Zinc Coated Mn-containing advanced high strength steel and method of manufacturing the same

A galvanized steel sheet, made of a steel base metal having a composition comprising, by weight percent:

• 0.08≤C≤0.3%
• 3.0≤Mn≤6.0%
• 0.5≤Si≤2.5%
• 0.003≤Al≤2.0%
• 0.01≤Mo≤0.5%
• 0.01≤Ti≤0.1%
• 0.01≤Nb≤0.08%
• 0.0002≤B≤0.005
• Cr≤1.0%
• S≤0.010%
• P≤0.025%
• N≤0.008%
• and inevitable impurities from the manufacturing process, a remainder being iron, the steel sheet including, as measured from an interface between the base metal and the galvanized plated layer:
• a) a decarburized layer starting at the interface with the plated layer wherein the carbon content is below 0.1 weight percent at a depth of 20 μm;
• b) the decarburized layer including a ferrite sublayer starting at the interface with the plated layer and having a depth of 3 μm or more, wherein the ferrite content is above 70% in terms of cross-sectional area fraction,
• c) the ferrite sublayer including an internal oxidation selective zone starting from the interface with the plated layer and having a depth of at least 2.5 μm and containing manganese oxides, silicon oxides and optionally aluminum oxides.

A method for producing a galvanized steel, made of a steel base metal having a composition comprising, by weight percent:

• 0.08≤C≤0.3%
• 3.0≤Mn≤6.0%
• 0.5≤Si≤2.5%
• 0.003≤Al≤2.0%
• 0.01≤Mo≤0.5%
• 0.01≤Ti≤0.1%
• 0.01≤Nb≤0.08%
• 0.0002≤B≤0.005
• Cr≤1.0%
• S≤0.010%
• P≤0.025%
• N≤0.008%
• and inevitable impurities from the manufacturing process, a remainder being iron, the method comprising the following steps of annealing heat treatment cycle before galvanizing:
• i. a pre-heating step in a direct fired furnace section, an oxidizer/fuel ratio being 1.00 or more;
• ii. a heating step in a radiant tube heating section up to a temperature of soaking of at least Ae3-10° C., Ae3 being determined by dilatometry, with a dew point of at least 0° C., under a reducing atmosphere containing at least 1% of hydrogen in volume, a balance being nitrogen;
• iii. a soaking step in a radiant tube soaking section at the temperature of soaking with a dew point of at least 0° C. and under a reducing atmosphere containing at least 1% of hydrogen in volume, the balance being nitrogen;
• iv. a cooling step with a dew point lower than −20° C. and under a reducing atmosphere containing at least 1% of hydrogen in volume, a balance being nitrogen;
• v. an overageing or a partitioning step;
• and
• vi. a galvanizing step.