JP6405894B2 - Hot rolling roll - Google Patents

Description

  The present invention relates to a hot rolling roll used for hot rolling of steel or the like, and more particularly to a hot rolling roll in which an outer layer material of a body portion is made of an iron-based alloy in which carbides are dispersed.

  As is well known, as a roll for hot rolling, an inner layer material (shaft material) having high toughness such as forged steel or ductile cast iron is used to ensure good wear resistance on the outer peripheral surface of the body portion in contact with the material to be rolled. On the outer surface, a composite roll in which an iron-based alloy in which carbides are dispersed, that is, a so-called high-speed alloy is formed as an outer layer material is frequently used.

  As a conventional technique related to a hot rolling roll using such a high-speed alloy as an outer layer material, in particular, a conventional technique related to a carbide of the outer layer material, techniques shown in Patent Documents 1 to 3 have been proposed.

Patent Document 1 discloses that heat is characterized in that MC type carbide is dispersed in an area ratio of 7 to 15% and eutectic carbide other than MC type carbide is dispersed in an area ratio of 15% or less in the structure of a high-speed alloy. A technique related to a roll material for hot rolling is disclosed.
Patent Document 2 discloses a technique relating to a roll material for hot rolling, characterized in that the hardness of the outer layer surface is a Shore hardness of 87 or more and the base hardness of the outer layer surface at 600 ° C. is a Vickers hardness of 350 or more. Is disclosed.
Patent Document 3 discloses a hot steel characterized in that 60000 or more of fine carbides having a particle size of 2 μm or less are dispersed in 1 mm ^{2 in} the structure of a high-speed alloy, and the average distance between adjacent fine carbides is 10 μm or less. A technique related to a roll material for rolling is disclosed.

JP 2000-51912 A JP 2004-114049 A JP-A-9-99306

Recently, the heat load of rolling rolls is increasing due to the reduction of idle time associated with increased production per unit time in hot rolling operations and the increased rolling load associated with increased production of high-strength steel. .
However, in the conventional roll material for hot rolling described in Patent Documents 1 to 3, particularly under high heat load operating conditions, roll surface roughness caused by selective wear of the base and thermal cracks that progress through the carbide and base. It is difficult to suppress both of them at the same time, and in particular, the rough surface of the roll due to thermal cracking has not been sufficiently studied, so that sufficient wear resistance and sufficient thermal crack resistance must be satisfied at the same time. Was difficult.

  The present invention has been made against the background described above, and it is an object of the present invention to provide a hot rolling roll having excellent wear resistance and thermal crack resistance at the same time.

  In order to solve the above-mentioned problems, the present inventors have conducted detailed experiments and examinations on the types, shapes and dimensions of carbides dispersed in a high-speed iron-based alloy, and their proportions. Among these, the MC type carbide having a specific shape and size has been newly found to have an action of preventing or suppressing the progress of cracks from the roll surface.

That is, in this type of high-speed iron-based alloy, as carbides, eutectic carbides represented by M ₇ C ₃ type, M ₂₃ C ₆ type, M ₂ C type, M ₆ C type, and non-eutectic crystals It is known that types of MC type carbides are produced and dispersed in a matrix. The former eutectic carbide crystallizes in a eutectic form when the iron-based alloy is solidified, and therefore exists in a net-like or branched streak state in the matrix. On the other hand, MC type carbides are dispersed in the form of particles or fine lumps by precipitation. Here, it is well known that all of these carbides contribute to the improvement of wear resistance, but the influence on the heat cracking resistance has not been elucidated so far.

  However, when the present inventors examined in detail the influence of the types and shapes of these carbides on the thermal crack resistance, MC type carbides, especially MC type carbides having a sphere or a shape close to that, are in the base. It has been newly found that it has the effect of preventing or suppressing the growth of cracks propagating through the metal. On the other hand, eutectic carbides rather promote the development of cracks and have an adverse effect on heat cracking resistance, and MC type carbides are also polygonal or flat (for example, oblong) It has been found that a shape having a shape far from the true sphere, such as a shape), a piece (flakes), or a surface having a large unevenness, has a small effect of preventing or suppressing the progress of cracks.

  And if MC type carbides having such a true sphere or a shape close thereto are dispersed to a certain degree or more, excellent heat cracking resistance is secured thereby, which may adversely affect heat cracking resistance. Even if a certain amount of eutectic carbide containing Mo, Cr or the like as a main constituent element exists, it is possible to ensure good thermal crack resistance. That is, even if the addition amount of the alloy element that generates eutectic carbide is increased, it is possible to ensure good thermal crack resistance. Therefore, by increasing the addition amount of the alloy element that generates eutectic carbide to some extent, Increase the hot hardness of the steel or increase the amount of eutectic carbides to reduce the area of the base (the interval between the eutectic carbides present in the net shape) (and therefore within the network of eutectic carbides present in the net shape). It has been found that the wear resistance can be improved by reducing the space of the material.

  Eventually, it was found that by dispersing a large number of MC type carbides having a spherical shape or a shape close to that to a certain degree, both the thermal crack resistance and wear resistance characteristics can be improved at the same time. It came to make.

Specifically, the roll for hot rolling of the basic aspect (first aspect) of the present invention is:
In the hot rolling roll in which the outer layer material constituting the outer peripheral portion of the roll body is made of an iron-based alloy in which carbides are dispersed;
The iron-based alloy of the outer layer material is
% By mass
C: 2.5-4.0%
Si: 0.3 to 2.0%,
Mn: 0.1 to 2.0%,
Cr: 5.0 to 15.0%,
Mo + 0.5 × W: 5.0 to 15.0%,
V: 7.0 to 15.0%,
It consists of the balance Fe and inevitable impurities,
And, on the surface of the outer layer material, as the carbide in the iron-based alloy, MC type carbide is dispersed in an area ratio of 10 to 20%, eutectic carbide is dispersed in an area ratio of 5 to 20%,
In addition, among the MC-type carbides, the equivalent area circle has a particle diameter of 5 to 40 μm on the surface and a circumference of 1.0 to 1.1 times the circumference of the equivalent area circle. There are 300 or more MC-type carbides per 1 mm ^{2 of} the surface.

  The above-mentioned equivalent area circle means a perfect circle having the same area as the area of MC type carbide grains appearing on the outer layer material surface. Therefore, the MC type carbide whose circumference is 1.0 times the circumference of the equivalent area circle means that the MC type carbide is a perfect circle on the surface of the outer layer material. The closer the magnification to the circumference of the equivalent area circle is to 1.0, the closer to a perfect circle. Therefore, the MC type carbide whose circumference is in the range of 1.0 to 1.1 times the circumference of the equivalent area circle is a perfect circle or a true circle on the surface of the outer layer material (two-dimensional shape). It means close MC type carbide.

Here, the shape of the MC-type carbide particles dispersed in the outer layer material is actually a three-dimensional solid, but the circumference viewed in two dimensions on the outer layer material surface is 1.0 of the circumference of the equivalent area circle. On average, the number of MC type carbides in the range of -1.1 times substantially correlates with the number of MC type carbides in the vicinity of the surface of the outer layer material or its shape. Therefore, here, MC type carbides (however, the equivalent area) whose circumference is in the range of 1.0 to 1.1 times the circumference of the equivalent area circle, using the two-dimensional circumference on the outer layer material surface as an index. The number of circles having a particle diameter in the range of 5 to 40 μm is defined as 300 or more per 1 mm ² .

Among the carbides dispersed in the outer layer iron-based alloy, it is MC type carbide, and the particle diameter of the equivalent area circle is in the range of 5 to 40 μm, and the circumference is 1 of the circumference of the equivalent area circle. Carbides in the range of 0.0 to 1.1 times have the effect of preventing the propagation of thermal cracks. Therefore, hereinafter, the MC type carbide having a particle size and shape satisfying such a particle size condition and a circumferential length condition will be referred to as a “crack-inhibiting MC type carbide”. Further, the presence of 300 or more crack-suppressing MC-type carbides per 1 mm ^{2 on the} surface of the outer layer material can reliably improve the heat crack resistance of the outer layer material. Further, if the area ratio of all MC type carbides including crack-suppressing MC type carbides is in the range of 10 to 20% and the area ratio of eutectic carbides is in the range of 5 to 20%, the heat cracking resistance Not only can the wear resistance be improved.
And in the present invention, as the component composition of the iron-based alloy, the iron-based alloy can satisfy the above-mentioned carbide dispersion conditions and also obtain the performance desired for the roll for rolling other than the heat crack resistance and wear resistance. The component composition is defined as described above.

The roll for hot rolling according to the second aspect of the present invention is the roll for hot rolling according to the first aspect,
The iron-based alloy further contains Nb: 0.1 to 5.0% by mass.

  The addition of Nb in this way is also effective in generating crack-suppressing MC type carbides.

Furthermore, the roll for hot rolling according to the third aspect of the present invention is the roll for hot rolling according to any one of the first and second aspects,
The iron-based alloy further comprises
% By mass
Ni: 0.1 to 10%,
Co: 0.1 to 10%,
B: 0.01 to 0.5%
It contains at least one selected from among the above.

  If one or more of Ni, Co, and B are added in this way, the hardness of the base in the outer layer iron-based alloy can be increased, and the wear resistance of the roll can be further improved.

The roll for hot rolling according to the fourth aspect of the present invention is the roll for hot rolling according to any one of the first to third aspects,
The iron-based alloy further contains at least one of Ti, Zr, Al, Mg, Ca, and Ce in a total mass% of 0.01 to 0.5%. .

  As described above, the addition of at least one of Ti, Zr, Al, Mg, Ca, and Ce contributes to uniform and fine generation of MC-type carbides, and as a result, the specific particle size and the specific shape as described above. This is effective for promoting the formation of crack-suppressing MC-type carbides having the following.

The roll for hot rolling according to the fifth aspect of the present invention is the roll for hot rolling according to any one of the first to fourth aspects,
Area ratio on the surface of the MC-type carbide having a particle diameter of an equivalent area circle within a range of 5 to 40 μm and a circumference within a range of 1.0 to 1.1 times the circumference of the equivalent area circle Is 0.7 times or more the area ratio of all MC type carbides.

  Thus, the area ratio of the MC type carbide in which the particle diameter of the equivalent area circle is in the range of 5 to 40 μm and the circumference is in the range of 1.0 to 1.1 times the circumference of the equivalent area circle, That is, by setting the area ratio of the crack-suppressing MC type carbide to 0.7 times or more of the area ratio of all MC type carbides, the heat crack resistance can be improved more reliably.

  The roll for hot rolling of the present invention is excellent in wear resistance and at the same time, is excellent in heat cracking resistance. Therefore, by using the roll for hot rolling of the present invention, it is worn even under operating conditions of high heat load. In addition, it is possible to suppress the rough surface of the roll and the life of the roll. Along with this, the effects such as improvement of the roll basic unit, improvement of the surface quality of the product and improvement of productivity can be achieved.

It is a schematic diagram which shows the metal microstructure of the outer-layer material surface in the roll for hot rolling of this invention, especially the dispersion | distribution condition of a carbide | carbonized_material. It is a schematic diagram for demonstrating an example of the definition of the area in the outer-layer material surface of the particle | grains of MC type carbide | carbonized_material. It is a schematic diagram for demonstrating an example of the definition of the circumference in the outer-layer material surface of the particle | grains of MC type carbide | carbonized_material.

  Hereinafter, embodiments of the present invention will be described in detail.

  The roll for hot rolling of the present invention basically uses a high-toughness iron-based material such as forged steel or ductile cast iron as an inner layer material constituting the shaft portion of the roll, and the outer surface, in particular, a cylinder in contact with the material to be rolled. Assuming a so-called composite roll in which an outer layer material made of an iron-based alloy is formed on the outer peripheral surface of the part, the component composition and structure of the iron-based alloy constituting the outer layer material are defined. First, the reasons for limiting the components of the outer layer iron-based alloy will be described.

<Ingredient composition>
[C: 2.5-4.0% by mass]
C combines with Cr, Mo and V to precipitate eutectic carbides such as M ₇ C ₃ type, M ₂₃ C ₆ type, M ₂ C type, M ₆ C type, and MC type carbides. , Improve wear resistance. If the amount of C is less than 2.5% by mass, the amount of carbide is reduced and sufficient wear resistance cannot be obtained. If it is more than 4.0% by mass, too much C is dissolved in the base, Toughness decreases. For these reasons, the C content is in the range of 2.5 to 4.0% by mass.

[Si: 0.3 to 2.0% by mass]
Si improves the castability of the iron-based alloy. If the amount of Si is less than 0.3% by mass, the effect cannot be obtained, and if it is less than 2.0% by mass, the matrix becomes brittle. For these reasons, the Si content is in the range of 0.3 to 2.0 mass%.

[Mn: 0.1 to 2.0% by mass]
Mn has an effect of deoxidation and desulfurization of the molten metal. If Mn is less than 0.1% by mass, this effect cannot be obtained, and if it is more than 2.0% by mass, the toughness of the matrix is lowered. For these reasons, the Mn content is set in the range of 0.1 to 2.0% by mass.

[Cr: 5.0 to 15.0 mass%]
Cr combines with C to form M ₇ C ₃ type and M ₂₃ C ₆ type carbides, that is, eutectic carbides, and partly dissolves in the matrix to improve hardenability, thereby improving resistance to resistance. It has the effect of improving wear. If the Cr content is less than 5.0% by mass, the effect cannot be obtained. If the Cr content is more than 15.0% by mass, the M ₇ C ₃ type carbide, which is an eutectic carbide, with respect to the total carbide crystal precipitation amount, M ₂₃ C ₆ The relative proportion of type carbides increases, the relative proportion of MC type carbides decreases, and as a result, the thermal crack resistance decreases. For these reasons, the Cr content is in the range of 5.0 to 15.0 mass%. The Cr amount is particularly preferably in the range of 6.0 to 13.0% by mass, even in the range of 5.0 to 15.0% by mass.

[Mo + 0.5W: 5.0 to 15.0% by mass]
Both Mo and W combine with C to form eutectic carbides of M ₂ C type and M ₆ C type carbides, and some of them dissolve in the base to increase the hot hardness of the base. Raise and improve wear resistance. If the total amount of Mo and 1/2 of the W amount, that is, Mo + 0.5W is lower than 5.0% by mass, this effect cannot be obtained, and if it is higher than 15.0% by mass, the total amount of precipitated carbide crystals The relative proportion of M ₂ C type and M ₆ C type carbides, which are eutectic carbides, increases, and the relative proportion of MC type carbides decreases. As a result, the thermal crack resistance decreases. For this reason, the amount of Mo + 0.5W is set in the range of 5.0 to 15.0% by mass. Here, as for Mo and W, as long as the amount of Mo + 0.5W satisfies the range of 5.0 to 15.0 mass%, either one may be added alone or both may be added simultaneously. When adding alone, it is preferable to satisfy the condition of Mo + 0.5W: 5.0 to 15.0% by mass by adding Mo. The reason is that if the addition amount of W alone is increased, too much W is dissolved in the matrix and the toughness of the matrix decreases. The amount of Mo + 0.5W is particularly preferably in the range of 6.5 to 12.5% by mass even within the range of 5.0 to 15.0% by mass.

[V: 7.0 to 15.0 mass%]
V combines with C to produce MC type carbides. And since MC type carbides (crack-suppressing type MC type carbides) having a specific particle size and a specific shape have an action of stopping the progress of cracks, they greatly contribute to improving wear resistance and thermal crack resistance. . If V is less than 7.0% by mass, this effect cannot be obtained, and if V is more than 15.0% by mass, aggregation and coarsening of MC-type carbides occur. In MC type carbides that are likely to be generated and coarsened, the effect of stopping the propagation of thermal cracks is reduced. As a result, the thermal cracking resistance is lowered, and as a result, the rough skin resistance is lowered.

  In addition to the above alloy elements, basically, Fe and inevitable impurities may be used, but Nb, which is an MC type carbide forming element, may be added as in the case of V if necessary. Further, in order to further improve the wear resistance through the improvement of the hardness of the base, it may contain at least one of Ni, Co and B. Furthermore, at least one of Ti, Zr, Al, Mg, Ca, and Ce may be contained for uniform and refinement of MC type carbide. The reason for addition of these elements and the reason for limiting the addition amount are as follows.

[Nb: 0.1 to 5.0% by mass]
Nb, like V, combines with C to produce MC-type carbides. And since the MC type carbide having the above-mentioned specific particle size and specific shape has an action of stopping the growth of cracks, Nb also contributes to improve wear resistance and heat crack resistance. Therefore, Nb may be added together with the addition of V. If the amount of Nb is less than 0.1% by mass, the effect of adding Nb cannot be obtained. On the other hand, if the amount of Nb exceeds 5.0% by mass, many MC-type carbides do not satisfy the above-mentioned conditions (thus preventing cracks). Type MC type carbide), and it becomes difficult to improve the thermal crack resistance. Therefore, the amount of Nb when Nb is added is in the range of 0.1 to 5.0% by mass.

[Ni: 0.1 to 10% by mass]
The addition of Ni improves the hardenability, increases the hardness of the base, and contributes to the improvement of wear resistance. If Ni is less than 0.1% by mass, the effect cannot be obtained. On the other hand, if Ni exceeds 10% by mass, the retained austenite increases too much, and the hardness decreases. For these reasons, the amount of Ni in the case of adding Ni is set in the range of 0.1 to 10% by mass.

[Co: 0.1 to 10% by mass]
The addition of Co is effective for improving the hot hardness of the base and improving the wear resistance. If Co is less than 0.1%, the effect cannot be obtained, while if it exceeds 10%, the toughness of the base is lowered. For these reasons, the amount of Co added in the case of adding Co is set in the range of 0.1 to 10% by mass.

[B: 0.01 to 0.5% by mass]
The addition of B is effective for making the crystal grains of the structure finer, improving the hardness of the matrix, and improving the wear resistance. If B is less than 0.01% by mass, the effect cannot be obtained, while if it exceeds 0.5% by mass, the material becomes brittle. For these reasons, the amount of B added is in the range of 0.01 to 0.5% by mass.

[At least one of Ti, Zr, Al, Mg, Ca, and Ce: 0.01 to 0.5 mass% in total]
Ti, Zr, Al, Mg, Ca, and Ce all precipitate MC type carbide finely and uniformly. Therefore, by adding any one or more of these, the effect of increasing the proportion of the carbide having the particle size and shape as described above, that is, the crack-suppressing MC-type carbide, as the MC-type carbide dispersed in the iron-based alloy. Is obtained. The effect is small if at least one of Ti, Zr, Al, Mg, Ca, Ce is less than 0.01% by mass in total, while if it exceeds 0.5% in total, it reacts violently with the molten metal for casting work. It becomes difficult. For these reasons, the total amount of addition in the case of adding one or more of Ti, Zr, Al, Mg, Ca, and Ce was set in the range of 0.01 to 0.5 mass%.

<Carbide conditions>
As schematically shown in FIG. 1, the metal structure of the outer layer material in the roll of the present invention is a matrix (matrix) 1 in which eutectic carbide 3 and MC type carbide 2 are dispersed. Yes. Here, the eutectic carbide 3 exists in a network (net-like) or branched form, while the MC-type carbide 2 is dispersed as particles of various shapes.
Each condition of the carbide related to the roll for hot rolling of the present invention, that is, the area ratio condition of the MC type carbide, the area ratio condition of the eutectic carbide, the particle size condition and the perimeter of the crack-inhibiting MC type carbide The condition, the number (dispersion density) condition of the crack-suppressing MC type carbides, and the area ratio condition of the crack-suppressing MC type carbides with respect to the area ratio of all MC type carbides will be described.

[Area ratio of MC type carbide: 10-20%]
In the component system of the iron-base alloy of the outer layer material of the present invention, MC type carbide is produced mainly by V, and at least one of Nb, Ti, Zr, Al, Mg, Ca, Ce, which is added if necessary. The above also contributes to the production of MC type carbides. The hardness of the MC type carbide is Hv 2500 to 3000, and has the highest hardness among the carbides. Therefore, by dispersing the MC type carbide as described above, it is effective in improving the wear resistance. That is, as described above, the crack-suppressing MC type carbide having a specific particle size and a specific shape is effective in improving the thermal crack resistance, but includes the crack suppressing MC type carbide having the specific particle size and the specific shape. In general, MC type carbides have improved wear resistance. Here, if the area ratio of all MC type carbides is smaller than 10% on the surface of the outer layer material, sufficient wear resistance cannot be obtained. On the other hand, if the area ratio of all MC type carbides is to be made larger than 20%, it becomes difficult to uniformly and finely disperse MC type carbides. Therefore, it becomes difficult to secure the number of crack-suppressing MC type carbides having the specific particle size and specific shape described above, and it is difficult to sufficiently improve the thermal crack resistance. For these reasons, the area ratio of all MC carbides on the surface of the outer layer material is set within a range of 10 to 20%. The area ratio of all MC type carbides is preferably within a range of 12 to 17%, even within the above range.

[Area ratio of eutectic carbide: 5 to 20%]
In the present invention, the eutectic carbide is a carbide such as M ₇ C ₃ type, M ₂₃ C ₆ type, M ₂ C type, M ₆ C type, and the like in the component system of the outer layer material iron-based alloy of the present invention. The eutectic carbide is mainly produced by Cr, Mo, and W, and V is also related to the production of the eutectic carbide depending on the balance of the content of Cr and Mo (+ W). Eutectic carbides crystallize in the form of nets or branches in the iron-based alloy and contribute to the improvement of wear resistance. If the area ratio of the eutectic carbide on the surface of the outer layer material is less than 5%, the wear resistance is not sufficient. On the other hand, if it is attempted to make the area ratio greater than 20%, the constituent elements of carbides are Cr, Mo (+ W), V The balance of the content of each of the steel changes, and the proportion of V contributing to the formation of the eutectic carbide increases, resulting in a decrease in the amount of crystal precipitation of the MC type carbide with the highest hardness. In addition to the improvement in wear resistance, it becomes difficult to secure the number of crack-inhibiting MC type carbides with the specific particle size and the specific shape as described above, and the heat crack resistance is sufficient. It becomes difficult to improve. For these reasons, the area ratio of the MC type carbide on the surface of the outer layer material is set in the range of 5 to 20%. The area ratio of the eutectic carbide is particularly preferably in the range of 8 to 17% even within the above range.

[Particle size of equivalent circle of MC type carbide to be crack-suppressed MC type carbide: 5 to 40 μm]
MC type carbide whose particle size of equivalent area circle is smaller than 5μm cannot be expected to prevent or suppress crack growth. If the proportion of MC type carbide larger than 40μm increases, the area ratio of MC type carbide is the same. In comparison, the interval between the MC type carbides increases, the probability that the cracks collide with the MC type carbides decreases, and the thermal crack resistance does not improve. For these reasons, the particle size of the equivalent area circle of MC type carbides that defines the number of crack-suppressing MC type carbides is specified to be in the range of 5 to 40 μm.

[Perimeter length of MC-type carbide used as crack-suppressing MC-type carbide: 1.0 to 1.1 times the circumference length of the equivalent area circle]
If the cross-sectional area of a particle is constant, the cross-sectional shape is the shortest when the cross-sectional shape is a perfect circle. As the unevenness increases, the circumference of the particle becomes longer than the circumference of the perfect circle. That is, for a certain particle, if a true circle having the same cross-sectional area as the equivalent cross-sectional area is defined as an equivalent area circle, the circumference of the particle is larger than the circumference of the equivalent area circle as the cross-sectional shape changes from the true circle. Becomes longer.

  In the case of the present invention, an equivalent area circle having the same area as the area of MC type carbide particles appearing on the surface of the outer layer material (three-dimensionally equivalent to the cross section of the particle) is assumed, and the MC type carbide particles When the circumference on the surface of the outer layer material is close to 1.0 times the circumference of the equivalent area circle, that is, when it is close to a perfect circle, the MC type carbide exhibits an action of suppressing the progress of cracks. On the other hand, as the circumferential length of the MC type carbide particles on the outer layer material surface increases from 1.0 times the circumferential length of the equivalent area circle, that is, the farther the cross-sectional shape is from the perfect circle, the MC type carbide becomes cracked. It will not show the action to suppress the progress. According to the experiments by the present inventors, in the MC type carbide in which the particle diameter of the equivalent area circle is in the range of 5 to 40 μm, the circumference is 1.0 to 1.1 times the circumference of the equivalent area circle. If it is within the range, it has been confirmed that it effectively suppresses the progress of cracks. The MC type carbide whose particle circumference on the surface of the outer layer material is longer than 1.1 times the circumference of the equivalent area circle does not exhibit the effect of stopping crack propagation. For these reasons, MC type carbide having a circumference within the range of 1.0 to 1.1 times the circumference of the equivalent area circle and a particle diameter of the equivalent area circle within the range of 5 to 40 μm. The crack-suppressing MC-type carbide is defined, and the number (distribution density) per unit area of the crack-suppressing MC-type carbide necessary for suppressing crack growth is specified as described below.

  Although the shape of the MC-type carbide has an influence on the thermal crack resistance as described above, the reason why the thermal crack resistance is better as it is closer to a perfect circle has not yet been clarified, but the closer it is to a perfect circle, the MC type It seems that when a carbide collides with a crack, the MC type carbide easily absorbs the energy of the crack propagation. That is, if the shape is far away from the perfect circle and becomes a flat shape, polygonal shape, or a shape with severe surface irregularities, the crack progresses along the interface between the carbide and the base, or the carbide itself It is considered that the effect of the MC type carbide absorbing the crack propagation energy is lost due to destruction.

[Number of crack-suppressing MC type carbides: 300 or more per 1 mm ² ]
MC type carbide having a circumference of 1.0 to 1.1 times the circumference of the equivalent area circle and a particle diameter of the equivalent area circle within the range of 5 to 40 μm on the surface of the outer layer material, that is, a crack If the number of restraining MC type carbides is less than 300 per 1 mm ² , the probability that the generated cracks will collide with the MC type carbides decreases, so that the thermal crack resistance is not sufficiently improved. For this reason, the number of crack-suppressing MC type carbides is set to 300 or more per 1 mm ² . In addition, although the upper limit of the number per 1 mm < ² > of crack suppression type MC type carbide is not specified, in the case of the iron base alloy specified by the present invention, it is usually about 600 or less.

[Area ratio of crack-suppressing MC-type carbide relative to the area ratio of all MC-type carbides: 0.7 times or more]
If the area ratio of MC type carbides is the same, it does not depend on the shape of MC type carbides (that is, regardless of the area ratio of crack-suppressing MC type carbides relative to the area ratio of all MC type carbides), wear resistance Sex is substantially equivalent. On the other hand, the thermal crack resistance is improved as the area ratio of the crack-suppressing MC type carbide is larger. If the area ratio of the crack-suppressing MC type carbide on the surface of the outer layer material is smaller than 0.7 times the area ratio of all the MC type carbides, the effect of improving the thermal crack resistance cannot be sufficiently obtained. For these reasons, the area of the crack-suppressing MC type carbide having a particle diameter of the equivalent area circle of 5 to 40 μm and a circumference of 1.0 to 1.1 times the circumference obtained from the equivalent area circle diameter. The rate is preferably 0.7 times or more the area ratio of all MC type carbides. In addition, the ratio of the area ratio of the crack-suppressing MC type carbide to the area ratio of all MC type carbides is particularly preferably 0.8 or more.

[Definition of equivalent area circle for particle size of MC type carbide]
In general, in the technical field dealing with dispersed particles, the concept of equivalent area circle has been frequently used in the past, and in the present invention, the conventional measurement method and definition of equivalent area circle may be followed. The measurement method and definition of the equivalent area circle related to the particle diameter are not particularly limited, but the geometric definition will be described below as an example.

As schematically shown in FIG. 2, a square lattice having a lattice pitch p of, for example, 0.5 μm is drawn on the surface of the outer layer material, and among the many squares (unit lattices) between the lattices, MC type carbide 2 The number of unit cells that the outer peripheral portion 2A crosses is n, and the number of unit cells on the inner side (inside the MC type carbide particles) is m. Then, the total number P (= m + n; the total number of unit cells hatched in FIG. ² ) is multiplied by the area q of one unit cell (q = p ² = 0.25 μm ^{2 in the} above example). This value is defined as the area S on the surface of the outer layer material of one MC type carbide particle. Then, assuming a perfect circle (equivalent area circle) having the area S, the diameter of the circle may be the particle diameter of the MC type carbide 2 in the equivalent area circle. However, in practice, the particle size of the MC type carbide in the equivalent area circle can be measured by image processing and calculation using an image obtained by photographing the surface of the outer layer material.

[Definition of circumference of MC type carbide]
For example, as schematically shown in FIG. 3, the circumference of the MC type carbide particles on the surface of the outer layer material is a square lattice having a lattice pitch p of, for example, 0.5 μm on the surface of the outer layer material as described above. Among the many squares (unit cell) between them, the number of unit cells (the unit cell shaded in FIG. 3) that the outer peripheral portion 2A of the MC-type carbide 2 crosses is assumed to be n. Here, the length of the outer peripheral portion of the MC type carbide crossing one unit cell can be estimated as the total total line length of the lines connecting the centers of adjacent unit cells. Therefore, the length between the centers of the unit cells adjacent in the vertical direction and the horizontal direction is estimated as p, and the length between the centers of the unit cells adjacent in the diagonal direction is estimated as √2 × p. Where a is the number of locations adjacent to a, and b is the number of locations adjacent in the diagonal direction (hence n = a + b), the total value of the value obtained by multiplying p by a and the value obtained by multiplying √2 × p by b, The circumference of one MC type carbide can be estimated.

[Production method of roll]
In manufacturing the hot rolling roll of the present invention, the surface hardness is adjusted to about Hs 80 to 90 by heat treatment or the like to maintain the wear resistance, and in order to ensure the strength of the roll, the inner layer has high toughness. It is a composite roll with ductile cast iron and forged steel.

  Here, as a method of forming the outer layer material on the outer side of the inner layer material (shaft member) made of a high toughness material, a continuous casting overlay method (CPC method), a centrifugal casting method, or the like can be applied. A cast overlay method is preferred. The reason is that according to the continuous casting overlay method, segregation in the roll radial direction of MC type carbide having a specific gravity smaller than that of the molten metal can be suppressed.

For the iron-based alloy of the outer layer material, the particle diameter of the equivalent area circle as described above is in the range of 5 to 40 μm and the circumference is in the range of 1.0 to 1.1 times the circumference of the equivalent area circle. There are no particular limitations on the specific means for generating 300 or more crack-suppressing MC-type carbides per 1 mm ² of the surface. For example, the average solidification rate in the roll radial direction during continuous casting or centrifugal casting May be adjusted so that carbides are generated uniformly and finely by setting the molten metal temperature to about 5 to 10 mm / min, or by rapidly cooling the molten metal at the time of casting at about 1550 to 1650 ° C. Conventionally, in order to keep the manufacturing cost low in terms of energy consumption and facility maintenance, the average solidification rate in the roll radial direction is 2 to 5 mm / min, and the molten metal temperature during casting is 1450 to 1550 ° C., The roll for hot rolling was not manufactured except such conditions.

  Further, as described above, the addition of Ti, Zr, Al, Mg, Ca, and Ce is effective in precipitating MC type carbides finely and uniformly. Therefore, in addition to the above process conditions, Adding one or more of Ti, Zr, Al, Mg, Ca, and Ce is also effective for generating many crack-suppressing MC-type carbides.

Below, the result of the confirmation experiment performed in order to confirm the effectiveness of this invention is demonstrated.
As the outer layer material, hot rolling rolls having the components and structures of Examples 1 to 35 and Comparative Examples 1 to 6 shown in Tables 1 to 4 were prepared. The roll dimensions were roll diameter: 800 mm, roll body length: 2000 mm, and roll shaft length: 4000 mm. Further, as the inner layer material (shaft member), chromium molybdenum steel (SCM440) was used, and a composite roll was formed in which the outer layer material was formed with a single wall of 50 mm on the surface of the body.

As a specific roll manufacturing method, the following method was applied.
That is, the molten metal composed of the above components soaked at 1600 ° C. by continuous casting is poured into the gap between the water-cooled mold and the inner layer material so that the average solidification rate in the roll radial direction becomes 7.5 mm / min. The molten metal is solidified to form an outer layer material around the inner layer material, and the integrated inner layer material and outer layer material are intermittently drawn downward, and the composite rolls of Examples 1-35 and Comparative Examples 1-5 Manufactured. On the other hand, the composite roll of Comparative Example 6 was poured into the gap between the water-cooled mold and the inner layer material by injecting a molten metal composed of the above components soaked at 1500 ° C., which is a conventional production condition, by continuous casting and overlaying. The melt is solidified so that the average solidification rate in the direction is 3.0 mm / min, the outer layer material is formed around the inner layer material, and the integrated inner layer material and outer layer material are intermittently drawn downward and manufactured. did.
After such continuous casting and overlaying, the roll body was heated to 1000 ° C. and soaked for a certain time, and then quenched to near normal temperature. Subsequently, tempering was performed once or twice at 550 ° C., and the Shore hardness was about Hs 85 to 95.

In addition, the carbide on the roll surface in the iron-based alloy of the outer layer material of each roll is, as a software for image analysis, from a structure observation image of FE-SEM (manufactured by Hitachi High-Technologies Corporation, SU-70 field emission scanning electron microscope). The image was analyzed by using Image-ProPlus manufactured by Media Cybernetics.
Hereinafter, a specific method for measuring carbide will be described.

The particle size and area ratio of MC type carbides in the equivalent area circle of all MC type carbides were examined as follows.
First, a square lattice of p = 0.5 μm is drawn on an observation image obtained by photographing a field of view of 450 μm × 600 μm, and for each MC type carbide, among the many squares (unit cell) between the lattices, the outer peripheral portion of the MC type carbide The number of unit cells traversed by n is n, and the number of unit cells on the inner side (inside the MC type carbide particles) is m. Then, the total number P (= m + n) multiplied by the area q (= p ² = 0.25 μm ² ) of one unit cell is used as the area S on the outer layer material surface of one MC type carbide particle. It was. Further, assuming a perfect circle (equivalent area circle) having the area S, the diameter of the circle was defined as the particle size of the MC type carbide in the equivalent area circle. Next, the ratio of the total area of all MC type carbides in the field of view of 450 μm × 600 μm (= 0.27 mm ² ) was defined as the total MC type carbide area ratio.
The area ratio of all eutectic carbides was also measured by the same method as that for the MC type carbides.

The area ratio of the crack-suppressing MC type carbide having the equivalent area circle particle size in the range of 5 to 40 μm and the circumference in the range of 1.0 to 1.1 times the circumference of the equivalent area circle is as follows: I investigated as follows.
In the same manner as described above, a square lattice having a lattice pitch p of, for example, 0.5 μm is drawn on an observation image obtained by photographing a field of view of 450 μm × 600 μm, and among the many squares (unit lattice) between the lattices, the outer periphery of the MC type carbide The number of unit cells crossed by the part was n. Then, the length between the centers of the unit cells adjacent in the vertical direction and the horizontal direction is estimated as p, and the length between the centers of the unit cells adjacent in the oblique direction is estimated as √2 × p. Where a is the number of locations adjacent to a and b is the number of locations adjacent to each other diagonally (n = a + b), and the total value of the value obtained by multiplying p by a and the value obtained by multiplying √2 × p by b is one The circumference of MC type carbide was used. Next, the ratio of the circumference obtained for each MC type carbide and the circumference obtained from the particle size of the MC type carbide in the equivalent area circle is taken, and the particle diameter of the equivalent area circle is in the range of 5 to 40 μm and The MC-type carbide having a circumference within the range of 1.0 to 1.1 times the circumference of the equivalent area circle is defined as a crack-suppressing MC-type carbide, and the area ratio of the crack-suppressing MC-type carbide is measured. The ratio of the area ratio of the crack-suppressing MC type carbide to the area ratio of all MC type carbides was determined.
Also, while sequentially performing the numbering in each MC type carbide each said operation, after counting the number of MC-type carbide except the crack suppressing type MC-type carbide and crack inhibitory MC type carbides, per 1 mm ² total Converted to the number of MC type carbides.
In addition, the number of visual fields of the observation image was 5, and the average of five visual fields was calculated | required about said each index value measured for every visual field.

  Each roll manufactured as described above is attached to the third stage stand of a finishing hot rolling mill having a seven-stage stand, the inlet side plate thickness: 13 mm, the outlet side plate thickness: 9 mm, the rolling pitch: rolling time 70 sec, After rolling 100 coils of normal steel (SPHC) under the conditions of an idle time of 30 sec, a plate width of 1250 mm, a rolling load of 1700 ton, and a roll speed of 165 mpm, the amount of wear was examined as an evaluation of the wear resistance of the rolling roll, The amount of grinding was measured as an evaluation of heat cracking resistance. The results are shown in Tables 3 and 4. The amount of wear was expressed as a value obtained by measuring the maximum wear depth and dividing the maximum wear depth by the roll diameter. The amount of grinding as an evaluation of heat cracking resistance is indicated by the amount of grinding of the outer peripheral surface of the roll until the cracks disappear completely by visual inspection.

  In Tables 3 and 4, “number of crack-suppressing MC” means the number of crack-suppressing MC-type carbides, and “crack-suppressing MC area ratio” means the area ratio of crack-suppressing MC-type carbides. To do. The “number of MC excluding crack suppression type” means the number of all MC type carbides minus the number of crack suppression type MC type carbides.

As shown in Tables 3 and 4, it was found that all of Examples 1 to 35 within the specified range of the present invention had a small wear amount and grinding amount after rolling. That is, Examples 1 to 35 were able to improve wear resistance and thermal crack resistance at the same time as compared with Examples of Comparative Examples 1 to 5.
Of these Examples 1 to 35, Example 8 is an example in which the area ratio of the crack-suppressing MC type carbide is less than 0.7 times the area ratio of all MC type carbides. In this case, the grinding amount was larger than that of the other examples and the thermal crack resistance was slightly inferior, but the grinding amount was smaller than that of the comparative example and the thermal crack resistance was good.

On the other hand, Comparative Example 1 in which the number of crack-suppressing MC type carbides having a particle diameter of an equivalent area circle of 5 to 40 μm and a circumference of 1.0 to 1.1 times the circumference of the equivalent area circle is small, eutectic It was confirmed that Comparative Example 2 having a large carbide area ratio had a larger amount of grinding than the examples and was inferior in heat crack resistance.
Furthermore, it was confirmed that the comparative example 3 with a small area ratio of all MC type carbides has a larger wear amount than the examples and is inferior in wear resistance.
In addition, it was confirmed that Comparative Example 4 in which the area ratio of the eutectic carbide is small also has a larger wear amount than the Examples and is inferior in wear resistance.
Furthermore, in Comparative Example 5 in which the area ratio of all MC carbides was too large, the MC type carbides were more severely aggregated and coarsened than in the examples, and therefore 300 crack-suppressed MC type carbides per 1 mm ^2. It was difficult to produce the rolls present above, and it was confirmed that the amount of grinding was larger than that of the Examples and the thermal crack resistance was inferior.
Further, in Comparative Example 6 manufactured under the conventional casting conditions, since the MC type carbides were more agglomerated and coarsened than in the Example, a roll having 300 or more crack-suppressing MC type carbides per 1 mm ^2. It was difficult to produce, and it was confirmed that the amount of grinding was larger than that of the Examples and the thermal crack resistance was inferior.

  The preferred embodiments and examples of the present invention have been described above. However, these embodiments and examples are merely examples within the scope of the present invention, and do not depart from the scope of the present invention. Thus, addition, omission, replacement, and other changes of the configuration are possible. That is, the present invention is not limited by the above description, is limited only by the scope of the appended claims, and can be appropriately changed within the scope.

1 Base 2 MC type carbide 3 Eutectic carbide

Claims (5)

In the hot rolling roll in which the outer layer material constituting the outer peripheral portion of the roll body is made of an iron-based alloy in which carbides are dispersed;
The iron-based alloy of the outer layer material is
% By mass
C: 2.5-4.0%
Si: 0.3 to 2.0%,
Mn: 0.1 to 2.0%,
Cr: 5.0 to 15.0%,
Mo + 0.5 × W: 5.0 to 15.0%,
V: 7.0 to 15.0%,
It consists of the balance Fe and inevitable impurities,
And, on the surface of the outer layer material, as the carbide in the iron-based alloy, MC type carbide is dispersed in an area ratio of 10 to 20%, eutectic carbide is dispersed in an area ratio of 5 to 20%,
In addition, among the MC-type carbides, the equivalent area circle has a particle diameter of 5 to 40 μm on the surface and a circumference of 1.0 to 1.1 times the circumference of the equivalent area circle. A roll for hot rolling characterized in that there are 300 or more MC type carbides per 1 mm ^{2 of} the surface. In the hot rolling roll according to claim 1,
The iron-based alloy further includes Nb: 0.1 to 5.0% by mass%, and a roll for hot rolling characterized by the above. In the hot rolling roll according to any one of claims 1 and 2,
The iron-based alloy further comprises
% By mass
Ni: 0.1 to 10%,
Co: 0.1 to 10%,
B: 0.01 to 0.5%
A roll for hot rolling characterized by containing one or more selected from among the above. In the hot rolling roll according to any one of claims 1 to 3,
The iron-based alloy further contains at least one kind of Ti, Zr, Al, Mg, Ca, and Ce in a total mass% of 0.01 to 0.5% for hot rolling, roll. In the hot rolling roll according to any one of claims 1 to 4,
The area ratios of the MC type carbides whose particle diameter of the equivalent area circle is in the range of 5 to 40 μm and whose circumference is in the range of 1.0 to 1.1 times the circumference of the equivalent area circle are all The roll for hot rolling characterized by being 0.7 times or more of the area ratio of the MC type carbide.

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