JP6840696B2 - Composite roll for rolling - Google Patents

Description

The present invention relates to a composite roll for rolling used in a hot strip mill in a hot rolling process.

The composite roll for rolling used in a hot strip mill for hot rolling is required to have excellent wear resistance, rough skin resistance, crack resistance and accident resistance in the outer layer in contact with the steel sheet. For this reason, a high-grade cast iron material or a high-alloy grain cast iron material is used as the outer layer material.

For example, in Patent Document 1, the chemical composition is wt%, C: 1.8% to 3.6%, Si: 1.0% to 3.5%, Mn: 0.1% to 2.0%, Ni: 0.5% to 10.0%, Cr: 2.0% to 10%, Mo: 0.1% to 10%, W: 0.1% to 10%, V, Nb: 1 or 2 types A high-grade cast iron material having a total of 1.5% to 10% and the balance substantially consisting of Fe has been proposed.

Cracks caused by rolling drawing accidents occur when the outer layer of the roll and the rolled material are seized and the outer layer receives a thermal shock. It is believed that the growth of this crack is hindered by graphite in the metallographic structure. Therefore, in the hot strip mill, in the finishing post-stage stand where drawing accidents are likely to occur, graphite is crystallized in the metal structure and a high alloy grain cast iron material with excellent seizure resistance is used to form MC type carbides. High-speed cast iron material with excellent wear resistance is not generally used.

Japanese Unexamined Patent Publication No. 06-256889

Graphite is formed by crystallization and precipitation of carbon in the alloy during the solidification process, and Cr, Mo, W, V, and Nb are all elements that inhibit graphitization. In particular, Cr is the most potent graphitization-inhibiting element among them, and if the amount of Cr added is large, the formation of graphite may be inhibited and the crack resistance may be lowered.

Further, V and Nb combine with carbon in the alloy to form MC-type carbides, respectively, but since VC has a smaller specific gravity than the molten metal, it segregates on the inner peripheral side of the outer layer during centrifugal casting. Since this segregated MC-type carbide is difficult to redissolve when the intermediate layer or the inner layer serving as the shaft core material is cast inside the outer layer, there is a risk of causing welding abnormality at the boundary with the shaft core material. Abnormal welding between the outer layer and the shaft core material causes a decrease in accident resistance.

On the other hand, since NbC has a higher specific gravity than the molten metal, it moves to the outer peripheral side of the outer layer during centrifugal casting and remains in the used layer of the product. As a result, rough skin is generated on the surface of the outer layer, and the surface pattern of the outer layer is transferred to the rolled steel sheet.

Since the MC-type carbide has high hardness, it contributes to the improvement of wear resistance of the outer layer. However, as described above, the MC-type carbide segregates on the outer peripheral side or the inner peripheral side of the outer layer, so that the MC-type carbide is reduced and the resistance is reduced. A layer with inferior wear resistance is generated.

An object of the present invention is to suppress segregation of MC-type carbides and adjust the amount of graphite to be crystallized to provide a composite for rolling with an outer layer having excellent wear resistance, rough skin resistance, crack resistance and accident resistance. To provide a role.

The composite roll for rolling of the present invention
A composite roll for rolling having an outer layer.
In terms of mass%, the outer layer has C: 2.2% to 3.2%, Si: 1.0% to 3.0%, Mn: 0.3% to 2.0%, Ni: 3.0. % To 7.0%, Cr: 0.5% to 2.5%, Mo: 1.0% to 3.0%, V: 2.5% to 5.0%, Nb: exceeding 0% 0.5% or less, balance Fe and unavoidable impurities
Condition (a): Nb% / V% <0.1,
Condition (b): 2.1 × C% + 1.2 × Si% -Cr% + 0.5 × Mo% + (V% + Nb% / 2) ≦ 13.0%
To be satisfied.

Further, the composite roll for rolling of the present invention is
A composite roll for rolling having an outer layer.
In terms of mass%, the outer layer has C: 2.2% to 3.2%, Si: 1.0% to 3.0%, Mn: 0.3% to 2.0%, Ni: 3.0. % To 7.0%, Cr: 0.5% to 2.5%, Mo: 1.0% to 3.0%, W: more than 0% and 3.0% or less, V: 2.5% ~ 5.0%, Nb: more than 0% and 0.5% or less, the balance Fe and unavoidable impurities.
Condition (a): Nb% / V% <0.1,
Condition (b'): 2.1 × C% + 1.2 × Si% -Cr% + 0.5 × (Mo% + W% / 2) + (V% + Nb% / 2) ≦ 13.0%
To be satisfied.

The outer layer can further contain B: 0% and 0.1% or less in mass%.

The outer layer preferably has a graphite area ratio of 0.5% to 5.0%.

Further, the outer layer preferably has an MC-type carbide area ratio of 4.0% to 11.0%.

The outer layer can be manufactured by centrifugal casting.

According to the present invention, segregation of MC-type carbides in the outer layer can be suppressed during centrifugal casting, MC-type carbides can be dispersed substantially evenly from the outer peripheral side to the inner peripheral side of the outer layer, and the amount of graphite crystallized can be adjusted. can do. Therefore, it is possible to improve the wear resistance, rough skin resistance, crack resistance and accident resistance of the composite roll for rolling. The composite roll for rolling of the present invention is suitable for application to a post-stage stand for hot finish rolling, which requires operational stability, in a hot strip mill.

FIG. 1 is a macrostructure photograph of the test material of Invention Example 9 in a longitudinal cross section. FIG. 2 is a macrostructure photograph of the test material of Comparative Example 4 in the longitudinal cross section. FIG. 3 is a microstructure photograph at a position 20 mm from the outer peripheral side of the test material of Invention Example 4. FIG. 4 is a microstructure photograph at a position 20 mm from the outer peripheral side of the test material of Invention Example 9. FIG. 5 is a microstructure photograph at a position 20 mm from the outer peripheral side of the test material of Comparative Example 4. FIG. 6 is a microstructure photograph of the vicinity of the inner peripheral side of the test material of Comparative Example 4.

The composite roll for rolling of the present invention can be composed of an outer layer to be rolled and a shaft core material composed of an intermediate layer and an inner layer or an inner layer inside the outer layer. Examples of the inner layer material constituting the inner layer include materials having toughness such as high-grade cast iron, ductile cast iron, and graphite steel, and examples of the adamite material as the intermediate layer material constituting the intermediate layer.

The outer layer can be manufactured, for example, by centrifugal casting. Centrifugal force casting may be of any of vertical type (rotational axis is in the vertical direction), inclined type (rotational axis is in the diagonal direction) and horizontal type (rotational axis is in the horizontal direction). The number of rotations of the mold in centrifugal casting is GNo. Is preferably 100 G or more. Of course, the outer layer can be manufactured by static casting or the like, but centrifugal casting is preferable in order to reduce the manufacturing cost.

The reasons for limiting the components of the outer layer of the present invention are as follows.

<Reason for limiting ingredients>
The reason for limiting the components of the outer layer material of the composite roll for rolling of the present invention will be described. In the following, unless otherwise specified, "%" is mass%.

C: 2.2% to 3.2%
C can crystallize graphite to improve wear resistance and crack resistance, and can crystallize high hardness MC-type carbides to increase hardness. The C content is 3.2% or less in order to reduce crystallized graphite and cementite, which is a eutectic carbide, suppress excessive crystallization of MC-type carbide as described later, and improve accident resistance. And. On the other hand, in order to secure the crystallization amount of graphite, the content of C is set to 2.2% or more. It is preferably 2.3% to 3.0%.

Si: 1.0% to 3.0%
Si is an element required as a deoxidizer for molten metal. In particular, in centrifugal casting, it is necessary to ensure the flowability of hot water. Further, in the case of a high alloy Glen cast iron material, Si is required as an element that promotes graphite crystallization (partly precipitated). Therefore, it is contained in an amount of 1.0% or more. However, if it exceeds 3.0%, the mechanical properties deteriorate and cause a decrease in crack resistance, so the upper limit is set to 3.0%. Preferably, it is 1.5% to 2.5%.

Mn: 0.3% to 2.0%
Mn is an element necessary for improving the soundness of the molten metal as a desulfurizing agent or a deoxidizing agent for the molten metal and for strengthening the matrix structure. Therefore, it is contained in an amount of 0.3% or more. However, if it is contained in excess of 2.0%, the mechanical properties are deteriorated and the crack resistance is lowered, so the content is set to 2.0% or less.

Ni: 3.0% to 7.0%
Ni is an effective element as an auxiliary element for graphite crystallization and for improving the hardenability of the base to promote bainite formation and strengthening the base. If it is less than 3.0%, such an effect is not sufficient, high hardness cannot be obtained, and wear resistance becomes insufficient. Therefore, the lower limit is set to 3.0%. On the other hand, if it is contained in excess of 7.0%, the amount of retained austenite increases, and the retained austenite is decomposed during hot rolling to reduce the rough skin resistance. Therefore, the upper limit is 7.0%. It is preferably 4.0% to 6.0%.

Cr: 0.5% to 2.5%
Cr mainly binds to C and dissolves in crystallized cementite, which contributes to the improvement of wear resistance. Also, some form precipitated carbides to strengthen the matrix. Therefore, 0.5% is contained. Cr is also contained in MC-type carbides and has an effect of lowering the crystallization temperature thereof. By lowering the temperature at which the MC-type carbide crystallizes, the crystallization of the MC-type carbide can be delayed, and when the MC-type carbide crystallizes, the viscosity of the molten metal increases due to the temperature decrease of the molten metal. Movement in the molten metal is suppressed. In this respect, it has the effect of suppressing segregation of MC-type carbides. On the other hand, since Cr is a very strong graphitization-inhibiting element, crystallization and precipitation of graphite are inhibited, the coefficient of friction increases, and seizure resistance also decreases. As a result, the passability of the rolled material is impaired, and the rolled material is seized or brittle on the roll surface, which causes a decrease in crack resistance. Therefore, the upper limit of Cr is 2 so that graphite can be crystallized even if a predetermined amount of V, which is a graphitization inhibitory element, is contained as in Cr (V: 2.5% to 5.0%). It shall be 5.5%. Preferably, it is 0.8% to 1.5%.

Mo: 1.0% to 3.0%
Mo mainly binds to C and dissolves in crystallized cementite, which contributes to the improvement of wear resistance. Further, since a part of the precipitate forms a precipitated carbide and has an effect of strengthening the matrix, it is contained in an amount of 1.0% or more. However, if it exceeds 3.0%, crystallization and precipitation of graphite are inhibited, which causes a decrease in seizure resistance and a decrease in crack resistance as described above. Therefore, the upper limit is 3.0% or less. Preferably, it is 1.2% to 2.5%.

V: 2.5% -5.0%
V has an action of solid-solving in the matrix to strengthen the matrix, and further, mainly bonds with C to form a high-hardness MC-type carbide, and improves the wear resistance of the outer layer. Therefore, V is added in an amount of 2.5% or more. On the other hand, when the amount of V added is large, MC-type carbides mainly composed of V, which has a smaller specific gravity than the molten metal, are formed, segregate on the inner peripheral surface side of the outer layer during centrifugal casting, and are provided inside the outer layer. The upper limit is set to 5.0% because it reduces the weldability with the inner layer and the intermediate layer. Preferably, it is 3.0% to 4.0%.

Nb: Exceeding 0% and 0.5% or less Since Nb is an element that binds to C and crystallizes MC-type carbide having extremely high hardness, it is added in excess of 0%. On the other hand, since the MC-type carbide mainly composed of Nb has a higher specific gravity than the molten metal, for example, when the outer layer is prepared by centrifugal casting, segregation occurs on the outer peripheral side. Therefore, in the present invention, as described later, since Nb% / V% <0.1 is limited, the upper limit of Nb is 0.5% or less, which is 1/10 of V. Preferably, it is 0.002% to 0.3%.

Residual Fe and Inevitable Impurities The balance of this outer layer material is substantially Fe, and impurities unavoidably mixed during melting are allowed to be contained as long as they do not affect the characteristics of the cast iron material. Since both P and S reduce the toughness of the material, the smaller the amount, the more preferable, and it is more preferable that both P and S are suppressed to 0.2% or less.

The outer layer component satisfies the following conditions in addition to the individual component ranges.

Condition (a) Nb% / V% <0.1
In this condition, in V and Nb forming MC-type carbides for enhancing wear resistance, MC-type carbides mainly composed of V having a small specific gravity with respect to the molten metal and Nb having a large specific gravity with respect to the molten metal are mainly used. By adjusting the balance of the MC-type carbides, the MC-type carbides themselves are conditions for suppressing segregation due to centrifugal casting. In this outer layer material containing a large amount of carbide-forming elements, in order to crystallize graphite, C needs to be contained in a large amount as the content of the carbide-forming elements Cr, Mo, and V increases. When the outer layer is formed by casting, if the amount of C is high, a large amount of MC-type carbide having a large difference in specific gravity from the molten metal is crystallized. Therefore, in the present invention, by reducing the amount of Cr as described above and limiting the amount to Nb% / V% <0.1, the crystallization of MC-type carbides mainly composed of Nb is suppressed, and V is mainly used. By solid-solving Nb in the MC-type carbide, the specific gravity of the MC-type carbide mainly composed of V can be increased to bring it closer to the molten metal, and segregation of the MC-type carbide can be prevented.

Condition (b) 2.1 × C% + 1.2 × Si% -Cr% + 0.5 × Mo% + (V% + Nb% / 2) ≦ 13.0%

Conventionally, in a high alloy grain cast iron material having an increased concentration of carbide-forming elements, a large amount of MC-type carbide is crystallized in order to improve wear resistance, and at the same time, a large amount of C and Si is contained in order to stably crystallize graphite. I was letting you. However, it has been found that even if the concentrations of V and Nb forming the MC-type carbide are increased, the excessive crystallization of the MC-type carbide can be suppressed by lowering the amount of C. This condition is a condition for controlling the amount of crystallization of MC-type carbide and suppressing segregation. In order to prevent segregation of MC-type carbides, it is necessary to bring the specific gravity of MC-type carbides closer to that of the molten metal. In the above condition (a), for this purpose, Nb is dissolved in the MC-type carbide mainly composed of V to suppress the crystallization of the MC-type carbide mainly composed of Nb. The atomic weight of V is 50.94, the atomic weight of Nb is 92.91, which is about twice that of V, and Nb is required to be twice as much as V in order to obtain the same effect on the formation effect of MC-type carbides. .. Therefore, in this condition (b), the V equivalent (V% + Nb% / 2) is used as a reference (coefficient 1.0) for V and Nb adjusted in the above condition (a), and C, Si, and Cr are used. , Mo amount was specified respectively. C is 2.1 times V equivalent as an element for forming graphite crystallization and MC-type carbide. Further, since Si is required as a graphite crystallization promoting element, it is set to 1.2 with respect to V equivalent in order to secure the graphite crystallization amount. Since Cr (atomic weight: 52.0) has an atomic weight close to V, the coefficient with respect to V equivalent is set to 1.0, but since it has the effect of suppressing segregation formation even if C is consumed to form carbides, other factors are used. As an effect opposite to that of the carbide-forming element, the coefficient of "-" was used. Since Mo (atomic weight: 95.94) has an atomic weight of about twice that of V and it is more difficult to form MC-type carbides than V, the coefficient with respect to V equivalent was set to 0.5. Based on various experimental data based on the action and effect on segregation of MC-type carbides, it was found that segregation of MC-type carbides can be suppressed by setting the upper limit of the sum of the left side of the condition (b) to 13.0%. .. The preferable upper limit of the condition (b) is 12.5%.

By satisfying the above conditions (a) and (b), segregation of high-hardness MC-type carbides in the outer layer can be suppressed, and MC-type carbides can be dispersed substantially evenly from the outer peripheral side to the inner peripheral side of the outer layer, and thus wear resistance. , Rough skin resistance and accident resistance can be improved.

The outer layer can further contain W. W has the same effect as Mo, but in the case of the centrifugal casting method, it tends to form lamination segregation, so W: preferably more than 0% and 3.0% or less. Preferably, it is 2.0% or less.

Then, when W is contained, the above condition (b) becomes the following condition (b').

Condition (b') 2.1 × C% + 1.2 × Si% -Cr% + 0.5 × (Mo% + W% / 2) + (V% + Nb% / 2) ≦ 13.0%

Since the atomic weight of Mo is 95.94 and the atomic weight of W is 183.8, which is about twice that of Mo, W is required twice as much as Mo when converted to Mo equivalent. Therefore, in this condition (b'), the Mo equivalent (Mo% + W% / 2) was set with respect to the above condition (b). The upper limit of the condition (b') is also preferable, which is 12.5%.

The outer layer preferably has an MC-type carbide area ratio of 4.0% to 11.0%. If the area ratio of the MC-type carbide is less than 4.0%, the effect of improving the wear resistance of the outer layer may be insufficient. Further, it is difficult to increase the area ratio of MC-type carbide to more than 11.0% due to the coexistence relationship with graphite.

Further, it is preferable to adjust the graphite area ratio of the outer layer to 0.5% to 5.0%. If the area ratio of graphite is less than 0.5%, the effect of improving the seizure resistance of the outer layer is insufficient, and cracks and seizure due to thermal shock occur in the event of a drawing accident. Seizure concentrates the load and significantly accelerates the growth of cracks. By crystallizing graphite in the outer layer, it is possible to suppress the growth of cracks generated by receiving a thermal shock in the event of a drawing accident, and it is possible to improve the crack resistance of the outer layer. For example, the amount of graphite crystallized can be adjusted by adding an inoculant such as Fe-Si or Ca-Si to the outer layer molten metal. On the other hand, if the area ratio of graphite exceeds 5.0%, the wear resistance and mechanical properties of the outer layer may be significantly deteriorated.

Further, the outer layer can further contain B. In this case, if the content exceeds 0.1%, the mechanical properties are significantly deteriorated, so the upper limit is set to 0.1%. It is preferably 0.05% or less.

In a high-frequency induction melting furnace, molten alloys of various components shown in Table 1 were melted and centrifugally cast to prepare an outer layer material. In Table 1, Invention Examples are Invention Examples 1 to 13, and Comparative Examples are Comparative Examples 1 to 9, respectively, and the composition of the molten metal component is described. In addition, the condition (a) and the condition (b) (however, the condition (b') when W is contained) were calculated together.

In Table 1, for each outer layer material, components that are not contained or components that may be contained but cannot be detected are indicated by "-". Further, in Table 1 and Table 2 showing the measurement and test results, the components and conditions that do not meet the scope of the present invention, the preferable measured values of the present invention, the values that deviate from the test results, or the values that are significantly inferior to the examples of the invention are included. Stars are added to enhance understanding visually.

In both the invention example and the comparative example, the outer layer material has a mold rotation speed GNo. Was 100 G to 200 G, and the casting temperature was 1250 ° C to 1360 ° C. The outer layer material has an outer diameter of 300 mm and a length of 200 mm, and an outer diameter of 570 mm to 800 mm and a length of 1100 mm to 2500 mm. Then, after tempering each outer layer material at 400 ° C. to 600 ° C. several times, a test material having a size of 200 mm × 200 mm was collected from each outer layer material. In addition, gamma heat treatment of 850 ° C. or higher, quenching, and tempering may be carried out.

With respect to the obtained test material, the presence or absence of segregation of MC-type carbide, graphite area ratio, MC-type carbide area ratio, seizure load and wear amount were measured as shown below. Regarding Comparative Examples 1 to 5 in which segregation of MC-type carbides described below was observed, the graphite area ratio and the like were measured only in Comparative Examples 3 and 4.

First, in order to measure segregation of MC-type carbides, graphite area ratio, and MC-type carbide area ratio, each test material was polished with sandpaper having a particle size of 240, and after polishing, the range of the total length in the longitudinal cross section was polished. The test material was subjected to etching treatment with an aqueous nitric acid solution, and a macrostructure photograph was taken. In addition, after taking a macrostructure photograph, alumina buffing was performed on the test material, and a microstructure photograph was taken. For reference, FIGS. 1 and 2 show macrostructure photographs of Invention Example 9 and Comparative Example 4, and FIGS. 3 to 5 show positions 20 mm from the outer peripheral side in the longitudinal cross section of Invention Example 4, Invention Example 9 and Comparative Example 4. Microstructure photograph, FIG. 6 shows a microstructure photograph in the vicinity of the inner peripheral side in the longitudinal cross section of Comparative Example 4. Since the MC-type carbide is hard, it protrudes more than other carbides when polishing with alumina buff, and the shadow of the protruding MC-type carbide is photographed in the microstructure photograph. In the microstructure photograph, the black mass is graphite.

<Segregation of MC type carbide>
Segregation of MC-type carbide was evaluated based on the obtained macrostructure photograph. With reference to FIGS. 1 and 2, in FIG. 1 (Invention Example 9), segregation of MC-type carbide is not observed in the longitudinal cross section of the test material. On the other hand, in FIG. 2 (Comparative Example 4), a deeply corroded region is observed on the inner peripheral side of the test material, that is, on the lower side of FIG. This is the segregation of MC-type carbides. Looking at the microstructure photographs (FIGS. 5 and 6) on the outer peripheral side and the inner peripheral side in the longitudinal cross section of Comparative Example 4, MC-type carbides are sparse on the outer peripheral side of FIG. From the fact that the MC-type carbides are dense on the peripheral side, it can be seen that the MC-type carbides are segregated on the inner peripheral side in Comparative Example 4. This is because MC-type carbides, which are lightweight with respect to the specific gravity of the molten metal, crystallize excessively and aggregate inward due to the difference in specific gravity during centrifugal casting. As in Comparative Example 4, when MC-type carbide segregates on the inner peripheral side of the outer layer material, the segregated layer is difficult to redissolve even when a shaft core material such as an intermediate layer or an inner layer is cast inside the segregation layer. It is not suitable as an outer layer of a composite roll for rolling because it remains and causes welding abnormalities and reduces accident resistance. In the test material of Invention Example 9 shown in FIG. 1, a structure different from that of the inner peripheral side is observed in a range of about 20 mm on the inner peripheral side, but this layer is a final solidified layer having a relatively low melting point. Therefore, although some shrinkage cavities are involved, there is no problem because the shaft core material is redissolved when cast and is welded soundly.

In the same manner, the presence or absence of segregation of MC-type carbide was examined from the macrostructure photograph of each test material. Then, based on the obtained tissue photographs, those without segregation of MC-type carbides were evaluated as "1 (none)", and those without segregation were evaluated as "2 (yes)". The results are shown in Table 2 “Segregation of MC-type carbides”.

With reference to Table 2, it can be seen that all of the invention examples were evaluated as “1 (none)” and no segregation of MC-type carbides was observed. This is because all of the invention examples satisfy the condition (a) and the condition (b) or (b'). In particular, by satisfying the mass ratio of the contents of V and Nb, which is the condition (a), Nb is dissolved in the MC-type carbide mainly composed of V, so that the specific gravity of the MC-type carbide is close to the specific gravity of the molten metal. It can be adjusted to. On top of that, by setting the alloy composition to satisfy the formula of the condition (b), the segregation of MC-type carbides can be greatly reduced. As a result, it can be seen that the wear resistance, rough skin resistance and accident resistance due to segregation of MC-type carbides of the composite roll for rolling having an outer layer of the invention example could be improved.

On the other hand, in Comparative Examples, Comparative Examples 1 to 5 were evaluated as "2 (Yes)". In Comparative Example 1, Comparative Example 2, and Comparative Example 5, the evaluation was "2 (yes)" because even if the concentrations of V and Nb forming the MC-type carbide were appropriate, these comparative examples as a whole were evaluated. This is because the amounts with respect to the C concentration and the Cr concentration did not satisfy the condition (b), and the excessively crystallized MC-type carbide was segregated. In particular, in Comparative Example 2 and Comparative Example 5, it can be seen that the component range of each element is included in the range of the present invention, but the segregation of MC-type carbides was caused by not satisfying the condition (b). This is because the MC type carbide having a large amount of V and being lightweight with respect to the specific gravity of the molten metal crystallized and segregated in Comparative Example 3. Further, in Comparative Example 4, the amount of V was small but the amount of Nb was large, and not only the condition (b) but also the condition (a) was not satisfied. As a result, MC-type carbides heavy with respect to the specific gravity of the molten metal were crystallized. This is because the ejection segregation occurred.

<Graphite area ratio (%)>
Next, with respect to the obtained test material, the microstructure on the outer peripheral side of the longitudinal cross section was photographed in the same manner as described above, and the amount of graphite was measured as the graphite area ratio. From the subsequent measurements and tests, Comparative Example 1, Comparative Example 2 and Comparative Example 5 in which segregation of MC-type carbide was confirmed were excluded.

3 to 5 show microstructure photographs of the inner peripheral side of the outer layer material for the test materials of Invention Example 4, Invention Example 9 and Comparative Example 4, respectively. The results are shown in Table 2 “Graphite area ratio”. With reference to Table 2, in each of the invention examples, the graphite area ratio can be adjusted to 0.5% to 5.0%. Further, in Comparative Example 4, Comparative Example 7, and Comparative Example 9, the graphite area ratio can be adjusted from 0.5% to 5.0%. By adjusting the graphite area ratio as described above, the growth of cracks generated in the outer layer material due to thermal shock can be prevented by the graphite, and the crack resistance of the outer layer material can be improved.

In both Comparative Example 6 and Comparative Example 8, no crystallization of graphite was observed. This is because the Cr% of Comparative Example 6 and Comparative Example 8 is high.

<MC type carbide area ratio (%)>
The amount of MC-type carbide was measured by the MC-type carbide area ratio from the microstructure photograph on the inner peripheral side of the test material (see, for example, FIGS. 3 to 5). The results are shown in Table 2 “MC type carbide area ratio”. With reference to Table 2, it can be seen that in each of the invention examples, the amount of MC-type carbides can be adjusted in the range of 4.0% to 11.0%, and the examples have excellent wear resistance. On the other hand, in Comparative Example 4, Comparative Example 7 to Comparative Example 9, since the MC type carbide area ratio is less than 4.0%, it can be seen that they are inferior in terms of wear resistance.

<Seizure load (N)>
The seizure load was measured with a high-speed Farex type wear tester (manufactured by Shinko Engineering Co., Ltd., product name: high-speed Farex type friction tester). In the test, a columnar test piece (diameter 10 mm, length 35 mm) was collected from each test material, and while rotating the test piece at 200 rpm, it was sandwiched between two SUS430 V blocks, and the load was increased at an ascending speed. While loading at 100 N / sec, increase the load at a speed of 15 N / 9 seconds until the load reaches 1500 N. The torque inflection point during this period was evaluated as the seizure load. The higher the seizure load, the better the seizure resistance. The test results are shown in Table 2 “Seizure load”. With reference to Table 2, it can be seen that all of the invention examples have a high seizure load and have sufficient seizure resistance as compared with the comparative examples. On the other hand, it can be seen that in Comparative Example 4, Comparative Example 7, and Comparative Example 9, the seizure load is not sufficient and the seizure resistance is inferior.

<Amount of wear (μm)>
The amount of wear was measured by the above-mentioned high-speed Farex type wear tester. In the test, a columnar test piece (diameter 10 mm, length 35 mm) was collected from each test material, and while rotating the test piece at 200 rpm, it was sandwiched between two SS400 V blocks to increase the load. The load was maintained at 100 N / sec for 30 minutes when the load reached 1000 N. After the test, the amount of wear on the contact surface between the test piece and the V block was measured. The results are shown in Table 2 “Amount of wear”. With reference to Table 2, it can be seen that all of the invention examples have a small amount of wear and have sufficient wear resistance. On the other hand, it can be seen that Comparative Example 4, Comparative Example 7 to Comparative Example 9 have a large amount of wear and are inferior in wear resistance. This is because the amount of MC-type carbide required to improve the wear resistance is less than 4.0% in terms of area ratio.

<Comprehensive evaluation>
With reference to each of the above measurements and test results, in all of the invention examples, since segregation of MC-type carbides is not observed, welding abnormality with the shaft core material can be suppressed, and accident resistance can be improved as a composite roll for rolling. I understand. Further, since the graphite area ratio and the MC type carbide area ratio are adjusted to the desired ranges, it can be seen that the seizure load and the amount of wear are also extremely good as the outer layer of the composite roll for rolling.

As described above, in the composite roll for rolling of the present invention, since the outer layer is excellent in wear resistance, crack resistance and accident resistance, the service life of the roll can be extended, thereby reducing the roll cost. Furthermore, it contributes to the stabilization of roll inventory management. The composite roll for rolling of the present invention is particularly suitable for application to a post-stage stand for hot finish rolling, which requires operational stability.

The above description is for explaining the present invention, and should not be construed as limiting or limiting the scope of the invention described in the claims. In addition, each part of the present invention is not limited to the above embodiment, and it goes without saying that various modifications can be made within the technical scope described in the claims.

Claims (4)

A method for manufacturing an outer layer for a composite roll for rolling.
The outer layer has a mold rotation speed GNo. Manufactured by centrifugal casting with a value of 100G to 200G.
The outer layer is
In terms of mass%, C: 2.2% to 3.2%, Si: 1.0% to 3.0%, Mn: 0.3% to 2.0%, Ni: 3.0% to 7. 0%, Cr: 0.5% to 2.5%, Mo: 1.0% to 3.0%, V: 2.5% to 5.0%, Nb: 0.5% over 0% Hereinafter, W: more than 0% and 3.0% or less, the balance Fe and unavoidable impurities, and
Condition (a): Nb% / V% <0.1,
Condition (b'): 2.1 × C% + 1.2 × Si% -Cr% + 0.5 × (Mo% + W% / 2) + (V% + Nb% / 2) ≦ 13.0%
A method for producing an outer layer for a composite roll for rolling, characterized in that MC-type carbides are dispersed substantially evenly from the outer peripheral side to the inner peripheral side of the outer layer by satisfying the above. Method for producing a rolled outer layer composite roll according to claim 1, 0.5 to 5.0% graphite area ratio in the melt to the addition of inoculant of Fe-Si or Ca-Si at the time of casting. The outer layer produced has an outer diameter of 570 mm to 800 mm.
The method for producing an outer layer for a composite roll for rolling according to claim 1 or 2. The outer layer produced is 1100 mm to 2500 mm in length.
The method for producing an outer layer for a composite roll for rolling according to any one of claims 1 to 3.