JP2013155390A - High strength hot rolled steel sheet having excellent fatigue property and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength hot rolled steel sheet having excellent fatigue properties, in which TS satisfies ≥490 MPa.SOLUTION: A steel sheet has a composition containing, by mass, 0.08 to 0.18% C, <0.5% Si, 0.8 to 1.8% Mn, ≤0.05% P, ≤0.005% S, ≤0.008% N, 0.01 to 0.1% Al and 0.01 to 0.1% Ti, and the balance Fe with inevitable impurities. The structure of the surface layer part to 100 μm from the surface of the steel sheet is composed of a ferrite phase as the main phase and a second phase in a fraction of ≤30%, and the average grain size of the ferrite phase is controlled to ≤10 μm. Further, at the surface layer part, ≥30% of Ti in the surface layer part is precipitated as Ti carbides, and the average grain size of the Ti carbides is controlled to ≤30 nm.

Description

  The main object of the present invention is a high-strength hot-rolled steel sheet used for automobile members, for example, structural members such as body members and frames, suspension members such as suspensions, and thick members such as truck frames, and the like. It relates to the manufacturing method.

  In recent years, high-strength steel sheets have been actively used to reduce the weight of automobile bodies. In other words, not only the skeleton members of automobile bodies but also suspension members and the like that generally use hot-rolled steel sheets are desired to be switched to high-tensile materials that exhibit excellent performance in order to reduce weight. .

When high tension is applied to an underbody member, a track frame, etc., in addition to formability, chemical conversion and corrosion resistance, it is essential to have high fatigue characteristics.
In general, it is known that the fatigue limit of a smooth material increases with the tensile strength of the base material, but fatigue properties such as punched holes, slit ends, and blank ends increase the tensile strength of the base material. However, it is known that it does not necessarily increase.

Conventionally, to increase the strength of hot rolled high tension,
a) Solid solution strengthening method in which Si or the like is dissolved in the ferrite phase
b) Precipitation strengthening method to form carbonitrides in the ferrite phase
c) Structure strengthening methods that generate martensite phase or bainite phase, and composite strengthening methods that combine these methods have been used.

Furthermore, various high-strength hot-rolled steel sheets have been developed according to required characteristics. For example, as a steel sheet that requires stretchability, a structure-strengthened composite steel sheet (DP steel sheet) consisting of a ferrite phase and a martensite phase is used, and as a steel sheet that requires stretch flangeability, the structure strengthens by a bainite phase. Steel plates that have been developed have been developed.
However, it has been extremely difficult to satisfy all of formability, chemical conversion / corrosion resistance, and fatigue characteristics with the steel sheet.

  On the other hand, in Patent Document 1, by weight, C: 0.18% or less, Si: 0.5 to 2.5%, Mn: 0.5 to 2.5%, P: 0.05% or less, S: 0.02% or less, and Al: 0.01 Containing 0.1 to 0.1%, Ti: 0.02 to 0.5% and Nb: 0.02 to 1.0%, either one or two, so that C, Ti and Nb satisfy a specific relationship, A low-yield ratio high-strength hot-rolled steel sheet having a structure composed of ferrite and martensite precipitated with Ti, Nb carbides, or ferrite, martensite and retained austenite with Ti and Nb carbides precipitated is described.

  According to the description of the above-mentioned Patent Document 1, since a high-density movable dislocation network is formed around the second phase composed of the martensite phase or the retained austenite phase, the steel sheet has a low yield ratio. Furthermore, the presence of the second phase prevents the propagation of fatigue cracks, so that the fatigue resistance is improved.

Japanese Patent No. 3219820

That is, the technique described in the above-mentioned Patent Document 1 is considered to aim at improving fatigue resistance properties by precipitation strengthening the ferrite phase and reducing the strength difference from the martensite phase. The martensite phase has different plastic deformability and deformation behavior, and the heterogeneous interface between the ferrite phase and the martensite phase is likely to be the starting point of fatigue cracks. Realization of characteristics is difficult.
In addition, the material described in the above-mentioned Patent Document 1 forms a composite structure by adding 0.5% or more of Si, but such addition of Si deteriorates surface properties and adversely affects fatigue properties. In addition to the effects, chemical conversion and corrosion resistance are deteriorated, so that chemical conversion and corrosion resistance cannot be satisfied to the extent required in recent years.

  The present invention is a high-strength hot-rolled steel sheet that secures formability, chemical conversion and corrosion resistance, which have been recently required as a member for automobiles, and has excellent fatigue properties, specifically, steel sheet strength. An object of the present invention is to provide a high-strength hot-rolled steel sheet having excellent fatigue characteristics (TS) of 490 MPa or more and a method for producing the same. The application is not particularly limited as long as the characteristics of the steel of the present invention are required.

As a result of intensive studies to achieve the above object, the inventors obtained various findings and completed the present invention.
That is, the gist configuration of the present invention is as follows.
(1) By mass%, C: 0.08 to 0.18%, Si: less than 0.5%, Mn: 0.8 to 1.8%, P: 0.05% or less, S: 0.005% or less, N: 0.008% or less, Al: 0.01 to 0.1 % And Ti: 0.01 to 0.1%, with the balance being Fe and inevitable impurities, the structure of the surface layer part from the steel sheet surface to 100 μm has a fraction of the main phase ferrite phase: 30% or less The ferrite phase has an average particle size of 10 μm or less, and more than 30% of the amount of Ti in the surface layer is precipitated as Ti carbide in the surface layer portion. A high-strength hot-rolled steel sheet with excellent fatigue characteristics characterized by a particle size of 30 nm or less.

(2) The fatigue characteristics according to (1), wherein the steel sheet further contains at least one selected from V: 0.005 to 0.1% and Nb: 0.005 to 0.1% by mass%. Excellent high-strength hot-rolled steel sheet.

(3) The steel sheet further contains at least one selected from Cu: 0.005-0.1%, Ni: 0.005-0.1%, Cr: 0.002-0.1% and Mo: 0.002-0.1% by mass%. A high-strength hot-rolled steel sheet having excellent fatigue properties as described in (1) or (2) above.

(4) The high-strength hot-rolled steel sheet having excellent fatigue characteristics according to any one of (1) to (3), wherein the steel sheet further contains B: 0.0002 to 0.005% by mass%. .

(5) The steel sheet further comprises at least one selected from Ca: 0.0005 to 0.005% and REM: 0.0005 to 0.03% by mass%, according to the above (1) to (4) A high-strength hot-rolled steel sheet excellent in fatigue characteristics as described in any one of the above.

(6) After heating the steel comprising the component composition according to any one of (1) to (5) above to 1150 to 1300 ° C., hot rolling is performed at an Ar ₃ transformation point to an (Ar ₃ transformation point). High fatigue resistance, characterized by being applied at a finishing temperature in the range of +60) ° C, with a rolling reduction of 25% or more at the final stand of finish rolling, and then winding at a winding temperature of 570-670 ° C Manufacturing method of high strength hot rolled steel sheet.

(7) After finishing rolling according to the above (6), cooling of the steel sheet is started within 2 seconds, and the steel sheet is further brought to a temperature below (winding temperature + 50 ° C.) within 10 seconds after finishing rolling. A method for producing a high-strength hot-rolled steel sheet having excellent fatigue characteristics, characterized by cooling.

  According to the present invention, a high-strength hot-rolled steel sheet excellent in fatigue characteristics with a durability ratio of 0.45 or more can be produced stably. Further, by using the high-strength hot-rolled steel sheet of the present invention, it is possible to effectively reduce the weight of structural members such as vehicle body members and frames and suspension members such as suspensions in automobiles.

It is the figure which showed the plane bending fatigue test piece.

Hereinafter, the present invention will be specifically described. First, the component composition of the steel sheet according to the present invention will be described. In addition, unless otherwise indicated, "%" display regarding a component composition shall mean "mass%".
C: 0.08-0.18%
C is an element effective not only for ensuring the strength necessary for the steel sheet but also for making the crystal grains finer. In order to obtain a steel plate strength of 490 MPa or more (hereinafter referred to as TS) and desired crystal grains, the C content needs to be 0.08% or more. On the other hand, when the amount of C exceeds 0.18%, the total elongation (hereinafter referred to as EL) and the hole expansion rate (hereinafter referred to as λ) decrease. Therefore, the C content is 0.08 to 0.18%. Preferably it is 0.08 to 0.16%.

Si: Less than 0.5%
If Si is added in an amount of 0.5% or more, the surface properties will be significantly reduced, and the fatigue, chemical conversion and corrosion resistance will be adversely affected. The lower limit is not particularly limited, and may be 0%.

Mn: 0.8-1.8%
Mn is an element that is effective in securing strength by solid solution strengthening and crystal grain refinement. In order to obtain TS of 490 MPa or more and crystal grains having a desired grain size, the Mn content needs to be 0.8% or more. On the other hand, when the amount of Mn exceeds 1.8%, workability is remarkably lowered. Therefore, the Mn content is 0.8 to 1.8%. Preferably it is 0.8 to 1.5%.

P: 0.05% or less When the amount of P exceeds 0.05%, EL or λ is lowered due to segregation of P. Therefore, P is set to 0.05% or less. Preferably, it is 0.03% or less. The lower limit is not particularly limited, and may be 0%.

S: 0.005% or less S is not more than 0.005% because it forms sulfides and lowers EL and λ. Preferably it is 0.003% or less. The lower limit is not particularly limited, and may be 0%.

N: 0.008% or less If the N content exceeds 0.008%, a large amount of nitride is generated in the production process, which deteriorates hot ductility. Therefore, the N content is 0.008% or less. Preferably it is 0.005% or less. The lower limit is not particularly limited, and may be 0%.

Al: 0.01-0.1%
Al is an important element as a deoxidizer for steel. In order to obtain the effect, Al content needs to be 0.01% or more. On the other hand, if the Al content exceeds 0.1%, casting becomes difficult, or a large amount of inclusions remain in the steel, resulting in deterioration of the material and surface properties. Therefore, the Al content is 0.01 to 0.1%.

Ti: 0.01-0.1%
Ti is one of the most important elements in the present invention. This is because the addition of 0.01% or more improves the fatigue characteristics through refinement of crystal grains of the steel sheet and an increase in yield point due to precipitation of carbides. On the other hand, addition exceeding 0.1% adversely affects workability, weldability, toughness, etc., so 0.1% or less.

In the present invention, the amount of Ti added is determined according to the required strength level of the steel sheet, but in the surface layer portion, of the added Ti, 30% or more of Ti existing in the surface layer portion is the surface layer portion. It is important to precipitate as Ti carbide.
The presence of this Ti carbide can provide the level of fatigue characteristics desired by the present invention.
At this time, the crystal grain size of the Ti carbide is 30 nm or less. This is because by ensuring the precipitation of fine Ti carbide in the surface layer portion, the fine ferrite grains can be strengthened and the target fatigue durability ratio of the present invention can be achieved.

The present invention may contain at least one selected from V: 0.005 to 0.1% and Nb: 0.005 to 0.1% in the steel, in addition to the above-described component composition.
V and Nb are both elements that have a recrystallization suppressing effect and contribute to the refinement of crystal grains or to the increase of the yield point by the formation of carbides. In any case, the above effect can be obtained by adding 0.005% or more. On the other hand, even if added over 0.1%, an effect commensurate with the cost cannot be obtained, so 0.1% or less. V and Nb are utilized secondary because they have a higher alloy cost than Ti and have a lower carbide precipitation yield than Ti.

In addition to the above component composition, the present invention includes at least one of Cu: 0.005-0.1%, Ni: 0.005-0.1%, Cr: 0.002-0.1%, and Mo: 0.002-0.1% in the steel. Can be contained.
Both Cu and Ni contribute to improving the strength of the steel sheet by adding 0.005% or more. On the other hand, if added over 0.1%, surface layer cracking may occur during hot rolling, so when added, the content is made 0.1% or less.
Cr and Mo are both carbide forming elements, and if added in an amount of 0.002% or more, they contribute to improving the strength of the steel sheet. On the other hand, addition exceeding 0.1% does not provide an effect commensurate with cost.

Furthermore, B: 0.0002 to 0.005% can be contained.
B is an element advantageous for refinement of crystal grains, and for that purpose, addition of 0.0002% or more is necessary. On the other hand, if the amount of B exceeds 0.005%, the effect of addition is saturated and an effect commensurate with the cost cannot be obtained. Accordingly, the B content is preferably in the range of 0.0002 to 0.005%.

Moreover, this invention can contain at least 1 sort (s) in Ca: 0.0005-0.005% and REM: 0.0005-0.03% other than the above-mentioned component composition.
Ca and REM are effective elements for controlling the shape of inclusions, and contribute to improving workability. In order to obtain such an additive effect, the Ca amount and the REM amount must be 0.0005% or more, respectively. On the other hand, when the Ca content exceeds 0.005% or the REM content exceeds 0.03%, inclusions in the steel increase and the material deteriorates. Therefore, it is preferable that the Ca content is 0.0005 to 0.005% and the REM content is 0.0005 to 0.03%.

Next, the microstructure of the steel sheet in the present invention will be described.
The hot-rolled steel sheet of the present invention has a structure of the surface layer part from the steel sheet surface to 100 μm as the second phase with a main phase ferrite phase and a fraction: 30% or less, and the average grain size of the ferrite phase is 10 μm or less, and 30% or more of the Ti amount in the precipitate is precipitated as Ti carbide, and has a microstructure in which the average particle size of the Ti carbide is 30 nm or less.
In the present invention, it is extremely important to control the structure of the surface layer portion (hereinafter simply referred to as the surface layer portion) from the steel sheet surface to 100 μm. This is because the target fatigue phenomenon is a fatigue crack from the surface, and after the fatigue crack is generated, it progresses and reaches the final fracture.

The inventors have conducted intensive research, and in that research, the phenomenon that progresses from the generation of fatigue cracks until it reaches about 100 μm, the fatigue phenomenon is almost controlled, and the material structure We depended on the fact that the fatigue crack length is up to about 100 μm, and that crack growth does not depend on the material structure when it is longer than that.
That is, it has been found that the structure control of the surface layer is the most important point for the purpose of obtaining a high-strength hot-rolled steel sheet having excellent fatigue characteristics.

The average grain size of the main phase ferrite phase is 10 μm or less The surface layer portion of the hot-rolled steel sheet of the present invention comprises a main phase ferrite phase and a second phase having a fraction of 30% or less.
Here, the average grain size of the main phase ferrite needs to be 10 μm or less. This is because if the particle size exceeds this range, the desired fatigue characteristics cannot be obtained.
On the other hand, if the second phase fraction exceeds 30%, the frequency at which fatigue cracks are generated at the interface between the main phase and the second phase increases, and desired fatigue characteristics cannot be obtained. Preferably the second phase fraction is 20% or less. Here, the second phase refers to a single phase or the entire plurality of phases such as pearlite, bainite, martensite, retained austenite, and coarse carbide. That is, in the present invention, the second phase is not particularly limited as long as it is a crystal phase other than ferrite, and all crystal phases other than ferrite, such as pearlite and bainite, are the second phase.

Here, the fraction (area ratio) of the ferrite phase or other phase (second phase) can be obtained as follows.
A test piece is taken from the test steel, and after the plate thickness cross section parallel to the rolling direction is polished, it is subjected to nital corrosion to obtain a test piece for a scanning electron microscope (SEM). Subsequently, SEM photographs at magnifications of 1000 and 3000 are taken for 10 fields with a size of 1 field of view of 30 × 30 μm over an area within 100 μm from the surface layer of the specimen. Then, the ferrite phase and other phases are extracted by image processing of the SEM photograph, and the area of the ferrite phase and other phases and the area of the observation field are measured by image analysis processing, and (area of each phase) / ( The area ratio can be calculated by the formula of (observation visual field area) × 100 (%).
Here, the ferrite phase is a portion observed in gray in a 1000 times SEM photograph, and the portion observed in white excluding the grain boundary is the second phase.

  Further, the average particle diameter of the main phase ferrite is calculated by image analysis of the above 1000 times and 3000 times SEM photographs to obtain the equivalent circle diameter (JIS G 0551: 2005). In the present invention, ferrite grains of 0.5 μm or less are not included in the above average value calculation.

Of the added Ti amount, 30% or more of the Ti amount present in the surface layer part is precipitated as a carbide having an average particle size of 30 nm or less, thereby strengthening the ferrite phase of the surface layer part and increasing the yield strength of the material. Desired fatigue characteristics. If the average particle size exceeds the above value or the amount of precipitation is insufficient, a sufficient effect cannot be obtained.
In addition, the average particle diameter of the precipitate of Ti is cut out a sample of 100 μm thickness from the steel sheet surface, a thin film for a transmission electron microscope (TEM) is produced by electrolytic polishing, and using a transmission electron microscope (TEM), Select about 10 fields of view at a magnification of 150,000 times, and observe 1 field of 1 × 1 μm. Then, the average particle diameter of the Ti precipitate is calculated by calculating the equivalent circle diameter (JIS G 0551: 2005) of the observed Ti precipitate. In the present invention, the precipitate evaluated as an equivalent circle diameter of 1 nm or less is not included in the calculation of the average value.

  Here, the desired fatigue characteristics in the present invention were subjected to a Schenck type plane bending fatigue test. The test conditions refer to characteristics when the fatigue strength at 2 million cycles is evaluated at a stress ratio R = -1 and a frequency: 25 Hz.

Furthermore, manufacturing conditions for obtaining a hot-rolled steel sheet according to the present invention will be described below.
Slab heating at 1150 to 1300 ° C. In the present invention, it is important to heat the slab to 1150 ° C. or higher to dissolve the carbide in the slab stage. This is because in the case of heating below 1150 ° C., the carbide in the slab does not dissolve, and the desired carbide cannot be obtained in the hot rolling process. On the other hand, when the heating temperature exceeds 1300 ° C., the structure of the surface layer is coarsened and the target structure cannot be obtained, the Si scale is generated and the surface properties are impaired, and the fatigue characteristics are also adversely affected.

Finishing temperature of Ar ₃ transformation point to (Ar ₃ transformation point +60) ° C. and rolling reduction at the final stand of finish rolling: 25% or more In the finish rolling in the present invention, Ar ₃ transformation point to (Ar ₃ transformation point +60 ) Must be carried out in the range of ° C. When finish rolling is finished at less than the Ar ₃ transformation point, coarse grains are formed in the surface layer, resulting in fatigue and workability being significantly reduced.On the other hand, when finishing at a temperature exceeding (Ar ₃ transformation point +60) ° C, it is desirable This is because the target fatigue characteristics cannot be realized without obtaining the crystal grains.
Here, the Ar ₃ transformation point is a transformation temperature obtained from a thermal expansion curve obtained by a processing formaser experiment at a cooling rate of 10 ° C./s and obtained from the change point.

  Further, in the present invention, the rolling reduction at the finish rolling final stand is extremely important. This is because when the rolling reduction is less than 25%, a desired surface layer structure cannot be obtained and fatigue characteristics are not improved. Preferably, the rolling reduction is 30% or more.

Winding temperature: 570-670 ° C
When the coiling temperature exceeds 670 ° C, coarse pearlite is generated, which reduces fatigue and workability. On the other hand, when the coiling temperature is less than 570 ° C., a desired amount of Ti carbide cannot be secured in the surface layer portion, and the target fatigue characteristics cannot be obtained.

Furthermore, if the following manufacturing conditions are added, further excellent fatigue characteristics can be obtained.
Cooling starts within 2 seconds after finishing rolling, then cools to a temperature below (winding temperature + 50 ° C) within 10 seconds. When cooling starts within 2 seconds after finishing rolling, the structure becomes more effective. It can be miniaturized and further improvement in fatigue characteristics can be expected. In addition, by cooling to (winding temperature + 50 ° C.) within 10 seconds after finishing rolling, it is easier to obtain the target Ti carbide, and improvement in fatigue characteristics can be expected.

Other manufacturing conditions can be performed according to normal conditions.
For example, steel having a desired composition as described above is melted in a converter, electric furnace, induction furnace or the like. Next, secondary refining is performed in a vacuum degassing furnace. The casting performed thereafter is preferably performed by a continuous casting method from the viewpoint of productivity and quality, but a method by split rolling is also possible.
The cast slab may be a normal slab having a thickness of about 200 to 300 mm or a thin slab having a thickness of about 30 mm. If a thin slab is used, rough rolling can be omitted. The slab after casting may be directly hot-rolled as it is or may be hot-rolled after being heated in a heating furnace.

  Moreover, the high-strength hot-rolled steel sheet of the present invention can be a plated steel sheet such as an electrogalvanized steel sheet, a hot-dip galvanized steel sheet, or an alloyed hot-dip galvanized steel sheet.

Hot-rolled steel sheets Nos. 1 to 13 were produced using the steel slab symbols A to I having the compositions shown in Table 1 and Ar ₃ transformation points under the hot rolling conditions shown in Table 2. The Ar ₃ transformation point in Table 1 was determined by the above-described machining formaster experiment.

Then, the average ferrite particle diameter and the second phase fraction within 100 μm from the surface were determined by using the image processing method of the SEM photograph described above. The amount of Ti carbide deposited within 100 μm from the surface was determined by the electrolytic extraction residue method in which a sample having a thickness of 100 μm was cut from the surface and the residue was collected using a 30 μm filter. Further, the amount of Ti within 100 μm from the surface was measured by ordinary chemical analysis (for example, JIS G 1253 spark discharge emission spectroscopic analysis method) using a sample having a thickness of 100 μm from the surface. Furthermore, mechanical properties were obtained by a method described below by collecting JIS No. 5 tensile test pieces from a direction perpendicular to the rolling direction.
Three tensile test pieces are subjected to a tensile test at a strain rate of 10 mm / min, and the yield strength (YS) and tensile strength (TS) are calculated from the average value of the three in accordance with JIS Z 2241. Asked.

In the fatigue test, the test piece shown in FIG. 1 was taken in a direction perpendicular to the rolling direction of the steel sheet, and a Schenck type plane bending fatigue test was performed. The test conditions were stress ratio R = -1, frequency: 25 Hz, fatigue strength at 2 million cycles was evaluated, and durability ratio (2 million cycle fatigue strength / tensile strength) was calculated. Here, the durability ratio ≧ 0.45 was set as a target of the present invention.
The results are shown in Table 3.

  It can be seen that in the inventive examples (Nos. 2, 4, 5 and 8 to 10) which satisfy all the conditions of the present invention, the durability ratio is 0.45 or more and excellent fatigue properties are exhibited.

Claims (7)

  In mass%, C: 0.08 to 0.18%, Si: less than 0.5%, Mn: 0.8 to 1.8%, P: 0.05% or less, S: 0.005% or less, N: 0.008% or less, Al: 0.01 to 0.1% and Ti : A steel plate containing 0.01 to 0.1%, the balance being Fe and inevitable impurities, and the structure of the surface layer portion from the steel plate surface to 100 μm is the main phase ferrite phase and the second phase with a fraction: 30% or less The ferrite phase has an average particle size of 10 μm or less, and more than 30% of the Ti content in the surface layer portion is precipitated as Ti carbide on the surface layer portion, and the average particle size of the Ti carbide is A high-strength hot-rolled steel sheet with excellent fatigue characteristics characterized by being 30 nm or less.   The high strength with excellent fatigue characteristics according to claim 1, wherein the steel sheet further contains at least one selected from V: 0.005 to 0.1% and Nb: 0.005 to 0.1% by mass%. Hot rolled steel sheet.   The steel sheet further contains at least one selected from Cu: 0.005-0.1%, Ni: 0.005-0.1%, Cr: 0.002-0.1% and Mo: 0.002-0.1% by mass%. A high-strength hot-rolled steel sheet having excellent fatigue properties according to claim 1 or 2.   The high-strength hot-rolled steel sheet having excellent fatigue characteristics according to any one of claims 1 to 3, wherein the steel sheet further contains B: 0.0002 to 0.005% by mass%.   5. The steel sheet according to claim 1, wherein the steel sheet further contains at least one selected from Ca: 0.0005 to 0.005% and REM: 0.0005 to 0.03% by mass%. High-strength hot-rolled steel sheet with excellent fatigue characteristics. The steel comprising the component composition according to any one of claims 1 to 5 is heated to 1150 to 1300 ° C, and then hot-rolled in a range of Ar ₃ transformation point to (Ar ₃ transformation point + 60) ° C. Of high-strength hot-rolled steel sheets with excellent fatigue characteristics, characterized in that the rolling reduction at the final stand of finish rolling is 25% or more, and then winding is performed at a winding temperature of 570 to 670 ° C. Method. Cooling of the steel sheet is started within 2 seconds after the finish rolling according to claim 6 is finished, and the steel sheet is further cooled to a temperature of (winding temperature + 50 ° C.) or less within 10 seconds after finishing the finish rolling. A method for producing a high-strength hot-rolled steel sheet having excellent fatigue characteristics.

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