JP2018161655A - Roll outer layer material for hot rolling and compound roll for hot rolling - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a roll outer layer material for hot rolling which has an excellent rough surface resistance in such a manner that an abrasion resistance is assured and a crack depth of a roll surface and pit-like defects are reduced, and a compound roll for hot rolling.SOLUTION: A roll outer layer material for hot rolling which is characterized in that a bearing force of compression of 0.2% at 600°C is 1250 MPa or more and 1480 MPa or less.SELECTED DRAWING: Figure 3

Description

  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 finishing mill of a steel sheet.

  In recent years, the use environment of rolls has become severe with the progress of hot rolling technology of steel plates, and the production amount 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 .3 to 7.0%, Nb: 0.6 to 1.3%, C, Mo, V, and Nb contents are adjusted so that Mo + V and C-0.24V-0.13Nb are within a specific 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. Moreover, the roll outer layer material for rolling described in Patent Document 5 is C: 1.0 to 2.6%, Cr: 4.0 to 10.0%, Mo: 5.0 to 10.0%, W : 0 to 5.0%, V: 3.0 to 8.0%, 12.0% ≦ 2Mo + W ≦ 20.0%, 2Mo / W ≧ 3.0, 0.2% ≦ C-0 For rolling, characterized by satisfying .24V ≦ 0.7%, the hardness of the roll outer layer surface being a Shore hardness of 87 or more, and the base hardness of the outer layer surface at 600 ° C. being a Vickers hardness of 350 or more A role has been proposed. By increasing the base hardness at 600 ° C., it is possible to provide a rolling roll having excellent wear resistance and crack resistance.

Japanese Patent Laid-Open No. 04-365836 JP 05-1350 A JP 2009-221573 A Patent No. 3968238 JP 2004-114049 A

  However, in recent years, rolling technology 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, conventional roll material designs that focus only on carbides sometimes fail to reduce the depth of cracks formed on the roll surface and reduce the occurrence of pit-like wrinkles. In addition, macroscale phenomena such as wear resistance and rough skin resistance that occur on the surface of the roll for rolling may not match the mechanical properties of the microscopic area such as hardness, and both wear resistance and rough skin resistance are achieved. There was a problem that the outer layer material for hot rolling could not be manufactured.

  The present invention has been made in view of the above circumstances, and is intended for hot rolling that ensures wear resistance, reduces crack depth on the roll surface, reduces pit-like wrinkles, and has excellent skin roughness resistance. An object is to provide a roll outer layer material and a composite roll for hot 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, the Cr content that forms M ₇ C ₃ and M ₂₃ C ₆ carbides is suppressed, and V and Nb that form MC carbides, or Mo that forms M ₂ C and M ₆ C carbides. It has been clarified that the rough skin resistance can be improved by increasing the content of W and W and adjusting the content of these alloy elements to the optimum conditions. In addition, wear on the surface of the roll for hot rolling preferentially wears a soft base compared to carbide, so it was considered effective to strengthen the base to improve wear resistance. . As a result of intensive studies by the present inventors, it is possible to dramatically improve the wear resistance by setting the 0.2% proof stress at 600 ° C. of the roll outer layer material to a specific value or more. Was revealed. The present invention has been completed with the above findings.

  First, the experimental results that became the basis of this study will be explained. In mass%, Si: 0.1-1.2%, Mn: 0.1-1.3%, C: 1.7-3.0%, Cr: 3.5-7.5%, Mo : 2.8 to 7.5%, V: 4.1 to 8.0%, Nb: 0.2 to 4.0%, Ni: 0.01 to 3.4%, Co: 0 to 5.0 %, W: Changed in the range of 0 to 3.6%, the molten metal having the remaining Fe and inevitable impurities was melted in a high frequency induction furnace, and a ring-shaped roll material corresponding to the roll outer layer material (outer diameter: 250 mmφ, wall thickness: 55 mm) was cast by centrifugal casting. The casting temperature was 1350 ° C. to 1510 ° C., and the centrifugal force was 170 G as a multiple of gravity. After casting, quenching treatment and tempering treatment were performed, and the hardness was adjusted to 80 to 86 HS. The quenching treatment was performed by heating to a heating temperature of 930 to 1100 ° C., holding for 10 to 35 hours, 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) is taken from the obtained ring-shaped roll material, and the fatigue resistance of the hot rolling work roll in an actual machine is 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 slide method between 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.

  FIG. 2 shows the relationship between the hot-rolled fatigue life and (% V +% Nb +% Mo +% W) /% Cr for the obtained results. FIG. 2 shows that the hot-rolled fatigue life is remarkably improved when (% V +% Nb +% Mo +% W) /% Cr is in the range of 2.50% to 5.00%. These elements are elements that make it easy to form carbides. By adjusting the content of these elements to an appropriate range, the outer layer material for hot rolling rolls, which has a slow crack propagation speed and excellent hot-rolling fatigue life, can be obtained. can get.

  Separately from the above thermal fatigue test piece, a hot wear test piece (outer diameter 60 mmφ, wall thickness 10 mm) was sampled and a hot wear test was carried out using the same equipment used in the thermal fatigue test. did. The hot wear test was performed by a two-spin rolling and sliding method of the hot wear test piece 1 and the mating piece 4. The hot wear test piece 1 is rotated at 700 rpm while being cooled with water, and the rotating test piece 1 is pressed with a mating piece (material: S45C, outer diameter: 190 mmφ, width: 15 mm) 4 with a load of 980 N. While applying, sliding was performed at a sliding rate of 9% for 120 minutes. After the test, the amount of wear of the hot wear test piece 1 was measured. Using the wear amount of the conventional example as a reference value, the ratio of the wear amount of each test piece to the reference value was evaluated by the wear ratio (= (wear amount of each test piece) / (wear amount of the reference piece)). The relationship between the wear ratio and the compression 0.2% yield strength at 600 ° C. is shown in FIG. It can be seen that when the compression 0.2% proof stress at 600 ° C. is 1250 MPa or more, the wear ratio is remarkably reduced and the wear resistance is improved. When the compression 0.2% yield strength is 1250 MPa or more, it is considered that the plastic deformation of the roll surface is suppressed and the wear resistance is improved.

The present invention has been completed based on the above findings, and the gist thereof is as follows.
[1] A roll outer layer material for hot rolling characterized by having a compression 0.2% proof stress at 1600 MPa of 1250 MPa or more and 1480 MPa or less.
[2] The roll outer layer material for hot rolling is in mass% as component composition, Si: 0.2 to 1.0%, Mn: 0.2 to 1.0%, Ni: 0.05 to 3 The roll outer layer material for hot rolling as set forth in [1], comprising 0.000% and Co: 0.2 to 3.0%.
[3] The roll outer layer material for hot rolling further has a component composition of mass%, C: 1.9 to 2.8%, Cr: 4.0 to 6.5%, Mo: 3.0 to 6.5%, V: 4.5 to 7.5%, Nb: 0.5 to 3.0%, W: 0.5 to 3.0%,
And the contents of V, Nb, Mo, W, Cr satisfy the following formula (1),
The roll outer layer material for hot rolling as set forth in [1] or [2], comprising the balance Fe and inevitable impurities.
2.50 ≦ (% V +% Nb +% Mo +% W) /% Cr ≦ 5.00 (1)
Here,% V,% Nb,% Mo,% W, and% Cr are the contents (mass%) of each element.
[4] A hot-rolling composite roll in which an outer layer and an inner layer are integrated by welding, wherein the outer layer is made of the hot-rolling roll outer layer material according to any one of [1] to [3]. A composite roll for hot rolling.

  ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to manufacture the roll outer layer material for hot rolling and the composite roll for hot rolling with which the propagation speed of the crack was reduced significantly. 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 graph showing the relationship between the hot-rolled fatigue life and (% V +% Nb +% Mo +% W) /% Cr) in the hot-rolled fatigue test. FIG. 3 is a diagram showing the relationship between the wear ratio in the hot wear test and the compression 0.2% yield strength at 600 ° C. FIG. 4 shows the results of observing the structure of the outer layer material for hot rolling according to the embodiment of the present invention.

  The roll outer layer material 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 is suitable for hot finish rolling. It is applied 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 cast iron) or forged steel, and the intermediate layer is a high carbon material of C: 1.5 to 3.0% by mass. Is preferred.

  In this invention, it is set as the roll outer layer material whose compression 0.2% yield strength in 600 degreeC is 1250 Mpa or more and 1480 Mpa or less. Improving the compression 0.2% proof stress at 600 ° C. is effective for improving the wear resistance, for the following reason. In hot rolling of steel, a work roll for hot finish rolling comes into contact with a steel plate heated to about 900 ° C. to 1100 ° C., and the temperature of the work roll surface rises instantaneously to about 600 ° C. Then, since the work roll surface is cooled to around 100 ° C. by being cooled with cooling water, the work roll is subjected to a heat cycle of heating and cooling. In general, since the strength (hardness) of a material decreases at a high temperature, it was considered that increasing the strength at 600 ° C. of the roll outer layer material is effective in improving the wear resistance, and the present invention has been reached.

  When the compression 0.2% yield strength at 600 ° C. is 1250 MPa or more, the wear resistance can be remarkably improved. Conventionally, in order to develop a roll outer layer material excellent in wear resistance, material development for improving the hardness has been carried out, but as a result of intensive studies by the present inventors, the wear resistance is improved only by increasing the hardness. Has been found to be inadequate, and improving the compression 0.2% proof stress is effective for improving the wear resistance. The hardness of a general hot rolling roll outer layer material is around 80 HS in Shore hardness. This corresponds to a Vickers hardness of around 700 HV, and the indentation size at this time is around 51 μm at a test force of 9.8 N (= 1 kgf). FIG. 4 shows an example of a structure photograph of the roll outer layer material. The diamond-shaped region in FIG. 4 corresponds to an indentation size of 51 μm. As can be seen in FIG. 4, the roll outer layer material for hot rolling has a non-uniform carbide size and distribution, and the indentation size is smaller than the non-uniform carbide size and distribution. May not represent the overall properties of the material. Therefore, paying attention to the compression 0.2% proof stress representing the characteristics of the whole material rather than the hardness, it has been found that improving the compression 0.2% proof stress is effective in improving the wear resistance, leading to the present invention. It was. Even if the compression 0.2% proof stress at 600 ° C. is 1480 MPa or more, not only the effect is saturated, but a large amount of alloying element is required, which is disadvantageous in terms of cost. Therefore, the upper limit is 1480 MPa or less. More preferably, it is 1250-1450 MPa.

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

  In the present invention, the component composition is, by mass, Si: 0.2 to 1.0%, Mn: 0.2 to 1.0%, Ni: 0.05 to 3.00%, Co: 0.2 It is preferable to contain -3.0%. The hot rolling roll outer layer material has a microstructure in which carbides are dispersed in the base, and the base is greatly softened at a high temperature, so that the strength of the entire material is lowered. Therefore, we thought that strengthening the base was effective in improving the strength of the entire material. Si, Mn, Ni, and Co are elements that do not form carbides and dissolve in the matrix, distort the crystal lattice by the solid solution, and improve the strength of the matrix. As a result, the compression 0.2% yield strength at 600 ° C. can be controlled to 1250 MPa to 1480 MPa.

Si: 0.2 to 1.0%
Si is an element that acts as a deoxidizer and improves the castability of the molten metal. Si is also an element that improves the strength of bainite or tempered martensite by dissolving in the matrix. 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%. In addition, Preferably, it is 0.3 to 0.8%.

Mn: 0.2 to 1.0%
Mn is an element that has the effect of fixing S as MnS and detoxifying S, and partly dissolves in the matrix and improves the hardenability and strength. 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%. In addition, Preferably, it is 0.3 to 0.8%.

Ni: 0.05 to 3.00%
Ni is an element that dissolves in the matrix 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 3.00%, 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 strength decreases, and the rolling fatigue resistance during hot rolling decreases, for example, cracks occur during hot rolling. Therefore, Ni is limited to a range of 0.05 to 3.00%. If Ni is contained in an amount of 0.2 to 3.00%, a hardened structure can be easily obtained even if the cooling rate during the heat treatment is low, and thus the content is preferably 0.2 to 3.00%.

Co: 0.2-3.0%
Co is an element having a function of being dissolved in the matrix and strengthening the matrix particularly at a high temperature to improve the high temperature strength and 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 3.0%, the effect is saturated, and an effect commensurate with the content cannot be expected, which is economically disadvantageous. For this reason, Co is limited to the range of 0.2 to 3.0%. In addition, Preferably, it is 0.5 to 2.5%.

  In the present invention, as a component composition, C: 1.9 to 2.8%, Cr: 4.0 to 6.5%, Mo: 3.0 to 6.5%, V: 4.% by mass. It is preferable to contain 5 to 7.5%, Nb: 0.5 to 3.0%, and W: 0.5 to 3.0%.

C: 1.9 to 2.8%
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. Since eutectic carbide affects the rolling use characteristics, if the C content is less than 1.9%, the amount of eutectic carbide is insufficient, the frictional force during rolling increases, and rolling becomes unstable. On the other hand, the content exceeding 2.8% excessively increases the coarsening of carbides and the amount of eutectic carbides, hardens and embrittles the outer layer material of the roll, promotes the generation and growth of fatigue cracks, and fatigue resistance. Reduce. For this reason, C is limited to a range of 1.9 to 2.8%. In addition, Preferably, it is 2.2 to 2.7%.

Cr: 4.0 to 6.5%
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 6.5%, coarse M ₇ C ₃ -based or M ₂₃ C ₆ -based eutectic carbides increase, so that fatigue resistance decreases. For this reason, Cr is limited to the range of 4.0 to 6.5%. In addition, Preferably, it is 4.5 to 6.0%.

Mo: 3.0-6.5%
Mo is an element that combines with C to form a hard carbide and improves wear resistance. Mo dissolves in hard MC-type carbides in which V, Nb, and C are bonded, strengthens the carbides, and also dissolves in the 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%. In addition, Preferably it is 4.0 to 6.0%.

V: 4.5-7.5%
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 4.5% or more. However, when the content exceeds 7.5%, the MC type carbide is coarsened, and thus various characteristics of the rolling roll are unstable. For this reason, V is limited to the range of 4.5 to 7.5%. In addition, Preferably, it is 5.0 to 7.2%.

Nb: 0.5-3.0%
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 MC carbide formation temperature rises to promote the growth of MC type carbides in the molten metal and deteriorate the rolling fatigue resistance during heat resistance. For this reason, Nb is limited to 0.5 to 3.0% of range. In addition, Preferably, it is 0.8 to 2.5%.

W: 0.5-3.0%
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, when the content exceeds 3.0%, 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 the range of 0.5 to 3.0%. In addition, Preferably, it is 1.0 to 2.5%.

In this invention, it is preferable to contain V, Nb, Mo, W, Cr in the above-mentioned range, and to adjust and contain so that the following (1) Formula may be satisfied.
2.50 ≦ (% V +% Nb +% Mo +% W) /% Cr ≦ 5.00 (1)
Here,% V,% Nb,% Mo,% W, and% Cr are the contents (mass%) of each element.

By satisfying the above formula (1), it is possible to reduce eutectic carbides (mainly M ₇ C ₃ and M ₂₃ C ₆ carbides) that are liable to generate / propagate cracks, and remarkably improve the rough skin resistance. It becomes possible.

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

  In the present invention, 85% or more of the base structure is preferably tempered martensite and / or bainite structure. When pearlite is increased, the crack resistance is inferior, and when retained austenite is increased, transformation may occur during rolling use, and cracks may be generated due to transformation expansion. Therefore, 85% or more of the base structure is tempered martensite and / or A bainite structure is preferable, and 90% or more is more preferable from the viewpoint of hot rolling fatigue resistance. In addition, as a remainder, a retained austenite and / or pearlite are mentioned.

  Below, the preferable manufacturing method of the composite roll for hot rolling of this invention is demonstrated. In addition to limiting the composition of the roll outer layer material (outer layer) to the above range, by adjusting the casting temperature, the heating temperature during quenching, and the holding time at the quenching temperature to the following ranges, the compression at 600 ° C. A roll outer layer material for hot rolling having a 2% proof stress of 1250 MPa to 1480 MPa can be obtained.

Casting temperature: liquidus temperature or higher, liquidus temperature + 140 ° C. or lower Casting temperature (casting temperature) is in the range of liquidus temperature or higher and liquidus temperature + 140 ° C. or lower. When the casting temperature is higher than the liquidus temperature + 140 ° C., the time required for solidification becomes longer. As a result, the alloy elements are easily segregated, and a coarse martensite and / or bainite structure is formed in the base portion having a relatively small amount of alloy elements, thereby reducing the strength. Further, when the casting temperature is lower than the liquidus temperature, solidification proceeds rapidly. As a result, a casting defect such as a dent is likely to be formed, and the quality deteriorates. Therefore, it is necessary to limit the casting temperature to the range of the liquidus temperature or higher and the liquidus temperature + 140 ° C. In view of ease of operation, the liquidus temperature is preferably + 20 ° C. or higher and the liquidus temperature + 120 ° C. or lower. The liquidus temperature may be measured, for example, by DSC (Differential Scanning Calorimeter), the endothermic or exothermic peak rising temperature (liquidus temperature) when cooled from the liquid phase. When the roll outer layer material is subjected to DSC measurement, a plurality of peaks are obtained. The peak for measuring the rising temperature (liquidus temperature) is a peak obtained at the time of solidification of austenite.

Heating temperature during quenching After casting, quenching is performed. The heating temperature (quenching temperature) at this time is set to a range of 980 ° C. or higher and 1100 ° C. or lower. When the quenching temperature is lower than 980 ° C., segregation formed during solidification cannot be reduced, and a coarse martensite and / or bainite structure is formed in the base portion having a relatively small amount of alloy elements, and the strength is increased. descend. On the other hand, when the quenching temperature is higher than 1100 ° C., carbides are excessively dissolved and austenite grains become coarse, so that coarse martensite and / or bainite structure is formed in the base portion, and the strength is lowered. Therefore, it is necessary to limit to the range of 980 ° C. or higher and 1100 ° C. or lower. In order to reliably eliminate segregation regardless of the components, the temperature is preferably 1000 ° C. or higher and 1100 ° C. or lower.

Holding time at quenching temperature The holding time at the quenching temperature is in the range of 16 hr to 35 hr. When the holding time is shorter than 16 hours, the segregation formed during solidification is not sufficiently eliminated, and a coarse martensite and / or bainite structure is formed in the base portion having a relatively small amount of alloy elements, resulting in a decrease in strength. . On the other hand, if the holding time is longer than 35 hours, the roll surface is significantly oxidized and the yield is deteriorated. Therefore, the holding time at the quenching temperature needs to be limited to a range of 16 hr or more and 35 hr or less. In order to surely eliminate segregation regardless of the component, it is preferably 18 hr or more, and preferably 25 hr or less from an economic viewpoint.

  The production method of the roll outer layer material other than the above is not particularly limited, and is preferably produced by a known casting method such as a centrifugal casting method or a continuous casting overlay method. Needless to say, the present invention is not limited to these methods.

  For example, when casting a roll outer layer material by a 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. Is poured so as to have a predetermined thickness, and centrifugal casting is performed at the above casting temperature. Here, the number of rotations of the mold is preferably in the range of 120 to 220G 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 fused and integrated.

  In addition, it is preferable to use spheroidal graphite cast iron excellent in castability and mechanical properties, worm-like graphite cast iron (CV cast iron), etc. for the inner layer to be statically cast. Since the outer layer and the inner layer are integrally welded to the centrifugal casting roll, 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%.

  Moreover, when forming an intermediate | middle layer, it is preferable to use graphite steel, high carbon steel, hypoeutectic cast iron, etc. as an intermediate | middle 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 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, wall thickness: 55 mm) by centrifugal casting. The casting temperature was 1350-1510 ° C., and the centrifugal force was 170 G as a multiple of gravity. After casting, the quenching temperature was reheated to 930 to 1100 ° C., air-cooled, and quenching was performed. The holding time was 10 to 35 hours. After the quenching treatment, the tempering treatment is carried out 2 or 3 times depending on the components so that the amount of retained austenite is less than 10% by volume at a tempering temperature: 530 to 570 ° C., and the hardness is 80 to 80%. Adjusted to 86HS. In addition, about the prior art example, casting temperature was 1500 degreeC and the centrifugal force was 170G by the multiple of gravity. After casting, a quenching treatment was performed by reheating to a quenching temperature of 1000 ° C. and quenching by air cooling. The holding time was 15 hours. After the quenching treatment, a tempering treatment was carried out twice at a tempering temperature of 530 ° C. for production. A compression test piece, a hardness test piece, a hot rolling fatigue test piece and a hot wear test piece are collected from the obtained ring-shaped test material, and subjected to a compression test, a hardness test, a hot rolling fatigue test, and A hot wear test was performed.

  The compression test method was as follows. A compression test piece (φ4 mm, 6 mm height) is collected from the obtained ring-shaped test material, and a crosshead speed of 0.09 mm / min, 0 until 0.2% proof stress is obtained using an Instron universal testing machine. In a strain region larger than 2% proof stress, a compression test was performed under a test condition of 0.8 mm / min. The test temperature was 600 ° C., and the test was started after the sample was heated to 600 ° C. and held for 5 minutes using a thermostatic chamber installed in a universal testing machine. After the test, a 0.2% offset stress was determined from the obtained stress-strain curve. Each sample was tested twice, and the average 0.2% offset stress was taken as the compression 0.2% yield strength of the sample.

  The hardness test was as follows. 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 rolling sliding 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.

  The hot wear test was as follows. A hot wear test piece (outer diameter 60 mmφ, wall thickness 10 mm) was collected, and a hot wear test was performed using the same apparatus as used in the thermal fatigue test. The hot wear test was conducted by a two-spin rolling and sliding method between a hot wear test piece and a counterpart material. While rotating the hot wear test piece at 700 rpm while cooling with water, pressing the mating piece (material: S45C, outer diameter: 190 mmφ, width: 15 mm) heated to 800 ° C. with a load of 980 N on the rotating test piece, Sliding rate: Rolled at 9% for 120 minutes. After the test, the amount of wear of the test piece is measured, and the ratio of the wear amount of each test piece with respect to the reference value is defined as the wear ratio (= (wear amount of each test piece) / (wear amount of the reference piece) based on the conventional example. )).

  The obtained results are shown in Table 2.

  In the example of the present invention, the hot-rolled fatigue life was remarkably increased, and an excellent hot-rolled fatigue life exceeding 350,000 times was shown. Moreover, it turns out that the abrasion ratio is remarkably reduced and the abrasion resistance is improved in any of the inventive examples in which the compression 0.2% proof stress at 600 ° C. is 1250 MPa or more and 1480 MPa or less. When the compression 0.2% yield strength at 600 ° C. is 1250 MPa or more, it is considered that the plastic deformation of the roll surface during hot rolling is suppressed and the wear resistance is improved.

  Therefore, according to the present invention, it is possible to produce a composite roll for hot rolling that is excellent in wear resistance and has a significantly reduced crack propagation speed. As a result, surface damage due to hot rolling such as rough skin and missing edges can be suppressed. Moreover, the effect that extension of a continuous rolling distance and improvement of a roll life can be achieved is also acquired.

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

Claims (4)

  A roll outer layer material for hot rolling characterized by having a compression 0.2% yield strength at 600 ° C. of 1250 MPa to 1480 MPa.   The roll outer layer material for hot rolling is, as a component composition, mass%, Si: 0.2 to 1.0%, Mn: 0.2 to 1.0%, Ni: 0.05 to 3.00%. Co: 0.2-3.0% is contained, The roll outer-layer material for hot rolling of Claim 1 characterized by the above-mentioned. The roll outer layer material for hot rolling further has a component composition of mass%, C: 1.9 to 2.8%, Cr: 4.0 to 6.5%, Mo: 3.0 to 6.5. %, V: 4.5-7.5%, Nb: 0.5-3.0%, W: 0.5-3.0%,
And the contents of V, Nb, Mo, W, Cr satisfy the following formula (1),
The outer layer material for rolls for hot rolling according to claim 1 or 2, comprising remaining Fe and inevitable impurities.
2.50 ≦ (% V +% Nb +% Mo +% W) /% Cr ≦ 5.00 (1)
Here,% V,% Nb,% Mo,% W, and% Cr are the contents (mass%) of each element.   A hot-rolling composite roll in which an outer layer and an inner layer are integrated by welding, wherein the outer layer is made of the outer layer material for hot rolling according to any one of claims 1 to 3. Composite roll.