Description
HIGH MN STEEL SHEET FOR HIGH CORROSION
RESISTANCE AND METHOD OF MANUFACTURING
GALVANIZING THE STEEL SHEET
Technical Field
! 1 ] The present invention relates to a high-manganese hot-dip coated steel sheet mainly
used for automobiles and a method of manufacturing the same, and, more particularly, to a high-manganese hot-dip coated steel sheet exhibiting high corrosion resistance and high workability as well as high ductility and high strength and a method of manufacturing the same.
[2]
Background Art
[3] Recently, regulation has been deepened on waste gas in automobiles due to the
exhaustion of fossil fuels and environmental-related problems. As a result, the reduction of weight in automobile bodies to increase automobile fuel efficiency has come to the fore.
[4]
[5] Various kinds of high-strength steel sheets have been developed to accomplish the
reduction in weight of automobile bodies. As the strength of the steel sheets increases, however, the steel sheets exhibit low ductility. As a result, the use of the steel sheets to be formed as automobile parts is limited.
[6]
[7J Much research has been carried out to epochally solve the reduction in ductility of
the high-strength steel sheets. As a result, technologies have been proposed for adding 7 to 35 weight % of Mn to a steel, such that the twin deformation of the steel is maintained when the steel is plastically deformed, thereby considerably improving the ductility of the steel while maintaining high strength of the steel (WO93/013233, JP1992-259325, WO99/001585, and WO02/101109).
[8]
[9] Meanwhile, Mn is an element exhibiting high ionization tendency. For this reason, a
steel having a high Mn content corrodes more rapidly than general steels.
[10] In order that a steel having a high Mn content may be applied to automobiles,
therefore, it is necessary to treat the steel such that the steel has high corrosion resistance.
[11] However, development has not taken place on an optimum coating layer and a hot-
dip coating process for providing a high-manganese steel containing approximately 5

to 35 weight % of Mn with high corrosion resistance. [12]
Disclosure of Invention Technical Problem
[13] It is an aspect of the present invention to provide a high-manganese hot-dip coated
steel sheet exhibiting high corrosion resistance and high workability as well as high ductility and high strength.
[14] It is another aspect of the present invention to provide a method of hot-dip coating a
high-manganese steel sheet to manufacture a high-manganese hot-dip coated steel sheet exhibiting high corrosion resistance and high workability as well as high ductility and high strength.
[15]
Technical Solution
[ 16] In accordance with one aspect, the present invention provides a high-manganese
hot-dip coated steel sheet exhibiting high corrosion resistance, comprising: a substrate steel sheet having a composition of (in weight %) 0.1 to 1.5 % of C, 5 to 35 % of Mn, and the remainder including Fe and other unavoidable impurities; and a hot-dip zinc coating layer formed on the substrate steel sheet, the hot-dip zinc coating layer containing only Zn, or an alloying hot-dip coating layer formed on the substrate steel sheet, the alloying hot-dip coating layer having a composition of (in weight %) 0.1 to 10 % of Mn, 5 to 15 % of Fe, and the remainder including Zn and other unavoidable impurities.
[17]
[18] In accordance with another aspect, the present invention provides a method of man-
ufacturing a high-manganese hot-dip coated steel sheet exhibiting high corrosion resistance, comprising: depositing a substrate steel sheet having a composition of (in weight %) 0.1 to 1.5 % of C, 5 to 35 % of Mn, and the remainder including Fe and other unavoidable impurities in a hot-dip zinc coating bath to form a hot-dip zinc coating layer at a surface of the substrate steel sheet.
[19]
[20] In accordance* with a further aspect, the present invention provides a method of
manufacturing a high-manganese hot-dip coated steel sheet exhibiting high corrosion resistance, comprising: depositing a substrate steel sheet having a composition of (in weight %) 0.1 to 1.5 % of C, 5 to 35 % of Mn, and the remainder including Fe and other unavoidable impurities in a hot-dip zinc coating bath to form a hot-dip zinc coating layer at a surface of the substrate steel sheet; and alloying heat-treating the subsu-ate steel sheet to form an alloying hot-dip coating layer, having a composition of

(in weight %) 0.1 to 10 % of Mn, 5 to 15 % of Fe, and the remainder including Zn and other unavoidable impurities, at the surface of the substrate steel sheet.
[21]
Advantageous Effects
[22] According to the present invention, a hot-dip zinc coating layer or an alloying hot-
dip coating layer of Zn-Fe-Mn is formed on a high-manganese steel sheet. Consequently, the present invention has the effect of providing a hot-dip coated steel sheet superior in corrosion resistance to a conventional hot-dip coated steel sheet while exhibiting high ductility and high strength.
[23]
Best Mode for Carding Out the Invention
[24] Hereinafter, the present invention will be described in detail.
[25] First, a substrate steel sheet for a hot-dip coated steel sheet according to the present
invention will be described.
[26] As a substrate steel sheet according to the present invention, a hot rolled steel sheet
or a cold rolled steel sheet may be used containing (in weight %) 0.1 to 1.5 % of C, 5 to 35 % of Mn, and the remainder including Fe and other unavoidable impurities (a first substrate steel sheet).
[27] 0.01 to 3 % of Al may be further added to the first substrate steel sheet (a second
substrate steel sheet).
[28]
[29] Also, one or more elements selected from a group consisting of less than 3 % of Si,
less than 9 % of Cr, less than 5 % of Cu, less than 4 % of Ni, less than 1 % of Mo, less than 1 % of Nb, less than 0.5 % of V, and less than 0.04 % of N may be further added to the first substrate steel sheet or the second substrate steel sheet (a third substrate steel sheet).
[30] Also, one or more elements selected from a group consisting of 0.005 to 0.05 % of
Sn, 0.005 to 0.05 % of Sb, 0.005 to 0.05 % of As, and 0.005 to 0.05 % of Te may be further added to the first substrate steel sheet, the second substrate steel sheet, or the third substrate steel sheet (a fourth substrate steel sheet).
[31] i
[32] Also, one or more elements selected from a group consisting of 0.0005 to 0.040 %
of B, 0.0005 to 0.1 % of Zr, 0.0005 to 0.1 % of Ti, 0.0005 to 0.040 % of La, 0.0005 to 0.040 % of Ce, and 0.0005 to 0.030 % of Ca may be further added to the first substrate steel sheet, the second substrate steel sheet, the third substrate steel sheet, or the fourth substrate steel sheet (a fifth substrate steel sheet).
The reason to select elements for the substrate steel sheet and to restrict the content

range of the elements will be described hereinafter in detail. [34]
[35] Carbon (C) is an element for stabilizing austenite phase.
[36] It is advantageous to increase the content of C. Preferably, more than 0.1 % of C is
added to accomplish the addition effect thereof.
[37] When the content of C exceeds 1.5 %, however, the. stability of the austenite phase
greatly increases with the result that the transition of deformation behavior occurs due
to slip deformation, and therefore, the workability of the substrate steel sheet
decreases.
[38] Consequently, it is preferable to restrict the upper limit content of C to 1.5 %.
[39]
[40] Manganese (Mn) is an indispensable element for stabilizing austenite phase. In
addition, Mn is an important element for acting as a Mn source for a coating layer
during alloying heat treatment after the completion of a coating process.
[41] When the content of Mn is less than 5 %, the diffusion of Mn from the substrate
steel sheet to the coating layer suddenly decreases during alloying heat treatment after
the completion of a coating process. For this reason, it is preferable to add more than 5
% of Mn.
[42] When the content of Mn exceeds 35 %, on the other hand, high-temperature
oxidation rapidly progresses at the surface of a steel due to a large quantity of Mn
during a reheating process for hot rolling the steel. As a result, the surface quality of
final products is deteriorated. Furthermore, the excessive addition of Mn increases the
manufacturing costs of the steel. For this reason, it is preferable to limit the content of
Mn to less than 35 %.
[43]
[44] Generally, aluminum (Al) is added to deoxidize a steel. In the present invention,
however, Al is added to improve the ductility of the steel.
[45] Specifically, Al is an element for stabilizing ferrite phase. In addition, Al increases
stacking fault energy at a slip plane of the steel to restrain the creation of e-martensite
phase and thus to improve the ductility of the steel.
[46]
[47] Furthermore, Al restrains the creation of £-martensite phase even when the content
of Mn is small. Consequently, Al contributes greatly to the minimization of Mn
content and the improvement of the workability of the steel.
[48] When the content of Al is less than 0.01 %, £-martensite is created, whereby the
strength of the steel increases; however, the ductility of the steel suddenly decreases.
For this reason, it is preferable to add more than 0.01 % of Al.

[50] When the content of Al exceeds 3 %, however, twin generation is restrained with
the result that the ductility of the steel decreases. In addition, the castability of the steel decreases during continuous casting, and surface oxidation excessively occurs during hot rolling. As a result, the surface quality of the products deteriorates. For this reason, it is preferable to restrict the upper limit content of Al to 3.0 %.
151] When silicon (Si) is excessively added, a silicon oxide layer is formed on the
surface of a steel with the result that the hot-dip coatability of the steel decreases.
[52] When an appropriate amount of Si is added to a steel containing a large amount of
Mn, however, a thin Si oxide layer is formed at the surface of the steel with the result that oxidation of the steel is restrained in the air. Consequently, a thick Mn oxide layer is prevented from being formed on the surface of a cold rolled steel sheet after rolling. In addition, corrosion of the cold rolled steel sheet is prevented after annealing. Consequently, it is possible to maintain superior surface quality of the cold rolled steel sheet.
[53]
[54] Furthermore, the creation of the thick Mn oxide layer is restrained during hot-dip
coating, the hot-dip coating characteristics are greatly improved. Besides, the tensile strength and the elongation of a steel increase.
[55] When the content of Si increases, however, Si oxide is formed at the surface of a
steel sheet during hot rolling with the result that pickling efficiency decreases, and therefore, the surface quality of the hot-rolled steel sheet is deteriorated.
[56]
[57] Also, Si is enriched at the surface of the steel sheet during high-temperature
annealing in a continuous annealing process and a continuous hot-dip coating process. Consequently, wettability of molten zinc to the surface of the steel sheet decreases during hot-dip coating. As a result, the coatability of the steel sheet decreases. In addition, the excessive addition of Si greatly decreases the weldability of the steel sheet.
[58] For this reason, the upper limit content of Si is limited to 3 %.
[59] Similarly to Si, chromium (Cr) is an element for forming a passive-state film in the
air to restrain th