JP7468800B2 - Steel plate and its manufacturing method - Google Patents

Description

The present invention relates to a steel material suitable for use in steel structures such as ships, marine structures, pressure vessels, line pipes, and offshore wind power generators. Specifically, the present invention relates to a thick, high-tensile steel plate having a plate thickness of more than 100 mm and excellent not only in the strength and toughness of the base material but also in the joint CTOD characteristics in multi-layer welds, and a method for manufacturing the same.

In recent years, steel structures such as ships, marine structures, pressure vessels, line pipes, and offshore wind turbines have become larger. As these structures become larger, there is an increasing demand for the steel materials used as base materials to be stronger and thicker.

In particular, when manufacturing thick steel plates with a thickness of over 100 mm, the cooling rate is slower in the center of the plate thickness due to the large thickness, and the crystal grains tend to become coarse. Therefore, when trying to manufacture thick steel plates with excellent strength and toughness in the center of the plate thickness, it is important to refine the crystal grains in the center of the plate thickness.

For example, Patent Document 1 proposes a technology for controlling the rolling conditions to refine the average effective crystal grain size of the microstructure at the center of the plate thickness and improve the toughness of the base material.

Conventionally, the Charpy test has been mainly used as a method for evaluating the toughness of steel. On the other hand, in recent years, an evaluation method called a Crack Tip Opening Displacement Test (hereinafter referred to as "CTOD test") has been often applied as a method for evaluating the fracture resistance with higher accuracy for thick steel plates used in steel structures.
This CTOD test evaluates the resistance to brittle fracture by performing three-point bending on a test specimen in which a fatigue pre-crack has been introduced in the toughness evaluation section and measuring the amount of crack opening (amount of plastic deformation) just before fracture.

Furthermore, when thick steel plates are applied to steel structures such as ships, marine structures, pressure vessels, line pipes, and offshore wind turbines, multi-layer welding is used.
The heat affected zone (hereinafter, also referred to as "HAZ") of this multi-layer welding is subjected to multiple different heat cycles from each welding pass, and is formed with a mixture of various structures. Among them, the HAZ structure (hereinafter, referred to as ICCGHAZ: Inter-Critically Reheated Coarse Grain Heat Affected Zone) in which island martensite (MA: Martensite-Austenite Constituent) is mixed in the coarse base structure formed by reheating the HAZ area (CGHAZ: Coarse Grain Heat Affected Zone) near the weld line, which has become a coarse structure by the preceding welding pass, to a two-phase region of ferrite + austenite by the subsequent welding pass, has particularly low toughness. In addition, when the crystal grain size of the base material structure is coarse, the toughness of the SCHAZ (Sub-Critically Reheated HAZ) may become a problem.

The BS standard (British Standards) EN10225-4 (2019) and API (American Petroleum Institute) standard RP (Recommended Practice)-2Z (2005), which specify joint CTOD test methods, require joint CTOD characteristics of the CGHAZ near the weld line and the SC/ICHAZ (Inter-Critically Reheated HAZ) boundary, which is the boundary between the untransformed and transformed regions of the base material at the time of welding.

In a CTOD test of a welded joint, the test is basically performed over the entire thickness, so when the CGHAZ is the subject of evaluation, the region where a fatigue pre-crack is introduced includes the ICCGHAZ structure. In other words, the joint CTOD properties obtained by a joint CTOD test are governed by the toughness of the most embrittled structure in the evaluation region, so the joint CTOD properties of the CGHAZ reflect not only the CGHAZ structure but also the toughness of the ICCGHAZ structure.
For this reason, in order to improve the joint CTOD characteristics in the CGHAZ, it is also necessary to improve the toughness of the ICCGHAZ structure.
In addition, the above-mentioned HAZ structure, in a structure generated in one welding pass during multi-layer (multi-pass) welding, has a positional relationship of CGHAZ, ICHAZ, and SCHAZ in the order of proximity to the weld line. In multi-layer welding, the ICCGHAZ is a structure generated by heating the CGHAZ to a two-phase region of ferrite and austenite due to the thermal history of the subsequent passes, and the position and frequency of generation of the ICCGHAZ structure can vary depending on the way the welding passes are stacked.

Conventionally, techniques for improving the toughness of weld heat-affected zones (HAZ) have involved the fine dispersion of TiN to suppress the coarsening of austenite grains in the CGHAZ, and the use of TiN as a ferrite transformation nucleus. However, the bond zone can be heated to a temperature range where TiN melts, and when the low-temperature toughness requirements of the weld are strict, it becomes difficult to meet the requirements with the above-mentioned effects alone.

Technologies that have also been used include the inhibition of austenite grain growth by dispersing REM-based oxysulfides produced by adding rare earth metals (REM), and the utilization of the ferrite nucleation ability of BN.

For example, Patent Document 2 discloses a technique in which REM is added in combination with Ti to disperse fine particles in steel, thereby suppressing the grain growth of austenite and improving the toughness of welds.
Furthermore, Patent Document 3 proposes a technique for improving HAZ toughness by utilizing BN (boron nitride) as ferrite transformation nuclei in a high heat input weld heat affected zone and refining the HAZ structure.

Patent Document 4 proposes a technology to counter the decline in toughness of ICCGHAZ by reducing the C and Si content to suppress the generation of MA, and further increasing the strength of the base material by adding Cu.

As described above, in order to achieve both thickening and high strength of steel plate, it is necessary to increase the amount of alloying elements added. However, adding a large amount of alloying elements leads to deterioration of the toughness of the multi-pass weld HAZ, and there is a problem that it becomes difficult to ensure the CTOD characteristics of the joint at low temperatures.
To address this problem, Patent Document 5 discloses a technique for improving low-temperature toughness by controlling the hardness of the center segregation portion.

Patent No. 6477993 Japanese Patent Application Laid-Open No. 60-184663 Japanese Patent Application Laid-Open No. 61-253344 Japanese Patent Application Laid-Open No. 05-186823 Patent No. 5846311

Here, the CTOD specification temperature in the standards that prescribe the CTOD characteristics of joints (for example, API standard RP-2Z) is usually -10°C.
However, in order to meet the increasing demand for energy in recent years and to secure new resources, the construction regions of marine structures and the like are shifting to cold regions and deep sea regions where resource mining has not been possible until now. This has led to an increasing demand for high-strength, thick-walled steel plates that can withstand CTOD specification temperatures that are even lower than the CTOD specification temperatures set by the API standard.

According to the inventors' investigations, the conventional techniques described in the above Patent Documents 1 to 5 were unable to fully satisfy the joint CTOD characteristics required for multi-pass welded joints for low-temperature specifications in thick steel plates having a plate thickness of more than 100 mm.

For example, Patent Document 1 proposes control of rolling conditions to refine the average effective grain size of the microstructure in the center of the plate thickness, but this cannot be applied to thick steel plates with a plate thickness of over 100 mm. In addition, to improve toughness in the center of the plate thickness, it is not enough to refine the average effective grain size alone; it is also necessary to refine the maximum effective grain size.

The technology proposed in Patent Document 2 to suppress the coarsening of the austenite structure in the HAZ by dispersing fine particles in the steel through the combined addition of REM and Ti is intended for steel materials with relatively low strength and low amounts of alloying elements. Therefore, in the case of steel materials with higher strength and higher amounts of alloying elements, the HAZ structure does not contain ferrite, and so it cannot be applied.

The technology proposed in Patent Document 3 is effective when the cooling rate in the heat-affected zone is slow, as in the case of high heat input welding, and the HAZ has a structure mainly composed of ferrite. However, in multi-layer welding, the heat input is relatively small, and in the case of thick steel plates with a plate thickness of over 100 mm, the amount of alloy components contained in the base material is relatively large. Therefore, in multi-layer welding of thick steel plates, the HAZ structure is mainly composed of bainite, and the above-mentioned effect of improving HAZ toughness cannot be obtained.

According to the technology proposed in Patent Document 4, it is possible to satisfy the CTOD characteristics at the normal specification temperature (-10°C). However, the CTOD characteristics of the joint at the lower specification temperature as described above have not been considered, and it is considered that the low-temperature CTOD specification cannot be satisfied only by improving the ICCGHAZ toughness by reducing the base alloy components such as C and Si.
In addition, reducing the alloy element content of the base material in order to improve the toughness of the ICCGHAZ is a technology that contradicts the need to ensure strength for thickening, and is therefore considered difficult to apply to thick steel plates used in marine structures, etc.

Patent Document 5 proposes a technology for satisfying the joint CTOD characteristics at normal specification temperatures (-10°C) for thick steel plates with a plate thickness of 100 mm or less. However, for extra-thick steel plates with a plate thickness of more than 100 mm, it has not yet achieved mechanical properties equivalent to those of steel plates with a plate thickness of 100 mm or less, and no consideration has been given to the joint CTOD characteristics at even lower specification temperatures as mentioned above. In addition, the CTOD characteristics at the SC/ICHAZ boundary have not been considered.

Thus, in thick steel plates over 100 mm thick that combine high strength and low-temperature toughness, it cannot be said that technology has been established to simultaneously improve the toughness of the CGHAZ, ICCGHAZ, and SCHAZ in the heat-affected zone of multi-layer welds, and it has been difficult to improve the CTOD characteristics of joints.

The present invention has been made in consideration of the above problems associated with the conventional technology, and an object of the present invention is to provide a thick steel plate having a thickness of more than 100 mm, which has high strength and excellent base material toughness at low temperatures, and excellent multi-pass welded joint CTOD characteristics, and a manufacturing method thereof.
In the present invention, "high strength" refers to a yield strength of 325 MPa or more at the center of the plate thickness in a tensile test. "excellent base material toughness at low temperatures" refers to an absorbed energy of 100 J or more at the center of the plate thickness in a Charpy test at -40°C. "excellent multi-pass welded joint CTOD characteristics" refers to a crack opening displacement of 0.4 mm or more at the notch position CGHAZ and at the SC/ICHAZ boundary when the test temperature is -20°C.

In order to solve the above problems, the inventors have conducted extensive research into a method for improving the CTOD characteristics of thick steel plates with a thickness of over 100 mm while simultaneously increasing the strength of the base material and improving low-temperature toughness. As a result, they have obtained the following findings.

Since strength and toughness are strongly correlated with grain size, in order to achieve both high strength and low-temperature toughness at the center of the plate thickness in thick steel plates over 100 mm, it is essential to refine the grains in these areas. When refining the grains, it is also important to refine the average effective grain size, but if even a portion of the fine grains contain coarse grains, the coarse grains will become the weakest part and the starting point of fracture. In other words, material properties are governed not only by the average grain size but also by the maximum grain size. Therefore, in addition to refining the grains, it is also essential to regulate the grain size.

With regard to the above, the inventors have discovered that the desired strength and toughness can be ensured by setting the average effective grain size of the base material structure at the center of the plate thickness to 20 μm or less, while setting the maximum effective grain size to 150 μm or less, as described below.

However, in the conventional technology, it was difficult to introduce strain into the center of the plate thickness during rolling, particularly in thick steel plates with a thickness of more than 100 mm, and it was difficult to refine the average effective crystal grain size of the base material structure in the center of the plate thickness to 20 μm or less.
Furthermore, even if strain is appropriately introduced into the center of the plate thickness and the average effective grain size is refined, if coarse grains are mixed in, the coarse grains can become the starting point of fracture.
It has been discovered that this problem can be solved by eliminating coarse grains having a maximum effective grain size of more than 150 μm.
However, with conventional techniques, it has been difficult to eliminate coarse crystal grains having a maximum effective crystal grain size of more than 150 μm, particularly in thick steel plates having a plate thickness of more than 100 mm.

As a result of the above investigation, it was found that these problems can be solved by the following method.
(1) In a temperature range where the plate thickness center temperature is equal to or higher than T ₁ ° C., the average reduction rate/pass (average value of reduction rate in each pass) is 3% or more, and the cumulative reduction rate (cumulative reduction amount/rolling start plate thickness) is 25% or more, so that sufficient strain can be introduced into the plate thickness center, and the crystal grains can be refined and homogenized by recrystallization. Next, while avoiding rolling in the partial recrystallization temperature range (T ₁ to T ₂ ° C.) where recrystallized grains and coarse recovery grains are generated, rolling is performed at a cumulative reduction rate of 30% or more in the non-recrystallization temperature range of T ₂ ° C. or less. By these rolling processes, even in thick steel plates with a plate thickness of more than 100 mm, the maximum effective crystal grain size in the plate thickness center can be made 150 μm or less, while the average effective crystal grain size can be made 20 μm or less.

In this way, for thick steel plates with a thickness of over 100 mm, the recrystallization temperature range above _T1 °C and the non-recrystallization temperature range below _T2 °C are clearly defined, and rolling at the partial recrystallization temperature range ( _T1 to _T2 °C) where recrystallized grains and coarse recovered grains are generated is avoided, and rolling is performed under appropriate conditions as described above at temperatures above _T1 °C and below _T2 °C, respectively, thereby making it possible to obtain the desired fine and regular grained structure in the center of the plate thickness.
In the present invention, the plate thickness center portion refers to a region having a thickness of 10% of the plate thickness from the center (1/2 position) of the plate thickness toward both surface directions of the steel plate.

(2) When the contents of Ca, O and S in steel are controlled to be within a range of 0 to 1.5 in terms of the atomic concentration ratio (ACR) represented by the following formula, the inclusions become composite inclusions containing Ca-based sulfides partially dissolved with Mn and Al-based oxides.
ACR = {[Ca] - (0.18 + 130 [Ca]) x [O]} / 1.25 / [S]
When manufacturing high-strength, thick steel plates, it is essential to add a large amount of alloying elements, so in the past, it was difficult to ensure joint CTOD properties at low temperatures in multi-pass weld HAZs.
As a result of the above investigation, it was found that by making the inclusions into two types of inclusions, i.e., composite inclusions containing sulfides containing Ca and Mn and oxides containing Al, such composite inclusions can be stably present even in the region near the weld line where the temperature is raised to high temperatures, and can fully suppress the coarsening of austenite grains. Furthermore, it was found that a Mn-poor layer is formed around the composite inclusions, which has a nucleation effect (transformation nucleation effect) of bainite, etc.
That is, when the above-mentioned complex inclusions having a nucleation effect are present in the austenite grains, nucleation occurs not only at the austenite grain boundaries but also within the austenite grains, so that the finally obtained HAZ structure becomes fine, and as a result, the HAZ toughness and the CTOD characteristics of the joint are improved.

We also found that in order to fully utilize the transformation nucleation effect of the above-mentioned composite inclusions, the size of such composite inclusions needs to be a circular equivalent diameter of 0.1 μm or more.

Furthermore, in order to fully utilize the refinement of the HAZ structure due to the transformation nucleation effect, it is necessary that at least one composite inclusion is present in the austenite grain of the HAZ when the welding temperature is increased. In particular, since the austenite grain size in the vicinity of the weld line reaches about 200 μm or more, it has been found that in order for the finally obtained HAZ structure to be sufficiently fine, the number density of the composite inclusions must be 25 pieces/ ^mm2 or more. On the other hand, since the toughness of the above-mentioned composite inclusions themselves is low, the presence of an excessive amount of composite inclusions rather reduces the HAZ toughness. In particular, it is necessary to appropriately control the number of composite inclusions in the plate thickness center where element segregation exists and the multi-layer welding HAZ toughness is poor. Therefore, it has been found that by setting the number density of the composite inclusions to 250 pieces/ ^mm2 or less, good multi-layer weld joint CTOD characteristics can be obtained.

(3) Usually, in the element segregation region at the center of the slab thickness, there is a problem that the alloy elements are concentrated, causing large inclusions to be dispersed at a low density.
As a result of the above-mentioned investigation, it was found that by rolling at an average reduction rate/pass of 3% or more and a cumulative reduction rate of 25% or more in _a high temperature range where the plate thickness center temperature is T1°C or more, the strain applied to the plate thickness center can be increased, coarse inclusions can be elongated and broken up, and fine inclusions can be dispersed at a high density. It was also found that such inclusions can ensure the effect of improving HAZ toughness.

(4) It is known that the toughness of the base material is dominant in the CTOD characteristics of joints at the SC/ICHAZ boundary. As a result of the above investigation, it was found that in order to satisfy the CTOD characteristics of joints at the SC/ICHAZ boundary at a test temperature of -20°C, it is necessary to improve the toughness of the base material by refining and regulating the crystal grains so that the maximum effective crystal grain size of the base material microstructure is 150 μm or less and the average effective crystal grain size is 20 μm or less.
Usually, in a thick steel plate having a thickness of more than 100 mm, the cooling rate at the center of the plate thickness is small, and the crystal grains become coarse. Therefore, the rolling conditions at the plate thickness center temperature of T ₁ ° C or higher, which is the recrystallization temperature range, are set to an average reduction rate/pass of 3% or more and a cumulative reduction rate of 25% or more, and rolling at the partial recrystallization temperature range (T ₁ to T ₂ ° C) where recrystallized grains and coarse recovery grains are generated is avoided, and the rolling conditions at the non-recrystallization temperature range T ₂ ° C or lower are set to a cumulative reduction rate of 30% or more. It has been found that these rolling conditions can sufficiently refine and regulate the structure at the center of the plate thickness, and can refine and regulate the crystal grains to the desired grain size.

The present invention has been completed based on the above findings and further investigations. That is, the gist of the present invention is as follows.
[1] In mass%, it contains C: 0.03 to 0.13%, Si: 0.60% or less, Mn: 0.9 to 2.7%, P: 0.050% or less, S: 0.0050% or less, Al: 0.002 to 0.100%, Ti: 0.002 to 0.055%, Nb: 0.005 to 0.070%, Ca: 0.0005 to 0.0200%, N: 0.0120% or less, and O: 0.0070% or less, with the balance being Fe and unavoidable impurities, and has a component composition that satisfies the following formulas (1) to (4):
1.50≦[Ti]/[N]≦5.00 ... (1)
0≦{[Ca]−(0.18+130[Ca])×[O]}/1.25/[S]≦1.50 … (2)
0.280%≦Ceq(=[C]+[Mn]/6+([Cu]+[Ni])/15+([Cr]+[Mo]+[V])/5)≦0.500% ... (3)
Pcm (= [C] + [Si] / 30 + ( [Mn] + [Cu] + [Cr]) / 20 + [Ni] / 60 + [Mo] / 15 + [V] / 10 + 5 [B]) ≦ 0.240% ... (4)
(The brackets in the formulas (1) to (4) above represent the content (mass%) of the element in the brackets, and are set to zero when the element is not contained.)
A steel plate having a thickness of more than 100 mm, in which, at the center of the plate thickness, an average effective crystal grain size is 20 μm or less and a maximum effective crystal grain size is 150 μm or less, and at a 1/2 position of the plate thickness, there are present 25 to 250 composite inclusions per ^mm2 , each of which contains a sulfide containing Ca and Mn and an oxide containing Al and has a circle equivalent diameter of 0.1 μm or more.

[2] The steel plate described in [1], wherein the chemical composition further includes, by mass%, one or more selected from the group consisting of Ni: 2.5% or less, Cu: 2.0% or less, Cr: 1.5% or less, Mo: 1.5% or less, V: 0.25% or less, W: 0.45% or less, B: 0.0045% or less, REM: 0.025% or less and Mg: 0.005% or less.

（ここで、ｓｏｌ．［Ｎｂ］は（７）式による）

（上記式（５）、（７）における［Ｃ］、［Ｎ］はそれぞれＣ、Ｎの質量％を、Ｔ_０は素材の加熱温度（℃）、ｓｏｌ．［Ｎｂ］はＮｂの固溶量（質量％）を表す。ただし、［Ｎｂ］を鋼板に含まれる総Ｎｂ含有量（質量％）とすると、ｓｏｌ．［Ｎｂ］≦［Ｎｂ］である。） [3] A method for producing a steel plate, comprising heating a material having the chemical composition described in [1] or [2] above to 990°C or lower and rolling it under conditions of an average reduction rate/pass of 3% or higher and a cumulative reduction rate of 25% or higher when the temperature at the center of thickness is equal to or higher than _T1 °C as defined by the following formula (5), and rolling it under conditions of a cumulative reduction rate of 30% or higher when the temperature at the center of thickness is equal to or lower than _T2 °C as defined by the following formula (6), and then cooling it to a cooling stop temperature of 600°C or lower at an average cooling rate of 1.0 to 50.0°C/s at the center of thickness.

(where sol.[Nb] is based on formula (7))

(In the above formulas (5) and (7), [C] and [N] respectively represent mass% of C and N, _T0 represents the heating temperature (°C) of the material, and sol. [Nb] represents the amount of Nb dissolved in solid solution (mass%). However, if [Nb] is the total Nb content (mass%) contained in the steel plate, sol. [Nb] ≦ [Nb].)

[4] A method for manufacturing the steel plate described in [3], in which after cooling to the cooling stop temperature, tempering is performed at a temperature of 700°C or less.

According to the present invention, it is possible to provide a thick steel plate having high strength, excellent base material toughness at low temperatures, and excellent CTOD characteristics of multi-pass welded joints, and a manufacturing method thereof.

The reasons for limiting each of the constituent elements of the present invention will now be described.
[Component composition]
First, the reasons for limiting the composition of the steel plate and the base material to the above ranges in the present invention will be explained. Note that "%" regarding the composition means "mass %" unless otherwise specified.
C: 0.03 to 0.13%
C is an element that improves hardenability and improves the strength of steel, and must be contained at 0.03% or more. On the other hand, if C is contained in excess of 0.13%, the hardness of the C-concentrated portion increases, and the CTOD characteristics of the joint deteriorate. Therefore, the C content is set to a range of 0.03 to 0.13%. Preferably, it is 0.04% or more and 0.12% or less. More preferably, it is 0.06% or more and 0.10% or less.

Si: 0.60% or less Si is also used as a deoxidizer and is an element that is inevitably contained as an impurity, but if the Si content exceeds 0.60%, the joint CTOD characteristics will deteriorate. Therefore, the upper limit of the Si content is limited to 0.60%, and preferably 0.50% or less. On the other hand, although there is no particular limit, an excessively low Si content will increase refining time and costs, so that the lower limit is preferably about 0.02%, and more preferably 0.04% or more.

Mn: 0.9 to 2.7%
Mn is an element that has the effect of improving the strength of the base material and the welded portion by improving the hardenability of the steel. To obtain such an effect, the addition of 0.9% or more is necessary. On the other hand, the addition of more than 2.7% not only reduces the weldability, but also causes excessive hardenability, which reduces the toughness of the base material and the welded portion, thereby deteriorating the joint CTOD characteristics. For this reason, the Mn content is set to the range of 0.9 to 2.7%. It is preferably 1.1% or more and 2.5% or less. It is more preferably 1.2% or more and 2.3% or less.

P: 0.050% or less P is an element that has a large effect of embrittling grain boundaries, and adding a large amount of it reduces HAZ toughness and joint CTOD characteristics. Therefore, the P content is limited to 0.050% or less, and preferably 0.030% or less. On the other hand, since it is desirable to reduce the P content as much as possible, the lower limit of the P content is not particularly limited, but excessive reduction in P content leads to an increase in refining time and an increase in costs. Therefore, the P content is preferably 0.001% or more, and more preferably 0.005% or more.

S: 0.0050% or less S is an element that reduces the CTOD characteristics of joints, so the upper limit of the S content is limited to 0.0050%. It is preferably 0.0030% or less. On the other hand, since it is desirable to reduce the S content as much as possible, the lower limit of the S content is not limited, but excessive reduction in S content leads to an increase in refining time and an increase in costs. Therefore, the S content is preferably 0.0001% or more, and more preferably 0.0005% or more.

Al: 0.002 to 0.100%
Al is an element necessary for forming complex inclusions to improve the toughness of multi-pass weld HAZ and improve joint CTOD properties, and must be added at 0.002% or more. On the other hand, if added in excess of 0.100%, the amount of complex inclusions becomes excessive and the joint CTOD properties in the low temperature range deteriorate. Therefore, the Al content is set to a range of 0.002 to 0.100%. Preferably, it is 0.005% or more and 0.090% or less. More preferably, it is 0.020% or more and 0.075% or less.

Ti: 0.002 to 0.055%
Ti precipitates in steel as TiN. The precipitated TiN has the effect of suppressing the coarsening of austenite grains in the base material and HAZ, refining the HAZ structure, and improving the joint CTOD characteristics. In order to obtain such an effect, an addition of 0.002% or more is necessary. On the other hand, if the Ti content exceeds 0.055%, the Ti nitrides coarsen, and the toughness of the welded heat affected zone decreases, and the joint CTOD characteristics deteriorate. Therefore, the Ti content is set to a range of 0.002 to 0.055%. Preferably, it is 0.005% or more and 0.050% or less. More preferably, it is 0.010% or more and 0.045% or less.

Nb: 0.005 to 0.070%
Nb is an element that widens the non-recrystallization temperature range of the austenite phase, and has the effect of efficiently performing non-recrystallization region rolling and obtaining a fine structure, thereby improving the strength and toughness of the base material. If Nb is not added, the non-recrystallization temperature _T2 becomes too low, and the rolling temperature of the non-recrystallization region rolling for grain refinement becomes too low. When rolling at a low temperature, the deformation resistance of the rolled material increases, and the load on the rolling machine increases, so that the number of rolling passes increases, leading to a decrease in production efficiency and making it difficult to increase the pass reduction rate. As a result, in thick steel plates with a plate thickness of more than 100 mm, it is not possible to introduce appropriate strain to 1/2t, making it difficult to obtain the desired characteristics. In order to obtain such an effect, the Nb content must be 0.005% or more. On the other hand, if the Nb content exceeds 0.070%, the joint CTOD characteristics decrease. Therefore, the Nb content is set to a range of 0.005 to 0.070%. Preferably, it is 0.010% or more and 0.060% or less. More preferably, it is 0.015% or more and 0.050% or less.

Ca: 0.0005 to 0.0200%
Ca is an element that improves the toughness of multi-pass weld HAZ by forming oxysulfides that are highly stable at high temperatures, and improves the CTOD characteristics of the joint. In order to obtain such an effect, a Ca content of 0.0005% or more is necessary. On the other hand, a Ca content of more than 0.0200% causes excessive precipitation of oxysulfides, which in turn reduces the CTOD characteristics of the joint. Therefore, the Ca content is set to 0.0005 to 0.0200%. Preferably, the Ca content is 0.0010% or more and 0.0170% or less. More preferably, the Ca content is 0.0015% or more and 0.0150% or less.

N: 0.0120% or less N is an element that reduces HAZ toughness and deteriorates joint CTOD characteristics, so the upper limit of the N content is limited to 0.0120%. On the other hand, since it is desirable to reduce the N content as much as possible, the lower limit of the N content is not limited, but excessive reduction in N content leads to an increase in refining time and an increase in costs. Therefore, the N content is preferably 0.0005% or more. More preferably, it is 0.0020% or more and 0.0110% or less. Even more preferably, it is 0.0030% or more and 0.0090% or less.

O: 0.0070% or less O is an element that reduces HAZ toughness and deteriorates joint CTOD characteristics, so the upper limit of the O content is limited to 0.0070%. On the other hand, since it is desirable to reduce the O content as much as possible, the lower limit of the O content is not limited, but excessive reduction of O content leads to an increase in refining time and an increase in cost. Therefore, the O content is preferably 0.0005% or more. More preferably, it is 0.0010% or more and 0.0060% or less. Even more preferably, it is 0.0015% or more and 0.0055% or less.

The composition of the steel plate in one embodiment of the present invention is made up of the above essential elements with the balance being Fe and unavoidable impurities.
In another embodiment of the present invention, in order to further improve the strength, base material toughness, joint toughness, and the like, in addition to the above-mentioned component composition, one or more optional elements selected from the group consisting of Ni, Cu, Cr, Mo, V, W, B, REM, and Mg can be further contained in the contents shown below.

Ni: 2.5% or less Ni is an element that can increase the strength of thick steel plates without significantly deteriorating the toughness of both the base material and the joint. On the other hand, if the Ni content exceeds 2.5%, the manufacturing cost and the environmental load increase become problems. Therefore, the Ni content is limited to 2.5% or less. More preferably, it is 2.0% or less. On the other hand, when Ni is added, it is preferable that it is 0.1% or more.

Cu: 2.0% or less Cu is an element that can increase the strength of thick steel plates without significantly deteriorating the base material and joint toughness, but if the Cu content exceeds 2.0%, surface cracking due to a Cu-enriched layer formed just below the scale becomes a problem. Therefore, the Cu content is limited to 2.0% or less, and more preferably to 1.8% or less. On the other hand, when Cu is added, it is preferably 0.05% or more, and more preferably 0.1% or more.

Cr: 1.5% or less Cr is an element that has the effect of improving the strength by improving the hardenability of steel, but if the Cr content exceeds 1.5%, the joint CTOD characteristics deteriorate, so the Cr content is limited to 1.5% or less. More preferably, it is 1.3% or less. On the other hand, when Cr is added, it is preferably 0.05% or more, and more preferably 0.1% or more.

Mo: 1.5% or less Mo is an element that has the effect of improving the strength by improving the hardenability of steel, but if the Mo content exceeds 1.5%, the joint CTOD characteristics deteriorate, so the Mo content is limited to 1.5% or less. More preferably, it is 1.3% or less. On the other hand, when Mo is added, it is preferably 0.05% or more, and more preferably 0.1% or more.

V: 0.25% or less V is an element that improves the strength of the base metal, but if the V content exceeds 0.25%, the HAZ toughness decreases and the joint CTOD characteristics deteriorate, so the V content is limited to 0.25% or less. More preferably, it is 0.20% or less. On the other hand, when V is added, it is preferably 0.01% or more, and more preferably 0.03% or more.

W: 0.45% or less W is an element that improves the strength of the base metal, but if the W content exceeds 0.45%, the HAZ toughness decreases and the joint CTOD characteristics deteriorate, so the W content is limited to 0.45% or less. More preferably, it is 0.40% or less. On the other hand, when W is added, it is preferably 0.05% or more, and more preferably 0.15% or more.

B: 0.0045% or less Although B is an element that can improve hardenability with a very small amount of content, thereby improving the strength of the steel plate, if the B content exceeds 0.0045%, the HAZ toughness decreases and the joint CTOD characteristics deteriorate, so the B content is limited to 0.0045% or less, more preferably 0.0040% or less. On the other hand, if B is added, it is preferably 0.0005% or more, and more preferably 0.0010% or more.

REM: 0.025% or less REM (rare earth metals) form oxysulfide inclusions to suppress the growth of austenite grains in the HAZ and improve HAZ toughness, but if the REM content exceeds 0.025%, the base material toughness and HAZ toughness decrease, and the joint CTOD characteristics deteriorate. Therefore, the REM content is limited to 0.025% or less. More preferably, it is 0.020% or less. On the other hand, when REM is added, it is preferably 0.001% or more, and more preferably 0.010% or more.

Mg: 0.005% or less Mg is an element that forms oxide-based inclusions to suppress the growth of austenite grains in the weld heat affected zone and improve the toughness of the weld heat affected zone, but if the Mg content exceeds 0.005%, the effect of adding it is saturated, and it is not possible to expect an effect commensurate with the content, which is economically disadvantageous. Therefore, the Mg content is limited to 0.005% or less, and more preferably 0.004% or less. On the other hand, when Mg is added, it is preferably 0.0005% or more, and more preferably 0.001% or more.

In the present invention, the chemical compositions of the above-mentioned steel plate and base material must further satisfy the following four conditions.
1.50≦[Ti]/[N]≦5.00 ... (1)
[Ti]/[N] controls the amount of solute N in the HAZ and the precipitation state of TiN. If [Ti]/[N] is less than 1.50, the presence of solute N that is not fixed as TiN deteriorates the HAZ toughness and deteriorates the joint CTOD characteristics. On the other hand, if [Ti]/[N] is greater than 5.00, the precipitation of coarse TiN deteriorates the HAZ toughness and deteriorates the joint CTOD characteristics. Therefore, the range of [Ti]/[N] is set to 1.50 to 5.00. Preferably, it is 1.80 or more and 4.50 or less. More preferably, it is 2.00 or more and 4.00 or less.

0≦{[Ca]−(0.18+130[Ca])×[O]}/1.25/[S]≦1.50 … (2)
{[Ca]-(0.18+130[Ca])×[O]}/1.25/[S] is the atomic concentration ratio (ACR) of Ca, O and S in steel. When the ACR is less than 0, the main form of the sulfide-based inclusions is MnS. Since MnS has a low melting point and melts near the weld line during welding, the effect of suppressing austenite grain coarsening near the weld line and the transformation effect during cooling after welding cannot be obtained, and the CTOD characteristics of the joint deteriorate. On the other hand, when the ACR exceeds 1.50, the main form of the sulfide-based inclusions is CaS. Since CaS does not form a Mn-poor layer necessary for becoming a transformation nucleus around CaS, the transformation nucleus effect cannot be obtained, and the CTOD characteristics of the joint deteriorate. Therefore, the range of the ACR is set to 0 or more and 1.50 or less. The range of the ACR is preferably 0.20 or more and 1.40 or less. More preferably, it is 0.40 or more and 1.20 or less.

Ceq: 0.280% or more, 0.500% or less When the carbon equivalent Ceq defined by the following formula (3) increases, the amount of structures with poor toughness such as island martensite and bainite in the HAZ structure increases, resulting in deterioration of HAZ toughness. That is, when Ceq is greater than 0.500%, the toughness of the HAZ base structure itself deteriorates, so that the necessary joint CTOD characteristics cannot be satisfied even if a HAZ toughness improvement technique using complex inclusions is used. On the other hand, when Ceq is less than 0.280%, the target strength cannot be secured. Therefore, the range of Ceq is set to 0.280 to 0.500%. Preferably, it is 0.300% or more, and 0.490% or less. More preferably, it is 0.320% or more, and 0.480% or less.
Ceq(%)=[C]+[Mn]/6+([Cu]+[Ni])/15+([Cr]+[Mo]+[V])/5 ... (3)

Pcm: 0.240% or less When the weld crack susceptibility index Pcm defined by the following formula (4) increases, the amount of structures with poor toughness, such as island martensite and bainite, in the HAZ structure increases, resulting in deterioration of the HAZ toughness. In other words, when Pcm exceeds 0.240%, the necessary joint CTOD characteristics cannot be obtained due to the deterioration of the toughness of the HAZ base structure itself. Therefore, Pcm is set to 0.240% or less. It is preferably 0.230% or less, more preferably 0.210% or less. On the other hand, the lower limit is not particularly limited, but if an attempt is made to reduce Pcm excessively, the value of Ceq becomes too low, so that it is preferably about 0.140%, and more preferably 0.155% or more.
Pcm(%)=[C]+[Si]/30+([Mn]+[Cu]+[Cr])/20+[Ni]/60+[Mo]/15+[V]/10+5[B]...(4)

In addition, the brackets [ ] in the above formulas (1) to (4) represent the content (mass%) of the element shown within the brackets, and are set to zero if the element is not contained.

[Average effective grain size]
Average effective grain size at the center of plate thickness (meaning a region having a thickness of 10% of the plate thickness in both directions of the steel plate from the 1/2 position of the plate thickness): 20 μm or less In the present invention, the average effective grain size of the microstructure at the center of plate thickness of a thick steel plate with a plate thickness of more than 100 mm is set to 20 μm or less. By refining the grains at the center of plate thickness where segregation is likely to exist as described above to improve the toughness of the base material, it is possible to improve the CTOD characteristics of the joint at the SC/ICHAZ boundary. On the other hand, since the smaller the average effective grain size, the more advantageous it is, the lower limit is not particularly limited, but it is usually about 1 μm.
Here, the "effective grain size" in the present invention is defined as the circle-equivalent diameter of a grain surrounded by a grain boundary having an orientation difference of 15° or more with adjacent grains, i.e., a high-angle grain boundary. The average effective grain size in the center of the sheet thickness can be measured by the method described in the examples below.

[Maximum effective grain size]
In the present invention, the maximum effective grain size of the microstructure in the thickness center is set to 150 μm or less. Even if the average effective grain size is 20 μm or less, if coarse grains with an effective grain size exceeding 150 μm are mixed in the thickness center, such coarse grains become fracture origins, leading to a decrease in the base material strength, base material toughness, and SCHAZ toughness in the thickness center. Therefore, the maximum effective grain size is set to 150 μm or less. The maximum effective grain size can be measured by the method described in the examples below.

[Composite inclusions]
In the present invention, the number density of composite inclusions containing sulfides containing Ca and Mn and oxides containing Al and having an equivalent circle diameter of 0.1 μm or more at the 1/2 sheet thickness position is limited to a range of 25 to 250 pieces/ ^mm2 .
When sulfides containing Mn are formed, Mn-poor regions are formed around the composite inclusions, making them effective as transformation nuclei. Furthermore, by including Ca in such sulfides, the melting point is increased, and they can remain at the temperature reached by the vicinity of the weld line of the HAZ. As a result, the austenite grain growth suppression effect and the transformation nuclei effect are exerted, and the joint CTOD characteristics are improved. In order to fully exert the above effects, the number density of the composite inclusions at the 1/2 plate thickness position must be 25 pieces/ ^mm2 or more. On the other hand, the presence of an excessive amount of composite inclusions rather deteriorates the joint CTOD characteristics. For this reason, the number density of composite inclusions having a circle equivalent diameter of 0.1 μm or more at the 1/2 plate thickness position is set to 250 pieces/ ^mm2 or less. Preferably, it is 30 pieces/ ^mm2 or more and 215 pieces/ ^mm2 or less, and more preferably, it is 50 pieces/ ^mm2 or more and 200 pieces/ ^mm2 or less. The number density can be measured by the method described in the examples described later.

Here, the measurement frequency for the average effective grain size, maximum effective grain size, and complex inclusions can be achieved by measuring one or two cross sections at the center of the thickness of any one steel plate among steel plates that have the same melting conditions and rolling conditions. As long as the melting method and rolling conditions of the material are not changed, the grain size and number density of complex inclusions can be produced with good reproducibility, so the measurement results at the above measurement frequency represent the whole.

[Production method]
Next, the reasons for limiting each condition in the manufacturing method of the thick steel plate in the present invention will be described below. In the following description, the temperature is the temperature at the center of the plate thickness unless otherwise specified. The plate thickness center temperature can be actually measured as in the examples described later, but in an actual manufacturing line, it may be calculated by heat transfer calculation from the steel plate surface temperature measured by a radiation thermometer.

Heating Conditions of the Material In the present invention, the method of melting the material is not particularly limited, and any of the known melting methods such as converter, electric furnace, and vacuum melting furnace are suitable. Such materials are produced, for example, by a continuous casting method. Furthermore, the molten steel used to melt such materials may be further subjected to secondary refining such as ladle refining.
The material having the above-mentioned composition and manufactured as described above is heated to 990°C or more and 1210°C or less. If the heating temperature is lower than 990°C, the conditions for hot rolling described later cannot be satisfied, and sufficient effects cannot be obtained. On the other hand, if the heating temperature is higher than 1210°C, the austenite grains become coarse, and the desired fine grain structure cannot be obtained after controlled rolling. For this reason, the heating temperature is set to a range of 990°C or more and 1210°C or less. It is preferably 1010°C or more and 1190°C or less, and more preferably 1030°C or more and 1170°C or less.

Hot rolling conditions In the present invention, it is important to control the rolling conditions in both the recrystallization temperature range defined by the _T1 temperature or higher according to the following formula (5) and the non-recrystallization temperature range defined by the _T2 temperature or lower according to the following formula (6) in hot rolling, and to avoid rolling at the partial recrystallization temperature range ( _T1 to _T2 °C) where recrystallized grains and coarse recovered grains are generated. Since the recrystallization temperature range and the non-recrystallization temperature range change depending on the contained components, it is also important to clarify the recrystallization temperature range and the non-recrystallization temperature range for each component composition of the steel using formulas (5) and (6).

（ここで、ｓｏｌ．［Ｎｂ］は（７）式による）

(where sol.[Nb] is based on formula (7))

（なお、式（５）、（７）における［Ｃ］、［Ｎ］はそれぞれＣ、Ｎの質量％を、Ｔ_０は素材の加熱温度（℃）、ｓｏｌ．［Ｎｂ］はＮｂの固溶量（質量％）を表す。ただし、［Ｎｂ］を鋼板に含まれる総Ｎｂ含有量（質量％）とすると、ｓｏｌ．［Ｎｂ］は、常にｓｏｌ．［Ｎｂ］≦［Ｎｂ］である。よって、式（７）の計算の結果がｓｏｌ．［Ｎｂ］＞［Ｎｂ］となる場合、式（７）のｓｏｌ．［Ｎｂ］は［Ｎｂ］の値を用いる。）

(Note that in formulas (5) and (7), [C] and [N] represent the mass% of C and N, respectively, _T0 represents the heating temperature (°C) of the material, and sol.[Nb] represents the amount of Nb dissolved in solid solution (mass%). However, if [Nb] is the total Nb content (mass%) contained in the steel plate, then sol.[Nb] always satisfies sol.[Nb]≦[Nb]. Therefore, when the result of calculation of formula (7) is sol.[Nb]>[Nb], the value of [Nb] is used as sol.[Nb] in formula (7).)

Rolling in the recrystallization temperature range defined by the sheet thickness center temperature being equal to or higher than the _T1 temperature is performed so that the average reduction/pass is 3% or more and the cumulative reduction is 25% or more.

The purpose of rolling in the recrystallization temperature range is to refine and granulate the structure of a thick steel plate having a thickness of more than 100 mm by recrystallization, and to refine and disperse coarse inclusions. In hot rolling, if rolling is performed in a partial recrystallization temperature range in the thickness center temperature range of T ₁ to T ₂ ° C., a mixed grain structure of recrystallized grains and coarse recovered grains is formed, and the desired granular structure cannot be obtained. Therefore, it is necessary to roll at the thickness center temperature T ₁ temperature or higher while avoiding rolling at the thickness center temperature of T ₁ to T ₂ ° C. In addition, when the average reduction rate/pass is less than 3%, sufficient strain cannot be introduced into the thickness center in a thick steel plate having a thickness of more than 100 mm, and the structure in the thickness center cannot be refined. In addition, even if the average reduction rate/pass is 3% or more, recrystallization does not proceed sufficiently when the cumulative reduction rate is less than 25%, and a uniform fine structure cannot be obtained. For this reason, the rolling in the recrystallization temperature range is set to an average reduction/pass of 3% or more and a cumulative reduction of 25% or more, preferably 30% or more, and more preferably 35% or more.

Thereafter, the sheet is rolled to a cumulative reduction rate of 30% or more in a non-recrystallization temperature range defined as a temperature at the center of the sheet thickness being equal to or lower than the _T2 temperature.

In rolling at the plate thickness center temperature in the non-recrystallization temperature range, recrystallization is difficult to occur in the steel structure, so the strain introduced by rolling is not consumed in recrystallization but is accumulated, and becomes the driving force for nucleation in the subsequent cooling process. As a result, the structure of the finally obtained thick steel plate can be refined. On the other hand, if the cumulative reduction rate at the plate thickness center temperature in the non-recrystallization temperature range is less than 30%, the crystal grain refinement effect is insufficient, and the average effective crystal grain size at the plate thickness center cannot be made 20 μm or less. For this reason, the rolling in the non-recrystallization temperature range is set to a cumulative reduction rate of 30% or more, preferably 35% or more, and more preferably 40% or more.
The conditions for rolling in the non-recrystallization temperature region are not particularly limited, but a larger average reduction/pass (average value of the reduction in each pass) is preferable, and specifically, the average reduction/pass is preferably 3% or more.

[cooling]
After the hot rolling is completed, the hot-rolled steel sheet is cooled. The cooling can be performed by any method as long as the following conditions are satisfied. For example, the cooling can be performed by water cooling.

Average cooling rate: 1.0 to 50.0 ° C./s
When the average cooling rate at the center of the plate thickness is less than 1.0°C/s, a coarse ferrite phase is generated in the base material structure, which reduces the base material strength and toughness, and deteriorates the CTOD characteristics of SC/ICHAZ. On the other hand, when the average cooling rate is greater than 50.0°C/s, the hard bainite phase increases, which increases the base material strength and deteriorates the CTOD characteristics of SC/ICHAZ. For this reason, the average cooling rate at the center of the plate thickness is set to a range of 1.0 to 50.0°C/s. It is preferably 1.2°C/s or more and 45°C/s or less, and more preferably 1.5°C/s or more and 40°C/s or less.
The temperature measurement range for the cooling rate is between 700 and 500°C when the cooling stop temperature is 500°C or lower, and between 700°C and the cooling stop temperature when the cooling stop temperature is higher than 500°C.

Cooling stop temperature: 600°C or less In the cooling, the hot-rolled steel sheet is cooled until the temperature at the center of the thickness reaches a cooling stop temperature of 600°C or less. If the cooling stop temperature is higher than 600°C, the structure after transformation becomes coarse, the strength of the base material is insufficient, the toughness of the base material decreases, and the CTOD characteristics of SC/ICHAZ deteriorate. For this reason, the cooling stop temperature is set to 600°C or less at the center of the thickness, preferably 580°C or less, more preferably 560°C or less. The lower limit of the cooling stop temperature is not particularly limited, but is preferably about 200°C.

[Tempering treatment]
Tempering temperature: 700°C or less After the cooling is stopped, a further tempering treatment can be performed as desired. The tempering treatment can further improve the toughness of the base material. At that time, if the tempering temperature is higher than 700°C, a coarse ferrite phase is generated, and the base material toughness and SCHAZ toughness deteriorate. Therefore, the tempering temperature is set to 700°C or less at the plate thickness center temperature. More preferably, it is set to 650°C or less. The lower limit of the tempering temperature is not particularly limited as long as the effect of improving the base material toughness is obtained, but it can be set to about 300°C.

In the manufacturing method according to the present invention, any items not described in this specification can be performed using conventional methods.

Next, the present invention will be described in more detail based on examples. The following examples are intended to illustrate preferred embodiments of the present invention, and the present invention is not limited to these examples.

Steel plates were manufactured under the manufacturing conditions shown in Table 2 using materials with the chemical composition shown in Table 1. Rolling to _{T 1} °C or higher was performed with an average reduction rate/pass of 3% or more. During hot rolling, thermocouples were attached to the center positions of the steel material being rolled in the longitudinal direction, width direction, and plate thickness direction, and the temperature at the plate thickness center was measured. Additionally, the surface temperature of the steel material was measured with a radiation thermometer.
For each of the obtained thick steel plates, the average effective crystal grain size, the maximum effective crystal grain size, the number density of complex inclusions containing sulfides containing Ca and Mn and oxides containing Al, the yield strength, the toughness and the CTOD characteristics were measured by the following methods.

[Average effective grain size/Maximum effective grain size]
From the obtained steel sheet, a sample was taken so that the center of the steel sheet in the longitudinal direction, width direction, and sheet thickness direction was the measurement position and the sheet thickness center was included. Next, the surface of the sample was mirror-polished, and then EBSP (electron backscatter diffraction pattern) analysis was performed under the following conditions. From the obtained crystal orientation map, the circle equivalent diameter of the structure surrounded by high-angle grain boundaries with an orientation difference of 15° or more with adjacent crystal grains was obtained, and the average value of the circle equivalent diameter in the following analysis region was defined as the average effective crystal grain size, and the maximum value of the circle equivalent diameter was defined as the maximum effective crystal grain size.
EBSP analysis conditions: Analysis area: 1 mm x 1 mm area at the center of plate thickness; Step size: 0.4 μm

[Number density of composite inclusions containing sulfides containing Ca and Mn and oxides containing Al]
Samples were taken from the center of the longitudinal direction, width direction, and thickness direction of the steel plate, and mirror polished with a diamond buff + alcohol. Then, using a field emission scanning electron microscope (FE-SEM: Field Emission-Scanning Electron Microscope), composite inclusions with a circle equivalent diameter of 0.1 μm or more present in an evaluation region of 1 mm × 1 mm, the center of which is the center of the region, were identified by EDX (Energy Dispersive X-ray spectroscopy) analysis, and the number density of the composite inclusions was also evaluated. In addition, the evaluation of the type of inclusion was performed by determining that the inclusion contained various elements in an atomic fraction of 3% or more of the chemical composition of the inclusion quantified by the ZAF method.

[Yield strength]
A tensile test was carried out in accordance with EN10002-1 to determine the yield strength (YS) at 1/2 the thickness (t) of the thick steel plate. For the tensile test, a round bar tensile test piece with a parallel part diameter of 14 mm and a parallel part length of 70 mm, taken parallel to the plate width direction from the 1/2 position of the plate thickness, was used. When an upper yield point appeared in the tensile test, the upper yield stress was taken as the yield strength, and when an upper yield point did not appear, the 0.2% proof stress was taken as the yield strength.

[Base material toughness]
Three V-notch test pieces as specified in JIS Z2242 were taken from the 1/2 position of the plate thickness of the steel plate so that the longitudinal direction of the test piece was perpendicular to the rolling direction of the steel plate, and the absorbed energy vE _-40°C at -40°C was measured in a Charpy impact test. Those having an average vE _-40°C of 100 J or more were deemed to have good base material toughness.

Next, multi-layer welded joints were fabricated using each of the thick steel plates. A joint CTOD test was performed on each of the resulting multi-layer welded joints to measure the crack opening displacement in the CGHAZ and the crack opening displacement in the SC/ICHAZ. The fabrication conditions for the multi-layer welded joints and the joint CTOD test conditions are described below.

[Joint CTOD test]
The welded joints used in the joint CTOD test were prepared by submerged arc welding (multi-layer welding) with a K groove shape and a heat input of 5.0 kJ/mm. The test method conformed to BS standard EN10225-4 (2019), and a test piece with a square cross section of t x t (t is plate thickness) was used to evaluate the crack opening displacement (CTOD value (δ)) at a test temperature of -20°C.

A test was conducted in which the notch was located at the CGHAZ on the linear side of the K groove, and a test was conducted in which the notch was located at the SC/ICHAZ boundary, and δ of the CGHAZ and δ of the SC/ICHAZ boundary were measured. Note that for each steel plate, the test was conducted using three test pieces for each notch position, and the minimum measured value was recorded as δ.
A larger CTOD value (δ) indicates that brittle fracture is less likely to occur.

Furthermore, after the test, it was confirmed that the tip of the fatigue pre-crack on the fracture surface of the test specimen was located in the CGHAZ and the SC/ICHAZ boundary as specified in EN10225-4 (2019). In the case of a joint CTOD test in multi-pass welding, even if the notch position is in the CGHAZ, a certain amount of ICCGHAZ is included, so the test results reflect the toughness of both the CGHAZ and the ICCGHAZ.
The measurement results are shown in Table 2.

As shown in Table 2, the steel plate (invention example) satisfying the conditions of the present invention had manufacturing conditions, average effective crystal grain size of the base material, maximum effective crystal grain size, and number density of composite inclusions containing sulfides containing Ca and Mn and oxides containing Al that all satisfied the ranges of the present invention. As a result, the yield strength was 325 MPa, and the vE _-40°C of the base material was 100 J or more, demonstrating excellent base material properties, and furthermore, the CTOD value of the CGHAZ and the CTOD value (δ) of the SC/ICHAZ boundary were both 0.40 mm or more at -20°C, providing excellent joint CTOD properties.
In contrast, the steel plates (comparative examples) not satisfying the conditions of the present invention were inferior to the steel plates of the above-mentioned invention examples in either or both of the base material properties and the joint CTOD properties.

Claims (4)

In mass percent,
C: 0.03 to 0.13%,
Si: 0.60% or less,
Mn: 0.9 to 2.7%,
P: 0.050% or less,
S: 0.0050% or less,
Al: 0.002 to 0.100%,
Ti: 0.004 to 0.055%,
Nb: 0.005 to 0.070%,
Ca: 0.0008 to 0.0200%,
N: 0.0120% or less and O: 0.0070% or less;
The balance is Fe and unavoidable impurities, and has a composition satisfying the following formulas (1) to (4):
1.50≦[Ti]/[N]≦5.00 ... (1)
0≦{[Ca]−(0.18+130[Ca])×[O]}/1.25/[S]≦1.50 … (2)
0.280%≦Ceq(=[C]+[Mn]/6+([Cu]+[Ni])/15+([Cr]+[Mo]+[V])/5)≦0.500% ... (3)
Pcm(=[C]+[Si]/30+([Mn]+[Cu]+[Cr])/20+[Ni]/60+[Mo]/15+[V]/10+5[B])≦ 0.231 % … (4)
(The brackets in the above formulas (1) to (4) represent the content (mass%) of the element in the brackets, and are set to zero when the element is not contained.)
A steel plate having a thickness of more than 100 mm, in which, at the center portion of the plate thickness, an average effective crystal grain size is 20 μm or less and a maximum effective crystal grain size is 150 μm or less, and at a 1/2 position of the plate thickness, there are 25 to 250 composite inclusions/ ^mm2 having a circle equivalent diameter of 0.1 μm or more which contain sulfides containing Ca and Mn and oxides containing Al. The composition further comprises, in mass%,
Ni: 2.5% or less,
Cu: 2.0% or less,
Cr: 1.3 % or less,
Mo: 1.3 % or less,
V: 0.20 % or less,
W: 0.40 % or less,
B: 0.0045% or less,
The steel plate according to claim 1, comprising at least one selected from the group consisting of REM: 0.022 % or less, and Mg: 0.005% or less. A method for producing a steel plate, comprising heating a raw material having the composition according to claim 1 or 2 to 990°C or higher and rolling it at an average reduction/pass of 3% or higher and a cumulative reduction of 25% or higher when the plate thickness center temperature is _T1 °C or higher as defined by the following formula (5), rolling it at a cumulative reduction of 30% or higher when the plate thickness center temperature is _T2 °C or lower as defined by the following formula (6), and then cooling it at an average cooling rate of 1.0 to 50.0°C/s at the plate thickness center to a cooling stop temperature of 600°C or lower to obtain a steel plate having a thickness of more than 100 mm, which has an average effective grain size of 20 μm or lower and a maximum effective grain size of 150 μm or lower at the plate thickness center, and which contains 25 to 250 composite inclusions/mm2 having a circle equivalent diameter of 0.1 μm or higher containing sulfides containing Ca and Mn and oxides containing Al at a ^1/2 plate thickness position .
(where sol.[Nb] is based on formula (7))
(In the above formulas (5) and (7), [C] and [N] respectively represent the C and N contents (mass%), _T0 represents the heating temperature (°C) of the material, and sol.[Nb] represents the amount of Nb in solid solution (mass%). However, if [Nb] is the total Nb content (mass%) contained in the steel plate, then sol.[Nb]≦[Nb].) The method for manufacturing steel sheet according to claim 3, in which, after cooling to the cooling stop temperature, tempering is performed at a temperature of 700°C or less.