Method for the hot forming of a steel component

Abstract

A method for hot forming a steel component is provided. The steel component is heated into a range of complete or partial austenitization in a heat treatment step. The heated steel component is both hot-formed and quench-hardened in a forming step. A first pretreatment step precedes the heat treatment step in terms of process, in which first pretreatment step the steel component is provided with a corrosion-resistant protective layer in order to protect against scaling in the heat treatment step. Before the heat treatment step is performed, a surface oxidation process occurs in a second pre-treatment step, in which a weakly reactive, corrosion-resistant oxidation layer is formed on the scale protection layer by means of which oxidation layer abrasive tool wear is reduced in the forming step.

Claims

1. A method comprising: heating a steel component into a range of complete or partial austenitization in a heat treatment step; performing a forming step in which the heated steel component is both hot-formed and quench-hardened; performing a first pretreatment step that precedes the heat treatment step in terms of process, wherein in the first pretreatment step, the steel component is provided with a corrosion-resistant anti-scale layer to protect against scaling in the heat treatment step; and performing a second pretreatment step before the heat treatment step, wherein a surface oxidation process occurs in the second pretreatment step in which a weakly reactive corrosion-resistant oxidation layer is formed on the anti-scale layer such that abrasive tool wear is reduced in the forming step, wherein the surface oxidation in the second pretreatment step is carried out by pickling passivation, and wherein, for the pickling passivation, the steel component is treated in a pickling bath with a pickling solution and then dried, wherein the pickling solution is an aqueous solution of a phosphoric acid, and wherein the surface oxidation takes place partially in the second pretreatment step with a formation of at least one surface section without the oxidation layer and a surface section with the oxidation layer, and wherein the surface sections have different surface roughnesses which, in the forming step, form different adhesion/friction coefficients with the forming tool surface, as a result of which the flow of material is controllable during the hot forming.

2. The method according to claim 1, wherein a third pretreatment step is performed prior to the heat treatment step, wherein in the third pretreatment step, a cover layer of a high melting point is formed in a dipping bath on the corrosion-resistant oxidation layer, and wherein melting of underlying layers in the subsequent heat treatment step is prevented via the cover layer.

3. The method according to claim 2, wherein the cover layer is a metal oxide layer, a titanium oxide layer, or a titanium-zirconium layer.

4. The method according to claim 2, wherein the oxidation layer and/or the cover layer have a melting point greater than 2000 C., a flexural strength greater than 300 MPa, a compressive strength greater than 2000 MPa, and a Vickers hardness greater than 1600 HV1.

5. The method according to claim 2, wherein the anti-scale layer, the oxidation layer, and the cover layer are applied to a substrate of the steel component before the heat treatment step, and wherein during the heat treatment step, further phases or layers including an AlFeSi phase, an AlFe zone, an AlFeSiMn Zone, an FeAl zone, and an aluminum oxide zone form by diffusion processes under the oxidation layer.

6. The method according to claim 2, wherein the cover layer is a titanium oxide layer or a titanium-zirconium layer.

7. The method according to claim 1, wherein the anti-scale layer is an aluminum-silicon layer, which is applied to the steel component in the first pretreatment step using a hot-dip coating process or a coil-coating process.

8. The method according to claim 1, wherein the anti-scale layer is an aluminum based layer, which is applied to the steel component in the first pretreatment step using a hot-dip coating process or a coil-coating process.

9. The method according to claim 1, wherein the anti-scale layer is a zinc or zinc-iron coating, which is applied to the steel component in the first pretreatment step using a hot-dip coating process.

10. The method according to claim 1, wherein the starting material or substrate of the steel component is a manganese-boron-alloyed quenched and tempered steel.

11. The method according to claim 1, wherein a total layer thickness before the heat treatment step is less than 20 m or greater than 33 m.

12. The method according to claim 1, wherein an austenitization temperature of the steel component is not achieved.

13. The method according to claim 1, wherein an austenitization temperature of the steel component is only partially achieved.

14. The method according to claim 1, wherein a critical cooling rate for forming a martensite structure of the steel component is not achieved or is only partially achieved.

15. The method according to claim 1, wherein the starting material or substrate of the steel component is 20MnB5, 22MnB5, 27MnB5 or 30MnB5.

16. A method comprising: heating a steel component into a range of complete or partial austenitization in a heat treatment step; performing a forming step in which the heated steel component is both hot-formed and quench-hardened; performing a first pretreatment step that precedes the heat treatment step in terms of process, wherein in the first pretreatment step, the steel component is provided with a corrosion-resistant anti-scale layer to protect against scaling in the heat treatment step; and performing a second pretreatment step before the heat treatment step, wherein a surface oxidation process occurs in the second pretreatment step in which a weakly reactive corrosion-resistant oxidation layer is formed on the anti-scale layer such that abrasive tool wear is reduced in the forming step, wherein the surface oxidation takes place partially in the second pretreatment step with a formation of at least one surface section without the oxidation layer and a surface section with the oxidation layer, and wherein the surface sections have different surface roughnesses which, in the forming step, form different adhesion/friction coefficients with the forming tool surface, as a result of which the flow of material is controllable during the hot forming.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus, are not limitive of the present invention, and wherein:

(2) FIG. 1 shows the layer structure on a finished steel component after hot forming;

(3) FIG. 2 shows in a simplified block diagram the process steps for producing the steel component shown in FIG. 1;

(4) FIGS. 3 to 6 show the layer structure on the surface of the steel component in different process steps;

(5) FIG. 7 shows the layer structure on a finished steel component in a view corresponding to FIG. 1; and

(6) FIG. 8 shows an exemplary embodiment in a view corresponding to FIG. 1.

DETAILED DESCRIPTION

(7) A coating system of a finished steel component 1, the system being formed by diffusion processes in the furnace, after hot forming is shown by way of example in FIG. 1. The base material (substrate) 3 of steel component 1 is, for example, 22MnB5. A diffusion zone 5, followed outwardly by further alloy layers, namely, an iron-aluminum-silicon zone 7, an iron-aluminum zone 9, an iron-aluminum-silicon-manganese zone 11, an iron-aluminum zone 13, and an aluminum oxide zone 15, an oxidation layer 17, and as a cover layer 19 a titanium oxide layer, is formed directly on base material 3.

(8) The laminar structure labeled by reference number 2 in FIG. 1 corresponds to a coating system as known in the prior art. In addition, the laminar structure is covered with oxidation layer 17 and with cover layer 19. These reduce, inter alia, the roughness of the metal surface of steel component 1, as a result of which the abrasive tool wear in the forming step and in the furnace transfer is reduced.

(9) The method for producing steel component 1 shown in FIG. 1 will be described hereinbelow with reference to FIGS. 2 to 6: Thus, in FIG. 2, base material 3 of steel component 1 is first subjected to a pretreatment I in preparation for the hot forming. Pretreatment I has, inter alia, the process steps Ia, Ib, and Ic shown in FIG. 2. In process step Ia, a hot-dip coating takes place in which aluminum-silicon layer 15 is applied to steel component base material 3. This serves as an anti-scale layer during the heat treatment. In the subsequent process step Ib, a pickling passivation takes place in which steel component 1 is treated with a pickling solution in a pickling bath and then air-dried at room temperature. The pickling solution can be, for example, an aqueous solution of an acid, a base, or pH neutral, for example, phosphoric acid, by means of which the weakly reactive and corrosion-resistant oxidation layer 17 forms on aluminum-silicon layer 15. Next, in a third process step Ic, a further hot-dip coating is carried out in which titanium oxide layer 19 is applied as the cover layer.

(10) In FIG. 3, steel component 1 is shown after the completed process step Ia, that is, with AlSi layer 15. FIG. 4 shows steel component 1 after process step Ib (that is, after pickling passivation) with the additional oxidation layer 17, whereas steel component 1 after process step Ic, namely, with the additional covering layer 19, is shown in FIG. 5.

(11) Subsequent to pretreatment I, steel component 1 is transferred to a heat treatment furnace in which heat treatment II is performed. For this purpose, steel component 1 is heated to a target temperature of, for example, at least 945 C., by way of example for a predefined process duration which may be in the range of, for example, 100 to a maximum of 4000 seconds. The coating system shown in FIG. 6 forms on the surface of steel component 1 by diffusion processes in the furnace. Steel component 1, which is still in the hot state, is then subjected to a hot forming III, in which steel component 1 is both hot-formed and quench-hardened.

(12) In the above exemplary embodiment, anti-scale layer 15 is an AlSi layer. Instead, anti-scale layer 15 may also be a zinc or zinc-iron coating. This can be applied to steel component 1 preferably in a hot-dip coating process.

(13) FIG. 7 shows a steel component 1 according to a second exemplary embodiment, the coating system of which is essentially identical to the coating system shown in FIG. 1. As an alternative to FIG. 1, cover layer 19 has been omitted in FIG. 7, so that oxidation layer 17 is exposed to the outside.

(14) A further steel component 1 in which oxidation layer 17 is likewise exposed to the outside is shown in FIG. 8. The surface of steel component 1 in FIG. 8 is divided into a surface section 21 without oxidation layer 17 and into a surface section 23 with oxidation layer 17. The two surface sections 21, 23 have different surface roughnesses, which form different adhesion/friction coefficients for the forming tool surface in the following forming step III, as a result of which the flow of material during hot forming can be controlled. Different surface sections 21, 23 of this kind can be adjusted, for example, via a masking of steel component 1 during passage through the pickling passivation (pickling plant).

(15) The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are to be included within the scope of the following claims.