JP5457852B2 - Method for producing Si-containing steel sheet - Google Patents

Description

  The present invention produces a Si-containing steel sheet that generates a stable and uniform scale over the entire surface of a steel sheet produced by hot rolling, eliminates uneven cooling, makes the surface temperature of the steel sheet uniform, and has good surface properties. For a useful method.

  In order to manufacture a thin steel plate by hot rolling, the slab is heated in a heating furnace, then descaled, subsequently rolled to a predetermined plate thickness by rough rolling and finish rolling, and a water cooling zone (water cooling zone) is arranged. The water is cooled to a predetermined temperature on a hot run table and wound into a coil. During water cooling at this time, the boiling state of the water-cooled surface varies depending on the surface properties of the steel sheet, which affects the accuracy of the coiling temperature and causes problems such as inducing variations in the mechanical properties of the steel sheet. .

In recent years, in a high-tensile steel plate that is widely used mainly for automobile applications, a large amount of Si is generally added to ensure strength. When normal hot rolling is performed on a steel plate containing a large amount of Si, a Si-containing oxide such as firelite (Fe ₂ SiO ₄ ) is generated. This firelight is generated at the steel material (base metal) / scale interface, and when it exceeds the melting temperature of the firelight (1173 ° C), depending on the oxidation conditions, it bites into the steel material in a wedge shape, and the adhesion between the scale and the steel material This will increase the descalability.

The firelight that remains unevenly obstructs the diffusion of Fe from the steel material, so the oxygen diffusion from the surface becomes dominant, and the hematite (Fe ₂ O ₃ ), which is a higher-order oxide, is formed on the outermost surface of the scale. Generate. Since this hematite is a hard and brittle oxide, the hematite is pulverized during rolling, resulting in surface properties with severe irregularities, and causes uneven cooling. In addition, firelight, which causes hematite, is an oxide with extremely low thermal conductivity compared to other oxides. Cause unevenness.

  For this reason, in order to obtain a steel sheet with good surface properties, it is extremely important to remove firelight that causes instability of the steel sheet surface temperature and to suppress the formation of hematite that deteriorates the surface properties. . That is, by removing firelite uniformly over the entire surface of the steel sheet to reduce the formation of hematite, it achieves good surface properties, and manufactures Si-containing steel sheets with less variation in steel sheet surface temperature and less cooling unevenness during cooling. Realization of the method is required.

  Under these circumstances, in descaling before rough rolling and finish rolling, realization of scale properties with good peelability that does not leave firelight and reduction of wrinkles (indentation wrinkles) caused by pressing the scales are realized. Therefore, various techniques have been proposed. As such a technique, for example, Patent Documents 1 to 6 propose a method for controlling the properties of the scale by appropriately controlling the conditions in the heating furnace and rolling.

  However, all of the technologies that have been proposed so far focus on the suppression of indentation flaws, and most of the fluctuations in the steel sheet surface temperature and cooling unevenness due to the formation of hematite caused by the remaining firelight. The situation is not taken into account.

Japanese Patent Laid-Open No. 4-247829 JP-A-6-192728 JP-A-6-346145 Japanese Patent Laid-Open No. 7-034137 JP 2000-273579 A JP 2005-297008 A

  The present invention has been made paying attention to the circumstances as described above, and the purpose thereof is useful for producing a Si-containing steel sheet having excellent surface properties by reducing variations in the steel sheet surface temperature and cooling unevenness. Is to provide a simple method.

  The method of the present invention capable of achieving the above object is C: 0.02 to 0.6% (meaning mass%, the same applies to the chemical components of steel), Si: 0.2 to 3.0% , Mn: 0.2 to 3.5%, P: 0.02% or less (not including 0%), S: 0.02% or less (not including 0%), Al: 0.15% or less ( Steel materials each containing 0%) in an atmosphere having a water vapor concentration in the heating furnace of 15 to 40% by volume and an oxygen concentration of 1.0% by volume or less at 1173 to 1300 ° C. for 15 to 40 minutes. It has a gist in that the atmosphere before heating and descaling, followed by rough rolling, and then immediately before finish rolling is oxidized as a nitrogen atmosphere having an oxygen concentration of 10% by volume or less.

  In the method of the present invention, it is preferable to oxidize in an atmosphere containing an oxygen concentration of 10% by volume or less and a water vapor concentration of 5 to 30% by volume immediately after the rough rolling and immediately before finish rolling.

  Regarding the steel materials to be processed by the method of the present invention, the above basic components are further added to (a) Cu: 1.0% or less (not including 0%) and / or Ni: 1.0% or less (not including 0%). ), (B) Cr: 1.0% or less (not including 0%), (c) Ti: 1.0% or less (not including 0%), V: 1.0% or less (including 0%) And Nb: at least one selected from the group consisting of 1.0% or less (not including 0%), (d) Mo: 1.0% or less (not including 0%), (e) B: 0.1% or less (not including 0%), (f) Ca: 0.005% or less (not including 0%) and / or Mg: 0.01% or less (not including 0%), etc. You may make it contain and the characteristic of a steel plate is further improved according to the component to contain.

  According to the present invention, by appropriately controlling the atmosphere in the heating furnace and immediately before rough rolling after the rough rolling, the firelite is uniformly removed over the entire surface of the steel sheet to reduce the formation of hematite. Therefore, a Si-containing steel sheet having excellent surface properties can be produced.

It is a schematic explanatory drawing which shows the apparatus structural example for implementing this invention method.

When a steel sheet containing Si is oxidized, firelite (FeO) is formed at the interface between the iron-based oxides such as wustite (FeO), magnetite (Fe ₃ O ₄ ), hematite (Fe ₂ O ₃ ) and the steel. Fe ₂ SiO ₄ ) is formed. Of these, firelight has a remarkably low thermal conductivity compared to magnetite and wustite. Therefore, non-uniform residue of firelight causes fluctuations in the surface temperature of the steel sheet and causes variations in material.

  For example, when oxidized at 1173 ° C. or higher, the firelite becomes a liquid phase and descaling properties are improved. However, depending on the oxidation conditions, the steel may bite into the steel material and may remain in the rolling process. . The remaining firelite hinders the diffusion of Fe from the steel material, the oxygen diffusion from the surface becomes dominant, and the hematite, which is a high-order oxide, is formed on the outermost surface of the scale formed in the portion where the firelight remains. Many will be generated.

Since hematite (Fe ₂ O ₃ ) is a hard and brittle oxide, if it is broken by rolling, the surface properties become uneven, resulting in uneven cooling. Therefore, it is necessary to remove as much as possible the firelight that induces variations in the steel sheet surface temperature, and to further suppress hematite on the surface layer of the scale as much as possible.

  The inventors of the present invention have studied from the viewpoint of eliminating firelight that is a cause of hematite generation and leading to instability of the surface temperature as much as possible, and suppressing hematite on the outermost surface of the scale as much as possible. As a result, by increasing the water vapor concentration in the atmosphere in the heating furnace and lowering the oxygen concentration, the intrusion of the liquid phase firelight is suppressed, the firelight is uniformly removed, and then generated during rolling In order to suppress the formation of hematite on the surface layer of the scale, the uniform scale without the formation of hematite on the outermost surface by rolling in a low concentration oxygen atmosphere or a low concentration oxygen atmosphere containing water vapor and then rolling Was found to be generated. As a result, variations in the surface temperature of the steel sheet due to non-uniform residue of firelight and deterioration of the surface properties due to hematite breakage during rolling can be avoided, and uneven cooling can be suppressed.

  In the method of the present invention, in order to improve the surface properties of the Si-containing steel, the conditions for heating the steel (slab, etc.) in the heating furnace are controlled, and after the rough rolling is finished after taking out from the heating furnace, finishing is performed. The atmosphere is controlled immediately before rolling, and after finishing rough rolling if necessary, the atmosphere immediately before finish rolling is more strictly controlled. The reasons for limiting these atmosphere controls are as follows. .

[Heating temperature in heating furnace: 1173 to 1300 ° C.]
Although the firelight has an effect of increasing the adhesion of the scale, when the heating temperature in the heating furnace is 1173 ° C. or higher, the firelight becomes a liquid phase and the descaling property is improved. Therefore, the heating temperature in the heating furnace needs to be 1173 ° C. or higher which is the melting temperature of firelight. On the other hand, if the heating temperature at this time becomes too high, the scale loss increases and the yield decreases, so the heating temperature condition is set to 1300 ° C. or lower. The minimum with this preferable heating temperature is 1190 degreeC, and a preferable upper limit is 1280 degreeC.

[Atmosphere in the heating furnace]
Although the descalability is improved by melting the firelight, if the oxygen partial pressure becomes too high, the melted firelite will bite into the surface of the steel sheet, worsening the descaling property, and causing uneven firelight. Will remain. Therefore, since the molten firelite has a high oxygen partial pressure and tends to bite into the steel sheet surface, the oxygen concentration in the atmosphere needs to be 1.0% by volume or less. Preferably it is 0.8 volume% or less.

  Note that the atmosphere in the heating furnace can promote inward oxidation (oxidation inside the steel formed by inward diffusion of oxygen) by increasing the water vapor concentration, and scale the bite-light bite. Thus, descaling property can be improved, which is preferable in that it can be removed uniformly. For this purpose, the water vapor concentration is preferably 15% by volume or more. However, if the water vapor concentration at this time becomes too high, the inward oxidation proceeds excessively and the scale loss increases.

[Holding time in heating furnace (heating time): 15 to 40 minutes]
The holding time in the heating furnace is 15 to 40 minutes. If the time is less than 15 minutes, the soaking of the slab is insufficient and unevenness occurs in scale generation. Preferably it is 20 minutes or more, More preferably, it is 25 minutes or more. However, if the holding time becomes too long, scale loss occurs, so it must be 40 minutes or less. Preferably it is 35 minutes or less, More preferably, it is 30 minutes or less.

  After the heat treatment is performed under the above conditions, descaling is performed. In this descaling, high-pressure water descaling is common, but mechanical descaling may be performed.

[Atmosphere from after rough rolling to just before finish rolling]
Usually, hot rolling is performed in an air atmosphere (that is, an oxygen concentration of about 20% by volume). However, if firelight remains on the surface of the steel sheet, the diffusion of Fe is hindered. Under partial pressure, surface oxidation becomes more dominant and hematite, which is a higher-order oxide, is formed unevenly thick on the outermost layer of the scale. This hematite is a hard and brittle oxide, and is easily broken during rolling, so that the surface texture becomes uneven and becomes a factor inhibiting uniform cooling. Therefore, it is preferable that the atmosphere has a low oxygen partial pressure so that hematite is not generated.

  Since it is a high temperature exceeding 1100 ° C after rough rolling to the finish rolling, hematite is generated on the outermost surface of the scale in an atmosphere with a high oxygen partial pressure (usually exposed to the atmosphere) It becomes easy to do. In order to suppress the formation of hematite, it is necessary to reduce the oxygen concentration in the atmosphere as low as possible. From this viewpoint, the oxygen concentration needs to be 10% by volume or less. Preferably it is 5 volume% or less.

  In the atmosphere from rough rolling to just before finish rolling, it is preferable to increase the water vapor concentration, thereby reducing the oxygen partial pressure of the scale surface layer, making it harder to produce hematite and stabilizing the surface properties. . From such a viewpoint, the water vapor concentration in the atmosphere from after rough rolling to immediately before finish rolling is preferably 5% by volume or more, and more preferably 10% by volume or more. However, if the water vapor concentration increases too much, the scale loss increases, so it is preferably 30% by volume or less, more preferably 25% by volume or less.

  In addition, the atmosphere control from after rough rolling to just before finish rolling is performed, for example, as shown in FIG. 1 in order to provide a furnace for controlling the atmosphere between rough rolling and finish rolling, and to adjust the atmosphere in this furnace. The gas may be introduced from the gas introduction hole (adjusted atmosphere gas introduction hole).

As for the steel material which is the object of the present invention, the chemical composition of the Si-containing steel is as long as it can remove firelite (Fe ₂ SiO ₄ ) caused by Si uniformly, suppress the formation of hematite, and can be used as a hot rolled steel sheet Although not particularly limited, for example, C: 0.02 to 0.6%, Si: 0.2 to 3.0%, Mn: 0.2 to 3.5%, P: 0.02% or less (0% Steel) containing S: 0.02% or less (not including 0%) and Al: 0.15% or less (not including 0%). The reason for adding each element is as follows.

[C: 0.02 to 0.6%]
C is an element effective for increasing the strength of a steel material (that is, a steel sheet), and is an element that affects elongation and stretch flangeability by changing the amount and transformation of a low-temperature transformation product. If the C content is less than 0.02%, it will not be possible to meet the needs of high strength for automobiles, while if it exceeds 0.6%, the weldability will be reduced. The preferable C content is 0.05% or more (particularly 0.1% or more) and 0.3% or less (particularly 0.2% or less).

[Si: 0.2-3.0%]
Si is an important element for securing the strength of the steel material. In the steel material which is the object of the present invention, the content is set to 0.2% as the minimum amount of Si necessary for securing the strength. However, when the Si content is excessive, the ductility may be deteriorated, and the content is set to 3.0% or less. A preferable Si content is 0.5% or more and 2.5% or less.

[Mn: 0.2 to 3.5%]
Mn is an element useful for ensuring the strength of the steel material, and in order to obtain the characteristics as a high-strength steel plate with excellent workability, it is necessary to contain at least 0.2% or more. However, if the Mn content is excessive, the elongation is reduced and the carbon equivalent is increased, and the weldability is deteriorated. A preferable Mn content is 0.5% or more and 3.0% or less.

[P: 0.02% or less (excluding 0%)]
P is an effective element for obtaining a high-strength steel sheet. However, if it exceeds 0.02%, the scale is easily peeled off during rolling, and the surface properties of the steel sheet become unstable. When it mixes as an impurity, it is necessary to stop the upper limit to 0.02%. The P content is preferably 0.01% or less. In addition, it is difficult to make P content in steel materials 0% on industrial production.

[S: 0.02% or less (excluding 0%)]
S is an element that causes hot cracking during hot rolling and significantly impairs spot weldability. In steel materials, it is fixed as a precipitate, but if the amount increases, elongation and stretch flangeability are deteriorated. Therefore, when it is mixed as an unavoidable impurity, the upper limit must be limited to 0.02%. The S content is preferably 0.01% or less. In addition, it is difficult to make S amount in steel materials 0% on industrial production.

[Al: 0.15% or less (excluding 0%)]
Al is an effective element for deoxidation in the steelmaking stage. However, if the Al content exceeds 0.15%, not only the production cost is increased, but also the surface properties are deteriorated. Therefore, the upper limit of the Al content is set to 0.15% or less. The Al content is preferably 0.1% or less.

  The balance other than C, Si, Mn, P, and S may be iron. In the case where the balance is iron, it is naturally allowed that unavoidable impurities (for example, impurities (O, N, etc.) brought in depending on the status of raw materials, materials, manufacturing equipment, etc.) are contained in the steel sheet. The steel material to be used in the present invention may contain various selective elements as necessary, and the characteristics of the steel material are further improved according to the kind of the contained element. The contents and reasons for limitation when these elements are contained are as follows.

[Cu: 1.0% or less (not including 0%) and / or Ni: 1.0% or less (not including 0%)]
Cu and Ni are effective elements for improving the strength of the steel material itself. In particular, Cu and Ni, which are harder to oxidize than Fe, are evenly concentrated on the surface, which is effective in preventing the deterioration of plating properties by changing the form of the oxide containing Si and Mn. However, excessive inclusion makes it economically unsuitable, so both should be 1.0% or less. A preferable content when these elements are contained is 0.003% or more. A more preferred upper limit is 0.8% or less.

[Cr: 1.0% or less (excluding 0%)]
Cr is an element effective in enhancing the hardenability of steel and strengthening the structure. Cr is an element that not only concentrates C in austenite to improve stability and generates martensite but also affects the plating properties by forming it on the surface of an oxide steel plate. In order to exhibit such an effect, it is preferable to contain 0.003% or more. However, even if the Cr content exceeds 1.0% and becomes excessive, the effect is saturated and the cost is disadvantageous. Therefore, the upper limit of the Cr content is set to 1.0% or less. More preferably, it is 0.8% or less.

[Ti: 1.0% or less (not including 0%), V: 1.0% or less (not including 0%) and Nb: 1.0% or less (not including 0%) At least one selected]
Ti, V, and Nb are all effective elements for forming carbides and increasing the strength of steel. Among these, Ti is effective in fixing C and N and improving the deep drawability (r value) of the steel material. In order to exhibit such an effect, it is preferable to contain 0.003% or more of all. However, if these contents exceed 1.0% and become excessive, the cost increases and processability is deteriorated. In addition, the more preferable upper limit of these is 0.5% or less.

[Mo: 1.0% or less (excluding 0%)]
Mo is an element effective in achieving solid solution strengthening of steel materials. However, the Mo content is 1.0.
If it exceeds 1%, the manufacturing cost will increase. In order to exert such effects, the Mo content is preferably 0.003% or more (more preferably 0.01% or more). In addition, the more preferable upper limit of Mo content is 0.5% or less.

[B: 0.1% or less (excluding 0%)]
B is an element that has the effect of improving the weldability of the steel material and improving the hardenability. However, when the B content exceeds 0.1% and becomes excessive, these effects are not only saturated, but ductility is deteriorated and workability is lowered. In order to exert such effects, the B content is preferably 0.0002% or more (more preferably 0.0004% or more). In addition, the upper limit with more preferable B content is 0.005% or less.

[Ca: 0.005% or less (not including 0%) and / or Mg: 0.01% or less (not including 0%)]
Ca and Mg have the effect | action which controls the form of an inclusion, raises ductility, and improves workability. However, when the content of Ca exceeds 0.005% and Mg exceeds 0.01%, inclusions in the steel increase, ductility deteriorates, and workability deteriorates. In addition, in order to exhibit such an effect, it is preferable to contain 0.0005% or more (more preferably 0.0007% or more) in any case. The upper limit of these contents is more preferably 0.004% or less for Ca and 0.008% or less for Mg.

  EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited by the following examples, and appropriate modifications are made within a range that can meet the above and the following purposes. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

[Example 1]
Steel slabs (steel types A to U) having the chemical composition shown in Table 1 below are melted, and the heating conditions shown in Table 2 below (heating furnace temperature, heating furnace holding time, heating furnace oxygen concentration, heating furnace steam concentration) ) And descaling was performed with high-pressure water of 10 MPa or more. At this time, in order to adjust the oxide composition of the outermost surface of the scale before finish rolling (suppress hematite), after adjusting the atmosphere (oxygen concentration) from rough rolling to finish rolling, rolling under normal rolling conditions Plate thickness: After making a steel plate of 3 mm, it was water-cooled and wound up at a target winding temperature of 450 ° C. The temperature was controlled with a radiation thermometer. In addition, the control of the water vapor concentration in the atmosphere was not performed from the rough rolling to the finish rolling (substantially less than 5% by volume).

The atmosphere control from rough rolling to finish rolling (see FIG. 1) was performed by spraying various gases on the steel surface at a flow rate of 50 L / min from a gas cylinder (nitrogen dilution) with a predetermined oxygen concentration. . Then, the ratio (L ₁ / L ₀ ) between the coil length L ₁ and the total coil length L ₀ within the range of ± 20 ° C. of the target coiling temperature is set to the correct ratio (± 20 It was determined as a degree of median (° C). The success criterion for this hit rate is 90% or more. The results are shown in Table 2 below together with the finish rolling temperature.

  From this result, it can be considered as follows. First, those satisfying the requirements specified in the present invention (Test Nos. 1, 4, 5, 7, 10, 12-14, 16, 17, 19-23, 25-29) are suppressed in the formation of hematite. Cooling unevenness is reduced, and the center ratio of the winding temperature is high. Moreover, it has confirmed that the obtained steel plate had the favorable surface property by these.

  On the other hand, those that do not satisfy the requirements specified in the present invention (test Nos. 2, 3, 6, 8, 9, 11, 15, 18, 24) have insufficient effect of suppressing hematite, resulting in uneven cooling. As a result, the hit ratio of the winding temperature is lowered.

  Specifically, Test No. In the cases of Nos. 2 and 11, the water vapor concentration in the heating furnace is lowered, the inward oxidation does not proceed, the descaling property is deteriorated, and the center ratio of the coiling temperature is slightly lowered. Test No. In the case of No. 3, the temperature in the heating furnace is low, the liquid phase of the firelight does not progress, the descaling property is deteriorated, and the hit ratio of the winding temperature is slightly lowered.

  Test No. For 6, 8, 15, and 24, the oxygen concentration is excessive from rough rolling to finish rolling, the effect of suppressing hematite becomes insufficient, cooling unevenness occurs, and the central ratio of the winding temperature is increased. It is falling.

  Test No. Nos. 9 and 18 have an excessive oxygen concentration in the heating furnace, making it easy for the molten firelite to bite into the surface of the steel sheet, resulting in poor descaling and a moderate percentage of the coiling temperature. It is falling.

[Example 2]
In order to further improve the composition of the outermost surface of the scale before finish rolling (suppress hematite), after adjusting the atmosphere (oxygen concentration, water vapor concentration) from rough rolling to finish rolling as shown in Table 3 below, normal rolling Rolling, cooling, and winding were performed under conditions. At this time, the heating temperature in the heating furnace was 1250 ° C., the holding time in the heating furnace was 25 minutes, the oxygen concentration in the heating furnace was 0.1% by volume, and the water vapor concentration was 25% by volume.

  Also, the atmosphere (oxygen concentration, water vapor concentration) from rough rolling to finish rolling can be controlled on the surface of the steel material at a flow rate of 50 L / min from a gas cylinder (nitrogen dilution) with a predetermined oxygen concentration, as in Example 1. The gas was sprayed. Then, the central ratio (± 20 ° C. central ratio) of the winding temperature in each test was determined in the same manner as in Example 1. The results are shown in Table 3 below together with the finish rolling temperature.

As is apparent from the results in Table 3, the oxygen concentration in the atmosphere from rough rolling to finish rolling was low and the water vapor concentration was controlled to an appropriate amount (Test Nos. 30, 32-38, 40-42). , 44, 45, 47 to 54), a uniform scale mainly composed of magnetite (Fe ₃ O ₄ ) reduced to the hematite limit is formed. . Of these, test no. Nos. 33 and 38 deviate from the preferable requirements defined in the present invention, and the correctness of the coiling temperature is slightly lower than the others.

  On the other hand, in the case where the oxygen concentration in the atmosphere from rough rolling to finish rolling is high (test Nos. 31, 39, 43, and 46), since the formation of hematite is not suppressed, the winding temperature The hit rate remains low.

Claims (8)

C: 0.02 to 0.6% (meaning mass%; the same applies to the chemical components of steel), Si: 0.2 to 3.0%, Mn: 0.2 to 3.5%, P: 0.02% or less (not including 0%), S: 0.02% or less (not including 0%), Al: 0.15% or less (not including 0%), and the balance being iron The steel material consisting of unavoidable impurities is heated at 1173 to 1300 ° C. for 15 to 40 minutes in an atmosphere in which the water vapor concentration in the heating furnace is 15 to 40% by volume and the oxygen concentration is 1.0% by volume or less. A method for producing a Si-containing steel sheet, characterized by performing rough rolling after scaling, and then oxidizing the atmosphere immediately before finish rolling as a nitrogen atmosphere having an oxygen concentration of 10% by volume or less.   The method for producing a Si-containing steel sheet according to claim 1, wherein oxidation is performed in an atmosphere containing an oxygen concentration of 10% by volume or less and a water vapor concentration of 5 to 30% by volume immediately after the rough rolling and immediately before the finish rolling. .   The Si-containing steel sheet according to claim 1 or 2, wherein the steel material further contains Cu: 1.0% or less (not including 0%) and / or Ni: 1.0% or less (not including 0%). Manufacturing method.   The method for producing a Si-containing steel sheet according to any one of claims 1 to 3, wherein the steel material further contains Cr: 1.0% or less (not including 0%).   The steel material further includes: Ti: 1.0% or less (not including 0%), V: 1.0% or less (not including 0%) and Nb: 1.0% or less (not including 0%) The manufacturing method of the Si containing steel plate in any one of Claims 1-4 containing at least 1 sort (s) chosen from the group which consists of.   The method for producing a Si-containing steel sheet according to any one of claims 1 to 5, wherein the steel material further contains Mo: 1.0% or less (not including 0%).   The method for producing a Si-containing steel sheet according to any one of claims 1 to 6, wherein the steel material further contains B: 0.1% or less (not including 0%).   The steel material further contains Ca: 0.005% or less (not including 0%) and / or Mg: 0.01% or less (not including 0%). A method for producing a Si-containing steel sheet.

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