CA1306391C

CA1306391C - Process for treatment of rolled steel to increase resistance to corrosion

Info

Publication number: CA1306391C
Application number: CA000576161A
Authority: CA
Inventors: Max Aicher
Original assignee: Stahlwerk Annahuette Max Aicher GmbH and Co KG
Current assignee: Stahlwerk Annahuette Max Aicher GmbH and Co KG
Priority date: 1987-09-01
Filing date: 1988-08-31
Publication date: 1992-08-18
Anticipated expiration: 2009-08-18
Also published as: CN1031724A; PT88371A; ES2028964T3; DK460688D0; DK460688A; PL158408B1; DD282246A5; PT88371B; EP0305914A2; NO176578B; NO883781D0; JPH0445591B2; EP0305914A3; SU1674689A3; DE3866998D1; EP0305914B1; NO883781L; PL274389A1; ATE70568T1; DE3729177C1

Abstract

ABSTRACT
For application of a coating of a non-ferrous metal, such as for example zinc, on the surface of rolled steel to increase resistance to corrosion it is proposed that the rolling steel be descaled in direct continuation of the rolling process from the rolling heat by the use of water under pressure and directly after descaling, to prevent re-formation of scale, that it be cooled rapidly by the use of water to a temperature suitable for hot application of the coating and that finally the coating be applied. The advantage of the invention lies on the one hand in the fact that only one single medium, i.e. water, is used for both descaling and prevention of re-formation of scale and at the same time for cooling purposes, where cooling can be carried out such that the rolling heat is utilizable in an ideal manner for the purpose of coating. This ensures that all process stages or treatment processes can take place in direct continuation of the rolling process, that is to say at rolling speed, so that the finish coated end product leaves the production line in the same way as a normal rolled product, not subjected to aftertreatment.

Description

- 2 - ~ ~ 2118~-28 The invention concerns a process for treatment of rolled steel, in particular concrete reinforcement bar steel, prestressing steel or the like for increasing the resistance to corrosion by application of a coating of a non-ferrous metal, preferably æinc, where the rolled steel is initially descaled from the rolling heat and then the coating applied to the hot rolled steel, reformation of scale being prevented.

Rolled steel products are used inter alia in the construction industry as structural steel, here primarily in the form of concrete reinforcement bar steel, as prestressing steel for prestressed concrete, as well as for guys for ground and rock anchors, etc.

When used as reinforcement for concrete structural components, as non-prestressed or prestressed reinforcement or as tension bar for a ground or rock anchor, a bond generally exists between the reinforcing steel and the concrete. This occurs in the case of reinforced concrete and in the case of instant bond reinforced concrete as a direct result of the reinforcing elements being embedded in the concrete and being closely surrounded by it, in the case of prestressed concrete with subsequent bond by virtue of the fact that the prestressing ducts necessary to maintain longitudinal mobility of the prestressing elements are subsequently injected with cement mortar.

In the manufacture of cement, an alkaline solution with a pH value of over 12 occurs as a result of the dissolution of the lime particles contained in the cement. This results in a protective layer of iron o~ide forming on the surface of the reinforcing elements which passivates the stesl surface and protects it from surface corrosion. For static as well as structural reasons, the reinforcing elements are usually located in the surface zones of the components, where they must maintain a certain minimum distance from the outer face, the "concrete cover". As a result of the air pollution and in the case of bridge structures, particularly as a result of the increasing use of de-icing salt to remove snow and ice, it is possible for chlorides dissolved in these media to penetrate to the steel in the case of inadequate and even frequently in the case of adequate concrete cover, where they ~..
~ , '3~ 2 1 1 g2 - 2 8 4 destroy th~ passivating layer and can thus attack the steel directly. A
similar situation applies to the tension bars of ground and rock anchors, where the corrosive effect on the steel is caused by ground moisture, water and acidic soil constituents. For this reason, a need exists for additional corrosion protection measures for such rolled steel products used in construction engineering.

Quite ganerally, a process is known for coating metals, particularly ferrous metals, with a thin layer of zinc as rust protection. In this connection, various processes are known. For the purpose of hot dip zinc coating, the parts to be protected are dipped in a bath of molten zinc, which can also, if necessary, be alloyed with aluminium. For electrolytic galvanizing, sulphate, sulphate chloride or potassium cyanide-caustic potash bathS are used. Processes are also known for applying metal coatings by spraying on the metal in liquid form, the metal being supplied to a spray gun in wire or powder form, rendered molten by oxygen-combustible gas mixtures and atomized by compressed air. All these processes are costly, particularly for treatment of mass-produced items of considerable length manufactured in one continuous operation, such as concrete reinforcement bar steels or prestressing steels and can be used not at all or only with involvement of considerable labour and expense. In addition, there is the fact that the products to be coated must of course be cleaned before application of such a coating. Rolled steel products, in particular, must have the layer of scale, adhering to them from the rolling process, removed; this is usually done by sand blasting, pickling or similar treatment.

In the case of rolled steel products produced by a continuous process, it has also become known practice to descale the rolling stock directly following the rolling process, preventing re-formation of scale and utilizing the rolling heat to provide it with a corrosion resistant &oating of, for example, zinc (US-PS 2 442 485). In this instance, descaling takes place in the final roll stand as the result of a pronounced reduction in the cross sectional area of the rolling stock combined with elongation, as a result of which the brittle scale breaks up, so that the rolling stock leaves the roll stand essentially free from scale. In order to maintain this state, the rolling stock, after leaving the roll stand at a temperature of approx. 980 to 760 degrees C, is passed through An enclosed chamber in ~ ~3(~:i3~1 - ~ ~ 21182-2 which the carbon monoxide is combusted to form carbon dioxide. Directly afterwards, the rolling stock passes through another chamber in which it is coated, for example, with ~inc in the form of zinc powde} which fuses on as a result of the heat still retained by the rolling stock. As however, when leaving this chamber, the coated rolling stock is still at a temperature of approx. 700 to 600 C, it must be cooled rapidly immediately after coating to prevent the zinc powder applied combusting to form zinc oxide for example.

This process is again relatively costly. Keeplng the descaled surface of the rolling stock clean by means of an atmosphere of deoxidizing gas requires not only storing, supplying ~he latter and maintaining a deoxidizing atmosphere in a suitably sealed chamber, but also maintaining a suitable temperature before the rolling stock is cooled down rapidly after coating by the use of other media.

In the light of this, the invention is based on the technical problem of being able to coat rolled steel products in a continuous process directly following the rolling process, utilizing the rolling heat, and at the same time keeping process and equipment expense to a minimum.

This problem is addressed according to the invention by the ~olled steel being impinged upon by water under high pressure for the purpose of descaling as a direct continuation of the rolling process, by the steel surface being cooled down rapidly to a temperature of less than 600 degrees C by the use of water directly after descaling to provent re-formation of scale,and by the coating being applied directly afterwards.

The advantage of the invention lies primarily in the fact that, for the purpose of descaling as well as for preventing re-formation of scale, and at tha same time for the purpose of cooling, only a single medium, i.e. water, is used, whereby cooling can be carried out such that the rolling heat can bé put to optimum use for the purpose of coating. This ensures that all process stages or treatment processes can take place as a direct continuation of the rolling process, that is to say at rolling speed, so tha~ the finish coated end product leaves the productlon line in the same way as a normal, untreated rolled product.

_ 5 By subjecting the steel surface to water at high pressure for the purpose of descaling, use is made of the phenomenon that the scale adhering to the rolled steel from the rolling process, which has been loosened during the course of deformation of the rolling stock taking place in th~ final roll stand, e.g. during transition from an oval to a circular cross section, immediately after the rolling stock has left the roll stand, can be completely removed with relative ease. The pressure of the water should be in excess of 200 bar, preferably 300 to 400 bar; it can, if necessary, be increased to about 1000 bar. This involves only very brief treatment, i.e.
passage of the rolled steel through a spray nozzle which results in only insignificant reduction in the temperature of the rolled steel.

Whilst descalin~ with water under pressure from rolling heat takes place at a temperature of about 900 to 1000 degress C, according to the invention, cooling is carried out as a continuation of this water treatment as a continuous process by using the same medium, such that the temperature drops as rapidly as possible below the scaling temperature of 600 degrees C, thus that no fresh scale can form and temperatures are reached at which the coatings can be applied. According to the invention, this can be effected by the rolled steel being passed through a water bath or being sprayed with water. Suitable temperatures for application of corrosion-resistant coating are for example with pure aluminium a temperature of approx. 560 to 570 degrees C and with pure zinc a temperature of approx. 415 degrees C;
intermedia~e stages can be used for alloys. Thus, according to the invention, it is directly possible to use any desired alLoy for the coatings, as all temperature ranges are passed through, from rolllng heat down to complete cooling.

Application of the coating can be effected in any desired manner~ The non-ferrous metal forming the coating can be sprayed on in powder form where lt uses on; the rolled steel can also however be passed through a bath containing the molten non-ferrous metal.

Whilst, after descaling, the rolled steel is protected by the water treatment from fresh scale formation, this protection, until complete cooling of the rolled stce] has taken place, is in fact provided after application of the coating by this coating itself. In some cases~ it is - 6 - ~ 3~ 21182-28~

advisable to apply to the coating of zinc, for example, a further layer of synthetic resin, e.g. epoxy resin, as additional protection. A process of this nature can also be readily integrated in the process according to the invention by utilizing the residual heat.

As the rolled steel according to the Lnvention is cooled after descaling by treatment with water, this treatment can also take place in the form of known hardening and tempering processes using water. One of these is, for e~ample, the process known as the "Tempcore" process for low-carbon and thus weldable concrete reinforcement bar steels, where the water treatment takes place such that directly after quenching a surface zone of martensite and bainite is present in the rolled steel, whilst the heat remaining in the steel does not cause tempering of the surface zone past the bainite stage during subsequent cooling. In the case of high-carbon steels suitable for use ~s prestressing steels, the water treatment is carried out, such that ehe steels are quenched from a final rolling temperature of between ô60 and 1060 degrees C in such a way that completely martensitic structure if~
formed in the skin layer and the temperature of the skin layer due to thermal equilibrium is 400 to 500 degrees C in the period between 2 and 6 seconds after commencement of the quenching process. In this way, the cnpling to be carried out according to the invention can still be utilized in a particularly advantageous manner to improve the strength of the steels.

A further advantage of the invention lies in the fact that all operations can be carried out in a continuous process directly following the rolling process in individual treatment stations located downstream of the final roll stand. Whilst the rolling process which is of necessity discontinuous on account of the rolling of billets has been largely standardized up to leaving the final roll stand, the process according to the invention displays of course particular advantages when the rolling process is made into continuous rolling by welding the billetsA This is rendered particularly successful by the known method of flying flash butt welding of the billets in the area between leaving the furnace or leaving a three-high roll stand and the initial pass of the finishing ~ill train.

Claims

1. Process for treatment of rolled steel to increase the resistance to corrosion wherein the rolled steel is impinged upon by water under pressure for the purpose of descaling from the rolling heat as a direct continuation of the rolling process, directly after descaling the steal surface is rapidly cooled down to a temperature below 600 degrees C by the use of water to prevent re-formation of scale on the hot rolled steel, and applying a coating of a non-ferrous metal.

2. Process in accordance with claim 1, wherein the coating is zinc.

3. Process in accordance with claim 1, wherein concrete reinforcement bar steel or prestressing steel are treated.

4. Process in accordance with claim 1, wherein the coating is zinc and wherein concrete reinforcement bar steel or prestressing steel are treated.

5. Process in accordance with any one of claim 1, 2, 3, or 4, wherein the water is applied at a pressure of over 200 up to about 1000 bar, preferably between 300 and 400 bar.

6. Process in accordance with claim 5, wherein the rolled steel is passed through a water bath.

7. Process in accordance with claim 5, wherein the rolled steel is sprayed with water.

8. Process in accordance with any one of claims 1, 6 or 7, wherein the coating is applied by spraying on non-ferrous metal in powder form.

9. Process in accordance with any one of claims 1, 6 or 7, wherein the rolled steel is passed through a bath with the molten non-ferrous metal for application of the coating.

10. Process in accordance with claim 8, wherein a layer of synthetic resin is additionally applied to the coating.

11. Process in accordance with claim 9 wherein a layer of synthetic resin is additionally applied to the coating.

12. Process in accordance with any one of claims 10 or 11, wherein a layer of epoxy resin is additionally applied to the coating.

13. Process in accordance with any one of claims 1, 6 or 7, wherein cooling of the rolled steel takes place in the form of quenching such that directly after quenching a surface zone of martensite and bainite is present, whilst the heat remaining in the steel core causes tempering of the surface zone during the subsequent cooling.