JP3844006B2 - Method for evaluating slidability of galvannealed steel sheets - Google Patents

Description

  The present invention relates to a method for evaluating the slidability of a plating-coated metal material.

As the plating layer of the plating coated metal material, there are a metal single phase plating layer and a plating layer having a plurality of types of alloy phases.
In particular, in a plated product having a plurality of types of alloy phases, it is known that various properties of the product are greatly influenced by the composition and amount of the alloy phase.
For this reason, control of the alloy phase is indispensable for improving the plating characteristics.

A plating layer of a galvannealed steel sheet having a large production volume among surface-treated steel sheets has a Zn and Fe alloy phase and is a representative example of a plating layer having a plurality of types of alloy phases.
In the above alloyed hot-dip galvanized steel sheet, the alloy phase that greatly affects the various characteristics of plating is the alloy phase of Zn and Fe (ζ phase, δ ₁ phase, Γ phase). In particular, the ζ phase has a great influence on the slidability of an alloyed hot-dip galvanized steel sheet suitable as an automobile body rust-proof steel sheet.

For the analysis of the alloy phase structure of plated steel sheets, as a physical method, observation of the cross section of the steel sheet with an optical microscope or a scanning electron microscope is generally used (Akihiko Nishimura, Junichi Inagaki, Kazuhide Nakaoka: Iron and steel, 8,101 (1986)).
According to such observation, the degree of development of each alloy phase can be obtained qualitatively and the average thickness data of each phase can be obtained quantitatively, but the problem is that the preparation and observation of the sample is complicated. It is.

Further, in recent years, with the sophistication of properties expected for plated products, a small amount of alloy phase that adversely affects the plating properties has become a problem.
That is, in the case of an alloyed hot-dip galvanized steel sheet, it is necessary to suppress the formation of the ζ phase and the Γ phase, but it is difficult to identify these minute alloy phases.
On the other hand, studies have been made to use the X-ray diffraction method to relate the diffraction intensity of each alloy phase to various characteristics of plating, and application to online measurement is being attempted.

  In other words, in alloyed hot-dip galvanized steel sheets, the relationship between the X-ray diffraction strength of each alloy phase and the slidability and powdering resistance during processing of the plated steel sheets has been reported (Masato Yamada, Aki Masuko, Toshio Hayashi, Naoki Matsuura) : Materials and Processes, 3,591 (1990)), and its application to online measurement of X-ray diffraction has been reported (Sawa Kawabe, Tadao Fujinaga, Satoshi Kimura, Kazuya Oshiba, Tadahiro Abe, Toshio Takahashi: Kawasaki Ironworks) , 18, 129 (1986)).

However, these methods are not a method for directly determining the absolute amount of each alloy phase. In order to quantify each alloy phase, a calibration curve is created using a standard sample whose content of each alloy phase is known, It is necessary to calculate the content from the strength ratio with the standard sample.
That is, for example, in the determination of a very small amount of ζ phase or Γ phase in an alloyed hot-dip galvanized steel sheet, measurement cannot be performed unless there is a standard sample whose content of ζ phase or Γ phase is known.

On the other hand, a constant current anodic electrolysis method (electrolytic peeling method) is used as a chemical method. In this method, the time of the potential flat portion corresponding to each phase is measured using a time-potential curve, and the amount of electricity is calculated. The thickness of each plated alloy phase is determined (SCBritton: J. Inst. Metals, 58, 211 (1936)).
However, in the case of the above method, the inflection point of the electric potential (electrolytic end point of each phase) is unclear in the alloyed hot-dip galvanized steel sheet with few ζ phases and Γ phases, and a very small amount such as ζ phase and Γ phase. Phase quantification is difficult.

In this method, it is difficult to uniformly dissolve each alloy phase in the plating layer.
In addition, when the above method is applied to galvannealed steel sheets, it is reported that the Γ phase with high iron concentration remains as a residue, so that there is a problem in converting the melting time of the flat part into the plating thickness as it is. (Susumu Kurosawa: Surface Technology, 45,234 (1994)).
Moreover, the shape of the time-current curve fluctuates depending on the surface state of the sample, and it was further difficult to determine the alloy phase present on the outermost surface of the plating, for example, the ζ phase of the galvannealed steel sheet.
Iron and steel, 8,101 (1986) Materials and Processes, 3,591 (1990) Kawasaki Steel Technical Report, 18,129 (1986) J. Inst. Metals, 58, 211 (1936) Surface technology, 45,234 (1994)

  An object of the present invention is to solve the above-mentioned problems of the prior art and to provide a method for evaluating the slidability of a galvannealed steel sheet.

The present invention, a galvannealed steel sheet as an anode, zinc sulfate - sodium chloride aqueous solution, the potential: -940 performed ~-920mV vs electrolysis operations within the potential of the SCE, the amount flowing electricity is 0.5C This is a slidability evaluation method for an galvannealed steel sheet characterized by having an alloyed galvanized steel sheet having a good slidability at / cm ² or less .
In the present invention, preferably the time at which current density is 5 .mu.A / cm ² and the end point of the electrolysis operation.

  Note that vs SCE described as a unit of potential indicates a potential with respect to a saturated calomel electrode.

  According to the present invention, the slidability of the galvannealed steel sheet can be evaluated.

Hereinafter, the present invention will be described in more detail.
The present inventors investigated the electrolytic behavior of the ζ phase of alloyed hot-dip galvanized steel sheets having various sliding properties. As a result, it has been found that an alloyed hot-dip galvanized steel sheet having a total electric quantity (current density × time) until the end of electrolysis of a certain amount or less has good slidability.
That is, the present invention uses an alloyed hot-dip galvanized steel sheet as an anode, performs an electrolysis operation in a zinc sulfate-sodium chloride aqueous solution within a potential range of −940 to −920 mV vs SCE, and slides depending on the amount of electricity flowing. It is a slidability evaluation method for an alloyed hot-dip galvanized steel sheet characterized by being evaluated as an alloyed hot-dip galvanized steel sheet having good mobility.

If the amount of electricity flowing at constant potential electrolysis is 0.5 C / cm ² or less, good characteristics can be obtained in various tests for evaluating slidability. As a test for evaluating the slidability, a cylindrical flat bottom cup drawing test can be exemplified. Constant-potential electrolysis is performed in a zinc sulfate-sodium chloride-based electrolytic solution with a plated plate (alloyed hot-dip galvanized steel plate) as an anode and a potential with respect to a saturated calomel electrode from -940 mV to -920 mV. The reason for setting the potential to −940 mV to −920 mV is to selectively electrolyze and quantify the portion of the alloyed hot-dip galvanized layer that has a large effect on slidability. Electrolysis is performed in a zinc sulfate-sodium chloride-based electrolyte because the chemical dissolution action of the plating layer is small and it is difficult to be affected by an oxide film formed on the surface. In addition, when changing electrolyte solution, since the electric potential which can selectively electrolyze the part which has a big influence on slidability among alloying hot dip galvanization layers changes, it is necessary to confirm by preliminary experiment.

If it is determined that the sliding properties good as with the electricity amount is 0.5 C / cm ² or less, if it is determined to have good sliding properties in the sliding property evaluation in a cylindrical flat bottom cup drawing test equivalent Judgment can be obtained. When evaluating and selecting an alloyed hot-dip galvanized steel sheet having better slidability, it is good to judge that the slidability is good based on being 0.3 C / cm ² or less.

In the present invention, the end point of the electrolysis operation is preferably the time when the current density reaches 5 μA / cm ² . If electrolysis is continued until the current density reaches 5 μA / cm ² , it can be applied to the evaluation of slidability as a substantial measurement result of electricity. Rather, the measurement of electricity when the current density exceeds 5 μA / cm ² not only increases the cost, but also measures the amount of electricity involved in the unintended electrolytic reaction, and may lead to erroneous measurement results of electricity. Sexuality increases.

Hereinafter, the present invention will be described more specifically based on examples.
An alloyed hot-dip galvanized steel sheet as a test material was produced by the following method.
After ultra-low carbon steel specimens were melted in a converter, slabs were formed by continuous casting. The slab was wound at 550 ° C. with a slab heating temperature of 1150 to 1250 ° C. and a final finishing temperature of the hot rolling process of 920 ° C. A hot rolled coil coil with a thickness of 3.2 mm was prepared, the black skin was removed by pickling, and then cold rolled to obtain a cold rolled sheet with a thickness of 0.8 mm. This steel plate was used as a plating original plate at an annealing temperature of 790 to 830 ° C. in a continuous galvanizing line. The intrusion plate temperature to the plating bath was 460 to 470 ° C, the bath temperature of the plating bath was 460 to 470 ° C, and the alloying temperature was 490 to 530 ° C. The coating amount on one side was 40 to 50 g / m ^2, and the coating amount on both sides was the same.

The alloyed hot-dip galvanized steel sheet produced as described above was punched into a 15 mmφ circle, and then subjected to constant potential electrolysis at −930 mV vs SCE. A 20 mass% zinc sulfate-10 mass% sodium chloride aqueous solution was used as the electrolytic solution. Electrolysis was performed until the current density reached 5 μA / cm ² or less, and the amount of electricity that flowed from the start of electrolysis was measured. The time required for electrolysis was about 10 to 20 minutes.
The slidability was evaluated for the alloyed hot-dip galvanized steel sheet whose electric quantity was measured. The alloyed hot-dip galvanized steel sheet was coated with 1.5 g / m ² of normal rust-preventive oil and then subjected to a 33 mmφ cylindrical flat bottom cup squeeze test to determine the limit squeeze ratio. The smaller the numerical aperture ratio, the better the slidability. The critical drawing ratio is 2.0% or more ... 1, 1.9-2.0% ... 2, 1.8-1.9% ... 3, 1.7-1.8% ... 4, 1.7% or less ... 5, and the results are shown in Table 1.

All plated steel sheets with an electric quantity of 0.5 C / cm ² or less showed good slidability with a slidability of “Evaluation 3” or less, whereas with “Sample 5” exceeding 0.5 C / cm ² , The slidability is inferior to “Evaluation 5”. In particular, all of the plated steel sheets having an electric quantity of 0.3 C / cm ² or less were “Evaluation 1” and showed particularly excellent slidability.
From the above results, it can be seen that the slidability of the galvannealed steel sheet can be evaluated by the method of the present invention.

Claims (2)

An alloyed hot-dip galvanized steel sheet is used as an anode, and an electrolysis operation is performed in a zinc sulfate-sodium chloride aqueous solution within a potential range of −940 to −920 mV vs SCE, and the amount of electricity flowing is 0.5 C / cm ² or less. A slidability evaluation method for alloyed hot-dip galvanized steel sheet, characterized in that the galvannealed steel sheet has good slidability . The slidability evaluation method according to claim 1, wherein the time point when the current density becomes 5 μA / cm ² is set as the end point of the electrolysis operation.