JP6669109B2 - Roll outer layer material for hot rolling and composite roll for hot rolling - Google Patents

Description

  The present invention relates to a composite roll for hot rolling, and more particularly to an outer layer material for a hot rolling roll and a composite roll for hot rolling that are suitable for a hot rolling finishing mill for a steel sheet.

  In recent years, the use environment of rolls has become severer with the progress of hot rolling technology of steel sheets, and the production of steel sheets with a large rolling load, such as high-strength steel sheets and thin-walled products, has also increased. For this reason, the work rolls for rolling often have surface roughness and missing defects due to fatigue of the rolling surface, and the demand for the surface roughness resistance and the chipping resistance is stronger than ever. At present, in hot rolling, a high-speed roll in which hard carbides are formed in a large amount by adding V in an amount of several percent to improve wear resistance is often used.

  As an outer layer material of such a high-speed roll, for example, in Patent Document 1, 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 the content of Nb and V is Nb, V and C. A roll outer layer material for rolling has been proposed which satisfies a specific relationship and further contains Nb and V in a specific range. By this, even if the centrifugal casting method is applied, segregation of hard carbide in the outer layer material is suppressed, and a roll outer layer material for rolling excellent in wear resistance and crack resistance is described. Patent Document 2 discloses that C: 1.5 to 3.5%, Cr: 5.5 to 12.0%, Mo: 2.0 to 8.0%, and V: 3.0 to 10.0. %, Nb: 0.5 to 7.0%, and Nb and V, when 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 has been proposed to be contained as follows. By this, even if the centrifugal casting method is applied, segregation of the hard carbide in the outer layer material is suppressed, wear resistance and crack resistance are improved, and it contributes greatly to improvement in hot rolling productivity.

  On the other hand, from the viewpoint of improving the quality and productivity of hot-rolled products, the use environment of hot-rolling rolls is becoming severer, and the continuous rolling amount of steel sheets is increasing. In addition, the demands on the surface quality of hot rolled products have become strict. Therefore, it has become a big problem to suppress fatigue damage on the roll surface such as rough skin rather than wear. With respect to 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%, and V: 5 0.3 to 7.0%, Nb: 0.6 to 1.3%, and the contents of C, Mo, V, and Nb are adjusted so that Mo + V and C−0.24V−0.13Nb fall within a specific range. In addition, a centrifugally cast composite roll has been proposed that has excellent fatigue resistance of the roll surface layer in a hot rolling environment. Patent Document 4 discloses that C: 1.3 to 2.2%, Si: 0.3 to 1.2%, Mn: 0.1 to 1.5%, and 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. An outer layer material for a rolling roll has been proposed which contains one or more species and the balance substantially consists of Fe and unavoidable impurities, and in which the size of carbide dispersed in the structure falls within a specific range. In Patent Document 4, it is possible to reduce pit-like flaws by reducing the amount of eutectic carbide which is easily formed as coarse carbide. Further, the roll outer layer material for rolling described in Patent Document 5 has 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 .24 V ≦ 0.7%, the hardness of the outer surface of the roll is not less than 87 Shore hardness, and the base hardness of the outer layer surface at 600 ° C. is not less than 350 Vickers hardness. Role has been proposed. By increasing the base hardness at 600 ° C., it is possible to provide a rolling roll excellent in wear resistance and crack resistance.

JP-A-04-36536 JP-A-05-1350 JP 2009-221573 A Patent No. 3962838 JP 2004-114049 A

  However, recent rolling technology has made remarkable progress toward higher quality and higher grade of rolled steel sheet. At the same time, the use environment of the rolls is becoming increasingly severe because the cost reduction for rolling is strictly pursued. For this reason, in the conventional design of the roll material focusing only on carbides, there have been cases where the depth of cracks formed on the roll surface and the occurrence of pit-like flaws cannot be reduced. In addition, macro-scale phenomena occurring on the roll surface for rolling such as abrasion resistance and rough surface resistance may not match the mechanical properties of minute regions such as hardness in some cases, and both abrasion resistance and rough surface resistance have been achieved. There has been a problem that the roll outer layer material for hot rolling cannot be manufactured.

  The present invention has been made in view of the above circumstances, and while ensuring abrasion resistance, reduces the crack depth on the roll surface and reduces pit-like flaws, and is used for hot rolling excellent in surface roughening resistance. It is an object to provide a roll outer layer material and a composite roll for hot rolling.

The present inventors have investigated in detail the locations of pit-shaped flaws formed on the surface of a hot rolling roll. As a result, the pit-shaped flaws are formed by cracks generated from eutectic carbides (mainly M ₂ C-based, M ₆ C-based, M ₇ C ₃ -based, and M ₂₃ C _6- based carbides) propagated through the base structure. It was revealed that the shape was missing.

Therefore, the content of Cr 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 it is possible to improve the surface roughness resistance by increasing the contents of W and W and setting the contents of these alloying elements to optimal conditions. In addition, the wear of the surface of the hot rolling roll is considered to be effective in strengthening the base in order to improve the wear resistance, since the soft base is preferentially worn as compared with the carbide. . As a result of extensive studies by the present inventors, it has been found that the wear resistance can be drastically improved by setting the 0.2% proof stress of the outer roll material at 600 ° C. to a specific value or more. Revealed. The present invention has been completed based on the above findings.

  First, the experimental results on which this study was based will be described. By mass%, Si: 0.1 to 1.2%, Mn: 0.1 to 1.3%, C: 1.7 to 3.0%, Cr: 3.5 to 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: 0 to 3.6%, and a molten metal having a composition consisting of the balance of Fe and unavoidable impurities is melted in a high-frequency induction furnace, and a ring-shaped roll material (outer diameter: (250 mmφ, wall thickness: 55 mm) was cast by a centrifugal casting method. The casting temperature was 1350 ° C. to 1510 ° C., and the centrifugal force was 170 G as a multiple of gravity. After casting, quenching and tempering were performed to adjust the hardness to 80 to 86 HS. Note that the quenching treatment was a treatment of heating to a heating temperature of 930 to 1100 ° C., holding for 10 to 35 hours, and air cooling. The tempering treatment was performed two or three times at a temperature of 530 to 570 ° C. depending on the components so that the amount of retained austenite was less than 10% by volume.

  A hot-rolled fatigue test specimen (outer diameter 60 mmφ, wall thickness 10 mm) was sampled from the obtained ring-shaped roll material, and the fatigue resistance of a hot-rolling work roll in an actual machine was evaluated with good reproducibility in Japanese Patent Application Laid-Open No. 2010-101752. A hot-rolling fatigue test showing that it was possible was performed. In addition, a notch (depth t: 1.2 mm, circumferential length L: 0.8 mm) as shown in FIG. It was introduced by the electric discharge machining (wire cut) method. The end of the rolling surface of the fatigue test piece was chamfered at 1.2C.

  As shown in FIG. 1, the hot-rolling fatigue test was performed by a rolling and sliding method of a two-disc 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. The opposing piece (material: S45C, outer diameter: 190 mmφ, width: 15 mm) 4 was pressed at a load of 980 N and rolled at a slip ratio of 9%. Rolling was performed until the two notches 5 introduced into the hot-rolling fatigue test piece 1 were broken, the rolling speeds until each notch was broken were determined, and the average value was defined as the hot-rolling fatigue life. When the hot rolling fatigue life exceeded 350,000 times, it was evaluated that the hot rolling fatigue life was remarkably excellent.

  FIG. 2 shows the relationship between the hot rolling fatigue life and (% V +% Nb +% Mo +% W) /% Cr for the obtained results. From FIG. 2, it can be seen that when (% V +% Nb +% Mo +% W) /% Cr is in the range from 2.50% to 5.00%, the hot-rolling fatigue life is significantly improved. These elements are elements that easily form carbides, and by adjusting the content of these elements to an appropriate range, the outer layer material for hot rolling rolls with a low crack propagation speed and excellent hot rolling fatigue life can be obtained. can get.

  Separately from the above-mentioned thermal fatigue test pieces, hot wear test pieces (outer diameter 60 mmφ, wall thickness 10 mm) were sampled and subjected to a hot wear test using the same apparatus as that used in the thermal fatigue test. did. The hot abrasion test was carried out by a rolling and sliding method of a two-disc disk including the hot abrasion test piece 1 and the counterpart piece 4. The hot abrasion test piece 1 is rotated at 700 rpm while cooling with water, and a mating piece (material: S45C, outer diameter: 190 mmφ, width: 15 mm) 4 heated to 800 ° C. is pressed against the rotating test piece 1 with a load of 980 N. Rolling was performed for 120 minutes at a slip ratio of 9% while applying. After the test, the wear amount of the hot abrasion 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)). FIG. 3 shows the relationship between the wear ratio and the 0.2% proof stress at 600 ° C. When the 0.2% proof stress at 600 ° C. becomes 1250 MPa or more, the wear ratio is significantly reduced, and it can be seen that the wear resistance is improved. When the 0.2% proof stress of the compression becomes 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 is as follows.
[1] A roll outer layer material for hot rolling, having a 0.2% proof stress at 600 ° C of 1250 MPa to 1480 MPa.
[2] The roll outer layer material for hot rolling has, as a component composition, Si: 0.2 to 1.0%, Mn: 0.2 to 1.0%, Ni: 0.05 to 3 by mass%. The roll outer layer material for hot rolling according to [1], comprising 0.000% and Co: 0.2 to 3.0%.
[3] The roll outer layer material for hot rolling further has, as a component composition, C: 1.9 to 2.8%, Cr: 4.0 to 6.5%, and Mo: 3.0 to 3.0% by mass. 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, and Cr satisfy the following expression (1);
The roll outer layer material for hot rolling as described in [1] or [2], comprising the balance of Fe and unavoidable impurities.
2.50 ≦ (% V +% Nb +% Mo +% W) /% Cr ≦ 5.00 (1)
Here,% V,% Nb,% Mo,% W, and% Cr are the contents (% by mass) of each element.
[4] A composite roll for hot rolling obtained by welding and integrating an outer layer and an inner layer, wherein the outer layer is made of the outer layer material of the roll for hot rolling described in any one of [1] to [3]. 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 in which the propagation speed of cracks is significantly reduced. As a result, surface damage due to hot rolling such as roughening or chipping can be suppressed, and there is an effect that a continuous rolling distance can be extended and a roll life can be improved.

FIG. 1 shows the configuration of the testing machine used in the hot rolling fatigue test, the hot rolling fatigue test specimen (fatigue test specimen), and the outer periphery of the hot rolling fatigue test specimen (fatigue test 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 relationship between hot rolling fatigue life and (% V +% Nb +% Mo +% W) /% Cr in a hot rolling fatigue test. FIG. 3 is a diagram showing a relationship between a wear ratio in a hot wear test and a 0.2% proof stress at 600 ° C. FIG. 4 is a result of observing the structure of the outer layer material of the roll 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 centrifugal casting method or a continuous casting overlaying method or the like, and can be used as it is as a ring roll or a sleeve roll. It is applied as an outer layer material of a composite roll for cold rolling. 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. Note that an intermediate layer may be provided 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 a 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 mass%. Is preferred.

  In the present invention, a roll outer layer material having a compression 0.2% proof stress at 600 ° C. of 1250 MPa to 1480 MPa is used. Improving the 0.2% proof stress at 600 ° C. is effective for improving the wear resistance, for the following reasons. In hot rolling of steel, a work roll for hot finish rolling comes into contact with a steel sheet heated to about 900 ° C. to 1100 ° C., and the temperature of the work roll surface instantaneously rises to about 600 ° C. Thereafter, the work roll surface is cooled down to around 100 ° C. by cooling with cooling water, so that the work roll undergoes a heat cycle of heating and cooling. In general, the strength (hardness) of a material decreases at a high temperature. Therefore, it was considered that increasing the strength of the outer layer material of the roll at 600 ° C. was effective for improving the wear resistance, and the present invention was reached.

  When the compression 0.2% proof stress at 600 ° C. is 1250 MPa or more, the wear resistance can be significantly improved. Conventionally, in order to develop a roll outer layer material with excellent wear resistance, materials have been developed to increase the hardness.However, as a result of intensive studies by the present inventors, improvement of the wear resistance was achieved only by increasing the hardness. Is insufficient, and it has been found that improving the 0.2% proof stress is effective for improving the wear resistance. The hardness of a general hot roll outer layer material is about 80 HS in Shore hardness. This corresponds to a Vickers hardness of about 700 HV, and the indentation size at this time is about 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 area in FIG. 4 corresponds to an indentation size of 51 μm. As can be seen in FIG. 4, the outer layer material of the hot rolling roll has a non-uniform carbide size and distribution, a smaller indentation size than the non-uniform carbide size and distribution, and a hardness measurement result. May not represent the properties of the entire material. Therefore, focusing on the 0.2% proof stress, which represents the properties of the entire material, rather than the hardness, it was found that improving the 0.2% proof stress is effective in improving the wear resistance, and led to the present invention. Was. Even if the 0.2% proof stress at 600 ° C. is 1480 MPa or more, the effect is not only saturated, but also a large amount of alloying elements must be added, which is disadvantageous in cost. Therefore, the upper limit is set to 1480 MPa or less. More preferably, it is 1250-1450 MPa.

  Next, the reasons 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. Hereinafter, mass% is simply described as% unless otherwise specified.

  In the present invention, as a component composition, in 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 about 3.0%. The outer layer material of the roll for hot rolling has a microstructure in which carbides are dispersed in the matrix, and when the temperature is high, the matrix is greatly softened, so that the strength of the entire material is reduced. Therefore, it was considered that strengthening the base was effective in improving the strength of the entire material. Si, Mn, Ni, and Co do not form carbides and are elements that form a solid solution in the matrix. The solid solution distorts the crystal lattice and improves the strength of the matrix. As a result, it is possible to control the 0.2% proof stress at 600 ° C. from 1250 MPa to 1480 MPa.

Si: 0.2 to 1.0%
Si is an element that acts as a deoxidizing agent and improves the castability of the molten metal. Si is also an element that forms a solid solution in the matrix to improve the strength of bainite or tempered martensite. In order to obtain such an effect, the content needs to be 0.2% or more. 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 may further embrittle the base structure. For this reason, Si is limited to 0.2 to 1.0%. In addition, preferably, it is 0.3 to 0.8%.

Mn: 0.2-1.0%
Mn is an element that has the effect of fixing S as MnS and rendering S harmless, and has the effect of partially dissolving in the matrix to improve quenchability and strength. In order to obtain such an effect, the content needs to be 0.2% or more. On the other hand, 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 be embrittled. 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 forms a solid solution in the matrix and improves the hardenability of the matrix. In order to obtain such an effect, the content needs to be 0.05% or more. 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. If the austenite remains, the strength is reduced and the hot rolling fatigue resistance is reduced, for example, cracks are generated during hot rolling. Therefore, Ni is limited to the range of 0.05 to 3.00%. When Ni is contained in an amount of 0.2 to 3.00%, a quenched structure is easily obtained even when the cooling rate during the heat treatment is low. Therefore, the content is preferably 0.2 to 3.00%.

Co: 0.2-3.0%
Co is an element that forms a solid solution in the matrix, strengthens the matrix particularly at high temperatures, and has an effect of improving high-temperature strength and fatigue resistance. In order to obtain such an effect, the content needs to be 0.2% or more. On the other hand, if the content exceeds 3.0%, the effect saturates, 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%, and V: 4. 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 has a function of increasing the hardness of the base by forming a solid solution and combining with the carbide forming element to form a hard carbide, thereby improving the wear resistance of the outer layer material of the roll. The eutectic carbide content changes according to the C content. Since the eutectic carbide affects the rolling use characteristics, if the C content is less than 1.9%, the amount of the eutectic carbide is insufficient, the frictional force at the time of rolling increases, and the rolling becomes unstable. On the other hand, a 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 improves fatigue resistance. Lower. For this reason, C is limited to the range of 1.9 to 2.8%. In addition, preferably, it is 2.2 to 2.7%.

Cr: 4.0 to 6.5%
Cr forms a eutectic carbide mainly by combining with C to improve wear resistance, reduce frictional force with a steel plate during rolling, reduce roll surface damage, and stabilize rolling. Is an element having In order to obtain such an effect, a content of 4.0% or more is required. On the other hand, when the content exceeds 6.5%, coarse M ₇ C ₃ or M ₂₃ C ₆ eutectic carbides increase, so that fatigue resistance is reduced. For this reason, Cr is limited to the range of 4.0 to 6.5%. In addition, Preferably, it is 4.5-6.0%.

Mo: 3.0 to 6.5%
Mo is an element that combines with C to form a hard carbide and improves wear resistance. Further, Mo forms a solid solution in a hard MC-type carbide in which V or Nb and C are combined to strengthen the carbide, and also forms a solid solution in a eutectic carbide, thereby increasing the fracture resistance of the carbide. Mo improves the wear resistance and fatigue resistance of the roll outer layer material through such an action. In order to obtain such an effect, the content needs to be 3.0% or more. On the other hand, when the content exceeds 6.5%, a hard and brittle carbide mainly composed of Mo is formed, and the rolling fatigue resistance during heat resistance is reduced, and the fatigue resistance is reduced. For this reason, Mo is limited to the range of 3.0 to 6.5%. In addition, Preferably, it is 4.0-6.0%.

V: 4.5 to 7.5%
V is an important element in the present invention in order to have both abrasion resistance and fatigue resistance as a roll. V forms an extremely hard carbide (MC type carbide), improves wear resistance, effectively acts to separate and disperse crystallize eutectic carbide, and improves heat-resistant rolling fatigue resistance. Is an element that remarkably improves fatigue resistance as a roll outer layer material. Such effects become remarkable when the content is 4.5% or more, but when the content exceeds 7.5%, the MC-type carbides are coarsened, so that various characteristics of the rolling roll become unstable. For this reason, V is limited to the range of 4.5 to 7.5%. In addition, Preferably, it is 5.0-7.2%.

Nb: 0.5 to 3.0%
Nb solid-dissolves in the 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 the MC-type carbide. When both Nb and Mo are solid-solved in the carbide, the wear resistance and further the fatigue resistance are significantly improved. Further, Nb is an element that promotes the division of the eutectic carbide, suppresses the destruction of the eutectic carbide, and improves the fatigue resistance of the roll outer layer material. Nb also has an action of suppressing segregation during centrifugal casting of MC type carbide. Such effects become remarkable when the content is 0.5% or more. On the other hand, if the content exceeds 3.0%, the growth temperature of MC carbides is increased to promote the growth of MC-type carbides in the molten metal, thereby deteriorating the hot rolling fatigue resistance. For this reason, Nb is limited to the range of 0.5 to 3.0%. In addition, preferably, it is 0.8 to 2.5%.

W: 0.5-3.0%
W is an element that forms a solid solution in the matrix, strengthens the matrix particularly at high temperatures, and has an effect of improving fatigue resistance, and forms an M ₂ C or M ₆ C-based carbide to form a wear resistant material. Improve. In order to obtain such an effect, the content needs to be 0.5% or more. 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 hot rolling fatigue resistance is reduced. For this reason, W is limited to the range of 0.5 to 3.0%. In addition, Preferably, it is 1.0-2.5%.

In the present invention, it is preferable that V, Nb, Mo, W, and Cr are contained in the above-described range, and further adjusted so as to satisfy the following formula (1).
2.50 ≦ (% V +% Nb +% Mo +% W) /% Cr ≦ 5.00 (1)
Here,% V,% Nb,% Mo,% W, and% Cr are the contents (% by mass) of each element.

By satisfying the above expression (1), eutectic carbides (mainly, M ₇ C ₃ and M ₂₃ C ₆ based carbides) that easily generate and propagate cracks can be reduced, and the surface roughness resistance is significantly improved. It becomes possible.

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

  Further, in the present invention, it is preferable that 85% or more of the base structure is a tempered martensite and / or bainite structure. When pearlite increases, crack resistance is inferior, and when retained austenite increases, transformation occurs during rolling use, and cracks may occur due to transformation expansion. Therefore, 85% or more of the base structure is tempered martensite and / or It is preferably a bainite structure, and more preferably 90% or more from the viewpoint of hot rolling fatigue resistance. In addition, a residual austenite and / or a pearlite are mentioned as a balance.

  Next, a preferred method for producing the composite roll for hot rolling of the present invention will be described. In addition to limiting the composition of the roll outer layer material (outer layer) to the above range, the casting temperature, the heating temperature during quenching, and the holding time at the quenching temperature are adjusted to the following ranges, so that 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: not less than liquidus temperature, not more than liquidus temperature + 140 ° C Casting temperature (casting temperature) is in the range not less than liquidus temperature and not more than liquidus temperature + 140 ° C. If the casting temperature is higher than the liquidus temperature + 140 ° C., the time required for solidification becomes longer. As a result, the alloy element is likely to segregate, and coarse martensite and / or bainite structure is formed in the base portion having a relatively small amount of the alloy element, thereby reducing the strength. Further, when the casting temperature is lower than the liquidus temperature, solidification proceeds rapidly, so that casting defects such as zigzag cavities are easily formed, and the quality is deteriorated. Therefore, it is necessary to limit the casting temperature to the range of the liquidus temperature or higher and the liquidus temperature + 140 ° C. Note that, from the viewpoint of easiness of operation, the liquidus temperature is preferably set to + 20 ° C. or higher and liquidus temperature + 120 ° C. or lower. The liquidus temperature may be measured by, for example, a DSC (differential scanning calorimeter), which measures the rising temperature (liquidus temperature) of an endothermic or exothermic peak when cooled from a liquid state. A plurality of peaks are obtained by DSC measurement of the outer layer material of the roll, but a peak for measuring a rising temperature (liquidus temperature) is a peak obtained during austenite solidification.

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

Holding time at quenching temperature The holding time at the quenching temperature is in the range of 16 hr or more and 35 hr or less. If the holding time is shorter than 16 hours, segregation formed at the time of solidification is not sufficiently eliminated, and a coarse martensite and / or bainite structure is formed in a base having a relatively small amount of alloying elements, and the strength is reduced. . On the other hand, when the holding time is longer than 35 hours, oxidation of the roll surface becomes remarkable, and the yield is deteriorated. Therefore, it is necessary to limit the holding time at the quenching temperature to a range of 16 hr or more and 35 hr or less. In order to surely eliminate the segregation irrespective of the components, it is preferably 18 hours or more, and from an economic viewpoint, it is preferably 25 hours or less.

  The method of manufacturing the outer layer material of the roll other than the above is not particularly limited, and it is preferable that the outer layer material be manufactured by a known centrifugal casting method or a continuous casting overlaying 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 metal of the above-described roll outer layer material composition is applied to a rotating mold whose inner surface is coated with a refractory 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 centrifugally cast at the above-mentioned casting temperature. Here, it is preferable that the number of rotations of the mold is such that the gravity multiple applied to the outer surface of the roll is in the range of 120 to 220G. In the case of forming the intermediate layer, it is preferable that during the solidification of the outer layer material of the roll or after it is completely solidified, the molten metal having the composition of the intermediate layer is poured while the mold is rotated and centrifugally cast. After the outer layer or the intermediate layer is completely solidified, it is preferable that the rotation of the mold is stopped and the mold is erected, and then the inner layer material is statically cast to form a composite roll. Thereby, the inner surface side of the roll outer layer material is redissolved, and a composite roll is formed by welding and integrating the outer layer and the inner layer, or the outer layer and the intermediate layer, or the intermediate layer and the inner layer.

  The inner layer to be statically cast is preferably made of spheroidal graphite cast iron or worm-like graphite cast iron (CV cast iron) having excellent castability and mechanical properties. In the centrifugally cast roll, since the outer layer and the inner layer 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 is weakened. For this reason, it is preferable that the mixing ratio of the outer layer component into the inner layer be suppressed to less than 6%.

  When the intermediate layer is formed, 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 similarly integrally welded, and the components of the outer layer are mixed into the intermediate layer in a range of 10 to 95%. From the viewpoint of suppressing the mixing amount of the outer layer component into the inner layer, it is important to reduce the mixing amount of the outer layer component into the intermediate layer as much as possible.

  The molten metal having the composition of the outer layer material of the roll 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 to 1510 ° C., and the centrifugal force was 170 G as a multiple of gravity. After the casting, the steel sheet was reheated to a quenching temperature of 930 to 1100 ° C., air-cooled, and quenched. The holding time was 10 to 35 hr. After the quenching treatment, the tempering treatment is performed twice or three times depending on the components at a tempering temperature of 530 to 570 ° C. so that the amount of retained austenite is less than 10% by volume, and the hardness is 80 to Adjusted to 86HS. In the conventional example, the casting temperature was 1500 ° C., and the centrifugal force was 170 G in multiple of gravity. After casting, the steel sheet was reheated to a quenching temperature of 1000 ° C., air-cooled and quenched. The holding time was 15 hours. After the quenching process, a tempering process was performed twice at a tempering temperature of 530 ° C. From the obtained ring-shaped test material, a compression test piece, a hardness test piece, a hot rolling fatigue test piece and a hot wear test piece were collected, and a compression test, a hardness test, a hot rolling fatigue test and A hot abrasion test was performed.

  The compression test method was as follows. From the obtained ring-shaped test material, a compression test piece (φ4 mm, height 6 mm) was sampled, and a crosshead speed of 0.09 mm / min, 0 to 0.2% proof stress was measured using an Instron universal testing machine. The compression test was performed under a test condition of 0.8 mm / min in a strain region larger than 0.2% proof stress. The test temperature was set to 600 ° C., and the sample was heated to 600 ° C. using a thermostat installed in a universal testing machine, held for 5 minutes, and then the test was started. After the test, 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% proof stress of the sample.

  The hardness test was as follows. The Vickers hardness HV50 of the obtained hardness test piece was measured with a Vickers hardness tester (test force: 50 kgf (490 N)) in accordance with JIS Z 2244, and the Shore hardness HS was calculated according to the JIS conversion table. Converted. The measurement points were 10 points each, and the highest value and the lowest value were deleted, an average value was calculated, and the average value was determined as the hardness of the test material.

  The hot rolling fatigue test method was as follows. Hot rolling fatigue test pieces (outer diameter 60 mmφ, wall thickness 10 mm, chamfered) were collected from the obtained ring-shaped test material. In the hot rolling fatigue test specimen, notches (depth t: 1.2 mm, circumferential length L: 0.8 mm) as shown in FIG. 1 were provided at two places (180 ° apart) on the outer peripheral surface. And 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-disc rolling sliding method between a test piece and a mating material. The test piece 1 was rotated at 700 rpm while being cooled with water with the cooling water 2, and the rotating test piece 1 was heated to 800 ° C. by the high frequency induction heating coil 3 (material: S45C, outer diameter: 190 mmφ, width: 15 mm). , C1 chamfer) 4 was rolled at a slip ratio of 9% while contacting with a load of 980 N. Then, the two rolling notches 5 introduced into the hot rolling fatigue test piece 1 were rolled until the notches were broken, the number of rolling revolutions until each notch was broken was determined, and the average value was defined as the hot rolling fatigue life. . When the hot rolling fatigue life exceeded 350,000 times, it was evaluated that the hot rolling fatigue life was remarkably excellent.

  The hot wear test was as follows. A hot abrasion test piece (outer diameter 60 mmφ, wall thickness 10 mm) was collected and subjected to a hot abrasion test using the same apparatus as that used in the thermal fatigue test. The hot abrasion test was carried out by a rolling and sliding method of a two-disc disk between a hot abrasion test piece and a mating material. The hot abrasion test piece was rotated at 700 rpm while being cooled with water, and a mating piece (material: S45C, outer diameter: 190 mmφ, width: 15 mm) heated to 800 ° C. was pressed against the rotating test piece with a load of 980 N. Rolling was performed at a slip ratio of 9% for 120 minutes. After the test, the wear amount of the test piece was measured, and based on the conventional example, the ratio of the wear amount of each test piece to the reference value was calculated as the wear ratio (= (wear amount of each test piece) / (wear amount of the reference piece) )).

  Table 2 shows the obtained results.

  In the examples of the present invention, the hot-rolling fatigue life was remarkably increased, and an excellent hot-rolling fatigue life exceeding 350 thousand times was exhibited. In addition, in each of the examples of the present invention in which the 0.2% proof stress at 600 ° C. is 1250 MPa or more and 1480 MPa or less, the wear ratio is significantly reduced, and it can be seen that the wear resistance is improved. It is considered that when the 0.2% proof stress at 600 ° C. is 1250 MPa or more, plastic deformation of the roll surface during hot rolling is suppressed, and wear resistance is improved.

  Therefore, according to the present invention, it is possible to manufacture a composite roll for hot rolling having excellent wear resistance and having a significantly reduced crack propagation speed. As a result, it is possible to suppress surface damage due to hot rolling such as rough surface and chipping. Further, it is possible to obtain an effect that the continuous rolling distance can be extended and the roll life can be improved.

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

Claims (2)

As the component composition, in 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% , Si: 0.2 to 1.0%, Mn: 0.2 to 1.0%, Ni: 0. 0.05 to 3.00%, Co: 0.2 to 3.0% ,
And the contents of V, Nb, Mo, W, and Cr satisfy the following expression (1);
The balance consists of Fe and unavoidable impurities,
A roll outer layer material for hot rolling, having a 0.2% proof stress at 600 ° C. of 1250 MPa to 1480 MPa.
2.50 ≦ (% V +% Nb +% Mo +% W) /% Cr ≦ 5.00 (1)
Here,% V,% Nb,% Mo,% W, and% Cr are the contents (% by mass) of each element. A composite roll for hot rolling, wherein an outer layer and an inner layer are welded and integrated, wherein the outer layer is made of the outer layer material of the roll for hot rolling according to claim 1 .