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 10 to 20 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. [Brief explanation of the drawings]

[0010] [Figure 1] The normal structure of a car can be seen in Figure 1, which is labelled as follows: 1: A-pillar, 2: B-pillar, 3: C-pillar, 4: Side sill, 5: Roof rail, 6: Body side. DETAILED DESCRIPTION OF THE INVENTION

[0011] 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).

[0012] 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.

[0013] Therefore, it is an object of the present invention to provide a motor vehicle manufactured with at least one press-hardened coated steel part, the waviness of which is 2.5-8 is reduced compared to prior art press hardened parts.

[0014] This object is achieved by a vehicle according to any one of claims 1 to 7.

[0015] To this end, the present invention discloses a method for producing a press-hardened coated steel part, comprising the following steps:

[0016] A) providing a steel plate having a thickness of 0.7 to 2.5 mm; B) coating the steel sheet by hot dipping in an aluminum-based liquid metal bath containing 8-12 wt. % silicon, maximum 3 wt. % iron, and not more than 0.1 wt. % unavoidable impurities from the manufacturing process, wherein the coating has a thickness of 10-20 μm per side; C) temper rolling the coated steel sheet to a total elongation of 0.1-1.2%, the 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 said blank to obtain a press hardened part.

[0017] In step A), any steel can be used for the frame of the invention. However, if steel with high mechanical strength is required, especially for structural parts of automobiles, steels can be used that have a tensile strength of more than 500 MPa, preferably 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.

[0018] 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.

[0019] 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%, 0.003%≦N≦0.010%, S≦0.005%, P≦0.025%, and the balance being iron and unavoidable impurities resulting from the production of the steel.

[0020] 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.

[0021] 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.

[0022] 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.

[0023] 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.

[0024] 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.

[0025] 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 automotive bodies, press-hardened parts thicker than 3.0 mm are not required.

[0026] 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 10 μm per side, the corrosion performance is insufficient. If the coating thickness is more than 20 μm per side, the waviness of the pressed parts will increase. 2.5-8 is too big.

[0027] 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, which applies a linear pressure along the generatrix in contact with the steel strip. The elongation in the temper mill is obtained by the relative difference between the material speed leaving the temper rolling stand and the material speed 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 large. In fact, the inventors have surprisingly found that if the elongation in temper rolling exceeds 1.2%, the waviness (Wa) of the press-hardened part becomes too large. 2.5-8 It was found that the waviness of the press-hardened parts was induced to exceed 0.41 μm. Without being bound by theory, it appears that reducing the temper rolling elongation also reduces the waviness of the press-hardened parts.

[0028] Preferably, the elongation in step C) is less than or equal to 0.9%, more preferably less than or equal to 0.7%, advantageously less than or equal to 0.5% or even less than or equal to 0.3%.

[0029] In the automobile according to the invention, the press-hardened parts have a waviness Wa of less than 0.41 μm, preferably less than 0.35 μm, or even less than 0.29 μm. 2.5-8 It has.

[0030] The press-hardened automotive parts according to the present invention are suitable for outer skin parts.

[0031] Thanks to the present invention, decorative parts that hide press-hardened parts can be suppressed. For example, the present invention makes it possible to suppress the body side 6.

[0032] The press-hardened automotive parts according to the present invention are also suitable for semi-visible parts, such as the A-pillar 1, B-pillar 2, C-pillar 3, side sill 4, or roof rail 5, which are only visible when the door is open.

[0033] For example, the invention is also suitable for semi-visible components contained in automobile hatchbacks that are only visible when the rear tailgate is open.

[0034] Press-hardened parts used in automobiles 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. Press-hardened parts can also have a microstructure containing at least 50% martensite and less than 40% bainite. This is the case for parts placed in automobiles where both resistance and deformation are required. Allowing deformation during a collision is a design technique for absorbing the impact energy. Finally, for parts with anti-intrusion functions, press-hardened parts can have a microstructure containing 5-20% martensite, up to 10% bainite, and at least 75% equiaxed ferrite.

[0035] The invention will now be described in terms of tests carried out for informational purposes only and not to be construed as limiting.

[0036] (Example) For all samples, the carbon steel coil used is 22MnB5, with the following composition: C=0.23%, Mn=1.2%, Si=0.25%, Cr=0.2%, Al=0.04%, Ti=0.04%, and B=0.003%.

[0037] 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.

[0038] 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 leaving the temper mill stand and the material velocity entering the stand.

[0039] As the trial progressed, 2.5-8The 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 low-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.

[0040] Example 1: Hot stamping test The steel plate is 200 x 250 mm in size. 2 The blanks were then cut into rectangular blanks. Each blank was then heated in a furnace at 900°C for 345-405 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.

[0041] For temper rolled steel sheets, the waviness (Wa) corresponding to each temper elongation 2.5-8 was measured. The results are disclosed in Table 1.

[0042] [Table 1]

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, said coating having a thickness of 10 to 20 μm per side.

2. 2. The vehicle of claim 1, wherein the at least one semi-visible part 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 0.41 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 0.35 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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