Hot-Dip Coating Method in a Zinc Bath for Strips of Iron/Carbon/Manganese Steel

Abstract

The subject of the invention is a method for the hot-dip coating, in a liquid bath based on zinc containing aluminum, of a running strip of iron-carbon-manganese austenitic steel, in which said strip is subjected to a heat treatment in a furnace in which an atmosphere that is reducing with respect to iron prevails, in order to obtain a strip covered with a thin manganese oxide layer, and then the strip covered with the thin manganese oxide layer is made to run through said bath, the aluminum content in the bath being adjusted to a value at least equal to the content needed for the aluminum to completely reduce the manganese oxide layer, so as to form, on the surface of the strip, a coating comprising an iron-manganese-zinc alloy layer and a zinc surface layer.

Description

The invention will now be illustrated by examples given by way of nonlimiting indication and with reference to the appended figures in which:

FIGS. 1, 2 and 3 are photographs of the surface of an iron-carbon-manganese austenitic steel strip that has undergone annealing with a dew point of −80° C., −45° C. and +10° C. under the conditions described below; and

FIG. 4 is an SEM micrograph showing a cross section through the oxide bilayer formed on an iron-carbon-manganese austenitic steel after recrystallization annealing with a dew point of +10° C. under the conditions described below; and

FIG. 5 is an SEM micrograph showing a cross section through the zinc-based coating formed after immersion in a zinc bath containing 0.18% aluminum by weight, on an iron-carbon-manganese austenitic steel annealed, with a −80° C. dew point, under the conditions described below.

1) Influence of the Dew Point on Coatability

Tests were carried out using specimens cut from a strip of iron-carbon-manganese austenitic steel which, after hot rolling and cold rolling, had a thickness of 0.7 mm. The chemical composition of this steel is given in Table 1, the contents being expressed in % by weight.

TABLE 1
Mn	C	Si	Al	S	P	Mo	Cr
20.77	0.57	0.009	traces	0.008	0.001	0.001	0.049

The specimens were subjected to recrystallization annealing in an infrared furnace, the dew point (DP) of which was varied from −80° C. to +10° C. under the following conditions:

gas atmosphere: nitrogen+15% hydrogen by volume;

heating rate V1: 6° C./s

heating temperature T1: 810° C.;

soak time M: 42 s;

cooling rate V2: 3° C./s; and

immersion temperature T3: 480° C.

Under these conditions, the steel was completely recrystallized and Table 2 gives the characteristics of the oxide bilayer comprising an (Fe,Mn)O amorphous continuous lower layer and an MnO upper layer, formed on specimens after the annealing, as a function of the dew point.

	TABLE 2
	−80° C. DP	−45° C. DP	+10° C. DP
Color of the surface of	yellow	green	blue
the strip
Mean diameter of the	50	100	300
MnO crystals (nm)	(discontinuous	(continuous layer)	(continuous
	layer)		layer)
Thickness of the	10	110	1500
bilayer (nm)

After having been recrystallized, the specimens were cooled down to a temperature T3 of 480° C. and immersed in a zinc bath comprising, by weight, 0.18% aluminum and 0.02% iron, the temperature T2 of which was 460° C. The specimens remained in contact with the bath for a contact time C of 3 seconds. After immersion, the specimens were examined to check whether a zinc-based coating was present on the surface of the specimen. Table 3 indicates the results obtained as a function of the dew point.

	TABLE 3
	−80° C. DP	−45° C. DP	+10° C. DP
Presence of the zinc-	yes	no	no
based coating

The inventors have demonstrated that if the oxide bilayer formed on the iron-carbon-manganese austenitic steels strip after recrystallization annealing was greater than 110 nm, the presence in the bath of 0.18% by weight of aluminum was insufficient to reduce the oxide bilayer and to give the strip sufficient wettability or zinc with respect to the steel in order to form a zinc-based coating.

2) Influence of the Aluminum Content in the Steel

Tests were carried out using specimens cut form an iron-carbon-manganese austenitic steel strip which, after hot rolling and cold rolling, had a thickness of 0.7 mm. The chemical compositions of the steels used are given in Table 4, the contents being expressed in % by weight.

	TABLE 4
	Mn	C	Si	Al
	Steel A	25.10	0.50	0.009	1.27
	*Steel B	24.75	0.41	0.009	traces
	*according to the invention

The specimens were subjected to recrystallization annealing in an infrared furnace, the dew point (DP) of which was −80° C. under the following conditions:

gas atmosphere: nitrogen+15% hydrogen by volume;

heating rate V1: 6° C./s;

heating temperature T1: 810° C.;

soak time M: 42 s;

cooling rate V2: 3° C./s; and

immersion temperature T3: 480° C.

Under these conditions, the steel is completely recrystallized and Table 5 gives the structures of the various oxide films that were formed on the surface of the steel after the annealing as a function of the composition of the steel.

	TABLE 5
	Oxide films	Steel A	*Steel B
	Sublayer	MnAl2O4	(Fe,Mn)O
	Upper layer	MnO•Al2O3	MnO
	*according to the invention

After having been recrystallized, the specimens were cooled to a temperature T3 of 480° C. and immersed in a zinc bath containing 0.18% aluminum and 0.02% iron, the temperature T2 of which was 460° C. The specimens remained in contact with the bath for a contact time C of 3 seconds. After immersion, the specimens were coated with a zinc-based coating.

To characterize the adhesion of this zinc-based coating formed on the specimens of steel A and steel B, an adhesive tape was applied to the coated steel and then torn off. Table 6 gives the results after tearing off the adhesive strip in this adhesion test. The adhesion was assessed by a gray level rating on the adhesive tape, starting from 0, for which the tape remains clean after tearing, up to the level 3, in which the gray level is the most intense.

TABLE 6
Steel A  Poor adhesion, gray level: 3
*Steel B Good adhesion, gray level: 0, no trace of zinc-based
         coating on the adhesive tape
*according to the invention

Claims

1. A method for the hot-dip coating, in a liquid bath based on zinc containing aluminum, said bath having a temperature T2, of a strip of iron-carbon-manganese austenitic steel comprising: 0.30%≦C≦1.05%, 16%≦Mn≦26%, Si≦1%, and Al≦0.050%, the contents being expressed by weight, said method comprising the steps consisting in: subjecting said strip to a heat treatment in a furnace in which an atmosphere that is reducing with respect to iron prevails, said heat treatment comprising a heating phase at a heating rate V1, a soak phase at a temperature T1 for a soak time M, followed by a cooling phase at a cooling rate V2, in order to obtain a strip covered on both its sides with a continuous sublayer of an amorphous iron manganese mixed oxide (Fe,Mn)O and with a continuous or discontinuous external layer of crystalline MNO manganese oxide; and thenmaking said strip covered with said oxide layers run through said bath in order to coat the strip with a zinc-based coating, the aluminum content in said bath being adjusted to a value at least equal to the content needed for the aluminum to completely reduce the crystalline MnO manganese oxide layer and at least partially reduce the amorphous (Fe,Mn)O oxide layer so as to form, on the surface of the strip, said coating comprising three iron-manganese-zinc alloy layers and one surface zinc layer.

2. The method as claimed in claim 1, characterized in that said atmosphere reducing with respect to iron is composed of a gas chosen from hydrogen and nitrogen—hydrogen mixtures.

3. The method as claimed in claim 2, characterized in that said gas comprises between 20 and 97% nitrogen by volume and between 3 and 80% hydrogen by volume.

4. The method as claimed in claim 3, characterized in that said gas comprises between 85 and 95% nitrogen by volume and between 5 and 15% hydrogen by volume.

5. The method as claimed in any one of claims 1 to 4, characterized in that said gas has a dew point between −80 and 20° C.

6. The method as claimed in claim 5, characterized in that said gas has a dew point between −80 and −40° C.

7. The method as claimed in claim 6, characterized in that said gas has a dew point between −60 and −40° C.

8. The method as claimed in any one of claims 1 to 7, characterized in that the heat treatment of the strip is carried out as a heating rate V1 of 6° C./s or higher, at a temperature T1 between 600 and 900° C. for a soak time M between 20 s and 60 s, and with a cooling rate V2 of 3° C./s or higher down to a strip immersion temperature T3 between (T2−10° C.) and (T2+30° C.).

9. The method as claimed in claim 8, characterized in that the temperature T1 is between 650 and 820° C.

10. The method as claimed in claim 9, characterized in that the temperature T1 does not exceed 750° C.

11. The method as claimed in one of claims 8 to 10, characterized in that the soak time M is between 20 and 40 s.

12. The method as claimed in any one of claims 1 to 11, characterized in that the heat treatment is carried out in a reducing atmosphere in such a way that an amorphous (Fe,Mn)O mixed oxide layer is formed with a thickness of between 5 and 10 nm, together with a crystalline MnO manganese oxide layer having a thickness between 5 and 90 nm, before the MnO layer is completely reduced by the aluminum of the bath.

13. The method as claimed in claim 12, characterized in that the crystalline MnO manganese oxide layer has a thickness between 5 and 50 nm.

14. The method as claimed in claim 13, characterized in that the crystalline MnO manganese oxide layer has a thickness between 10 and 40 nm.

15. The method as claimed in nay one of claims 1 to 14, characterized in that the liquid zinc-based bath contains between 0.15 and 5% aluminum by weight.

16. The method as claimed in any one of claims 1 to 15, characterized in that the temperature T2 of the liquid zinc-based bath is between 430 and 480° C.

17. The method as claimed in any one of claims 1 to 16, characterized in that the strip is in contact with the liquid zinc-based bath for a contact time C between 2 and 10 s.

18. The method as claimed in claim 17, characterized in that the contact time C is between 3 and 5 s.

19. The method as claimed in any one of claims 1 to 18, characterized in that the carbon content of the steel is between 0.40 and 0.70% by weight.

20. The method as claimed in any one of claims 1 to 19, characterized in that the manganese content of the steel is between 20 and 25% by weight.

21. The method as claimed in any one of claims 1 to 20, characterized in that, after the austenitic steel strip has been coated with the coating comprising three iron-manganese-zinc alloy layers and surface zinc layer, said coated strip is subjected to a heat treatment so as to completely alloy said coating.

22. An iron-carbon-manganese austenitic steel strip that can be obtained as claimed in any one of claims 1 to 20, the chemical composition of which comprises, the contents being expressed by weight: 0.30%≦C≦1.05%16%≦Mn≦26%Si≦1%Al≦0.050%S≦0.030%P≦0.080%N≦0.1%,and, optionally, one or more elements such as Cr≦1%Mo≦0.40%Ni≦1%Cu≦5%Ti≦0.50%Nb≦0.50%V≦0.50%,

23. An iron-carbon-manganese austenitic steel strip that may be obtained as claimed in claim 21, the chemical composition of which comprises, the contents being expressed by weight: 0.30%≦C≦1.05%16%≦Mn≦26%Si≦1%Al≦0.050%S≦0.030%P≦0.080%N≦0.1%and, optionally, one or more elements such as Cr≦1%Mo≦0.40%Ni≦1%Cu≦5%Ti≦0.50%Nb≦0.50%V≦0.50%,

24. The steel strip as claimed in either of claims 22 and 23, characterized in that the silicon content is less than 0.5% by weight.

25. The steel strip as claimed in any one of claims 22 to 24, characterized in that the carbon content is between 0.40 and 0.70% by weight.

26. The steel strip as claimed in any one of claims 22 to 25, characterized in that the manganese content is between 20 and 25% by weight.