JPS63126683A - Welding method for steel having excellent toughness of weld metal - Google Patents

Abstract

PURPOSE:To improve the low temp. toughness of a weld metal by using a steel contg. specific components in the case of executing high-energy density welding with an electron beam or the like. CONSTITUTION:The basic components are composed, by weight %, of 0.01-0.20% C, <=0.85 Si, 0.30-2.0% Mn, <=0.025% P, <=0.020% S, <=0.007% Al, and <=0.010% O. The steel further contains 1 or >=22 kinds among <=4.5% Ni, <=1% Cr, <=0.5% Mo, <=0.10% Nb, eta0.10% V, <=1.5% Cu, and <=0.002% B, and contains the balance iron and unavoidable impurity elements. The high- energy density welding is executed by using such steel. The weld metal having the excellent toughness is thereby obtd.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to a welding method using a high energy density heat source such as an electron beam or a laser, and particularly relates to a welding method for obtaining good toughness in steel weld metal. It is something.

(Prior art) High energy density welding is a highly efficient, low heat input welding method that can achieve deep penetration with a narrow bead width compared to conventional arc welding. It is being applied to welding large steel structures such as ships and pressure vessels. In this high-energy-density welding method, in which melting and solidification occur extremely rapidly, gas components in the molten metal (carbon monoxide gas generated by the reaction between oxygen and carbon contained in the weld metal) are released. In particular, there are extremely strict restrictions on the oxygen content allowed in steel materials to be welded, and the value depends on the plate thickness of the joining parts. Although it varies depending on the situation, it is generally about 1) 00 pp or less. This oxygen content is about 100 to 400 ppm, which is required to obtain a microstructure containing a large amount of fine acicular ferrite that exhibits good toughness in weld metal by submerged arc welding, M A G welding, and covered electrode welding. much less than the oxygen content of The reason why this level of oxygen content is necessary for the refinement of the Mi weave is that the numerous nonmetallic inclusions that finely precipitate during the transformation from austenite to ferrite during the cooling process of welding serve as nuclei for the transformation. However, in high energy density welding, the number of nonmetallic inclusions is insufficient and a fine structure cannot be obtained. Therefore, for example, welding academic journal, Vol. 54, 2, published in March 1985,
As can be seen on pages 105 and 106 of the issue, the toughness of the weld metal in the as-welded state is not necessarily good, which poses a serious problem when applying the progeny energy density welding method.

Therefore, methods for improving the toughness of weld metal have been studied.

One option is post-weld heat treatment, but the additional steps can offset the benefits of high-energy density welding. In addition, in Japanese Patent Publication No. 56-50793, in order to improve the toughness of the as-welded state, 100 to 300 ppm was added to the part of the steel material that will be melted during electron beam welding in advance.
We propose a method of supplying a substance equivalent to the components of low-alloy steel obtained by arc welding or slag welding, which contains an amount of oxygen.

As a result, the microstructure of the weld metal becomes a structure consisting mainly of fine acicular ferrite, which improves toughness, but as mentioned earlier, if there is excess oxygen in the weld metal, welding defects are likely to occur. Therefore, it is thought that there are technical problems in actual welding work.

(Problems to be Solved by the Invention) In view of the above, the present invention aims to provide a welding method that dramatically improves the low-temperature toughness of weld metal obtained by high-energy density welding means in an as-welded state. do.

(Means for Solving the Problems) Based on the above-mentioned current situation, some of the present inventors have proposed that the weld metal will not melt or become coarse even at high temperatures during electron beam welding. By using steel that contains fine nonmetallic inclusions evenly dispersed, we aim to refine the structure by introducing these fine nonmetallic inclusions into the weld metal as transformation nuclei of acicular ferrite. Patent application was made in 1986 for technology that improves the toughness of welded metal even in large quantities.
No. 182982.

However, as a result of a detailed study by the present inventors on the transformation behavior of weld metal by high-energy density welding, we found that 5o1) contained in weld metal. When the amount of Al is very small, the number of nonmetallic inclusions in the weld metal is far smaller than the number of nonmetallic inclusions in the weld metal that was conventionally thought to be necessary to obtain a Mi texture consisting mainly of fine acicular ferrite, i.e. It was found that the same microstructure could be obtained even with the number of non-metallic inclusions that inevitably exist in weld metal produced by high-energy density welding.

Therefore, based on the above study results, the inventors of the present invention intentionally introduced non-metallic inclusions in order to adjust the content of 5oj2゜Al in the weld metal to an appropriate range in the high energy density welding method. The present invention was based on the conclusion that it is possible to refine the microstructure of a weld metal with a low oxygen content and obtain a weld metal with excellent toughness in the as-welded state without the above-mentioned conditions.

That is, the gist of the present invention is that C: 0.01 to 0.01% by weight.
20%, Si: 0.8% or less, Mn: 0.30-2.
0%, P: 0.025% or less, S: 0.020% or less, A6: 0.007% or less, O: 0.010% or less, or further Ni: 4.5% or less , Cr: 1% or less, Mo: 0.5% or less, Nb:
0.10% or less, V2 0.10% or less, Cu: 1.
5% or less, B: 0.002% or less, and the toughness of weld metal characterized by high energy density welding using steel containing iron and unavoidable impurity elements. There is an excellent method of welding steel.

The present invention will be explained in detail below.

(Function) First, the high energy density welding method referred to in the present invention is one that has a higher degree of energy concentration in its heat source than conventional arc welding methods such as electron beam welding and laser welding. Also sol.

8β is the amount of β expressed by the following formula.

Soj! , AA = (Total AI2 amount) - (insoC8l) - (Hej2 amount as A IIN) Next, the reason why the components of the steel targeted by the present invention are limited as described above is as follows. .

First, C is added as an effective component to improve the strength of weld metal, but excessive addition of more than 0.2% deteriorates toughness and makes weld cracking more likely, so the upper limit is set at 0.20%. %. Furthermore, if the content is lower than 0.01%, it becomes difficult to secure the necessary strength as a weld metal, so the lower limit was set at 0601%.

Si is added mainly to ensure strength, but 0.8
Excessive addition of more than 1% will reduce weldability and toughness, so the upper limit was set at 0.8%.

Furthermore, although Mn is important for ensuring the strength and toughness of weld metal, adding less than 0.30% does not have sufficient effects, and adding more than 2.0% has a negative effect on toughness. Therefore, it was set in the range of 0.30 to 2.0%.

On the other hand, since P and S cause weld cracking especially in high energy density welding, the upper limit of each is set to 0.
0.025% and 0.020%.

Next, 81 was set to 0.007% or less for the following reason. In other words, when ferrite is generated from austenite during the cooling process after steel is welded, the presence of nonmetallic inclusions in the weld metal, like austenite grain boundaries, is energetically advantageous for nucleation of ferrite. These are the places where metamorphosis begins.

A1 dissolved in the weld metal acts as a repellent between C and 5o1). When All exists in excess of 0.005%, the exclusion of C from ferrite undergoing transformation to untransformed austenite is promoted, and the concentration of C in austenite is more marked than when sol and A1 is small. Become. As a result, the transformation temperature of untransformed austenite decreases, fine needle-like ferrite becomes unable to transform, and bainite with a lower transformation temperature is produced.

Therefore, in order to obtain a microstructure consisting mainly of fine acicular ferrite when SOβ and A1 are present, a large number of non-metallic inclusions must exist in the weld metal, and the non-metallic inclusions must contain carbon atoms in the untransformed austenite. It is essential that ferrite nucleation begins almost simultaneously before the concentration of ferrite occurs and ferrite nucleation becomes difficult. Conventionally, the number of nonmetallic inclusions corresponding to an oxygen content of 100 to 400 ppm has been required in the weld metal in order to obtain a fine structure for the reasons described above. On the other hand, when sol#1 is 0.005% or less, the concentration of C in untransformed austenite is not significant even after ferrite nucleation from nonmetallic inclusions in the weld metal, and In the process, nucleation of ferrite begins from untransformed austenite that has been sufficiently cooled to an extent other than nonmetallic inclusions. As a result, even a small amount of non-metallic inclusions that are unavoidably present in weld metal produced by high-energy density welding is sufficient to cause ferrite nucleation and form a microstructure consisting mainly of fine acicular ferrite. be.

Sol of plow weld metal based on the above completely new knowledge.

In order to keep A1 to 0.005% or less, in the present invention, the AAI contained in the steel used for high energy density welding is reduced to 0.0%.
07% or less. The reason why the limit on the amount of Al contained in the steel is set to the total amount of Al rather than the amount of sol or Al is as follows. In other words, Al often exists in steel as an oxide or a nitride, but after both are decomposed at high temperatures during high-energy density welding, the oxide is decomposed due to the reduction of oxygen that occurs during welding. Furthermore, due to the rapid cooling rate during general high-energy density welding, not all of the A1 becomes oxides and nitrides, and most of the nitrides are sob! , Aj2 will remain in the weld metal.

Next, as explained above, since O is an element that leads to the occurrence of weld defects such as weld cracks and blowholes in high energy density welding methods, its upper limit was set as o', oio%.

The above are the basic components of the steel used in the present invention, but in addition to these, Ni: 4.5% or less, Cr: 1% or less, Mo
: 0.5% or less, Nb: 0.10% or less, V: 0
．． 10% or less, Cu: 1.5% or less, B: 0.
Even if the steel contains one or more of the following: 0.002% or less, the effects obtained by lowering the soj and Aj2 of the weld metal are effective.

First, Ni is an element that increases the strength and toughness of weld metal at the same time, but its effect is saturated at concentrations exceeding 4.5%.
Furthermore, since the strength may become excessive, the upper limit was set at 4.5%.

Next, Cr is an element that increases the hardenability of weld metal, but if the concentration exceeds 1%, the strength will be excessive, so the upper limit should be set to 1%.
%.

Like Cr, Mo is an element that improves hardenability, but
If it exceeds 0.5%, the strength of the weld metal becomes excessive, so the upper limit was set at 0.5%.

Nb and V are added to improve the strength and toughness of steel materials by increasing hardenability and forming carbides, but if they exceed 0.1% in weld metal, hardenability becomes excessive and toughness decreases. Therefore, the upper limit is 0.1 for each
%.

Cu, like Nj, is an element that increases the strength and toughness of the weld metal, but if it exceeds 1.5%, the strength becomes excessive, so the upper limit was set at 1.5%.

B segregates at austenite grain boundaries in the weld metal and contributes to improving toughness by suppressing the precipitation of grain boundary ferrite, but excessive addition of more than 0.002% conversely deteriorates toughness and The upper limit was set at 0.002%.

Next, in the present invention, a method for obtaining a weld metal with excellent toughness in a high energy density welding method using the above-mentioned steel will be described.

First, the present invention can be achieved by using a steel material having the chemical composition described in the present invention as a joining member, performing I-type opening processing on the end face, and performing butt welding by high energy density welding. However, Mn causes loss due to massive explosion during high energy density welding, so
It should be taken into consideration that the value in the weld metal is lower than the content in the steel material. Moreover, in this method, the weld metal has a low soj! Of course, it is essential to use a steel material with a low AN content that satisfies H2 conversion, that is, an N material obtained by vacuum deoxidation, Si-Mn deoxidation, etc., as a joining member. According to the publication, ordinary steel materials with relatively high ^β deoxidation can also be used as joining members.

In other words, even in a joining member with a high Al content, welding can be carried out by supplying an insert member made of a steel material with a low Al content, which is the object of the present invention, and a welding wire, and diluting the amount of Al in the weld metal. 301 of metal. It is possible to achieve the present invention by reducing the amount of Al. For example, a method of fixing an insert member made of a steel material with a low AN content between joining members subjected to I-type opening processing and melting this insert member together with both joining members, and a welding wire made of a steel material with a low AN content. The weld metal has a low SO6. It becomes possible to convert into A/. It is also possible to use a steel material with a low A6 content for one of the joining members.

Of course, it goes without saying that the chemical compositions of the steel materials, insertion members, and welding wires used in these cases, other than the AI salt, must also fall within the scope of the present invention.

Furthermore, although the main purpose of the present invention is to obtain a weld metal with high toughness in an as-welded state in a high energy density welding method, the weld metal also has good toughness even when heat treatment is required after welding. Needless to say, it shows toughness.

Next, the effects of the present invention will be described in more detail with reference to Examples.

(Example 1) Table 1 is a table showing the chemical composition of the prototype steel, and the plate thickness is 501 mm.
We have produced prototypes of steel ranging from 40 kg to 80 kg. These steel plates were subjected to I-bevel processing, and butt welding was performed on pairs of steels of the same type without a root gap using the electron beam welding conditions shown in Table 2. The a/b value shown in the same table means the ratio of the distance from the center of the convergent lens to the surface of the non-weld object to the distance from the center of the electron beam convergent lens to the focal position of the electron beam, and the "Between" means the direction of welding progress and the direction perpendicular thereto, respectively. Table 3 shows the chemical composition of each weld metal part. A Charpy impact test piece was taken from the center of the plate thickness from each welded part in the as-welded state as shown in Figure 1, and the impact test was conducted in the range of -80 to 0°C with the number of repetitions at the same temperature being 3. Ta. The results are also listed in Table 3.

Numbers 2 and 2 of the present invention in Table 3 are examples of electron beam welding metal using 40 kg class steel. Soj! .

Since the amount of AAO is kept sufficiently low, the microstructure of the weld metal is mainly composed of fine acicular ferrite, and exhibits an extremely excellent toughness value.

Example 3 of the present invention is an example of electron beam welding metal using a steel material containing A1 at the upper limit of the range of the present invention.

5oj7. Because the amount of Al is high, the microstructure is a mixture of fine acicular ferrite and some upper bainite, and the toughness value is somewhat lower than that of Invention Examples 1 and 2. The toughness value of the weld metal still shows a sufficiently good value as long as the amount generated is low.

On the other hand, Comparative Examples 1) and 12 have almost the same chemical composition as the weld metals of Inventive Examples 1 and 2, respectively, but because they used steel containing a large amount of Aj2 beyond the scope of the present invention, welding was difficult. As a result of excessive amounts of sol and A1 contained in the metal, the microstructure differs greatly from the microstructures of Examples 1 and 2 of the present invention, resulting in a structure consisting mainly of upper bainite, which has not been observed in electron beam welded metal parts in the past. This shows extremely low toughness values.

Examples 4 and 5 of the present invention are examples of electron beam welded metals of 40 kg class steel containing Ni and Nb in the basic component system, respectively. The amount of steel used is appropriate and their toughness is comparable to that of Inventive Examples 1 and 2.

Example 6 of the present invention is an example of electron beam welding metal using 50 kg class steel. SOβ and AN of the weld metal are within appropriate ranges and exhibit good toughness values.

Invention Example 7 is an example of weld metal of 50kg steel containing Nt+-V. so7! ,//! The amount of is low, and exhibits an excellent toughness value equivalent to that of Inventive Example 1. On the other hand, in Comparative Example 13, although the chemical composition of the weld metal is almost the same as in Inventive Example 7, the amount of AfO is much higher than the range of the present invention, so that fine acicular ferrite is formed. It becomes a coarse bainite structure, which is completely different from the main microstructure of Invention Example 7, and its toughness value is very low.

Examples 8 to 10 of the present invention each have Nb − in the basic component system.
60 kg class steel containing V-Cu, Ni-Cr-Mo,
8 containing N+-Cr-Mo-Nb-V-Cu-B
This is an example of electron beam welding metal using 0 kg class steel. The amount of Al in the steel used was appropriate, and the so# and Al of the weld metal were
As a result, the micro-Mi texture is mainly composed of fine needle-like ferrite and exhibits an excellent toughness value.

(Example 2) Among the prototype steels with the chemical compositions shown in Table 1, steel materials 1 and 6 used in Invention Example 1 and Comparative Example 1 in Table 3 were selected, and the end faces were subjected to I-type opening processing. , Butt welding of pairs of dissimilar steel materials without a root gap was performed under the electron beam welding conditions shown in Table 2. The incident position of the electron beam was set at the butt position. Charpy impact test pieces were taken from the as-welded welds in the same manner as described in Example 1, and the Charpy impact test was conducted at the same temperature with a repetition rate of 3. Table 4 shows the obtained toughness values and the chemical composition of the weld metal together with those of Comparative Example 1) shown in Table 3.

In the example of the present invention in which steel materials 1 and 6 are butt welded, the amount of Al contained in steel material 6 is diluted by steel material 1 with a low Aρ content, so that the so l of the weld metal,
The A j2 content was reduced, and unlike the microstructure of the comparative example, which consisted mainly of upper bainite, the microstructure changed to a structure mainly composed of fine acicular ferrite, although some bainite remained. Therefore, its toughness value shows good values even at -40°C.

(Example 3) Among the prototype steels with the chemical compositions shown in Table 1, steel material 2 used in Invention Example 2 in Table 3 was processed to prototype an insert member with a plate thickness of 3. As shown in FIG. 2, this insert member was inserted between the I-shaped openings of the steel material 7 used in Comparative Example 12 in Table 3, and butt welding was performed to melt the insert member together with the steel material 7. The welding conditions were the electron beam welding conditions shown in Table 2. A Charpy impact test piece was taken from the as-welded weld in the same manner as shown in Example 1, and the Charpy impact test was performed at the same temperature with three repetitions. Table 5 shows the obtained toughness values and chemical compositions of the weld metals. At the same time, the toughness value and chemical composition of the weld metal of Comparative Example 12 shown in Table 3 are also listed.

In the example of the present invention in which the steel material 2 is inserted into the butt weld of the steel material 7 as an insert member, the Al contained in the steel material 7 in large amounts is diluted by the melting of the insert member with a low Al content, and as a result, 5 ojl of the weld metal ! .

The amount of Fβ decreased, and the microstructure changed to a structure mainly composed of fine acicular ferrite, which is significantly different from the microstructure of the comparative example, which was mainly composed of upper bainite. Due to this improvement in the structure of the weld metal, the toughness value in the example of the present invention is significantly improved.

(Example 4) Among the prototype steels with the chemical compositions shown in Table 1, steel material 1 used in Invention Example 1 in Table 3 was processed to produce a 1.2 mm diameter welding wire with a low Al content. was produced. Also,
Steel material 6 used in Comparative Example 1) in Table 3 was 12.
After the thickness was reduced to 7+n, the end face was subjected to I-type opening processing to prepare a butt weld with a root gap of 0.5 am. Laser welding was performed while feeding the welding wire to this butt weld. Laser welding conditions are: laser output 14kW, welding speed 750mm/min
, focal length 200 Tsuru, a/b (lI!O,g,
Wire feeding speed 5m/min.

i1e gas flow m 151 /min. At the same time, butt welding without wire feeding and without root gap was performed under the laser welding conditions described above using steel material 6 which had been subjected to I-type opening processing after friction reduction to 12.7 mm as a comparative example.

A Charpy impact test piece was taken from the as-welded weld in the same manner as shown in Example 1, and the Charpy impact test was performed at the same temperature with three repetitions. Table 6 shows the obtained toughness values and chemical compositions of the weld metals.

In the example of the present invention in which a wire with a low Al content was fed during laser welding, the 8β contained in the steel material 6 was diluted and the weld metal sol, Al2) was reduced, resulting in a good toughness value. shows. On the other hand, in a comparative example in which no wire was fed during laser welding, the s
oj? , All is high, the microstructure is an upper bainite structure containing some martinsite, and the toughness is very low.

(Effects of the Invention) As is clear from the above embodiments, according to the present invention, it is possible to significantly improve the toughness of high energy density weld metal, which has been a problem in the past, and to improve the toughness of high energy density welding metals such as electron beam welding. It is possible to obtain weld metal with extremely excellent toughness, and it is suitable for pressure vessels, offshore structures,
The industrial effects are extremely significant in the field of welding large structures such as line pipes.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the procedure for collecting Charpy impact test pieces from welded parts. FIG. 2 is a diagram showing a third embodiment of a welding method in which an insertion member is inserted between two joining members. 1: Weld metal, 2: Charpy impact test piece, 3: Notch position, 4.5: Joining member, 6: Insert member, 7: Outline of weld metal. Figure 1 Figure 2

Claims (2)

[Claims] (1) C: 0.01-0.20%, Si: 0.0% by weight.
8% or less, Mn: 0.30-2.0%, P: 0.025
% or less, S: 0.020% or less, Al: 0.007% or less, O: 0.010% or less as basic components, and high energy density welding is performed using steel containing iron and inevitable impurity elements. A method of welding steel with excellent toughness of the weld metal. (2) C: 0.01-0.20%, Si: 0.
8% or less, Mn: 0.30-2.0%, P: 0.025
% or less, S: 0.020% or less, Al: 0.007% or less, O: 0.010% or less, and further Ni
: 4.5% or less, Cr: 1% or less, Mo: 0.5% or less, Nb: 0.10% or less, V: 0.10% or less, Cu:
A weld metal characterized by performing high energy density welding using steel containing one or more of B: 1.5% or less, B: 0.002% or less, and the balance containing iron and unavoidable impurity elements. A method of welding steel with excellent toughness.