STEEL SHEET FOR SHIELDING MAGNETIC FIELD AND METHOD FOR MANUFACTURING SAME

Abstract

The present invention relates to a steel sheet for shielding a magnetic field, which is used for a medical magnetic resonance imaging (MRI) room wall body and the like, and a method for manufacturing the same.

Claims

1. A steel sheet for shielding magnetic field, comprising: by weight %, 0.02% or less of carbon (C), 0.001% to 0.05% of silicon (Si), 0.01% to 0.2% of manganese (Mn), 0.001% or 0.05% of aluminum (Al), 0.005% or less niobium (Nb), 0.07% or less of titanium (Ti), 0.01% or less of nitrogen (N), 0.015% or less of phosphorus (P), 0.005% or less of sulfur (S), and a remainder of iron (Fe) and inevitable impurities, wherein a microstructure has ferrite as main structure, wherein the ferrite has an average grain size of 100 m or greater.

2. The steel sheet for shielding magnetic field of claim 1, wherein the steel sheet has a maximum size of a precipitate existing at a boundary of or within the ferrite grain is 100 nm or less.

3. The steel sheet for shielding magnetic field of claim 1, wherein the steel sheet has a magnetic flux density of 1 T (Tesla) or higher under a magnetic field intensity of 300 A/m.

4. The steel sheet for shielding magnetic field of claim 1, wherein steel sheet has yield strength of 190 MPa or less, tensile strength of 220 MPa to 300 MPa, elongation of 40% or more, and a yield ratio of 0.8 or less.

5. A method for manufacturing a steel sheet for shielding magnetic field, comprising: reheating a steel slab comprising by weight %, 0.02% or less of carbon (C), 0.001% to 0.05% of silicon (Si), 0.01% to 0.2% of manganese (Mn), 0.001% or 0.05% of aluminum (Al), 0.005% or less niobium (Nb), 0.07% or less of titanium (Ti), 0.01% or less of nitrogen (N), 0.015% or less of phosphorus (P), 0.005% or less of sulfur (S), and a remainder of iron (Fe) and inevitable impurities at a temperature of Ac3 to 1250 C.; hot rolling the reheated slab and finish hot rolling at a temperature of Ar3 or higher to obtain a steel sheet; and air cooling the steel sheet to room temperature.

6. The method for manufacturing a steel sheet for shielding magnetic field of claim 5, further comprising normalizing heat treatment of maintaining at a temperature of Ar3 or higher for at least (1.3t+30) minutes, and then, of furnace cooling or air cooling.

7. The method for manufacturing a steel sheet for shielding magnetic field of claim 6, further comprising stress relief annealing of heat treating at a temperature of 800 C. to 900 C. for at least (1.3t+30) minutes after the normalizing heat treatment, and then, of furnace cooling.

8. The method for manufacturing a steel sheet for shielding magnetic field of claim 5, wherein, during the hot rolling, rough rolling is performed at a temperature of Tnr to 1250 C. before finish hot rolling.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0011] FIG. 1 is a graph illustrating magnetic flux density according to magnetic field intensity of Inventive Examples 2, 5 and 7 and Comparative Example 2.

[0012] FIG. 2 is a graph illustrating relative permeability according to the magnetic flux density of Inventive Examples 2, 5 and 7 and Comparative Example 2.

BEST MODE FOR INVENTION

[0013] The present inventors conducted extensive research on the above technical problems, and as a result, controlled alloying elements and a manufacturing process to improve shielding performance of a DC magnetic field to coarse grains and facilitate smooth movements of a magnetic domain when the DC magnetic field is applied, thereby providing a steel material having a size of 15 mmt capable of preventing the DC magnetic field released from hospital MRI devices being applied to the outside an MRI room and affecting the human body and external electronic devices.

[0014] First, a composition range of the steel sheet of the present disclosure is described. The steel sheet of the present disclosure may contain, by weight % (hereinafter, %), 0.02% or less of carbon (C), 0.001% to 0.05% of silicon (Si), 0.01% to 0.2% of manganese (Mn), 0.001% or 0.05% of aluminum (Al), 0.005% or less of niobium (Nb), 0.07% or less of titanium (Ti), 0.01% or less of nitrogen (N), 0.015% or less of phosphorus (P), 0.005% or less of sulfur (S), and a remainder of iron (Fe) and inevitable impurities.

[0015] C: 0.02% or Less

[0016] Carbon (C) significantly reduces permeability by elastically deforming a lattice of a solid solution. In addition, as ferrite or carbide formed due to C interferes a movement of a magnetic domain wall so that increases an iron loss, it is preferable not to contain carbon as little as possible. Accordingly, C is preferably contained of 0.02% or less, more preferably 0.005% or less.

[0017] Si: 0.001% to 0.05%

[0018] Silicon (Si) is mainly used as a deoxidizer. When Si is contained, solubility of C is reduced, thereby lowering magnetic field shielding characteristics. In this regard, it is preferable to contain 0.05% or less. However, Si less than 0.001% results in insufficient deoxidization. Accordingly, an amount of Si is preferably 0.001% to 0.05%, more preferably 0.001% to 0.05%.

[0019] Mn: 0.01% to 0.2%

[0020] Manganese (Mn) is an element binding to S to form MnS, which is a factor itself increasing brittleness. In order to reduce the brittleness due to S, it is preferable to contain 0.01% or more. However, since MnS formed at a grain boundary during high temperature heat treatment suppress grain coarsening, it is preferable that an amount of Mn not exceed 0.2%.

[0021] Al: 0.001% to 0.05%

[0022] Aluminum (Al) is an element for deoxidizing molten steel inexpensively. To sufficiently obtain such an effect, it is preferable to contain 0.001% or more. In the case of an amount exceeding 0.05%, however, Al binds to N to form AlN, thereby suppressing grain coarsening. In this regard, it is preferable to contain Al in an amount of 0.05% or less.

[0023] Nb: 0.005% or Less

[0024] Niobium (Nb) is an element precipitated in the form of NbC or Nb(C,N) to significantly improve strength of a base material and a weld zone. Further, Nb employed during reheating at a high temperature suppresses recrystallization of austenite and transformation of ferrite or bainite thereby refine a structure. In this regard, Nb is an element added to secure strength of a conventional hot rolled steel sheet. Meanwhile, Nb has an adverse effect on magnetic shielding characteristics due to grain refinement, and thus, it is preferable that 0.005% or less of Nb be contained.

[0025] Ti: 0.07% or Less

[0026] Titanium (Ti) has an effect of inhibiting grain growth as a result of reacting mainly with nitrogen when heated and thus is preferable to be added to a conventional carbon steel to improve strength and toughness. In the present disclosure, however, not only strength and toughness are not important factors but also Ti is a detrimental element ingrain coarsening, it is preferable to contain Ti in an amount of 0.07% or less.

[0027] N: 0.01% (100 ppm) or Less

[0028] Nitrogen (N) is an element forming TiN when simultaneously added with Ti and forming AlN by binding to Al when Ti is not added. In the case in which TiN, AlN, or the like, is formed at a grain boundary or within a grain, grain coarsening is suppressed during heat treatment at a high temperature.

[0029] Accordingly, N is preferably contained in an amount of 0.01% (100 ppm) or less, more preferably 0.005% (50 ppm) or less.

[0030] P: 0.015% or Less

[0031] Phosphorus (P) is an element advantageous in improving strength and corrosion resistance but may significantly increase brittleness of a material. In this regard, it is desirable that an amount thereof be managed to be as low as possible. Accordingly, it is preferable that the amount of P be 0.015% or less.

[0032] S: 0.005% or Less

[0033] Sulfur (S) is an element, which significantly increases brittleness by forming MnS, or the like, and thus is desirable to manage an amount as low as possible. Accordingly, it is preferable that an amount of S be 0.005% or less.

[0034] The steel sheet of the present disclosure contains iron (Fe) in addition to the above mentioned alloy elements. However, undesired impurities may be inevitably incorporated from an environment or a raw material during a conventional manufacturing process and thus cannot be excluded. Such impurities are known to any one of ordinary skill in the art, and will thus not be mentioned in detail.

[0035] The steel sheet of the present disclosure has a main structure of ferrite and preferably contains 95 area % or more of ferrite, more preferably 99 area % or more.

[0036] It is preferable that an average grain size of the ferrite be 100 m or greater. In the case in which the grain size is less than 100 m after hot rolling, air cooling, normalizing heat treatment, and stress relief annealing, a movement of a magnetic domain is not smooth so that a magnetic flux density is not sufficiently high when a magnetic field is induced on a shielding steel sheet. Thereby, a shielding effect is not sufficient when a DC magnetic field is applied to an MRI of 7 T or higher.

[0037] Meanwhile, the steel sheet of the present disclosure may contain precipitates at a grain boundary or within a grain of the ferrite. The precipitates may be AlN, TiN, MnS, or the like. As the precipitates serve to suppress grain growth due to a pinning effect during the normalizing heat treatment after rolling, it is preferable that a maximum size of the precipitate not exceed 100 nm, more preferably 20 nm.

[0038] It is preferable that the steel sheet has yield strength of 190 MPa or less, tensile strength of 220 MPa to 300 MPa, elongation of 40% or more, and a yield ratio of 0.8 or less.

[0039] In the case of high magnetic field MRI equipment of 7 T or higher, magnetic field intensity of a DC magnetic field induced on a shielding steel sheet forms an MRI room wall body is conventionally 200 A/m to 300 A/m and is commonly designed to be applied with a magnetic flux density of 1.3 T to 1.5 T. Accordingly, the steel sheet of the prevent disclosure is preferably has a magnetic flux density of 1 T (Tesla) or higher under magnetic field intensity of 300 A/m, more preferably 1.3 T or higher for better DC magnetic field shielding performance.

[0040] A method for manufacturing the steel sheet of the present disclosure will be described in detail. The steel sheet of the present disclosure may be manufactured by heating a steel material (a steel slab) satisfying the alloy composition described above and hot rolling. If necessary, stress relief annealing may be performed, in addition to a normalizing heat treatment. Each process will be described in detail.

[0041] As an example, a steel material satisfying the previously described alloy composition, a steel slab, is prepared and heated. The heating is preferably performed in the temperature range of Ac3 to 1250 C. The Ac3 can be calculated using Equation 1 below. When the heating temperature of the steel slab is less than Ac3, transformation of austenite to ferrite is initiated during rolling, thereby adversely affecting magnetic characteristics due to refinement of grain size of ferrite. Meanwhile, it is preferable that the heating be performed at a temperature not exceeding 1250 C. in consideration of economical feasibility.

Ac3( C.)=937.2436.5C+56Si19.7Mn+136.3Ti19.1Nb+198.4Al[Equation 1]

[0042] Each component symbol refers to a content thereof (wt %).

[0043] The heated steel material is hot rolled. The hot rolling process specifically involves rough rolling of the reheated slab and finish rolling to obtain a hot rolled steel sheet. The rough rolling is preferably performed in the temperature range of Tnr to 1250 C. When the slab is rolled at a temperature below Tnr, grains are refined, thereby making it ineffective in improving magnetic shielding performance. Meanwhile, the Tnr can be derived from Equation 2 below.

Tnr( C.)=887+464C+(6445Nb/644Nb)+890Ti+363Al357Si[Equation 2]

[0044] Each component symbol refers to a content thereof (wt %).

[0045] Meanwhile, when the finish rolling is performed in a temperature range below Ar3, at which the austenite begins to transform to ferrite, refined ferrite forms nuclei at a ferrite grain boundary, thereby reducing an average grain size so that adversely affecting magnetic characteristics. Accordingly, it is preferable that the finish rolling be performed at a temperature of Ar3 or higher.

[0046] Hot rolled steel sheet is then cooled. The cooling is not particularly managed. As a preferred example, air cooling is performed, the air cooling is performed until the temperature reaches room temperature.

[0047] The cooled steel sheet is normalizing heat treated of maintaining at a temperature of Ac3 or higher for (1.3t+30) minutes, and then, furnace cooling or air cooling. The normalizing heat treatment performed for a long period of time at a temperature of Ar3 or higher is effective in improving magnetic shielding characteristics due to additional grain coarsening. It requires at least (1.3t, t: thickness) minutes to allow an thick steel plate to reach a target temperature, and at least 30 minutes of maintaining after the target temperature is reached to distribute a uniform temperature from a surface of the steel sheet to a center thereof.

[0048] To remove stress inside the steel sheet which has been normalizing heat treated, stress relief annealing of heat treating at 800 C. to 900 C. for (1.3t+30) minutes, and then, furnace cooling may be performed. Stress remaining in the hot rolled steel sheet, which has been rough rolled and finish rolled, may interfere movements of the magnetic domain and significantly reduce the magnetic characteristics. In this regard, stress relief annealing of heat treating at 800 C. to 900 C. for (1.3t+30, t: thickness of steel sheet) minutes after the normalizing heat treatment, and then air cooling or furnace cooling, may be additionally performed to maximize the magnetic shielding characteristics.

MODE FOR INVENTION

[0049] The prevent disclosure will be described in more detail with reference to the Examples. The Examples, however, are merely for understanding of the present disclosure and should not be construed as limiting the present disclosure. The scope of the present disclosure is determined by subject matter described in the claims and reasonably inferred therefrom.

Examples

[0050] A steel slab having a thickness of 300 mm and the composition of Table 1 is prepared, and a sheet material is manufactured under the condition of Table 2 below. A unit of the alloy composition of Table 1 is wt %, and a remainder of iron (Fe) and inevitable impurities are included.

TABLE-US-00001 TABLE 1 classification C Si Mn P S T-Al Nb Ti N Steel 1 0.0015 0.002 0.053 0.0070 0.0038 0.0322 0.0014 0.0686 0.0022 Steel 2 0.0195 0.001 0.195 0.0072 0.0032 0.0230 0.0001 0.0001 0.0018 Steel 3 0.0016 0.003 0.032 0.0057 0.0045 0.0076 0.0005 0.0003 0.0019 Steel 4 0.1500 0.010 0.500 0.0150 0.0150 0.0400 0.0001 0.0001 Steel 5 0.0300 0.010 0.200 0.0100 0.0100 0.035 0.0001 0.0001 Steel 6 0.0015 0.002 0.069 0.0092 0.0043 0.035 0.0001 0.0289 0.0095

TABLE-US-00002 TABLE 2 Normalizing Normalizing Cooling Stress Slab Finish heat heat after relief Stress heating rolling treatment treatment Normalizing annealing relief temp temp Thickness temp time heat temp time Steel ( C.) ( C.) (mm) ( C.) (min) treatment ( C.) (min) note Steel 1 1180 980 25 950 62.5 air IE 1 cooling Steel 1 1180 980 25 950 62.5 furnace IE 2 cooling Steel 1 1180 980 25 IE 3 Steel 2 1180 980 25 950 62.5 air IE 4 cooling Steel 2 1180 980 25 950 62.5 furnace IE 5 cooling Steel 3 1140 920 25 910 100 air IE 6 cooling Steel 3 1140 920 25 910 100 air 840 120 IE 7 cooling Steel 4 1180 860 25 CE 1 Steel 5 1180 890 20 CE 2 Steel 6 1180 920 2.5 CE 3 *IE: Inventive Example **CE: Comparative Example

[0051] For the steel sheet manufactured under the conditions of Table 2, a grain size and mechanical properties were evaluated and indicated in Table 3 below. The grain size was observed in a thickness direction of the steel sheet using an optical microscope. Meanwhile, the mechanical properties, such as yield strength, tensile strength, elongation, and the like, were evaluated using a tensile tester by taking a full thickness sample in a rolling direction at room temperature.

TABLE-US-00003 TABLE 3 Yield Tensile Grain size strength strength Elongation classification (m) (MPa) (MPa) (%) Yield ratio IE 1 357.0 102 258 76.8 0.40 IE 2 386.0 86 253 77.3 0.34 IE 3 121.5 113 263 76.4 0.43 IE 4 320.5 180 289 65.8 0.63 IE 5 345.5 172 281 70.2 0.61 IE 6 137.1 149 270 66.0 0.62 IE 7 168.3 120 265 71.5 0.45 CE 1 21.1 232 409 42.1 0.57 CE 2 22.4 194 304 44.1 0.61 CE 3 35.2 204 272 40.2 0.75 * IE: Inventive Example ** CE: Comparative Example

[0052] As shown in Table 3, Inventive Examples satisfying the alloy composition and manufacturing process of the present disclosure have a grain size of 100 m or greater, yield strength of 190 MPa or less, tensile strength of 220 MPa to 300 MPa, elongation of 40% or more, and a yield ratio of 0.8 or less.

[0053] Meanwhile, in the case of Inventive Example 3, the same composition as and similar manufacturing processes of Inventive Examples 1 and 2 are applied, however, a normalizing heat treatment was not performed. Thereby, the grain size of Inventive Example 3 is relatively small as compared to Inventive Examples 1 and 2.

[0054] Meanwhile, Comparative Examples 1 and 2 employs a steel containing C in an amount exceeding the amount suggested in the present disclosure, so that they contain grains having a refined size as compared to those of Inventive Examples 1 to 7, and thus have yield strength and tensile strength beyond the ranges suggested in the present disclosure due to grain refinement. Comparative Example 3 has a composition satisfying the range suggested in the present disclosure, however, a thickness of a final product after hot rolling the slab is small (a total reduction amount is increased), which is beyond the strength range suggested in the present disclosure.

[0055] Meanwhile, a magnetic flux density of the manufactured steel sheet was measured with respect to magnetic field intensity and indicated in Table 4 below.

TABLE-US-00004 TABLE 4 Magnetic flux density to magnetic field intensity (T) B2 (200 B3 (300 B5 (500 B25 (2500 classification A/m) A/m) A/m) A/m) IE 1 1.14 1.36 1.50 1.66 IE 2 1.33 1.44 1.51 1.65 IE 3 0.88 1.11 1.34 1.65 IE 4 0.74 1.04 1.32 1.64 IE 5 0.78 1.16 1.44 1.66 IE 6 1.03 1.23 1.38 1.64 IE 7 1.41 1.48 1.54 1.65 CE 1 0.02 0.11 0.36 1.55 CE 2 0.22 0.52 0.86 1.63 CE 3 0.74 0.98 1.34 1.59 * IE: Inventive Example ** CE: Comparative Example

[0056] As indicated in Table 4 above, Inventive Examples 1 to 7 satisfying the alloy composition and manufacturing method of the present disclosure have a magnetic flux density of 1.0 T or higher at magnetic field intensity of B3.

[0057] Meanwhile, Inventive Example 3, among all Inventive Examples, does not involve additional normalizing heat treatment after hot rolling and cooling and showed relative lower values as compared to Inventive Examples 1 and 2, which are in similar conditions. In contrast, Inventive Example 7 is a result of performing stress relief annealing and thus has a highest excellent magnetic flux density compared to the other conditions.

[0058] Meanwhile, Comparative Examples 1 to 3 have a magnetic flux density of 0.11 T to 0.98 T at magnetic field intensity of B3, indicating inappropriateness to be used as a material for an MRI shielding room of 7 T or higher.

[0059] FIG. 1 is a graph illustrating magnetic flux density according to magnetic field intensity of Inventive Examples 2, 5 and 7 and Comparative Example 2. In a certain magnetic field intensity region, the magnetic flux density was found to be high in the order of Examples 7, 2, and 5, while Comparative Example 2 showed a lowest magnetic flux density. It can be seen that the magnetic characteristics are the highest when the stress relief annealing and normalizing heat treatment are performed after hot rolling and the composition and components suggested in the present disclosure are satisfied.

[0060] FIG. 2 is a graph illustrating relative permeability according to the magnetic flux density of Inventive Examples 2, 5 and 7 and Comparative Example 2. Permeability is a value indicating a degree of magnetization of a medium in a given magnetic field, while relative permeability indicates a ratio of permeability of a medium to vacuum permeability and is represented as the equation magnetic flux density/magnetic field intensity/1.257. The relative permeability of FIG. 2 illustrates a result of each sample indicated in FIG. 1 calculated using the equation with respect to the magnetic flux density. The graph indicates that a higher relative magnetic permeability enables easier magnetization of a shielding material when a magnetic field is applied and more effective shielding performance.