KR100712796B1 - A thick steel plate having superior toughness in high input heat welding joint - Google Patents

Abstract

An object of the present invention is to provide a high strength thick steel sheet of 590 to 780 MPa class, which is excellent in weld seam toughness (HAZ toughness) even when high heat input welding is performed.

In the thick steel sheet of the specific component, the total content of C, Mn, Cu, and Ni is further controlled by specific parameters, and the average number of fine Ti-containing oxides having an average particle diameter of 0.05 to 1 µm is 10000 / cm ² or more. At the same time, the average number of coarse Ti-containing oxides having an average particle diameter of 2 µm or more is set to 2000 pieces / cm ² or less, so that even in a high strength steel sheet of 590 to 780 MPa class, the toughness is particularly low from 3 to 3 from the melting line. Significantly improves the toughness of the entire HAZ area at the time of high heat input welding, including the weld heat affected zone near 5mm.

Description

High-strength welded joint steel plate with excellent toughness {A THICK STEEL PLATE HAVING SUPERIOR TOUGHNESS IN HIGH INPUT HEAT WELDING JOINT}

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

Conventionally, it has been attempted to ensure HAZ toughness by dispersing an oxide containing Ti in a base material so as to secure the HAZ toughness of a thick steel sheet, and by forming ferrite from inside the particles when cooling the HAZ portion to refine the structure.

For example, Japanese Patent Publication No. 1995-824 (claims) discloses one or two kinds of Ti oxide of 0.1 to 3.0 µm or a composite of Ti oxide and Ti nitride, thereby making it possible to make It has been described to improve the HAZ toughness by controlling the γ → α transformation during cooling to generate intraparticle ferrite.

In addition, Japanese Patent Publication No. 1991-67467 (claims) and Japanese Patent Publication No. 199-59134 (claims) include coarse primary deoxidation products (Al, Ca, REM, etc.) produced in molten steel. By suppressing the production of deoxidation products by strong deoxidation elements and by uniformly dispersing secondary deoxidation products generated by deoxidation by weak deoxidation elements (Ti, Si, Nb, V, and Ta). It is proposed to secure the toughness of the (HAZ section).

Japanese Unexamined Patent Publication No. 1997-3599 (claims) discloses 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 mu m. It is described to improve HAZ toughness of steel materials at the time of welding by disperse | distributing uniformly.

However, in such a technique, it is difficult to say that excellent HAZ toughness is ensured even when a large heat input welding is performed larger than the heat input generally performed recently.

In contrast, Japanese Patent Laid-Open No. 2003-313628 (claims, paragraphs 0031 and 0030) discloses a high temperature of a HAZ region heated to 1400 ° C. or higher in order to improve HAZ toughness in high heat input welded joints. In order to suppress the growth (coarsening) of the austenite particles and to promote the reheating of the austenite grains, oxides containing 3% or more of Ca and 1% or more of Al are used as the composition of the oxide particles produced in the steel. Use is described.

However, Japanese Laid-Open Patent Publication No. 2003-313628 (claims, paragraph 0031, and paragraph 0030) are intended for thick steel sheets of 50 to 60 kilo class (less than 590 MPa). For this reason, in the case of the high strength 590-780 MPa grade thick steel plate above, in the oxide means by Unexamined-Japanese-Patent No. 2003-313628 (claim, Paragraph 0031, and Paragraph 0030), at the time of high heat input welding. Toughness cannot be secured over the entire area of the HAZ. According to the findings of the present inventors, in the above-mentioned Japanese Patent Laid-Open Publication No. 2003-313628 (claims, paragraph 0031, and paragraph 0030), in particular, the toughness of the weld heat affected portion near the melting line of 3 to 5 mm cannot be secured. .

This is because, in the weld heat affected zone near 3 to 5 mm from the melting line, the hardenability is reduced during the high heat input welding in a region where the γ particle size is relatively small (about 10 μm in diameter), and the bainite structure tends to coarsen, resulting in grain boundary generation. Because of the high nucleation frequency of bainite, it is believed that bainite is not formed in the particles.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a high strength thick steel sheet of 590 to 780 MPa class having excellent weld seam toughness (HAZ toughness) even when high heat input welding is performed.

In order to achieve this object, the gist of the thick steel sheet excellent in the high heat input welding seam toughness of the present invention is mass%, C: 0.01 to 0.07%, Si: 1.0% or less (does not include 0%), Mn: 1.0 to 3.0%, Al: less than 0.010% (not including 0%), Ti: 0.005 to 0.050% and O: 0.0010 to 0.0100%, respectively, further Cu: 0.1 to 1.5% and Ni: 0.1 to A steel sheet containing any one or more of 2.5%, further comprising 20C (%) + 1.5Mn (%) + Cu (%) + Ni (%) of 4.5 to 7.0% and the balance being substantially iron and inevitable impurities. , when the average particle diameter is the average number of the Ti-containing oxide of 0.05 to 1㎛ have observed times multiplier 1000 at the same time and 10,000 / cm ² or more, when the average number of the Ti-containing oxide having an average particle size of less than 2㎛ have observed fold magnification: 200 It is 2000 pieces / cm <^2> or less.

In the present invention, the Ti-containing oxide is an oxide containing 10% by mass or more of Ti when the composition analysis of steel inclusions is performed using a measuring device such as an EPMA device or an FE-SEM / EDX device described later. It is called. Therefore, the oxide whose Ti content is less than 10 mass% in the interference | inclusion in steel is not called the Ti containing oxide of this invention. In addition, said "average particle diameter" means the circle equivalent particle diameter of the oxide containing Ti.

As described above, in the case of thick steel plates of 590 to 780 MPa class having high strength, Japanese Patent Laid-Open Publication No. 1997-3599 (claims) and Japanese Patent Publication No. 2003-313628 (claims, paragraph 0031) In the Ca or Al-based oxide, as described above, the toughness cannot be secured over the entire area of the HAZ during the high heat input welding.

In contrast, in the present invention, a large number of fine oxides are produced after the number of coarse oxides is suppressed among the Ti-containing oxides present in the steel. In this manner, when a large number of fine oxides is generated, intragranular bainite is easily generated in the HAZ portion at the time of cooling after the high heat input welding. As a result, even in the case of a high strength 590 to 780 MPa class thick steel sheet, the toughness of the entire HAZ region at the time of high heat input welding, including a weld heat influence part near 3 to 5 mm from the above-described melting line, in which toughness tends to be particularly low. Can be greatly improved.

In addition, in this invention, content of C, Mn, Cu, and Ni which is an element which stabilizes austenite (element which raises hardenability) in total not only in the vicinity of a melting line but also ensures toughness in the whole HAZ area | region is summed up. To control.

Therefore, the thick steel sheet of the present invention is suitable for welding structures such as ships, offshore structures, bridges, building structures, etc., having a strength of 590 to 780 MPa, and excellent welding seams even when welding with high heat input when used as the welding structure. Demonstrate toughness.

<Ti containing oxide>

First, in the present invention, the significance of the oxide containing Ti in the thick steel sheet structure will be described below. In the present invention, as described above, after suppressing the number of coarse oxides in the Ti-containing oxides, many fine oxides are produced. Specifically, the average number of fine Ti-containing oxides having an average particle diameter of 0.05 to 1 µm is 10000 particles / cm ² or more when observed at a magnification of 1000 times. On the other hand, when the average number of coarse Ti containing oxide whose average particle diameter is 2 micrometers or more is observed by 200 times the magnification, it becomes 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. The Ti content is less than 10% by mass of the oxide having a small Ti content, the effect of forming bainite in the HAZ particles at the time of high heat input welding of fine Ti-containing oxide, or as a nucleus of ferrite of coarse Ti-containing oxide. There is no action to inhibit the formation of useful bainite.

As described above, the ferrite is not grown in the HAZ particles at the time of high heat input welding. In the present invention, bainite is formed in the HAZ particles at the time of high heat input welding. For this reason, in this invention, many said fine Ti containing oxides exist in steel previously.

By forming bainite in HAZ particles during high heat input welding as in the present invention, in HAZ near a melting line having a relatively small γ particle diameter, coarsening of bainite structure due to hardenability at high heat input welding or grain boundary generation bay Suppresses nucleation of knight. And even in the case of the high strength 590-780 MPa grade thick steel plate, especially the toughness of the whole HAZ area | region at the time of high heat input welding including the welding heat influence part near 3-5 mm from said melting line which toughness tends to fall. Significantly improve.

At the same time, the coarse Ti-containing oxide is previously suppressed in the present invention, in order to ensure the number of the present Ti-containing oxides and to not inhibit the formation of bainite in the particles. More specifically, coarse Ti-containing oxides having a thickness of 2 µm or more have a strong ferrite generating ability, and therefore tend to form ferrite nuclei at high temperatures such as high heat input welding, thereby suppressing formation of useful bainite. In addition, such coarse Ti-containing oxide tends to be a breakdown point in the fine bainite structure, and also serves to inhibit the micronization effect by bainite.

In order to reliably exert the above-mentioned effect and to obtain excellent weld seam toughness, in the present invention, the number of the presence of fine Ti-containing oxides having an average particle diameter of 0.05 to 1 μm and coarse Ti-containing oxides of 2 μm or more is the optimal average. It is set as the number range. However, when both Ti-containing oxides deviate from the optimum average number range, in the case of high strength 590 to 780 MPa grade thick steel sheets, the weld heat influence part near the melting line of 3 to 5 mm is particularly likely to decrease the toughness. In addition, the toughness of the whole HAZ area | region at the time of high heat input welding cannot be improved.

Since the fine Ti-containing oxide promotes the generation of bainite in the particles as the number thereof increases, it is preferable to present at least 20000 pieces / cm ² and more preferably at least 40000 pieces / cm ² . From the viewpoint of the above-mentioned effect, there is no upper limit to the number of fine Ti-containing oxides, but it is considered that the number of fine Ti-containing oxides that can be precipitated in an ordinary thick plate manufacturing process is about 1 × 10 ⁸ / cm ² . .

The fine Ti-containing oxide may contain Si, Ca, Mg, or the like as an alloying element other than Ti. Especially, Mn is especially preferable as an element contained with Ti. More preferably, it is preferable that Ti + Mn occupied by the all-alloy element which comprises an oxide is 60% or more (more preferably 70% or more).

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

In this way, after the number of coarse Ti-containing oxides is suppressed, a large number of fine Ti-based inclusions are generated, and intragranular bainite is easily generated in the HAZ portion during cooling after welding, and HAZ toughness can be greatly improved. On the other hand, the steel sheet of the present invention, as described above, it is preferable that the intragranular bainite is preferentially generated in the HAZ portion during cooling after welding, and even when bainite or ferrite is somewhat generated at the grain boundary during the cooling, the HAZ toughness Allows the production of these bainite or ferrite to the extent that no improvement is impeded.

<Measurement of Ti-containing Oxide>

A field emission scanning electron microscope (FE-SEM), a scanning electron microscope (SEM), an electron probe micron analyzer (EPMA) apparatus, etc. can be applied to observation of Ti containing oxide at the magnification of the said prescription | regulation. In addition, in order to make the FE-SEM a clearer distinction between the steel matrix phase and each Ti-containing oxide, EDX (Kevex, Sigma energy dispersive X-ray detector: energy dispersive X-ray spectrometer) may be used for analysis (FE-SEM / EDX).

Among these, EPMA apparatus is used suitably for coarse Ti containing oxide observation with an average particle diameter of 2 micrometers or more at 200 times the magnification. In addition, when observing the fine Ti containing oxide whose average particle diameter at the magnification 1000 times is 0.05-1 micrometer, suitably, a field emission scanning electron microscope (FE-SEM) is used. As described above, in the present invention, the magnification is changed in accordance with the size of the Ti-containing oxide in order to increase the accuracy and reproducibility of the Ti-containing oxide number measurement.

For example, when FE-SEM / EDX is used for fine Ti-containing oxide observation, first, a composition analysis of inclusions having an average particle diameter of 0.05 to 1.0 µm in steel is performed, and inclusions of Ti-containing oxides containing 10% by mass or more of Ti are included. Find the ratio for. Next, 1000 times of reflection electron images are used in an area of 0.1 mm ² , and if any example of 0.01 mm ² is taken, 10 fields of view are taken, and as software for image analysis, manufactured by MEDIACYBERNETICS, Inc. The number of inclusions with an average particle diameter of 0.05-1.0 micrometer is measured by the image analysis apparatus using Image-ProPlus etc. 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 fields by the ratio of the Ti-containing oxide and making it 1000 times.

<Composition of thick steel plate>

The composition (unit: mass%) of the thick steel sheet of this invention is demonstrated below including the reason for limitation of each element. By controlling the above structure (number of fine Ti inclusions and coarse inclusions) of the thick steel sheet of the present invention to increase the HAZ toughness at the time of high heat input welding and at the same time obtain the base material characteristics such as high strength. It is effective to make the composition of a steel plate into the range shown below, and to manufacture 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 0.01% (without 0%), Ti: 0.005 to 0.050% And O: 0.0010 to 0.0100%, respectively, further containing at least one of Cu: 0.1 to 1.5% and Ni: 0.1 to 2.5%, and also 20C (%) + 1.5 Mn (%) + Cu (% ) + Ni (%) is 4.5 to 7.0%, the balance is a steel sheet which is substantially iron and unavoidable impurities.

Such a thick steel sheet composition is particularly important because it is important to adjust the base material component in order to generate intragranular bainite in the HAZ portion during cooling after welding. Specifically, in the above composition, the amount of C is relatively high, and intraparticle bainite is easily produced.

In the high heat input welding, the cooling rate of the HAZ portion is slowed, so ferrite is easily generated, and even if the formation of ferrite is suppressed, bainite is easily generated from grain boundaries. By increasing the C amount of the HAZ portion relatively, it is believed that the formation of ferrite is suppressed and the bainite formation at the grain boundary is also suppressed, and the bainite production is promoted in the particles.

On the other hand, in order to suppress the formation of bainite at the grain boundaries, selective inclusion of alloying elements with strong carbide generating ability such as Nb or V is also effective. Nb and V segregate around the Ti oxide to prevent the formation of bainite around the Ti oxide.

In addition, in the present invention, as described above, the intra-particle bainite is an element for stabilizing austenite (element that increases hardenability) in order to ensure good toughness not only in the vicinity of the generated melting line but also throughout the entire HAZ region. Content of C, Mn, Cu, and Ni is controlled by the total amount like the following parameter. By controlling the total content of these elements, even if the cooling rate of the HAZ part is slow by high heat input welding, hardenability becomes high and the bainite produced in particle | grains can be refined.

Hereinafter, the reason which prescribed | regulated each element amount is explained in full detail.

C: 0.01 to 0.07%

C (carbon) is an element necessary for securing the strength of the base metal, and is required at least 0.01%. In addition, when the amount of C is increased, generation of intraparticle bainite is promoted. These effects are exhibited at 0.01% or more, preferably 0.02% or more. In general, when the amount of C is excessive, on the contrary, the content of crack cracking and HAZ toughness is deteriorated, and therefore suppressed 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 0.05%.

Si: 1.0% or less (does not contain 0%)

Si strengthens solid solution and contributes to securing the base metal strength. It is also an element useful as a preliminary deoxidizer. This effect is preferably exhibited at 0.05% or more. On the other hand, if the content exceeds 1.0% and more strictly exceeds 0.5%, the base metal toughness and the HAZ toughness decrease together, so the upper limit of Si is 1.0%, preferably 0.5%. In addition, the range of Si content becomes like this. Preferably it is 0.05 to 1.0%, More preferably, you may be 0.05 to 0.5%.

Mn: 1.0 to 3.0%

Mn not only acts to improve the hardenability of the steel, but also has the effect of promoting the production of bainite in the particles and improving the HAZ toughness. 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, when it contains excessively, since HAZ toughness deteriorates, an upper limit is 3.0%, Preferably it is 2.5% or less. Therefore, the Mn content is in the range of 1.0 to 3.0%, preferably 1.2 to 2.5%.

Al: less than 0.010% (does not include 0%)

Al is required as a strong deoxidation element, preferably 0.001% or more, more preferably 0.002% or more. On the other hand, when Al is contained in excess of 0.010% or more and strictly exceeding 0.005%, the ratio of Al in Ti containing oxide will increase and the effect which promotes generation | occurrence | production of bainite in particle | grains will reduce. For this reason, Al content is less than 0.010%, Preferably it is 0.001 to 0.010%, More preferably, you may be 0.002 to 0,005% of range.

Ti (total amount): 0.005 to 0.050%

As described above, Ti is a main element having the effect of forming a fine Ti-containing oxide (part of nitride), promoting the production of bainite in the particles, and improving HAZ toughness. In order to exhibit such an effect effectively, it is good to contain 0.005% or more, Preferably it is 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 and, on the contrary, the HAZ toughness and the base metal toughness deteriorate together. Therefore, the Ti content is suppressed to 0.05% or less, preferably 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%.

At least one of Cu: 0.1 to 1.5% and Ni: 0.1 to 2.5%

Cu and Ni together improve the hardenability, strengthen the matrix, and refine the HAZ structure, contributing to the improvement of the base metal toughness and the HAZ toughness. In order to exert these effects, at least one of Cu and Ni is contained in 0.1% or more, preferably 0.2% or more. On the other hand, when Cu and Ni are contained excessively, hardening of HAZ becomes remarkable and rather, HAZ toughness falls. 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 Ni equal to or more than chemical equivalent Containing is recommended.

O: 0.0010 to 0.0100%

O (oxygen) forms an Ti-containing oxide and, as described above, is an element effective for promoting the generation of bainite in the particles of HAZ. In order to exhibit such an effect, it is preferable to contain 0.0010% or more, Preferably it is 0.0015% or more, More preferably, it is 0.0020% or more. On the other hand, when the oxygen content is excessive, coarse Ti-containing oxides are likely to be produced, which deteriorates HAZ toughness. Therefore, oxygen content needs to be suppressed to 0.0100% or less, Preferably you may be 0.0030% or less.

20 C (%) + 1.5 Mn (%) + Cu (%) + Ni (%) = 4.5 to 7.0%

In the present invention, the C, Mn, Cu, and Ni described above are controlled by the total amount as described above in order to ensure good toughness not only in the vicinity of the melting line but also in the entire HAZ region. By controlling the contents of C, Mn, Cu, and Ni, which is an element stabilizing austenite (element that increases the hardenability), as described above, the hardenability becomes high even if the cooling rate of the HAZ portion is slowed in high heat input welding. The bainite produced in the particles can be refined.

At 4.5% or more of the above parameters, the hardenability increase effect can be exerted, and the intravenous generated bainite structure can be refined, so that the toughness can be improved over the entire HAZ region. If this parameter is less than 4.5%, even if the respective contents of C, Mn, Cu, and Ni are within a prescribed range, the hardenability increase effect cannot be exhibited, and the intravenous generated bainite structure cannot be made fine. Toughness cannot be improved over the entire HAZ region.

On the other hand, when this parameter exceeds 7.0%, even if the respective contents of C, Mn, Cu, and Ni are within the prescribed range, the amount of MA (martensite) formed in the MA bainite structure increases, On the contrary, toughness is likely to decrease throughout the entire HAZ region.

Below, the element selectively contained is demonstrated.

Any one or two or more of Cr: 0.1 to 2.0%, Mo: 0.1 to 1.0%, Nb: 0.10% or less (does not contain 0%), and V: 0.10% or less (does not contain 0%)

Cr, Mo, Nb, and V all improve hardenability and raise base material strength. In addition, Cr has an effect of increasing hardenability to refine HAZ structure and contributing to improvement of HAZ toughness. Nb and V increase the temper softening resistance, and also have the effect of increasing the base material strength. In order to exert these effects, one or two or more of Cr: 0.1% or more, Mo: 0.1% or more, Nb: preferably 0.01% or more, and V: preferably 0.01% or more are selectively contained. .

On the other hand, when they contain excessively, HAZ toughness will fall rather, respectively. Cr results in an increase in martensite and lowers the HAZ toughness. Mo hardens HAZ and lowers HAZ toughness. Nb and V suppress the formation of intraparticle bainite, and deteriorate HAZ toughness. Therefore, the upper limit is made into Cr: 2.0%, Mo: 1.0%, Nb: 0.10%, and V: 0.10%, respectively.

B: 0.0005 to 0.0050%

B is dissolved in steel and has an effect of improving hardenability. In addition, the HAZ portion also exhibits the effect of inhibiting ferrite generation at grain boundaries and promoting bainite production in the particles. 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, on the contrary, hardenability will fall on the contrary, formation of the intraparticle bainite will be suppressed, and HAZ toughness will deteriorate. Therefore, the amount of B is made 0.0005 to 0.0050%, Preferably it is 0.0005 to 0.0030%.

Zr: 0.05% or less (does not contain 0%), Mg: 0.0050% or less (does not contain 0%), Ca: 0.0050% or less (does not contain 0%), and REM: 0.0050% or less (0% It does not include any one kind or two or more kinds

Zr, Mg, Ca, and REM have the effect of improving the HAZ toughness, but when they are contained in excess, respectively, rather, they deteriorate the HAZ toughness.

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

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

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 an effect of forming nitrides with Ti to promote the formation of bainite in the particles, thereby improving HAZ toughness, and N content of up to 0.0090% is allowed.

P (phosphorus) and S (sulfur) are also elements that exist as unavoidable impurities, and have adverse effects 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).

<Manufacturing method>

The thick steel sheet of this invention can be manufactured by a conventional method itself. However, it demonstrates below including preferable manufacturing conditions for controlling the number of fine Ti containing oxide and coarse Ti containing oxide in the above-mentioned aspect.

Japanese Patent Publication No. 1991-67467 (claim) or Japanese Patent Publication No. 199-59134 (claim) disclose a method of controlling the size and number of oxides. Controlling or controlling the cooling rate at the time of solidification is described. However, coarse Ti-containing oxides of 2 µm or more noted in the present invention cannot be reduced. Moreover, only by reducing the amount of oxygen in the molten steel, it is not possible to ensure the number of fine Ti-containing oxides defined in the present invention.

In the present invention, a solvent is prepared by a conventional solvent method such as a converter, and then a steel material having a predetermined dimension (slab) by a conventional casting method such as a continuous casting method, while increasing the number of fine Ti-containing oxides. In order to reduce Ti-containing oxide with respect to the inclusions, it is preferable to strictly manage the dissolved oxygen amount of the steel before the Ti addition and the holding time from the addition of Ti to the solvent in this solvent step.

Specifically, in the solvent step, when adding Ti, 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 manner, the fine Ti-containing oxide can be produced, and the fine Ti-containing oxide in the amount specified in the present invention can be secured.

In order to generate more fine Ti containing oxides, the dissolved oxygen amount in molten steel is preferably made 20 ppm or more, more preferably 25 ppm or more. On the other hand, when the amount of dissolved oxygen in molten steel before Ti addition is excessive, coarse Ti containing oxide and other oxides are easy to produce | generate, and it is unpreferable. Therefore, Ti is added after suppressing the amount of dissolved oxygen in molten steel to 100 ppm or less. Preferably, Ti is added after suppressing the amount of dissolved oxygen in molten steel to 70 ppm or less. As described above, in order to control the amount of dissolved oxygen in the molten steel in the solvent step, deoxidation by Mn addition, vacuum C (carbon) deoxidation, and deoxidation by Si addition may be performed alone or in combination as appropriate.

In addition, after Ti addition, it hold | maintains for 10 to 50 minutes in a stationary state. Thus, after Ti addition, 2 micrometers or more of Ti containing oxides which are likely to become the nuclei of ferrite in a particle are floating-separated and removed, ensuring the fine Ti containing oxide of the suitable size which acts effectively as a nucleation of the bainite in a particle | grain. can do. In order to reliably remove the said coarse Ti containing oxide, it is good to hold for 10 minutes or more, Preferably it is 15 minutes or more, More preferably, it is 20 minutes or more.

In addition, the said holding | maintenance means holding | maintaining at about 1550-1650 degreeC, as implemented in a normal solvent.

On the other hand, when such holding time is too long, fine inclusions aggregate and coarsen, and it is not preferable because the amount of fine Ti-containing oxide defined in the present invention cannot be secured. Therefore, 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 simultaneously so as to the final component amounts, and then maintained as described above, followed by injection.

By solvent in this way, coarse Ti containing oxide can be reduced, ensuring the appropriate amount of fine Ti containing oxide determined by this invention.

A steel raw material (slab) is made into a structure which is subjected to hot rolling after heating, inhibits the development of the aggregate structure along the rolling direction, and recrystallizes at the end of the hot rolling, according to the usual thick steel sheet production method. The steel sheet after completion of hot rolling is quenched. Thereafter, tempering of the steel sheet is performed to obtain a product thick steel sheet.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the present invention is not limited by the following Examples, and of course, the present invention is not limited by the following Examples. Possible and they are all included in the technical scope of this invention.

Example

As shown in Table 3 (invention example) and Table 4 (comparative example), the steels of the chemical composition shown in Table 1 (invention example) and Table 2 (comparative example), the amount of dissolved oxygen before the addition of Ti in the solvent step The holding time in the stationary state after Ti addition was changed in various ways, it was melted and cast, and the slab was obtained. The slab obtained was heated to 1100 ° C., and then rolled and cooled to obtain a steel sheet having a sheet thickness of 50 mm. In addition, in order to adjust the strength and toughness of a base material, the tempering process to 500-650 degreeC was implemented.

The sample was taken from the steel plate obtained in this way, and the base material characteristics, HAZ toughness, and the magnitude | size of the Ti containing oxide which exist in a base material were measured. These results are shown in Table 3 and Table 4.

<Material properties>

JIS No. 4 test pieces were taken from each steel sheet manufactured as described above, and a tensile test in accordance with JISZ2241 was performed to carry out tensile strength of the steel sheet and a Charpy impact test in accordance with J1SZ2242 to measure the toughness (vE-20) of the steel sheet. .

Here, the tensile strength of 590 MPa or more and the Charpy impact value of 150 J or more were evaluated to have excellent base material properties of high strength and high toughness, and to ensure such excellent base material properties, HAZ toughness was evaluated as described below. It was.

<Welding joint toughness>

Thermal cycles corresponding to the thermal history of the HAZ near the melting line (+0.5 mm) when SAW welding the test pieces (size 12.5 mm x 32 mm x 55 mm) drawn out from the steel sheets manufactured above were SAW welded with a large input heat of 100 kJ / mm. Carried out. That is, it heated at 1400 degreeC, hold | maintained at this temperature for 5 second, and then performed the heat cycle which cools from 800 degreeC to 500 degreeC for 730 second. In addition, a heat cycle corresponding to the thermal history of the HAZ in the region of about 3 to 5 mm from the melting line at the time of the high heat input welding was also performed. That is, it heated at 1200 degreeC, hold | maintained at this temperature for 5 second, and performed the heat cycle which cools for 730 second from 800 degreeC to 500 degreeC. From each of these test pieces, the Charpy test piece was sampled, respectively, and vE0 (1400 degreeC and 1200 degreeC) was measured. And the case where vE0 was 150 J or more respectively evaluated the weld joint toughness excellent.

<Ti containing oxide>

The magnitude | size of the Ti containing oxide which exists in a base material was measured with the following method.

<Measurement position (sample collection position)>

The sample was taken in the position of 1/4 of plate | board thickness so that the cross section parallel to a rolling direction can be observed. Using the obtained sample, as described later, 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 was measured, respectively.

<Measurement of the number of coarse Ti-containing oxides having an average particle diameter 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 device 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 magnitude | size of Ti containing oxide calculated | required the round equivalent particle diameter and made it the average particle diameter value (it is the same below).

<Measurement of the number of fine Ti-containing oxides having an average particle diameter of 0.05 to 1.0 mu m>

The composition analysis of 20 inclusions with an average particle diameter of 0.05-1.0 micrometer was performed using said FE-SEM / EDX apparatus, and the ratio of the Ti containing oxide containing 10 mass% or more of Ti was calculated | required. Next, in an area of 0.1mm ^2, by using a 1000-fold reflection electron image, recording an arbitrary 10 fields of view of 0.01mm ² and, by the image analysis device, measuring the number average particle diameter of 0.05 to inclusion of 1.0㎛ The number of Ti-containing oxides having an average particle diameter of 0.05 to 1.0 µm per 1 cm ² was obtained by multiplying the total number of the 10 fields by the ratio of the Ti-containing oxide and further setting it to 1000 times. These results are also shown in Table 3 and Table 4.

As apparent from Tables 1 and 3, Inventive Examples 1 to 20, including the parameters of 20 C (%) + 1.5 Mn (%) + Cu (%) + Ni (%), satisfy the composition of the present invention and It was prepared by the preferred solvent method together with the dissolved oxygen amount and the retention time.

For this reason, when the average number of Ti containing oxides whose average particle diameter is 0.05-1 micrometer is observed by 1000 times the magnification, when it is 10000 piece / cm <^2> or more, and when the average number of Ti oxides whose average particle diameter is 2 micrometers or more is observed by 200 times magnification, 2000 pieces / cm ² or less. As a result, base material strength of 590-780 MPa grade and base material toughness of 210J or more were obtained. Moreover, toughness of 150J or more was also obtained with vE0 of 1400 degreeC and 1200 degreeC of thermal cycling characteristics.

These results, even in the case of high strength 590 to 780 MPa class thick steel sheet, including all the heat affected zone of the weld heat around 3 to 5 mm from the above-described melting line, especially in the toughness tends to be lowered, the whole HAZ area at the time of high heat input welding It shows that the toughness can be greatly improved.

In contrast, as is apparent from Tables 2 and 4, Comparative Examples 21 to 29 and 32 deviate from the scope of the present invention in the component composition of the elements.

In addition, in Comparative Examples 30 and 31, the parameters of 20 C (%) + 1.5 Mn (%) + Cu (%) + Ni (%) are outside the scope of the present invention.

In addition, although the composition of the comparative examples 33-35 exists in the range of this invention, it was manufactured out of the conditions of the solvent method in which dissolved oxygen amount and holding time are preferable.

As a result, the comparative example is relatively low in the base material strength and the base material toughness compared to the invention example, and even if the base material strength and the base material toughness are similar to the invention example, the thermal cycle characteristics are both significantly lower in common at vE0 of 1400 ° C and 1200 ° C.

These results show that the comparative examples which deviate from the scope of the present invention include the heat of 590 to 780 MPa class high strength steel sheet, particularly including the weld heat influence part around 3 to 5 mm from the melting line. It indicates that the toughness of the region cannot be improved.

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

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

In Comparative Example 23, since the amount of Mn is excessive, the definition of 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, coarse Ti-containing oxides increase, resulting in inferior thermal cycle characteristics (HAZ toughness).

In Comparative Example 25, since the amount of Cu is excessive, the definition of 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 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 fine Ti-containing oxide is too small, resulting in poor thermal cycle characteristics (HAZ toughness).

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

In Comparative Example 29, since the amount of oxygen is excessive and the amount of dissolved oxygen at the time of solvent is also excessive, there are too many coarse Ti-containing oxides, too few fine Ti-containing oxides, and the thermal cycle characteristics (HAZ toughness) are deteriorated.

Since the amount of boron is excessive in Comparative Example 32, the specification of the Ti-containing oxide is satisfied, but the base metal toughness and heat cycle characteristics (HAZ toughness) are inferior.

In Comparative Example 30, the parameters of 20 C (%) + 1.5 Mn (%) + Cu (%) + Ni (%) are less than 4.5%, which deviates from the scope of the present invention. For this reason, in spite of containing C, Mn, Cu, and Ni within the prescribed amount ranges, the thermal cycle characteristics at 1200 ° C are inferior, and the toughness of the HAZ 3 to 5 mm portion is not improved. This is because the above hardenability increasing effects of C, Mn, Cu, and Ni are not exhibited, and the intravenous generated bainite structure cannot be refined.

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

In Comparative Example 33, since the amount of dissolved oxygen before the addition of Ti in the solvent step is too large, the number of coarse Ti-containing oxides is mostly precipitated and exceeds the prescribed number. For this reason, toughness is remarkably low about 102J at vE0 of 1400 degreeC in heat cycle characteristics.

In Comparative Example 34, since the holding time in the stationary state after the addition of Ti in the solvent step is too short, most of the coarse Ti-containing oxides are precipitated, exceeding the prescribed number, and there are relatively few fine Ti-containing oxides on one side. For this reason, toughness is remarkably low about 85J in vEO of 1400 degreeC in heat cycle characteristics.

In Comparative Example 35, since the holding time in the stationary state after the addition of Ti in the solvent step is too long, most of the coarse Ti-containing oxides are precipitated, exceeding the prescribed number, and on the other hand, there are relatively few fine Ti-containing oxides. For this reason, toughness is remarkably low about 99J at vE0 of 1400 degreeC in heat cycle characteristics.

From the above results, in the case of a thick steel sheet of 590 to 780 MPa class having a critical significance, that is, a high strength of the component composition of the present invention and the Ti-containing oxide definition, in particular, the inclusion of a weld heat influence part near 3 to 5 mm from the melting line is included. The critical significance of improving the toughness of the entire HAZ area during thermal welding is supported.

As described above, the present invention can provide a thick steel sheet of 590 to 780 MPa class which is excellent in weld seam toughness (HAZ toughness) even when high heat input welding is performed. For this reason, the thick steel plate of this invention can be applied for high strength welded structures, such as a ship, an offshore structure, a bridge, and a building structure.

Claims (4)

By mass%, 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% (not containing 0%), Ti: 0.005 to 0.050% and O: 0.0010 to 0.0100%, respectively, further containing any one or more of Cu: 0.1 to 1.5% and Ni: 0.1 to 2.5%, and also 20C (%) + 1.5Mn (%) + Cu (%) + Ni (%) is 4.5 to 7.0%, the remainder is iron and inevitable impurities, When the average number of Ti-containing oxides having an average particle diameter of 0.05 to 1 µm and containing 10% by mass or more of Ti is 1000 times the magnification, the average particle diameter is 10000 / cm ² or more, and the average particle diameter is 2 µm or more and 10 mass% of Ti. When the average number of Ti containing oxide contained above is observed by 200 times the magnification, it is 2000 piece / cm <^2> or less, The steel plate excellent in the high heat input weld joint toughness characterized by the above-mentioned. The method of claim 1, Furthermore, any one or two of Cr: 0.1 to 2.0%, Mo: 0.1 to 1.0%, Nb: 0.10% or less (0%) and V: 0.10% or less (0%) are not included. The thick steel plate excellent in the high heat input welding joint toughness characterized by containing the above. The method of claim 1, B: 0.0005 to 0.0050% of a thick steel sheet excellent in high heat input weld joint toughness. The method of claim 1, Additionally Zr: 0.05% or less (does not contain 0%), Mg: 0.0050% or less (does not contain 0%), Ca: 0.0050% or less (does not contain 0%), and REM: 0.0050% or less ( A thick steel sheet excellent in high heat input weld joint toughness, characterized by containing any one or two or more kinds thereof.