EP3211103B1 - Method for manufacturing a motor vehicle component with at least two different strength areas - Google Patents

Description

The present invention relates to a method for producing a motor vehicle component with at least two areas with mutually different strengths and a protective layer according to the features in claim 1.

It is known from the prior art to produce motor vehicle components by means of sheet metal forming. On the one hand, sheet metal outer skin components are produced, for example an engine hood or a roof skin. In the case of a self-supporting body, however, motor vehicle structural components are also produced. These are, in particular, motor vehicle pillars, roof pillars, rocker panels, cross members or longitudinal members, as well as other structural components built into the motor vehicle body.

In the course of the increased safety requirements for the motor vehicle body itself as well as the legal requirements for lower fuel consumption and lower CO2 emissions, hot forming and press hardening technology has established itself from the state of the art. For this purpose, sheet metal components made of a hardenable steel alloy are first heated to a temperature above AC3, so that the material structure is austenitized. In this warm state, the blank is then reshaped and, after reshaping has been completed, cooled down so quickly that the material structure is hardened. In particular, martensite is formed in the process.

As a result, it is possible to manufacture components with smaller wall thicknesses, which reduces the component weight, but at the same time with at least the same or higher strength.

From the DE 102 08 216 C1 It is also known to produce components with areas of different strength from one another during the press forming.

Also from the EP 2 905 346 A1 a method is known in which a partially different temperature profile is impressed on a formed sheet steel component.

The components made of a hardenable steel alloy are, however, at the same time susceptible to corrosion, which is why it is also known from the prior art to provide hot-formed and press-hardened components with a corrosion protection layer.

The object of the present invention is to provide a possibility to manufacture motor vehicle components inexpensively with corrosion protection, which specifically have sharply edged areas with mutually different strengths.

The aforementioned object is achieved according to the invention with a method according to the features in patent claim 1.

Advantageous design variants of the method are described in the dependent claims.

• Bereitstellen von vorbeschichteten Platinen, insbesondere Platinenzuschnitten, aus einer härtbaren Stahllegierung,
• Homogenes Erwärmen auf eine Aufwärmtemperatur, größer gleich AC3 Temperatur,
• Halten der Aufwärmtemperatur, so dass die Vorbeschichtung mit der Platine durchlegiert,
• Homogene Zwischenkühlung der durchlegierten Platine auf eine Zwischenkühltemperatur zwischen 450°C und 700°C,
• Partielles Erwärmen der Platine von der Zwischenkühltemperatur in Bereichen erster Art auf mindestens AC3 Temperatur und Halten von Bereichen zweiter Art auf im Wesentlichen der Zwischenkühltemperatur
• Warmumformen und Presshärten der partiell temperierten Platine zu dem Kraftfahrzeugbauteil, wobei in Bereichen erster Art eine Zugfestigkeit größer 1400 MPa und in Bereichen zweiter Art eine Zugfestigkeit kleiner 1050 MPa sowie ein dazwischen liegender Übergangsbereich mit einer Breite kleiner 50mm eingestellt werden.

The method according to the invention for producing a motor vehicle component with at least two areas with different strengths from one another and a corrosion protection layer is characterized by the following method steps:

• Provision of pre-coated blanks, in particular blanks, made of a hardenable steel alloy,
• Homogeneous heating to a warm-up temperature, greater than or equal to AC3 temperature,
• Maintaining the warm-up temperature so that the pre-coating alloyed through with the board,
• Homogeneous intermediate cooling of the fully alloyed blank to an intermediate cooling temperature between 450 ° C and 700 ° C,
• Partial heating of the board from the intermediate cooling temperature in areas of the first type to at least AC3 temperature and maintaining areas of the second type at essentially the intermediate cooling temperature
• Hot forming and press hardening of the partially tempered blank to the motor vehicle component, with a tensile strength greater than 1400 MPa in areas of the first type and a tensile strength of less than 1050 MPa in areas of the second type as well as an intermediate transition area with a width of less than 50mm.

The method thus initially provides for the provision of precoated starting material made of a hardenable steel alloy. This can be a steel material unwound from a coil, which has already been separated into blanks, or it can also be cut directly from blanks. A blank has an approximately near-net-shape trim that the component should have after hot forming.

This raw material is pre-coated. In particular, this is an aluminum silicon coating. The hardenable steel alloy is preferably a boron-manganese steel.

First of all, it is now provided that the starting material is heated to a heating temperature that is greater than or equal to the AC3 temperature of the iron-carbon diagram of the hardenable steel alloy. This warming-up temperature is also preferably maintained for a period of time, in particular for 90 s to 300 s. The pre-coating is alloyed with the board. This is also called diffusion of the Called pre-coating in the surface of the board. The coating preferably has a layer thickness between 20 μm and 40 μm. In particular, a pronounced intermetallic phase is formed. The homogeneous heating to the warm-up temperature is carried out in particular in a continuous furnace.

Once the warming-up temperature has been reached and, in particular, the holding phase of the warming-up temperature has been completed, homogeneous intermediate cooling of the fully alloyed blank with the pre-coating to an intermediate cooling temperature takes place. The intermediate cooling temperature is between 450 ° C. and 700 ° C., but it is at least lower than the warming-up temperature and therefore particularly preferably lower than AC1. The intermediate cooling temperature +/- 50 ° C. is also preferably kept for a holding time. One or more material structures can be set in a targeted manner through the intermediate cooling and in particular due to the temperature range of the intermediate cooling. If the intermediate cooling temperature is selected at approx. 500 ° C, the material structure changes predominantly into bainite, which after quench hardening has a tensile strength of 750 MPa to 1050 MPa. If the intermediate cooling temperature is selected at approx. 600 ° C, a predominantly ferritic / pearlitic structure is formed, with a tensile strength of approx. 500 MPa to 750 MPa after quench hardening. For example, to set a bainitic material structure to an intermediate cooling temperature of approx. 500 ° C with a cooling rate between 3 to 15 ° C / sec. cooled down. The subsequent holding time is preferably 30 s to 90 s. In order to obtain a ferritic / pearlitic material structure, a cooling rate of 3 to 15 ° C / sec. cooled to a temperature of approx. 600 ° C. and this intermediate cooling temperature was also maintained for a time of 30 s to 90 s.

So that now areas of the motor vehicle component have different strengths from one another and in particular some areas have high-strength or ultra-high-strength properties with a tensile strength greater than 1300 MPa, in particular greater than 1400 MPa, the homogeneously intercooled and alloyed plate is partially from the intermediate cooling temperature +/- 50 ° C in areas first Type and therefore heated in areas to at least AC3 temperature. The remaining areas are called areas of the second type, which are essentially kept at the intermediate cooling temperature +/- 50 °. Warming the areas first Type to at least AC3 temperature, preferably to 930 ° C. to 980 ° C., is preferably carried out in such a way that the areas of the first type completely austenitize. If this heating of the areas of the first type is carried out to at least AC3 temperature, the blank, which is partially tempered in areas with different temperatures, is transferred to a hot forming and press hardening tool, hot formed in this tempered state and then press hardened. A tensile strength greater than 1400 MPa is thus set in the areas of the first type and a tensile strength Rm less than 1050 MPa in the areas of the second type.

According to the invention it is further provided that a transition area between the areas of the first type and the second type has a width of less than 50 mm. In particular, this can be achieved in that the partial heating of the areas of the first type to at least AC3 temperature is carried out in a particularly short time, in particular at a heating rate greater than 30 ° C./sec. The time for heating is preferably less than 20 s, in particular less than 15 s, particularly preferably less than 10 s. The heat conduction that occurs in the blank from areas of the first type to areas of the second type takes place only to a small extent due to the shortness of the time, so that a sharp-edged transition area with the subsequent hot forming and press hardening is achieved. The cycle time for hot forming and press hardening is preferably approx. 10 s to 20 s, in particular 15 s. Furthermore, a relatively short transfer time between the completion of the intermediate cooling or the end of the holding time of the intermediate cooling and the hot forming and press hardening tool is realized. The transfer time is preferably 2 s to 15 s.

The homogeneous heating to the warm-up temperature is particularly preferably carried out in a continuous furnace. The homogeneous intermediate cooling to the intermediate cooling temperature as well as the optional maintenance of the intermediate cooling temperature is also carried out in a continuous furnace. This continuous furnace for intermediate cooling is preferably designed as a continuous furnace module and, in particular, is connected directly to the continuous furnace for heating to the warm-up temperature. Alternatively, the intermediate cooling can also be carried out in a chamber furnace. As an alternative, it would also be possible to use a separate cooling station. Will continue A continuous furnace module is used for the entire homogeneous heating and homogeneous intermediate cooling, a cooling station or cooling plates being integrated in the continuous furnace module in order to carry out the intermediate cooling.

With the method according to the invention, structural components for motor vehicles in particular can thus be produced which are intended to have small-area, strip-like and / or island-like soft areas, and consequently areas of the second type. These can be trigger strips or side wall islands, for example, so that targeted deformation points are first deformed in the event of a vehicle crash. Coupling points, in particular coupling flanges of the components for coupling two motor vehicle components to one another with areas of the second type, thus soft areas, can be designed so that in the event of a motor vehicle crash and deformation, the coupling points are prevented from tearing off in these areas and the tendency to crack along later joints is reduced .

With the method according to the invention, it is also possible to set a width of the transition region smaller than 40 mm, in particular smaller than 30 mm and particularly preferably smaller than 25 mm. Areas with very sharp edges and different strengths can thus be delimited.

The areas of the second type, in particular the soft areas, are preferably designed to cover or occupy only a small area in relation to the total area of the motor vehicle component. The predominant part of the motor vehicle component should therefore have a hardened material structure, thus areas of the first type. More than 70%, in particular more than 80% and particularly preferably more than 90% of the motor vehicle component preferably has areas of the first type.

Furthermore, particularly preferably, the intermediate cooling to the intermediate cooling temperature can be carried out in several stages and thus at least in two stages. A first stage of the intermediate cooling has a higher cooling rate compared to a second stage with a lower cooling rate. This means that the temperature decreases more strongly in the first stage of the intermediate cooling. In the second stage of the intermediate cooling, the temperature is less than one decreased for a longer period. The at least two-stage intermediate cooling can then in turn be followed by a holding phase at the intermediate cooling temperature.

Depending on how the intermediate cooling is carried out, a predominantly bainitic structure or a predominantly ferritic / pearlitic structure is set. However, a mixed structure of ferrite, pearlite and bainite can also be set during the intermediate cooling.

Following the intermediate cooling, the partial heating is then carried out by, in particular, contact heating of the areas of the first type. At the same time, the areas of the second type are kept in particular at essentially the intermediate cooling temperature. The partial heating takes place particularly preferably by contact heating. For this purpose, contact plates are placed on the surface of the fully alloyed board. There is a conduction, i.e. heat conduction from the contact plate into the board. For this purpose, the contact plate preferably has a temperature that is greater than or equal to the AC3 temperature. The contact plate itself is heated by induction, by thermal radiation, in particular by torch heating. A heating means, for example a heating cartridge or heating wire, can also be assigned to the contact plate. However, it is also possible that the contact plate itself is designed as an electrical resistance heater. By applying an electrical voltage to the contact plate, the contact plate heats up itself. If the contact plate is placed on the board, heat is conducted from the contact plate into the board, at least in the areas of the first type to be austenitized.

Alternatively, it is possible for the partial heating to be carried out in an oven which has at least two zones. It is also possible to integrate cooling plates or temperature control plates in an oven or to place them on the board so that the cooling plates of the areas of the second type are kept at the intermediate cooling temperature and areas of the first type are heated to a temperature greater than or equal to AC3 in the oven. The oven can be designed as a continuous oven, but also as a chamber oven, deck oven or buffer oven.

Again, as an alternative, it is possible for the areas of the first type to be heated directly by means of laser radiation. This is particularly useful if particularly large areas of the second type are provided, which consequently should not be heated to above AC3.

In particular, with the method according to the invention it is thus possible to set a tensile strength between 750 MPa and 1050 MPa in the softer areas, thus areas of the second type, which corresponds to a bainitic structure with a martensitic component. It is also possible to set a tensile strength between 600 MPa and 750 MPa in the softer areas, which corresponds to a proportion of ferrite / pearlitic structure.

In particular, motor vehicle components are thus produced as structural components. These are preferably motor vehicle pillars, very particularly preferably A-pillars or B-pillars. However, side members can also be produced. Furthermore, bars, in particular roof bars or rockers can be produced. However, chassis components can also be produced with the method according to the invention. In particular, coupling flanges, desired deformation points, coupling areas, hole edges, trigger strips and / or side wall islands are formed as areas of the second type and therefore softer areas.

It is particularly preferred to use a multi-drop tool as the hot forming and press hardening tool. In particular a double or quadruple dropping tool. This means that two components are formed at the same time during one movement and, after the forming process has been completed, the two components are also press hardened at the same time. In the case of a four-fold mold, four blanks are formed into components at the same time during a closing movement and all four components are then press-hardened.

Furthermore, particularly preferably, two individual temperature control stations can be used for a double hot forming and press hardening tool. Both a cooling station for intermediate cooling and a partial heating station for partial heating to via AC3 can be referred to as a temperature control station. This means that for a double hot forming and Press hardening tool two individual intermediate cooling stations and / or two individual heating stations can be used. For a four-fold hot forming and press hardening tool, two double-falling tempering stations can be used, i.e. two double-falling cooling stations and two double-falling partial heating stations.

Figur 1

eine erfindungsgemäße Warmformlinie zur Durchführung des Verfahrens mit Kontakterwärmung,

Figur 2

eine alternative Ausgestaltungsvariante zu Figur 1 mit Zweizonenofenerwärmung,

Figur 3

eine Veranschaulichung des Übergangsbereiches und

Figur 4

ein Zeit-Temperaturdiagramm zur Durchführung des Verfahrens.

The temperature control stations preferably work in the press cycle of the hot forming and press hardening tool.

Figure 1

a hot forming line according to the invention for carrying out the method with contact heating,

Figure 2

an alternative design variant Figure 1 with two-zone heating,

Figure 3

an illustration of the transition area and

Figure 4

a time-temperature diagram for carrying out the method.

In the figures, the same reference numerals are used for the same or similar components, even if a repeated description is omitted for reasons of simplicity.

Figure 1 shows a hot forming line 1 according to the invention for carrying out the method according to the invention. First, a blank 2 is provided in the form of a blank and here in particular for a B-pillar. This passes through a continuous furnace 3, the blank 2 being heated to a temperature greater than or equal to AC3 temperature in a first heating zone 4 of the continuous furnace 3. Thus, at the latest at the end 5 of the heating zone 4 of the continuous furnace 3, the blank 2 has the heating temperature. However, it can also have the warm-up temperature before reaching the end 5 and then maintains the warm-up temperature for the rest of the time of the warm-up zone 4. The pre-coating alloyed through with the blank 2 so that at the end 5 of the heating zone 4 the coating was completely alloyed through with the blank 2. This is followed by an intermediate cooling zone 6, in which the blank 2 is cooled to a temperature between 450 ° C. and 700 ° C., but at least less than the heating temperature. At the end 7 of the intermediate cooling zone 6, the homogeneously intermediate cooled blank 8 has the intermediate cooling temperature.

The homogeneously intercooled board 8 is then transferred to a contact heating station 9, whereby by closing the contact heating station 9, the board 2 is partially heated to a temperature in areas of the first type 10 of at least AC3 by area-wise contact with the contact plates 9a. In areas of the second type 11, the board 2 has a temperature which essentially corresponds to the intermediate cooling temperature +/- 50 ° C. In particular, this is achieved in that the area of the first type 10 has direct system contact with contact plates 9 a of the contact heating station 9. The regions of the second type 11 do not lie directly on the contact plates 9a, so a recess 9d is arranged between them as an insulating air gap 9b. The contact plates 9a are themselves heated by a heating means 9c, for example an inductor. The areas of the first type 10 and the areas of the second type 11 on the temperature-controlled blank 12 are to be equated with the areas of the first type 10 with high strength and the areas of the second type 11 with a comparatively lower strength after hot forming and press hardening.

The partially tempered blank 12 is then immediately transferred to a hot forming and press hardening tool 13 and formed by hot forming and press hardening to form the motor vehicle component 14 with two areas with mutually different strengths. The production of a B-pillar is illustrated here, wherein the blank is adapted to the final contour of the B-pillar after the forming and the B-pillar has a hat-shaped profile in cross section after the forming. However, it is also possible to manufacture spars, side members and other structural components for motor vehicles using the method according to the invention.

Figure 1 also shows a hot forming and press hardening tool 13, shown here in particular as a double-sided tool. This means that with one closing movement two components are formed and press hardened at the same time will. A tool that falls four times can preferably also be set. The contact heating station 9 can also be designed to be double, preferably quadruple.

Figure 2 shows an alternative design variant Figure 1 In contrast to the contact heating station 9, a zone furnace 15 is used here. The zone furnace 15 has a first zone 16 with a higher temperature, in particular greater than or equal to AC3 temperature, and a second zone 17 with a lower temperature, the lower temperature corresponding to the intermediate cooling temperature +/- 50 ° C. In the zone furnace 15, for example, a bulkhead 18 or the like can be arranged so that the board 8, which is at the intermediate cooling temperature, is correspondingly tempered in areas different from one another. This also produces a partially tempered blank 12 with an area of the first type 10 and an area of the second type 11, which is then hot-formed and press-hardened. The zone furnace 15 does not have to be a two-zone furnace; it can also be designed as a multiple zone furnace, depending on the geometric specification of the position of the areas of the first type 10 and second type 11. The zone furnace 15 can be operated as a continuous furnace. However, it can also be designed as a multi-storey oven, in particular to save space. It can also be designed as a multi-storey continuous furnace. In the first zone 16, the furnace particularly preferably has a significantly higher interior temperature, in particular greater than 1000 ° C.

Figure 3 shows an illustration of the areas of the first and second type 10, 11 and a transition area 19 in between. The transition area 19 extends with a width between the area of the first type 10 and the area of the second type 11. According to the invention, the width is preferably less than 50 mm. The area of the second type 11 is designed here as an island area or inland area. It is therefore completely enclosed by the area of the first type 10. In the context of the invention, the area of the first type 10 preferably has a tensile strength greater than 1400 MPa, in particular greater than 1500 MPa. The tensile strength should be limited to approx. 2000 MPa. However, should it be possible to achieve greater tensile strengths using a steel alloy, this would also be within the meaning of this invention.

Figure 4 shows a schematic sequence of the method according to the invention, the temperature T to be set being shown in degrees Celsius on the Y-axis and the time in seconds being shown on the X-axis, but unfortunately not true to scale. First of all, at point in time S0, the circuit board 2 is provided at room temperature. This is then brought into the continuous furnace 3 and heated to the warm-up temperature, shown here at approx. AC3, by time S1. The heating curves shown as examples can in reality run linear, progressive, degressive or in mixed forms. These are shown here by straight lines and not to scale for illustration purposes only. The time for heating is approx. 300 to 400 s, in particular 320 to 380 s, preferably 350 to 370 s and in particular 360 s. This can also include maintaining the warm-up temperature up to time S2. At time S2, the homogeneously heated and fully alloyed blank 8 is transferred to homogeneous intermediate cooling and homogeneously cooled to the intermediate cooling temperature. This is carried out in a time preferably between 30 s and 200 s, preferably 50 s to 100 s. The homogeneously intercooled temperature thus leaves the intercooling station at time S3 and is transferred to a partial heating station, for example to a contact heating station 9. This is shown at time S4. The transfer time from S3 to S4 is preferably as short as possible. The heating step from the intermediate cooling temperature to the partial heating temperature is shown from time S3 to S5. From S4, the beginning of the partial temperature control to S5, switching off the partial temperature control, it generally takes less than 20 s, in particular less than 15 s, preferably less than 10 s, very particularly preferably 8 s. At the point in time S5, the partially tempered blank 12 is then transferred to the hot forming and press hardening tool 13 and hot formed and press hardened. The areas of the first type 10 are quenched by the heating temperature that is greater than or equal to the AC3 temperature and the areas of the second type 11 are quenched by the intermediate cooling temperature +/- 50 ° C., shown here in the area of AC1. Press hardening is ended at time S6, the temperature of the press hardened component being between room temperature, therefore approx. 20 ° C and 200 ° C, when it is removed from the press shop.

Reference number :

1 -

Hot forming line

2 -

circuit board

3 -

Continuous furnace

4 -

Warm-up zone for 3

5 -

End to 4

6 -

Intermediate cooling zone for 3

7 -

End to 6

8th -

homogeneously intercooled board

9 -

Contact heating station

9a -

Contact plate

9b -

Air gap

9c -

Heating means

9d -

deepening

10 -

Area of the first kind

11 -

Area of the second kind

12 -

partially tempered board

13 -

Hot forming and press hardening tool

14 -

Automotive component

15 -

Zone furnace

16 -

first zone to 15

17 -

second zone to 15

18 -

Bulkhead to 15

19 -

Transition area between 10 and 11

20 -

Width to 19

Claims (12)

1. Method for producing a motor vehicle component (14) having at least two regions with differing strengths and a protective layer, characterised by the following method steps:

   - provision of pre-coated plates (2), in particular plate blanks, made of a hardenable steel alloy,

   - homogeneous heating to a heating temperature greater than or equal to AC3 temperature,

   - maintaining the heating temperature so that the pre-coating alloys with the plate (2),

   - homogeneous intermediate cooling of the alloyed plate (2) to an intermediate cooling temperature between 450 and 700°C,

   - partial heating of the plate (2) in regions of the first type (10) from the intermediate cooling temperature to at least AC3 temperature and maintaining regions of the second type (11) substantially at the intermediate cooling temperature,

   - hot-forming and press-hardening of the partially tempered plate (12) to form the motor vehicle component (14), a tensile strength greater than 1400 MPa being set in regions of the first type (10) and a tensile strength less than 1050 MPa being set in regions of the second type (11) and in a transition region (19) lying therebetween.
2. Method according to claim 1, characterised in that the homogeneous heating to heating temperature is carried out in a continuous furnace (3) and/or in that the homogeneous intermediate cooling to intermediate cooling temperature is carried out in a continuous oven (3) or in a chamber oven.
3. Method according to claim 1 or 2, characterised in that a transition region (19) with a breadth (20) less than 50 mm, in particular less than 40 mm, preferably less than 30 mm, particularly preferably less than 25 mm, is set.
4. Method according to any one of claims 1 to 3, characterised in that an AlSi coating is used as a pre-coating.
5. Method according to any one of claims 1 to 4, characterised in that the homogeneous intermediate cooling is carried out in several stages.
6. Method according to the preceding claim, characterised in that a first stage of the intermediate cooling is carried out with a higher cooling rate compared to a second or further stages with a lower cooling rate.
7. Method according to one of claims 1 to 6, characterised in that a predominantly bainitic structure is set with the intermediate cooling or that a predominantly ferritic/pearlitic structure is set with the intermediate cooling.
8. Method according to any one of claims 1 to 7, characterised in that the partial heating is carried out by contact heating, in particular by contact plates (9a) or rollers.
9. Method according to any one of claims 1 to 8, characterised in that the partial heating is carried out in an oven having at least two zones (16,17) of different temperature.
10. Method according to any one of claims 1 to 9, characterised in that the hot forming and press hardening is carried out in a double or quadruple hot-forming and press-hardening tool (13) and in particular a double or quadruple contact heating tool (9) is utilised.
11. Method according to any one of claims 1 to 10, characterised in that a tensile strength between 750 and 1050 MPa or a tensile strength between 600 and 750 MPa is set in regions of the second type (11).
12. Method according to any one of Claims 1 to 11, characterised in that as motor vehicle component structural components are produced, in particular motor vehicle pillars, longitudinal beams, spars or sills, or in that chassis components are produced.

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