Steel sheet with excellent powdering resistance after press hardening and manufacturing method thereof

Abstract

The object of the present invention is to provide a coated steel sheet suitable for producing press-hardened parts that provide cathodic protection and have good powdering resistance and good corrosion performance during press-hardening. The present invention relates to a method for producing hardened parts starting from a steel sheet coated with a metallic coating. The parts have good properties in terms of corrosion resistance and powdering resistance. The present invention is particularly well suited for the production of motor vehicles.

Description

[Technical Field]

[0001] The present invention relates to a method for producing hardened parts starting from steel sheets coated with a metallic coating, which parts have good properties in terms of corrosion resistance and powdering resistance. The invention is particularly well suited for the production of motor vehicles. [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 manufacture of such parts involves the following main steps: -Coating steel sheets by hot dip galvanizing - Trimming or cutting to obtain blanks Alloying the steel substrate with the coating and heating the blank to obtain the austenitization of the steel. -Press hardening of parts to obtain a primarily martensitic structure.

[0003] Diffusion of iron from the steel substrate through the coating produces a high melting temperature intermetallic alloy. Blanks bearing such coatings can be heated in the temperature range where austenitization of the metal substrate occurs, allowing further hardening by quenching.

[0004] Steel press-hardened parts for automotive manufacturing can be deep-drawn at high temperatures to achieve the desired microstructure. After quenching in the forming tool, the material properties can achieve a tensile strength of 500-2000 mPa and a tensile elongation of 5-15%.

[0005] The part to be hardened can be coated with a zinc-based coating or an aluminum-based coating.

[0006] Zinc-based coatings are commonly used because they can prevent corrosion by providing barrier protection and cathodic protection.

[0007] However, when such galvanized steel sheets are formed at high temperatures, the outer oxide, including zinc, is stripped from the sheet being formed. This results in the formation of oxide powder, which then agglomerates. After stamping a certain number of parts, the forming tool must be wiped and cleaned to remove the agglomerated powder. This powdering necessitates the shutdown of the press-hardening line, resulting in reduced productivity. If not removed from the forming tool, the deposits from the agglomerated powder will eventually cause the sheet to flake off or destroy the stamping tool, significantly increasing line downtime.

[0008] Aluminum-based coatings have good suitability for press-curing at high temperatures and painting. They provide effective barrier protection. However, they do not provide effective cathodic protection.

[0009] Patent EP 3239336 aims to provide press-hardened parts, which can minimize the problem of plating layers peeling off from the plated object and adhering to the mold surface during hot press molding. However, the disclosed coating does not provide cathodic protection. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] European Patent No. 3239336 Summary of the Invention [Problem to be solved by the invention]

[0011] It is an object of the present invention to provide a coated steel sheet that provides cathodic protection and is suitable for producing press-hardened parts that have good powdering resistance during press-hardening and good corrosion performance.

[0012] This object is achieved by a steel sheet according to claim 1 or 2.

[0013] Another object of the present invention is to provide a method for producing the same according to claim 3.

[0014] The object of the present invention is also achieved by providing a component as set forth in any one of claims 4 to 8.

[0015] A final object of the invention is the use of such a component as claimed in claim 9.

[0016] To illustrate the invention, various embodiments and test articles will be described by way of non-limiting examples, with particular reference to the following figures: [Brief explanation of the drawings]

[0017] [Figure 1] 1 shows the uniform layer structure observed in the cross section of the metal coating after heat treatment on a 1.5 mm thick steel sheet (specimen 2) with a coating containing 8% by weight of zinc according to the invention. [Figure 2] 1 shows the non-uniform layer structure observed in the cross section of a metal coating after heat treatment on a 0.8 mm thick steel sheet (specimen 4) for a coating not according to the invention containing 15 wt. % zinc. [Figure 3] 1 shows the distribution of magnesium oxide (MgO) particles having a size of 5 μm or more on the surface of a metal coating containing 15 wt. % zinc, not according to the invention, after curing. The MgO particles are shown as black circular areas. [Figure 4] 1 shows the distribution of MgO particles having a size of 5 μm or more on the surface of a metal coating containing 8% by weight of zinc according to the present invention after curing. [Figure 5] Parts having a linear profile and a "hat-shaped" cross section are shown, and such parts have been tested in the examples of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0018] The present invention relates to a steel sheet coated with a metallic coating containing, by weight percentage, 7.5 to 9.0% zinc, 2.0 to 4.0% silicon, 1.1 to 4.0% magnesium, a maximum of 3.0% iron as residual elements, and a maximum of 0.02% unavoidable impurities, with the remainder being aluminum.

[0019] Preferably, the coating contains 1.5 to 2.5% by weight of magnesium.

[0020] Optionally, the coating comprises an additional element selected from Ni, Zr, Hf, Sr, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, or Bi, wherein the weight content of each additional element is less than 0.3 wt%.

[0021] In a preferred embodiment, up to 100 ppm by weight of calcium is added.

[0022] Finally, the coating may contain up to 0.01 wt% of unavoidable impurities.

[0023] The steel sheet according to the present invention can be produced by hot dip galvanizing in a bath, the temperature of which is set at 600 to 700°C, preferably 620 to 650°C.

[0024] The coating weight is 50 to 500 g / m² for both sides of the steel sheet. 2 , in some cases 80-150g / m 2 , preferably 100 to 120 g / m 2 The range is set during the gas knife wiping process.

[0025] Before being coated, the steel sheet according to the invention can be obtained by hot rolling and optionally cold rolling, depending on the desired thickness, which can be for example between 0.5 and 3.0 mm.

[0026] The steel substrate to be coated can have any composition depending on the final properties required. If the steel is to be press hardened, the composition is preferably as follows:

[0027] The method according to the invention comprises the following steps:

[0028] A) providing a steel sheet according to the invention; B) cutting the coated steel sheet to obtain blanks; C) a heat treatment step of the blank at a temperature between 840 and 950°C in order to obtain a fully austenitic microstructure in the steel; D) transferring the blank to a press tool; E) press hardening the blank to obtain a part; F) Cooling the part obtained in step E) to obtain a press-hardened coated steel sheet.

[0029] Other features and advantages of the present invention will become apparent from the following detailed description of the invention.

[0030] In step A, any steel can be advantageously used in the frame of the present invention, provided that it is coated with a metal coating containing, by weight, 7.5 to 9.0% zinc, 2.0 to 4.0% silicon, 1.1 to 4.0% magnesium, and up to 3.0% of any element selected from iron, Pb, Ni, Zr, or Hf, the content of each element being less than 0.3% by weight, optionally containing up to 100 ppm calcium and up to 0.02% unavoidable impurities, the remainder being aluminum.

[0031] However, if steels with high mechanical strength are required, in particular for components in the structure of motor vehicles, steels with a tensile strength of more 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%, the remainder being unavoidable impurities from the production of iron and steel.

[0032] For example, the steel sheet is 22MnB5 having 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 remainder being unavoidable impurities from the production of iron and steel.

[0033] 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%; the remainder being unavoidable impurities resulting from the production of iron and steel.

[0034] Alternatively, the steel plate 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%, the remainder being unavoidable impurities resulting from the production of iron and steel.

[0035] 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%, with the balance being unavoidable impurities from the production of iron and steel.

[0036] 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%, less than 0.1% Cu, Ni and Mo, the remainder being unavoidable impurities resulting from the production of iron and steel.

[0037] 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%; the balance being unavoidable impurities from the production of iron and steel.

[0038] Alternatively, the steel plate 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 balance being unavoidable impurities from the production of iron and steel.

[0039] The steel sheet is cut into blanks in step B. The coated steel blanks may have a non-uniform thickness. This is the case of so-called "tailor-rolled blanks", which result from cutting a sheet obtained by a rolling process with a variable force along the length of the sheet. Or it may be the case of so-called "tailor-welded blanks", which result from welding at least two sub-blanks of different thicknesses.

[0040] In step C, the blank is heat treated at a temperature between 800 and 970°C, preferably between 840 and 950°C. The blank is maintained for a rest time of 1 to 15 minutes so that it has a fully austenitic structure. During the heat treatment, the coating forms an alloy layer that is highly resistant to corrosion and wear.

[0041] In step D, after heat treatment, the blank is then transferred to a press hardening tool.

[0042] In step E, press hardening is carried out at a temperature of 600-830°C.

[0043] In step F, the part is cooled either in the hot forming tool or after transfer to a specific cooling tool. The cooling rate is controlled depending on the steel composition so that the final microstructure after press hardening matches the target mechanical properties. After press hardening, the part can be tempered to reach the target microstructure and mechanical properties.

[0044] In a preferred embodiment, the steel microstructure comprises at least 95% martensite, in terms of volume fraction.

[0045] In another embodiment, the steel microstructure comprises, by volume fraction, at least 50% martensite and less than 40% bainite after compression hardening.

[0046] In another embodiment, the steel microstructure comprises, by volume fraction, 5-20% martensite, up to 10% bainite, and at least 75% equiaxed ferrite after compression hardening.

[0047] The part obtained in step F has a surface oxide layer on its exterior, which contains aluminum, zinc, and magnesium from the coating and iron from the steel substrate, which diffused through the coating during the heat treatment.

[0048] Thus, the coated parts according to the invention are obtained by press hardening, but can also be achieved by any suitable combination of cold stamping and press hardening.

[0049] When the hardened part leaves the stamping tool at the end of step F, some powder from the outer oxide layer of the coating may remain on the tool. Forming at high temperatures enhances formability and reduces rebound from the stamping tool. However, the press-hardening process may be limited by coating spallation. If the powder weight on the surface of the hardened part is less than 0.9 g / m 2 If this is exceeded, powdering of the coating can cause excessive wear on the stamping tool and lead to line stoppages.

[0050] The inventors have carried out several tests on the coated parts obtained in step F which show the effect of the zinc content of the metal coating.

[0051] When the coating contained excessive zinc, it was observed that the surface became rough and the layer structure became non-uniform, as can be seen in Figure 2 compared to Figure 1. This is believed to contribute to poor powdering resistance.

[0052] During heat treatment, most oxidizable elements form oxides on the surface. This is the case for magnesium or calcium. Magnesium oxide is a very hard particle compared to the surrounding zinc oxide phase. It is believed that hard MgO particles of a certain size can embrittle the outer oxide layer, thereby causing powdering.

[0053] The inventors have surprisingly found that the surface density of MgO particles is related to the amount of zinc in the coating: too much zinc in the coating reduces the surface density of MgO particles with diameters of 5 μm or more to 100 particles / mm 2 Exceeds.

[0054] If less than 5% by weight of zinc is present in the coating, corrosion protection is not sufficient.

[0055] When more than 8% by weight of magnesium is present in the coating, the surface density of MgO particles with diameters of 5 μm or more must be less than 100 particles / mm 2 Exceeds.

[0056] If less than 1.1 wt. % magnesium is present in the coating, other service properties such as corrosion resistance are not achieved.

[0057] The present invention will now be described by way of example and not by way of limitation. [Example]

[0058] For all samples, the steel plate used is 22MnB5. The steel composition is as follows: C = 0.23%; Mn = 1.2%; Si = 0.25%; Cr = 0.2%; Al = 0.04%; Ti = 0.04%; B = 0.003%.

[0059] All coatings were deposited by hot dip galvanizing, with the bath temperature set at 620 or 650°C.

[0060] The coated steel sheets were then cut into blanks and heat treated in a furnace at 900 °C for 5–6 min, depending on the thickness of the steel sheet. A sampling is shown in Table 1.

[0061] The heated blank was then transferred and quenched in a tool die to obtain a microstructure containing at least 75% martensite in terms of surface fraction.

[0062] Outer surface and morphology of MgO particles To characterize the morphology of the oxide layer on the surface of the press-hardened parts, the surfaces were observed by a field electron gun-scanning electron microscope at ×500 magnification.

[0063] Energy dispersive spectroscopy (EDS) was then used to identify the presence of various elements in the photographs, particularly magnesium. 2 Several EDS images were generated covering an area of ​​100 μm. In the images representing elemental magnesium, image analysis software was used to determine small MgO particles with diameters of 5 μm or greater. The MgO particles were then counted manually or automatically. Their surface densities are disclosed in Table 1.

[0064] Laboratory tests to evaluate oxide deposition and powdering For this experiment, the steel plates were cut to the following dimensions before heat treatment: 400 × 500 mm 2 A rectangular blank having a diameter of 1 / 4 was cut.

[0065] After heating, each blank was transferred to a forming tool consisting of a complementary punch and die. The tool had no additional binder to hold the blank in place during forming. The punch and die were cooled with circulating water. The temperature set point for the cooling water circuit was 17°C.

[0066] The resulting part has a linear profile and a "hat" shaped cross section. The part is made up of five segments. Figure 3 shows the different regions of the part along the hat shaped section: the "top of the hat" 11, the two walls 12 and 13, and the two lower flanges 14 and 15.

[0067] After press hardening, a portion of the top of the "hat" is cut out. The powdering of the coating is then evaluated by measuring the weight of the powdering on the "top of the hat" 11. The scale for the weighing operation was obtained from the manufacturer Sartorius and its accuracy is 0.1 mg.

[0068] Adhesive tape 2525 from supplier 3M is cut into strips of 50 mm x 50 mm and a place of the same size is marked on the sample for testing.

[0069] For all samples, the following procedure was performed.

[0070] a) Weigh the adhesive coupon on a laboratory balance b) Stitch the adhesive coupon to the sample at the desired location c) Remove the adhesive coupon from the sample and reweigh it. - If the weight difference is greater than 0.1 mg, start the procedure again from step a) with a new coupon at the same location on the sample. - If the weight difference is less than 0.1 mg, all measurable powder is recovered d) g / m 2 The sum of the weight differences (after stitching - before stitching) is calculated to obtain the powdering weight in units of 0.00025 m². 2 ) is divided by The results are summarized in Table 1.

[0071] [Table 1]

Claims

1. A steel sheet, wherein the metal coating comprises, in weight percentages, 7.5 to 9.0% zinc, 2.0 to 4.0% silicon, 1.1 to 4.0% magnesium, maximum 3.0% iron, optional elements selected from Pb, Ni, Zr or Hf, each of which has a content by weight of less than 0.3%, optionally maximum 100 ppm calcium and maximum 0.02% unavoidable impurities, the balance being aluminum, and the coating weight of the metal coating is 50 to 500 g / m based on the total of both sides of the steel sheet. ２ That is, steel plate.

2. The coating weight of the coating is 80 to 150 g / m2 in total on both sides of the steel plate. ２ The steel sheet according to claim 3, wherein

3. 1. A method of manufacturing a hardened part coated with a corrosion protection coating, comprising the steps of: A) providing a coated steel sheet according to claim 1 or 2, B) cutting the coated steel sheet to obtain blanks; C) heat treating the blank at a temperature of 840-950°C to obtain a fully austenitic microstructure in the steel; D) transferring the blank to a press tool; E) press hardening said blank to obtain a part; F) cooling the part obtained in step E) to obtain a press-hardened coated steel part. A method comprising:

4. 3. A coated steel sheet according to claim 1, which is obtained by press hardening the coated steel sheet according to claim 1 or 2, and which has an oxide layer on its outer surface and an amount of magnesium oxide particles having a diameter of more than 5 μm of 100 particles / mm ２ Press-hardened and coated steel parts exceeding 1000kJ / s.

5. 5. The press-hardened, coated steel part of claim 4, wherein the microstructure comprises at least 95% martensite by volume fraction.

6. 5. The press-hardened, coated steel part of claim 4, wherein the microstructure comprises, by volume fraction, at least 50% martensite and less than 40% bainite.

7. 5. A press-hardened, coated steel part according to claim 4, wherein the microstructure comprises, by volume fraction, 5-20% martensite, a maximum of 10% bainite and at least 75% equiaxed ferrite.

8. The amount of metal powder resulting from the removal of the metal coating, as measured by weighing the adhesive tape removed from the surface of the part, is 0.9 g / m ２ A press-hardened, coated steel part according to any one of claims 4 to 7, wherein the hardness is less than 1 / 2.

9. Use of a press-hardened and coated steel part according to any one of claims 4 to 8 for the manufacture of a motor vehicle.

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