High manganese steel for low temperature applications having excellent surface quality and a manufacturing method thereof

1. A high manganese steel for low temperature applications, comprising:

0.3 wt % to 0.8 wt % of C, 18 wt % to 26 wt % of Mn, 0.01 wt % to 1 wt % of Si, 0.01 wt % to 0.5 wt % of Al, 0.1 wt % or less of Ti (excluding 0%), 1 wt % to 4.5 wt % of Cr, 0.1 wt % to 0.9 wt % of Cu, 0.03 wt % or less of S (excluding 0%), 0.3 wt % or less of P (excluding 0%), 0.001 wt % to 0.03 wt % of N, 0.004 wt % or less of B (excluding 0%), and a remainder of Fe and other inevitable impurities,

wherein a microstructure comprises an austenite single phase structure,

an average grain size of the austenite is 50 μm or less, and

a number of an austenite grain having a grain size of 50 μm or more is less than 1 per Cm 2 .

2. The high manganese steel of claim 1 , wherein an average grain size of the austenite structure is 20 μm to 30 μm.

3. The high manganese steel of claim 1 , wherein, in the austenite structure, a number of austenite grains having a grain size of 30 μm or more is less than 1 per cm 2 .

4. The high manganese steel of claim 1 , wherein the high manganese steel has rolling direction impact toughness of 100 J or higher at −196° C.

5. The high manganese steel of claim 1 , wherein the high manganese steel has an anisotropy index of 0.6 or higher, wherein the anisotropy index is a ratio of thickness direction impact toughness at −196° C. to rolling direction impact toughness at −196° C.

6. The high manganese steel of claim 1 , wherein the high manganese steel has yield strength of 400 MPa or higher.

7. The high manganese steel of claim 1 , wherein the high manganese steel is manufactured by a manufacturing method comprising preparing a slab having the composition of claim 1 , reheating the slab and hot rolling the reheated slab,

wherein a recrystallization structure having less than 1 grain having a grain size of 150 μm or more is formed per cm 2 on a surface layer portion (a region of the slab surface layer portion up to 2 mm from the surface in a slab thickness direction) of the slab before reheating.

8. The high manganese steel of claim 7 , wherein an average grain size of the surface layer portion of the slab before reheating is 100 μm or less.

9. The high manganese steel of claim 7 , wherein the slab before reheating has a cross-section reduction rate of at least 60% at 1100° C.

10. The high manganese steel of claim 1 , wherein the high manganese steel has a thickness of 8.0 mm to 40 mm.

11. A method of manufacturing a high manganese steel for low temperature applications according to claim 1 , the method comprising:

preparing a slab comprising 0.3 wt % to 0.8 wt % of C, 18 wt % to 26 wt % of Mn, 0.01 wt % to 1 wt % of Si, 0.01 wt % to 0.5 wt % of Al, 0.1 wt % or less of Ti (excluding 0%), 1 wt % to 4.5 wt % of Cr, 0.1 wt % to 0.9 wt % of Cu, 0.03 wt % or less of S (excluding 0%), 0.3 wt % or less of P (excluding 0%), 0.001 wt % to 0.03 wt % of N, 0.004 wt % or less of B (excluding 0%), and a remainder of Fe and other inevitable impurities;

deformation application involving applying a deformation to the slab such that a recrystallization microstructure is formed on a surface layer portion of the slab;

air cooling involving air-cooling the slab on which the recrystallization microstructure is formed on the surface layer portion thereof to room temperature;

reheating involving heating the air-cooled slab to 1100° C. to 1250° C.;

hot rolling involving finish-rolling the reheated slab at 850° C. to 950° C. to obtain a hot-rolled steel; and

accelerated cooling involving accelerated-cooling the hot-rolled steel at a cooling speed of 10° C./sec or more to an accelerated cooling termination temperature of 600° C. or less.

12. The method of claim 11 , wherein the deformation application is performed by rough rolling under a high reduction condition at 1000° C. to 1200° C.

13. The method of claim 11 , wherein the deformation application is performed by high temperature forging at 1000° C. to 1200° C.

14. The method of claim 11 , wherein the deformation application is performed such that a number of grains having a grain size of at least 150 μm on the surface layer portion (a region of the slab surface layer portion up to 2 mm from the surface in a slab thickness direction) is less than 1 per cm 2 by rough rolling under a high reduction condition at 1000° C. to 1200° C.

15. The method of claim 11 , wherein an average grain size of the surface layer portion of the slab after the deformation application is 100 μm or less.

16. The method of claim 11 , wherein the deformation application is performed such that a thickness reduction rate is 15% to 50% for an initial slab.

17. The method of claim 11 , wherein, in the hot rolling, a final pass rolling temperature during hot finish rolling is 850° C. or above and less than 900° C. when a final thickness of the steel is 18 mm or above, and a final pass rolling temperature during hot finish rolling is 900° C. to 950° C. when a final thickness of the steel is less than 18 mm.

18. The method of claim 11 , wherein, in the hot rolling, a reduction ratio is 40% or more of a total reduction rate at a temperature below a non-recrystallization temperature (Tnr) when a final thickness of the steel is 18 mm or above.

19. The method of claim 11 , wherein the hot-rolled steel has a thickness of 8 mm to 40 mm.