KR102391652B1 - Surbmerged arc weld joint having excellent low temperature toughness and method of manufacturing the same - Google Patents

Abstract

An embodiment of the present invention provides a submerged arc welded joint having excellent low-temperature toughness and a manufacturing method thereof The submerged arc welded joint comprises: 0.045 to 0.10 wt% of carbon (C); 0.01 to 0.6 wt% of silicon (Si); 1.0 to 2.0 wt% of manganese (Mn); 0.5 to 0.9 wt% of nickel (Ni); 0.2 to 1.0 wt% of molybdenum (Mo); 0.1 to 0.3 wt% of copper (Cu); equal to or less than 0.01 wt% of titanium (Ti); equal to or less than 0.005 wt% of niobium (Nb); equal to or less than 0.02 wt% of vanadium (V); equal to or less than 0.015 wt% of phosphorus (P); equal to or less than 0.015 wt% of sulfur (S); 0.0020 to 0.016 wt% of nitrogen (N); and the remainder consisting of Fe and other inevitable impurities, wherein impact energy at the temperature of -46℃ is equal to or greater than 40 J, and tensile strength is equal to or greater than 550 MPa.

Description

Submerged arc weld joint with excellent low-temperature toughness and manufacturing method thereof

The present invention relates to a submerged arc weld joint having excellent low-temperature toughness and a method for manufacturing the same.

In general, a pipe fitting refers to a pipe material used when a branched pipe from the main pipe is required by changing the pipe direction or changing the pipe diameter. There are mainly Elbow, Tee, Reducer, Cap, etc., and it is mainly used by heat-treating fitting steel. Recently, as fittings have become larger in diameter, high-strength fitting materials are being applied, and in the case of A860-70, tensile strength of 550 MPa or more is required. In the case of large-diameter fittings, they are manufactured by welding rather than seamless, and submerged arc welding is mainly applied. Fittings go through a heat treatment process according to the material applied after welding. For A860-70 (tensile strength≥550 MPa), quenching and tempering heat treatment processes are applied.

On the other hand, the weld joint formed by welding undergoes rapid heating and rapid cooling during the heat treatment process. is deteriorated Therefore, securing the mechanical properties of the weld joint after heat treatment in the fitting structure is emerging as an important technical task.

One aspect of the present invention is to provide a submerged arc welded joint having excellent low-temperature toughness and a method for manufacturing the same.

One embodiment of the present invention is by weight %, carbon (C): 0.045 to 0.10%, silicon (Si): 0.01 to 0.6%, manganese (Mn): 1.0 to 2.0%, nickel (Ni): 0.5 to 0.9% , molybdenum (Mo): 0.2 to 1.0%, copper (Cu): 0.1 to 0.3%, titanium (Ti): 0.01% or less, niobium (Nb): 0.005% or less, vanadium (V): 0.02% or less, phosphorus ( P): 0.015% or less, sulfur (S): 0.015% or less, nitrogen (N): 0.0020 to 0.016%, the remainder including Fe and other unavoidable impurities, the impact energy at -46°C is 40J or more, and tensile strength Provided is a submerged arc welded joint with excellent low-temperature toughness of 550 MPa or more.

Another embodiment of the present invention is by submerged arc welding steel materials with a heat input of 1.0 ~ 6.0 KJ / mm, by weight, carbon (C): 0.045 ~ 0.10%, silicon (Si): 0.01 ~ 0.6%, Manganese (Mn): 1.0 to 2.0%, Nickel (Ni): 0.5 to 0.9%, Molybdenum (Mo): 0.2 to 1.0%, Copper (Cu): 0.1 to 0.3%, Titanium (Ti): 0.01% or less, Niobium (Nb): 0.005% or less, vanadium (V): 0.02% or less, phosphorus (P): 0.015% or less, sulfur (S): 0.015% or less, nitrogen (N): 0.0020 to 0.016%, balance Fe and other unavoidable obtaining a weld joint containing impurities; heating the weld joint to 890 to 930°C; water cooling the heated weld joint; and tempering the water-cooled weld joint at 600 to 670°C.

According to one aspect of the present invention, it is possible to provide a submerged arc weld joint having excellent low-temperature toughness and a method for manufacturing the same.

Hereinafter, a submerged arc welded joint having excellent low-temperature toughness according to an embodiment of the present invention will be described. First, the alloy composition will be described. However, the content of the alloy composition to be described below means wt% unless otherwise specified.

Carbon (C): 0.045-0.10%

The C is an element necessary to secure the strength and hardness of the weld metal, and for the above-described effect, the C is preferably included in an amount of 0.045% or more. However, when the content of C exceeds 0.10%, the strength is increased due to the generation of carbides after heat treatment, and the impact toughness of the weld joint may be greatly reduced. Therefore, the content of C is preferably in the range of 0.045 to 0.10%. The lower limit of the C content is more preferably 0.05%, and even more preferably 0.055%. The upper limit of the C content is more preferably 0.095%, and even more preferably 0.085%.

Silicon (Si): 0.01~0.6%

The Si is added for the deoxidation effect of the weld metal, and when the content of Si is less than 0.01%, the deoxidation effect in the weld metal is insufficient and the fluidity of the weld metal is reduced. On the other hand, when it exceeds 0.6%, there is a problem in that the transformation of martensite (MA constituent) in the weld metal is promoted, and the hardenability is increased, so that the impact toughness is lowered. Accordingly, the Si content is preferably in the range of 0.01 to 0.6%.

Manganese (Mn): 1.0~2.0%

The Mn is an essential element for improving deoxidation, desulfurization, and strength. When the content of Mn is less than 1.0%, there may be a problem in that the grain size of the grains increases after heat treatment (Quenching & Tempering). On the other hand, when it exceeds 2.0%, there is a problem in that the strength is increased and the toughness is lowered. Accordingly, the Mn content is preferably in the range of 1.0 to 2.0%. The upper limit of the Mn content is more preferably 1.9%, even more preferably 1.8%, and most preferably 1.7%.

Nickel (Ni): 0.5-0.9%

The Ni is an element that improves the strength and toughness of a matrix by solid solution strengthening. For the above effect, it is preferable that 0.5% or more is included, but when it exceeds 0.9%, hardenability is greatly increased and an excessive difference in strength with the base material may occur. Accordingly, the Ni content is preferably in the range of 0.5 to 0.9%. The upper limit of the Ni content is more preferably 0.8%.

Molybdenum (Mo): 0.2~1.0%

The Mo is an element that improves the strength of the matrix, and it is preferable to include 0.2% or more in order to obtain this effect, but when it exceeds 1.0%, the effect is not only saturated, but also welding hardenability is greatly increased and martensite is There is a problem of reducing toughness by accelerating transformation. Therefore, the Mo content is preferably in the range of 0.2 to 1.0%.

Copper (Cu): 0.1~0.3%

The Cu is an element advantageous for securing strength and toughness due to the solid solution strengthening effect by being dissolved in the matrix, and for this effect, it is preferable to include 0.1% or more. However, when the content exceeds 0.3%, there is a problem in that the toughness is reduced by increasing the hardenability at the weld joint. Accordingly, the Cu content is preferably in the range of 0.1 to 0.3%.

Titanium (Ti): 0.01% or less

The Ti is generally known as an element that improves strength by forming carbides. However, in the present invention, the Ti is not intentionally added, and the size of the carbide is refined by controlling the content of Ti contained as an impurity to a very small amount, and the toughness is improved by reducing the fraction. To this end, the Ti content is preferably 0.01% or less. The Ti content is more preferably 0.006% or less, even more preferably 0.005% or less, and most preferably 0.003% or less.

Niobium (Nb): 0.005% or less

The Nb is generally known as an element that improves strength by forming carbides. However, in the present invention, the Nb is not intentionally added, and the size of the carbide is refined by suppressing the content of Nb contained as an impurity to a small amount, and its fraction is also reduced to improve toughness. For this, the Nb content is preferably 0.005% or less. The Nb content is more preferably 0.004% or less, even more preferably 0.003% or less, and most preferably 0.002% or less.

Vanadium (V): 0.02% or less

V is generally known as an element that improves strength by forming carbides. However, in the present invention, the V is not intentionally added, and the size of the carbide is refined by suppressing the content of V contained as an impurity to a small amount, and its fraction is also reduced to improve toughness. For this, the content of V is preferably 0.02% or less. The V content is more preferably 0.015% or less, and even more preferably 0.010% or less.

Phosphorus (P): 0.015% or less

The P is an impurity that promotes high-temperature cracking, and it is preferable to manage it as low as possible, and manage it to 0.015% or less to secure low-temperature impact toughness.

Sulfur (S): 0.015% or less

The S is an impurity that promotes high-temperature cracking, and it is preferable to manage it as low as possible, and manage it to 0.015% or less to secure low-temperature impact toughness.

Nitrogen (N): 0.0020 to 0.016%

The N is an element that causes an increase in free nitrogen and increases strength. For the above-described effect, the N content is preferably 0.0020% or more, but when it exceeds 0.016%, dislocations may be fixed and toughness may be reduced. Therefore, the content of N is preferably in the range of 0.0020 to 0.016%. The lower limit of the N content is more preferably 0.0040%, even more preferably 0.0060%, and most preferably 0.0090%.

In addition to the above-described steel composition, the remainder may include Fe and unavoidable impurities. The unavoidable impurities may be unintentionally mixed in a typical steel manufacturing process, and this cannot be entirely excluded, and those skilled in the ordinary steel manufacturing field can easily understand the meaning. In addition, the present invention does not entirely exclude the addition of compositions other than the above-mentioned steel composition.

On the other hand, it is preferable that the alloy composition of the above-described weld joint has Ceq defined by the following formula 1 of 0.29 to 0.72. If the Ceq is less than 0.29, the strength of the weld joint compared to the base material may be too low, whereas, if it exceeds 0.72, low-temperature impact toughness may be reduced. Accordingly, the Ceq preferably has a range of 0.29 to 0.72.

[Equation 1] Ceq = C + Mn/6 + (Ni+Cu)/15 + (Cr+Mo+V)/5

(However, in [Equation 1], C, Mn, Ni, Cu, Cr, Mo, and V mean the content (wt%) of each element.)

The weld joint of the present invention provided as described above is characterized by having excellent low-temperature toughness through QT heat treatment, and the impact energy at -46°C is 40J or more, and the tensile strength may be 550MPa or more.

Hereinafter, a method for manufacturing a submerged arc weld joint having excellent low-temperature toughness according to an embodiment of the present invention will be described.

First, the steel is prepared. In the present invention, the alloy composition or manufacturing conditions of the steel are not particularly limited, and all kinds of steels commonly used in the art may be used.

Then, the prepared steel material is submerged arc welding with a heat input of 1.0 ~ 6.0KJ / mm to obtain a weld joint having the above-described alloy composition. If the amount of heat input during welding is less than 1.0KJ/mm, welding itself may not be possible, and if it exceeds 6.0KJ/mm, the grains of the weld joint are coarsened, thereby impairing strength and impact toughness. Therefore, the amount of heat input preferably has a range of 1.0 ~ 6.0KJ / mm. As for the upper limit of the said heat input amount, it is more preferable that it is 5.0 KJ/mm.

Then, the welding joint is heated to 890 ~ 930 ℃. When the heating temperature is less than 890°C, it is difficult to secure a fully austenite structure, and when it exceeds 930°C, it is difficult to secure strength and toughness after heat treatment due to coarsening of the austenite grain size. Therefore, the heating temperature is preferably in the range of 890 to 930 ℃.

Then, after cooling the heated weld joint with water, it is tempered at 600 ~ 670 ℃. When the tempering temperature is less than 600 ℃, recovery of dislocations generated after water cooling does not occur sufficiently, and when it exceeds 670 ℃, the effective grain size increases due to lath bonding of martensite and bainite, and the strength is increased by coarsening of carbides. There is a problem with falling. Therefore, the tempering temperature is preferably in the range of 600 ~ 670 ℃.

Hereinafter, the present invention will be described in more detail through examples. However, the following examples are only examples for explaining the present invention in more detail, and do not limit the scope of the present invention.

(Example)

After butt-welding the steel with a heat input of 3.0 kJ/mm using submerged welding, heating to the conditions of Table 2 below, cooling with water, and tempering to prepare a weld joint having an alloy composition shown in Table 1 below. At this time, the holding time after heating was 30 minutes. After measuring the tensile strength and low-temperature toughness of the welded joint thus prepared, it is shown in Table 2 below.

Tensile strength was measured by manufacturing a specimen according to ASTM E08 and then performing a tensile test.

Low-temperature toughness was measured at -46°C using a Charpy impact tester.

Kang type No. Alloy composition (wt%) C Si Mn Ni Mo Cu Ti Nb V P S N Ceq Invention lecture 1 0.055 0.288 1.43 0.78 0.242 0.175 0.004 0.004 0.009 0.01 0.002 0.007 0.41 Invention lecture 2 0.066 0.279 1.43 0.82 0.229 0.241 0.004 0.004 0.01 0.013 0.002 0.007 0.42 Invention lecture 3 0.062 0.268 1.42 0.81 0.227 0.239 0.003 0.004 0.009 0.013 0.002 0.005 0.42 Invention lecture 4 0.06 0.025 1.1 0.52 0.8 0.12 0.004 0.002 0.009 0.014 0.002 0.007 0.45 Comparative lecture 1 0.058 0.276 1.45 0.81 0.227 0.24 0.004 0.005 0.01 0.018 0.004 0.005 0.42 Comparative lecture 2 0.062 0.303 1.2 0.81 0.225 0.24 0.004 0.004 0.01 0.017 0.015 0.008 0.38 Comparative lecture 3 0.055 0.288 1.42 0.79 0.228 0.213 0.004 0.004 0.01 0.017 0.002 0.008 0.41 Comparative lecture 4 0.06 0.345 1.5 0.8 0.23 0.215 0.004 0.005 0.01 0.019 0.002 0.007 0.43 Comparative steel 5 0.06 0.289 1.41 0.8 0.229 0.219 0.003 0.005 0.009 0.016 0.002 0.008 0.41 Comparative lecture 6 0.055 0.288 1.43 0.78 0.242 0.175 0.015 0.008 0.01 0.01 0.002 0.008 0.41 Comparative lecture 7 0.03 0.225 1.1 0.3 0.12 0.12 0.008 0.1 0.02 0.014 0.002 0.007 0.27 Comparative steel 8 0.042 0.261 1.41 0.67 0.11 0.21 0.005 0.01 0.04 0.013 0.005 0.008 0.37

division Kang type No. Heating temperature (℃) Tempering temperature (℃) Tensile strength (MPa) Impact energy (J, @-46℃) Invention Example 1 Invention lecture 1 890 670 675 121 Invention example 2 Invention lecture 2 910 620 710 83 Invention example 3 Invention lecture 3 910 620 702 62 Invention example 4 Invention lecture 4 890 670 620 72 Comparative Example 1 Invention lecture 1 940 610 530 50 Comparative Example 2 Invention lecture 1 880 650 524 46 Comparative Example 3 Invention lecture 1 860 650 510 52 Comparative Example 4 Invention lecture 1 910 590 685 32 Comparative Example 5 Comparative lecture 1 910 595 670 35 Comparative Example 6 Comparative lecture 2 910 620 610 25 Comparative Example 7 Comparative lecture 3 910 620 715 16 Comparative Example 8 Comparative lecture 4 910 620 735 26 Comparative Example 9 Comparative Steel 5 910 620 701 32 Comparative Example 10 Comparative lecture 6 910 620 692 39 Comparative Example 11 Comparative lecture 7 910 620 701 37 Comparative Example 12 Comparative steel 8 910 620 706 32

As can be seen from Tables 1 and 2, in the case of Inventive Examples 1 to 4 that satisfy the alloy composition and manufacturing conditions proposed by the present invention, the impact energy at -46°C is 40J or more, and the tensile strength is 550MPa or more. , it can be seen that excellent strength and low-temperature toughness are secured.

On the other hand, in the case of Comparative Examples 1 to 12, which do not satisfy the alloy composition or manufacturing conditions proposed by the present invention, it can be seen that the strength or low-temperature toughness is at a low level.

Claims (4)

By weight%, carbon (C): 0.045 to 0.10%, silicon (Si): 0.01 to 0.6%, manganese (Mn): 1.0 to 2.0%, nickel (Ni): 0.5 to 0.9%, molybdenum (Mo): 0.2 ~1.0%, copper (Cu): 0.1 to 0.3%, titanium (Ti): 0.01% or less, niobium (Nb): 0.005% or less, vanadium (V): 0.02% or less, phosphorus (P): 0.015% or less, Sulfur (S): 0.015% or less, nitrogen (N): 0.0020 to 0.016%, the remainder including Fe and other unavoidable impurities,
A submerged arc welded joint with excellent low-temperature toughness with an impact energy of 40J or more at -46℃ and a tensile strength of 550MPa or more.
The method according to claim 1,
The weld joint is a submerged arc weld joint having excellent low-temperature toughness having a Ceq of 0.29 to 0.72, which is defined by Equation 1 below.
[Equation 1] Ceq = C + Mn/6 + (Ni+Cu)/15 + (Cr+Mo+V)/5
(However, in [Equation 1], C, Mn, Ni, Cu, Cr, Mo, and V mean the content (wt%) of each element.)
Submerged arc welding of steel material with heat input of 1.0~6.0KJ/mm, by weight%, carbon (C): 0.045~0.10%, silicon (Si): 0.01~0.6%, manganese (Mn): 1.0~ 2.0%, Nickel (Ni): 0.5 to 0.9%, Molybdenum (Mo): 0.2 to 1.0%, Copper (Cu): 0.1 to 0.3%, Titanium (Ti): 0.01% or less, Niobium (Nb): 0.005% or less , Vanadium (V): 0.02% or less, Phosphorus (P): 0.015% or less, Sulfur (S): 0.015% or less, Nitrogen (N): 0.0020~0.016%, balance Welded joints containing Fe and other unavoidable impurities to obtain;
heating the weld joint to 890 to 930°C; and
A method of manufacturing a submerged arc weld joint having excellent low-temperature toughness, comprising: cooling the heated weld joint with water, and then tempering at 600 to 670°C.
4. The method according to claim 3,
The welding joint is a method of manufacturing a submerged arc welded joint having excellent low-temperature toughness having a Ceq of 0.29 to 0.72, which is defined by Equation 1 below.
[Equation 1] Ceq = C + Mn/6 + (Ni+Cu)/15 + (Cr+Mo+V)/5
(However, in [Equation 1], C, Mn, Ni, Cu, Cr, Mo, and V mean the content (wt%) of each element.)