KR20190078210A - Soft magnetic steel having excellent free cutting characteristics and method of manufacturing therof - Google Patents

Abstract

Disclosed is a soft magnetic steel material with an excellent cutting property. According to an embodiment of the present invention, the soft magnetic steel material with an excellent cutting property comprises: 0.001-0.01 wt% of C; 0.3-0.6 wt% of Mn; 0.03-0.5 wt% of Si; 0.02-0.05 wt% of Sn; 0.015-0.03 wt% of S; 0.025 wt% or less (excluding 0 wt%) of P; and the remainder of Fe and other unavoidable impurities. A fine structure is a ferrite single phase, and satisfies the following formula (1) represented by 15 < Mn/S < 21.

Description

TECHNICAL FIELD [0001] The present invention relates to a soft magnetic steel material having excellent machinability and a manufacturing method thereof. [0002]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a soft magnetic steel material, and more particularly, to a soft magnetic steel material having excellent machinability and electromagnetic characteristics and a method of manufacturing the same.

In order to improve energy efficiency according to environmental regulations, it is required to improve the electromagnetic characteristics of steel materials for electric parts such as automobiles. The steel material to be the material of electric parts requires electromagnetic properties such as high magnetic permeability and magnetic flux density, and is required to have excellent machinability for miniaturization and high precision parts production.

A wire rod for a solenoid valve used in an automatic fuel injection device requires a high response rate and requires a high magnetic flux density related to the force when the valve is opened and closed. Therefore, pure iron based steels such as JS-SUYB steels have been used in the past, and such pure iron steels have problems in terms of workability in order to heat cutting workability due to rigid ductility. Therefore, in the field where conventional precision machining is required, precision machining property is given by utilizing free machining steel for heating electromagnetic characteristics. General free-machining steel has an advantage in machining and machinability, but its magnetic properties are poor due to limited movement of magnetic domains by inclusions such as carbide and MnS. Despite the electromagnetic characteristics, the reason for using free cutting steel was to manufacture precision parts by minimizing machining error limit through cutting.

There has been an invention for improving the electromagnetic characteristics of the free cutting steel in order to solve the problem that the electromagnetic characteristics are heated. The present invention is intended to commercialize a soft magnetic material having excellent machinability by adding Pb to a pure iron-based soft magnetic material, but has a problem of environmental pollution due to the addition of Pb.

As the development of environmentally friendly Pb-free free cutting steels due to environmental pollution problems has been required, sulfur soft magnetic steels using MnS have been developed by adding S to soft magnetic steels. Sulfur soft magnetic steel has improved machinability, but has the problem that the electromagnetic properties are heated by MnS inclusions.

In addition, a soft magnetic steel having excellent machinability with improved machinability and <100> texture has been developed by adding Sn + Sb to the low melting point Sn of grain boundaries. However, the steel through the addition of the low melting point element is complex, so that it is melted due to the adiabatic heat generated during molding and is segregated into the grain boundary. Accordingly, there is a demand for development of a soft magnetic steel having excellent electromagnetic characteristics and machinability and a manufacturing method thereof without environmental pollution.

Embodiments of the present invention can be applied to a soft magnetic steel material having excellent electromagnetic characteristics and machinability that can improve electromagnetic characteristics by developing Sn-added <100> texture and improve machinability according to grain boundary segregation of Sn, Method.

According to an embodiment of the present invention, there is provided a soft magnetic steel having excellent machinability, comprising 0.001 to 0.01% of C, 0.3 to 0.6% of Mn, 0.03 to 0.5% of Si, 0.02 to 0.05% of Sn, 0.015 to 0.03%, P: not more than 0.025% (excluding 0), the remainder Fe and other unavoidable impurities, and the microstructure is a ferrite single phase and satisfies the following formula (1).

(1): 15 < Mn / S < 21

According to an embodiment of the present invention, the average diameter of the MnS in the soft-magnetic steel having excellent machinability is 1 to 3 占 퐉, and the area fraction of MnS may be 0.08 to 0.15%.

Also, according to an embodiment of the present invention, the soft magnetic steel having excellent machinability may include an aggregate structure in which the <100> aggregate fraction is higher than the <111> aggregate fraction in the rolling direction.

Also, according to an embodiment of the present invention, the soft magnetic steel having excellent machinability may have a magnetic flux density B ₅₀ of 1.65 Tesla or more at a magnetic field H = 5,000 A / m.

Also, according to an embodiment of the present invention, the soft magnetic steel having excellent machinability may have a number of free-machining sites expressed by the following formula (2): 105% or more.

Equation (2): number of free spots = (a ₁ * CF _A / CF _B + a ₂ * SR _A / SR _B + a ₃ * TL _B / TL _A ) * 100 (%)

A is the steel material, B is the steel material with the alloy composition excluding the Sn alloy component in A steel, CF is the cutting force, SR is the surface roughness, TL is the tool life Life). Further, a ₁ = 0.6, a ₂ = 0.2, and a ₃ = 0.2.

A method of manufacturing a soft magnetic steel having excellent machinability according to an embodiment of the present invention includes: 0.001 to 0.01% of C, 0.3 to 0.6% of Mn, 0.03 to 0.5% of Si, 0.02 to 0.05% of Sn, , 0.015 to 0.03% of S, 0.025% or less of P (exclusive of 0), the balance Fe and other unavoidable impurities, and satisfying the following formula (1) at 1100 to 1260 캜; Hot-rolling the heated billet to obtain a steel material; And cooling the hot-rolled steel at a cooling rate of 0.1 to 10 ° C / second.

(1): 15 < Mn / S < 21

According to an embodiment of the present invention, the hot rolling temperature may be 800 to 1100 ° C.

Also, according to an embodiment of the present invention, the steel material may be in the form of a wire rod or a bar.

According to the disclosed embodiment, by adding Sn to the soft magnetic steel, electromagnetic characteristics and machinability can be improved at the same time. There is no environmental pollution problem due to Pb, and Sn having a low melting point forms a thin film on the tool surface, There is an effect that improvement can be achieved. Therefore, a soft magnetic steel material excellent in machinability of the present invention can be utilized as a component material such as a core and an iron core of a solenoid valve which requires precision parts machining.

Also, in consideration of the production cost caused by the addition of Sn, it is possible to provide a low-cost soft magnetic steel by controlling the addition amount of Sn to a low level and controlling it with proper MnS to supplement the cutting property.

FIG. 1 is a photograph of MnS having an average diameter of 3 μm or less distributed in a steel material according to an embodiment of the present invention.

The following embodiments are provided to fully convey the spirit of the present invention to a person having ordinary skill in the art to which the present invention belongs. The present invention is not limited to the embodiments shown herein but may be embodied in other forms. For the sake of clarity, the drawings are not drawn to scale, and the size of the elements may be slightly exaggerated to facilitate understanding.

Throughout the specification, when an element is referred to as "comprising ", it means that it can include other elements as well, without excluding other elements unless specifically stated otherwise.

The singular forms &quot; a &quot; include plural referents unless the context clearly dictates otherwise.

Hereinafter, embodiments according to the present invention will be described in detail with reference to the accompanying drawings. First, a description will be given of a soft magnetic steel material excellent in machinability, and then a method of manufacturing a soft magnetic steel material having excellent machinability will be described.

General free cutting steels are poor in electromagnetic characteristics, and soft magnetic materials with excellent electromagnetic properties are poor in machinability. The inventors of the present invention confirmed that it is possible to simultaneously improve the conflicting characteristics when Mn is appropriately distributed in the ferrite matrix by properly adding Sn to the soft magnetic material, and the present invention has been accomplished.

Specifically, Sn is an element segregating in grain boundaries, which suppresses aggregate structure in the <111> direction which hinders electromagnetic characteristics and increases aggregate structure in the <100> direction which is an easy magnetization direction, thereby improving electromagnetic characteristics.

In addition, Sn improves cutting performance. Generally, excellent machinability means that a minimum production cost or a maximum production rate can be realized in machining. That is, it means that the tool life is long, the cutting force is low, and the surface roughness is excellent. Sn is segregated in grain boundaries to induce brittleness of the grain boundaries and can be processed with low cutting power. Especially, Sn is formed by forming a thin film on the tool surface by melting low melting point of Sn due to temperature rise due to cutting, Thereby contributing to improvement in tool life.

On the other hand, MnS is a nonmetallic inclusive material, which induces brittleness during cutting and can be machined with low cutting force, so that cutting force is improved and surface roughness, which is caused by high ductility of low carbon steel, is improved. On the other hand, since MnS inclusions act as inclusions and cause the electromagnetic characteristics to become dull, optimization of size and distribution control is necessary.

According to one aspect of the present invention, there is provided a soft magnetic steel having excellent cutting performance, comprising 0.001 to 0.01% of C, 0.3 to 0.6% of Mn, 0.03 to 0.5% of Si, 0.02 to 0.05% of Sn, 0.03%, P: not more than 0.025% (excluding 0), the balance Fe and other unavoidable impurities.

Hereinafter, the reason for limiting the numerical value of the content of the component component in the embodiment of the present invention will be described. Unless otherwise stated, the unit is wt%.

The content of C is 0.001 to 0.01%.

Carbon (C) is an element that is solidified in steel or forms carbide, etc., and is preferably controlled to be extremely low carbon for excellent magnetic properties of soft magnetic steel. However, if it is added in an amount exceeding 0.01%, the magnetic properties due to formation of carbides will be deteriorated. When the amount is less than 0.001%, the productivity of the steel is lowered because RH (Rheinstahl Hüttenwerke & Heraus) . Therefore, the content of C is preferably 0.001 to 0.01% by weight.

The content of Mn is 0.3 to 0.6%.

Manganese (Mn) not only acts as a deoxidizer but also increases the resistivity and lowers the iron loss. In addition, MnS is formed by binding with S in the steel to improve machinability. It is preferable to add Mn of 0.3% or more. However, when the content is excessive, there is a problem that MnS is coarsened and the magnetic susceptibility and the magnetic flux density are inferior, so that the upper limit can be limited to 0.6%.

The content of Si is 0.03 to 0.5%.

Silicon (Si) is an element which also acts as a deoxidizing agent and has an effect of suppressing magnetic property deterioration due to oxygen in steel, and it is preferable to add Si of 0.03% or more. However, when the content thereof is excessive, the upper limit can be limited to 0.5% in consideration of the fact that the saturation magnetic flux density is lowered.

The content of Sn is 0.02 to 0.05%.

Tin (Sn) is an important component in the present invention, and is an element segregating in grain boundaries. It inhibits the diffusion of nitrogen through grain boundaries, suppresses the <111> directional texture that hinders electromagnetic characteristics, To improve the electromagnetic characteristics.

In addition, Sn improves cutting performance. Generally, excellent machinability means that a minimum production cost or a maximum production rate can be realized in machining. That is, it means that the tool life is long, the cutting force is low, and the surface roughness is excellent. Sn is segregated in grain boundaries to induce brittleness of the grain boundaries and can be processed with low cutting power. Especially, Sn is formed by forming a thin film on the tool surface by melting low melting point of Sn due to temperature rise due to cutting, Thereby contributing to the improvement of the tool life. It is preferable to add it by 0.02% or more.

However, if the content is excessive, segregation may occur at the boundary of the casting structure during continuous casting, which may cause a performance crack, resulting in a decrease in productivity and an increase in the production cost, so that the upper limit can be limited to 0.05% have.

The content of S is 0.015 to 0.03%.

Sulfur (S) is an element which binds with manganese in the steel to form MnS. In order to improve machinability, MnS fraction should be ensured. It is preferable to add at least 0.015%. However, when the content is excessive, there is a problem that the magnetic flux density sharply decreases due to the increase in the amount of interposition, so that the upper limit can be limited to 0.03%.

The content of P is 0.025% or less.

Phosphorus (P) is an impurity which is inevitably contained. It reduces the toughness and delayed fracture resistance of the steel and promotes the occurrence of surface defects. Therefore, it is desirable to control the content as low as possible. Therefore, it is important to manage the upper limit, and in the present invention, the upper limit of the P content is preferably limited to 0.025 wt%.

When the content of P is less than 0.001% by weight, the production cost of the refining process is greatly increased. Therefore, the content of P is more preferably controlled to 0.001 to 0.025%.

The remainder of the present invention is iron (Fe). However, in the ordinary manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably incorporated, so that it can not be excluded. These impurities are not specifically mentioned in this specification, as they are known to any person skilled in the art of manufacturing.

According to one embodiment of the present invention, the soft magnetic steel having excellent machinability satisfying the above-described alloy composition can satisfy the following formula (1).

(1): 15 < Mn / S < 21

When S is excessively contained in comparison with Mn, there is a problem that S which is not bonded with Mn precipitates into FeS and causes performance crack, and Mn / S value is preferably more than 15. On the other hand, when the content of Mn is relatively large, coarse MnS is formed to inhibit the magnetic susceptibility and the magnetic flux density, so that the Mn / S value is preferably controlled to be less than 21.

The average diameter of the MnS in the steel is 1 to 3 占 퐉, and the area fraction of MnS may be 0.08 to 0.15%. When the average diameter is less than 1 탆, brittle fracture is caused at the MnS boundary, so that it is difficult to improve the desired machinability in the present invention. When the average diameter is more than 3 탆, the electromagnetic characteristics are weakened. When the MnS area fraction is less than 0.08%, improvement in machinability is difficult and when it exceeds 0.15%, the electromagnetic characteristics are poor.

In addition, it is preferable that the microstructure of the soft magnetic steel having excellent machinability according to the embodiment of the present invention is a ferrite single phase. Ferrite and other non-ferrite pearlite are present, the electromagnetic characteristics are inferior. When C above the solubility in Fe is added to form pearlite or carbide, electromagnetic characteristics are poor.

In addition, it is preferable that the steel has an aggregate structure in which the <100> aggregate fraction is higher than the <111> aggregate fraction in the rolling direction. The orientation in the < 100 > direction, which is the direction of easy magnetization, can be improved to simultaneously improve iron loss and magnetic flux density.

In addition, the steel material according to the present invention may have a magnetic flux density B ₅₀ of 1.65 Tesla or more when the magnetic field H = 5,000 A / m is applied.

Further, the steel material according to the present invention may have a number of free-machining sites represented by the following formula (2): 105% or more.

Equation (2): number of free spots = (a ₁ * CF _A / CF _B + a ₂ * SR _A / SR _B + a ₃ * TL _B / TL _A ) * 100 (%)

A is the steel material, B is the steel material with the alloy composition excluding the Sn alloy component in A steel, CF is the cutting force, SR is the surface roughness, TL is the tool life Life). Further, a ₁ = 0.6, a ₂ = 0.2, and a ₃ = 0.2.

As shown in the above equation (2), the free machining index can be expressed by relative comparison of the machinability of the workpiece B to be evaluated with respect to the workpiece A.

In the above relational expression, a ₁ , a ₂ , and a ₃ are factors indicating the relative importance of machinability evaluation criteria such as cutting force, surface roughness, and tool life. Generally, consumers can weight according to the purpose of use. In the present invention, a ₁ , a ₂ , and a ₃ are set to 0.6, 0.2, and 0.2, respectively.

Further, the steel material according to the present invention is preferably in the form of a wire rod or a bar.

Next, a method of manufacturing a soft magnetic steel material having excellent machinability according to another aspect of the present invention will be described.

A method of manufacturing a soft magnetic steel having excellent machinability according to an embodiment of the present invention includes: 0.001 to 0.01% of C, 0.3 to 0.6% of Mn, 0.03 to 0.5% of Si, 0.02 to 0.05% of Sn, 0.015 to 0.03% of S, 0.025% or less of P (excluding 0), the balance Fe and other unavoidable impurities at 1100 to 1260 캜; Hot-rolling the heated billet to obtain a steel material; And cooling the hot-rolled steel material at a cooling rate of 0.1 to 10 占 폚 / sec.

The reason for limiting the numerical value of the alloy element content is as described above.

The billet satisfying the above-mentioned component system is heated to 1100 to 1260 占 폚. By heating the steel strip in the temperature range described above, the remaining segregation, carbides and inclusions can be effectively dissolved.

When the heating temperature of the billet exceeds 1260 占 폚, the austenite grains become coarse and the fraction of the grain boundaries becomes small, so that the grain boundary of the Sn is worsened and the hot rolling property can be deteriorated. On the other hand, if it is less than 1100 ° C, the above-mentioned effect due to heating may not be sufficient. Here, the term "steel" means all semifinished products such as blooms and billets, which can be produced from a wire rod or a bar.

Thereafter, hot rolling is performed. The hot rolling temperature may be 800 to 1100 &lt; 0 &gt; C.

When the rolling temperature is lower than 800 ° C, the electromagnetic characteristics may be deteriorated due to the fine ferrite crystal grains. On the other hand, when the rolling temperature exceeds 1100 ° C, the mechanical properties of the wire rod and the steel rod are impaired due to the coarsening of the austenite grains.

Thereafter, the hot-rolled steel is cooled. Upon cooling, the cooling rate may be from 0.1 to 10 DEG C / second.

If the cooling rate is less than 0.1 ° C / sec, the mechanical properties of the wire rod and the steel rod due to grain growth are deteriorated. On the other hand, if it exceeds 10 DEG C / second, internal cracking due to generation of low-temperature structure and grain boundary brittleness may occur.

Further, after the cooling step, it may further include a step of winding to facilitate storage and movement of the cooled steel material.

The soft magnetic steel material having excellent machinability thus produced satisfies the following formula (1).

(1): 15 < Mn / S < 21

Hereinafter, preferred embodiments of the present invention will be described in more detail.

Example

A steel of the component system described in Table 1 below was cast. Then, after reheating at 1150, the hot-rolled bars were cooled at a rate of 5 / sec at a temperature of 1050 to prepare bars having a diameter of 50 mm.

division C Mn Si Sn S P Mn / S Example 1 0.003 0.3 0.1 0.035 0.016 0.01 18.8 Example 2 0.003 0.3 0.4 0.035 0.016 0.01 18.8 Example 3 0.003 0.45 0.1 0.035 0.025 0.01 18.0 Example 4 0.002 0.45 0.4 0.035 0.025 0.01 18.0 Example 5 0.008 0.4 0.1 0.035 0.025 0.01 16.0 Example 6 0.007 0.4 0.4 0.035 0.025 0.01 16.0 Comparative Example 1 0.003 0.4 0.1 0.035 0.016 0.01 25.0 Comparative Example 2 0.003 0.45 0.1 0.07 0.025 0.01 18.0 Comparative Example 3 0.007 0.65 0.1 0.035 0.035 0.01 18.6 Comparative Example 4 0.012 0.3 0.1 0.035 0.016 0.01 18.8 Comparative Example 5 0.008 0.25 0.4 0.035 0.01 0.01 25.0 Comparative Example 6 0.003 0.25 0.1 0.035 0.025 0.01 10.0 Comparative Example 7 0.002 0.45 0.4 0.01 0.025 0.01 18.0

Table 2 shows the electromagnetic characteristics, the number of free cutting spots, the average size of MnS precipitates, the presence of cracks in the performance, and the texture of the aggregates.

The electromagnetic characteristics were obtained by measuring ring solenoid specimens with an outer diameter of 45 mm, an inner diameter of 35 mm and a thickness of 10 mm from the bar. The copper wire was wound 20 times from the primary 100 times to measure HH from the BH (magnetic flux density - magnetic field strength) curve to H = 5000 A / m The magnetic flux density was defined as B ₅₀ .

Cryopreservation indices were obtained by turning 50 samples of 50 mm in diameter and 500 mm in length.

Tissue was cut by C section of the bar and XRD inverse pole figure was measured to show the direction showing the highest intensity.

division B ₅₀
(Tesla) Free cutting
Indices(%) MnS fraction MnS average size Surface cracking caused by Sn Key developmental organization Example 1 1.73 106 0.09 1.8 X <100> Example 2 1.7 106 0.09 1.9 X <100> Example 3 1.71 110 0.13 2.4 X <100> Example 4 1.68 111 0.13 2.5 X <100> Example 5 1.67 113 0.13 2.1 X <100> Example 6 1.66 115 0.13 2.5 X <100> Comparative Example 1 1.64 104 0.09 3.1 X <100> Comparative Example 2 No experiment No experiment No experiment No experiment O No experiment Comparative Example 3 1.61 116 0.18 3.3 X <100> Comparative Example 4 1.64 108 0.09 1.8 X <100> Comparative Example 5 1.68 99 0.07 1.8 X <100> Comparative Example 6 1.69 102 0.16 3.2 X <100> Comparative Example 7 1.64 103 0.09 1.8 X <111>

Referring to Tables 1 and 2, Examples 1 to 6 show that the composition and Mn / S ratio of the present invention are satisfied and <100> texture is developed, and the magnetic flux density (B ₅₀ ) is 1.65 Tesla or more, Was 105% or more. When the Sn content of the present invention is satisfied, <100> aggregate structure develops in the rolling direction, so that the magnetic flux density is improved. In addition, it can be seen that the Sn segregated in the grain boundaries lowers the cutting force and increases the tool life and improves the cutting performance.

On the other hand, Comparative Example 1 satisfied the range of the composition proposed by the present invention, but it was confirmed that the Mn / S ratio exceeded 21 at 25.0 to form coarse MnS, and the magnetic flux density and free-cutting indexes were favorable.

In Comparative Example 2, the Sn content was excessively added in an amount of 0.07%, and the surface grain boundary strength of the surface layer was lowered, resulting in surface cracking.

In Comparative Example 3, it was confirmed that the Mn and S contents were excessively added to 0.65% and 0.035% to form not only coarse MnS but also a high fraction of MnS and a high magnetic flux density.

In Comparative Example 4, the C content was excessively added to 0.012%, and the magnetic flux density was weakened by forming carbide (including cementite).

In Comparative Example 5, the S content was added in a small amount of 0.01%, and the free-formability index was inferior due to the low MnS fraction.

In Comparative Example 6, the Mn / S ratio was less than 15 at 10.0, and not only the coarse MnS was formed, but the fraction of MnS was also high, and thus the freeze-drying ability index was inferior.

In Comparative Example 7, a Sn content of 0.01% was added so that the <100> texture could not be formed and the magnetic flux density was low, and the Sn-segregated Sn did not act as a lubricant, resulting in a deficiency of the free-machining index.

While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited thereto. Those skilled in the art will readily obviate modifications and variations within the spirit and scope of the appended claims. It will be understood that various changes and modifications may be made therein without departing from the spirit and scope of the invention.

Claims (8)

0.001 to 0.01% of C, 0.3 to 0.6% of Mn, 0.03 to 0.5% of Si, 0.02 to 0.05% of Sn, 0.015 to 0.03% of S, 0.025% or less of P (excluding 0) And other unavoidable impurities,
The microstructure is composed of a ferrite single phase,
A soft magnetic steel material excellent in machinability satisfying the following formula (1).
(1): 15 < Mn / S < 21 The method according to claim 1,
A soft magnetic steel material having an average diameter of MnS in a steel material of 1 to 3 占 퐉 and an area fraction of MnS of 0.08 to 0.15%. The method according to claim 1,
Wherein the steel has an aggregate structure in which the <100> aggregate fraction is higher than the <111> aggregate fraction in the rolling direction. The method according to claim 1,
A soft magnetic steel having excellent magnetic properties with a magnetic flux density B ₅₀ of 1.65 Tesla or more at a magnetic field H = 5,000 A / m. The method according to claim 1,
A soft magnetic steel material excellent in machinability wherein the number of free-machining centers represented by the following formula (2) is 105% or more.
Equation (2): number of free spots = (a ₁ * CF _A / CF _B + a ₂ * SR _A / SR _B + a ₃ * TL _B / TL _A ) * 100 (%)
CF is the cutting force, SR is the surface roughness, TL is the tool life (in millimeters), A is the steel material, B is the steel material having the alloy composition excluding the Sn alloy component in A steel, CF is the cutting force, Tool Life), and a ₁ = 0.6, a ₂ = 0.2, and a ₃ = 0.2. 0.001 to 0.01% of Mn, 0.3 to 0.6% of Mn, 0.03 to 0.5% of Si, 0.02 to 0.05% of Sn, 0.015 to 0.03% of S, 0.025% or less of P (exclusive of 0) Fe and other unavoidable impurities and satisfying the following formula (1) at 1100 to 1260 占 폚;
Hot-rolling the heated billet to obtain a steel material; And
And cooling the hot-rolled steel material at a cooling rate of 0.1 to 10 ° C / second.
(1): 15 < Mn / S < 21 The method according to claim 6,
Wherein the hot rolling is performed in a temperature range of 800 to 1100 占 폚. The method according to claim 6,
Wherein the steel material is in the form of a wire rod or a bar steel.

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