Steel plate with variable thickness and reduced risk of delayed fracture after press hardening and method for manufacturing same

Abstract

1. A coated steel sheet having a variable thickness in the rolling direction, with one portion rolled at a rolling ratio of 1 to 60% and at least another portion rolled at a different rolling ratio, wherein the coating comprises optional elements selected from zinc, silicon, magnesium, maximum 3.0 wt% iron, Ni, Zr, Hf, Sr, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr or Bi, each of which has a content by weight of less than 0.3 wt%, optionally maximum 100 ppm calcium, and maximum 0.02 wt% unavoidable impurities, the balance being aluminum, and wherein the coating has a thickness of 50 to 500 g / m2 on both sides combined before flexible rolling. 2 Coated steel sheets with variable thicknesses having coating weights of

Description

[Technical Field]

[0001] The present invention relates to a method for producing press-hardened steel parts having variable thickness and minimal risk of delayed fracture. The present invention is particularly well suited to the manufacture of motor vehicles.

[0002] It is known that in certain applications, particularly in the automotive field, metal structures need to be further lightened and strengthened in the event of an impact, and for this purpose steel sheets with improved mechanical properties are usually used, which are formed by austenitizing and subsequent press hardening.

[0003] Susceptibility to delayed cracking increases with mechanical strength after press hardening because high residual stresses tend to remain after deformation. Combined with atomic hydrogen, which may be present in the steel, these stresses tend to result in delayed cracking, which means that cracks will occur after a certain time from the deformation itself. Hydrogen can gradually accumulate by diffusion into crystal lattice defects, such as matrix / inclusion interfaces, twin boundaries, and grain boundaries. It is these latter defects that can become harmful when hydrogen reaches a critical concentration after a certain time. This delay arises from the distribution of residual stresses and the kinetics of hydrogen diffusion, which has a low coefficient of diffusion at room temperature. Furthermore, hydrogen localized at grain boundaries weakens their cohesion and promotes the appearance of delayed intergranular cracks.

[0004] It is known that press hardening is important for hydrogen absorption, increasing the sensitivity to delayed fracture. Absorption can occur during the austenitizing heat treatment, which is a heating step prior to the press forming itself. The saturation of hydrogen into the steel actually depends on the metallurgical phase. Furthermore, at high temperatures, furnace water dissociates into hydrogen and oxygen at the surface of the steel.

[0005] In the automotive field, it is also known to design parts with variable thickness, so that mechanical resistance is only present in areas where it is needed, and weight is not added where it is not needed. Lightweighting of motor vehicles is essential for reasons of energy consumption and exhaust emissions.

[0006] Variable thickness parts are usually produced by continuous flexible rolling, where the thickness of the resulting plate varies in the rolling direction. This occurs due to the load applied to the plate via the rollers during the rolling process, as described in patent EP 1074317. Flexible rolling is characterized by the intentional change of the roll gap during the rolling operation. The purpose of flexible rolling is to produce a rolled plate with a cross section optimized for load and weight. The thickness of such variable thickness steel plates is derived from the rolling process. Hereinafter, the rolling ratio is defined by the following formula:

[0007]

number

[0008] The resulting parts with variable thickness are known to absorb more hydrogen during the austenitizing heat treatment than standard parts with uniform thickness.

[0009] Patent application EP 3489386 discloses a coated steel sheet that undergoes particularly low hydrogen absorption during the press hardening process, resulting in a surface that is simple and allows for good further processing. The proposed solution is a coated steel substrate for hot working, comprising a first coating containing at least 85% by weight of aluminum and a second coating on top of the first coating, the second coating being a copper-containing coating. The method according to the present application is particularly suitable for flexible rolled strip material, since the thinner rolled substrate portions also increase resistance to hydrogen absorption after application of the nanocrystalline zinc-copper coating. However, this solution requires a second coating on top of the aluminum-based coating, which results in an additional process step and the corresponding costs and complexity.

[0010] Regarding hydrogen uptake during the heating step, not only does the rolling ratio increase the amount of hydrogen, but so do the parameters used to heat the blank: the longer the dwell time or the higher the furnace dew point, the higher the amount of hydrogen in the press-hardened part. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] European Patent No. 1074317 [Patent Document 2] European Patent No. 3489386 Summary of the Invention [Problem to be solved by the invention]

[0012] It is therefore an object of the present invention to provide a steel sheet with variable thickness suitable for press hardening, which can be used to manufacture parts with variable thickness that have limited hydrogen absorption, independent of the heating time used for press hardening, and in particular to make available parts with excellent resistance to delayed fracture.

[0013] This object is achieved by the steel sheets according to claims 1-3.

[0014] Another object of the present invention is to provide a method for producing the same according to claim 4.

[0015] The object of the present invention is also achieved by providing components as set forth in claims 5 to 9.

[0016] A final object of the invention is the use of such a component as claimed in claim 10.

[0017] The present invention relates to a steel sheet coated with a metallic coating comprising zinc, silicon, magnesium, maximum 3.0% of optional elements selected from iron, Pb, Ni, Zr, Hf, Sr, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr or Bi, each element in a content by weight of less than 0.3%, optionally maximum 100 ppm calcium and maximum 0.02% unavoidable impurities, the balance being aluminium.

[0018] Preferably, the coating contains, by weight, 1.0-11.0% zinc, 1.0-7.0% silicon, 1.0-8.0% magnesium, maximum 3.0% iron, and maximum 0.02% unavoidable impurities, the balance being aluminum.

[0019] Advantageously, the coating contains, by weight, 6.0 to 10.0% zinc, 1.0 to 4.0% silicon, 1.0 to 4.0% magnesium, maximum 3.0% iron, and maximum 0.01% unavoidable impurities, the balance being aluminum.

[0020] In another embodiment, the coating comprises, by weight percentage, 7.5 to 9.0% zinc, 2.0 to 4.0% silicon, and 1.5 to 2.5% magnesium, with the balance being aluminum.

[0021] 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 600-700°C, preferably 620-650°C.

[0022] The coating weight, controlled by the wiping process, is 50 to 500 g / m² on both sides of the steel sheet. 2 , in some cases 80-150g / m 2 , preferably 100 to 120 g / m 2 It could be.

[0023] 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 1.0 and 4.0 mm.

[0024] The substrate to be coated can have any composition depending on the required mechanical properties. When steel is used for press hardening, its composition is preferably as follows:

[0025] After being coated, the coated steel sheet is subjected to a flexible rolling operation, after which the steel sheet has a variable thickness in the rolling direction.

[0026] The flexible rolling is preferably a cold rolling operation. The reduction ratio is 1-60%, preferably 5-50%. The resulting re-rolled material is then a tailor-rolled steel plate. The tailor-rolled steel plate is then cut to obtain a tailor-rolled blank. The flexible rolling operation is usually carried out in one step using a single-stand reversing rolling mill. At this time, the thickness of the coating is also reduced. After rolling, the plate can have a thickness reduced to 0.8 mm or even 0.6 mm.

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

[0028] A. Providing a coated steel sheet having a variable thickness according to the present invention; B. Cutting the rolled steel plate to obtain tailor-rolled blanks; C. Heat treating the tailor rolled blank to obtain a fully austenitic microstructure in the steel; D. transferring the tailor rolled blank to a press tool; E. Press hardening the tailor rolled blank to obtain a part with variable thickness; F. Cooling the part with variable thickness obtained in step E) to obtain a press-hardened part with variable thickness.

[0029] Any steel can be advantageously used in the frame of the invention in step A. However, if a steel with high mechanical strength is required for a component of the structure of a motor vehicle, a steel with a tensile strength of more than 500 MPa, preferably 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 being unavoidable impurities from the production of iron and steel.

[0030] 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.

[0031] 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.

[0032] 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.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] 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 period of 1 to 15 minutes to achieve a fully austenitic structure. During the heat treatment, the pre-coating forms an alloy layer that is highly resistant to corrosion and wear. The furnace atmosphere influences the amount of hydrogen absorbed by the steel sheet during heat treatment. For example, at a dew point of 20°C, hydrogen absorption can be significant, but heat treatment in a dry atmosphere is known to be much less dangerous.

[0038] In step D, after heat treatment, the blank is then transferred to a press hardening tool.

[0039] In step E, press hardening is preferably carried out at a temperature of 600-830°C.

[0040] In step F, the part is cooled in the press hardening 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 targeted mechanical properties. After press hardening, the part can be further tempered to reach the targeted microstructure and mechanical properties.

[0041] In a preferred embodiment, the steel microstructure comprises at least 95% martensite, in terms of volume fraction.

[0042] In another embodiment, the steel microstructure comprises, by volume fraction, at least 50% martensite and less than 40% bainite after compression hardening.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] A hydrogen content of 0.6 ppm or less after heat treatment in an atmosphere with a dew point of 20° C. is considered satisfactory. Conversely, a hydrogen content of more than 0.6 ppm may induce the risk of subsequent delayed fracture.

[0047] The inventors have found that the composition of the metallic coating influences the hydrogen absorption of the rolled material, and the coating composition according to the invention makes it possible to keep the hydrogen content of the press-hardened part below 0.6 ppm, whatever the rolling ratio.

[0048] It is believed that a surface oxide layer can act as a barrier to hydrogen, especially if said oxide layer contains zinc, magnesium, and if it has a minimum thickness.

[0049] The oxide layer comprises elements from the coating. According to the invention, the oxide layer contains zinc and magnesium from the aluminum-based coating, and the oxide layer has a minimum thickness of 0.4 μm. Preferably, the oxide layer has a minimum thickness of 0.5 μm, advantageously 0.6 μm.

[0050] The present invention will now be described in terms of tests that have been carried out for informational purposes only and are not limiting. [Example]

[0051] For all samples, the steel plate used is 22MnB5, with the following composition by weight: C = 0.22%; Mn = 1.2%; Si = 0.25%; Cr = 0.2%; Al = 0.041%; Ti = 0.04%; B = 0.003%.

[0052] All coatings were deposited by hot dip in a single bath.

[0053] After coating deposition, some samples were left unrolled and some others were rolled by 50%, ie, their thickness was reduced to half of what it was before rolling.

[0054] After the rolling step, the samples were heated in a furnace at 900°C with a dew point of +20°C for 5 and 12 minutes.

[0055] After press hardening, two different measurements were made: the amount of hydrogen and the thickness of the outer oxide layer.

[0056] The hydrogen content absorbed by the steel sheets during the heat treatment was measured by thermal desorption using a Thermal Desorption Analyzer (TDA). For this purpose, each sample was placed in a quartz chamber and slowly heated in an infrared furnace under a nitrogen flow. The released hydrogen / nitrogen mixture was collected by a leak detector, and the hydrogen concentration was measured by a mass spectrometer.

[0057] The oxide layer was measured by cross-sectional observation under a microscope, and the minimum value along the cross section is reported.

[0058] The results are shown in Table 1.

[0059] Table 1

[0060] [Table 1] Specimens 4 to 6, whose coating composition is not according to the invention, contain more than 0.60 ppm of hydrogen, which poses a risk of delayed fracture. Their oxide layer thickness is too thin. Specimens 1 to 3 contain a maximum of 0.40 ppm of hydrogen and a thick oxide layer. They show that parts coated with a coating according to the invention, with variable thicknesses obtained by rolling one part at a reduction ratio of 30% and another part at a reduction ratio of 50%, solve the fracture risk problem.

Claims

1. 1. A coated steel sheet having a variable thickness in the rolling direction, with one portion rolled at a rolling ratio of 1 to 60% and at least another portion rolled at a different rolling ratio, wherein the coating comprises optional elements selected from zinc, silicon, magnesium, maximum 3.0 wt.% iron, Ni, Zr, Hf, Sr, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr or Bi, each of which has a content by weight of less than 0.3 wt.%, optionally maximum 100 ppm calcium, and maximum 0.02 wt.% unavoidable impurities, the balance being aluminum, and wherein the coating has a thickness of 50 to 500 g / m2 on both sides combined before flexible rolling. ２ Coated steel sheets with variable thicknesses having coating weights of

2. 2. The coated steel sheet having variable thickness according to claim 1, wherein the coating comprises, by weight percentage, 1.0 to 11.0% zinc, 1.0 to 7.0% silicon, 1.0 to 8.0% magnesium, maximum 3.0% iron, optionally maximum 100 ppm calcium and maximum 0.02% unavoidable impurities, the balance being aluminum.

3. 3. The coated steel sheet having variable thickness according to claim 2, wherein the coating comprises, by weight percentage, 6.0 to 10.0% zinc, 1.0 to 4.0% silicon, 1.0 to 4.0% magnesium, maximum 3.0% iron, optionally maximum 100 ppm calcium and maximum 0.01% unavoidable impurities, the balance being aluminum.

4. A press hardening method comprising: A. Providing a coated steel sheet according to any one of claims 1 to 3; B. Cutting the coated plate having variable thickness to obtain tailor rolled blanks; C. heat treating the tailor rolled blank to obtain a fully austenitic microstructure in the steel; D. transferring the tailor rolled blank to a press tool; E. Press hardening the tailor rolled blank to obtain a part having variable thickness; F. A press hardening method comprising cooling the part having variable thickness obtained in step E) to obtain a press hardened part having variable thickness.

5. 4. A coated press-hardened steel part with variable thickness obtained by press-hardening a coated steel sheet with variable thickness according to any one of claims 1 to 3, wherein the press-hardened part is covered with a surface oxide layer comprising aluminium, zinc and magnesium from the coating and iron from the steel substrate, the oxide layer comprising oxides of zinc and magnesium and having a minimum thickness of 0.40 μm, the thickness of the oxide layer being measured by microscopic observation of a cross section.

6. 6. The coated press hardened steel part having variable thickness according to claim 5, wherein said oxide layer has a minimum thickness of 0.50 μm, said oxide layer thickness being measured by cross-sectional examination under a microscope.

7. 7. A coated press-hardened steel part with variable thickness according to claim 5 or 6, wherein the microstructure of the press-hardened part comprises at least 95% martensite by volume fraction.

8. 7. A coated press-hardened steel part with variable thickness according to claim 5 or 6, wherein the microstructure of the press-hardened part comprises, by volume fraction, at least 50% martensite and less than 40% bainite.

9. 7. A coated press-hardened steel part with variable thickness according to claim 5 or 6, wherein the microstructure of the press-hardened part comprises 5-20% martensite, maximum 10% bainite, and at least 75% equiaxed ferrite.

10. Use of a component according to any one of claims 5 to 9 for the manufacture of a motor vehicle.

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