EP3561115B1

EP3561115B1 - Thick steel plate having excellent low-temperature impact toughness and ctod characteristic and manufacturing method therefor

Info

Publication number: EP3561115B1
Application number: EP17885144.0A
Authority: EP
Inventors: Woo-Gyeom KIM; Kyung-Keun Um; Ki-Hyun Bang
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2016-12-23
Filing date: 2017-12-22
Publication date: 2022-07-13
Anticipated expiration: 2037-12-22
Also published as: JP6824415B2; EP3561115A4; WO2018117727A1; CN110100026B; CN110100026A; KR101899694B1; EP3561115A1; JP2020509206A; KR20180074470A

Description

[Technical Field]

The present disclosure relates to a thick steel sheet having excellent low temperature impact toughness and CTOD properties which may be preferably applied to an offshore structural steel material, and a method of manufacturing the same.

[Background Art]

The North Pole has been considered as a land storing future energy sources, and the development of petroleum and gas sources have been conducted, centering on countries neighboring the Arctic Circle. The development of energy sources in the Arctic will be accelerated due to the exhaustion of energy source of land, offshore regions, and deep sea regions.
A steel material applied to offshore structure facilities for mining, drilling, storing energy sources developed in the polar regions may need to have toughness at a low temperature of -60°C or lower, and may need to have a CTOD value representing fatigue fracture properties at -60°C. Also, strength and thickness of a steel material have been increased as the sizes of facilities have increased and facilities have been integrated.
As for resistance against brittleness fracture, generally, there may be resistance against the formation of brittleness cracks and resistance against the propagation of brittleness cracks. The formation of brittleness cracks may refer to, after fatigue cracks started in a defect portion in a structure is grown to a certain size, the generation of brittleness cracks from the grown fatigue cracks when high external stress is applied. The resistance properties of the material which may prevent the formation of brittleness cracks may be referred to as resistance against the formation of brittleness cracks, and the resistance is tested using a CTOD (crack tip opening displacement) testing method prescribed in BS 7448 standard or ASTM 1290 standard. Thus, excellent CTOD properties may indicate that resistance against the formation of brittleness cracks may be excellent.
A large amount of research and development have been conducted to secure low temperature impact toughness and CTOD properties. For example, reference 1 discloses a method of manufacturing a steel sheet to maintain excellent CTOD properties by maintaining a certain level of reduction ratios of final three passes during rolling the steel sheet including a rolling process in which a reduction ratio is low, such as a width widening rolling.
However, reference 1 has a problem in which it may be difficult to sufficient low temperature toughness and CTOD properties.
Also, as the use environments have become harsh, there has been demand for development of a technique which may secure excellent impact toughness even at an extremely low temperature of about -80°C, and there has been demand for development of a thick steel sheet having excellent CTOD properties and strength and a method of manufacturing the same.

(Prior Art)

(Reference 1) Korean Laid-Open Patent Publication No. 10-2010-0066757
Further, KR 2009 0070485 A discloses a steel sheet comprising, by weight%, 0.06% to 0.12% of C, 0.005 to 0.08% of Si, 1.0 to 2.0% of Mn, 0.01% or less of P, 0.003% or less of S, 0.001 to 0.01% of Al, 0.5 to 2.0% of Ni, 0.001 to 0.02% of Ti, 0.005 to 0.03% of Nb, 0.05 to 0.4% of Cu, 0.002 to 0.006% of N, and a balance of Fe and inevitable impurities.

[Disclosure]

[Technical Problem]

An aspect of the present disclosure is to provide a thick steel sheet having excellent low temperature impact toughness and CTOD properties which may be preferably applied to an offshore structural steel material, and a method of manufacturing the same.
However, aspects of the present disclosure are not limited thereto. Additional aspects will be set forth in part in the description which follows, and will be apparent from the description to those of ordinary skill in the related art.

[Technical Solution]

The present invention relates to a thick steel sheet having excellent low temperature impact toughness and CTOD properties comprising, by weight%, 0.02 to 0.055% of C, 0.005 to 0.08% of Si, 1.0 to 2.0% of Mn, 0.01% or less of P, 0.003% or less of S, 0.001 to 0.01% of Al, 0.5 to 2.0% of Ni, 0.001 to 0.02% of Ti, 0.005 to 0.03% of Nb, 0.05 to 0.4% of Cu, 0.002 to 0.006% of N, and a balance of Fe and inevitable impurities, wherein the thick steel sheet satisfies Equation 1 and Equation 2 below, $3.0 \leq Mn + 2 Ni \leq 4 .3$
$0.05 \leq C + Si + 10 Al \leq 0 .25$
where each element symbol indicates a content of each element by weight%, and wherein a microstructure consists of ferrite of 95 area% or higher, and a sum of martensite-austenite MA and cementite of 2 area% or lower, and wherein the thick steel sheet has yield strength of 420MPa or higher, impact toughness of 200J or higher at -80°C, and a CTOD of 0.5mm or higher at -60°C, and wherein the thick steel sheet has tensile strength of 500MPa or higher, an elongation rate of 25% or higher, and impact toughness of 400J or higher at -60°C, and wherein the thick steel sheet has a thickness of 50 to 100mm.
The present invention further relates to a method of manufacturing a thick steel sheet having excellent low temperature impact toughness and CTOD properties, the method comprising heating a slab at 1020 to 1150°C, the slab comprising, by weight%, 0.02 to 0.055% of C, 0.005 to 0.08% of Si, 1.0 to 2.0% of Mn, 0.01% or less of P, 0.003% or less of S, 0.001 to 0.01% of Al, 0.5 to 2.0% of Ni, 0.001 to 0.02% of Ti, 0.005 to 0.03% of Nb, 0.05 to 0.4% of Cu, 0.002 to 0.006% of N, and a balance of Fe and inevitable impurities, and satisfying Equation 1 and Equation 2 below, $3.0 \leq Mn + 2 Ni \leq 4.3$
$0.05 \leq C + Si + 10 Al \leq 0.25$
where each element symbol indicates a content of each element by weight%; recrystallization-region rolling the heated slab at 900°C or higher; obtaining a thick steel sheet by performing a non-recrystallization-region rolling process for a finish rolling temperature to be Ar3 to 850°C after the recrystallization-region rolling; cooling the thickness steel sheet to 250°C or lower at a cooling speed of 2 to 15°C/sec; and tempering the cooled thick steel sheet at 500 to 650°C, and wherein the recrystallization-region rolling is performed at each of the reduction ratios of the final two passes to be within 15 to 20%, and wherein the non-recrystallization-region rolling is performed for a thickness of the thick steel sheet to be 50 to 100mm.
In Equation 1 and Equation 2, each element symbol indicates a content of each element by weight%.
The above-described technical solutions do not list all the features of the present disclosure. Various features and advantages and effects thereof of the present disclosure will further be understood with reference to specific embodiments described below.

[Advantageous Effects]

According to the present invention, excellent yield strength may be secured with respect to a thick steel sheet having a thickness of 50mm or greater, excellent impact toughness may be secured even at an extremely low temperature of about -80°C, and a thick steel sheet having impact toughness and CTOD properties at -60°C and a method of manufacturing the same may be provided, which are effects of the present disclosure.

[Description of Drawings]

FIG. 1 is an image of a microstructure of inventive example 1; and
FIG. 2 is graphs illustrating yield strength in accordance with an Mn+2Ni value and a CTOD value at -60°C.

[Best Mode for Invention]

In the description below, preferable embodiment of the present invention will be described. However, embodiments of the present disclosure may be modified in various manners, and the scope of the present invention may not be limited to the embodiments described below, but is limited by the appended claims. Also, the embodiments may be provided to more completely described the present invention to a person having ordinary skill in the art.

Thick Steel Sheet Having Excellent Low Temperature Toughness and CTOD Properties

Hereinafter, a thick steel sheet having excellent low temperature impact toughness and CTOD properties will be described in detail.
A thick steel sheet having excellent low temperature impact toughness and CTOD properties may include, by weight%, 0.02 to 0.06% of C, 0.005 to 0.08% of Si, 1.0 to 2.0% of Mn, 0.01% or less of P, 0.003% or less of S, 0.001 to 0.01% of Al, 0.5 to 2.0% of Ni, 0.001 to 0.02% of Ti, 0.005 to 0.03% of Nb, 0.05 to 0.4% of Cu, 0.002 to 0.006% of N, and a balance of Fe and inevitable impurities, and may satisfy Equation 1 and Equation 2 below, and a microstructure may include ferrite of 95 area% or higher, and a sum of MA and cementite of 2 area% or lower. $3.0 \leq Mn + 2 Ni \leq 4 .3$
$0.05 \leq C + Si + 10 Al \leq 0 .25$
(in Equation 1 and Equation 2, each element symbol indicates a content of each element by weight%).
An alloy composition of the present disclosure will be described in detail. A unit of a content of each element may be wt% unless otherwise indicated.

C: 0.02 to 0.055%

C is an element which may be effective for strengthening solid solution, and may improve strength by forming Nb, and the like, and carbide.
When a content of C is lower than 0.02%, the above-described effect may be insufficient. When a content of C exceeds 0.055% the formation of MA may be facilitated, and pearlite may be formed such that impact and fatigue properties at a low temperature may be deteriorated. Thus, a preferable content of C may be 0.02 to 0.055%.
A more preferable lower limit content of C may be 0.025%, and an even more preferable lower limit content of C may be 0.03%. A more preferable upper limit content of C may be 0.05%.

Si: 0.005 to 0.08%

Si is an element which may deoxidize molten steel auxiliary to Al, and may help improving yield strength and tensile strength, but may adversely affect impact and fatigue properties at a low temperature.
When a content of Si exceeds 0.08%, Si may interfere with dispersion of C such that the formation of MA may be facilitated, which may adversely affect impact and fatigue properties at a low temperature. Also, to control a content of Si to be 0.005% or less, a processing time of a steel making process may greatly increase, such that productivity may decrease. Thus, a preferable content of Si may be 0.005 to 0.08%.
A more preferable lower limit content of Si may be 0.01%, a more preferable upper limit content of Si may be 0.07%, and an even more preferable upper limit content of Si may be 0.055%.

Mn: 1.0 to 2.0%

Mn may have a great effect in increasing strength by strengthening solid solution, and thus, 1.0% or higher of Mn may be added. However, when a content of Mn is excessive, toughness may degrade due to the formation of an MnS inclusion, and the segregation of a central portion. Thus, a preferable upper limit content of Mn may be 2.0%.

P: 0.01% or less

P is an element which may cause grain boundary segregation, and may thus be a cause of weakening steel. Thus, a content of P may need to be controlled to be low as possible as P is one of impurities, and it may be preferable to control a content of P to be 0.01% or less. It may be substantially impossible to control a content of P to be 0%, and thus, 0% may not be included.

S: 0.003% or less

S may be a factor which may form an MnS inclusion by being combined with Mn, and the MnS inclusions may degrade low temperature toughness. Thus, a content of S may need to be controlled to be low as possible as S is one of impurities. It may be preferable to control a content of S to be 0.003% or less to secure low temperature toughness and low temperature fatigue properties. It may be substantially impossible to control a content of S to be 0%, and thus, 0% may not be included.

Al: 0.001 to 0.01%

Al is a main deoxidizer in the present disclosure, and it may be required to add 0.001% or higher of Al. When a content of Al exceeds 0.01%, Al may be a cause of gradation of low temperature toughness due to an increase of a fraction and a size of Al₂O₃ inclusion. Also, similarly to Si, Al may facilitate the formation of an MA phase in a base material and a welding heat affected portion such that low temperature toughness and low temperature fatigue properties may degrade. Thus a preferable content of Al may be 0.001 to 0.01%.

Ni: 0.5 to 2.0%

An increased content of Ni may not greatly improve strength, but Ni may improve strength and toughness at the same time.
When a content of Ni is lower than 0.5%, the above-described effect may be insufficient. When a content of Ni exceeds 2.0%, Ni may facilitate the formation of MA due to an increase of hardenability such that impact and CTOD toughness may be deteriorated.

Ti: 0.001 to 0.02%

Ti is an element which may form a precipitation by being combined with oxygen or nitrogen, and may accordingly prevent a structure from being coarse, and may contribute to refinement and improvement of toughness.
When a content of Ti is lower than 0.001%, the above-described effect may be insufficient. When a content of Ti exceeds 0.02%, Ti may become a cause of fracture due to a coarse precipitation.

Nb: 0.005 to 0.03%

Nb is an element which may prevent recrystallization during rolling or cooling by precipitating solute or carbonitride and may thus refine a structure and may increase strength.
When a content of Nb is lower than 0.005%, the above-described effect may be insufficient. When a content of Nb exceeds 0.03%, C concentration may occur due to C affinity, which may facilitate the formation of an MA phase and may accordingly degrade toughness and fracture properties at a low temperature.

Cu: 0.05 to 0.4%

Cu is an element which may not greatly degrade impact properties, and may improve strength by solid solution and precipitation.
When a content of Cu is lower than 0.05%, the above-described effect may be insufficient. When a content of Cu exceeds 0.4%, there may be a risk of surface cracks on a steel sheet by Cu heat impact.

N: 0.002 to 0.006%

N is an element which may refine an austenite structure during reheating by forming a precipitation along with Ti, Nb, Al, and the like, and may thus be helpful for improving strength and toughness. It may be preferable to add 0.002% or higher of N.
When a content of N exceeds 0.006%, surface cracks may be created at a high temperature, and residual N which remains after a precipitation is formed may be present in an atomic state and may decrease toughness. Thus, a preferable content of N may be 0.002 to 0.006%.
A remainder other than the above-described composition is Fe. However, in a general manufacturing process, inevitable impurities may be inevitably added from raw materials or a surrounding environment, and thus, impurities may not be excluded. A person skilled in the art may be aware of the impurities, and thus, the descriptions of the impurities may not be provided in the present disclosure.
An alloy composition of the present disclosure satisfies the above-described element contents, and also satisfies Equation 1 and Equation 2 below. $3.0 \leq Mn + 2 Ni \leq 4.3$
$0.05 \leq C + Si + 10 Al \leq 0.25$
(in Equation 1 and Equation 2, each element symbol indicates a content of each element by weight%).
Compliance with Equation 1 and Equation 2 secures excellent low temperature impact toughness and CTOD properties without degradation of strength, and the equations are designed in consideration of correlation affecting an MA preventing effect and strength.
To prevent MA in accordance with Equation 2, contents of C, Si, and Al may be controlled, and to compensate for degradation of strength caused by the control of contents of the elements, Mn and Ni may need to be added in accordance with Equation 1.
When a value of Equation 1 is lower than 3.0, an effect of improving strength may be insufficient. When the value exceeds 4.3, low temperature impact toughness and CTOD properties may degrade.
A preferable value of Equation 2 may be 0.05 or higher for steel making processes such as deoxidation, and the like. When a value of Equation 2 is lower than 0.05, it may be difficult to secure strength. When the value exceeds 0.25, a large amount of MA phase may be formed such that low temperature impact toughness and CTOD properties may degrade.
A microstructure of a thick steel sheet of the present disclosure will be described in detail.
A microstructure of the thick steel sheet connsists of ferrite of 95 area% or higher, and a sum of MA and cementite of 2 area% or lower.
When ferrite is 95 area% or lower, impact toughness at -80°C and CTOD properties at -60°C may degrade.
To secure low temperature impact toughness and CTOD properties, a structure of a base material and a fraction of MA may be important. When C is integrated and concentrated during rolling and cooling, hardenability may increase, and C may be transformed to martensite having high hardenability or may remain as austenite, which may be referred to as martensite-austenite (MA) . MA may be vulnerable to fracture due to properties of high hardness, and may cause stress to be concentrated when soft ferrite around MA is transformed, and may accordingly work as initiation of fracture.
Also, cementite may have a characteristic similar to that of MA, and may be a hard phase having hardenability higher than that of base material acicular ferrite, and cementite may deteriorate low temperature impact toughness and CTOD properties.
Thus, to secure excellent low temperature impact toughness and CTOD properties, it is important to control a sum of MA and cementite to be 2 area% or lower.
Ferrite has a grain size of 20 µm or less measured in equivalent circle diameter. When the grain size exceeds 20 µm, dislocation in ferrite may increase such that fracture propagation may easily occur such that low temperature impact toughness and CTOD properties may be deteriorated. The smaller the grain size, it may be more advantageous to securing low temperature impact toughness and CTOD properties, and thus, a lower limit content of ferrite may not be particularly limited.
Ferrite may also include polygonal ferrite and acicular ferrite, and specific fractions thereof may not be limited.
The thick steel sheet may have yield strength of 420MPa or higher, impact toughness of 200J or higher at -80°C, and a CTOD of 0.5mm or higher at -60°C. By securing such properties, the thick steel sheet may be appropriately applied to an offshore structural steel material used in an extremely low temperature environment. More preferably, a CTOD may be 1.0mm or higher at -60°C.
The thick steel sheet may have tensile strength of 500MPa or higher, an elongation rate of 25% or higher, and impact toughness of 400J or higher at -60°C.
The thickness steel sheet may have a thickness of 50 to 100mm.

Method of Manufacturing Thick Steel Sheet Having Excellent Low Temperature Impact Toughness and CTOD Properties

In the description below, a method of manufacturing a thick steel sheet having excellent low temperature impact toughness and CTOD properties, another aspect of the present disclosure, will be described in detail.
The method of manufacturing a thick steel sheet having excellent low temperature impact toughness and CTOD properties may include heating a slab comprising the above-described alloy composition at 1020 to 1150°C; recrystallization-region rolling the heated slab at 900°C or higher; obtaining a thick steel sheet by performing a non-recrystallization-region rolling process for a finish rolling temperature to be Ar3 to 850°C after the recrystallization-region rolling; cooling the thickness steel sheet to 250°C or lower at a cooling speed of 2 to 15°C/sec; and tempering the cooled thick steel sheet at 500 to 650°C.
Each process will be described in detail in the description below.

Heating Slab

The slab satisfying the above-described alloy composition is heated at 1020 to 1150°C.
When the slab heating temperature exceeds 1150°C, an austenite grain may become coarse such that toughness may degrade. When the slab heating temperature is lower than 1020°C, sufficient solid solution of Ti, Nb, and the like may not be possible such that strength may degrade.

Recrystallization-Region Rolling

The heated slab is recrystallization-region rolled at 900°C or higher. When the temperature is lower than 900°C, it may be difficult to implement sufficient recrystallization of austenite.
The recrystallization-region rolling is performed at each of the reduction ratios of the final two passes to be within 15 to 20%, which is to secure a uniform and fine final microstructure.

Non-Recrystallization-Region Rolling

After the recrystallization-region rolling, a thick steel sheet is obtained by performing a non-recrystallization-region rolling for a finish rolling temperature to be Ar3 to 850°C after the recrystallization-region rolling.
When the finish rolling temperature is lower than Ar3, a temperature of a surface of the thick steel sheet may be that of a two-phase region, and a two phase structure may be formed at a thickness of surface to 1/4t such that impact toughness may be deteriorated. When the finish rolling temperature exceeds 850°C, grain refinement may be insufficient such that strength and toughness may be deteriorated.
The non-recrystallization-region rolling is performed for a thickness of the thick steel sheet to be 50 to 100mm.

Cooling

The thick steel sheet is cooled to 250°C or lower at a cooling speed of 2 to 15°C/sec.
When the cooling speed exceeds 15°C/sec, there may be a difference in properties due to a difference in cooling speed between a surface and a central portion of the thick steel sheet. When the cooling speed is lower than 2°C/sec, distribution of acicular ferrite may decrease, and distribution of polygonal ferrite may increase.
When a cooling terminating temperature exceeds 250°C, target strength may not be satisfied.

Tempering

The cooled thick steel sheet is tempered at 500 to 650°C. Dislocation in an MA phase and ferrite may be factors which may greatly affect low temperature impact toughness and CTOD properties. Thus, the tempering may be performed to dissolve an MA phase and to decrease dislocation in ferrite.
When the tempering temperature is lower than 500°C, the above-described effect may be insufficient. When the temperature exceeds 650°C, carbide may be formed, which may degrade toughness.

[Mode for Invention]

In the description below, an example embodiment of the present disclosure will be described in greater detail.

(Embodiment)

Molten steel having a composition indicated in Table 1 below was prepared, and a slab was manufactured using a continuous casting process. The slab was heated, recrystallization-region rolled, non-recrystallization-region roll, and cooled and tempered under manufacturing conditions as in Table 2 below, and a thick steel sheet having a thickness of 80mm was manufactured. The recrystallization-region rolling was performed for each of reduction ratios of final two passes to be 18%.
A microstructure, mechanical properties, low temperature impact toughness and CTOD properties of the thick steel sheet were measured and listed in Table 3 below.
The microstructure was observed using a scanning electron microscope (SEM) and a transmission electron microscope (TEM), and a sum (a secondary phase) of MA and cementite was analyzed and listed in Table 3. A portion other than the secondary phase was ferrite including polygonal ferrite and acicular ferrite.
As for a grain size of ferrite, an average value measured in equivalent circle diameter was listed in Table 3.
Yield strength, tensile strength, and an elongation rate were measured through a tensile test.
Low temperature impact toughness was measured through a charpy impact test at -80°C and -60°C.

As for CTOD properties, a sample was processed to have a size of 60mm × 120mm × 300mm in perpendicular to a rolling direction in accordance with BS 7448 standards, fatigue cracks were inserted such that a fatigue crack length became 50% of a sample width, and a CTOD test was carried out at -60°C. The CTOD test was performed three times for each steel sheet, and a minimum value among the test values obtained from the three times of tests was listed in Table 3.

[Table 1]

Clas sifi cati on	Ste el Typ e	C	Si	Mn	P*	S*	Al	Ni	Ti	Nb	Cu	N*	Equa tion 1	Equa tion 2
Inve ntiv e stee l	A	0.0 35	0.0 46	1.9 2	77	17	0.0 05	0.9 1	0.00 91	0.0 06	0.2 8	36	3.74	0.13 1
Inve ntiv e stee l	B	0.0 38	0.0 45	1.9 5	84	19	0.0 07	0.9 5	0.01 2	0.0 07	0.2 5	38	3.85	0.15 3
Inve ntiv e stee l	C	0.0 36	0.0 39	1.9 6	75	20	0.0 08	0.9 4	0.00 98	0.0 07	0.2 6	37	3.84	0.15 5
Inve ntiv e stee l	D	0.0 38	0.0 48	1.9 3	65	21	0.0 07	0.9 2	0.00 10	0.0 06	0.2 5	35	3.77	0.15 6
Comp arat ive stee l	E	0.0 84	0.0 68	1.9 5	84	18	0.0 07	0.9 4	0.00 11	0.0 06	0.2 7	41	3.83	0.22 2
Comp arat ive stee l	F	0.0 42	0.0 59	1.4 5	82	17	0.0 08	0.3 4	0.00 99	0.0 06	0.2 6	40	2.13	0.18 1
Comp arat ive stee l	G	0.0 37	0.0 64	2.2 3	91	20	0.0 07	1.4 3	0.00 98	0.0 07	0.2 6	35	5.09	0.17 1
Comp arat ive stee l	H	0.0 38	0.1 2	1.9 4	94	16	0.0 15	0.9 3	0.01 1	0.0 08	0.2 5	32	3.8	0.30 8

In Table 1 above, a unit of each element content is weight%. Unites of P*, S*, and N* are ppm.
Equation 1 is a value of Mn+2Ni,
Equation 2 is value of C+Si+10Al, and each element symbol indicates a content of each element by weight %.

[Equation 2]

Classification	Steel Type	Slab Heating Temperature (°C)	Recrystallization-Region rolling Terminating Temperature (°C)	Non-Recrystallization-Region Rolling Terminating Temperature (°C)	Cooling Terminating Temperature (°C)	Cooling Speed (°C/s)	Tempering Temperature (°C)
Inventive example 1	A	1105	1021	775	224	3.4	552
Inventive example 2	B	1108	1028	773	246	3.3	553
Inventive example 3	C	1112	1030	768	187	2.8	551
Inventive example 4	D	1110	1023	781	213	2.6	550
Comparative example 1	A	1121	1022	875	229	3.1	551
Comparative example 2	B	1105	1032	780	195	3.0	Not conducted
Comparative example 3	C	1112	1023	785	524	2.8	553
Comparative example 4	E	1098	1030	802	195	3.1	553
Comparative example 5	F	1103	1035	799	188	3.0	550
Comparative example 6	G	1108	1025	786	201	2.8	547
Comparative example 7	H	1106	1029	782	235	3.0	554

[Table 3]

Classification	Steel Type	Secondary Phase (area%)	Grain Size (µm)	Yield Strength (MPa)	Tensile Strength (MPa)	Elongation Rate (%)	Impact Toughness (-60°C, J)	Impact Toughness (-80°C, J)	CTOD (-60°C, mm)
Inventive example 1	A	1.3	17.6	429	528	29	439	427	1.03
Inventive example 2	B	1.1	18.9	435	548	29	415	348	2.42
Inventive example 3	C	1.5	17.5	434	543	28	442	298	1.25
Inventive example 4	D	1.3	18.6	428	529	30	435	310	1.18
Comparative example 1	A	1.8	18.3	445	538	27	354	54	0.24
Comparative example 2	B	1.5	38.3	432	541	28	284	28	0.31
Comparative example 3	C	3.6	22.6	417	507	29	225	38	0.85
Comparative example 4	E	3.8	19.5	452	585	26	129	18	0.13
Comparative example 5	F	1.7	18.5	375	481	31	413	62	0.51
Comparative example 6	G	5.6	19.8	486	612	25	83	15	0.11
Comparative example 7	H	4.6	18.3	434	535	29	102	43	0.07

Inventive examples which satisfied the overall alloy composition and the manufacturing conditions suggested in the present disclosure secured yield strength of 420MPa or higher, impact toughness of 200J or higher at -80°C, and a CTOD value of 0.5mm or higher at -60°C, which indicate that low temperature impact toughness and CTOD properties were excellent.
FIG. 1 is an image of a microstructure of inventive example 1. A small amount of MA and cementite were formed, and a grain size was also fine.
Comparative examples 1 to 3 satisfied the alloy composition suggested in the present disclosure, but did not satisfy the manufacturing conditions.
As for comparative examples 1 and 2, impact toughness at -80°C and CTOD properties at -60°C were deteriorated. As for comparative example 3, impact toughness at -80°C degraded, and it was difficult to secure strength.
Comparative examples 4 to 7 satisfied the manufacturing conditions suggested in the present disclosure, but did not satisfy the alloy composition.
As for comparative example 4, a content range of C exceeded the suggested range, and as for comparative example 5, the Mn+2Ni range exceeded the suggested range such that, although strength was excellent, impact toughness at -80°C and CTOD properties at -60°C greatly degraded.
As for comparative example 6, a M+2Ni range was lower than the suggested range, and strength and impact toughness at -80°C were deteriorated.
As for comparative example 7, a range of C+Si+10Al exceeded the suggested range, and impact toughness at -80°C and CTOD properties at -60°C were greatly deteriorated.
FIG. 2 is graphs illustrating yield strength in accordance with an Mn+2Ni value and a CTOD value at -60°C. As indicated in FIG. 2, to secure yield strength of 420MPa or higher and a CTOD value of 0.5mm or higher, 3.0≤Mn+2Ni≤4.3 may need to be satisfied. When the Mn+2Ni value is lower than 3.0, strength decreased. When the value exceeds 4.3, a CTOD value at -60°C greatly decreased.
While exemplary embodiments have been shown and described above, the scope of the present disclosure is not limited thereto, and 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 invention as defined by the appended claims.

Claims

A thick steel sheet having excellent low temperature impact toughness and CTOD properties, comprising:
by weight%, 0.02 to 0.055% of C, 0.005 to 0.08% of Si, 1.0 to 2.0% of Mn, 0.01% or less of P, 0.003% or less of S, 0.001 to 0.01% of Al, 0.5 to 2.0% of Ni, 0.001 to 0.02% of Ti, 0.005 to 0.03% of Nb, 0.05 to 0.4% of Cu, 0.002 to 0.006% of N, and a balance of Fe and inevitable impurities,

wherein the thick steel sheet satisfies Equation 1 and Equation 2 below, $3.0 \leq Mn + 2 Ni \leq 4.3$
$0.05 \leq C + Si + 10 Al \leq 0.25$
where each element symbol indicates a content of each element by weight%, and

wherein a microstructure consists of ferrite of 95 area% or higher, and a sum of martensite-austenite MA and cementite of 2 area% or lower, and

wherein the thick steel sheet has yield strength of 420MPa or higher, impact toughness of 200J or higher at -80°C, and a CTOD of 0.5mm or higher at -60°C, and

wherein the thick steel sheet has tensile strength of 500MPa or higher, an elongation rate of 25% or higher, and impact toughness of 400J or higher at -60°C, and

wherein the thick steel sheet has a thickness of 50 to 100mm,

wherein the impact toughness was measured through a charpy impact test at -80°C and -60°C, and for CTOD properties, a sample was processed in accordance with BS 7448 standards, as defined in the description.
The thick steel sheet of claim 1, wherein ferrite has a grain size of 20 µm or less measured in equivalent circle diameter.
The thick steel sheet of claim 1, wherein ferrite comprises polygonal ferrite and acicular ferrite.
A method of manufacturing a thick steel sheet having excellent low temperature impact toughness and CTOD properties, the method comprising:
heating a slab at 1020 to 1150°C, the slab comprising, by weight%, 0.02 to 0.055% of C, 0.005 to 0.08% of Si, 1.0 to 2.0% of Mn, 0.01% or less of P, 0.003% or less of S, 0.001 to 0.01% of Al, 0.5 to 2.0% of Ni, 0.001 to 0.02% of Ti, 0.005 to 0.03% of Nb, 0.05 to 0.4% of Cu, 0.002 to 0.006% of N, and a balance of Fe and inevitable impurities, and satisfying Equation 1 and Equation 2 below, $3.0 \leq Mn + 2 Ni \leq 4.3$
$0.05 \leq C + Si + 10 Al \leq 0.25$
where each element symbol indicates a content of each element by weight%;

recrystallization-region rolling the heated slab at 900°C or higher;

obtaining a thick steel sheet by performing a non-recrystallization-region rolling process for a finish rolling temperature to be Ar3 to 850°C after the recrystallization-region rolling;

cooling the thickness steel sheet to 250°C or lower at a cooling speed of 2 to 15°C/sec; and

tempering the cooled thick steel sheet at 500 to 650°C, and

wherein the recrystallization-region rolling is performed at each of the reduction ratios of the final two passes to be within 15 to 20%, and

wherein the non-recrystallization-region rolling is performed for a thickness of the thick steel sheet to be 50 to 100mm.