Method to produce a hot formed part, and part thus formed

Description

[0001] The invention relates to a method for manufacturing a coated steel part having very high mechanical properties using hot forming techniques, and a part thus formed.

[0002] Hot forming of parts is well-known in the art and is used in the automotive industry to produce complex parts with a reduced weight.

[0003] It has been found that hot forming of parts for instance using Zn or Zn alloy coated steel is susceptible to the forming of micro-cracks due to the hot forming process, wherein the micro-cracks are present not only in the coating, but also in the part itself. The micro-cracks in the hot formed part are seen as undesirable.

[0004] It is an object of the invention to provide a method for manufacturing a coated steel part having very high mechanical properties using hot forming techniques that avoid or at least diminish the occurrence of micro-cracks in the hot formed part.

[0005] It is another object of the invention to provide a method for manufacturing a coated steel part having very high mechanical properties using hot forming techniques that is faster than the known methods.

[0006] According to the invention one or more of the above objects are reached using a method for manufacturing a coated steel part having very high mechanical properties using hot forming techniques, comprising the following steps:

• providing a coated steel blank comprising a steel substrate with a coating layer
• heating the blank to a temperature of 800° C or more in a furnace
• transporting the heated blank from the furnace and placing the blank in a hot forming press
• hot forming the blank in the press to form the coated steel part
• quenching the coated steel part

wherein the heated blank has a temperature of at most 780° C when placed in the hot forming press.

[0007] The inventors have found that the forming of micro-cracks is strongly influenced by the temperature of the blank when it is placed in the hot forming press and the temperature at which the hot formed part is formed in the hot forming press. At high temperatures, such as used in the art, many micro-cracks are formed in the substrate, which often have a depth of more than 10 µm. Lowering the temperature at which the coated blank is placed in the hot forming press reduced the micro-cracks in number and depth. It has been found that cooling the heated blank to a temperature of at most 780° C when placed in the hot forming press already reduces the number and depth of the micro-cracks to a considerable extent.

[0008] Preferably, the blank is heated to a temperature of 850° C or more, more preferably a temperature of 880° C or more, and still more preferably to a temperature of 900° C or more. Usually the coated blank is heated to a temperature such that the substrate is fully and homogenously austenitised before it is placed in the hot forming press.

[0009] According to a preferred embodiment the heated blank has a temperature of at least 500° C when placed in the hot forming press, more preferably a temperature of at least 600° C, and still more preferably a temperature of at least 650° C. The temperature of the blank can be below the Arl temperature when placed into the blank, but preferably the blank is not cooled too far because the cooling rate is very critical at low temperatures.

[0010] Preferably the heated blank has a temperature of at most 740° C when placed in the hot forming press, and more preferably a temperature of at most 700° C. The temperature of the blank should be as low as possible to prevent (large) micro-cracks. Thus, it is optimal if the temperature of the blank is between 650° C and 700° C when the blank is placed in the hot forming press.

[0011] According to a preferred embodiment the heated blank is at least partially cooled between the furnace and the hot forming press by forced cooling, preferably with an average cooling velocity of at least 30° C/sec, more preferably with an average cooling velocity of at least 40° C/sec, still more preferably with an average cooling velocity of at least 60° C/sec, most preferably with an average cooling velocity of at least 80° C/sec. Forced cooling effectively reduces the blank temperature at a cooling rate that is higher than the cooling rate obtainable when the blank is cooled in open air. A high cooling velocity is preferred, so the time in which the blank is kept between the furnace and the hot forming press can be as short as possible. This is advantageous so as to realise desired high strength of the formed part and so as to reduce oxidation of the coating on the blank.

[0012] Preferably the heated steel blank is transported between the furnace and the hot forming press in a time span of at most 10 seconds, more preferably in a time span of at most 8 seconds, and still more preferably in a time span of at most 6 seconds. A short transportation time is also advantageous to reduce the cycle time to produce a hot formed part.

[0013] According to a preferred embodiment the heated blank is cooled between the furnace and the hot forming press using accelerated cooling making use of gas, such as cooling with forced air flow, cooling with forced N₂ flow, or cooling with forced CO₂ flow or cooled CO₂ using solid CO₂. Use of a gas flow to cool the blank provides a high cooling capacity which can be easily controlled.

[0014] According to another preferred embodiment the heated blank is cooled between the furnace and the hot forming press using liquid or solid material, such as liquid or solid CO₂ particles or metal cooling plates. Liquid or solid material will provide a fast cooling rate, but is more difficult to control. Solid CO₂ particles, for instance have the advantage of a fast cooling capacity and the additional advantage that oxide layers on the coating are at least partially removed at the same time, but solid CO₂ particles are more difficult to supply at the right time period. Metal cooling plates that are brought into contact with the heated blank provide a very fast cooling rate, but a tight control of the cooling time is required.

[0015] Preferably the coating on the blank is a metallic coating, preferably a coating layer of zinc or a zinc alloy. Zinc or zinc alloy can provide an active corrosion protection.

[0016] In this regard, it is preferred when the zinc alloy consists of 0.3 - 4.0% Mg and 0.05 - 6.0% Al, optionally at most 0.2% of one or more additional elements, unavoidable impurities, the remainder being zinc. It has been found that such a zinc alloy provides a better corrosion resistance to the hot formed part.

[0017] It is an option when the zinc or zinc alloy layer is pre-diffused in the substrate. The pre-diffused coating makes it possible to heat the blank fast to a temperature above the Ac3 temperature.

[0018] Preferably the zinc or zinc alloy coated steel blank contains an average of 10 - 75 % iron in the coating after heating the blank in the furnace, preferably an average of 25 - 55 %. This amount of iron in the coating ensures that the (initial) zinc or zinc alloy coating does not evaporate at temperatures above the Ac3 temperature.

[0019] According to a preferred embodiment the hot forming press is internally cooled such that the blank is cooled with a cooling velocity of at least 50° C/sec during the first second after the press is fully closed, preferably with a cooling velocity of at least 100° C/sec during the first second. This very fast cooling rate after the hot forming press is fully closed makes that the austenitic structure of the heated blank is transformed fully or almost fully to a martensitic structure.

[0020] Preferably the hot forming press is fully closed in at most one second. This fast closing of the hot forming press is advantageous so as to fast cool also the portions of the blank that are not in immediate contact with the press.

[0021] According to another preferred embodiment of the invention there is provided a hot formed part manufactured according to the above method, wherein the substrate of the part is free of micro-cracks having a depth of more than 15 µm, and preferably the substrate is free of micro-cracks having a depth of more than 10 µm. Parts having less deep micro-cracks are preferred over parts having deeper micro-cracks.

[0022] According to another preferred embodiment of the invention there is provided a hot formed part manufactured according to the above method, wherein the micro-cracks in the substrate have an average depth of at most 3 µm in five surface area's of the blank with a size of 1 cm². Parts having on average shallow micro-cracks are preferred over parts having deep micro-cracks on average.

Claims

1. Method for manufacturing a coated steel part having very high mechanical properties using hot forming techniques, comprising the following steps:

• providing a coated steel blank comprising a steel substrate with a coating layer

• heating the blank to a temperature of 800° C or more in a furnace

• transporting the heated blank from the furnace and placing the blank in a hot forming press

• hot forming the blank in the press to form the coated steel part

• quenching the coated steel part

wherein the heated blank has a temperature of at most 780° C when placed in the hot forming press.

2. Method according to claim 1, wherein the blank is heated to a temperature of 850° C or more, preferably a temperature of 880° C or more, more preferably to a temperature of 900° C or more.

3. Method according to claim 1 or 2, wherein the heated blank has a temperature of at least 500° C when placed in the hot forming press, preferably a temperature of at least 600° C, and more preferably a temperature of at least 650° C.

4. Method according to any one of the preceding claims, wherein the heated blank has a temperature of at most 740° C when placed in the hot forming press, and preferably a temperature of at most 700° C.

5. Method according to any one of the preceding claims, wherein the heated blank is at least partially cooled between the furnace and the hot forming press by forced cooling, preferably with an average cooling velocity of at least 30° C/sec, more preferably with an average cooling velocity of at least 40° C/sec, still more preferably with an average cooling velocity of at least 60° C/sec, most preferably with an average cooling velocity of at least 80° C/sec.

6. Method according to any one of the preceding claims, wherein the heated steel blank is transported between the furnace and the hot forming press in a time span of at most 10 seconds, preferably in a time span of at most 8 seconds, and more preferably in a time span of at most 6 seconds.

7. Method according to claims 5 or 6, wherein the heated blank is cooled between the furnace and the hot forming press using accelerated cooling making use of gas, such as cooling with forced air flow, cooling with forced N₂ flow, or cooling with forced CO₂ flow or cooled CO₂ using solid CO₂.

8. Method according to claims 5 or 6, wherein the heated blank is cooled between the furnace and the hot forming press using liquid or solid material, such as liquid or solid CO₂ particles or metal cooling plates.

9. Method according to any one of the preceding claims, wherein the coating on the blank is a metallic coating, preferably a coating layer of zinc or a zinc alloy, and more preferably the zinc alloy consists of 0.3 - 4.0% Mg and 0.05 - 6.0% Al, optionally at most 0.2% of one or more additional elements, unavoidable impurities, the remainder being zinc.

10. Method according to claim 9, wherein the zinc or zinc alloy layer is pre-diffused in the substrate.

11. Method according to claim 9 or 10, wherein the zinc or zinc alloy coated steel blank contains an average of 10 - 75 % iron in the coating after heating the blank in the furnace, preferably an average of 25 - 55 %.

12. Method according to any one of the preceding claims, wherein the hot forming press is internally cooled such that the blank is cooled with a cooling velocity of at least 50° C/sec during the first second after the press is fully closed, preferably with a cooling velocity of at least 100° C/sec during the first second.

13. Method according to any one of the preceding claims, wherein the hot forming press is fully closed in at most one second.

14. Hot formed part manufactured according to the method of any one of claims 1 - 13, wherein the substrate of the part is free of micro-cracks having a depth of more than 15 µm, and preferably the substrate is free of micro-cracks having a depth of more than 10 µm.

15. Hot formed part manufactured according to the method of any one of claims 1 - 13, wherein the micro-cracks in the substrate have an average depth of at most 3 µm in five surface area's of the blank with a size of 1 cm².