WO2020085858A1 - Cryogenic austenitic high-manganese steel having excellent shape, and manufacturing method therefor - Google Patents

Abstract

The cryogenic austenitic high-manganese steel having an excellent shape, according to one aspect of the present invention, comprises 0.2-0.5 wt% of C, 23-28 wt% of Mn, 0.05-0.5 wt% of Si, at most 0.03 wt% of P, at most 0.005 wt% of S, at most 0.5 wt% of Al, 2.5-4.5 wt% of Cr, and 0.0005-0.01 wt% of B, with the remainder being Fe and other unavoidable impurities, and also comprises at least 95 area% of austenite as a microstructure, wherein the Charpy impact toughness at -196°C is at least 30 J (based on a thickness of 5 mm), and the maximum difference in height between a crest and a trough formed within an area of 2 m along the direction of rolling may be at most 10 mm.

Description

Austenitic high-manganese steel for excellent cryogenic shape and its manufacturing method

The present invention relates to an austenitic high-manganese steel and a method for manufacturing the same, and more particularly, to an ultra-low-temperature austenite-based high-manganese steel for excellent cryogenic toughness and a method for manufacturing the same.

Austenitic high-manganese steel materials have high toughness by stabilizing austenite even at ambient or cryogenic environments by adjusting the content of manganese (Mn) and carbon (C), which are elements that increase the stability of austenite, for LNG storage. It has particularly suitable properties as a material for cryogenic structures such as tanks and tanks for transporting LNG.

However, high manganese (Mn) steel has high deformation resistance at high temperatures, and in particular, in the case of thin materials, it is difficult to secure a uniform shape in the longitudinal direction according to a rolling pass, a rolling reduction, and the like. When the shape of the hot rolled material is inferior, cooling safety is lowered, and there is a possibility of causing equipment damage in a process such as transfer. In addition, when the longitudinal shape of the hot rolled material is inferior, it is not preferable in terms of economical efficiency and productivity since subsequent work such as shape correction work must be performed. In addition, since there are technical limitations in securing a uniform shape even after additional shape correction work after cooling, a high-manganese steel material having excellent shape uniformity and a method of manufacturing the same are required without involving additional work such as shape fixing. This is true.

(Advanced technical literature)

(Patent Document 1) Republic of Korea Patent Publication No. 10-1994-0002370 (published on February 17, 1994)

According to an aspect of the present invention, an austenite-based high-manganese steel material having excellent shape and a method of manufacturing the same can be provided.

The subject of this invention is not limited to the above-mentioned content. Those skilled in the art will have no difficulty in understanding the additional subject matter of the present invention from the general contents of this specification.

The austenite-based high-manganese steel material having excellent shape according to an aspect of the present invention is in weight%, C: 0.2 to 0.5%, Mn: 23 to 28%, Si: 0.05 to 0.5%, P: 0.03% or less , S: 0.005% or less, Al: 0.5% or less, Cr: 2.5 to 4.5%, B: 0.0005 to 0.01%, the balance includes Fe and other unavoidable impurities, and includes austenite of 95 area% or more as a microstructure, Charpy impact toughness of -196 ℃ is more than 30J (based on 5mm thickness), and the maximum height difference between the floor and the valley formed in the region within 2m along the rolling direction may be within 10mm.

The grain size of the austenite may be 5 ~ 150㎛.

The yield strength of the steel material may be 350 MPa or more, tensile strength 700 MPa or more, and elongation 30% or more.

The austenite-based high-manganese steel material having excellent shape according to an aspect of the present invention is in weight%, C: 0.2 to 0.5%, Mn: 23 to 28%, Si: 0.05 to 0.5%, P: 0.03% or less , S: 0.005% or less, Al: 0.5% or less, Cr: 2.5 to 4.5%, B: 0.0005 to 0.01%, slab containing residual Fe and other inevitable impurities is first heated to a temperature range of 1050 to 1300 ° C , First heating the slab at a final rolling temperature of 800 to 1100 ° C at a total rolling reduction of 35 to 80% to provide an intermediate material, and secondarily heating the intermediate material to a temperature range of 1050 to 1300 ° C, The secondary heated intermediate material is subjected to secondary hot rolling at a finish rolling temperature of (Tnr-120) to Tnr ° C to provide a hot rolled material, and the hot rolled material is cooled to a temperature of 600 ° C. or less at a cooling rate of 1 to 100 ° C./s. Cooling to the range, it can be prepared by controlling the total rolling reduction of the intermediate material in the temperature range of (Tnr-120) ~ Tnr ℃ during the second hot rolling to 5-25%.

The hot-rolled material that has been cooled may have a maximum height difference between a floor and a valley formed in a region within 2 m along the rolling direction within 10 mm.

The solving means of the above problems does not list all the features of the present invention, and various features of the present invention and the advantages and effects thereof may be understood in more detail with reference to specific embodiments below.

According to one preferred aspect of the present invention, it is possible to provide an austenite-based high-manganese steel material having excellent cryogenic toughness and excellent shape.

Figure 1 (a) is a view for helping to understand the bone and floor formed in the steel material in the present invention, Figure 1 (b) is a picture of a steel material according to an example of the present invention.

The present invention relates to an austenitic high-manganese steel for excellent cryogenic shape and a method for manufacturing the same, hereinafter, to describe preferred embodiments of the present invention. The embodiments of the present invention may be modified in various forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. These embodiments are provided to those skilled in the art to further detail the present invention.

Hereinafter, the steel composition of the present invention will be described in more detail. Hereinafter, unless otherwise indicated,% representing the content of each element is based on weight.

The austenite-based high-manganese steel material having excellent shape according to an aspect of the present invention is in weight%, C: 0.2 to 0.5%, Mn: 23 to 28%, Si: 0.05 to 0.5%, P: 0.03% or less , S: 0.005% or less, Al: 0.5% or less, Cr: 2.5 to 4.5%, B: 0.0005 to 0.01%, balance Fe and other inevitable impurities.

Carbon (C): 0.2 ~ 0.5%

Carbon (C) is not only an austenite stabilizing element, but also an effective element for securing strength by solid solution strengthening. Therefore, the present invention can limit the lower limit of the carbon (C) content to 0.2% in order to secure low-temperature toughness and strength. That is, when the carbon (C) content is less than 0.2%, the stability of austenite is insufficient to obtain a stable austenite at cryogenic temperatures, and it is easy to process organic transformation into ε-martensitic and α'-martensitic due to external stress. This is because it can reduce the toughness and strength of steel materials. On the other hand, when the carbon (C) content exceeds a certain range, the toughness of the steel may be rapidly deteriorated due to the precipitation of carbides, and the strength of the steel may be excessively high, thereby significantly reducing the workability of the steel. (C) The upper limit of the content can be limited to 0.5%. Therefore, the carbon (C) content of the present invention may be 0.2 to 0.5%, the preferred carbon (C) content may be 0.3 to 0.5%, and the more preferable carbon (C) content may be 0.3 to 0.45%.

Manganese (Mn): 23-28%

Manganese (Mn) is an element that effectively contributes to the stabilization of austenite, so the present invention can limit the lower limit of the manganese (Mn) content to 23% to achieve this effect. That is, the present invention can effectively increase the stability of austenite because it contains 23% or more of manganese (Mn), thereby suppressing the formation of ferrite, ε-martensite and α'-martensite, thereby improving the low-temperature toughness of steel. It can be secured effectively. On the other hand, when the manganese (Mn) content exceeds a certain level range, the effect of increasing the stability of austenite is saturated, while the manufacturing cost is greatly increased, and surface oxidation may be deteriorated due to excessive internal oxidation during hot rolling. , The present invention can limit the upper limit of the manganese (Mn) content to 28%. Therefore, the manganese (Mn) content of the present invention may be 23 to 28%, and a more preferable manganese (Mn) content may be 23 to 25%.

Silicon (Si): 0.05 ~ 0.5%

Silicon (Si) is an element that is indispensably added in trace amounts as a deoxidizer, such as aluminum (Al). However, when silicon (Si) is excessively added, an oxide is formed at a grain boundary to reduce high temperature ductility, and there is a fear that surface quality may be lowered by causing cracks, etc., so that the present invention has an upper limit of the silicon (Si) content. It can be limited to 0.5%. On the other hand, an excessive cost is required to reduce the Si content in the steel, and the present invention can limit the lower limit of the silicon (Si) content to 0.05%. Therefore, the silicon (Si) content of the present invention may be 0.05 to 0.5%.

Phosphorus (P): 0.03% or less

Phosphorus (P) is not only an inevitably introduced impurity element, but also an element that is easily segregated and causes cracking during casting or deteriorates weldability. Therefore, the present invention can limit the upper limit of the phosphorus (P) content to 0.03% in order to prevent deterioration of castability and deterioration of weldability.

Sulfur (S): 0.005% or less

Sulfur (S) is not only an inevitably introduced impurity element, but also an element that causes hot embrittlement defects by inclusion formation. Therefore, the present invention can limit the upper limit of the sulfur (S) content to 0.005% to suppress the occurrence of hot embrittlement.

Aluminum (Al): 0.5% or less

Aluminum (Al) is a representative element added as a deoxidizer. However, aluminum (Al) may form precipitates by reacting with carbon (C) and nitrogen (N), and the hot workability may be deteriorated by these precipitates, and the present invention provides an upper limit of the aluminum (Al) content. It can be limited to 0.5%. The more preferable content of aluminum (Al) may be 0.05 to 0.5%.

Chromium (Cr): 2.5-4.5%

Chromium (Cr) is a winso that stabilizes austenite up to a range of an appropriate addition amount, thereby contributing to the improvement of impact toughness at low temperatures, and is employed in austenite to increase the strength of steel. In addition, chromium is also an element that improves the corrosion resistance of steel materials. Therefore, the present invention can add more than 2.5% chromium (Cr) to achieve this effect. However, chromium (Cr) is a carbide-forming element, and is also an element that forms a carbide at the austenite grain boundary to reduce low-temperature impact, so the present invention takes into account the content relationship with carbon (C) and other elements added together The upper limit of the chromium (Cr) content may be limited to 4.5%. Therefore, the chromium (Cr) content of the present invention may be 2.5 to 4.5%, and a more preferable chromium (Cr) content may be 3 to 4%.

Boron (B): 0.0005 ~ 0.01%

Boron (B) is a grain boundary strengthening element for strengthening the austenite grain boundary, and is an element capable of effectively lowering the high temperature cracking sensitivity of steel materials by strengthening the austenite grain boundary even with a small amount added. Therefore, in order to achieve this effect, the present invention can limit the lower limit of the boron (B) content to 0.0005%. On the other hand, when the content of boron (B) exceeds a certain range, it causes segregation at the austenite grain boundary, thereby increasing the sensitivity of high temperature cracking of the steel, so the surface quality of the steel may be lowered. ) The upper limit of the content can be limited to 0.01%. Therefore, the boron (B) content of the present invention may be 0.0005 to 0.01%, and the more preferable content of boron (B) may be 0.002 to 0.006%.

The austenite-based high-manganese steel for excellent cryogenic shape according to one aspect of the present invention may contain the balance of Fe and other unavoidable impurities in addition to the above-described components. However, in the normal manufacturing process, unintended impurities may be inevitably mixed from the raw material or the surrounding environment, and thus cannot be excluded. Since these impurities are known to anyone skilled in the art, they are not specifically mentioned in this specification. In addition, addition of effective ingredients other than the above composition is not excluded.

The austenite-based high-manganese steel material having excellent shape according to an aspect of the present invention may include 95% by area or more of austenite as a microstructure, thereby effectively securing cryogenic toughness of the steel material. The average grain size of austenite may be 5 to 150 μm. The average grain size of austenite that can be implemented in the manufacturing process is 5 µm or more, and when the average grain size is greatly increased, the strength of the steel material may be lowered, so the grain size of austenite may be limited to 150 µm or less.

The austenite-based high-manganese steel material having excellent shape according to an aspect of the present invention may include carbide and / or ε-martensite as a structure that can exist in addition to austenite. When the fraction of carbide and / or ε-martensite exceeds a certain level, the toughness and ductility of the steel may be rapidly reduced. In the present invention, the fraction of carbide and / or ε-martensite is less than 5 area%. Can be limited.

The austenite-based high-manganese steel material having excellent shape according to an aspect of the present invention may have a yield strength of 350 MPa or more, a tensile strength of 700 MPa or more, and an elongation of 30% or more. In addition, the austenite-based high-manganese steel material having excellent shape according to one aspect of the present invention has a Charpy impact toughness of 30 J or more (based on a thickness of 5 mm) of -196 ° C, and thus can have excellent cryogenic properties.

The austenite-based high-manganese steel for excellent cryogenic shape according to an aspect of the present invention has a difference in height between the floor and the valley formed in the steel in the region within 2 m for the rolling direction even if a separate calibration operation is not performed after the steel is manufactured. Since it is within a maximum of 10 mm, excellent shape uniformity can be secured. Figure 1 (a) is a view for helping to understand the bone and floor formed in the steel material in the present invention, Figure 1 (b) is a picture of a steel material according to an example of the present invention.

Hereinafter, the manufacturing method of the present invention will be described in more detail.

The austenite-based high-manganese steel material having excellent shape according to an aspect of the present invention is in weight%, C: 0.2 to 0.5%, Mn: 23 to 28%, Si: 0.05 to 0.5%, P: 0.03% or less , S: 0.005% or less, Al: 0.5% or less, Cr: 2.5 to 4.5%, B: 0.0005 to 0.01%, slab containing residual Fe and other inevitable impurities is first heated to a temperature range of 1050 to 1300 ° C , First heating the slab at a final rolling temperature of 800 to 1100 ° C at a total rolling reduction of 35 to 80% to provide an intermediate material, and secondarily heating the intermediate material to a temperature range of 1050 to 1300 ° C, The secondary heated intermediate material is subjected to secondary hot rolling at a finish rolling temperature of (Tnr-120) to Tnr ° C to provide a hot rolled material, and the hot rolled material is cooled to a temperature of 600 ° C. or less at a cooling rate of 1 to 100 ° C./s. Cooling to the range, it can be prepared by controlling the total rolling reduction of the intermediate material in the temperature range of (Tnr-120) ~ Tnr ℃ during the second hot rolling to 5-25%.

Primary slab heating

Since the composition of the slab provided in the manufacturing method of the present invention corresponds to the steel composition of the austenitic high-manganese steel described above, the description of the steel composition of the slab is described for the steel composition of the austenitic high-manganese steel described above. Instead.

The slab provided with the above-described steel composition may be primary heated in a temperature range of 1050 to 1300 ° C. When the primary heating temperature is less than a predetermined range, a problem that excessive rolling load may occur during primary hot rolling or a problem that an alloy component is not sufficiently dissolved may occur, and the present invention provides a lower limit of the primary heating temperature range. It can be limited to 1050 ℃. On the other hand, when the primary heating temperature exceeds a certain range, there is a fear that the grains are excessively grown and the strength is lowered or the hot rollability of the steel material is deteriorated by heating the steel above the solidus temperature of the steel material. The upper limit of the silver slab primary heating temperature range may be limited to 1300 ° C.

1st hot rolling

The primary hot rolling process includes a rough rolling process and a finish rolling process, and the primary heated slab may be sized in the primary hot rolling to be provided as an intermediate material. The total rolling reduction of the primary hot rolling may be 35 to 80%, and the finishing rolling of the primary hot rolling is preferably performed in a temperature range of 800 to 1100 ° C. When the finish rolling temperature of the primary hot rolling is less than a certain range, excessive rolling load due to an increase in the rolling load may be a problem, and when the finish rolling temperature of the primary hot rolling exceeds a certain range, the grains grow coarsely and target This is because strength cannot be obtained.

Primary heating of intermediate materials

In order to charge the intermediate material to the heating furnace, the intermediate material may be cut to an appropriate length according to the thickness of the intermediate material, and preferably, the intermediate material may be cut to a length of 1500 to 4000 mm. This is because when the length of the intermediate material is less than 1500 mm, tracking in the heating furnace is difficult, and when the length of the intermediate material exceeds 4000 mm, bending may occur along the longitudinal direction.

The intermediate material may be secondarily heated in a temperature range of 1050 to 1300 ° C. If the secondary heating temperature is less than a certain range, a problem may occur that excessive rolling load occurs during secondary hot rolling, or a problem that an alloy component is not sufficiently dissolved may occur, and the present invention provides a lower limit of the secondary heating temperature range. It can be limited to 1050 ℃. On the other hand, when the secondary heating temperature exceeds a certain range, the crystal grains grow excessively and the strength is lowered, or the hot rollability of the steel material may be deteriorated by heating the steel above the solidus temperature of the steel bar. The upper limit of the secondary heating temperature range of the silver intermediate may be limited to 1300 ° C.

2nd hot rolling

The secondary hot rolling process includes a rough rolling process and a finish rolling process, and the secondary reheated intermediate material may be provided as an intermediate material by secondary hot rolling. At this time, the finish rolling is preferably performed in a temperature range of (Tnr-120) to Tnr ° C. Here, Tnr can be derived by Equation 1 below.

[Equation 1]

Tnr (℃) = 840 + 150 * C + 2.5 * Mn + 3.5 * Cr-50 * Si

(Where C, Mn, Cr, and Si mean the weight percent of each component)

When the finish rolling temperature of the secondary hot rolling is less than (Tnr-120) ℃, the strength tends to rise and impact toughness tends to deteriorate, and when the finish rolling temperature of the secondary hot rolling exceeds Tnr ℃, Since the strength decrease is concerned, the present invention can limit the finish rolling temperature of the secondary hot rolling to the range of (Tnr-120) to Tnr ° C.

In addition, the present invention can control the total rolling reduction of the intermediate material in the temperature range of (Tnr-120) to Tnr ° C during the secondary hot rolling to 5 to 25%. If the total rolling reduction of the intermediate material in the temperature range of (Tnr-120) to Tnr ° C is less than 5%, the desired shape correction effect cannot be achieved, and the total rolling reduction of the intermediate material in the temperature range of (Tnr-120) to Tnr ° C. This is because if the amount exceeds 25%, the impact toughness may decrease due to excessive pressure reduction.

Cooling

The second hot-rolled hot rolled material can be accelerated to a cooling stop temperature of 600 ° C. or less at a cooling rate of 1 to 100 ° C./s. If the cooling rate is less than a certain range, the ductility of the steel may be reduced due to carbides precipitated at the grain boundary during cooling, and thus deterioration of abrasion resistance may be a problem. Therefore, the present invention can limit the cooling rate of the hot rolled material to 1 ° C / s or more. have. The lower limit of the preferred cooling rate may be 10 ° C / s, and the cooling method may be accelerated cooling. However, the faster the cooling rate is, the more advantageous the effect of suppressing the precipitation of carbides, but considering the circumstances in which the cooling rate exceeding 100 ° C / s in normal cooling is difficult to implement due to the characteristics of the facility, the present invention sets the upper limit of the cooling rate to 100 ° C. Can be limited to / s.

In addition, even if the hot-rolled material is cooled by applying a cooling rate of 10 ° C./s or more, when the cooling is stopped at a high temperature, there is a high possibility that carbides are generated and grown, so the present invention limits the cooling stop temperature to 600 ° C. or less. You can.

The austenitic high-manganese steel material prepared as described above contains 95% by area or more of austenite, yield strength of 350MPa or more, tensile strength of 700MPa or more, elongation of 30% or more, and Charpy of 30J or more (based on 5mm thickness) at -196 ° C. Impact toughness can be provided.

In addition, the austenite-based high-manganese steel manufactured as described above can ensure excellent shape uniformity because the height difference between the floor and the valley formed in the steel is within 10 mm or less in the region within 2 m along the length direction of the steel.

Hereinafter, the present invention will be described in more detail through examples. However, it is necessary to note that the embodiments described below are only intended to further illustrate the present invention and are not intended to limit the scope of the present invention.

(Example)

A slab having an alloy composition of Table 1 and a thickness of 250 mm was produced. Each slab was first heated in a temperature range of 1200 ° C, and then first hot rolled at a total rolling reduction rate of 50 to 60% at a finish rolling temperature of 1000 ° C to prepare an intermediate material. Each intermediate material was subjected to secondary heating and secondary hot rolling under the conditions of Table 2 to prepare a hot-rolled material specimen, yield strength, tensile strength, elongation, Charpy impact toughness at -196 ° C, and The shape uniformity was measured and is shown in Table 3 below. At this time, the shape uniformity was described by measuring the maximum height difference between the floor and the valley formed in the 2 m region along the rolling direction of the specimen. Here, the tensile properties were tested at room temperature in accordance with ASTM A370, and the impact toughness was measured at -196 ° C by processing into 5mm thick impact specimens according to the conditions of the same standard.

division	Alloy composition (% by weight)
	C	Mn	Si	P	S	Al	Cr	B
Steel type 1	0.41	24.5	0.28	0.016	0.0031	0.27	3.21	0.0015
Steel type 2	0.46	25.9	0.24	0.015	0.0029	0.31	3.62	0.0015
Steel type 3	0.61	26.9	0.25	0.016	0.0033	0.28	3.91	0.0021

Steel	division	Furnace temperature (℃)	Extraction temperature (℃)	Final width (mm)	Tnr (℃)	Tnr-120 (℃)	Second rolling end temperature (℃)	Total reduction rate under Tnr (%)
Gangjong 1	1-1	1205	1192	2410	960	840	955	4
	1-2						780	20
	1-3						880	22
	1-4						920	16
Gangjong 2	2-1	1190	1180	2410	974	854	806	27
	2-2						980	0
	2-3						880	20
	2-4						915	18
Gangjong 3	3-1	1220	1210	2510	1000	880	988	4
	3-2						940	8.8
	3-3						825	25

Steel	division	Wave height (mm)	YS (MPa)	Ts (MPa)	El (%)	Impact toughness (J, @ -196 ℃)	division
Gangjong 1	1-1	18	368	822	68	46	Comparative example
	1-2	4	334	692	40	31
	1-3	3	465	842	52	40	Inventive Example
	1-4	6	456	856	54	44
Gangjong 2	2-1	3.5	585	954	40	25	Comparative example
	2-2	13	344	737	69	49
	2-3	6	472	861	56	45	Inventive Example
	2-4	8	456	848	59	47
Gangjong 3	3-1	19	504	928	39	21	Comparative example
	3-2	6	584	969	45	19
	3-3	5	625	977	38	15

As shown in Table 2 and Table 3, the alloy composition and manufacturing process of the present invention secures the desired physical properties and shape uniformity of the present invention in the case of a satisfactory invention example, whereas the alloy composition or manufacturing process of the present invention is not satisfied. In the case of the comparative example, it can be confirmed that the present invention does not secure the desired physical properties or shape uniformity.

Although the present invention has been described in detail through the above embodiments, other types of embodiments are possible. Therefore, the technical spirit and scope of the claims set forth below are not limited to the embodiments.

Claims (5)

1. In weight percent, C: 0.2-0.5%, Mn: 23-28%, Si: 0.05-0.5%, P: 0.03% or less, S: 0.005% or less, Al: 0.5% or less, Cr: 2.5-4.5%, B: 0.0005 to 0.01%, the balance of Fe and other inevitable impurities,

   Microstructure contains at least 95% of austenite,

   Charpy impact toughness of -196 ℃ is more than 30J (based on 5mm thickness),

   Austenitic high-manganese steel with excellent shape with a maximum difference in height between the floor and the valley formed within 2 m along the rolling direction within 10 mm.
2. According to claim 1,

   The austenite has a crystal grain size of 5 to 150 µm, an austenite-based high-manganese steel material having excellent shape.
3. According to claim 1,

   The yield strength of the steel is 350MPa or more, the tensile strength is 700MPa or more, and the elongation is 30% or more.
4. In weight percent, C: 0.2-0.5%, Mn: 23-28%, Si: 0.05-0.5%, P: 0.03% or less, S: 0.005% or less, Al: 0.5% or less, Cr: 2.5-4.5%, B: 0.0005 ~ 0.01%, the slab containing the residual Fe and other unavoidable impurities is first heated to a temperature range of 1050 ~ 1300 ℃,

   The heated slab is first hot rolled at a final rolling temperature of 800 to 1100 ° C at a total rolling reduction of 35 to 80% to provide an intermediate material,

   Secondary heating the intermediate material to a temperature range of 1050 ~ 1300 ℃,

   The second heated intermediate material is second hot-rolled at a finish rolling temperature of (Tnr-120) to Tnr ° C. to provide a hot rolled material,

   Cooling the hot rolled material to a temperature range of 600 ℃ or less at a cooling rate of 1 ~ 100 ℃ / s,

   The second hot rolling (Tnr-120) ~ Tnr ℃ in the temperature range of the total rolling reduction of the intermediate material is 5 to 25%, the method of manufacturing austenite-based high-manganese steel for excellent cryogenic shape.
5. According to claim 4,

   The method of manufacturing the austenite-based high-manganese steel material having excellent shape in which the maximum height difference between the floor and the valley formed in a region within 2 m along the rolling direction is 10 mm or less, wherein the cooled hot-rolled material is finished.

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