Method for producing a motor vehicle component with at least two regions of different strengths

Abstract

A method for producing a motor vehicle component with at least two regions of different strengths and a protective layer, consisting of the following process steps: —providing precoated blanks made of a steel alloy, which can be hardened, —homogeneously heating to a heating temperature, which is at least greater than or equal to the AC1 temperature, preferably greater than or equal to the AC3 temperature, —holding the heating temperature, so that the precoating alloys with the blank, —homogeneously intercooling the alloyed blank to an intercooling temperature between 450 deg. C. and 700 deg. C., partially heating the blank from the intercooling temperature to at least the AC3 temperature in regions of the first type and holding regions of the second type at substantially intercooling temperature, —hot forming and press hardening the partially tempered blank so as to form the motor vehicle component, wherein a tensile strength of greater than 1400 MPa is produced in regions of the first type, a tensile strength of less than 1050 MPa is produced in regions of the second type, and a transition region is produced between said regions.

Claims

1. A method for producing a motor vehicle component with at least two regions of different strengths and a protective layer, said method comprising the following process steps: providing precoated precut blanks, made of a steel alloy, which can be hardened; homogeneously heating to a heating temperature, which is at least greater than or equal to the AC3 temperature; holding the heating temperature for 90 to 300 seconds, so that the precoating alloys with the blank to provide a coating having a layer thickness of between 20 μm and 40 μm; homogeneously intercooling the alloyed blank to an intercooling temperature between 450 deg. C. and 700 deg. C. with a cooling rate of 3 to 15 K/s, holding the intercooling temperature for 30 to 90 seconds and attaining a predominantly ferritic-pearlitic or ferritic-pearlitic bainitic microstructure, whereby the homogeneously heating and intercooling is performed in one single continuous furnace with a heating station and a cooling station; partially heating the blank in less than 20 from the intercooling temperature to at least the AC3 temperature in regions of a first type and holding regions of a second type at the intercooling temperature to form a partially tempered blank; and hot forming and press hardening the partially tempered blank so as to form the motor vehicle component, wherein a tensile strength of greater than 1400 MPa is produced in regions of the first type, a tensile strength of between 600 and 750 MPa and a ferritic-pearlitic microstructure is produced in regions of the second type, and a transition region of less than 50 mm is produced between said regions.

2. The method according to claim 1, wherein an AlSi coating is used as a precoating.

3. The method according to claim 1, wherein the homogeneous intercooling is carried out in multiple stages.

4. The method according to claim 3, wherein a first stage of the intercooling is carried out at a higher cooling rate compared to a second stage or further stages at a lower cooling rate.

5. The method, according to claim 1, wherein with the intercooling a predominantly bainitic microstructure is produced.

6. The method according to claim 1, wherein the partial heating is carried out by contact heating.

7. The method according to claim 6, wherein the contact heating is effected by contact plates or rollers.

8. The method, according to claim 1, wherein the partial heating is carried out in a furnace comprising at least two zones of different temperatures.

9. The method according to claim 1, wherein the hot forming and press hardening is carried out in a two-fold or four-fold falling hot forming and press hardening tool, and wherein a two-fold falling or four-fold falling contact heating tool is used.

10. The method according to claim 1, wherein in regions of the second type a tensile strength between 750 and 1050 MPa is produced.

11. The method according to claim 1, wherein structural components including motor vehicle pillars, longitudinal members, rails or sills, or body components are produced as a motor vehicle component.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 an inventive hot forming line for carrying out the method with contact heating,

(2) FIG. 2 an alternative design variant of FIG. 1 with two zone furnace heating,

(3) FIG. 3 an illustration of the transition region and

(4) FIG. 4 a time-temperature diagram for carrying out the method.

DETAILED DESCRIPTION OF THE INVENTION

(5) The same reference numerals are used for the same or similar components in the figures, even if a repeated description is omitted for reasons of simplicity.

(6) FIG. 1 shows an inventive hot forming line 1 for carrying out the method of the present invention. First, a blank 2 is provided in the form of a precut blank and here, in particular, for a B-pillar. This blank passes through a continuous furnace 3, wherein in a first heating zone 4 of the continuous furnace 3, the blank 2 is heated to a temperature of greater than or equal to AC1, in particular, greater than or equal to the AC3 temperature. Consequently, no later than at the end 5 of the heating zone 4 of the continuous furnace 3, the blank 2 exhibits the heating temperature. However, it can also exhibit the heating temperature before reaching the end 5 and then retains the heating temperature for the rest of the time in the heating zone 4. In this case the precoating alloys with the blank 2, so that at the end 5 of the heating zone 4 the coating completely alloys with the blank 2.

(7) This heating zone is followed by an intercooling zone 6, in which the blank 2 is cooled to a temperature between 450 deg. C. and 700 deg. C., but at least less than the heating temperature. At the end 7 of the intercooling zone 6, the homogeneously intercooled blank 8 exhibits the intercooling temperature.

(8) Then the homogeneously intercooled blank 8 is transferred to a contact heating station 9, wherein by closing the contact heating station 9, the blank 2 is partially heated by area contact with the contact plates 9a to a temperature of at least AC3 in regions of the first type 10. In regions of the second type 11, the blank 2 has a temperature that corresponds in essence to the intercooling temperature+/−50 deg. C. In particular, this temperature is reached in that the region of the first type 10 has a direct bearing contact with the contact plates 9a of the contact heating station 9. The regions of the second type 11 do not lie directly on the contact plates 9a; that is, a recess 9d is arranged in-between as an insulating air gap 9b. The contact plates 9a themselves are heated by a heating means 9c, for example, an inductor. After hot forming and press hardening, the regions of the first type 10 and the regions of the second type 11 on the tempered blank 12 should be equated with the regions of the first type 10 having high strength and the regions of the second type 11 having a comparatively lower strength.

(9) Then the partially tempered blank 12 is transferred directly to a hot forming and press hardening tool 13 and formed by hot forming and press hardening into the motor vehicle component 14 having two regions of different strengths. Illustrated here is the production of a B-pillar, wherein, after forming, the precut blank is adapted to the final contour of the B-pillar; and, after the forming process, the B-pillar has a hat-shaped profile in the cross section. However, it is also possible to produce rails, longitudinal members as well as other structural components of the motor vehicle with the method of the present invention.

(10) Furthermore, FIG. 1 shows a hot forming and press hardening tool 13, shown here, in particular, as a two-fold falling tool. This means that with a closing movement, two components are simultaneously formed and press hardened. It may also be preferred to use a four-fold falling tool. The contact heating station 9 can also be designed in a two-fold falling, preferably four-fold falling manner.

(11) FIG. 2 shows an alternative design variant of FIG. 1, wherein in contrast to the contact heating station 9, a zone furnace 15 is used herein. The zone furnace 15 has a first zone 16 with a higher temperature, in particular, greater than or equal to the AC3 temperature and a second zone 17 with a lower temperature, with the lower temperature corresponding to the intercooling temperature of +/−50 deg. C. For example, a bulkhead 18 or the like can be arranged in the zone furnace 15, so that the blank 8, which is at an intercooling temperature, is tempered accordingly in different regions. In this case, too, a partially tempered blank 12 having a region of the first type 10 and a region of the second type 11 is produced; and this blank is subsequently 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 regions of the first type 10 and the second type 11. The zone furnace 15 can be operated as a continuous furnace. However, it can also be designed so as to be multiple storied, in particular, for saving space as a multi-level furnace. It can also be designed as a multi-story continuous furnace. In the first zone 16 it is particularly preferred that the furnace have a significantly higher interior temperature, in particular, greater than 1,000 deg. C.

(12) FIG. 3 shows an illustration of the regions of the first and second type 10, 11 and a transition area 19 between the two regions. The transition region 19 extends with a width between the region of the first type 10 and the region of the second type 11. The width is, according to the invention, preferably less than 50 mm. In this case the region of the second type 11 is designed as an island region or inland region. Consequently it is completely enclosed by the region of the first type 10. In accordance with the invention, the region of the first type 10 has preferably a tensile strength of greater than 1400 MPa, in particular, greater than 1500 MPa. The tensile strength should be limited to approximately 2000 MPa. If, however, it were possible to achieve greater tensile strengths by means of a steel alloy, then this would also be within the scope of this invention.

(13) FIG. 4 shows in schematic form the sequence of the method of the present invention, wherein the temperature T, which is to be produced, is shown in degrees centigrade on the Y axis; and the time in seconds is shown on the X axis, but unfortunately not to scale. First, at the time S0 the blank 2 is provided at room temperature. Then this blank is brought into the continuous furnace 3 and heated to the heating temperature, here shown at approximately AC3, until the time S1. The heating processes, shown by way of example, can in reality be linear, progressive, digressive or in mixed forms. They are shown here by means of straight lines and not to scale only for illustrative purposes. The time for heating is about 300 to 400 seconds, in particular, 320 to 380 seconds, preferably 350 to 370 seconds and, in particular, 360 seconds. This time can also already include the holding of the heating temperature up to the time S2. At time S2 the homogeneously heated and alloyed blank 8 is transferred to the homogeneous intercooling and is cooled homogeneously to the intercooling temperature. This is carried out in a period of time preferably between 30 seconds and 200 seconds, preferably 50 seconds to 100 seconds. Thus, the homogeneously intercooled temperature leaves the intercooling station at time S3 and is passed to a partial heating station, for example, into 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 intercooling temperature to partial heating temperature is shown from time S3 to S5. From S4, beginning of the partial tempering to S5, stopping the partial tempering, it usually takes less than 20 seconds, in particular, less than 15 seconds, preferably less than 10 seconds, even more preferably 8 seconds. At time S5 the partially tempered blank 12 is then transferred to the hot forming and press hardening tool 13 and is hot formed and press hardened. In so doing, the regions of the first type 10 are quenched by the heating temperature, i.e. greater than or equal to the AC3 temperature, and the regions of the second type 11 are quenched by the intercooling temperature+/−50 deg. C., shown here in the range of AC1. At time S6 the press hardening is completed, wherein the temperature of the press hardened component is between room temperature, i.e., about 20 deg. C. and 200 deg. C., upon removal from the press shop.

REFERENCE NUMERALS

(14) 1—hot forming line 2—blank 3—continuous furnace 4—heating zone with respect to 3 5—end with respect to 4 6—intercooling zone with respect to 3 7—end with respect to 6 8—homogeneously intercooled blank 9—contact heating station 9a—contact plate 9b—air gap 9c—heating means 9d—recess 10—region of the first type 11—region of the second type 12—partially tempered blank 13—hot forming and press hardening tool 14—motor vehicle component 15—zone furnace 16—first zone with respect to 15 17—second zone with respect to 15 18—bulkhead with respect to 15 19—transition region between 10 and 11 20—width with respect to 19