KR20110128154A - Thick steel plate - Google Patents

Abstract

PURPOSE: A thick steel plate is provided to spray the oxide having a predetermined chemical composition by appropriately controlling the chemical composition through the obtaining of a good HAZ toughness. CONSTITUTION: A thick steel plate comprises C: 0.02-0.15 mass % Si: 0-0.50 mass % Mn: 1.0-3.0 mass % P: 0-0.03 mass % S: 0-0.015 mass % Al: 0-0.07 mass % Ti: 0.010-0.08O mass % Ca: 0.0005-0.010 mass % N: 0.002-0.020 mass %. REM: 0.0001-0.02 mass % Zr: 0.0001-0.02 mass %, at least one side component except for the thick steel plate, in which the remains is the iron and inevitable impurity. It more contain one kind or greater selected from 0.0001-0.02 mass % the oxygen satisfies the condition.

Description

Thick Plate {THICK STEEL PLATE}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to thick steel sheets used in welding structures such as bridges, high-rise buildings, ships, and the like. More specifically, the welding heat affected zone (hereinafter, referred to as HAZ) when welding is performed in a high heat input. It has a thick steel sheet excellent in toughness).

In recent years, the characteristics required for steel materials used for welding structures such as bridges, high-rise buildings, ships, and the like tend to become increasingly strict with the increase in the size of these welding structures. In particular, among many of the required properties, there is a tendency to become more stringent with respect to the demand for toughness.

Steel materials used for welding structures and the like have a problem in that the toughness of HAZ formed under heat influence at the time of welding bonding is easily deteriorated. Since the deterioration of the HAZ toughness tends to be remarkable as the heat input amount during welding increases, it is considered that the heat input amount during welding may be suppressed as much as possible in order to improve the HAZ toughness. However, with respect to steel materials used for building structures and the like, from the viewpoint of improving the work efficiency of welding, high heat input welding, for example, high heat input welding having a heat input amount of 100 kJ / mm or more tends to be required.

There is a high Ni steel sheet as a representative example of the technique of increasing the HAZ toughness in the case of high heat input welding, but, for example, Patent Document 1 discloses a steel sheet having improved strength and toughness by adding about 2% of Ni. . However, since Ni is an extremely expensive element and is difficult to handle, it is not preferable to increase the amount of Ni to be added, and industrially, it can be said that it is preferable to suppress the addition amount of Ni as low as possible.

There is such a background, and as described in Patent Literature 1, various techniques for increasing HAZ toughness after reducing the amount of Ni added as low as possible, for example, the amount of Ni added to 1.5% or less, are also various. It is considered. Since the HAZ during the high heat input welding is maintained for a long time in the high temperature austenite region by heat input, and then cooled, the coarse ferrite in the growth and slow cooling process of the γ grain during high temperature holding in the heat input (α) The generation of) granules is likely to occur, resulting in coarsening of the structure, which causes a decrease in the HAZ toughness at the time of high heat input welding. In view of this, it is necessary to suppress coarsening of the structure at the time of high heat input welding, and it is a necessary subject to develop a technique for stably maintaining a high level of HAZ toughness at the time of high heat input welding. .

As a main means for suppressing the coarsening of the structure at the time of high heat input welding and securing the HAZ toughness, the γ grain growth pinning by inclusion particles such as oxides, nitrides, and sulfides, and intragranular α production starting from inclusion particles The technique regarding the refinement | miniaturization of the tissue by this etc. is proposed. As a proposal example of such a technique, the technique of patent documents 2-4 exists. In Patent Documents 2 to 4, fine Ti-containing nitrides are dispersed and precipitated in steel to act as γ grain growth pinning particles, thereby suppressing coarsening of austenite grains generated in HAZ during high heat input welding, and deteriorating HAZ toughness. Inhibiting is disclosed. However, when Ti-containing nitride is welded by the high heat input currently requested | required, the amount which will lose | disappears may become large, and sufficient water density may not be secured, and stable HAZ toughness cannot be obtained.

On the other hand, as patent documents 5-7, the technique of using the oxide type interference | inclusion stabilized at high temperature as (gamma) grain growth pinning particle is proposed. However, since oxide-based inclusions have a lower water density than Ti-containing nitrides, and it is difficult to obtain a sufficient pinning effect, it is not possible to sufficiently cope with welding in a large heat input where the heat input amount is 100 kJ / mm or more. Needs improvement.

That is, although Patent Documents 5 and 6 describe that good HAZ characteristics can be obtained by the presence of Ti-REM-Ca-Al-based oxides or oxides containing REM or Zr, the estimated heat input remains at a low level. , Good HAZ characteristics cannot be obtained in high heat input welding of 100 kJ / mm or more. In addition, Patent Document 7 describes a technique using an oxide containing REM and Zr similarly to Patent Document 6, but similarly to the above, the assumed heat input amount remains at a low level, and in high heat input welding of 100 kJ / mm or more. It cannot be said that good HAZ characteristics are obtained.

In addition, Patent Document 8 describes a technique of obtaining high HAZ toughness by using both an oxide-based inclusion and a Ti-containing inclusion as γ grain growth pinning particles. However, considering the recent trend of increasing heat input, there is a limit to the use of γ grain growth pinning mainly containing Ti-containing inclusions, and it is necessary to quickly establish means for improving HAZ toughness in large heat input by oxide-based inclusions. can do.

In addition, as a technique for acting as a starting point of intragranular α production starting from oxide-based inclusion particles, a technique using a composite oxide containing Mn and REM and MnS described in Patent Document 9 has been proposed. A technique for promoting intragranular α production by controlling the inclusion shape at 10 has been proposed above. These techniques are constructed on the premise that inclusions having low interfacial energy (intracellular α / inclusion) are effective for intragranular α production. However, in the technique of patent document 9, the heat input amount initially assumed is small, and it does not reach until it fully ensures high heat input HAZ toughness.

In addition, in addition to excellent HAZ toughness, steel materials used in building structures and the like also require strength of the base material, resistance ratio, base material toughness, and the like. In recent years, with high-rise and large-size buildings, there has been an increase in the use of high-strength steel as a building steel.

For example, Patent Document 11 proposes a technique for realizing a resistance ratio in a steel sheet having a tensile strength of 590 MPa or more by dispersing fine carbonitride and securing a predetermined amount or more of ferrite. However, the technique concerning this patent document 11 is not a technique which paid attention to HAz toughness at the time of high heat input welding in which a heat input amount becomes 100 kJ / mm or more. In addition, Patent Document 12 also proposes a technique for realizing a resistance ratio in a steel sheet having a tensile strength of 590 MPa or more by dispersing an oxide-based inclusion and securing a predetermined amount or more of ferrite. However, the heat input target is small.

As described above, in addition to securing the HAZ toughness in the case of the high heat input welding in which the heat input amount is 100 kJ / mm or more, other characteristics (improvment of the strength and toughness of the base material, or resistance ratio) can also be improved. It is a phenomenon that there is not yet developed about existing technology.

Japanese Patent Application Publication No. 2006-118007 Japanese Patent Application Laid-Open No. 2001-98340 Japanese Patent Application Publication No. 2004-218010 Japanese Patent Application Laid-open No. 61-253344 Japanese Patent Application Publication No. 2001-20031 Japanese Patent Application Publication No. 2007-100213 Japanese Patent Application Publication No. 2007-247005 Japanese Patent Application Publication No. 2008-223062 Japanese Patent Application Laid-open No. Hei 7-252586 Japanese Patent Application Publication No. 2008-223081 Japanese Patent No. 2901890 Japanese Patent Application Publication No. 2007-247004

This invention is made | formed in view of the said prior art, and it aims at providing the thick steel plate which exhibits the outstanding HAZ toughness also at the time of high heat input welding.

More specifically, even when the high heat input welding is performed, the HAZ toughness in the case of evaluating as the average value is excellent, and the resistance ratio in the high strength region having a tensile strength of 490 MPa or more can be realized, and good base material toughness can be ensured. It is one object of the present invention to provide a thick steel sheet (hereinafter sometimes referred to as "thick steel sheet according to the first aspect") having excellent toughness of the weld heat affected zone.

Another object of the present invention is a thick steel sheet which is excellent in the average value of the high heat input HAZ toughness and exhibits a resistance ratio and a good base material toughness even in a high strength region of 590 MPa or more (hereinafter, referred to as a "thick steel sheet according to the second aspect"). In some cases).

Still another object of the present invention is a thick steel sheet (hereinafter referred to as "thick steel sheet according to the third aspect") that can improve the minimum value of the HAZ toughness in the region to be evaluated as well as the average value of the above-mentioned high heat input HAZ toughness. In some cases).

Still another object of the present invention is a thick steel sheet capable of improving the average value of the HAZ toughness and the minimum value of the HAZ toughness when high heat input welding is performed even in a high strength region having a tensile strength of 780 MPa class. It may be called "the thick steel plate about four forms").

The thick steel sheet of the present invention which has attained the above object has a steel component and structure as follows as a basic configuration. That is, C: 0.02 to 0.15 mass%, Si: 0 to 0.50 mass%, Mn: 1.0 to 3.0 mass%, greater than P: 0 mass%, 0.03 mass% or less, S: greater than 0.01 mass%, 0.015 mass% or less, Al: 0 to 0.07% by mass, Ti: 0.010 to 0.080% by mass, Ca: 0.0005 to 0.010% by mass, N: 0.002 to 0.020% by mass, REM: 0.0001 to 0.02% by mass, Zr: 0.0001 to 0.02% by mass It is a thick steel plate which further contains 1 or more types chosen from%, and remainder is iron and an unavoidable impurity, and constituent elements except oxygen are 10% <Ti, Al <20%, 5% <Ca < An oxide satisfying 40% and satisfying at least one of 5% <REM <50% and 5% <Zr <40% exists so as to satisfy the following conditions (1) and (2). It is a thick steel plate.

(1) 200 or more mm <2> of the said oxides whose circular equivalent diameter is less than 2 micrometers,

(2) The said oxide whose circular equivalent diameter is 2 micrometers or more is 100 piece / mm <2> or less.

The equivalent circular diameter described in the present invention is obtained by considering the size of an oxide or the like and obtaining a diameter of a circle assumed to be equal in area, and is a transmission electron microscope (TEM) or a scanning electron microscope (SEM). It can obtain | require by observing with () and image analysis.

The thick steel sheet according to the first aspect of the present invention, which was invented based on such a basic configuration, has a Si content of 0 to 0.46 mass% and a Mn content of 1.0 to 2.0 mass%, and has a circular equivalent diameter in the oxide. Among these 2 micrometers or more, 30-70 pieces / mm <2> of oxides with a circular equivalent diameter of 2 micrometers or more and less than 5 micrometers exist, and also present oxides with less than 30 pieces / mm <2> of oxides with a circular equivalent diameter of 5 micrometers or more, and contain Ti It is comprised so that 5 * 10 <^6> pieces / mm <2> or more of Ti containing nitride containing a nitride and having a circular equivalent diameter of 100 nm or less may exist in the said Ti containing nitride.

In addition, the thick steel plate which concerns on the said 2nd aspect invented based on the said basic structure is Si content of 0.01-0.35 mass%, Mn content 1.0-2.0 mass%, and the said oxide is Al / ( An oxide that satisfies the relationship of REM + Zr) &lt; 0.7, wherein the equivalent circular diameter is less than 2 µm, 300 or more per 1 mm 2, the Ti-containing nitride, and the Ti-containing nitride has a circular equivalent diameter of 100 nm or less. It is comprised so that a containing nitride may exist 5 * 10 <^6> pieces / mm <2> or more.

In addition, the thick steel plate which concerns on the said 3rd aspect invented based on the said basic structure is 0-0.25 mass% of Si content, 1.0-2.0 mass% of Mn content, and both REM and Zr are REM It is contained in the range of 0.0003-0.02 mass%, Zr: 0.0003-0.02 mass%,

A = 10 ⁴ × {[Ti]-0.7 × ([O] -0.09 × [Al]-0.16 × [Ca]-0.07 × [REM]-0.04 × [Zr])} × [N]

The value A obtained from the formula of satisfies 0.4 ≦ A ≦ 2.4, and the oxide is an oxide that satisfies 3% <N, 2 <Ti / N, and has a circular equivalent diameter of less than 2 µm. It is comprised so that it may become 300 or more per mm <2>. In addition, in said Formula A, [] shows content (mass%) of each element.

In addition, the thick steel plate which concerns on the said 4th aspect invented based on the said basic structure is Si content of 0.02-0.50 mass%, Mn content 1.4-3.0 mass%, and both REM and Zr are REM It is contained in the range of 0.0003-0.02 mass% and Zr: 0.0003-0.02 mass%, and further contains Cr: 0.5-2.0 mass%,

D = 62 × [Mn] + 27 × [Ni] + 111 × [Cr]

While the D value obtained from the formula satisfies 238 <D <388, in the above oxides, the equivalent circular diameter is less than 2 µm, 300 or more per mm 2, and the equivalent circular diameter is 2 µm or more, 30-70 pieces / mm <2> of oxides with a circular equivalent diameter of 2 micrometers or less and less than 5 micrometers exist, and it is comprised so that there may be less than 30 pieces / mm <2> of oxides with a circular equivalent diameter of 5 micrometers or more. In addition, in said D formula, [] shows content (mass%) of each element.

The thick steel sheet according to the fourth aspect of the present invention is an oxide that satisfies 10% <REM + Zr <70% among the oxides, and that the mass ratio Ti / Ca of Ti and Ca satisfies more than 1 and less than 1.4, and has a circular equivalent diameter. It is preferable that 300 or less mm <2> or more of oxides less than 2 micrometers exist.

It is preferable that the thick steel plate which concerns on said 1st thru | or 3rd aspect further includes at least 1 group chosen from the following (a)-(c).

(a) 1 or more types selected from the group consisting of Ni: 1.50 mass% or less, Cu: 1.50 mass% or less, Cr: 1.50 mass% or less, and Mo: 1.50 mass% or less,

(b) at least one selected from the group consisting of Nb: 0.10 mass% or less and V: 0.10 mass% or less,

(c) B: 0.005 mass% or less.

Moreover, it is preferable that the thick steel plate which concerns on said 4th aspect mentioned above contains another at least 1 group chosen from the following (a ')-(c') as another element.

(a ') 1 or more types chosen from the group which consists of Ni: 1.50 mass% or less, Cu: 1.50 mass% or less, and Mo: 1.50 mass% or less,

(b ') 1 or more types chosen from the group which consists of Nb: 0.10 mass% or less, V: 0.10 mass% or less,

(c ') B: 0.005 mass% or less.

In the thick steel sheet of the present invention, the chemical composition is appropriately controlled as in the basic configuration described above, and an oxide having a predetermined chemical composition is dispersed in an appropriate amount for each of the sizes, and in the thick steel sheet according to the first aspect of the present invention, it is added thereto. Thus, since the fine Ti-containing nitride is secured by a predetermined amount or more, even when a large heat input welding in which the heat input amount is 100 kJ / mm or more is performed, good HAZ toughness can be ensured. In addition, it is possible to realize a resistance ratio in a high strength region having a tensile strength of 490 MPa or more, and to secure good base material toughness.

In the thick steel sheet according to the second aspect of the present invention, since the chemical composition of the oxide is further controlled in addition to the basic configuration, a finer Ti-containing nitride is ensured for a predetermined or more, so that even when a high heat input welding of 100 kJ / mm or more is performed, a good HAZ is obtained. Toughness can be ensured, and high-strength thick steel plate which can suppress the variation of HAZ toughness and exhibits a resistance ratio and good base material toughness can be realized.

In the thick steel sheet according to the third aspect of the present invention, since the chemical composition of the steel and the chemical composition of the oxide are further controlled in addition to the basic configuration, the minimum value is also added to the average value of the HAZ toughness even when the high heat input welding is performed. Can be improved.

In the thick steel sheet according to the fourth aspect of the present invention, in addition to the basic configuration, the chemical composition of the steel is further controlled. Therefore, even in the case of high heat welding of a high strength steel sheet having a tensile strength of 80 kilo class, the average value of the HAZ toughness In addition, the minimum value can also be improved.

First, the basic configuration of the present invention will be described.

The present invention relates to a thick steel sheet used for welding structures such as bridges, high-rise buildings, and ships, and in particular, to develop a thick steel sheet having excellent toughness in welding heat affected zones when welding is performed at a high heat input of 100 kJ / mm or more of heat input. In the meantime, the inventors first focused on the fact that high HAZ toughness can be secured by oxide inclusions (hereinafter, simply referred to as oxides), and studied from various angles.

The HAZ toughness improvement technique of the thick steel plate using this oxide has an advantage and a fault. The advantage is that the oxide becomes the starting point of intragranular α production and refines the HAZ structure, thereby improving the HAZ toughness. On the contrary, the drawback is that the oxide itself becomes the starting point of destruction, adversely affecting the HAZ toughness.

Conventionally, from such a viewpoint, when using an oxide as a starting point of intragranular α production, the development of a thick steel sheet is mainly made up of the technical idea that an oxide having a diameter of less than 2 µm is mainly used, and an oxide having a larger size is reduced as much as possible. This was done.

However, according to recent research and development, it is theoretically confirmed and reported that the larger the size of the oxide to be introduced into the thick steel sheet from the viewpoint of the origin of intragranular α production, the higher the intragranular α production capacity is. In addition, it is theoretically confirmed and reported that the upper limit of the size of the oxide (coarse second phase) for securing toughness is relaxed as the toughness evaluation temperature is higher.

From the results of such research and development, the inventors have followed the conventional technical idea after relaxing the upper limit of the size of the oxide that adversely affects the HAZ toughness in the case of a thick steel sheet used in a welded structure having a toughness evaluation temperature. In other words, the oxide dispersed in the thick steel sheet is mainly composed of an oxide having a circular equivalent diameter of less than 2 μm, and then introduces a relatively large oxide having a high intragranular α-producing ability into the inside, thereby making it possible to refine the HAZ structure. It was considered that the HAZ toughness can be improved, and the examination was advanced.

Next, the inventors advanced further from the viewpoint of the resistance 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 production. In addition, by increasing the number of nucleation sites by the introduction of oxide, the base material is converted to bainite at a relatively high temperature as compared with the case where no oxide is introduced. On the other hand, the oxide itself is a starting point of destruction. There is also a problem of lowering the toughness of the base metal. In addition, the problem is known to increase as the strength of the base material increases, and it can be said that it is important not to lower the base metal toughness in introducing an oxide into the high strength steel.

As a means to improve base material toughness, it is generally considered effective to devise a steel structure microstructure by devising a heat treatment of steel materials. However, heat treatment cannot be said to be an industrially preferable means in order to complicate the process. Therefore, the inventors considered that it is effective to promote the formation of intragranular α by introducing an oxide into the steel material as well as improving the base metal toughness and the HAZ toughness.

However, the base metal structure has a smaller sphere γ particle diameter than the HAZ structure. Since the size of the tissue is always determined according to the balance between the α production from the mouth and the α production from the old γ grain boundary, the smaller the sphere γ particle diameter of the base metal tissue is, the larger the force of α production from the grain boundary becomes. It can be said. Therefore, the structure refinement | miniaturization effect by intraoral alpha production is hard to be exhibited compared with HAZ in a base material. Therefore, in order to promote the formation of intragranular α by the oxide-based inclusion control and to refine the base metal structure, it can be said that it is necessary to control the composition and size of the oxide to be introduced more appropriately.

The inventors tested the control of the oxide from the viewpoint of improving the HAZ toughness and securing the properties of the base metal based on the technical ideas examined from the above various angles. As a result, the constituent elements excluding oxygen satisfy 10% <Ti, Al <20%, 5% <Ca <40% at mass%, while 5% <REM <50%, 5% <Zr <40 An oxide satisfying at least one of% is introduced, and among these oxides, 200 oxides / mm 2 or more of an oxide having a circular equivalent diameter of less than 2 μm, and 100 oxides / mm 2 or less of an oxide having a diameter of 2 μm or more of circular equivalent, respectively. It was found that the effect of refining HAZ tissues by intragranular α production was remarkably expressed, thereby confirming that even better HAZ toughness was obtained.

In addition, when there are only less than 200 oxides / mm <2> of oxides whose original equivalent diameter is less than 2 micrometers among these oxides, HAZ structure refinement | miniaturization effect by intragranular alpha production will not fully be expressed. Regarding the oxide having a circular equivalent diameter of less than 2 µm, the presence of preferably 220 pieces / mm 2 or more, more preferably 250 pieces / mm 2 or more is recommended. Although the upper limit of the number is not specifically defined, it is about 1,000 pieces / mm <2> or less actually.

Among the oxides, the original equivalent diameter is 2 µm or more, which promotes brittle fracture and deteriorates the HAZ toughness. Therefore, as few as possible is better, the diameter is set to 100 / mm 2 or less.

In the oxide, the mass of Ti, Al, Ca, REM, Zr in the oxide is 10% <Ti, Al <20%, 5% <Ca <40%, 5 to the total mass of the constituent elements excluding oxygen. It is good to be in the range of% <REM <50% and / or 5% <Zr <40%. In addition to the elements of Ti, Al, Ca, REM, and Zr, constituent elements other than oxygen include elements such as Si and Mn. In addition, this oxide exists normally in the form of the complex oxide containing elements other than said oxygen.

<Chemical composition>

Next, the chemical component composition in the thick steel plate of this invention is demonstrated. The thick steel sheet of the present invention can improve the HAZ toughness and the base metal toughness with the above-described oxides, but in addition to this, by adjusting the content of each chemical component (element) appropriately, HAZ toughness and base material toughness can be achieved, and another characteristic (resistance ratio etc.) can be achieved. Therefore, in the thick steel sheet of this invention, it is also a requirement that content of each chemical component exists in the range demonstrated below. Among these chemical components, the contents of Al, Ca, Ti, REM, and Zr constituting the oxide include the amounts constituting the oxide, as apparent from the effect thereof.

C: 0.02-0.15 mass%

C is an essential element for securing the strength of the steel sheet. When content of C is less than 0.02 mass%, necessary strength cannot be ensured. The minimum with preferable content of C is 0.03 mass%, and a more preferable minimum is 0.04 mass%. On the other hand, when C content becomes excess, a lot of hard island martensite MA will produce | generate, and will cause the toughness of a base material to deteriorate. Therefore, content of C needs to be 0.15 mass% or less. The upper limit with preferable content of C is 0.12 mass%, and a more preferable upper limit is 0.10 mass%.

Si: 0.50 mass% or less (including 0%)

Si is an element useful for securing strength by solid solution strengthening and an element useful for securing the number density of Ti-containing nitrides. Moreover, depending on the strength class of a steel plate, it is a useful element also in order to ensure alpha phase. However, when Si content becomes excess, (alpha) phase will introduce | transduce excessively and it will become impossible to ensure required tensile strength TS. Therefore, the upper limit of content of Si is made into 0.50 mass%. Moreover, a preferable upper limit is 0.46 mass%, a more preferable upper limit is 0.42 mass%, a more preferable upper limit is 0.35 mass%, and an especially preferable upper limit is 0.25 mass%. In addition, in this invention, although the minimum in particular does not specifically limit, Si may become an essential element according to the plate | board thickness of the thick steel plate made into the object. In other words, when the ingot size is large, the cooling rate is slow. Therefore, the number density of the Ti-containing nitride can be ensured without adding Si. However, when the ingot size is small, the cooling rate is faster. In some cases, the number density of the Ti-containing nitride may be secured by increasing. The minimum with preferable content of Si in that case is 0.01 mass%. Moreover, the minimum with more preferable content of Si is 0.02 mass%, still more preferable minimum is 0.05 mass%, and especially preferable minimum is 0.08 mass%.

Mn: 1.0-3.0 mass%

Mn is an element useful for securing the strength of the steel sheet, and in order to effectively exhibit such an effect, it is necessary to contain Mn by 1.0 mass% or more. The minimum with preferable content of Mn is 1.3 mass%, and a more preferable minimum is 1.4 mass%. On the other hand, when it contains exceeding 3.0 mass% excessively, since the intensity | strength of HAZ may rise too much and toughness may deteriorate, content of Mn shall be 3.0 mass% or less. The upper limit with preferable content of Mn is 2.5 mass%, a more preferable upper limit is 2.0 mass%, a more preferable upper limit is 1.8 mass%, and an especially preferable upper limit is 1.6 mass%.

P: 0.03 mass% or less (does not contain 0%)

Since P is an impurity element that tends to cause grain boundary breakage and adversely affects toughness, the content thereof is preferably as small as possible. From the viewpoint of securing the toughness of the base material and the HAZ, the content of P needs to be suppressed to 0.03% by mass or less, preferably 0.02% by mass or less, and more preferably 0.01% by mass or less. The lower limit of the content of P is not particularly defined, but it is difficult to industrially make P in the steel 0%.

S: 0.015 mass% or less (does not contain 0%)

Since S is an element which forms Mn sulfide and deteriorates the toughness of a base material, it is preferable that the content is as few as possible. From the viewpoint of securing the base metal toughness, the content of S needs to be suppressed to 0.015 mass% or less, preferably 0.010 mass% or less, and more preferably 0.008 mass% or less. The lower limit of the content of S is not particularly defined, but it is difficult to make S in the steel 0% industrially.

Al: 0.07 mass% or less (including 0%)

Al is an element useful for forming an oxide effective for the production of α in the mouth by adding Ti or Ca and prior to REM or Zr. In order to exhibit such an effect effectively, it is preferable to contain 0.003 mass% or more, More preferably, you may be 0.010 mass% or more. However, if the content is excessive, coarse oxide is formed and the toughness of the base metal and the HAZ deteriorates. Therefore, it is necessary to suppress the content to 0.07% by mass or less. The upper limit with preferable content of Al is 0.05 mass%, a more preferable upper limit is 0.04 mass%, and a more preferable upper limit is 0.03 mass%.

Ti: 0.010 to 0.080 mass%

Ti is an element that contributes to the improvement of HAZ toughness by forming an oxide effective for the generation of intragranular α by adding Al before Ca, REM or Zr. In order to exhibit such an effect effectively, it is necessary to contain 0.010 mass% or more. The minimum with preferable content of Ti is 0.012 mass%, and a more preferable minimum is 0.015 mass%. On the other hand, when the content of Ti is excessive, many coarse oxides are generated to deteriorate the HAZ toughness. Therefore, it is necessary to suppress the content to 0.080% by mass or less. The upper limit with preferable content of Ti is 0.060 mass%, and a more preferable upper limit is 0.040 mass%.

Ca: 0.0005-0.010 mass%

Ca is added after 3 to 20 minutes after the addition of Ti, thereby forming an oxide effective for the generation of intragranular α and contributing to the improvement of HAZ toughness. In order to exhibit these effects effectively, it is necessary to contain 0.0005 mass% or more. The minimum with preferable content of Ca is 0.0008 mass%, and a more preferable minimum is 0.0010 mass%. On the other hand, when the content of Ca is excessive, coarse oxide is formed and the toughness of the base metal and the HAZ is deteriorated. Therefore, it is necessary to suppress the content to 0.010% by mass or less. The upper limit with preferable content of Ca is 0.008 mass%, and a more preferable upper limit is 0.007 mass%.

N: 0.002-0.020 mass%

N is a useful element for securing the toughness of the base metal and HAZ by dissolving at high temperature and forming the remaining Ti-containing nitride. In order to exhibit such an effect effectively, it is necessary to contain 0.002 mass% or more. The minimum with preferable content of N is 0.003 mass%, and a more preferable minimum is 0.004 mass%. On the other hand, when the content of N becomes excessive, the amount of solid solution N increases and the toughness of the base material and the HAZ deteriorates due to strain aging, so it is necessary to suppress the content to 0.020% by mass or less. The upper limit with preferable content of N is 0.018 mass%, and a more preferable upper limit is 0.013 mass%.

1 or more types chosen from REM: 0.0001-0.02 mass% and Zr: 0.0001-0.02 mass%

REM (rare earth element) and Zr are elements which contribute to the improvement of HAZ toughness by forming an oxide effective for the generation of intragranular α by adding Ti after addition of Ti and prior to addition of Ca. Although these effects increase as their content increases, in order to exhibit these effects effectively, it is necessary to contain all 0.0001 mass% or more. The minimum with preferable content of REM and Zr is 0.0003 mass%, more preferable minimum is 0.0005 mass%, and still more preferable minimum is 0.0010 mass%. On the other hand, when these elements are excessively contained, oxides coarsen and deteriorate the toughness of the base material and HAZ. Therefore, it is necessary to suppress them to 0.02% by mass or less. The upper limit with preferable these content is 0.015 mass%, and a more preferable upper limit is 0.01 mass%. In addition, REM (rare earth element) represents a lanthanoid element (15 elements from La to Ln in a periodic table), Sc, and Y. The REM may be in the form of a misch metal (eg, containing about 50% Ce and about 25% La).

The above is an essential containing element prescribed | regulated by this invention. Moreover, the element mentioned later may further be included and remainder is iron and an unavoidable impurity. As an unavoidable impurity, mixing of elements, such as Sn, As, Pb, etc. which are carried in by the situation of a raw material, a material, manufacturing facilities, etc. is permissible.

Based on the basic structure of this invention demonstrated above, the thick steel plate which concerns on the 1st aspect of this invention is demonstrated concretely. In addition, description is abbreviate | omitted about the content common to the said basic structure.

About the component composition of the thick steel plate which concerns on the 1st aspect, Si, Mn is 0.46 mass% or less (including 0%), and Mn content is 1.0-2.0 mass% as described in the said basic structure. The effect to contain is remarkably exhibited.

In order to make such a component composition, 30-70 pieces / mm <2> of oxides with a circular equivalent diameter of 2 micrometers or more and less than 5 micrometers exist among the said oxide whose circular equivalent diameter is 2 micrometers or more, and the circular equivalent diameter is At least 30 oxides / mm 2 of oxide having a diameter of 5 μm or more is present, and Ti-containing nitrides are contained, and in the Ti-containing nitrides, Ti-containing nitrides having a diameter equivalent to 100 nm or less are present at 5 × 10 ⁶ atoms / mm 2 or more. It is a characteristic characteristic of the structure of the thick steel plate which concerns on this form of this.

30 to 70 pieces / mm2 of oxides having a circular equivalent diameter of 2 µm or more and less than 5 µm and less than 30 pieces / mm2 of oxides having a circular equivalent diameter of 5 µm or more, respectively, thereby miniaturizing the effect of HAZ tissue refinement by intragranular α production. It is remarkably expressed.

When only 30 oxides / mm <2> of oxides with a circular equivalent diameter of 2 micrometers or more and less than 5 micrometers exist, it is hard to express compared with the case where 30 or more / mm <2> of HAZ structure refinement | miniaturization effect by intragranular α production exists. About the oxide whose circular equivalent diameter is 2 micrometers or more and less than 5 micrometers, it is recommended that it exists preferably 50 pieces / mm <2> or more, More preferably, 60 pieces / mm <2> or more.

In addition, among these oxides, oxides having a mass ratio Ti / Ca of Ti and Ca of more than 1 and less than 1.4 are partially liquefied during high temperature heating of HAZ, and crystallized with a crystal structure favorable for intragranular α formation during subsequent cooling. In addition to the reduction of the intergranular α / γ interfacial energy, it is possible to realize even lower (intergranular α / inclusion) interfacial energy, which is particularly effective because the generation of intragranular α is extremely actively promoted.

In addition to controlling the above oxides, it is also necessary for the thick steel sheet according to the first aspect to contain fine Ti-containing nitrides or more in an appropriate number (fine number density) in the thick steel sheet. Pinning of lip growth can be performed.

In the Ti-containing nitride defined in the first aspect, not only TiN but also a part of Ti of TiN, specifically, Ti, an element having an atomic ratio of 50% or less by other nitride forming elements (Nb, Zr, V, etc.) Substituted with) is also included.

This Ti-containing nitride is very fine in comparison with the above oxide, and can be finely dispersed in steel before heat input by welding. Thus, for example, the Ti-containing nitride is formed by a high heat input welding in which the heat input amount is 100 kJ / mm or more. Even if most are lost, it is still possible to leave a sufficient number of Ti-containing nitrides. In this manner, a sufficient number of Ti-containing nitrides can still be left even after the high heat input welding, so that pinning of the spherical growth of HAZ can be effectively exhibited.

In the thick steel sheet according to the first aspect of the present invention, in order to secure HAZ toughness, the number density of the fine Ti-containing nitrides having a circular equivalent diameter of 100 nm or less introduced into the steel was 5 × 10 ⁶ / mm 2 or more. It is because the pinning effect of spherical-γ grain growth is weak when it is less than 5x10 <^6> piece / mm <2>.

Moreover, the preferable number density of the fine Ti containing nitride whose circular equivalent diameter introduce | transduced into steel materials is 5.4x10 <^6> piece / mm <2> or more, and more preferable number density is 6 * 10 <^6> piece / mm <2> or more. Although the upper limit of the number is not specifically defined, In practice, it is about 15 * 10 <^6> pieces / mm <^2> or less. In the first aspect, the lower limit of the circular equivalent diameter of the fine Ti-containing nitride introduced into the steel is not particularly specified, but from the measurement limit of the transmission electron microscope (TEM) to measure, the lower limit of the actual circular equivalent diameter is 10 nm. It is enough.

<Production Requirements>

In order to manufacture the thick steel plate which concerns on the 1st aspect of this invention which satisfy | fills the requirements mentioned above, it is preferable to manufacture a thick steel plate by suitably controlling the manufacturing requirements at the time of a solvent and the time of casting. Hereinafter, the manufacturing requirements will be described in detail for each item.

(Amount of dissolved oxygen in molten steel at the time of solvent)

At the time of a solvent, the amount of dissolved oxygen in molten steel is made into 0.00%-0.01% of range by mass deoxidation using Mn and if necessary, Si further. When the dissolved oxygen amount in molten steel is less than 0.002%, it becomes impossible to ensure the required amount of oxide which has a suitable composition used as the starting point of intragranular α production. On the other hand, when the dissolved oxygen amount exceeds 0.01%, a coarse oxide having a circular equivalent diameter of 2 µm or more is formed more than necessary, adversely affecting the HAZ toughness. The minimum with preferable dissolved oxygen amount is 0.0025%, and a preferable upper limit is 0.008%.

(Addition amount of Ti, REM, Zr)

In order to secure the predetermined | prescribed or more oxide with a circular equivalent diameter of 2 micrometers or more and less than 5 micrometers, it is preferable to adjust the addition amount of Ti, REM, Zr which are comparatively difficult to control among oxide formation elements, and the addition amount balance suitably. The amount of Ti added is more than 160 ppm and the total amount of REM and Zr is more than 55 ppm, and then the amount of balance is adjusted so that the value obtained from the formula of [Ti] / [REM] + [Zr] is 0.8 or more and less than 11.8. Just do it.

However, in the previous formula, [] is a value which shows the addition amount of each element etc. in mass%. In addition, [Ti], [REM], and [Zr] do not necessarily correspond with Ti amount, REM amount, and Zr amount in the steel material finally obtained. This is because these elements may be evaporated during manufacture or included in slag and removed.

(Addition order of oxide forming element)

The oxide-forming elements of Al, Ti, REM, Zr, and Ca need to be added in order of Al → Ti → (REM, Zr) → Ca. When each element is added in the order other than this addition order, it becomes impossible to ensure the required amount of oxide which has an appropriate composition used as the starting point of intragranular α production. In particular, since Ca has a property of having a very strong deoxidation force, when it is added prior to Ti or Al, all of oxygen associated with Ti or Al may disappear, thereby forming an oxide having a component composition defined in the first embodiment. You won't be able to. In addition, when adding both REM and Zr, the addition order may be ahead, and you may add simultaneously.

In addition, in addition of Al, Ti, REM, Zr, and Ca, as long as it mentions later, time from Ti addition to Ca addition may just be in a prescribed range, and it does not specifically limit about another space | interval.

(Time t1 from Ti addition to Ca addition)

The time t1 (minute) from Ti addition to Ca addition is 3 to 20 minutes. If the time t1 (minute) from the Ti addition to the Ca addition is shorter than 3 minutes, oxide formation by the element added before Ca addition does not proceed sufficiently, so that the required amount of oxide having an appropriate composition as a starting point of intragranular α production is secured. You can't. In addition, when this time t1 (minute) becomes longer than 20 minutes, generation | occurrence | production of an oxide will advance until Ca addition, and the composition of an oxide does not become desired, and the oxide which has an appropriate composition which can become a starting point of alpha production in a mouth The required amount cannot be secured. Moreover, the minimum with preferable time t1 (minute) from Ti addition to Ca addition is 5 minutes, and a preferable upper limit is 15 minutes.

(Time t2 from the addition of Ca to the start of casting)

The time t2 (minutes) from Ca addition to the start of casting is defined as a time satisfying between ta and tb obtained from the following equations (1) and (2), that is, ta (minutes) <t2 (minutes) <tb (minutes). .

[Equation 1]

[Equation 2]

However, in said Formula (1) and Formula (2), [] is the value which showed the addition amount of each element etc. in mass%.

In addition, [Ca], [Ti], [Al], [REM], and [Zr] do not necessarily coincide with the amount of Ca, Ti, Al, REM and Zr in the finally obtained steel. This is because these elements may be evaporated during manufacture or included in slag and removed.

The time t2 (minutes) from the addition of Ca to the start of casting is the time required for Ca to deprive oxygen from other oxides produced before Ca addition to form a desired oxide, and this time t2 (minutes) is equal to or less than ta (minutes). If it becomes, the formation reaction of Ca containing oxide after Ca addition will not fully advance, and it will become impossible to ensure the required amount of oxide which has a suitable composition which can become a starting point of intragranular alpha production. In addition, when this time t2 (minute) becomes tb (minute) or more, the formation reaction of Ca containing oxide after Ca addition will progress excessively, and it will become impossible to ensure the required amount of the oxide which has a suitable composition used as the starting point of intragranular α production. . Although t2 (minute) needs to satisfy ta (minute) <t2 (minute) <tb (minute) in this way, it is preferable from a viewpoint of productivity that it is as short time as possible in this range. In addition, Equations 1 and 2 for calculating ta and tb are obtained by repeated numerous experiments in consideration of the oxide-forming ability of each element.

The above is the condition of the time t2 (minutes) from Ca addition to the start of casting to secure the required amount of oxide, but in the thick steel sheet according to the first aspect of the present invention, 5 x 10 ⁶ Ti-containing nitrides having a diameter of 100 nm or less in the original equivalent. It is necessary to secure more than / mm2. For that purpose, the relationship between tx (minutes), t2 (minutes), ta (minutes), and tb (minutes), which is obtained from Equation 3 below, is also defined as ta (minutes) <tx (minutes) <t2 (minutes) < It is necessary to satisfy tb (minutes).

&Quot; (3) &quot;

However, in said Formula (3), [] is the value which showed the addition amount of each element etc. in mass%.

When Si is added, it is added from before deoxidation, and it is in the state oxidized until just before deoxidation. In order to secure the amount of solid solution Si more than a predetermined amount, since oxidized Si needs to be reduced enough by the strong deoxidation element which has affinity with oxygen rather than Si, it is set to tx (minute) <t2 (minute). On the other hand, since it is necessary to secure a predetermined amount of the oxide properly adjusted so as to be the starting point of the intragranular α, the time required for the reduction of the Si oxide needs to be shorter than the predetermined time, and t2 (minute) &lt; tb (minute )

(Cooling time t3 at the time of casting)

Cooling time t3 (second) in 1500-1450 degreeC at the time of casting shall be 300 second or less. When the cooling time t3 (seconds) exceeds 300 seconds, the amount of coarse oxide having a circular equivalent diameter of 5 µm or more increases, resulting in deterioration of the HAZ toughness. In addition, the cooling time t3 (minute) at the time of casting becomes like this. Preferably it is 280 second or less. Although the minimum of t3 (minute) is not specifically limited, Usually, it is preferable that it is about 190 second.

(Addition amount of oxide forming element)

The addition amount (mass%) of Ca in an oxide formation element, ie, [Ca], is an addition amount (mass) that satisfies A1 and B1 values obtained from Equations 4 and 5 below, i.e., A1 ≤ [Ca] ≤ B1. %). In addition, Equations 4 and 5 below which A1 and B1 values are obtained are obtained from repeated experiments.

&Quot; (4) &quot;

[Equation 5]

However, in the above-described equations (4) and (5), [Of] is the dissolved oxygen amount (mass%) before Ca addition, [Ti], [REM], and [Zr] are the addition amounts (mass%) of the molten steel of each element, respectively. )to be.

When the amount of Ca added is less than the A1 value, most of the added Ca is consumed as a single oxide of Ca, so that an oxide having an appropriate composition for becoming the starting point of intragranular α production cannot be ensured. On the other hand, when Ca addition amount exceeds B1 value, the ratio (mass%) of Ti in an oxide will become small, and also in this case, it becomes impossible to ensure the oxide which has a suitable composition for setting as the starting point of intragranular alpha production.

(Adjustment of Si addition amount and total addition amount of element with higher deoxidation ability than Si)

In the first aspect, it is necessary to secure 5 x 10 ⁶ nitrides / mm 2 or more of Ti-containing nitride having a circular equivalent diameter of 100 nm or less, but for this purpose, it is preferable to sufficiently secure the amount of solid solution Si that increases the amount of Ti activity in steel materials. . As described above, when Si is added, since it is added before deoxidation by Ca, Ti, Al, REM, and Zr, and is oxidized in a certain amount in the steelmaking process, in order to secure a desired amount of solid solution Si in steel, it is deoxidized than Si. It is necessary to make the sum total of the addition amount of a high-performance element, ie, Al, Ti, Ca, REM, Zr into 0.020% or more by mass%.

The preferable addition amount at the time of adding Si is 0.02% or more in mass%, and more preferable addition amount is 0.10% or more. On the other hand, preferable total addition amount of Al, Ti, Ca, REM, Zr is 0.025% or more, and more preferable total addition amount is 0.030% or more.

In producing the thick steel sheet according to the first aspect of the present invention, it is preferable to manufacture the thick steel sheet by appropriately controlling the production requirements at the time of solvent and casting described above, but after the solvent and casting, After hot rolling, it is recommended to perform quenching after performing quenching and heating in the austenite ferrite two-phase region.

Quenching after the said hot rolling may perform quenching (RQ) offline using a hot rolling material other than direct quenching (DQ) which quenchs immediately after hot rolling. In addition, in the case of DQ processing, retry is not possible. Therefore, strict temperature control of the quenching start temperature is required than in the case of RQ processing.

In addition, after quenching such as RQ treatment by heating in the austenite ferrite two-phase region, it is preferable to temper at a temperature not higher than the ferrite transformation start temperature (Ac1) to adjust the strength of the steel.

Next, the thick steel plate which concerns on the 2nd aspect of this invention is demonstrated concretely. In addition, description is abbreviate | omitted about the content common to the said basic structure.

About the component composition of the thick steel plate which concerns on 2nd aspect, the effect which makes Si and Mn demonstrated in the said basic structure becomes remarkable that the Si content makes 0.01-0.35 mass% and Mn content 1.0-2.0 mass%. Exerted.

After setting it as such a component composition, the said oxide is an oxide which satisfy | fills the relationship of Al / (REM + Zr) <0.7, and it is 300 or more per 1mm <2> and whose Ti equivalent diameter is less than 2 micrometers, and contains Ti containing nitride In addition, it is the characteristic feature of the structure of the thick steel plate which concerns on 2nd aspect of this invention that it comprised so that 5 * 10 <^6> pieces / mm <2> or more of Ti containing nitrides with a circular equivalent diameter of 100 nm or less may exist among said Ti containing nitrides.

Among the above oxides, the oxide satisfying the relationship of Al / (REM + Zr) <0.7 is liable to be liquefied during high temperature heating of HAZ, and has a crystal structure favorable for intragranular α production in the subsequent cooling process. In addition to the reduction of intragranular α / γ interfacial energy, a much lower (intergranular α / inclusion) interfacial energy can be realized, and as a result, generation of intragranular α is promoted more actively. As a result, in the thick steel sheet according to the second aspect of the present invention, by promoting intragranular α production in the base material, the base material structure can be refined to secure the base material toughness, and the resistance yield ratio can be realized, and the HAZ toughness, yield ratio and Any property of base metal toughness can be secured in a balanced manner.

In addition, with respect to the number of oxides satisfying the above-mentioned composition range, 300 or more per 1 mm 2 exist in a circular equivalent diameter of less than 2 µm, which can further promote intragranular α production.

Among the oxides, the number per 1 mm 2 of the equivalent circular diameter of less than 2 μm is preferably 350 or more, and more preferably 400 or more. Although the upper limit of the number is not specifically defined, In practice, it will be about 1,000 pieces / mm <2> or less.

Moreover, in the thick steel plate which concerns on the 2nd aspect of this invention, the number per 1 mm <2> of the said oxide whose circular equivalent diameter is 2 micrometers or more becomes like this. Preferably it is 85 or less, More preferably, it is 70 or less.

In the calculation of the value of Al / (REM + Zr) of the oxide of the second aspect, when the oxide does not contain REM or Zr, the content may be calculated as 0.

In the thick steel sheet according to the second aspect of the present invention, in addition to the oxide control, the thick steel sheet further has a feature of peening spherical grains of HAZ by fine Ti-containing nitride. The Ti-containing nitride in the second embodiment not only contains TiN but also replaces a part of Ti of TiN (about 50% or less by atomic ratio) with another nitride forming element (for example, Nb, Zr, V, etc.). It is also intended to include a nitride. Compared before the heat input of welding, since the Ti-containing nitride is finely dispersed and the water density can be secured very much in comparison with the oxide inclusions, even if most of the Ti-containing nitride is lost by the heat input welding, the Ti content is still present. The nitride can be further left to ensure the water density, and the remaining fine Ti-containing nitride can effectively exhibit the spherical γ-pinning effect. In order to reduce the variation of the HAZ toughness, that is, to maintain not only the average value but also the minimum value of the HAZ at a high level, the Ti-containing nitride is 5 × 10 ⁶ or more per 1 mm 2 having a circular equivalent diameter of 100 nm or less. It is because an effective pinning effect cannot be exhibited when it is less than 5x10 <^6> per 1mm <2>. The number of Ti-containing nitrides having a circular equivalent diameter of 100 nm or less is preferably 5.4 × 10 ⁶ or more, more preferably 6.5 × 10 ⁶ or more per ¹ mm 2. Although the upper limit of the number is not specifically defined, In practice, it is about 15 * 10 <^6> pieces / mm <^2> or less. Although the lower limit of the circular equivalent diameter of Ti containing nitride made into the thick steel plate which concerns on the 2nd aspect of this invention is not specifically limited, From the measurement limit of TEM used by the Example mentioned later, the minimum of a circular equivalent diameter is about 10 nm. .

In order to adjust the oxide of the 2nd form by which the chemical composition was suitably controlled as mentioned above for every size, it is preferable to suitably control the conditions at the time of a solvent and a casting. Namely, the amount of dissolved oxygen in molten steel during the solvent, (ii) the order of addition of oxide forming elements (Ti, Al, Ca, REM, Zr), and (iii) the addition of Ti to Ca in the oxide forming elements. It is effective to appropriately control the time of (iV), the time from the last oxide formation element (Ca) to the start of casting, and the cooling time in a predetermined temperature range at the time of (v) casting. In addition, in order to ensure a predetermined or more that the original equivalent diameter of the oxide of the second aspect is less than 2 µm, it is also important to control the addition amounts of the oxide-based forming elements in addition to (i) to (v). . Hereinafter, (i)-(vi) is explained in full detail.

(i) About dissolved oxygen amount in molten steel at the time of solvent

First, it is preferable to make dissolved oxygen amount 0.002 to 0.01% by deoxidation using Mn and Si at the time of a solvent. If the amount of dissolved oxygen is less than 0.002%, the required amount of the oxide of the second form cannot be secured. On the other hand, if the amount of dissolved oxygen exceeds 0.01%, coarse oxide is formed to adversely affect toughness. The dissolved oxygen amount is more preferably 0.0025% or more and 0.008% or less.

(ii) the order of addition of oxide forming elements

Oxide forming elements of Al, Ti, REM, Zr, and Ca are preferably added in the order of Al → Ti → (REM, Zr) → Ca. If it adds in order other than this, it becomes impossible to ensure the required amount of oxide which has a suitable composition as a starting point of intragranular alpha production. In particular, since Ca has an extremely strong deoxidation force, when it is added before Ti or Al, all of the oxygen associated with Ti or Al may disappear, and thus, an oxide having the desired composition cannot be formed. In addition, when adding both REM and Zr, either of the order of addition of REM and Zr may be ahead, and may add simultaneously.

In addition, in addition of Al, Ti, REM, Zr, and Ca, as long as it mentions later, time from Ti addition to Ca addition may just be in a prescribed range, and it does not specifically limit about another space | interval.

(iii) time from Ti addition to Ca addition in said oxide formation element

It is preferable that the time t1 (minute) from Ti addition to Ca addition in the said oxide formation element shall be 3-20 minutes. When t1 becomes shorter than 3 minutes, oxide generation by the element added before Ca addition does not fully advance, and the required amount of oxide of the appropriate composition which becomes a starting point of intragranular α production will not be obtained. In addition, when t1 exceeds 20 minutes, the formation of oxide proceeds excessively until Ca addition, and the composition of the oxide does not become desired, and the required amount of oxide having an appropriate composition that can be the starting point of intragranular α production is not obtained. . t1 (minutes), More preferably, they are 5 minutes or more and 15 minutes or less.

(iv) time until the start of casting after addition of the last oxide forming element (Ca)

It is preferable to control so that time t2 (minute) from addition of Ca to starting casting may be ta <t2 <tb in the relationship of ta and tb shown by following formula (1) and (2), respectively.

[Equation 1]

[Equation 2]

In the above formulas (1) and (2), [Ca], [Ti], [Al], [REM] and [Zr] are added amounts of Ca, Ti, Al, REM and Zr to the molten steel, respectively (mass%). Indicates. In addition, the said [Ca], [Ti], [Al], [REM], and [Zr] do not necessarily correspond with Ca amount, Ti amount, Al amount, REM amount, and Zr amount in the obtained steel materials. This is because these elements may be evaporated or included in the slag and removed.

The time t2 (minute) from the addition of Ca to the start of casting is the time required for Ca to deprive oxygen from oxides other than those produced before the addition of Ca to form oxides. The subsequent formation reaction of the Ca-containing oxide does not proceed sufficiently, and it is impossible to secure an oxide having an appropriate composition that can be the starting point of intragranular α production. Moreover, when t2 becomes tb or more, the formation reaction of Ca containing oxide after Ca addition will progress excessively, and also this will be unable to ensure the oxide which has a suitable composition for becoming a starting point of intragranular alpha production. From the viewpoint of productivity, it is preferable that t2 satisfies the above range and is as short as possible. Equations 1 and 2 are equations obtained from numerous experiments in consideration of the oxide forming ability of the elements.

(v) About cooling time in predetermined temperature range at the time of casting

It is preferable to make cooling time t3 (second) in 1500-1450 degreeC at the time of casting into 300 second or less. When t3 exceeds 300 seconds, the amount of coarse oxides, i.e., oxides having a diameter of 2 µm or more, increases, resulting in deterioration of the HAZ toughness. t3 becomes like this. More preferably, it is 280 second or less. The lower limit of t3 is not particularly limited, but is usually about 190 seconds.

(vi) About controlling the addition amounts of the oxide-based forming elements to each other

It is preferable to adjust Ca addition amount to the range of A1 <= Ca <= B1 in relationship with A1 value and B1 value calculated by following formula (4) and (5). The A1 value and B1 value defined in the following Equations 4 and 5 are obtained from numerous experiments.

&Quot; (4) &quot;

[Equation 5]

In formulas (4) and (5), [Of] is the dissolved oxygen amount (mass%) before Ca addition, [Ti], [REM], and [Zr] are Ti, REM and Zr addition amounts to molten steel, respectively. %).

If the amount of Ca added is less than the A1 value, most of the added Ca is consumed as a single oxide of Ca, so that an oxide having an appropriate composition for becoming the starting point of intragranular α production cannot be ensured. On the other hand, when Ca addition amount exceeds B1 value, the ratio of Ti in an oxide will become small, and this also cannot secure the oxide which has an appropriate composition for becoming the starting point of intragranular alpha production.

In addition, Al addition amount, REM addition amount, and Zr addition amount may be controlled so that [Al] / ([REM] + [Zr]) <1.0. When the value of [Al] / ([REM] + [Zr]) is 1.0 or more, the ratio of Al in the oxide is increased, and as a result, inclusions having high interfacial energy (intracellular α / inclusion) increase, resulting in intracellular α generation ability. It will fall. The value of [Al] / ([REM] + [Zr]) is more preferably 0.9 or less.

In addition to the preferable conditions for obtaining the oxide, in order to secure a predetermined or more Ti-containing nitride of the second aspect, (vii) the amount of added Si and the total amount of addition of elements higher in deoxidation than Si are adjusted; viii) It is desirable to more strictly control the time from the addition of Ca to the start of casting in addition to the requirements of (iv) above.

(vii) About adjustment of the amount of addition Si and the total amount of addition of the element whose deoxidation ability is higher than Si

In order to secure a predetermined amount of Ti-containing nitride of the second aspect, it is effective to secure an amount of solid solution Si that increases the amount of Ti in the steel. Since the amount of Si is added before deoxidation (deoxidation by Ca, Ti, Al, REM and Zr), and a certain amount is oxidized in the steelmaking process, in order to secure the above-mentioned solid solution Si, the amount of added Si is made 0.01% or more. At the same time, in order to reduce the Si in the oxidized state, it is effective to make the sum of the added amounts of elements higher in deoxidation than Si, that is, Al, Ti, Ca, REM, Zr, to be 0.020% or more. The amount of added Si is more preferably 0.02% or more, and still more preferably 0.10% or more. The sum total of the addition amount of Al, Ti, Ca, REM, Zr becomes like this. More preferably, it is 0.025% or more, More preferably, it is 0.030% or more.

(viii) for the time from the addition of Ca to the start of casting

The time t2 (minutes) from the addition of Ca to the start of casting has been described in the above (iv) that it is preferable to set ta <t2 <tb in order to secure the required amount of the oxide of the second aspect. From a viewpoint, it is effective to control so that the relationship of ta <tx <t2 <tb is satisfied in relationship with tx shown by following formula (3).

&Quot; (3) &quot;

Si is added before deoxidation and is oxidized until just before deoxidation. In order to secure a predetermined amount or more of solid solution Si, since it is necessary to reduce oxidized Si sufficient time by the strong deoxidation element which has affinity with oxygen than Si, it is preferable to set it as t2> tx. On the other hand, in order to achieve compatibility with the oxide of the second aspect (an oxide properly adjusted to be the starting point of intragranular α production), the time required for the reduction of the Si oxide needs to be less than or equal to predetermined, and t2 &lt; Let tb be.

Next, the thick steel plate which concerns on the 3rd form of this invention is demonstrated concretely. In addition, description is abbreviate | omitted about the content common to the said basic structure.

About the component composition of the thick steel plate which concerns on 3rd aspect, Si content is 0.25 mass% or less (including 0%), Mn content is 1.0-2.0 mass%, and both REM and Zr are REM: 0.0003 It is remarkable that the effect of containing these elements demonstrated in the said basic structure as what is contained in the range of -0.02 mass% and Zr: 0.0003-0.02 mass% is exhibited remarkably. And,

A = 10 ⁴ × {[Ti]-0.7 × ([O] -0.09 × [Al]-0.16 × [Ca]-0.07 × [REM]-0.04 × [Zr])} × [N]

The value A obtained from the formula of satisfies 0.4 ≦ A ≦ 2.4, and the oxide is an oxide that satisfies 3% <N, 2 <Ti / N, and has a circular equivalent diameter of less than 2 µm. What is comprised so that it may become 300 or more per mm <2> is a structural characteristic of the thick steel plate which concerns on 3rd form of this invention. In addition, in said Formula A, [] shows content (mass%) of each element.

First, in the thick steel sheet according to the third aspect of the present invention, the value A obtained from the above formula A must satisfy 0.4 ≦ A ≦ 2.4, but when this value A is lower than 0.4, sufficient TiN cannot be obtained. This is because when the HAZ toughness is lowered and A value is larger than 2.4, coarse TiN is crystallized in molten steel to adversely affect HAZ toughness.

In addition, the present inventors stabilize TiN by complex precipitation of TiN on the surface of the oxide described in the basic configuration, while the intra-alpha promoted action by the oxide and the intra-alpha promoted action of the composite precipitated TiN on the oxide, It has been found to be synergistically promoted to further promote intragranular α production, thus leading to obtaining a thick steel sheet according to the third aspect.

And in these composite inclusions, excellent HAZ toughness is obtained by disperse | distributing 300 or more mm <2> or more oxides whose original equivalent diameter is less than 2 micrometers.

(More than 300 oxides / mm 2 with a circular equivalent diameter of less than 2 μm)

By making the circular equivalent diameter of an oxide less than 2 micrometers, HAZ toughness can be promoted by intra-alphanumeric promotion. If the original equivalent diameter of the oxide is 2 µm or more, TiN is in a state of covering the entire surface of the oxide particles, so that the oxide does not contribute to intragranular α production, thereby reducing the intragranular α production amount. In addition, the composition of the oxide is outside the range of 3% <N, 5% <Ca <40%, 5% <REM <50%, 5% <Zr <40%, and 2 <Ti / N in mass%. If the thick steel sheet according to the third aspect of the present invention is not expected to produce sufficient intragranular α. In addition, when there are less than 300 oxides / mm <2> of oxides whose diameter is less than 2 micrometers, since the origin of intragranular α production is insufficient, the amount of intracellular α production similarly does not increase as expected in the thick steel sheet according to the third aspect. , Sufficient HAZ toughness cannot be obtained. Although the upper limit of the number is not specifically defined, In practice, it will be about 1,000 pieces / mm <2> or less.

(Production method)

In order to manufacture the thick steel plate which concerns on the 3rd aspect of this case which satisfy | fills the said requirements, it is necessary to manufacture a thick steel plate so that the following manufacturing requirements may be satisfied.

The production requirement is that after the deoxidation using Mn, Si, etc. at the time of a solvent, the dissolved oxygen amount in molten steel is made into 0.002-0.01% by mass%, and after (Al->) Ti-> (REM, Zr)-> Ca, Each element is added while controlling so that time t1 from Ti addition to Ca addition may be 3-20 minutes in order, and it maintains the time t2 (minute) from Ca addition to casting start in the range of 40-90 minutes. In addition, the cooling time t3 in the temperature range of 1500-1450 degreeC at the time of casting is 300 seconds or less, and when rolling the obtained ingot, the heating temperature Th before rolling is 1050-1200 degreeC, and the heating time t4. It is good to set it as 2 to 5 hours, and to make the microcrystal area | region reduction rate at the time of rolling into 40% or more. Next, the reason for regulation of these manufacturing requirements is explained in detail.

Dissolved oxygen content in molten steel before (Al) Ti addition: 0.002 to 0.01%

When the dissolved oxygen amount in molten steel before (Al) Ti addition is lower than 0.002%, it becomes impossible to ensure the required amount of oxide which has a suitable composition used as the starting point of intragranular α production. Moreover, when dissolved oxygen amount is higher than 0.01%, coarse oxide with a circular equivalent diameter of 2 micrometers or more increases, and HAZ toughness will deteriorate.

In the case of a solvent, (Al →) Ti → (REM, Zr) → Ca is added in the order

When each element is added in the order other than this addition order, it becomes impossible to ensure the required number of oxides which have an appropriate composition used as the starting point of intragranular α production. In particular, since Ca has a very strong deoxidation power, when it is added before Ti or Al, all oxygen associated with Ti or Al is lost.

In addition, in addition of Al, Ti, REM, Zr, and Ca, as long as it mentions later, time from Ti addition to Ca addition may just be in a prescribed range, and it does not specifically limit about another space | interval.

Time t1 from Ti addition to Ca addition is from 3 to 20 minutes

When the time t1 from the Ti addition to the Ca addition is shorter than 3 minutes, the reaction of the oxide prior to the Ca addition does not proceed sufficiently, so that the required number of oxides having an appropriate composition serving as the starting point of intragranular α production cannot be secured. If the time t1 is longer than 20 minutes, the reaction of the oxide prior to the addition of Ca proceeds excessively, and it becomes impossible to secure the required number of oxides having an appropriate composition as the starting point of intragranular α production.

Time t2 from the addition of Ca to the start of casting is 40 to 90 minutes

In order to obtain TiN complex precipitated on the oxide, it is necessary to reduce the oxide type Ti in the solvent step to increase the solid solution Ti. If the time t2 is shorter than 40 minutes, subsequent composite precipitation TiN cannot be sufficiently secured. On the other hand, when the time t2 is longer than 90 minutes, the reaction of the oxide after Ca addition proceeds excessively (REM and Zr are reduced excessively), so that an oxide having an appropriate composition which becomes the starting point of α production in the mouth cannot be obtained. do.

The cooling time t3 at 1500 to 1450 ° C. during casting within 300 seconds

When the cooling time t3 at 1500-1450 degreeC at the time of casting exceeds 300 second, coarse TiN which adversely affects toughness is determined by component segregation at the time of solidification, and HAZ toughness will deteriorate. The lower limit of t3 is not particularly limited, but is usually about 190 seconds.

Heating temperature Th before rolling is 1050-1200 degreeC, and heating time t4 is 2-5 hours

In the step of heating before rolling, a composite precipitation of TiN on the oxide surface is obtained. When heating temperature Th before rolling is less than 1050 degreeC, or heating time t4 is less than 2 hours, composite precipitation of TiN becomes inadequate. On the other hand, when the heating temperature Th prior to rolling is higher than 1200 degreeC, the composite precipitation amount of TiN will become inadequate and a predetermined oxide will not be acquired as needed. Moreover, when heating time t4 is longer than 5 hours, the composite precipitation amount of TiN will be saturated, but a base material structure will coarsen and it will adversely affect the toughness of a base material.

ㆍ 40% or more of microcrystalline region reduction rate

By applying the reduction in the microcrystalline region, strain is introduced around the oxide, and the strain becomes a driving source, thereby further obtaining composite precipitated TiN. If this microcrystalline area | region reduction rate is less than 40%, sufficient strain will not be introduce | transduced and composite precipitation TiN will run short.

Next, the thick steel plate which concerns on this form 4 is demonstrated concretely. In addition, description is abbreviate | omitted about the content common to the said basic structure.

About the component composition of the thick steel plate which concerns on 4th aspect, Si content is 0.02-0.50 mass%, Mn content is 1.4-3.0 mass%, and both REM and Zr are REM: 0.0003-0.02 mass%, Zr The effect of containing these elements demonstrated in the said basic structure as for what is contained in the range of: 0.0003-0.02 mass% is exhibited remarkably. And further, it contains Cr: 0.5-2.0 mass%,

D = 62 × [Mn] + 27 × [Ni] + 111 × [Cr]

While the D value obtained from the formula satisfies 238 <D <388, in the above oxides, the equivalent circular diameter is less than 2 µm, 300 or more per mm 2, and the equivalent circular diameter is 2 µm or more, 30-70 pieces / mm <2> of oxides with a circular equivalent diameter of 2 micrometers or more and less than 5 micrometers exist, and less than 30 pieces / mm <2> of oxides with a circular equivalent diameter of 5 micrometers or more exist in the thick steel plate which concerns on 4th aspect of this invention. It is a characteristic of construction. In addition, in said D formula, [] shows content (mass%) of each element.

In the thick steel sheet according to the fourth aspect of the present invention, containing 0.5 to 2.0 mass% of Cr is an effective element for securing high strength of 780 MPa of tensile strength, and therefore it is necessary to add 0.5 mass% or more. However, when excessively added, an excessive increase in the strength of the HAZ is caused, which adversely affects the toughness of the HAZ. Therefore, it is necessary to suppress the content to 2.0% by mass or less. The minimum with preferable content of Cr is 0.6 mass%, and a more preferable minimum is 0.7 mass%. Moreover, a preferable upper limit is 1.8 mass%, and a more preferable upper limit is 1.6 mass%.

In addition, in the thick steel sheet according to the fourth aspect of the present invention, the D value obtained from the above D expression needs to satisfy 238 <D <388, but when the D value is lower than 238, the generation of intragranular α at the oxide starting point is sufficient. If the D-value is greater than 388, the strength of the HAZ is excessively increased and the HAZ toughness cannot be secured, because the HAZ toughness improvement effect expected in the thick steel sheet according to the fourth aspect is not obtained. Because.

As described above, the thick steel sheet according to the fourth aspect was subjected to high heat input welding in a high strength steel sheet having a tensile strength of 780 MPa class by performing balance control of alloying elements in addition to the control of oxide composition and size. Although it becomes possible to improve the minimum value of the HAZ toughness at the time, the high strength steel plate of tensile strength 780 Mpa class specifically refers to the high strength steel plate whose tensile strength exceeds 780 Mpa.

The reason why an oxide having a circular equivalent diameter of less than 2 µm is 300 particles / mm 2 or more is for obtaining the amount of generation of intragranular α to the extent that the HAZ toughness expected in the thick steel sheet according to the fourth aspect of the present invention is exhibited. Although the upper limit of the number is not specifically defined, In practice, it will be about 1,000 pieces / mm <2> or less.

An oxide having a circular equivalent diameter of 2 µm or more and less than 5 µm has a higher ability to generate intragranular α compared to an oxide having a circular equivalent diameter of less than 2 µm, and thus has a tensile strength of 780 MPa by dispersing 30 / mm 2 or more. Also in the high-strength steel sheet of the class, there is a remarkable effect on the refinement of the HAZ structure. On the other hand, when the number density exceeds 70 pieces / mm 2, the adverse effect as the brittle fracture starting point is present, so it is necessary to control the number to 70 pieces / mm 2 or less.

An oxide having a circular equivalent diameter of 5 µm or more has a great adverse effect on the toughness of HAZ as a starting point for brittle fracture, and therefore it is necessary to control it to less than 30 / mm 2.

In addition, the thick steel plate which concerns on 4th aspect of this invention is an oxide in which 10% <REM + Zr <70% is satisfy | filled among the said oxides, and the mass ratio (Ti / Ca) of Ti and Ca is more than 1 and less than 1.4, and a circular equivalent diameter is It is more preferable that 300 or less mm <2> or more of oxides less than 2 micrometers exist.

(Production method)

In order to manufacture the thick steel plate which concerns on the 4th aspect of this invention which satisfy | fills the requirements mentioned above, it is necessary to manufacture a thick steel plate so that the following manufacturing requirements may be satisfied.

The production requirements are, in the order of Al → Ti → (REM, Zr) → Ca, after the amount of dissolved oxygen in molten steel is 0.002 to 0.01% by mass deoxidation using Mn and Si at the time of solvent. While controlling so that time t1 from Ti addition to Ca addition is 3-20 minutes, each element is added, and time t2 (minute) from Ca addition to casting start is ta (minute) <t2 (minute) The cooling time t3 in the temperature range of 1500-1450 degreeC at the time of casting is kept within the range which satisfy | fills <tb (min), and it is within 300 second. In addition, the addition amount of Ti, REM, and Zr at this time must make the value calculated | required by [Ti] / ([REM] + [Zr]) 0.8 or more and less than 11.8. In addition, in a previous formula, [] shows the addition amount (mass%) of molten steel of each element. The reason for regulation of these production requirements is described in detail below.

In addition, ta (minute) and tb (minute) shown previously can be calculated | required from the following formulas.

However, [Ca], [Ti], [Al], [REM] and [Zr] represent the addition amounts (mass%) of Ca, Ti, Al, REM and Zr to the molten steel, respectively.

In addition, the said [Ca], [Ti], [Al], [REM], and [Zr] do not necessarily correspond with Ca amount, Ti amount, Al amount, REM amount, and Zr amount in the obtained steel materials. This is because these elements may be evaporated or included in the slag and removed.

The constituent elements excluding oxygen were 10% <Ti, Al <20%, 5% <Ca <40%, 5% <REM <50%, 5% <Zr <40%, and 10% by mass. In order to satisfy | fill% <REM + Zr <70% and the thing which is 300 pieces / mm <2> or more of oxides whose diameter is less than 2 micrometers among oxides whose mass ratio (Ti / Ca) of Ti and Ca is more than 1 and less than 1.4, It is good to control the addition amount [Ca] of Ca in the range of A1 <= Ca <= B1 calculated | required based on the following formula. In addition, the value of Al and B1 calculated | required based on the following formulas is calculated | required by experiment.

However, [Of] represents the dissolved oxygen amount (mass%) before Ca addition, [Ti], [REM], and [Zr] represents the addition amount (mass%) of Ti, REM, and Zr to molten steel, respectively.

That is, when Ca addition amount [Ca] is less than A1 value, since most of Ca added is consumed as an oxide of Ca single | piece | group, the oxide which becomes a starting point of intragranular α production (an oxide whose constituent element satisfy | fills the said requirement) is sufficient. Will not be obtained. When the amount of Ca added [Ca] exceeds the B1 value, the Ti / Ca ratio in the oxide is less than 1, so that the required number of oxides as the starting point of intragranular α production cannot be secured.

Dissolved oxygen in molten steel before Al addition: 0.002 to 0.01%

When the dissolved oxygen amount in molten steel before Al addition is lower than 0.002%, the required amount of oxide having an appropriate composition serving as a starting point of intragranular α production can be ensured. Moreover, when dissolved oxygen amount is higher than 0.01%, coarse oxide with a circular equivalent diameter of 2 micrometers or more increases, and HAZ toughness will deteriorate.

At the time of solvent, Al → Ti → (REM, Zr) → Ca added in order

When each element is added in the order other than this addition order, it becomes impossible to ensure the required number of oxides which have an appropriate composition used as the starting point of intragranular α production. In particular, since Ca has a very strong deoxidation power, when it is added before Ti or Al, all oxygen associated with Ti or Al is lost.

In addition, in addition of Al, Ti, REM, Zr, and Ca, as long as it mentions later, time from Ti addition to Ca addition may just be in a prescribed range, and it does not specifically limit about another space | interval.

Time t1 from Ti addition to Ca addition is from 3 to 20 minutes

When the time t1 from the Ti addition to the Ca addition is shorter than 3 minutes, the reaction of the oxide prior to the Ca addition does not proceed sufficiently, so that the required number of oxides having an appropriate composition serving as the starting point of intragranular α production cannot be secured. If the time t1 is longer than 20 minutes, the reaction of the oxide prior to the addition of Ca proceeds excessively, and it becomes impossible to secure the required number of oxides having an appropriate composition which becomes the starting point of IGB production.

The time t2 (minutes) from Ca addition to the start of casting satisfies ta (minutes) &lt; t2 (minutes) &lt; tb (minutes)

The time t2 from the addition of Ca to the start of casting is a requirement that affects the production state of the oxide (the time Ca takes oxygen from other oxides to form an oxide), and when this time t2 becomes less than ta (minutes), Ca is added. Subsequent oxide reaction does not advance sufficiently, and it becomes impossible to ensure the required number of oxides having an appropriate composition serving as a starting point of intragranular α production. In addition, when this time t2 becomes tb (minute) or more, reaction of the oxide after Ca addition will progress excessively and it will become impossible to ensure the required number of oxides with an appropriate composition which becomes a starting point of intragranular alpha production. In addition, the formula which calculates ta and tb is calculated | required by experiment in consideration of being easy to become the oxide of each element.

The cooling time t3 at 1500 to 1450 ° C. during casting within 300 seconds

When the cooling time t3 at 1500-1450 degreeC at the time of casting exceeds 300 second, the production amount of coarse oxide with a circular equivalent diameter of 5 micrometers or more will increase, and HAZ toughness will deteriorate. The lower limit of t3 is not particularly limited, but is usually about 190 seconds.

Addition amount of Ti, REM, Zr: The value obtained by [Ti] / ([REM] + [Zr]) is 0.8 or more and less than 11.8.

When the value calculated by [Ti] / ([REM] + [Zr]) is less than 0.8, the addition amount of REM and Zr which are strong deoxidation elements increases compared with Ti which is a plundering element. In such an amount of addition, since the free oxygen concentration in molten steel falls and the oxide growth rate at the time of subsequent Ca addition falls, 30 or more mm <2> or more oxides whose original equivalent diameters are 2 micrometers or more and less than 5 micrometers cannot be ensured. . On the other hand, when the value determined by [Ti] / ([REM] + [Zr]) becomes 11.8 or more, on the contrary, the oxide whose circular equivalent diameter is 2 micrometers or more and less than 5 micrometers will exceed 70 pieces / mm <2>.

In addition, in manufacturing the thick steel plate which concerns on the 4th aspect of this invention, although it does not specifically limit about the manufacturing method other than the requirements in the solvent or casting step demonstrated above, After heating the obtained casting piece, it hot-rolls, After quenching, it is recommended to reheat the austenite ferrite two-phase region again and to perform a quenching tempering treatment.

It is preferable that the thick steel plate which concerns on said 1st thru | or 3rd aspect contains at least 1 group chosen from following (a)-(c) as another element.

(a) 1 or more types selected from the group consisting of Ni: 1.50 mass% or less, Cu: 1.50 mass% or less, Cr: 1.50 mass% or less, and Mo: 1.50 mass% or less,

(b) at least one selected from the group consisting of Nb: 0.10 mass% or less and V: 0.10 mass% or less,

(c) B: 0.005 mass% or less

The reason for adding these elements is demonstrated below.

1 or more types chosen from the group which consists of Ni: 1.50 mass% or less, Cu: 1.50 mass% or less, Cr: 1.50 mass% or less, and Mo: 1.50 mass% or less

Ni, Cu, Cr and Mo are all effective elements for improving the strength-toughness balance of the steel sheet, and the effect is increased as their content increases. In order to exhibit such an effect effectively, it is preferable to contain all 0.05% or more, and it is more preferable to contain 0.10% or more. However, Ni is an expensive element and is preferably suppressed to 1.50% or less from the viewpoint of cost, and preferably to 1.20% or less. In addition, when Cu, Cr, and Mo are contained in excess, there is a possibility of causing excessive increase in strength and deterioration of the toughness of the base metal and HAZ. Therefore, it is preferable to suppress all to 1.50% or less, and to 1.20% or less. It is more preferable to do.

1 or more types chosen from the group which consists of Nb: 0.10 mass% or less and V: 0.10 mass% or less

Nb and V precipitate as carbonitride and suppress coarsening of (gamma) grains, and are effective elements for improving base material toughness. Although the effect increases as their content increases, in order to exhibit such an effect effectively, it is preferable to contain all 0.002% or more, and it is more preferable to contain 0.005% or more. However, excessively containing them may cause coarsening of the HAZ structure and deteriorate the HAZ toughness. Therefore, it is preferable to suppress them all at 0.10% or less, more preferably 0.08% or less, even more preferably 0.05 It is made into% or less.

B: 0.005 mass% or less

B is an element effective in improving the toughness of the base material and HAZ by suppressing the generation of coarse grain boundary α. Although the effect increases as the content increases, in order to effectively exhibit such an effect, it is preferable to contain B 0.0010% or more, and it is more preferable to contain 0.0015% or more. However, when the content of B becomes excessive, there is a risk of causing precipitation of BN at the austenite grain boundary and deterioration of the toughness of the base metal and the HAZ. The upper limit with more preferable content of B is 0.004%, and a more preferable upper limit is 0.003%.

Moreover, it is preferable that the thick steel plate which concerns on said 4th aspect mentioned above contains another at least 1 group chosen from the following (a ')-(c') as another element.

(a ') 1 or more types chosen from the group which consists of Ni: 1.50 mass% or less, Cu: 1.50 mass% or less, and Mo: 1.50 mass% or less,

(b ') 1 or more types chosen from the group which consists of Nb: 0.10 mass% or less, V: 0.10 mass% or less,

(c ') B: 0.005 mass% or less

The reason for adding these elements is demonstrated below.

1 or more types chosen from the group which consists of Ni: 1.50 mass% or less, Cu: 1.50 mass% or less, and Mo: 1.50 mass% or less

Ni, Cu, and Mo are all effective elements for improving the strength-toughness balance of the steel sheet, and the effect is increased as their content increases. In order to exhibit such an effect effectively, it is preferable to contain all 0.05% or more, and it is more preferable to contain 0.10% or more. However, Ni is an expensive element and is preferably suppressed to 1.50% or less from the viewpoint of cost, and preferably to 1.20% or less. In addition, when Cu and Mo are contained in excess, there is a risk of excessive increase in strength and deterioration of the toughness of the base metal and HAZ. Therefore, it is preferable to suppress all to 1.50% or less, and to make it 1.20% or less. More preferred.

1 or more types chosen from the group which consists of Nb: 0.10 mass% or less and V: 0.10 mass% or less

Nb and V precipitate as carbonitride and suppress coarsening of (gamma) grains, and are effective elements for improving base material toughness. Although the effect increases as their content increases, in order to exhibit such an effect effectively, it is preferable to contain all 0.002% or more, and it is more preferable to contain 0.005% or more. However, excessively containing them may cause coarsening of the HAZ structure and deteriorate the HAZ toughness. Therefore, it is preferable to suppress them all at 0.10% or less, more preferably 0.08% or less, even more preferably 0.05 It is made into% or less.

B: 0.005 mass% or less

B is an element effective in improving the toughness of the base material and HAZ by suppressing the generation of coarse grain boundary α. Although the effect increases as the content increases, in order to effectively exhibit such an effect, it is preferable to contain B 0.0010% or more, and it is more preferable to contain 0.0015% or more. However, when the content of B becomes excessive, there is a risk of causing precipitation of BN at the austenite grain boundary and deterioration of the toughness of the base metal and the HAZ. The upper limit with more preferable content of B is 0.004%, and a more preferable upper limit is 0.003%.

Although this is the thick steel plate which concerns on the 1st-4th aspect, generally, a thick steel plate means the steel plate whose plate | board thickness is 3.0 mm or more as defined by JIS2402. Although the thick steel sheet which concerns on 1st-4th aspect of this invention exhibits the outstanding characteristic especially in the thick steel plate of the plate thickness which is 50 mm or more especially, these are only the preferable forms, and this invention is a board of 3 mm or more and less than 50 mm. It does not exclude the application to thick steel plates of thickness.

[Example]

Hereinafter, although an Example is given and this invention is demonstrated further more concretely, this invention is not limited by the following example of course, It is also possible to add a change suitably in the range which may be suitable for the meaning of this invention. If possible, they are all included in the technical scope of the present invention.

[Example of the thick steel plate which concerns on the first form]

In the Example, first, the steel of each component composition shown in Tables 1-3 is melted by the vacuum melting furnace (No.1-54: 50kgVIF, No.55-57: 150kgVIF), and the By casting a cast piece using molten steel and hot rolling using the cast piece, in the case of Nos. 1 to 54, a hot rolled plate having a sheet thickness of 50 mm is used. A hot rolled sheet having a thickness of 80 mm was obtained. The hot rolled sheet was quenched by heating to an austenite-ferrite two-phase region, and then tempered at 50 ° C to obtain a thick steel sheet for testing.

In manufacturing this thick steel sheet for testing, each controlled condition is shown in Tables 4 to 6. The conditions are the amount of dissolved oxygen [Of], Al, Ti, (REM, Zr) in the molten steel before Al addition, the order of addition of Ca, the time t1 from the Ti addition to the Ca addition, the time t2 from the Ca addition to the start of casting, the casting It is each addition amount of cooling time t3, Al, Ti, REM, Zr, Ca in 1500-1450 degreeC of city.

In addition, A1 and B1 values for determining the appropriate amount of Ca addition, the total amount of Al, Ti, Ca, REM, and Zr contained in the thick steel plate, and the original equivalent diameters for securing a predetermined or more oxide of 2 µm or more and less than 5 µm It also shows whether or not preferable conditions {[Ti]> 160 ppm, ([REM] + [Zr])> 55 ppm and (0.8 ≦ [Ti] / [REM] + [Zr]) ≦ 11.8)} are also satisfied. .

In addition, in Tables 1-3, REM was added in the form of the misch metal which contains about 50% Ce and about 25% La by mass%. In addition, in Tables 1-3, "-" shows that the element is not added.

In addition, in Table 4-Table 6, addition order of Al, Ti, (REM, Zr), Ca is "(circle)" when adding Al → Ti → (REM, Zr) → Ca in the order other than that. When added in order, it represents with "x". In addition, since No. 33 did not add Ca, the addition order was shown by "-".

In addition, about time t2 from Ca addition to a start of casting, it is "(circle)" that satisfy | filling said ta (minutes) <tx (minutes) <t2 (minutes) <tb (minutes) "*" Represented by In addition, regarding Ca addition amount [Ca], what satisfy | filled the relationship of said A1 <= Ca <= B1 was shown as "(circle)", and "x" which did not satisfy.

Using each thick steel sheet manufactured according to the above requirements, the number density of oxides (oxide-based inclusions) of various sizes, the number density of Ti-containing nitrides, the production rate of α in the matrix, the tensile strength (TS), and the yield ratio (YR) , Base material toughness and HAZ toughness were determined by measurement. These measurement results are shown in Tables 7-9.

(Measurement of the Number Density of Oxides with a Circle Equivalent Diameter of Less Than 2 µm)

From the surface of each thick steel plate, the test piece is cut out from the position of depth t / 4 (t: plate thickness) (taken so that the axis of the test piece passes through the position of t / 4), and a cross section parallel to the rolling direction and the plate thickness direction, It observed using the field emission scanning electron microscope "SUPRA 35" (brand name) (henceforth FE-SEM) by Carl Zeiss. The observation conditions were magnification: 5000 times, observation visual field: 0.0024 micrometer <^2> , observation place: 20 places. By image analysis, the area of each oxide in this observation visual field was measured, and the circular equivalent diameter of each oxide was computed from the area. In addition, it was confirmed by EDX (energy dispersion type X-ray detector) that each oxide satisfy | fills said component composition. And the number N1 of oxides whose original equivalent diameter becomes less than 2 micrometers among oxides which satisfy | fills said component composition was calculated | required in conversion of the number density of 1 mm <2>. However, since the reliability of EDX was not enough about the oxide whose circular equivalent diameter becomes 0.2 micrometer or less, it removed from analysis.

(Measurement of Number Density of Oxides with Original Equivalent Diameters of 2 µm or More and Less than 5 µm)

From the surface of each thick steel plate, the test piece is cut out from the position of depth t / 4 (t: plate thickness) (taken so that the axis of the test piece passes through the position of t / 4), and a cross section parallel to the rolling direction and the plate thickness direction, Observation was made using FE-SEM. The observation conditions were magnification: 1000 times, observation visual field: 0.06 micrometer <^2> , observation place: 20 places. By image analysis, the area of each oxide in this observation visual field was measured, and the circular equivalent diameter of each oxide was computed from the area. In addition, it was confirmed by EDX that each oxide satisfy | fills said component composition. And the number N2 of oxides whose circular equivalent diameter becomes 2 micrometers or more and less than 5 micrometers among oxides which satisfy | fills said component composition was calculated | required in conversion of the number density of 1 mm <2>.

(Measurement of the Number Density of Oxides with a Circular Equivalent Diameter of 5 µm or More)

From the surface of each thick steel plate, the test piece is cut out from the position of depth t / 4 (t: plate thickness) (taken so that the axis of the test piece passes through the position of t / 4), and a cross section parallel to the rolling direction and the plate thickness direction, Observation was made using FE-SEM. The observation conditions were magnification: 1000 times, observation visual field: 0.06 micrometer <^2> , observation place: 20 places. By image analysis, the area of each oxide in this observation visual field was measured, and the circular equivalent diameter of each oxide was computed from the area. In addition, it was confirmed by EDX that each oxide satisfy | fills said component composition. And the number N3 of oxides whose circular equivalent diameter becomes 5 micrometers or more among the oxide which satisfy | fills the above-mentioned component composition was calculated | required in conversion of the number density equivalent to 1mm <2>.

(Measurement of Number Density of Ti-containing Nitride)

The site | part of the position of depth t / 4 (t: plate 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 visual field: 2 micrometers x 2 micrometers, observation place: 5 places. By image analysis, the area of each Ti-containing nitride in this observation visual field was measured, and the circular equivalent diameter of each Ti-containing nitride was computed from the area. In addition, as Ti containing nitride, what detected the peak of Ti and N when it analyzed by EDX was made into Ti containing nitride. And the number (N4) of Ti containing nitride whose circular equivalent diameter becomes 100 nm or less was calculated | required in conversion of the number density of 1 mm <2>.

(Measurement of In-situ α Production Rate of the Base Material)

From the surface of each thick steel plate, the test piece is cut out from the position of depth t / 4 (t: plate thickness) (taken so that the axis of the test piece passes through the position of t / 4), and a cross section parallel to the rolling direction and the plate thickness direction, Observation was carried out using an optical microscope. The observation conditions were made into magnification: 400 times, observation visual field: 0.04 mm <2>, observation location: 20 places. For an oxide of 2 m or more, at least one central axis of the lath phase α extending radially in one or more directions starting from the oxide has a difference of 15 ° or more from the mean of the central axis of the Lath phase α group around the oxide. In this case, the oxide was determined to be the starting point of intragranular α, and the (number of oxides starting from intragranular α) / (the total number of oxides) were taken as the intragranular α production rate of the base material.

(Evaluation of tensile strength and yield ratio)

From the position of the depth t / 4 (t: sheet thickness) from the surface of each thick steel sheet, a No. 4 test piece of JIS Z 2201 was taken in a direction perpendicular to the rolling direction, and subjected to a tensile test by the method described in JIS Z 2241. , Tensile strength (TS) and yield strength (YS) were measured, respectively. The yield ratio (YR) was calculated by the calculation called YS / TS. In the present Example, the obtained TS evaluated that 490 Mpa or more and YR are less than 80% were excellent in mechanical properties.

(Evaluation of Base Metal Toughness)

From the position of the depth t / 4 (t: plate thickness) from the surface of each thick steel plate, take three Charpy impact test specimens (No. 4 specimen of JIS Z 2201) (so that the axis of the specimen passes the position of said t / 4) Collection), the Charpy impact test was conducted at 0 ° C, the absorbed energy (vE ₀ ) was measured to obtain their average value, and the average value was used as the base material toughness of each thick steel sheet. In the present embodiment, is not less than the average value of the obtained vE ₀ 200J, was evaluated that the base material is excellent in toughness.

(Evaluation of HAZ Toughness)

From the position of the depth t / 4 (t: plate thickness) from the surface of each thick steel plate, take three Charpy impact test specimens (No. 4 specimen of JIS Z 2201) (so that the axis of the specimen passes the position of said t / 4) Collection) to reproduce a HAZ heat cycle V notch Charpy impact test. Reproduction HAZ heat cycle conditions simulate the thermal history of the bond part in the electrogas arc welding of 100 kJ / mm of heat input: holding time at 1400 degreeC for 45 second, and cooling time at 800-500 degreeC for 800 second. It was. About each test piece which gave this heat cycle, the absorption energy (vE0) in ₀ degreeC was measured, and the average value of three test pieces was calculated | required. In this embodiment, that the average value of the obtained vE ₀ or greater of reference 70J required for the construction of steel, it was evaluated HAZ toughness is excellent.

Nos. 1 to 23 and 47 to 57 are examples of inventions that satisfy the requirements of the second aspect. The chemical composition, the oxide and the Ti-containing nitride are suitably dispersed, and the tensile strength (TS) and yield ratio (YR) , The HAZ toughness when the base material toughness and the heat input amount were 100 kJ / mm, all satisfied the evaluation criteria of this example. That is, Nos. 1 to 23 and 47 to 57 are excellent in HAZ toughness at the time of high heat input welding in which the heat input amount is 100 kJ / mm or more, and at the same time, it is possible to realize a resistance ratio in a high strength region of 490 MPa or more. In addition, it can be said that it is a thick steel plate excellent in the toughness of the weld heat-affected zone which can also secure good base metal toughness.

On the other hand, Nos. 24 to 46 are comparative examples which do not satisfy any of the requirements of the second aspect, and the tensile strength (TS), yield ratio (YR), base material toughness and heat input amount were 100 kJ / mm. It can be seen that either of the HAZ toughness in the case does not satisfy the evaluation criteria.

Further, Tables 4 to 6 show preferable conditions for securing a predetermined or more oxide having a circular equivalent diameter of 2 µm or more and less than 5 µm {[Ti]> 160 ppm, ([REM] + [Zr])> 55 ppm, (0.8 ≦ [ Ti] / [REM] + [Zr]) ≤ 11.8)} is represented by (circle), but No. 31 and No. 36 which do not satisfy this condition have a circular equivalent diameter of 2 µm or more. It turns out that the number of oxides which are less than 5 micrometers is low after No.33 which does not add Ca.

[Examples about thick steel sheet according to the second aspect of the present]

Steels of various component compositions (excluding Si, Al, Ti, REM, Zr, and Ca) shown in Tables 10 and 11 were subjected to the conditions shown in Tables 12 and 13 in a vacuum melting furnace (VIF: 50 kg). Dissolved oxygen, Al, Ti, REM, Zr, Ca addition order, time t1 from Ti addition to Ca addition, time t2 from Ca addition to start of casting and amount of Si, Al, Ti, REM, Zr, Ca addition ), And the molten steel of the chemical composition shown in Table 10, Table 11, and cast the molten steel so that the cooling time t3 at 1500 to 1450 ℃ at the time of casting to the conditions shown in Table 12, Table 13 A cast piece was obtained (cross-sectional shape: 120 mm x 120 mm). The cast piece was heated to hot roll to obtain a hot rolled sheet having a plate thickness of 50 mm.

In addition, in Table 10 and Table 11, "-" shows that the element is not added. In Table 12 and Table 13, [REM] was added in the form of a misch metal containing about 50% Ce and about 25% La. "Additional order" in Table 12 and Table 13 was represented by "(circle)" when Al-> Ti-> (REM, Zr)-> Ca, and in all other cases, it showed by "x". Time t2 from Ca addition to casting start was represented by "(circle)" and other than "x" when ta <tx <t2 <tb was satisfied. In addition, in Table 12 and Table 13, when the amount of added Ca satisfies the requirement of A1≤ [Ca] ≤B1, it was indicated by "(circle)" beside Ca addition amount. In addition, since No.32 is an example of no Ca addition, the addition order in Table 13, and the column of t1 and t2 were shown with "-".

Further, the hot rolled sheet was quenched by heating to an austenite-ferrite two-phase region, and then tempered at 500 ° C.

For each steel sheet obtained as described above, the number density of oxide, number density of Ti-containing nitride, tensile strength, yield ratio, base material toughness, and HAZ toughness were measured in the following manner.

(1) Measurement of the number density of oxides

Equivalent diameter  Oxides less than 2 μm

From the surface of each steel plate, the test piece is cut out from the depth t / 4 position (t: sheet thickness) (collected so that the axis of the test piece passes the t / 4 position), and a cross section parallel to the rolling direction and the sheet thickness direction is cut out. Observation magnification: 5000 times, Observation visual field: 0.0024 mm <2>, It observed in 20 places using the electrolytic-radiation scanning electron microscope "SUPRA 35 (brand name) made by Corporation. And the area of each oxide in the visual field was measured by image analysis, and the circular equivalent diameter of each oxide was computed from this area. In addition, it was determined by EDX (energy dispersive X-ray detector) whether each oxide satisfy | filled the composition of the above-mentioned 2nd form of oxide. And the composition of the oxide of the 2nd aspect was satisfy | filled, and the number N1 of oxides whose original equivalent diameter becomes less than 2 micrometers was calculated | required in conversion per 1 mm <2>.

Equivalent diameter  2㎛ or more oxide

Except for making the observation magnification 1000 times and making the observation visual field 0.06 mm <2>, it carried out similarly to the measurement of the number density of the oxide of less than 2 micrometers in circular equivalents, and satisfy | filled the composition of the oxide of a 2nd form, and 2 micrometers in circular equivalent diameters The number N2 of oxides described above was calculated in terms of 1 mm 2.

(2) Measurement of Number Density of Ti-containing Nitride

The t / 4 position (t: plate | board thickness) of each steel plate was observed with the transmission electron microscope (TEM) on the conditions of observation magnification: 60,000 times, observation visual field: 2 micrometers x 2 micrometers, and five observation places. And the area of each Ti containing nitride in the visual field was measured by image analysis, and the circular equivalent diameter of each Ti containing nitride was computed from the area. In addition, when Ti and N peaks were detected when it analyzed by EDX, it was set as Ti containing nitride. And the number (N3) of Ti containing nitride whose circular equivalent diameter becomes 100 nm or less was calculated | required in conversion per 1 mm <2>.

(3) Measurement of intramolecular α production rate of base material

Cut the specimen from the depth t / 4 position (t: sheet thickness) from the surface of each steel sheet (taken so that the axial center of the specimen passes through the position of t / 4), and cross-section parallel to the rolling direction and the sheet thickness direction is taken through an optical microscope. It observed using the conditions of observation magnification: 400 times, observation visual field: 0.04 mm <2>, observation place: 20 points. If the central axis of the lath phase α having the inclusion as one end has an angle of 15 ° or more with the central axis of the α surrounding the inclusion, for the inclusion of 2 µm or more, the inclusion is judged to be the starting point of the α in the mouth. The number of inclusions) / (the total number of inclusions) serving as the starting point of the α production was defined as the intra-α production rate.

(4) tensile strength and yield ratio

From the t / 4 position (t: sheet thickness) of each steel plate, the No. 4 test piece of JIS Z 2201 was taken in the direction orthogonal to a rolling direction, the tensile test was performed according to JIS Z 2241, and the tensile strength (TS), Yield strength (YS) was measured, respectively, and yield ratio (YR) = YS / TS was computed. And TS evaluated that 590 Mpa or more and YR were less than 80% were excellent in mechanical characteristics.

(5) Measurement of base metal toughness

Charpy impact test specimens (No. 4 test specimen of JIS Z 2201) were taken from the t / 4 position (t: sheet thickness) of each steel sheet (collected so that the axis of the specimen passes through the t / 4 position in the rolling direction), and Charpy at 0 ° C. An impact test was carried out to measure absorbed energy (vE ₀ ). The measurement was performed about three test pieces, and the average value was made into the base material toughness of each steel plate. vE ₀ is equal to or greater than 200J, it evaluated that the base metal toughness is excellent.

(6) measurement of HAZ toughness

Charpy impact test specimens (No. 4 specimen of JIS Z 2201) were taken from the t / 4 position (t: sheet thickness) of each steel sheet (collected so that the axial center of the specimen passed the t / 4 position in the rolling direction) to reproduce the HAZ thermal cycle. V notch Charpy impact test was done. Reproduction HAZ heat cycle conditions were set to hold time at 1400 degreeC: 45 seconds, and cooling time to 800-500 degreeC: 800 degreeC, and simulated the thermal history of the bond part in the electroslug welding of 100 kJ / mm heat input. . The absorbed energy (vE ₀ ) at 0 ° C. was measured for the test pieces to which the thermal cycle was applied. The measurement was performed about three test pieces, and the average value and minimum value were calculated | required. And, the average value of vE ₀ 100J or more, it was evaluated excellent HAZ toughness is not less than the minimum value is 70J.

The results are shown in Table 14 and Table 15.

Nos. 1 to 22 and 46 to 53 are examples satisfying the requirements specified in the second aspect, and since the chemical composition, oxide and Ti-containing nitride are properly controlled, the HAZ toughness is excellent (both in average and minimum) at the same time. Excellent base material toughness and mechanical properties (high strength and resistance ratio).

On the other hand, Nos. 23 to 45 are examples in which the base material toughness and the HAZ toughness are lowered because they do not satisfy the component composition or preferable manufacturing conditions.

Since No.23 had a large amount of C, many hard island martensite (MA) was produced, and base metal toughness and HAZ toughness deteriorated. In addition, since the amount of C increased, hardenability became high and YR became high because the bainite structure formed at a comparatively low temperature was mainly used. Since No. 24 had a small amount of Si, Ti activity could not be raised, Ti-containing nitride was reduced, and base metal toughness and HAZ toughness were deteriorated. Since No. 25 had many Si amounts, many hard island martensite (MA) was produced and the base material toughness and HAZ toughness deteriorated. Since No. 26 had much Mn amount, intensity | strength rose too much and the base material toughness and HAZ toughness deteriorated. In addition, since the amount of Mn was large, the hardenability became high and YR increased because the bainite structure formed at a relatively low temperature was mainly used. No. 27 and 28 had a large amount of P and S, respectively, so that the base metal toughness and the HAZ toughness deteriorated. Since No. 29 had a large amount of Al, coarse oxide was formed and the base material toughness and HAZ toughness deteriorated. Since No. 30 had a small amount of Ti, the number of Ti-containing nitrides and the number of Ti-containing nitrides in the oxide equivalent to less than 2 µm could not be secured, resulting in deterioration of base metal toughness and HAZ toughness. No. 31 is an example in which the amount of Ti was large, and the number of oxides having a diameter equivalent to 2 µm or more in the oxide was increased, and the base metal toughness and the HAZ toughness deteriorated. Since No. 32 had little Ca amount, the number of things less than 2 micrometers in circular equivalent diameter in an oxide could not be ensured, and base material toughness and HAZ toughness deteriorated. Since No. 33 had a large amount of Ca, the thing of less than 2 micrometers of original equivalent diameters and 2 micrometers or less could not be adjusted to an appropriate number, and base metal toughness and HAZ toughness deteriorated. Since No. 34 had a large amount of N, the toughness of the base material and HAZ was deteriorated by deformation aging. Nos. 35 and 36 each had a large amount of REM and Zr, so that the number of oxides having a diameter equivalent to 2 µm or more in the oxides increased, resulting in deterioration of base metal toughness and HAZ toughness. No. 37 and 38 had a large amount of Cu and Cr, respectively, so that the strength increased excessively, resulting in deterioration of the base metal toughness and the HAZ toughness. Moreover, hardenability became high because there were many amounts of Cu and Cr, and YR became high because the bainite structure formed at a comparatively low temperature is mainly used.

No. 39 is not preferable to the addition order of Al, Ti, REM, Zr, Ca, No. 41, 42 is not appropriate time t1 from Ti addition to Ca addition, No. 42, 43 is added Ca Since the time t2 until the start of casting afterwards was not appropriate, the number of oxides with a diameter of less than 2 µm per circular equivalent could not be secured, and the toughness of the base metal and the HAZ was deteriorated. Since No. 44 had long cooling time t3 at 1500-1450 degreeC at the time of casting, the number of oxides with a diameter equivalent to 2 micrometers or more increased, and the toughness of a base material and HAZ deteriorated. In No. 45, since the time t2 from the addition of Ca to the start of casting was tx or less, the number of Ti-containing nitrides could not be secured, and the toughness of the base metal and the HAZ was deteriorated.

[Example of thick steel plate which concerns on 3rd form]

First, the steel of each component composition shown in Table 16 and Table 17 is melted by the vacuum melting furnace (VIF: 150 kg), and the casting piece (cross-sectional shape: 150 mm x 250 mm) is cast using this molten steel, Moreover, the hot rolling board of 80 mm of plate | board thickness was obtained by performing hot rolling using this casting piece. In addition, it adjusted by changing slab thickness in order to change the unrecrystallization reduction ratio P shown in Table 18 and Table 19.

In manufacturing this hot rolled sheet (thick steel sheet), each controlled condition is shown in Table 18 and Table 19. The conditions are the amount of dissolved oxygen [Of] in molten steel before Al (Ti) addition, the addition order of Al, Ti, REM, Zr, Ca, the time t1 from the Ti addition to the Ca addition, the time t2 from the Ca addition to the start of casting. , Heating temperature Th before rolling, heating time t4 before rolling, and uncrystallized reduction rate P.

In Table 16 and Table 17, REM was added in the form of a misch metal containing about 50% Ce and about 25% La by mass%. In addition, in Table 16 and Table 17, "-" shows that the element is not added.

In addition, in Table 16 and Table 17, the order of addition of Al, Ti, REM, Zr, and Ca is "(circle)" and the order other than that in the case of (Al-> Ti-> (REM, Zr)-> Ca. Is represented by "x".

Using each hot rolled sheet (thick steel sheet) manufactured according to the above requirements, the number density (N1) of inclusions having a diameter of less than 2 µm, the number density (N2) of inclusions having a diameter of 2 µm or more, and HAZ toughness It calculated | required by the following measurement. These measurement results are shown in Table 20 and Table 21.

(Measurement of the Number Density of Oxides with a Circle Equivalent Diameter of Less Than 2 µm)

From the surface of each thick steel plate, the test piece is cut out from the position of depth t / 4 (t: plate thickness) (taken so that the axis of the test piece passes through the position of t / 4), and a cross section parallel to the rolling direction and the plate thickness direction, It observed using the field emission scanning electron microscope "SUPRA 35" (brand name) (henceforth FE-SEM) by Carl Zeiss. The observation conditions were magnification: 5000 times, observation visual field: 0.0024 micrometer <^2> , observation place: 20 places. By image analysis, the area of each interference | inclusion in this observation visual field was measured, and the circular equivalent diameter of each interference | inclusion was computed from the area. In addition, it was confirmed by EDX (energy dispersion type X-ray detector) that each inclusion satisfy | fills said component composition. The acceleration voltage at the time of component composition measurement by EDX is 15 kV, and the measurement time is 100 second. And the number (N1) of the inclusions whose circular equivalent diameter becomes less than 2 micrometers was calculated | required in conversion into the number density of 1 mm <2>. However, since the reliability of EDX was not enough about the inclusion whose circular equivalent diameter becomes 0.2 micrometer or less, it removed from analysis.

(Measurement of the Number Density of Oxides with a Circle Equivalent Diameter of 2 µm or More)

From the surface of each thick steel plate, the test piece is cut out from the position of depth t / 4 (t: plate thickness) (taken so that the axis of the test piece passes through the position of t / 4), and a cross section parallel to the rolling direction and the plate thickness direction, Observation was made using FE-SEM. The observation conditions were magnification: 1000 times, observation visual field: 0.06 micrometer <^2> , observation place: 20 places. By image analysis, the area of each interference | inclusion in this observation visual field was measured, and the circular equivalent diameter of each interference | inclusion was computed from the area. In addition, it was confirmed by EDX (energy dispersion type X-ray detector) that each inclusion satisfy | fills said component composition. The acceleration voltage at the time of component composition measurement by EDX is 15 kV, and the measurement time is 100 second. And the number N2 of inclusions whose circular equivalent diameter becomes 2 micrometers or more was calculated | required in conversion to the number density of 1 mm <2>.

(Evaluation of HAZ Toughness)

From each thick steel plate, the test piece for welding joints was extract | collected, and after performing V line processing, electrogas arc welding was performed at the heat input amount: 50 kJ / mm. From these test pieces, the Charpy impact test piece (V notch test piece of JIS Z 2202) which processed the notch part in the HAZ of the weld line (bond) of the position of depth t / 4 (t: plate thickness) from the surface of each thick steel plate is 3 Each sample was taken out, the Charpy impact test was performed at _-40 degreeC, the absorption energy (vE- ₄₀ ) was measured, and those average value and minimum value were calculated | required. From this measurement result, it was evaluated that the average value of vE- ₄₀ exceeded 180 J and the minimum value exceeded 120 J as being excellent in HAZ toughness.

In addition, the Charpy impact test was carried out even under the same conditions as those described above except that electrogas arc welding was performed at a heat input amount of 60 kJ / mm, and the absorbed energy (vE- ₄₀ ) of the three test pieces was measured, and the average value thereof was measured. Was obtained. From this measurement result, it was evaluated that the average value of vE- ₄₀ exceeded 120 J as being excellent in HAZ toughness.

Nos. 1 to 30 are examples of inventions satisfying the requirements of the third aspect, wherein the HAZ toughness (average value and minimum value) when the chemical composition, dispersion of the inclusions, etc. are properly formed, and the heat input amount is 50 kJ / mm, and It turns out that HAZ toughness (average value) in the case of making heat input into 60 kJ / mm is excellent. That is, it can be said that No.1-30 is a thick steel plate excellent in the toughness of a weld heat affected zone.

On the other hand, Nos. 31 to 50 are comparative examples which do not satisfy any of the requirements of the third aspect, and the HAZ toughness (average value and minimum value) and the heat input amount when the heat input amount is 50 kJ / mm are 60 kJ /. It turns out that either of HAZ toughness (average value) in the case of being mm does not satisfy | fill evaluation criteria.

[Example of thick steel plate which concerns on 4th form]

First, the steel of each component composition (in addition, Al, Ti, REM, Zr, and Ca describes the mass% after addition) shown in Table 22 and Table 23 by a solvent using a vacuum melting furnace (VIF: 150 kg). Then, the cast piece (cross-sectional shape: 150 mm x 250 mm) was cast using this molten steel, and the hot rolled plate of 50 mm thickness was obtained by performing hot rolling using this cast piece.

In manufacturing this hot rolled sheet (thick steel sheet), each controlled condition is shown in Table 24 and Table 25. FIG. The conditions are the amount of dissolved oxygen [O], Al, Ti, REM, Zr, and Ca in molten steel before Al addition, [Ti] / ([REM] + [Zr]), and the time from Ti addition to Ca addition. t1, time t2 from Ca addition to casting start, cooling time t3 in 1500-1450 degreeC at the time of casting, Ca addition amount [Ca].

In addition, in Table 22 and Table 23, REM was added by mass form in the form of the misch metal containing about 50% Ce and about 25% La. In addition, in Table 22 and Table 23, "-" shows that the element is not added.

In addition, in Table 22 and Table 23, when addition order of Al, Ti, REM, Zr, and Ca is order of Al → Ti → (REM, Zr) → Ca when it is "(circle)" and other order Is represented by "x". In addition, about time t2 from Ca addition to a start of casting, it is represented by "(circle)" and what does not satisfy that said ta (minute) <t2 (minute) <tb (minute) is represented by "x".

In addition, regarding Ca addition amount [Ca], what satisfy | filled the relationship of said A1 <= Ca <= B1 was shown as "(circle)", and "x" which did not satisfy.

Using each hot rolled sheet (thick steel sheet) manufactured according to the above requirements, the number density, HAZ toughness, and tensile strength of oxides (oxide-based inclusions) of various sizes were determined by measurement. These measurement results are shown in Table 24 and Table 25.

(Measurement of the Number Density of Oxides with a Circle Equivalent Diameter of Less Than 2 µm)

From the surface of each thick steel plate, the test piece is cut out from the position of depth t / 4 (t: plate thickness) (taken so that the axis of the test piece passes through the position of t / 4), and a cross section parallel to the rolling direction and the plate thickness direction, It observed using the field emission scanning electron microscope "SUPRA 35" (brand name) (henceforth FE-SEM) by Carl Zeiss. The observation conditions were magnification: 5000 times, observation visual field: 0.0024 micrometer <^2> , observation place: 20 places. By image analysis, the area of each oxide in this observation visual field was measured, and the circular equivalent diameter of each oxide was computed from the area. In addition, it was confirmed by EDX (energy dispersion type X-ray detector) that each oxide satisfy | fills said component composition. And the number N1 of oxides whose diameter of a circular equivalent becomes less than 2 micrometers was calculated | required in conversion into the number density of 1 mm <2>. However, since the reliability of EDX was not enough about the oxide whose circular equivalent diameter becomes 0.2 micrometer or less, it removed from analysis.

In the measured oxides, the constituent elements excluding oxygen contained 10% <Ti, Al <20%, 8% <Ca <40%, 5% <REM <50%, and 5% <Zr <40 in mass%. % And more than 10% &lt; REM + Zr &lt; 70%, and the number (NA) of oxides having a diameter equivalent to less than 2 µm in a number density of equivalent to 1 mm &lt; 2 &gt; It calculated | required in conversion.

(Measurement of Number Density of Oxides with Original Equivalent Diameters of 2 µm or More and Less than 5 µm)

From the surface of each thick steel plate, the test piece is cut out from the position of depth t / 4 (t: plate thickness) (taken so that the axis of the test piece passes through the position of t / 4), and a cross section parallel to the rolling direction and the plate thickness direction, Observation was made using FE-SEM. The observation conditions were magnification: 1000 times, observation visual field: 0.06 micrometer <^2> , observation place: 20 places. By image analysis, the area of each oxide in this observation visual field was measured, and the circular equivalent diameter of each oxide was computed from the area. In addition, it was confirmed by EDX (energy dispersion type X-ray detector) that each oxide satisfy | fills said component composition. And the number (N2) of oxides whose circular equivalent diameter becomes 2 micrometers or more and less than 5 micrometers was calculated | required in conversion of the number density of 1 mm <2>.

(Measurement of the Number Density of Oxides with a Circular Equivalent Diameter of 5 µm or More)

From the surface of each thick steel plate, the test piece is cut out from the position of depth t / 4 (t: plate thickness) (taken so that the axis of the test piece passes through the position of t / 4), and a cross section parallel to the rolling direction and the plate thickness direction, Observation was made using FE-SEM. The observation conditions were magnification: 1000 times, observation visual field: 0.06 micrometer <^2> , observation place: 20 places. By image analysis, the area of each oxide in this observation visual field was measured, and the circular equivalent diameter of each oxide was computed from the area. In addition, it was confirmed by EDX (energy dispersion type X-ray detector) that each oxide satisfy | fills said component composition. And the number N3 of oxides whose circular equivalent diameter becomes 5 micrometers or more was calculated | required in conversion to the number density of 1 mm <2>.

(Evaluation of HAZ Toughness)

After cutting the test piece of 12.5 x 32 x 55 mm from the position of depth t / 4 (t: plate thickness) from the surface of each thick steel plate, and holding it at 1400 degreeC for 5 seconds, the cooling time of 800 degreeC-500 degreeC is 200 The speed was controlled to cool to seconds. This is a thermal cycle that simulates each joint submerged arc welding (heat input amount: 50 kJ / mm). From these test pieces, three Charpy impact test pieces (V notch test pieces of JIS Z 2202) were taken, and a Charpy impact test was conducted at 0 ° C to measure absorbed energy (vE ₀ ). From each of these three Charpy impact test measurement results, it was evaluated that the minimum absorption energy (vE ₀ ) (min) exceeded 70 J as being excellent in HAZ toughness.

(Evaluation of tensile strength)

From the position of the depth t / 4 (t: sheet thickness) from the surface of each thick steel sheet, a No. 4 test piece of JIS Z 2201 was taken at right angles to the rolling direction, and subjected to a tensile test of JIS Z 2241 to tensile strength (TS). Was obtained. Satisfying TS> 780 MPa was evaluated as being excellent in strength.

Nos. 1 to 32 are examples of the invention that satisfy the requirements of the fourth aspect, and it is understood that chemical composition, oxide dispersion, and the like are appropriately made, and the HAZ toughness and strength are excellent. In other words, Nos. 1 to 32 can be said to be thick steel plates excellent in the strength and toughness of HAZ after large heat input.

Also, in general, HAZ toughness and tensile strength tend to show negative correlation. Among the invention examples satisfying the requirements of the fourth aspect, when comparing Nos. 2, 6, and 32 with tensile strengths of 810 to 820 MPa, only No. 6 in which the oxide number NA did not satisfy the requirements of claim 2 was compared. , E ₀ (min) is less than 110 kJ.

On the other hand, No. 33-64 is a comparative example which does not satisfy the requirements of any of the requirements of a 4th aspect, It turns out that at least one of HAZ toughness and intensity | strength does not satisfy | fill evaluation criteria.

Although this application was demonstrated in detail with reference to the specific embodiment, it is clear for those skilled in the art that various changes and correction can be added without deviating from the mind and range of this invention.

This application is a Japanese patent application (Japanese Patent Application No. 2010-088347) of an application on April 7, 2010, a Japanese patent application (Japanese Patent Application No. 2010-116485) of an application on May 20, 2010, and September 29, 2010. Japanese patent application (Japanese Patent Application No. 2010-218205) of one application, Japanese patent application (Japanese Patent Application No. 2010-219511) of an application on September 29, 2010, Japanese patent application of March 1, 2011 (Japan Patent Application No. 2011-044075, the contents of which are incorporated herein by reference.

Claims (8)

C: 0.02 to 0.15 mass%, Si: 0 to 0.50 mass%, Mn: 1.0 to 3.0 mass%, greater than P: 0 mass%, 0.03 mass% or less, S: greater than 0 mass%, 0.015 mass% or less, Al: 0 to 0.07 mass%, Ti: 0.010 to 0.08 mass%, Ca: 0.0005 to 0.010 mass%, N: 0.002 to 0.020 mass%,
It is a thick steel plate which further contains at least 1 type selected from REM: 0.0001-0.02 mass% and Zr: 0.0001-0.02 mass%, and remainder is iron and an unavoidable impurity,
The constituent elements excluding oxygen satisfy 10% <Ti, Al <20%, 5% <Ca <40% in mass%, and at least among 5% <REM <50% and 5% <Zr <40% An thick steel sheet, characterized in that an oxide satisfying either one exists to satisfy the following conditions (1) and (2).
(1) 200 or more mm <2> of the said oxides whose circular equivalent diameter is less than 2 micrometers,
(2) The said oxide whose circular equivalent diameter is 2 micrometers or more is 100 piece / mm <2> or less. The Si content is 0-0.46 mass%, Mn content is 1.0-2.0 mass%,
In the said oxide, in a circle equivalent diameter is 2 micrometers or more,
30-70 pieces / mm <2> of oxides with a circular equivalent diameter of 2 micrometers or more and less than 5 micrometers exist,
Further, there are less than 30 oxides / mm 2 of oxide having a diameter of 5 µm or more,
The thick steel plate which contains Ti containing nitride and has 5 * 10 <^6> pieces / mm <2> or more of Ti containing nitrides with a circular equivalent diameter of 100 nm or less in the said Ti containing nitrides. The Si content is 0.01-0.35 mass%, Mn content is 1.0-2.0 mass%,
The oxides are also oxides satisfying the relationship of Al / (REM + Zr) <0.7, and the equivalent diameter is less than 2 µm, 300 or more per mm 2,
The thick steel plate which contains Ti containing nitride and has 5 * 10 <^6> pieces / mm <2> or more of Ti containing nitrides with a circular equivalent diameter of 100 nm or less in the said Ti containing nitrides. The Si content is 0 to 0.25 mass%, Mn content is 1.0 to 2.0 mass%, and both REM and Zr are in the range of REM: 0.0003-0.02 mass%, Zr: 0.0003-0.02 mass%. Including,
A = 10 ⁴ × {[Ti]-0.7 × ([O] -0.09 × [Al]-0.16 × [Ca]-0.07 × [REM]-0.04 [Zr])} × Losing A value satisfies 0.4≤A≤2.4,
The said steel oxide is an oxide which satisfy | fills 3% <N and 2 <Ti / N further, The thick steel plate whose 300 equivalent diameter is less than 2 micrometers per 300 mm <2>.
However, in said formula, [] shows content (mass%) of each element. The Si content is 0.02 to 0.50 mass%, Mn content is 1.4 to 3.0 mass%, and both REM and Zr are in the range of REM: 0.0003-0.02 mass% and Zr: 0.0003-0.02 mass%. It contains and contains Cr: 0.5-2.0 mass% further,
The D value obtained from the formula D = 62 × [Mn] + 27 × [Ni] + 111 × [Cr] satisfies 238 <D <388,
Among the oxides, the equivalent diameter is less than 2 µm and 300 or more per mm 2, and the equivalent diameter is 2 µm or more.
30-70 pieces / mm <2> of oxides with a circular equivalent diameter of 2 micrometers or more and less than 5 micrometers exist,
Moreover, the thick steel plate in which less than 30 piece / mm <2> of oxides with a circular equivalent diameter of 5 micrometers or more exist.
However, in said formula, [] shows content (mass%) of each element. The oxide according to claim 5, wherein the oxide satisfies 10% <REM + Zr <70% and further satisfies a mass ratio Ti / Ca of Ti and Ca of more than 1 and less than 1.4, and has an original equivalent diameter of less than 2 µm. The thick steel sheet which exists more than 300 pieces / mm <2>. The thick steel sheet according to any one of claims 1 to 4, wherein the thick steel sheet further includes at least one group selected from the following (a) to (c).
(a) 1 or more types selected from the group consisting of Ni: 1.50 mass% or less, Cu: 1.50 mass% or less, Cr: 1.50 mass% or less, and Mo: 1.50 mass% or less,
(b) at least one selected from the group consisting of Nb: 0.10 mass% or less and V: 0.10 mass% or less,
(c) B: 0.005 mass% or less. The thick steel sheet according to claim 5 or 6, wherein the thick steel sheet includes at least one group selected from the following (a ') to (c') as another element.
(a ') 1 or more types chosen from the group which consists of Ni: 1.50 mass% or less, Cu: 1.50 mass% or less, and Mo: 1.50 mass% or less,
(b ') 1 or more types chosen from the group which consists of Nb: 0.10 mass% or less, V: 0.10 mass% or less,
(c ') B: 0.005 mass% or less.