EP4640885A1

EP4640885A1 - Steel sheet and method for manufacturing same

Info

Publication number: EP4640885A1
Application number: EP23907633.4A
Authority: EP
Inventors: Tae-Il SO; Sang-Deok Kang; Jae-Young Cho
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2022-12-21
Filing date: 2023-12-15
Publication date: 2025-10-29
Also published as: KR20240098514A; WO2024136348A1; JP2025540884A; CN120359318A

Abstract

The present invention relates to a steel sheet having excellent strength and toughness which can be used in offshore and onshore wind power generators, etc., and a method for manufacturing same.

Description

Technical Field

The present disclosure relates to a steel sheet having excellent strength and toughness which can be used in offshore and onshore wind power generators, or the like, and a method for manufacturing the same.

Background Art

As wind turbines for offshore and onshore wind power generators become larger, the load-resisting capacity of wind towers needs to be improved, and demand for a thick steel material with improved strength is increasing. However, normally, when a steel material is thickened, the strength decreases and the required thickness is additionally applied, creating a vicious cycle. Therefore, when a thick steel material also has the yield strength and tensile strength required for a thin steel material, it is possible to reduce a shell thickness according to the increase in design strength, which has many advantages. In addition, as the number of cases of wind towers being installed and operated in extreme condition locations is increasing, impact toughness guarantees are also required.
In order to realize high strength and excellent low-temperature impact toughness of a steel material, grain refinement is essential, and a rolling process is one of the representative methods of grain refinement. When rolling is performed at a temperature at which recrystallization may be performed, new austenite fine grains are created using internal stress generated by rolling force as driving force. Meanwhile, rolling in a non-recrystallized temperature region causes grains to be stressed, forming a band structure in a rolling direction, and many dislocations are generated therein, so when austenite is phase transformed, more nucleation points may be provided, thereby including a grain refinement effect
However, as the thickness of the steel material increases, rolling force that can be applied by rolling is limited, so it is difficult to form fine grains through rolling, toward an internal structure, especially, a central portion of the steel material, and accordingly, the yield strength and tensile strength decrease significantly, making it difficult to satisfy the target mechanical properties.
Meanwhile, it is often difficult to obtain grains of a sufficiently small size through only slab heating and rolling, which are processes in which austenite grain refinement mainly occurs. In particular, the higher the temperature of a rolled steel material, the lower the deformation resistance during rolling, so slab heating is mainly performed at a temperature much higher than the Ae3 temperature for easy rolling, and at that time, austenite grains grow significantly. When a grain refinement effect through rolling is not sufficient, an additional austenite grain refinement effect may be expected through a reheat treatment, which is usually performed through a normalizing heat treatment.
A normalizing heat-treated steel material has traditionally been used as a material for wind towers, but when heat treatment is applied in the manufacturing process, the manufacturing cost increases significantly, making it less commercially feasible compared to an as-rolled steel material or a Thermo Mechanical Controlled Process (TMCP) steel material.
Normalized Rolling (NR) is a manufacturing method in which hot rolling is performed in a temperature range having properties similar to that of the normalizing heat-treated steel sheet without a normalizing heat treatment and then air cooled, which is referred to as Normalized Rolling (NR). When the properties similar to that of the normalizing heat treated steel material may be obtained through optimal component design or establishment of manufacturing conditions, the manufacturing cost due to omitting heat treatment can be reduced, making it possible to provide a commercially useful steel material.
Patent Document 1 has proposed a method for manufacturing a steel material having excellent impact toughness without a normalizing heat treatment. However, although the steel material of Patent Document 1 may be advantageous in terms of securing low-temperature impact toughness due to a low content of carbon, it is difficult to satisfy sufficient strength, and as the thickness of the steel material increases due to the lack of consideration for finishing rolling conditions to secure the strength, the strength decreases significantly, making it difficult to satisfy sufficient yield strength.
(Patent Document 1) Korean Patent Publication No. 10-1917453

Summary of Invention

Technical Problem

An aspect of the present disclosure relates to a steel sheet having excellent strength and impact toughness, and having excellent strength and impact toughness even when a normalizing heat treatment is omitted, and a method for manufacturing the same.
An object of the present disclosure is not limited to the above description. The object of the present disclosure will be understood from the entire content of the present specification, and a person skilled in the art to which the present disclosure pertains will understand an additional object of the present disclosure without difficulty.

Solution to Problem

According to an aspect of the present disclosure, provided is a steel sheet, the steel sheet comprising by weight %, more than 0.10 to 0.17% of carbon (C), 0.2 to 0.5% of silicon (Si), 1.2 to 1.6% of manganese (Mn), 0.012% or less of phosphorus (P), 0.003% or less of sulfur (S), 0.015 to 0.045% of aluminum (Al), 0.03 to 0.05% of niobium (Nb), 0.06% or less of vanadium (V), 0.005 to 0.017% of titanium (Ti), 0.002 to 0.01% of nitrogen (N), with a remainder of Fe and inevitable impurities, the steel sheet satisfying the following Relational Expression 1,
wherein a microstructure of the steel sheet comprises, by area fraction, 60 to 85% of ferrite and a remainder of pearlite and inevitable structures,

$[C] + [Mn] / 6 + [V] / 5 \leq 0.43$
where [C], [Mn], and [V] are contents (weight %) of each component.
A thickness of the steel sheet may be 100 mm or less.
An average grain size of the ferrite may be 50 µm or less.
The steel sheet may have a yield strength of 355 MPa or more and a tensile strength of 470 MPa or more, evaluated perpendicularly to a rolling direction, at a t/4 point, where t is a thickness of the steel sheet (mm).
The steel sheet may have a Charpy impact absorption energy of 100 J or more at -20°C, evaluated in a rolling direction, at a t/4 point, where t is a thickness of the steel sheet (mm).
According to another aspect of the present disclosure, provided is a method for manufacturing a steel sheet, the method comprising preparing a steel slab comprising by weight %, more than 0.10 to 0.17% of carbon (C), 0.2 to 0.5% of silicon (Si), 1.2 to 1.6% of manganese (Mn), 0.012% or less of phosphorus (P), 0.003% or less of sulfur (S), 0.015 to 0.045% of aluminum (Al), 0.03 to 0.05% of niobium (Nb), 0.06% or less of vanadium (V), 0.005 to 0.017% of titanium (Ti), 0.002 to 0.01% of nitrogen (N), with a remainder of Fe and inevitable impurities, wherein the following Relational Expression is satisfied;
heating the steel slab to a temperature within a range of 1150 to 1200°C for 3 hours or more;

hot rolling of rough rolling the heated steel slab, and finishing rolling at a finishing rolling start temperature satisfying the following Relational Expression 2; and
cooling after the hot rolling,

$[C] + [Mn] / 6 + [V] / 5 \leq 0.43$
where [C], [Mn], and [V] are contents (weight %) of each component. 800°C ≤ finishing rolling start temperature of rolling ≤ 857°C + (464 × [C]) + (6445 × [Nb]) - (644 × [Nb]^0.5) + (732 × [V]) - (230 × [V]^0.5) + (890 × [Ti]) + (363 × [Al]) - (357 × [Si])
where [C], [Nb], [V], [Ti], [Al], and [Si] are contents (weight %) of each component.

The hot rolling may be performed with a total reduction ratio of 45% or more and the number of rolling passes of 8 or less.
The finishing rolling end temperature may be Ar3 or higher.
The cooling may be air cooling.
After the cooling, a heat treatment operation may be further included at a temperature within a range of 830 to 930°C for 1.3t+30 minutes or more, where t is a thickness of the steel material (mm).

Advantageous Effects of Invention

As set forth above, according to an aspect of the present disclosure, a steel sheet having excellent strength and low-temperature impact toughness that can be used in offshore and onshore wind generators, or the like, may be provided, and normalizing heat treatment may be omitted, thereby improving cost efficiency.
The various and beneficial advantages and effects of the present disclosure are not limited to the above-described content, and may be more easily understood through description of specific embodiments of the present disclosure.

Brief Description of Drawings

FIGS. 1 (a) and 1 (b) are photographs obtained by observing a microstructure at a t/4 point of Inventive Example 1 and Comparative Example 10, respectively, in the embodiments of the present disclosure.
FIG. 2 is a graph illustrating a change in yield strength according to a finishing rolling start temperature, to manufacture a hot-rolled steel sheet with a thickness of 100 mm using a steel type 2 in an embodiment of the present disclosure.

Best Mode for Invention

The terms used in this specification are used to describe the present disclosure and are not intended to limit the present disclosure. In addition, as used herein, singular forms include plural forms unless the relevant definition clearly indicates the contrary.
The meaning of "including" or "comprising" used in the specification specifies a configuration and does not exclude the presence or addition of another configuration.
Unless otherwise defined, all terms, including technical terms and scientific terms used in this specification, have the same meaning as that which could be commonly understood by those skilled in the art in the technical field to which the present disclosure pertains. Terms defined in the dictionary are interpreted as having meanings consistent with related technical literature and current disclosure.
The inventors of the present disclosure have recognized the need for development of a technology for securing the properties required for the material as wind power structural steel used in onshore and offshore wind towers becomes larger and economic efficiency is required.
In particular, for steel materials for wind structures having a thickness greater than a certain level, the inventors of the present disclosure have conducted in-depth research on a method to secure both high strength and low-temperature impact toughness. As a result, the inventors of the present disclosure have confirmed that steel materials for wind structures having target properties may be provided by controlling a composition of components and a relationship between some components in alloy design, and at the same time, optimizing manufacturing conditions, and thus the present disclosure was provided.
Hereinafter, the present disclosure will be described in detail.
First, an alloy composition of a steel sheet according to an aspect of the present disclosure will be described in detail. Unless otherwise particularly stated in the present disclosure, a content of each element is by weight and a ratio of the structure is by area.
The steel sheet may include by weight%, more than 0.10 to 0.17% of carbon (C), 0.2 to 0.5% of silicon (Si), 1.2 to 1.6% of manganese (Mn), 0.012% or less of phosphorus (P), 0.003% or less of sulfur (S), 0.015 to 0.045% of aluminum (Al), 0.03 to 0.05% of niobium (Nb), 0.06% or less of vanadium (V), 0.005 to 0.017% of titanium (Ti), and 0.002 to 0.01% of nitrogen (N).

Carbon (C): more than 0.10% to 0.17%

Carbon (C) is an effective element for improving the strength of steel. To obtain such an effect sufficiently, C may be included in an amount of more than 0.10%. However, when a content of C exceeds 0.17%, it may be advantageous in securing strength, but there is a problem in that a band-shaped pearlite fraction increases significantly, which may significantly inhibit low-temperature impact toughness. When the content of C is less than 0.10%, it may not be sufficient in securing strength. The content of C is preferably 0.100% to 0.170%.

Silicon (Si): 0.2 to 0.5%

Silicon (Si) is not only used as a deoxidizer, but is also an element which is advantageous in improving the strength of steel. In order to sufficiently obtain the effect described above, Si may be included in an amount of 0.2% or more. However, when a content of Si exceeds 0.5%, there is a concern that excessive formation of martensite austenite constituent (MA) may occur, resulting in poor low-temperature impact toughness. The content of Si is preferably 0.20 to 0.50%.

Manganese (Mn): 1.2 to 1.6%

Manganese (Mn) is an element which is advantageous in improving the strength of steel due to a solid solution strengthening effect. In order to obtain such an effect sufficiently, Mn may be included in an amount of 1.2% or more. However, when a content of Mn exceeds 1.6%, Mn combines with sulfur (S) in steel to form Mns, thereby reducing the low-temperature impact toughness. Therefore, Mn may be included in an amount of 1.2 to 1.6%, preferably 1.20 to 1.60%, and more preferably, 1.45 to 1.6%.

Phosphorus (P): 0.012% or less

Phosphorus (P) is an element which is advantageous in improving the strength of steel and securing corrosion resistance, but P can significantly reduce impact toughness of steel, so it is preferable to limit a content of P to be as low as possible. In the present disclosure, even when a maximum content of P is 0.012%, there is no problem in securing target properties, so the content of P may be set to be 0.012% or less. However, 0% may be excluded considering the level that is inevitably added.

Sulfur (S): 0.003% or less

Sulfur (S) is an element which combines with Mn in steel to form MnS, or the like, thereby significantly reducing low-temperature impact toughness. Therefore, it is advantageous to limit a content of S to be as low as possible. In the present disclosure, even when a maximum content of S is 0.003%, there is no problem in securing target properties, so the content of S may be set to be 0.003% or less. However, 0% may be excluded, considering the level that is inevitably added.

Aluminum (Al): 0.015 to 0.045%

Aluminum (Al) is an element that can deoxidize molten steel inexpensively, and to sufficiently obtain the above-described effect, Al may be included in an amount of 0.015% or more. However, when a content of Al is excessive and exceeds 0.045%, not only may nozzle clogging occur during continuous casting, but also a significant decrease in impact toughness may be caused due to the formation of oxidizing inclusions, which is not preferable.

Niobium (Nb): 0.03 to 0.05%

Niobium (Nb) is precipitated in the form of NbC or Nb(C,N), which greatly improves the strength of a base material, and when reheated at a high temperature, dissolved Nb suppresses the recrystallization of austenite and the transformation of ferrite or bainite, thereby obtaining a structural refinement effect. However, when a content of Nb becomes excessive, undissolved Nb is formed in the form of TiNb(C,N), which becomes a factor which inhibits low-temperature impact toughness, so an upper limit of the content of Nb is preferably limited to 0.05%. Therefore, in the present disclosure, Nb may be included in an amount of 0.03 to 0.05%, preferably 0.030 to 0.050%, and more preferably 0.035 to 0.045%.

Vanadium (V): 0.06% or less (including 0%)

Vanadium (V) has a lower temperature at which V is dissolved than other alloying elements, and V forms VC during an air cooling process after hot rolling, which greatly contributes to increasing strength, so that a strength improvement effect may be obtained. However, when a content of V exceeds 0.06%, there is a problem that hardness of polygonal ferrite becomes excessively high and a fraction of a hard phase such as MA increases, resulting in a significant decrease in low-temperature impact toughness. The content of V is preferably 0.060% or less.

Titanium (Ti): 0.005 to 0.017%

Titanium (Ti) forms TiN when added together with N, thereby reducing the occurrence of surface cracks due to the formation of AlN precipitates, so it is effective that Ti is added in an amount of 0.005% or more. However, when a content of Ti exceeds 0.017%, coarse TiN is formed during reheating of a steel slab, which acts as a factor which inhibits low-temperature impact toughness. Therefore, it is effective that the content of Ti is 0.005 to 0.017%, preferably 0.0050 to 0.0170%, and more preferably 0.01 to 0.015%.

Nitrogen (N) : 0.002 to 0.01%

Nitrogen (N) is an element which is advantageous in suppressing grain growth due to heat affect during welding by forming TiN, when added together with Ti. In order to sufficiently obtain the above-described effect when adding Ti, N may be included in an amount of 0.002% or more. However, when a content of Ti exceeds 0.01%, coarse TiN is formed, which inhibits low-temperature impact toughness, which is not preferable. The content of Ti is preferably 0.0020 to 0.010%.
The remaining component of the present disclosure is iron (Fe). However, since in the common manufacturing process, unintended impurities may be inevitably incorporated from raw materials or the surrounding environment, the component may not be excluded. Since these impurities are known to any person skilled in the common manufacturing process, the entire contents thereof.
The steel sheet may satisfy the following Relational Expression 1. $[C] + [Mn] / 6 + [V] / 5 \leq 0.43$
where [C], [Mn], and [V] are contents (weight %) of each component.
In order to secure low-temperature impact toughness along with the target level of strength, in the present disclosure, in adding a certain amount of elements, advantageous in improving such properties, and it is necessary to appropriately control the contents of the elements. When a value of Relational Expression 1 above exceeds 0.43, it may be advantageous in securing strength, but there is a concern that the physical properties after welding may be significantly deteriorated. In addition, when a large amount of alloying elements are included, economic feasibility is deteriorated due to increased cost, it is effective that Relational Expression 1 above has a value of 0.43 or less, and it is more effective that Relational Expression 1 above has a value of 0.430 or less.
A microstructure of the steel sheet may include, by area %, 60 to 85% of ferrite and a remainder of pearlite and inevitable structures.
In this case, an average grain size of the ferrite may be 50 µm or less.
When a fraction of the ferrite is less than 60%, the strength may be excessively high and low-temperature impact toughness may be reduced, and when the fraction of ferrite exceeds 85%, it may be difficult to secure the strength. When the average grain size of the ferrite is less than 50 µm, it is not easy to secure the yield strength and low-temperature impact toughness suggested in the present disclosure.
In addition, the steel material of the present disclosure may have a yield strength of 355 MPa or more, a tensile strength of 470 MPa or more, and an average Charpy impact absorption energy (CVN) value of 100 J or more at - 20°C, evaluated perpendicularly to a rolling direction at a t/4 point in a thickness direction thereof, where t is a thickness of the steel material (mm), and may have appropriate strength and excellent low-temperature impact toughness.
Meanwhile, the steel material of the present disclosure may secure excellent strength and impact toughness even without a normalizing heat treatment. In the prior art, a normalizing heat treatment was performed to improve yield strength and low-temperature impact toughness. When a normalizing heat treatment is performed, the yield strength and low-temperature impact toughness are improved through an additional grain refinement effect during a transformation process from austenite to ferrite. The steel material of the present disclosure may secure sufficient yield strength and low-temperature impact toughness even without performing the normalizing heat treatment.
Next, a method for manufacturing a steel sheet according to another aspect of the present disclosure will be described in detail. A steel slab satisfying both the alloy components described above and Relational Expression 1 may be prepared, and the steel slab may be manufactured by performing the processes of heating - hot rolling - cooling.
Each process condition is described in detail.

Steel slab heating

It is preferable to perform a process of heating and homogenizing the steel slab, and a heating process may be performed at a temperature within a range of 1150 to 1200°C for 3 hours or more.
When a heating temperature of the steel slab is lower than 1150°C, precipitates (carbides and nitrides) formed within the slab are not sufficiently re-dissolved, so the formation of precipitates decreases in a process after hot rolling, and ultimately, it becomes difficult to satisfy the yield strength and tensile strength suggested in the present disclosure. On the other hand, when the heating temperature exceeds 1200°C, there is a concern that austenite grains may be coarsened, which may deteriorate the properties of the steel. In addition, when the heating time of the steel slab is 3 hours or less, the target temperature may not reach to the central portion, so that it may be difficult that the precipitates (carbides and nitrides) are re-dissolved.

Hot rolling

A steel slab heated according to the above may be hot rolled to manufacture a hot-rolled steel sheet. In this case, the heated steel slab may be rough rolled in a temperature range of 900 to 1100°C, and then finishing rolled in a temperature range shown in Relational Expression 2, and finishing hot rolled at Ar3 or higher. 800°C ≤ finishing rolling start temperature ≤ 857°C + (464 × [C]) + (6445 × [Nb]) - (644 × [Nb]^0.5) + (732 × [V]) - (230 × [V]^0.5) + (890 × [Ti]) + (363 × [Al]) - (357 × [Si])
where [C], [Nb], [V], [Ti], [Al], and [Si] are contents (weight %) of each component.
When the temperature during the rough rolling is lower than 900°C, there is a problem in that a temperature at which subsequent finishing rolling starts becomes too low. When the finishing rolling start temperature is lower than 800°C, there is a problem in that a temperature at which subsequent finishing hot rolling starts becomes too low. When finishing rolling starts at a temperature higher than that of the temperature in Relational Expression 2, sufficient rolling force is not transmitted in a non-recrystallized region, so a size of polygonal ferrite does not become sufficiently be reduced, and as a result, it is difficult to secure the yield strength and impact toughness suggested in the present disclosure.
In addition, when the finishing hot rolling temperature is lower than Ar3, a rolling load increases, which may cause quality defects such as surface cracks.
In the present disclosure, Ar3 may be represented as follows.
Ar3 = 910 - 310C - 80Mn - 20Cu - 55Ni - 80Mo + 119V + 124Ti - 18Nb + 179Al, where each element represents a weight content.
The finishing rolling may be performed with a total reduction ratio of 45% or more, and the number of rolling passes of 8 or less. When the total reduction ratio is less than 45%, austenite may not be sufficiently compressed during rolling, which may cause a final ferrite structure to be coarsened, resulting in a decrease in strength and impact toughness. When the number of rolling passes exceeds 8, reduction per pass decreases, which can also cause coarsening of ferrite.

Cooling

Cooling is performed after the hot rolling, and cooling is not particularly limited in the present disclosure. For example, air cooling may be performed to room temperature.

Normalizing heat treatment

For the steel sheet, a normalizing heat treatment may be performed as needed. In this case, the heat treatment temperature and time may be a temperature within a range of 830 to 930°C for 1.3t+30 minutes or more, where t is a thickness of the steel material (mm).

Mode for Invention

Hereinafter, the present disclosure will be specifically described through the following Examples. However, it should be noted that the following examples are only for describing the present disclosure by illustration, and not intended to limit the scope of rights of the present disclosure. The reason is that the scope of rights of the present disclosure is determined by the matters described in the claims and reasonably inferred therefrom.

(Example)

A steel slab having a thickness of 300 mm was manufactured by continuously casting molten steel having a composition (weight %, the remainder of Fe and inevitable impurities) shown in Table 1, the steel slab was heated under the conditions of Table 2, then rough rolled at a temperature of 900°C or higher, and then finishing rolled under the conditions of Table 2, to manufacture a hot-rolled steel sheet.

[Table 1]

DIVISION	C	Si	Mn	P	S	Al	Nb	V	Ti	N	RELATIONAL EXPRESSION 1
STEEL TYPE 1	0.160	0.45	1.55	0.01	0.003	0.040	0.040	0.000	0.012	0.0035	0.418
STEEL TYPE 2	0.150	0.45	1.53	0.01	0.003	0.035	0.040	0.040	0.013	0.0035	0.413
STEEL TYPE 3	0.155	0.40	1.55	0.01	0.002	0.035	0.040	0.025	0.012	0.0037	0.418
STEEL TYPE 4	0.150	0.40	1.50	0.01	0.003	0.035	0.025	0.000	0.012	0.0036	0.400
STEEL TYPE 5	0.180	0.40	1.55	0.01	0.003	0.040	0.040	0.010	0.015	0.0034	0.440
STEEL TYPE 6	0.150	0.40	1.55	0.01	0.002	0,035	0.040	0.030	0.013	0.0035	0.414
STEEL TYPE 7	0.155	0.45	1.50	0.01	0.002	0.040	0.035	0.035	0.012	0.0035	0.412

Relational Expression 1 is calculated as follows.

[C] + [Mn] / 6 + [V] / 5 \leq 0.43

where [C], [Mn], and [V] are contents (weight %) of each component.

[Table 2]

STEEL TYPE	STEEL SLAB HEATING TEMPERATURE (°C)	STEEL SLAB HEATING TIME (min.)	FINISHING ROLLING START TEMPERATURE (°C)	CUMULATIVE REDUCTION RATIO (%)	THE NUMBER OF FINISHING ROLLING PASSES	ROLLING END TEMPERATURE (°C)	RELATIONAL EXPRESSION 2	WHETHER RELATIONAL EXPRESSION 2 IS SATISFIED	THICKNESS OF STEEL SHEET (mm)	REFERENCE
STEEL TYPE 1	1162	242	890	55	8	860	925	0	50	INVENTIVE EXAMPLE 1
	1160	241	875	50	7	850		0	75	INVENTIVE EXAMPLE 2
	1161	240	830	50	6	825			100	INVENTIVE EXAMPLE 3
STEEL TYPE 2	1160	240	900	55	8	870	903	0	50	INVENTIVE EXAMPLE 4
	1158	241	900	50	7	875		0	75	INVENTIVE EXAMPLE 5
	1155	242	880	50	6	870		0	100	INVENTIVE EXAMPLE 6
STEEL TYPE 3	1164	240	915	55	8	885	920	0	50	INVENTIVE EXAMPLE 7
	1161	240	910	50	7	895		0	75	INVENTIVE EXAMPLE 8
	1158	242	890	50	6	880		0	100	INVENTIVE EXAMPLE 9
STEEL TYPE 4	1162	240	910	55	8	880	866	X	50	COMPARATIVE EXAMPLE 1
	1161	240	900	50	7	875		X	75	COMPARATIVE EXAMPLE 2
	1158	240	880	50	6	870		X	100	COMPARATIVE EXAMPLE 3
STEEL TYPE 5	1164	241	880 910	55	8	880	939	0	50	COMPARATIVE EXAMPLE 4
	1162	242	900	50	7	875		0	75	COMPARATIVE EXAMPLE 5
	1160	240	880	50	6	870		0	100	COMPARATIVE EXAMPLE 6
STEEL TYPE 6	1164	241	910	55	11	870	919	0	50	COMPARATIVE EXAMPLE 7
	1160	241	900	50	10	865		0	75	COMPARATIVE EXAMPLE 8
	1156	240	880	50	9	860		0	100	COMPARATIVE EXAMPLE 9
STEEL TYPE 7	1160	242	910	40	8	880	881	X	50	COMPARATIVE EXAMPLE 10
	1158	240	900	35	7	875		X	75	COMPARATIVE EXAMPLE 11
	1156	241	890	30	6	870		X	100	COMPARATIVE EXAMPLE 12

Here, Relational Expression 2 illustrates a value calculated as follows. 800°C ≤ finishing rolling start temperature ≤ 857°C + (464 × [C]) + (6445 × [Nb]) - (644 × [Nb]^0.5) + (732 × [V]) - (230 × [V]^0.5) + (890 × [Ti]) + (363 × [Al]) - (357 × [Si])
where [C], [Nb], [V], [Ti], [Al], and [Si] are contents (weight %) of each component.
A microstructure of the steel sheets manufactured as shown in Tables 1 and 2 above was observed, and the mechanical properties were evaluated. A microstructure was observed using an optical microscope, and then a fraction and a diameter of ferrite were measured using an analysis program. In this case, the microstructure was measured at a t/4 point, where t is a thickness of the steel sheet (mm), in a thickness direction of each steel material, and the results are shown in Table 3 below.
The mechanical properties were evaluated at a t/4 point in a thickness direction of each steel material, and at this time, tensile specimens were collected at each point in the thickness direction in a direction perpendicular to a rolling direction, and a tensile strength (TS), yield strength (YS), and elongation (El) were measured, and impact test specimens were collected from the JIS No. 4 standard test specimens at the t/4 point in the thickness direction in the rolling direction, and an average impact toughness (CVN) at -20°C was measured, which is shown in Table 4.

Meanwhile, in order to investigate the change in mechanical properties before and after a normalizing heat treatment of Inventive Examples and Comparative Examples, a hot-rolled steel sheet was subjected to a normalizing heat treatment at 890°C for 1.3t+30 minutes, where t is a thickness of the steel sheet (mm) and then air-cooled to room temperature, and the results of the evaluation of mechanical properties before and after the heat treatment are shown in Table 4 below.

[Table 3]

DIVISION	THICKNESS OF STFFI SHFFT (mm)	FERRITE (%)	PEARLITE (%)	DIAMETER OF FERRITE (µm)
INVENTIVE EXAMPLE 1	50	70	30	22
INVENTIVE EXAMPLE 2	75	70	30	28
INVENTIVE EXAMPLE 3	100	65	35	36
INVENTIVE EXAMPLE 4	50	75	25	25
INVENTIVE EXAMPLE 5	75	75	25	31
INVENTIVE EXAMPLE 6	100	70	30	39
INVENTIVE EXAMPLE 7	50	75	25	25
INVENTIVE EXAMPLE 8	75	75	25	30
INVENTIVE EXAMPLE 9	100	70	30	37
COMPARATIVE EXAMPLE 1	50	80	20	51
COMPARATIVE EXAMPLE 2	75	75	25	56
COMPARATIVE EXAMPLE 3	100	75	25	62
COMPARATIVE EXAMPLE 4	50	55	45	24
COMPARATIVE EXAMPLE 5	75	55	45	32
COMPARATIVE EXAMPLE 6	100	55	45	36
COMPARATIVE EXAMPLE 7	50	75	25	52
COMPARATIVE EXAMPLE 8	75	75	25	54
COMPARATIVE EXAMPLE 9	100	70	30	57
COMPARATIVE EXAMPLE 10	50	75	25	51
COMPARATIVE EXAMPLE 11	75	75	25	60
COMPARATIVE EXAMPLE 12	100	70	30	67

[Table 4]

DIVISION	As-rolled				Normalized (890°C)
DIVISION	YIELD STRENGTH (MPa)	TENSILE STRENGTH (MPa)	ELONGATION (%)	IMPACT TOUGHNESS (J@-20°C)	YIELD STRENGTH (MPa)	TENSILE STRENGTH (MPa)	ELONGATION (%)	IMPACT TOUGHNESS (J@-20°C)
INVENTIVE EXAMPLE 1	410	557	30	180	404	554	32	221
INVENTIVE EXAMPLE 2	401	545	29	173	398	540	32	209
INVENTIVE EXAMPLE 3	374	541	30	162	369	541	31	178
INVENTIVE FXAMPLF 4	388	252	32	252	383	550	34	286
INVENTIVE EXAMPLE 5	367	540	32	239	371	536	32	272
INVENTIVE EXAMPLE 6	360	525	30	223	372	525	33	244
INVENTIVE EXAMPLE 7	395	554	33	200	375	550	34	204
INVENTIVE EXAMPLE 8	370	542	33	194	367	543	34	200
INVENTIVE EXAMPLE 9	366	540	31	188	368	542	32	190
EXAMPLE 1 COMPARATIVE	354	536	31	94	343	533	33	122
COMPARATIVE EXAMPLE 2		528	30	88	340	522	31	108
COMPARATIVE EXAMPLE 3	327	511	30	85	325	510	32	92
COMPARATIVE EXAMPLE 4	432	572	28	76	415	565	29	99
COMPARATIVE EXAMPLE 5	417	560	27	60	409	555	30	78
COMPARATIVE EXAMPLE 6	409	544	27	54	402	540	30	67
COMPARATATIVE EXAMPLE 7	367	540	31	91	354	538	33	102
COMPARATIVE EXAMPLE 8	354	532	30	82	356	530	32	99
COMPARATIVE EXAMPLE 9	339	517	31	50	332	519	32	88
COMPARATIVE EXAMPLE 10	342	529	32	92	335	524	33	111
COMPARATIVE EXAMPLE 11	333	520	31	86	334	519	33	100
COMPARATIVE EXAMPLE 12	317	508	31	69	315	509	32	85

Table 4 illustrates tensile properties and low-temperature impact toughness before and after normalizing. In the case of Inventive Examples 1 to 9, the component range, Relational Expressions 1 and 2, and microstructure characteristics, suggested in the present disclosure are all satisfied, so that both tensile properties and low-temperature impact toughness are satisfied, and in particular, it can be confirmed that the yield strength suggested in the present disclosure is 355 MPa or more even as the thickness increases.
Specifically, in the case of Inventive Examples 1 to 9, when comparing the results after as rolled and normalizing heat treatment, the impact toughness slightly increased after the heat treatment, which is different from the physical properties before the heat treatment, but still satisfies the impact toughness suggested in the present disclosure, which indicated that the physical properties may be sufficiently secured by the method of the present disclosure (Normalized Rolling method) even without an additional normalizing heat treatment. Normalized Rolling is a rolling process which is performed by managing rolling conditions so that the same effect as that obtained by normalizing may be obtained (Rolling process in which the final deformation is carried out at a certain temperature range leading to material condition equal to values obtained by normalizing).
On the other hand, in the case of Comparative Examples 1 to 3, which are conditions in which the content of Nb and Relational Expression 2 are outside the values suggested in the present disclosure, and it can be confirmed that Nbc precipitates may not be sufficiently precipitated due to coarsened ferrite grains and a low content of Nb, so the yield strength, tensile strength, and impact toughness suggested in the present disclosure may not be satisfied. In the case of Comparative Examples 4 to 6, which are component systems in which the content of C and Relational Expression 1 are outside the range suggested in the present disclosure, and it can be confirmed that the yield/tensile strength may be sufficient due to excessive addition of the content of C, but the impact toughness is deteriorated.
Comparative examples 7 to 9 are cases in which all the component ranges suggested in the present disclosure are satisfied but the total number of rolling passes is excessive during finishing rolling, and it can be seen that even if a fraction of ferrite and pearlite satisfies the values suggested in the present disclosure, a size of ferrite grains is large so the yield strength and impact toughness are not satisfied. In the case of Comparative Examples 10 to 12, the component range is satisfied, Relational Expression 2 and a total reduction ratio during finishing rolling are outside the values suggested in the present disclosure, and it can be confirmed that the yield strength and impact toughness are not satisfied due to the coarsening of the size of ferrite grains.
In the case of Comparative Examples 1 to 12, the impact toughness was improved after normalizing heat treatment, but the results did not satisfy the yield strength or impact toughness.
FIGs. 1 (a) and (b) are photographs illustrating an optical microstructure at a t/4 point of Inventive Example 1 and Comparative Example 10 in the thickness direction, respectively. When a finishing rolling start temperature does not satisfy Relational Expression 2, it can be predicted that it will be difficult to secure the yield strength and impact toughness intended to be suggested in the present disclosure since ferrite grains are significantly coarsened due to austenite growth during rolling. In the case of Inventive Example 1 satisfying Relational Expression 2, it can be confirmed that it has a very fine ferrite.
FIG. 2 is a graph illustrating a change in yield strength according to a finishing rolling start temperature for a hot-rolled steel sheet with a thickness of 100 mm, using a steel type 2, satisfying the component range suggested in the present disclosure, and it can be confirmed that in a temperature range satisfying Relational Expression 2, a result satisfying the yield strength suggested in the present disclosure is shown, and when the temperature is too high, a decrease in the yield strength occurs.

Claims

A steel sheet, comprising by weight%:
more than 0.10 to 0.17% of carbon (C), 0.2 to 0.5% of silicon (Si), 1.2 to 1.6% of manganese (Mn), 0.012% or less of phosphorus (P), 0.003% or less of sulfur (S), 0.015 to 0.045% of aluminum (Al), 0.03 to 0.05% of niobium (Nb), 0.06% or less of vanadium (V), 0.005 to 0.017% of titanium (Ti), 0.002 to 0.01% of nitrogen (N), with a remainder of Fe and inevitable impurities, the steel sheet satisfying the following Relational Expression 1,

wherein a microstructure of the steel sheet comprises, by area fraction, 60 to 85% of ferrite and a remainder of pearlite and inevitable structures, $[C] + [Mn] / 6 + [V] / 5 \leq 0.43$

where [C], [Mn], and [V] are contents (weight %) of each component.
The steel sheet of claim 1, wherein a thickness of the steel sheet is 100 mm or less.
The steel sheet of claim 1, wherein an average grain size of the ferrite is 50 µm or less.
The steel sheet of claim 1, wherein the steel sheet has a yield strength of 355 MPa or more and a tensile strength of 470 MPa or more, evaluated perpendicularly to a rolling direction, at a t/4 point, where t is a thickness of the steel sheet (mm).
The steel sheet of claim 1, wherein the steel sheet has a Charpy impact absorption energy of 100J or more at - 20°C, evaluated in a rolling direction, at a t/4 point, where t is a thickness of the steel sheet (mm).
A method for manufacturing a steel sheet, comprising:
preparing a steel slab comprising by weight %, more than 0.10 to 0.17% of carbon (C), 0.2 to 0.5% of silicon (Si), 1.2 to 1.6% of manganese (Mn), 0.012% or less of phosphorus (P), 0.003% or less of sulfur (S), 0.015 to 0.045% of aluminum (Al), 0.03 to 0.05% of niobium (Nb), 0.06% or less of vanadium (V), 0.005 to 0.017% of titanium (Ti), 0.002 to 0.01% of nitrogen (N), with a remainder of Fe and inevitable impurities, wherein the following Relational Expression is satisfied;

heating the steel slab to a temperature within a range of 1150 to 1200°C for 3 hours or more;

hot rolling of rough rolling the heated steel slab, and finishing rolling at a finishing rolling start temperature satisfying the following Relational Expression 2; and

cooling after the hot rolling, $[C] + [Mn] / 6 + [V] / 5 \leq 0.43$

where [C], [Mn], and [V] are contents (weight %) of each component. 800°C ≤ finishing rolling start temperature of rolling ≤ 857°C + (464 × [C]) + (6445 × [Nb]) - (644 × [Nb]^0.5) + (732 × [V]) - (230 × [V]^0.5) + (890 × [Ti]) + (363 × [Al]) - (357 × [Si])

where [C], [Nb], [V], [Ti], [Al], and [Si] are contents (weight %) of each component.
The method for manufacturing a steel sheet of claim 6, wherein the hot rolling is performed with a total reduction ratio of 45% or more, and the number of rolling passes of 8 or less.
The method for manufacturing a steel sheet of claim 6, wherein the finishing rolling end temperature is Ar3 or higher.
The method for manufacturing a steel sheet of claim 6, wherein the cooling is air cooling.
The method for manufacturing a steel sheet of claim 6, further comprising, after the cooling, performing a heat treatment at a temperature within a range of 830 to 930°C for 1.3t+30 minutes or more, where t is a thickness of the steel material (mm).