KR102357082B1 - High strength steel sheet having excellent heat affected zone toughness and method for manufacturing the same - Google Patents

Abstract

The present invention relates to a steel sheet having excellent weld heat affected zone toughness while having high strength characteristics and a method for manufacturing the same. In more detail, the steel sheet according to one aspect of the present invention may be used for manufacturing structures such as offshore structures, building structures, and ships.

Description

High-strength steel sheet with excellent toughness in the heat-affected zone of welding and its manufacturing method

The present invention relates to a high-strength steel sheet and a method for manufacturing the same, and more particularly, to a high-strength steel sheet having excellent welded heat-affected zone toughness and a method for manufacturing the same.

In recent years, the use of high-strength steel sheet for the manufacture of structures such as offshore structures, building structures, and ships has been actively carried out. This is because the weight of the structure can be lowered by reducing the weight of the steel by using the high-strength steel.

However, in general, high-strength steel is disadvantageous in terms of welding because the amount of alloying elements added is high. In particular, since the toughness of the heat-affected zone in the weld is greatly reduced, the use of high-strength steel sheet is limited.

Accordingly, various techniques have been proposed in order to prevent the deterioration of the toughness of the weld heat affected zone in the high-strength steel sheet.

For example, Patent Document 1 aims to increase the strength of the steel sheet through precipitation strengthening due to the large amount of copper added, and thus suppresses the amount of carbon added that greatly reduces the toughness of the heat-affected zone of the weld, and accordingly, the toughness of the heat-affected zone of the weld is reduced. Suggests ways to improve it.

However, in Patent Document 1, as a large amount of copper is added, surface cracks may occur due to copper in the steel sheet manufacturing process, resulting in a problem in which the surface quality of the steel sheet is greatly reduced.

Patent Document 2 proposes a method of improving the toughness of the weld heat-affected zone by appropriately controlling the content of calcium, oxygen, and sulfur to utilize the CaS inclusions and increasing the manganese content.

However, calcium, oxygen, and sulfur have a disadvantage in that it is difficult to precisely control them in the steelmaking process.

Patent Document 3 proposes a method of achieving ultra-low carbonization in order to utilize the hardenability of boron, and thus simultaneously securing strength and toughness of the weld heat-affected zone.

However, in order to effectively utilize boron, it is necessary to precisely control not only boron, but also nitrogen, titanium, aluminum and oxygen. Problems can arise.

Republic of Korea Patent Publication No. 10-2001-0111625 Republic of Korea Patent Publication No. 10-2013-0035277 Republic of Korea Patent Publication No. 10-2015-0020181

According to one aspect of the present invention, it is an object of the present invention to provide a steel sheet having excellent weld heat-affected zone toughness and a method for manufacturing the same while having high strength characteristics.

The subject of the present invention is not limited to the above. A person of ordinary skill in the art will have no difficulty in understanding the further problems of the present invention from the overall content of the present specification.

One aspect of the present invention, by weight, C: 0.02 to 0.05%, Si: 0.05 to 0.2%, Mn: 2.0 to 2.4%, P: 0.012% or less, S: 0.003% or less, Al: 0.01 to 0.04% , Cu: 0.2~0.4%, Ni: 2.1~2.5%, Nb: 0.005~0.015%, Ti: 0.005~0.015%, N: 0.002~0.006%, Ca: 0.0005~0.002%, balance Fe and unavoidable impurities and Pcm defined in the following Relation 1 is 0.19 to 0.21, and contains 0.5 to 1.5 area% of cementite and residual ferrite as a microstructure, and the average ferrite particle diameter measured by the EBSP method is 4 μm or less, and the yield strength is It is possible to provide a high-strength steel sheet excellent in toughness of the weld heat-affected zone of 500 MPa or more.

[Relational Expression 1]

Pcm = [C]+[Si]/30+([Mn]+[Cu]+[Cr])/20+[Ni]/60+[Mo]/15+[V]/10+5[B]

(Here, [C], [Si], [Mn], [Cu], [Cr], [Ni], [Mo], [V] and [B] are weight percent of the corresponding alloying element.)

The steel sheet may have an average shock absorption energy of 100J or more at -50°C, and a minimum FL CTOD value of 0.25 mm or more at -20°C.

Another aspect of the present invention, by weight, C: 0.02 to 0.05%, Si: 0.05 to 0.2%, Mn: 2.0 to 2.4%, P: 0.012% or less, S: 0.003% or less, Al: 0.01 to 0.04 %, Cu: 0.2~0.4%, Ni: 2.1~2.5%, Nb: 0.005~0.015%, Ti: 0.005~0.015%, N: 0.002~0.006%, Ca: 0.0005~0.002%, balance Fe and unavoidable impurities Reheating a steel slab having a Pcm defined in Relation 1 below, which satisfies 0.19 to 0.21 at a temperature in the range of 1050 to 1150° C.; Primary rolling the reheated slab at a temperature of 900° C. or higher; Second rolling the first rolled steel sheet to a rolling end temperature of 700 ~ 850 ℃; water cooling the secondary rolled steel sheet to 300° C. or less at a cooling rate of 3° C./s or more; And it is possible to provide a method of manufacturing a high strength steel sheet excellent in the weld heat-affected zone toughness comprising the step of tempering heat treatment of the cooled steel sheet at 500 ~ 600 ℃ can be provided.

[Relational Expression 1]

Pcm = [C]+[Si]/30+([Mn]+[Cu]+[Cr])/20+[Ni]/60+[Mo]/15+[V]/10+5[B]

(Here, [C], [Si], [Mn], [Cu], [Cr], [Ni], [Mo], [V] and [B] are weight percent of the corresponding alloying element.)

It may include the step of air cooling the water-cooled steel sheet to room temperature.

The cumulative reduction ratio of the secondary rolling may be 40% or more.

According to one aspect of the present invention, a steel sheet having excellent weld heat-affected zone toughness while having high strength characteristics and a method for manufacturing the same may be provided. In more detail, the steel sheet according to one aspect of the present invention may be used for manufacturing structures such as offshore structures, building structures, and ships.

Hereinafter, preferred embodiments of the present invention will be described. Embodiments of the present invention may be modified in various forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. The present embodiments are provided to explain the present invention in more detail to those skilled in the art to which the present invention pertains.

As the heat input of welding increases during the manufacture of steel structures, the volume of the heat-affected zone increases, and the grain coarsening region immediately adjacent to the weld metal increases, thereby reducing the toughness, thereby limiting the amount of heat input from welding in order to secure safety.

However, in the case of welding a thick steel plate with a low heat input of 5 kJ/mm or less, since welding is not performed in one pass, multi-layer welding is essential. When multi-layer welding is performed, the structure of the heat-affected zone created by the heat of welding in the previous pass is changed by the heat of welding in the next pass, so that various localized areas are formed in the heat-affected zone.

As a result of studying various localized regions within the weld heat-affected zone of a multi-layer weld, many researchers have found that the localized region with the weakest toughness is the intercritically reheated coarse-grained region. This local region is a region where coarse grains are formed in the previous pass and then reheated to the ideal region where ferrite and austenite coexist in the next pass and then cooled. Therefore, humanity is the weakest. Since the local area is very small compared to the entire heat-affected zone of the weld, it is not easily revealed in tests that evaluate average toughness such as impact tests, but easily samples the weakest areas in the specimen, such as in the Crack Tip Opening Displacement (CTOD) test. The exam can cause big problems.

As a result of investigations of specimens with very low CTOD values compared to the average during CTOD test, many researchers confirmed that brittle cracks started in the localized region of abnormal reheating and coarsening of grains.

As a result of the present inventor's study on a method to increase the toughness of the local region, the abnormal region reheating grain coarsening region basically belongs to the grain coarsening region, so it has coarse old austenite grain boundaries, and bays inside the old austenite grain boundaries It was found that the microstructure has a knight or acyclic ferrite type. The acicular ferrite microstructure is excellent in toughness because an acyclic ferrite is formed by using a specific inclusion as a nucleus, but it is industrially very difficult to make the entire grain coarsening region into an ashicular ferrite structure. Therefore, it was confirmed that it was necessary to increase the toughness of the bainite structure formed in addition to the acicular ferrite.

In the present invention, it is confirmed that the toughness of the local region can be increased by adding more than a certain amount of Ni to improve the toughness of bainite generated in the grain coarsening region by reheating the abnormal region, and by optimizing the manufacturing conditions to complete the present invention reached

Hereinafter, the present invention will be described in detail.

Hereinafter, the steel composition of the present invention will be described in detail.

In the present invention, unless otherwise specified, percentages indicating the content of each element are based on weight.

The steel sheet according to an aspect of the present invention is, by weight, C: 0.02 to 0.05%, Si: 0.05 to 0.2%, Mn: 2.0 to 2.4%, P: 0.012% or less, S: 0.003% or less, Al: 0.01 to 0.04%, Cu: 0.2 to 0.4%, Ni: 2.1 to 2.5%, Nb: 0.005 to 0.015%, Ti: 0.005 to 0.015%, N: 0.002 to 0.006%, Ca: 0.0005 to 0.002%, balance Fe and unavoidable impurities may include

Carbon (C): 0.02~0.05%

Carbon (C) is a curable element, and in order to secure the strength targeted in the present invention, it should be added in an amount of 0.02% or more. However, since carbon (C) inhibits toughness by increasing the fraction of Martensite-Austenite (M-A) structure in the abnormal region reheating grain coarsening region, it is preferable to limit its content to 0.05% or less.

Accordingly, the content of carbon (C) may be 0.02 to 0.05%, more preferably 0.02 to 0.035%.

Silicon (Si): 0.05~0.2%

Silicon (Si) is an element contributing to deoxidation and securing strength, and is preferably added in an amount of 0.05% or more. However, it is preferable to limit the content of the retained austenite to 0.2% or less because it prevents the transformation of the retained austenite into cementite and helps to form an M-A structure in the grain coarsening region of the abnormal reheating.

Accordingly, the content of silicon (Si) may be 0.05 to 0.2%.

Manganese (Mn): 2.0-2.4%

Manganese (Mn) is an element capable of increasing the hardenability of steel without significantly reducing the toughness of the heat-affected zone of the weld in a low C content state. It is preferable to add at least 2.0% to secure strength. However, if it exceeds 2.4%, a problem of inhibiting toughness may occur by promoting segregation in the thickness center of the steel sheet.

Accordingly, the content of manganese (Mn) may be 2.0 to 2.4%, more preferably 2.1 to 2.3%.

Phosphorus (P): 0.012% or less

Phosphorus (P) is an impurity that is unavoidably mixed in steel during the steelmaking process, and since it reduces the toughness of the heat-affected zone of the weld, it is preferable to limit its content to 0.012% or less.

Accordingly, the content of phosphorus (P) may be 0.012% or less, more preferably 0.008% or less.

Sulfur (S): 0.003% or less

Sulfur (S) is an impurity that is unavoidably mixed in steel during the steelmaking process, and it is preferable to limit it to 0.003% or less because it greatly reduces the CTOD toughness of the heat-affected zone of the weld.

Therefore, the content of sulfur (S) may be 0.003% or less, more preferably 0.002% or less.

Aluminum (Al): 0.01~0.04%

Aluminum (Al) is an element contributing to deoxidation, and is preferably added in an amount of 0.01% or more in order to prevent the formation of coarse oxides. However, if the content exceeds 0.04%, the formation of the M-A structure becomes remarkable in the grain coarsening region of the abnormal reheating, which may cause a problem of greatly reducing the toughness.

Accordingly, the content of aluminum (Al) may be 0.01 to 0.04%, more preferably 0.02 to 0.03%.

Copper (Cu): 0.2~0.4%

Copper (Cu) is an element that does not significantly decrease the toughness of the heat-affected zone while increasing the strength of the steel, and is preferably added in an amount of 0.2% or more. However, if the content exceeds 0.4%, there is a risk of deterioration of the surface quality of the steel sheet due to cracks caused by copper (Cu).

Accordingly, the content of copper (Cu) may be 0.2 to 0.4%.

Nickel (Ni): 2.1 to 2.5%

Nickel (Ni) is the most important alloying element in the present invention. The inventors of the present invention conducted many experiments on a way to secure the toughness of the localized region of grain coarsening by reheating abnormal region existing in the heat-affected zone of the multi-layer welded part. As a result, it was confirmed that nickel (Ni) is the most effective element. . In addition, in order to secure the target toughness of the weld heat-affected zone in the present invention, it is preferable to add 2.1% or more. However, when the content exceeds 2.5%, the above-described effect is saturated, and nickel (Ni) is an expensive element, so there may be a problem in that the manufacturing cost increases.

Accordingly, the content of nickel (Ni) may be 2.1 to 2.5%, more preferably 2.1 to 2.3%.

Niobium (Nb): 0.005 to 0.015%

Niobium (Nb) expands the non-recrystallization region of austenite during rolling to refine the final structure to increase strength and toughness, so it is preferable to add 0.005% or more. However, if the content exceeds 0.015%, it may promote the formation of M-A structure in the heat-affected zone of the weld, thereby impairing the toughness.

Accordingly, the content of niobium (Nb) may be 0.005 to 0.015%.

Titanium (Ti): 0.005-0.015%

Titanium (Ti) is combined with N to form TiN particles with excellent high-temperature stability to prevent excessive growth of austenite due to welding heat during welding, thereby improving the toughness of the grain coarsening region, so it is preferable to add 0.005% or more . However, when the content exceeds 0.015%, the TiN grains become coarse and cannot prevent the growth of austenite.

Accordingly, the content of titanium (Ti) may be 0.005 to 0.015%.

Nitrogen (N): 0.002 to 0.006%

Nitrogen (N) combines with Ti to form TiN particles to prevent large austenite from growing in the grain coarsening region, so it is preferable to add 0.002% or more. However, if the content exceeds 0.006%, it may cause surface cracks in the steel slab during continuous casting.

Accordingly, the content of nitrogen (N) may be 0.002 to 0.006%, more preferably 0.003 to 0.005%.

Calcium (Ca): 0.0005~0.002%

Calcium (Ca) is preferably added in an amount of 0.0005% or more because it binds with S and prevents the formation of easily elongated MnS during rolling, thereby preventing deterioration of the toughness of the heat-affected zone of the weld. However, if the content exceeds 0.002%, the above-described effect is saturated, there is a problem that the load increases in the steelmaking process.

Accordingly, the content of calcium (Ca) may be 0.0005 to 0.002%, more preferably 0.001 to 0.002%.

The steel sheet of the present invention may include the remaining iron (Fe) and unavoidable impurities in addition to the above-described composition. Since unavoidable impurities may be unintentionally incorporated in a normal manufacturing process, they cannot be excluded. Since these impurities are known to anyone skilled in the art of steel manufacturing, all of them are not specifically mentioned in the present specification.

The steel sheet of the present invention may satisfy the alloy composition described above, and Pcm defined in Relation 1 may satisfy 0.19 to 0.21.

Pcm is one of the indexes indicating the possibility of low-temperature cracking, and can be used as an index indicating the hardenability of steel in steel containing 0.05% or less of C, such as the steel of the present invention. Pcm can be expressed by the following relational formula (1).

[Relational Expression 1]

Pcm = [C]+[Si]/30+([Mn]+[Cu]+[Cr])/20+[Ni]/60+[Mo]/15+[V]/10+5[B]

(Here, [C], [Si], [Mn], [Cu], [Cr], [Ni], [Mo], [V] and [B] are weight percent of the corresponding alloying element.)

If Pcm is less than 0.19, it becomes difficult to secure the target strength in the present invention due to insufficient hardenability of the steel.

Hereinafter, the steel microstructure of the present invention will be described in detail.

In the present invention, unless otherwise specified, % indicating the fraction of microstructure is based on the area.

The steel sheet of the present invention satisfying the above-described alloy composition may contain 0.5 to 1.5% cementite and the remainder ferrite as a microstructure. At this time, it is preferable that the average particle diameter of the ferrite measured by the EBSP method is 4 µm or less.

Cementite preferably contains 0.5 to 1.5% as a microstructure.

If the cementite content is less than 0.5%, it is difficult to secure the strength, and if the area ratio exceeds 1.5%, the formation of M-A structure in the abnormal region reheating grain coarsening heat affected zone is promoted, making it difficult to secure toughness.

It is preferable to include ferrite as the remainder structure.

Ferrite is a structure formed while the bainite or martensite structure generated after accelerated cooling is unwrapped by tempering, and should be included as the remaining structure except for cementite in order to satisfy the toughness targeted in the present invention.

It is preferable that the average particle diameter of ferrite measured by the EBSP method is 4 µm or less.

Ferrite is formed by tempering bainite or martensite formed in the water cooling process, and is very fine, so it is difficult to clearly decompose grain boundaries with an optical microscope. Instead, when the boundary with an azimuth difference of 15 degrees or more measured by the EBSP (Electron Back-Scattered Pattern) method is regarded as an effective grain boundary, it is preferable that the average grain diameter of ferrite measured at the thickness center of the steel sheet is 4 μm or less. When the average ferrite particle diameter exceeds 4 μm, it is difficult to secure the toughness at the center of the thickness targeted in the present invention.

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

The steel sheet according to one aspect of the present invention may be manufactured by reheating, rolling, cooling and heat treatment of a steel slab satisfying the above-described alloy composition.

slab reheat

It is preferable to reheat the steel slab satisfying the above alloy composition at a temperature in the range of 1050 to 1150 °C.

When the reheating temperature exceeds 1150 ° C, the austenite structure becomes coarse before rolling, and in the case of a thick steel sheet of 70 mm or more, it is difficult to sufficiently refine the austenite structure at the center of the thickness even after rolling, so it is difficult to secure the toughness at the center of the thickness. However, if the temperature is less than 1050 ℃, the rolling load is increased and a sufficient amount of reduction per pass cannot be given during rolling, so pores that may exist in the thickness center of the steel slab cannot be compressed, so the thickness direction ductility of the thickness center portion may be reduced have.

The reheating time is not particularly limited, but in general, if it is 1 minute or more per 1 mm of the thickness of the steel slab, it can be sufficiently heated to the center of the thickness.

primary rolling

It is preferable to first roll the reheated steel slab at a temperature of 900° C. or higher. If the primary rolling temperature is less than 900°C, the rolling load increases and the reduction per pass becomes insufficient, which makes it difficult to compress the pores existing in the thickness center of the steel slab, making it difficult to secure ductility in the thickness direction.

secondary rolling

It is preferable to perform secondary rolling of the primary rolled steel sheet at a rolling end temperature of 700 to 850°C and a cumulative reduction ratio of 40% or more. When the secondary rolling end temperature exceeds 850°C, it is difficult to secure the toughness of the thickness center, and when the temperature is less than 700°C, the rolling load becomes excessively high, making industrial application difficult.

If the cumulative rolling reduction of the secondary rolling is less than 40%, the austenite structure at the center of the thickness may not be sufficiently refined, making it difficult to secure the toughness at the center of the thickness of the steel sheet.

Cooling

It is preferable to water-cool the rolled steel sheet to 300° C. or less, at a cooling rate of 3° C./s or more, and then to air-cool to room temperature.

If the cooling end temperature exceeds 300°C or the cooling rate is less than 3°C/s, coarse upper bainite is formed in the thickness center of the steel sheet, making it difficult to secure the impact toughness at the center thickness of the steel sheet.

The steel sheet cooled to 300° C. or less can be air-cooled to room temperature.

heat treatment

It is preferable to heat-treat the cooled steel sheet by tempering at a temperature in the range of 500 to 600°C. Tempering heat treatment is to supplement the material properties in the thickness direction of the steel sheet after cooling. However, when the temperature exceeds 600° C., the carbide generated through the tempering heat treatment becomes coarse, making it difficult to secure strength.

The steel sheet of the present invention prepared as described above has a yield strength of 500 MPa or more, a tensile strength of 600 MPa or more, an average shock absorption energy at -50°C of 100J or more, and a FL (Fusion Line) minimum CTOD value of 0.25 at -20°C. mm or more, during low heat input multi-layer welding, the toughness of the weld affected zone may be excellent and high strength characteristics may be provided.

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

(Example)

After making molten steel having an alloy composition shown in Table 1 below, a steel slab was manufactured through a continuous casting method. The steel slab thus prepared was reheated, rolled, cooled and heat treated under the conditions shown in Table 2 to prepare a 75 mm thick steel sheet. At this time, after cooling, it was air-cooled to room temperature.

steel grade Alloy composition (wt%) Pcm C Si Mn P S Al Cu Ni Nb Ti N Ca A 0.026 0.15 2.34 0.006 0.002 0.033 0.21 2.46 0.008 0.012 0.0038 0.0012 0.200 B 0.037 0.17 2.27 0.005 0.003 0.028 0.35 2.14 0.006 0.009 0.0027 0.0008 0.209 C 0.042 0.11 2.15 0.003 0.002 0.032 0.37 2.16 0.009 0.013 0.0046 0.0017 0.208 D 0.045 0.08 2.05 0.004 0.001 0.031 0.25 2.27 0.011 0.011 0.0033 0.0013 0.201 E 0.039 0.06 2.11 0.005 0.003 0.022 0.22 2.11 0.013 0.008 0.0048 0.0011 0.193 F 0.024 0.09 2.36 0.009 0.001 0.027 0.38 2.38 0.007 0.012 0.0031 0.0009 0.204 G 0.056 0.15 2.13 0.003 0.002 0.034 0.26 2.29 0.008 0.009 0.0033 0.0015 0.219 H 0.028 0.03 2.37 0.005 0.003 0.004 0.37 2.35 0.013 0.011 0.0028 0.0014 0.208 I 0.046 0.09 2.05 0.006 0.002 0.027 0.21 1.87 0.009 0.009 0.0037 0.0011 0.193 J 0.033 0.17 2.27 0.007 0.002 0.016 0.28 2.21 0.022 0.013 0.0045 0.0009 0.203 K 0.042 0.13 2.35 0.004 0.002 0.022 0.36 2.36 0.008 0.011 0.0035 0.0015 0.221

[Relational Expression 1]

Pcm = [C]+[Si]/30+([Mn]+[Cu]+[Cr])/20+[Ni]/60+[Mo]/15+[V]/10+5[B]

(Here, [C], [Si], [Mn], [Cu], [Cr], [Ni], [Mo], [V], and [B] are weight percent of the corresponding alloying element.)

Psalm number steel grade reheat primary rolling secondary rolling Cooling heat treatment Temperature (℃) Termination temperature (℃) Termination temperature (℃) Cumulative rolling reduction (%) Termination temperature (℃) speed (℃/s) tempering temperature (℃) One A 1068 932 764 55 206 3.6 552 2 B 1123 981 798 45 251 3.9 548 3 C 1098 993 832 50 287 4.2 519 4 D 1135 987 735 50 265 3.4 583 5 E 1052 926 718 55 230 3.1 561 6 F 1087 959 789 40 276 3.7 540 7 G 1099 946 754 45 250 3.3 539 8 H 1112 992 729 50 255 3.4 576 9 I 1125 965 769 55 288 3.2 581 10 J 1067 955 771 45 223 3.8 546 11 K 1098 931 769 55 271 3.4 567 12 C 1183 970 804 55 261 3.7 552 13 D 1091 955 876 35 259 3.9 548 14 E 1103 979 805 55 349 2.4 568 15 F 1092 936 756 50 255 3.7 626

To analyze the microstructure of the manufactured steel sheet, a specimen was collected from the thickness center of the steel sheet. After the collected specimen was polished and etched with a nital corrosion solution, the cementite area ratio was measured using an image analyzer connected to an optical microscope. The average particle diameter of ferrite was measured using the EBSP method. The observation interval of the Kikuchi pattern was 0.2 µm, and the observation area was 200 µm x 200 µm. The average ferrite particle diameter was defined as a boundary where the difference in azimuth angle between the measured crystals was 15 degrees or more, and the results are shown in Table 3 along with the cementite area ratio.

Tensile specimens were taken from the manufactured steel sheet at 1/4 of the thickness of the steel sheet, and then, the yield strength and tensile strength were measured through a tensile test at room temperature. In addition, after taking three impact specimens at 1/2 the thickness of the steel plate, a Charpy impact test was performed at -50°C to obtain the average absorbed energy.

To evaluate the toughness of the heat-affected zone of welding, submerged arc multi-layer welding was performed on the steel sheet at a heat input of 5 kJ/mm. After inserting a fatigue crack in the FL (Fusion Line), which is the boundary between the weld metal and the heat-affected zone, CTOD tests were performed 5 times at -20°C according to the BS 7448 standard.

The minimum CTOD values measured at room temperature yield strength and tensile strength from the tensile test, the average absorbed energy at -50°C from the impact test, and the minimum CTOD value measured at -20°C from the FL CTOD test are shown in Table 3 below.

Psalm number steel grade microstructure mechanical properties division cementite area ratio
(%) Ferrite average particle size (㎛) Yield strength (MPa) Tensile strength (MPa) -50℃ Average shock absorption energy (J) -20℃ FL CTOD Minimum (mm) One A 0.6 2.9 527 623 287 0.85 Invention Example 1 2 B 1.2 3.7 538 621 236 0.68 Invention example 2 3 C 1.4 3.9 547 637 178 0.55 Invention example 3 4 D 1.4 3.4 511 607 251 0.41 Invention Example 4 5 E 1.3 3.8 518 609 290 0.85 Invention Example 5 6 F 0.7 3.2 522 616 266 1.03 Invention example 6 7 G 1.8 3.7 558 641 217 0.11 Comparative Example 1 8 H 0.8 3.3 531 624 236 0.07 Comparative Example 2 9 I 1.6 3.5 514 504 264 0.18 Comparative Example 3 10 J 1.1 3.1 528 619 289 0.13 Comparative Example 4 11 K 1.2 3.6 549 653 198 0.09 Comparative Example 5 12 C 1.3 4.6 532 625 78 0.48 Comparative Example 6 13 D 1.4 4.9 530 618 51 0.39 Comparative Example 7 14 E 1.3 4.3 491 604 95 0.77 Comparative Example 8 15 F 1.2 3.8 486 591 153 0.95 Comparative Example 9

Inventive Examples 1 to 6 satisfy the alloy composition and manufacturing method proposed in the present invention, yield strength of 500 MPa or more, tensile strength of 600 MPa or more, average absorbed energy of impact test of 100 J or more, and FL CTOD minimum value of 0.25 mm or more. It exhibited excellent weld heat-affected zone toughness while having high strength characteristics.

Comparative Examples 1 to 5 did not satisfy the alloy composition or Pcm range proposed in the present invention, so that the FL CTOD value did not exceed 0.25 mm, which is the minimum value targeted in the present invention.

In Comparative Examples 6 and 7, the reheating temperature and the secondary rolling end temperature were out of the scope of the present invention, respectively, the strength and FL CTOD were good, but the average particle diameter of ferrite exceeded 4 μm, and the impact toughness of the -50°C thick center was poor. couldn't

In Comparative Examples 8 and 9, respectively, the cooling conditions and the tempering temperature did not satisfy the ranges of the present invention, so the strength or thickness center impact toughness was not good. In the case of Comparative Example 9, the tempering temperature was higher than the range of the present invention, so cementite was coarsened, and strength could not be secured.

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

Claims (5)

By weight%, C: 0.02 to 0.05%, Si: 0.05 to 0.2%, Mn: 2.0 to 2.4%, P: 0.012% or less, S: 0.003% or less, Al: 0.01 to 0.04%, Cu: 0.2 to 0.4% , Ni: 2.1 to 2.5%, Nb: 0.005 to 0.015%, Ti: 0.005 to 0.015%, N: 0.002 to 0.006%, Ca: 0.0005 to 0.002%, the remainder including Fe and unavoidable impurities,
Pcm defined in the following relation 1 is 0.19 to 0.21,
It contains 0.5 to 1.5 area% of cementite and the remainder of ferrite as a microstructure,
The average particle diameter of ferrite measured by the EBSP method is 4 μm or less,
High-strength steel sheet with excellent weld heat-affected zone toughness with a yield strength of 500 MPa or more.

[Relational Expression 1]
Pcm = [C]+[Si]/30+([Mn]+[Cu]+[Cr])/20+[Ni]/60+[Mo]/15+[V]/10+5[B]
(Here, [C], [Si], [Mn], [Cu], [Cr], [Ni], [Mo], [V], and [B] are wt% of the corresponding alloying element.)
The method of claim 1,
The steel sheet is a high-strength steel sheet having an average impact absorption energy of 100J or more at -50°C, and excellent toughness in a welded heat-affected zone having a minimum FL CTOD value of 0.25 mm or more at -20°C.
By weight%, C: 0.02 to 0.05%, Si: 0.05 to 0.2%, Mn: 2.0 to 2.4%, P: 0.012% or less, S: 0.003% or less, Al: 0.01 to 0.04%, Cu: 0.2 to 0.4% , Ni: 2.1 to 2.5%, Nb: 0.005 to 0.015%, Ti: 0.005 to 0.015%, N: 0.002 to 0.006%, Ca: 0.0005 to 0.002%, the remainder including Fe and unavoidable impurities, in the following relation 1 Reheating the steel slab with a defined Pcm of 0.19 to 0.21 at a temperature in the range of 1050 to 1150° C.;
First rolling the reheated slab at a temperature of 900° C. or higher;
Second rolling the first rolled steel sheet at a rolling end temperature of 700 ~ 850 °C and a cumulative reduction ratio of 40% or more;
water cooling the secondary rolled steel sheet to 300° C. or less at a cooling rate of 3° C./s or more; and
A method of manufacturing a high-strength steel sheet excellent in toughness of a welded heat-affected zone, comprising the step of tempering heat treatment of the cooled steel sheet at 500 to 600°C.

[Relational Expression 1]
Pcm = [C]+[Si]/30+([Mn]+[Cu]+[Cr])/20+[Ni]/60+[Mo]/15+[V]/10+5[B]
(Here, [C], [Si], [Mn], [Cu], [Cr], [Ni], [Mo], [V], and [B] are wt% of the corresponding alloying element.)
4. The method of claim 3,
A method of manufacturing a high-strength steel sheet having excellent welded heat-affected zone toughness, further comprising the step of air cooling the water-cooled steel sheet to room temperature.


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