JP4356949B2 - Thick steel plate with excellent toughness in weld heat affected zone - Google Patents

Description

  The present invention relates to a thick steel plate applied to a welded structure such as a bridge, a high-rise building, and a ship, and in particular, a heat-affected zone after high heat input welding of 50 kJ / mm or more (hereinafter simply referred to as “HAZ”). A thick steel plate having excellent toughness.

  In recent years, with the increase in size of the above various welded structures, it is inevitable to weld thick steel plates having a plate thickness of 50 mm or more. For this reason, in all fields, high heat input welding of 50 kJ / mm or more is directed from the viewpoint of improving welding construction efficiency.

  However, if high heat input welding is performed, the HAZ is heated to a high temperature austenite region and then gradually cooled, so the structure of the HAZ part (particularly near the bond part of the HAZ part) becomes coarse and the toughness of that part deteriorates. There is a problem that it is easy to do. It has been a long-standing problem to ensure such good toughness in the HAZ portion (hereinafter sometimes referred to as “HAZ toughness”).

  Various techniques for preventing the deterioration of the HAZ toughness during high heat input welding have been proposed so far. As a representative example of such technology, as shown in Patent Document 1, for example, fine TiN is dispersed and precipitated in a steel material, thereby suppressing austenite grain coarsening that occurs in HAZ when high heat input welding is performed. However, a steel material that suppresses the deterioration of the HAZ toughness has been proposed. However, in such post-surgery, when the weld metal is heated to a high temperature of 1400 ° C. or higher, the TiN disappears from the solid solution due to the heat received during welding, particularly in the portion of the HAZ that is close to the weld metal (bond portion). There is a problem that deterioration of the film cannot be sufficiently suppressed.

  For this reason, for example, Patent Document 2 proposes to improve HAZ toughness by using MgO to finely disperse TiN and using it as an alternative to TiN that dissolves during high heat input welding. . However, in the case of using an oxide, there is a problem that uniform fine dispersion that is comparable to TiN is difficult and variations in characteristics are likely to occur.

  Further, in Patent Document 3, in order to suppress coarse TiN having a particle size exceeding 0.1 μm, by optimizing the distribution of fine TiN having a particle size of 0.01 to 0.1 μm, A technique for improving the HAZ toughness has also been proposed. However, sufficient HAZ toughness cannot be ensured only by optimizing the distribution of fine TiN.

  By the way, the present inventors have previously proposed a steel material in which the HAZ toughness does not deteriorate even when subjected to high-temperature heat effects during welding. In this technology, a large amount of N is added to the steel, and the balance of addition of Ti and B is appropriately controlled to increase the amount of TiN that remains in an insoluble state after welding, thereby improving the HAZ toughness. It is.

In addition, the present inventors actively include Nb in TiN-based inclusions present in the steel for welding, control the Ti / Nb ratio, and have an inclusion having a particle size of 0.01 to 0.25 μm. A technique for securing HAZ toughness in a wide heat input range by setting the number of objects to 1.0 × 10 ⁴ or more per 1 mm ² is also proposed (for example, Patent Document 5).
Japanese Patent Publication No.55-26164 JP-A-10-298708 JP 2001-98340 A JP-A-2005-200716 JP 2004-2181010 A

  However, in the field of welding, the actual situation is that further improvement of the HAZ toughness is required. In addition to the average value of HAZ toughness (Charpy absorbed energy), it is also necessary to meet the user needs to further improve the minimum value to improve the toughness balance in the vicinity of the bond.

  The present invention has been made in view of such a situation, and the purpose thereof is a thick steel plate that is excellent in HAZ toughness even when large heat input welding with a heat input of 50 kJ / mm or more is performed. An object of the present invention is to provide a thick steel plate capable of raising the minimum toughness near the bond of HAZ, improving the average value of toughness, and improving the toughness balance near the bond.

The thick steel plate according to the present invention that has solved the above problems is C: 0.03 to 0.10% (meaning “mass%”; the same applies hereinafter), Si: 0.2% or less (0% Mn: 1.0 to 2.0%, P: 0.03% or less (not including 0%), S: 0.01% or less (not including 0%), Al: 0.01 to 0.10%, Nb: 0.005 to 0.020%, Ti: 0.015 to 0.040%, N: 0.0060 to 0.0120%, B: 0.0010 to 0.0050%, respectively. containing, Rutotomoni such a balance of iron and inevitable impurities, the following (1) equation relationship satisfied, and the following Ti-containing nitride 0.05μm circle equivalent diameter 1 mm ² per 5.0 × 10 ⁶ cells Of these, the number of Ti-containing nitrides having an equivalent circle diameter of 0.01 to 0.03 μm is 75% of the total Ti-containing nitrides. It has a gist in that it occupies the above.
[Ti] × 16 [Si] × (12-40 [C]) <0.38 (%) (1)
However, [Ti], [Si] and [C] indicate the contents (mass%) of Ti, Si and C, respectively.

  The “equivalent circle diameter” refers to the diameter of a circle that is assumed to have the same area by paying attention to the size of the Ti-containing nitride, and is a transmission electron microscope (TEM) observation surface. Of the nitrides identified above. Further, the Ti-containing nitride targeted in the present invention includes not only TiN but also a part of Ti (at an atomic ratio of about 50% or less) to other nitride-forming elements (for example, Nb, Zr, V, etc.). This also includes the nitride substituted with ().

  In the thick steel plate of the present invention, if necessary, (a) one or more elements selected from the group consisting of Cu, Ni and Cr: 0.1 to 1.5% in total, (b) Mo and / or V : 0.5% or less in total (excluding 0%), (c) Zr: 0.005 to 0.050%, (d) one or more elements selected from the group consisting of Ca, Mg and REM: It is also useful to contain 0.0005 to 0.0050% in total, and the inclusion of such elements further improves the properties of the thick steel plate depending on the type.

  According to the present invention, while satisfying the relationship of the above formula (1), the chemical composition of the steel sheet is kept within an appropriate range, and the dispersion state (number / density) of fine Ti-containing nitride is appropriately controlled. As a result, fine Ti-containing nitrides that do not dissolve and disappear in the steel during high heat input welding can be dispersed in the steel, so that the toughness of the weld heat affected zone (HAZ) is improved while ensuring a good toughness balance. A thick steel plate could be realized.

  The present inventors have succeeded in increasing the Ti-containing nitride (hereinafter, sometimes represented by TiN) that remains undissolved even at a high temperature during welding (Patent Document 4). Based on the above, studies were made to further improve the HAZ toughness.

At the time of welding, fine TiN dissolves and coarse TiN exhibits a behavior that causes grain growth (Ostwald growth). The present inventors pay attention to such a behavior, and in order to make the TiN distribution fine and uniform even after grain growth by dispersing as much TiN as possible as much as possible, the equivalent circle diameter is 0. It has been found that the TiN of 0.05 μm or less may be controlled to be 5.0 × 10 ⁶ or more per 1 mm ² .

  In addition, the Ostwald growth as described above is promoted when the size distribution (variation) of TiN is large, and attention is also paid to the fact that the structure after welding tends to be non-uniform. The idea was obtained that the fine TiN content should be uniformly dispersed so that the proportion of the fine TiN becomes a certain amount or more. Specifically, if the number of fine TiN having an equivalent circle diameter of 0.01 to 0.03 μm occupies 75% or more of the total TiN, it is possible to prevent the structure after welding from becoming uneven. It turns out.

  In the steel plate of the present invention, fine TiN is mainly dispersed by the control described later. Therefore, even if partially coarse TiN (for example, TiN larger than 0.05 μm in equivalent circle diameter) is included, such coarse TiN does not significantly affect the properties of the steel sheet, so “total TiN” is such The purpose is to include coarse TiN. The proportion of fine TiN of 0.01 to 0.03 μm with respect to the total TiN (hereinafter sometimes referred to as “occupancy”) is preferably 77% or more, more preferably 80% or more. It is.

  By the way, in order to disperse a large amount of TiN, it is necessary to increase the contents of Ti and N (0.015% or more for Ti and 0.0060% or more for N). However, TiN is likely to be generated around 1500 ° C. at the time of steel casting, and TiN generated in this temperature range is more likely to become coarser due to an increase in Ti and N. As a result, it becomes difficult to achieve the proper distribution of TiN as described above.

  Therefore, the present inventors have further studied to reduce the amount of TiN generated in the high temperature range during casting. It can be estimated that TiN is likely to be generated in a high temperature range because the Ti activity is high. And, under the idea of lowering the activity of Ti, when examined, the amount of TiN generated in the high temperature range can be reduced if the relationship between Si and Ti that increase the activity of Ti is appropriately controlled. The idea of being able to do it was obtained.

  It has also been found that it is effective to reduce the temperature range of the “δ region” represented by the phase diagram of steel as a means for suppressing high temperature coarsening of TiN. The present inventors have been studying to improve the HAZ toughness of steel sheets for some time, and as part of that research, the same Ti and N additions were made by reducing the temperature range of the δ region represented in the steel phase diagram. It has been found that TiN can be finely dispersed even in an amount, and since its technical significance has been recognized, it has been filed earlier (Japanese Patent Application No. 2006-31457). In the present invention, such knowledge is also applied.

  The “δ region” means a region including δ iron in the iron phase diagram. The “region including δ iron” includes not only a region including δ iron but also a region including a δ phase and another phase state, such as a two-phase region of δ + γ. The “temperature range in the δ range” refers to a temperature range including δ iron (difference between the upper limit temperature and the lower limit temperature in the δ range). Here, in the steel having a specific composition, for example, when there is a temperature range of only δ iron and a temperature range of δ + γ iron, the sum of these temperature ranges becomes the temperature range of the δ region. The temperature range of the δ region can be calculated by inputting the chemical composition of the steel material into comprehensive thermodynamic calculation software (Themo-calc, available from CRC Research Institute).

  Since the diffusion rate of Ti is fast in the above-mentioned δ iron, it is considered that when the temperature range in the δ region is wide, the time during which δ iron is present becomes longer and coarse Ti-containing nitrides are easily formed. In the above technique, in the calculation of Thermo-calc, the amount of each chemical component on the temperature range in the δ region is examined by changing only one chemical component amount based on the specific component, and C, Si, It was found that Mn, Nb, etc. could be involved, and a predetermined relational expression having these components as elements was obtained. Of the above components, C is particularly useful for reducing the temperature range in the δ region.

  In addition to the “delta region temperature range reduction effect” by such C, the “TiN amount reduction effect” in the high temperature region by appropriately controlling the relationship between Si and Ti described above, these components (C , Si and Ti) further investigated the effects on HAZ toughness by experiments. As a result, when the above components satisfy the relationship of the following formula (1), the “δ region temperature range reduction effect” by C and the TiN amount reduction effect in the high temperature region are effectively exhibited, and the HAZ toughness becomes extremely good. I found out to get.

[Ti] × 16 [Si] × (12-40 [C]) <0.38 (%) (1)
However, [Ti], [Si] and [C] indicate the contents (mass%) of Ti, Si and C, respectively.

  If the amount of each chemical component is within an appropriate range, the value [[Ti] × 16 [Si] × (12−40 [C]) in the left side of the above equation (1): hereinafter referred to as “Z value”] is small. The above-mentioned “δ region temperature range reduction effect” and / or “TiN amount reduction effect” are effectively exhibited, and the HAZ toughness is improved. The upper limit of this Z value is 0.38 (%), but the preferable upper limit is 0.35 (%), more preferably 0.30 (%) or less. The lower limit of the Z value is determined from the appropriate amount of each chemical component and is about 0.0 (%).

  Next, the component composition in the steel material (base material) of the present invention will be described. As described above, the steel sheet of the present invention is excellent if the content of each chemical component (element) is not within an appropriate range even if the chemical component composition satisfies the relational expression (1). HAZ toughness cannot be achieved. Therefore, in the thick steel sheet of the present invention, the distribution state of the Ti-containing nitride is good and the chemical components satisfy the above formula (1), and the amounts of the respective chemical components are described below. It is also necessary to be within the proper range. The reasons for limiting the ranges of these components are as follows.

[C: 0.03-0.10%]
C is an element indispensable for securing the strength of the steel sheet, and is an element effective for reducing the temperature range in the δ region in the steel phase diagram as described above. If the C content is less than 0.03%, not only the strength of the steel sheet cannot be secured, but also the effect of reducing the δ region is not exhibited, and the Ti-containing nitride becomes coarse. Preferably it is 0.04% or more, More preferably, it is 0.05% or more. However, if it exceeds 0.10%, a large number of island martensite phases (MA phases) are generated in the HAZ during welding, leading to deterioration of the toughness of the HAZ. Therefore, C must be suppressed to 0.10% or less, preferably 0.09% or less.

[Si: 0.2% or less (including 0%)]
Si is an element useful for securing the strength of the steel sheet by solid solution strengthening. However, when it is excessively contained, the Ti-containing nitride is increased by increasing the activity of Ti even if the above formula (1) is satisfied. Will lead to coarsening. From such a viewpoint, the Si content needs to be 0.2% or less, and preferably 0.15% or less. In addition, when the further high toughness is calculated | required by HAZ, it is good to suppress Si to 0.3% or less. Further, from the viewpoint of securing HAZ toughness, the Si content may be 0%.

[Mn: 1.0 to 2.0%]
Mn is an element useful for ensuring the strength of the steel sheet, and in order to effectively exhibit such effects, it is necessary to contain 1.0% or more. Preferably it is 1.2% or more. However, if the content exceeds 2.0% excessively, the strength of the HAZ increases excessively and the toughness deteriorates, so the Mn content is set to 2.0% or less. Preferably it is 1.8% or less.

[P: 0.03% or less (excluding 0%)]
P, which is an impurity element, easily causes grain boundary fracture and adversely affects the HAZ toughness. Therefore, the amount is preferably as small as possible. From the viewpoint of ensuring HAZ toughness, the P content needs to be suppressed to 0.03% or less, and preferably 0.02% or less. However, industrially, it is difficult to make P in steel 0%.

[S: 0.01% or less (excluding 0%)]
S is an impurity that promotes hot cracking of HAZ, and its amount is preferably as small as possible. From the viewpoint of securing HAZ toughness, the S content needs to be suppressed to 0.01% or less, and preferably 0.008% or less. However, industrially, it is difficult to reduce S in steel to 0%.

[Al: 0.01 to 0.10%]
Al is useful as a deoxidizing element. In order to exhibit such an effect, it is necessary to contain 0.01% or more, preferably 0.01% or more. However, if the Al content becomes excessive, the HAZ toughness deteriorates, so it is necessary to keep it at 0.10% or less, preferably 0.05 or less.

[Nb: 0.005 to 0.020%]
Nb is an element that contributes to improving the toughness of both the HAZ and the base material. In order to exhibit such an effect effectively, Nb needs to be contained in an amount of 0.005% or more, preferably 0.007% or more. However, if excessively contained, the HAZ bainite structure is coarsened and toughness is deteriorated, so it should be suppressed to 0.020% or less. Preferably, the content is 0.015% or less.

[Ti: 0.015-0.040%]
Ti is an element that contributes to improving the toughness of HAZ by forming fine nitrides with N. In order to exhibit such an effect effectively, it is necessary to contain Ti 0.015% or more, preferably 0.018% or more. However, if added excessively, the Ti-containing nitride becomes coarse and deteriorates the toughness of the HAZ, so it should be suppressed to 0.040% or less. Preferably it is 0.035% or less.

[N: 0.0060 to 0.0120%]
N is an element useful for securing HAZ toughness by forming a nitride (Ti-containing nitride) that remains undissolved at a high temperature. By setting the N content to 0.0060% or more, the Ti-containing nitride that remains undissolved at a high temperature increases. However, when the N content is excessive, the solid solution N amount increases and the HAZ toughness deteriorates. Therefore, N must be suppressed to 0.012% or less, preferably 0.0110% or less.

[B: 0.0010 to 0.0050%]
B combines with N in the steel in the process of cooling the HAZ heated during welding, and precipitates BN to improve the HAZ toughness. In order to exhibit such an effect effectively, it is necessary to make it contain 0.0010% or more. Preferably it is 0.0015% or more. However, if the B content is excessive, the HAZ bainite structure is coarsened and the toughness deteriorates, so it is necessary to make it 0.005% or less. Preferably it is 0.0040% or less.

  The contained elements specified in the present invention are as described above, and the balance is iron and unavoidable impurities, and elements brought in depending on the situation of raw materials, materials, manufacturing equipment, etc. (for example, Sn, As) , Pb, etc.) can be permitted. Further, it is also effective to positively contain the following elements, and the characteristics of the steel sheet are further improved according to the types of components contained.

[One or more elements selected from the group consisting of Cu, Ni and Cr: 0.1 to 1.5% in total]
Cu, Ni and Cr are all elements that effectively act to enhance the hardenability and increase the strength of the steel sheet. In order to exert such an effect, it is preferable to contain 0.1% or more of these by one kind or two or more kinds (total). More preferably, it is 0.15% or more. However, if the content of these elements becomes excessive, the HAZ toughness deteriorates due to an increase in the MA phase in the HAZ, so it is preferable to keep it to 1.5% or less. More preferably, it is 1.0% or less.

[Mo and / or V: 0.5% or less in total (excluding 0%)]
Mo and V are elements that effectively act to enhance the hardenability and increase the strength of the steel sheet, like Cu, Ni and Cr. Such an effect increases as the content thereof increases. However, if the content of these elements becomes excessive, the HAZ toughness deteriorates due to an increase in the MA phase in the HAZ, and therefore it is preferable to suppress the content to 0.5% or less. More preferably, it is 0.4% or less.

[Zr: 0.005 to 0.050%]
Zr is an element that contributes to improving the toughness of HAZ by forming fine nitrides with N, like Ti. In order to exhibit such an effect effectively, it is preferable to contain Zr 0.005% or more, more preferably 0.008% or more. However, if added excessively, the nitride becomes coarse and deteriorates the toughness of the HAZ, so it should be suppressed to 0.050% or less. Preferably it is 0.030% or less.

[One or more elements selected from the group consisting of Ca, Mg and REM: 0.0005 to 0.0050% in total]
Ca, Mg, and REM are elements that contribute to improving the toughness of HAZ by refining and spheroidizing the shapes of inclusions (oxides, sulfides, etc.) that are inevitably mixed in the steel material. In order to exhibit such an effect effectively, it is preferable to contain 0.0005% or more of these by 1 type or 2 types or more (total). More preferably, it is 0.0008% or more. However, if the content of these elements becomes excessive, inclusions become coarse and the HAZ toughness deteriorates, so it is preferable to keep the content to 0.0050% or less. More preferably, it is 0.0030% or less.

  In the present invention, REM (rare earth element) means a lanthanoid element (15 elements from La to Ln), Sc (scandium) and Y (yttrium).

  In the present invention, in order to control the Ti-containing nitriding as described above, it is effective to use molten steel satisfying the above component composition and control the cooling rate during casting. That is, it is recommended to form a slab by cooling the temperature range of 1500 to 1300 ° C. at 10 ° C./min or more. In order to control the cooling rate in this way, means for reducing the slab thickness or increasing the amount of cooling water can be used.

  The present invention relates to a thick steel plate. In this field, a thick steel plate generally refers to one having a plate thickness of 3.0 mm or more as defined by JIS. However, the thickness of the thick steel plate of the present invention is preferably 50 mm or more, more preferably 60 mm or more. That is, the thick steel plate of the present invention exhibits good HAZ toughness even with high heat input welding with a heat input of 50 kJ / mm or more. Therefore, even if the plate thickness is large, it is efficiently welded by increasing the heat input. It can be done.

  The steel plate of the present invention thus obtained can be used as a material for structures such as bridges, high-rise buildings, ships, etc., and deteriorates the toughness of the weld heat affected zone not only in small to medium heat input welding but also in large heat input welding. Can be prevented.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the following examples are not intended to limit the present invention, and may be implemented with appropriate modifications within a range that can meet the purpose described above and below. These are all possible and are within the scope of the present invention.

The steel having the composition shown in Table 1 below is melted by a normal melting method, and this molten steel is cooled while controlling the cooling rate at the time of casting (a temperature range of 1500 to 1300 ° C.) (slab shape: cross-sectional shape:
200 mm × 250 mm), heated to 1100 ° C. and hot-rolled to obtain a hot-rolled plate having a thickness of 50 mm, and air-cooled after rolling to obtain a test plate. In Table 1, REM was added in the form of a misch metal containing about 50% La and about 25% Ce. In Table 1, "-" indicates that no element is added.

  For each test plate produced as described above, the number density of Ti-containing nitrides (the number of equivalent-circle diameters of 0.05 μm or less and the equivalent-circle diameter of 0.01 to 0.03 μm) was as follows. Occupancy ratio), tensile strength of the thick steel plate, and HAZ toughness. These results are shown in Table 2 below together with the Z value [= [Ti] × 16 [Si] × (12-40 [C])] and the cooling rate during casting.

[Measurement of number density of Ti-containing nitride]
The t (plate thickness) / 4 portion of each steel plate was observed with a transmission electron microscope (TEM) under the conditions of an observation magnification of 60,000, an observation field of view 2 × 2 (μm), and five observation locations. Then, the area of each Ti-containing nitride in the field of view was measured by image analysis, and the equivalent circle diameter of each nitride was calculated from this area. In addition, it was discriminate | determined by EDX (energy dispersive X-ray detector) that it was Ti containing nitride.

The number of Ti-containing nitrides having an equivalent circle diameter of 0.05 μm or less is calculated per 1 mm ² , and the total Ti content of the fine Ti-containing nitride having an equivalent circle diameter of 0.01 to 0.03 μm The number ratio (occupancy:%) with respect to nitrides (including those with an equivalent circle diameter exceeding 0.05 μm) was calculated.

[Tensile test]
Sample No. 4 of JIS Z 2201 was taken from the t (plate thickness) / 4 part of each steel plate in a direction perpendicular to the rolling direction, and subjected to a tensile test in accordance with JIS Z 2241, yielding 0.2% yield. Stress (YS) was measured. And the thing whose YS was 400 Mpa or more was evaluated as a pass.

[Evaluation of HAZ toughness]
Sample No. 4 of JIS Z 2201 was sampled in the direction perpendicular to the rolling direction from the t (plate thickness) / 4 part of each steel plate, and a heat cycle test simulating large heat input welding was performed to obtain HAZ toughness. evaluated. At this time, in the heat cycle test, the test piece was heated to 1400 ° C. and held for 60 seconds, and then the temperature range of 800 to 500 ° C. was cooled over about 500 seconds, whereby the heat input of welding was equivalent to 65 kJ. A cycle was given. In accordance with JIS Z 2242, a Charpy impact test was performed at −40 ° C., and the absorbed energy (vE ₋₄₀ ) was measured. At this time, absorption energy (vE- ₄₀ ) was measured about five test pieces, and the average value and the minimum value were calculated | required. Then, those having an average value of vE- ₄₀ of 200 J or more were evaluated as being excellent in HAZ toughness, and those having a minimum value of vE- ₄₀ of 120 or more were evaluated as being improved in stabilization.

  Tables 1 and 2 can be considered as follows (note that the following numbers indicate the steel numbers in Tables 1 and 2). No. Nos. 1 to 15 are examples that satisfy the requirements defined in the present invention, in which a chemical component composition, Z value, and fine dispersion of Ti-containing nitride are appropriately performed, and a steel sheet having good toughness in the weld heat affected zone is obtained. You can see that it is obtained.

  In contrast, no. 16 to 28 are examples that deviate from any of the requirements defined in the present invention, and the toughness of the weld heat affected zone is inferior. Details are as follows.

  No. No. 16 is such that the C content in the steel sheet exceeds the range specified in the present invention, and the HAZ toughness is deteriorated even when the form of the Ti-containing nitride is good.

  No. No. 17 is that the Si content in the steel sheet exceeds the range specified in the present invention, the form of the Ti-containing nitride is inferior (fine Ti-containing nitride is not obtained), good HAZ toughness is not obtained. No. No. 18 is one in which the Mn content in the steel sheet exceeds the range specified in the present invention, and the HAZ toughness is deteriorated even when the form of the Ti-containing nitride is good.

  No. Nos. 19 and 20 are those in which the content of P or S in the steel sheet exceeds the range specified in the present invention, and the HAZ toughness is deteriorated even if the form of the Ti-containing nitride is good. No. No. 21 exceeds the range specified by the present invention in the Nb content in the steel sheet, and even if the form of the Ti-containing nitride is good, the HAZ toughness is deteriorated.

  No. No. 22 is that the Ti content in the steel sheet is less than the range specified in the present invention, and sufficient number density and occupation ratio of the Ti-containing nitride are not achieved, and the HAZ toughness is deteriorated. No. No. 23, the Ti content in the steel sheet exceeds the range specified in the present invention, the Ti-containing nitride is coarsened, sufficient number density and occupancy are not achieved, and the HAZ toughness deteriorates is doing.

  No. No. 24 is that the N content in the steel sheet is less than the range defined in the present invention, and a sufficient number density of the Ti-containing nitride is not achieved, and the HAZ toughness is deteriorated. No. No. 25 is that the N content in the steel sheet exceeds the range specified in the present invention, and the solute N increases and the HAZ toughness is deteriorated.

  No. No. 26 is that the B content in the steel sheet is less than the range defined in the present invention, and the HAZ toughness is deteriorated even though the form of the Ti-containing nitride is good. No. In No. 27, the Z value in the chemical component is out of the range defined in the present invention, and sufficient number density and occupation ratio of the Ti-containing nitride are not achieved, and the HAZ toughness is deteriorated. No. No. 28 is that the B content in the steel sheet exceeds the range specified in the present invention, and the HAZ toughness is deteriorated even if the Ti-containing nitride has a good form.

  No. No. 29 has a low cooling rate during casting, Ti-containing nitrides are coarsened, sufficient number density and occupancy of Ti-containing nitrides are not achieved, and HAZ toughness is deteriorated. No. No. 30, the Z value in the chemical component and the cooling rate at the time of casting are both out of the proper range, the Ti-containing nitride is remarkably coarsened, and sufficient number density and occupancy are not achieved. , HAZ toughness is deteriorated.

Claims (5)

C: 0.03 to 0.10% (meaning “mass%”; the same applies hereinafter), Si: 0.2% or less (including 0%), Mn: 1.0 to 2.0%, P: 0 0.03% or less (excluding 0%), S: 0.01% or less (not including 0%), Al: 0.01 to 0.10%, Nb: 0.005 to 0.020%, Ti : 0.015~0.040%, N: 0.0060~0.0120% , B: a .0010 to .0050% respectively contain, Rutotomoni such a balance of iron and inevitable impurities, the following (1 ) There are 5.0 × 10 ⁶ or more Ti-containing nitrides satisfying the relationship of the formula and having an equivalent circle diameter of 0.05 μm or less per 1 mm ² , of which 0.01 to 0.03 μm in equivalent circle diameter. The toughness of the weld heat affected zone characterized in that the number of Ti-containing nitrides accounts for 75% or more of the total Ti-containing nitride. Steel plate with excellent properties.
[Ti] × 16 [Si] × (12-40 [C]) <0.38 (%) (1)
However, [Ti], [Si] and [C] indicate the contents (mass%) of Ti, Si and C, respectively.   The thick steel plate according to claim 1, further comprising at least one element selected from the group consisting of Cu, Ni and Cr: 0.1 to 1.5% in total.   Furthermore, Mo and / or V: The thick steel plate of Claim 1 or 2 containing 0.5% or less (0% is not included) in total.   The thick steel plate according to any one of claims 1 to 3, further comprising Zr: 0.005 to 0.050%.   Furthermore, the thick steel plate in any one of Claims 1-4 which contains 0.0005 to 0.0050% of 1 or more types of elements chosen from the group which consists of Ca, Mg, and REM in total.