JPH079052B2 - Wear-resistant composite roll and manufacturing method thereof - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a wear-resistant composite roll for hot or cold rolling and a method for manufacturing the same.

BACKGROUND ART Composite rolls made of cast iron manufactured by centrifugal casting are widely used as rolling rolls. These rolls use a cast iron material in which a large amount of carbide with high wear resistance is crystallized in the outer layer,
The structure has an inner layer made of tough gray cast iron or ductile cast iron, but this manufacturing method limits the range of materials that can be used for the outer and inner layers.

Carbides formed by elements such as W, V, Nb, Ti, Ta, Zr, and Hf have a Vickers hardness of 2000 or more, and if these carbides are contained in the outer layer material, it is effective in improving the wear resistance of the roll. However, it is practically impossible to manufacture a composite roll in which the outer layer in which the carbides are crystallized and the inner layer are soundly welded together by the centrifugal casting method.

The reason is that the carbides formed by these elements have a different specific gravity from the molten metal, and are therefore prone to segregation due to the centrifugal force during casting. Also, many of these elements have a strong tendency to oxidize, and are prone to oxidation when melted, cast, or cooled in air.
Furthermore, in centrifugal casting, the inner layer is made of gray cast iron or ductile cast iron containing graphite to obtain toughness. However, if the outer layer contains a large amount of elements that tend to turn white, as described above, the outer layer components will dissolve slightly into the inner layer, resulting in poor graphitization of the inner layer and embrittlement. In particular, carbides will be concentrated near the boundary with the inner layer, making the inner layer embrittlement and prone to peeling of the outer layer starting from the boundary.

Furthermore, the tensile strength of the inner layer of twisted cast iron or ductile cast iron is limited to about 55 kg/ ^mm² , and the elongation is less than 1%. To achieve a higher value, it is necessary to use a steel-based material for the inner layer, but this is also difficult to achieve with centrifugal casting. The reason for this is that the inner layer has a higher melting point than the outer layer, so when the inner layer is cast, the outer layer melts and the resulting mixed state forms the final solidification layer, and casting defects are likely to occur at this boundary.

Therefore, it has been impossible to produce a composite roll by centrifugal casting in which a large amount of carbides of the above elements are crystallized in the outer layer, the outer layer and the inner layer are soundly welded together, and the inner layer has a tensile strength of 55 kg/ ^mm2 or more and an elongation of 1.0% or more.

In addition, by rolling a large amount at once, we aim to streamline the rolling process and improve the dimensional accuracy of the rolled material.
It has become necessary to significantly improve the wear resistance of rolling rolls. At the same time, the bending stress on the roll shaft increases due to the bending of the roll shaft in the direction opposite to the deflection caused by rolling in order to improve the dimensional accuracy of the rolled material, and a large rolling force is applied in one rolling stand to complete the rolling with fewer stands. This has led to the need to improve the strength of the roll shaft. However, in rolls with a structure in which the outer layer material and the shaft material are shrink-fitted or assembled, there are problems such as slippage between the outer layer and the shaft during rolling and the outer layer being prone to cracking, so the outer layer and the shaft must be completely metallically bonded.

In order to satisfy these requirements simultaneously, the outer layer is made of W,
The material must be composed of components that crystallize large amounts of carbides of elements such as V, Nb, Ti, Ta, Zr, and Hf, the shaft must be made of strong steel, and the outer layer and the shaft must be completely metallically bonded.However, for the reasons mentioned above, this cannot be produced by casting methods such as centrifugal casting, and the method of diffusion bonding between solids is not practical for relatively large items such as rolling mill rolls, as the manufacturing equipment would be extremely expensive.

Under the above circumstances, a so-called cast cladding method has attracted attention, in which a composite roll is manufactured by depositing and solidifying a molten metal of an outer layer material around a steel shaft material made of cast steel or forged steel that has been formed in advance, and composite rolls have been proposed that are manufactured by combining various outer layer materials and shaft materials using this method.

In the case of a composite roll in which an outer layer and a core material are metallurgically bonded, the outer layer has a weight percent of C: 2.0 to
3.5%, Si: 0.5-1.5%, Mn: 0.4-1.5%, Cr: 8-25%, M
The present invention discloses a composite roll for hot rolling having excellent resistance to seizure and roughness, characterized in that the roll is made of high chromium cast iron containing 0.5 to 3.0% O, 10% or less V, and 1.5% or less Ni, the core is made of cast steel or forged steel having a tensile strength of 55 kg/mm2 or ^more and an elongation of 1.0% or more, the outer layer is formed by pouring a molten metal of the outer layer components onto the outer periphery of a previously prepared core material to metallurgically bond them together, the bonding strength at the boundary between the outer layer and the core material is at least equal to or greater than the strength of the weaker of the outer layer material and the core material, the hardness of the outer layer is 70 or more Shore hardness, and the decrease in hardness up to 100 mm below the surface is 3 or less Shore hardness.

In addition, the Patent Publication No. 60-180609 discloses a composite roll in which the outer layer is made of high chromium cast iron, the core is made of cast steel and forged steel, and the surface hardness is 90 or more on the Shore hardness scale, and the outer layer is formed by pouring molten metal of the outer layer components onto the outer periphery of a previously prepared core material to metallurgically bond the outer layer, and the high chromium cast iron composition of the outer layer is
By weight, C2.5-3.5%, Si0.5-1.5%, Mn0.4-1.5%, N
i 0.5-3.0%, Cr 8-25%, Mo 1.0-5.0%, balance substantially
The core material is made of Fe, and the strength and elongation of the core material are 55 kg/m
The present invention discloses a high chromium cast iron roll for cold rolling, characterized in that the surface roughness of the ^roll is 1.0% or more and the bond strength at the boundary between the outer layer and the core material is at least equal to or greater than the strength of the weaker of the outer layer material and the core material.

However, in these rolling composite rolls, the outer layer is formed of high chromium cast iron, and although it has good wear resistance, it is not necessarily sufficient to meet the increasingly high level of demands in recent years.

In addition, Japanese Patent Publication No. 51-24969 discloses a steel containing 0.2 to 0.6 weight percent carbon, 2 weight percent or less nickel, 2 to 6 weight percent chromium,
5-12 wt.% in an iron-based matrix containing at least one element selected from the group consisting of 1-6 wt.% molybdenum, 1-6 wt.% tungsten, and up to 10 wt.% cobalt
The publication discloses an ultra-wear resistant steel characterized by the crystallization of carbides formed by combining vanadium, 3 to 10 weight percent of niobium, and an amount of carbon required to combine them. It is stated that this ultra-wear resistant steel can be used for rolling mill rolls, etc., but there is no disclosure that it can be used as a composite roll.

In view of the above, the object of the present invention is to provide a method for manufacturing a semiconductor device using W, V, Nb, Ti, Ta, Zr, Hf
To provide a wear-resistant composite roll in which an outer layer is made of a material of a component system that crystallizes hard carbides such as iron, and a shaft is made of a tough steel-based material, the outer layer and the shaft are metallically joined, and the outer layer is free from carbide segregation, and a method for casting the same in the atmosphere at a relatively low cost.

DISCLOSURE OF THE INVENTION The wear-resistant composite of the present invention contains 1.5 to 3.5% by weight of C, 2 to 4% by weight of Cr, ...
7% Cr, 9% or less Mo, and W, V, Nb, Ti, Ta, Zr
and Hf, and a carbide of one or more of these elements, is formed on a steel shaft by casting an outer layer of an iron-based alloy, the outer layer having a Shore hardness of 70 or more, and the shaft having a tensile strength and elongation of 55 kg/ ^mm2 or more and 1.0% or more,
The bonding strength at the boundary between the outer layer and the shaft is equal to or greater than the strength of the weaker of the outer layer and the shaft.

The method for producing the wear-resistant composite roll of the present invention is also characterized in that W, V,
A method for metallically joining, to a steel shaft, an outer layer made of an iron-based alloy of which components crystallize from the molten metal during solidification, a carbide of one or more elements selected from the group consisting of Nb, Ti, Ta, Zr, and Hf, by loosely fitting the steel shaft material coaxially in a space provided inside a combination mold consisting of a refractory frame surrounded by an induction heating coil and a cooling mold installed coaxially below the frame, and injecting molten iron-based alloy into the gap formed between the shaft and the mold,
The method is characterized in that the surface of the molten metal is sealed with flux, the molten metal is heated and stirred and maintained at a temperature within a range from the primary crystallization temperature to 100°C higher than that, the shaft is moved downward coaxially with the mold, the molten metal is brought into contact with the cooling mold, and solidified and welded to the shaft, thereby forming the outer layer continuously around the shaft.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic cross-sectional view of an apparatus used to carry out the present invention, Figure 2 is a schematic diagram showing a rolling wear tester used for wear testing of the outer layer of a roll, Figure 3 is a graph showing the wear profiles of test rolls for the rolling wear tester of the present invention and comparative examples, Figure 4 is a micrograph showing the carbide distribution state in the metal structure of the outer layer of a wear-resistant composite roll according to one embodiment of the present invention, Figure 5 is a micrograph showing the bonding state between the outer layer and the shaft of a wear-resistant composite roll according to one embodiment of the present invention, and Figure 6 is a micrograph of the outer layer material of the wear-resistant roll of the present invention when cast in a conventional static casting mold.

BEST MODE FOR CARRYING OUT THE INVENTION The wear-resistant composite roll of the present invention comprises an outer layer made of an iron-based alloy and a steel shaft metallically bonded to the outer layer. The iron-based alloy contains components that crystallize one or more of carbides of W, V, Nb, Ti, Ta, Zr, and Hf from the molten metal during solidification. In particular, the iron-based alloy contains, by weight, 1.5 to 3.5% C, 0.3 to 3.0% Si, 0.3 to 1.5% Mn, 2 to 7% Cr,
It preferably consists of up to 9% Mo, up to 20% W, 3-15% V and the balance essentially Fe.

C is necessary for the formation of carbides to improve wear resistance.
If the content is less than 1.5%, the amount of crystallized carbide is small, and wear resistance is insufficient. Furthermore, in terms of the balance with V, if the content is below the lower limit, carbides precipitate in a network pattern at grain boundaries, and the objectives of the present invention cannot be achieved in terms of toughness and surface roughening resistance. On the other hand, if the content of C exceeds 3.5%, the balance with V is disrupted, the uniformly distributed structure of VC is lost, and surface roughening resistance and toughness are impaired.

Si is an element necessary as a deoxidizer, and is effective in replacing expensive elements such as W and Mo by dissolving in M ₆ C carbide, thereby saving them. If the amount is less than 0.3%, there is no deoxidizing effect, and casting defects tend to occur in cast iron materials.
If it exceeds 10%, embrittlement is likely to occur.

Mn has a deoxidizing effect and also fixes the impurity S as MnS. If the amount is less than 0.3%, the deoxidizing effect is poor. However, if the amount exceeds 1.5%, retained austenite is likely to occur, and sufficient hardness cannot be maintained stably.

If the Cr content is less than 2% _, the hardenability is poor, and if it exceeds 7%, the amount of chromium-based carbides becomes excessive, which is undesirable. That is, Cr-based carbides such as _M23C6 have lower hardness than MC, _M4C3 , _M6C , _{and M2C} , and reduce wear resistance.

Mo is necessary to obtain hardenability and high-temperature hardness.
%, the balance between C, V and _M0 leads to an increase in _M6C -based carbides, which is undesirable in terms of toughness and surface roughening resistance, so the upper limit of the _M0 content is 9%.

W is necessary for maintaining high temperature hardness, but if it exceeds 20%, M ₆ C-based carbides increase, which is undesirable in terms of toughness and surface roughening resistance, so the upper limit is set to 20%.

V is an essential element for forming MC carbides, which are effective in improving wear resistance. Therefore, if it is less than 5%, it will not have a sufficient effect, and if it is more than 15%, it will be difficult to distribute the MC carbides uniformly in balance with the above range of C.

The iron-based alloy for the outer layer material of the present invention contains Ni, Co, Nb in addition to the above elements.
may be contained singly or in combination.

Ni has the effect of improving hardenability. Therefore, it can be added in an amount of 5% or less. However, if it is added in an amount greater than that, it will increase the amount of retained austenite, causing problems such as cracking and roughness during rolling, so it is added up to a maximum of 5%.

Co is a useful element in terms of temper softening resistance and secondary hardening, but if it exceeds 5%, hardenability deteriorates.

Nb, like V, forms MC carbides and has the effect of improving wear resistance, but if it exceeds 5%, oxidation becomes severe and dissolution in the atmosphere becomes difficult.

Ni, Co, and Nb can be added individually,
Two or more of them may be added in combination.

In addition, MC carbides are formed to improve wear resistance.
One or more of Ta, Zr, Hf and Ti may be added as appropriate.

Furthermore, one or more of the rare earth elements La, Ce, and Nd may be added as appropriate. These rare earth elements are added together with Nb to form Nb-rare earth carbides, which are finely and uniformly dispersed.

The iron-based alloy for the outer layer material used in the present invention also has a N content
The content of N is preferably 0.15% or less. That is, N is usually contained in the material of the present invention in an amount of 0.005 to 0.10%, and is effective in improving tempered hardness. However, if it is contained in excess, the material becomes embrittled, so the upper limit of the content is set to 0.15% or less.

Other than the above elements, iron-based alloys consist essentially of iron, with the exception of impurities, the main impurities being P and S.
is 0.1% or less to prevent embrittlement, and S is 0.08%
It should be the following:

The shaft of the composite roll of the present invention is made of steel, and may be either cast steel or forged steel. Its tensile strength is 55 kg/ ^mm2 or more, and its elongation is
It must be 1.0% or more. This is because, when used as a rolling roll, a large rolling force is applied and the roll must be able to withstand the bending force applied to both ends of the shaft to correct deflection during rolling. The shaft must also be firmly bonded to the outer layer made of the iron-based alloy. To achieve this, the bond strength at the interface between the two must be equal to or greater than the mechanical strength of the weaker of the outer layer or the shaft.

In order to form an outer layer with high bonding strength on the outer periphery of a steel shaft in this manner, the following method is used.

The manufacturing method is basically the same as that shown in Japanese Patent Application Laid-Open No. 61-60256, in which an outer layer is continuously formed around the steel using a high frequency coil.

Fig. 1 shows an example of an apparatus that can be used to carry out the method of the present invention. This apparatus has a combination mold 10 consisting of a funnel-shaped refractory frame 1 having a peripheral wall with a tapered portion and a parallel portion, and a cooling mold 4 installed coaxially below the frame.

The refractory frame 1 has an annular induction heating coil 2 disposed around its outer periphery, and a ring-shaped buffer mold 3 disposed coaxially below the coil 2 and having an inner hole with the same diameter as the bottom of the refractory frame 1.
The cooling mold 4 below it is a buffer mold 3.
The cooling mold 4 has an inner diameter approximately the same as that of the cooling mold 14 and is coaxial with the cooling mold 14. Cooling water is continuously introduced into the mold from an inlet 14 of the cooling mold 4 and an outlet 14'.
is emitted from

The roll shaft 5 is set inside the combined mold 10 constructed as above. A closing member (not shown) having an outer diameter approximately the same as that of the outer layer is fixed to the lower end of the shaft 5 or at a position appropriately spaced from the lower end as necessary, and its lower part is attached to the lifting mechanism (not shown) of the shaft 5.
Molten metal 7 is poured into the space between the shaft and the closure member, and the surface of the molten metal is sealed with molten flux 6 to prevent it from coming into contact with air. The molten metal 7 is then heated and stirred by heating coil 2 to prevent solidification. The molten metal 7 flows in the direction indicated by arrow A in the figure, causing a stirring motion. Next, the closure member fixed to shaft 5 is gradually lowered along with the shaft. As the shaft and closure member are lowered, the molten metal 7 also descends, and solidification of the molten metal 7 begins on the surface of buffer mold 3 and water-cooled mold 4. During this solidification, the shaft and outer layer are completely metallically bonded. The surface of the molten metal in the basin also descends as the shaft and closure member are lowered, but new molten metal is injected as needed to maintain the liquid level. The lowering and pouring are then repeated sequentially, causing the molten metal to gradually solidify from below, forming outer layer 8.

The composite roll thus obtained is further quenched,
The desired hardness of the outer layer can be obtained by applying heat treatment such as tempering. The surface hardness of the outer layer of the obtained composite roll is 70 or more Shore hardness, the tensile strength of the shaft is 55 kg/ ^mm2 or more, and the elongation is 1.0% or more. Since the outer layer and the shaft are metallically bonded,
The bonding strength at the boundary is equal to or greater than the strength of the weaker of the outer layer and the shaft.

The present invention is further illustrated by the following examples.

Example 1 The molten metal for the outer layer having the composition shown in Table 1 was poured into a mold having a diameter of 70 mm and a height of 80 mm.
The material was poured into _a sand mold to cast a small roll material for rolling wear tests. This material was quenched from 1000 to 1100°C and quenched for 50
After the steel was subjected to a heat treatment by tempering at 0 to 550°C, a sleeve-shaped test roll having an outer diameter of 60 mm, an inner diameter of 35 mm and a length of 40 mm was prepared.

The hardness of the outer layer surface of each test roll was measured using a Shore hardness tester, and the results are shown in Table 2. Next, a rolling wear test was carried out on these test rolls. The rolling wear tester, as shown in Figure 2, consisted of a rolling mill 21 and an upper roll 22 incorporated in the rolling mill 21.
22 and a lower roll 23, a heating furnace 24 for preheating the rolled material S,
The apparatus comprises a cooling water tank 25 for cooling the rolled material S, a winder 26 for applying a constant tension during rolling, and a tension controller 27 for adjusting the tension. The test conditions were as follows:

Rolled material: SUS 304, thickness 1 mm, width 15 mm, rolling distance: 800 m, rolling temperature: 900°C, reduction: 25%, rolling speed: 150 m/min, roll cooling: water cooling. The wear depth on the surface of the test rolls was measured using a stylus surface roughness meter (SURFCOM). The results obtained for Sample No. 1 are shown in Figure 3(A). The wear depth for each roll was averaged across the rolling width to determine the average wear depth, and the results are shown in Table 2.

Comparative Examples 1 and 2 Test rolls were prepared in the same manner as in Example 1 using high chromium cast iron (Comparative Example 1) and alloy grain roll material (Comparative Example 2) as conventional materials. However, heat treatments were applied that were suited to these materials. Wear tests were conducted in the same manner as in Example 1, and the measured values of wear depth were recorded as shown in Figure 3(B).
The results of hardness measurements are shown in Table 2.

Example 2 Composite rolls were manufactured using the apparatus shown in Figure 1, using shafts with diameters and materials shown in Tables 3 and 4, and molten metal for the outer layer with a composition shown in Tables 3 and 4. The preheating temperatures of the shafts and the molten metal for the outer layer were as shown in Table 3. The surface of the molten metal was sealed with molten flux to prevent it from coming into contact with air. The dimensions of the composite rolls obtained in this manner are as shown in Table 3. Each composite roll was subjected to a heat treatment consisting of quenching from 1000 to 1100°C and tempering at 500 to 550°C.

No cracks or other defects were observed in the composite roll due to the heat treatment described above.
The hardness of the outer layer surface was measured using a Shore hardness tester, and the results are shown in Table 4.

The composite roll (sample No. 1) was cut at a position 200 mm from the barrel end, spanning both the outer layer and the shaft in the diametric direction, and a tensile test was carried out. The fracture occurred on the outer layer iron-based alloy side, and the tensile strength was 64 kg/ ^mm2 .

In addition, samples were cut out from each part of the roll and the structure was observed. Figure 4 shows the metal structure of the outer layer of the composite roll.
As is clear from the micrograph of FIG. 4, hard vanadium carbide is finely and uniformly dispersed in granular form, resulting in a good cast structure.

For comparison, an example of the structure of a steel with the same composition cast in a normal static mold is shown in Figure 6. It can be seen that vanadium carbide has formed in relatively large chunks and that its distribution is uneven.

Figure 5 shows the results of an observation of the structure of the boundary between the outer layer and the shaft. In the figure, the outer layer is on the left and the shaft is on the right. It can be seen that there is no phenomenon such as carbide concentration at the boundary, and there are no casting defects, making for a good bond.

Example 3 A composite roll was manufactured using the outer layer material having the composition shown in Table 5 below, and the apparatus shown in Figure 1. The shaft material was SCM 440. The manufacturing conditions were the same as in Example 2. However, the conventional example was an alloy grain roll cast by the conventional centrifugal casting method.

Furthermore, the composite roll of the present invention is quenched from 1050°C and
The composite roll thus obtained had the following dimensions:

Body diameter: 312 mm Body length: 500 mm Shaft outer diameter: 230 mm The hardness of the above rolls was as shown in Table 6.

These rolls were used to roll flat steel bars. The number of rolled bars and the amount of wear are shown in Table 6.

The amount of wear is measured at the center of the roll barrel with an outside micrometer and is expressed as the amount of reduction in roll diameter.

As is clear from the above results, the composite roll of the present invention has high surface hardness and excellent wear resistance, and also has excellent shaft mechanical strength and sufficient bonding strength between the outer layer and the shaft. In contrast, the roll having an outer layer of a conventional composition outside the scope of the present invention does not have sufficient wear resistance.

Furthermore, the manufacturing method of the present invention makes it possible to manufacture a composite roll using a shaft made of cast steel or forged steel, which is free from segregation of crystallized carbides, has excellent wear resistance, and is free from defects such as voids.

Industrial Applications The composite roll of the present invention has good mechanical strength and excellent wear resistance, and can therefore be widely used as a roll for cold rolling and hot rolling.

Claims (12)

[Claims] Claim 1: 1.5 to 3.5% C, 2 to 7% Cr by weight,
1. A wear-resistant composite roll characterized in that an outer layer of an iron-based alloy containing 9% or less of Mo, and one or more elements selected from the group consisting of W, V, Nb, Ti, Ta, Zr, and Hf, and carbides of one or more of these elements, is cast onto a steel shaft, the surface hardness of the outer layer being 70 or more in Shore hardness, the tensile strength and elongation of the shaft being 55 kg/ ^mm2 or more and 1.0% or more, respectively, and the bond strength at the boundary between the outer layer and the shaft being equal to or greater than the strength of the weaker of the outer layer and the shaft. 2. The wear-resistant composite roll according to claim 1, wherein the carbide is uniformly distributed in the outer layer. 3. The wear-resistant composite roll according to claim 1 or 2, wherein the iron-based alloy is in a weight ratio of 1.5 to 1.5.
3.5% C, 0.3-3.0% Si, 0.3-1.5% Mn, 2-7
% Cr, 9% or less Mo, 20% or less W, and 3 to 15% V. 4. The wear-resistant composite roll according to claim 3, wherein the iron-based alloy further comprises 5% or less by weight.
A wear-resistant composite roll containing Ni. 5. The wear-resistant composite roll according to claim 3, wherein the iron-based alloy further comprises 5% or less by weight.
A wear-resistant composite roll containing Co. 6. The wear-resistant composite roll according to claim 3, wherein the iron-based alloy further comprises 5% or less by weight.
A wear-resistant composite roll containing Nb. 7. The wear-resistant composite roll according to claim 3, wherein the iron-based alloy further comprises 5% or less by weight.
A wear-resistant composite roll comprising Ni and 5% or less of Co. 8. The wear-resistant composite roll according to claim 3, wherein said iron or its alloy further comprises 5% or less by weight.
A wear-resistant composite roll comprising Ni and 5% or less of Nb. 9. The wear-resistant composite roll according to claim 3, wherein the iron-based alloy further comprises 5% or less by weight.
A wear-resistant composite roll comprising Co and 5% or less of Nb. [Claim 10] A wear-resistant composite roll according to claim 3, wherein the iron-based alloy further contains, by weight ratio, 5% or less of Ni, 5% or less of Co, and 5% or less of Nb. [Claim 11] A method for manufacturing a wear-resistant composite roll in which an outer layer made of an iron-based alloy of which components crystallize from the molten metal during solidification, a carbide of one or more elements selected from the group consisting of W, V, Nb, Ti, Ta, Zr, and Hf, is metallically bonded to a steel shaft, the method comprising: loosely fitting the steel shaft coaxially into a space inside a combination mold consisting of a refractory frame surrounded by an induction heating coil and a cooling mold installed coaxially below the frame; pouring molten iron-based alloy into the gap formed between the shaft and the mold; sealing the surface of the molten metal with flux; heating and stirring the molten metal at a temperature within a range from the primary crystallization temperature to 100°C higher; moving the shaft downward coaxially with the mold; bringing the molten metal into contact with the cooling mold, whereby the molten metal is solidified and welded to the shaft, thereby forming the outer layer continuously around the shaft. [Claim 12] A method for manufacturing a wear-resistant composite roll as described in claim 11, characterized in that the outer layer made of the iron-based alloy contains, by weight, 1.5 to 3.5% C, 2 to 7% Cr, and 9% or less Mo.