The invention relates to a process for manufacturing press hardened parts of coated steel having an improved appearance, more particularly intended to be used for the manufacture of exposed or semi-exposed parts for automobiles, without however being limited thereto.

In recent years the use of coated steels in hot-stamping processes for the shaping of parts has become important, especially in the automotive industry. Fabrication of such parts may include the following main steps:

- Coating of steel sheets, by hot dipping

- Trimming or cutting for obtaining blanks

- Heating the blanks to transform the steel microstructure into austenite.

- Hot forming followed by rapid cooling of the part to obtain predominantly a martensitic structure.

Press hardened steel parts intended for the manufacture of automobiles are generally coated with an aluminum-based metallic coating, which sustains both the austenitizing heat treatment and the subsequent press hardening step itself. After hot deformation and quenching of the part, the coating provides protection against corrosion. Said coating is deposited by hot-dip coating in a liquid bath.

Press hardened steel parts intended for the manufacture of automobiles can be deep drawn at high temperatures and are quenched in the forming tools to reach the targeted microstructure. In terms of material properties, tensile strength from 500 to 2000 MPa and tensile elongation from 5 to 15 % can be achieved. Press hardened steel parts offer the major advantage of combining good formability with very high strength.

Press hardened parts are then assembled, to form a body in white, which is then coated with at least one paint coat, thereby providing greater corrosion protection. Compared to the aspect achieved by cold stamped galvanized steel material, the surface aspect of press hardened steel parts remains poor. Paint layers tend to reduce surface irregularities. But even after painting, press hardened parts can’t be used for visible outer parts because of surface defects and corresponding detrimental appearance. This is because press hardened coated steel parts have various defects, such as wavy surfaces. After painting, the parts would have an unacceptable appearance, for example locally similar to “orange peel”.

The waviness W of the surface is a gentle, pseudoperiodic, geometric irregularity of quite a long wavelength (0.8 to 10 mm), distinguished from the roughness R, which corresponds to geometric irregularities of short wavelengths (< 0.8 mm).

In the present invention, the arithmetic mean Wa of the waviness profile, expressed in pm, is used to characterize the surface waviness of the sheet, and the waviness measurements with cut-off thresholds of 2.5 mm to 8.0 mm are denoted by Wa2.5-8.

The aim of the invention is therefore to provide a coated press hardened steel part, the waviness Wa2.s-8 of which is reduced compared to press hardened parts of the prior art, such press hardened parts having a better appearance.

This object is achieved by providing a manufacturing method for such a part according to anyone of claims 1 to 5.

Additionally, the object of the invention is achieved by providing the automotive parts according to anyone of claims 6 to 12.

Finally, the last object of the invention is the use of such part in an automotive vehicle according to anyone of claims 13 to 15.

The invention will now be illustrated by means of indicative examples given for information purposes only, and without limitation, with reference made to the accompanying figures in which:

- Figure 1 shows a part having a linear profile and a cross-section in a “hat shape”, such part has been tested in the examples of the present disclosure.

For this purpose, a first subject of the invention consists of a process for manufacturing press hardened coated steel parts, comprising the following steps: A) Supplying a steel sheet having a thickness from 0.5 to 2.5 mm,

B) Coating said steel sheet by hot dipping into a liquid metallic bath containing by weight, 8 to 12 % of Silicon, up to 3 % Iron, and unavoidable impurities up to 0.1 %, the balance being Aluminum, wherein said coating thickness from 20 to 40 pm per side of said steel sheet,

C) Temper rolling the coated steel sheet at an elongation rate from 0.1 to 1.2 %, the temper rolling elongation being defined by the speed difference between the material in and the material out of the temper rolling stand,

D) Cutting said coated, temper rolled steel sheet to obtain a blank,

E) Heating said blank at a temperature from 800 to 970°C, to obtain a fully austenitic microstructure in the steel,

F) Transferring the blank into a press tool,

G) Press hardening of the part obtained at step by cooling to obtain a press-hardened part.

In step A, any steel can be advantageously used in the frame of the invention. However, in case steel having high mechanical strength is needed, for parts of structure of automotive vehicle, steel having a tensile resistance superior to 500MPa, advantageously between 500 and 2000MPa before or after heattreatment, can be used. The weight composition of steel sheet is preferably as follows: 0.03% < C < 0.50% ; 0.3% < Mn < 3.0% ; 0.05% < Si < 0.8% ; 0.015% < Ti

< 0.2% ; 0.005% < Al < 0.1 % ; 0% < Cr < 2.50% ; 0% < S < 0.05% ; 0% < P < 0.1 % ; 0% < B < 0.010% ; 0% < Ni < 2.5% ; 0% < Mo < 0.7% ; 0% < Nb < 0.15% ; 0% < N

< 0.015% ; 0% < Cu < 0.15% ; 0% < Ca < 0.01 % ; 0% < W < 0.35%, the remainder being iron and unavoidable impurities from the manufacture of steel.

For example, the steel sheet is 22MnB5 with the following weight composition: 0.20% < C < 0.25%; 0.15% < Si < 0.35%; 1.10% < Mn < 1.40%; 0% < Cr < 0.30%; 0.020% < Ti < 0.060%; 0.020% < Al < 0.060%; 0.002% < B < 0.004%, the remainder being iron and unavoidable impurities from the manufacture of steel.

In another embodiment, the steel sheet has the following weight composition: 0.24% < C < 0.38%; 0.40% < Mn < 3%; 0.10% < Si < 0.70%; 0.015% < Al < 0.070%; Cr < 2%; 0.25% < Ni < 2%; 0.015% < Ti < 0.10%; Nb < 0.060%; 0.0005% < B < 0.0040%; the remainder being iron and unavoidable impurities resulting from the manufacture of steel.

Alternatively, the steel sheet can have the following weight composition: 0.30% < C < 0.40%; 0.5% < Mn < 1.0%; 0.40% < Si < 0.80%; 0.1 % < Cr < 0.4%; 0.1 % < Mo < 0.5%; 0.01 % < Nb < 0.1 %; 0.01 % < Al < 0.1 %; 0.008% < Ti < 0.003%; 0.0005% < B < 0.003%; 0.0% < P < 0.02%; 0.0% < Ca < 0.001 %; 0.0% < S < 0.004 %; 0.0% < N < 0.005 %, the remainder being iron and unavoidable impurities resulting from the manufacture of steel.

In another embodiment, the steel sheet has the following weight composition: 0.040% < C < 0.100%; 0.80% < Mn < 2.00%; 0% < Si < 0.30%; 0% < S < 0.005%; 0% < P < 0.030%; 0.010% < Al < 0.070%; 0.015% < Nb < 0.100%; 0.030% < Ti < 0.080%; 0% < N < 0.009%; 0% < Cu < 0.100%; 0% < Ni < 0.100%; 0% < Cr < 0.100%; 0% < Mo < 0.100%, the balance being iron and unavoidable impurities from the manufacture of steel.

In another embodiment, the steel sheet has the following weight composition: 0.06% < C < 0.1 %, 1 % < Mn < 2%, Si < 0.5%, Al <0.1 %, 0.02% < Cr < 0.1 %, 0.02%

< Nb < 0.1 %, 0.0003% < B < 0.01 %, N < 0.01 %, S < 0.003%, P < 0.020% less than 0,1 % of Cu, Ni and Mo, the remainder being iron and unavoidable impurities resulting from the manufacture of steel.

In another embodiment, the steel sheet has the following weight composition: 0.015% < C < 0.25%; 0.5% < Mn < 1.8%; 0.1 % < Si < 1.25%; 0.01 % < Al < 0.1 %; 0.1 % < Cr < 1 .0%; 0.01 % < Ti < 0.1 %; 0% < S < 0.01 %; 0.001 % < B < 0.004%; 0%

< P < 0.020%; 0% < N < 0.01 %; the balance being iron and unavoidable impurities from the manufacture of steel.

Alternatively, the steel sheet has the following weight composition: 0.2% < C

< 0.34%; 0.5% < Mn < 1 .24%; 0.5% < Si < 2.0%; 0% < S < 0.01 %; 0% < P < 0.020%; 0% < N < 0.01 %, the balance being iron and unavoidable impurities from the manufacture of steel. Steel sheet can be obtained by hot rolling and optionally cold rolling depending on the desired thickness. Thickness below 0.5 mm may tear off during hot forming process. Press hardened parts thicker than 2.5 mm are not needed fir the visible parts of an automotive body.

In step B), the steel sheet is then hot dip coated in a molten bath and subsequently wiped by air knifes to adjust the coating thickness. If the coating thickness is below 20 pm per side, the corrosion performance is not sufficient. If the coating thickness is above 40 pm per side, the waviness Wa2.s-8 of the stamped part is too high.

In step C), the steel sheet is then temper-rolled. The temper rolling operation occurs on a single stand temper rolling mill, wherein the steel strip is rolled between the two working rolls of said mill. A pressure force is applied on the steel strip by the work rolls, which in turn exert a lineic pressure along the generatrix in contact with the strip. The temper rolling elongation rate at the temper rolling mill is given by the relative difference of the material speed rolling out of the temper rolling stand minus the material flow rolling into said stand. If the elongation rate is below 0.1 %, punctual surface defects will be visible on the steel sheet on the final press hardened part as well. If the elongation rate is above 1 .2 %, the waviness Wa of the press hardened part will be too high. Indeed, inventors have surprisingly found that a temper rolling elongation rate of 1 .3 %, 1 .4 % or more results, in a waviness Wa2.s- 8 of the press hardened part of more than 0.60 pm. Without to be bound by theory, it seems that decreasing the temper rolling elongation rate also decreases the waviness of the press hardened part.

Preferably, the temper rolling elongation rate in step C) is below 0.9 %, more preferably below 0.7%, advantageously below 0.5%. In another embodiment, the elongation rate in step C) is below 0.3 %.

A second subject of the invention consists of a press hardened coated steel part, which is obtained in step G.

The waviness of a deformed part depends on its deformation, explicitly the strain and the deformation mode. In the case of a visible part, the maximum waviness on the part must be considered for its visual aspect. The maximum acceptable waviness for a visible part is 0.60 pm. For parts having a higher waviness, the appearance is deteriorated.

According to the present invention, the part obtained in step G has a waviness Wa2.5-8 below 0.60 pm, preferably below 0.55 pm, advantageously below 0.50 pm, or even below 0.45 pm. In another embodiment, the part obtained in step G has a waviness Wa2.s-8 below 0.40 pm

Such press hardened parts with improved appearance may be used for outer parts. They are also suitable for so-called semi-visible parts. These require a lower quality of appearance. These semi-visible parts are visible only when the openings of the vehicle are not closed. For instance, when the front door is open, one can see the parts behind and surrounding the hole of the door: A-Pillar, roof rail, B-Pillar, side sill. These different semi-visible parts are usually designed with material and thicknesses which are different from each other as they may have different functions. However, the expectation regarding their appearance is identical. The same applies when rear door opens with the semi-visible C-pillar, and also when the rear tailgate opens with the semi-visible hatchback.

For this reason, a fourth subject of the invention is the use of such press hardened parts in regions of an automobile, especially where they are the most suitable.

The press hardened obtained in step H) can have various types of microstructure, depending on the targeted mechanical properties, especially the yield strength and tensile strength. For instance, the press hardened part can have a steel microstructure comprising, in terms of volume fraction, at least 95% of martensite, when a high resistance is needed. The press hardened part can also have a microstructure comprising at least 50% of martensite and less than 40 % of bainite. This is the case for parts located in the automobile where both resistance and deformation are needed. Allowing deformation in the event of a crash is a design technique to absorb the crash energy. Finally, the press hardened part can have a microstructure comprising from 5 to 20 % of martensite, up to 10 % of bainite and at least 75 % of equiaxed ferrite for parts having an anti-intrusion function. The invention will now be explained in trials carried out for information only. They are not limiting.

Examples

For all samples, carbon steel coils used are 22MnB5. The composition of the steel is as follows: C = 0.23 %; Mn = 1.2%; Si = 0.25%; %; Cr = 0.2%; Al = 0.04%; Ti = 0.04%; B = 0.003 %.

All steel coils were continuously rolled to desired thickness. After rolling, they were annealed and continuously coated with a coating deposited by hot dipping in a metallic bath. This coating comprises 9% by weight of Silicon, 3% by weight of iron, the balance being aluminum.

After hot-dip aluminizing, the steel coils were temper rolled at different elongation rates. The temper rolling operation occurred on a single stand temper rolling mill, wherein the steel strip was rolled between the two working rolls of said mill. The elongation rate at the temper rolling mill is given by the relative difference of the material speed rolling out of the temper rolling stand minus the material speed rolling into said stand.

At the end of the test, the Wa2.s-8 waviness values is measured. This measurement consists in acquiring by mechanical palpation, without skid, a profile of the sheet of a length of 40 mm, measured in the direction transversal to the direction of rolling. The long-wave components corresponding to the form are separated using a Gaussian filter with a cutoff of 8 mm. The waviness Wa is then isolated from the short-wave components, including roughness Ra by a Gaussian filter with a cutoff of 2.5 mm. The gaussian filters used are defined in the standard ISO 16610-21 :2012.

Example 1 : Undeformed hot-stamping test

The steel sheets were cut into rectangular blanks having the following dimension: 200x250 mm ² . Then each blank was heated in a furnace at 900°C for 340 to 490 seconds, depending on the material thickness. After heating, each blank was transferred into a flat tool composed of two plates. The plates were cooled with circulating water. Temperature set point of the cooled water circuit was 17°C. Tool pressing force between the two plates was 50 T. The waviness Wa2.5-8 corresponding to each temper elongation rate was measured on the temper rolled steel sheets after heat treatment. Results are disclosed in table 1.

Table 1 - Temper-rolling, austenitisation heat treatment and quenching

*examples according to the invention, underlined values are not according to the invention

Example 2: Deformed hot-stamping test

For this experience, three steel sheets were used: a first one having been temper rolled at 0.2 % elongation rate and a second having been temper rolled at 1 .3% elongation rate. After having measured the waviness Wa2.s-8, the steel sheets were cut into rectangular blanks having the following dimension: 400x500 mm ² . Then each blank was heated in a furnace at 900°C for 390 seconds.

After heating, each blank was transferred into a forming tool composed of a punch and a die of complementary shape. The tool had no additional binder to hold the blank during forming. The punch and the die were cooled with circulating water. Temperature set point of the cooled water circuit was 17°C.

The resulting part has a linear profile and a cross-section in a “hat shape”. Figure 1 gives an indication of the different zones of said part, along its hat-shaped section. Said section is made of five segments: the top of the “hat” (11 ), two walls 12 and 13, and two bottom flanges 14 and 15.

The resulting, deformed, and quenched part was then cut into five samples along the parts’ 4 radii, corresponding to the zones labelled 11 to 15. Then the waviness Wa2.s-8was measured on each part zone. Results are disclosed in table 2.

Table 2 - Waviness measurement on sheet and hot deformed part *examples according to the invention, underlined values are not according to the invention.