JP5348386B2 - Thick high-strength steel sheet with excellent low yield ratio and brittle crack resistance and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel plate which has a low yield ratio and excellent brittle crack generation resistance, and is suitable for a high strength line pipe having a pipe thickness of &ge;20 mm and a tensile strength of &gt;600 MPa, and to provide a method for producing the same. <P>SOLUTION: The steel is composed of a Cu-Ni-Nb-Ti system as the fundamental component system and, as necessary, comprises one or two kinds selected from Mo, Cr, V, B, Ca, rare earth metals, Zr and Mg, and the balance Fe with inevitable impurities, and in which the Vickers hardness Hvm of the central part in the plate thickness part satisfies Hvm&le;1.05 Hva to the average Hva of the Vickers hardness in the plate thickness direction, a microstructure is essentially composed of bainite, and insular martensite is dispersed in the bainite at an area ratio of 5 to 15% as a second phase. The steel having the above composition is reheated to a specified temperature, is subjected to hot rolling including a cumulative draft of &ge;30% in the temperature range of &le;1,000 to &ge;950&deg;C, is subjected to accelerated cooling after the completion of the rolling, is reheated to a specified temperature, and is air-cooled. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to a thick high-strength steel sheet and a method for producing the same, and is suitable for a high-strength line pipe having a pipe thickness of 20 mm or more and a tensile strength exceeding 600 MPa, and an excellent deformability with a yield ratio of 80% or less and a base material CTOD (Critical Tip Open Displacement) The critical opening displacement amount δc (mm) of the test is 0.20 mm or more at a test temperature of −20 ° C., which is excellent in brittle crack resistance.

  In recent years, line pipes used for transportation of natural gas and crude oil have been strengthened year by year in order to improve transportation efficiency by increasing pressure and to improve local welding construction efficiency by thinning.

  At the same time, there has been a demand for high deformability to prevent the occurrence of ductile cracks even if large deformations occur in the line pipe due to large earthquakes and ground deformation in the frozen land zone.

  As an index of high deformability, the yield ratio (YR) obtained by dividing the yield strength by the tensile strength is used, and the lower the YR, the higher the limit strain for crack generation.

  It is known that a low YR is obtained by making the microstructure of a steel material a two-phase structure of a soft ferrite phase and a hard phase in which hard bainite and martensite are appropriately dispersed. Is a method for obtaining a structure in which a hard phase is appropriately dispersed in a soft phase as described above, and quenching from a two-phase region of ferrite and austenite between quenching (Q) and tempering (T) ( A heat treatment method for applying Q ′) is disclosed.

  Patent Document 2 discloses that a low YR can be achieved by a ferrite + bainite + martensite structure in which a soft phase is processed ferrite.

  Further, Patent Document 3 discloses that island-like martensite is dispersed in bainite to achieve both low YR and high Charpy absorbed energy.

On the other hand, in order to improve the safety of the pipeline system, even if an operational abnormality occurs, it does not cause brittle fracture in the base metal part, so a CTOD test is performed and it is required to have a high limit opening displacement amount δc Increasing cases are also taking place.
JP-A-55-97425 Japanese Patent Laid-Open No. 08-209291 Japanese Patent Application Laid-Open No. 2006-265577

However, in the technique described in Patent Document 1, it is necessary to perform heat treatment a number of times, which decreases productivity and increases manufacturing cost.

  High toughness by the technique described in Patent Document 2 is obtained by actively developing a texture of ferrite in order to improve the ductility-brittle fracture surface ratio. The fracture surface of a Charpy impact test piece is separated. And the absorbed energy (Charpy impact value) in the Charpy impact test is rather lowered.

  Although low YR and Charpy absorbed energy by the technique described in Patent Document 3 can be compatible, when the base material CTOD test is performed on a thick material having a plate thickness exceeding 20 mm, the critical opening displacement amount δc value is significantly reduced. . That is, it has been difficult to achieve both low YR and brittle crack initiation characteristics particularly in thick-walled high-strength steel.

  Therefore, an object of the present invention is to provide a thick steel plate that achieves both high strength, low YR, and brittle crack resistance without increasing productivity and manufacturing cost, and a method for manufacturing the same.

The inventors have conducted intensive research on the cause of the remarkable decrease in the base metal CTOD characteristics as the thickness of the steel sheet is increased, particularly when manufacturing a bainite-island martensite duplex steel sheet.

  As a result, with the concentration of Mn and P in the segregated part, a large number of island martensites are generated in the central part of the plate thickness, the hardness of the central part of the plate thickness is increased, and brittle cracks are easily generated at that site. Since it becomes easy to generate | occur | produce, it discovered that the limiting opening displacement amount (delta) c in a base material CTOD test fell.

  Furthermore, if the plate thickness central portion hardness obtained by the Vickers hardness test can be 1.05 times or less of the average hardness value in the plate thickness direction, an island shape is formed in the bainite structure in order to obtain a low YR. It has been found that even when martensite is dispersed, the occurrence of brittle fracture is suppressed and the CTOD characteristics are improved.

The present invention has been made based on the obtained knowledge and further studies, that is,
1. % By mass
C: 0.04 to 0.08%
Si: 0.05 to 0.2%
Mn: 1.5 to 2.0%
P ≦ 0.006%
S ≦ 0.0006%
Al: 0.01 to 0.05%
Cu: 0.1 to 0.7%
Ni: 0.1 to 1.0%
Nb: 0.01 to 0.06%
Ti: 0.005-0.020%
N: 0.001 to 0.006%
Comprising the balance Fe and inevitable impurities,
The Vickers hardness Hvm at the center of the plate thickness is the average Hva of the Vickers hardness in the plate thickness direction.
Hvm ≦ 1.05Hva
Satisfied,
The microstructure is characterized in that the area ratio of bainite is 85% or more, and the island-like martensite is dispersed in the bainite at an area ratio of 5 to 15% with respect to the entire microstructure as the second phase.
Thick high-tensile steel plate with low yield ratio and excellent brittle crack resistance.
2. Furthermore, in mass%,
Mo: 0.01 to 1%
Cr: 0.01 to 1%
V: 0.01 to 0.1%
B: 0.0005 to 0.005%
A thick high-tensile steel sheet having a low yield ratio and excellent brittle cracking characteristics, according to claim 1, wherein
3. Furthermore, in mass%,
Ca: 0.0005 to 0.01%
REM: 0.0005 to 0.02%
Zr: 0.0005 to 0.03%
Mg: 0.0005 to 0.01%
A thick high-tensile steel sheet having a low yield ratio and excellent brittle cracking characteristics, characterized by containing one or more of the following.
After reheating the steel slab having the composition according to any one of 4.1 to 3 and manufactured by a continuous casting method to 1000 to 1100 ° C, hot rolling is started, and 1000 ° C or less 950 Rolling is performed so that the cumulative rolling reduction is ≧ 30% in the temperature range of ℃ or higher, and the cumulative rolling reduction is ≧ 70% in the temperature range of less than 950 ° C. The Vickers hardness Hvm at the center of the plate thickness is Vickers hardness in the plate thickness direction , characterized in that after cooling is stopped in the temperature range of 450 to 650 ° C., reheating to 600 to 700 ° C. and air cooling is performed. Hvm ≦ 1.05Hva is satisfied with respect to the average Hva, and the area ratio of bainite is 85% or more in the microstructure, and the area ratio of island-like martensite in the bainite is 5 to 15% with respect to the entire microstructure. low yield ratio are dispersed in One brittle manufacturing method of cracking characteristics superior thick high tensile steel.

  According to the present invention, a tensile strength of 600 MPa or more which achieves a low yield ratio of 80% or less exceeding 20 mm and a high limit opening displacement δc exceeding 0.2 mm in the base material CTOD test (test temperature—20 ° C.). This makes it possible to produce a thick, high-strength steel sheet, which is extremely useful industrially.

    In the present invention, the component composition, microstructure and manufacturing method are defined.

  [Component composition]% is mass%.

C: 0.04 to 0.08%
C contributes to an increase in strength by being supersaturated in a low temperature transformation structure. In order to obtain this effect, addition of 0.04% or more is necessary. However, if the addition exceeds 0.08%, island martensite in the center segregation portion of the plate thickness increases, and the base material CTOD characteristics deteriorate. Therefore, the upper limit is made 0.08%.

Si: 0.05 to 0.2%
Si is effective in increasing the strength of the base material and HAZ because addition of 0.05% or more causes solid solution strengthening regardless of transformation strengthening. However, if it exceeds 0.2%, island martensite is likely to be formed in the base material and HAZ. In particular, this effect is remarkable in a region where Mn and P are concentrated, such as a plate thickness central segregation portion, and the upper limit is set to 0.20% because the base material CTOD value is deteriorated through an increase in hardness of the segregation portion. To do.

Mn: 1.5 to 2.0%
Mn acts as a hardenability improving element. Furthermore, by adding a large amount, there is an effect of reducing the amount of C that can be dissolved in the ferrite phase, and when the bainite transformation is performed by accelerated cooling from the austenite region of steel, the C concentration in the untransformed austenite region is increased. The amount of island martensite produced can be increased.

  As will be described later, in order to make the area ratio of island martensite 5% or more, it is necessary to add at least 1.5% or more. On the other hand, in the continuous casting process, the concentration in the central segregation part is remarkably increased, and if the addition exceeds 2.0%, the base material CTOD characteristics are deteriorated, so the upper limit is made 2.0%.

P: ≦ 0.006%
P exists as an inevitable impurity in steel. In particular, segregation at the center segregation part is an element, which causes an increase in island-like martensite and remarkably deteriorates the base material CTOD characteristic, so the upper limit is made 0.006%. Preferably, it is 0.004% or less.

S: ≦ 0.0006%
S is also present as an inevitable impurity in the steel. In particular, the upper limit is set to 0.0006% because it exists as inclusions and lowers the cleanliness of steel and adversely affects the base material CTOD characteristics. Preferably, it is 0.0004% or less.

Al: 0.01 to 0.05%
Al acts as a deoxidizing element. A sufficient deoxidation effect can be obtained with addition of 0.01% or more, but if added over 0.05%, the cleanliness of the steel, including the segregation part, is reduced and the toughness is reduced. .05%.

Cu: 0.1 to 0.7%
Cu acts as a hardenability improving element when added in an amount of 0.1% or more, and can be used as a substitute for adding a large amount of Mn. However, if added over 0.7%, Cu dissolved in supersaturation precipitates during reheating after accelerated cooling, and in particular, the yield strength of steel increases due to precipitation hardening, which makes it difficult to achieve low YR. Therefore, the upper limit is made 0.7%.

Ni: 0.1 to 1.0%
Ni is also a useful element because it acts as a hardenability improving element and does not cause toughness deterioration when added. In order to obtain this effect, addition of 0.1% or more is necessary, but since it is an expensive element, the upper limit is made 1.0%.

Nb: 0.01 to 0.06%
Nb forms carbides to prevent temper softening of the weld heat-affected zone (HAZ) that is subjected to two or more thermal cycles, and to provide the HAZ strength necessary for a steel plate for high-strength line pipes with a tensile strength exceeding 600 MPa. It is an element necessary for obtaining.

  Moreover, there is also an effect of expanding the austenite non-recrystallized region at the time of hot rolling to the high temperature side, and in order to make the non-recrystallized region up to 950 ° C., addition of 0.01% or more is necessary. On the other hand, if added over 0.06%, the toughness of the HAZ is significantly impaired, so the upper limit is made 0.06%.

Ti: 0.005-0.020%
Ti forms nitrides and is effective in reducing the amount of solute N in the steel. Precipitated TiN suppresses austenite grain coarsening due to the pinning effect, contributing to improved toughness of the base metal and HAZ. To do.

  Addition of 0.005% or more is necessary to obtain the required pinning effect, but if added over 0.020%, carbides are formed, and the toughness deteriorates significantly due to precipitation hardening, The upper limit is 0.020%.

N: 0.001 to 0.006%
N is usually present as an inevitable impurity in steel, but TiN is added to form TiN that suppresses the coarsening of austenite grains as described above. In order to obtain the required pinning effect, it is necessary to be present in the steel in an amount of 0.001% or more, but when it exceeds 0.006%, it was heated to 1450 ° C. or more in the vicinity of the weld, particularly in the vicinity of the melting line When TiN decomposes in the region, the solute N has a bad influence, so the upper limit is made 0.006%.

  The above is the basic component composition of the present invention, but when further improving the characteristics, it is possible to add one or more of Mo, Cr, V, B, Ca, REM, Zr, and Mg.

Mo, Cr, V, B:
Mo, Cr, V, and B can be added alone or in combination of two or more for the purpose of increasing the strength.

Mo: 0.01 to 1%
Mo acts as a hardenability improving element when added in an amount of 0.01% or more, and can be used as a substitute for adding a large amount of Mn. However, since it is an expensive element and the increase in strength is saturated even if it is added in excess of 1%, when it is added, the upper limit is made 1%.

Cr: 0.01 to 1%
Cr also acts as a hardenability improving element when added in an amount of 0.01% or more, and can be used as a substitute for adding a large amount of Mn. However, if added over 1%, the HAZ toughness deteriorates remarkably, so when added, the upper limit is made 1%.

V: 0.01 to 0.1%
V is precipitation-hardened during multiple welding thermal cycles due to the combined addition with Nb, contributing to the prevention of HAZ softening. When added in an amount of 0.01% or more, the effect of preventing softening is manifested. However, if added over 0.1%, precipitation hardening remarkably deteriorates the HAZ toughness. To do.

B: 0.0005 to 0.005%
B segregates at the austenite grain boundaries and suppresses ferrite transformation, thereby contributing particularly to prevention of HAZ strength reduction. In order to obtain this effect, addition of 0.0005% or more is required, but even if added over 0.005%, the effect is saturated, so when added, the upper limit is 0.005%. To do.

Ca, REM, Zr, Mg
Ca, REM, Zr, and Mg control the morphology of MnS, which is a non-metallic inclusion in steel, or form oxides or nitrides, and suppress the coarsening of austenite grains mainly in the weld heat affected zone by the pinning effect. Etc., for the purpose of improving the toughness of the steel including the weld.

Ca: 0.0005 to 0.01%
Ca is an element effective for controlling the form of sulfide in steel, and the addition of 0.0005% or more suppresses the generation of MnS harmful to toughness. However, if added over 0.01%, a CaO-CaS cluster is formed and the toughness is deteriorated. Therefore, when added, the upper limit is made 0.01%.

REM: 0.0005 to 0.02%
REM is also an element effective for controlling the form of sulfide in steel, and by adding 0.0005% or more, the generation of MnS harmful to toughness is suppressed. However, since it is an expensive element and the effect is saturated even if it is added over 0.02%, when it is added, the upper limit is made 0.02%.

Zr: 0.0005 to 0.03%
Zr forms carbonitrides in steel and brings about a pinning effect that suppresses the coarsening of austenite grains, particularly in the weld heat affected zone. In order to obtain a sufficient pinning effect, addition of 0.0005% or more is necessary. However, when the addition exceeds 0.03%, the cleanliness in the steel is remarkably lowered and the toughness is lowered. Therefore, when added, the upper limit is made 0.03%.

Mg: 0.0005 to 0.01%
Mg produces fine oxides in the steel during the steelmaking process, and in particular, has a pinning effect that suppresses the austenite grain coarsening in the weld heat affected zone. In order to obtain a sufficient pinning effect, addition of 0.0005% or more is necessary, but if added over 0.01%, the cleanliness in the steel is lowered and the toughness is lowered. When added, the upper limit is made 0.01%.

[Microstructure and hardness]
The microstructure is mainly composed of bainite, and the second phase is a structure in which island martensite is uniformly dispersed in the bainite so as to have an area ratio of 5 to 15%. It should be noted that cementite and residual γ are allowed to be included in the microstructure as long as the strength and toughness are not impaired. The amount of cementite and residual γ is desirably less than 10% in area ratio individually or in combination.

  When the structure is mainly composed of ferrite due to insufficient cooling rate of accelerated cooling, it becomes difficult to achieve a tensile strength of 600 MPa or more. On the other hand, when the martensite structure is formed, although the strength can be sufficiently secured, the toughness is lowered, so that the structure is mainly composed of bainite.

  In order to achieve a low yield ratio, island martensite is uniformly dispersed and generated as a harder phase (second phase) than bainite as a parent phase in bainite. If the area ratio of island martensite is less than 5%, the yield ratio is not sufficiently lowered. Therefore, the lower limit of the area ratio of island martensite is set to 5%. On the other hand, if the area ratio exceeds 15%, the toughness of the base metal is remarkably deteriorated, so the upper limit is made 15%.

  Furthermore, in order to achieve the limit opening displacement δc ≧ 0.20 mm at the test temperature of −20 ° C. in the base material CTOD test, the plate thickness central portion hardness Hvm obtained in the Vickers hardness test (load 10 kg), The average value Hva of the Vickers hardness in the thickness direction is set to Hvm ≦ 1.05 × Hva.

  In the continuous cast material, the amount of island martensite is increased in the segregated portion of C, Mn, P, and S inevitably formed in the central portion of the plate thickness, and the hardness is increased as compared with other regions. If it exceeds 1.05 times the average hardness of the total thickness of the plate, the critical opening displacement δc is remarkably reduced, so the hardness of the central portion of the thickness is defined. Preferably, Hvm ≦ 1.02 × Hva.

[Production method]
The thick steel plate of the present invention can be manufactured by the following manufacturing method.

Casting method: Continuous casting method From the viewpoint of economy and productivity as steel for line pipes, the production of steel slabs for thick steel plates is a continuous casting method.

Billet heating temperature: 1000-1100 ° C
When hot rolling is performed, the steel slab is heated to 1000 ° C. or higher in order to austenite. On the other hand, if heating is performed at a temperature exceeding 1100 ° C., the crystal grains become extremely coarse and adversely affect the base metal Charpy and base material CTOD characteristics, so the upper limit is set to 1100 ° C.

Hot rolling: Cumulative rolling reduction at 1000 ° C. or lower and 950 ° C. or higher ≧ 30%
In order to reduce segregation of C, Mn, P, S, etc. in the center of the plate thickness, the steel of the present invention is specified as a cumulative rolling reduction ratio ≧ 30% in hot rolling at 1000 ° C. or less and 950 ° C. or more in production conditions. .

  When the steel slab by continuous casting is austenitized, Mn, P, S, etc. segregate at the austenite grain boundaries at the center thickness segregation part, and when non-recrystallization zone rolling and accelerated cooling are performed, the grain boundary segregated Mn, P, S is inherited by the transformed organization.

  Therefore, hot rolling is performed at a cumulative reduction of ≧ 30% at 1000 ° C. or lower and 950 ° C. or higher, which is an austenite recrystallization region, and the recrystallization is promoted to reduce segregation of Mn, P and S to austenite grain boundaries. .

    When the cumulative rolling reduction in the temperature range is less than 30%, recrystallization becomes partial. Therefore, the cumulative rolling reduction is set to 30% or more, and the entire steel piece has a recrystallized structure. In addition, although there is no restriction | limiting in the upper limit of a rolling reduction from a viewpoint of recrystallization, it sets it to 70% at the maximum from a viewpoint of steel-plate size and ensuring the rolling reduction required for rolling in the below-mentioned non-recrystallization area | region.

Hot rolling: Cumulative rolling reduction below 950 ° C. ≧ 70%
At a temperature below 950 ° C., which is an austenite non-recrystallized region, cumulative large pressure reduction is performed, austenite grains are expanded, and bainite that is transformed and formed by subsequent accelerated cooling is refined.

  In the steel according to the present invention, in order to reduce the yield ratio, hard island martensite is dispersed in bainite. Therefore, it is necessary to sufficiently increase the toughness of the bainite phase as a parent phase. If the cumulative rolling reduction is less than 70%, bainite is not sufficiently refined and the toughness is lowered by island martensite. Therefore, the cumulative rolling reduction is set to 70% or more. Preferably, the cumulative rolling reduction is 75% or more.

Accelerated cooling: cooling start temperature ≧ 700 ° C., cooling rate: 20-80 ° C./s, cooling stop temperature: 450-650 ° C.
In order to achieve a high strength of a tensile strength of 600 MPa or more, the microstructure needs to be a bainite-based structure. For this reason, accelerated cooling is performed after hot rolling. When the cooling start temperature is less than 700 ° C., proeutectoid ferrite is generated from the austenite grain boundaries in the air cooling process after hot rolling until the start of cooling, and the base material strength is reduced. Therefore, the lower limit temperature for starting accelerated cooling Is 700 ° C.

  When the cooling rate is less than 20 ° C./s, since the transformation is performed at a relatively high temperature, sufficient strength cannot be obtained.

  On the other hand, in the case of a cooling rate exceeding 80 ° C./s, it is difficult to control to a cooling stop temperature, which will be described later, and martensitic transformation occurs in the vicinity of the surface and the base material toughness is significantly reduced. Let s.

  In the present invention, the definition of the cooling stop temperature for accelerated cooling is important for obtaining a desired microstructure. In order to transform C-concentrated untransformed austenite, which is generated by reheating treatment after the stop of accelerated cooling, into island-like martensite during subsequent air cooling, it is cooled in the temperature range where untransformed austenite exists during bainite transformation. Need to stop.

  If the cooling stop temperature is less than 450 ° C., the bainite transformation is completed, so that island-like martensite is not generated during air cooling, and a low yield ratio cannot be achieved. On the other hand, if the temperature exceeds 650 ° C., C is consumed by the pearlite that precipitates during cooling, and no island-like martensite is generated, so the upper limit is set to 650 ° C.

Reheating after stopping cooling: Reheating temperature: 600-700 ° C
By reheating immediately after accelerated cooling, C can be concentrated in untransformed austenite and island martensite can be generated in the subsequent air cooling process. When the time until the start of reheating is long, untransformed austenite decreases due to the temperature drop during that time, and the amount of island martensite generated in the air cooling process after heating decreases, so reheating can be performed within 300 seconds. desirable. Preferably, it is within 100 seconds.

  Furthermore, if the reheating temperature is less than 600 ° C., C concentration to austenite does not occur sufficiently, and the required amount of island martensite cannot be ensured. On the other hand, when the reheating temperature exceeds 700 ° C., the bainite transformed by accelerated cooling is austenitized again and sufficient strength cannot be obtained, so the reheating temperature is specified to be 600 ° C. or more and 700 ° C. or less.

  It is not necessary to set the temperature holding time at the reheating temperature. Further, in the cooling process after reheating, island martensite is generated regardless of the cooling rate, and thus the cooling condition after reheating is not particularly defined, but basically it is preferably air cooling.

  In addition, although it does not specifically limit about the steel making method of steel, In order to ensure the economical efficiency as steel for line pipes, the steel making process by a converter method is desirable.

  Steel plates A to I were produced under the hot rolling / accelerated cooling / tempering conditions shown in Table 2 using steel having the chemical composition shown in Table 1.

  A sample for microstructural observation is collected from the center of the plate width of the obtained steel plate, and after mirror-polishing the plate thickness section parallel to the rolling longitudinal direction, island-like martensite is revealed using a two-step etching method. It was.

  Thereafter, a 10-view microstructure photograph was randomly taken at a magnification of 2000 using a surface scanning microscope (SEM), and the area ratio of island martensite in the photograph was measured with an image analyzer.

  Next, a full thickness tensile test piece based on API-5L from each steel plate, a full thickness sample for Vickers hardness test of JIS Z2244 (1998 revised edition), B (plate thickness) × 2B size 3 based on BS7448 A point-bending CTOD test piece and a V-notch Charpy impact test piece of JIS Z2202 (1980 revised edition) were collected from the center position of the plate thickness, and a steel plate tensile test, a Vickers hardness test in the plate thickness direction, and a three-point bend CTOD test. And Charpy impact test was performed to evaluate strength and toughness. The load of the Vickers hardness test was 10 kg, the measurement was 1 mm pitch, the test temperature of the three-point bending CTOD test and the Charpy impact test was −20 ° C., and the average value of the three samples was obtained.

  Table 3 summarizes the results of image analysis of the microstructure of the base metal and the results of the strength / toughness investigation. In Table 3, Hvm is the Vickers hardness at the center of the plate thickness, Hva is the average value of all Vickers hardnesses measured at a pitch of 1 mm in the plate thickness direction, and CTOD-20 (mm) is the limit opening at -20 ° C. The displacement amount δc (mm) is shown. The Vickers hardness at the center position of the plate thickness was the average value of Vickers hardness measured at 10 points at a pitch of 1 mm in the thickness direction ± 0.5 mm around the plate thickness 1/2.

  Invention Examples 1 to 6 are all within the chemical composition of steel sheet, rolling / accelerated cooling / reheating condition range of the present invention, and the thickness central hardness is 1.05 relative to the average hardness in the sheet thickness direction. The microstructure is composed of bainite and island martensite, and the area ratio of island martensite is 5 to 15%. Therefore, the target tensile strength is 600 MPa or more, the yield ratio is ≦ 80%, Charpy. The absorbed energy was 300 J or more, and CTOD-20 was 0.20 mm or more.

  On the other hand, in Comparative Example 7 in which the cumulative rolling reduction and cooling stop temperature at less than 950 ° C. were below the lower limit of the present invention, the YR, Charpy absorbed energy, and CTOD values were below the target. In Comparative Example 8 in which the slab heating temperature and the reheating temperature after cooling were outside the scope of the present invention, the YR and CTOD values were below the target.

  Comparative Example 9 in which the cumulative rolling reduction at 1000 ° C. or lower and 950 ° C. or higher was lower than the lower limit of the present invention was caused by segregation of Mn, P, and S at austenite grain boundaries because recrystallization of austenite was insufficient. As a result of the increase in hardness due to the increase in island-shaped martensite, the hardness at the center of the plate thickness greatly exceeded the average hardness, and as a result, the CTOD value fell below the target.

  In Comparative Examples 10, 11 and 13 in which the Mn content, P content, and Si content of the base material component exceeded the upper limit of the present invention, all of the alloy elements were segregated at the center of the plate thickness or island martensite was formed by segregation. As a result of the promotion and the fact that the hardness at the center of the plate thickness greatly exceeded the average hardness, both Charpy absorbed energy and CTOD value were below the target.

  In Comparative Example 12, in which S of the base material component exceeded the upper limit of the present invention, since there were many inclusions in the steel, both Charpy absorbed energy and CTOD value were below the target.

Claims (4)

% By mass
C: 0.04 to 0.08%
Si: 0.05 to 0.2%
Mn: 1.5 to 2.0%
P ≦ 0.006%
S ≦ 0.0006%
Al: 0.01 to 0.05%
Cu: 0.1 to 0.7%
Ni: 0.1 to 1.0%
Nb: 0.01 to 0.06%
Ti: 0.005-0.020%
N: 0.001 to 0.006%
Comprising the balance Fe and inevitable impurities,
The Vickers hardness Hvm at the center of the plate thickness is the average Hva of the Vickers hardness in the plate thickness direction.
Hvm ≦ 1.05Hva
Satisfied,
The microstructure is characterized in that the area ratio of bainite is 85% or more, and the island-like martensite is dispersed in the bainite at an area ratio of 5 to 15% with respect to the entire microstructure as the second phase.
Thick high-tensile steel plate with low yield ratio and excellent brittle crack resistance. Furthermore, in mass%,
Mo: 0.01 to 1%
Cr: 0.01 to 1%
V: 0.01 to 0.1%
B: 0.0005 to 0.005%
The thick-walled, high-tensile steel sheet having a low yield ratio and excellent brittle cracking characteristics, according to claim 1. Furthermore, in mass%,
Ca: 0.0005 to 0.01%
REM: 0.0005 to 0.02%
Zr: 0.0005 to 0.03%
Mg: 0.0005 to 0.01%
The thick-walled high-tensile steel sheet having a low yield ratio and excellent brittle cracking characteristics according to claim 1 or 2, characterized by containing at least one of the following. The steel slab having the composition according to any one of claims 1 to 3 and manufactured by a continuous casting method is reheated to 1000 to 1100 ° C, and then hot rolling is started, and 1000 ° C or less 950 Rolling is performed so that the cumulative rolling reduction is ≧ 30% in the temperature range of ℃ or higher, and the cumulative rolling reduction is ≧ 70% in the temperature range of less than 950 ° C. The Vickers hardness Hvm at the center of the plate thickness is Vickers hardness in the plate thickness direction , characterized in that after cooling is stopped in the temperature range of 450 to 650 ° C., reheating to 600 to 700 ° C. and air cooling is performed. Hvm ≦ 1.05Hva is satisfied with respect to the average Hva, and the area ratio of bainite is 85% or more in the microstructure, and the area ratio of island-like martensite in the bainite is 5 to 15% with respect to the entire microstructure. in the dispersion to have low yield And method for producing a superior thick high tensile steel in brittle cracking characteristics.