JPH07126795A - Work roll for hot rolling - Google Patents

Abstract

PURPOSE:To develop a work roll for hot rolling steel excellent in wear resistance, surface roughening resistance, thermal crack resistance and slip resistance by laminating an intermediate layer consisting of high carbon cast steel and outer shell layer consisting of high carbon/high chromium cast steel around the axial center material made of ductile cast iron. CONSTITUTION:At the time of manufacturing the work roll for hot rolling by a hot rolling device such as a hot strip mill, first, the centrifugal casting of the outer shell layer 1 made of high carbon/high chromium cast steel contg. by wt., 0.8-1.2% C, 0.5-1.5% Si, 0.5-1.5% Mn, 3.0-12.0% Cr, 1.0-3.0% Mo, 0.5-2.0% V and 0.5-2.0% Ni is executed by using a horizontal centrifugal casting machine and the intermediate layer 2 made of high carbon cast steel contg. 1.0-2.0% C, 1.5-2.5% Si, l.O% Mn and <1.0% Ni is cast inside the outer shell layer. A mold is made to stand on its end, the axial center material 3 consisting of a tough ductile cast iron is integrated by pouring casting and, after that, quenching and tempering treatment is executed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to improvements in wear resistance, surface roughening resistance, heat crack resistance, slip resistance and the like of work rolls used for hot rolling of steel materials such as hot strip mills.

[0002]

Work rolls used in hot strip mills such as hot strip mills have a toughness that can withstand bending of rolling load and wear in order to smoothly perform hot rolling operation and ensure quality of rolled material. It is required to have resistance to rough skin, heat cracks, etc. Adamantite cast iron, nickel grain cast iron, ductile cast iron, etc. are typical roll materials that have been used so far, but they have advantages and disadvantages, wear resistance, rough surface resistance,
It is hard to say that all the properties such as heat crack resistance can be satisfied.

In recent years, as a work roll for hot rolling, high C-
High Cr cast iron rolls have been widely used, and
In JP 58-30382, as shown in FIG. 1, a roll is a laminated structure composed of an outer shell layer 1, an intermediate layer 2 and a mandrel 3, and the outer shell layer 1 is made of 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 is proposed. The roll having this laminated structure is made of high C-high Cr cast iron of the outer shell layer 1 (M ₇ C ₃ type hard Cr in the matrix).
The roll surface has a high degree of wear resistance by virtue of having a structure in which a large amount of system carbides are dispersed), while the shaft core material 3 made of ductile cast iron guarantees toughness that resists bending of rolling load, and the outer shell From layer 1 (containing a large amount of carbide forming elements such as Cr) to shaft core material 3 (ductile cast iron)
The intermediate layer 2 is provided as a barrier layer for preventing deterioration (decrease in toughness) of the mandrel 3 due to diffusion and mixing of a carbide forming element such as r, so that required toughness is maintained. There is.

[0004]

The above-mentioned laminated structure roll in which the outer shell layer of the hot rolling roll is made of high C-high Cr cast iron is applied to a rough rolling stand having a large thermal and mechanical load. Exhibits excellent wear resistance over conventional rolls such as adamite cast iron rolls. However, the resistance to rough skin, thermal cracking, chipping and the like cannot be said to be sufficient, and there is a problem that slippage with the material to be rolled easily occurs. According to the study by the present inventors, the above-mentioned problems regarding surface characteristics are
High C-high Cr cast iron forming the outer shell layer is M ₇ C ₃ type C
This is due to having a coarse structure containing a large amount of r carbide. That is, rough skin is likely to occur because the texture is coarse and resistance to tissue flow under hot rolling conditions with large thermal and mechanical loads is low, and thermal cracking is likely to occur in the texture. This is because the amount of hard M ₇ C ₃ type Cr carbide (area ratio) occupying 20% to 30% is high, so that sufficient toughness cannot be secured, and a large amount of hard carbide is included. Therefore, the roll surface roughness is small in the process of using the actual machine, which causes slipping 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 phenomenon of peeling and falling off from the roll surface is likely to occur during the rolling process under high pressure and high heat load conditions. Recent hot rolling operations are
Due to the demand for improved productivity, the trend of high pressure and high heat load is progressing, and the above problems are becoming more prominent. The present invention, with respect to the hot rolled roll having the above-mentioned laminated structure, enhances the above-mentioned characteristics including toughness and wear resistance capable of withstanding operating conditions under high pressure and high heat load, and improves roll life and quality of rolled material. -It is intended to provide an improved hot rolling roll that enables stabilization.

[0005]

A roll for hot rolling according to the present invention is a laminate comprising an outer shell layer and an intermediate layer laminated by centrifugal casting, and a shaft core material cast inside the intermediate layer. The outer shell layer has a structure of C: 0.8 to 1.2%, S
i: 0.5 to 1.5%, Mn: 0.5 to 1.5%, C
r: 3.0 to 12.0%, Mo: 1.0 to 3.0%,
V: 0.5 to 2.0%, Ni: 0.5 to 2.0%, the balance being cast steel having a molten metal composition consisting essentially of Fe,
The intermediate layer has C: 1.0 to 2.0% and Si: 1.5 to 2.
5%, Mn: 1.0% or less, Ni: 1.0% or less, the balance being cast steel having a molten metal composition consisting essentially of Fe,
The shaft core material is made of ductile cast iron.

[0006]

The cast steel forming the outer shell layer of the hot-rolled roll of the present invention has a small amount of C and a small amount of Cr, which is a carbide forming element, as compared with the high C-high Cr cast iron.
Is adjusted to a composition containing a certain amount. M ₇ C ₃ type carbide [(Fe, Cr) formed by combining Cr and C
₇ C ₃ ] has a coarse chrysanthemum petal-like crystallized form as described above, whereas the crystallized form of MC-type VC carbide formed by combining V and C is fine and uniform. . In the outer shell layer of the roll of the present invention, the dendrite growth is suppressed and the austenite crystal grains are miniaturized, as an effect of finely crystallizing VC carbides during the temperature-lowering solidification process of the casting melt. Further, along with the refinement of the crystal grains, the M ₇ C ₃ type Cr carbide generated at the grain boundaries is also refined. As a micronization effect of this structure, high resistance to tissue flow under high pressure / high heat load is actually imparted, and good resistance to rough skin is exhibited.
Also, since the amount of C is regulated to a small amount, the amount of carbides generated is small, and the ratio (area ratio) in the structure is about 8% or less. Therefore, it is rich in toughness and has resistance to the generation and propagation of cracks due to thermal stress. It has high resistance to heat cracking. Further, as an effect of suppressing the amount of carbides occupying in the structure, an appropriate surface roughness required to avoid slip with the material to be rolled is imparted in the process of using the roll in an actual machine. Moreover, since the carbide has a fine crystallized morphology, it has excellent chipping resistance in the process of actual use.

The roll of the present invention has a high degree of abrasion resistance while suppressing the amount of carbide in the outer shell layer to a small amount as described above. This is because VC carbide has a micro Vickers hardness Hv of about 2400 to 3400 and Cr carbide [(F
e, Cr) ₇ C ₃ ], which is significantly higher than Hv of about 1800 to 2500, and the effect of finely dispersing this high hardness carbide in the matrix is that although the total amount of carbide is small, high C-
This makes it possible to ensure a high level of wear resistance equivalent to or higher than that of high Cr cast iron. In addition, if the carbide forming element diffuses and mixes into the shaft core material made of ductile cast iron from the outer shell layer (including carbide forming elements such as Cr, Mo, V), it causes the deterioration of the toughness of the shaft core material. However, in the present invention, since the Cr content of the outer shell layer is relatively low, the diffusion and mixing from the outer shell layer is so small, and the diffusion and invasion are effectively suppressed and prevented by using the intermediate layer as a barrier, Therefore, the strength and toughness of the shaft core material against the bending of a high rolling load are sufficiently secured.

With respect to the roll of the present invention, the reasons for limiting the components of the molten cast steel of the outer shell layer will be described below. C: 0.8 to 1.2% C combines with Cr, Mo, V, etc. to form a carbide, which contributes to the improvement of wear resistance. If it is less than 0.8%, the amount of carbide formed is insufficient and the wear resistance cannot be sufficiently enhanced. On the other hand, if it exceeds 1.2%, the toughness is lowered due to excessive formation of carbides, and the heat crack resistance is impaired.

Si: 0.5 to 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%
If it exceeds%, embrittlement is caused and the heat crack resistance is lowered.

Mn: 0.5-1.5% Mn is a deoxidizing agent for the molten cast steel and is an impurity of S.
As MnS, at least 0.5% is required as an element for fixing and detoxifying. However, if it exceeds 1.5%,
Residual austenite is likely to occur, 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 the improvement of wear resistance. At least 3.0% is required to obtain this effect. The effect is increased by increasing the amount of addition, but if it exceeds 12.0%, the amount of Cr-based carbides is unnecessarily increased, and the amount of fine VC carbides with high hardness is insufficiently generated, resulting in a decrease in toughness. As a result, problems such as deterioration of the rough skin resistance are caused.

Mo: 1.0 to 3.0% Mo is effective as a solid solution in the matrix to improve hardenability and prevent temper embrittlement, and also has an effect of improving high temperature characteristics. It forms a Mo-based carbide and contributes to the improvement of wear resistance. To secure these effects, at least 1.0% is required. The effect increases with increasing the amount of addition, but if it exceeds 3.0%, it becomes supersaturated, and addition beyond that is not only a waste, but also leads to stabilization of retained austenite, causing a lack of hardness and wear resistance. Become.

V: 0.5 to 2.0% V is an element which contributes most to the improvement of wear resistance by forming a fine and high hardness VC carbide by combining with C. The MC type carbide is significantly finer than the M ₇ C ₃ type Cr carbide crystallized in the matrix of high C-high Cr cast iron, and the effect of the fine crystallization is that the growth of dendrites is suppressed. At the same time that the austenite grains are refined,
The M ₇ C ₃ type Cr-based carbides generated at the grain boundaries are also refined. By making the structure fine and uniform, remarkable improvement effects of surface characteristics such as resistance to roughening of the surface are achieved. At least 0.5% is required to make the effect of adding V sufficient. The effect is increased by increasing the amount added, but 2.0%
If it exceeds, the amount of carbide produced becomes excessive, and in the centrifugal casting step, the specific gravity separation of the carbide becomes remarkable due to centrifugal force, and the effect of improving the above-mentioned surface characteristics is impaired.

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 apt to occur,
It causes a lack of hardness.

Next, the intermediate layer formed at the layer interface between the outer shell layer and the mandrel will be described. As described above, the intermediate layer has a function of preventing the carbide forming elements such as Cr, Mo and V from diffusing and mixing into the shaft core material from the outer shell layer, and therefore, the cast steel used for this casting. The molten metal does not contain carbide forming elements such as Cr, Mo, V, etc.
Amount in the range that is inevitably mixed in the melting technology (about 0.05%
Limited to the following). Further, 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 becomes higher, the amount of remelting of the outer shell layer caused by the heat effect of the molten metal at the time of casting increases, so the amount of Cr, Mo, V, etc. in the cast intermediate layer increases. This is because it causes an increase in the amount of these elements diffused and mixed into the mandrel and deterioration of its toughness. The reasons for limiting the components of the molten cast steel forming the 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 becomes difficult to prevent and prevent remelting of the outer shell layer. On the other hand, the upper limit of 2.0% is to prevent deterioration of toughness associated with the formation of carbides when the carbide-forming elements diffuse and mix.

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 lowered.

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

Ni: 1.0% or less Ni is an element effective in graphitizing and strengthening the matrix. Crystallization of graphite alleviates the volume contraction accompanying the solidification of the molten metal and is effective in preventing shrinkage cavities. This effect can be obtained in an amount of 1.0% or less, and addition beyond that is unnecessary.

Ductile cast iron is applied to the shaft core formed through the intermediate layer. Ductile cast iron is a material well known as a roll material for hot rolling having a toughness, 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
%, Balance Fe.

The work roll for hot rolling of the present invention is manufactured by a two-step casting process of centrifugal force casting and decant casting. First, in a horizontal centrifugal casting machine, cast steel molten metal to be the outer shell layer is cast into the mold, the outer shell layer is formed on the inner surface of the mold by the action of centrifugal force, and after the outer shell layer has almost solidified to the inner surface, the intermediate layer Then, a cast steel molten metal is cast to form an intermediate layer on the inner surface of the outer shell layer. Then, while making the mold upright,
A sand mold is added to the upper and lower ends of the mold, and the molten core of ductile cast iron is cast (placed and cast) into the inner space of the intermediate layer to form the mandrel. The laminated cast body (roll) obtained by the above casting is subjected to quenching / tempering treatment as heat treatment for refining. The quenching treatment is achieved by heating and holding at a temperature of 900 to 1000 ° C. and then rapidly cooling (forced air cooling), and the tempering treatment is performed at a temperature of 4
It is achieved by heating and holding at 50 to 600 ° C. and then gradually cooling (air cooling). After the heat treatment, predetermined machining is performed to finish the product roll.

[0022]

【Example】

[1] Manufacture of test roll After manufacturing a laminated cast body shown in FIG. 1 by centrifugal force casting in which an outer shell layer and an intermediate layer are laminated and by pouring casting in which a shaft core material is formed, and subjecting to heat treatment Then, finish machining is applied to obtain a test roll A (invention example) and a roll B (comparative example). Roll size is 1200 mm outer diameter, 25 body length
The surface finish roughness of the body is 6S. Table 1 shows the molten metal composition (Wt%) and the layer thickness (mm) of each layer of the test rolls A and B. The grade of the outer shell layer of the test roll B is high C-high Cr cast iron (Japanese Patent Publication Sho 58-30382).
(Corresponding to the material of the outer shell layer described in the publication).

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

[3] Test using actual machine Test rolls A and B were used as work rolls of a rough rolling stand (which is the rolling stand with the largest thermal and mechanical load) in the hot strip mill, and several testes using the actual machine were carried out. (The rolling amount of each time is 25000 to 4500, respectively.
It was subjected to 0 tons) and the results shown in Table 2 were obtained. The surface roughness (R _max ) and wear amount in the table are averages of the measured values after the test use. As is clear from the table, the roll B of the comparative example has rough skin on the body (outer shell layer surface) and chipping off,
And, while slips etc. with the rolled material have occurred,
In Roll A of the invention example, the skin is not roughened, chipped off, slipped, and the abrasion loss was reduced. As shown in FIG. 2, the excellent resistance to rough skin, slip resistance, chipping resistance, etc. is due to the fact that the structure is fine and the amount of carbide is moderately suppressed. The reason why it has a high degree of wear resistance despite being suppressed is that the hard VC carbides are finely and uniformly dispersed, and the difference from the roll B of the comparative example is 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, a slight increase in carbide forming elements such as Cr, Mo and V (Cr: 0.09% → 0.49%, Mo: 0.02% → 0.12
%, V: trace → 0.05%), and the 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 of the present invention has been improved so as to be able to substantially completely prevent wear resistance, surface roughening resistance, heat crack resistance, chipping resistance, and slip with the material to be rolled. In addition to having surface characteristics, it has sufficient toughness to withstand high-load bending, and can be used stably under the recent severe conditions of high pressure and high heat load. It contributes to reduction of maintenance, improvement of line efficiency, and improvement of rolling quality.

[Brief description of drawings]

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

FIG. 2 is a drawing-substituting micrograph showing a metal structure of an outer shell layer of roll A (invention example) in Examples [(1) is a magnification of 50, and (2) is a magnification of 400].

FIG. 3 is a drawing-substituting micrograph showing a metal structure of an outer shell layer of Roll B (Comparative Example) in Examples [(1) is a magnification of × 50, (2) is a magnification of × 400].

[Explanation of symbols]

 1: Outer shell layer, 2: Middle layer, 3: Shaft core material.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Office reference number FI technical display location C22C 37/08 Z 38/00 301 L 302 E 38/56

Claims (1)

[Claims] 1. A laminated structure comprising an outer shell layer laminated by centrifugal casting, an intermediate layer, and a mandrel cast inside the intermediate layer, wherein the outer shell layer has a C: 0. 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 to 2.0
%, Ni: 0.5 to 2.0%, the balance being cast steel having a molten metal composition substantially consisting of Fe, the intermediate layer being C: 1.0 to 2.0%, Si: 1.5 to 2 ．
5%, Mn: 1.0% or less, Ni: 1.0% or less, the balance is made of cast steel having a molten metal composition substantially consisting of Fe, and the mandrel is made of ductile cast iron. Work roll for rolling.