JP5943061B2 - Surface-treated steel sheet for heat treatment, method for producing heat-treated steel, and heat-treated steel - Google Patents

Description

  The present invention relates to a surface-treated steel sheet suitable for applications in which heat treatment such as quenching is performed, a method for producing a heat-treated steel material, and a heat-treated steel material.

  In recent years, in automobiles, in order to further improve fuel economy performance and collision safety performance by reducing the weight of the vehicle body due to environmental regulations, weight reduction by increasing the strength of automotive steel materials has been promoted. In general, the workability of steel materials tends to decrease as the strength increases. Therefore, in order to increase the strength of members such as automobile chassis and undercarriage parts, using high-strength steel sheets as their materials significantly limits the degree of freedom in designing these members, or for press processing. It may be necessary to introduce high-spec processing machines.

On the other hand, there is a technique for manufacturing a high-strength member by processing a steel material into a desired shape and then quenching. In addition, recently, the steel material is heated and hot-worked in a temperature range of Ac ₃ points or higher, and rapidly cooled (quenched) almost simultaneously with the hot-working or immediately after the hot-working. A technique for producing a heat-treated steel material (in the case where the hot working is press forming is generally called hot pressing (forming), hot pressing or hot stamping) is becoming widespread. In this hot press, a technique of using a zinc-based plated steel material (mainly a steel plate) as a steel material before heating has been developed for the purpose of suppressing scale generation due to heating and improving the corrosion resistance of members.

  Such heat-treated steel materials, particularly in the aforementioned automotive applications, are electrodeposited through chemical conversion treatment such as zinc phosphate treatment after being assembled on the vehicle body, for example, by spot welding. Alternatively, intermediate coating and top coating are performed. At this time, in the heat-treated steel material obtained by hot pressing a zinc-based plated steel sheet, when subjected to a cyclic corrosion test, wrinkled bulges are easily generated in the coating film.

For example, Patent Documents 1 to 3 disclose inventions that improve coating film adhesion and post-coating corrosion resistance of a heat-treated steel material obtained by hot pressing a zinc-based plated steel sheet.
In Patent Document 1, when a thick zinc oxide layer is formed on the surface of a heat-treated steel obtained by hot pressing a galvanized steel sheet, the coating film adhesion and the corrosion resistance after coating are adversely affected. An invention is disclosed in which the zinc oxide layer is removed by shot blasting or the thickness of the zinc oxide layer is reduced before coating.

  According to the invention disclosed in Patent Document 1, it is possible to improve the coating film adhesion and the post-coating corrosion resistance of the heat-treated steel, but in order to carry out the present invention, a shot blasting process and equipment therefor However, depending on the shape and part of the heat-treated steel (for example, the inner surface of the tubular member), it may be difficult to perform shot blasting.

  Patent Document 2 discloses an invention in which a hot-dip galvanized steel sheet whose surface is coated with a silicone resin film is used as a steel sheet for hot pressing, and Patent Document 3 discloses a barrier layer containing P and Si (P An invention is disclosed in which a hot-dip galvanized steel sheet coated with a phosphate is exemplified as Si and colloidal silica is exemplified as Si as a steel sheet for hot pressing.

  According to the inventions disclosed in Patent Documents 2 and 3, since the zinc plating layer is covered with the above-described layer, the evaporation of zinc is suppressed, whereby the adhesion and coating of the intermediate coating film and the top coating film are applied. It is said that post-corrosion resistance is good.

JP 2004-323897 A JP 2007-63578 A JP 2007-291508 A

  However, when the present inventors re-examined a heat-treated steel material obtained by using a hot-dip galvanized steel sheet coated with a silicone resin film as a steel sheet for hot pressing, disclosed in Patent Document 2, as will be described later. In addition, it was found that the corrosion resistance after coating in the cyclic corrosion test in which the wet and dry environment is repeated is good, but the corrosion resistance is not necessarily good in an environment immersed in salt water.

  For this reason, the heat-treated steel material obtained by the invention disclosed in Patent Document 2 is used, for example, in a part or member where water is likely to accumulate due to its structure (for example, a bag-like structure part under the door, a closed cross-section member in the engine compartment, etc.) It is not suitable for use as it is, and in order to apply this heat-treated steel material to these parts and members, it is necessary to devise a design so that water does not accumulate.

The present invention includes a zinc-based plating layer on the element areas steel, 0 further thereon, which contains a particulate silica, which is formed from the treatment liquid further contains a silane coupling agent, a silica coating weight. A surface-treated steel sheet for heat treatment, comprising a surface-treated layer containing 24-0.47 g / m ² .
In the surface-treated steel sheet for heat treatment according to the present invention, the particle size of the granular silica is preferably 10 nm or more and 500 nm or less, and more preferably 10 nm or more and 50 nm or less.

  In addition, the present invention provides a surface-treated steel sheet for heat treatment according to the present invention, (i) hot-worked in a predetermined temperature range and formed into a predetermined shape, and simultaneously with hot-working or after hot-working, A heat-treated steel material is produced by heat treatment, or (ii) a heat-treated steel material is produced by heat-treating the formed member at a predetermined temperature after forming the formed member into a predetermined shape. It is a manufacturing method of the heat-treated steel materials made into.

  According to the present invention, a heat-treated surface-treated steel sheet for obtaining a heat-treated steel material having good corrosion resistance not only in an environment where it is repeatedly dried and wet but also in an environment where it is immersed in salt water, and a heat-treated steel material using this heat-treated steel sheet for heat treatment A manufacturing method and the heat-treated steel material obtained by this can be provided.

FIG. 1 is a surface SEM image (BSE image) and a cross-sectional SEM image in the vicinity of the surface of the heat-treated steel material in the examples.

Hereinafter, modes for carrying out the present invention will be described.
1. Heat-treating the surface-treated steel sheet according to the heat treatment for the surface treated steel sheet present invention includes a zinc-based plating layer on the base steel sheet, further thereon, the particulate silica of 60% non-volatile content (especially otherwise indicated herein Unless otherwise indicated, “%” means “mass%”) . It is provided with a surface treatment layer having a deposition amount of 0.4 to ² g / m ² formed from a treatment liquid containing the above.

(1) Base steel plate The chemical composition of the base steel plate is not particularly limited, but is preferably a chemical composition capable of obtaining high strength by quenching. For example, when obtaining a heat-treated steel material having a tensile strength of 980 Mpa, the base steel plate is C: 0.1% to 0.3%, Si: 0.01% to 0.5%, Mn: 0.5% to 3.0%, P: 0.003% to 0.05%, S: 0.05% or less, Cr: 0.1% to 0.5%, Ti: 0.01 %: 0.1% or less, Al: 1% or less, B: 0.0002% or more and 0.004% or less, N: 0.01% or less, and the balance consisting of Fe and impurities. For quenching having a chemical composition containing one or more selected from Cu: 1% or less, Ni: 2% or less, Mo: 1% or less, V: 1% or less, and Nb: 1% or less An example is made of steel.

(2) Zinc-based plating The zinc-based plating layer formed on the base steel plate may be pure zinc plating or alloy plating. The plating method may be any method such as electroplating or hot dipping. Preferably, an alloyed hot-dip galvanized steel sheet is better than a pure zinc-based hot-dip galvanized steel sheet (GI) if it is a hot-dip plating method so that the evaporation and flow of zinc due to heating can be reduced. Is preferred, and electroplating may be used.

(3) Surface treatment layer The surface treatment layer formed from the process liquid containing a granular silica is further provided on a zinc plating layer.

  Here, “granular silica” is not present in a state of being dissolved in the treatment liquid such as a silane coupling agent, but is dispersed in the treatment liquid as a solid having a primary particle size of several nm or more. Means a substance mainly composed of an oxide of Si. By using silica that is dispersed in the treatment liquid in such a solid state, compared to the case where the Si component is supplied in a dissolved state such as a silane coupling agent, the durability in an environment immersed in salt water is improved. An excellent heat-treated steel material can be provided.

  Specifically, the particle size of the granular silica is preferably 10 nm or more and 500 nm or less. From the viewpoint of corrosion resistance after coating, it is advantageous that the silica particle size is smaller, but those having a particle size of less than 10 nm are difficult to obtain and disadvantageous in terms of cost. The preferred particle size of silica is 10 nm or more and 30 nm or less.

The surface treatment layer provided in the heat-treated surface-treated steel sheet according to the present invention is formed from a treatment liquid containing 60% or more of granular silica with respect to the total nonvolatile content, and the adhesion amount of this surface treatment layer is 0.4 g / m ² or more and 2 g / m ² or less. That is, the surface treatment layer according to the present invention has an adhesion amount of 0.4 g / m ² or more and 2 g / m ² or less, and the content of the silica component based on granular silica in the surface treatment layer is 60% or more. is there.

If the adhesion amount of the surface treatment layer is less than 0.4 g / m ² , the growth of the zinc oxide layer cannot be sufficiently suppressed, and the performance of the surface-treated steel material after coating may be adversely affected. On the other hand, when the adhesion amount of the surface treatment layer exceeds ² g / m ² , the cost increases, the cohesive force of the surface treatment layer becomes weak, and the coating film formed thereon is likely to be peeled off. Accordingly, the adhesion amount of the surface treatment layer is set to 0.4 g / ^{m 2} or more 2 g / ^{m 2} or less.

  Colloidal silica is typically exemplified as the treatment liquid containing granular silica, and a specific commercial product is exemplified by SNOWTEX (registered trademark) series manufactured by Nissan Chemical Co., Ltd.

  For the formation of the surface treatment layer, colloidal silica may be applied as it is on the galvanized steel sheet, but in order to improve the stability of the treatment liquid and the adhesion of the surface treatment layer, a resin or a silane coupling agent may be used. It is preferable to use a mixed processing solution. Moreover, unless the effect of this invention based on containing a granular silica is inhibited, a process liquid may contain pigments, such as carbon black, and microparticles | fine-particles, such as an alumina and a zirconia, for example. In this case, the granular silica is 60% or more of the nonvolatile content (that is, in the surface treatment layer) in the treatment liquid as described above. In view of the recent trend in which consideration for environmental protection is strongly demanded, the surface treatment layer according to the present invention does not contain hexavalent chromium, and of course, does not contain chromium content including trivalent chromium. Chromium free is preferred.

The surface treatment layer may be formed by applying a treatment liquid containing granular silica to the surface of the galvanized steel sheet, followed by drying and baking.
The coating method need not be limited to a specific method, either by immersing the base steel sheet in the processing liquid, or spraying the processing liquid on the surface of the base steel sheet and then attaching it by a roll or gas spray so that a predetermined amount of adhesion is achieved. Examples thereof include a method of controlling the amount and a method of applying with a roll coater or a bar coater.

  The drying and baking method may be any method that can volatilize the dispersion medium (mainly water), and is not limited to a specific method. When heated at an excessively high temperature, there is a concern that the uniformity of the surface treatment layer is lowered, and conversely, when heated at an excessively low temperature, there is a concern about a decrease in productivity. Therefore, in order to stably and efficiently produce the heat-treated surface-treated steel sheet according to the present invention having excellent characteristics, it is preferable to heat at a temperature of about 100 ° C. for about 10 seconds.

  In addition, it is economical and preferable that the surface treatment layer is formed in-line in the production line of the plated steel sheet, but it may be formed in a separate line or formed after blanking for forming. May be.

2. Next, a method for producing a heat-treated steel using the surface-treated steel sheet for heat treatment according to the present invention will be described. Since the manufacturing method according to the present invention is typically aimed at increasing the strength of heat-treated steel by quenching, in the following description, the case where the heat treatment is quenching is taken as an example, but the present invention is limited to quenching. Instead, the heat treatment may be post-heating removal such as annealing.

The method for producing the heat-treated steel according to the present invention is as follows:
(A) A method of cold-treating (or warm-working) a surface-treated steel sheet for heat treatment according to the present invention to a member having a predetermined shape, and then heating and quenching to a predetermined temperature of _three or more points of Ac (B) the present invention Including any method of hot working the surface-treated steel sheet for heat treatment in a predetermined temperature range, quenching at the same time as the hot working or immediately after the hot working,
(C) A method of hot forming a surface-treated steel sheet for heat treatment according to the present invention to a shape in the middle of the final shape and then hot forming the processed to the final shape (D) For heat treatment according to the present invention It also includes a method of cold-working the surface-treated steel sheet into a tubular shape to obtain a steel pipe, bending the steel pipe hot, and quenching immediately after the processing.

  In the heating in these methods, when it is desired to increase the strength of the entire heat-treated steel, the target member or a blank made of surface-treated steel for heat treatment before forming may be heated, and the heat-treated steel may be partially increased. In order to increase the strength, it is only necessary to partially heat a target member or a blank made of surface-treated steel for heat treatment before forming by induction heating, electron beam, or the like.

  The atmosphere during heating is not particularly limited, and for example, heating may be performed in the air.

3. Heat-treated steel material The heat-treated steel material according to the present invention thus obtained is subjected to chemical conversion treatment such as water washing and phosphate treatment, followed by electrodeposition coating and baking. Coating and baking, or intermediate coating and baking and top coating and baking are performed.

The heat-treated steel material according to the present invention has good corrosion resistance after painting.
In addition, when observing the appearance after the zinc phosphate treatment of the heat-treated steel material obtained by the production method according to the present invention, a phenomenon in which the surface of the heat-treated steel material is partially attached with chemical conversion crystals, so-called skeins. As a result, chemical conversion crystals may not always be formed uniformly. However, even such a heat treated steel material has good corrosion resistance after coating. The reason for this is not necessarily clear, but is estimated as follows.

  When zinc oxide grows thick on the surface of the heat-treated steel by heat treatment, the zinc oxide layer itself is considered to be brittle or porous, and in a corrosive environment, moisture or the like enters and corrosion under the coating progresses. It is considered that the film is easily peeled off.

  On the other hand, when the steel sheet for heat treatment of the present invention having a surface treatment layer containing a silica component based on granular silica is used, the surface treatment layer suppresses the growth of zinc oxide on the surface of the heat treatment steel material when heated. Therefore, it is considered that the occurrence of the above situation can be advantageously suppressed.

  Furthermore, the surface treatment layer after the heat treatment has fine defects, and zinc oxide derived from the zinc oxide film below the surface treatment layer partially grows and appears on the surface of the surface treatment layer through the defects. . This fine defect portion is considered to be caused by a film defect due to a difference in thermal expansion coefficient of each film layer, cracks that are supposed to be caused by curing or heat shrinkage, or combustion of a resin component in the film. Zinc oxide has relatively good reactivity with zinc phosphate solution, so even if the surface of the heat-treated steel is not uniform, the adhesion of the coating film is coupled with the anchor effect due to cracks. Is considered to be good.

  As described above, the surface-treated steel material according to the present invention is formed with a coating mainly containing Si oxide and having fine cracks, and zinc oxide appears on the surface through the cracks.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited by the following examples, and appropriate modifications are made within a range that can conform to the gist described in the present specification. In addition, it is also possible to carry out, and they are all included in the technical scope of the present invention.

(I) Production of surface-treated steel sheet for heat treatment As a steel sheet for hot pressing, C: 0.2%, Si: 0.3%, Mn: 1.3%, P: 0.01%, S: 0.002 %, Sol. Alloyed hot-dip galvanized steel sheet with a base steel sheet of a cold-rolled steel sheet having a chemical composition comprising Al: 0.044%, Cr: 0.2%, the balance Fe and impurities and a thickness of 1.6 mm (plating adhesion) Amount: 50 g / m ² per side, Fe content in the plating layer: 12%) One surface (this surface is the evaluation surface) was applied with the treatment liquid shown in Table 1 with a bar coater, and the maximum reached A surface-treated steel sheet for heat treatment was produced by drying using an oven under conditions such that the temperature was maintained at 100 ° C. for 8 seconds. The adhesion amount of the treatment liquid was adjusted by the count of the bar coater so that the total adhesion amount of the non-volatile content in the treatment liquid became a numerical value shown in Table 1.

In addition, the numerical value displayed in the column of “content of silica etc.” in Table 1 is the ratio (unit: mass%) of the nonvolatile content of “(i) silica etc.” below to the total nonvolatile content in the treatment liquid.
Each component (symbol) in Table 1 is as follows. As will be described later, a treatment liquid containing alumina (sol) as a particulate material other than silica was also examined. In this case, alumina was regarded as “(i) silica or the like”.

(I) Silica and the like: Silica / alumina ST-C: Colloidal silica (Nissan Chemical Co., Ltd. Snowtex (registered trademark) C), particle size 10 to 20 nm (catalog value)
ST-S: Colloidal silica (Nissan Chemical Co., Ltd. Snowtex (registered trademark) S), particle size 8-11 nm (catalog value)
ST-20L: Colloidal silica (Nissan Chemical Co., Ltd. Snowtex (registered trademark) 20L), particle size 40-50 nm (catalog value) and ST-O: Colloidal silica (Nissan Chemical Co., Ltd. Snowtex (registered trademark) O) ), Particle size 10-20nm (catalog value)
ST-OS: Colloidal silica (Nissan Chemical Co., Ltd. Snowtex (registered trademark) OXS), particle size 8-11 nm (catalog value)
ST-OL: Colloidal silica (Nissan Chemical Co., Ltd. Snowtex (registered trademark) OXL), particle size 40-50 nm (catalog value)
Alumina sol 200: Alumina sol (Nissan Chemical Corporation Alumina Sol 200)
(Ii) Carbon Black: Mitsubishi Chemical Corporation Mitsubishi (registered trademark) carbon black # 1000
(Iii) Silane coupling agent: Chisso Corporation Silaace S510
(Iv) Resin A: Urethane resin emulsion (Daiichi Kogyo Seiyaku Co., Ltd. Superflex (registered trademark) 150)
B: Silicone resin (Shin-Etsu Chemical Co., Ltd. Straight Silicone Varnish KR282)
(V) Chromic acid: CrO ₃ (industrial reagent)
In the treatment liquid, it is partially reduced by a reducing agent (ethylene glycol).

(II) Heat treatment method The above-mentioned surface-treated steel sheet for heat treatment was held for 4 minutes in a gas furnace set at 900 ° C. in an oxidizing atmosphere (air-fuel ratio 1.1), so that the temperature of the steel plate reached approximately 900 ° C. Thereafter, the steel sheet was taken out into the atmosphere, immediately sandwiched between water-cooled molds, and rapidly cooled to produce a heat-treated steel material.

  With respect to the heat-treated steel materials thus obtained, the surface resistance was measured and the coating film adhesion and the corrosion resistance after coating were investigated by the methods listed below. As a reference example, the same investigation was performed without heating the galvannealed steel sheet (symbol: GA in Table 2).

(I) Zinc oxide film layer Using an X-ray diffractometer, when the heat-treated steel material was X-ray diffracted from the surface, the peak intensity of the ZnO peak (in the case of CoKα rays, 2θ = near 37 °) was measured and counted. ZnO strength is less than 25%, ◯ when 25% or more and less than 50%, and △ when 50% or more and less than 90% of the reference material, X was defined as equivalent to or higher than that of the reference material.

(Ii) Coating film adhesion, post-coating corrosion resistance The heat treated steel material was degreased and surface-adjusted under the conditions described below, followed by zinc phosphate treatment and electrodeposition coating in this order under the following conditions.
-Degreasing conditions Fine-cleaner 4380 (made by Nihon Parkerizing) 200g / L liquid (50 degreeC) is immersed for 2 minutes, and is washed with water.
-Surface adjustment conditions Immerse in Percolen Z (manufactured by Nihon Parkerizing) 1 g / L liquid (room temperature) for 10 seconds.
-Chemical conversion treatment PB-L3080 (manufactured by Nihon Parkerizing) Spray at a liquid temperature of 43 ° C for 2 minutes.
・ Electrodeposition paint: Epoxy resin-based cationic electrodeposition paint Voltage: 200V
Baking conditions: Bake in an oven at 170 ° C. for 20 minutes.

Coating thickness: 15 μm
The following three types of tests were conducted to examine the adhesion and the post-coating corrosion resistance of the samples applied up to electrodeposition coating.

(Ii-1) Coating film adhesion; Primary adhesion A grid pattern is given to the coating film on the evaluation surface, a tape (CT405A-24) manufactured by Nichiban Co., Ltd. is applied and peeled off, and coating film peeling is not recognized at all. Was evaluated as x (defect).

(Ii-2) Corrosion resistance after coating 1; warm salt water immersion test A sample in which the cut surface was cut with a cutter knife (load 500 gf) was painted for 240 hours in a 5% NaCl aqueous solution (50 ° C.). Thereafter, the sample was taken out, and a tape (CT405A-24) manufactured by Nichiban Co., Ltd. was applied to the cut and peeled off, and the width of the coating film peeled from the cut part was measured. The peel width of less than 1 mm was evaluated as ◯ (excellent), 1 mm or more and less than 2 mm was evaluated as Δ (good), and 2 mm or more was evaluated as x (defective).

  In parallel, a sample whose end face was sealed with a polyester tape was immersed in warm water (salt water) under the same conditions as described above. Thereafter, a sample was taken out, and in the same manner as in (ii-1), a cross-cut was applied to the coating of the evaluation surface, and the peeling state was investigated.

(Ii-3) Corrosion resistance after coating 2; cycle corrosion test The coating on the evaluation surface was cut with a cutter knife (load 500 gf), and a cycle corrosion test 180 cycles under the following cycle conditions was performed.

[Cycle conditions]
SST (5% NaCl, 35 ° C. atmosphere) 2 hr → dry (60 ° C.) 2 hr → wet (50 ° C., 98% RH) 4 hr was carried out as one cycle.

Thereafter, the presence or absence of ridge-like blisters (photographed) generated in an area of about 1 cm width from the cut portion was observed.
The results are summarized in Table 2.

As shown in Table 2, the samples according to the present invention (reference numerals Si1 to Si10) had good primary adhesion and post-coating corrosion resistance, and exhibited performance close to or equivalent to GA without heating.
On the other hand, those containing alumina instead of silica (reference symbol Al) and those subjected to chromate treatment not containing silica (reference symbols Cr1 to 3) did not have good post-coating corrosion resistance.

The one coated with a urethane-based resin not containing silica (symbol U) had good performance in the hot salt water immersion test, but blisters were generated in the cycle corrosion test.
The chromate-treated one containing silica (reference numeral Cr-Si1-3) and the one coated with silicone resin (reference numeral S) had good post-coating corrosion resistance in the cyclic corrosion test, which was a weak point of GA. The performance in the salt water immersion test was unsatisfactory.

Furthermore, about some heat-treated steel materials, the structure of the steel material surface vicinity was investigated. FIG. 1 shows a surface SEM image (BSE image) and a cross-sectional SEM image near the surface of these heat-treated steel materials.
When viewed from the cross section, the surface of the heat-treated steel material Si3 according to the present invention had a two-layer structure of lower layer: zinc oxide coating-upper layer: silicon-containing coating.

  Furthermore, when this was observed from the surface, the Si-containing film had a large number of cracks, and a lower zinc oxide film was observed from the cracks. That is, in the surface treatment layer, many fine cracks that are supposed to be caused by hardening or heat shrinkage are formed, and through these fine cracks, zinc oxide derived from the zinc oxide film below the surface treatment layer is partially It grew and appeared on the surface of the surface treatment layer.

  In addition, the surface of the heat-treated steel material Si10 according to the present invention had a zinc oxide layer exposed on a surface. The details of this mechanism are unknown, but it is thought that zinc oxide was ejected from the cracks in the Si-containing coating or that there were fine surface defects rather than cracks in the Si-containing coating itself.

  Such cracks and fine surface defects in the Si-containing coating, that is, fine defects, existed in the heat treated steel materials Si1, 2, 4 to 9 other than the heat treated steel materials Si3 and 10 described above.

  On the other hand, in the heat-treated steel material S, the silicon-containing film considered to be derived from the silicone resin film before the heat treatment covered the entire surface. On the other hand, in heat-treated steel materials C, U, and GA, a thick zinc oxide film was formed on the surface.

Claims (5)

A zinc-based plating layer is provided on the base steel plate, and further, a granular silica is further formed thereon, and further formed from a treatment liquid containing a silane coupling agent, 0.24 to 0.47 g as a silica adhesion amount. A surface-treated steel sheet for heat treatment, comprising a surface treatment layer containing / m ² .   A surface-treated steel sheet for heat treatment, wherein the particle size of the granular silica according to claim 1 is 10 nm or more and 500 nm or less.   A surface-treated steel sheet for heat treatment, wherein the particle size of the granular silica according to claim 1 is 10 nm or more and 50 nm or less.   The heat-treated surface-treated steel sheet according to claim 1 is hot-worked in a predetermined temperature range to be formed into a predetermined shape, and is heat-treated simultaneously with or after the hot-working. A method for producing heat-treated steel. A method for producing a heat-treated steel material, comprising: forming a surface-treated steel sheet for heat treatment according to claim 1 into a predetermined shape to form a formed member, and then heat-treating the formed member in a predetermined temperature range .