JP2000160283A - Rolling mill roll excellent in wear resistance and heat crack resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot roll excellent in heat crack resistance and wear resistance even at the time of severe hot rolling. SOLUTION: In this rolling mill roll, the surface layer part to be in contact with a rolling stock has a composition consisting of 0.1-1.2% C, 0.1-2.5% Si, 0.1-2.5% Mn, 1.0-5.0% Cr, 0.1-4.0% Mo, 0.1-5.0% V, 0.1-5.0% Co, 0-7.3% Nb, 0-7.8% W, and the balance Fe with inevitable impurities and in which V(%)+2/3Nb(%)<=5.0% and W(%)+2Mo(%)<=8.0% are satisfied and also has a metallic structure in which amount of carbide is regulated to <=5% 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 roll for rolling which is excellent in wear resistance and heat crack resistance even in hot rolling under severe conditions.

[0002]

BACKGROUND OF THE INVENTION The present invention is particularly applicable to a continuous hot rolling process in which the heat load is severe, for example, from a continuous casting machine or a continuous casting machine to a hot rolling process, particularly a continuous casting in-line rolling mill. The present invention relates to a suitable roll for rolling.

[0003] The roll used when continuously rolling a rolling material having a simple sectional shape such as a slab, a billet, or a round bar as described above has a high surface pressure and a low roll surface due to low speed rolling. Are exposed to high temperatures exceeding 1000 ° C. for a long time. For this reason, higher wear resistance and higher heat crack resistance are particularly required for rolling rolls.

Conventionally, as a roll for hot rolling, ductile cast iron, adamite cast steel or adamite cast iron has been used when the thermal influence is severe and particularly toughness is required, and especially when wear resistance is required. High-chromium cast iron, high-alloy grain cast iron, high-carbon tool steel, etc. have been used.

[0005] Ductile cast iron has spheroidal graphite and cementite in its structure, and adamite cast steel or adamite cast iron has a network-like cementite structure. However, there is a problem that heat cracking resistance and abrasion resistance are not good.

High chromium cast iron contains M ₇ C ₃ type double carbide in an area ratio of 20 to 35%, and high alloy gray cast iron contains M ₃ C type carbide in an area ratio of 25 to 40% and graphite in an area ratio of 5 to 5%. % Or less. These rolls have the characteristic of being excellent in wear resistance as a roll for hot rolling, but have poor heat crack resistance, and have a problem that the roll surface is easily roughened and the surface appearance of the product is easily damaged. In either case, it is necessary to increase the frequency of roll exchange, and there is a common problem that rolling productivity and manufacturing cost increase.

[0007] JP-A-2-240634, JP-A-2-240634
-25205, JP-A-2-88745, JP-A-3-126838, JP-A-3-219047
JP, JP-A-4-141553, JP-A-5-1
JP-A-48584 and JP-A-5-271875 disclose a roll made of high-carbon tool steel (high-speed steel) having a high carbon content.

[0008] The high-speed steel contains, in its metal structure, 10 to 20% by area ratio of a double carbide such as MC type high hardness carbide mainly composed of V, M ₂ C type, M ₆ C type and the like.
W, Co, and the like have a matrix with a high resistance to high-temperature softening in which a solid solution or fine carbide is precipitated in the matrix. It is said that HSS exhibits particularly excellent wear resistance when used as a roll for hot rolling, because HSS has a carbide of high hardness at its base.

However, when a rolling roll made of HSS is used as a hot rolling roll in which a heat cycle from a high temperature to a low temperature is periodically applied, and a high pressure acts on the surface thereof at a high temperature, the roll surface becomes high. Is susceptible to thermal cracking, which propagates deeply. Since the surface roughening progresses from the thermal crack as a starting point and generates surface defects on the rolled material,
There is a problem that the rolling roll must be repaired or replaced while the line is stopped.

JP-A-57-123959 discloses C
A roll material containing 10 to 20% of r and having excellent wear resistance and heat crack resistance is disclosed. However, since this roll material contains a large amount of Cr carbide in the metal structure, it has poor toughness and cannot sufficiently suppress the generation and propagation of thermal cracks. Further, since the carbide is mainly composed of a Cr carbide having a low high-temperature hardness, the wear resistance was not sufficient.

Japanese Patent Application Laid-Open No. 6-336643 discloses C,
A roll material in which the content of Cr, Mo, W and the like is limited is disclosed. Although this roll material suppresses Cr carbide and improves heat crack resistance, it has the problem of insufficient wear resistance. In order to achieve high productivity in hot rolling and high quality of a rolled product, a roll for rolling having excellent wear resistance and excellent heat crack resistance has been required.

[0012]

SUMMARY OF THE INVENTION An object of the present invention is to provide a hot rolling roll which is excellent in heat crack resistance and wear resistance even in severe hot rolling.

[0013]

Means for Solving the Problems The present inventor has conducted various studies and studies on a method for achieving both abrasion resistance and heat crack resistance in a rolling roll, and as a result, has obtained the following findings.

Based on a high-alloy steel having excellent wear resistance, the metal structure has a carbide content of 5% or less and is substantially a base-based metal structure, thereby providing both wear resistance and heat crack resistance. What can be done, for that, V equivalent: V (%)
+ 2 / 3Nb (%) and W equivalent: W (%) + 2Mo (%)
I knew I had to regulate.

Mo, V, W, Nb which are carbide forming elements
, The appearance of coarse carbides is suppressed, and the solid solution strengthening and precipitation strengthening in the matrix can be realized by quenching and tempering heat treatment. As a result, not only the wear resistance at high temperatures is improved, but also high heat crack resistance, which does not crack even when subjected to a large thermal shock, is obtained.

The present invention has been completed on the basis of these findings, and the gist of the present invention resides in the following rolling roll having excellent wear resistance and heat crack resistance.

The chemical composition of the surface layer in contact with the material to be rolled is as follows: C: 0.1-1.2%, Si: 0.1-2.5% by weight.
%, Mn: 0.1 to 2.5%, Cr: 1.0 to 5.0
%, Mo: 0.1 to 4.0%, V: 0.1 to 5.0%,
Co: 0.1 to 5.0%, Nb: 0 to 7.3%, and V
(%) + ２/3 Nb (%) ≦ 5.0%, W: 0 to 7.8
%, And W (%) + 2Mo (%) ≦ 8.0%, the balance being substantially composed of Fe and unavoidable impurities, and having a metal structure whose carbide amount is 5% or less in area ratio. Rolls for rolling with excellent wear resistance and heat crack resistance.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described in detail below. The percentages described below mean weight%.

Chemical composition of the surface layer in contact with the material to be rolled; C: C is combined with carbide-forming elements such as Cr, Mo, W, V, and Nb to precipitate and harden the steel, thereby making the steel resistant at high temperatures. Contributes to improved wear. C content is 0.
If it is less than 1%, the effect of improving the wear resistance is insufficient. Therefore, C is contained in an amount of 0.1% or more. It is preferably at least 0.3%. On the other hand, if the amount of carbide is excessively increased, the toughness and heat crack resistance of the steel decrease. In order to prevent this, the C content is set to 1.2% or less. Preferably it is 1.0% or less.

Si: Si has the effect of improving the deoxidation and flowability of steel and facilitating the casting operation. In order to obtain this effect, 0.1% or more of Si is contained. Preferably 0.
2% or more. If the Si content is excessively increased, toughness and mechanical properties deteriorate. In order to avoid this, the content is set to 2.5% or less. Preferably it is 2.0% or less.

Mn: Mn has an action of deoxidizing steel,
In order to obtain a sufficient deoxidizing effect, the content is set to 0.1% or more. It is preferably at least 0.2%. On the other hand, if the Mn content is excessively increased, the toughness deteriorates. Therefore, the upper limit of the Mn content is set to 2.5%. Preferably it is 2.0%.

Cr: Cr has the effect of forming a solid solution in the matrix to strengthen the matrix. To obtain this effect,
1.0% or more. Preferably it is 1.5% or more. If Cr is excessively contained, the carbides become coarse and the amount thereof becomes excessive, resulting in a decrease in toughness and heat crack resistance.
Therefore, the Cr content is set to 5.0% or less. Preferably it is 4.5% or less. Mo: Mo has a function of improving the high-temperature softening resistance by forming a solid solution in the matrix and increasing the high-temperature strength. To obtain this effect, Mo is contained in an amount of 0.1% or more. Preferably 0.
5% or more. If the Mo content is too large, M
O combines with C to form an M ₆ C-type or M ₂ C-type composite carbide, and the toughness and the heat crack resistance are reduced. In order to avoid this, the Mo content is set to 4.0% or less. Preferably it is 3.5% or less.

V: V combines with C to form an MC-type high-hardness composite carbide to enhance wear resistance, and further serves to refine crystal grains to improve toughness. In order to obtain these effects, V is contained at 0.1% or more. Preferably it is 0.5% or more. On the other hand, if the V content is excessively increased, the amount of carbide increases, and toughness and heat crack resistance decrease. In order to avoid this, the V content is set to 5.0% or less. Preferably it is 4.5% or less.

Co: Most of Co has a function of improving the hardness and the high-temperature softening resistance of the matrix and improving the wear resistance by forming a solid solution in the matrix. In order to obtain these effects, Co is contained at 0.1% or more. It is preferably at least 0.2%. On the other hand, if Co is excessively contained, precipitation of carbides is promoted, and oxidation resistance and toughness at high temperatures are undesirably reduced. Therefore, the Co content is 5.
0% or less. Preferably it is 4.5% or less.

Nb: Like V, Nb combines with C to form a high-hardness carbide and has an effect of improving wear resistance. It also has the effect of making the structure finer and improving toughness and heat crack resistance. Therefore, although not an essential element, Nb may be contained in order to obtain these effects. According to the research results of the inventor, when the value calculated by V (%) + 2 / 3Nb (%) exceeds 5.0, the toughness and heat crack resistance of the steel may be reduced. This is because the relationship defined by the V equivalent has an effect equivalent to that of V and a large amount of coarse carbides is likely to appear. In order to avoid such a problem, even when Nb is contained, the upper limit is V + 2 /
It is preferable that 3Nb ≦ 5.0%. More preferably, 4.
5% or less.

W: W combines with C to form a high-hardness composite carbide to enhance wear resistance, or to form a solid solution in the matrix to increase tempering softening resistance. W is not an essential element, but may be contained in order to obtain the above-mentioned effects. However, the value calculated by W (%) + 2Mo (%) is 8.
If it exceeds 0%, the toughness and heat crack resistance of the steel may decrease. In addition, a large amount of coarse carbides tends to appear. In order to avoid such inconvenience, even when W is contained, the upper limit is W (%) + 2Mo (%) ≦ 8.
It is good to make it 0%. It is more preferably at most 7.0%.

The reason why the upper limit of the W content is determined in relation to the Mo content is that these elements have an equivalent effect due to the relationship defined by the W equivalent.

P and S: Since P and S cause deterioration of mechanical properties, the smaller the better, the better. However, all were 0.08%
If the amount is less than the above, no significant adverse effect is exerted. The balance other than the above components is Fe and inevitable impurities.

In the present invention, at least the metal structure of the steel in the surface layer in contact with the material to be rolled must have an area ratio of carbide of 5% or less.

In the present invention, the rolling roll means an integral roll or a composite roll comprising a core and a sleeve or an outer shell layer. The surface layer portion in contact with the material to be rolled is a portion where the material to be rolled is in contact with the integrated roll, and means a portion of the outer shell layer where the material to be rolled is in contact with the composite roll. It is sufficient that at least the surface layer portion has the chemical composition and metal structure specified by the present invention, and the entire roll may have the above-described chemical composition and structure structure.For example, in the case of a composite roll, only the outer shell layer is used. May be the contents described above. In the latter case,
As the core body, known ones can be applied, but in particular, ductile cast iron, ordinary cast iron, graphite cast iron, spheroidal graphite cast iron, forged steel,
A tough material such as cast steel is preferred.

The roll for rolling according to the present invention comprises a centrifugal casting method,
A composite roll manufacturing method such as a continuous overlay method using high-frequency heating, a method for forming an outer layer by hot isostatic pressure using powder metallurgy, an overlay method using electroslag melting, and electroslag remelting. It can be manufactured by a known method such as a method of manufacturing an integrated roll in which a molten metal is statically cast and forged.

The roll for rolling according to the present invention can be used for all rolls that are used hot, such as a roll for hot-rolled steel sheet, a roll for hot skin pass mill, a roll for strip steel, and a roll for various pipes. . In addition, it has general characteristics required as a rolling roll for warm rolling, cold rolling, and low-temperature rolling, and is applicable. In particular, 1
The material to be rolled, which has a temperature of 000 to 1250 ° C., is applied to a rolling roll of a hot rolling mill which gives a high degree of work at a low speed, and exhibits remarkable wear resistance and heat crack resistance.

[0033]

EXAMPLE An outer layer material having various chemical compositions shown in Table 1 was subjected to centrifugal casting to form a composite roll (body diameter 150 mm,
(Body length 300 mm, total length 510 mm). In the core material, C: 3.0%, Si: 1.1%, Mn: 0.5%,
Ductile cast iron containing Ni: 1.1%, Cr: 0.1%, Mo: 0.25% and the balance substantially consisting of Fe was used.

Each roll was subjected to two heat treatments of quenching at 1050 to 1150 ° C. and tempering at 500 to 580 ° C. to adjust the surface hardness to 70 to 75 Hs.

A hot coil rolling experimental device (a four-stage rolling mill) simulating rolling conditions when these rolls are applied to an in-line hot rolling mill in a continuous rolling process from continuous casting to hot rolling. , Reinforcement roll body diameter 3
00mm) as a roll for rolling, 3m thick
m, a sheet width of 40 mm and a length of 800 m were hot-rolled (rolling reduction 50%, workpiece temperature 1150 ° C., rolling speed 2)
0 m / min).

A test piece for a microscope was collected from the surface of the body of the roll before hot rolling, carbides appeared by etching, and the area ratio was measured by an image processing method.

The abrasion resistance was evaluated by measuring the maximum abrasion depth from the roll profile after rolling. The heat crack resistance was evaluated by measuring the maximum crack depth generated in the roll. The maximum crack depth was evaluated by repeating the die check and the roll polishing, and the amount of polishing (radial depth) until the crack was no longer confirmed by the roll polishing. The heat crack resistance is calculated by the arithmetic mean roughness Ra specified in JIS-B0601.
Was measured and evaluated according to the degree of surface roughness. A roll having good heat crack resistance has few bases and the roll surface after rolling is smooth, so that the heat crack resistance can be evaluated by measuring the change in Ra.

Table 1 shows the results of evaluating the chemical composition of the outer layer of each roll, the area ratio of carbides, and the abrasion resistance, heat crack resistance, and rough surface resistance of each roll used in the rolling test.

[0039]

[Table 1]

Having a chemical composition within the range defined by the present invention;
Test numbers 1 to 13 in which the area ratio of carbide is 5% or less,
Compared with Test Nos. 14 to 17 which deviated from the conditions specified by the present invention, the balance of abrasion resistance, heat crack resistance, and rough surface resistance was excellent. In particular, Test Nos. 1 to 11 containing Nb and W in a preferable range are Test Nos. 12 and 1
In comparison with No. 3, heat crack resistance and skin roughness resistance were excellent. On the other hand, the Cr content is large, and V + ２Nb and W + 2Mo
Test number 14, both of which are out of the scope of the present invention, contains a large amount of fragile and low-temperature hardness Cr carbide, so that the abrasion resistance,
Heat cracking resistance and skin roughness resistance were significantly inferior.
In Test Nos. 15 and 16 in which V + 2 / 3Mo or W + 2Mo was out of the range of the present invention, the wear resistance was good because the high-hardness carbide was present in an area ratio exceeding 5%, but the heat crack resistance and the heat resistance were high. Skin roughness was significantly inferior. V + 2
Test No. 17, in which both / 3Nb and W + 2Mo were out of the range of the present invention and the carbide area ratio exceeded 5%, had poor heat crack resistance and rough surface resistance.

[0041]

The rolling roll of the present invention has excellent abrasion resistance, heat crack resistance, and surface roughening resistance. Therefore, even when used in hot rolling in a severe environment, the roll is hardly abraded and roughened. The effects of extending the life of the roll, reducing the frequency of replacing the roll, and improving the unit consumption of the roll can be obtained. As a result, quality improvement and high productivity of the rolled product can be realized.

Claims (1)

[Claims] 1. The chemical composition of a surface layer in contact with a material to be rolled,
C: 0.1-1.2%, Si: 0.1-2.
5%, Mn: 0.1 to 2.5%, Cr: 1.0 to 5.0
%, Mo: 0.1 to 4.0%, V: 0.1 to 5.0%,
Co: 0.1 to 5.0%, Nb: 0 to 7.3%, and V
(%) + ２/3 Nb (%) ≦ 5.0%, W: 0 to 7.8
%, And W (%) + 2Mo (%) ≦ 8.0%, the balance being substantially composed of Fe and unavoidable impurities, and having a metal structure whose carbide amount is 5% or less in area ratio. Rolls for rolling with excellent wear resistance and heat crack resistance.