EP2220259B9 - Method for the production of coated and hardened components made of steel, and coated and hardenable steel strip therefor - Google Patents

Description

The invention relates to a method for producing hardened components made of hardenable steel and a hardenable steel strip for this purpose.

It is known to produce components from a hardenable steel and in particular hardened components. Hardenable steels are to be understood below as steels in which a phase transformation of the base material results during the heating and subsequent cooling, the so-called quench hardening from the previous microstructure transformation and optionally during the quench hardening further microstructural transformations results in a material which is significantly harder or higher Has tensile strengths as the starting material.

From the DE 24 52 486 C2 For example, the method of so-called press-hardening is known, in which a board made of a hardenable steel material is heated above the so-called austenitizing temperature and placed in a heated state in a mold and formed in this mold and cooled at the same time, whereby on the one hand, the final geometry of the desired component and on the other hand gives the desired hardness or strength. This procedure is widely used.

From the EP 1 651 789 A1 a method is known in which a hardened component made of hardenable steel sheet with a cathodic corrosion protection, wherein the component is cold-formed already in a metallic coated state, so that it is 0.5 to 2% smaller than the nominal final dimension of the finished hardened component. Subsequently, this component is heated and placed in a tool which corresponds exactly to the final dimensions of the desired component. Due to the thermal expansion, the coated component has expanded to exactly this final dimension and is held on all sides in the so-called mold and cooled, whereby the hardening is brought about.

Moreover, from the EP-A 0 971 044 a method is known in which a sheet of a hardenable steel and with a metallic coating is heated to a temperature above the Austenitisierungstemperatur and then placed in a hot forming tool, where the heated sheet is formed and simultaneously cooled and cured by cooling.

It is disadvantageous in the abovementioned methods for hot forming that microcracks occur in the steel substrate, regardless of whether a metallic coating is present on the steel substrate or not, especially during hot forming, even of cold preformed but unfinished components.

These micro-cracks occur especially in the areas that are reshaped and especially in the areas with high degrees of deformation. These microcracks are located on the surface and / or in the metallic coating and may partially extend relatively far into the base material. This has the disadvantage that such cracks under load of the component can continue to grow and represent a pre-damage of the component, which can lead to failure under load.

Metallic coatings on steels have long been known in the form of aluminum, aluminum alloy coatings, particularly aluminum-zinc alloy coatings, zinc coatings and zinc alloy coatings.

Such coatings have the task of protecting the steel material from corrosion. In the case of aluminum coatings, this is done by what is known as barrier protection, in which the aluminum creates a barrier against the entry of corrosive media.

For zinc coatings, protection is provided by the so-called cathodic effect of zinc.

Such coatings have hitherto been used in particular for normal-strength steel alloys, in particular for the motor vehicle industry, the construction industry, but also in the household appliance industry.

They can be applied to the steel material by hot dip coating, PVD or CVD or electrodeposited.

The use of higher-strength steel grades has also tried to coat the latter with such hot-dip coatings.

From the DE 10 2004 059 566 B3 For example, a process for hot dip coating a strip of high strength steel is known in which the strip in a continuous furnace in first A reducing atmosphere is heated to a temperature of about 650 ° C. At this temperature, the alloying constituents of the higher-strength steel should diffuse to the surface of the strip in only small amounts. The predominantly made of pure iron surface is converted by a very short heat treatment at an overlying temperature of up to 750 ° C in a continuous furnace integrated reaction chamber with an oxidizing atmosphere in an iron oxide layer. This iron oxide layer is intended to prevent diffusion of the alloying constituents to the surface of the strip in a subsequent annealing treatment at a higher temperature in a reducing atmosphere. In the reducing atmosphere, the iron oxide layer is converted into a purer iron layer on which the zinc and / or aluminum is optimally adhered in the hot dip. The oxide layer applied by this method should have a maximum thickness of 300 nm. In practice, the layer thickness is usually set to about 150 nm.

The object of the invention is to provide a method for producing hardened components made of hardenable steel, with which the forming behavior, in particular the hot forming behavior, is improved.

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

It is another object to provide a steel strip which has improved formability, especially hot workability.

The object is achieved with a steel strip having the features of claim 10.

Advantageous developments are characterized in the dependent claims.

The invention provides to oxidize a hot or cold rolled steel strip on the surface, then make a metallic coating and for the purpose of producing the component from the corresponding coated sheet - so necessary - to cut a board to heat this board to them by heating austenitize at least partially so that in a subsequent forming and cooling of the board, an at least partially cured microstructure or partially cured component is formed. Surprisingly, the superficial oxidation of the strip of hardenable steel during the heating for the purpose of Austenitisierens and / or forming and cooling superficially a ductile layer is formed, which can reduce the stresses during forming so well that it no longer comes to microcracking , The metallic coating serves to protect against superficial decarburization, although this metallic coating can of course also perform other tasks, such as corrosion protection.

Instead of a metallic coating, during the heating for the purpose of austenitizing also a protective gas atmosphere can be produced, in particular, a superficial oxidation, for. B. to about 700 ° C in an oxidizing atmosphere, brought about and the further heating under inert gas atmosphere are carried out so that further oxidation and / or decarburization is omitted.

Should it be necessary, the oxidation of the steel strip for the purpose of applying the metallic coating on the surface be reduced to achieve a reactive surface.

Under no circumstances, however, is the oxide layer, as in conventional pre-oxidation, largely eliminated for the purpose of galvanizing. In addition, the oxidation according to the invention is carried out to a far greater extent than the pre-oxidation according to the prior art. The prior art pre-oxidation takes place up to a maximum thickness of 300 nm, the oxidation according to the invention to a much greater extent, so that even after a reduction has been carried out, an oxidized layer, preferably at least 300 nm thick, remains.

The oxidation according to the invention apparently not only superficially creates an iron-oxide layer which, of course, also contains oxides of the alloying elements, it also seems to be the case that the alloying elements are partially oxidized even below this layer.

After curing, a component produced by the process according to the invention shows on the surface between the steel substrate and the coating a thin, in the micrograph FIG. 4 as a whitish layer. Currently, the most likely cause of this ductile layer is oxidized alloying elements, which were not available in the superficially oxidized phase for the phase transformation during curing or have delayed or obstructed this transformation. However, the exact mechanisms have not yet been elucidated.

Surprisingly, it has been found that such oxidation, which is not necessary for the actual coating with a coating metal, leads to increased ductility of the hardened substrate in the surface region, even after the metal coating. In a surprising way can be achieved with an oxidation, which forms an iron oxide layer with a layer thickness> 300 nm, a sheet which also during hot forming and during the heat treatment for the purpose of curing, for example for a suitable steel type 22MnB5 above 850 ° C, or the respective Austenitisierungstemperatur, microcracking is deformable.

Figur 1:

stark schematisiert den erfindungsgemäßen Verfahrensablauf;

Figur 2:

ein Diagramm zeigend die Verbesserung beim Biegewinkel bei der Erfindung gegenüber dem Stand der Technik;

Figur 3:

stark schematisiert einen Schichtaufbau nach der Erfindung im Vergleich zum Stand der Technik nach dem Härten;

Figur 4:

eine mikroskopische Schliffaufnahme der Oberfläche des erfindungsgemäßen Stahlbandes;

Figur 5:

eine mikroskopische Schliffaufnahme eines nicht erfindungsgemäßen Vergleichsbeispiels;

Figur 6:

eine rasterelektronische Schliffaufnahme eines erfindungsgemäßen Vergleichsbeispiels;

Figur 7:

ein Auschnitt aus der rasterelektronischen Schliffaufnahme Figur 6 mit einem Linien-Zink-Konzentrationsprofil aus einer energiedispersiven Röntgenanalyse (EDX).

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

FIG. 1:

highly schematic of the process sequence according to the invention;

FIG. 2:

a graph showing the improvement in the bending angle in the invention over the prior art;

FIG. 3:

strongly schematized a layer structure according to the invention in comparison with the prior art after curing;

FIG. 4:

a microscopic micrograph of the surface of the steel strip according to the invention;

FIG. 5:

a microscopic micrograph of a comparative example not according to the invention;

FIG. 6:

a scanning electron micrograph of a comparative example according to the invention;

FIG. 7:

an excerpt from the scanning electron micrograph FIG. 6 with a line-zinc concentration profile from an energy-dispersive X-ray analysis (EDX).

In FIG. 1 the method according to the invention is shown with reference to a method sequence, for example for a hot-dip-coated steel strip, in particular a galvanized steel strip of the 22MnB5 type with an overlay Z140.

In the FIG. 1 and 3 layer thicknesses shown are not shown to scale, but distorted to scale for the purpose of better representability.

A bright steel strip 1 is subjected to oxidation before the hot-dip coating, so that the strip 1 is provided with an oxide layer 2.

This oxidation is carried out at temperatures between 650 ° and 800 ° C. While for a conventional pre-oxidation, which would be necessary for a hot-dip galvanizing, the oxide layer thickness of 150 nm would be fully sufficient, the oxidation is carried out according to the invention so that the oxide layer thickness is> 300 nm. In order to apply the metallic hot-dip coating, for example galvanizing or aluminizing, a partial reduction of the oxides at the surface is carried out in the next step, so that a very thin reduced layer 4 is produced, which consists essentially of pure iron. Below this remains a residual oxide layer 3.

Under the oxide layer 3, the oxidation presumably results in a region of "internal oxidation" 3a. In this region 3a, it is apparent that the alloying elements are partially oxidized or partially present in oxidized form.

Subsequently, the hot-dip coating is carried out with the coating metal, so that a layer 5 of the coating metal on the residual oxide layer 3 results. Now around the hardened component To achieve the tape 1 is heated to the Austenitisierungstemperatur and at least partially austenitized, which inter alia alloy the metallic coating 5 and the surface of the belt 1 together. The oxide layer 3 is in this case partially or completely consumed by diffusion processes between the band 1 and the metallic coating 5, or is not detectable in the high-temperature treatment.

In the case of a galvanically applied metallic coating, the deposition onto the oxide layer can take place without prior reduction or with reduction, if appropriate, however, a pickling is also carried out.

In order to obtain the hardened component or partially cured component depending on Austenitisierungsgrad, then the forming and cooling takes place in a tool, wherein the layer 6, where appropriate, the phases converted and also a phase transformation takes place in the band 1. After curing, between the strip 1 and the metallic coating 6, a micrograph ( FIG. 4 ) bright, ductile layer 7 is observed, which is apparently responsible for the fact that the final product is a micro-crack-free, hardened component. This ductile layer 7 presumably already forms during heating for the purpose of hardening and is therefore already present during hot forming.

The most probable cause of this light-colored layer 7 is that the oxidation carried out before the metallic coating has oxidized the alloying elements necessary for hardening, such as manganese, in the near-surface region and are not available for conversion or hinder conversion. so that the steel strip in the very thin near-surface area this forms ductile layer 7, which is apparently sufficient to compensate for the near-surface stresses so that no deformation and crack propagation occur during forming.

It is also assumed that the area 3a of the "internal oxidation" of the alloying elements is also important for this purpose.

The advantage of the process is also evident after curing, or can also be detected after curing, if a correspondingly produced or hardened sheet is subjected to a three-point bending test, for example. This can also positively influence the crash behavior.

In this three-point bending test two supports with a diameter of 30 mm are arranged at a distance of twice the sheet thickness. The hardened sheet is placed on top and then loaded with a bending bar with a radius of 0.2 mm at the same distance from the supports.

Time is measured, the distance from touching the flexural strength value with the sample and the force.

The force and displacement or a force-bending angle curve are recorded, the angle being calculated from the path. The test criterion is the bending angle at the force maximum.

The comparison is in the FIG. 2 for a steel type 22MnB5 with a coating Z140, from which it can be seen that a significantly greater bending angle can be achieved in the cured cold sample by the ductile layer produced according to the invention.

The invention and the prior art are also in FIG. 3 Once again, according to which the prior art after curing, although a metallic coating is present, which adheres to the cured substrate, but a ductile layer is not present.

In the invention, between the hardened substrate and the coating after the hardening reaction, the ductile layer 7 is located.

The average layer thickness of this layer is greater than 0.3 microns, which layer may be continuous, but need not be completely continuous, to bring about the success of the invention.

FIG. 6 shows a scanning electron micrograph of a comparative example according to the invention. It can be seen that the zinc content decreases abruptly from about 40% Zn content to below 5% Zn due to the diffusion processes in the direction of the base material martensite.

The grains of the iron-zinc layer have near the base material only a very low zinc content, these appear in the cross-section whitish Fe-rich layer acts as a ductile intermediate layer between the other laminates.

FIG. 7 shows an excerpt FIG. 6 with a line-zinc concentration profile from an energy-dispersive X-ray analysis (EDX). It is once again clearly evident that the zinc content decreases in the direction of the base material.

The FIGS. 4 and 5 each show a cross-section of a hardened steel strip of the invention ( FIG. 4 ) and the prior art ( FIG. 5 ), where clearly the substrate in the cut 1, lying over the converted metallic coating 6 and lying between the ductile layer 7 are visible.

FIG. 5 shows a layer structure according to the prior art, in which a galvanized strip 101 has a steel substrate 102 made of high-strength steel, on which a zinc-iron layer 103 is applied. A ductile layer is missing.

According to the invention, the metallic coating can be selected from all common metallic coatings, since it is only a matter of counteracting a decarburization. Therefore, the coatings may be pure aluminum or aluminum-silicon coatings as well as alloy coatings of aluminum and zinc (Galvalume) as well as coatings of zinc or substantially zinc. But other coatings of metals or alloys are suitable if you can experience the high temperatures during curing in the short term.

The coatings may, for. B. by electroplating or hot dip coating or PVD or CVD method may be applied.

The oxidation can hereby be brought about conventionally in that the strip passes through a directly heated preheater in which gas burners are used and an increase of the oxidation potential in the atmosphere surrounding the strip can be produced by a change of the gas-air mixture. As a result, the oxygen potential can be controlled and lead to oxidation of the iron on the strip surface. In this case, the control is carried out in such a way that an oxidation is achieved which is significantly above the oxidation in the prior art. In a subsequent furnace section, in contrast to the prior art, the iron oxide layer formed or an optionally achieved internal oxidation of the steel only superficially or partially reduced.

Moreover, it is possible to anneal the strip in a known RTF preheater under a protective gas atmosphere, wherein in this case the oxidation or pre-oxidation is also carried out considerably more than it would be necessary. The oxidation strength can be adjusted here in particular by the supply of an oxidizing agent.

In addition, it has been found that moistening the furnace atmosphere, i. H. a very strong water vapor-containing atmosphere (stronger than usual) alone or together with other oxidizing agents achieved the desired effect. It is essential to the invention that the optionally subsequent reduction is carried out only in such a way that a residual oxidation remains. In a heat treatment alone with a water vapor-containing atmosphere, the internal oxidation state of the steel is not completely returned.

The control of the oxidation can be controlled via the atmosphere, via an oxidant concentration of an optionally added further oxidizing agent, the treatment time, the temperature, the temperature profile and the water vapor content in the furnace chamber.

Such a treated band, as in the Figures 3 and 4 is excellent, and free of microcracks in the steel substrate cold form, heat and press hardening or reshaping but also hot forming and press hardening.

It has been found that the implementation of the invention oxidation - in contrast to the edge decarburization in uncoated Steel material - has no negative effects on the achievable ultimate strength of the material.

An advantage of the invention is the provision of a method and a steel strip which make it possible, in a simpler and safer way, to considerably increase the quality of formed and hardened components made of higher-strength steel.

Reference numerals:

1

steel strip

2

oxide

3

residual oxide layer

4

thin reduced layer

5

metallic coating

6

metallic coating

7

bright, ductile layer

101

galvanized band

102

steel substrate

103

Zinc-iron layer

Claims (12)

1. Method for producing a hardened component from a hardenable steel, wherein the steel strip is subjected to a temperature increase and, in the process, an oxidizing treatment in a furnace, so that a superficial oxide layer is produced, and a coating with a metal or a metal alloy is then carried out, and, in order to produce an at least partially hardened component, the strip is heated and at least partially austenitized and then cooled off and thus hardened, wherein, in order to produce a superficial ductile layer (7), the oxides at the surface are partially reduced prior to the coating with a metal or a metal alloy, so that a very thin reduced layer (4) is produced, which is located on the residual oxide layer (3), wherein an area of an inner oxidation (3a) is located beneath the oxide layer (3) in the strip, in which the steel-alloy elements are present in a partially oxidized form.
2. Method according to claim 1, characterised in that a reducing treatment is carried out after producing the superficial oxide layer (3) in order to reverse the oxidation superficially, and a reducing layer (4) is produced on the layer (3), and a coating with a metal or a metal alloy is subsequently carried out, wherein, however, the oxidation and the reduction are carried out such that, after the superficial reduction and the coating, an oxide layer (3) remains between the coating and the steel strip.
3. Method according to claim 1 or 2, characterised in that the metallic coating is formed as a hot-dip coating with a molten metal or a molten metal alloy or by electrodeposition of one or more metals on the strip or by PVD and/or CVD methods.
4. Method according to any one of the preceding claims, characterised in that the oxidizing treatment is carried out by means of an oxidizing furnace chamber atmosphere and/or a water-vapour containing furnace chamber atmosphere.
5. Method according to any one of the preceding claims, characterised in that the degree of oxidation and the oxide layer thickness is adjusted by the content of oxidizing agents in the treatment atmosphere and/or the duration of the treatment and/or the temperature level and/or the water-vapour concentration in the furnace chamber.
6. Method according to any one of the preceding claims, characterised in that coating is carried out with aluminium or an alloy substantially containing aluminium, or with an alloy from aluminium and zinc, and/or a different zinc alloy substantially containing zinc, and/or zinc and/or other coating metals.
7. Method according to any one of the preceding claims, characterised in that the furnace chamber in which the oxidation and/or reduction is carried out is directly or indirectly heated.
8. Method according to any one of the preceding claims, characterised in that the furnace chamber in which the oxidation and/or reduction is carried out is heated by means of gas and/or oil burners and/or convectively, or that the steel strip is heated inductively.
9. Method according to any one of the preceding claims, characterised in that the oxidation is carried out such that an oxidation layer thickness of more than 300 nm is achieved at the end of the oxidation, and the subsequent reduction is carried out such that the oxide layer is partially reduced from the surface.
10. Steel strip from a hardenable steel, comprising a steel substrate (1) and a metallic coating applied thereon, wherein an oxidation layer (3) of the steel substrate (1) is present in the boundary area in which the metallic coating (5) is formed overlying the steel substrate (1), and a reduction layer (4) is present on the oxidation layer (3).
11. Steel strip according to claim 10, characterised in that the metallic coating (5) consists of aluminium or substantially aluminium, an aluminium alloy, an aluminium-zinc-alloy, a zinc alloy substantially containing zinc, a zinc-iron alloy or substantially zinc.
12. Use of a steel strip according to any one of the claims 10 to 11 for producing press-hardened components in which the component is cold-formed, austenitized and then quench hardened, or austenitized, formed, and quench hardened.

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