JP2003073774A - Plated steel sheet for hot press - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a technology that can form a high-tension steel sheet such as that with carbon content of 0.3% or less by hot press, which is a material to be hardly press formed, without requiring post-treatment for imparting corrosion resistance, and that can simultaneously acquire corrosion resistance. SOLUTION: The plated steel sheet for hot press has a galvanized layer or a zinc-based alloy plated layer with coating weight of 90 g/m<2> or less per one side thereon, which has a barrier layer formed by galvannealing on the surface layer, consisting of such an oxide layer of zinc that can prevent zinc from vaporizing when heated prior to the hot-press forming.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel material for hot pressing, and more particularly to a steel sheet and steel material for hot pressing used for manufacturing underbody, 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.

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

Under these circumstances, many hot press proposals have been made 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.

For example, as an advantage of hot pressing, it is possible to perform heat treatment together with press forming. 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 the productivity is low and the cost is significantly increased.

There is a method of hot press forming in order to form a high strength steel plate 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 standpoint of improving rust resistance or corrosion resistance, as proposed in Japanese Patent Laid-Open No. 6-240414, for example, in automobile parts such as an 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 molding, there is a problem of deterioration of press moldability due to the addition of the alloy components.

An object of the present invention is to provide a steel material for hot pressing which can ensure a predetermined corrosion resistance even when hot pressing a so-called difficult press molding material and does not cause deterioration in appearance.

Further, a specific object of the present invention is to enable hot press forming of, for example, a high-strength steel sheet, which is a difficult press forming material, without requiring post-treatment for ensuring corrosion resistance, and at the same time ensure corrosion resistance. It is to provide the technology.

[0016]

The inventors of the present invention have earnestly studied from various angles about the means for solving the above problems, and as a result, instead of directly pressing the difficult press-molding material as described above, the deformation resistance The idea is to perform press forming at high temperature in order to reduce corrosion, and at the same time, perform hot press forming using a plated steel sheet that is originally excellent in corrosion resistance in order to ensure excellent corrosion resistance without post-treatment. Obtained. Then, based on this, it was conceived to apply the hot press to a zinc-based plated steel sheet having a sacrificial anticorrosion action for the steel sheet in a corrosion resistant wet environment. But,
Hot pressing means heating at a temperature of 700 to 1000 ° C., which is a temperature above the melting point of zinc-based plating metal, and when heated to such a high temperature, the plating layer is It was predicted that they would melt and then be washed away from the surface, or they would melt and evaporate and not remain, or that even if they remained, the surface properties would be extremely poor.

However, further various investigations were repeated thereafter, and it was found that some change might be observed due to alloying between the plating layer and the steel sheet by heating, and various plating was conducted as a preliminary test. With composition and various atmospheres, actually heat to a temperature of 700 to 1000 ° C,
Next, when hot pressing was performed, contrary to the predictions made so far, it was found that hot pressing could be performed without problems for some materials.

Therefore, the conditions for obtaining good surface properties even when hot pressing is performed after heating at a temperature of 700 to 1000 ° C. were found, and a zinc oxide film was found on the surface of the plating layer.
It was found that it was entirely formed as a kind of barrier layer for preventing evaporation of zinc in the lower layer. This barrier layer is
It is necessary to be formed to some extent before heating prior to hot pressing, and it is speculated that the formation will also proceed by heating to 700 to 1000 ° C after that.

Further, when the plating layer was analyzed,
It has been found that the alloying has progressed considerably, whereby the plating layer has a high melting point to prevent the evaporation of zinc from the surface of the plating layer, and to suppress the formation of iron oxide in the steel sheet. Moreover, it has been found that the plated layer thus heated has good adhesion between the plated layer and the steel sheet which is the base material after hot press forming.

By using the above steel sheet as a galvanized steel sheet, it was found that hot press forming could be performed even with a high tensile steel sheet, and further studies were conducted to determine whether it can be used as a practical technique. After confirming the effect, and convinced that it was a practical technique, the present invention was completed.

As described above, in the present invention, the reason why the plating layer remains even when heated to a temperature range near the melting point of the plating layer is that the oxide film layer having heat resistance and better adhesion than the plating layer on the surface of the plating layer. Is considered to be formed as a barrier layer that blocks the evaporation of zinc. Further, in order for such action to be sufficiently exhibited, the fact that the alloying of the plating layer and the steel sheet progresses and the melting point of the plating layer becomes high also has an effect, and preferably the action of both of these causes It is presumed that even when heated to 950 ° C, which is higher than the boiling point of the constituent zinc, the oxidation of the steel sheet substrate is suppressed.

The present invention completed based on such knowledge is as follows. (1) A steel sheet for hot pressing, having a zinc or zinc-based alloy plating layer on the surface of the steel sheet, the surface of which has a barrier layer for preventing evaporation of zinc during heating.

(2) The steel sheet for hot pressing according to (1) above, wherein the barrier layer is a zinc oxide layer. (3) The steel sheet for hot pressing according to the above (1) or (2), wherein the coating amount of the plating layer on one surface is 90 g / m ² or less.

(4) A steel sheet for hot pressing in which the plated layer is directly provided on the surface of the steel sheet, wherein the Fe content in the plated layer is 80% by mass or less, and (1) to (3) above. The steel sheet for hot pressing according to.

(5) The steel sheet for hot pressing according to any one of the above (1) to (4), wherein the steel sheet is a difficult press forming material. (6) The carbon content of the steel sheet, which is a difficult press molding material, is 3.0.
The steel sheet for hot pressing according to the above (5), which is at most% by mass.

(7) A steel material for hot pressing, comprising a zinc or zinc-based alloy plating layer on the surface of the steel material, the surface of which has a barrier layer for preventing evaporation of zinc during heating.

[0027]

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 to form an oxide film on the surface, which acts as a barrier layer, and even when heated to 900 ° C. or higher, the surface is Evaporation of the zinc-based plating layer of
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 at the time of hot dip galvanizing, and plating adhesion after plating. From the viewpoint of practical use, hardened steel that has high strength and high hardness when rapidly cooled after hot forming, for example, a high-strength steel sheet having a steel chemical composition as shown in Table 1 below 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 sheet 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.

In particular, 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 and the like. So, in that case, the bar material,
You may use a wire material, a pipe material, etc. as a raw material.

[0034]

[Table 1]

Zinc-based plating layer / barrier layer In the present invention, to provide a zinc-based plating layer having a barrier layer, for example, after performing a normal hot dip galvanizing treatment, heating in an oxidizing atmosphere, that is, usually Alloying treatment may be performed. Such alloying treatment is performed by reheating in a gas furnace or the like. At that time, not only oxidation of the surface of the plating layer but also metal diffusion between the plating layer and the steel sheet of the base material is performed. Normally, the heating temperature at this time is 550 to 650.
℃.

As a specific plating operation according to the present invention, 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 following systems are disclosed as zinc alloy plating.

For example, zinc-iron alloy plating, zinc-12% nickel alloy plating, zinc-1% cobalt alloy plating, 55
% Aluminum-zinc alloy plating, zinc-5% aluminum alloy plating, zinc-chromium alloy plating, zinc-aluminum-magnesium alloy plating, tin-8% zinc alloy plating, zinc-manganese alloy plating, and the like.

The coating weight is preferably 90 g / m ² or less.
If it exceeds this, the formation of the zinc oxide layer as the barrier 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, Will secure about 20g / m ² or more. In case of more severe heating such as higher heating temperature, 40 ~
The performance is good in the range of 80 g / m ² .

The composition of the zinc-based plating layer is not particularly limited,
Al, Mn, Ni, Cr, Co, Mg,
It may be a zinc alloy plating layer in which an alloying element such as Sn or Pb is added in an appropriate amount according to the purpose. Be, B, Si, P, S, T that may be inevitably mixed with other raw materials
Some of i, V, W, Mo, 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. Usually, the hot dip galvanizing bath contains Al, and in the case of the present invention,
The Al content in the plating film should be in the range of 0.08 to 0.4%. More preferably, it is 0.08 to 0.3%. In order to increase the Fe content in the plating film, the Al concentration should be low.

Heating / Hot Press Forming of Steel Sheet A zinc-based plated steel sheet having a barrier layer on the surface layer prepared as described above is then heated to a predetermined temperature and press formed. In the case of the present invention, since hot press forming is performed, it is usually heated to 700 to 1000 ° C., but depending on the type of the raw material steel plate, there are those in which the press formability is considerably good,
In that 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, 700 to 1000 ° C as described above
Heat to.

As a heating method in this case, an electric furnace, a gas furnace, flame heating, electric heating, high frequency heating, induction heating and the like can be mentioned. The atmosphere during heating is also not particularly limited, but in the case of a material in which a barrier layer is formed in advance, there is no particular limitation as long as it does not adversely affect the maintenance of such a barrier layer.

In the case of quenching steel, the heating temperature at this time is such that, if the steel is hardened steel, it is heated to the quenching temperature that gives the target hardness and then held for a certain period of time to perform press molding 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.

By the way, according to the present invention, an oxide film acting as a barrier layer for preventing the evaporation of zinc during heating is formed on the surface of the zinc-based plating layer. 0.01 to 5.0 μm is sufficient.

Further, the Fe content in the plated layer after the heat treatment affects the melting point of the plated film, so that it is advantageous that the Fe content is 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 barrier layer and the Fe content pose a problem during hot press forming. Therefore, as described above, the barrier layer is previously formed by the alloying treatment at the time of forming the plating layer, and the pressing is further performed. When the heating is performed before the forming, it is preferable that the heating condition during the alloying treatment is a condition that takes into consideration the heating treatment immediately before the press forming.

The hot-pressing steel sheet according to the present invention thus heated and having the barrier layer formed on the surface thereof is then subjected to hot-press forming. Particularly, the hot-press forming at this time is performed. There is no limitation, and press molding which is usually performed may be performed. As a feature of hot press forming, quenching is performed at the same time as forming. Therefore, 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 in more detail with reference to examples.

[0050]

[Example] [Example 1] In this example, a hot-dip galvanized steel sheet of steel type A shown in Table 2 having a plate thickness of 1.0 mm was alloyed at 650 ° C, and then 950 ° C x 950 ° C in a heating furnace in an air atmosphere. After heating for 5 minutes, it was taken out of the heating furnace, and hot press forming of a cylindrical drawing was performed under the same high temperature condition. The hot press molding conditions at this time were a drawing height of 25 mm, a shoulder radius R5 mm, a blank diameter of 90 mm, a punch diameter of 50 mm, and a die diameter of 53 mm. The adhesion state of the plated layer after forming was evaluated as formability by visually judging the presence or absence of peeling of the plated layer. In this example, the temperature of the steel sheet reached 900 ° C in about 2 minutes.

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 either the rust width or the swelling width of the coating film, whichever is larger. Omm or more and less than 4 mm is good.
Defects 4 mm or more: Evaluation symbol x. The results of these tests are summarized in Table 2.

As a comparative example, cold-rolled steel sheets and stainless steel sheets were heated at 950 ° C. for 5 minutes and then hot-pressed in the same manner, and the above-mentioned characteristic evaluation was performed.
The results are summarized in Table 2, and when the alloyed hot-dip galvanized steel sheet is used, good characteristics are shown, but when a stainless steel sheet or a cold-rolled steel sheet is used, an oxide is formed and blackened. The oxide peeled off, and indentation flaws occurred during press molding. Moreover, the coating film adhesion and corrosion resistance were also unacceptable.

[0057]

[Table 2]

Example 2 In this example, the same test as in Example 1 was repeated for steel type A, but as shown in Table 3,
The Fe content in the plating film was changed by changing the coating weight and the alloying conditions immediately after plating. In this example, prior to hot press forming of an alloyed plated steel sheet (A) air atmosphere heating furnace 950 ° C ×
Heating was performed for 5 minutes and (B) heating in an air atmosphere heating furnace at 850 ° C. for 3 minutes. In Examples No. 9 to 23, the Fe content of the plating layer is changed.This is because the alloying treatment temperature (500 to 800 ° C) and time (30 minutes or less) are set before heating prior to hot pressing. It was done by changing. Also, No.18-23
In the above, the heating time prior to hot pressing was extended from 3 minutes to 6 minutes, and hot pressing was performed under more severe conditions.

The results are summarized in Table 3. In all cases, the results after heating were good in appearance, moldability, coating adhesion and corrosion resistance.

[0060]

[Table 3]

Example 3 In this example, the same tests as in Example 1 were repeated for each steel type in Table 1, and the test pieces obtained were evaluated for formability, coating film adhesion and corrosion resistance. The results are summarized in Table 4.

In all the examples, the results after heating were good in appearance, moldability, coating film adhesion and corrosion resistance.

[0063]

[Table 4]

Example 4 A steel plate having the composition of steel type A shown in Table 1 and a thickness of 1.0 mm was used and plated in a laboratory. The electroplating was performed in a laboratory using a plating bath used in an actual production line. Hot-dip plating was performed by reproducing the bath used in the actual production line in the laboratory. The alloying treatment of zinc-iron plating is 550
A method of immersing in a molten salt bath at ℃ was used. The obtained plated steel sheet was hot-formed and evaluated in the same manner as in Example 1. The heating prior to the hot pressing was performed in an atmospheric furnace at 850 ° C. for 3 minutes.

The obtained results are shown in Table 5. Good characteristics were obtained regardless of the plating method and the composition of the plating layer.

[0066]

[Table 5]

As can be seen from these results, according to the present invention, in any case, a material having excellent press-formability can be obtained, and a film having excellent coating film adhesion and corrosion resistance can be obtained. I understand.

[0068]

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.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) C23C 2/40 C23C 2/40 8/12 8/12 28/00 28/00 B (72) Inventor Yoshikawa Yukihiro 4-5-3 Kitahama, Chuo-ku, Osaka-shi, Osaka Prefecture F-term in Sumitomo Metal Industries, Ltd. (reference) 4K027 AA02 AA05 AA22 AB02 AB05 AB09 AB14 AB43 AB44 AB46 AB48 AC12 AC52 AE23 4K044 AA02 AB02 BA10 BA12 BB03 BC02 BC04 BC05 BC09 CA11 CA12 CA18

Claims (7)

[Claims] 1. A steel sheet for hot pressing, comprising a zinc or zinc-based alloy plating layer on the surface of the steel sheet, the surface layer of which has a barrier layer for preventing evaporation of zinc during heating. 2. The steel sheet for hot pressing according to claim 1, wherein the barrier layer is a zinc oxide layer. 3. The amount of adhesion of the plating layer per surface is 90.
The steel sheet for hot pressing according to claim 1 or 2, which has a g / m ² or less. 4. A heat-pressing steel sheet having the plating layer directly provided on the surface of the steel sheet, wherein the Fe content in the plating layer is 80% by mass or less. Steel plate for hot pressing. 5. The steel sheet for hot pressing according to claim 1, wherein the steel sheet is a difficult press forming material. 6. The steel sheet for hot pressing according to claim 5, wherein the carbon content of the steel sheet which is a difficult press forming material is 3.0 mass% or less. 7. A steel material for hot pressing, comprising a zinc or zinc-based alloy plated layer having a barrier layer for preventing evaporation of zinc during heating on the surface of the steel material.