EP2655674B1 - Method for forming and hardening coated steel sheets - Google Patents

Description

The invention relates to a method for forming and hardening coated steel sheets 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. Then this heated blank is transferred to a molding tool and in this molding tool in a single-stage forming step The finished component is reshaped and at the same time cooled by the cooled molding tool at a speed that is above the critical hardening speed. 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.

In the direct process, i.e. hot forming of press-hardening steels with zinc coating, the forming tools are significantly soiled. This is apparently not only due to abrasion but much more to the sublimation of zinc vapors that evaporate from the liquid zinc phases during forming. The consequences of zinc deposits building up in the forming tool range from surface damage to the hot-formed component in the form of grooves to system downtimes due to jammed components in the forming tool or the risk of tool breakage due to double-part press-off if jammed components are not detected in time. The required regular removal of zinc deposits reduces the productivity of the hot forming plant due to the necessary production downtime.

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.

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 is at least equal to the critical cooling rate and that the average cooling rate of the molded component from the MS point at 200 ° C is in the range of 25 ° C / s to 150 ° C / s. Also JP 2007 182 608 discloses a method for hot forming coated boron-manganese steels.

The object of the invention is to create a method for reshaping and hardening metal-coated steel sheets in which the contamination of the tools is reduced to the level that is inevitable due to abrasion.

The object is achieved with the features of claim 1. Advantageous further developments are characterized in the subclaims.

The inventors have recognized that metallic buildup, such as Zn buildup on hot forming tools, which goes beyond the level of inevitable abrasion, has a severe adverse effect on productivity in the direct process. The cause suspected by the inventors is mainly due to evaporating liquid metallic phases, such as Zn phases in the hot forming of steels with a zinc coating.

According to the invention, it is therefore provided that the hot forming of steels with zinc coating is carried out below the peritectic temperature of the iron-zinc system (melt, ferrite, gamma 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 Bohr magnesium steel (22 MnB5) in such a way that quench hardening is achieved through a delayed conversion of the austenite into martensite and thus the presence of austenite even at the lower temperature is carried out below 800 ° C or lower, so that at the moment in which the steel is formed, no liquid Zinc phases are present, from which zinc could evaporate and deposit on the tools.

The desired forming temperature is between 450 ° C and 800 ° C, preferably between 450 ° C and 700 ° C and more preferably between 450 ° C and 600 ° C.

Figur 1:

stark schematisiert einen Versuchsaufbau;

Figur 2:

schematisch das Anhaftungspotential einer metallischen Beschichtung am Werkzeug am Beispiel Zink;

Figur 3:

Bilder zeigend das Werkzeug bei drei aufeinanderfolgende Umformversuchen die ohne Zwischenkühlung erfolgten;

Figur 4:

Bilder zeigend das Werkzeug bei drei aufeinanderfolgende Umformversuchen die mit erfindungsgemäßer Zwischenkühlung vor dem Umformen erfolgten;

Figur 5:

Ein Bild zeigend das Werkzeug nach den Versuchen ohne und mit erfindungsgemäßer Zwischenkühlung und das Werkzeug in gereinigtem Ausgangszustand.

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

Figure 1:

an experimental setup;

Figure 2:

schematically the adhesion potential of a metallic coating on the tool using zinc as an example;

Figure 3:

Pictures show the tool in three successive forming attempts that were carried out without intermediate cooling;

Figure 4:

Pictures show the tool in three successive forming attempts which were carried out with intermediate cooling according to the invention before forming;

Figure 5:

A picture shows the tool after the tests with and without intermediate cooling according to the invention and the tool in a cleaned initial state.

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) 0.001-0.06 Phosphorus (P) <0.01 Sulfur (S) <0.01 Molybdenum (Mo) <1

Remaining iron and impurities caused by 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. is only transformed below the peritectic. This also means that at the moment when the sheet metal to be formed comes into contact with the tool, there are no longer any liquid zinc phases that can be deposited on the tool surface.

In Figure 1 you can see the experimental setup. The steel sheet used is a 1.5 mm thick steel sheet made of an alloy described above which is coated with a Z140 layer. The furnace temperature for heating and austenitizing the sheet is around 910 ° C. The baking time of the trays is set so that the trays reach a temperature of 870 ° C and are then held for 45 seconds. For the tests, the sheets were either brought into the forming tool and formed there, or removed from the furnace after heating, fed to an intermediate cooling station and, after cooling, transferred to the tool as quickly as possible, where they were formed and quench-hardened. The intercooling is carried out so that a forming temperature between 450 ° C and 800 ° C, preferably between 450 ° C and 700 ° C and more preferably between 450 ° C and 600 ° C is achieved.

In Figure 2 one can see schematically the adhesion potential of a metallic coating on the tool using zinc as an example. However, it also applies accordingly to other metallic coatings. At the turning points you can see the temperature ranges in which liquid phases are transformed into solid phases and below which transformation is possible with less adherence.

Figure 3 shows the clearly visible contamination of the tool during forming without intermediate cooling. After just three forming steps, the degree of contamination is so high that an impairment of the surface quality of the hardened steel components is foreseeable with continued forming steps. Here, the zinc components adhering to the tool through first evaporation and then adhesion and welding can tear parts out of the zinc layer of the following components by welding, which has a negative effect on the corrosion protection. Conversely, zinc components adhering to the tool can be transferred in the same way to the steel component and there impair the surface quality and the paintability of the component.

In the Figures 4 and 5 On the other hand, it can be seen that the tool remains essentially unaffected apart from absolutely insignificant and harmless minor zinc abrasions in the tool.

In addition, according to the invention, after the board has been heated up, a holding phase can be carried out in the temperature range of the peritectic Provide so that the solidification of the zinc coating is promoted and promoted before it is formed.

With the invention it is thus possible to reliably achieve a cost-effective hot forming process for steel sheets coated with metallic coatings such as zinc or zinc alloys or aluminum or aluminum governments in which, on the one hand, quench hardening is brought about and, on the other hand, adhesions on the tool are reduced or avoided.

Claims (3)

1. Method for forming and hardening coated sheet steels, 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, cooled and thereby hardened,
   characterized in that
   in order to avoid zinc adhesions on the forming tool, the steel material is adjusted in a conversion-delayed manner, such that forming takes place at a forming temperature which lies within the range of 500°C to 800°C and is below the peritectic temperature of the zinc/iron graph,
   and 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 of between 600°C to 800°C and kept at this temperature, in order to achieve solidification of the zinc layer, and after a predefined holding time is moved into the forming tool and formed there at 500°C to 800°C, 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) 0.001 - 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 the steel material contains the elements boron, manganese and carbon and, optionally, chromium and molybdenum, as conversion retarders.
3. Method according to Claim 1 or 2, characterized in that a steel material having 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.

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