METHOD FOR GALVANNEALING STEEL MATERIALS

Abstract

Method for use when galvannealing a steel material (1), in which the material (1), in a first step, is preheated to a first process temperature and is coated with a layer of a liquid alloying metal (3), in a second step is further heated to a second, higher process temperature, and in a third step is kept at the second process temperature during a predetermined time period so that the alloying metal coating at least partially is caused to alloy with the steel material (1). The heating in the second step is caused to be carried out by one or several DFI burners (5).

Description

The present invention relates to a method for use when galvannealing steel materials.

Galvannealing is a process in which steel material is both galvanised and annealed. The galvanising typically takes place by dipping the steel material in a bath of liquid zinc. The steel material may be preheated before it is dipped and/or may be heated by contact with the liquid zinc.

Thereafter, the steel material is further heated, up to a temperature where annealing takes place. As the material is kept at this higher temperature, the zinc coating forms an alloy at the surface of the steel material, which alloy offers attractive properties in terms of corrosion resistance, etc.

Conventionally in such a process, either induction heating or heating in an air gas furnace is used for further heating the material. Both of these strategies involve problems.

Induction heating is indeed efficient, but it is sensitive for the dimensions and the geometrical configuration of the heated material. Moreover, zinc is not heated as well as steel, why temperature gradients may arise. Finally, devices for induction heating are typically very costly.

Heating in an air gas furnace does not lead to any dimension or material geometry related problems, but on the other hand it is substantially less efficient as compared to induction heating. Because of the low emission factor of zinc, it is also difficult to increase the heat transfer to the material, why the rate of production is limited.

The present invention solves the above described problems.

Thus, the invention relates to a method when galvannealing a steel material, in which the material, in a first step, is preheated to a first process temperature and is coated with a layer of a liquid alloying metal, in a second step is further heated to a second, higher process temperature, and in a third step is kept at the second process temperature during a predetermined time period so that the alloying metal coating at least partially is caused to alloy with the steel material, and is characterised in that the heating in the second step is caused to be carried out by one or several DFI burners.

In the following, the invention will be described in detail, with reference to exemplifying embodiments of the invention and to the appended drawings, in which:

FIG. 1 is a cross-sectional overview showing the various component parts used in a process in which a conventional galvannealing process is carried out.

FIG. 2 is a cross-sectional overview showing the various component parts used in a process in which a galvannealing process according to the present invention is carried out.

In FIG. 1, it is illustrated how a steel product 101 in the form of an elongated strip is transported along various process steps in a conventional, continuous process for galvannealing. In a first step, the steel product is conveyed through a bath 102 in which an alloying metal in the form of liquid zinc 103 is present. Accordingly, the steel strip 101 thus dipped is coated with a layer of liquid zinc.

In a second step, the steel strip 101 is transported past a pair of air knives 104, removing surplus zinc from the surface of the strip 101.

In a third step, the strip is conveyed through a gas or induction furnace 105, which boosts the temperature of the steel strip 101 so that annealing is initiated.

Thereafter, the annealing is completed during a certain time period by the strip 101 being transported through a holding furnace 106 in which the temperature of the steel material 101 is kept constant.

The process illustrated in FIG. 2 is similar to that of FIG. 1. In a first step, the metal strip 1 runs through a bath 3 with liquid zinc 2, and thereafter past a pair of air knives 4.

However, instead of the furnace 105, one or several DFI burners 5 are used in a second step in order to further heat the steel strip 1 to its annealing temperature. The DFI burners 5 are arranged at such a distance from the steel material 1 so that their respective flames strike the surface of the material 1. This guarantees very good heat transfer efficiency.

Thereafter, the strip 1 is transported, in a third step, through a holding furnace 6 during a certain predetermined time period, to allow the annealing to be completed.

By using DFI burners 5 instead of a conventional furnace or an induction furnace 105 in order to further boost the temperature of the steel material 1, a number of advantages are achieved.

Firstly, heating using DFI burners is rapid and efficient, and substantially more efficient than a conventional heating furnace. The reason for this is that zinc has a low emission factor, which gives a low heat transfer rate between the furnace atmosphere and the zinc coated metal surface in a conventional furnace. This problem does not arise with DFI burners.

Secondly, heating with DFI burners is not as sensitive for the dimensions of the material 1 and its mechanical and geometrical configuration, which is the case with, for example, conventional induction heaters.

Thirdly, heating with DFI burners is a cheaper alternative to induction heaters, the latter requiring a more complex installation than what is necessary for corresponding DFI heating.

The temperature of the steel strip 1 when leaving the zinc bath 2, in the following referred to as “the first process temperature”, is preferably between 350° C. and 450° C., according to a preferred embodiment above about 420° C., at which temperature zinc melts.

The heating using the DFI burners 5 is preferably so intense that the final temperature of the steel material 1, in the following referred to as “the second process temperature”, is achieved within a few seconds. This means that those parts of the surface structure of the steel material that take part in the alloying process with the alloying metal essentially and in their entirety have a temperature which at least amounts to the second process temperature within a few seconds. The second process temperature is preferably between 50 and 150° C. warmer than the first process temperature.

In order to achieve maximum efficiency for the DFI burners 5, it is preferred that the oxidant being used for the combustion of the fuel is comprised of at least 80 percentages by weight of oxygen. The fuel may be any suitable fuel, such as natural gas or propane.

In order to avoid overheating of the surface of the steel material 1, it is preferred that it is in continuous motion in relation to the DFI burners 5. For example, this may be achieved by the process being of a continuous type, in which the steel material 1 is continuously transported along the process line and thereafter at all the time has a certain velocity in relation to the components being arranged on the line, notably the DFI burners 5.

It is also possible to arrange several consecutive DFI burners along the process line, so that the surface of the steel material 1 obtains a thermal impulse when passing each DFI burner, and thereafter has time to cool down somewhat before passing the next DFI burner and there receiving additional thermal energy. In this way, the heat has time to transfer through heat conduction from the surface of the steel material 1 to the central parts of the steel material 1 between the thermal impulses received from the DFI burners. Preferably, in this case the DFI burners are arranged at such a distance from each other so that the surface of the steel material 1 has time to cool down between two consecutive DFI burners to such an extent so that when passing the next DFI burner it will not be heated above a certain predetermined temperature. The predetermined temperature is suitably a temperature at which the risk for material deterioration is unacceptably high, most preferably maximally 560° C.

It is also possible to arrange two or several groups of DFI burners in a corresponding manner, where each group of DFI burners simultaneously heats the steel material 1 from different sides.

It is preferred that the predetermined time period during which the steel material 1 is kept at annealing temperature in the furnace 6 is at least a number of seconds, however this time period can naturally be adapted to the present prerequisites, the used steel material and alloying metal, and so forth. Preferably, the steel material 1 is kept at an essentially constant temperature during a time period which is sufficiently long in order to allow at least partial alloying between the alloying metal 3 and the steel material 1.

Above, preferred embodiments have been described. However, many modifications may be made to the described embodiments without departing from the spirit of the invention.

Thus, other alloying metals than zinc may be used for coating the surface of the steel material 1 in liquid form. Examples of such metals are aluminium and mixtures of aluminium and zinc. In these cases, it is also realised that the first process temperature may be adapted to the melting point, or to any other essential material property, of the currently used alloying metal.

Of course, it is also possible to apply the liquid alloying metal 3 onto the steel material 1 in other ways than by dipping, as long as the application takes place mechanically and as long as the alloying metal is in liquid form.

Furthermore, the steel material 1 does not have to be in the form of an elongated steel strip. The method is also useful for other elongated steel products, such as wire and rods.

Moreover, the air knives 4 may in certain applications be replaced with blowing action from the DFI burners 5. In other words, the DFI burners 5 may remove surplus alloying metal using the striking of the flames against the surface of the steel material 1, whereby the air knives 4 are no longer necessary.

Thus, the invention shall not be limited by the described embodiments, but may be varied within the frame of the appended claims.

Claims

1-13. (canceled)

14. Method for use when galvannealing a steel material (1) in the form of an elongated steel product which is transported along a process line, in which the material (1), in a first step, is preheated to a first process temperature of between 350 and 450° C. and is coated with a layer of a liquid alloying metal (3), in a second step is further heated to a second process temperature which is caused to be between 50 and 200° C. warmer than the first process temperature, and in a third step is kept at the second process temperature during a predetermined time period of at least a number of seconds so that the alloying metal coating at least partially is caused to alloy with the steel material (1), characterised in that the heating in the second step is caused to be carried out by several DFI burners (5), arranged one after another along the process line at such a distance from the steel material (1) so that their respective flames strike the surface of the material (1), in that at least one DFI burner (5) is caused to be driven with an oxidant which to at least 80 percentages by weight is comprised of oxygen, in that the steel material (1) is held in continuous motion in relation to each DFI burner (5), and in that the DFI burners (5) are arranged at such a distance from each other so that the surface of the steel material (1) has time to cool down between two consecutive DFI burners to such an extent so that when passing the next consecutive DFI burner it will not be heated above a certain predetermined temperature at which the risk for material deterioration is unacceptably high.

15. The method according to claim 14, characterised in that the certain predetermined temperature is not more than 565° C.

16. The method according to claim 14, characterised in that the steel material (1) is a steel strip.

17. The method according to claim 14, characterised in that the additional heating is caused to be so intense that it goes on only a few seconds before the second process temperature is reached.

18. The method according to claim 14, characterised in that the flame from at least one DFI burner (5) is caused to remove any surplus alloying metal from the surface of the material (1).

19. The method according to claim 14, characterised in that the alloying metal (3) comprises zinc.

20. The method according to claim 14, characterised in that the alloying metal (3) comprises aluminum.