Steel sheet with variable thickness and reduced risk of delayed fracture after press hardening, and method for manufacturing the 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 manufacturing press-hardened steel parts having variable thickness and a minimum risk of delayed fracture. The present invention is particularly well-suited for the manufacture of motor vehicles.

[0002] In particular applications in the automotive field, it is known that there is a need to further lightweight and strengthen metal structures in preparation for impacts. For this purpose, steel sheets having improved mechanical properties are commonly used, and such steel sheets are formed by austenitization and subsequent press hardening.

[0003] The susceptibility to delayed cracking increases in accordance with the mechanical strength after press hardening, because high residual stresses tend to remain after deformation. Combined with atomic hydrogen that may be present in the steel sheet, these stresses tend to cause delayed cracking, which means that cracking occurs after a certain time of 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 the latter defects that can become harmful when hydrogen reaches a critical concentration after a certain time. This delay results from the field of the residual stress distribution and the kinetics of hydrogen diffusion, and the diffusion coefficient of hydrogen at room temperature is low. Furthermore, hydrogen localized at grain boundaries weakens their cohesion and promotes the appearance of delayed intergranular cracking.

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

[0005] In the automotive field, it is also known to design components with variable thickness, so that they exist only where mechanical resistance is needed, and do not add weight where it is not needed. Lightweighting of automobiles is essential for reasons of energy consumption and exhaust emissions.

[0006] Parts with variable thickness are typically manufactured by continuous flexible rolling, and the thickness of the resulting sheet is variable in the rolling direction. This occurs in relation to the load applied to the sheet through the rollers during the rolling process, as described in European Patent No. 1074317. Flexible rolling is characterized by the intentional modification of the roll gap during the rolling operation. The purpose of flexible rolling is to produce rolled sheets with optimized load and weight cross-sections. The thickness of such steel sheets with variable thickness is derived from the rolling process. Hereinafter, the rolling ratio is defined by the following formula:

[0007]

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[0008] It is known that components with variable thickness absorb more hydrogen during austenitization heat treatment than standard components with uniform thickness.

[0009] Patent application European Patent No. 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 wt% aluminum and a second coating on the first coating, wherein the second coating is a copper-containing coating. The method according to this application is particularly suitable for flexible rolled strip materials, as thinner portions of the rolled substrate also increase resistance to hydrogen absorption after the 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 corresponding cost and complexity.

[0010] Regarding hydrogen incorporation during the heating step, the rolling ratio not only increases the amount of hydrogen but also increases the parameters used to heat the blank. The longer the resting time or the higher the dew point of the furnace, the greater 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 [Overview of the project] [Problems that the invention aims to solve]

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

[0013] This objective is achieved by the steel plates described in claims 1 to 3.

[0014] Another object of the present invention is to provide the manufacturing method described in claim 4.

[0015] Furthermore, the object of the present invention is achieved by providing the components described in claims 5 to 9.

[0016] The final object of the present invention is the use of such a component as described in claim 10.

[0017] The present invention relates to a steel sheet coated with a metal coating containing zinc, silicon, magnesium, up to 3.0% iron, and an optional element selected from Pb, Ni, Zr, Hf, Sr, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, or Bi, wherein the weight content of each element is less than 0.3%, optionally up to 100 ppm calcium, and up to 0.02% unavoidable impurities, with the remainder being aluminum.

[0018] Preferably, the coating contains, by weight, 1.0 to 11.0% zinc, 1.0 to 7.0% silicon, 1.0 to 8.0% magnesium, up to 3.0% iron, and up to 0.02% unavoidable impurities, with the remainder being aluminum.

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

[0020] In another embodiment, the coating contains, 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 to 600 to 700°C, preferably 620 to 650°C.

[0022] The coating weight controlled by the wiping process can be 50 to 500 g / m for the total of both sides of the steel sheet, 2 and in some cases 80 to 150 g / m, 2 preferably 100 to 120 g / m. 2 It can be.

[0023] The steel sheet according to the present invention can be obtained by hot rolling and optionally cold rolling according to the desired thickness before being coated. The desired thickness can be, for example, between 1.0 and 4.0 mm.

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

[0025] After coating, the coated steel sheet is subjected to a flexible rolling operation, and thereafter the steel sheet has a variable thickness in the rolling direction.

[0026] The flexible rolling is preferably a cold rolling operation. The rolling ratio is 1 to 60%, preferably 5 to 50%. At that time, the obtained re-rolled material is a tailor-rolled steel sheet. Then, the tailor-rolled steel sheet is cut to obtain a tailor-rolled blank. The flexible rolling operation is usually carried out in one step with a single-stand reversible rolling mill. At that time, the thickness of the coating also becomes thinner. After rolling, the sheet can have a thickness reduced to 0.8 mm or even 0.6 mm.

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

[0028] A. A step of providing a coated steel plate having a variable thickness according to the present invention, B. Steps to cut rolled steel sheets to obtain tailor-rolled blanks, A step of heat-treating a C. Taylor rolled blank to obtain a perfect austenitic microstructure in steel. Steps to transfer the D. Taylor rolled blank to the press tool, Steps include: Press-hardening an E. Taylor rolled blank to obtain a part with variable thickness; F. A step in which the part having variable thickness obtained in step E) is cooled to obtain a press-hardened part having variable thickness.

[0029] In step A, any steel can be advantageously used in the frame of the present invention. However, if a steel with high mechanical strength is required for a component of the structure of an automobile, a steel having a tensile resistance of more than 500 MPa, and preferably between 500 and 2000 MPa before or after heat treatment, can be used. The weight composition of the steel sheet 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%, with the remainder being unavoidable impurities from the manufacture of iron and steel.

[0030] For example, the steel sheet is 22MnB5, having 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%, with the remainder being unavoidable impurities from the manufacture of iron and steel.

[0031] In another embodiment, the steel sheet 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 being unavoidable impurities arising from the manufacture of iron and steel.

[0032] 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 unavoidable impurities arising from the manufacture of iron and steel.

[0033] In another embodiment, the steel sheet 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%, with the remainder being unavoidable impurities from the manufacture of iron and steel.

[0034] In another embodiment, the steel sheet 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, with the remainder being unavoidable impurities resulting from the manufacture of iron and steel.

[0035] In another embodiment, the steel sheet 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 remainder being unavoidable impurities from the manufacture of iron and steel.

[0036] Alternatively, the steel plate 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%, with the remainder being unavoidable impurities from the manufacture of iron and steel.

[0037] In step C, the blank is heat-treated at a temperature of 800-970°C, preferably 840-950°C. The blank is held for a resting time of 1-15 minutes to have 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 affects the amount of hydrogen absorbed by the steel sheet during heat treatment. For example, hydrogen absorption can be significant at a dew point of 20°C, but heat treatment in a dry atmosphere is known to be far less dangerous.

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

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

[0040] In step F, the part is cooled within the press hardening tool or after transfer to a specific cooling tool. The cooling rate is controlled according to the steel composition so that the final microstructure after press hardening matches the target mechanical properties. After press hardening, the part can be further tempered to achieve the target microstructure and mechanical properties.

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

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

[0043] In another embodiment, the steel microstructure, after compression hardening, comprises, in terms of volume fraction, 5–20% martensite, up to 10% bainite, and at least 75% equiaxed ferrite.

[0044] Therefore, the coated parts according to the present invention are obtained by press curing, but can also be achieved by any suitable combination of cold stamping and press curing.

[0045] The parts obtained in step F have their outer surfaces covered with a surface oxide layer. This oxide layer contains aluminum, zinc, and magnesium from the coating, as well as iron from the steel substrate. The iron diffused through the coating during the heat treatment.

[0046] After heat treatment in an atmosphere with a dew point of 20°C, a hydrogen content of 0.6 ppm or less is considered satisfactory. Conversely, a hydrogen content exceeding 0.6 ppm may induce a risk of delayed breakdown later on.

[0047] The inventors have found that the composition of the metal coating affects the hydrogen absorption of the rolled material. The coating composition according to the present invention makes it possible to maintain the hydrogen content of press-hardened parts at less than 0.6 ppm, regardless of the rolling ratio.

[0048] The surface oxide layer is considered to be able to act as a barrier against hydrogen, especially when the oxide layer contains zinc and magnesium, and when it has a minimum thickness.

[0049] The oxide layer contains elements derived from the coating. According to the present invention, the oxide layer contains zinc and magnesium from an 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, and more preferably 0.6 μm.

[0050] The present invention will now be described using test samples that were used for informational purposes only. These samples are not limiting. [Examples]

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

[0052] The entire coating was deposited by hot-dip galvanizing in a single molten tank.

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

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

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

[0056] The hydrogen content absorbed by the steel sheet during 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 flow of nitrogen. The released hydrogen / nitrogen mixture was acquired with a leak detector, and the hydrogen concentration was measured with a mass spectrometer.

[0057] The oxide layer was measured by observing its cross-section under a microscope. The minimum value along the cross-section is reported.

[0058] The results are shown in Table 1.

[0059] Table 1

[0060] [Table 1] Test specimens 4-6, whose coating composition is not according to the present invention, contain more than 0.60 ppm of hydrogen, which induces a risk of delayed fracture. Their oxide layers are too thin. Test specimens 1-3 contain up to 0.40 ppm of hydrogen and a thick oxide layer. They demonstrate that parts coated with the coating according to the present invention, which has variable thickness obtained by rolling one part at a 30% rolling ratio and the other part at a 50% rolling ratio, solve the fracture risk problem.

Claims

1. A coated steel sheet having a thickness variable in the rolling direction, comprising 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 contains, by weight percentage, 7.5 to 9.0% zinc, 2.0 to 4.0% silicon, 1.5 to 2.5% magnesium, up to 3.0 wt% iron, and any selected element chosen from Ni, Zr, Hf, Sr, Sb, Pb, Ti, Mn, Sn, La, Ce, Cr, or Bi, with each of the selected elements having a weight content of less than 0.3 wt%, optionally up to 100 ppm calcium, and up to 0.02 wt% unavoidable impurities, with the remainder being aluminum, and the coating having a thickness of 50 to 500 g / m² relative to the total of both sides before flexible rolling. ２ It has a coating weight of, The coated steel sheet has a variable thickness, and after being subjected to heat treatment at a temperature of 800 to 970°C for 1 to 15 minutes in an atmosphere having a dew point of 20°C, the hydrogen content measured using a Thermal Desorption Analyzer is 0.6 ppm or less.

2. A press curing method, the following: A. The step of providing the coated steel plate according to claim 1, B. The step of cutting the coated plate having the variable thickness to obtain a tailor-rolled blank, C. A step of heat-treating the Taylor-rolled blank to obtain a perfect austenite microstructure in steel. D. A step of transferring the tailor-rolled blank to a press tool. E. The step of press-hardening the tailor-rolled blank to obtain a part having a variable thickness, F. A press hardening method comprising the step of cooling the part having variable thickness obtained in step E) to obtain a press hardened part having variable thickness.

3. A coated press-hardened steel part having a variable thickness, obtained by press-hardening a coated steel sheet having a variable thickness as described in claim 1, wherein the press-hardened steel part is covered with a surface oxide layer containing aluminum, zinc, and magnesium derived from the coating and iron derived from the steel substrate, the oxide layer contains zinc oxide and magnesium oxide and has a minimum thickness of 0.40 μm, and the thickness of the oxide layer is measured by observation of the cross-section using a microscope.

4. A coated press-hardened steel part having variable thickness according to claim 3, wherein the oxide layer has a minimum thickness of 0.50 μm, and the thickness of the oxide layer is measured by observation of the cross-section using a microscope.

5. A coated press-hardened steel part having a variable thickness according to claim 3, wherein the microstructure of the press-hardened part contains at least 95% martensite by volume fraction.

6. A coated press-hardened steel part having a variable thickness according to claim 3, wherein the microstructure of the press-hardened part comprises, by volume fraction, at least 50% martensite and less than 40% bainite.

7. A coated press-hardened steel part having a variable thickness according to claim 3, wherein the microstructure of the press-hardened part comprises 5 to 20% martensite, up to 10% bainite, and at least 75% equiaxed ferrite.

8. Use of the component according to any one of claims 3 to 7 for the manufacture of an automatic vehicle.

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