Main points of aluminum alloy hot rolling process (Ⅲ)

Surface treatment of ingots

Ingot surface treatment can be divided into three methods: mechanical treatment, chemical treatment and surface coating.

1 Mechanical treatment of ingot surface

The surface of the ingot is completely or partially stripped of a layer, and the surface defects of the ingot are mechanically eliminated by methods such as milling and planing.

A wound edge
Aluminum-magnesium alloy ingots with magnesium content greater than 3%, high-zinc aluminum alloy ingots, and 2xxx series alloy ingots that have been press-pressed will produce segregation on the edge when they are cooled during casting and are enriched with Fe, Mg, Si and other alloying elements form very hard particles, and the edge is easily broken and cracked during hot rolling, which affects normal rolling. Therefore, this type of ingot needs to be planed before hot rolling (the best side milling effect). Generally, the thickness of the surface quenching zone is about 5mm, so the edge planing depth is generally controlled in the range of 5-10mm. After edge planing, there should be no obvious burrs, the knife marks are uniform, and the depth of the knife marks is not more than 3.0mm.
The curvature of the ingot facet is too large, which is not conducive to the side rolling of the hot rolling roll. In severe cases, it is easy to cause the rolled strip to have camber bending and cannot be corrected. Therefore, when the ingot facet curvature exceeds the control range (3mm/m) , can also be corrected by the method of planing.

B Milling
The aluminum alloy ingot produced by the semi-continuous casting method often has defects such as segregation nodules, slag inclusions, scarring and cracks on the surface. epidermis. Due to the high content of alloying elements in the segregation nodules, the plasticity is very low during hot rolling, and the metal that is easily crushed or fractured, and the dust and dirt contained in it are pressed into the surface of the plate, which will cause the metal to be crushed or crushed. Non-metallic, press-in defects. If the surface defects are not milled out, the product quality will be deteriorated.
The milling surface of the ingot is carried out on a special machine tool, including single-sided milling, double-sided milling, double-sided milling with side milling, etc. At present, the equipment used by most small and medium-sized enterprises in China is single-sided milling. The milling process needs to be turned over once. The production efficiency is low, and the milling surface is prone to mechanical damage. Most large aluminum processing factories abroad use double-sided milling or double-sided milling with side milling. , high production efficiency and good surface quality. According to the different lubrication and cooling methods used in milling, it can be divided into wet milling and dry milling. Wet milling uses emulsion for cooling and lubrication. The concentration of emulsion is generally 2% – 20%. The method of etching and washing removes the residual dirt on the surface; dry milling means that no cooling lubricant is added when milling the surface, and oil mist lubrication is used. The advantage is that the surface is clean and free of dirt, and the furnace can be heated after milling.
Generally speaking, all other aluminum and aluminum alloy ingots need to be milled, except for general-purpose pure aluminum plates with low surface quality requirements, whose ingots can be etched instead of milled. The amount of milling on the surface of the ingot should be determined according to the principles of the alloy characteristics, the level of melting and casting technology, and the use of the product. Among them, the casting technology used is the most important factor in determining the amount of milling surface. For example, the current advanced electromagnetic casting technology, LHC (low head carbon) casting technology, etc., the thickness of the quenching zone on the surface of the ingot is less than 1mm. The amount of milling is greatly reduced. The determination of the ingot surface milling amount should take into account both production efficiency and economic benefits. When milling the surface, the milling depth should be appropriate, and the minimum depth of milling on each side depends on the surface condition of the ingot. In general, the ingot milling surface volume is 5-30mm, and the maximum milling surface volume does not exceed 40mm. Depending on the alloy composition, the upper limit of duralumin or deep surface defects is taken, and the lower limit of pure aluminum or shallow surface defects. The minimum milling depth of the ingot is also different.

C Main points of mechanical process control of ingot surface treatment
(1) Before edge creation and milling, the equipment should be checked in detail, and the production can be carried out after confirming that it is normal.

(2) The temperature of the ingot for planing shall not exceed 100°C.
(3) The maximum cutting amount of each planing edge shall not exceed 5mm, and the planing amount of each edge shall not exceed 10mm. The black skin or defects on the surface must be planed, and the small surface must be smoothed.
(4) The temperature of the ingot milling surface shall not exceed 40℃.
(5) The milling surface ingot must be firmly clamped, and the milling chips in the milling surface are uniform.
(6) After milling, the depth of the tool marks shall not exceed 0.1mm, the thickness difference of the ingots shall not exceed 3mm, and the mechanical bumps shall not exceed 3mm, and the ingots shall be stacked according to the melting order.
(7) The ingots after milling should avoid bumps and bumps during handling and storage, keep the storage environment clean, and avoid being polluted by dust and oil. Generally, the storage time should not exceed 24 hours.
(8) The ingot must be clamped firmly during the work, and the milling is uniform.
(9) Continuously spray the emulsion to the milling cutter and the ingot when milling the surface.
(10) Correctly select the moving speed of the bed surface, and replace the milling cutter in time. After milling, the surface of the ingot is not allowed to have sticking aluminum, peeling, pores, slag inclusions, surface cracks and looseness.
(11) After milling the surface, wipe off the emulsion and aluminum chips to prevent cratering and corrosion. The depth of the tool mark after milling the surface should not exceed 0.1mm, and the thickness difference of the ingot should not exceed 3.0mm.
(12) Stacked according to melting times and milling surfaces. Hanging ingots should be stable and lightly placed. The depth of mechanical bumps after milling is not allowed to exceed 3.0mm.

2. Ingot surface chemical treatment

A Principle of chemical treatment of ingot surface
The chemical treatment of the surface of the ingot refers to the process of removing the oil and dirt on the surface by chemical methods, so that a new bright oxide film is formed on the surface of the ingot, also known as etching. The purpose of etching and milling of ingots and aluminum-clad plates is to use chemical corrosion to remove oil and dirt on their surfaces and make them clean.
Hard aluminum alloy ingots, such as 2A06, 2A11, 2A12, 2014, etc., should be etched and washed before heating after milling. Industrial pure aluminum ingots with good casting surface quality and no special requirements on the surface of the plate can be directly processed without milling. Etch wash. The ingot after etching should not be stored for too long (generally no more than 24h). If the storage time is too long and dust falls, it must be etched again before heating, or wiped with aviation gasoline.
High zinc, high magnesium aluminum alloy ingots, such as 7A04, 7A09, 5A05, 5A06 and other alloy ingots, cannot be etched. Some of these alloys turn black after etching (alloys containing high zinc and aluminum), and some are prone to white spots (alloys containing high magnesium), all of which will deteriorate product quality. The surface of the ingot must be cleaned with aviation gasoline before heating the above alloy.
The aluminum-clad plate should be etched and washed, and the surface should be cleaned, so that the aluminum-clad plate and the ingot can be firmly welded during hot rolling to prevent defects such as bubbles after annealing.

B ingot surface chemical treatment process
The process of etching and washing is: alkaline washing (NaOH concentration 15%-25%, temperature 60-80°C, soaking time 2-10min) → room temperature flowing water washing → pickling (HNO, concentration 15%-30%, room temperature, soaking Time 2 ~ 4min) → room temperature flowing water washing → hot water washing (temperature not lower than 70 ℃) → drying or drying.

C The main points of process control of chemical treatment of ingot surface are:
(1) The ventilation device of acid and alkali tank should be opened during etching and washing production.
(2) When the ingot is loaded into the furnace, wipe the surface with a towel soaked with aviation gasoline, and wait for the gasoline on the ingot to be completely volatilized before loading into the furnace.
(3) It is allowed to use a scraper or a wire brush to repair the surface defects caused by etching or milling, and the depth of the ingot should not exceed 3mm; the aluminum-clad plate should not exceed 5% of the actual thickness, and the defects after treatment with the scraper should be smooth. .
(4) For ingots that have been stored for a long time, if the dust is too thick or the oil stains are heavy and cannot be removed by etching and washing, the surface needs to be re-milled, and it is not allowed to install the furnace only by wiping the surface.
(5) After surface treatment, ingots, aluminum-clad plates and clips that have been stored for more than 24 hours should be reprocessed according to the original method.
(6) The etched ingots, aluminum-clad plates and clips are not allowed to have defects that affect the welding quality, such as water marks, rust, alkali marks and dirty fingerprints.
(7) The surface-treated ingots should be lifted and placed lightly to prevent collision damage. The depth of the collision injury and the depth after repair are not allowed to exceed 3mm.
(8) When the surface segregation tumor of the pure aluminum ingot is not milled more than 5mm or there are defects such as delamination and metal tumor, it needs to be milled and put into production.

3. Ingot surface coating

The surface coating of the ingot means that the surface or both sides of the ingot is mechanically lined with pure metal or alloy plates similar in size to the ingot. It is then heated, hot rolled or cold rolled with the ingot until the finished product. The outer metal is called the cladding layer, and the inner metal is called the matrix. The sheet produced by this method is essentially a bimetallic product.

A Ingot surface coating
Aluminum cladding is to place the aluminum clad plate on both sides of the milled ingot, and the aluminum clad plate and the ingot are firmly welded together by hot rolling. Surface-coated aluminum can be divided into two categories: process-coated aluminum and anti-corrosion-coated aluminum. In order to improve the processing performance of aluminum alloys (such as hot-rolled surface cracks), aluminum cladding is called process aluminum cladding; in order to improve the corrosion resistance of aluminum alloy products, aluminum cladding is called anti-corrosion aluminum cladding. Anti-corrosion clad aluminum is divided into normal clad aluminum and thickened clad aluminum. Thickened aluminum cladding is a plate used under special conditions, requiring high corrosion resistance, and the method of thickening the cladding layer is adopted.
The principle of selecting the material for the aluminum-clad layer of the hard aluminum alloy is: under the action of the corrosive medium, the aluminum-clad layer becomes the anode material for the substrate (ingot), and plays a protective role in an electrochemical manner. When the aluminum cladding layer encounters local damage (such as scratches, abrasions, etc.), it can also play a stable protective role on the base metal. Hard aluminum alloys mainly refer to 2xxx series, 6xxx series, 7xxx series and aluminum alloys with high magnesium content (such as 5A06). 7xxx series alloys use 7A01 clad aluminum plates containing zinc, and other alloys use 1A50 clad aluminum plates.

B Ingot side covered with aluminum
In order to improve the thermoplasticity of hard aluminum alloys, my country has pioneered a new process of side-coated aluminum. The side-coated aluminum process is: 7A01, 1ASO thickness of 7~9mm, the width of which is slightly smaller than the thickness of the ingot, is used for side-coated edge (aluminum strip), after etching and washing, before the ingot is heated and the aluminum-coated plate is placed at the same time. Both sides of the ingot. After heating, a certain processing rate and vertical roll hemming are given in the first few passes of the rolling mill, so that the upper and lower sides and the two sides of the ingot are welded together with an aluminum-clad layer. Reducing the hot-rolled cracked edges greatly improves the process plasticity of hard aluminum alloys. This is because the edge cracks of the product have been filled with pure aluminum with high plasticity during hot-rolling and are firmly welded with the matrix to form a reliable. atresia. When rolling is continued, the formed cracks are not easy to expand, thus improving the process plasticity of hot rolling and cold rolling, reducing the number of intermediate annealing during cold rolling, reducing the number of strip breaks in cold rolling, and correspondingly improving the yield and Productivity.

C The main points of ingot surface coating process control
The key points of process control are:
(1) Surface treatment should be carried out before the ingot is coated with aluminum.
(2) The alloy and specifications of the ingot-clad aluminum plate should be compatible with the process requirements and ordering standards. (3) The aluminum-clad plate should be placed on the ingot and aligned with the bite end.
(4) For the alloy ingots that need to be placed into the bent-sided aluminum plate, the clamped edge should be placed in the furnace. (5) When the aluminum-clad plate is not bent, the clips are placed on the ingot separately.

 

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