US20200318215A1

US20200318215A1 - Steel sheet for shielding magnetic field and method for manufacturing same

Info

Publication number: US20200318215A1
Application number: US16/955,562
Authority: US
Inventors: Tae-Il So; Ho-Kyung SHIM; Chon-Kue KIM; Jong-In Choi
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2017-12-22
Filing date: 2018-12-19
Publication date: 2020-10-08
Also published as: JP2021507988A; CN111492725A; EP3731614B1; CN111492725B; EP3731614A4; WO2019124982A1; KR101977507B1; EP3731614A1

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.

Description

TECHNICAL FIELD

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

BACKGROUND ART

A magnetic resonance imaging (MRI) device mainly used in hospitals is a device applying radio waves to a human body located within a strong magnetic field and measuring the reflected magnetic field to observe brains, internal organs, and other internal organs. The MRI device utilizes a superconducting magnet to generate a static magnetic field, and there have recently been high magnetic field MRI devices of 7 T (T: Tesla) or higher developed to obtain high-resolution images.
When a leakage magnetic field occurs outside an MRI room due to a high magnetic field generated from the MRI device, malfunctioning of electronic equipment is caused by disturbing a signal system of a high-precision electronic device. Further, as there have been studies undertaken to show a high magnetic field has a detrimental effect on the human body, it is necessary to use a magnetic shielding material to prevent such high magnetic field from leaking to the outside. More preferably, a magnetic shielding material, which has high performance and is economical, is required. In this regard, it is essential to select an optimal shielding material and thickness such that a magnetic flux density outside a conventional MRI shielding room is less than 5 Gauss (=0.5 mT).
In particular, permeability characteristics indicating a high magnetic flux density according to a specific magnetic field intensity in a material are important in shielding a DC magnetic field, and it is necessary to have a low coercive force. In order to decrease coercive force while increasing permeability, precipitates such as carbon, nitrogen, carbides or nitrides in steel, which hinder movements of a magnetic domain wall, should be minimized, and grain growth should be promoted while reducing grain boundaries. An addition of impurities to steel forms finely dispersed precipitates during solidification or during a heat treatment process after rolling, thereby forming a new magnetic domain which reduces static magnetic energy and intervenes movements of the magnetic domain wall. In addition, when impurities enter iron as an intrusion type rather than a substitution type, a greater adverse effect is imposed on magnetic characteristics. The intrusive element reduces permeability by elastically deforming a lattice of a solid solution, and carbon is a representative element.
Patent Document 1 has proposed a hot-rolled pickled thick steel sheet having excellent magnetic field shielding characteristics; however, the steel sheet of Patent Document 1 is different from that of the present disclosure in that it lacks magnetic shielding performance because magnetic characteristics are only provided by hot-rolling, coiling and pickling without a heat treatment and is a material for shielding an AC magnetic fields. Recently, there is increasing demand for steel materials having excellent magnetic shielding ability capable of shielding a DC magnetic field such as an MRI room. (Patent Document 1) Korea Patent Publication No. 10-2005-0129244

DISCLOSURE

Technical Problem

An aspect of the present disclosure is to provide a steel sheet having excellent magnetic field shielding performance by controlling alloying elements and heat treatment conditions, and a method for manufacturing the same.
A technical problem of the present disclosure is not limited thereto, and other problems not mentioned will be clearly understood by those skilled in the art from the following description.

Technical Solution

An aspect of the present disclosure relates to 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 μm or greater.
An another aspect relates to a method for manufacturing a steel sheet for shielding magnetic field, including 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.

Advantageous Effects

The present disclosure can provide a steel sheet having excellent magnetic field shielding performance, capable of suppressing an impact on the human body and external electronic devices when applying a DC magnetic field and applying the DC magnetic field from an MRI device to outside of an MRI room by enabling smooth movements of magnetic domains.

BRIEF DESCRIPTION OF DRAWINGS

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.

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

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.
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.
C: 0.02% or Less
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.
Si: 0.001% to 0.05%
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%.
Mn: 0.01% to 0.2%
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%.
Al: 0.001% to 0.05%
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.
Nb: 0.005% or Less
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.
Ti: 0.07% or Less
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.
N: 0.01% (100 ppm) or Less
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.
Accordingly, N is preferably contained in an amount of 0.01% (100 ppm) or less, more preferably 0.005% (50 ppm) or less.
P: 0.015% or Less
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.
S: 0.005% or Less
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.
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.
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.
It is preferable that an average grain size of the ferrite be 100 μm or greater. In the case in which the grain size is less than 100 μm 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.
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.
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.
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.
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.
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.)=937.2−436.5C+56Si−19.7Mn+136.3Ti−19.1Nb+198.4Al [Equation 1]
Each component symbol refers to a content thereof (wt %).
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+363Al−357Si [Equation 2]
Each component symbol refers to a content thereof (wt %).
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.
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.
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.
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

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

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 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 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

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 3

		Yield	Tensile
	Grain size	strength	strength	Elongation
classification	(μm)	(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

As shown in Table 3, Inventive Examples satisfying the alloy composition and manufacturing process of the present disclosure have a grain size of 100 μm 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.
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.
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.
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 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

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.
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.
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.
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.
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.

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 μm 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.