EP4455351A1

EP4455351A1 - Steel plate having high strength and excellent low-temperature impact toughness, and method for manufacturing same

Info

Publication number: EP4455351A1
Application number: EP22911661.1A
Authority: EP
Inventors: Woo-Gyeom KIM; Sang-Ho Kim; Young-Sub BYUN
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2021-12-21
Filing date: 2022-11-29
Publication date: 2024-10-30
Also published as: JP7848328B2; WO2023121027A1; KR20230094375A; US20240417836A1; CN118302559A; EP4455351A4; JP2024545693A

Abstract

The present invention pertains to a steel plate and a method for manufacturing same. More specifically, the present invention pertains to a steel plate having high strength and excellent low-temperature impact toughness, and a method for manufacturing same.

Description

Technical Field

The present disclosure relates to a thick steel plate and a method for manufacturing the same, and more specifically, to a thick steel plate having high strength and excellent low-temperature impact toughness, and a method for manufacturing same.

Background Art

Since the 2000s, interest has been focused on environmental issues and renewable energy to reduce greenhouse gas emissions. Renewable energy is a term which refers to a combination of new energy (hydrogen, fuel cells, and the like) and renewable energy (solar heat, wind power, bio, and the like), and thereamong, wind power generation is attracting attention as a next-generation energy source, as an eco-friendly power generation method that does not generate waste and does not cause pollution.Among types of wind power generation, onshore wind power, installed on land, is limited in mission and a space for optimal wind formation, so recently, offshore wind power, installed in the sea, has been growing rapidly, especially in Europe.
Although offshore wind power was activated later than onshore wind power, the relative advantage of offshore wind power over onshore wind power is increasingly highlighted as the level of technology has advanced due to various advantages such as low concerns about wind speed, noise generation, and the ability to secure a large catchment area. In particular, offshore wind power has the great advantage of being able to increase power generation capacity per wind turbine, compared to onshore wind power. In other words, average power generation capacity is about twice that of onshore wind power, and average capacity of new turbine installations per unit based on European standards is rapidly increasing from 4MW in 2015 to 7.2MW in 2019, and is expected to exceed 10MW within 2-3 years.
Accordingly, the strength of the applied thick steel plate gradually increased, and most thick steel plates had a yield strength of 325 MPa based on a thickness of 80 mm, but in the 20s, thick steel plates having yield strengths of 380 and 410 MPa were being applied. A substructure of the offshore wind power is largely divided into monopile and jacket, and the jacket-type substructure is divided into a pinpile-type or a suction bucket-type depending on a method of fixation to the seafloor.
In the case of the monopile substructure, it is divided into a monopile portion inserted into the seafloor, and a transition piece portion connecting the monopile and a tower portion. In this structure, a load increases the most, and high-strength steel can be mainly applied to a connection portion of the monopile and the transition piece, which is a joint portion. The important support area of this offshore wind power substructure is formed of a thick steel plate which not only has high strength but can also guarantee extremely thick and low-temperature toughness.

Summary of Invention

Technical Problem

An aspect of the present disclosure is to provide a thick steel plate having high strength and excellent low-temperature impact toughness, and a method for manufacturing the same.
The subject of the present invention is not limited to the above. The subject of the present invention will be understood from the overall content of the present specification, and those of ordinary skill in the art to which the present invention pertains will have no difficulty in understanding the additional subject of the present invention.

Solution to Problem

According to an aspect of the present disclosure, a steel plate may be provided, the steel plate comprising by weight%: 0.04 to 0.08% of carbon (C), 0.1 to 0.35% of silicon (Si), 1.4 to 1.8% of manganese (Mn), 0.01 to 0.035% of sol.Al (Al), 0.2 to 0.5% of nickel (Ni), 0.1 to 0.3% of chromium (Cr), 0.05 to 0.15% of molybdenum (Mo), 0.015 to 0.035% of niobium (Nb), 0.005 to 0.02% of titanium (Ti), 0.002 to 0.006% of nitrogen (N), 0.01% or less of phosphorous (P), 0.003% or less of sulfur (S), with a balance of iron (Fe) and other inevitable impurities,
wherein an R value defined in Relational expression 1 below is 0.85 to 1.35,
a microstructure at a point equal to 1/4 of a thickness thereof has a mixed structure of acicular ferrite and bainite as a main structure, and comprises by area fraction, 2% or less of a sum of residual cementite and MA,
an average grain size of acicular ferrite at the point equal to 1/4 of the thickness is 25 µm or less. $R = [Ni] + 3 [Mo] + 2 [Cr]$

where [Ni], [Mo], and [Cr] are a weight percent of each element.
The steel plate may include by area fraction, 40 to 60% of acicular ferrite and 40 to 60% of bainite, as the microstructure at the point equal to 1/4 of the thickness.
The steel plate may include by area fraction, 1% or less of a sum of cementite and MA.
The steel plate may have an acicular ferrite grain size at the point equal to 1/4 of the thickness of 15 to 25 µm.
The steel plate may have a thickness of 50 to 100 mm.
The steel plate may have a yield strength of 460 MPa or more, a tensile strength of 580 MPa or more, and impact toughness of 100 J or more at -50°C.
According to another aspect of the present disclosure, a method for manufacturing a steel plate may be provided, the method comprising: reheating a steel slab comprising by weight%: 0.04 to 0.08% of carbon (C), 0.1 to 0.35% of silicon (Si), 1.4 to 1.8% of manganese (Mn), 0.01 to 0.035% of sol.Al (Al), 0.2 to 0.5% of nickel (Ni), 0.1 to 0.3% of chromium (Cr), 0.05 to 0.15% of molybdenum (Mo), 0.015 to 0.035% of niobium (Nb), 0.005 to 0.02% of titanium (Ti), 0.002 to 0.006% of nitrogen (N), 0.01% or less of phosphorous (P), 0.003% or less of sulfur (S), with a balance of iron (Fe) and other inevitable impurities, wherein an R value defined in the following Relational expression 1 is 0.85 to 1.35;

rolling the reheated steel slab in a recrystallization zone at a temperature range of 900°C or higher with an amount of reduction of final pass of 15 to 25 mm;
rolling the steel plate rolled in the recrystallization zone in a non-recrystallization zone at a rolling finish temperature of Ar3+20 to Ar3+60; and
water cooling the rolled steel sheet in the non-recrystallization zone to a temperature range of 400°C or lower based on the point equal to 1/4 of the thickness.

R = [Ni] + 3 [Mo] + 2 [Cr]

The reheating may be performed at a temperature range of 1020 to 1100°C, and
when rolling is performed in the non-recrystallized zone, a cumulative reduction ratio may be 30 to 50%.
During the cooling, cooling may be performed at a cooling rate of 5 to 10°C/s based on the point equal to 1/4 of the thickness.
The steel plate may have a thickness of 50 to 100 mm.

Advantageous Effects of Invention

According to an aspect of the present disclosure, a thick steel plate having high strength and excellent low-temperature impact toughness and a method for manufacturing the same may be provided.
According to an aspect of the present disclosure, a thick steel plate that has excellent strength and low-temperature impact toughness and may be applied as an extremely thick steel material for offshore wind power, and may also be used as a structural steel material for infrastructure industries such as construction and bridges, and a method for manufacturing the same may be provided.

Brief description of drawings

FIG. 1 is a photograph of a microstructure of an Inventive Example according to an aspect of the present disclosure observed at 500x magnification using an optical microscope.

Best Mode for Invention

Hereinafter, preferred embodiments of the present disclosure will be described. Embodiments of the present disclosure may be modified in various forms, and the scope of the present disclosure should not be construed as being limited to the embodiments described below. The present embodiments are provided to those skilled in the art to further elaborate the present disclosure.
Hereinafter, the present disclosure will be described in detail.
Hereinafter, a steel composition of the present disclosure will be described in detail.
In the present disclosure, unless otherwise specified, % indicating the content of each element is based on weight.
Steel according to an aspect of the present disclosure may comprise, by weight%: 0.04 to 0.08% of C, 0.1 to 0.35% of Si, 1.4 to 1.8% of Mn, 0.01 to 0.035% of sol.Al, 0.2 to 0.5% of Ni, 0.1 to 0.3% of Cr, 0.05 to 0.15% of Mo, 0.015 to 0.035% of Nb, 0.005 to 0.02% of Ti, 0.002 to 0.006% of N, 0.01% or less of P, 0.003% or less of S, with a balance of iron (Fe) and other inevitable impurities.

Carbon (C): 0.04 to 0.08%

Carbon (C) is an element which causes solid solution strengthening, and exist as a carbonitride due to Nb, or the like, and a content of C may be limited to 0.04% or more. On the other hand, when the carbon (C) content exceeds 0.08%, it may not only promote the formation of MA, but also generate pearlite, which may deteriorate impact characteristics at low temperatures, and there is a risk in that welding properties may be deteriorated when welding a structure. A more preferable upper limit of the C content may be 0.07%.

Silicon (Si): 0.1 to 0.35%

Silicon (Si) is an element which assists Al in deoxidizing molten steel and is necessary to secure yield strength and tensile strength, and a content of silicon (Si) may be 0.1% or more. However, if the Si content exceeds 0.35%, there may be a problem of inhibiting diffusion of C to encourage the formation of MA. More preferably, 0.15% or more of silicon (Si) may be included, and even more preferably, 0.25% or less of silicon (Si) may be included.

Manganese (Mn): 1.4 to 1.8%

Since manganese (Mn) has a significant effect of increasing strength through solid solution strengthening, 1.4% or more of manganese (Mn) is preferably added. On the other, when a content of manganese (Mn) is excessive, it may cause a decrease in toughness due to formation of MnS inclusions and central segregation, so an upper limit of the Mn content may be limited to 1.8%.

Aluminum (sol.Al): 0.01 to 0.035%

Aluminum (sol.Al) is a major deoxidizing agent in steel, and it is preferable to add 0.01% or more of aluminum (sol.Al) to obtain the effect. However, a content of aluminum (sol.Al) exceeds 0.035%, it may cause a decrease in low-temperature toughness due to an increase in a fraction and size of Al₂O₃ inclusions. In addition, similar to Si, there may be a risk of deteriorating low-temperature toughness characteristics by promoting the creation of a base material and MA in a weld heat-affected zone. More preferably, 0.015% or more of aluminum (sol.Al) may be included, and even more preferably, 0.03% or less of aluminum (sol.Al) may be included.

Nickel (Ni): 0.2 to 0.5%

Nickel (Ni) is an element improving strength without deteriorating impact toughness, and may increase strength by promoting the formation of an appropriate amount of acicular ferrite, so it is preferable that 0.2% or more of nickel (Ni) is added. On the other hand, when a content of nickel (Ni) exceeds 0.5%, an Ar3 temperature may drop to form bainite, and accordingly, there may be a risk that impact toughness in an extremely thick material may decrease. A more preferable lower limit of the nickel (Ni) content may be 0.3%.

Chromium (Cr): 0.1 to 0.3%

Chromium (Cr) is a carbide-forming element, which is advantageous for securing strength, but in an extremely thick steel material, since coarse carbides may be formed depending on a cooling rate of steel, which may impede impact toughness, a content of chromium (Cr) may be limited to 0.1 to 0.3%. A more preferable lower limit of the Cr content may be 0.15%.

Molybdenum (Mo): 0.05 to 0.15%

Molybdenum (Mo) is an element which effectively increases strength with an addition of a small amount of Mo, and since Mo forms Mo-C-based precipitates to improve the strength, it is preferable that 0.05% or more of Mo is added. However, since coarsening of precipitates may occur due to excessive addition of molybdenum (Mo), an upper limit of the Mo content may be limited to 0.15%. A more preferable lower limit of the Mo content may be 0.08%, and a more preferable upper limit of the Mo content may be 0.12%.

Niobium (Nb): 0.015 to 0.035%

Niobium (Nb) is an element which suppresses recrystallization during rolling or cooling by forming a solid solution or precipitating carbonitrides, to make the structure finer and increase strength, and Nb may be added in an amount of 0.015% or more. However, C concentration occurs due to C affinity and promotes MA production, which can reduce toughness and fracture characteristics at low temperatures, so an upper limit of the Nb content may be limited to 0.035%. A more preferable lower limit of the Nb content may be 0.02%, and a more preferable upper limit of the Nb content may be 0.03%.

Titanium (Ti): 0.005 to 0.02%

Titanium (Ti) may combine with oxygen or nitrogen to form precipitates. The precipitates contribute to refinement by suppressing coarsening of the structure, and play a role in improving toughness, so it is preferable that Ti is added in an amount of 0.005% or more. However, if a content of Ti exceeds 0.02%, there is a risk that it may cause destruction due to coarsening of the precipitates. A more preferable lower limit of the T content may be 0.01%, and a more preferable upper limit of the T content may be 0.018%.

Nitrogen (N): 0.002 to 0.006%

Nitrogen (N) forms precipitates with Ti, Nb, Al, and the like, and when reheated, N may help improve strength and toughness by refining an austenite structure. However, when N is contained excessively, it may cause surface cracks and form precipitates at high temperatures, and the remaining nitrogen (N) exists in an atomic state and may reduce toughness, so a content of nitrogen (N) may be limited to 0.002 to 0.006%.

Phosphorus (P): 0.01% or less

Phosphorus (P) is an element causing grain boundary segregation and can cause embrittlement, so an upper limit of a content of phosphorous (P) may be limited to 0.01%. However, 0% may be excluded considering an avoidably added level.

Sulfur (S): 0.003% or less

Sulfur (S) may mainly combine with Mn to form MnS inclusions, which can be a factor impairing low-temperature impact toughness. Therefore, in order to secure low-temperature toughness and low-temperature fatigue properties, an upper limit of a content of S may be limited to 0.003%. However, 0% may be excluded considering an avoidably added level.
The steel of the present disclosure may include remaining iron (Fe) and unavoidable impurities in addition to the above-described composition. Since unavoidable impurities may be unintentionally incorporated in a common manufacturing process, the component may not be excluded. Since these impurities are known to any person skilled in the common manufacturing process, the entire contents thereof are not particularly mentioned in the present specification.
In particular, copper (Cu) may be added as an impurity, but in the present disclosure, a content of copper (Cu) may be limited to less than 0.05%.
The steel according to an aspect of the present disclosure may have an R value of 0.85 to 1.35 defined in Relational expression 1 below.
In the present invention, Relational expression 1 is proposed to secure strength and low-temperature toughness at -50 °C at the same time. Relational expression 1 relates to a component formula for securing strength and toughness, and the desired level of strength and low-temperature toughness may be secured by controlling an R value of Relational expression 1. When the R value of Relational expression 1 is less than 0.85, there is a problem in that the desired yield strength may not be secured due to insufficient solid solution strengthening, precipitation strengthening, hardenability, and the like, and when the R value of Relational expression 1 exceeds 1.35, hard structures such as MA, bainite, and the like may be formed, resulting in inferior impact toughness. $R = [Ni] + 3 [Mo] + 2 [Cr]$

where [Ni], [Mo], and [Cr] are a weight percent of each element.
Hereinafter, a microstructure of steel of the present disclosure will be described in detail.
In the present disclosure, unless specifically stated otherwise, % indicating a fraction of microstructure is based on area.
The steel according to an aspect of the present disclosure may include, by area fraction: 40 to 60% of acicular ferrite, 40 to 60% of bainite, and 3% or less of a sum of residual cementite and MA as a microstructure at a point equal to 1/4 of a thickness thereof.
In the present disclosure, in order to implement impact toughness at -50 °C at the point equal to 1/4 of the thickness, a size, dislocation density, and the like of acicular ferrite are important, and it is preferable to minimize cementite and MA. More preferably, 1% or less of a sum of cementite and MA may be included. In the present disclosure, a point equal to 1/4 of the thickness is t/4, where t is a thickness of the steel plate.
The steel according to an aspect of the present disclosure have an average grain size of acicular ferrite at a point equal to 1/4 of the thickness of 25 µm or less.
In the present disclosure, the average grain size of acicular ferrite may be limited to 25 um or less to ensure low-temperature impact toughness. If the average grain size of acicular ferrite exceeds 25um, there may be a problem in that a shock absorption energy value decreases at -50°C. Meanwhile, due to the characteristics of a thick steel plate with a thickness of 50 mm or more, targeted in the present disclosure, there is a limitation in refinement of the grains, so a lower limit of the size may be limited to 15 µm.
Hereinafter, a method for manufacturing steel of the present disclosure will be described in detail.
The steel according to an aspect of the present disclosure may be manufactured by reheating, rolling, and cooling a steel slab satisfying the above-described alloy composition.

Reheating

A steel slab satisfying the alloy composition of the present disclosure may be reheated to a temperature range of 1020 to 1100°C.
If a reheating temperature is higher than 1100°C, austenite grains may become coarse, which may reduce toughness due to the development of bainite structure due to increased hardenability. On the other hand, if the reheating temperature is lower than 1020°C, Ti, Nb, and the like may not be sufficiently dissolved, which may result in a decrease in strength.

Rolling in a recrystallization zone

The reheated steel slab may be rolled in a recrystallization zone with an amount of reduction of final pass of 15 to 25 mm in a temperature range of 900°C or higher.
In the present disclosure, the operation of rolling in a recrystallization zone is performed for completely recrystallizing austenite, and suppressing the refinement and growth of austenite. Rolling in the recrystallization zone is preferably performed at a temperature range of 900°C or higher for complete austenite recrystallization, and an amount of reduction of final pass may be 15 to 25 mm for initial austenite refinement. If the amount of reduction of final pass is less than 15 mm, it may be difficult to secure the desired level of refinement. Meanwhile, during rolling, an upper limit of the amount of reduction of final pass may be limited to 25 mm, in consideration of productivity according to equipment specifications.

Rolling in a non-recrystallization zone

The steel plate rolled in the recrystallization zone may be rolled in a non-recrystallization zone at a rolling finish temperature of Ar3+20 to Ar3+60 with a cumulative reduction ratio of 30 to 50%.
In the present disclosure, it is preferable that the rolling finish temperature is closer to directly above an Ar3 temperature in order to refine an acicular ferrite size. When the rolling finish temperature in the non-recrystallization zone is lower than Ar3+20, a surface thereof before cooling may be a two phase of austenite and ferrite, and when cooling is performed, there may be a problem in that a hard phase is formed on the surface. On the other hand, the rolling finish temperature in the non-recrystallization zone is higher than Ar3+60°C, there may be a problem in which grains cannot be refined during rolling in the non-recrystallized zone.
In addition, in the case of thick materials with a thickness of 50 mm or more, it is preferable that a cumulative reduction ratio is 30 to 50%. When the cumulative reduction ratio is less than 30%, an amount of rolling in the non-recrystallization zone may be reduced, causing a problem such as pancaking of the structure and in that it is difficult to be refined, and when the cumulative reduction ratio exceeds 50%, there may be a risk that low-temperature impact toughness is deteriorated due to the insufficient amount of rolling in the non-recrystallized zone. $Ar3 = 910 - 310 [C] - 80 [Mn] - 20 [Cu] - 15 [Cr] - 55 [Ni] - 80 [Mo]$

where [C], [Mn], [Cu], [Cr], [Ni], and [Mo] are a weight percent of each element.

Cooling

The steel plate rolled in the non-recrystallization zone may be cooled to a temperature range of 400°C or lower at a cooling rate of 5 to 10°C/s based on a point equal to 1/4 of a thickness thereof.
It is preferable for the thick steel plate targeted in the present disclosure to secure impact toughness around the point equal to 1/4 of the thickness. Therefore, the cooling rate limited in the present disclosure may be based on the point of 1/4 of the thickness. When a cooling finish temperature is higher than 400°C, or a cooling rate exceeds 10°C/s, the formation of MA may be promoted, so the impact toughness may be deteriorated. On the other hand, when the cooling rate is less than 5°C/s, it may be difficult to secure the desired level of strength. In the present disclosure, water cooling can be used as a cooling method.
The steel of the present disclosure manufactured as described above has a thickness of 50 to 100 mm, yield strength of 460 MPa or more, tensile strength of 580 MPa or more, and impact toughness of 100J or more at -50°C, and high strength and excellent low-temperature impact toughness may be provided.
Hereinafter, the present disclosure will be described in more detail through examples. It should be noted that the following examples are only for understanding of the present invention, and are not intended to specify the scope of the present invention. The scope of the present invention may be determined by the matters described in the claims and the matters reasonably inferred therefrom.

Mode for Invention

(Example)

After preparing molten steel having the alloy composition shown in Table 1 below, a slab was manufactured using continuous casting. The slab was reheated, rolled, and cooled under the conditions shown in Table 2 below to manufacture a steel plate. A rolling temperature in a recrystallization zone, not disclosed in Table 2, was applied equally at a temperature of 900°C or higher, and when rolling was performed in a non-recrystallization zone, a cumulative reduction ratio was applied equally within a range of 30 to 50%. In addition, Table 1 showed the calculated Ar3 temperature and R value of Relational expression 1 according to the alloy composition of each steel type.

[Table 1]

St ee l ty pe	Alloy composition (wt%)												Ar3 (°C )	Rela tion al expr essi on 1
St ee l ty pe	C	Si	Mn	P	S	Al	Ni	Cr	Mo	Ti	Nb	N	Ar3 (°C )	Rela tion al expr essi on 1
A	0.05 6	0.14	1.62	0.007 5	0.001 8	0.02 5	0.35	0.21	0.12	0.01 3	0.02 4	0.003 7	731	1.13
B	0.06 3	0.18	1.58	0.008 2	0.001 9	0.02 4	0.36	0.18	0.09	0.01 2	0.02 6	0.004 1	734	0.99
C	0.06 5	0.19	1.55	0.006 8	0.002 0	0.02 6	0.42	0.23	0.1	0.01 2	0.02 1	0.004 2	731	1.18
D	0.06 2	0.15	1.61	0.008 3	0.001 5	0.01 9	0.12	0.07	0.03	0.01 1	0.02 8	0.003 6	752	0.35
E	0.05 7	0.20	1.59	0.082	0.019	0.02 4	0.62	0.38	0.18	0.01 2	0.02 5	0.003 9	711	1.92
F	0.06	0.18	1.63	0.009 1	0.002 2	0.02 5	0.41	0.23	0.1	0.01 2	0.02 6	0.040	727	1.17
G	0.05 7	0.18	1.61	0.009	0.001 8	0.02 3	0.4	0.25	0.11	0.01 2	0.02 5	0.003 8	729	1.23
H	0.06 1	0.15	1.63	0.008 2	0.001 8	0.02 2	0.48	0.26	0.13	0.01 1	0.02 5	0.004 2	720	1.39

R = [Ni] + 3 [Mo] + 2 [Cr]

where [C], [Mn], [Cu], [Cr], [Ni], and [Mo] are a weight percent of each element.

Ar3 = 910 - 310 [C] - 80 [Mn] - 20 [Cu] - 15 [Cr] - 55 [Ni] - 80 [Mo]

[Table 2]

Spe cim en No.	Steel type	Reheating	Rolling in a recrystalliza tion zone	Rolling in a non-recrystallizati on zone	Cooling
Spe cim en No.	Steel type	temperatu re(°C)	Amount of reduction of final pass (mm)	Finish temperature (°C)	Finish temperature (° C)	rate(°C/s)
1	A	1086	21.4	789	365	7.1
2	B	1080	22.5	791	345	8.2
3	C	1093	18.6	786	384	7.8
4	D	1088	19.3	794	356	6.8
5	E	1085	18.7	765	374	7.1
6	F	1087	20.4	787	507	7.5
7	G	1090	17.4	829	394	6.9
8	G	1088	10.3	775	342	7.0
9	H	1092	20.4	778	354	6.8

In Table 3 below, a microstructure of the manufactured steel plate was measured and described. The microstructure was measured at a point equal to 1/4 of a thickness of steel, and fractions of acicular ferrite (AF), cementite, and MA were respectively observed, and the remaining fraction thereof was observed to be bainite. In addition, an acicular ferrite grain size at the point equal to 1/4 of the thickness of the steel was measured and shown. The microstructure was measured using an optical microscope at 500x magnification.

In addition, in Table 3, physical property values for each manufactured specimen were measured and showed. A yield strength (YS), tensile strength (TS), and elongation (El) were evaluated through a tensile test, and a circular specimen was collected at the point equal to 1/4 of the thickness in a direction perpendicular to rolling according to the EN-ISO 6892-1 standard, and an average of two tests was measured. In addition, an impact toughness value at - 50°C was measured. For the impact toughness, a specimen was collected in a direction parallel to rolling at the point equal to 1/4 of the thickness according to the EN ISO 148-1 standard and an average of three tests was measured.

[Table 3]

Spe cim en No.	Stee l type	Microstructure			Mechanical properties				Divis ion
Spe cim en No.	Stee l type	AF (%)	Cementite and MA(%)	AF grain size (µm)	Yield strengt h (MPa)	Tensile strengt h (MPa)	elongat ion (%)	Impact toughness (-50°C,J)	Divis ion
1	A	55	0.7	21	520	618	24	338	Inven tive Examp le 1
2	B	58	0.6	22	491	612	26	376	Inven tive Examp le 2
3	C	49	0.8	21	498	618	25	316	Inven tive Examp le 3
4	D	69	0.8	23	445	568	27	274	Compa rativ e Examp le 1
5	E	27	0.7	20	534	654	21	26	Compa rativ e Examp le 2
6	F	42	2.6	21	459	598	27	28	Compa rativ e Examp le 3
7	G	55	0.7	34	441	581	31	28	Compa rativ e Examp le 4
8	G	56	2.6	33	438	596	26	31	Compa rativ e Examp le 5
9	H	34	3.2	20	482	635	21	32	Compa rativ e Examp le 6

As shown in Table 3, in Inventive Example satisfying the alloy composition and manufacturing conditions of the present disclosure, the microstructure characteristic proposed in the present disclosure were satisfied, and the physical properties desired in the present disclosure were secured. FIG. 1 is a photograph of a microstructure of Inventive Example according to an embodiment of the present disclosure observed at 500x magnification using an optical microscope.
On the other hand, Comparative Examples 1 and 2 are examples not satisfying the alloy composition of the present disclosure, and in Comparative Examples 1 and 2, a desired level of strength or impact toughness in the present disclosure was not secured. Specifically, in Comparative Example 1 in which a value of Relational expression 1 was below the range of the present disclosure, a bainite fraction was reduced, causing a decrease in strength. In Comparative Example 2 in which the value of Relational expression 1 exceeded the range of the present disclosure, bainite was excessively formed, resulting in poor impact toughness.
Comparative Example 3 is an example in which a cooling end temperature was outside the range of the present disclosure, and in Comparative Example 3, large amounts of fractions of cementite and MA were generated, resulting in poor impact toughness properties.
In Comparative Example 4, it can be seen that the rolling end temperature in the non-recrystallization zone was high, so that the yield strength decreased due to the lack of grain refinement and at the same time, the impact toughness at -50°C was inferior.
In Comparative Example 5, initial austenite refinement was not achieved well due to insufficient reduction in the amount of reduction of final pass when rolling is performed in the recrystallization zone, so it can be confirmed that a decrease in the yield strength and impact toughness, due to an increase in a final ferrite grain size, formation of cementite and MA due to increased hardenability, and the like.
Comparative Example 6 is a case satisfying the component range proposed in the present disclosure, but not satisfying the value of Relational expression 1, and in Comparative Example 6, the impact toughness decreased sharply due to a decrease in acicular ferrite and an excessive increase in bainite.
While example embodiments have been shown and described above, it will be apparent to those skilled in the art that modifications and variations could be made without departing from the scope of the present disclosure as defined by the appended claims.

Claims

A steel plate, comprising by weight%:
0.04 to 0.08% of carbon (C), 0.1 to 0.35% of silicon (Si), 1.4 to 1.8% of manganese (Mn), 0.01 to 0.035% of sol.Al (Al), 0.2 to 0.5% of nickel (Ni), 0.1 to 0.3% of chromium (Cr), 0.05 to 0.15% of molybdenum (Mo), 0.015 to 0.035% of niobium (Nb), 0.005 to 0.02% of titanium (Ti), 0.002 to 0.006% of nitrogen (N), 0.01% or less of phosphorous (P), 0.003% or less of sulfur (S), with a balance of iron (Fe) and other inevitable impurities,

wherein an R value defined in Relational expression 1 below is 0.85 to 1.35,

a microstructure at a point equal to 1/4 of a thickness thereof has a mixed structure of acicular ferrite and bainite as a main structure, and comprises by area fraction, 2% or less of a sum of residual cementite and MA, and

an average grain size of acicular ferrite at the point equal to 1/4 of the thickness is 25 µm or less, $R = [Ni] + 3 [Mo] + 2 [Cr]$

where [Ni], [Mo], and [Cr] are a weight percent of each element.
The steel plate of claim 1, wherein the steel plate comprises by area fraction, 40 to 60% of acicular ferrite and 40 to 60% of bainite as the microstructure at the point equal to 1/4 of the thickness of the steel plate.
The steel plate of claim 1, wherein the steel plate comprises by area fraction, 1% or less of a sum of cementite and MA.
The steel plate of claim 1, wherein the steel plate has an average grain size of acicular ferrite at the point equal to 1/4 of the thickness of the steel plate of 15 to 25 µm.
The steel plate of claim 1, wherein the steel plate has a thickness of 50 to 100 mm.
The steel plate of claim 1, wherein the steel plate has a yield strength of 460MPa or more, a tensile strength of 580MPa or more, and an impact toughness of 100J or more at -50°C.
A method for manufacturing a steel plate, comprising:
reheating a steel slab comprising by weight%: 0.04 to 0.08% of carbon (C), 0.1 to 0.35% of silicon (Si), 1.4 to 1.8% of manganese (Mn), 0.01 to 0.035% of sol.Al (Al), 0.2 to 0.5% of nickel (Ni), 0.1 to 0.3% of chromium (Cr), 0.05 to 0.15% of molybdenum (Mo), 0.015 to 0.035% of niobium (Nb), 0.005 to 0.02% of titanium (Ti), 0.002 to 0.006% of nitrogen (N), 0.01% or less of phosphorous (P), 0.003% or less of sulfur (S), with a balance of iron (Fe) and other inevitable impurities, wherein an R value defined in the following Relational expression 1 is 0.85 to 1.35;

rolling the reheated steel slab in a recrystallization zone at a temperature range of 900°C or higher with an amount of reduction of final pass of 15 to 25 mm;

rolling the steel plate rolled in the recrystallization zone in a non-recrystallization zone at a rolling finish temperature of Ar3+20 to Ar3+60; and

water cooling the steel plate rolled in the non-recrystallization zone to a temperature range of 400°C or lower based on the point equal to 1/4 of the thickness of the steel plate, $R = [Ni] + 3 [Mo] + 2 [Cr]$

where [Ni], [Mo], and [Cr] are a weight percent of each element.
The method for manufacturing a steel plate of claim 7, wherein the reheating is performed at a temperature range of 1020 to 1100°C, and
when rolling is performed in the non-recrystallized zone, a cumulative reduction ratio is 30 to 50%.
The method for manufacturing a steel plate of claim 7, wherein, during the cooling, cooling is performed at a cooling rate of 5 to 10°C/s, based on the point of 1/4 of the thickness of the steel plate.
The method for manufacturing a steel plate of claim 7, wherein the steel plate has a thickness of 50 to 100 mm.