EP2655673B1 - 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 method, 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, in a one-step molding step, it is formed into the finished component and in the process through the cooled one The mold is cooled at the same time at a rate that is above the critical hardening rate. 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 a bit easier to implement here, but only allows shapes that can actually be created 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, or the much more frequently required coatings based on zinc are considered as a corrosion protection layer in automobile construction. 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, as 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 process, i.e. hot forming of press-hardening steels with zinc coating, micro (10 µm to 100 µm) or even macro cracks occur 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, microcracks can also occur in the coating, which are also undesirable, but not nearly as pronounced.

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

An overview is given 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. 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 a small amount 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 of the coating when heated quickly exceeds, 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, which consists 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 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. Also JP 2007 182608 A discloses a method for producing a hardened steel component with a coating of zinc, which is a direct method for hot forming.

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 by liquid zinc, which penetrates the steel in the area of the grain boundaries, is also known as so-called "liquid metal embrittlement".

In contrast to the direction taken in the prior art because of the "liquid metal embrittlements", the indirect method is also provided for simple geometries, the invention takes a more favorable route in that the direct method is used in which a zinc or zinc alloy coated plate is heated and is reshaped and quench hardened after heating.

As was recognized according to the invention, as far as possible no zinc melt should come into contact with austenite during the forming phase, that is to say when stress is introduced. According to the invention, it is therefore provided that the deformation is carried out below the peritectic temperature of the iron-zinc system (melt, ferrite, T phase). In order to still be able to guarantee quench hardening, the composition of the steel alloy is set within the scope of the usual composition of a magnesium drill steel (22 MnB5) so that quench hardening is achieved through a delayed conversion of the austenite into martensite and thus the presence of austenite even in the lower Temperature is carried out below 780 ° C or lower, so that at the moment in which mechanical tension is applied to the steel, which in connection with a zinc melt and austenite would lead to "liquid metal embrittlement", just no or very few liquid zinc phases are present. Thus it succeeds by means of a corresponding the alloying elements adjusted boron-manganese steel to achieve sufficient quench hardening without provoking excessive or damaging crack formation.

Figur 1:

eine Tabelle zeigend die Ofenverweildauer von mit einer 140 g/m² betragenden Zinkschicht beschichteten Stahlplatinen mit unterschiedlichen Transferzeiten ins Umformwerkzeug und damit verbundenen repräsentativen Risstiefen;

Figur 2:

die Zeit-Temperaturkurve bei der Abkühlung zwischen Ofen und Umformung;

Figur 3:

stark vergrößerte Bilder zeigend die Proben mit den unterschiedlichen Transferzeiten;

Figur 4:

Querschnittschliffdarstellungen der Proben nach Figur 3;

Figur 5:

das Zink-Eisen-Diagramm.

The invention is explained with reference to a drawing, it shows:

Figure 1:

a table showing the furnace dwell time of steel blanks coated with a zinc layer of 140 g / m ² with different transfer times into the forming tool and associated representative crack depths;

Figure 2:

the time-temperature curve during cooling between furnace and forming;

Figure 3:

greatly enlarged images showing the samples with the different transfer times;

Figure 4:

Cross-sectional micrographs of the samples Figure 3 ;

Figure 5:

the zinc-iron diagram.

According to the invention, a conventional boron-manganese steel for use as a press-hardening steel material is adjusted with regard to the conversion of the austenite into other phases in such a way that the conversion shifts into deeper ranges.

Steels of the general alloy composition are therefore suitable for the invention (all data in% by mass): C [%] Si [%] Mn [%] P [%] S [%] Al [%] Cr [%] Ti [%] [%] B N [%] 0.22 0.19 1.22 0.0066 0.001 0.053 0.26 0.031 0.0025 0.0042 The remainder is iron and impurities from the melting process

The alloying elements boron, manganese, carbon and optionally chromium and molybdenum are used as transformation retarders in such steels.

Steels of the general alloy composition are therefore suitable for the invention (all data in% by mass): Carbon (C) 0.08-0.6 Manganese (Mn) 0.8-3.0 Aluminum (Al) 0.01-0.07 Silicon (Si) 0.01-0.5 Chromium (Cr) 0.02-0.6 Titanium (Ti) 0.01-0.05 Nitrogen (N) 0.003-0.1 Boron (B) 0.0005-0.06 Phosphorus (P) <0.01 Sulfur (S) <0.01 Molybdenum (Mo) <1 The remainder is iron and impurities from the melting process

Steel arrangements as follows have proven particularly suitable (all data in% by mass): Carbon (C) 0.08-0.30 Manganese (Mn) 1.00-3.00 Aluminum (Al) 0.03-0.06 Silicon (Si) 0.15-0.20 Chromium (Cr) 0.2-0.3 Titanium (Ti) 0.03-0.04 Nitrogen (N) 0.004-0.006 Boron (B) 0.001-0.06 Phosphorus (P) <0.01 Sulfur (S) <0.01 Molybdenum (Mo) <1 The remainder is iron and impurities from the melting process

By adjusting the alloying elements acting as conversion retarders, quench hardening, i.e. H. rapid cooling with a cooling rate above the critical hardening rate can be safely achieved even below 780 ° C. This means that in this case work is carried out below the peritectic of the zinc-iron system, i.e. H. mechanical tension is only applied below the peritectic. This also means that at the moment when mechanical stress is applied, there are no longer any liquid zinc phases that can come into contact with the austenite.

In Figure 1 one can see that these different starting temperatures during hardening were achieved through different transfer times from the furnace to the forming press. With a transfer time of 3 seconds, one can see strongly developed deep cracks with a representative crack depth of 200 µm. Transfer times of 5 seconds and 7 seconds show that both the crack strength and the crack depth are visibly decreasing, while with a transfer time of 9 seconds the depth and width of the cracks have clearly decreased. This was not to be expected in this form, since, despite the known phenomenon of the liquid metal embrittlement, the person skilled in the art would have assumed that a very soft, ductile and more or less liquid metallic cover layer with many liquid phases can follow the forming process better than an already solid one metallic layer.

In addition, after the blank has been heated up, a holding phase can be provided according to the invention in the temperature range of the peritectic, so that the solidification of the zinc coating is promoted and driven before it is subsequently reshaped.

With the invention it is thus possible to reliably achieve a cost-effective hot forming process for steel sheets coated with zinc or zinc alloys, in which, on the one hand, quench hardening is brought about and, on the other hand, micro and macro cracking, which leads to component damage, is reduced or avoided.

Claims (3)

1. Method for producing a hardened steel component with a coating of zinc or a zinc alloy, wherein a blank is stamped from a sheet coated with the zinc or zinc alloy, the stamped blank is heated to a temperature ≥Ac₃ and kept at this temperature for a predefined time where appropriate, in order to allow austenite formation, after which the heated blank is moved into a forming tool, formed in said forming tool and cooled in said forming tool at a speed that lies above the critical hardening speed, and thereby hardened,
   wherein
   the steel material is adjusted in a conversion-delayed manner, such that with a forming temperature which lies within the range of 600°C to 800°C, in particular 730°C to 782°C, and is below the peritectic temperature of the iron/zinc graph, quench-hardening takes place through conversion of the austenite into martensite, wherein a steel material having the following analysis is used (all figures in % by mass): Carbon (C) 0.08-0.6 Manganese (Mn) 0.8-3.0 Aluminium (Al) 0.01-0.07 Silicon (Si) 0.01-0.5 Chromium (Cr) 0.02-0.6 Titanium (Ti) 0.01-0.05 Nitrogen (N) 0.003-0.1 Boron (B) 0.0005-0.06 Phosphor (P) <0.01 Sulphur (S) <0.01 Molybdenum (Mo) < 1 Residual iron and smelting-related impurities.
2. Method according to Claim 1, characterized in that a steel material with the following analysis is used (all figures in % by mass): Carbon (C) 0.08-0.30 Manganese (Mn) 1.00-3.00 Aluminium (Al) 0.03-0.06 Silicon (Si) 0.15-0.20 Chromium (Cr) 0.2-0.3 Titanium (Ti) 0.03-0.04 Nitrogen (N) 0.004-0.006 Boron (B) 0.001-0.06 Phosphor (P) <0.01 Sulphur (S) <0.01 Molybdenum (Mo) <1 Residual iron and smelting-related impurities.
3. Method according to one of the preceding claims, characterized in that the blank is heated in a furnace to a temperature >Ac₃ and kept at this temperature for a predefined time, after which the blank is left to cool to a temperature between 600°C and 800°C, in particular 730°C and 782°C, and kept at this temperature in order to achieve hardening of the zinc layer and moved into the forming tool after a predefined time, where it is formed.

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