JP5042744B2 - Electroslag welding method - Google Patents

Description

  The present invention relates to an electroslag welding method for thick steel plates, and more particularly to an electroslag welding method applied to welding a diaphragm and a skin plate constituting a four-sided box column used as a building steel frame.

  In general, the electroslag welding method can perform one-pass welding with large heat input, so that it is possible to perform welding more efficiently than other welding methods, and is used as a steel frame in welded structures such as buildings and bridges. It is often used for vertical welding of skin plates and diaphragms (reinforcing goods) that constitute box columns. On the other hand, the electroslag welding method has a welding heat input of about 500 kJ / cm or more, which is larger than that of general arc welding. Therefore, the cooling rate of the weld metal formed by welding is low, and the toughness of the weld metal is ensured. There is a problem that it is not easy. This is because the cooling rate of the weld metal is slow, so in the cooling process after the weld metal solidifies, a coarse proeutectoid ferrite (hereinafter abbreviated as austenite) (hereinafter abbreviated as austenite). It is believed that ferrite is sometimes referred to as α) and that the intragranular structure is coarsened, which causes a reduction in the toughness of the weld metal.

  In welded structures such as buildings and bridges, there is a great social demand for higher toughness of the welded parts in order to avoid collapse due to brittle fracture during an earthquake. In response to this request, the toughness of the weld heat affected zone (hereinafter sometimes referred to as HAZ (Heat Affected Zone)) as the steel material applied to buildings, bridges, etc., is the absorbed energy of the 2 mm V notch Charpy impact test at 0 ° C. High HAZ toughness steels that guarantee 70J and higher have been developed. In addition, in order to increase the safety of welded structures, weld metal is required to have toughness equal to or better than that of steel, and conventionally, a technology for improving the toughness of weld metal formed in welds by electroslag welding. Has been studied in various ways.

  For example, when performing electroslag welding of steel having a tensile strength of 500 to 600 MPa class with a welding heat input of 800 kJ / cm or more, C, Si, Mn, Mo, Ni, Ti, B are used for the purpose of improving the toughness of the weld metal. , N and O contents have been proposed (see, for example, Patent Document 1). According to this Patent Document 1, a Charpy absorbed energy at 0 ° C. of 110 J or more is obtained by welding a diaphragm having a thickness of 50 mm and a skin plate having a thickness of 80 mm, or a diaphragm having a thickness of 85 mm and a skin plate having a thickness of 85 mm. It is disclosed that a weld metal was obtained.

  In addition, when performing electroslag welding of steel having a tensile strength of 400 to 600 MPa class with a welding heat input of 500 kJ / cm or more, C, Si, Mn, P, S, N, and O are used for the purpose of improving the toughness of the weld metal. A backing metal for high heat input electroslag welding in which the content of is optimized is proposed (see, for example, Patent Document 2). According to Patent Document 2, it is disclosed that a weld metal of 70 J or more is obtained with Charpy absorbed energy at 0 ° C. by welding a diaphragm having a thickness of 50 mm and a skin plate having a thickness of 50 mm.

  However, electroslag welding is super heat input welding as described above, and the welding rate of steel (base metal) and backing metal into the weld metal (hereinafter sometimes simply referred to as dilution rate) is large. Not only the chemical composition but also the chemical composition due to the dilution of diaphragm and skin plate steel and backing metal. Also, in electroslag welding, the welding heat input is large as described above, and the welding heat input is changed according to the diaphragm plate thickness. Therefore, the cooling rate of the weld metal varies depending on the diaphragm plate thickness conditions. Further, even if the plate thickness of the diaphragm is the same, if the plate thickness of the skin plate is reduced, the heat capacity of the skin plate is reduced, so that the cooling rate of the weld metal is further reduced.

  In this way, electroslag welding involves the penetration of steel (base metal) and backing metal into the weld metal depending on the amount of welding heat input that changes according to the diaphragm thickness and the thickness ratio of the skin plate to the diaphragm. Since the rate and the cooling rate of the weld metal change greatly, in the adjustment of the composition of the weld metal using only the welding wire or the backing metal proposed in Patent Document 1 or Patent Document 2, the structure and characteristics of the weld metal are changed. There was a limit to control.

  Moreover, the conventional welding wire for large heat input electroslag welding proposed by the above-mentioned Patent Document 1 or the like has a high welding heat input of a diaphragm having a plate thickness of 50 mm or more, and the welding metal has a low cooling rate. By improving the hardenability and suppressing the formation of grain boundary ferrite harmful to toughness, the effect of improving the toughness of the weld metal is obtained. However, under the same welding heat input conditions (same diaphragm plate thickness conditions), if the plate thickness ratio of the skin plate to the diaphragm is changed, the cooling rate of the weld metal changes greatly, so the effect of improving the toughness of the same weld metal is achieved. In order to ensure, it is necessary to design the chemical composition of the welding wire for each combination of diaphragm thickness and skin plate thickness.

  That is, when the welding metal cooling rate after welding is reduced, the diaphragm plate thickness is thick, the welding heat input is large, and the skin plate thickness is thin, it is attempted to suppress the formation of intergranular ferrite in the weld metal. Inevitably, the welding wire necessarily has a high alloy composition. However, when using this high alloy composition welding wire to weld a thin plate and a thick plate to the plate, the cooling rate of the weld metal after welding is relatively large. Since the hardenability becomes excessive and a hard bainite-based structure is formed, the toughness of the weld metal deteriorates.

  On the other hand, when welding a diaphragm with a large plate thickness and a thin skin plate using a welding wire with a relatively low hardenability alloy composition that meets the conditions of a thin plate and a thick plate plate, The hardenability of the weld metal is greatly insufficient, the formation of coarse grain boundary ferrite in the weld metal cannot be suppressed, and the toughness deteriorates.

  Further, the backing metal for large heat input electroslag welding proposed by the above-mentioned Patent Document 2 is welded from the backing metal under the condition that the welding heat input of the diaphragm having a plate thickness of 50 mm or more is high and the cooling rate of the weld metal is low. The chemical composition of the backing metal is designed in consideration of the component penetration (dilution) into the metal, thereby improving the hardenability of the weld metal and suppressing the formation of grain boundary ferrite harmful to toughness. However, the dilution rate of the backing metal is only about 10%, and even if the welding heat metal condition (same diaphragm plate thickness condition) changes the plate thickness ratio of the skin plate to the diaphragm, the weld metal Therefore, it is difficult to ensure the effect of improving the toughness in a wide combination of diaphragm and skin plate thickness, simply by adjusting the chemical composition of the backing metal.

  As described above, a wide range of diaphragm and skin plate thickness combinations in electroslag welding, specifically, the diaphragm plate thickness is 25 mm to 100 mm, and the skin plate thickness is 20 mm to 100 mm (however, the skin plate In the present situation, no technology has been established that can stably and satisfactorily ensure the toughness of weld metal in various combinations of plate thicknesses (which are equal to or less than the plate thickness of the diaphragm).

  In particular, in the prior art, the welded metal after welding is used in the combination condition of the steel plate in which the thickness of the diaphragm that increases the heat input of welding is 50 mm or more, and further, the thickness of the skin plate is 2/3 or less of the thickness of the diaphragm. Since the cooling rate of the steel became slower, it was difficult to ensure good weld metal toughness.

JP 2002-79396 A JP 2004-216384 A

  The present invention is an electroslag welding applied to the welding of a diaphragm and a skin plate that constitute a four-sided box column used as a construction steel frame in particular, in view of the above-described conventional technology, and the thickness of the diaphragm is 50 mm. In addition, the skin plate thickness is 2/3 or less of the diaphragm plate thickness, the welding heat input is high, the dilution ratio of the components from the steel plate and backing metal is high, and the cooling rate of the weld metal There is provided an electroslag welding method capable of stably securing a weld metal having a tensile strength of 400 to 780 MPa and a toughness of 0 J at a Charpy absorbed energy vE0 of 70 J or more even under a combination condition of steel plate thicknesses that slow down. For the purpose.

  According to the inventors' investigation, the conventional high heat input electroslag welding wire or backing metal is used to suppress the microstructure of the weld metal, for example, to suppress grain boundary ferrite harmful to toughness, and It was confirmed that the cooling rate range in which the toughness of the weld metal can be ensured is limited by making the structure fine acicular ferrite or fine bainite preferable for improving toughness. In particular, when the diaphragm plate thickness is 50 mm or more, and the plate thickness of the skin plate is 2/3 or less of the diaphragm plate thickness, the welding heat input is high, and the dilution ratio of components from the steel plate and backing metal And the cooling rate of the weld metal is extremely slow. Therefore, considering the component adjustment by the welding wire and the component dilution from the backing metal, the toughness of the weld metal is 70J or more with Charpy absorbed energy vE0 at 0 ° C. It is practically difficult to ensure.

  In the microstructure control of the weld metal in consideration of the component adjustment by the welding wire and the component dilution from the backing metal, the inventors have combined the conditions of the diaphragm and skin plate thickness in a wide range and the cooling of the weld metal. It was necessary to design the components of the welding wire and backing metal depending on the speed, and because of lack of practicality, the factors controlling the weld metal toughness, which have a small dependence on the microstructure, were investigated by experiments.

As a result, if the basic component composition of the weld metal is within a predetermined range, even if the microstructure of the weld metal changes depending on the cooling rate under the combined conditions of the diaphragm and skin plate thickness in a wide range, Considering each contribution (dilution rate) of welding wire, steel plate (diaphragm, skin plate) and backing metal to weld metal, optimizing the amount of C and N at 0 ° C It has been found that it is possible to ensure the toughness of a weld metal of 70 J or more with Charpy absorbed energy vE0.
The present invention has been made based on these findings, and the gist thereof is as follows.

(1) The end face of the steel plate 1 having a plate thickness of 50 mm or more is butted against the surface of the steel plate 2 having a plate thickness of 2/3 or less of the steel plate 1 with a gap, and the back surface disposed on both sides of the gap In an electroslag welding method in which a groove is formed with a metal, and the gap is filled with flux and a welding wire is fed through the power supply nozzle, and the power supply nozzle is pulled up in the vertical direction and welded. ,
As a chemical component of the welding wire,
C: 0.005 to 0.1%
Si: 0.01 to 1%,
Mn: 0.1 to 2.5%
P: 0.02% or less,
S: 0.01% or less,
Al: 0.002 to 0.1%,
Ti: 0.002 to 0.3%,
B: 0.0003 to 0.015%,
N: 0.001 to 0.015%, and
O: limited to 0.01% or less,
Using the welding wire with the balance being inevitable impurities and Fe,
As a chemical component of the backing gold,
C: 0.02 to 0.25%,
Si: 0.01 to 1.5%,
Mn: 0.1 to 2.5%
P: 0.02% or less,
S: 0.01% or less,
N: 0.001 to 0.015%, and
O: limited to 0.01% or less,
Using the backing made of the inevitable impurities and Fe in the balance,
As a chemical component of the steel plate 1 and the steel plate 2 in mass%,
C: 0.02 to 0.2%,
Si: 0.01 to 1%,
Mn: 0.1 to 2.5%
P: 0.02% or less,
S: 0.01% or less,
Al: 0.002 to 0.1%,
N: 0.001 to 0.015%, and
O: limited to 0.01% or less,
Welding steel plate 1 and steel plate 2 with the balance being inevitable impurities and Fe,
Furthermore, X value calculated | required by the following (1) formula based on the mass% of C each contained in the said welding wire, the steel plate 1, the steel plate 2, and a backing metal is 0.09 or less, and the said An electroslag characterized in that the Y value obtained by the following formula (2) based on the welding wire, the steel plate 1, the steel plate 2, and the mass% of N contained in the backing metal is 0.007 or less. Welding method.
X = 0.6 × C _W + 0.2 × (C _S1 + C _S2 ) + 0.1 × C _F (1)
However, the _{C W,} C _{S1, C S2} and shows _{C F} respectively welding wire, the steel plate 1, the steel plate 2, and the mass% of C contained in in the backing metal.
Y = 0.2 × N _W + 0.15 × (N _S1 + N _S2 ) + 0.1 × N _F (2)
However, the _{N W,} N _{S1, N S2} and, _{N F,} each welding wire, the steel plate 1, the steel plate 2, and shows the mass% of N, contained in the backing metal.

( 2 ) The welding wire is further in mass%,
Mo: 0.01 to 2.5%,
Cr: 0.01 to 1.5%
W: 0.01 to 1.5%
Cu: 0.01 to 1.5%,
Ni: 0.01-6%,
Nb: 0.002 to 0.1%,
V: 0.002 to 0.5% and
Ta: One or more of 0.002 to 0.5% are contained, The electroslag welding method as described in said ( 1) characterized by the above-mentioned.

( 3 ) The welding wire is further in mass%,
Ca: 0.0002 to 0.01%,
Mg: 0.0002 to 0.01%, and
REM: The electroslag welding method according to (1) or (2) above, containing one or more of 0.0002 to 0.01%.

( 4 ) The steel plate 1 and the steel plate 2 are further in mass%,
Ti: 0.002 to 0.05%,
B: 0.0003 to 0.015%,
Mo: 0.01 to 1.5%,
Cr: 0.01 to 1.5%
W: 0.01 to 1.5%
Cu: 0.01 to 1.5%,
Ni: 0.01-6%,
Nb: 0.002 to 0.1%,
V: 0.002 to 0.5% and
The electroslag welding method according to any one of (1) to ( 3 ), wherein one or more of Ta: 0.002 to 0.5% are contained.

( 5 ) The steel plate 1 and the steel plate 2 are further in mass%,
Ca: 0.0002 to 0.01%,
Mg: 0.0002 to 0.01%, and
REM: One or more of 0.0002 to 0.01% are contained, The electroslag welding method in any one of said (1)-( 4 ) characterized by the above-mentioned.

( 6 ) The backing money is further mass%,
Al: 0.002 to 0.1%,
Ti: 0.002 to 0.05%,
B: 0.0003 to 0.015%,
Mo: 0.01 to 1.5%,
Cr: 0.01 to 1.5%
W: 0.01 to 1.5%
Cu: 0.01 to 1.5%,
Ni: 0.01-6%,
Nb: 0.002 to 0.1%,
V: 0.002 to 0.5% and
The electroslag welding method according to any one of (1) to ( 5 ), wherein one or more of Ta: 0.002 to 0.5% are contained.

( 7 ) The backing money is further mass%,
Ca: 0.0002 to 0.01%,
Mg: 0.0002 to 0.01%, and
REM: One or more of 0.0002 to 0.01% are contained, The electroslag welding method in any one of said (1)-( 6 ) characterized by the above-mentioned.

  According to the present invention, electroslag welding is applied to welding of a diaphragm and a skin plate that constitute a four-sided box column used as a construction steel frame in particular, and the thickness of the diaphragm is 50 mm or more. The plate thickness of the steel plate is 2/3 or less of the plate thickness of the diaphragm, the welding heat input is high, the dilution ratio of the components from the steel plate and the backing metal is high, and the cooling rate of the weld metal is slow The electroslag welding method can stably provide a weld metal having a tensile strength of 400 to 780 MPa and a toughness of 0 J at a Charpy absorbed energy vE0 of 70 J or more even under the above combination conditions. If the basic composition of the weld metal is within a predetermined range by the application of the present invention, the microstructure of the weld metal changes depending on the cooling rate under a combination condition of the diaphragm and skin plate thickness in a wide range. However, considering the contribution (dilution rate) of the welding wire, steel plate (diaphragm, skin plate), and backing metal to the weld metal, welding is performed by optimizing the amount of C and N respectively. Since the metal toughness can be stably secured, the industrial contribution of the present invention is extremely large.

  Hereinafter, the best mode for carrying out the present invention will be described. In general, a method of electroslag welding a box column in a building steel frame is generally performed as follows. The box column is composed of four skin plates made of a thick steel material for welded structure. Then, the end face of the diaphragm as a reinforcing material is abutted against the skin plate surface inside the box column with a predetermined gap (groove width) between the skin plate surface, and two sheets are provided on both sides of the gap. A backing metal is placed in contact with the skin plate surface and the diaphragm end face. When welding, the power supply nozzle is filled with the flux and inserted into the groove space surrounded by the skin plate surface, the diaphragm end face and the backing metal, and the wire is fed into the power supply nozzle. Weld while pulling up in the vertical direction.

  Since the non-consumable nozzle type electroslag welding of this box column is mostly used for construction, the steel materials used as skin plates and diaphragms are generally hot rolled steel for welded structures (specified in JIS standards) ( For example, SM material 490, SM520, SM570, etc.) may be mentioned. In particular, when toughness and lamellar resistance are required, rolled steel for building structures (for example, SN400, SN490, SA440, etc.) is used. As the welding wire, for example, JIS Z3353 YES52 or JIS Z3353 YES62 equivalent is used according to the strength and toughness requirements for the weld metal. On the other hand, since the backing metal is not a structural member, a 490 MPa class flat steel (flat bar) is usually used regardless of the strength level of the steel, but steel equivalent to the skin plate and diaphragm is used. It can be done.

  In the ordinary electroslag welding method, the present invention provides an end face having a thickness of 50 mm or more corresponding to a diaphragm as a surface of the steel plate 2 corresponding to a skin plate having a thickness equal to or less than the thickness of the steel sheet 1. A gap is formed with a gap between the gaps, and a backing metal disposed on both sides of the gap, and a flux is filled in the gap space and a welding wire is fed through a power supply nozzle. It is assumed that the power supply nozzle is pulled up and welded in the vertical direction. In the usual electroslag welding method, the welding heat input is set according to the plate thickness of the steel plate 1 corresponding to the diaphragm. When the plate thickness of the steel plate 1 is 50 mm or more, the welding heat input becomes high, and the weld metal Therefore, the toughness of the weld metal tends to decrease due to the formation of grain boundary ferrite and the coarsening of the grain structure.

  In the present invention, the toughness of the weld metal is reduced due to a decrease in the cooling rate under the condition that the thickness of the steel plate 1 is 50 mm or more, and further, the steel plate 2 (corresponding to a skin plate) is 2/3 or less of the steel plate 1. As shown below, the basic components of welding wire, steel plate 1 (corresponding to diaphragm), steel plate 2 (corresponding to skin plate), and backing metal are specified as shown below. , The welding wire that largely controls the toughness of the weld metal, steel plate 1 (corresponding to the diaphragm), steel plate 2 (corresponding to the skin plate), and the amount of C and N of the backing metal contribution to each weld metal ( Considering the dilution ratio), the feature is to optimize.

  First, a welding wire, a steel plate 1 (corresponding to a diaphragm), a steel plate 2 (corresponding to a skin plate), and a backing, which do not depend on the microstructure which is the first characteristic requirement of the present invention, but largely control the toughness of the weld metal. Considering the contribution (dilution rate) of each of the gold C and N to the weld metal, the limitation of the optimization conditions will be described below.

The present inventors use welding wires, steel plates and backing metal having various chemical compositions, and the steel plate 1 corresponding to the diaphragm as a reinforcing material, which is the T-shaped joint shown in FIG. Electroslag welding is also performed by changing various combinations of plate thicknesses with the steel plate 2 corresponding to the plate, and the relationship between the chemical composition of each of the welding wire, the steel plate 1, the steel plate 2, and the backing metal and the weld metal toughness. Examined. As a result, each of the welding wire, the steel plate 1, the steel plate 2, and the backing metal is within the range of the basic chemical composition as defined in the present invention, and C in the weld metal obtained from the following formula (1). The X value, which is an index related to the amount, is 0.09 or less, preferably 0.08 or less, and the Y value, which is an index related to the N content in the weld metal, obtained from the following formula (2) is 0.007 or less. By using welding wire, steel plate 1, steel plate 2, and backing metal that satisfy the conditions, the microstructure of the weld metal changes due to a decrease in cooling rate in a wide range of diaphragm and skin plate thickness combinations. It was found that a weld metal with a tensile strength of 400 to 780 MPa and a toughness of 0 J with a Charpy absorbed energy vE0 of 70 J or more can be stably secured even under such conditions.
X = 0.6 × C _W + 0.2 × (C _S1 + C _S2 ) + 0.1 × C _F (1)
However, the _{C W,} C _{S1, C S2} and shows _{C F} respectively welding wire, the steel plate 1, the steel plate 2, and the mass% of C contained in in the backing metal.
Y = 0.2 × N _W + 0.15 × (N _S1 + N _S2 ) + 0.1 × N _F (2)
However, the _{N W,} N _{S1, N S2} and, _{N F,} each welding wire, the steel plate 1, the steel plate 2, and shows the mass% of N, contained in the backing metal.

  In the above formulas (1) and (2), electroslag welding is performed under the combined conditions of the thickness of various diaphragms and skin plates, and the C amount of each of the welding wire, the steel plate 1, the steel plate 2, and the backing metal Investigating the relationship between the N content and the weld metal toughness, and by experimental regression, the contribution of the respective C content and N content of the welding wire, steel plate 1, steel plate 2, and backing metal composition to the weld metal toughness It is derived by determining the rate (dilution rate or metal yield).

  The dilution rate of C and N amounts from the welding wire, steel plate 1, steel plate 2 and the weld metal from the backing metal is affected by the weld joint shape, welding conditions, etc., but is a diaphragm that determines welding heat input. Since it is almost governed by the plate thickness, the above formulas (1) and (2) also change depending on the plate thickness of the diaphragm. In the present invention, from the experimental results of the present inventors, it is difficult to ensure the toughness of the weld metal in the prior art. The thickness of the steel plate 1 (corresponding to the diaphragm) is 50 mm or more, and further, the steel plate 2 (corresponding to the skin plate). ) In the plate thickness range up to about 100 m where electroslag welding can be applied industrially under the condition that the thickness of the steel plate 1 is 2/3 or less, the above equations (1) and (2) are experimentally derived from the regression equation. It is preferable to derive. When the plate thickness of this steel plate 1 (corresponding to the diaphragm) is 50 mm or more and 100 mm or less, there is little variation in the dilution rate from the welding wire, the steel plate 1, the steel plate 2, and the backing metal to the weld metal. Variation in correlation with toughness is reduced.

FIG. 2 shows a 2 mm V notch Charpy impact absorption energy (vE) at 0 ° C. of a weld metal center obtained by electroslag welding using a welding wire, steel plate 1, steel plate 2, and backing metal each having a different composition. ₀ ) and the X value (an index related to the amount of C in the weld metal). The Y value of the weld metal (an index related to the amount of N in the weld metal) was 0.004 to 0.007.

FIG. 3 shows a 2 mm V notch Charpy impact absorption energy (vE) at 0 ° C. of a weld metal center obtained by electroslag welding using a welding wire, steel plate 1, steel plate 2, and backing metal each having a different composition. ₀ ) and a Y value (an index related to the amount of N in the weld metal). The X value of the weld metal (an index related to the amount of C in the weld metal) was 0.006 to 0.009.

  2 and 3, both of the thicknesses of the steel plate 1 (corresponding to the diaphragm) are 50 mm and 60 mm, and the thickness of the steel plate 2 (corresponding to the skin plate) is the same as that of the steel plate 1. Using 4 types of 25 mm and 35 mm, which are 50 mm and 60 mm, and 2/3 or less of the plate thickness of the steel plate 1, 8 conditions (= 2 × 4) combining the plate thicknesses of the steel plate 1 and the steel plate 2 respectively. It was done.

  Further, the steel plate 1 and the steel plate 2 are adjusted to a chemical composition having a tensile strength of 490 to 780 MPa, and the welding wire is also adjusted to a chemical composition corresponding to a weld metal having a tensile strength of 490 to 780 MPa, and within the respective chemical composition ranges. It was changed with. The backing metal was also changed within the range of the same chemical composition using a commercially available flat bar, a steel plate 1 having a tensile strength of 490 to 780 MPa, a steel plate cut from the steel plate 2, and the like.

  Further, the sampling position of the 2 mm V notch Charpy impact test piece 5 at 0 ° C. at the center of the weld metal was sampled so that the center of the weld metal was at the position of the notch 6 as shown in FIG.

From the relationship graph between the X value of the weld metal (an index related to the amount of C in the weld metal) and toughness shown in FIG. 2, the smaller the X value of the weld metal, the better the toughness and Thus, it can be seen that a tough weld metal having a 2 mmV notch Charpy impact absorption energy (vE ₀ ) at 0 ° C. of 70 J or more can be obtained.

Further, from the relationship graph between the Y value of the weld metal (an index related to the amount of N in the weld metal) and the toughness shown in FIG. It can be seen that high toughness with a 2 mmV notch Charpy impact absorption energy (vE ₀ ) at 0 ° C. of 70 J or more can be obtained under the above conditions.

2 and 3, as described above, the thickness of the steel plate 1 (corresponding to the diaphragm) is 50 mm or more, and the combination conditions of the thicknesses of the steel plate 1 and the steel plate 2 (corresponding to the skin plate) are also included. Due to the wide range, the cooling rate of the weld metal is also different, and the microstructure of the resulting weld metal develops grain boundary ferrite, which is harmful to toughness, from a bainite main structure transformed at a low temperature preferable for toughness and an overall acicular ferrite structure. Even a mixed structure of grain boundary ferrite and acicular ferrite was included. As described above, in any of the weld metal structures in a wide range in which a macro structure undesirable for the toughness of the weld metal is generated, the toughness of the weld metal is strongly controlled by the C amount and the N amount of the weld metal. By setting the X value, which is an index related to the C amount of metal, to 0.09 or less and the Y value, which is an index related to the N amount of weld metal, to 0.007 or less, the toughness of the weld metal is 2 mmV notch at 0 ° C. The Charpy impact absorption energy (vE ₀ ) can be stably improved to 70 J or more.

From the above, in the present invention, the sheet thickness of the steel plate 1 (corresponding to the diaphragm) is 50 mm or more, and further, the steel plate 2 (corresponding to the skin plate) is 2/3 or less of the steel plate 1 with the cooling rate. Under conditions where the toughness of the weld metal is likely to decrease due to the decrease, the amount of C and N in the weld metal, which largely controls the toughness of the weld metal, is optimally controlled without depending on the microstructure, and the toughness of the weld metal at 0 ° C. In order to stably improve to 70 J or more at 2 mm V notch Charpy impact absorption energy (vE ₀ ), the X value that is an index related to the C amount of the weld metal obtained by the above formula (1) is set to 0.09 or less, and The Y value, which is an index related to the N amount of the weld metal obtained by the above equation (2), is set to 0.007 or less.

  The present invention adjusts the composition of the components of the welding wire, the steel plate 1, the steel plate 2, and the backing metal so as to satisfy the above-described X value that is an indicator relating to the C amount of the weld metal and Y that is an indicator relating to the N amount. In order to obtain the intended strength and toughness of the weld metal, the chemical composition of the welding wire, steel plate 1, steel plate 2, and backing metal is limited as follows. There is a need. The reason for limitation will be described below. In the following description, “%” means “mass%” unless otherwise specified.

First, the reason for limiting the component composition of the welding wire in the present invention will be described.
In the present invention, the chemical composition of the welding wire is mass%, C: 0.005 to 0.1%, Si: 0.01 to 1%, Mn: 0.1 to 2.5%, P: 0.00. 02% or less, S: 0.01% or less, Al: 0.002-0.1%, Ti: 0.002-0.3%, B: 0.0003-0.015%, N: 0.001 And 0.015% or less, and O: 0.01% or less, if necessary, Mo: 0.01-2.5%, Cr: 0.01-1.5%, W: 0.01-1.5%, Cu: 0.01-1.5%, Ni: 0.01-6%, Nb: 0.002-0.1%, V: 0.002-0.5% And Ta: 0.002 to 0.5%, or one or more of them, and if necessary, Ca: 0.0002 to 0.01%, Mg: 0.0002 to 0 .01%, And, REM: contain one or two or more of from 0.0002 to 0.01 percent, the balance being inevitable impurities and Fe.
The reason for limiting the content of each chemical component of the welding wire will be described below.

[C in the welding wire]
C of the welding wire is a component necessary for improving the strength of the weld metal, and if it is intended to ensure the strength of the weld metal up to a tensile strength of 780 MPa class, it is necessary to contain 0.005% or more in the welding wire. is there. However, if C in the welding wire exceeds 0.1%, the amount of C in the weld metal also becomes excessive, and the X value that is an index relating to the amount of C in the weld metal obtained by the above equation (1). Of 0.09 or less is not easy, and the toughness of the weld metal is deteriorated. Therefore, in the present invention, the amount of C in the welding wire is limited to 0.005 to 0.1%. In addition, since the one where the amount of C in a weld metal is low is preferable, when trying to obtain the high toughness whose absorbed energy of the Charpy impact test in 0 degreeC is 100 J or more, for example, the upper limit of C content of a welding wire is set to 0. More preferably, it is limited to less than 0.02%.

[Si of welding wire]
Si in the welding wire is a component that acts as a deoxidizing element and reduces the amount of oxygen as an impurity of the weld metal. In the present invention, it is necessary to contain 0.01% or more. Si is effective in increasing the strength of the weld metal by solid solution strengthening. However, if the content exceeds 1% in the wire, the hardness of the weld metal is excessively increased, and the increase of island martensite harmful to toughness is promoted to deteriorate the toughness of the weld metal. The upper limit was 1%.

[Mn of welding wire]
Mn of the welding wire has an improvement in the strength of the weld metal and a deoxidizing action. However, if the content in the welding wire is less than 0.1%, a sufficient deoxidizing action and a sufficient strength of the welding metal can be obtained. Moreover, since the oxygen content of the weld metal becomes high, the toughness of the weld metal is deteriorated. Therefore, the lower limit of the content in the wire is 0.1%. On the other hand, if the Mn content in the wire exceeds 2.5%, the weld metal structure becomes a coarse bainite structure and the toughness is likely to deteriorate. Therefore, in the present invention, the Mn content in the welding wire Is set to 2.5%.

[P of welding wire]
P in the welding wire is an impurity element, and it is preferable to reduce the content in the welding wire as much as possible in order to reduce the content in the weld metal. However, the upper limit is 0.02 as an allowable amount from the viewpoint of ensuring toughness. %. In addition, in order to suppress reliably the toughness deterioration of the weld metal by P, it is more preferable to make P content in a welding wire 0.01% or less.

[S of welding wire]
Since S of the welding wire is also an impurity element and degrades both the ductility and toughness of the weld metal, the content in the welding wire needs to be reduced as much as possible. As the upper limit that is practically acceptable without significant deterioration in ductility and toughness, the content is 0.01% or less. In order to surely suppress the ductility and toughness deterioration of the weld metal due to S, the S content in the welding wire is more preferably 0.005% or less.

[Al of welding wire]
Al of the welding wire works as a deoxidizing element and is effective in controlling the amount of oxygen in the weld metal. In order to effectively contribute to deoxidation of the weld metal, it is necessary to contain 0.002% or more in the welding wire. On the other hand, if Al is excessively contained in the weld metal, there is a high possibility that coarse inclusions are formed, and the formation of acicular ferrite in the weld metal is suppressed and the structure becomes coarse, so that the toughness Deteriorates. Since the upper limit of Al in the welding wire for preventing these adverse effects is 0.1%, in the present invention, the range of Al in the welding wire is 0.002 to 0.1%.

[Ti of welding wire]
Ti of the welding wire has an effect as a deoxidizing element like Si and Al, and forms a relatively fine oxide containing Ti and contributes to refinement of the structure as an acicular ferrite nucleus in the weld metal. Therefore, it is an effective element for improving toughness. As a lower limit at which the effect is clearly generated, it is necessary to contain 0.002% or more in the welding wire. However, when the Ti content in the welding wire exceeds 0.3%, it becomes a starting point of brittle fracture in the weld metal. In order to form such coarse oxides and nitrides and deteriorate the toughness of the weld metal, in the present invention, the Ti content in the welding wire is set to 0.002 to 0.3%.

[B of welding wire]
When an appropriate amount of B in the welding wire is contained in the weld metal, it enhances hardenability and suppresses coarse grain boundary ferrite, and exhibits a remarkable effect in improving toughness. It is an essential element in order to ensure a certain level of microstructural refinement. In order to contain an appropriate amount of B in the weld metal, it is most effective to make it contained in the welding wire having the largest contribution to the weld metal composition. Therefore, in the present invention, an appropriate amount of B is contained in the welding wire. Is an essential requirement. In order for B to be contained in the weld metal to reliably exhibit the effect of refining the structure, the B content in the welding wire needs to be 0.0003% or more. On the other hand, if the B content in the welding wire exceeds 0.015%, B in the weld metal becomes excessive and tends to be a coarse upper bainite structure. Moreover, it may promote hot cracking of the weld metal. Therefore, in the present invention, the B content of the welding wire is set to 0.0003 to 0.015%.

[N of welding wire]
N in the welding wire is preferably an impurity element in the weld metal, so it is preferable to reduce it as much as possible. However, it is industrially recommended that the N content in the welding wire is less than 0.001%. Due to difficulties, the lower limit is made 0.001%. When the N content of the welding wire exceeds 0.015%, depending on the N amount of the steel plate and the backing metal, the Y value, which is an index related to the N amount in the weld metal obtained by the above formula (2), is obtained. The invention cannot be satisfied and N in the weld metal is in a solid solution state, which deteriorates the toughness of the ferrite matrix and further fixes B as a nitride, thereby suppressing the pro-eutectoid ferrite transformation at the austenite grain boundary of B in the weld metal. Reduce the effect. Therefore, in the present invention, the content in the welding wire is 0.001 to 0.015%.

[O of welding wire]
If a large amount of O in the welding wire is present in the wire, the manufacturability of the welding wire is hindered. Further, in weld metal in electroslag welding, O is basically an impurity element, which is not preferable because it deteriorates the ductility and toughness of the weld metal. Therefore, it is preferable to reduce the amount of O in the welding wire as much as possible. However, in the present invention, the upper limit of the content is 0.01% as a range in which the weld wire manufacturability and the weld metal quality do not deteriorate. To do.

  The above is the reason for limiting the essential components in the chemical composition of the welding wire. For the purpose of adjusting the material of the weld metal, one of Mo, Cr, W, Cu, Ni, Nb, V, and Ta is used. A seed | species or 2 or more types can be contained.

[Mo of welding wire]
Mo is an element effective for improving toughness by increasing the hardenability and refining the bainite or acicular ferrite of the weld metal structure, and is also an element effective for improving the strength by solid solution strengthening and precipitation strengthening. In order to acquire this effect, when making it contain in a welding wire, it is necessary to make it contain 0.01% or more. However, if the content exceeds 2.5%, the weld metal is excessively cured and the toughness of the weld metal is remarkably deteriorated. Therefore, in the present invention, the upper limit of the content is set to 2.5%.

[Cr of welding wire]
Since Cr has substantially the same effect as Mo, the lower limit when it is contained in the wire is 0.01%, but the toughness deterioration when it is excessively contained is more remarkable than Mo. The upper limit of the content is 1.5%.

[W of welding wire]
Since W also has substantially the same effect as Cr, the content as a welding wire is set to 0.01 to 1.5%.

[Cu of welding wire]
Cu is an austenite stabilizing element, and is an element effective for improving the strength and toughness through microstructure refinement by enhancing the hardenability of the weld metal. When Cu is contained in the welding wire, the content of the welding wire needs to be 0.01% or more in order to reliably improve the hardenability of the weld metal. On the other hand, if the content in the welding wire is more than 1.5%, hot cracking tends to occur, which is not preferable because the productivity of the welding wire deteriorates. In the present invention, the upper limit is limited to 1.5% from the content for preventing the productivity of the welding wire from deteriorating. In addition, when Cu plating is performed on the surface of the welding wire, the plating content is also included in the content.

[Ni of welding wire]
Ni is a very useful element that, when contained in a weld metal at a certain level or more, can increase toughness by the solid solution toughening effect, and at the same time increase the strength by improving hardenability and solid solution strengthening. In the case where Ni is contained in the welding wire, the Ni content in the welding wire needs to be 0.01% or more in order to clearly demonstrate the effect of Ni in the weld metal. On the other hand, if the Ni content in the welding wire exceeds 6%, the yield stress of the weld metal is remarkably lowered, and it is difficult to ensure the required strength. Therefore, in the present invention, when Ni is contained in the welding wire, the Ni content in the welding wire is 0.01 to 6%.

[Nb of welding wire]
When Nb is contained in the weld metal, it is effective in improving the strength of the weld metal due to the effect of improving hardenability and precipitation strengthening. When Nb is contained in the welding wire, the Nb content in the welding wire needs to be 0.002% or more in order to reliably exhibit this effect. On the other hand, when the amount of Nb in the welding wire exceeds 0.1%, the strength of the weld metal becomes excessive, and coarse Nb precipitates are formed. Absent. Therefore, in the present invention, when Nb is contained in the welding wire, the range of Nb content is limited to 0.002 to 0.1%.

[V of welding wire]
When V is contained in the weld metal, it is effective for improving the strength of the weld metal by precipitation strengthening. When V is contained in the welding wire, the V content in the welding wire needs to be 0.002% or more in order to reliably exhibit this effect. On the other hand, if the V content in the welding wire exceeds 0.5%, the strength of the weld metal becomes excessive and the toughness of the weld metal is significantly deteriorated, which is not preferable. Therefore, when V is contained in the welding wire in the present invention, the range of the V content in the welding wire is limited to 0.002 to 0.5%.

[Ta of welding wire]
When Ta is contained in the weld metal as in V, it is effective for improving the strength of the weld metal by precipitation strengthening. When Ta is contained in the welding wire, the Ta content in the welding wire needs to be 0.002% or more in order to reliably exhibit this effect. On the other hand, if the Ta content in the welding wire exceeds 0.5%, the strength of the weld metal becomes excessive, and the toughness deterioration of the weld metal becomes significant. Therefore, when Ta is contained in the welding wire in the present invention, the range of Ta content in the welding wire is limited to 0.002 to 0.5%.

  In the present invention, when it is necessary to further improve the ductility and toughness of the weld metal, one or more of Ca, Mg, and REM are further included in the welding wire as necessary. be able to.

[Ca, Mg, and REM of welding wire]
Ca, Mg, and REM are all effective for improving ductility and toughness by changing the structure of sulfides and reducing the size of sulfides and oxides in the weld metal. When these elements are contained in the welding wire, the lower limit content for exhibiting the effect is 0.0002%. On the other hand, if it is contained excessively, it causes coarsening of sulfides and oxides, leading to deterioration of ductility and toughness, and also the possibility of deterioration of weld bead shape and weldability. Is also 0.01%.

  Next, the reasons for limiting the component compositions of the steel plate 1 (corresponding to a diaphragm as a reinforcing material) and the steel plate 2 (corresponding to a skin plate of a four-sided box column) in the present invention will be described.

  In the present invention, each of the component compositions of the steel plate 1 (corresponding to a diaphragm as a reinforcing material) and the steel plate 2 (corresponding to a skin plate of a four-sided box column) is mass%, and C: 0.02 to 0.2%. , Si: 0.01 to 1%, Mn: 0.1 to 2.5%, P: 0.02% or less, S: 0.01% or less, Al: 0.002 to 0.1%, N: 0.001-0.015% is contained, O: It restrict | limits to 0.01% or less, Ti: 0.002-0.05%, B: 0.0003-0.015% as needed, Mo: 0.01-1.5%, Cr: 0.01-1.5%, W: 0.01-1.5%, Cu: 0.01-1.5%, Ni: 0.01- 6%, Nb: 0.002 to 0.1%, V: 0.002 to 0.5%, and Ta: 0.002 to 0.5%. Further, if necessary, one or more of Ca: 0.0002 to 0.01%, Mg: 0.0002 to 0.01%, and REM: 0.0002 to 0.01% are added. And the balance is made of inevitable impurities and Fe.

  In normal electroslag welding, the dilution ratio from steel plate 1 (corresponding to a diaphragm as a reinforcing material) and steel plate 2 (corresponding to a skin plate of a four-sided box column) to the weld metal is approximately the same. The effect on the toughness of the weld metal can be obtained equally even if the composition of any of the components 1 and 2 is the same. Needless to say, the chemical composition of the steel plate 1 and the steel plate 2 may be different from each other as long as they are within the range of the chemical composition defined in the present invention.

  Below, the reason for limitation of content of each chemical component of the steel plate 1 and the steel plate 2 is demonstrated.

[C of steel plate 1 and steel plate 2]
C of the steel plate 1 and the steel plate 2 has a tensile strength of 400 to 780 MPa, and it is necessary to contain 0.02% or more for securing the strength of the steel plate. On the other hand, if more than 0.2% is contained in the steel plate, the deterioration of the toughness of the steel plate, the weld heat affected zone toughness, and further the resistance to weld cracking will be impaired, and the safety as structural steel will be impaired. As a result, the C content of the weld metal becomes excessive, and depending on the composition of the welding wire and backing metal, the X value, which is an index of the C amount of the weld metal obtained by the above formula (1), cannot be satisfied within the scope of the present invention. Since there is a concern that the toughness of the weld metal is deteriorated, in the present invention, the upper limit of the C content of the steel plate 1 and the steel plate 2 is set to 0.2%.

[Si of steel plate 1 and steel plate 2]
Si in the steel plate 1 and the steel plate 2 is an element effective as a deoxidizing element and for securing the strength of the steel plate. If the content is less than 0.01%, deoxidation becomes insufficient and it is disadvantageous for securing the strength. On the other hand, an excessive content exceeding 1% forms a coarse oxide and causes the ductility and toughness of the steel sheet to deteriorate. In addition, the Si content in the weld metal may be excessive and impair the toughness. Then, the range of Si content in a steel plate shall be 0.01-1%.

[Mn of steel plate 1 and steel plate 2]
Mn in the steel plate 1 and the steel plate 2 is an element necessary for enhancing the hardenability of the steel plate and ensuring strength and toughness, and it is necessary to contain at least 0.1% or more. However, an excessive content exceeding 2.5% generates a hard phase, so that the toughness of the steel sheet is remarkably deteriorated, and the toughness and crackability of the weld heat affected zone are also deteriorated. Furthermore, since it also adversely affects weld metal toughness, the upper limit of the Mn content in the steel plate 1 and the steel plate 2 is set to 2.5%.

[P of steel plate 1 and steel plate 2]
P in the steel plate 1 and the steel plate 2 is an impurity element and is preferably reduced as much as possible with respect to the properties of the steel plate and the weld metal. However, the upper limit is 0.02% as an allowable amount in terms of securing toughness. It was.

[S of steel plate 1 and steel plate 2]
Since S of the steel plate 1 and the steel plate 2 is also an impurity element and deteriorates both the ductility and toughness of the steel plate and the weld metal, reduction is necessary. As the upper limit that is practically acceptable without significant deterioration in ductility and toughness, the content is 0.01% or less.

[Al of steel plate 1 and steel plate 2]
The Al in the steel plate 1 and the steel plate 2 is an element effective for deoxidation of the steel plate, refinement of the grain size of the heated austenite, etc., and in order to exert the effect, it is necessary to contain 0.002% or more in the steel plate. If over 0.1% is included, a coarse oxide is formed, and the toughness and ductility of the steel sheet are extremely deteriorated. Also, the amount of Al in the weld metal becomes excessive, resulting in increased toughness. Since harmful upper bainite may be formed and the toughness of the weld metal may be deteriorated, in the present invention, the Al content in the steel plate 1 and the steel plate 2 is limited to a range of 0.002% to 0.1%.

[N of steel plate 1 and steel plate 2]
N in the steel plate 1 and the steel plate 2 is combined with Al and Ti and effectively works to refine the austenite grains and contributes to improving the toughness of the steel plate. However, in order to clarify the effect, the N content is 0.001% or more. On the other hand, if the N content in steel plate 1 and steel plate 2 exceeds 0.015%, depending on the N amount of the welding wire or backing metal, the N amount of the weld metal determined by the above equation (2) Y value that is an index related to the present invention is not satisfied within the scope of the present invention, N in the weld metal is in a solid solution state greatly deteriorates the toughness of the ferrite matrix, and further B in the weld metal is fixed as a nitride, The effect of inhibiting pro-eutectoid ferrite transformation at the austenite grain boundaries of B is reduced, and the toughness is deteriorated. Therefore, in the present invention, the upper limit of the N amount in the steel plate 1 and the steel plate 2 is set to 0.015%.

[O of steel plate 1 and steel plate 2]
O in steel plate 1 and steel plate 2 is an impurity element and adversely affects the ductility and toughness of the steel plate due to the adverse effects of oxides. Also, the amount of O in the weld metal is increased to similarly deteriorate the ductility and toughness of the weld metal. Therefore, it is limited to 0.01% or less.

  The above is the effect and the reason for limitation on the essential requirements in the chemical composition of the steel plate 1 corresponding to the diaphragm and the steel plate 2 corresponding to the skin plate, but depending on the properties required for the steel plate 1 and the steel plate 2, One or more of Ti, B, Mo, Cr, W, Cu, Ni, Nb, V, and Ta can be contained. Also in the selectable element, it is necessary to limit each composition range as follows.

[Ti of steel plate 1 and steel plate 2]
Ti in the steel sheet 1 and the steel sheet 2 is an element effective for improving the toughness of the steel sheet by refining the austenite crystal grains by forming TiN. It is necessary to contain 0.002% or more. On the other hand, if it exceeds 0.05%, coarse oxides and nitrides are formed to deteriorate the toughness and ductility of the steel sheet, so the upper limit is made 0.05%.

[B of steel plate 1 and steel plate 2]
B in the steel plate 1 and the steel plate 2 is an element that enhances hardenability in a very small amount, and is an element effective for increasing the strength of the steel plate. Further, when an appropriate amount of B is contained in the steel plate, it is also contained in the weld metal by dilution, and is effective in suppressing the grain boundary ferrite of the weld metal. When B is contained in the steel plate 1 and the steel plate 2 as necessary, B must be contained in the steel plate in an amount of 0.0003% or more in order to clearly exhibit these effects. On the other hand, if it is included in the steel sheet in excess of 0.015%, a coarse precipitate is often formed in the heating stage at the time of steel slab production or steel sheet production, so the effect of improving the hardenability becomes insufficient. In addition, there is an increased risk of toughness degradation due to cracks and precipitates in the steel slab. Therefore, in the present invention, when B is contained in the steel plate 1 and the steel plate 2, the range of B in the steel plate 1 and the steel plate 2 is 0.0003 to 0.015%.

[Mo of steel plate 1 and steel plate 2]
Since Mo in the steel plate 1 and the steel plate 2 is an element effective for improving the strength of the steel plate by improving the hardenability and precipitation strengthening, it can be contained in the steel plate 1 and the steel plate 2 as necessary for improving the strength. In order to exhibit a clear effect, 0.01% or more is necessary in the steel sheet. On the other hand, when Mo is excessively contained exceeding 1.5%, the strength is excessively increased and the toughness of the steel sheet is deteriorated. Therefore, in the present invention, when steel sheet 1 and steel sheet 2 contain Mo. The Mo content in the steel plate 1 and the steel plate 2 is 0.01 to 1.5%.

[Cr of steel plate 1 and steel plate 2]
Since Cr of the steel plate 1 and the steel plate 2 has the same effect as Mo, the Cr content in the steel plate 1 and the steel plate 2 is limited to 0.01 to 1.5% for the same reason as Mo.

[W of steel plate 1 and steel plate 2]
Since W of steel plate 1 and steel plate 2 has the same effect as Mo and W, W content in steel plate 1 and steel plate 2 is limited to 0.01 to 1.5% for the same reason.

[Cu of steel plate 1 and steel plate 2]
Cu in the steel plate 1 and the steel plate 2 is an element effective for improving the strength of the steel plate mainly by the effect of improving hardenability and solid solution strengthening. % Or more must be contained. On the other hand, if the steel plate contains more than 1.5%, there is a problem in hot workability, so the Cu content in the steel plate 1 and the steel plate 2 is limited to 0.01 to 1.5%.

[Ni in steel plate 1 and steel plate 2]
Ni in steel plate 1 and steel plate 2 is an element that can essentially increase the toughness of the steel plate matrix, and when used properly, it can improve strength and toughness simultaneously without greatly depending on the microstructure, improving mechanical properties. Is a very effective element. When Ni is contained in the steel sheet, it is necessary to contain 0.01% or more in order to exert the effect. As the content in the steel sheet increases, the strength-toughness balance is improved, but the effect is saturated even if the content exceeds 6%, so the upper limit is made 6% in consideration of economy.

[Nb of steel plate 1 and steel plate 2]
Nb of the steel plate 1 and the steel plate 2 is an element effective for increasing the strength of the steel plate in a small amount by precipitation strengthening and transformation strengthening, and also effective for improving the toughness of the steel plate by refining the heated austenite grain size. In the case where Nb is contained in the steel plate 1 and the steel plate 2 in anticipation of the effect, it is necessary to contain 0.002% or more in order to exert the effect. However, if it is excessively contained exceeding 0.1%, the toughness of the steel sheet is deteriorated, and there is also a concern that excessive Nb is contained in the weld metal due to dilution, and the toughness of the weld metal is deteriorated. In this invention, Nb content of the steel plate 1 and the steel plate 2 is limited to 0.002 to 0.1% of range.

[V of steel plate 1 and steel plate 2]
V in the steel plate 1 and the steel plate 2 is an element effective for increasing the strength of the steel plate by a small amount mainly by precipitation strengthening. When V is contained in the steel plate 1 and the steel plate 2, 0.002 is necessary to exert the effect. % Or more is necessary. However, if it is contained excessively exceeding 0.5%, coarse precipitates are formed and the toughness of the steel sheet is deteriorated, and excessive V is also contained in the weld metal due to dilution, and the toughness of the weld metal. In the present invention, the V content of the steel plate 1 and the steel plate 2 is limited to a range of 0.002 to 0.5%.

[Ta of steel plate 1 and steel plate 2]
The Ta of the steel plate 1 and the steel plate 2 is also an element effective for increasing the strength of the steel plate mainly by precipitation strengthening, and when the steel plate 1 and the steel plate 2 contain Ta, 0.002% The above is necessary. However, if it exceeds 0.5% and contains excessively, coarse precipitates are formed and the toughness of the steel sheet is deteriorated, and excessive Ta is also contained in the weld metal due to dilution, and the toughness of the weld metal In the present invention, the Ta content of the steel plate 1 and the steel plate 2 is limited to a range of 0.002 to 0.5%.

  In the present invention, when it is necessary to further improve the ductility and toughness of the steel plate 1 and the steel plate 2, one or more of Ca, Mg, and REM are further added to the steel plate 1 and the steel plate as necessary. 2 can be contained.

[Ca, Mg, and REM of steel plate 1 and steel plate 2]
Ca, Mg, and REM are all effective in improving the ductility and toughness of the steel sheet by changing the sulfide structure and reducing the size of the sulfide and oxide in the steel sheet. When these elements are contained in the steel plate 1 and the steel plate 2, the lower limit content for exhibiting the effect is 0.0002%. On the other hand, if it is contained excessively, it causes coarsening of sulfides and oxides, leading to deterioration of ductility and toughness, and also the possibility of deterioration of weld bead shape and weldability. Is also 0.01%.

  Next, the reason for limiting the component composition of the backing metal in the present invention will be described.

  Since the backing metal affects the strength and toughness of the weld metal due to dilution, it is necessary to limit the content of the components in order to ensure the material of the backing metal and to secure weld cracking resistance. .

  That is, in the present invention, the chemical components of the backing gold are in mass%, C: 0.02-0.25%, Si: 0.01-1.5%, Mn: 0.1-2.5% , P: 0.02% or less, S: 0.01% or less, N: 0.001 to 0.015%, O: Restricted to 0.01% or less, and if necessary, Al: 0 0.002-0.1%, Ti: 0.002-0.05%, B: 0.0003-0.015%, Mo: 0.01-1.5%, Cr: 0.01-1.5 %, W: 0.01-1.5%, Cu: 0.01-1.5%, Ni: 0.01-6%, Nb: 0.002-0.1%, V: 0.002- 0.5% and Ta: 0.002 to 0.5% of one or two or more kinds, if necessary, Ca: 0.0002-0.01%, Mg: 0 .0002-0. 1% and, REM: contain one or two or more of from 0.0002 to 0.01 percent, the balance being inevitable impurities and Fe.

  The reason for limiting the content of each chemical component of the backing gold will be described below.

[C of backing money]
The C of the backing metal has the effect of increasing the C content of the weld metal by dilution to increase the hardenability of the weld metal and refine the structure, thereby improving the toughness of the weld metal. In order to exert the effect, it is necessary to contain 0.02% or more of C in the backing material. However, if C is excessively contained in the weld metal, it is difficult to set the index X value relating to the C amount of the weld metal obtained by the above formula (1) within the scope of the present invention depending on the C content of the welding wire or steel plate. In such a case, the hardness of the weld metal becomes excessive and deteriorates the toughness of the weld metal, and the low-temperature cracking susceptibility of the back metal increases, which is not preferable. The upper limit of the amount is 0.25%.

[Si of backing gold]
The backing metal Si is effective in deoxidizing the weld metal and increasing its strength by increasing the Si content of the weld metal by dilution. In order to exert these effects, it is necessary to contain 0.01% or more in the backing metal. However, if it exceeds 1.5%, the hardness of the weld metal is excessively increased. In addition, in order to promote the increase of island martensite harmful to toughness and deteriorate the toughness of the weld metal, and further increase the cracking sensitivity of the backing metal, in the present invention, the Si content of the backing metal is reduced. The range is 0.01-1.5%.

[Mn of backing gold]
The Mn of the backing metal is also effective for deoxidizing the weld metal and increasing the strength by increasing the Mn content of the weld metal by dilution. If the content in the backing metal is less than 0.1%, the effect is not clear. On the other hand, if the content exceeds 2.5% in the backing metal, the hardness of the weld metal is increased excessively, Since the possibility of degrading toughness is high, in order to further increase the cracking sensitivity of the backing metal, the range of the backing metal content is set to 0.1 to 2.5% in the present invention.

[P of backing money]
P in the backing metal is an impurity element. If the content in the weld metal increases due to dilution, the toughness of the weld metal is deteriorated. Therefore, the content of the backing metal is reduced as much as possible to reduce the weld metal properties by dilution. It is an element that should suppress adverse effects. In consideration of the dilution ratio to the weld metal, as an upper limit that can adversely affect the weld metal toughness, in the present invention, the P content in the backing metal is limited to 0.02% or less.

[Singe money]
S in the backing metal is also an impurity element. When the content in the weld metal increases due to dilution, the toughness and ductility of the weld metal is deteriorated. It is an element that should suppress adverse effects on the body. In consideration of the dilution ratio to the weld metal, as an upper limit that can adversely affect the weld metal toughness and ductility, the S content in the backing metal is limited to 0.01% or less in the present invention.

[N of backing money]
N in the backing metal is also an impurity element, and depending on the N content of the welding wire or steel plate, the index Y value related to the N amount of the weld metal obtained by the above formula (2) is excessively deviating from the scope of the present invention. Therefore, it is an element that should reduce the content in the backing metal as much as possible and suppress the adverse effects on the weld metal properties due to dilution. In the present invention, the N content in the backing metal is set to 0.015% as an upper limit where the limitation on the N content of the welding wire and the steel plate does not become extremely strict, and as an upper limit that can adversely affect the backing metal itself. The following. The lower the N content in the backing metal, the better. However, since there is a limit to the amount that can be industrially reduced, in the present invention, the range of the N content in the backing metal is 0.001 to 0.015. Limited to%.

[Our money of backing money]
O in the backing metal is also an impurity element, and if the content in the weld metal increases due to dilution, the toughness and ductility of the weld metal deteriorates. It is an element that should suppress adverse effects on the body. In consideration of the dilution ratio to the weld metal, as an upper limit that can adversely affect the weld metal toughness and ductility, the O content in the backing metal is limited to 0.01% or less in the present invention.

  The above is the reason for the limitation on the essential requirements in the chemical composition of the backing metal. For the microstructure and property control of the weld metal, Al, Ti, B, Mo, Cr, W, Cu, Ni are optionally used. , Nb, V, and Ta can be contained alone or in combination.

[Back gold Al]
When Al is contained in the weld metal by dilution, it is a beneficial element that acts as a deoxidizing element in the weld metal and reduces the amount of O as an impurity of the weld metal. When Al is contained in the backing metal, it is necessary to contain 0.002% or more in the backing metal in order to exert the effect. However, if the content exceeds 0.1% in the backing, coarse oxides and nitrides may be formed in the backing, which may adversely affect the material of the backing. The upper limit of the content is 0.1%.

[Ti of backing gold]
When Ti in the backing metal is also contained in the weld metal by dilution, it acts as a deoxidizing element in the weld metal and has the effect of reducing the amount of O as an impurity of the weld metal, and also has a fine ashes in the weld metal. It contributes to Ti oxides and the like that form nuclei of curled ferrite, and is effective in improving the toughness of the weld metal. When Ti is contained in the backing metal aiming at these effects, it is necessary to contain 0.002% or more in the backing metal in order to exhibit the effect clearly. However, if the content exceeds 0.05% in the backing, coarse Ti oxides and nitrides are formed in the backing, which is undesirable because it deteriorates the properties and surface properties of the backing. In the present invention, when Ti is contained in the backing metal, the content is limited to 0.002 to 0.05%.

[Binner money B]
When B in the backing metal is contained in the weld metal by dilution, it has the effect of improving the strength and toughness of the weld metal through improving the hardenability of the weld metal. When B is contained in the backing metal, in order to sufficiently obtain the effect, B must be contained in the backing metal in an amount of 0.0003% or more. However, if the B content exceeds 0.015% and a large amount of B is contained in the backing, excess B forms coarse precipitates and deteriorates the toughness of the weld metal. In order to inhibit this, the upper limit of the content is made 0.015%.

[Mo of backing gold]
When Mo in the backing metal is contained in the weld metal by dilution, it is an element effective in improving the toughness of the weld metal by increasing the hardenability of the weld metal and refining the weld metal structure bainite or acicular ferrite. In addition, it is an element effective for improving the strength by solid solution strengthening and precipitation strengthening. In order to obtain Mo by including Mo in the backing metal, it is necessary to contain 0.01% or more of Mo in the backing metal. However, if the backing metal is contained in excess of 1.5%, the weld metal may be excessively hardened and the toughness of the weld metal may be deteriorated, and the crack sensitivity of the backing metal is increased. The upper limit of the Mo content is 1.5%. Therefore, in the present invention, when Mo is contained in the backing metal, the range is limited to 0.01 to 1.5%.

[Cr of backing gold]
Since the backing metal Cr has the same effect as Mo, the Cr content in the backing metal is limited to 0.01 to 1.5% for the same reason as Mo.

[The W of the backing money]
Since the backing W has the same effect as Mo and W, the W content in the backing is limited to 0.01 to 1.5% for the same reason as Mo and W.

[Cu of backing gold]
When Cu in the backing metal is contained in the weld metal by dilution, it has the effect of increasing the hardenability of the weld metal and making the structure finer, thereby improving the toughness of the weld metal. is there. In order for Cu to be contained in the backing metal to exert the effect, it is necessary to contain 0.01% or more in the backing metal. However, if Cu is contained in the backing metal in excess of 1.5%, the hardness of the weld metal may be excessively increased and the toughness may be deteriorated. In order to degrade weld cracking, the upper limit is made 1.5%.

[Ni of backing gold]
When Ni in the backing metal is contained in the weld metal by dilution, it is effective for improving the toughness of the weld metal. In order for Ni to be contained in the backing metal to reliably exhibit the effect, it is necessary to contain 0.01% or more in the backing metal. However, if it contains more than 6% in the backing metal, there is a concern that hot cracking of the weld metal is promoted, which is not preferable, and also deteriorates the productivity and welding crack resistance of the backing metal. The upper limit is 6%.

[Nb of backing money]
When Nb in the backing metal is contained in the weld metal by dilution, it is effective for improving toughness by increasing the hardenability of the weld metal, suppressing intergranular ferrite, and refining intragranular acicular ferrite. Moreover, it is also effective as a strength adjusting element by transformation strengthening and precipitation strengthening. In order for Nb to be contained in the backing metal to exhibit these effects, it is necessary to contain at least 0.002% or more in the backing metal. On the other hand, if it exceeds 0.1% and is contained in the backing metal excessively, the hardness of the weld metal becomes excessive and the toughness deteriorates significantly, and the welding crack resistance of the backing metal deteriorates. Therefore, in the present invention, the range when Nb is contained in the backing metal is limited to 0.002 to 0.1%.

[Back money V]
When V in the backing metal is contained in the weld metal by dilution, it is effective as a strength adjusting element mainly by precipitation strengthening. In order for V to be contained in the backing metal to exhibit this effect, it is necessary to contain at least 0.002% or more in the backing metal. On the other hand, if it exceeds 0.5% and is contained in the backing metal excessively, the hardness of the weld metal becomes excessive and the toughness deterioration increases, and the welding crack resistance of the backing metal deteriorates. Therefore, in the present invention, the range when V is contained in the backing metal is limited to 0.002 to 0.5%.

[Tank of backing gold]
When Ta in the backing metal is contained in the weld metal by dilution, like V, it is effective mainly as a strength adjusting element by precipitation strengthening. In order to exhibit this effect by including Ta in the backing metal, it is necessary to contain at least 0.002% or more in the backing metal. On the other hand, if it exceeds 0.5% and is contained in the backing metal excessively, the hardness of the weld metal becomes excessive and the toughness deterioration increases, and the welding crack resistance of the backing metal deteriorates. Therefore, in the present invention, the range when Ta is contained in the backing metal is limited to 0.002 to 0.5%.

  In order to control the structure and properties of the weld metal in the chemical composition of the backing metal, Al, Ti, B, Mo, Cr, W, Cu, Ni, which can be contained in the backing metal as necessary. The reason for the limitation is one or more of Nb, V, and Ta, but in the present invention, it is necessary to control the structure and ductility of the weld metal, or to secure the manufacturability and material of the backing metal. Therefore, if necessary, the backing metal can contain one or more of Ca, Mg, and REM.

[Back metal Ca, Mg, and REM]
Ca, Mg, and REM each have the same effect and act as a deoxidizing element in the weld metal and reduce the amount of oxygen as an impurity, which is effective in improving the toughness and ductility of the weld metal. Further, it is effective for improving the ductility of the backing metal. In particular, when the amount of S in the backing metal is large, it is effective for ensuring ductility to contain appropriate amounts of these elements. When these elements are contained in the backing metal, it is necessary to contain 0.0002% or more of each in the backing metal in order to reliably obtain the effect. However, if the content exceeds 0.01% in the backing metal, a coarse oxide is formed in the weld metal, which may deteriorate the toughness and ductility. Therefore, in the present invention, Ca, Mg, REM When both are contained in backing metal, the range is made 0.0002 to 0.01%.

  The effects of the present invention will be further described below with reference to examples. Using weld wires, steel plates and backing metal of various chemical compositions, weld joints are produced by T-shaped joints as shown in FIG. 1, and the toughness of the weld metal at the center of the groove is measured at 0 ° C. in a 2 mm V notch Charpy impact test. Evaluation was based on absorbed energy.

  Table 1 shows the chemical composition of the welding wire used. All are trial solid wires with a diameter of 1.6 mm, wire numbers WA1 to WA7 satisfy the chemical composition of the wire of the present invention, and wire numbers WB1 to WB6 are requirements related to the chemical composition of the welding wire of the present invention. It is a comparative welding wire that does not satisfy the above.

  Table 2 shows the chemical composition of the steel plate used as the steel plate 2 corresponding to the skin plate. Table 3 shows the chemical composition of the steel plate used for the steel plate 1 corresponding to the diaphragm. The chemical compositions in Tables 2 and 3 were ingots or slabs, and steel plates with various plate thicknesses were manufactured by hot rolling using the ingots or slabs and used for joint production. That is, in order to confirm the change in the toughness of the weld metal in various combinations of plate thickness, the steel plate 2 has three types of plate thicknesses of 25 mm, 35 mm, and 50 mm for each composition, and the steel plate 1 has two types of plate thicknesses of 50 mm and 60 mm. did.

  In Table 2, steel plate numbers SA1 to SA8 satisfy the component requirements related to the steel plate 2 of the present invention, and the steel plate numbers SB1 to SB4 do not satisfy the component requirements related to the steel plate 2 of the present invention. It is.

  In Table 3, the steel plate numbers DA1 to DA8 satisfy the component requirements related to the steel plate 1 of the present invention, and the steel plate numbers DB1 to DB4 do not satisfy the component requirements related to the steel plate 1 of the present invention. It is.

  Table 4 shows the chemical composition of the backing metal used to produce the joint. A commercially available flat bar or a thick steel plate was used. In Table 4, the backing metal numbers BA1 to BA6 are examples satisfying the component requirements relating to the backing metal of the present invention, and the backing metal numbers BB1 to BB4 are the component requirements relating to the backing metal of the present invention. This is an example not satisfying.

  Various combinations of the materials shown in Tables 1 to 4 were made to produce a joint as shown in FIG. 1, and the welding conditions at that time were set as shown in Table 5 for each plate thickness of the steel plate 1 corresponding to the diaphragm. That is, as the plate thickness of the steel plate 1 increases, the groove cross-sectional area increases, and it is necessary to increase the welding heat input accordingly. In addition, since the groove is rectangular, the welding nozzle is pulled up during welding for the purpose of spreading the weld metal into the groove, and is oscillated along the surface of the steel plate 2. 1 is adjusted appropriately according to the plate thickness.

  In the joint after welding, a 2 mm V notch Charpy impact test piece is taken from the center of the weld metal as shown in FIG. 4 and subjected to an impact test at 0 ° C. The toughness is obtained by the absorbed energy at 0 ° C. of the 2 mm V notch Charpy impact test. evaluated.

  Table 6 shows weld metal toughness in electroslag welded joints when various combinations of materials and steel plate thicknesses are changed. For each combination of materials, four types of combinations of the steel plate 1 and the steel plate 2 were changed, and the toughness change between the respective plate thickness combinations was compared with each toughness level. That is, the plate thickness combinations of the steel plate 1 (diaphragm) and the steel plate 2 (skin plate) are (a) 50 mm × 50 mm, (b) 60 mm × 50 mm, (c) 50 mm × 25 mm, (d) 60 mm × 35 mm, As the heat input, 79.0 to 93.0 kJ / mm is significantly changed. Further, when the plate thickness of the steel plate 2 is 25 mm and 35 mm, which is thinner than 2/3 of the steel plate 1 corresponding to the diaphragm, the steel plate 2 The cooling rate of the weld metal after welding is smaller than when the plate thickness is 50 mm, which is the same as or slightly thinner than that of the steel plate 1. Therefore, conventionally, it has been difficult to ensure stable weld metal toughness under all of the above conditions (a) to (d) with the same steel plate, wire, and backing metal combination. That is, if a material matched to the welding conditions of the thin material is used, even if the conditions (a) and (b) have a low toughness, or (a) and (b), the toughness can be secured, In (c) and (d), the toughness may be deteriorated due to a significant lack of hardenability. Conversely, if the materials matched to (c) and (d) are used, the conditions of (a) and (b) Due to excessive hardenability, a hard bainite phase is generated and toughness deteriorates. In addition, under the conditions (c) and (d), it is very difficult to achieve vE0 ≧ 70 J with the conventional means for refining the structure by optimizing the weld metal composition.

As shown in Table 6, the joint A1 (joints by a combination of steel A1- 3, A1-4, hereinafter the same) satisfying the present invention - joint A10 (joint A10-3, A10-4,) in all of the steel sheet Under the plate thickness combination conditions, the weld metal has a 0 ° C Charpy impact absorption energy (vE0) of 70 J or more, and the toughness difference between the steel plate combinations is small. According to the present invention, the welding wire is changed for each plate thickness combination. It is clear that high toughness can be obtained stably.
Note that joints A1 (joints A1-1, A1-2, and so on depending on the combination of steel plates) to joint A10 (joints A10-1 to A10-2) are reference examples.

  On the other hand, since the joints B1 to B8 in Table 6 do not satisfy the requirements of the present invention, the toughness of the weld metal is higher than that of the present invention in some or all of the steel plate thickness combinations of (a) to (d). Inferior, it is apparent that vE0 ≧ 70J required for high toughness cannot be obtained with one material combination (wire, steel plate, backing metal) regardless of the steel plate thickness.

  That is, in the case of the joint B1 of the comparative example (joints B1-1 to B1-4 depending on the combination of steel plates, the same applies hereinafter), the C amount of the welding wire is excessive, and the C amount of the steel plates 1 and 2 is also excessive. In addition, the amount of C in the backing metal is excessive, and as a result, the X value, which is an index related to the amount of C in the weld metal, is extremely excessive, so that the toughness is extremely inferior in any steel plate thickness combination. Moreover, since the C amount of the steel plates 1 and 2 is excessive, the toughness of the welding heat-affected zone of the steel plate cannot satisfy vE0 ≧ 70J, which is not preferable from the viewpoint of the safety of the entire structure.

  In the joint B2 of the comparative example, since the N amount of all of the welding wires, the steel plates 1, 2 and the backing metal deviates from the present invention, the Y value which is an index related to the N amount of the weld metal is excessive. Of course, although the C amount of each material satisfies the present invention, the X value that is an index related to the C amount of the weld metal is excessive, so the toughness of the weld metal depends on the combination of the steel plate thickness. Regardless, it is generally inferior. Among them, the weld metal toughness under welding conditions with large heat input is inferior.

  The joint B3 of the comparative example relates to the C amount of the weld metal because B is not added to the welding wire, Mn is excessive in both the steel plates 1 and 2, and P is excessive in the backing metal. The X value as an index and the Y value as an index related to the N amount of the weld metal satisfy the present invention, but the toughness of the weld metal is generally inferior regardless of the combination of the steel plate thicknesses.

  In the joint B4 of the comparative example, Ti is not added to the welding wire, P is excessive in the steel plate 1, and S is excessive in both the steel plate 2 and the backing metal. Since the X value, which is an index related to the amount of C, is too large, the toughness of the weld metal is generally inferior regardless of the combination of the steel plate thicknesses. Among them, the weld metal toughness under welding conditions with large heat input is inferior.

  The joint B5 of the comparative example is an example in which the steel plates 1 and 2 and the backing metal satisfy the present invention, but only the welding wire deviates from the present invention and O is excessive. Since the welding wire O is excessive and the X value, which is an index relating to the C amount of the weld metal, is also excessive, the deterioration of the toughness of the weld metal is remarkable especially under the condition that the thickness of the steel plate 2 is large. In addition, since the amount of O of the welding wire is excessive, there is a problem that when wire-drawing from a material to the welding wire, disconnection frequently occurs and cracks are generated, which is not preferable.

  The joint B6 of the comparative example has the same steel plates 1 and 2 and backing metal as the joint B6 and satisfies the present invention, but the Mn amount of the welding wire is excessive, and the index relating to the C amount of the weld metal Since the X value is too large, the toughness of the weld metal is generally poor. Since the welding metal has a high chemical composition with high hardenability because the amount of Mn in the welding wire is large, the weld metal toughness of the steel plate 2 having a smaller thickness and lower heat input is even lower.

  As for the joint B7 of the comparative example, each of the welding wire, the steel plates 1 and 2 and the backing metal satisfies the present invention, but as a result of the combination, X is an index relating to the C amount of the weld metal. This is an example in which the value deviates from the present invention and becomes excessive. Therefore, it is assumed that the toughness of the weld metal is good when the steel plate 2 is thinner, for example, 25 mm. However, the thickness of the steel plate 2 is large and the heat input becomes large. When it is large and the plate thickness of the steel plate 1 is small, the steel plate 1 is greatly deteriorated, and good toughness cannot be achieved without depending on the combination of plate thicknesses.

  In the joint B8 of the comparative example, the chemical composition of the welding wire satisfies the present invention, but the steel plates 1, 2 and the backing metal do not satisfy the present invention. This is an example in which the Y value that is an index is excessively deviating from the present invention. Therefore, it is assumed that the toughness of the weld metal is good when the steel plate 2 is thinner, for example, 25 mm. However, the thickness of the steel plate 2 is large and the heat input becomes large. When it is large and the plate thickness of the steel plate 1 is small, the steel plate 1 is greatly deteriorated, and good toughness cannot be achieved without depending on the combination of plate thicknesses.

  Also from the above examples, according to the present invention, a wide range of diaphragm and skin plate thickness combinations in electroslag welding, specifically, the diaphragm plate thickness is 25 mm to 100 mm, and the plate thickness of the skin plate. Even in various combinations of 20 mm to 100 mm (however, the plate thickness of the skin plate is equal to or less than the plate thickness of the diaphragm), a single welding wire can be used without having to design components that greatly change the chemical composition of the welding wire. In all cases, vE0 ≧ 70 J can be secured stably for the toughness of the weld metal. In particular, the conventional technology cannot achieve such high toughness, the cooling rate of the weld metal after welding becomes slower, the diaphragm thickness is 50 mm or more, and the skin plate thickness is 2/3 or less of the diaphragm thickness. Good toughness of the weld metal can be secured under the combination conditions.

It is sectional drawing which shows the groove shape of the welded joint used for the experiment and Example of FIG. 2, FIG. It is a figure which shows the relationship between the parameter X and weld metal toughness. It is a figure which shows the relationship between the parameter Y and weld metal toughness. It is a figure which shows the sampling procedure of the 2 mmV notch Charpy impact test piece in the experiment of FIG. 2, FIG. 3, and an Example.

Explanation of symbols

1 Steel plate 1 (diaphragm)
2 Steel plate 2 (skin plate)
3 Back metal 4 Groove width 5 Impact test piece 6 Notch

Claims (7)

The end face of the steel plate 1 having a thickness of 50 mm or more is butted against the surface of the steel plate 2 having a thickness of 2/3 or less of the thickness of the steel plate 1 with a gap, and a backing metal disposed on both sides of the gap In the electroslag welding method of forming a groove by the above, and filling the groove space with a flux and feeding a welding wire through the power supply nozzle while pulling up the power supply nozzle in the vertical direction and welding,
As a chemical component of the welding wire,
C: 0.005-0.1%,
Si: 0.01 to 1%,
Mn: 0.1 to 2.5%
P: 0.02% or less,
S: 0.01% or less,
Al: 0.002 to 0.1%,
Ti: 0.002 to 0.3%,
B: 0.0003 to 0.015%,
N: 0.001 to 0.015%, and
O: limited to 0.01% or less,
Using the welding wire with the balance being inevitable impurities and Fe,
As a chemical component of the backing gold,
C: 0.02-0.25%,
Si: 0.01 to 1.5%,
Mn: 0.1 to 2.5%
P: 0.02% or less,
S: 0.01% or less,
N: 0.001 to 0.015%, and
O: limited to 0.01% or less,
Using the backing made of the inevitable impurities and Fe in the balance,
As a chemical component of the steel plate 1 and the steel plate 2 in mass%,
C: 0.02 to 0.2%,
Si: 0.01 to 1%,
Mn: 0.1 to 2.5%
P: 0.02% or less,
S: 0.01% or less,
Al: 0.002 to 0.1%,
N: 0.001 to 0.015%, and
O: limited to 0.01% or less,
Welding steel plate 1 and steel plate 2 with the balance being inevitable impurities and Fe,
Furthermore, X value calculated | required by the following (1) formula based on the mass% of C each contained in the said welding wire, the steel plate 1, the steel plate 2, and a backing metal is 0.09 or less, and the said An electroslag characterized in that the Y value obtained by the following formula (2) based on the welding wire, the steel plate 1, the steel plate 2, and the mass% of N contained in the backing metal is 0.007 or less. Welding method.
X = 0.6 × C _W + 0.2 × (C _S1 + C _S2 ) + 0.1 × C _F (1)
However, the _{C W,} C _{S1, C S2} and shows _{C F} respectively welding wire, the steel plate 1, the steel plate 2, and the mass% of C contained in in the backing metal.
Y = 0.2 × N _W + 0.15 × (N _S1 + N _S2 ) + 0.1 × N _F (2)
However, the _{N W,} N _{S1, N S2} and, _{N F,} each welding wire, the steel plate 1, the steel plate 2, and shows the mass% of N, contained in the backing metal. The welding wire is further in mass%,
Mo: 0.01 to 2.5%,
Cr: 0.01 to 1.5%
W: 0.01 to 1.5%
Cu: 0.01 to 1.5%,
Ni: 0.01-6%,
Nb: 0.002 to 0.1%,
V: 0.002 to 0.5% and
The electroslag welding method according to claim 1, wherein one or more of Ta: 0.002 to 0.5% are contained. The welding wire is further in mass%,
Ca: 0.0002 to 0.01%,
Mg: 0.0002 to 0.01%, and
The electroslag welding method according to claim 1 or 2 , comprising one or more of REM: 0.0002 to 0.01%. The steel plate 1 and the steel plate 2 are further in mass%,
Ti: 0.002 to 0.05%,
B: 0.0003 to 0.015%,
Mo: 0.01 to 1.5%,
Cr: 0.01 to 1.5%
W: 0.01 to 1.5%
Cu: 0.01 to 1.5%,
Ni: 0.01-6%,
Nb: 0.002 to 0.1%,
V: 0.002 to 0.5% and
The electroslag welding method according to any one of claims 1 to 3 , comprising one or more of Ta: 0.002 to 0.5%. The steel plate 1 and the steel plate 2 are further in mass%,
Ca: 0.0002 to 0.01%,
Mg: 0.0002 to 0.01%, and
The electroslag welding method according to any one of claims 1 to 4 , comprising one or more of REM: 0.0002 to 0.01%. The backing money is further in mass%,
Al: 0.002 to 0.1%,
Ti: 0.002 to 0.05%,
B: 0.0003 to 0.015%,
Mo: 0.01 to 1.5%,
Cr: 0.01 to 1.5%
W: 0.01 to 1.5%
Cu: 0.01 to 1.5%,
Ni: 0.01-6%,
Nb: 0.002 to 0.1%,
V: 0.002 to 0.5% and
The electroslag welding method according to any one of claims 1 to 5 , wherein one or more of Ta: 0.002 to 0.5% are contained. The backing money is further in mass%,
Ca: 0.0002 to 0.01%,
Mg: 0.0002 to 0.01%, and
The electroslag welding method according to any one of claims 1 to 6 , comprising one or more of REM: 0.0002 to 0.01%.

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