JP5208450B2 - Cr-containing steel with excellent thermal fatigue properties - Google Patents

Description

The present invention relates to a Cr-containing steel excellent in thermal fatigue characteristics, which is optimal for use in exhaust system members that particularly require high-temperature strength and oxidation resistance.

Exhaust system members such as automobile exhaust manifolds, front pipes, and center pipes pass high-temperature exhaust gas exhausted from the engine, so the materials that make up the exhaust members have various characteristics such as oxidation resistance, high-temperature strength, and thermal fatigue characteristics. Is required.

Conventionally, cast iron is generally used for automobile exhaust members, but stainless steel exhaust manifolds are used from the viewpoints of stricter exhaust gas regulations, improved engine performance, and lighter vehicle body. Became. In recent years, the exhaust gas temperature has increased to about 800 to 900 ° C., and there is a demand for a material having oxidation resistance, high temperature strength, and thermal fatigue characteristics in an environment used at a high temperature for a long time.

Among stainless steels, austenitic stainless steel has excellent heat resistance and workability, but due to its large thermal expansion coefficient, thermal fatigue failure occurs when it is applied to a member that repeatedly receives heating and cooling, such as an exhaust manifold. Is likely to occur.

On the other hand, since ferritic stainless steel has a smaller coefficient of thermal expansion than austenitic stainless steel, it has excellent thermal fatigue characteristics. Also, steel with adjusted alloy addition amount such as Cr, Mo, Nb is used to increase the high temperature strength according to the exhaust gas temperature. As the exhaust gas temperature increases, the amount of these alloys added has increased, but the thermal fatigue life, which is the most important characteristic, has not necessarily increased. Further, excessive increase in the amount of alloy added leads to an increase in cost, and may have a disadvantage that is not economical.

  Patent Document 1 below discloses ferritic stainless steel excellent in oxidation resistance, high temperature strength, and thermal fatigue characteristics for an automobile exhaust manifold. The Cr content is a relatively low Cr content of 11 to 14%, and is a technique for improving oxidation resistance, high temperature strength, and thermal fatigue characteristics at 900 ° C. or more by adding Si. Among them, the thermal fatigue property is made under the condition of 50% restraint at 200 to 900 ° C., and the invention steel has a long thermal fatigue life, but the restraint rate is low or the temperature becomes about 800 ° C. However, sufficient characteristics could not be obtained under the condition that the cycle to be performed was long. This is considered to be due to the lack of high-temperature strength and high-temperature ductility when the material was subjected to aging treatment, that is, when the material was aged. In addition, in Patent Document 1, there is an example of combined addition of Ti and Nb. In this case, a phenomenon that a crack occurs during the processing of an actual molded part (secondary processing crack) occurs, and the member cannot be molded. In some cases, the thermal fatigue characteristics are significantly deteriorated due to micro cracks.

  Patent Document 2 below discloses an invention in which Si / Mn is controlled so that the proof stress at 900 ° C. is 15 Mpa or more and the high temperature characteristics are improved. Also in this case, it was not sufficient in a long-time use environment only by specifying the tensile strength of the product plate at 900 ° C. Further, since Mn is added in an amount of 0.7 to 1.3%, the ductility is low, and there is a problem that the thermal fatigue life is reduced due to the formability when processing into a member and the high temperature ductility is reduced.

Patent Document 3 below discloses that the 0.2% yield strength after holding at 900 ° C. for 1 hour by adjusting the components is 18 MPa or more. Here, by holding at high temperature for 1 hour or more,
Although it has been a technical idea to improve the strength under the use environment, when subjected to a thermal cycle, the thermal fatigue life may not be improved only by improving the high temperature strength.

In Patent Documents 4 to 6 below, steel containing B is disclosed as a ferritic stainless steel having excellent high-temperature characteristics, but it is added from the viewpoint of workability improvement. The impact was not clear. The role of B in improving workability is to improve the secondary workability by improving the grain boundary strength due to grain boundary segregation, but in the present invention, by adding B, the precipitates are refined to increase the high temperature strength. It is an improvement. Further, V is added to Patent Documents 4 to 6, but there are cases where V is added from the viewpoint of improving the corrosion resistance of the welded portion and improving the workability by fixing C or N.
Japanese Patent Laid-Open No. 07-145453 JP 09-279316 A JP 2002-105605 A JP-A-9-279312 JP 2000-169943 A Japanese Patent Laid-Open No. 10-204590

In particular, it is a relatively inexpensive material with excellent oxidation resistance, high temperature strength, and thermal fatigue characteristics in an environment where the exhaust temperature is close to 800 ° C and is used for a long time at high temperatures or repeatedly subjected to heating and cooling. It is to provide a Cr-containing steel.

In order to solve the above-mentioned problems, the present inventors investigated the relationship between the components and the high temperature deformation characteristics regarding the oxidation resistance, high temperature strength and thermal fatigue characteristics of the Cr-containing steel. In particular, considering the environment subject to thermal cycling, we carefully investigated how the deformation characteristics in the low temperature range affect the thermal fatigue life in addition to the deformation characteristics in the high temperature range. And as a result of repeating various examinations in order to achieve this purpose, the following knowledge was obtained. As this feature, Cr and Si are added mainly from the viewpoint of oxidation resistance,
From the viewpoint of improving the high temperature characteristics, by adjusting each component in the new component added with Nb-Ti composite and further adding V and B, the strength and ductility during long-time use are ensured and the thermal fatigue characteristics are greatly improved.

The gist of the present invention for solving the above problems is as follows.
(1) In mass%, C: 0.01% or less, N: 0.015% or less, Si: 0.8 to 1.0%, Mn: 0.2 to 1.5%, P: 0.0. 03% or less, S: 0.01% or less, Ni: 0.2% or less, Cu: 0.2% or less, Cr: 13-15%, Mo: 0.1% or less, Nb: 0.3-0 0.55%, Ti: 0.05-0.2%, V: 0.01-0.2%, Al: 0.015-1.0%, B: 0.0002-0.0010% And (Nb + 1.9 × Ti) / (C + N) ≦ 40, a Cr-containing steel excellent in thermal fatigue characteristics, characterized in that the balance consists of Fe and inevitable impurities.
(2) 0.2% proof stress at 800 ° C. after aging treatment at 800 ° C. for 100 hours or more is 2
The Cr-containing steel having excellent thermal fatigue properties as described in (1), wherein the drawing value at 0 MPa or more and a drawing value at 200 ° C. is 35% or more.
(3) The amount of solid solution Nb + the amount of solid solution Ti after the aging treatment at 800 ° C. for 100 hours or more is 0
. The Cr-containing steel excellent in thermal fatigue properties according to (1) or (2), characterized by being at least 08%.
(4) The Cr-containing steel excellent in thermal fatigue properties according to any one of (1) to (3), wherein Mn / Ti ≧ 3 is satisfied.

  According to the present invention, it is possible to provide Cr-containing steel having excellent thermal fatigue characteristics without adding particularly expensive alloy elements, and it is particularly effective for environmental measures by applying it to exhaust system members such as automobiles. An effect is obtained.

  The reason for limitation of the present invention will be described below.

C deteriorates moldability and corrosion resistance and causes a decrease in high-temperature strength. Therefore, the smaller the content, the better. Therefore, the content is set to 0.015% or less. However, excessive reduction leads to an increase in refining costs, so 0.001 to 0.005% is further desirable.

N, like C, deteriorates moldability and corrosion resistance and causes a decrease in high-temperature strength. Therefore, the smaller the content, the better. Therefore, the content was made 0.015% or less. However, excessive reduction leads to an increase in refining costs, so 0.001 to 0.010% is further desirable.

Si is an important element for improving oxidation resistance and high temperature characteristics in the present invention. Oxidation resistance and high-temperature strength improve with an increase in Si content, and the effect is manifested at 0.8% or more. Further, Si promotes the precipitation of intermetallic compounds mainly composed of Fe and Nb called a Laves phase at a high temperature. If the Laves phase precipitates excessively, the solid solution N necessary to ensure high temperature strength
The amount of b is reduced. Moreover, when Si is added excessively, the room temperature workability deteriorates, and the ductility during long-time use is lowered, resulting in a decrease in the thermal fatigue life. From these viewpoints, the upper limit was made 1.0%. Furthermore, it is 0.8 to 0.9% desirably.

Mn is an element that is added as a deoxidizer and improves the high-temperature strength, and the effect is manifested from 0.2% or more. In addition, it has been found that, in a composite additive steel with Ti, oxidation of Ti is suppressed during continuous oxidation by addition of Mn, and oxidation resistance is improved. On the other hand, addition over 1.5% lowers the ductility and forms MnS to lower the corrosion resistance. Moreover, excessive addition brings about deterioration of oxidation resistance. Therefore, it was set to 0.2 to 1.5%. Furthermore, when considering high temperature ductility and scale adhesion, 0.3 to 1.0% is desirable.

  Since P is a solid solution strengthening element like Mn and Si, the lower the content, the better. Therefore, the upper limit is preferably 0.03%. However, excessive reduction leads to an increase in refining costs, so the lower limit is preferably 0.01%. Furthermore, if considering the refining cost and corrosion resistance, 0.012 to 0.025% is desirable.

S is an element that deteriorates the corrosion resistance and oxidation resistance, but since the effect of improving the workability by combining with Ti or C is manifested from 0.0001%, the lower limit was made 0.0001%. On the other hand, the upper limit is set to 0.01% because the high-temperature strength decreases because it combines with Ti and C by appropriate addition to reduce the amount of solid solution Ti and causes coarsening of precipitates. Furthermore, if considering refining costs and high-temperature oxidation characteristics, 0.0010 to 0.0090% is desirable.

Cr is an essential element for securing oxidation resistance in the present invention. If it is less than 13%, the effect is not exhibited, and if it exceeds 15%, the workability is lowered or the toughness is deteriorated. Furthermore, considering high temperature ductility and manufacturing cost, 13.2 to 14.5%
Is desirable.

Ni is effective for improving toughness and resistance to high temperature salt corrosion. However, it is an austenite-forming element, has an adverse effect on oxidation resistance, and is expensive.

Cu is effective for improving the high-temperature strength, but lowers the ductility and adversely affects the oxidation resistance, so the content was made 0.2% or less.

Mo is effective for improving corrosion resistance, suppressing high-temperature oxidation, and improving high-temperature strength by solid solution strengthening. However, it is not more than 0.2% because of high cost ductility and high cost. More desirably, it is 0.1% or less.

Nb is an element necessary for improving the high-temperature strength by solid solution strengthening and precipitate refinement strengthening. In addition, C and N are fixed as carbonitrides, contributing to the development of the recrystallization texture that affects the corrosion resistance and r value of the product plate. These effects appear from 0.3% or more. On the other hand, in the environment of use, when it precipitates as a Lafes phase depending on the temperature, the effect is saturated even if added excessively because it loses the solid solution strengthening ability. Moreover, excessive addition will reduce the ductility in a low temperature range and will shorten a thermal fatigue life. In the present invention, the amount of dissolved Nb is ensured by composite addition with Ti. In this case, the action is saturated at 0.55%, so the content is set to 0.3 to 0.55%. Furthermore, if considering moldability, intergranular corrosion property and production cost, 0.32 to 0.45% is desirable.

Ti is an element that combines with C, N, and S to improve corrosion resistance, intergranular corrosion resistance, and deep drawability. In addition, in the combined addition with Nb, addition of an appropriate amount brings about improvement in high-temperature strength and high-temperature ductility after being exposed at high temperature for a long time, thereby improving thermal fatigue characteristics. These effects are manifested from 0.05% or more, but addition of more than 0.2% increases the amount of solid solution Ti and deteriorates the formability, as well as generation of surface flaws, lower toughness, and oxidation resistance. In order to bring about deterioration of property, it was made 0.05 to 0.2%. Furthermore, if considering the manufacturability, 0.05 to 0.15% is desirable.

  V, when added in an amount of 0.01% or more, forms fine carbonitrides and causes a precipitation strengthening action, thereby contributing to an improvement in high temperature strength. On the other hand, addition of over 0.2% lowers the low temperature ductility and conversely reduces the thermal fatigue life, so the upper limit was made 0.2%. Furthermore, if considering the manufacturing cost and manufacturability, 0.08 to 0.15% is desirable.

  In addition to being added as a deoxidizing element, Al is an element that improves high-temperature strength by oxidation resistance and solid solution strengthening, and is an essential element in the present invention. The effect is manifested from 0.015%, but addition of more than 1.0% hardens or deteriorates surface defects and weldability, so the content was made 0.015 to 1.0%. Furthermore, 0.03 to 0.7% considering the refining cost is desirable.

B is an element that improves the secondary workability during product press working. In particular, Ti-added steel is an essential element in the present invention because secondary processing cracks are likely to occur. In addition, it has been found that the component system to which Nb and Ti are added and Si is added as in the present invention contributes to the improvement of the high temperature strength. In general, B is (Fe, Cr) 23 (C, B) at high temperatures.
6 or Cr2B is formed or segregated at grain boundaries, but in the component system to which Si is added, these precipitates are not precipitated, and it has been found that there is an effect of finely depositing the above-mentioned Laves. . The Laves phase brings about a decrease in the amount of solid solution Nb and usually coarsens, so there is almost no high-temperature strengthening ability after long-term aging in particular, but because B precipitates finely,
It has precipitation strengthening ability and contributes to improvement of high temperature strength. As a factor for causing B to finely precipitate the Laves phase, it is presumed that the interfacial energy is lowered due to grain boundary segregation, and the grain boundary is hardly precipitated. These effects are manifested at 0.0002% or more. However, excessive addition deteriorates the hardness and intergranular corrosion, and also causes weld cracks. Therefore, the content was made 0.0002 to 0.0010%.
Furthermore, if considering moldability and manufacturing cost, 0.0003 to 0.0007% is desirable.
In the case of combined addition of Ti and Nb, it has been found that if both of them are added excessively, the solid solution Ti and the solid solution Nb increase and the room temperature ductility is remarkably lowered. In this study, as shown in FIG. 1, (Nb + 1.
By setting 9 × Ti) / (C + N) ≦ 50, the elongation at break at room temperature can be secured at 32% or more. Here, JIS No. 13 B specimens were sampled in the rolling direction with respect to 14% Cr steel having a Ti, Nb, C, and N amount and a plate thickness of 2 mm, a tensile test was performed, and the elongation at break was measured. Elongation at break is 3
If it is 2% or more, it is at a level where cracking and constriction do not occur even if press working from the plate material to the exhaust member or bending or pipe expansion after processing into the pipe shape.

It was also found that ductility is important in addition to high temperature strength after aging for improving the thermal fatigue life. Here, the thermal fatigue test will be described. The product plate is made by electro-welding welding (outer diameter 38
. 1 mm) and subjected to a thermal fatigue test. The thermal fatigue test was conducted with a computer-controlled electrohydraulic control fatigue tester. The temperature cycle to be applied is 120se from 200 to 800 ° C.
The temperature was raised at c and held at 800 ° C. for 30 seconds, then cooled to 300 ° C. in 120 seconds,
Furthermore, it was set as the pattern cooled to 90 degreeC in 90 seconds. Heating was performed with a high-frequency induction heating coil, and cooling was performed by supplying air into the test tube. The restraint rate was set so as to give a compressive strain so as to be a constant ratio with respect to the free expansion amount. That is, for example, in the case of 50% restraint, compressive strain was mechanically applied so that the amount of expansion was half of the free expansion. The chemical components of the test materials are shown in Table 1. Steel A is a compatible steel of the present invention, and steel B is a comparative steel. Here, the steel B is a heat-resistant stainless steel plate that is used for general purposes. From FIG. 2, the steel of the present invention has a longer life than the comparative example at any constraint. This is because high ductility is maintained in the low temperature region of the thermal cycle, in addition to the fact that there is almost no decrease in strength even when aging deterioration of high temperature strength, that is, long-term thermal cycling is applied. While undergoing a thermal cycle, a compressive load is applied to the material at a high temperature, and creep deformation or stress relaxation phenomenon occurs at 800 ° C. Therefore, an increase in 0.2% proof stress at 800 ° C. It is considered effective for extension. Meanwhile, 800
In the cooling process from ℃ to 200 ℃, the material is given tensile stress. This tensile stress is much larger than the compressive stress in the high temperature region, and when damage (defect) occurs in the thermal cycle, the plastic deformation at this portion is remarkable. Therefore, it is considered that an increase in ductility (drawing) at 200 ° C. of the material has an effect of suppressing damage progress in the cooling process. 3 and 4 show the tensile strength and drawing at high temperature after aging treatment at 800 ° C. The steel of the present invention has a high strength at a high temperature strength of 20 MPa or higher and a drawing value at 200 ° C. of 35% or higher even when subjected to an aging treatment at 800 ° C. for 10 hours or longer. This means that the strength at the highest temperature is high and the ductility at the lowest temperature is high even when subjected to a long thermal cycle treatment in the thermal fatigue process. As a result, it is considered that the thermal fatigue life is improved at any of the restraints as shown in FIG. In the conventional invention, the technical idea was only to improve the strength at the maximum temperature when subjected to a thermal cycle, but in the present invention, the thermal fatigue life is remarkably improved by improving the ductility at the minimum temperature. I found it.

The improvement of the aperture value at 200 ° C. is considered to be due to the fact that the elongation at break and the aging deterioration described above were suppressed. That is, the present invention has revealed that the balance of addition of TI and Nb is important. On the other hand, for improvement of high temperature strength at 800 ° C., solid solution N
The amount of b and the amount of solid solution Ti have an influence. FIG. 5 shows the amount of solid solution Nb + solution Ti after aging at 800 ° C.
The relationship between the amount and the high temperature strength at 800 ° C. is shown. When the solid solution Nb amount + solid solution Ti amount is 0.08% or more, the high-temperature strength at 800 ° C. is 20 MPa or more. Higher temperature strength than 20MPa
In order to obtain the above and improve the thermal fatigue life, the amount of solid solution Nb + the amount of solid solution Ti was set to 0.08% or more.

In the present invention, by adding an appropriate amount of Ti and Nb in combination, the high temperature strength after long-term aging and high temperature ductility are improved to improve thermal fatigue properties. There is a deterioration effect. When the steel containing Si, Cr, Mn, and Ti shown in the present invention is continuously oxidized in the atmosphere, the scale has a spinel oxide mainly containing TiO ₂ , Cr, and Mn in the outer layer, and Cr _{2 in} the inner layer. O ₃ is formed. As the amount of Ti increases, the Cr ₂ O ₃ film of the inner layer becomes thicker and the oxidation resistance deteriorates. The present inventors examined the influence of Mn, and when Mn was increased, the amount of TiO _{2 in} the outer layer was reduced, and the growth of the Cr ₂ O ₃ film in the inner layer was suppressed, whereby the oxidation resistance was increased. Found to improve. FIG. 6 shows the Cr ₂ O ₃ film thickness of the inner layer after continuous oxidation with Ti / Mn at 900 ° C. for 200 hours. When the thickness of the Cr ₂ O ₃ inner layer scale exceeds 5 μm, scale peeling occurs and the oxidation resistance is inferior. However, when Mn / Ti ≧ 3, the thickness of the Cr ₂ O ₃ inner layer scale is thin and the acid resistance Excellent in chemical properties. Ti is diffused outward through the inner layer of Cr ₂ O _3, results outward diffusion of Ti is inhibited by Mn, believed to growth of the inner layer of Cr ₂ O ₃ film is suppressed. In order to obtain good oxidation resistance, it is important to suppress the growth of the Cr ₂ O ₃ film of the inner layer, and the Cr ₂ O ₃ inner layer scale formed when continuously oxidized in the atmosphere at 900 ° C. for ₂₀₀ hours. In order to make the thickness 5 μm or less, Mn / Ti ≧ 3.

Steel having the component composition shown in Table 2 was melted and cast into a slab, and the slab was hot-rolled to form a hot rolled coil having a thickness of 5 mm. Then, the hot-rolled coil is annealed and pickled, cold-rolled to a thickness of 2 mm,
Annealed and pickled to give product plates. The annealing temperature of the cold-rolled sheet was set to 980 to 1050 ° C. in order to make the grain size number about 6 to 8. From the product plate thus obtained, a high-temperature tensile test piece was collected and subjected to a tensile test at 200 ° C and 800 ° C. Further, after aging treatment at 800 ° C. for 100 hours, a high-temperature tensile test was conducted in the same manner as described above. Further, the product plate was pipe-formed (outer diameter 38.1 mm) by electro-welding and subjected to a thermal fatigue test. The applied temperature cycle is 20
After raising the temperature from 0 to 800 ° C. in 120 seconds and holding at 800 ° C. for 30 seconds, 300 °
The pattern was cooled to 120 ° C. in 120 seconds and further cooled to 200 ° C. in 90 seconds, and the constraint rate was 50%. For evaluation of oxidation resistance, a test piece with a width of 20 mm and a length of 25 mm was cut from the product plate, polished to # 600 with emery paper, and then subjected to a continuous oxidation test in the atmosphere at 900 ° C. for 200 hours. It was. The thickness of the Cr ₂ O ₃ inner layer scale was determined by observing a cross section with an SEM (scanning electron microscope).

As is apparent from Table 2, when a steel having the component composition defined in the present invention is produced by the usual method as described above, it has a higher room temperature elongation than the comparative example and is excellent in workability. Recognize. Further, the high temperature strength satisfies the above range and is excellent in thermal fatigue characteristics. In the comparative example, Steel No. Since 12 and 13 have high C and N, the elongation at break at normal temperature is low, and the drawing value at high temperature is also low. Moreover, high temperature strength is low by carbonitride formation. Steel No. No. 14 is low in Si and thus has a low high temperature strength after aging. Steel Nos. 15, 17, 18, 19, and 20 have high Mn, S, Ni, Cu, and Cr, respectively, so that the room temperature workability is poor and the aperture value after aging is low. Steel No. In No. 16, S is outside the upper limit, the amount of solid solution Ti + Nb after aging is low, and the high temperature strength after aging is low. Steel No. In 21, 22, 23, 24, 25, and 26, Mo, Nb, Ti, V, Al, and B are out of the upper limit. Although these contribute to high temperature strength, they have poor room temperature workability and have a short thermal fatigue life due to low drawing at 200 ° C.

  In terms of oxidation resistance, the inner layer scale thickness of the steel of the present invention is as good as 5 μm or less. In the comparative example, Si deviates from the scope of the present invention, and steel No. 1 with a small Mn / Ti. 14, 17, 23, 24, and 26 have an inner layer scale thickness of more than 5 μm, and are inferior in oxidation resistance.

In addition, although it does not prescribe | regulate especially about the manufacturing method of a steel plate, what is necessary is just to select hot-rolling conditions, hot-rolled sheet thickness, a hot-rolled sheet and cold-rolled sheet annealing temperature, atmosphere, etc. suitably. Further, temper rolling or tension leveler may be applied after cold rolling and annealing. Further, the product plate thickness may be selected according to the required member thickness maintenance.

It is a figure which shows the relationship between (Nb + 1.9Ti) / (C + N) and breaking elongation at normal temperature. It is a figure which shows the influence of Ti addition on a thermal fatigue life. Yield strength at 800 ° C. after aging at 800 ° C. It is a figure which shows the aperture value in 200 degreeC after aging at 800 degreeC. It is a figure which shows the aperture_diaphragm | restriction in 200 degreeC after aging at 800 degreeC. It is a diagram showing the _Cr 2 _{O 3} thickness relationship at the time of 200h continuous oxidation test in Mn / Ti and 900 ° C..

Claims (4)

In mass%
C: 0.01% or less,
N: 0.015% or less,
Si: 0.8 to 1.0%,
Mn: 0.2 to 1.5%
P: 0.03% or less,
S: 0.01% or less,
Ni: 0.2% or less,
Cu: 0.2% or less,
Cr: 13-15%,
Mo: 0.1% or less,
Nb: 0.3 to 0.55%,
Ti: 0.05 to 0.2%,
V: 0.01-0.2%
Al: 0.015-1.0%,
B: contains 0.0002 to 0.0010%,
And (Nb + 1.9 × Ti) / (C + N) ≦ 50, a Cr-containing steel excellent in thermal fatigue characteristics, characterized in that the balance consists of Fe and inevitable impurities. 0.2% proof stress at 800 ° C after aging treatment at 800 ° C for 100 hours or more is 20MP
The Cr-containing steel excellent in thermal fatigue characteristics according to claim 1, wherein the drawing value at a is 200% or more and 35% or more. The amount of solute Nb + the amount of solute Ti after aging treatment at 800 ° C. for 100 hours or more is 0.08.
The Cr-containing steel excellent in thermal fatigue characteristics according to claim 1 or 2, wherein the Cr-containing steel is excellent in thermal fatigue characteristics.   4. The Cr-containing steel excellent in thermal fatigue properties according to claim 1, wherein Mn / Ti ≧ 3 is satisfied. 5.

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