SUPERPLASTIC MEDIUM MANGANESE STEEL AND METHOD OF PRODUING THE SAME
A superplastic medium manganese steel according to the present invention preferably has a composition containing 4 to 8 wt. % of manganese (Mn) and 3 wt. % or less (excluding 0 wt. %) of aluminum (Al), with the remainder being iron (Fe) and inevitable impurities. In another embodiment, a superplastic medium manganese steel according to the present invention preferably has a composition containing 4 to 8 wt. % of manganese (Mn) and 3 wt. % or less (excluding 0 wt. %) of silicon (Si), with the remainder being iron (Fe) and inevitable impurities.
1 . A superplastic medium manganese steel having a composition containing 4 to 8 wt. % of manganese (Mn) and 3 wt. % or less (excluding 0 wt. %) of aluminum (Al), with the remainder being iron (Fe) and inevitable impurities.
2 . A superplastic medium manganese steel having a composition containing 4 to 8 wt. % of manganese (Mn) and 3 wt. % or less (excluding 0 wt. %) of silicon (Si), with the remainder being iron (Fe) and inevitable impurities.
3 . The superplastic medium manganese steel of claim 1 , wherein the composition further contains 0.2 wt. % or less (excluding 0 wt. %) of niobium (Nb).
4 . The superplastic medium manganese steel of claim 1 , wherein the composition further contains 0.03 wt. % or less (excluding 0 wt. %) of boron (B).
5 . The superplastic medium manganese steel of claim 1 , wherein the composition further contains 0.2 wt. % or less (excluding 0 wt. %) of carbon (C).
6 . The superplastic medium manganese steel of claim 3 , wherein the composition further contains 0.2 wt. % or less (excluding 0 wt. %) of carbon (C).
7 . The superplastic medium manganese steel of claim 4 , wherein the composition further contains 0.2 wt. % or less (excluding 0 wt. %) of carbon (C).
8 . The superplastic medium manganese steel of claim 1 , wherein the medium manganese steel is annealed in a temperature range of a ferrite-austenite dual-phase region to form ferrite and austenite.
9 . The superplastic medium manganese steel of claim 8 , wherein the temperature range of the ferrite-austenite dual-phase region ranges from 600 to 900° C.
10 . The superplastic medium manganese steel of claim 8 , wherein grains of the ferrite and austenite formed in the temperature range of the ferrite-austenite dual-phase region have an average grain size of 2 μm or less.
11 . A method of producing a superplastic medium manganese steel, the method comprising the steps of:
(S1) melting either a medium manganese steel having a composition containing 4 to 8 wt. % of manganese (Mn) and 3 wt. % or less (excluding 0 wt. %) of aluminum (Al), with the remainder being iron (Fe) and inevitable impurities, or a medium manganese steel having a composition containing 4 to 8 wt. % of manganese (Mn) and 3 wt. % or less (excluding 0 wt. %) of silicon (Si), with the remainder being iron (Fe) and inevitable impurities, and then homogenizing the medium manganese steel;
(S2) hot-rolling the homogenized medium manganese steel;
(S3) cooling the hot-rolled steel;
(S4) cold-rolling the cooled steel; and
(S5) annealing the cold-rolled steel at a predetermined elevated temperature.
12 . The method of claim 11 , wherein a temperature for the homogenizing in step (S1) is 1200° C., and the melting in step (S1) is performed at a temperature equal to or higher than the temperature for the homogenizing.
13 . The method of claim 11 , wherein the hot rolling in step (S2) is performed at a temperature in a range of 1000 to 1200° C.
14 . The method of claim 11 , wherein step (S3) is performed by at least one cooling method selected from among water quenching, oil quenching and air cooling.
15 . The method of claim 11 , wherein the cold rolling in step (S4) is performed at a reduction ratio of 90% or less (excluding 0%).
16 . The method of claim 15 , wherein the reduction ratio ranges from 60 to 80%.
17 . The method of claim 11 , wherein the cold rolling in step (S4) is performed at room temperature.
18 . The method of claim 11 , wherein a dual phase formed in step (S5) is ferrite and austenite.
19 . The method of claim 18 , wherein a temperature for the annealing in step (S5) is in a temperature range of a ferrite-austenite dual-phase region.
20 . The method of claim 19 , wherein the temperature range of the ferrite-austenite dual-phase region ranges from 600 to 900° C.
21 . The method of claim 19 , wherein each of the ferrite and the austenite has an average grain size of 2 μm.
22 . The superplastic medium manganese steel of claim 2 , wherein the composition further contains 0.2 wt. % or less (excluding 0 wt. %) of niobium (Nb).
23 . The superplastic medium manganese steel of claim 2 , wherein the composition further contains 0.03 wt. % or less (excluding 0 wt. %) of boron (B).
24 . The superplastic medium manganese steel of claim 2 , wherein the composition further contains 0.2 wt. % or less (excluding 0 wt. %) of carbon (C).
25 . The superplastic medium manganese steel of claim 22 , wherein the composition further contains 0.2 wt. % or less (excluding 0 wt. %) of carbon (C).
26 . The superplastic medium manganese steel of claim 23 , wherein the composition further contains 0.2 wt. % or less (excluding 0 wt. %) of carbon (C).
27 . The superplastic medium manganese steel of claim 2 , wherein the medium manganese steel is annealed in a temperature range of a ferrite-austenite dual-phase region to form ferrite and austenite.
28 . The superplastic medium manganese steel of claim 27 , wherein the temperature range of the ferrite-austenite dual-phase region ranges from 600 to 900° C.
29 . The superplastic medium manganese steel of claim 27 , wherein grains of the ferrite and austenite formed in the temperature range of the ferrite-austenite dual-phase region have an average grain size of 2 μm or less.