JP2000303135A - Composite roll for hot rolling made by centrifugal casting excellent in accident resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a composite roll for hot rolling made by centrifugal casting, having excellent wear resistance and surface roughening resistance, excellent in accident resistance and suitable for a final stand in hot finish rolling. SOLUTION: As to this roll, an external layer has a compsn. contg., by weight, 1.5 to 2.6% C, 7 to 15% Cr, 2.5 to 10% Mo, 3 to 10% V and 0.5 to 5% Nb and simultaneously satisfying 0.27<=Mo(%)/Cr(%)<=0.8 and Cr(%)/C(%)>=3.3, and in which the average thermal expansion coefficient in the range from room temp. to 300 deg.C is controlled to <=11.5×10-6/ deg.C, and an internal layer is composed of spherical graphite cast iron contg. 2.5 to 4.0% C, 1.5 to 3.5% Si, 0.3 to 2.0% Mn, 0.3 to 0.8% Cr, 0.5 to 4.0% Mo, 0.1 to 1.0% V, <=0.5% Nb, <=1.5% Ni and 0.02 to 0.08% Mg or moreover contg. <=10% Co as well, and in which the average thermal expansion coefficient in the range from room temp. to 500 deg.C is controlled to <=13.5×10-6/ deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a centrifugally castable composite roll, and more particularly to a composite roll for hot rolling suitable for a hot-rolling finishing stand.

[0002]

2. Description of the Related Art In recent years, the development of hot rolling technology has been remarkable, and accordingly, there has been a strong demand for improvement of the properties of a hot rolling roll used, especially wear resistance. In response to such a demand for improved wear resistance, a high-performance roll (high-speed steel roll) has an outer layer having a composition similar to that of a high-speed tool steel, has hard carbide precipitated, and has significantly improved wear resistance.
Has been developed and put into practical use.

For example, JP-A-8-73977 discloses that, by weight, C: 2.5-4.0%, Si: 1.5% or less, Mn: 1.2%.
%: Cr: 6.0-20%, Mo: 2.0-12%, V: 3.0-
10.0%, Nb: 0.6-5.0%, 10 <6.5C-1.3V-
A high-speed hot-rolling roll outer layer material having a composition satisfying 0.7Nb ≦ 2Cr-2 has been proposed. It is said that the composite roll using the outer layer material has a low coefficient of friction and is excellent in abrasion resistance, skin roughening resistance and banding resistance.

Japanese Patent Application Laid-Open No. 10-183289 discloses that C: 2.4 to 2.9%, Si: 1% or less, Mn: 1% or less, Cr: 12 to 18%, Mo: 3 to 9 by weight. %, V: 3 to 8%, Nb:
Contains 0.5-4%, 0.27 ≦ Mo / Cr <0.7, and C +
A roll for hot rolling having a high-speed outer layer having a composition satisfying 0.2Cr ≦ 6.2 has been proposed. This hot-rolling roll is said to have less segregation of carbides and extremely excellent wear resistance.

Japanese Patent Application Laid-Open No. Hei 10-192916 discloses that the thermal expansion coefficient at room temperature to 100.degree.
× 10 ^-6 / ° C or less or thermal conductivity of 0.12 cal / (cm
(Sec. ° C.) A composite roll for hot rolling in which an outer layer of a high-speed material is welded to the outer periphery of the following shaft material has been proposed.
This roll is said to be excellent in abrasion resistance and skin abrasion resistance and has a small thermal crown.

However, the use environment of hot rolling rolls is becoming increasingly severe from the viewpoint of quality improvement and efficient production of rolled products. Demands for hot rolling work rolls are further increasing, such as the demand for heat rolls is increasing. For example, in the final stand in hot finish rolling, especially during thin rolling, a so-called draw-rolling accident often occurs in which the steel sheet is rolled in a folded state due to rolling troubles and the like, and the use of high-speed rolls is restricted. Have to be forced. When such a drawing rolling accident occurs, a very large heat load and surface pressure are instantaneously applied to the work roll surface, so that coarse cracks, so-called drawn cracks, may be formed on the work roll surface. Many. When rolling is continued on the work roll with the drawn cracks still formed, fatigue cracks propagate from the drawn cracks by the action of repeated rolling stress, eventually leading to breakage of the work roll.

It is considered that many of the rolling accidents are caused by a decrease in the roll gap due to an increase in the thermal crown of the work roll. By reducing the roll gap, the sheet passing property is reduced, and meandering of the steel sheet,
Rolling troubles such as belly extension and ear extension of the steel sheet occur, and the steel sheet is folded and rolled. Here, a high-speed steel roll excellent in abrasion resistance has a problem in that the consumption amount of the roll is reduced, so that the thermal crown is apparently large and the threadability is reduced. In addition, there is also a problem that a drawing crack is easily generated in a high-speed roll,
The application of the high-speed roll to the final stand of the hot finish rolling was limited.

For example, JP-A-8-73977,
The composite roll for hot rolling manufactured by the technique described in 10-183289 Publication has high resistance to crack generation due to strengthening of carbide, but contains a large amount of hard carbide,
If excessive reduction rolling is encountered, the cracks develop carbides and become coarse, and at the same time, the crack progresses significantly due to the presence of high residual stress in the outer layer of the roll due to the difference in the thermal expansion coefficient between the inner layer and the outer layer. Accelerated, a roll surface breakage accident called spalling may occur, and there is a problem that it does not have the properties until it can be used stably as a roll for the subsequent stand of hot finish rolling. .

Further, the composite roll for hot rolling manufactured by the technique described in Japanese Patent Application Laid-Open No. 10-192916 reduces the thermal expansion coefficient and the heat transfer coefficient of the shaft and suppresses the thermal expansion of the shaft. In order to reduce the thermal crown of the roll by doing, the suppression of the thermal expansion of the outer layer portion of the roll, which is the most thermally expanded during rolling, is insufficient, and the rolling is performed so that excellent threadability can be ensured. However, there was a problem that the apparent thermal crown could not be reduced and the occurrence of rolling trouble could not be completely suppressed.

[0010] From such a background, there has been a strong demand for a high-speed roll that can be used stably even in a post stand of hot finish rolling.

[0011]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned problems of the prior art, and has excellent abrasion resistance and rough surface resistance which can be stably applied to the final stand of hot finish rolling. It is an object of the present invention to provide a composite roll for hot rolling made by centrifugal casting, which is excellent in accident resistance such as good sheet passing property and drawing crack resistance.

[0012]

In order to achieve the above-mentioned object, the present inventors have studied the effect of the amount of alloying elements on the wear resistance, surface roughening resistance and accident resistance of the outer layer material of a high-speed roll. Diligently studied. As a result, the contents of Cr and Mo were increased, and V and Nb were added in a complex manner.
It has been found that by optimizing the amount of Mo and the amount of Mo, it is possible to simultaneously reduce the coefficient of thermal expansion, and improve the abrasion resistance, rough surface resistance and accident resistance.

However, if the coefficient of thermal expansion of the outer layer material is reduced in order to ensure good sheet passing properties and improve accident resistance, the inner layer material shrinks during cooling during the heat treatment of the roll, based on the shrinkage of the outer layer material. The amount of shrinkage becomes very large, which increases the internal stress of the roll during heat treatment and the residual stress of the roll after heat treatment. The increase in stress during the heat treatment causes a roll breakage accident during the heat treatment, and the increase in the residual stress after the heat treatment increases the shearing force of the roll surface layer when encountering the reduction rolling, and causes the reduction of the reduction crack. It causes formation and coarsening. Therefore, the present inventors have come to the conclusion that it is necessary to reduce the thermal expansion coefficient of the inner layer material from the viewpoint of stably producing a composite roll having a low thermal expansion coefficient of the outer layer and a low residual stress. .

The composite roll according to the present invention, after quenching,
Tempering treatment that also serves as stress relaxation treatment at a temperature around 500 ° C
It is carried out several times. From this, the present inventors think that it is effective to reduce the average thermal expansion coefficient of the inner layer material from room temperature to 500 ° C. in order to reduce the stress during the roll heat treatment and the residual stress after the heat treatment. Reached. The present invention has been completed based on the above findings.

That is, a composite roll for hot rolling having an inner layer welded and integrated with an outer layer manufactured by centrifugal casting, wherein the outer layer has a weight ratio of C: 1.5 to 2.6% and Si: 0.1%.
~ 2.0%, Mn: 0.1 ~ 2.0%, Cr: 7 ~ 15%, Mo: 2.
5-10%, V: 3-10%, Nb: 0.5-5%, or further contains one or two selected from Co: 10% or less, Ni: 3% or less. Next (1), (2)
Formula 0.27 ≦ Mo (%) / Cr (%) ≦ 0.8 (1) Cr (%) / C (%) ≧ 3.3 (2) (where, Cr, Mo, C: content of each element) (%)) At the same time, has a composition comprising the balance of Fe and unavoidable impurities, and has an average coefficient of thermal expansion from room temperature to 300 ° C. of 1
1.5 × 10 ⁻⁶ / ° C or less, 500 ° C from room temperature of the inner layer
Is a composite roll for hot rolling excellent in accident resistance characterized in that the average thermal expansion coefficient up to 13.5 × 10 ⁻⁶ / ° C. or less, and in the present invention, the inner layer has a weight ratio of , C:
2.5 ~ 4.0%, Si: 1.5 ~ 3.5%, Mn: 0.3 ~ 2.0%,
Cr: 0.3-0.8%, Mo: 0.4-4.0%, V: 0.1-1.0
%, Nb: 0.5% or less, Ni: 1.5% or less, Mg: 0.02 to 0.08
% Or Co: 10% or less, with the balance being
Preferably, it has a composition comprising Fe and unavoidable impurities, and has an average coefficient of thermal expansion from room temperature to 500 ° C. of 13.5 × 10 ⁻⁶ / ° C. or less.

Further, the present invention relates to an outer case manufactured by centrifugal casting.
An intermediate layer between the layer and the inner layer, and an outer layer
A composite roll for hot rolling in which the inner layer is welded and integrated.
Thus, the outer layer is, by weight ratio, C: 1.5 to 2.6%, Si:
0.1 to 2.0%, Mn: 0.1 to 2.0%, Cr: 7 to 15%, Mo:
2.5 to 10%, V: 3 to 10%, Nb: 0.5 to 5%.
Or select from Co: 10% or less, Ni: 3% or less.
(1), (2)
The formula is satisfied at the same time, and the balance consists of Fe and unavoidable impurities.
Average thermal expansion from room temperature to 300 ° C
Coefficient is 11.5 × 10 ^-6/ ° C or less, from the room temperature of the inner layer
Average thermal expansion coefficient up to 500 ℃ 13.5 × 10 ^-6/ ℃ or less
Composite roll for hot rolling with excellent accident resistance
In the present invention, the inner layer has a weight ratio of
And C: 2.5 ~ 4.0%, Si: 1.5 ~ 3.5%, Mn: 0.3 ~
2.0%, Cr: 0.3-0.8%, Mo: 0.5-4.0%, V: 0.
1 to 1.0%, Nb: 0.5% or less, Ni: 1.5% or less, Mg: 0.02
0.08% or Co: 10% or less
Having a composition consisting of the balance Fe and unavoidable impurities,
And the average coefficient of thermal expansion from room temperature to 500 ℃ is 13.5 × 10^-6
/ ° C or lower. In the present invention,
The intermediate layer contains C: 1 to 3% by weight, and is more preferable.
Or, by weight ratio, C: 1.5 to 3%, Si: 0.5 to 3.5
%, Mn: 0.2-2%, Cr: 1.0-6.0%, Mo: 1.0-6.
0%, V: 3% or less, Nb: 2% or less, Ni: 2% or less
Or further Co: 8% or less, with the balance Fe and
Alloy material with a composition consisting of
Is preferred. In the present invention, the intermediate layer may be
The average thermal expansion coefficient of the outer layer and the average thermal expansion coefficient of the inner layer
Or the average thermal expansion coefficient of the inner layer
Alloy material having an average coefficient of thermal expansion equivalent to the
Is preferred.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The reasons for limiting the composition of the outer layer of the composite roll for hot rolling according to the present invention will be described. In addition,% in a composition is a weight ratio. C: 1.5 to 2.6% C is an element essential for carbide formation for improving the abrasion resistance of the roll, and is limited to the range of 1.5 to 2.6% in the present invention. By adjusting the amount of C within the range of the present invention, crystallization of a large amount of eutectic carbide is suppressed, and simultaneously the wear resistance and the draw crack resistance (thermal shock resistance) are improved. C content
If it is less than 1.5%, the amount of carbides becomes insufficient and the wear resistance deteriorates. On the other hand, if it exceeds 2.6%, the amount of carbides becomes excessive and the thermal shock characteristics deteriorate, and at the same time, the coefficient of thermal expansion increases.
Incidentally, from the viewpoint of lowering the coefficient of thermal expansion, preferably 1.5
~ 2.4%.

Si: 0.1 to 2.0% Si acts not only as a deoxidizing agent but also as a solid solution in a matrix, like Cr, to enhance high-temperature oxidation resistance. Such an effect is observed at a content of 0.1% or more. However, if the content exceeds 2.0%, the effect is saturated, and it is economically disadvantageous. For this reason, Si is limited to the range of 0.1 to 2.0%.

Mn: 0.1 to 2.0% Mn is useful for fixing S in molten steel as MnS and removing the adverse effect of S. It also has the effect of improving the hardenability. In order to obtain such an effect, it is necessary to contain 0.1% or more. However, when the content exceeds 2.0%, a large amount of austenite remains to deteriorate the roll characteristics. For this reason, Mn was limited to the range of 0.1 to 2.0%.

Cr: 7 to 15% Cr is an essential element for the appearance of a strong Cr-based carbide which improves wear resistance and surface roughening resistance. Also, Cr
Has the effect of forming a solid solution in the matrix to effectively lower the coefficient of thermal expansion. Further, Cr has a strong effect of lowering the thermal conductivity, suppresses heat transfer from the outer layer to the inner layer, and has an important effect of suppressing an increase in thermal crown due to thermal expansion of the inner layer. Such an effect is recognized when the content of Cr is 7% or more. If the Cr content is less than 7%, the above-described effects are insufficient, and it becomes impossible to achieve an improvement in sheet passing properties and an improvement in wear resistance. On the other hand, if it exceeds 15%, the amount of carbides becomes excessive, and the thermal shock resistance decreases. In addition, more preferably, 7
It is in the range of ~ 13%.

Cr / C: ≧ 3.3 Cr / C affects the coefficient of thermal expansion and is an important factor in the present invention, and is limited to 3.3 or more. If Cr / C is less than 3.3, the amount of Cr dissolved in the matrix is insufficient, so that a predetermined coefficient of thermal expansion cannot be achieved. Preferably, Cr / C is 3.5 to 7.
5 range.

Mo: 2.5 to 10% Mo is concentrated in Cr carbide and MC carbide to form these Cr
It has the effect of strengthening the system carbides and significantly improving the surface roughness resistance, abrasion resistance and thermal shock resistance. Mo also effectively acts to lower the coefficient of thermal expansion. These effects are:
It is recognized at a content of 2.5% or more. However, if the content exceeds 10%, a large amount of fragile Mo-based carbides appears, and the rough surface resistance and wear resistance are significantly deteriorated. Therefore, Mo is 2.5 ~
Limited to 10% range.

0.27 ≦ Mo (%) / C (%) ≦ 0.8 In the present invention, the Mo content is further adjusted according to the Cr content so that Mo / Cr is in the range of 0.27 to 0.8. Mo / Cr is 2.7
If it is less than 10, the strengthening of the Cr-based carbide is insufficient, and on the other hand, it is not more than 0.1.
Above 8, Mo that is more fragile than reinforced Cr-based carbides
A large amount of carbides of the system appears, and the surface roughness and abrasion resistance are significantly deteriorated.

V: 3 to 10% V has an effect of forming a hard MC carbide and improving wear resistance. Therefore, in order to obtain a certain level of wear resistance, it is an essential element for a high-speed roll.
Such an effect is observed at a content of 3% or more,
%, The melting point of the molten metal is increased and the fluidity of the molten metal is reduced, making centrifugal casting difficult. For this reason, V is limited to the range of 3 to 10%. In addition, Preferably, it is the range of 3-7%.

Nb: 0.5 to 5% Nb forms a hard MC carbide similarly to V,
Has the effect of more effectively enriching in MC carbides, and the coexistence of Cr and Mo within the scope of the present invention significantly improves wear resistance. Further, it has the effect of granulating MC carbides to increase the resistance to crack formation. Such an effect is 0.5
%, But the effect is saturated even if it exceeds 5%, and the crystallization temperature of MC type carbide is remarkably increased, resulting in remarkable coarsening of MC type carbide and segregation of carbide. Encourage. Preferably, the content is 0.5 to 3%.

Co: 10% or less In the present invention, Co can be added as needed. Co is dissolved in the matrix and has the effect of increasing the amount of other elements dissolved in the matrix to further strengthen the matrix. Co also has the effect of lowering the coefficient of thermal expansion. Even if the content exceeds 10%, the effect is saturated, so the upper limit is preferably 10%.

Ni: 3% or less In the present invention, Ni can be added as needed. Ni has an effect of improving hardenability and is a useful element for controlling the transformation behavior in heat treatment. However, since it also has the effect of deteriorating wear resistance, it is preferably limited to 3% or less. If the Ni content exceeds 3%, the effect of improving hardenability is saturated.

Further, in the present invention, W and B may be contained within the following range. W: 1% or less W is an element forming a hard carbide, and may be contained if it is 1% or less. However, if it exceeds 1%, the rough surface resistance and the thermal shock resistance deteriorate. B: 0.05% or less B has an effect of binding N and stabilizing N,
If it is 0.05% or less, it may be contained. In the present invention, Cr,
Since a large amount of alloying elements such as V and Nb is contained, a large amount of N is inevitably mixed into the molten metal in ordinary atmospheric melting, and there is a concern that N may have an adverse effect. B has the effect of forming BN together with N and stabilizing N, so that B may be contained at 0.05% or less. However, if it exceeds 0.05%, the material becomes brittle.

The outer layer of the composite roll of the present invention is the balance of Fe and inevitable impurities other than the above components. The outer layer of the composite roll of the present invention is an outer layer having a low coefficient of thermal expansion of 11.5 × 10 ⁻⁶ / ° C. or less from room temperature to 300 ° C. In particular, the reduction of the thermal expansion coefficient of the outer layer
It significantly reduces the thermal crown of the roll and works most effectively to improve the rollability of the roll. When the average coefficient of thermal expansion from room temperature to 300 ° C. exceeds 11.5 × 10 ⁻⁶ / ° C., the thermal crown of the roll tends to greatly increase, and it becomes impossible to secure excellent sheet passing property.

In the composite roll for hot rolling of the present invention, the abrasion resistance is improved as compared with the conventional high-speed roll.
Roll consumption is reduced. For this reason, when the outer layer expands similarly to the conventional roll, it looks as if the roll expanded compared to the conventional case. That is, it seems that the thermal crown has apparently increased by an amount corresponding to the decrease in the wear amount. However, in the roll of the present invention, the thermal expansion coefficient from room temperature to 300 ° C. is limited to a small value of 11.5 × 10 ⁻⁶ / ° C. or less. The apparent thermal crown is also reduced as compared to the roll. For this reason,
The roll for hot rolling of the present invention improves the sheet passing property.

The fact that the outer layer having the composition within the range of the present invention has abrasion resistance, thermal shock resistance and low thermal expansion coefficient will be described based on the results of experiments conducted by the present inventors. A melt for a roll outer layer having the composition shown in Table 1 was melted, and a roll outer layer material (thickness: 80 mm) having a diameter of 690 mm was formed by centrifugal casting. After heating these outer layer materials to 1050 ° C, they are quenched,
Tempering was performed twice at 500 to 550 ° C. The outer layer material N
oO-11 is NiG cast iron mainly used as a work roll for the final stand after the hot finish rolling.

Next, test specimens were taken from the heat-treated outer layer material and subjected to a wear test, a thermal shock test and a thermal expansion test. The abrasion test was performed on the test piece (S45C) and the test piece.
The test was performed by the disk sliding wear method. While rotating at a rotational speed of 600 rpm, the mating material is heated to 800 ° C., and the test piece is water-cooled,
The test piece and the mating material were rolled while being pressed against each other with a load of 100 kg with a slip ratio of 10%. After repeating this test four times by updating the mating material (number of rolling: 72000 times), the abrasion loss of the test piece was measured.

In the thermal shock test, a disk-shaped counterpart material (S45C) that was heated and rotated was pressed against a 25 mm-thick plate-shaped test piece, and water cooling was performed immediately after the pressing was completed. The temperature of the mating material was varied between 800 ° C and 1050 ° C (50 ° C pitch). In this thermal shock test, the higher the temperature of the mating material, the greater the thermal load on the test piece and the more likely it is for the thermal shock crack to enter the test piece.Therefore, the higher the temperature at which the crack occurs, the better the thermal shock resistance. means.

The thermal expansion test is performed using a test piece having a length of 5 mmφ × 20 mm and a room temperature (20 ° C.) to 300 ° C. and a room temperature (20 ° C.) to 500 ° C.
The average coefficient of thermal expansion at ℃ was measured. Table 2 shows the results.

[0035]

[Table 1]

[0036]

[Table 2]

In the range of the outer layer composition of the present invention, the wear amount was reduced to about 1/10 compared to the conventional outer layer material No. 11 (NiG cast iron), and extremely excellent wear resistance was obtained. It has nature. In addition, the thermal expansion coefficient from room temperature to 300 ° C. has an extremely small thermal expansion coefficient of 11.5 × 10 ⁻⁶ or less, and it is expected that the thermal crown can be reduced to improve the sheet passing property.

Further, in the range of the composition of the outer layer of the present invention, the thermal shock crack initiation temperature in the thermal shock test is as high as 1050 ° C. or more, the thermal shock resistance is excellent, and the suppression of drawing crack can be achieved. When the outer layer composition of the present invention is out of the range, the thermal expansion coefficient is large, or any of the wear resistance and the thermal shock resistance is deteriorated. The outer layer material No.O-8 has a large thermal expansion coefficient, abrasion resistance, because the Cr content and Cr / C are out of the range of the present invention.
Thermal shock resistance is significantly degraded.

Since the outer layer material No. O-9 has a C content outside the range of the present invention, the thermal shock resistance is remarkably deteriorated. Outer layer material N
Since oO-10 contains a large amount of W and the amount of Nb is out of the range of the present invention, the coefficient of thermal expansion is large, and the wear resistance and thermal shock resistance are significantly deteriorated. In the present invention, after the molten metal having the above-described composition is formed into an outer layer by a centrifugal casting method, the inner layer is further subjected to stationary casting to form a composite roll in which the outer layer and the inner layer are welded and integrated.

For the inner layer to be subjected to stationary casting, it is preferable to use spheroidal graphite cast iron having excellent castability and mechanical properties. Further, in order to stably produce a composite roll having low residual stress, room temperature is used. From 500 to 500 ℃
The composition should be 13.5 × 10 ⁻⁶ / ° C or less. Room temperature to 500 ℃
To reduce the average coefficient of thermal expansion between Cr and Mo or Co
Is effective.

Next, a preferred composition of the inner layer will be described. C: 2.5 to 4.0% C is added in order to improve castability and make graphite appear. When the C content is less than 2.5%, the amount of graphite is small, castability is deteriorated, and shrinkage cavities are easily generated. Meanwhile, 4.0%
If it exceeds, coarse graphite appears and the material of the inner layer becomes brittle.
For this reason, C is preferably limited to the range of 2.5 to 4.0%.

Si: 1.5 to 3.5% Si is added for improving castability and graphitizing, like C. If the Si content is less than 1.5%, the graphitization becomes insufficient and the crystallization of carbides increases. For this reason, the inner layer becomes hard and brittle. On the other hand, if the content exceeds 3.5%, the effect of graphitization is saturated, and graphite with a deformed shape is likely to appear, and the inner layer strength is reduced. For this reason, Si is preferably limited to the range of 1.5 to 3.5%.

Mn: 0.3 to 2.0% Mn has the effect of fixing S in the molten metal as MnS and removing the adverse effect of S. Such an effect is observed at a content of 0.3% or more, but if it exceeds 2%, the material of the inner layer becomes brittle. Cr: 0.1 to 0.8% Cr has the effect of stabilizing the pearlite structure, improves the strength of the inner layer, and has the effect of lowering the coefficient of thermal expansion. This effect is observed at a content of 0.1% or more, but is not less than 0.1%.
If the content exceeds 8%, the crystallization of eutectic carbides increases and the material of the inner layer becomes hard and brittle.

Mo: 0.5 to 4.0% Mo has the effect of lowering the coefficient of thermal expansion of the inner layer and also has the effect of increasing the strength. To obtain this effect, the content must be 0.5% or more. However, if it exceeds 4.0%, the amount of carbide crystallization increases and the material of the inner layer becomes hard and brittle. V: 0.1 to 1% V forms an MC carbide, suppresses the appearance of fragile cementite, and has the effect of increasing the inner layer strength. To obtain this effect, the content of 0.1% or more is required. On the other hand, when the content exceeds 1%, the material of the inner layer is hardly embrittled. For this reason, V is preferably limited to the range of 0.1 to 1%.

Nb: 0.5% or less Nb has the effect of forming MC carbide similarly to V, suppressing the appearance of fragile cementite, and increasing the inner layer strength. However, if the content exceeds 0.5%, the material of the inner layer becomes hard and brittle. For this reason, Nb is preferably limited to 0.5% or less.

Ni: 1.5% or less Ni has the effect of suppressing the appearance of carbides and improving the strength. However, the effect is saturated even if the content exceeds 1.5%, so that Ni is preferably limited to 1.5% or less. Mg: 0.02 to 0.08% Mg is an element essential for spheroidizing graphite to improve strength. In order to spheroidize graphite, a content of 0.02% or more is required. If it is less than 0.02%, the degree of spheroidization of graphite decreases. On the other hand, if it exceeds 0.08%, it becomes easy to turn into white iron, and inclusion defects such as dross increase. For this reason, Mg is preferably limited to the range of 0.02 to 0.08%.

Co: 10% or less Co has an effect of lowering the coefficient of thermal expansion, and can be contained if necessary. However, even if the content exceeds 10%, the effect is saturated. For this reason, it is preferable that Co is limited to 10% or less. The inner layer is composed of the balance Fe and unavoidable impurities other than the above components. As unavoidable impurities,
It is preferable that P: 0.05% or less and S: 0.03% or less.

[0048] The inner layer of the composite roll of the present invention is formed at room temperature to 5
The thermal expansion coefficient at 00 ° C is 13.5 × 10 ^-6/ ° C or lower.
The heat treatment of the composite roll of the present invention is performed after quenching.
Tempering at 00-600 ° C is performed once or multiple times
You. Thermal expansion difference during or after heat treatment of outer layer and inner layer
Average thermal expansion from room temperature to 500 ° C to reduce
The coefficient is 13.5 × 10^-6/ ° C or lower. Room temperature to 500 ℃
Average thermal expansion coefficient of 13.5 × 10^-6/ ℃,
Difference in shrinkage between inner layer and outer layer during heat treatment in roll production
Becomes excessively large, so that an excessive tensile stress
Excessive compressive stress is generated in the layer. Excessive tension in the inner layer
The force causes a roll breakage accident in heat treatment and the outer layer becomes excessive.
Compressive residual stress may cause coarse drawing cracks.
You. Therefore, in the present invention, the temperature of the inner layer is increased from room temperature to 500 ° C.
Thermal expansion coefficient is 13.5 × 10^-6/ ° C or lower. This
As a result, the stress generated in the inner and outer layers can be
Breakage accidents and generation of coarse drawing cracks can be suppressed. What
In conventional spheroidal graphite cast iron, at room temperature to 500 ° C
Thermal expansion coefficient is 13.5 × 10^-6/ ° C in many cases.

The fact that the inner layer having a composition within the range of the present invention has a low coefficient of thermal expansion will be described based on the results of experiments conducted by the present inventors. A molten metal having the composition shown in Table 3 was melted to form a cast product having a thickness of 50 mm, and then heated to 1050 ° C. and cooled (slowly cooled) to obtain a base structure of pearlite.
Specimens were collected from these castings and subjected to a tensile test and a thermal expansion test. Table 4 shows the results.

[0050]

[Table 3]

[0051]

[Table 4]

The inner layer material of the present invention (inner layer materials No. I-1 to No. I-5)
Means that the average coefficient of thermal expansion from room temperature to 500 ° C is
Since it is 13.5 × 10 ⁻⁶ / ° C. or less, it is possible to suppress occurrence of excessive stress in the heat treatment of the composite roll. Moreover, despite increasing the amounts of Cr and Mo, which are elements of white iron, it is possible to maintain sufficient tensile strength of 500 MPa or more as an inner layer material. on the other hand,
In castings outside the range of the inner layer composition of the present invention, the inner layer material N
oI-6 has Cr and Mo amounts, No.I-7 has Mo and V amounts outside the range of the present invention, and has an average thermal expansion coefficient of 13.5 × 10
Exceeding ^-6 / ° C, there is a concern that excessive stress may occur during the heat treatment of the composite roll.

In the composite roll of the present invention, an intermediate layer is provided between the outer layer and the inner layer, and the inner layer and the outer layer can be welded and integrated. After manufacturing the outer layer by centrifugal casting,
It is preferable to form it inside the outer layer by a centrifugal casting method. The presence of the intermediate layer acts as a buffer layer that bridges the gap between physical properties and mechanical properties such as the coefficient of thermal expansion and Young's modulus of the outer layer and the inner layer, and the components of the outer layer, especially the carbide-forming element (Cr), This is effective for preventing the inner layer from being excessively mixed into the inner layer and becoming hard and brittle.

The intermediate layer is preferably made of an alloy material containing 1 to 3% of C, for example, hypereutectoid steel, graphite steel or cast iron material. Even more preferably, it is recommended to have a coefficient of thermal expansion between the outer layer and the inner layer or equivalent to that of the inner layer for roll production or roll use. Next, a preferred composition of the intermediate layer will be described.

C: 1 to 3% C is dissolved in the matrix to secure the strength. If the C content is less than 1%, the effect is insufficient. On the other hand, if the C content exceeds 3%, carbides increase and toughness decreases. For this reason,
C is limited to 1-3%. Preferably, 1.5 to 3%
It is. Si: 0.5 to 3.5% Si is required to be 0.5% or more in order to suppress hard embrittlement and ensure castability. However, the effect is saturated even if the content exceeds 3.5%. For this reason, the content of Si is preferably set to 0.5 to 3.5%.

Mn: 0.2 to 2% Mn has the effect of improving the strength. To secure the strength, the content of 0.2% or more is required, but if it exceeds 2%, the effect is saturated. For this reason, Mn is preferably set to 0.2 to 2%. Cr: 1.0 to 6.0% Cr is an element effective in lowering the coefficient of thermal expansion.
In order to approach the thermal expansion coefficient of the outer layer or the inner layer, the content is preferably 1.0% or more. On the other hand, the content exceeding 6%
Excessive carbides and brittle material. Therefore, the content of Cr is preferably set to 1.0 to 6.0%.

Mo: 1.0-6.0% Mo is also an element effective in lowering the coefficient of thermal expansion simultaneously with Cr. In order to approach the thermal expansion coefficient of the outer layer or the inner layer, it is preferable to contain 1% or more. On the other hand, when the content exceeds 6%, the amount of carbide becomes excessive and the material becomes brittle. For this reason, Mo is preferably limited to 1.0 to 6.0%.

V: 3% or less, Nb: 2% or less Both V and Nb have an effect of improving the strength of the intermediate layer. However, if V contains more than 3% and Nb exceeds 2%,
Micro shrinkage cavities are easily formed in the intermediate layer, and the strength of the intermediate layer is reduced. Therefore, V is 3% or less, and Nb is 2%.
It is preferable to limit to the following.

Ni: 2% or less Ni is effective in improving hardenability, but the effect is saturated even if it exceeds 2%. For this reason, Ni is preferably limited to 2% or less. Co: 8% or less Co has a function of lowering the coefficient of thermal expansion, and can be contained as necessary. Even if the content exceeds 8%, the effect is not changed, so it is preferable to limit the content to 8% or less.

The intermediate layer was composed of the remainder Fe except for the above components.
And unavoidable impurities. As the inevitable impurities, it is preferable that P: 0.05% or less and S: 0.03% or less. Further, Al, Ti, Zr or the like may be added as a deoxidizing agent. The intermediate layer in the composite roll of the present invention has an average thermal expansion coefficient between the average thermal expansion coefficient of the outer layer and the average thermal expansion coefficient of the inner layer, or an average thermal expansion coefficient equivalent to the average thermal expansion coefficient of the inner layer. Preferably, it is

[0061]

EXAMPLE A composite roll having a body diameter of 690 mm and a body length of 2400 mm was produced by the following procedure. In a mold rotating with a centrifugal force of 140G,
A molten metal having a composition shown in Table 5 was cast so that the outer layer had a thickness of 80 mm. Immediately after the outer layer solidifies, an intermediate layer corresponding to a thickness of 40 mm is cast on the inner surface of the outer layer, and the inner surface of the outer layer is
It was redissolved by 20 mm and was integrally welded to the intermediate layer. After the intermediate layer was solidified, the rotation of the mold was stopped, and the outer layer-intermediate layer-inner layer was integrally welded by casting the inner layer material. After cooling until the surface temperature of the outer layer became 80 ° C. or lower, the mold was disassembled.

The obtained composite roll was quenched from 1050 ° C., and subsequently subjected to a heat treatment of tempering at 500 to 550 ° C. three times or more, so that the hardness of the outer layer was 77 to 82 Hs. After heat treatment,
Test materials for the outer layer and the intermediate layer were collected from the end of the roll body. For the outer layer, a sample for chemical composition analysis and a test material for thermal expansion test were collected, and for the intermediate layer, a sample for chemical composition analysis was collected from the center of the wall thickness. As for the inner layer, a sample for chemical composition analysis and a test material for thermal expansion test were collected from the center of the roll shaft end.

Table 5 shows the chemical composition of the outer layer, the intermediate layer, and the inner layer of each composite roll, and the thermal expansion coefficients of the outer layer and the inner layer.

[0064]

[Table 5]

Both the outer layer and the inner layer of the present invention have a low coefficient of thermal expansion. On the other hand, Comparative Examples outside the scope of the present invention have a high coefficient of thermal expansion. roll
In No. 4, the outer layer was within the range of the present invention, but the inner layer was out of the range of the present invention. This breakage is caused by the generation of excessive thermal stress based on the difference in thermal strain between the inner layer and the outer layer. In addition, even if rolls can be manufactured without breakage during heat treatment, during hot rolling, there is a high possibility of breakage due to thermal expansion of the rolls,
It is considered that a very large crack is likely to be generated when the reduction rolling occurs.

Except for the roll No. 4, the composite rolls of the rolls No. 1 to No. 3 were loaded as working rolls of the F7 stand of the hot finish rolling mill, and test rolling was performed. Note that roll N
In test o.5, test rolling in a hot-rolling finishing mill was not performed because the roll barrel end was broken during the heat treatment. The test rolling was performed at a rolling distance of about 200 km. Example of the present invention (Roll No. 1, N
In o.2), there was no occurrence of rolling troubles such as meandering, ear extension, and belly extension of the steel sheet, and extremely good sheet permeability was confirmed. On the other hand, in the comparative example (roll No. 3) (conventional high-speed roll), the steel plate was stretched in antinodes, and deterioration of the sheet passing property was recognized.

Immediately after rolling, the roll is pulled out of the mill, and the transition of the roll surface temperature and the transition of the outer diameter of the roll are measured for 4 hours, and the apparent thermal expansion coefficient of the entire roll (apparent thermal expansion coefficient) is measured. I asked. The apparent coefficient of thermal expansion was calculated assuming that the temperature inside the roll was the same as the surface temperature. The result is shown in FIG.

FIG. 1 shows that the example of the present invention (roll No. 1 and No. 2)
Then, a comparative example (roll No. 3) (conventional high-speed roll)
The apparent coefficient of thermal expansion is greatly reduced as compared with. In the composite roll of the present invention, since the outer layer contains a large amount of Cr, heat conduction to the inner layer is suppressed, and the apparent coefficient of thermal expansion is extremely small. From this, it is clear that the composite roll of the present invention has an extremely large thermal crown suppressing effect.

Further, the rolled surface of the composite roll of the present invention was good, and no rough surface occurred.

[0070]

According to the present invention, even in the final stand of hot finish rolling, it has good threading properties, can prevent rolling troubles such as reduction rolling, and can perform stable hot finish rolling. Further, the wear resistance of the roll is improved, the roll consumption is reduced, and the unit consumption of the roll is improved. Further, there is an effect that breakage during roll production can be prevented, and stable production of rolls becomes possible. Further, the composite roll of the present invention may have a structure in which the intermediate layer is omitted, that is, a roll having two structures of an outer layer and an inner layer.

The composite roll of the present invention is preferably applied to a stand after hot finish rolling. However, since the composite roll is excellent in abrasion resistance and surface roughening resistance, it can be used for a hot roll finishing pre-roll or a hot roll rough mill. Needless to say, it can be applied as a roll.

[Brief description of the drawings]

FIG. 1 is a graph showing the apparent thermal expansion coefficient of each composite roll.

Claims (7)

[Claims] 1. A composite roll for hot rolling, comprising an intermediate layer between an outer layer and an inner layer produced by centrifugal casting, wherein the outer layer and the inner layer are welded and integrated via the intermediate layer, The outer layer is, by weight ratio, C: 1.5 to 2.6%, Si: 0.1 to 2.0%, Mn: 0.1 to 2.0%, Cr: 7 to 15%, Mo: 2.5 to 10%, V: 3 to 10%, Nb: 0.5 to 5%, and simultaneously satisfy the following expressions (1) and (2).
It has a composition consisting of the balance Fe and unavoidable impurities, and has an average thermal expansion coefficient from room temperature to 300 ° C. of 11.5 × 10 ⁻⁶ /
C. or lower, and the inner layer has an average coefficient of thermal expansion from room temperature to 500 ° C. of 13.5 × 10 ⁻⁶ / ° C. or lower, a composite roll for hot rolling excellent in accident resistance. 0.27 ≦ Mo (%) / Cr (%) ≦ 0.8 (1) Cr (%) / C (%) ≧ 3.3 (2) Here, Cr, Mo, C: content of each element ( %) 2. The method according to claim 1, wherein the outer layer further contains, in addition to the composition, one or two selected from Co: 10% or less and Ni: 3% or less by weight. The composite roll for hot rolling according to claim 1, which is excellent in accident resistance. 3. The method according to claim 1, wherein the inner layer comprises, by weight, C: 2.5 to 4.0%, Si: 1.5 to 3.5%, Mn: 0.3 to 2.0%, Cr: 0.3 to 0.8%, Mo: 0.5 to 4.0%, V: 0.1 to 1.0%, Nb: 0.5% or less, Ni: 1.5% or less, Mg: 0.02 to 0.08%, the composition consisting of the balance of Fe and unavoidable impurities, and the average from room temperature to 500 ° C Thermal expansion coefficient is 13.5
The composite roll for hot rolling excellent in accident resistance according to claim 1 or 2, wherein the composite roll is not more than × 10 ^-6 / ° C. 4. The composite for hot rolling excellent in accident resistance according to claim 3, wherein the inner layer further contains Co: 10% or less by weight in addition to the composition. roll. 5. The method according to claim 1, wherein the intermediate layer has a weight ratio of C: 1 to 3%.
The composite roll for hot rolling excellent in accident resistance according to any one of claims 1 to 4, wherein the composite roll is a contained alloy material. 6. The intermediate layer has a weight ratio of C: 1.5-3%, Si: 0.5-3.5%, Mn: 0.2-2%, Cr: 1.0-6.0%, Mo: 1.0-6.0%, V: 6. The accident resistance according to any one of claims 1 to 5, wherein the composition contains: 3% or less, Nb: 2% or less, Ni: 2% or less, and has a balance of Fe and unavoidable impurities. Excellent roll for hot rolling. 7. The hot-rolled composite with excellent accident resistance according to claim 6, wherein the intermediate layer further contains Co: 8% or less by weight in addition to the composition. roll.