JP4265583B2 - Cold-rolled steel sheet having excellent toughness after quenching and method for producing the same - Google Patents

Description

  The present invention relates to a thin steel plate used for a structural part of an automobile and a manufacturing method thereof.

  Currently, high-strength steel sheets of 980 MPa or more are used as automotive structural parts such as door impact beams and center pillars from the viewpoint of light weight and high durability. However, since these parts have severe formability, when the above-described high-strength steel sheet is used, there are many problems of cracks and defective shapes, and the material cost is high.

  In recent years, with such a problem as a background, forming is performed using a thin steel plate having a low strength of 440 MPa, and high strength is achieved by induction hardening or the like. As such an example, in Non-Patent Document 1, the center pillar reinforcement, the front cross member, and the like are strengthened by induction hardening using steel plates of 440 MPa and 390 MPa, respectively. A new method has been developed in which the induction coil is supported by the robot when induction hardening is performed on a part with a three-dimensional surface, and the hardening is performed while precisely moving the coil along the part shape. Yes.

  As a technique for increasing the strength by post-heat treatment, Patent Document 1 discloses a method of partial strengthening by laser irradiation.

Patent Document 2 discloses a technique for strengthening by high energy density beam irradiation.
Materia, Vol. 37, No. 6 (1998) JP-A-60-238424 JP-A-7-126807

  However, the technique described in Non-Patent Document 1 requires enormous capital investment in order to reduce the variation in quenching conditions.

  In the technique described in Patent Document 1, the laser irradiation portion is very small, and it takes a long time to increase the strength of the member. In addition, the capital investment is enormous, resulting in an increase in cost.

  Since the technique described in Patent Document 2 only performs local strengthening, the obtained strength level is only about 710 MPa. Also, no mention is made of toughness.

  Thus, the present condition is that the steel plate which is excellent in quenching stability and excellent in toughness after quenching has not yet been proposed.

  Therefore, an object of the present invention is to provide a thin steel sheet that is less affected by quenching conditions and has excellent toughness after quenching, and a method for producing the same.

  In order to achieve the above object, the present inventors have conducted intensive research and found the following.

  1) The component composition greatly affects the hardenability when the heating temperature is 1000 ° C. or less, particularly 950 ° C. or less, and the addition of C and B is essential.

  2) The toughness after quenching is greatly affected by the grain size and microstructure of the precipitates. In B-containing steel, the BN morphology greatly changes the austenite grain size during heating, and BN precipitates finely. In this case, BN dissolves during heating and the austenite grain size becomes extremely coarse, so that the Charpy impact absorption energy is lowered.

  3) When the austenite grain size is small in the microstructure, ferrite is partially formed during cooling, and cracks tend to extend at the interface between ferrite and austenite and the toughness decreases.

  4) In addition, when B / N (14B / 10.8N) is large and B / N (14B / 10.8N) is small, the effect of quenching conditions, such as the time until cooling after high-frequency heating, varies. In addition, ferrite is generated during cooling after high-frequency heating, and the toughness decreases as in the case where the austenite grain size is reduced.

The present invention has been made on the basis of such knowledge, in mass% as a steel component, C: 0.10-0.37%, Si: 1% or less, Mn: 2.5% or less, P: 0.1% or less, S: 0.03% or less, sol.Al: 0.01 to 0.1%, N: 0.0005 to 0.0050%, B: 0.0003 to 0.0050%, 14B / 10.8N: 0.50 or more satisfied, consisting of remaining Fe and inevitable impurities , with precipitates in steel A cold-rolled steel sheet having excellent toughness after quenching, characterized in that a certain BN has an average grain size of 0.14 μm or more and a prior austenite grain size after quenching of 2 to 25 μm.

  In this invention, it is also possible to provide a cold-rolled steel sheet having excellent toughness after quenching, characterized by containing at least 1% of Ni, Cr, and Mo in mass% as a steel component in total. .

  Furthermore, it can also be set as the cold-rolled steel plate which is excellent in the toughness after hardening characterized by mass% as a steel component and containing Nb: 0.1% or less.

The invention of the production method capable of obtaining the thin steel sheet of the above invention is a method of hot rolling the steel having the above steel components at a coiling temperature of 620 ° C. or more and 720 ° C. or less, pickling, and then cold-pressing rate 30 It is a method for producing a cold-rolled steel sheet having excellent toughness after quenching, characterized in that it is cold-rolled at% or higher and then annealed at 640 ° C. or higher and the Ac ₁ transformation point or lower.

Further, in this invention of the manufacturing method, hot rolling at a coiling temperature of 620 ° C. or more and 720 ° C. or less, pickling, spheroidizing annealing at 640 ° C. or more and Ac ₁ transformation point or less, and a cold pressure ratio of 30% or more cold rolled, then, may be a method for manufacturing a cold-rolled steel sheet excellent in toughness after quenching, characterized by annealing below ₁ transformation point 600 ° C. or higher Ac.

  ADVANTAGE OF THE INVENTION According to this invention, the thin steel plate which is excellent in the hardenability in low temperature for a short time, has high Charpy impact absorption energy, and is excellent in toughness after quenching with a small fluctuation | variation by quenching conditions can be obtained. Furthermore, since the thin steel sheet can be obtained stably and at low cost, it provides an industrially useful effect as a high-strength member, and is optimal, for example, as an automobile structural component.

  First, the reasons for limitation of the steel components of the steel sheet will be described.

C: 0.10-0.37%
C is an important element for obtaining strength after quenching, and at least 0.10% or more is necessary to obtain 980 MPa or more. However, if added over 0.37%, the toughness is remarkably lowered although the strength is obtained. Therefore, in the present invention, the addition range of C is set to 0.10 to 0.37%. In order to obtain excellent toughness, 0.30% or less is preferable.

Si: 1% or less
Si is an element that improves the hardenability and increases the strength by solid solution strengthening. However, if added in excess of 1%, the band structure that is a segregation zone in the hot-rolled sheet becomes significant, and the toughness deteriorates. Therefore, in the present invention, the Si addition range is 1% or less. Moreover, 0.5% or less is preferable for obtaining excellent toughness.

Mn: 2.5% or less
Mn is an element that improves hardenability and increases strength by solid solution strengthening. However, when the content exceeds 2.5%, the formation of a manganese band, which is a segregation band, becomes remarkable and the toughness deteriorates. Therefore, in the present invention, the Mn addition range is 2.5% or less. Further, 1.5% or less is preferable for obtaining excellent toughness.

P: 0.1% or less
P is an element that improves hardenability and increases strength by solid solution strengthening. However, P is also an element that segregates at grain boundaries and lowers toughness. Grain boundary segregation is suppressed by addition of B, but addition exceeding 0.1% of P still causes grain boundary embrittlement and deteriorates toughness. Therefore, in the present invention, the addition range of P is 0.1% or less. Moreover, 0.05% or less is preferable for obtaining excellent toughness.

S: 0.03% or less
S is an element that must be reduced in order to form sulfides and reduce toughness. If the content exceeds 0.03%, the toughness deteriorates remarkably, so it must be suppressed to 0.03% or less. Therefore, in the present invention, the S addition range is 0.03% or less. In order to obtain excellent toughness, 0.02% or less is preferable.

sol.Al:0.01-0.1%
sol.Al is an element used as a deoxidizer to improve the cleanliness of steel. Addition of less than 0.01% reduces cleanliness, increases inclusions, and decreases toughness. On the other hand, when the content exceeds 0.1%, the formation of AlN becomes remarkable, the austenite at the time of quenching becomes fine, and ferrite is formed at the time of cooling, so that the toughness is deteriorated. Therefore, in the present invention, the addition range of sol.Al is set to 0.01 to 0.1%. In order to obtain excellent toughness, 0.03% to 0.07% is preferable.

N: 0.0005-0.0050%
N is an important element that forms BN, suppresses the grain growth of austenite during heating, and improves toughness, and at least 0.0005% or more is necessary. On the other hand, when the content exceeds 0.0050%, formation of not only BN but also AlN becomes remarkable, austenite at the time of quenching becomes fine, ferrite is formed at the time of cooling, and toughness deteriorates. Therefore, in the present invention, the addition range of N is 0.0005 to 0.0050%.

B: 0.0003-0.0050%
B is an important element that enhances hardenability and improves toughness by forming BN and suppressing austenite coarsening. However, when the addition amount is less than 0.0003%, a sufficient effect cannot be obtained. On the other hand, addition over 0.0050% increases the hot rolling load, lowers operability, and lowers workability. Therefore, in the present invention, the addition range of B is 0.0003 to 0.0050%. In addition, 0.0010 to 0.0030% is preferable to obtain an extremely excellent effect.

14B / 10.8N: 0.50 or more
14B / 10.8N is a ratio that greatly influences the fluctuation of quenching conditions (stability stability). Therefore, the effect of 14B / 10.8N on the toughness after quenching was investigated.

  As base components, C = 0.16%, Si = 0.01%, Mn = 0.75%, P = 0.015%, S = 0.012%, sol.Al = 0.040%, N = 0.0020-0.0028%, B = 0.0003-0.0028% , 14B / 10.8N = 0.19 to 1.30, and then heat the steel at a heating temperature of 1200 ° C., a hot rolling finishing temperature of 880 ° C., an intermediate temperature of 710 ° C., and a winding temperature of 640 ° C. After rolling and pickling, a cold rolled sheet of 1.2 mmt was manufactured at a cold pressure rate of 50% and an annealing temperature of 700 ° C. × 2 min. Subsequently, the toughness after induction hardening was evaluated about the obtained sample.

  In the induction hardening, heating and hardening were performed while moving the high-frequency coil on a flat plate (width 35 mm x length 300 mm). FIG. 1 shows an embodiment of induction hardening. The heating temperature at this time was a low temperature of 900 ° C., and the heating time was 4 seconds for the energization time up to 900 ° C.

  In addition, the cooling start time was 0.5 seconds for the immediate cooling that is normally performed, and three patterns of 1.5 seconds and 3 seconds were performed to evaluate the quenching stability.

  As evaluation after induction hardening, the Charpy impact test was implemented. The Charpy impact test was performed with a test piece shape as shown in FIG. 2 at a test temperature of −50 ° C. and n = 3.

  The obtained results are shown in FIG. FIG. 3 is a diagram showing the influence of the cooling start time and 14B / 10.8N on the Charpy impact absorption energy. From FIG. 3, it is understood that high Charpy impact absorption energy can be obtained stably even when 14B / 10.8N is 0.50 or more and the cooling start time is 3 seconds.

  In addition, when 14B / 10.8N is less than 0.50, a sufficient amount of solute B during quenching heating is not ensured, and when the cooling start time after heating is delayed, ferrite is generated and toughness is reduced.

  Therefore, 14B / 10.8N is set to 0.50 or more in order to reduce the variation in production and stably obtain high toughness.

Ni, Cr, Mo: 1% or more when added, totaling 1% or less
Ni, Cr and Mo are elements for improving hardenability, and one or more of them may be added. However, excessive addition causes an increase in cost, so at least one of Ni, Cr, and Mo is made 1% or less in total.

  In the present invention, 0.1% or less of Nb may be added in order to suppress coarsening of austenite grains during heating.

In the present invention, the balance consists of Fe and inevitable impurities.

  Next, the reasons for limiting the precipitate will be described.

BN average particle size: 0.14 μm or more
BN is a precipitate that suppresses coarsening of austenite grains during quenching heating. If the BN average particle size is less than 0.14 μm, it dissolves when heated at 900 ° C. or higher, and austenite grain growth cannot be suppressed. Therefore, in the present invention, the BN average particle size is 0.14 μm or more. The upper limit is preferably 1 μm or less from the viewpoint of toughness.

  Next, the reason for limiting the microstructure will be described.

Old austenite grain size after quenching: 2-25μm
The prior austenite grain size after quenching, that is, the prior austenite grain size before transformation measured after quenching has a great influence on toughness. When the prior austenite grain size is less than 2 μm, some ferrite is formed during cooling after heating, and the toughness is reduced due to stress concentration at the ferrite-austenite interface. On the other hand, in the case of coarse grains exceeding 25 μm, grain boundary embrittlement becomes prominent and the toughness is lower than that of the conventional JSC980Y (Iron Standard). Therefore, in the present invention, the prior austenite grain size after quenching is 2 to 25 μm.

  Next, the reason for limiting the manufacturing method will be described.

Winding temperature: 620 ° C or higher and 720 ° C or lower The hot rolling temperature is 620 ° C or higher from the viewpoint of BN formation. Below 620 ° C, BN becomes finer and dissolves during quenching heating, austenite becomes coarse and toughness deteriorates. On the other hand, when the temperature exceeds 720 ° C., the lamella spacing of pearlite increases, hardenability decreases, and cementite remains undissolved during quenching and toughness decreases. Therefore, in this invention, the coiling temperature in hot rolling shall be 620 degreeC or more and 720 degrees C or less. In addition, the upper limit of the coiling temperature is preferably 700 ° C. or less because of a decrease in pickling property due to increased scale formation.

Spheroidizing annealing temperature after hot rolling: 640 ° C or higher and Ac ₁ transformation point or lower After pickling hot rolled steel sheet, cementite can be spheroidized and spheroidizing annealing can be performed to obtain excellent workability and hardenability . When the annealing temperature is less than 640 ° C., the cementite is insufficiently spheroidized and the effect cannot be obtained. On the other hand, when the annealing temperature exceeds the Ac ₁ transformation point, it partially becomes austenite and coarse pearlite is generated during cooling, resulting in a decrease in workability and a decrease in hardenability. In addition, cementite remains undissolved during quenching and the toughness decreases. Therefore, in the present invention, when spheroidizing annealing is performed after hot rolling, the annealing temperature is set to 640 ° C. or more and Ac ₁ transformation point or less.

Rolling ratio during cold rolling: 30% or more If the rolling reduction ratio (cold rolling ratio) of cold rolling is less than 30%, unrecrystallized parts remain after annealing, and cementite spheroidization becomes insufficient and soft The processability is deteriorated because it cannot be obtained. Therefore, the cold pressure ratio of cold rolling is set to 30% or more. The upper limit of the cold pressure ratio is not particularly defined, but is preferably 80% or less so as not to increase the load on the rolling mill.

Annealing temperature after cold rolling: 640 ° C. or more and Ac ₁ transformation point or less For annealing after cold rolling, when spheroidizing annealing after hot rolling is omitted, spheroidizing annealing is performed here. The annealing temperature of spheroidizing annealing after cold rolling is set to 640 ° C. or more and Ac ₁ transformation point or less, similarly to the spheroidizing annealing after hot rolling described above.

When spheroidizing annealing after hot rolling is performed, recrystallization annealing is performed here. If the annealing temperature of recrystallization annealing after cold rolling is less than 600 ° C., unrecrystallized portions remain and the workability deteriorates. On the other hand, when the annealing temperature exceeds the Ac ₁ transformation point, it partially becomes austenite and coarse pearlite is generated during cooling, resulting in a decrease in workability and a decrease in hardenability. In addition, cementite remains undissolved during quenching and the toughness decreases. Therefore, in the present invention, when performing recrystallization annealing after cold rolling, the annealing temperature is set to 600 ° C. or more and Ac ₁ transformation point or less.

  The target thin steel sheet may be either a hot-rolled steel sheet or a cold-rolled steel sheet, but in the present invention, it is a cold-rolled steel sheet. When manufacturing this invention steel plate, raw material steel is smelted by a converter, an electric furnace, etc., for example. The slab can be manufactured by any of the ingot-bundling rolling method, continuous casting method, thin slab casting method, and strip casting method.

  The hot rolling process may be either a method of rolling after slab heating, a method of performing a heat treatment for a short time after continuous casting, or a method of rolling immediately after omitting the heating step, but in order to give excellent surface quality It is preferable to sufficiently remove not only the primary scale but also the secondary scale generated during hot rolling. In addition, you may heat with a bar heater during hot rolling.

The finish rolling finish temperature is preferably not less than the Ar ₃ transformation point in view of the uniformity of the structure. For the purpose of homogenizing the structure, rapid cooling at 200 ° C./second or more may be performed within 1 second after finish rolling.

  The rolling rate (cold pressure rate) during cold rolling is preferably 80% or less. In the case of a high cold pressure ratio that exceeds 80%, the rolling load becomes too high and the productivity is lowered. The cold rolling at this time may be either tandem rolling or reverse rolling.

  The annealing temperature in the recrystallization annealing performed after the cold rolling needs to be at least 650 ° C. from the viewpoint of recrystallization, and if it exceeds 850 ° C., the pearlite becomes coarse and the workability deteriorates. Therefore, the annealing temperature is preferably 650 to 850 ° C. In addition, as a method of performing recrystallization annealing, any of continuous annealing, box annealing, or continuous heat treatment preceding hot dip galvanizing treatment may be used.

  The thin steel sheet according to the present invention may be used after appropriately being subjected to a surface treatment (chemical conversion treatment, hot dip galvanizing, galvannealed alloying).

[Reference Example 1]
Steel having chemical composition of steel Nos. 1 to 12 shown in Table 1 was melted, and then hot rolling or hot rolling-annealing was performed according to the manufacturing conditions shown in Table 2 to produce 2.4 mmt hot rolled sheets. . The hot-rolled sheets thus manufactured were examined for tensile tests (JIS No. 5, C direction (perpendicular to the rolling direction)), average particle size measurement of BN, and induction hardening characteristics.

  The average particle size of BN was measured by extracting BN by the replica method, photographing the precipitate with a transmission electron microscope, and using a microanalyzer for 500 samples.

  In the induction hardening, heating and hardening were performed while moving the high-frequency coil on a flat plate (width 35 mm x length 300 mm). FIG. 1 shows an embodiment of induction hardening. The heating temperature at this time was a low temperature of 900 ° C., and the heating time was 4 seconds for the energization time up to 900 ° C.

  The cooling start time was 0.5 seconds for the immediate cooling that is normally performed, and two patterns of 3 seconds for evaluating the quenching stability.

  As evaluation after induction hardening, a tensile test (JIS No. 5, C direction (perpendicular to the rolling direction)), Charpy impact test, and prior austenite particle size measurement were performed. The Charpy impact test was performed with a test piece shape as shown in FIG. 2 at a test temperature of −50 ° C. and n = 3. In addition, the hot-rolled sheet was ground to a thickness of 1.2 mm to have the same shape as a cold-rolled sheet described later. In addition, the Charpy impact test value passed 0.4 kgm or more of the JSC980Y level tested on the same conditions.

  For the prior austenite particle size, after polishing and corroding the plate thickness section of the sample, the microstructure was photographed with an optical microscope, and the average particle size was measured using a microanalyzer.

  The results obtained from the above are shown in Table 3.

  From Table 3, No. A, B, C, E, and G, whose components, 14B / 10.8N, BN average particle size, and prior austenite particle size are within this range, have a strength of 980 MPa or more as characteristics after quenching. It was revealed that Charpy impact absorption energy of JSC980Y or more (0.4 kgm or more) was stably obtained regardless of the cooling start time after quenching, and excellent toughness was obtained.

  In particular, No. A and B with low C, Si, Mn, P, S, sol.Al 0.03-0.07%, B 0.0010-0.0030%, Charpy impact absorption energy is 0.5kgm or more It was found that toughness was obtained.

  On the other hand, No.H having a low C outside the scope of the present invention has low strength, No.I having a high C outside the scope of the present invention, and No.J having a high Si and P outside the scope of the present invention, Mn, and S. No. K high outside the range of the present invention has low Charpy impact absorption energy and deteriorates toughness.

  No. L having a high sol.Al, N outside the scope of the present invention has a low prior austenite grain size outside the scope of the present invention, and when the cooling start time is slow, the Charpy impact absorption energy is low and the toughness is deteriorated.

  In No. M, in which B is outside the scope of the present invention and the BN average particle diameter is outside the scope of the present invention, the prior austenite grain size is coarsened outside the scope of the present invention, and the toughness is deteriorated. No. N having a small 14B / 10.8N outside the scope of the present invention has low Charpy impact absorption energy and deteriorates toughness when the cooling start time is slow.

  In No. D, which has a high coiling temperature outside the range of the present invention, cementite remains undissolved during quenching, has low Charpy impact absorption energy, and deteriorates toughness. In No. F, which has a high annealing temperature outside the range of the present invention, pearlite is partially generated, and Charpy impact absorption energy is low and toughness is deteriorated.

  Steel having chemical composition of steel Nos. 1 to 12 shown in Table 1 was melted, and then hot-rolled-cold-rolled-annealed according to the manufacturing conditions shown in Table 4 to produce 1.2 mmt cold-rolled sheets .

  With respect to the cold-rolled sheet thus produced, the tensile test, the BN average particle diameter measurement, and the induction hardening characteristics were investigated in the same manner as in Reference Example 1. The results are shown in Table 5.

  From Table 5, as in the case of hot-rolled steel sheets, No. a, c, d, e, and h whose components, 14B / 10.8N, BN average particle size, and prior austenite particle size are within this range are after quenching It has a strength of 980MPa or more as a characteristic of the above, and it is clear that Charpy impact absorption energy of JSC980Y or more (0.4kgm or more) is stably obtained regardless of the cooling start time after quenching, and excellent toughness is obtained. It became.

  In particular, C, Si, Mn, P, S is low, sol.Al is 0.03-0.07%, B is 0.0010-0.0030% No. a, c, d has Charpy impact absorption energy of 0.5 kgm or more, It was found that extremely excellent toughness was obtained.

  On the other hand, No. b having a small BN average particle size outside the range of the present invention has a small prior austenite particle size and a low cooling start time, resulting in low Charpy impact absorption energy and poor toughness.

  No.i having a low C outside the scope of the present invention is low in strength, No.j having a high C outside the scope of the present invention, No.k having a high Si and P outside the scope of the present invention, Mn, S No. 1 high outside the scope of the present invention has low Charpy impact absorption energy and deteriorates toughness.

  No.m having a high sol.Al, N outside the scope of the present invention has a low prior austenite grain size outside the scope of the present invention, and when the cooling start time is slow, the Charpy impact absorption energy is low and the toughness is deteriorated.

  No. n having a low B outside the scope of the present invention and a BN average grain size outside the scope of the present invention has a prior austenite grain size that is outside the scope of the present invention and has deteriorated toughness. No.o having a small 14B / 10.8N outside the scope of the present invention has low Charpy impact absorption energy and deteriorates toughness when the cooling start time is slow.

  No. f having a high coiling temperature outside the range of the present invention has cementite remaining undissolved during quenching, has low Charpy impact absorption energy, and deteriorates toughness. No.g having a high annealing temperature after cold rolling outside the scope of the present invention partially generates pearlite, has low Charpy impact absorption energy, and deteriorates toughness.

It is a figure which shows one embodiment of induction hardening. It is a figure which shows one embodiment of the test piece shape in a Charpy impact test. It is a figure which shows the influence of the cooling start time and 14B / 10.8N which give to the Charpy impact absorption energy.

Claims (5)

The steel component is mass%, C: 0.10 to 0.37%, Si: 1% or less, Mn: 2.5% or less, P: 0.1% or less, S: 0.03% or less, sol.Al: 0.01 to 0.1%, N: 0.0005 Containing ~ 0.0050%, B: 0.0003 ~ 0.0050%, satisfying 14B / 10.8N: 0.50 or more, balance Fe and inevitable impurities, and the average particle size of BN as precipitates in steel is 0.14 μm or more A cold-rolled steel sheet having excellent toughness after quenching, wherein the prior austenite grain size after quenching is 2 to 25 μm.   2. The cold-rolled steel sheet having excellent toughness after quenching according to claim 1, further comprising at least 1% of Ni, Cr, and Mo in mass% as a steel component.   3. The cold-rolled steel sheet having excellent toughness after quenching according to claim 1, wherein the steel component further contains mass% and Nb: 0.1% or less. The steel having the steel component according to any one of claims 1 to 3, is hot-rolled at a coiling temperature of 620 ° C or higher and 720 ° C or lower, pickled, and then cold-rolled at a cold pressure ratio of 30% or higher. Then, by annealing at 640 ° C. or more and below the Ac ₁ transformation point, the average particle size of BN as a precipitate in the steel is 0.14 μm or more, and the prior austenite particle size after quenching is 2 to 25 μm. A method for producing a cold-rolled steel sheet having excellent toughness after quenching, characterized by obtaining a cold-rolled steel sheet. The steel having the steel component according to any one of claims 1 to 3, after hot rolling at a coiling temperature of 620 ° C or more and 720 ° C or less, pickling, and then spherical at 640 ° C or more and Ac ₁ transformation point or less By annealing and cold rolling at a cold pressure ratio of 30% or higher, and then annealing at 600 ° C or higher and the Ac ₁ transformation point or lower, the average grain size of BN, which is a precipitate in steel, is 0.14 μm or higher. And the manufacturing method of the cold-rolled steel plate excellent in the toughness after quenching characterized by obtaining the cold-rolled steel plate whose prior austenite grain size after quenching is 2-25 micrometers.

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