Hot-dip coated steel sheet

Abstract

The present invention relates to a method for the manufacture of a hot-dip coated steel sheet coated with a zinc or an aluminum based coating including the provision of a specific steel sheet, a recrystallization annealing with specific heating, soaking and cooling sub-steps using an inert gas and a hot-dip coating; the hot dip coated steel sheet and the use of the hot-dip coated steel sheet.

Claims

1. A method for manufacturing a hot-dip coated steel sheet, the method comprising: A. providing a steel sheet having 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%, Al≤0.20%, V<0.005% and on a purely optional basis, one or more elements including: 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%, a remainder of the composition being made of iron and inevitable impurities resulting from processing; B. recrystallization annealing the steel sheet in a full radiant tube furnace comprising a heating section, a soaking section, and a cooling section, and optionally an equalizing section, the recrystallization annealing including the following sub-steps: i. heating the steel sheet from ambient temperature to a temperature T1 between 700 and 900° C. in the heating section having an atmosphere A1 including from 0.1 to 15% by volume of H.sub.2 and an inert gas whose a dew point DP1 is between −18° C. and +8° C., ii. soaking the steel sheet from T1 to a temperature T2 between 700 and 900° C. in the soaking section having an atmosphere A2 identical to A1 with a dew point DP2 equal to DP1, and iii. cooling the steel sheet from T2 to T3 between 400 and 700° C. in the cooling section having an atmosphere A3 including from 1 to 30% H.sub.2 by volume and an inert gas whose a dew point DP3 is below or equal to −30° C., and iv. optionally, equalizing the steel sheet from a temperature T3 to a temperature T4 between 400 and 700° C. in the equalizing section having an atmosphere A4 including from 1 to 30% H.sub.2 by volume and an inert gas whose a dew point DP4 is below or equal to −30° C.; and C. hot-dip coating of the annealed steel sheet in a bath based on zinc or based on aluminum.

2. The method as recited in claim 1 wherein in step A), the steel sheet includes less than 0.30% by weight of Si.

3. The method as recited in claim 1 wherein in step A), the steel sheet includes above 0.0001% by weight of V.

4. The method as recited in claim 1 wherein in steps B.i) and B.ii), A1 includes between 1 and 10% by volume of H.sub.2.

5. The method as recited in claim 1 wherein in steps B.i) and B.ii), DP1 is between −15° C. and +5° C.

6. The method as recited in claim 1 wherein in step B.ii), T2 is equal to T1.

7. The method as recited in claim 1 wherein in steps B.i) and B.ii), T1 and T2 are between 750 and 850° C.

8. The method as recited in claim 1 wherein in steps B.iii) and the optional sub-step B.iv), A3 is identical to A4, DP4 being equal to DP3.

9. The method as recited in claim 1 wherein the optional sub-step B.iv) is performed and T4 is equal to T3.

10. The method as recited in claim 1 wherein in steps B.i), B.ii) and B.iii) and the optional sub-step B.iv), the inert gas is chosen from the group consisting of: N2, Ar, He and Xe.

11. The method as recited in claim 1 wherein the bath in step C) is based on zinc, the hot-dip coating creates a zinc-based coating, and the zinc-based coating includes from 0.01 to 8.0% by weight of Al, optionally from 0.2 to 8.0% by weight of Mg, less than 5.0% Fe, a remainder of the zinc-based coating being Zn.

12. The method as recited in claim 1 wherein the bath in step C) is based on aluminum, the hot-dip coating creates an aluminum-based coating, and the aluminum-based coating includes less than 15% Si, less than 5.0% Fe, optionally 0.1 to 8.0% Mg and optionally 0.1 to 30.0% Zn, a remainder of the aluminum-based coating being Al.

13. The method as recited in claim 1 wherein the chemical composition of the steel does not include Bismuth (Bi).

14. The method as recited in claim 1 wherein the equalizing sub-step is performed.

15. A method for manufacturing part of an automotive vehicle comprising the method for manufacturing a hot-dip coated steel sheet as recited in claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) To illustrate the invention, various embodiments and trials of non-limiting examples will be described, particularly with reference to the following Figure:

(2) FIG. 1 illustrates one method of the prior art disclosed in the patent application JP2011153367.

(3) FIG. 2 illustrates one example of the method according to the present invention.

DETAILED DESCRIPTION

(4) The following terms will be defined: “vol. %” means the percentage by volume, “wt. %” means the percentage by weight.

(5) The invention relates to a method for the manufacture of a hot-dip coated steel sheet coating with a zinc or an aluminum based coating, comprising: A. The provision of a steel sheet having 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%, Al≤0.20%, V<0.005% and on a purely optional basis, one or more elements such as 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%,
the remainder of the composition being made of iron and inevitable impurities resulting from the elaboration, B. The recrystallization annealing of said steel sheet in a full radiant tube furnace comprising a heating section, a soaking section, a cooling section, optionally an equalizing section comprising the sub-following steps: i. the heating of said steel sheet from ambient temperature to a temperature T1 between 700 and 900° C. in the heating section having an atmosphere A1 comprising from 0.1 to 15% by volume of H.sub.2 and an inert gas whose a dew point DP1 is between −18° C. and +8° C., ii. the soaking of the steel sheet from T1 to a temperature T2 between 700 and 900° C. in the soaking section having an atmosphere A2 identical to A1 with a dew point DP2 equal to DP1, iii. the cooling of the steel sheet from T2 to T3 between 400 and 700° C. in the cooling section having an atmosphere A3 comprising from 1 to 30% H.sub.2 by volume and an inert gas whose a dew point DP3 is below or equal to −30° C., iv. optionally, the equalizing of the steel sheet from a temperature T3 to a temperature T4 between 400 and 700° C. in the equalizing section having an atmosphere A4 comprising from 1 to 30% H.sub.2 by volume and an inert gas whose a dew point DP4 is below or equal to −30° C. and C. The hot-dip coating of the annealed steel sheet in a bath based on zinc or based on aluminum.

(6) Without willing to be bound by any theory, it seems that the method according to the present invention allows for a high improvement of the wettability and the coating adhesion of the steel sheet having a specific chemical composition. Indeed, on the contrary to prior art methods such as the one disclosed in JP2011153367 (FIG. 1) and as illustrated in FIG. 2, the inventors have found that the recrystallization annealing according to the present invention performed in a full Radiant Tube Furnace (RTF) wherein the heating and soaking section have the same atmosphere with DP being −18° C. and +8° C., such atmosphere comprising from 0.1 to 15% by volume of H.sub.2 allows for the production of a hot-dip coated steel sheet having a specific oxides repartition allowing a high wettability and coating adhesion. In particular, the oxides including MnO, FeO and Mn.sub.2SiO.sub.4 are formed during the recrystallization annealing externally at the steel sheet surface and also internally allowing a high wettability and coating adhesion. Preferably, the external oxides are present in form of nodules at the sheet sheet surface.

(7) If the recrystallization annealing of the above specific steel sheet is not performed 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., there is a risk to form oxides such as MnO, FeO and Mn.sub.2SiO.sub.4, such oxides being mainly or only external. Moreover, there is a risk that these oxides form a thick continuous layer at the steel sheet surface decreasing significantly the wettability and the coating adhesion of the steel sheets.

(8) Moreover, if the heating and soaking sections do not have the same atmosphere and if the dew point is above 8° C., there is a risk to form external oxides MnO and FeO and internal oxide such as Mn.sub.2SiO.sub.4. Especially, there is a risk that MnO and mainly FeO are formed in a form of a continuous layer at the steel sheet surface decreasing the wettability and the coating adhesion of the steel sheet.

(9) Regarding the chemical composition of the steel, the carbon amount is between 0.05 and 0.20% by weight. If the carbon content is below 0.05%, there is a risk that the tensile strength is insufficient. Furthermore, if the steel microstructure contains retained austenite, its stability which is necessary for achieving sufficient elongation, can be not obtained. In a preferred embodiment, the carbon content is in the range between 0.05 and 0.15%.

(10) Manganese is a solid solution hardening element which contributes to obtain high tensile strength. Such effect is obtained when Mn content is at least 1.5% in weight. However, above 3.0%, Mn addition can contribute to the formation of a structure with excessively marked segregated zones which can adversely affect the welds mechanical properties. Preferably, the manganese content is in the range between 1.5 and 2.9% to achieve these effects. This makes it possible to obtain satisfactory mechanical strength without increasing the difficulty of industrial fabrication of the steel and without increasing the hardenability in the welds.

(11) Silicon must be comprised between 0.1 and 0.45%, preferably between 0.1 to 0.30% and more preferably between 0.1 to 0.25% by weight of Si to achieve the requested combination of mechanical properties and weldability: silicon reduces the carbides precipitation during the 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. It seems that if Si amount is above 0.45%, other oxides are formed at the steel sheet surface decreasing the wettability and the coating adhesion.

(12) Aluminum must be below or equal to 0.20%, preferably below 0.18 by weight. With respect to the stabilization of retained austenite, aluminum has an influence that is relatively similar to the one of the silicon. However, aluminum content higher than 0.20% in weight would increase the Ac3 temperature, i.e. the temperature of complete transformation into austenite in the steel during the annealing step and would therefore make the industrial process more expensive.

(13) Chromium makes it possible to delay the formation of pro-eutectoid ferrite during the cooling step after holding at the maximal temperature during the annealing cycle, making it possible to achieve higher strength level. Thus, the chromium content is between 0.10 and 0.60%, preferably between 0.10 and 0.50% by weight for reasons of cost and for preventing excessive hardening.

(14) Vanadium also plays an important role within the context of the invention. According to the present invention, the amount of V is below 0.005% and preferably 0.0001≤V≤0.005%. Preferably, V forms precipitates achieving hardening and strengthening.

(15) The steels may optionally contain elements such as P, Nb, B, Mo, Ni, Ti, S, Cu, Co, N achieving precipitation hardening.

(16) P and S are considered as a residual element resulting from the steelmaking. P can be present in an amount <0.04% by weight. S can present in an amount below or equal to 0.01% by weight.

(17) Titanium and Niobium are also elements that may optionally be used to achieve hardening and strengthening by forming precipitates. However, when the Nb amount is above 0.05% and/or Ti content is greater than 0.06%, there is a risk that an excessive precipitation may cause a reduction in toughness, which has to be avoided.

(18) The steels may also optionally contain boron in quantity comprised below or equal to 0.003%. By segregating at the grain boundary, B decreases the grain boundary energy and is thus beneficial for increasing the resistance to liquid metal embrittlement.

(19) Molybdenum in quantity below or equal to 0.2% is efficient for increasing the hardenability and stabilizing the retained austenite since this element delays the decomposition of austenite.

(20) The steel may optionally contain nickel, in quantity below or equal to 0.1% so to improve the toughness.

(21) Copper can be present with a content below or equal to 0.1% for hardening the steel by precipitation of copper metal.

(22) Preferably, the chemical composition of the steel does not include Bismuth (Bi). Indeed, without willing to be bound by any theory, it is believed that if the steel sheet comprises Bi, the wettability decreases and therefore the coating adhesion.

(23) Preferably, in steps B.i) and B.ii), A1 comprises between 1 and 10% by volume of H2 and more preferably, A1 comprises between 2 and 8% by volume of H2, A2 being identical to A1.

(24) Advantageously, in steps B.i) and B.ii), DP1 is between −15° C. and +5° C., and more preferably, DP1 is between −10 and +5° C., DP2 being equal to DP1.

(25) In a preferred embodiment, in step B.i), the steel sheet is heated from ambient temperature to T1 with a heating rate above 1° C. per second and for example between 2 and 5° C. per second.

(26) Preferably, in step B.i), the heating is performed during a time t1 between 1 and 500 seconds and advantageously between 1 and 300 s.

(27) Advantageously, in step B.ii), the soaking is performed during a time t2 between 1 and 500 seconds and advantageously between 1 and 300 s.

(28) Preferably, in step B.ii), T2 is equal to T1. In this case, in steps B.i) and B.ii), T1 and T2 are between 750 and 850° C., T2 being equal to T1. In another embodiment, it is possible that T2 is below or above T1 depending on the steel sheet chemical composition and microstructure. In this case, in steps B.i) and B.ii), T1 and T2 are between 750 and 850° C. independently from each other.

(29) Preferably, in step B.iii), A3 comprises from 1 to 20% by weight of H2 and more preferably, from 1 to 10% by weight of H2.

(30) Preferably, in step B.iii), DP3 is below or equal to −35° C.

(31) In a preferred embodiment, in step B.iii), the cooling is performed during a time t3 between 1 and 50 seconds.

(32) Advantageously, in step B.iii), the cooling rate is above 10° C. per second and preferably between 15 and 40° C. per second.

(33) Advantageously, in step B.iv), A4 comprises from 1 to 20% and more preferably, from 1 to 10% by weight of H2.

(34) Preferably, in step B.iv), DP4 is below or equal to −35° C.

(35) In a preferred embodiment, in step B.iv), the equalizing is performed during a time t4 between 1 and 100 seconds and for example between 20 and 60 seconds.

(36) Advantageously, in steps B.iii) and B.iv), A3 is identical to A4, DP4 being equal to DP3.

(37) Preferably, in step B.iv), T4 is equal to T3. In this case, in steps B.iii) and B.iv), T3 and T4 are between 400 and 550° C. or between 550 and 700° C., T4 being equal to T3. In another embodiment, it is possible that T4 is below or above T3 depending on the steel sheet chemical composition and microstructure. In this case, in steps B.iii) and B.iv), T3 and T4 are between 400 and 550° C. or between 550 and 700° C. independently from each other.

(38) Preferably, in steps B. i) to B. iv), the inert gas is chosen from: N.sub.2, Ar, He and Xe.

(39) Preferably in step C), the zinc-based coating comprises from 0.01 to 8.0% by weight of Al, optionally from 0.2 to 8.0% by weight of Mg, less than 5.0% Fe, the remainder being Zn. More preferably, the zinc-based coating comprises between 0.01 and 0.40% by weight of Al, the balance being Zn. In this case, the bath temperature is between 400 and 550° C. and preferably between 410 and 460° C.

(40) In another preferred embodiment, the aluminum-based coating comprises less than 15% Si, less than 5.0% Fe, optionally 0.1 to 8.0% Mg and optionally 0.1 to 30.0% Zn, the remainder being Al. In this case, the temperature of this bath is between 550 and 700° C., preferably between 600 and 680° C.

(41) The invention also relates to a hot-dip coated steel sheet coated with a zinc or an aluminum based coating obtainable from the method according to the present invention, comprising external oxides comprising FeO, Mn2SiO4 and MnO at the steel surface underneath the zinc or aluminum based coating and internal oxides comprising FeO, Mn2SiO4 and MnO in the steel sheet. Preferably, the external oxides comprising FeO, Mn2SiO4 and MnO are in a form of nodules at the steel surface.

(42) Preferably, the steel microstructure comprises bainite, martensite, ferrite and optionally austenite. In one preferred embodiment, the steel microstructure comprises from 1 to 45% of martensite, from 1 to 60% of bainite, the balance being austenite. In another preferred embodiment, the steel microstructure comprises from 1 to 25% of fresh martensite, from 1 to 10% of ferrite, from 35 to 95% of martensite and lower bainite and less than 10% of austenite.

(43) In a preferred embodiment, the surface of steel sheet is decarburized. Preferably, the depth of the decarburization is up to 100 μm, preferably up to 80 μm, from the surface steel sheet. In this case, without willing to be bound by any theory, it is believed that the steel sheet has a better resistance to LME due to the reduction of carbon amount into the steel sheet. Indeed, it seems that carbon is an element highly sensitive to liquid metal embrittlement LME. Additionally, better bendability and better crash behavior.

(44) Finally, the invention relates to the use of the hot-dip coated steel sheet for the manufacture of a part of an automotive vehicle.

(45) The invention will now be explained in trials carried out for information only. They are not limiting.

EXAMPLES

(46) In this example, DP steels having the following composition in weight percentage were used:

(47) TABLE-US-00001 C Mn Si Cr Al Mo Ti P S Cu Ni Nb V B N 0.072 2.52 0.255 0.30 0.15 0.1 0.017 0.013 0.001 0.015 0.021 0.025 0.004 0.0020 0.006

(48) All Trials being DP steels were annealed from ambient temperature in a full RTF furnace according to the conditions of Table 1.

(49) Then, all Trials were hot-dip coated in a zinc bath containing 0.117 wt. % of Aluminum.

(50) Finally, the trials were analyzed by naked eyes, scanning electron microscope and Auger spectroscopy. For the wettability, 0 means that the coating is continuously deposited and 1 means that the coating is not continuously deposited. For the coating aspect, 0 means that the coating has no surface defect and 1 means that surface defects such as bare spots are observed in the coating. Results are shown in the Table 1 below.

(51) TABLE-US-00002 Presence of Coat- FeO, Mn2SiO4, Heating section (A1) Soaking section (A2) Cooling section (A3) Equalizing (A4) ing MnO Oxides DP1 T1 % t1 DP2 T2 % t2 DP3 T3 % t3 DP4 T4 % t4 Wetta- as- At the steel In the Trials (° C.) (° C.) H2 (s) (° C.) (° C.) H2 (s) (° C.) (° C.) H2 (s) (° C.) (° C.) H2 (s) bility pect surface steel 1 +18 780 5 209 +18 780 5 72 −40 460 5 10 −40 460 5 35 1 1 no no 2 +15 780 5 209 +15 780 5 72 −40 460 5 10 −40 460 5 35 1 1 no no 3 +10 780 5 209 +10 780 5 72 −40 460 5 10 −40 460 5 35 1 1 no no  4* +5 780 5 209 +5 780 5 72 −40 460 5 10 −40 460 5 35 0 0 yes yes  5* 0 780 5 209 0 780 5 72 −40 460 5 10 −40 460 5 35 0 0 yes yes  6* −10 780 5 209 −10 780 5 72 −40 460 5 10 −40 460 5 35 0 0 yes yes  7* −15 780 5 209 −15 780 5 72 −40 460 5 10 −40 460 5 35 0 0 yes yes 8 −20 780 5 209 −20 780 5 72 −40 460 5 10 −40 460 5 35 1 1 yes no 9 −30 780 5 209 −30 780 5 72 −40 460 5 10 −40 460 5 35 1 1 yes no 10  −40 780 5 209 −40 780 5 72 −40 460 5 10 −40 460 5 35 1 1 yes no *Examples according to the present invention

(52) Trials 4 to 7 according to the present invention show a high wettability and therefore a high coating adhesion and the surface aspect of the coating was significantly good. For these trials, FeO, Mn2SiO4 and MnO oxides were present in form of nodules at the steel surface and in the steel sheet.

(53) For Trials 8 to 10, MnO, FeO and Mn2SiO4 oxides formed a thick continuous layer at the steel sheet surface decreasing significantly the wettability and the coating adhesion of the steel sheets.

(54) For Trials 1 to 3, external oxides MnO and FeO were present in form of continuous layer at the steel surface decreasing the wettability and the coating adhesion of the steel sheet. Mn2SiO4 was present as internal oxide.