JPH11293377A - Chromium-base alloy for hearth member of heating furnace - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an alloy excellent in castability, creep resistance at a high temperature, or the like, by providing a composition containing respectively specified percentages of Cr, at least one element selected from the group consisting of Ti, Mn, Mo, W, and Zr, and at least one element selected from the group consisting of C, N, and B and having the balance Ni with inevitable impurities. SOLUTION: An alloy, having a composition containing 50-80 wt.% Cr, 2-10 wt.% of at least one element selected from the group consisting of Ti, Mn, Mo, W, and Zr, and 0.2-2 wt.% of at least one element selected from the group consisting of C, N, and B and having excellent properties at >=1300 deg.C, is provided. Ni has a function of improving the castability and oxidation resistance of Cr. Ti, or the like, have a function of forming, together with C, or the like, at least one kind selected from the group consisting of carbides, nitrides, and borides. The resultant carbides, or the like, are finely and dispersedly precipitated in the α-phase to improve compressive strength and creep resistance. By this method, the inexpensive alloy for hearth, free from casting crack and having superior productivity, can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Cr-based alloy suitable for a hearth member of a heating furnace, and more particularly, to an excellent castability, high-temperature compressive strength at 1300 ° C. or higher, creep resistance and oxidation resistance. To a Cr-based alloy for heating furnace hearth members.

[0002]

2. Description of the Related Art In order to perform hot plastic working such as hot rolling and hot forging, heat treatment is performed on steel pieces such as slabs and billets in the atmosphere. In the heat treatment, the material to be heated is charged from the end of the heating furnace, heated at a required temperature during the transfer in the furnace, and extracted from the other end. The material to be heated is transferred while being carried on a driven water-cooled movable rail. A skid button is attached to the water-cooled movable rail at the top of the peripheral surface of the water-cooled skid pipe at regular intervals in the axial direction of the skid pipe.
Therefore, the skid button receives various loads from the material to be heated in an oxidizing atmosphere.

[0003] For this reason, furnace hearth members such as skid buttons are required to be made of a material having excellent properties such as durability and stability, specifically, high-temperature compressive strength, creep resistance and oxidation resistance. You. Here, the high-temperature compressive strength and the creep resistance mean the deformation resistance to a load received at a high temperature, the high-temperature compressive strength is an index of the deformation rate at a constant compressive force at a high temperature, and the creep resistance is a constant tensile force at a high temperature. The time taken to break due to deformation (elongation) due to the pressure is used as an index. The oxidation resistance means oxidation resistance to a high-temperature oxidizing atmosphere, and the oxidation rate in a high-temperature atmosphere is used as an index.

[0004] The water cooling of the skid pipe in the above is intended to alleviate the heat effect in the heating furnace, maintain the deformation resistance, and prevent the surface from being oxidized and damaged. Since the skid pipe is water-cooled, the material to be heated carried by the skid button is locally cooled via a contact surface with the skid button, and a low-temperature portion called a skid mark is generated there. Since the skid mark has an adverse effect on product dimensions, surface properties, heating furnace fuel cost, productivity, and the like, reduction of the skid mark is one of the important tasks of the heating furnace.

However, in recent years, high-temperature operation in which the temperature inside the heating furnace is 1300 ° C. or higher has become popular for the purpose of improving the heating furnace operation efficiency and the like. Has become. On the other hand, to further enhance the water cooling of the skid pipe,
The above-mentioned important problem of skid mark reduction is made more difficult, which is not preferable. In other words, the requirements for the characteristics of the heating furnace hearth member such as the high-temperature compressive strength, creep resistance and oxidation resistance are becoming more stringent.

Conventionally, a furnace hearth member has been made of an alloy steel containing Cr, Co and Ni (for example, 27Cr-40Co-
17Ni-Fe, 30Cr-10Co-25Ni-Fe
Etc.) have been used. However, the above-mentioned alloy steel has poor properties. In the above examples, the number before the element symbol indicates the content (% by weight) of the element.

Therefore, the use of ceramics has been studied. However, although ceramics have excellent high-temperature compressive strength and oxidation resistance, they have a problem of high price, and thus have not yet been widely used.

[0008] For this reason, recently, a Cr-based alloy containing very large amount of Cr, which has the best high temperature compressive strength and creep resistance, has been developed. Some of these Cr-based alloys have high-temperature compressive strength comparable to ceramics. However, since the above Cr-based alloy has a high ductile brittle transition temperature of Cr, when it is cast after melting,
Casting cracks easily occur. That is, castability is poor (productivity is poor). Further, since it contains a very large amount of Cr, which has the worst oxidation resistance, the oxidation resistance during use is significantly reduced due to oxidation during use. Therefore, the Cr-based alloy has much room for improvement in cost and characteristics.

[0009]

SUMMARY OF THE INVENTION In view of the above circumstances, it is an object of the present invention to provide a Cr-based heating furnace hearth member excellent in castability, high-temperature compressive strength at 1300 ° C. or higher, creep resistance and oxidation resistance. To provide an alloy. In addition,
Hereinafter, the high-temperature compression strength, creep resistance, and oxidation resistance at 1300 ° C. or higher are simply referred to as compression strength, creep resistance, and oxidation resistance, respectively.

[0010]

In order to achieve the above-mentioned object, the present invention provides a method for producing a steel containing 50 to 80% by weight of Cr, Ti, Mn,
At least one member selected from the group consisting of Mo, W and Zr (hereinafter, simply referred to as "Ti etc.") is 2 to 10% by weight, and at least one member selected from the group consisting of C, N and B (hereinafter referred to as "Ti"). , Simply “C etc.”) is 0.2 to 2
It is a Cr-based alloy for a heating furnace hearth member, which contains wt% and the balance is Ni and unavoidable impurities.

[0011]

DETAILED DESCRIPTION OF THE INVENTION In the present invention, Ni improves the castability and oxidation resistance of Cr. It is Ni,
(1) lowering the ductile-brittle transition temperature of Cr; (2) forming an (α + γ) two-phase structure, unlike an additive element such as Co or Fe, which forms a brittle σ phase (intermetallic compound). This is because (3) the formation of a Cr oxide film having a high vapor pressure is suppressed, and a Ni oxide film having extremely good adhesion to a base material is formed, which prevents embrittlement due to precipitation.

If the Cr content is less than 50% by weight, the Ni content is increased and the oxidation resistance is improved, but the compressive strength and creep resistance are remarkably reduced, making it impractical. On the other hand, if it exceeds 80% by weight, the Ni content is small, and the effect of Ni is not sufficient. In particular, the castability is reduced, which leads to a reduction in productivity and an increase in cost.

In the present invention, Ti or the like is an element that, together with C and the like, forms at least one selected from the group consisting of carbides, nitrides, and borides (hereinafter, simply referred to as “carbides”). The generated carbides and the like are finely dispersed and precipitated in the α phase.

[0014] The finely dispersed carbides and the like are compared with a Cr-Ni binary alloy to which Ti, C and the like are not added.
Improves compressive strength and creep resistance. And T
The oxidation resistance of a Cr-Ni binary alloy to which i and the like and C and the like are not added is maintained at the same level. In other words, finely dispersed carbides and the like do not adversely affect oxidation resistance. T
When i or the like is less than 2% by weight or C or the like is less than 0.2% by weight, the compressive strength and the creep resistance cannot be sufficiently improved. When the content of Ti or the like exceeds 10% by weight, not only the compressive strength and the creep resistance cannot be further improved, but also the oxidation resistance is significantly reduced. When C or the like exceeds 2% by weight, not only carbides and the like which are dispersed and precipitated become coarse, but also form a solid solution in the α phase, thereby causing embrittlement.

[0015]

Examples [Examples 1 to 15, Comparative Examples 1 to 6 and Conventional Example 1] Cr-based alloys having various compositions were prepared by casting molten metal produced in the air in a high-frequency melting furnace in a sand mold. Examples, Comparative Examples) and alloy steels (conventional examples) were prepared. These Cr-based alloys and alloy steels were subjected to chemical composition analysis, as well as to a castability test, a high-temperature compression test, a creep resistance test, and an oxidation resistance test. Each test method is as follows.

(1) Castability Test The presence or absence of casting cracks on the entire surface of the chamfered gold was visually observed. At this time, the symbol “に” indicates that no casting crack was observed on the entire surface, and the symbol “×” indicates that even a slight crack was observed.

(2) High-temperature compression test A test piece (30 mm diameter, 50 mm
Length) was placed on a fixed base of the test balance, and a vertical load of 30 MPa was applied to the test balance while being heated and maintained at 1350 ° C. The test time was 50 hours. After the test, the pre-test length (L ₀ (= about 50 mm)) and the post-test length (L) of the test piece were measured, and the compressive deformation rate (D) was determined by the following equation (Equation 1).

[0018]

D (% / hr) = 2 (L ₀ −L) / L ₀

(3) Creep resistance test A test piece (6 mm diameter, 80 mm length) produced by machining was heated and held at 1200 ° C., a tensile stress of 10 MPa was applied, and the time until fracture was measured.

(4) Oxidation resistance test A test piece (8 mm diameter, 50 mm length) produced by machining was heated and maintained at 1350 ° C. for 100 hours in the atmosphere.
After the test, the oxide scale on the surface of the test piece was removed, the weight of the test piece after the test was measured, and the weight loss (g / m ² / hr) was determined from the change in the weight of the test piece before and after the test.

Table 1 shows the results of chemical composition analysis, and Table 2 shows other test results (excluding the castability test results). In addition, the castability test result was "O" in all the tests.

[0022]

[Table 1]

[0023]

[Table 2]

Examples 1 to 15 have significantly better compressive strength, creep resistance and oxidation resistance than conventional example 1 (high Co heat resistant alloy steel), which is a conventional skid button material, in a well-balanced manner. Excellent castability.

In Comparative Examples 1 to 3, C and the like were too small.
Compressive strength and creep resistance are reduced. In Comparative Examples 4 to 6, since C and the like are too large, carbides and the like that are dispersed and precipitated form a solid solution in the α phase, which causes embrittlement. As a result, the creep resistance is significantly reduced. Examples 16 to 72 Tests were performed in the same manner as in Examples 1 to 15 except that the composition of Cr was changed to 50% by weight (Examples 16 to 30). The tests were performed in the same manner as in Examples 1 to 15, except that the composition of Cr was changed to 80% by weight (Examples 31 to 45). A test was performed in the same manner as in Examples 1 to 12, except that the composition of Ti, Mn, Mo, and W was changed from 10% by weight to 2% by weight, respectively (Examples 46 to 5).
7). Further, tests were performed in the same manner as in Examples 1 to 15 except that the compositions of C, N and B were changed from 0.5% by weight to 0.2% by weight, respectively (Examples 58 to 72).

As a result, in all the examples, the castability was excellent. Further, in these examples, the compressive strength, creep resistance and oxidation resistance were determined in Examples 1 to 15.
(Table 2).

[0027]

From the above, it can be seen that the Cr-based alloy for a heating furnace hearth of the present invention has the following effects. That is,
(1) Since it has excellent balance of compressive strength, creep resistance and oxidation resistance, it can sufficiently cope with high-temperature operation of a heating furnace. (2) In production, the Cr-based alloy of the present invention can be produced with good productivity and at low cost because not only is it easy to melt, but it can be cast without causing casting cracks (has excellent castability). Can be widely used for practical use.

Claims (1)

[Claims] 1. The method according to claim 1, wherein the content of Cr is 50 to 80% by weight, Ti, Mn,
2 to 10% by weight of at least one selected from the group consisting of Mo, W and Zr, and 0.2 to 2% by weight of at least one selected from the group consisting of C, N and B, with the balance being Cr-based alloy for heating furnace hearth members made of Ni and unavoidable impurities.