WO2017111437A1 - Lean duplex stainless steel and method for manufacturing same - Google Patents

Abstract

Lean duplex stainless steel and a method for manufacturing the same are disclosed. Lean duplex stainless steel according to an embodiment of the present invention comprises, by weight, 0.05% to 0.1% of carbon (C), 2.0% to 4.0% of silicon (Si), 4.0% to 8.0% of manganese (Mn), 13.0% to 15.0% of chromium (Cr), 0.05% to 0.15% of nitrogen (N), and the balanced amount of iron (Fe) and inevitable impurities. Therefore, alloy components, such as Cr, Ni, and Mo, are minimized in or excluded from the composition of the duplex stainless steel, thereby minimizing production cost, affording an elongation of 30% or higher, and guaranteeing corrosion resistance at a level of general-purpose 400-series steel.

Description

Lean Duplex Stainless Steel and Manufacturing Method thereof

The present invention relates to a lean duplex stainless steel and a method of manufacturing the same, which minimizes the content of expensive alloy elements such as Ni and Mo in the component system, and controls thermal martensite transformation and plastic organic martensite transformation, thereby controlling the austenite phase and the ferrite phase. It relates to a lean duplex stainless steel having a phase structure and a method for producing the same.

In general, austenitic stainless steel having good processability and corrosion resistance contains iron (Fe) as a base metal, and contains chromium (Cr) and nickel (Ni) as main raw materials, and molybdenum (Mo) and copper (Cu), etc. It is developed in various steel grades to suit various purposes by adding other elements of.

300-based stainless steel, which has excellent corrosion resistance and workability, includes expensive raw materials such as Ni and Mo. As an alternative, 400-based stainless steel has been discussed, but the formability does not reach 300-based stainless steel. This exists. In the case of thick plate with less molding than heat / cold rolled material, it can be applied to corrosion resistance of 400 series depending on the usage environment.However, it is used as a thick plate due to the thermal shock characteristics and deterioration of welding part of 400 series stainless steel. There are many restrictions.

On the other hand, the duplex stainless steel mixed with the austenitic phase and the ferrite phase has all the advantages of the austenitic and ferritic systems, and various types of duplex stainless steel have been developed to date.

U.S. Patent No. 6,644,611 (2000.8.1) discloses "ferritic-austenitic stainless steels having low nickel high tensile elongation." This is iron as the base metal, C: 0.04% or less, Si: 0.4 to 1.2%, Mn: 2 to 4% or less, Ni: 0.1 to 1.0%, Cr: 18 to 22%, Cu: 0.05 to 4.0%, S: 0.03% or less, P: 0.1% or less, N: 0.1-0.3%, Mo: 3.0% or less and other unavoidable impurities, including two phases of austenite and ferrite, among which the austenite phase is 30- 70%, Creq = Cr (%) + Mo (%) + 1.5Si (%), Nieq = Ni (%) + 0.33Cu (%) + 0.5Mn (%) + 30C (%) + 30N ( Creq / Nieq ranges from 2.3 to 2.75 in the equation defined by%), IM = 551-805 (C + N) (%)-8.52Si (%)-8.57Mn (%)-12.51Cr (%) In the equation defined by -36 Ni (%)-34.5Cu (%)-14Mo (%), IM is characterized in that in the range of 40 to 115.

On the other hand, one of the most widely used duplex stainless steels used in high corrosion resistance environment is AL2205 (UNS S31803 or S32205) with 22% Cr, 5.5% Ni, 3% Mo, 0.16% N components.

The steels provide excellent corrosion resistance in various corrosive environments, and thus have better corrosion resistance than austenitic, such as AISI 304 and 316.

However, since the duplex stainless steel contains expensive elements such as Ni and Mo, not only the manufacturing cost is increased but also the Ni and Mo are consumed, and thus the price competitiveness with other steel grades is inferior.

Recently, in order to solve such a problem, lean duplexes are further improved by excluding expensive alloying elements such as Ni and Mo among duplex stainless steels and adding low-cost alloying elements in place of these elements. duplex) There is increasing interest in stainless steel.

Lean duplex stainless steel has the same corrosion resistance as 304 and 316 steel, which is roughly classified as austenitic stainless steel, and has low Ni content, so it is economical and high in strength and easy to secure. It is in the spotlight as a molten steel.

Such lean duplex steels include, for example, S32304 (typical component 23Cr-4Ni-0.13N) standardized in ASTMA240, S32101 (typical component 21Cr-1.5Ni-5Mn-0.22N) standardized in ASTMA240.

While lean duplex steels can reduce manufacturing costs by eliminating expensive elements, they have corrosion resistance equivalent to or higher than that of 304, 304L and 316 steels. However, in some cases, lean duplex steels are overspecified. There is also a demand for duplex steel having corrosion resistance at the system level.

The steel grade corresponding to this requirement is not developed at present, and 400 series stainless steel is a structural factor, which has a low DBTT characteristic, which is very weak in impact characteristics, and is difficult to use as a thick plate due to the coarsening of the welded HAZ part structure. .

In addition, when the lean duplex steel component simply reduces Cr and Ni components to reduce corrosion resistance, the austenite phase stability at room temperature is lowered, and the austenite phase is transformed into a martensite phase during cooling. do. That is, it is difficult to realize a duplex steel structure having a two-phase structure of ferrite phase and austenite phase at 20% Cr or less, and the martensite phase formed during the cooling process lowers the elongation of the material and causes an impossible process such as a tubing process. .

(Patent Document 0001) US Registered Patent No. 6096441 (August 2000)

Embodiments of the present invention to provide a lean duplex stainless steel having a two-phase structure of austenite and ferrite phase by minimizing the content of expensive alloy elements such as Ni, Mo, etc. in the component system of the duplex stainless steel and Si, N component control.

In addition, embodiments of the present invention to provide austenite phase stability at room temperature to secure the elongation, to provide a method for producing a lean duplex stainless steel that can ensure corrosion resistance of the 400-based universal steel level.

Lean duplex stainless steel according to an embodiment of the present invention, in weight percent, carbon (C) 0.05 to 0.1%, silicon (Si) 2.0 to 4.0%, manganese (Mn) 4.0 to 8.0%, chromium (Cr) 13.0 To 15.0%, nitrogen (N) 0.05 to 0.15%, balance iron (Fe) and other unavoidable impurities.

In addition, according to an embodiment of the present invention, the ferrite fraction (Ferrite fraction, FF (%)) according to the following formula (1) is 60 to 80%, modified Md ₃₀ (Modified Md according to the following formula (2) ₃₀ , MM (° C.)) may be 110 ° C. or less.

FF (%) = 398-146 * C + 9.07 * Si-0.58 * Mn-22.5 * Cr-416N ------ Equation (1)

MM (° C.) = 551- [462 * (C + N) / (1-0.01 * FF)]-9.2 * Si-8.1 * Mn-13.7 * Cr ------ Formula (2)

In addition, according to an embodiment of the present invention, the chromium (Cr) may comprise 13.5 to 14.5%.

In addition, according to an embodiment of the present invention, the stainless steel may have a Cr equivalent weight of 13.0 to 16.0 according to the following Formula (3).

Cr equivalent = Cr + 0.48Si + 1.5Mo ------ Formula (3)

In addition, according to an embodiment of the present invention, nickel (Ni) may include 0.05% or less.

In addition, according to one embodiment of the present invention, the stainless steel may have a Ni equivalent weight of 5.0 or less according to the following formula (4).

Ni equivalent = Ni + 18N + 30C + 0.1Mn-0.01Mn ² ------ Formula (4)

In addition, according to an embodiment of the present invention, the elongation of the stainless steel may be 30% or more.

Method for producing a lean duplex stainless steel according to an embodiment of the present invention, in weight percent, carbon (C) 0.05 to 0.1%, silicon (Si) 2.0 to 4.0%, manganese (Mn) 4.0 to 8.0%, chromium (Cr ) Hot rolling a lean duplex stainless steel slab containing 13.0 to 15.0%, nitrogen (N) 0.05 to 0.15%, balance iron (Fe) and other unavoidable impurities, and annealing the hot rolled steel sheet at a temperature of 1,050 to 1,150 ° C. And water-cooling.

Further, according to one embodiment of the present invention, the hot rolled steel sheet may be annealed for 10 to 60 minutes.

Embodiments of the present invention can save resources by minimizing or excluding alloy components such as Cr, Ni, Mo, etc. in the component system of the duplex stainless steel can minimize the manufacturing cost of the duplex stainless steel.

In addition, by increasing the austenite phase stability at room temperature through the control of Si and N components, an elongation of 30% or more can be secured, and corrosion resistance of the 400 series universal steel can be secured.

1 is a Schaeffler's diagram for explaining the component system of lean duplex stainless steel according to embodiments of the present invention.

2 is a photograph of a microstructure of a lean duplex stainless steel according to an embodiment of the present invention using a transmission electron microscope (TEM).

3 is a graph for explaining the correlation between the stress-elongation of lean duplex stainless steel according to an embodiment of the present invention.

Figure 4 is a graph showing the official potential value of lean duplex stainless steel according to an embodiment of the present invention.

Lean duplex stainless steel according to an embodiment of the present invention, in weight percent, carbon (C) 0.05 to 0.1%, silicon (Si) 2.0 to 4.0%, manganese (Mn) 4.0 to 8.0%, chromium (Cr) 13.0 To 15.0%, nitrogen (N) 0.05 to 0.15%, balance iron (Fe) and other unavoidable impurities.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following embodiments are presented to sufficiently convey the spirit of the present invention to those skilled in the art. The present invention is not limited to the embodiments presented herein but may be embodied in other forms. The drawings may omit illustrations of parts not related to the description in order to clarify the present invention, and may be exaggerated to some extent in order to facilitate understanding.

Lean duplex stainless steel according to an embodiment of the present invention, in weight percent, carbon (C) 0.05 to 0.1%, silicon (Si) 2.0 to 4.0%, manganese (Mn) 4.0 to 8.0%, chromium (Cr) 13.0 To 15.0%, nitrogen (N) 0.05 to 0.15%, balance iron (Fe) and other unavoidable impurities.

The amount of carbon (C) is 0.05 to 0.1%. Carbon (C) is an austenite phase forming element and is an effective element for increasing the material strength by solid solution strengthening. Carbon (C) should be added at least 0.05% in order to contribute to the austenite phase stability. Such carbon (C), when excessively added, forms segregated and coarse carbides in the center of the material manufacturing, adversely affecting the post-hot rolling-annealing-cold rolling-cold annealing process, and the ferrite-austenite phase boundary. In order to maximize corrosion resistance, it is desirable to add it within 0.1% or less in order to reduce corrosion resistance by easily combining with carbide forming elements such as chromium (Cr), which are effective for corrosion resistance. Do.

The amount of silicon (Si) is 2.0 to 4.0%. Silicon (Si) is a ferrite phase forming element and is an element that is concentrated in ferrite during annealing. The content of chromium (Cr) in the lean duplex stainless steel according to the embodiments of the present invention is lower than that of the conventional lean duplex stainless steel, so that 2.0% to 4.0% of the lean duplex stainless steel may be added. However, in the case where silicon (Si) is added in excess of 4.0%, the hardness of the ferrite phase is sharply increased to deteriorate the workability and impact characteristics. Therefore, it is desirable to limit the content of silicon (Si) to 2.0 to 4.0%.

The amount of manganese (Mn) is 4.0 to 8.0%. Manganese (Mn) is an element that increases melt flow control, deoxidizer and nitrogen solubility, and is an austenite forming element. Manganese (Mn) is added in place of expensive nickel (Ni). If the manganese (Mn) is less than 4%, austenite stability is lowered at room temperature, transformed to martensite during cooling, making it difficult to maintain two-phase tissue. When the manganese (Mn) is more than 8%, the austenite phase fraction becomes excessively difficult to control the phase fraction. Therefore, it is preferable to limit the content of manganese (Mn) to 4.0 to 8.0%.

The amount of chromium (Cr) is 13.0 to 15.0%. Chromium (Cr) is minimized in terms of manufacturing cost reduction of the duplex stainless steel, it is desirable to limit to less than 15.0% to deviate from the existing lean duplex stainless steel component range. However, it is preferable to add more than 13% to ensure corrosion resistance of the duplex stainless steel. Therefore, it is preferable to limit the content of chromium (Cr) to 13.0 to 15.0%. More preferably, the lean duplex stainless steel may include chromium (Cr) in an amount of 13.5 to 14.5%.

The amount of nitrogen (N) is 0.05 to 0.15%. Nitrogen (N), together with nickel (Ni) in duplex stainless steel, contributes greatly to the stabilization of the austenite phase, and is one of the elements concentrated on the austenite phase during annealing.

Therefore, by increasing the content of nitrogen (N), it is possible to improve the corrosion resistance and strength incidentally, but since the solid solubility of nitrogen (N) can be changed according to the content of the added manganese (Mn), the content control This is necessary.

When the nitrogen (N) content in the manganese (Mn) range of the present invention exceeds 0.15%, blow holes and pin holes, etc., occur during casting due to excess nitrogen solubility. Surface defects and edge cracks are caused during rolling. Therefore, it is preferable to limit the content of nitrogen (N) to 0.05 to 0.15%.

For example, the lean duplex stainless steel according to the exemplary embodiment of the present invention may include 0.05% or less of nickel (Ni). Nickel (Ni) is an element that contributes greatly to stabilization of the austenite phase with nitrogen (N) in duplex stainless steel.

When the amount of nickel (Ni) is greater than 0.05%, there is a problem that the production cost increases by increasing the content of nickel (Ni), which is an expensive metal.

For example, the lean duplex stainless steel according to an embodiment of the present invention has a ferrite fraction (FF (%)) of 60 to 80% according to Equation (1), and according to Equation (2). modified _{Md 30 (modified Md 30, MM} (℃)) has a number equal to or less than 110 ℃.

FF (%) = 398-146 * C + 9.07 * Si-0.58 * Mn-22.5 * Cr-416N ------ Equation (1)

MM (° C.) = 551- [462 * (C + N) / (1-0.01 * FF)]-9.2 * Si-8.1 * Mn-13.7 * Cr ------ Formula (2)

For example, when the austenite phase is transformed into the martensite phase during the cooling process after the annealing heat treatment, or suddenly transformed into the martensite phase during deformation, the modified Md ₃₀ according to Equation (2) exceeds 110 ° C. The elongation of lean duplex stainless steel is shown to be less than 30%, specifically about 10 to 15% of the workability is very weak.

For example, the stainless steel may have a Cr equivalent weight of 13.0 to 16.0 according to the following Formula (3).

Cr equivalent = Cr + 0.48Si + 1.5Mo ------ Formula (3)

For example, the stainless steel may have a Ni equivalent weight of 5.0 or less according to the following Formula (4).

Ni equivalent = Ni + 18N + 30C + 0.1Mn-0.01Mn ² ------ Formula (4)

1 is a Schaeffler's diagram for explaining the component system of lean duplex stainless steel according to embodiments of the present invention.

1, that is, lean duplex stainless steel according to an embodiment of the present invention is reduced Cr equivalent and Ni equivalent as the content of chromium (Cr) and nickel (Ni), the existing lean duplex steel It can be seen that the Cr and Ni equivalents are located in a lower region.

In general, in this region, the austenite phase stability at room temperature is lowered. Therefore, after the annealing heat treatment, as the austenite phase is transformed into the martensite phase in the cooling process, workability and impact characteristics are sharply degraded.

However, in the present invention, to compensate for such low chromium (Cr), nickel (Ni) content, and to suppress the transformation phenomenon, by controlling the content of silicon (Si) to 2.0% or more to secure the austenite phase stability, In addition, the content of nitrogen (N) was controlled to 0.15% or less to suppress the martensite transformation driving force.

Accordingly, by increasing the austenite phase stability at room temperature of the lean duplex stainless steel of the present invention, it is possible to secure an elongation of 30% or more, and to secure corrosion resistance at the level of 400 series universal steel.

Method for producing a lean duplex stainless steel according to an embodiment of the present invention, in weight percent, carbon (C) 0.05 to 0.1%, silicon (Si) 2.0 to 4.0%, manganese (Mn) 4.0 to 8.0%, chromium (Cr ) Hot rolled lean duplex stainless steel slab containing 13.0 to 15.0%, nitrogen (N) 0.05 to 0.15%, balance iron (Fe) and other unavoidable impurities, and annealing the hot rolled steel sheet at a temperature of 1,050 to 1,150 ° C. , Water-cooled to produce lean duplex stainless steel.

The lean duplex stainless steel slab of the above composition may be thick rolled by a conventional method, and the hot rolled steel sheet may have a thickness of 5 to 20 mm.

For example, the hot rolled steel sheet is annealed for 10 to 60 minutes at a temperature of 1,050 to 1,150 ℃.

Under the component and heat treatment conditions, the martensitic phase transformation does not occur during the cooling of the microstructure and the ferrite phase and the austenite phase are maintained in two-phase structure, so that the ferrite phase fraction is maintained at 60 to 80% and the crystal Md ₃₀ is 110 ° C. or less. It can have a value of.

Through the following examples will be described in more detail the present invention.

Invention steel  And Comparative steel

The lean duplex stainless steel slabs including the component system according to the inventive steels and the comparative steels of Table 1 below were prepared, followed by thick plate rolling to prepare thick rolled specimens of 10 mmt.

Table 1 below shows the composition of the low cost lean duplex stainless steel, which is the steel grade of the present invention. In particular, chromium (Cr), which significantly affects the corrosion resistance level of STS 409 steel or more, is fixed in the range of 13.5 to 14.5%, manganese (Mn) within the range of 5.5 to 6.5%, and silicon (Si) and nitrogen (N) Prepared by varying the content.

	C	Si	Mn	Cr	N
Inventive Steel 1	0.048	2.47	5.93	13.87	0.051
Inventive Steel 2	0.048	3.04	6.12	14.02	0.053
Inventive Steel 3	0.047	3.07	6.1	13.94	0.097
Comparative Steel 1	0.064	1.02	6.01	13.98	0.048
Comparative Steel 2	0.047	1.99	5.87	13.95	0.047
Comparative Steel 3	0.05	4.1	6.08	14.1	0.052
Comparative Steel 4	0.051	3.03	6.04	13.87	0.151

Thereafter, the thick plate rolled specimens were maintained at an annealing temperature of 1,100 ° C. for 30 minutes and then cooled to evaluate materials, property changes, tensile properties, and corrosion resistance.

	Martensite phase transformation	Ferrite Prediction Fraction (%)	Modified Md 30 Value (℃)	Elongation (%)	Other
Inventive Steel 1	×	76.7	94.2	32.5	-
Inventive Steel 2	×	77.5	73.8	31.9	-
Inventive Steel 3	×	61.4	109.8	31.2	-
Comparative Steel 1	○	59.9	172.4	10.2	-
Comparative Steel 2	×	72.4	136.9	15.3	-
Comparative Steel 3	×	85.5	-53.7	-	Brittle fracture occurrence
Comparative Steel 4	×	39.6	129.3	30.8	Edge cracking

In Table 2, the tissues of the inventive steels 1 to 3 and the comparative steels 1 to 4 were observed to confirm the transformation onto the martensite phase. In addition, the ferrite predicted fraction and the modified Md ₃₀ value were calculated with reference to the compositions and formulas (1) and (2) in Table 1 above. Regarding the elongation, the tensile test was carried out by taking ASTM sub-size tensile test specimens in the rolling direction, specifying the temperature at the time of the tensile test at room temperature and the strain rate of 20 mm / min.

2 is a photograph of a microstructure of a lean duplex stainless steel according to an embodiment of the present invention using a transmission electron microscope (TEM).

2 shows the microstructure of inventive steel 1 of the present invention. As shown in the tissue photograph of Figure 2, when prepared in the component range of the present invention, it can be seen that the austenite phase is maintained in the austenite phase, without transformation into a martensite phase.

Referring to Tables 1 and 2, when the content of silicon (Si) is less than 1.0 to 2.0%, the austenite phase transforms into a martensite phase during cooling, or a sudden transformation into a calcined organic martensite phase during deformation (modified Md ₃₀ was greater than 110 ℃) elongation was 10 to 15% level was confirmed that the workability is very weak. In addition, when the content of silicon (Si) is more than 4.0%, brittle fracture is induced during specimen processing due to the increase in the ferrite phase fraction and hardness, preferably limited to 4.0% or less.

Therefore, when the content of silicon (Si) is maintained at 2.0 to 4.0%, the phase stability of austenite is increased, so that the modified Md ₃₀ is maintained at 110 ° C. or less, thereby ensuring processability of 30% or more of elongation.

When the nitrogen (N) content is more than 0.15%, since the decrease of the real rate due to the edge crack bundle during rolling occurs, it is preferable to limit it to 0.15% or less.

3 is a graph for explaining the correlation between the stress-elongation of lean duplex stainless steel according to an embodiment of the present invention.

3 is a graph of tensile properties of inventive steel 1 and comparative steel 2 of the present invention.

ASTM sub-size tensile specimens were taken in the rolling direction, and the tensile test was carried out at the temperature of the tensile test at room temperature and the deformation rate of 20 mm / min.

When the content of silicon (Si) is less than 2.0%, it was found that it is impossible to secure a desired elongation due to the rapid transformation into the plastic organic martensite phase during deformation. Therefore, when the silicon (Si) content is maintained at 2.0 to 4.0%, it was found that the austenite phase stability is increased to secure an elongation of 30% or more.

Figure 4 is a graph showing the official potential value of lean duplex stainless steel according to an embodiment of the present invention.

4 is a graph comparing the official potential value of the invention steel 1 of the present invention with the official potential value of the 400 series general-purpose steel STS 409 steel, STS 430 steel.

Inventive steel 1, STS 409 steel, STS 430 steel is shown in Figure 4 by measuring the official potential of each specimen in 1.0% NaCl solution. Accordingly, it was confirmed that the lean duplex stainless steel according to the embodiment of the present invention has corrosion resistance between STS 409 steel and STS 430 steel.

As described above, the exemplary embodiments of the present invention have been described, but the present invention is not limited thereto, and a person skilled in the art does not depart from the spirit and scope of the following claims. It will be understood that various changes and modifications are possible in the following.

Lean duplex stainless steel and its manufacturing method according to embodiments of the present invention can be applied to steel materials for industrial equipment, such as freshwater equipment, pulp, paper, chemical equipment.

Claims (9)

1. By weight%, carbon (C) 0.05 to 0.1%, silicon (Si) 2.0 to 4.0%, manganese (Mn) 4.0 to 8.0%, chromium (Cr) 13.0 to 15.0%, nitrogen (N) 0.05 to 0.15%, balance Lean duplex stainless steel containing iron (Fe) and other unavoidable impurities.
2. The method of claim 1,

   Ferrite fraction (FF (%)) according to the following formula (1) is 60 to 80%, and modified Md ₃₀ (Modified Md ₃₀ , MM (℃)) according to the formula (2) is 110 ℃ or less Lean duplex stainless steel.

   FF (%) = 398-146 * C + 9.07 * Si-0.58 * Mn-22.5 * Cr-416N ------ Equation (1)

   MM (° C.) = 551- [462 * (C + N) / (1-0.01 * FF)]-9.2 * Si-8.1 * Mn-13.7 * Cr ------ Formula (2)
3. The method of claim 1,

   Lean duplex stainless steel, characterized in that containing 13.5 to 14.5% chromium (Cr).
4. The method of claim 1,

   The stainless steel is a lean duplex stainless steel, characterized in that Cr equivalent weight of 13.0 to 16.0 according to the following formula (3).

   Cr equivalent = Cr + 0.48Si + 1.5Mo ------ Formula (3)
5. The method of claim 1,

   Nickel (Ni) Lean duplex stainless steel, characterized in that it contains 0.05% or less.
6. The method of claim 1,

   The stainless steel is a lean duplex stainless steel, characterized in that the Ni equivalent according to the following formula (4) is 5.0 or less.

   Ni equivalent = Ni + 18N + 30C + 0.1Mn-0.01Mn ² ------ Formula (4)
7. The method of claim 1,

   Lean duplex stainless steel, characterized in that the elongation of the stainless steel is 30% or more.
8. By weight%, carbon (C) 0.05 to 0.1%, silicon (Si) 2.0 to 4.0%, manganese (Mn) 4.0 to 8.0%, chromium (Cr) 13.0 to 15.0%, nitrogen (N) 0.05 to 0.15%, balance Hot rolling a lean duplex stainless steel slab containing iron (Fe) and other unavoidable impurities;

   Annealing the hot rolled steel sheet at a temperature of 1,050 to 1,150 ° C; And

   Process for producing a lean duplex stainless steel comprising the step of water cooling.
9. The method of claim 8,

   Method for producing a lean duplex stainless steel, characterized in that the hot-rolled steel sheet for annealing for 10 to 60 minutes.

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