Classifications

• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
  • C23C8/06—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
  • C23C8/08—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases only one element being applied
  • C23C8/20—Carburising
  • C23C8/22—Carburising of ferrous surfaces
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
  • C23C8/80—After-treatment

Definitions

• the present invention relates to a carburized and quenched member having excellent fatigue strength and dimensional accuracy, and a method for manufacturing the same.
• gears which are power transmission parts for automatic transmissions, are often made of carburized and quenched members in order to increase both surface hardness and toughness.
• Conventional carburized and quenched members are formed into a desired shape using case hardened steel (JIS: SCM420H, SCR422OH, SNCM220) and then cast in a carburized atmosphere. In general, it was produced by gas carburizing and then quenching in oil.
• the carburized and quenched members are required to reduce costs and improve performance more than ever.
• One of the issues with carburized and quenched members is to further improve the strength after carburizing and quenching, while at the same time improving dimensional accuracy by suppressing quenching distortion more than ever.
• the present invention has been made in view of the above-mentioned conventional problems, and is intended to provide a carburized and quenched member capable of achieving high strength while sufficiently suppressing quenching distortion, and a method of manufacturing the same. is there. Disclosure of the invention
• the first aspect of the present invention is to contain F e as a main component, 0.1 to C 1 0 to 0. 5 0 wt%, 3 1 0.5 0 to 1.5 0 wt 0/0 containing
• alloy steel with a hardenability of one end quenching test and a J force of S35 to 50 (at 12.5 mm) was used as the material.
• a carburized layer is formed by carburizing in an antioxidant atmosphere.
• a method for producing a carburized and quenched member characterized in that quenching is performed under the following conditions.
• the hardenability J in the above one-side quenching test is obtained by the one-side quenching test method (commonly called Jomier one-side quenching test method) specified in JIS: GO561.
• Value. (At 12.5 mm) is the value of the hardenability J at 12.5 mm from the water-cooled end face of the bar specimen for the Jomini one-end quenching test. Is what it means.
• the C content and the Si After forming a carburized layer by carburizing treatment in an antioxidant atmosphere using a specific alloy steel with a hardenability J within the above specified range, the conditions of the monotonic cooling and the specified quench quench H Quench to satisfy both conditions. That is, by ensuring all of these material properties and manufacturing conditions, it is possible to obtain, for the first time, a carburized and quenched member with high strength while sufficiently suppressing quenching distortion. is there.
• the above-mentioned C content it is possible to secure appropriate toughness and strength of the non-carburized portion (inside) after carburizing and quenching. it can.
• the C content is less than 0.1% by weight, the above effect cannot be obtained, and if the C content exceeds 0.5% by weight, the hardness before quenching becomes too high, and the processing cost is reduced.
• the toughness may increase and the toughness may decrease.
• the transformation stress increases in the non-carburized portion after carburizing and quenching, and the transformation stress increases, and large quenching strain causes a decrease in component accuracy.
• Si is positively contained in the component, and the content is set to 0.50 to 1.5% by weight.
• the carburizing treatment is performed in an antioxidant atmosphere.
• the content of Si is 0.50 weight. If the ratio is less than / 0 , the effect of the above improvement is small, and in particular, there is a problem that the effect of preventing grain boundary oxidation during carburizing treatment is reduced. On the other hand, if the content exceeds 1.5% by weight, the above-mentioned improvement effect is saturated and there is a problem that uniform austenite before quenching is difficult.
• a more preferable range of the Si content is within a range of more than 0.5% by weight and 0.7% by weight or less.
• the hardenability J of the above material is 35 to 50 (at 12.5 mm). limit. As a result, an excellent quenching effect can be obtained even if the range of the above-mentioned quenching and quenching and the degree H are limited to the above-mentioned ranges.
• the quenchability J is less than 35, the quenching process after carburizing cannot provide a sufficient quenching effect to the carburized layer and the non-carburized part (inner part), and the desired quenching effect cannot be obtained. High strength cannot be achieved. Therefore, more preferably, the hardenability J is set to 38 or more.
• the hardenability J exceeds 50, there is a problem that the transformation stress increases due to an increase in the microstructure transformation rate inside the non-carburized portion, and quenching strain tends to occur. . Also, the higher the hardenability J, the higher the hardness before carburizing and quenching, and the lower the workability such as plastic workability and cutting workability before carburizing. Therefore, in order to prevent this reduction in workability, the hardenability J is more preferably set to 45 or less.
• the quenching quenching intensity H is 0.0 1 to 0.0 limited to 8 (c m '1).
• the quenching quenching degree H is less than 0.01 (cm- 1 )
• the quenching process after carburizing is performed in the same manner as when the quenchability J is less than 35. A sufficient quenching effect cannot be given to the layer and the non-carburized portion (inside), and the desired high strength cannot be achieved.
• the above quenching treatment must be performed not only in the above range of quenching and quenching H but also under the condition of monotonically cooling from point A1 to point Ms as described above.
• Cooling monotonically means that reheating is not performed during cooling, that is, the material temperature during cooling does not rise. Therefore, when the above condition of monotonous cooling is satisfied, the material temperature continues to drop, or the temperature drops during the process. Even if the cooling stops, this includes the case where the temperature remains constant and never rises, and then falls again. Changes in the cooling rate are, of course, permitted.
• the cooling condition can be selected so as not to cover the nose region of the S curve shown in the so-called isothermal transformation curve in the carburized portion. As a result, a sufficient martensitic transformation can be ensured.
• the C content, the Si content, the hardenability J, the carburizing treatment in an antioxidant atmosphere, the conditions of the monotonic cooling, and the specific By providing all of the quenching treatments that satisfy both the conditions of quenching and quenching degree H, it is possible to obtain a carburized quenched member with high strength while sufficiently suppressing quenching distortion. If one of the requirements is missing, the intended purpose cannot be achieved. The inventors of the present invention have found these for the first time through numerous experiments.
• a second aspect of the present invention relates to a carburized and quenched member manufactured by the above-described manufacturing method, wherein the surface hardness of the carburized layer is 700 to 900 Hv, and the non- A carburized and quenched member characterized in that the internal hardness of the carburized part is 250 to 45 OH v.
• This carburized and quenched member adopts the above-mentioned superior manufacturing method and adjusts the component range processing conditions to limit the surface hardness of the carburized layer and the internal hardness of the non-carburized portion to the specific ranges described above. Things.
• the stress distribution applied to the member obtained by combining the stress acting on the member due to the additional load acting on the member and the stress concentration near the member surface caused by the unevenness of the member shape, holes, etc.
• static strength tensile strength, bending strength, torsional strength, etc.
• dynamic strength surface fatigue strength, bending fatigue strength, torsional fatigue strength, etc.
• the surface hardness of the carburized layer is less than 700 HV, the strength may not be secured against stress concentration near the member surface. Another problem is that the wear resistance on the outermost surface is insufficient. On the other hand, if the surface hardness exceeds 90 OHv, carbides such as cementite may be generated on the surface layer, which may lead to insufficient strength, especially toughness.
• Figure 1 is an explanatory diagram showing a rotating bending fatigue test piece.
• Figure 2a is a plan view of the evaluation gear.
• Figure 2b is a cross-sectional view of the evaluation gear.
• the carburizing treatment is preferably performed in a reduced-pressure atmosphere reduced to 1 to 30 hPa.
• a reduced-pressure atmosphere reduced to 1 to 30 hPa.
• the value of the reduced pressure in the reduced-pressure atmosphere is less than 1 hPa, there is a problem in that it is excessive for suppressing the oxidation, and the equipment for reducing the pressure has a high-pressure specification, which raises the cost.
• the carburizing treatment is performed in an atmosphere containing an inert gas as a main component. Also in this case, the antioxidant atmosphere can be easily formed.
• the inert gas include a nitrogen gas and an argon gas.
• the carburizing treatment is preferably performed so that the surface carbon content of the carburized layer is 0.6 to 1.5% by weight.
• the surface carbon concentration of the carburized layer affects the surface hardness of the carburized and quenched material. If the surface carbon content of the carburized layer is less than 0.6% by weight, there is a problem that the surface hardness is insufficient. If the content exceeds 1.5% by weight, there is a problem that the hardenability of the matrix is remarkably reduced due to an increased amount of carbide precipitation and the surface hardness is insufficient.
• the grain boundary oxidation generated from the surface of the above material be 3 zx m or less. That is, it is preferable to control the above-mentioned grain boundary oxidation to 3 ⁇ m or less from the surface by adjusting the composition of the material, the antioxidant atmosphere during carburization, the heating temperature, the heating time, and the like.
• grain boundary oxides lowers the grain boundary strength, so that the strength is lower than that of general carburized and quenched layers (parts), and when grain boundary oxides are formed deeper than 3 ⁇ In this case, there is a possibility that the abrasion resistance may decrease due to insufficient strength or hardness of the member.
• the surrounding alloying elements are also incorporated into the grain boundary oxide by chemical compound reaction. As a result, the hardenability improving element in the carburized quenched layer around the grain boundary oxide is taken in and consumed by the grain boundary oxide, and a dead zone of the additive is formed around the grain boundary oxide layer. Insufficient hardenability of the carbon quenching layer itself may result in insufficient hardness and insufficient strength.
• the material preferably has a surface compressive residual stress of 300 to 800 MPa. That is, it is preferable that the residual compressive stress on the surface be adjusted to 300 MPa or more by adjusting the component composition of the material, the antioxidant atmosphere during the carburization, the heating temperature, the heating time, and the like. As a result, the tensile stress near the surface can be reduced The working stress can be reduced by the compressive residual stress in the vicinity of the surface, and the dynamic strength (surface fatigue strength, bending fatigue strength, torsional fatigue strength) can be particularly improved. On the other hand, if the surface compressive residual stress exceeds 80 OMPa, the cooling rate during quenching must be increased beyond the limit in order to increase the amount of martensite. As a result, large quenching distortion occurs, and the dimensional accuracy of the members cannot be secured.
• the residual surface compressive stress can be obtained by quenching the carburized layer to generate martensite and generating a compressive stress field by volume expansion accompanying the transformation.
• the amount of martensite formation is small, that is, when the amount of retained austenite is large or when the structure of troostite is large, a sufficient compressive residual stress field cannot be formed. Therefore, reducing the retained austenite (specifically, to 25% or less) and reducing the troostite structure (specifically, to 10% or less) are necessary for such compression residual It works advantageously from the viewpoint of enhancing stress effect.
• the absorption of volume expansion during martensitic transformation can be explained by the fact that when the amount of martensite is small, the surrounding residual austenite or troostite structure undergoes plastic deformation and progresses, resulting in stress relaxation and increased surface compressive residual stress. Does not contribute much. However, when the amount of martensite increases and the retained austenite or troostite structure decreases as described above, the density of dislocations introduced by plastic deformation increases, and slip deformation is constrained. To increase.
• the quenching process it is preferable to perform quenching under the condition of monotonically cooling in the range of the quenching degree H from the temperature in the austenite region to 300 ° C. Good. As a result, a sufficient quenching effect can be obtained.
• the quenching quenching intensity H in the cooling between the temperature of the austenite region to 3 0 0 ° C is less than 0. 0 1 (c m '1 ), becomes hardened insufficient, the desired hardened structure, Characteristics cannot be secured, resulting in insufficient member strength.
• quenching quenching rate H in the cooling from the temperature in the austenitic region to 300 ° C exceeds quenching quenching rate H of 0.08 (cm " 1 ), the quenching is excessive.
• the structural transformation stress and thermal stress increase, quenching strain increases, and the accuracy of parts may decrease.
• the quenching is performed by gas cooling.
• the above-mentioned quenching degree H can be relatively easily secured.
• the gas cooling is performed with an inert gas.
• an inert gas Preferably, safety during quenching can be ensured.
• the inert gas is a nitrogen gas.
• Nitrogen gas is preferably used as the inert gas because of its availability, cost, and ease of handling during mass production operations.
• the carburized layer preferably has a residual austenite area ratio of 25% or less. If the retained austenite area ratio exceeds 25%, the residual austenite is transformed into martensite due to the processing stress after the carburizing and quenching process, or the applied stress during use of the member. In such a case, distortion is generated by the transformation stress at that time, and there is a possibility that the accuracy of parts may be reduced.
• the area ratio of retained austenite is more desirably 20% or less. In order to reduce the area ratio of retained austenite, the retained austenite can be forcibly turned into martensite, for example, by shot peening, to reduce the area ratio.
• the ratio is 10% or less.
• the above-mentioned troostite is formed in the carburized layer after carburizing and quenching. Since the structure is incompletely quenched and the hardness is low, if the area ratio of the structure exceeds 10%, the strength of the component may decrease due to low-strength troostite.
• the internal structure of the carburized and quenched member is preferably bainite. More specifically, the area ratio of bainite in the cross-sectional structure is desirably 50% or more. In bainite, unlike martensite, transformation proceeds while iron atoms forming the lattice partially diffuse. Therefore, compared to martensite, the occurrence of distortion due to transformation is small, and the hardness is higher than the pearlite generated when the cooling rate is further reduced, so that the strength of the inner non-carburized part is appropriately increased. be able to.
• the inner layer is composed mainly of bainite
• a structure mainly composed of bainite can be obtained by setting the cooling quenching degree H in the range of 0.01 to 0.08 (cm- 1 ). It is desirable to select the composition as follows. This makes it possible to obtain parts with both strength and toughness.
• the carburized and quenched member is a carburized gear.
• Gears are components that require a variety of strict conditions, and the excellent characteristics obtained by the above manufacturing method are very effective.
• Example 1 As Example 1, the results of an experiment performed to confirm the effects of the present invention will be described.
• Steel 11 and Steel 12 are steel grades having the composition newly developed in this example, and Steel 13 and Steel 14 are steel grades corresponding to JIS case hardening steel SCM420 and SN CM 815, respectively.
• Table 1 shows the results. This property is a property of the material that is not related to the manufacturing method described later.
• Steel 11 and Steel 12 are alloy steels applicable to the material of the present invention in terms of material and hardenability J.
• steel 13 has hardenability J and Si content outside the range of the present invention
• steel 14 has Si content outside the range of the present invention.
• test spur gear 4 (equivalent round bar diameter: 10.5 mm ⁇ ) was manufactured.
• the test pieces and gears made from steels 11, 12, and 14 were subjected to low-pressure carburizing (vacuum carburizing) and gas quenching under the conditions of “Method 1” shown in Table 2.
• “Production method 1” has a quenching quenching rate of 0.05 (cm ′ 1 ) after carburizing treatment and satisfies the requirements of the production method of the present invention. is there.
• the following tests were performed on the test pieces prepared as described above.
• the hardness distribution (internal hardness) of the cross section of a round bar specimen with a diameter of 25 mm was examined using a Vickers hardness tester.
• the surface hardness (surface hardness) of the carburized and quenched material was measured at 0.02 mm from the surface.
• the area ratio of troostite at the same position was measured by image analysis of a scanning electron micrograph.
• the maximum depth of the oxide layer of the grain boundary acid underlayer was determined from the surface metallographic structure using an optical microscope.
• the surface carbon concentration was measured at 50 / m from the surface using an X-ray macro analyzer.
• the retained austenite area ratio was measured on the member surface using an X-ray diffractometer using the Co-Ka line.
• the surface residual stress was measured by an X-ray stress meter using the half-width half-point method using Fe-rays.
• carburized and quenched material “steel 11 12 + process 1” obtained by treating steel 11 and copper 12 by process 1 (hereinafter the combination of steel type and process is referred to as “steel type 12”). + Manufacturing method), the hardness at the center is 25 OHv or more. Both the surface layer and the central structure are martensite, and there is no remarkable incompletely quenched structure.
• gear accuracy and dimensional accuracy of the gears were evaluated as follows.
• a dedicated precision gear accuracy measuring machine was used to measure the amount of error in each direction of gear pressure and the amount of error in the torsion angle direction on each of the left and right tooth surfaces.
• the tooth space height was measured over the entire circumference, and the value obtained by subtracting the minimum value from the maximum value was calculated as the tooth groove runout.
• the C content, Si content, and hardenability J were used as a material for the specific alloy steel within the above specific range, and after forming a carburized layer by carburizing in an antioxidant atmosphere, In the case of “Steel 11, 12+ Production Method 1” quenched under the conditions of the specific quenching quenching degree H, it can be seen that high strength can be achieved while quenching distortion is sufficiently suppressed. '
• alloy steel has Fe as a main component and C: 0.12 to 0.22% by mass, Si: 0.5 to 1.5% by mass, and Mn: 0.25 to 0% as subcomponents. . 45 mass 0/0, N i: 0. 5 ⁇ 1. 5 mass%, C r: 1. 3 ⁇ 2. 3 wt%, B: 0. 0 ⁇ 1 ⁇ 0. 003 wt%, T i: 0.02 to 0.06% by mass, Nb: 0.02 to 0.12% by mass, A1: 0.005 to 0.05% by mass should be set.
• N of steel types 11 and 12 is 87.6 and 93.4, respectively.
• N is larger than 95. If N exceeds 95, the hardness of the rolled steel and the hardness of the normalized steel significantly increase, and it becomes impossible to obtain machinability and cold workability. Therefore, when emphasis is placed on manufacturability, it is necessary to control the composition of the steel so that this component parameter N is 95 or less.
• bainite is not generated at a cooling rate of at least 0.1 ° C / sec or less, and the cooling rate is at least 12 ° CZ seconds or more.
• No ferrite is formed in the region.
• the range of the cooling rate can be specified by measuring the continuous cooling transformation diagram (CCT diagram) of steel at various cooling rates.
• ferrite is formed in the steel composition at a cooling rate of at least 12 ° C / sec (hereinafter referred to as the upper limit cooling rate) so that the carburized layer can be sufficiently quenched even by gas cooling. Set to disappear. If ferrite is formed even if the cooling rate is increased beyond 12 ° CZ seconds, martensite will not be sufficiently formed in the carburized layer due to gas cooling, leading to insufficient hardness.
• bainite should not be formed at a cooling rate of at least 0.1 ° C / sec or less. 0. If bainite is formed even at a cooling rate of 1 ° CZ or less, the inner layer, which is not affected by the carburized layer, is deeply quenched and the strain increases.
• bainite is not formed at a cooling rate of less than 0.1 ° c / sec, the formation of bainite is sufficiently suppressed in the range of the actual annealing cooling rate, and the workability of ferrite + pearlite is increased. A rich organization can be obtained. Therefore, in the annealed state, that is, in the range in which the cooling rate from austenite is equivalent to natural cooling or air cooling, a sufficiently low material hardness is obtained to improve the workability, and machining before carburizing and quenching is performed. It can be done easily.
• the composition so that the inner layer is composed mainly of bainite by setting the cooling rate to 0.1 to 10 ° CZs. Is desirable.
• steel with the chemical composition shown in Table 6 (steels 21 to 24 and steels 31 to 38) was smelted, then slab-formed, then slab-rolled and bar-rolled to a diameter of 7 Omm. Round bars were manufactured.
• test pieces and gears were sorted and processed by three types of manufacturing methods (manufacturing methods 3 to 5).
• Manufacturing method 3 is characterized by gas carburizing and oil quenching. Heating in a carburizing gas atmosphere at 930 ° C for 5 hours ⁇ 850 ° CX for 1 hour ⁇ 130 ° C oil quenching ⁇ 180 ° C for 1 hour Carburizing and tempering are performed under the following conditions. In this case, the degree of quenching and quenching H is 0.15 (cm- 1 ).
• Manufacturing method 4 is characterized by vacuum carburizing and gas cooling. Heating at 0 ° C for 5 hours ⁇ Diffusion for 1 hour at 850 ° C-cooling with nitrogen gas ⁇ Carburizing and tempering under the conditions of tempering at 180 ° C for 1 hour. In this case quenching steep Hiyado H 0. A 0 5 (c m '1) .
• Process 5" is a modification of the nitrogen gas cooling in the process 4 in hardening 1 3 0 ° C oil, quenching quenching intensity H in this case is 0. 1 5 (c m ' 1). The same measurements and tests as in Example 1 were performed on each test piece and gear that had been processed by the above method.
• steel types 31 to 38 have low flexural fatigue strength or surface fatigue strength, and oil-cooled parts have large variations in accuracy due to quenching strain, and there are many practical problems.
• Steel grades 31 to 34 have an incomplete quenched structure due to the generation of grain boundary oxidation during gas carburization, and the resulting surface hardness is low, resulting in low strength.
• quenching is more rapid than oil cooling with gas cooling, and cooling unevenness is large, so that the accuracy variation due to quenching distortion has increased.
• steel types 21 to 24 all have high surface hardness, have an appropriate internal hardness, and can suppress distortion to a small level, thus achieving both high strength and low distortion. is there.
• the alloy steel has Fe as the main component, the subcomponent and C: 0.1 to 0.5 mass 0 S i: 0.5 to: L. 0 mass%, Mn: 0.3 to 1.0 mass%, C r: 0. 1 ⁇ 1 0 mass 0 I P:.. 0. 003 ⁇ 0 01 5 mass%, S: 0. 00 5 ⁇ 0. 03 mass%, A 1: 0. . 01-0 06 wt%, N: with containing 0.005 to 0.03 wt 0/0, Mo:. 0. 3 ⁇ 1 3 wt%, N i:. 0. 1-1 0 wt% It is preferable to set so as to contain at least one kind. As a further auxiliary component, V: 0. 05 ⁇ : L.
• T i 5 mass 0/0, Nb:. 0. 02 ⁇ 0 2 mass 0/0, T i:. 0. 01 ⁇ 0 1 or more 2 wt% , Or B: 0.0005 to 0.005 mass 0 /. , T i:. 0. 005 ⁇ 0 1 mass 0 do or, B:. 0. 0 005 ⁇ 0 005 mass%, T i: also contain 0.1 1-0 2 mass 0/0. No. As other elements, in terms of mass%, selected from the group consisting of Ca: 0.01% or less, Mg: 0.01% or less, Zr: 0.05% or less, Te: 0.1% or less. At least one kind may be contained.

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