JPH11279680A - Outer layer material for hot rolling roll - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an outer layer material for a hot rolling roll, minimal in coefficient of friction and excellent in wear resistance as well as in surface roughing resistance. SOLUTION: This material has a composition containing, by weight, 2.5-3.5% C, <=1.5% Si, <=1.2% Mn, 6.0-14.0% Cr, 0.01-0.3% Ti, 3.0-10.0% V, and 0.6-5.0% Nb, further containing C, Cr, Nb, and V in the amounts satisfying 10<=6.5C-1.3 V-0.7Nb<=2Cr-2 (where C, V, Nb, and Cr represent respective contents of the elements C, V, Nb, and Cr), and having the balance Fe with inevitable impurities.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a roll outer layer material for hot rolling which has both surface roughness resistance and wear resistance and has no accident such as breakage during manufacturing or rolling use.

[0002]

2. Description of the Related Art A hot-rolled steel sheet is prepared by heating a slab manufactured by continuous casting or lumping in a heating furnace or receiving a hot slab as it is, and hot-rolling it in a rough rolling mill and a finishing rolling mill. It is made into a steel strip, wound around a coil by a coiler, cooled, and then subjected to various refining processes in various refining lines. The finishing mill is usually composed of 5 to 7 stands of a quadruple rolling mill, but recently, most mills have adopted 7 stands in order to increase production capacity and increase the size of coils.

[0003] Conventionally, the outer layer material of the work roll of the finishing mill is composed of high Cr cast iron or adamite in the former stage (up to the third stand in the case of 5 or 6 stands, up to 4 stands in the case of 7 stands), and in the latter stage. For Ni, cast iron has been used. Further, as the inner layer material of the work roll after the finish rolling, gray cast iron or ductile cast iron having good toughness has been used in both the first and second stages. Then, composite rolls manufactured mainly by centrifugally casting the outer layer material and the inner layer material have been used.

[0004] However, demands for severer rolling conditions and improved productivity have led to a demand for a roll for rolling having even more wear resistance. In response to such a demand, for example, JP-A-60-124407 and JP-A-61-177355 propose the use of high-V cast iron as the outer layer material of a centrifugal casting roll. However, when high V cast iron is manufactured by centrifugal casting as an outer layer material of a roll, there is a problem that V carbide having a small specific gravity is centrifuged and characteristics in the outer layer material of the roll become uneven in the thickness direction.

With respect to the above-mentioned problems, JP-A-4-365836, JP-A-5-1350 and JP-A-5-339673 disclose:
A method of preventing segregation of the outer layer material of the centrifugal casting roll by adding Nb to the high-V cast iron in a complex manner has been proposed.

[0006]

However, according to the findings of the present inventors, Japanese Patent Laid-Open Nos.
No. 1350, the technology described in JP-A-5-339673 is advantageous in that the roll outer layer material made of a high V-Nb material is advantageous in that the wear resistance is remarkably improved. Carbides and Nb carbides (MC carbides) protrude from the roll surface in a convex shape, and the spike action increases the coefficient of friction between the material to be rolled and the roll surface.

[0007] For this reason, in the rolling using a roll having such an outer layer material, (a) the rolling load becomes excessive,
(B) secondary scale is generated on the surface of the material to be rolled due to frictional heat, and the surface of the material to be rolled is roughened. (C) Rolling under high pressure causes excessive frictional heat and damages the roll surface,
The problem of causing the rough surface of the material to be rolled has become remarkable. In particular, when such a roll is used in the pre-finishing stage, the above problem does not appear when used in the post-finishing stage.

The roll roll outer layer material disclosed in JP-A-4-365836, JP-A-5-35050, and JP-A-5-339673 has not only abrasion resistance but also crack resistance. However, in the case of the roll for the pre-finishing stage in which the excessive rolling load and the formation of the secondary scale on the surface of the material to be rolled are problematic, the roll resistance for the finishing stage is higher than that of the post-finishing roll which may cause a drawing accident. The demand for cracking is low.

An object of the present invention is to provide a roll outer layer material for hot rolling which has a low coefficient of friction, excellent skin resistance, and excellent wear resistance.

[0010]

Means for Solving the Problems In order to achieve the above object, the present inventors diligently studied the structure and added alloy elements in order to reduce the wear coefficient of the outer layer material of the rolling roll.
The present inventors first investigated in detail the relationship between the structure of the outer layer material of the roll and the wear. The structure of the outer layer material of the roll containing high V or high V-Nb is composed of granular carbide (mainly V, Nb carbide), eutectic carbide (mainly Cr, Mo, Fe carbide), and a matrix. In the structure of the roll outer layer material described above, the base having low hardness is selectively worn, and the granular carbide and eutectic carbide having high hardness remain in a convex shape. It was found that the increase in the coefficient of friction was due to the fact that the granular carbides acted as projections due to their shape, and that a large step was created between the matrix and the matrix, which occupies a considerable area in the tissue. Therefore, in order to reduce the coefficient of friction of the outer layer material in the hot rolling roll, particularly in the hot finishing pre-roll, change the shape of the granular carbide or reduce the level difference between the granular carbide and other portions. That is all you need to know.

The present inventors increased the eutectic carbide having high hardness existing in the structure of the roll outer layer material, and further added Ti for forming granular carbide, and increased the amount of granular carbide to increase the amount of granular carbide. It has been conceived that it is possible to reduce the level difference between the roll and the base portion, and to reduce the friction coefficient of the outer layer material of the roll. The invention according to claim 1, wherein the C:
2.5-3.5%, Si: 1.5% or less, Mn: 1.2% or less, Cr:
6.0-14.0%, Mo: 2.0-8.0%, Ti: 0.01-0.3%,
V: 3.0 to 10%, Nb: 0.6 to 5.0%, and
(1) Formula 10 ≦ 6.5 C−1.3 V−0.7 Nb ≦ 2Cr−2 (1) (where C, V, Nb, Cr: content of each element of C, V, Nb, Cr (weight) %)), And the outer layer material for hot rolling rolls, characterized by the balance of Fe and inevitable impurities. The outer layer material for hot rolling rolls is preferably used in a stage before hot rolling finishing.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The reasons for limiting the content of each alloy element in the outer layer material and the limiting formulas for the amounts of V and Nb will be described. C: 2.5 to 3.5% C is an element indispensable for forming hard carbide for improving the wear resistance of the outer layer material of the roll. If it is less than 2.5%, the amount of carbides is insufficient, the abrasion resistance is inferior, and the friction coefficient is increased, which causes roughening of the skin. On the other hand, if it exceeds 3.5%, not only the effect of reducing the friction coefficient is saturated, but also the wear resistance is deteriorated. For this reason, C is limited to 2.5 to 3.5%.

Si: 1.5% or less Si is added as a deoxidizing agent and an element necessary for ensuring castability, but if it exceeds 1.5%, its effect is saturated. for that reason,
The upper limit of Si is set to 1.5%. Mn: 1.2% or less Mn is added for removing S as MnS and for strengthening the tissue. However, if it exceeds 1.2%, the effect is saturated.
Therefore, the upper limit of Mn is set to 1.20%.

Cr: 6.0 to 14.0% Cr is an element necessary for forming carbides and improving wear resistance, and is added in an amount of 6.0% or more, but the wear resistance effect is saturated even if it exceeds 14.0%. In addition, the Cr content is limited to the range of 6.0 to 14.0% due to deterioration of seizure resistance. Mo: 2.0 to 8.0% Mo forms carbides like Cr and is effective in improving abrasion resistance, and also improves the hardenability of the matrix and the resistance to temper softening, and is effective in strengthening the matrix structure. Therefore, 2.0% or more is necessary, but if it exceeds 8.0%, the effect is saturated.
Therefore, Mo is limited to the range of 2.0 to 8.0%.

Ti: 0.01-0.3% Ti quickly bonds with C in the molten metal to form TiC, and a large amount of fine-grained (V, Nb, Ti) composite MC is crystallized using the Ti as a nucleus. Therefore, it effectively works to improve wear resistance and reduce the coefficient of friction. However, the addition of less than 0.01% has no effect, and the effect is exhibited in the addition range of 0.01 to 0.3%. However, when the content exceeds 0.3%, centrifugal separation of TiC easily occurs, and segregation in the thickness direction of the outer layer material of the roll easily occurs.

V: 3.0 to 10.0% V is hard MC or M ₄ C ₃ most effective for improving abrasion resistance.
It is an essential element for forming carbides, and its effect is required to be 3.0% or more in order to exhibit its effect. However, if it exceeds 10.0%, the effect is saturated and the V alloy to be added at the time of melting is inferior in melting. The above problems arise. Therefore, V is 3.
It was limited to the range of 0 to 10.0%.

Nb: 0.6-5.0% Nb is an element which forms a hard MC type carbide like V and is effective in improving wear resistance. To achieve this effect, V
It is necessary that the content be 0.6% or more, but if it exceeds 5.0%, the effect is saturated and production problems such as poor melting of the Nb alloy during melting occur. Therefore, Nb is 0.6
Limited to the range of ~ 5.0%. However, if Nb alone is added, it becomes a coarse massive carbide and the effect cannot be obtained.

In the present invention, the contents of C, V, Nb and Cr are expressed by the following equation (1): 10 ≦ 6.5 C-1.3 V−0.7 Nb ≦ 2Cr-2 (1) where C, V, Nb , Cr must satisfy the content (% by weight) of each element of C, V, Nb, and Cr. The friction coefficient of the outer layer material is, as shown in FIG. 1, a formula assuming the amount of C consumed by V and Nb. In the region where 6.5 C-1.3 V-0.7 Nb is 10 or more, the friction coefficient is as high as that of high Cr cast iron. (0.28). On the other hand, when the amount of Cr decreases, the amount of Cr distributed in the eutectic carbide decreases, and the wear resistance tends to decrease significantly. As shown in FIG. 2, the amount of wear of the outer layer material is significantly reduced by setting 6.5 C-1.3 V-0.7 Nb to 2Cr-2 or less, assuming the amount of C consumed by V and Nb. From this, 6.5 C
−1.3 V−0.7Nb is 10 or more and 6.5 C−1.3 V−0.7
By setting Nb to 2Cr-2 or less, a low friction coefficient and high wear resistance can be obtained.

The outer layer material of the roll for hot rolling according to the present invention is preferably formed as a three-layer composite roll into which an intermediate layer and a shaft material are cast as an inner layer. The roll outer layer material is manufactured by centrifugal casting. The applied centrifugal force is preferably about 140G. In centrifugal casting, a molten metal having the above composition is melted and cast into a cooled mold. After the solidification has progressed and the entire outer layer has once solidified, the intermediate layer is cast. The cast intermediate layer partially redissolves the inner surface side of the outer layer, and then the outer layer and the intermediate layer are solidified by cooling. Next, the shaft material is left standing cast to form a composite three-layer roll.

[0020]

Next, an embodiment of the present invention will be described. Materials equivalent to the roll outer layer material having the chemical composition shown in Table 1 were melted and centrifugally cast to obtain test materials. After heating and quenching these test materials at 1000 ° C., they were tempered at 550 ° C. Abrasion test pieces were collected from these test materials, and abrasion tests were performed. The wear test is
A two-disk sliding wear test was performed on a mating material of φ190 × 15 mm and a test piece of φ50 × 10 mm. Heat the mating material to 900 ° C, press the mating material against the test piece with a load of 100 kgf, and cool the test piece.
The sample was rotated at 800 rpm and tested for 120 minutes under the condition of a slip ratio of 3.9%. After the test, the wear loss was investigated. Also, the coefficient of friction was investigated during the test. Table 2 shows the results.

[0021]

[Table 1]

[0022]

[Table 2]

From Table 2, it can be seen that the example of the present invention has a coefficient of friction similar to that of high Cr cast iron and has high wear resistance. On the other hand, the test pieces No. B-1 to B-5 which deviated from the range of the present invention (6.5 C-1.3 V-0.7 Nb was 10 or more) had a friction coefficient higher than that of the high Cr cast iron material of Comparative Example D. When used as a roll, the surface roughness of the material to be rolled becomes a problem. The scope of the present invention (6.5 C-
1.3 V-0.7 Nb is less than 2Cr-2)
-C-5 has deteriorated wear resistance.

According to the present invention, an outer layer material for a hot rolling roll having a low coefficient of friction, excellent abrasion resistance and excellent surface roughening resistance is obtained, and is suitable for use in the first stage of hot rolling finishing.

[0025]

According to the present invention, the abrasion resistance of the outer layer is uniform in the thickness direction, the coefficient of friction is low, and therefore the surface roughness is excellent, and at the same time, the abrasion resistance is excellent. Rolls for hot rolling, which have a very low possibility of breakage in the inside, can be manufactured by centrifugal casting, which is excellent in productivity and economy, and has a remarkable industrial effect.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing a relationship between C, V, and Nb exerted on a friction coefficient.

FIG. 2 is an explanatory diagram showing the relationship between Cr, C, V, and Nb affecting the amount of wear.

Claims (2)

[Claims] C: 2.5 to 3.5%, Si: 1.5% or less, Mn: 1.2% or less, Cr: 6.0 to 14.0%, Mo: 2.0 to 8.0%, Ti: 0.01 to 0.3%, V : An outer layer material for hot rolling, comprising: 3.0 to 10.0%, Nb: 0.6 to 5.0%, and satisfying the following expression (1), the balance being Fe and unavoidable impurities. 10 ≦ 6.5 C−1.3 V−0.7 Nb ≦ 2Cr−2 (1) where C, V, Nb, Cr: Content of each element of C, V, Nb, Cr (% by weight) 2. The hot-rolling roll outer layer material according to claim 1, wherein the hot-rolling roll outer layer material is a hot-rolling roll outer layer material used in a preceding stage of hot-rolling finishing.