JP2009293393A - Pump tappet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a pump tappet capable of attaining a long service life. <P>SOLUTION: The pump tappet 21 includes a shaft 22, and a roller bearing 31 arranged on the outer diameter side of the shaft 22 and rotatably supported on the shaft 22. The roller bearing 31 comprises an outer ring 32 abutting on a cam 12a, and a plurality of rollers 33 arranged between the outer ring 32 and the shaft 22. At least one of the roller 33 and the shaft 22 has a nitrogen enriched layer and has the grain size number of an austenite crystal grain size exceeding No.10, the residual amount of austenite being 11-25 vol.% and a nitrogen content of 0.1-0.5 wt.%. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a pump tappet, and more particularly to a pump tappet including a roller bearing.

  Some engines such as automobiles are provided with a high-pressure pump that injects fuel at a high pressure. The high-pressure pump converts the rotational movement of the camshaft provided with the cam into the reciprocating linear movement of the pump plunger, sends the gas by the reciprocating linear movement of the pump plunger, increases the pressure in the high-pressure chamber, and injects it into the fuel chamber And supply fuel. As a constituent member of the high-pressure pump, there is a pump tappet that transmits the rotational motion of the camshaft to the pump plunger as a reciprocating linear motion. There are several types of pump tappets, such as a roller-containing tappet including rollers and a mushroom-shaped tappet, depending on the shape of the contact portion with the cam.

  Here, a technique related to a tappet for a pump including a roller bearing is disclosed in DE 10 2005 047 234 A1 (Patent Document 1). FIG. 18 is a cross-sectional view of the pump tappet shown in Patent Document 1. Referring to FIG. 18, the tappet as the roller push rod 101 shown in Patent Document 1 includes a push rod housing 102 and a push rod roller 103 (roller bearing) fixed to the push rod roller 102 and supported by a needle. . The roller push rod 101 is driven by a three-stage cam 105 of a cam shaft 104 that rotates clockwise. At the same time, the roller push rod 101 is guided in the push rod guide hole 106 in the axial direction indicated by the arrow XVIII in FIG. 18 to drive a pump plunger 107 of a fuel high pressure pump (not shown).

The push rod roller 103 includes an outer ring 108 that contacts the three-stage cam 105, a shaft 109 disposed on the inner diameter side of the outer ring 108, and a plurality of needle rollers 110 disposed between the outer ring 108 and the shaft 109. The push rod roller 103 is a full roller type, that is, a type in which only a plurality of needle rollers 110 are disposed between the outer ring 108 and the shaft 109.
DE 10 2005 047 234 A1

  Modern high-pressure pumps are required to increase the pressure of fuel in a short time. In order to meet such a demand for high pressure in a short time, a roller bearing that is a constituent member of a pump tappet as shown in Patent Document 1 is required to be able to withstand high speed, that is, withstand high rotation. . Here, in use under high-speed rotation, the life of the pump tappet may be shortened from the viewpoint of durability of the roller or the shaft.

  An object of the present invention is to provide a pump tappet capable of extending the service life.

  The pump tappet according to the present invention transmits the rotational motion of the camshaft provided with the cam to the pump plunger as a reciprocating linear motion, and performs the reciprocating linear motion together with the pump plunger. The pump tappet includes a shaft, a roller bearing disposed on the outer diameter side of the shaft and rotatably supported on the shaft, and a case for housing the shaft and the roller bearing. The roller bearing includes an outer ring that contacts the cam and a plurality of rollers disposed between the outer ring and the shaft. Here, at least one of the roller and the shaft has a nitrogen-enriched layer, the austenite grain size number exceeds 10, and the residual austenite amount is 11% by volume or more and 25% by volume or less. And the nitrogen content is 0.1 wt% or more and 0.5 wt% or less.

  With such a configuration, the fatigue life of at least one of the roller and the shaft can be improved, the durability of the bearing can be improved, and the life of the pump tappet can be extended. In addition, such a roller and a shaft can be manufactured with the manufacturing method mentioned later.

  The austenite crystal grain size may be obtained by a normal method prescribed in JIS, or an average grain size corresponding to the crystal grain size number may be obtained by an intercept method and converted. Note that the austenite crystal grains do not change even in the surface layer portion where the nitrogen-enriched layer exists or in the inner part thereof. Therefore, the target position of the above crystal grain size number range is the surface layer portion and the inside. Here, the austenite crystal grains are crystal grains of austenite that has undergone phase transformation during quenching heating, and this means what remains as a past history even after transformation to martensite by cooling. .

  The amount of retained austenite is a value in the surface layer of 50 μm after rolling, and can be measured, for example, by comparing the diffraction intensities of martensite α (211) and retained austenite γ (220) by X-ray diffraction. it can. In addition, using the fact that the austenite phase is a non-magnetic material and the ferrite phase is a ferromagnetic material, it can also be measured by determining the magnetizing force with a magnetic balance or the like. In addition, it can measure easily using a commercially available measuring apparatus.

  The nitrogen-enriched layer is a layer having an increased nitrogen content formed on the surface layer, and can be formed by a process such as carbonitriding, nitriding, or nitriding. The nitrogen content of the nitrogen-enriched layer is a value in the surface layer of 50 μm of the rolling surface after grinding, and can be measured by, for example, EPMA (Electron Probe Micro-Analysis: wavelength dispersion type X-ray microanalyzer).

  In addition, both the roller and the shaft have a nitrogen-enriched layer, the austenite grain size number exceeds 10, and the residual austenite amount is 11% by volume or more and 25% by volume or less, and The nitrogen content may be 0.1 wt% or more and 0.5 wt% or less. With this configuration, the durability of the bearing can be further improved, and the life of the pump tappet can be extended.

  According to the pump tappet according to the present invention, the fatigue life of at least one of the roller and the shaft can be improved, the durability of the bearing can be improved, and the life of the pump tappet can be extended.

  Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a cross-sectional view showing a part of a high-pressure pump including a pump tappet (hereinafter simply referred to as “tappet”) according to an embodiment of the present invention. 2 and 3 are sectional views of tappets included in the high-pressure pump shown in FIG. FIG. 4 is a schematic perspective view of the tappet shown in FIGS. 2 and 3. FIG. 5 is a view of the tappet shown in FIG. 4 as viewed from the direction of the arrow V in FIG. 6 is a view of the tappet shown in FIG. 4 as viewed from the direction of the arrow VI in FIG. 7 is a view of the tappet shown in FIG. 4 as viewed from the direction of arrow VII in FIG. 8 is a view of the tappet shown in FIG. 4 as viewed from the direction of arrow VIII in FIG. 2 corresponds to the II-II cross section shown in FIG. 5, and FIG. 3 corresponds to the III-III cross section shown in FIG. In addition, in the figure shown below, illustration of hatching of the cross section of a roller is abbreviate | omitted from a viewpoint of easy understanding.

  With reference to FIGS. 1-8, the structure of the high pressure pump containing the tappet which concerns on one Embodiment of this invention is demonstrated first. A high pressure pump 11 including a tappet according to an embodiment of the present invention has a cam 12a provided on the outer diameter side thereof, a camshaft 12 that rotates in the direction of arrow A in FIG. The tappet 21 that abuts and transmits the rotary motion of the camshaft 12 as a reciprocating linear motion to the pump plunger 13 (hereinafter simply referred to as “plunger”) and performs the reciprocating linear motion, and the reciprocating linear motion abutting the tappet 21. A plunger 13 as a rod-shaped member that moves, a high-pressure chamber (not shown) that feeds gas in accordance with the reciprocating linear motion of the plunger 13, abuts against the tappet 21, and moves the plunger 13 inward. And a housing 15 that houses the tappet 21, the plunger 13, and the spring 14.

  The tappet 21, the plunger 13, and the spring 14 are arranged so as to be accommodated in the opening hole 16 provided in the housing 15. The tappet 21 is guided by the inner diameter surface 16a of the opening hole 16 in the vertical direction in FIG. 1, that is, in the direction of the arrow I in FIG.

  The camshaft 12 and the tappet 21 are arranged so that the outer diameter surface 12b of the cam 12a and the outer diameter surface 32a of the outer ring 32 provided in the roller bearing 31 included in the tappet 21 abut. One end 13 a of the plunger 13 is disposed so as to abut on an intermediate bottom 23 c provided in a case 23 included in the tappet 21. The spring 14 is disposed such that one end portion 14a thereof abuts against a spring seat 17 provided on the lower side of the intermediate bottom 23c.

  The spring 14 has an elastic force in the downward direction, that is, in the direction opposite to the direction indicated by the arrow I in FIG. The tappet 21 is urged upward by the elastic force of the spring 14 through the plunger 13, that is, in the direction indicated by the arrow I in FIG. 1.

  The tappet 21 and the plunger 13 are moved in the vertical direction, that is, in the direction of the arrow I in FIG. Reciprocating linear motion in the direction of. The reciprocating linear motion here is a motion in the direction of arrow I in FIG. 1 or in the opposite direction. The tappet 21 performs a reciprocating linear motion in the direction of the arrow I in FIG. When the camshaft 12 rotates at a high speed, the speed of the reciprocating linear motion of the tappet 21 and the plunger 13 also increases.

  The high pressure chamber is disposed on the other end side (not shown) of the plunger 13. The reciprocating linear motion of the plunger 13 can increase the pressure of the fuel supplied to the high pressure chamber.

  Next, the configuration of the tappet 21 according to an embodiment of the present invention will be described. The tappet 21 includes a shaft 22, a roller bearing 31 that is disposed on the outer diameter side of the shaft 22 and is rotatably supported on the shaft 22, and a case 23 that houses the shaft 22 and the roller bearing 31.

  The case 23 includes a cylindrical peripheral wall 23a and an intermediate bottom 23c provided at an intermediate position of the inner diameter surface 23b of the peripheral wall 23a so as to partition the space in the vertical direction. Of the case 23, the intermediate bottom 23 c is in contact with the plunger 13. The peripheral wall 23a and the intermediate bottom 23c have a predetermined thickness.

  A pair of support holes 23d and 23e that support the shaft 22 are provided on one end side of the peripheral wall 23a. The shaft 22 is arranged so that the shaft 22 is inserted into the pair of support holes 23d and 23e. A roller bearing 31 is disposed on the outer diameter side of the shaft 22. Thus, the case 23 accommodates the shaft 22 and the roller bearing 31 in the space 23f from the intermediate bottom 23c toward the one end side of the peripheral wall 23a.

  A part of the plunger 13 is accommodated in the space 23g from the intermediate bottom 23c toward the other end side of the peripheral wall 23a. Specifically, the one end portion 13a of the plunger 13 is disposed so as to contact the central portion in the radial direction of the intermediate bottom 23c, and the one end portion 13a of the plunger 13 is accommodated. The one end portion 14a of the spring 14 is also accommodated in the space 23g.

  The middle bottom 23c is provided with four oil holes 25 penetrating in the thickness direction (see FIGS. 7 and 8). The four oil holes 25 are provided so as to avoid a contact portion between the one end portion 13a of the plunger 13 and the intermediate bottom 23c. By using this oil hole 25, the lubricating oil supplied to the tappet 21 can be passed between the space 23f and the space 23g.

  The case 23 is provided with a through hole 23i that penetrates from the outer diameter surface 23h to the inner diameter surface 23b of the peripheral wall 23a. A positioning pin 24 is fitted in the through hole 23i. The positioning pin 24 is a cylindrical member. The positioning pin 24 is configured such that a part thereof protrudes from the outer diameter surface 23h when fitted into the through hole 23i. A concave groove 16b extending in the direction of arrow I in FIG. 1 is provided on the inner diameter surface 16a of the opening hole 16 of the housing 15 so as to be recessed from the inner diameter surface 16a. When the tappet 21 is accommodated in the opening hole 16, the protruding portion of the positioning pin 24 is fitted into the concave groove 16b. Thereby, the case 23 in the opening hole 16 and the positioning of the tappet 21 in the circumferential direction are performed. Thereby, rotation to the circumferential direction of the tappet 21 in the opening hole 16 can be prevented.

  Next, the configuration of the roller bearing 31 included in the tappet 21 will be described. FIG. 9 is an enlarged view of a portion indicated by IX of a two-dot chain line in FIG. A roller pitch circle 31a is indicated by a one-dot chain line in FIG. FIG. 10 is a view of a part of the roller bearing 31 seen from the direction of the arrow X shown in FIG. FIG. 11 is a cross-sectional view showing a part of the roller bearing 31, and is a XI-XI cross section in FIG. 1 to 11, the roller bearing 31 includes an outer ring 32, a plurality of rollers 33 disposed between the outer ring 32 and the shaft 22, and a cage 34 that holds the plurality of rollers 33. .

  The retainer 34 includes a pair of annular portions 34 a and 34 b and a plurality of column portions 34 d that connect the pair of annular portions 34 a and 34 b so as to form a pocket 34 c that accommodates the rollers 33. The column portion 34d has a shape that extends straight in the axial direction, that is, in the cross section shown in FIG.

  Similar to the rollers 33, the retainer 34 is disposed between the outer ring 32 and the shaft 22. The plurality of rollers 33 are accommodated and held in the respective pockets 34 c provided in the cage 34. The retainer 34 is configured such that the outer diameter guide, that is, the inner diameter surface 32 b of the outer ring 32 disposed on the outer diameter side of the retainer 34 and the outer diameter surface 34 e of the retainer 34 are in radial contact. Further, the retainer 34 is provided with an oil groove 34f so as to be recessed inward from the outer diameter surface 34e. The oil groove 34f is provided at the center of the column portion 34d and has a shape extending in the circumferential direction.

  Due to the rotational movement of the camshaft 12, the outer ring 32 and the rollers 33, which are constituent members of the roller bearing 31, are also rotated. When the camshaft 12 rotates at a high speed, the rotation of the rollers 33 also increases. Here, in the roller bearing 31 which is a constituent member of the tappet 21, the position of the roller 33 in the roller bearing 31 at the time of high speed rotation can be stabilized by the cage 34. Then, the skew of the roller 33 can be suppressed and the lateral running of the roller bearing 31 can be prevented. Accordingly, it is possible to reduce the risk of poor lubrication of the roller bearing 31 and wear of the roller bearing 31 during high-speed rotation.

  In addition, the high-pressure pump 11 includes the tappet 21 that can reduce the risk of poor lubrication of the roller bearing 31 and wear of the roller bearing 31 during high-speed rotation. High pressure can be achieved.

  Here, the retainer 34 is an outer diameter guide, and the retainer 34 and the outer ring 32 are provided on the outer diameter surface 34e of the retainer 34 because an oil groove 34f that is recessed inward from the surface is provided. And the radial position of the cage 34 can be stabilized. Further, the oil permeability of the inner diameter surface 34g of the cage 34 and the outer diameter surface 22a of the shaft 22 is improved, and the lubricity between the outer diameter surface 34e of the cage 34 and the inner diameter surface 32b of the outer ring 32 is improved and retained. Wear of the vessel 34 and the outer ring 32 can be suppressed. Therefore, the life of the cage 34, the rollers 33, the outer ring 32, and the shaft 22 can be extended. The oil groove may be provided so as to extend with an inclination in the axial direction, or may be provided so as to extend in a curved manner. A plurality of oil grooves may be provided.

  The cage 34 is an inner diameter guide, and an oil groove may be provided on the inner diameter surface 34 g of the cage 34. By carrying out like this, the holder | retainer 34 and the shaft 22 can be made to contact and the radial position of the holder | retainer 34 can be stabilized. Further, the oil permeability of the outer diameter surface 34e of the cage 34 and the inner diameter surface 32b of the outer ring 32 is improved, and the lubricity between the inner diameter surface 34g of the cage 34 and the outer diameter surface 22a of the shaft 22 is improved and retained. Wear of the vessel 34 and the shaft 22 can be suppressed. Therefore, the life of the cage 34, the rollers 33, the outer ring 32, and the shaft 22 can be extended.

  The cage 34 provided in the roller bearing 31 may be made of resin. By carrying out like this, the holder | retainer 34 itself can be made lightweight and the weight of the tappet 21 can be lightened generally. Therefore, the force required for the reciprocating linear motion, here, the force required for the vertical movement of the tappet 21 can be reduced. Further, when the cage 34 is made of resin, it can be manufactured by injection molding or the like, so that mass production is facilitated and it can be manufactured at low cost. Examples of the material of the cage 34 include nylon 66, nylon 46, polyphenylene sulfide (PPS), polyether ether ketone (PEEK), and the like. If necessary, the resin may be filled with carbon fiber, glass fiber, carbon black, or the like.

  In FIG. 10, the dimension of the clearance between the rolling surface 33a of the roller 33 accommodated in the pocket 34c and the side wall surface 34h of the column portion 34d is exaggerated and greatly illustrated from the viewpoint of easy understanding. .

  In addition, the roller and shaft which comprise the above-mentioned tappet are manufactured by forging, cutting, etc., steel members, for example, SUJ2 and SCM420 (all are JIS standards).

  Here, at least one member of the rollers and shafts constituting the tappet is subjected to a heat treatment of a low-temperature secondary quenching method to be described below, so that the nitrogen richness of at least one member of the rollers and shafts is increased. The grain size number of the austenite crystal grains of the stratified layer is in a range exceeding # 10, the amount of retained austenite is 11% by volume to 25% by volume, and the nitrogen content is 0.1% by weight to 0.5% % Or less.

  By adopting the above-described configuration for at least one of the rollers and shafts constituting the tappet, the fatigue life of at least one of the rollers and shafts can be improved, and the durability of the bearings can be improved. The life of the tappet can be extended.

Next, heat treatment including carbonitriding performed on at least one member of the roller and the shaft will be described. FIG. 12 is a diagram for explaining an example of a two-stage heat treatment method for obtaining a member having the above-described configuration. Moreover, FIG. 13 is a figure explaining the modification of the two-step heat processing method shown in FIG. Figure 12 is a heat treatment pattern showing a method for entering primary quenching and secondary sintering, FIG. 13, the hardenability middle material is cooled to below the A ₁ transformation point temperature, then finally reheated It is the heat processing pattern which shows the method of quenching. In either case, the above-described member can be obtained. In these figures, to diffuse the carbon and nitrogen into a green body of the processing T ₁ steel also after a sufficient penetration of carbon, cooled to below the A ₁ transformation point. Next, in the process T ₂ of the in the figure, then reheated to a temperature lower than the processing T _1, subjected to oil quenching from there. Note that the heat treatment methods shown in FIGS. 12 and 13 are collectively referred to as two-step heat treatment.

  The above heat treatment can improve the cracking strength and reduce the aging dimensional change rate while carbonitriding the surface layer portion rather than ordinary quenching, that is, carbonitriding as it is after the carbonitriding treatment. According to the heat treatment method described above, it is possible to obtain a microstructure in which the grain size of austenite crystal grains is ½ or less of the conventional one. The member subjected to the above heat treatment has a long life in rolling fatigue characteristics, can improve the cracking strength, and can also reduce the aging rate of dimensional change.

  14 and 15 are diagrams showing the microstructure of the member, particularly austenite grains. FIG. 14 shows a member subjected to the above heat treatment method, and FIG. 15 shows a conventional member. That is, FIG. 14 shows the austenite grain size of the bearing steel to which the heat treatment pattern shown in FIG. 13 is applied. For comparison, FIG. 15 shows the austenite grain size of the bearing steel by the conventional heat treatment method. FIGS. 16 and 17 are diagrams showing the austenite grain boundaries illustrated in FIGS. 14 and 15 described above. From the structure showing the austenite crystal grain size, the conventional austenite grain size is a number of 10 or less in the JIS standard grain size number, and according to the above-described two-stage heat treatment method, the 12th fine grain can be obtained.

  Thus, by performing the above-described two-stage heat treatment, it is possible to improve the fatigue life of at least one member of the roller and the shaft, improve the durability of the bearing, and extend the life of the pump tappet. .

  Note that both the roller and the shaft may be subjected to the heat treatment described above to have the above-described configuration. That is, both the roller and the shaft have a nitrogen-enriched layer, the austenite grain size number exceeds 10, and the residual austenite amount is 11% by volume or more and 25% by volume or less, and The nitrogen content may be 0.1 wt% or more and 0.5 wt% or less. With this configuration, the durability of the bearing can be further improved, and the life of the pump tappet can be extended.

  In the above-described embodiment, the cage includes a pair of annular portions and a plurality of column portions, but is not limited thereto, and is divided into a plurality of members instead of an integrated type, A spacer-type cage disposed between the rollers may be used.

  In the above embodiment, the column portion has a shape extending straight in the axial direction. However, the present invention is not limited to this, and the column portion may be bent in the radial direction. It may be shaped like an M-type cage. Furthermore, it is good also as providing the roller stop part which prevents the fall-off | omission of the roller of the radial direction on the side wall surface of a pillar part.

  In the above-described embodiment, the roller bearing is configured to include a cage that holds a plurality of rollers. However, the configuration is not limited thereto, that is, a configuration in which the roller bearing does not include a cage, that is, a full-roller type. The same applies to roller bearings.

  As mentioned above, although embodiment of this invention was described with reference to drawings, this invention is not limited to the thing of embodiment shown in figure. Various modifications and variations can be made to the illustrated embodiment within the same range or equivalent range as the present invention.

  The tappet according to the present invention is included in a high-pressure pump that supplies fuel to an engine such as an automobile, and is effectively used as a part for an automobile.

It is sectional drawing which shows a part of high-pressure pump containing the tappet which concerns on one Embodiment of this invention. It is sectional drawing of the tappet contained in the high pressure pump shown in FIG. It is sectional drawing of the tappet contained in the high pressure pump shown in FIG. FIG. 4 is a perspective view of the tappet shown in FIGS. 2 and 3. It is the figure which looked at the tappet shown in FIG. 4 from the direction of arrow V in FIG. It is the figure which looked at the tappet shown in FIG. 4 from the direction of arrow VI in FIG. It is the figure which looked at the tappet shown in FIG. 4 from the direction of arrow VII in FIG. It is the figure which looked at the tappet shown in FIG. 4 from the direction of arrow VIII in FIG. It is an enlarged view of the part shown by IX of the tappet shown in FIG. It is the figure which looked at a part of roller bearing contained in the tappet shown in FIG. 2 from the direction of the arrow X in FIG. FIG. 10 is a partial cross-sectional view of a roller bearing included in the tappet illustrated in FIG. 2, and is a view cut along a XI-XI cross section in FIG. 9. It is a figure explaining the two-step heat processing method. It is a figure explaining the modification of the two-step heat processing method. It is a figure which shows the microstructure in the tappet structural member to which the heat processing pattern shown in FIG. 12 is applied, especially an austenite grain. It is a figure which shows the microstructure in the conventional tappet structural member, especially an austenite grain. FIG. 15 is a schematic diagram of the microstructure shown in FIG. 14 and illustrates the illustrated austenite grain boundaries. FIG. 16 is a schematic diagram of the microstructure shown in FIG. 15, illustrating the illustrated austenite grain boundaries. It is sectional drawing which shows the tappet as a conventional roller pushrod.

Explanation of symbols

  11 High pressure pump, 12 Cam shaft, 12a Cam, 12b, 22a, 23h, 32a, 34e Outer diameter surface, 13 Plunger, 13a, 14a End, 14 Spring, 15 Housing, 16 Open hole, 16a, 23b, 32b, 34 g inner diameter surface, 16 b concave groove, 17 spring seat, 21 tappet, 22 shaft, 23 case, 23 a peripheral wall, 23 c intermediate bottom, 23 d, 23 e support hole, 23 f, 23 g space, 23 i through hole, 24 positioning pin, 25 oil hole , 31 Roller bearing, 31a Roller pitch circle, 32 Outer ring, 33 Roller, 33a Rolling surface, 34 Cage, 34a, 34b Annular part, 34c Pocket, 34d Pillar part, 34f Oil groove, 34h Side wall surface.

Claims (2)

A pump tappet that transmits the rotational motion of a camshaft provided with a cam to a pump plunger as a reciprocating linear motion, and performs a reciprocating linear motion together with the pump plunger,
A shaft, a roller bearing disposed on the outer diameter side of the shaft and rotatably supported on the shaft, and a case for housing the shaft and the roller bearing;
The roller bearing includes an outer ring that contacts the cam, and a plurality of rollers disposed between the outer ring and the shaft,
At least one of the roller and the shaft has a nitrogen-enriched layer, the austenite grain size number exceeds 10, and the residual austenite amount is 11% by volume or more and 25% by volume or less, And the tappet for pumps whose nitrogen content is 0.1 to 0.5 weight%. Both the roller and the shaft have a nitrogen-enriched layer, the austenite grain size number exceeds 10, and the residual austenite amount is 11% by volume or more and 25% by volume or less, and The pump tappet according to claim 1, wherein the nitrogen content is 0.1 wt% or more and 0.5 wt% or less.

Images