Temporary corrosion protection layer

Abstract

The present invention relates to a method for producing a component made of a steel product coated with an Al-Si protective coating, comprising the following steps: - providing a substrate consisting of a steel produced coated with an Al-Si protective coating, - heating the substrate to a temperature T1 such that the Al-Si protective coating is only partially pre-alloyed with Fe of the steel product, - cooling the pre-alloyed substrate to room temperature, - applying a corrosion protection oil to the surface of the pre-alloyed substrate, wherein the corrosion protection oil consists of a composition containing fatty acid ester, - transporting the pre-alloyed substrate to which the corrosion protection oil has been applied, - heating the pre-alloyed substrate to which the corrosion protection oil has been applied to a temperature T2 such that the Al-Si protective coating is fully alloyed with Fe of the steel product and the corrosion protection oil is removed without leaving residue, and - shaping the re-heated substrate to form the component.

Description

Technical field

[0001] The present invention relates to a method for manufacturing a component from a steel product coated with an Al-Si protective coating. Technical background

[0002] Steel products, such as steel strips or steel sheets, are nowadays provided with an Al-Si protective coating by means of hot-dip aluminizing to protect them from corrosive influences.

[0003] To prevent localized flaking of the protective coating during the forming process of a desired component, the steel products are generally alloyed with the iron of the base material. This necessitates longer annealing times.

[0004] From DE 10 2008 006 771 B3 it is known that a pre-alloyed Al-Si protective coating leads to a reduced heating time compared to an unpre-treated Al-Si protective coating.

[0005] GB 1 500 807 A describes properties of esters.

[0006] Despite the protective coating present on such pre-alloyed steel products, in practice it has been shown that weather conditions, for example during storage and / or transport, lead to corrosion (red rust) on the surface. Summary of the invention

[0007] The invention is therefore based on the objective of providing a method that overcomes the disadvantages of the prior art.

[0008] This problem is solved by a method with the features of claim 1.

[0009] According to the invention, the method for manufacturing a component from a steel product coated with an Al-Si protective coating comprises the following steps: Providing a substrate consisting of a steel product coated with an Al-Si protective coating, heating the substrate to a temperature T1 such that the Al-Si protective coating is only partially pre-alloyed with the Fe of the steel product, cooling the pre-alloyed substrate to room temperature, applying a corrosion protection oil to the surface of the pre-alloyed substrate, wherein the corrosion protection oil consists of a composition containing fatty acid esters, transporting the pre-alloyed substrate coated with the corrosion protection oil, heating the pre-alloyed substrate coated with the corrosion protection oil to a temperature T2 such that the Al-Si protective coating is completely alloyed with the Fe of the steel product and the corrosion protection oil is removed without residue, and forming the reheated substrate into the component.

[0010] Surprisingly, it has been shown that - in addition to the extra temporary corrosion protection - the pre-alloyed substrate treated with the corrosion protection oil leaves no residues after reheating for the forming process that adversely affect the material performance and thus do not adversely affect further process steps within the production chain.

[0011] Furthermore, it was surprisingly shown that the heating time of the pre-alloyed substrate coated with corrosion protection oil to temperature T 2 could be significantly reduced.

[0012] In the process according to the invention, a substrate consisting of a steel product coated with an Al-Si protective coating is first provided. In this case, the steel product is a steel sheet or steel strip coated with an Al-Si protective coating. Typically, the steel product is coated by hot-dip aluminizing.

[0013] In the next process step, the substrate is heated to a temperature T1 such that the Al-Si protective coating is only partially pre-alloyed with the Fe of the steel product. The substrate, thus only partially alloyed, exhibits a toughness that allows it to be cut or trimmed without damaging the protective coating.

[0014] Heating the substrate to temperature T1 can be carried out in a bell furnace, chamber furnace or continuous furnace.

[0015] Preferably, such incompletely alloyed Al-Si protective coatings have an Fe content of 25–50 wt.%. In a particularly preferred embodiment, the Al-Si protective coating consists of 10 wt.% Si, 25–50 wt.% Fe, and the remainder Al.

[0016] According to the invention, after the pre-alloyed substrate has cooled to room temperature, a corrosion protection oil is applied to the surface, wherein the corrosion protection oil consists of a composition containing fatty acid esters. The application of the corrosion protection oil to the pre-alloyed substrate can be carried out, for example, by spraying or immersion in a bath containing the corrosion protection oil. Alternatively, the corrosion protection oil can be applied by means of a roller application process.

[0017] Alternatively, the pre-alloyed substrate can be immersed in a bath containing the corrosion protection oil before cooling to room temperature, in order to cool it and provide it with temporary corrosion protection in one process step.

[0018] The pre-alloyed substrate, coated with corrosion protection oil, is then transported. The term "transport" as used here encompasses all types of transport operations in which the pre-alloyed substrate is moved from a first location, for example a steel producer, to a second location, for example a manufacturing plant of a steel processing company or a storage facility.

[0019] In a further step of the process according to the invention, the pre-alloyed substrate provided with the corrosion protection oil is heated to a temperature T2 such that the Al-Si protective coating is completely alloyed with the Fe of the steel product and the corrosion protection oil is removed without leaving any residue. Thus, neither deformed carbon chains remain on the surface nor are corrosive or toxic combustion residues formed during the heating process.

[0020] Heating the substrate to temperature T2 can be done inductively, conductively or by means of thermal radiation in a continuous oven.

[0021] The reheated substrate is then reshaped into the desired component.

[0022] Preferably, this involves hot forming. Furthermore, the component is preferably an automobile body or parts thereof.

[0023] According to a preferred embodiment, the temperature T2 corresponds to a temperature range of 850 °C to 1000 °C. More preferably, the temperature T2 corresponds to 880 °C to 930 °C.

[0024] According to another preferred embodiment, heating the pre-alloyed substrate provided with the corrosion protection oil to temperature T 2 comprises the following process steps: Heating the substrate to a temperature range T2 of 850 °C to 1000 °C, preferably 880 °C to 930 °C, maintaining the substrate in the temperature range T2, and cooling the substrate to a temperature range T3 of 550 °C to 780 °C, preferably 600 °C to 700 °C.

[0025] Preferably, the heating time to T2 is 60 to 210 s, preferably 90 to 180 s. The heating time of the substrate depends on the thickness of the substrate and must be adjusted individually, depending on the specific substrate used.

[0026] Preferably, the holding time in the temperature range T2 is 60 to 600 s, preferably 30 to 120 s.

[0027] Cooling is preferably carried out at a cooling rate in the range of 5 to 25 K / s, preferably in the range of 10 to 20 K / s.

[0028] Preferably, the substrate is cooled during its transfer into a tool in which it undergoes a forming process.

[0029] During the forming process, further cooling takes place, and then the material hardens when it is fully interlocked with the tool.

[0030] Preferably, heating to T2 takes place under a protective atmosphere. Dry air or a protective gas, such as nitrogen gas, can be used as the protective atmosphere.

[0031] In another preferred embodiment, the temperature T 1 corresponds to a temperature range of 550° to 750°C, preferably from 550° to 700°C.

[0032] According to the invention, the composition contains at least 98 wt.% of the fatty acid ester. In another preferred embodiment, the composition preferably contains 98.5–99 wt.% of the fatty acid ester. With such a composition, the gaseous combustion residues consist of CO₂ and H₂O and can be discharged from the furnace chamber together with the exhaust air without further complex measures.

[0033] In a particularly preferred embodiment, the fatty acid ester is a C 8 - C 16, more preferably a C 11 - C 17 compound.

[0034] Preferably, the composition has a sulfur content in the range of 1 - 2 wt.%, more preferably in the range of 1 - 1.5 wt.%.

[0035] Preferably, the composition has a saponification value in the range of 150 - 265 mg KOH / g, more preferably in the range of 165 - 195 mg KOH / g.

[0036] In another preferred embodiment, the corrosion protection oil is applied to the substrate in an amount of 0.5 to 2 g / m², more preferably 0.7 to 1.7 g / m².

[0037] Preferably, the composition of the corrosion protection oil contains no grease.

[0038] Preferably, the composition contains no additives or inhibitors.

[0039] According to a particularly preferred embodiment, the corrosion protection oil is not removed from the substrate coated with the corrosion protection oil by means of a cleaning step before it is heated to temperature T2. This eliminates, among other things, the need for a complex cleaning device within the process. Furthermore, the entire process is not only more cost-effective, since the process times are shorter compared to processes with a cleaning step, but also more environmentally friendly.

[0040] According to a further aspect, the present invention relates to the use of a corrosion protection oil consisting of a composition containing fatty acid esters, wherein the composition contains at least 98 wt.% of the fatty acid ester, as temporary corrosion protection during the storage and / or transport of pre-alloyed substrates consisting of a steel product coated with an Al-Si protective coating, which, after storage, is heated to a temperature T 2 such that the Al-Si protective coating is completely alloyed with Fe of the steel product and the corrosion protection oil is removed without residue, and which is then formed into a component as a reheated substrate. Examples

[0041] The invention will be explained in more detail below using examples.

[0042] A substrate consisting of a 1.5 mm thick steel sheet of grade 22MnB5 was hot-dip coated with a 25 µm thick Al-Si protective coating. The coating contains 10 wt% Si, 3 wt% Fe, and the remainder Al. The steel product coated with the Al-Si protective coating was pre-alloyed as a pre-fabricated blank at 700 °C in a convection oven. The Al-Si protective coating of the pre-alloyed steel blank now contains 30 wt% Fe, 10 wt% Si, and the remainder Al. Subsequently, 0.5 g / m² of a corrosion protection oil is applied by roller coating. The corrosion protection oil used is a fatty acid derivative of a natural oil and contains no other additives or inhibitors. After transport and storage in an exposed location, these blanks are further processed. No surface changes or corrosion damage were detected prior to further processing.For further processing, the circuit boards are fed into a hot forming furnace by an industrial robot and austenitized at 925 °C for 2.5 minutes to the point where they can subsequently be formed and hardened in a cooled die. Measurements at the hot forming furnace revealed no further emissions in the furnace atmosphere other than CO₂, H₂O, and the already existing nitrogen in the furnace atmosphere. No residues of the applied oil were detected on the press-hardened component either.

Claims

1. Method for producing a component made of a steel product coated with an Al-Si protective coating, comprising the following steps: - providing a substrate consisting of a steel product coated with an Al-Si protective coating, - heating the substrate to a temperature T1 such that the Al-Si protective coating is only partially pre-alloyed with Fe of the steel product, - cooling the pre-alloyed substrate to room temperature, - applying a corrosion protection oil to the surface of the pre-alloyed substrate, wherein the corrosion protection oil consists of a composition containing fatty acid ester, wherein the composition contains at least 98 % by weight fatty acid ester - transporting the pre-alloyed substrate to which the corrosion protection oil has been applied, - heating the pre-alloyed substrate to which the corrosion protection oil has been applied to a temperature T2 such that the Al-Si protective coating is fully alloyed with Fe of the steel product and the corrosion protection oil is removed without residue, and - shaping the re-heated substrate to form the component.

2. Method according to Claim 1, wherein the temperature T2 corresponds to a temperature range from 850 °C to 1000 °C, preferably 880 °C to 930 °C.

3. Method according to Claim 1, wherein the heating of the pre-alloyed substrate to which the corrosion protection oil has been applied to the temperature T2 comprises the following method steps: - heating the substrate to the temperature range T2 from 850 °C to 1000 °C, preferably 880 °C to 930 °C, - keeping the substrate in the temperature range T2, and - cooling the substrate to a temperature range T3 from 550 °C to 750 °C, preferably 600 °C to 700 °C.

4. Method according to Claim 3, wherein the heating to T2 lasts 60 to 210 s, preferably and depending on the sheet thickness 90 to 180 s.

5. Method according to Claim 3 or 4, wherein keeping in the temperature range T2 lasts 30 to 600 s, preferably 30 to 120 s.

6. Method according to any one of Claims 3 to 5, wherein cooling after pre-alloying takes place at a cooling rate in the range from 2 to 25 K / s, preferably in the range from 8 to 20 K / s.

7. Method according to any one of the preceding Claims 1 to 6, wherein the heating to T2 takes place in a protective atmosphere.

8. Method according to any one of the preceding Claims 1 to 7, wherein the temperature T1 corresponds to a temperature range from 550 °C to 780 °C preferably from 600 °C to 700 °C.

9. Method according to any one of the preceding Claims 1 to 8, wherein the composition contains at least 98.5 % - 99.8 % by weight fatty acid ester.

10. Method according to any one of the preceding Claims 1 to 9, wherein the fatty acid ester is a C8-C16, preferably a C11-C17 compound.

11. Method according to any one of the preceding Claims 1 to 10, wherein the composition has a sulphur content in the range from 0.1 - 2 % by weight, preferably in the range from 0.5 - 1 % by weight.

12. Method according to any one of the preceding Claims 1 to 11, wherein the composition has a saponification number in the range from 150 - 265 mg KOH / g, preferably in the range from 165 - 195 mg KOH / g.

13. Method according to any one of the preceding Claims 1 to 12, wherein the corrosion protection oil is applied to the substrate in a quantity from 0.5 to 2 g / m2, preferably 0.7 to 1.7 g / m2.

14. Use of a corrosion protection oil consisting of a composition containing fatty acid ester, wherein the composition contains at least 98 % by weight fatty acid ester, as temporary corrosion protection during the storage and / or transport of pre-alloyed substrates consisting of a steel product coated with an Al-Si protective coating, which product is heated after storage to a temperature T2 such that the Al-Si protective coating is completely alloyed with Fe of the steel product and the corrosion protection oil is removed without residue, and the substrate in reheated form is subsequently shaped to form the a component.