JP5110989B2 - Large steel plate for high heat input welding with excellent brittle crack propagation stopping characteristics - Google Patents

Description

  The present invention relates to a thick steel plate applied to a welded structure such as a ship or an offshore structure, and particularly has excellent toughness of a heat affected zone (HAZ) after high heat input welding and brittle crack propagation. The present invention relates to a thick steel plate having excellent stopping characteristics.

  In recent years, for example, container ships have been increased in size, and thick steel plates having a thickness of 50 mm or more are sometimes used. In order to efficiently weld such thick steel plates, large heat input or super large heat input welding (hereinafter, may be represented by “high heat input welding”) such that the heat input amount is 40 kJ / mm or more. There is a need to do.

  However, if high heat input welding is performed, the HAZ is heated to a high temperature austenite region and then gradually cooled, so that the structure becomes coarse and the HAZ toughness is remarkably deteriorated. For these reasons, conventionally, the amount of welding heat input has been limited.

  In order to achieve good HAZ toughness by such high heat input welding, for example, Patent Document 1 reduces the C content in the thick steel plate and limits the content of P that is inevitably mixed. In addition, it is proposed to control the contents of Nb and B within an appropriate range. Further, Patent Document 2 suppresses the formation of coarse ferrite by positively containing Nb in TiN-based inclusions present in welding steel. However, in these techniques, when TiN is insufficient or when TiN is sufficient, the TiN is coarsened or the pinning effect due to CaO oxide or the like is not sufficient, so that HAZ toughness is further improved. There is room for. Moreover, the brittle crack propagation stop characteristic of the base material steel plate mentioned later is not considered.

  Patent Document 3 proposes that a steel material contains a relatively large amount of N and appropriately controls the content balance of Ti and B. However, even in such a technique, the precipitation amount of TiN or BN is not sufficient or fine, and the ferrite becomes coarse due to low hardenability without addition of Nb, so that the HAZ toughness is further improved. There is room for. Moreover, the brittle crack propagation stop characteristic of the base material steel plate mentioned later is not considered.

  On the other hand, large-scale structures such as ships, offshore structures, low-temperature storage tanks, line pipes, and construction / civil engineering structures have a large impact on the economy and the environment due to accidents caused by brittle fracture, and a high degree of safety is required. It is like that. Therefore, steel materials used for these structures are often required to have low temperature toughness. Particularly recently, from the viewpoint of preventing breakage even when a crack has occurred in a structure due to an accident, etc., it is excellent in brittle crack propagation stop characteristics at low temperatures, so-called arrest characteristics. It is requested.

  In recent years, it has also been proposed that a technique for making the structure of the surface layer portion of steel material ultrafine is effective in improving the brittle crack propagation stop characteristics. As such a technique, for example, Patent Document 4 discloses that when a brittle crack propagates, a shear lip (plastic deformation region) generated in a steel surface layer portion is effective in improving the brittle crack propagation stop characteristic. However, in such a technique, only the steel surface layer portion is once cooled and then reheated, and by processing during recuperation, a structure effective for brittle crack propagation stop characteristics is obtained. It is necessary to implement a process that is considered to be difficult to control. In addition, ferrite is processed and recrystallized to obtain a finer structure. However, processed recrystallized ferrite is prone to growth and lacks structural stability. There is also a problem that non-uniformity of the material tends to occur.

  In Patent Document 5, for the purpose of uniform hardness distribution in the thickness direction, a steel material containing V is used to directly cool a heated slab to give a temperature difference to generate ferrite. A method has been proposed in which rolling is started from the beginning and reheated again in the temperature range near the transformation point during or after the rolling. In this technique, precipitation hardening of V is applied only to the center portion of the plate thickness, the hardness distribution in the plate thickness direction can be made uniform, and the brittle crack propagation stop characteristic is improved. However, in order to appropriately precipitate the V compound, it is necessary to further complicate the process, and this is not necessarily directly related to the solution of the “structure stabilization” as described above.

As described above, various technologies for improving the brittle crack propagation stopping characteristics have been proposed so far, but it is difficult to stably achieve the desired structure on an industrial scale. The fact is that it is desired to establish a technology capable of stably realizing the brittle crack propagation stopping characteristics even on a scale. In these techniques, no consideration is given to improving the HAZ toughness during high heat input welding.
Japanese Patent Laid-Open No. 2003-166033 JP 2004-2181010 A JP-A-2005-200716 JP-A-4-141517 JP-A-8-253812

  The present invention has been made under such circumstances, and an object of the present invention is to provide a thick steel plate that exhibits good HAZ toughness even in high heat input welding and has excellent brittle crack propagation stop characteristics.

The thick steel plate of the present invention that can achieve the above object is C: 0.030 to 0.15% (meaning mass%, the same shall apply hereinafter), Si: 1.0% or less (not including 0%), Mn: 0.8 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 %, Ti: 0.015-0.03%, B: 0.0010-0.0035%, N: 0.0050-0.01%, Ca: 0.005% or less (excluding 0%), O: each containing 0.01% or less (excluding 0%), satisfying the following formulas (1) and (2), the balance being Fe and inevitable impurities, and a bainite fraction of 95 areas % or more with a tissue, have a summary to point dislocation density of bainite ([rho ^1/2) is ^{1.0 × 10 6 ~5.0 × 10 7} (m -1) Is shall.
1.5 ≦ [Ti] / [N] ≦ 4.0 (1)
40 ≦ X value ≦ 160 (2)
X value = 500 [C] +32 [Si] +8 [Mn] -9 [Nb]
+14 [Cu] +17 [Ni] -5 [Cr] -25 [Mo] -34 [V]
(In the formula, [] represents the content (% by mass) of each element.)

  In the thick steel plate of the present invention, from the viewpoint of good low temperature toughness and HAZ toughness, (a) the temperature range of the δ region is 40 ° C. or less, and (b) the position of the depth t / 4 (t = plate thickness) ), It is preferable to satisfy the requirements such that the average particle diameter of the Ti-based charcoal / nitride is 43 nm or less. The “Ti-based carbon / nitride” is intended to include any of carbides, nitrides, and carbonitrides containing Ti.

  In the thick steel plate of the present invention, if necessary, (a) Nb: 0.035% or less (not including 0%), (b) Cu: 2.0% or less (not including 0%), Ni: One or more selected from the group consisting of 2.0% or less (excluding 0%) and Cr: 2.0% or less (not including 0%), (c) Mo: 1.0% or less (0% (D) V: 0.1% or less (not including 0%), (e) one or more selected from the group consisting of Mg, Sr, and Ba: 0.01% or less in total (0 %), (F) rare earth element: 0.01% or less (not including 0%), (g) one or more selected from the group consisting of Zr, Ta and Hf: 0.05% or less in total (0) not included, (h) Co: not more than 2.5% (not including 0%) and / or W: not more than 2.5% (not including 0%), etc. It is also useful to have, and the properties of the steel sheet are further improved depending on the components contained.

  According to the present invention, the amount and structure of each component fall within an appropriate range, the chemical component composition is adjusted so as to satisfy the above formulas (1) and (2), and the dislocation density of bainite is controlled. As a result, a thick steel plate having excellent HAZ toughness even in high heat input welding and excellent in brittle crack propagation stopping characteristics was realized.

  The present inventors tried to achieve good HAZ toughness even in high heat input welding by refining Ti-based charcoal / nitride. The dispersion state of conventional Ti-based charcoal / nitride has been considered to be determined only by the balance of addition of Ti and N if the cooling rate during solidification of molten steel is constant. However, as a result of intensive studies by the present inventors, it is possible to finely disperse Ti-based carbon / nitride even with the same Ti and N addition amount by reducing the temperature range in the δ region represented in the phase diagram of steel. I found.

  The X value that defines the relationship of the above equation (2) is a function related to the temperature range in the δ region. The present inventors have tried to improve the HAZ toughness and found the relationship of the above formula (2). First, the background will be described. The “δ region” means a region including δ iron in the steel phase diagram. The “region including δ iron” includes not only a region including δ iron but also a region including δ iron and other states 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 is 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 sheet into comprehensive thermodynamic calculation software (Thermo-calc, available from CRC Research Institute).

Since the diffusion rate of Ti is fast in this δ iron, it is considered that if the temperature range in the δ region is wide, the time during which the δ iron exists becomes longer, and coarse Ti-based carbon / nitride is likely to be formed. Therefore, the refinement of the Ti-based carbon / nitride was studied by adjusting the chemical composition and reducing the temperature range in the δ region. For this purpose, in Thermo-calc calculation, the influence of each chemical component on the temperature range in the δ region was examined by changing only one of the chemical component amounts based on the specific component. As a result of such studies, an X value correlated with the temperature range of the δ region and expressed as a function of the chemical composition was determined:
X value = 500 [C] +32 [Si] +8 [Mn] -9 [Nb]
+14 [Cu] +17 [Ni] -5 [Cr] -25 [Mo] -34 [V]
(In the formula, [] represents the content (% by mass) of each element.)

  The coefficient in the above formula of the X value corresponds to the amount of change in the temperature range in the δ region when each chemical component is changed from the specific component steel. Specifically, for example, when the coefficient of [C] is “500”, when the C content is increased by 0.01%, the temperature range in the δ region decreases by about 5 ° C. in the calculation of Thermo-calc. Means. The X value and the temperature range in the δ region are in an inversely proportional relationship (the relationship that the temperature range in the δ region decreases as the X value increases).

  In addition to the basic components (C, Si, Mn) of the thick steel plate of the present invention, the elements that define the X value include elements that are included as necessary (Nb, Cu, Ni, Cr, Mo, V, etc.) When these elements are not included, the X value is calculated assuming that these items are not present, and when these elements are included, the X value may be calculated from the above formula.

  Based on this idea, steel sheets with various X values were manufactured and investigated, and by increasing the X value, the average particle diameter of Ti-based charcoal / nitride can be refined and the HAZ toughness improved. I found out that I could make it.

  And it discovered that the low temperature toughness of a steel plate was further improved by increasing X value. This phenomenon is presumed to be due to a decrease in the average particle size of the Ti-based carbon / nitride by increasing the X value.

As described above, the thick steel plate of the present invention has a chemical component composition represented by the following formula (2):
40 ≦ X value ≦ 160 (2)
X value = 500 [C] +32 [Si] +8 [Mn] -9 [Nb]
+14 [Cu] +17 [Ni] -5 [Cr] -25 [Mo] -34 [V]
(In the formula, [] represents the content (% by mass) of each element.)
There is a major feature in that However, the present invention is limited to the above estimation reasons (decrease in the average particle diameter of charcoal / nitride due to decrease in temperature range of δ region, improvement of HAZ toughness and low temperature toughness due to decrease in average particle diameter, etc.) Rather, the scope of the invention is defined by the claims. That is, a thick steel plate that satisfies the structural requirements defined in the claims is included within the scope of the present invention.

  If the amount of each chemical component is within an appropriate range, the average particle diameter of the Ti-based carbon / nitride, the HAZ toughness, and the base material toughness improve as the X value increases. The lower limit of the X value is 40 (preferably 45, more preferably 50). The upper limit of the X value is determined from an appropriate amount of each chemical component and is 160 (preferably 100 or less, more preferably 75 or less). From the viewpoint of suppressing generation of a hard phase MA structure (mixed structure of martensite and austenite), the preferable upper limit of the X value is 75 or less.

In the thick steel plate of the present invention, the Ti-based carbon / nitride is made fine by adjusting the chemical composition so that the X value is 40 or more. However, when the balance between the Ti content and the N content is lost, the toughness of the steel sheet, particularly the HAZ toughness, is deteriorated. Specifically, when the ratio of Ti content [Ti] to N content [N] ([Ti] / [N]) exceeds 4.0, the Ti-based carbon / nitride becomes coarse, and the HAZ toughness Decreases. On the other hand, if it is less than 1.5, the low temperature toughness and the HAZ toughness are reduced by the influence of excess N. Therefore, in the thick steel plate of the present invention, in addition to the above formula (2) that defines the X value, the following formula (1):
1.5 ≦ [Ti] / [N] ≦ 4.0 (1)
One of the characteristics is that the Ti content [Ti] and the N content [N] are balanced so as to satisfy the above. The preferable lower limit of [Ti] / [N] is 2.0, and the preferable upper limit is 3.5.

  From the viewpoint of toughness, the Ti-based carbon / nitride in the thick steel plate of the present invention is preferably fine. Therefore, the Ti-based carbon / nitride in the thick steel plate of the present invention is preferably 43 nm or less, more preferably 40 nm or less, and still more preferably 35 nm or less.

  The value of the average particle diameter of the Ti-based carbon / nitride in the present invention is a value measured as follows. First, a position at a depth t / 4 (t = plate thickness) as a portion representing the thermal history of a steel plate is observed with a transmission electron microscope (TEM) at an observation magnification of 60,000 times or more and an observation field of view of 2.0 μm × 2. Observation is performed under conditions of 0 μm or more and five or more observation places. Then, the area of each charcoal / nitride in the field of view is measured, and the equivalent circle diameter of each charcoal / nitride is calculated from this area. The value obtained by arithmetic average (arithmetic mean) of the equivalent circle diameters of each charcoal / nitride is defined as the average particle diameter of the Ti-based charcoal / nitride in the present invention.

  In addition, discrimination | determination whether it is Ti type | system | group charcoal / nitride is decided by the component used as the main body of each charcoal / nitride particle | grain. That is, “Ti-based charcoal / nitride” means that the ratio of Ti is 50% by mass or more when the total mass of the remaining elements excluding carbon and nitrogen is 100%. The amount of element can be determined by an energy dispersive X-ray detector (EDX). However, since too fine carbon / nitride cannot be measured, “Ti-based carbon / nitride” in the present invention is limited to 5 nm or more.

  In steel plates, in addition to having good HAZ toughness as described above, in large structures such as ships, offshore structures, low-temperature storage tanks, line pipes, buildings, bridges, etc. The impact on the environment is large and a high level of safety is required. The present inventors have also studied from the viewpoint of achieving the improvement of the brittle crack propagation stopping characteristics together. As a result, in order to improve the brittle crack propagation stop characteristics, it is a structure mainly composed of bainite (bainite fraction is 95 area% or more), and if the dislocation density of bainite is controlled to be within a predetermined range, It has been found that the brittle crack propagation stop property can be improved.

In the thick steel sheet of the present invention, excellent brittle crack propagation stoppage is achieved by controlling the dislocation density (ρ ^1/2 ) of bainite within a range of 1.0 × 10 ^{6 to} 5.0 × 10 ⁷ (m ⁻¹ ). Characteristics can be realized. It is known that metal materials break at a stress lower than the expected ideal fracture strength. It is also known that such a phenomenon is due to the presence of various defects inside the metal material. Examples of such defects include inclusions, precipitates, heterogeneous particles, dislocations, and the like, and the toughness of the base material is improved by controlling these structure factors.

  The present inventors consider that these defect factors also affect the brittle crack propagation stopping characteristics, and the brittle crack propagation stopping characteristics may be improved by performing structure control focusing on the dislocation density. It turns out. By controlling the dislocation density to an appropriate range, the mechanism of improving the brittle crack propagation stopping property has not been fully clarified, but if the dislocation density is too high, it is probably due to dislocation coalescence. It is presumed that the stress concentration at the crack tip part makes it easy to form a new cleavage fracture surface, and therefore it is difficult to stop the propagation of the brittle crack.

In order to exert the above effects, it is necessary to control the dislocation density (ρ ^1/2 ) of bainite within a range of 1.0 × 10 ^{6 to} 5.0 × 10 ⁷ (m ⁻¹ ). If the dislocation density (ρ ^1/2 ) of bainite is less than 1.0 × 10 ⁶ (m ⁻¹ ), the strength of the base material cannot be secured, and conversely, the dislocation density (ρ ^1/2 ) is 5.0 × 10 ^7. If (m ^-1 ) is exceeded, the brittle crack propagation stop property will deteriorate instead. The preferable lower limit of the dislocation density is 5.0 × 10 ⁶ (m ⁻¹ ), more preferably 1.0 × 10 ⁷ (m ⁻¹ ), and still more preferably 1.5 × 10 ⁷ (m ⁻¹ ) [or 2.0 × 10 ⁷ (m ⁻¹ ) or more], and a preferable upper limit is 4.0 × 10 ⁷ (m ⁻¹ ) [more preferably 3.0 × 10 ⁷ (m ⁻¹ )].

  For measurement of the dislocation density, an X-ray diffraction method which is a general method may be employed. For example, based on the method described in “CAMP-ISIJ VoL.17 (2004) P396-399”, the dislocation density can be obtained from the half width of the (200) plane. In the present invention, a 25 mm square test piece was collected, and a rolled surface and a horizontal surface at a ¼ portion in the thickness direction were electrolytically polished to obtain a measurement surface.

  In the present invention, it is necessary to have a structure mainly composed of bainite. Further, by defining the dislocation density as described above, a good brittle crack propagation stopping characteristic is exhibited. However, in order to exhibit such an effect, 100 area% does not necessarily need to be a bainite structure, and it is sufficient if the bainite fraction is 95 area% or more. Examples of structures other than bainite include martensite, ferrite, and pearlite. In the present invention, the bainite fraction was measured according to the following method.

[Measurement method of bainite fraction]
A 2 cm square test piece taken from the t / 4 (t: plate thickness) position of each steel plate is mirror-polished, etched with a nital etchant (2% nitric acid-ethanol solution), and the structure is observed with an optical microscope. The bainite fraction was calculated by an image analyzer (manufactured by Media Cybernetics: Imega-Pro Plus) for a photograph taken at (magnification: 100 times) and n = 10 (times). At this time, all lath structures other than ferrite were regarded as bainite.

  The thick steel plate of the present invention satisfies the relationship of the above formulas (1) and (2), and has excellent HAZ toughness and brittle crack propagation stopping characteristics by controlling the dislocation density of bainite. It will be a thing. However, even if these requirements are satisfied, the above effects cannot be achieved unless the content of each chemical component (each element) is within an appropriate range. Therefore, in the thick steel sheet of the present invention, in addition to the above formulas (1) and (2) and the dislocation density of bainite satisfying the specified range, the amount of each chemical component is in an appropriate range as described below. It is also characterized by being inside. Hereinafter, chemical components will be described individually.

[C: 0.030 to 0.15%]
C is an element necessary for ensuring the strength of the steel sheet, and is an effective element for reducing the temperature range in the δ region in the steel phase diagram. If the C content is less than 0.030%, those effects are not exhibited. On the other hand, when the C content exceeds 0.15%, the hard second-phase MA structure is excessively increased, and the base material toughness and the HAZ toughness are lowered. Therefore, the C content is set to 0.030 to 0.15%. The preferable lower limit of the C content is 0.040% (more preferably 0.050% or more), and the preferable upper limit is 0.10% (more preferably 0.070% or less).

[Si: 1.0% or less (excluding 0%)]
Si is an effective element for ensuring the strength of the steel sheet, and for that purpose, it is preferable to contain 0.10% or more (more preferably 0.20% or more). However, when Si is excessively contained, a large amount of MA structure is generated, and the base material toughness and the HAZ toughness are lowered. Therefore, the upper limit thereof needs to be 1.0%. The upper limit with preferable Si amount is 0.8%, More preferably, it is 0.50%, More preferably, it is 0.40%.

[Mn: 0.8 to 2.0%]
Mn is an element effective for improving the hardenability and ensuring the strength of the steel sheet. When the Mn content is less than 0.8%, the effect of securing the strength is not sufficiently exhibited. On the other hand, if the Mn content exceeds 2.0%, the base metal toughness and the HAZ toughness are lowered. Therefore, the Mn content is set to 0.8 to 2.0%. The minimum with preferable Mn content is 1.00%, More preferably, it is 1.20%, More preferably, it is 1.50%. On the other hand, the preferable upper limit of the amount of Mn is 1.80%, more preferably 1.60%.

[P: 0.03% or less (excluding 0%)]
Since the impurity element P adversely affects the base material toughness and the HAZ toughness, the amount is preferably as small as possible. Therefore, the amount of P is 0.03% or less, preferably 0.010% or less. However, industrially, it is difficult to reduce the P content in steel to 0%.

[S: 0.01% or less (excluding 0%)]
S is an element that forms MnS and lowers the ductility, and the adverse effect is large particularly in high-strength steel. Therefore, the amount is preferably as small as possible. Therefore, the amount of S is 0.01% or less, preferably 0.005% or less. However, industrially, it is difficult to reduce the amount of S in steel to 0%.

[Al: 0.01 to 0.10%]
Al is an element having an effect of improving the base material toughness by deoxidation and refinement of the microstructure. In order to sufficiently exhibit such an effect, Al is contained in an amount of 0.01% or more. However, if Al is excessively contained, the base metal toughness and the HAZ toughness are lowered, so the upper limit is made 0.10%. The minimum with preferable Al content is 0.020%. On the other hand, the preferable upper limit is 0.060%, more preferably 0.040% or less.

[Ti: 0.015-0.03%]
Ti is an effective element for improving HAZ toughness by forming fine nitrides with N and suppressing the coarsening of austenite grains of HAZ during welding (so-called pinning effect). In order to sufficiently exhibit such an effect, Ti is contained by 0.015% or more. However, if the Ti content becomes excessive, the HAZ toughness deteriorates on the contrary, so the upper limit of the Ti content was set to 0.03%. The Ti content is preferably 0.017% or more and 0.020% or less.

[B: 0.0010 to 0.0035%]
B produces HAZ, especially in the vicinity of the bond part, in the vicinity of the bond part, and generates intragranular ferrite with BN as the nucleus, and also has a fixing action of solute N, which is important for improving HAZ toughness. It is an element. In this invention, in order to fully exhibit the effect, B is contained more than the content in a normal thick steel plate, 0.0010% or more. However, if the B content is excessive, a coarse bainite structure is formed during high heat input welding, and the HAZ toughness deteriorates. Therefore, the upper limit of the B content is set to 0.0035%. A preferable lower limit of the B content is 0.0015% (more preferably 0.0020% or more), and a preferable upper limit is 0.0030% (more preferably 0.0025% or less).

[N: 0.0050 to 0.01%]
N is an element that combines with Ti to form fine carbonitrides, suppresses the coarsening of austenite grains during high heat input welding, and improves the HAZ toughness. If the N content is too small, the above effect is not sufficiently exhibited, so the lower limit was set to 0.0050%. On the other hand, if the N content is excessive, the base material toughness and the HAZ toughness are adversely affected, so the upper limit was set to 0.01%. The minimum with preferable N content is 0.0055%, More preferably, it is 0.0060% or more. Moreover, the upper limit with preferable N content is 0.0090%, More preferably, it is 0.0080% or less.

[Ca: 0.005% or less (excluding 0%)]
Ca is an element having an effect of improving HAZ toughness. Specifically, Ca improves HAZ toughness by reducing anisotropy due to inclusion shape control of spheroidizing MnS. On the other hand, HAS toughness is improved by forming CaS and CaO and suppressing the coarsening of HAZ austenite grains. In order to sufficiently exhibit such an effect, it is preferable to contain 0.0005% or more of Ca in the steel sheet. However, if the Ca content is excessive, the base metal toughness and the HAZ toughness are deteriorated. Therefore, the upper limit is set to 0.005%. The upper limit with preferable Ca amount is 0.0030%, More preferably, it is 0.0025%.

[O: 0.01% or less (excluding 0%)]
O is an element that reacts with Al, Ca, Mg and the like to form a stable oxide at a high temperature and effectively works to prevent coarsening of the prior austenite grains of HAZ. Such an effect increases as the content increases. However, when the content is excessive, the cleanliness decreases and the HAZ toughness decreases. Therefore, the upper limit is set to 0.01%.

  The thick steel plate of the present invention basically comprises Fe and inevitable impurities in addition to the above components. However, the present invention does not exclude thick steel plates containing other elements, and within the scope of the present invention is a thick steel plate containing other component elements as long as the effects of the present invention are not impaired. Is also included.

  For example, in the thick steel plate of the present invention, in addition to the above components, if necessary, (a) Nb: 0.035% or less (excluding 0%), (b) Cu: 2.0% or less (0 %), Ni: 2.0% or less (not including 0%) and Cr: 2.0% or less (not including 0%), (c) Mo: 1.0% or less (not including 0%), (d) V: 0.1% or less (not including 0%), (e) one or more selected from the group consisting of Mg, Sr, and Ba 0.01% or less (not including 0%), (f) rare earth element: 0.01% or less (not including 0%), (g) one or more selected from the group consisting of Zr, Ta and Hf: 0.05% or less in total (excluding 0%), (h) Co: 2.5% or less (excluding 0%) and / or W: 2.5% or less (0% It is also effective to contain (not contained), etc., and the characteristics of the steel sheet are further improved according to the type of the ingredient to be contained.

[Nb: 0.035% or less (excluding 0%)]
Nb is an effective element for improving the hardenability of the substrate and increasing the strength of the steel sheet, and is contained as necessary. However, when the Nb content is excessive, the base material toughness and the HAZ toughness are lowered, so the upper limit was set to 0.035%. Nb is preferably contained in an amount of 0.005% or more, more preferably 0.010% or more in order to exert the effect. A more preferable upper limit of the Nb content is 0.025%, and further preferably 0.020% or less.

[Selected from the group consisting of Cu: 2.0% or less (not including 0%), Ni: 2.0% or less (not including 0%), and Cr: 2.0% or less (not including 0%) One or more
Cu, Ni, and Cr are all elements that increase the hardenability and contribute to improving the strength, and can be added as necessary. Among these, Cu is considered to have the effect of reducing the temperature range in the δ region in the same way as C and miniaturizing the Ti-based carbonitride. Ni is also an effective element for reducing the temperature range in the δ region. In order to fully exhibit such an effect, it is recommended that the content thereof is preferably 0.20% or more, more preferably 0.40% or more. However, if these amounts are excessive, the base material toughness and the HAZ toughness tend to decrease, so the upper limit was set at 2.0%. Preferably it is 1.0% or less.

[Mo: 1.0% or less (excluding 0%)]
Mo is an element effective for improving hardenability and improving strength and preventing temper embrittlement, and can be added as necessary. In order to sufficiently exhibit such an effect, it is recommended that the Mo content is preferably 0.05% or more, more preferably 0.10% or more. However, when the Mo content is excessive, the base metal toughness and the HAZ toughness deteriorate, so the upper limit was set to 1.0%. The Mo content is more preferably 0.50% or less.

[V: 0.1% or less (excluding 0%)]
V is an element having an effect of enhancing hardenability and temper softening resistance by addition of a small amount, and can be added as necessary. In order to sufficiently exhibit such effects, it is recommended that the V amount is preferably 0.01% or more, more preferably 0.02% or more. However, if the amount of V is excessive, the base metal toughness and the HAZ toughness deteriorate, so the upper limit was set to 0.1%. The amount of V is preferably 0.05% or less.

[One or more selected from the group consisting of Mg, Sr, and Ba: Total 0.01% or less (excluding 0%)]
Mg, Sr and Ba are effective elements for improving the HAZ toughness by generating fine oxides in the thick steel plate and suppressing the coarsening of the austenite grains of the HAZ. In order to fully exhibit such an effect, it is preferable to contain 0.0003% or more of these one or more (total). However, if these contents are excessive, the base metal toughness and the HAZ toughness are deteriorated, so the upper limit was made 0.01%. A more preferable upper limit is 0.0040%, still more preferably 0.0020%.

[Rare earth elements: 0.01% or less (excluding 0%)]
Rare earth elements (REM) exist as oxides or sulfides, have the effect of improving HAZ toughness by promoting the austenite grain refinement of HAZ and promoting ferrite transformation, and can be effectively utilized as necessary. However, if REM is excessively contained, the HAZ toughness is deteriorated, so the upper limit is preferably made 0.01%. A more preferable upper limit of REM is 0.003%. In addition, REM used by this invention contains all of the lanthanoid series rare earth elements, and what is necessary is just to contain these 1 or more types.

[One or more selected from the group consisting of Zr, Ta and Hf: 0.05% or less in total (excluding 0%)]
Zr, Ta, and Hf are elements that are effective in improving HAZ toughness because they form carbon and nitrides similarly to Ti and suppress the coarsening of austenite grains of HAZ during welding. In order to exert such effects sufficiently, it is preferable to contain one or more of the above elements in a total amount of 0.001% or more. However, if these contents are excessive, the base material toughness and the HAZ toughness are decreased. Therefore, when these elements are contained, the total amount is preferably 0.05% or less, and more preferably 0.03% or less.

[Co: 2.5% or less (not including 0%) and / or W: 2.5% or less (not including 0%)]
Co and W are elements having an effect of improving hardenability and increasing the strength of the steel sheet. In order to sufficiently exhibit such an effect, it is preferable to contain one or both of these at 0.2% or more. However, if these amounts are excessive, the base material toughness and the HAZ toughness deteriorate, so the upper limit of these amounts was set to 2.5%.

In producing the thick steel plate of the present invention, the steel satisfying the requirements of the chemical composition, [Ti] / [N] and X value as described above is melted by an ordinary melting method, and the molten steel is cooled. For example, after heating to a range of 950 to 1300 ° C., hot rolling is performed, and the cumulative rolling reduction in the temperature range of T value ± 50 ° C. defined by the following formula (3) is 40% or more. Then, rolling may be finished so that the cumulative rolling reduction at 750 to 800 ° C. becomes 10 to 30%, and then cooled.
T value = 750 + 4000 [Nb] +32600 [B] +250 [Mo] +400 [V] (3)

  The T value defined by the above equation (3) serves as an index for adjusting the bainite dislocation density. In addition, among the elements that define the T value, components (B, Mo, V) that are contained as necessary in the thick steel plate of the present invention are also included, but when these elements are not included, these items are absent. When the T value is calculated as an object and these elements are included, the T value may be calculated from the above equation (3).

  The reason why the cumulative rolling reduction in the temperature range of T value ± 50 ° C. defined by the above formula (3) is 40% or more is from the viewpoint of securing the dislocation density, and when the rolling reduction at this time is less than 40% Low dislocation density. The cumulative rolling reduction at 750 to 800 ° C. is set to 10 to 30% from the viewpoint of securing dislocations. If the rolling reduction at this time is less than 10%, a low dislocation density is obtained. It becomes density.

  Since the thick steel plate of the present invention controls the X value to narrow the temperature range in the δ region, the molten steel is cooled under normal conditions (for example, from 1500 ° C. to 1100 ° C. at 0.1 to 2.0 ° C. / By cooling at a cooling rate of seconds) to form a slab, a sufficiently small average particle diameter of Ti-based carbon / nitride can be formed. However, in order to form finer charcoal / nitrides, it is preferable to change the cooling water amount and cooling method of the casting machine to improve the cooling rate during solidification.

  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 plate thickness of the steel plate of the present invention is preferably 20 mm or more, preferably 40 mm or more, and more preferably 60 mm or more. This is because the thick steel plate of the present invention exhibits good HAZ toughness even with high heat input or super high heat input welding where the heat input is 40 kJ / mm, so even if the plate thickness is thick, the heat input is increased. This is because it can be efficiently welded.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples as a matter of course, and appropriate modifications are made within a range that can meet the purpose described above and below. Of course, it is also possible to implement them, and they are all included in the technical scope of the present invention.

  Steels having the compositions shown in the following Tables 1 to 4 were melted by a normal melting method, and this molten steel was cooled from 1500 ° C. to 1100 ° C. at a cooling rate of 0.1 to 2.0 ° C./min to form a slab. Thereafter, it was heated to 1100 ° C. and hot-rolled, and optionally tempered to produce a high-tensile steel plate having a thickness of 60 mm. At this time, the cumulative rolling reduction in the temperature range of T value ± 50 ° C. defined by the formula (3) and the cumulative rolling reduction at 750 to 800 ° C. were controlled. In the following Tables 1 to 4, the [Ti] / [N] calculated from the chemical composition of the steel sheet, the X value, the value of the temperature range of the δ region calculated from Thermo-calc (described as “δ region” in the table) ) And T values are also shown.

  For the steel sheet produced as described above, the average particle diameter of the Ti-based carbon / nitride, the tensile strength of the steel sheet, the base material toughness, the HAZ toughness, and the brittle crack propagation stopping characteristics were measured by the following method. The area ratio and dislocation density of bainite were measured by the methods described above. These results are shown in Tables 5 to 8 below together with the production method (cumulative rolling reduction in the temperature range of T value ± 50 ° C., cumulative rolling reduction at 750 to 800 ° C.).

[Average particle diameter of Ti-based charcoal / nitride]
The position at the depth t / 4 (t = plate thickness) was observed with a transmission electron microscope (TEM) under the conditions of an observation magnification of 60,000 times, an observation field of view 2.0 μm × 2.0 μm, and five observation locations. Then, the area of each charcoal / nitride in the field of view was measured, and the equivalent circle diameter of each charcoal / nitride was calculated from this area. The average equivalent diameter of each carbonitride was arithmetically averaged (arithmetic average), and the average particle diameter of the Ti-based carbonitride / nitride in each steel sheet was calculated.

[Tensile strength of steel sheet]
At a depth t / 4 (t = plate thickness), a JIS No. 4 test piece was sampled so that the longitudinal direction of the test piece would be the plate width direction (C direction) of the steel plate, and a tensile test was performed to The strength was measured.

[Base material toughness]
At the position of depth t / 4 (t = plate thickness), V-notch standard test pieces defined in JIS Z 2242 are sampled so that the longitudinal direction of the test pieces is the rolling direction (L direction) of the steel sheet, A Charpy impact test (impact blade radius: 2 mm) was performed at the temperature, and the absorbed energy (vE _-40 ) at -40 ° C was measured.

[HAZ toughness]
Welding (electrogas arc welding) was performed at a heat input of 40 kJ / mm, and a JIS No. 4 specimen was taken from the site shown in FIG. 1 (the notch position was 0.5 mm HAZ side from the bond) and Charpy impact test (impact The blade radius was 2 mm), and the absorbed energy (vE- ₄₀ ) was measured. In the present invention, those having a tensile strength of 200 J or more were regarded as acceptable.

[Evaluation of brittle crack propagation stop characteristics]
The brittle crack propagation stop property was tested according to the method specified in the steel type certification test method of the Japan Welding Association. That is, the test was performed by obtaining a temperature Tk (° C.) at which the brittle crack propagation performance (Kca value) is 600 N / mm ^1.5 by a test (ESSO test) in which a 50 mm square test piece was notched to a depth of 29 mm. In the present invention, Tk ≦ −40 ° C. is regarded as acceptable.

  From these results, it can be considered as follows. First, test no. 1 to 46 satisfy the requirements defined in the present invention, but it is understood that all the properties (tensile strength of steel plate, base metal toughness, HAZ toughness and brittle crack propagation stop property) are good.

  In contrast, test no. In the case of 47 to 75, any of the requirements defined in the present invention is lacking, and any of the characteristics is deteriorated.

It is the schematic which shows the position which extract | collected the test piece for HAZ toughness (vE- ₄₀ ) measurement.

Claims (11)

C: 0.030 to 0.15% (meaning of mass%, the same shall apply hereinafter), Si: 1.0% or less (excluding 0%), Mn: 0.8 to 2.0%, P: 0.0. 03% or less (not including 0%), S: 0.01% or less (not including 0%), Al: 0.01 to 0.10%, Ti: 0.015 to 0.03%, B: 0.0010 to 0.0035%, N: 0.0050 to 0.01%, Ca: 0.005% or less (not including 0%), O: 0.01% or less (not including 0%) , Nb: 0.035% or less (excluding 0% ), respectively, satisfies the following formulas (1) and (2), the balance is Fe and inevitable impurities, and the bainite fraction is 95 area % or more with a tissue, wherein a dislocation density of bainite ([rho ^1/2) is ^{5.0 × 10 6 ~5.0 × 10 7} (m -1) High heat input welding steel plate excellent in brittle crack propagation stopping characteristics.
1.5 ≦ [Ti] / [N] ≦ 4.0 (1)
40 ≦ X value ≦ 160 (2)
X value = 500 [C] +32 [Si] +8 [Mn] -9 [Nb]
+14 [Cu] +17 [Ni] -5 [Cr] -25 [Mo] -34 [V]
(In the formula, [] represents the content (% by mass) of each element.) C: 0.030 to 0.15% (meaning of mass%, the same shall apply hereinafter), Si: 1.0% or less (excluding 0%), Mn: 0.8 to 2.0%, P: 0.0. 03% or less (not including 0%), S: 0.01% or less (not including 0%), Al: 0.01 to 0.10%, Ti: 0.015 to 0.03%, B: 0.0010 to 0.0035%, N: 0.0050 to 0.01%, Ca: 0.005% or less (not including 0%), O: 0.01% or less (not including 0%) Further, Cu: 2.0% or less (not including 0%), Ni: 2.0% or less (not including 0%), and Cr: 2.0% or less (not including 0%) And the following formulas (1) and (2) are satisfied, the balance is composed of Fe and inevitable impurities, and the bainite fraction is 9 With an area% or more organizations, the dislocation density of bainite ([rho ^1/2 ) Is 5.0 × 10 ⁶ ~ 5.0 × 10 ⁷ (m ^-1 A thick steel plate for high heat input welding with excellent brittle crack propagation stopping characteristics.
1.5 ≦ [Ti] / [N] ≦ 4.0 (1)
40 ≦ X value ≦ 160 (2)
X value = 500 [C] +32 [Si] +8 [Mn] -9 [Nb]
+14 [Cu] +17 [Ni] -5 [Cr] -25 [Mo] -34 [V]
(In the formula, [] represents the content (% by mass) of each element.) C: 0.030 to 0.15% (meaning of mass%, the same shall apply hereinafter), Si: 1.0% or less (excluding 0%), Mn: 0.8 to 2.0%, P: 0.0. 03% or less (not including 0%), S: 0.01% or less (not including 0%), Al: 0.01 to 0.10%, Ti: 0.015 to 0.03%, B: 0.0010 to 0.0035%, N: 0.0050 to 0.01%, Ca: 0.005% or less (not including 0%), O: 0.01% or less (not including 0%), Nb: 0.035% or less (not including 0%), Cu: 2.0% or less (not including 0%), Ni: 2.0% or less (not including 0%), and Cr: Each contains at least one selected from the group consisting of 2.0% or less (excluding 0%), satisfies the following formulas (1) and (2), and the balance is Fe and Consists avoid impurities, and with bainite fraction is 95 area% or more of the tissue, the dislocation density of bainite ([rho ^1/2 ) Is 5.0 × 10 ⁶ ~ 5.0 × 10 ⁷ (m ^-1 A thick steel plate for high heat input welding with excellent brittle crack propagation stopping characteristics.
1.5 ≦ [Ti] / [N] ≦ 4.0 (1)
40 ≦ X value ≦ 160 (2)
X value = 500 [C] +32 [Si] +8 [Mn] -9 [Nb]
+14 [Cu] +17 [Ni] -5 [Cr] -25 [Mo] -34 [V]
(In the formula, [] represents the content (% by mass) of each element.) The thick steel plate for high heat input welding according to any one of claims 1 to 3, wherein the temperature range of the δ region is 40 ° C or lower. The thick steel plate for high heat input welding according to any one of claims 1 to 4 , wherein an average particle diameter of the Ti-based carbon / nitride is 43 nm or less at a position at a depth t / 4 (t = plate thickness).   Furthermore, it contains Mo: 1.0% or less (excluding 0%), The thick steel plate for high heat input welding in any one of Claims 1-5.   The thick steel plate for high heat input welding according to any one of claims 1 to 6, further comprising V: 0.1% or less (not including 0%).   Furthermore, 1 type or more chosen from the group which consists of Mg, Sr, Ba: It is what contains 0.01% or less in total (excluding 0%), The large heat input in any one of Claims 1-7 Thick steel plate for welding.   Furthermore, the thick steel plate for high heat input welding in any one of Claims 1-8 which contains rare earth elements: 0.01% or less (0% is not included).   Furthermore, 1 type or more chosen from the group which consists of Zr, Ta, and Hf: 0.05% or less (it does not contain 0%) in total is contained, The large input in any one of Claims 1-9 Thick steel plate for heat welding.   Furthermore, Co: 2.5% or less (0% is not included) and / or W: 2.5% or less (not including 0%) is contained. Heavy steel plate for high heat input welding.

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