Method for producing a steel strip with improved bonding of metallic hot-dip coatings

Patent metadata

Field	Value
Publication number	US-2022220598-A1
Application number	US-202017600245-A
Country	US
Kind code	A1
Filing date	Mar 27, 2020
Priority date	Apr 1, 2019
Publication date	Jul 14, 2022
Grant date	—

Abstract

A cold-rolled or hot-rolled steel strip having a metal coating, the steel strip having iron as the main constituent and, in addition to carbon, an Mn content of 4.1 to 8.0 wt. % and optionally one or more of the alloy elements Al, Si, Cr, B, Ti, V, Nb and/or Mo. The surface of the uncoated steel strip is cleaned, a layer of pure iron is applied to the cleaned surface, an oxygen-containing iron-based layer is applied to the layer of pure iron and contains more than five mass percent oxygen. The steel strip is then annealed and, to attain a surface consisting substantially of metallic iron, is subjected to a reduction treatment in a reducing furnace while being annealed. The steel strip is then coated with the metallic coating by hot dipping. Uniform and reproducible adhesion conditions are hereby achieved for the metallic coating on the steel strip surface.

First claim

1 . Method for producing a cold-rolled or hot-rolled steel strip having a metallic coat, the steel strip comprises iron as a main constituent and, in addition to carbon, an Mn content of 4.1 to 8.0 wt. % and optionally one or more of the alloy elements Al, Si, Cr, B, Ti, V, Nb and/or Mo, wherein the surface of the uncoated steel strip is cleaned, a layer of pure iron with an average iron content of more than 96 wt. % is applied onto the cleaned surface, onto the layer of pure iron an oxygen-containing, iron-based layer is applied which contains more than 5 mass percent of oxygen, then the steel strip together with the oxygen-containing, iron-based layer is subjected to annealing treatment and is reduction-treated during the course of the annealing treatment in a reducing furnace atmosphere and the steel strip thus treated and subjected to annealing treatment is then hot-dip coated with the metallic coat.

2 . Method as claimed in claim 1 , characterised in that an average thickness of the pure iron layer is formed to be 0.05 to 0.5 μm and an average thickness of the oxygen-containing, iron-based layer is formed to be 0.1 to 0.6 μm.

3 . Method as claimed in claim 2 , characterised in that an average thickness of the pure iron layer is 0.1 to 0.4 μm and an average thickness of the oxygen-containing, iron-based layer is from 0.2 to 0.5 μm.

4 . Method as claimed in at least one of claims 1 to 3 , characterised in that the average thickness of the oxygen-containing, iron-based layer is greater than the average thickness of the pure iron layer.

5 . Method as claimed in at least one of claims 1 to 4 , characterised in that the oxygen-containing, iron-based layer with a proportion of oxygen of more than 5 to 40 wt. % is applied to the pure iron layer.

6 . Method as claimed in claim 5 , characterised in that the oxygen-containing, iron-based layer with a proportion of oxygen of more than 10 to 30 wt. %, advantageously more than 12 to 25 wt. %, is applied to the pure iron layer.

7 . Method as claimed in at least one of claims 1 to 6 , characterised in that the pure iron layer is deposited electrolytically or by deposition from the vapour phase and the oxygen-containing, iron-based layer is deposited electrolytically.

8 . Method as claimed in at least one of claims 1 to 7 , characterised in that the steel strip comprises the following composition in wt. %:
C: 0.03% to 0.35%,
Mn: 4.1% to 8.0%,
Si: 0.008% to 2.5%,
Al: 0.001% to 2.0%,
optionally
Cr: 0.01% to 0.7%,
B: 0.001% to 0.08%,
Ti: 0.005% to 0.3%,
V: 0.005% to 0.3%,
Nb: 0.005% to 0.2%,
Mo: 0.005% to 0.7%,
P:<_0.10%,
S:<_0.010%,
with the remainder being iron and unavoidable impurities.

9 . Method as claimed in at least one of claims 1 to 8 , characterised in that the annealing treatment is carried out in a radiant tube furnace as a continuous annealing furnace, at an annealing temperature of 550° C. to 880° C. and an average heating rate of 1 K/s to 100 K/s, with a reducing annealing atmosphere, consisting of 2 to 40% H 2 and 98 to 60% N2 and a dew point in the annealing furnace between +15 and −70° C. and a holding time of the steel strip at an annealing temperature between 30 s and 650 s with optional subsequent cooling to a holding temperature between 200° C. and 600° C. for up to 500 s with subsequent optional inductive heating to a temperature above the melting bath temperature of the metallic coat at 400° C. to 750° C. and subsequently hot-dip coating of the steel strip with the metallic coat is carried out.

10 . Method as claimed in at least one of claims 1 to 9 , characterised in that the ratio of the partial pressures of steam and hydrogen during the annealing in the radiant tube furnace is in the range of 0.00077>pH 2 O/pH 2 >0.00021, advantageously 0.00254>pH 2 O/pH 2 >0.00021.

11 . Method as claimed in at least one of claims 1 to 10 , characterised in that the following are used as metallic coats: aluminium-silicon (AS, AlSi), zinc (Z), zinc-aluminium (ZA, galfan), zinc-iron (ZF, galvannealed), zinc-aluminium-magnesium (ZM, ZAM) or aluminium-zinc (AZ, galvalume).

12 . Steel strip comprising, in addition to carbon, iron as a main constituent, an Mn content of 4.1 to 8.0 wt. % and optionally one or more of the alloy elements Al, Si, Cr, B, Ti, V, Nb and/or Mo with a metallic coat applied by means of hot-dipping, characterised in that, in the transition region between the metallic coat and the steel strip surface, a predominantly ferritic edge zone with more than 60 vol. % ferrite is formed, wherein the predominantly ferritic edge zone has a thickness of 0.15 to 1.1 gm and, as seen from the steel strip surface, consists of a pure iron layer with an average iron content of more than 96 wt. % and an oxygen-containing, iron-based layer containing more than 5 mass percent of oxygen thereon.

13 . Steel strip as claimed in claim 12 , characterised in that the predominantly ferritic edge zone has a thickness of between 0.3 and 0.9 μm.

14 . Steel strip as claimed in at least one of claims 12 and 13 , characterised by the following composition in wt. %:
C: 0.03% to 0.35%,
Mn: 4.1% to 8.0%,
Si: 0.008% to 2.5%,
Al: 0.001% to 2.0%,
optionally
Cr: 0.01% to 0.7%,
B: 0.001% to 0.08%,
Ti: 0.005% to 0.3%,
V: 0.005% to 0.3%,
Nb: 0.005% to 0.2%,
Mo: 0.005% to 0.7%,
P:<_0.10%,
S:<_0.010%,
with the remainder being iron and unavoidable impurities.

15 . Steel strip as claimed in at least one of claims 12 to 14 , characterised by a metallic coat consisting of aluminium-silicon (AS, AlSi), zinc (Z), zinc-aluminium (ZA), zinc-aluminium-iron (ZF/galvannealed), zinc-magnesium-aluminium (ZM, ZAM) or aluminium-zinc (AZ).

16 . Steel strip as claimed in claim 15 , characterised in that in the case of a metallic coat based on zinc, the zinc coat contains 0.1 to 1 wt. % Al.

17 . Steel strip as claimed in claim 15 , characterised in that in the case of a metallic coat based on zinc, the zinc coat contains 0.1 to 6 wt. % Al and 0.1 to 6 wt. % Mg.

18 . Steel strip as claimed in claim 15 , characterised in that in the case of a metallic coat based on zinc, the zinc coat contains 5 to 15 wt. % Fe.

19 . Use of a steel strip produced according to at least one of claims 1 to 11 or of a steel strip according to at least one of claims 12 to 18 for the production of parts for motor vehicles.

Assignees

• Salzgitter Flachstahl Gmbh

Inventors

• Köhler Kai
• Köpper Nils
• Luther Friedrich
• Debeaux Marc

Classifications

• C21D8/02

  during manufacturing of plates or strips (C21D8/12 takes precedence) · CPC title

• C21D6/005

  containing Mn · CPC title

• C23C28/023 Primary

  only coatings of metal elements only · CPC title

• C22C38/38

  with more than 1.5% by weight of manganese · CPC title

• C21D1/70

  while heating or quenching · CPC title