DE102008027460B9 - Method for producing a sheet steel component with regions of different ductility - Google Patents

Abstract

A method for producing a hardened sheet steel component having regions of different ductility, wherein the behavior of the steel sheet is changed during heating so that the heat absorption capacity of the steel sheet is influenced during the curing for curing depending on the desired degree of hardness, for high degrees of hardness a good heat absorption behavior and less hard A reduced heat absorption behavior can be realized areas and thereby the structure over the surface of the component or over the surface of the board is made variable, the setting of the microstructure and the adjustment of the heat absorption behavior is controlled by the surface emissivity.

Description

The invention relates to a method for producing a sheet steel component with regions of different ductility.

In the automotive industry, there are still great efforts to reduce fuel consumption and associated emissions. In this case, the desire of the automotive industry in particular to reduce the weight of the vehicle with new materials is at the forefront. In principle, two ways can be taken. On the one hand, here an objective can be the production of steel alloys with a lower specific weight, which otherwise maintain the previous favorable properties. On the other hand, a reduction in weight can be achieved by reducing the component cross section through the use of high and higher strength steel materials.

It is also known to produce thermoformed components for example, structural components such as B-pillars, struts and side members with distributed over the mold component uniform properties. This is done by a complete heating of the output board or the mold component with a subsequent hot forming step or curing. In various applications of motor vehicle technology, molded components are intended to have high strength over certain regions, and in turn to have higher ductility in relation to other regions. In this case, it is already known to treat a component via heat treatments in such a way that it locally has regions of higher strength or higher ductility.

That's how it shows DE 197 43 802 C2 a method to produce a molded component with areas of different ductility by an output board is only partially heated before or after pressing or deliberately reheated in a previous homogeneous heating in the areas with desired higher ductility. Preferably, the partial heating is done inductively.

The DE 200 14 361 U1 describes a B-pillar which also has areas of different strength. The preparation of the B-pillar is carried out in the thermoforming process, starting from a molding board or a preformed longitudinal profile of this is austenitized in the oven and then converted / cured in a cooled tool. In the oven, large areas of the workpiece can be isolated against the effect of temperature, in these areas the austenitizing temperature is not reached and thus does not adjust in the tool during curing no martensitic structure.

The DE 102 56 621 B3 describes a method in which an output board or a preformed component during transport through a continuous furnace simultaneously passes through at least two adjacent to each other in the direction of passage zones of the continuous furnace with different temperature levels. A zone 1 of the continuous furnace is set to a temperature A and another zone 2 to a temperature B which is higher than temperature A. As a result, the semi-finished product is heated in the areas where it passes through the continuous furnace in zone 1 to temperature A. and in the areas in which it passes through zone 2 to temperature B. Subsequently, the thus differently heated semi-finished product is subjected to a thermoforming process and / or hardening process, whereby in the previously heated to temperature A region 1 of the component in relation to the adjusted to temperature B heated area 2 of the component ductile structure.

The DE 102 08 216 C1 describes a method in which an output board or a preformed component is brought to Austenitisierungstemperatur in a continuous furnace and areas of the first type of the board or the preformed component, which are ductile in the later end component, active from a certain cooling start temperature z. B. quenched with compressed air, wherein the active quenching is stopped when a predetermined stop temperature is reached. Subsequently, these areas are held approximately isothermally for the transformation of the austenite into a structure with a high ferrite and / or pearlite content. Meanwhile, in areas of the second kind, which have relatively lower ductility in the final component, a hardening temperature is reached which is at least high enough for sufficient martensite formation to take place in these areas during a hardening process. While in the areas of the first kind an isothermal transformation takes place, the areas of the second kind are predominantly or entirely kept in the austenite area. An excessive temperature drop can be counteracted here, for example, with a heating device or with a reflecting mirror.

Some of these methods have some problems in their implementation or require technical aids such as insulating materials, inductive heating devices or active cooling for the successful implementation in order to set the desired different microstructure ranges in the finished component. Isolation by encapsulation in the oven is technically complex because in each cycle every single part needs its own insulation. Furthermore, an oven with different temperature zones in the direction of flow, especially for preformed components can be technically difficult to realize.

The object of the invention is to provide a method for producing a sheet steel component with which areas of different ductility can be produced simply and inexpensively.

The object is achieved with the features of claim 1.

Advantageous developments are characterized in the subclaims.

In the method according to the invention, a steel sheet component with regions of different ductility is produced by deliberately changing the behavior of the steel sheet during heating or cooling in desired surface areas. As is known, steel changes its crystalline structure during heating or cooling, so that different phases, eg, heating, and / or cooling, depending on the temperature-time profile. As ferrite or perlite or austenite. In known manner, this phase formation is used to influence certain properties of a product made from these steels. An example of this is the hardening, in particular the hardening of boron-manganese steels.

In known processes, for example, sheet metal components are produced from such steels, heated to a temperature at which austenite forms in the microstructure, or even a complete transformation into austenite is achieved and then cooled rapidly, so that a hardening structure is formed which allows strengths to be achieved to achieve about 1,500 MPa. In the case of incomplete austenitization, i. H. lower temperatures and / or reduced holding times, the microstructure can be homogeneously brought only to a certain part of the absolutely achievable hardness. Usually, these methods are carried out either on the already formed sheet metal component or a metal sheet is heated to the desired temperature, then hot formed and cooled after hot reduction so quickly that the desired structure is achieved.

In the method according to the invention, the microstructure over the surface of the component or over the surface of the board is made variable, d. H. When heating in the continuous furnace and / or cooling of the semifinished product during transfer from the continuous furnace into the press, areas having different microstructures and thus different ductilities or hardnesses are produced. According to the invention, this is achieved in that the sheet metal plate or the sheet steel component is formed with different surfaces.

Different surfaces in the sense of the invention means that surfaces are present which absorb or reflect heat differently during heating or emit heat differently when cooled. This property is called emissivity. By adjusting the strength of the emissivity can be controlled according to the invention very sensitively the heat absorption and heat dissipation behavior of the corresponding area and thus the corresponding structure both during heating and during cooling.

An inventive approach for influencing the emissivity is the use of a metallic coating on the steel component or the output board and in particular a metallic coating with zinc or based on zinc. As is known, coatings of zinc or of zinc on surfaces of steel components during heating in the border region between steel zinc alloy layers, depending on the height and duration of application of the temperature and a superalloy can be achieved. It has been found that coatings with different compositions based on zinc and iron and different zinc layer thicknesses lead to different surface absorbencies or degrees of absorption. This is due to the different degrees of alloying reaction of the zinc with the underlying iron, which results in different levels of formation of surface oxides, in particular zinc oxide compounds. Zinc oxide compounds typically exhibit a slightly greenish to dark brown coloration in the optically visible range, and accordingly the surface emissivity increases. For example, higher zinc deposits and / or thick Fe-Al inhibitor layers result in low surface emissivities during heat treatment, while low zinc deposits and / or thin Fe-Al inhibitor layers lead to high surface emissivities. An inhibiting layer is a layer which occurs as a result of aluminum addition in the zinc bath between the steel substrate and the zinc layer during the continuous hot-dip coating and, if appropriate, subsequent heat treatment.

However, according to the invention, the surface emissivity can also be carried out by additional inorganic and organic coatings on a zinc layer and / or aluminum-silicon layer - so-called fire-aluminized layers - and / or the bare metal or by roughening the surface.

According to the invention, the surface emissivity is adjusted via the starting zinc layer thickness in the hot-dip coating or an electrolytic galvanizing of the steel strip. The starting zinc layer thickness can be adjusted by means of variable adjustment of the stripping pressure or additional electromagnetic fields in hot dip galvanizing. For electrolytic Coatings, the layer thickness to be applied is controlled by the strength of the electrolytically active current.

This adjustment of the layer thickness already during the coating of the strip is carried out in such a way that the corresponding activation, for example the stripping pressure or the parameters influencing the electrolysis, is achieved via previously known cross-sectional images of boards to be produced from a strip and thus also the regions of different emissivity determined therewith , is determined. Thus, a corresponding steel strip is already formed during the coating position and exact position with different coating thicknesses, which result in the areas of different emissivity after cutting the board and optionally the forming at a temperature treatment.

The steel strip produced in this way is subsequently platinum-plated in such a way that the board from which the component is later produced by direct or indirect hot forming has at least two regions of different initial zinc layer thickness, the more ductile microstructure in the finished component taking advantage of the surface emissivity for structural adjustment when heating in the continuous furnace in the range of high initial zinc coating or taking advantage of surface emissivity for microstructure adjustment during cooling in the time of transfer of the board or the preformed component from the furnace to the press in the range of lower starting zinc coating comes to rest.

The surface treatment of the steel strip to produce different surface emissivities before and / or during the furnace transit time can also be done in different ways.

This can according to the invention by a matting treatment, a skin pass, d. H. a micro-contouring of the surface, or an additional inorganic and / or organic coating take place.

According to the invention, additional inorganic and / or organic coatings can be, for example, phosphate-containing substances which can be applied to the steel strip but also to the blank or the preformed component, for example by dipping, spraying or vapor deposition.

It is thus possible to form the semifinished product areas, which are to have a high surface emissivity in the final state, in order to allow a rapid achievement of the austenitizing temperature on the one hand when heating in the continuous furnace or, on the other hand, a rapid cooling after removal of the furnace and thus the formation of a mixed structure. even before the actual hardening or cooling process starts. The semi-finished regions can be given a matted, low-reflection or dressed surface or be provided with a temporary dark protective layer or, for example, with a metal oxide and / or metal nitride surface, which enables particularly good absorption or release of heat radiation.

In the invention, it is advantageous that it is possible to form curable steels, which must be subjected to a heat treatment for curing, with areas of different ductility, this training different ductilities inline on the belt, which is a significant cost savings in addition but also high process reliability guaranteed. In addition, it is advantageous that a harmonious gradual transition in hardness is achieved, in contrast to taylored welded blanks consisting of several sheets of different hardness properties, which have abrupt transitions.

Furthermore, the semifinished product area, which is to have a hardened, martensitic structure in the end component, by an infrared-reflective element and / or an additional heating element, for example in the form of a steel plate or molded part as an integral part of the semi-finished carrier system in the passage after removal from the oven be hindered by too rapid cooling, since in this case a free cooling in the ambient air is prevented.

It is further possible according to the invention to use the differently present or differently formed in the continuous furnace surface emissivities insofar that in a zone continuous furnace, for example, only in the last separate zone in a semi-finished part with high surface emissivity the austenitizing temperature is exceeded, while in other semi-finished products with significantly lower Surface austenitization the Austenitisierungstemperatur is not reached at kiln removal and thus arise in the subsequent direct and / or indirect hot forming again areas of different ductility.

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

1 : the temperature-time profile of a 1 mm thick board with different initial zinc coating in a 900 ° C radiation furnace;

2 : Areas of different emissivity of a board after furnace removal with low and high initial zinc deposit;

3 : the temperature-time profile of a 1 mm thick board with a homogeneous, high initial zinc coating, wherein a region to produce a different emissivity with a zinc phosphate layer is coated in a radiation oven at 900 ° C;

4 : Areas of different emissivity of a board before and after heating with a zinc phosphate covered area 1b and a low initial zinc pad area 2 B ;

5 : the temperature-time profile of a 1 mm thick board with different initial zinc coating after removal from the radiation furnace at 900 ° C;

6 : the temperature-time profile of a 1 mm thick board with a homogeneous, low initial zinc coating, wherein a region for generating a different emissivity with a boron nitride base solution is coated after removal from a radiation furnace at 900 ° C;

7 : Areas of different emissivity of a board after furnace removal with low initial zinc deposit 1c and an area with an additional boron nitride base coating 2c ;

1 shows the temperature-time course of a 1 mm thick board ( 2 ) with different zinc coating in a 900 ° C radiation oven. While the area 1a For example, if the board coated with a Z140 g / m ² hot dip coating rapidly heats up to austenitizing temperature due to the formation of surface oxides, the temperature of the other area increases 2a The board, which is coated with a hot-dip coating of Z275 g / m ² , comparatively slowly and is - while the other area is already fully austenitic - still below the Austenitisierungstemperatur. Only after a longer residence time in the oven, it comes to an equalization of the two temperatures. If the board so heated between two water-cooled steel plates hardened, even before the area 2a reaches the austenitizing temperature, two different structural areas are established. While area 1a becomes full-hearted, shows in the area 2a a mixed structure of preferably ferrite, pearlite and bainite with typical mechanical properties of R _p0.2 > 300-400 MPa, R _m > 500-700 MPa and A> 16%.

3 shows the temperature-time course of a 1 mm thick board ( 4 ) with a homogeneous zinc coating of Z275 g / m ² - the subregion 1b covered with a zinc phosphate layer - in a 900 ° C radiation oven. While the area 1b The board, which is coated with a Z275 g / m ² hot dip coating and a zinc phosphate layer, rapidly heats up to austenitizing temperature due to the formation of surface oxides and the optically darker phosphate-containing layer, raises the temperature of the area 2 B The board, which is coated with a hot-dip coating of Z275 g / m ² , comparatively slowly and is - while the other area is already fully austenitic - still below the Austenitisierungstemperatur. Only after a longer residence time in the oven, it comes to an equalization of the two temperatures. If the board so heated between two water-cooled steel plates hardened, even before the area 2 B reaches a temperature of about 820 ° C, set up two different structural areas. While area 1b becomes full-hearted, shows in the area 2 B a mixed structure of preferably ferrite, pearlite and bainite with typical mechanical properties of R _p0.2 > 300-400 MPa, R _m > 500-700 MPa and A> 16%.

5 shows the temperature-time course of a 1 mm thick board ( 2 ) with different zinc coating after removal from a radiation oven at 900 ° C, the entire board was heated to approximately furnace temperature, that is fully austenitic. While the area 1a The board coated with a Z140 g / m ² hot dip coating and rapidly cooling due to the surface oxides formed, ie with high surface emissivity, on the ambient air, decreases the temperature of the other area 2a the board, which is coated with a hot-dip coating of Z275 g / m ² , comparatively slowly from. Is the board between water-cooled steel plates so hardened that the area 1a has passed through the temperature range between about 700 ° C and about 600 ° C to form a mixed structure and that the range 2a a temperature of about 700 ° C has not fallen below, set up two different structural areas. While area 2a Vollmartensitisch becomes evident in the area 1a a mixed structure of preferably ferrite, pearlite and bainite with typical mechanical properties of R _p0.2 > 300-400 MPa, R _m > 500-700 MPa and A> 16%.

6 shows the temperature-time profile of a 2 mm thick board ( 7 ) with a homogeneous Zinc deposit of Z140 g / m ² - the subregion 2c covered with a boron nitride layer - after removal from a radiation oven at 900 ° C, the entire board was heated to approximately furnace temperature, that is fully austenitic. While the area 1c The board, which is only coated with a Z140 g / m ² hot dip coating, and rapidly cools due to the surface oxides formed, ie with a high surface emissivity, on the ambient air, decreases the temperature of the other area 2c the board, which is coated with boron nitride and leads to a comparatively low surface emissivity after furnace removal, relative to area 1c slowly. Is the board between water-cooled steel plates so hardened that the area 1c has passed through the temperature range between about 700 ° C and about 600 ° C to form a mixed structure and that the range 2c a temperature of about 700 ° C has not fallen below, set up two different structural areas. While area 2c Vollmartensitisch becomes evident in the area 1c a mixed structure of preferably ferrite, pearlite and bainite with typical mechanical properties of R _p0.2 > 300-400 MPa, R _m > 500-700 MPa and A> 16%.

The invention is not limited to hardenable steels, for example of the type 22MnB5. The method described is such. B. applicable to dual-phase steels, complex phase steels but also TRIP (Transformation Induced Plasticity) or TWIP steels (Twinning Induced Plasticity).

As shown, it is possible with the invention to make the structure over the surface of a component or over the surface of a board variable, it being possible to produce during heating and cooling, areas with different microstructures and thus different ductility or hardness.

In this case, the inventive method is particularly favorable, because it is possible, the surface of the steel strip inline, d. H. in a running tape process. With the steel strips, it is possible to determine from the beginning where the areas of different ductility should be positioned. Accordingly, with variable wiping nozzles or electric fields in these areas, the zinc coating can be adjusted as desired. In the further process, the corresponding cuts and punches can then be made.

Alternatively or in addition, the predetermined regions of different ductility with coatings, roughenings, etc. can also be formed two-dimensionally over the output board or over the preformed component. This procedure is particularly suitable for the selective local coating and thus adjusting the surface emissivity, which can be adjusted locally on the finished component, the ductility as required. One possibility is, for example, the additional inorganic and / or organic coatings by selective brushing or rolling, d. H. a transfer via a rotating roller to apply to the metal surface. The application may also be by selective spraying, spraying or printing by means of a suitable spraying, spraying or printing device (for example by screen printing) or a suitable printer system or in another known manner.

By means of the described procedure, desired ductile and less ductile regions of virtually any shape can be applied very simply, quickly and precisely with simple coating systems at low cost. The described methods furthermore enable a high degree of flexibility and are particularly suitable for three-dimensional preformed steel components with a wide variety of geometries. Thus, in a simple manner, three-dimensional stampings can be selectively provided with coatings for targeted adjustment of surface emissivity.

Since these processes should not take place only with the finished board or the component to be formed, but already at a very early stage, the effort is minimized and thus the costs are reduced.

Claims (10)

A method for producing a hardened sheet steel component having regions of different ductility, wherein the behavior of the steel sheet is changed during heating so that the heat absorption capacity of the steel sheet is influenced during the curing for curing depending on the desired degree of hardness, for high degrees of hardness a good heat absorption behavior and less hard A reduced heat absorption behavior can be realized areas and thereby the structure over the surface of the component or over the surface of the board is made variable, the setting of the microstructure and the adjustment of the heat absorption behavior is controlled by the surface emissivity. A method for producing a hardened sheet steel component with regions of different ductility, wherein the behavior of the steel sheet is changed during cooling such that the heat transfer capacity of the steel sheet is influenced during transfer to subsequent Abhühleinrichtungen and / or cooling, depending on the desired degree of hardness, for high degrees of hardness a reduced heat dissipation behavior and for less hard areas a good heat transfer behavior can be realized and thereby the structure over the surface of the component or over the surface of the board is made variable, the setting of the structure and the adjustment of the heat dissipation behavior is controlled by the surface emissivity. A method according to claim 1 or 2, characterized in that the surface emissivity of the steel sheet is realized by metallic supports, wherein the metallic supports are applied with different thicknesses on the steel sheet, so that during heating depending on the thickness of the metallic supports and / or by the Reaction of the overlay with the steel sheet surfaces with varying degrees of emissivity are created. Method according to one of claims 1 to 3, characterized in that the surface emissivity are formed by coatings on metallic and / or inorganic and / or organic base on the steel sheet and / or a first metallic layer on the steel sheet. Method according to one of the preceding claims, characterized in that the surface emissivity is brought about by mechanical treatments of the surface, the mechanical treatment being a roughening, a skin-piercing step, a jet step with particles, a grinding or polishing step. Method according to one of the preceding claims, characterized in that the areas of the sheet steel component, which are to have a high surface emissivity in the final state, are formed highly absorbent, to achieve a rapid reaching the Austenitisierungstemperatur necessary for curing or on the other hand a rapid cooling when heated in the oven after furnace removal and thus to allow the formation of a mixed structure. Method according to one of the preceding claims, characterized in that the metallic coating is a zinc layer, zinc alloy layer, a zinc-aluminum layer, an aluminum-zinc layer or an aluminum-silicon layer or another aluminum alloy layer. Method according to one of the preceding claims, characterized in that to achieve different ductility the steel strip has at least two regions of different starting metal layer thicknesses, wherein the more ductile in the finished component structure area in exploiting the surface emissivity for microstructure adjustment during heating in the furnace in the region of high output metal overlay or when exploiting the surface emissivity for microstructure adjustment upon cooling in the time of transfer of the board or preformed component from the furnace to the press is in the range of the lower initial metal coverage. Method according to one of the preceding claims, characterized in that the areas of high surface emissivity are particularly highly absorbent, wherein in addition to metal layers these areas receive a matted, poorly reflective or dressed surface or with a temporary absorbent layer, for example with a metal oxide surface and / or Metallnitidoberfläche are provided. Method according to one of claims 1 to 8, characterized in that the areas of low surface emissivity are particularly highly reflective, wherein in addition to metal layers, these areas are provided with a temporary reflective layer, for example with a metal oxide and / or metal nitride surface.

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