JP2002294487A - Electrolytic zinc-base plated steel-sheet with high strength - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electrolytic zinc-base plated steel-sheet with high strength suitable for a material of an automotive inner and outer sheet, superior in formability; adhesiveness of the plated film, corrosion resistance after forming, and further a continuous spot-welding property. SOLUTION: The steel sheet includes 0.05-2% Si, 0.1-3% Mn, and 0.1-3% Al, satisfies Si(%)+0.5Mn(%)+Al(%)>=0.5, has a plated layer on at least its one side, and has a metal oxide layer of 0.05-1 μm thick containing at least one kind among Si, Mn, and Al, in the boundary between the plated layer and the steel sheet as a base material.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a material for automobiles, home electric appliances, building materials, etc., and more particularly to a material suitable for inner and outer panels of automobiles. The present invention relates to a high-strength electrogalvanized steel sheet having excellent continuous hitting properties.

[0002]

2. Description of the Related Art High-strength steel sheets are widely used as cold-rolled steel sheets for use in vehicle body components, exteriors, and the like to reduce the body. The characteristics of such high-strength steel sheets for automobiles are required to have sufficient strength, formability in press working, and spot weldability in assembling automobile bodies to ensure the safety of automobiles. I have.

For example, Japanese Patent Application Laid-Open No. 5-70886 discloses a high-strength steel sheet in which the chemical composition and structure of steel are specified, and a method for producing the same. Recently, the corrosion resistance of these high-strength steel sheets has been emphasized, and the plating properties of the steel sheets themselves have become important. Already, in the field of hot-dip plating, 340 MPa (35
The plating properties of high-strength steel sheets of kgf / mm ² ) class or higher have been studied. For example, Japanese Patent Application Laid-Open No. 9-41111 discloses a Si-added steel. However, if an element that easily forms an oxide such as Si, Mn, or Al is added to the base material component to increase the strength, problems such as deterioration of plating adhesion due to the oxide generated on the base material surface are caused. is there.

[0004]

A steel sheet containing Si, Mn, Al or the like as a base material component of a high-strength steel sheet and having these metal oxides in the surface layer is generally called a difficult-to-plate material. Also in the case of electroplating, sufficient plating adhesion cannot be obtained when ordinary plating pretreatment is performed.

[0005] The present invention solves these problems in the prior art, and is mainly a high-quality material excellent in plating adhesion, corrosion resistance after molding, and continuous spotting during spot welding, which is suitable as a material for automobile inner and outer panels. It is an object to provide a high-strength galvanized steel sheet.

[0006]

Means for Solving the Problems In order to study electroplating on a high-strength steel sheet, the present inventors first performed electrogalvanizing using a steel sheet containing Si, Mn and Al as a base material. It was confirmed that there was a case where plating adhesion deteriorated and a case where it did not. A detailed investigation of the cause of the poor plating adhesion of this base material revealed that these elements were formed as oxides on the surface during annealing. Therefore, in a material in which metal oxides such as Si, Mn, and Al were formed on the surface of a steel sheet, attention was paid to the effect of plating pretreatment conditions on the adhesion of an electrogalvanized film,
As a result of conducting detailed research focusing on electrolytic pickling,
New findings were obtained as described below.

(1) When the adhesion of the plating film is poor, the interface between the base material of the steel sheet and the plating layer has a thickness exceeding 1 μm on the interface.
Residual metal oxide layers such as i, Mn, and Al were observed.

(2) Deterioration of the adhesion of the plating film is caused by the presence of these metal oxide layers, which weakens the metal bond between the base material and the plating layer. As a result, the bond between the base material surface and the plating layer is weakened. It was presumed that it was.

(3) In order to grasp the cause of the poor adhesion in more detail, the present inventors used a high-strength cold-rolled steel sheet of 340 MPa class as a base material, fixed the electrolytic pickling time, After electrolytic pickling by changing the amount of electricity per unit surface area during washing (hereinafter simply referred to as “electrical density”), the thickness of the oxide layer of Si, Mn and Al on the surface of the base material is changed to the secondary ion mass. Using the analytical method (SIMS), the base material bulk Si,
It was determined from the amount of sputtering until the strength of Mn and Al was reached. The result is shown in FIG.

According to FIG. 1, the metal oxide layer contains Si, Mn and Al as main components, and the thickness of the oxide layer is the largest at about 0.65 μm for Mn oxide, and decreases in the order of Si oxide and Al oxide. I knew I was going to do it.

(4) After pickling, electroplating was performed under the same conditions to produce an electrogalvanized steel sheet. The adhesion of the plating films of these steel sheets was investigated by the following method. A disk-shaped blank with a diameter of 90 mm was sampled from an electro-galvanized steel sheet, deep-drawn into a cylindrical shape with a diameter of 50 mm and a depth of 28 mm, and a transparent adhesive tape was stuck on its side wall surface.
After peeling off, the area ratio of the peeled piece adhered to the pressure-sensitive adhesive tape was visually determined, and evaluated using the following “contact code”.

1: No peeling, 2: 10% or less, 3: 10% or more and less than 30%, 4: 30% or more.

FIG. 2 shows the relationship between the thickness of the metal oxide layer after electrolytic pickling of the steel sheet obtained in the above experiment and the adhesion cord. From FIG. 2, when the thickness of the metal oxide layer was 1 μm or less, the adhesion code was 1 or 2, indicating that the plating adhesion was good.

(5) Although the details of the mechanism of this phenomenon are unknown, when the thickness of the oxide layer such as Si, Mn, or Al existing on the surface is reduced, a number of vacancies are generated in the oxide layer, It is presumed that the portion effectively functions as a site for forming a metal bond, and that the adhesion between the plating layer and the base material is improved.

(6) The present inventors further studied the relationship between the thickness of the metal oxide layer and the continuous hitting property during spot welding. The continuous hitting property was investigated by the following method.

[0016] Two electrogalvanized test pieces are stacked,
Using an electrode rod with a dome-shaped tip and a diameter of 5 mm, a welding force of 300 kgf, 60 Hz, current of 27,000 A for 6 cycles, spot welding for 20 seconds at 2 second intervals, 40 seconds each The resting condition was repeated and performed continuously. Three welding test pieces were taken at every 100 shots, a tensile test was performed in a direction parallel to the plate surface, and the diameter of the nugget at the fractured weld was measured. The nugget diameter was 4t ^0.5 (t: mm The number of continuous hits until the thickness became equal to or less than the indicated steel sheet thickness) was determined.

FIG. 2 shows the relationship between the thickness of the metal oxide layer obtained in the above experiment and the continuous hitting property, together with the result of the adhesion described above. The continuous hitting property was indicated by a continuous hitting number code (A: extremely good to X: bad). As the thickness of the metal oxide layer increases, the continuous hitting property improves, and the thickness of the oxide layer is 0.05
When the particle size is not less than μm, the evaluation of the continuous hitting property is extremely good as “◎”. This phenomenon is presumed to be due to the fact that the electrical resistance of the galvanized film was increased due to the presence of an oxide layer such as Si, Mn, or Al on the surface of the base material, and as a result, the continuous spotting property during spot welding was improved.

From these results, it has been clarified that the presence of an oxide layer having an appropriate thickness can simultaneously improve the adhesion and the continuous hitting property during spot welding.

The present invention has been made on the basis of the above-mentioned findings, and is described as follows. "In mass%, Si: 0.05 to 2%, M
n: 0.1 to 3% and Al: 0.1 to 3%,
And a steel sheet having a plating layer on at least one side of a steel sheet satisfying Si (%) + 0.5 Mn (%) + Al (%) ≧ 0.5,
The interface between the plating layer and the base material of the steel sheet contains at least one of Si, Mn, and Al, and has a thickness of 0.05 to 1 μm.
A high-strength electrogalvanized steel sheet having a metal oxide layer of: ”. Here, the electrogalvanized steel sheet refers to a plating bath containing one or more metal ions of Co, Fe, Ni, Cr and Mn in addition to zinc ions in the plating bath. Steel plate plated with C
one or two of o, Fe, Ni, Cr and Mn
It refers to a steel sheet containing a total of 0.01 to 15% of a kind or more.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The reasons for limiting the chemical composition of the steel sheet base material and the thickness of the metal oxide layer at the interface between the plating layer and the base material are as follows.
It is as follows. % Means mass%.

(1) Si: 0.05 to 2% Si is a component that enhances the strength and ductility of the base material.
If the content is less than 0.05%, the strength and ductility of the base material are insufficient. On the other hand, if the content exceeds 2%, the ductility as a high-strength steel sheet for automobiles is impaired, so the upper limit is 2%. When particularly high strength and ductility are required, a content of 0.2 to 1% is desirable.

(2) Mn: 0.1 to 3% Mn is a component that enhances the strength and ductility of the base material like Si. If the content is less than 0.1%, the strength and ductility of the base material are insufficient. On the other hand, if the content exceeds 3%, the upper limit is 3% in order to deteriorate ductility. Mn is
Since an additional element for increasing the strength is expensive, it is desirable from the viewpoint of cost to minimize the amount of addition. The desirable content is 2% or less, and more preferably 1.5% or less when secondary processing embrittlement is a problem for automobiles.

(3) Al: 0.1-3% Al is also a component that increases the strength and ductility of the base material, and is particularly effective for increasing the collision resistance of automobiles. Si, M
If the content is less than 0.1% as in the case of n, the strength and ductility of the base material are insufficient. On the other hand, if the content exceeds 3%, the surface properties deteriorate during steelmaking, so the upper limit is set to 3%. Further, scale unevenness may occur during pickling of the hot-rolled sheet and the surface properties may deteriorate, so the upper limit of the desirable content is 1.5%.

(4) Si (%) + 0.5 Mn (%) + Al (%)
≧ 0.5 The contents of Si, Mn and Al are as described in the above (1),
In order to satisfy (2) and (3) and further increase the strength, it is necessary that Si (%) + 0.5Mn (%) + Al (%) be 0.5 or more. Desirably, it is 2.0 or more.

(5) Thickness of metal oxide layer: 0.05 to 1
μm As described above, a metal oxide layer containing at least one of Si, Mn, and Al is required at the interface between the base material and the plating layer. 0.05 to improve spot weldability
μm or more is required. On the other hand, in order to improve plating adhesion, the thickness must be 1 μm or less. Desirable thickness of the metal oxide layer is 0.1 to 0.5 μm.

The elements contained in the metal oxide layer are mainly Si, Mn or Al.
Although trace amounts of r, Ti, Nb, etc. may be contained, there is no problem even if trace components of 1% or less with respect to the total composition of the metal oxide layer are contained.

As the chemical composition other than Si, Mn and Al of the base material, C is 0.01 to 0.25%, and P is 0.1 to 0.2%.
10% or less, S is 0.01% or less, N is 0.000
1 to 0.0030%, Cr is 0.5% or less, Ti is
0.10% or less, Nb is 0.10% or less, B is 0.1% or less.
It is desirable that the content is not more than 01% and the balance is Fe and impurities.

An example of a pretreatment method for producing a high-strength electrogalvanized steel sheet having a metal oxide layer of 0.05 to 1 μm with respect to the above base material will be described below.

1) As a pretreatment for plating, electrolytic pickling is performed while increasing the density of electricity.

2) Further, a pickling accelerator such as an alkylphenol-based or higher alcohol-based acid is added to the pre-treated pickling solution, and the electric charge density is increased to perform electrolytic pickling.

3) Further, before the electrolytic pickling, the electric quantity density is increased to perform electrolytic degreasing.

4) In the hot rolling step, the hot rolling winding temperature is
A low temperature of 650 ° C. or less is desirable.

5) As cooling after continuous annealing of the cold-rolled steel sheet,
It is desirable to use rapid cooling such as gas cooling or mist cooling. 6) As a plating pretreatment, a method of mechanically removing the base material surface layer by grinding may be used together.

The pretreatment method is not particularly limited as long as the method for suppressing the formation of the metal oxide layer is adopted as much as possible. However, as defined above, the oxide layer is
It is necessary to leave a thickness of at least μm. In addition, when a plurality of the above treatment methods are combined, the effect of controlling the thickness of the metal oxide layer is further increased. From the viewpoint of cost, the chemical treatment methods 1) and 2) are effective.

The electro-zinc plating bath for plating may be any of an acidic bath (eg, a sulfate bath, a chloride bath, etc.) and an alkaline bath (eg, a cyanide bath, etc.).

The plating bath contains a zinc source or one or more metal sources of Co, Fe, Ni, Cr and Mn in addition to the zinc source. To 15%. The zinc source or these metal sources added to the plating bath are added in the form of sulfates, acetates, carbonates, molybdates, hypophosphites, or organometallic salts to achieve the target composition. do it. In the method of the present invention, any plating film may be applied as long as it is an electro-zinc plating film. For example, a dispersed electroplating film of ceramic fine particles may be used.

The electrogalvanized steel sheet of the present invention is effective not only for automobiles but also for home appliances and building materials. The electrogalvanized steel sheet of the present invention can be used for various applications as described above as it is, but depending on the application and use environment, a chromate treatment, an organic thin film film (for example, , An epoxy resin-based film having a thickness of 2 μm or less). Further, this plating film may be applied to one side of the base material or may be applied to both sides depending on the use and the required rust-preventive site.

As described above, the steel containing a predetermined amount of Si, Mn and Al is used as the base material, and the metal oxide layer having the optimum thickness is present at the interface between the base material and the plating layer, so that the plating adhesion and the forming property can be improved. It has become possible to provide a high-strength electrogalvanized steel sheet having excellent post-corrosion resistance and further excellent continuous spotting properties during spot welding.

[0039]

EXAMPLE 1.4 mm thick having the chemical composition shown in Table 1.
After degreasing using a high-tensile cold-rolled steel sheet, the current density was set to 15 A / dm ^{2 in} an 8% by weight sulfuric acid solution at 50 ° C.
After electrolytic pickling for about 12 seconds, the following three plating baths (Z
n plating, Zn-Ni plating and Zn-Co plating)
Then, plating was performed so that the adhesion amount became 30 g / m ² and 60 g / m ² . The plating bath temperature was 50 ± 2 ° C., the current density was 60 A / dm ² , and the flow rate of the plating solution was 1 m / sec.

[0040] 1) Zn plating bath _{ZnSO 4 · 7H 2 O: 350g} / l, Na 2 SO 4: 75g / l, pH: 1.8 2) Zn-Ni plating bath _{ZnSO 4 · 7H 2 O: 100g} / l _{, NiSO 4 · 6H 2 O:} 270g / l, Na 2 SO 4: 75g / l, pH: 1.8 3) Zn-Co plating bath _{ZnSO 4 · 7H 2 O: 430g} / l, CoSO 4 · 7H 2 O _{: 74g / l, Na 2 SO} 4: 75g / l, pH: 1.8

[0041]

[Table 1] Table 2 shows the results of evaluating the thickness of the metal oxide layer, the adhesion of the plating film, the corrosion resistance after molding, and the spot weldability after the electroplating by the methods described below.

[0042]

[Table 2] (1) Thickness of metal oxide layer The thickness of the metal oxide layer at the interface between the plating and the base material was determined by plating the sample after plating in a 6% hydrochloric acid solution containing a pickling accelerator (alkylphenol type). After separation from the material, the surface of the base material was analyzed by secondary ion mass spectrometry (SIMS).
The strength of Si, Mn, and Al in the depth direction was determined from the sputtering time required to reach the strengths of Si, Mn, and Al in the base material bulk, respectively.

(2) Adhesion of plating film A disk-shaped blank having a diameter of 90 mm was sampled from an electrogalvanized steel sheet, deep-drawn into a cylinder having a diameter of 50 mm and a depth of 28 mm, and a transparent adhesive was applied to the side wall surface. After attaching the tape, the tape was peeled off, and the area ratio of the peeled piece adhered to the pressure-sensitive adhesive tape was visually determined, and evaluated by the above-mentioned adhesion code (1 to 4). Adhesion cords 1 and 2 were accepted and 3 and 4 were rejected.

(3) Corrosion resistance after molding Using the above deep drawing molded sample, the area ratio of red rust occurrence after salt spray test (5% NaCl, 35 ° C., 60 hours) was measured, and classified into the following five stages. Was evaluated.

◎: No occurrence of red rust (pass) ○: less than 5% (pass) △: 5% or more and less than 10% (pass) ×: 10% or more, less than 30% (fail) XX: 30% or more (Fail) (4) Continuous spotting after spot welding Two test pieces subjected to electro-zinc plating were stacked, and a dome-shaped electrode rod having a diameter of 5 mm and a pressure of 300 kgf was used. The spot welding was performed under the conditions of 60 Hz, 27000 A, and 6 cycles. The conditions of continuous 20 points at 2 second intervals and a pause of 40 seconds were repeated.
Three welding test pieces were taken at every 100 spots, a tensile test was conducted in the welding direction of the plate surface, the diameter of the nugget at the broken weld was measured, and the number of continuous spots at which the nugget diameter sharply decreased was determined by the method described above. It was evaluated and evaluated in the following five stages.

◎: 3000 points or more (pass) ○: 2000 points or more and less than 3000 points (pass) △: 1500 points or more and less than 2000 points (fail) ×: 1000 points or more and less than 1500 points (fail) × ×: Less than 1000 points (fail) As can be seen from the performance evaluation results in Table 2, the steel sheets (test Nos. 1 to 4) satisfying all the conditions specified in the present invention have excellent adhesion of the plating film, and after forming. Was good in both corrosion resistance and spot weldability, and passed (〇) in the overall evaluation. Steel sheets (test Nos. 5 to 10) whose metal oxide layer thickness was out of the range specified in the present invention were rejected (x) in the comprehensive evaluation.

[0047]

The high-strength electro-galvanized steel sheet of the present invention is excellent in formability, plating adhesion, corrosion resistance after forming, and continuous spotting during spot welding. And the like, particularly as a material for the inner and outer panels of an automobile.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating an example of a profile of a change in the concentration of Si, Mn, and Al in a depth direction from a surface of a general high-strength steel sheet base material after electrolytic pickling.

FIG. 2 is a view showing the relationship between the thickness of a metal oxide layer, plating adhesion, and continuous hitting properties.

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Masahiko Hori 4-5-33 Kitahama, Chuo-ku, Osaka-shi, Osaka Sumitomo Metal Industries, Ltd. F-term (reference) 4K024 AA05 AA17 AA19 AA20 AB06 BA02 BB02 BB15 BB18 BC01 DA02 DB10 EA06 GA01 GA04 GA07 GA14

Claims (1)

[Claims] (1) In terms of mass%, Si: 0.05 to 2%, Mn:
A steel sheet containing 0.1 to 3% and Al: 0.1 to 3% and having a plating layer on at least one surface of a steel sheet satisfying Si (%) + 0.5Mn (%) + Al (%) ≧ 0.5 A steel sheet, wherein Si, Mn and A
1 at least one of which has a thickness of 0.05 to 1
A high-strength galvanized steel sheet having a μm metal oxide layer.