JP2004353005A - Steel bar for steering rack - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel bar for a steering rack, which has superior impact resistance in addition to improved abrasion resistance, and prevents propagation/penetration of cracks. <P>SOLUTION: The steel bar for the steering rack includes 0.50-0.60% C, 0.05-0.5% Si, 0.2-1.5% Mn, 0.0005-0.003% B, 0.005-0.05% Ti, 0.0005-0.1% Al, 0.002-0.02% N and 1.5% or less Cr. Besides, the quenched/tempered structure of a D/4 part is adjusted so as to satisfy the following conditions (1) to (3). (1) The total of a tempered bainite structure and a tempered martensite structure occupies 30-100% (by area percentage). (2) A reproduced pearlite structure occupies 0-50% (by area percentage). (3) The total of the tempered bainite structure, the tempered martensite structure and the reproduced pearlite structure occupies 50-100% (by area percentage). <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【００４７】
【表２】

【００４８】
表１〜２のうち実験例２０〜２８は成分設計が不適切な例である。すなわち実験例２０ではＣ量が不足しているため、耐摩耗性が不十分である。実験例２１は逆にＣ量が多いため耐衝撃特性が不十分である。実験例２２ではＭｎ量が少なく焼入性に劣る為、焼戻しベイナイト組織及び焼戻しマルテンサイト組織の合計量が不足しており、曲げ変形能が不十分である。実験例２３はＭｎ量が過剰となっているため、高周波焼入れの際に硬化層が深くなってしまい、曲げ変形能が不十分となる。実験例２４〜２７では、Ｂ又はＴｉ量が不適切であるために耐衝撃性が不十分である。実験例２８はＣｒ量が過剰であるため、高周波焼入れの際に硬化層が深くなってしまい、曲げ変形能が不十分となる。
【００４９】
また実験例２９〜３６から明らかなように、成分設計が適切であっても、組織が不適切であると諸特性が不十分となる。すなわち実験例２９〜３１では焼戻しベイナイト組織及び焼戻しマルテンサイト組織の合計量が不足しており、曲げ変形能が不十分となる。実験例３２〜３４では再生パーライト組織が多すぎるため、曲げ変形能が不十分となる。実験例３５〜３６では、焼戻しベイナイト組織、焼戻しマルテンサイト組織、及び再生パーライト組織の合計量が不足しているため、曲げ変形能が不十分となる。
【００５０】
これらに対して実験例１〜１９は、成分設計及び組織の両方が適切であるため、耐衝撃特性及び耐摩耗性の両方に優れており、しかも亀裂の進展・貫通を防止できる。
【００５１】
【発明の効果】
本発明のステアリングラック用棒鋼によれば、成分及び組織の両方が適切に制御されているため、耐摩耗性が改善されながらも耐衝撃特性にも優れており、しかも亀裂の進展・貫通を防止することができる。
【図面の簡単な説明】
【図１】図１は実験例で用いる試験ディスクの形状を示す概略斜視図である。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a steel bar useful for manufacturing a rack (steering rack) used for a steering gear of an automobile, and a method for manufacturing the same.
[0002]
[Prior art]
Automobile steering is classified into a type that is power assisted by hydraulic pressure (hydraulic power steering) and a type that is power assisted by electric power (electric power steering). BACKGROUND ART Hydraulic power steering is a conventional mainstream, and assists a steering force by pressure oil discharged from an oil pump that operates using an engine output. However, the hydraulic power steering uses a part of the engine output as a power source, and thus has a problem in that the fuel efficiency of the vehicle is reduced. On the other hand, the electric power steering assists the steering force by an electric motor operated by electric energy from a battery. Compared with the hydraulic power steering, the electric power steering can improve the fuel efficiency of an automobile. Its spread is increasing.
[0003]
By the way, the steering rack is one of the main components constituting the steering, and also plays a role of a framework of the automobile. Moreover, if the steering rack breaks, the steering wheel cannot be operated, so it is designated as an important security component, and high reliability and strength characteristics are required. That is, the steering rack is excellent in 1) excellent impact resistance such that the vehicle does not break even if it receives an impact when it gets on a curb or the like, 2) does not break even when bending stress acts (crack resistance), and 3). The wear resistance of the rack teeth is required.
[0004]
Conventionally, such a steering rack is made of S45C steel (see Patent Documents 1 and 2), medium carbon steel (see Patent Documents 3 and 4), and the like. By forming a surface hardened layer by induction hardening. In addition to increasing the wear resistance, the strength (cracking resistance) against bending stress is also increased (see Patent Documents 1 and 2).
[0005]
However, even if the bending strength is increased by induction hardening, once an excessive load acts and a crack is generated in the induction hardened layer, the crack propagates and penetrates into the inside, leading to breakage, and the handle operation becomes difficult. Becomes impossible. Further, in electric power steering, which has been widely used in recent years, the contact surface pressure between the steering rack and the pinion gear tends to be higher than that in hydraulic power steering, and S45C steel (Patent Documents 1 and 2) has wear resistance. Run out. Although the amount of carbon in medium carbon steels (Patent Documents 3 and 4) is not clear, even if the wear resistance is increased by using a large amount of C, the impact resistance decreases.
[0006]
Patent Document 5 describes that when induction hardening is performed by adding B, brittle fracture does not occur even when an excessive load is applied, and fracture is prevented by bending deformation. The steel for a steering rack disclosed in Patent Document 5 is a steel whose quenching and tempering treatment is omitted, and has a substantially ferrite-pearlite structure.
[0007]
[Patent Document 1]
JP-A-62-178472 (page 3, upper left column, third line, prior art column)
[Patent Document 2]
JP-A-62-180018 (page 15, lower left column, line 15, conventional technology column)
[Patent Document 3]
JP-A-2000-153336 (paragraph 0013, second line)
[Patent Document 4]
JP 2001-79639 A (paragraph 0015, second line)
[Patent Document 5]
JP-A-10-8189 (paragraph 0006, column of Examples)
[0008]
[Problems to be solved by the invention]
It should be noted that, even after preparing a steel omitting B-added quenching and tempering treatment in accordance with the method of Patent Document 5 and performing a bending test after induction hardening, the inventors of the present invention found that cracks propagated and penetrated to break. (See Experimental Example 31 described later).
[0009]
SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned circumstances, and an object of the present invention is to improve the abrasion resistance and also have an excellent impact resistance, and furthermore, a steering that can prevent crack propagation and penetration. An object of the present invention is to provide a steel bar for a rack and a method of manufacturing the same.
[0010]
[Means for Solving the Problems]
The present inventors have conducted intensive studies in order to solve the above-mentioned problems. As a result, even if the amount of C is increased, the addition of B can prevent a decrease in impact resistance, and furthermore, a tempered bainite structure, a tempered pearlite structure. By controlling the regenerated pearlite structure within a predetermined range, the bending deformability can be enhanced, and even if a crack is generated, it is possible to prevent the propagation and penetration of the crack, and the present invention has been completed.
[0011]
That is, the steel bar for a steering rack according to the present invention has C: 0.50 to 0.60% (meaning mass%, the same applies hereinafter), Si: 0.05 to 0.5%, Mn: 0.2 to 0.2%. It contains 1.5%, B: 0.0005 to 0.003%, and Ti: 0.005 to 0.05%, and further contains 0.0005 to 0.1% of Al and 0.1 to 0.05% as other elements. Containing both 002-0.02% N and / or 1.5% or less (excluding 0%) Cr, with the balance consisting of Fe and unavoidable impurities,
The gist is that the quenched and tempered structure at a depth D / 4 (D indicates the diameter of the bar) from the surface of the bar is adjusted as in 1), 2) and 3) below. is there.
[0012]
1) The total of the tempered bainite structure and the tempered martensite structure is 30 to 100% (area percentage).
2) Regenerated pearlite structure of 0 to 50% (area percentage)
3) Tempered bainite structure, tempered martensite structure, and regenerated pearlite structure in total of 50 to 100% (area percentage)
The steel bar for a steering rack may further contain free-cutting elements (S, Pb, Bi, Te, Mg, Ca, REM, Zr, etc.).
[0013]
The steel bar for a steering rack of the present invention is obtained by quenching a steel bar obtained by rolling a slab from a temperature of 780 ° C. or higher to obtain a bainite structure and a martensite structure in a portion having a depth of D / 4 in a total of 30 to 100% (area). Percentage)
It can be manufactured by placing it in a furnace heated to an atmosphere temperature of 660 to 720 ° C., performing a tempering treatment for a short time of 20 minutes or less, and cooling to room temperature.
[0014]
In the present specification, the term “quenching” refers to quenching during quenching and tempering of a steel bar after rolling, and is used in distinction from the term “induction quenching”.
[0015]
BEST MODE FOR CARRYING OUT THE INVENTION
The steel bar for a steering rack of the present invention has C: 0.50 to 0.60% (meaning mass%, the same applies hereinafter), Si: 0.05 to 0.5%, Mn: 0.2 to 1.5. %, B: 0.0005 to 0.003%, and Ti: 0.005 to 0.05%, and as other elements, 0.0005 to 0.1% Al and 0.002 to 0%. It contains both 0.02% N and / or 1.5% or less (excluding 0%) Cr. The balance is Fe and inevitable impurities.
[0016]
The reasons for limiting the above components are as follows.
[0017]
The reason why the content of C is set to 0.50% or more is to sufficiently enhance wear resistance when a steering rack (for example, a steering rack for electric power steering) is used. The preferred C content is 0.52% or more. However, when the content of C is too large, the impact resistance of the steering rack is deteriorated. Therefore, the content of C is set to 0.60% or less, preferably 0.58% or less.
[0018]
The reason why the content of Si is set to 0.05% or more is to deoxidize the steel material. The preferred Si content is 0.10% or more, particularly 0.15% or more. However, if the content of Si is too large, the machinability at the time of forming the rack teeth decreases. Therefore, the content of Si is set to 0.5% or less, preferably 0.35% or less, and more preferably 0.30% or less.
[0019]
The reason why the Mn content is set to 0.2% or more is not only to enhance the strength of the steel material, but also to enhance the hardenability and facilitate the introduction of the bainite structure, thereby bending the steel material into a steering rack. This is to enhance the deformability. The preferred Mn content is 0.5% or more, particularly 0.7% or more. However, if the content of Mn is too large, the hardened layer formed by induction hardening becomes too deep, and the bending deformability decreases. Therefore, the content of Mn is set to 1.5% or less, preferably 1.3% or less, and more preferably 1.2% or less.
[0020]
The content of B is set to 0.0005% or more in order to secure impact resistance even in the steel of the present invention in which the C content is increased. The preferred B content is 0.0007% or more. However, when the content of B is too large, a harmful B-based compound is generated, and the impact resistance is rather deteriorated. Therefore, the content of B is set to 0.003% or less, preferably 0.0025% or less, and more preferably 0.0020% or less.
[0021]
Ti is effective in suppressing the generation of BN by forming TiN in combination with N in steel, and ensuring the above-mentioned effect of B. Therefore, the content of Ti is 0.005% or more, preferably 0.010% or more, and more preferably 0.012% or more. However, if the content of Ti is too large, the impact resistance of the steering rack is rather deteriorated. Therefore, the content of Ti is set to 0.05% or less, preferably 0.04% or less, and more preferably 0.035% or less.
[0022]
The reason why Al and N are contained is that by forming AlN, austenite grains during induction hardening can be refined. The content of Al is 0.0005% or more, preferably 0.010% or more, and more preferably 0.020% or more. The N content is 0.002% or more, preferably 0.003% or more, and more preferably 0.004% or more. However, if the contents of Al and N are excessively large, the impact resistance decreases. Therefore, the content of Al is set to 0.1% or less, preferably 0.08% or less, and more preferably 0.05% or less. The N content is 0.02% or less, preferably 0.01% or less, and more preferably 0.007% or less.
[0023]
The inclusion of Cr is for improving the hardenability. Although the lower limit of the Cr content is not particularly limited, it is, for example, about 0.05%, preferably about 0.08%, and more preferably about 0.10%. However, if the content of Cr is too large, the hardened layer formed by induction hardening becomes too deep, and the bending deformability becomes insufficient. Therefore, the content of Cr is, for example, 1.5% or less, preferably 1.0% or less, and more preferably 0.50% or less.
[0024]
The steel bar for a steering rack of the present invention may contain a free-cutting element (S, Pb, Bi, Te, Mg, Ca, REM, Zr, etc.) as necessary. The amounts of these free-cutting elements are, for example, S: 0.06% or less (excluding 0%), Pb: 0.3% or less (excluding 0%), Bi: 0.2% or less (0%). %: Te: 0.1% or less (excluding 0%), Mg: 0.01% or less (excluding 0%), Ca: 0.01% or less (excluding 0%) , REM: about 0.01% or less (excluding 0%) and Zr: about 0.3% or less (excluding 0%). These free-cutting elements can be added alone or in combination of two or more.
[0025]
In the steel bar for a steering rack of the present invention, the quenched and tempered structure at a depth D / 4 (D indicates the diameter of the steel bar) from the surface of the steel bar is adjusted as in the following 1), 2) and 3). ing.
[0026]
1) The total of the tempered bainite structure and the tempered martensite structure [hereinafter, sometimes referred to as “TB + TM”] is 30 to 100% (area percentage).
2) Regenerated pearlite structure of 0 to 50% (area percentage)
3) The total of the tempered bainite structure, the tempered martensite structure, and the regenerated pearlite structure [hereinafter, sometimes referred to as TB + TM + RP] is 50 to 100% (area percentage).
Hereinafter, the reason for performing these organization controls will be described.
[0027]
The tempered bainite structure and the tempered martensite structure are structures introduced by quenching and tempering a rolled steel bar, and prevent the growth and penetration of cracks generated in the induction hardened layer when used as a steering rack. It is effective for In other words, the steering rack is very hard at the induction hardened part (surface layer), so that when subjected to a large bend, the induction hardened layer near the root of the steering tooth (usually near the D / 4 part) tends to crack, If a tempered bainite structure or a tempered martensite structure remains at the boundary between the part and the part that has not been induction hardened, the cracks generated in the induction hardened layer can be prevented from developing inside, and cracks in the steering rack itself can be prevented. Can be prevented. Therefore, the total (TB + TM) of the bainite structure and the tempered martensite structure in the D / 4 portion is set to 30% or more. It is preferably at least 40%, more preferably at least 50%.
[0028]
The recycled pearlite structure is a structure introduced in the tempering step, and is distinguished from the pearlite structure of as-rolled steel. Unlike the tempered bainite structure and the tempered martensite structure, the regenerated pearlite structure does not help to prevent the growth and penetration of cracks, and the bending deformability decreases when only the regenerated pearlite structure increases. Therefore, the reproduced pearlite structure is set to 50% or less. It is preferably at most 40%, more preferably at most 30%. Also, when the regenerated pearlite structure is reduced, the impact resistance tends to be further improved.
[0029]
Further, even if a tempered bainite structure and a tempered martensite structure are introduced by quenching and tempering, if a ferrite-pearlite structure or a soft ferrite structure remains from the as-rolled material, crack propagation and penetration cannot be prevented. Therefore, the structure from the as-rolled material needs to be reduced, in other words, the tempered bainite structure, the tempered martensite structure, and the regenerated pearlite structure need to be increased. The total (TB + TM + RP) of the three structures introduced by these quenching / tempering steps is 50% or more, preferably 60% or more, and more preferably 70% or more.
[0030]
The diameter of the steel bar for a steering rack of the present invention is not particularly limited, but is generally about 10 to 40 mm, preferably about 15 to 38 mm, and more preferably about 20 to 36 mm in consideration of processing into a steering rack.
[0031]
For the steel bars for steering racks as described above, for example, rolling a slab having the above-mentioned composition, quenching the resulting steel bars to introduce a bainite structure and a martensite structure, and then performing a high-temperature, short-time tempering treatment. Can be manufactured by
[0032]
In such a manufacturing method, the quenching heating temperature is set to 780 ° C. or higher, preferably 800 ° C. or higher. If the heating temperature for quenching is too low, TB + TM + RP after tempering tends to be small. In addition, a soft ferrite layer is generated, and the strength of the steering rack is insufficient. The upper limit of the heating temperature is, for example, about 860 ° C, preferably about 850 ° C. If the heating temperature is too high, the bending of the steel bar during quenching tends to increase.
[0033]
The cooling conditions for the quenching are such that the total amount of bainite structure and martensite structure introduced by the quenching (total amount in D / 4 parts; area percentage) is at least the total amount of the above-mentioned tempered bainite structure and martensite structure. It must be set to be the same. It is convenient to use controlled cooling for such cooling. The conditions of the controlled cooling can be appropriately set according to the composition of the steel and the like. For example, it is desirable to cool a temperature range of 800 to 300 ° C. at a cooling rate of 30 to 80 ° C./sec.
[0034]
The tempering temperature (furnace temperature) is about 660 to 720 ° C, preferably about 680 to 700 ° C, and the tempering time (residence time including the temperature raising process) is about 20 minutes or less, preferably about 15 minutes or less. If the tempering temperature is too high or the tempering time is too long, the regenerated pearlite structure becomes too large.
[0035]
The steel bar for a steering rack according to the present invention has improved abrasion resistance, excellent impact resistance, and excellent bending deformability. Therefore, a steering rack (particularly a steering rack for an electric power steering). Is extremely useful.
[0036]
【Example】
Hereinafter, the present invention will be described more specifically with reference to Examples. However, the present invention is not limited to the following Examples, and may be appropriately modified within a range that can be adapted to the purpose of the preceding and the following. It is of course possible to carry out them, and all of them are included in the technical scope of the present invention.
[0037]
Experimental Examples 1-36
Steel materials having the components shown in Tables 1 and 2 were melted and rolled into steel bars having a diameter of 30 mm. Next, after heating to the temperature shown in Tables 1-2, it was quenched by controlled cooling to room temperature. In the controlled cooling, the structure of the steel bar was controlled by changing the amount of water and the cooling time. The cooled steel bars were tempered by staying in a furnace heated to the ambient temperature shown in Tables 1 and 2 for the time shown in Tables 1 and 2. The steel bars after tempering were allowed to cool.
[0038]
The structure of D / 4 part of the quenched steel bar and the structure of D / 4 part of the tempered steel bar were observed with an electron microscope (magnification: 5000 times), and the martensite structure and bainite structure, the tempered martensite structure, and the tempered structure. The area ratio of the bainite structure and the regenerated pearlite structure was determined.
[0039]
In addition, the following tests were conducted to examine the crack resistance (bending deformability), impact resistance, and abrasion resistance when the tempered steel bars were used as steering racks.
[0040]
[Bending test]
After the tempered steel bar was drawn to a diameter of 27.5 mm, it was cut to form rack teeth. The depth of the rack teeth is about D / 4. Next, a steering rack was prepared by induction hardening the teeth under the following conditions.
[0041]
Coil used for induction hardening conditions: Surface hardening (diameter 40 mm, thickness 2 mm)
Voltage: 4.0 kV
Current: 4.5A
Frequency: 40kHz
Heating method: Moving quenching (moving speed 3.0 mm / sec)
Cooling: Using a steering rack obtained from a mixed solvent of soluble oil and water, a three-point bending test was performed in which the distance between fulcrums was 400 mm and the pressed part was on the opposite side of the teeth of the steering rack, and evaluated according to the following criteria.
[0042]
×: The crack generated in the induction hardened layer propagated and penetrated to the inside, and the steering rack broke into two parts. ○: The crack stopped halfway and did not lead to breakage [impact test].
After the tempered steel bar was drawn to a diameter of 27.5 mm, a JIS No. 3 U notch test piece was cut out from the D / 4 part, and the surface layer on the notch forming side was induction hardened. The induction quenching conditions were the same as in the bending test except that the moving speed was 3.5 mm / sec. The obtained test piece was subjected to a Charpy impact test (test temperature: room temperature) according to JIS Z2242 to determine an impact value.
[0043]
[Wear test]
A disk having the same characteristics as the tempered steel bar obtained in the experimental example was prepared. That is, a steel material having the same composition as the steel bar of the experimental example was melted, forged to a diameter of 53 mm by hot forging, cut into a disk having a thickness of 15 mm, and then quenched and tempered under the same conditions as in each experimental example. Was.
[0044]
Next, as shown in FIG. 1, it was machined into a two-stage semicircle (upper stage diameter: 44 mm, upper stage thickness: 3 mm; lower stage diameter: 50 mm, lower stage thickness: 5 mm), and the surface layer of the upper stage was induction hardened. . The induction quenching conditions were the same as in the bending test except that the moving speed was 2.5 mm / sec. A pin-on-disc wear test was performed on the obtained test disc to measure the loss on wear of the test piece. The detailed conditions of the wear test are as follows.
[0045]
Lubrication: Dry test specimen surface roughness: Ra 0.25 μm
Circumferential speed: 0.05 m / sec Surface pressure: 0.05 GPa
Pin: SUJ2 [diameter 5 mm, hardness (HRC) 64]
The results are shown in Tables 1 and 2.
[0046]
[Table 1]

[0047]
[Table 2]

[0048]
In Tables 1 and 2, Experimental Examples 20 to 28 are examples in which the component design is inappropriate. That is, in Experimental Example 20, the amount of carbon was insufficient, so that the wear resistance was insufficient. On the contrary, in Experimental Example 21, the impact resistance was insufficient due to the large amount of C. In Experimental Example 22, since the amount of Mn was small and the hardenability was poor, the total amount of the tempered bainite structure and the tempered martensite structure was insufficient, and the bending deformability was insufficient. In Experimental Example 23, since the amount of Mn was excessive, the hardened layer was deepened during induction hardening, and the bending deformability was insufficient. In Experimental Examples 24 to 27, the impact resistance was insufficient because the amount of B or Ti was inappropriate. In Experimental Example 28, since the amount of Cr was excessive, the hardened layer was deepened during induction hardening, and the bending deformability was insufficient.
[0049]
Moreover, as is clear from Experimental Examples 29 to 36, even if the component design is appropriate, if the structure is inappropriate, various characteristics become insufficient. That is, in Experimental Examples 29 to 31, the total amount of the tempered bainite structure and the tempered martensite structure is insufficient, and the bending deformability is insufficient. In Experimental Examples 32 to 34, since the regenerated pearlite structure was too large, the bending deformability was insufficient. In Experimental Examples 35 to 36, since the total amount of the tempered bainite structure, the tempered martensite structure, and the regenerated pearlite structure was insufficient, the bending deformability was insufficient.
[0050]
On the other hand, in Experimental Examples 1 to 19, since both the component design and the structure are appropriate, both the impact resistance and the wear resistance are excellent, and furthermore, the propagation and penetration of cracks can be prevented.
[0051]
【The invention's effect】
ADVANTAGE OF THE INVENTION According to the steel bar for steering racks of this invention, since both a component and a structure are controlled appropriately, it is excellent in impact resistance while improving abrasion resistance, and also prevents crack propagation and penetration. can do.
[Brief description of the drawings]
FIG. 1 is a schematic perspective view showing the shape of a test disk used in an experimental example.

Claims (3)

C: 0.50 to 0.60% (meaning mass%, the same applies hereinafter), Si: 0.05 to 0.5%, Mn: 0.2 to 1.5%, B: 0.0005 to 0 0.003% and Ti: 0.005 to 0.05%,
Still other elements include both 0.0005-0.1% Al and 0.002-0.02% N, and / or 1.5% or less (excluding 0%) Cr. ,
The balance is a bar made of Fe and unavoidable impurities,
A steering rack characterized in that the quenched / tempered structure at a depth D / 4 (D indicates the diameter of the bar) from the surface of the bar is adjusted as described in 1), 2) and 3) below. For steel bars.
1) The tempered bainite structure and the tempered martensite structure are 30 to 100% in total (area percentage).
2) Regenerated pearlite structure of 0 to 50% (area percentage)
3) The total of the tempered bainite structure, the tempered martensite structure, and the regenerated pearlite structure is 50 to 100% (area percentage). Further, S: 0.06% or less (excluding 0%), Pb: 0.3% or less (excluding 0%), Bi: 0.2% or less (excluding 0%), Te: 0. 1% or less (excluding 0%), Mg: 0.01% or less (excluding 0%), Ca: 0.01% or less (excluding 0%), REM: 0.01% or less (0%) %, And at least one selected from the group consisting of Zr: 0.3% or less (not including 0%). Rolling a billet containing the element according to claim 1 or 2,
After the obtained steel bar is quenched from a temperature of 780 ° C. or higher to make the bainite structure and the martensite structure in a portion having a depth of D / 4 to a total of 30 to 100% (area percentage),
A method for producing a steel bar for a steering rack, wherein the steel bar is placed in a furnace heated to an ambient temperature of 660 to 720 ° C., subjected to a short tempering treatment for 20 minutes or less, and cooled to room temperature.

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