examples obtained. The wear test was performed using a two-disk sliding rolling method with a test piece 1 and a mating piece 4, as shown in Figure 1. The test piece 1 was rotated at 700 rpm while being cooled with cooling water 2, and a mating piece 4 (outer diameter 190 mmφ, width 15 mm, C1 chamfer) heated to 800 ° C. by a high-frequency induction heating coil 3 was applied with a load of 686 N in the load direction 7 to the rotating test piece 1, and the test piece was rolled in contact with the mating piece 4. The rotation direction 5 of the test piece 1 and the rotation direction 6 of the mating piece 4 are the rotation directions in which the tangents at the contact points of the test piece 1 and the mating piece 4 are in the same direction. The abrasion test was carried out for 135 minutes, with the mating piece replaced with a new one every 45 minutes (31,500 rotations of the test piece), for a total of three tests (94,500 rotations of the test piece), and the mass loss of the test piece before and after the test, i.e., the amount of wear, was measured.

The friction coefficient measurement method using a hot rolling wear tester was as follows. As in the hot rolling wear test, hot rolling wear test pieces (outer diameter 60 mmφ, width 10 mm, with C1 chamfer) were taken from the ingots of each of the present invention examples and each of the comparative examples obtained. The wear test was performed by a two-disk sliding rolling method between a test piece 1 and a mating piece 4, as shown in FIG. 1. The test piece 1 was rotated at 76 rpm while being water-cooled with cooling water 2, and a mating piece 4 (outer diameter 190 mmφ, width 15 mm, C1 chamfer) heated to 1000 ° C. by a high-frequency induction heating coil 3 was applied with a load of 686 N in the load direction 7 to the rotating test piece 1, and the test piece was rolled while being in contact with the mating piece 4. The rotation direction 5 of the test piece 1 and the rotation direction 6 of the mating piece 4 are the rotation directions in which the tangents at the contact points of the test piece 1 and the mating piece 4 are in the same direction. The friction coefficient measurement method was performed for 5 hours, and the mating piece was replaced with a new one every hour, and the test was performed a total of 5 times. The torque and load during the test were measured, and the friction coefficient was calculated from the following formula (5).
μ=T/P×L (5)
Here, μ is the friction coefficient, T is the torque (kgf·m), P is the load (kgf), and L is the radius of the test piece (m).

The above test is designed to simulate continuous hot rolling operations, so the mating piece is heated to 1000°C and the friction coefficient and seizure state are evaluated after 300 minutes.

After the heat treatment, each sample was polished to a mirror finish, etched with nital solution, and then observed for its structure using a digital microscope. The photographs were taken in a field where 200 or more eutectic cells could be confirmed. In addition, an image analysis tool (ImageJ) was used to perform binarization of the photographs at a measurement magnification of 200 times. Since there is a difference in brightness between the base structure and the carbide in the photograph, the base structure and the carbide can be classified and their areas can be calculated by performing binarization. Five photographs of each sample were taken, and the average value of the area ratio of the carbide was calculated. The diameter of the carbide was measured from the digital microscope image taken at 200 times, and the diameter of the carbide was used as the grain size of the carbide. In addition, when the shape of the carbide was elliptical, a perpendicular bisector was drawn to the longest line segment in the carbide, and the length between the two points where the perpendicular bisector intersected with the grain boundary of the carbide was used as the grain size of the carbide.

In order to calculate the eutectic cell size, the outer layer material of each heat-treated sample was mirror-polished and then etched with nital solution, and the structure was observed at a measurement magnification of 100 times using a digital microscope. Three fields of view were photographed for each sample, and a total of six straight lines were drawn on each obtained image. As shown in the following formula (3), the eutectic cell size was calculated from the value obtained by dividing the number of intersections between carbides with a grain size of more than 5.0 μm and the straight lines by the length of the straight lines. The eutectic cell size of each sample was obtained from the average value of the three fields of view.
((2574/N1)+(2574/N2)+(2574/N3)+(2574/N4)+(2574/N5)+(2574/N6))/6 ... (3)
Here, the straight line length is 2574 μm, and Ni is the number of intersections between the i-th straight line and carbides having a grain size exceeding 5.0 μm.

The results are shown in Table 2.

In Table 2, the amount of wear was rated as pass (meaning good wear resistance) when it was 0.46 g or less, and as fail when it was greater than 0.46 g. The coefficient of friction was rated as pass when it was in the range of 0.12 to 0.30, and as fail when it was less than 0.12 or greater than 0.30. When the coefficient of friction was less than 0.12, the friction coefficient during the test was small, so the slip resistance was insufficient, and when the coefficient of friction was 0.12 or greater, the slip resistance was deemed excellent. When there was no seizure or roughness, the resistance to roughness was deemed excellent. As is clear from Table 2, the comparative example did not excel in either or both of the wear resistance and slip resistance, but it was confirmed that the examples of the present invention had both excellent wear resistance and slip resistance. When it was greater than 0.30, it is considered that seizure occurred due to the high friction coefficient during the test. By optimizing the carbide area ratio and the ratio of each carbide type, wear resistance was improved, and the eutectic cell size was adjusted, and the unevenness intervals on the roll surface were controlled, making it possible to maintain a high friction coefficient and simultaneously improving slip resistance. Furthermore, the surface of the test piece in the present invention showed good resistance to roughening without any seizure or roughening.

Therefore, according to the present invention, it is possible to manufacture a hot rolling roll outer layer material and a composite roll that are excellent in abrasion resistance, slip resistance, and surface roughness resistance. As a result, the life of the hot rolling roll is improved, and time lost during rolling interruptions due to roll trouble is reduced, thereby improving the rolling efficiency of the hot rolling roll and improving the productivity of hot rolled steel sheets.

1: Test piece 2: Cooling water 3: High frequency induction heating coil 4: Counterpart 5: Rotation direction of test piece 6: Rotation direction of counterpart 7: Load direction

Claims (2)

In mass percent,
C: 1.1 to 2.6%,
Si: 0.15 to 2.50%,
Mn: 0.15 to 2.50%,
Ni: 0.1 to 6.0%,
Cr: 1.5 to 10.0%,
Mo: 3.5 to 12.5%,
V: 2.5 to 7.5%,
W: 0.1 to 6.0%,
P: 0.01 to 0.04%,
S: 0.001 to 0.015%;
The contents of Si, Mn, Ni, Cr, Mo, V, and W satisfy the following formula (1), the contents of C, Cr, Mo, V, and W satisfy the following formula (2), and the balance is Fe and unavoidable impurities,
A roll outer layer material for hot rolling, characterized in that carbides having a grain size of 1.0 μm or more are present in an area ratio of 7.5 to 18.0%, the eutectic cell size is 65 to 100 μm, and the Shore hardness at 600° C. is 44.0 HS or more and 52.0 HS or less.
50.0≦([%Cr]×[%Mo]×[%V]×[%W])/([%Si]×[%Mn]×[%Ni])≦150.0 ... (1)
0.90≦[%C]×((0.177×[%V])/(0.099×[%Cr]+0.063×[%Mo]+0.033×[%W]))≦3.00 ... (2)
In formulas (1) and (2), [%C], [%Si], [%Mn], [%Ni], [%Cr], [%Mo], [%V], and [%W] are the contents (mass%) of each element. A composite roll for hot rolling having two layers, an outer layer and an inner layer, or three layers, an outer layer, an intermediate layer and an inner layer, wherein the outer layer is
C: 1.1 to 2.6%,
Si: 0.15 to 2.50%,
Mn: 0.15 to 2.50%,
Ni: 0.1 to 6.0%,
Cr: 1.5 to 10.0%,
Mo: 3.5 to 12.5%,
V: 2.5 to 7.5%,
W: 0.1 to 6.0%,
P: 0.01 to 0.04%,
S: 0.001 to 0.015%;
The contents of Si, Mn, Ni, Cr, Mo, V, and W satisfy the following formula (1), the contents of C, Cr, Mo, V, and W satisfy the following formula (2), and the balance is Fe and unavoidable impurities,
A composite roll for hot rolling, characterized in that carbides having a grain size of 1.0 μm or more are present in an area ratio of 7.5 to 18.0%, the eutectic cell size is 65 to 100 μm, and the Shore hardness at 600° C. is 44.0 HS or more and 52.0 HS or less.
50.0≦([%Cr]×[%Mo]×[%V]×[%W])/([%Si]×[%Mn]×[%Ni])≦150.0 ... (1)
0.90≦[%C]×((0.177×[%V])/(0.099×[%Cr]+0.063×[%Mo]+0.033×[%W]))≦3.00 ... (2)
In formulas (1) and (2), [%C], [%Si], [%Mn], [%Ni], [%Cr], [%Mo], [%V], and [%W] are the contents (mass%) of each element.

Images