Method for hot-dip coating of a steel flat product

Abstract

A method which allows process-stable hot-dip coating of Ni-alloy steel flat products in a cost- and resource-effective manner, including the following steps: a) provision of a steel flat product obtained by cold- or hot-rolling; b) within 1-30 s, heating the steel flat product to a holding temperature between 700 and 1100° C., under a heating atmosphere of N2; c) holding the steel flat product at the holding temperature for a holding duration of 10-120 s under a holding atmosphere of N2; d) cooling the steel flat product from the holding temperature to a strip inlet temperature of 430-800° C.; and e) passing the steel flat product through an inlet zone, in which an inert or reducing inlet atmosphere predominates, and passing the steel flat product through a melt bath, wherein TP1>TP2>TP4.

Claims

1. A method for hot-dip coating of a steel flat product with a metallic protective coating, comprising: a) providing a steel flat product obtained by cold- or hot-rolling, which is produced from a steel containing at least 2.0 wt. % Ni and at least 5.0 wt. % Cr; b) heating of the steel flat product, for a period of 1-30 s, to a holding temperature between 700 and 1100° C., wherein the heating takes place under a heating atmosphere, wherein a dew point of the heating atmosphere is −15° C. to +30° C. and the heating atmosphere contains N.sub.2 and unavoidable contaminants, and optionally one or more of the following constituents by volume: H.sub.2: 1-50%, CO: 0.1-2.0%, CO.sub.2: 5.0-15.0%; c) holding the heated steel flat product at the holding temperature for a holding duration of 10-120 s under a holding atmosphere which consists of N.sub.2 and unavoidable contaminants, 1.0-50.0 vol. % H.sub.2 and up to 1.0 vol. % O.sub.2, and wherein a dew point of the holding atmosphere is −30° C. to 0° C.; d) cooling the steel flat product from the holding temperature to a strip inlet temperature of 430-800° C.; e) passing the steel flat product through an inlet zone, in which the steel flat product is held under an inert or reducing inlet atmosphere until entry into a melt bath, and subsequent passage through the melt bath in which the steel flat product is hot-dip coated with a metallic coating; wherein the dew point of the heating atmosphere is higher than the dew point of the holding atmosphere, and the dew point of the holding atmosphere is higher than the dew point of the inlet atmosphere.

2. The method according to claim 1, wherein the holding temperature is 700 to 850° C.

3. The method according to claim 1, wherein the heating atmosphere further contains 1.0-5.0 vol. % H.sub.2.

4. The method according to claim 1, wherein the heating is carried out in a directly heated furnace zone and the heating atmosphere further contains 1-50 vol. % H.sub.2, 0.1-2.0 vol. % CO and 5.0-15.0 vol. % CO.sub.2.

5. The method according to claim 1, wherein the H.sub.2 content of the holding atmosphere is 1.0-5.0 vol. %.

6. The method according to claim 1, wherein the dew point of the holding atmosphere is −30° C. to −10° C.

7. The method according to claim 1, wherein the O.sub.2 content of the holding atmosphere is maximum 0.1 vol. %.

8. The method according to claim 1, wherein following cooling to the strip inlet temperature, the steel flat product undergoes an overageing treatment in which the steel flat product is held for 1-30 s under an overageing atmosphere at the strip inlet temperature, that the overageing atmosphere contains N.sub.2 and unavoidable contaminants, and optionally 1-50 vol. % H.sub.2, and wherein a dew point of the overageing atmosphere is −50° C. to −25° C.

9. The method according to claim 8, wherein the H.sub.2 content of the overageing atmosphere is 1.0-5.0 vol. %.

10. The method according to claim 8, wherein the dew point of the heating atmosphere is greater than the dew point of the holding atmosphere is greater than the dew point of the overageing atmosphere is greater than or equal to the dew point of the inlet atmosphere.

11. The method according to claim 1, wherein the inlet atmosphere consists of N.sub.2 and unavoidable contaminants and optionally 1.0-50.0 vol. % H.sub.2, and wherein a dew point of the inlet atmosphere is −80° C. to −25° C.

12. The method according to claim 1, wherein the steel flat product is immersed in the melt bath for 1-10 s.

13. The method according to claim 1, wherein the steel from which the steel flat product is produced contains by weight: Cr: 5.0-30.0% Ni: 2.0-30.0% Mn: ≦6.0% Mo: ≦5.0% Si: ≦2.0% Cu: ≦2.0% Ti: ≦1.0% Nb: ≦1.0% V: ≦0.5% N: ≦0.2% Al: ≦2.0% C: ≦0.5%, and the balance being iron and unavoidable contaminants.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is explained below in more detail with reference to exemplary embodiments.

(2) FIG. 1 shows diagrammatically a hot-dip coating plant A comprising a passage furnace 1 which has furnace zones 2, 3, 4, 5, 6 adapted to perform the method according to the invention.

DESCRIPTION OF THE INVENTION

(3) The sheet steel product S to be hot-dip coated passes in succession through furnace zones 2-6 in the advance direction F, without interruption. The steel flat product S first enters furnace zone 2 in which it is heated within a heating duration of 20 seconds to a holding temperature T1 under a heating atmosphere Atm1.

(4) Then after furnace zone 2, the steel flat product S passes through furnace zone 3 in which it is held at the holding temperature T1 for a duration of 45 s under a holding atmosphere Atm2.

(5) Furnace zone 3 is followed by furnace zone 4, in which the steel flat product S is cooled to the strip inlet temperature T2 within 10 seconds. The cooling is carried out under the overageing atmosphere Atm3 which predominates in the furnace zone 5 following furnace zone 4.

(6) In furnace zone 5, the steel flat product S is subjected to overageing treatment for a duration of 20 s under an overageing atmosphere Atm3 at the strip inlet temperature T2.

(7) After furnace zone 5, the steel flat product S enters the furnace zone 6, formed as an inlet which opens with its free end into a melt bath B which is filled in a melt bath boiler 7. In furnace zone 6, the steel flat product S is held under an inlet atmosphere Atm4 at the strip inlet temperature T2.

(8) The steel flat product conducted via the furnace zone 6 into the melt bath B is there diverted in the known manner over a reversing roller sitting in the melt bath B, and then passes through a scraper device (not shown here) with which the thickness of the metal coating present on the steel flat product S emerging from the melt bath B is adjusted, and finally, in a manner known in itself, is guided via a cooling section to a winding device (also not shown here) at which it is wound into a coil.

(9) The steel flat product S given the metallic protective coating in the hot-dip coating plant A is typically a cold-rolled steel strip in roll-hardened state.

(10) For 18 experiments V1-V18 from three different steels S1-S3, the alloy constituents of which are given in table 2 in wt. %, cold-rolled strip steels were produced which were then fed in roll-hardened state into the hot-dip coating plant A.

(11) TABLE-US-00002 TABLE 2 Steel C Ni Cr Mn Mo N S1 0.03 13.0 18.5 2.0 2.5 — S2 0.15 9.5 19.0 2.0 0.8 0.11 S3 0.07 10.5 17.0 2.0 — — Remainder Fe and unavoidable contaminants

(12) Table 3 shows, for each of the experiments V1-V18, the holding temperature T1 reached in the furnace zone 2, the composition of the heating atmosphere Atm1 predominating in furnace zone 2, the dew point TP1 of the heating atmosphere Atm1 set, the composition of the holding atmosphere Atm2 predominating in the furnace zone 3, the respective dew point TP2 of the holding atmosphere Atm2, the strip inlet temperature reached after cooling in furnace zone 4, the composition of the overageing atmosphere Atm3 predominating in the furnace zone 5, the dew point TP3 of the overageing atmosphere Atm3, the composition of the inlet atmosphere Atm4 predominating in the furnace zone 6 formed as an inlet, the dew point TP4 of the inlet atmosphere Atm4, the temperature T3 of the melt bath B and the composition of the melt bath B.

(13) By setting a pressure drop between the atmospheres Atm3-Atm4 predominating in the furnace zones 2-6, in the passage furnace 1 a gas flow G flowing against the advance direction F is maintained, which prevents a contamination of the respectively drier atmosphere Atm2, Atm3, Atm4 by the damper atmosphere Atm1, Atm2, Atm3 previously passed by the steel flat product S.

(14) Table 4 evaluates the results of the experiments V1-V18. It is clear that the exemplary embodiments 4, 5, 6, 11, 12 which do not conform to the invention, in which the dew points TP1, TP2, TP3, TP4 set do not follow the definition according to the invention TP1>TP2>TP3≧TP4, and where applicable showed further deviations from the requirements of the invention, and the exemplary embodiments 15 and 17 in which one of the dew points TP1 and TP2 lay outside the range predefined according to the invention, gave only unsatisfactory coating results. In contrast, the experiments carried out in the manner according to the invention each gave optimum coating results.

(15) TABLE-US-00003 TABLE 3 T1 Atm1 TP1 Atm2 TP2 T2 Atm3 Experiment Steel [° C.] [vol.-%] [° C.] [vol.-%] [° C.] [° C.] [vol.-%] 1 S1 780 N.sub.2 + 5% H.sub.2  0 N.sub.2 + 5% H.sub.2 −20 485 N.sub.2 + 5% H.sub.2 2 S1 750 N.sub.2 + 5% H.sub.2 +20 N.sub.2 + 5% H.sub.2 −20 485 N.sub.2 + 5% H.sub.2 3 S1 800 N.sub.2 + 5% H.sub.2 −10 N.sub.2 + 5% H.sub.2 −20 480 N.sub.2 + 5% H.sub.2 4 S1 650 N.sub.2 + 5% H.sub.2 −10 N.sub.2 + 5% H.sub.2 −30 485 N.sub.2 + 5% H.sub.2 5 S1 800 N.sub.2 + 5% H.sub.2    −20 *) N.sub.2 + 5% H.sub.2 −20 485 N.sub.2 + 5% H.sub.2 6 S1 800 N.sub.2 + 5% H.sub.2    −30 *) N.sub.2 + 5% H.sub.2 −30 490 N.sub.2 + 5% H.sub.2 7 S2 780 N.sub.2 + 5% H.sub.2  0 N.sub.2 + 5% H.sub.2 −20 490 N.sub.2 + 5% H.sub.2 8 S2 750 N.sub.2 + 5% H.sub.2  0 N.sub.2 + 5% H.sub.2 −20 490 N.sub.2 + 5% H.sub.2 9 S2 800 N.sub.2 + 5% H.sub.2  +5 N.sub.2 + 5% H.sub.2 −20 485 N.sub.2 + 5% H.sub.2 10 S2 850 N.sub.2 + 5% H.sub.2 +10 N.sub.2 + 5% H.sub.2 −25 485 N.sub.2 + 5% H.sub.2 11 S2 800 N.sub.2 + 5% H.sub.2    −20 *) N.sub.2 + 5% H.sub.2 −30 485 N.sub.2 + 5% H.sub.2 12 S1 800 N.sub.2 + 5% H.sub.2    −30 *) N.sub.2 + 5% H.sub.2 −30 485 N.sub.2 + 5% H.sub.2 13 S3 780 N.sub.2 + 5% H.sub.2 +10 N.sub.2 + 5% H.sub.2 −20 480 N.sub.2 + 5% H.sub.2 14 S3 750 N.sub.2 + 5% H.sub.2  0 N.sub.2 + 5% H.sub.2 −20 690 N.sub.2 + 5% H.sub.2 15 S3 800 N.sub.2 + 5% H.sub.2 −10 N.sub.2 + 5% H.sub.2    −45 *) 485 N.sub.2 + 5% H.sub.2 16 S3 850 N.sub.2 + 5% H.sub.2 −10 N.sub.2 + 5% H.sub.2 −30 680 N.sub.2 + 5% H.sub.2 17 S3 800 N.sub.2 + 5% H.sub.2    −20 *) N.sub.2 + 5% H.sub.2 −30 485 N.sub.2 + 5% H.sub.2 18 S3 800 N.sub.2 + 5% H.sub.2    −30 *) N.sub.2 + 5% H.sub.2 −30 485 N.sub.2 + 5% H.sub.2 Composition of the TP3 Atm4 TP4 T3 melt bath B Experiment [° C.] [vol.-%] [° C.] [° C.] [wt.-%] 1 −30 N.sub.2 + 5% H.sub.2 −40 465 Zn + 0.18% Al 2 −30 N.sub.2 + 5% H.sub.2 −50 465 Zn + 0.18% Al 3 −30 N.sub.2 + 5% H.sub.2 −50 470 Zn + 0.12% Al 4 −30 N.sub.2 + 5% H.sub.2 −30 465 Zn + 0.18% Al 5 −20 N.sub.2 + 5% H.sub.2 −20 465 Zn + 0.18% Al 6 −30 N.sub.2 + 5% H.sub.2 −30 470 Zn + 0.18% Al 7 −30 N.sub.2 + 5% H.sub.2 −40 460 Zn + 0.18% Al 8 −30 N.sub.2 + 5% H.sub.2 −50 460 Zn + 0.18% Al 9 −30 N.sub.2 + 5% H.sub.2 −50 460 Zn + 0.22% Al 10 −30 N.sub.2 + 5% H.sub.2 −30 470 Zn + 0.18% Al 11 −20 N.sub.2 + 5% H.sub.2 −20 465 Zn + 0.18% Al 12 −30 N.sub.2 + 5% H.sub.2 −30 465 Zn + 0.18% Al 13 −30 N.sub.2 + 5% H.sub.2 −40 465 Zn + 0.9% Al + 0.9% Mg 14 −30 N.sub.2 + 5% H.sub.2 −50 680 Al + 11.5% Si 15 −50 N.sub.2 + 5% H.sub.2 −50 465 Zn + 0.18% Al 16 −35 N.sub.2 + 5% H.sub.2 −50 670 Al + 11.5% Si 17 −40 N.sub.2 + 5% H.sub.2 −50 470 Zn + 0.18% Al 18    −20 *) N.sub.2 + 5% H.sub.2    −20 *) 470 Zn + 0.18% Al *) outside definitions according to the invention

(16) TABLE-US-00004 TABLE 4 According to the Experiment Result invention 1 good yes 2 good yes 3 good yes 4 wetting defects no 5 wetting defects no 6 wetting defects no 7 good yes 8 good yes 9 good yes 10 good yes 11 wetting defects no 12 wetting defects no 13 good yes 14 good yes 15 wetting defects no poor adhesion 16 good yes 17 wetting defects no 18 wetting defects no