JP3068861B2 - Stainless steel for engine gasket excellent in moldability and method of manufacturing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a material for a gasket of an internal combustion engine (referred to as an engine). In particular, the present invention relates to a metal gasket material that exhibits excellent characteristics in an engine gasket that secures sealing properties by tightening a beaded portion.

[0002]

2. Description of the Related Art One of the components constituting an engine is a gasket interposed at a joint. This gasket
The characteristics required to maintain the airtightness of the joint surface must be provided under the high temperature, high pressure and high vibration characteristic of the engine, and over a long period of time in which the temperature and pressure changes are repeated. Conventionally, asbestos or the like has been generally used as a material for an engine gasket, particularly from the viewpoint of heat resistance. However, metal gaskets are being used in response to the recent trend toward higher performance engines and non-asbestos regulations. As a material for metal gaskets, work-hardening metastable austenitic stainless steel, for example, SUS301 steel, which can easily obtain high strength by cold rolling, is mainly used. The mode of use is as follows:
A bead is formed in a thin plate of required shape of about 0.4 mm, and this is interposed on the joint surface around the combustion chamber and around water and oil grooves.
Generally, gas, water and oil are sealed by the high surface pressure generated when the bead is tightened.

[0003]

The metal gasket used for the cylinder head of the engine needs a particularly good gas sealing property because it has a high pressure when the engine cycle is compressed. For this reason, the bead forming height must be increased to increase the surface pressure, and the material strength must be sufficiently high. Stainless steel that can handle this is SUS3
Series 01 steel (SUS301, 304, Type301L, etc.).

[0004] However, in order to obtain high strength, it is necessary to perform strong cold working with SUS301 series steel, and as a result, the formability is reduced, and cracks are generated in the bead outside R during bead forming. There was a problem. On the other hand, when the strength is reduced in order to enhance the formability, it is necessary to further increase the bead forming height in order to enhance the sealing performance with a high surface pressure. If the bead forming height is increased at that time, microcracks tend to occur at the bead shoulder R (inner R, outer R), and cracks occur during use in the microcracks as starting points. There was a problem of lowering. The present invention is intended to solve such a problem, and an object of the present invention is to develop a material having excellent characteristics as a metal gasket for an engine.

[0005]

According to the present invention, the weight%
C: 0.03% or less Si; 1.0% or less, Mn: 2.5% or less, Ni: 4.0 to 10.0%, Cr: 13.0 to 20.0%, N: 0.06 to 0.30%, S: 0.01% or less, O: 0.007% And M = 330− (480 × C%) − (2 × Si%) − (10 × Mn%) − (14 × Ni%) − (5.7 × Cr%) − (320 × N% )), The amount of each component is adjusted so that the M value is 30 or more, and the balance is Fe and unavoidable impurities, the stainless steel for engine gaskets having excellent moldability, and this steel, and i. ). At least one of Mo of 3.0% or less or Cu of 0.5-3.0%, ii). At least one of Ti, Nb or V of 0.1-1.0%,
Containing the elements of group i and group ii alone or in combination, and M = 330− (480 × C%) − (2 × Si%) − (10 × Mn%) − (14 × Ni%) − ( 5.7 × Cr%) − (5 × Mo%) − (14 × Cu%) − (320 × N%) Each component amount is adjusted so that the M value is 30 or more, and the remainder is Fe and inevitable Provided is a stainless steel for an engine gasket which is excellent in molding workability and is composed of a metallic impurity.

[0006] Prior to annealing before tempering, these steels are subjected to cold rolling at a reduction ratio of more than 50% so as to reduce the average grain size to 10 µm or less. By adopting a manufacturing method of performing pre-annealing and performing temper rolling, various characteristics required for an engine gasket can be advantageously provided. Further, after temper rolling, even higher strength is exhibited by performing an aging treatment of maintaining the temperature in a temperature range of 300 ° C. to 600 ° C., preferably 350 ° C. to 550 ° C. for 10 seconds or more. This aging treatment may be performed before or after forming into a gasket. Pre-temper annealing is preferably performed in a temperature range of 700 ° C. or more and 1000 ° C. or less.

First, the outline of the reasons for limiting the content ranges of the respective components in the steel of the present invention will be described.

[Detailed Description of the Invention] C is an austenite forming element, and δ formed at high temperature
It is extremely effective in suppressing ferrite and strengthening the martensite phase induced by cold working. However, in order to obtain better formability after temper rolling, if the reinforcement by cold working is too remarkable, the formability becomes poor. If C is too high, carbide precipitation may be accompanied by annealing before tempering or depending on the aging treatment conditions. For this reason, C is set to 0.03% or less.

Although Si is effective as a deoxidizing agent, its effect is the same as in the case of 1.0% even if it is added in an amount of 1.0% or more. The lower limit is not particularly limited, but is preferably 0.2% or more from the viewpoint of the deoxidizing effect.

Mn works effectively as a deoxidizing agent, but is an element that controls the stability of the austenite phase, and its use is considered in balance with other elements. In the steel of the present invention, its utilization can be achieved at an Mn amount of up to 2.5%. However, in the steel of the present invention, high strength and formability are considered important.
In particular, M
It is preferable that the n content is less than 0.5% to minimize the formation of inclusions such as MnS.

[0011] Cr is an essential component for ensuring corrosion resistance. At least 13% or more is required to provide the intended corrosion resistance and heat resistance as an engine gasket. However, since Cr is a ferrite-forming element, if it is too high, a large amount of δ-ferrite will be formed at high temperatures. Therefore, it is necessary to add an austenite-forming element (C, N, Ni, Mn, etc.) in an appropriate amount to suppress the δ ferrite phase, but if a large amount of austenite-forming element is added, the austenite phase at room temperature becomes stable. However, high strength cannot be obtained by cold working or after further aging treatment. For this reason, the upper limit of Cr is set to 20%.

Ni is an essential component for obtaining an austenite phase at high temperature and room temperature. However, in the case of the steel of the present invention, Ni is converted to a metastable austenite phase at room temperature to obtain better formability. It is necessary to induce a moderate martensitic phase so that high strength can be obtained.
In the present invention, when Ni is lower than 4%, a large amount of δ
A ferrite phase is formed, and martensite other than an austenite phase is hardly formed at room temperature. On the other hand, if it exceeds 10%, it becomes difficult to induce a martensite phase in cold working. For this reason, the Ni content is set to 4.0 to 10.0%, more preferably
5.0 to 8.0%. Further, from the viewpoints of durability and heat resistance of the gasket, addition of Ni of 4.0% or more is advantageous. However, the effect becomes saturated even if it exceeds 10%. For this reason, Ni is 4.0-1.0%, preferably 5.0%.
To 8.0%.

Mo increases the base hardness of the steel and increases the hardness after aging treatment, so that Mo effectively acts to obtain high strength. However, since it is a ferrite former, if it is added in large amounts, it will crystallize out the δ-ferrite phase, which may cause a reduction in strength. Therefore, the upper limit is set to 3.0%.

[0014] Cu provides the effect of hardening the steel by interaction with Si during the aging treatment. However, if the content is too small, the effect is small, and if the content is too large, the hot workability is impaired and causes cracking. Therefore, the content is set to 0.5 to 3.0%.

Ti, Nb, and V effectively act to increase the hardness after the aging treatment. To achieve this effect
Requires addition of 0.1% or more. However, if added more than necessary, a large amount of non-metallic inclusions are formed, leading to a decrease in fatigue strength and deterioration of surface properties.
0% or less.

N, like C, is an austenite-forming element and an element effective for hardening the austenite phase and the martensite phase. In addition, since precipitates are less likely to be formed than C, it is effective in terms of durability.
Therefore, at least 0.06% is added instead of C. However, if added in a large amount, it causes blowholes, so the content should be 0.30% or less, more preferably 0.06 to 0.20%.

S generates MnS under coexistence with Mn,
Since this causes a reduction in workability such as ductility and bending, the content is set to 0.010% or less. Note that, depending on the shape of the gasket, Mn and S are preferably lower in a thin plate or in a severely processed area, and the Mn content is preferably less than 0.5% and the S content is preferably 0.004% or less.

O is an element that easily forms non-metallic inclusions that are the starting points of fatigue fracture, and is particularly remarkable when an element having a strong affinity for O, such as Al or Ti, is included. For this reason, O is preferably as low as possible, but up to 0.007% is allowable in the steel of the present invention. Therefore, O is set to 0.007% or less.

M value; C, Si, Mn, Ni, C for 30 or more
r, Mo, Cu and N are contained in the above range.
Each component is adjusted so that the M value according to the following equation (1) is 30 or more. M = 330− (480 × C%) − (2 × Si%) − (10 × Mn%) − (14 × Ni%) − (5.7 × Cr%) − (5 × Mo%) − (14 × Cu%) %) − (320 × N%) (1) The constants of the components in the formula (1) were confirmed in the laboratory during the development of the material of the present invention. This M value is an index of austenite stability. In order to obtain the intended high strength after cold rolling or aging treatment when the M value is less than 30, a strength of 70% or more at room temperature in cold rolling is required. Since it is necessary to perform processing, the ductility of the material is reduced, and the bead formability as an engine gasket is reduced. Therefore, the M value needs to be 30 or more.

Thus, in order to enhance the formability after cold working as much as possible, the components of the steel of the present invention are balanced so that an appropriate amount of a martensite phase generated by cold working is produced by low cold working. Further, after the aging treatment, as high strength as possible can be obtained. In addition, during the low-temperature or short-time recrystallization annealing described later (at the time of fine crystal treatment), C is lowered in order to avoid precipitation of Cr carbides, and the decrease in strength (the decrease in work hardening) accompanying this decrease in C is compensated for by the addition of N. In addition, it is characterized in that the age hardening due to the addition of N and the increase in the age hardening degree due to the fine crystal treatment are effectively utilized, so that higher strength can be developed. From the facts that the α 'phase can be finely and densely distributed even at a low temper reduction ratio due to fine crystallization, and that the addition of N greatly increases the strength due to aging, the temper reduction ratio is kept low. The feature is that the moldability is improved by this.

In addition to these components, Ca or REM (rare earth element) commonly used as a deoxidizing agent or desulfurizing agent, B (having 0.01% or less) having an effect of improving hot workability, etc. are contained as necessary. And can contain impurities that are unavoidably mixed. However, Al is not used when high strength and high fatigue strength are required, or it is desirable to use Al in such an amount that nonmetallic inclusions are not formed in steel.

The steel of the present invention whose component composition is adjusted to the above range substantially exhibits an austenite structure in a solution treatment state. A uniform fine crystal structure can be obtained by applying cold rolling exceeding 50% to the steel in this structure state and performing annealing before tempering at a temperature of 700 ° C to 1000 ° C. When temper rolling is performed in the state of the fine crystal structure, excellent characteristics as an engine gasket material can be obtained. In addition, aging treatment that maintains the temperature in the temperature range of 300 ° C to 600 ° C for 10 seconds or more before or after forming the gasket improves the hardness and thus the strength, further improving the durability as an engine member. Can be done.

Hereinafter, the manufacturing conditions will be described. In a conventional method of manufacturing this type of steel sheet, the crystal grain size is generally about 25 μm. The present inventors have found that the occurrence of microcracks during bead forming of an engine gasket is related to the crystal grain size. As shown in the examples below, during the bead forming after temper rolling, microcracks are generated from the crystal grain boundaries in the bead processed part and the slip band generated due to processing strain, shortening the life as a gasket. ing. According to the present invention, first, by applying a reduction in area of more than 50% in the cold rolling before performing the pre-tempering annealing, a short pre-tempering annealing is performed uniformly as shown in the examples described later. In addition, by obtaining fine crystal grains of 10 μm or less and by increasing the strength level in the pre-temper annealing state, sufficient strength characteristics can be obtained even at a low temper rolling reduction. A material free from rough skin and micro cracks can be obtained. Therefore, the life of the engine gasket can be significantly prolonged. In addition, the increase in strength due to aging treatment after temper rolling is larger than that of conventional materials by adding N and fine crystal treatment. If the same strength is obtained after aging treatment, the strength level after temper rolling can be lowered. It is possible, and it can be made to have excellent moldability.

Here, annealing before temper rolling is 700 ° C. or more and 1000 ° C.
It is better to do the following. This is because it takes a long time to obtain fine crystal grains below 700 ° C and is not industrial.
If the temperature is higher than ℃, recrystallization and grain growth are remarkable, and it is difficult to stably obtain crystal grains of 10 μm or less. This annealing can be carried out on a continuous annealing line on an industrial scale.

The temper rolling reduction is governed by the strength level after annealing before temper rolling and the stability of the austenite phase, and can be variously changed in accordance therewith, but is essentially shown in Examples. As described above, the target strength is achieved at a lower rolling reduction than that of conventional steel, and usually an appropriate value is 20 to 50%. For the aging treatment, a temperature range of 300 ° C. or more and 600 ° C. or less is preferable in order to obtain strength characteristics as an engine gasket. At temperatures below 300 ° C, it takes a long time to achieve the target strength level and it is not economical. At temperatures above 600 ° C, significant recovery progresses before the strength increases, and it is required as a metal gasket. This is because the required strength cannot be obtained. The aging time is 10 seconds or more. If the time is shorter than this, sufficient strength characteristics cannot be obtained. The upper limit of the processing time is preferably about one hour from the viewpoint of manufacturing cost.

As described above, according to the present invention, in addition to adopting the above-mentioned composition, the steel strip is subjected to a cold working process of more than 50% before the tempering pre-annealing, and to 700 ° C. 1000
Recrystallized grain size of 10μm or less in the temperature range of ℃, practically 1
Pre-temper annealing in a continuous annealing furnace under the processing conditions of ~ 5μm and temper rolling to obtain a strength equal to or higher than that of the conventional material, An engine gasket material having excellent workability can be obtained. In addition, since the steel of the present invention exhibits a metastable austenite phase in a solid solution state, the steps before annealing before tempering can be manufactured in the same manner as conventional materials. However, in order to obtain stable fine crystal grains,
It is necessary to perform cold rolling exceeding 50%.

[0027]

EXAMPLES Steels of the present invention (M1 to 10), conventional steels (A) and comparative steels (a, b) having the chemical component values (% by weight) shown in Table 1 were melted in a normal atmospheric melting furnace and heated. After cold rolling,
Annealing and pickling were performed, and a sample was taken with a sheet thickness of 0.25 mm after final temper rolling. Further, the steel sheet was subjected to an aging treatment at 400 ° C. for 30 minutes, and this was used as a sample after the aging treatment.
Table 2 shows the details of the cold rolling rate, pre-tempering annealing conditions, and temper rolling rate of each steel immediately before annealing.

FIG. 1 shows the effect of annealing time on the fine recrystallization characteristics when M1 of the steel of the present invention was used as a test material.
35% cold rolling reduction before annealing (annealing before tempering in the present invention)
The recrystallization characteristics are different between those subjected to (●) and those subjected to 55% (○). Cold rolling rate before annealing is 55
%, The hardness rapidly softened from around 10 minutes, and it was found that the crystals recrystallized sufficiently in 20 minutes. However, 35
% Cold-rolled material requires about 300 minutes to soften, and also partially recrystallized, resulting in a mixed structure containing unrecrystallized parts, making it difficult to obtain a uniform and fine recrystallized structure. That is, in order to easily obtain uniform recrystallized grains in a short time on an industrial production scale, it is recognized that it is necessary to provide sufficient cold working before annealing.

Table 2 summarizes the manufacturing conditions of each steel in Table 1, the crystal grain size, the formability before aging treatment, the tensile properties, the tensile properties after aging treatment, and the fatigue test results. It is shown. ΔTS in Table 2 represents a difference in tensile strength (TS) before and after the aging treatment. It is desirable that the metal gasket material has high strength, and the target value is at least around 170 kg / mm ² in tensile strength after aging treatment.

The moldability F and R in the table indicate that no microcracks occurred in the inner R portion (F) and the outer R portion (R) when each sample was processed into the shape shown in FIG. The samples were evaluated as ○, those with microcracks were evaluated as Δ, and those with cracks were evaluated as X.

In the vibration test, in order to investigate the influence of the surface condition during the forming process on the fatigue characteristics, a test piece having a bead shape formed on a plate in a circular shape (a hole of about 50 mm was made in a flat plate, A bead is formed on the outer circumference of the test piece 1), and as shown in FIGS. 3 (a) and 3 (b), this test piece 1 From the state
After the vibration test in which the tightening was repeated in the state of (b) 1 million times was performed, it was examined whether or not the through crack occurred.

[0032]

[Table 1]

[0033]

[Table 2]

[0034]

The following can be seen from the results in Table 2. Each of the fine crystal-treated materials of the invention steels (M1 to 10) according to the present invention does not generate microcracks or cracks during forming and has sufficient tensile strength after aging treatment. However, even in the case of the steel of the present invention (M1), in order to obtain a high strength after aging treatment as shown as a comparative material in the conventional method with the reduction ratio before tempering and the annealing condition before tempering, the tempering rolling before aging treatment is required. It is necessary to increase the rate to about 60%, so that microcracks occur at both the inner and outer R portions during molding. In addition, the temper rolling rate is 45% to enhance the formability.
When it is lowered to a lower level, the outer R portion can be formed without cracking, but microcracks occur in the inner R portion (F). Therefore, sufficient characteristics as a metal gasket cannot be obtained. Furthermore, the steel of the present invention (M3) also shows a low rolling reduction in the pre-temper annealing, but in this case, even if the fine crystal treatment was applied, the grains became mixed and the inner R
In (F), fine microcracks are generated. The comparative steel (a) has a low N value. However, since C and N are low, work hardening is small, and to obtain high strength after aging treatment,
It is necessary to increase the temper rolling rate before the aging treatment. For this reason, cracks occur at the outer R portion. The comparative steel (b) has a low M value specified in the present invention and is out of the range specified in the present invention. However, since the work hardening is small, it is difficult to obtain high strength after aging treatment. ) With similar results. In addition, if the conventional steel (A) also attempts to obtain a tensile strength of about 170 kg / mm ² after the aging treatment, microcracks and cracks occur in both the inner and outer R portions after forming before the aging treatment. Further, even if the temper rolling reduction is reduced so that the tensile strength after aging treatment is about 160 kg / mm ² , (F) microcracks occur in the inner R portion because the crystal grain size is large.

4 to 6 are scanning electron micrographs of the surface of the R portion after the forming process. FIG. 4 shows the inner R portion (FIG. 4) when the steel (M1) of the present invention was manufactured by the conventional method. F), and many microcracks are observed. FIG.
Is the inner R when steel of the present invention (M1) is manufactured by the method of the present invention.
In part (F), generation of microcracks is not recognized. FIG. 6 shows the outer R portion (R) of the comparative steel (b), in which a large crack is generated due to a high cold rolling reduction.

It is apparent from the results of the vibration test that the surface condition at the time of these forming processes affects the fatigue characteristics. While none of the steels according to the present invention cracked even after one million vibration tests, the ones with microcracks or cracks at the inner R and outer R parts, such as the comparative material, conventional steel and comparative steel, did not In each case, penetration cracks occurred, and it was recognized that these greatly affected the fatigue life.

As described above, the steel of the present invention has a large increase in strength due to aging as compared with the conventional metal gasket material SUS301, so that the strength before aging treatment can be reduced and the formability is excellent. In particular, when the manufacturing method according to the present invention is adopted, remarkable characteristics in molding workability can be obtained, and the life when used as a metal gasket can be significantly improved. In addition, the manufacturing method itself is not costly, so that it can be manufactured economically and advantageously.

[0038]

[Brief description of the drawings]

FIG. 1 shows the relationship between the annealing time at 700 ° C., the hardness, and the grain size of the materials having the rolling reductions before annealing (temper annealing) of 35% and 55% of the M1 steel shown in the examples. FIG.

FIG. 2 is a cross-sectional view showing the shape of a test piece used for a test of moldability.

Fig. 3 Before (a) and under load (b) in the vibration test
It is a schematic sectional drawing which shows the test state of.

FIG. 4 is a photomicrograph of the surface of a molded metal test piece at the R portion.

FIG. 5 is a photomicrograph showing the surface of a molded metal test piece at an R portion.

FIG. 6 is a photomicrograph of the surface of a molded metal test piece at the R portion.

[Explanation of symbols]

 1 metal test piece 2 load fluctuation flange

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Toshihiko Takemoto 4976 Nomuraminamicho, Shinnanyo-shi, Yamaguchi Prefecture Inside Nippon Steel Research Institute (72) Inventor Shigeto Hayashi 4976 Nomuraminamicho, Shinnanyo-shi, Yamaguchi Japan New Steel Works, Ltd.Steel Research Laboratories (72) Inventor Hideki Tanaka 4976 Nomura Minamicho, Shinnanyo-shi, Yamaguchi Prefecture Nissin Steel Works, Ltd.Steel Research Laboratories (72) Inventor Shibata Shinji 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Motor (72) Inventor Chiyoshi Toshima Maeda 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Motor Corporation (56) References JP-A-63-206429 (JP, A) JP-A-1-92342 (JP) , A) (58) Field surveyed (Int. Cl. ⁷ , DB name) C22C 38/00-38/40 C21D 8/02

Claims (7)

(57) [Claims] 1. In weight%, C: 0.03% or less Si; 1.0% or less, Mn: 2.5% or less, Ni: 4.0 to 10.0%, Cr: 13.0 to 20.0%, N: 0.06 to 0.30%, S; 0.01 %; O; 0.007% or less, and M = 330− (480 × C%) − (2 × Si%) − (10 × Mn%) − (14 × Ni%) − (5.7 × Cr%) )-(320 × N%) A stainless steel for engine gaskets, whose components are adjusted so that the M value in accordance with the equation of (320 × N%) is 30 or more, and whose balance is Fe and unavoidable impurities and has excellent moldability. 2. In% by weight, C: 0.03% or less Si; 1.0% or less, Mn: 2.5% or less, Ni: 4.0 to 10.0%, Cr: 13.0 to 20.0%, N: 0.06 to 0.30%, S; 0.01 %, O; 0.007% or less; and i). At least one of Mo of 3.0% or less or Cu of 0.5 to 3.0%, ii). At least one of Ti, Nb or V of 0.1 to 1.0%. seed,
And M = 330− (480 × C%) − (2 × Si%) − (10 × Mn%) − (14 × Ni%) − ( 5.7 × Cr%) − (5 × Mo%) − (14 × Cu%) − (320 × N%) Each component amount is adjusted so that the M value is 30 or more, and the remainder is Fe and inevitable Stainless steel for engine gaskets, which is excellent in moldability and is composed of chemical impurities. 3. Mn: 0.5% or less, S: 0.004% or less,
The stainless steel according to claim 1 or 2, wherein N is 0.06 to 0.20%. 4. In% by weight, C: 0.03% or less Si; 1.0% or less, Mn: 2.5% or less, Ni: 4.0 to 10.0%, Cr: 13.0 to 20.0%, N: 0.06 to 0.30%, S; 0.01 %; O; 0.007% or less, and M = 330− (480 × C%) − (2 × Si%) − (10 × Mn%) − (14 × Ni%) − (5.7 × Cr%) ) − (320 × N%) The content of each component is adjusted so that the M value according to the formula of (320 × N%) becomes 30 or more, and the balance of stainless steel containing Fe and unavoidable impurities is reduced by the surface reduction rate before annealing before tempering. A stainless steel sheet for engine gaskets with excellent formability characterized by cold rolling exceeding 50%, pre-annealing so that the average crystal grain size is 10 μm or less, and then temper rolling. Manufacturing method. 5. In% by weight, C: 0.03% or less Si; 1.0% or less, Mn: 2.5% or less, Ni: 4.0 to 10.0%, Cr: 13.0 to 20.0%, N: 0.06 to 0.30%, S; 0.01 %, O; 0.007% or less; and i). At least one of Mo of 3.0% or less or Cu of 0.5 to 3.0%, ii). At least one of Ti, Nb or V of 0.1 to 1.0%. seed,
And M = 330− (480 × C%) − (2 × Si%) − (10 × Mn%) − (14 × Ni%) − ( 5.7 × Cr%) − (5 × Mo%) − (14 × Cu%) − (320 × N%) Each component amount is adjusted so that the M value is 30 or more, and the remainder is Fe and inevitable Stainless steel consisting of chemical impurities is subjected to cold rolling with a reduction in area of more than 50% prior to annealing before tempering, annealing before tempering so that the average crystal grain size is 10 μm or less, and then temper rolling. A method for producing a stainless steel sheet for an engine gasket, which is excellent in formability, characterized by applying a method. 6. The method according to claim 4, wherein the pre-temper annealing is performed in a temperature range of 700 ° C. or more and 1000 ° C. or less. 7. The temper-rolled steel is at least 300 ° C. and 600 ° C.
The method according to claim 4, 5 or 6, wherein an aging treatment is carried out for 10 seconds or more in the following temperature range.