JPWO2012115025A1 - Manufacturing method of cold working mold - Google Patents

Abstract

In mass%, C: 0.6-1.2%, Si: 0.8-2.5%, Mn: 0.4-2.0%, S: 0.03-0.1%, Cr: 5.0 to 9.0%, Mo and W alone or in combination (Mo + 1 / 2W): 0.5 to 2.0%, Al: 0.04 to less than 0.3%, balance Fe and inevitable impurities A cold work tool steel ingot is made into a raw material by hot working, and after quenching and tempering the raw material to adjust the hardness to 58 to 62 HRC, cutting is performed to finish the shape of the mold This is a manufacturing method of a cold working mold. Preferably, the hardness after tempering is 60 HRC or more. The cold tool steel may further contain Ni of 1.0% or less, Cu of 1.0% or less, V of 1.0% or less, and Nb of 0.5% or less.

Description

  The present invention relates to a method for manufacturing a cold working mold for molding, for example, home appliances, mobile phones, and automobile-related parts.

  In a cold working die used for press forming such as bending, drawing and punching of a plate at room temperature, in order to improve its wear resistance, quenching and tempering (hereinafter referred to as “tempering”) of 55 HRC or higher. Steel materials that can achieve hardness have been proposed (Patent Documents 1 to 3). When such a high hardness steel material is used, it is difficult to cut into a mold shape after tempering. Therefore, normally, after carrying out rough processing in the annealing state with low hardness after hot-working a steel ingot, it tempers to use hardness of 55HRC or more. In this case, since heat treatment deformation occurs in the mold due to tempering, after tempering, final cutting is performed again to correct the deformation and the final mold shape is adjusted. The main cause of the heat treatment deformation of the mold due to the tempering is that the volume of the steel material expands due to the transformation of the steel material that was a ferrite structure into a martensite structure in the annealed state.

In addition to the above steel materials, many pre-hardened steels that have been tempered and supplied in advance to the hardness to be used have been proposed. In pre-hardened steel, there is no need for tempering after cutting all the way to the final mold shape, so heat treatment deformation of the mold due to tempering can be excluded, and the above-mentioned finishing cutting can also be omitted. It is an effective technology. With regard to this technology, by optimizing the amount of undissolved carbide that decreases the machinability in the hardened steel material, excellent machinability while ensuring a temper hardness exceeding 55 HRC. The cold tool steel which has is proposed (patent document 4). On the other hand, an oxide ((FeO) ₂ · SiO ₂ , Fe ₂ SiO ₄ or (FeSi) having a melting point of 1200 ° C. or lower is used to suppress tool wear caused by friction between the cutting tool and the steel material during cutting. There is also proposed a cold tool steel in which self-lubricating properties are imparted by adding an element that forms a) Cr ₂ O ₂ and forming the oxide on the mold surface by heat generated during cutting. (Patent Document 5)

JP 2008-189982 A JP 2009-132990 A JP 2006-193790 A JP 2001-316769 A JP 2005-272899 A

  Recently, the conditions for using cold working dies are becoming stricter, and cold tool steels are required to be able to achieve a quenching and tempering hardness of 58 HRC or higher, or even 60 HRC or higher. Therefore, in the case of pre-hardened steel, not only the above-mentioned hardness of 58 HRC or more, but also the hardness of 60 HRC or more can be obtained stably, and it has excellent machinability in its high hardness state. preferable. The cold tool steel disclosed in Patent Document 4 is an excellent prehardened steel that achieves both machinability during cutting and wear resistance as a mold. However, with regard to the wear resistance, in addition to the small amount of undissolved carbide that is defined, the quenching temperature is also limited, so this can be obtained when the tempered hardness is 60 HRC or higher. The component range is very limited. Nb and V, which are preferably added in Patent Document 4 for the purpose of suppressing crystal grain growth during quenching heating, are elements that easily form insoluble MC carbide at the quenching temperature. It is. Since MC carbide is hard, the component composition disclosed in Patent Document 4 has a problem that the machinability after tempering is significantly reduced.

  Further, the cold tool steel disclosed in Patent Document 5 uses a low melting point oxide as a self-lubricating film, but if the cutting temperature does not increase to the melting point of the oxide, a lubricating effect cannot be obtained. On the contrary, when the cutting temperature is excessively increased, the viscosity of the oxide is remarkably lowered, and there is a problem that the function as the lubricating film may not be performed.

  The object of the present invention is based on a component composition that can stably achieve a high tempering hardness of 60 HRC or higher as well as 58 HRC or higher, and preferably even if the amount of undissolved carbide formed is further increased. Provided is a method of manufacturing a cold working die that cuts cold work tool steel with remarkably improved machinability after tempering at a temper hardness of 58 to 62 HRC without depending on temperature. That is.

The inventor has intensively studied a method for improving the machinability of cold tool steel. As a result, Al ₂ O ₃ which is a high melting point oxide is positively introduced, and a composite lubricating protective film composed of this and MnS which is a highly ductile inclusion is formed on the surface of the cutting tool by heat during cutting. I found a technique. And the steel material which can achieve the refining hardness of 60HRC or more as well as 58HRC or more and can form this composite lubricating protective film has an optimum component range, and this could be specified. The present invention has been reached.

That is, the present invention is mass%,
C: 0.6-1.2%
Si: 0.8 to 2.5%
Mn: 0.4 to 2.0%,
S: 0.03-0.1%,
Cr: 5.0-9.0%,
Mo and W are single or composite (Mo + 1 / 2W): 0.5 to 2.0%,
Al: 0.04 to less than 0.3%,
The steel ingot of the cold tool steel composed of the remaining Fe and inevitable impurities is hot-worked into a raw material, and the raw material is quenched and tempered to adjust the hardness to 58 to 62 HRC, and then cut. A method for manufacturing a cold working mold, wherein the mold is finished in the shape of a mold. As a specific example, there is a method of manufacturing a cold working mold in which a material subjected to hot working is annealed and then quenched and tempered. As another specific example, quenching is a method for manufacturing a cold working mold, which is direct quenching performed in the cooling process after the hot working. Preferably, the hardness after tempering is 60 HRC or more.

  The cold tool steel according to the present invention may contain 1.0% or less of Ni, or 1.0% or less of Cu.

  And the cold tool steel which concerns on this invention may contain 1.0% or less of V, or also 0.5% or less of Nb further.

  According to the present invention, since machinability improving means widely applicable to a large number of component compositions has been adopted, the hardness is adjusted to a hardness of 60 HRC or more as well as 58 HRC or more, and the amount of undissolved carbide is further increased. Even if an alloy design with a large amount of steel is used, it is possible to obtain a cold work tool steel that greatly improves the machinability after tempering without depending on the cutting temperature. Therefore, it is possible to freely select the tempered hardness of the cold tool steel and the amount of undissolved carbide according to various functions. Then, if this cold tool steel is tempered to a hardness of 58 to 62 HRC and then subjected to cutting, problems related to heat treatment deformation and refinishing can be solved, and a mold can be manufactured. In particular, this is an indispensable technique for the practical application of a cold working die using pre-hardened cold tool steel.

Sample No. which is an example of the present invention. 3 is a digital microscope photograph showing the rake face and flank face of the cutting tool used in the cutting process of No. 3. The upper side of the drawing shows the rake face, and the lower side of the drawing shows the flank face. Sample No. which is an example of the present invention. 5 is a digital microscope photograph showing a rake face and a flank face of a cutting tool used in the cutting process of No. 5. The upper side of the drawing shows the rake face, and the lower side of the drawing shows the flank face. Sample No. which is an example of the present invention. The digital microscope photograph which showed the rake face and flank face of the cutting tool used for 15 cutting. The upper side of the drawing shows the rake face, and the lower side of the drawing shows the flank face. Sample No. which is a comparative example. 22 is a digital microscope photograph showing a rake face and a flank face of a cutting tool used for 22 cutting operations. The upper side of the drawing shows the rake face, and the lower side of the drawing shows the flank face. Sample No. which is a comparative example. A digital microscope photograph showing the rake face and flank face of a cutting tool used for 30 cutting operations. The upper side of the drawing shows the rake face, and the lower side of the drawing shows the flank face. When deposits formed on the surface of the cutting tool in FIG. 1A (Sample No. 3) were analyzed by EPMA (electron beam microanalyzer), Al (upper left), O (upper right), Mn (lower left), S (right) FIG. It is a map figure of Al, O, Mn, and S when the deposit formed on the surface of the cutting tool of Drawing 1B (sample No. 5) is analyzed by EPMA (electron beam microanalyzer), respectively. It is a map figure of Al, O, Mn, and S when the deposit formed on the surface of the cutting tool of Drawing 1C (sample No. 15) is analyzed by EPMA (electron beam microanalyzer), respectively. It is a map figure of Al, O, Mn, and S when the deposit formed on the surface of the cutting tool of Drawing 1D (sample No. 22) is analyzed by EPMA (electron beam microanalyzer), respectively. It is a map figure of Al, O, Mn, and S when the deposit formed on the surface of the cutting tool of Drawing 1E (sample No. 30) is analyzed by EPMA (electron beam microanalyzer), respectively. It is a cross-sectional TEM (transmission electron microscope) photograph which showed the deposit | attachment of FIG. 2A (sample No. 3) with TiN coating. It is a cross-sectional TEM (transmission electron microscope) photograph which showed the adhesion thing of Drawing 2D (sample No. 22) with TiN coating. It is the cross-sectional TEM (transmission electron microscope) photograph which showed the deposit | attachment of FIG. 2E (sample No. 30) with TiN coating.

The feature of the present invention is that the machinability after tempering depends on the cutting temperature even when a large amount of undissolved carbide is formed to improve the temper hardness and control the crystal grain size. A good cold tool steel is realized, and the tempered cold tool steel is being cut. Specifically, in addition to obtaining a tempered hardness of 58 HRC or higher, preferably 60 HRC or higher, in order to suppress wear of the cutting tool, Al ₂ O ₃ which is a high melting point oxide and high ductility interposition are used. It is to temper cold tool steel, which is a component design of a steel material, before cutting, so that a composite lubricating protective film of MnS, which is a product, is formed on the surface of the cutting tool.

  First, the present inventor has examined means for improving machinability that can widely correspond to the component composition of cold tool steel. As a result, we focused on the effectiveness of self-lubrication. And when self-lubricating action effect using a low melting point oxide like patent document 5 was examined, it discovered that this had the subject depending on cutting temperature. In other words, the low melting point oxide having self-lubricating properties is a composite oxide containing Fe and Cr that are generally contained in a large amount in a steel material. It fluctuates greatly and a stable lubricating effect cannot be obtained.

Therefore, in the present invention, when a method for improving the machinability of the cold tool steel without using the low melting point oxide has been intensively studied, on the contrary, Al ₂ O ₃ which is a high melting point oxide is actively introduced. Thus, the present inventors have found a method of forming a composite lubricating protective film composed of this and MnS, which is a highly ductile inclusion, on the surface of the cutting tool by heat during cutting. The effect of this composite lubricating protective film does not vary in response to a wide range of cutting temperatures, and even when an element that forms a hard MC carbide such as Nb or V is added, good machinability can be secured. And the steel material which can achieve the refining hardness of 60HRC or more as well as 58HRC or more and can form this composite lubricating protective film has an optimum component range, and this could be specified. The present invention has been reached. Hereinafter, the component composition of the cold tool steel according to the production method of the present invention will be described.

C: 0.6 to 1.2% by mass (hereinafter simply expressed as%)
C is an important element that forms carbides in the steel and imparts hardness to the cold tool steel. If the amount of C is too small, the amount of carbide formed is insufficient, and it is difficult to impart a hardness of 58 HRC or more, preferably 60 HRC or more. On the other hand, if the content is excessive, the toughness tends to decrease due to an increase in the amount of undissolved carbide when quenched. Therefore, the C content is set to 0.6 to 1.2%. Preferably they are 0.7% or more and / or 1.1% or less. 1.0% or less is more preferable.

・ Si: 0.8-2.5%
Si is an important element that dissolves in steel and imparts hardness to cold tool steel. In addition to being stronger in oxidation tendency than Fe and Cr, it is an element that easily forms a corundum-based oxide with Al ₂ O _3. There is an important effect of suppressing the formation of Cr-based oxides and promoting the formation of an Al ₂ O ₃ protective film. However, if it is too much, hardenability and toughness are significantly reduced. Therefore, Si is set to 0.8 to 2.5%. Preferably they are 1.0% or more and / or 2.0% or less. 1.2% or more is more preferable.

Mn: 0.4 to 2.0%
Mn is an important element of the present invention, and acts as a good lubricating film on the Al ₂ O ₃ protective film formed on the cutting tool surface. And it is an austenite formation element, and it dissolves in steel and improves hardenability. However, if the added amount is too large, a large amount of retained austenite remains after tempering, which causes aging changes during use as a mold. Further, since the easily form a Fe and Cr and a low melting oxide, is a factor that inhibits the function of the Al ₂ O ₃ protective coating. Therefore, in the present invention, it was set to 0.4 to 2.0%. Preferably they are 0.6% or more and / or 1.5% or less.

・ S: 0.03-0.1%
S is an important element of the present invention, and acts as a good lubricating film on the Al ₂ O ₃ protective film formed on the cutting tool surface. That is, a sufficient amount of S contained in the steel material forms MnS. Then, MnS is added to ductile, for compatibility with Al ₂ O ₃ is good, deposited on Al ₂ O ₃ protective coating serves these as a good composite lubricating protective coating. Addition of 0.03% or more is necessary in order to sufficiently exhibit such a lubricating action, but since S deteriorates the toughness of steel, the upper limit is made 0.1%. Preferably it is 0.04% or more and / or 0.08% or less.

・ Cr: 5.0-9.0%
Cr imparts hardness to the cold tool steel by forming M ₇ C ₃ carbide in the tempered structure. In addition, a part of the material is present as insoluble carbide during quenching heating, and has an effect of suppressing the growth of crystal grains. And by making Cr 5.0% or more, the amount of carbide | carbonized_material formed increases and the hardness of 58 HRC or more, Preferably 60 HRC or more can fully be achieved. Furthermore, when various coating processes are performed on the surface of the cold working mold, the ability to form a VC film by a TD process or a TiC film by a CVD process is improved. Cr is an element effective in securing corrosion resistance.

On the other hand, Cr, which is a main component of cold tool steel, tends to form a low melting point oxide. That is, when Cr is excessively contained, it becomes a factor that inhibits the function of the Al ₂ O ₃ protective film. As a result, it becomes a factor that hinders the function of the composite lubricating protective film composed of Al ₂ O ₃ and MnS, which is a feature of the invention. Therefore, it is important to adjust Cr after containing a sufficient amount of Al described later. And the function of said composite lubricating protective film is exhibited by adjusting S amount corresponding to this. For this reason, it is important that Cr is 5.0 to 9.0%. Preferably it is 6.0% or more, More preferably, it is 7.0% or more.

Mo and W are single or composite (Mo + 1 / 2W): 0.5 to 2.0%
Mo and W are elements that improve the hardness by precipitation strengthening (secondary hardening) of fine carbides during tempering during tempering. However, at the same time, since the decomposition of residual austenite that occurs during tempering is delayed, when it is excessively contained, residual austenite tends to remain in the tempered structure. Further, since Mo and W are expensive elements, the amount of addition should be reduced as much as possible for practical use. Therefore, the addition amount of these elements is 0.5 to 2.0% in the relational expression (Mo + 1 / 2W).

Al: 0.04 to less than 0.3% Al is an important element of the present invention. That is, a sufficient amount of Al contained in the steel material forms Al ₂ O ₃ , which is a high melting point oxide, on the cutting tool surface by heat generated during the cutting process. Since the melting point of Al ₂ O ₃ is about 2050 ° C., which is much higher than the cutting temperature, Al ₂ O ₃ functions as a protective film for the cutting tool. And by containing 0.04% or more, the protective film of sufficient thickness is formed and a tool life improves. However, when a large amount of Al is added, since much Al ₂ O ₃ is formed as inclusions in the steel material, the machinability of the steel material is lowered. For this reason, the upper limit of the amount of Al added is less than 0.3%. Preferably they are 0.05% or more and / or 0.15% or less.

-Preferably, Ni: 1.0% or less Ni is an element which improves the toughness and weldability of steel. Further, since tempering during tempering precipitates as Ni ₃ Al and has the effect of increasing the hardness of the steel, it is effective to add it according to the amount of Al contained in the cold tool steel according to the present invention. However, Ni is an expensive metal, and the amount to be added should be reduced as much as possible for practical use. Therefore, Ni in the present invention is preferably 1.0% or less even when added.

-Preferably, Cu: 1.0% or less Cu precipitates as (epsilon) -Cu in the tempering at the time of tempering, and has the effect of raising the hardness of steel. However, Cu is an element that causes hot brittleness of a steel material. Accordingly, Cu in the present invention is preferably 1.0% or less even when added. In addition, since hot brittleness due to Cu can be suppressed by adding substantially the same amount of Ni, when the cold tool steel according to the present invention contains Ni, the regulation value can be relaxed according to the amount. it can.

-Preferably, V: 1.0% or less V has the effect of forming various carbides and increasing the hardness of the steel. In addition, the formed insoluble MC carbide has an effect of suppressing the growth of crystal grains. In particular, by adding it in combination with Nb, which will be described later, the MC carbide that has not been dissolved yet during quenching heating becomes fine and uniform, and has the function of effectively suppressing crystal grain growth. On the other hand, MC carbide is hard and causes a decrease in machinability. Therefore, in the present invention, the above-described composite lubricating protective film is formed on the tool surface at the time of cutting, which is important in that good machinability can be ensured even if many MC carbides are formed in the steel material. Has characteristics. However, excessive V addition excessively forms coarse MC carbides and also reduces the toughness of the cold tool steel. Therefore, V is preferably 1.0% or less even when added. More preferably, it is 0.7% or less.

-Preferably, Nb: 0.5% or less Nb has the function which forms MC carbide | carbonized_material and suppresses the coarsening of a crystal grain. However, if added excessively, coarse MC carbides are excessively formed, and the toughness of the steel decreases. Therefore, even when added, the content is preferably 0.5% or less. More preferably, it is 0.3% or less.

  The present invention is characterized in that a cold work tool steel having the above component composition is tempered to a hardness of 58 to 62 HRC and then cut. The cold tool steel according to the present invention can stably obtain a tempered hardness of 58 HRC or higher by quenching and tempering. A hardness of 60 HRC or higher can also be achieved. And since it has excellent machinability in this high hardness state, it is not necessary to perform quenching and tempering after cutting in an annealed state. Alternatively, since it is not necessary to go through the annealing state itself, direct quenching using a cooling process after hot working the steel ingot can be applied to quenching. And even if it is a case where this direct hardening is applied, the machinability improvement effect similar to the case where the hardening after annealing is applied can be acquired. Therefore, by using the cold tool steel according to the present invention as pre-hardened steel, heat treatment deformation due to tempering is excluded, and finishing cutting and further annealing steps related to material production are omitted. Can do. In the present invention, the upper limit of the refining hardness is set to 62 HRC in order to sufficiently maintain mechanical properties other than the hardness of the cold tool steel and to perform the cutting process stably.

  Moreover, although the metal mold | die which consists of the manufacturing method of the metal mold | die for cold processing of this invention has the outstanding dimensional accuracy and abrasion resistance, it is abrasion resistance, maintaining high dimensional accuracy by performing surface PVD process. It is also possible to improve further.

  The material was melted using a high frequency induction melting furnace to produce a steel ingot having the chemical components shown in Table 1. Next, hot forging was performed on these so that the forging ratio was about 10, and after cooling, annealing was performed at 860 ° C. And after performing the quenching process by air cooling from 1030 degreeC to these annealed materials, it is tempered to the target hardness of 60HRC by two tempering processes at 500-540 degreeC, and the test for evaluating machinability A piece was made.

  The machinability test was performed by plane cutting using an insert PICOmini manufactured by Hitachi Tool Co., Ltd. as a cutting edge exchangeable tool corresponding to cutting of a hard material. The insert is made of cemented carbide as a base material and TiN coating is applied to the surface. Cutting conditions were a cutting speed of 70 m / min, a rotation speed of 1857 / min, a feed speed of 743 mm / min, a feed amount per blade of 0.4 mm / blade, a cutting depth of 0.15 mm, a cutting width of 6 mm, and a blade count of 1. .

The machinability was evaluated based on the following two points. First, the formation amount of the composite lubricating protective film composed of Al ₂ O ₃ and MnS on the cutting tool surface was evaluated. This amount of formation was analyzed using EPMA from the rake face side at a cutting distance of 0.8 m immediately after the start of cutting, and was used as the average count number of Al and S at this time. Then, the cutting distance was extended to 8 m, and the amount of tool wear at this time was measured using an optical microscope. These evaluation results are shown in Table 2.

  In the cutting of cold tool steel according to the present invention, a composite lubricating protective film is formed on the surface of the cutting tool, and tool wear is suppressed. And even when Nb and V which form insoluble carbides are added, good machinability is maintained. On the other hand, the cutting of cold tool steel that does not satisfy the present invention has a larger amount of tool wear than the present invention.

  1A to E show Sample No. FIGS. 2A to E are digital microscope photographs showing flank and rake surfaces of cutting tools used in 3, 5, 15, 22, and 30, and FIGS. 2A to E show EPMA of deposits formed on the surfaces of FIGS. (The high concentration part of each element is shown in white). In Table 2, sample Nos. With high average counts of Al and S were obtained. 3, 5 and 15 also confirmed that a large amount of Al and S was adhered over a wide range of the tool in the EPMA analysis of FIGS. In comparison with this, sample No. Sample No. 22 The average count numbers of Al and S were lower than 3, 5, and 15, and the adhesion range of Al and S was narrow. In addition, sample No. 1 with a low Al and S content in the steel. No. 30 also had a low average count of these elements, and Al and S were hardly detected by EPMA analysis (most of the Fe and Cr that seemed to be transferred from the test piece were detected).

  In FIGS. 1A to 1C showing the wear state of the cutting tool, sample No. It can be seen that deposits are remarkably adhered to the 3, 5 and 15 tool rake faces, and that tool wear is suppressed on both the flank face and the rake face. Moreover, tool wear is progressing uniformly and stably. In contrast, sample no. The tool wear amount of No. 22 is the same as that of Sample No. The tool was close to 3 times, and chipping occurred in the tool. And sample no. The tool surface of 30 is also sample No. Like 22 the damage was severe.

Further, FIGS. It is the cross-sectional TEM image which showed the deposit confirmed by the tool surface in 3, 22, and 30 with the TiN coating under it. In the figure, reference numeral 1 is a protective film for sample preparation, 2 is a deposit on cutting, 3 is a TiN plastic deformation region, and 4 is a TiN undeformed region. In conformity with the above results, the sample Nos. The deposit of No. 3 is thicker, and as the count number decreases, the sample No. 3 increases. In 22, the deposit was thinly transferred. Sample No. At 30, almost no deposits were observed. And sample no. Like sample 3, sample no. Al ₂ O ₃ and MnS were also attached to the surface of the tool No. 22, but the thickness was thin and chipping occurred as described above. Sample No. 3 exhibits a high lubrication protection function because the TiN coating on the tool surface, which is usually plastically deformed by frictional stress during cutting, has a thick deposit. 3 is suppressed (the plastic deformation region is the narrowest).

Claims (8)

% By mass
C: 0.6-1.2%
Si: 0.8 to 2.5%
Mn: 0.4 to 2.0%,
S: 0.03-0.1%,
Cr: 5.0-9.0%,
Mo and W are single or composite (Mo + 1 / 2W): 0.5 to 2.0%,
Al: 0.04 to less than 0.3%,
Hot working on the steel ingot of the cold tool steel consisting of the balance Fe and inevitable impurities,
After quenching and tempering the material to adjust the hardness to 58-62 HRC,
A method for producing a cold working mold, wherein the mold is finished by cutting.   The method for manufacturing a cold working die according to claim 1, wherein the material subjected to the hot working is annealed and then subjected to the quenching and tempering.   The method of manufacturing a cold working mold according to claim 1, wherein the quenching is direct quenching performed in a cooling process after the hot working.   The said cold tool steel is the mass%, and further contains Ni: 1.0% or less, The manufacturing method of the metal mold | die for cold work in any one of Claim 1 thru | or 3 characterized by the above-mentioned.   The method for manufacturing a cold working die according to any one of claims 1 to 4, wherein the cold tool steel further contains Cu: 1.0% or less in terms of mass%.   The method for manufacturing a cold working die according to any one of claims 1 to 5, wherein the cold tool steel further contains, by mass%, V: 1.0% or less.   The cold work steel manufacturing method according to any one of claims 1 to 6, wherein the cold tool steel further contains, by mass%, Nb: 0.5% or less.   The method for manufacturing a cold working die according to any one of claims 1 to 7, wherein the hardness after tempering is 60 HRC or more.

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