JP4311740B2 - Thick steel plate with high heat input welded joint toughness - Google Patents

Description

  The present invention relates to a thick steel plate excellent in high heat input weld joint toughness, and relates to a high strength thick steel plate of 590 to 780 MPa class that exhibits excellent weld joint toughness even when high heat input welding is performed.

  Conventionally, in order to ensure the HAZ toughness of the thick steel plate, an oxide containing Ti is dispersed in the base material, and ferrite is generated from within the grains when the HAZ part is cooled to refine the structure, thereby reducing the HAZ toughness. Attempts have been made to ensure

  For example, in Patent Document 1, coarse particles in the HAZ part are precipitated by precipitating either one or two of a 0.1 to 3.0 μm Ti oxide or a composite of Ti oxide and Ti nitride. It is described that the γ → α transformation during cooling in the chemical conversion region is controlled to produce intragranular ferrite and to improve the HAZ toughness.

  In Patent Document 2 and Patent Document 3, the production of coarse primary deoxidation products (deoxidation products by strong deoxidation elements such as Al, Ca, and REM) generated in molten steel is suppressed and weak. It is shown that the toughness of the weld heat affected zone (HAZ zone) can be secured by uniformly dispersing the secondary deoxidation product produced by deoxidation with deoxidizing elements (Ti, Si, Nb, V, Ta). ing.

  In Patent Document 4, a composite oxide of Ti and Al having a Ti composition ratio of 5% or more, an Al composition ratio of 95% or less, and a particle diameter of 0.01 to 1.0 μm is uniformly dispersed in steel. It is described that the HAZ toughness of the steel material during welding is improved.

  However, in these techniques, it is difficult to say that excellent HAZ toughness is ensured even when high heat input welding having a larger heat input, which has become common in recent years, is performed.

On the other hand, Patent Documents 5 and 6 describe austenite grain growth (coarsening) at a high temperature in the HAZ region heated to 1400 ° C. or higher in order to improve HAZ toughness in a high heat input welded joint. In order to suppress and promote reheated austenite refinement, it is described that an oxide containing 3% or more of Ca and 1% or more of Al is used as a composition of oxide particles generated in steel. Yes.
Japanese Patent Publication No. 7-824 (Claims) Japanese Patent Publication No. 3-67467 (Claims) Japanese Patent Publication No. 3-59134 (Claims) JP-A-9-3599 (Claims) JP2003-316628A (Claims, paragraph 0031) JP2003-316628A (Claims, paragraph 0030)

  However, Patent Documents 5 and 6 are intended for thick steel plates of 50 to 60 kilo class (less than 590 MPa). For this reason, when it becomes a 590-780MPa class thick steel plate having higher strength than this, the oxide means according to Patent Documents 5 and 6 may not ensure toughness over the entire area of the HAZ during high heat input welding. Arise. According to the knowledge of the present inventors, in Patent Documents 5 and 6 described above, in particular, the toughness of the weld heat affected zone in the vicinity of 3 to 5 mm from the melt line cannot be ensured.

  This is because, in the weld heat affected zone near 3 to 5 mm from the melt line, in a region where the γ grain size is relatively fine (grain size of about 10 μm), the hardenability is reduced and the bainite structure is coarse during high heat input welding. It is presumed that, because the nucleation frequency of grain boundary generation type bainite is high, intragranular bainite is not formed.

  The present invention has been made in view of such circumstances. The purpose of the present invention is to provide a high-strength thick steel plate of 590 to 780 MPa class that is excellent in weld joint toughness (HAZ toughness) even when subjected to high heat input welding. Is to provide.

In order to achieve this object, the gist of the thick steel plate excellent in the high heat input welded joint toughness of the present invention is mass%, C: 0.01 to 0.07%, Si: 1.0% or less (0 %), Mn: 1.0 to 3.0%, Al: less than 0.010% (not including 0%), Ti: 0.005 to 0.050%, O: 0.0010 to 0 0.0100% , Cu: 0.1-1.5% , Ni: 0.1-2.5%, and 20C (%) + 1.5Mn (%) + Cu (%) + Ni (%) When the average number of Ti-containing oxides having an average particle diameter of 0.05 to 1 μm is observed at a magnification of 1000 times, the steel sheet is 4.5 to 7.0% and the balance is iron and inevitable impurities. in conjunction with at 10000 / cm ² or more, the average number of average particle size 2μm or more Ti-containing oxides were observed at 200 magnifications 2,000 pieces / cm ² or less.

  In the present invention, the Ti-containing oxide refers to Ti when a composition analysis of inclusions in steel is performed using a measuring apparatus such as an EPMA apparatus or an FE-SEM / EDX apparatus, which will be described later. This refers to an oxide containing 10% by mass or more. Therefore, among the inclusions in steel, an oxide having a Ti content of less than 10% by mass is not said to be a Ti-containing oxide of the present invention. The “average particle diameter” refers to the equivalent-circle particle diameter of an oxide containing Ti.

  As described above, in the case of a high-strength 590-780 MPa thick steel plate, the Ca and Al-based oxides according to Patent Documents 4 and 5 have toughness over the entire area of the HAZ during large heat input welding as described above. There are cases where it cannot be secured.

  On the other hand, in this invention, many oxides are produced | generated after suppressing the number of coarse oxides among the oxides containing Ti which exist in steel. When the number of fine oxides generated is large in this way, intragranular bainite is likely to be generated in the HAZ part during cooling after high heat input welding. As a result, even in the case of a high-strength 590-780 MPa class thick steel plate, the toughness is particularly likely to decrease, and the entire HAZ during high heat input welding including the weld heat affected zone in the vicinity of 3-5 mm from the above-described melting line. Can significantly improve the toughness.

  Furthermore, in the present invention, C, Mn, Cu, elements that stabilize austenite (elements that enhance hardenability) in order to ensure good toughness not only in the vicinity of the melting line but also in the entire HAZ region. The Ni content is controlled in total.

  Therefore, the steel plate of the present invention is optimal for welded structures such as ships, offshore structures, bridges, and building structures having a strength of 590 to 780 MPa, and has a large heat input when used as the welded structures. Exhibits excellent welded joint toughness even when welding with.

(Ti-containing oxide)
First, the significance of the oxide containing Ti in the steel plate structure in the present invention will be described below. In the present invention, as described above, a large number of fine oxides are generated while suppressing the number of coarse oxides among the Ti-containing oxides. Specifically, the average number of average particle size of 0.05~1μm fine Ti-containing oxides, and 10,000 / cm ² or more when observed at a magnification 1000 times. On the other hand, when the average number of coarse Ti-containing oxides having an average particle diameter of 2 μm or more is observed at a magnification of 200 times, the average number is set to 2000 pieces / cm ² or less.

  In the present invention, the Ti-containing oxide refers to an oxide containing 10% by mass or more of Ti as described above. For oxides with a Ti content of less than 10% by mass and a low Ti content, the effect of forming bainite from within the HAZ grains during large heat input welding of a fine Ti-containing oxide, and a coarse Ti-containing oxide There is no effect of suppressing the formation of useful bainite as a ferrite nucleus.

  In the present invention, bainite is formed from within the HAZ grains during large heat input welding, instead of growing ferrite from within the HAZ grains during large heat input welding as in the prior art described above. For this reason, in this invention, many said fine Ti containing oxides exist beforehand in steel.

  As in the present invention, by forming bainite from within the HAZ grains during high heat input welding, the bainite structure due to a decrease in hardenability during high heat input welding in the HAZ in the vicinity of the melting line having a relatively fine γ particle diameter Suppresses coarsening and nucleation of grain boundary bainite. And even in the case of a high-strength 590-780 MPa thick steel plate, the toughness is particularly likely to decrease, including the heat affected zone in the vicinity of 3 to 5 mm from the above-mentioned melt line, and the entire HAZ during large heat input welding. Greatly improve toughness.

  At the same time, in order to ensure the number of the fine Ti-containing oxides and not to inhibit the formation of bainite from the grains, the present invention suppresses the coarse Ti-containing oxides in advance. More specifically, a coarse Ti-containing oxide of 2 μm or more has a strong ability to produce ferrite, so it tends to form ferrite nuclei at high temperatures, such as during high heat input welding, and suppresses the formation of useful bainite. End up. In addition, such a coarse Ti-containing oxide is likely to be a starting point of destruction in a fine bainite structure, and also plays a role of inhibiting the refinement effect by bainite.

  In order to reliably exhibit the above-mentioned effects and obtain excellent weld joint toughness, in the present invention, a fine Ti-containing oxide having an average particle diameter of 0.05 to 1 μm and a coarse Ti-containing oxide of 2 μm or more are used. The existence number is set to the optimum average number range. Therefore, when any Ti-containing oxide deviates from the above-mentioned optimum average number range, in the case of a high strength 590-780 MPa class thick steel plate, especially in the vicinity of 3-5 mm from the melt wire where the toughness tends to decrease. It becomes impossible to improve the toughness of the entire HAZ during high heat input welding including the heat affected zone.

Since the fine Ti-containing oxide promotes the formation of intragranular bainite as the number increases, it is desirably 20000 / cm ² or more, more desirably 40000 / cm ² or more. From the viewpoint of the above effects, there is no upper limit to the number of fine Ti-containing oxides, but the number of fine Ti-containing oxides that can be precipitated in a normal plate manufacturing process is 1 × 10 ⁸ / cm ^2. The degree is considered the upper limit.

  The fine Ti-containing oxide may contain Si, Ca, Mg or the like as an alloy element other than Ti. Among these, Mn is particularly suitable as an element contained together with Ti. More preferably, Ti + Mn occupying 60% or more (more preferably 70% or more) of all alloy elements constituting the oxide is preferable.

On the other hand, the above-mentioned coarse Ti-containing oxide has a smaller number, preferably 1000 pieces / cm ² or less, more preferably, in order to achieve the above-mentioned suppressing effect, such as to suppress the formation of ferrite. 500 pieces / cm ² or less.

  In this way, if a large number of fine Ti-based inclusions are generated while suppressing the number of coarse Ti-containing oxides, intragranular bainite is likely to be generated in the HAZ part during cooling after welding, greatly increasing HAZ toughness. Can be improved. In the steel sheet of the present invention, as described above, intragranular bainite may be preferentially produced in the HAZ part during cooling after welding, and some bainite and ferrite may be produced from the grain boundary during the cooling. Yes, the generation of these bainite and ferrite is allowed in a range that does not inhibit the improvement of HAZ toughness.

(Measurement of Ti-containing oxide)
A field emission scanning electron microscope (FE-SEM), a scanning electron microscope (SEM), an EPMA (electron probe microanalyzer) apparatus, or the like can be applied to the observation of the Ti-containing oxide at the specified magnification. In addition, EDX (manufactured by Kevex, Sigma energy dispersive X-ray spectrometer) is used for FE-SEM to more clearly distinguish between the steel matrix phase and each Ti-containing oxide. Additional analysis may be performed (FE-SEM / EDX).

  Among these, observation of a coarse Ti-containing oxide having an average particle diameter of 2 μm or more at a magnification of 200 is preferably performed with an EPMA apparatus. In addition, a field emission scanning electron microscope (FE-SEM) is preferably used for observing a fine Ti-containing oxide having an average particle size of 0.05 to 1 μm at a magnification of 1000 times. Thus, in this invention, in order to improve the precision and reproducibility of Ti-containing oxide number measurement, the magnification is changed according to the size of the Ti-containing oxide.

For example, when FE-SEM / EDX is used for observation of fine Ti-containing oxides, first, composition analysis of inclusions having an average particle diameter of 0.05 to 1.0 μm present in steel is performed, and 10 mass of Ti is obtained. The ratio with respect to the inclusion of the Ti-containing oxide containing at least% is obtained. Use Next, in the region of 0.1 mm ^2, using a 1000-fold reflection electron image, by photographing an arbitrary example 10 fields of 0.01 mm ^2, as a software image analysis, etc. MEDIACYBERNETICS Ltd. Image-ProPlus The number of inclusions having an average particle diameter of 0.05 to 1.0 μm is measured using an image analysis apparatus. Then, the number of Ti-containing oxides having an average particle diameter of 0.05 to 1.0 μm per 1 cm ^{2 is} obtained by multiplying the total number of these 10 visual fields by the ratio of the Ti-containing oxide and further multiplying by 1000.

(Composition of thick steel plate)
The composition (unit:% by mass) of the thick steel plate of the present invention will be described below including the reasons for limiting each element. As a premise of controlling the structure of the steel plate of the present invention (the number of fine Ti-based inclusions and coarse inclusions) to increase the HAZ toughness during large heat input welding and obtaining the base material characteristics such as high strength, The composition of the steel plate of the present invention is within the range shown below, and it is effective to manufacture it by a prescribed method.

That is, C: 0.01 to 0.07%, Si: 1.0% or less (not including 0%), Mn: 1.0 to 3.0%, Al: less than 0.010% (0% Not included), Ti: 0.005 to 0.050% , O: 0.0010 to 0.0100% , Cu: 0.1 to 1.5%, Ni: 0.1 to 2.5%, respectively And 20C (%) + 1.5Mn (%) + Cu (%) + Ni (%) is 4.5 to 7.0%, and the balance is iron and inevitable impurities.

  The reason why such a thick steel plate composition is used is that, in particular, in order to generate intragranular bainite at the HAZ part during cooling after welding, it is important to adjust the base material component. Specifically, in the above composition, the amount of C is relatively increased so that intragranular bainite is easily generated.

  In high heat input welding, the cooling rate of the HAZ part is slow, so that ferrite is easily generated, and even if the generation of ferrite is suppressed, bainite is easily generated from the grain boundary. However, when the C content in the HAZ part is relatively increased, it is presumed that the generation of ferrite is suppressed and the bainite generation from the grain boundary is also suppressed, and the bainite generation from within the grains is promoted.

  In order to suppress the formation of bainite from the grain boundaries, it is also effective to selectively contain an alloy element having a strong carbide generating ability such as Nb or V. Nb and V segregate around the Ti oxide and prevent bainite from being formed using the Ti oxide as a nucleus.

  Furthermore, in the present invention, the intragranular bainite is an element that stabilizes austenite (an element that enhances hardenability) in order to ensure good toughness not only in the vicinity of the generated melting line but also throughout the HAZ, As described above, the contents of C, Mn, Cu, and Ni are controlled by the total amount as in the following parameters. By controlling the total content of these elements, even if the cooling rate of the HAZ part is slowed down by high heat input welding, the hardenability is improved and the bainite generated from within the grains can be refined.

Hereinafter, the reason for defining the amount of each element will be described in detail.
C: 0.01 to 0.07%.
C (carbon) is an element necessary for ensuring the strength of the base material, and is required to be at least 0.01%. Moreover, when the amount of C is increased, the generation of intragranular bainite is promoted. These effects are exhibited at 0.01% or more, preferably 0.02% or more. On the other hand, if the amount of C is excessive, the weld crack resistance and the HAZ toughness deteriorate, on the other hand, it is limited to 0.07% or less, preferably less than 0.05%. Therefore, the C content is in the range of 0.01 to 0.07%, preferably 0.02 to less than 0.05%.

Si: 1.0% or less (excluding 0%).
Si strengthens the solid solution and contributes to securing the strength of the base material. Moreover, it is an element useful as a preliminary deoxidizer. This effect is preferably exhibited when the content is 0.05% or more. On the other hand, if it exceeds 1.0%, more strictly exceeds 0.5%, and excessively contained, both the base metal toughness and the HAZ toughness are lowered, so the upper limit of Si is 1.0%, preferably 0.5%. Further, the range of the Si content is preferably 0.05 to 1.0%, more preferably 0.05 to 0.5%.

Mn: 1.0 to 3.0%.
Mn has the effect of improving the hardenability of the steel and also has the effect of improving the HAZ toughness by promoting the formation of intragranular bainite. In order to exhibit such an effect effectively, it is necessary to contain 1.0% or more, preferably 1.2% or more. On the other hand, since an HAZ toughness will deteriorate if it is contained excessively, the upper limit is made 3.0%, preferably 2.5% or less. Therefore, the range of Mn content is 1.0 to 3.0%, preferably 1.2 to 2.5%.

Al: Less than 0.010% (excluding 0%).
Al is necessary as a strong deoxidizing element, and is preferably contained in an amount of 0.001% or more, more preferably 0.002% or more. On the other hand, if Al is contained in excess of 0.010% or more, strictly exceeding 0.005%, the proportion of Al in the Ti-containing oxide increases and the effect of promoting the formation of intragranular bainite is reduced. To do. For this reason, the Al content is set to a range of less than 0.010%, preferably 0.001 to 0.010%, more preferably 0.002 to 0.005%.

Ti (total amount): 0.005 to 0.050%.
As described above, Ti is an important element that also has an effect of improving the HAZ toughness by forming fine Ti-containing oxides (partially nitrides), promoting the formation of intragranular bainite. In order to exhibit such an effect effectively, it is 0.005% or more, preferably 0.008% or more. On the other hand, when the amount of Ti becomes excessive, the Ti-containing oxide becomes coarse, or the Ti-containing oxide becomes excessive. On the other hand, both the HAZ toughness and the base material toughness deteriorate, so 0.05% or less, Preferably, it is suppressed to 0.030% or less. Therefore, the total Ti content is in the range of 0.005 to 0.050%, preferably in the range of 0.008 to 0.030%.

Cu: 0.1 to 1.5%, Ni: 0.1 to 2.5 %.
Both Cu and Ni improve the hardenability, strengthen the matrix, refine the HAZ structure, and contribute to the improvement of the base metal toughness and the HAZ toughness. In order to exert these effects, Cu and Ni are contained in an amount of 0.1% or more, preferably 0.2% or more. On the other hand, when Cu and Ni are contained excessively, the hardening of HAZ becomes remarkable, and on the contrary, the HAZ toughness is lowered. Therefore, the upper limit of Cu is 1.5%, preferably 1.2%, and the upper limit of Ni is 2.5%, preferably 2.2%.

  Moreover, when it contains Cu exceeding 0.5%, it is preferable to contain Ni more than half of Cu content (mass%) from a viewpoint of preventing the hot cracking during rolling, More preferably It is recommended to contain more than a chemical equivalent of Ni.

O: 0.0010 to 0.0100%.
O (oxygen) is an element that forms a Ti-containing oxide and is effective for promoting the formation of HAZ intragranular bainite as described above. In order to exhibit such an effect, the content is preferably 0.0010% or more, preferably 0.0015% or more, and more preferably 0.0020% or more. On the other hand, when the oxygen content is excessive, a coarse Ti-containing oxide is easily generated, and the HAZ toughness is deteriorated. Therefore, the oxygen content needs to be suppressed to 0.0100% or less, preferably 0.0030% or less.

20C (%) + 1.5Mn (%) + Cu (%) + Ni (%) = 4.5-7.0%.
Furthermore, in the present invention, for the above-described C, Mn, Cu, and Ni, in order to ensure good toughness not only in the vicinity of the melting line but also in the entire HAZ region, as in the above specific parameter, the total amount Control. By controlling the content of C, Mn, Cu, and Ni, which are elements that stabilize austenite (elements that enhance hardenability) as specified above, the cooling rate of the HAZ part can be increased by high heat input welding. Even if it becomes late, hardenability becomes high and the bainite produced | generated from the inside of a grain | grain can be refined | miniaturized.

  When the above parameter is 4.5% or more, the effect of increasing hardenability can be exhibited, the intragranular bainite structure can be refined, and the toughness can be improved over the entire HAZ region. If this parameter is less than 4.5%, even if the contents of C, Mn, Cu, and Ni are within the specified range, the effect of increasing the hardenability cannot be exhibited, and the intragranular bainite structure is refined. Toughness cannot be improved over the entire HAZ.

  On the other hand, when this parameter exceeds 7.0%, even if the contents of C, Mn, Cu, and Ni are within the specified ranges, the amount of MA (martensite) formed in the MA bainite structure is small. On the contrary, there is a high possibility that the toughness is lowered over the entire HAZ.

Hereinafter, elements to be selectively contained will be described.
Cr: 0.1 to 2.0%, Mo: 0.1 to 1.0%, Nb: 0.10% or less (not including 0%), V: 0.10% or less (not including 0%) ) Any one kind or two kinds or more.
Cr, Mo, Nb, and V all improve the hardenability and increase the strength of the base material. Cr also has the effect of enhancing the hardenability, making the HAZ structure finer, and contributing to the improvement of HAZ toughness. Nb and V also have the effect of increasing the temper softening resistance and increasing the base material strength. In order to exert these effects, Cr: 0.1% or more, Mo: 0.1% or more, Nb: preferably 0.01% or more, V: preferably 0.01% or more, either one or two A seed or more is selectively contained.

  On the other hand, if these are excessively contained, the HAZ toughness is lowered on the contrary. Cr causes an increase in martensite and decreases the HAZ toughness. Mo significantly hardens the HAZ and lowers the HAZ toughness. Nb and V suppress the formation of intragranular bainite and degrade the HAZ toughness. Accordingly, the upper limits are set to Cr: 2.0%, Mo: 1.0%, Nb: 0.10%, and V: 0.10%, respectively.

B: 0.0005 to 0.0050%.
B has an effect of improving the hardenability by dissolving in steel. Moreover, in the HAZ part, the effect of suppressing the formation of ferrite from grain boundaries and promoting the formation of bainite from within the grains is also exhibited. In order to exert such an effect, 0.0005% or more is selectively contained. On the other hand, when there is too much B content, while hardenability will fall rather, formation of an intragranular bainite will be suppressed and HAZ toughness will deteriorate. Therefore, the B amount is 0.0005 to 0.0050%, preferably 0.0005 to 0.0030%.

Zr: 0.05% or less (not including 0%), Mg: 0.0050% or less (not including 0%), Ca: 0.0050% or less (not including 0%), REM: 0.0050 % Or less (excluding 0%), one kind or two kinds or more.
Zr, Mg, Ca, and REM have an effect of improving the HAZ toughness, but if they are contained excessively, the HAZ toughness is deteriorated.

  For example, Zr improves HAZ toughness by fixing nitrogen. Ca spheroidizes inclusions such as sulfides such as MnS, reduces the anisotropy of the inclusion shape, and improves the HAZ toughness. Mg and REM improve HAZ toughness by refining inclusions such as sulfides such as MnS. However, when these Ca, Mg, and REM are each contained excessively, the Ti-containing oxide is coarsened, so that the HAZ toughness is deteriorated.

  For this reason, when one or more kinds are selectively contained, Zr: 0.05% or less, preferably 0.005 to 0.03%, Mg: 0.0050% or less, preferably Is 0.0040% or less, Ca: 0.0050% or less, preferably 0.0040% or less, and REM: 0.0050% or less, preferably 0.0040% or less.

Next, impurities will be described below.
N (nitrogen) is an impurity in the present invention because it degrades both the base metal toughness and the HAZ toughness. However, N also has the effect of forming Ti and nitrides to promote the formation of intragranular bainite and improving the HAZ toughness, and N is allowed to contain up to 0.0090%.

  P (phosphorus) and S (sulfur) are elements present as inevitable impurities, and have an adverse effect such as lowering weldability and base metal toughness. Therefore, it is preferable to suppress P to 0.020% or less (preferably 0.010% or less) and S to 0.010% or less (preferably 0.005% or less).

(Production method)
The thick steel plate of the present invention can be produced by a conventional method. However, a description will be given below including preferable manufacturing conditions for controlling the number of fine Ti-containing oxides and coarse Ti-containing oxides as described above.

  In Patent Document 2 and Patent Document 3 described above, as a method for controlling the size and number of oxides, it is shown that the amount of dissolved oxygen or the like is controlled before casting, or the cooling rate or the like during solidification is controlled. . However, it is not possible to reduce the coarse Ti-containing oxide of 2 μm or more, which is noticed in the present invention. In addition, the number of fine Ti-containing oxides defined in the present invention cannot be ensured only by reducing the amount of oxygen in the molten steel.

  Even in the present invention, the steel material (slab) having a predetermined size is produced by a normal casting method such as a continuous casting method after being melted by a normal melting method such as a converter, while increasing the number of fine Ti-containing oxides. In order to reduce Ti-containing oxides in coarse inclusions, it is necessary to strictly manage the amount of dissolved oxygen in the steel before the addition of Ti and the holding time after the addition of Ti until casting in this melting stage. preferable.

  Specifically, when adding Ti at the melting stage, first, the amount of dissolved oxygen in the molten steel is controlled within a range of 20 to 100 ppm. By controlling the amount of dissolved oxygen in this way, the fine Ti-containing oxide can be generated, and the amount of fine Ti-containing oxide specified in the present invention can be ensured.

  In order to produce more fine Ti-containing oxides, the dissolved oxygen content in the molten steel is preferably 20 ppm or more, and more preferably 25 ppm or more. On the other hand, if the amount of dissolved oxygen in the molten steel before the addition of Ti is excessive, coarse Ti-containing oxides and other oxides are likely to be generated, which is not preferable. Therefore, Ti is added after suppressing the amount of dissolved oxygen in molten steel to 100 ppm or less. Preferably, Ti is added after the amount of dissolved oxygen in the molten steel is suppressed to 70 ppm or less. As described above, in order to control the amount of dissolved oxygen in the molten steel at the melting stage, deoxidation by adding Mn, vacuum C (carbon) deoxidation, and deoxidation by adding Si may be performed alone or in combination as appropriate.

  Moreover, after adding Ti, it is kept for 10 to 50 minutes in a stationary state. In this way, by holding after addition of Ti, a Ti-containing oxide having a thickness of 2 μm or more, which tends to be a formation nucleus of intragranular ferrite, while ensuring a fine Ti-containing oxide of a suitable size that effectively acts as a formation nucleus of intragranular bainite. Objects can be removed by floating. In order to reliably remove the coarse Ti-containing oxide, it is preferable to hold for 10 minutes or more, preferably 15 minutes or more, and more preferably 20 minutes or more.

  In addition, the said holding means holding between about 1550-1650 degreeC as it is performed by normal melting.

  On the other hand, if the holding time is too long, fine inclusions aggregate and become coarse, and it is not preferable because a fine Ti-containing oxide in an amount defined by the present invention cannot be secured. Accordingly, the holding time is 50 minutes or less. Preferably it is 40 minutes or less.

  In actual operation, Ti, Si, Mn and C may be added at the same time so as to have the final component amount, then held as described above, and then cast.

  By melting by such a method, coarse Ti-containing oxides can be reduced while securing an appropriate amount of fine Ti-containing oxides defined in the present invention.

  The steel material (slab) is a structure that is recrystallized at the end of the hot rolling by performing hot rolling after heating, preventing the development of the texture along the rolling direction, in accordance with the normal method for producing thick steel plates. To do. The steel sheet after hot rolling is subjected to water quenching. Thereafter, the steel plate is tempered to obtain a product thick steel plate.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to the following examples, and the present invention is not limited to the following examples. Of course, it is also possible to implement them, and they are all included in the technical scope of the present invention.

  Steels having chemical composition shown in Table 1 (Invention Example) and Table 2 (Comparative Example) are shown in Table 3 (Invention Example) and Table 4 (Comparative Example). The slab was obtained by melting and casting variously with the holding time in the stationary state after adding Ti. After heating the obtained slab to 1100 ° C., rolling and cooling were performed to obtain a steel plate having a thickness of 50 mm. Moreover, in order to adjust the intensity | strength and toughness of a base material, the tempering process to 500-650 degreeC was performed.

  A sample was collected from the steel sheet thus obtained, and the base material characteristics, HAZ toughness, and the size of the Ti-containing oxide present in the base material were measured. These results are shown in Tables 3 and 4.

(Base material characteristics)
A JIS No. 4 test piece was collected from each of the manufactured steel sheets, subjected to a tensile test according to JISZ2241, and subjected to a tensile strength of the steel sheet, and a Charpy impact test according to JISZ2242, to measure the toughness (vE-20) of the steel sheet. .

  Here, those having a tensile strength of 590 MPa or more and a Charpy impact value of 150 J or more were evaluated as having excellent base material properties such as high strength and high toughness, and thus excellent base materials. About the thing with which the characteristic was ensured, HAZ toughness was evaluated as follows.

(Welded joint toughness)
In the heat history of the HAZ near the melting line (+0.5 mm) when the specimen (size 12.5 mm × 32 mm × 55 mm) cut out from each of the manufactured steel plates was subjected to SAW welding with a large heat input of 100 kJ / mm The corresponding thermal cycle was applied. That is, it was heated to 1400 ° C., held at this temperature for 5 seconds, and then subjected to a heat cycle of cooling from 800 ° C. to 500 ° C. in 730 seconds. Further, a thermal cycle corresponding to the thermal history of the HAZ in the region of about 3 to 5 mm from the melt line during the high heat input welding was also performed. That is, the sample was heated to 1200 ° C., held at this temperature for 5 seconds, and then subjected to a heat cycle of cooling from 800 ° C. to 500 ° C. in 730 seconds. From these test pieces, Charpy test pieces were collected and vE0 (1400 ° C. and 1200 ° C.) was measured. Each case where vE0 was 150 J or more was evaluated as having excellent weld joint toughness.

(Ti-containing oxide)
The size of the Ti-containing oxide present in the base material was measured by the following method.
<Measurement position (sample collection position)>
A sample was taken so that a cross section parallel to the rolling direction could be observed at a position of 1/4 of the plate thickness. Using the obtained samples, the number of coarse Ti-containing oxides having an average particle diameter of 2 μm or more and fine Ti-containing oxides having an average particle diameter of 0.05 to 1.0 μm were measured as follows.

<Measurement of the number of coarse Ti-containing oxides having an average particle size of 2 μm or more>
An area of 100 mm ² (10 mm × 10 mm) was observed at a magnification of 200 times using the EPMA apparatus described above, and the number of coarse Ti-containing oxides having an average particle diameter of 2 μm or more was measured. In addition, the size of the Ti-containing oxide was determined as the average particle size by obtaining the equivalent particle size (hereinafter the same).

<Measurement of the number of fine Ti-containing oxides having an average particle size of 0.05 to 1.0 [mu] m>
Using the above-mentioned FE-SEM / EDX apparatus, the composition analysis of 20 inclusions having an average particle diameter of 0.05 to 1.0 μm was performed, and the ratio of Ti-containing oxide containing 10% by mass or more of Ti was obtained. . Next, in the region of 0.1 mm ^2, taken any 10 fields of 0.01 mm ² using a 1000-fold reflection electron image, by an image analyzer, inclusions of an average particle diameter 0.05~1.0μm The total number of the 10 fields of view is multiplied by the ratio of the Ti-containing oxide and further multiplied by 1000 to obtain a Ti-containing oxide having an average particle diameter of 0.05 to 1.0 μm per 1 cm ^2. The number of objects was determined. These results are also shown in Tables 3 and 4.

  As is apparent from Tables 1 and 3, Invention Examples 1 to 20 satisfy the present invention composition including the parameter of 20 C (%) + 1.5 Mn (%) + Cu (%) + Ni (%), and dissolved oxygen. Both the amount and the holding time are produced by a preferable melting method.

For this reason, the average number of Ti-containing oxides having an average particle diameter of 0.05 to 1 μm is 10000 / cm ² or more when observed at a magnification of 1000 times, and Ti oxides having an average particle diameter of 2 μm or more. The average number is 2000 pieces / cm ² or less when observed at a magnification of 200 times. As a result, a base material strength of 590 to 780 MPa class and a base material toughness of 210 J or more are obtained. In addition, the thermal cycle characteristics have a toughness of 150 J or more for both 1400 ° C and 1200 ° C vE0.

  These results show that even in the case of a high-strength 590-780 MPa class thick steel plate, the toughness is particularly likely to deteriorate, including the weld heat-affected zone in the vicinity of 3-5 mm from the above-mentioned melting line, during high heat input welding. It shows that the toughness of the entire HAZ can be greatly improved.

  On the other hand, as is clear from Tables 2 and 4, in Comparative Examples 21 to 29 and 32, the component composition of any element is out of the scope of the invention.

  In Comparative Examples 30 to 31, the parameter of 20C (%) + 1.5 Mn (%) + Cu (%) + Ni (%) is out of the scope of the invention.

  In Comparative Examples 33 to 35, although the component composition is within the range of the invention, the amount of dissolved oxygen or the retention time is manufactured outside the preferable conditions of the melting method.

  As a result, in the comparative example, the base material strength and base material toughness are relatively low as compared with the invention examples, or the base material strength and base material toughness are comparable to those of the invention examples, the thermal cycle characteristics are 1400 ° C. and 1200 ° C. Both vE0 are significantly lower in common.

  These results show that when these comparative examples that deviate from the scope of the present invention are high-strength 590-780 MPa class thick steel plates, particularly including a heat-affected zone in the vicinity of 3-5 mm from the melting line, a large heat input. It shows that the toughness of the entire HAZ during welding cannot be improved.

  For example, in Comparative Example 21, since the amount of C is excessive, the definition of the Ti-containing oxide is satisfied, but the thermal cycle characteristics (HAZ toughness) are extremely low.

  In Comparative Example 22, since the amount of Mn is insufficient, the strength of the base material cannot be ensured, and the definition of the Ti-containing oxide is satisfied, but the thermal cycle characteristics (HAZ toughness) are also inferior.

  In Comparative Example 23, since the amount of Mn is excessive, the definition of the Ti-containing oxide is satisfied, but the thermal cycle characteristics (HAZ toughness) are inferior.

  In Comparative Example 24, since the amount of Al is excessive, the amount of coarse Ti-containing oxide is excessive, and the thermal cycle characteristics (HAZ toughness) are inferior.

  In Comparative Example 25, since the amount of Cu is excessive, the definition of the Ti-containing oxide is satisfied, but the thermal cycle characteristics (HAZ toughness) are inferior.

  In Comparative Example 26, since the amount of Ni is excessive, the definition of the Ti-containing oxide is satisfied, but the thermal cycle characteristics (HAZ toughness) are inferior.

  In Comparative Example 27, since the amount of Ti is excessive, the amount of fine Ti-containing oxide is too small, and the thermal cycle characteristics (HAZ toughness) are inferior.

  In Comparative Example 28, since the amount of oxygen is insufficient and the amount of dissolved oxygen is small at the time of melting, the amount of fine Ti-containing oxide is too small, and the thermal cycle characteristics (HAZ toughness) are inferior.

  In Comparative Example 29, the amount of oxygen is excessive, and the amount of dissolved oxygen is excessive at the time of melting, so that there are too many coarse Ti-containing oxides and few fine Ti-containing oxides. HAZ toughness) is inferior.

  In Comparative Example 32, since the boron amount is excessive, the provision of the Ti-containing oxide is satisfied, but the base material toughness and the thermal cycle characteristics (HAZ toughness) are inferior.

  In Comparative Example 30, the parameter of 20C (%) + 1.5 Mn (%) + Cu (%) + Ni (%) is less than 4.5% and is out of the scope of the invention. For this reason, in spite of containing C, Mn, Cu, and Ni within the specified amount ranges, the thermal cycle characteristics at 1200 ° C. are inferior, and the toughness of the HAZ 3-5 mm portion cannot be improved. This is because the hardenability increasing effect of C, Mn, Cu, and Ni cannot be exhibited, and the intragranular bainite structure cannot be refined.

  In Comparative Example 31, the above parameter exceeds 7.0%, which is out of the scope of the invention. For this reason, in spite of containing C, Mn, Cu, and Ni within the specified amount ranges, the thermal cycle characteristics at 1200 ° C. are inferior, and the toughness of the HAZ 3-5 mm portion cannot be improved. This is because the amount of MA (martensite) formed in the MA bainite structure increases, and on the contrary, the toughness is reduced over the entire HAZ.

  In Comparative Example 33, since the amount of dissolved oxygen before addition of Ti in the melting stage is too large, a large number of coarse Ti-containing oxides are precipitated and exceed the specified number. For this reason, the thermal cycle characteristic is remarkably low, about 102 J at vE0 of 1400 ° C.

  In Comparative Example 34, since the holding time in a stationary state after addition of Ti in the melting stage is too short, a large amount of coarse Ti-containing oxide precipitates and exceeds the specified number, while a fine Ti-containing oxide is compared. Less. For this reason, the thermal cycle characteristic is remarkably low, about 85 J at vE0 of 1400 ° C.

  In Comparative Example 35, since the holding time in a stationary state after addition of Ti in the melting stage is too long, a large amount of coarse Ti-containing oxide is precipitated and exceeds the prescribed number, while the fine Ti-containing oxide is compared. Less. For this reason, the thermal cycle characteristic is about 99J at vE0 of 1400 ° C., and the toughness is remarkably low.

  From the above results, in the case of a high-strength 590-780 MPa class thick steel plate as defined by the component composition of the present invention and the Ti-containing oxide, particularly including a weld heat affected zone in the vicinity of 3-5 mm from the melting line. In addition, the critical significance of improving the toughness of the entire HAZ during high heat input welding is supported.

As described above, according to the present invention, it is possible to provide a 590-780 MPa class thick steel plate excellent in weld joint toughness (HAZ toughness) even when high heat input welding is performed. For this reason, this invention steel plate is applicable for high-strength welded structures, such as a ship, a marine structure, a bridge, and a building structure.

Claims (4)

In mass%, C: 0.01 to 0.07%, Si: 1.0% or less (excluding 0%), Mn: 1.0 to 3.0%, Al: less than 0.010% (0 %), Ti: 0.005 to 0.050% , O: 0.0010 to 0.0100% , Cu: 0.1 to 1.5%, Ni: 0.1 to 2.5% Steel sheets each containing 20C (%) + 1.5 Mn (%) + Cu (%) + Ni (%) is 4.5 to 7.0%, the balance being iron and inevitable impurities, with a particle diameter of 10,000 pieces / cm ² or more when the average number of Ti-containing oxide 0.05~1μm was observed at a magnification 1,000 times, the average number of average particle size 2μm or more Ti-containing oxide ratio excellent thickness in high heat input welded joint toughness, characterized in that when observed at 2000 / cm ² or less at 200-fold Plate.   The thick steel plate is further Cr: 0.1-2.0%, Mo: 0.1-1.0%, Nb: 0.10% or less (excluding 0%), V: 0.10% The thick steel plate excellent in toughness of the high heat input welded joint according to claim 1, comprising any one or more of the following (not including 0%):   The thick steel plate excellent in the high heat input weld joint toughness according to claim 1 or 2, wherein the thick steel plate further contains B: 0.0005 to 0.0050%.   The thick steel plate is further Zr: 0.05% or less (not including 0%), Mg: 0.0050% or less (not including 0%), Ca: 0.0050% or less (not including 0%) ), REM: 0.0050% or less (excluding 0%), which is excellent in high heat input welded joint toughness according to any one of claims 1 to 3 Thick steel plate.