NON-MAGNETIC HIGH MANGANESE STEEL SHEET WITH HIGH STRENGTH AND MANUFACTURING METHOD THEREOF

Abstract

The present invention relates to a non-magnetic high manganese steel sheet with high-strength, which has superior strength and moldability, and at the same time, can obtain superior non-magnetic characteristics, and a method for manufacturing the same.

Description

TECHNICAL FIELD

The present disclosure relates to a non-magnetic high manganese steel sheet having a high degree of strength for use as a material for heavy electrical machinery such as switchboards and transformers.

BACKGROUND ART

In general, materials for equipment such as switchboards and transformers are required to have high degrees of strength as well as good non-magnetic properties.

In the related art, stainless steel having high nickel and chromium contents and satisfying the requirements for high strength and non-magnetivity is used in such applications. However, such stainless steel is expensive and may not have sufficient strength.

Ferritic or martensitic stainless steel may be used as alternatives to satisfy the requirement for high strength. However, ferritic and martensitic stainless steels have high-quality magnetic properties that cause eddy currents and thus the loss of electrical currents. In addition, ferritic or martensitic stainless steel is very expensive.

Therefore, materials that are free of the limitations of such stainless steels while having high strength and non-magnetic properties are required.

DISCLOSURE

Technical Problem

Aspects of the present disclosure may provide a non-magnetic high manganese steel sheet having high degrees of strength and formability and good non-magnetic properties, and a method of manufacturing the steel sheet.

Technical Solution

Advantageous Effects

According to the present disclosure, a high manganese steel sheet having high austenite stability and non-magnetic properties is provided. Aluminum (Al) is added to the steel sheet to prevent carbon from forming carbides and to thus further increase the stability of austenite. Therefore, the steel sheet has a high degree of formability as well as a high degree of strength. The steel sheet has a sufficient degree of rigidity and thus can be used to form a structural member of a large transformer.

DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B are microstructure images of Inventive Example 1-7 and Comparative Example 1-4, respectively.

FIG. 2 is an XRD graph in which curves A and B show phase-stability measurement results of Inventive Steel 2-1 and Comparative Steel 2-1, respectively.

FIGS. 3A and 3B are microstructure images of Inventive Steel 2-1 and Comparative Steel 2-1, respectively.

BEST MODE

Eddy current loss occurring when a material is placed in a magnetic field is closely related to the magnetic properties of the material. More eddy current is generated in a material having better magnetic properties, and thus more eddy current loss is generated. In general, the magnetism of a material is proportional to the permeability (μ) of the material. That is, the higher the permeability, the higher the magnetism. Permeability is defined as μ=B/H where H denotes a magnetic field and B denotes an induced magnetic field. That is, if the permeability of a material is reduced, the magnetism of the material is reduced, and thus when the material is placed in a magnetic field, loss caused by eddy currents in the surface of the material may be reduced to increase energy efficiency. Therefore, if non-magnetic steel sheets are used as materials for electric equipment such as switchboards or transformers, energy loss may be reduced.

The inventors have conducted in-depth research and have invented a high manganese steel having a high degree of strength and good non-magnetic properties by adding manganese (Mn) and carbon (C) to improve the stability of austenite. According to embodiments of the present disclosure, steel sheets having good non-magnetic properties as well as high degrees of strength and elongation (formability) are provided by controlling the contents of carbon and manganese to improve the phase stability of austenite, and adding aluminum to suppress the formation of deformation-induced ε-martensite and the generation of dislocation-induced slip deformation.

The embodiments of the present disclosure will now be described in detail. First, steel sheet will now be described in detail according to an embodiment of the present disclosure. The steel sheet of the embodiment has the following composition (hereinafter, % refers to weight %).

Carbon (C): 0.4% to 0.9%

Carbon (C) is an element for forming austenite in steel. It may be preferable that the content of carbon (C) in the steel sheet be 0.4% or greater. However, if the content of carbon (C) is greater than 0.9%, carbides may excessively precipitate to worsen the non-magnetic properties and castability of the steel sheet. Therefore, it may be preferable that the content of carbon (C) in the steel sheet be within the range of 0.4% to 0.9%.

Manganese (Mn): 10% to 25%

Manganese (Mn) is a key element for stabilizing austenite. In the embodiment of the present disclosure, the content of manganese (Mn) in the steel sheet is 10% or greater. If the content of manganese (Mn) is less than 10%, α′-martensite may be formed to worsen the non-magnetic properties of the steel sheet. On the other hand, if the content of manganese (Mn) is greater than 25%, the manufacturing costs of the steel sheet may be markedly increased, and oxidation may be markedly increased in the steel sheet to worsen the surface quality of the steel sheet when the steel sheet is heated in a hot-rolling process. Therefore, it may be preferable that the content of manganese (Mn) be within the range of 10% to 25%.

Aluminum (Al): 0.01% to 8.0%

Aluminum (Al) is an element effective in preventing the formation of carbides and controlling the fraction of twins for improving formability, in the embodiment of the present disclosure, since carbon (C) is dissolved to stabilize austenite, aluminum (Al) is used as a key element for preventing the formation of carbides and thus improving non-magnetic properties. To this end, the content of aluminum (Al) is set to be 0.01% or greater. However, if the content of aluminum (Al) is greater than 8.0%, the manufacturing cost of the steel sheet may be increased, and oxides may be excessively formed to worsen the quality of the steel sheet. Therefore, it may be preferable that the content of aluminum (Al) be within the range of 0.01% to 6.0%.

Silicon (Si): 0.01% to 2.0%

Silicon (Si) is an element having no significant influence on stacking fault energy. Silicon (Si) is generally used as a deoxidizer, and about 0.01% of silicon (Si) is included in steel in a general steel making process. Since excessive costs are incurred in removing silicon (Si), the content of silicon. (Si) in the steel sheet may be about 0.01%. in addition, if the content of silicon (Si) exceeds 2.0%, manufacturing costs are increased, and oxides are excessively generated to worsen the surface quality of the steel sheet. Therefore, it may be preferable that the content of silicon in the steel sheet be within the range of 0.01% to 2.0%.

Titanium (Ti): 0.05% to 0.2%

Titanium (Ti) is an element reacting with nitrogen in the steel sheet to precipitate nitrides and facilitate the formation of twins. Titanium (Ti) is added to the steel sheet to improve the strength and formability of the steel sheet. In addition, titanium (Ti) improves the strength of the steel sheet by forming precipitates. To this end, it may be preferable that the content of titanium (Ti) be 0.05% or greater. However, if the content of titanium (Ti) is greater than 0.2%, precipitates may be excessively formed to generate cracks in the steel sheet during a cold-rolling process and thus to worsen the formability and weldability of the steel sheet. Therefore, it may be preferable that the content of titanium (Ti) be within the range of 0.05% to 0.2%.

Boron (B): 0.0005% to 0.005%

A low content of Boron (B) enhances the grain boundaries of a slab, and thus it may be preferable that the content of boron (B) be 0.0005% or greater. However, if the content of boron (B) is excessive, manufacturing costs may be increased, and thus it may be preferable that the content of boron (B) be within the range of 0.0005% to 0.05%.

Sulfur (5): 0.05% or less (excluding 0%)

The content of sulfur (5) may be adjusted to be 0.05% or less for controlling the amounts of inclusions. if the content of sulfur (5) in the steel sheet is greater than 0.05%, the steel sheet may exhibit hot brittleness, and thus it may be preferable that the upper limit of the content of sulfur (5) be set to be 0.05%.

Phosphorus (P): 0.8% or less (excluding 0%)

Phosphorus (P) easily segregates and leads to cracks during a casting process. Therefore, it may be preferable that the content of phosphorus (F) be set to be 0.8% or less. If the content of phosphorus (F) in the steel sheet is greater than 0.8%, the castability of the steel sheet may deteriorate, and thus it may be preferable that the upper limit of the content of phosphorus (F) be 0.08%.

Nitrogen (N): 0.003% to 0.01%

Nitrogen is inevitably included in the steel sheet because of a reaction with air during a steel making process. Excessive manufacturing costs may be incurred to reduce the content of nitrogen (N) to lower than 0.003%, and if the content of nitrogen (N) exceeds 0.01%, nitrides may be formed to worsen the formability of the steel sheet. Therefore, it may be preferable that the content of nitrogen (N) be within the range of 0.003% to 0.01%.

The steel sheet may include iron (Fe) and inevitable impurities as the remainder of constituents.

In the embodiment of the present disclosure, it may be preferable that the microstructure of the steel sheet have 1 volume % or less of carbides. In the embodiment of the present disclosure, carbon (C) may be dissolved in the steel sheet in an atomic state to stabilize austenite. That is, if carbon (C) is present in the steel sheet in the form of carbides, the stability of austenite of the steel sheet may be decreased, and the permeability of the steel sheet may be increased to worsen non-magnetic properties of the steel sheet. Therefore, it may be preferable that the steel sheet have a low content of carbides, for example, 1 volume % or less. Particularly, it may be preferable that the content of carbides in the steel sheet be 1 volume % or less even after a heat treatment. The heat treatment includes a heat treatment during a manufacturing process of the steel sheet and a heat treatment during the use of the steel sheet.

In the embodiment of the present disclosure, the steel sheet has austenite in the microstructure thereof, and although energy such as heat is applied to the steel sheet, the steel sheet may maintain the austenite component thereof and thus retain non-magnetic properties. That is, in the embodiment of the present disclosure, the steel sheet may have austenite and a low content of carbides (1 volume % or less) according to heat-treatment conditions.

In the embodiment of the present disclosure, when the content of aluminum (Al) in the steel sheet is within the range of 1.3% to 8.0%, it may be preferable that the stacking fault energy (SFE) of the steel sheet be 30 mJ/cm² or greater. The term “stacking fault energy” refers to energy in an interface between partial dislocations. In the embodiment of the present disclosure, the stacking fault energy of the steel sheet is controlled by adjusting the content of aluminum (Al), and by this the phase stability of austenite is improved.

If the stacking fault energy of the steel sheet is appropriate, dislocations and twins in the steel sheet may be harmoniously formed, and thus the phase stability of the steel sheet may be improved. However, if the stacking fault energy is too low, immobile dislocations may be formed to lower the phase stability of the steel sheet, and if the stacking fault energy of the steel sheet is too high, deformation of the steel sheet proceeds only in the form of dislocations to result in the strength of the steel sheet. Therefore, in the embodiment of the present disclosure, an optimal range of stacking fault energy of the steel sheet is proposed so that the steel sheet is provided with appropriate strength and phase stability.

If the stacking fault energy of the steel sheet is lower than 30 mJ/cm², twins may be generated, and thus the strength of the steel sheet may be increased. In this case, however, ε-martensite is formed in the steel sheet. Although ε-martensite has a hexagonal closed packed structure and non-magnetic properties, ε-martensite may be easily transformed into α-martensite. Therefore, for the steel sheet to maintain non-magnetic properties and have a high degree of strength by the formation of twins, it may be preferable that the stacking fault energy of the steel sheet be 30 mJ/cm² or greater.

The stacking fault energy of the steel sheet may be measured by various methods such as X-ray measurement methods, transmission electron microscope methods, and thermodynamic calculation methods. For example, a thermodynamic calculation method using thermodynamic data that is easy and effective in reflecting the effects of components may be used to measure the stacking fault energy of the steel sheet.

In the embodiment of the present disclosure, the steel sheet may have a tensile strength of 800 MPa or greater and an elongation of 15% or greater. That is, the steel sheet may have high degrees of strength and formability.

Hereinafter, a method of manufacturing the steel sheet will be described in detail according to an embodiment of the present disclosure.

A steel slab having the above-described composition is reheated to 1100° C. to 1250° C. If the reheating temperature is too low, an excessive load may be applied to the steel slab during a hot-rolling process. Therefore, it may be preferable that the reheating temperature be 1100° C. or higher. If the reheating temperature is high, hot-rolling may be easily performed. However, since steel having a high content of manganese (Mn) usually undergoes excessive internal oxidation and deterioration in surface quality, it may be preferable that the upper limit of the reheating temperature of the steel slab be 1250° C.

After the reheating process, the steel slab is hot-rolled, and then finish-rolled at a temperature range of 800° C. to 1000° C. so as to form a hot-rolled steel sheet. If the finish rolling (finish hot rolling) is performed at a high temperature, the steel slab may be easily finish-rolled because of low resistance to deformation, but the surface quality of the steel sheet may deteriorate. Therefore, it may be preferable that the finish rolling he performed at 1000° C. or lower. On the other hand, if the finish rolling is performed at a too low temperature, an excessive load may be applied to the steel slab. Therefore, it may be preferable that the finish rolling be performed at 800° C. or higher.

After the hot rolling process, the steel sheet is coiled. The steel sheet may be coiled within the temperature range of 400° C. to 700° C. After the coiling process, generally, the steel sheet may be cooled at a low cooling rate. A large amount of cooling water may be used to start the coiling process at a low temperature, and in this case, an excessive load may be applied to the steel sheet during cooling. Therefore, the coiling start temperature may be set to be 400° C. or higher. If the coiling temperature of the steel sheet is too high, an oxide film formed on the steel sheet may react with the matrix of the steel sheet, and thus the steel sheet may not be easily treated in a later pickling process. Therefore, it may be preferable that the coiling temperature be 700° C. or lower.

Between the hot rolling process and the coiling process, the steel sheet may be water-cooled.

The steel sheet hot-rolled as described above is cold-rolled to form a cold-rolled steel sheet. Generally, the reduction ratio of the steel sheet in the cold-rolling process may be determined by the thickness of a final product. In the embodiment of the present disclosure, since recrystallization occurs in the steel sheet during a heat treatment process after the cold-rolling process, a force inducing recrystallization may be appropriately controlled. In detail, if the reduction ratio of the steel sheet in the cold-rolling process is too low, the strength of the steel sheet may be lowered, and thus the reduction ratio may be set to be 30% or higher. On the other hand, if the reduction ratio is too high, the strength of the steel sheet may be increased, but a heavy load may be applied to a rolling mill. Thus, it may be preferable that the reduction ratio be 60% or lower.

After the cold-rolling process, a continuous annealing process is performed. It may be preferable that continuous annealing process be performed within the temperature range of 650° C. to 900° C. Although it is preferable that the continuous annealing process is performed at 650° C. or higher for enabling sufficient recrystallization, if the process temperature of the continuous annealing process is excessively high, oxides may be formed on the steel sheet. In addition, the steel sheet may not be processed smoothly with the previous/next steel sheet. Therefore, it is preferable that the continuous annealing process be performed at 900° C. or lower.

MODE FOR INVENTION

Hereinafter, examples of the present disclosure will be described in detail. The following examples are for illustrative purposes and are not intended to limit the scope and spirit of the present disclosure.

Embodiment 1

Steel slabs having the following compositions were reheated to 1200° C., and a finish hot rolling was performed on the steel slabs at 900° C. to form steel sheets. Thereafter, the steel sheets were coiled at 500° C. and then cold-rolled with a reduction ratio of 50%. The cold-rolled steel sheets were continuously annealed at 800° C.

TABLE 1
No.	C	Mn	Si	P	S	Al	Ti	B	N
1	0.61	17.96	0.01	0.09	0.004	0.01	0.066	0.002	0.0097
2	0.61	18.30	0.01	0.09	0.003	1.50	0.086	0.002	0.0087
3	0.61	18.50	0.01	0.09	0.003	2.69	0.083	0.003	0.0065
4	0.61	14.54	0.01	0.10	0.005	0.01	0.077	0.002	0.0098
5	0.61	15.10	0.01	0.09	0.006	1.51	0.085	0.002	0.0081
6	0.61	15.54	0.01	0.09	0.005	1.97	0.085	0.002	0.0069
7	0.61	11.58	0.01	0.10	0.005	0.01	0.068	0.002	0.0095
8	0.61	11.63	0.01	0.10	0.006	1.46	0.087	0.002	0.0039
9	0.61	12.41	0.01	0.10	0.004	1.95	0.092	0.002	0.0069

The yield strength, tensile strength, and elongation of each of the steel sheets were measured a shown in Table 2 so as to inspect physical properties of the steel sheets.

TABLE 2
No.	Yield strength (MPa)	Tensile strength (MPa)	Elongation (%)
1	484.1	1105.6	60.4
2	498.3	960.1	59.3
3	498.8	848.9	49.7
4	509.3	1124.1	51.3
5	479.5	976	57.6
6	488.2	938.9	58.4
7	485.6	837.8	16.1
8	491.9	899.5	30.3
9	477.6	914.6	40.7

In addition, the steel sheets were inspected by measuring the fraction of inclusions, the fraction of carbides according to heat treatment conditions, and relative permeability under a magnetic field of 25 kA/M. The heat treatment conditions were determined by simulating heat treatments that might be performed during manufacturing process of the steel sheets or the use of the steel sheets.

The term “relative permeability” refers to the ratio of the permeability of a specific medium to the permeability of vacuum. In the examples, the ratio of the permeability of each of the steel sheets to the permeability of vacuum or air was measured as the relative permeability (μ_r). The measurement was carried out using a vibrating sample magnetometer (VSM) by recording a magnetic field applied to a sample through a Hall probe and electromotive force generated by Faraday's law when the sample was vibrated to measure the magnetization of the sample using the recorded values. VSMs are devices operating according the above-described operational principle to measure the magnetization of a sample by vibrating the sample to generate electromotive force, detecting the electromotive force using a search coil, and calculating the magnetization of the sample using the electromotive force. VSMs enable simple and rapid measurements of magnetic properties of materials as a function of a magnetic field, temperature, and time within a magnetic flux range up to 2 teslas (T) and a temperature range of 2 K to 1273 K. In addition, various types of samples such as powder, thin films, single crystals, and liquids can be inspected using VSMs, and thus VCMs are widely used for measuring the magnetic properties of materials.

TABLE 3
	Heat.	Inclusion	Carbide
	treatment	fraction	fraction	Relative
No.	conditions	(%)	(%)	permeability	Notes
1	400° C., 1 hr	0.065	1.18	1.07	*CS1-1
2	400° C., 1 hr	0.091	0.57	1.01	**IS1-1
3	400° C., 1 hr	0.129	0.08	1.01	IS1-2
4	400° C., 1 hr	0.122	1.26	1.09	CS1-2
5	400° C., 1 hr	0.108	0.1	1.01	IS1-3
6	400° C., 1 hr	0.087	0.05	1.01	IS1-4
7	400° C., 1 hr	0.117	1.02	1.07	CS1-3
8	400° C., 1 hr	0.075	0.1	1.01	IS1-5
9	400° C., 1 hr	0.136	0.01	1.02	IS1-6
1	650° C., 5 hrs	0.065	1.35	1.11	CS1-4
2	650° C., 5 hrs	0.091	0.85	1.07	IS1-7
3	650° C., 5 hrs	0.129	0.14	1.05	IS1-8
4	650° C., 5 hrs	0.122	1.47	1.11	CS1-5
5	650° C., 5 hrs	0.108	0.46	1.08	IS1-9
6	650° C., 5 hrs	0.087	0.25	1.06	IS1-10
7	650° C., 5 hrs	0.117	2.12	1.37	CS1-6
8	650° C., 5 hrs	0.075	0.91	1.09	IS1-11
9	650° C., 5 hrs	0.136	0.51	1.05	IS1-12
*CS: Comparative Sample,
**IS: Inventive Sample

Referring to Table 3, if a heat treatment is performed at 400° C. for 1 how: on steel sheets having a carbide fraction of 1 volume % or less, the permeability of the steel sheets is 1.05 or lower. That is, the steel sheets have good non-magnetic properties. In addition, even though a more severe heat treatment is performed at 600° C. for 5 hours on steel sheets having a carbide fraction of 1 volume % or less, the permeability of the steel sheets is less than 1.10.

Microstructures of Inventive Sample 1-7 and Comparative Sample 1-3 are shown in FIGS. 1A and 1B, respectively. As shown in FIGS. 1A and 1B, Inventive Sample 1-7 has a low carbide fraction, and Comparative Sample 1-3 not satisfying requirements of the present disclosure has a carbide fraction of greater than 1 volume % and poor non-magnetic properties.

Thus, it may be understood that a carbide fraction of 1 volume % or less leads to good non-magnetic properties.

Embodiment 2

Steel slabs having the following compositions (weight %) were reheated to 1200° C., and a finish hot rolling was performed on the steel slabs at 900° C. to form steel sheets. Thereafter, the steel sheets were coiled at 500° C. and then cold-rolled at a reduction ratio of 50%. The cold-rolled steel sheets were continuously annealed at 800° C.

TABLE 4
Sample No.	C	Mn	P	S	Al	Si	Ti	B	N
1	0.61	18.0	0.091	0.004	0.01	0.01	0.0662	0.0021	0.0097
2	0.61	18.3	0.087	0.0034	1.49	0.01	0.0857	0.0023	0.0087
3	0.60	18.3	0.087	0.0024	1.93	0.01	0.0856	0.0023	0.0078
4	0.61	18.5	0.090	0.0027	2.68	0.01	0.0833	0.0025	0.0065
5	0.61	14.5	0.097	0.0051	0.02	0.01	0.0766	0.0021	0.0098
6	0.61	15.1	0.094	0.0055	1.51	0.01	0.0854	0.0024	0.0081
7	0.61	15.5	0.094	0.0049	1.97	0.01	0.0846	0.0024	0.0069
8	0.61	11.6	0.101	0.0053	0.01	0.01	0.0684	0.002	0.0095
9	0.61	11.6	0.102	0.0057	1.45	0.01	0.0868	0.0023	0.0039
10	0.61	12.4	0.098	0.0039	1.94	0.01	0.0915	0.0022	0.0069
11	0.61	18.3	0.092	0.0041	0.51	0.01	0.0662	0.0021	0.0037
12	0.62	18.4	0.091	0.0042	1.02	0.01	0.0857	0.0023	0.0087
13	0.61	18.2	0.093	0.0041	1.21	0.01	0.0856	0.0023	0.0078
14	0.61	18.3	0.092	0.0044	4.52	0.01	0.0833	0.0025	0.0065
15	0.61	18.4	0.091	0.0045	6.02	0.01	0.0766	0.0021	0.0098
16	0.62	18.1	0.092	0.0041	7.513	0.01	0.0854	0.0024	0.0081
17	0.61	14.3	0.096	0.0052	0.51	0.01	0.0846	0.0024	0.0069
18	0.61	14.5	0.097	0.0051	1.01	0.01	0.0684	0.002	0.0095
19	0.61	14.4	0.095	0.0053	1.23	0.01	0.0662	0.0021	0.0097
20	0.62	14.5	0.096	0.0052	4.51	0.01	0.0857	0.0023	0.0087
21	0.62	14.4	0.097	0.0054	6.03	0.01	0.0856	0.0023	0.0078
22	0.61	14.2	0.096	0.0052	7.54	0.01	0.0833	0.0025	0.0065
23	0.61	11.4	0.102	0.0053	0.52	0.01	0.0766	0.0021	0.0098
24	0.62	11.6	0.101	0.0052	1.01	0.01	0.0854	0.0024	0.0081
25	0.61	11.3	0.103	0.0054	1.22	0.01	0.0846	0.0024	0.0069
26	0.62	11.3	0.102	0.0055	4.53	0.01	0.0684	0.002	0.0095
27	0.61	11.4	0.101	0.0052	6.01	0.01	0.0868	0.0023	0.0039
28	0.61	11.6	0.101	0.0053	7.51	0.01	0.0915	0.0022	0.0069

The yield strength (YS), tensile strength (TS), and elongation of each of the cold-rolled steel sheets were measured as shown in Table 2 in addition, the stacking fault energy (SFE) and relative permeability of each of the steel sheets were measured as shown in Table 5. The relative permeability was measured in the same conditions as in Example 1 except that a magnetic field of 50 kA/m was applied.

TABLE 5
Sample	YS	UTS	Elongation	SFE	Relative
No.	(MPa)	(Mpa)	(%)	(mJ/m2)	permeability	Note
1	484.1	1105.6	60.4	24.57	1.07	*CS2-1
2	498.3	960.1	59.3	37.00	1.01	**IS2-1
4	498.8	848.9	49.7	46.68	1.01	IS2-2
5	509.3	1124.1	51.3	20.71	1.06	CS2-2
6	479.5	976	57.6	32.97	1.02	IS2-3
7	488.2	938.9	58.4	36.94	1.02	IS2-4
8	485.6	837.8	16.1	19.97	1.08	CS2-3
9	491.9	899.5	30.3	31.34	1.04	IS2-5
10	477.6	914.6	40.7	35.13	1.02	IS2-6
11	—	—	—	29.11	1.04	CS2-4
12	—	—	—	33.20	1.03	IS2-7
13	—	—	—	34.92	1.03	IS2-8
14	—	—	—	60.82	1.00	IS2-9
15	—	—	—	72.50	1.00	IS2-10
IS	—	—	—	84.07	1.00	IS2-11
17	—	—	—	25.48	1.05	CS2-5
18	—	—	—	28.51	1.05	CS2-6
19	—	—	—	30.23	1.04	IS2-12
20	—	—	—	56.02	1.00	IS2-13
21	—	—	—	67.56	1.00	IS2-14
22	—	—	—	79.20	1.00	IS2-15
23	—	—	—	24.60	1.06	CS2-7
24	—	—	—	28.67	1.05	CS2-8
25	—	—	—	29.30	1.04	CS2-9
26	—	—	—	55.03	1.00	IS2-16
27	—	—	—	66.64	1.00	IS2-17
28	—	—	—	78.14	1.00	IS2-18
*CS: Comparative Sample,
**IS: Inventive Sample

As shown in Table 5, inventive samples of the present disclosure have a stacking fault energy (SFE) of 30 mJ/m² or greater and a low degree of relative permeability. That is, the inventive samples have good non-magnetic properties and a high degree of phase stability.

However, one of the stacking fault energy and relative permeability of each of comparative examples was not satisfactory.

FIG. 2 is a graph showing XRD curves A and B of Inventive Sample and Comparative Sample 2-1, respectively. Curves A and B of FIG. 2 show the phase stability of the samples and effects of the stacking fault energy of the samples. FIGS. 3A and 3B show microstructures of Inventive Sample 1-1 and Comparative Sample 1-1, respectively. Referring to FIGS. 2, 3A, and 3B, it may be understood that the inventive samples of the present disclosure have twins uniformly formed throughout the entire regions thereof and thus high phase stability. However, since the comparative samples have low stacking fault energy, the formation of twins increases after deformation, twins are not present on some crystal surfaces.

Claims

1. A non-magnetic high manganese steel sheet with high strength, the steel sheet comprising, by weight %, C: 0.4% to 0.9%, Mn: 10% to 25%, Al: 0.01% to 8.0%, Si: 0.01% to 2.0%, Ti: 0.05% to 0.2%, Si: 0.01% to 2.0%, B: 0.0005% to 0.005%, S: 0.05% or less (excluding 0%), P: 0.8% or less (excluding 0%), N: 0.003% to 0.01%, and the balance of Fe and inevitable impurities.

2. The steel sheet of claim 1, wherein the steel sheet has a microstructure comprising 1 volume % or less of carbides.

3. The steel sheet of claim 1, wherein the steel sheet has a relative permeability of 1.10 or less in a magnetic field of 25 kA/m.

4. The steel sheet of claim 1, wherein if the content Al in the steel sheet ranges from 1.3% to 8.0%, the steel sheet has a stacking fault energy of 30 mJ/cm2 or more.

5. The steel sheet of claim 4, wherein the steel sheet has a relative permeability of 1.05 or less in a magnetic field of 50 kA/m.

6. The steel sheet of claim 1, wherein the steel sheet has a tensile strength of 800 MPa or greater and an elongation of 15% or greater.

7. A method of manufacturing a non-magnetic high manganese steel sheet having high strength, the method comprising:

reheating a steel slab to a temperature within a range of 1100° C. to 1250° C., the steel slab comprising, by weight %, C: 0.4% to 0.9%, Mn: 10% to 25%, Al: 0.01% to 8.0%, Si: 0.01% to 2.0%, Ti: 0.05% to 0.2%, Si: 0.01% to 2.0%, B: 0.0005% to 0.005%, S: 0.05% or less (excluding 0%), P: 0.8% or less (excluding 0%), N: 0.003% to 0.01%, and the balance of Fe and inevitable impurities;

performing a hot-rolling process by hot-rolling the reheated steel slab and finish-rolling the steel slab at a temperature within a temperature range of 800° C. to 950° C., so as to form a hot-rolled steel sheet;

coiling the hot-rolled steel sheet at a temperature within a temperature range of 400° C. to 700° C.;

cold-rolling the steel sheet with a reduction ratio of 30% to 60%; and

continuously annealing the cold-rolled steel sheet at a temperature within a temperature range of 650° C. to 900° C.