Method for the hot-dip coating of a flat steel product containing 2-35 wt.% of Mn, and a flat steel product

Abstract

A method by which a flat steel product containing 2-35 wt. % of Mn can be provided with a coating of Zn which adheres well by annealing at an annealing temperature T.sub.a of 600-1100 C. for an annealing time of 10-240 s under an annealing atmosphere which has a reducing effect on the FeO present on the flat steel product and an oxidizing effect on the Mn contained in the steel substrate thereby forming a layer of Mn mixed oxide which covers the flat steel product at least in sections and then cooling the flat steel product to a temperature for bath entry and conveying it through a bath of molten Zn saturated within iron at a temperature of 420-520 C., within a dip time of 0.1-10 s.

Claims

1. A flat steel product comprising a steel substrate having an Mn content of 2-35 wt. % and a Zn protective coating formed by zinc or a zinc alloy which provides protection against corrosion, wherein the protective Zn coating comprises a layer of Mn mixed oxide which substantially covers and adheres to the flat steel product and a layer of Zn which shields the flat steel product and the layer of Mn mixed oxide adhering thereto from the surroundings and wherein the Mn mixed oxide is MnO.Fe.sub.Metal.

2. The flat steel product according to claim 1, wherein the protective Zn coating further comprises a layer of FeMn.sub.2Al.sub.5 arranged between the layer of Mn mixed oxide and the layer of Zn.

3. The flat steel product according to claim 1, wherein the protective Zn coating further comprises a layer of FeMnZn which is situated between the layer of Mn mixed oxide and the layer of Zn.

4. The flat steel product according to claim 1, wherein the protective Zn coating takes the form of a coating of ZnMg alloy.

5. The flat steel product according to claim 1, wherein the flat steel product is produced by a method comprising: a) providing the flat steel product; b) annealing the flat steel product; at an annealing temperature Ta of 600-1100*C, for an annealing time of 10-240 s under an annealing atmosphere which has a reducing effect on the FeO present on the flat steel product and an oxidising effect on the Mn contained in the steel substrate and which annealing atmosphere contains 0.01-85 vol. % of H2, H.sub.2O and the remainder N.sub.2 and unavoidable impurities present for technical reasons and which has a dew point lying between 70 C. and +60 C., the H.sub.2O/H2 ratio being:
8.Math.10.sup.15xT.sub.a.sup.3.529<H.sub.2O/H20.957, thereby producing on the flat steel product a 20-400 nm thick layer of MnO.Fe.sub.Metal which covers the flat steel product at least in sections, c) cooling the annealed flat steel product to a temperature for bath entry; d) conveying the flat steel product which has been cooled to the temperature for bath entry through a bath of molten Zn saturated within iron and which is at a temperature of 420-520 C., within a dip time of 0.1-10 s, the flat steel product thus being hot-dip coated with a protective coating of Zn providing protection against corrosion, the bath of molten Zn containing, as well as the main constituent zinc and unavoidable impurities, 0.05-8 wt. % of Al and/or up to 8 wt. % of Mg, and, optionally, Si<2%, Pb<0.1%, Ti<0.2%, Ni<1%, Cu<1%, Co<0.3%, Mn<0.5%, Cr<0.2%, Sr<0.5%, Fe<3%, B<0.1%, Bi<0.1%, Cd<0.1%; and, e) cooling the flat steel product provided with the Zn coating which emerges from the bath of molten metal.

6. A method for producing the flat steel according to claim 1, comprising: a) providing the flat steel product; b) annealing the flat steel product; at an annealing temperature Ta of 600-1100*C, for an annealing time of 10-240 s under an annealing atmosphere which has a reducing effect on the FeO present on the flat steel product and an oxidising effect on the Mn contained in the steel substrate and which annealing atmosphere contains 0.01-85 vol. % of H2, H.sub.2O and the remainder N.sub.2 and unavoidable impurities present for technical reasons and which has a dew point lying between 70 C. and +60 C., the H.sub.2O/H2 ratio being:
8.Math.10.sup.15xT.sub.a.sup.3.529<H.sub.2O/H20.957, thereby producing on the flat steel product a 20-400 nm thick layer of MnO.Fe.sub.Metal which covers the flat steel product at least in sections, c) cooling the annealed flat steel product to a temperature for bath entry; d) conveying the flat steel product which has been cooled to the temperature for bath entry through a bath of molten Zn saturated within iron and which is at a temperature of 420-520 C., within a dip time of 0.1-10 s, the flat steel product thus being hot-dip coated with a protective coating of Zn providing protection against corrosion, the bath of molten Zn containing, as well as the main constituent zinc and unavoidable impurities, 0.05-8 wt. % of Al and/or up to 8 wt. % of Mg, and, optionally, Si<2%, Pb<0.1%, Ti<0.2%, Ni<1%, Cu<1%, Co<0.3%, Mn<0.5%, Cr<0.2%, Sr<0.5%, Fe<3%, B<0.1%, Bi<0.1%, Cd<0.1%; and, e) cooling the flat steel product provided with the Zn coating which emerges from the bath of molten metal.

7. The method according to claim 6, wherein the flat steel product is made available in the form of a cold-rolled steel strip.

8. The method according to claim 6, wherein the annealing is preceded by an annealing step in which the flat steel product is kept at an annealing temperature of 200-1100 C. for an annealing time of 0.1 to 60 s under an atmosphere which is oxidative to Fe and Mn and which contains 0.0001-5 vol. % of H2 and, optionally, 200-5500 vol. ppm of O2 and which has a dew point in the range from 60 C. to +60 C.

9. The method according to claim 6, wherein the thickness of the layer of MnO.Fe.sub.Metal obtained after annealing is 40-400 nm.

10. The method according to claim 6, wherein the layer of MnO.Fe.sub.Metal substantially covers the whole of the surface of the flat steel product after the annealing of the flat steel product.

11. The method according to claim 6, wherein the dip time in the bath of molten Zn is 0.1-5 s.

12. The method according to claim 6, wherein the bath of molten Zn contains both Al and Mg.

13. The method according to claim 12, wherein the Al content of the bath is smaller than the Mg content thereof.

14. The method according to claim 6, wherein the temperature for bath entry of the flat steel product is 360-710 C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be explained in detail by exemplary embodiments below. In the drawings:

(2) FIG. 1 is a schematic view in section of a flat steel product provided with a Zn coating containing Al.

(3) FIG. 2 is a taper microsection of a specimen of a flat steel product provided with a Zn coating.

(4) FIG. 3 is a schematic view in section of a flat steel product provided with a ZnMg coating.

(5) FIG. 4 is a taper microsection of a specimen of a flat steel product provided with a ZnMg coating.

DETAILED DESCRIPTION OF THE INVENTION

(6) Cold-rolled steel strip was produced in a known way from a high-manganese steel of the composition given in Table 1.

(7) TABLE-US-00001 TABLE 1 C Mn P Si V Al Cr Ti Nb 0.634 22.2 0.02 0.18 0.2 0.01 0.08 0.001 0.001 Remainder iron and unavoidable impurities Figures are in wt. %

(8) A first specimen of the cold-rolled steel strip was then annealed in an annealing process carried out in a single stage.

(9) For this purpose, the specimen of steel strip was heated at a heating rate of 10 K/s to an annealing temperature T.sub.a of 800 C. at which the specimen was then held for 30 seconds. The annealing took place in this case under an annealing atmosphere of which 5 vol. % comprised H.sub.2 and 95 vol. % comprised N.sub.2 and whose dew point was +25 C. The annealed steel strip was then cooled at a cooling rate of 20 K/s to a temperature for bath entry of 480 C., at which it was first subjected to an over-ageing treatment for 20 seconds. The over-ageing treatment took place in this case under the unchanged annealing atmosphere. Without leaving the annealing atmosphere, the steel strip was then conveyed into a bath of molten zinc saturated with Fe which was at a temperature of 460 C. and which contained, as well as Zn, unavoidable impurities and Fe, 0.23 wt. % of Al in addition. After a dip time of 2 seconds, the steel strip, which had now been hot-dip coated, was conveyed out of the bath of molten metal and was cooled to room temperature.

(10) In a second test, a second specimen of the cold-rolled steel strip of the composition shown in Table 1 was annealed in a two-stage process and then hot-dip coated in a method sequence which it likewise passed through continuously.

(11) For this purpose, the steel strip was first heated at a heating rate of 10 K/s to 600 C. and was held at this annealing temperature for 10 seconds. The annealing atmosphere contained in this case 2000 ppm of O.sub.2 and the remainder N.sub.2. Its dew point was 30 C.

(12) Immediately following this, in a second annealing step, the steel strip was heated to an annealing temperature T.sub.a of 800 C., at which it was kept for 30 seconds under an annealing atmosphere containing 5 vol. % of H.sub.2 and the remainder N.sub.2 whose dew point was 30 C. While still under the annealing atmosphere, the steel strip was then cooled at a cooling temperature of approximately 20 K/s to 480 C. and was subjected for 20 seconds to an over-ageing treatment. Following this the steel strip was conveyed, at a temperature for bath entry of 480 C., into a bath of molten metal saturated with Fe which was at a temperature of 460 C. and which once again contained 0.23 wt. % of Al together with other elements in the form of inactive trace impurities and the remainder zinc. After a dip time of 2 seconds, the fully hot-dip coated flat steel product was then conveyed out of the bath of molten metal and was cooled to room temperature.

(13) FIG. 1 is a schematic view of the structure of the coating Z which was obtained in this way on the steel substrate S. It shows that, lying on the steel substrate S, there is a layer M (M=MnO.Fe) of manganese mixed oxide Mn.sub.yO.sub.x, on which there has formed an intermediate Fe(Mn).sub.2Al.sub.5 layer F (F=MnO.Fe(Mn).sub.2Al.sub.5) or, when the Al contents of the bath of molten metal are a maximum of 0.15 wt. %, a layer of FeMnZn, which is screened off in turn from the surroundings by a Zn layer Zn ( phase). The thickness of the layer M of Mn mixed oxide is 20-400 nm in this case while the thickness of the intermediate Fe(Mn).sub.2Al.sub.5 layer F is 10-200 nm. The total thickness of the coating layers M and F is thus 20-600 nm. The zinc layer Zn on the other hand is appreciably thicker at 3-20 m.

(14) Shown in FIG. 2 is a taper microsection of a specimen which was produced in the manner described above. Clearly apparent are the steel substrate S, together with the layer M lying thereon of manganese mixed oxide Mn.sub.yO.sub.x containing interstitial metallic iron, the intermediate Fe(Mn).sub.2Al.sub.5 layer F lying on the layer M of mixed oxide, and the Zn layer lying on the intermediate layer F.

(15) To check the success of the procedure according to the invention, twenty additional tests 1-20 were carried out in each of which the bath of molten metal contained 0.23 wt. % of Al as well as Zn and unavoidable impurities. The degree of wetting and the adhesion of the zinc were examined visually on each of the specimens so obtained. The principle of testing applied was the notch impact test under the German iron and steel testing standard (SEP 1931). The testing parameters and the results of the tests are given in Table 2.

(16) Moreover, a further sixteen tests 21-36 were also performed in which the bath of molten metal contained 0.11 wt. % of Al as well as Zn and unavoidable impurities. As opposed to the barrier layer demonstrated in the above test which took the form of an Fe(Mn).sub.2Al.sub.5 layer, an FeMnZn barrier layer formed when the bath of molten metal had this lower Al content. The degree of wetting and the adhesion of the zinc were likewise examined on each of the specimens so obtained. The testing parameters and the results of the tests are given in Table 3.

(17) On the basis of further specimens of the high-manganese steel strip cold-rolled from the steel of the composition shown in Table 1, the effect of the dew point of the given annealing atmosphere on the results of the coating process was examined. For this purpose, the specimens were each subjected to an annealing process in which they were similarly heated at a heating rate of 10 K/s to an annealing temperature T.sub.a of 800 C. The specimen was then held at this annealing temperature for 60 seconds. The annealing took place in each case under an annealing atmosphere which each time comprised 5 vol. % of H.sub.2 and 95 vol. % of N.sub.2, with the respective dew points of the annealing atmosphere being varied between 55 C. and +45 C.

(18) After the heat treatment, the annealed steel strip was, as in the series of tests described above, cooled at a cooling rate of 20 K/s to a temperature for bath entry of 480 C., at which it was first subjected to an over-ageing treatment for 20 seconds. The over-ageing treatment took place in this case under the unchanged annealing atmosphere. Without leaving the annealing atmosphere, the steel strip was then conveyed into a bath of molten zinc saturated with Fe which was at a temperature of 460 C. and which contained in respective cases, as well as Zn, unavoidable impurities and Fe, either a combination of 0.4 wt. % of Al and 1.0 wt. % of Mg, or 0.14 wt. %, 0.17 wt. % or 0.23 wt. % of Al alone. After a dip time of 2 seconds, the steel strip, which had now been hot-dip coated, was conveyed out of the bath of molten metal and was cooled to room temperature.

(19) FIG. 3 is a schematic view of the structure of the coating Z which was obtained in this way on the steel substrate S. It shows that, lying on the steel substrate S, there is a layer M (M=MnO.Fe) of manganese mixed oxide Mn.sub.yO.sub.x, on which there has formed an intermediate Fe(Mn).sub.2Al.sub.5 layer F (F=MnO.Fe(Mn).sub.2Al.sub.5) or, when the Al contents of the bath of molten metal are a maximum of 0.15 wt. %, a layer of FeMnZn, which is screened off in turn from the surroundings by a ZnMg layer. The thickness of the layer M of Mn mixed oxide is 20-400 nm while the thickness of the intermediate Fe(Mn).sub.2Al.sub.5 layer F is 10-200 nm. The total thickness of the coating layers M and F is thus 20-600 nm. The zinc layer ZnMg on the other hand is appreciably thicker at 3-20 m.

(20) Shown in FIG. 4 is a taper microsection of a specimen which was produced in the manner described above. Clearly apparent are the steel substrate S, together with the layer M lying thereon of manganese mixed oxide Mn.sub.yO.sub.x containing interstitial metallic iron, the intermediate Fe(Mn).sub.2Al.sub.5 layer F lying on the layer M of mixed oxide, and the ZnMg layer lying on the intermediate layer F.

(21) As well as the above-mentioned variation in the dew points of the annealing atmosphere, a variation was also made in the Al and Mg contents of the bath of molten metal in twenty-one tests 37 to 57 which were carried out to check the success of the procedure according to the invention. The degree of wetting and the adhesion of the zinc were examined visually on each of the specimens so obtained. The principle of testing applied in this case too was the notch impact test under Stahl-Eisen Prfblatt SEP 1931. The testing parameters and the results of the tests are given in Table 4.

(22) It was found that, in the combined presence of Al and Mg and with the dew point set to the range from 50 C. to +60 C., zinc-based coatings could be produced reliably on substrates of high-manganese steel even by the annealing process carried out in a single stage.

(23) To allow a comparison to be made, three more respective specimens V1-V3 and V4-V6 were obtained from cold-rolled steel strip composed of an Al TRIP steel VS1 and from a likewise cold-rolled Si TRIP steel VS2. The compositions of steels VS1 and VS2 are given in Table 5.

(24) TABLE-US-00002 TABLE 5 C Mn P Si V Al Cr Ti Nb VS1 0.22 1.1 0.02 0.1 0.002 1.7 0.06 0.1 0.001 VS2 0.18 1.8 0.02 1.8 0.002 0 0.06 0.01 0.001 Remainder iron and unavoidable impurities Figures are in wt. %.

(25) The comparative specimens V1-V6 too were heat treated in the manner described above for the specimens according to the invention before they were hot-dip coated in the bath of molten metal. In this case, the bath of molten metal contained, as well as Zn and unavoidable impurities, 0.4 wt. % of Al and 1 wt. % of Mg in the case of each specimen. In this case too, the degree of wetting and the adhesion of the zinc were examined on each of the specimens V1-V6 which had been coated in this way. The testing parameters and the results of the tests are listed in Table 6. It was found that, due to the lower manganese contents of steels VS1 and VS2, an MnO structure did not form in the layer of mixed oxide on the surface of the steel substrate. Consequently, a covering layer of Fe(Mn).sub.2 likewise failed to form as a primer. As a result, sufficient reduction of MnO by dissolved magnesium did not occur in the bath of molten metal and it was thus impossible for adequate wetting and hence adequate adhesion of the coating to be obtained in the comparative specimens.

(26) TABLE-US-00003 TABLE 2 1.sup.st stage of annealing 2.sup.nd stage of annealing In Annealing Annealing O.sub.2 Annealing Annealing H.sub.2 Dew accordance Test temp. time content temp. T.sub.a time content point Wetting Adhesion with the no. [ C.] [s] [ppm] [ C.] [s] [%] [ C.] by zinc of zinc invention 1 Single stage 800 60 5 50 No No No 2 800 60 5 30 No No No 3 800 60 5 15 Severely- No No disrupted 4 800 60 5 5 Severely No No disrupted 5 800 60 5 5 Severely No No disrupted 6 800 60 5 +15 Disrupted Limited No 7 800 60 5 +25 Yes Yes Yes 8 800 60 5 +45 Yes Yes Yes 9 500 10 2000 800 30 5 30 Disrupted Yes Yes at points 10 600 10 2000 800 60 5 30 Yes Yes Yes 11 700 10 2000 800 30 5 15 Disrupted Yes Yes at points 12 800 10 2000 800 30 5 15 Disrupted Yes Yes at points 13 500 10 2500 800 30 5 15 Disrupted Yes Yes at points 14 600 10 2500 800 30 5 30 Yes Yes Yes 15 700 10 2500 800 30 5 30 Yes Yes Yes 16 800 10 2500 800 30 5 30 Yes Yes Yes 17 500 6 2500 800 30 5 30 Disrupted Yes Yes at points 18 600 6 2500 800 30 5 30 Yes Yes Yes 19 700 6 2500 800 30 5 30 Yes Yes Yes 20 800 6 2500 800 30 5 30 Yes Yes Yes

(27) TABLE-US-00004 TABLE 3 1.sup.st stage of annealing 2.sup.nd stage of annealing In Annealing Annealing O.sub.2 Annealing Annealing H.sub.2 Dew accordance Test temp. time content temp. T.sub.a time content point Wetting Adhesion with the no. [ C.] [s] [ppm] [ C.] [s] [%] [ C.] by zinc of zinc invention 21 Single stage 800 60 5 50 No No No 22 800 60 5 30 No No No 23 800 60 5 15 Severely No No disrupted 24 800 60 5 5 Severely No No disrupted 25 800 60 5 +5 Severely No No disrupted 26 800 60 5 +15 Disrupted Limited No 27 800 60 5 +25 Yes Yes Yes 28 800 60 5 +45 Yes Yes Yes 29 500 10 2000 800 30 5 30 Disrupted Yes Yes at points 30 600 10 2000 800 60 5 30 Yes Yes Yes 31 700 10 2000 800 30 5 15 Disrupted Yes Yes at points 32 800 10 2000 800 30 5 15 Disrupted Yes Yes at points 33 500 10 2500 800 30 5 15 Disrupted Yes Yes at points 34 600 10 2500 800 30 5 30 Yes Yes Yes 35 700 10 2500 800 30 5 30 Yes Yes Yes 36 800 10 2500 800 30 5 30 Yes Yes Yes

(28) TABLE-US-00005 TABLE 4 Annealing Bath of molten metal In Annealing Holding H.sub.2 Dew Mg Al accordance Test temp. T.sub.a time content point content content Wetting Adhesion with the no. [ C.] [s] [%] [ C.] [wt. %] [wt. %] by zinc of zinc invention 37 800 60 5 +5 1 0.4 Yes Yes Yes 38 800 60 5 +15 1 0.4 Yes Yes Yes 39 800 60 5 +25 1 0.4 Yes Yes Yes 40 800 60 5 +45 1 0.4 Yes Yes Yes 41 800 60 5 50 0.14 No No No 42 800 60 5 30 0.14 No No No 43 800 60 5 15 0.14 No No No 44 800 60 5 50 0.17 No No No 45 800 60 5 30 0.17 No No No 46 800 60 5 15 0.17 No No No 47 800 60 5 50 0.23 No No No 48 800 60 5 30 0.23 No No No 49 800 60 5 15 0.23 No No No 50 800 60 5 55 1 0.9 Disrupted No No at points 51 800 60 5 30 1 0.9 Yes Yes Yes 52 800 60 5 15 1 0.9 Yes Yes Yes 53 800 60 5 5 1 0.9 Yes Yes Yes 54 800 60 5 55 5 1 Disrupted No No at points 55 800 60 5 30 5 1 Yes Yes Yes 56 800 60 5 15 5 1 Yes Yes Yes 57 800 60 5 5 5 0.4 Yes Yes Yes

(29) TABLE-US-00006 TABLE 6 Annealing Bath of molten metal In Annealing Holding H.sub.2 Dew Mg Al accordance Test temp. T.sub.a time content point content content Wetting Adhesion with the no. Steel [ C.] [s] [%] [ C.] [wt. %] [wt. %] by zinc of zinc invention V1 VS1 800 60 5 50 1 0.4 No No No V2 VS1 800 60 5 30 1 0.4 No No No V3 VS1 800 60 5 15 1 0.4 No No No V4 VS2 800 60 5 50 1 0.4 No No No V5 VS2 800 60 5 30 1 0.4 No No No V6 VS2 800 60 5 15 1 0.4 No No No