JP3223430B2 - Method for producing Al-Mg alloy plate with excellent pitting resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an aluminum alloy having excellent pitting corrosion resistance.
-A method for producing a Mg alloy plate, in particular, a method for producing a material which is used in an environment where chloride ions are present, such as a can lid of a ship or a food can, has strength and is suitable for applications in which through holes due to pitting corrosion should be avoided. About.

[0002]

2. Description of the Related Art Conventionally, Al-Mg based alloys have moderate strength and excellent formability and weldability, so that they are used for structural materials for ships and vehicles, and packaging for can lids and the like. Widely used as a material.

However, these materials may cause pitting corrosion due to local dissolution in a severe corrosive environment in which chlorine ions are contained in an amount of several hundred ppm or more. For example, penetration holes caused by such pitting may cause seawater to enter when used in a hull, and may lead to serious defects such as leakage of contents when used in a can.

Conventionally, as a method for preventing through holes due to pitting, a method of cladding a skin material that is electrochemically less than the core material as a sacrificial anode (Japanese Patent Publication No. 2-25783, Japanese Patent Publication No. No. 29585) and a method of preventing pitting corrosion by adding Zn (Japanese Patent Publication No. 62-1987).
No. 3231) has been proposed.

[0005]

However, the method of cladding a skin material that is electrochemically lower than the core material as a sacrificial anode requires a complicated manufacturing process and makes it difficult to control the remaining material. There are drawbacks. Further, there is a disadvantage that adding a component element leads to an increase in material cost.

[0006] Accordingly, the present invention provides an Al-M
It is an object of the present invention to provide a method for producing an Al-Mg alloy plate having improved pitting corrosion resistance by controlling the distribution of the precipitation amount of a g-based compound.

[0007]

Means for Solving the Problems To achieve the above object, the present inventors, in order to prevent through-holes caused by pitting of an aluminum alloy, have to make the surface layer of the material more electrochemically than the center of the sheet thickness. Based on the idea that it should be made base, we have conducted extensive research on a method of precipitating a β phase (Al ₈ Mg ₅ ) known as a base precipitate on the surface layer of the material.
It has been found that this can be realized by controlling the rolling strain rate and the rolling end temperature at a temperature not higher than the precipitation temperature, and have completed the present invention.

[0008] The present invention based on the above-mentioned findings shows that M
g: 3.0-6.0%, Mn: 0.2-1.0
%, Cr: one or two of 0.05 to 0.4%, Fe: 0.5% or less as an impurity, Si: 0.
5% or less, the aluminum alloy ingot consisting of the balance Al and unavoidable impurities, after the homogenization treatment, start hot rolling, at least the rolling conditions in the final rolling stand,
In the range where the temperature of the rolled material is equal to or lower than the temperature represented by the formula T (° C.) = 120 + 27.1 Mg (%), the strain rate per pass is set to 2 / sec or more, and the rolling end temperature is set to 15
A method for obtaining a hot-rolled sheet at 0 to 280 ° C. and, if necessary, cold rolling and stabilization after completion of hot rolling to produce an Al—Mg alloy sheet having high strength and excellent pitting corrosion resistance. The method is summarized.

[0009]

The structure and operation of the present invention will be described. Generally, the eutectic temperature of an Al-Mg alloy is 450
At that time, the Mg solid solubility limit at that time is 14.9%, and as the temperature decreases, the Mg solid solubility limit also decreases. Therefore, if hot rolling is performed at a temperature lower than the solid solubility limit, Mg may precipitate. Are known.

Therefore, the β phase (Al ₈ Mg ₅ ), which is known as an electrochemically noble precipitate, is rolled in a temperature range according to the Mg content in order to change the precipitation state in the thickness direction. It is necessary that the strain rate per pass be 2 / sec or more and the rolling end temperature be 150 to 280 ° C.

Here, the strain rate per rolling pass was calculated using the following equation. ε '= [1/60] · [ V R / √ (R' · h)] · [2√r / (2-r)] where, epsilon '; average strain rate (s ^-1) h; enters Side plate thickness (m) R '; Roll diameter (m) r; Reduction ratio (decimal number) V _R ; Rolling speed (m / min.)

The reasons for limiting the manufacturing conditions and alloy composition range in the present invention will be described. Rolling conditions: If the temperature (T ° C.) exceeds the range calculated by the formula: T (° C.) = 120 + 27.1 Mg (%), the formation of β phase hardly occurs, so that the difference in the amount of precipitation in the thickness direction hardly occurs. .

[0013] The reason why the strain rate per rolling pass is 2 / sec or more is that the number of dislocations formed by rolling is larger than the number of dislocations that disappear by recovery.
When the strain rate per rolling pass is less than 2 / sec, dislocations formed by rolling disappear one after another (recovery), so that precipitation of β phase on the dislocations becomes difficult to occur, and the amount of precipitation in the thickness direction of the sheet is reduced. The difference hardly occurs.

When the rolling end temperature is set to 150 to 280 ° C., dislocations in the surface portion of the sheet thickness are more likely to remain than those in the central portion of the sheet thickness, β phase precipitates on the dislocations, and the surface portion of the sheet thickness becomes more intense. It becomes electrochemically low. In other words, the surface layer part of the thickness acts as a sacrificial anode, and a through-hole due to pitting is avoided.

When the rolling end temperature exceeds 280 ° C., the dislocations formed by rolling recover more quickly, so that precipitation of the β phase hardly occurs, and the surface layer of the sheet thickness cannot have a sacrificial anode effect. Although the effects of the present invention can be obtained even if the rolling end temperature is lower than 150 ° C., the deformation resistance of the material increases, which is industrially disadvantageous.

Cold rolling and stabilization treatment: Al-Mg based aluminum alloy has excellent strength even when hot rolled.
Stabilization processing is performed as post-processing.

For example, in the case of a material for cans, cold rolling and stabilization of 50% or more are performed to increase the strength, and in the case of a material for ships, the strength is increased and the thickness of the member is reduced. In some cases, about 20% of cold rolling and stabilization processing are performed in order to achieve this.

The stabilization treatment is 120 to 200 industrially.
This is a method of treating at several degrees Celsius for several hours, and Al-Mg-based aluminum alloys are softened after a long time at room temperature when cold-rolled, so that they are heated to a low temperature in advance to slightly reduce mechanical strength and increase elongation. Therefore, it is possible to prevent the mechanical properties from changing over time.

Next, the reason why the composition range of the material is limited will be described. Mg: Mg is an element necessary for increasing the strength of the material, and its content needs to be changed according to the use (for ships or can lids). However, if it is less than 3%, the strength and elongation decrease, and if it exceeds 6%, cracks tend to occur during hot rolling.

Mn, Cr: Mn and Cr increase the strength,
Formability is improved by making the structure finer. In any case, if the amount is less than the lower limit, the above-mentioned effect is not obtained. If the amount exceeds the upper limit, the compound is crystallized as a coarse compound at the time of casting, and elongation and formability are reduced.

Fe: Fe, when Mn is contained, forms a coarse Al-Fe-Mn intermetallic compound and deteriorates formability, so that Fe is preferably smaller. However, 0.5
% Or less, a coarse Al-Fe-Mn intermetallic compound is not formed, and this effect is small.

Si: Since Si precipitates at the grain boundaries and deteriorates the formability, it is preferable that Si is small. However, 0.5
%, There is no practical problem.

[0023]

EXAMPLES The present invention will be described specifically with reference to Examples.
Thus, the present invention is not limited. Example 1 An aluminum alloy shown in Table 1 was melted and ingot, and 500 ° C.
A homogenization treatment of × 8 hr was performed, and the sheet was rolled to a thickness of 5 mm under the conditions shown in Table 2. As for the strain rate of the rolling, hot rolling was performed under the conditions shown in Table 2, with the roll diameter of the rolling mill being 325 mm.

[0024]

[Table 1]

After the obtained test material was subjected to a stabilization treatment at 170 ° C. for 2 hours, mechanical properties were measured and a pitting resistance test was performed. The mechanical properties were measured using JIS No. 5 test pieces. Since this material is intended for marine use, its proof stress is 2
20 MPa or more was defined as the material of the present invention.

The pitting corrosion evaluation test was carried out using 0.1 M NaCl
In an aqueous solution, constant current electrolysis was performed using a test material adjusted to a surface area of 1 cm ² as an anode and platinum as a cathode. The current density was 0.5 mA / cm ² , the electrolysis period was 30 days, and after the electrolysis was completed, the maximum depth of the pit portion was measured with an optical microscope and a microstructure photograph. As a criterion for evaluation, a material having a maximum pitting corrosion depth of 2 mm or less was defined as a material of the present invention. Table 2 shows the results.

[0027]

[Table 2]

In Example No. of the present invention, Nos. 1 to 9 are obtained by performing the material components and hot rolling conditions within the scope of the present invention.
A yield strength of 25 to 255 MPa is obtained, and the maximum pitting corrosion depth is as good as 1.4 to 1.6 mm.

However, in Comparative Example No. 10 and 11 are
Since the strain rate during rolling was as low as 0.6 / sec, the maximum pitting corrosion depth was increased to 2.9 mm and 3.3 mm. N
o. 12 and 13 are hot rolling end temperatures of 320 ° C., 3
Since it is as high as 40 ° C., the maximum pit depth is 3.2 mm and 3.
It became as large as 5 mm.

No. 14 is Mg content 2.4%, Cr
Since the content is as small as 0.01%, the yield strength is 18%.
It became as low as 5 MPa. No. In No. 15, since the Mn content was as low as 0.16% and the Cr content was as low as 0.03%, the yield strength was as low as 210 MPa. No. Sample No. 16 had a high Mg content of 7.1%, and thus cracks occurred during hot rolling. Therefore, the subsequent test was stopped.

Example 2 In the same manner as in Example 1, the aluminum alloys shown in Table 1 were melted and ingot, and homogenized at 500 ° C. for 8 hours.
Hot rolling was performed under the conditions shown in Table 3. Thereafter, cold rolling is performed, and further stabilization treatment (170 ° C. × 2 hours) and baking at 205 ° C. × 10 minutes in an oil bath (for cans)
Substantial treatment was applied.

The mechanical properties of the obtained material were tested in the same manner as in Example 1. In the pitting corrosion evaluation test, the following tests were performed to promote corrosion. That is, the test material is subjected to U-bending (the bending radius is the same as the plate thickness), and then a 3% -NaCl aqueous solution (pH = 3.
0), alternate immersion (40 ° C. × 30 minutes immersion, 50 ° C. × 3)
(0 minute drying), and after 10 days, the bent portion of the test material was observed, and the presence or absence of through holes due to pitting was observed.
Table 3 shows the results. Since this material is intended for cans, the proof stress after baking is 315 MPa.
The above is the material of the present invention.

[0033]

[Table 3]

The material of the present invention has Nos. 17 to 21 were obtained by subjecting the components of the material and the conditions of hot rolling, cold rolling and stabilizing treatment to within the scope of the present invention. After a treatment equivalent to paint baking, a yield strength of 318 to 342 MPa was obtained. The through-hole due to pitting was not recognized in the bending corrosion test, and was good.

However, in Comparative Example No. 22 and 23 are
Since the strain rate during rolling is as low as 0.6 / sec, the yield strength is 31
It was as low as 0 and 295 MPa, and in both cases, through holes due to pitting were observed in the U-bending corrosion test.

No. In Nos. 24, 25 and 26, the rolling end temperature was as high as 320 ° C. 24 and 25 have a proof stress of 255
In each case, through-holes due to pitting were observed in the U-bending corrosion test.

No. 27 has a Mn content of 0.16% and C
Since the r content is as small as 0.03%, the yield strength is 3%.
It is as low as 10 MPa. No. 28 has a Mg content of 7.1%
Therefore, cracks occurred during hot rolling, and the subsequent test was stopped.

[0038]

Since the present invention is constructed as described above, the obtained material has a high strength and does not generate through holes due to pitting even when used in an environment where chloride ions are present. This is very useful industrially.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-4-63255 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22F 1/04-1/057 B21B 3 / 00 C22C 21/00-21/18

Claims (2)

(57) [Claims] 1. An alloy containing 3.0 to 6.0% of Mg by mass%, 0.2 to 1.0% of Mn, and 0.05 to 0.4 of Cr.
%, One or two of which contain Fe as an impurity:
After 0.5% or less, Si: 0.5% or less, the aluminum alloy ingot consisting of the remaining Al and unavoidable impurities is subjected to homogenization treatment and then hot-rolled. In the range where the temperature of the material is equal to or lower than the temperature represented by the following formula, the strain rate per pass is set to 2 / sec or more, and the rolling end temperature is set to 150 to 28.
0 ° C. followed by stabilization
Al-M with proof stress of 20MPa or more and excellent pitting corrosion resistance
g Alloy plate manufacturing method. T (° C.) = 120 + 27.1 Mg (%) 2. The composition contains 3.0 to 6.0% of Mg by mass%, 0.2 to 1.0% of Mn, and 0.05 to 0.4 of Cr.
%, One or two of which contain Fe as an impurity:
After 0.5% or less, Si: 0.5% or less, the aluminum alloy ingot consisting of the remaining Al and unavoidable impurities is subjected to homogenization treatment and then hot-rolled. In the range where the temperature of the material is equal to or lower than the temperature represented by the following formula, the strain rate per pass is set to 2 / sec or more, and the rolling end temperature is set to 150 to 28.
A method for producing an Al-Mg alloy plate having a proof stress of 315 MPa or more after coating baking and excellent in pitting corrosion resistance, wherein the Al-Mg alloy sheet is subjected to cold rolling and stabilization treatment after rolling at 0 ° C. T (° C.) = 120 + 27.1 Mg (%)