The two most common types of reheating furnaces are walking beam furnaces and pusher-type furnaces. Scale (oxide) is formed during the heating and it must be removed. There will also be a decarburisation of the surface. Particularly in the case of alloyed steels, for example high speed cutting steel, these effects will cause high costs because of loss of material. It is an object of the invention to reduce the formation of scale and the decarburisation and it is fulfilled in principle by the separation of a final zone from the direct heating zone and the conveying of a protective gas through conduits that lead through the direct heating zone and end in the final zone so that a preheated protective gas is released into the final zone. The invention has been given the characteristics that are defined in the claims.

The invention will be described with reference to the drawings that show schematically and fragmentary a walking beam furnace as an example of the invention.

Figure 1 is a side section through a walking beam furnace, and the section is taken along line 1-1 in figure 2.

Figure 2 is a horizontal section taken along the line 2-2 in figure 1.

Figure 3 corresponds to figure l, but it shows some details modified.

The walking beam furnace shown in the figures is divided into three zones, namely the two direct heating zones 12,13 and the final zone (the temperature equalising zone) 14. Baffles 15,16 separate these zones. A swingable door 17 is suspended in hinges. A plurality of semis (seventeen semis in the form of slabs are shown) are in the furnace and they are moved from the in-feed end at the left in the figures to the output end at the right in the figures by means of non- illustrated walking beams in a conventional way. Only the first slab 20 and the last slab 21 have been denoted. The walking beams elevate the slabs about a decimetre when moving the slabs.

The door is controlled by a hydraulic jack or a corresponding device on the outside of the furnace housing. The arrangements for the input and for the output of the slabs are not shown.

An air-lock should be arrange at the output end to keep oxygen out of the final zone 14. The conventional oxy-fuel burners that eject the hot combustion gases into the direct heating zone are not illustrated. Electric radiation heaters 22 are shown in the final zone 14.

Conduits in the form of tubes 23,24 extend through the direct heating zones 12,13 along the sidewalls and they end in the final zone 14. The tubes 23,24 extend through a sealing threshold 25 against which the swingable door 17 seals. The walking beams lift the slabs over the threshold. A protective gas is supplied to the final zone 14 through the tubes 23,24 and since the tubes extend through the direct heating zones 12,13, the gas will be preheated when it reaches the final zone 14. The protective gas can suitably consist of nitrogen or mainly of nitrogen. It can be advantageous to add a small amount of hydrogen or another combustible gas to the protective gas so that any oxygen in the final zone will be burnt and give a slightly reducing atmosphere in the final zone.

Figure 3 shows the same furnace as do the figures 1 and 2, but instead of the electric radiation heaters, there are radiating tubes 30 that extend from the outside of the furnace through the entire final zone 14 and end in the direct heating zone 13. A non-illustrated burner is mounted to each tube 30 at the outside of the furnace and the combustion gases are conveyed through the tubes into the direct heating zone 13. In this way, the combustion gases heat the tubes 30 so that the tubes form radiation heaters.

Usually, the slabs have a thickness of more than 20 cm and a width of about 1 m. The temperature at the surface should not increase in the final zone, but a temperature equalisation should take place. The main purpose of the radiation heaters 22,30 is therefore to maintain the surface temperature during the temperature equalisation. The extremely low oxygen content in the final zone will reduce the formation of scale considerably and the profit from the reduced scale formation will surprisingly be much greater than the additional cost for a production according to the invention. This is particularly true for some alloyed steels, for example high speed steel. It has been proven that the scale formation can be reduced by 50% when the invention is applied as described.