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Title:
EQUIPMENT AND METHOD FOR HOMOGENIZATION OF CAST PRODUCTS
Document Type and Number:
WIPO Patent Application WO/2021/032850
Kind Code:
A1
Abstract:
Equipment and method for homogenization of elongated cast products (P) from an aluminium alloy, said products being either extrusion ingots, sheet ingot or slabs of different dimensions, where the equipment (furnace) comprises a conveyor for transporting the products through it, a first heating zone and a following residing zone and a cooling chamber. According to the invention, the products are pre-heated in a separate pre-heating step before entering the heating zone, preferably up to 400 ˚C. The heat is generated by induction heating coils (1', 2', 3', 4') that can be operated intermittently.

Inventors:
RIST SNORRE (NO)
ROMFO ALF INGE (NO)
ÅSHOLT PETTER (NO)
Application Number:
EP2020/073380
Publication Date:
February 25, 2021
Filing Date:
August 20, 2020
Export Citation:
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Assignee:
NORSK HYDRO AS (NO)
International Classes:
F27B9/06; C21D9/00; C21D9/70; F27B9/36; F27D99/00
Foreign References:
US3386717A1968-06-04
US20100043699A12010-02-25
CN102796976A2012-11-28
CN105734466A2016-07-06
CN105714223A2016-06-29
US3386717A1968-06-04
Attorney, Agent or Firm:
HYDRO IPD (NO)
Download PDF:
Claims:
Claims

1. Equipment for homogenization of elongated cast products (P) from aluminium alloys, said products being either extrusion ingots or sheet ingot(slabs) of different dimensions, where the equipment comprises a conveyor for transporting the products through it, a first heating zone and a following residing zone and a cooling chamber, characterised in that upfront the heating zone the equipment comprises a pre-heating equipment (1) that heats the products from ambient temperature to an elevated temperature before entering the heating zone, the pre-heating equipment (1) comprises induction heating coils (G, 2’, 3’, 4’).

2. Equipment according to claim 1 , characterised in that the induction heating coils (G, 2’, 3’, 4’) are arranged above the cast products.

3. Equipment according to claims 1 - 2, characterised in that the movement of the products through the equipment is done by walking beams in a stepwise manner.

4. Equipment according to claims 1 - 3, characterised in that the arrangement of pre-heating equipment is installed as a brownfield installation compatible with an existing homogenization equipment.

5. Method for homogenization of elongated cast products (P) from aluminium alloys by an equipment and more specific a furnace, said products being either extrusion ingots or sheet ingot (slabs) of different dimensions, where the equipment comprises a conveyor for transporting the products through it in a stepwise manner and a first heating zone and a following residing zone and a cooling chamber, characterised in that the products are pre-heated from ambient temperature to an elevated temperature in a separate pre-heating step before entering the heating zone, wherein the heating is generated by induction heating coils (G, 2’, 3’, 4’). 6. Method according to claim 5, characterised in that the pre-heating elevates the temperature in the products to approximately 4009C.

7. Method according to claims 5 - 6, characterised in that the heating coils ( , 2’, 3’, 4’) are operated intermittently.

8. Method according to claims 5 - 7, characterised in that the pre-heating of the products increases the capacity of the homogenization equipment by 20 - 40 %.

9. Method according to claims 5 - 8, characterised in that the pre-heating step is provided for as a brownfield equipment installation in an existing furnace, preferably being provided with a feed system upfront the inlet of the furnace, by arranging induction heating coils above said feed system and wherein a heat control system for the energization of the induction coils is integrated with an existing control system of the said furnace.

Description:
Equipment and method for homogenization of cast products

The present invention relates to equipment and a method for continuous homogenization of products like extrusion ingot or sheet ingots (slabs), said products being made from aluminium alloys. Extrusion ingots are most commonly used for extruded products, but also as raw material (workpiece) for forged and machined products. The present invention is also relevant for these kind of applications.

Said products are commonly cast in a semi continuous manner by delivery of liquid aluminium from a melting / casting furnace to a casting apparatus.

The casting operation is commonly followed by a homogenization process to improve the metallic microstructure of the cast products.

The homogenization process is done by heating the products up to a certain temperature in a furnace and holding this temperature for a certain period. At the end of this period the products are cooled in a controlled manner to ambient temperature. Commonly, gas or electricity separately or in a combination can be applied as heat sources.

US 3386717 relates to a process and apparatus for heat treating aluminium ingots upfront heating in a direct fired furnace. A surface portion of the ingot is heated above a dew point to prevent water from condensing out on the surface of the ingot and forming a stain as it permeates through the amorphous oxide film present thereon. It is indicated that the surface temperature should be increased up to 150°F, which corresponds to approximately 65 °C. Heating can be done by an induction furnace.

There are two dominating technologies for homogenizing; batch homogenizing where all ingots are heated, held and cooled simultaneously and continuous homogenizing where ingots are travelling trough the process in a sequential manner. The present invention is focusing on the latter technology. In a continuous homogenizing line, the products are arranged onto a conveyor in front of a homogenization equipment and moved one by one into a first heating zone. The products are moved in a stepwise manner in the heating zone by means of walking beams and are in this manner moved successively from one position to the next one.

Commonly the equipment comprises three zones; heating zone (HZ), residing zone (RZ) and a cooling chamber. The heating zone and the residing zone are interconnected while the cooling chamber is commonly arranged at the side of the residing zone. A conveyor interconnects the residing zone and the cooling chamber and transports the products from the residing zone to the cooling chamber.

The residing period of the products in the equipment is determined by the specifications set by the customer. This specified period is defined by the necessary time to obtain desired metallurgical effects like transformation and dissolution of intermetallic particles and eliminating concentration gradients of alloying elements.

The residing period may vary, but commonly the period is 120 to 240 minutes. In an equipment having 65 positions in the residing zone, given a residing period of 135 minutes, the step frequency through the equipment will be approximately one step per each 125 seconds.

The maximum capacity of the equipment is reached when all positions in the residing zone are occupied. However, if the heating zone is lacking capability for heating the products to the right temperature, the normal procedure is to keep the product resting in the last position of the heating zone until the right temperature has been reached. As a consequence, there will be open positions in the residing zone since the stepwise movement of the residing zone is normally done independently of the influx of products from the heating zone.

Operating the equipment with empty positions in the residing zone will result in lost capacity and unnecessary high energy consumption. One way of avoiding this situation is to adjust the energy supply and the heating capacity in the heating zone. A challenge for many casthouses is that the homogenizing process represents a bottle neck, thus the capacity of the casting equipment cannot be fully exploited. Increasing the heating capacity in the homogenizing unit would enable an increase in the total production capacity.

The equipment for operating the homogenization process can be replaced with larger equipment, but this kind of equipment is very space demanding and an expansion of the building facilities may become necessary, if at all possible both practically and economically.

By the present invention this challenge can be remedied.

As an example, installation of one unit that can preheat the products to approximately 400 S C upfront the products entering the heating zone has shown that the residing time in this zone can be significantly reduced (ref. fig. 1 ). This will free many positions of the heating zone, that can be added to the residing zone. The residing period at each step in the residing zone can then be lowered and the capacity of the residing zone can be largely increased to meet the productivity of the casting operation.

According to the present invention there can be arranged a separated pre-heating step upfront the heating zone where the heating is generated by induction heating coils.

A further advantage of using induction preheating contrary to heating for instance with hot air is avoiding overheating of the ingot surface, thereby reducing the risk for surface remelting and oxidation (black billets).

These and further advantages can be achieved in accordance with the present invention as defined in the accompanying claims.

The present invention will be described in further detail in the following by means of examples and with reference to the drawings, where:

Fig. 1 shows a series of plots of the temperature versus time of a product in a traditional homogenizing line without pre-heating, Fig. 2 shows temperature development during induction heating in two positions along the length of a billet versus time,

Fig. 3 shows the temperature development in different positions in the end surface of a billet versus time from the same test as in figure 2 Fig. 4 shows billets on a conveyor under flat-coil induction heaters upfront a heating zone.

As can be seen from Fig. 1 , the graph discloses the temperature versus time of a product in a traditional homogenizing line without pre-heating. The graph shows several measurements of the temperature versus time of a product in a heating and residing chamber. The time up to 400 S C is marked in the drawing.

According to the graph it is clearly shown that the pre-heating allows a shorter residing period in the heating zone. As an example, the heating period up to the target temperature, constituting about half of the total time, can be reduced by approximately 25% by doing the preheating of the product to 400 S C, this time period is marked in figure 1. Thus, the residing time in the heating zone can be substantially shortened by implementing the invention, and this can be exploited by increased capacity in the holding zone.

The pre-heating is preferably done by induction heating of the individual product as shown in Fig. 4 (will be described in detail later).

According to the invention, the residing time of the products in the heating zone can be reduced and the capacity of the residing zone can be increased correspondingly, by the suggested pre-heating. The existing equipment can be operated with the same number of positions as before, where some positions has shifted from the heating zone to the residing zone. The total residing period of the products in the homogenizing line can in one embodiment be reduced by 35 minutes when preheated up to 400 S C, which will represent a significant increase in capacity (20-40%).

By the application of induction for preheating the products before the homogenization step, it is demonstrated that a significant increase of capacity is obtained. The invention can be relatively simply introduced as a brownfield installation in existing continuous homogenization equipment without need to enlarge the footprint (floor space) or amending the building structure.

Commonly, this kind of heat treatment furnaces have a feeding system, often a conveyor, for feeding cast products to the inlet of the furnace, and the provision of the induction heating system including the induction coils can be arranged in this area. The coils can be fixed to a rack or a frame, preferably above the said feeding system. A heat control system for the energization of the induction coils can be integrated with an existing control system of the furnace

The products will not be exposed to very high temperature gradients during the pre heating cycle. The same applies as it goes further into the heating zone of the homogenization equipment. The exposure of the surface of the product will be less as the product will not be exposed to the excessive heating from gas burners in the heating zone.

In Fig. 2 the temperature development during induction heating in two positions along the length of a billet versus time is shown. The heating cycles comprise 90 seconds heating followed by 90 seconds of resting which is repeated until 450 S C is reached.

In Fig. 3 the temperature development in different positions in the end surface of a billet versus time from the same test as in figure 2 is shown. The measurement positions are distributed along the radius from the side (SP1 ) to the centre (SP4). The measurements indicate that the heating is giving a homogenous rise in the temperature throughout the billet’s cross section.

In Fig. 4 it is shown one embodiment of the invention where the preheating equipment 1 comprises induction coils 1 ’, 2’, 3’, 4’ of the so called flat-coil type that are arranged above the products P (here billets 11 , 12, 13, 14) as they are moved transversally into the equipment preferably by an existing conveyor system, without need for substantial modifications of the conveyor system. Application of different type of coils can be realized in case the products are arranged for movement lengthwise into and through the equipment.