KR101351266B1 - 900MPa HIGH STRENGTH WELDING JOINT HAVING EXCELLENT LOW TEMPERATURE TOUGHNESS - Google Patents

Abstract

The present invention relates to a high strength welded 900MPa class weldability excellent.
Welded part according to one aspect of the present invention carbon (C): 0.05 ~ 0.07%, silicon (Si): 0.2 ~ 0.4%, manganese (Mn): 1.6 ~ 1.9%, nickel (Ni): 2.6 ~ 3.5%, molybdenum (Mo): 0.4-0.6%, chromium (Cr): 0.3-0.5%, titanium (Ti); 0.07 ~ 0.10%, Boron (B): 0.004 ~ 0.006%, Aluminum (Al): 0.020 ~ 0.025%, Copper (Cu): 0.03 ~ 0.04%, Phosphorus (P): 0.016% or less, Sulfur (S): 0.007 % Or less, Oxygen (O): 0.035 to 0.040%, Nitrogen (N): 0.003 to 0.009%, balance Fe and inevitable impurities, 15-20 area% intragranular acicular ferrite, 21-30 area% martens It may have an internal organization of sites and remaining bainite.

Description

900MPa class high strength welded part with excellent low temperature toughness {900MPa HIGH STRENGTH WELDING JOINT HAVING EXCELLENT LOW TEMPERATURE TOUGHNESS}

The present invention relates to a high strength welded 900MPa class weldability excellent.

Welded structures, such as offshore structures, buildings, ships, line pipes, bridges, etc. are gradually increasing in size. As a result, high-rise buildings, ultra-long bridges, ultra-large container ships, extra-large tankers, ultra-large LNG carriers, and large-diameter line pipes are being developed one after another.

Accordingly, the demand for the development of ultra-high strength steel for manufacturing the super-large structure is also increasing day by day, and accordingly, a new alloy component system of steel is designed, and an appropriate manufacturing method is developed, for example, strength. Ultra high strength steel of 900MPa level or more has been provided.

However, even if ultra high strength steel is developed, the super high strength steel should be welded in order to manufacture a structure, and the welded part has a history that is completely different from the thermal history received by the steel (base material). Strength is not necessarily achieved at the weld. Therefore, for this purpose, the strength of the weld itself needs to be sufficient.

However, since the welded portion is welded to join the steel and then solidified, the tissue is different from the base metal having the structure controlled by thermomechanical deformation, and as a result, the low temperature toughness is very weak. Higher strength welds become stronger, so it is not easy to develop welds that meet both strength and low temperature toughness at the same time.

Republic of Korea Patent Publication No. 2011-0050039 as a technique for securing the strength and toughness of the welded portion. However, the technique is not suitable for use in ultra high strength welded structures because the tensile strength of the weld is only 600MPa class.

According to one aspect of the present invention, a welded portion having a strength of 900 MPa or more and excellent low temperature toughness is provided.

The problem of the present invention is not limited to the above description, and can be sufficiently understood from the general description of the specification.

Welded part according to one aspect of the present invention carbon (C): 0.05 ~ 0.07%, silicon (Si): 0.2 ~ 0.4%, manganese (Mn): 1.6 ~ 1.9%, nickel (Ni): 2.6 ~ 3.5%, molybdenum (Mo): 0.4-0.6%, chromium (Cr): 0.3-0.5%, titanium (Ti); 0.07 ~ 0.10%, Boron (B): 0.004 ~ 0.006%, Aluminum (Al): 0.020 ~ 0.025%, Copper (Cu): 0.03 ~ 0.04%, Phosphorus (P): 0.016% or less, Sulfur (S): 0.007 % Or less, Oxygen (O): 0.035 to 0.040%, Nitrogen (N): 0.003 to 0.009%, balance Fe and inevitable impurities, 15-20 area% intragranular acicular ferrite, 21-30 area% martens It may have an internal organization of sites and remaining bainite.

At this time, the carbon equivalent represented by the following formula is preferably 0.59 to 0.64.

[Equation 1]

Carbon equivalent = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4

Where C, Si, Mn, Ni, Cr, and Mo are the values in percent by weight of the element, respectively.

And, the martensite is less than 26 area% is more advantageous for securing high toughness.

In addition, it is effective that the internal structure further contains residual austenite of 3.2 area% or less.

The welding part of this invention mentioned above is a welding part excellent in low-temperature toughness whose Charpy impact energy value in said -40 degreeC is 47J or more.

The welded part of the present invention not only has a high strength as the composite structure of bainite and martensite, but also can secure high low temperature toughness by appropriately controlling the steel composition, which is useful for producing a high strength structure.

1 is a schematic diagram showing a cross-sectional shape of a welded portion formed in one embodiment of the present invention.

Hereinafter, the present invention will be described in detail. It is important to note that the welding part of the present invention includes a weld part and a reheating part where the weld part absorbs heat again by multi-layer welding as a part that is melted and solidified by welding.

The present inventors have found that when the composition of the welded portion is controlled as follows and the structure is controlled in a specific form, high strength of 900 MPa or more, as well as high impact toughness at -40 ° C, can be ensured and the present invention has been achieved.

First, the advantageous composition of the weld part of this invention is demonstrated. It is to be noted that the content of each element described below is based on weight% unless otherwise specified.

Carbon (C): 0.05 ~ 0.07%

Carbon is added at 0.05% by weight or more in order to secure strength as a powerful hardening element. However, when excessively added, the weldability is lowered and low temperature cracking is more likely to occur, so the content of carbon is preferably 0.07% by weight or less.

Silicon (Si): 0.2 ~ 0.4%

Since silicon is an element added to steel in order to deoxidize steel, 0.2 weight% or more needs to be added. If the amount is not sufficient, the residual amount of dissolved oxygen may increase, causing blow holes, deterioration of toughness and deterioration of molten metal fluidity. In addition, silicon is preferably added in an amount of 0.2% by weight or more in order to improve the strength of the welded portion as an element for improving hardenability. However, when excessively added, it is not preferable because non-martensite phase martensite (also referred to as Martensite-Austenite Constituent, MA) is formed in the tissue, thereby deteriorating impact toughness. Therefore, it is preferable to limit it to 0.4 weight% or less.

Manganese (Mn): 1.6 ~ 1.9%

Manganese also deoxidizes the steel, but also plays a role in securing strength and toughness, so that the manganese is preferably added at least 1.6% by weight. However, when excessively added, low temperature cracks are likely to occur, so the upper limit of the content of manganese is preferably set to 1.9% by weight.

Nickel (Ni): 2.6 ~ 3.5%

Nickel is an element that improves both the strength and toughness of the weld metal, and therefore it is preferably added at 2.6% by weight or more. When added excessively, the hardenability increases more than necessary, thereby increasing the possibility of hot cracking.

Molybdenum (Mo): 0.4 ~ 0.6%

Molybdenum improves the low temperature impact toughness by improving the strength and reducing the size of crystal grains. Therefore, 0.4 weight% or more needs to be added for this. However, when the content exceeds 0.6% by weight, island martensite is formed, which is not only deteriorated in impact toughness but also may cause welding low temperature cracking.

Chromium (Cr): 0.3 ~ 0.5%

Chromium contributes to the increase in strength, so 0.3 wt% or more is added. However, when excessively added, coarse carbides are formed at the grain boundaries and the toughness decreases. Therefore, it is preferable to limit it to 0.5 weight% or less.

Titanium (Ti); 0.07-0.10%

Titanium combines with oxygen to form titanium oxide, which becomes the nucleation site of intragranular ferrite. However, if it exceeds 0.1, the titanium oxide becomes coarse and the toughness is lowered.

Boron (B): 0.004 ~ 0.006%

Boron is an element which greatly improves hardenability. It is preferable to add 0.004% by weight or more because segregation at the grain boundary suppresses grain boundary ferrite transformation and promotes intragranular acicular ferrite transformation. On the other hand, if it exceeds 0.006% by weight, the curing ability is excessively increased, so that welding low temperature cracking may occur and the toughness is lowered.

Aluminum (Al): 0.020 ~ 0.025%

Since aluminum acts as a deoxidizer, it is required at least 0.020% by weight to reduce the amount of oxygen in the weld metal. However, when exceeding 0.025, coarse Al ₂ O ₃ is formed, thereby decreasing toughness by itself, preventing the formation of titanium oxide which promotes transformation of grain boundary acicular ferrite.

Copper (Cu): 0.03 ~ 0.04%

If copper is added 0.03% by weight or more, strength can be secured by solid solution strengthening effect. However, if it exceeds 0.04% by weight, welding workability is lowered and welding hot cracking is generated.

Phosphorus (P): 0.016% or less

Phosphorus is an inevitable impurity, so keep it as low as possible. It degrades welding workability and reduces toughness. It segregates in the austenite grain boundary to promote welding hot cracking.

Sulfur (S): 0.007% or less

It is inevitable to keep the impurities as low as possible. It may lower toughness and form a low melting point compound such as FeS, causing hot cracking.

Oxygen (O): 0.035 ~ 0.040%

The oxygen is needed to form titanium oxide. Titanium oxide becomes an intraoral needle ferrite nucleation site. If less than 0.035% by weight, too little titanium oxide is formed, if exceeding 0.040% by weight of titanium oxide is coarse, a large amount of oxides, such as FeO is formed to reduce the low-temperature toughness of the weld metal. Here, oxygen content means total oxygen analyzed by an oxygen / nitrogen analyzer.

Nitrogen (N): 0.003 ~ 0.009%

TiN and Al can be combined with TiN to form precipitates and AlN precipitates to obtain precipitation strengthening effects. However, if the content exceeds 0.009% by weight, the dissolved nitrogen content of the weld metal is increased, so the toughness is greatly reduced.

Since the weld of the present invention is a weld of steel, the remainder other than the above components is Fe, and may include inevitable impurities that may be mixed in other steels or welding wires.

Therefore, the welded portion of the present invention in weight%, carbon (C): 0.05 ~ 0.07%, silicon (Si): 0.2 ~ 0.4%, manganese (Mn): 1.6 ~ 1.9%, nickel (Ni): 2.6 ~ 3.5% , Molybdenum (Mo): 0.4-0.6%, chromium (Cr): 0.3-0.5%, titanium (Ti); 0.07 ~ 0.10%, Boron (B): 0.004 ~ 0.006%, Aluminum (Al): 0.020 ~ 0.025%, Copper (Cu): 0.03 ~ 0.04%, Phosphorus (P): 0.016% or less, Sulfur (S): 0.007 % Or less, oxygen (O): 0.035 to 0.040%, nitrogen (N): 0.003 to 0.009%, balance Fe and inevitable impurities.

In addition, according to another aspect of the present invention, the weld is preferably limited to the carbon equivalent (Ceq) as follows in addition to the above-described composition limitation. That is, the carbon equivalent is expressed by C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 (wherein C, Si, Mn, Ni, Cr, and Mo are It is preferable that the value is 0.59-0.64 as a content (weight-% value). If the carbon equivalent is too low, it is difficult to obtain sufficient strength. On the contrary, if the carbon equivalent is too high, the fraction of the low temperature structure such as bainite and martensite may increase beyond the range defined herein, resulting in deterioration of low temperature toughness.

It is preferable that the welded portion of the present invention has the following relationship based on the above-described carbon equivalent tensile strength. According to the results of the present inventors, when the tensile strength satisfies the relationship of Equation 1 below, the Charpy impact energy value at −40 ° C. may have excellent low temperature toughness of 47 J or more.

[Equation 1]

Tensile Strength (MPa) = 850.3 × Ceq + a

However, a has a value of 380-415 as a constant value here, More preferably, it has a value of 395.2-400.2.

It is preferable that the welded part of the present invention having the above-mentioned advantageous composition consists of intragranular acicular ferrite, bainite and martensite. Intraoral needle-like ferrite is a tissue necessary to secure both strength and low temperature toughness at the same time, and preferably contains 15 area% or more. However, since intragranular acicular ferrite alone is difficult to secure high strength of 900 MPa or more, the upper limit of intragranular acicular ferrite is limited to 20 area%, and the remaining tissues except for intragranular acicular ferrite may be composed of bainite and martensite. have. At this time, the martensite is more preferably contained 21 area% or more in order to obtain sufficient strength. However, martensite is relatively fragile in tissue, so if it is included in a large amount, it may damage the toughness of the entire weld. Therefore, the content of martensite in the weld is preferably 30 area% or less, more preferably 26 area% or less.

Accordingly, the internal structure of the weld according to one aspect of the present invention may consist of a predetermined amount of intragranular acicular ferrite and martensite and the balance substantially bainite.

In addition, according to another aspect of the present invention, the above-described advantageous tissue of the present invention may further include residual austenite in a ratio of 3.2 area% or less (not including 0%). When the residual austenite is included in small amounts to improve the strength of the welded part, a beneficial effect may be obtained. However, since the residual austenite is present in the form of martensite, the toughness may be degraded when added in excess. Therefore, the fraction of the retained austenite is preferably 3.2 area% or less. The more advantageous residual austenite content is from 2.0 to 3.2 area%.

The above-described advantageous welding part of the present invention can be sufficiently manufactured by a method well known in the art to which the present invention belongs, and thus a detailed manufacturing method is not particularly discussed in the present invention. However, one method advantageous to the application of the present invention is gas-shielded arc welding. The gas-shielded arc welding can be advantageously applied by lowering the oxygen partial pressure in the atmosphere gas to prevent oxidation of the weld and control the composition.

Hereinafter, the present invention will be described in more detail by way of examples. However, it is necessary to note that the following examples are intended to more specifically illustrate the present invention and are not intended to limit the scope of the present invention.

(Example)

In order to form a weld, an improved part was formed in the form shown in FIG. 1, followed by butt gas-shielded arc welding. Welding conditions applied during welding were set as described in Table 1 below. As can be seen in the figure, a V-shaped improvement was formed between the base metal plate, and attached to the base metal plate to cover the bottom of the butt weld using a backing strip having the same composition as the base material.

Base material Root gap Improving
Angle Lamination electric current Voltage welding
speed Heat input Protective gas Protective gas flow AH36,
20mm thickness 13mm 45 ° 15 pass 320A 31 V 26 ~ 33cm / min 18-23 kJ / cm CO ₂ 20 l / min

As a result of welding under the above conditions, as shown in FIG. 1, a welded part in which seven weld metals were formed was obtained. Table 2 shows the composition of the welded part used in this example. The content of each element in the table is expressed in weight percent.

division C Si Mn Ni Mo Cr Ti B Al Cu P S O N Example 1 0.062 0.29 1.6 2.9 0.48 0.34 0.07 0.005 0.02 0.032 0.014 0.006 0.0376 0.0044 Example 2 0.06 0.31 1.8 3 0.5 0.34 0.1 0.006 0.023 0.039 0.016 0.007 0.0372 0.0062 Example 3 0.059 0.31 1.8 3.3 0.48 0.34 0.1 0.006 0.023 0.037 0.015 0.007 0.04 0.007 Example 4 0.058 0.31 1.7 2.9 0.47 0.33 0.09 0.006 0.021 0.037 0.015 0.007 0.0351 0.0051 Example 5 0.058 0.27 1.6 2.9 0.47 0.33 0.07 0.005 0.021 0.038 0.015 0.007 0.0359 0.0031 Example 6 0.06 0.28 1.6 2.8 0.52 0.3 0.09 0.006 0.02 0.037 0.015 0.006 0.0358 0.0089 Example 7 0.061 0.27 1.7 2.9 0.54 0.32 0.1 0.006 0.021 0.033 0.014 0.006 0.0369 0.0063 Example 8 0.06 0.3 1.7 3.2 0.49 0.31 0.09 0.005 0.02 0.033 0.014 0.007 0.0377 0.0085 Comparative Example 1 0.057 0.29 1.7 2 0.44 0.31 0.07 0.005 0.024 0.031 0.015 0.006 0.0373 0.0064 Comparative Example 2 0.058 0.28 1.7 2.1 0.46 0.33 0.07 0.005 0.024 0.033 0.015 0.006 0.0394 0.0059 Comparative Example 3 0.06 0.31 1.7 2.9 0.82 0.33 0.09 0.005 0.023 0.035 0.016 0.006 0.0382 0.0054 Comparative Example 4 0.055 0.31 1.8 2.9 0.9 0.31 0.07 0.005 0.024 0.031 0.015 0.007 0.0379 0.0048 Comparative Example 5 0.059 0.35 1.7 2.8 0.48 0.33 0.17 0.006 0.024 0.034 0.015 0.006 0.0181 0.0044 Comparative Example 6 0.06 0.34 1.8 2.7 0.49 0.32 0.18 0.005 0.022 0.032 0.016 0.006 0.0183 0.0053 Comparative Example 7 0.06 0.33 1.8 3 0.49 0.33 0.08 0.029 0.024 0.034 0.014 0.007 0.0356 0.0058 Comparative Example 8 0.061 0.32 1.8 3.1 0.43 0.35 0.08 0.03 0.023 0.034 0.015 0.006 0.0364 0.0057

Table 3 shows the carbon equivalent, microstructure, tensile strength, and Charpy impact energy of each welded part having the composition shown in Table 2. The structure of the welded part can be changed by adjusting the heat input amount and cooling condition within the range of the welding conditions of Table 1 described above. Tensile test pieces for measuring tensile strength were prepared according to ASTM Standard E8, and impact test specimens were prepared according to ASTM Standard E23. Among the microstructures, the remaining tissues not listed in the table were confirmed to be bainite.

division Ceq (%) The tensile strength
(MPa) -40 ℃ Charpy impact energy (J) In-mouth needle bed ferrite fraction (area%) Martensite fraction (area%) Residual austenite fraction (area%) Example 1 0.601 910 51 19 23 2 Example 2 0.641 942 48 16 26 3.2 Example 3 0.642 945 50 17 26 3.1 Example 4 0.610 919 48 17 24 2.5 Example 5 0.592 901 49 17 21 2.2 Example 6 0.598 904 55 19 21 2.1 Example 7 0.627 929 51 18 24 2.3 Example 8 0.620 924 57 18 25 2.4 Comparative Example 1 0.574 887 55 25 18 2.4 Comparative Example 2 0.587 892 54 22 18 2.4 Comparative Example 3 0.700 964 43 10 34 3.2 Comparative Example 4 0.727 970 38 9 33 3.3 Comparative Example 5 0.613 918 12 14 23 3.1 Comparative Example 6 0.628 929 13 13 24 2.8 Comparative Example 7 0.637 935 11 13 25 2.7 Comparative Example 8 0.629 929 13 13 25 2.8

As can be seen in Table 3, Comparative Example 1 and Comparative Example 2 is a case where the content of nickel and the carbon equivalent (Ceq) is lower than the range specified in the present invention, sufficient tensile strength could not be obtained. In Comparative Examples 1 and 2, it can also be confirmed that the fraction of martensite necessary for securing strength of 900 MPa or more is only 18 area%.

Comparative Examples 3 and 4 are the case where the content of molybdenum and carbon equivalent is higher than the range limited by the present invention, and the fraction of martensite is 34 and 33 area%, respectively, and the Charpy impact energy is less than 47J targeted here. This defect can be seen. In particular, in Comparative Examples 3 and 4, it can be confirmed that the fraction of intragranular acicular ferrite required for securing toughness is below the range specified in the present application.

In Comparative Examples 5 and 6, the titanium content is higher than the limit of the present application, and it can be seen that the value of the Charpy impact energy is significantly lowered.

In addition, Comparative Examples 7 and 8 show that the value of boron is excessive, and in this case, the strength is high but the value of Charpy impact energy is poor.

However, Examples 1 to 8 are cases in which the content range, the carbon equivalent, and the fraction of the tissue of each component all meet the ranges defined by the present invention. As a result, all of the strengths of 900 MPa or more and -40 ° C Charpy impact energy of 47 J or more are as a result. It can be seen that the value is satisfied.

Thus, the advantageous effects of the present invention could be confirmed.

Claims (5)

Carbon (C): 0.05 ~ 0.07%, Silicon (Si): 0.2 ~ 0.4%, Manganese (Mn): 1.6 ~ 1.9%, Nickel (Ni): 2.6 ~ 3.5%, Molybdenum (Mo): 0.4 ~ 0.6%, Chromium (Cr): 0.3-0.5%, titanium (Ti); 0.07 ~ 0.10%, Boron (B): 0.004 ~ 0.006%, Aluminum (Al): 0.020 ~ 0.025%, Copper (Cu): 0.03 ~ 0.04%, Phosphorus (P): 0.016% or less, Sulfur (S): 0.007 % Or less, Oxygen (O): 0.035 to 0.040%, Nitrogen (N): 0.003 to 0.009%, balance Fe and inevitable impurities, 15-20 area% intragranular acicular ferrite, 21-30 area% martens 900MPa class high strength welded part with excellent low temperature toughness with internal structure of site and remaining bainite.
According to claim 1, 900MPa class high strength welded parts excellent in low temperature toughness of 0.59 ~ 0.64 carbon equivalent represented by the following formula.

[Equation 1]
Carbon equivalent = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4
Where C, Si, Mn, Ni, Cr, and Mo are the values in percent by weight of the element, respectively.
According to claim 1, The martensite is 900MPa class high strength weldability excellent in low temperature toughness of 26 area% or less.
According to claim 1, wherein the internal structure is 900MPa class high strength weldability excellent excellent low-temperature toughness further comprises a residual austenite of 3.2 area% or less.
The 900MPa class high strength welding part according to any one of claims 1 to 4, wherein the welding part has excellent low temperature toughness of a Charpy impact energy value of −47 ° C. or higher at −40 ° C. 6.

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