JP5818343B2 - Thick steel plate with excellent toughness in weld heat affected zone - Google Patents

Description

  The present invention relates to a thick steel plate mainly used for a building structure, and more specifically, a welding heat-affected zone (hereinafter referred to as “welding heat affected zone”) when welding is performed with a large heat input such that the heat input is 100 kJ / mm or more. , Also referred to as HAZ).

  In recent years, the characteristics required for steel materials used for construction structures and the like tend to become increasingly severe with the increase in the height and size of buildings. In particular, among the many required properties, the toughness requirements tend to be stricter.

  Steel materials used for construction structures and the like are often welded for joining, but HAZ formed under the influence of heat particularly during welding joining has a problem that toughness tends to deteriorate. This deterioration in HAZ toughness tends to appear more prominently as the heat input during welding becomes larger. Therefore, it is considered that the heat input during welding should be suppressed as much as possible in order to improve the HAZ toughness. However, for steel materials used for construction structures, from the viewpoint of improving the work efficiency of welding, conversely, high heat input welding, particularly high heat input welding in which the heat input is 100 kJ / mm or more is required. It tends to be.

  As a representative example of the technology for increasing the HAZ toughness when performing high heat input welding, there is a high Ni steel plate. For example, Patent Document 1 improves the strength and toughness by adding about 2% Ni. Steel plates are disclosed. However, since Ni is an extremely expensive and difficult to obtain element, it is not preferable to increase the amount of Ni to be added, and it is preferable to keep the amount of Ni added as low as possible industrially. .

  Against such a background, as described in Patent Document 1, the Ni addition amount is made as low as possible without increasing the Ni content, for example, the Ni addition amount is 1.5% or less, and the HAZ toughness is increased. Various techniques have been studied. The HAZ during high heat input welding is held in the high temperature austenite (γ) region for a long time by heat input and then gradually cooled, so in the process of γ grain growth and slow cooling during the high temperature holding in the above heat input. Coarse ferrite (α) grains are generated and the structure is likely to be coarsened, which causes a reduction in HAZ toughness during high heat input welding. For these reasons, it is necessary to suppress the coarsening of the structure during high heat input welding, and it is necessary to develop a technology that stably maintains the HAZ toughness during high heat input welding at a high level. .

  The main means for securing the HAZ toughness by suppressing the coarsening of the structure during high heat input welding are γ grain growth pinning with inclusion particles such as oxides, nitrides and sulfides, inclusion particles Techniques relating to the refinement of the structure by intragranular α generation starting from the above have been proposed. As a proposal example of such a technique, there are techniques described in Patent Documents 2 to 4. These Patent Documents 2 to 4 suppress the coarsening of austenite grains generated in HAZ during high heat input welding by dispersing and precipitating fine Ti-containing nitrides in steel materials and acting as γ grain growth pinning particles. However, it is disclosed to suppress the deterioration of the HAZ toughness. However, the amount of Ti-containing nitride disappears when welding is performed with a large heat input as required in recent years, and the number density may not be sufficiently secured, and stable HAZ toughness is obtained. It is not possible.

  On the other hand, Patent Documents 5 to 7 propose techniques for using oxide inclusions that are stable at high temperatures as γ-growth pinning particles. However, since oxide inclusions have a smaller number density than Ti-containing nitrides and it is difficult to obtain a sufficient pinning effect, the oxide inclusions have a large heat input such that the heat input is 100 kJ / mm or more. It is not possible to cope with welding and further improvement is required.

  That is, although it is described in Patent Documents 5 and 6 that good HAZ characteristics can be obtained by the presence of an oxide containing Ti-REM-Ca-Al-based oxide or REM or Zr, it is assumed. The amount of heat input remains at a low level, and it cannot be said that good HAZ characteristics can be obtained by high heat input welding with a heat input of 100 kJ / mm or more. Further, Patent Document 7 describes a technique using an oxide containing REM or Zr as in Patent Document 6, but the assumed heat input amount remains at a low level as described above, and is 100 kJ / It cannot be said that good HAZ characteristics can be obtained by high heat input welding with a thickness of at least mm.

  Further, Patent Document 8 describes a technique for obtaining high HAZ toughness by using both oxide inclusions and Ti-containing inclusions as γ grain growth pinning particles. However, in view of the recent trend of increasing heat input, there is a limit to the use of γ grain growth pinning mainly of Ti-containing inclusions, and means for improving HAZ toughness with large heat input by oxide inclusions. It can be said that it needs to be established immediately.

  In addition, as a technique for acting as an origin of intragranular α generation starting from oxide inclusion particles, a technique using a composite oxide containing Ti and REM described in Patent Document 9 and MnS has been proposed. In addition, the inventors have previously proposed a technique of promoting intragranular α production by controlling the inclusion shape in Patent Document 10. These techniques are constructed on the premise that inclusions with low interfacial energy are effective for intragranular α production (intragranular α / inclusions). However, in the technique described in Patent Document 9, the assumed heat input amount is small in the first place, and the large heat input HAZ toughness has not been sufficiently ensured.

  In addition, steel materials used for construction structures and the like are required to have excellent strength, low yield ratio, and base material toughness in addition to the requirement for excellent HAZ toughness. In recent years, with the increase in the height and size of buildings, there has been an increase in the use of high-tensile steel materials as steel materials for buildings.

  For example, Patent Document 11 proposes a technique for realizing a low yield ratio in a steel sheet having a tensile strength of 590 MPa or more by dispersing fine carbonitrides and securing a certain amount or more of ferrite. However, the technique according to Patent Document 11 is not a technique that pays attention to the HAZ toughness when high heat input welding is performed such that the heat input amount is 100 kJ / mm or more. Patent Document 12 also proposes a technique for realizing a low yield ratio in a steel sheet having a tensile strength of 590 MPa or more by dispersing oxide inclusions and securing a certain amount or more of ferrite. The target heat input is small.

  As explained above, while ensuring the HAZ toughness when large heat input welding is performed such that the heat input amount is 100 kJ / mm or more, the strength and toughness of the base material are improved, and further a low yield ratio is realized. As for the technology, it has not been developed yet.

JP 2006-118007 A JP 2001-98340 A JP 2004-2181010 A JP-A-61-253344 Japanese Patent Laid-Open No. 2001-20031 Japanese Patent Laid-Open No. 2007-1001000 JP 2007-247005 A JP 2008-223062 A Japanese Patent Laid-Open No. 7-252586 JP 2008-223081 A Japanese Patent No. 2901890 JP 2007-247004 A

  The present invention has been made in view of the above-described conventional situation. Even when high heat input welding is performed such that the heat input is 100 kJ / mm or more, the HAZ toughness is excellent and the tensile strength is 490 MPa. It is an object of the present invention to provide a thick steel plate that can realize a low yield ratio in the above-described high strength region and that can also ensure good base metal toughness and has excellent toughness of a weld heat affected zone.

Invention of Claim 1 is the mass%, C: 0.02-0.15%, Si: 0.02-0.46% , Mn: 1.0-2.0%, P: 0.03 %: Not including 0%, S: not exceeding 0.015% (not including 0%), Al: not exceeding 0.05% (not including 0%), Ti: 0.010 to 0.080% Ca: 0.0005-0.010%, N: 0.002-0.020%, REM: 0.0001-0.02% and / or Zr: 0.0001-0.02%, The balance is thick steel plate with iron and inevitable impurities, and the constituent elements excluding oxygen are 10% <Ti, Al <20%, 5% <Ca <40%, 5% <REM <50 in mass%. % And / or 5% <Zr <40%, and among the oxides, an acid having an equivalent circle diameter of less than 2 μm Product is 200 / mm ² or more, the oxide 30 to 70 pieces / mm ² of less than the circle equivalent diameter of 2μm or more 5 [mu] m, with a circle equivalent diameter of oxide over 5 [mu] m is present less than 30 / mm ^2, Ti The weld heat-affected zone is characterized in that a nitride-containing nitride is contained, and among the Ti-containing nitrides, Ti-containing nitrides having an equivalent circle diameter of 100 nm or less are present at 5 × 10 ⁶ pieces / mm ² or more. It is a thick steel plate with excellent toughness.
Invention of Claim 2 is the mass%, C: 0.02-0.15%, Si: 0.46% or less (0% is not included), Mn: 1.0-2.0%, P : 0.03% or less (not including 0%), S: 0.015% or less (not including 0%), Al: 0.05% or less (not including 0%), Ti: 0.010 0.080%, Ca: 0.0005-0.010%, N: 0.002-0.020%, REM: 0.0001-0.02% and / or Zr: 0.0001-0.02% And at least one selected from the group consisting of Ni: 1.50% or less, Cu: 1.50% or less, Cr: 1.50% or less, and Mo: 1.50% or less. In which the balance is iron and unavoidable impurities and the constituent elements excluding oxygen are 10% <Ti, Al < 0%, 5% <Ca <40%, 5% <REM <50% and / or 5% <Zr <40%, and an equivalent circle diameter of 2 μm. oxides of 200 / mm ² or more and less than oxides of less than the circle equivalent diameter of 2μm or more 5μm 30-70 pieces / mm ^2, an equivalent circle diameter of more oxide 5μm is present less than 30 / mm ² And a Ti-containing nitride, and among the Ti-containing nitrides, there are 5 × 10 ⁶ pieces / mm ² or more of Ti-containing nitrides having an equivalent circle diameter of 100 nm or less. It is a thick steel plate with excellent toughness of the affected part.

  In addition, including the above description, the equivalent circle diameter described in the present invention refers to the diameter of a circle that is assumed to have an equal area by paying attention to the size of an oxide or the like. It can be obtained by observing with an electron microscope (TEM) or a scanning electron microscope (SEM) and analyzing the image.

The invention according to claim 3 is further selected from the group consisting of Ni: 1.50% or less, Cu: 1.50% or less, Cr: 1.50% or less, and Mo: 1.50% or less by mass%. It is a thick steel plate excellent in the toughness of the welding heat affected zone of Claim 1 characterized by including 1 or more types.

Invention of claim 4, further containing, by mass%, Nb: 0.10% or less and / or V: according to any one of claims 1 to 3, characterized in that it contains 0.10% It is a thick steel plate with excellent toughness of the weld heat affected zone.

The invention according to claim 5 further comprises, in mass%, B: 0.005% or less, and the thickness excellent in toughness of the weld heat affected zone according to any one of claims 1 to 4 It is a steel plate.

  According to the steel plate of the present invention, the chemical composition is appropriately controlled, an oxide having a predetermined chemical composition is dispersed in an appropriate amount for each size, and more than a predetermined amount of fine Ti-containing nitride is secured. Therefore, even when high heat input welding is performed in which the heat input is 100 kJ / mm or more, good HAZ toughness can be ensured. Further, a low yield ratio can be realized in a high strength region where the tensile strength is 490 MPa or more, and good base material toughness can be secured. Therefore, the thick steel plate of the present invention can be effectively used as a steel material for high-rise buildings and the like for which high heat input welding is required such that the heat input is 100 kJ / mm or more.

  In developing a steel plate with excellent weld heat-affected zone toughness when welding with a large heat input such that the heat input is 100 kJ / mm or more, The inventors focused on the fact that high HAZ toughness can be ensured by oxide inclusions (hereinafter also simply referred to as oxides), and studied from various angles.

  This technology for improving the HAZ toughness of thick steel plates using oxides has advantages and disadvantages. The advantage is that the HAZ toughness is improved by reducing the HAZ structure by making the oxide an intragranular α formation starting point, and conversely, the disadvantage is that the HAZ toughness is adversely affected by the oxide itself becoming the starting point of fracture. It is to affect.

  Conventionally, from this point of view, when an oxide is used as the starting point of intragranular α formation, the equivalent circle diameter is mainly composed of an oxide having an equivalent circle diameter of less than 2 μm, and oxides larger than that are reduced as much as possible. The development of thick steel plates was undertaken with technical ideas.

  However, according to recent research and development, it was theoretically confirmed that the larger the size of the oxide introduced into the thick steel plate, the higher the ability to produce intra-granular α, from the viewpoint of the origin of intra-granular α formation. It has been theoretically confirmed and reported that the upper limit of the size of the oxide (coarse second phase) for ensuring toughness is relaxed as the toughness evaluation temperature is higher.

  From such research and development results, the inventors relaxed the upper limit of the size of the oxide that adversely affects the HAZ toughness in the case of a thick steel plate used for a building structure whose toughness evaluation temperature is relatively high. In the above, following the conventional technical idea, that is, the oxide dispersed inside the thick steel plate is mainly composed of an oxide having an equivalent circle diameter of less than 2 μm as in the conventional case, and has an intragranular α-forming ability. Considering that the introduction of a high oxide having a relatively large size can refine the HAZ structure and improve the HAZ toughness.

  Next, the inventors further studied from the viewpoint of a low yield ratio of the base material. As described above, the oxide can contribute to the improvement of HAZ toughness by being the starting point of intragranular α formation. Moreover, it can be said that by increasing the number of nucleation sites by introducing an oxide, the base material undergoes a bainite transformation at a relatively high temperature as compared with the case where no oxide is introduced, so that the yield ratio is relatively low. On the other hand, there is also a problem that the oxide itself becomes a starting point of destruction and the base material toughness is lowered. Moreover, it is known that the defect becomes larger as the strength of the base material increases, and it can be said that it is important not to lower the base material toughness when introducing an oxide into high strength steel.

  As means for improving the base material toughness, it is generally considered effective to refine the steel structure by devising heat treatment of the steel. However, heat treatment makes the process complicated and is not an industrially preferable means. Therefore, the inventors considered that it is effective to improve the toughness of the base metal by promoting the generation of intra-granular α by introducing an oxide into the steel material, similarly to the improvement of the HAZ toughness.

  However, the parent material structure has a smaller prior γ grain size compared to the HAZ structure. Since the size of the structure is always determined by the balance between α production from within the grains and α production from the old γ grain boundaries, the fact that the old γ grain size of the matrix structure is small means that the power of α production from the grain boundaries It can be said that the direction is relatively large. For this reason, the effect of refining the structure due to intragranular α formation is less likely to be exhibited in the base material compared to HAZ. Therefore, it can be said that it is necessary to more appropriately control the composition and size of the oxide to be introduced in order to promote the formation of intra-granular α by controlling oxide inclusions and to refine the matrix structure.

The inventors tried to control the oxide from the viewpoint of improving the HAZ toughness and securing the properties of the base material based on the technical ideas examined from various angles as described above. As a result, the constituent elements excluding oxygen are 10% <Ti, Al <20%, 5% <Ca <40%, 5% <REM <50% and / or 5% <Zr <40% in mass%. A satisfactory oxide is introduced, and among these oxides, an oxide having an equivalent circle diameter of less than 2 μm is 200 / mm ² or more, and an oxide having an equivalent circle diameter of 2 μm or more and less than 5 μm is 30 to 70 / mm ^2, an equivalent circle diameter of 30 pieces / mm less than ² an oxide of more than 5 [mu] m, by so as to present each found that HAZ structure refining effect by intragranular α generation is remarkably expressed, more It was confirmed that excellent HAZ toughness was obtained.

Among the oxides, there are less than 200 oxides / mm ^{2 with an} equivalent circle diameter of less than 2 μm and / or less than 30 oxides / mm ^{2 with an} equivalent circle diameter of 2 μm or more and less than 5 μm. In this case, the effect of refining the HAZ structure due to intragranular α generation is not sufficiently exhibited. For oxides with an equivalent circle diameter of less than 2 μm, it is recommended that the number is preferably 220 / mm ² or more, more preferably 250 / mm ² or more. Further, it is recommended that the oxide having an equivalent circle diameter of 2 μm or more and less than 5 μm is preferably 50 / mm ² or more, more preferably 60 / mm ² or more.

  In the oxide, the mass of Ti, Al, Ca, REM, and Zr in the oxide is 10% <Ti, Al <20%, 5% <Ca <with respect to the total mass of the constituent elements excluding oxygen. It may be in the range of 40%, 5% <REM <50% and / or 5% <Zr <40%. Constituent elements excluding oxygen are allowed to contain elements such as Si and Mn in addition to elements of Ti, Al, Ca, REM, and Zr. Further, this oxide usually exists in the form of a complex oxide containing an element other than oxygen described above.

  Among these oxides, oxides having a mass ratio of Ti and Ca of more than 1 and less than 1.4 are partly in a liquid phase during high-temperature heating of HAZ, and in the intragranular α in the subsequent cooling process. Since it has a crystal structure advantageous for generation and crystallizes, in addition to reducing (intragranular α / γ) interfacial energy, it is possible to realize a lower (intragranular α / inclusion) interfacial energy. It can be said that the production of α is particularly effective because it is actively promoted.

  As described above, since the thick steel plate of the present invention contains an appropriate number (number density) of oxides whose component composition is appropriately controlled for each size, the HAZ toughness is sufficiently enhanced. Yes.

  In addition to the control of the oxide, the inventors also need to contain a suitable number (number density) of fine Ti-containing nitride in the thick steel plate. By such Ti-containing nitride, It has been found that the old γ grain growth of HAZ can be pinned. The Ti-containing nitride defined in the present invention includes not only TiN but also a part of Ti of TiN, specifically, an element having an atomic ratio of 50% or less in place of Ti and other nitride-forming elements. Those substituted with (Nb, Zr, V, etc.) are also included.

  This Ti-containing nitride is very fine as compared with the oxide, and since it can be finely dispersed in the steel before heat input by welding, even if the heat input is 100 kJ / mm or more Even if much of the Ti-containing nitride disappears due to such high heat input welding, a sufficient number of Ti-containing nitrides can still remain. In this way, a sufficient number of Ti-containing nitrides can still remain after high heat input welding, so that pinning of the old γ grain growth of HAZ can be effectively exhibited.

In the present invention, in order to ensure HAZ toughness, the number density of fine Ti-containing nitrides having a circle-equivalent diameter of 100 nm or less introduced into the steel material is set to 5 × 10 ⁶ pieces / mm ² or more. If it is less than 5 × 10 ⁶ particles / mm ² , the pinning effect of old γ grain growth cannot be exhibited effectively.

The preferable number density of the fine Ti-containing nitride having an equivalent circle diameter of 100 nm or less introduced into the steel material is 5.4 × 10 ⁶ pieces / mm ² or more, and the more preferable number density is 6 × 10 ^6. Pieces / mm ² or more. In the present invention, the lower limit of the equivalent circle diameter of the fine Ti-containing nitride introduced into the steel material is not particularly defined, but from the measurement limit of the transmission electron microscope (TEM) for performing the measurement, the lower limit of the actual equivalent circle diameter is determined. Can be said to be about 10 nm.

<Manufacturing requirements>
The thick steel plate of the present invention that satisfies the above requirements, in particular, the constituent elements excluding oxygen are 10% <Ti, Al <20%, 5% <Ca <40%, 5% <REM <50% in mass%. And / or contains an oxide satisfying 5% <Zr <40%, and among the oxides, the number of equivalent circle diameters is 200 / mm ² or more and the equivalent circle diameter is 2 μm or more. oxides of less than 5μm 30-70 pieces / mm ^2, a circle with equivalent diameter more oxide 5μm is present less than 30 / mm ^2, an equivalent circle diameter of less Ti-containing nitride 100nm is 5 × 10 ⁶ In order to manufacture a thick steel plate having a number of pieces / mm ² or more, it is preferable to manufacture a thick steel plate by appropriately controlling manufacturing requirements during melting and casting. Hereinafter, the manufacturing requirements will be described in detail for each item.

(Amount of dissolved oxygen in molten steel at the time of melting)
At the time of melting, the amount of dissolved oxygen in the molten steel is set in a range of 0.002 to 0.01% by mass deoxidation using Mn and, if necessary, further Si. When the amount of dissolved oxygen in the molten steel is less than 0.002%, a necessary amount of oxide having an appropriate composition that becomes a starting point of intragranular α formation cannot be secured. On the other hand, when the amount of dissolved oxygen exceeds 0.01%, a coarse oxide having an equivalent circle diameter of 2 μm or more is formed more than necessary, which adversely affects the HAZ toughness. The preferable lower limit of the dissolved oxygen amount is 0.0025%, and the preferable upper limit is 0.008%.

(Ti, REM, Zr addition amount)
In order to secure an oxide having an equivalent circle diameter of 2 μm or more and less than 5 μm to a predetermined level or more, the addition amount of Ti, REM, and Zr, which are relatively difficult to control among oxide forming elements, and the balance of the addition amount are appropriately adjusted. It is preferable. The addition amount of Ti is over 160 ppm, the total addition amount of REM and Zr is over 55 ppm, and the addition amount balance is 0.8 or more, which is obtained from the formula [Ti] / [REM] + [Zr]. What is necessary is just to adjust so that it may become less than 11.8.

  However, in the above formula, [] is a value representing the amount of each element added in mass%. [Ti], [REM], and [Zr] do not necessarily match the Ti amount, REM amount, and Zr amount in the finally obtained steel material. This is because these elements may evaporate during the production or may be contained and removed in the slag.

(Order of addition of oxide-forming elements)
The oxide forming elements of Al, Ti, REM, Zr, and Ca need to be added in the order of Al → Ti → (REM, Zr →) Ca. If each element is added in an order other than the addition order, a necessary amount of oxide having a suitable composition that is the starting point of intragranular α formation cannot be secured. In particular, since Ca has a characteristic that it has a very strong deoxidizing power, if it is added prior to Ti or Al, all of the oxygen associated with Ti or Al may be lost. Cannot be formed. In addition, when adding both REM and Zr, whichever may be added first, you may add simultaneously.

(Time t1 from Ti addition to Ca addition)
Time t1 (min) from Ti addition to Ca addition is 3 to 20 minutes. When the time t1 (min) from the addition of Ti to the addition of Ca is shorter than 3 minutes, the oxide formation by the element added prior to the addition of Ca does not proceed sufficiently, and an appropriate composition that is the starting point of intragranular α formation The required amount of oxides having the above cannot be secured. If this time t1 (minute) is longer than 20 minutes, the formation of oxide excessively progresses until Ca is added, and the oxide composition does not become the desired one, and becomes the starting point of intragranular α formation. The required amount of oxide having an appropriate composition cannot be secured. In addition, the preferable minimum of time t1 (min) from Ti addition to Ca addition is 5 minutes, and a preferable upper limit is 15 minutes.

(Time t2 from Ca addition to casting start)
The time t2 (min) from the addition of Ca to the start of casting satisfies ta and tb obtained from the following formulas (1) and (2), that is, ta (min) <t2 (min) <tb (min). Time.

ta = 4-10 [Ca] / ([Ti] +2 [Al] +5 [REM] +2 [Zr] +0.01 Formula (1)
tb = 25-40 [Ca] / ([Ti] +2 [Al] +5 [REM] +2 [Zr] +0.01 Formula (2)
However, in the above-described formulas (1) and (2), [] is a value representing the addition amount of each element or the like in mass%. [Ca], [Ti], [Al], [REM], and [Zr] do not necessarily match the Ca amount, Ti amount, Al amount, REM amount, and Zr amount in the steel material finally obtained. . This is because these elements may evaporate during the production or may be contained and removed in the slag.

  The time t2 (min) from the addition of Ca to the start of casting is the time required for Ca to form a desired oxide by depriving oxygen from other oxides generated before the addition of Ca. This time t2 When (min) is less than ta (min), the formation reaction of Ca-containing oxide after Ca addition does not proceed sufficiently, and a necessary amount of oxide having an appropriate composition that can serve as a starting point for intragranular α formation is secured. become unable. Moreover, when this time t2 (min) becomes tb (min) or more, the formation reaction of the Ca-containing oxide after Ca addition proceeds excessively, and an oxide having an appropriate composition that becomes the starting point of intragranular α formation is obtained. The necessary amount cannot be secured. Thus, t2 (min) needs to satisfy ta (min) <t2 (min) <tb (min), but from the viewpoint of productivity, it should be as short as possible within this range. Is desirable. The expressions (1) and (2) for determining ta and tb are expressions obtained by repeating many experiments in consideration of the oxide forming ability of each element.

The above is the condition of the time t2 (min) from the addition of Ca for securing the required amount of oxide to the start of casting. In the present invention, 5 × 10 ⁶ Ti-containing nitrides having an equivalent circle diameter of 100 nm or less / Mm ² or more needs to be secured. For this purpose, the relationship between tx (min) obtained from the following formula (3) and t2 (min), ta (min), and tb (min) is ta (min) <tx (min) <t2 (min ) <Tb (minutes) must be satisfied.

tx = 6-{[Si] / ([Ti] +2 [Al] +5 [Ca] +5 [REM] +2 [Zr] +0.01)} Expression (3)
However, in the above-described formula (3), [] is a value representing the amount of each element added in mass%.

  When Si is added, it is added before deoxidation, and is in an oxidized state immediately before deoxidation. In order to secure a predetermined amount or more of solid solution Si, it is necessary to reduce oxidized Si for a sufficient time with a strong deoxidizing element having an affinity for oxygen larger than that of Si. Therefore, tx (min) <t2 ( Minutes). On the other hand, since it is necessary to secure a predetermined amount of oxide appropriately adjusted so as to be the starting point of intra-granular α, it is necessary to make the time required for the reduction of the Si oxide shorter than the predetermined time, t2 ( Min) <tb (min).

(Cooling time t3 during casting)
The cooling time t3 (seconds) at 1500 to 1450 ° C. during casting is 300 seconds or less. When the cooling time t3 (seconds) exceeds 300 seconds, the amount of coarse oxides having an equivalent circle diameter of 5 μm or more increases, and the HAZ toughness deteriorates. The cooling time t3 (min) during casting is preferably 280 seconds or less. The lower limit of t3 (min) is not particularly limited, but usually about 190 seconds are necessary.

(Addition amount of oxide-forming elements)
Among the oxide-forming elements, the amount of Ca added (mass%), that is, [Ca] is between A value and B value obtained from the following formulas (4) and (5), that is, A ≦ [Ca] ≦ B. The amount of addition is satisfactory (mass%). The following formulas (4) and (5) for calculating the A value and the B value are formulas obtained by repeating many experiments.

A = 2.25 × [Of] (4)
B = [Of] × [Ti] / (0.25 [REM] +0.12 [Zr]) (5)
However, in the above formulas (4) and (5), [Of] is the amount of dissolved oxygen (mass%) before addition of Ca, and [Ti], [REM], and [Zr] are additions of each element to the molten steel, respectively. Amount (% by mass).

  If the amount of Ca added is less than the value A, most of the added Ca will be consumed as a single oxide of Ca, and it will not be possible to secure an oxide having an appropriate composition to be the starting point for intragranular α formation. . On the other hand, when the Ca addition amount exceeds the B value, the ratio (mass%) of Ti in the oxide becomes small, and in this case as well, an oxide having an appropriate composition for starting the intragranular α formation can be secured. Disappear.

(Adjustment of Si addition amount and total addition amount of elements with higher deoxidation ability than Si)
In the present invention, it is necessary to secure 5 × 10 ⁶ pieces / mm ² or more of Ti-containing nitride having an equivalent circle diameter of 100 nm or less. For this purpose, the amount of solute Si that increases the activity of Ti is contained in the steel material. It is preferable to ensure sufficiently. As described above, when Si is added, it is added before deoxidation by Ca, Ti, Al, REM, and Zr, and is oxidized by a certain amount in the steelmaking process. In order to secure in steel, it is necessary to make the total addition amount of elements having higher deoxidation capacity than Si, that is, Al, Ti, Ca, REM, Zr, 0.020% or more by mass%. It is.

  When Si is added, a preferable addition amount is 0.02% or more by mass%, and a more preferable addition amount is 0.10%. On the other hand, the preferable total addition amount of Al, Ti, Ca, REM, and Zr is 0.025% or more, and the more preferable total addition amount is 0.030% or more.

  In manufacturing the steel plate of the present invention, it is preferable to manufacture the steel plate by appropriately controlling the manufacturing requirements at the time of melting and casting described above, but the steel plate was obtained after melting and casting. It is recommended that the slab be heated and hot-rolled, then quenched, and further heated in the austenite / ferrite two-phase region before quenching and tempering.

  The quenching after the hot rolling may be performed by off-line quenching (RQ) using a hot-rolled material in addition to direct quenching (DQ) in which quenching is performed immediately after hot rolling. In the case of the DQ process, since the process cannot be performed again, stricter temperature control of the quenching start temperature is required than in the case of the RQ process.

  In addition, after heating to the austenite / ferrite two-phase region and quenching such as RQ treatment, it is preferable to adjust the strength of the steel material by tempering at a temperature equal to or lower than the ferrite transformation start temperature (Ac1) if necessary. .

<Chemical component composition>
Next, the chemical component composition in the thick steel plate of the present invention will be described. The steel plate of the present invention can improve the HAZ toughness and the base metal toughness by the oxide and Ti-containing nitride described above, but in addition to this, the content of each chemical component (element) is appropriately adjusted. As a result, the high HAZ toughness and base metal toughness targeted by the present invention can be achieved, and a low yield ratio can be achieved. Therefore, in the thick steel plate of the present invention, it is also a requirement that the content of each chemical component is within the range described below. Among these chemical components, the contents of Al, Ca, Ti, REM, and Zr constituting the oxide include the amount constituting the oxide, as is apparent from the effects thereof. In addition, all the content (%) of the following chemical component shows the mass%.

C: 0.02-0.15%
C is an essential element for ensuring the strength of the steel sheet. If the C content is less than 0.02%, the required strength cannot be ensured. A preferable lower limit of the C content is 0.03%, and a more preferable lower limit is 0.04%. On the other hand, when the C content is excessive, a large amount of hard island martensite (MA) is generated, leading to deterioration of the toughness of the base material. Therefore, the C content needs to be 0.15% or less. The upper limit with preferable content of C is 0.12%, and a more preferable upper limit is 0.10%.

Si: 0.46% or less (including 0%)
Si is an element useful for securing strength by solid solution strengthening, and is also an element useful for securing the number density of Ti-containing nitrides. Further, depending on the strength class of the steel sheet, it is an element useful for securing the α phase. However, if the Si content is excessive, the α phase is excessively introduced and the required tensile strength (TS) cannot be ensured. Therefore, the upper limit of the Si content is 0.46%. Moreover, a preferable upper limit is 0.42%, a more preferable upper limit is 0.35%, and a more preferable upper limit is 0.25%. In the present invention, the lower limit of the Si content is not particularly specified, but Si may be an essential element depending on the thickness of the target thick steel plate. That is, when the ingot size is large, the cooling rate is slow, so it is possible to ensure the number density of the Ti-containing nitride without adding Si, but when the ingot size is small, the cooling rate is low. Therefore, it is necessary to secure the number density of the Ti-containing nitride by increasing the Ti activity by adding Si. In that case, the preferable lower limit of the Si content is 0.01%. A more preferred lower limit of the Si content is 0.05%, and a more preferred lower limit is 0.08%.

Mn: 1.0-2.0%
Mn is an element useful for securing the strength of the steel sheet, and it is necessary to contain Mn in an amount of 1.0% or more in order to effectively exhibit such effects. A preferable lower limit of the Mn content is 1.3%, and a more preferable lower limit is 1.4%. On the other hand, if the content exceeds 2.0%, the HAZ strength increases excessively and the toughness deteriorates, so the Mn content is set to 2.0% or less. A preferable upper limit of the Mn content is 1.8%, and a more preferable upper limit is 1.6%.

P: 0.03% or less (excluding 0%)
Since P is an impurity element that easily causes grain boundary fracture and adversely affects toughness, its content is preferably as small as possible. From the viewpoint of ensuring the toughness of the base material and the HAZ, the P content needs to be suppressed to 0.03% or less, preferably 0.02% or less, more preferably 0.01% or less. The lower limit of the P content is not particularly defined, but it is difficult to make P in steel 0% industrially.

S: 0.015% or less (excluding 0%)
Since S is an element that forms Mn sulfide and degrades the toughness of the base material, its content is preferably as small as possible. From the viewpoint of securing the base material toughness, the S content needs to be suppressed to 0.015% or less, preferably 0.010% or less, more preferably 0.008% or less. The lower limit of the S content is not particularly specified, but it is difficult to industrially make S in steel 0%.

Al: 0.05% or less (excluding 0%)
Al is an element useful for forming an oxide effective for the generation of intra-grain α by being added prior to Ti, Ca, REM, and Zr. In order to exhibit such an effect effectively, it is necessary to contain 0.003% or more preferably, and more preferably 0.010% or more. However, if the content is excessive, a coarse oxide is generated and the toughness of the base material and the HAZ deteriorates, so it is necessary to keep it to 0.05% or less. The upper limit with preferable Al content is 0.04%, and a more preferable upper limit is 0.03%.

Ti: 0.010 to 0.080%
Ti is an element that contributes to the improvement of HAZ toughness by forming an oxide effective for generating intra-granular α by adding prior to Ca, REM, and Zr after addition of Al. In order to exhibit such an effect effectively, it is necessary to make it contain 0.010% or more. A preferable lower limit of the Ti content is 0.12%, and a more preferable lower limit is 0.15%. On the other hand, if the Ti content is excessive, a large amount of coarse oxide is generated and the HAZ toughness is deteriorated, so it is necessary to keep it to 0.080% or less. The upper limit with preferable Ti content is 0.060%, and a more preferable upper limit is 0.040%.

Ca: 0.0005 to 0.010%
Ca is an element that contributes to the improvement of HAZ toughness by forming an oxide effective for the generation of intra-granular α by adding 3 to 20 minutes after the addition of Ti. In order to exhibit such an effect effectively, it is necessary to contain 0.0005% or more. A preferable lower limit of the Ca content is 0.0008%, and a more preferable lower limit is 0.0010%. On the other hand, if the Ca content is excessive, coarse oxides are produced and the toughness of the base material and the HAZ deteriorates, so it is necessary to keep it to 0.010% or less. The upper limit with preferable Ca content is 0.008%, and a more preferable upper limit is 0.007%.

N: 0.002 to 0.020%
N is an element useful for securing the toughness of the base material and the HAZ by forming a Ti-containing nitride that remains undissolved at a high temperature. In order to exhibit such an effect effectively, it is necessary to contain 0.002% or more. A preferable lower limit of the N content is 0.003%, and a more preferable lower limit is 0.004%. On the other hand, if the N content is excessive, the solid solution N amount increases and the toughness of the base metal and the HAZ deteriorates due to strain aging, so it is necessary to keep it to 0.020% or less. A preferable upper limit of the N content is 0.018%, and a more preferable upper limit is 0.013%.

REM: 0.0001-0.02% and / or Zr: 0.0001-0.02%
REM (rare earth element) and Zr are elements that contribute to the improvement of HAZ toughness by forming an oxide effective for the generation of intragranular α by adding Ti prior to the addition of Ca. These effects increase as the content thereof increases, but in order to effectively exhibit these effects, it is necessary to contain 0.0001% or more of all. A preferable lower limit of the content of REM and Zr is 0.0005%, and a more preferable lower limit is 0.0010%. On the other hand, if these elements are contained excessively, the oxide becomes coarse and deteriorates the toughness of the base material and the HAZ, so both of them must be suppressed to 0.02% or less. A preferable upper limit of these contents is 0.015%, and a more preferable upper limit is 0.01%. Note that REM (rare earth element) refers to lanthanoid elements (15 elements from La to Ln in the periodic table), Sc and Y.

  The above are the essential elements specified in the present invention, and the balance is iron and inevitable impurities. As an inevitable impurity, mixing of elements such as Sn, As, and Pb brought in depending on the situation of raw materials, materials, manufacturing equipment, etc. is allowed. Further, it is also effective to positively contain the following elements, and the characteristics of the thick steel plate are further improved depending on the kind of chemical components (elements) contained.

One or more selected from the group consisting of Ni: 1.50% or less, Cu: 1.50% or less, Cr: 1.50% or less, Mo: 1.50% or less Ni, Cu, Cr, and Mo are: Both are effective elements for improving the strength-toughness balance of the steel sheet, and the effect increases as the content thereof increases. In order to exhibit such an effect effectively, it is preferable to contain 0.05% or more, more preferably 0.10% or more. However, Ni is an expensive element, and is preferably suppressed to 1.50% or less and more preferably 1.20% or less from the viewpoint of cost. Moreover, Cu, Cr, and Mo cause excessive increase in strength when they are excessively contained, and deteriorate the toughness of the base material and HAZ. Therefore, it is preferable to suppress all to 1.50% or less, More preferably, it is made 20% or less.

Nb: 0.10% or less and / or V: 0.10% or less Nb and V precipitate as carbonitrides and are effective in improving the base material toughness by suppressing the coarsening of γ grains. It is an element. The effect increases as the content thereof increases. However, in order to effectively exhibit such an effect, it is preferable to contain 0.002% or more, more preferably 0.005% or more. . However, if they are contained excessively, the HAZ structure is coarsened and the HAZ toughness is deteriorated. Therefore, it is preferable to suppress them to 0.10% or less, more preferably 0.08% or less, and still more preferably. 0.05% or less.

B: 0.005% or less B is an element effective in improving the toughness of the base material and the HAZ by suppressing the formation of coarse grain boundaries α. The effect increases as the content increases, but in order to effectively exhibit such an effect, B is preferably contained in an amount of 0.0010% or more, and more preferably 0.0015% or more. However, if the B content is excessive, it causes precipitation of BN at the austenite grain boundaries and degrades the toughness of the base material and the HAZ. Therefore, it is preferably suppressed to 0.005% or less. A more preferable upper limit of the B content is 0.004%, and a further preferable upper limit is 0.003%.

  Although this invention is invention regarding a thick steel plate, generally a thick steel plate shows the steel plate whose plate | board thickness is 3.0 mm or more as defined by JIS2402. However, the thick steel plate of the present invention was invented for large heat input welding of a thick steel plate having a thickness of 50 mm or more, and the target steel plate is a steel plate having a thickness of 50 mm or more. Although it may be possible, these are merely preferred embodiments and do not exclude application of the present invention to a thick steel plate having a thickness of 3 mm or more and less than 50 mm.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, and the present invention is implemented with appropriate modifications within a range that can meet the gist of the present invention. These are all included in the technical scope of the present invention.

  In Examples of the present invention, first, steels having respective component compositions shown in Tables 1 to 3 were melted in a vacuum melting furnace (No. 1 to 54: 50 kg VIF, No. 55 to 57: 150 kg VIF), By casting a slab using molten steel and further performing hot rolling using the slab, no. In the case of 1 to 54, a hot-rolled sheet having a thickness of 50 mm In the case of 55-57, a hot-rolled sheet having a thickness of 80 mm was obtained. The hot-rolled sheet was heated to the austenite / ferrite two-phase region and quenched, and then tempered at 50 ° C. to obtain a thick steel plate for testing.

  Tables 4 to 6 show the controlled conditions in producing the thick steel plate for testing. The conditions are as follows: dissolved oxygen amount [Of] in molten steel before Al addition, Al, Ti, (REM, Zr), Ca addition sequence, time t1 from Ti addition to Ca addition, from Ca addition to casting start. The time t2, the cooling time t3 at 1500 to 1450 ° C. during casting, and the respective addition amounts of Al, Ti, REM, Zr, and Ca.

  It should be noted that A value and B value for obtaining an appropriate Ca addition amount, a total addition amount of Al, Ti, Ca, REM, and Zr contained in the thick steel plate, and an oxide having an equivalent circle diameter of 2 μm or more and less than 5 μm are predetermined. Preferred conditions for ensuring the above ([Ti]> 160 ppm, ([REM] + [Zr])> 55 ppm, (0.8 ≦ [Ti] / [REM] + [Zr]) ≦ 11.8)) Whether it is satisfied or not is also shown.

  In Tables 1 to 3, REM was added in the form of a misch metal containing about 50% Ce and about 25% La by mass%. In Tables 1 and 2, “-” indicates that the corresponding element is not added.

  In addition, in Tables 4 to 6, the addition order of Al, Ti, (REM, Zr), and Ca is “O” when added in the order of Al → Ti → (REM, Zr) → Ca, otherwise The time of addition in order is indicated by “x”. No. Since 33 did not add Ca, the order of addition was indicated by “−”.

  As for the time t2 from the addition of Ca to the start of casting, “○” indicates that the above-mentioned ta (min) <tx (min) <t2 (min) <tb (min) is satisfied, and “×” indicates that the time is not satisfied. ". As for the Ca addition amount [Ca], “◯” indicates that the relationship of A ≦ [Ca] ≦ B described above is satisfied, and “×” indicates that the relationship is not satisfied.

  Using each steel plate manufactured according to the above requirements, the number density of oxides (oxide inclusions) of various sizes, the number density of Ti-containing nitrides, the rate of formation of intragranular α in the base material, and the tension Strength TS, yield ratio YR, base metal toughness and HAZ toughness were determined by measurement. These measurement results are shown in Tables 7 to 9.

(Measurement of number density of oxides with equivalent circle diameter less than 2 μm)
A test piece is cut out from the surface of each thick steel plate at a depth of t / 4 (t: thickness) (taken so that the axis of the test piece passes through the position of t / 4), and in the rolling direction and the thickness direction. The parallel cross section was observed using a field emission scanning electron microscope “SUPRA35 (trade name)” (hereinafter referred to as FE-SEM) manufactured by Carl Zeiss. The observation conditions were as follows: magnification: 5000 times, observation visual field: 0.0024 μm ² , observation location: 20 locations. The area of each oxide in this observation field was measured by image analysis, and the equivalent circle diameter of each oxide was calculated from the area. In addition, it was confirmed by EDX (energy dispersive X-ray detector) that each oxide satisfies the above-described component composition. Of the oxides satisfying the above component composition, the number (N1) of oxides having an equivalent circle diameter of less than 2 μm was calculated by converting the number density to 1 mm ² . However, oxides having an equivalent circle diameter of 0.2 μm or less were excluded from the analysis because the reliability of EDX was not sufficient.

(Measurement of number density of oxides with equivalent circle diameter of 2 μm or more and less than 5 μm)
A test piece is cut out from the surface of each thick steel plate at a depth of t / 4 (t: thickness) (taken so that the axis of the test piece passes through the position of t / 4), and in the rolling direction and the thickness direction. Parallel cross sections were observed using FE-SEM. The observation conditions were as follows: magnification: 1000 times, observation visual field: 0.06 μm ² , observation location: 20 locations. The area of each oxide in this observation field was measured by image analysis, and the equivalent circle diameter of each oxide was calculated from the area. It was confirmed by EDX that each oxide satisfied the above-described component composition. Of the oxides satisfying the above component composition, the number (N2) of oxides having an equivalent circle diameter of 2 μm or more and less than 5 μm was calculated by converting the number density to 1 mm ² .

(Measurement of number density of oxides with equivalent circle diameter of 5 μm or more)
A test piece is cut out from the surface of each thick steel plate at a depth of t / 4 (t: thickness) (taken so that the axis of the test piece passes through the position of t / 4), and in the rolling direction and the thickness direction. Parallel cross sections were observed using FE-SEM. The observation conditions were as follows: magnification: 1000 times, observation visual field: 0.06 μm ² , observation location: 20 locations. The area of each oxide in this observation field was measured by image analysis, and the equivalent circle diameter of each oxide was calculated from the area. It was confirmed by EDX that each oxide satisfied the above-described component composition. Then, among the oxides satisfying the above component composition, the number (N3) of oxides having an equivalent circle diameter of 5 μm or more was determined by converting into a number density equivalent to 1 mm ² .

(Measurement of number density of Ti-containing nitride)
The site | part of the position of depth t / 4 (t: board thickness) from the surface of each thick steel plate was observed using the transmission electron microscope (TEM). The observation conditions were magnification: 60000 times, observation field: 2 μm × 2 μm, and observation location: 5 locations. The area of each Ti-containing nitride in this observation field was measured by image analysis, and the equivalent circle diameter of each Ti-containing nitride was calculated from the area. As for Ti-containing nitride, Ti-containing nitride was detected when Ti and N peaks were detected when analyzed by EDX. Then, the number (N4) of Ti-containing nitrides having an equivalent circle diameter of 100 nm or less was determined by converting into a number density equivalent to 1 mm ² .

(Measurement of intragranular α production rate of base material)
A test piece is cut out from the surface of each thick steel plate at a depth of t / 4 (t: thickness) (taken so that the axis of the test piece passes through the position of t / 4), and in the rolling direction and the thickness direction. Parallel sections were observed using an optical microscope. The observation conditions were as follows: magnification: 400 times, observation visual field: 0.04 mm ² , observation location: 20 locations. With respect to an oxide of 2 μm or more, at least one central axis of the lath-like α extending radially in one or more directions starting from the oxide is 15 ° or more with the central axis average of the lath-like α group around the oxide When there is a difference, it is determined that the oxide is the starting point of intragranular α, and the (number of oxides starting from intragranular α) / (total number of oxides) is the intragranular α production rate of the base material. did.

(Evaluation of tensile strength and yield ratio)
From the position of depth t / 4 (t: thickness) from the surface of each thick steel plate, No. 4 test piece of JIS Z 2201 was sampled in the direction perpendicular to the rolling direction and pulled as described in JIS Z 2241. The test was carried out and the tensile strength TS and the yield strength YS were measured. The yield ratio YR was calculated by a calculation called YS / TS. In this example, it was evaluated that the obtained TS was 490 MPa or more and the YR was less than 80% as having excellent mechanical properties.

(Evaluation of base metal toughness)
Three Charpy impact test pieces (No. 4 test piece of JIS Z 2201) were collected from the position of the depth t / 4 (t: plate thickness) from the surface of each thick steel plate (the axis of the test piece was the t / 4), the Charpy impact test was performed at 0 ° C., the absorbed energy (vE ₀ ) was measured to obtain an average value thereof, and the average value was defined as the base material toughness of each thick steel plate. . In this example, the average value of the obtained vE ₀ was evaluated to be excellent in the base material toughness of 200 J or more.

(Evaluation of HAZ toughness)
Three Charpy impact test pieces (No. 4 test piece of JIS Z 2201) were collected from the position of the depth t / 4 (t: plate thickness) from the surface of each thick steel plate (the axis of the test piece was the t / 4), and a reproducible HAZ thermal cycle V-notch Charpy impact test was conducted. The reproduced HAZ heat cycle conditions were such that the holding time at 1400 ° C. was 45 seconds, the cooling time at 800 to 500 ° C. was 800 seconds, and the thermal history of the bond part in electrogas arc welding with a heat input of 100 kJ / mm was simulated. About each test piece which gave this thermal cycle, the absorbed energy (vE0) in ₀ degreeC was measured, and the average value of three test pieces was calculated | required. In this example, an average value of the obtained vE ₀ of 70 J or more, which is a standard required for building steel, was evaluated as having excellent HAZ toughness.

  No. 1 to 23 and 47 to 57 are examples of the invention that satisfy the requirements of the present invention, in which chemical composition, oxide and Ti-containing nitride are appropriately dispersed, and tensile strength TS, yield ratio YR, matrix The HAZ toughness when the material toughness and heat input are 100 kJ / mm all satisfy the evaluation criteria of this example. That is, no. 1 to 23 and 47 to 57 have excellent HAZ toughness when high heat input welding is performed so that the heat input becomes 100 kJ / mm or more, and a low yield ratio is realized in a high strength region where the tensile strength is 490 MPa or more. Furthermore, it can be said that it is a thick steel plate excellent in the toughness of the weld heat-affected zone that can ensure good base material toughness.

In contrast, no. 25 to 46 are comparative examples that do not satisfy any of the requirements of the present invention, and are any of the HAZ toughness when the tensile strength TS, the yield ratio YR, the base material toughness, and the heat input is 100 kJ / mm. It can be seen that the evaluation criteria are not satisfied.

  In Tables 4 to 6, preferable conditions ([Ti]> 160 ppm, ([REM] + [Zr])> 55 ppm for securing an oxide having a circle equivalent diameter of 2 μm or more and less than 5 μm to a predetermined value or more, ( 0.8 ≦ [Ti] / [REM] + [Zr]) ≦ 11.8)). 31 and no. No. 36 is the oxide content with an equivalent circle diameter of 2 μm or more and less than 5 μm. It turns out that it is low following 33.

Claims (5)

In mass%, C: 0.02-0.15%, Si: 0.02-0.46% , Mn: 1.0-2.0%, P: 0.03% or less (excluding 0%) ), S: 0.015% or less (not including 0%), Al: 0.05% or less (not including 0%), Ti: 0.010 to 0.080%, Ca: 0.0005 to 0 0.010%, N: 0.002-0.020%, REM: 0.0001-0.02% and / or Zr: 0.0001-0.02%, with the balance being iron and inevitable impurities A thick steel plate,
Oxidation satisfying 10% <Ti, Al <20%, 5% <Ca <40%, 5% <REM <50% and / or 5% <Zr <40% in terms of mass%, except for oxygen Among the above-mentioned oxides, oxides having an equivalent circle diameter of less than 2 μm are 200 / mm ² or more, oxides having an equivalent circle diameter of 2 μm or more and less than 5 μm are 30 to 70 / mm ² , While there are less than 30 oxides / mm ² with an equivalent circle diameter of 5 μm or more,
A welding heat-affected zone containing Ti-containing nitrides, and among the Ti-containing nitrides, Ti-containing nitrides having an equivalent circle diameter of 100 nm or less are present at 5 × 10 ⁶ pieces / mm ² or more. Steel plate with excellent toughness. In mass%, C: 0.02 to 0.15%, Si: 0.46% or less (excluding 0%) , Mn: 1.0 to 2.0%, P: 0.03% or less (0 %), S: 0.015% or less (not including 0%), Al: 0.05% or less (not including 0%), Ti: 0.010 to 0.080%, Ca: 0 .0005 to 0.010%, N: 0.002 to 0.020%, REM: 0.0001 to 0.02% and / or Zr: 0.0001 to 0.02%, and further, mass% And at least one selected from the group consisting of Ni: 1.50% or less, Cu: 1.50% or less, Cr: 1.50% or less, Mo: 1.50% or less, with the balance being iron and A steel plate that is an inevitable impurity,
Oxidation satisfying 10% <Ti, Al <20%, 5% <Ca <40%, 5% <REM <50% and / or 5% <Zr <40% in terms of mass%, except for oxygen Among the above-mentioned oxides, oxides having an equivalent circle diameter of less than 2 μm are 200 / mm ² or more, oxides having an equivalent circle diameter of 2 μm or more and less than 5 μm are 30 to 70 / mm ² , While there are less than 30 oxides / mm ² with an equivalent circle diameter of 5 μm or more,
A welding heat-affected zone containing Ti-containing nitrides, and among the Ti-containing nitrides, Ti-containing nitrides having an equivalent circle diameter of 100 nm or less are present at 5 × 10 ⁶ pieces / mm ² or more. Steel plate with excellent toughness.   Furthermore, it contains at least one selected from the group consisting of Ni: 1.50% or less, Cu: 1.50% or less, Cr: 1.50% or less, and Mo: 1.50% or less by mass%. The thick steel plate excellent in the toughness of the weld heat affected zone according to claim 1. Furthermore, it is excellent in the toughness of the welding heat affected zone according to any one of claims 1 to 3, characterized by containing Nb: 0.10% or less and / or V: 0.10% or less in mass%. Thick steel plate. Furthermore, the thick steel plate excellent in the toughness of the welding heat affected zone in any one of Claims 1 thru | or 4 characterized by containing B: 0.005% or less by mass%.