AUSTRALIA FB RICE & CO Patent and Trade Mark Attorneys Patents Act 1990 KABUSHIKI KAISHA KOBE SEIKO SHO COMPLETE SPECIFICATION STANDARD PATENT Invention Title: High cellulose type covered electrode The following statement is a full description of this invention including the best method of performing it known to us:- HIGH CELLULOSE TYPE COVERED ELECTRODE BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a high cellulose type covered electrode for use in butt circumferential welding of fixed pipes of a pipeline for transporting natural gas and petroleum, and the like. 5 Description of the Related Art For butt welding of fixed pipes of a pipeline for transporting natural gas and petroleum, the following short circuit may occur: short circuit between the core wire of an electrode and a base metal is caused at a position not intended 10 by the welding worker during root pass welding, which interrupts welding. At the short-circuit occurring site, lack of penetration of molten metal often occurs. Therefore, it is necessary to grind the short-circuit occurring site by means of a grinder, and then, to restart welding. This causes reduction of 15 the welding performance efficiency. Further, at the weld resuming site, welding defects such as blow holes and slag inclusion tend to occur in the weld metal. Accordingly, improvement of the short-circuit resistance leads to shortening of the welding excluding working time and reduction of the 20 welding defect occurrence frequency. As a result, the effect of reducing the psychological burden of the welding worker is also expectable. Further, reduction of the short-circuit frequency between the core wire of the electrode and the base metal due to short-circuit occurrence enables use of the entire electrode to - 1 /Athe end. This leads to contribution of conservation of natural resources. The high cellulose type covered electrode has a strong arc force, and generates a less amount of slag, and hence is suitable 5 for downward welding. Alternatively, penetration welding is also easy because of its arc characteristics. Further, at the stage of root pass in downward welding of fixed pipes, a very high welding rate can be obtained. For this reason, in Europe and America, the high cellulose type covered electrode has been 10 adopted for long in the welding scene of pipelines or the like. For example, JP-A No. 60-162592 discloses a high cellulose type covered electrode with sufficient arc strength, penetration, spread of the crater, and slag removability resulting from addition of Na 2
CO
3 and MnCO 3 to the coating material, and improved 15 in performances of the welding joint part. Further, JP-A No. 63-220994 discloses a high cellulose type covered electrode with MgO contained in the coating material as an essential component, which is less likely to cause dripping of the molten metal during manipulation of electrode for enhancing 20 the welding performance efficiency, further prevents the reduction of performance due to excessive slag formation, and has been improved in performances of the welding joint part. Further, JP-A No. 4-138895 discloses a high cellulose type covered electrode which can provide a weld metal having excellent 25 toughness resulting from addition of B to the coating material. However, the related art technologies have the following problems. The improvement of the short-circuit resistance in root pass -2welding leads to the decrease in number of joints between beads, so that the occurrence of defects formed at the weld part is suppressed. This can improve the performances of the welding joint part. The objects of welding using the high cellulose type 5 covered electrodes described in the foregoing respective Patent Documents mainly include second- or subsequent-layer hot, filler, and cover passes. Thus, there has not been proposed heretofore any means for improving the weldability of the root pass in downward welding of the fixed pipes, particularly, the short 10 circuit resistance which is indispensable for ensuring sufficient penetration of the molten metal and stable penetration bead formation. Improvement of the short-circuit resistance is effective not only for root pass, but also for the improvement of the weldability in all passes, and the improvement of the weld 15 metal. SUMMARY OF THE INVENTION The present invention was completed in view of such problems. It is therefore an object of the present invention to provide a 20 high cellulose type covered electrode which is less likely to undergo short-circuit even in root pass welding, is excellent in weldability in downward welding, and further, has excellent mechanical characteristics due to less susceptibility to defects at the weld part. 25 A high cellulose type covered electrode in accordance with the present invention includes a coating material including a cellulose-containing raw material and a bond, the raw material being kneaded with the bond; and a mild steel core wire, the -3coating material being coated around the outer circumference of the mild steel core wire. The coating material contains, based on the total mass of the coating material, MgO: 0.1 to 10 mass%, iron oxide (FeO equivalent value): 5 to 20 mass%, TiO 2 : 5 to 15 5 mass%, Fe as a metal or an alloy (in the case of an alloy, Fe equivalent value): 5 to 15 mass%, a carbonate compound (CO 2 equivalent value): 0.5 to 5.0 mass%, A1 2 0 3 : 0.3 to 5.0 mass%, ZrO 2 : 0.3 to 5.0 mass%, cellulose : 20 to 40 mass%, at least one selected from a group consisting of water glass, silicate mineral, 10 metal Si, and Si alloy in an amount of 10 to 30 mass% in terms of SiO 2 , and Mn as a metal, an alloy, or an oxide in an amount of 2 to 10 mass% (in the case of an alloy or an oxide, Mn equivalent value). The coverage of the coating material with respect to the core wire is 12 to 21 % based on the total mass of the electrode. 15 The term "coverage of the coating material with respect to the core wire" herein used means the ratio of the total mass of the coating material relative to the total mass of the electrode. Further, preferably, the high cellulose type covered electrode includes water glass as the bond, wherein the mole 20 ratio of the water glass is 2.7 to 4.0. The term "mole ratio of water glass" herein used means the ratio of the number of moles of SiO 2 relative to the total number of moles of Na 2 0, K20, and Li 2 O in the components of water glass. Still further, preferably, the high cellulose type covered 25 electrode includes water glass as the bond, and includes an alkali metal compound: 0.5 to 5.0 mass% (oxide equivalent value) based on the total mass of the coating material as a component of the water glass. -4- In accordance with the present invention, while keeping the high melting rate due to addition of cellulose, the resulting arc with a sufficient intensity can be obtained with stability. Therefore, also in downward welding of root pass, excellent 5 short-circuit resistance and weldability can be obtained. Further, by specifying the composition to be added into the coating material, it is possible to obtain a high cellulose type covered electrode capable of providing a weld metal which is less likely to cause defects at the weld part, and has excellent 10 mechanical characteristics. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a view showing a weld part by a covered electrode; and 15 FIG. 2 is a view showing base metal pipes in butt circumferential welding. DESCRIPTION OF THE PREFERRED EMBODIMENTS The present inventors focused attention on the amounts of Fe, 20 cellulose, and Al 2 03 to be added in the coating material for the purpose of improving the short-circuit resistance, and the coverage of the coating material. In the foregoing respective Patent Documents, no study is made on the amount of Fe to be added in the coating material for the purposes of achieving the 25 improvement of the weldability, and the compatibility between the efficiency and the weldability. The present inventors focused attention on the following fact. Excessive addition of Fe into the coating material weakens the resulting arc, which - 5 deteriorates the stability thereof. Accordingly, short circuit becomes more likely to occur at the weld part. Thus, a decrease in amount of Fe added into the coating material reduces spreading of the crater at the weld part, and the welding performance 5 efficiency such as the compatibility of the molten metal. Thus, various optimum amounts of Fe added were studied. However, when an iron powder is added until favorable welding performance efficiency can be obtained, unfavorably, the short-circuit resistance is reduced. 10 Under such circumstances, the present inventors focused attention on cellulose contributing to the stability of the resulting arc, and studied various addition amounts thereof. As for cellulose which is a main component of the coating material, when the content thereof in the coating material is small, the 15 resulting arc is weakened. As a result, not only the short circuit resistance is reduced, but also, it becomes impossible to obtain a high melting rate which is a feature of the high cellulose type covered electrode. On the other hand, when the cellulose content in the coating material is large, the resulting 20 arc becomes too strong. Therefore, not only a penetration bead is not formed, but also it becomes difficult to obtain the objective weld metal performances in hot, filler, and cover passes of the second and subsequent layers. However, an increase in amount of cellulose added to enhance the resulting arc 25 strength did not lead to a sufficient improvement of the short circuit resistance. Under such circumstances, the present inventors variously studied the coverage of the coating material, and found out the - 6 following. In order to obtain the objective weld metal, the amount of alloy components to be added into the coating material and the coverage are balanced. Further, the amounts of Fe and cellulose to be added were adjusted. As a result, while keeping 5 a high melting rate, the resulting arc having a sufficient intensity can be obtained with stability. Further, excellent short-circuit resistance can also be obtained in downward welding of the root pass. Further, the present inventors found out the following. Addition of A1 2 0 3 which is a slag forming agent under 10 control of the amount increases the viscosity of the slag which has been reduced by the increase in amount of cellulose added. This results in favorable spreadability of the crater at the weld part. Below, the reasons for the numerical limitations of the 15 present invention will be described. Each content shown below is the content based on the total mass of the coating material. "MgO content: 0.1 to 10 mass%" MgO contributes to the viscosity and the fluidity of the slag. When the content of MgO is less than 0.1 mass%, in manipulation 20 of the electrode performed for the purpose of improving the welding performance efficiency, due to the insufficient viscosity of the molten metal, dripping of the molten metal and the slag, reduction of the pit resistance, and reduction of the welding joint part performances become more likely to occur. On the 25 other hand, when the content of MgO exceeds 10 mass%, the fluidity of the slag becomes excessive, resulting in reduction of the downward weldability. Therefore, the content of MgO is 0.1 to 10 mass%. -7- "Iron oxide content (FeO equivalent value): 5 to 20 mass%" Iron oxide has effects of making the slag porous for improving the slag removability, and preventing the occurrence of pits due to excessive deoxidation. When the content of iron 5 oxide is less than 5 mass% in terms of FeO, the effects of improving the slag removability 'and suppressing the occurrence of pits formation cannot be obtained sufficiently. Whereas, when the content exceeds 20 mass%, the fluidity of the slag becomes excessive, resulting in reduction of the downward weldability. 10 Therefore, the content of iron oxide (FeO equivalent value) is 5 to 20 mass%. "TiO 2 content: 5 to 15 mass%" TiO 2 contributes to the stability and intensity of arc. When the content of TiO 2 is less than 5 mass%, the occurrence of arc 15 becomes instable, so that defects such as incomplete fusion become more likely to occur at the weld part. Whereas, when the content exceeds 15 mass%, the arc strength is decreased, resulting in reduction of the welding performance efficiency. 20 Thus, the downward weldability becomes difficult. Therefore, the content of TiO 2 is 5 to 15 mass%. "Fe content: 5 to 15 mass%" Fe is added in the form of an iron powder and/or an alloy such as Fe-Mn or Fe-Si. The term "Fe content" herein used 25 denotes the content of Fe thus added, except for the amount of Fe contained in the form of iron oxide. Fe has effects of improving the weldability, the spreadability of the crater at the weld part, and the compatibility of the weld metal with the base metal. However, -8when the Fe content in the coating material is less than 5.0 mass%, these effects cannot be obtained sufficiently. On the other hand, when the Fe content in the coating material exceeds 15 mass%, the intensity and stability of the arc are reduced. 5 Accordingly, the continuity of the arc is reduced, resulting in reduction of the short-circuit resistance. Therefore, the Fe content is 5 to 15 mass%. "Carbonate compound (CO 2 equivalent value) : 0.5 to 5.0 mass%"
CO
2 is mainly added in the form of lime, and acts as a shield 10 gas generator and a slag forming agent. When the content of CO 2 in equivalent value is less than 0.5 mass%, the atmosphere shielding property at the arc part is reduced. Thus, the alloy elements are not properly supplied to the fusion zone, so that the joint part performance is reduced. Further, the oxygen 15 content in the weld metal increases, resulting in reduction of the toughness of the weld metal. On the other hand, when the content of CO 2 exceeds 5.0 mass%, the amount of the slag formed increases, resulting in reduction of the performance of downward welding. Therefore, the content of CO 2 is 0.5 to 5.0 mass%. 20 Incidentally, as a carbonate compound, CaCO 3 , MgCO 3 , BaCO 2 , or the like is used. "Al 2 0 3 content: 0.3 to 5.0 mass%" A1 2 0 3 increases the stability of the arc and the viscosity of the slag, and acts as a slag forming agent. Whereas, A1 2 0 3 25 increases the spreadability of the crater in the fusion zone. When the content of A1 2 0 3 is less than 0.3 mass%, the stability of the arc, the viscosity of the slag, and the spreadability of the crater cannot be obtained sufficiently. Whereas, when the -9 content exceeds 5.0 mass%, the amount of the slag formed increases, and the slag removability is also reduced. Accordingly, the weldability is reduced. Therefore, the content of A1 2 0 3 is 0.3 to 5.0 mass%. 5 "Zr0 2 content: 0.3 to 5.0 mass%" ZrO 2 has effects of increasing the convergence of the resulting arc, and the luster of the bead surface. Further, ZrO 2 improves the compatibility of the weld metal with the base metal. When the content of Zr0 2 is less than 0.3 mass%, the arc 10 convergence, the luster of the bead surface, and the compatibility of the weld metal with the base metal cannot be obtained sufficiently. Whereas, when the content exceeds 5.0 mass%, the resulting slag becomes dense, resulting in reduction of the removability. Therefore, the content of Zr0 2 is 0.3 to 5.0 15 mass%. "Cellulose content: 20 to 40 mass%" Cellulose contributes to the stability of the resulting arc, and allows the resulting arc with a sufficient intensity to be obtained with stability. Thus, cellulose has an effect of 20 improving the short-circuit resistance. When the content of cellulose is less than 20 mass%, the arc is weak, and becomes instable, so that the short-circuit resistance is also reduced. Whereas, when the content of cellulose exceeds 40 mass%, the resulting arc strength increases, so that penetration bead 25 becomes less likely to be formed. Therefore, the content of cellulose is 20 to 40 mass%. "Content of at least one from a group consisting of water glass, silicate mineral, metal Si, and Si alloy (SiO 2 equivalent - 10 value) : 10 to 30 mass%" SiO 2 increases the arc strength, and improves the spreadability of the crater, and the compatibility of the weld metal with the base metal. When the SiO 2 equivalent content of 5 water glass, silicate mineral, metal Si, and Si alloy is less than 10 mass%, the increase in arc strength, the spreadability of the crater, and the compatibility of the weld metal with the base metal cannot be obtained sufficiently. Whereas, when the content exceeds 30 mass%, the amount of the slag formed becomes excessive, 10 and the fluidity of the slag becomes excessive. Accordingly, the performance of the downward welding is reduced. Therefore, the content of at least one or more from a group consisting of water glass, silicate mineral, metal Si, and Si alloy (SiO 2 equivalent value) is 10 to 30 mass%. 15 "Mn in the form of metal, alloy, or oxide (in the case of an alloy or oxide, Mn equivalent value) : 2 to 10 mass%" Mn is a component indispensable as a deoxidizer, and further contributes to the increase in strength of the weld metal. When the Mn equivalent content of Mn oxide, metal Mn, and Mn alloy is 20 less than 2 mass%, it becomes impossible to obtain a proper weld metal due to insufficient deoxidation. When the content exceeds 10 mass%, excessive deoxidation occurs, so that pits become more likely to be formed in the bead surface. Therefore, the content of Mn in the form of metal, alloy, or oxide (in the case of an 25 alloy or oxide, Mn equivalent value) is 2 to 10 mass%. "Alkali metal compound: 0.5 to 5.0 mass%" The alkali metal compound is added, if required, in order to improve the arc stability and the blow hole resistance. The - 11 alkali metal compound is one component of water glass. When the content of the alkali metal compound is less than 0.5 mass%, effects of improving the arc stability and the blow hole resistance cannot be obtained sufficiently. Whereas, when the 5 content exceeds 5.0 mass%, the spreadability of the arc is reduced. Accordingly, the bead width becomes narrow, so that beads excessively swell. Further, the moisture resistance becomes more likely to be deteriorated. Therefore, the alkali metal compound is in an amount of 0.5 to 5.0 mass% when added. 10 The term "alkali metal compound content" herein used means the value of each content of Na, K, and Li contained in the form of a compound thereof in terms of each corresponding oxide thereof(NazO, K 2 0, or Li 2 0). "Coverage of the coating material with respect to the core 15 wire: 12 to 21 % based on the total mass of the electrode" The increase in coverage results in expansion of the range for adjusting the addition ratio of the alloy components, which results in expansion of the range for addition of cellulose to the coating material. When the coverage is less than 12 %, the 20 function of coating as a protective tube becomes insufficient. In addition, there results in an increase in tendency of the occurrence of burning of electrode coating as follows: coating in the vicinity of the exposed core of the electrode of the welding rod is burnt down, so that the electrode becomes unusable. On 25 the other hand, when the coverage exceeds 21 %, the convergence of the arc is reduced. Thus, the penetration bead becomes less likely to be formed. Further, in welding of the second and subsequent layers, the resulting arc weakens, and the amount of - 12 the slag generated also increases. Accordingly, the downward weldability is reduced. Therefore, the coverage of the coating material with respect to the electrode is 12 to 21 %. "Mole ratio of water glass: 2.7 to 4.0" 5 The term "mole ratio of water glass" herein used is the ratio determined as: (Number of moles of SiO 2 ) /{ (number of moles of Na 2 0) + (Number of moles of K 2 0) + (Number of moles of Li 2 0)) 10 in the components of water glass. When the mole ratio of the water glass added as a bond is less than 2.7, the moisture resistance of the coating material is reduced. Accordingly, the intensity and stability of the resulting arc are reduced, resulting in reduction of the weldability. On the other hand, 15 when the mole ratio of water glass to the coating material exceeds 4.0, the viscosity becomes high. Accordingly, in the drying step of the electrode, the coating material becomes more likely to undergo cracking in the surface thereof, resulting in reduction of the productivity of the electrode. Therefore, the 20 mole ratio of the water glass to the coating material is 2.7 to 4.0. [Examples] Below, Examples showing the effects of a high cellulose type covered electrode of this embodiment will be shown together with 25 Comparative Examples. A coating material was applied around the outer circumference of each steel core wire having the composition and dimensions shown in Table 1, thereby to manufacture a covered electrode. - 13 - Welding was carried out using the electrode. As the base metal, pipes with an outer diameter of 1240 mm, and a wall thickness of 16.6 mm, and having the composition of Table 2 were arranged facing each other with a root gap of the 5 groove of 1.6 mm therebetween as shown in FIG. 2. Arc welding was carried out on the groove. Incidentally, the wall thickness at the root part of the groove was set at 1.5 to 2.0 mm, and the groove angle was set at 60*. 10 [Table 1] Core Mass% Wire Length Diameter mm C Si Mn P S Cu mm 2.4 300 0.09 0.03 0.35- 0.020 0.023 0.20 3.2 350 or or 0.65 or or or 4.0 350 less less less less less 15 [Table 2] Mass% C Si Mn P S 0.31 or less - 1.35 or 0.030 or less 0.030 or less less 20 On the steel core wires having the composition shown in Table 1, coating materials having various compositions were coated, resulting in test materials of covered electrodes of Examples and Comparative Examples. Incidentally, as the bond for use in 25 coating of the coating materials onto the steel core wire, water glass was used. The mole ratio of the water glass to the coating material was. variously changed. The compositions of the covered electrodes in respective Examples and Comparative Examples are - 14 shown in Tables 3-1 to 3-2. [Table 3-1] (Di No V) E No MgO SiO2 T102 Fe FeO Mn AJ203 Zr02 C02 _ C)0 3:) 1 0.9 17.2 11.9 9.9 10.1 6.5 28.4 1.5 1.9 2.1 2.2 18.0 3.5 2 0.1 21.0 10.4 7.8 10.5 9.9 29.1 1.9 2.5 2.5 2.5 16.0 3.3 3 9.8 15.4 8.0 6.0 14.4 3.2 30.9 2.1 2.1 3.0 2.1 14.0 3.4 4 1.4 10.5 6.7 11.2 9.3 7.5 39.8 2.2 2.2 4.8 1.6 12.0 3.4 5 2.3 29.7 14.2 6.3 12.0 8.0 20.2 0.5 0.8 0.5 2.6 18.0 3.4 6 0.7 24.4 5.3 11.4 15.0 5.1 25.4 1.2 0.4 5.0 2.8 14.0 3.2 7 1.2 16.0 9.3 8.5 8.0 7.1 37.0 2.7 4.3 4.0 1.9 12.0 3.3 8 5.9 27.1 6.0 5.1 10.5 5.5 22.9 1.8 4.9 3.1 2.9 19.0 3.2 9 0.9 19.0 12.1 14.9 5.4 6.4 27.8 4.5 1.6 1.9 1.9 16.0 3.5 10 1.3 16.7 10.5 10.2 19.9 4.9 30.0 0.3 2.2 1.5 2.3 17.0 3.3 11 0.7 24.4 5.3 11.4 15.0 5.1 25.4 1.2 0.4 5.0 4.3 14.0 2.0 12 0.1 21.0 10.4 7.8 10.5 9.9 29.1 1.9 2.5 2.5 1.9 16.0 4.3 13 0.0 17.0 12.0 13.0 13.0 7.0 30.0 0.8 1.5 3.0 1.8 15.0 3.6 14 12.0 14.9 8.8 9.7 8.9 5.5 29.1 1.2 1.7 2.0 2.0 16.0 3.8 15 1.5 9.0 14.0 12.2 13.6 7.0 33.9 2.0 1.7 1.9 0.8 16.0 3.9 16 0.6 35.0 6.7 9.0 6.5 4.5 27.0 1.5 0.5 1.5 4.6 17.0 2.9 17 1.2 16.5 4.0 8.0 14.0 6.1 33.4 1.7 4.4 3.6 2.9 19.0 3.0 18 0.2 15.6 19.0 7.8 13.9 6.7 28.1 2.3 1.8 0.7 2.5 18.0 2.8 19 0.3 17.9 10.9 4.2 12.1 7.2 30.4 3.4 2.4 2.2 1.9 18.0 3.5 20 0.6 18.8 13.4 21.0 10.1 8.9 22.0 1.0 0.5 0.6 2.5 19.0 3.9 21 7.2 17.0 12.4 11.1 3.0 9.3 26.3 4.5 1.9 2.2 2.8 18.0 3.6 22 0.9 20.9 10.9 11.0 23.0 5.3 23.9 0.5 0.3 0.6 2.7 17.0 4.0 23 1.3 23.2 10.4 10.0 17.3 1.5 31.4 0.9 0.4 0.6 2.9 17.0 3.4 24 5.6 23.4 9.7 9.6 7.3 13.0 25.6 0.9 0.4 0.9 3.5 16.0 2.8 25 9.3 21.8 9.3 8.8 11.6 4.9 15.0 4.9 4.9 0.6 3.6 16.0 2.9 26 0.2 15.2 8.6 9.9 8.9 6.2 45.0 1.9 1.1 0.5 1.1 15.0 3.5 27 3.6 21.9 8.7 13.9 10.1 5.5 31.0 0.2 0.6 1.9 2.6 14.0 3.2 28 2.0 21.0 9.9 10.9 12.0 5.6 26.1 6.5 1.9 1.2 2.8 16.0 3.0 29 7.7 23.4 9.4 9.8 10.5 6.3 28.3 0.9 0.2 0.7 2.0 17.0 4.0 30 6.5 16.9 13.0 9.7 8.7 7.0 25.5 1.4 7.0 1.9 2.4 18.0 3.3 31 2.1 16.6 11.5 9.6 7.9 7.2 33.9 3.2 2.0 0.3 3.0 18.0 2.9 - 15 - 32 0.9 17.3 12.5 10.2 9.7 7.8 32.0 0.3 1.2 7.1 2.2 17.0 3.4 33 0.9 17.2 11.9 9.9 10.1 6.5 28.4 1.5 1.9 2.1 4.0 8.0 2.8 34 0.9 17.2 11.9 9.9 10.1 6.5 28.4 1.5 1.9 2.1 1.9 25.0 3.0 (Table 3-2] Alkali metal. Si0 2 equivalent Mn equivalent Fe equivalent C02 equivalent (Oxie equivalent) No. Silicat Na K Water e Metal Si Mn Metal Mn Iron Fe Ca Mg Ba Na K Li glass minera Si alloy oxide Mn alloy powder alloy C03 C03 C03 und und ound 1 74 9.8 - - - - 6.5 8.9 1.0 1.3 0.8 2.1 0.1 2 8.0 10.9 0.3 0.1 0.1 3.5 6.3 6.8 1.0 2.4 0.1 2.3 0.3 3 7.0 8.3 0.1 . - - 3.2 5.5 0.5 1.3 1.7 - 2.1 - 4 5.2 5.3 - 4.3 3.2 10.0 1.2 2.3 2.5 - 1.4 0.2 8.5 20.7 0.3 0.2 - 1.2 6.8 5.4 0.9 - 0.3 0.2 2.4 0.3 6 8.8 15.6 - - 5.1 - 11.4 - 3.6 0.5 0.9 2.1 0.2 0.3 6.2 9.8 - - 2.0 5.1 7.0 1.5 3.4 0.6 - 1.9 8 8.9 18.2 - . - 5.5 3.5 1.6 - 2.9 0.2 2.5 0.5 9 6.3 12.7 - - 6.4 10.1 4.8 0.6 0.3 1.0 1.8 0.1 10 7.4 9.3 - 4.9 - 10.2 - 0.9 0.6 - 2.3 - 11 8.3 16.1 - - - 5.1 - 11.4 - 3.4 0.8 0.8 2.9 0.5 0.5 12 8.0 12.5 0.2 0.3 - - 9.9 5.6 2.2 2.1 0.2 0.2 1.9 - 3 6.1 10.8 - 0.1 2.0 - 5.0 11.7 1.3 3.0 - - 1.8 - 14 7.5 7.4 1.2 4.3 7.0 2.7 0.3 1.7 -2.0 0.1 15 3.1 5.9 - 0.4 - 6.6 10.2 2.0 1.5 0.4 - 0.8 - 16 12.8 20.2 1.0 1.0 - - 4.5 8.1 0.9 1.5 - - 2.5 1.0 0.7 I7 8.3 8.2 - - - - 6.1 6.8 1.2 1.9 1.5 0.2 2.8 0.1 18 6.7 8.9 - . - 2.0 4.7 7.4 0.4 0.7 - - 2.5 - 19 6.5 11.4 - - 0.6 5.2 1.4 4.0 0.2 1.2 0.1 0.9 1.9 20 9.3 9.5 - . - 0.9 8.0 18.0 3.0 0.6 - - 2.5 21 99 7.1 - 0.3 - 9.0 7.9 3.2 0.2 2.0 - 2.1 0.2 0.3 22 10.3 10.6 - - - 5.3 9.0 2.0 0.6 - - 2.7 - 23 9 1 10.9 1.5 1.1 ~ - 1.5 9.1 0.9 0.5 0.1 - 2.9 - 24 9.6 13.3 - 0.5 - 8.6 4.4 8.4 1.2 - 0.9 - 3.0 0.8 25 10.0 11.8 - - - 4.9 7.5 1.3 0.5 - 0.1 3.5 0.1 26 10. . - - 6.2 8.3 1.6 - - 0.5 1.1 - 27 79 12.9 0.5 0.6 5.5 13.0 0.9 1.0 0.6 0.3 2.6 - 28 8.0 13.0 - - - - 5.6 9.6 1.3 0.8 0.3 0.1 2.8 - 25 7.7 15.4 0.2 0.1 6.3 - 9.8 - - 0.7 - 2.0 - - 16 - 30 7 9.2 - 0.9 6.1 8.2 1.5 0.6 0.4 0.9 2.3 0.2 31 8.1 - 0.5 0.2 6.5 7.7 1.9 0.3 - - 2.8 0.4 32 7.1 10.2 - _ - 7.8 - 10.2 - 5.3 1.5 0.3 2.2 33 10.8 6.4 - _ 1.5 - 5.0 9.3 0.6 1.9 0.2 - 3.9 0.1 34 54 11.8 - _ - 6.5 9.9 - 1.8 0.3 - 1.9 5 By the use of the covered electrodes of Examples and Comparative Examples including the coating materials having the compositions shown in Tables 3-1 to 3-2, coated thereon, arc welding was carried out on the base metal pipes shown in Table 1 10 and FIG. 2. Thus, the arc strength, arc stability, slag dripping resistance, and short-circuit resistance during welding performance were evaluated. Whereas, the slag removability and the stability of the penetration bead after welding were visually evaluated. Incidentally, for the short-circuit resistance, the 15 difference in weldability noticeably appears at welding positions from a position 90 degrees clockwise from vertically above the pipes to vertically below. For this reason, the performance according to the welding position was particularly evaluated. Then, for respective items, there are shown, as compared with the 20 related art, each sample which is particularly superior as @ , each sample which is superior as 0, each sample which is equal to, or a little inferior to the related art as A, and each sample which is particularly inferior as X in Table 3. Then, the weld parts welded by the use of the covered 25 electrodes of respective Examples and Comparative Examples were subjected to weld part rating by radiographic testing specified in JIS Z3104, and absorption energy measurement by Charpy impact examination specified in JIS Z3128. Then, each weld part rating - 17 by radiographic testing is shown in Table 3. Whereas, for the absorption energy of each weld part by Charpy impact examination, each sample with a vE- 29 -c of 40 J or more is rated as good. Each sample which has been rated as good is shown as 0 , and each 5 sample which has been rated as not good is shown as X in Table 4. [Table 4] Weldability Coating material Weld metal ________ characteristics ___ Stability No Short- Releasea Of Moisture Dropping X-ray Impact Arc Arc Dripping circuit bility of penetrate resistanc resistanc prope proper strength stability of slag resistance on bead e e rty ty sa appearan ce 1 @ @ @ @ @ @ @ @ Grade 2 @ @ @ @ @ @ @ @ Grade 0 3 Grade 4 @ @ @ @ @ @ @ Grade0 5 @ @ @ @ @ @ @ @ Grade 6 @ @ @ @ @ @ @ @ Grade 7 @ @ @ @ @ @ @ @ Grade 8 @ @ @ @ @ @ @ @ Grade 9 @ © @ @ @ @ @ @ Grade 0 10 @ @ @ @ @ @ @ @ Grade 11 0 0 @ 0 @ 0 0 @ Grade 12 @ @ @ @ 0 @ @ A Grade x A x x A A @ Grade 14 x A x x A A © © GradeX 15 A x A x x @ Grade 16 @ @ 0 x Grade o 17 A A x_ 0 0 @ Grade 18 x A Grade 11 0 0 0 0 © 1 0 19 0 A A A x L Grade 20 x x x x A A Grade 21 @ @ A A x x @ @ Grade 2 O 0 A A A @ @ Grade - 18 - 23 A x O x O x @ @ Grade X 24 0 0 0 A 0 O @ @ Grade X _ _ 2 25 x x 0 x A x @ 2 0 26 @ 0 A @ A x @ Grade o 27 O x O O0_ A @ @ Grade o 28 0 0 A x 0 © @ Grade o 29 @ 0 @ x 0 0 @ @ Grade 0 30 0 0 x 0 x. A @ @ Grade 0 3 Grade 32 Q x x 0 0 Grade 33 x x A x © x @ Grade x 34Q © x @ 0 0 @ @ Grade 0 Nos. 1 to 12 are examples satisfying the requirements of the present invention. As for all samples of Nos. 1 to 12, excellent weldability, coating material characteristics, and the defect resistance and impact property of the weld part were able to be 5 obtained. As for Comparative Example No. 13, the content of MgO in the flux is less than the scope of the present invention. Thus, dripping of the weld metal occurred, and the defect resistance of the weld part was reduced. As for Comparative Example No. 14, 10 the content of MgO exceeds the scope of the present invention. Thus, the fluidity of the slag became excessive, so that the molten pool was difficult to form. As for Comparative Example No. 15, the content of SiO 2 in the flux is less than the scope of the present invention. Thus, the arc strength was small, so that the 15 short-circuit resistance and the stability of the penetration bead were reduced. As for Comparative Example No. 16, the content of SiO 2 exceeds the scope of the present invention. Thus, the arc strength became excessive, so that dripping of the slag - 19 occurred, resulting in reduction of the penetration bead. As for Comparative Example No. 17, the content of TiO 2 in the flux is less than the scope of the present invention. Thus, occurrence of the arc became unstable, resulting in reduction of 5 the short-circuit resistance. As for Comparative Example No. 18, the content of TiO 2 exceeds the scope of the present invention. Thus, the arc strength was reduced, resulting in reduction of the short-circuit resistance. As for Comparative Example No. 19, the Fe content in the flux is less than the scope of the present 10 invention. Thus, the spreadability of the crater in the weld part, and the compatibility of the weld metal with the base metal were reduced. As for Comparative Example No. 20, the Fe content exceeds the scope of the present invention. Thus, the arc strength was weak, resulting in reduction of the short-circuit 15 resistance. As for Comparative Example No. 21, the FeO content in the flux is less than the scope of the present invention. Thus, the slag removability was reduced, so that seizure of slag occurred. As for Comparative Example No. 22, the FeO content exceeds the 20 scope of the present invention. Thus, the fluidity of the slag became excessive, so that dripping of the slag occurred, resulting in reduction of downward weldability. As for Comparative Example No. 23, the content of Mn in the flux is less than the scope of the present invention. This resulted in 25 insufficient deoxidation, so that pits occurred in the weld part. As for Comparative Example No. 24, the content of Mn exceeds the scope of the present invention. This results in excessive deoxidation, so that the impact property was reduced. - 20 - As for Comparative Example No. 25, the content of cellulose in the flux is less than the scope of the present invention. Thus, the arc strength was weak, so that the arc became unstable, resulting in reduction of the short-circuit resistance. As for 5 Comparative Example No. 26, the content of cellulose exceeds the scope of the present invention. Thus, the resulting arc strength became higher, resulting in reduction of the stability of the penetration be.ad. As for Comparative Example No. 27, the content of A1 2 0 3 in the flux is less than the scope of the present 10 invention. Thus, the stability of the arc was reduced, so that the amount of the slag formed was insufficient, resulting in reduction of the stability of the penetration bead. As for Comparative Example No. 28, the content of A1 2 0 3 exceeds the scope of the present invention. Thus, the amount of the slag formed 15 increased. Further, the slag removability was reduced, so that slag seizure occurred. As for Comparative Example No. 29, the content of ZrO 2 in the flux is less than the scope of the present invention. Thus, the convergence of the arc was reduced, resulting in reduction of the 20 short-circuit resistance. As for Comparative Example No. 30, the content of ZrO 2 exceeds the scope of the present invention. Thus, the resulting slag became dense, resulting in reduction of the removability. As for Comparative Example No. 31, the content of
CO
2 in the flux is less than the scope of the present invention. 25 Thus, the shielding property from atmosphere at the weld part was reduced, resulting in reduction of the welding joint part performance and impact property. As for Comparative Example No. 32, the content of CO 2 exceeds the scope of the present invention. - 21 - Thus, the amount of the slag formed increased, so that dripping of the slag occurred, resulting in reduction of the short-circuit resistance. As for Comparative Example No. 33, the coverage of the 5 coating material is less than the scope of the present invention. Thus, the stability and the short-circuit resistance of the arc were reduced. As for Comparative Example No. 34, the coverage exceeds the scope of the present invention. Thus, the increase in amount of slag formed results in occurrence of dripping of the 10 slag. This made downward welding difficult. Out of Examples 1 to 12 satisfying the scope of the present invention, Examples 1 to 10 are Examples in which the mole ratio of the water glass to the coating material satisfies 2.7 to 4.0. As for Example 11, the mole ratio of the water glass is less than 15 the scope of the present invention. As compared with Examples 1 to 10 satisfying the scope of the present invention, the moisture resistance of the coating material was reduced, so that the intensity and the stability of the resulting arc were reduced. As for Example 12, the mole ratio of the water glass exceeds the 20 scope of the present invention. Thus, the coating material underwent cracking in the surface thereof. - 22 -