JP7158312B2 - Hot rolling roll outer layer material, hot rolling composite roll, and method for manufacturing hot rolling roll outer layer material - Google Patents

Description

The present invention relates to a hot rolling roll outer layer material and a hot rolling composite roll, and in particular, a hot rolling roll outer layer material and a hot rolling composite roll suitable as work rolls for a hot rolling mill for steel sheets, and The present invention relates to a method for manufacturing a roll outer layer material for hot rolling.

In recent years, with the progress in hot rolling technology for steel sheets, the environment in which rolls are used has become more severe, and the production of high-strength steel sheets, thin-walled steel sheets, and other steel sheets that require a large rolling load is increasing. For this reason, the quality level required for rolling work rolls is increasing, and high-performance rolling work rolls (high-speed rolls) free from casting defects such as segregation, porosity, and porosity are desired.

As an outer layer material of such a rolling work roll, for example, Patent Document 1 discloses C: 1.5 to 3.5%, Ni: 5.5% or less, Cr: 5.5 to 12.0%, Mo: 2.0 to 8.0%, V: 3.0 to 10.0%, Nb: 0.5 to 7.0%, and Nb and V, Nb, V and C content satisfies a specific relationship and the ratio of Nb to V is within a specific range. As a result, the segregation of hard carbides in the outer layer material is suppressed even if the centrifugal casting method is applied, and the outer layer material for rolling rolls is excellent in wear resistance and crack resistance. Further, in Patent Document 2, C: 1.5 to 3.5%, Cr: 5.5 to 12.0%, Mo: 2.0 to 8.0%, V: 3.0 to 10.0 %, Nb: 0.5 to 7.0%, Nb and V, the contents of Nb, V and C satisfy a specific relationship, and the ratio of Nb and V is within a specific range A roll outer layer material for rolling containing so that As a result, segregation of hard carbides in the outer layer material is suppressed even if the centrifugal casting method is applied, and wear resistance and crack resistance are improved, contributing greatly to the improvement of hot rolling productivity. Further, in Patent Document 3, C, Cr, and Mo are optimally adjusted, [%C] + 0.2 [%Cr] ≤ 6.2, 0.27 ≤ [%Mo] / [%Cr] < 0 A rolling roll outer layer material having a component system satisfying .7 has been proposed. This is said to significantly reduce the significant segregation of carbides.

Since high speed steel rolls contain a large amount of alloying elements, remarkable carbide segregation and band-like (layered) segregation called lamination segregation tend to occur during the centrifugal casting process. In regions where such segregation occurs, deep heat cracks are formed, and rolling troubles called chipping and spalling are likely to occur. In Patent Document 4, in order to improve the surface roughening resistance and crack resistance by making the structure of the outer shell layer of the centrifugal casting roll fine and uniform, the supply temperature (casting temperature) of the molten metal (molten metal) to the mold is A method for producing rolling rolls is disclosed in which the temperature range is maintained from the primary crystal formation temperature T _c (° C.) to T _c +90 (° C.) and the average lamination speed is controlled at 2 to 40 mm/min.

Further, in Patent Document 5, C: 1.5 to 3.5%, Si: 0.1 to 2.0%, Mn: 0.1 to 2.0%, Cr: 5 to 25%, Mo: 2 to 12%, V: 3 to 10%, Nb: 0.5 to 5%, Mo and Cr, the content of Mo and Cr satisfies a specific relationship, and the amount of carbide in the radial direction A rolling roll outer layer material has been proposed in which the difference between the maximum value and the minimum value of is 20% or less of the average value. As a result, the outer layer material for rolling rolls is free from structural segregation and has excellent surface roughening resistance and mailability.

JP-A-04-365836 JP-A-05-1350 JP-A-10-183289 Japanese Patent No. 2778896 Japanese Patent Application Laid-Open No. 2000-239779

As mentioned above, when rolling rolls with lamination segregation are used, rolling troubles such as chipping and spalling occur during rolling, and minute unevenness existing in the segregation areas deteriorates the surface quality of the rolled material. There was a problem such as causing it, and it lowered productivity. In addition, in recent years, the surface quality of the material to be rolled required by customers has become even more stringent. There has been a demand for rolling rolls with excellent surface quality.

The present invention has been made in view of the above circumstances, and solves the problems of the prior art, and is free from structural segregation, has excellent surface quality, and has excellent roll characteristics for hot rolling. An object of the present invention is to provide a method of manufacturing a composite roll for rolling and a roll outer layer material for hot rolling.

When trying to produce a metal material by centrifugal casting, the dendrites or carbides that crystallize in the molten metal during the solidification process are separated by centrifugal separation due to the difference in specific gravity from the molten metal (phases that are heavier than the molten metal are on the outer periphery, and phases that are lighter than the molten metal are on the center side. movement) phenomenon occurs. On the other hand, the lamination segregation presents a morphology in which dendrite-enriched layers and carbide-enriched layers are alternately overlapped and segregated in a band shape. The cause of band-like segregation is considered to be the shear flow at the solid-liquid interface (solid-liquid coexisting phase) during the solidification process in centrifugal casting. Ohnaka et al. (Casting Engineering, Vol. 69 (1997), No. 3, pp. 240-246) reported that the occurrence of band-like segregation in horizontal centrifugal casting is influenced by gravity (1 G). According to this idea, it is difficult to avoid lamination segregation as long as horizontal or oblique centrifugal casting, in which gravity acts in the direction of mold rotation, is performed.

In the conventional centrifugal casting method, the mold rotation speed is controlled as constant as possible to prevent external forces from acting as much as possible, based on the basic idea that the molten metal should be solidified quietly by avoiding vibrations and shearing forces as much as possible. In addition, casting operation is performed by suppressing casting vibration. However, it was impossible to avoid lamination segregation by this method. In addition, in the method disclosed in Patent Document 4, since the casting speed is extremely low, the solidification becomes unstable, and double skin defects and spatter-like defects tend to occur on the surface of the outer layer. In addition to the very low casting speed, the casting temperature is as low as Tc to _Tc + 90° _C , and the control range is narrow. It is also difficult to operate easily and stably.

In view of the fact that lamination segregation cannot be eliminated with the conventional basic idea, the present inventors
If lamination segregation is generated by the influence of gravity, we thought that if we positively apply acceleration in the direction of rotation to the molten metal, it would be possible to generate lamination segregation by the shear force generated there. rice field. Based on this idea of reversal, as a result of intensive investigation of methods for suppressing lamination segregation, it was found that by continuously or intermittently changing the mold rotation speed to give acceleration in the direction of mold rotation, countless lamination segregations were generated. is also possible in principle, and it has been found that if countless lamination segregations are generated, macroscopically uniform structure can be obtained. Furthermore, according to detailed studies by the present inventors, a predetermined hardness difference can be obtained by adjusting the supply rate (kg / s) of the molten outer layer material when casting into the centrifugal casting mold within a specific range. We have obtained unprecedented knowledge that lamination segregation can be remarkably reduced and a roll outer layer material for hot rolling with excellent surface quality can be obtained.

The present invention has been completed based on the above findings, and the gist thereof is as follows.
[1] In mass%, C: 1.5 to 3.0%, Si: 0.1 to 2.0%, Mn: 0.1 to 2.0%, Cr: 5.0 to 15.0% , Mo: 2.0 to 12.0%, V: 3.0 to 10.0%, Nb: 0.5 to 5.0%, and the content of Mo and Cr is the following formula (1) and having a composition consisting of the balance Fe and unavoidable impurities,
It has a carbide amount distribution in which the change rate S of the carbide amount represented by the following formula (2) is 20% or less, and further satisfies the hardness difference represented by the following formula (3) of 3.0 or less. Roll outer layer material for hot rolling.
0.35≦[%Mo]/[%Cr]≦0.70 (1)
Here, [%Mo] and [%Cr] are the content (% by mass) of each element.
S=(X _max -X _min )×100/X _ave (2)
here,
X _max : Maximum value (area %) in the carbide amount distribution in the region from the outer surface to 30 mm in the radial direction of the roll
X _min : Minimum value (area %) in the carbide amount distribution in the region from the outer surface to 30 mm in the radial direction of the roll
X _ave : average value (area %) in the carbide amount distribution in the region from the outer surface to 30 mm in the radial direction of the roll
is.
ΔHS = HS _Xmax - HS _Xmin (3)
here,
HS _Xmax : Shore hardness at the position of the maximum value in the carbide amount distribution in the region from the outer surface to 30 mm in the roll radial direction HS _Xmin : The position of the minimum value in the carbide amount distribution in the region from the outer surface to 30 mm in the roll radial direction is the Shore hardness at
[2] Any one or more of Co: 5.0% or less, Ni: 3.0% or less, and W: 5.0% or less in mass% in the outer layer material for hot rolling rolls The roll outer layer material for hot rolling according to [1] containing.
[3] A hot rolling composite roll having a three-layer structure of an outer layer, an intermediate layer and an inner layer or a two-layer structure of an outer layer and an inner layer, wherein the outer layer is for hot rolling according to [1] or [2] A composite roll for hot rolling comprising a roll outer layer material.
[4] When the molten metal having the composition described in [1] or [2] is poured and centrifugally cast, the centrifugal force on the outer surface of the roll outer layer material changes at 1.0 G / s or more, A method for producing an outer layer material for hot rolling rolls, characterized in that the number of rotations of a centrifugal casting mold is changed two or more times, and the supply rate of the molten metal is 80 to 200 kg/s.

According to the present invention, it is possible to obtain a hot rolling roll outer layer material and a hot rolling composite roll that are free from structural segregation and have excellent surface quality.

FIG. 1 is a diagram for explaining the distribution of carbide content in the present invention. FIG. 2 is a diagram for explaining fluctuations in the number of rotations of the mold, and FIG. 2(a) is an axial cross-sectional view of the molten outer layer material for schematically explaining the centrifugal force and shear force acting on the molten outer layer material. , and FIG. 2(b) is a waveform diagram of the rotation speed of the mold. FIG. 3 is a waveform diagram showing an example of a mold rotation speed change pattern. FIG. 4 is a diagram showing the rotation direction of the mold and the supply direction of molten metal in centrifugal casting. 5A and 5B are schematic diagrams showing the positions of sampled specimens for measuring the amount of carbide from the sleeve roll, FIG. 5A being a front view of the sleeve roll viewed from the axial direction, and FIG. It is a schematic diagram of the test piece which carried out. FIG. 6 is a diagram showing the distribution of the amount of carbide in Examples, and FIG. 6(b) is a diagram showing the distribution of the carbide content of No. 1 in Table 1. FIG. 5 is a diagram showing the distribution of the amount of carbide in No. 5. FIG. FIG. 7 is a schematic diagram showing positions for measuring hardness in a test piece for measuring the amount of carbide taken from a sleeve roll. 8A and 8B are schematic diagrams showing the position of a test piece taken from a sleeve roll, FIG. 8A is a front view of the sleeve roll viewed from the axial direction, and FIG. FIG. 8(c) is a right side view of the sleeve roll seen from direction B in FIG. 8(a). FIG. 9 is a diagram schematically showing the configuration of the testing machine used in the wear test and the shape of the test piece for the wear test (wear test piece). FIG. 10 shows the configuration of the testing machine used in the hot rolling fatigue test, the hot rolling fatigue test test piece (hot rolling fatigue test piece), and the hot rolling fatigue test test piece (hot rolling fatigue test Fig. 3 is a diagram schematically showing the shape and dimensions of the notch introduced into the outer peripheral surface of the piece). FIG. 11 is a schematic diagram of a rolling test roll according to the present invention.

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

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

C: 1.5-3.0%
C dissolves into a solid solution to increase the hardness of the matrix (martensite and/or bainite), and combines with carbide-forming elements to form hard carbides, thereby improving the wear resistance of the roll outer layer material. have If the C content is less than 1.5%, the amount of carbide is insufficient, resulting in deterioration of wear resistance. On the other hand, if the content exceeds 3.0%, the carbide is coarsened and the amount of eutectic carbide is excessively increased, making the roll outer layer material hard and embrittled, promoting the occurrence and growth of fatigue cracks, and improving fatigue resistance. lower the Therefore, C is limited to 1.5 to 3.0%. In addition, preferably, it is 1.7 to 2.7%.

Si: 0.1-2.0%
Si is an element that acts as a deoxidizing agent and improves castability of the molten metal. In addition, Si dissolves in the matrix and has the effect of strengthening the matrix. In order to obtain such an effect, the content of 0.1% or more is required. On the other hand, even if the content exceeds 2.0%, the effect is saturated and the effect corresponding to the content cannot be expected, which is economically disadvantageous and may embrittle the matrix structure. Therefore, Si is limited to 0.1 to 2.0%. Incidentally, it is preferably 0.2 to 1.5%.

Mn: 0.1-2.0%
Mn is an element that has the effect of fixing S as MnS and detoxifying S, and partly forming a solid solution in the matrix structure and having the effect of improving the hardenability. Moreover, Mn dissolves in the matrix and has the effect of strengthening the matrix (solution strengthening). In order to obtain such an effect, the content of 0.1% or more is required. On the other hand, even if the content exceeds 2.0%, the effect is saturated and the effect commensurate with the content cannot be expected, and the material may become embrittled. Therefore, Mn is limited to 0.1 to 2.0%. Incidentally, it is preferably 0.2 to 1.6%.

Cr: 5.0-15.0%
Cr combines with C to form mainly eutectic carbide, which improves wear resistance, reduces frictional force with the steel sheet during rolling, reduces surface damage of rolls, and stabilizes rolling. is an element having In order to obtain such effects, a content of 5.0% or more is required. On the other hand, if the content exceeds 15.0%, the amount of coarse eutectic carbide increases, which reduces the fatigue resistance. Therefore, Cr is limited to 5.0 to 15.0%. Incidentally, it is preferably 5.5 to 13.0%.

Mo: 2.0-12.0%
Mo is an element that combines with C to form hard carbides and improves wear resistance. In addition, Mo dissolves in hard MC-type carbides in which V, Nb and C combine to strengthen the carbides, and also dissolves in eutectic carbides to increase the fracture resistance of these carbides. Through such actions, Mo improves the wear resistance and fatigue resistance of the roll outer layer material. In order to obtain such an effect, the content of 2.0% or more is required. On the other hand, if the content exceeds 12.0%, Mo-based hard and brittle carbides are formed, which lowers hot rolling contact fatigue resistance and fatigue resistance. Therefore, Mo is limited to 2.0 to 12.0%. Incidentally, preferably, it is 3.0 to 9.0%.

V: 3.0-10.0%
V is an element that provides both wear resistance and fatigue resistance as a roll. V forms extremely hard carbides (MC-type carbides) to improve wear resistance, and acts effectively to split and dispersely crystallize the eutectic carbides, thereby improving hot rolling contact fatigue resistance. , is an element that remarkably improves the fatigue resistance of the roll outer layer material. Such an effect becomes remarkable at a content of 3.0% or more. On the other hand, if the content exceeds 10.0%, the MC-type carbides become coarse, which destabilizes various properties of the roll for rolling. In addition, the amount of eutectic melt is reduced, and coarse cavities are formed. Therefore, V is limited to 3.0 to 10.0%. Incidentally, it is preferably 4.0 to 9.0%.

Nb: 0.5-5.0%
Nb solid-solves in MC-type carbide to strengthen the MC-type carbide, and improves wear resistance, particularly fatigue resistance, through the action of increasing the fracture resistance of MC-type carbide. When both Nb and Mo are solid-solved in the carbide, wear resistance and further fatigue resistance are significantly improved. Further, Nb is an element that promotes the division of eutectic carbide, has the effect of suppressing breakage of eutectic carbide, and improves the fatigue resistance of the roll outer layer material. In addition, Nb also has the effect of suppressing segregation during centrifugal casting of MC type carbide. Such an effect becomes remarkable at a content of 0.5% or more. On the other hand, if the content exceeds 5.0%, the growth of MC-type carbides in the molten metal is accelerated, deteriorating rolling contact fatigue resistance during hot temperatures. In addition, the amount of eutectic melt is reduced, and coarse cavities are formed. Therefore, Nb is limited to 0.5 to 5.0%. In addition, preferably, it is 0.8 to 2.5%.

Furthermore, in the present invention, the contents of Mo and Cr must satisfy the following formula (1).
0.35≦[%Mo]/[%Cr]≦0.70 (1)
Here, [%Mo] and [%Cr] are the content (% by mass) of each element.
When Mo and Cr are contained in the above range and the value of [%Mo]/[%Cr] is 0.35 or more, Mo dissolves in the MC type carbide to strengthen the solid solution, resulting in wear resistance. improve sexuality. On the other hand, when the value of [%Mo]/[%Cr] exceeds 0.70, the amount of Mo that does not contribute to solid-solution strengthening increases, and brittle eutectic carbide is formed. Abrasion resistance deteriorates. More preferably, 0.40≦[%Mo]/[%Cr]≦0.65.

In the present invention, it is preferable to further contain one or more of Co: 5.0% or less, Ni: 3.0% or less, and W: 5.0% or less.

Co: 5.0% or less Co is an element that dissolves in the matrix and has the effect of improving fatigue resistance. Even if the content exceeds 5.0%, the effect saturates and the effect corresponding to the content cannot be expected, which is economically disadvantageous. Therefore, when Co is contained, it is limited to a range of 5.0% or less. In addition, preferably, it is 3.5% or less. In order to obtain such effects, the content is preferably 0.2% or more, more preferably 0.3% or more.

Ni: 3.0% or less Ni is an element that forms a solid solution in the matrix, lowers the transformation temperature of austenite during heat treatment, and improves the hardenability of the matrix. If the content exceeds 3.0%, the transformation temperature of austenite becomes too low, and austenite tends to remain after heat treatment. When austenite remains, cracks occur during hot rolling, and the hot rolling fatigue resistance is lowered. Therefore, when Ni is contained, it is limited to a range of 3.0% or less. In addition, even if the cooling rate during heat treatment is slow, the martensite and/or bainite structure can be obtained, so that the operation is easy, so the content is preferably 0.2% or more, more preferably 0.3% or more. is.

W: 5.0% or less W is an element that forms a solid solution in the matrix, strengthens the matrix and improves fatigue resistance, and forms M ₂ C or M ₆ C-based carbides, Improve wear resistance. When the content exceeds 5.0%, not only is the effect saturated, but also coarse M ₂ C or M ₆ C carbides are formed, which lowers the hot rolling contact fatigue resistance. Therefore, when W is contained, it is limited to a range of 5.0% or less. In order to obtain such effects, the content is preferably 0.2% or more, more preferably 0.5% or more.

The balance is Fe and unavoidable impurities. In addition, Al, S, P, Cu, Ca, Sb, Ti, Zr, B, etc. are mentioned as an unavoidable impurity. These are mixed from raw materials and refractories during melting. These unavoidable impurities are Al: 0.3% or less, S: 0.05% or less, P: 0.05% or less, Cu: 0.20% or less, Ca: 0.01% or less, Sb: 0 .01% or less, Ti: 0.05% or less, Zr: 0.05% or less, B: 0.008% or less, even if the total amount of these unavoidable impurities is 0.5% or less The total amount should be 0.5% or less because it does not adversely affect wear resistance and thermal fatigue resistance. In addition, more preferably, it is 0.4% or less.

Next, in addition to the above components, the present invention is characterized by having a carbide amount distribution in which the difference between the maximum value and the minimum value is 20% or less of the average value in the region from the outer surface to 30 mm in the radial direction of the roll. .

As described above, the outer layer material for hot rolling rolls manufactured by the centrifugal casting method has structural segregation such as lamination segregation, and the segregation pattern is transferred to the steel sheet surface, degrading the surface quality of the steel sheet. In addition, such structural segregation deteriorates roll properties such as surface roughening resistance and crack resistance. According to the findings of the present inventors, segregation patterns and surface roughening are more likely to occur as the change in the amount of carbide in the radial direction of the roll increases. That is, if there is a large change in the amount of carbide in the radial direction of the roll, a difference in the amount of wear is likely to occur in the axial direction of the roll, and small unevenness is formed on the outer surface of the roll, and this unevenness becomes a segregation pattern or surface roughness that can be seen visually. Become.

The change in the amount of carbide in the radial direction of the roll is measured by measuring the distribution of the amount of _carbide in the _radial direction, as shown in FIG. _max −X _min ) and the average carbide amount X _ave of the entire distribution (=ΔX/X _ave ×100 (%)). This ΔX/X _ave ×100 (%) is defined as the “change rate S of the amount of carbide”. The present inventors investigated the relationship between this difference, the segregation pattern, and the occurrence of rough skin. As a result, when the change rate S of the amount of carbide exceeds 20%, a large segregation pattern and rough surface occur, but when it is 20% or less, no segregation pattern and rough surface occur, and such a distribution condition is in the radial direction of the roll. It has been found that if the region from the outer surface to 30 mm is satisfied, it can be used sufficiently as a roll for rolling. Therefore, in the present invention, the carbide amount distribution represented by the following equation (2) is such that the change rate S of the carbide amount is 20% or less in the region up to 30 mm from the outer surface in the radial direction of the roll.
S=(X _max -X _min )×100/X _ave (2)
here,
X _max : Maximum value (area %) in the carbide amount distribution in the region from the outer surface to 30 mm in the radial direction of the roll
X _min : Minimum value (area %) in the carbide amount distribution in the region from the outer surface to 30 mm in the radial direction of the roll
X _ave : average value (area %) in the carbide amount distribution in the region from the outer surface to 30 mm in the radial direction of the roll
is.

The method for measuring the amount of carbide is not particularly limited, but in the present invention, the cross section of the outer layer material (the cross section perpendicular to the roll axis) is corroded with a nital liquid to reveal the metal structure. The area ratio (%) of carbide was measured by , and this was taken as the amount of carbide. In addition, when obtaining the distribution in the radial direction, the measurement position is a position with a 1 mm pitch in the radial direction from the outer surface, and the value obtained by averaging the values measured at any four points in the circumferential direction at the same position is the carbide at the same position. The relationship between the measurement position and the carbide amount data was graphed as shown in FIG. 1, and the maximum and minimum values were obtained. Moreover, the average value of the distribution was obtained by averaging the entire carbide amount data. The radial measurement pitch is not limited to 1 mm, and can be selected as appropriate. However, if the measurement pitch is too large, the maximum or minimum value cannot be accurately evaluated, so 0.5 to 3 mm. is preferred.

The desired carbide content distribution in the hot rolling roll outer layer material of the present invention is obtained by varying the rotation speed of the mold (centrifugal casting mold) during casting of the molten outer layer material, as described later.

Moreover, in recent years, the required level of the surface quality of the material to be rolled has become more severe. The surface quality level required for hot rolling rolls is even higher, and the optimization of the carbide content distribution is not sufficient. By setting the hardness difference of the outer layer material at the position to 3.0 or less in Shore hardness, the unevenness of the surface of the outer layer material for the hot rolling roll during rolling is significantly reduced, and the surface quality is remarkably excellent. It was found that it can be used as a roll outer layer material. That is, in the present invention, the hardness difference represented by the following formula (3) is to satisfy 3.0 or less.
ΔHS = HS _Xmax - HS _Xmin (3)
here,
HS _Xmax : Shore hardness at the position of the maximum value in the carbide amount distribution in the region from the outer surface to 30 mm in the roll radial direction HS _Xmin : The position of the maximum value in the carbide amount distribution in the region from the outer surface to 30 mm in the roll radial direction is the Shore hardness at

A desired hardness difference in the hot rolling roll outer layer material of the present invention can be obtained by controlling the speed at which the molten outer layer material is supplied to the mold, as will be described later.

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

When casting the hot rolling roll outer layer material, first, a rotating mold whose inner surface is coated with a refractory mainly made of zircon or the like with a thickness of 1 to 5 mm is coated with the above hot rolling roll outer layer material composition. (simply referred to as outer layer material molten metal) is poured so as to have a predetermined thickness, and is centrifugally cast.

In the present invention, a desired amount of carbide can be obtained by varying the rotation speed of the mold (centrifugal casting mold) during casting of the molten outer layer material. In the conventional horizontal or oblique centrifugal casting method in which centrifugal casting is performed at a constant mold rotation speed, shear force acts on the solid-liquid coexisting region due to the influence of gravity (1 G), which inevitably causes coarse segregation such as lamination segregation. Generate. On the other hand, in the present invention, as shown in FIG. 2(a), the rotation speed n of the mold 1 rotating about the axis O during casting is forcibly varied. Specifically, as shown in FIG. 2(b), the centrifugal force (expressed as a multiple of gravity, the same shall apply hereinafter) applied to the outer surface of the cast material (roll outer layer material) changes at 1.0 G/s or more. , the step of increasing or decreasing the number of rotations n of the mold is performed two or more times. As a result, the centrifugal force Gn applied to the casting material can be varied, and the shearing force Gv can be applied to the casting material continuously or intermittently (Gv≠0). Therefore, the shearing action of the solid-liquid coexisting phase and the stirring action of the unsolidified region can be arbitrarily imparted, and therefore the dispersal of the structural segregation and the uniform stirring of the segregated components can be effectively promoted.

The variation pattern of the mold rotation speed is not particularly limited. Any pattern that can apply the shearing force Gv, such as a pattern of continuous variation (FIG. 3(b)) or a pattern of intermittent variation (FIG. 3(c)), may be used. If the speed at which the centrifugal force changes is less than 1.0 G/s, it is difficult to overcome the action of lamination segregation due to gravity. Also, if the number of fluctuations is less than 2, the effect of dispersing the structure segregation and uniform stirring of the segregated components cannot be obtained, so the lower limit is limited to 2. From the viewpoint of obtaining the effect of dispersing the structure segregation and uniforming the stirring of the segregated components, the number of fluctuations is preferably 5 times or more, and the upper limit is preferably 100 times.

Further, in the present invention, a desired difference in hardness can be obtained by setting the rate at which the molten outer layer material is supplied to the mold (=melt supply rate) at 80 to 200 kg/s. As shown in FIG. 4, in centrifugal casting, a molten outer layer material 3 is poured into a rotating mold 1 from a melt supply pipe 2 and solidified while centrifugal force is applied.

In centrifugal casting, since the outer layer material melt 3 is supplied in the same direction as the longitudinal direction of the mold 1, the outer layer material melt 3 when supplied to the mold 1 has a speed in a direction different from the rotation of the mold 1. is doing. Therefore, if the molten metal supply speed V _M is too high, the influence of the flow in the direction different from the rotation direction of the mold 1 becomes stronger, causing local disturbance of solidification (for example, a region where MC carbides are locally abundant is formed). As a result, the proportion of carbides produced locally varies, resulting in a large difference in hardness. If the supply speed of the outer layer material molten metal 3 is 80 to 200 kg/s, such a phenomenon does not occur, and as a result of uniform solidification in the radial direction, the hardness difference between the maximum value and the minimum value of the carbide amount distribution becomes Shore hardness of 3.0 or less is obtained. If the supply speed of the molten outer layer material is lower than 80 kg/s, the temperature of the molten outer layer material decreases before the molten outer layer material is poured into the mold, and a solid phase is generated, resulting in uneven solidification. can not meet the hardness difference. From the viewpoint of uniform solidification, the preferred molten outer layer material feed rate is 90 to 180 kg/s. The feed rate of the molten outer layer material is the difference t between the time when the molten outer layer material starts pouring the total weight W _M (kg) of the molten outer layer material into the mold and the time when the molten outer layer material is completely poured into the mold. It is a value (=W _M /t) divided by (s).

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

In addition, it is preferable to use spheroidal graphite cast iron, worm-like graphite cast iron (CV cast iron), etc., which are excellent in castability and mechanical properties, for the inner layer to be statically cast. Since the outer layer and the inner layer of the centrifugally cast roll are integrally welded, about 1 to 8% of the components of the outer layer material are mixed into the inner layer. When carbide-forming elements such as Cr and V contained in the outer layer material are mixed into the inner layer, the inner layer becomes brittle. Therefore, it is preferable to suppress the mixing ratio of the outer layer components to the inner layer to less than 6%.

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

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

The composite roll for hot rolling of the present invention is preferably subjected to heat treatment after casting. The heat treatment is preferably carried out twice or more by heating to 950 to 1100° C. followed by air cooling or blast air cooling, and further heating and holding at 450 to 570° C. and then cooling. Cooling is preferably performed at an average cooling rate of 5 to 100°C/h.

The hardness of the composite roll for hot rolling of the present invention is preferably 78-90 HS (Shore hardness), more preferably 80-88 HS. If the hardness is lower than 78HS, wear resistance deteriorates, and if the hardness exceeds 90HS, cracks formed on the surface of the hot rolling roll during hot rolling become difficult to remove by grinding. It is preferable to adjust the heat treatment temperature and heat treatment time after casting so that such hardness can be stably secured. The heat treatment time varies depending on the heat treatment temperature, but it is preferable to set the tempering time at 950 to 1100° C. for 30 minutes to 50 hours and for the tempering at 450 to 570° C. for 5 to 50 hours.

A molten metal obtained by melting the materials shown in Table 1 is supplied to a mold of a horizontal centrifugal casting machine under the casting conditions shown in Table 1, and a sleeve roll having an outer diameter of 250 mm, an inner diameter of 170 mm, and a thickness of 70 mm (corresponding to the outer layer material for the rolling roll). was cast. The casting temperature was 1500° C. under all conditions. During casting, the mold rotation speed n was varied in the inventive examples. The following two types of fluctuation patterns were used, and a periodic function n=α+β cos (2π×t/T) [rpm] (t is the time (s) from the start of casting) of period T (s) was adopted.
<Variation pattern 1>
α=1000rpm
β=40rpm
Variation range of n: 1040-960 rpm
T = 10s
Average increase/decrease speed of n: 80/(10/2)=16 rpm/s
<Variation pattern 2>
α=900rpm
β=20rpm
Variation range of n: 920-880 rpm
T = 8s
Average increase/decrease speed of n: 40/(8/2) = 10 rpm/s
At this time, regarding the increase/decrease speed of the centrifugal force on the outer surface of the sleeve roll, in the variation pattern 1, the centrifugal force is 151 G and 129 G at n=1040 rpm and 960 rpm, respectively, so the average is about 4.4 G/s (=(151 −129)/(10/2)). In the variation pattern 2, the centrifugal force is 113 G and 108 G at n=920 rpm and 880 rpm, respectively, so the average is about 1.3 G/s (=(113-108)/(8/2)).

On the other hand, in the comparative example, n is constant (900 rpm) as before, or α = 950 rpm, β = 10 rpm (that is, the fluctuation range of n is 960 to 940 rpm), T = 20 s (that is, the average increase/decrease speed of n is 20/(20 /2)=2 rpm/s) and a periodic function with an average of about 0.5 G/s (=(129−124)/(20/2)).

After casting, quenching from 1000 ° C. and tempering at 500 ° C., a 30 × 20 × 40 mm test piece for measuring the amount of carbide (30 mm is the circumferential direction of the sleeve roll) , 20 mm in the thickness direction of the sleeve roll, and 40 mm in the radial direction of the sleeve roll (see FIG. 5)). After polishing a 30 × 40 mm surface of a test piece for measuring the amount of carbides, the metal structure revealed by etching with nital was observed, and the distribution of the amount of carbides in the area from the outer surface to 30 mm in the radial direction of the cross section was measured as described above. (1 mm pitch in the radial direction and the average value of four test pieces) and arranged in the graph shown in FIG. 6(b) is a diagram showing the distribution of carbide content of No. 5 in Table 1.), and the rate of change of these distributions was obtained. Table 2 shows the results. Table 2 shows the maximum value X _max and minimum value X _min in the carbide amount distribution, the difference between the maximum value and the minimum value, the average value of the entire carbide amount distribution (overall average), and the change rate of the carbide amount. there is

In addition, using four test pieces for measuring the amount of carbide, the Vickers hardness HV50 was measured with a Vickers hardness meter (test force: 50 kgf (490 N)) in accordance with the provisions of JIS Z 2244, and the JIS conversion table was obtained. converted to Shore hardness HS. The measurement positions were the positions showing the maximum and minimum values in the carbide content distribution. A total of 10 arbitrary points were measured for 4 test pieces for measuring the amount of carbide at each measurement position (see Fig. 7), and the average value excluding the highest and lowest measured Shore hardness values was calculated. and the difference in hardness was obtained.

In addition, a sleeve roll having an outer diameter of 250 mm, an inner diameter of 170 mm, and a thickness of 70 mm having the composition shown in Table 1 was manufactured, quenched from 1000°C, and tempered at 500°C. and hot-rolling fatigue tests.

The abrasion test method was as follows. A wear test piece (outer diameter 60 mm, thickness 10 mm, chamfered) was taken from a position 10 mm inside from the outer surface of the obtained sleeve roll (see FIG. 8).
As shown in FIG. 9, the wear test was carried out by a two-disc sliding rolling method using a test piece and a mating member. The test piece 4 is rotated at 700 rpm while cooling with cooling water 5, and the rotating test piece 4 is heated to 800 ° C. with a high-frequency induction heating coil 6 (material: S45C, outer diameter: 190 mm, width: 15 mm, C1 chamfering) 7 was brought into contact with a load of 980 N and rolled at a slip ratio of 9%. The wear test was performed for 300 minutes, and the test was performed by replacing the mating piece with a new one every 50 minutes. Based on the conventional example, the ratio of the amount of wear of each test piece to the reference value was evaluated by the wear ratio (= (amount of wear of each test piece) / (amount of wear of the reference piece)), and the wear ratio was 0.97. It was determined that the wear resistance in the above cases was equal to or higher than that of the conventional example, and that the wear resistance was inferior in the case of less than 0.97.

A hot-rolling fatigue test piece (outer diameter: 60 mm, thickness: 10 mm) was taken from a position within 10 mm from the outer surface of the obtained sleeve roll, and a hot-rolling fatigue test was performed (see FIG. 8). In the fatigue test piece, notches (depth t: 1.2 mm, circumferential length L: 0.8 mm) as shown in FIG. Introduced by electrical discharge machining (wire cutting). Also, the edge of the rolling surface of the fatigue test piece was chamfered by 1.2C. As shown in FIG. 10, the hot-rolling fatigue test was performed by a two-disc rolling-sliding method of a test piece having a notch (hot-rolling fatigue test piece) and a heated mating member. That is, the test piece (hot-rolled fatigue test piece) 8 is cooled with cooling water 5 and rotated at 700 rpm, and the rotating test piece 8 is heated to 800 ° C. by a high-frequency induction heating coil 6 (material: S45C, Outer diameter: 190 mm, width: 15 mm) 7 was pressed with a load of 980 N and rolled at a slip ratio of 9%. The hot-rolling fatigue test piece 8 was rolled until the two notches 9 were broken, and the number of rolling revolutions until each notch was broken was determined, and the average value was taken as the hot-rolling fatigue life. Then, when the hot-rolling fatigue life exceeded 350,000 times, it was evaluated as being remarkably excellent in hot-rolling fatigue life (excellent in fatigue resistance).

In addition, a sleeve roll having an outer diameter of 250 mm, an inner diameter of 170 mm, and a thickness of 70 mm having the composition shown in Table 1 was used as a material, quenched from 1000 ° C., tempered at 500 ° C., and then ground 10 mm in the radial direction from the outer surface. An outer layer material for a roll (outer diameter 230 mm x width 40 mm) having the surface of the outer surface as the outer surface was sampled and shrink-fitted to a shaft material made of carbon steel forged steel to produce a roll for rolling test. This composite roll is installed in a 4Hi hot rolling mill, and a steel plate (width 20 mm, thickness 1.5 mm x length 20 m) having a tensile strength of 590 MPa at room temperature is used as a material to be rolled, and heated to 950 ° C. The material was subjected to hot rolling with a thickness reduction rate of 20%. This hot rolling operation was performed 5 times for each roll, and the surface roughness (arithmetic mean roughness Ra) of the roll after rolling 5 times was measured with a laser displacement meter. With an arbitrary position on the surface of the roll as a reference, the measurement positions were four points in the circumferential direction at intervals of 90° from the reference. Arithmetic average roughness Ra was measured at four points, and the average value was calculated to evaluate the presence or absence of segregation. It was determined that segregation was present when the surface roughness was greater than 2.0 μm, slight segregation was present when the surface roughness was 1.0 to 2.0 μm, and no segregation was present when the surface roughness was less than 1.0 μm.

Here, the overall evaluation is as follows: "X" for segregation, "○" for excellent wear resistance and fatigue resistance and slight segregation, and "◎" for no segregation. Only "double-circle" was made into the pass.

It can be seen that the inventive examples are outer layer materials for centrifugally cast rolling rolls that have wear resistance and fatigue resistance and are free from segregation.

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

1 mold 2 molten metal supply pipe 3 outer layer molten metal 4 test piece (wear test piece)
5 cooling water 6 high frequency induction heating coil 7 mating piece 8 test piece (hot rolling fatigue test piece)
9 notches

Claims (4)

% by mass, C: 1.5 to 3.0%, Si: 0.1 to 2.0%, Mn: 0.1 to 2.0%, Cr: 5.0 to 15.0%, Mo: 2.0 to 12.0%, V: 3.0 to 10.0%, Nb: 0.5 to 5.0%, and the Mo and Cr contents satisfy the following formula (1) , having a composition consisting of the balance Fe and unavoidable impurities,
It has a carbide amount distribution in which the change rate S of the carbide amount represented by the following formula (2) is 20% or less, and further satisfies the hardness difference represented by the following formula (3) of 3.0 or less. Roll outer layer material for hot rolling.
0.35≦[%Mo]/[%Cr]≦0.70 (1)
Here, [%Mo] and [%Cr] are the content (% by mass) of each element.
S=(X _max -X _min )×100/X _ave (2)
here,
X _max : Maximum value (area %) in the carbide amount distribution in the region from the outer surface to 30 mm in the radial direction of the roll
X _min : Minimum value (area %) in the carbide amount distribution in the region from the outer surface to 30 mm in the radial direction of the roll
X _ave : average value (area %) in the carbide amount distribution in the region from the outer surface to 30 mm in the radial direction of the roll
is.
ΔHS = HS _Xmax - HS _Xmin (3)
here,
HS _Xmax : Shore hardness at the position of the maximum value in the carbide amount distribution in the region from the outer surface to 30 mm in the roll radial direction HS _Xmin : The position of the minimum value in the carbide amount distribution in the region from the outer surface to 30 mm in the roll radial direction is the Shore hardness at The hot rolling roll outer layer material contains, by mass %, any one or more of Co: 5.0% or less, Ni: 3.0% or less, and W: 5.0% or less. Item 1. The roll outer layer material for hot rolling according to Item 1. A hot rolling composite roll having a three-layer structure of an outer layer, an intermediate layer, and an inner layer or a two-layer structure of an outer layer and an inner layer, wherein the outer layer comprises the hot rolling roll outer layer material according to claim 1 or 2. A composite roll for hot rolling characterized by: A centrifugal casting mold so that the rate of increase and decrease of centrifugal force on the outer surface of the roll outer layer material is 1.0 G / s or more when pouring the molten metal having the composition according to claim 1 or 2 and performing centrifugal casting. 3. The method for manufacturing the outer layer material for hot rolling rolls according to claim 1 , wherein the number of revolutions of the roller is changed 40 times or more, and the feed rate of the molten metal is set to 80 to 137 kg/s.

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