Heat-resistant alloy for hearth metal member

Abstract

The present invention provides a Co-free heat-resistant alloy for a hearth metal member that has properties superior to or equal to those of Co-containing heat resistant steel. The heat-resistant alloy for a hearth metal member according to the present invention is a heat-resistant alloy used in a hearth metal member of a steel heating furnace, the heat resistant alloy containing: 0.05% to 0.5% of C; more than 0% and 0.95% or less of Si, where 0.05%≤C+Si≤1.0%; more than 0% and 1.0% or less of Mn; 40% to 50% of Ni; 25% to 35% of Cr; 1.0% to 3.0% of W; and 10% or more of Fe and inevitable impurities as the balance, with all percentages being in mass %. The heat-resistant alloy for a hearth metal member may further contain 0.05% to 0.5% of Ti and/or 0.02% to 1.0% of Zr, with all percentages being in mass %.

Claims

1. A heat-resistant alloy for a hearth metal member of a steel heating furnace, the heat-resistant alloy consisting of: 0.2% to 0.5% of C; more than 0% and 0.95% or less of Si, where 0.20<C+Si≤1.0%; more than 0% and 1.0% or less of Mn; 40% to 50% of Ni; 25% to 35% of Cr; 1.0% to 2.0% of W; 0.05% to 0.5% of Ti; optionally, 0.02% to 1.0% of Zr; and 10% or more of Fe and inevitable impurities as the balance, with all percentages being in mass %.

2. The heat-resistant alloy for a hearth metal member according to claim 1, wherein the inevitable impurities consist of at least one selected from the group consisting of 0.2% or less of N, 0.2% or less of 0, and 0.1% or less of H, with all percentages being in mass %.

3. A hearth metal member of a steel heating furnace, wherein the hearth metal member partially or entirely consists of the heat-resistant alloy for a hearth metal member according to claim 2.

4. A hearth metal member of a steel heating furnace, wherein the hearth metal member partially or entirely consists of the heat-resistant alloy for a hearth metal member according to claim 1.

5. The heat-resistant alloy for a hearth metal member according to claim 1, wherein the heating zone of the hearth metal member is from 1100° C. to 1300° C.

6. The heat-resistant alloy for a hearth metal member according to claim 1, wherein the inevitable impurities consist of 0.03% or less of P and/or 0.03% or less of S, with all percentages being in mass %.

7. A hearth metal member of a steel heating furnace, wherein the hearth metal member partially or entirely consists of the heat-resistant alloy for a hearth metal member according to claim 6.

8. The heat-resistant alloy for a hearth metal member according to claim 6, wherein the inevitable impurities consist of at least one selected from the group consisting of 0.2% or less of N, 0.2% or less of 0, and 0.1% or less of H, with all percentages being in mass %.

9. A hearth metal member of a steel heating furnace, wherein the hearth metal member partially or entirely consists of the heat-resistant alloy for a hearth metal member according to claim 8.

10. The heat-resistant alloy for a hearth metal member according to claim 1, wherein there is present 0.02% to 1.0% of Zr, with all percentages being in mass %.

11. A hearth metal member of a steel heating furnace, wherein the hearth metal member partially or entirely consists of the heat-resistant alloy for a hearth metal member according to claim 10.

12. The heat-resistant alloy for a hearth metal member according to claim 10, wherein the inevitable impurities consist of at least one selected from the group consisting of 0.2% or less of N, 0.2% or less of 0, and 0.1% or less of H, with all percentages being in mass %.

13. A hearth metal member of a steel heating furnace, wherein the hearth metal member partially or entirely consists of the heat-resistant alloy for a hearth metal member according to claim 12.

14. The heat-resistant alloy for a hearth metal member according to claim 10, wherein the inevitable impurities consist of 0.03% or less of P and/or 0.03% or less of S, with all percentages being in mass %.

15. A hearth metal member of a steel heating furnace, wherein the hearth metal member partially or entirely consists of the heat-resistant alloy for a hearth metal member according to claim 14.

16. The heat-resistant alloy for a hearth metal member according to claim 14, wherein the inevitable impurities consist of at least one selected from the group consisting of 0.2% or less of N, 0.2% or less of 0, and 0.1% or less of H, with all percentages being in mass %.

17. A hearth metal member of a steel heating furnace, wherein the hearth metal member partially or entirely consists of the heat-resistant alloy for a hearth metal member according to claim 16.

Description

DESCRIPTION OF EMBODIMENTS

(1) The heat-resistant alloy for a hearth metal member according to the present invention can be produced by casting the component elements described above and performing heat treatment and machining so as to shape the alloy into a desired shape. The hearth metal member may be, for example, a skid button or a skid rail, Here, the hearth metal member may be completely made of the heat-resistant alloy of the present invention, or may be partially made of the heat-resistant alloy of the present invention depending on the hearth structure, the furnace operation conditions, or the like. For example, only a portion that comes into contact with the slab may be formed using the heat-resistant alloy of the present invention.

(2) As will be shown in examples below, the heat-resistant alloy for a hearth metal member according to the present invention has a solidus temperature of about 1300° C. to 1400° C. Accordingly, the heat-resistant alloy of the present invention is preferably used in the preheating zone and the heating zone of a heating furnace, and it is more desirable that the heat-resistant alloy of the present invention is used in the heating zone operating at about 1100° C. to 1300° C.

(3) The heat-resistant alloy for a hearth metal member according to the present invention is free of Co, and thus will not be regulated under the Japanese Industrial Safety and Health Act. Also, as will be shown in examples given below, the heat-resistant alloy of the present invention has a high solidus temperature and high high-temperature strength in terms of oxidation resistance, compressive deformation resistance rate, and the like. Accordingly, it is very useful as an alternative to Co-containing heat resistant steel used in hearth metal members.

EXAMPLES

(4) Heat-resistant alloys having compositions shown in Table 1 were used to produce molten metals through atmospheric melting in a high-frequency induction melting furnace, and the molten metals were subjected to casting to obtain samples. In the samples shown in Table 1, Inventive Examples 1 to 5 are examples according to the present invention, and Comparative Examples 1 to 7 are comparative examples. Also, for comparison, a sample containing Co was produced as Reference Example.

(5) TABLE-US-00001 TABLE 1 C + Fe C Si Si Mn P S Ni Cr W Mo Co Ti Zr N O (remainder) Inventive 0.2 0.5 0.7 0.6 0.005 0.003 46.0 33.0 2.0 0.1 0.1 17.5 Example 1 Inventive 0.2 0.3 0.5 0.3 0.001 0.004 45.0 33.0 2.0 0.001 0.050 19.2 Example 2 Inventive 0.2 0.3 0.5 0.4 0.007 0.006 45.0 33.0 2.0 0.05 0.001 0.044 19.0 Example 3 Inventive 0.2 0.3 0.5 0.5 0.001 0.005 46.0 33.0 2.0 0.1 0.001 0.061 17.9 Example 4 Inventive 0.2 0.3 0.5 0.5 0.001 0.005 45.0 33.0 2.0 0.1 0.1 0.001 0.050 18.8 Example 5 Comp. Ex. 1 0.1 0.8 0.9 0.7 0.007 0.001 44.3 20.1 2.0 32.1 Comp. Ex. 2 0.1 1.5 1.6 2.0 0.012 0.003 44.3 34.1 2.0 16.1 Comp. Ex. 3 0.1 1.1 1.2 0.7 30.0 44.8 2.9 20.4 Comp. Ex. 4 0.1 1.1 1.2 2.1 19.7 45.0 2.9 29.1 Comp. Ex. 5 0.4 0.7 1.1 0.6 0.005 0.003 45.0 32.5 2.0 0.1 0.0 18.7 Comp. Ex. 6 0.4 0.6 1.0 0.6 0.013 0.003 46.2 30.3 3.6 0.1 0.0 18.2 Comp. Ex. 7 0.4 0.5 1.3 0.5 0.009 0.007 42.1 43.2 2.3 0.1 0.0 10.9 Ref. Ex. 0.1 1.3 1.4 1.2 0.011 0.014 16.4 26.5 1.0 38.3 15.2

(6) Then, the solidus temperature, the tensile strength, the tensile elongation, the compressive deformation ratio, and the oxidation reduction rate that is an indicator of oxidation resistance were measured for each sample, and an evaluation was made. The results are shown in Tables 2 to 5.

(7) The solidus temperature is a value measured at a heating rate of 3° C./min. The results are shown in Table 2.

(8) The tensile strength was measured at temperatures of 600° C., 800° C., 900° C., and 1100° C. in accordance with JIS Z2241. The results are shown in Table 2 as actually measured values.

(9) The tensile elongation was measured at temperatures of 600° C., 800° C., 900° C., and 1100° C. in accordance with JIS 22241, and the ratio of the length of each sample at break relative to the original length of the sample was calculated as a percentage (%). The results are shown in Table 3 as actually measured values.

(10) The compressive deformation ratio was measured using a plurality of cylindrical test pieces (each having a height of 50 mm and a diameter of 30 mm) obtained by cutting each sample. More specifically, in an electric furnace at an internal temperature of 1300° C., the test pieces were fixed upright on a fixing table, and a compressive load of 9.81 N/mm.sup.2 was repeatedly applied to the test pieces while maintaining the temperature of the test pieces at 1230° C. to 1260° C. The repetitive application of a load was performed as follows. The operation (a total of 12 seconds) of applying the load for 5 seconds and applying no load for 5 seconds, with each transition time between the application of the load and the application of no load being set to 1 second, was defined as one cycle, and the cycle was repeatedly performed on each test piece 2000 times. This test was performed on two to four test pieces, and then the ratio of change in height and the ratio of change in diameter of each test piece were measured before and after the test, and the average of each ratio of change (%) was calculated. The results are shown in Table 4 as actually measured values.

(11) The oxidation reduction rate was also measured using round-rod shaped test pieces (each having a length of 50 mm and a diameter of 10 mm) obtained by cutting each sample. More specifically, each test piece was kept in an atmosphere at temperatures of 1200° C., 1252° C., and 1302° C. for 100 hours, and then a weight change of the test piece due to oxidation was measured to obtain the oxidation reduction rate (mm/year). The results are shown in Table 5 as actually measured values.

(12) The results of the above-described tests are shown in Tables 2 to 5. A blank space in the tables indicates that measurement was not performed on the sample.

(13) The solidus temperature was measured using all samples. As shown in Table 2, it can be seen that all samples had a solidus temperature (actually measured value) above 1300° C. On the other hand, in a heating furnace, in order to achieve stable operation particularly in the heating zone and the soaking zone, the alloy is required to have a solidus temperature greater than 1300° C. by 50° C. to 60° C. or more. Accordingly, the following evaluation criteria for solidus temperature was used: a sample that had a solidus temperature of 1400° C. or higher, which was close to that of Reference Example, was rated as “A”; a sample that had a solidus temperature of 1380° C. or higher was rated as “B”; a sample that had a solidus temperature of 1360° C. or higher was rated as “C”; and a sample that had a solidus temperature less than 1360° C. was rated as “D”. As a result, as shown in Table 2, none of the samples of Inventive Examples and Comparative Examples was rated as “A”, but the samples of Inventive Examples were rated as either “B” or “C”. In Comparative Examples, the sample of Comparative Example 1 was rated as “C”, and the other samples were rated as “D”.

(14) TABLE-US-00002 TABLE 2 Solidus Temp. (° C.) (actually Tensile strength measured Total Individual score Rating value) Rating score 600° C. 800° C. 900° C. 1100° C. Inventive C 1,363 B 1 −1 1 0 1 Example 1 Inventive C 1,374 Example 2 Inventive B 1,381 Example 3 Inventive B 1,383 C 0 −1 0 0 1 Example 4 Inventive B 1,382 Example 5 Comp. Ex. 1 C 1,377 C 0 −1 −1 1 1 Comp. Ex. 2 D 1,334 A 3 1 1 1 0 Comp. Ex. 3 D 1,322 A 4 1 1 1 1 Comp. Ex. 4 D 1,336 B 2 −1 1 1 1 Comp. Ex. 5 D 1,340 B 2 −1 1 1 1 Comp. Ex. 6 D 1,342 C 0 −1 0 1 Comp. Ex. 7 D 1,348 B 1 −1 1 1 Ref. Ex. 1,412 Tensile strength Comparison with Reference Example Actually measured value (N/mm.sup.3) 600° C. 800° C. 900° C. 1100° C. 600° C. 800° C. 900° C. 1100° C. Inventive −7% 8% 2% 19% 330 244 167 63 Example 1 Inventive Example 2 Inventive Example 3 Inventive −12% 0% −3% 9% 310 226 158 58 Example 4 Inventive Example 5 Comp. Ex. 1 −26% −14% 16% 8% 261 194 189 57 Comp. Ex. 2 12% 31% 26% 4% 394 296 206 55 Comp. Ex. 3 11% 41% 34% 9% 392 318 218 58 Comp. Ex. 4 −24% 10% 10% 9% 267 249 179 58 Comp. Ex. 5 −8% 22% 18% 45% 326 275 193 77 Comp. Ex. 6 −27% 2% 8% 256 166 57 Comp. Ex. 7 −19% 20% 9% 286 196 58 Ref. Ex. 353 226 163 53

(15) The tensile strength was measured using all samples excluding those of Inventive Examples 2, 3, and 5. Also, for the samples of Inventive Example 2, and Comparative Examples 6 and 7, the tensile strength was measured only at some measurement temperatures. Each measured value of tensile strength (actually measured values) was scored relative to the actually measured value of Reference Example obtained at each measurement temperature based on the following scale: “−1” was given when the difference was less than −5%, “0” was given when the difference was within ±5%, and “+1” was given when the difference was greater than +5%. The individual scores at each measurement temperature are shown in Table 2. Then, a rating of “A” was given when the total score was +3 or greater and there was no minus value. A rating of “B” was given when the total score was greater than 0. A rating of “C” was given when the total score was 0. A rating of “D” was given when the total score was less than 0. The results are collectively shown in Table 2.

(16) As shown in Table 2, in terms of tensile strength, the samples of Comparative Examples 2 and 3 were rated as “A”, the samples of Inventive Example 1 and Comparative Examples 4, 5, and 7 were rated as “B”, and the other samples were rated as either “C” or “D”.

(17) The tensile elongation was measured using all samples excluding those of Inventive Example 3. For the samples of Inventive Examples 2 and 5 and Comparative Examples 6 and 7, the tensile elongation was measured only at some measurement temperatures. Each measured value of tensile elongation (actually measured values) was scored relative to the actually measured value (14%) of Reference Example obtained at 600° C. based on the following scale: “−1” was given when the actually measured value was less than 14%, and “+1” was given when the actually measured value was 14% or more. Generally, the tensile strength increases as the temperature increases. Accordingly, at measurement temperatures of 800° C. or higher, evaluation was performed relative to the same value (14%). The individual scores at each measurement temperature are shown in Table 3, Then, a rating of “B” was given when the total score was greater than 0 and there was no minus value, and a rating of “C” was given when the total score was less than 0 or there was a minus value. The results are collectively shown in Table 3.

(18) TABLE-US-00003 TABLE 3 Tensile elongation Individual score Actually measured value (%) Rating Total score 600° C. 800° C. 900° C. 1100° C. 600° C. 800° C. 900° C. 1100° C. Inventive B 4 1 1 1 1 27.7 21.3 22.8 20.6 Example 1 Inventive B 3 1 1 1 25.9 23.5 21.2 Example 2 Inventive Example 3 Inventive B 4 1 1 1 1 26.3 19.8 26.6 24.5 Example 4 Inventive B 1 1 24.2 Example 5 Comp. Ex. 1 B 4 1 1 1 1 34.5 22.1 26.4 31.8 Comp. Ex. 2 C 0 −1 −1 1 1 2.4 7.9 15.4 40.6 Comp. Ex. 3 C 0 −1 −1 1 1 1.9 4.7 15.9 42.1 Comp. Ex. 4 B 4 1 1 1 1 39.4 18.7 29.3 22.7 Comp. Ex. 5 C 2 −1 1 1 1 9.3 17.7 18.4 19.2 Comp. Ex. 6 C 2 1 −1 1 1 14.6 18.4 19.2 Comp. Ex. 7 C −2 −1 −1 −1 1 3.2 13.4 18.8 Ref. Ex. 14.0 21.3 11.6 25.3

(19) As shown in Table 3, in terms of tensile elongation, the samples of Inventive Examples 1, 2, 4 and 5 and Comparative Examples 1 and 4 were rated as “B”, and the other samples were rated as “C”.

(20) The compressive deformation ratio was measured using all samples. Each measured value of the compressive deformation ratio (actually measured values) was scored relative to the compressive deformation ratio (actually measured value) in the height or diameter direction of Reference Example based on the following scale: “+2” was given when the difference was less than −50%, “+1” was given when the difference was less than −5%, “0” was given when the difference was within ±5%, and “−1” was given when the difference was greater than +5%. The individual scores in the height and diameter directions are shown in Table 4, Then, a rating of “A” was given when the total score was +3 or greater and there was no minus value. A rating of “B” was given when the total score was greater than 0. A rating of “C” was given when the total score was 0. A rating of “D” was given when the total score was less than 0. The results are collectively shown in Table 4.

(21) TABLE-US-00004 TABLE 4 Compressive deformation ratio Comparison with Actually measured Individual score Reference Example value (%) Rating Total score Height Diameter Height Diameter Height Diameter Inventive A 4 2 2 −87% −70% 0.6 3.4 Example 1 Inventive A 4 2 2 −66% −63% 1.6 4.2 Example 2 Inventive A 4 2 2 −82% −60% 0.9 4.6 Example 3 Inventive A 4 2 2 −73% −71% 1.3 3.3 Example 4 Inventive A 4 2 2 −84% −77% 0.8 2.7 Example 5 Comp. Ex. 1 A 4 2 2 −83% −75% 0.8 2.9 Comp. Ex. 2 D −2 −1 −1 259% 165% 16.5 30.0 Comp. Ex. 3 D −2 −1 −1 188% 117% 13.3 24.6 Comp. Ex. 4 B 2 1 1 −45% −38% 2.5 7.0 Comp. Ex. 5 A 4 2 2 −92% −88% 0.4 1.3 Comp. Ex. 6 B 3 2 1 −72% −48% 1.3 5.9 Comp. Ex. 7 B 3 2 1 −72% −48% 1.3 5.9 Ref. Ex. 4.6 11.3

(22) As shown in Table 4, in terms of compressive deformation ratio, the samples of Inventive Examples 1 to 5 and Comparative Examples 1 and 5 were rated as “A”, the samples of Comparative Examples 4, 6 and 7 were rated as “B”, and other samples were rated as “D”.

(23) The oxidation reduction rate was measured using all samples. However, for the samples of Inventive Examples 2 to 5, measurement was performed only at some measurement temperatures. Each measured value of the oxidation reduction rate (actually measured value) was scored relative to the actually measured value of Reference Example obtained at each measurement temperature based on the following scale: “+2” was given when the difference was less than −50%, “+1” was given when the difference was less than −5%, “0” was given when the difference was within ±5%, and “−1” was given when the difference was greater than +5%. The individual scores at each measurement temperature are shown in Table 5, Then, a rating of “B” was given when the total score was greater than 0. A rating of “C” was given when the total score was 0. A rating of “D” was given when the total score was less than 0 and there were two or more minus values. The results are collectively shown in Table 5.

(24) TABLE-US-00005 TABLE 5 Oxidation reduction rate Comparison with Reference Actually measured value Total Individual score Example (mm/year) Rating score 1200° C. 1252° C. 1302° C. 1200° C. 1252° C. 1302° C. 1200° C. 1252° C. 1302° C. Inventive B 2 −1 1 2 50% −35% −79% 0.83 2.17 2.69 Example 1 Inventive B 1 −1 2 130% −86% 1.28 1.87 Example 2 Inventive B 1 −1 2 186% −75% 1.59 3.22 Example 3 Inventive B 1 −1 2 213% −77% 1.74 2.92 Example 4 Inventive B 1 −1 2 271% −71% 2.06 3.72 Example 5 Comp. Ex. 1 D −3 −1 −1 −1 221% 2002% 1136% 1.79 69.54 160.34 Comp. Ex. 2 B 2 −1 1 2 154% −14% −68% 1.41 2.86 4.18 Comp. Ex. 3 C 0 −1 −1 2 278% 15% −58% 2.10 3.82 5.44 Comp. Ex. 4 B 3 0 1 2 4% −35% −76% 0.58 2.16 3.10 Comp. Ex. 5 B 2 −1 1 2 105% −28% −65% 1.14 2.38 4.52 Comp. Ex. 6 D −3 −1 −1 −1 314% 55% 27% 2.30 5.14 16.50 Comp. Ex. 7 B 2 −1 1 2 120% −3% −63% 1.22 3.21 4.80 Ref. Ex. 0.56 3.31 12.97

(25) As shown in Table 5, the samples of Inventive Examples 1 to 5 and Comparative Examples 2, 4, 5 and 7 were rated as “B”, and other samples were rated as “D”.

(26) Then, the ratings “A” to “D” of each sample obtained above were again scored as follows: “+2” was given to a rating of “A”, “+1” was given to a rating of “B”, “0” was given to a rating of “C”, and “−1” was given to a rating of “D”. The ratings and scores (within parentheses) of each sample are shown in Table 6. Then, the overall rating of each sample was determined based on the scores. In the overall rating, a rating of “A” was given when the total score was greater than 3 and there was no minus value, a rating of “B” was given when the total score was 3, a rating of “C” was given when the total score was 0 to 2, and a rating of “D” was given when the total score was less than 0 or there were two or more minus values. The overall ratings are shown in Table 6.

(27) TABLE-US-00006 TABLE 6 Compressive Tensile deformation Oxidation Solidus Tensile strength elongation ratio reduction rate Overall rating Inventive C (0) B (1) B (1) A (2) B (1) A Example 1 Inventive C (0) B (1) A (2) B (1) A Example 2 Inventive B (1) A (2) B (1) A Example 3 Inventive B (1) C (0) B (1) A (2) B (1) A Example 4 Inventive B (1) A (2) B (1) A Example 5 Comp. Ex. 1 C (0) C (0) B (1) A (2) D (−1) C Comp. Ex. 2 D (−1) A (2) C (0) D (−1) B (1) D Comp. Ex. 3 D (−1) A (2) C (0) D (−1) C (0) D Comp. Ex. 4 D (−1) B (1) B (1) B (1) B (1) B Comp. Ex. 5 D (−1) B (1) C (0) A (2) B (1) B Comp. Ex. 6 D (−1) C (0) C (0) B (1) D (−1) D Comp. Ex. 7 D (−1) B (1) C (0) B (1) B (1) C

(28) As shown in Table 6, all of the samples of Inventive Examples were rated as “A” in the overall rating, from which it can be seen that they have properties superior to or equal to those of the Co-containing heat resistant steel of Reference Example. That is, it can be seen that the heat-resistant alloys of Inventive Examples are very useful as an alternative to Co-containing heat resistant steel used in hearth metal members.

(29) On the other hand, all of the samples of Comparative Examples were rated as any one of “B” to “D” in the overall rating. The following factors are considered to be the cause thereof.

(30) In Comparative Example 1, the amount of C, the amount of Si, and the total amount of C and Si (C+Si) were within the ranges of the present invention, and thus the solidus temperature was high. However, the amount of Cr was less than the range of the present invention, and thus sufficient oxidation resistance (oxidation reduction rate) was not obtained.

(31) In Comparative Example 2, the amount of Si and the total amount of C and Si (C+Si) exceeded the ranges of the present invention, and thus the solidus temperature was low. Accordingly in the oxidation resistance test, sufficient oxidation resistance was observed, but the alloy may melt or the oxidation amount may increase when the temperature rises due to an anomaly in the heating furnace.

(32) In Comparative Examples 3 and 4, the amount of Si and the total amount of C and Si (C+Si) exceeded the ranges of the present invention, and the solidus temperature was low. Also, the amount of Cr exceeded the range of the present invention, and thus sufficient ductility (tensile elongation) was not obtained. Furthermore, in Comparative Example 4, the amount of Ni was less than the range of the present invention, and the tensile strength was low.

(33) In Comparative Example 5, the amount of C, the amount of Ni, and the amount of Cr were within the ranges of the present invention, but the total amount of C and Si (C+Si) exceeded the range of the present invention, and thus the solidus temperature was low and the tensile elongation was low.

(34) In Comparative Example 6, the amount of C, the amount of Si, and the total amount of C and Si (C+Si) were within the ranges of the present invention. However, the amount of W exceeded the range of the present invention, and thus the oxidation resistance was low.

(35) In Comparative Example 7, the amount of C, the amount of Si, and the total amount of C and Si (C+Si) were within the ranges of the present invention. However, the amount of Cr exceeded the range of the present invention, sufficient ductility was not obtained.

(36) The foregoing description is given merely to describe the present invention. Accordingly, it should not be construed as limiting the invention recited in the appended claims or narrowing the scope of the present invention. Also, the constituent elements of the present invention are not limited to those described in the examples given above, and it is of course possible to make various modifications within the technical scope defined in the appended claims.