KR20140119898A - Hot-rolled steel and method of manufacturing the same - Google Patents

Abstract

The present invention relates to a hot-rolled steel sheet satisfying the API 5L X70 PSL2 standard (tensile strength (TS): 600 MPa or more and yield strength (YP): 555 MPa or more)
A method for manufacturing a hot-rolled steel sheet according to the present invention comprises: 0.03 to 0.07 weight% of carbon (C), 0.2 weight% or less of silicon (Si), 0.5 to 1.5 weight% of manganese (Mn) (Ni), 0.07 to 0.1 wt% of niobium (Nb), 0.01 to 0.02 wt% of titanium (Ti), 0.2 to 0.45 wt% of chromium (Cr), 0.2 to 0.35 wt% of copper, : Reheating a steel slab composed of 0.1 to 0.3% by weight of Ca, 0.001 to 0.003% by weight of Ca and the balance of Fe and other unavoidable impurities to a slab reheating temperature (SRT) of 1200 to 1250 占 폚; A hot rolling step in which the reheated slab is primary rolled to 900 to 950 占 폚 for RDT (Roughing Delivery Temperature) and then secondarily rolled to 800 to 850 占 폚 for FRT (Finish Rolling Temperature); And cooling and winding the hot-rolled steel.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a hot-rolled steel sheet,

More particularly, the present invention relates to an API (American Pertoleum Institute) hot-rolled steel sheet having excellent corrosion resistance through control of alloy components and process conditions and a method for manufacturing the same.

Until now, the development of genetic resources has been actively promoted mainly in temperate and tropical regions where the development environment is good. However, due to the depletion of resources in the region along with the development of the petroleum industry in recent years, Is being developed.

In this regard, crude steel pipes for transporting petroleum and natural gas, which have been developed as large-scale projects, from the oil field to the inspection site for long distances are also required to be redeveloped, which can be applied in severe environments of these extremes.

The quality characteristics of pipe steels used under extreme environmental conditions are not only strength but also resistance to brittleness which can be easily generated at low temperatures and HIC (hydrogen organic cracking) properties such as hydrogen sulfide.

However, in the case of currently used pipe steels, there is no specific criterion for impact toughness.

A related prior art is Korean Patent Laid-Open Publication No. 2002-0070282 (published on September 5,2002), which discloses a high-strength hot-rolled steel sheet and a manufacturing method thereof.

It is an object of the present invention to provide a hot-rolled steel sheet which is used as a material for an oil pipeline for transporting crude oil, and which is durable even in the corrosive environment of H2S, and a method of manufacturing the same.

Another object of the present invention is to provide a hot-rolled steel sheet satisfying a tensile strength (TS) of 600 MPa or more and a yield strength (YP) of 555 MPa or more by controlling an alloy component and process conditions and a method of manufacturing the same.

A method for manufacturing a hot-rolled steel sheet for achieving the above object is characterized by comprising: 0.03 to 0.07% by weight of carbon (C), 0.2% by weight or less of silicon (Si), 0.5-1.5% by weight of manganese (Mn) 0.05 to 0.35 weight% of niobium (Nb), 0.0 to 0.1 weight% of titanium oxide, 0.2 to 0.45 weight% of chromium (Cr), 0.2 to 0.35 weight% of copper (Cu) Reheating a steel slab composed of 0.1 to 0.3% by weight of Ni, 0.001 to 0.003% by weight of Ca and balance of Fe and other unavoidable impurities to a slab reheating temperature (SRT) of 1200 to 1250 占 폚; A hot rolling step in which the reheated slab is primary rolled to 900 to 950 占 폚 for RDT (Roughing Delivery Temperature) and then secondarily rolled to 800 to 850 占 폚 for FRT (Finish Rolling Temperature); And cooling and winding the hot-rolled steel.

According to another aspect of the present invention, there is provided a hot-rolled steel sheet comprising 0.03 to 0.07% by weight of carbon (C), 0.2% by weight or less of silicon (Si), 0.5 to 1.5% (Al): 0.02 to 0.05 wt%, Nb: 0.07 to 0.1 wt%, Ti: 0.01 to 0.02 wt%, Cr: 0.2 to 0.45 wt%, Cu: (TS) of 600 MPa or more and a yield of 500 MPa or more, and is composed of 0.35 wt% of nickel, 0.1-0.3 wt% of nickel, 0.001-0.003 wt% of calcium and the balance of iron and other unavoidable impurities. And a strength (YP) of 555 MPa or more.

The hot-rolled steel sheet and the manufacturing method thereof according to the present invention satisfy the API 5L X70 PSL2 standard (tensile strength (TS): 600 MPa or higher and yield strength (YP): 555 MPa or higher), and hydrogen organic cracking resistance can be secured .

1 is a flowchart showing a method of manufacturing a hot-rolled steel sheet according to an embodiment of the present invention.
Fig. 2 shows the results of C-scan after the HIC test of the hot-rolled steel sheet according to Example 1 and Comparative Example 2 of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS The advantages and features of the present invention, and the manner of achieving them, will be apparent from and elucidated with reference to the embodiments described hereinafter in conjunction with the accompanying drawings. The present invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Is provided to fully convey the scope of the invention to those skilled in the art, and the invention is only defined by the scope of the claims. Like reference numerals refer to like elements throughout the specification.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a hot-rolled steel sheet according to a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings.

Hot-rolled steel sheet

The hot-rolled steel sheet according to the present invention is intended to have a tensile strength (TS) of 600 MPa or more and a yield strength (YP) of 555 MPa or more.

For this purpose, the hot-rolled steel sheet according to the present invention comprises 0.03 to 0.07% by weight of carbon (C), 0.2% by weight or less of silicon (Si), 0.5-1.5% by weight of manganese (Mn) (Ni), 0.07 to 0.1 wt% of niobium (Nb), 0.01 to 0.02 wt% of titanium (Ti), 0.2 to 0.45 wt% of chromium (Cr), 0.2 to 0.35 wt% of copper ): 0.1 to 0.3% by weight, calcium (Ca): 0.001 to 0.003% by weight, and the balance of iron (Fe) and other unavoidable impurities.

It may further comprise at least one of phosphorus (P): 0.01 wt% or less, sulfur (S): 0.001 wt% or less, and nitrogen (N): 0.006 wt% or less.

Hereinafter, the role and content of each component included in the hot-rolled steel sheet according to the present invention will be described.

Carbon (C)

In the present invention, carbon (C) is added for securing strength and controlling microstructure.

The carbon (C) is preferably added at a content ratio of 0.03 to 0.07% by weight based on the total weight of the hot-rolled steel sheet according to the present invention. When the content of carbon (C) is less than 0.03 wt%, it may be difficult to secure strength. On the contrary, when the content of carbon (C) exceeds 0.07% by weight, the strength of the steel increases, but the low-temperature impact toughness and weldability deteriorate.

silicon( Si )

In the present invention, silicon (Si) is added as a deoxidizer to remove oxygen in the steel in the steelmaking process. Silicon (Si) also has a solid solution strengthening effect.

However, when the content of silicon (Si) is more than 0.2% by weight, the weldability of the steel is lowered and the scale of the reheating step and the hot rolling step are generated, which may cause problems in the surface quality and may hinder the plating ability after welding .

Therefore, silicon (Si) is preferably added at a content ratio of 0.2% by weight or less based on the total weight of the hot-rolled steel sheet according to the present invention.

On the other hand, the hot-rolled steel sheet according to the present invention preferably includes silicon (Si) and manganese (Mn) in a range satisfying the following formula (1).

6 < / RTI > [Mn] / [Si] ≤ 10 wherein [

If the weight ratio of silicon to manganese (Mn) is less than 6 in Equation (1), Mn-Si-O inclusions can not be sufficiently discharged to the outside, and cracks may occur in the welded portion. On the other hand, if the weight ratio of silicon to manganese (Mn) exceeds 10, center segregation may occur and the corrosion resistance of the steel may be greatly reduced.

manganese( Mn )

Manganese (Mn) is an element that is effective in securing strength by improving the hardenability of steel as a solid solution strengthening element. In addition, by retarding the ferrite and pearlite transformation as the austenite stabilizing element, the corrosion resistance of the steel is significantly lowered.

The content of manganese (Mn) is preferably 0.5 to 1.5% by weight based on the total weight of the hot-rolled steel sheet according to the present invention. If the content of manganese (Mn) is less than 0.5 wt%, it may be difficult to secure strength even if the content of carbon (C) is high. On the contrary, when the content of manganese (Mn) exceeds 1.5% by weight, the amount of MnS-based nonmetallic inclusions increases, which can cause defects such as cracking during welding.

aluminum( Al )

Aluminum (Al) reacts with nitrogen (N) to form fine AlN precipitates, thereby contributing to improvement of strength by precipitation strengthening as well as grain refinement.

Aluminum (Al) is preferably added in a content ratio of 0.02 to 0.05% by weight based on the total weight of the hot-rolled steel sheet according to the present invention. When the content of aluminum (Al) is less than 0.02% by weight, the amount of AlN precipitates is reduced and it may be difficult to secure sufficient strength. On the contrary, when the content of aluminum (Al) exceeds 0.05% by weight, difficulty is encountered in the performance process, which lowers the productivity and raises the yield strength excessively.

Niobium ( Nb )

Niobium (Nb) is one of the elements that have a great influence on the strength. It precipitates carbonitride precipitates in the steel or improves the strength of the steel through solid solution strengthening in iron (Fe). Further, by increasing the recrystallization temperature during rolling, suppressing recrystallization, the non-recrystallized reverse pressurization is increased, and the crystal grains of the final microstructure can be miniaturized and the low temperature toughness can be greatly improved.

Niobium (Nb) is preferably added in a content ratio of 0.07 to 0.1% by weight based on the total weight of the hot-rolled steel sheet according to the present invention. When the content of niobium (Nb) is less than 0.07% by weight, the effect of adding niobium can not be exhibited properly. On the contrary, when the content of niobium (Nb) exceeds 0.1 wt%, it is difficult to be reused and the material and corrosion resistance can be deteriorated.

titanium( Ti )

Titanium (Ti) has the effect of improving the toughness and strength of steel by refining the texture of the welded part by inhibiting the growth of austenite crystal grains during welding by generating precipitates of Ti (C, N) having high stability at high temperatures.

Titanium (Ti) is preferably added in an amount of 0.01 to 0.02% by weight based on the total weight of the hot-rolled steel sheet according to the present invention. When the content of titanium (Ti) is less than 0.01% by weight, there arises a problem that aging hardening occurs because of the remaining solid carbon and nitrogen employed without precipitation. On the other hand, when the content of titanium (Ti) exceeds 0.02% by weight, corrosion resistance of the steel can be lowered by forming coarse precipitates.

chrome( Cr )

Chromium (Cr) is an element that suppresses center segregation by increasing diffusion of manganese (Mn) in the manufacture of slabs, and forms a low-temperature transformation phase upon accelerated cooling after hot rolling to provide excellent strength and corrosion resistance.

Cr (Cr) is preferably added in a content ratio of 0.2 to 0.45% by weight based on the total weight of the hot-rolled steel sheet according to the present invention. If the content of chromium (Cr) is less than 0.2% by weight, the effect of the addition can not be exhibited properly. On the contrary, when the content of chromium (Cr) exceeds 0.45% by weight, inclusions of MC2 and M23C6 may be formed and corrosion resistance may be inhibited.

Copper( Cu )

Copper (Cu), when added together with nickel (Ni), exhibits excellent corrosion resistance and inhibits the permeation of hydrogen (H).

Copper (Cu) is preferably added in an amount of 0.2 to 0.35% by weight based on the total weight of the hot-rolled steel sheet according to the present invention. When the content of copper (Cu) is less than 0.2% by weight, the effect of adding copper can not be exhibited properly. On the contrary, when the content of copper (Cu) exceeds 0.35% by weight, there arises a problem of inducing heat-induced embrittlement.

nickel( Ni )

Nickel (Ni) is an element that improves the strength of steel through solid solution strengthening and improves the hardenability of steel.

Nickel (Ni) is preferably added at a content ratio of 0.1 to 0.3% by weight based on the total weight of the hot-rolled steel sheet according to the present invention. If the content of nickel (Ni) is less than 0.1% by weight, the effect of adding nickel can not be exhibited properly. On the contrary, when the content of nickel (Ni) is more than 0.3 wt% and added in a large amount, there arises a problem of inducing a hot brittleness.

calcium( Ca )

Calcium (Ca) is added for the purpose of improving the electrical resistance weldability by inhibiting the formation of MnS inclusions which inhibit corrosion resistance and weldability by forming CaS inclusions.

The calcium (Ca) is preferably added in an amount of 0.001 to 0.003% by weight or less of the total weight of the hot-rolled steel sheet according to the present invention. If the content of calcium is less than 0.001% by weight, the above effects can not be exhibited properly. On the contrary, when the content of calcium (Ca) exceeds 0.003% by weight, generation of CaO inclusions is excessive and performance is deteriorated.

Phosphorus (P), sulfur (S)

Phosphorus (P) contributes partly to strength improvement, but it is an element that lowers corrosion resistance by slab center segregation. The lower the content, the better. Therefore, in the present invention, the content of phosphorus (P) is limited to 0.01% by weight or less based on the total weight of the steel material.

Sulfur (S), together with phosphorus (P), is an element that is inevitably contained in the production of hot-rolled steel sheet, which deteriorates the toughness and weldability of steel and forms non-metallic inclusions of MnS. Accordingly, in the present invention, the content of sulfur (S) is limited to 0.001% by weight or less based on the total weight of the hot-rolled steel sheet.

Nitrogen (N)

Nitrogen (N) binds with niobium (Nb) or the like to form a denitrified material. However, when the amount of nitrogen added increases, the amount of dissolved nitrogen is increased to decrease the impact characteristics and elongation of the steel.

However, when the content of nitrogen (N) is added in excess of 0.006 wt% of the total weight, the effect of precipitate formation is saturated, and the amount of dissolved nitrogen distributed in the weld heat affected zone increases, .

Therefore, nitrogen (N) was limited to a content ratio of 0.006% by weight or less of the total weight of the hot-rolled steel sheet according to the present invention.

Hot-rolled steel sheet manufacturing method

1 is a flowchart schematically showing a method of manufacturing a hot-rolled steel sheet having excellent low temperature toughness according to an embodiment of the present invention.

Referring to FIG. 1, the illustrated hot-rolled steel sheet manufacturing method includes a slab reheating step S110, a hot rolling step S120, and a cooling and winding step S130. At this time, the slab reheating step (S110) is not necessarily performed, but it is more preferable to perform the reheating step (S110) in order to obtain effects such as the reuse of the precipitate.

In the hot rolled steel sheet manufacturing method according to the present invention, the steel slab in the semi-finished product state to be subjected to the hot rolling process is composed of 0.03 to 0.07% by weight of carbon (C), 0.2% by weight or less of silicon (Si) (Al): 0.02 to 0.05 wt%, niobium (Nb): 0.07 to 0.1 wt%, titanium (Ti): 0.01 to 0.02 wt%, chromium (Cr): 0.2 to 0.45 wt% (Fe) and other inevitable impurities, in an amount of 0.2 to 0.35 wt%, nickel (Ni): 0.1 to 0.3 wt%, calcium (Ca): 0.001 to 0.003 wt%

It may further comprise at least one of phosphorus (P): 0.01 wt% or less, sulfur (S): 0.001 wt% or less, and nitrogen (N): 0.006 wt% or less.

Reheating slabs

In the slab reheating step S110, the slab having the above composition is reheated for 2 to 3 hours at the SRT (Slab Reheating Temperature) of 1200 to 1250 ° C. The slab having the above composition can be obtained through a continuous casting process after obtaining a molten steel having a desired composition through a steelmaking process. It is preferable to reheat the slab at a relatively high temperature so that niobium (Nb) can be sufficiently solidified.

When the slab reheating temperature is lower than 1200 ° C, the segregated components are not sufficiently reused during casting, and the precipitate dissolution of niobium (Nb) and the like is not sufficiently achieved. On the contrary, when the reheating temperature exceeds 1250 DEG C, the austenite grain size increases and it may be difficult to obtain strength.

Hot rolling

In the hot rolling step (S120), the reheated plate is hot-rolled. The hot rolling step (S120) in the present invention can be carried out by primary rolling and secondary rolling.

In the primary rolling process, the reheated plate is first rough-rolled. At this time, the finish rolling temperature (Roughing Delivery Temperature: RDT) of primary rolling is 900 to 950 ° C. If the finish rolling temperature of the primary rolling is less than 900 캜, there may arise problems such as generation of blind spots due to abnormal reverse rolling. On the other hand, when the finish rolling temperature of the primary rolling exceeds 950 DEG C, the austenite grains are coarsened and the ferrite grains are not finely refined after the transformation, which makes it difficult to ensure strength.

Also, the primary rolling can be performed so that the cumulative rolling reduction in the non-recrystallized region is 60% or more. If the cumulative rolling reduction of the primary rolling is less than 60%, it is difficult to obtain a uniform but fine structure, which may cause a significant variation in strength and impact toughness.

In the secondary rolling process, the primary rolled plate is rolled in an austenite non-recrystallized region. At this time, finishing delivery temperature (FDT) of secondary rolling is preferably 800 to 850 캜. If the finish rolling temperature of the secondary rolling is less than 800 ° C, abnormal reverse rolling occurs to form an uneven structure, which may significantly reduce the low temperature impact toughness. On the other hand, when the finish rolling temperature of the secondary rolling exceeds 850 DEG C, there is a problem that the ductility and toughness are excellent but the strength is rapidly lowered.

At this time, the secondary rolling may be performed so that the cumulative rolling reduction in the non-recrystallized region is 70% or more. If the cumulative rolling reduction of the secondary rolling is less than 70%, the strain can not be sufficiently applied to the center of the thickness, so that it may be difficult to obtain fine crystal grains after cooling.

Cooling and Coiling

In the cooling and winding step (S130), the hot rolled plate is cooled to a CT (Coiling Temperature): 550 ° C to 650 ° C and is wound.

In the cooling process of the present invention, by cooling the rolled plate to 550 to 650 ° C, the austenite to ferrite structure transformation takes place, and a high strength steel can be produced by forming a fine precipitate phase at the time of transformation. At this time, the cooling rate is preferably 15 to 25 DEG C / sec.

If the coiling temperature (CT) is less than 550 占 폚 or the cooling rate is less than 15 占 폚 / sec, there is a problem that strength is lowered due to formation of coarse microstructure. On the other hand, when the coiling temperature CT exceeds 650 占 폚 or the cooling rate exceeds 25 占 폚 / sec, the bainite fraction increases and the strength increases, but toughness and corrosiveness are hardly ensured.

Accordingly, the hot-rolled steel sheet formed by the manufacturing method according to the present invention may have a tensile strength (TS) of 600 MPa or more and a yield strength (YP) of 555 MPa or more.

Example

Hereinafter, the configuration and operation of the present invention will be described in more detail with reference to preferred embodiments of the present invention. It is to be understood, however, that the same is by way of illustration and example only and is not to be construed in a limiting sense.

The contents not described here are sufficiently technically inferior to those skilled in the art, and a description thereof will be omitted.

1. Specimen Manufacturing

Specimens according to Examples 1 to 3 and Comparative Examples 1 and 2 were prepared with the composition shown in Tables 1 and 2 and the process conditions shown in Table 3.

 [Table 1] (unit:% by weight)

[Table 2] (unit:% by weight)

[Table 3]

2. Evaluation of mechanical properties

Table 4 shows the results of evaluation of mechanical properties of the specimens prepared according to Examples 1 to 3 and Comparative Examples 1 and 2.

[Table 4]

Referring to Tables 1 to 4, the specimens prepared according to Examples 1 to 3 satisfy all of the tensile strength (TS) of not less than 600 MPa and yield strength (YP) of not less than 555 MPa corresponding to the target value of the present invention .

On the other hand, in the case of specimens prepared according to Comparative Example 1 in which manganese (Mn) was added in a large amount as compared with Example 1 and reheating was performed at a temperature lower than the temperature range suggested by the present invention, The tensile strength TS and the yield strength YP are both lower than the target value because the niobium (Nb) and the like are not sufficiently dissolved in the precipitate.

Also, in the case of the specimen produced according to Comparative Example 2 in which the amount of niobium (Nb) added was smaller than that in Example 1 and the cooling rate and the coiling temperature exceeded the range suggested by the present invention, the yield strength (YP) Value, but the grain size of the final microstructure is not properly miniaturized and the tensile strength TS is less than the target value.

2 shows C-scan results of the specimens according to Example 1 and Comparative Example 2 after the HIC test, and Table 5 below shows the HIC characteristic evaluations of Example 1 and Comparative Example 2, respectively.

[Table 5]

Referring to FIGS. 2 and 5, the specimen prepared according to Example 1 has a crack length ratio (Crack Ratio), a crack height ratio (CTR) and a crack sensitivity ratio (CSR) in the HIC (Hydrogen Induced Cracking) %, It can be seen that no crack occurred.

On the other hand, in the case of Comparative Example 2 in which manganese (Mn) was added at a content ratio lower than the range suggested by the present invention and the cooling rate and coiling temperature exceeded the range suggested by the present invention, cracks were observed in the HIC characteristic evaluation Able to know.

As described above, the hot-rolled steel sheet and the manufacturing method thereof according to the present invention can provide a method of manufacturing an API high-strength hot-rolled steel sheet excellent in corrosion resistance satisfying the X70 standard having durability even in the corrosive environment of H2S.

Accordingly, the hot-rolled steel sheet produced according to the present invention may have a tensile strength (TS) of 600 MPa or more and a yield strength (YP) of 555 MPa or more.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments. Such changes and modifications are intended to fall within the scope of the present invention unless they depart from the scope of the present invention. Accordingly, the scope of the present invention should be determined by the following claims.

S110: Slab reheating step
S120: Hot rolling step
S130: Cooling and winding step

Claims (8)

(a) from 0.03 to 0.07% by weight of carbon (C), up to 0.2% by weight of silicon (Si), from 0.5 to 1.5% by weight of manganese (Mn) (Cu): 0.2-0.35 wt.%, Nickel (Ni): 0.1-0.3 wt.%, , 0.001 to 0.003% by weight of calcium (Ca), and the balance of iron (Fe) and other unavoidable impurities to a slab reheating temperature (SRT) of 1200 to 1250 캜;
(b) a hot rolling step in which the reheated slab is primary rolled to 900 to 950 占 폚 for RDT (Roughing Delivery Temperature), followed by secondary rolling to FRT (Finish Rolling Temperature) at 800 to 850 占 폚; And
(c) cooling and winding the hot-rolled steel.
The method according to claim 1,
In the step (a)
Wherein the steel slab further comprises at least one of phosphorus (P): 0.01 wt% or less, sulfur (S): 0.001 wt% or less, and nitrogen (N): 0.006 wt% .
The method according to claim 1,
In the step (a)
Wherein the silicon (Si) and the manganese (Mn) are added in the range satisfying the following formula (1).
6 < / RTI > [Mn] / [Si] ≤ 10 wherein [
The method according to claim 1,
In the step (c)
The cooling
15 to 25 캜 / sec.
The method according to claim 1,
In the step (c)
The cooling
CT (Coiling Temperature): 550 to 650 占 폚.
(Si): 0.2 wt% or less, Mn: 0.5 to 1.5 wt%, Al: 0.02 to 0.05 wt%, Niobium (Nb): 0.07 to 0.07 wt% 0.1 to 0.3 wt% of nickel (Ni), 0.1 to 0.3 wt% of copper (Cu), 0.1 to 0.3 wt% of titanium (Ti), 0.01 to 0.02 wt% of chromium (Cr) Ca): 0.001 to 0.003% by weight, and the balance of iron (Fe) and other unavoidable impurities,
A tensile strength (TS) of 600 MPa or more, and a yield strength (YP) of 555 MPa or more.
The method according to claim 6,
The steel sheet
0.01% by weight or less of phosphorus (P), 0.001% by weight or less of sulfur (S), and 0.006% by weight or less of nitrogen (N).
The method according to claim 6,
Wherein the silicon (Si) and the manganese (Mn) are added in the range satisfying the following formula (1).
6 < / RTI > [Mn] / [Si] ≤ 10 wherein [

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