Method for producing a component having improved elongation at break properties

Abstract

The invention relates to a process for producing a component having improved elongation at break properties, in which a component is firstly produced, preferably in a hot forming or press curing process, and the component is heat treated after hot forming and/or press curing, where the heat treatment temperature T and the heat treatment time t essentially satisfy the numerical relationship T900.Math..sub.t.sup.0.087, where the heat treatment temperature T is in C. and the heat treatment time t is in seconds. The invention also relates to a component, in particular an automobile body component or the chassis of a motor vehicle, which has been produced by such a process. The invention further relates to the use of such a component as part of an automobile body or a chassis of a motor vehicle.

Claims

1. Method for manufacturing a component for a body part or a chassis of a motor vehicle with improved elongation at break properties, in which a component is first produced by one of a hot forming and press curing process, and in which the component is tempered after the one of hot forming and press curing processes characterised in that a tempering temperature T and a tempering time t substantially satisfy the numerical relationship T900.Math.t.sup.0.087, wherein the tempering temperature T is expressed in C. and the tempering time tin seconds and wherein the tempering temperature is at least 500 C. and lower than AC.sub.1 temperature.

2. Method according to claim 1, characterised in that the tempering time at a tempering temperature of approximately 500 C. is at least 20 minutes, at a tempering temperature of approximately 550 C. at least 5 minutes, and at a tempering temperature of approximately 600 C. at least 3 minutes.

3. Method according to claim 1, characterised in that the tempering temperature is at least 500 C. and the tempering time is selected to be high enough that the elongation at break value A80 of the component is increased by approximately 15%.

4. Method according to claim 1, characterised in that the component substantially consists of a manganese-boron steel.

5. Method according to claim 1, characterised in that the component is coated or uncoated.

6. Method according to claim 1, characterised in that prior to tempering, the component is coated with an inorganic, an organic and/or an inorganic-organic coating.

7. Method according to claim 1, characterised in that the component is coated with a corrosion protection coating.

8. Method according to claim 1, characterised in that prior to tempering, the component is coated electrolytically and/or by hot-dip processing.

9. Method according to claim 1, characterized in that the tempering temperature T is lower than 700 C.

10. Method according to claim 1, characterized in that the tempering temperature is at least 500 C. and the tempering time is selected to be high enough that the elongation at break value A80 of the component is increased by approximately 20%.

11. Method according to claim 1, characterized in that the tempering temperature is at least 500 C. and the tempering time is selected to be high enough that the elongation at break value A80 of the component is increased by approximately 25%.

12. Method according to claim 1, characterized in that the tempering temperature is at least 550 C. and the tempering time is selected to be high enough that the elongation at break value A80 of the component is increased by approximately 15%.

13. Method according to claim 1, characterized in that the tempering temperature is at least 550 C. and the tempering time is selected to be high enough that the elongation at break value A80 of the component is increased by approximately 20%.

14. Method according to claim 1, characterized in that the tempering temperature is at least 550 C. and the tempering time is selected to be high enough that the elongation at break value A80 of the component is increased by approximately 25%.

15. Method according to claim 1, characterized in that the tempering temperature is at least 600 C. and the tempering time is selected to be high enough that the elongation at break value A80 of the component is increased by approximately 15%.

16. Method according to claim 1, characterized in that the tempering temperature is at least 600 C. and the tempering time is selected to be high enough that the elongation at break value A80 of the component is increased by approximately 20%.

17. Method according to claim 1, characterized in that the tempering temperature is at least 600 C. and the tempering time is selected to be high enough that the elongation at break value A80 of the component is increased by approximately 25%.

18. Method according to claim 1, characterized in that the component substantially consists of a manganese-boron tempering steel.

19. Method according to claim 1, characterized in that the component substantially consists of 22MnB5 tempering steel.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further features and advantages of the present invention will be explained in more detail in the description of an exemplary embodiment wherein reference is made to the attached drawings. The drawing shows as follows:

(2) FIG. 1 is an exemplary embodiment of the method according to the invention for producing a component with improved elongation at break properties;

(3) FIG. 2 is a diagram with the parameters for the tempering process;

(4) FIG. 3a is a diagram showing the influence of the tempering time on the material properties of a component at a tempering temperature of 450 C.;

(5) FIG. 3b is a diagram similar to FIG. 3a for a tempering temperature of 500 C.;

(6) FIG. 3c is a diagram similar to FIG. 3a for a tempering temperature of 550 C.;

(7) FIG. 3d is a diagram similar to FIG. 3a for a tempering temperature of 600 C.;

(8) FIG. 4 is four cross-sectional views of coated components following various tempering treatments and

(9) FIG. 5 is a vehicle frame of a motor vehicle with exemplary embodiments of components according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

(10) FIG. 1 shows an exemplary embodiment of a method for producing a component with improved elongation at break properties. From a bar 2, which is for example made from manganese-boron steel, initially in a hot forming and press curing process 4, a component 6 is produced. The component 6 is for example a side rail of a motor vehicle's body work. As a result of the hot forming and press curing process the material of the component 6 has a substantially martensitic structure and thus a high level of hardness. The component 6 is then tempered in a tempering step 8. The tempering can for example take place in an oven provided for the purpose, in which the component 6 is maintained by way of example for approximately 10 minutes at approximately 550 C. Compared with the component 6 the tempered component 10 has an elongation at break value A.sub.80 that is 60% higher. The hardness of the tempered component 10 is not excessively reduced compared with the component 6.

(11) FIG. 2 shows a diagram with the parameters for the tempering process. The tempering time t in seconds is plotted against the abscissa and the tempering temperature T in C. against the ordinate. The solid line curve corresponds to the numerical relationship T=900.Math..sub.t.sup.0.087, wherein the tempering temperature T is expressed in C. and the tempering time t in seconds.

(12) For the selection of the tempering temperature T and the tempering time t all pairs of values that are located in the diagram above the plotted curve and below the AC.sub.1-temperature are suitable. Out of practical considerations here a tempering time t of between 180 and 1200 s is taken into account in particular. Thus at lower tempering times the necessary tempering temperatures are too high and at high tempering times on the other hand the production time is too long.

(13) In FIGS. 3a to 3d the influence of the tempering temperature and of the tempering time on the material properties of components is shown. The components are strips of 22MnB5 steel of 1.47 mm in thickness with an aluminium-silicon coating (AS). In a first step the samples were heated for 6 minutes at 920 C. and austenetised and then press cured for 15 seconds at a pressure of 6 bar in a cooling tool. In a second step the components obtained in this way were tempered at differing tempering temperatures in the forced-air oven for various tempering times.

(14) FIG. 3a shows for this, measurements of the yield strength R.sub.p0.2 12, the tensile strength R.sub.m 14 and measurements of the elongation at break A.sub.80 16 for comparative components V and for components E produced with exemplary embodiments of the method according to the invention. All measurements were carried out according to DIN. The strength Rm in Mpa is plotted against the ordinate on the left-hand side and the elongation at break A.sub.80 against the ordinate on the right-hand side in percent. The comparative component V.sub.0 was not tempered following complete austenitisation and press curing, V.sub.11 was tempered for 5 minutes following press curing, V.sub.12 for 10 minutes, V.sub.13 for 20 minutes and V.sub.14 for 30 minutes at 450 C. Using an exemplary embodiment of the method according to the invention component E.sub.15 was tempered for 60 minutes at 450 C. It is clear from the diagram that the elongation at break value initially drops during tempering and then as the tempering time increases rises even to above the elongation at break value directly after press curing. Thus the elongation at break value of the component E.sub.15 exceeds that of the un-tempered component V.sub.0 by approximately 13%. The yield point shows a slight retraction as the tempering time increases while this is greater for the tensile strength.

(15) FIG. 3b shows a diagram similar to that of FIG. 3a for a tempering temperature of 500 C. The comparative component V.sub.21 was tempered at 500 C. following press curing for 5 minutes and V.sub.22 for 10 minutes. The components E.sub.23, E.sub.24 and E.sub.25 produced using exemplary embodiments of the method according to the invention were tempered for 20, 30 and 60 minutes respectively at 500 C. The diagram shows that the elongation at break value at this temperature for the component E.sub.23 tempered for 20 minutes already exceeds the elongation at break value of the component V.sub.0 by almost 30%.

(16) FIG. 3c shows a diagram similar to that of FIG. 3a for a tempering temperature of 550 C. The components E.sub.32, E.sub.33, E.sub.34 and E.sub.35 produced using exemplary embodiments of the method according to the invention were tempered for 10, 20, 30 and 60 minutes respectively at 550 C.

(17) FIG. 3d shows a diagram similar to that of FIG. 3a for a tempering temperature of 600 C. The components E.sub.41, E.sub.42, E.sub.43, E.sub.44 and E.sub.45 produced using exemplary embodiments of the method according to the invention were tempered for 5, 10, 20, 30 and 60 minutes respectively at 600 C. At this tempering temperature the elongation at break value of the component E.sub.41 already exceeds the elongation at break value of component V.sub.0 by approximately 66%.

(18) From diagrams 3a to 3d it can be seen that with long tempering times the elongation at break value of the components increases more sharply or that the tensile strength and the yield strength of the components fall more quickly the higher the tempering temperature. It is therefore advantageous to select the tempering temperature so that in the time available for the tempering process the necessary increase in the elongation at break value is achieved. In selecting the parameters for the tempering process it is also crucial that a sensible compromise is found between the increase in elongation at break and the reduction in hardness of the material. It was noted among other things that the elongation at break, when the tempering time is increased, initially rises very quickly before transitioning to a slow increase or even saturation. Through the selection according to the invention of the tempering time at a specified tempering temperature the elongation at break value can be sufficiently increased and the yield strength and stability values reduced. The result is that components can be provided with optimised mechanical characteristic values in terms of yield strength, tensile strength and elongation values.

(19) FIG. 4 shows cross-sections of the components V.sub.12, V.sub.22, E.sub.32 and E.sub.42 described above. The tempering time for all components is 5 minutes. In the cross-sections the core material 20 of the respective component and the AS coatings 21 applied to this can be seen. With all AS coatings there are clear phase limits within the AS coating 21, which can be applied with up to five alloy coatings 22, 24, 26, 28, 30. In step a) the core material 20 of the component V.sub.12 exhibits the structure of tempered martensite. For the components E.sub.32 and E.sub.42 tempered using an exemplary embodiment of the method according to the invention the granularity of this structure has clearly increased. A conversion of the martensitic structure has thus been achieved without the martensite being converted into other types of structure. In this way an excessive reduction in the stability of the components is prevented.

(20) FIG. 5 shows a vehicle frame 30, which has side rails in the roof area 32 and side rails in the floor area 34. For these side rails 32, 34 components produced by a method according to the invention are used. Since these components have a high elongation at break A.sub.80 value and thus in the event a crash, in particular in a head-on crash or rear shunt and the tensile loadings resulting from these, demonstrate high stability, the stability of the vehicle frame 30 is thereby guaranteed.