EP2513346B1 - Method for producing an easily deformable flat steel product - Google Patents

Description

The invention relates to a process for producing a readily deformable, a C content of 0.1 to 0.4 wt .-% having flat steel product, in which the flat steel product is subjected in a continuous furnace to an annealing treatment.

Flat steel products of the type in question are required in particular for the production of bodywork and chassis parts for automobiles. The highest demands are placed on the flat steel products with regard to their forming properties. This applies to both cold and hot workability.

A special difficulty is the hot forming of galvanized flat steel products to high- or high-strength steel components. In such steel components, usually based on zinc or a zinc alloy protective coating ensures sufficient cathodic corrosion protection.

However, a steel sheet provided with a metallic anticorrosive coating must be used for hot working and, if necessary, subsequently or in combination with hot working Hardening be heated to a temperature above the melting temperature of the metal of the protective coating, so there is a risk of so-called "liquid metal embrittlement". This embrittlement of the steel occurs when molten metal of the coating penetrates into the notches forming during the deformation on the surface of the respective flat steel product. The liquid metal entering the steel substrate deposits there at the grain boundaries and thus reduces the maximum absorbable tensile and compressive stresses.

Particularly critical is the risk of liquid metal embrittlement in higher and high strength steels, which have only a limited ductility and consequently tend to form near-surface cracks during their transformation.

From the JP 60-159120 A It is well known that the bending properties of a steel sheet can be improved by a decarburization treatment which produces a near-surface 20-100 μm thick surface layer having a C content reduced from the core portion of the steel sheet. However, in this prior art, this measure is not related to steel sheets coated with a metallic protective layer, nor does it concern higher or high strength steels having C contents of at least 0.1% by weight.

The decarburization tendency of a carbon-containing steel alloy results from the oxidation behavior of the dissolved carbon. Because of his big Agility, the carbon dissolved in the lattice tends to effusion during a heat treatment. Decarburization, which occurs depending on the C potential of the gas phase under which the heat treatment takes place, with or without simultaneous scaling, is therefore one of the oldest problems in the production and processing of steel.

In principle, decarburization is carried out according to the Boudouard equilibrium reactions according to the following reaction processes:

[C] + 1/2 O ₂ <-> CO

[C] + O ₂ <-> CO ₂

[C] + CO ₂ <-> CO ₂

[C] + H ₂ <-> CH ₄

with [C] = dissolved carbon

In large-scale annealing plants with a typical protective gas atmosphere, which contains both hydrogen, nitrogen and water vapor, the following equilibrium reaction forms:

H2 + 1/2 O ₂ <-> H ₂ O

Hydrous gas atmospheres are particularly reactive towards carbon. Therefore, another heterogeneous equilibrium reaction, which is particularly significant in practice, is added to the mentioned decarburization reactions:

[C] + H ₂ O <-> CO + H ₂

When used in a targeted way, decarburization can improve certain properties of a steel product.

To be able to use this knowledge effectively in practice, is in the GB 1 189 464 proposed the "open coil" method. In this process, a hard-rolled, cold-rolled strip is wound loosely into a coil such that a free space exists between the individual winding layers of the coil. The annealing gas flowing through the clearances during the subsequent annealing treatment in a bell-type furnace then uniformly passes over the entire steel surface, so that a uniform decarburization result over the entire length of the treated steel strip is achieved. However, the annealing treatment thus carried out takes several hours.

An economical feasible process for decarburization annealing of steel strip in a continuous furnace under a reducing annealing atmosphere is disclosed in US Pat DE-OS 2,105,218 described. According to this known method, the respective steel strip is annealed at an annealing temperature of less than 780 ° C for a sufficiently long annealing time until the carbon content in the steel strip at the exit from the continuous furnace is less than 0.01%. Subsequently, the steel strip can be provided with a hot dip coating to improve its corrosion resistance. The steel sheet obtained in this way has a particularly good deformability. However, its strength values do not meet the requirements of today regularly on flat steel products be made from which components for automobile bodies are to be formed.

Also on the "open coil" procedure is at one of the WO 2009/024472 A1 Known proposal has been resorted to, according to the one for a made of a tool steel steel strip, which is particularly intended for the production of cutting tools and the like and has a C content of at least 0.4 wt .-%, a Randschichtentkohlung has been proposed to a To combine high hardness with a good ductility. In the area of the decarburized boundary layer, the steel strip treated accordingly has an increased deformability compared to the base material, which reduces the risk of brittle fracture under high external load.

Unlike in the WO 2009/024472 A1 In the applications considered, the skilled person generally endeavors to avoid glow-induced decarburization or surface decarburization as far as possible in the case of high-strength and ultra-high-strength steels intended for the production of high-strength components. It has been widely believed that decarburization adversely affects the mechanical properties of materials that are important for these applications.

Following this idea is in the DE 102007061489 A1 a method has been proposed in which on a steel sheet for forming a favorable ductile boundary layer is produced by a selective oxidation of the solidifying alloying elements is performed. In the process, every decarburization is deliberately counteracted.

In addition to the above-described prior art is from the WO 2004/090187 A1 a steel sheet is known, which have a superior connection of the coating to the steel substrate and thereby should have high strengths with good deformability. For this purpose, the steel sheet in question from 0.05 to 0.4 wt .-% C, 0.2 to 3.0% by weight of Si and 0.1 to 2.5 wt .-% Mn, balance Fe and consist of unavoidable impurities and be coated with a Zn layer having contents of 0.01 to 1 wt .-% A1. It should be present at the interface between the steel substrate and the Zn coating over a depth of the steel substrate of 2 microns oxide particles belonging to the group of Al, Si and Mn oxides. This is to be achieved by setting a specific ratio of the partial pressures PH2O and PH2 in the annealing atmosphere, under which the respective steel substrate is maintained at a temperature of 600-900 ° C. for a holding time of 30 seconds to 10 minutes.

Against the background of the prior art explained above, the object of the invention was to provide a method which makes it possible in an economical manner to produce a readily deformable, high-strength or very high-strength steel flat product.

With regard to the manufacturing method, the above-stated object has been achieved according to the invention in that, in the production of a flat steel product, the steps specified in claim 1 are completed.

Advantageous embodiments of the invention are specified in the claims dependent on the respective independent claims and are explained below.

The process according to the invention for producing a readily deformable flat steel product which has a C content of 0.1-0.4% by weight, in particular less than 0.4% by weight, is based on the idea of the relevant flat steel product undergo an annealing treatment in a continuous furnace, which leads to surface layer decarburization. For this purpose, the annealing treatment according to the invention is carried out under an annealing atmosphere containing 0.1 to 25 vol .-% H ₂ , H ₂ O and the balance N ₂ and technically unavoidable impurities. The dew point of the annealing atmosphere is in the range of -20 ° C and +60 ° C. At the same time, the ratio H ₂ O / H _{2 must be set} at most equal to 0.957 in the annealing atmosphere in order to achieve an optimally decarburizing effect.

The flat steel product is further heated according to the invention in the course of the annealing to a holding temperature of 600-1100 ° C., at which it is kept under the atmosphere composed according to the invention for a holding time of 10 to 360 s.

As a result, the flat steel product obtained after the annealing according to the invention has a ductile boundary layer 10 to 200 μm thick, adjacent to its free surface, with a ductility which is greater than the ductility of the inner core layer of the flat steel product covered by the boundary layer.

Contrary to the conviction existing in the prior art, with the invention it is possible to achieve the desired combination of properties of high strength and good ductility in a steel sheet which contains 0.1-0.4% by weight, in particular up to 0.38% by weight. Containing carbon, by an annealing treatment, which leads to a Randentkohlung of the steel material. This edge decarburization causes a ductilization of the near-surface microstructure, which counteracts the otherwise form-related crack failure of the material.

The invention is thus based on the idea of producing a surface decarburization of cold-rolled steel flat products intended for cold or hot forming, i. Steel strip or sheet to be carried out so that the obtained after the annealing, the flat product has a ductile, typically ferritic, near-surface edge region of certain thickness on the first grain layers, which improves the forming properties of the steel product for both cold and for hot forming. In particular, the risk of cracking or notching on the surface of the steel product during its transformation is minimized.

It is essential for the method according to the invention that although the edge decarburization of the near-surface microstructure can take place simultaneously with annealing conditioning of the steel surface for a subsequent application of a corrosion protection layer, it has a decoupled reaction mechanism.

• mit [Me] = jeweiliges Metall
• x,y = stöchiometrische Koeffizienten.

Thus, the edge decarburization of the near-surface microstructure takes place according to the following relationship:

[C] + H ₂ O <-> CO + H ₂

with [C] = dissolved carbon,
whereas the oxidation / reduction reaction of the surface proceeds as follows:

x [Me] + yH ₂ 0 <-> [Me _x O _y ] + yH ₂

• with [Me] = respective metal
• x, y = stoichiometric coefficients.

Surprisingly, it is possible to achieve the desired Entkohlungstiefe even with very short conditioning times when using the annealing conditions according to the invention. Thus, the inventive method is characterized in particular by the fact that it can be carried out in a particularly economical manner using a continuous furnace. This makes it possible to incorporate the method according to the invention into continuous production processes which require high strip speeds, as is the case, for example, in fire-coating installations in which steel strips are heat-treated in a continuous pass and hot-dip coated with a corrosion protection coating.

Accordingly, a particularly advantageous embodiment of the invention provides that the flat steel product after the annealing with a metallic protective layer is coated. The invention makes use in particular of this variant of the method according to the invention that the risk of liquid metal embrittlement can be minimized by selectively modifying the near-surface region of the flat steel product for the liquid metal embrittlement susceptible temperature range can be shifted so that this does not coincide with the typical temperature range for hot forming.

In the event that the inventive production process is preceded by a subsequent hot-dip coating, the annealing carried out according to the invention is carried out simultaneously with the surface conditioning for the downstream surface refinement by controlling the near-surface carbon effusion via a heterogeneous annealing gas-metal reaction.

In this case, it is particularly advantageous to use the method according to the invention in a fire-coating plant, since the annealing treatment in this case can include surface decarburization, surface conditioning and recrystallization of the base material and then the hot-dip coating can be carried out in-line following the annealing treatment in a continuous process sequence.

In the course of the surface refinement, preferably carried out by hot-dip coating, of a flat steel product produced according to the invention, coating systems known per se on the steel substrate can be used which are based on Zn, Al, Zn-Al, Zn-Mg, Zn-Ni, Al-Mg, Al-Si or Zn-Al-Mg.

Alternatively or in addition to in-line hot dipping refinement, a steel strip which has been provided in a continuous annealing with a ductile decarburized surface layer according to the invention can subsequently be given a metallic, an inorganic-metallic or an organometallic coating by electrolytically coating e.g. coated with a Zn, a ZnNi or a ZnFe coating, by PVD or CVD deposition or by another metal-organic or metal-inorganic coating process.

According to a process variant which is particularly important in practice, the invention thus provides for hot-dip coating of the flat steel product in a work step which is carried out continuously following the annealing treatment. In this case, the hot-dip coating can be carried out in a manner known per se as a fire coating, in particular hot-dip galvanizing. In order to ensure optimal adhesion of the coating on the steel substrate, oxidation of the surface of the flat steel product can be carried out before the fire coating.

In order to further optimize the mechanical properties, the annealing treatment according to the invention may be followed by an over-aging treatment carried out in a conventional manner.

As explained above, a flat steel product made by using a method according to the invention has a C content of 0.1-0.4% by weight and a 10-200 μm thick ductile edge layer facing the core layer of the present invention Flat steel product has increased ductility.

The thickness of the ductile layer can be determined in a customary manner in accordance with the procedure defined in DIN EN ISO 3887. Thus, the total decarburization depth is the distance from the surface to the point where the content of carbon is equal to that of the unaffected core region. In this way, a hardness in the decarburized surface layer region which is not higher than 75% of the hardness of the core region, ie $$\mathrm{Hv}\left(\mathrm{decarbonized}\right)/\mathrm{Hv}\left(\mathrm{core\; area}\right)=3/\mathrm{4th}$$

The ductile surface layer of a flat steel product produced according to the invention is characterized, at least near its free surface, typically by a ferritic microstructure. This applies to a multiphase base material in which a ferritic near-surface microstructure occurs in the region of the decarburized edge layer according to the invention, just as for a single-phase, typically ferritic steel, in which decarburization according to the invention results in ductileization of the near-surface ferrite.

A flat steel product produced according to the invention is suitable in the same way for cold and hot forming, wherein its particular advantages in particular in the hot forming of provided with a metallic protective layer, in particular a galvanized steel sheets or bands show. The steels according to the invention intended for cold forming typically have a tensile strength of 500-1500 MPa. According to the invention, steels which have a tensile strength of 900 to 200 MPa after hot working can be used for the hot forming.

In the case where a steel flat product produced according to the invention is to be formed into a component by hot forming, the flat steel product according to the invention may first be heated to a heating temperature above its Ac1 temperature and then hot formed into the component.

If, for example, hardening is to follow the hot working, the steel flat product according to the invention can also be heated without problems to a heating temperature which is at least equal to the Ac3 temperature of the flat steel product. Even with such a high heating temperature, the risk of embrittlement is minimized in a steel flat product produced according to the invention even if the flat steel product is provided with a metallic coating whose melting temperature is less than or equal to the heating temperature. The ductility of the surface layer achieved by the surface-layer decarburization according to the invention prevents cracking and thus ensures that no molten metal of the coating can penetrate into the core region of the steel substrate.

The method according to the invention thus improves in particular the forming properties of surface-treated high / ultra-high strength flat steel products for both cold and hot forming, wherein flat steel products coated with a metallic protective coating according to the invention are particularly advantageously suitable for hot forming. This is achieved in that, according to the invention, by means of a targeted annealing gas-metal reaction in a continuous furnace, an edge decarburization is induced, through which a ductile, typically ferritic, boundary layer is formed. This shields the solid, brittle steel base material against surface crack propagation during forming.

Fig. 1

einen senkrechten Anschliff einer erfindungsgemäß randschichtentkohlten Stahlprobe;

Fig. 2

einen senkrechten Anschliff einer konventionell geglühten Vergleichsprobe;

Fig. 3

GDOES-Tiefenprofile des Kohlenstoffgehaltes der in den Figuren 1 und 2 dargestellten Proben;

Fig. 4

die Ergebnisse von Drei-Punkt-Biegeversuchen mit den in Fig. 1 und 2 dargestellten Proben.

The invention will be explained in more detail by means of exemplary embodiments. Show it:

Fig. 1

a vertical section of an inventively randschichtentkohlten steel sample;

Fig. 2

a vertical section of a conventionally annealed comparison sample;

Fig. 3

GDOES depth profiles of the carbon content of the Figures 1 and 2 represented samples;

Fig. 4

the results of three-point bending tests with the in Fig. 1 and 2 presented samples.

To test the effects achieved by the method according to the invention, hard-cold-rolled strip samples are in each case a multi-phase steel "MP" and a steel used commonly for hot forming "WU" has been produced. The compositions of steels MP and WU are given in Table 1. Table 1 stole C Mn P Si V al Cr Ti B Nb [Wt .-%] MP 0.22 1.7 0.02 0.1 0.00 1.7 0.06 0.1 0.005 0.001 WU 0.22 1.22 0.01 0.25 0.00 0.02 0.13 0.03 0.005 0,003 Remaining iron and unavoidable impurities

Two samples made of the steels MP and WU have been subjected to an annealing treatment according to the invention in a continuous furnace for surface-layer decarburization. The annealing parameters used in this case are given in the "Inventive" column of Table 2 below.

For comparison, two more samples made of steels MP and WU in the continuous furnace have been subjected to conventional annealing, as is commonly done in preparation for hot dip galvanizing.

In order to optimize the mechanical properties of the samples, an overaging treatment has additionally been performed. This has no influence on the formation of the decarburized surface layer, but was merely optional to improve the properties of the tape.

The parameters used in the overaging treatment, which are the same for both experiments, are also given in Table 2. Table 2 step According to the invention Conventional annealing heating 10 K / s 10 K / s holding temperature 800 ° C 800 ° C hold time 120 s 60 s Annealing atmosphere Dew point 5% H ₂ 5% H ₂ 95% N ₂ 95% N ₂ +5 ° C -30 ° C Cooling rate after holding 20 K / s 20 K / s Aging treatment Temperature of the overaging treatment 480 ° C 480 ° C Duration of overaging treatment 20 s 20 s Atmosphere of overaging treatment dew point 5% H ₂ 5% H ₂ 95% N ₂ 95 N ₂ +5 ° C -30 ° C Cool to room temperature

In Fig. 1 the micrograph of the sample produced from the steel MP and annealed according to the invention is shown. It can be clearly seen that as a result of the procedure according to the invention, a decarburised, near-surface microstructure region (boundary layer "R") has been established.

In contrast, the micrograph of the sample likewise produced from steel MP but subjected to a conventional annealing treatment shows no decarburized area ( Fig. 2 ).

GDOES measurements of the carbon content have additionally been carried out on the samples produced from the steel MP according to the invention and conventionally annealed. The GDOES measurement method ("GDOES" = Glow Discharge Optical Emission Spectrometer) is a standard method for the rapid detection of a concentration profile of coatings. It is for example in the VDI Lexicon Materials, ed. by Hubert Gräfen, VDI-Verlag GmbH, Dusseldorf 1993.

The result of the GDOES measurements is in Fig. 3 summarized, wherein the dashed line represents the carbon distribution of the conventionally treated sample and the solid line, the carbon distribution of the invention treated sample.

Also Fig. 3 clearly shows that the sample treated according to the invention has a pronounced decarburized edge layer R whose thickness is approximately 40 μm. By contrast, such a surface layer is not present in the conventionally treated sample.

On the basis of microhardness measurements, it was possible to prove that the decarburized edge region R which was decarburized in the sample MP produced from the steel MP and heat-treated according to the invention has a microhardness of 163 HV and the non-decarburized core region K has a hardness of 255 HV. The% ratio Hv _R / Hv _K of hardness Hv _{R of} the decarburized edge region R to hardness Hv _{K of} the core region K was thus 64%, which was well below that according to the invention for this ratio predetermined value of 75%.

Subsequent to the annealing, the samples were surface-refined, zinc being electrolytically applied to the samples.

Subsequently, a three-point bending test was carried out on the coated samples both before and after press hardening. The results of the tests are for the samples made of the steel MP in Fig. 4 summarized. The bending angle Bw of the sample produced according to the invention is symbolized therein by the black and the bending angle Bw of the conventionally produced sample by the white bar. Here, too, it becomes clear that the samples produced and treated according to the invention have significantly better forming and bending properties than the conventionally processed samples.

For the heat-treated, galvanized and shaped samples produced from the steel WU, comparable results could be demonstrated for the inventive and conventionally annealed specimens.

Claims (8)

1. Method for producing a readily formable flat steel product which has a C content of from 0.1 to 0.4% by weight, in which the flat steel product is subjected in a continuous furnace to an annealing treatment, characterised in that the annealing treatment is carried out under an annealing atmosphere which contains from 0.1 to 25% by vol. of H₂, H₂O, with the balance being N₂ and technically unavoidable impurities as the balance and which has a dew point between -20°C and +60°C, the relationship H₂O/H₂ of the annealing atmosphere being a maximum of 0.957 and in that, in the course of the annealing treatment, the flat steel product is heated to a holding temperature which is from 600 to 1100°C and at which it is held for a holding time which is from 10 to 360 seconds, so that the flat steel product obtained after the annealing treatment has a ductile edge layer which is from 10 to 200 µm thick and which adjoins its free surface and which has a ductility greater than the ductility of the inner core layer of the flat steel product that is covered by the edge layer.
2. Method according to claim 1, characterised in that the flat steel product is coated with a metal protective layer after the annealing treatment.
3. Method according to claim 2, characterised in that the flat steel product is hot-dip-coated in an operating sequence which is carried out continuously following the annealing treatment.
4. Method according to claim 2, characterised in that the flat steel product is hot-coated after the annealing treatment.
5. Method according to claim 4, characterised in that oxidation of the surface of the flat steel product is carried out before the hot coating.
6. Method according to either claim 2 or claim 3, characterised in that the flat steel product is coated with a metal/organic coating.
7. Method according to either claim 2 or claim 3, characterised in that the flat steel product is coated with a metal/inorganic coating.
8. Method according to any one of the preceding claims, characterised in that the C content of the flat steel product is less than 0.38% by weight.

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