Heat-resistant alloy steel for hearth metal members of steel material heating furnaces

A heat-resistant alloy comprising, as expressed in % by weight, 0.03 to 0.1% of C, 0.2 to 0.7% of Si, 0.2 to 0.7% of Mn, 42 to 60% of Ni, 25 to 35%0 of Cr, 8 to 20% of W, over 0% to not more than 8% Mo, over 0% to not more than 5% of Co, and the balance substantially Fe. The alloy has improved resistance to compressive deformation and oxidation resistance for use in oxidizing atmospheres having a high temperature of 1250.degree. C.

FIELD OF THE INVENTION 
The present invention relates to heat-resistant alloy steels having 
improved high-temperature characteristics and useful for skid buttons and 
like hearth metal members which are support members for the steel 
materials to be heated in heating furnaces. 
BACKGROUND OF THE INVENTION 
Steel materials such as slabs or billets are placed into a heating furnace 
prior to hot plastic working (for example, hot rolling or hot forging) and 
subjected to a specified heat treatment. Heating furnaces of the walking 
beam conveyor type have skid beams (fixed beams and movable beams) adapted 
to be internally cooled with water and arranged longitudinally of the 
furnace. The skid beams have attached thereto heat-resistant alloy blocks 
(skid buttons) arranged at a predetermined interval and serving as hearth 
metal members. The steel material placed into the furnace is transported 
within the furnace as supported by the skid buttons on the fixed beams and 
those on the movable beams alternately. 
The hearth metal members must have oxidation resistance so as to be free of 
corrosion (oxidation wear) due to the high-temperature oxidizing internal 
atmosphere of the furnace, and such resistance to compressive deformation 
that the members will not readily deform even if repeatedly subjected to 
the compressive load of the heavy steel material to be heated. The 
materials conventionally used for hearth metal materials include high 
alloy steels such as high Ni-high Cr alloy steels (JIS G5122 SCH22, etc.) 
and Co-containing Ni-Cr alloy steels (e.g., 50Co--20Ni--30Cr--Fe). Also 
proposed as improved hearth alloy materials are 0.3-0.6% C--40-60% 
Ni--25-35% Cr--8-15% W--Fe alloys (Japanese post-examination publication 
SHO54-18650), 0.2-1.5% C+N--15-60% Ni--15-40% Cr--3-10% W--Fe alloys 
(Japanese post-examination publication SHO 63-44814), 1.0% 
.gtoreq.C--26-38% Cr--10-25% W--Ni alloys (U.S. Pat. No. 3,403,998), etc. 
Some of these alloys are already in actual use. 
The operating temperature of steel material heating furnaces is elevated 
year after year for the treatment of a wide variety of steel materials, 
improvements in the quality of treated materials and savings in energy. It 
is common practice to operate the furnace at a high temperature of 
1250.degree. C. or higher, and the internal furnace temperature is likely 
to exceed 1300.degree. C. Higher oxidation resistance and improved 
resistance to compressive deformation are required of the hearth metal 
members in order to carry out the high-temperature operation efficiently 
and safely. 
However, the conventional heat-resistant alloys fail to fully withstand 
such high-temperature operations. Although it may be attempted to cool the 
hearth metal members more effectively by the internal water-cooling 
structure of the skid beams, the attempt leads to an increased heat loss 
due to the cooling water and uneven heating of the steel material to be 
treated as supported by the hearth metal members (occurrence of so-called 
"skid marks") and can not be a substantial countermeasure. 
An object of the present invention is to provide a heat-resistant alloy 
steel having improved high-temperature characteristics in order to solve 
the above problem encountered with hearth metal members. 
SUMMARY OF THE INVENTION 
The present invention provides a heat-resistant alloy steel having a high 
melting point for hearth metal members of steel material heating furnaces, 
the alloy steel having a chemical composition consisting essentially of , 
as expressed in % by weight, 0.03 to 0.1% of C, 0.2 to 0.7% of Si, 0.2 to 
0.7% of Mn, 42 to 60% of Ni, 25 to 35% of Cr, 8 to 20% of W, over 0% to 
not more than 8% of Mo, over 0% to not more than 5% of Co, and the balance 
substantially Fe.

DETAILED DESCRIPTION OF THE INVENTION 
Given below are reasons for limiting the components of the heat-resistant 
alloy steel of the invention as above. The contents of elements are 
expressed in % by weight. 
C: 0.03-0.1% 
With heat-resistant alloy steels, it is common practice to cause C to 
combine, for example, with Cr or Fe and to give improved strength at high 
temperatures by the dispersion strengthening effect of the carbide 
precipitated, whereas the carbide becomes dissolved in the matrix at high 
temperatures of over 1250.degree. C. at which the present steel is to be 
used, failing to contribute to the improvement of strength. Further it is 
desired to reduce the C content in affording alloy steels of high melting 
point because C exerts a great influence on the melting point of the alloy 
steel. According to the present invention, therefore, the C content is 
limited to not greater than 0.1% to obtain a high melting point, while the 
strengthening elements, such as W, Mo and Co, to be described below are 
added in combination so as to ensure the required strength at high 
temperatures. Although a lower C content is more advantageous in giving 
the alloy a higher melting point, the alloy which is prepared by a melting 
procedure becomes more costly. Further since the reduction of the C 
content below 0.03% entails no substantial benefit, this value is taken as 
the lower limit. 
Si: 0.2-0.7% 
Si serves as a deoxidizer in the alloy preparation process, affords 
improved castability and should be present in an amount of at least 0.2%. 
Increases in the Si content result in a lower melting point although 
effective for improving the oxidation resistance of the alloy, so that the 
upper limit should be 0.7%. 
Mn: 0.2-0.7% 
Mn is a deoxidizing-desulfurizing element and also contributes to the 
formation of a stabilized austenitic structure. However, an increase in 
the amount of the element lowers the melting point of the alloy. For this 
reason, at least 0.2% to not more than 0.7% of Si should be present. 
Ni: 42-60% 
Ni is the basic element of heat-resistant alloy steels, forms an austenitic 
structure, further forms a stabilized oxide film to give enhanced 
corrosion resistance when present conjointly with Cr, and has an effect to 
give improved high-temperature strength when present in combination with 
Cr, W or the like, affording enhanced resistance to compressive 
deformation. To ensure this effect, the Ni content should be at least 42% 
to not higher than 60%. 
Cr: 25-35% 
Cr is an element contributing to improvements in oxidation resistance and 
high-temperature strength. At least 25% of Cr needs to be present to 
obtain this effect. The upper limit should be 35% since presence of an 
excess of Cr results in impaired castability and lower high-temperature 
strength. 
W: 8-20% 
W affords improved compressive strength. At least 8% of W should be present 
to obtain this effect. The effect increases with an increase in the W 
content but nearly levels off when the content exceeds 20%. Excessive 
contents also adversely affect the oxidation resistance and castability of 
the alloy. The upper limit should therefore be 20%. 
Mo: over 0% to not more than 8% 
Mo is an element producing a favorable effect on the high-temperature 
compressive strength of the alloy and the elevation of the melting point 
thereof. This effect becomes more pronounced when Mo is added in 
combination with Co. Although an increase in the Mo content leads to an 
enhanced effect, use of up to 8% of the element achieves a satisfactory 
result, and greater amounts entail impaired economy, so that 8% is the 
upper limit. The preferred content is 0.5 to 5%. 
Co: over 0% to not more than 5% 
Co, like Mo, is favorable in imparting improved high-temperature 
compressive strength and higher melting point to the alloy, and this 
effect increases when Co is present conjointly with Mo. An increased Co 
content produces an enhanced effect, whereas Co is an expensive element 
and should therefore be present in an amount of up to 5% in view of the 
effect available and economy. The amount is preferably 0.5 to 3%. 
The hearth member of the heat-resistant alloy steel of the invention is 
prepared by machining this material as cast to the required shape. The 
alloy steel of the invention has high strength and high resistance to 
oxidation to withstand operations at high temperatures of over 
1250.degree. C. The solidus of the steel indicates that the material has 
an exceedingly high melting point of at least 1300.degree. C. The high 
melting points makes possible a design of hearth structure wherein the 
forced cooling from the skid beams is attenuated and the resulting 
reduction in the internal heat loss of the furnace. 
The hearth metal member need not always be made entirely from the 
heat-resistant alloy steel of the invention. Depending on the construction 
of the hearth or furnace operating conditions, the member can be of a 
structure of superposed layers which comprises a block of conventional 
material providing a base portion of the member (i.e. , portion in contact 
with the skid beam and subjected to a relatively great forced cooling 
effect) , and an upper portion made from the steel of the invention and 
joined to the base portion. 
EXAMPLES 
A molten alloy prepared in a high-frequency melting furnace was cast, and 
the resulting cast material was machined to prepare test pieces. Table 1 
shows the chemical compositions of the specimen alloys thus prepared, and 
the solidi, high-temperature compressive deformation resistance and 
oxidation resistance of the alloys determined. With reference to the 
table, the solidus (c) is a measurement obtained at a rate of rise of 
temperature of 3 min, and the amount of high-temperature deformation (%) 
and oxidation loss (mm/year) were measured by the following tests. 
High-Temperature Compression Test! 
As shown in FIG. 1, a solid cylindrical test piece (b) was placed upright 
on a base (a), and a compressive load was applied to the test piece (b) by 
pressing a pressure jig (c) against the top face of the test piece. As 
shown in FIG. 2, the jig was held pressed for a predetermined period of 
time, and the test piece b was thereafter relieved of the load. This cycle 
was repeated a specified number of times, and the test piece b was 
thereafter checked to calculate the amount D of resulting compressive 
deformation from the following equation. 
EQU D=(L1 -L0)/L0.times.100(%) 
Size of test piece: 30 (diameter).times.50 L (mm) 
Test temperature: 1300.degree. C. 
Compressive load: 24.5 MPa 
Number of cycles: 2000 
Oxidation Test! 
A solid cylindrical test piece was held in a heating furnace (natural 
atmosphere) for a predetermined period of time and thereafter checked for 
the variation in weight due to oxidation to calculate the rate of 
oxidation loss (mm/year). 
Size of test piece: 8 (diameter).times.50 L (mm) 
Test temperature: 1250.degree. C. 
Test time: 100 hr 
TABLE 1 
__________________________________________________________________________ 
High-temperature 
High-temperature 
Specimen 
Alloy Composition (wt %) Solidus 
Compressive 
Oxidation Loss 
No. C Si Mn Ni Cr W Mo Co Fe (.degree.C.) 
Deformation (%) 
(mm/year) 
__________________________________________________________________________ 
1 0.05 
0.32 
0.42 
50.4 
29.8 
12.7 
1.02 
0.97 
Bal. 
1335 
4.35 1.21 
2 0.06 
0.33 
0.40 
50.3 
29.6 
10.1 
2.09 
1.91 
Bal. 
1337 
4.22 0.94 
3 0.05 
0.28 
0.41 
49.8 
30.1 
10.1 
1.02 
0.21 
Bal. 
1333 
4.73 0.99 
4 0.05 
0.29 
0.42 
50.1 
29.8 
10.1 
1.05 
2.91 
Bal. 
1325 
4.11 0.89 
5 0.05 
0.31 
0.40 
50.3 
30.2 
10.3 
3.12 
1.98 
Bal. 
1332 
3.98 1.21 
6 0.06 
0.32 
0.43 
50.6 
29.9 
9.8 
5.01 
2.02 
Bal. 
1329 
3.85 1.34 
11 0.24 
0.30 
0.42 
50.2 
29.8 
12.8 
-- -- Bal. 
1310 
8.85 1.36 
12 0.07 
0.11 
0.41 
50.2 
30.2 
13.1 
-- -- Bal. 
1347 
7.01 1.75 
13 0.05 
0.30 
0.65 
49.9 
29.9 
12.9 
-- -- Bal. 
1324 
7.55 1.33 
14 0.06 
0.33 
0.98 
50.1 
30.0 
13.1 
-- -- Bal. 
1319 
8.12 1.35 
15 0.07 
0.28 
0.41 
50.3 
29.7 
7.4 
-- -- Bal. 
1327 
15.21 0.72 
16 0.05 
0.34 
0.45 
49.4 
30.1 
12.4 
1.02 
-- Bal. 
1335 
6.02 1.35 
17 0.07 
0.29 
0.46 
50.4 
30.6 
12.5 
4.89 
-- Bal. 
1342 
5.57 1.50 
18 0.05 
0.34 
0.45 
49.7 
30.4 
9.9 
1.21 
-- Bal. 
1338 
6.84 1.28 
19 0.05 
0.32 
0.41 
49.8 
29.9 
12.9 
-- 0.55 
Bal. 
1321 
7.40 1.14 
20 0.05 
0.30 
0.47 
50.1 
30.4 
12.9 
-- 2.50 
Bal. 
1324 
6.65 0.92 
21 0.44 
0.31 
0.39 
49.8 
31.2 
13.1 
-- -- Bal. 
1302 
9.81 1.37 
22 0.48 
0.13 
0.15 
51.2 
31.5 
17.2 
-- -- Bal. 
1312 
10.81 2.45 
23 0.14 
0.30 
0.45 
50.0 
29.9 
12.9 
5.03 
2.10 
Bal. 
1308 
6.15 1.40 
24 0.23 
0.29 
0.43 
50.3 
30.0 
9.5 
2.10 
1.98 
Bal. 
1309 
6.52 1.35 
__________________________________________________________________________ 
In Table 1, No. 1 to No. 6 are examples of the invention, and No. 11 to No. 
24 are comparative examples. 
Of the comparative examples (No. 11 to No. 24), No. 11 to No. 20 are low 
C-high Ni--W alloys like the examples of the invention, and No. 21 and No. 
22, which are heat-resistant alloys not containing the combination of Mo 
and Co, are conventional materials. No. 21 is a material corresponding to 
the alloy disclosed in Japanese post-examination publication SHO 54-18650, 
and No. 22 is a material corresponding to the alloy disclosed in U.S. Pat. 
No. 3,403,998. No. 23 and No. 24 are heat-resistant alloys containing 
larger amount of C. No. 24 is also a material corresponding to the alloy 
disclosed in Japanese post-examination publication SHO 63-44814. 
A comparison between the examples of the invention No. 1 to No. 6 and the 
conventional materials No. 21 and No. 22 shows that as compared with the 
conventional materials, the examples of the invention are exceedingly 
higher in melting point and improved in resistance to compressive 
deformation and oxidation resistance. The comparative examples No. 11 to 
No. 20, although higher than the conventional materials in melting point, 
are not improved in both compressive deformation resistance and oxidation 
resistance and still remain to be improved unlike the materials of the 
invention. The comparative examples No. 23 and No. 24 are lower with 
respect to melting point and inferior in compresive deformation 
resistance. 
The heat-resistant alloy steel of the present invention has high 
compressive deformation resistance, improved oxidation resistance and an 
exceedingly high melting point which are required of the hearth metal 
members for use in steel material heating furnaces. These improved 
high-temperature characteristics render the alloy steel useful for the 
hearth metal members to be subjected to high-temperature furnace operating 
conditions in recent years, ensuring improved durability, easier 
maintenance, stabilized furnace operation and higher furnace operation 
efficiency. The high melting point of the alloy steel mitigates the forced 
cooling of hearth metal members, diminishing the heat loss due to the 
removal of heat to the outside of the furnace and achieving savings in 
energy.