Manufacturing method for steel pressed parts with low environmental impact

Abstract

1. A method for producing a press hardened part, comprising the steps of: A) providing a steel sheet provided with a coating, the coating containing, in weight percent, 7.5 to 8.5% zinc, 2.7 to 3.5% silicon, 1.0 to 3.0% magnesium, a maximum of 3.0% iron as a residual element, and any element selected from Ni, Zr, Hf, Sr, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr or Bi, the weight content of each element being less than 0.3%, a maximum of 0.02% of unavoidable impurities, and the remainder being aluminum; B) cutting the coated steel sheet to obtain blanks; C) heating the coated steel sheet in a furnace at a temperature set to 900-950°C for a time period of 2.5-4.0 minutes; D) transferring to a press tool; E) press hardening to obtain a press hardened part; A method comprising:

Description

[Technical Field]

[0001] The present invention relates to a method for producing steel press hardened parts, which method is effective in terms of energy saving and environmental impact during the production process, and which is particularly well suited for the production of automotive vehicles. [Background technology]

[0002] In recent years, the use of coated steels in hot stamping and press hardening processes for manufacturing parts has become important, especially in the automotive industry. The manufacture of such parts can include the following main steps:

[0003] coating the steel sheet by hot dip galvanizing in a metal bath; trimming or cutting to obtain blanks; heating the blank to obtain alloying of the steel substrate with the coating and austenitization of the steel; A process in which parts are press hardened to obtain a primarily martensitic microstructure.

[0004] Due to press hardening and the resulting microstructure, the part has a higher and well-controlled yield strength than would be possible without press hardening. Such parts can be used in automotive crash modules.

[0005] The forced heating step directly impacts the energy consumption of the production process and the CO2 emissions generated by the process. Global goals to reduce the effects of climate change and global warming require a reduction in the overall CO2 emissions of the production process. A shorter duration of the heating step results in a lower energy consumption associated with the production of the present process.

[0006] During heating, the steel plate, including the coating, is heated above the temperature at which the steel microstructure fully transforms to austenite (above the Ac3 temperature). After heating, the fully austenitized steel plate is transferred to a press-hardening tool, which performs both the press-forming at high temperature and the subsequent press-hardening by quenching the steel plate within the press tool. The press-hardening quench is achieved by controlling the cooling rate so that the part hardens. In most cases, the desired microstructure is predominantly martensite.

[0007] WO 2017017521 discloses a method for producing phosphate-treatable parts starting from a steel sheet coated with an aluminum-based metallic coating, the metallic coating comprising 4.0 to 20.0% by weight of zinc, 1.0 to 3.5% by weight of silicon, optionally 1.0 to 4.0% by weight of magnesium, and optionally an additional element selected from Pb, Ni, Zr, or Hf, the weight content of each additional element being less than 0.3% by weight, the remainder being aluminum and unavoidable impurities and residual elements, and the Zn / Si ratio being between 3.2 and 8.0. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] International Publication No. 2017 / 017521 Summary of the Invention [Problem to be solved by the invention]

[0009] It is an object of the present invention to provide a method for producing press hardened parts with a reduced CO2 footprint without compromising paint adhesion properties while ensuring the proper microstructure is achieved. [Means for solving the problem]

[0010] The object of the present invention is achieved by a method according to claim 1.

[0011] The object of the invention is also achieved by a method according to claims 2 and 3.

[0012] The steel sheet used in the present invention is coated with a metal coating, which contains, in weight percent, 7.5 to 8.5% zinc, 2.7 to 3.5% silicon, 1.0 to 3.0% magnesium, a maximum of 3.0% iron as a residual element, and any element selected from Ni Zr, Hf, Sr, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr or Bi, the weight content of each element being less than 0.3%, unavoidable impurities being a maximum of 0.02%, and the remainder being aluminum.

[0013] Preferably, the coating contains 1.5 to 3.0% magnesium by weight.

[0014] Preferably, the coating contains up to 2.0% by weight iron.

[0015] In a preferred embodiment, up to 100 ppm by weight of calcium is added.

[0016] Preferably, the coating may contain up to 0.01% by weight of unavoidable impurities.

[0017] The steel sheet used in the present invention can be produced by hot-dip galvanizing in a bath, the temperature of which is set at 600-700°C, preferably 620-650°C. When the coating is applied by hot-dip coating, iron results from the dissolution of the steel sheet in the hot-dip coating bath and may fluctuate during production.

[0018] The coating weight is 50 to 500 g / m², based on the total weight of both sides of the steel sheet. 2 , in some cases 80-150g / m 2 , preferably 90 to 120 g / m 2 is set during the gas knife wiping method.

[0019] Before coating, 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 0.5 to 3.0 mm, preferably 0.7 to 2.0 mm, or even 1.0 to 1.5 mm.

[0020] The method according to the invention comprises the following steps: A) providing a steel sheet provided with a coating, the coating containing, in weight percent, 7.5 to 8.5% zinc, 2.7 to 3.5% silicon, 1.0 to 3.0% magnesium, a maximum of 3.0% iron as a residual element, and any element selected from Ni, Zr, Hf, Sr, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr or Bi, the weight content of each element being less than 0.3%, unavoidable impurities being a maximum of 0.02%, and the remainder being aluminum; B) cutting the coated steel sheet to obtain blanks; C) heating the coated steel sheet in a furnace at a temperature set to 900-950°C for a time period of 2.5-4.0 minutes; D) transferring the heated coated steel sheet to a press tool; E) press hardening the coated steel sheet to obtain a press hardened part.

[0021] In step A), the steel plate used is made of a heat-treatable steel as specified in European standard EN 10083. This may have a tensile strength of more than 500 MPa, advantageously between 500 MPa and 2000 MPa, before or after heat treatment.

[0022] 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 are unavoidable impurities from the production of iron and steel.

[0023] 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%, with the remainder being unavoidable impurities from the production of iron and steel.

[0024] 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 remainder being unavoidable impurities resulting from the production of iron and steel.

[0025] 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%, and the remainder being unavoidable impurities resulting from the production of iron and steel.

[0026] 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 remainder being unavoidable impurities from the production of iron and steel.

[0027] 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.

[0028] 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 are unavoidable impurities from the production of iron and steel.

[0029] 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%, with the remainder being unavoidable impurities from the production of iron and steel.

[0030] In step B), the steel sheet is cut into blanks. The coated steel blanks may have a non-uniform thickness. This is the case for so-called "tailor rolled blanks", which are obtained by cutting a sheet obtained by rolling with a variable force along the length of the sheet. Or this can be the case for so-called "tailor welded blanks", which are obtained by welding at least two sub-blanks of different thicknesses.

[0031] In step C), the blank is heat treated. If the heat treatment is longer than 4.0 minutes, the energy consumed during the press hardening process is too high and the CO2 footprint is too high.

[0032] Heat treatment for less than 2.5 minutes results in incomplete austenitization of the steel sheet before press hardening. Because the steel sheet is not fully austenitized, even if the cooling rate is sufficient, the microstructure of the part will differ from the target microstructure. In this case, the microstructure of the final part will not be primarily martensite. It may also highlight excessive amounts of ferrite, bainite, or retained austenite.

[0033] The inventors have surprisingly found that the method according to the invention can achieve paint adhesion properties in heating times of 2.15 minutes or more, regardless of the steel grade.

[0034] In step D), the coated steel sheet is transferred into a press-forming tool and hot-formed at temperatures between 600 and 900°C. After forming, the steel part is either quenched in the press-forming tool or transferred to a specific quenching tool. Quenching is performed at a cooling rate faster than the critical cooling rate. The resulting microstructure is predominantly martensite. The amount of ferrite, bainite, or retained austenite is limited to a volume fraction depending on the steel grade.

[0035] The present invention will now be described by way of example and not by way of limitation. [Example]

[0036] In the examples, three different compositions of steel grades with different sheet thicknesses are tested with different coating compositions and coating weights, as shown in Table 2.

[0037] Steel Grade A has a composition by weight of 0.23% carbon, 1.19% manganese, 0.26% silicon, 0.18% chromium, 0.03% aluminum, 0.04% titanium and 0.002% boron. The target microstructure of the steel grade after full austenitizing heat treatment and subsequent press hardening comprises, by volume fraction, more than 95% martensite and less than 5% ferrite + bainite.

[0038] Steel Grade B has a composition by weight of 0.33% carbon, 0.62% manganese, 0.57% silicon, 0.37% chromium, 0.04% aluminum, 0.01% titanium and 0.002% boron. The target microstructure of Steel Grade B after full austenitizing heat treatment and subsequent press hardening comprises, by volume fraction, more than 95% martensite and less than 5% ferrite + bainite.

[0039] Steel Grade C has a composition by weight of 0.076% carbon, 1.60% manganese, 0.36% silicon, 0.08% chromium, 0.04% aluminum, 0.02% titanium and 0.003% boron. The target microstructure of Steel Grade C after full austenitizing heat treatment and subsequent press hardening comprises, by volume fraction, more than 50% martensite and less than 45% ferrite + bainite.

[0040] <Microstructure and thickness of the interdiffusion layer> The specimens were heated and press hardened according to the parameters collected in Table 2. During press hardening, the cooling rate was faster than the critical cooling rate.

[0041] The phase contents in terms of volume fractions are determined by the following method: a sample is cut from the press-hardened steel part, polished, and etched with a reagent known per se, such as Nital reagent, to reveal the microstructure. The cross section is then examined by optical or scanning electron microscope, for example a scanning electron microscope equipped with a field emission electron gun ("FEG-SEM") at a magnification of more than 5000 times, coupled to an electron backscatter diffraction (EBSD) device.

[0042] The results are summarized in Table 2.

[0043] <Paint adhesion> This test is used to determine paint adhesion on cured parts.

[0044] After press hardening, the parts evaluated to determine the microstructure were subjected to a phosphating step, which was achieved by immersion in a bath at 50°C for 3 minutes. The components of the phosphating bath were Gardobond® products from the supplier Chemetall. Their concentrations are disclosed in Table 1.

[0045] [Table 1]

[0046] The samples were then wiped with water and dried with hot air.

[0047] After the phosphate treatment, a 20 μm e-coating layer was deposited on specimens 1 to 5. For this purpose, all specimens were immersed for 180 seconds at 30 °C in a bath containing an aqueous solution of Pigment paste® W9712-N6 and Resin blend® W7911-N6 from PPG Industries. A current of 200 V was applied. The panels were then wiped and cured in an oven at 180 °C for 35 minutes.

[0048] The painted parts are then immersed in a sealed box containing demineralized water at 50°C for 10 days in accordance with the NF EN ISO 2409 standard. After immersion, a grid is created using a cutter. The paint is then peeled off with adhesive tape.

[0049] The paint removed is assessed visually: 0 means excellent, i.e., no paint has been removed; 5 means very poor, i.e., a lot of paint has been removed. A satisfactory result is 0 or 1; a score of 2 or higher indicates insufficient paint adhesion. Three to six samples were run for each test item, and the results are shown in Table 2 as the average for each test item.

[0050] [Table 2]

[0051] Test piece 1, which uses the heat treatment according to the invention rather than the coating according to the invention, does not exhibit sufficient paint adhesion. Test piece 1 does not achieve the targeted microstructure.

[0052] Specimens 3, 5 and 8, which had a coating according to the present invention but had a shorter heat treatment, did not achieve the target microstructure.

[0053] Specimens 2, 4, 6, 7 and 9 according to the invention exhibit paint adhesion that is equal to or better than the comparative examples, as well as the targeted microstructure, regardless of the steel grade.

Claims

1. 1. A method for producing a press hardened part, comprising the steps of: A) providing a steel sheet provided with a coating, the coating containing, in weight percent, 7.5 to 8.5% zinc, 2.7 to 3.5% silicon, 1.0 to 3.0% magnesium, a maximum of 3.0% iron as a residual element, and any element selected from Ni, Zr, Hf, Sr, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr or Bi, the weight content of each element being less than 0.3%, maximum 0.02% unavoidable impurities, and the remainder being aluminum; B) cutting the coated steel sheet to obtain blanks; C) heating the coated steel sheet in a furnace at a temperature set at 900-950°C for a time period of 2.5-4.0 minutes; D) transferring the heated coated steel sheet to a press tool; E) press hardening the coated steel sheet to obtain a press hardened part; A method comprising:

2. 2. The method for producing press-hardened parts according to claim 1, wherein the coating of step A) comprises, in weight percent, 1.5 to 3.0% magnesium.

3. 3. The method for manufacturing press-hardened parts according to claim 1, wherein the coating of step A) contains, in weight percent, a maximum of 2% iron.

Images