JP2007270258A - Aluminum-based hot pressed steel material having excellent adhesion and corrosion resistance after coating - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an aluminum-based hot pressed steel material having excellent adhesion and corrosion resistance after coating appropriately for use in various members such as chassis members and skeleton reinforcing members of an automobile for which high strength is required. <P>SOLUTION: The hot pressed high strength steel material has a coating ground film formed of a metal oxide of one or more of Zr, Ti and Si, or a mixture composed of the metal oxide and metal hydroxide, in which an Fe-Al based plating layer exists on the steel sheet containing, by weight, 0.1 to 0.5% C, 0.05 to 0.5% Si, and 0.5 to 3% Mn, and two or more phases of Fe<SB>3</SB>Al, FeAl, FeAl<SB>2</SB>, Fe<SB>2</SB>Al<SB>5</SB>, and FeAl<SB>3</SB>coexist down to depth 2 μm near the surface layer of the plating layer; the average Al content of the plating layer down to the depth 2 μm near the surface layer is 10 to 75%, and further the surface layer of the plating layer contains an F element at a thickness 5 to 100 nm. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  TECHNICAL FIELD The present invention relates to a hot press member in which aluminum-based plating is applied to various members that require high strength, such as an underbody member and a skeleton reinforcing member of an automobile.

  In recent years, demands for saving resources and energy for automobiles are increasing, and the weight reduction of automobile bodies for reducing fuel consumption is required. On the other hand, there is an effort to increase the strength of the vehicle body due to an increase in the demand for passenger safety. Increasing the strength of the vehicle body can be achieved by increasing the plate thickness or applying reinforcing parts, but this means an increase in the amount of steel used, leading to an increase in the weight of the automobile body. Application of high-strength steel sheets is a means of satisfying these mutually conflicting requirements. Thereby, it becomes possible to obtain a member having the same strength with a thinner plate thickness and a smaller shape, leading to a reduction in the amount of steel used. In recent years, application of high strength steel sheets of 780 MPa class in which the tensile strength of steel materials is about three times that of mild steel has begun. However, two major problems have been pointed out for the application of high-strength steel sheets. The first is a decrease in press formability. High-strength steel is greatly inferior in mechanical properties such as elongation value and r-value of steel compared to mild steel, so that allowable values for elongation deformation and drawing deformation during pressing are lowered, and cracking is likely to occur. The second is a defect in shape freezing after pressing, which is called a springback phenomenon. This is a problem in terms of both quality and cost due to a decrease in dimensional accuracy and the addition of a rework process. Such a problem at the time of press working of a high-strength steel sheet becomes conspicuous as the strength of the steel sheet increases, and it is expected that it will be difficult to solve by the conventional material development concept. Accordingly, there is a strong demand for the development of a steel sheet that achieves both high strength and high formability based on different technical ideas.

  There is a hot press technique as another technique for achieving both. This technology eliminates the problem of formability of high-strength steel sheets by press-molding high-carbon steel heated to a high temperature of about 900 ° C and softened, and is quenched by rapid cooling by contact with a press die after molding. It is cured to obtain a desired strength. This technique is excellent from the viewpoint of press formability, but a new problem has been pointed out. For example, since high temperature heating up to about 900 ° C. is involved, oxides are generated on the steel sheet surface, which may fall off during pressing and damage the press die or the steel sheet surface. In addition, the oxide remaining on the surface of the steel sheet has poor adhesion to the surface of the steel sheet and the coating material, and causes peeling of the coating film and deterioration of corrosion resistance. Therefore, it is necessary to add a pickling or shot blasting process for removing oxides before coating the processed product, resulting in an increase in cost and a decrease in shape accuracy. Moreover, since the press member obtained in this way is not rust-proof plated, it cannot be said that it has sufficient corrosion resistance. As a method of suppressing corrosion on the surface and improving corrosion resistance, a technique of applying aluminum plating to a steel sheet is disclosed. For example, there are JP2003-183802A, JP2003-193187A, and JP2004-244704A. These techniques exhibit excellent oxidation resistance due to the action of aluminum plating, suppress the growth of the oxide film, and further improve the corrosion resistance. However, problems have also been pointed out with this aluminized steel. That is, the hot press material is generally subjected to cationic electrodeposition coating. However, the aluminum plating material after hot pressing hardly forms a coating pretreatment film by a phosphate treatment, which is a general chemical conversion treatment agent before electrodeposition coating. This is presumably because a chemically stable oxide film is formed on the surface of the aluminum plating layer. Therefore, the adhesion between the electrodeposition coating film and the aluminum plating surface becomes insufficient, and the coating film peeling and swelling of the coating film particularly in a wet environment tend to increase, and the corrosion resistance may not be exhibited.

Japanese Patent Laid-Open No. 2003-183802 JP 2003-193187 A JP 2004-244704 A

  The reason why the phosphate treatment film, which is the coating base treatment of the aluminized steel sheet after hot pressing, is not formed is due to the presence of a chemically stable oxide film layer formed on the aluminized surface and FeAl alloy phases of various compositions. This is because the reactivity of the phosphate bath is insufficient. The oxide film formed on the surface of the aluminum plating itself has a role to suppress rapid oxidation of the plating layer, and can be said to be an inevitable film. Similarly, various FeAl alloy phases cannot be avoided because they are inevitably generated by heat treatment. On the other hand, improving the reactivity of the phosphate bath is realistic because there is a risk that the reactivity of other steel types to be treated at the same time will be excessive, leading to increased costs due to increased sludge and reaction accelerators. There is no improvement in bathing.

  Therefore, the object of the present invention is not a conventional phosphate-based film as a coating ground treatment, but an aluminum system excellent in adhesion and corrosion resistance after painting after coating the base with a new component system and treatment liquid film. The aim is to provide hot pressed steel.

  As a result of intensive studies from various angles on the means for solving such a problem, the present steel materials after hot press treatment of aluminum-plated steel materials include metal oxides of Zr, Ti, Si, It has been found that a hydroxide-treated film is suitable as a paint base-treated film. Furthermore, if the system contains F ions as a chemical system for film formation, even if there is a chemically strong oxide film or AlFe alloy phase on the plating surface layer, the reactivity is ensured and film formation is possible. I found out. The formation of the oxide film is caused by the hydrolysis equilibrium reaction of the metal fluorocomplex present in the solution of the present invention. This is necessary for the formation reaction of the fluorinated Al complex that is important for shifting this equilibrium reaction to the oxide formation side. It was found that the Al ions to be supplied by the dissolution reaction of the FeAl alloy phase on the plating surface layer. Furthermore, it was also found that the presence of FeAl alloy phases having different compositions activates the etching reaction and remarkably improves the supply capacity of Al ions.

The present invention has been made on the basis of the above findings, and the features thereof are as follows.
(1) Fe-Al alloy plating layer is present on a steel sheet containing C: 0.1 to 0.5%, Si: 0.05 to 0.5%, Mn: 0.5 to 3% by weight. Two or more phases of Fe ₃ Al, FeAl, FeAl ₂ , Fe ₂ Al ₅ , and FeAl ₃ are mixed up to a depth of 2 μm, and the average Al content of the plating layer up to a depth of 2 μm near the surface layer The rate is 10% or more and 75% or less by weight%, and the surface layer of the plating layer has a thickness of 5 to 100 nm and contains one or more metal oxides or metals of Zr, Ti, Si containing F element A hot-pressed high-strength steel material excellent in adhesion and corrosion resistance after coating, characterized in that a coating base film of a mixture of oxide and metal hydroxide is formed.
(2) Hot-pressed high-strength steel material excellent in adhesion and corrosion resistance after coating according to (1) above, wherein the plating layer further contains 1 to 15% by weight of Si. .
(3) The plating layer thickness is 3 μm or more and 50 μm or less, the coating base coating thickness is 5 nm or more and 100 nm or less, and the cationic electrodeposition coating layer thickness is 3 μm or more and 30 μm or less. A hot-pressed high-strength steel material excellent in adhesion and corrosion resistance after coating according to (1) and (2).

  ADVANTAGE OF THE INVENTION According to this invention, the high strength member for motor vehicles which was excellent in the adhesiveness and corrosion resistance after coating, and the intensity | strength of steel materials can be obtained.

The details of the present invention and the reasons for limitation will be described below.
The base steel material has a definition of a component for exhibiting quenching hardenability after hot pressing. Since the strength after quenching is mainly determined by the carbon content (C), when high strength is required, the C content is preferably 0.1% by mass or more and 0.5% by mass or less. If the amount is less than 0.1% by mass, sufficient strength cannot be obtained, and if it exceeds 0.5% by mass, the strength is not only saturated but also a weld crack is likely to occur. If Si is too low, fatigue characteristics will be lowered, so addition of 0.05% by mass or more is desirable. However, since Si forms a stable oxide film on the surface of the steel sheet during recrystallization annealing and inhibits the plating wettability during hot dipping, the upper limit is set to 0.5% by mass. Mn is known as an element that enhances the hardenability of the steel sheet, and is also an element effective in preventing hot brittleness caused by sulfur (S) inevitably mixed. Therefore, addition of 0.5% by mass or more is necessary. However, if added over 3% by mass, the impact properties after quenching will decrease, so it is desirable to make this the upper limit. Moreover, you may add Ti, Nb, Mo, V, Ni, and Cu in steel as needed.

Next, the FeAl alloy plating layer of the present invention will be described. Plated steel sheets to be used for hot pressing may be manufactured by the existing hot-dip aluminum plating process. In the plating bath, Si is added in an amount of 1 to 15% for the purpose of suppressing formation of an alloy layer of base iron and aluminum. Also good. If Si is less than 1%, the effect of suppressing the alloy layer is small, and if it exceeds 15%, the effect is saturated and at the same time dross floats on the bath surface, which is not preferable. Furthermore, Ni, Cu, Cr, Zn, and Mg may be added to the plating layer. The plated layer defined in the present invention is formed in the process of heating a steel plate subjected to aluminum plating to about 900 ° C. in order to use it for hot pressing. The plated layer after hot pressing undergoes an alloying reaction with the base iron in the course of heat treatment, and various FeAl alloys are formed. In general, an FeAl alloy phase having a large Fe content is formed from the plating surface toward the base iron, but the type and phase distribution of the alloy phase can be controlled by changing the heating conditions. The distribution and structure of the alloy layer are important in order to greatly influence the formation reaction of the chemical conversion treatment film layer described later, and are the core technology of the present invention. This formation reaction proceeds with the process of acid-dissolving the FeAl alloy phase on the plating surface, but when a different phase of FeAl alloy is present, a rapid and dense film with good adhesion and corrosion resistance after coating is formed. It is formed. Specifically, two or more of the alloy phases of Fe ₃ Al, FeAl, FeAl ₂ , Fe ₂ Al ₅ , and FeAl ₃ may be present together. Even if only one of them exists, the formation of the chemical conversion film proceeds, but the film formation rate is slow, and the adhesion and corrosion resistance after coating are not sufficient. The presence of the FeAl phase needs to exist in a range from the plating layer to a depth of 2 μm. The part deeper than 2 μm does not contribute to the formation reaction of the chemical conversion treatment layer. That is, since the surface of the plating layer has cracks, cracks, irregularities, etc., it is essential that the portion having a depth of up to 2 μm is defined by the above-described alloy phase. The aluminum content of the plating layer surface layer composed of such a plurality of alloy phases needs to be 10% or more and 75% or less. If it is less than 10%, the supply of Al necessary for the chemical conversion treatment reaction is not sufficient, and the film formation rate is lowered, which is not preferable. Moreover, it becomes a FeAl alloy with large Fe content, and corrosion resistance falls. If Al exceeds 75%, the chemical conversion treatment reaction becomes too active, and a powdery chemical conversion treatment film is formed, resulting in a decrease in adhesion. The film thickness of the plating layer is preferably 3 μm to 50 μm or less after hot pressing. If it is less than 3 μm, the corrosion resistance is insufficient. Also, the alloying reaction proceeds excessively during the heating process, making it impossible to obtain a desirable alloy phase. On the other hand, if the thickness exceeds 50 μm, the improvement in corrosion resistance is saturated, the plating layer tends to sag during the heating process, and the melting point of the unalloyed phase portion is low, which causes contamination of equipment due to elution in the furnace. Absent.

  The plating layer having such a structure can be obtained by heating a steel material to about 900 ° C. in a heating furnace. As the heating furnace, an existing superheated furnace such as an electric furnace, a direct-fired furnace, an induction superheated furnace, a hot air furnace, and an infrared image furnace can be applied. Moreover, the atmospheric gas at the time of a heating may be air, an inert gas, or a mixture thereof. In order to obtain a desirable alloy phase structure, it is necessary to optimize the rate of temperature rise, the maximum temperature reached, the holding time at a specific temperature, and the like.

  Next, the chemical conversion treatment film will be described. This film is composed of one or more metal oxides of Zr, Ti, and Si, or a mixture of metal oxide and metal hydroxide, and fluorine (F) is mixed in the film. This film is formed by immersing the plated steel material in a solution containing a metal fluoro complex and fluorine ions. Although the formation reaction is not clear, the presumed reaction is shown in the formulas (1) and (2). Here, M means Zr, Ti, Si.

When the dissolution reaction of Al by hydrofluoric acid, which is a driving reaction, occurs, the fluoride ion concentration in the solution decreases due to the formation of an aluminum fluoride complex. As a result, the equilibrium reaction of the formula (1) that is a precipitation reaction occurs. It will shift to the right and MO _n will deposit. At that time, since F ions are released, F is taken into the film. The presence of Al on the plating surface is essential in order to cause a driving reaction actively, and the FeAl alloy composition is important. Further, when there are a plurality of alloy phases, the reaction activity on the surface becomes non-uniform, and a more active dissolution reaction occurs. As a result of intensive studies on the optimal structure of the plating layer surface, it is clear that the type, existence ratio, and depth of the FeAl alloy phase greatly affect the film formation rate of the chemical conversion coating and the adhesion and corrosion resistance after coating. The optimum value was obtained. Even if a metal oxide is used alone, the performance is sufficiently exhibited, but two or more kinds may be mixed. Moreover, although the production | generation of a metal oxide has mainly arisen, there exist some which are produced | generated in the form of the hydroxide. Therefore, a chemical conversion treatment film consists of the site | part in which a metal oxide exists independently, and the site | part which exists in the mixed state of a metal hydroxide. Further, a deposit of a water-soluble resin, a silane coupling agent, or a metal such as Co or Mn may co-deposit in the film. When the thickness of the chemical conversion treatment layer is less than 5 nm, the covering property is insufficient, and the adhesion after coating and the corrosion resistance are not satisfied. On the other hand, if the thickness exceeds 100 nm, the chemical conversion film tends to be precipitated in a powder form, and therefore, adhesion and corrosion resistance after coating are deteriorated, which is not preferable.

  Cationic electrodeposition coating is generally used as a coating for automotive parts. Although the thickness of the coating layer varies depending on the part shape, sufficient corrosion resistance cannot be expressed unless the thickness is 3 μm or more even in a thin part. On the other hand, if it exceeds 30 μm, the corrosion resistance is saturated, the voltage is increased to ensure the film thickness, and coating film defects such as the occurrence of pinholes are liable to occur, which is not preferable.

Next, the present invention will be described in more detail based on examples.
A cold-rolled steel plate (thickness 1.6 mm) having the steel components shown in Table 1 manufactured through normal hot rolling and cold rolling steps was used as a base steel plate, and hot-dip aluminum plating was performed. Plating is an Al bath containing 8% by mass of Si. After the steel plate is immersed in the plating bath using a cleaning, non-oxidizing furnace and reducing furnace type manufacturing equipment, the amount of plating applied is controlled by nitrogen gas wiping after drawing. Cooled down. The Al-plated steel sheet thus manufactured was cut into 300 mm × 400 mm, put into an electric furnace (furnace temperature 950 ° C.) in an atmospheric atmosphere, taken out after alloying treatment under the heating conditions shown in Table 2, and a hat bent shape After being set in a press die and allowed to cool to a predetermined temperature, press working was performed. The steel sheet was quenched by contact with the die by press working and quench hardening occurred. Thereafter, the sample was taken out after being held in a mold until it reached 50 ° C. The temperature increase rate in the heat treatment was the average of the temperature increase rates until the steel sheet was changed from 300 ° C to 800 ° C. The holding time was the total time at a temperature of 800 ° C. or higher.

  This sample was applied to a treatment liquid mainly composed of complex ions containing 6 times the molar ratio of Zr, Ti and Si metals and two or more kinds of fluorine ions and ammonium ions with respect to the metals, and from 60 seconds to 120 seconds. After dipping and obtaining a predetermined adhesion amount, it was taken out, washed with water and dried. The concentration and pH of each component were as shown in Table 2.

  For coating adhesion and corrosion resistance, the wall portion of the hat bending press-processed product was cut out and subjected to electrodeposition coating. For electrodeposition coating, U-80 manufactured by Nippon Paint Co., Ltd. was used. The film thickness was adjusted to a predetermined film thickness by adjusting the voltage during electrodeposition. After electrodeposition coating, it was washed with water and baked at 150 ° C. for 20 minutes in a hot air oven.

  The evaluation of adhesion was carried out according to a cross-cut tape test of JIS K 5400 after a test piece of 150 mm × 70 mm was immersed in distilled water at 50 ° C. for 240 hours and taken out. That is, a grid of 1 mm intervals was put with 10 cutters, and the cellotape (registered trademark) was adhered and peeled off. If the missing part of the grid was within 5% of the total area, it was evaluated as ◯, when the area of the defective part was 5 to 15%, Δ when it was more than that, and ○ as acceptable.

  Evaluation of corrosion resistance performed the crosscut which reaches | attains a base iron with a cutter to the test piece of 150 mm x 70 mm, and used for the corrosion test after that. The corrosion test was evaluated with 100 cycles of JASO M609 consisting of salt spray, drying and wetting processes. One cycle of this corrosion test consists of JIS Z 2371 salt spray for 2 hours, drying at 60 ° C. and 30% relative humidity for 4 hours, and wet at 50 ° C. and 98% relative humidity for 2 hours. In the evaluation, the maximum bulge width on one side of the cross-cut portion was 2 mm or less, ○ was 4 mm or less, Δ was 4 mm or more, and ○ was passed.

  The evaluation results are shown in Tables 3 and 4.

  In Examples 1 to 4, the heating condition was changed to change the surface alloy phase structure and the surface Al content within the scope of the present invention. In Examples 5 to 8, the steel type was changed within the scope of the present invention. In Examples 9 to 12, the Si content in the plating layer was changed within the scope of the present invention. In Examples 13 to 17, the film thickness of the chemical conversion treatment film was changed within the scope of the present invention. In Examples 18 to 22, the thickness of the plating layer was changed within the scope of the present invention. In Examples 23 to 25, the thickness of the electrodeposition coating layer was changed within the scope of the present invention. In Examples 26 to 37, the oxide species of the chemical conversion coating layer was changed within the scope of the present invention. In all these examples, excellent results were shown in paint adhesion, corrosion resistance, and steel material characteristics. On the other hand, in Comparative Examples 1 to 11 outside the scope of the present invention, it was found that any of the coating film adhesion, corrosion resistance, and steel material characteristics was insufficient and could not be used as a hot pressed steel material.

  From the above, the hot-pressed Al-plated coating material obtained in the present invention has good adhesion and corrosion resistance after painting, and can be applied to structural members and reinforcing members of automobile bodies. is there. If the material of the present invention is applied to a vehicle body, it becomes possible to simultaneously achieve a high strength and light weight of the vehicle body, leading to protection of irreplaceable human life and protection of the natural environment by reducing fuel consumption.

Claims (3)

A steel sheet containing C: 0.1 to 0.5% by weight, Si: 0.05 to 0.5%, Mn: 0.5 to 3% has an Fe-Al-based plating layer, and the vicinity of the surface layer of the plating layer has a depth of 2 μm. In this part, two or more phases of Fe ₃ Al, FeAl, FeAl ₂ , Fe ₂ Al ₅ , FeAl ₃ are mixed, and the average Al content of the plating layer up to a depth of 2 μm near the surface layer is weight%. 10% or more and 75% or less, and the surface layer of the plating layer has a thickness of 5 to 100 nm and contains F element and one or more metal oxides or metal oxides of Zr, Ti, Si, and A hot-pressed high-strength steel material excellent in adhesion and corrosion resistance after coating, characterized in that a coating base film of a mixed metal hydroxide is formed.   The hot-pressed high-strength steel material excellent in adhesion and corrosion resistance after coating according to claim 1, further comprising 1 to 15% by weight of Si in the plating layer.   The film thickness of the plating layer is 3 μm or more and 50 μm or less, the thickness of the coating base film is 5 nm or more and 100 nm or less, and the thickness of the cationic electrodeposition coating layer is 3 μm or more and 30 μm or less. 2. Hot-pressed high-strength steel material excellent in adhesion and corrosion resistance after painting as described in 2.