JP2005307261A - Thick high strength steel plate having large heat input weld heat affected zone toughness - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a thick high strength steel plate having a Ceq of 0.35 to 0.40, and, e.g., having a plate thickness of 50 to 80 mm and a tensile strength in a class of 490 to 620 MPa, and having excellent HAZ (heat affected zone) toughness in high heat input welding at 20 to 100 kJ/mm. <P>SOLUTION: The steel plate has a composition comprising, by mass, 0.03 to 0.14% C, 0.01 to 0.30% Si, 0.8 to 2.0% Mn, ≤0.02% P, ≤0.005% S, 0.003 to 0.050% Nb, 0.005 to 0.040% Al, 0.005 to 0.030% Ti, 0.0010 to 0.0050% B, 0.0030 to 0.0080% N, and 0.0010 to 0.0050% O, and in which the Ceq in formula (1) is 0.35 to 0.40, and also, Nb, Ti, N, and B satisfy expression (2), and further comprising one or more kinds selected from 0.0005 to 0.0030% Ca, 0.0005 to 0.0050% Mg, and 0.005 to 0.030% rare earth metals, and comprising multiple oxides with a diameter of the equivalent circle of 0.005 to 0.5 μm by 100 to 3,000 pieces/mm<SP>2</SP>; formula (1): Ceq=C+Mn/6+(Ni+Cu)/15+(Cr+Mo+V)/5, and expression (2): 0.29Ti+1.3B≤N≤0.29Ti+1.3B+0.15Nb. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a thick high-strength steel sheet excellent in low-temperature toughness of a welded heat affected zone (hereinafter referred to as HAZ) used for ships, offshore structures, middle-high-rise buildings, bridges, and the like. Further, the present invention relates to a steel plate having a thickness of 50 mm or more and a base metal tensile strength of 490 to 620 MPa class and having excellent low temperature HAZ toughness with a welding heat input of 20 to 100 kJ / mm.

  In recent years, demands for material properties of steel materials for welding used in large structures such as ships, offshore structures, high-rise buildings, and bridges have increased. In particular, in these structures, the use of steel plates having a thickness exceeding 50 mm and whose base material has a tensile strength of 570 MPa has been increasing.

  Also, in order to promote the efficiency of welding, one-pass welding by high heat input welding methods such as electrogas welding method and electroslag welding method is considered for welding such thick high strength steel plates. As with the toughness of the base material itself, the demand for HAZ toughness is becoming stricter.

  Many proposals that focus on the HAZ toughness of steel materials to which the high heat input welding method is applied have been made so far. For example, Patent Document 1 proposes a method in which composite precipitates of Ti nitride and MnS are utilized as ferrite transformation nuclei, and the HAZ structure is refined by increasing the ferrite generation sites to improve the HAZ toughness. . Further, Patent Document 2 proposes a method of improving HAZ toughness by utilizing a composite precipitate of Ti nitride and B nitride as a precipitation nucleus of grain boundary ferrite. However, when the required strength of the steel material is high, the hardenability is improved and the steel material does not function sufficiently.

  Further, as disclosed in Patent Document 3 and the like, there is also a method of improving toughness by pinning and suppressing austenite grains that grow and coarsen during welding with Ti nitride finely dispersed in steel. However, Ti nitride is almost dissolved in the vicinity of the boundary (hereinafter also referred to as a weld bond portion) with a weld metal having a maximum ultimate temperature exceeding 1400 ° C. of HAZ, so that the effect of improving toughness is reduced. There's a problem. Therefore, it is difficult for steel materials using such Ti nitrides to suppress the coarsening of austenite grains to achieve the strict requirements for HAZ toughness in recent years and the required characteristics of HAZ toughness in super-high heat input welding.

  As a method for improving the toughness in the vicinity of the weld bond portion, steel containing Ti oxide is used in various fields such as thick plates and section steel. For example, as exemplified in Patent Document 4 and Patent Document 5 in the thick plate field, steel containing Ti oxide is very effective in improving the toughness of a high heat input weld, and is applicable to high-tensile steel. Promising. This principle is based on Ti oxide, which is stable even at the melting point of steel, and Ti nitride, MnS, etc. are precipitated in the middle of the temperature drop after welding, and fine ferrite is formed using these as sites, resulting in toughness. The generation of coarse ferrite that is harmful to steel is suppressed, and deterioration of toughness can be prevented.

  However, such a Ti oxide has a problem that the number dispersed in steel cannot be increased too much. The cause is considered to be coarsening or aggregation of Ti oxides, and if the number of Ti oxides is increased, coarse Ti oxides of 5 μm or more, so-called inclusions are increased. This inclusion of 5 μm or more should be avoided because it is harmful because it becomes a starting point of destruction of the structure or causes a decrease in toughness. Therefore, in order to achieve further improvement in HAZ toughness, it is necessary to utilize an oxide that is less likely to be coarsened or aggregated and is more finely dispersed than Ti oxide.

  Further, as a method of dispersing such Ti oxide in steel, there are many methods by adding Ti to molten steel which does not substantially contain a strong deoxidizing element such as Al. However, it is difficult to control the number and dispersion of Ti oxides in the steel simply by adding Ti to the molten steel. Furthermore, the number and dispersion of precipitates such as TiN and MnS are controlled. It is also difficult. Therefore, in steel in which Ti oxide is dispersed only by Ti deoxidation, for example, the number of Ti oxides cannot be obtained sufficiently, or the toughness in the thickness direction of the thick plate varies. .

  Furthermore, in the method disclosed in Patent Document 4 and the like, the upper limit of the Al amount is limited to a very small amount of 0.007% in order to easily generate a Ti oxide. Therefore, when the amount of Al in the steel material is small, the toughness of the base material may be lowered due to a shortage of the AlN precipitate amount. Further, when a steel sheet having a small Al amount is welded using a commonly used welding material, the toughness of the weld metal may be lowered.

With respect to such a problem, in Patent Document 6 and Patent Document 7, Ti—Al composite oxide or composite oxide of Ti, Al, and Ca that is generated by Al addition immediately after Ti addition or Al / Ca composite addition is disclosed. A technology that utilizes the above has been proposed. Such a technique makes it possible to greatly improve the high heat input welding HAZ toughness.
Japanese Patent Laid-Open No. 03-264614 Japanese Patent Laid-Open No. 04-143246 Japanese Patent Publication No. 55-026164 JP 61-079745 A JP-A-62-103344 Japanese Patent Laid-Open No. 06-293937 JP-A-10-183295

  In recent years, particularly in the shipbuilding industry, steel plates having a plate thickness of 50 to 80 mm and a base material strength of 490 to 620 MPa in terms of tensile strength have been applied. When manufacturing this thick high-strength steel sheet by the current manufacturing method, it is necessary to increase hardenability by increasing the amount of alloy elements from the viewpoint of securing strength. When the hardenability due to the increase of the alloy element is expressed by the carbon equivalent (Ceq) indicating the hardenability of the chemical component as well as the weldability of the steel material, 0.35 or more in Ceq is required to secure the strength. . The value of Ceq is related to the hardness of the welded HAZ, and the higher this value, the harder the welded HAZ.

  Moreover, in welding of such thick steel plates, large heat input welding with a heat input of 20 to 100 kJ / mm is applied in view of work efficiency. Conventionally, since Ceq was not so high, the HAZ toughness was improved by applying the conventional methods described in Patent Documents 5 to 7. However, when Ceq is as high as 0.35 or higher, the hardenability is improved and the HAZ hardness is also increased. Therefore, in the application of the conventional method, sufficient HAZ toughness cannot be obtained particularly in the vicinity of the weld bond portion. .

  Therefore, the present invention is a steel plate having a Ceq of 0.35 to 0.40, for example, a plate thickness of 50 to 80 mm and a tensile strength of 490 to 620 MPa, and a large heat input welding with a heat input of 20 to 100 kJ / mm. The object is to provide a thick high-strength steel sheet having excellent HAZ toughness.

  As described above, as a means for improving the HAZ toughness, it has been considered that the HAZ structure is refined due to the formation of ferrite transformation nuclei and the growth of austenite grains at a high temperature is suppressed as described above. Among them, the suppression of coarsening of austenite grains due to fine dispersion of oxide is the case where the weld bond is heated to a high temperature of 1400 ° C. for a long time by high heat input welding with a heat input of 20 to 100 kJ / mm as described above. Also works and is effective. However, even if the austenite grain coarsening is suppressed, a steel material having a Ceq of 0.35 to 0.40 cannot obtain sufficient toughness because the hardness of the HAZ after high heat input welding is increased. The HAZ hardness is high because the hardenability of HAZ is high because Ceq is high, and the amount of intergranular ferrite occupying the HAZ structure is small, and the austenite grains are mainly composed of upper bainite. It is.

  Therefore, the inventors diligently studied a method for improving the HAZ toughness by generating a large number of ferrite forming nuclei in the austenite grains and suppressing the increase in the HAZ hardness and refining the structure. First, in order to investigate precipitates that can become intragranular ferrite formation nuclei, a thermal cycle simulating the high heat input welding shown in FIG. 1 was used using various steel materials having Ceq of 0.35 to 0.40. And the formation of intragranular ferrite in the reproduced HAZ structure was investigated. As a result, it was confirmed that in some of the B-added steel, ferrite was generated in the grains even under such a high Ceq condition.

  Then, based on the said knowledge, the production | generation nucleus of the intragranular ferrite of the steel material which intragranular ferrite produces | generates was investigated by TEM observation. As a result, it was confirmed that most of them were precipitated with Ti nitride (hereinafter referred to as Ti (Nb) nitride) in which a part of Ti was substituted with Nb as nuclei. Furthermore, as a result of further investigation on the Ti (Nb) nitride, the Ti (Nb) nitride is not only precipitated alone, but also contains fine composite oxides mainly composed of submicron-sized Al or Ca. It was confirmed that it was precipitated. The B nitride that forms ferrite is deposited in a polycrystalline state on the outer periphery so as to surround these Ti (Nb) nitride and composite oxide and Ti (Nb) nitride composites. It was confirmed that the size was the main thing.

Therefore, in order to generate a large number of ferrite-forming nuclei, the inventors studied diligently on conditions for depositing a large number of B nitrides.
First, a composite oxide serving as a Ti (Nb) nitride precipitation site was examined. As a result, the size of the composite oxide is 0.5 μm or less, and Ti (Nb) nitride is generated not only in oxides mainly composed of Al and Ca but also in composite oxides containing Mg and REM. It was confirmed.

  Next, conditions for generating a large number of these complex oxides were examined. First, in order to deposit a large amount of oxide, it is a method of increasing the amount of elements constituting these oxides, but it is difficult to increase the number while forming a coarse oxide of micron size simply by increasing the amount. .

  Therefore, a method for increasing the number of oxides by miniaturizing oxides was studied. In this method, the oxygen concentration is adjusted to be low in order to prevent coarsening of the oxide, and then the deoxidizing element is introduced in order from the deoxidizing element having a relatively weak deoxidizing power to the strong deoxidizing element. Since a plurality of deoxidizing elements are administered in order of decreasing deoxidizing power under the concentration adjustment, a large number of fine oxides can be generated.

And, as a result of examining the deoxidation method based on this idea, the oxygen concentration in the molten steel was controlled to 0.0010 to 0.0050% using a deoxidizing element of C or Si in the deoxidizing process at the time of molten steel, Then, after deoxidizing with a weak deoxidizing element such as Ti, and further deoxidizing with Al, one or more steel types of strong deoxidizing elements Ca, Mg, and REM are added, and the equivalent circle diameter is 0.005. -0.5 μm composite oxide (for example, Al oxide, etc. In addition, Ca—Mg oxide, Ti—Mg oxide, REM oxide and composite oxides thereof are also produced by adding Ca, Mg, REM. It was found that 100-3000 / mm ² can be contained. Note that when the oxygen concentration exceeds 0.0050%, coarse oxides are generated, and the number of oxides may be drastically reduced.

  Next, detailed examination was made on the balance of Ti, N, and B, which are constituent elements of Ti (Nb) nitride and B nitride. First, the amount of N was adjusted to be stoichiometrically equal to the amount of B from the point of B nitride formation. In this case, the austenite grains in the reproduced HAZ structure became a bainite-based structure, and almost no intragranular ferrite could be confirmed. Therefore, when the amount of N was further increased and stoichiometrically approximately equal to the sum of the amounts of B and Ti, the formation of a large number of ferrite was confirmed in the austenite grains. Further, when the amount of N was further increased, the formation of a large number of ferrite was confirmed in the austenite grains.

  Therefore, as a result of investigating the toughness of those having a reproducible HAZ structure, it has been found that those in which a large number of ferrites are formed in the austenite grains have excellent toughness even when compared with those containing bainite. Further, it has been found that among those producing intragranular ferrite, the case where N is added in a larger amount than the case where the amount of N is stoichiometrically equal to the total amount of B and Ti is found to be superior. However, if added excessively, the toughness decreases.

  The reason for the change in toughness due to the increase in the amount of N is that the amount of N is the sum of the amounts of B, Ti, and Nb, based on the fact that a portion of Ti in the deposited Ti nitride is replaced by Nb. The reproducibility of HAZ toughness was compared and examined by dividing it into a stoichiometrically large and a small amount. As a result, it was found that when the amount of N is stoichiometrically greater than the total amount of B, Ti, and Nb, the reproduced HAZ toughness is greatly reduced.

From the above examination results, it was found that the amounts of Nb, Ti, N, and B in the steel satisfy the formula [2] is a necessary condition for improving the HAZ toughness. That is, if the amount of N is stoichiometrically larger than the amounts of Ti and B and less than the amounts of Nb, Ti and B, there is hardly any solid solution B or solid solution N that causes a decrease in toughness. It has been found that Ti (Nb) nitride and B nitride necessary for the formation of intragranular ferrite can be generated and toughness can be improved.
0.29Ti + 1.3B ≦ N ≦ 0.29Ti + 1.3B + 0.15Nb [2] Formula

  Furthermore, regarding the components to be added contained in the steel, the HAZ toughness when Ni, Cu, Cr, Mo, V was added was also examined from the viewpoint of securing strength and improving corrosion resistance. As a result, the Ceq value represented by the formula [1] is 0.15 to 3.0%, 0.1 to 0.6%, 0.1 It has been found that HAZ toughness is not significantly reduced if the addition is in the range of -0.5%, 0.03-0.2%, 0.005-0.050%.

  In addition, the manufacturing method of the steel plate of this invention is not restrict | limited in particular, What is necessary is just to manufacture according to a well-known method. For example, after the molten steel adjusted to the above-mentioned preferred component composition is made into a slab by a continuous casting method, it is heated to 1000 to 1250 ° C. and then hot rolled.

The present invention has been completed based on the above findings. That is, the gist of the present invention is as follows.
(1) In mass%,
C: 0.03-0.14%, Si: 0.01-0.30%,
Mn: 0.8 to 2.0%, P: 0.02% or less,
S: 0.005% or less, Nb: 0.003-0.050%,
Al: 0.005-0.040%, Ti: 0.005-0.030%,
B: 0.0010 to 0.0050%, N: 0.0030 to 0.0080%,
O: 0.0010 to 0.0050%
Containing
[1] The value of Ceq represented by the formula is 0.35 to 0.40, and
The mass relationship of Nb, Ti, N and B satisfies the formula [2],
Furthermore, in mass%,
Ca: 0.0005 to 0.0030%, Mg: 0.0005 to 0.0050%,
REM: 0.005-0.030%
Of high heat input welding, characterized in that it contains 100-3000 pieces / mm ² of complex oxide containing one or more of them and having an equivalent circle diameter of 0.005-0.5 μm. Thick high-strength steel sheet with excellent toughness.
Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo + V) / 5 .. [1] Formula 0.29Ti + 1.3B ≦ N ≦ 0.29Ti + 1.3B + 0.15Nb (2) Formula

(2) Furthermore, in mass%,
Ni: 0.1-3.0%, Cu: 0.1-0.6%,
Cr: 0.1-0.5%, Mo: 0.03-0.2%,
V: 0.005 to 0.050%
Thick high-strength steel sheet having excellent high heat input welding heat-affected zone toughness as described in (1) above.

  The present invention supplies steel sheets that satisfy severe toughness requirements for the destruction of ships, offshore structures, mid-to-high-rise buildings, etc., and has an extremely large effect on this type of industrial field. Further, it means the safety of structures. The contribution to society is very large.

The reason for limiting the composition of the steel material used in the present invention will be described. Hereinafter, the mass% in the composition is simply expressed as%.
C is an effective component for improving the strength of steel. The lower limit is 0.03%, and excessive addition produces a large amount of carbides and MA, and the HAZ toughness is remarkably reduced. Therefore, the upper limit is 0.14%. It was.

  Si is a component necessary for securing the strength of the base material, deoxidation, and the like, and is added in an amount of 0.01% or more. .

  Mn needs to be added in an amount of 0.8% or more as an effective component for securing the strength and toughness of the base material, but the upper limit is set to 2.0% within an allowable range such as toughness and crackability of the welded portion. .

  P is more desirable as the content is smaller, but in order to reduce this industrially, it takes a great deal of cost, so the content range was set to 0.02% or less.

  The content of S is preferably as small as possible. However, since it takes a great deal of cost to reduce this industrially, the content range is set to 0.005% or less.

  Nb is an element effective for improving the strength of the base material by improving the hardenability. In the present invention, Nb is a nucleus for generating intragranular ferrite in the HAZ structure in high heat input welding. This is necessary to produce Ti (Nb) nitride, which is a BN precipitation site. However, in order to improve the strength of the base material, the addition of less than 0.003% does not provide a sufficient increase in strength, and excessive addition exceeding 0.050% significantly reduces the toughness of the base material. The range was 0.003% to 0.050%.

  Al is an important deoxidizing element, so 0.005% or more is added. However, if it is present in a large amount, the surface quality of the slab is deteriorated and coarse oxides are generated. did.

  Ti is an important deoxidizing element, and at the same time, in the present invention, Ti (Nb) nitride, which is a BN precipitation site serving as a nucleus for generating intragranular ferrite in the HAZ structure in high heat input welding. In order to produce the above, 0.005% or more is added. However, excessive addition leads to the formation of coarse oxides, so 0.030% was made the upper limit.

  B is added in an amount of 0.0010% or more in order to form B nitride. However, excessive addition degrades the toughness of the base metal, so 0.0050% was made the upper limit.

  N is necessary to produce B nitride and Ti (Nb) nitride. The amount of Nb, Ti, and B need to be added in an amount that satisfies the relationship of the formula [2]. Therefore, 0.0030% or more must be added, but excessive addition is coarse. In this case, 0.0080% was made the upper limit.

O needs to be suppressed to 0.0050% or less in order to generate fine oxides in the steel, but a sufficient oxide cannot be obtained at 0.0010% or less, so the range is 0.0010% or more. 0.0050% or less.

  In order to increase the number of oxides in the steel, one or more of Ca, Mg, and REM must be added after deoxidation of Ti and Al. In consideration of the generation of coarse inclusions, the respective ranges are Ca: 0.0005 to 0.0030%, Mg: 0.0005 to 0.0050%, REM: 0.005 to 0.030%. It was.

  Ni is added in an amount of 0.1% or more as necessary in order to improve the strength and corrosion resistance of the steel material, but the upper limit is set to 3.0% in view of economy.

  Cu is added in an amount of 0.1% or more as necessary in order to improve the strength and corrosion resistance of the steel material. However, if it exceeds 0.6%, MA tends to be formed and the HAZ toughness is lowered. The upper limit was 6%.

  Cr is added in an amount of 0.1% or more as necessary in order to improve the corrosion resistance of the steel material, but excessive addition causes a reduction in HAZ toughness due to MA formation, so 0.5% was made the upper limit.

  Mo is an effective element for improving the strength and corrosion resistance of the base metal, and is added in an amount of 0.03% as necessary. However, excessive addition causes a decrease in HAZ toughness due to MA formation. % Was the upper limit.

  V is an element effective for improving the strength of the base metal and is added as required in an amount of 0.005%. However, excessive addition causes a decrease in HAZ toughness due to MA formation, so 0.050% is added. The upper limit.

  A steel piece having a chemical composition shown in Table 1 was continuously cast to produce a rope piece. As for the deoxidation method, for those other than C5, the dissolved oxygen of the molten steel was adjusted to 0.0010% to 0.0050% with Si before introducing Ti, and then deoxidized with Ti first and subsequently deoxidized with Al. Thereafter, one or more of Ca, Mg, and REM were added for deoxidation. C5 is Al, and none of Ca, Mg, and REM after deoxidation is added. After reheating these at 1100 to 1250 ° C., steel plates having a thickness of 50 to 80 mm were manufactured by the following two types of rolling methods. One is a method in which the surface temperature is rolled in a temperature range of 750 to 900 ° C., and then the plate surface after water cooling is cooled in water until it is reheated within a temperature range of 200 to 400 ° C. (described as TMCP in Table 2). The other is a manufacturing method (indicated as DQ-T in Table 2) after hot rolling, water cooling to room temperature, and tempering in the range of 500-600 ° C.

Table 2 shows the manufacturing conditions, plate thickness, and mechanical properties of the steel plate. Table 2 also shows the number of fine oxides having an equivalent circle diameter of 0.005 to 0.5 μm, measured at an arbitrary location on the steel sheet. The number of oxides is observed by producing an extraction replica from an arbitrary part of the steel sheet, and observing it with an electron microscope at a magnification of 10,000 at 100 fields or more (observation area of 10,000 μm ² or more). Elemental analysis was performed on each particle having a diameter of 0.5 μm, and the number of oxides was counted. In steel materials other than C5, a fine oxide having an equivalent circle diameter of 0.01 to 0.5 μm is dispersed in an amount of 100 to 3000 / mm ² within the range of the present invention.

  These steel plates were subjected to vertical one-pass welding by facing the steel plates using electrogas welding (EGW) or electroslag welding (ESW) having a welding heat input of 20 to 100 kJ / mm. And in HAZ located in a plate | board thickness center part (t / 2), notch was put into two places, 1 mm away from FL, and the Charpy impact test was done at -40 degreeC. Table 2 shows welding conditions and HAZ toughness. In this Charpy impact test, a full size test piece of JIS No. 2 mmV notch was used.

  D1 to D10 are steels of the present invention. Since the chemical composition of steel is appropriately controlled, the high heat input HAZ toughness at −40 ° C. is satisfactory while satisfying a predetermined base material performance. Further, since the number of fine oxides is 100 or more and the amounts of Nb, Ti, N, and B satisfy the formula [2], the high heat input HAZ toughness at −40 ° C. It is a high value exceeding 100J.

  On the other hand, since C1-5 of the comparative steels are all outside the scope of the present invention, the high heat input HAZ toughness is insufficient. That is, C1 and C4 do not satisfy the formula [2], C2 is outside the scope of the present invention, C3 is outside the scope of the present invention, and C5 is Ca, Mg, Since none of the REMs is contained and deoxidation is not appropriate, the number of fine oxides is outside the scope of the present invention.

It is a figure which shows the welding thermal cycle equivalent to 45 kJ / mm.

Claims (2)

% By mass
C: 0.03-0.14%,
Si: 0.01-0.30%,
Mn: 0.8 to 2.0%,
P: 0.02% or less,
S: 0.005% or less,
Nb: 0.003 to 0.050%,
Al: 0.005 to 0.040%,
Ti: 0.005 to 0.030%,
B: 0.0010 to 0.0050%,
N: 0.0030 to 0.0080%,
O: 0.0010 to 0.0050%
Containing
[1] The value of Ceq represented by the formula is 0.35 to 0.40, and
The mass relation of Nb, Ti, N and B satisfies the formula [2],
Furthermore, in mass%,
Ca: 0.0005 to 0.0030%,
Mg: 0.0005 to 0.0050%,
REM: 0.005-0.030%
High heat input welding heat-affected zone characterized in that it contains 100-3000 pieces / mm ² of composite oxide containing one or more of them and having an equivalent circle diameter of 0.005-0.5 μm Thick high-strength steel sheet with excellent toughness.
Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo + V) / 5 .. [1] Formula 0.29Ti + 1.3B ≦ N ≦ 0.29Ti + 1.3B + 0.15Nb (2) Formula Furthermore, in mass%,
Ni: 0.1 to 3.0%,
Cu: 0.1 to 0.6%,
Cr: 0.1 to 0.5%,
Mo: 0.03-0.2%,
V: 0.005 to 0.050%
The thick high-strength steel sheet excellent in high heat input welding heat-affected zone toughness according to claim 1, characterized by containing 1 type or 2 types or more.

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