EP2580358A1 - Method for producing a hot-formed and heat-treated steel component that is coated with a metal anti-corrosion coating from a sheet steel product - Google Patents

Abstract

The invention relates to a method for producing a steel component that is coated with a metal protective coating from a sheet steel product comprising at least 0.4 wt % Mn. In order to economically generate a high-strength steel component, while minimizing the risk of the development of metal-induced cracks, according to the invention the sheet steel product is annealed in a continuous furnace under an annealing atmosphere containing up to 25% by volume H2, 0.1 - 10% by volume NH3, H2O, the remainder being N2 as well as process-related inevitable impurities, at a dew point ranging between -50ºC and -5ºC and at a holding temperature of 400 - 1100ºC for a holding period of 5 - 600 s. The annealed sheet steel product has a nitride layer (N) 5 - 200 pm thick, the particle size of which is finer than the particle size of the interior core layer (K) of the sheet steel product. After it has been coated with a metal protective layer, a blank is separated from the annealed sheet steel product and is soaked to an austenitizing temperature of 780 - 950ºC subsequent to an optional preforming step, hot-formed to form the steel component and cooled so quickly that a tempered martensitic structure forms in the sheet steel product.

Description

 METHOD FOR PRODUCING A STEEL COMPONENT MADE OF A STEEL FLAXED PRODUCT AND THERMALLY COATED WITH A METALLIC CORROSION PROTECTION COATING

The invention relates to a method for producing a hot-formed and hardened, coated with a metallic Korrosionsschut zbeschichtung steel component made of a flat steel product having an Mn content of at least 0.4 wt .-%.

As in the article "Potentials for lightweight body construction", published in the trade fair newspaper of ThyssenKrupp Automotiv AG for the 61st International Motor Show in

Frankfurt, 15.-25. Sept. 2005, it is reported

Thermoforming in practice, in particular for the

Manufacture of high-strength body parts made from boron-alloyed steels. A typical example of such a steel of the type in question here is the steel known under the name 22MnB5, which can be found in the steel key 2004 under the material number 1.5528.

A steel comparable to steel 22MnB5 is known from JP 2006104526A. This known steel contains besides Fe and unavoidable impurities (in% by weight) 0.05 - 0, 55% C, max. 2% Si, 0.1-3% Mn, max. 0.1% P and max. 0.03% S. To improve through hardenability, additional contents of 0.0002 - 0.005% B and 0.001 - 0.1% Ti can be added to the steel. The respective Ti content serves for setting the nitrogen present in the steel. In this way, the boron present in the steel can develop its strength-increasing effect as completely as possible.

According to JP 2006104526 Ä become such from the

first made of sheet steel, which then at a temperature above the Ac ₃ temperature, typically in the range of 850 - 950 ° C.

to be preheated. In the subsequent taking place in the pressing tool, emanating from this temperature range rapid cooling forms in the molded from the respective sheet metal part that the

ensuring high strength

martensitic structure. Favorably, it has the effect that the sheet metal parts heated to the stated temperature level can be shaped to complex-shaped components at relatively low forming forces. this applies

especially for such sheet metal parts, the

made of high strength steel and with a

Corrosion protection coating are provided.

A special difficulty is the hot forming of galvanized flat steel products to high- or high-strength steel components. Must have a provided with a metallic corrosion protection coating steel sheet for hot forming and, where appropriate or in combination with the hot working hardening be heated to a temperature which is above the melting temperature of the metal of Schut zbeschichtung, there is a risk of the so-called

"Liquid metal embrittlement". This embrittlement of the steel occurs when the molten metal of the

Coating in which penetrates during the deformation of the surface of the respective flat steel product notches. The liquid metal entering the steel substrate deposits there at the grain boundaries and

thus reduces the maximum absorbable train and

Compressive stresses.

Particularly critical is the danger of

Liquid metal embrittlement in flat steel products made of higher strength and high strength Mn containing steels. These steels have only a limited ductility and, as a result, tend during their deformation to form near-surface, near the grain boundary cracks.

From DE-OS 18 13 808 it is well known that the corrosion and oxidation resistance of a steel sheet can be improved by a nitriding, by a near-surface, 2.5 - 19 μπι thick surface layer with a relation to the core region of the steel sheet increased nitrogen Content is generated. The nitriding layer has good adhesion.

From DE 691 07 931 T2 it is further known that in a near-surface region of

low-carbon steels existing, for the construction of Automotive body certain higher carbon steel products C or N contents by a carburizing or

Nitriding treatment can be generated to the

To improve the machinability of the relevant flat steel products.

These measures are in the prior art not associated with higher or high strength steels having Mn contents of at least 0.4 wt .-%, with typical Mn contents of the present invention processed steels in the range of 0.4 - 0, 6 wt .-%, in particular 0.6 to 3.0 wt .-%, are.

The C content of the invention processed

 Flat-rolled steel is typically more than

 0.06 wt .-% and less than 0.8 wt .-%, in particular less than 0.45 wt .-%, on.

Examples of the steels processed according to the invention can be adjusted to their respective properties up to 0.2 wt .-% Ti, up to 0.005 wt .-% B, up to 0.5 wt% Cr, up to 0.1 wt. -% V or up to 0.03 wt .-% Nb included.

Nitrous oxide or internal nitriding requires the presence of diffusible nitrogen. This condition is met when the nitrogen is present in the statu nascendi.

Usually, the nitriding is carried out by annealing the respective flat steel product in an ammonia-containing H ₂ -N ₂ Glühgasatmosphäre. There are ammonia and Nitrogen as a nitrogen donor available.

Ammonia gas splits at atmospheric pressure and

Temperatures above 400 ° C while doubling its

Volume in nitrogen and hydrogen. The

Dissociation of ammonia gas can be described by the following reaction equation:

2NH ₃ -> 2 [N] + 3H ₂

Against the background of the prior art explained above, the object of the invention was to provide a method which minimizes it

Reduced risk of metal-induced crack formation in an economical manner allows to produce a high strength steel component.

This object is achieved by the fact that in the production of a high-strength steel component specified in claim 1 operations are completed.

Advantageous embodiments of the invention are set forth in the claims dependent on the respective independent claims and are explained below in detail as the general inventive idea. The method according to the invention for producing a metal with a corrosion protection coating

coated steel component, is based on the idea of performing a nitriding treatment on the flat steel product prior to its hot forming, in which W

Flat steel product is a finely structured surface layer is produced. On the one hand, this surface layer improves the forming properties of the surface-refined

 Steel product for hot forming.

On the other hand, the embroidered in accordance with the invention edge region of the flat steel product proves

 surprisingly helpful in avoiding

 Metal embrittlement of the steel sheet at the

 Hot forming. So the nitriding zone causes a

 significant increase in grain boundaries /

 Phase interfaces during the hot forming process, which the crack failure of the material as a result of penetrating into the structure of the steel substrate

 Metal material of the coating counteracts. Moreover, an unusually high iron diffusion sets in the coating. As a result, especially in the processing of zinc based coatings, the

 Coating thermally stable.

In order to utilize the above-summarized positive influences of the surface layer nitriding carried out according to the invention, the method according to the invention comprises the following working steps:

- It becomes a steel flat product made of a steel

 provided having an Mn content of at least 0.4 wt .-%. If here from one

 Flat steel product is the talk, then are with it

 in general steel sheets, bands, boards or

the same meant. Such a flat steel product may be in the hot or cold rolled state in processed according to the invention. It is also possible to make different steel blanks

 then assemble in accordance with the invention processed flat steel product, wherein one of

 Steel blanks of a steel according to claim 1

 specified type.

The flat steel product is heated in a continuous furnace under an annealing atmosphere containing up to 25% by volume of H ₂ ,

0.1 - 10 vol .-% NH3, H ₂ 0 and the remainder N ₂ as well as technically unavoidable impurities and contains a lying between -50 ° C and -5 ° C dew point. The holding temperature at which the

 Flat steel product is held for a 5 - 600 s lasting hold time, it is 400 - 1100 ° C. As a result, by this nitriding annealing treatment on the

 Flat steel product a 5 - 200 μκν thick, ductile nitride layer adjacent to its free surface

 present, whose grain size is finer than the grain size of the inner, covered by the surface layer, formed by the base material of the flat steel product core layer.

- After the generation of the nitriding layer is annealed in the above manner

 Flat steel product coated with a metallic protective layer. The invention makes this the

 To seize the knowledge that the danger of a

 Minimize liquid metal embrittlement by selectively modifying the near surface area of the flat steel product for the

Liquid metal embrittlement prone temperature range so it can be postponed that this does not coincide with the typical temperature range for hot forming.

- From the one with the metallic protective layer

 coated flat steel product become boards

 divided.

- if the forming takes place in two or more stages,

 At this point, the board can optionally be preformed. The preforming can go so far that after preforming the shape of the board corresponds approximately completely to the shape of the finished component. Typically, preforming occurs at cold or below the austenitizing temperature

 heated, half-warm board. In the case of single-stage forming by thermoforming alone, preforming is eliminated.

- For the hot forming, the board is on a

 780 - 950 ° C austenitizing temperature warmed through.

- Thereafter, the thermoforming of the durchberwäritvten board to the finished steel component takes place.

- The steel component obtained is then subjected to a cooling, starting from which

Austenitizing temperature is accelerated cooled. The cooling of the steel component is carried out such that forms in the flat steel product hardness structure. Hot forming and hardening can be carried out "in one step". In this case, the thermoforming and the curing are performed in one go together in a tool. On the other hand, in the two - stage process, the working steps "forming" and "production of the

Compensation or hardness structure "carried out separately.

Surprisingly, when using the

According to the invention predetermined annealing conditions, the

desired nitration depth even with very short

To achieve conditioning times. 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 use the method according to the invention in

continuous production processes

Incorporate, which require high belt speeds, as is the case for example in fire-coating plants, in which in continuous flow

Steel strips heat treated and with a

Anticorrosive coating are hot-dip coated.

Iron surfaces present in the reaction space catalyze the dissociation. Some of the nitrogen atoms liberated at the moment of decomposition can diffuse into the iron material.

Nitrogen transmission takes place in several substeps: • Transport to the workpiece surface Adsorption on the surface

 Penetration of the surface (absorption)

 Diffusion into the workpiece interior

Due to the increased nitrogen solubility in austenite, it is desirable to perform the annealing intercritically, i. in the two-phase region α / γ-Fe. Regardless of whether the subsequent coating with the metallic

Accordingly, the result of the nitriding treatment can be carried out under the usual practice in practice

 Conditions on particularly economic and

 be optimized in an environmentally sound way by complying with at least one of the following conditions:

- The H ₂ content of the annealing atmosphere is at most

 10 vol.%,

the NH ₃ content of the annealing atmosphere is at most 5% by volume,

- the dew point of the annealing atmosphere is -40 ° C to

 -15 ° C,

the holding temperature of the annealing is 680-840 ° C,

- The holding time of the annealing is 30 - 120 s.

Decisive for the success of the invention is that in the course of the annealing treatment according to the invention

Setting nitriding surface layer whose grain size is significantly finer than the grain size of the annealing in the course unembroidered core layer of the flat steel product. Practical tests have shown that according to DIN EN ISO 643, the particle size index of the nitriding layer to

at least 2 smaller than the particle size index of the base material (core layer) of the annealed

Flat steel product before warming and thermoforming of the board.

In the course of the process according to the invention, a nitrided boundary layer is deliberately adjusted. The thickness of these fine-structured, if necessary only

partially recrystallized, nitriding layer is determined by the determined according to DIN 50190-3 Nitrierhärtiefe. Hereinafter, the nitriding hardness depth is the distance from the surface to the point of the steel substrate where the hardness of the core hardness is + 50HV. In this way turns in the nitrided, near-surface

Surface layer of the flat steel product a hardness that is at least 25% higher than the hardness of

Core area, i. Hv (nitrided) / Hv (core area) 1.25.

Typically, one is according to the invention

processed flat steel product the thickness of

embroidered edge layer after the annealing treatment> 5 m and <200 pm.

A particularly advantageous embodiment of the invention is characterized in that the

Coating of the flat steel product with the metallic protective layer by a hot dip coating is carried out in a continuous on the annealing treatment - -

following completion of the course of work. In this case, the annealing treatment carried out according to the invention takes place simultaneously with the surface conditioning for the downstream surface finishing via a heterogeneous annealing gas-metal reaction.

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 the edge nitriding, surface conditioning and recrystallization of the base material and then the

Hot dip coating in a continuous

Procedure can be performed in-line following the annealing treatment. It is conceivable in principle, the traversed by the flat steel product

Oven line to flood over its entire length with NH ₃ -containing gas. However, in order not to expose all the components of the continuous furnace to the embroidering atmosphere, it may also be advantageous to divide one section of the furnace section from the other sections of the furnace and only this partitioned section containing the NH ₃ -containing

Atmosphere to apply.

In order to ensure optimum adhesion of the coating to the steel substrate in the case of a hot-dip coating of the annealed flat steel product, in particular as a fire-coating, it is possible to use before the

Fire coating carried out an oxidation of the surface of the flat steel product. In the course of the surface refinement, preferably carried out by hot-dip coating, of a

According to the invention produced flat steel product can be applied to the steel substrate per se known coating systems which on Zn, Al, Zn-Al, Zn-Mg, Zn-Ni, Zn-Fe, Al-Mg, Al-Si, Zn-Al-Mg or Zn-Al-Mg-Si are based. Subsequent to the hot-dip coating, further heat treatment steps can be carried out in order to emboss the metallic protective coating in a specific way. If necessary, continuous annealing after hot dip coating, e.g. a galvanic treatment.

Alternatively or in addition to in-line

In hot dipping refinement, a flat steel product on which a fine-structured nitriding layer has been formed in a continuous annealing according to the invention can be given a metallic, a metallic-inorganic or a metallic-organic coating by applying it electrolytically, 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.

In order to further optimize the mechanical properties, the annealing treatment according to the invention can be carried out in a conventional manner

 Connect overaging treatment.

Have thermoformed from a treated steel flat product according to the invention and then cured components Tensile strengths of 800 - 2000 MPa, in particular

900 - 2000 MPa, up.

The nitriding layer produced according to the invention makes it possible to easily heat the flat steel product according to the invention to an austenitizing temperature at which the flat steel product is largely complete

has austenitic structure. Even at such a high temperature is generated according to the invention

 Flat steel product even then the risk of embrittlement minimized when the flat steel product with a

Metallic coating is provided, whose

 Melting temperature is less than or equal to

 Heating temperature is. The fine graininess of the surface layer achieved by the nitriding according to the invention prevents cracking and thus ensures that no metal of the coating penetrates into the core region or

 Base material of the steel substrate can penetrate.

The inventive generation of a

 finely structured, nitrided nitrile layer is thus in the preferred directly, i. without prior

 Preforming the board performed hot working that of a metallic coating, in particular a zinc coating, which otherwise occurs as a result of diffusion of the coating metal on the grain boundaries

 Solid metal embrittlement prevented. Likewise, the procedure according to the invention as a result of the resulting from the nitriding, with respect to the

Fe / coating metal ratio advantageous Coating training the formation of solder cracks and thus counteracts liquid metal embrittlement.

The invention is based on

Embodiments explained in more detail. Show it:

1 shows a vertical section of a nitriding-annealed steel sample according to the invention;

FIG. 2 shows a vertical section of a non-annealed, hard-rolling comparative sample; FIG.

Fig. 3 GDOES depth profiles of the nitrogen content of the samples shown in Figs. 1 and 2;

FIG. 4 shows a vertical section of the tensile zone region of a steel component formed from the steel sample according to FIG. 1; FIG.

5 shows a vertical section of the tensile zone region of a steel component formed from the hard steel sample according to FIG. 2.

In order to test the effects achieved by the method according to the invention, hard-cold-rolled cold-rolled strip samples of a multi-phase steel "MP" and of a steel "WU" usually used for hot-forming were produced in each case. The compositions of steels MP and WU are given in Table 1.

Two samples made of the steels MP and WU are in a continuous furnace for surface layer nitriding Under the annealing treatment according to the invention. The annealing parameters used are given in Table 2.

For comparison, two more samples made of steels MP and WU in the continuous furnace are one

been subjected to conventional annealing, as they are usually used to prepare a

Hot-dip galvanizing is performed.

FIG. 1 shows the micrograph of the sample produced from the steel WU and heat-treated according to the invention. It can be clearly seen that as a result of the procedure according to the invention

Finely structured, near-surface microstructure

{Nitriding layer "N") has been set.

In contrast, the micrograph of the hard-rolled sample, also produced from steel WU, shows no such

Nitriding layer (Fig. 2).

At each consisting of the steel WU

In addition, GDOES measurements of the nitrogen content have been carried out in accordance with the invention annealed or hard-rolled samples. 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 Grafen, VDI-Verlag GmbH, Dusseldorf 1993

describe. The result of the GDOES measurements is shown in FIG. 3

summarized, wherein the dashed line the

Nitrogen distribution of the hard specimen and the

solid line the nitrogen distribution of the

According to the invention treated sample represents.

Also, Fig. 3 clearly shows that the sample treated according to the invention has a pronounced embroidered

Nitriding N, whose thickness is about 20 μπι.

On the basis of microhardness measurements, it could be demonstrated that the sample heat-treated according to the invention, produced from steel WU, was embroidered

Nitriding region N has a microhardness of 340 HV and the non-nitrided core region (base material) K has a hardness of 180 HV. The ratio Hv _N / Hv _K from hardness Hv _{N of} the embroidered nitriding layer N to hardness Hv _K of

Core area K was thus about 1.9 and thus significantly above the invention for this ratio

predetermined value of 1.25.

Following the annealing was a

 Surface refinement of the samples by zinc

 has been applied to the samples electrolytically with a layer thickness of 10 pm.

Subsequently, the samples consisting of the steel WU by means of the so-called single-stage or direct

 Hot forming process to a steel component formed and press hardened. The samples are at an austenitizing time of 6 minutes at a

Austenitizing temperature of 880 ° C durchgewarmt and then thermoformed in a hot press forming tool to a component for an automobile body.

After thermoforming, the components obtained have been cooled in a manner known per se so fast that hardened structure has formed.

The comparison of FIGS. 4 and 5 makes it clear that no cracking has occurred in the region of the tensile zone in the component produced in accordance with the invention, while in the component produced in a conventional manner distinct intergranular crack formation

determine is.

For the annealed, galvanized and deformed specimens produced from the steel MP, it was possible for the

According to the invention and the conventionally annealed samples comparable results can be detected.

The inventive method thus improves the forming properties of surface-refined

Flat steel products for hot forming. For this purpose, by a targeted gas-metal reaction during the annealing process before the surface finishing in a continuous process or piecemeal Randaufstickung generated as a result of which a finely structured, nitrogen-containing nitride layer N is established. This embroidered edge layer N increases the one

Fe diffusion into the coating and hinders the transport of the embrittle "coating metal", ie in particular zinc, to the grain boundaries during the annealing process performed prior to the hot working. As a result, components are obtained in which the steel substrate is largely completely free of cracks.

Steel C Mn P Si V Al Cr Ti B Nb

 [Wt. -%]

 MP 0, 22 1.7 0, 02 0.1 0, 002 1.7 0.06 0.1 0, 005 0, 001

WU 0, 22 1, 22 0.017 0.25 0, 005 0, 025 0, 13 0, 03 0, 005 0, 003

Remaining iron and unavoidable impurities

 Table 1

 Table 2

Claims

PATENT APPLICATIONS

1. A method for producing a metallic corrosion protection coating

 Steel component made of a flat steel product which has an n-content of at least 0.4% by weight, comprising the following working steps:

- providing the flat steel product;

Annealing of the flat steel product in one

 Continuous furnace

- Under an annealing atmosphere containing up to 25 vol .-% H ₂ , 0.1 - 10 vol .-% NH ₃ , H ₂ 0 and the balance N ₂ and technically unavoidable impurities and the one between -50 ° C. and -5 ° C dew point,

- at a holding temperature of 400 - 1100 ° C,

- for a holding time of 5 to 600 s,

- So that obtained after the annealing treatment

Flat steel product has a nitride layer (N), which is 5-200 μm thick and has a free surface, whose grain size is finer than that Grain size of the inner core layer (K) of the flat steel product covered by the surface layer;

Coating the annealed flat steel product with a metallic protective layer;

- dividing a board from the flat steel product;

- optional preforming of the board;

- heating the board to a austenitizing temperature of 780-950 ° C,

- Thermoforming of the heated board to the

 Steel component,

- Accelerated cooling of the steel component such that forms in the flat steel product hardness structure.

2. The method of claim 1, d a d u r c h

characterized in that the H ₂ content of the annealing atmosphere is at most 10% by volume.

3. The method according to any one of the preceding claims, characterized in that the NH ₃ content of the annealing atmosphere is at most 5 vol .-%.

4. Method according to one of the preceding claims, characterized in that the dew point of the annealing atmosphere is -40 ° C to -15 ° C.

5. Method according to one of the preceding claims, characterized in that the holding temperature of the annealing is 680-840 ° C, d.

6. Method according to one of the preceding claims, characterized in that the holding time of the annealing is 30-120 s.

7. Method according to one of the preceding claims, characterized in that the particle size index of the nitrided layer (N) of the annealed layer determined in accordance with DIN EN ISO 643

 Steel flat product before warming and

 Thermoforming of the board by at least 2 is smaller than the particle size index of the base material (K).

8. Method according to one of the preceding claims, characterized in that the coating of the flat steel product with the metallic protective layer by a

 Hot dip coating takes place in one

 continuously following the annealing treatment

completed workflow is completed.

9. The method of claim 8, d a d u r c h

 e n c e s t e s, e n s before the

 Hot dip coating an oxidation of the surface of the flat steel product is performed.

10. The method according to claim 8 or 9, d a d u r c h

 g e n c i n e s, the s

 Flat steel product continuously after

 Hot dip coating is diffusion annealed.

11. The method according to any one of claims 1 to 7,

 The coating of the flat steel product with the metallic, metallic-organic or metallic-inorganic protective layer by electrolytic coating or PVD or CVD deposition is carried out.

12. The method according to any one of the preceding claims, characterized in that the metallic protective layer is a Zn, an AI, a Zn-Al, a Zn-Mg, a Zn-Ni, an AI-Mg, an Al Si, a Zn-Al-Mg or a Zn-Al-Mg-Si coating is.

13. Method according to one of the preceding claims, characterized in that the heating is set during the heating

Austenitizing temperature 860 - 950 ° C is.

14. Method according to one of the preceding claims, characterized in that the thermoforming and the cooling of the component obtained by the thermoforming in one go

 be performed.

15. Method according to one of the preceding claims, characterized in that the obtained component of a blast treatment

 is subjected.