Motor vehicle with press-hardened visible steel component

Abstract

The invention relates to a motor vehicle in which at least one outer skin part or at least one semi-visible part is made of a coated press-hardened steel, the coating of which prior to heating and press-hardening contains, by weight, from 8% to 12% silicon, no more than 3% iron and no more than 0.1% unavoidable impurities, the balance being aluminum, and in which the at least one outer skin part or the at least one semi-visible part is made from a coated press-hardened steel, the coating of which prior to heating and press-hardening contains, by weight, from 8% to 12% silicon, no more than 3% iron and no more than 0.1% unavoidable impurities. The coating on each side has a thickness of 20 [mu] m to 40 [mu] m.

Description

Technical Field

[0001] The present invention relates to a process for producing coated steel press-hardened parts having an improved appearance, and more particularly, but not limited to, the invention is intended for use in producing exposed or semi-exposed parts for automotive applications. Background Art

[0002] In recent years, the use of coated steels in hot stamping processes for component forming has become important, especially in the automotive industry. The manufacture of such components may include the following main steps:

[0003] -Coating of steel sheets by hot dipping

[0004] - Trimming or cutting to obtain blanks

[0005] -Heating the blank to transform the steel microstructure into austenite.

[0006] -Hot forming followed by rapid cooling of the component to obtain a predominantly martensitic structure.

[0007] Press-hardened steel parts intended for use in the manufacture of automobiles are usually coated with an aluminum-based metal coating that can itself withstand both the austenitizing heat treatment and the subsequent press-hardening step. After hot deformation and quenching of the part, the coating provides corrosion protection. The coating is deposited by hot-dip coating in a liquid bath.

[0008] Press-hardened steel parts intended for the manufacture of automobiles can be deep drawn at high temperatures and quenched in the forming tool to achieve the target microstructure. In terms of material properties, tensile strengths of 500 MPa to 2000 MPa and tensile elongations of 5% to 15% can be achieved. The main advantage of press-hardened steel parts is the combination of good formability with very high strength.

[0009] The press-hardened parts are then assembled to form a body in white which is then coated with at least one paint layer, thereby providing improved corrosion protection.

[0010] The surface appearance of press-hardened steel parts is still poor compared to the appearance obtained by cold stamping galvanized steel materials. The paint layer tends to reduce surface irregularities. But even after painting, press-hardened parts cannot be used for outer skin parts due to surface defects and the corresponding detrimental appearance. This is because press-hardened coated steel parts have various defects, such as wavy surfaces. After painting, the parts will have an unacceptable appearance, for example, locally similar to "orange peel".

[0011] With the exception of outer skin parts, semi-visible parts are only visible when the vehicle's doors are open. Press-hardened parts are not suitable for making semi-visible parts.

[0012] For the reasons mentioned above, the press-hardened parts are not directly exposed to the customer's sight but are covered by additional metal parts with better appearance. The additional parts have no or little mechanical function but have a decorative function. The additional parts act as a barrier and make the press-hardened parts invisible to the naked eye. For example, the part called body side is usually made as one large piece with a very good visual appearance. This decorative body side part acts as a barrier and covers the structural parts from the front wheel to the rear wheel. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The invention will be described by means of indicative examples given for information purposes only and without limitation, with reference to the accompanying drawings, in which:

[0014] - Figure 1 Components having a linear profile and a "hat-shaped" cross-section are shown, and such components have been tested in examples of the present disclosure.

[0015] - Figure 2 Shown in Figure 1 The common structure of a car can be seen in the figure, where the following are marked:

[0016] 1: A-pillar

[0017] 2: B-pillar

[0018] 3: C-pillar

[0019] 4: Side beam

[0020] 5: Roof rails

[0021] 6: Side of the car body Summary of the invention

[0022] The waviness W of the surface is a gentle, pseudo-periodic geometric irregularity with a longer wavelength (0.8 mm to 10 mm), which is distinguished from the roughness R corresponding to geometric irregularities with a shorter wavelength (<0.8 mm).

[0023] In the present invention, the arithmetic mean Wa of the waviness profile expressed in μm is used to characterize the surface waviness of the sheet, and the waviness measurement value with a cutoff threshold of 2.5 mm to 8.0 mm is represented by Wa. 2.5-8 To express.

[0024] The object of the present invention is therefore to provide a motor vehicle manufactured by means of coated press-hardened steel components, the waviness Wa of the coated press-hardened components being reduced compared to the press-hardened components of the prior art. 2.5-8 Reduce.

[0025] This object is achieved by a motor vehicle according to any one of claims 1 to 7 .

[0026] To this end, the present invention discloses a process for producing a press-hardened coated steel component, the process comprising the following steps:

[0027] A) providing a steel plate having a thickness of 0.5 mm to 2.5 mm,

[0028] B) coating said steel sheet by hot dipping into a liquid metal bath containing by weight 8 to 12% silicon, not more than 3% iron and not more than 0.1% unavoidable impurities, the balance being aluminum, wherein the coating thickness on each side of said steel sheet is 20 μm to 40 μm,

[0029] C) temper rolling the coated steel sheet with an elongation of 0.1% to 1.2%, the temper rolling elongation being defined by the speed difference between the material entering the temper rolling stand and the material leaving the temper rolling stand,

[0030] D) cutting the coated skin-pass rolled steel sheet to obtain blanks,

[0031] E) heating the blank at a temperature of 800° C. to 970° C. to obtain a fully austenitic microstructure in the steel,

[0032] F) transferring the blank into the pressing tool,

[0033] G) press-hardening the component obtained in the step by cooling to obtain a press-hardened component.

[0034] Any steel can be advantageously used within the framework of the invention in step A. However, in the case where a steel with high mechanical strength is required, for parts of the structure of a motor vehicle, a steel with a tensile strength better than 500 MPa, advantageously between 500 and 2000 MPa, before or after heat treatment can be used. The weight composition of the steel plate 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%, and the remainder is iron and unavoidable impurities from the manufacture of steel.

[0035] For example, the steel plate 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%, and the remainder is iron and unavoidable impurities from the manufacture of steel.

[0036] In another embodiment, the steel plate 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 is iron and unavoidable impurities produced during the manufacture of steel.

[0037] Alternatively, the steel plate may 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%, with the remainder being iron and unavoidable impurities resulting from the manufacture of the steel.

[0038] In another embodiment, the steel plate 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.

[0039] In another embodiment, the steel plate 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, and the rest is iron and unavoidable impurities generated by the manufacture of steel.

[0040] In another embodiment, the steel plate 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.

[0041] 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.

[0042] The steel sheets can be obtained by hot rolling and optionally cold rolling, depending on the desired thickness. Thicknesses below 0.5 mm may tear during the hot forming process. For visible parts of automobile bodies, press-hardened parts with a thickness of more than 2.5 mm are not required.

[0043] In step B), the steel sheet is then hot-dip coated in a molten bath and subsequently wiped with an air knife to adjust the coating thickness. If the coating thickness is less than 20 μm per side, the corrosion performance is insufficient. If the coating thickness is more than 40 μm per side, the waviness Wa of the stamped part is 2.5-8 Too high.

[0044] In step C), the steel plate is subsequently subjected to skin-pass rolling. The skin-pass rolling operation takes place on a single-stand skin-pass mill, wherein the steel strip is rolled between two working rolls of said mill. Pressure is applied to the steel strip by the working rolls, which in turn apply a linear pressure along a generatrix in contact with the steel strip. The skin-pass rolling elongation at the skin-pass mill is given by the relative difference of the material speed rolled out of the skin-pass rolling stand minus the material flow rate rolled into said stand. If the elongation is below 0.1%, noticeable surface defects will also be seen on the steel plate on the final press-hardened part. If the elongation is above 1.2%, the waviness Wa of the press-hardened part will be too high. In fact, the inventors have surprisingly found that skin-pass rolling elongations of 1.3%, 1.4% or more result in a waviness Wa of the press-hardened part. 2.5-8 Without being bound by theory, reducing the skin pass elongation also appears to reduce the waviness of the press hardened part.

[0045] Preferably, the skin-pass rolling elongation in step C) is lower than 0.9%, more preferably lower than 0.7%, advantageously lower than 0.5%. In another embodiment, the elongation in step C) is lower than 0.3%.

[0046] The waviness of a deformed component depends on its deformation, specifically on the strain and the deformation mode. In the case of visible components, the maximum waviness on the component must be considered for its visual appearance. For visible components, the maximum acceptable waviness is 0.60 μm. For components with higher waviness, their appearance is deteriorated.

[0047] In the vehicle according to the invention, the press-hardened component has 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 component has a waviness Wa of less than 0.40 μm. 2.5-8 .

[0048] The press-hardened component in the automobile according to the present invention is suitable for an outer skin component.

[0049] Thanks to the invention, decorative cold stamped parts which hide the press-hardened parts from the customer's eyes can be omitted. For example, the invention allows the decorative body side 6 to be omitted.

[0050] The press-hardened parts in a motor vehicle according to the invention are also suitable for semi-visible parts.

[0051] For example, the invention is applicable to the following semi-visible parts of a motor vehicle which are only visible when the vehicle door is open: A-pillar 1 , B-pillar 2 , C-pillar 3 , side beam 4 or roof rail 5 .

[0052] For example, the present invention is applicable to a hatchback type of automobile including semi-visible parts that are visible only when the rear tailgate is open.

[0053] The press-hardened components in the automobile according to the present invention can have various types of microstructures, depending on the target mechanical properties, in particular yield strength and tensile strength. For example, when high resistance is required, the press-hardened component can have a steel microstructure containing at least 95% martensite by volume fraction. The press-hardened component can also have a microstructure containing at least 50% martensite and less than 40% bainite. This is the case for components located in the automobile that require both resistance and deformation. Allowing deformation during a collision is a design technique that absorbs collision energy. Finally, for components with an anti-intrusion function, the press-hardened component can have a microstructure containing 5% to 20% martensite, no more than 10% bainite and at least 75% equiaxed ferrite. DETAILED DESCRIPTION

[0054] The invention will now be explained in experiments which were carried out for reference only. They are non-limiting.

[0055] Example

[0056] For all samples, two carbon steel compositions were used.

[0057] The steel composition A is as follows in percentage by weight: C=0.23%; Mn=1.2%; Si=0.25%; %; Cr=0.2%; Al=0.04%; Ti=0.04%; B=0.003%.

[0058] The steel composition B is as follows in percentage by weight: C=0.06%; Mn=0.34%; SI=0.014%; Cr=0.03%; Al=0.04%; Ti=0.001%; ​​and B=0.0001%.

[0059] All steel coils are continuously rolled to the desired thickness. After rolling, the steel coils are annealed and continuously coated with a coating deposited by hot dipping in a metal bath. The coating contains 9 wt% silicon, 3 wt% iron, the balance being aluminum.

[0060] After hot-dip aluminizing, the steel coils are skin-pass rolled at different elongation rates. The skin-pass rolling operation takes place on a single-stand skin-pass mill, where the steel strip is rolled between two working rolls of the mill. The elongation rate at the skin-pass mill is given by the relative difference of the material speed rolled out of the skin-pass rolling stand minus the material speed rolled into the stand.

[0061] At the end of the test, measure Wa 2.5-8Waviness value. This measurement consists in acquiring, by mechanical palpation without sliding, the profile of a sheet of length 40 mm measured in a direction transverse to the rolling direction. The long-wave component corresponding to the form is separated using a Gaussian filter with a cut-off value of 8 mm. The waviness Wa is then separated from the short-wave component including the roughness Ra by a Gaussian filter with a cut-off value of 2.5 mm. The Gaussian filter used is defined in standard ISO16610-21:2012.

[0062] Example 1: Undeformed hot stamping test

[0063] The steel plates were cut into rectangular blanks with the following dimensions: 200×250 mm 2 Each blank is then heated in a furnace at 900°C for 340 seconds to 490 seconds, depending on the material thickness. After heating, each blank is transferred to a flat tool consisting of two plates. The plates are cooled by circulating water. The temperature set point of the cooling water circuit is 17°C. The tool pressing force between the two plates is 50T.

[0064] After press hardening, the microstructure of each sample was analyzed by microscope on the cross section. The samples had the following microstructure by area fraction:

[0065] Steel composition A: at least 95% martensite.

[0066] Steel composition B: at least 88% ferrite.

[0067] The waviness Wa corresponding to each temper elongation rate was measured for the skin-pass rolled steel sheet after heat treatment. 2.5-8 The results are disclosed in Table 1.

[0068] Table 1 - Skin-pass rolling, austenitizing heat treatment and quenching

[0069]

[0070] *According to the example of the present invention, Underlined values Not according to the invention.

[0071] Example 2: Deformation hot stamping test

[0072] For this practice, three steel plates were used, all of which had steel composition A: the first steel plate had been skin-pass rolled at an elongation of 0.2% and the second steel plate had been skin-pass rolled at an elongation of 1.3%. 2.5-8 Afterwards, the steel plates were cut into rectangular blanks with the following dimensions: 400×500mm 2Each blank was then heated in a furnace at 900°C for 390 seconds.

[0073] After heating, each blank is transferred to a forming tool consisting of a punch and die of complementary shape. The tool has no additional adhesive to hold the blank during forming. The punch and die are cooled by circulating water. The temperature set point of the cooling water circuit is 17°C.

[0074] The resulting part has a linear profile and a "hat-shaped" cross-section. Figure 1 An indication is given of the different areas of the component along its hat-shaped cross section. The cross section consists of five sections: the "hat" top (11), two walls 12 and 13 and two bottom flanges 14 and 15.

[0075] The resulting deformed and quenched component was then cut into five specimens along four radii corresponding to the regions marked 11 to 15 of the component.

[0076] The waviness Wa is then measured in each component area 2.5-8 The results are disclosed in Table 2.

[0077] Table 2 - Waviness measurements on plates and thermally deformed parts

[0078]

[0079] *According to the example of the present invention, Underlined values Not according to the invention.

Claims

1. A car, wherein: At least one outer skin component or at least one semi-visible component is made of coated press-hardened steel, the coating of which before heating and press hardening contains by weight 8% to 12% silicon, not more than 3% iron and not more than 0.1% unavoidable impurities, the balance being aluminum, and wherein the coating on each side has a thickness of 20 μm to 40 μm.

2. The automobile according to 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 rail.

3. The automobile according to claim 1 or 2, wherein: The component has a waviness Wa of less than 0.60 μm after press hardening. 2.5-8 .

4. The automobile according to claim 3, wherein: The component has a waviness Wa of less than 0.40 μm after press hardening. 2.5-8 .

5. The vehicle according to any one of claims 1 to 4, wherein: The microstructure of the press-hardened component comprises at least 95% martensite by volume.

6. The vehicle according to any one of claims 1 to 4, wherein: The microstructure of the press-hardened component comprises, by volume fraction, at least 50% martensite and less than 40% bainite.

7. The vehicle according to any one of claims 1 to 4, wherein: The microstructure of the press-hardened component comprises 5% to 20% martensite, no more than 10% bainite and at least 75% equiaxed ferrite.

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