JPH108212A - Roll for hot rolling - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a roll for hot rolling, minimal in frictional resistance with a material to be rolled and having an effect of preventing the occurrence of scale defects at the surface of the material to be rolled. SOLUTION: This roll is constituted so that at least the outer layer of the roll is composed of a high carbon high speed steel having a composition containing, by weight, 1.5-3% C, 0.5-5% Cr, 0.5-8% Mo, 1-8% V, >1-8% W, 0.1-5% Nb, and 0.01-1% B and spheroidal MC type carbide, which has <=15μm grain diameter and in which the ratio between the major axis and minor axis of each grain is regulated to <=2, exists in the amount of 5-20% by area ratio.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot rolling roll used for hot rolling of steel, and more particularly, to a scale having a low friction property and generating scale flaws on the surface of the steel caused by the rolling roll. The present invention relates to a hot-rolling roll capable of suppressing the occurrence of heat.

[0002]

2. Description of the Related Art The outer shell layer of a conventionally used roll for hot rolling of steel is made of high chromium cast iron or high alloy grain cast iron having the following chemical composition and excellent wear resistance. ing. High chromium cast iron, in weight%, C: 2.3-2.9
%, Si: 0.4 to 0.9%, Mn: 0.8 to 1.2
%, Ni: 0.5 to 1.5%, Cr: 15 to 20%, M
o: contains 0.7 to 2%, the balance being substantially Fe, is intended to include 20 to 35 percent by area ratio of the M ₇ C ₃ type carbide.

[0003] The high-alloy grain cast iron has a C: 3.2 to
3.4%, Si: 0.7 to 0.9%, Mn: 0.8 to
1.2%, Ni: 4.2 to 4.6%, Cr: 1.5 to
1.9%, Mo: 0.3 to 0.6%, the balance substantially consisting of Fe, and M ₃ C-type carbide in an area ratio of 25 to
It contains 40%.

Recently, in order to further improve wear resistance,
JP-A-2-220634, JP-A-2-25205, JP-A-2
-88745, JP-A-3-26838, JP-A-3-219047
And high-speed steels as disclosed in Japanese Patent Application Laid-Open No. HEI 4-176840 and the like (hereinafter referred to as high-carbon high-speed steels) have been used. Since such a roll material has a high hardness carbide in its structure, it exhibits excellent hot wear resistance.

Japanese Patent Application Laid-Open No. Hei 5-217855 discloses a roll material for hot sheet rolling in which the wear resistance and the surface roughening resistance are improved by limiting the amount and hardness of carbides in the structure. Further, Japanese Patent Publication No. 7-78267 discloses that the organization is 30 to 150
It has a crystal grain size of μm and is surrounded by M ₆ C-type eutectic carbide crystallized at the grain boundaries, and has a metal structure having MC-type primary crystal carbide in the grains. A rolling roll is disclosed. In JP-A-5-305312,
In order to prevent the MC type carbide having a small specific gravity from separating to the inner layer side during centrifugal casting during centrifugal casting, a roll using a high-carbon high-speed steel as an outer layer, which defines V and Nb content limiting conditions, is disclosed. Have been.

Further, Japanese Patent Application Laid-Open No. 58-213856,
No. 21042 discloses a roll for rolling in which a sintered alloy layer of a high-carbon high-speed steel is formed on the body surface by powder metallurgy. However, this powder metallurgy method can obtain a spherical carbide and a fine metal structure, but has disadvantages in that quality defects such as porosity are present and the production cost is increased.

[0007]

The high-carbon high-speed steel roll described above has excellent wear resistance, but has high frictional resistance with the material to be rolled during hot rolling. Therefore, the rolling load increases, and the performance as a rolling mill decreases. In addition, this roll causes a surface defect of the material to be rolled called a scale flaw. The scale flaw is a defect in which an oxide (scale) layer on the surface of a material to be rolled is bitten on the surface of a steel sheet during hot rolling, and deteriorates the quality of a product. The cause is presumed to be fine irregularities on the roll surface caused by abrasion of the roll surface. For this reason, the roll profile must be changed before the roll profile deteriorates due to the wear of the roll surface, and the roll grinding amount increases, resulting in an increase in manufacturing cost, and solving the problem has become an important issue. I have.

It is an object of the present invention to provide a roll for hot rolling which has a small frictional resistance with a material to be rolled and has an effect of suppressing the occurrence of scale flaws on the surface of the material to be rolled.

[0009]

Means for Solving the Problems The present inventors have proposed means for reducing the frictional resistance of a high-carbon high-speed steel roll with a material to be rolled and for preventing the occurrence of scale flaws between the roll and material to be rolled. The effect of roll material on the coefficient of friction was studied. As a result, the following findings were obtained.

[0010] 1) The reason that the friction coefficient increases during rolling is that the base in the structure of the roll surface is selectively worn and the high hardness M
This is because the C-type carbides protrude from the roll surface and increase the friction between the roll and the material to be rolled.

2) Elongated fine M formed on the roll surface
The projections of the C-type carbide prevent the roll from slipping between the roll and the steel sheet, the deformation of the scale becomes uneven, and the scale bites into the base material to generate scale flaws.

3) By dispersing fine MC-type carbide having a ratio of major axis to minor axis of 2 or less (hereinafter referred to as spherical MC carbide), the height of carbide projections formed on the roll surface during rolling is reduced. And the frictional resistance during rolling can be reduced, and the occurrence of scale flaws can be suppressed.

The present invention has been made based on such knowledge, and the gist of the invention is that “at least the outer shell layer of the roll is
C: 1.5-3%, Cr: 0.5-5%, M by weight%
o: 0.5 to 8%, V: 1 to 8%, W: more than 1 to 8%, N
b: High-carbon high-speed steel containing 0.1 to 5% and B: 0.01 to 1%.
In the following, a spherical M in which the ratio of the major axis to the minor axis of each grain is 2 or less.
A roll for hot rolling, wherein the C-type carbide is present in an area ratio of 5 to 20%. "

Here, the outer shell layer refers to a surface layer of a roll which comes into contact with a material to be rolled at the time of rolling, and can be manufactured by selecting a mold of any thickness. Although the thickness of the outer shell layer is not limited, it is needless to say that the outer shell layer needs to have a thickness at least not to be worn and lost even after long-time rolling use.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION The reasons for limiting the chemical composition of the outer shell layer of the hot rolling roll of the present invention and the shape and area ratio of MC type carbide in the outer shell layer as described above are described below.

(1) Chemical composition (Hereinafter, all chemical composition percentages are expressed as% by weight.) C: 1.5 to 3% C combines with Cr, Mo, W, V, etc. to form a high hardness composite. A carbide (composite carbide of M ₆ C, M ₂ C, M ₇ C ₃ or the like mainly composed of MC type) is formed to increase the wear resistance of the roll. If the content is less than 1.5%, the amount of carbide generated will be insufficient and the hot wear resistance will be reduced. On the other hand, if it exceeds 3%, the amount of carbide generated will increase too much, and toughness and heat crack resistance will decrease.
Therefore, the C content is set to 1.5 to 3%. Preferably, it is 1.6 to 2.6%.

Cr: 0.5 to 5% Cr forms a solid solution in the matrix to strengthen the matrix and improve the heat crack resistance, and also combines with C to precipitate a fine high-hardness compound and to provide wear resistance. Has the effect of increasing the toughness and toughness. However, Cr
If the content is less than 0.5%, these effects are small. On the other hand, if it exceeds 5%, carbides become coarse. Therefore, Cr
The content was 0.5 to 5%. Preferably, 2-4.5
%.

Mo: 0.5 to 8% Mo is dissolved in a matrix to improve the high-temperature softening resistance.
It has the effect of forming fine and high-hardness complex carbides such as M ₆ C and M ₂ C by combining with C to improve warm wear resistance. However, when the content is less than 0.5%, these effects are small.
%, The toughness and the heat crack resistance deteriorate. Therefore, the Mo content is set to 0.5 to 8%. Preferably, it is 2 to 6%.

V: 1 to 8% V combines with C to form an MC-type carbide, which is useful for improving wear resistance and accident resistance. However, if it is less than 1%, the effect of improving the wear resistance is small, while if it exceeds 8%, the amount of carbides increases and the heat crack resistance decreases. Therefore, the V content was set to 1 to 8%. Preferably, it is 2 to 6.5%.

W: more than 1% to 8% W combines with C to form M ₆ C, M ₂ C type fine high-hardness complex carbides to increase wear resistance, and to form a solid solution in the matrix. Increases tempering softening resistance and heat resistance. However, when the content is less than 1%, these effects are small. On the other hand, when the content exceeds 8%, coarse carbides are increased to deteriorate toughness and heat crack resistance, and segregation is liable to occur. Therefore, the W content is set to more than 1 to 8%. Preferably, it is 1.5 to 6%.

Nb: 0.1 to 5% Nb is one of the elements that play an important role in the present invention, and combines with C together with V to form a fine MC-type carbide, and has a fine metal structure. And contribute to the reduction of the friction coefficient of the roll. However, when the content is 0.1% or less, these effects are small, and when the content exceeds 5%, the material becomes brittle. Therefore, the content of Nb is 0.1.
1 to 5%. Preferably, it is 0.1 to 4%.

B: 0.01 to 1% B is one of the elements that play an important role in the present invention, and spheroidizes the MC type carbide to reduce the friction coefficient of the roll and improve the wear resistance. . However, if the content is 0.01% or less, these effects are small, while if it exceeds 1%, the melting point of the material decreases, and the high-temperature strength decreases. Therefore, the content of B is set to 0.01 to 1%.
Preferably, it is 0.01 to 0.7%.

Other elements: As other elements, T
i, Al, Si, Mn, Co, and the like can be contained as necessary. Their actions and suitable contents are as follows. Ti: 0.01-1%, Al: 0.01-0.5% 1
Species or two: Ti and Al are elements to be contained as necessary, form oxides at the time of casting, and serve as nuclei for crystallization of spherical and fine MC-type carbides. However, if the content is too small, these effects are small. On the other hand, if the content is too large, the material tends to become brittle, and the molten metal is easily oxidized at the time of casting, which causes defects such as cavities and segregation. Therefore, when these elements are contained, the content is preferably within the above range. .

Si: 0.2-2.5% Si is preferably contained as a deoxidizing agent. If it is less than 0.2%, deoxidizing action cannot be expected, while if it exceeds 2.5%, the roll material is Tend to be brittle and the cracks at the time of the occurrence of a rolling accident tend to be deep.

Mn: 0.3 to 1.5% Mn has a deoxidizing effect, and if its content is less than 0.3%, a sufficient deoxidizing effect cannot be obtained, while it exceeds 1.5%. And the toughness tends to decrease.

Co: 0.2-10.0% Most of Co forms a solid solution with the matrix to increase the hardness of the matrix, increase the high-temperature softening resistance, and improve the high-temperature hardness and wear resistance. . However, when the content is less than 0.2%, these effects are small. On the other hand, when the content is more than 10%, sufficient hardness cannot be obtained, and the wear resistance tends to deteriorate.

P and S: P and S are inevitably contained impurity elements, but the mechanical properties are deteriorated. Therefore, the contents of P and S are preferably small. However, in any case, if it is 0.08% or less, there is not so much adverse effect.

(2) Spherical MC carbide MC carbide: There are various types of carbides.
C carbides have high hardness in a high temperature range and are therefore suitable for increasing the hardness of the roll, and are present in the outer shell layer of the roll to improve the wear resistance of the roll.

Shape of carbide: The hardness of the MC type carbide of the high carbon high speed steel roll is 3 to 4 times higher than that of the matrix (tempered martensite), and wear proceeds mainly in the matrix. Therefore, the MC type carbides protrude from the roll surface and increase the friction, which causes an increase in the rolling load.

MC in a conventional roll of high-carbon high-speed steel
The type carbide has a rod-like elongated shape. Therefore, if the carbide is present on the roll surface layer with its length direction facing the center axis direction of the roll, even if the base is worn out when the roll is used and the carbide is projected, it easily falls off the roll surface. do not do. Therefore, the height of the projections increases, and the friction with the material to be rolled increases accordingly. In order to prevent this, in the roll of the present invention, the shape of the carbide formed on the roll surface during rolling is made spherical so that the protrusion height does not increase. That is, even if the carbides are in a protruding state during rolling by being formed into a sphere, they fall off the roll surface when they slightly protrude from the base, so that the height of the protrusions does not increase.

If the ratio of the major axis to the minor axis of the MC type carbide is larger than 2, it becomes too elongated, and even if the base is worn out and the carbide particles protrude from the roll surface, the base becomes rooted in the base and is hard to fall off. Therefore, it tends to become a large projection. on the other hand,
If the ratio is 2 or less, the projections fall off before becoming large projections, so that large projections are hardly generated. Therefore, the ratio of the major axis to the minor axis of the MC type carbide was set to 2 or less.

Particle size of carbide: MC type carbide has high hardness and is indispensable for enhancing the abrasion resistance of the roll. However, if the particle size is too large, mechanical properties such as strength and toughness of the roll deteriorate, and segregation tends to occur during casting. Therefore, the particle size of the MC type carbide needs to be 15 μm or less. It is desirable that carbide particles of 5 to 15 μm be mainly contained. The particle size of MC carbide refers to the major axis of the carbide.

Amount of carbide: The high-temperature hardness of the roll increases as the MC-type carbide increases. However, if the area ratio is less than 5%, sufficient high-temperature hardness cannot be obtained, resulting in insufficient hot abrasion resistance. On the other hand, if it exceeds 20%, the toughness is reduced, so that the thermal shock resistance is deteriorated. Therefore, the content of MC type carbide having the shape and particle size specified in the present invention is set to 5 to 20% in area ratio.

The area ratio is the area ratio of carbides per unit area of the roll surface. The area ratio can be determined by cutting out a part of the surface of the cast roll, polishing it, observing it with a microscope, and performing image processing.

Next, the amount and size of the carbides are almost determined by the chemical components, but the solidification rate after casting also has some influence. Carbides tend to coarsen when the solidification rate is low, whereas they tend to be fine when the solidification rate is high. By adjusting the solidification from about 300 seconds to about 600 seconds from the start to the end of the solidification, it is possible to control the amount of generated carbides.

The roll of the present invention may be any type of cast iron in which "at least the outer shell layer of the roll" has the above-mentioned chemical composition and carbide. "At least the outer shell layer of the roll" means that the outer shell layer of the roll must have the above chemical composition and carbide, and the other parts may have the same composition as the outer shell layer. This means that a material different from the outer shell layer such as graphite cast iron, spheroidal graphite cast iron, and forged steel may be used. Therefore, the whole roll has the above-mentioned chemical composition and spherical M
The roll may be an integrated roll having a C-type carbide, or may be a composite roll in which only the outer layer has the above chemical composition. In addition, the outer shell layer refers to the surface layer of the roll, and the thickness is not particularly limited as long as the thickness does not disappear due to abrasion due to friction with the material to be rolled, the effect does not change. do not do. In the case of a composite roll, the thickness of the outer shell layer is preferably about 50 to 150 mm.

When the roll for hot rolling of the present invention is formed into a composite roll, it can be manufactured by a centrifugal casting method or a casting method, and the integrated roll can be manufactured by a static casting method, a forging method or the like. .

In the case where carbides are enriched at the boundary between the outer shell layer and the inner layer and the bonding strength between the two is reduced, a low carbon alloy cast iron or the like is used as an intermediate layer between the outer shell layer and the inner layer. It is good to provide.

[0039]

EXAMPLE A high-carbon high-speed steel having the chemical composition shown in Table 1 was used as an outer shell layer and a ductile cast iron was used as an inner layer.
mm, a body length of 1800 mm and a total length of 3800 mm were produced by a centrifugal casting method. The thickness of all the outer layer materials was 100 mm.

In order to form rolls having different MC type carbide contents and shapes in the outer shell layer, the chemical composition was adjusted by various changes.

[0041]

[Table 1]

The structure of the cast roll is adjusted,
Heat treatment for removing residual stress and adjusting hardness was performed under the following conditions.

Quenching: 1000 to 1100 ° C. Tempering: 500 to 550 ° C. A part of the surface layer of each roll obtained as described above is cut out, and MC-type carbide is extracted by an electrolytic method. The diameter ratio was measured using an optical microscope. Also,
The roll surface of the cut specimen was polished, and the area ratio of MC type carbide was determined by image processing. Table 2 shows the results.

[0044]

[Table 2]

In the roll of the present invention, the MC type carbide is finer and spherical compared to the roll of the comparative example, and the area ratio of the carbide is in the range of 5 to 20%.

These rolls are incorporated into a hot rolling finishing mill, and have a thickness of 15 to 20 mm and a width of 1000 to 1250.
mm carbon steel sheet, rolling reduction 40-50%, rolling temperature 950
Hot rolled at 1000 ° C. From the rolling load during rolling,
The coefficient of friction was determined. In addition, the maximum wear depth in the roll profile after rolling 2,000 tons with each roll was measured, and the roll surface properties at that time were visually observed. The rolled hot-rolled steel sheet was visually inspected for the occurrence of scale flaws on the surface. The results are also shown in Table 2.

As is clear from the table, the rolling load of the roll of the present invention is lower than that of the roll of the comparative example, and the coefficient of friction shows a stable value. Further, it can be seen that the roll of the present invention has a small maximum wear depth and good wear resistance. The roll of the present invention suppressed the occurrence of scale flaws. (1) The effect of suppressing fine projections on the roll surface. (2) The generation of steel sheet scale due to the reduction in processing heat and frictional heat due to a reduction in rolling load. This is considered to be a synergistic effect of the reduction in the amount.

In particular, it can be seen that Example 12 of the present invention is a roll excellent in both abrasion resistance and low friction properties and having a high scale flaw suppression effect.

In Comparative Examples 16 to 22, MC-type carbides were insufficiently spheroidized and had a high coefficient of friction. In Comparative Example 19, since the C content was low, the wear resistance was insufficient due to insufficient hardness. Comparative Example 20 is insufficient in abrasion resistance because the amount of MC type carbide is small. In Comparative Examples 21 and 22, the toughness was deteriorated due to the excessive amount of the alloying elements, and the surface of the roll was observed to be rough due to a large number of missings originating from heat cracks.

[0050]

The roll for hot rolling of the present invention is excellent in wear resistance and low friction, and has an effect of suppressing the occurrence of scale flaws on the surface of the material to be rolled.
An excellent roll that greatly contributes to the production of hot rolled steel sheets.

 ──────────────────────────────────────────────────続 き Continued on front page (72) Inventor Shohachiro Tamura 1850 Minato, Wakayama City, Wakayama Prefecture Sumitomo Metal Industries Wakayama Works

Claims (1)

[Claims] (1) At least the outer shell layer of the roll has C: 1.5 to 3%, Cr: 0.5 to 5%, Mo: 0.5% by weight.
-8%, V: 1-8%, W: more than -8%, Nb: 0.1
MC type consisting of high-carbon high-speed steel containing up to 5% and B: 0.01 to 1% and having a grain size of 15 μm or less in the structure and a ratio of the major axis to minor axis of 2 or less A roll for hot rolling, wherein carbides are present in an area ratio of 5 to 20%.