EP1660693B1 - Method for producing a hardened profile part - Google Patents

Abstract

Method for production of a hardened profile part from a hardenable steel alloy having cathodic corrosion protection by: (a) application of a Zn coating to a hardenable steel alloy sheet; (b) roll profiling of the coated sheet with shaping into a roll-shaped profile strand; (c) heating of the coated steel sheet to the hardening temperature with admission of atmospheric oxygen, formation of a superficial oxide skin from oxygen-affine elements; and (d) cooling. Independent claims are included for: (1) a corrosion protection Zn layer for a steel sheet; (2) a hardened and profiled component in hardenable steel with a cathodically protected layer obtained as above.

Description

The invention relates to a method for producing a hardened profile component with cathodic corrosion protection and to a hardened metallic profile component with cathodic corrosion protection.

Low alloy steel sheets, especially for bodywork, are not resistant to corrosion after being produced by suitable forming steps, either by hot rolling or cold rolling. This means that after a relatively short time and due to the humidity at the surface, oxidation occurs.

It is known to protect steel sheets from corrosion with corresponding anti-corrosion layers. According to DIN-50900, Part 1, corrosion is the reaction of a metallic material with its environment, which causes a measurable change in the material and can lead to a deterioration in the function of a metallic component or an entire system. In order to avoid corrosion damage, steel is usually protected so that it can withstand the corrosion loads during the required service life. The avoidance of corrosion damage can be achieved by influencing the properties of the reactants and / or by changing the reaction conditions, separating the metallic material from the corrosive Medium done by applied protective layers and by electrochemical measures.

According to DIN 50902, a corrosion protection layer is a layer produced on a metal or in the near-surface region of a metal, which consists of one or more layers. Multi-layer coatings are also referred to as corrosion protection systems.

Possible corrosion protection layers are, for example, organic coatings, inorganic coatings and metallic coatings. The purpose of metallic corrosion protection layers is to transfer the properties of the support material to the steel surface for as long as possible. Accordingly, the choice of an effective metallic corrosion protection requires the knowledge of the corrosion-chemical relationships in the system steel / coating metal / attacking medium.

The coating metals can be electrochemically nobler or electrochemically less noble than steel. In the first case, the respective coating metal protects the steel only through the formation of protective layers. One speaks of a so-called barrier protection. As soon as the surface of the coating metal has pores or was injured, a "local element" forms in the presence of moisture, in which the base partner is attacked by the metal to be protected. The more noble coating metals include tin, nickel and copper.

Less precious metals form protective coatings on one side; on the other hand, being less noble than steel, they are additionally attacked by leaks in the layer. In the case of a breach of such a coating layer, the steel is accordingly not attacked, but by the formation of local elements first corrodes the less noble coating metal. One speaks of a so-called galvanic or cathodic corrosion protection. For example, zinc is one of the less noble metals.

Metallic protective layers are applied by various methods. Depending on the metal and process, the connection of the steel surface is chemical, physical or mechanical and ranges from alloy formation and diffusion to adhesion and mere mechanical clamping.

The metallic coatings should have similar technological and mechanical properties as steel and behave similarly to steel or mechanical deformations or plastic deformations. The coatings should therefore not be damaged during forming and also affected by forming operations.

When applying hot-dip coatings, the metal to be protected is immersed in molten metal melts. As a result of the hot dip, corresponding alloy layers are formed at the phase boundary steel-coating metal. An example of this is the hot dip galvanizing.

In hot dip galvanizing, the steel strip is passed through a zinc bath, the zinc bath having a temperature of around 450 ° C. Hot-dip galvanized products have high corrosion resistance, good weldability and formability, and their main applications are the construction, automotive and household appliance industries.

In addition, the production of a coating of a zinc-iron alloy is known. For this purpose, these products are after hot dip galvanizing at temperatures above the zinc melting point, mostly subjected to a diffusion annealing between 480 ° C and 550 ° C. The zinc-iron alloy layers grow and absorb the overlying zinc layer. This process is called "galvannealing". The zinc-iron alloy thus produced also has a high corrosion resistance, good weldability and formability. Main applications are the automotive and home appliance industry. In addition, other coatings of aluminum-silicon, zinc-aluminum and aluminum-zinc can be produced by hot dipping.

Furthermore, the production of electrodeposited metal coatings is known, i. the electrolytic, so under current passage deposition of metallic coatings of electrolytes.

The electrolytic coating is also possible with such metals, which can not be coated by melt-dip process. Conventional layer thicknesses in electrolytic coating are usually between 2.5 and 10 microns, they are thus generally lower than in enamel dip coatings. Some metals, e.g. Zinc, also allow thick film coatings with electrolytic coating. Electrolytically galvanized sheets are mainly used in the automotive industry, because of the high surface quality, these sheets are used above all in the outer skin area. They have good formability, weldability and storability as well as good paintable and matt surfaces.

In particular, in the automotive industry there is an effort to make the body shell always easier. This is partly due to the fact that lighter vehicles consume less fuel, on the other hand vehicles are equipped with more and more additional functions and additional aggregates, which increases the weight bring with it, which should be compensated by a lighter body shell.

At the same time, however, the safety requirements for motor vehicles are increasing, with the body being responsible for the safety of persons in a motor vehicle and their protection in the event of accidents. Accordingly, there is a requirement for lighter body heights to bring about increased safety in case of accident. This can only be achieved by using materials with increased strength, in particular in the area of the passenger compartment.

In order to achieve the required strengths, it is necessary to use steel grades which have improved properties of a mechanical nature or to treat the steel grades used so that they have the required mechanical properties.

In order to form steel sheets with increased strength, it is known to form steel components in one step and to harden at the same time. This process is also called "press hardening". Here, a steel sheet is heated to a temperature above the Austenitisierungstemperatur, usually above 900 ° C, and then formed in a cold tool. In this case, the tool deforms the hot steel sheet, which cools very rapidly due to the surface contact with the cold mold, so that the hardening effects known per se occur with steel. In addition, it is known to first reshape the steel sheet and then to cool and harden the formed sheet steel component in a sizing press. In contrast to the former method, it is advantageous that the sheet is formed in a cold state and thus more complex shapes are possible. In both methods, however, the surface of the plate is scaled by heating, so that after forming and hardening the sheet surface must be cleaned, for example by sandblasting. Then the sheet is trimmed and possibly punched necessary holes. In this case, it is disadvantageous that the sheets have a very high hardness in the mechanical processing and therefore the processing is complicated and in particular a high tool wear exists.

WO 03/035922 A1 & EP 1439240 A1 ) discloses a method for producing a hardened profile component made of steel with cathodic corrosion protection, wherein a galvanized steel sheet is warmed up, then heated to an austenitizing temperature and then hot-press molded. The Zn coating melt may contain elements such as 0.08-0.4% Al.

The US 6,564,604 B2 The object of the invention is to provide steel sheets which are subsequently subjected to a heat treatment, and a method for producing parts by press-hardening these coated steel sheets. In this case, it should be ensured despite the increase in temperature that the steel sheet is not decarburized and the surface of the steel sheet is not oxidized before, during and after the hot pressing or the heat treatment. For this purpose, an alloyed intermetallic mixture should be applied to the surface before or after punching, which should provide protection against corrosion and decarburization and also can provide a lubricating function. In one embodiment, this document proposes to use a conventional, apparently electrolytically applied zinc layer, wherein this zinc layer is to convert with the steel substrate in a subsequent Austenitisieren the sheet substrate in a homogeneous Zn Fe Fe alloy layer. This homogeneous layer structure is confirmed by microscopic images. Contrary to previous assumptions, this coating is said to have a mechanical resistance that prevents it from melting. In practice, however, such an effect does not show. In addition, the use of zinc or zinc alloys should provide cathodic protection of the edges when cuts are present. In this embodiment, however, is a disadvantage that with such a coating - contrary to the information in this document - but to the Edge hardly a cathodic corrosion protection and in the area of the sheet surface, in case of damage to the layer, only a poor corrosion protection is achieved.

In the second example the US 6,564,604 B2 a coating is specified which consists of 50% to 55% aluminum and 45% to 50% zinc with possibly small amounts of silicon. Such a coating is not new in itself and known under the brand name Galvalume®. It is stated that the coating metals zinc and aluminum with iron should form a homogeneous zinc-aluminum-iron alloy coating. In the case of this coating, it is disadvantageous that sufficient cathodic corrosion protection is no longer achieved here, but the predominant barrier protection which is achieved with this is not sufficient when used in the press hardening process, since partial surface damage to the surface is unavoidable. In summary, the method described in this document is unable to solve the problem that, in general, zinc-based cathodic corrosion coatings are not suitable for protecting steel sheets which are to be subjected to a heat treatment after coating and may also be subjected to another shaping or Ümformschritt.

From the EP 1 013 785 A1 a method for producing a sheet metal component is known, wherein the sheet on the surface should have an aluminum layer or an aluminum alloy layer. A sheet provided with such coatings is to be subjected to a press hardening process, giving as possible coating alloys an alloy with 9-10% silicon, 2-3.5% iron, balance aluminum with impurities and a second alloy with 2-4% iron and the rest aluminum with impurities. Such coatings are known per se and correspond to the coating of a hot-dip aluminized steel sheet. In such a coating is disadvantageous in that only a so-called barrier protection is achieved. The moment that such a barrier layer is damaged or cracked in the Fe-Al layer, the base material, in this case the steel, is attacked and corroded. A cathodic protective effect is absent.

Furthermore, it is disadvantageous that even such a hot-dip coated coating during the heating of the steel sheet to the austenitizing temperature and the subsequent press hardening step is so far chemically and mechanically claimed that the finished component has an insufficient corrosion protection layer. As a result, it can be stated that such a hot-dip aluminized layer is suitable for press-hardening complex geometries, i. for heating a steel sheet to a temperature higher than the austenitizing temperature is not well suited.

From the DE 102 46 614 A1 For example, a method for producing a coated structural component for vehicle construction is known. This method is intended to solve the problems of the aforementioned European patent application 1 013 785 A1 to solve. In particular, it is stated that the immersion process according to the European patent application 1 013 785 A would form an intermetallic phase already during the coating of the steel, this alloy layer being hard and brittle between the steel and the actual coating and being torn during cold forming. As a result, microcracks would form to a degree that the coating itself detaches from the base material and thus loses its protective function. The DE 102 46 614 A1 therefore proposes a coating as a metal or a metal alloy by means of a galvanic coating process in organic, non-aqueous solution, wherein a particularly suitable and therefore preferred coating material is aluminum or an aluminum alloy. Alternatively, zinc or zinc alloys would be suitable. Such a coated sheet can then be cold preformed and hot finished molded. In this method, however, has the disadvantage that an aluminum coating, even if it was applied electrolytically, no longer offers corrosion protection in case of damage to the surface of the finished component, since the protective barrier has been broken. In the case of an electrodeposited zinc coating, it is disadvantageous that during heating for hot forming, the zinc is largely oxidized and no longer available for cathodic protection. Under a protective gas atmosphere, the zinc evaporates.

From the DE 101 20 063 C2 a method for the production of metal profile components for motor vehicles is known. In this known method for the production of metal profile components for motor vehicles, starting material provided in strip form is fed to a roll forming unit and formed into a rolled section. After leaving the roll forming unit, at least partial areas of the rolled section are to be heated inductively to a temperature required for hardening and then quenched in a cooling unit. After this, the rolled sections are cut to form the profile components. A particular advantage of roll forming is to be seen in the low production costs due to the high processing speed and low compared to a press tool costs. A special heat-treatable steel is used for the profile component. According to an alternative of this method, partial regions of the starting material may also be inductive, prior to entry into the rolling profiling unit, to those required for hardening Temperature heated and quenched before cutting the rolled section in a cooling unit. In the second alternative, it is disadvantageous that the cutting must take place already in the hardened state, which is problematic due to the high hardness of the material. Furthermore, it is disadvantageous that in the prior art already described the cut-to-length profile components have to be cleaned or descaled and after the ignition a corrosion piece coating has to be applied, whereby such corrosion piece coatings usually do not give a very good cathodic corrosion protection.

The object is to provide a method for producing a hardened profile component with a cathodic corrosion protection, wherein the cathodic corrosion protection is formed so that even the starting material has a protective layer which does not convert during the further processing in a negative way.

The object is achieved by a method having the features of claim 1. Advantageous developments are characterized in the subclaims.

Another object is to provide a cathodic anti-corrosion layer for curable profile components.

The object is achieved with a corrosion protection layer having the features of claim 29. Advantageous developments are characterized in the dependent claims.

Another object is to provide a hardened profile component with cathodic corrosion protection.

The object is achieved by a method having the features of claim 45. Advantageous developments are characterized in the dependent claims.

The inventive method provides, on a hardenable steel sheet, a coating of a mixture consisting essentially of zinc and an oxygen affinity element, such as magnesium, silicon, titanium, calcium and aluminum with a content of 0.1 to 15 wt .-% of the Apply oxygen-affine element and heat the coated steel sheet at least partially with the access of oxygen to a temperature above the Austenitisierungstemperatur the sheet metal alloy and before to reform, wherein the sheet is cooled after sufficient heating and the cooling rate is such that hardening of the sheet metal alloy takes place. As a result, a hardened component is obtained from a steel sheet having a good cathodic corrosion protection.

The corrosion protection according to the invention for steel sheets, which are first formed and in particular roll-profiled and then subjected to a heat treatment and deformed and thereby hardened, is a cathodic corrosion protection which is essentially based on zinc. According to the invention, 0.1% to 15% of one or more oxygen-containing elements such as magnesium, silicon, titanium, calcium, aluminum, boron and manganese or any mixture or alloy thereof are added to the zinc forming the coating. It has been found that such small amounts of an oxygen affinity element as magnesium, silicon, titanium, calcium, aluminum, boron and manganese cause a surprising effect in this particular application.

According to the invention, at least Mg, Al, Ti, Si, Ca, B, Mn are suitable as oxygen-affine elements. When aluminum is mentioned below, this is representative of the other elements mentioned.

It has surprisingly been found that, despite the small amount of an oxygen-affine element, in particular aluminum, an essentially Al ₂ O ₃ or an oxide of the oxygen-affine element (MgO, CaO, TiO, SiO ₂ , B ₂ O ₃ , MnO) existing, very effective and healing, superficial protective layer. This very thin oxide layer protects the underlying Znhaltige corrosion protection layer even at very high temperatures from oxidation. That is, during the special processing of the galvanized sheet in the press hardening process, an approximately two-layer corrosion protection layer is formed, which consists of a cathodically highly effective layer with a high proportion of zinc and a very thin oxidation protection layer of one or more oxides (Al ₂ O ₃ , MgO , CaO, TiO, SiO ₂ , B ₂ O ₃ , MnO) is protected against oxidation and evaporation. This results in a cathodic corrosion protection layer with a superior chemical resistance. This means that the heat treatment has to take place in an oxidizing atmosphere. Although under protective gas (oxygen-free atmosphere) oxidation can be avoided, the zinc would evaporate due to the high vapor pressure.

To provide a sheet with the corrosion protection according to the invention, in a first step, a zinc alloy with a content of aluminum in weight percent of greater than 0.1 but less than 15%, in particular less than 10%, more preferably less than 5% on a Steel sheet, in particular an alloyed steel sheet are applied, where in a second step, the sheet is formed inline as a strand and heated in the presence of atmospheric oxygen to a temperature above the Austenitisierungstemperatur the sheet metal alloy and then cooled at an increased rate.

It is assumed that in the first step of the process, namely when the sheet is coated on the sheet surface or in the proximal region of the layer, a thin barrier phase is formed, in particular Fe ₂ Al ₅ -x Zn _x , which forms the Fe-Zn Diffusion in a liquid metal coating process, which takes place in particular at a temperature up to 690 ° C, hindered. Thus, in the first process step, the sheet is formed with a zinc-metal coating with an addition of aluminum, which is effective only towards the sheet surface, as in the proximal region of the support an extremely thin barrier phase, which is effective against rapid growth of an iron-zinc compound phase, having. In addition, it is conceivable that only the presence of aluminum lowers the iron-zinc diffusion tendency in the region of the boundary layer.

If, in the second step, heating of the sheet provided with a zinc-aluminum-metal layer to the austenitizing temperature of the sheet metal material with access of atmospheric oxygen occurs, the metal layer on the sheet is liquefied for the time being. At the distal surface, the oxygen-containing aluminum from the zinc reacts with atmospheric oxygen to form solid oxide, thereby causing a decrease in the aluminum metal concentration, which causes a steady diffusion of aluminum towards depletion, that is to the distal region. This Tonerdeanreicherung, at the air exposed layer area now acts as oxidation protection for the layer metal and as Abdampfungssperre for the zinc.

In addition, during heating, the aluminum is withdrawn from the proximal blocking phase by continuous diffusion towards the distal region and is available there for the formation of the superficial Al ₂ O ₃ layer. Thus, the formation of a sheet metal coating is achieved, which leaves a cathodically highly effective layer with a high zinc content.

Well suited is, for example, a zinc alloy with a content of aluminum in weight percent of greater than 0.2 but less than 4, preferably greater than 0.26 but less than 2.5 wt .-%.

Conveniently, in the first step, the zinc alloy layer is applied to the sheet surface passing through a liquid metal bath at a temperature higher than 425 ° C, but lower than 690 ° C, especially at 440 ° C to 495 ° C, followed by cooling of the coated sheet, not only the proximal barrier phase can be effectively formed, or a very good diffusion inhibition can be observed in the region of the barrier layer, but it also takes place to improve the thermoforming properties of the sheet material.

An advantageous embodiment of the invention is given in a method in which a hot or cold rolled steel strip having a thickness of for example greater than 0.15 mm and having a concentration range of at least one of the alloying elements within the limits in wt .-% carbon to 0.4, preferably 0.15 to 0.3 silicon to 1.9, preferably 0.11 to 1.5 manganese to 3.0, preferably 0.8 to 2.5 chrome to 1.5, preferably 0.1 to 0.9 molybdenum to 0.9, preferably 0.1 to 0.5 nickel to 0.9, titanium to 0.2 preferably 0.02 to 0.1 vanadium to 0.2 tungsten to 0.2, aluminum to 0.2, preferably 0.02 to 0.07 boron to 0.01, preferably 0.0005 to 0.005 sulfur Max. 0.01, preferably max. 0.008 phosphorus Max. 0.025, preferably max. 0.01 Rest iron and impurities is used.

It has been found that the surface structure of the cathodic corrosion protection according to the invention is particularly favorable for a high adhesion of paints and varnishes.

The adhesion of the coating to the steel sheet article can be further improved if the surface layer has a zinc-rich intermetallic iron-zinc-aluminum phase and an iron-rich iron-zinc-aluminum phase, the iron-rich phase having a zinc to iron ratio of at most 0, 95 (Zn / Fe ≦ 0.95), preferably from 0.20 to 0.80 (Zn / Fe = 0.20 to 0.80), and the zinc rich phase has a zinc to iron ratio of at least 2.0 (Zn / Fe ≥ 2.0), preferably from 2.3 to 19.0 (Zn / Fe = 2.3 to 19.0).

The strip-form provided starting material with the coating according to the invention is fed to a roll forming unit and formed into a rolled section, wherein the rolled section is deformed during roll forming and subsequently cut to length in a cutting unit to the profile components. According to the invention, at least partial areas of the rolled section after leaving the rolling profiling unit or heated to a temperature required for curing prior to entry into the roll forming unit and quenched prior to being cut to length in a cooling unit. The required heating takes place, for example, inductively.

In a further advantageous embodiment, starting material provided in strip form is fed to a roll forming unit and converted into a rolled section in the roll forming unit, wherein the rolled section is deformed during roll forming and then the rolled section is cut to length in a cutting unit to the profile components. Subsequently, the already cut to length profiles are stored in a profile memory with separation and then subjected to the hardening step by heating and cooling.

A further advantageous embodiment provides for subjecting the separated profiles to an intermediate heat stage prior to curing under oxygen access, wherein in the intermediate heat stage an advantageous change in the corrosion protection layer takes place and only then to a temperature required for curing. The latter can be done with band material as well as with cut profiles.

Basically, open and closed profiles can be produced by inductive high frequency welding, laser welding, spot welding, seam welding, projection welding and rolling technology.

Figur 1:

schematisch eine Vorrichtung mit Induktionsspule und Abkühlring zum Herstellen von gehärteten Profilbauteilen;

Figur 2:

schematisch eine Vorrichtung zum Herstellen der erfindungsgemäßen Bauteile,

Figur 3:

eine weitere Ausführungsform einer Vorrichtung zum Herstellen der Profilbauteile;

Figur 4:

schematisch den Temperaturzeitverlauf beim Herstellen des erfindungsgemäßen Profilbauteils;

Figur 5:

den Temperaturzeitverlauf bei einer weiteren vorteilhaften Ausführungsform des Verfahrens zum Herstellen des erfindungsgemäßen Profilbauteils;

Figur 6:

die lichtmikroskopische Aufnahme des Querschnitts eines erfindungsgemäß hergestellten Profilbauteils mit erfindungsgemäßer Phasenzusammensetzung;

Figur 7:

REM-Aufnahme des Querschliffs einer geglühten Probe eines erfindungsgemäßen kathodischen korrosionsgeschützten Blechs;

Figur 8:

den Potentialverlauf für das Blech nach Figur 7;

Figur 9:

die REM-Aufnahme des Querschliffs einer geglühten Probe eines erfindungsgemäßen mit einem kathodischen Korrosionsschutz versehenen Blechs;

Figur 10:

den Potentialverlauf des Blechs nach Figur 9;

Figur 11:

REM-Aufnahme des Querschliffs eines nicht erfindungsgemäß beschichteten und behandelten Blechs;

Figur 12:

den Potentialverlauf des nicht erfindungsgemäßem Blechs nach Figur 11;

Figur 13:

REM-Aufnahme des Querschliffs der Oberfläche eines erfindungsgemäß beschichteten und wärmebehandelten Blechs;

Figur 14:

den Potentialverlauf des Blechs nach Figur 13;

The invention will now be described by way of example with reference to a drawing, in which:

FIG. 1:

schematically an apparatus with induction coil and cooling ring for producing hardened profile components;

FIG. 2:

schematically a device for producing the components according to the invention,

FIG. 3:

a further embodiment of an apparatus for producing the profile components;

FIG. 4:

schematically the temperature-time profile during the manufacture of the profile component according to the invention;

FIG. 5:

the temperature time course in a further advantageous embodiment of the method for producing the profile component according to the invention;

FIG. 6:

the photomicrograph of the cross section of a profile component according to the invention produced with phase composition according to the invention;

FIG. 7:

SEM image of the cross section of an annealed specimen of a cathodic corrosion-protected sheet according to the invention;

FIG. 8:

the potential curve for the sheet after FIG. 7 ;

FIG. 9:

the SEM image of the cross section of a calcined sample of a cathode according to the invention provided with a cathodic corrosion protection;

FIG. 10:

the potential course of the sheet after FIG. 9 ;

FIG. 11:

SEM image of the cross section of a sheet not coated and treated according to the invention;

FIG. 12:

the potential curve of the non-inventive sheet after FIG. 11 ;

FIG. 13:

SEM image of the cross section of the surface of a sheet coated and heat-treated according to the invention;

FIG. 14:

the potential course of the sheet after FIG. 13 ;

A profile component according to the invention with cathodic protection against corrosion was subsequently produced, as explained below, subsequently subjected to a heat treatment for hardening the profile component and rapid cooling. Subsequently, the sample was analyzed for optical and electrochemical properties. Assessment criteria were the appearance of the annealed sample and the protection energy. The protection energy is the measure for the electrochemical protection of the layer, which is determined by galvanostatic detachment.

The electrochemical method of galvanostatic dissolution of the metallic surface coatings of a material allows to classify the mechanism of corrosion protection of the layer. The potential-time behavior of a corrosion-protective layer is determined for a given constant current flow. For the measurements, a current density of 12.7 mA / cm ^{2 was} specified. The measuring arrangement is a three-electrode system. The counterelectrode used was a platinum network, the reference electrode consisting of Ag / AgCl (3M). The electrolyte consists of 100 g / l ZnSO ₄ .5H ₂ O and 200 g / l NaCl dissolved in deionized water.

If the potential required for dissolving the layer is greater than or equal to the steel potential, which can be easily determined by pickling or abrading the surface coating, this is called pure barrier protection without active cathodic protection. The barrier protection is characterized by the fact that it separates the base material from the corrosive medium.

Example 1 (according to the invention)

A steel sheet is hot dip galvanized with a melt consisting of 95% zinc and 5% aluminum. The coated steel sheet is then roll-profiled in a profiling device. After annealing, the sheet shows a silvery-gray surface with no defects. In cross section ( FIG. 7 ) shows that the coating consists of a light phase and a dark phase, wherein the phases are Zn-Fe-Al-containing phases. The bright phases are more zinc-rich, the dark phases more iron-rich. Some of the aluminum reacted with atmospheric oxygen during the calcination and formed a protective Al ₂ O ₃ skin.

At the beginning of the measurement, the galvanostatic dissolution shows a potential of about -0.7 V required for the resolution. This value is significantly below the potential of the steel. After a measuring time of approx. 1,000 seconds, a potential of approx. -0.6 V arises. This potential is also clearly below the steel potential. After a measurement time of approximately 3,500 seconds, this part of the layer is used up and the necessary potential for dissolving the layer approaches the steel potential. This coating thus offers after the annealing in addition to the barrier protection a cathodic corrosion protection. The potential is up to a measuring time of 3,500 seconds at a value of ≤ -0.6 V, so that a considerable cathodic protection is maintained over a long time, even if the sheet was fed to the austenitizing temperature. The potential time diagram is in FIG. 8 shown.

Example 2 (according to the invention)

The sheet is passed through a melt or through a zinc bath, with a zinc content of 99.8% and an aluminum content of 0.2%. The coated steel sheet is then roll profiled in a profiling. Aluminum present in the zinc coating reacts with atmospheric oxygen during the calcination and forms a protective Al ₂ O ₃ skin. Through constant diffusion of the oxygen-affinity aluminum to the surface, this protective skin is maintained and expanded. After inductive heating of the sheet shows a silvery-gray surface without defects. From the originally about 15 microns thick zinc coating develops during the annealing due to diffusion, a about 20 to 25 microns thick layer, said layer ( FIG. 9 ) consists of a gray appearing phase with a composition Zn / Fe of about 30/70 and a bright area with the composition Zn / Fe of about 80/20. On the surface of the coating, an increased aluminum content is detectable. Due to the detection of oxides at the surface, it can be concluded that a thin Al ₂ O ₃ protective layer is present.

At the beginning of the galvanostatic dissolution, the annealed material has a potential of approx. -0.75 V. After a measuring time of approx. 1,500 seconds, the potential required for the resolution increases to ≤ -0.6 V. The phase lasts up to a measuring time of approx. 2,800 seconds. Then the required potential increases to steel potential. In this case too, in addition to barrier protection, there is cathodic corrosion protection. The potential is up to a measurement time of 2,800 seconds at a value of ≤ -0.6 V. Thus, such a material has thus over a very long time a cathodic protection against corrosion. The potential time diagram is FIG. 10 refer to.

Example 3 (not according to the invention)

From a galvanized in the melt-dip sheet metal a profile component is produced in a roll profiler. This anticorrosive layer contains some aluminum in the zinc bath, of the order of about 0.13%. The profile component is heated to a temperature of about 500 ° C prior to austenitizing. Here, the zinc layer is completely converted into Zn-Fe phases. The zinc layer is thus wholly, i. converted to Zn-Fe phases to the surface. This results in zinc-rich phases on the steel sheet, all of which are formed with a Zn-Fe ratio of> 70% zinc. This anticorrosive layer contains some aluminum in the zinc bath, of the order of about 0.13%.

The profile component with the aforementioned fully converted coating is inductively heated to> 900 ° C. The result is a yellow-green surface.

The yellow-green surface indicates oxidation of the Zn-Fe phases during annealing. An aluminum oxide protective layer is undetectable. The reason for the absence of an aluminum oxide protective layer can be explained by the fact that in the annealing treatment the aluminum does not migrate as quickly to the surface due to solid Zn-Fe phases and can protect the Zn-Fe coating from oxidation. When heating this material at temperatures around 500 ° C is still no liquid zinc-rich phase, because this forms only at higher temperatures of 782 ° C. If 782 ° C are reached, thermodynamically there is a liquid zinc-rich phase in which the aluminum is freely available. Nevertheless, the surface layer is not protected against oxidation.

Possibly, the corrosion protection layer is already partially oxidized at this time and no opaque aluminum oxide skin can form any more. The layer shows wavy rugged in cross-section and consists of Zn and Zn-Fe oxides ( FIG. 11 ). In addition, the surface of said material is much larger due to the highly crystalline acicular surface finish of the surface, which could also be detrimental to the formation of a covering and thicker aluminum oxide protective layer. The said non-inventive coating forms a brittle layer which is provided with numerous cracks, both transversely and longitudinally to the coating. As a result, in the course of the heating, both decarburization and oxidation of the steel substrates can take place, especially with cold preformed components.

In the galvanostatic dissolution of this material, a potential of approx. + 1V is applied for the resolution under constant current flow at the beginning of the measurement, which then settles to a value of approx. + 0.7V. Here, too, the potential lies well above the steel potential during the entire dissolution ( FIG. 12 ). Consequently, under these annealing conditions, it is also necessary to speak of a pure barrier protection. Also in this case no cathodic corrosion protection could be determined.

Example 4 (according to the invention)

A profile component made of a sheet metal with a galvanizing as in Example 3 is subjected after the roll forming a particular short, inductive heat treatment, at about 490 ° C to 550 ° C, the zinc layer is only partially converted into Zn-Fe phases. The process is carried out in such a way that the phase transformation is only partially carried out and therefore not yet converted zinc with aluminum on the surface is present and thus free aluminum as oxidation protection for the zinc layer is available.

The profile component with the heat-treated coating according to the invention and only partially converted into Zn-Fe phases is subsequently heated inductively to the required austenitizing temperature. The result is a surface that is gray and without defects. A SEM / EDX examination of the cross section ( FIG. 13 ) shows an approximately 20 microns thick surface layer, wherein from the originally about 15 microns thick zinc coating of the coating has formed in the inductive annealing due to diffusion, an about 20 microns Zn-Fe layer, said layer with the typical for the invention two-phase structure a "leopard pattern" with a gray phase in the image with a composition Zn / Fe of about 30/70 and light areas with the composition Zn / Fe of about 80/20. In addition, individual areas with zinc contents ≥ 90% zinc are present. On the surface, a protective layer of aluminum oxide is detectable.

In the galvanostatic detachment of the surface coating of a rapidly heated sheet-metal plate with the hot-dip galvanized layer according to the invention and in contrast to Example 2 only incompletely before the press-hardening results, since at the beginning of the measurement the potential necessary for the resolution is about -0.94 V. and thus comparable to the potential necessary for the dissolution of an unannealed zinc coating. After a measuring time of approx. 500 seconds, the potential rises to a value of -0.79 V, far below the steel potential. After approx. 2,200 seconds measuring time, ≤ -0.6 V are necessary for the detachment, with the potential subsequently rising to -0.38 V and then approaching the steel potential ( FIG. 14 ). In the case of the material according to the invention, which has been heated up quickly and incompletely heat-treated prior to press-hardening, both barrier protection and very good cathodic corrosion protection can therefore be achieved form. Even with this material, the cathodic protection against corrosion can be maintained over a very long measuring time.

The examples show that only the corrosion-protected metal sheets used according to the invention for roll forming still offer a cathodic corrosion protection with a cathodic corrosion protection energy> 4 J / cm ² even after the heat treatment.

For the evaluation of the quality of the cathodic corrosion protection, not only the time during which the cathodic corrosion protection can be maintained must be considered, but also the difference between the potential required for the dissolution and the steel potential must be considered. The larger this difference, the more effective is the cathodic protection against corrosion even with poorly conducting electrolytes. The cathodic corrosion protection is negligible with a voltage difference of 100 mV to the steel potential in poorly conducting electrolytes. Although there is still a cathodic corrosion protection even with a smaller difference to the steel potential, if a current is detected when using a steel electrode, but this is negligible for practical aspects, since the corrosive medium must conduct very well, so this contribution to the cathodic Corrosion protection can be used. This is practically not the case under atmospheric conditions (rainwater, humidity, etc.). Therefore, the difference between the potential required for the dissolution and the steel potential was not used for the evaluation, but a threshold value of 100 mV below the steel potential was used. Only the difference up to this threshold was taken into account for the evaluation of the cathodic protection.

As an evaluation criterion for the cathodic protection of the respective surface coating after annealing, the area between the potential curve at the galvanostatic dissolution and the specified threshold value of 100 mV was set below the steel potential ( FIG. 8 ). Only the area below the threshold is taken into account. The overlying surface contributes negligibly little or not at all to the cathodic corrosion protection and is therefore not included in the evaluation.

The area thus obtained is multiplied by the current density, the protection energy per unit area with which the base material can be actively protected against corrosion. The greater this energy, the better the cathodic corrosion protection. While a sheet with the known aluminum-zinc layer of 55% aluminum and 44% zinc, as it is also known from the prior art, only a protection energy per unit area of about 1.8 J / cm ² , which is Protection energy per unit area in accordance with the invention coated profile components to> 7J / cm ² .

As a cathodic corrosion protection in the context of the invention is subsequently determined that at 15 microns thick coatings and the process and experimental conditions described at least a cathodic corrosion protection energy of 4 J / cm ^{2 is} present.

It is typical of the coatings according to the invention that in addition to the superficial protective layer of an oxide of the oxygen-affine element (s) used, in particular Al ₂ O ₃ after the heat treatment for the press-hardening, the layers according to the invention in cross-section show a typical "leopard pattern" that results from a zinc-rich, intermetallic Fe-Zn-Al phase and an iron-rich Fe-Zn-Al phase, wherein the iron-rich phase has a zinc to iron ratio of at most 0.95 (Zn / Fe≤0.95), preferably from 0.20 to 0.80 (Zn / Fe = 0.20 to 0.80) and the zinc-rich phase has a zinc to iron ratio of at least 2.0 (Zn / Fe≥2.0), preferably from 2.3 to 19.0 (Zn / Fe = 2.3 to 19.0). It has been found that only when such a biphasic construction is achieved is sufficient cathodic protection still present. Such a biphasic structure, however, only arises if the formation of an Al ₂ O ₃ protective layer on the surface of the coating has previously taken place. In contrast to a known coating according to the US 6,564,604 B2 , which has a homogeneous structure in terms of structure and texture, where Zn Fe needles are to be present in a zinc matrix, an inhomogeneous structure of at least two different phases is achieved here. This inhomogeneous layer structure, which manifests itself in the leopard pattern, is evidently also responsible for an improved ductility and thus stability of the layer.

A zinc layer which has been deposited electrolytically on the steel sheet surface is not in itself capable of providing a corrosion protection according to the invention, even after a heating step above the austenitizing temperature.
The anticorrosive coatings according to the invention were mentioned for profiling a profiled strand or for roll forming and the subsequent hardening of such a profiled strand or profiled strand sections.

Nevertheless, the coatings according to the invention or, according to the invention, for a sheet-metal component which must be subjected to a heating step are also suitable for other processes in which a steel sheet should first be provided with a corrosion protection layer, and the thus coated steel sheet is then subjected to a heating step for curing the same and before the heating, in the heating or after heating, a deformation of the sheet is to take place. The fundamental advantage of the layer is that a heated component does not have to be descaled after heating and, moreover, that a very good cathodic corrosion protection layer with a very high corrosion protection energy is available.

Whenever profiles or tubes are mentioned below, this also always refers to tubes, open profiles and generally rolled profiles.

In one embodiment of the method according to the invention, the profile component according to the invention is produced in that a band is first passed through a forward punch and then inserted into the profiling machine. In the profiling machine, the strip is bent to a desired profile. After bending in the profiling machine, the necessary welding is carried out in a welding device. After the profile has been formed in-line in this way, it is subsequently carried out by a heating device, the heating device being, for example, an induction coil. With the induction coil or the heating device, the profile is heated at least partially to an austenitizing temperature necessary for hardening. Subsequently, the cooling takes place. As cooling in this case a special cooling is used which prevents the partially liquid surface layer is fused. This causes high cooling rates with low fluid pressure. The special cooling mimics the immersion of the profile in a water bath, with a very large amount of water at low pressure on all sides led to the profile. To one The surface treatment of the sheet according to the invention can be preceded by an additional heating device for the induction heating device which serves to heat the sheet to the austenitizing temperature, which leads the sheet to the first heating stage at approximately 550 ° C. This can be, for example, an induction heating device to which - in order to comply with the necessary periods of time - an isolated region, for example an insulated tunnel region, is connected.

The cooling is followed by a calibration device, which subjects the heated and quenched profile strand to a calibration, whereupon the profile strand is then cut to length using a cutting unit.

In a further advantageous embodiment, band is drawn off from a strip preparation section and punched in a forward punch in a soft state and then profiled or bent and shaped accordingly in a profiling machine. Optionally, a welding device may also follow the profiling. The thus preformed profile strand is then cut with a cutting unit or cutting device to the appropriate lengths and transferred to a profile memory with separation. In the profile memory, a multiplicity of profiles, in particular a multiplicity of profiles of different cross sections which are also formed differently, are stored. From the profile memory with separation the desired profiles are subtracted and fed via a drive roller set the hardness level. In particular, the individual profiles are heated with an inductive heating already described to the temperature necessary for curing and subsequently quenched in the form already described, that is gently. Subsequently, the cured profiles can be retrofitted on a straightening scaffold. In an advantageous Embodiment, a heat treatment of the coating is performed prior to heating to the temperature required for curing. For this heat treatment, the profile is first heated to a temperature necessary for the heat treatment in particular 550 ° C. This heating can be done relatively quickly in an inductive heating stage, wherein, if necessary, the heat of the component for a certain time in an insulating region, for example, an insulated tunnel through which the profiles are performed is maintained.

In a further advantageous embodiment of this method, the profiled and finished molded profile strands are cut to standard profile lengths and then transferred to the profile memory with separation, the profile memory stores there exclusively tubes and profiles of a certain length, for example 6m. Depending on the required profile, the profiles are then removed individually and fed to the appropriate further treatment. Even with these profiles, if necessary, a hole pattern can already be arranged.

In all the above-mentioned inventive method, the profiling and in particular the arrangement of the hole pattern can be such that the thermal expansion is fully taken into account during the heat treatment and / or heating to the temperature necessary for curing, so that the component after quenching with respect to the dimensional and Position tolerances is made accurately.

In the invention, it is advantageous that a profile component made of sheet steel is provided, which has a cathodic protection against corrosion, which reliably remains even when the sheet is heated above the austenitizing temperature. It is also advantageous that the components do not have to be reworked after curing.

Claims (34)

1. Method for producing a hardened profiled component from a hardenable steel alloy with galvanic protection, wherein:

   a) a coating is applied to a sheet of a hardenable steel alloy, wherein

   b) the coating substantially consists of zinc, and

   c) the coating further contains one or more oxygen-affine elements totalling 0.1 % by weight to 15 % by weight of the total coating, and

   d) the coated sheet steel is then roll profiled in a profiling apparatus, so that the sheet metal strip is formed to produce a roll formed profiled strip, and

   e) the coated sheet steel is then brought at least regionally, with the addition of atmospheric oxygen, to an austenitising temperature required for hardening and heated up to a structural change required for hardening, wherein

   f) a surface skin is formed on the coating from an oxide of the oxygen-affine element(s), and

   g) following the required heating, the sheet metal is cooled, the cooling rate being chosen such that a hardening of the sheet metal alloy is obtained, wherein

   h) the profiled strip is cut to length into profiled strip sections before or after the hardening process,

   i) holes, recesses, punchings and/or a required hole pattern is/are produced in the profiled strip before the profiling and cutting-to-length process and before the heating to the temperature required for hardening, wherein

   j) the cooling process is carried out with water, wherein a large volume of water is applied to the component to be hardened at a low pressure.
2. Method according to claim 1, characterised in that the profiled strip profiled in the profiling apparatus is welded in a downstream welding apparatus.
3. Method according to any of the preceding claims, characterised in that the profiled strip or the profiled strip sections is/are, prior to being heated to the temperature required for hardening, heated in a heating-up step to and held at a temperature which allows the partial formation of iron-zinc phases in the coating.
4. Method according to any of the preceding claims, characterised in that the profiled strip or the profiled strip sections is/are heated to a temperature of 850°C to 950°C at a heating rate of 50°C to 100°C per second, held at this temperature for at least 5 seconds and cooled at a cooling rate of 25°C to 45°C per second.
5. Method according to any of the preceding claims, characterised in that in the heating process the profiled strip or the profiled strip sections is/are held for at least 10 seconds at 500°C to 600°C, in particular 530°C to 580°C, followed by further heating.
6. Method according to any of the preceding claims, characterised in that the profiled strip and/or the profiled strip sections is/are heated inductively and/or by means of radiation.
7. Method according to any of the preceding claims, characterised in that magnesium and/or silicon and/or titanium and/or calcium and/or aluminium and/or manganese and/or boron is/are used as oxygen-affine elements in the mixture.
8. Method according to any of the preceding claims, characterised in that the coating is applied in a hot dipping process, using a mixture containing substantially zinc together with the oxygen-affine element(s).
9. Method according to claim 1 and/or 2, characterised in that 0.2 % by weight to 5 % by weight of the oxygen-affine elements are used.
10. Method according to any of the preceding claims, characterised in that 0.26 % by weight to 2.5 % by weight of the oxygen-affine elements are used.
11. Method according to any of the preceding claims, characterised in that aluminium is substantially used as oxygen-affine element.
12. Method according to any of the preceding claims, characterised in that the coating mixture is chosen such that the layer forms in the heating process a surface oxide skin of oxides of the oxygen-affine element(s) and the coating forms at least two phases, these being a zinc-rich phase and an iron-rich phase.
13. Method according to any of the preceding claims, characterised in that the iron-rich phase has a maximum zinc-to-iron ratio of 0.95 (Zn/Fe ≤ 0.95), preferably of 0.20 t0 0.80 (Zn/Fe = 0.20 to 0.80), and the zinc-rich phase has a zinc-to-iron ratio of at least 2.0 (Zn/Fe > 2.0), preferably of 2.3 to 19.0 (Zn/Fe = 2.3 to 19.0).
14. Method according to any of the preceding claims, characterised in that the iron-rich phase has a zinc-to-iron ratio of approximately 30 : 70 and the zinc-rich phase is formed with a zinc-to-iron ratio of approximately 80 : 20.
15. Method according to any of the preceding claims, characterised in that the layer further contains individual regions with zinc contents > 90 %.
16. Method according to any of the preceding claims, characterised in that the coating is formed such that it develops a cathodic protection effect of at least 4 J/cm² at a thickness of 15 µm following the heating process.
17. Method according to any of the preceding claims, characterised in that the coating process is carried out with the mixture of zinc and the oxygen-affine element(s) in a continuous pass through a liquid metal bath at a temperature of 425°C to 690°C, followed by the cooling of the coated sheet metal.
18. Method according to any of the preceding claims, characterised in that the coating process is carried out with the mixture of zinc and the oxygen-affine elements in a continuous pass through a liquid metal bath at a temperature of 440°C to 495°C, followed by the cooling of the coated sheet metal.
19. Method according to any of the preceding claims, characterised in that the sheet metal is heated inductively.
20. Method according to any of the preceding claims, characterised in that the sheet metal is heated in the radiation furnace.
21. Method according to any of the preceding claims, characterised in that the forming and hardening of the component is carried out by means of a roll forming apparatus, wherein the coated sheet metal is at least partially heated to the austenitising temperature and roll-formed before, during and/or after this process and is, following the roll-forming process, cooled at a cooling rate which causes a hardening of the sheet metal alloy.
22. Corrosion protection layer for roll-formed sheet steel profiles subjected to a hardening step, wherein the corrosion protection layer, following its application to the sheet steel, is subjected to a heat treatment with the addition of oxygen, wherein in the heating process the profiled strip or the profiled strip sections is/are held for at least 10 seconds at 500°C to 600°C, in particular 530°C to 580°C, followed by further heating, wherein the coating substantially consists of zinc, and the coating further contains one or more oxygen-affine elements totalling 0.1 % by weight to 15 % by weight of the total coating, wherein the corrosion protection layer has a surface oxide skin of oxides of the oxygen-affine element(s) and the coating forms at least two phases, these being a zinc-rich phase and an iron-rich phase.
23. Corrosion protection layer according to claim 26, characterised in that the corrosion protection layer contains magnesium and/or silicon and/or titanium and/or calcium and/or aluminium and/or manganese and/or boron as oxygen-affine elements in the mixture.
24. Corrosion protection layer according to claim 26 and/or 27, characterised in that the corrosion protection layer is a corrosion protection layer applied in a hot dipping process.
25. Corrosion protection layer according to any of claims 26 to 31, characterised in that the oxygen-affine elements are present in a total quantity of 0.1 to 15.0 % by weight.
26. Corrosion protection layer according to any of claims 26 to 31, characterised in that these oxygen-affine elements are present in a total quantity of 0.02 to 0.5 % by weight of the total coating.
27. Corrosion protection layer according to any of claims 26 to 33, characterised in that the oxygen-affine elements are present in a total quantity of 0.6 to 2.5 % by weight.
28. Corrosion protection layer according to any of claims 26 to 34, characterised in that aluminium is substantially present as an oxygen-affine element.
29. Corrosion protection layer according to any of claims 26 to 35, characterised in that the iron-rich phase has a maximum zinc-to-iron ratio of 0.95 (Zn/Fe ≤ 0.95), preferably of 0.20 t0 0.80 (Zn/Fe = 0.20 to 0.80), and the zinc-rich phase has a zinc-to-iron ration of at least 2.0 (Zn/Fe > 2.0), preferably of 2.3 to 19.0 (Zn/Fe = 2.3 to 19.0).
30. Corrosion protection layer according to any of claims 26 to 36, characterised in that the iron-rich phase has a zinc-to-iron ratio of approximately 30 : 70 and the zinc-rich phase has a zinc-to-iron ratio of approximately 80 : 20.
31. Corrosion protection layer according to any of claims 26 to 37, characterised in that the corrosion protection layer further contains individual regions with zinc contents > 90 %.
32. Corrosion protection layer according to any of claims 26 to 38, characterised in that the corrosion protection layer has a cathodic protection energy of at least 4 J/cm² at a thickness of 15 µm.
33. Hardened profiled component from a hardenable steel alloy with galvanic protection, wherein:

    a) a coating is applied to a sheet of a hardenable steel alloy, wherein

    b) the coating substantially consists of zinc, and

    c) the coating further contains one or more oxygen-affine elements totalling 0.1 % by weight to 15 % by weight of the total coating, and

    d) the coated sheet steel is then roll profiled in a profiling apparatus, so that the sheet metal strip is formed to produce a roll formed profiled strip, and

    e) the coated sheet steel is then brought at least regionally, with the addition of atmospheric oxygen, to an austenitising temperature required for hardening and heated up to a structural change required for hardening, wherein

    f) a surface skin is formed on the coating from an oxide of the oxygen-affine element(s), and

    g) following the required heating, the sheet metal is cooled, the cooling rate being chosen such that a hardening of the sheet metal alloy is obtained, wherein

    h) the profiled strip is cut to length into profiled strip sections before or after the hardening process,

    i) holes, recesses, punchings and/or a required hole pattern is/are produced in the profiled strip before the profiling and cutting-to-length process and before the heating to the temperature required for hardening, wherein

    j) the cooling process is carried out with water, wherein a large volume of water is applied to the component to be hardened at a low pressure,

    k) wherein the corrosion protection layer has a cathodic protection energy of at least 4 J/cm² at a thickness of 15 µm.
34. Hardened steel component according to claim 40, wherein the component is formed from a hot- or cold-rolled steel strip with a thickness of > 0.15 mm and with a concentration range of at least one of the alloying elements within the following limits in % by weight: carbon up to 0.4, preferably 0.15 to 0.3 silicon up to 1.9, preferably 0.11 to 1.5 manganese up to 3.0, preferably 0.8 to 2.5 chromium up to 1.5, preferably 0.1 to 0.9 molybdenum up to 0.9, preferably 0.1 to 0.5 nickel up to 0.9 titanium up to 0.2, preferably 0.02 to 0.1 vanadium up to 0.2 tungsten up to 0.2 aluminium up to 0.2, preferably 0.02 to 0,07 boron up to 0.01, preferably 0.0005 to 0.005 sulphur max. 0.01, preferably max. 0.008 phosphorus max. 0.025, preferably max. 0.01 rest iron and impurities.

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