Galvannealed steel sheet

Abstract

Provided is a method for producing an alloyed hot-dip galvannealed steel sheet that shortens the alloying treatment time and makes it possible to carry out the method industrially. [Solution] This manufacturing method includes the steps of heating and soaking a steel sheet having a predetermined chemical composition range from ambient temperature to temperatures T1 and T2 of 700 to 900°C in a heating section having an atmosphere containing 0.1 to 15% by volume of H2 and an inert gas with a dew point DP1 of -18 to +8°C, cooling the steel sheet to 400 to 700°C (T3, T4) in a cooling section having an atmosphere containing 1 to 30% by volume of H2 and an inert gas with a dew point DP3 of -30°C or lower, and optionally performing a soaking step within that temperature range, recrystallization annealing of the steel sheet in a fully equipped radiant tube furnace with a heating section, soaking section, cooling section, and optionally performing a soaking step within that temperature range, hot-dip galvanizing of the annealed steel sheet in a zinc bath, and alloying treatment at a temperature T5 of 460 to 600°C for a time t5 of 1 to 45 seconds.

Description

[Technical field]

[0001] The present invention relates to a method for producing a galvannealed steel sheet and a galvannealed steel sheet. The present invention is particularly suitable for the automobile industry. [Background technology]

[0002] It is known to use high tensile steels for the manufacture of automotive vehicles with the aim of reducing the weight of the vehicles. The mechanical properties of such steels must be improved, for example for the manufacture of structural parts. It is known to add alloying elements to improve the mechanical properties of steels. For this reason, high tensile or ultra-high tensile steels with high mechanical properties are manufactured and used, including TRIP (Transformation-Induced Plasticity) steels, DP (Dual Phase) steels, HSLA (High-Strength Low Alloyed), TRIPLEX DUPLEX.

[0003] Typically, DP steels have a ferritic-martensite microstructure. This results in a microstructure consisting of a soft ferritic matrix containing islands of martensite as a second phase (martensite increases tensile strength). The overall behavior of DP steels is governed, among other things, by the phase volume fractions and morphologies (grain size, aspect ratios, etc.) as well as the chemical composition of the steel. DP steels have a high ultimate tensile strength (UTS, enabled by martensite) along with a low initial yield stress (imposed by the ferritic phase) and high initial strain hardening. These characteristics make DP steels an ideal material for automotive related plate forming operations.

[0004] The advantages of DP steels are low yield strength, low yield strength to tensile strength ratio, high premature strain hardening rate, good uniform elongation, high strain rate sensitivity and good fatigue resistance.

[0005] These steels are usually coated with a metallic coating that improves properties such as corrosion resistance, phosphating ability, etc. The metallic coating can be deposited by hot-dip galvanizing after annealing of the steel sheet. This can then be followed by an alloying treatment, in which the iron of the steel sheet diffuses in the direction of the zinc coating, to obtain a zinc-iron alloy on the steel sheet, called galvannealed steel sheet. This galvannealed steel sheet has good welding behavior.

[0006] However, especially for DP steels, during the annealing carried out in the continuous annealing line, alloying elements that have a higher affinity for oxygen (compared to iron), such as manganese (Mn), aluminum (Al), silicon (Si) or chromium (Cr), oxidize, leading to the formation of oxides on the surface, e.g. manganese oxide (MnO) or silicon oxide (SiO 2 These oxides, which are oxides of the metal oxides (such as niobium oxide, ...

[0007] Galvannealing DP steels is a difficult task, since the oxides formed on the steel sheet during annealing can interfere with the Fe-Zn formation. In fact, the oxides formed can delay galvannealing, since the diffusion of iron is hindered. Galvannealing depends, among other things, on the distribution of oxides in the steel sheet, the morphology of the oxides and sometimes the nature of the oxides formed, at the steel sheet surface.

[0008] EP2415896 includes a galvanized layer and is 20 g / m 2 ~120g / m 2The present invention discloses a method for manufacturing a high-tensile galvanized steel sheet arranged on a steel plate having a mass per unit area of ​​0.01% to 0.18% C, 0.02% to 2.0% Si, 1.0% to 3.0% Mn, 0.001% to 1.0% Al, 0.005% to 0.060% P, and 0.01% or less S, with the balance being Fe and unavoidable impurities, comprising annealing and galvanizing the steel sheet in a continuous galvanizing line. The temperature range having a furnace temperature of A°C to B°C is performed at an atmospheric dew point temperature of -5°C or higher in the heating process, where 600≦A≦780 and 800≦B≦900. The dew point temperature of the atmosphere in the annealing furnace other than the range of A°C to B°C is not particularly limited, and is preferably within the range of -50°C to -10°C. EP2415896 also discloses a method further comprising alloying the steel sheet after galvanization by heating the steel sheet to a temperature of 450-600°C such that the Fe content in the galvanized layer is in the range of 7-15 wt.%.

[0009] The galvannealed steel sheet obtained by the above method has a surface portion of the steel sheet located directly under the coating layer and within 100 μm from the surface of the base steel sheet, and has an oxide content of at least one selected from the group consisting of Fe, Si, Mn, Al, P, B, Nb, Ti, Cr, Mo, Cu and Ni of 0.010 g / m per unit area. 2 ~0.50g / m 2 and has a structure or microstructure in which crystalline silicon oxide, crystalline manganese oxide, or crystalline silicon-manganese composite oxide precipitates in base metal grains that are present in a region within 10 μm below the plating layer and within 1 μm from the grain boundary.

[0010] However, by using the above method, there is a risk that a significant layer of outer oxides such as FeO is formed on the steel sheet surface, in which case it is difficult to reduce all the outer oxides, which reduces the wettability and the coating adhesion of zinc on the steel surface, and there is a risk that the diffusion of iron into the zinc coating during the alloying treatment is significantly delayed, so that in this case it is not effective to carry out the alloying treatment in order to obtain a galvannealed steel sheet.

[0011] JP2008156734 is a method for producing a high-tensile hot-dip galvanized steel sheet, The method includes subjecting a steel composed of the composition according to claim 1 or 2 to hot rolling, pickling and cold rolling, and subjecting the obtained steel sheet to a hot-dip galvanizing treatment to produce a hot-dip galvanized steel sheet, In the hot rolling, the slab heating temperature is set to 1150 to 1300°C, the finish rolling temperature is set to 850 to 950°C, and the coiling temperature is set to 400 to 600°C; In the pickling, the bath temperature is set to 10° C. or more and less than 100° C., and the concentration of hydrochloric acid is set to 1 to 20%; and In the hot dip galvanizing process, the hydrogen concentration in the atmosphere in the heat treatment furnace from the temperature rise process to 600°C or more through the annealing temperature to the cooling process to 450°C is set to 2 to 20%, the dew point of the atmosphere is set to -60 to -10°C, and the cold rolled steel sheet is kept at the annealing temperature of 760 to 860°C for 10 to 500 seconds, and then cooled at an average cooling rate of 1 to 30°C / second. The method can also include an alloying treatment in the temperature range of 450-600°C for 10-120 seconds to obtain a galvannealed steel sheet.

[0012] It is stated that inside the steel sheet, Si-based and Mn-based oxides are formed at the grain boundaries and within the grains.

[0013] However, the examples do not mention the alloying treatment time. Also, since the oxides are close to the surface of the steel sheet, there is a risk that the presence of such oxides will form a discontinuous oxide film in the steel sheet and inhibit the diffusion of iron into the zinc coating. Therefore, there is a risk of delaying the alloying treatment.

[0014] JP2000212648 describes a one-step method for producing high-tensile hot-dip galvanized steel sheet having excellent workability and coating adhesion, subjecting a steel slab containing not more than 0.10 wt. % P to hot rolling and then pickling or subjecting said steel slab to cold rolling; a step of heating in an atmosphere in which the heating temperature T is 750° C. or more and 1000° C. or less, the following formula (2) is satisfied, the dew point t of the atmospheric gas satisfies the following formula (3), and the hydrogen concentration of the atmospheric gas is 1 to 100 volume %; and then a step of subjecting the steel sheet to hot-dip galvanizing; Methods including: 0.85 ≦ {[P(weight%)+(2 / 3)]*1150} / {T(℃)} ≦ 1.15(2); 0.35 ≦ {[P(weight%)+(2 / 3)]*(-30)} / {t(℃)} ≦ 1.8(3) Disclose.

[0015] The method also discloses a further alloying treatment to obtain a galvannealed steel sheet.

[0016] All examples (Examples 18 to 26) of JP2000212648 in which a one-step heat treatment method is implemented include a thermal reduction treatment, in which the heating temperature T is 810 to 850°C, and the dew point is extremely dry ( ≦ -35°C) or very humid ( ≧ 35℃), which allows for good coating adhesion. A thermal reduction treatment at 750-1000℃ is followed by an alloying treatment.

[0017] The only comparative example (Comparative Example 10) of the one-step process of JP2000212648 is carried out using a steel sheet having a very small amount of Si and Cr. In this case, the one-step heat treatment process includes a thermal reduction treatment with a heating temperature T of 820°C and a dew point of 0°C. This is followed by an alloying treatment carried out at 480°C. However, the P-based oxides were not reduced, resulting in poor coating adhesion and poor appearance after alloying.

[0018] JP2011117040 discloses a steel plate base material having a chemical composition containing, by mass%, 0.01-0.25% C, 0.3-2.0% Si, 0.030-3.0% Mn, 0.050% or less P, 0.010% or less S, 0.0060% or less N, and 0.5% or less sol.Al, with the balance being Fe and impurities, and a plating layer containing, by mass%, 8.0-15% Fe and 0.15-0.50% Al on the surface of the steel plate base material. Disclosed is a galvannealed steel sheet further comprising a single oxide of Si, Mn or Al, an oxide containing two or more of these, or a composite oxide containing two or more of these and Fe, wherein the single oxide, the oxide, or the composite oxide is present in the steel sheet base material within a depth of 2 μm from the interface between the coating layer and the steel sheet base material, and the single oxide, the oxide, or the composite oxide has a maximum grain size of 0.10 μm or less.

[0019] JP2011117040 is a method for producing a galvannealed steel sheet, a hot rolling process of hot rolling a steel slab having a chemical composition including, by mass%, 0.01-0.25% C, 0.3-2.0% Si, 0.030-3.0% Mn, 0.050% or less P, 0.010% or less S, 0.0060% or less N, and 0.5% or less sol.Al, and coiling the resulting hot-rolled steel sheet at a coiling temperature of 650°C or less; A pickling process for pickling the hot-rolled steel plate; A cold rolling process in which the hot-rolled steel plate that has been pickled in the pickling process is cold-rolled at a rolling reduction of 50% or more; and After the cold rolling step, the cold-rolled steel sheet is annealed in a reduction annealing furnace in a continuous hot-dip galvanizing line in a nitrogen-hydrogen atmosphere having a hydrogen concentration of 1 to 30% by volume and a dew point of -30°C to 10°C in a temperature range of 700°C or higher in order to reduce the steel sheet surface; and a hot-dip galvanizing step in which the steel sheet is continuously subjected to hot-dip galvanizing; and With alloying treatment; Also disclosed is a method comprising:

[0020] Nevertheless, a number of oxides with completely different properties may be formed during annealing, including single oxides of Si, Mn or Al, oxides containing two or more of these, or composite oxides containing two or more of these and Fe. Oxides, especially oxides containing Al and composite oxides containing two or more of these and Fe, may form in the form of a continuous layer, thus reducing the coating adhesion and retarding the galvannealing.

[0021] JP2011153367 discloses a method for producing a galvannealed steel, which includes annealing, hot-dip galvanizing, and alloying treatment for a steel sheet containing, by mass%, C: 0.03-0.20%, Mn: 0.03-3.0%, Si: 0.1-2.5%, S: 0.01% or less, P: 0.1% or less, sol.Al: 1.0% or less, N: 0.01% or less, and Bi: 0.0001-0.05%, and in the heating to the recrystallization temperature in the annealing, the annealing is performed in an annealing furnace during heating in a range of at least 650°C to the recrystallization temperature, with a dew point of -25 to 0°C.

[0022] However, the presence of bismuth in steel can reduce the mechanical properties of the steel and there is a risk of reducing coating adhesion and retarding the galvannealing of high and ultra-high strength steels.

[0023] Furthermore, as shown in FIG. 1 of JP2011153367, this method uses N 2 -10% by volume H 2 It starts with purging the furnace with gas. The gas is switched to a predefined high dew point gas at the beginning of heating. Indeed, when the temperature of the plate reaches 650°C, the furnace is again purged with a high dew point gas having a predefined dew point, for example -10°C. Then, when the temperature of the plate reaches 860°C, which is above the recrystallization temperature, the gas is again switched to the original low dew point gas, i.e. -60°C, before the temperature of the plate immersed in the plating bath reaches 460°C.

[0024] For this reason, the method requires three purges.

[0025] First, the method begins with a gas with a dew point of -60°C. One is to use a gas with a dew point of -10°C when the steel sheet temperature reaches 650°C during annealing; and The other uses a gas with a low dew point of -60°C during annealing when the steel sheet temperature reaches 850°C.

[0026] This process is extremely difficult to control on an industrial scale, especially in a continuous annealing line.

[0027] Therefore, in addition to the recrystallization annealing method, the chemical composition and microstructure of the steel, the nature of the oxides formed during recrystallization annealing and the redistribution of the oxides are also important attributes to be taken into consideration in order to improve the galvannealing rate of DP steels.

[0028] As a result, there is a need to find ways to improve the wetting and coating adhesion of high strength and ultra high strength steels, especially DP steels that contain amounts of alloying elements. [Prior art documents] [Patent documents]

[0029] [Patent Document 1] European Patent Application Publication No. 2415896 [Patent Document 2] JP 2008-156734 A [Patent Document 3] JP 2000-212648 A [Patent Document 4] JP 2011-117040 A [Patent Document 5] JP 2011-153367 A Summary of the Invention [Problem to be solved by the invention]

[0030] It is therefore an object of the present invention to provide a galvannealed steel sheet having a chemical composition containing alloying elements, which reduces the alloying treatment time and allows industrial implementation. Another object is to obtain a galvannealed steel sheet having high quality, i.e., good diffusion of iron into the steel. Finally, it is an object to provide an easy-to-implement method for the manufacture of said galvannealed steel sheet. [Means for solving the problem]

[0031] This object is achieved by providing a method according to claim 1. The method may include any of the features of claims 2-13.

[0032] Another object is achieved by providing a galvannealed steel sheet according to claim 14. This galvannealed steel sheet may include any of the features of claims 15 to 17.

[0033] Finally, this object is achieved by providing the use of a galvannealed steel sheet according to claim 18.

[0034] Other features and advantages of the present invention will become apparent from the following detailed description of the invention.

[0035] To illustrate the invention, various embodiments and trials of non-limiting examples will now be described with particular reference to the following drawings: [Brief description of the drawings]

[0036] [Figure 1] FIG. 1 illustrates one prior art method disclosed in JP2011153367. [Diagram 2] FIG. 2 illustrates an example of a method of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0037] The following terms are defined:

[0038] "Volume %" means percentage by volume; "Wt. %" means percentage by weight.

[0039] The present invention relates to a method for producing a galvannealed steel sheet, A. Chemical composition by weight: 0.05≦C≦0.20%, 1.5≦Mn≦3.0%, 0.10≦Si≦0.45%, 0.10≦Cr≦0.60%, Al≦0.20%, V<0.005% and, purely arbitrarily, P < 0.04%, Nb≦0.05%, B≦0.003%, Mo≦0.20%, Ni≦0.1%, Ti≦0.06%, S≦0.01%, Cu≦0.1%, Co≦0.1%, N≦0.01% and Providing a steel sheet, the balance of said composition being made up of iron and inevitable impurities resulting from smelting; B. The following sub-processes: i. 0.1 to 15% by volume H 2 heating the steel sheet from ambient temperature to a temperature T1 of 700 to 900°C in a heating section having an atmosphere A1 containing an inert gas having a dew point DP1 of -18°C to +8°C; ii. soaking the steel sheet from T1 to a temperature T2 of 700-900°C in a soaking section having an atmosphere A2 identical to A1 with a dew point DP2 equal to DP1; iii. 1 to 30% by volume H 2 and an inert gas having a dew point DP3 of -30°C or less, in a cooling section A3 having an atmosphere A3 containing the inert gas and the inert gas having a dew point DP3 of -30°C or less, from T2 to T3 of 400-700°C; iv. 1 to 30% by volume H 2 an optional equalization step of the steel sheet from a temperature T3 to a temperature T4 of 400-700 ° C in an equalization compartment having an atmosphere A4 containing an inert gas having a dew point DP4 of -30 ° C or less; recrystallization annealing of the steel sheet in a complete radiant tube furnace equipped with a heating section, a soaking section, a cooling section, and optionally an equalization section, comprising: C. Hot-dip galvanizing the annealed steel sheet in a zinc bath; and D. alloying treatment carried out at a temperature T5 of 460-600°C for a time t5 of 1-45 seconds; The present invention relates to a method comprising the steps of:

[0040] Without wishing to be bound by any theory, it appears that the method of the present invention allows for a high degree of improvement in wettability and coating adhesion of steel sheets having a specific chemical composition. Moreover, it is possible to carry out the alloying process in a shortened time using the method of the present invention. Indeed, contrary to conventional methods such as the method disclosed in JP2011153367 (FIG. 1) and as illustrated in FIG. 2, the inventors have found that the recrystallization annealing according to the present invention, which is carried out in a full radiant tube furnace (RTF) with heating and soaking zones having the same atmosphere with DP of -18°C and +8°C, such atmosphere containing 0.1-15% by volume of H2, allows for the production of galvannealed steel sheets with a specific oxide redistribution, allowing for high wettability and having high quality. In particular, MnO, FeO and Mn 2 SiO 4 Oxides containing are formed on the steel sheet surface and in the interior during recrystallization annealing, allowing high wettability and coating adhesion. Preferably, these outer oxides are present on the steel sheet surface in the form of nodules, so that during the alloying process, the iron of the steel can easily diffuse in the direction of the coating in a shortened time.

[0041] If the recrystallization annealing of the particular steel sheet is not carried out according to the present invention, in particular if the heating and soaking sections do not have the same atmosphere and if the dew point is below -18°C, MnO, FeO and MnO may be present. 2 SiO 4There is a risk of forming oxides such as those mainly or only on the outside. Furthermore, there is a risk that these oxides will form a thick continuous layer on the steel sheet surface, significantly reducing the wettability. In this case, it is not effective to carry out an alloying treatment in order to obtain a galvannealed steel sheet.

[0042] Furthermore, if the heating and soaking compartments do not have the same atmosphere and if the dew point is above 8°C, external oxides such as MnO and FeO as well as Mn 2 SiO 4 There is a risk of forming internal oxides such as MnO, FeO and mainly FeO on the steel sheet surface in the form of a continuous layer, which reduces the wettability. In this case, it is not effective to carry out an alloying treatment in order to obtain a galvannealed steel sheet.

[0043] Regarding the chemical composition of the steel, the carbon content is between 0.05 and 0.20% by weight. If the carbon content falls below 0.050%, there is a risk of insufficient tensile strength. Furthermore, if the microstructure of the steel contains residual austenite, it will not be possible to obtain the stability required to achieve sufficient elongation. In a preferred embodiment, the carbon content is in the range of 0.05 to 0.15%.

[0044] Manganese is a solid solution hardening element that contributes to obtaining high tensile strength. Such an effect is obtained when the Mn content is at least 1.5% by weight. However, above 3.0%, Mn additions can contribute to the formation of structures with excessively pronounced segregation zones, which can adversely affect the weld mechanical properties. Preferably, the manganese content is in the range of 1.5-2.9% to achieve these effects. This makes it possible to obtain good mechanical strength without increasing the difficulties of industrial production of the steel and without increasing the hardenability during welding.

[0045] In order to achieve the required combination of mechanical properties and wettability, silicon must be present in an amount of 0.1-0.45% by weight of Si, preferably 0.1-0.30% by weight, more preferably 0.1-0.25% by weight. That is, silicon reduces carbide precipitation during annealing after cold rolling of the sheet due to its low solubility in cementite and due to the fact that this element increases the activity of carbon in austenite. If the amount of Si exceeds 0.45%, other oxides are likely to form on the steel sheet surface, reducing wettability and coating adhesion.

[0046] Aluminum should not exceed 0.20% by weight, preferably below 0.18% by weight. With regard to the stabilization of the retained austenite, aluminum has an effect relatively similar to that of silicon. However, an aluminum content higher than 0.20% by weight may increase the Ac3 temperature, i.e. the temperature of complete transformation to austenite in the steel during the annealing process, and therefore may make the industrial process more expensive.

[0047] Chromium makes it possible to delay the formation of proeutectoid ferrite during the cooling step after the maximum temperature during the annealing cycle, making it possible to achieve higher strength levels, for which reason the chromium content is between 0.10 and 0.60% by weight, preferably between 0.10 and 0.50% by weight, for cost reasons and to prevent excessive hardening.

[0048] Vanadium also plays an important role in the context of the present invention. In the present invention, the amount of V is less than 0.005%, preferably 0.0001≦V≦0.005%. Preferably, V forms precipitates to achieve hardening and strengthening.

[0049] This steel may optionally contain elements such as P, Nb, B, Mo, Ni, Ti, S, Cu, Co, and N to achieve precipitation hardening.

[0050] P and S are considered residual elements resulting from steelmaking. P can be present in amounts less than 0.04 wt.%. S can be present in amounts up to 0.01 wt.%.

[0051] Titanium and niobium are also elements that may optionally be used to achieve hardening and strengthening by forming precipitates, however, when the Nb content is above 0.05% and / or the Ti content is above 0.06%, there is a risk that excessive precipitation may cause a decrease in toughness, which must be avoided.

[0052] This steel may also optionally contain boron in an amount not exceeding 0.003%. By segregating at grain boundaries, B reduces the grain boundary energy and is therefore beneficial in increasing the resistance to liquid metal embrittlement.

[0053] Since the element molybdenum retards the decomposition of austenite, an amount of molybdenum of 0.2% or less is effective in increasing the hardenability and stabilizing the retained austenite.

[0054] This steel may optionally contain nickel in an amount up to 0.1% to improve toughness.

[0055] Copper may be present in a content up to 0.1% in order to harden the steel by precipitation of copper metal.

[0056] Preferably, the chemical composition of this steel does not contain bismuth. Indeed, without wishing to be bound by any theory, it is believed that the inclusion of Bi in the steel sheet reduces wettability and therefore coating adhesion.

[0057] Preferably, in steps Bi) and B.ii), A1 contains 1-10% by volume of H2, more preferably A1 contains 2-8% by volume of H2 and A2 is identical to A1.

[0058] Advantageously, in steps Bi) and B.ii), DP1 is between −15 and +5° C., more preferably DP1 is between −10 and +5° C. and DP2 is equal to DP1.

[0059] In a preferred embodiment, in step Bi) the steel sheet is heated from ambient temperature to T1 at a heating rate of more than 1° C. / s, for example between 2 and 5° C. / s.

[0060] Preferably, in step Bi), the heating is carried out for a time t1 comprised between 1 and 500 seconds, advantageously between 1 and 300 seconds.

[0061] Advantageously, in step B.ii), the soaking is carried out for a time t2 comprised between 1 and 500 seconds, advantageously between 1 and 300 seconds.

[0062] Preferably, in step B.ii), T2 is equal to T1. In this case, in steps Bi) and B.ii), T1 and T2 are between 750 and 850°C and T2 is equal to T1. In another embodiment, depending on the chemical composition and microstructure of the steel sheet, T2 can be below or above T1. In this case, in steps Bi) and B.ii), T1 and T2 are, independently of each other, between 750 and 850°C.

[0063] Preferably, in step B.iii), A3 comprises 1 to 20% by weight of H2, more preferably 1 to 10% by weight of H2.

[0064] Preferably, in step B.iii), DP3 is at -35°C or lower.

[0065] In a preferred embodiment, in step B.iii), the cooling is carried out for a time t3 comprised between 1 and 50 seconds.

[0066] Advantageously, in step B.iii), the cooling rate is greater than 10° C. / s, preferably between 15 and 40° C. / s.

[0067] Advantageously, in step B.iv), A4 comprises between 1 and 20% by weight of H2, and better still between 1 and 10% by weight of H2.

[0068] Preferably, in step B.iv), DP4 is at -35°C or below.

[0069] In a preferred embodiment, in step B.iv), the equalization is carried out for a time t4 comprised between 1 and 100 seconds, for example between 20 and 60 seconds.

[0070] Advantageously, in steps B.iii) and B.iv), A3 is identical to A4 and DP4 is equal to DP3.

[0071] Preferably, in step B.iv), T4 is equal to T3. In this case, in steps B.iii) and B.iv), T3 and T4 are 400-550°C or 550-700°C and T4 is equal to T3. In another embodiment, depending on the chemical composition and microstructure of the steel sheet, T4 can be below or above T3. In this case, in steps B.iii) and B.iv), T3 and T4 are, independently of each other, 400-550°C or 550-700°C.

[0072] Preferably, in steps Bi) to B.iv), the inert gas is selected from N2, Ar, He and Xe.

[0073] Preferably, in step C), the zinc-based coating contains 0.01 to 0.4% by weight of Al, the remainder being Zn.

[0074] Advantageously, in step D), T5 is between 470 and 570°C, more preferably between 470 and 530°C.

[0075] Preferably, in step D), t5 is from 1 to 35 seconds, for example, from 1 to 20 seconds.

[0076] In a preferred embodiment, the alloying process is carried out in an atmosphere A5 that comprises air.

[0077] The present invention also relates to a galvannealed steel sheet, the zinc coating comprising 5-15% by weight of Fe and oxides comprising FeO, Mn2SiO4 and MnO, the balance being zinc, the zinc coating being alloyed through diffusion of iron from the steel sheet such that the steel sheet comprises an internal oxide comprising FeO, Mn2SiO4 and MnO in the steel sheet. Preferably, the oxides comprising FeO, Mn2SiO4 and MnO present in the zinc or aluminum coating are in the form of nodules.

[0078] Preferably, the coating has a thickness of 1 to 15 μm.

[0079] Preferably, the microstructure of the steel comprises bainite, martensite, ferrite and optionally austenite. In one preferred embodiment, the microstructure of the steel comprises 1-45% martensite, 1-60% bainite and the remainder austenite. In another preferred embodiment, the microstructure of the steel comprises 1-25% fresh martensite, 1-10% ferrite, 35-95% martensite and lower bainite and less than 10% austenite.

[0080] In a preferred embodiment, the surface of the steel sheet is decarburized. Preferably, the depth of decarburization is up to 100 μm from the surface steel sheet, preferably up to 80 μm. In this case, without wishing to be bound by any theory, it is believed that the steel sheet has better resistance to LME due to the reduced amount of carbon in the steel sheet. Indeed, carbon appears to be an element that is highly susceptible to liquid metal embrittlement LME. Furthermore, better bendability and better crushing behavior.

[0081] Finally, the invention relates to the use of a galvannealed steel sheet for the manufacture of parts of motor vehicles.

[0082] The invention is now described in tests carried out for informational purposes only and are not intended to be limiting. EXAMPLES

[0083] In this example, a DP steel having the following composition in weight percentage was used.

[0084] [Table 1]

[0085] All specimens, being DP steels, were annealed from ambient temperature in a full RTF furnace according to the conditions in Table 1.

[0086] All specimens were then hot-dip galvanized in a zinc bath containing 0.117% of aluminum.

[0087] After deposition of the coating, the specimens were analyzed by visual inspection, scanning electron microscopy and Auger spectroscopy. With regard to wettability, 0 means that the coating is deposited continuously, 1 means that the coating is not deposited continuously. If the wettability was 0, i.e. very good, the specimens were alloyed to obtain galvannealed steel sheets. If the wettability was 1, i.e. very poor, there was no need to alloy, since the quality of the coating was very poor due to the presence of a large number of undesirable oxides on the steel surface.

[0088] The results are shown in Table 1 below.

[0089] [Table 2]

[0090] Specimens 4-7 according to the invention as well as examples 11 and 12 show excellent wettability. Nevertheless, for specimens 4-7 the alloying time was significantly reduced compared to specimens 11 and 12. Furthermore, the surface aspect of the coating was significantly better for the examples of the invention.

Claims

1. A method for producing a galvannealed steel sheet, comprising the steps of: A. The following chemical composition in weight percent: 0.05≦C≦0.20%, 1.5≦Mn≦3.0%, 0.10≦Si≦0.45%, 0.10≦Cr≦0.60%, 0<Al≦0.20%, 0<V<0.005% and, optionally, P < 0.04%, Nb≦0.05%, B≦0.003%, Mo≦0.20%, Ni≦0.1%, Ti≦0.06%, S≦0.01%, Cu≦0.1%, Co≦0.1%, N≦0.01% and Providing a steel sheet, the balance of said composition being made up of iron and inevitable impurities resulting from smelting; B. The following sub-steps: i. 0.1 to 15 volume % H ２ and an inert gas having a dew point DP1 of -15°C to +5°C. ii. soaking the steel sheet from T1 to a temperature T2 between 700 and 900°C in a soaking section having an atmosphere A2 identical to A1 with a dew point DP2 equal to DP1; iii. 1 to 30% by volume of H ２ and an inert gas having a dew point DP3 of -30°C or less, cooling the steel sheet from T2 to T3 of 400-700°C in a cooling section having an atmosphere A3 comprising the inert gas and an inert gas having a dew point DP3 of -30°C or less; iv. 1 to 30% by volume H ２ an optional equalization step of said steel sheet from a temperature T3 to a temperature T4 of 400-700 ° C. in an equalization compartment having an atmosphere A4 comprising an inert gas and an inert gas having a dew point DP4 of -30 ° C. or less; recrystallization annealing of the steel sheet in a complete radiant tube furnace equipped with a heating section, a soaking section, a cooling section, and optionally an equalization section, comprising: C. Hot dip galvanizing the annealed steel sheet in a zinc bath; and D. An alloying treatment carried out at a temperature T5 between 460 and 600° C. for a time t5 between 1 and 45 seconds; A method comprising:

2. 2. The method according to claim 1, wherein in step A) the steel sheet contains less than 0.30 wt.% Si.

3. 3. The method according to claim 1 or 2, wherein in step A) the steel sheet contains more than 0.0001 wt. % V.

4. In steps B. i) and B. ii), A1 is 1 to 10% by volume of H ２ and A2 is identical to A1.

5. The method according to any one of claims 1 to 4, wherein in step B.ii), T2 is equal to T1.

6. The method according to any one of claims 1 to 5, wherein in steps B.i) and B.ii), T1 and T2 are 750 to 850°C.

7. The method according to any one of claims 1 to 6, wherein in step B.iii) and optional sub-step B.iv), A3 is identical to A4 and DP4 is equal to DP3.

8. The method according to any one of claims 1 to 7, wherein in steps B.i) to B.iii) and optional sub-step B.iv), the inert gas is selected from N2, Ar, He and Xe.

9. A method according to any one of the preceding claims, wherein the zinc-based coating comprises 0.01-0.4 wt% Al, the balance being Zn.

10. 10. The method according to claim 1, wherein in step D), T5 is between 470 and 570°C.

11. The method according to any one of claims 1 to 10, wherein in step D), t5 is 1 to 35 seconds.

12. The method according to any one of the preceding claims, wherein the chemical composition of the steel is bismuth-free.

13. A method for producing a galvannealed steel sheet, comprising the steps according to any one of claims 1 to 12, wherein the galvannealed steel sheet has a zinc coating of 5 to 15 wt % Fe, FeO, Mn ２ SiO ４ and MnO, with the balance being zinc and optionally aluminum, and the steel plate contains FeO, Mn ２ SiO ４ The zinc coating is alloyed through the diffusion of iron from the steel sheet to include inner oxides including FeO, MnO, and MnO. ２ SiO ４ and MnO are in the form of nodules, the coating has a thickness of 1 to 15 μm, the microstructure of the steel comprises 1 to 45% martensite, 1 to 60% bainite, and the balance is austenite, or comprises 1 to 25% fresh martensite, 1 to 10% ferrite, 35 to 95% martensite and lower bainite and less than 10% austenite, and the steel sheet is decarburized to a depth of a maximum of 100 μm from the surface of the steel sheet.

14. A method for using a galvannealed steel sheet, comprising carrying out the method according to any one of claims 1 to 13 to produce a galvannealed steel sheet, and using the galvannealed steel sheet for the manufacture of a part of a motor vehicle.

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