JP3006984B2 - Work roll for hot rolling rough rolling stand - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to improvement of abrasion resistance, surface roughening resistance, heat crack resistance, slip resistance, etc. of a work roll for rough rolling used for hot rolling of steel materials such as a hot strip mill. .

[0002]

2. Description of the Related Art Work rolls in hot rolling mills such as hot strip mills , especially those with large thermal and mechanical loads
The work roll used in the rough rolling stand has a toughness that can withstand the bending of the rolling load and a resistance to abrasion, rough skin, thermal cracks, etc., in order to smoothly perform the hot rolling operation and ensure the quality of the material to be rolled. It is required to have the property. Adamite cast iron, nickel grain cast iron, ductile cast iron, etc. are typical roll materials that have been used in the past, but each has its strengths and weaknesses, and all of its properties such as wear resistance, surface roughening resistance, and heat crack resistance. It is hard to say that it can be satisfied.

In recent years, as a work roll for hot rolling, high C-
High-Cr cast iron rolls have been widely used.
No. 58-30382 discloses that, as shown in FIG. 1, a roll is a laminated structure composed of an outer shell layer 1, an intermediate layer 2, and a shaft core material 3, and the outer shell layer 1 has a high C-high Cr cast iron. And a roll in which the shaft core material 3 is formed of ductile cast iron via the intermediate layer 2 has been proposed. The roll having the laminated structure is made of a high C-high Cr cast iron of the outer shell layer 1 (M ₇ C ₃ type hard Cr
(Having a structure in which a large amount of carbides are dispersed) so that the roll surface has a high degree of wear resistance, while the core material 3 made of ductile cast iron guarantees the toughness against bending of the rolling load and the outer shell. From layer 1 (which contains a large amount of carbide forming elements such as Cr) to shaft core material 3 (ductile cast iron)
An intermediate layer 2 is provided as a barrier layer for preventing deterioration (decrease in toughness) of the shaft core material 3 due to diffusion and mixing of a carbide-forming element such as r so that required toughness is maintained. I have.

[0004]

The above-mentioned roll having a laminated structure in which the outer shell layer of the hot rolling roll is formed of high C-high Cr cast iron can be applied to a rough rolling stand having a large thermal and mechanical load. And exhibits better wear resistance than conventional rolls such as adamite cast iron rolls. However, there is a problem that resistance to rough surface, heat crack, chipping, and the like is not sufficient, and a slip with a material to be rolled easily occurs. According to the studies of the present inventors, the above-mentioned problems related to surface properties are as follows.
High C- high Cr cast iron forming the outer shell layer, M ₇ C ₃ type C
This is due to having a coarse structure containing a large amount of r carbide. In other words, skin roughness is likely to occur because the structure is coarse and the resistance to tissue flow under hot rolling conditions with large thermal and mechanical loads is low. The amount (area ratio) of hard M ₇ C ₃ type Cr carbide in the steel is as high as about 20 to 30%, so that sufficient toughness cannot be secured, and a large amount of hard carbide is contained. Therefore, the surface roughness of the roll in the process of using the actual machine is small, and this is a cause of the slip with the material to be rolled. Moreover, since the M ₇ C ₃ type Cr carbide has a so-called chrysanthemum petal-like coarse shape, a chipping phenomenon of peeling and falling off from the roll surface in a rolling process under high pressure and high heat load conditions is apt to occur. Recent hot rolling operations are
A demand for improvement in productivity and the like has led to a trend toward high pressure and a high heat load, and the above problem has become more remarkable. The present invention relates to a hot-rolled roll having the above-mentioned laminated structure, wherein a roll of a rough rolling stand having the largest thermal and mechanical load is provided.
The above characteristics such as toughness and abrasion resistance that can withstand the operating conditions of high pressure and high heat load required for crawl are improved, and the service life of the roll and the quality of the material to be rolled are improved.
It is an object of the present invention to provide an improved hot roll capable of stabilizing.

[0005]

Means for Solving the Problems Hot rolling rough rolling of the present invention
The stand work roll has a laminated structure including an outer shell layer and an intermediate layer laminated by centrifugal casting, and a shaft core material cast inside the intermediate layer.
0.8-1.2%, Si: 0.5-1.5%, Mn:
0.5 to 1.5%, Cr: 3.0 to 12.0%, Mo:
1.0-3.0%, V: 0.5% or more, less than 2.0%,
Ni: 0.5 to 2.0%, the balance being cast steel having a molten metal composition substantially composed of Fe, the amount of carbide (area ratio) in the structure of the outer shell layer is 8% or less, and the intermediate layer is C:
1.0 to 2.0%, Si: 1.5 to 2.5%, Mn:
1.0% or less, Ni: 1.0% or less, balance substantially Fe
, And the shaft core is made of ductile cast iron.

[0006]

The cast steel forming the shell layer of the hot-rolled roll of the present invention has a lower C content and a lower Cr content as a carbide-forming element than V-C cast iron.
Is adjusted to a composition containing a certain amount of M ₇ C ₃ type carbide formed by the combination of Cr and C [(Fe, Cr)
₇ C ₃ ] exhibits a chrysanthemum petal-like coarse crystallization form as described above, whereas the MC-type VC carbide formed by combining V and C has a fine and uniform crystallization form. . The outer shell layer of the roll of the present invention suppresses the growth of dendrites and refines austenite crystal grains as an effect that VC carbides are finely crystallized in the process of cooling and solidifying the molten cast metal. Further, as the crystal grains are refined, M ₇ C ₃ type Cr carbide generated at the grain boundaries is also refined. As an effect of the micronization of the structure, high resistance to the flow of the structure in actual use under a high pressure and a high heat load is imparted, and good surface roughness resistance is exhibited.
In addition, since the amount of C is regulated to a small amount, the amount of carbide generated is small, and the ratio (area ratio) in the structure is about 8% or less. Therefore, the toughness is high, and the resistance to crack generation and propagation due to thermal stress is high. High heat resistance and excellent heat crack resistance. Further, as an effect of suppressing the amount of carbide in the structure, an appropriate surface roughness required for avoiding slip with the material to be rolled in the actual use process of the roll is imparted. In addition, since the carbide has a fine crystallization form, it is also excellent in chipping resistance in the process of using the actual machine.

The roll of the present invention has high abrasion resistance while suppressing the amount of carbide in the outer shell layer to a small amount as described above. This is because the VC carbide has a micro Vickers hardness Hv of about 2400 to 3400 and a Cr carbide [(F
e, Cr) ₇ C ₃ ], which is remarkably higher than the Hv of about 1800 to 2500. The effect of this high-hardness carbide being finely dispersed in the matrix is that despite the fact that the total amount of carbides is small, high C-
It is possible to secure a high level of wear resistance equal to or higher than that of high Cr cast iron. In addition, if the carbide forming element diffuses and mixes from the outer shell layer (including carbide forming elements such as Cr, Mo, and V) into the shaft core made of ductile cast iron, it may cause the toughness of the shaft core to be impaired. However, in the present invention, since the Cr content of the outer shell layer is relatively low, the diffusion and intrusion from the outer shell layer are correspondingly small, and the diffusion and intrusion are effectively suppressed and prevented using the intermediate layer as a barrier, Accordingly, the strength and toughness of the shaft core material for resisting bending under a high rolling load are sufficiently ensured.

Hereinafter, the reasons for limiting the components of the molten cast steel in the outer shell layer of the roll of the present invention will be described. C: 0.8 to 1.2% C combines with Cr, Mo, V and the like to form carbides, and contributes to improvement of wear resistance. If the content is less than 0.8%, the amount of carbide generated is insufficient, and the wear resistance cannot be sufficiently increased. On the other hand, when it exceeds 1.2%, the toughness is reduced due to the excessive generation of carbides, and the heat crack resistance is impaired.

Si: 0.5-1.5% Si is added as a deoxidizing agent for molten cast steel. If the amount is less than 0.5%, the deoxidizing effect is insufficient, while 1.5%
%, It causes embrittlement and lowers thermal crack resistance.

Mn: 0.5 to 1.5% Mn is used as a deoxidizing agent for molten cast steel and is an impure S component.
Requires at least 0.5% as an element for fixing and detoxifying MnS. However, if it exceeds 1.5%,
Retained austenite is likely to be generated, resulting in insufficient hardness and wear resistance.

Cr: 3.0 to 12.0% Cr forms a solid solution in the matrix to strengthen the matrix, and partially forms a Cr-based carbide to contribute to improvement in wear resistance. At least 3.0% is required to achieve this effect. The effect is increased by increasing the amount of addition, but if it exceeds 12.0%, the generation amount of Cr-based carbides increases unnecessarily, and the generation amount of fine VC carbides having high hardness is insufficient. This causes inconveniences such as a decrease in resistance to rough skin.

Mo: 1.0 to 3.0% Mo forms a solid solution in the matrix to improve hardenability and prevent temper brittleness, and has an effect of improving high-temperature properties. It forms Mo-based carbides and contributes to improvement of wear resistance. To ensure these effects, at least 1.0% is required. The effect increases with the increase in the amount of addition, but if it exceeds 3.0%, it becomes supersaturated, and further addition is not only useless, but also causes stabilization of retained austenite and causes insufficient hardness and wear resistance. Become.

V: 0.5% or more and less than 2.0% V is an element that most contributes to improvement of wear resistance by forming fine and high-hardness VC carbides by combining with C. The MC type carbide is remarkably finer than the M ₇ C ₃ type Cr carbide crystallized in the matrix of high C-high Cr cast iron, and as a result of the fine crystallization, the growth of dendrites is suppressed. , While the austenite crystal grains are refined,
M ₇ C ₃ type Cr-based carbides generated at grain boundaries are also refined. Due to the fineness and uniformity of the structure, a remarkable improvement effect of surface characteristics such as skin roughness resistance is achieved. At least 0.5% is necessary for the effect of adding V to be sufficient. Although the effect is seen to increase with the increase in
If the amount is too large, the amount of carbide generated becomes excessive, and the specific gravity separation of the carbide due to centrifugal force becomes significant in the centrifugal casting step, thereby impairing the effect of improving the surface characteristics. this
Therefore, it is set to less than 2.0%.

Ni: 0.5 to 2.0% Ni has the effect of forming a solid solution in the matrix to enhance the hardenability. If it is less than 0.5%, the effect cannot be obtained, while if it exceeds 2.0%, retained austenite is liable to occur,
It causes insufficient hardness.

Next, the intermediate layer formed at the layer interface between the outer shell layer and the shaft core will be described. The intermediate layer is a layer that serves to prevent carbide-forming elements such as Cr, Mo, and V from diffusing and mixing into the mandrel from the outer shell layer as described above. The molten metal does not contain carbide forming elements such as Cr, Mo, V, etc.
The amount inevitably mixed in the melting technology (about 0.05%
Below). Also, the intermediate layer needs to be castable at a casting temperature lower than the melting point of the outer shell layer. When the casting temperature is increased, the amount of remelting of the outer shell layer caused by the heat effect at the time of casting the molten metal increases, so that the amount of Cr, Mo, V, etc. of the intermediate layer to be cast increases. This is because these elements cause an increase in the amount of diffusion and mixing of these elements into the shaft core material and a deterioration in toughness thereof. The reasons for limiting the components of the molten cast steel forming this intermediate layer are as follows.

C: 1.0 to 2.0% C has the effect of lowering the melting point of cast steel and lowering the casting temperature. If it is less than 1.0%, the casting temperature is high, and it is difficult to suppress and prevent remelting of the outer shell layer. On the other hand, the upper limit is set to 2.0% in order to prevent the deterioration of toughness due to the formation of carbide when the carbide forming element is diffused and mixed.

Si: 1.5 to 2.5% Si requires 1.5% or more as a deoxidizing agent. However, if it exceeds 2.5%, embrittlement is caused and the tensile strength of the intermediate layer is reduced.

Mn: 1.0% or less Mn has a deoxidizing effect and an effect of fixing impurities S as MnS. This effect can be obtained in amounts of 1.0% or less, and does not require further addition.

Ni: 1.0% or less Ni is an element effective for graphitization and strengthening of the matrix. The crystallization of graphite is effective in reducing volume shrinkage accompanying solidification of the molten metal and preventing shrinkage cavities. This effect can be obtained in an amount of 1.0% or less, and no additional addition is necessary.

As the core material formed through the intermediate layer, ductile cast iron is used. Ductile cast iron is a material known as a tough roll material for hot rolling, and its chemical composition is C: 2.5 to 4.0%, Si: 2.5 to
3.5%, Mn: 0.5% or less, Ni: 0.5 to 1.0
%, Cr: 0.5% or less, Mg: 0.02 to 0.08
%, With the balance being Fe.

The work roll for hot rolling of the present invention is manufactured by a two-stage casting process of centrifugal casting and cast-in-place casting. First, in a horizontal centrifugal casting machine, a cast steel melt serving as an outer shell layer is cast in a mold, and an outer shell layer is formed on the inner surface of the mold by the action of centrifugal force. Then, an intermediate layer is formed on the inner surface of the outer shell layer. Then, while erecting the mold,
A sand mold is added to the upper and lower ends of the mold, and a ductile cast iron melt is cast (place casting) in the inner space of the intermediate layer to form a shaft core material. The laminated cast body (roll) obtained by the casting is subjected to a quenching / tempering treatment as a heat treatment for refining. The quenching treatment is achieved by heating and holding at a temperature of 900 to 1000 ° C., followed by rapid cooling (forced air cooling).
This is achieved by gradually cooling (air cooling) after heating and holding at 50 to 600 ° C. After the heat treatment, predetermined machining is performed to finish the product roll.

[0022]

【Example】

[1] Production of test rolls The laminated cast body shown in FIG. 1 is produced by centrifugal casting in which the outer shell layer and the intermediate layer are laminated and cast-in-place casting in which the shaft core material is formed, and is subjected to heat treatment. Then, finish machining is performed to obtain a test roll A (inventive example) and a roll B (comparative example). The roll size is 1200 mm in outer diameter of the trunk and 25 in the trunk length.
00 mm and the surface finish roughness of the trunk is 6S. Table 1 shows the composition (Wt%) and the thickness (mm) of the cast molten metal of each layer of the test rolls A and B. The material of the outer shell layer of the test roll B is high C-high Cr cast iron (Japanese Patent Publication No. 58-30382).
No. 2).

[2] Surface Hardness of Test Roll and Structure of Outer Shell Layer (1) Surface Hardness Roll A (Inventive Example): H _S 78.5 ± 0.6 Roll B (Comparative Example): H _S 72.3 ± 0.7 (2) Structure of outer shell layer FIGS. 2 and 3 show roll A (inventive example) and roll B
The metal structure (etching: 5% nitric acid alcohol, 10 seconds) of the outer shell layer of (Comparative Example) is shown (magnification: (1): ×
100, (2): × 400). The black part in the figure is the matrix, and the white part is the carbide. Roll B shown in FIG. 3 (comparative example)
Keeps the dendritic remnants grown during the casting and solidification process,
The carbide grains are also large and have a coarse structure. On the other hand, the roll A (invention example) in FIG. 2 has a fine structure in which dendritic growth is suppressed. The amount of carbide occupying each structure is as follows. Roll A (Invention Example): 4.8 area% (FIG. 2) Roll B (Comparative Example): 29.6 area% (FIG. 3)

[3] Use test of actual machine The test rolls A and B are used as work rolls of a rough rolling stand (a roll stand having the largest thermal and mechanical load) in a hot strip mill, and several actual machine use tests are performed. (Each rolling amount is 25000-4500
0 tons) to obtain the results shown in Table 2. The surface roughness ( _Rmax ) and the amount of wear in the table are the average of the measured values after the test use. As is clear from the table, the roll B of the comparative example has rough skin and chipping of the trunk (the surface of the outer shell layer),
And slip with the material to be rolled, etc.
In the case of the roll A of the invention, there is no occurrence of rough skin, chipping and slippage, and the wear loss is also reduced. As shown in FIG. 2, the excellent surface roughness resistance, slip resistance, chipping resistance and the like are due to the fact that the structure is fine and the amount of carbide is moderately suppressed. Despite being suppressed, it has high abrasion resistance because hard VC carbides are finely and uniformly dispersed, and the difference from the roll B of the comparative example is as follows. It is obvious. In addition, as a result of analyzing the chemical composition of the shaft core material after the end of the use test, the increase of carbide forming elements such as Cr, Mo, and V was trace (Cr: 0.09% → 0.49%, Mo: 0.02% → 0.12).
%, V: trace → 0.05%), and its tensile strength is about 43%.
It was confirmed that a sufficient level of 5 N / mm ² was maintained.

[0025]

[Table 1]

[0026]

[Table 2]

[0027]

The work roll for hot rolling according to the present invention has improved abrasion resistance, rough surface resistance, heat crack resistance, chipping resistance, and almost complete prevention of slip with the material to be rolled. It has surface properties and has sufficient toughness to withstand high-load bending, so it can withstand thermal and mechanical loads.
As rough rolling stand work roll larger, can be stably used in harsh use environments of recent high pressure-high heat load, reduce improve roll maintenance service life, improvement in line efficiency, and rolling quality It contributes to the improvement of the quality.

[Brief description of the drawings]

FIG. 1 is an axial sectional view showing a laminated structure of rolls.

FIG. 2 is a micrograph instead of a drawing showing a metal structure of an outer shell layer of a roll A (inventive example) in an example (FIG. (1) is a magnification of × 50, and (2) is a magnification of × 400).

FIG. 3 is a micrograph instead of a drawing showing a metal structure of an outer shell layer of a roll B (comparative example) in an example (FIG. (1) is a magnification of × 50, and (2) is a magnification of × 400).

[Explanation of symbols]

 1: outer shell layer, 2: middle layer, 3: core material.

──────────────────────────────────────────────────続 き Continuation of front page (51) Int.Cl. ⁷ Identification code FI C22C 37/08 C22C 37/08 Z 38/00 301 38/00 301L 302 302E 38/56 38/56 (56) References JP JP-A-2-258949 (JP, A) JP-A-5-271855 (JP, A) JP-A-3-254354 (JP, A) JP-A-3-122249 (JP, A)

Claims (1)

(57) [Claims] 1. A laminated structure comprising an outer shell layer and an intermediate layer laminated by centrifugal force casting, and a shaft core material cast inside the intermediate layer. 8 to 1.2%, Si: 0.5 to 1.
5%, Mn: 0.5 to 1.5%, Cr: 3.0 to 12.
0%, Mo: 1.0 to 3.0%, V: 0.5% or more,
Less than 2.0%, Ni: 0.5 to 2.0%, the balance substantially consisting of cast steel having a molten metal composition of Fe, and the amount of carbide (area ratio) in the structure of the outer shell layer is 8% or less. Yes, the intermediate layer is C: 1.0-2.0%, Si: 1.5-2.
5%, Mn: 1.0% or less, Ni: 1.0% or less, balance is substantially composed of cast steel having a molten metal composition of Fe, and the shaft core is composed of ductile cast iron. Work roll for rolling rough rolling stand .