Method for manufacturing zinc-plated steel sheet with liquid metal embrittlement resistance

Abstract

The object of the present invention is to provide coated steel sheets that exhibit high LME resistance behavior. The object of the present invention is to make available a method that is particularly easy to implement industrially for obtaining assemblies that are prone to LME resistance, in particular after hot press forming and / or welding. The present invention provides a method for producing coated steel sheets, which comprises the following successive steps: A. coating a steel sheet with a first coating made of nickel and having a thickness between 600 nm and 1400 nm; B. Recrystallization annealing the coated steel sheet at a temperature between 820 and 1200 ° C; C. Coating the steel sheet obtained in step B) with a second nickel-free zinc-based coating.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a zinc-based coated steel sheet, and is particularly suitable for the manufacture of motor vehicles.

Background Art

[0002] Zinc-based coatings are commonly used because, like cathodic protection, they enable protection against corrosion thanks to the barrier. This barrier effect is obtained by applying a metal coating to the steel surface. In this way, the metal coating prevents contact between the steel and the corrosive atmosphere. The barrier effect is independent of the nature of the coating and the substrate. In contrast, sacrificial cathodic protection is based on the fact that zinc is less noble than steel. Therefore, when corrosion occurs, zinc is preferentially consumed compared to steel. Cathodic protection is essential in areas where the steel is directly exposed to a corrosive atmosphere, such as cut edges where the surrounding zinc is consumed before the steel.

[0003] However, when a heating process, such as hot press hardening or welding, is performed on such a zinc-coated steel sheet, cracks propagating from the steel / coating interface are observed in the steel. In fact, sometimes, the presence of cracks in the coated steel sheet after the above operations reduces the mechanical properties of the metal, such as ductility. These cracks appear due to the presence of high temperature, tensile stress, in addition to contact with a liquid metal (such as zinc) having a low melting point, and non-uniform diffusion of the molten metal at the substrate particles and grain boundaries. The name of such a phenomenon is known as liquid metal embrittlement (LME) and is also called liquid metal-assisted cracking (LMAC).

[0004] Patent Application JPS589965 discloses a surface-treated steel sheet obtained by electroplating one of Ni, Cr, Zn, Zn-Ni alloy, or Sn-Ni alloy on both sides of a steel sheet, heating in a non-oxidizing atmosphere to form a diffusion layer of the plated metal in the steel substrate, and subjecting one surface of the obtained plated steel sheet to hot dip galvanizing to form a zinc plating layer. It is mentioned that the coating weight of the zinc plating layer can be reduced, which is extremely advantageous from the viewpoints of weldability and economy.

[0005] In fact, the above patent application shows that the surface-treated steel sheet has weldability improved only by a decrease in the zinc coating weight. Also, there is no mention of the improvement of LME resistance, especially with respect to high-strength steels having alloying elements including Mn, Al, and Si.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0007] Accordingly, an object of the present invention is to provide a coated steel sheet showing high LME resistance behavior. The present invention aims to make available a method that is particularly easy to implement industrially, in order to obtain an assembly that easily acquires LME resistance, especially after hot press forming and / or welding.

Means for Solving the Problems

[0008] The first object is achieved by providing the method according to claim 1. This method can also include any of the characteristics described in claims 2 to 11.

[0009] The second object is achieved by providing the coated steel sheet according to claim 12 or 13.

[0010] The third object is achieved by providing the spot weld joint according to claim 14. The spot weld joint can also include the characteristics described in claims 15 to 17.

[0011] Finally, the fourth object is achieved by providing the use of the steel sheet or assembly according to claim 18.

Modes for Carrying Out the Invention

[0012] Other features and advantages of the present invention will become apparent from the following detailed description of the invention.

[0013] The term "steel" or "steel sheet" refers to steel sheets, coils, plates having a composition which allows the parts to achieve a tensile strength of up to 2500 MPa, more preferably up to 2000 MPa, for example a tensile strength of at least 500 MPa, preferably at least 980 MPa, advantageously at least 1180 MPa or even at least 1470 MPa.

[0014] The present invention comprises the following successive steps: A. Coating a steel sheet with a first coating consisting of nickel and having a thickness between 600 nm and 1400 nm, wherein the steel sheet has the following composition by weight: 0.10 <C<0.40%、 1.5 <Mn<3.0%、 0.7 <Si<3.0%、 0.05 <Al<1.0%、 0.75<(Si+Al)<3.0%, and on a purely voluntary basis, Nb≦0.5%, B≦0.010%, Cr≦1.0%, Mo≦0.50%, Ni≦1.0%, Ti≦0.5% Contains one or more elements such as The remainder of the composition is made up of iron and unavoidable impurities resulting from production; B. Recrystallization annealing the coated steel sheet at a temperature between 820 and 1200 ° C; C. Coating the steel sheet obtained in step B) with a second nickel-free zinc-based coating. The present invention relates to a method for producing a coated steel sheet comprising the steps of:

[0015] Although not intended to be bound by any theory, it appears that in order to obtain a steel sheet having a specific composition with high resistance to LME, it is an essential feature to deposit a first coating of nickel on the steel sheet before recrystallization annealing. During recrystallization annealing, Ni diffuses towards the substrate steel sheet, enabling the formation of an Fe-Ni alloy layer. In fact, the Ni-rich layer concentrates in the surface and subsurface regions of the steel sheet, thus preventing the intrusion of molten zinc into the steel during any heating process such as welding. Thus, by applying the above method according to the present invention, a barrier layer or buffer layer for preventing LME can be obtained.

[0016] When the first coating made of nickel has a thickness of less than 600 nm, the LME resistance behavior of the above-mentioned specific coated steel sheet may be significantly reduced. In fact, there does not seem to be sufficient Ni in the surface and subsurface regions of the steel sheet to provide a sufficient barrier against LME.

[0017] In the case of the above steel composition, when the first coating made of nickel has a thickness exceeding 1400 nm, after recrystallization annealing, the amount of iron in the Fe-Ni alloy layer formed in the subsurface and surface regions is very small and is insufficient to form a barrier during subsequent hot-dip galvanizing. The presence of a larger amount of Ni causes a significant amount of Ni to diffuse into the steel substrate during recrystallization annealing. On the other hand, since there is no barrier layer, Ni also diffuses into the zinc coating. The presence of a larger amount of Ni in the coating causes the LME resistance behavior to decrease. Also, along with the presence of a larger amount of Ni in the coating, since there is no barrier layer, the quality of the zinc coating is insufficient.

[0018] The first coating consists of nickel, i.e., the nickel content exceeds 99 wt% and less than 1% is inevitable impurities.

[0019] The first coating can be deposited by any deposition method known to those skilled in the art. The first coating can be deposited by physical vapor deposition, electroplating, or roll coating. Preferably, the first coating is deposited by electroplating.

[0020] Preferably, in step A), the first coating has a thickness between 600 and 950 nm. Preferably, in step A), the first coating has a thickness between 600 and 750 nm or between 750 and 950 nm.

[0021] Preferably, in step B), the recrystallization annealing is continuous annealing including continuous preheating, heating, soaking, and cooling steps.

[0022] Advantageously, the recrystallization annealing is carried out in an atmosphere containing 1 to 30% H2 at a dew point between -60 and +30°C or a dew point below 60°C. For example, the atmosphere contains 1 to 10% H2 at a dew point between -60°C and -30°C. In another embodiment, the recrystallization annealing is carried out with 1 to 30% H2 at a dew point between -30 and +30°C. Preferably, the recrystallization annealing is carried out with 1 to 30% H2 at a dew point between -10 and +10°C. In fact, without intending to be bound by any theory, this dew point is considered to further improve the coating property of the steel sheet according to the present invention without any significant reduction in any mechanical property.

[0023] Advantageously, in step C), the second layer contains more than 50%, more preferably more than 75%, and advantageously more than 90% zinc. The second layer can be deposited by any deposition method known to those skilled in the art. The second layer can be deposited by hot dip galvanizing, physical vapor deposition, or electrogalvanizing.

[0024] For example, the zinc-based coating contains 0.01 to 8.0% Al, optionally 0.2 to 8.0% Mg, and the balance is Zn.

[0025] Preferably, the zinc-based coating is deposited by a hot-dip galvanizing method. In this embodiment, the molten bath may also contain inevitable impurities and residual elements from the supply of the ingot or the movement of the steel sheet in the molten bath. For example, optionally, the impurities are selected from Sr, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, Zr, or Bi, and the content by weight of each additional element is less than 0.3 wt%. The residual elements from the supply of the ingot or the movement of the steel sheet in the molten bath can be iron with a content of up to 5.0%, preferably up to 3.0 wt%.

[0026] In a preferred embodiment, the second layer consists of zinc. When the coating is deposited by a hot-dip galvanizing method, the proportion of Al is included between 0.15 and 0.40 wt% in the bath. Also, the iron present in the first coating after recrystallization annealing reacts with aluminum to form a barrier layer. Therefore, it provides reactive wetting behavior during hot-dip galvanizing.

[0027] According to the method of the present invention, a steel sheet coated with a diffusion alloy layer containing iron and nickel formed by diffusion of nickel in steel, and a steel sheet covered with a zinc-based layer directly above such a layer can be obtained. The diffusion alloy layer is considered to act as a barrier layer against LME.

[0028] Preferably, the steel sheet has a microstructure containing 1 to 50% retained austenite, 1 to 60% martensite, and optionally at least one component selected from bainite, ferrite, cementite, and pearlite. In this case, the martensite may or may not be tempered.

[0029] In a preferred embodiment, the steel sheet has a microstructure containing 5 to 45% retained austenite.

[0030] Preferably, the steel sheet has a microstructure containing 1 to 60%, more preferably 10 to 60% tempered martensite.

[0031] Advantageously, the steel sheet has a microstructure containing 10 to 40% bainite, and such bainite contains 10 to 20% lower bainite, 0 to 15% upper bainite, and 0 to 5% carbide-free bainite.

[0032] Preferably, the steel sheet has a microstructure containing 1 to 25% ferrite.

[0033] Preferably, the steel sheet has a microstructure containing 1 to 15% un-tempered martensite.

[0034] Advantageously, the steel sheet has decarburized layers with a maximum depth of 40 μm, preferably a maximum of 30 μm, and more preferably a maximum of 20 μm, on both sides of the subsurface region. Decarburization is defined in standard ISO3887:2017. In fact, without intending to be bound by any theory, it is considered that the decarburized layer further improves the LME resistance without significantly degrading the mechanical properties of the steel sheet.

[0035] Preferably, an internal oxide layer with a thickness of 5 μm or less is present in the steel sheet. Without intending to be bound by any theory, this layer is considered to result in good coating properties of the zinc coating because a continuous inhibiting layer Fe2Al5 representing good reactive wettability is formed.

[0036] After the production of the steel sheet, it is known to weld and assemble at least two metal sheets to manufacture some parts of a vehicle. Therefore, a spot weld joint is formed during the welding of at least two metal sheets, and the spot is the connection between the at least two metal sheets.

[0037] To manufacture the spot weld joint according to the present invention, the welding is performed with an effective welding current between 3 kA and 15 kA, the force applied to the electrode is between 150 and 850 daN, and the diameter of the active surface of the electrode is between 4 and 10 mm.

[0038] As a result, a spot welding joint of at least two metal plates including at least a steel plate provided with the coated steel plate according to the present invention is obtained. The above joint has less than 2 cracks having a size exceeding 100 μm, and the longest crack has a length of less than 450 μm.

[0039] Preferably, the second metal plate is a steel plate or an aluminum plate. More preferably, the second metal plate is a steel plate according to the present invention.

[0040] In another embodiment, the spot welding joint includes a third metal plate that is a steel plate or an aluminum plate. For example, the third metal plate is a steel plate according to the present invention.

[0041] The steel plate or the spot welding joint according to the present invention can be used for manufacturing parts for motor vehicles.

[0042] From this, the present invention is described in tests conducted only for the purpose of information. They are not limiting.

Examples

[0043] <Example 1: Optimization of the thickness of the Ni coating regarding the LME resistance behavior> For all samples, the steel plate used had a composition of C = 0.37 wt%, Mn = 1.9 wt%, Si = 1.9 wt%, Cr = 0.35 wt%, Al = 0.05 wt% and Mo = 0.1 wt%.

[0044] In Test 1, the steel was annealed in an atmosphere containing 5% H2 and 95% N2 at a dew point of -45°C. The annealing was carried out at 900°C for 132 seconds. Then, after quenching at 210°C, carbon enrichment was carried out at 410°C for 88 seconds. Finally, the steel plate was cooled to room temperature. Zinc plating was applied to the annealed steel plate by the electro-galvanizing method.

[0045] In Tests 2 to 6, first, Ni with thicknesses of 150, 400, 650, 900 nm, and 1600 nm respectively was deposited on a fully hardened steel plate before annealing by electroplating. Then, the pre-coated steel plate was annealed in an atmosphere containing 5% H2 and 95% N2 at a dew point of -45°C. The annealing was carried out at 900°C for 132 seconds. At the end of annealing, the steel plate was cooled to a quenching temperature of 210°C and then heated again to a carbon enrichment temperature of 410°C. Carbon enrichment was carried out for 88 seconds, and then it was heated again to a zinc plating temperature of 460°C. A zinc coating was applied by the hot-dip coating method using a liquid zinc bath containing 0.20 wt% Al maintained at 460°C. The purpose of the above tests was to determine the optimal Ni coating thickness that provides excellent LME resistance behavior. The LME sensitivity of the coated steel was evaluated by the resistance spot welding method. For this purpose, for each test, three coated steel plates were welded together by resistance spot welding. The type of electrode was ISO type B with a face diameter of 6 mm, the electrode force was 5 kN, the water flow rate was 1.5 g / min, and the welding cycle was reported in Table 1.

[0046]

Table 1

[0047] The LME crack resistance behavior was evaluated using the three-layer lamination conditions. Then, as reported in Table 2, the number of cracks with a crack length exceeding 100 μm was evaluated using an optical microscope.

[0048]

Table 2

[0049] Tests 4 and 5 according to the present invention show excellent resistance to LME compared to Tests 1, 2, 3, and 6. In fact, the number of cracks exceeding 100 μm is 2 or less, and the longest crack has a length of less than 450 μm. This results in a reduction in the amount of heat input during spot welding, which is the cause of a significant reduction in the number of cracks formed by LME.

[0050] Also, for Tests 1, 4, and 5, the LME crack resistance behavior was evaluated using the two-layer lamination conditions. Under these conditions, two coated steel sheets were welded together by resistance spot welding. Then, as reported in Table 3, the number of cracks exceeding 100 μm was evaluated using an optical microscope.

[0051]

Table 3

[0052] Tests 4 and 5 according to the present invention show excellent resistance to LME compared to Test 1. In fact, the number of cracks exceeding 100 μm is 1, and the length of the longest crack is 300 μm. This results in a reduction in the amount of heat input during spot welding, which is the cause of a significant reduction in the number of cracks formed by LME.

[0053] From the above tests, excellent LME resistance behavior was observed when the Ni coating thickness was maintained between 600 and 1400 nm. To further enhance the LME resistance, the subsurface region of the steel sheet was modified by the formation of a decarburized layer. Example 2 represents the combined effect of a Ni coating with a specific thickness and the decarburized layer.

[0054] <Example 2: Effect of Ni Coating and Decarburization under the Steel Surface on LME Resistance Behavior> To prevent any decarburization, in Test 7, the steel was annealed in an atmosphere containing 5% H2 and 95% N2 at a dew point of -80°C. The annealing was carried out at 900°C for 132 seconds. Then, the steel was quenched at 210°C and carbon enriched at 410°C for 88 seconds. Finally, the steel sheet was cooled to room temperature. Zinc plating was applied to the annealed steel sheet by the electro-galvanizing method.

[0055] In Tests 8 and 9, first, Ni with a thickness of 900 nm was deposited on the fully hardened steel sheet by electroplating before annealing. Then, the pre-coated steel sheet was annealed in an atmosphere containing 5% H2 and 95% N2 at a dew point of -80°C for Test 8, which contained no decarburized layer in the subsurface region of the steel. In Test 9, the annealing dew point was maintained at -10°C, with 5% H2 and 95% N2. Tests 8 and 9 were annealed at 900°C for 132 seconds. At the end of annealing, the steel sheet was cooled to a quenching temperature of 210°C and then reheated to a carbon enrichment temperature of 410°C. Carbon enrichment was carried out for 88 seconds. Finally, the steel sheet was cooled to room temperature. Zinc plating was applied to the annealed steel sheet by electro-galvanizing.

[0056] Table 4 compares the thickness of the decarburized layer when the steel was annealed at different dew points with and without an Ni coating. By controlling the annealing dew point, the thickness of the decarburized layer was limited without impairing the mechanical properties of the steel.

[0057]

Table 4

[0058] The LME susceptibility of the above-coated steels (Tests 7, 8, and 9) was evaluated by resistance spot welding. For this purpose, three coated steel sheets were welded together by resistance spot welding for each test. The electrode type was ISO type B with a face diameter of 6 mm, the electrode force was 5 kN, and the water flow rate was 1.5 g / min. The welding cycle is reported in Table 5.

[0059]

Table 5

[0060] The LME crack resistance behavior was evaluated for Tests 7, 8, and 9 using a two-layer lamination condition. Under this condition, two coated steel sheets were welded together by resistance spot welding. Then, as reported in Table 6, the number of cracks exceeding 100 μm was evaluated using an optical microscope.

[0061]

Table 6

[0062] Tests 8 and 9 according to the present invention show high resistance to LME as compared with Test 7. Further, for Test 9, excellent LME resistance behavior was observed in the steel sheet due to the combined effect of the Ni layer having a specific thickness and the decarburized layer.

Claims

1. A method for producing a coated steel sheet, comprising the following successive steps: A. Coating a steel sheet with a first coating consisting of nickel and having a thickness between 600 nm and 950 nm, said steel sheet having the following composition by weight: 0.10<C<0.40%, 1.5<Mn<3.0%, 0.7<Si<3.0%, 0.05<Al<1.0%, 0.75<(Si+Al)<3.0%, and optionally, Nb≦0.5%, B≦0.010%, Cr≦1.0%, Mo≦0.50%, Ni≦1.0%, and Ti≦0.5% and comprising one or more elements selected from the group consisting of: The remainder of the composition is made up of iron and unavoidable impurities resulting from production; B. Recrystallization annealing the coated steel sheet at a temperature between 820 and 1200°C, with a dew point between -10 and +10°C, and 1 to 10% H ２ performing recrystallization annealing in an atmosphere containing C. Coating the steel sheet obtained in step B) with a second nickel-free zinc-based coating. A method comprising:

2. The method according to claim 1, wherein in step A), the first coating has a thickness between 600 and 750 nm.

3. The method according to claim 1, wherein in step A), the first coating has a thickness between 750 and 950 nm.

4. The method according to any one of claims 1 to 3, wherein in step B), the recrystallization annealing is a continuous annealing.

5. The method according to any one of claims 1 to 4, wherein in step C), the second layer contains more than 50% zinc.

6. The method according to claim 5, wherein in step C), the second layer contains more than 75% zinc.

7. The method according to claim 6, wherein in step C), the second layer contains more than 90% zinc.

8. The method according to claim 7, wherein in step C), the second layer consists of zinc.

9. The method according to any one of claims 1 to 8, wherein the coated steel sheet includes a diffusion coating containing nickel and iron formed by diffusion of nickel in the steel, the diffusion layer is directly covered by a zinc-based coating containing no nickel, and the decarburized layer having a maximum depth of 40 μm from the steel sheet surface is included.

10. A method for manufacturing a spot weld joint of at least two metal sheets including the first metal sheet and the second metal sheet, wherein the first metal sheet is at least obtained from the method according to any one of claims 1 to 9, the spot weld joint has two or less cracks having a size exceeding 100 μm, and the longest crack has a length of less than 450 μm.

11. The method according to claim 10, wherein the second metal sheet is a steel sheet or an aluminum sheet.

12. The method according to claim 11, wherein the second metal sheet is a steel sheet obtained from the method according to any one of claims 1 to 9.

13. The method according to any one of claims 10 to 12, wherein the spot weld joint includes a third metal sheet which is a steel sheet or an aluminum sheet.

14. Use of a coated steel sheet obtained from the method according to claim 9 or a spot weld joint obtained from the method according to any one of claims 10 to 13 for the manufacture of a motor vehicle.

Images