KR102045647B1 - Welded joint having exceleent low temperature impact toughness, and method for manufacturing the same - Google Patents

Abstract

The present invention relates to a welded joint having excellent low-temperature impact toughness that can be used in an offshore plant, a land plant, and the like and a method of manufacturing the same.

Description

WELDED JOINT HAVING EXCELEENT LOW TEMPERATURE IMPACT TOUGHNESS, AND METHOD FOR MANUFACTURING THE SAME}

The present invention relates to a welded joint having excellent low-temperature impact toughness that can be used in an offshore plant, a land plant, and the like and a method of manufacturing the same.

Parts such as power plants, shipyards and chemical plants are manufactured using high strength, high toughness steel. In order to manufacture the part, it can be manufactured in a seamless manner without a welding process, but most are manufactured by welding. One preferred example is butt-welding.

Parts, structures, etc. manufactured through the welding need to secure the impact toughness of the welded joint. The welded joint undergoes rapid quenching and quenching due to the welding heat source. At this time, the metal histologically causes the phase transformation to coarse grains or weak tissues, which inevitably degrades the quality characteristics of the base metal.

Therefore, in securing low temperature toughness of a part or structure, a weld joint is always the core, and securing a low temperature toughness of a weld joint has emerged as an important technical problem.

One aspect of the present invention is to provide a welded joint having excellent low-temperature impact toughness and a method of manufacturing the same by optimizing the composition and microstructure of the welded joint.

The subject of this invention is not limited to the above-mentioned matter. Further objects of the present invention are described in the general description, and those skilled in the art will have no difficulty understanding the additional objects of the present invention from the contents described in the specification of the present invention.

One aspect of the present invention is, by weight%, carbon (C): 0.04-0.12%, silicon (Si): 0.3-0.5%, manganese (Mn): 1.3-2.0%, nickel (Ni): 1.0-2.5% , Molybdenum (Mo): 0.1-0.5%, Copper (Cu): 0.3-0.5%, Titanium (Ti): 0.01-0.1%, Niobium (Nb): 0.02-0.04%, Vanadium (V): 0.04-0.06% , Phosphorus (P): 0.02% or less (excluding O), sulfur (S): 0.01% or less (excluding O), oxygen (O): 0.025 to 0.040%, nitrogen (N): 0.005 to 0.008%, boron (B): Provide an electrogas welded joint having excellent low temperature impact toughness containing 0.0025% to 0.004%, remainder Fe and unavoidable impurities.

Another embodiment of the present invention is carried out by electrogas welding in weight%, weight%, carbon (C): 0.04 ~ 0.12%, silicon (Si): 0.05 ~ 0.5%, manganese (Mn): 1.3 ~ 2.0% , Nickel (Ni): 1.0-2.5%, molybdenum (Mo): 0.1-0.5%, copper (Cu): 0.3-0.5%, titanium (Ti): 0.01-0.1%, niobium (Nb): 0.01-0.04% , Vanadium (V): 0.01-0.06%, Phosphorus (P): 0.02% or less, Sulfur (S): 0.01% or less, Oxygen (O): 0.025-0.060%, Nitrogen (N): 0.005-0.008%, Boron (B): manufacturing a welded joint including 0.0015% to 0.004%, balance Fe and inevitable impurities;

Heating the weld joint;

Quenching the heated welded joint; And

It relates to a method of manufacturing a welded joint having excellent low temperature impact toughness comprising the step of heat-treating the cooled welded joint.

According to the present invention, it is possible to provide a welded joint having excellent low-temperature impact toughness and a method of manufacturing the same.

1 is a cross-sectional photograph of a welded joint manufactured by one embodiment of the present invention.
Figure 2 is an optical microscope microstructure photograph of a welded joint prepared in one embodiment of the present invention.
Figure 3 is an optical microscope microstructure photograph of a welded joint prepared in another embodiment of the present invention.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

First, the composition of the welded joint will be described in detail. The welding joint of the present invention is a weight%, carbon (C): 0.04 to 0.12%, silicon (Si): 0.05 to 0.5%, manganese (Mn): 1.3 to 2.0%, nickel (Ni): 1.0 to 2.5% , Molybdenum (Mo): 0.1-0.5%, Copper (Cu): 0.3-0.5%, Titanium (Ti): 0.01-0.1%, Niobium (Nb): 0.01-0.04%, Vanadium (V): 0.01-0.06% , Phosphorus (P): 0.02% or less (excluding O), sulfur (S): 0.01% or less (excluding O), oxygen (O): 0.025 to 0.060%, nitrogen (N): 0.005 to 0.008%, boron (B): 0.0015 to 0.004%, the remainder containing Fe and inevitable impurities.

Carbon (C): 0.04-0.12 wt% (hereinafter%)

C is an element necessary to secure the strength and hardness of the weld metal, and for this purpose, C is preferably included at least 0.04%. However, if the content of C exceeds 0.12%, there is a problem in that the crack susceptibility during welding increases and the impact toughness of the welded joint is greatly reduced due to the generation of carbide, low temperature phase, etc., preferably not more than 0.12%.

Silicon (Si): 0.05-0.5%

The Si is added for the deoxidation effect of the weld metal, if the content is less than 0.05%, the deoxidation effect in the weld metal is insufficient and the fluidity of the weld metal is lowered. On the other hand, if the content exceeds 0.5%, there is a problem that promotes the transformation of the phase martensite in the weld metal, and increases the hardenability, thereby lowering the impact toughness. Therefore, the upper limit is preferably not more than 0.5%.

Manganese (Mn): 1.3-2.0%

The Mn is an essential element for improving deoxidation and strength, and precipitates in the form of MnS around the TiO oxide to promote the formation of acicular ferrite, which is advantageous for improving the toughness of the Ti composite oxide. In addition, Mn forms a substituted solid solution in the matrix structure to strengthen the matrix by solid solution to secure strength and toughness, Mn is preferably included at least 1.3% for this effect. In addition, when the Mn is less than 1.3%, there is a problem that the effective ferrite grain size increases after the heat treatment (Quenching & Tempering). However, if it exceeds 2.0%, there is a problem that the toughness is lowered by creating a low-temperature transformation tissue, it is preferable not to exceed 2.0%.

Nickel (Ni): 1.0 ~ 2.5%

Ni is an element that improves the strength and toughness of the matrix by solid solution strengthening. It is necessary to include 1.0% or more for the above effects, but when excessively exceeding 2.5%, the hardenability may be greatly increased or hot cracking may occur, which is not preferable.

Molybdenum (Mo): 0.1 ~ 0.5%

The Mo is an element that improves the known strength, and in order to obtain such an effect, it is preferable to include 0.1% or more, but when it exceeds 0.5%, the effect is saturated, the weld hardenability is greatly increased, and the transformation of martensite is increased. There is a problem of accelerating the toughness.

Copper (Cu): 0.3-0.5%

The Cu is an element which is advantageous in securing a strength and toughness due to solid solution at the base, and it is preferable to contain 0.3% or more for such an effect. However, if the content exceeds 0.5%, there is a problem of decreasing the toughness by increasing the hardenability in the welded joint, it is preferable not to exceed 0.5%.

Titanium (Ti): 0.01 ~ 0.1%,

Ti is an element that combines with oxygen (O) to not only form fine Ti oxide, but also forms fine TiN precipitate to promote formation of acicular ferrite, thereby improving strength and toughness. Thus, in order to obtain the effect of the fine TiO oxide and TiN composite precipitate by Ti, it is preferable to contain 0.01% or more of season. However, if excessively more than 0.1%, coarse oxides and precipitates are formed and thus the toughness is lowered, so it is preferable not to exceed 0.1%.

Niobium (Nb): 0.01-0.04%,

Nb is an element added for the purpose of improving quenching property, and in particular, it is effective in obtaining bainite structure because it has an effect of lowering the Ar3 temperature and widening the bainite formation range even in a low cooling rate range. During austenizing, it may be precipitated by solute dragging or fine carbide to give a pinning effect. In order to obtain such an effect, Nb is preferably contained at 0.01% or more. However, if the content exceeds 0.04%, it can promote the formation of phase martensite (MA) structure in the welded joint during welding, and the welded joint due to the formation of M _x C during PWHT (Post Weld Heat Treatment) Since impact toughness falls, it is not preferable.

Vanadium (V): 0.01-0.06%

V is an element that promotes ferrite transformation by combining with N to form a VN precipitate. In order to acquire such an effect, it is preferable to contain V 0.01% or more. However, if the content exceeds 0.06%, a hardened phase such as carbide is formed on the welded joint, which is not preferable because it adversely affects the toughness of the welded joint.

Phosphorus (P): 0.02% or less, Sulfur (S): 0.01% or less

P and S are impurities that promote high temperature cracking and are preferably managed as low as possible.

Oxygen (O): 0.025 to 0.060%

The O is an element that forms Ti oxide by reacting with Ti during solidification of the weld joint, and the Ti oxide promotes transformation of acicular ferrite in the weld joint. If the content of O is less than 0.025%, Ti oxide may not be properly distributed in the welded joint. If the content of O is more than 0.060%, coarse Ti oxide and other oxides such as FeO are generated, which is not preferable.

Nitrogen (N): 0.005-0.008%

The N is bonded to Ti and crystallized at a high temperature at a time when the molten pool is solidified to TiN serves to inhibit the austenite grain growth by the pinning effect, containing 0.005% or more to obtain the above effect desirable. However, if the content exceeds 0.008%, the potential may be fixed by lowering the dislocation, which is not preferable.

Boron (B): 0.0015-0.004%

The B is to induce the formation of nitride to fix free nitrogen to play a role of suppressing the impact toughness caused by solid solution nitrogen, it is preferable to include at least 0.0015%. On the other hand, when the B is excessive, since it becomes a factor that lowers the impact toughness while being dissolved in a matrix, it is preferable not to exceed 0.004%.

Ti and N preferably satisfy Ti / N ≦ 3.14. Ti / N is a factor that determines the fraction and size of TiN precipitates. The TiN particles do not decompose even at a high temperature, and are dispersed inside the material during the temperature rising and solidification of the molten metal, thereby inhibiting the growth of the austenite size. In order to suppress grain boundary growth before heat treatment, it is preferable to distribute fine TiN particles, and for this purpose, the smaller the value of TiN is, the more preferable. To this end, in the present invention, the Ti / N is preferably 3.14 or less. Ti and N refer to the content (wt%) of each element.

It is preferable that B and N satisfy B / N <0.5. Free N is fixed through the B / N. The reason for adding a part of B in the welded joint is to strengthen the hardenability of the welded joint by dilution. However, when the specific value is exceeded, the toughness is reduced by increasing the hardenability in addition to fixing free nitrogen. May result. In the present invention, the B / N is preferably 0.5 or less. The B and N means the content (% by weight) of each element.

On the other hand, the welded joint of the present invention preferably Ceq value 0.41 ~ 0.76% defined by the following relational formula (1). If the Ceq is less than 0.41%, the strength of the welded joint is too low compared to the base material, whereas if the Ceq is more than 0.76%, low-temperature impact toughness may be lowered, which is not preferable.

[Relationship 1]

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

(The C, Mn, Ni, Cu, Cr, Mo and V is the content (wt%) of each element)

The welded joint of the present invention contains Fe and inevitable impurities. The addition of an effective ingredient other than the above composition is not excluded.

The welded joint of the present invention comprises fine tempered martensite, tempered bainite, and a small amount of ferrite (20% or less). The tempered martensite and tempered bainite are the main structures, and the tempered martensite and tempered martensite are 80% or more, preferably 90% or more by area fraction. In addition, it is preferable that a former austenite particle size is 150-200 micrometers.

The welded joint of the present invention can secure a shock absorbing energy of 100J or more even at low temperature of -40 ° C.

Hereinafter, an embodiment of manufacturing the welded joint of the present invention will be described in detail. Method for producing a welded joint of the present invention can be produced in various ways, not limited to the method described below.

Preferably, the welded joint of the present invention is intended for welding high strength steel such as power plants, shipyards, and plants. It does not specifically limit about the base material of the said welding joint part.

For example, the base material that can be applied in the present invention is a weight%, carbon (C): 0.2% or less (excluding 0%), manganese (Mn): 1.0 ~ 1.45%, silicon (Si): 0.15 ~ 0.40%, aluminum (Al): 0.06% or less (excluding 0%), phosphorus (P): 0.03% or less, sulfur (S): 0.01% or less, high-strength carbon steel containing residual Fe and other unavoidable impurities (e.g. ASTM A960, A860, etc.) Can be. The high carbon carbon steel may further include Cr, Ni, Mo, Cu, Ti, V, etc. in addition to the above alloy composition in terms of improving physical properties of the steel, but is not limited thereto.

The prepared base metal is welded. The welding method is not particularly limited, but is preferably performed by butt-welding, and for example, it is preferable to use electro-gas arc welding.

At this time, it is preferable to form a weld joint by 1-pass welding. 1 is a photograph showing a cross section of a welded joint formed through the one pass welding. Productivity can be improved during one pass welding.

The welded joint of the present invention may be applied to a pipe fitting or the like, and for this purpose, the base metal may be cut and rolled into a pipe shape, and then welded to form a pipe shape. The pipe fitting refers to a pipe material used when changing the direction of the pipe or changing the diameter to require the pipe branched from the main pipe. Mainly, there are elbow, tee, reducer and cap.

On the other hand, the microstructure of the welded joint after welding is preferably 40-50% bainite, acicular ferrite 40% or more, grain boundary ferrite 5-10% by area fraction. In addition, it is preferable that the old austenite particle size at this time is 250-300 micrometers. In general, it is preferable that the microstructure of the weld seam be similar to the microstructure of the base metal or have a higher strength, and it is common to make fine needle-like ferrite through an optimal component system suitable for the rapid cooling rate of the weld seam. In the present invention, in order to secure a welded joint having a higher strength than the base material, it is preferable to have a microstructure of some of the ferrite in the bainite and acicular ferrite excitation. On the other hand, in order to recover the impact toughness after the heat treatment described later it is preferable that the old austenite particle size of 250 ~ 300㎛ in the welded joint in the welded state.

It is preferable to heat-process after the said welding. In one example, the heat treatment may be for hot forming after the SR (Stress Relief) treatment. The SR (Stress Relief) treatment is a process for removing the internal residual stress generated during the welding process, it can be performed according to ASME standards for each steel grade or thickness. Preferably, the recrystallization temperature is performed at a temperature section at which the mechanical properties do not change. The hot forming may be performed by mounting the SR-treated base material to a mold and then compressing it. As a preferred example, the tube may be heated to a temperature of about 900 ° C. or more and then pressed to form a tube in a desired shape, but is not limited thereto. The mold is preferably selected according to the shape (for example, fitting) to be manufactured.

After the heat treatment, it is preferable to perform quenching and tempering heat treatment. Through the quenching, the weld seam forms martensite tissue, and the subsequent tempering treatment can remove internal stress and soften the tissue.

The quenching process is preferably cooled at a cooling rate of 5 ~ 15 ℃ / s in a state heated to a temperature of 900 ℃ or more for the hot forming, to ensure an appropriate microstructure when cooling at the cooling rate, strength reduction Can be prevented.

The tempering process is performed to reduce the brittleness of the steel hardened by martensite generated after quenching and to improve toughness, and the residual stress of carbides generated during quenching, segregation of impurities, lattice deformation of martensite, etc. Can be removed. The tempering step is preferably carried out at a temperature range of 590 ~ 670 ℃. If the temperature exceeds 670 ℃ there is a problem that the strength, hardness is lowered, whereas if it is less than 590 ℃ there is a problem of inferior toughness.

Hereinafter, the Example of this invention is described in detail. The following examples are only for the understanding of the present invention and are not intended to limit the present invention.

(Example)

As a base material, a 50 mm-thick ASTM A860-70 steel material was prepared and subjected to Electro Gas Welding to prepare a welded joint. The electrogas welding was performed at a heat input amount of about 300 KJ / cm, wherein the protective gas was welded using 100% carbon dioxide (CO ₂ ). The composition of the welded joint thus prepared is shown in Table 1 below.

After the heat treatment, the welded joints were subjected to quenching and tempering heat treatment. At this time, the heating conditions were heated to a temperature of 920 ℃, quenched at a cooling rate of 10 ℃ / sec, and air-cooled after tempering heat treatment at a temperature of 620 ℃. The particle size and microstructure of the welded joint before the heat treatment were observed and shown in Table 2, and after the tempering heat treatment, the particle size of the welded joint was observed and shown in Table 2. After the tempering heat treatment, the microstructure of the welded joint was more than 90% by area fraction of tempered martensite and tempered bainite, and some ferrite was observed.

For the welded joint after the tempering heat treatment prepared as described above, the Charpy impact absorption energy at -40 ° C was measured, and the results are shown in Table 2 together.

division Nb Ti N B Ti / N B / N Comparative Example 1 0.02 0.005 0.005 0.0015 One 0.3 Comparative Example 2 0.02 0.005 0.005 0.002 One 0.4 Comparative Example 3 0.02 0.005 0.005 0.003 One 0.6 Comparative Example 4 0.02 0.005 0.005 0.004 One 0.8 Comparative Example 5 0.02 0.005 0.005 0.005 One One Comparative Example 6 0.02 0.005 0.005 0.006 One 1.2 Inventive Example 1 0.02 0.01 0.005 0.0015 2 0.3 Inventive Example 2 0.02 0.01 0.005 0.002 2 0.4 Comparative Example 7 0.01 0.01 0.005 0.0015 2 0.3 Comparative Example 8 0.01 0.01 0.005 0.002 2 0.4 Comparative Example 9 0 0.01 0.005 0.002 2 0.4 Comparative Example 10 0.02 0.01 0.005 0.003 2 0.6 Inventive Example 3 0.02 0.01 0.008 0.0015 1.25 0.19 Inventive Example 4 0.02 0.01 0.008 0.002 1.25 0.25 Inventive Example 5 0.02 0.01 0.008 0.003 1.25 0.38 Inventive Example 6 0.02 0.01 0.008 0.004 1.25 0.50 Comparative Example 11 0.02 0.01 0.008 0.005 1.25 0.63 Comparative Example 12 0.02 0.01 0.008 0.006 1.25 0.75 Comparative Example 13 0.02 0.01 0.009 0.0015 1.11 0.17 Comparative Example 14 0.02 0.02 0.009 0.002 1.11 0.22 Comparative Example 15 0.02 0.02 0.005 0.0015 4 0.30 Comparative Example 16 0.02 0.02 0.005 0.002 4 0.40 Comparative Example 17 0.02 0.02 0.005 0.003 4 0.60 Inventive Example 7 0.02 0.02 0.008 0.0015 2.5 0.19 Inventive Example 8 0.02 0.02 0.008 0.002 2.5 0.25 Inventive Example 9 0.02 0.02 0.008 0.003 2.5 0.38 Inventive Example 10 0.02 0.02 0.008 0.004 2.5 0.50 Comparative Example 18 0.01 0.02 0.008 0.003 2.5 0.38 Comparative Example 19 0 0.02 0.008 0.004 2.5 0.50 Comparative Example 20 0.02 0.02 0.008 0.006 2.5 0.75 Comparative Example 21 0.02 0.03 0.005 0.0015 6 0.30 Comparative Example 22 0.02 0.03 0.005 0.002 6 0.40

The unit of Table 1 is weight percent, the content of the composition not shown in Table 1 is as follows. C (0.05%), Si (0.5%), Mn (1.5%), Ni (1.0%), Mo, 0.1%), Cu (0.3%), V (0.04), P (0.005%), S (0.003 %), The rest are Fe and inevitable impurities.

division Microstructure after welding (area%) Former Austenite Particle Size (㎛) -40 ° C Charpy Shock Absorber Energy (J) Couch ferrite Intergranular ferrite Bainite After welding After tempering Comparative Example 1 45 8 47 267 154 88 Comparative Example 2 50 7 43 278 189 89 Comparative Example 3 22 6 72 288 175 96 Comparative Example 4 18 9 73 256 177 80 Comparative Example 5 19 9 72 264 164 71 Comparative Example 6 20 16 64 275 166 70 Inventive Example 1 46 7 47 267 159 127 Inventive Example 2 43 7 50 255 183 130 Comparative Example 7 41 9 50 259 211 91 Comparative Example 8 47 10 43 287 248 85 Comparative Example 9 53 6 41 267 209 82 Comparative Example 10 51 9 40 288 161 92 Inventive Example 3 49 8 43 267 171 112 Inventive Example 4 44 7 49 257 166 120 Inventive Example 5 46 5 49 287 177 135 Inventive Example 6 49 10 41 283 185 102 Comparative Example 11 32 16 52 271 155 82 Comparative Example 12 33 17 50 266 163 86 Comparative Example 13 31 15 54 271 153 55 Comparative Example 14 29 11 60 259 175 52 Comparative Example 15 35 19 46 378 268 75 Comparative Example 16 41 14 45 355 288 76 Comparative Example 17 46 16 38 369 267 82 Inventive Example 7 44 8 48 259 167 110 Inventive Example 8 43 7 50 255 164 106 Inventive Example 9 42 8 50 246 169 101 Inventive Example 10 46 8 46 278 188 134 Comparative Example 18 45 8 47 277 234 76 Comparative Example 19 43 9 48 269 229 88 Comparative Example 20 33 14 53 284 159 60 Comparative Example 21 47 11 42 467 368 54 Comparative Example 22 48 12 40 455 384 52

As can be seen from the results of Table 2, it can be seen that the welded joints satisfying the conditions of the present invention can secure excellent low-temperature impact toughness even at cryogenic temperatures of -40 ° C.

Claims (14)

By weight%, carbon (C): 0.04-0.12%, silicon (Si): 0.05-0.5%, manganese (Mn): 1.3-2.0%, nickel (Ni): 1.0-2.5%, molybdenum (Mo): 0.1 0.5%, copper (Cu): 0.3-0.5%, titanium (Ti): 0.01-0.1%, niobium (Nb): 0.01-0.04%, vanadium (V): 0.01-0.06%, phosphorus (P): 0.02 % Or less, sulfur (S): 0.01% or less, oxygen (O): 0.025 to 0.060%, nitrogen (N): 0.005 to 0.008%, boron (B): 0.0015 to 0.004%, balance Fe and inevitable impurities , Ti and N satisfies Ti / N ≤ 3.14, B and N satisfies B / N ≤ 0.5, and the weld joint is a microstructure of the tempered martensite and tempered bainite as the main structure And a weld joint having excellent low temperature impact toughness including ferrite in a second phase.
delete delete delete The method according to claim 1,
The old austenite grain size of the microstructure is 150 ~ 200㎛ weld joints excellent in low temperature impact toughness.
The method according to claim 1,
The weld joint is a weld joint excellent in low-temperature impact toughness of 0.41 ~ 0.76% Ceq value defined by the following relational formula (1).
[Relationship 1]
Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo + V) / 5
(The C, Mn, Ni, Cu, Cr, Mo and V is the content (wt%) of each element)
The method according to claim 1,
The welded joint is excellent welded low-temperature impact toughness having a shock energy of more than 100J at a low temperature of -40 ℃.
Electro gas welding is carried out in weight%, in weight%, carbon (C): 0.04 to 0.12%, silicon (Si): 0.05 to 0.5%, manganese (Mn): 1.3 to 2.0%, nickel (Ni): 1.0 to 2.5%, molybdenum (Mo): 0.1-0.5%, copper (Cu): 0.3-0.5%, titanium (Ti): 0.01-0.1%, niobium (Nb): 0.01-0.04%, vanadium (V): 0.01- 0.06%, phosphorus (P): 0.02% or less, sulfur (S): 0.01% or less, oxygen (O): 0.025-0.060%, nitrogen (N): 0.005-0.008%, boron (B): 0.0015-0.004% Preparing a welded joint including residual Fe and unavoidable impurities, the bainite having 40 to 50%, acicular ferrite and at least 40% and grain boundaries of 5 to 10%;
Heating the weld joint to a temperature of 900 ° C. or higher;
Quenching the heated welded joint at a cooling rate of 5 to 15 ° C./sec; And
Tempering heat treatment of the cooled welded joint at a temperature range of 590 to 670 ° C; Method of manufacturing a welded joint excellent in low temperature impact toughness comprising a.
The method according to claim 8,
The prior austenite grain size of the welded joint before heating is 250 ~ 300㎛ manufacturing method of the welded joint excellent in low-temperature impact toughness.
delete delete delete delete The method according to claim 8,
The electro-gas welding is a method of manufacturing a welded joint having excellent low-temperature impact toughness to perform one-pass welding.

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