KR102200243B1 - Offshore structural steel weldment for high heat input welding - Google Patents

Abstract

High heat input marine structural steel welded joints with excellent low-temperature toughness are provided.
The high heat input of the offshore structural steel welded joint of the present invention, by weight, carbon (C): 0.07 to 0.15%, silicon (Si): 0.10 to 0.25%, manganese (Mn): 1.5 to 1.65%, phosphorus (P) ): 0.008% or less, S: 0.002% or less, Aluminum (Al): 0.015 to 0.025%, Copper (Cu): 0.2 to 0.3%, Nickel (Ni): 0.5 to 1.0%, Niobium (Nb): 0.017% or less (Excluding 0%), Titanium (Ti): 0.010 to 0.025%, Nitrogen (N): 0.002 to 0.005%, including the balance Fe and inevitable impurities, in the range of welding heat input 5 kJ/mm to 8.75 kJ/mm, The phase fraction of cornerstone ferrite and bainite in the coarse heat affected zone (CGHAZ) is 1: 4.8-33.5; The coarse heat affected zone (CGHAZ) includes a M/A (Martensite-Austenite) phase having a size of 0.7 μm or less and a phase fraction of 0.04 area% or less for the entire tissue; In addition, the ideal zone heat affected zone (ICHAZ) is characterized by including an M/A phase having a size of 0.9 μm or less and a phase fraction of 0.3 area% or less for the entire tissue.

Description

Offshore structural steel weldment for high heat input welding with excellent low-temperature toughness

The present invention relates to an offshore structural steel welded joint having excellent low-temperature toughness, and more particularly, to an offshore structural steel welded joint having excellent low-temperature impact toughness and CTOD characteristics when applying high heat input welding of 5kJ/mm or more.

Offshore structures are large structures that drill, refine, store, and produce crude oil and gas, and must maintain stability for a long time in physical environments such as waves and storms and at low temperatures. Therefore, the steel material applied to such offshore structures must have excellent strength and low-temperature toughness, and, in addition, the properties of the weld including the heat-affected area must be guaranteed to be equal to that of the base material.

As an example of such an offshore structural steel manufacturing technology, the invention described in Patent Document 1 is mentioned. In the patent document 1, C: 0.02 to 0.07% by weight, Si: 0.2% or less, Mn: 1.2 to 1.7%, P: 0.012% or less, S: 0.003% or less, Al: 0.005 to 0.02%, Cu: Alloy components of 0.1 to 0.5%, Ni: 0.1 to 1.0%, Ti: 0.007 to 0.013%, and N: 0.002 to 0.006% were used to produce a thick plate having a yield strength of 380 MPa through an appropriate rolling and cooling process. However, the alloy components developed in Patent Document 1 satisfy the -40℃ impact toughness pass criteria (average minimum 36J, individual minimum 26J) and -10℃ CTOD (Crack tip opening displacement) test pass criteria (0.25mm or more). There is a problem that the range of heat input for welding is limited to a maximum of 5.0kJ/mm.

Due to the recent downturn in the offshore economy, ship and plant manufacturers are striving to improve productivity through extreme cost reduction efforts, and among them, the importance of improving welding productivity is emerging.

Korean Patent Publication No. KR2010-0067509

Therefore, the present invention is to solve the above problems, through the control of the alloy component of the weld joint, the low-temperature toughness pass criterion (EN10225) at a maximum heat input of 8.75kJ/mm, that is, -40℃ impact toughness (average minimum 46J , Individual minimum 32J) and -10℃ CTOD (0.25mm or more) to obtain a welded joint with excellent yield strength of 420MPa class high heat input marine structural steel welded joints.

The present invention for achieving the above object,

In wt%, carbon (C): 0.07 to 0.15%, silicon (Si): 0.10 to 0.25%, manganese (Mn): 1.5 to 1.65%, phosphorus (P): 0.008% or less, S: 0.002% or less, aluminum (Al): 0.015 to 0.025%, Copper (Cu): 0.2 to 0.3%, Nickel (Ni): 0.5 to 1.0%, Niobium (Nb): 0.017% or less (excluding 0%), Titanium (Ti): 0.010 ~0.025%, nitrogen (N): 0.002 ~ 0.005%, the balance contains Fe and inevitable impurities,

In the range of welding heat input 5kJ/mm ~ 8.75kJ/mm,

The phase fraction of cornerstone ferrite and bainite in the coarse heat affected zone (CGHAZ) is 1: 4.8 to 33.5;

The coarse heat affected zone (CGHAZ) includes a M/A (Martensite-Austenite) phase having a size of 0.7 μm or less and a phase fraction of 0.04 area% or less for the entire tissue; And

The ideal heat-affected zone (ICHAZ) is a welded joint of high heat input for offshore structural steel with excellent low-temperature toughness, characterized in that it includes an M/A phase with a size of 0.9㎛ or less and a phase fraction of 0.3 area% or less for the entire structure. About.

According to the present invention having the above-described configuration, the strength of the weld is guaranteed by adding an appropriate amount of carbon, silicon, manganese, copper, and nickel in the offshore structural steel, and the nickel content is increased by increasing the content of Ti and N together with limiting the niobium content. By increasing it, it is possible to obtain a welded metal part that satisfies the yield strength of 420 MPa or higher, low temperature impact toughness and CTOD value even when welding with a heat input of 8.75 kJ/mm, which is 75% higher than the existing limit heat input and 5.0 kJ/mm.

1 is a 5kJ/mm, 8.75kJ/mm, 10kJ/mm, and 15kJ/mm heat input in an embodiment of the present invention in a temperature range of 800 to 500°C, 7°C/sec, 4°C/sec, 3.5°C/sec, and The figure shows the result of measuring the phase transformation temperature and phase fraction during cooling using a dilatometer tester when cooling at a cooling rate of 2.3℃/sec (during actual welding, the value measured using a thermocouple on the specimen).
2 is a graph showing the size change of the M/A phase according to the amount of heat input in CG and ICHAZ in an embodiment of the present invention.
3 is a graph showing the change in the M/A phase fraction according to the amount of heat input in CG and ICHAZ in an embodiment of the present invention.

Hereinafter, the present invention will be described.

The high heat input of the offshore structural steel welded joint of the present invention, by weight, carbon (C): 0.07 to 0.15%, silicon (Si): 0.10 to 0.25%, manganese (Mn): 1.5 to 1.65%, phosphorus (P) ): 0.008% or less, S: 0.002% or less, Aluminum (Al): 0.015 to 0.025%, Copper (Cu): 0.2 to 0.3%, Nickel (Ni): 0.5 to 1.0%, Niobium (Nb): 0.017% or less (Excluding 0%), titanium (Ti): 0.010 ~ 0.025%, nitrogen (N): 0.002 ~ 0.005%, the balance contains Fe and inevitable impurities. Hereinafter, each alloying element and the reason for its content limitation will be described. Herein, "%" is disclosed as "% by weight" unless otherwise specified.

Carbon (C): 0.07~0.15%,

C is an element that is very useful for securing the strength of steel, but it increases the hardness of the microstructure constituting the heat-affected zone, promotes the M/A structure, and generates coarse needle-shaped Beadmanstaton ferrite at the grain boundary during high heat input welding. It greatly reduces toughness. For the above reasons, in the present invention, the carbon content is limited to 0.15% or less.

Silicon (Si): 0.10~0.25%

Si contributes to securing the hardenability of steel and plays a role in deoxidation during steel making, so it should be added at least 0.05%, but when added in a certain amount, the formation of non-metallic inclusions or M/A structure becomes remarkable, so the upper limit is set to 0.25%.

Manganese (Mn): 1.5-1.65%

Mn, together with Cu and Ni, increases the strength of the base metal and is an element that has relatively little influence on the deterioration of the toughness of the heat-affected zone and is an important element in the present invention. If the Mn content is high, it is easy to secure the strength and delay the formation of the M/A structure, so it should be added at least 1.5%, but if the amount exceeds 1.65%, the hardness of the heat-affected zone increases too much, especially the toughness of the IC heat-affected zone. Since it is greatly reduced, the upper limit is limited to 1.65%.

Employed aluminum (Sol. Al): 0.015~0.025%

Al is a strong deoxidizing agent and exhibits a grain refinement effect by depositing AlN. However, when excessively added, it is preferable to add less than 0.025% because it forms dendritic carbides to make the steel fragile, generates surface cracks of the playing slab, and impairs impact toughness.

Copper (Cu): 0.2~0.3%

When copper (Cu) is added by 0.20% or more, it exhibits a solid solution strengthening effect, thereby increasing the strength, hardness and corrosion resistance of the steel. However, if excessively added, the oxidation rate is lower than that of Fe during hot processing, so it penetrates into the interior after scattering on the surface and causes red heat embrittlement, so it is preferable to control the upper limit of the content to 0.3%.

Nickel (Ni): 0.5~1.0%

Ni, similar to Cu, is useful as an element that increases the strength of steel and has little decrease in toughness in the heat-affected zone. In particular, it must be added to suppress surface cracks that may occur when Cu is added. 0.5% or more should be added to secure the strength and toughness of the heat-affected zone of the steel, but 1.0% is the upper limit because the effect is saturated and the manufacturing cost is greatly increased as an expensive element above a certain amount.

Titanium (Ti): 0.010 to 0.025%

Titanium combines with nitrogen to form TiN, which exhibits high thermal stability at high temperatures, reducing coarsening of austenite in the CG heat-affected zone, so it should be added at least 0.010%. However, if it is added in excess of 0.025%, TiN is coarsened and the suppressing effect of coarsening of austenite grains decreases, so the addition amount is limited to 0.025% or less.

Niobium (Nb): 0.017% or less (excluding 0%)

Nb is an element that combines with N to form NbN precipitates and promotes ferrite transformation in the heat-affected zone of the general heat input.In terms of precipitation of fine NbN, a content of 0.005% or more was required, but (Ti, Nb)(C,N ), it is desirable to limit the content to 0.017% or less in order to improve the toughness of the coarse heat-affected zone during high heat input welding because the high temperature stability of the precipitate is lowered.

Nitrogen (N): 0.002~0.005%

Since N is combined with Ti to form TiN, it contributes to the improvement of the toughness of the CG heat-affected zone, so it should be added at least 0.002%.However, if too much is added, it may cause surface cracks on the slab during continuous casting, so the upper limit is limited to 0.005%. do.

Phosphorus (P): 0.008% or less

P is an impurity that is inevitably mixed into the steel and causes grain boundary embrittlement, which lowers the toughness of the base metal and the heat-affected zone, so it is limited to 0.02% or less.

Sulfur (S): 0.002% or less

Sulfur is an impurity that is inevitably mixed into the steel, and since it lowers the toughness of the thickness center of the steel sheet, 0.002% is the upper limit.

The offshore structural steel welded joint for high heat input welding of the present invention having the above composition is limited in carbon and solid solution aluminum, so that the coarsened grained heat affected zone (CGHAZ) and the intercritical heat affected zone :ICHAZ) can minimize the M/A (Martensite-Austenite) phase.

In addition, when the super grain boundary ferrite, which increases with the increase in heat input due to the low carbon content, is suppressed, the formation of needle-shaped bead-only staton ferrite is suppressed, so that the low-temperature toughness is increased compared to the high carbon steel material of the same carbon equivalent.

In addition, by limiting the niobium content and increasing the content of Ti and N to increase the high-temperature stability of (Ti,Nb)(C,N), the growth of old austenite grains is limited, thereby suppressing the formation of a brittle phase in the coarse heat-affected zone and decrease in toughness. can do.

That is, in the present invention, in the range of 5kJ/mm to 8.75kJ/mm of heat input for welding, the phase fraction of the cornerstone ferrite and bainite of the coarse heat-affected zone (CGHAZ) is provided with a controlled weld joint of 1: 4.8 to 33.5. can do. As the welding heat input from 5 kJ/mm to 8.75 kJ/mm increases, the content of cornerstone ferrite increases from 2.9% to 21%, while the fraction of bainite decreases, and accordingly, the average hardness decreases from 222Hv to 204Hv. However, when the welding heat input exceeds 8.75kJ/mm (10.0kJ/mm, 15.0kJ/mm), the phase fraction of ferrite and bainite becomes 1: 4.7 or less, and at the same time, excessive needle-shaped Beadmans Staten ferrite formation. Due to this, the toughness of the weld may not be guaranteed.

In addition, in the present invention, the coarse heat-affected zone (CGHAZ) may include a Martensite-Austenite (M/A) phase having a size of 0.7 μm or less and a phase fraction of 0.04 area% or less for the entire tissue. In addition, the ideal zone heat affected zone (ICHAZ) may include an M/A phase having a size of 0.9 μm or less and a phase fraction of 0.3 area% or less for the entire tissue. When these conditions are satisfied, high heat input offshore structural steel welded joints with high heat input of 420 MPa class satisfying -40℃ impact toughness (average minimum 46J, individual minimum 32J) and -10℃ CTOD (0.25mm or more) in high heat input welding Can be obtained.

On the other hand, in the present invention, the cooling rate that directly affects the phase transformation is slowed as the heat input increases.In terms of the generation of the M/A phase, the peak temperature in ICCGHAZ, where the coarse heat-affected zone, known to be the most vulnerable among the welding heat-affected zones, is reheated to the ideal zone In the case of the same, as the heat input increases (as the cooling rate decreases), the generation of ferrite and cementite instead of the M/A phase becomes dominant, so the effect of the increase in the heat input amount on the toughness can be said to be positive.

Hereinafter, the present invention will be described in detail through examples.

(Example)

Steel composition (% by weight) C Si Mn P S Sol.Al Cu Ni Nb Ti N(ppm) Invention example 0.074 0.22 1.6 0.008 0.002 0.023 0.23 0.61 0.012 0.015 40

Using a steel material having an emphasizing component as shown in Table 1, thread welding was performed while varying the amount of heat input to 5kJ/mm, 8.75kJ/mm, 10kJ/mm, and 15kJ/mm.

Figure 1 is 5kJ/mm, 8.75kJ/mm, 10kJ/mm and 15kJ/mm cooling of 7℃/sec, 4℃/sec, 3.5℃/sec and 2.3℃/sec in the temperature range of 800~500℃ for each heat input This figure shows the result of measuring the phase transformation temperature and phase fraction during cooling when cooling at speed.

As shown in FIG. 1, the Ts temperature (the temperature at which the primary ferrite is generated) increased as the amount of heat input increased, and the bainite start temperature (Bs) and the transformation end temperature (Tf) are as shown in FIG. The final phase fraction (cornerstone ferrite: bainite) was confirmed to be 2.9: 97.1, 13.2: 66.8, 17.5: 82.5, 21: 79 at welding speeds of 5, 8.75, 10, and 15.0 kJ/mm, respectively. That is, it was confirmed that the content of cornerstone ferrite increased as the amount of heat input for welding increased, and accordingly, it was confirmed that the average hardness also decreased from 222Hv to 195Hv. And it was confirmed that most of the observed ferrite shapes were grain boundary ferrites, and the width increased as the amount of heat input increased. And it was confirmed that the coarse needle-shaped Beadman Staten ferrite significantly increased under the condition of cooling rate in the temperature range of 800~500℃ and heat input of 10.0kJ/mm (3.5℃/sec).

2 is a graph showing the change in the size of the M/A phase according to the amount of heat input in CG and ICHAZ, and FIG. 3 is a graph showing the change in the fraction of the M/A phase according to the amount of heat input in CG and ICHAZ.

As shown in Fig. 2-3, it can be seen that the size and fraction of the M/A phase deteriorating the low-temperature toughness decreases as the amount of heat input increases from 5.0 to 8.75 kJ/mm in common for CG and ICHAZ.

Table 2 below shows the results of a tensile test for the total thickness of each heat input including the weld. It can be seen that the requirements of EN S460 steel meet all the criteria of a minimum yield strength of 400 MPa and a tensile strength of 500 to 660 MPa.

Heat input YS(MPa) TS(MPa) EL(%) 5.0kJ/mm 444.62 575.52 27.42 8.75kJ/mm 432.67 567.26 23.28

Table 3 shows the -40°C impact toughness test according to the heat input in the steel thickness (Face, Center, Root) and notch location (welded part (WM), coarse heat affected part (CGHAZ), single-phase heat affected part (SCHAZ)) It is a result, and it can be confirmed that all the criteria of at least 46J, which is the requirement of ENS460 steel, are satisfied under all conditions.

5.0kJ/mm,
Impact absorption energy (J) 8.75kJ/mm,
Impact absorption energy (J) FACE (surface) WM 114.6 109.83 CG 132.4 105.96 SC 222.3 244.8 CENTER(t/2) WM 105.2 179.59 CG 152.7 243.35 SC 183.3 178.99 ROOT (base) WM 126 135.45 CG 213.1 104.02 SC 277.5 294.04

Table 4 below is the result of -10°C CTOD test according to the heat input at the notch position (welded part (WM), coarse heat affected part (CGHAZ), single phase heat affected part (SCHAZ)), and at least 0.25 mm or more in all conditions It can be seen that it shows excellent CTOD results.

Heat input Notch CTOD(BS7448), mm 5.0kJ/mm Weld Metal 0.39 CGHAZ 1.47 SCHAZ 0.4 8.75kJ/mm Weld Metal 1.0 CGHAZ 2.42 SCHAZ 1.11

As described above, in the amount of heat input from welding 5kJ/mm to 8.75kJ/mm, the phase fraction of cornerstone ferrite and bainite in the coarse heat affected zone (CGHAZ) is 1: 4.8 to 33.5, and the coarse heat affected zone (CGHAZ) Includes the M/A phase whose size is 0.7㎛ or less and the phase fraction for the whole tissue is 0.04 area% or less, and the ideal zone heat affected zone (ICHAZ) is 0.9㎛ or less and the phase fraction for the whole tissue. By providing a welded joint including an M/A phase of 0.3% by area or less, stable welded toughness can be secured.

In the detailed description of the present invention, a preferred embodiment of the present invention has been described, but a person of ordinary skill in the art to which the present invention pertains can, of course, make various modifications without departing from the scope of the present invention. Therefore, the scope of the present invention is limited to the described embodiments and should not be determined, and should not be determined by the claims to be described later, as well as those equivalent thereto.

Claims (1)

In wt%, carbon (C): 0.07 to 0.15%, silicon (Si): 0.10 to 0.25%, manganese (Mn): 1.5 to 1.65%, phosphorus (P): 0.008% or less, S: 0.002% or less, aluminum (Al): 0.015 to 0.025%, Copper (Cu): 0.2 to 0.3%, Nickel (Ni): 0.5 to 1.0%, Niobium (Nb): 0.017% or less (excluding 0%), Titanium (Ti): 0.010 ~0.025%, nitrogen (N): 0.002 ~ 0.005%, the balance contains Fe and inevitable impurities,
In the range of welding heat input 5kJ/mm ~ 8.75kJ/mm,
The phase fraction of cornerstone ferrite and bainite in the coarse heat affected zone (CGHAZ) is 1: 4.8-33.5;
The coarse heat affected zone (CGHAZ) includes a M/A (Martensite-Austenite) phase having a size of 0.7 μm or less and a phase fraction of 0.04 area% or less for the entire tissue; And
The ideal heat-affected zone (ICHAZ) is a welded joint of high heat input marine structural steel having excellent low-temperature toughness, characterized in that it includes an M/A phase having a size of 0.9 μm or less and a phase fraction of 0.3 area% or less for the entire structure.

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