In a continuously operated material sintering process, there is conducted hot gas through the material bed that is formed on the belt of the material to be sintered, which gas reacts with the material bed at a high temperature, for instance when the ferroalloy pellets form a material bed within the temperature range of 1 ,300 - 1 ,600° C, so that the soft particles to be sintered are hardened and are thus easy to be processed further. After the sintering proper, the sintered particles are cooled, and cooling gases are conducted in the material bed during the cooling process. Thus the belt is subjected to both heating and cooling within an essential- ly short length, which essentially raises the criteria for high-standard qualities required of the belt.

From the US patent 4,316,718, there is known an endless, perforated conveyor belt used for heat-treating a bed of material by feeding hot gas through said mate- rial bed. Gas can be fed through the bed either from above the bed or from underneath it. When gas is fed from above, it first penetrates the material bed and proceeds through the elongate slots provided in the conveyor belt. The slots are made in the centre of the conveyor belt in adjacent rows that are spaced apart. The rows made of elongate slots are mutually arranged so that the rows fall trans- versally to the proceeding direction of the conveyor belt. When the belt wears off in the heat treatment of the material bed, the endless conveyor belt must always be replaced as a whole. This increases the expenses of the heat treatment of the material bed, because a perforated conveyor belt as such is expensive to manufacture. In addition, the replacing of a whole conveyor belt takes a lot of time, which cuts the working time of the apparatus.

As disclosed e.g. in documents WO 01/55659 A1 and WO 2009/022059 A1 , the conveyor belt of a strand sintering furnace is formed from a number of rectangular steel plate elements that are sequentially welded to each other by weld seams. Each plate element includes a plurality of holes arranged into a plurality of groups of perforations to enable the flow-through of the gas used in the sintering process.

During operation, the conveyor belts are subjected to static and dynamical loads, corrosive environment and elevated temperature. Dynamical loads, i.e. fatigue loads, cause damage that commonly limits the lifetime of the belt. Cyclic loads (fatigue loads) are generated when the belt rotates around the deflector rolls. Because the perforations act as stress raisers, fatigue cracks are typically initiated and start to grow. This leads to damage, especially in the perforated regions. The choosing of the material of the conveyor belt is also critical because the conditions vary a lot both concerning the temperature, the gases, environment of use etc. In addition un-uniform temperature distribution leads to thermal stresses which induce plastic deformation which manifests itself as for example waviness of the belt. The object of the present invention is to eliminate the drawbacks of the prior art and to achieve a conveyor belt suited for a continuously operated thermal treatment of a material bed, i.e. for a continuously operated conveyor-type sintering of the material bed, which conveyor belt is economical in manufacturing costs and can also be replaced in parts. Additionally the object of the present invention is higher strength better corrosion resistance an on some cases two-phase micro- structure eliminate the above-mention problems.

The essential novel features of the invention are apparent from the appended claims.

According to the invention, the conveyor belt for a continuously operated conveyor-type thermal treatment, i.e. sintering, of a material bed and the conveyor belt being provided with perforations in order to allow the gases that are used for heat- ing and possibly cooling the material bed to flow through the material bed and the conveyor belt, and said conveyor being based on elements connected to each other. The conveyor belt is made of a perforated, at least one-part element made of a metal piece and allowing the gas to flow through, and that the perforations are arranged in perforation zones alternating with perforation-free zones, and that the area of the perforations, i.e. area of the perforation zones, is about 1 - 60% of the total area of the conveyor belt. The conveyor belt used in a continuously operated conveyor-type thermal treatment, i.e. sintering, of a material bed is made of interconnected, at least one-part elements, and in each element, at least in the part that is in contact with the material bed formed on the conveyor belt, there are made perforations for conducting the gas needed in the thermal treatment for heating and possibly cooling the material bed and the conveyor belt to flow through the conveyor belt via said perforations. The elements constituting the conveyor belt are advantageously made of metal, such as ferritic, austenitic, stainless steel or acid-proof steel by rolling, so that the rolling direction is either trans- versal or parallel to the proceeding direction of the conveyor belt. The conveyor belt perforations are mutually arranged so that said perforations advantageously form a row that is either transversal or parallel to the proceeding direction of the conveyor belt or forms an angle with respect to the proceeding direction of the conveyor belt. According to the invention the conveyor belt element is a one-part metal object or a multi-part metal object A single element of the conveyor belt according to the invention compiled of elements consists of one or several pieces. In particular, if the element is essentially a uniform piece along the whole width of the conveyor belt, the element is usually made of one piece, which is installed in the conveyor belt transversally to the proceeding direction of the conveyor belt. Even then several one-piece elements can be interconnected transversally to the proceeding direction of the conveyor belt prior to attaching the arrangement compiled of several pieces as part of the conveyor belt.

A conveyor belt element can also include several pieces arranged in parallel to the proceeding direction of the conveyor belt, which pieces are first interconnected in the lengthwise direction and further attached as part of the conveyor belt transversally to the proceeding direction thereof. When the element is formed of pieces that are arranged in parallel to the proceeding direction of the conveyor belt, said pieces can be made either essentially equally thick, or for instance such that at the outer edges of the conveyor belt, there are installed pieces that are either thicker or thinner than the pieces installed in the middle of the conveyor belt. According to the invention the perforations made in the conveyor belt element can be essentially identical in shape or the perforations made in the conveyor belt element are mutually different in shape.

According to the invention the perforations made in the conveyor belt element are placed in parallel to the proceeding direction of the conveyor belt.

According to the invention the shapewise essentially identical perforations are arranged in a row that forms an angle of 30-60 degrees with respect to the proceeding direction of the conveyor belt. According to the invention the shapewise essentially identical perforations made in the conveyor belt element and placed on different sides of the centre line of the conveyor belt element are positioned mutually symmetrically with respect to the proceeding direction of the conveyor belt.

According to the invention the area of the perforations (perforation zones) is about 5 - 25 %, preferably 5 - 19 %, more preferably 5 - 15 % of the total area of the conveyor belt. The number of the perforations provided in the conveyor belt element according to the invention is such that as regards the area of the element consisting of one or several pieces, the total area of the perforations is 1 - 60%, advantageously 5 - 25%. Moreover, said perforations are arranged in groups in the lateral direction of the conveyor belt, so that at both edges of the conveyor belt, there is provided an perforation-free zone having the width of 20-25% of the total width of the conveyor belt. In addition, the perforations provided in the middle section of the conveyor belt, in between the two perforation-free edge zones, are positioned so that in between two perforated zones, there is provided a perforation-free zone having a width that is 30-200% of the perforate zone width. The area of perforations affects the stiffness of the belt and the gas flow through the belt and therefore the area of perforations is adjusted by customer requirement. In the perforated zone, the perforations are placed in one or several rows in the lateral direction of the conveyor belt. As such, the rows formed by the perforations can follow either the lateral or the lengthwise direction of the conveyor belt, or they can form an angle of about 30-60 degrees with respect to the proceeding direction of the conveyor belt.

In shape, the perforation made in the conveyor belt element can be for example circular or oval, or it can have some other shape. A perforation that represents some other shape than circular is advantageously placed, with respect to the conveyor belt, so that the larger dimension of said perforation is positioned in the lateral direction of the conveyor belt. The perforation can also be placed in the conveyor belt so that the smaller dimension of said perforation is positioned in the lateral direction of the conveyor belt. When the perforation represents some other shape than circular, the ratio of the perforation dimensions is advantageously within the range 0.1 - 0.5. If the perforations are oval the end of perforated zone can be staggered.

In a conveyor belt according to the invention, the perforated zone is made of mu- tually identical perforations that are advantageously arranged in one or several rows, and in each row the perforations are spaced apart. A perforated zone may also be formed of mutually different perforations, advantageously so that mutually identical perforations are arranged in their own rows, and other different perforations again in their own rows. The obtained different rows can advantageously be arranged for instance in an alternating fashion.

A conveyor belt element according to the invention is connected to another element in order to compile the conveyor belt proper advantageously in a mechanical fashion, such as by welding the conveyor belt in the lateral direction of the con- veyor belt, i.e. transversally to the proceeding direction thereof. The junction can also be realised so that at one lateral edge of the conveyor belt element, there is installed at least one mechanical connecting piece, and at another lateral edge of the conveyor belt element, there is installed another mechanical connecting piece in order to create the junction.

A conveyor belt element according to the invention is, the material of the conveyor belt element is chosen from stainless steel grades including: ferritic choromium- alloyed stainless steel, austenitic-martensitic precipitation hardened stainless steel, austenic stainless steel, duplex stainless steel or any combination of them. A conveyor belt element according to the invention is, the material of the conveyor belt element is chosen from duplex stainless steel grades including: lean duplex, standard duplex, super duplex and hyper duplex or any combination of them. The basic idea of duplex is to produce a chemical composition that leads to an approximately equal mixture of ferrite and austenite. The important elements in stainless steels can be classified into ferritisers and austenitisers. Each element favours one structure or the other: Ferritisers - Cr (chromium), Si (silicon), Mo (molybdenum), W (tungsten), Ti (titanium), Nb (niobium)

Austenitisers - C (carbon), Ni (nickel), Mn (manganese), N (nitrogen), Cu (copper) A suitable combination of ferritizing alloying elements and austitizing alloyed elements leads to duplex microstructure,

Duplex stainless steels have a mixed microstructure of austenite and ferrite, the aim usually being to produce a 50/50 mix, although in commercial alloys the ratio may be 40/60. Duplex stainless steels have roughly twice the strength compared to austenitic stainless steels and also improved resistance to localized corrosion, particularly pitting, crevice corrosion and stress corrosion cracking. They are characterized by high chromium (19-32%) and molybdenum (up to 5%) and lower nickel contents than austenitic stainless steels.

The properties of duplex stainless steels are achieved with an overall lower alloy content than similar-performing super-austenitic grades, making their use cost-effective for many applications. Duplex grades are characterized into groups based on their alloy content and corrosion

resistance. • Lean duplex refers to grades such as UNS S32101 (LDX 2101 ), S32304, and S32003.

• Standard duplex is 22% chromium with UNS S31803/S32205 known as 2205 being the most widely used.

· Super duplex is by definition a duplex stainless steel with a Pitting Resistance Equivalent Number (PREN) > 40, where PREN = %Cr + 3.3x(%Mo + 0.5x%W) + 16x%N. Usually super duplex grades have 25% chromium or more and some common examples are S32760 (Zeron 100 via Rolled Alloys), S32750 (2507) and S32550 (Ferralium),.

· Hyper duplex refers to duplex grades with a PRE > 48 and at the moment only UNS S32707 and S33207 are available on the market.

The invention is explained in more detail with reference to the appended drawings, where figure 1 is a top-view illustration of a preferred embodiment of the invention, figure 2 is a top-view illustration of another preferred embodiment of the invention, and figure 3 is a top-view illustration of a third preferred embodiment of the invention.

According to figure 1 , in a conveyor belt designed for a continuously operated sintering of a material bed, a conveyor belt element 1 consists of a one-part metal object made of for instance ferritic steel, in which element there are made perforations 2 in order to allow the gases that are used in the sintering process to flow through the element 1 . The perforations 2 are made in the element in zones 3, so that in the element 1 , there is arranged a perforation-free edge zone 4 at both edges of said conveyor belt element 1 . In similar fashion, in between the perforat- ed zones 3, there is placed a perforation-free zone 5. The perforations 2 are ar- ranged in the element 1 , so that the perforations 2 form several rows in parallel to the proceeding direction 6 of the conveyor belt element 1 .

Figure 2 illustrates a conveyor belt element 1 1 designed for a continuously operat- ed sintering of a material bed, said element 1 1 consisting of a one-part metal object made for instance of austenitic steel and provided with perforations in zones 12. Said zones include perforations 13 and 14 with different shapes. The shape- wise mutually identical perforations 13 are positioned in a row in similar fashion as the mutually identical perforations 14. The perforations 13 and 14 are arranged at an angle of 45 degrees with respect to the proceeding direction 15 of the conveyor belt element 1 1 .

According to figure 3, a conveyor belt element 21 designed for a continuously operated sintering of a material bed consists of a multi-part metal object, length- wise in the proceeding direction of the conveyor belt, with parts 22, 23, 24 and 25 interconnected by welding. The material of the parts 22 and 25 is for instance austenitic steel, whereas the material of the parts 23 and 24 is for instance acid- proof steel. The parts 22 and 25 of the metal object constitute the edge parts of the element 21 . The parts 23 and 24 of the metal object are provided with perfora- tions 26 and 27 that allow gases to flow through the element 21 . The perforations 26 are mutually identical in shape, and respectively the perforations 27 are mutually identical in shape. The perforations 26, and respectively the perforations 27, are arranged in rows that form an angle of 45 degrees with the proceeding direction of the conveyor belt. On the other hand, the perforation rows 26 and respectively 27, placed on different sides of the centre line 29 of the conveyor belt element 21 , are positioned mutually symmetrically.