JPH0729132B2 - Composite roll manufacturing method and rolling mill - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for manufacturing a composite roll and a rolling mill, and in particular, wear resistance in which a core material is low alloy steel and an outer layer material is high speed steel,
The present invention relates to a method for producing a composite roll having excellent crack resistance and axial strength, and a rolling mill.

[Conventional technology]

Since high speed steel has excellent wear resistance and crack resistance, it has attracted attention as a work roll material for rolling.

As a known example of rolls using high speed steel, ordinary casting method and ES
Although an integral roll by the R method is known, heat treatment and forging become very difficult because the segregation and huge carbides are more likely to be generated as the steel ingot size increases.
Currently, it is impossible to manufacture rolls of 300 mm or more.

On the other hand, a composite roll having different characteristics on the roll surface and the shaft portion is conceivable. For example, as described in JP-A-59-23846, it is generally manufactured by a centrifugal casting method. According to the centrifugal casting method, it is possible to manufacture a composite roll using various high alloy steels as the outer layer material, but in order to secure the weldability of the outer layer material and the roll core material, a cast iron system is used. It is general. That is, when the outer layer material such as the present invention is a high-speed steel, when a composite roll having a roll core material of a low alloy steel is manufactured by a centrifugal casting method, their solidification temperatures are high, and welding is incomplete. , The joint is peeled off during use, which is not practical. Temporarily, in order to improve the weldability,
Even if cast iron with a low solidification temperature is used as the roll core material, the requirements cannot be satisfied because the axial strength of the manufactured composite roll is low, and there is no economic merit.

On the other hand, the heat treatment method of the composite roll is a method of performing strain relief treatment at a temperature of Ac ₁ point or lower, as described in Japanese Patent Publication No. 59-1772, quenching after heating only the outer layer material to Ac ₁ point or higher,
Although a method of performing tempering treatment has been adopted, there is a problem that the quench hardening depth is restricted. Moreover, in the case of the integral roll by the ordinary casting method or the ESR method, the method of quenching and tempering after the whole heating up to the Ac ₁ point or less was adopted.

[Problems to be Solved by the Invention]

In the recent technical field of steel plate rolling, a new type of mill has been introduced to save energy, improve productivity and improve steel plate quality, and work rolls have been used under increasingly severe rolling conditions. . In particular, since a new type of mill applies bending force to the roll, it is required that the shaft and neck of the roll have high strength and toughness. Since ordinary cast iron, spheroidal graphite cast iron or graphite steel is used as the core material, it was difficult to sufficiently satisfy the requirements.

On the other hand, the demand for difficult-to-machine materials such as silicon steel sheets and steel sheets with high surface quality is increasing more and more, and it is required that the rolls have higher wear resistance. In order to improve wear resistance, it is possible to increase hardness, but the composite roll produced by the conventional centrifugal casting method has low reliability in joining the core material and the outer layer material, and cast iron is used as the core material. For example, if severe heat treatment is applied, the roll cannot withstand the heat and transformation stress that occurs, causing peeling and cracking of the joint.Therefore, the roll is strain-relieved without quenching and tempering. Was applied. Further, even if quenching and tempering are performed, in order to prevent local transformation stress from being generated at the joint boundary portion, a method has been adopted in which the vicinity of the joint portion is heated to Ac ₁ point or less. As a result, quantitative restrictions were added to the hardness and depth obtained.

On the other hand, in order to obtain the required strength and toughness, the integral roll produced by the ordinary casting method or the ESR method is forged to improve the mechanical properties. However, as the ingot size increases, segregation and huge eutectic carbides are more likely to be generated, and the ingot size that can be forged is restricted.
The above rolls could not be manufactured. Further, in the conventional quenching of the integral roll, a method in which the entire roll is heated and held at Ac ₁ point or higher and then rapidly cooled is taken, but on the roll surface, tensile residual stress due to transformation stress occurs, and the surface crack inside the roll is generated. It is not preferable because it promotes transmission.

An object of the present invention is to provide a method for producing a composite roll and a rolling mill which are excellent in wear resistance, crack resistance and axial strength.

[Means for Solving the Problems]

In order to achieve the above object, the present invention comprises a roll core material made of low alloy steel, and an outer layer material made of high-speed steel formed by electroslag welding using a water-cooled mold on the outer periphery of the roll core material. A method for manufacturing a composite roll having a layer composed of a ferrite phase at a bonding boundary portion between the roll core material and an outer layer material, wherein the roll core material is wt% and C is 0.25 to 0.
8%, Si 0.15 to 0.6%, Mn 0.3 to 1.2%, Ni 2.0 or less, Cr 0.5 to 3.0%, Mo 0.15 to 0.7%, the balance consisting of steel containing Fe and unavoidable impurities, On the surface of the core material,
The outer layer material consisting of the high speed steel having an upper limit of 5.1% and a lower limit of 1.2% or more with respect to the Cr content of the low alloy steel of the core material is welded and integrated by an electroslag remelting method to perform the joining. A step of forming a layer made of a ferrite phase at the boundary, a step of austenizing the outer layer material by induction heating the composite roll to 1150 to 1240 ° C, and cooling after the heating and quenching the outer layer material to form martensite. A step of forming a structure, a step of subjecting the roll surface to a Shore hardness (Hs) of 90 or more after the sub-zero treatment, and a step of performing a tempering treatment after the sub-zero treatment, It is characterized in that the thickness of the layer composed of the ferrite phase is 5 to 200 μm.

In the method for producing a composite roll, it is preferable that the composite roll has a Shore hardness (Hs) of 90 or more up to a depth of at least 40 mm from the surface of the roll.

Further, in the method for producing a composite roll, it is preferable that the composite roll has a compressive residual stress applied to the surface thereof.

In addition, in the method for producing the composite roll, the diameter of the roll body is particularly preferably 300 mm or more.

Examples of the rolling mill equipped with the above-mentioned composite roll include cold, finishing work roll for hot strip mill, roll for hot skin pass, roll for finishing wire rod, and the like.

[Action]

The present inventors have examined the relationship between heat treatment cracking of the composite roll and the internal residual stress, and as a result, local tensile stress in the radial direction occurs at the bonding boundary between materials having different thermal expansion and transformation characteristics, which is It was found to cause cracking. It was found that forming a ferrite layer with a small yield stress at the joint boundary is effective for suppressing local tensile stress. That is, it has been found that it is effective to make the ferrite layer function as a stress relaxation material and to absorb the generated stress by the deformation of the ferrite layer.

According to the manufacturing method of the present invention, basically, the ferrite layer is C at the bonding boundary from the roll core material side to the outer layer material side.
However, as a result of various studies on the relationship between the combination of the outer layer material and the roll core material with respect to the formation of the ferrite layer, the present inventors control the formation of the ferrite layer by the core material and the outer layer material. It was found that there was a difference in the amount of C and Cr. It was found that the difference in Cr content between the two is particularly important.

An example of the experimental result will be described in detail below. In the experiment, the outer layer material was 1.4% C-4.2% Cr-5.4% Mo-4.6% W-5.3%
The thickness of the ferrite layer was measured after varying the C and Cr amounts of the roll core material to V-6.5% Co and joining them.

FIG. 4 shows the relationship between the thickness of the ferrite layer and the C content of the core material. The Cr content was fixed at 1.5%. As is clear from the figure, when the C content is 0.8% or more, the required ferrite layer thickness is not obtained, and when the C content is 0.8% or less, the ferrite layer thickness increases as the C content decreases.

Next, the amount of Cr was changed with the amount of C kept constant at 0.4%. Fig. 5 shows the results. When the Cr content becomes higher than that of the outer layer material, the ferrite layer does not have the required thickness, but rather the phenomenon that C is enriched from the outer layer material side to the core material side is recognized. It was The thickness of the ferrite layer increases as the Cr content decreases.

Based on the above findings, the present inventors have found that in order to form a ferrite layer at the bonding boundary, the C content of the core material is 0.8%, and the Cr content of the core material is lower than that of the outer layer material. C is 0.
It was found that 25 to 0.8% and Cr of 0.5 to 3.0% are suitable.

At the same time, the outer layer material has a Mo content with an upper limit of 5.1% and a lower limit of 1.2% or more higher than the Cr layer of the low alloy steel of the core material.
It has been found that system, W and V high speed steels are suitable. The present invention prevents heat treatment cracking by setting the thickness of the layer made of the ferrite phase formed at the joint boundary portion between the roll core material and the outer layer material to 5 to 200 μm, and the ferrite having such a thickness is used. In order to form a phase, it is necessary to adjust the amount of Cr in the core material and the amount of Cr in the outer layer material. If the upper limit of the Cr content of the outer layer material is 5.1% by weight, a ferrite phase can be formed as shown in FIG.

In order to form a ferrite phase, the fifth
As shown in the figure, the difference in the amount of Cr between the core material and the outer layer material is that if the amount of Cr in the outer layer material is 4.2% and the amount of Cr in the core material is not more than 3%, a ferrite phase with a thickness of 5 μm or more will be formed. Therefore, it is necessary to set the difference in Cr content between the two to be 1.2% or more. Based on this, the lower limit of the Cr content in the outer layer material is set to 1.2% or more higher than the Cr content in the core material.

By using such a combination of the core material and the outer layer material, C moves from the core material side with low Cr to the outer layer material side with high Cr during electroslag welding of both and the subsequent heat treatment, and the boundary between the two. A ferrite layer is formed by reducing the carbon content.

The electroslag remelting method for producing the composite roll of the present invention is described in, for example, Japanese Patent Publication No. 61-54097. FIG. 1 schematically shows the structure of a composite roll of high speed tool steel according to the present invention. 1 is an outer layer material made of high speed tool steel, 2 is a roll core material made of low alloy steel, 3 is a joining boundary,
Reference numeral 4 indicates a ferrite layer. Hereinafter, the range of components of the roll core material constituting the composite roll of the present invention and the reason for limiting the components will be described.

Since the composite roll manufactured by the manufacturing method according to the present invention basically forms a ferrite layer by the diffusion of C from the core material side to the outer layer material side at the bonding boundary, the chemical composition of the roll core material has this point. It is necessary to select it in consideration.

C is determined in balance with Cr so as to obtain the target tensile strength, but as described above, if the content is 0.25% or less, the required axial strength cannot be obtained, and if it is 0.8% or more, the amount of carbide increases and the toughness increases. And the ferrite layer is not sufficiently formed at the junction boundary. Therefore, C is 0.25-0.8%
Stipulated in.

Si and Mn are added for the purpose of deoxidation, but 0.15 to 0.6% and 0.3 to 1.2% are added, respectively, as long as the properties are not affected.

Ni is added to enhance the toughness, but if 2.0% or more, the matrix becomes hard and the toughness deteriorates, so 2% or less is desirable.

Cr is determined by the target amount of carbide while keeping balance with C. If it is 0.5% or less, the amount of carbide is small and the axial strength is insufficient, and if it is 3.0% or more, the amount of carbide is too large and the toughness is deteriorated. Not preferable. Therefore, Cr is 0.5 to 3.0
Specify as%.

Mo has the same effect as Ni, but if it is 0.15% or less, it has no effect, and if it is 0.7% or more, the matrix becomes hard and the toughness deteriorates, so 0.15 to 0.7% is specified.

In addition to the above composition, the roll core material contains the balance Fe and unavoidable impurities.

Next, the heat treatment method will be described.

The austenitization of the composite roll is carried out by induction heating, but the austenitizing temperature is 1150 to 1240 ° C. At 1150 ° C or lower, the austenitization of high-speed steel is not sufficient, and when it becomes 1240 ° C or higher, This is because the amount of retained austenite after quenching increases and stabilizes, so that the desired hardness cannot be obtained. In the present invention, cooling (quenching) is carried out at a cooling rate sufficient to make the matrix structure of the outer layer material a martensite structure, but rapid cooling such as water cooling, mist cooling or wind blast cooling is adopted depending on the components of the outer layer material. To be

A sub-zero treatment (deep cooling treatment) must be performed in order to retain a region with a Shore hardness (Hs) of 90 or more up to at least 40 mm from the roll surface. Therefore, immediately after the above quenching treatment, subzero treatment is performed to decompose residual austenite and transform it to martensite,
Increase hardness and stabilize the structure. The sub-zero treatment temperature is selected in the temperature range of −30 to −150 ° C. according to the composition of the outer layer material so as to obtain the target hardness.

The ferrite layer at the joint boundary between the roll core material and the outer layer material acts as stress relaxation to prevent heat treatment cracking, but when the thickness of the ferrite layer after heat treatment is 5 μm or less, there is no effect, and when it is 200 μm or more, the strength of the roll is increased. Is lowered, which is not preferable. Therefore, the thickness of the ferrite layer is 5-200μ
It is defined as m. 900-110 after electroslag remelting
It is effective to retain the ferrite layer by heating and holding it in the range of 0 ° C. to sufficiently diffuse C.

Next, a tempering treatment is carried out in order to strengthen the roll, but it is preferable that the tempering temperature be repeated several times within a range of 100 to 550 ° C.

As described above, the reason why rapid quenching and sub-zero treatment, which were not possible with conventional composite rolls by the centrifugal casting method and integral rolls by the ordinary casting method or ESR method, became possible is the joining of the roll core material and the outer layer material. It can be mentioned that a ferrite layer is formed at the boundary. That is, the local tensile stress in the radial direction generated at the bonding boundary is absorbed and relaxed by the deformation of the ferrite layer. In addition, low-alloy steel with excellent toughness and higher tensile strength than the outer layer material is used for the roll core material, and the roll core material and outer layer material are completely welded and integrated by electroslag remelting, and heat and transformation that occur This is because it can sufficiently withstand stress.

In the conventional centrifugal casting composite roll, when subjected to heat treatment to give the desired residual stress of compression to the roll surface,
Since the reliability of joining the roll core material and the outer layer material is poor and the roll core material is cast iron having low strength, cracking occurs without being able to withstand the heat and transformation stress generated.

On the other hand, in the conventional integrated roll, as described above, forging is performed in order to improve the mechanical properties, and then quenching is performed by heating the whole body, but segregation, microcavity or huge co-rolling occurs as the ingot size increases. Forging was difficult due to the formation of crystallized carbides, and the rolls could not be increased in size. On the other hand, the outer layer material of the present invention has a fine and sound texture because it solidifies unilaterally while being cooled by the water-cooled mold and the core material. As a result, the strength required for rolls was obtained even without forging, and the rolls with a diameter of 300 mm or more were enlarged.

If various rolling mills are manufactured using the above composite roll, the rolling mill can withstand severe rolling strips. Specific examples thereof include cold work rolls for hot strip mills, rolls for hot skin passes, and rolls for finishing wire rods.

〔Example〕

 Examples of the present invention will be described below.

Example 1 Inside a mobile water-cooled mold having a diameter of 380 mm and a height of 500 mm, the diameter was
270 mm, height 2500 mm 0.4% C-0.25% Si-0.6% Mn-1.0
1.4% C-4.2% Cr-5.4% Mo-4.6% W-5.3% V is used as the outer layer material by arranging the roll core made of% Cr-0.3% Mo steel.
Electroslag remelting was performed using a consumable electrode of cylindrical V-based high-speed tool steel made of -6.5% Co and having an outer diameter of 352 mm and an inner diameter of 300 mm. The voltage was 40 V and the current was 7 kA.

In the obtained composite roll, the roll core material and the outer layer material are completely welded and integrated by electroslag remelting, and it is considered that the joining boundary has high joining reliability.

After that, the composite roll was cooled by blast wind at 1200 ° C after low-frequency gradual induction heating, then subjected to sub-zero treatment at -70 ° C, 530 ° C x 3h.
This was repeated 3 times.

FIG. 2 schematically shows the microstructure of the joint boundary portion after the heat treatment. Since the ferrite layer 4 is formed in the joint boundary portion 3, even if the severe heat treatment as described above is performed, cracking occurs. Did not happen.

FIG. 3 shows the hardness distribution in the radial direction of the composite roll after the heat treatment. It can be seen that the hardness is high and the hardening depth is large.

Comparative Example 1 Roll core material is 0.9% C-0.5% Si-0.4% Mn-3.0%
Electroslag remelting was performed under the same conditions as in Example 1 except that Cr-0.2% Mo steel was used, and then heat treatment was performed. As is clear from the structural composition diagram of the joining boundary portion schematically shown in FIG. In addition, no ferrite layer was observed at the boundary,
As a result, heat treatment cracking occurred.

Comparative Example 2 An integral roll of the same material as the outer layer material of Example 1 and having a diameter of 180 mm was used.
Quenching was performed at 200 ° C., and then tempering at 530 ° C. for 3 hours was repeated 3 times.

Table 1 shows the residual stress in the axial direction on the roll surface. It can be seen that the comparative example has tensile stress, whereas the example 1 has compressive stress and is excellent in crack resistance.

Example 2 A diameter of 470 mm and a height of 500 mm was applied to the inside of a mobile water-cooled mold, and the diameter of
0mm, height 2700mm 0.3% C-0.25% Si-0.75% Mn-1.8
1.5% C-5.1% Cr-5.3% Mo-4.9% W, which is the outer layer material, with the roll core made of% Ni-0.8% Cr-0.5% Mo steel.
Electroslag remelting was performed using a consumable electrode of cylindrical V-based high speed tool steel made of -4.9% V-5.0% Co and having an outer diameter of 352 mm and an inner diameter of 300 mm. The voltage was 40 V and the current was 9 kA.

After that, the roll was subjected to low frequency gradual induction heating at 1200 ° C., then cooled with blast wind, then subjected to subzero treatment at −70 ° C., and tempering at 530 ° C. for 3 hours was repeated 3 times.

The composite roll was mounted in a cold mill finishing No. 2 stand and various test rolling was performed. The material to be rolled is ordinary steel.
Table 2 shows the results of comparing the use results of the roll with the conventional 0.9% C-3% Cr forged steel integrally quenched roll, and it can be seen that the roll of the present invention has excellent wear resistance. Here, the amount of consumption means the amount of reduction in the diameter direction of the roll per one time until the rolling of a certain amount of the rolled material is completed, and the amount of rolling is the rolled material under the same conditions. It is the weight of the rolled material that can be rolled until the diameter of the roll wears by 1 mm.

[Effects of the Invention] According to the manufacturing method of the present invention, since a ferrite layer having an appropriate thickness can be easily interposed at the bonding boundary between the roll core material and the outer layer material, the ferrite layer is a stress cast material. Thus, it is possible to easily provide a composite roll having excellent axial strength, abrasion resistance and crack resistance. In particular,
When the present invention is applied to a large-sized composite roll having a diameter of 300 mm or more, the effect is remarkable.

The rolling mill of the present invention can withstand severe rolling conditions.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view showing the structure of a composite roll according to the present invention, FIG. 2 is a schematic view showing the structure of the joint boundary portion of the present invention composite roll after heat treatment, and FIG. 3 is a radius of the present composite roll. 4 is a characteristic diagram showing a hardness curve in the direction, FIG. 4 is a characteristic diagram showing the relationship between the C content of the roll core material and the depth of the ferrite layer, 5
Fig. 6 is a characteristic diagram showing the relationship between the amount of Cr in the roll core and the depth of the ferrite layer. Fig. 6 shows a conventional example of 0.9% in the roll core.
It is a schematic diagram which shows the structure constitution after the heat processing of the joining boundary part of the high speed tool steel composite roll which used C-0.5% Si-0.4% Mn-3.0% Cr-0.2% Mo steel. 1 ... Outer layer material, 2 ... Roll core material, 3 ... Bonding boundary, 4
…… Ferrite layer.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Reference number within the agency FI Technical display location C22C 38/44 (72) Inventor Osamu Shitamura 2-832 Horiguchi, Katsuta City, Ibaraki Hitachi, Ltd. (72) Inventor, Masami Shimizu, 832, Horiguchi, Katsuta City, Ibaraki Prefecture, Hitachi Ltd., Katsuta Factory, Hitachi, Ltd. (56) References JP 59-23846 (JP, A) JP 58-25866 ( JP, A) JP 53-28057 (JP, A) JP 60-180608 (JP, A) JP 59-1772 (JP, B2) JP 61-54097 (JP, B2)

Claims (5)

[Claims] 1. A roll core material made of low alloy steel, and an outer layer material made of high speed steel formed on the outer periphery of the roll core material by electroslag welding using a water-cooled mold. A method for producing a composite roll having a layer composed of a ferrite phase at a boundary between the outer layer material and the outer layer material, wherein the roll core material is wt%, C is 0.25 to 0.8%, and Si is 0.1.
5-0.6%, Mn 0.3-1.2%, Ni 2.0 or less, Cr 0.5-3.
0%, Mo is 0.15 to 0.7%, the balance is made of steel containing Fe and unavoidable impurities, the upper limit of the core material surface is 5.1% and the lower limit is 1.2% with respect to the Cr content of the low alloy steel of the core material. Higher Cr
A step of forming a layer made of a ferrite phase at the joint boundary by welding and integrating an outer layer material made of the high-speed steel having an amount by an electroslag remelting method, and induction heating the composite roll to 1150 to 1240 ° C. A step of austenitizing the outer layer material, a step of quenching the outer layer material after heating to form a martensite structure, and a subzero treatment after the quenching to obtain a Shore surface hardness (Hs) of 90 or more. And a step of performing a tempering treatment after the sub-zero treatment,
And a thickness of the layer made of the ferrite phase after the tempering treatment is 5 to 200 μm. 2. The method for producing a composite roll according to claim 1, wherein the composite roll has a Shore hardness (Hs) of 90 or more up to a depth of at least 40 mm from the roll surface. 3. The method of manufacturing a composite roll according to claim 1, wherein the composite roll is provided with a compressive residual stress on the surface thereof. 4. The method for manufacturing a composite roll according to claim 2, wherein the diameter of the roll body is 300 mm or more. 5. A rolling mill provided with the composite roll manufactured by the manufacturing method according to claim 1.