WO2021094404A1

WO2021094404A1 - Method for producing a part of steel or aluminium

Info

Publication number: WO2021094404A1
Application number: PCT/EP2020/081808
Authority: WO
Inventors: Chiel Matthias DANE; Jan Paul Penning
Original assignee: Tata Steel Ijmuiden B.V.
Priority date: 2019-11-13
Filing date: 2020-11-11
Publication date: 2021-05-20
Also published as: EP4058283A1

Abstract

The invention relates to a method for producing a part of steel or aluminium, for instance for automotive purposes. According to the invention the method comprising the following steps: - providing a steel or aluminium sheet; - providing a coating on one or both sides of the sheet, wherein the coating consists of one or more polymer layers, wherein the polymer is a thermoplastic polymer; - and forming the coated sheet into a part by pressing, stamping, deep drawing, roll forming, bending, hydroforming or stretching after the polymer layer(s) have been provided on the sheet.

Description

METHOD FOR PRODUCING A PART OF STEEL OR ALUMINIUM

The invention relates to a method for producing a part of steel or aluminium, for instance for automotive purposes.

In the automotive industry, but also in other industries, flat sheets or blanks are cut from strip material produced by the steel or aluminium manufacturers. These flat sheets or blanks have to be (de)formed before they can be used in cars or other products, for instance by pressing, deep drawing, bending or roll forming.

Especially for deep drawing, but also in other techniques, the sheet or blank has to be deformed three-dimensionally in a pressing tool. During pressing, high friction can exist between the sheet and the tooling; the resulting friction forces reduce the forming capacity. To reduce the friction, usually an oil layer is applied on the sheet or blank.

However, the application of such an oil layer is an additional cost factor, and moreover the oil layer has normally to be removed from the pressed part before the part can undergo further processing steps, such as painting.

Moreover, it is increasingly seen as advantageous when the part can be attached to other parts or other material in an easy way. A part that is degreased is not seen as the best starting point for attaching.

It is therefore an object of the invention to provide a method for producing a part of steel or aluminium that is easy to form or deform with the standard forming and deformation techniques.

It is another object of the invention to provide a method for producing a part of steel or aluminium that is cheaper to form or deform with the standard forming and deformation techniques.

It is furthermore an object of the invention to provide a method for producing a part of steel or aluminium that makes it easier to attach the formed part to another object or material.

According to the inventors at least one of these objects is reached by using the method of the invention, wherein the method comprising the following steps: providing a steel or aluminium sheet; providing a coating on one or both sides of the sheet, wherein the coating consists of one or more polymer layers, wherein the polymer is a thermoplastic polymer; and forming the coated sheet into a part by pressing, stamping, deep drawing, roll forming, bending, hydroforming or stretching after the polymer layer(s) have been provided on the sheet. By providing a polymer coating on one or both sides of the sheet, the sheet will act as a lubricant between the sheet and the tool for deforming the sheet. In this way, the friction between the sheet and the tool is reduced. The polymer coating remains on the part produced using the tool.

Preferably, each layer consists of a polymer from the group of:

• polyolefins;

• polyamides;

• polyesters

• polycarbonates.

It has been found by the inventors that at least one of the thermoplastic polymers in each of these four groups can be used as a layer, both when the coating consists of one layer and when the coating consists of two or more layers.

Preferably the polymer is chosen such that:

• a polyolefin is selected from the group of polyethylenes, polypropylenes, polyethylene copolymers, polypropylene copolymers, chemically modified polyolefins such as maleic anhydride grafted polyethylene or polypropylene, or is a combination of two or more of these polyolefins;

• a polyamide is selected from the group of polyamide 6 (PA6), polyamide 6.6 (PA6.6), polyamide 4.6 (PA4.6) or is a combination of two or more of these polyamides;

• a polyester is selected from the group polyethylene terephthalate (PET), polybutylene terephthalate (PBT), a copolyester containing isophthalic acid (I PA) and/or cyclohexanedimethanol (CHDM), or is a combination of two or more of these polyesters;

• a polycarbonate is a polycarbonate of Bisphenol A.

From a chemical point of view, these groups of polyolefins, polyamides, polyesters and polycarbonates are suitable as a layer on a steel or aluminium sheet that is to be deformed, in view of their adherence properties on steel and aluminium, and/or in view of their lubricating properties relative to the deformation tool.

Now the polymer coating is suitable to act as a lubricant, it is advantageous when the coated sheet is formed free from an applied lubricant or oil. In this case, there is no need to apply a layer of an oil or other lubricant on the strip or sheet, and thus also no need to remove this layer after the sheet has been formed or deformed into a part. Thus, not using an applied lubricant or oil means a reduction in processing steps and thus a reduction in cost. Furthermore, there are less environmental issues. It is known to the inventors that a so-called “oil-free press shop” is seen as advantageous by the industry.

When using the polymer coated sheet according to the invention for deep drawing, for instance, the sheet is deformed between a punch and a die, while the circumference of the sheet or blank is kept under a compression force by a blank holder. The distance between the punch and the die is larger than the thickness of the polymer coated sheet, that is the combined thickness of the sheet and the polymer coating. In this way the polymer coating can act as a lubricant. The polymer coating with remain adhered to the sheet and deforms together with the sheet. Usually, the distance between the punch and the die is approximately 1.2 times the thickness of the polymer coated sheet, so between 1.1 and 1.4 times the thickness of the polymer coated sheet.

Preferably the steel sheet is cut from a hot-rolled steel strip or from a hot-rolled and subsequently cold-rolled steel strip. Using a hot-rolled or cold-rolled strip to cut the sheet from is the easiest and most cost-effective way to provide a sheet for the present method. Such a strip can be produced in the required composition and with the required properties. The steel sheet can for instance be a formable steel sheet for outer panels for automotive purposes, a high strength steel sheet for the chassis of a car, or a steel sheet for the white goods industry. Such sheet are known as such to the skilled person.

According to a preferred embodiment the hot rolled or cold rolled strip has a width of less than 600 mm. Steel strips having this width are suitable to cut out sheets or blanks that can be formed by for instance deep drawing.

The hot rolled or cold rolled steel strip preferably has a thickness of at least 0,66 mm, preferably a thickness of at least 0,68 mm. Usually the strip has a maximum thickness of 4 mm, because thicker sheets or blanks are difficult to form and took heavy for use.

For instance for automotive purposes the high strength steel sheet used can be a HSLA steel, a AHSS or a UHSS, such as a DP steel, a CP steel or a TRIP steel, which usually have a thickness between 0,8 and 2,5 mm. Such steel types are known to the person skilled in the art and ned no further elucidation. The same holds for steel sheet for outer panels which are made from a mils steel such as DX or BH steel, which sheets usually have a thickness between 0,7 and 1 ,5 mm. For other purposes standard cold rolled steel sheets or blanks can be used.

According to a preferred embodiment the steel sheet consists of a steel having a UTS of at least 600 MPa, more preferably a UTS of at least 800 MPa, preferably the steel sheet being a AHSS or UHSS such as DP, CP, TRIP, TWIP steel. Such high strength steels have to be deep drawn for use in the automotive industry, and the use of the polymer coated steel sheet according to the invention is very advantageous for these purposes.

According to a preferred embodiment the steel sheet is coated with a metal or metal alloy layer that is present between the steel sheet and the one or more polymer layers, preferably the steel sheet is coated with a zinc or zinc alloy layer, more preferably the zinc or zinc alloy layer being a EG or GA or Gl or ZM applied layer and/or the steel sheet is treated with an adhesion promotor or conversion coating, and wherein the steel sheet coated with a GA or Gl or ZM layer is preferably coated by hot dip coating. For automotive purposes, it is almost always a requirement that the steel sheet is coated with a metal or metal alloy to reduce or prevent corrosion. A conversion coating or adhesion promotor is often required when a polymer coating must be applied on a metal or metal alloy that is present on the steel sheet.

According to another preferred embodiment the aluminium sheet consists of AA5xxx, AL6xxx or AA7xxx. These aluminium alloys are frequently used in the automotive industry for outer panels or chassis components.

In a preferred embodiment the polymer coating consists of more than one layer, in which the layer contacting the metal sheet is a (co)polyester or a chemically modified polyolefin or a polycarbonate. A (co)polyester or a chemically modified polyolefin or a polycarbonate is very suitable as layer in direct contact with the metal sheet in view of its bonding properties.

According to a further preferred embodiment the polymer coating consists of more than two layers, in which the outermost layer, facing away from the metal sheet, is a (co)polyester or a chemically modified polyolefin or a polycarbonate. A (co)polyester or a chemically modified polyolefin or a polycarbonate is also very suitable as outermost layer in view of its adhesion properties to other materials.

It is preferred that, when the polymer coating consists of more than one layer, the outermost layer, facing away from the metal sheet, contains certain additives to reduce the friction coefficient of the coating, such as insoluble ‘anti-block’ particles and organic slip additives. Using such additives, which are known per se, improves the lubricating effect of the polymer coating by reducing the friction coefficient thereof.

In another preferred embodiment the polymer coating consists of one layer that is a (co)polyester or a chemically modified polyolefin or a polycarbonate. Using only one layer as a coating simplifies the production process of the strip or sheet to a certain extent.

Preferably a polymer coating has a layer contacting the metal sheet with a thickness between 2 and 10 pm, and the polymer coating has a thickness between 10 and 200 pm. Preferably the polymer coating has a thickness between 10 and 90 pm , more preferably a thickness between 10 and 70 pm. These thicknesses provide the polymer coating with a good strength during deformation, such that disruption of the coating is reduced. Especially thinner polymer coatings are less likely to deform during the forming process.

According to a further aspect of the invention after the forming of a part from the polymer coated sheet, a further layer is attached to the part, which further layer at least partly consists of a polymer and optionally further consists of flexible material such a fibres, to form a steel-polymer hybrid part.

The use of a polymer coating on the part that is formed in accordance with the invention thus has the further advantage that a further layer can be easily attached to the formed part. This is easy to realise because the coating on the part consists of a polymer, and the further layer at least partly consists of a polymer as well. Adhesion between the coating and the further layer can thus be realised by choosing suitable polymers for both the coating and the further layer.

In another further aspect of the invention a further layer is attached to the polymer coated sheet before the forming of a part, which further layer at least partly consists of a polymer and optionally further consists of flexible material such a fibres, to form a steel-polymer hybrid part.

By attaching a further layer to the polymer coated sheet before the forming thereof, the part that is formed is directly a hybrid part, without any further processing step. Since the adhering of a further layer to a flat sheet is much easier than the adhering to a formed sheet, this is an advantageous method. Of course the further layer has to be able to withstand the forming step.

In any of the above inventive methods it is possible, after the forming of a part from the polymer coated sheet, to apply a polymer three-dimensional body against the polymer layer of the part, preferably by injection moulding in a die for forming the part.

Since the part that is formed has a side that is provided with an at least partially polymeric layer, it is possible to apply the three-dimensional body to the part. Injection moulding is a suitable way to apply the three-dimensional body, because a very good adherence can be obtained.

According to an advantageous embodiment the part is painted after the preceding steps have been performed. This is often needed when the part is used for automotive purposes, but also for other purposes.

No examples are presented, but in view of the above elucidation of the claims the person skilled in the art will have no problem in performing experiments using the method as claimed in the attached claims and explained above. No further description of the use of the invention is thus needed to reproduce the technical information as provided, and the description of the invention provides the person skilled in the art with all information in a manner that is clear and complete to carry out the invention. Thus, even now no examples are presented, this description discloses the invention to fulfil all requirements of the European Patent Convention.

Claims

1. Method for producing a part of steel or aluminium, for instance for automotive purposes, the method comprising the following steps: providing a steel or aluminium sheet; providing a coating on one or both sides of the sheet, wherein the coating consists of one or more polymer layers, wherein the polymer is a thermoplastic polymer; and forming the coated sheet into a part by pressing, stamping, deep drawing, roll forming, bending, hydroforming or stretching after the polymer layer(s) have been provided on the sheet.

2. Method according to claim 1, wherein each layer consists of a polymer from the group of:

• polyolefins;

• polyamides;

• polyesters

• polycarbonates.

3. Method according to claim 2, wherein

• a polyester is selected from the group polyethylene terephthalate (PET), polybutylene terephthalate (PBT), a copolyester containing isophthalicacid (I PA) and/or cyclohexanedimethanol (CHDM), or is a combination of two or more of these polyesters

• a polycarbonate is a polycarbonate of Bisphenol A.

4. Method according to any one of the preceding claims, wherein the coated sheet is formed free from an applied lubricant or oil.

5. Method according to any one of the preceding claims, wherein the steel sheet is cut from a hot-rolled steel strip or from a hot-rolled and subsequently cold-rolled steel strip, the hot rolled or cold rolled strip preferably having a width of less than 600 mm and/or having a thickness of at least 0,66 mm, preferably a thickness of at least 0,68 mm.

6. Method according to any one of the preceding claims, wherein the steel sheet is coated with a metal or metal alloy layer that is present between the steel sheet and the one or more polymer layers, preferably the steel sheet is coated with a zinc or zinc alloy layer, more preferably the zinc or zinc alloy layer being a EG or GA or Gl or ZM or PVD applied layer and/or wherein the steel sheet is treated with an adhesion promotor or conversion coating, and wherein the steel sheet coated with a GA or Gl or ZM layer is preferably coated by hot dip coating.

7. Method according to any one of the preceding claims, wherein the steel sheet consists of a steel having a UTS of at least 600 MPa, preferably a UTS of at least 800 MPa, preferably the steel sheet being a AHSS or UHSS such as DP, CP, TRIP, TWIP steel.

8. Method according to any one of the preceding claims, wherein the polymer coating consists of more than one layer, in which the layer contacting the metal sheet is a (co)polyester or a chemically modified polyolefin or a polycarbonate.

9. Method according to any one of the preceding claims, wherein the polymer coating consists of more than two layers, in which the outermost layer, facing away from the metal sheet, is a (co)polyester or a chemically modified polyolefin or a polycarbonate.

10. Method according to any one of the preceding claims, wherein the polymer coating consists of more than one layer, in which the outermost layer, facing away from the metal sheet, contains certain additives to reduce the friction coefficient of the coating, such as insoluble ‘anti-block’ particles and organic slip additives.

11. Method according to any one of the claims 1 - 7, wherein the polymer coating consists of one layer that is a (co)polyester or a chemically modified polyolefin or a polycarbonate.

12. Method according to any one of the preceding claims, wherein a polymer coating has a layer contacting the metal sheet with a thickness between 2 and 10 pm, and wherein the polymer coating has a thickness between 10 and 200 pm, preferably a thickness between 10 and 90 pm, more preferably a thickness between 10 and 70 pm.

13. Method according to any one of the claims 1 - 12, wherein after the forming of a part from the polymer coated sheet, a further layer is attached to the part, which further layer at least partly consists of a polymer and optionally further consists of flexible material such a fibres, to form a steel-polymer hybrid part.

14. Method according to any one of claims 1 - 12, wherein a further layer is attached to the polymer coated sheet before the forming of a part, which further layer at least partly consists of a polymer and optionally further consists of flexible material such a fibres, to form a steel-polymer hybrid part.

15. Method according to any one of claims 1 - 14, wherein after the forming of a part from the polymer coated sheet, a polymer three-dimensional body is applied against the polymer layer of the part, preferably by injection moulding in a die for forming the part.

16. Method according to any one of claims 1 - 15, wherein the part is painted after the preceding steps have been performed.