JP2003126920A - Hot press processing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot press processing method which improves corrosion resistance and painting adhesion of press processing products without deterioration of surface condition. SOLUTION: Prior to hot press processing on plated steels, the plated steel is maintained at more than 400 deg.C and at a temperature below a transformation point Ac3 for 5 to 1,000 seconds.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot press working method, and more particularly to a hot press working method for parts used for manufacturing undercarriage, chassis, reinforcing parts and the like for automobiles.

[0002]

2. Description of the Related Art In recent years, in order to reduce the weight of automobiles, efforts have been made to increase the strength of steel materials and reduce the thickness of the steel materials used. However, when press forming a steel sheet as a steel material, for example, when drawing is considered, as the strength of the steel sheet used increases, the contact pressure with the mold increases during draw forming, causing galling of the steel sheet and rupture of the steel sheet. In order to reduce such problems as much as possible, there is a problem that when the blank holding pressure is lowered in order to increase the flow of material into the mold during drawing of a steel sheet, the shape after forming varies.

Shape stability, so-called springback, also occurs, and there are countermeasures against this, for example, by using a lubricant, but the effect is small for high-strength steel sheets of 780 MPa class or higher.

As described above, there are many problems in the press forming of high strength steel as a difficult-to-machine material. In addition, hereinafter, this kind of material is referred to as "difficult press molding material".

[0005]

By the way, as a technique for press-molding such a difficult press-molding material, a method of heating the material to be molded in advance and molding it is conceivable. These are so-called hot press molding and warm press molding. Hereinafter, they are simply referred to as hot press molding.

However, since hot press forming is a forming method for working a heated steel sheet, surface oxidation is unavoidable, and even if the steel sheet is heated in a non-oxidizing atmosphere, for example, press forming from a heating furnace is performed. Because of this, iron oxide is formed on the surface if it is exposed to the air when taken out. There is a problem that the iron oxide is dropped during the pressing and adheres to the mold to lower the productivity, or such an oxide film remains on the product after pressing and the appearance is deteriorated. Moreover, if such an oxide film remains, the adhesion of the coating film to the steel sheet will be poor when coating in the next step. Further, when the scale remains, the adhesion of the coating film is poor due to the poor adhesion between the scale and the steel sheet even if the scale is applied in the next step.

Therefore, after hot press forming, it is necessary to perform shot blasting to remove the iron oxide layer forming such a scale, but this inevitably increases the cost.

Further, even if a low alloy steel or stainless steel is used to prevent the formation of such a scale during heating, the scale generation cannot be completely prevented, and the cost is much higher than that of ordinary steel.

In order to solve such a problem, Japanese Patent Laid-Open No. 2000-
Japanese Patent No. 38640 proposes a steel sheet coated with aluminum in order to have the oxidation resistance of the base steel sheet during hot forming, but such a steel sheet also causes a significant cost increase when compared with ordinary steel. Become.

As a countermeasure against such a problem of surface oxidation during hot press forming, it is theoretically effective to make both the atmosphere during heating and the atmosphere of the entire pressing process non-oxidizing, but it is considerably high in terms of equipment. It will be a cost.

Under these circumstances, many proposals have been made for hot pressing even today, but at present, they have not reached the practical stage. An overview of the current technology proposed as a patent application is as follows.

[0012] For example, as an advantage of hot pressing, it is possible to perform heat treatment together with press molding, and at the same time, surface treatment is also performed at the same time.
-116900 gazette. Of course, such a technique also has the problem of surface oxidation as described above, but it is difficult to evenly apply a surface treatment agent such as a rust preventive agent to a mold having a complicated shape. It is also difficult to uniformly transfer the surface treatment agent previously applied to the product to the product during press molding. Of course, it is obvious that a rust preventive treatment such as a plating treatment is individually performed as a treatment after the press molding, but it is obvious that such a method has a low productivity and causes a large cost increase.

There is a method of hot press forming in order to form a high strength steel sheet as described above, but a step of removing the produced iron oxide is necessary, and even if the iron oxide is removed, At present, steel sheets alone are inferior in rust resistance.

From the viewpoint of improving rust prevention or corrosion resistance, as proposed in Japanese Patent Laid-Open No. 6-240414, for example, in automobile parts such as impact bar in a door, corrosion that has penetrated into the door Moisture of the factor may corrode the unpainted part of the hardened steel pipe, so elements such as Cr and Mo are added to the steel components of the steel material forming such hardened steel pipe to improve the corrosion resistance. There are also examples. However, such measures not only increase the cost by adding Cr and Mo, but also in the case of materials for press forming, there is a problem of deterioration of press formability due to addition of their alloy components.

An object of the present invention is to provide a hot pressing method excellent in practicality. More specifically, an object of the present invention is to provide a hot pressing method in which the corrosion resistance and the coating adhesion of a pressed product are improved without deteriorating the surface properties.

[0016]

Means for Solving the Problems Here, the inventors of the present invention have diligently studied means for solving the problems from various angles, and have repeatedly studied various heating operations in hot pressing using a plated steel material. As a result, it was found that it is effective to improve the corrosion resistance and the press workability by maintaining the intermediate temperature up to the hot press working temperature to further promote the mutual diffusion of Fe and the plating layer.

That is, hot pressing means heating at a temperature of 700 to 1000 ° C., which is higher than the melting point of the zinc-based plated metal, and heated to such a high temperature. In this case, it was predicted that the plating layer would melt and flow away from the surface, or would not melt and evaporate to remain, or that the surface properties would be significantly inferior even if it remained.

Thus, instead of directly heating the hot-dip galvanized steel sheet, the hot-dip galvanized (GI) steel sheet is changed to an alloyed hot-dip galvanized (GA) steel sheet to promote the diffusion of Fe / Zn. Unexpectedly, we learned that the problems described above can be avoided and further improvement in corrosion resistance can be achieved, and we have developed a method for hot-pressing difficult-to-process high-strength steel materials.

Here, in the present invention, by further improving the heating operation at that time, the appearance of the finally obtained material after pressing, corrosion resistance and coating adhesion are further improved.

The present invention completed based on the above knowledge is as follows. (1) A hot press working method, characterized in that, prior to hot press working of the plated steel material, the temperature of the plated steel material is kept at a temperature of 400 ° C. or more and Ac ₃ transformation point or less for a time of 5 to 1000 seconds.

(2) The hot pressing method according to (1) above, wherein hot pressing is performed at a temperature of 700 to 1000 ° C. after the heat retention. (3) The plated steel material is a zinc-based plated steel material (1)
Alternatively, the hot pressing method described in (2).

[0022]

BEST MODE FOR CARRYING OUT THE INVENTION Next, the reasons for limiting the present invention as described above will be described in detail. In the present specification, “%” that defines the steel composition and the plating composition is “mass%”.

According to the present invention, the hot dip galvanized steel sheet is heated in an oxidizing atmosphere, and then, for example, 900
Even if heated to a temperature of not less than 0 ° C., evaporation of the zinc-based plating layer on the surface is prevented, and hot pressing can be performed after heating. In addition, since the zinc-based plating film is provided after the press forming, the zinc-based plating film itself already has excellent corrosion resistance, and it is possible to exhibit the excellent effect that no rust-preventing treatment is required as a post-treatment.

Base Steel Material The base steel material for hot pressing according to the present invention is not particularly limited as long as it has good plating wettability during hot dip galvanizing and adhesion of plating after plating. From the viewpoint of practical use, hardened steel, such as high-strength steel sheet, which has high strength and high hardness when rapidly cooled after hot forming is particularly preferable.

For example, even in the case of steel types such as Si-containing steels and stainless steels having problems in plating wettability and plating adhesion, the present invention can be improved by improving the plating adhesion by using a plating adhesion improving method such as pre-plating treatment. Can be used for.

The strength of the steel material after quenching is mainly the carbon content.
(C) Since it depends on the amount, if a high-strength molded product is required, the C content is preferably 0.1% or more and 3.0% or less. At this time, if the upper limit is exceeded, the toughness may decrease.

Particularly, in the case of the present invention, high-tensile steel plate, Si, which is a difficult press-molding material which is said to be difficult to press-mold,
Practical significance is high for steel plates for machine structures, high hardness steel plates, etc. to which Mn, Ni, Cr, Mo, V, etc. are added.

The form of the press-molding base material as a material is generally a plate material, but as the form of the hot press to which the present invention is applied, there are bending, drawing, bulging, hole-expanding, flange-forming, etc. So, in that case, the bar material,
You may use a wire material, a pipe material, etc. as a raw material.

Zinc-based plating layer In the case of the zinc-based plated steel sheet according to the present invention, as a specific plating operation, the steel sheet is immersed in a molten zinc and zinc alloy plating bath and pulled up. The amount of plating adhered is controlled by adjusting the pulling speed and the flow rate of the wiping gas blown from the nozzle. The alloying treatment is performed by additionally heating in a gas furnace or an induction heating furnace after the plating treatment. In this plating operation, plating may be carried out by either a continuous coil plating method or a cut plate single plate plating method.

Of course, if a plating layer having a predetermined thickness can be obtained, the plating layer may be provided by any method such as electroplating, thermal spray plating, vapor deposition plating and the like.
The composition of the zinc-based plating layer is not particularly limited, and even if it is a pure zinc plating layer, alloy elements such as Al, Mn, Ni, Cr, Co, Mg, Sn, and Pb were added in appropriate amounts according to the purpose. It may be a zinc alloy plating layer. Be, B, Si, P, S, Ti, V, W, M that may be inevitably mixed with other raw materials
Some of o, Sb, Cd, Nb, Cu, Sr, etc. may be contained.

However, a pure zinc-plated layer or an alloyed zinc-plated layer is preferable because of its low cost. Examples of the zinc alloy plating include the following systems.

Zinc-iron alloy plating, zinc-cobalt alloy plating, zinc-chromium alloy plating, zinc-aluminum-magnesium alloy plating, zinc-manganese alloy plating and the like.

The coating weight is preferably 90 g / m ² or less per side. If it exceeds 90 g / m ² , the formation of the zinc oxide layer becomes non-uniform and there is a problem in appearance. The lower limit is not particularly limited, but if it is too thin it will not be possible to ensure the required corrosion resistance after press forming, or it will not be possible to form the zinc oxide layer necessary to suppress the oxidation of the steel sheet during heating, Is about 20 g / m ² or more, preferably 30 g / m ² or more. In the case of more severe heating such as higher heating temperature, the performance is desirably in the range of 40 to 80 g / m ² .

Usually, the hot dip galvanizing bath contains Al, and in the case of the present invention as well, the Al content in the plating film is
It should be in the range of 0.08 to 0.4%. More preferably 0.
08 to 0.3%. In order to increase the Fe content in the plating film, the Al concentration should be low.

Insulation conditions The heating methods required for insulation include furnace heating, direct energization, induction heating, high frequency heating, direct heating, etc., but any method can be used as long as the predetermined heating pattern defined by the present invention can be realized.

The temperature rising rate is preferably in the range of 5 to 200 ° C./sec. If the heating rate is too low, the productivity will be low.
If it exceeds ℃ / sec, the heating equipment such as electric heating will be large and the cost will be high.

Considering that the maximum temperature reached during the heating period requires heating to a temperature higher than the phase transformation temperature and quenching at a temperature higher than the phase transformation temperature by a press die in order to quench-harden. Just decide.

According to the present invention, when the temperature is raised, held and soaked prior to the hot press working, the heating is temporarily performed over the Ac ₃ transformation point during such heating period. Also 400
It is held for 5 to 1000 seconds in the temperature range of ℃ or higher and Ac ₃ transformation point or lower, but its effect is to make Fe diffusion sufficient,
For example, in the case of galvanized steel, hot dip galvanizing (G
I) The purpose is to change from steel to alloyed hot-dip galvanized (GA) steel and finally form a solid solution phase of iron and zinc. This solid solution phase plays an important role in determining the product quality after pressing. That is, if the solid solution phase is not formed, peeling of the plating film occurs during press working, for example, during molding, which deteriorates the appearance after pressing.

As described above, the maximum heating temperature during the heating period is not particularly limited, but it is an intermediate temperature retention during the heating period, and the holding time in the temperature range of 400 ° C. or higher and Ac ₃ transformation point or lower is less than 5 sec. In this case, the solid solution phase is not sufficiently formed, and the zinc plating component evaporates during press working or heating prior to it, resulting in poor coating film adhesion and corrosion resistance.

Further, if the temperature is kept over 1000 seconds, the diffusion of iron will proceed too much and the zinc concentration on the plating surface will decrease, and iron oxide will be generated, which not only impairs the appearance after pressing but also adheres the coating film. And corrosion resistance are also reduced.

Heating / Hot Pressing of Steel Sheet A zinc-based plated steel sheet having a plating layer on the surface layer prepared as described above is then subjected to predetermined heat treatment including heat retention (holding) at an intermediate temperature as described above. Pressing (forming) is performed by heating to the inter-pressing temperature. In the case of the present invention, since hot press working (molding) is performed, the final temperature is usually 700-
Although it is heated to 1000 ° C, depending on the type of raw steel sheet, there are some that have fairly good press formability, in which case heating to a slightly lower temperature is sufficient. In the case of the present invention, depending on the steel type, the case of heating in a heating region of a so-called warm press is also included, but since the effect of the present invention is effectively exerted when applied to a so-called difficult press molding material, Is heated to 700-1000 ° C as described above.

The heating temperature prior to the press forming at this time is, if it is a hardened steel, it is heated to the hardening temperature to obtain the target hardness and then held for a certain period of time to perform the press forming at a high temperature. Quench quickly. Under normal steel types and conditions,
At this time, the maximum temperature reached during heating is about 700 ℃
It may be in the range of 1000 ° C.

Further, the Fe content in the plated layer after the heat treatment affects the melting point of the plated film, and is therefore preferably high. Even if the Fe content is high, press forming at room temperature decreases the workability of the plating film as the Fe content in the film increases.
It was around 13%. However, in the present invention, hot press forming can be performed even if the Fe content is high because the steel sheet and the plating film are softer than normal temperature. Fe content is 80
% Or less. The Fe content is preferably in the range of 5 to 80%, more preferably 10 to 30%. If the Fe content is less than the lower limit, the oxide film after heating becomes nonuniform, and if it exceeds the upper limit, it takes time to form a Zn—Fe alloy, which lowers the productivity and increases the cost.

The Fe content is a problem during hot press forming. Therefore, when heating is performed before press forming in advance as described above, the heating condition at that time is just before press forming. It is preferable that the heat treatment is performed under the conditions.

According to the present invention, the hot-pressed steel sheet thus heated is then subjected to hot-press forming. The hot-press forming at this time is not particularly limited.
Press molding that is usually performed may be performed. As a feature of hot press molding, quenching is performed simultaneously with molding,
It is preferable to use a steel type that enables such quenching. Of course, the press die may be heated and the quenching temperature may be changed to control the product characteristics after pressing.

Next, the function and effect of the present invention will be described more specifically with reference to Examples.

[0047]

[Examples] [Example 1] In this example, various platings shown in Table 2 were applied to a steel plate having a thickness of 1.0 mm and an Ac ₃ transformation point temperature of 820 ° C and having the components shown in Table 1. These steel sheets are
A tunnel furnace having three zones of a heating zone, a holding zone, and a soaking (heating) zone was used, and temperature rising, heat retention, and soaking were performed in each zone.

A thermocouple was attached to each of the steel plates and heated while measuring the temperature. Table 3 shows the furnace temperature of each zone. First, insert the steel plate into the heating zone, when the temperature reaches the set temperature of the holding zone, move the steel sheet from the heating zone to the holding zone, keep it at a predetermined temperature for a predetermined time, move to the soaking zone and take out after a predetermined time, Cylindrical drawing was performed. Here, the intermediate heat retention time shown in Table 2 represents the total time in the temperature range of 400 ° C. or higher and Ac ₃ transformation point or lower when heating or heat retention is performed in these heating zones, heat retaining zones and soaking zone. .

As a comparative example, a steel sheet was rapidly heated to 920 ° C. by a direct current heating method. Heating rate is approximately 210 ° C
It was / sec. This is a heating pattern D. A cylindrical drawing forming test was performed on the plated steel sheet thus heated. In other words, a circular blank with a diameter of 90 mm is replaced with a punch diameter of 50 m.
m, punch shoulder R5mm, die diameter 52.4mm, die shoulder R5m
Molded at m.

Molding height is 25 mm, plate pressing force (BHF) is 1 ton
F The surface condition after molding was visually evaluated to evaluate the appearance after pressing. Passing without peeling: marked with ◯, peeling failed: marked with x.

With respect to the hot press-formed products thus obtained, the coating film adhesion and the post-coating corrosion resistance (simply referred to as corrosion resistance) were evaluated in the following manner. Coating film adhesion test The test piece cut out from the cylindrical squeezing body obtained in this example was treated with PBL-3080 manufactured by Nippon Parkerizing Co., Ltd. under the usual chemical conversion treatment conditions using zinc phosphate, and then Kansai Paint's electrodeposition paint GT.
-10 was electro-deposition coated with a voltage of 200 V with slope energization, and baked at a baking temperature of 150 ° C for 20 minutes. Coating thickness is 20 μm
Met.

The test piece was immersed in ion-exchanged water at 50 ° C for 240
It was taken out after a lapse of time, scratches were made in a grid pattern with a width of 1 mm with a cutter knife, a peeling test was conducted with a polyester tape made by Nichiban, and the number of remaining masses of the coating film was compared to evaluate the coating film adhesion. The total number of cells was 100.

The evaluation criteria were 90 to 100 remaining masses as good: evaluation code ◯, 0 to 89 bad: bad evaluation code x. Corrosion resistance test after painting Test pieces cut out from the cylindrical diaphragm obtained in this example were subjected to zinc phosphate treatment with PBL-3080 manufactured by Nippon Parkerizing Co., Ltd. under ordinary chemical conversion treatment conditions, and then Kansai Paint's electrodeposition paint GT- 10 was electrodeposited by applying a voltage of 200 V with slope energization, and baked at a baking temperature of 150 ° C for 20 minutes. Coating thickness is 20
It was μm.

After scratching the coating film of the test piece to reach the substrate with a cutter knife, a salt spray test prescribed in JIS Z2371 was carried out for 480 hours. The coating swelling width or rust width from the scratched portion was measured to evaluate the corrosion resistance after coating.

The evaluation criterion is the larger one of the rust width and the swelling width of the coating film, and less than 4 mm is evaluated as good: evaluation symbol ◯, and 4 mm or more is defective: evaluation symbol x. These results are summarized in Table 2, and the reference numerals 1 to 13 of the examples of the present invention show good characteristics, but the reference numerals 14 to 18 deviating from the conditions of the present invention show the appearance after pressing and the adhesion of the coating film. And corrosion resistance were unacceptable.

[0056]

[Table 1]

[0057]

[Table 2]

[0058]

[Table 3]

[0059]

As described above, according to the present invention, it becomes possible to perform hot press forming of a difficult press forming material such as a high-tensile steel plate and a stainless steel plate. At that time, an atmosphere control facility for a heating furnace is provided. In addition to the above, the process for removing the steel sheet oxide during press forming is not necessary and the production process can be simplified. Further, since the galvanized layer has a sacrificial anticorrosion effect, the corrosion resistance of the press-formed product is also improved.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Kazuhito Imai             4-53 Kitahama, Chuo-ku, Osaka City, Osaka Prefecture             Sumitomo Metal Industries, Ltd. (72) Inventor Shuntaro Sudo             1 Toyota Town, Toyota City, Aichi Prefecture Toyota Auto             Car Co., Ltd. (72) Inventor Masanobu Ichikawa             343 Higashijima, Nogata Town, Nisshin City, Aichi Prefecture (72) Inventor Nobuaki Yamazaki             177-10 Ishihito, Fujioka-cho, Nishikamo-gun, Aichi

Claims (3)

[Claims] 1. A hot press working method characterized in that, prior to hot press working of a plated steel material, the temperature of the plated steel material is kept at a temperature of 400 ° C. or higher and Ac ₃ transformation point or lower for a time of 5 to 1000 seconds. 2. The hot pressing method according to claim 1, wherein hot pressing is performed at a temperature of 700 to 1000 ° C. after the heat retention. 3. The hot pressing method according to claim 1, wherein the plated steel material is a zinc-based plated steel material.