JPH036985B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing an Al-Zn-Mg alloy having excellent weldability and stress corrosion cracking resistance. In general, Al--Zn--Mg alloys have excellent mechanical properties and weldability, and are therefore widely used in railway vehicles and various land structures. However, as this type of high-strength Al alloy increases in strength, stress corrosion cracking becomes more likely to occur.
Al-Zn-Mg alloys are no exception; when the Mg and Zn contents are increased to increase strength, stress corrosion cracking resistance deteriorates. Further, Al-Zn-Mg alloy is the material with the highest strength among Al alloys that can be welded, but as the Mg and Zn contents increase, weldability also deteriorates. This is also the reason why the development of high-strength welded structural materials is hindered. However, to date, stress corrosion cracking has been reduced by improving the ingredients and manufacturing conditions.
Although there is no longer a possibility of stress corrosion cracking occurring in the parallel and perpendicular directions of plates and sections, there is a possibility of stress corrosion cracking occurring in the plate thickness direction and at welded parts depending on the usage conditions. In recent years, the use of thick-walled materials has increased in order to increase the size of structures and rationalize design and construction, and the stress generated in the plate thickness direction and welded parts is large, resulting in stress corrosion resistance and cracking. There is a strong demand for improved sexual performance. The present invention solves various problems in high-strength Al alloys as explained above.
-Provides a method for producing a Zn-Mg alloy. The manufacturing method of the Al-Zn-Mg alloy with excellent weldability and stress corrosion cracking resistance according to the present invention is characterized by Zn3.0-8.0wt%, Mg0.3-3.0wt%,
Ti0.005~0.20wt%, B0.005~0.05wt%, Ni0.2~
Contains 1.5wt%, and Mn0.05-0.40wt%,
An ingot containing one or more selected from 0.05 to 0.40 wt% of Cr and 0.05 to 0.25 wt% of Zr, with the balance consisting of Al and impurities, with the crystal grain size refined to 1500 μm or less, is After homogenization treatment at a temperature of ~550°C, hot working is performed at a temperature of 350~500°C with a processing rate of 60% or more, and the ratio of the short axis to the long axis of the grain after the final heat treatment is 1. : 5 or more, and the short axis length is 80 μm or less. The method for producing an Al-Zn-Mg alloy with excellent weldability and stress corrosion cracking resistance according to the present invention improves the stress corrosion cracking resistance without impairing weldability. This is a type of brittle fracture that occurs at grain boundaries, and the initial cause is said to be preferential elution of grain boundaries due to the potential difference between grain boundaries and within grains, and as the Mg and Zn contents increase, the strength increases. As a result, the potential difference between the grain boundaries and the grain increases, making stress corrosion cracking more likely to occur. However, the inclusion of Ni prevents preferential elution of grain boundaries and improves stress corrosion cracking resistance. There is.
Although the Ni content improves stress corrosion cracking resistance, weldability does not deteriorate even if the Ni content increases. Ti and B are important elements for refining the structure and also contribute to improving weldability. Mn, Cr, and Zr are elements to be included to stabilize the structure, and it is necessary to include one or more selected from these elements to improve weldability. In addition, as the Mg and Zn contents increase, the melting temperature at the grain boundaries decreases, making it easier for cracks to occur at the grain boundaries due to temperature increases during welding and shrinkage stress during solidification. Refine to 1500 μm or less and heat at a temperature of 400 to 550°C, e.g. 1 to 24
60% at a temperature of 350-500℃ after homogenization treatment for an hour
By performing the above hot working, the ratio of the short axis to the long axis of the crystal grains is 1:5 or more, and the length of the short axis is 80 μm.
Weldability is improved by setting it to less than m. The method of manufacturing an Al-Zn-Mg alloy having excellent weldability and stress corrosion cracking resistance according to the present invention (hereinafter sometimes simply referred to as the manufacturing method according to the present invention) will be explained. First, used in the manufacturing method according to the present invention
The components and component ratios of the Al-Zn-Mg alloy will be explained. Zn is the most important element for improving strength; if the content is less than 3.0wt%, sufficient strength cannot be obtained, and if the content exceeds 8.0wt%, stress corrosion cracking will occur. It becomes easier to do. Therefore, the Zn content is set to 3.0 to 8.0 wt%. Like Zn, Mg is an important element for improving strength; if the content is less than 0.3wt%, sufficient strength cannot be obtained, and if the content exceeds 3.0wt%, stress corrosion cracking is likely to occur. Become. Therefore, the Mg content is set to 0.3 to 3.0 wt%. Ti and B are important elements for refining the structure of the ingot, and if the Ti content is less than 0.005wt% and the B content is less than 0.005wt%, it will not be effective in refining the grains, and Ti0. If the content exceeds 20wt% and B0.05wt%, giant compounds may occur. Therefore, the Ti content is 0.005 to 0.20 wt% and the B content is 0.005 to 0.05 wt%. Although Ni significantly improves stress corrosion cracking resistance,
If the content is less than 0.2 wt%, no such effect will be produced, and if the content exceeds 1.5 wt%, weldability will deteriorate. Therefore, the Ni content is 0.2~1.5wt%
shall be. Mn, Cr, and Zr are elements necessary to stabilize the structure, and the crystal grains are finely controlled by a combination of homogenization and hot working, but the content is less than 0.05wt% Mn and 0.05wt% Cr. %, Zr less than 0.05wt%, this effect is not present, and Mn0.40wt%, Cr0.40wt%,
If Zr is contained in excess of 0.25wt%, giant compounds may occur. Therefore, the Mn content is
0.05~0.40wt%, Cr content is 0.05~0.40wt%, Zr
The content is 0.05-0.25wt%. Al−Zn with such components and component ratios
-The grain size of the ingot made by melting and casting Mg alloy is
Because it contains Ti and B, it is refined to 1500μm or less, and the crystal grain size is 1500μm or less.
If it is larger, the grain size of the product will increase and weldability will deteriorate, so the grain size of the ingot must be 1500μ or less. Next, heat treatment of the manufacturing method according to the present invention will be explained. The above ingot is heated at a temperature of 400 to 550℃, for example, 1
The homogenization process is carried out for ~24 hours, but the
At temperatures below 550°C, the precipitation of Mn, Cr, and Zr is insufficient and the crystal grains of the product become enlarged. At temperatures above 550°C, the precipitates of Mn, Cr, and Zr begin to dissolve again, resulting in poor casting. Even if the crystal grains of the lump are fine, the crystal grain size of the product increases and weldability deteriorates. After this homogenization process, the temperature is 350-500℃ (preferably
60% or more (preferably 80%) at a temperature of 400-450℃)
(above)), Mn,
Dislocations are finely and uniformly distributed in a metastable form using Cr and Zr precipitates as nuclei, and the ratio of the short axis to the long axis is set to 1:5 in the recrystallization process in the subsequent process of solution treatment, quenching, etc.
or more, and the length of the short axis is controlled to be 80 μm or less. However, hot processing such as hot rolling, hot extrusion, and hot forging becomes difficult at low temperatures below 350°C, and at high temperatures exceeding 500°C, there is a possibility of hot cracking, resulting in product damage. The crystal grain size increases and weldability deteriorates. In addition, if the processing rate is less than 60%, the crystal grain size of the product will increase, and the ratio of the short axis to the long axis of the final product will be less than 1:5 and the short axis will be 80μ.
If the size exceeds m, weldability will be poor. Examples of the method for producing an Al-Zn-Mg alloy having excellent weldability and stress corrosion cracking resistance according to the present invention will be described together with comparative examples. Example Al- with the components and component ratios shown in Table 1
An ingot produced by melting and casting a Zn-Mg alloy by a conventional method was treated under the following conditions. (A) Conditions for manufacturing method of Al-Zn-Mg alloy with excellent weldability and stress corrosion cracking resistance according to the present invention After homogenization treatment at a temperature of 450°C for 24 hours,
By performing 90% hot rolling at a temperature of 450℃, 25
mmt plate material was manufactured. (B) Comparative conditions After 24 hours of homogenization at a temperature of 570℃, 450℃
25 by performing 90% hot rolling at a temperature of 500℃
mmt plate material was manufactured. These plates were solution-treated at a temperature of 450°C for 30 minutes, then water-cooled and then heated at a temperature of 120°C.
The 24-hour statute of limitations has expired. Table 2 shows the results of investigating the properties of this plate material. It can be seen that the material manufactured by the manufacturing method according to the present invention has better weldability and stress corrosion cracking resistance than the material manufactured under comparative conditions.

【table】

[Table] (1) Grain size: Observation of a cross section parallel to the longitudinal direction of plates and shapes. (2) Stress corrosion cracking resistance: Stress is applied in the thickness direction using a C-Ring test piece, and 3g/l at 100℃
NaCl-30g/l K ₂ Cr ₂ O ₇ -36g/l
Immersed in CrO ₃ mixed aqueous solution. Oα: No cracking at α minute, ×α: Cracking occurred at α minute. (3) Slit type crack test: A slit type weld crack test piece with a thickness of 12 mm was used. Cracking % = (Crack length / Total weld length) × 100 Filler metal 5356 Current 280A Voltage 30V (4) Microfissure: Observe the vicinity of the weld of butt welded material. Thickness 6mm Filler metal 5356 Current 260A Voltage 30V Test conditions 100℃ 3g/l NaCl-36g/l CrO ₃
-30 g/l K ₂ Cr ₂ O ₇ mixed aqueous solution was immersed and cracks were observed. The method for producing an Al-Zn-Mg alloy with excellent weldability and stress corrosion cracking resistance according to the present invention has the above-mentioned structure, so compared to conventional materials,
It has the effects of excellent weldability and also excellent stress corrosion cracking resistance.

Claims (1)

[Claims] 1 Zn3.0~8.0wt%, Mg0.3~3.0wt%, Ti0.005
~0.20wt%, B0.0005~0.05wt%, Ni0.2~1.5wt
%, and Mn0.05~0.40wt%, Cr0.05
Contains one or more selected from ~0.40wt%, Zr0.05~0.25wt%, and the remainder consists of Al and impurities.
After homogenizing the aluminum alloy ingot whose grain size has been refined to 1500μm or less at a temperature of 400 to 550℃, hot processing is performed at a temperature of 350 to 500℃ at a processing rate of 60% or more. The ratio of the short axis to the long axis of the crystal grains after the final heat treatment is 1:5 or more, and the length of the short axis is
A method for producing an Al-Zn-Mg alloy with excellent weldability and stress corrosion cracking resistance, characterized by a thickness of 80 μm or less.