EP2656187B1 - Method for producing hardened structural elements - Google Patents

Description

The invention relates to a method for producing hardened, corrosion-protected components with the features of claim 1.

It is known that so-called press-hardened components made of sheet steel are used, particularly in automobiles. These press-hardened components made of sheet steel are high-strength components that are used in particular as safety components in the bodywork area. By using these high-strength steel components, it is possible to reduce the material thickness compared to normal-strength steel and thus to achieve low body weights.

In press hardening, there are basically two different ways of producing such components. A distinction is made between the so-called direct and indirect method.

In the direct process, a sheet steel blank is heated above the so-called austenitizing temperature and, if necessary, kept at this temperature until a desired degree of austenitizing is reached. This heated blank is then transferred to a molding tool and in this molding tool it is formed into the finished component in a single-stage forming step and, in this case, by the cooled molding tool at a speed that exceeds the critical hardening speed is cooled. The hardened component is thus produced.

In the case of the indirect process, the component is first formed almost completely, if necessary in a multi-stage forming process. This formed component is then likewise heated to a temperature above the austenitizing temperature and, if necessary, kept at this temperature for a required time.

This heated component is then transferred and inserted into a molding tool which already has the dimensions of the component or the final dimensions of the component, possibly taking into account the thermal expansion of the preformed component. After the particularly cooled tool has been closed, the preformed component is therefore only cooled in this tool at a speed above the critical hardening speed and thereby hardened.

The direct method is somewhat easier to implement here, but only allows shapes that can actually be produced with a single forming step, i.e. relatively simple profile shapes.

The indirect process is somewhat more complex, but it is also able to produce more complex shapes.

In addition to the need for press-hardened components, the need arose not to produce such components from uncoated sheet steel, but to provide such components with a corrosion protection layer.

Only aluminum or aluminum alloys, which are used to a lesser extent, are used as a corrosion protection layer in automobile construction in question or the much more frequently required coatings based on zinc. Zinc has the advantage that zinc not only provides a barrier protection layer like aluminum, but also a cathodic protection against corrosion. In addition, zinc-coated press-hardened components fit better into the overall corrosion protection concept of the vehicle body, since these are fully galvanized in today's common construction. In this respect, contact corrosion can be reduced or excluded.

However, disadvantages could be found in both processes, which are also discussed in the prior art. In the direct method, i.e. The hot forming of press-hardening steels with zinc coating leads to micro (10 µm to 100 µm) or even macro cracks in the material, whereby the micro cracks appear in the coating and the macro cracks even extend through the entire sheet metal cross-section. Such components with macro cracks are unsuitable for further use.

In the indirect process, i.e. Cold forming with subsequent hardening and residual forming can also lead to microcracks in the coating, which are also undesirable, but not nearly as pronounced.

With the exception of one component in Asia, zinc-coated steels have been used in a direct process, i.e. not used for hot forming. Rather, steels with an aluminum-silicon coating are used here.

An overview can be found in the publication "Corrosion resistance of different metallic coatings on press hardened steels for automotive", Arcelor Mittal Maiziere Automotive Product Research Center F-57283 Maiziere-Les-Mez. In this Publication states that it results in an aluminized boron-manganese steel for the hot forming process, which is sold commercially under the name Usibor 1500P. In addition, for the purpose of cathodic corrosion protection, steels precoated with zinc are sold for the hot forming process, namely the galvanized Usibor GI with a zinc coating that contains small amounts of aluminum and a so-called galvanealed coated Usibor GA, which contains a zinc layer with 10% iron.

It should be noted that the zinc-iron phase diagram shows that above 782 ° C there is a large area that contains liquid zinc as long as the iron content is less than 60%. However, this is also the temperature range in which the austenitized steel is hot worked. However, it is also pointed out that if the deformation takes place above 782 ° C, there is a great risk of stress corrosion due to liquid zinc, which penetrates the grain boundaries of the base steel, which leads to macro cracks in the base steel. In addition, if the iron content is less than 30% in the coating, the maximum temperature for forming a safe product without macro cracks is lower than 782 ° C. This is the reason why this is not a direct forming process, but an indirect forming process. This is to circumvent the problem described.

Another way to get around this problem is to use galvannealed coated steel, which is due to the fact that the iron content of 10% that already exists at the beginning and the absence of an Fe ₂ Al ₅ barrier layer, the critical value of 60% iron in rapidly exceeds the coating when heated, which avoids the presence of liquid iron during the hot forming process.

From the EP 1 439 240 B1 a method for hot forming a coated steel product is known, the steel material having a zinc or zinc alloy coating which is formed on the surface of the steel material and the steel base material with the coating is heated to a temperature of 700 ° C to 1000 ° C and hot formed, wherein the coating has an oxide layer consisting mainly of zinc oxide before the steel base material with the zinc or zinc alloy layer is heated in order to then prevent the zinc from evaporating when heated. A special procedure is provided for this. From the EP 2 159 292 a method for hot forming a coated steel product is also known.

From the EP 1 642 991 B1 a method for hot forming a steel is known in which a component made of a given boron-manganese steel is heated to a temperature at the Ac ₃ point or higher, is kept at this temperature and then the heated steel sheet is formed into the finished component, wherein the molded component is quenched by cooling from the molding temperature during molding or after molding in such a way that the cooling rate to the MS point corresponds to at least the critical cooling rate and that the average cooling rate of the molded component from the MS point to 200 ° C is in the range of 25 ° C / s to 150 ° C / s.

The object of the invention is to create a method for the production of sheet steel components provided with a corrosion protection layer, in which the formation of cracks is reduced or eliminated and nevertheless sufficient protection against corrosion is achieved.

The object is achieved with the features of claim 1.

Advantageous further developments are characterized in the subclaims.

The above-described effect of crack formation due to liquid zinc, which penetrates the steel in the area of the grain boundaries, is also known as what is known as "liquid metal embrittlement".

According to the invention, the object is achieved in that it was recognized that the combination of the base material in the austenitized form, i.e. At high temperatures, the presence in this state of liquid zinc phases and the introduction of stress through deformation must be avoided in order to avoid the stresses induced thereby and thus cracks.

This is achieved according to the invention in that a barrier layer is arranged between the austenitized base material and the liquid zinc phases. The barrier layer between the base material (austenite) and the zinc phases, which are liquid in this temperature range, on the one hand buffers microcracks, whereby the formation of a thick barrier layer also consumes liquid phases.

Such a barrier layer is, for example, a zinc ferrite barrier layer from the reaction between zinc and iron, which dissolves pure zinc via a solid phase solution, the layer growing as a result using up zinc and forming a stable zinc ferrite mixed crystal.

This effect occurs both with pure zinc layers, zinc-aluminum alloy layers and zinc-magnesium alloy layers, which are therefore also suitable.

According to the invention, zinc-nickel layers are also possible as the first or sole corrosion protection layer because a zinc-nickel layer does not develop any liquid zinc phases during the process.

According to the invention, the reduction in liquid zinc or the rapid build-up of an effective barrier layer can be achieved by quickly completing the formation of the barrier layer by reducing the available amount of zinc and thus avoiding a remaining liquid phase of zinc. This can include can be achieved by reducing the zinc layer thickness.

According to the invention, however, in this case the zinc-iron reaction can also be accelerated and thus a faster and greater barrier layer thickness can be achieved if the chemistry of the zinc layer is interfered with. Conventional zinc layers, which are applied in the quick-dip galvanizing process, contain a certain amount of aluminum, which forms an inhibiting layer between the carrier material (steel) on the one hand and the zinc layer on the other, preventing a strong reaction between the substrate and the coating. The addition of aluminum can be reduced in a targeted manner in order to promote precisely this rapid formation of a thick zinc-iron layer. For this purpose, aluminum is reduced in the liquid zinc coating and, if necessary, the coating is fed to a galvanealing reaction before forming to form zinc-iron phases in order to dissolve this inhibiting layer. Such a coating then does not cause any liquid zinc layers during direct forming, which could have harmful interaction with the austenite.

In addition, it is possible to heat-treat a conventional zinc layer with a low content of aluminum for longer than usual during production in order to extend it over a longer period Annealing time to form a thicker barrier layer that protects the material during the direct forming process.

Figur 1:

eine Tabelle mit der typischen chemischen Zusammensetzung der untersuchten Stahlproben;

Figur 2:

ein Diagramm zeigend den Zusammenhang zwischen Risstiefe und Ofenverweildauer bei einer vor dem Umwandlung stehenden Glühbehandlung;

Figur 3:

ein Diagramm zeigend die kritischen Intervalle der Ofenverweildauer;

Figur 4:

eine Tabelle zeigend die Ofenverweildauer zusammen mit Bildern, zeigend die Rissausbildung abhängig von der Ofenverweildauer;

Figur 5:

Proben gemäß Figur 4 in einem Querschnitt zeigend die Risstiefe abhängig von der Ofenverweildauer;

Figur 6:

die Ferrit-Schichtausbildung durch längere Ofenverweildauer;

Figur 7:

das Zink-Eisen-Zustandsdiagramm.

The invention is explained merely by way of example with reference to a drawing. It shows:

Figure 1:

a table with the typical chemical composition of the steel samples examined;

Figure 2:

a diagram showing the relationship between crack depth and furnace dwell time in an annealing treatment prior to conversion;

Figure 3:

a diagram showing the critical intervals of the oven residence time;

Figure 4:

a table showing the oven dwell time together with pictures showing the crack formation depending on the oven dwell time;

Figure 5:

Samples according to Figure 4 showing in a cross section the depth of the crack as a function of the furnace dwell time;

Figure 6:

the formation of the ferrite layer due to longer furnace dwell times;

Figure 7:

the zinc-iron phase diagram.

According to the invention, a zinc-ferrite layer can be formed with a longer furnace dwell time and the associated longer annealing treatment of a zinc coating, which effectively prevents the "liquid metal embrittlement", even if austenite is present and stresses are introduced.

In this way it is possible according to the invention to also carry out the direct method instead of using the more complex, indirect process.

In Figure 1 shows the analysis of a typical steel which was used for the method according to the invention. It will be understood that the remainder of the analysis consists of iron and fusion-related, inevitable impurities.

In Figure 2 the relationship between the furnace dwell time, the presence of liquid phases and the crack depth is shown.

It can be seen from the diagram that the curves for the different curves rise sharply after a certain period of time in the furnace, which is related to the formation of liquid zinc phases. At the same time, this induces an increasing crack depth. A kink can also be seen in all curves, in which the crack depth does not increase any further, but instead the observed crack depth decreases after this furnace dwell time. Then there is again a relatively sharp kink and a curve progression towards smaller crack depths with increasing furnace dwell time. It can be stated here that with a pure zinc layer of 120 g / m ² very long furnace dwell times are necessary, while with a zinc-iron layer with 120 g layer, both the absolutely achievable crack depth is lower and, with a considerably shorter furnace dwell time, a strong one Decrease in crack depth can be observed.

In contrast to a zinc-iron layer of 120 g / m ² , with a zinc-iron layer of only 80 g / m ² the achievable crack depth compared to a zinc-iron layer is 120 g / m ² in considerably reduced and also the time to observe reduced crack depths again considerably shortened.

From these observations, there are observed critical intervals of the furnace dwell time, which are approximately 90 s to 140 s with a zinc coating of 80 g / m ² , approximately 100 s to 155 s with a zinc coating of 100 g / m ² and with a zinc coating of 120 g / m ^{2 is} even 90 s to over 200 s.

In contrast to this, the critical intervals of the furnace dwell time with zinc-iron coatings of 80 g / m ² , 100 g / m ² and 120 g / m ^{2 are} significantly shorter, with the critical intervals, especially with a zinc-iron coating of 80 g / m ² between 45 s and 70 s and a zinc-iron layer of 120 g / m ² with 50 s to 105 s are also significantly narrower.

This can be seen from the fact that with the already pre-reacted zinc-iron coatings, in which an iron aluminate barrier layer is not present, the further zinc-iron reaction takes place so quickly that only a few liquid phases are available for a liquid metal embrittlement.

The direct influence of the oven dwell times can be seen in Figure 4 recognize, whereby the table shows that three similar zinc coatings with 140 g / m ² at similar temperatures from 870 ° C to max. 910 ° C for 185 s, 325 s and 475 s. In this experiment, the components heated in this way were transferred to a molding tool with a transfer time of 3 s and formed there directly in the hot state.

Different crack depths of max. 200 µm for the shortest oven dwell time to 20 µm for the longest oven dwell time.

The pictures clearly show the significant differences.

These are also used in Figure 5 again particularly clearly by making cross-sections of the different samples Figure 4 can be seen. Accordingly, not only the depth of the crack, but also the width of the crack is reduced considerably with increasing furnace dwell time. In addition, it can be seen that the cracks in the sample with the oven dwell time are only present in the coating, while the cracks in the other samples extend into the base material.

It can thus be shown that with the method according to the invention it is possible to maintain the direct forming process and thus, in particular, to produce components with simpler geometry at low cost, if it is ensured that as little as possible liquid zinc is present in the sensitive temperature range during the forming. The compliance with certain temperature-time parameters according to the invention thus makes it possible to continue working with previous methods.

Claims (7)

1. Method for producing a hardened steel component with a coating of zinc or a zinc alloy, wherein a hardenable steel material is coated with a zinc layer or a layer of a zinc alloy, from which blanks are stamped out of the hardenable steel material, the blanks are heated to a temperature at the AC3 point or above and, after a desired hold time, are reshaped in the heated condition in a forming die, wherein the reshaped steel sheet blank is cooled down by the forming die at a speed exceeding the critical hardening speed and is hardened thereby, characterised in that the blank is held, depending on the thickness of the zinc or zinc alloy layer, before the reshaping, at a temperature above 782°C until a barrier layer of zinc ferrite has been formed between the steel and the coating of zinc or zinc alloy and the molten zinc forming the zinc ferrite layer has increased and has become thick enough that, during reshaping, liquid zinc phases do not react with the steel, wherein, with zinc layers between 80 g/m² and 120 g/m², the furnace holding time does not fall below 120 sec. to 210 sec., or with zinc-iron (galvanealed) layers, with coatings of 80 g/m² to 120 g/m², the furnace holding time does not fall below 75 sec. to 100 sec..
2. Method according to claim 1, characterised in that the zinc-ferrite barrier coat is a zinc-ferrite barrier coat resulting from the reaction between zinc and iron which releases pure zinc via a solid state solution wherein the growing layer consumes zinc and forms a stable zinc-ferrite solid solution.
3. Method according to claim 1, characterised in that the coating on the steel is a zinc coating with an aluminium proportion of 0.1 % to 5 %.
4. Method according to any one of the preceding claims, characterised in that the coating on the steel is applied electrolytically and/or by hot dip coating.
5. Method according to any one of the preceding claims, characterised in that the coating on the steel comprises a thin electrolytically deposited zinc layer and a zinc layer or zinc-aluminium layer deposited on it.
6. Method according to claim 5, characterised in that, before the hot dip galvanising, the electrolytically applied zinc layer was converted into a zinc-ferrite layer.
7. Method according to any one of the preceding claims, characterised in that the coating is a zinc-nickel coating, a zinc-aluminium coating, a zinc-iron coating, a zinc-chromium coating, a pure zinc coating or a zinc-magnesium coating.

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