JP6662174B2 - Steel plate - Google Patents

Description

  The present invention relates to a thick steel plate. In particular, the present invention relates to a thick steel plate having excellent low-temperature toughness of a heat affected zone (hereinafter, referred to as “HAZ”) used in offshore structures such as oil and natural gas drilling equipment at sea.

  Steel plates used in various welded steel structures such as buildings, bridges, shipbuilding, line pipes, construction machinery, marine structures, tanks, etc. have a demand for toughness from the viewpoint of improving the safety and reliability against fracture of welds. The severity is increasing year by year, and as with the toughness of the base steel sheet, it is required that the HAZ also ensure more excellent toughness.

  In the HAZ, the heating temperature during welding increases as the melting line is approached, and particularly in the region heated to 1400 ° C. or higher near the melting line, austenite (γ) grains are significantly coarsened, and the HAZ structure after cooling is reduced. Are coarsened and toughness is deteriorated. This tendency is more remarkable as the welding heat input increases. In recent years, in order to reduce the number of welding passes and reduce welding cost, large heat input welding is performed using a high efficiency welding method in which welding heat input is increased. Therefore, there is a problem that the HAZ toughness is inevitably reduced.

  In order to solve these problems, various measures have been taken to improve HAZ toughness when large heat input welding is performed.

  As a method of improving HAZ toughness, for example, a method of controlling the crystal grain size in HAZ is known. As a method of controlling the crystal grain size, specifically, a method of suppressing the coarsening of austenite grains in the heating process of welding by dispersing a large amount of fine pinning particles in steel, a core of ferrite transformation. By dispersing particles to be formed into steel, a method of promoting intragranular transformation in the cooling process of welding and subdividing the inside of the grains can be cited.

For example, in Patent Literature 1, 1 × 10 ⁶ / mm ³ or more of composite inclusions composed of an oxide composed of Mg, Mn, and Al and MnS and having a particle size of less than 0.6 μm are dispersed and generated in a steel material. This discloses a steel material capable of suppressing coarsening of prior austenite grains and consequently ensuring excellent toughness even when a large heat input welding of 300 kJ / cm or more is performed.

  In Patent Literature 2, even when welding is performed with a large heat input (200 kJ / cm), Mn oxides and Al oxides, which are likely to become precipitation nuclei of MnS particles, are finely and numerously dispersed in steel. , A thick steel plate having good HAZ toughness is disclosed.

In Patent Document 3, welding is performed by controlling the total number density of TiN particles, MnS particles, and composite particles having a circle equivalent diameter of 0.5 to 2.0 μm contained in a steel sheet to 20 to 200 particles / mm ^2. When the steel sheet is heated by, the growth of austenite grains is suppressed by a pinning effect, or when the steel sheet is cooled after welding, it becomes a nucleus where ferrite is transformed, thereby refining the structure, A steel plate having a thickness of 10 to 35 mm capable of improving the HAZ toughness during large heat input welding is disclosed.

JP 2014-5527 A JP-A-5-271864 Japanese Patent Application Laid-Open No. 2015-98642

  In recent years, thick steel plates used for welding structures such as marine structures are required to be thick and have high strength. However, since such a thick steel plate is assembled by welding, it is a problem to secure the characteristics of the welded portion. In particular, a thick steel plate having a thickness of 50 mm or more has a problem that it is difficult to secure the toughness of the HAZ at a low temperature because the heat input during welding increases.

  The present invention has been made in view of the above circumstances, and has an object to provide a thick steel sheet excellent in low-temperature toughness of HAZ at the time of large heat input welding.

  The present inventors have conducted intensive studies to solve the above-mentioned problems, and as a result, have obtained the following knowledge.

  When heated to around 1400 ° C., coarse γ grains grow in the HAZ due to crystal grain growth. The growth of such coarse γ grains contributes to a decrease in toughness. Therefore, as a means for securing HAZ toughness, it is effective to reduce the number of fracture units by refining crystal grains. Conventionally, as a method for refining crystal grains, (a) a method utilizing a pinning effect of suppressing the γ grain boundary growth with TiN or the like, and (b) a fine method starting from inclusions existing in γ grains. A method has been proposed in which intragranular ferrite is grown and crystal grains are refined.

  The present inventors have dispersed fine TiN particles in steel by controlling the balance of Ti, Al, O and N in the steel making process. As a result, it was possible for the TiN particles to suppress the γ grain growth in the HAZ (pinning effect) and to suppress the coarse γ grain growth.

  On the other hand, TiN particles are easily dissolved at around 1400 ° C., and the pinning effect is reduced. As a result, coarse gamma grains easily grow. Therefore, the present inventors also utilized intragranular transformation caused by inclusions. In order to effectively grow intragranular ferrite in γ grains during welding, it is essential to control inclusions that form intragranular ferrite generation nuclei. In particular, in the case of a thick steel plate having a thickness of 50 mm or more, it is difficult to control the composition and the number of inclusions in the thickness direction due to the difference in cooling rate between the surface and the inside. Then, when the mechanism of intragranular ferrite growth was clarified, the following was found.

  [1] At the time of welding cooling, a driving force for diffusing Mn from the matrix into the inclusion is generated due to the Mn concentration gradient formed when MnS is compositely precipitated around the inclusion.

  [2] Mn is absorbed into atomic vacancies existing inside the Ti-based oxide.

  [3] A Mn-deficient layer having a reduced Mn concentration is formed around the inclusions, and the ferrite growth start temperature in this portion increases.

  [4] During cooling, ferrite grows preferentially from inclusions.

  Based on these assumptions, the present inventors have found that the amount of MnS complex of inclusions serving as intragranular ferrite nuclei affects the growth of intragranular ferrite. That is, a Ti-based oxide having many vacancies therein for absorbing Mn becomes a core of the inclusion. When the amount of the MnS complex in the inclusion is large, a larger Mn concentration gradient is formed around the inclusion, so that the Mn diffusion driving force is increased, and as a result, a Mn deficiency layer is easily formed. On the other hand, if the amount of the MnS complex in the inclusions is small, it is difficult to form a Mn concentration gradient around the inclusions, and as a result, it is difficult to form a Mn-deficient layer.

  Based on the above mechanism, in the present invention, the coarse grain growth is suppressed by TiN particles, the composite form of the Ti-based composite oxide is controlled, and the generation of intragranular ferrite starting from inclusions in the grains is promoted. As a result, a fine HAZ structure was formed.

  The present invention has been made on the basis of the above findings, and the gist of the present invention lies in the following thick steel plates (1) and (2).

(1) A steel plate having a thickness of 50 to 100 mm,
Chemical composition in mass%
C: 0.01 to 0.10%,
Si: 0.10 to 0.25%,
Mn: 1.00 to 2.50%,
P: 0.0100% or less,
S: 0.0010 to 0.0100%,
Ti: 0.005 to 0.030%,
Al: 0.003% or less,
Ni: 0.50 to 1.50%,
O: 0.0010 to 0.0050%,
N: 0.0100% or less,
Cu: 0 to 0.50%,
Cr: 0 to 0.50%,
Mo: 0 to 0.50%,
V: 0 to 0.10%,
Nb: 0 to 0.05%, and
The balance: Fe and impurities, and
In the steel, including a composite inclusion in which MnS exists around the Ti oxide,
An area ratio of the MnS in a cross section of the composite inclusion is 10% or more and less than 90%,
The ratio of the MnS at the interface of the composite inclusion is 10% or more;
The number density of the composite inclusions having a particle size of 0.5 to 5.0 μm is 10 to 100 / mm ² , and
A thick steel plate wherein X obtained from the following (i) is 0.04 to 9.70.

However, in the above formula (i), the meaning of each symbol is as follows.
Ti_TiO (mass%): Ti content of Ti oxide in total Ti content O (mass%): O content in steel Mn_MnS (mass%): Mn content of MnS in total Mn content R1 (%): average value of MnS area ratio in cross section of composite inclusion R2 (%): average value of MnS ratio at interface of composite inclusion

(2) The chemical composition is represented by mass%
Cu: 0.25 to 0.5%,
Cr: 0.01-0.5%,
Mo: 0.01-0.5%,
V: 0.03 to 0.1%, and
Nb: 0.01-0.05%,
The steel plate according to the above (1), comprising at least one member selected from the group consisting of:

  ADVANTAGE OF THE INVENTION According to this invention, the thick steel plate excellent in the low temperature toughness of HAZ at the time of large heat input welding can be provided.

  Hereinafter, each requirement of the present invention will be described in detail.

(A) Chemical composition The effects of each element and the reasons for limiting the content are as follows. In the following description, “%” for the content means “% by mass”.

C: 0.01 to 0.10%
C is an element having an effect of increasing the strength of the base material and the HAZ. In order to ensure strength, the C content needs to be 0.01% or more. On the other hand, if C is excessively contained, the HAZ tends to form a hard structure, so that the toughness of the HAZ decreases. Therefore, the C content is set to 0.10% or less. From the viewpoint of securing the strength of the base material and the HAZ and securing the low-temperature toughness of the HAZ, the C content is preferably 0.02% or more, and more preferably 0.08% or less. .

Si: 0.10 to 0.25%
Since Si acts as a deoxidizing agent during the production of steel, it is effective in controlling the amount of oxygen, and is an element that forms a solid solution in steel to increase the strength. To obtain the above effect, the Si content is set to 0.10% or more. On the other hand, when Si is excessively contained, the toughness of the base material is reduced, and the HAZ tends to form a hard structure, so that the toughness of the HAZ is reduced. Therefore, the Si content is set to 0.25% or less. The Si content is preferably 0.13% or more, and more preferably 0.18% or less, from the viewpoint of controlling the oxygen content to an appropriate value and securing the low-temperature toughness of the HAZ.

Mn: 1.00 to 2.50%
Mn is an element effective for securing strength. To obtain the above effect, the Mn content is set to 1.00% or more. On the other hand, if Mn is excessively contained, Mn is likely to segregate, and HAZ is likely to locally form a hard structure. As a result, the toughness of the HAZ decreases. Therefore, the Mn content is set to 2.50% or less. From the viewpoint of suppressing the formation of coarse ferrite and preventing segregation, the Mn content is preferably 1.20% or more, and more preferably 2.00% or less.

P: 0.0100% or less P is an impurity element. The reduction in the P content suppresses a decrease in grain boundary strength in HAZ. Therefore, the P content is set to 0.0100% or less.

S: 0.0010-0.0100%
S is an element for complex precipitation of MnS. Therefore, the S content is set to 0.0010% or more. On the other hand, if S is excessively contained, coarse elemental MnS precipitates, so that the toughness of the HAZ decreases. Therefore, the S content is set to 0.0100% or less. The S content is preferably 0.0020% or more, and more preferably 0.0050% or less, from the viewpoint of complex precipitation of MnS and securing the low-temperature toughness of the HAZ.

Ti: 0.005 to 0.030%
Ti is an element essential for generating a Ti-based oxide. In order to obtain a sufficient inclusion density, the Ti content is set to 0.005% or more. On the other hand, if Ti is contained excessively, carbides such as TiC are likely to be generated, so that the toughness of the HAZ decreases. Therefore, the Ti content is set to 0.030% or less. From the viewpoint of ensuring a sufficient inclusion density and securing the toughness of the HAZ, the Ti content is preferably 0.009% or more, and more preferably 0.020% or less.

Al: 0.003% or less Al is an impurity element. By increasing the Al content, generation of a Ti-based oxide is suppressed. Therefore, the Al content is set to 0.003% or less.

Ni: 0.50 to 1.50%
Ni is an element that improves toughness. From the viewpoint of ensuring low-temperature toughness, the Ni content is 0.50% or more. On the other hand, since Ni is an austenite stabilizing element, if it is contained excessively, it becomes difficult to generate intragranular ferrite. Therefore, the Ni content is set to 1.50% or less. The Ni content is preferably 0.60% or more in order to secure stable toughness even when the number of inclusions serving as intragranular transformation nuclei is small. In order to promote the formation of intragranular ferrite, the Ni content is preferably 1.00% or less.

O: 0.0010 to 0.0050%
O is an element indispensable for generating a Ti-based composite oxide. In order to obtain a sufficient inclusion density, the O content is 0.0010% or more. On the other hand, when O is excessively contained, it becomes easy to form a coarse oxide that can serve as a fracture starting point. Therefore, the O content is set to 0.0050% or less. Further, from the viewpoint of suppressing the formation of coarse inclusions, the O content is preferably 0.0030% or less.

N: 0.0100% or less N is an element that contributes to refinement of crystal grains by combining with Ti to generate TiN. However, when N is excessively contained, the amount of Ti required for TiN precipitation increases, and it becomes difficult to form a Ti oxide. Therefore, the N content is set to 0.0100% or less. In order to stably secure the amount of Ti forming the Ti oxide, the N content is preferably 0.0080% or less.

Cu: 0 to 0.5%
Cu may be included because it has the effect of increasing the strength. However, when Cu is excessively contained, hot embrittlement occurs, which leads to deterioration in the quality of the slab surface. Therefore, the Cu content is set to 0.5% or less. The Cu content is preferably 0.25% or more to further increase the strength.

Cr: 0 to 0.50%
Since Cr has an effect of increasing the strength, Cr may be contained. However, when Cr is contained excessively, the toughness of the HAZ decreases. Therefore, the Cr content is set to 0.50% or less. The Cr content is preferably 0.30% or less. Further, the Cr content is preferably 0.01% or more, and more preferably 0.10% or more, in order to further increase the strength.

Mo: 0 to 0.50%
Mo is an element whose strength is remarkably increased when contained in a small amount, and therefore may be contained. However, when Mo is excessively contained, the toughness of HAZ is significantly reduced. Therefore, the Mo content is set to 0.50% or less. The Mo content is preferably 0.30% or less. Further, the Mo content is preferably 0.01% or more to further increase the strength.

V: 0 to 0.10%
V is an element effective for improving the strength and toughness of the base material, and may be contained. However, when V is excessively contained, carbides such as VC are formed, leading to a decrease in toughness. Therefore, the V content is set to 0.10% or less. The V content is preferably 0.05% or less from the viewpoint of suppressing a decrease in toughness due to carbide formation. In order to further improve the strength and toughness of the base material, the V content is preferably 0.03% or more.

Nb: 0 to 0.05%
Nb is an element effective for improving the strength and toughness of the base material, and therefore may be contained. However, when Nb is excessively contained, carbides such as NbC are easily generated, which leads to a decrease in toughness. Therefore, the Nb content is set to 0.05% or less. The Nb content is preferably at most 0.03%. Further, the Nb content is preferably 0.01% or more in order to further improve the strength and toughness of the base material.

  The steel plate according to the present invention contains the above-mentioned elements, and the remainder has a chemical composition of Fe and impurities. “Impurities” are components that are mixed in due to various factors in ore, scrap, and other raw materials and manufacturing processes when steel is industrially manufactured, and are acceptable as long as they do not adversely affect the present invention. Means

(B) Composite Inclusion The thick steel plate of the present invention includes a composite inclusion in which MnS exists around a Ti oxide in steel.

Area ratio of MnS in cross section of composite inclusion: 10% or more and less than 90% In the present invention, a composite inclusion appearing on an arbitrary cut surface is analyzed, and the area ratio of MnS in the cross-sectional area of the composite inclusion is measured. By doing so, the amount of MnS in the composite inclusion is defined. If the area ratio of MnS in the cross section of the composite inclusion is less than 10%, the amount of MnS in the composite inclusion is small, and a sufficient Mn deficiency layer cannot be formed. As a result, it becomes difficult to generate intragranular ferrite. On the other hand, when the proportion of MnS in the cross section of the composite inclusion is 90% or more, the composite inclusion mainly comprises MnS, and the proportion of the Ti-based oxide decreases. As a result, the Mn absorption ability decreases, and a sufficient Mn deficiency layer cannot be formed, so that it is difficult to generate intragranular ferrite.

MnS ratio at interface of composite inclusion: 10% or more Since MnS needs to absorb Mn from around the composite inclusion, it must be present at the interface of the composite inclusion. If the proportion of MnS at the interface of the composite inclusion is less than 10%, Mn cannot be sufficiently absorbed from the periphery of the composite inclusion, so that a Mn-deficient layer cannot be formed. As a result, it becomes difficult to generate intragranular ferrite.

Particle size of composite inclusions: 0.5 to 5.0 μm
When the particle size of the composite inclusions is less than 0.5 μm, the amount of Mn that can be absorbed from the periphery of the composite inclusions is small, and as a result, it is difficult to form a Mn-deficient layer required for the generation of intragranular ferrite. On the other hand, when the particle size of the composite inclusion is larger than 5.0 μm, the composite inclusion becomes a starting point of fracture.

Number density of composite inclusions: 10 to 100 / mm ²
In order to generate stable intragranular ferrite, it is necessary that at least one of each composite inclusion is contained in the old γ. Therefore, the number density of the composite inclusions is set to 10 pieces / mm ² or more. On the other hand, if the amount of the composite inclusions is excessively large, it tends to be a fracture starting point. Therefore, the number density of the composite inclusions is set to 100 / mm ² or less.

  In the thick steel plate of the present invention, X obtained from the following equation (i) is 0.04 to 9.70.

However, in the above formula (i), the meaning of each symbol is as follows.
Ti_TiO (mass%): Ti content of Ti oxide in total Ti content O (mass%): O content in steel Mn_MnS (mass%): Mn content of MnS in total Mn content R1 (%): average value of MnS area ratio in cross section of composite inclusion R2 (%): average value of MnS ratio at interface of composite inclusion

  In the formula (i), the first term represented by (Ti_TiO / O) represents the balance between the amount of Ti and the amount of O to become a Ti oxide. The first term is calculated by subtracting, from the total Ti content, the Ti amount required for TiN generation calculated from the N content in the steel. The larger the value of the first term, the more easily a Ti oxide is formed. When the value in this section is negative, no Ti oxide is formed.

  In the formula (i), the second term represented by (Mn_MnS) represents the amount of Mn to be MnS. The second term is calculated from the S content in steel. The larger the value of the second term, the more MnS is likely to be compounded.

  In the formula (i), in the third term represented by [(R1 + R2) / 100], R1 represents the average value of the area ratio of MnS in the cross section of the composite inclusion, and R2 represents the MnS content at the interface of the composite inclusion. Indicates the average value of the ratio. The larger the value of the third term, the greater the number of MnS-composite inclusions.

  X obtained from the above formula (i) is a formula showing the easiness of forming a Ti oxide compounding MnS and the degree of MnS compounding of the formed composite inclusion. As the value of X is larger, a composite inclusion in which a larger amount of MnS is formed is formed, and a finer structure is more likely to be formed at the welded portion. As a result, a steel material having excellent toughness is obtained.

  If X obtained from the above formula (i) is less than 0.04, the Ti amount required for forming the Ti oxide, the S amount and Mn amount required for forming MnS, or the proportion of MnS is insufficient. . That is, a state in which inclusions effective for intragranular transformation are not formed. Therefore, from the viewpoint of forming an effective Ti oxide, X needs to be 0.04 or more.

  If the value of X obtained from the formula (i) exceeds 9.70, an excessive amount of Ti oxide is formed, so that the particles are easily aggregated. As a result, coarse inclusions are formed and serve as fracture starting points. In addition, since inclusions of MnS alone are easily formed, intragranular transformation is not promoted. As a result, a coarse microstructure increases, which causes a deterioration in CTOD characteristics. Therefore, X needs to be 9.70 or less.

  From the viewpoint of stably forming a Ti-based oxide, X is preferably 0.50 or more, and more preferably 5.00 or less. Further, from the viewpoint of forming a Ti-based oxide stably and appropriately controlling the ratio of MnS, X is more preferably 1.00 or more, and more preferably 4.00 or less.

  Since the thick steel plate of the present invention has the above-described composite inclusions, even if the plate thickness is 50 mm or more, it is excellent in low-temperature toughness in HAZ. That is, when trying to weld a thick steel plate having a thickness of 50 mm or more with a low number of passes, it is necessary to increase the heat input during welding, but the thick steel plate of the present invention is excellent even when large heat input welding is performed. Low temperature HAZ toughness. The thick steel plate of the present invention has excellent low-temperature HAZ toughness even when the plate thickness is large, but when the plate thickness is large, control of the composite inclusion becomes difficult. As a result, it becomes difficult to manufacture a thick steel plate satisfying the composite inclusion defined in the present invention. Therefore, the thickness of the thick steel plate is preferably set to 100 mm or less.

  In addition, the thick steel plate of the present invention has a yield stress (YP) of 400 to 500 MPa.

  The steel plate of the present invention has a tensile strength (TS) of 600 to 700 MPa.

(C) Manufacturing Method There is no particular limitation on the method for manufacturing a steel plate according to the present invention. For example, a slab having the chemical composition described above is heated, then hot-rolled, and finally cooled. Can be manufactured.

  Hereinafter, the present invention will be described more specifically with reference to Examples, but the present invention is not limited to these Examples.

<Manufacture of rolled base material>
Test No. 1 shown in Table 1 Steels having the chemical compositions of Examples 1 to 31 and Comparative Examples 1 to 13 were melted in an actual manufacturing process. In actual production, before the RH, Ar gas was blown into the molten steel from above, and the slag on the molten steel surface reacted with the molten steel to adjust the total Fe amount in the slag. Here, the flow rate of Ar gas was adjusted to 100 to 200 L / min, and the blowing time was adjusted to 5 to 15 min. Thereafter, the components were adjusted by adding each element by RH, and a 300 mm thick slab was cast by continuous casting. The slab after casting was heated in a heating furnace in a range of 1000 to 1100 ° C. After the heating, rolling was performed at 760 ° C. or more until the thickness reached 2 t with respect to the final finished plate thickness t, and then, rolling was performed at a temperature range of 730 to 750 ° C. until the final finished plate thickness t. After the rolling, the sample was cooled with water at a temperature of −2 to −3 ° C./sec to 200 ° C. or less to prepare a test material.

<Calculation of MnS area ratio in cross section of composite inclusion>
<Calculation of MnS ratio at interface of composite inclusion>
As the test piece for analyzing the composite inclusions, a sample taken from a ｔt portion when the plate thickness of the test material was t was used. For the composite inclusions, the area ratio of MnS and the ratio of MnS at the interface of the composite inclusions were measured from a mapping image obtained by plane analysis of the composite inclusions using an electron probe microanalyzer (EPMA). More specifically, the MnS area ratio was calculated by measuring the cross-sectional area of the entire composite inclusion and the cross-sectional area of the MnS portion occupying the entire composite inclusion from an image. The MnS ratio at the interface of the composite inclusion was calculated by measuring the peripheral length of the Ti oxide in the composite inclusion and the length of the MnS interface in contact with the Ti oxide from an image. In order to reduce the variation in the measurement, the MnS area ratio and the ratio of MnS at the interface of the composite inclusion were determined by analyzing 20 pieces of each test material by EPMA and calculating an average value. Table 1 shows the results.

<Calculation of number density of composite inclusion>
The number of composite inclusions was determined by an automatic inclusion analyzer combining SEM-EDX, and from the shape measurement data of the detected composite inclusions, the composite inclusions having a particle size in the range of 0.5 to 5.0 μm were determined. , The number density was calculated. Table 1 shows the results.

<Tensile test>
A JIS No. 4 tensile test piece was sampled from a 1/4 t portion where the thickness of the prepared test material is t, and a tensile test was performed at room temperature. The yield stress (YP) and tensile strength of the rolled base material (TS) was measured. Table 2 shows the results.

<CTOD test>
A test piece for CTOD test was collected from the prepared test material at n = 3. Each test piece was subjected to groove processing, and multi-layer welding was performed by submerged arc welding (SAW) at a heat input of 5.0 kJ / mm. Notch processing was performed on the HAZ of the created welded joint, and a CTOD test was performed at a test temperature of −40 ° C. in accordance with the BS7448 standard. The quality of the test results was determined based on the following criteria. Of the following criteria, a test piece with a judgment of ◎ or ○ was judged to be acceptable. Table 2 shows the results.
◎: All three test pieces are over-gauge ：: Of the three test pieces, 0 to 2 are over-gauge and not over-gauge, all CTOD values are 0.4 mm or more ×: 3 CTOD value of one or more test specimens is less than 0.4 mm

  Note that gauge over means that the attached clip gauge is fully opened to the limit. Further, the CTOD value of the joint at -40 ° C., which is normally required, is 0.4 mm or more because the CTOD value is 0.4 mm or more.

  In Examples 1 to 31, the results of the CTOD test passed since all the requirements defined in the present invention were satisfied.

  In Example 9, although the CTOD test result passed, the YP and TS were low because the C content was close to the lower limit specified in the present invention.

  In Example 10, although the CTOD test result passed, the YP and TS were low because the Si content was close to the lower limit specified in the present invention.

  In Example 11, although the CTOD test result passed, the YP and TS were low because the Mn content was close to the lower limit specified in the present invention.

  In Example 13, since the S content was close to the lower limit specified in the present invention, the MnS composite amount decreased, and the MnS area ratio in the cross section of the composite inclusion and the MnS ratio in the interface of the composite inclusion decreased. As a result, in the CTOD test, only one test piece did not exceed the gauge.

  In Example 14, since the Ni content was close to the lower limit specified in the present invention, the toughness was reduced. As a result, in the CTOD test, only one test piece did not exceed the gauge.

  In Example 15, the number content of the composite inclusion was low because the Ti content was close to the lower limit specified in the present invention. As a result, in the CTOD test, only one test piece did not exceed the gauge.

  In Example 16, since the O content was close to the lower limit specified in the present invention, the number density of the composite inclusion was low. As a result, in the CTOD test, only one test piece did not exceed the gauge.

  In Example 17, since the C content was close to the upper limit specified in the present invention, the hard structure increased. Therefore, in the CTOD test, all the test pieces were not over-gauge, but the CTOD value was 0.4 mm or more.

  In Example 18, since the Si content was close to the upper limit specified in the present invention, the hard structure was increased. Therefore, in the CTOD test, all the test pieces were not over-gauge, but the CTOD value was 0.4 mm or more.

  In Example 19, segregation occurred because the Mn content was close to the upper limit specified in the present invention. Therefore, in the CTOD test, all the test pieces were not over-gauge, but the CTOD value was 0.4 mm or more.

  In Example 20, since the P content was close to the upper limit specified in the present invention, the toughness was reduced due to segregation. Therefore, in the CTOD test, all the test pieces were not over-gauge, but the CTOD value was 0.4 mm or more.

  In Example 21, since the S content was close to the upper limit specified in the present invention, the toughness was reduced due to segregation. Therefore, in the CTOD test, all the test pieces were not over-gauge, but the CTOD value was 0.4 mm or more.

  In Example 22, since the Ni content was close to the upper limit specified in the present invention, the formation of intragranular transformed ferrite was suppressed and the toughness was reduced. Therefore, in the CTOD test, all the test pieces were not over-gauge, but the CTOD value was 0.4 mm or more.

  In Example 23, since the Ti content was close to the upper limit specified in the present invention, the toughness was reduced due to an increase in carbides such as TiC. Therefore, in the CTOD test, all the test pieces were not over-gauge, but the CTOD value was 0.4 mm or more.

  In Example 24, since the Al content was close to the upper limit specified in the present invention, inclusions serving as nuclei for forming intragranular ferrite were reduced, and as a result, toughness was lowered. Therefore, in the CTOD test, all the test pieces were not over-gauge, but the CTOD value was 0.4 mm or more.

  In Example 25, since the N content was close to the upper limit specified in the present invention, TiN was increased, and as a result, toughness was reduced. Therefore, in the CTOD test, all the test pieces were not over-gauge, but the CTOD value was 0.4 mm or more.

  In Example 26, since the O content was close to the upper limit specified in the present invention, the toughness was reduced due to an increase in coarse oxides. Therefore, in the CTOD test, all the test pieces were not over-gauge, but the CTOD value was 0.4 mm or more.

  In Example 27, since the Cu content was within the range specified in the present invention, the CTOD test result passed, but the toughness was relatively low.

  In Example 28, since the Cr content was within the range specified by the present invention, the CTOD test result passed, but the toughness was relatively low.

  In Example 29, since the Mo content was within the range specified in the present invention, the CTOD test result passed, but the toughness was relatively low.

  In Example 30, since the V content was within the range specified by the present invention, the CTOD test result passed, but the toughness was relatively low.

  In Example 31, since the Nb content was within the range specified in the present invention, the CTOD test result passed, but the toughness was relatively low.

  In Comparative Example 1, since the C content was outside the range specified in the present invention, the hard structure increased, and as a result, the toughness decreased. Therefore, in the CTOD test, there was a test piece having a CTOD value of less than 0.4 mm.

  In Comparative Example 2, since the Si content was outside the range specified in the present invention, the hard structure was increased, and as a result, the toughness was reduced. Therefore, in the CTOD test, there was a test piece having a CTOD value of less than 0.4 mm.

  In Comparative Example 3, since the Mn content was outside the range specified in the present invention, segregation increased, and as a result, toughness decreased. Therefore, in the CTOD test, there was a test piece having a CTOD value of less than 0.4 mm.

  In Comparative Example 4, since the Ni content was outside the range specified in the present invention, the matrix toughness was reduced. Therefore, in the CTOD test, there was a test piece having a CTOD value of less than 0.4 mm.

  In Comparative Example 5, since the Ti content was out of the range specified in the present invention, the toughness decreased due to an increase in coarse TiC. Therefore, in the CTOD test, there was a test piece having a CTOD value of less than 0.4 mm.

In Comparative Example 6, since the Al content was outside the range specified in the present invention, the toughness was lowered due to coarse Al ₂ O ₃ increase. Therefore, in the CTOD test, there was a test piece having a CTOD value of less than 0.4 mm.

  In Comparative Example 7, since the N content was outside the range specified in the present invention, coarse aggregation of TiN occurred, and as a result, the toughness was reduced. Therefore, in the CTOD test, there was a test piece having a CTOD value of less than 0.4 mm.

  In Comparative Example 8, since the O content was outside the range specified in the present invention, coarse oxides increased, and as a result, toughness was reduced. Therefore, in the CTOD test, there was a test piece having a CTOD value of less than 0.4 mm.

  In Comparative Example 9, since the Cu content was outside the range specified in the present invention, the strength was increased, and as a result, the toughness was decreased. Therefore, in the CTOD test, there was a test piece having a CTOD value of less than 0.4 mm.

  In Comparative Example 10, since the Cr content was out of the range specified in the present invention, the strength was increased, and as a result, the toughness was decreased. Therefore, in the CTOD test, there was a test piece having a CTOD value of less than 0.4 mm.

  In Comparative Example 11, since the Mo content was outside the range specified in the present invention, the strength was increased, and as a result, the toughness was decreased. Therefore, in the CTOD test, there was a test piece having a CTOD value of less than 0.4 mm.

  In Comparative Example 12, since the V content was outside the range specified in the present invention, in addition to the increase in strength, a large amount of VC was precipitated. As a result, toughness decreased. Therefore, in the CTOD test, there was a test piece having a CTOD value of less than 0.4 mm.

  In Comparative Example 13, since the Nb content was outside the range specified in the present invention, a large amount of NbC was precipitated, and as a result, the toughness was reduced. Therefore, in the CTOD test, there was a test piece having a CTOD value of less than 0.4 mm.

  ADVANTAGE OF THE INVENTION According to this invention, the thick steel plate excellent in the low temperature toughness of HAZ at the time of large heat input welding can be provided. Therefore, the steel plate of the present invention can be suitably used for a welded structure such as an offshore structure, in particular, a steel plate having a thickness of 50 to 100 mm.

Claims (2)

A steel plate having a thickness of 50 to 100 mm,
Chemical composition in mass%
C: 0.01 to 0.10%,
Si: 0.10 to 0.25%,
Mn: 1.00 to 2.50%,
P: 0.0100% or less,
S: 0.0010 to 0.0100%,
Ti: 0.005 to 0.030%,
Al: 0.003% or less,
Ni: 0.50 to 1.50%,
O: 0.0010 to 0.0050%,
N: 0.0100% or less,
Cu: 0 to 0.50%,
Cr: 0 to 0.50%,
Mo: 0 to 0.50%,
V: 0 to 0.10%,
Nb: 0 to 0.05%, and
The balance: Fe and impurities, and
In the steel, including a composite inclusion in which MnS exists around the Ti oxide,
An area ratio of the MnS in a cross section of the composite inclusion is 10% or more and less than 90%,
The ratio of the MnS at the interface of the composite inclusion is 10% or more;
The number density of the composite inclusions having a particle size of 0.5 to 5.0 μm is 10 to 100 / mm ² , and
A thick steel plate wherein X obtained from the following formula (i) is 0.04 to 9.70.

However, in the above formula (i), the meaning of each symbol is as follows.
Ti_TiO (mass%): Ti content of Ti oxide in total Ti content O (mass%): O content in steel Mn_MnS (mass%): Mn content of MnS in total Mn content R1 (%): average value of MnS area ratio in cross section of composite inclusion R2 (%): average value of MnS ratio at interface of composite inclusion The chemical composition is expressed in mass%;
Cu: 0.25 to 0.5%,
Cr: 0.01-0.5%,
Mo: 0.01-0.5%,
V: 0.03 to 0.1%, and
Nb: 0.01-0.05%,
The thick steel plate according to claim 1, comprising at least one selected from the group consisting of: