JP2018154884A - Cold tool steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a cold tool steel having high hardness applicable to a cold tool used for molding a high-tensile material, capable of securing high impact value and maintaining machinability needed during manufacturing the cold tool, and having high strength and excellent in impact value and machinability.SOLUTION: A cold tool steel contains C:0.55 to 0.85%, Si:1.00 to 2.50%, Mn:0.30 to 1.50%, P:0.030% or less, S:0.100% or less, Cr:5.00 to 8.00%, Mo:1.00 to 2.00%, Ti:0.05 to 0.20%, B:0.0005 to 0.0050%, Al:0.005 to 0.150%, N:0.0250% or less and satisfies the formula 1:1.4×[Si]-2.7×[Mn]+1.7×[Mo]+1.5×[C]≥4. In a cross section structure after a hardening and tempering, coarse carbide with circle equivalent diameter of 15 μm or more is 50 or less in a field of view with 0.4 mm.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a cold tool steel used for a tool for cold working of a steel material.

  Development of high-tensile steel plates (high-tensile materials) that can ensure the same or higher strength with thinner plate thickness than before in order to realize collision safety and higher strength of the automobile skeleton while reducing the weight of automobiles. And its use is increasing. However, high-tensile materials can contribute to weight reduction, but also increase deformation resistance during molding such as deep drawing and bending, increasing the burden on the mold for molding. Characteristics such as wear resistance are deficient. For this reason, there has been a strong demand for improving the performance of cold tools (such as molds) for processing high-tensile materials.

JP 2009-132990 A

  In order to ensure the wear resistance required when forming high-tensile materials, it is necessary to increase the hardness. Generally, increasing the hardness improves the wear resistance, but machinability and impact value (toughness). ) Decreases, the problem cannot be solved simply by increasing the hardness. For example, the tool steel described in Patent Document 1 regulates the area ratio of coarse carbides and secures a hardness of about 60 HRC by adding various alloys, but the impact value is a comparison with SKD11. However, although there is some improvement, the degree of improvement is not sufficient.

  The present invention has been made in view of such a background, and has a high hardness applicable to a cold tool (mold, etc.) used for forming a high-tensile material, on which a high impact value can be secured. In addition, it is possible to maintain the machinability necessary when manufacturing a cold tool, and it is a high strength cold tool steel excellent in impact value and machinability, particularly in comparison with SKD11 in a high hardness region of 60 HRC or more. An object of the present invention is to provide a cold work tool steel capable of securing an impact value of at least twice.

One embodiment of the present invention is mass%, C: 0.55 to 0.85%, Si: 1.00 to 2.50%, Mn: 0.30 to 1.50%, P: 0.030% Hereinafter, S: 0.100% or less, Cr: 5.00 to 8.00%, Mo: 1.00 to 2.00%, Ti: 0.05 to 0.20%, B: 0.0005 to 0 .0050%, Al: 0.005 to 0.150%, N: 0.0250% or less, with the balance consisting of Fe and inevitable impurities,
Satisfying the following formula 1,
Formula 1: 1.4 × [Si] −2.7 × [Mn] + 1.7 × [Mo] + 1.5 × [C] ≧ 4 (where, [X] is the content of element X) (Mass%).),
And in the cross-sectional structure after quenching and tempering treatment, there are 50 or less coarse carbides in an equivalent circle diameter of 50 μm or less in the cold tool steel in the field of view of 0.4 mm ² .

  By making the said cold tool steel into the chemical component of the said specific range, the number of the said coarse carbide | carbonized_material observed in the cross-sectional structure | tissue after a quenching and tempering process can be suppressed to the said specific range. It is known that the presence of coarse carbide contributes to improvement in wear resistance when the hardness is relatively low (region of less than 60 HRC). However, it has been found that in the high hardness region of 60 HRC or higher, the effect of improving wear resistance by coarse carbides is hardly seen as will be described later. Focusing on this point, as mentioned above, by suppressing the number of coarse carbides to the above specific range, improving toughness and wear resistance while maintaining machinability without problems in the production of cold tools such as dies. The effect can be extracted in the high hardness region.

  For chemical components, it is essential to satisfy the above relational expression 1: 1.4 × [Si] −2.7 × [Mn] + 1.7 × [Mo] + 1.5 × [C] ≧ 4. Yes. Thereby, the hardness after the quenching and tempering treatment can be secured to 60 HRC or more. In addition, by synthesizing appropriate amounts of both Ti and B elements at the same time, a synergistic effect can be exhibited as compared with the case of the effect of addition alone. That is, not only the grain refinement effect by grain refinement by addition of Ti and the suppression of grain boundary segregation of P by addition of B is obtained, but the toughness is remarkably improved by the combined addition of both. Furthermore, as described above, since the formation of coarse carbides can be suppressed, not only the toughness can be improved but also excellent machinability can be realized.

Explanatory drawing which shows the relationship between the value of Formula 1 in Example 1, and hardness (HRC). Explanatory drawing which shows the effect of Ti and B compound addition which has on the relationship between hardness and an impact value in Example 1. FIG. Explanatory drawing which shows the influence of the presence or absence of a coarse carbide | carbonized_material on the relationship between hardness and specific wear amount in Example 1. FIG.

First, the reasons for limiting the chemical components of the cold tool steel will be described.
C: 0.55-0.85%
C (carbon) is an element necessary for ensuring hardness, and is contained by 0.55% or more. On the other hand, if C is added excessively, the toughness and machinability are lowered, so the upper limit of the C content is set to 0.85%.

Si: 1.00-2.50%,
Si (silicon) is an element effective for securing the hardness of the steel and increasing the temper softening resistance, and is contained in an amount of 1.00% or more in order to obtain the effect. On the other hand, if Si is added excessively, the toughness and hot workability are reduced, so the upper limit of the Si content is 2.50%.

Mn: 0.30 to 1.50%,
Mn (manganese) is an element effective for improving machinability by forming MnS. Therefore, Mn is contained by 0.30% or more. On the other hand, when Mn is added excessively, the temper softening resistance is lowered, so the upper limit of the Mn content is 1.50%.

P: 0.030% or less,
P (phosphorus) is an element inevitably contained, but if contained in a large amount, the toughness may be lowered. Therefore, the upper limit of the P content is set to 0.030%.

S: 0.100% or less,
S (sulfur) is an element that is inevitably contained, but is an element that can improve machinability with a small amount of positive addition. However, when it is contained in a large amount, there is a risk of a decrease in toughness and a reduction in surface roughness after wire cutting and machining. Therefore, the upper limit of the S content is set to 0.100%.

Cr: 5.00 to 8.00%,
Cr (chromium) is effective in securing high hardness after the tempering treatment, and is contained in an amount of 5.00% or more in order to obtain the effect. On the other hand, when Cr is added excessively, the toughness and machinability are lowered, and the adhesion of the film when the film is formed by PVD or CVD may be lowered. Therefore, the upper limit of the Cr content is 8.00%. And

Mo: 1.00 to 2.00%,
Mo (molybdenum) is effective in securing tempering softening resistance, and is contained in an amount of 1.00% or more in order to obtain the effect. On the other hand, if Mo is added excessively, the toughness and machinability may be lowered, so the upper limit of the Mo content is 2.00%.

Ti: 0.05-0.20%,
As described above, Ti (titanium) is an element that is effective for refining crystal grains and synergistically improves toughness in a high hardness region by compound addition with B. To obtain these effects , 0.05% or more. On the other hand, if Ti is added in excess, the toughness may be lowered, so the upper limit of the Ti content is 0.20%.

B: 0.0005 to 0.0050%,
B (boron) is an element that, as described above, has an effect of improving the grain boundary strength by suppressing grain boundary segregation of P, and synergistically improves toughness in a high hardness region by compound addition with Ti. In order to acquire an effect, it contains 0.0005% or more. On the other hand, even if B is added excessively, the above effect is saturated, so the upper limit of the B content is set to 0.0050%.

Al: 0.005 to 0.150%,
Al (aluminum) is an element effective for deoxidation and crystal grain refinement, and is contained in an amount of 0.005% or more in order to obtain the effect. On the other hand, if Al is added excessively, the toughness may decrease due to an increase in oxide inclusions, so the upper limit of the Al content is set to 0.150%.

N: 0.0250% or less,
N (nitrogen) is an element that is unavoidably contained. However, when it is contained in a large amount, the toughness is reduced, or TiN is increased to reduce TiC having a pinning effect. There is a risk of coarsening. Therefore, the upper limit of the N content is set to 0.0250%.

  Furthermore, in order to obtain the crystal grain refining effect by pinning using carbonitride, in addition to the said essential addition element, the said cold tool steel can further add arbitrary elements as follows. That is, one or more of V: 0.20% or less, Nb: 0.20% or less, W: 0.50% or less, Ta: 0.20% or less can be contained.

V: 0.20% or less, Nb: 0.20% or less,
As described above, V (vanadium) and Nb (niobium) are optional additional elements that can be expected to have an effect of improving toughness in a high hardness region due to the effect of crystal grain refinement. Therefore, it is preferable to add as needed. On the other hand, even if Nb is added in a large amount, the effect is saturated, and V may cause the toughness and machinability to deteriorate due to excessive addition. Therefore, the upper limit of both elements is 0.20%.

W: 0.50% or less, Ta: 0.20% or less,
W (tungsten) and Ta (tantalum) are also elements that form carbonitrides and contribute to grain refinement by the pinning effect. However, since the effect is saturated and the cost is increased even if it is contained too much, the upper limit of the W content is 0.50% and the upper limit of the Ta content is 0.20%.

  In order to use carbonitride (Ti is carbide) for crystal grain refinement, a coarse precipitate having no pinning effect must be once dissolved and finely precipitated again. In the case of the present invention, solid solution is performed by heating during hot rolling or by forging a steel ingot, and precipitation is performed by spheroidizing annealing for improving workability. As described above, Ti is an element having a large toughness improving effect due to the synergistic effect of crystal grain refinement and combined addition with B, but V, Nb, W, and Ta can also be used for crystal grain refinement.

  Moreover, in addition to the said essential addition element, the said cold tool steel can further add the following arbitrary elements. That is, one or more of Mg: 0.10% or less, Te: 0.10% or less, and Bi: 0.05% or less can be contained.

Mg: 0.10% or less, Te: 0.10% or less, Bi: 0.05% or less,
By adding these Mg (magnesium), Te (tellurium), and Bi (bismuth), the effect of further improving the machinability can be obtained. On the other hand, since the effect is saturated even if it is added excessively, the upper limit of the content is made 0.10% for Mg and Te and 0.05% for Bi.

  Next, Formula 1: 1.4 × [Si] −2.7 × [Mn] + 1.7 × [Mo] + 1.5 × [C] ≧ 4 (wherein [X] in the formula is the element X Is a necessary condition for securing a hardness of 60 HRC or more on the premise of the chemical component composition described above, and has been derived by many experiments. It is. Therefore, it is important to limit the chemical components so as to satisfy Formula 1 after adding each component so as to satisfy the basic chemical component composition range described above in order to achieve high hardness. It is.

The cold tool steel has 50 or less coarse carbides having an equivalent circle diameter of 15 μm or more in a 0.4 mm ² field of view in the cross-sectional structure after quenching and tempering. The cold tool steel of the present invention is basically designed to prevent coarse carbides from being generated, but even if it is generated, it is controlled within the range of 50 or less in the field of view of 0.4 mm ^2. Meaning. And by ensuring this requirement, excellent machinability and toughness can be ensured as described above. In addition, the cross-sectional structure | tissue which observes a coarse carbide | carbonized_material is performed in the state after performing the quenching tempering process which is heat processing essential as a cold tool. As the quenching condition and the tempering condition, an optimum condition within a range described later may be adopted.

  The cold tool steel of the present invention is mainly used as a mold for plate forming such as high tension steel. Then, after the base material is manufactured by hot rolling or steel ingot forging, spheroidizing annealing for improving workability (for example, heating to 850 ° C. for about 5 hours and then gradually cooling to about 550 ° C.) After being machined into a mold shape, quenching and tempering can be performed to produce a mold having high hardness and high impact value. And the quenching conditions in this case can employ | adopt the conditions of air-cooling, after heating and holding a to-be-processed material at 1000-1050 degreeC.

  Further, the tempering after quenching can be selected from low-temperature tempering and high-temperature tempering described below, and tempering in an intermediate temperature range becomes brittle and is not selected. In the case of low temperature tempering, for example, the material to be treated can be air-cooled after being held at 150 ° C. to 200 ° C. for 60 minutes. In the case of high temperature tempering, for example, the material to be treated is 450 ° C. to 550 ° C. It can be set as the conditions of air-cooling after hold | maintaining at 60 degreeC for 60 minutes. Therefore, as the tempering process, either the above-described low-temperature tempering or high-temperature tempering can be adopted, and this is whether or not a wire cut is used as processing, and whether or not a film forming process by CVD or PVD is performed. Etc. can be selected. For example, a tempering process at a high temperature is effective when performing a wire cut process and when performing a film formation process by CVD or PVD. On the other hand, when these processes are unnecessary, a tempering process at a low temperature is effective.

  According to the tempering treatment at high temperature, secondary hardening due to carbide precipitation can be expected, high hardness characteristics equivalent to those in the case of tempering treatment at low temperature can be maintained, and wire cutting treatment can be made possible by reducing residual austenite, There is a merit that film formation at a high temperature such as CVD or PVD becomes possible. On the other hand, the tempering treatment at a low temperature has an advantage that a slightly higher impact value characteristic can be obtained than the case of the tempering treatment at a high temperature. Therefore, tempering conditions can be set according to the characteristics required for the cold tool.

Example 1
Examples of the cold tool steel will be described together with a plurality of samples for comparison. First, a plurality of steels (sample Nos. 1 to 38) having chemical components shown in Table 1 were prepared as test materials. Among these, sample no. 1 to 20 are steels satisfying the conditions of the present invention. 21 to 36 are comparative steels that do not satisfy some conditions. 37 is a conventional steel that has been developed in the past and is already on the market. 38 is JIS standard SKD11. These steels are melted in an electric furnace to produce steel ingots, hot forged to the steel ingots, held at 850 ° C. for 4 hours, and then gradually cooled to 550 ° C. at a cooling rate of 20 ° C./hour. After improving the machinability by spheroidizing annealing, it was processed into a block-shaped test piece of 150 mm × 90 mm × 40 mm. The test piece was subjected to a quenching treatment that was heated to the quenching temperature shown in Table 2 and held for 30 minutes and then air-cooled, and then held at the tempering temperature shown in the same table for 60 minutes and then air-cooled. did. And the following various tests were implemented.

<Charpy impact test>
After the spheroidizing annealing described above, a 10R notch Charpy test piece was cut out from the test piece immediately before performing the quenching and tempering treatment. The cutting direction was aligned so that the longitudinal direction of the block-shaped test piece described above was the longitudinal direction of the Charpy test piece. And in order to avoid the influence of surface oxidation, the quenching and tempering process mentioned above was implemented in the vacuum, and it was set as the test piece of this test. The Charpy impact test was performed at room temperature.

The Charpy impact value with a 10R notch specimen at a hardness of 60 HRC of SKD11 is about 25 J / cm ² , and if the impact value is 60 J / cm ² or more at 60 HRC or more exceeding the double value, the impact value is very excellent. Actually, the impact value tends to decrease as the hardness increases. Further, in the present invention, the object is to ensure wear resistance that does not cause a problem even if a high-tensile material is molded. Therefore, the hardness is basically required to be 60 HRC or more. Therefore, as a target impact value for each hardness in a hardness range of 60 HRC or more, impact value evaluation value = Charpy impact value (J / cm ² ) − (− 10 × hardness (HRC) +660) (J / cm ² ) A reference was made that a positive value should be used, and this value is shown in Table 2 for the result of a test piece having a hardness of 60 HRC or more, and a positive case was indicated as ◯ and a negative case as x. Moreover, about the result of the test piece in which hardness became less than 60 HRC, that is described below about the thing whose numerical value is clearly inferior from FIG.

<Hardness measurement>
The flat part on the side having the notch surface in the test piece used in the Charpy test was polished using # 220 polishing paper, and then the HRC hardness of the surface was measured. The measurement results are shown in Table 2.

<Cutting test (drill test)>
Riken Steel Co., Ltd. drill, blade diameter: 5.0 mm, groove length: 42 mm, full length: 92 mm, shank diameter: 5.0 mm, thinning: X shape, tip angle: 135 °, helix angle 30 °, material: high cobalt A drill test was performed using high speed steel (with homo treatment). The test conditions were as follows: cutting speed: 20 m / min, rotation speed: 1273 rpm, feed: 0.05 mm / rev (feed speed: 64 mm / min), hole depth 25 mm (blind hole), cutting oil: yes Block-shaped test pieces (150 mm × 90 mm × 40 mm) after spheroidizing annealing were processed up to 200 holes. The spheroidized annealing material of conventional steel SKD11 (sample 38) was also evaluated at the same time, and the outer peripheral corner wear width of the drill used was compared. In consideration of the fact that the machinability can be determined to be possible to process into a mold without any problem if it is equal to or higher than that of SKD11, which is a conventional steel, from the past results, the outer peripheral corner wear width is SKD11 (sample 38). If it was equal or higher, it was judged that machinability was poor (x), and if it was small, machinability was good (◯).

<Confirmation of coarse carbide>
By obtaining the number of coarse carbides having a circle-equivalent diameter of 15 μm or more in a field of view of 0.4 mm ² using a cross-sectional structure image obtained by imaging the cross-sectional structure after performing quenching and tempering heat treatment using an optical microscope. Do. The determination that the equivalent circle diameter is 15 μm or more can be made by using image processing software. When there are 51 or more coarse carbides with an equivalent circle diameter of 15 μm or more in the field of view of 0.4 mm ² , it is judged that there are coarse carbides and there are no more than 50 coarse carbides in terms of coarse carbides. The result is shown by “◯”, which means “Yes” or “No problem” in 2.

As can be seen from Tables 1 and 2, Sample No. The steel of 1-20 has the specific chemical composition described above, and all the values of Formula 1 are 4.0 or more. In addition, all coarse carbides result in 50 or less in a visual field of 0.4 mm ² . Therefore, these steels have a hardness value of 60 HRC or more, an extremely excellent impact value, and a machinability that does not have a problem that enables die machining, which is equal to or higher than that of the conventional SKD11. It was confirmed that

  On the other hand, sample No. as a comparative steel. 21-36 and Sample No. as conventional steel. In 37 and 38, none of the results having all the required characteristics were obtained. Specifically, sample no. No. 21 had too low C content, did not satisfy Formula 1, and could not obtain sufficient hardness. Sample No. No. 22 had a C content that was too high, and even though high hardness could be secured, the presence of coarse carbides exceeding the reference number was confirmed. As a result, the impact value was lowered and the machinability was also lowered.

  Sample No. In No. 23, the Si content was too low, the formula 1 was not satisfied, and sufficient hardness was not obtained. Sample No. In No. 24, the Si content was too high, and the impact value was low even though high hardness could be secured.

  Sample No. In No. 25, the Mn content was too low, the formula 1 was not satisfied, and not only a sufficient hardness was not obtained, but also MnS effective for improving the machinability was not sufficiently formed and the machinability was lowered. Sample No. No. 26 had an excessively high Mn content, did not satisfy Formula 1, and did not have sufficient hardness.

  Sample No. In No. 27, the Cr content was too low, the formula 1 was not satisfied, and sufficient hardness was not obtained. Sample No. No. 28, the Cr content was too high, the formula 1 was not satisfied, and not only a sufficient hardness was not obtained, but also the presence of a predetermined number or more of coarse carbides was confirmed. Decreased.

  Sample No. In No. 29, the Mo content was too low, the formula 1 was not satisfied, and sufficient hardness was not obtained. Sample No. No. 30 had a high Mo content and was able to achieve high hardness in order to satisfy Equation 1, but no impact value and machinability results that satisfy the target were obtained.

  Sample No. Nos. 31 to 33 have low contents of Ti and B, which are elements for improving the impact value of the present invention, and the effect of improving the impact value is insufficient. However, the impact value satisfying the target was not obtained.

  Sample No. 34-36 did not satisfy Formula 1 although individual chemical components were within the above-mentioned range, and sufficient hardness was not obtained.

  Sample No. Although 37 is not a JIS steel, it is a conventional steel that has been developed and marketed in the past, but has a low hardness and a low impact value. Sample No. 38 is a conventional steel (SKD11), which has a low hardness and a large inferior impact value.

  FIG. 1 is a plot of all the samples shown in Table 1 and Table 2 with the value of Equation 1 on the horizontal axis and the hardness on the vertical axis. It turns out that 60HRC can be ensured reliably by the value of Formula 1 being 4 or more.

  FIG. 2 is a plot of hardness on the horizontal axis and impact value on the vertical axis for all samples shown in Tables 1 and 2. Those with the appropriate chemical composition were shown as black plot points, and those with the wrong chemical composition were shown as white plot points. From this result, it can be clearly understood that the impact value of the present invention is increased in a hardness region of 60 HRC or more due to a synergistic effect by combined addition of Ti and B.

(Example 2)
In this example, the influence on the wear resistance by the hardness of the steel material and the presence or absence of coarse carbides was investigated. First, as shown in Table 3, developed steel (E1 to E4) corresponding to the examples in the present application, SKD11 (S1 to S4) which are conventional steels, commercially available A steel (A1 to A4), commercially available B steel (B1) ˜B4) and commercially available C steels (C1 to C4), four samples were prepared for each of five types of steel, and a total of 20 types of samples were prepared. Among them, the conventional steels SKD11 and B are steels whose components are designed to generate coarse carbides, and the others are steels whose components are designed not to easily generate coarse carbides.

  Each sample was manufactured by the same manufacturing method as in Example 1, and the final tempering temperature was changed in the range of 500 ° C. to 530 ° C. to adjust the hardness. For each sample, the hardness was measured in the same manner as in Example 1. And about abrasion resistance, it carried out using the Ogoshi type abrasion tester as follows.

<Abrasion resistance test>
The test with the Ogoshi type abrasion tester is a test in which the test piece is worn by pressing the side surface of the flat test piece while rotating the ring member. As a ring member, SCM415 was used. The test conditions were as follows: test distance: 400 m, wear rate: 0.74 m / s, load: 6.3 kgf (final load), and no lubricant. The wear width of the test piece was measured, and the specific wear amount (Ws) of each steel type was compared. The definition of specific wear (Ws) is shown below.
Specific wear amount Ws = (Bb ³ ) / (8rPL) mm ² / kgf,
Here, B: thickness of rotating ring (mm), r: radius of rotating ring (mm), b: wear scar width (mm), P: final load (kgf), L: friction distance (mm).

  The obtained hardness and specific wear amount are shown in Table 3 and shown in FIG. Fig. 3 shows hardness on the horizontal axis and specific wear on the vertical axis, the mark shape of the plot points is changed for each steel type, and the results of the SKD11 and B steels where coarse carbides are formed are plotted in white. The other steel types are indicated by black plot points. And the line which roughly connects the plot points of the steel types where coarse carbides are formed is shown by a dotted line, and the line which roughly connects the plot points of steel types where coarse carbides are hard to be formed is shown by a solid line.

  As can be seen from the figure, within the range where the hardness is less than 60 HRC, if the hardness is the same, the steel type with coarse carbide has better wear resistance. It can be seen that the presence or absence of carbide has little effect on the wear resistance. From this result, it can be clearly understood that at 60 HRC or more, it is advantageous to pursue the improvement of toughness and machinability by eliminating coarse carbides.

Claims (3)

In mass%, C: 0.55-0.85%, Si: 1.00-2.50%, Mn: 0.30-1.50%, P: 0.030% or less, S: 0.100 % Or less, Cr: 5.00 to 8.00%, Mo: 1.00 to 2.00%, Ti: 0.05 to 0.20%, B: 0.0005 to 0.0050%, Al: 0 0.005 to 0.150%, N: 0.0250% or less, with the balance being Fe and inevitable impurities,
Satisfying the following formula 1,
Formula 1: 1.4 × [Si] −2.7 × [Mn] + 1.7 × [Mo] + 1.5 × [C] ≧ 4 (where, [X] is the content of element X) (Mass%).),
And cold work tool steel in which the cross-sectional structure after quenching and tempering treatment has 50 or less coarse carbides having an equivalent circle diameter of 15 μm or more in a visual field of 0.4 mm ² .   Instead of a part of the remaining part, V: 0.20% or less, Nb: 0.20% or less, W: 0.50% or less, Ta: 0.20% or less The cold tool steel according to claim 1, which is contained.   2. It replaces with a part of said remainder, Mg: 0.10% or less, Te: 0.10% or less, Bi: 0.05% or less 1 type or 2 types or more are contained, It is characterized by the above-mentioned. Or the cold tool steel of 2.

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