Automobiles with press-hardened visible steel parts

Abstract

1. A motor vehicle, wherein at least one outer skin part or at least one semi-visible part is made of coated press-hardened steel, the coating on the steel before heating and press-hardening comprising 8 to 12% by weight of silicon, a maximum of 3% by weight of iron, and a maximum of 0.1% by weight of unavoidable impurities, the balance being aluminum, and the coating having a thickness of 20 to 40 μm per side.

Description

[Technical Field]

[0001] The present invention relates to a method for producing press-hardened coated steel parts having improved appearance, and is more particularly, but not exclusively, intended for use in the production of exposed or semi-exposed automotive parts. [Background technology]

[0002] In recent years, the use of coated steels in hot stamping processes for forming parts has become increasingly important, especially in the automotive industry. The main steps in the manufacture of such parts are:

[0003] -Coating of steel sheets by hot dipping - Trimming or cutting to obtain blanks - Heating the blank to transform the steel microstructure to austenite -Hot forming of the part to obtain a predominantly martensitic structure followed by rapid cooling.

[0004] Hot-stamped steel parts intended for automotive manufacturing are generally coated with an aluminum-based metallic coating to support both the austenitizing heat treatment and the subsequent press-hardening process itself. After hot deformation and hardening of the part, the coating provides protection against corrosion. The coating is applied by hot-dip coating in a liquid bath.

[0005] Press-hardened steel parts intended for automotive manufacturing can be deep-drawn at high temperatures and quenched in the forming tool to achieve the desired microstructure. Regarding material properties, tensile strengths of 500-2000 MPa and tensile elongations of 5-15% can be achieved. Press-hardened steel parts offer the significant advantage of combining good formability with very high strength.

[0006] The press-hardened parts are then assembled to form a blank body which is then coated with at least one paint to provide greater corrosion protection.

[0007] Compared to the surface appearance achieved by cold-stamped galvanized steel, the surface appearance of press-hardened steel parts remains unsatisfactory. A paint layer helps reduce surface irregularities. However, even after painting, press-hardened parts cannot be used for outer skin parts due to surface defects and the corresponding poor appearance. This is because press-hardened coated steel parts have various defects, such as a wavy surface. After painting, the parts have an unacceptable appearance, for example, locally resembling a "yellow peel" appearance.

[0008] In addition to outer skin parts, semi-visible parts are only visible when the vehicle door is open. Press hardened parts are not suitable for manufacturing semi-visible parts.

[0009] For the reasons explained above, hardened parts are not directly exposed to the customer's gaze, but are instead covered by additional metal parts with a better appearance. These additional parts have no or little mechanical function, but only a decorative function. They act as a screen, hiding the hardened parts from view. For example, a part called a body side is usually made from one large part with a very good visual aspect. This decorative body side part acts as a screen, covering the structural parts from the front wheels to the rear wheels.

[0010] The present invention is illustrated by way of illustrative examples, given for information purposes only, and not by way of limitation, with reference to the accompanying drawings, in which: [Brief explanation of the drawings]

[0011] [Figure 1] Parts having a "hat-shaped" linear profile and cross section are shown, and such parts were tested in the examples of the present disclosure. [Figure 2] The typical structure of a car is shown in Figure 1 with the following labels:

[0012] 1: A-pillar 2: B-pillar 3: C-pillar 4: Side sill 5: Roof rails 6: Body side DETAILED DESCRIPTION OF THE INVENTION

[0013] The surface waviness W is a geometric irregularity with a gentle quasi-periodicity and a fairly long wavelength (0.8 to 10 mm), and is different from the roughness R, which corresponds to a geometric irregularity with a short wavelength (<0.8 mm).

[0014] In the present invention, the arithmetic mean of the waviness profile, Wa, expressed in μm, is used to characterize the waviness of the steel sheet surface, and measurements taken with a cut-off threshold of 2.5 mm to 8.0 mm are taken as Wa 2.5-8 It is shown as follows.

[0015] Therefore, an object of the present invention is to provide an automobile manufactured with coated press-hardened steel parts, the waviness of which is 2.5-8 is reduced compared to prior art press hardened parts.

[0016] This object is achieved by a vehicle according to any one of claims 1 to 7.

[0017] To this end, the present invention discloses a method for producing coated press hardened steel parts, comprising the following steps:

[0018] A) providing a steel plate having a thickness of 0.5 to 2.5 mm; B) coating the steel sheet by hot dipping it into a liquid metal bath containing 8-12 wt. % silicon, at most 3 wt. % iron, and at most 0.1 wt. % unavoidable impurities, the balance being aluminum, wherein the coating has a thickness of 20-40 μm per side of the steel sheet; C) temper rolling the coated steel sheet to an elongation of 0.1-1.2%, the temper rolling elongation being defined by the speed difference between the material inside the temper rolling stand and the material outside the temper rolling stand; D) cutting the coated temper rolled steel sheet to obtain blanks; E) heating the blank at a temperature between 800°C and 970°C to obtain a fully austenitic microstructure in the steel; F) transferring the blank to a press tool; G) Press hardening the part obtained in the cooling process to obtain a press hardened part.

[0019] Any steel can be advantageously used for the frame of the invention in step A. However, if steel with high mechanical strength is required for the structural parts of the vehicle, steels can be used that have a tensile strength of more than 500 MPa, advantageously between 500 and 2000 MPa, before and after heat treatment. The steel sheet preferably has the following composition by weight: 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%, with the remainder being iron and unavoidable impurities resulting from the production of steel.

[0020] For example, the steel sheet is 22MnB5 with the following composition by weight: 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 balance being iron and unavoidable impurities resulting from the manufacturing of the steel.

[0021] In another embodiment the steel sheet has the following composition by weight: 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%, and the balance being iron and unavoidable impurities resulting from the production of the steel.

[0022] Alternatively, the steel sheet may have the following composition by weight: 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%, and the remainder being iron and unavoidable impurities resulting from the production of the steel.

[0023] In another embodiment, the steel sheet has the following composition by weight: 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%, and the balance being iron and unavoidable impurities resulting from the production of the steel.

[0024] In another embodiment the steel sheet has the following composition by weight: 0.06%≦C≦0.1%, 1%≦Mn≦2%, Si≦0.5%, AI≦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%, Cu, Ni and Mo are less than 0.1%, and the balance is iron and unavoidable impurities resulting from the production of the steel.

[0025] In another embodiment the steel sheet has the following composition by weight: 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%, and the remainder being iron and unavoidable impurities resulting from the production of the steel.

[0026] Alternatively, the steel sheet has the following composition by weight: 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 remainder being iron and unavoidable impurities resulting from the production of the steel.

[0027] Depending on the desired thickness, the steel sheets can be obtained by hot rolling and optionally cold rolling. Thicknesses less than 0.5 mm can cause tearing during the hot forming process. For visible parts of the car body, press-hardened parts thicker than 2.5 mm are not required.

[0028] Then, in step B), the steel plate is hot-dip coated in a molten bath, followed by wiping with an air knife to adjust the coating thickness. If the coating thickness is less than 20 μm per side, the corrosion performance is not sufficient. If the coating thickness is more than 40 μm per side, the waviness of the pressed parts will increase. 2.5-8 is too expensive.

[0029] Then, in step C), the steel sheet is temper rolled. The temper rolling operation is performed in a single stand temper mill, where the steel strip is rolled between two work rolls of the mill. The work rolls apply pressure to the steel strip, thereby applying a linear pressure along the generatrix in contact with the steel strip. The temper rolling elongation in the temper mill is given by the relative difference between the material velocity leaving the temper rolling stand minus the material flow entering the stand. If the elongation is less than 0.1%, point-like surface defects are also visible on the steel sheet on the final press-hardened part. If the elongation exceeds 1.2%, the waviness (Wa) of the press-hardened part becomes too high. In fact, the inventors have surprisingly found that if the elongation in temper rolling is 1.3%, 1.4% or more, the waviness (Wa) of the press-hardened part becomes too high. 2.5-8 It was found that the waviness of the press hardened part is greater than 0.60 μm. Without being bound by theory, it appears that reducing the temper rolling elongation also reduces the waviness of the press hardened part.

[0030] Preferably, the temper rolling elongation in step C) is less than 0.9%, more preferably less than 0.7% and advantageously less than 0.5%. In another embodiment, the elongation in step C) is less than 0.3%.

[0031] The waviness of a deformed part depends on its deformation, specifically the strain and deformation mode. For visible parts, the maximum waviness on the part must be considered for its appearance. The maximum allowable waviness for visible parts is 0.60 μm. The more waviness a part has, the worse its appearance will be.

[0032] In the automobile according to the invention, the press-hardened parts have a waviness Wa of less than 0.60 μm, preferably less than 0.55 μm, advantageously less than 0.50 μm or even less than 0.45 μm. 2.5-8 In another embodiment, the part has a waviness Wa of less than 0.40 μm. 2.5-8 It has.

[0033] The press-hardened automotive parts according to the present invention are suitable for outer skin parts.

[0034] Thanks to the present invention, decorative cold stamped parts that would otherwise be hidden from the customer's view of the hardened part can be suppressed. For example, the present invention makes it possible to suppress decorative body sides 6.

[0035] The press-hardened automotive parts according to the invention are also suitable for semi-visible parts.

[0036] For example, the invention is suitable for semi-visible parts of a motor vehicle that are only visible when the door is open, i.e. the A-pillar 1, B-pillar 2, C-pillar 3, side sill 4 or roof rail 5.

[0037] For example, the invention is suitable for semi-visible components contained in a car hatchback that are only visible when the rear tailgate is open.

[0038] Press-hardened automotive parts according to the present invention can have various types of microstructures depending on the desired mechanical properties, particularly yield strength and tensile strength. For example, if high resistance is required, the press-hardened part can have a steel microstructure containing at least 95% martensite by volume. The press-hardened part can also have a microstructure containing at least 50% martensite and less than 40% bainite. This is the case for parts located in automobiles where both resistance and deformation are required. Allowing deformation during a crash is a design technique for absorbing the impact energy. Finally, for parts with anti-intrusion functions, the press-hardened part can have a microstructure containing 5-20% martensite, up to 10% bainite, and at least 75% equiaxed ferrite.

[0039] The invention will now be described in terms of tests carried out for informational purposes only and not to be construed as limiting.

[0040] (Example) For all samples, two carbon steel compositions are used.

[0041] The composition A of steel is, in weight percent, C=0.23%, Mn=1.2%, Si=0.25%, Cr=0.2%, Al=0.04%, Ti=0.04%, and B=0.003%.

[0042] Steel composition B is, in weight percent, C=0.06%, Mn=0, 34%, Si=0.014%, Cr=0.03%, Al=0.04%, Ti=0.001%, and B=0.0001%.

[0043] All steel coils were continuously rolled to the desired thickness. After rolling, they were annealed and continuously coated with a coating deposited by hot dipping in a metal bath. The coating contained 9% by weight silicon, 3% by weight iron, and the balance aluminum.

[0044] After hot dip aluminization, the steel coils were temper rolled to various elongations. The temper rolling operation was performed in a single stand temper mill, where the steel strip was rolled between two work rolls of the mill. The elongation in the temper mill was determined by the relative difference between the material velocity exiting the temper mill stand and the material velocity entering the stand.

[0045] At the end of the exam, Wa 2.5-8 The waviness values are measured. This measurement consists of obtaining the waviness profile of a 40 mm long steel plate by mechanical palpation without skids, measured perpendicular to the rolling direction. The long-wave component corresponding to the forming is separated using a Gaussian filter with a cutoff of 8 mm. The waviness Wa is then separated from the short-wave component containing the roughness Ra by a Gaussian filter with a cutoff of 2.5 mm. The Gaussian filter used is specified in the standard ISO 16610-21:2012.

[0046] (Example 1: Undeformed hot stamping test) The steel plate is 200 x 250 mm in size. 2The blanks were then cut into rectangular blanks. Each blank was then heated in a furnace at 900°C for 340 to 490 seconds, depending on the material thickness. After heating, each blank was transferred to a flat tool consisting of two plates. The plates were cooled with circulating water. The temperature set point of the cooling water circuit was 17°C. The tool pressure between the plates was 50 T.

[0047] After press hardening, the microstructure of each specimen was analyzed by cross-sectional microscopy. The samples have the following microstructure by area fraction:

[0048] Steel composition A: At least 95% martensite.

[0049] Steel composition B: at least 88% ferrite.

[0050] For temper-rolled steel sheets after heat treatment, the waviness (Wa) corresponding to each temper elongation rate 2.5-8 was measured. The results are disclosed in Table 1.

[0051] [Table 1]

[0052] (Example 2: Deformation hot stamping test) In this experiment, three steel plates were used, all with steel composition A, the first temper rolled to an elongation of 0.2% and the second temper rolled to an elongation of 1.3%. 2.5-8 After measuring, the steel plate is 400 x 500 mm 2 Each blank was then heated in a furnace at 900°C for 390 seconds.

[0053] After heating, each blank was transferred to a forming tool consisting of a punch and die of complementary shape. The tool did not have any additional binder to hold the blank in place during forming. The punch and die were cooled with circulating water. The temperature set point of the cooling water circuit was 17°C.

[0054] The resulting part has a "hat-shaped" linear profile and cross section. Figure 1 shows the various zones of the part along the hat-shaped section. The section is made up of five sections: the top of the "hat" (11), two walls 12 and 13, and two lower flanges 14 and 15.

[0055] The resulting deformed and quenched part was then cut into five samples corresponding to zones labeled 11–15 along the four extents of the part.

[0056] Next, the swell 2.5-8 was measured for each part zone. The results are disclosed in Table 2.

[0057] [Table 2]

Claims

1. 1. A motor vehicle, wherein at least one outer skin part or at least one semi-visible part is made of coated press-hardened steel, the coating of said steel before heating and press-hardening comprising 8 to 12% by weight of silicon, a maximum of 3% by weight of iron, and a maximum of 0.1% by weight of unavoidable impurities, the balance being aluminum, and said coating having a thickness of 20 to 40 μm per side.

2. 10. The vehicle of claim 1, wherein the at least one outer skin component or the at least one semi-visible component is selected from the group consisting of an A-pillar, a B-pillar, a C-pillar, a roof rail, and a side sill.

3. The part has a waviness Wa of less than 0.60 μm after press hardening. ２．５－８ 3. A vehicle according to claim 1 or 2, comprising:

4. The part has a waviness Wa of less than 0.40 μm after press hardening. ２．５－８ 4. The vehicle of claim 3, wherein:

5. 5. The motor vehicle of claim 1, wherein the microstructure of the press-hardened part comprises at least 95% martensite by volume.

6. 5. The motor vehicle of claim 1, wherein the microstructure of the press-hardened part comprises, by volume fraction, at least 50% martensite and less than 40% bainite.

7. 5. The motor vehicle of claim 1, wherein the microstructure of the press-hardened part comprises 5-20% martensite, a maximum of 10% bainite, and at least 75% equiaxed ferrite.

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