EP2240622A1

EP2240622A1 - Method for producing a component from a steel product provided with an al-si coating and intermediate product of such a method

Info

Publication number: EP2240622A1
Application number: EP09705444A
Authority: EP
Inventors: Friedhelm Macherey; Franz-Josef Lenze; Michael Peters; Manuela Ruthenberg; Sascha Sikora
Original assignee: ThyssenKrupp Steel Europe AG
Current assignee: ThyssenKrupp Steel Europe AG
Priority date: 2008-01-30
Filing date: 2009-01-29
Publication date: 2010-10-20
Anticipated expiration: 2029-01-29
Also published as: ATE520798T1; US20110056594A1; WO2009095427A1; JP5666313B2; KR20100108608A; PL2240622T3; DE102008006771B3; JP2011514440A; CA2713381A1; ES2368820T3; CN101932747B; EP2240622B1; MX2010008390A; PT2240622E; CA2713381C; US8349098B2; CN101932747A; KR101539077B1

Abstract

The present invention relates to a method for producing a component from a steel product coated with an Al-Si protective coating, and to an intermediate product, which is generated in the course of such a method and can be used for the production of components of the type in question. In the course of the method, the steel product coated with the Al-Si coating is subjected to a first heating step, wherein the temperature and the duration of the heat treatment are set such that the Al-Si coating is only partially pre-alloyed with Fe of the steel product, in a second heating step the steel product is heated to a heating temperature above the Ac1 temperature, the steel product at said heating temperature having an at least partially austenitic structure, wherein the temperature and duration of the second heating step are set such that the Al-Si coating in the course of the second heating step is completely alloyed with Fe of the steel product, the steel product heated to the heating temperature is shaped into the component, and the resulting component is cooled in a controlled manner in order to produce a hardness structure.

Description

A method of manufacturing a component from a steel product provided with an Al-Si coating and

Intermediate of such a process

The invention relates to a method for producing a component from a steel product coated with an Al-Si protective coating. Moreover, the invention also relates to an intermediate product which arises in the course of such a process and can be used for the production of components of the type in question.

Steel products of the type in question are typically steel bands or sheets, which are provided in a manner known per se, for example by fire aluminizing, with an Al-Si coating. However, it can also be preformed semifinished products, which are preformed, for example, from sheet metal and then molded to the respective component.

Due to the Al-Si coating, the component formed from the respective steel product is protected against corrosion during its practical use. The corrosion protection effect, in particular the protection against scaling, however, already provides the Al-Si protective coating immediately after the coating of the steel substrate and keeps it in the course of the forming process at. This applies in particular when the shaping is carried out as so-called "press-hardening".

In press-hardening, the starting material to be formed is brought to a temperature at which a at least partially austenitic microstructure is present, and deformed in the warm state before molding. Either already during the thermoforming process or immediately thereafter, the resulting component is then accelerated cooled to form Hartegefuge. As a starting material for the press-hardening flat products, such as sheet metal blanks, or already pre-or final molded semi-finished products can be used.

During press-hardening, the Al-Si coating prevents tinder from forming on the steel product, which would severely hamper the forming process. In this way, it is possible to form components from high-strength, treatable steel, which are exposed in practice particularly high loads.

A steel good typically used for this purpose is known in the art as "22MnB5". From steel goods of this type, for example, body parts of motor vehicles are manufactured, which must have a high strength with low flat product thickness and, consequently, comparatively low weight. Likewise, however, other steel good, such as deep-drawing steels known under the trade name "DX55D", according to DIN EN 10327 composite type, and microalloyed steels alloyed according to DIN EN 10292, under the Designation "HX300 / 340 LAD", commercially available type. It is also possible to use the starting products, which are composed in the manner of tailored blanks / patchwork blanks of several sheets.

In order for the Al-Si coating to adhere to the steel substrate so strongly that it does not crack or flake off during forming, it is necessary to subject the Al-Si coated steel product to a heat treatment prior to forming, in which iron is precipitated the steel substrate is alloyed into the Al-Si coating. The aim is to pass through the coating over its entire thickness to ensure that there are no breaks or chips in the upper layers of the coating, which are adjacent to the free outer surface of the coated flat product. The type or degree of alloying of Al-Si coatings also has an influence on the weldability and paintability of the components produced by press-hardening.

A method of the type described above is described in EP 1 380 666 A1. In this method, a coated with an Al-Si coating steel sheet is first heated to 900 ⁰ C to 950 ⁰ C over a period of 2 to 8 minutes. Subsequently, the coated steel sheet to 700 - 800 ⁰ C cooled draw forming temperature hot forged at this temperature. Subsequently, the shaped steel member is cooled rapidly to a temperature below 300 ⁰ C to produce a martensitic Gefuge in the obtained steel part. The heat treatment of the coating provided steel substrate is carried out so that by diffusion of the iron from the steel substrate after the heat treatment, the iron content in the coating in a range between 80% and 95%. In this way, a thermoformed component is to be obtained in which a good weldability and good formability combined with a high corrosion protection.

A problem in carrying out the heat treatment required for the alloying out is that in addition to the setting of a sufficient heating temperature, a certain amount of oven resting time must be maintained. The duration over which the respective steel product must be kept in the oven results from the heating rate of the substrate and the required alloying of the substrate with the Al-Si layer. The prior art is a bake time of five to 14 minutes.

In practice, heating of the steel products provided with Al-Si-Uberzugen before the hot working carried out radiation furnace are used. In this context, a fundamental investigation into the heating behavior of steel products provided with Al-Si coatings has revealed that in such furnaces the reflection of heat radiation at the surface of the respective coating leads to a reduced heating rate in comparison to uncoated or organically or inorganically coated materials. As a result, a relatively long period of time for the warming must be accepted. This long period of time leads to long process times in the processor of the aluminum-Si coated products, which not only extends the cycle times during component production, but also increases the expenditure on equipment for the furnace required for the heating.

Technically, it would also be possible to heat the steel base material of the flat products faster with its coating by inductive or conductive heating. Also, the heating could be accelerated by forced convection of heat radiation. In the case of accelerated heating, however, there is a risk that the alloying process in the Al-Si coating layer proceeds more slowly than the heating rate, with the result that the Al-Si layer is not completely or incorrectly alloyed. In extreme cases, it may even happen that the Al-Si layer flows away from the steel product.

From DE 10 2004 007 071 B4 an attempt is known to shorten the process time in the processor of the provided with an Al-Si coating flat products characterized in that the alloying of the coating and the heating of the flat steel product is carried out to heating temperature in two separate steps. This procedure makes it possible to perform the alloying process with the manufacturer of the Al-Si coated steel flat product. In the processor, the heating of the steel flat product provided with the then already alloyed coating can be heated, for example by means of induction or conduction optαmal done a short time, without having to take into account the formation of the coating. Accordingly, when using the known method, it is basically possible to store flat steel products, which are already provided with a completely alloyed coating by the manufacturer, in an intermediate store, from which they can then be retrieved in case of need for further processing by the processor at short notice.

However, the above-described proposal proves to be problematic in that the fully alloyed coating itself is exposed to a corrosive attack both during storage of the pre-produced flat steel products in the intermediate storage facility and in the course of the processing steps performed by the processor. This problem arises from the iron content present on the free surface of the galled coating. In order to suppress such a surface corrosion, extensive protective measures are required, which consume the achieved with the decoupling of alloy and Pressharten advantages for the most part again. In addition, it may be difficult to cut the flat product boards coated with the galled-over coating, which may be necessary before hot-forming, because Al-Si layers alloyed with aluminum are hard and brittle. Against the background of the prior art explained above, the object of the invention was to provide a method which enables shortened processing times in the case of processors of Al-Si-coated steel products without the risk of corrosive attacks or disadvantages in a subsequent cutting of the coated flat products must be accepted.

According to the invention, this object has been achieved by the method specified in claim 1. Advantageous embodiments of this method are given in the claims based on claim 1.

The steel product processed according to the invention can be a flat steel product, such as a steel sheet or steel strip, or a semifinished product preformed, for example, from a steel sheet, which is completely deformed during the hot pressing hardening carried out according to the invention. Also, in accordance with the invention, it is possible to process a plurality of blanks composed in the manner of tailored blanks / patchwork blanks.

In the method according to the invention, a two-stage heat treatment also takes place, it also being in accordance with the state of the art in the first heating stage for alloying iron from the steel substrate into the Al-Si coating.

In contrast to the prior art, however, this first alloying step is carried out by setting a suitable temperature and duration of treatment so that the Al-Si coating after the first heating step is only partially alloyed with iron of the steel product. Subsequently, the steel product provided with the imperfectly alloyed coating according to the invention can be cooled to room temperature and stored until it is supplied to the respective component for further processing. Since the Al-Si coating is only partially alloyed in the first heating step, the Al-Si coating has low susceptibility to corrosion even after the first heating step, so that its storage, transport and other work steps carried out in advance of the second heat treatment are problem-free can be carried out without the need for additional measures.

At the same time, according to the invention, in the course of the first heating step, only partially by a covered coating maintains a toughness, which makes it possible, even after the first heating step, to cut or cut the flat products obtained with simple cutting operations, without causing lasting damage Cover layer comes.

Prior to its transformation to the component, the flat product obtained after the first heating step and provided with a pre-alloyed coating according to the invention undergoes a second heating step. This second heating step is usually carried out at the final processor, while the first heat treatment step to be completed will usually be at the producer of the steel products. The second heating step is usually completed immediately before the hot forming. In the course of the second heating stage, the steel product provided in the manner according to the invention with only a prealloyed Al-Si coating is heated to the heating temperature required for the subsequent curing, which is above the Acl-teroperatur at which the steel product has an at least partially frosted structure. If necessary, it is also possible to set a heating temperature which corresponds to or lies above at least the Ac3 temperature in order to set a largely complete austenitic structure in the starting product to be shaped.

The temperature and duration of the second heating step are erfmdungsgemaß adjusted so that the Al-Si coating is completely alloyed in the course of the second heating step with Fe of the steel product.

Surprisingly, it has been found in this connection that the coating according to the invention which is only partially alloyed with the steel substrate has a degree of reflection which, compared to the heating of flat products provided with completely alloyed Al-Si-Fe coatings, has a considerably higher heating rate when heated in a radiant oven

Warming allowed without running off the coating.

An interim product obtained in the manner according to the invention is thus characterized in that it has only one with the iron of the steel substrate incompletely pre-alloyed Al-Si coating is provided.

After the second heating step, the starting material, which is now provided with a fully alloyed Si-Al-Fe coating, is then converted in a manner known per se into a suitable thermoforming tool to form the desired component. The resulting component may be a finished-molded component or a semi-finished product which is subsequently subjected to further deformation steps.

Already during the hot forming or immediately afterwards, the thermoformed component is finally cooled in a controlled manner in order to produce hard joints in the steel substrate. The working steps "thermoforming" and "cooling" can be carried out in particular in the manner known from "compression molding".

The procedure according to the invention thus makes it possible, in a cost-effective and at the same time particularly efficient manner, to provide an aluminized component produced by press molding within shorter processing times. It is not the cost of the usually carried out at the producer of the steel product heating step due to the fact that the process time and the

Treatment temperature for the partial Al alloy of the Al-Si layer with the iron of the steel substrate is shortened compared to the prior art, but also the usually at the processor of the inventive incomplete alloyed alloy Si coating carried out second heating step in a shorter process time can run at a correspondingly reduced energy consumption and minimal equipment.

The fact that a lower Fe content is present after the first heating step carried out according to the invention in the Al-Si layer than in the component obtained after hot-pressing hardening, in which there is only a minimal risk of corrosion, allows the steel product to be interposed between the first and second the second heating step to cool to room temperature and store before it is then sent for further processing. The anticorrosion effect of the present after the first heating step, only partially alloyed Al-Si layer is so large that the steel product between the first and the second heating step easily transported in air, for example between the plant of the producer of the steel product and the factory of the final processor leaves.

Practical experiments have shown that the temperature of the first heating is at least 500 ⁰ C, but at the same time at most equal to the A _c i temperature of the steel product. In practice, therefore, suitable temperatures for the first heating step, in particular in the range of 550 - 723 ⁰ C, in particular 550 - 700 ⁰ C. By heating in this temperature range, the mechanical technological characteristics of the steel product are not deteriorates and the Grunαgefαge remains intact in its components.

The time to be planned for the first heating step at these heating temperatures amounts to Al-Si coating thicknesses in the initial state of 10 to 30 μm (corresponding to 80 to 150 g / m ² ) for 4 to 24 hours when heated in a hood furnace. It is also conceivable to heat in a continuous furnace or chamber furnace, the heating times in each case be less than one hour.

In each case, the temperature and duration of the first treatment step are in each case adjusted such that the Al-Si coating, measured starting from the steel substrate, is alloyed with Fe over at least 50%, in particular 70-99%, preferably 90-99%, of its thickness ,

Depending on the furnace technology available from the manufacturer of the steel product, the first heating step may be carried out in a hood annealing furnace, a chamber furnace or a continuous annealing furnace. In the case of processing a flat steel product, it is possible to obtain a master alloy in a continuous furnace, which is arranged directly inline at the outlet of a coating plant, similar to a plant for Galvannealmg the case and the heating in a temperature range between 600 and 723 ⁰ C takes place. Likewise, the obtained according to the invention, provided with a partially alloyed Al-Si coating steel product in the second heating step in a continuous furnace to the required heating temperature to be heated. In this case, the second heating can be effected inductively, conductively or by means of thermal radiation.

The invention will be explained in more detail with reference to an exemplary embodiment.

Samples of a 1.5 mm thick steel sheet were examined, which besides iron and unavoidable impurities (in wt%) C: 0.226%, Si: 0.25%, Mn: 1.2%, Cr: 0.137%, Mo : 0.002%, Ti: 0.034%, B: 0.003%, and had been provided with a 20 μm (corresponding to 120 g / m ² ) thick Al-Si coating by conventional fire aluminizing.

The samples were subjected to a heat treatment corresponding to the first heating step of the process according to the invention in a test oven simulated for a period of eight hours in a test-tube furnace. A first part of samples has been annealed at 500 ⁰ C, a second part at 550 ⁰ C and a third part at 600 ⁰ C. In addition, further samples in six minutes at 950 ⁰ C have passed through the continuous furnace. This represents a typical heat treatment for press-hardening in which the Al-Si coating layer is alloyed. After the respective anneals, the samples were cooled to room temperature. The resulting samples each had a not completely alloyed Al-Si plating layer down to the Warm-treated at 950 ⁰ C sample.

Subsequently, the previously annealed and cooled samples are in a second heating step was heated in a radiation oven to a heating temperature of 950 ⁰ C in which the steel substrate Austenitgefuge possessed. The heating rates were recorded, ie it was monitored how quickly the samples were heated to the target temperature of 950 ⁰ C.

In Diag. 1, the temperature T of the respective samples over the annealing time t is entered. Additional is in Diag. 1, the sample annealed for a sample not annealed in an upstream first heating step (curve "- ⁰ C / - s").

It can be seen that in the examined samples optimal heating rates result when the samples have been annealed in the first heating step in the hood furnace for 8 h at 550 ⁰ C or 600 ⁰ C. An equally good heating behavior was observed for the samples annealed in the continuous furnace at 950 ⁰ C for six minutes.

The poorer heating behavior of the previously annealed at 500 ° C for 8 h samples is due to the fact that these samples in the upper, non-alloyed layer of the Al-Si coating, the reflection of the radiation behave exactly as in conventional Al-Si coating. Excess in delivery state without previous heat treatment.

With the process according to the invention, the times required for the alloying out in the austenitizing furnace before the hot working can be significantly shortened. It was thus shown that a time saving of at least 90 s is expected compared to the conventional procedure can be. With this time gain, the required for the heating before the hot forming oven can be designed smaller. During maintenance of the conventional size furnace, cooling takes place to room temperature in about 10 days, whereas cooling of the furnace size possible by the invention can be expected to save at least 2 to 3 days in time.

Claims

PATENT APPLICATIONS

1. A method for producing a component from a coated with an Al-Si Schutzüberzug steel product, wherein in the course of the process

the steel product coated with the Al-Si coating is subjected to a first heating step in which the temperature and the duration of the heat treatment are adjusted such that the Al-Si coating is only partially pre-alloyed with Fe of the steel product,

in a second heating step, the steel product is heated to a heating temperature above the Acl temperature at which the steel product has an at least partly spherical structure, the temperature and duration of the second heating cycle being adjusted such that the Al-Si coating is in the course of the second heating step is completely alloyed with Fe of the steel product,

- The heated to the heating temperature steel product is formed into the component and

- The resulting component is cooled in a controlled manner to produce Hartegefuge.

2. The method of claim 1, wherein the steel product is cooled to room temperature between the first and second heating modes.

3. The method of claim 2, wherein the steel product is transported in air between the first and second heating steps.

4. The method according to any one of the preceding claims, characterized in that the temperature of the first warming step amounts to at least 500 ⁰ C and at the same time is at most equal to the A _C i-temperature of the steel product.

5. A method according to any one of the preceding claims, wherein the temperature of the first heating step is d

550 - 723 ⁰ C, in particular 550 - 700 ⁰ C, amounts.

6. The method according to any one of the preceding claims, characterized in that the first heating step is carried out in a hood annealing furnace.

7. The method according to any one of claims 1 to 5, wherein a first heating step is carried out in a continuous furnace.

8. A process according to any one of the preceding claims, wherein the heating temperature to which the steel product is heated in the second heating step corresponds at least to the Ac3 temperature.

9. Method according to one of the preceding claims, characterized in that the second heating step is carried out in a continuous furnace.

10. A process according to any one of claims 1 to 8, wherein a second heating step is carried out in a chamber furnace.

11. Method according to one of the preceding claims, characterized in that the steel product consists of a tempered steel.

12. The method according to any one of the preceding claims, characterized in that the steel product is a flat steel product, such as a steel sheet or a steel strip.

13. The method of claim 1, wherein the steel product is a preformed semifinished product.