EP2655675B1 - 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 in automobiles in particular. 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 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 it is formed into the finished component in a single-stage forming step, and in the process through the cooled one Mold simultaneously cooled 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 somewhat easier to implement here, but only enables shapes that can actually be created with a single forming step, i.e. relatively simple profile shapes.

The indirect method is a bit 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 zinc-based coatings are suitable 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 both processes, however, disadvantages could be found which are also discussed in the prior art. In the direct process, i.e. the 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.

Up to now, zinc-coated steels have not been used in the direct process, i.e. hot forming, with the exception of one component in Asia. 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 it is stated that there is 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 in which liquid zinc-iron phases occur as long as the iron content is low, in particular 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 presumably penetrates the grain boundaries of the base steel, which leads to macro cracks in the base steel. In addition, with iron contents 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 intended to circumvent the problem described.

Another possibility to circumvent this problem is to use galvannealed coated steel, which is due to the fact that the iron content of 10% already existing at the beginning and the absence of an Fe ₂ Al ₅ barrier layer lead to a more homogeneous formation of the coating of predominantly rich in iron Phases leads. This results in a reduction or avoidance of zinc-rich, liquid phases.

In " 'STUDY OF CRACKS PROPAGATION INSIDE THE STEEL ON PRESS HARDENED STEEL ZINC BASED COATINGS', Pascal Drillet, Raisa Grigorieva, Gregory Leuillier, Thomas Vietoris, 8th International Conference on Zinc and Zinc Alloy Coated Steel Sheet, GALVATECH 2011 - Conference Proceedings, Genova (Italy ), 2011 "it is pointed out that galvanized sheet metal cannot be processed directly.

From the EP 1 439 240 B1 a method for hot forming a coated steel product is known, wherein the steel material has 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 worked 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 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 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.

From the EP 1 651 789 B1 the applicant is known a method for producing hardened components from sheet steel, in which case molded parts from a steel sheet provided with a cathodic corrosion protection are cold formed and a heat treatment for the purpose of austenitization follows, with an end trimming of the molded part before, during or after the cold forming Molded part and necessary punchings or the creation of a hole pattern are made and the cold forming as well as the trimming and the punching and arrangement of the hole pattern on the component are 0.5% to 2% smaller than the dimensions that the finally hardened component should have, whereby the cold-formed molding for heat treatment is then heated to a temperature at least partially with the admission of atmospheric oxygen, which enables austenitization of the steel material and the heated component is then transferred to a tool and a so-called formh Curing is carried out in which the component is cooled and thereby hardened by applying and pressing (holding) the component using the form hardening tools and the cathodic corrosion protection coating consists essentially of zinc and also one or more elements with an affinity for oxygen. As a result, an oxide skin is formed on the surface of the anti-corrosion coating from the elements with an affinity for oxygen during heating, which protects the cathodic anti-corrosion layer, in particular the zinc layer. In addition, the method takes into account the thermal expansion of the component due to the scaling down of the component in relation to its final geometry, so that neither calibration nor reshaping is necessary during hot-stamping.

From the WO 2010/109012 A1 the applicant is known a method for producing partially hardened steel components, wherein a blank made of a hardenable steel sheet is subjected to a temperature increase which is sufficient for quench hardening and the blank is transferred to a forming tool after reaching a desired temperature and possibly a desired holding time by the The blank is formed into a component and quenched hardened at the same time, or the blank is cold formed and the component obtained by the cold forming is then subjected to a temperature increase, the temperature increase being carried out so that a temperature of the component is reached that is necessary for quench hardening is necessary and the component is then transferred to a tool, in which the heated component is cooled and thereby quenched and hardened, with during the heating of the board or the component for the purpose of increasing the temperature to one for hardening necessary temperature in the areas that should have a lower hardness and / or a higher ductility, absorption masses are applied or are spaced with a small gap, the absorption mass with regard to its expansion and thickness, its thermal conductivity and its heat capacity and / or with regard to its emissivity are dimensioned in such a way that the thermal energy acting on the component in the ductile area flows through the component through into the absorption mass, so that these areas remain cooler and in particular do not or only partially reach the temperature required for hardening, so that these areas cannot be hardened or only partially hardened.

From the DE 10 2005 003 551 A1 a method for hot forming and hardening of a steel sheet is known, in which a steel sheet is _{heated to a temperature above the Ac 3} point, then is cooled to a temperature in the range from 400 ° C to 600 ° C and is only reshaped after this temperature range has been reached. However, this document does not deal with the problem of cracks or a coating, nor is martensite formation described. The aim of the invention is the formation of intermediate structures, so-called bainite.

The object of the invention is to create a method for producing sheet steel components provided with an anti-corrosion layer, in which the formation of cracks is reduced or eliminated and, nevertheless, adequate 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" or "liquid metal assisted cracking".

In contrast to the direction taken in the prior art of providing the indirect method even with simple geometries because of the "liquid metal embrittlements", the invention takes a more favorable route by using the direct method in which a plate coated with zinc or a zinc alloy 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 intended to carry out the forming under 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 a manganese-boron steel (22MnB5) so that quench hardening is carried out, and the presence of austenite also in the case of the delayed conversion of austenite into martensite Lower temperature below 780 ° C or lower is reached, so that at the moment in which mechanical stress is introduced into the steel by deformation, which in connection with a zinc melt and austenite would lead to "liquid metal embrittlement", just none or only very few liquid zinc phases are still present. It is thus possible to achieve sufficient quench hardening by means of a boron-manganese steel adjusted according to the alloying elements without provoking excessive or damaging crack formation.

In particular, the cooling can take place with air nozzles, whereby the control of air nozzles for blowing can take place via pyrometers, which, for example, are available outside the press and the furnace in a separate system as well as the corresponding nozzles.

The cooling options are not limited to air nozzles, cooled tables can also be used on which the circuit boards are positioned accordingly, so that the circuit boards come to rest on cooled areas of the table and are brought into thermally conductive contact, for example by pressing or sucking.

The use of a cooling press is also conceivable, in which the press geometry is very simple and easy thanks to the flat plates is favorable, the areas of the tool in which the board is to be cooled are correspondingly liquid-cooled. Blanks that have been heated over the entire surface can thus be cooled over the entire surface in corresponding devices, with the entire surface cooling being able to take place both via the described tables and via the intermediate presses described as well as simply by spraying, blowing or dipping.

Figur 1:

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

Figur 2:

das Zink-Eisen-Diagramm;

Figur 3:

Querschnittschliffdarstellungen der Oberfläche von Proben mit und ohne Zwischenkühlung;

Figur 4:

ZTU-Schaubild mit vereinfachter Darstellung des Abkühlverlaufs.

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

Figure 1:

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

Figure 2:

the zinc-iron diagram;

Figure 3:

Cross-sectional views of the surface of samples with and without intermediate cooling;

Figure 4:

ZTU diagram with a simplified representation of the cooling process.

According to the invention, a conventional boron-manganese steel (e.g. 22MnB5) for use as a press-hardening steel material is adjusted with regard to the transformation of the austenite into other phases in such a way that the transformation shifts to deeper areas and martensite can be formed.

Steels of this 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 conversion retarders in such steels.

Steels of the general alloy composition are also 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) 0.01-0.08 Nitrogen (N) <0.02 Boron (B) 0.002-0.02 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) 0.01-0.20 Chromium (Cr) 0.02-0.3 Titanium (Ti) 0.03-0.04 Nitrogen (N) <0.007 Boron (B) 0.002-0.006 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 that act as conversion retarders, quench hardening, i. 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 addition, according to the invention, after the blank has been heated, a holding phase can be provided in the temperature range of the peritectic, so that the solidification of the zinc coating is promoted and promoted before it is subsequently reshaped.

In Figure 1 one recognizes a favorable temperature profile for an austenitized steel sheet, whereby it can be seen that after the heating to a temperature above the austenitizing temperature, a certain cooling already takes place by the corresponding placing in a cooling device. This is followed by a rapid intermediate cooling step. The intermediate cooling step is advantageously carried out at cooling rates of at least 15 K / s, preferably at least 30 K / s, more preferably at least 50 K / s. The blank is then transferred to the press and reshaped and hardened.

In Figure 3 you can see the difference in the formation of cracks. Without intermediate cooling, cracks are formed that reach into the steel material; with intermediate cooling, only superficial cracks occur in the coating, but these are not critical.

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 (5)

1. Method of producing a hardened steel component with a coating of zinc or of a zinc alloy, wherein a blank is stamped out of a sheet coated with the zinc or zinc alloy, the stamped out blank is heated to a temperature ≥Ac₃ and, if necessary, held at this temperature for a predetermined time to allow austenite to form, after which the heated blank is transferred into a forming tool, is formed in the forming tool at a speed which exceeds the critical hardening speed, is cooled down and is hardened thereby, in which the steel material is delayed in its conversion such that, at a forming temperature in the range of 450°C to 700°C and below the peritectic temperature of the zinc-iron system, quench hardening to convert the austenite into martensite takes place, upon which, after heating and before the conversion, active cooling takes place wherein the blank or parts of the blank is/are cooled down at a cooling speed >15K/sec wherein a steel material is used with the following analysis (all data 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) 0.01-0.08 Nitrogen (N) < 0.02 Boron (B) 0.002-0.02 Phosphorus (P) < 0.01 Sulphur (S) <0.01 Molybdenum (Mo) < 1 the remainder being iron and impurities caused by the melting, and progress of the cooling and/or the temperature at the which the material is loaded into the forming tool is monitored by means of sensors, in particular pyrometers, and the cooling is controlled accordingly where the blank is then heated in a furnace to a temperature >Ac₃ and is held for a predetermined time, wherein the blank is cooled down to a temperature between 500°C and 600°C in order to harden the layer of zinc, after which it is then transferred into the forming tool where it is formed.
2. Method according to claim 1 characterised in that a steel material with the following analysis is used (all data on % by mass): Carbon (C) 0.08-0.30 Manganese (Mn) 1.00-3.00 Aluminium (Al) 0.03-0.06 Silicon (Si) 0.01-0.20 Chromium (Cr) 0.02-0.3 Titanium (Ti) 0.03-0.04 Nitrogen (N) < 0.007 Boron (B) 0.002-0.006 Phosphorus (P) < 0.01 Sulphur (S) <0.01 Molybdenum (Mo) < 1 the remainder being iron and impurities caused by the melting.
3. Method according to any one of the preceding claims, characterised in that the active cooling is performed such that the cooling down rate is >30 K/sec.
4. Method according to claim 3, characterised in that the active cooling is performed such that the cooling takes place at a rate greater than 50 K/sec.
5. Method according to any one of the preceding claims, characterised in that the active cooling is executed by blowing with air or gas, spraying with water or with other cooling fluids, immersing in water or in other cooling fluids or the active cooling is achieved by placing cooler solid bodies on the blank.

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