Component made of metallic composite material and method for the manufacture of the component by hot forming

Abstract

A component made of metallic composite material having high corrosion resistance and scale resistance. The metallic composite material contains as a core material an uncoated hardenable steel on which surface a corrosion resistance and scaling resistance layer is provided using heat resistant stainless steel, and has a yield strength Rp.sub.0,2 of at least 1000 MPa and a tensile strength R.sub.m of at least 1500 MPa for the core material and a critical scaling resistance temperature in air for the layer material is at least 850° C.

Claims

1. A component made of a metallic composite material having high corrosion resistance and scale resistance, wherein the metallic composite material comprises as a core material an uncoated hardenable steel on which surface a corrosion resistant and scaling resistant layer is provided using heat resistant stainless steel as a layer material, wherein the core material comprises in mass %: up to 0.48% C, up to 0.4% N, 10.5-18% Cr, up to 8% Ni, up to 18% Mn, up to 3.0% Mo, up to 1.0% Si, up to 0.65% Cu, up to 0.005 B, and the remainder being Fe and inevitable impurities and has a yield strength Rp.sub.0,2 of at least 1000 MPa and a tensile strength R.sub.m of at least 1500 MPa, and wherein the layer material is austenitic heat resistant stainless steel comprising by mass % 18-25% Cr, 10-19% Ni, Mn≥0.5% and Si≥0.4% or ferritic heat resistant stainless steel comprising by mass % 13-18% Cr, Mn≥0.5%, Si≥1.0% and C≤0.1%.

2. The component according to claim 1, wherein, a ratio of a total thickness of the layer material to a thickness of the core material is at most 0.4.

3. The component according to claim 1, wherein, a ratio of a total thickness of the layer material to a thickness of the core material is at most 0.2.

4. The component according to claim 1, wherein a ratio of a thermal conductivity of the layer material to a thermal conductivity of the core material is at most 2.

5. A method for the manufacture of the component of claim 1, comprising: welding a core material of uncovered hardenable steel and a layer material of heat resistant stainless steel together in order to produce a metallic composite material, wherein the layer material covers outer surfaces of the core material, heat treating the composite material at the austenization temperature for at most 5 minutes, hot forming the heat treated composite material in a hot forming tool to have a desired shape of the component, and cooling the hot formed component to the room temperature.

6. The method according to claim 5, wherein welding is carried out by laser welding.

7. The method according to claim 5, wherein welding is carried out by keyhole welding.

8. The method according to claim 5, wherein hot forming is carried out in an atmosphere comprising at least one shielding gas.

9. The method according to claim 8, wherein nitrogen is used as the shielding gas and a top part of the component has a nitride hardened surface with high wear resistance.

10. The method according to claim 5, wherein the layer material for the composite material of the component is made of austenitic heat resistant stainless steel comprising by mass % 18-25% Cr, 10-19% Ni, Mn≥0.5% and Si≥0.4%.

11. The method according to claim 5, wherein the layer material for the composite material of the component is made of ferritic heat resistant stainless steel comprising by mass % 13-18% Cr, Mn≥0.5%, Si≥1.0% and C≤0.1%.

12. The method according to claim 5, wherein a heating rate during the hot forming process of the composite material is at least 50K/s.

13. The method according to claim 5, wherein the composite material is removed from the hot forming tool during the cooling process when a martensitic start temperature (M.sub.s) of the layer material is reached.

14. The method according to claim 5, wherein a connection of the layer material to the core material is achieved by preconditioning the layer material by electrobrightening with an electrolytic solution comprising a 96% sulfuric acid and 85% orthophosphoric acid.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention is described in more detail referring to the drawings where

(2) FIG. 1 illustrates as one preferred embodiment of the invention a cut piece of the composite material schematically from the side view,

(3) FIG. 2 illustrates as another preferred embodiment of the invention a cut piece of the composite material schematically from the side view,

(4) FIG. 3 illustrates still another preferred embodiment of the invention schematically seen as a cross-sectional view, and

(5) FIG. 4 illustrates the surface of the layer material after electrobrightening and the surface of the core material schematically from the side view.

DESCRIPTION OF THE INVENTION

(6) The invention achieves a new kind of corrosion and scaling protection layer for a hardenable steel during a hot forming process. The structure is shown in FIG. 1. The core material 1 is covered on its surfaces with a layer material 2.

(7) According to the FIG. 2 the core material 11 is covered on its surfaces with a nitride hardening surface 12. The nitride hardening surface 12 is made of the layer material 13, which is positioned between the core material 11 and the nitride hardening surface 12.

(8) FIG. 3 shows the composite material of the invention used in a tubular shape. The core material 21 is covered on its outer surface and on its inner surface with a layer material 22. FIG. 3 also show a laser welder 23, the laser beam 24 and the weld seam 25 achieved with laser welding.

(9) On the basis of the non-scaling of the layer material during the hot forming process of the composite material of the invention it is achieved a better paintability for a cathodic dip coating process with a better paint adhesion, resistance against lattice cut and stone chipping influenced corrosion attack as well as a better behavior against cathodic disbanding and a better corrosion resistance. Further, using the composite material and the manufacturing method it is not required to make a post-treatment of the surface with brushing, grinding or polishing to remove unwanted oxides or scale.

(10) FIG. 4 illustrates the layer material 31 and the core material 32. The defined roughness of the layer material surface 33 is achieved by electrobrightening in order to ensure a high corrosion resistance in wet corrosion areas and to ensure a high paintability with dip coatings. The defined roughness of the layer material surface 34 is also achieved by electrobrightening in order to ensure a high adhesion force with the core material 32. The reference number 35 shows the undefined, uncoated and not prepared surface of the hardenable core material 32.

(11) The invention has the solution field in the wet corrosion areas of automotive passenger cars or commercial vehicles. The layer material made of stainless steel has a much higher melting point than the hot forming austenization temperature and, therefore, no adhesion is achieved with the ceramic kiln roller of the roller head furnaces. Further, cost-intensive anticorrosion primer, often used for sheets in the automotive industry, is not necessary.

(12) The layer material made of heat resistant stainless steel in accordance with the invention means high temperature strength and thus also high stiffness of the hardened sheet at temperatures above 250° C. The heating of the composite material up to the austenization temperature can be accelerated because no Fe-phases, such as FeAl, are generated on the surface of the layer material. Further, when the core material is a martensitic stainless material, the forming tool after heating can be opened at high temperature that increases the clock frequency, i.e. speeds up the production, during the component production. This means cost savings for the manufacturer. Based on the properties of the layer material the usage of conductive or inductive processes is possible in heating.

(13) The high electrochemical corrosion potential of the layer material in accordance with the invention enables an enduring corrosion protection instead of a zinc coating which only represents a sacrificial anode. The high electrochemical corrosion potential is important especially for wet corrosion areas of an automotive car body, such as a passenger car as well as a transporter or other commercial vehicles.

(14) In case that the layer material of the invention is of austenitic stainless steel, the austenitic microstructure results in a ductility of the weld seam. This means that the layer material has high weldability with no brittle phases or hardness increase, better fatigue behavior in the weld seam.

(15) In accordance with the method of the invention during the heating process of the composite material it is, if desired, to use an atmosphere of a shielding gas or an atmosphere of a combination of shielding gases, such as nitrogen, argon or helium. The shielding gas atmosphere increases the wear resistance of the layer material, which is practical, when the composite material of the invention is used in wearing circumstances, such as agricultural or mining solutions.

(16) When nitrogen is used as a shielding gas in connection with the method of the invention, it is, if desired, possible to generate a specific gas nitriding so that as illustrated in FIG. 2 the top part of the layer material has a nitride hardening surface having high wear resistance.