WO2018047444A1 - Roll outer layer material for hot rolling and composite roll for hot rolling - Google Patents

Abstract

The purpose of the present invention is to provide a roll outer layer material for hot rolling and a composite roll for hot rolling, whereby wear resistance is ensured, pit-shaped flaws on the roll surface are reduced, and excellent surface roughing resistance is obtained. A roll outer layer material characterized by having a composition containing, in terms of mass%, 2.0-3.0% C, 0.2-1.0% Si, 0.2-1.0% Mn, 4.0-7.0% Cr, 3.0-6.5% Mo, 5.0-7.5% V, 0.5-3.0% Nb, 0.05-3.0% Ni, 0.2-5.0% Co, and 0.5-5.0% W, the content of C, Cr, Mo, V, Nb, Ni, and W satisfying expression (1), and the remainder comprising Fe and unavoidable impurities, at least 85% of the matrix structure of the roll outer layer material being a tempered martensite and/or bainite structure, and the minor axis of the tempered martensite or bainite being 0.5-3.0 µm. (1): 0.05 ≤ (%C - %V × 0.177 - %Nb × 0.129 - %Cr × 0.099 - %Mo × 0.063 - %W × 0.033) + (%Ni) ≤ 4.0. In expression (1), the terms %C, %V, %Nb, %Cr, %Mo, %W, and %Ni are the content (mass%) of each element.

Description

Roll outer layer material for hot rolling and composite roll for hot rolling

The present invention relates to a composite roll for hot rolling, and more particularly to a roll outer layer material for hot rolling and a composite roll for hot rolling suitable for use in a hot rolling finish mill of a steel plate.

In recent years, with the progress of hot rolling technology for steel plates, the usage environment of rolls has become severe, and the production volume of steel plates with high rolling load such as high-strength steel plates and thin-walled products has also increased. For this reason, the work roll for rolling often has rough skin and chipping due to fatigue of the rolling surface, and the demand for rough skin resistance and chipping resistance is stronger than ever. At present, high-speed rolls in which a large amount of hard carbide is formed by adding several percent of V in hot rolling to improve wear resistance are frequently used.

As an outer layer material of such a high-speed roll, for example, Patent Document 1 discloses that 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 include Nb, V and C content. A rolling roll outer layer material that satisfies a specific relationship and further contains a ratio of Nb and V within a specific range has been proposed. Thereby, even if the centrifugal casting method is applied, segregation of hard carbides in the outer layer material is suppressed, and the outer layer material of the roll for rolling is excellent in wear resistance and crack resistance. 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%, and Nb and V, the contents of Nb, V and C satisfy a specific relationship, and the ratio of Nb to V is within a specific range A roll outer layer material for rolling containing so as to have been proposed. Thereby, even if the centrifugal casting method is applied, segregation of hard carbide in the outer layer material is suppressed, wear resistance and crack resistance are improved, and it is said that it contributes greatly to productivity improvement of hot rolling.

On the other hand, from the viewpoint of improving the quality and productivity of hot-rolled products, the usage environment of hot-rolling rolls has become severe, and the amount of continuous rolling of steel sheets has increased. Furthermore, the requirements for the surface quality of hot-rolled products are becoming stricter. Therefore, it has become a big subject to suppress fatigue damage on the roll surface such as rough skin rather than wear. For such a problem, Patent Document 3 discloses that C: 2.2 to 2.6%, Cr: 5.0 to 8.0%, Mo: 4.4 to 6.0%, V: 5 Adjust the C, Mo, V, and Nb contents so that Mo + V and C-0.24V-0.13Nb are within the specified range. Thus, there has been proposed a centrifugal cast composite roll that is excellent in fatigue resistance of the roll surface layer in a hot rolling environment. In Patent Document 4, C: 1.3 to 2.2%, Si: 0.3 to 1.2%, Mn: 0.1 to 1.5%, Cr: 2.0 to 9.0 %, Mo: 9.0% or less, V: 4.0 to 15.0%, and W: 20.0% or less, Ni: 5.0% or less, Co: 10.0% or less A roll outer layer material for rolling has been proposed that contains seeds or two or more types, the balance being substantially composed of Fe and inevitable impurities, and the size of carbides dispersed in the structure being within a specific range. In Patent Document 4, it is possible to reduce pit-like wrinkles by reducing the amount of eutectic carbide that is easily formed as coarse carbide.

Japanese Patent Laid-Open No. 04-365836 JP 05-1350 A JP 2009-221573 A Patent No. 3968238

However, the rolling technology in recent years has made remarkable progress toward high quality and high quality rolled steel sheets. At the same time, the rolling environment is becoming increasingly severe because of the rigorous pursuit of cost reduction for rolling. For this reason, in the conventional roll material design focusing only on carbides, the occurrence of pit-like wrinkles has sometimes been reduced.

The present invention has been made in view of the above circumstances, and while ensuring the wear resistance, reducing the pit-like wrinkles on the roll surface, the roll outer layer material for hot rolling and the hot It aims at providing the composite roll for rolling.

The present inventors investigated in detail the location of the occurrence of pit-like wrinkles formed on the surface of the hot rolling roll. As a result, the pit-shaped soot is formed by cracks generated from eutectic carbides (mainly M ₂ C, M ₆ C, M ₇ C ₃ and M ₂₃ C ₆ carbides) propagating through the base structure. It was clarified that the shape was lost. Therefore, in order to reduce pit-like wrinkles, it is considered effective to reduce the propagation speed of cracks propagating through the base structure in addition to focusing on conventional carbide types and sizes. Was completed. In other words, as a result of studying various factors that affect the rolling fatigue resistance and base structure size of the outer layer material of the roll, adjustment of the component range of each element, and each element to satisfy a specific relationship The present inventors have obtained an unprecedented knowledge that fatigue resistance during hot rolling is remarkably improved by adjusting the content. Furthermore, the knowledge that fatigue resistance at the time of hot rolling is remarkably improved by controlling the size of the base structure was also obtained.

First, the experimental results that became the basis of this research will be explained. In mass%, Si: 0.1 to 1.5%, Mn: 0.1 to 1.5%, C: 1.6 to 3.5%, Cr: 3.5 to 9.0%, Mo : 2.1 to 7.0%, V: 4.1 to 8.5%, Nb: 0.3 to 4.6%, Ni: 0.02 to 3.6%, Co: 0.3 to 8 0.0%, W: A ring-shaped roll material corresponding to the roll outer layer material by changing the range of 0.2 to 8.0% and melting the molten metal having the balance Fe and inevitable impurities in a high-frequency induction furnace (Outer diameter: 250 mmφ, width: 65 mm, wall thickness: 55 mm) was cast by a centrifugal casting method. The casting temperature was 1450 ° C. to 1530 ° C., and the centrifugal force was such that the outer peripheral portion of the ring-shaped roll material had a gravity multiple of 180 G. After casting, quenching and tempering were performed, and the hardness was HS 78-86. The quenching treatment was performed by heating to a heating temperature of 1070 ° C. and air cooling. Further, the tempering treatment was performed twice or three times depending on the components at a temperature of 530 to 570 ° C. so that the amount of retained austenite was less than 10% by volume.

A hot-rolled fatigue test piece (outer diameter 60 mmφ, wall thickness 10 mm) was taken from the obtained ring-shaped roll material, and the fatigue resistance of the hot rolling work roll in an actual machine was evaluated with good reproducibility in JP 2010-101752. A hot rolling fatigue test was conducted to show that it was possible. As the fatigue test piece, a notch (depth t: 1.2 mm, circumferential length L: 0.8 mm) as shown in FIG. 1 was used at two locations on the outer peripheral surface, and a 0.2 mmφ wire was used. It was introduced by the electric discharge machining (wire cut) method. Further, the end of the rolling surface of the fatigue test piece was chamfered with 1.2C.

As shown in FIG. 1, the hot-rolling fatigue test was performed by a two-spindle rolling sliding method of a test piece having a notch (hot-rolling fatigue test piece) and a heated counterpart material. That is, as shown in FIG. 1, a test piece (hot rolled fatigue test piece) 1 was rotated at 700 rpm while being cooled with cooling water 2, and the rotating test piece 1 was heated to 800 ° C. by a high frequency induction heating coil 3. While the mating piece (material: S45C, outer diameter: 190 mmφ, width: 15 mm) 4 was pressed with a load of 980 N, it was rolled at a slip ratio of 9%. Rolling was performed until the two notches 5 introduced into the hot-rolled fatigue test piece 1 were broken, and the rolling rotation speed until each notch was broken was determined, and the average value was defined as the hot-rolled fatigue life. And when the hot-rolled fatigue life exceeded 350,000 times, it was evaluated that the hot-rolled fatigue life was remarkably excellent.

The structure of the obtained ring-shaped roll material was observed. For tissue observation, a 10 × 10 × 5 mm (5 mm is the thickness direction of the ring) tissue observation specimen is taken at an arbitrary position within 10 mm from the outer surface of the ring-shaped roll material, and the 10 × 10 mm surface is a mirror surface Polishing was performed, and corrosion was performed with nital (5% by volume nitric acid + ethanol) for about 10 seconds.

Moreover, in order to measure the short diameter (short axis length) of tempered martensite or bainite, a measurement test piece (5 mm × 10 mm × 5 mm) at an arbitrary position within 10 mm from the outer surface of the obtained ring-shaped roll material The surface of 5 mm × 10 mm was mirror-polished and EBSD measurement was performed. The measurement was performed by electron beam backscatter diffraction (EBSD) in an area of 10000 μm ² or more at an acceleration voltage of 15 kV and a step size of 0.1 μm. A boundary line is drawn at a position where the azimuth difference between adjacent measurement points is 15 ° or more, and as shown in FIG. 12, the region surrounded by the boundary line is a single crystal, and the major axis is 5 μm or more on the measurement surface. The minor axis of 20 crystals was measured and the average value was calculated.

Regarding the obtained results, the hot rolling fatigue life and (% C-% V × 0.177-% Nb × 0.129-% Cr × 0.099-% Mo × 0.063-% W × 0.033) FIG. 3 shows the relationship with + (% Ni), and FIG. 4 shows the relationship between the hot-rolled fatigue life and the minor axis of tempered martensite or bainite.

FIG. 3 shows that (% C−% V × 0.177−% Nb × 0.129−% Cr × 0.099−% Mo × 0.063−% W × 0.033) + (% Ni) is 0. It can be seen that when it is 0.05 or more or 4.0 or less, the hot-rolled fatigue life is remarkably improved. Here, V, Cr, Mo, Nb, and W are elements that are easy to produce carbides. (% C-% V × 0.177-% Nb × 0.129-% Cr × 0.099-% Mo × 0 .063-% W × 0.033) represents the amount of carbon dissolved in the matrix. Therefore, (% C-% V × 0.177-% Nb × 0.129-% Cr × 0.099-% Mo × 0.063-% W × 0.033) + (% Ni) It is the sum of the amount of dissolved carbon and the amount of Ni, and by adjusting this value to an appropriate range, a roll outer layer material having a slow crack propagation speed in the base and an excellent hot-rolled fatigue life can be obtained. Further, by satisfying the above component range and controlling the crystal size of the tempered martensite or bainite in the matrix structure within the range shown in FIG. 4, the hot-rolled fatigue life can be remarkably improved.

The present invention has been completed based on the above findings, and the gist thereof is as follows.
[1] By mass%, C: 2.0 to 3.0%, Si: 0.2 to 1.0%, Mn: 0.2 to 1.0%, Cr: 4.0 to 7.0% , Mo: 3.0 to 6.5%, V: 5.0 to 7.5%, Nb: 0.5 to 3.0%, Ni: 0.05 to 3.0%, Co: 0.2 -5.0%, W: 0.5-5.0%, and the contents of C, Cr, Mo, V, Nb, Ni, W satisfy the following formula (1), and the balance Fe and It has a composition consisting of inevitable impurities, and 85% or more of the base structure is a tempered martensite and / or bainite structure, and the minor axis of the tempered martensite or bainite is 0.5 to 3.0 μm. Roll outer layer material for hot rolling.
0.05 ≦ (% C−% V × 0.177−% Nb × 0.129−% Cr × 0.099−% Mo × 0.063−% W × 0.033) + (% Ni) ≦ 4 .0 (1)
Here,% C,% V,% Nb,% Cr,% Mo,% W, and% Ni are the contents (mass%) of each element.
[2] A hot-rolling composite roll in which an outer layer and an inner layer are welded and integrated, wherein the outer layer is made of the hot-rolling roll outer layer material described in [1]. roll.

According to the present invention, it is possible to manufacture a hot rolling roll outer layer material and a hot rolling composite roll in which the propagation speed of cracks is significantly reduced. As a result, surface damage due to hot rolling such as rough skin and missing edges can be suppressed, and there is an effect that continuous rolling distance can be extended and roll life can be improved.

Fig. 1 shows the configuration of the testing machine used in the hot rolling fatigue test, the outer periphery of the hot rolling fatigue test specimen (fatigue test specimen), and the hot rolling fatigue test specimen (fatigue specimen) It is explanatory drawing which shows typically the shape and dimension of the notch introduced into the surface. FIG. 2 is a diagram showing a result of measuring the outer layer material for hot rolling according to the embodiment of the present invention by EBSD. FIG. 3 shows the hot-rolled fatigue life in the hot-rolled fatigue test and (% C-% V × 0.177-% Nb × 0.129-% Cr × 0.099-% Mo × 0.063-% W × 0 .033) + (% Ni). FIG. 4 is a diagram showing the relationship between the hot-rolled fatigue life in the hot-rolled fatigue test and the minor axis of tempered martensite or bainite.

The roll outer layer material for hot rolling of the present invention is manufactured by a known casting method such as a centrifugal casting method or a continuous casting overlaying method, and can be used as it is as a ring roll or a sleeve roll, but for hot finish rolling. It is preferably used as an outer layer material of a composite roll for hot rolling. The composite roll for hot rolling according to the present invention includes an outer layer and an inner layer welded and integrated with the outer layer. An intermediate layer may be disposed between the outer layer and the inner layer. That is, instead of 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 the inner layer is spheroidal graphite cast iron (ductile iron) or forged steel, and the intermediate layer is a high carbon material of C: 1.5 to 3.0% by mass. Is preferred.

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

C: 2.0 to 3.0%
C forms a solid solution to increase the base hardness and combines with a carbide forming element to form a hard carbide, thereby improving the wear resistance of the roll outer layer material. The amount of eutectic carbide varies depending on the C content. Eutectic carbides affect the rolling service characteristics. For this reason, if the C content is less than 2.0%, the amount of eutectic carbide is insufficient, the frictional force during rolling increases, rolling becomes unstable, and the amount of C dissolved in the base structure is low. Reduces rolling fatigue resistance during heat resistance. On the other hand, the content exceeding 3.0% excessively increases the coarsening of carbide and the amount of eutectic carbide, hardens and embrittles the roll outer layer material, promotes the generation and growth of fatigue cracks, and fatigue resistance. Reduce. For this reason, C is limited to a range of 2.0 to 3.0%. Preferably, the content is 2.1 to 2.8%.

Si: 0.2 to 1.0%
Si is an element that acts as a deoxidizer and improves the castability of the molten metal. In order to obtain such an effect, a content of 0.2% or more is required. On the other hand, if the content exceeds 1.0%, the effect is saturated and an effect commensurate with the content cannot be expected, which is economically disadvantageous, and further, the base structure may become brittle. For this reason, Si is limited to 0.2 to 1.0%. Preferably, the content is 0.3 to 0.7%.

Mn: 0.2 to 1.0%
Mn is an element having the effect of fixing S as MnS and detoxifying S, and partly dissolving in the base structure and improving the hardenability. In order to obtain such an effect, a content of 0.2% or more is required. On the other hand, even if the content exceeds 1.0%, the effect is saturated and an effect commensurate with the content cannot be expected, and further, the material may become brittle. For this reason, Mn is limited to 0.2 to 1.0%. Preferably, the content is 0.3 to 0.8%.

Cr: 4.0 to 7.0%
Cr combines with C to mainly form eutectic carbides, improves wear resistance, reduces friction with the steel sheet during rolling, reduces roll surface damage, and stabilizes rolling. It is an element having In order to obtain such an effect, the content of 4.0% or more is required. On the other hand, if the content exceeds 7.0%, coarse eutectic carbides increase, and fatigue resistance is lowered. For this reason, Cr is limited to the range of 4.0 to 7.0%. Preferably, the content is 4.3 to 6.5%.

Mo: 3.0 to 6.5%
Mo is an element that combines with C to form a hard carbide and improves wear resistance. In addition, Mo dissolves in hard MC-type carbides in which V, Nb, and C are bonded, strengthens the carbides, and also dissolves in eutectic carbides to increase the fracture resistance of these carbides. Through such an action, Mo improves the wear resistance and fatigue resistance of the outer roll layer material. In order to obtain such an effect, the content of 3.0% or more is required. On the other hand, if the content exceeds 6.5%, Mo-based hard and brittle carbides are produced, which reduces the rolling fatigue resistance during heat resistance and lowers the fatigue resistance. For this reason, Mo is limited to the range of 3.0 to 6.5%. Preferably, the content is 3.5 to 6.0%.

V: 5.0-7.5%
V is an important element in the present invention in order to combine wear resistance and fatigue resistance as a roll. V forms extremely hard carbide (MC type carbide), improves wear resistance, and effectively acts to sever and disperse eutectic carbide, thereby improving rolling fatigue resistance during heat resistance. Further, it is an element that remarkably improves fatigue resistance as a roll outer layer material. Such an effect becomes remarkable when the content is 5.0% or more. On the other hand, if the content exceeds 7.5%, the MC type carbides are coarsened, so that various properties of the rolling roll are made unstable. For this reason, V is limited to the range of 5.0 to 7.5%. Preferably, the content is 5.2 to 7.0%.

Nb: 0.5 to 3.0%
Nb improves the wear resistance, particularly fatigue resistance, through the action of strengthening the MC type carbide by solid solution in the MC type carbide and increasing the fracture resistance of the MC type carbide. When both Nb and Mo are dissolved in the carbide, the improvement in wear resistance and further fatigue resistance becomes significant. Moreover, Nb is an element which has the effect | action which promotes the division | segmentation of a eutectic carbide and suppresses destruction of a eutectic carbide, and improves the fatigue resistance of a roll outer layer material. Nb also has the effect of suppressing segregation during centrifugal casting of MC type carbide. Such an effect becomes remarkable when the content is 0.5% or more. On the other hand, if the content exceeds 3.0%, the growth of MC type carbide in the molten metal is promoted, and the rolling fatigue resistance during heat resistance is deteriorated. Therefore, Nb is limited to a range of 0.5 to 3.0%. Preferably, the content is 0.8 to 1.5%.

Ni: 0.05-3.0%
Ni is an element that dissolves in the matrix, lowers the transformation temperature of austenite during heat treatment, and improves the hardenability of the matrix. In order to acquire such an effect, 0.05% or more of content is required. On the other hand, if the content exceeds 3.0%, the transformation temperature of austenite becomes too low and the hardenability is improved, so that austenite tends to remain after heat treatment. When austenite remains, the rolling fatigue resistance during hot rolling is reduced, for example, cracks are generated during hot rolling. Therefore, Ni is limited to the range of 0.05 to 3.0%. Even if the cooling rate during the heat treatment is slow, it is preferably 0.2 to 3.0% from the viewpoint of ease of operation that the crystal size of the matrix structure can be made fine.

Co: 0.2-5.0%
Co is an element that has a function of solid-dissolving in the matrix, strengthening the matrix, particularly at high temperatures, and improving fatigue resistance. In order to obtain such an effect, a content of 0.2% or more is required. On the other hand, even if the content exceeds 5.0%, the effect is saturated and an effect commensurate with the content cannot be expected, which is economically disadvantageous. Therefore, Co is limited to the range of 0.2 to 5.0%. Preferably, it is 0.5 to 3.0%.

W: 0.5-5.0%
W is an element having a function of solid-dissolving in the matrix, strengthening the matrix at a high temperature and improving fatigue resistance, and forming M ₂ C or M ₆ C-based carbides, and wear resistance. To improve. In order to acquire such an effect, 0.5% or more of content is required. On the other hand, if the content exceeds 5.0%, not only the effect is saturated, but also coarse M ₂ C or M ₆ C-based carbides are formed, and the rolling fatigue resistance during heat resistance is reduced. For this reason, W is limited to a range of 0.5 to 5.0%. Preferably, it is 1.0 to 3.5%.

In the present invention, C, Cr, Mo, V, Nb, Ni, and W are contained in the above-described range, and further adjusted and contained so as to satisfy the following expression (1).
0.05 ≦ (% C−% V × 0.177−% Nb × 0.129−% Cr × 0.099−% Mo × 0.063−% W × 0.033) + (% Ni) ≦ 4 .0 (1)
Here,% C,% V,% Nb,% Cr,% Mo,% W, and% Ni are the contents (mass%) of each element.

(% C−% V × 0.177−% Nb × 0.129−% Cr × 0.099−% Mo × 0.063−% W × 0.033) + (% Ni) above (1) By adjusting so as to satisfy the formula, the number of breakage rolling is remarkably increased, and the rolling fatigue resistance during heat resistance is remarkably improved. (% C-% V x 0.177-% Nb x 0.129-% Cr x 0.099-% Mo x 0.063-% W x 0.033) + (% Ni) is rolling during heat resistance It is an important factor that becomes a driving force for improving fatigue resistance, and if it falls outside the range of the above formula (1), the rolling fatigue resistance during heat resistance deteriorates. V, Cr, Mo, Nb, and W are elements that are easy to produce carbides. (% C-% V × 0.177-% Nb × 0.129-% Cr × 0.099-% Mo × 0.063- % W × 0.033) represents the amount of carbon dissolved in the base. Therefore, (% C-% V × 0.177-% Nb × 0.129-% Cr × 0.099-% Mo × 0.063-% W × 0.033) + (% Ni) It is the sum of the amount of dissolved carbon and the amount of Ni, and by adjusting this value to an appropriate range, a roll outer layer material having a slow crack propagation speed in the base and an excellent hot-rolled fatigue life can be obtained. Therefore, in the present invention, (% C−% V × 0.177−% Nb × 0.129−% Cr × 0.099−% Mo × 0.063−% W × 0.033) + (% Ni) Is adjusted to satisfy the above expression (1).

The balance other than the above components is composed of Fe and inevitable impurities.

In the present invention, 85% or more of the base structure is tempered martensite and / or bainite structure, and it is preferable that the minor axis of tempered martensite or bainite is 0.5 to 3.0 μm. When the fraction of retained austenite and pearlite structure is large, the rolling fatigue resistance during heat resistance decreases, so that it is preferable that the base structure contains 85% or more of tempered martensite and / or bainite structure. It is more preferable that it is contained 90% or more from the viewpoint of property. In addition, as a remainder, a retained austenite and / or pearlite are mentioned. In order to make 85% or more of the matrix structure tempered martensite and / or bainite, it may be controlled by repeating the cooling process after heating and holding at 500 to 570 ° C.

In addition, in a component system in which the minor axis of tempered martensite or bainite is smaller than 0.5 μm, the transformation temperature becomes too low, and it becomes difficult to reduce the amount of retained austenite even if tempering is repeated. Cracking during hot rolling due to austenite may occur, and the hot rolling fatigue resistance is reduced. On the other hand, if the minor axis of tempered martensite or bainite exceeds 3.0 μm, the crack propagation rate of the base structure is high, and the rolling fatigue resistance during heat resistance decreases. Therefore, it is preferable to limit the minor axis of tempered martensite or bainite of the base structure to a range of 0.5 to 3.0 μm. From the viewpoint of rolling fatigue resistance during heat resistance, the thickness is preferably in the range of 0.5 to 2.0 μm. In order to obtain this minor axis, the components and the cooling rate may be controlled so that the transformation temperature of the matrix is in the range of 200 to 400 ° C.

Next, a preferred method for producing the composite roll for hot rolling according to the present invention will be described.

In the present invention, the roll outer layer material is preferably produced by a known centrifugal casting method or a casting method such as a continuous casting overlay method. Needless to say, the present invention is not limited to these methods.

When casting a roll outer layer material by the centrifugal casting method, first, a molten mold having the above-described roll outer layer material composition is applied to a rotating mold whose inner surface is coated with a refractory material mainly composed of zircon or the like in a thickness of 1 to 5 mm. It is poured and centrifugally cast so as to have a predetermined thickness. Here, the number of rotations of the mold is preferably in the range of 120 to 220 G in the gravity multiple applied to the outer surface of the roll. When the intermediate layer is formed, it is preferable that the outer layer layer material is solidified during solidification or completely solidified, and then the molten metal having the intermediate layer composition is poured and centrifugally cast while rotating the mold. After the outer layer or the intermediate layer is completely solidified, it is preferable to stop the mold rotation and stand the mold, and then statically cast the inner layer material to form a composite roll. Thereby, the inner surface side of the roll outer layer material is redissolved to form a composite roll in which the outer layer and the inner layer, or the outer layer and the intermediate layer, and 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. excellent in castability and mechanical properties for the inner layer to be statically cast. In the centrifugal casting roll, the outer layer and the inner layer are integrally welded, so that the component of the outer layer material is mixed in the inner layer by about 1 to 8%. When carbide forming elements such as Cr and V contained in the outer layer material are mixed into the inner layer, the inner layer is weakened. For this reason, it is preferable to suppress the mixing rate of the outer layer component into the inner layer to less than 6%.

Further, when forming the intermediate layer, it is preferable to use graphite steel, high carbon steel, hypoeutectic cast iron or the like as the intermediate layer material. The intermediate layer and the outer layer are integrally welded in the same manner, and the outer layer component is mixed in the intermediate layer in the 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.

The composite roll for hot rolling of the present invention is preferably subjected to heat treatment after casting. In the heat treatment, it is preferable to perform the step of heating to 950 to 1100 ° C. by air cooling or blast air cooling, and further the step of cooling after heating and holding at 500 to 570 ° C. is performed twice or more. At this time, by adjusting the cooling rate in accordance with the components so that the transformation temperature is in the range of 200 to 400 ° C., it is possible to obtain the above-mentioned preferred minor axis size. In addition, since the amount of tempered martensite and / or bainite in the base structure changes depending on the number of repetitions of the cooling step after heating and holding at 500 to 570 ° C., 85% or more of the base structure is tempered martensite and / or Alternatively, the number of repetitions may be set so as to be bainite. The preferred hardness of the composite roll for hot rolling of the present invention is 79 to 88 HS (Shore hardness), and more preferred is 80 to 86 HS. If the hardness is lower than 80 HS, the wear resistance is deteriorated. Conversely, if the hardness exceeds 86 HS, cracks formed on the surface of the hot rolling roll during hot rolling are 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 melt of the roll outer layer material composition shown in Table 1 was melted in a high-frequency induction furnace and made into a ring-shaped test material (ring roll; outer diameter: 250 mmφ, width: 65 mm, wall thickness: 55 mm) by centrifugal casting. The casting temperature was 1450 to 1530 ° C., and the centrifugal force was such that the outer peripheral portion of the ring-shaped roll material was 180 G in the multiple of gravity. After casting, the quenching temperature is reheated to 1070 ° C., air-cooled, and quenching quenching and tempering are performed at a temperature of 530 to 570 ° C. so that the residual austenite amount is less than 10% by volume. Performed 2 or 3 times depending on the ingredients and the hardness was adjusted to 78-86 HS. From the obtained ring-shaped test material, a hardness test piece, a hot rolling fatigue test piece, and a test piece for EBSD measurement were collected, and a hardness test, a hot rolling fatigue test, and a structure observation test were performed.

About the obtained hardness test piece, Vickers hardness HV50 is measured with a Vickers hardness tester (test force: 50 kgf (490 N)) in accordance with the provisions of JIS Z 2244, and Shore hardness HS is calculated with a JIS conversion table. Converted. The measurement points were 10 points each, the maximum value and the minimum value were deleted, the average value was calculated, and the hardness of the test material.

The hot rolling fatigue test method was as follows. A hot rolling fatigue test piece (outer diameter 60 mmφ, wall thickness 10 mm, chamfered) was collected from the obtained ring-shaped test material. In the hot rolling fatigue test piece, notches (depth t: 1.2 mm, circumferential length L: 0.8 mm) as shown in FIG. 1 are provided at two locations on the outer peripheral surface (positions 180 ° apart). And introduced by an electric discharge machining (wire cut) method using a 0.20 mmφ wire. As shown in FIG. 1, the hot rolling fatigue test was performed by a two-disk sliding rolling method between a test piece and a counterpart material. The test piece 1 was rotated at 700 rpm while being cooled with cooling water 2, and the rotating test piece 1 was heated to 800 ° C. with the high frequency induction heating coil 3 (material: S45C, outer diameter: 190 mmφ, width: 15 mm). , C1 chamfer) 4 was brought into contact with a load of 980 N while rolling at a slip ratio of 9%. And it rolled until the two notches 5 introduced into the hot rolling fatigue test piece 1 broke, each rolling rotation speed until each notch broke was calculated | required, and the average value was made into the hot rolling fatigue life. . And when the hot-rolled fatigue life exceeded 350,000 times, it was evaluated that the hot-rolled fatigue life was remarkably excellent.

For tissue observation, a 10 × 10 × 5 mm (5 mm is the thickness direction of the ring) tissue observation specimen is taken at an arbitrary position within 10 mm from the outer surface of the ring-shaped roll material, and the 10 × 10 mm surface is a mirror surface Polishing was performed, and corrosion was performed with nital (5% by volume nitric acid + ethanol) for about 10 seconds.

The short diameter (short axis length) of tempered martensite or bainite is obtained by taking an EBSD measurement test piece (5 mm × 10 mm × 5 mm) from an arbitrary position within 10 mm from the outer surface of the obtained ring-shaped roll material, A surface of 5 mm × 10 mm was mirror-polished and determined by EBSD measurement. EBSD measurement was performed in an area of 10000 μm ² or more at an acceleration voltage of 15 kV and a step size of 0.1 μm. Using the obtained data, as shown in FIG. 2, a boundary line is drawn at a position where the azimuth difference between adjacent measurement points is 15 ° or more, and the region surrounded by the boundary line is taken as one crystal, and the measurement surface Above, the minor axis of 20 crystals whose major axis is 10 μm or more was measured, and the average value was calculated.

Table 2 shows the results obtained.

In the examples of the present invention, the hot-rolled fatigue life was remarkably increased, and an excellent hot-rolled fatigue life exceeding 350,000 times was exhibited. As a result of the structure observation, it was confirmed that 85% or more of the base structure was tempered martensite and / or bainite structure.

Therefore, according to the present invention, it is possible to manufacture a composite roll for hot rolling in which the propagation speed of cracks is significantly reduced. As a result, surface damage due to hot rolling such as rough skin and missing edges can be suppressed, so that the effect of extending the continuous rolling distance and improving the roll life can also be obtained.

1 Test piece (hot rolled fatigue test piece)
2 Cooling water 3 High frequency induction heating coil 4 Opposite piece 5 Notch

Claims (2)

% By mass, C: 2.0 to 3.0%,
Si: 0.2 to 1.0%,
Mn: 0.2 to 1.0%,
Cr: 4.0 to 7.0%,
Mo: 3.0 to 6.5%,
V: 5.0-7.5%,
Nb: 0.5 to 3.0%,
Ni: 0.05 to 3.0%,
Co: 0.2-5.0%
W: 0.5-5.0%
And the content of C, Cr, Mo, V, Nb, Ni, W satisfies the following formula (1), has a composition consisting of the remainder Fe and inevitable impurities, and is 85% or more of the base structure Is a tempered martensite and / or bainite structure, the tempered martensite or bainite having a minor axis of 0.5 to 3.0 μm.
0.05 ≦ (% C−% V × 0.177−% Nb × 0.129−% Cr × 0.099−% Mo × 0.063−% W × 0.033) + (% Ni) ≦ 4 .0 (1)
Here,% C,% V,% Nb,% Cr,% Mo,% W, and% Ni are the contents (mass%) of each element. A composite roll for hot rolling in which an outer layer and an inner layer are welded and integrated, wherein the outer layer is made of the outer layer material for hot rolling according to claim 1.

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