JP5276871B2 - Low yield specific thickness steel plate with excellent toughness of weld heat affected zone - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a low-yield ratio thick steel plate which exhibits excellent HAZ toughness not only in the case high heat input welding in which welding heat input is 20 kJ/mm is performed but also in the case welding in which a heat input volume is the relatively small one of 5 kJ/mm is performed. <P>SOLUTION: The thick steel plate satisfies a prescribed chemical componential composition, and further satisfies inequalities (1) and (2), and in which the volume fraction of ferrite is 5 to 50%, the diameter of the average equivalent circle of the ferrite is &le;100 &mu;m and the average hardness of a hard phase satisfies HV 150 to 400: inequality (1): 1.0&le;[Ti]/[N]&le;2.5 (wherein [Ti] and [N] represent the contents (mass%) of Ti and N, respectively), and (2): 2.0&le;1000&times;([Ca]+2&times;[S]+3&times;[O])&le;13.0 (wherein, [Ca], [S] and [O] represent the contents (mass%) of Ca, S and O, respectively). <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to a steel plate applied to a welded structure such as a building, a ship, and an offshore structure, and is particularly referred to as a portion (hereinafter referred to as “HAZ”) that is affected by heat when performing super-high heat input welding. The present invention relates to a low yield specific thickness steel plate having excellent toughness.

  Structures in various fields such as ships, buildings, and offshore structures are generally constructed by joining steel materials by welding, but steel materials used in such structures have a viewpoint of ensuring safety. Therefore, it is required that not only the steel material strength but also the toughness of the welded portion is good.

  In recent years, with the increase in size of welded structures, from the viewpoint of improving the construction efficiency of the structure and reducing the construction cost, improvement in welding construction efficiency is required, and an increase in welding heat input is directed. In particular, there is a tendency that high heat input welding is performed such that the welding heat input is 20 kJ / mm or more.

  In carrying out the high heat input welding as described above, the HAZ [base metal side number of the weld metal-base metal interface (bond part) is affected by the heat of the weld base metal (steel plate as the welded material). The toughness at the position of mm] becomes a problem. In this HAZ, the base material is exposed to a high temperature just below the melting point during welding, the austenite grains in the metal structure tend to be coarse, and the cooling rate is also slowed due to the increase in welding heat input, so a coarse structure is likely to be formed. For these reasons, there has been a problem that the HAZ toughness tends to decrease.

  Various steel plates have been proposed so far to suppress HAZ toughness deterioration when the high heat input welding method is adopted. For example, Patent Documents 1 and 2 finely disperse TiN in the steel plate. A technique for improving the HAZ toughness has been proposed by precipitating MnS and suppressing the prevention of austenite grain coarsening. Patent Documents 3 and 4 propose a technique for improving the toughness in the vicinity of the weld bond portion by using finely precipitated Ti oxide as a nucleation site for ferrite transformation.

  Patent Document 5 proposes a technique for improving HAZ toughness by using BN precipitated from TiN or the like in the cooling process during welding as a nucleation site for ferrite transformation.

  By the way, it is also known that the HAZ toughness deteriorates when the amount of the solid solution N is excessive, and it is common to reduce the N to improve the HAZ toughness (for example, Non-Patent Document 1). Moreover, in patent document 6, from the viewpoint of reducing solid solution N thoroughly, by containing Ti and sufficient quantity of Al, and also using Ca oxide as a fine oxide, HAZ in super-high heat input welding is used. Techniques for improving toughness have also been proposed.

On the other hand, Patent Document 7 also proposes a technique for improving HAZ toughness in high heat input welding by utilizing CaS.
Japanese Patent Laid-Open No. 2-2501717 JP-A-2-254118 JP-A-60-245768 JP-A-61-79745 JP-A-61-253344 JP 2001-107177 A JP 2001-356379 A Journal of the Japan Welding Society, vol. 13, no. 4, P758-766 (issued in November 1985)

  However, none of the techniques that have been proposed so far has fundamentally improved the HAZ toughness, and each has the following problems.

  In the technology of finely dispersing TiN in steel (Patent Documents 1, 2, and 5), when high heat input welding is performed, the vicinity of the weld bond portion is heated to a high temperature for a long time. The actual situation is that it is dissolved and the coarsening of crystal grains cannot be suppressed, and good HAZ toughness cannot be obtained.

  In addition, the technique for finely depositing Ti oxide (Patent Documents 3 and 4) has a problem that the HAZ toughness cannot be improved because it is difficult to uniformly disperse the oxide in the steel. . In the technique for reducing the solid solution N (Patent Document 6 and Non-Patent Document 1), if excessive Ti is contained, the amount of solid solution Ti increases, and conversely, an embrittled structure is generated. is there.

  Further, in the technology using CaS (Patent Document 7), since CaS becomes relatively coarse, the HAZ toughness has not been sufficiently improved. In this technology, the use of TiN is also considered, but the synergistic effect with the ferrite forming ability of TiN is not fully utilized, and the effect of improving the HAZ toughness in high heat input welding is not sufficient. It is not considered.

  On the other hand, with the increase in the size of steel structures such as buildings, the steel materials used are required to have high strength from the viewpoint of the need to reduce the weight of the steel materials used. Further, from the viewpoint of the safety of the structure, a low yield ratio is required. For example, a thick steel sheet exhibiting a low yield ratio has a large allowable stress before fracture, even when stress above the yield point is applied, and also has a large uniform elongation. Even if it encounters an earthquake, it has the advantage of absorbing seismic energy and not causing destruction.

  However, in the conventional technique, there is a problem that reheating treatment is required and the number of steps increases, so that the production efficiency is lowered and the manufacturing cost is increased.

  The present invention has been made to solve such problems in the prior art, and its purpose is, of course, when super-high heat input welding is performed in which the welding heat input is 20 kJ / mm or more, For example, even when welding is performed with a relatively high heat input such as a heat input of 5 kJ / mm or more, a low yield specific thickness steel plate with excellent HAZ toughness can be produced online with high production efficiency. It is to provide.

The thick steel plate of the present invention that can achieve the above-mentioned object (hereinafter sometimes simply referred to as “steel plate”) is C: 0.03 to 0.150% (meaning mass%, the same applies hereinafter), Si: 0 .50% or less (including 0%), Mn: 1.0 to 2.0%, P: 0.015% or less (not including 0%), S: 0.005% or less (not including 0%) ), Al: 0.005-0.06%, Ti: 0.008-0.030%, N: 0.0050-0.010%, Ca: 0.0010-0.0035%, and O: 0 0.003% or less (excluding 0%) respectively,
The ferrite fraction is 5 to 50 area%, the average equivalent circle diameter of the ferrite is 100 μm or less, and the average hardness of the hard phase is HV 150 to 400, which is defined by the following formulas (1) and (2). The point is to satisfy each of the relations.

1.0 ≦ [Ti] / [N] ≦ 2.5 (1)
However, [Ti] and [N] indicate the contents (mass%) of Ti and N, respectively.
2.0 ≦ 1000 × ([Ca] + 2 × [S] + 3 × [O]) ≦ 13.0 (2)
However, [Ca], [S] and [O] indicate the contents (mass%) of Ca, S and O, respectively.

  In the steel sheet of the present invention, if necessary, (a) B: 0.0035% or less (not including 0%), (b) Cu: 2.0% or less (not including 0%), Ni: 2. One or more selected from the group consisting of 0% or less (not including 0%) and Cr: 1.50% or less (not including 0%), (c) Mo: 0.5% or less (including 0%) (D) Nb: 0.035% or less (not including 0%) and / or V: 0.10% or less (not including 0%), (e) Mg: 0.005% or less (0 %), (F) Zr: 0.1% or less (not including 0%) and / or Hf: 0.05% or less (not including 0%), (g) Co: 2.5% Or less (not including 0%) and / or W: 2.5% or less (not including 0%), (h) REM: 0.010% or less (not including 0%), etc. Is effective also contain, it is possible to further improve the properties of the steel sheet in accordance with the components to which they are contained.

  In the steel sheet of the present invention, with respect to the elements that affect the HAZ toughness, the chemical composition is strictly defined while satisfying a predetermined relational expression, and the steel structure is controlled as described above. It can exhibit good HAZ toughness and realize a low yield ratio. Such a low yield specific thickness steel plate excellent in HAZ toughness is extremely useful as a material for various building structures and the like.

  In order to achieve the above-mentioned problems, the present inventors have studied from various angles the factors affecting the HAZ toughness and the yield ratio when high heat input welding is performed. As a result, firstly, the HAZ toughness of the steel sheet is greatly influenced by the presence or absence of the formation of an embrittled structure. It was found that this can be prevented by fine dispersion of transformation nuclei that promote transformation. Conventionally, these are insufficient, and it is considered that the toughness of the HAZ could not be stably improved.

  Therefore, the present inventors have further studied under the idea of effectively using CaS, TiN, and MnS produced using them as nuclei in the solidification stage during casting for fine dispersion of ferrite transformation nuclei. . CaS and TiN exist alone or in a composite precipitate with MnS, but in order to finely disperse them and disperse many ferrite-forming nuclei, the chemical composition of the steel sheet is adjusted appropriately. Thus, it has been clarified that it is effective to satisfy the relationship of the following expressions (1) and (2).

  Conventionally, it has been generally attempted to reduce N due to a decrease in toughness due to solute N (Non-Patent Document 1). However, in the present invention, [Ti] / [N It is important to reduce the effect of solid solution N that increases when the ratio is made low (positively high N), and TiN itself is finely dispersed to improve the HAZ toughness. From these viewpoints, the following formulas (1) and (2) are defined. The reasons for defining these formulas are as follows.

1.0 ≦ [Ti] / [N] ≦ 2.5 (1)
However, [Ti] and [N] indicate the contents (mass%) of Ti and N, respectively.

  In order to finely disperse TiN and generate many ferrite nuclei, it is necessary to keep the balance of addition of Ti and N within this range. By adjusting to the said range, a ferrite production | generation nucleus can be increased and the favorable HAZ toughness in super-large heat input can be ensured. If the value of [Ti] / [N] (hereinafter referred to as “P value”) exceeds 2.5, TiN becomes coarse, and if it is less than 1.0, the TiN generation amount itself decreases. From such a viewpoint, the above equation (1) is defined. In addition, the preferable minimum of the value of [Ti] / [N] is 1.5, and a preferable upper limit is 2.0.

2.0 ≦ 1000 × ([Ca] + 2 × [S] + 3 × [O]) ≦ 13.0 (2)
However, [Ca], [S] and [O] indicate the contents (mass%) of Ca, S and O, respectively.

  It is shown that there is a strong tendency to disperse in the order of Ca, S and O under the range of chemical components defined in the present invention. By setting the value of [1000 × ([Ca] + 2 × [S] + 3 × [O])] (hereinafter referred to as “Q value”) in the range of 2.0 to 13.0, super large heat input Thus, a large number of ferrite-forming nuclei effective for securing the HAZ toughness can be introduced, and good HAZ toughness can be obtained.

  In addition, the present inventors show a low yield ratio, ensure a tensile strength of 440 MPa or more, and ensure an excellent base metal toughness. It was clarified that it was effective to use a composite structure whose hardness was controlled as follows. Hereinafter, the reason why these are defined in the present invention will be described in detail.

[Ferrite fraction: 5 to 50 area%]
In the present invention, if the fraction of ferrite in the entire structure is too small, that is, if the ratio of the soft phase is small, the yield ratio becomes high, which is not preferable. Therefore, in the present invention, the lower limit of the ferrite fraction is set to 5 area%. Preferably it is 20 area% or more. On the other hand, if the fraction of ferrite occupying the entire structure is too large, high strength cannot be ensured and the base material toughness is lowered, which is not preferable. Therefore, in the present invention, the upper limit of the ferrite fraction is set to 50 area%. Preferably it is 40 area% or less. The ferrite fraction is determined by the method shown in the examples described later.

[Average equivalent circle diameter of ferrite: 100 μm or less]
If the average equivalent circle diameter of ferrite is too large, the base material toughness deteriorates, which is not preferable. Therefore, in the present invention, the upper limit of the average equivalent circle diameter of the ferrite is set to 100 μm. Preferably it is 40 micrometers or less. The present invention does not define the lower limit of the average equivalent circle diameter of the ferrite, but the lower limit is approximately 10 μm. The average equivalent circle diameter of the ferrite was determined by the method shown in the examples described later.

[Average hardness of hard phase: HV 150 to 400]
If the average hardness of the hard phase (hereinafter sometimes simply referred to as “hardness”) is too small, the yield ratio increases, which is not preferable. Therefore, in the present invention, the lower limit of the hardness of the hard phase is set to HV150. Preferably it is HV220 or more. On the other hand, if the hardness of the hard phase is too large, the yield ratio increases and the toughness decreases, which is not preferable. Therefore, in the present invention, the upper limit of the hardness of the hard phase is set to HV400. Preferably it is HV300 or less, More preferably, it is HV250 or less. The hard phase is composed of one or more of bainite, martensite, and pearlite. The hardness of the hard phase is determined by the method shown in the examples described later.

  The steel sheet of the present invention is composed of a mixed structure mainly composed of ferrite and the hard phase. The “main body” means 70 area% or more, and residual γ (residual austenite), cementite and the like may be included as the remaining structure.

  In the steel plate of the present invention, it is also an important requirement to control the chemical component composition within an appropriate range in order to sufficiently exhibit the above characteristics. The reasons for limiting the range including the elements (Ti, N, Ca, S and O) involved in the above formulas (1) and (2) are as follows.

[C: 0.03-0.150%]
C is an element necessary for ensuring the strength of the steel sheet (welding base metal), and it is necessary to contain 0.03% or more in order to ensure the desired strength. However, if C is contained excessively, the HAZ toughness is reduced instead. For these reasons, the upper limit needs to be 0.150%. In addition, the minimum with preferable C content is 0.05%, and a preferable upper limit is 0.08%.

[Si: 0.50% or less (including 0%)]
Si is an effective element for securing the strength of the steel sheet, and is contained if necessary. However, if it is contained excessively, a large amount of island martensite phase (MA phase) precipitates in the steel material (base material), and the HAZ toughness deteriorates. For these reasons, the upper limit was made 0.50%. In addition, the minimum with the preferable amount of Si is 0.1%, and a preferable upper limit is 0.4%.

[Mn: 1.0 to 2.0%]
Mn is an element effective in improving the hardenability and ensuring the strength of the steel sheet, and in order to exert such an effect, it is necessary to contain 1.0% or more of Mn. However, if Mn is contained excessively, the HAZ toughness of the steel sheet deteriorates, so the upper limit is made 2.0%. A preferable lower limit of the amount of Mn is 1.3%, and a preferable upper limit is 1.8%.

[P: 0.015% or less (excluding 0%)]
P is an impurity which is inevitably mixed, and adversely affects the toughness of the base material and HAZ. From such a viewpoint, P is suppressed to 0.015% or less. A preferable upper limit of the amount of P is 0.01%.

[S: 0.005% or less (excluding 0%)]
S is an element that effectively forms ferrite in the HAZ part by forming CaS in the steel sheet during solidification during casting and forming MnS on the CaS after welding. Such an effect increases as the content increases, but if it is contained in excess of 0.005%, the toughness of the base material and the HAZ deteriorates. In addition, in order to exhibit the said effect by S, it is preferable to make it contain 0.0005% or more, and a preferable upper limit is 0.0020% and a more preferable upper limit is 0.0010%. Thus, in order to reduce the amount of S, the desulfurization time may be relatively long (for example, 25 minutes or more).

[Al: 0.005 to 0.06%]
Al is an element effective as a deoxidizer, and also exhibits an effect of improving the base material toughness by refining the microstructure of the steel sheet. In order to exert such an effect, the Al amount needs to be 0.005% or more. However, when Al is contained excessively, a large amount of island martensite phase (MA phase) is precipitated on the steel plate (base material), and the HAZ toughness deteriorates. For these reasons, the upper limit was made 0.06%. The preferable lower limit of the Al amount is 0.01% (more preferably 0.02%), and the preferable upper limit is 0.04%.

[Ti: 0.008 to 0.030%]
Ti is an element that forms nitrides, suppresses coarsening of prior austenite grains during high heat input welding, and improves HAZ toughness. In order to exert such an effect, the Ti amount needs to be 0.008% or more. However, if Ti is excessively contained, coarse inclusions are precipitated, and the HAZ toughness is deteriorated. Therefore, the upper limit is made 0.030%. In addition, the minimum with preferable Ti content is 0.01%, and a preferable upper limit is 0.025%.

[N: 0.0050 to 0.010%]
In order to ensure high toughness in the high heat input welding HAZ, it is effective to prevent TiO from coarsening by precipitating TiN in the prior austenite grains. In order to exert such an effect, the N amount needs to be 0.0050% or more. However, if the amount of N becomes excessive and exceeds 0.010%, coarse TiN precipitates and the HAZ toughness decreases. For these reasons, the upper limit was made 0.010%. In addition, the minimum with the preferable amount of N is 0.006%, and a preferable upper limit is 0.009% (more preferably 0.008%).

[Ca: 0.0010 to 0.0035%]
Ca is an element that contributes to the improvement of HAZ toughness by controlling the form of sulfide. In order to exhibit such an effect, it is necessary to contain 0.0010% or more. However, if the content exceeds 0.0035%, the HAZ toughness deteriorates. In addition, the preferable minimum of the amount of Ca is 0.0015% (more preferably 0.0020%), and a preferable upper limit is 0.003%.

[O: 0.003% or less (not including 0%)]
O is contained as an unavoidable impurity and exists as an oxide in steel. However, if the content exceeds 0.003%, coarse CaO is generated and the HAZ toughness deteriorates. For these reasons, the upper limit of the O content is set to 0.003%. The upper limit with preferable O content is 0.0020%, More preferably, it is 0.0015% or less.

  In the steel sheet of the present invention, in addition to the above components, the steel plate is composed of iron and inevitable impurities (for example, Sb, Se, Te, etc.), but may contain trace components (allowable components) to the extent that the properties are not impaired. Such a steel sheet is also included in the scope of the present invention. If necessary, (a) B: 0.0035% or less (excluding 0%) (b) Cu: 2.0% or less (excluding 0%), Ni: 2.0% or less (0% 1) or more selected from the group consisting of Cr: 1.50% or less (not including 0%), (c) Mo: 0.5% or less (not including 0%), (d) Nb : 0.035% or less (not including 0%) and / or V: 0.10% or less (not including 0%), (e) Mg: 0.005% or less (not including 0%), ( f) Zr: 0.1% or less (not including 0%) and / or Hf: 0.05% or less (not including 0%), (g) Co: 2.5% or less (not including 0%) ) And / or W: 2.5% or less (excluding 0%), (h) REM: 0.010% or less (excluding 0), etc. It is. The reasons for limiting the range when these components are contained are as follows.

[B: 0.0035% or less (excluding 0%)]
B is an element effective in improving HAZ toughness, in addition to generating intragranular ferrite with BN as the nucleus in the vicinity of the bond part of the super high heat input HAZ, and also having a fixing action of solute N. . However, when the amount of B becomes excessive, the structure of the bond portion becomes a coarse bainite structure, so that the HAZ toughness is deteriorated. For these reasons, when B is contained, the upper limit is preferably made 0.0035%. A more preferable range is 0.0010 to 0.0025%.

[Selected from the group consisting of Cu: 2.0% or less (not including 0%), Ni: 2.0% or less (not including 0%) and Cr: 1.50% or less (not including 0%) One or more
Cu, Ni and Cr are all effective elements for improving the hardenability and improving the strength, and are contained as necessary. However, if the content of these elements is excessive, the HAZ toughness is reduced instead, so that Cu and Ni are 2.0% or less (more preferably 1% or less), and Cr is 1.50% or less (more Preferably it is 1% or less. A preferable lower limit for exhibiting the above effect is 0.20% (more preferably 0.40%).

[Mo: 0.5% or less (excluding 0%)]
Mo is effective for improving the hardenability and ensuring the strength, and is appropriately used for preventing temper brittleness. Such an effect increases as the content thereof increases. However, if the Mo amount becomes excessive, the HAZ toughness deteriorates, so that the content is preferably 0.5% or less. More preferably, it is 0.30% or less.

[Nb: 0.035% or less (not including 0%) and / or V: 0.10% or less (not including 0%)]
Nb and V exhibit the effect of improving the hardenability and improving the base material strength. V also has the effect of increasing the temper softening resistance. However, since HAZ toughness deteriorates when contained in a large amount, Nb is 0.035% or less (more preferably 0.030% or less), and V is 0.10% or less (more preferably 0.05% or less). It is good to do. The contents for effectively exhibiting these effects are 0.005% or more for Nb and 0.01% or more for V.

[Mg: 0.005% or less (excluding 0%)]
Since Mg has the effect of improving HAZ toughness by forming MgO and suppressing the coarsening of austenite grains in the HAZ, it is contained as necessary. However, when the amount of Mg becomes excessive, inclusions become coarse and the HAZ toughness deteriorates. Therefore, it is preferable to make the content 0.005% or less (more preferably 0.0035% or less).

[Zr: 0.1% or less (excluding 0%) and / or Hf: 0.05% or less (0%
Is not included)]
Zr and Hf are elements effective for improving the HAZ toughness by forming a nitride with N as in the case of Ti, refining the austenite grains of the HAZ during welding. However, if excessively contained, the HAZ toughness is reduced. Therefore, when these elements are contained, Zr is set to 0.1% or less and Hf is set to 0.05% or less.

[Co: 2.5% or less (not including 0%) and / or W: 2.5% or less (not including 0%)]
Co and W are contained as necessary because they have the effect of improving the hardenability and increasing the strength of the base material. However, since the HAZ toughness deteriorates if contained excessively, the upper limit is set to 2.5%.

[REM: 0.010% or less (excluding 0)]
REM (rare earth element) is an element that contributes to improving the toughness of HAZ by refining and spheroidizing the shape of inclusions (oxides, sulfides, etc.) inevitably mixed in the steel material, Contains if necessary. Such an effect increases as the content thereof increases. However, if the content of REM becomes excessive, inclusions become coarse and the HAZ toughness deteriorates. Therefore, the content is preferably suppressed to 0.010% or less. In the present invention, REM means a lanthanoid element (15 elements from La to Ln), Sc (scandium) and Y (yttrium).

  Although this invention does not prescribe | regulate to the manufacturing method of the said steel plate, in order to ensure the composite structure prescribed | regulated stably, the steel which satisfy | fills the said amount of chemical components, (1) Formula and (2) Formula, It is very effective to appropriately control the rolling reduction amount and the cooling rate in hot rolling after melting by a normal melting method and cooling the molten steel to form a slab. Specifically, for example, hot rolling is performed after heating in the range of 950 to 1300 ° C. In this hot rolling, rolling is performed so that the cumulative reduction ratio from Ar3 + 100 ° C to Ar3 + 50 ° C is 10 to 30%. Then, the rolling is completed so that the cumulative reduction ratio of Ar3 + 50 ° C. to Ar 3 is 5 to 30%, and then cooling between Ar 3 to 400 ° C. at a cooling rate of 1 to 50 ° C./sec is performed. A structure can be easily obtained (Ar3 can be obtained from the formula (3) shown in the examples described later). If the steel sheet is cooled at a cooling rate within this range, the above composite structure can be secured stably regardless of the sheet thickness within the same composition range, and online without reheating to the two-phase temperature range. This is preferable because the steel sheet of the present invention can be produced without reducing the production efficiency. However, the method is not limited to the above method, and other means may be used as long as a yield ratio of 80% or less can be obtained.

  In addition, although the steel plate which is the subject of the present invention is basically a thick steel plate having a thickness of 20 mm or more, it has the same characteristics even in a thickness less than that, and is the subject of the present invention. Is included. In addition, the heat input when welding the steel sheet of the present invention is assumed to be 20 kJ / mm or more. When welding is performed with such a super-high heat amount, good HAZ toughness is exhibited. For example, even if the amount of heat input is 5 kJ / mm or more, good HAZ toughness is exhibited.

  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 any design modifications may be made in accordance with the gist of the present invention. It is included in the range.

  Steels having the compositions shown in the following Tables 1 to 3 were melted by a normal melting method, and the 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. . And it hot-rolled after heating to the range of 950-1300 degreeC. In this hot rolling, the cumulative rolling reduction from Ar3 + 100 ° C. to Ar3 + 50 ° C. is set to the value shown in Table 4, Table 5 or Table 6, and then the cumulative rolling reduction from Ar3 + 50 ° C. to Ar3 is set to Table 4, Table 5. 5 or Table 6 is rolled so that the value is as shown in Table 6, and cooling between Ar 3 and 400 ° C. is performed at the cooling rate shown in Table 4, Table 5 or Table 6 to obtain a thick steel plate (plate thickness 40 mm). Obtained.

  In Table 2, REM was added in the form of a misch metal containing about 50% La and about 25% Ce. In Tables 1 to 3, “-” indicates that no element was added. Tables 1 to 3 also show the P value ([Ti] / [N]) and the Q value [1000 × ([Ca] + 2 × [S] + 3 × [O])] defined in the present invention.

In Tables 4 to 6, Ar3 is determined from the following equation (3).
Ar3 (° C.) = 910−230 × [C] + 25 × [Si] −74 × [Mn] −56 × [Cu] −16 × [Ni] −9 × [Cr] −5 × [Mo] −1620 × [Nb]
... (3)

  With respect to the various steel sheets thus obtained, the following methods were used to measure the ferrite fraction and ferrite particle size (average equivalent circle diameter), measure the average hardness (HV) of the hard phase, and evaluate the tensile properties and base material toughness. At the same time, welding was performed under the following conditions to create a welded portion and evaluate the HAZ toughness.

[Measurement of ferrite fraction and ferrite particle size (average equivalent circle diameter)]
Each steel plate is mirror-polished on a 2 cm square test piece TD surface taken from the position t (t is the plate thickness; the same shall apply hereinafter) / 4 position, and then etched with a nital etchant (2% nitric acid-ethanol solution). The structure was observed with a microscope (magnification 100 times, n number = 10), and the ferrite grain size (average value) was calculated based on the method of the comparative method defined in JIS G 0551, and the average equivalent circle diameter of the ferrite was obtained. The ferrite fraction (area%) was determined using image analysis software (Image Pro Plus, manufactured by Micromedia).

[Measurement of average hardness (HV) of hard part]
A 2 cm square test piece TD surface taken from the t / 4 position of each steel plate is mirror-polished, etched with a nital etchant (2% nitric acid-ethanol solution), and then subjected to hard phase Vickers using the method defined in JIS Z 2244. It was measured using a hardness tester. As described above, the hard phase is composed of one or more of bainite, martensite, and pearlite, and can be recognized as a portion other than ferrite by an optical microscope. However, since it is difficult to measure them individually, it is difficult to measure them individually. When two or more kinds are present, the average hardness of the hard phase was measured in a form including all of them. The measurement was performed at room temperature and the load was 10 g, and the average hardness was calculated by 5-point measurement of each steel type (maximum and minimum were not counted, but 3-point average).

[Tensile properties of steel sheet]
A JIS Z 2201 No. 4 test piece was collected from t / 4 of the steel sheet, subjected to a tensile test in accordance with the procedure of JIS Z 2241, measured for tensile strength (TS), and yield ratio was determined. In the present invention, the tensile strength (TS): 440 MPa or more and the yield ratio: 80% or less were accepted.

[Base material toughness]
At the t / 4 position, a V-notch standard test piece (size: 10 mm × 10 mm × 55 mm) defined in JIS Z 2242 is taken so that the longitudinal direction of the test piece is the rolling direction of the steel sheet (L direction), and −15 A Charpy impact test was conducted at ℃, and a V Charpy impact value (vE- ₁₅ ) at -15 ℃ was measured, with a V Charpy impact value (vE- ₁₅ ) of 150 J or more passed.

[HAZ toughness test]
As a HAZ toughness evaluation method simulating a thermal cycle when electroslag welding (30 kJ / mm) is performed, a heating temperature is maintained at 1400 ° C. for 30 seconds, and then a cooling time is 800 to 500 ° C. (Tc): 500 seconds. After heat-treating each test steel plate in the thermal cycle, Charpy absorbed energy (V notch) at a temperature of −15 ° C. was measured. In addition, as a test piece, a rod shape having a size of 10 mm × 10 mm × 55 mm collected from the position of the plate thickness t / 4 and having a V notch having a depth of 2 mm on one side of the central portion was used. At this time, the V Charpy impact value (vE- ₁₅ ) was determined to be 150 J or more.

Also, a heat treatment simulating welding with a heat input equivalent to 5 kJ / mm (heating temperature: 1400 ° C.
The V Charpy impact value (vE _-15 ) was measured in the same manner as described above for those subjected to second holding, Tc = 120 seconds. The V Charpy impact value (vE- ₁₅ ) at this time also passed 150J or more. These results are also shown in Tables 4-6.

  From these results, it can be considered as follows. First, test no. Nos. 1 to 43 satisfy the requirements defined in the present invention, and the strength of the steel sheet (base material) satisfies the target, exhibits a low yield ratio, and is excellent in HAZ toughness. is there. It can also be seen that these exhibit sufficient HAZ toughness even under welding conditions where the heat input is 5 kJ / mm.

  In contrast, test no. Nos. 51 to 87 lack any of the requirements defined in the present invention, and any of the characteristics is deteriorated. Among these, test No. 51-67, because the chemical composition is outside the range specified in the present invention (test No. 64 has a large P value), the strength cannot be secured, the yield ratio becomes high, or the HAZ toughness is inferior. As a result. In addition, Test No. 68 and 80 were inferior in HAZ toughness because the chemical composition was satisfactory, but the P value was outside the range specified in the present invention. Test No. In Nos. 69 to 78, although the chemical composition was satisfactory, the Q value was out of the range defined in the present invention, and in this case, the HAZ toughness was also inferior. Test No. Since 81 to 87 are not manufactured under the recommended conditions and the steel structure does not satisfy the prescribed conditions, any of the characteristics is deteriorated. Specifically, test no. 81, 83, and 86 have a high yield ratio because the ferrite fraction is too small or no ferrite exists. Test No. Nos. 82 and 84 are inferior in the base material toughness because the ferrite grain size is too large. Test No. No. 85 has a ferrite fraction that is too high to ensure high strength, has a high yield ratio, and is inferior in toughness. Test No. No. 87 has a high yield ratio because the hardness of the hard phase exceeds HV400, and the base metal toughness and HAZ toughness are also inferior.

Claims (8)

C: 0.03 to 0.150% (meaning mass%, the same shall apply hereinafter),
Si: 0.50% or less (including 0%),
Mn: 1.0-2.0%,
P: 0.015% or less (excluding 0%),
S: 0.005% or less (excluding 0%),
Al: 0.005 to 0.06%,
Ti: 0.008 to 0.030%,
N: 0.0050 to 0.010%,
Ca: 0.0010 to 0.0035%,
O: 0.003% or less (excluding 0%) , and
B: 0.0010 to 0.0035%
Each of which contains iron and inevitable impurities,
The ferrite fraction is 5 to 50 area%, the average equivalent circle diameter of the ferrite is 100 μm or less, and the average hardness of the hard phase is HV 150 to 400,
A low-yield specific thickness steel plate excellent in toughness of a weld heat-affected zone, characterized by satisfying the relationship defined by the following formulas (1) and (2).
1.0 ≦ [Ti] / [N] ≦ 2.5 (1)
However, [Ti] and [N] indicate the contents (mass%) of Ti and N, respectively.
2.0 ≦ 1000 × ([Ca] + 2 × [S] + 3 × [O]) ≦ 13.0 (2)
However, [Ca], [S] and [O] indicate the contents (mass%) of Ca, S and O, respectively. Cu: 2.0% or less (not including 0%), Ni: 2.0% or less (not including 0%), and Cr: 1.50% or less (not including 0%) The thick steel plate according to claim 1, which contains at least one kind. The thick steel plate according to claim 1 or 2 , which contains Mo: 0.5% or less (not including 0%). The thick steel plate according to any one of claims 1 to 3 , which contains Nb: 0.035% or less (not including 0%) and / or V: 0.10% or less (not including 0%). . The thick steel plate according to any one of claims 1 to 4 , which contains Mg: 0.005% or less (excluding 0%). The thick steel plate according to any one of claims 1 to 5 , which contains Zr: 0.1% or less (not including 0%) and / or Hf: 0.05% or less (not including 0%). . Co: 2.5% or less (excluding 0%) and / or W: 2.5% or less (not including 0%) Thick steel plate according to any one of claims 1 to 6 . The thick steel plate according to any one of claims 1 to 7 , which contains REM: 0.010% or less (not including 0%).