JP2023148712A - High strength thick steel plate and manufacturing method thereof - Google Patents

Abstract

To provide a steel plate having yield strength of 500 MPa or more, excellent host toughness before and after SR treatment, and when performing multilayer welding, having excellent joint CTOD characteristics, and a manufacturing method thereof.SOLUTION: A plate thickness is 60 to 100 mm, the yield strength is 500 MPa or more, the tensile strength is 570 MPa or more, C, Si, Mn, Ti, Cu, Ni, Nb, N, O, P, S, Al, Mg, Ca, and B are within a predetermined range, the remainder is a component consisting of Fe and impurities, the plate thickness t/4 part has a predetermined multiphase structure, an average equivalent circle diameter of the crystal grains at the center of the plate thickness is 50 μm or less, the maximum hardness HVmax at the center of the plate thickness is 250HV or less, a degree of segregation of Si, Mn, P, Cu, and Ni at the center of the plate thickness is within a predetermined range, and a particle number density (EIGFD) of Ti-containing inclusions contained in the steel structure is 20 particles/mm2 or more.SELECTED DRAWING: Figure 7

Description

The present invention is mainly aimed at steel plates for offshore structures where low-temperature toughness is required for welded joints, but it is not limited to this application, but can be applied to welded structures for a wide range of applications such as ships, buildings, bridges, tanks, etc. This relates to high-strength thick steel plates.

Welding is used to assemble not only marine structures but also various structures such as ships, buildings, bridges, and tanks into a predetermined shape. When the thickness of the steel plate used increases, it is common to perform multiple passes of welding. HAZ (Heat Affected Zone) refers to a region on the steel plate side of a welded joint where the metallographic structure of the base material created by processing heat treatment is thermally affected by reheating during welding.

It is known that the most brittle part of the weld HAZ is the base metal part in contact with the weld metal, which is CGHAZ (Coarse Grain Heat Affected Zone), which has a coarse metal structure. Since CGHAZ is reheated to near the melting line of the steel, austenite grains generated during reheating grow. During subsequent cooling, coarse austenite grains undergo re-transformation, resulting in a coarser metal structure.

In CGHAZ, since coarse austenite grains are cooled, quenching occurs easily and the hardness increases compared to the base material. In addition, when CGHAZ is reheated to a two-phase region of ferrite and austenite in subsequent welding passes, MA (Martensite-Austenite Constituent), which is hard and becomes the starting point of brittle fracture, is generated.

When strength is required for steel sheets, alloying elements such as carbon are added. When the amount of alloying elements added increases with increasing strength and thickness, the hardness of the HAZ increases and the number of brittle phases such as MA increases. The grain size of the structure, the size of the embrittlement phase, and the hardness are known as factors that affect HAZ toughness. Since CGHAZ has a coarse metal structure, contains many embrittlement phases, and is hard, it becomes the most brittle part of the weld HAZ. To ensure the safety of structures, control of the structure of CGHAZ in multilayer welded joints is important.

In order to improve the toughness of CGHAZ, Patent Document 1 describes a technology for refining the structure by dispersing TiN particles in steel to suppress the growth of austenite grains in CGHAZ that is reheated to the vicinity of the melting line and to refine the transformed structure. A technique has been described for achieving both high strength and HAZ toughness of steel sheets. However, depending on the welding conditions, if the reheating temperature of CGHAZ is 1350° C. or higher, TiN may melt, and the effect of grain refinement due to grain growth suppression cannot be obtained sufficiently, resulting in a decrease in toughness.

In addition, as a means for refining the structure of CGHAZ, there is a technique of finely dispersing Ti oxides in steel and using them as transformation nuclei to generate intra-granular ferrite (IGF). For example, as shown in Patent Document 2, a steel plate with excellent HAZ toughness at low temperatures has been developed.

Furthermore, high-strength thick steel plates are sometimes subjected to SR (Stress Relief) treatment for the purpose of removing residual stress generated during welding. From the viewpoint of the stability of the mechanical properties of the steel material, it is required that the change in the mechanical properties of the steel material be small before and after the SR treatment. In order to ensure the toughness of the base material after SR treatment, it is necessary to reduce impurity elements such as P and S. However, the concentration of impurity elements is high in the center segregation area, and furthermore, the concentration of alloying elements causes local damage. Therefore, it is difficult to obtain the same base material toughness as before SR treatment after SR treatment with high strength thick steel plates containing many alloying elements.

JP2012-207237A Japanese Patent Application Publication No. 7-278653

The purpose of the present invention is to provide a steel plate having a yield strength of 500 MPa or more, good base metal toughness before and after SR treatment, and good joint CTOD characteristics when multilayer welding is performed, and a method for manufacturing the same. shall be.

The present inventors conducted intensive research on a steel plate that achieves both a base material strength with a yield strength of 500 MPa or more, good base material toughness, and joint toughness.
1) Promotion of IGF transformation by Ti-containing inclusions containing Ti oxide and refinement of CGHAZ structure,
2) Refinement of coarse structures peculiar to CGHAZ, such as grain boundary ferrite (hereinafter referred to as GBF) and ferrite side plates (hereinafter referred to as FSP) generated through transformation from austenite grain boundaries, by controlling the balance of C and Mn;
3) Balancing the strength of the base material by optimizing the steel composition and reducing the hardness and stress concentration sources at the center of the plate thickness,
4) Reducing impurity elements by suppressing the degree of segregation in the center of the plate thickness and ensuring base material toughness after SR treatment by limiting the maximum hardness HVmax,
It was found that this method is effective in solving the above problems.

1) Conventionally, inclusions have often been regarded as a single entity, and their size, shape, and/or number have been treated as factors that affect the properties of steel sheets. According to the experimental results of the inventors, it has been found that the composition of the inclusions themselves is significantly involved in the formation of intragranular transformed ferrite in the HAZ. Specifically, Ti-containing inclusions containing Ti oxides (TiO, Ti ₂ O ₃ ) also include other Al oxides, Mg oxides, Ca oxides, etc., and contain them in a composite manner. As the content ratio of Ti oxide in Ti-containing inclusions increases, (i-1) the formation of intra-granular ferrite (IGF) is promoted, and (i-2) grain boundary ferrite (Grain Boundary ferrite) is promoted. It has been found that the formation of Ferrite (GBF) and Ferrite Side Plate (FSP) (both are brittle structures) is suppressed, and the low-temperature toughness in the HAZ is significantly improved.

2) The metal structures constituting CGHAZ include IGF, GBF, FSP, bainite, etc. Among these, FSP is a coarse structure having the same crystal orientation, and is a structure that reduces fracture toughness. In addition to refinement by IGF generation using Ti oxide, the HAZ structure can be further refined by suppressing coarse FSP generated from austenite grain boundaries. Normally, the growth of ferrite transformation is controlled by the diffusion of carbon, but by controlling the composition of C and Mn, the growth can be controlled by the diffusion of the alloying elements, and the growth rate can be significantly suppressed. . As a result of the study, it was found that ferrite transformation can be suppressed if [Mn]≧−3.8 [C]+2.1 is satisfied.

3) In order to ensure base material strength, alloys are added to ensure hardenability, but toughness decreases. It has been discovered that the target strength and toughness can be secured by reducing stress concentration sources such as MnS, which are the starting point of fracture, and further reducing the hardness at the center of the plate thickness. Specifically, it has been found that toughness can be ensured by satisfying a maximum hardness HVmax of 250 HV or less at the center of the plate thickness.

4) The greater the number of impurity elements and the harder the material, the lower the toughness of the base material after SR treatment. In addition to the high concentration of impurity elements, the central part of the sheet thickness has a high concentration of alloying elements, resulting in a hard structure with high hardenability, and is the part that becomes most brittle in the SR treatment. By performing homogenization treatment (SP (Soaking Process) treatment) in which the slab is held at high temperature for a long period of time, the concentration of impurity element P in the center of the plate thickness is eliminated, and the quenching of Si, Mn, Cu, Ni, etc. It has been found that the toughness of the base metal after SR treatment can be ensured by reducing the degree of segregation of elements that improve insertion properties and suppressing hardness.

The present invention was completed based on the above findings and further studies. The gist of the invention is as follows.

<1>
The plate thickness is 60 to 100 mm, the yield strength is 500 MPa or more, and the tensile strength is 570 MPa or more,
In mass%,
C: 0.020-0.120%,
Si: 0.05-0.30%,
Mn: 1.70-3.00%,
Ti: 0.005-0.018%,
Cu: 0.05-1.50%,
Ni: 0.05-2.00%,
Nb: 0.005-0.025%,
N: 0.0015-0.0060%,
O: 0.0010 to 0.0045%,
Contains
P: 0.015% or less,
S: 0.0050% or less,
Al: 0 to 0.004%,
Mg: 0 to 0.0010%,
Ca: 0-0.0010%,
B: 0 to 0.0015%,
and
Ceq. calculated by the following formula (1). The value is 0.460≦Ceq. and further satisfies formula (2), and has a chemical composition with the remainder consisting of Fe and impurities,
The ferrite fraction in the plate thickness t/4 portion is 0 to 15 area%, and the remainder is a multi-phase structure consisting of one or more types of bainite and martensite,
The average equivalent circle diameter of the top 10 crystal grains from the maximum area of the region surrounded by large-angle grain boundaries with a crystal orientation difference of 15° at the center of the plate thickness is 50 μm or less,
The maximum hardness HVmax at the center of the plate thickness is 250HV or less,
The degree of segregation at the center of the plate thickness is [Si]max/[Si]≦1.9 and [Mn]max/[Mn]≦2.0 and [P]max/[P]≦4.0 and [Cu] max/[Cu]≦2.1 and [Ni]max/[Ni]≦1.8,
Furthermore, Ti-containing inclusions, which are contained in the steel structure and contain Ti and one or more of Al, Mg, Si, Ca, and Mn, and have an equivalent circle diameter of 0.5 μm or more and 5.0 μm or less For particles, the Ti content percentage (TCP) of each particle of Ti-containing inclusions was calculated using formula (3) based on the mass ratio of elements measured by EDS, and group A with TCP of 40% or more, TCP of 40% When classified into Group B with less than 20% and 20% or more, the particle number density (EIGFD) of Ti-containing inclusions effective for intragranular transformation shown in formula (4) is 20 particles/mm ² or more. High strength thick steel plate.
Ceq=[C]+[Mn]/6+[Cu]/15+[Ni]/15... Formula (1)
[Mn]≧-3.8[C]+2.1... Formula (2)
TCP=[Ti]/([Ti]+[Al]+[Mg]+[Ca])...Equation (3)
EIGFD=(XA×0.8)+(XB×0.5) … Formula (4)
In the formulas (1) and (2), [C] is mass% of C, [Mn] is mass% of Mn, [Cu] is mass% of Cu, and [Ni] is mass% of Ni. It is.
In the formula (3), [Ti], [Al], [Mg], and [Ca] are the Ti, Al, Mg, and Ca contents (mass%) obtained from EDS analysis of Ti-containing inclusions, If not included, substitute 0.
In the above equation (4), XA and XB are measured values of the inclusion number density (pieces/mm ² ) classified into group A and group B, respectively.

<2>
Furthermore, in mass%,
Mo: 0.50% or less,
Cr: 0.50% or less,
V: 0.03% or less,
Contains one or more of the following,
The high-strength thick steel plate according to <1>, wherein the Ceq value is calculated by the following formula (1)' instead of the formula (1).
Ceq. =[C]+[Mn]/6+[Cu]/15+[Ni]/15+[Cr]/5+[Mo]/5+[V]/5... Formula (1)'
In the formula (1)', [C] is mass% of C, [Mn] is mass% of Mn, [Cu] is mass% of Cu, [Ni] is mass% of Ni, [Cr ] is the mass % of Cr, [Mo] is the mass % of Mo, and [V] is the mass % of V.

<3>
A steel piece having the composition described in either <1> or <2> above and produced by a continuous casting method is subjected to homogenization treatment at 1200°C or higher for 10 hours or more, and then cooled to 950°C to 1100°C. After being reheated at 670°C to 800°C and roughly rolled to an average rolling reduction of 7.5% or more per pass to a thickness of 135 to 210 mm, the rolling reduction was rolled from 670°C to 800°C to an average rolling reduction of 6% or more per pass. A method for producing a high-strength thick steel plate, which comprises performing finish rolling so that the steel plate is 0.0% or more, and then cooling at a cooling rate such that the central part of the plate thickness is 10° C./s or less.

<4>
The method for manufacturing a high-strength thick steel plate according to item <3> above, characterized in that after cooling at a cooling rate such that the central portion of the plate thickness is 10° C./s or less, heat treatment is performed at 300° C. or higher and 670° C. or lower.

The present invention has excellent CTOD characteristics in the multi-layer welded HAZ part of low to medium heat input welding, and the yield strength of the base metal is 500 MPa or more, the tensile strength is 570 MPa or more, and it has good properties before and after SR treatment. It becomes possible to manufacture high-strength thick steel plates with a thickness of 60 mm or more that have the same toughness as the base material. This makes it possible to increase the size and weight of steel structures used in extremely harsh environments, such as offshore structures, and to reduce costs by reducing the amount of steel used.

It is a figure showing the relationship between equation (2) of the steel plate and joint HAZ toughness after SR according to the present embodiment. FIG. 3 is a diagram showing the influence of homogenization heat treatment on the maximum hardness HVmax at the center of the plate thickness and the relationship between them and the base material toughness after SR according to the present embodiment. FIG. 2 is an explanatory diagram of a state in which a sample for metallographic observation is processed at a position including the central part of the plate thickness (portion of plate thickness t/2). FIG. 6 is an explanatory diagram of a state in which a portion having the highest average Mn concentration is specified centering on the plate thickness t/2 portion, and a viewing area of 1 mm×1 mm is specified. In a 1 mm x 1 mm square viewing area, a 20 x 20 μm square section is scanned in the vertical and horizontal directions (rolling direction and plate thickness direction), and at each position, the Si of the measurement point included in the area of the square section, FIG. 3 is an explanatory diagram of a state in which each average value of the region is determined from each mass % of Mn, P, Cu, and Ni. FIG. 2 is an explanatory diagram illustrating areas of square parts where the average values of Si, Mn, P, Cu, and Ni are the maximum values. FIG. 2 is a schematic diagram of inclusions in a steel plate according to the present embodiment. FIG. 3 is a diagram showing a thermal cycle when investigating the ability to generate intragranular transformed ferrite. This is an example of determining the presence or absence of intragranular transformation originating from inclusions in the steel sheet according to the present embodiment. 1 is a graph illustrating an example of compositional analysis results and IGF generation behavior of all inclusions present in a 1 mm x 1 mm region of a t/4 plate thickness portion after thermal cycling of a steel plate according to the present embodiment. 1 is a graph illustrating an example of compositional analysis results and IGF generation behavior of all inclusions present in a 1 mm x 1 mm region of a t/4 plate thickness portion after thermal cycling of a steel plate according to the present embodiment. 2 is a graph illustrating an example of a TCP calculation result of Ti-containing inclusions present in a 1 mm x 1 mm region of a t/4 plate thickness portion after a thermal cycle of a steel plate according to the present embodiment. It is a figure which shows the relationship between Formula (4) and SR successor HAZ toughness of the steel plate based on this embodiment.

Embodiments of the present invention will be described below.

[Plate thickness]
The present invention targets high-strength thick steel plates suitable for large welded structures such as ships, buildings, bridges, and tanks, and particularly relates to steel plates with a plate thickness of 60 mm to 100 mm.

[Yield strength, tensile strength]
The yield strength shall be 500 MPa or more. As offshore structures become larger, steel plates for offshore structures are required to have even higher strength and thickness. Therefore, for steel plates with a thickness of 60 to 100 mm, yield strength of 500 MPa or more and tensile strength of 570 MPa or more are used as indicators.

[Chemical composition]
The chemical composition of the steel plate according to this embodiment will be explained.
The steel plate according to this embodiment has C: 0.020 to 0.120%, Si: 0.05 to 0.30%, Mn: 1.70 to 3.00%, and Ti: 0.005% by mass. ~0.018%, Cu: 0.05~1.50%, Ni: 0.05~2.00%, Nb: 0.005~0.025%, N: 0.0015~0.0060%, Contains O: 0.0010 to 0.0045%, P: 0.015% or less, S: 0.0050% or less, Al: 0 to 0.004%, Mg: 0 to 0.0010%, Ca : 0 to 0.0010%, B: 0 to 0.0015%, and Ceq. calculated by the following formula (1). The value is 0.460≦Ceq. It also satisfies formula (2), and the remainder consists of Fe and impurities.
Ceq.=[C]+[Mn]/6+[Cu]/15+[Ni]/15... Formula (1)
[Mn]≧-3.8[C]+2.1... Formula (2)
In the formulas (1) and (2), [C] is mass% of C, [Mn] is mass% of Mn, [Cu] is mass% of Cu, and [Ni] is mass% of Ni. It is.

In addition, in the following description of chemical components, mass % is expressed as %. Furthermore, in the following description, when a range is indicated by connecting the upper limit and lower limit of the element content with "~", unless otherwise noted, it means the range that includes the upper limit and the lower limit. Therefore, when expressed as 0.01% to 0.20% by mass, the range means 0.01% by mass or more and 0.20% by mass or less.

C: 0.020-0.120%
C is an element that increases the strength of the base material. If the C content is less than 0.020%, the effect of improving the strength of the base material is small, so 0.020% is set as the lower limit. A more preferable lower limit of the C content is 0.030%. On the other hand, when C is contained in excess of 0.120%, the HAZ toughness decreases because cementite and a mixture of martensite and austenite (referred to as MA), which becomes the starting point of brittle fracture, increase. do. Therefore, the upper limit of the C content is set to 0.120%. In particular, for HAZ toughness and low-temperature toughness in high heat input welding, even a relatively small amount of small cementite or MA can easily become a starting point for brittle fracture and reduce HAZ toughness. Strict regulation is preferable. The upper limit of the C content is preferably 0.110%, more preferably 0.100%, even more preferably 0.090%, and even more preferably 0.080%.

Si: 0.05-0.30%
Si functions as a deoxidizing agent and also contributes to increasing strength. Particularly in the case of Ti-deoxidized steel, Si is often added to increase the deoxidizing ability, and the lower limit was set at 0.05%. A more preferable lower limit of the Si content is 0.06%. On the other hand, if it is contained excessively, MA, which is a hard embrittled structure, is likely to be generated in the microstructure of the HAZ. Since this MA deteriorates the toughness of the HAZ, it is desirable to limit the Si content, and the upper limit was set to 0.30%. The upper limit of Si content is preferably 0.23%, more preferably 0.15%.

Mn: 1.70-3.00%
Mn is an effective component for ensuring the strength and toughness of the base metal, and also has the effect of promoting intragranular transformation by combining with S and precipitating compositely as MnS on Ti oxides, so Mn is 1.70%. Contain the above. In order to exhibit these effects, the lower limit of the Mn content is more preferably 1.80%, and even more preferably 1.90%. Containing a large amount of Mn leads to segregation and the formation of hard phases, reducing HAZ toughness. In particular, after SR, grain boundary embrittlement is promoted, thereby degrading the toughness of the base material and HAZ. The upper limit was set at 3.00% within an allowable range. A more preferable upper limit of the Mn content is 2.90%, still more preferably 2.80%.

Ti: 0.005-0.018%
Ti is one of the important elements in the present invention, forming Ti-containing inclusions containing Ti oxides and promoting the formation of intragranular transformed ferrite in the HAZ. In addition, it forms nitrides and refines the microstructure through the pinning effect of γ grain boundaries, contributing to improved toughness. If Ti is less than 0.005%, a sufficient number of Ti-containing inclusions and nitride particles may not be obtained, so the lower limit is set to 0.005%. In order to generate a larger number of particles, the lower limit of the Ti content is preferably 0.008%, more preferably 0.009%, even more preferably 0.010%. On the other hand, if a large amount is added, the coarsely grown nitrides will become a starting point for brittle fracture and the toughness will deteriorate. It also promotes grain boundary embrittlement after SR, deteriorating the toughness of the base material and HAZ. In order to suppress the formation of coarse nitrides, the upper limit of Ti is preferably 0.018%. The upper limit of Ti is more preferably 0.016%.

Cu: 0.05% to 1.50%
Unless added in excess, Cu improves the strength and toughness of the base metal without adversely affecting the toughness of the weld heat affected zone. In order to exhibit these effects, the content should be 0.05% or more, but since adding too much will impair HAZ toughness and weldability, the upper limit was set at 1.50%.

Ni: 0.05% to 2.00%
Like Cu, Ni improves the strength and toughness of the base metal without adversely affecting the toughness of the weld heat affected zone unless added in excess. In order to exhibit these effects, the content should be at least 0.05% or more. In addition to being an expensive element, too much content impairs HAZ toughness and weldability, so the upper limit of addition for industrial production is set at 2.00%.

Nb: 0.005% to 0.025%
Nb is effective for improving the strength of the base material. In order to obtain this effect, it is necessary to contain 0.005% or more. On the other hand, excessive addition has an adverse effect on HAZ toughness. It also promotes grain boundary embrittlement after SR, deteriorating the toughness of the base material and HAZ. Therefore, the upper limit is set at 0.025%. More preferably, the upper limit is 0.020%. More preferably, the upper limit is 0.018%.

N: 0.0015-0.0060%
N is an element that forms nitrides and is an essential element for obtaining the γ grain pinning effect by nitrides, and the lower limit for this purpose was set at 0.0015%. The lower limit is preferably 0.0018%, more preferably 0.0020%. On the other hand, if the N content is high, coarse nitrides such as AlN and TiN are likely to be generated. These coarse particles may become a starting point for brittle fracture, leading to a decrease in HAZ toughness. Therefore, the upper limit of the N content is set to 0.0060%. A preferable upper limit of the N content is 0.0055%, more preferably 0.0050%.

O: 0.0010-0.0045%
O is an element that forms oxides, and is one of the important elements in generating Ti-containing inclusions that become intragranular transformed ferrite nuclei. In order to obtain a dispersion of Ti-containing inclusions, the lower limit was set to 0.0010%. On the other hand, if the content is large, coarse oxides are likely to be generated. Since coarse oxides become a starting point for fracture and reduce HAZ toughness, the upper limit of the O content is set to 0.0045%. The upper limit of the O content is preferably 0.0040% or less, and more preferably 0.0035%.

P: 0.015% or less P is an element that causes grain boundary embrittlement and is harmful to toughness. In particular, in the present invention, since segregation to grain boundaries is promoted during the SR treatment and subsequent slow cooling, it is a very harmful element from the viewpoint of ensuring the toughness of the base material and HAZ after SR. Therefore, it is desirable that the P content be small. In the present invention, since the homogenization heat treatment lowers the P content in the central segregation area and the micro segregation area, it is possible to relax the content to some extent. If more than 0.015% of P is contained, the base material and HAZ toughness after SR will be significantly reduced, so the upper limit of the P content is limited to 0.015%. Preferably it is 0.013% or less, more preferably 0.011% or less. Although it is not necessary to particularly limit the lower limit of the P content, it is technically not easy to reduce the P content to 0%, so it may be set to exceed 0%. The lower limit of the P content may be 0.001%.

S: 0.0050% or less S is an element that generates inclusions such as MnS, and if coarse drawn MnS is generated in the center of the plate thickness, it will reduce the toughness (HAZ, base material) and the elongation in the thickness direction. let Therefore, the upper limit of the S content is set to 0.0050%. The preferable upper limit of the S content is 0.0040%. In order to improve HAZ toughness, the upper limit of the S content may be set to 0.0030% or 0.0025%. Although it is not necessary to particularly limit the lower limit of the S content, it is technically not easy to reduce the S content to 0%, so it may be set to exceed 0%. On the other hand, when aiming to obtain more stable intragranular transformation by precipitating MnS in a composite manner in intragranular transformation nuclei, the lower limit of the S content is preferably 0.0005%. In order to generate a larger amount of MnS, the lower limit of the S content may be set to 0.0010%.

Al: 0-0.004%
Al is an element that functions as a deoxidizing agent and reduces the amount of dissolved oxygen in molten steel. However, when a large amount of Al is contained, the Ti-containing oxide loses its function as an intragranular transformed ferrite nucleus, and the HAZ toughness deteriorates. do. Therefore, the upper limit of the Al content was set to 0.004%. The upper limit of the preferable Al content is 0.003%. There is no need to particularly limit the lower limit of the Al content, and the lower limit may be set to 0%.

Mg: 0-0.0010%
Mg is an element that functions as a deoxidizing agent and a desulfurizing agent and reduces the amount of dissolved oxygen and S in molten steel. However, as Mg oxide increases, Ti oxide, which is most effective for generating intragranular transformed ferrite, decreases, so it is preferable to suppress the generation of Mg oxide as much as possible. Therefore, the upper limit of the Mg content is set to 0.0010%. Preferably, the upper limit of the Mg content may be set to 0.0005%. There is no need to particularly limit the lower limit of the Mg content, and the lower limit may be set to 0%.

Ca: 0-0.0010%
Ca is an element that functions as a deoxidizing agent and a desulfurizing agent and reduces the amount of dissolved oxygen and S in molten steel. However, if a large amount of Ca-containing oxide or Ca-containing sulfide is contained in the composite oxide, the function of the Ti-containing oxide as an intragranular transformed ferrite nucleus is lost, and the HAZ toughness is deteriorated. Therefore, the upper limit of Ca content is set to 0.0010%. Preferably, the upper limit of the Ca content may be set to 0.0005%. There is no need to particularly limit the lower limit of the Ca content, and the lower limit may be set to 0%.

B: 0-0.0015%
B is an element that significantly increases the hardenability and improves the strength and toughness of the base material and HAZ, and may be included. However, if it is added in a large amount, the strength may vary widely, and the toughness may become unstable accordingly. Therefore, the upper limit of the B content was set to 0.0015%. A preferable upper limit of the B content is 0.0013%, and a more preferable upper limit is 0.0010%. The lower limit of the B content may be 0%, but since it is technically not easy to reduce the B content to 0%, it may be set to exceed 0%. In order to obtain the effect of increasing strength, the B content is preferably 0.0003% or more. More preferably, the B content is 0.0005% or more.

Ceq. ≧0.460
The Ceq value calculated by the following formula (1) is an index indicating the hardenability of steel components, and Ceq. The higher the value, the higher the strength of the steel plate. Ceq. It is necessary to make it 0.460 or more.
Ceq.=[C]+[Mn]/6+[Cu]/15+[Ni]/15... Formula (1)
In formula (1), [C] is the mass % of C, [Mn] is the mass % of Mn, [Cu] is the mass % of Cu, and [Ni] is the mass % of Ni.

[Mn]≧-3.8[C]+2.1... Formula (2)
In formula (2), [C] is mass % of C, and [Mn] is mass % of Mn. When the balance between Mn and C is controlled so as to satisfy the above formula (2), it is possible to suppress the generation of coarse structures such as FSP that transform from coarse austenite grain boundaries in CGHAZ, which adversely affect toughness. FIG. 1 shows that if formula (2) is satisfied, the joint CTOD at -10° C. is 0.6 mm or more, indicating that the joint has good CTOD characteristics.

The steel sheet according to the present embodiment basically contains the above chemical components, but in order to improve the mechanical properties and HAZ toughness of the steel sheet (base material), some of the Fe may be added as necessary. Alternatively, one or more of Mo: 0.50% or less, Cr: 0.50% or less, and V: 0.03% or less may be contained as optional components.

Mo: 0.50% or less Mo is an element that improves hardenability and increases the strength of the base material, and may be included. However, if Mo is contained in an amount exceeding 0.50%, a hard structure may be generated in the HAZ and the HAZ toughness may be reduced, so the upper limit of the Mo content is limited to 0.50%. Preferably, the upper limit of the Mo content is 0.40%, more preferably 0.30%. Although Mo may be mixed as an impurity from scrap etc. during the production of molten steel, there is no need to particularly limit its lower limit, and it may be 0%. In order to improve the strength of the base material, the Mo content is preferably 0.02% or more. More preferably, the Mo content is 0.04% or more.

Cr: 0.50% or less Cr is an element that increases the strength of the base material by improving hardenability and precipitation strengthening, and may be included. However, when Cr is contained in an amount exceeding 0.50%, MA tends to be generated in the HAZ, resulting in a decrease in HAZ toughness. Therefore, the upper limit of the Cr content is limited to 0.50%. Preferably, the upper limit of the Cr content is 0.40%, more preferably 0.30%. Although Cr may be mixed as an impurity from scrap etc. during the production of molten steel, there is no need to particularly limit its lower limit, and it may be 0%. In order to improve the strength of the base material, the Cr content is preferably 0.02% or more. More preferably, the Cr content is 0.10% or more.

V: 0.03% or less V is an element that improves hardenability, and is an element that forms carbides and nitrides and is effective in increasing the strength of the base material, so V may be included. . However, if V is contained in an amount exceeding 0.03%, precipitation of carbonitrides in the HAZ becomes significant and the HAZ toughness may decrease, so the V content is limited to 0.03% or less. Preferably, the V content is 0.025% or less. Although V may be mixed in as an impurity from scrap etc. during the production of molten steel, there is no need to particularly limit its lower limit, and it may be 0%. In order to improve the strength of the base material, the V content is preferably 0.01% or more.

When the high-strength thick steel plate of the present invention contains one or more of these as optional components, the Ceq value is calculated by the following formula (1)'.
Ceq. =[C]+[Mn]/6+[Cu]/15+[Ni]/15+[Cr]/5+[Mo]/5+[V]/5... Formula (1)'
In formula (1)', [C] is mass% of C, [Mn] is mass% of Mn, [Cu] is mass% of Cu, [Ni] is mass% of Ni, [Cr] is the mass % of Cr, [Mo] is the mass % of Mo, and [V] is the mass % of V.

The remainder of the chemical components of the steel plate according to this embodiment are iron (Fe) and impurities. Impurities refer to components that are mixed in from raw materials such as ore, scrap, and other factors during the industrial production of steel materials, and are allowed within the range that does not adversely affect the steel materials according to this embodiment. do.

[Metal structure]
If the ferrite fraction exceeds 15%, the yield strength of the base material cannot satisfy 500 MPa, so the ferrite fraction in the metal structure of the plate thickness t/4 portion is set to 15% or less, and the remainder is made of bainite and/or martensite. The metal structure consists of: If the ferrite fraction exceeds 15%, or even if the ferrite fraction is 15% or less, the remainder is not a metal structure consisting of bainite and/or martensite, the yield strength of the base material is 500 MPa. become unsatisfied. Therefore, the ferrite fraction in the plate thickness t/4 portion is 15 area % or less, and the remainder is made of one or more types of bainite and martensite. Note that the ferrite fraction can be measured from a 500x optical microscope observation field by processing a sample for metallographic observation from a cross section in the plate thickness direction parallel to the rolling direction of the plate thickness t/4 portion of the steel plate.

[Average circular equivalent diameter ≦50μm]
The finer the metal structure, the better the toughness of the base material. If the average circular equivalent diameter of the crystal grains surrounded by large-angle grain boundaries with a misorientation of 15° or more at the center of the plate thickness (plate thickness t/2 part) is 50 μm or less, good base material toughness can be obtained at low temperatures. The upper limit of the average equivalent circle diameter was set to 50 μm. Note that the average equivalent circle diameter is the average of the equivalent circle diameters (diameters) of the top 10 crystal grains from the largest area. Since it is crystal grains with a large grain size that cause brittle fracture, the equivalent circle diameters of the top 10 crystal grains were defined based on the maximum area. Note that when measuring the equivalent circle diameter of a metal structure, the entire observation field is measured without considering the structure classification such as ferrite or bainite. The grain size is determined by measuring EBSD (Electro Back Scattering Diffraction) by processing a sample for metallographic observation from a cross section in the thickness direction parallel to the rolling direction, centered on the plate thickness t/2 portion of the steel plate, and measuring, for example, crystallization. It can be determined by averaging the equivalent circle diameters of the top 10 largest areas defined by boundaries with an orientation difference of 15° or more.

[Maximum hardness HVmax at center of plate thickness is 250HV or less]
As for base material toughness, the harder the base material is, the lower the toughness is due to SR treatment. In order to maintain toughness after SR, the maximum hardness of the central part of the plate thickness (t/2 part of the plate thickness) needs to be 250 HV or less. FIG. 2 shows the influence of homogenization heat treatment on the maximum hardness HVmax at the center of the plate thickness and the relationship between these and the toughness of the base material after SR. By performing the homogenization heat treatment, the maximum hardness HVmax at the center of the plate thickness can be made 250 HV or less. The hardness at the center of the plate thickness was determined by processing a sample for metallographic observation from a cross section in the plate thickness direction parallel to the rolling direction, centered at the plate thickness t/2, and performing optical microscopic observation at the plate thickness t/2. For example, five fields of view are photographed at random, and the hardness of each two locations is measured by a Vickers test with a load of 25 g, and the maximum hardness is defined as the maximum hardness HVmax at the center of the plate thickness.

[Segregation degree of Si, Mn, P, Cu, Ni]
Next, the segregation of Si, Mn, P, Cu, and Ni will be explained.
The steel plate according to the present embodiment has a degree of segregation in the central part of the plate thickness such that [Si]max/[Si]≦1.9 and [Mn]max/[Mn]≦2.0 and [P]max/[P] ≦4.0, [Cu]max/[Cu]≦2.1, and [Ni]max/[Ni]≦1.8.
Since the alloying elements are concentrated in the center of the plate thickness, the hardenability is locally high and the structure becomes hard, resulting in a decrease in toughness. Furthermore, when the impurity elements become concentrated, the toughness decreases significantly in the SR treatment. In order to ensure base material toughness, the degree of segregation at the center of the plate thickness must be [Si]max/[Si]≦1.9, [Mn]max/[Mn]≦2.0, and [P]max/[ P]≦4.0, [Cu]max/[Cu]≦2.1, and [Ni]max/[Ni]≦1.8.

The degree of segregation of each element can be determined by EPMA (Electron Probe Micro Analysis) measurement according to the following procedures (a), (b), (c), and (d). Furthermore, the degree of segregation at the center of the plate thickness is determined in a viewing area of 1 mm x 1 mm, which is determined based on the plate thickness t/2 portion.

(a)
First, as shown in FIG. 3, a metallographic observation sample 11 is processed at a position including the central part of the plate thickness (plate thickness t/2 part) in a cross section in the plate thickness direction parallel to the rolling direction of the steel plate 10. As shown in FIG. 4, in the cross section in the plate thickness direction that appears in the sample 11 for metallographic observation, lines with a length of 10 mm are analyzed by EPMA measurement at a pitch of 50 μm in the plate thickness direction with the plate thickness t/2 part as the center. The analysis is performed in a range 12 of ±5 mm from the center of the plate thickness to measure the mass % of Mn. Then, the plate thickness position range 13 in which the Mn concentration average value obtained by averaging each line analysis Mn concentration average in the plate thickness direction ±0.5 mm is the highest is specified. Among the plate thickness position range 13, a range in which the average Mn concentration is high in the range of ±0.5 mm in the rolling direction is determined, and a viewing area 14 of 1 mm x 1 mm is specified as follows.
(b)
That is, in the cross section in the plate thickness direction appearing in the sample 11 for metallographic observation, the center in the plate thickness direction of the region 13 where the average Mn concentration is high is the longitudinal center position, and the Mn concentration in the range of ±0.5 mm in the rolling direction of 13 A 1 mm x 1 mm square viewing area 14 whose horizontal center is the rolling direction center of the area where the average value is high is specified. In this viewing area 14, a surface analysis is performed by EPMA measurement at a pitch of 2 μm in each of the longitudinal and lateral directions (rolling direction and plate thickness direction), and each mass % of Si, Mn, P, Cu, and Ni is measured.
(c)
Next, as shown in FIG. 5, in the 1 mm x 1 mm square viewing area 14, the 20 x 20 μm square portion 15 is scanned in the vertical and horizontal directions (rolling direction and plate thickness direction), and at each position, a square portion 15 is scanned. Each average value of the region of the portion 15 is determined from each mass % of Si, Mn, P, Cu, and Ni at the measurement points included in the region of the portion 15. Then, the average value of each mass % of Si, Mn, P, Cu, and Ni in the square portion 15 where the average value is the maximum is determined by the respective maximum values ([Si]max, [Mn]max, [P]max , [Cu]max, [Ni]max). For example, as shown in FIG. 6, if the average value of Si determined from each mass % of Si at measurement points included in the area of square portion 15-1 becomes the maximum value, then The average value of Si determined from the mass % of Si at each measurement point included in the region is the maximum value [Si]max. Similarly, if the average value of Mn determined from each mass % of Mn of the measurement points included in the area of the square part 15-2 is the maximum value, each measurement included in the area of the square part 15-2 The average value of Mn determined from the mass % of Mn at a point becomes the maximum value [Mn]max. Similarly, if the average value of P determined from each mass % of P at the measurement points included in the area of the square part 15-3 is the maximum value, each measurement included in the area of the square part 15-3 The average value of P determined from the mass % of P at a point is the maximum value [P]max. Similarly, if the average value of Cu determined from each mass % of Cu at measurement points included in the area of the square part 15-4 is the maximum value, each measurement included in the area of the square part 15-4 The average value of Cu determined from the mass % of Cu at a point becomes the maximum value [Cu]max. Similarly, if the average value of Ni determined from each mass % of Ni at measurement points included in the area of the square part 15-5 is the maximum value, each measurement included in the area of the square part 15-5 The average value of Ni determined from the mass % of Ni at a point becomes the maximum value [Ni]max.
(d)
The respective maximum values ([Si] max, [Mn] max, [P] max, [Cu] max, [Ni] max) are calculated as the analytical values of the steel plate for each component ([Si], [Mn], [ P], [Cu], [Ni]) divided by ([Si]max/[Si], [Mn]max/[Mn], [P]max/[P], [Cu]max/[Cu ], [Ni]max/[Ni]) is the degree of segregation of each component at the center of the plate thickness.

[Ti-containing inclusions]
The steel plate according to this embodiment is premised on being a steel plate manufactured by a manufacturing method including deoxidation using Ti. The present inventors conducted detailed investigation and research on the relationship between HAZ structure and toughness. As a result, it was found that in order to improve HAZ toughness, it is necessary to promote ferrite transformation that occurs within prior austenite grains. In order to promote the formation of intragranular transformed ferrite, it is effective to disperse inclusion particles that serve as intragranular transformation nuclei, and as the oxide species, Ti oxide (TiO, Ti ₂ O ₃ ) is preferable as described above. However, in actual equipment manufacturing, even when Ti is deoxidized, a large amount of complex oxides containing elements such as Al, Mg, and Ca may be contained, and depending on their abundance ratio, sufficient intragranular transformation may not occur. found.

In view of the above circumstances, the present inventors investigated the particle composition and microstructure of Ti-containing inclusions that serve as intragranular transformation nuclei, and found that the probability of occurrence of intragranular transformation changes depending on the composition of each Ti-containing inclusion. It was confirmed. Furthermore, by optimizing the manufacturing conditions in the steelmaking process, particles of Ti-containing inclusions with a predetermined Ti content ratio are generated in the steel with a number density within a predetermined range, and HAZ toughness can be improved. We also examined the contributing conditions.

In the steel material according to the present embodiment, it is essential to generate intragranular transformed ferrite in order to ensure HAZ toughness, and for this purpose, the dispersion state of each particle of Ti-containing inclusions containing Ti oxide is defined. The Ti-containing inclusions may contain one or more of Al oxide, Mg oxide, Si oxide, Ca oxide, Ca sulfide, and Mn sulfide in addition to Ti oxide. Further, impurity elements such as Zr, Y, Hf, REM, Sn, Sb, Te, Se, Bi, Pb, etc., which are contained in extremely small amounts in steel, may be mixed into the Ti-containing inclusions. FIG. 7 shows a schematic diagram of particles of Ti-containing inclusions (composite inclusions containing Ti, Al, Mg, Ca, and Mn). Mn sulfide (MnS) is partially precipitated around the Ti-containing inclusion particles.

The present inventors have found that the mass or mass ratio of Al oxide, Mg oxide, Ca oxide, Ca sulfide, etc. to Ti-containing inclusions greatly influences the quality of HAZ toughness. Ti oxide is characterized by containing cation vacancies, and forms a Mn-depleted layer around particles of Ti-containing inclusions during solidification and cooling, thereby contributing to the promotion of intragranular transformed ferrite formation. Since the formation temperature of Al oxide, Mg oxide, Ca oxide, Ca sulfide, etc. is higher than that of Ti oxide, their presence in the formation of Ti-containing inclusion particles may inhibit the formation of the Mn-depleted layer. Conceivable. Therefore, the promotion of intragranular transformed ferrite formation is more effective as the proportion of Ti oxide in the Ti-containing inclusions increases, while the proportion of other Al oxides, Mg oxides, Ca oxides, Ca sulfides, etc. The higher the number, the lower the effect. Note that it is considered that oxides such as Mn oxide and Si oxide, which are generated at a lower temperature than Ti oxide, do not necessarily inhibit the formation of a Mn-depleted layer.

The contents of Ti, Al, Mg, and Ca contained in particles of Ti-containing inclusions can be determined by EDS mapping (energy dispersive X-ray spectroscopy) during cross-sectional observation using a scanning electron microscope (SEM). elemental mapping) can be measured for the entire particle containing Ti-containing particles, and determined as the average value.

The particle size (equivalent circle diameter) of Ti-containing inclusions effective for intragranular transformed ferrite is 0.5 to 5.0 μm. If the particle size of the Ti-containing inclusions is small, it becomes difficult to generate intragranular transformed ferrite, so the lower limit was set to 0.5 μm. Further, since coarse particles of Ti-containing inclusions themselves may become a starting point of brittle fracture and reduce toughness, the upper limit was set to 5.0 μm. Note that a suitable method for measuring the particle size is to measure a photograph of the particles of the Ti-containing inclusion using a SEM, and then use image analysis to determine the equivalent circular diameter from the cross-sectional area.

Ti-containing inclusions may be observed by preparing a micro sample from a steel material heated to 1350 to 1400° C., held for about 3 to 30 seconds, and then rapidly cooled. This is because, for example, if alloy carbonitrides or the like are generated, it is difficult to measure the number of Ti-containing inclusion particles with a size of 0.5 μm or more and 5.0 μm or less, which are the observation targets. A sample with less carbonitrides can be produced by heating to a high temperature to dissolve precipitates other than those to be observed, followed by rapid cooling, or by applying a thermal cycle in which ferrite is generated during rapid cooling. Ti-containing inclusions containing Ti oxides are stable even when heated to high temperatures, and their morphology hardly changes during cooling. Therefore, even when subjected to such thermal cycles, the measurement results of the number of particles of Ti-containing inclusions remains almost unchanged. Further, particles of Ti-containing inclusions may be observed either in a state where a structure such as nital corrosion is exposed or by mirror polishing.

In determining the composition of Ti-containing inclusions, particles of Ti-containing inclusions with a low Ti content are first excluded. For this determination, [Ti]/([Ti]+[ Al]+[Mg]+[Ca]+[Mn]+[S]+[Si]). Ti-containing inclusions in which this [Ti]/([Ti]+[Al]+[Mg]+[Ca]+[Mn]+[S]+[Si]) is 10% or more Those with less than 10% are excluded. Note that [Ti], [Al], [Mg], [Ca], [Mn], [S], and [Si] are Ti, Al, Mg, Ca, Mn, and Ti obtained from EDS analysis of inclusions, respectively. This is the S and Si content (mass%), and if it is not contained, 0 is substituted.

Next, the proportion of Ti oxide (TCP) contained in the Ti-containing inclusions is calculated based on equation (3).
TCP=[Ti]/([Ti]+[Al]+[Mg]+[Ca])...Equation (3)

Then, the intragranular transformation ability of each particle of the Ti-containing inclusion is determined based on the Ti content ratio contained in the Ti-containing inclusion. By calculating the Ti content percentage (TCP) of each Ti-containing inclusion particle based on the analysis value of each particle by oxysulfide EDS, it is possible to determine whether the Ti-containing inclusion particle has a high probability of undergoing intragranular transformation. I can judge.

Specifically, when TCP is 40% or more, intragranular transformation is most likely to occur (group A), and when TCP is less than 40% and 20% or more, intragranular transformation ability is slightly inferior (group B), and TCP is less than 20%. Therefore, almost no intragranular transformation can be expected. Here, TCP can be calculated using the following equation (3).
TCP=[Ti]/([Ti]+[Al]+[Mg]+[Ca])...Equation (3)
In the above formula (3), [Ti], [Al], [Mg], and [Ca] are the Ti, Al, Mg, and Ca contents (mass%) obtained from EDS analysis of Ti-containing inclusions, respectively. , if not included, substitute 0.

Note that sulfides and nitrides such as MnS and TiN may be precipitated in a composite form in particles of Ti-containing inclusions. If these compounds are included in the composition calculation of Ti-containing inclusions, the oxide composition cannot be evaluated correctly, so when calculating the contents of Ti, Al, Mg, and Ca from the EDS mapping analysis results mentioned above, , regions where it is determined that S and N are relatively high and sulfides and nitrides such as MnS and TiN are present are excluded.

Using the TCP described above, it is possible to estimate the extent to which intragranular transformation occurs in the tissue. According to the studies conducted by the present inventors, the probability of intragranular transformation for Ti-containing inclusions in Group A is approximately 80%, the probability of intragranular transformation for Ti-containing inclusions in Group B is approximately 50%, and the probability of intragranular transformation for Ti-containing inclusions in Group B is approximately 50%. It is known that intragranular transformation hardly occurs with inclusions. Furthermore, even within the same group, intragranular metamorphosis may or may not occur. This is thought to be because the ease with which intragranular transformation occurs differs for each particle of the Ti-containing inclusions depending on the composite form of the Ti-containing inclusions, the micro-segregation of the steel material, and the like. Considering these intragranular transformation probabilities, when the particle number density (EIGFD) of Ti-containing inclusions effective for intragranular transformation is 20 pieces/mm ² or more, the effect of improving HAZ toughness can be obtained. Here, EIGFD can be calculated using the following equation (4).
EIGFD=(XA×0.8)+(XB×0.5) … Formula (4)
In Equation (4), XA and XB are measured values of the number density (particles/mm ² ) of Ti-containing inclusion particles classified into Group A and Group B, respectively.

Regarding the measurement of the particle number density XA, XB of Ti-containing inclusions of each classification, it is basically desirable to observe and map a continuous 1 mm ² area in the observation field and calculate it. However, if the number of particles of Ti-containing inclusions is large, the number will be 100 or more, so it becomes a difficult task to perform map analysis of all the particles of Ti-containing inclusions one by one. Therefore, the particle composition of at least 30 particles of Ti-containing inclusions having an equivalent circle diameter of 0.5 μm to 5.0 μm may be measured in a continuous measurement field, and the number density may be determined from the abundance ratio thereof.

[Production method]
Next, a method for manufacturing steel materials according to this embodiment will be explained.
[Smelting process]
When controlling Ti-containing inclusions in steel, it is effective to control the melting process. Specifically, it is necessary to increase the mass % of Ti oxide in the Ti-containing inclusions and lower the mass % of Al oxide, Mg oxide, Ca oxide, and Ca sulfide. Usually, molten steel is deoxidized with an Al-based deoxidizing material, but in the manufacturing method of this embodiment, it is necessary to reduce the mass % of Al oxide, so it is preferable not to use an Al-based deoxidizing material.

In addition, in the refining process using RH, which is a secondary refining facility, in order to ensure the necessary molten steel temperature, the temperature of the molten steel is raised by adding metal Al to the molten steel and by a heating reaction caused by oxygen spraying. However, in the manufacturing method of this embodiment, it is necessary to reduce the mass % of Al oxide, so the heating reaction is not performed in the refining step.

Therefore, in the manufacturing method of this embodiment, it is necessary to secure in advance a molten steel temperature that does not require heat raising treatment. The required molten steel temperature varies depending on the steel type, refining equipment and process, casting conditions, etc., but in the manufacturing method of this embodiment, the molten steel temperature is maintained at 1590 ° C. or higher, which is a general temperature that does not require heat raising treatment. is preferred.

Furthermore, in the melting process, by preventing the contamination of Al, Mg, and Ca as impurities, it is possible to suppress the formation of Al oxides, Mg oxides, Ca oxides, and Ca sulfides in Ti-containing inclusions. can. Normally, the furnace materials, flux, and slag used in operations contain Al, Mg, and Ca, so some degree of contamination is unavoidable. It is necessary to suppress the amounts of Al, Mg, and Ca present in the steel as much as possible. The concentration of the impurities Al, Mg, and Ca in the additive raw materials cannot be clearly defined because it depends on the target addition amount, but in the molten steel sample after composition adjustment at RH, Al: 0.0040% or less, Mg: 0 The upper limit may be adjusted so that Ca: 0.0010% or less and Ca: 0.0010% or less. More preferably, the upper limits may be adjusted so that Al: 0.0035% or less, Mg: 0.0005% or less, and Ca: 0.0005% or less.

After adjusting the components at RH, the Al oxide, Mg oxide, and Ca oxide present in the molten steel can also be removed by refluxing the molten steel. By ensuring a reflux time of 3 minutes or more after the above-mentioned component adjustment at RH, the proportions of Al oxide, Mg oxide, Ca oxide, and Ca sulfide in the Ti-containing inclusions can be lowered. As mentioned above, in a molten steel sample that has been subjected to composition adjustment at RH and reflux treatment, Al: 0.0040% or less, Mg: 0.0010% or less, and Ca: 0.0010% or less. More preferably, Al: 0.0035% or less, Mg: 0.0005% or less, and Ca: 0.0005% or less.

Homogenization heat treatment, heating, rolling, and heat treatment conditions after casting depend on the target mechanical properties of the steel material, such as controlled rolling/controlled cooling, quenching/tempering directly after rolling, quenching/tempering after cooling once after rolling, etc. Tempering, etc. may be selected as appropriate. A representative example of the manufacturing process is shown below.

[Homogenization processing]
In the present invention, securing toughness after SR is extremely important, and the homogenization treatment to eliminate element concentration (segregation) at the center of the plate thickness, which is applied as necessary, is necessary for the present invention manufactured by the continuous casting method. It is necessary to heat a steel piece having a predetermined composition at a temperature of 1200° C. or higher for 10 hours or more. If the homogenization treatment is not performed at 1200° C. or higher for 10 hours or more, the alloy elements such as Mn and P concentrated at the center of the plate thickness will not be sufficiently diffused, and the degree of segregation at the center of the plate thickness will not be sufficiently reduced. The higher the temperature, the more the alloying elements will diffuse, so homogenization treatment at 1250° C. or higher is desirable. However, in order to avoid a decrease in yield due to oxidation and a decrease in productivity due to unnecessarily long treatment, the temperature is preferably 1350° C. or less and 70 hours or less.

The steel slab that had been subjected to the above homogenization treatment and cooled was reheated at 950°C to 1100°C and roughly rolled to an average rolling reduction of 7.5% or more per pass to a thickness of 135 to 210 mm. Afterwards, finish rolling is performed from 670°C to 800°C to an average reduction rate of 6.0% or more per pass, followed by cooling at a cooling rate that makes the plate thickness t/2 part 10°C/s or less. do.

[Steel billet heating temperature: 950°C or higher and 1100°C or lower]
When hot rolling, if the steel billet heating temperature is less than 950° C., coarse inclusions that are generated during solidification and have an adverse effect on toughness may remain without solid solution in the matrix. In addition, as the rolling load increases, depending on the capacity of the rolling mill, rolling may become insufficient and center porosity may remain, resulting in internal defects, or the diffusion of the C element may be insufficient, resulting in defects at the center of the plate thickness. This may reduce toughness at the center of the plate thickness, such as by increasing hardness. Therefore, the steel billet heating temperature is set to 950°C or higher. Preferably it is 980°C or higher. On the other hand, if the heating temperature of the steel billet exceeds 1100° C., Ti nitrides become coarse and the effect of improving the toughness of the weld heat affected zone cannot be expected. In addition, the initial austenite grains become coarse and the hardenability of the steel increases, which increases the hardness at the center of the plate thickness and deteriorates the toughness at the center of the plate thickness. Considering suppression of coarsening of crystal grains, the temperature is more preferably 1070°C or less.

[Rough rolling: rolling reduction per pass (7.5% or more on average), thickness 135 to 210 mm]
Rough rolling immediately after heating is usually performed at about 900° C. to 1200° C., and is a step in which center porosity, which is an internal defect in the slab, is crushed, and heated γ is recrystallized and refined by rolling. If the reduction rate per pass is less than 7.5% on average, the reduction in the t/2 portion will be insufficient, and the center porosity will remain and γ recrystallization will be insufficient, making it impossible to ensure toughness. . Similarly, in order to fully obtain the effect of the rough rolling, the upper limit of the rough rolling end thickness is set to 210 mm. On the other hand, if the thickness at the end of rough rolling is less than 135 mm, the amount of reduction in finish rolling in the subsequent process cannot be ensured, so the lower limit was set to 135 mm. Note that the upper limit of the rolling reduction per pass of rough rolling is, for example, about 20% from the viewpoint of equipment capacity and the like.

[Final rolling temperature range: 670°C to 800°C, reduction rate per pass (6.0% or more on average)]
Finish rolling after rough rolling is necessary to cool the steel to the non-recrystallization temperature range and then roll it to introduce processing strain into the γ grains, which will later refine the ferrite transformation structure and ensure toughness. It is a process. This is also the process of finishing the plate to a final thickness of 60 to 100 mm. If the final finish rolling is performed at a temperature lower than 670° C., a large amount of processed ferrite is produced, and the toughness of the base material deteriorates. Furthermore, the ferrite fraction increases, and the strength may not be satisfactory. On the other hand, when rolling at a temperature higher than 800° C., the effect of refining the final structure is not sufficient and the toughness of the base material deteriorates. Furthermore, as the hardenability of the steel increases, the hardness of the center segregation area increases, which may deteriorate the toughness of the central part of the sheet thickness. Furthermore, if the reduction rate per pass of finish rolling is less than 6.0% on average, processing strain cannot be effectively introduced up to the t/2 portion, and the effect of refining the metal structure is not sufficient. The lower limit of the average rolling reduction rate per pass shall be 6.0% or more. The upper limit of the reduction rate per pass of finish rolling is, for example, about 15% from the viewpoint of equipment capacity and the like.

[Cooling rate after rolling: 10°C/s or less]
Cooling after rolling is performed at a rate of 10° C./s or less, since if the cooling rate at the plate thickness t/2 portion exceeds 10° C./s, it will be difficult to control the temperature during cooling. Furthermore, if the cooling rate is fast, the hardness at the center of the plate thickness increases, which may deteriorate the toughness at the center of the plate thickness. However, in order to easily ensure the strength of the base material, the cooling rate is preferably 1° C./s or more.

In the manufacturing method of the present invention, the cooled steel plate may be heated to a temperature range of 300° C. or higher and 670° C. or lower and subjected to heat treatment (SR treatment) in order to improve the characteristics of the steel sheet.

[Tempering temperature range: 300°C or higher and 670°C or lower]
After the above-mentioned cooling, heat treatment (SR treatment) may be performed for the purpose of improving the strength-toughness balance of the base material of the steel plate. In order to improve the strength balance by heat treatment, it is necessary to heat treat at 300°C to 670°C. If the temperature is lower than 300°C, a sufficient tempering effect cannot be obtained. In addition, heat treatment at a temperature higher than 670° C. causes carbonitrides to coarsen, resulting in a decrease in strength and toughness.

Next, an example of the present invention will be described. The conditions in the example are examples of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is based on this example of conditions. It is not limited. The present invention can adopt various conditions as long as the purpose of the present invention is achieved without departing from the gist of the present invention.

(Example 1)
[Test steel]
Steel plates of various compositions were manufactured using a converter-continuous casting-plate rolling process, and their material properties were investigated. Molten steel having a temperature of 1590°C or higher and having 53 types of chemical compositions shown in Tables 1-1 and 1-2 was continuously cast to obtain continuous cast pieces with a thickness of 240 mm or 300 mm.

[Smelting process]
The Al, Mg, and Ca contents of component analysis samples taken after RH of each slab are shown in Tables 2-1 and 2-2.

[Hot rolling]
Hot-rolled continuous cast pieces were manufactured under the homogenization heat treatment, heating, rolling, cooling, and heat treatment conditions shown in Tables 2-1 and 2-2. The plate thickness was 60 to 100 mm. Some of the steel plates were subjected to heat treatment (SR treatment) after water cooling. The laboratory-rolled steel sheet produced in this manner was used as a test steel. Table 2 shows the rolling results for each steel plate. The material was investigated before and after SR (Stress Relief) treatment. The SR conditions (heat treatment temperature) are also shown in Tables 2-1 and 2-2.

[Evaluation of steel plate strength]
In order to examine the tensile strength of the steel plate (base material), a tensile test was conducted at room temperature using a tensile test piece taken in a direction perpendicular to the rolling direction at a 1/4 plate thickness portion. The tensile test piece shape is a JIS No. 4 tensile test piece. Table 3 shows the strength (yield stress, tensile stress) of the steel plate (base material). Table 3 also shows the strength of the steel sheet material after the SR treatment.

The toughness of the base material was tested by taking three 10 mm x 10 mm full size V-notch Charpy test pieces at -40°C in the direction perpendicular to the rolling direction from the t/2 thickness section of the steel plate. It was evaluated based on the average value of absorbed energy at -40°C, and 100J or more was considered to be good base material toughness. After the SR treatment, 10 mm x 10 mm full-size V-notch Charpy specimens were taken from the t/2 thickness section of the steel plate in the direction perpendicular to the rolling direction in the same manner as before the SR treatment, and three test pieces were conducted at -40°C. did. A steel plate with an average value of absorbed energy at -40°C of 100 J or more and a reduction amount ΔvTrs of vTrs due to SR treatment of 15°C or less was defined as a steel plate with excellent base material toughness after SR treatment.

Joint toughness was determined by a CTOD test in accordance with the BS7448 standard. Welding was performed by multilayer submerged arc welding with a heat input of 3.5 kJ/mm. A three-point bending test was conducted at -10°C using the straight side of the rectangular groove as the notch introduction position for the CTOD test. A steel plate with a CTOD value of 0.50 or more at -10°C was defined as a steel plate with good joint toughness. After the SR treatment, a three-point bending test was conducted at -10°C with the straight side of the rectangular groove as the notch introduction position for the CTOD test, as before the SR treatment. Even after SR, a steel plate with a CTOD value of 0.50 or more at -10°C was defined as a steel plate with good toughness after SR.

[Evaluation of steel sheet structure, etc.]
The ferrite fraction was measured from a 500x optical microscope observation field by processing a sample for metallographic observation from a cross section in the plate thickness direction parallel to the rolling direction of the t/4 plate thickness portion of the steel plate. When observed under an optical microscope, ferrite is a blocky structure observed with white contrast, and inside the grain there are cementite observed with black dot contrast, lath structure and processed structure observed with black linear contrast, etc. It does not contain much, and basically shows a uniform contrast. However, even in the case of ferrite, dotted or linear black contrast may exist within the grains as long as there are only a few. Furthermore, the ferrite grain boundaries exhibit a smooth curve with clear black contrast, and the prior austenite grain boundaries become unclear. However, depending on the orientation difference between crystal grains, the ferrite grain boundaries may be unclear. Note that the remainder other than ferrite is a multi-phase structure consisting of one or more types of bainite and martensite. Calculation of the tissue fraction is basically done visually, and a method of marking ferrite parts on a micrograph and binarizing it can be applied.

The grain size was determined by processing a sample for metallographic observation from a cross section in the thickness direction parallel to the rolling direction, centered on the thickness t/2 portion of the steel plate, and using EBSD (Electro Back Scattering Diffraction) with a 500 μm x 500 μm field of view at 1 μm pitch. It was determined by averaging the equivalent circle diameters of the top 10 largest areas defined by boundaries with a crystal orientation difference of 15° or more.

The hardness at the center of the plate thickness was determined by processing a sample for metallographic observation from a cross section in the plate thickness direction parallel to the rolling direction, centered at the plate thickness t/2 part, and observing it with an optical microscope at 500x magnification at the t/2 part. Five fields of view were randomly photographed, and the hardness of each two locations was measured using a Vickers test with a load of 25 g. The maximum hardness at 10 measurement points was defined as the maximum hardness HVmax at the center of the plate thickness.

The degree of segregation was determined by EPMA (Electron Probe Micro Analysis) measurement according to the above-mentioned procedure, and the degree of segregation at the center of the plate thickness was [Si]max/[Si]≦1.9, [Mn]max/[Mn]≦2. .0, [P]max/[P]≦4.0, [Cu]max/[Cu]≦2.1, and [Ni]max/[Ni]≦1.8, respectively. If satisfied, mark ○ in the table; if not, mark × in Tables 3-1 and 3-2.

[Composition, size, and number analysis of Ti-containing inclusions]
A heat cycle test piece (12 mm x 12 mm x 120 mm) was taken from the position of 1/4 of the plate thickness of the sample steel, and a reproduced heat cycle test simulating welding (after holding at 1400 °C for 3 seconds, 200 °C at 20 °C/sec) was taken. Cool to ℃. After holding at 200℃ for 3 seconds, heat up to 720℃ at 20℃/second. After holding at 720℃ for 3 seconds, cool to 200℃ at 12℃/second. After holding at 200℃ for 3 seconds, heat up to 720℃. The temperature was raised to 500° C. at a rate of 500° C./sec. After being held at 500° C. for 3 seconds, the sample was cooled to 20° C. at a rate of 12° C./sec) to prepare a sample for inclusion investigation. In the soaking zone of the simulated thermal cycle test piece, elemental analysis was performed using SEM-EDS on Ti-containing inclusions of 0.5 μm or more, which can be analyzed as particles, within an observation field of 1.0 mm x 1.0 mm. Table 4 shows the EIGFD calculated by dividing the Ti-containing inclusions in the observation field according to the above-mentioned composition classification.

Tables 1-1 and 1-2 show the steel components of the developed steel and comparison steel, Tables 2-1 and 2-2 show the manufacturing conditions of the steel plates, and Tables 3-1 and 3-2 show the mechanical properties of the steel plates. Table 4 shows the analysis results of Ti-containing inclusions in the steel sheet. All of the steel plates of the present invention have a base material strength of yield strength of 500 MPa or more and tensile strength of 570 MPa or more, vE-40 of 100 J or more, change in vTRs before and after SR ΔvTrs is within 15°C, and -10°C CTOD test value is 0.50 or more.

On the other hand, slab No. For the 33 steel plates of comparative examples using slabs 21 to 53, one or both of the chemical composition and the manufacturing method were outside the scope of the present invention, and the base material strength, base material toughness, before or after SR, One or more characteristics of the joint CTOD have not been achieved.

Slab No. Slabs 21 to 38 have a component range outside the scope of the present invention.

Slab No. Slab No. 21 has a high C content, so the hardness of the quenched structure is high, so the maximum hardness HVmax at the center of the plate thickness is high, and the toughness of the base metal and joint is inferior.
Slab No. Slab No. 22 has a high Si content, so the amount of MA generated increases and grain boundary carbides become finer, resulting in inferior toughness of the base material and joint.
Slab No. Slab No. 23 has a high Mn content, so the Mn concentration in the center segregation area is high, the maximum hardness HVmax at the center of the plate thickness is high, and the toughness of the joint is inferior.
Slab No. Slab No. 24 has a low Mn content, so the hardenability decreases, and an upper bainite structure with poor toughness is formed, resulting in poor joint toughness.

Slab No. Slab No. 25 has a low Ti content, so the proportion of Ti ₂ O ₃ that becomes the generation nucleus of intragranular transformed ferrite is low, and the required EIGFD is not satisfied.
Slab No. Slab No. 26 has a high Ti content, so there is a possibility that coarse inclusions that cause brittle cracks are formed, and the HAZ toughness is inferior.

Slab No. Slab No. 27 has a high content of Cu and Ni, so the concentration of Ni and Cu in the center segregation area is high, the maximum hardness HVmax at the center of the plate thickness is high, and the toughness of the joint after SR is low.
Slab No. Slab No. 28 has a high Nb content, so the Nb concentration in the center segregation area is high, the maximum hardness HVmax at the center of the plate thickness is high, and the joint toughness is inferior.
Slab No. In slab No. 29, since the O content is high, the oxide inclusions in the steel become coarse and the toughness of the joint is inferior.

Slab No. Slab No. 30 has a high P content, which increases the segregation of P at prior γ grain boundaries and ferrite grain boundaries, making grain boundary embrittlement more likely to occur, resulting in inferior joint toughness.
Slab No. In slab No. 31, since the S content is high, the sulfide inclusions in the steel become coarse and the toughness of the joint is inferior.
Slab No. Slabs Nos. 32 to 34 have high contents of Al, Mg, and Ca, and therefore have a low proportion of Ti ₂ O ₃ , which forms the nucleus for intragranular transformed ferrite, and do not satisfy the required EIGFD.
Slab No. Slab No. 35 has a high B content, so the hardenability is excessive and many hard structures are formed, the maximum hardness HVmax at the center of the plate thickness is high, and the toughness of the joint is inferior.

Slab No. Slab No. 36 has a high Mo content, so its hardenability is excessive and a large number of hard structures are formed, the maximum hardness HVmax at the center of the plate thickness is high, and the toughness of the joint after SR is low.
Slab No. Slab No. 37 has a high Cr content, so its hardenability is excessive, a large amount of hard structure is formed, and the toughness of the joint is inferior.
Slab No. Slab No. 38 has a high V content, so its hardenability is excessive and a large amount of hard structure is formed, resulting in inferior joint toughness.

Slab No. In slab No. 39, Al heating was performed in the melting process, and the proportion of Ti ₂ O ₃ that becomes the generation nucleus of intragranular transformed ferrite was low, and the required EIGFD was not satisfied.

Slab No. For slabs 40 to 44, the homogenization heat treatment was insufficient or not performed, and the hardness of the center segregation area was high, making it impossible to secure the base metal and joint toughness after SR.

Slab No. In slab No. 45, since the heating temperature was low, undissolved elements such as Nb and B remained, resulting in insufficient strength.
Slab No. Slab No. 46 has a high heating temperature and is outside the scope of the present invention.

Slab No. Slab No. 47 had a low average reduction rate per pass in finish rolling, and the effect of forming a fine grain structure due to the introduction of transformation nuclei in rolling in the non-recrystallized region was not sufficiently obtained, resulting in a crystal orientation difference of 15 at the center of the thickness. The average equivalent circular diameter of the top 10 crystal grains from the maximum area of the region surrounded by large-angle grain boundaries of ° is too large, and the toughness of the base material after SR is low.

Slab No. In slab No. 48, the transfer thickness was too large, and the effect of forming a fine grain structure due to the refinement of pre-transformed austenite grains in rolling in the recrystallization zone could not be sufficiently obtained, resulting in a large crystal orientation difference of 15° at the center of the thickness. The average equivalent circle diameter of the top ten crystal grains from the maximum area of the region surrounded by grain boundaries is too large, and the toughness of the base material after SR is low.
Slab No. In slab No. 49, the transfer thickness was too small, and the effect of forming a fine grain structure due to the introduction of transformation nuclei during rolling in the non-recrystallized region was not sufficiently obtained, resulting in large-angle grain boundaries with a crystal orientation difference of 15° at the center of the thickness. The average equivalent circle diameter of the top 10 crystal grains from the maximum area of the enclosed region is too large, and the toughness of the base material after SR is low.

Slab No. In the slab No. 50, the average reduction rate per pass of finish rolling was low, and the effect of forming a fine grain structure due to the introduction of transformation nuclei in rolling in the non-recrystallized region was not sufficiently obtained, and the maximum hardness HVmax at the center of the plate thickness was The toughness of the base material after SR is low.
Slab No. Slab No. 51 has a high final finish rolling temperature and is outside the scope of the present invention.

Slab No. In slab No. 52, the cooling rate was too high and too many hard structures were generated, so the maximum hardness HVmax at the center of the plate thickness was high and the toughness of the base material after SR was low.
Slab No. In slab No. 53, the heat treatment temperature after cooling was high, the fine carbides aggregated and coarsened excessively due to tempering, and the yield stress and base material toughness after SR were low.

FIG. 9 shows a comparison of the presence or absence of intragranular transformation for the inventive example and the comparative example. In the present invention example, it is determined that intragranular transformation has clearly occurred from Ti-containing inclusions (○). On the other hand, in the comparative example, it is judged that no obvious intragranular transformation has occurred from the Ti-containing inclusions (Δ), or the shape of the metal structure is unclear. FIG. 10 shows the relationship between the composition of Ti-containing inclusions (each content % of Ti, Al, Ca, Mg, Mn, S, and Si) and the presence or absence of IGF generation. FIG. 11 shows the relationship between TCP=[Ti]/([Ti]+[Al]+[Mg]+[Ca]) and the presence or absence of IGF generation. Figure 12 shows the relationship between EIGFD = (XA x 0.8) + (XB x 0.5) and joint HAZ toughness (-10°C CTOD) after SR treatment.

As described above, the present invention has excellent CTOD characteristics in the multi-layer welded HAZ part of low to medium heat input welding, and the yield strength of the base material is 500 MPa or more, and the yield strength of the base metal is 500 MPa or more, and It becomes possible to manufacture a high-strength, thick steel plate with a thickness of 60 mm or more that has good base material toughness. This makes it possible to increase the size and weight of steel structures used in extremely harsh environments, such as offshore structures, and to reduce costs by reducing the amount of steel used. Therefore, the present invention has high industrial applicability.

10 Steel plate 11 Sample for metallographic observation 12 ±0.5 mm range 13 Area with the highest average Mn concentration 14 Viewing area of 1 mm x 1 mm 15 Square area of 20 x 20 μm

Claims (4)

The plate thickness is 60 to 100 mm, the yield strength is 500 MPa or more, and the tensile strength is 570 MPa or more,
In mass%,
C: 0.020-0.120%,
Si: 0.05-0.30%,
Mn: 1.70-3.00%,
Ti: 0.005-0.018%,
Cu: 0.05-1.50%,
Ni: 0.05-2.00%,
Nb: 0.005-0.025%,
N: 0.0015-0.0060%,
O: 0.0010 to 0.0045%,
Contains
P: 0.015% or less,
S: 0.0050% or less,
Al: 0 to 0.004%,
Mg: 0 to 0.0010%,
Ca: 0-0.0010%,
B: 0 to 0.0015%,
and
Ceq. calculated by the following formula (1). The value is 0.460≦Ceq. and further satisfies formula (2), and has a chemical composition with the remainder consisting of Fe and impurities,
The ferrite fraction in the plate thickness t/4 portion is 0 to 15 area%, and the remainder is a multi-phase structure consisting of one or more types of bainite and martensite,
The average equivalent circle diameter of the top 10 crystal grains from the maximum area of the region surrounded by large-angle grain boundaries with a crystal orientation difference of 15° at the center of the plate thickness is 50 μm or less,
The maximum hardness HVmax at the center of the plate thickness is 250HV or less,
The degree of segregation at the center of the plate thickness is [Si]max/[Si]≦1.9 and [Mn]max/[Mn]≦2.0 and [P]max/[P]≦4.0 and [Cu] max/[Cu]≦2.1 and [Ni]max/[Ni]≦1.8,
Furthermore, Ti-containing inclusions, which are contained in the steel structure and contain Ti and one or more of Al, Mg, Si, Ca, and Mn, and have an equivalent circle diameter of 0.5 μm or more and 5.0 μm or less For particles, the Ti content percentage (TCP) of each particle of Ti-containing inclusions was calculated using formula (3) based on the mass ratio of elements measured by EDS, and group A with TCP of 40% or more, TCP of 40% When classified into Group B with less than 20% and 20% or more, the particle number density (EIGFD) of Ti-containing inclusions effective for intragranular transformation shown in formula (4) is 20 particles/mm ² or more. High strength thick steel plate.
Ceq=[C]+[Mn]/6+[Cu]/15+[Ni]/15... Formula (1)
[Mn]≧-3.8[C]+2.1... Formula (2)
TCP=[Ti]/([Ti]+[Al]+[Mg]+[Ca])...Equation (3)
EIGFD=(XA×0.8)+(XB×0.5) … Formula (4)
In the formulas (1) and (2), [C] is mass% of C, [Mn] is mass% of Mn, [Cu] is mass% of Cu, and [Ni] is mass% of Ni. It is.
In the formula (3), [Ti], [Al], [Mg], and [Ca] are the Ti, Al, Mg, and Ca contents (mass%) obtained from EDS analysis of Ti-containing inclusions, If not included, substitute 0.
In the formula (4), XA and XB are measured values of the inclusion number density (inclusions/mm ² ) classified into group A and group B, respectively. Furthermore, in mass%,
Mo: 0.50% or less,
Cr: 0.50% or less,
V: 0.03% or less,
Contains one or more of the following,
The high-strength thick steel plate according to claim 1, wherein the Ceq value is calculated by the following formula (1)' instead of the formula (1).
Ceq. =[C]+[Mn]/6+[Cu]/15+[Ni]/15+[Cr]/5+[Mo]/5+[V]/5... Formula (1)'
In the formula (1)', [C] is mass% of C, [Mn] is mass% of Mn, [Cu] is mass% of Cu, [Ni] is mass% of Ni, [Cr ] is the mass % of Cr, [Mo] is the mass % of Mo, and [V] is the mass % of V. A steel billet having the composition according to any one of claims 1 or 2 and produced by a continuous casting method is homogenized at 1200°C or higher for 10 hours or more, cooled, and then reheated at 950°C to 1100°C. Then, rough rolling is performed so that the average rolling reduction per pass is 7.5% or more, and the thickness is 135 to 210 mm, and then the rolling reduction is 6.0% on average per pass from 670 ° C to 800 ° C. A method for producing a high-strength thick steel plate, which comprises performing finish rolling as described above, and then cooling the center of the plate at a cooling rate of 10° C./s or less. 4. The method for producing a high-strength thick steel plate according to claim 3, wherein the steel plate is cooled at a cooling rate such that the central portion of the plate thickness is 10° C./s or less, and then heat-treated at a temperature of 300° C. or more and 670° C. or less.

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