EP2656187A2

EP2656187A2 - Method for producing hardened structural elements

Info

Publication number: EP2656187A2
Application number: EP11811026.1A
Authority: EP
Inventors: Harald Schwinghammer; Thomas Kurz; Siegfried Kolnberger; Martin Rosner
Original assignee: Voestalpine Stahl GmbH
Current assignee: Voestalpine Stahl GmbH
Priority date: 2010-12-24
Filing date: 2011-12-22
Publication date: 2013-10-30
Anticipated expiration: 2031-12-22
Also published as: CN103392014B; US20140020795A1; KR101582922B1; HUE054465T2; CN103392014A; EP2655672A2; CN103547686B; ES2858225T3; WO2012085247A3; WO2012085256A3; US20140027026A1; HUE054867T2; EP2656187B1; CN103384726A; KR20130132566A; CN103415630A; EP2655672B1; WO2012085247A2; WO2012085253A3; US10640838B2

Abstract

The invention relates to a method for producing a hardened structural steel element comprising a zinc or zinc alloy coating. According to the method, a hardenable steel material is coated with a zinc layer or a zinc alloy layer, blanks are stamped out from the hardenable steel material, and the blanks are heated to a temperature at the AC3 point or above and are formed after a desired holding time in a forming tool when still hot, the formed steel blank being cooled by the forming tool at a rate above the critical quenching rate, thereby being hardened. Depending on the thickness of the zinc layer or the thickness of the zinc alloy layer the blank is held prior to forming at a temperature of above 782°C until a barrier layer of zinc ferrite is formed between the steel and the zinc or zinc alloy coating and the forming zinc ferrite layer absorbs liquid zinc and reaches a thickness which prevents the reaction of liquid zinc phases with the steel.

Description

Method for producing hardened components

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

It is known that especially in automobiles so-called press-hardened components made of sheet steel are used. These press-hardened components made of sheet steel are high-strength components that are used in particular as safety components of the bodywork sector. The use of these high-strength steel components makes it possible to reduce the material thickness compared to a 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 in the so-called direct and indirect procedure.

In the direct method, a sheet steel plate is heated above the so-called austenitizing temperature and, if appropriate, kept at this temperature until a desired degree of austenitization is achieved. Subsequently, this heated board is transferred to a mold and formed in this mold in a one-step forming step to the finished component and thereby simultaneously by the cooled mold at a speed over the critical hardness is, cooled. Thus, the hardened component is produced.

In the indirect process, the component is first, if necessary, in a multi-stage forming process, the component formed almost completely finished. This formed component is then also heated to a temperature above the Austenitisierungstempe- temperature and optionally held for a desired time required at this temperature.

Subsequently, this heated component is transferred to a mold and inserted, which already has the dimensions of the component or the final dimensions of the component, where appropriate, taking into account the thermal expansion of the preformed component. After closing the particular cooled tool thus the preformed component is cooled only in this tool at a speed above the critical hardness and hardened thereby.

The direct method is somewhat simpler to implement, but allows only shapes that are actually to be realized with a single forming step, i. relatively simple profile shapes.

The indirect process is a bit more complex, but it is also able to realize more complex shapes.

In addition to the need for press-hardened components, the need has arisen for such components not to be uncoated

To produce steel, but to provide such components with a corrosion protection layer.

In the automotive industry, the corrosion protection layer used is only the aluminum or aluminum used to a lesser extent. alloys or the much more frequently requested coatings based on zinc. Zinc has the advantage here that zinc not only provides a barrier protection layer such as aluminum, but cathodic corrosion protection. In addition, zinc-coated press-hardened components fit better into the overall corrosion protection concept of vehicle bodies, since they are fully galvanized in today's common construction. In this respect, contact corrosion can be reduced or eliminated.

In both methods, however, disadvantages could be found, which are also discussed in the prior art. In the direct method, i. The hot forming of press-hardening steels with zinc coating leads to micro- (10 μπι to ΙΟΟμπι) or even macrocracks in the material, the microcracks appear in the coating and the macrocracks even reach through the complete sheet metal cross-section. Such components with macrocracks are unsuitable for further use.

In the indirect process, i. Cold forming with subsequent hardening and remolding may also result in micro-cracks in the coating, which are also undesirable, but not nearly as pronounced.

Zinc-coated steels are currently - with the exception of one component in the Asian region - in the direct process, i. the hot forming not used. Instead, 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 produces an aluminized boron-manganese steel commercially available under the name Usibor 1500P for the hot-forming process. In addition, for the purposes of cathodic protection against corrosion, zinc-coated steels are sold for the hot forming process, namely the zinc-plated Usibor Gl with a zinc coating containing small amounts of aluminum and a so-called galvealed-coated Usibor GA containing a zinc layer with 10% iron.

It should be noted that the zinc-iron phase diagram shows that above 782 ° C a large area is created containing 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 thermoformed. It should also be noted, however, that if the forming takes place above 782 ° C, there is a great risk of stress corrosion by liquid zinc, which penetrates into the grain boundaries of the base steel, resulting in macrocracks in the base steel. In addition, with iron levels less than 30% in the coating, the maximum temperature for forming a safe product with no macrocracks is less than 782 ° C. This is the reason why hereby no direct forming process is operated, but that indirect forming process. This is intended to circumvent the problem described.

Another way around this problem is to use galvannealed coated steel, which is because the already existing 10% iron content and the absence of a Fe ₂ Al ₅ barrier layer the critical value of 60% iron in when heated, quickly overshadows the coating, avoiding the presence of liquid iron during the hot working process. From EP 1 439 240 B1 a method for hot forming a coated steel product is known, wherein steel material has a zinc or zinc alloy coating formed on the surface of the steel material and the steel base material with the coating to a temperature of 700 ° C to 1000 ° C is heated and hot formed, wherein the coating has an oxide layer, which consists mainly of zinc oxide, before the steel base material is heated with the zinc or zinc alloy layer, then to prevent evaporation of the zinc during heating. For this purpose, a special procedure is provided.

From 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, kept at this temperature and then 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 manner that the cooling rate to MS point 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 provide a method for producing provided with a corrosion protective layer sheet steel components, in which the cracking is reduced or eliminated and yet sufficient corrosion protection is achieved.

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

The above-described effect of liquid zinc cracking, which penetrates the steel in the vicinity of the grain boundaries, is also known as so-called "liquid metal embrittlement".

According to the invention, the object is achieved by recognizing that the combination of the base material in the austenitized form, i. At high temperatures, the presence in this state of liquid zinc phases and the entry of stress by forming must be avoided in order to avoid the stresses induced thereby and thus cracks.

This is inventively achieved in that a barrier layer is disposed 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, the formation of a thicker barrier layer additionally consuming liquid phases.

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

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

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

According to the invention, the reduction of liquid zinc or the rapid construction of an effective barrier layer can be formed by rapidly closing the formation of the barrier layer by reducing the available amount of zinc and thus avoiding a residual liquid phase of zinc. This can i.a. be achieved by a reduction of the zinc coating thickness.

In accordance with the invention, acceleration of the zinc-iron reaction and thus a faster and larger barrier layer thickness can also be achieved in this case if the zinc layer chemistry is interfered with. Conventional zinc layers applied in the rapid dip galvanizing process have a certain amount of aluminum, which forms an inhibiting layer between the support material (steel) on the one hand and the zinc layer on the other hand, thereby preventing a strong reaction of substrate and coating. The addition of aluminum can be purposefully reduced to promote precisely this rapid formation of a thick zinc-iron layer. For this purpose, aluminum is reduced in the liquid zinc coating and optionally the coating before forming a Galvanealing reaction to form zinc-iron phases supplied to dissolve this inhibitor layer. Such a coating then does not cause any liquid zinc layers to directly interact with the austenite in detrimental interaction.

In addition, it is possible to heat treat a conventional zinc layer with low levels of aluminum longer than usual during production, to extend over a prolonged Annealing a thicker barrier layer, which protects the material during the direct forming process to form.

The invention will be explained by way of example only with reference to a drawing. It shows:

Figure 1: a table with the typical chemical composition of the examined steel samples;

FIG. 2 is a graph showing the relationship between crack depth and furnace residence time in a pre-conversion annealing treatment;

FIG. 3 shows a diagram showing the critical intervals of FIG

Oven residence time;

Figure 4 is a table showing the oven residence time along with images showing crack formation as a function of oven residence time;

FIG. 5 shows samples according to FIG. 4 in a cross section showing the

Crack depth depending on the oven residence time;

FIG. 6: the ferrite layer formation through longer furnace residence time;

FIG. 7: the zinc-iron state diagram.

According to the invention, a zinc ferrite layer can be formed with a longer furnace residence time and, consequently, a longer annealing treatment of a zinc coating, which effectively prevents the "liquid metal embrittlement" even if on the one hand austenite is present and stresses are introduced. As a result, it is possible according to the invention to carry out the direct method instead of avoiding the more complex, indirect process.

Figure 1 shows the analysis of a typical steel used in the method of the invention. It is understood that the remainder of the analysis consists of iron and unavoidable, unavoidable impurities.

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

It can be seen from the diagram that the curves increase sharply after a certain oven residence time in the different curves, which is related to the formation of liquid zinc phases. This simultaneously induces an increasing crack depth. Also in all curves, a kinking can be seen, in which the crack depth does not increase further, but the observed crack depth decreases after this Ofenverweingauer. Then, in turn, results in a relatively sharp kink and a curve to lower crack depths with increasing furnace residence time. It can be stated here that very long furnace residence times are necessary for a pure zinc deposit of 120 g / m ² , whereas for a zinc-iron layer with 120 g of support both the absolutely achievable crack depth is lower and a strong one in a considerably shorter furnace residence time Slimming crack depth can be observed.

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

From these observations observed observed critical intervals of Ofenverweingauer, which at a zinc coating of 80 g / m ² about 90 s to 140 s, with a zinc coating of 100 g / m ² about 100 s to 155 s and a zinc coating of 120 g / m ^{2 is} even 90 s to over 200 s.

In contrast, the critical intervals of the furnace residence time for zinc-iron deposits of 80 g / m ² , 100 g / m ² and 120 g / m ^{2 are} significantly lower, with the critical intervals, especially in a zinc-iron overlay of 80 g / m ² between 45 s and 70 s and a zinc-iron overlay of 120 g / m ² with 50 s to 105 s are also significantly narrower.

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

The direct influence of the furnace residence times can be seen in Figure 4, which can be read in the table that three similar zinc coatings with 140 g / m ² at similar temperatures of 870 ° C to max. 910 ° C for 185 s, 325 s and 475 s. In this experiment, the so heated components were transferred with a transfer time of 3 s in a mold and formed there directly in the hot state.

Depending on the oven residence time, different crack depths of max. 200 μπι at the lowest Ofenverweildauer to 20 μπι at the longest Ofenverweilauer. The pictures clearly show the obvious significant differences.

These are again particularly clear in FIG. 5 in that cross sections of the different samples from FIG. 4 can be seen. Accordingly, not only the crack depth but also the crack width is significantly reduced with increasing furnace residence time. In addition, it can be seen that in the sample with the furnace residence time, the cracks are present only in the coating, while the cracks in the other samples reach into the base material.

It can thus be shown that it is possible with the method according to the invention to maintain the direct forming process and thus to manufacture simpler components cost-effectively, especially in geometry, if it is ensured that as little zinc liquid as possible is present in the sensitive temperature interval during the forming. The observance of certain temperature-time parameters according to the invention thus makes it possible to continue working with previous methods.

Claims

claims

A method of producing a hardened steel component having a coating of zinc or a zinc alloy, wherein a hardenable steel material is coated with a zinc layer or a zinc alloy layer from which sheets of steel are punched out of the hardenable steel material, heating the boards to a temperature at or above the AC3 point and after a desired hold time in a forming tool, are hot worked, wherein the formed sheet steel blank is cooled by the forming tool at a rate above the critical hardening rate and thereby hardened, characterized in that the blank depends on the thickness of the zinc layer or the thickness the zinc alloy layer is kept at a temperature of above 782 ° C before forming, that forms a barrier layer of zinc ferrite between the steel and the coating of zinc or a zinc alloy, and the zinc ferrite layer forming t absorbs liquid zinc and is formed so thick that react during forming no liquid zinc phases with the steel.

2. The method according to claim 1, characterized in that the coating on the steel is a zinc coating which, prior to heating for the purpose of hot forming with a ner temperature treatment is converted into a zinc-iron coating.

3. The method according to claim 1, characterized in that the coating on the steel is a zinc coating with an aluminum content of 0.1% to 5%.

4. The method according to any one of the preceding claims, characterized in that the coating is applied to the steel electrolytically and / or by hot dip coating.

5. The method according to any one of the preceding claims, characterized in that the coating on the steel comprises a thin electrodeposited zinc layer and a zinc layer or zinc aluminum layer deposited thereon.

6. The method according to claim 5, characterized in that before the hot-dip galvanizing the electrolytically spent zinc layer has been converted into a zinc ferrite layer.

7. The method according to any one of the preceding claims, characterized in that the coating is a zinc-nickel coating, a zinc-aluminum coating, a zinc-iron coating, a zinc-chromium coating, a Reinz inkbeschichtung or a zinc Magnesium coating is.

8. The method according to any one of the preceding claims, characterized in that in zinc deposits between

80 g / m ² and 120 g / m ^2, the oven residence time does not fall below 120 s to 210 s.

9. The method according to any one of the preceding claims, characterized in that in zinc-iron (Galvanealed) - layers with runs of 80 g / m ² to 120 g / m ^2, the furnace residence time does not fall below 75 s to 100 s.

10. The method according to any one of the preceding claims, characterized in that the zinc coating or the zinc alloy coating of the coating with a basis weight of 60 g / m ² to 140 g / m ^{2 is} formed.